RESEARCH - Jay Chapman · 2019. 10. 28. · Afghanistan Pakistan India Tajikistan Kyrgyzstan Figure...
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1 RESEARCH Cretaceous shortening and exhumation history of the South Pamir terrane James B. Chapman 1, *, Alexander C. Robinson 2 , Barbara Carrapa 1 , Dustin Villarreal 2 , James Worthington 1 , Peter G. DeCelles 1 , Paul Kapp 1 , Mustafo Gadoev 3 , Ilhomjon Oimahmadov 3 , and George Gehrels 1 1 DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721, USA 2 DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF HOUSTON, HOUSTON, TEXAS 77204, USA 3 INSTITUTE OF GEOLOGY, EARTHQUAKE ENGINEERING AND SEISMOLOGY, TAJIK ACADEMY OF SCIENCES, DUSHANBE, TAJIKISTAN ABSTRACT Despite Miocene extension and exhumation of middle to lower crust in a series of gneiss domes and interpreted Cenozoic delamination of the lower crust, the crust in the modern Pamir Mountains is among the thickest in the world. Cenozoic shortening, crustal thickening, and prograde metamorphism in the Pamir have been associated with India-Asia collision. However, new mapping in the South Pamir terrane indicates relatively minor, distributed shortening since the Jurassic, which occurs in a thrust belt overprinted by late Cenozoic transpres- sion. The thrust belt connects with the Rushan-Pshart suture zone, a Mesozoic terrane boundary. New detrital zircon U-Pb and detrital zircon fission track ages of synorogenic clastic rocks exposed in the footwall of thrust faults in the South Pamir thrust belt provide maximum deposition ages (76–112 Ma), which are interpreted to document Cretaceous shortening prior to India-Asia collision. Furthermore, zircon (U-Th)/He and apatite (U-Th)/He data from the South Pamir terrane generally record cooling ages of ca. 102–44 Ma, suggesting limited Cenozoic exhumation. These results (1) are consistent with widespread Cretaceous deformation throughout the Pamir-Tibet orogen with limited Cenozoic upper crustal shortening in the South Pamir terrane, (2) together with previous studies, allow for the possibility that the upper crust of the Pamir orogen was characterized by net extension during the Cenozoic rather than net shortening, and (3) are consistent with models that relate Cenozoic crustal thickening to the insertion of Indian lower crust beneath the Pamir. Lower crustal thickening of the South Pamir terrane is difficult to reconcile with the prograde metamorphic history of gneiss domes in the South Pamir terrane and may require a relatively shallow (<15–20 km) shear zone separating lower crustal contraction from upper crustal extension. LITHOSPHERE GSA Data Repository Item 2018167 https://doi.org/10.1130/L691.1 1. INTRODUCTION The Pamir Mountains form a high-elevation orogenic plateau and high- relief mountain range that is equivalent and contiguous with the Tibetan plateau to the east. Similar to the Tibetan plateau, the modern thickness of continental crust in the Pamir is ~70 km (Mechie et al., 2012; Schneider et al., 2013). Unlike the Tibetan plateau, the Pamir Mountains contain widespread exposures (~30% of the surface area) of middle to lower crust within a series of extensional gneiss domes (Robinson et al., 2004, 2007; Schmidt et al., 2011; Stübner et al., 2013a; Stearns et al., 2013, 2015; Rutte et al., 2017a) (Fig. 1). The middle to lower crustal exposures in the Pamir gneiss domes record chiefly Eocene to Oligocene ages that have been interpreted to represent prograde metamorphism associated with crustal thickening in response to India-Asia collision (Robinson et al., 2007; Schmidt et al., 2011; Stearns et al., 2013; 2015; Smit et al., 2014; Hacker et al., 2017; Rutte et al., 2017a). Previous studies have suggested at least 300 km of Cenozoic internal shortening in the Pamir (Burtman and Molnar, 1993). However, both the timing and magnitude of this interpreted shortening may be incorrect. For example, the Shakhdara-Alichur gneiss dome was previously interpreted as a basement-involved thrust nappe (Shvol’man, 1978; Burtman and Molnar, 1993), but subsequent analysis has shown that it is extensional in origin (Stübner et al., 2013a). Aside from the mechanisms involved in the formation of the gneiss domes, the ages of the contractional structures within the Pamir are poorly constrained. Robinson (2015) hypothesized that many of these structures may be of Cretaceous age and that Pamir crust was in part thickened prior to India- Asia collision, during Mesozoic Cordilleran (i.e., Andean-style) orogen- esis and accretion of Gondwanan terranes. There is evidence along strike in Tibet for precollisional (Mesozoic) crustal shortening and thickening in a retroarc position (Allégre et al., 1984; Murphy et al., 1997; Kapp et al., 2005, 2007a, 2007b; Raterman et al., 2014), which may provide a template for understanding the Mesozoic tectonic evolution of the Pamir. Key to understanding the mechanisms involved in crustal thickening and orogenic growth in the Pamir is establishing the timing and magnitude of upper-crustal shortening. This study investigates the Mesozoic to recent geological history of the South Pamir terrane, the largest of several accreted terranes that com- prise the modern Pamir (Burtman and Molnar, 1993; Schwab et al., 2004) (Fig. 1). The South Pamir terrane hosts the Karakoram and South Pamir batholiths, which are remnants of a Cretaceous continental arc associ- ated with northward subduction of oceanic lithosphere (Debon et al., 1987; Crawford and Searle, 1992; Schwab et al., 2004; Ravikant et al., 2009; Aminov et al., 2017) (Fig. 2). The South Pamir terrane also hosts © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY-NC license. James Chapman http://orcid.org/0000-0002-1145-4687 *Corresponding author
Transcript of RESEARCH - Jay Chapman · 2019. 10. 28. · Afghanistan Pakistan India Tajikistan Kyrgyzstan Figure...
LITHOSPHERE | Volume 10 | Number 3 | www.gsapubs.org 1
RESEARCH
Cretaceous shortening and exhumation history of the South Pamir terrane
James B. Chapman1,*, Alexander C. Robinson2, Barbara Carrapa1, Dustin Villarreal2, James Worthington1, Peter G. DeCelles1, Paul Kapp1, Mustafo Gadoev3, Ilhomjon Oimahmadov3, and George Gehrels1
1DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, TUCSON, ARIZONA 85721, USA2DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF HOUSTON, HOUSTON, TEXAS 77204, USA3INSTITUTE OF GEOLOGY, EARTHQUAKE ENGINEERING AND SEISMOLOGY, TAJIK ACADEMY OF SCIENCES, DUSHANBE, TAJIKISTAN
ABSTRACT
Despite Miocene extension and exhumation of middle to lower crust in a series of gneiss domes and interpreted Cenozoic delamination of the lower crust, the crust in the modern Pamir Mountains is among the thickest in the world. Cenozoic shortening, crustal thickening, and prograde metamorphism in the Pamir have been associated with India-Asia collision. However, new mapping in the South Pamir terrane indicates relatively minor, distributed shortening since the Jurassic, which occurs in a thrust belt overprinted by late Cenozoic transpres-sion. The thrust belt connects with the Rushan-Pshart suture zone, a Mesozoic terrane boundary. New detrital zircon U-Pb and detrital zircon fission track ages of synorogenic clastic rocks exposed in the footwall of thrust faults in the South Pamir thrust belt provide maximum deposition ages (76–112 Ma), which are interpreted to document Cretaceous shortening prior to India-Asia collision. Furthermore, zircon (U-Th)/He and apatite (U-Th)/He data from the South Pamir terrane generally record cooling ages of ca. 102–44 Ma, suggesting limited Cenozoic exhumation. These results (1) are consistent with widespread Cretaceous deformation throughout the Pamir-Tibet orogen with limited Cenozoic upper crustal shortening in the South Pamir terrane, (2) together with previous studies, allow for the possibility that the upper crust of the Pamir orogen was characterized by net extension during the Cenozoic rather than net shortening, and (3) are consistent with models that relate Cenozoic crustal thickening to the insertion of Indian lower crust beneath the Pamir. Lower crustal thickening of the South Pamir terrane is difficult to reconcile with the prograde metamorphic history of gneiss domes in the South Pamir terrane and may require a relatively shallow (<15–20 km) shear zone separating lower crustal contraction from upper crustal extension.
LITHOSPHERE GSA Data Repository Item 2018167 https://doi.org/10.1130/L691.1
1. INTRODUCTION
The Pamir Mountains form a high-elevation orogenic plateau and high-relief mountain range that is equivalent and contiguous with the Tibetan plateau to the east. Similar to the Tibetan plateau, the modern thickness of continental crust in the Pamir is ~70 km (Mechie et al., 2012; Schneider et al., 2013). Unlike the Tibetan plateau, the Pamir Mountains contain widespread exposures (~30% of the surface area) of middle to lower crust within a series of extensional gneiss domes (Robinson et al., 2004, 2007; Schmidt et al., 2011; Stübner et al., 2013a; Stearns et al., 2013, 2015; Rutte et al., 2017a) (Fig. 1). The middle to lower crustal exposures in the Pamir gneiss domes record chiefly Eocene to Oligocene ages that have been interpreted to represent prograde metamorphism associated with crustal thickening in response to India-Asia collision (Robinson et al., 2007; Schmidt et al., 2011; Stearns et al., 2013; 2015; Smit et al., 2014; Hacker et al., 2017; Rutte et al., 2017a). Previous studies have suggested at least 300 km of Cenozoic internal shortening in the Pamir (Burtman and Molnar, 1993). However, both the timing and magnitude of this interpreted shortening may be incorrect. For example, the Shakhdara-Alichur gneiss dome was previously interpreted as a basement-involved thrust nappe
(Shvol’man, 1978; Burtman and Molnar, 1993), but subsequent analysis has shown that it is extensional in origin (Stübner et al., 2013a). Aside from the mechanisms involved in the formation of the gneiss domes, the ages of the contractional structures within the Pamir are poorly constrained. Robinson (2015) hypothesized that many of these structures may be of Cretaceous age and that Pamir crust was in part thickened prior to India-Asia collision, during Mesozoic Cordilleran (i.e., Andean-style) orogen-esis and accretion of Gondwanan terranes. There is evidence along strike in Tibet for precollisional (Mesozoic) crustal shortening and thickening in a retroarc position (Allégre et al., 1984; Murphy et al., 1997; Kapp et al., 2005, 2007a, 2007b; Raterman et al., 2014), which may provide a template for understanding the Mesozoic tectonic evolution of the Pamir. Key to understanding the mechanisms involved in crustal thickening and orogenic growth in the Pamir is establishing the timing and magnitude of upper-crustal shortening.
This study investigates the Mesozoic to recent geological history of the South Pamir terrane, the largest of several accreted terranes that com-prise the modern Pamir (Burtman and Molnar, 1993; Schwab et al., 2004) (Fig. 1). The South Pamir terrane hosts the Karakoram and South Pamir batholiths, which are remnants of a Cretaceous continental arc associ-ated with northward subduction of oceanic lithosphere (Debon et al., 1987; Crawford and Searle, 1992; Schwab et al., 2004; Ravikant et al., 2009; Aminov et al., 2017) (Fig. 2). The South Pamir terrane also hosts
© 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY-NC license.
James Chapman http://orcid.org /0000 -0002 -1145 -4687*Corresponding author
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the Shakhdara-Alichur extensional gneiss dome, the largest gneiss dome in the Pamir and one of the largest in the world (Stübner et al., 2013a) (Fig. 1). As a result, lower crustal deformation in the Shakhdara-Alichur gneiss dome can be compared directly to upper crustal deformation in the hanging wall of the gneiss dome. We conducted geological mapping and integrated stratigraphic, geochronological, thermochronological, and structural analyses to assess the geological evolution of the South Pamir terrane and its role in the growth of the Pamir orogen. The results provide new insight into the timing of upper crustal shortening and thickening in the region with implications for (1) mechanisms for crustal thickening in Cordilleran-style and continental collisional orogens, (2) the extent of coupling between the upper and lower crust during convergence, and (3) the tectonic evolution of the Pamir.
2. GEOLOGIC SETTING
The South Pamir terrane is bounded to the north by the Rushan-Pshart suture zone and the Central Pamir terrane (Burtman and Molnar, 1993). Upper Triassic to Lower Jurassic intrusive rocks on the northern margin of the South Pamir terrane (Fig. 3) may record the closure of the Rushan-Pshart ocean basin that separated the Central and South Pamir terranes (Schwab et al., 2004). The Central Pamir and South Pamir terranes are both Gondwanan fragments that were accreted to each other and to the southern margin of Asia during the Cimmerian orogeny in the Late Tri-assic to Early Jurassic (Angiolini et al., 2013). Prior to the accretion of the Central and South Pamir terranes, the southern Asian margin was occupied by a Triassic continental arc with a well-developed accretionary complex (Xiao et al., 2002; Schwab et al., 2004). Commonly, the upper Paleozoic southern Asian margin is referred to as the Kunlun “terrane,” the arc/accretionary complex is referred to as the Karakul-Mazar “terrane,” and collectively they are termed the North Pamir “terrane” (Xiao et al., 2002; Schwab et al., 2004; Robinson et al., 2012) (Fig. 1). Although the
para-autochthonous North Pamir is not a terrane sensu stricto, we retain the terrane designation here to facilitate comparisons with previous stud-ies (e.g., Burtman and Molnar, 1993).
When viewed down-plunge, from west to east, the South Pamir terrane in Tajikistan presents a crustal section with the lower crust exposed in the core of the Shakhdara gneiss dome to the west (Schmidt et al., 2011; Stübner et al., 2013a) and non-metamorphosed sedimentary and volcanic rocks of the upper crust exposed in the east (Vlasov et al., 1991) (Figs. 1 and 2). The South Pamir terrane is truncated on its eastern margin by the Karakoram fault (Robinson, 2009) and on its southern margin by the Tirich-Kilik suture, which separates the South Pamir terrane from the Karakoram terrane (Zanchi et al., 2000; Zanchi and Gaetani, 2011) (Fig. 2). Metamorphic ages suggest that the Karakoram terrane was accreted to the South Pamir terrane during the Late Jurassic to Early Cretaceous (Zanchi et al., 2000; Hildebrand et al., 2001), although sedimentological evidence suggests accretion by the Early Jurassic (Zanchi and Gaetani, 2011; Angiolini et al., 2013; Gaetani et al., 2013). The South Pamir terrane is equivalent to the Qiangtang terrane in the Tibetan plateau (Robinson, 2009; Robinson et al., 2012; Angiolini et al., 2013).
2.1. South Pamir Stratigraphy and Map Units
The oldest exposed rocks in the area investigated (Figs. 2–4) are Perm-ian to Triassic metasedimentary deposits (Vlasov et al., 1991) (map unit P-Tr). These rocks are predominantly dark-gray to black phyllite and slate with tan to dark-gray limestone and dolostone that cumulatively represent a rift-to-drift succession (Angiolini et al., 2015). The Permian-Triassic rocks were complexly deformed, uplifted, and eroded during the Cim-merian orogeny (Angiolini et al., 2013). Locally overlying the Permian-Triassic unit in angular unconformity is the Lower Jurassic Darbasatash Group (map unit Jr
D), which consists of nonmarine, red, coarse-grained
sandstone and conglomerate (Dronov et al., 2006; Angiolini et al., 2013).
