Left-Lateral Sense Offset of Upper Proterozoic to Paleozoic Features

ELSEVIER Earth and Planetary Science Letters 173 (1999) 183±194 www.elsevier.com/locate/epsl Left-lateral sense offset of Upper Proterozoic to Paleozoic features across the Gobi Onon, Tost, and Zuunbayan faults in southern Mongolia and implications for other central Asian faults Melissa A. Lamb a,Ł, Andrew D. Hanson b, Stephan A. Graham a, Gombosuren Badarch c, Laura E. Webb d a Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA b Texaco Exploration, West Africa Group-Nigeria, P.O. Box 430, Bellaire, TX 77402-430, USA c Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, 63 Peace Avenue, Ulaan Baatar 210357, Mongolia d DeÂpartement de MineÂralogie, UniversiteÂ GeneÁve, 13 rue des MaraõÃchers, 1211 Geneva 4, Switzerland Received 15 April 1999; revised version received 14 September 1999; accepted 15 September 1999 Abstract We present a preliminary study of Upper Proterozoic to Paleozoic features adjacent to, and offset across, three faults in southern Mongolia. Restoration of four offset stratigraphic units adjacent to the Gobi Onon fault indicates a left-lateral sense offset of 70±95 km. The Tost fault has a left-lateral sense offset of 95±125 km de®ned by three offset features. The Zuunbayan fault has seven offset features that display left-lateral sense offsets of 185±235 km. Deformation on these faults apparently occurred during the Mesozoic prior to the Late Cretaceous based upon the age of offset features, the age of overlap sequences, and inferred timing relationships with regional contractional structures. Although Cenozoic overprinting and reactivation has occurred along parts of these faults, thus complicating their restorations, we present a testable hypothesis which predicts possible connections between these faults and faults within China. 1999 Elsevier Science B.V. All rights reserved. Keywords: left-lateral faults; Mongolia; Mesozoic; deformation 1. Introduction ognizable on satellite images, geologic maps, and in the ®eld. Although progress has been made in under- Major strike-slip faults, including the Altun Tagh standing the current character and recent history of and Ruoqiang faults in China and the Gobi Tien some of these faults [5,7], their pre-Pleistocene his- Shan and Gobi Altai faults in Mongolia, are promi- tories generally remain unconstrained. Several im- nent features of central Asia [1±6] (Fig. 1a). Many of portant aspects of the history of these faults are these faults are demonstrably active and easily rec- unknown: What is the history of timing of movement? Has the sense of movement changed through Ł Corresponding author. Present address: 5371 Grand Avenue, time? What is the magnitude of total slip? To what White Bear Lake, MN 55110, USA. Tel.: C1-651-762-8249; tectonic events are their origins and slip histories E-mail: [email protected] related? 0012-821X/99/$ ± see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0012-821X(99)00227-7 184 M.A. Lamb et al. / Earth and Planetary Science Letters 173 (1999) 183±194 Fig. 1. (a) Map of central Asia, including southern Mongolia and portions of Inner Mongolia, Gansu, Qinghai and Xinjiang provinces of China. Shaded areas denote mountainous regions; white areas denote modern basins. Partially based on Yanshin [26], GBGMR [40], XBGMR [38], and NMBGMR [33]. Note that many place names have more than one commonly used English spelling, e.g. Tien vs. Tian and Altun vs. Altyn. We have used the spelling that is used by the government of the country in which the feature is located. (b) Major blocks and terranes of central Asia, after Zhou and Graham [15]. AB D Alashan block; ATF D Altun Tagh fault; NCB D North China block; QB D Qaidam basin; RXF D Ruoqiang Xingxingxia fault system; SGC D Songpan±Ganzi complex; SMV D southern Mongolia Paleozoic volcanic arcs; TB D Tarim basin; YB D Yangtze (South China) block; ZF D Zuunbayan fault. M.A. Lamb et al. / Earth and Planetary Science Letters 173 (1999) 183±194 185 These faults play major roles in many tectonic 2. Methods reconstructions of Asia [1,2,8±13]. Therefore, an- swers to these questions are critical to a complete Our identi®ed offset markers are based on data understanding of the tectonic development of cen- collected as part of two separate basin analysis tral Asia and ®gure prominently in consideration studies, one in southern Mongolia (ML, GB, SG, of intracontinental deformation and escape tecton- LW) and one in southern Tarim, northern Qaidam, ics [11,14]. These large intracontinental strike-slip and the Altun Mountains (AH, SG, GB). Both faults occur in regions along or near sutures between studies included documenting sedimentologic and major Paleozoic terranes of central Asia, such as stratigraphic features, interpreting depositional en- the North China block, Tarim block, Qaidam ter- vironments, ®eld-checking existing geologic maps, rane, and the southern Mongolian Paleozoic volcanic observing structural relations between units, and col- arcs (Fig. 1b). The amalgamation of the terranes and lecting and