Early extensional detachments in a contractional orogen · Draft Early extensional detachments in a...
Transcript of Early extensional detachments in a contractional orogen · Draft Early extensional detachments in a...
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Early extensional detachments in a contractional orogen:
coherent, map-scale, submarine slides (mass transport complexes) on the outer slope of an Ediacaran collisional
foredeep, eastern Kaoko belt, Namibia
Journal: Canadian Journal of Earth Sciences
Manuscript ID cjes-2015-0164.R1
Manuscript Type: Article
Date Submitted by the Author: n/a
Complete List of Authors: Hoffman, Paul; 1216 Montrose Ave Bellefroid, Eric; Yale University, Geology and Geophysics Johnson, Benjamin; University of Victoria, School of Earth and Ocean Sciences Hodgskiss, Malcolm ; McGill University, Earth and Planetary Sciences Schrag, Daniel; Harvard University, Earth and Planetary Sciences Halverson, Galen; McGill University, Earth and Planetary Sciences
Keyword: submarine landslide, mass transport complex, extensional detachment,
slope failure, continental margin
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cjes-‐2015-‐0164 (revised, 12 Jan 16) 1
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CJES Special Issue: Uniformitarianism and Plate Tectonics 3
A Tribute to Kevin C. Burke and John F. Dewey 4
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Early extensional detachments in a contractional orogen: coherent, 7
map-‐scale, submarine slides (mass transport complexes) on the outer 8
slope of an Ediacaran collisional foredeep, eastern Kaoko belt, Namibia 9
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Paul F. Hoffman1, Eric J. Bellefroid2, Benjamin W. Johnson3, 12
Malcolm S. W. Hodgskiss4, Daniel P. Schrag5 and Galen P. Halverson4 13 14
11216 Montrose Avenue, Victoria, BC V8T 2K4 15 2Department of Geology and Geophysics, Yale University, New Haven, CT 06520-‐8109 16
3School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8P 5C2 17 4Department of Earth and Planetary Sciences, McGill University, Montreal, QC H3A 0E8 18
5Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138 19
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Corresponding author: [email protected] 21
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Abstract: The existence of coherent, large-‐scale, submarine landslides on modern 22
continental margins implies that their apparent rarity in ancient orogenic belts is 23
due to non-‐recognition. Two map-‐scale, coherent, pre-‐orogenic, normal-‐sense 24
detachment structures of Ediacaran age are present in the Kaoko belt, a well-‐25
exposed arc-‐continent collision zone in northwestern Namibia. The structures occur 26
within the Otavi Group, a Neoproterozoic carbonate shelf succession. They are 27
brittle structures, evident only through stratigraphic omissions of 400 m or more, 28
that ramp down to the west with overall ramp angles of 1.1° and 1.3° with respect to 29
stratigraphic horizons. The separations of matching footwall and hangingwall 30
stratigraphic cut-‐offs require horizontal translations >20 km for each detachment. 31
One of the detachments is remarkably narrow (5 km) in the up-‐dip direction, just 32
one fourth of its translation. The other detachment is stratigraphically dated at the 33
shelf-‐foredeep transition, when the passive margin was abortively subducted 34
westward, in the direction of submarine sliding. Trenchward sliding on the 35
foreslope occurred concurrently with deep karstification of the autochthonous 36
carbonate succession to the east, presumably due to forebulge uplift and/or 37
conjectural basin-‐scale base-‐level fall. We expect that similar detachments exist in 38
other orogenic belts, and failure to recognize them can lead to misinterpretations of 39
stratigraphy, sedimentary facies and paleogeography. 40
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Keywords: submarine landslide, mass transport complex, extensional detachment, 42
slope failure, continental margin, collisional foredeep 43
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Introduction 46
Coherent, large-‐scale, submarine landslides exist on many modern continental 47
margins (Dingle 1977; Bugge et al. 1987; Kayen and Lee 1991; Hampton et al. 1996; 48
Rowan et al. 2004; Frey-‐Martinez et al. 2005; Masson et al. 2006; Butler and Paton 49
2010; Morley et al. 2011; Armandita et al. 2015). Bathymetric and/or seismic 50
reflection studies show them to consist of up-‐dip extended domains and down-‐dip 51
shortened domains, characterized by breakaway-‐ and thrust-‐duplexes respectively 52
(Fig. 1). Although stratigraphically confined, the presence of growth strata (Fig. 1A) 53
show some examples to have developed slowly by creep. Their apparent rarity in 54
ancient orogenic belts is more likely due to non-‐recognition than non-‐occurrence. 55
The discovery of the Ombonde detachment (Hoffman and Hartz 1999) in the 56
Kaoko belt of Namibia (Fig. 2) provided an Ediacaran example for which the net 57
displacement, paleotectonic setting and longitudinal (i.e., slip-‐parallel) cross-‐section 58
of the extended domain (Fig. 1) are well constrained by plunge projection of surface 59
maps and stratigraphic logs (Hoffman and Hartz 1999; Hoffman and Halverson 60
2008). What the Ombonde detachment did not reveal was its transverse (i.e., slip-‐61
normal) dimension, and an implicit shortened domain (Fig. 1) in the down-‐dip 62
direction, which is obscured by post-‐orogenic cover. The shortened domain of a 63
possibly analogous detachment, the ‘Saturn slide’ (Clifford 1962, 2008), was 64
recognized earlier within correlative strata of the adjacent Outjo zone (Fig. 3). 65
Here we describe a second low-‐angle extensional detachment in the Kaoko belt 66
(Fig. 3). Like the Ombonde detachment, 90 km to the south, the Ombepera 67
detachment predates orogenic shortening. It is a brittle structure within a carbonate 68
platform succession, locally associated with a dilational breccia zone in the 69
hangingwall, but lacking recognizeable shear fabrics. The structure is only evident 70
through stratigraphic mapping. The average ramp angle between the detachment 71
and stratigraphic planes is 1.3°, similar to the Ombonde detachment (1.1°). The 72
characteristic stratigraphic omission is somewhat greater than the 395 m of missing 73
strata across the Ombonde detachment, and the separation of stratigraphic cut-‐offs 74
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in the footwall and hangingwall of the Ombepera detachment imply >20 km of 75
displacement. 76