40°N
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Tajik Basin andThrust Belt
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NorthPamir
Karakoram
Kohistan
Indian PlateCHM
F
Tanymas suture
Rushan-Pshart suture
Shyok suture
MPT
MPT
In dus suture
Karakoram Fault
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ral Pamir
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Kongur Shan
Tirich-Kilik suture
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ush
China
Afghanistan
Pakistan
India
Tajikistan
Kyrgyzstan
Figure 1. Overview map of the Pamir Moun-tains. CHMF—Chaman fault, MPT—main Pamir thrust. Gneiss dome locations adopted from Stearns et al. (2015).
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Shorkul Fault
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internationalbordertown
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Tajikistan China
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Figure 3Figure 4
Alichur Gneiss Dome
Karakoram Batholith
Muskol Gneiss Dome
Tirich-Kilik suture
SarezGneissDome
South PamirBatholith
ShatputGneiss Dome
Wakhan corridor
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Mamazair
Kyzylrabot
Bash Gumbez
Alichur Alichur Shear Zone Jarty Gumbez
Tokhtamish
Taxkorgan
South Muskol Shear Zone
Aksu fault
Figure 2. Regional geologic map of the eastern Pamir. Map location is shown in Figure 1. The geologic map is compiled from results of this study, Vlasov et al. (1991), Lindsay (2007), Burg (2011), Zanchi and Gaetani (2011), and Robinson et al. (2004, 2007, 2012). Labeled circles are samples of the Mamazair conglomerate or Murghab Basin strata analyzed in this study.
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Jura
ssic Upper Jurassic (?), finely bedded
gray shale-siltsone and thin limestone
recorded in field
adopted from Vlasov et al., 1991
measured using remote sensing
sample location
topographic contour, 200m interval
6 km
Triassic undifferentiated, fine-grained black phyllite to slate
Jurassic undifferentiated, light brownto yellow limestone pebblemeta-conglomerate, sandstone, andcalc-silicate phyllite, found in CentralPamir terrane only
(Meta) Sedimentary Rocks
Rushan-Pshart suture
Cre
tace
ous Mamazair conglomerate, red
sandstone to pebble conglomerate
Basalt, inferred Cretaceous age
Paleozoic undifferentiated
karst breccia
Alichur Shear Zone
Karasu Fault
A
A’
Murghab
Mamazair Fault
Figure 8
sample 14-73U-Pb: 198 ± 7 MaZHe: 17 ± 3 Ma
sample 14-60U-Pb: 109 ± 5 MaZHe: 20 ± 7 Ma
sample 14-89U-Pb: 104 ± 3 MaZHe: 102 ± 11 MaAHe: 56 ± 17 Ma
sample GUM-02U-Pb peak: 112 MaZFT peak: 103 Ma
sample 14-56U-Pb peak: 84 Ma
no zircon
Mamazair
JartyGumbez
Kb
Km
Jr
Tr
Pz
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Trv
Jru
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Upper Gurumdi Gr., massive tan tolight gray fossiliferous limestone, karst
Lower Gurumdi Gr., flaggy, lightbrown siltstone and thin limestone
Darbasatash Gr., dark red carbonate-chert conglomerate and siltstone
Triassic (?) meta-volcanic rocks
P-TrPermian Triassic undifferentiated,dark gray-black, dark red, dark greenphyllite to slate, limestone, dolomite
Tria
ssic
Kg
Q Quaternary undifferentiated
Cretaceous granitoid
Kg
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Igneous Rocks
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ital
Valle
y
sample 13P101U-Pb: 120 ± 4 MaZHe: 34 ± 4 Ma
sample 16-22ZHe: 15 ± 1 Ma
Murghab Basin strata, orange togreen siltstone and dolostone, redsandstone to cobble conglomerate
Kmb
sample 13P99ZHe: 69 ± 13 Ma
Murghab River Valley
sutu
re z
one
sample 16-23AHe: 46 ± 16 Ma
high-angle normal fault,barb on downthrown block
low-angle normal fault,hat on hanging wall
thrust fault, tick markon hanging wall
trace of fold axial plane, solidarrow indicates plunge direction
U-Pb: (igneous age)
U-Pb peak: (youngest agepopulation in detrital sample)
ZFT peak: (youngest agepopulation in detrital sample)
faults and fold axes dashedwhere inferred or covered
P-Tr
Jr-K
Jr-K
Jr-KEtchki Tushar and Bakalash Fm.from Leven (1995), found only inRushan-Pshart suture zone
KmbKmb
Figure7b
Figure 7a
Aksu Fault
?
Figure 3. Geologic map of part of the South Pamir terrane, based on field mapping. Map location is shown in Figure 2. Sample information is presented in Table 1. Fm.—Formation; Gr.—Group; ZFT—zircon fission-track; AHe— apatite (U-Th)/He thermochro-nology; ZHe— zircon (U-Th)/He thermochronology.
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The Darbasatash Group is overlain conformably by Jurassic limestone belonging to the Gurumdi Group (Dronov et al., 2006; Angiolini et al., 2013). For mapping purposes, we informally divided the Gurumdi Group into lower (Jr
Gl) and upper (Jr
Gu) units (Figs. 3, 5A, and 5B). The lower
unit consists of tan to dark-brown, thinly bedded limestone and siltstone that vary in thickness from a few tens of meters to ~300 m. The upper unit consists of light-gray to white, massive, fossiliferous limestone up to ~400 m thick. We did not distinguish specific Jurassic formations dur-ing mapping. The fossiliferous upper unit is interpreted to be equivalent to the Gurumdi Group reef facies of Dronov et al. (2006). A fine-grained unit of thinly bedded gray to tan limestone, shale, and siltstone locally overlies the upper Gurumdi Group and was mapped as upper Jurassic (Jr
u), although its age is unknown.Unconformably overlying the Jurassic section in the South Pamir ter-
rane are nonmarine red to orange conglomerate and coarse-grained sand-stone previously mapped as undifferentiated Paleogene(?) (Vlasov et al., 1991). We refer to this informal lithostratigraphic unit as the Mamazair conglomerate (K
m), named after the village of Mamazair, following strati-
graphic nomenclature rules defined by the International Stratigraphic Code (Salvador, 1994). North of the Rushan-Pshart suture zone, in the Central Pamir terrane, red conglomerate and sandstone mapped as undifferentiated Paleogene(?) by Vlasov et al. (1991) unconformably overlie Jurassic and older rocks. We adopt the nomenclature of Rutte et al. (2017a, 2017b) who referred to this unit as the “Murghab Basin” (map unit K
mb). Sparse
paleontological data at the base of the Murghab Basin strata suggest that the unit is of Maastrichtian age (Yushin et al., 1964). Additional rocks mapped as undifferentiated Paleogene(?) by Vlasov et al. (1991) are pres-ent north of the Muskol gneiss dome in the northern Central Pamir terrane (Fig. 2), but these rocks were not investigated in this study. Leven (1995) described Jurassic?-Cretaceous metasedimentary rocks of the Bakalash Formation in the Rushan-Pshart suture zone, between the South and Cen-tral Pamir terranes (Fig. 3). The Bakalash Formation was not investigated in this study, but could be correlative with either the Murghab Basin or the
Mamazair conglomerate. Igneous rocks, including granitoid and andesite are exposed in the South Pamir terrane and are mapped according to their radiometric ages (Figs. 3 and 4).
3. GEOLOGIC MAPPING
Geologic mapping of the South Pamir terrane at 1:50,000 scale, con-ducted over about eight weeks spanning the 2014 and 2015 field seasons, focused on a transect from the Shakhdara-Alichur gneiss dome in the south to the Rushan-Pshart suture zone in the north (Figs. 3, 6, and 7), and a region near the Kyzylrabot village in the southeast Pamir (Fig. 4). The regional geologic map in Figure 2 is adopted from previous geologic mapping in the Pamir (Vlasov et al. 1991; Robinson et al., 2004, 2007, 2012; Lindsay, 2007; Stübner et al., 2013a; Rutte et al., 2017a), Karakoram (Zanchi and Gaetani, 2011), and Kohistan (Burg, 2011). Compiled maps were locally supplemented with mapping based on spectral-band combina-tions from Landsat data and satellite imagery, which were mainly used to fill in gaps between previously published maps, check the geometry of unit boundaries and contacts, and reconcile differences between existing maps.
3.1. Stratigraphy and Sedimentology
New mapping in the South Pamir terrane is generally consistent with previous mapping in the region (Vlasov et al., 1991). One of the main features mapped is the post-Cimmerian angular unconformity above the Permian-Triassic rocks and below the Darbasatash Group, which was recently recognized and defined by Angiolini et al. (2013). The post-Cimmerian unconformity is the primary stratigraphic feature used to distinguish the Jurassic carbonate section from carbonate units within the Permian-Triassic section (Figs. 5A and 5B). The Darbasatash Group varies in thickness from 0 to ~200 m and appears to fill in local topo-graphic lows on the post-Cimmerian unconformity surface. In some areas, the Darbasatash Group contains a basal, clast-supported, well-rounded
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Kyzylrabot
sample 14-76U-Pb: 105 ± 3 Ma
sample 14-74U-Pb: 111 ± 3 Ma
sample 14-77U-Pb: 106 ± 3 Ma
sample 14-88U-Pb: 105 ± 3 Ma
sample 14-87U-Pb: 105 ± 3 MaZHe: 96 ± 10 MaAHe: 44 ± 19 Ma
sample DV-7-16-15-2U-Pb peaks: 97 Ma (n=5), 104 Ma (n=17)
ZFT peaks: 111 Ma, 264 Ma
sample 14-83/85U-Pb peaks: 76 Ma (n=3), 104 Ma (n=208)
Aksu River
5 km
Km
sample DV-7-15-15-7U-Pb peak: 157 Ma (n=5)
Km
Jr
interbeddedrhyolite tuff
Figure 4. Geologic map of the southeastern part of the South Pamir terrane, near the village of Kyzylrabot, based on field mapping. Map location is shown in Figure 2. Kv—Cretaceous volcanic rocks, Jr—undifferentiated Jurassic rocks, Qf—Quaternary fluvial deposits; otherwise the map units, symbols, and labels are the same as in Figure 3. Sample information is presented in Table 1.
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CHAPMAN ET AL. | Crustal shortening in the Pamir RESEARCH
Karasu Fault
up-section
bedd
ing
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Darbasatash Gr.
upper unit Gurumdi Gr.
lower unitGurumdi Gr.
Permian-Triassic
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Gr.
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Karst breccia
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basalt flow
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Unit 3
Unit 2
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azai
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bedding
car
Mamazaircong.
A B
C D
E
F
G H
I J K L
Figure 5. Photographs from the South Pamir terrane. (A) Cimmerian angular unconformity (ang. uncf.) beneath the Darbasatash Group (Gr.) (ridge loca-tion: 37.73N, 73.83E). (B) Representative stratigraphy of the South Pamir terrane (peak location: 38.0N, 74.07E). (C) Basal conglomerate (cong.) in the Darbasatash Group (hammer height is ~42 cm; location: 37.77N, 73.245E). (D) Distinctive red and yellow clasts in the Darbasatash Group (pen height is ~14 cm; location: 74.05N, 37.75E). (E) Monomict limestone karst breccia in the upper Gurumdi Group (hammer height ~33 cm; location: 37.95N, 74.0E). (F) Paleokarst in the upper Gurumdi Group (ridge location: 37.92N, 74.06E). (G) Faulted strata of the Murghab Basin and basalt flow; Jurassic(?) rocks are conglomerates of the Central Pamir terrane (hillside location: 38.26N, 74.06E). (H) Growth strata in the Mamazair conglomerate displaying progres-sive bedding rotation and internal angular unconformities (ball-stick symbols show bedding dip; filled symbols are upright bedding; hollow symbols are overturned; hillside location: 37.843N, 74.096E). (I) Similar location to H. Folded Mamazair conglomerate in the footwall of the Mamazair fault (M. Gadoev for scale; outcrop in foreground location: 37.842N, 79.091E). (J) Recycled conglomerate clasts in the Mamazair conglomerate (hammer height ~33 cm; location same as H). (K) Informal units of the Mamazair conglomerate in the footwall of the Kyzylrabot fault (hillside location: 37.47N, 74.76E). (L) Volcanic pebble-cobble conglomerate in the Mamazair conglomerate. Part of Unit 3 shown in photo K (hammer height ~33 cm; location similar to photo K). Fm.—Formation.
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CHAPMAN ET AL. | Crustal shortening in the Pamir RESEARCH
pebble-to-cobble conglomerate up to ~10 m thick, composed of pur-ple and green quartzite clasts (Fig. 5C). Elsewhere in the map area, the Darbasatash Group consists of dark red sandstone to subrounded pebble conglomerate and is characterized by “ketchup and mustard” colored sandstone (red) and dolostone (yellow) clasts (Fig. 5D). Previous petro-graphic analyses of sandstones of the Darbasatash Group by Angiolini et al. (2013) indicate that it was derived in part from a volcanic arc. The Darbasatash Group is interpreted to have been deposited subaerially.
The lower unit of the Jurassic Gurumdi Group is in depositional contact with the Darbasatash Group and consists of several tan to gray tabular limestone beds up to several meters thick separated by relatively thin (~1 m) shale beds, giving the outcrops a flaggy appearance (Fig. 5B). The upper unit of the Gurumdi Group consists of massive white to light-gray fossiliferous limestone and contains widespread features indicative of dis-solution and paleokarst (cf. James and Choquette, 1988). Sheet, pod-like, and irregularly shaped bodies of clast-supported, angular to subrounded, monomict, light- to dark-gray limestone pebble to granule breccia with red clay to red siltstone matrix are common throughout the map area (Fig. 5E). Limestone clasts are almost exclusively derived from the local host rock.
Breccias that are laterally continuous for up to a few km are interpreted as mantling karst breccias and paleosols (e.g., north of Jarty Gumbez, Fig. 3). Pod-like and irregular exposures are up to ~150 m in diameter and are interpreted as collapse and intraformational karst breccias (Fig. 5F).
3.1.1. Mamazair ConglomerateThroughout much of the map area in the South Pamir terrane, the
Gurumdi Group is the youngest sedimentary unit present, apart from Quaternary deposits (Vlasov et al., 1991). However, in a few locations, the Mamazair conglomerate unconformably overlies the upper unit of the Gurumdi Group or older units. We made a concerted effort to visit and sample every known location of the Mamazair conglomerate in the southeastern Pamir (Fig. 2).
