doi:10.1130/G25448A.1 2009;37;371-374 Geology

  Nicholas J. Van Buer, Elizabeth L. Miller and Trevor A. Dumitru  

the northwestern Basin and RangeEarly Tertiary paleogeologic map of the northern Sierra Nevada batholith and 

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© 2009 Geological Society of America

GEOLOGY, April 2009 371

INTRODUCTIONAn extensive regional unconformity, termed the basal Tertiary uncon-

formity, is currently exposed at higher elevations in the northern part of the Sierra Nevada and in numerous mountain ranges across much of the northern Basin and Range province in the western United States. This unconformity separates underlying plutonic and country rocks of the upper Mesozoic Sierra Nevada batholith from immediately overlying Eocene-Miocene stratifi ed volcanic and sedimentary rocks. As such, the unconformity represents the Earth’s surface in this region at the time of the fundamental tectonic transition from uplift and erosion of the extinct Sierra Nevada magmatic arc to deposition and preservation of the over lying strata. In this paper we compile a regional map of the rock units present immediately beneath the unconformity (Fig. 1B) and then remove post-unconformity Basin and Range extension. This yields a paleogeologic map of the region’s land surface in early to middle Tertiary time (Fig. 1D).

Our reconstruction of the preextensional surface geology across a broad region provides useful and incontrovertible data bearing on several debated topics regarding the tectonic and topographic evolution of this region. It clarifi es the pre-Tertiary extent and geology of the northern part of the Sierra Nevada batholith prior to Basin and Range faulting (Fig. 1A). It also helps address the preextensional topography of the region by illus-trating variations in depth of erosion beneath the unconformity. Estimates of batholith and backarc crustal thickness based on restoration of regional cross sections (e.g., Chase et al., 1998; DeCelles et al., 1995), mineral thermobarometry (e.g., Camilleri et al., 1997; Lewis et al., 1999), and paleotopography using oxygen isotopes (e.g., Mulch et al., 2006) all sug-gest that thick crust (50–65 km) and high elevations (2–4 km) persisted to the beginning of Tertiary extension. Conversely, restoration of known amounts of Tertiary extension together with geophysical data on Moho depths in northwestern Nevada suggest normal (~38 km) thickness crust prior to extension (e.g., Colgan et al., 2006; Lerch et al., 2007). Analysis of our early Tertiary paleogeologic map represents an independent approach to addressing the controversial paleogeology and paleotopog raphy across the region. Although the construction of such maps has proven extremely useful for understanding pre–Basin and Range structure in the Sevier thrust belt (Armstrong, 1968) and in east-central Nevada (Gans and Miller, 1983), until now it has not been comprehensively applied across the north-ern Sierra Nevada and adjacent Basin and Range.

BASAL TERTIARY UNCONFORMITYThe striking unconformity at the base of the predominately volcanic

Tertiary section in the northern Sierra Nevada forms a mostly unfaulted, continuous surface developed across granitoids and associated metamor-phosed wall rocks (e. g., Lindgren, 1911) (Fig. 1B). In the Basin and Range, younger extensional faulting offsets and tilts this same uncon-formity, exposing it in many individual fault-bound ranges (Fig. 1B). In detail, the unconformity is somewhat diachronous, with basal overlying rocks varying in age from early Eocene (gold-bearing gravels in the Sierra paleochannels) to Miocene volcanic rocks in both the Sierra Nevada and Basin and Range. The relief of the unconformity is generally quite mod-erate, but is as much as 0.5–1.6 km in deep paleochannels that preserve Eocene–Miocene fi lls in many places throughout our map area (Fig. 1D) (Henry, 2008). The limited stratigraphic thicknesses of Eocene–Miocene rocks suggest that deposition was minor during this interval, whereas the widespread preservation of these same rocks suggests that erosion was also minor during this time span compared to the deep erosion of the batholith between the end of magmatism ca. 85 Ma and the early develop-ment of the unconformity ca. 40 Ma.

PALEOGEOLOGIC MAPThe greater geologic complexity of the Basin and Range province

relative to the Sierra Nevada (Fig. 1A) is primarily a function of Basin and Range normal faulting, which has exposed a wide range of crustal levels at the surface today. Prior to Basin and Range extension, the map distribu-tion and continuity of units would have been far simpler. To illustrate this, we fi rst constructed the map in Figure 1B showing the modern surface traces of the basal Tertiary unconformity as wormy lines color keyed to the subjacent rock units. We compiled these unconformity data at a scale of 1:500,000 from numerous local and county geologic maps. In areas with no unconformity exposure, but shallow preextensional paleodepths, we inferred the subjacent unit from modern map patterns. Only plutons >10 km in diameter are shown.