The timing of the Ombonde detachment is tightly constrained. Movement 77
postdates the entire Otavi Group carbonate shelf sequence and predates the basal 78
foredeep clastics of the disconformably overlying Mulden Group (Fig. 4)(Hoffman 79
and Hartz 1999). We assume contemporaneity of the two detachments based on 80
structural homology. Like the Ombonde detachment, only the extended domain of 81
the Ombepera detachment is exposed. Unlike the Ombonde detachment, its 82
transverse extent is directly constrained by surface mapping. This, and mutual 83
verification of the phenomenon, make the Ombepera detachment a structure of 84
interest. Failure to recognize the detachment would have led to serious 85
misinterpretations of the stratigraphy and sedimentary facies relations. 86
We begin by describing the basic stratigraphy of Neoproterozoic cover on the 87
southwestern Congo craton and its relation to the Kaoko and Damara orogenic belts, 88
in the context of centre-‐west Gondwana amalgamation. Next we review the salient 89
map-‐scale features of the Ombonde detachment that bear on its geometry, 90
displacement and age. Then we describe how the Ombepera detachment came to be 91
recognized in its type area, and how its interpretation was tested and confirmed by 92
up-‐dip projection. Finally, we identify the need for additional mapping to investigate 93
marked widening and/or flattening of the Ombepera detachment surface in the 94
down-‐dip direction 95
96
Neoproterozoic cover of the southwestern Congo craton 97
Paleoproterozoic crust of the present southwestern promontory of the Congo 98
craton (Fig. 2) is blanketed by a Neoproterozoic sedimentary succession composed 99
of three groups (Fig. 4). The basement-‐hugging Nosib Group (SACS 1980) consists of 100
subfeldspathic arenite and rudite deposited on a southward-‐sloping alluvial 101
braidplain. The conformably overlying Otavi Group (SACS 1980) is a 2-‐ to 4-‐km-‐102
thick carbonate shelf sequence with a well-‐defined shelf-‐break and a distally-‐103
tapered southern foreslope (Hoffman and Halverson 2008). A presumed western 104
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shelf margin is occluded by the Sesfontein thrust (Fig. 3), an eastward-‐vergent, 105
crustal-‐scale, thrust fault carrying the Central Kaoko zone, where the Otavi Group is 106
an off-‐shelf facies. 107
Internally, the Otavi Group consists of three subgroups (Fig. 4), divided by 108
unconformities that underlie each of the two Cryogenian glacial-‐periglacial 109
assemblages (Hoffmann and Prave 1996). The Chuos and Ghaub formations 110
represent the prolonged Sturtian (717-‐659 Ma) and briefer Marinoan (ca 645-‐635 111
Ma) glaciations, respectively. The Rasthof and Maieberg formations represent the 112
respective postglacial cap-‐carbonate sequences (Hoffman and Halverson 2008). 113
The Tonian Ombombo Subgroup (Hoffmann and Prave 1996) consists of cyclical, 114
shallow-‐marine dolomite with authigenic chert, intercalated with alluvial and 115
deltaic clastic rocks. Clastic input resulted from intermittent uplift and erosion of 116
northward-‐inclined dip-‐slopes, interpreted as the footwalls of spaced, south-‐117
dipping, normal faults (Hoffman and Halverson 2008). The age of the Nosib-‐Otavi 118
transgression and the onset of rift faulting must be significantly older than 760 ± 1 119
Ma (Halverson et al. 2005), the U-‐Pb zircon date of a bentonite near the top of the 120
carbonate-‐rich Devede Formation (Fig. 4) in the middle Ombombo Subgroup. 121
Intermittent rift-‐shoulder uplift, inferred from low-‐angle unconformities and 122
cannibalistic sedimentation, persisted into the Cryogenian Abenab Subgroup (SACS 123
1980) up to the base of the Ombaatjie Formation (Fig. 4), which marks the rift-‐to-‐124
shelf transition (Hoffman and Halverson 2008) at an estimated 650 Ma. Thereafter, 125
the shelf aggraded conformably and preferentially relative to the foreslope and 126
basin in the Outjo zone (Fig. 3) until the demise of carbonate sedimentation at the 127
top of the Ediacaran Tsumeb Subgroup (SACS 1980), the thickest and most 128
continuous of the Otavi subgroups. 129
The Mulden Group (SACS 1980) paraconformably overlies the Otavi Group and 130
forms an upward-‐shoaling, marine to nonmarine sequence of compositionally-‐131
immature clastics, culminating with crossbedded arenites (Renosterberg 132
Formation) deposited by high-‐discharge southeastward-‐flowing rivers (Hoffman 133
and Halverson 2008). On the southwest flank of the Kamanjab inlier (Fig. 3), paleo-‐134
outliers of Otavi Group carbonate <900 m thick are buried paraconformably by 135
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Mulden Group clastics which, at their base, were deposited directly upon the 136
crystalline basement (Frets 1969). The karstic outliers shed dolomite-‐chert debris-‐137
fans and conglomerates into the more far-‐travelled Mulden Group detritus. Locally, 138
the present drainages (e.g. upper Huab River at 20°06'10"S, 14°36'04"E) have re-‐139
incised Mulden-‐age paleovalleys (Frets 1969). The Mulden Group is interpreted as a 140
trench-‐foredeep assemblage associated with abortive westward subduction of the 141
Congo margin beneath the Dom Feliciano–Ribeira arc (Fig. 2)(Oyhantçabal et al. 142
2009; Chemale et al. 2012). Alternatively, it was a retro-‐arc foreland basin 143
developed above an east-‐dipping subduction zone (Goscombe and Gray 2007, 2008; 144
Konopásek et al. 2014). In our view, the largely amagmatic development of the rifted 145
western Congo margin (Guj 1970; Stanistreet and Charlesworth 1999; Goscombe et 146
al. 2005; Miller 2008) is more consistent with the first alternative. There is no 147
evidence for subduction-‐related magmatism in the Central Kaoko zone (see below), 148
in contrast with the Western Kaoko zone (Fig. 3). 149
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Kaoko belt 151
Neoproterozoic cover and basement of the southwestern Congo craton are 152
deformed in an arcuate fold belt that manifests crustal shortening in the mutually 153
perpendicular Kaoko and Damara orogenic belts (Fig. 2). In the Eastern Kaoko zone 154
(Fig. 3), a train of tight, thin-‐skinned folds and subordinate thrusts formed above a 155
décollement within the Beesvlakte Formation (Ombombo Subgroup). This thin-‐156
skinned, eastward-‐directed, thrust-‐fold belt was subsequently refolded by broader, 157
coaxial, basement-‐involved structures, a relationship best observed at the northern 158
plunge of the Kamanjab basement inlier (Fig. 3). The northward and southward 159