The largest contiguous outcrop of the Mamazair conglomerate is located ~40 km south-southeast of the town of Murghab and ~20 km east of the village of Mamazair, in the footwall of the Mamazair thrust fault (Figs. 3 and 6). The Mamazair conglomerate is exposed in an open, upright to overturned syncline and rests unconformably on the upper Gurumdi Group (Fig. 6). In a measured section of the Mamazair conglomerate at
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covered
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'E
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ang. uncf.
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Darbasatash Gr.
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ssic
Lower Gurumdi Gr.
Upper Gurumdi Gr.
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tace
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Quat. slope wash
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overturned bedding
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Mamazair Fault
Q
Q
Kmr
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Kgo
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JGu
JGu
Jl
Qa
Q
Kmo
JGuJl
Kmo Kmr
Kmr
Mam
azairconglom
erate
Mamazairconglomerate
Figure 6. Geologic map, cross section, and measured section of the Mamazair conglomerate located in the footwall of the Mamazair fault. Map location is shown in Figure 3. Quat.—Quaternary; LS—limestone; Gr.—Group; ang. uncf.—angular unconformity; cong.—conglomerate; VE—ver-tical exaggeration; c—clay; s—silt; vf—very fine; f—fine; m—medium; c—coarse; vc—very coarse; g—granule; p—pebble; c—cobble; b—boulder.
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CHAPMAN ET AL. | Crustal shortening in the Pamir RESEARCH
this location, it is ~420 m thick and contains at least two intraformational angular unconformities (Figs. 5H, 5I, and 6). The younger unconformity delineates a prominent color change from dark red (older) to pale orange (younger) (Figs. 5H and 6). The lower, red half of the section is not pres-ent in the southern limb of the syncline, which suggests it was eroded or not deposited (Fig. 6). The predominant lithofacies in the lower, red section comprise well-organized, horizontally stratified, clast-supported, rounded to subangular, pebble-to-cobble conglomerate. Individual beds are up to ~2 m thick and massive. Thin (10s of cm) and laterally dis-continuous lenses of red, coarse-grained sandstone are locally present between conglomerate beds. Gray limestone clasts are most abundant, but red sandstone, dolostone, chert, and quartzite are also common. Granitic and volcanic clasts are absent. The limestone and dolostone clasts are similar in appearance to exposures of Permian-Triassic carbonate rocks exposed in the hanging wall of the Mamazair thrust fault and elsewhere in the region. In the lower 200 m of the section (closest to the Mamazair thrust fault), the maximum clast size reaches ~1 m and recycled clasts of the Mamazair conglomerate are common (Fig. 5J). Dominant lithofacies in the upper, orange section include stratified clast-supported granule-to-pebble conglomerate and massive, coarse-grained sandstone with faint planar lamination. The conglomerate is arranged in 20–70-cm-thick beds, stacked into 2–3-m-thick bedsets, and capped by sharp surfaces marking a transition to sandstone. The intraformational angular unconformities in the Mamazair conglomerate at the Mamazair fault locality are progres-sively tilted so that the older unconformities dip more steeply than the younger unconformities (Figs. 5H, 5I, and 6). The oldest unconformities are overturned along with the older conglomerate beds. Bedding in the youngest part of the section has the shallowest dip angles (~15°) and is upright. The angular unconformities, recycled clasts, and progressive tilting of bedding attitudes are interpreted to represent growth strata and
suggest that the rocks were deposited in a growing footwall syncline dur-ing slip on the Mamazair thrust fault.
Approximately 90 km southeast of the Mamazair locality (Figs. 2 and 4), the Mamazair conglomerate is exposed in the footwall of the Kyzylrabot thrust fault where it consistently dips moderately toward the north-northeast (Fig. 4). The Kyzylrabot section can be divided into four informal units (units 1–4; lowest to highest) (Fig. 5K). Unit 1 is exposed along the small creek valley that drains into the Aksu River, located west of the Kyzylrabot village. Unit 1 consists of brown and red, fine- to medium-grained sandstone with occasional beds of matrix-supported, subangular, quartzite granule conglomerate up to 1.5 m thick. Trough cross-stratification is locally present in the sandstone beds. Unit 2 is an easily weathered, light orange and pink, clast-supported, angular, lime-stone pebble conglomerate with interbedded arkosic, pink, coarse-grained sandstone (Fig. 5K). Volcanic and granitic clasts are absent. Unit 3 is a dark-purple to brown, clast-supported, subangular, andesite and limestone clast, pebble-cobble conglomerate (Fig. 5K and 5L). Volcanic clasts are similar in appearance to andesite exposed ~20 km to the west (Fig. 4). Unit 4 is a dark-red, matrix-supported, angular, limestone and sandstone clast, granular to pebbly conglomerate with interbedded sandstone layers. Limestone clasts are dominant in the lower part of the unit but decrease in abundance up-section, where red sandstone clasts are more common. No volcanic or granitic clasts are present in Unit 4. Also in the Kyzylrabot area, ~5 km east of the lake Salon-kul (Fig. 4), the Mamazair conglomer-ate rests unconformably on Cretaceous volcanic rocks and contains an ~1-m-thick rhyolitic tuff near the base of the section. The Mamazair con-glomerate here consists of interbedded orange, coarse-grained sandstone and orange-red, subangular, clast-supported, rhyolite pebble-to-cobble conglomerate. The stratigraphic position and scarcity of limestone clasts is most similar to Unit 1 described above.
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Figure 7. Mapping results and interpretation of Landsat imagery for parts of the South Pamir thrust belt, using a spectral band combination where dark red colors appear as green. Quaternary units have been colored yellow. A single topographic contour line (4600 m) is shown to help assess structural relief on the Cimmerian unconformity located between map units JrGl (lower unit of the Gurumdi Group) and JrD (Darbashatash Group). Paired white (right) and black (left) colors in panel B are clouds (white) and their shadows (black). Map units, symbols, and labels are the same as in Figure 3. Map locations for A and B shown in Figure 3.
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Directly north of the Kyzylrabot fault, the Mamazair conglomerate is preserved in the footwall of the Karasu fault (Fig. 4), where it is a red, medium- to coarse-grained arkosic sandstone up to a few tens of meters thick. Along strike of the Karasu fault to the northwest, ~20 km south of Murghab, the Mamazair conglomerate rests unconformably on the upper Gurumdi Group in the footwall of the fault, which is steeply north-dipping at this location (Fig. 3). This exposure consists of several tens of meters of massive, clast-supported, angular carbonate pebble conglomerate with a fine-grained pale red siltstone matrix. The local stratigraphic relation-ships are unclear, and it is possible that this exposure is a sheet breccia or mantling karst breccia. The final exposure of the Mamazair conglomerate we examined is located in the southern Taxkorgan valley (China) where it occurs in angular unconformity with the Gurumdi Group in the core a syncline (Fig. 2). The Mamazair conglomerate here consists of up to a few tens of meters of well-bedded red, coarse-grained sandstone with trough cross-bedding preserved locally.
3.1.2. Murghab BasinStrata of the Murghab Basin are exposed along the north side of the
Rushan-Pshart suture zone, in the southernmost part of the Central Pamir terrane (Rutte et al., 2017a; Fig. 3). In the southern Akbaital valley, ~10 km northeast of Murghab, the Murghab Basin strata rest unconformably on Paleozoic(?) schist. The base of the Murghab Basin unit here consists of ~150 m of thinly bedded (<50 cm) pale-green to orange shale, carbon-ate-rich fine-grained sandstone, and dolostone. Ripple cross-lamination is present in most sandstone layers. The unit is overlain by ~175 m of massive, red, clast-supported cobble conglomerate that is capped by an ~100-m-thick basalt flow. Radiometric and thermochronologic dating of the basalt have been inconclusive (Rutte et al., 2017b). The basalt and surrounding sedimentary section are offset by a small south-dipping normal fault at this location (Figs. 3 and 5G). Overlying the basalt is an unknown thickness (>300 m) of interbedded dark-red sandstone, gran-ule-pebble conglomerate, and siltstone. This upper unit locally contains stacked calcic paleosols with carbonate nodules and calcrete layers (e.g., Mack et al., 1993).
3.2. Structural Geology
Structural features in the South Pamir terrane can be grouped into four categories based on the timing of deformation and structural style. These are (1) Cimmerian deformation, (2) the South Pamir thrust belt (SPTB), (3) gneiss dome (north-south–directed) extension, and (4) dextral strike-slip deformation and limited east-west extension. Rutte et al. (2017a) referred to the region containing the SPTB and overprinted by dextral
strike-slip faults as the Murghab-Aksu-Southeast Pamir thrust-wrench belt. In addition to geologic maps (Figs. 3, 4, 6, and 7), structural fea-tures in the South Pamir terrane are presented in cross section A–A′ (see Figure 3 for line of section), which was constructed based on structural measurements and observations in the field (Fig. 8).
3.2.1. Cimmerian DeformationPermian-Triassic rocks, below the Cimmerian unconformity, are com-
plexly deformed and record deformation associated with the Cimmerian orogeny (Fig. 5A). Angiolini et al. (2013) suggested that Cimmerian deformation is characterized by N-S to NNW-SSE–trending folds and high-angle reverse faults. The deformed Paleozoic rocks shown at depth in Figure 8 are schematic and are consistent with north-verging, tight folds shown below the Cimmerian unconformity in Rutte et al. (2017a). No attempt was made in this study to rigorously map structures associated with the Cimmerian orogeny, and the role of Cimmerian deformation in constructing the Pamir remains an outstanding question (Villarreal et al., 2015). Discontinuous exposures of carbonate units within the Permian-Triassic section are oriented subparallel to foliation/cleavage suggestive of locally transposed bedding. In some locations, the Permian-Triassic sec-tion exhibits type 1 fold interference patterns (basin and dome structures; Ramsay, 1967), which likely record re-folding of Cimmerian structures during Cretaceous or later episodes of shortening.
3.2.2. South Pamir Thrust Belt (SPTB)The SPTB consists of a series of thrust faults and folds with axial sur-
faces that strike E-W to SE-NW (Fig. 3). These structures, including the Mamazair and Kyzylrabot thrust faults, offset Jurassic limestone and have locally preserved syntectonic Mamazair conglomerate in their footwalls. The Aksu fault (Figs. 3 and 8) is not well exposed but is here interpreted as a gently north-dipping (blind?) thrust fault based on structural relief on the Cimmerian unconformity, gently north-dipping (20–25°) bedding in the Jurassic carbonate section ~12 km east of Murghab, and a tight syncline ~10 km south of Murghab interpreted as a footwall syncline. Bedding of Jurassic strata in the hanging walls of other thrust faults in the SPTB also generally dips gently to the north-northeast, consistent with displacement above moderate to gently dipping thrust ramps (Fig. 8). The inferred moderate dips of major thrust faults in the SPTB suggest a relatively shallow basal décollement (<10 km depth) that is estimated to be in the Permian-Triassic section, consistent with the interpretation of Rutte et al. (2017a) for the Murghab-Aksu-Southeast Pamir thrust-wrench belt. Major folds in the SPTB are upright and open with the dip of bed-ding in Jurassic units rarely exceeding 40°, except adjacent to the Karasu fault (Figs. 3 and 8). When viewed down-plunge (to the southeast), the
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Kg P-TrJr
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Figure 8. Present-day cross section A–A′ through the South Pamir terrane, location is shown in Figure 3. Inset box at top shows cross-section restora-tion corresponding to Late Jurassic time, assuming minimal initial relief of the Gurumdi Group. Jc—undifferentiated Jurassic carbonate rocks. Other map units and labels are the same as in Figure 3. Pre-Cenozoic deformation is schematic. No vertical exaggeration. Inset at left is a lower-hemisphere, equal area stereoplot of poles to bedding (black dots) in Jurassic carbonate rocks that have been contoured (colors) using a modified Kamp technique. A complete table of bedding measurements and station locations from the South Pamir is located in Data Repository File DR6.
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CHAPMAN ET AL. | Crustal shortening in the Pamir RESEARCH
northwest trend of the Mamazair fault, Karasu fault, and unnamed faults in the Murghab area suggest they root into the Rushan-Pshart suture zone (Fig. 2). Many thrust faults south of the Rushan-Pshart suture in the SPTB exhibit top-to-the-south displacement with south-vergent subsidiary anti-clines, although north-directed faults are present in the southeast Pamir (Fig. 2). Vertical displacement on individual faults in the SPTB, based on structural relief of the Cimmerian unconformity, decreases toward the south, and the southernmost thrust faults tip out over relatively short distances (≤30 km) along strike (Figs. 3 and 6).
East of Kyzylrabot, through-going thrust faults are not obvious and the SPTB is characterized by a series of folds (Fig. 2). It is unclear if deformation in the SPTB extends south of the Alichur gneiss dome. The Rushan-Pshart suture zone (the region south of the Rushan-Pshart fault and north of the Murghab River) is interpreted to be part of the SPTB (i.e., part of the South Pamir terrane). Rutte et al. (2017a) interpreted the northern limit to the Rushan-Pshart suture zone as a north-dipping thrust fault that places Permian-Triassic rocks on Murghab Basin strata (Figs. 3 and 8). Assuming minimal initial structural relief, line-length balancing of the Jurassic Gurumdi Group is used to restore cross section A–A′ (inset, Fig. 8). Whereas Burtman and Molnar (1993) suggested up to 50 km (~50%) of internal shortening within the SPTB, our initial assessment of shortening suggests significantly less shortening (≤10 km), on the order of several percent over the ~100 km length of the cross section and less if the entire South Pamir terrane is included (south of the Alichur gneiss dome).
3.2.3. Extension and Strike-Slip FaultingOther than the normal-sense shear zones bounding the Shakhdara-
Alichur gneiss dome, we observed little evidence for structures that might accommodate north-south–directed extension in the South Pamir terrane, consistent with the observations of (Stübner et al., 2013a). There are no deep extensional (supra-detachment) basins adjacent to the gneiss dome and few east-west–striking normal faults in the hanging wall of the gneiss dome with significant displacement (Figs. 2 and 3). The scarcity of nor-mal faults in the hanging wall of the gneiss domes in the South Pamir terrane contrasts with models for extensional provinces and core com-plexes, where numerous normal fault arrays are present in the hanging wall (Wernicke and Burchfiel, 1982; Lister and Davis, 1989).
Although extensional structures oriented parallel to the Alichur gneiss dome are scarce, approximately north-south striking normal faults bound-ing north-south–trending valleys are more common in the South Pamir terrane (Fig. 3; cf. Schurr et al., 2014). These normal faults offset older reverse faults associated with the SPTB. For example, a prominent normal fault is interpreted to juxtapose upper Jurassic limestone against Permian-Triassic rocks near the village of Mamazair (Fig. 3). Offsets of Jurassic strata across these faults are generally <1 km. Similar sets of young, north-south–striking normal faults have previously been interpreted to be active throughout much of the South Pamir terrane (Stübner et al., 2013b; Schurr et al., 2014) and reflect the modern stress field (Ischuk et al., 2013).