To construct a paleogeologic map from Figure 1B, the effects of Cenozoic normal faulting were removed, at least in an approximate way. Extension in the northern Basin and Range over the past 50 m.y. has been episodic and heterogeneous, with different regions extending at different times (and/or multiple times) and by different amounts (e.g., Miller et al., 1999; Stockli, 2000; Surpless et al., 2002; McQuarrie and Wernicke, 2005; Colgan et al., 2006). Inception of major Basin and Range faulting began

Geology, April 2009; v. 37; no. 4; p. 371–374; doi: 10.1130/G25448A.1; 1 fi gure.© 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

Early Tertiary paleogeologic map of the northern Sierra Nevada batholith and the northwestern Basin and Range

Nicholas J. Van Buer*, Elizabeth L. Miller, Trevor A. DumitruDepartment of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA

ABSTRACTThe northern Sierra Nevada and adjacent Basin and Range, western United States, are

marked by a widespread regional unconformity at the base of the Eocene-Miocene volcanic and sedimentary section that overlies the Mesozoic batholith and its wall rocks. To help address controversial questions about the origin, uplift, and erosion of the batholith, we compiled a subcrop geologic map of the unconformity prior to Tertiary extension. This simple but under-utilized technique reveals the distribution of rock units that could have contributed detritus to the Eocene-Miocene river channels crossing the Sierra Nevada and demonstrates that the Mesozoic Sierra Nevada batholith was continuous to the northeast across the northwestern Basin and Range. More speculatively, the subcrop map implies that Late Cretaceous–Eocene erosional stripping may have been greatest above the axis of the batholith and decreased to the east; thus, drainage in this area may have been eastward and then switched westward in Eocene-Miocene time.

*E-mail: vanbuer@stanford.edu.

372 GEOLOGY, April 2009

Elko

H

P

T

No data

(no data)

(no data)

No data

Modoc Plateau

Owyhee Plateau

100 km

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B

116° W121° W38° N

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OR

NIA

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DA100 km

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A

121° W 116° W

42°

38°

Legend for overview map (A), unconformity map (B) and paleogeologic map (D)

WLZ

Modoc Plateau

5–10%, at 0–14 Ma

(7.5%)

NW Nevada15–25%

at 0–12 Ma

(20%)

OwyheePlateau<5% since 16 Ma

(0%)

NE Nevada

vada

15–20%at 0–12 Ma

older Eocene?(20%)

Central Nevada15–100%

at 16 Ma, older Oligocene(50%)

Yerington

>>100% at14 Ma, 5–10% at 0–5 Ma(150%)

Sierra Front

5–10% at 0–10? Ma

(7.5%)

N

100 km

38° N

42° N

121° W 116° W

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▼▼▼▼ Paleozoic folds and thrusts

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▼ Mesozoic folds and thrusts

Paleo-river canyons

(both from Henry, 2008)

Symbols for paleogeologic map (D)

A B D

Granitic rocks of the Sierra Nevada Batholith

Mesozoic wall rocks of the batholith

Upper Paleozoic wall rocks and Golconda allochthon

Lower Paleozoic wall rocks and Roberts Mts. allochthon

Cenozoic cover (no data)

Unconformity (color coded by basement)

Mzg

Mzm

uPz

lPz

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uPzuPz

uPzuPz

lPz

lPz

lPz

uPz

PMs

Mzg

0

5

A A′

Same scale as above map; no vertical exaggeration

Approximate depth of erosionkm

Paleozoic to Mesozoic miogeoclinal sequence (anticlines expose rocks as old as Ordovician; synclines as young as Jurassic)