plunges characteristic of Kaoko belt structures are at least in part a product of late-‐160
stage refolding as a far-‐field consequence of collisions in the Damara belt. 161
The Central Kaoko zone (CKz, Fig. 3) underwent sinistral transpression, dynamic 162
metamorphism, and rapid exhumation at 580-‐570 Ma, creating a system of 163
eastward-‐vergent, basement-‐cored, thrust nappes with northwest-‐plunging 164
stretching lineations (Dürr and Dingeldey 1996; Goscombe et al. 2003; Goscombe 165
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and Gray 2008). Metamorphic P-‐T-‐t paths are clockwise with limited coeval granitic 166
magmatism (Goscombe et al. 2003; Will et al. 2004; Jung et al. 2014). The CKz is 167
juxtaposed against the Western Kaoko, or Coastal, zone (WKz), by a continuous 168
high-‐grade ultramylonite zone, the Purros suture (Fig. 3), having sinistral 169
subhorizontal shear-‐sense indicators (Goscombe et al. 2003; 2007; Konopásek et al. 170
2005; 2008). The WKz has a magmatic and metamorphic history distinct from the 171
CKz (Goscombe et al. 2005; Goscombe and Gray 2007; Konopásek et al. 2008) and is 172
taken to represent the leading edge of the Dom Feliciano–Ribeira (DF-‐R) magmatic 173
arc of Uruguay and southern Brazil (Fig. 2), against which the Kaoko belt was 174
juxtaposed prior to South Atlantic opening (Oyhantçabal et al. 2009, 2011; 175
Konopásek et al. 2014). Deformation and exhumation of the CKz at 580-‐570 Ma 176
(Goscombe et al. 2005; Jung et al. 2014) was broadly coeval with a transition from 177
subduction-‐related to slab-‐failure magmatism and exhumation in the DF-‐R arc 178
(Oyhantçabal et al. 2009, 2011; Faleiros et al. 2011; Tupinambá et al. 2012; Alves et 179
al. 2013; Heilbron et al. 2013). Closure of the Adamastor paleocean, situated at this 180
latitude between the rifted Congo margin and DF-‐R arc, occurred 20-‐30 Myr after 181
abortive continental subduction of the opposite (eastward) polarity in the West 182
Gondwana belt (Fig. 2)(Ganade de Araujo et al. 2014), and >10 Myr before final 183
closure of the Brasiliano paleoocean, between the DF-‐R arc and the Amazonia craton 184
(Fuck et al. 2008; Bandeira et al. 2012; McGee et al. 2015a, b). 185
186
Damara belt 187
The Damara belt (Fig. 2) is an intact collision zone, riddled with bodies of post-‐188
collisional syenogranite, separating the Congo and Kalahari cratons. It is a 189
compound belt with two basinal domains, the Outjo and Khomas zones (Fig. 3), 190
divided by a central microcontinent, the southern Central Zone (Miller 2008) or 191
Swakop terrane (Hoffmann 1987). Terminal collision in the Damara belt occurred 192
when northward subduction within the more southerly Khomas zone led to collision 193
around 550 Ma between the arc-‐bearing Swakop terrane and the north-‐facing 194
passive margin (Witvlei Group) of the Kalahari craton. The fossiliferous late 195
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Ediacaran and early Cambrian Nama Group of the Kalahari craton is a compound 196
foredeep related to collisions incurred by outward-‐dipping subduction in the 197
Damara belt to the north and the Gariep belt to the west (Fig. 2). 198
Miller (2008) and most previous workers consider the more northerly Outjo zone 199
to have been floored by stretched continental crust of the Congo craton, the Swakop 200
terrane (Fig. 3) being interpreted as a horst block forming an outer basement high at 201
the rifted continental margin. However, subduction rarely if ever initiates at a 202
passive margin (Cloetingh et al. 1982), and we therefore favour a more complex 203
development in which the Swakop terrane collided first with a passive Congo 204
margin via southward subduction within the Outjo zone. The Outjo zone is 205
coextensive with the largest-‐amplitude and longest-‐wavelength aeromagnetic 206
anomaly trough in the country. We postulate that northward subduction in the 207
Khomas zone initiated through subduction zone ‘flip’ (McKenzie 1969; Suppe 1984), 208
consequent to a Swakop-‐Congo collision. We suggest that the N-‐S shortening event 209
recently dated by 40Ar/39Ar at ~590 Ma in the Outjo zone (Lehmann et al. 2015) is a 210
manifestation of Swakop-‐Congo collision in the Damara belt, broadly synchronous 211
with Congo-‐DF-‐R arc collision in the Kaoko belt. 212
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Significance of the Otavi–Mulden group transition 214
Assuming the Kaoko belt is an arc-‐continent collision zone, the Otavi–Mulden 215
group transition (Fig. 4) records the passage of the upper Otavi Group carbonate 216
shelf margin over the outer forebulge of a west-‐dipping subduction zone and its 217
descent on the outer slope of the trench. Diachronous (southward-‐younging) 218
closure of the Adamastor paleocean and growth of the resulting collisional orogen 219
(Stanistreet et al. 1991) complies with paleocurrent evidence for southeastward 220
directed fluvial sediment transport (Renosterberg Formation) on the Congo 221
foreland (Hoffman and Halverson 2008). Rapid exhumation of the Kaoko belt at 222
580-‐570 Ma (Goscombe et al. 2005; Jung et al. 2014) implies an age of ~585 Ma or 223
older for the shelf-‐to-‐foredeep transition in the southern Kunene Region (Fig. 3). 224
225
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Ombonde detachment 226
The Ombonde detachment (Hoffman and Hartz 1999) is exposed for a strike 227
length of 51 km (22 km east-‐west) in the south-‐plunging Grootberg syncline (Fig. 5), 228
a basement-‐involved structure in the autochthonous foreland of the Kaoko belt. The 229
east limb of the syncline is steeply-‐dipping (50-‐70°) but the west limb is gentler and 230
crests in a broad anticline before disappearing beneath Early Cretaceous traps 231
(Etendeka Group). Paleogeographically, the Otavi Group is situated on the Makalani 232
dip-‐slope (Fig. 5C), where rift-‐shoulder uplift and erosion caused progressive 233
southward truncation of the Ombombo Subgroup, the Nosib Group, and the Rasthof 234
and Gruis formations. The Gruis Formation undergoes a remarkable facies change, 235
from peri-‐littoral carbonate cycles in the north to alluvial fanglomerate of exclusive 236
basement derivation 20 km to the south. Where intersected by the Ombonde 237
detachment, the Gruis Formation is a rheologically weak facies characterized by 238
marlstone and fine-‐grained clastic rocks. Other rheologically weak units are 239
limestones of the lower Rasthof, lower Ombaatjie and lower Maieberg formations, 240
while strong units are dolomites of the upper Rasthof, upper Ombaatjie, upper 241
Maieberg and Elandshoek-‐Hüttenberg (upper Tsumeb) formations. The Mulden 242