Offset river terraces document active dextral strike-slip displacement on the Karasu fault, which connects the Rushan-Pshart suture zone to the dextral Karakoram fault (Figs. 2 and 3) (Strecker et al., 1995). The Karasu fault appears to have cut or reactivated a thrust fault associated with the SPTB and a vertical component of offset across the fault (up to a few km) is likely related to earlier thrust belt deformation (Fig. 8). Other dextral strike-slip faults may be active in the South Pamir terrane as well (Schurr et al., 2014; Rutte et al. 2017a). An unnamed fault north of the Karasu fault and south of the Aksu fault (Figs. 3 and 7B) dips steeply north and may accommodate primarily strike-slip motion as suggested by Rutte et al. (2017a). Large changes in vertical displacement (>1 km) of the Cimmerian unconformity over short distances (<10 km) along strike
of this unnamed fault and the steep dip of the fault is consistent with strike-slip motion, and may have reactivated an older thrust fault. Rutte et al. (2017a) also suggested that the easternmost Aksu fault (Figs. 3 and 8) is a Neogene or younger dextral strike-slip fault, which could have reactivated an older thrust fault. We did not observe evidence for strike-slip motion on faults (e.g., subhorizontal slickenlines, offset geologic markers) south of the Karasu fault in the SPTB (Figs. 3, 4, and 7A), but overall low magnitudes of slip on faults in this region may have obscured a component of strike-slip motion.
4. GEOCHRONOLOGY AND THERMOCHRONOLOGY RESULTS
A detailed description of methods used for zircon U-Pb geochronology, zircon fission track thermochronology, and zircon and apatite (U-Th)/He thermochronology can be found in Data Repository File DR11.
4.1. Zircon U-Pb Geochronology
4.1.1. Igneous RocksFour igneous rocks were analyzed for zircon U-Pb geochronology
(10–30 grains analyzed per sample). Age estimates for igneous samples are presented on the geologic maps in Figures 3 and 4, a summary of sample locations and age information is presented in Table 1, and com-plete zircon U-Pb data are presented in Data Repository File DR2. The analyzed rocks are porphyritic andesite (samples: 14-74, 14-76, 14-77, 14-88) from near the village of Kyzylrabot in the southeastern Pamir and have middle Cretaceous (ca. 105 Ma) zircon U-Pb ages (Table 1; Fig. 4). Additional zircon U-Pb ages of igneous rocks presented in Figures 3 and 4 (shown only for samples with new thermochronological data) are from Chapman et al. (2018).
4.1.2. Sedimentary RocksEight sandstone samples were analyzed for detrital zircon U-Pb geo-
chronology (100–400 grains analyzed per sample) from the Mamazair conglomerate and Murghab Basin strata (Fig. 9A). Six samples (14-83, 14-85, GUM-02, DV-7-16-15-2, DV-7-15-15-7, and AR-5-27-00-5) are from the Mamazair conglomerate. All of the Mamazair conglomerate samples, except AR-5-27-00-5, were collected from the footwalls of thrust faults in the South Pamir terrane (Figs. 3 and 4). Detrital zircon U-Pb data is presented in a series of probability density plots in Figure 9A. Sample GUM-02 was collected in the footwall of the Mamazair thrust fault and has a minimum age population of ca. 112 Ma (Fig. 9A). Samples 14-83 and DV-7-16-15-2 were collected ~40 m below (in the footwall) the Kyzy-lrabot thrust fault and sample 14-85 was collected ~160 m stratigraphi-cally below sample 14-83 (Fig. 4). The detrital age spectra for samples 14-83 and 14-85 are nearly identical and are combined into a single age probability function in Figure 9A. Sample 14-83/85 has a minimum age population of ca. 76 Ma and a large detrital age population at ca. 104 Ma. Sample DV-7-16-15-2 has a minimum age population of ca. 97 Ma. Sample DV-7-15-15-7 was collected in the footwall of the Karasu fault and has a minimum age population of ca. 154 Ma. The easternmost sample analyzed, AR-5-27-00-5, was collected from the core of a syncline and has a minimum age population of ca. 141 Ma. Several Mamazair conglomerate
1 GSA Data Repository Item 2018167, File DR1: Analytical methods for zircon U-Pb, zircon fission track, zircon U-Th/He, and apatite U-Th/He; File DR2: Zircon U-Pb data for igneous and sedimentary rocks; File DR3: Detrital zircon fission track data for sedimentary rocks and zeta calibration; File DR4: Zircon and apatite (U-Th)/He data; File DR5: Compiled thermochronology data; File DR6: Structural measurements and locations, is available at http://www.geosociety.org/datarepository/2018, or on request from [email protected].
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samples contain detrital age populations in the 140–150 Ma range in addition to younger age populations (Fig. 9A). Samples DV-7-15-15-2 and AR-5-27-00-5 may have been deposited before the other samples.
Two samples (15-28 and 14-56) were collected from Murghab Basin strata just north of the Rushan-Pshart suture zone in a footwall syncline in the Central Pamir terrane (Figs. 2 and 3). Sample 14-56 has a mini-mum age population of ca. 84 Ma and sample 15-28 has a minimum age population of ca. 106 Ma (Table 1; Fig. 9a).
4.2. Zircon Fission-Track Thermochronology
A subset of zircon grains analyzed for detrital U-Pb geochronology, described above, were analyzed for zircon fission-track (ZFT) analysis (“double-dating”). Three samples were selected for detrital ZFT analysis (30–100 grains per sample). ZFT data are shown as kernel density estimate (KDE) plots in Figure 9B, and constituent age populations in each sample were identified and deconvolved using DensityPlotter (Vermeesch, 2012). Complete ZFT data are presented in Data Repository File DR3. Sample GUM-02 yielded ZFT single-grain ages ranging from 59 ± 13 Ma to 754 ± 188 Ma and has one prominent detrital ZFT age population of 103 ± 5 Ma, which is similar to the youngest zircon U-Pb age population (112 Ma; Fig. 9B). All of the individual zircons with Cretaceous and younger ZFT ages from sample GUM-02 have corresponding (doubled-dated)
Cretaceous U-Pb ages. ZFT single-grain ages in sample 15-28 range from 108 ± 23 Ma to 1054 ± 225 Ma and the sample exhibits two detrital ZFT age populations of 235 ± 19 Ma (65% of grains) and 160 ± 26 Ma (22% of grains) (Fig. 9B). The small number of zircon grains with Cretaceous U-Pb ages (n = 3) explains the lack of a recognizable younger ZFT age population. Four (out of 100) zircons in sample 15-28 yielded Creta-ceous ZFT cooling ages (134–108 Ma). These four individual analyses had U-Pb ages ranging from Cretaceous to Proterozoic. The ca. 235 Ma detrital ZFT age population in sample 15-28 is similar to detrital zircon U-Pb age populations in samples 15-28 and 14-56 (Fig. 9B). Single-grain ZFT ages from sample 14-83 range from 68 ± 20 Ma to 715 ± 168 Ma. Sample 14-83 has two detrital ZFT age populations of 111 ± 5 Ma (44% of grains) and 264 ± 18 Ma (45% of grains) (Fig. 9B). The youngest detrital ZFT age population (ca. 111 Ma) in sample 14-83 compares favorably to the prominent ca. 104 Ma zircon U-Pb detrital age population in the same sample (Fig. 9). Individual zircons with Cretaceous ZFT detrital ages in sample 14-83 have zircon U-Pb ages ranging from Cretaceous to Proterozoic (Data Repository File DR3). The older ZFT age popula-tion (ca. 264 Ma) in sample 14-83 is similar to the ca. 235 Ma ZFT age population in sample 15-28.
4.3. (U-Th)/He Thermochronology Results
To constrain the magnitude of exhumation in the South Pamir terrane, six granite samples (with Jurassic to Cretaceous zircon U-Pb crystallization ages) and two sandstone samples (with Triassic to Cretaceous detrital zir-con U-Pb maximum depositional ages) were collected for zircon (U-Th)/He (ZHe) and apatite (U-Th)/He (AHe) thermochronology. Weighted mean ZHe and AHe sample ages (based on three to five single-grain analyses) are presented in Table 1 and shown in Figures 3 and 4. Single-grain ZHe and AHe data are presented in Data Repository File DR4. Uncertainties (2σ) for average ages are calculated using internal (weighted analyti-cal uncertainty) and external (standard deviation of aliquot ages) errors. Three samples (granitoids 14-73, 14-60, and 16-22) were collected in the Rushan–Pshart suture zone and have weighted-mean ZHe cooling ages of ca. 15–20 Ma. Five samples (Cretaceous granitoids 13P101 and 14-87, Cretaceous dike 14-89, Cretaceous sandstone 16-23, and Triassic sandstone 13P99) were collected from the interior of the South Pamir terrane. Weighted mean average ZHe ages from samples in the South Pamir terrane range from 34 to 102 Ma and weighted mean average AHe ages range from 44 to 56 Ma (Table 1). The mean ZHe ages for samples 14-87 and 14-89 are within 10% of the zircon U-Pb age. All other (U-Th)/He ages are >10% younger than their crystallization or depositional age. Sample 16-22, a Paleozoic(?) schist, yielded a ZHe cooling age of 15.1 ± 0.7 Ma (Fig. 3; Table 1). Sample 16-23, a sandstone clast collected from the Mamazair conglomerate, yielded an AHe cooling age of 46 ± 16 Ma, similar to the AHe cooling age recorded from the igneous sample 14-89 collected nearby (56 ± 17 Ma). All of the mean AHe ages have large uncer-tainties, which may reflect a complicated cooling history, although none of the samples exhibited age versus effective uranium concentration (eU) trends (Data Repository File DR4), and ZHe age uncertainties from the same samples were lower than the AHe uncertainties (e.g., sample 14-89).
5. DISCUSSION
5.1. Exhumation in the South Pamir Terrane
Two igneous ZHe samples (14-87 and 14-89) from the SPTB in this study have Cretaceous cooling ages that are as much as 10 m.y. younger than the zircon U-Pb ages from the same samples (Table 1; Figures 3
TABLE 1. SAMPLE INFORMATION AND AGE SUMMARY OF GEOCHRONOLOGIC AND THERMOCHRONOLOGIC DATA
Sample Latitude (DD)
Longitude (DD)
Maximum depositional age
U-Pb (Ma)
ZFT (Ma)
Detrital Samples
Murghab Basin15-28 38.2719 73.6290 106 23014-56 38.2594 74.0476 84 117
Mamazair ConglomerateGUM-02 37.8396 74.0888 112 11214-83/85 37.4755 74.7648 76 111DV-7-16-15-2 37.4750 74.7660 97DV-7-15-15-7 37.5500 74.6900 157AR-5-27-00-5 37.2191 75.3657 141
Latitude (DD)
Longitude (DD)
U-Pb age (Ma)
AHe age (Ma)
ZHe age (Ma)
Bedrock Samples
Rushan-Pshart Suture Zone14-73 38.2081 74.0284 198 ± 7 17 ± 314-60 38.1289 73.8556 109 ± 5 20 ± 716-22 38.2495 74.0225 15 ± 1
South Pamir Thrust Belt13P101 37.9027 73.7301 120 ± 4 34 ± 414-89 37.7840 73.9593 104 ± 3 56 ± 17 102 ± 1116-23 46 ± 1613P99 37.7576 73.7743 69 ± 1314-87 37.4931 74.6112 105 ± 3 44 ± 19 96 ± 1014-74 37.5505 74.4665 111 ± 314-76 37.5230 74.3929 105 ± 314-77 37.5487 74.5235 106 ± 314-88 37.5102 74.5779 105 ± 3
Note: Maximum depositional ages are based on the youngest zircon U-Pb or zircon fission-track (ZFT) population (Fig. 9). DD—decimal degrees, AHe—apatite (U-Th/He), ZHe—zircon (U-Th/He). Complete zircon U-Pb data are located in Data Reposi-tory File DR2 (see text footnote 1). Complete ZFT data are located in Data Repository File DR3. Complete AHe and ZHe data are located in Data Repository File DR4.
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and 4). These ZHe ages are interpreted to represent cooling following magmatic emplacement and suggest less than 4 km of exhumation or burial during the Cenozoic. The other bedrock ZHe and AHe cooling ages in the SPTB range from Late Cretaceous to late Eocene and may record a regional exhumation signal associated with the initiation of the India-Asia collision (e.g., Rohrmann et al., 2012). An interpretive cooling path for the southern SPTB is presented in Figure 10A. The thermochronologic data from the SPTB are consistent with limited Cenozoic exhumation in much of the South Pamir terrane as suggested by the low-relief landscape, preservation of the low-angle Cimmerian unconformity, and uneroded remnants of a volcanic carapace in the Kyzylrabot area (Fig. 4). The results from this study are also consistent with Stübner et al. (2013b) who calculated cooling rates as low as ~0.1 mm/yr since the mid-Cretaceous in the South Pamir terrane, outside of the Shakhdara gneiss dome.
Figure 10B is a compilation of thermochronologic data from this study (10 new cooling ages, most located in the SPTB) and previous studies (Data Repository File DR5) plotted against the distance south from the main Pamir thrust, which bounds the Pamir Mountains to the north (Fig. 1). The y-axis in Figure 10B is closure temperature divided by cooling age for each sample, a proxy for exhumation rate, but not a well-defined cool-ing rate, which should be calculated from multiple thermochronometers.
Not surprisingly, inferred exhumation rates in the SPTB are relatively low compared to the Pamir gneiss domes and the Rushan-Pshart suture zone (Stübner et al., 2013b; Rutte et al., 2017b) and are more akin to exhumation rates in the North Pamir terrane (Amidon and Hynek, 2010; Sobel et al., 2013), which is not interpreted to have experienced internal shortening during the Cenozoic (Burtman and Molnar, 1993; Robinson et al., 2004, 2007).
The Rushan-Pshart suture zone is characterized by Miocene low-temperature thermochronologic cooling ages, yielding relatively high Miocene to Recent exhumation rates (Figs. 3 and 10; Table 1) that are interpreted to reflect rock uplift and erosion within the suture zone. The new data support the conclusions of Rutte et al. (2017b), who suggested low (<2 °C/m.y.) cooling rates in the Rushan-Pshart suture zone from the Late Cretaceous to the Miocene and accelerated cooling rates (up to ~30 °C/m.y.) from the Miocene to the present.
5.2. Age of the Mamazair Conglomerate and Murghab Basin Strata
The Murghab Basin strata and the Mamazair conglomerate have pre-viously been associated with Cenozoic deformation and growth of the Pamir orogen during India-Asia collision (Burtman and Molnar, 1993;
14-83/85
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asin 235 Ma
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zircon fissiontrack ages
zircon U-Pb ages
n=99
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160 Ma
Detrital Zircon U-Pb Results Detrital Zircon Fission Track Results
A B
Figure 9. (A) Normalized age probability curves for detrital zircon U-Pb samples from the Mamazair conglomerate and Murghab Basin strata. Parentheses next to individually labeled peak ages show the number of analyses that comprise that peak. Ages for the Karakul-Mazar mag-matic arc are from Schwab et al. (2004). (B) Blue filled curves are kernel density estimates (KDE) for zircon fission track (ZFT) ages for samples of the Mamazair conglomerate and Murghab Basin strata, plotted with an adaptive bandwidth. ZFT age populations identified and deconvolved using DensityPlotter (Vermeesch, 2012). Black unfilled curves are age probability curves of detrital zircon U-Pb data from the same ZFT sample (double-dated). Sample locations are shown in Figures 2–4 and listed in Table 1.