Paleo-continental divide

2004 Stanford seismic line (Lerch et al., 2007)colored by inferred basement

N

N

M O D O CP L A T E A U

S I E

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A E

V A D

A

B A S I NA N D

R A N G E

M O D O CP L A T E A U

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o

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GEOLOGY, April 2009 373

at 17 Ma in central Nevada, 12 Ma in northern Nevada, and is believed to be younger than 8 Ma along the eastern Sierra Nevada (Fig. 1C). A number of separate extensional domains have been defi ned, where per-cent extension has been estimated through the construction of balanced cross sections or consideration of Tertiary unconformity data (Fig. 1C). The strain domain map is highly generalized, but at the scale of Figure 1B can be used to qualitatively restore the subunconformity units into their approximate position prior to Cenozoic faulting (Fig. 1D). The choice of percent extension was based on conservative estimates, and there are parts of the map where data are unavailable (e.g., the Carson Sink). Because the total magnitude of extension is not large (0%–50%, except for local regions; Fig. 1C), minor changes to our assumed extensional domains have little effect on the resulting map. Extension was restored in the direc-tion of fault slip, based on geologic mapping and/or perpendicular to the modern range-bounding normal faults for each domain (parallel to the arrows shown in Fig. 1C). Restoration also accounted for ~25–60 km of right-lateral movement (decreasing to the north) across the Walker Lane (Wesnousky, 2005; Faulds et al., 2005).

DISCUSSION AND CONCLUSIONSSeveral fundamental relations are apparent from Figure 1D. First,

this map provides the framework necessary for understanding detrital sig-natures preserved in the Eocene–Miocene channels of the Sierra Nevada. According to Figure 1D, these channels should primarily contain detritus derived from the batholith and its overlying Tertiary volcanic units, upper Paleozoic to lower Mesozoic strata of the miogeocline, Paleozoic eugeo-clinal rocks of the Roberts Mountains and Golconda allochthons, and weakly metamorphosed Mesozoic basinal rocks from western Nevada. If the paleodrainages were at some time different than depicted in Figure 1D, this should also be refl ected in the detrital signatures. Regardless, channel deposits should not contain material derived from stratigraphically deeper rocks of the miogeocline or from the plutons that intrude these depths, and preliminary detrital zircon studies of Eocene channel deposits in the Sierra suggest that this is the case (Cecil and Ducea, 2007).

Second, the continuity of the Sierra Nevada batholith across north-western Nevada becomes apparent after Cenozoic cover and extension are removed. Unlike the southern Sierra Nevada batholith, which trends north-northwest along the truncated edge of the North American craton, parallel to the modern continental margin, the northern part of the batho-lith trends north-northeast and intrudes Mesozoic backarc basin deposits,

bending inland from the continental margin. The buried northwest edge of the batholith (Fig. 1D) is imaged geophysically as a boundary between lower (to the east) and higher (to the west) seismic velocities in the upper 15 km of the crust (Fig. 1B; Lerch et al., 2007). Petrologic and geo-chronologic data indicate that the granitic rocks crossing the northwestern Basin and Range correlate with the ca. 120–85 Ma plutonic belts of the Sierra Nevada batholith (Smith et al., 1971; Barton et al., 1988; Lerch et al., 2007; Van Buer, 2008). The magnitude of Tertiary extension in the northwestern Basin and Range is well constrained (Colgan et al., 2006) and is insuffi cient to account for the eastward bend in the Sierra Nevada batholith (Figs. 1C, 1D). This suggests that the bend is a primary fea-ture of the Cretaceous arc. Northward shallowing of the subduction angle and/or batholith-age or younger extension beneath the Modoc plateau (Nilsen, 1993) could explain the bend.

Third, it appears that more material was eroded from the axis of the batholith than from the backarc region to the east during Late Cretaceous–Eocene time (Fig. 1D). Nearly all traces of coeval volcanic cover were eroded from the northern Sierra Nevada batholith. Granitoid emplace-ment depths at the unconformity are 3–8 km (e.g., Ague, 1997). Narrow, andalusite-bearing contact aureoles associated with satellite plutons indi-cate <7–11 km depths for the batholith in northwestern Nevada (Compton , 1960; Colgan 2006). Many Mesozoic plutons are currently exposed to the east of those shown in Figure 1B, but they intrude crustal levels that remained at depth during the development of the Tertiary uncon formity and were only exposed by Basin and Range faulting and tilting (e.g., Smith et al., 1991; Gans and Miller, 1983). From the east side of the batholith to the Luning-Fencemaker thrust system (Fig. 1D), the metamorphic grade of deformed Mesozoic basinal deposits varies from low to medium green-schist facies (Wyld et al., 2003). East of the Luning-Fencemaker belt, the Paleozoic rocks of the Golconda and Roberts Mountains allochthons are mostly unmetamorphosed, although locally overprinted by Mesozoic deformation and metamorphism (Miller et al., 1992). Mesozoic deforma-tion of these older tectonic units is suffi ciently limited at the regional scale that the Paleozoic thrust architecture appears fundamentally unmodi-fi ed at the scale of our paleogeologic map (Fig. 1D). East of the Roberts Mountains thrust (Fig. 1D), upper Paleozoic and lower Mesozoic miogeo-clinal strata beneath the Tertiary unconformity have undergone little or no burial-induced metamorphism, as determined by their conodont alteration indices (e.g., Gans et al., 1990). Sevier-age folds in the miogeocline east of the Roberts Mountains thrust expose rocks as deep as Ordovician in anti-clines, but essentially unburied Jurassic strata are preserved in synclines, consistent with erosion varying from 0 to 4 km. Overall, these observa-tions indicate that depths of postbatholith erosion beneath the Tertiary unconformity decrease eastward from the axis of the batholith.