Group is represented by compositionally immature sandstone with metre-‐scale 243
crossbedding, locally with a basal chert-‐dolomite conglomerate containing clasts of 244
Elandshoek Formation stromatolite. 245
The Ombonde detachment was first identified in 1994 by Tony Prave and Dawn 246
Sumner on the east limb of the syncline (location h, Fig. 5A) by virtue of the fact that 247
~400 m of well-‐known strata (Ombaatjie and Maieberg formations) are missing 248
across a brittle, bedding-‐parallel, mappable, fault surface. The fault was 249
subsequently mapped throughout the syncline, revealing a ramp-‐flat-‐ramp 250
geometry (Fig. 5B) that cuts down-‐section westward from the lower Elandshoek 251
Formation to granitic basement in the footwall, and from the upper Hüttenberg 252
Formation to the Rasthof Formation in the hangingwall. The detachment is 253
commonly a single fault, but a 2.5-‐km-‐long extensional horse occurs at the exposed 254
keel of the syncline (f, Fig. 5A) and smaller horses occur elsewhere. Fault-‐related 255
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vein systems are uncommon, but stratigraphic cut-‐offs (Fig. 5B) provide essential 256
constraints on the fault-‐plane geometry and net slip. 257
The western ramp is exposed over the crest of the broad anticline (a-‐c, Fig. 5A). 258
The fault ramps down-‐section westward from the lower Elandshoek to the Rasthof 259
Formation in the hangingwall, and from the Gruis Formation to the basement in the 260
footwall. The hangingwall cut-‐off of the topmost Maieberg Formation is exposed on 261
both limbs of a second-‐order anticlinal crossfold (c, Fig. 5A), across which the 262
projected cut-‐off strikes 357°, implying a dip direction of 267° at that location. A dip 263
direction between 260° and 280° is required by the distribution of cut-‐offs 264
throughout the syncline in order for the detachment to ramp continuously down-‐265
section in one direction. It is noteworthy that the hangingwall is not structurally 266
extended in the panel that parallels the inferred slip direction (a-‐c, Fig. 5A). 267
Stratigraphic cut-‐offs in the hangingwall of the western ramp (a-‐c, Fig. 5A) are 268
matched by cut-‐offs in the footwall of the eastern ramp (h-‐j, Fig. 5A). The horizontal 269
separations of the corresponding cut-‐offs are 16.0, 16.4 and 17.3 km for the top of 270
the Gruis, Ombaatjie and Maieberg formations, respectively. Corrected for an 271
estimated 25% synclinal shortening, the respective horizontal separations are 20.0, 272
20.5 and 21.6 km (Fig. 5B). Because stratigraphic thicknesses are known from 273
measured sections (Hoffman and Halverson 2008), ramp angles between the 274
detachment surface and stratigraphic horizons can be calculated. The ramp angle 275
averaged through the Rasthof Formation in the footwall is 14°, while the respective 276
ramp angles through the Ombaatjie and Maieberg formations are 10° and 9° in the 277
hangingwall, and 8° and 9.5° in the footwall. The uncorrected footwall ramp angle 278
averaged through the entire Abenab Subgroup (395 m thick), including the long flat 279
within the Gruis Formation, is a mere 1.3°, or 1.1° corrected for synclinal shortening. 280
Compaction is slight in most Otavi Group carbonates; only in limestone (Fig. 4) are 281
stylolites common. If post-‐detachment compaction were 20%, at a maximum, the 282
overall ramp angle would increase by 0.2°. 283
The age of the Ombonde detachment is tightly constrained. High on the eastern 284
ramp, the detachment carries uppermost Otavi Group in the hangingwall onto upper 285
Maieberg Formation in the footwall (j, Fig. 5A). Fault slip must therefore post-‐date 286
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the entire Otavi Group. The critical minimum age constraint comes from basal 287
Mulden Group (Renosterberg Formation) paleovalleys, the largest of which is 288
located on the west limb of the syncline near the axial trace (d-‐e, Fig. 5A). This 289
paleovalley trends northeast-‐southwest and is filled by crudely bedded chert-‐290
dolomite conglomerate that dips ~20° toward the southeast, structurally 291
conformable with the underlying Otavi Group. Stromatolite clasts derived from the 292
upper Tsumeb Subgroup are prominent in the conglomerate, the structural attitude 293
of which precludes a Karoo (Carboniferous to Early Cretaceous) or younger age. On 294
the east wall of the paleovalley, the conglomerate forms a buttress unconformity 295
against the hangingwall of the detachment. The axial drainage of the syncline covers 296
the contact between the paleovalley and the detachment (e, Fig. 5A). On the west 297
wall of the paleovalley, the basal conglomerate bevels the detachment without 298
disturbance and variably abuts the Elandshoek Formation of the hangingwall and 299
both the Gruis and Rasthof formations of the footwall (d, Fig. 5A). Close to where the 300
paleovalley truncates the detachment, a small window in the hangingwall exposes 301
the Gruis Formation of the footwall. Erosional incision and truncation of the 302
detachment fault by the basal Mulden Group conglomerate (Fig. 5B), which is 303
conformably overlain by sandstone of the Renosterberg Formation, demonstrate 304
that the detachment must be pre-‐Mulden as well as post-‐Otavi in age. 305
High on the eastern ramp, a set of small paleogullies at the base of the Mulden 306
Group incises the uppermost Otavi Group in the hangingwall of the detachment (j, 307
Fig. 5A). The floors of these paleogullies reach the footwall Maieberg Formation. The 308
paleogullies confirm a post-‐Otavi, pre-‐Mulden group age for the detachment. 309
Hoffman and Hartz (1999) inferred that the Ombonde detachment formed as a 310
submarine slide on the outer slope of a trench that became a collisional foredeep 311
when the descending plate passed from oceanic to continental as the Congo margin 312
was abortively subducted at ~590 Ma. They postulated that coherent sliding of the 313
low-‐angle detachment was facilitated by pore-‐fluid overpressure, resulting from a 314
Messinian-‐type sea-‐level fall in the foredeep. As evidence for base-‐level fall, they 315
cited deep karstic erosion beneath the Mulden Group in the southwest of the Otavi 316
platform, incision of the conglomerate-‐filled paleovalley across the detachment, and 317
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the fluvial paleoenvironment of the Mulden Group (Renosterberg Formation) in the 318
area of the detachment. 319
One troubling fact regarding the Ombonde detachment emerged only over time. 320