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Rutte et al., 2017a). However, the results of this study do not yield evi-dence for a Cenozoic age for these deposits, and instead suggest older depositional ages.
Samples DV-7-15-15-2 and AR-5-27-00-5 (both interpreted to be cor-relative with the Mamazair conglomerate) from the southeastern SPTB have Upper Jurassic to Lower Cretaceous zircon U-Pb minimum age populations interpreted as maximum depositional ages (141–154 Ma). Because of the widespread exposures of mid-Cretaceous igneous rocks in the region and abundant mid-Cretaceous detrital zircon U-Pb ages pres-ent in other samples of the Mamazair conglomerate from the same area (Figs. 2, 4 and 9), we interpret samples DV-7-15-15-2 and AR-5-27-00-5 to have been deposited prior to the mid-Cretaceous. Growth strata were not observed where these samples were collected, and it is unclear if these strata are syntectonic or were deposited prior to SPTB deformation.
Detrital zircon U-Pb and detrital ZFT minimum age populations for the rest of the Mamazair conglomerate samples cluster in the mid-Cretaceous (112–76 Ma) (Fig. 9) and are interpreted as maximum depositional ages. Cenozoic igneous rocks are exposed in the Central and South Pamir ter-ranes, and Cenozoic leucogranite and migmatite are exposed within the Pamir gneiss domes (Ducea et al., 2003; Schwab et al., 2004; Jiang et al., 2012; Stübner et al., 2013a; Carrapa et al., 2014a; Stearns et al., 2015; Chapman et al., 2018), which could have contributed detrital zircon to the Mamazair conglomerate or Murghab Basin strata if these formations are Cenozoic in age. Furthermore, detrital zircon U-Pb ages from modern streams draining the Central and South Pamir, and from Oligocene-Miocene sediments from the northeast margin of the Pamir, yield a strong Eocene signal (Lukens et al., 2012; Bershaw et al., 2012; Carrapa et al., 2014a) that is not present in the Mamazair conglomerate and Murghab Basin samples.
The detrital ZFT data from the Mamazair conglomerate indicate that the unit was not buried deeply enough to exceed a temperature of ~240 °C for significant periods of time and exhibit a detrital (not reset) thermo-chronologic signal. None of the samples have Cenozoic detrital ZFT age populations (Fig. 9B), and very few (not statistically significant) single-grain ZFT analyses produced Cenozoic ages (Data Repository File DR3). In contrast, samples from the Pamir gneiss domes and Rushan-Pshart suture zone have Cenozoic ZFT exhumation ages (Stübner et al., 2013b;
Stearns et al., 2015; Rutte et al., 2017b). Crustal thickening in the Pamir is interpreted to have occurred during the late Eocene to Oligocene (Smit et al., 2014; Stearns et al., 2015; Rutte et al., 2017b), which might also be expected to have produced a detrital ZFT signature in the Mamazair and Murghab basin samples.
Finally, interbedded volcaniclastic and tuffaceous beds in the Mamazair conglomerate from the Kyzylrabot area also support a mid- to Late Cre-taceous depositional age (Figs. 4, 5K, and 5L), as the age of the volcanic units in the Kyzylrabot area were recently reclassified from Paleogene(?) to mid-Cretaceous (Aminov et al., 2017; Chapman et al., 2018).
We emphasize that because the new detrital zircon U-Pb and detrital ZFT ages are maximum depositional ages, the Mamazair conglomerate and Murghab Basin could be Cenozoic in age. However, we adopt a Late Cretaceous age consistent with the available detrital zircon U-Pb data, detrital ZFT data, previous biostratigraphic age control (Maastrichtian; Yushin et al., 1964), locally interbedded tuffaceous beds, and similarly aged red-colored units that occur in the Karakoram (the Tupopo and Darbaand Formation; Gaetani et al., 1993).
5.3. Cretaceous Tectonics in the South Pamir Terrane
The upper Gurumdi Group contains extensive dissolution features and paleokarst breccia, which indicate that the South Pamir terrane was uplifted above sea level during the Late Jurassic to Early Cretaceous (Figs. 3, 5E, and 5F). Robinson (2015) suggested Upper Jurassic deformation and uplift of the South Pamir terrane based on Late Jurassic 40Ar/39Ar cooling ages in the Rushan-Pshart suture zone presented in Schwab et al. (2004) and the absence of Upper Jurassic marine rocks (Pashkov and Budanov, 1990). Rutte et al. (2017b) suggested that the Late Jurassic 40Ar/39Ar cooling ages in the Rushan-Pshart zone are magmatic cooling ages associated with nearby Jurassic intrusions (Schwab et al., 2004).
Either shortly after or contemporaneously with uplift and karst weath-ering, continental arc magmatism migrated into the Pamir, related to north-ward directed subduction of oceanic lithosphere (Debon et al., 1987; Crawford and Searle, 1992; Fraser et al., 2001; Schwab et al., 2004; Ravikant et al., 2009). The oldest Cretaceous igneous rocks identified in
0
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sure
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oolin
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e (°
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40Ar/39Ar white mica40Ar/39Ar biotitezircon fission trackzircon (U-Th)/Heapatite fission trackapatite (U-Th)/He
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ture
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India-Asiacollision?
BA
Figure 10. (A) Interpretive cooling path shown for the Cretaceous igneous rocks in South Pamir thrust belt using apatite (U-Th)/He (AHe) and zircon (U-Th)/He (ZHe) data from samples 14-87, 14-89, 16-23, and 13P99 (Table 1; Figs. 3 and 4). Effective closure temperature was calculated using CLOSURE (Brandon et al., 1998) with an initial assumption of a 2 °C/m.y. cooling rate. The average equivalent spherical radius of the grains analyzed was used as the effective diffusion-domain size. A 20% uncertainty was applied to each calculated closure temperature. (B) Compiled thermochronologic data from the Pamir (this study; Data Repository File DR5) projected onto a north-south transect. Data is plotted as sample closure temperature/sample cooling age, which is a proxy for exhumation rate. Dodson closure temperatures (Dodson, 1973) were applied when a regional cooling rate was available, otherwise closure temperatures were adopted from Reiners and Brandon (2006) for a 10 °C/ m.y. cooling rate. The specific closure temperature used in the plot does not significantly affect the results, which are intended to show relative differences in exhumation rates only. Cooling rates for various regions are presented in Stübner et al. (2103b) and Rutte et al. (2017b).
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the South Pamir terrane are ca. 120 Ma (Fig. 3) and in the Kyzylrabot area, mid-Cretaceous (ca. 105 Ma) volcanic rocks were deposited uncon-formably on Jurassic carbonate rocks (Aminov et al., 2017; Chapman et al., 2018; this study) (Fig. 4).
The Mamazair conglomerate is primarily exposed in the footwalls of thrust faults that form the SPTB. Syndepositional deformation recorded by growth strata and recycled clasts in the Mamazair conglomerate link the timing of deposition and SPTB deformation (Figs. 5H, 5J, and 6). If our interpretation of the age of the Mamazair conglomerate is correct, this suggests that post-Cimmerian (post–Late Triassic) upper crustal shorten-ing in the SPTB occurred during the Late Cretaceous. This interpretation supports a growing body of evidence for Cretaceous shortening throughout the Pamir: (1) the Tajik and Tarim basins and northern Pamir foreland contain thick, coarse-grained, clastic Cretaceous deposits that were derived from the Pamir (Hamburger et al., 1992; Sobel, 1999; Burtman, 2000; Bershaw et al., 2012; Carrapa et al., 2015); (2) the North Pamir terrane experienced Cretaceous Barrovian metamorphism and shortening, includ-ing reactivation and southward-directed thrusting along the Tanymas fault (Robinson et al., 2004; Robinson, 2015); and (3) the Karakoram terrane experienced Cretaceous metamorphism and crustal thickening (Searle et al., 1999; Fraser et al., 2001; Hildebrand et al., 2001).
5.4. Comparison of the South Pamir Terrane to Tibet
The Mesozoic geology of the Pamir Mountains and Tibetan plateau share many characteristics. Like the South Pamir terrane, the laterally equivalent Qiangtang terrane was uplifted above sea level during the Late Jurassic to Early Cretaceous (Dewey et al., 1988; Leeder et al., 1988; Kapp et al., 2005). Early Cretaceous igneous rocks are prevalent in the northern Lhasa terrane and southern Qiangtang terrane, with a peak in magmatism at ca. 110 Ma (Zhu et al., 2009, 2016; Sui et al., 2013), similar to the South Pamir terrane (Schwab et al., 2004; Aminov et al., 2017; Chapman et al., 2018). Although the Yarlung suture zone and parts of the Gangdese arc underwent significant (>6 km?) Cenozoic exhumation (Copeland et al., 1987, 1995; Carrapa et al., 2014b), the northern Lhasa terrane and Qiangtang terrane experienced minimal (<3 km) post-Eocene burial and/or exhumation north of the suture zone (Kapp et al., 2005; DeCelles et al., 2007; Hetzel et al., 2011; Rohrmann et al., 2012; Wang et al., 2014), similar to the South Pamir terrane outside of the gneiss domes and suture zones (Fig. 10B).
In central Tibet, the majority of upper-crustal shortening was concen-trated along the Bangong suture zone during the Jurassic-Early Cretaceous and propagated southward into the northern Lhasa thrust belt during the Late Cretaceous (Murphy et al., 1997; Kapp et al., 2005, 2007b; DeCelles et al., 2007; Volkmer et al., 2007, 2014; Raterman et al., 2014). The timing of shortening, the relationship to Mesozoic suture zones, and the overall southward vergence of the northern Lhasa thrust belt is similar to the South Pamir thrust belt (Figs. 3 and 8). One difference between the South Pamir terrane and southern Tibet is in the magnitude of Cretaceous shortening, which exceeded 50% within the Bangong suture zone and northern Lhasa terrane, but was only moderate in the South Pamir terrane. There are also Cretaceous red beds in the Qiangtang terrane, Bangong suture zone, and northern Lhasa terrane that are associated with Cretaceous deformation (DeCelles et al., 2007; Kapp et al., 2007b; Leier et al., 2007; Raterman et al., 2014; Sun et al., 2015) and may be equivalent to the Murghab Basin strata and Mamazair conglomerate in the Pamir.
5.5. Crustal Thickening in the Pamir
Based on work by Burtman and Molnar (1993), Schmidt et al. (2011) provided a summary of total Cenozoic shortening for the Pamir Mountains
in order to examine models for crustal thickening. Schmidt et al. (2011) showed that if the precollisional thickness of the Pamir crust was similar to global averages (35–40 km), 400–600 km of plane-strain shortening is required to account for the present-day crustal thickness of the Pamir. The results of this study, combined with other recent investigations of the Pamir Mountains, enable a revised estimate of total shortening. Cenozoic shortening estimates are summarized below, from north to south.
No Cenozoic upper crustal shortening has been documented within the North Pamir terrane (Burtman and Molnar, 1993; Amidon and Hynek, 2010; Robinson, 2015). Rutte et al. (2017a) recently interpreted the Cen-tral Pamir terrane as a stack of thrust sheets overlying a series of large ductile fold nappes and reported >95 km of Cenozoic internal shortening. The present study estimates <10 km of Cenozoic shortening in the South Pamir terrane. Thus, the total amount of internal Cenozoic shortening documented in the North, Central, and South Pamir terranes is ~100 km, which is similar to Robinson’s (2015) interpretation that the total Cenozoic upper crustal shortening of the entire Pamir is ~80 km.
In contrast, Stübner et al. (2013a) estimated up to ~90 km of Cenozoic extension associated with the Shakhdara-Alichur gneiss dome, and Rutte et al. (2017a) suggested ≤75 km of Cenozoic extension in the Central Pamir gneiss domes, yielding ~165 km of total extension in the Pamir. Although the magnitude of Cenozoic extension in the Pamir is not pre-cisely constrained, these estimates raise the possibility that the upper crust of the Pamir interior could be characterized by net extension rather than net shortening (~165 km extension minus ~100 km of shortening) during the Cenozoic.
The above analysis and the paucity of internal Cenozoic shortening in the South Pamir terrane documented in this study raise the question of how the Pamir Mountains (and South Pamir terrane in particular) obtained their current crustal thickness. Three end-member models have been pre-viously proposed to explain the current crustal thickness: (1) Cenozoic shortening within the Pamir Mountains, (2) Mesozoic shortening within the Pamir Mountains, and (3) underthrusting of lithosphere external to the Pamir (e.g., Indian and/or Asian lithosphere).
5.5.1. Cenozoic Internal Shortening ModelCenozoic shortening associated with India-Asia collision is a common
explanation for the thick crust in the modern Pamir Mountains (Burtman and Molnar, 1993; Robinson et al., 2007; Schmidt et al., 2011; Stearns et al., 2013, 2015; Hacker et al., 2017; Rutte et al., 2017a). Thrust sheets and internal shortening in the Central Pamir terrane, documented by Rutte et al. (2017a, 2017b), explains the modern thickness of the crust in the Central Pamir terrane as well as the prograde metamorphic history of the Central Pamir gneiss domes (Smit et al., 2014; Stearns et al., 2015; Hacker et al., 2017). The Shakhdara-Alichur gneiss dome in the South Pamir ter-rane has a similar metamorphic history to the Central Pamir gneiss domes (Stübner et al., 2013b; Smit et al., 2014; Stearns et al., 2015; Hacker et al., 2017) and ideally, a similar model for crustal thickening could be applied to the South Pamir terrane. However, the results of this study suggest low magnitudes of internal shortening, which could not have buried the rocks in the Shakhdara-Alichur gneiss dome to mid-to-lower crustal depths (≥ 50 km) as indicated by thermobarometry (Hacker et al., 2017). In addi-tion, the Shakhdara-Alichur gneiss dome does not display ductile nappe (shortening) structures like the Central Pamir gneiss domes (Stübner et al., 2013a; Rutte et al., 2017a). Internal crustal thickening also does not explain the current thickness of the North Pamir terrane crust.