Greater erosion along the batholith axis implies greater net uplift and, presumably, higher topography in this region generated during its emplacement history. This relationship supports the idea that the ances-tral Sierra Nevada formed a paleodivide (e.g., Wakabayashi and Sawyer, 2001; DeGraaff-Surpless et al., 2002), with rivers draining both westward, into the Great Valley, and also eastward or northward (Fig. 1D). Modest preextensional crustal thicknesses (~38 km) in northwestern Nevada (e.g., Colgan et al., 2006) as compared to the southern Sierra Nevada (~45–65 km; e.g., Wernicke et al., 1996), however, suggest that this northern topographic divide may not have been as high as the contemporaneous southern Sierra Nevada. By Eocene time, however, paleorivers draining westward from a divide in east-central Nevada had already been incised and were beginning to accumulate gravels and volcanogenic strata, as documented in detail by Henry (2008) (Fig. 1D). The divide or crest of the western U.S. at this time, as shown by Henry (2008), is depicted as a dashed line in Figure 1C, and is considerably to the east. Where studied by Henry (2008), Eocene–Miocene river systems persisted until disruption by Basin and Range faulting and carried debris from as far east as central

Figure 1. A: Plutonic rocks of northern Sierra Nevada batholith (red) and their country rocks (green) extend across northern Basin and Range but are dismembered by normal faulting and partly covered by Tertiary strata. B: Database of extent and nature of basal Ter-tiary unconformity and underlying rocks across northern Nevada and northern Sierra. Line squiggles represent exposed traces of the unconformity today; color coding represents main rock types immediately beneath unconformity. Colored fi elds represent post-extensional view of pre-unconformity paleogeologic map. H—Honey Lake; P—Pyramid Lake; T—Lake Tahoe. C: Generalized estimates of percent extension and its age east of Sierra Nevada, based on fault attitudes, tilt of Tertiary unconformity, and palinspastic restora-tion of cross sections. Northwestern Nevada—Colgan et al. (2006); Singatse-Wassuk Range—Surpless et al. (2002) and Proffett and Dilles (1984); central Nevada—Smith et al. (1991); Modoc-Owyhee Plateau—Wells and Heller (1988). Actual values used for restora-tion are in parentheses. WLZ—Walker Lane zone. D: Restored early Tertiary paleogeologic map with extension removed. LFTB—Luning-Fencemaker thrust belt; GT—Golconda thrust; RMT—Roberts Moun-tains thrust. Graph at bottom shows relative amounts of erosion at the batholith and backarc region inferred along schematic west to east cross section along line A-A′.

374 GEOLOGY, April 2009

Nevada across the Sierra (Fig. 1D). These relations suggest a major shift in paleogeography within the time interval bracketed by the youngest plutons of the batholith (ca. 85 Ma) and the oldest fl uvial deposits (ca. 45 Ma) of the Eocene–Miocene river systems of the Sierra (Fig. 1D) (e.g., Lindgren, 1911; Henry, 2008). This shift was followed by the major switch from ero-sion to deposition as marked by the unconformity. Future work that better defi nes the timing of this regional paleotopographic change will be very useful for constraining geodynamic models of the change from batholith emplacement to shallow-slab subduction to renewed magmatism and the onset of Basin and Range extension.

ACKNOWLEDGMENTSThis paper represents part of an ongoing effort by the structural geology and

tectonics research group at Stanford University aimed at compiling and interpret-ing data on the Tertiary unconformity across the northern Great Basin and Sierra Nevada. During this time span, our research was partially funded by National Sci-ence Foundation (NSF) grants EAR-9417937, EAR-9725371, EAR-0229854; Earthscope NSF 0346245; American Chemical Society–Petroleum Research Fund 39063-AC8, and NSF Tectonics 0809226.

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Manuscript received 28 August 2008Revised manuscript received 17 November 2008Manuscript accepted 1 December 2008

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