After its nature and age were established by mapping in 1996, seventeen field 321
seasons elapsed before another example of the phenomenon was found in the Kaoko 322
belt. Such structures should not occur singly. 323
324
Ombepera detachment 325
Ninety kilometres to the northwest of the Grootberg syncline, the internal part of 326
the thin-‐skinned Eastern Kaoko zone includes a train of three synclines (S1-‐3, Fig. 327
6). Reconnaissance mapping and stratigraphic logging identified a problem in the 328
central syncline. The distinctive ‘cap dolostone’ of the younger Cryogenian 329
(Marinoan) glaciation, the normally ever-‐present Keilberg Member at the base of the 330
Maieberg Formation (Fig. 4), could not be found. In contrast, the cap dolomite was 331
easily recognized in the eastern and western synclines, in the latter atop glacigenic 332
carbonate diamictite of the Ghaub Formation. The conundrum of the central 333
syncline (S2, Fig. 6) remained unresolved until the area was revisited in the last 334
week of the 2014 field season. 335
Measuring the entire succession in the central syncline prompted a postulate 336
(Fig. 7) that the Gruis, Ombaatjie and lower Maieberg formations are missing across 337
a cryptic, normal-‐sense detachment that predates folding and thrusting, analogous 338
to the Ombonde detachment. The postulate was corroborated by carbon-‐isotope 339
chemostratigraphy. Strata directly overlying the Rasthof Formation are isotopically 340
depleted, δ13C = –2 to –5‰ VPDB (Section 2, Fig. 7). This is incompatible with the 341
Gruis and Ombaatjie formations, which are characteristically enriched (δ13C = +1 to 342
+8‰ VPDB), but is consistent with the Maieberg Formation (Fig. 4H). 343
In the western syncline (S1, Fig. 6), the upper Ombaatjie, Ghaub and lower 344
Maieberg formations, including the cap dolostone, appear on the hangingwall of the 345
inferred detachment (Section 1, Fig. 7), consistent with westward stratigraphic 346
downcutting analogous to the Ombonde detachment. The estimated thickness of 347
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missing strata, 450-‐700 m, implies a steeper ramp angle and/or larger displacement 348
compared with the Ombonde detachment. 349
In 2015, we tested a prediction of the detachment hypothesis: an up-‐ramp 350
extension of the detachment in the eastern syncline, near the village of Otjize (Fig. 351
6). On air photos and satellite imagery (Fig. 8), a channel-‐like salient of Tsumeb 352
Subgroup dolostone, ~5.0 km wide in a N-‐S direction, is seen to cut out much of the 353
underlying, strongly-‐cyclical, Ombaatjie Formation. This is evident on both limbs of 354
the syncline. Alternative interpretations of the channel-‐like structure can be 355
discriminated on the ground. If the structure was a (Ghaub) glacial-‐age paleovalley 356
(Fig. 8A), the cap dolostone would occur in the channel axis as well as outside the 357
channel. Up to 85 m of Ghaub-‐age local relief is known elsewhere on the carbonate 358
shelf (Halverson et al. 2002; Hoffman 2011) and more than 400 m occurs at the 359
southern shelf break on Fransfontein Ridge (Hoffman et al. 2014). In those areas, the 360
thickness of the Maieberg Formation as a whole reciprocates with relief on the 361
Ghaub glacial surface (Fig. 8A). Alternatively, if the structure is a post-‐Otavi Group 362
detachment (Fig. 8B), the cap dolostone (Keilberg Member) would be missing in the 363
channel, and both the footwall and hangingwall should contain strata younger than 364
their equivalents in the central syncline. This is because an Ombonde-‐type 365
detachment should ramp up-‐section in an eastward direction (Fig. 5B). 366
Fig. 9 compares measured sections through the upper Abenab and lower Tsumeb 367
subgroups in the eastern syncline. Sections 4 and 6 are from the axial part of the 368
channel-‐like structure on opposite limbs of the syncline (Fig. 6). Sections 3 and 7 are 369
from south of the structure on opposite limbs, and section 5 is from north of the 370
structure in the synclinal hinge zone (Fig. 6). The cap dolostone (Keilberg Member) 371
does not occur within the confines of the channel-‐like structure (Fig. 8), where 372
dolostone grainstone of the upper Maieberg Formation is in fault contact with marly 373
limestone of the lower Ombaatjie Formation. Missing are ~215 m of the upper 374
Ombaatjie Formation, and between 215 and 350 m of the lower Maieberg and 375
Ghaub formations. The total missing interval, 430-‐565 m, compares favourably in 376
thickness with that in the central and western synclines, 450-‐700 m. 377
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There is no discernible difference in stratigraphic level of the missing interval 378
across the eastern syncline (sections 4 and 6, Fig. 9). Footwall and hangingwall 379
stratigraphies in the eastern syncline are compared with those in the central and 380
western synclines in a longitudinal section of the detachment along the axial zone 381
(Fig. 7). An unfaulted section from east of the detachment is included for 382
comparison (section 8, Fig. 7). From east to west, the detachment ramps down-‐383
section from the lower Ombaatjie to the lower Rasthof formation in the footwall, 384
and from the upper Maieberg to the upper Ombaatjie formation in the hangingwall. 385
Like the Ombonde detachment, the Ombepera detachment ramps down-‐section 386
westward, which is therefore the inferred direction of normal-‐sense displacement. 387
The hypothesized Ombepera detachment is validated by its predicted up-‐ramp 388
extension in the eastern syncline. 389
We cannot determine the magnitude of displacement from the separation of 390
matching stratigraphic cut-‐offs, because the footwall and hangingwall have no 391
exposed cut-‐offs in common (Fig. 7). However, we can calculate a ramp angle of 2.0°, 392
uncorrected for tectonic shortening, given the stratigraphic climbs of ~516 and 393
~528 m in the footwall and hangingwall respectively (Fig. 7), over a horizontal 394
distance in the dip direction of 15 km. Corrected for an estimated 40% shortening 395
due to folding and thrusting, the original ramp angle was 1.3°, slightly steeper than 396
the Ombonde detachment (1.1°). As the footwall and hangingwall of the three 397
synclines have no stratigraphic cut-‐offs in common, the tectonically corrected 398
distance of 25 km between the four sections (Fig. 6) is a minimum bound on the 399
magnitude of displacement of the Ombepera detachment. Inescapably, strata in the 400
hangingwall of the western syncline restore palinspastically to the east of footwall 401
strata in the eastern syncline (Fig. 7). 402