5.5.2. Mesozoic Internal Shortening ModelThe crust of the Pamir Mountains may have been significantly thick-
ened during the Mesozoic (Robinson et al., 2004, 2012; Robinson, 2015),
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similar to some models for crustal thickening in Tibet (Murphy et al., 1997; Kapp et al., 2005, 2007a; Raterman et al., 2014). Robinson et al. (2004, 2012) presented evidence from the North Pamir terrane for Cretaceous shortening and crustal thickening, which could help explain the modern crustal thickness. Results of the current study suggest that internal short-ening occurred in the South Pamir terrane during the Mesozoic, but the magnitude of this shortening was not sufficient to significantly thicken the South Pamir crust. This model also does not explain the Eocene to Miocene prograde metamorphic ages recorded within the Pamir gneiss domes that indicate protracted Cenozoic burial (Robinson et al., 2004, 2007; Schmidt et al., 2011; Stearns et al., 2013, 2015; Hacker et al., 2017). Smit et al. (2014) proposed that Eocene Lu-Hf garnet (prograde) meta-morphic ages from the Pamir gneiss domes may reflect high mantle heat flow associated with breakoff and foundering of the Neotethyan oceanic slab, which would not require Cenozoic burial, although the protracted nature of prograde metamorphism within the gneiss domes (20–25 m.y.; Stearns et al., 2015; Hacker et al., 2017) is generally inconsistent with a short-lived thermal pulse following slab breakoff (Mahéo et al., 2002).
5.5.3. Underthrusting ModelLithosphere external to the Pamir may have been underthrust and incor-
porated into the Pamir orogen, helping to compress and thicken the crust (Butler and Coward, 1989; Kufner et al., 2016; Chapman et al., 2017), again analogous to some models for Tibet (Zhao and Morgan, 1987; Nel-son et al., 1996; DeCelles et al., 2002; Kapp and Guynn, 2004; Nábělek et al. 2009; Gao et al., 2016). The northern margin of the Pamir and the eastern Tajik fold-thrust belt record ~30 km of Cenozoic shortening (Leith and Alvarez, 1985; Hamburger et al., 1992; Bekker, 1996; Coutand et al., 2002; Li et al., 2012; Chapman et al., 2017). Some studies suggest that the Tajik-Tarim (Asian) lower crust was underthrust southward beneath the Pamir and incorporated into the orogen (rather than subducted), which could account for Cenozoic thickening and surface uplift in the North Pamir terrane without significant exhumation (Amidon and Hynek, 2010; Kufner et al., 2016; Chapman et al., 2017). On the southern margin of the Pamir, the Pakistan Himalayan fold-thrust belt, south of the Indus-Yarlung suture zone, records 400–600 km of upper crustal shortening (Butler and Coward, 1989). Underthrust Indian mantle lithosphere is interpreted to directly underlie much the Pamir crust today (Schneider et al., 2013; Sippl et al., 2013; Kufner et al., 2016), which implies that the overlying Indian lower crust has been incorporated into the upper plate. In addition to Indian lithosphere, studies of the northern Karakorum sedimentary sequences suggest significant (but unconstrained) upper crustal shortening during the Cenozoic in the Karakoram terrane (Zanchi and Gaetani, 2011). Part of the Karakoram terrane could also have been thrust beneath the South Pamir terrane and thickened the crust (e.g., Ducea et al., 2003).
An attractive aspect of the end-member model for underthrusting of Indian, Asian, or Karakoram lithosphere to thicken the Pamir lower crust is that it could drive extension in the mid-to-upper crust, resulting in the formation of the Pamir gneiss domes (Robinson et al., 2007; Stübner et al., 2013b). This model suggests mechanical decoupling (a mid-crustal shear zone) between a contractional lower crust and an extensional upper crust in the Pamir (Robinson et al., 2007; Stübner et al., 2013b). Similar mechanisms have been proposed to explain extension in the upper crust in Tibet (DeCelles et al., 2002; Liu and Yang, 2003; Kapp and Guynn, 2004; Copley et al., 2011; Styron et al., 2015). However, northward underthrust-ing of Indian lithosphere or the Karakoram terrane will not cause upper crustal thrust stacking or burial in the South Pamir terrane and it is difficult to explain the prograde metamorphic history of the Shakhdara-Alichur gneiss dome with this model. Hacker et al. (2017) reported that the rocks in the Pamir gneiss domes originated at ≤15–20 km depth prior to burial.
For the underthrusting model to be viable, the mid-crustal shear zone envi-sioned for the Pamir (Robinson et al., 2007; Stübner et al., 2013b) would need to exist above (shallower than) this depth. Deep seismic reflection data from the Tibetan Plateau suggest that Indian lower crustal rocks are incorporated into the Himalayan-Tibetan orogen at depths ≥25 km (Gao et al., 2016), although it is uncertain if this geometry is applicable to the Pamir, located ~1000 km away along strike.
5.5.4. Crustal Thickening SummaryThe results of this study, in conjunction with previous studies, sug-
gest that crustal thickening was heterogeneous in time, space, and style within the Pamir Mountains. Crustal thickening in the North Pamir ter-rane is best explained by Cretaceous upper crustal shortening (Robinson et al., 2004, 2012) and Cenozoic underthrusting of Tajik-Tarim Basin lithosphere and lower crust (Chapman et al., 2017). Crustal thickening in the Central Pamir terrane is best explained by Cenozoic stacking of upper crustal thrust sheets and mid-crustal ductile nappes (Rutte et al., 2017a). Despite the difficulties in explaining the prograde metamorphic history of the Shakhdara-Alichur gneiss dome, the results of this study are most consistent with models that propose that underthrusting of Indian lithosphere thickened the lower crust of the South Pamir terrane (Kufner et al., 2016). Underthrusting of the Karakoram terrane and/or Central Pamir terrane lithosphere also could have thickened the South Pamir terrane lower crust (e.g., Robinson et al., 2012); however, this possibil-ity still does not resolve the prograde (burial) history of the Shakhdara-Alichur gneiss dome. Although the timing of prograde metamorphism is comparable between the Central Pamir terrane gneiss domes and the Shakhdara-Alichur gneiss dome in the South Pamir terrane, Hacker et al. (2017) estimated that the rate of burial of rocks in the Shakhdara-Alichur gneiss dome was twice as fast (2–8 km/m.y.) and reached nearly twice the depth (≥50 km) of the rocks in the Central Pamir gneiss domes. These differences may indicate contrasting mechanisms for burial in the South Pamir terrane compared to the Central Pamir terrane.
6. SUMMARY AND CONCLUSIONS
Geologic mapping in the South Pamir terrane documents distributed deformation in an overall south-vergent thrust belt, the South Pamir thrust belt (SPTB) that offsets otherwise undeformed Jurassic shallow marine limestone and shale of the Gurumdi Group. Field observations of the SPTB indicate relatively minor amounts of shortening since the Jurassic. Paleokarst dissolution features and karst breccia in the upper Gurumdi Group suggest that the South Pamir was uplifted above sea level dur-ing the Late Jurassic to Early Cretaceous. Unconformably overlying the Gurumdi Group in the South Pamir terrane are coarse, clastic, synorogenic sedimentary rocks of the Mamazair conglomerate, which are exposed primarily in the footwall of thrust faults in the SPTB. Growth strata and recycled conglomerate clasts indicate that deformation in the SPTB was synchronous with deposition of the Mamazair conglomerate. New detrital zircon U-Pb and detrital ZFT data from the Mamazair conglomerate are consistent with it having been deposited during the mid- to Late Creta-ceous, with maximum depositional ages between 112 Ma and 76 Ma, but do not preclude a younger depositional age. These results suggest that the majority of upper crustal shortening in the South Pamir terrane could have occurred during the Cretaceous rather than the Cenozoic as previously proposed (Vlasov et al., 1991; Burtman and Molnar, 1993). Mid-Cretaceous zircon U-Pb ages (105–122 Ma) from intrusive rocks in the South Pamir terrane and interbedded volcaniclastic and tuffaceous units in the Mamazair conglomerate suggest that arc magmatism was in-part coeval with shortening. New AHe and ZHe data from rocks in the
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South Pamir terrane generally record Late Cretaceous to Eocene cooling ages and indicate minimal cooling and exhumation during the Cenozoic outside of gneiss domes and suture zones.
The record of Cretaceous shortening in the South Pamir terrane is consistent with previous studies that recognized Cretaceous deforma-tion in the North Pamir terrane (Robinson et al., 2004, 2007, 2012) and supports models for crustal thickening prior to the India-Asia collision (Murphy et al., 1997; Kapp et al., 2005; Robinson, 2015). The lack of internal Cenozoic shortening in the South Pamir terrane, however, is dif-ficult to reconcile with Cenozoic prograde metamorphic ages recorded in the Shakhdara-Alichur gneiss dome that imply Cenozoic crustal thicken-ing (Schmidt et al., 2011; Stearns et al., 2013, 2015; Smit et al., 2014; Hacker et al., 2017). Total Cenozoic internal shortening in the Pamir may be less than total extension (Stübner et al., 2013a; Rutte et al., 2017a), and the Pamir crust may have been thinned by an additional 10–15 km by delamination of the lower crust (Kufner et al., 2016; Chapman et al., 2017). The results of the study are consistent with models for Cenozoic crustal thickening in the South Pamir terrane by insertion of Indian lower crust into the Pamir orogen (Kufner et al., 2016) and models for a mid-crustal shear zone that decouples upper crustal extension and lower crustal contraction (Robinson et al., 2007; Stübner et al., 2013a).
ACKNOWLEDGMENTSSupport was provided by National Science Foundation (NSF) EAR-1450899 (A.R.), NSF EAR-1419748 (P.K.), NSF EAR-0929777 and 1338583 (G.G., Arizona Laserchron Center), and research grants from the American Philosophical Society (J.C.), the American Association of Petroleum Geologists (J.C.), the Geological Society of America (J.C.), and Exxon Mobil Corporation (B.C.).
REFERENCES CITEDAllégre, C.O., Courtillot, V., Tapponnier, P., Hirn, A., Mattauer, M., Coulon, C., Jaeger, J.J., Ach-
ache, J., Schärer, U., Marcoux, J., and Burg, J.P., et al., 1984, Structure and evolution of the Himalaya-Tibet orogenic belt: Nature, v. 307, p. 17–22, https:// doi .org /10 .1038 /307017a0.
Amidon, W.H., and Hynek, S.A., 2010, Exhumational history of the north central Pamir: Tec-tonics, v. 29, https:// doi .org /10 .1029 /2009TC002589.
Aminov, J., Ding, L., Mamadjonov, Y., Dupont-Nivet, G., Aminov, J., Zhang, L.Y., Yoqubov, S., Aminov, J., and Abdulov, S., 2017, Pamir Plateau formation and crustal thickening before the India-Asia collision inferred from dating and petrology of the 110–92 Ma Southern Pamir volcanic sequence: Gondwana Research, v. 51, p. 310–326, https:// doi .org /10 .1016 /j .gr .2017 .08 .003.
Angiolini, L., Zanchi, A., Zanchetta, S., Nicora, A., and Vezzoli, G., 2013, The Cimmerian geopuzzle: New data from South Pamir: Terra Nova, v. 25, p. 352–360, https:// doi .org /10 .1111 /ter .12042.
Angiolini, L., Zanchi, A., Zanchetta, S., Nicora, A., Vuolo, I., Berra, F., Henderson, C., Mala-spina, N., Rettori, R., Vachard, D., and Vezzoli, G., 2015, From rift to drift in South Pamir (Tajikistan): Permian evolution of a Cimmerian terrane: Journal of Asian Earth Sciences, v. 102, p. 146–169, https:// doi .org /10 .1016 /j .jseaes .2014 .08 .001.
Bekker, Y.A., 1996, Tectonics of the Afghan-Tajik depression: Geotectonics, v. 30, p. 64–70.Bershaw, J., Garzione, C.N., Schoenbohm, L., Gehrels, G., and Tao, L., 2012, Cenozoic evolu-
tion of the Pamir plateau based on stratigraphy, zircon provenance, and stable isotopes of foreland basin sediments at Oytag (Wuyitake) in the Tarim Basin (west China): Jour-nal of Asian Earth Sciences, v. 44, p. 136–148, https:// doi .org /10 .1016 /j .jseaes .2011 .04 .020.
Brandon, M.T., Roden-Tice, M.K., and Garver, J.I., 1998, Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State: Geological Society of America Bulletin, v. 110, p. 985–1009, https:// doi .org /10 .1130 /0016 -7606 (1998)110 <0985: LCEOTC>2 .3 .CO;2.
Burg, J.P., 2011, The Asia–Kohistan–India collision: Review and discussion, in Brown, D., and Ryan, P., eds., Arc-Continent Collision: Berlin, Heidelberg, Springer, p. 279–309, https:// doi .org /10 .1007 /978 -3 -540 -88558 -0_10.
Burtman, V.S., 2000, Cenozoic crustal shortening between the Pamir and Tien Shan and a reconstruction of the Pamir-Tien Shan transition zone for the Cretaceous and Palaeo-gene: Tectonophysics, v. 319, p. 69–92, https:// doi .org /10 .1016 /S0040 -1951 (00)00022 -6.
Burtman, V.S., and Molnar, P., 1993, Geological and geophysical evidence for deep subduc-tion beneath the Pamir: Geological Society of America Special Paper 281, 76 p., https:// doi .org /10 .1130 /SPE281.
Butler, R.W., and Coward, M.P., 1989, Crustal scale thrusting and continental subduction dur-ing Himalayan collision tectonics on the NW Indian plate, in Şengör, A.M.C., ed., Tec-tonic Evolution of the Tethyan Region: Netherlands, Springer, p. 387–413, https:// doi .org /10.1007 /978 -94 -009 -2253 -2_17.
Carrapa, B., Mustapha, F.S., Cosca, M., Gehrels, G., Schoenbohm, L.M., Sobel, E.R., DeCelles, P.G., Russell, J., and Goodman, P., 2014a, Multisystem dating of modern river detritus from Tajikistan and China: Implications for crustal evolution and exhumation of the Pamir: Lithosphere, v. 6, p. 443–455, https:// doi .org /10 .1130 /L360 .1.
Carrapa, B., Orme, D.A., DeCelles, P.G., Kapp, P., Cosca, M.A., and Waldrip, R., 2014b, Miocene burial and exhumation of the India-Asia collision zone in southern Tibet:
Response to slab dynamics and erosion: Geology, v. 42, p. 443–446, https:// doi .org /10 .1130 /G35350 .1.
Carrapa, B., DeCelles, P.G., Wang, X., Clementz, M.T., Mancin, N., Stoica, M., Kraatz, B., Meng, J., Abdulov, S., and Chen, F., 2015, Tectono-climatic implications of Eocene Paratethys regression in the Tajik basin of central Asia: Earth and Planetary Science Letters, v. 424, p. 168–178, https:// doi .org /10 .1016 /j .epsl .2015 .05 .034.
Chapman, J.B., Carrapa, B., Ballato, P., DeCelles, P.G., Worthington, J., Oimahmadov, I., Ga-doev, M., and Ketcham, R., 2017, Intracontinental subduction beneath the Pamir Moun-tains: Constraints from thermokinematic modeling of shortening in the Tajik fold-and-thrust belt: Geological Society of America Bulletin, v. 129, p. 1450-1471, https:// doi .org /10 .1130 /B31730 .1.