analyzing samples. Analyses included the location of their margins most likely contributed point-counting of sandstones for provenance, XRD to the development and localization of the faults, and XRF analyses of igneous samples for geochem- but the exact cause and effect relationships between ical study and tectonic indicators, 40Ar=39Ar dating amalgamation and faulting are unknown. Although in poorly dated sequences, and fossil identi®cation previous studies have contributed greatly to our un- for age determination and paleogeographic=climatic derstanding of the geologic history of the individual indicators. In sum, we use these original data to terranes, the nature and timing of their amalgama- de®ne distinctive petrotectonic features which have tion remains controversial (e.g. Zhou and Graham been offset by strike-slip faulting. We also discuss [15]). As a result, much work is currently focused possible offset markers that we have identi®ed from on understanding the strike-slip faults of this region geologic maps, from other available data, and from [16±21]. previous workers. Many studies speci®cally focus on the Altun Tagh To simplify the use of long and unfamiliar names, fault within China, in part because it has been more offset markers are assigned letters, as shown in accessible than the faults of southern Mongolia. Few Figs. 2 and 3. Although none of our offset fea- studies address the relationship of faults across the tures are de®nitive piercing points in the sense of China±Mongolia border, and yet such relations are Crowell [22], they probably formed as continuous crucial to understanding the Phanerozoic history of belts during the Paleozoic, and we believe can be Asia. Previous studies that have addressed structural treated as piercing points, because of their distinc- relations across the international boundary were typ- tive petrotectonic characteristics and the consistent ically restricted by access issues that were in place amount of offsets that they record. until recently, and thus, few authors have conducted ®eldwork in both countries. In this paper, we identify numerous geologic fea- 3. Offset geologic features tures of Late Proterozoic±Paleozoic age that we infer to have been offset across three poorly known faults 3.1. Gobi Onon fault in southern Mongolia: the Gobi Onon, Tost, and Zuunbayan faults (Figs. 2 and 3), and discuss con- The Gobi Onon fault has been recognized as a straints on the timing of deformation. Although these fault by previous workers [23±26] and mapped as a faults have been previously recognized, this is the left-lateral fault by Suvorov [24,25]. Suvorov [24,25] ®rst attempt to constrain their sense of offset, timing offered no estimates of the amount of displacement. and amount of offset using ®eld data from south- We recognize four offset markers on either side ern Mongolia. Building on these data, we present of this fault. Additionally, we suggest that the Gobi a plausible and internally consistent, albeit prelimi- Onon fault has been offset by approximately 10 km nary, model of how these faults relate to the more of younger left-lateral slip on the Gobi Altai fault widely known faults in China. (Fig. 2). Our ®rst offset marker, A=A00, is an east± west trending belt of Vendian±Cambrian limestone, 186 M.A. Lamb et al. / Earth and Planetary Science Letters 173 (1999) 183±194 M.A. Lamb et al. / Earth and Planetary Science Letters 173 (1999) 183±194 187 that yield similar Triassic ages (our unpublished 40Ar=39Ar dates from potassium feldspars). Offset marker D=D00 is a belt of Silurian and De- vonian metasedimentary rocks, metavolcanics, ultra- ma®c rocks, chert, and pillow lavas interbedded with volcaniclastic sandstones, and Lower Carboniferous interbedded siltstones, sandstones and debris-¯ow conglomerates. Small gabbroic bodies of uncertain age, possibly Silurian to Carboniferous, occur as structural inclusions within the Carboniferous sec- tion on both sides of the fault as well. On the western side of the fault, this belt includes deposits at Nomin Gobi (D) and on the eastern side, Nemegt, Zolen and Gurvan Sayhan ranges (D00, Figs. 1 and 2 [28,29]). A restoration of 70±95 km of left-lateral sepa- ration along the Gobi Onon fault (Fig. 3) returns these markers to probable end-Paleozoic, pre-offset positions. 3.2. Tost fault The Tost fault has been recognized by previous workers [23,26,30]. We recognize three markers off- Fig. 3. Estimated amount of left-lateral offset for each geologic set by the Tost fault in a left-lateral sense (Fig. 2). 00 feature discussed in the text. Shaded area denotes our preferred The ®rst, E=E , is an east±west trending belt range of the amount of displacement for each fault. of Carboniferous to Lower Permian arc-related volcanics, including andesite, dacite, rhyolite, and alka- line volcanics [26,28,31,32], that occur near Tsagan marble, and minor quartzites [26], the southern front Uul (E) in the west and Noyon Uul and Tost Uul (E00) of which best evinces the magnitude of offset.

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