403
Discussion and remaining problems 404
Arc-‐continent collision having been a common process for billions of years 405
(Brown and Ryan 2011), early extensional detachments of the type described here 406
should occur in other orogenic belts, regardless of whether sharp base-‐level fall is 407
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essential to their origin. Moreover, most examples on existing margins occur on 408
passive margins, so incipient subduction is not required for their development. Yet 409
to our knowledge, few coherent map-‐scale extensional detachments (mass transport 410
complexes) have been described from paleomargins in ancient orogenic belts. 411
Failure to recognize the Ombepera detachment could have led to serious errors in 412
the interpretation of shelf development. The anomalously thin and argillaceous 413
Abenab Subgroup of the western syncline (Fig. 7) could have been interpreted as 414
evidence for a western shelf margin, although no such margin is apparent in the 415
Ombombo or Tsumeb subgroups in the same area. According to the detachment 416
hypothesis, the Ombaatjie Formation is thin in the western syncline because it is 417
truncated from below. Its more terrigenous character, relative to footwall sections 418
in the eastern syncline, relates to its more inboard, not outboard, palinspastically 419
location on the shelf. 420
The anticlinorium between the western and central synclines includes a west-‐421
facing panel in which the apparent thickness of the Rasthof Formation attenuates 422
northward from 350 to 96 m in a strike distance of just 4.4 km (location a, Fig. 6). In 423
the absence of structural disturbance, the attenuation would imply a north-‐facing 424
shelf margin of upper Rasthof age. In light of the detachment hypothesis, the 425
attenuation could represent instead a lateral footwall ramp. Closely-‐spaced 426
measured sections imply top-‐down truncation, consistent with a footwall ramp 427
having an average ramp angle of 3.3°. On the other hand, the detachment could 428
merely track the Rasthof-‐Gruis formation rheological boundary across a primary 429
shelf margin. In this case, sedimentary facies should be distinct from those in the 430
shelf interior. 431
The lateral ramps bounding the Ombepera detachment are just ~5.0 km apart at 432
the base of the Tsumeb Subgroup in the footwall of the eastern syncline (Fig. 8). The 433
narrowness of the detachment recalls the South Makassar Strait mass transport 434
complex (Fig. 1C) and other submarine landslides (Frey-‐Martinez et al. 2005; 435
Masson et al. 2006; Armandita et al. 2015). Narrowness may help to account for 436
their infrequent recognition on ancient continental margins, including the belated 437
recognition of a second detachment in the Kaoko belt. 438
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However, reconnaissance mapping and air-‐photo interpretation have not 439
revealed any lateral ramp above the Rasthof Formation for 20 km southward or 40 440
km northward of the detachment axis in the central and western synclines. 441
Additional mapping is required to investigate the possibility that channel-‐like lateral 442
ramps develop preferentially where the detachment surface intersects specific 443
stratigraphic intervals. 444
445
Conclusions 446
A second map-‐scale, coherent, very-‐low-‐angle, detachment structure of Ediacaran 447
age has been found in the Kaoko belt of northwestern Namibia. Like the previously 448
described Ombonde detachment, the Ombepera detachment is a brittle structure 449
that predates orogenic shortening, ramps downward stratigraphically to the west, 450
and underwent normal-‐sense displacement of substantial magnitude (>20 km). The 451
detachment is surprisingly narrow (5 km N-‐S), 25% of its displacement, at the level 452
of the Cryogenian-‐Ediacaran boundary. By analogy with the Ombonde detachment, 453
the stratigraphic age of which is tightly constrained, we interpret the Ombepera 454
structure as the extended domain of a coherent submarine slide (mass transport 455
complex), mobilized on the outer slope of a trench-‐foredeep basin during incipient 456
collision between the Congo craton and the Dom Feliciano–Ribeira arc. Accordingly, 457
the average ramp angle of 1.3° for the Ombepera detachment, relative to the host 458
stratigraphy, would have been augmented at the time of failure by a few degrees of 459
incline related to lithospheric flexure. We expect that submarine slides of this type 460
should occur in other orogenic belts regardless of age. Failure to recognize them can 461
lead to misinterpretation of sedimentary facies and stratigraphic relations. 462
463
Acknowledgements 464
PFH first met John Dewey in April 1971 at a Canadian (then Alberta) Society of 465
Petroleum Geology short course on “The New Global Tectonics” at Banff (Alberta), 466
presented by Dewey and the late William R. Dickinson. He met Kevin Burke a few 467
months later at a night club in downtown Toronto. Together, they convinced him 468
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that, in tectonics, the present is the key to past. Field work was authorized by the 469
Geological Survey of Namibia and supported in 2010-‐14 by the Earth System 470
Evolution Project of the Canadian Institute for Advanced Research (CIFAR). Roy 471
McG. Miller drew our attention to Tom Clifford’s Saturn slide. We thank Stephen T. 472
Johnston, Francis A. Macdonald and William A. Thomas for thoughtful comments on 473
the manuscript, and Andrew Hynes and John W. F. Waldron for constructive 474
reviews. Ali Polat invited the paper. 475
476
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656
Figure captions 657
Fig. 1. Interpretations of seismic reflection profiles (redrawn from Butler and Paton 658
2010; Armandita et al. 2015) showing Late Cretaceous (A) and early Pliocene (B) 659
submarine landslides (mass transport complexes) within fine-‐grained terrigeous 660
and carbonate sediments respectively on the passive continental margins of 661
southwest Africa and southeast Borneo. The scale is the same for each profile and 662
the Borneo margin is geographically ‘reversed’ for kinematic comparison. The 663
Namibian margin faces the South Atlantic Ocean; the Borneo margin (Paternoster 664
Platform) faces the South Makassar Straits Basin and the West Sulawesi thrust-‐fold 665
belt. The slides include extended domains characterized by normal-‐fault duplexes 666
(green lines) and shortened domains by thrust duplexes (blue lines), both soled on a 667