Chapman, J.B., Scoggin, S.H., Kapp, P., Carrapa, B., Ducea, M.N., Worthington, J., Oimah-madov, I., and Gadoev, M., 2018, Mesozoic to Cenozoic magmatic history of the Pamir: Earth and Planetary Science Letters, v. 482, p. 181–192, https:// doi .org /10 .1016 /j .epsl .2017 .10 .041.
Copeland, P., Harrison, T.M., Kidd, W.E.A., Ronghua, X., and Yuquan, Z., 1987, Rapid early Mio-cene acceleration of uplift in the Gangdese Belt, Xizang (southern Tibet), and its bearing on accommodation mechanisms of the India-Asia collision: Earth and Planetary Science Letters, v. 86, p. 240–252, https:// doi .org /10 .1016 /0012 -821X (87)90224 -X.
Copeland, P., Harrison, T.M., Pan, Y., Kidd, W.S.F., Roden, M., and Zhang, Y., 1995, Thermal evolution of the Gangdese batholith, southern Tibet: A history of episodic unroofing: Tectonics, v. 14, p. 223–236, https:// doi .org /10 .1029 /94TC01676.
Copley, A., Avouac, J.P., and Wernicke, B.P., 2011, Evidence for mechanical coupling and strong Indian lower crust beneath southern Tibet: Nature, v. 472, p. 79, https:// doi .org /10 .1038 /nature09926.
Coutand, I., Strecker, M.R., Arrowsmith, J.R., Hilley, G., Thiede, R.C., Korjenkov, A., and Omu-raliev, M., 2002, Late Cenozoic tectonic development of the intramontane Alai Valley (Pamir‐Tien Shan region, central Asia): An example of intracontinental deformation due to the Indo‐Eurasia collision: Tectonics, v. 21, https:// doi .org /10 .1029 /2002TC001358.
Crawford, M.B., and Searle, M.P., 1992, Field relations and geochemistry of pre-collisional (India-Asia) granitoid magmatism in the central Karakoram, northern Pakistan: Tectono-physics, v. 206, p. 171–192, https:// doi .org /10 .1016 /0040 -1951 (92)90375 -G.
Debon, F., Le Fort, P., Dautel, D., Sonet, J., and Zimmerman, J.L., 1987, Granites of western Karakorum and northern Kohistan (Pakistan): A composite mid-Cretaceous to upper Ce-nozoic magmatism: Lithos, v. 20, p. 19–40, https:// doi .org /10 .1016 /0024 -4937 (87)90022 -3.
DeCelles, P.G., Robinson, D.M., and Zandt, G., 2002, Implications of shortening in the Hima-layan fold‐thrust belt for uplift of the Tibetan Plateau: Tectonics, v. 21, https:// doi .org /10.1029 /2001TC001322.
DeCelles, P.G., Kapp, P., Ding, L., and Gehrels, G.E., 2007, Late Cretaceous to middle Tertiary basin evolution in the central Tibetan Plateau: Changing environments in response to tectonic partitioning, aridification, and regional elevation gain: Geological Society of America Bulletin, v. 119, p. 654–680, https:// doi .org /10 .1130 /B26074 .1.
Dewey, J.F., Shackleton, R.M., Chengfa, C., and Yiyin, S., 1988, The tectonic evolution of the Ti-betan Plateau: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 327, p. 379–413, https:// doi .org /10 .1098 /rsta .1988 .0135.
Dodson, M.H., 1973, Closure temperature in cooling geochronological and petrological sys-tems: Contributions to Mineralogy and Petrology, v. 40, p. 259–274, https:// doi .org /10.1007 /BF00373790.
Dronov, V.L., Melnikova, G.K., Salibajev, G.Ch., Bardashev, I.A., Minajiev, V.E., and Mucha-batov, M.M., 2006, Stratigraphic Dictionary of the Pamirs: Technische Univeristaat Ber-gakademie, Freiberg, 252 p.
Ducea, M.N., Lutkov, V., Minaev, V.T., Hacker, B., Ratschbacher, L., Luffi, P., Schwab, M., Geh-rels, G.E., McWilliams, M., Vervoort, J., and Metcalf, J., 2003, Building the Pamirs: The view from the underside: Geology, v. 31, p. 849–852, https:// doi .org /10 .1130 /G19707 .1.
Fraser, J.E., Searle, M.P., Parrish, R.R., and Nobel, S.R., 2001, Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains: Geological So-ciety of America Bulletin, v. 113, p. 1443–1455, https:// doi .org /10 .1130 /0016 -7606 (2001)113 <1443: CODMAM>2 .0 .CO;2.
Gaetani, M., Jadoul, F., Erba, E., and Garzanti, E., 1993, Jurassic and Cretaceous orogenic events in the North Karakoram: Age constraints from sedimentary rocks, in Treloar, P.J., and Searle, M.P., eds., Himalayan Tectonics: Geological Society of London Special Pub-lication 74, p. 39–52, https:// doi .org /10 .1144 /GSL .SP .1993 .074 .01 .04.
Gaetani, M., Nicora, A., Henderson, C., Cirilli, S., Gale, L., Rettori, R., Vuolo, I., and Atudorei, V., 2013, Refinements in the Upper Permian to Lower Jurassic stratigraphy of Karakorum, Pakistan: Facies, v. 59, p. 915–948, https:// doi .org /10 .1007 /s10347 -012 -0346 -9.
Gao, R., Lu, Z., Klemperer, S.L., Wang, H., Dong, S., Li, W., and Li, H., 2016, Crustal-scale du-plexing beneath the Yarlung Zangbo suture in the western Himalaya: Nature Geoscience, v. 9, p. 555–560, https:// doi .org /10 .1038 /ngeo2730.
Hacker, B.R., Ratschbacher, L., Rutte, D., Stearns, M.A., Malz, N., Stübner, K., Kylander‐Clark, A.R.C., Pfänder, J.A., and Everson, A., 2017, Building the Pamir–Tibet plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Pamir: 3. Thermobarometry and petrochronology of deep Asian crust: Tectonics, v. 36, p. 1743–1766, https:// doi .org /10 .1002 /2017TC004488.
Hamburger, M.W., Sarewitz, D.R., Pavlis, T.L., and Popandopulo, G.A., 1992, Structural and seismic evidence for intracontinental subduction in the Peter the First Range, central Asia: Geological Society of America Bulletin, v. 104, p. 397–408, https:// doi .org /10 .1130 /0016 -7606 (1992)104 <0397: SASEFI>2 .3 .CO;2.
Hetzel, R., Dunkl, I., Haider, V., Strobl, M., von Eynatten, H., Ding, L., and Frei, D., 2011, Pe-neplain formation in southern Tibet predates the India-Asia collision and plateau uplift: Geology, v. 39, p. 983–986, https:// doi .org /10 .1130 /G32069 .1.
Hildebrand, P.R., Nobel, S.R., Searle, M.P., Waters, D.J., and Parrish, R.R., 2001, Old origin for an active mountain range: geology and geochronology of the eastern Hindu Kush,
LITHOSPHERE | Volume 10 | Number 3 | www.gsapubs.org 17
CHAPMAN ET AL. | Crustal shortening in the Pamir RESEARCH
Pakistan: Geological Society of America Bulletin, v. 113, p. 625–639, https:// doi .org /10 .1130 /0016 -7606 (2001)113 <0625: OOFAAM>2 .0 .CO;2.
Ischuk, A., Bendick, R., Rybin, A., Molnar, P., Khan, S.F., Kuzikov, S., Mohadjer, S., Saydullaev, U., Ilyasova, Z., Schelochkov, G., and Zubovich, A.V., 2013, Kinematics of the Pamir and Hindu Kush regions from GPS geodesy: Journal of Geophysical Research. Solid Earth, v. 118, p. 2408–2416, https:// doi .org /10 .1002 /jgrb .50185.
James, N.P., and Choquette, P.W., eds., 1988, Paleokarst: Springer Science & Business Media., 416 p, https:// doi .org /10 .1007 /978 -1 -4612 -3748 -8.
Jiang, Y.H., Liu, Z., Jia, R.Y., Liao, S.Y., Zhou, Q., and Zhao, P., 2012, Miocene potassic gran-ite–syenite association in western Tibetan Plateau: Implications for shoshonitic and high Ba–Sr granite genesis: Lithos, v. 134, p. 146–162, https:// doi .org /10 .1016 /j .lithos .2011 .12 .012.
Kapp, P., and Guynn, J.H., 2004, Indian punch rifts Tibet: Geology, v. 32, p. 993–996, https:// doi .org /10 .1130 /G20689 .1.
Kapp, P., Yin, A., Harrison, T.M., and Ding, L., 2005, Cretaceous-Tertiary shortening, basin de-velopment, and volcanism in central Tibet: Geological Society of America Bulletin, v. 117, p. 865–878, https:// doi .org /10 .1130 /B25595 .1.
Kapp, P., DeCelles, P.G., Gehrels, G.E., Heizler, M., and Ding, L., 2007a, Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet: Geo-logical Society of America Bulletin, v. 119, p. 917–933, https:// doi .org /10 .1130 /B26033 .1.
Kapp, P., DeCelles, P.G., Leier, A.L., Fabijanic, J.M., He, S., Pullen, A., Gehrels, G.E., and Ding, L., 2007b, The Gangdese retroarc thrust belt revealed: GSA Today, v. 17, no. 7, p. 4–9, https:// doi .org /10 .1130 /GSAT01707A .1.
Kufner, S.K., Schurr, B., Sippl, C., Yuan, X., Ratschbacher, L., Ischuk, A., Murodkulov, S., Schneider, F., Mechie, J., and Tilmann, F., 2016, Deep India meets deep Asia: Litho-spheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia): Earth and Planetary Science Letters, v. 435, p. 171–184, https:// doi .org /10 .1016 /j .epsl .2015 .11 .046.
Leeder, M.R., Smith, A.B., and Jixiang, Y., 1988, Sedimentology, palaeoecology and pal-aeoenvironmental evolution of the 1985 Lhasa to Golmud Geotraverse: Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, v. 327, p. 107–143, https:// doi .org /10 .1098 /rsta .1988 .0123.
Leier, A.L., DeCelles, P.G., Kapp, P., and Gehrels, G.E., 2007, Lower Cretaceous strata in the Lhasa terrane, Tibet, with implications for understanding the early tectonic history of the Tibetan Plateau: Journal of Sedimentary Research, v. 77, p. 809–825, https:// doi .org /10 .2110 /jsr .2007 .078.
Leith, W., and Alvarez, W., 1985, Structure of the Vakhsh fold-and-thrust belt, Tadjik SSR: geo-logic mapping on a Landsat image base: Geological Society of America Bulletin, v. 96, p. 875–885, https:// doi .org /10 .1130 /0016 -7606 (1985)96 <875: SOTVFB>2 .0 .CO;2.
Leven, J.E., 1995, Permian and Triassic of the Rushan-Pshart Zone (Pamir): Rivista Italiana di Paleontologia e Stratigrafia, v. 101, p. 3–16.
Li, T., Chen, J., Thompson, J.A., Burbank, D.W., and Xiao, W., 2012, Equivalency of geologic and geodetic rates in contractional orogens: New insights from the Pamir Frontal Thrust: Geophysical Research Letters, v. 39, L15305, https:// doi .org /10 .1029 /2012GL051782.
Lindsay, C.R., 2007, Geologic Map of Quadrangles 3772, 3774, 3672, and 3674, Gaz-Khan (313), Sarhad (314), Kol-I-Chaqmaqtin (315), Khandud (319), Deh-Ghulaman (320), and Ertfah (321) quadrangles, Afghanistan: U.S. Geological Survey Open File Report 2005-1097-A, https://pubs.usgs.gov/of/2005/1097/A/.
Lister, G.S., and Davis, G.A., 1989, The origin of metamorphic core complexes and detach-ment faults formed during Tertiary continental extension in the northern Colorado River region, USA: Journal of Structural Geology, v. 11, p. 65–94, https:// doi .org /10 .1016 /0191 -8141 (89)90036 -9.
Liu, M., and Yang, Y., 2003, Extensional collapse of the Tibetan plateau: Results of three‐di-mensional finite element modeling: Journal of Geophysical Research. Solid Earth, v. 108, https:// doi .org /10 .1029 /2002JB002248.
Lukens, C.E., Carrapa, B., Singer, B.S., and Gehrels, G., 2012, Miocene exhumation of the Pamir revealed by detrital geothermochronology of Tajik rivers: Tectonics, v. 31, TC2014, https:// doi .org /10 .1029 /2011TC003040.
Mack, G.H., James, W.C., and Monger, H.C., 1993, Classification of paleosols: Geological So-ciety of America Bulletin, v. 105, p. 129–136, https:// doi .org /10 .1130 /0016 -7606 (1993)105 <0129: COP>2 .3 .CO;2.
Mahéo, G., Guillot, S., Blichert-Toft, J., Rolland, Y., and Pêcher, A., 2002, A slab breakoff model for the Neogene thermal evolution of South Karakorum and South Tibet: Earth and Plan-etary Science Letters, v. 195, p. 45–58, https:// doi .org /10 .1016 /S0012 -821X (01)00578 -7.
Mechie, J., Yuan, X., Schurr, B., Schneider, F., Sippl, C., Ratschbacher, L., Minaev, V., Gadoev, M., Oimahmadov, I., Abdybachaev, U., and Moldobekov, B., 2012, Crustal and uppermost mantle velocity structure along a profile across the Pamir and southern Tien Shan as derived from project TIPAGE wide-angle seismic data: Geophysical Journal International, v. 188, p. 385–407, https:// doi .org /10 .1111 /j .1365 -246X .2011 .05278 .x.
Murphy, M.A., Yin, A., Harrison, T.M., Durr, S.B., Chen, Z., Ryerson, F.J., Kidd, W.S.F., Wang, X., and Zhou, X., 1997, Significant crustal shortening in south-central Tibet prior to the Indo-Asian collision: Geology, v. 25, p. 719–722, https:// doi .org /10 .1130 /0091 -7613 (1997)025 <0719: DTIACA>2 .3 .CO;2.
Nábělek, J., Hetényi, G., Vergne, J., Sapkota, S., Kafle, B., Jiang, M., Su, H., Chen, J., and Huang, B.S., 2009, Underplating in the Himalaya-Tibet collision zone revealed by the Hi-CLIMB experiment: Science, v. 325, p. 1371–1374, https:// doi .org /10 .1126 /science .1167719.
Nelson, K.D., Zhao, W., Brown, L.D., Kuo, J., Che, J., Liu, X., Klemperer, S.L., Makovsky, Y., Meissner, R., Mechie, J., and Kind, R., 1996, Partially molten middle crust beneath south-ern Tibet: Synthesis of project INDEPTH results: Science, v. 274, p. 1684–1688, https:// doi .org /10 .1126 /science .274 .5293 .1684.
Pashkov, B.R., and Budanov, V.I., 1990, Tectonics of the SE-SW Pamir Junction: Geotecton-ics, v. 24, p. 246–253.
Ramsay, J.G., 1967, Folding and Fracturing of Rocks: New York, McGraw-Hill, 568 p.