basal detachment (red). Note syn-‐slide growth strata on the Namibian margin. In the 668
South Makassar Strait slide, the strain domains migrated down-‐dip over time, 669
resulting in thrusting succeeded by cospatial normal faulting. Planform of the South 670
Makassar slide mass (C) shows its width to be only a fraction of its maximum 671
translation, unlike detachments of tectonic origin. The barbed line in C indicates the 672
steep frontal ramp in B, not the toe of the detachment. 673
Fig. 2. Cratons (clear), Ediacaran–early Cambrian orogenic belts (shaded), and 674
Mesozoic–Cenozoic belts (hachured) of southwest Gondwana. Dotted lines are the 675
present shorelines of South America, southern Africa and West Antarctica in a pre-‐676
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24
drift reconstruction. SF – Sao Francisco, RP – Rio Plata, P – Pampean, PP – 677
Paranápanema, Zam. – Zambesi belt (modified from Fuck et al. 2008). Black stars 678
indicate the Dom Feliciano – Ribeira magmatic arc, which collided with the Congo 679
craton to form the Kaoko belt. 680
Fig. 3. Tectonic map of northwest Namibia showing elements of the Congo and 681
Kalahari cratons, and the Kaoko and Damara orogenic belts (adapted from 682
Hoffmann 1987). Boxes in the Eastern Kaoko zone (EKz) frame the Ombonde (Fig. 683
5) and Ombepera (Fig. 6) detachment areas. Dashed magenta line is the boundary of 684
the Kunene Region. 685
Fig. 4. Stratigraphic column of the Neoproterozoic Damara Supergroup in the 686
Eastern Kaoko zone Namibia (Hoffman and Halverson 2008). (A) Geologic periods, 687
(B) Groups, (C) Subgroups, (D) Formations and Members, (E) radiometric 688
chronology, (F) tectonic phase, (G) generalized lithology, (H) carbonate C-‐isotope 689
composition in per mil relative to the VPDB (Vienna PeeDee Belemnite) standard. 690
Fig. 5. (A) Geologic map of the Grootberg syncline (location, Fig. 3). Red line is the 691
Ombonde detachment (ticks on hangingwall), a brittle, low-‐angle, normal-‐sense 692
detachment that ramps stratigraphically downward from east to west and omits 693
~400 m of stratigraphic section at any location (Hoffman and Hartz 1999). (B) 694
Unfolded W-‐E cross-‐section showing ramp-‐flat geometry of the detachment (red 695
line), matching footwall and hangingwall cutoffs (e.g., arrows b and h, keyed to 696
locations on map), and paleovalley (d) filled by dolomite-‐chert conglomerate (basal 697
Renosterberg Formation) that is incised across the detachment. Minimum age of the 698
detachment is the sub-‐Renosterberg paleovalley (d). Its maximum age is the top of 699
the Tsumeb Subgroup at location (i). (C) Unfolded N-‐S cross-‐section before the 700
detachment showing stratigraphic truncation of the basal Nabis Formation (g), basal 701
Rasthof Formation (i) and basal Gruis Formation (k). Truncations are related to 702
episodic uplift and down-‐to-‐the-‐north rotation of the Makalani dip-‐slope, the 703
footwall of an inferred south-‐dipping, syn-‐rift, normal fault, older and unrelated to 704
the Ombonde detachment. 705
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25
Fig. 6. Geologic map of a part of the eastern Kaoko belt (location, Fig. 3), showing 706
the Ombepera detachment (red line with ticks on the hangingwall) exposed in three 707
synclines (S1-‐3). Numbered lines indicate measured stratigraphic sections displayed 708
graphically in Fig. 7 and 9. The detachment widens (N-‐S direction) from the eastern 709
syncline to the central and western synclines. The Ombaatjie Formation (dark 710
green) in the hangingwall of the western syncline restores palinspastically to the 711
east of the same formation in the footwall of the eastern syncline. Abrupt thinning of 712
the Rasthof Formation at location a is tentatively interpreted as a lateral ramp. 713
Fig. 7. Axial (W-‐E) section of the Ombepera detachment (red line), with graphic logs 714
of measured sections of the Ombombo, Abenab and lower Tsumeb subgroups 715
(section 1-‐6 locations, Fig. 6; section 8 located 57 km due East of Section 6). The 716
detachment ramps stratigraphically downward from east to west in both the 717
hangingwall and footwall, and 430-‐700 m of strata are omitted across the 718
detachment in each section. Inset in section 2 shows depleted C-‐isotope values in 719
the hangingwall, diagnostic of the Maieberg Formation (Fig. 4H). Keilberg Member 720
cut-‐off in the hangingwall at magenta x corresponds to footwall cut-‐off at magenta y. 721
Ghaub-‐Keilberg diamictite-‐cap dolostone interval is missing in sections 2, 4 and 6. 722
Field notebook section numbers given alongside columns. 723
Fig. 8. Oblique satellite image (GoogleEarth) looking northwestward at the eastern 724
syncline near the village of Otjize (Fig. 6). The light dotted line is the Ombepera 725
detachment where it cuts out the lower Maieberg and upper Ombaatjie formations. 726
Measured sections are numbered as in Fig. 7 and 9. Insets A and B show 727
downcuttings of different origin: (A) Ghaub glacial-‐age paleovalley and (B) post-‐728
Maieberg extensional detachment. Observed truncation of the lower Maieberg 729
Formation including its basal Keilberg Member favours interpretation B. 730
Fig. 9. Transverse (N-‐S) sections of the Ombepera detachment (red line) on both 731
limbs of the Otjize syncline (S3 in Fig. 6), with graphic logs of measured 732
stratigraphic sections (locations, Fig. 6) of the upper Abenab and lower Tsumeb 733
subgroups (Fig. 4). The upper Ombaatjie and lower Maieberg formations are missing 734
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26
in the axial zone of the detachment (sections 4 and 6), compared with sections to the 735
north (section 5) and south (sections 3 and 7) of the detachment. 736
737
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Draftex tended domain
shor tened domain
p o s t - s l i d e s e c t i o n
b a s e o f s y n - s l i d e g r o w t h s t ra t a
p o s t - r i f t s e c t i o n
s y n - r i f t s e c t i o n
0
1
2
3
4
5sec.
(tw
o-w
ay tr
avel
tim
e)
approximately 2x vertical exaggeration
10 km
SW NE
redrawn from Butler and Paton (2010)original seismic line: CGGVeritas and the Virtual Seismic Atlas
NWSE1
2
3
4
5
sec.
(tw
o-w
ay tr
avel
tim
e)
redrawn from Armandita et al. (2015)
Namibian continental margin
S outh M ak assar St rait Bas i n, I ndonesia
Late Miocene
Late Pliocene - Pleistocene
f luid escape
shor
tene
d d
omai
n
exte
nded
dom
ain
SE
NW
10 km
Paternoster PlatformWest Sulawesi thrust-fold bel t
10 km
A
B
C
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DraftCongo
KalahariAmaz
onia
WAfr
RPP
PP
SF
Damara
Kao
koG
ariep
West
Gondwan
a
East
Afr
ica
n
Zam.