Raterman, N.S., Robinson, A.C., and Cowgill, E.S., 2014, Structure and detrital zircon geo-chronology of the Domar fold-thrust belt: Evidence of pre-Cenozoic crustal thickening of the western Tibetan Plateau, in Nie, J., Horton, B.K., Hoke, G.D., eds., Toward an Im-proved Understanding of Uplift Mechanisms and the Elevation History of the Tibetan Plateau: Geological Society of America Special Paper 507, p. 89–114, https:// doi .org /10 .1130 /2014 .2507 (05).
Ravikant, V., Wu, F.Y., and Ji, W.Q., 2009, Zircon U–Pb and Hf isotopic constraints on petrogen-esis of the Cretaceous–Tertiary granites in eastern Karakoram and Ladakh, India: Lithos, v. 110, p. 153–166, https:// doi .org /10 .1016 /j .lithos .2008 .12 .013.
Reiners, P.W., and Brandon, M.T., 2006, Using thermochronology to understand orogenic ero-sion: Annual Review of Earth and Planetary Sciences, v. 34, p. 419–466, https:// doi .org /10 .1146 /annurev .earth .34 .031405 .125202.
Robinson, A.C., 2009, Geologic offsets across the northern Karakorum fault: Implications for its role and terrane correlations in the western Himalayan-Tibetan orogen: Earth and Planetary Science Letters, v. 279, p. 123–130, https:// doi .org /10 .1016 /j .epsl .2008 .12 .039.
Robinson, A.C., 2015, Mesozoic tectonics of the Gondwanan terranes of the Pamir plateau: Journal of Asian Earth Sciences, v. 102, p. 170–179, https:// doi .org /10 .1016 /j .jseaes .2014 .09 .012.
Robinson, A.C., Yin, A., Manning, C.E., Harrison, T.M., Zhang, S.-H., and Wang, X.-F., 2004, Tectonic evolution of the northeastern Pamir: Constraints from the northern portion of the Cenozoic Kongur Shan extensional system: Geological Society of America Bulletin, v. 116, p. 953–974, https:// doi .org /10 .1130 /B25375 .1.
Robinson, A.C., Yin, A., Manning, C.E., Harrison, T.M., Zhang, S.-H., and Wang, X.-F., 2007, Cenozoic evolution of the eastern Pamir: implications for strain accommodation mecha-nisms at the western end of the Himalayan-Tibetan orogen: Geological Society of America Bulletin, v. 119, p. 882–896, https:// doi .org /10 .1130 /B25981 .1.
Robinson, A.C., Ducea, M., and Lapen, T.J., 2012, Detrital zircon and isotopic constraints on the crustal architecture and tectonic evolution of the northeastern Pamir: Tectonics, v. 31, https:// doi .org /10 .1029 /2011TC003013.
Rohrmann, A., Kapp, P., Carrapa, B., Reiners, P.W., Guynn, J., Ding, L., and Heizler, M., 2012, Thermochronologic evidence for plateau formation in central Tibet by 45 Ma: Geology, v. 40, p. 187–190, https:// doi .org /10 .1130 /G32530 .1.
Rutte, D., Lothar, R., Schneider, S., Stübner, K., Stearns, M.A., Gulzar, M.A. and Hacker, B.R., 2017a, Building the Pamir‐Tibet Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 1. Geometry and kinematics: Tectonics, v. 36, p. 342–384, https:// doi .org /10 .1002 /2016TC004293.
Rutte, D., Ratschbacher, L., Khan, J., Stübner, K., Hacker, B.R., Stearns, M.A., Enkelmann, E., Jonckheere, R., Pfänder, J.A., Sperner, B. and Tichomirowa, M., 2017b, Building the Pamir‐Tibet Plateau—Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 2. Timing and rates: Tectonics, v. 36, p. 385–419, https:// doi .org /10 .1002 /2016TC004294.
Salvador, A., ed., 1994, International Stratigraphic Guide: A Guide to Stratigraphic Classifica-tion, Terminology, and Procedure: Boulder, Colorado, Geological Society of America, 214 p.
Schmidt, J., Hacker, B.R., Ratschbacher, L., Stübner, K., Stearns, M., Kylander-Clark, A., Cottle, J.M., Alexander, A., Webb, G., Gehrels, G., and Minaev, V., 2011, Cenozoic deep crust in the Pamir: Earth and Planetary Science Letters, v. 312, p. 411–421, https:// doi .org /10 .1016 /j .epsl .2011 .10 .034.
Schneider, F.M., Yuan, X., Schurr, B., Mechie, J., Sippl, C., Haberland, C., Minaev, V., Oimah-madov, I., Gadoev, M., Radjabov, N., and Abdybachaev, U., 2013, Seismic imaging of subducting continental lower crust beneath the Pamir: Earth and Planetary Science Let-ters, v. 375, p. 101–112, https:// doi .org /10 .1016 /j .epsl .2013 .05 .015.
Schurr, B., Ratschbacher, L., Sippl, C., Gloaguen, R., Yuan, X., and Mechie, J., 2014, Seismotec-tonics of the Pamir: Tectonics, v. 33, p. 1501–1518, https:// doi .org /10 .1002 /2014TC003576.
Schwab, M., Ratschbacher, L., Siebel, W., McWilliams, M., Minaev, V., Lutkov, V., Chen, F., Stanek, K., Nelson, B., Frisch, W., and Wooden, J.L., 2004, Assembly of the Pamirs: Age and origin of magmatic belts from the southern Tien Shan to the southern Pamirs and their relation to Tibet: Tectonics, v. 23, https:// doi .org /10 .1029 /2003TC001583.
Searle, M.P., Khan, M.A., Fraser, J.E., Gough, S.J., and Jan, M.Q., 1999, The tectonic evolution of the Kohistan‐Karakoram collision belt along the Karakoram Highway transect, north Pakistan: Tectonics, v. 18, p. 929–949, https:// doi .org /10 .1029 /1999TC900042.
Shvol’man, V.A., 1978, Relicts of the Mesotethys in the Pamirs: Himalayan Geology, v. 8, p. 369–378.
Sippl, C., Schurr, B., Yuan, X., Mechie, J., Schneider, F.M., Gadoev, M., Orunbaev, S., Oimah-madov, I., Haberland, C., Abdybachaev, U., and Minaev, V., 2013, Geometry of the Pamir‐Hindu Kush intermediate‐depth earthquake zone from local seismic data: Journal of Geophysical Research. Solid Earth, v. 118, p. 1438–1457, https:// doi .org /10 .1002 /jgrb .50128.
Smit, M.A., Ratschbacher, L., Kooijman, E., and Stearns, M.A., 2014, Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry: Geol-ogy, v. 42, p. 1047–1050, https:// doi .org /10 .1130 /G35878 .1.
Sobel, E.R., 1999, Basin analysis of the Jurassic–Lower Cretaceous southwest Tarim basin, northwest China: Geological Society of America Bulletin, v. 111, p. 709–724, https:// doi .org /10 .1130 /0016 -7606 (1999)111 <0709: BAOTJL>2 .3 .CO;2.
Sobel, E.R., Chen, J., Schoenbohm, L.M., Thiede, R., Stockli, D.F., Sudo, M., and Strecker, M.R., 2013, Oceanic-style subduction controls late Cenozoic deformation of the North-ern Pamir orogen: Earth and Planetary Science Letters, v. 363, p. 204–218, https:// doi .org /10 .1016 /j .epsl .2012 .12 .009.
Stearns, M.A., Hacker, B.R., Ratschbacher, L., Lee, J., Cottle, J.M., and Kylander-Clark, A., 2013, Synchronous Oligocene–Miocene metamorphism of the Pamir and the north Hi-malaya driven by plate-scale dynamics: Geology, v. 41, p. 1071–1074, https:// doi .org /10 .1130 /G34451 .1.
Stearns, M.A., Hacker, B.R., Ratschbacher, L., Rutte, D., and Kylander‐Clark, A.R.C., 2015, Ti-tanite petrochronology of the Pamir gneiss domes: Implications for middle to deep crust
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CHAPMAN ET AL. | Crustal shortening in the Pamir RESEARCH
exhumation and titanite closure to Pb and Zr diffusion: Tectonics, v. 34, p. 784–802, https:// doi .org /10 .1002 /2014TC003774.
Strecker, M.R., Frisch, W., Hamburger, M.W., Ratschbacher, S., Semiletkin, S., Zamoruyev, A., and Sturchio, N., 1995, Quaternary deformation in the Eastern Pamirs, Tadzhikistan and Kyrgyzstan: Tectonics, v. 14, p. 1061–1079, https:// doi .org /10 .1029 /95TC00927.
Stübner, K., Ratschbacher, L., Rutte, D., Stanek, K., Minaev, V., Wiesinger, M., and Gloaguen, R., 2013a, The giant Shakhdara migmatitic gneiss dome, Pamir, India‐Asia collision zone: 1. Geometry and kinematics: Tectonics, v. 32, p. 948–979, https:// doi .org /10 .1002 /tect .20057.
Stübner, K., Ratschbacher, L., Weise, C., Chow, J., Hofmann, J., Khan, J., Rutte, D., Sperner, B., Pfänder, J.A., Hacker, B.R., and Dunkl, I., 2013b, The giant Shakhdara migmatitic gneiss dome, Pamir, India‐Asia collision zone: 2. Timing of dome formation: Tectonics, v. 32, p. 1404–1431, https:// doi .org /10 .1002 /tect .20059.
Styron, R., Taylor, M., and Sundell, K., 2015, Accelerated extension of Tibet linked to the northward underthrusting of Indian crust: Nature Geoscience, v. 8, p. 131, https:// doi .org /10 .1038 /ngeo2336.
Sui, Q.L., Wang, Q., Zhu, D.C., Zhao, Z.D., Chen, Y., Santosh, M., Hu, Z.C., Yuan, H.L., and Mo, X.X., 2013, Compositional diversity of ca. 110 Ma magmatism in the northern Lhasa Ter-rane, Tibet: Implications for the magmatic origin and crustal growth in a continent–con-tinent collision zone: Lithos, v. 168, p. 144–159, https:// doi .org /10 .1016 /j .lithos .2013 .01 .012.
Sun, G., Hu, X., Sinclair, H.D., BouDagher-Fadel, M.K., and Wang, J., 2015, Late Cretaceous evolution of the Coqen Basin (Lhasa terrane) and implications for early topographic growth on the Tibetan Plateau: Geological Society of America Bulletin, v. 127, p. 1001–1020, https:// doi .org /10 .1130 /B31137 .1.
Vermeesch, P., 2012, On the visualisation of detrital age distributions: Chemical Geology, v. 312–313, p. 190–194, https:// doi .org /10 .1016 /j .chemgeo .2012 .04 .021.
Villarreal, D., Robinson, A., Oimahmadov, I., MacDonald, B., Carrapa, B., and Gadoev, M., 2015, Assessing pre-Cenozoic shortening of the southern Pamir: American Geophysical Union Fall Meeting, San Francisco, California, 14–18 December, Abstract T21B-2825.
Vlasov, N., Yu, G., Dyakov, A., and Cherev, E.S., eds., 1991, Geological map of the Tajik SSR and adjacent territories, 1:500,000: Saint Petersburg, Russia, Vsesojuznoi Geologic In-stitute of Leningrad.
Volkmer, J.E., Kapp, P., Guynn, J.H., and Lai, Q., 2007, Cretaceous‐Tertiary structural evolu-tion of the north central Lhasa terrane, Tibet: Tectonics, v. 26, https:// doi .org /10 .1029 /2005TC001832.
Volkmer, J.E., Kapp, P., Horton, B.K., Gehrels, G.E., Minervini, J.M., and Ding, L., 2014, North-ern Lhasa thrust belt of central Tibet: Evidence of Cretaceous–early Cenozoic shortening
within a passive roof thrust system?: Geological Society of America Special Paper 507, p. 59–70, https:// doi .org /10 .1130 /2014 .2507 (03).
Wang, C., Dai, J., Zhao, X., Li, Y., Graham, S.A., He, D., Ran, B., and Meng, J., 2014, Outward-growth of the Tibetan Plateau during the Cenozoic: A review: Tectonophysics, v. 621, p. 1–43, https:// doi .org /10 .1016 /j .tecto .2014 .01 .036.
Wernicke, B., and Burchfiel, B.C., 1982, Modes of extensional tectonics: Journal of Structural Geology, v. 4, p. 105–115, https:// doi .org /10 .1016 /0191 -8141 (82)90021 -9.
Xiao, W.J., Windley, B.F., Chen, H.L., Zhang, G.C., and Li, J.L., 2002, Carboniferous-Triassic subduction and accretion in the western Kunlun, China: Implications for the collisional and accretionary tectonics of the northern Tibetan Plateau: Geology, v. 30, p. 295–298, https:// doi .org /10 .1130 /0091 -7613 (2002)030 <0295: CTSAAI>2 .0 .CO;2.
Yushin, I.P., Sass, M.E., Karapetov, S.S., Altukhov, S.M., Teplov, I.C., Raeakov, C.R., Harkov, S.I., and Davidchenko, A.G., 1964, J-43-VIII, XIV, XV, XX, XXI, 1:200,000 maps of the Tajik SSR: Saint Petersburg, Russia, Russian Geological Research Institute.
Zanchi, A., and Gaetani, M., 2011, The geology of the Karakoram range, Pakistan: The new 1:100,000 geological map of Central-Western Karakoram: Italian Journal of Geosciences, v. 130, p. 161–262.
Zanchi, A., Poli, S., Fumagalli, P., and Gaetani, M., 2000, Mantle exhumation along the Tirich Mir Fault Zone, NW Pakistan: Pre-mid-Cretaceous accretion of the Karakoram terrane to the Asian margin, in Khan, M.A., et al., eds., Tectonics of the Nanga Parbat Syntaxis and the Western Himalaya: Geological Society of London Special Publication 170, p. 237–252, https:// doi .org /10 .1144 /GSL .SP .2000 .170 .01 .13.
Zhao, W.L., and Morgan, W.J., 1987, Injection of Indian crust into Tibetan lower crust: A two‐dimensional finite element model study: Tectonics, v. 6, p. 489–504, https:// doi .org /10 .1029 /TC006i004p00489.
Zhu, D.C., Mo, X.X., Niu, Y., Zhao, Z.D., Wang, L.Q., Liu, Y.S., and Wu, F.Y., 2009, Geochemical investigation of Early Cretaceous igneous rocks along an east–west traverse throughout the central Lhasa Terrane, Tibet: Chemical Geology, v. 268, p. 298–312, https:// doi .org /10 .1016 /j .chemgeo .2009 .09 .008.
Zhu, D.C., Li, S.M., Cawood, P.A., Wang, Q., Zhao, Z.D., Liu, S.A., and Wang, L.Q., 2016, As-sembly of the Lhasa and Qiangtang terranes in central Tibet by divergent double subduc-tion: Lithos, v. 245, p. 7–17, https:// doi .org /10 .1016 /j .lithos .2015 .06 .023.
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