Alto Paraguay
1000 km
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Draft18o
20o
12o 14o 16o 18o
22o
24o
100 km
NaukluftAllochthon
Kamanjab inlier
O t a v i f o l d b e l t
Kunen e R.A N G O L A
Kalahar i Desert
Omaheke Desert
C o n g o c r a t o n
Rehoboth Inlier
Windhoek
K
homas
zone
Hakos marg
in
Swakop
te
rrane W
Kz
Etendeka
Witvlei zone
EK
z CK
z
Epupa inlier
Er
Namib Desert
KALAHARI CRATONNeoproterozoic - Cambrian cover
Hakos margin: deformed north-facing margin
1.0 - 1.4 Ga basement: Namaqua Belt
Khomas zone: south-facing accretionary prism
Swakop terrane: arc-bearing microcontinent
Outjo zone: Swakop-Congo collision(?) zoneDAMARA BELT (ca 550 Ma)
CONGO CRATON Neoproterozoic cover
1.8 - 2.0 Ga basement
KAOKO BELT (ca 590 Ma)
Central zone: deformed margin
Western (Coastal) zone: accreted forearc
CKz
WKz
Carb. - Cenozoic cover
SKz Southern zone: deformed deep-sea fan
Eastern zone: foreland thrust-fold beltEKz
geosuture thrust (teeth on hangingwall)
134 Ma igneous rocks
L EGEND plateau
S o u t h
A t l a n t i c
O ce a n
Fig. 5
Fig. 6
ST
PS
Outjo zone
SKz
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DraftHuttenberg
Elandshoek
Maieberg
Ombaatjie
Rasthof
Gruis
Okakuyu
Devede
Beesvlakte
Sesfontein
Renosterberg
Tsu
meb
Ab
enab
Om
bo
mb
o
MU
LDE
N
Ghaub
Chuos
Nabis
NO
SIB
ED
IAC
AR
AN
CR
YO
GE
NIA
NTO
NIA
N SY
N-R
IFT
SH
ELF
(P
OS
T-R
IFT
)FO
RE
DE
EP
Keilberg Mb
dolomite
limestone
marlstone
sandstone
siltstone
diamictitecarbonatediamictite
OTAVI
Legend
635
659717
760
~590
746
(Ma)
-5 0 5 10δ13Ccarb (o/oo VPDB)
-10
A B C D E F G
200
0 (m)
H
400 Isotope dataat Ongongo (46 km SSE of Otj ize).
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Draft
CenozoicCretaceousMulden Gp Renosterberg Fm
upper TsumebMaieberg FmOmbaatjie FmGruis FmRasthof FmBeesvlaakte FmNabis FmbasementOrosirian
Nosib Group
Ombombo
Abenab Subgp
Tsumeb Subgp
Etendeka Group
Ota
vi G
rou
p
0 2 4 6 8 10 kmOmbonde detachmentoblique reverse faultvehicle track
14o05'E 14o10'E 14o15'E
14o10'E14o05'E
14o15'E
19o30'S
19o35'S
19o40'S
19o45'S
KF KF
20.5 km
West East
North South
3.0 km
0.3 km
paleovalley
paleovalley
Ombonde detachment
Makalani dip-slope
Grootberg syncline
10 km
O mbonde R.
A
B
C
Kalahari Group
a b c
d
e f
b h
i
g
h
j
kig
d
j
k
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Draft
7
6
5
4
3
2
12
0 1 2 3
km
N
Se
sfo
nte
in
Th
rust
Okamborombonga
Okaaru
Otji kukutu
Otjo-matemba
Cont ract ional st ruct ures
Ombepera detachment
Graded road C43 O tj i taimo Ri ver
h a n g i n g wa l l
travertine
Ediacaran Tsumeb Subgroup Cryogenian Abenab Subgroup Tonian Ombombo Subgroup
Elandshoek-Huttenberg fms Ombaatjie Formation Okakuyu Formation
Maieberg Formation Gruis Formation Devede Formation
Cryogenian Tsumeb Subgroup Rasthof Formation Beesvlakte Formation
Ghaub (glacial) Formation Chuos (glacial) Formation
“
Geology by P.F. Ho�man & G.P. Halverson
13o35’00”E 13o40’00”E 13o45’00”E
18o45
’00”
S18
o50
’00”
S
Ombepera
Otjize
S1
S2
S3
a
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Draft
hangingwall
footwall
Datum = base Rasthof Fm
P210
6
P144
1/42
J141
5E1
414
P144
3
P144
4
100
200
300
0
400
-100
-400
-300
-200
-700
-600
-500
600
500
P650
1
P165
9
P650
4P6
500
700
800
900
1100
1000
1200
-800
P150
22
West East
100
200
300
0
400
-100
-400
-300
-200
-700
-600
-500
600
500
700
800
900
1100
1000
1200
-800
(m)
G11
03
1
62 4
strati�ed polymictic diamictitesiltstone w ice-rafted debrisconglomeratequar tz sandstonequar tz siltstoneargillite
tepeed microbialaminiteooid/intraclast grainstonepeloidal grainstone (cap dolomite)columnar or mounded stromatolitethrombolitic microbialaminiterollup microbialaminiteribbonitemarly ribboniterhythmite (dolomite)rhythmite (limestone)
L E G E N DTerrigenous
Carbonate
massive carbonate diamictitestrati�ed carbonate diamictitemassive polymictic diamictite
Rasthof
Gruis
Ombaatjie
Okakuyu
Devede
Chuos
8
P131
7P2
095
Elandshoek
upperMaieberg
middleMaieberg
middleMaieberg
u. Mbg
GhaubKeilberg
Ghaub
Keilberg
Ombaa- tjie
(m)
x
y
57 km4 km7 km4.5 km
P132
6
-10 -5 0δ13C
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Draft Ar
Ab
Tml
Te
Tmu
Tmu
Te
Ar
AbAg Ag
PALEOVALLEY DETACHMENT
N
Otjize
lateral ramp
lateral rampm ove m e n t
d irection
Keilberg MbC43
Ar
Ab
Ac
Ag
Tml
Od
7
6
54
Tml/Tmu - lower/upper Maieberg Fm Ab - Ombaatjie Fm Ag - Gruis Fm Ar - Rasthof Fm Ac - Chuos FmOd - Devede Fm
Ar
Od
Tmu
Tmu
Tmu
A B
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P15024Otj itaimo R.
P6502-3 Okaaru
P9504-7Otjomatemba
P15023Otjikukutu
Ombepera detachment
0
50
100
150
200
250
300
350
400
450
500
0
50
100
150
200
250
300
350
400
450
500
P15022 Otjize
-50
-100
-50
-100
Gruis Fm
Ombaatj ie Fm
M aieberg
Fm
up
per
Mai
eber
g F
m
up
per
Mai
eber
g F
m
SW N SE
Stra
tigr
aphi
c he
ight
(m) r
elat
ive
to th
e ba
se o
f the
Om
baat
jie F
m
3
45
67
Rasthof
Fm
Kei lberg Mb
CW CE
Ghaub Fm
rhythmite (seaf loor cement)
cap swaley dolopelarenite
cap tubestone stromatolite
littoral microbialaminite
intraclast-ooid grainstone
columnar stromatolite
ribbonite
marly ribbonite
limestone rhythmite
carbonate diamictite, rudite
terrigenous sandstone
argillite, terrigenous siltstone
Datum = baseOmbaatj ie Fm
cycle b8
b7
b5-6
b1-4
9 km 6 km 6 km 4 km
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