Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx

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Journal of Asian Earth Sciences

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Full length article Plate interior polyphase fault systems and sedimentary basin evolution: A case study of the East Gobi Basin and East Gobi Fault Zone, southeastern Mongolia ⁎ Matthew J. Heumanna, , Cari L. Johnsonb, Laura E. Webbc a Apache Corporation, 2000 Post Oak Blvd #100, Houston, TX 77056, USA b Geology and Geophysics Department, University of Utah, 115 South 1460 East FASB 383, Salt Lake City, UT 84112, USA c Department of Geology, University of Vermont, 180 Colchester Ave., Burlington, VT 05405, USA

ARTICLE INFO ABSTRACT

Keywords: Structural interpretation of 2-D seismic reflection data and subsurface-outcrop correlations reveal six distinct Tectonics phases of deformation recorded in the Paleozoic basement rocks and Mesozoic-Cenozoic basin fill of the East Basin analysis Gobi Basin (EGB), southeastern Mongolia. These phases include arc accretion and arc-continent collision in late Strike-slip Paleozoic time, Late Triassic sinistral shear-zone development, Early Jurassic fold and thrust belt style Intracontinental deformation shortening, Middle Jurassic-Lower Cretaceous extension and rift basin development, middle Cretaceous shortening with basin inversion and regional unconformity development, and Late Cretaceous-Oligocene thermal subsidence with renewed Paleogene left-lateral strike slip faulting across the fault zone. The five post-amalgamation deformation phases are localized along the East Gobi Fault Zone, suggesting that preexisting structures and boundary conditions exert fundamental controls on the long term evolution of intracontinental basins such as the EGB. Subsurface geophysical data and outcrop correlations demonstrate that the main subbasins of the EGB contain major differences in basement metamorphic and structural fabrics, basin fill patterns, and distinct Mesozoic-Cenozoic fault generations. Potential causes related to far-field boundary conditions and other driving factors are suggested for each of the major deformation phases.

1. Introduction Jian et al., 2010). Several studies provided evidence for multiple phases of deformation in this region from Mesozoic-early Cenozoic, including Localized deformation along plate-interior fault systems is an sinistral shear, contraction, and extension (e.g., Johnson, 2004, 2015; important mechanism in intraplate deformation (Johnston and Webb and Johnson, 2006; Webb et al., 2010; Heumann et al., 2012; Schweig, 1996; Célérier et al., 2005). These fault systems typically Graham et al., 2012). Building on this and related regional work, this have polyphase histories recorded in structural elements and associated study details the manifestation and timing of these events, offering sedimentary basin fill (e.g., Tarim and Ordos basins; Graham et al., insight into an under-studied class of polyphase fault systems. The 1993; Ritts et al., 2009). In southern Mongolia, the East Gobi Basin and results illustrate how such systems record the behavior of plate interiors its main defining structural element, the East Gobi Fault Zone (EGFZ), as they respond to evolving plate boundary conditions and other together form an example of a long-lived basin and fault system that driving factors. records multiple phases of deformation and subsidence (Fig. 1; Johnson This study presents data collected from the correlation of field and Ritts, 2012). Unlike end-member strike-slip fault systems defined mapping to 2-D seismic reflection surveys in southeastern Mongolia. by a single tectonic setting (Sylvester, 1988; Mann, 2007), the EGFZ The deformation and subsidence phases and fault generations identified evolved from a plate margin to a plate interior tectonic setting over a here shed light on models for Mesozoic-Cenozoic deformation in east- period of ∼300 million years. The record of this dynamic tectonic central Asia. A better understanding of the full deformation history of history underscores the role of pre-existing structures and crustal the EGFZ is necessary for evaluating the role of analogous polyphase, fabrics as mechanisms for the distribution of strain (Sykes, 1978). intracontinental fault and basin systems. The importance of preexisting The EGFZ formed within accreted arc complexes of the Central fabrics is also emphasized as a control on subsequent Mesozoic- Asian Orogenic Belt, or Altaids (Fig. 1; Şengör et al., 1993; Jahn, 1999; Cenozoic intracontinental deformation.

⁎ Corresponding author. E-mail address: [email protected] (M.J. Heumann). http://dx.doi.org/10.1016/j.jseaes.2017.05.017 Received 6 December 2016; Received in revised form 10 May 2017; Accepted 10 May 2017 1367-9120/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Heumann, M.J., Journal of Asian Earth Sciences (2017), http://dx.doi.org/10.1016/j.jseaes.2017.05.017 M.J. Heumann et al. Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx

(Johnson, 2004); and (4) Late Cretaceous to early Tertiary left-lateral strike-slip faulting (Webb and Johnson, 2006). Prost (2004) suggested an additional deformation phase of right-lateral strike-slip faulting lasting from Late Cretaceous–Holocene time. This study further inves- tigates these phases by documenting their subsurface and outcrop structural expressions and associated basin fill, and considers evidence for additional events not previously recognized. The terms Zuunbayan fault (Lamb et al., 1999; Graham et al., 2001) and the Alxa-East Mongolia fault system (Yue and Liou, 1999) have been used to broadly define prominent structures in the study area. Both names predate more detailed field and subsurface mapping, and each refers to multiple faults of the EGFZ rather than a single structure. One important revision is the recognition of the North Zuunbayan fault as distinct from the ‘main’ Zuunbayan fault (Johnson, 2004). The North Zuunbayan Fault forms the northern structural boundary of the Zuunbayan subbasin of the main East Gobi Basin, i.e., the northern margin of the Tavan Har basement uplift, and this structure was active during the latest phase of deformation observed in the study area (Fig. 3; Webb and Johnson, 2006). The Zuunbayan fault forms the southern boundary of the Zuunbayan subbasin and is only identified in Fig. 1. Digital elevation model and major tectonic elements of China and Mongolia the subsurface because it is overlapped by Upper Cretaceous-Recent (relative to the East Gobi Fault Zone (EGFZ) and East Gobi Basin (EGB)). Black lines strata (Fig. 3; Lamb et al., 1999; Johnson, 2004). Other individual fault correspond to political boundaries and documented regional-scale faults, fault zones fi fi (dashed where inferred) and sutures. Dashed box identifies the approximate location of segments have been identi ed throughout the study area, speci cally Fig. 2. Circled stars indicate the approximate location of Beijing and Ulaanbaatar. around Tavan Har (Fig. 3; Webb and Johnson, 2006). These individual Abbreviations are as follows: BS – Beishan Orogen; CAOB – Central Asia Orogenic Belt; structures (such as the Zuunbayan and North Zuunbayan faults) are EGFZ – East Gobi Fault Zone; NU – Noyon Uul. cited here as necessary for clarity, but for the purposes of this study the whole system is broadly characterized as the EGFZ. 2. Geologic setting The Unegt and Zuunbayan subbasins are located on either side of the North Zuunbayan fault at Tavan Har in the northeastern part of the Accreted arc terranes and synorogenic intrusive suites comprise the East Gobi Basin (Fig. 3). The Unegt subbasin is the larger in areal extent basement for much of southern Mongolia (Lamb and Badarch, 1997; of the two subbasins, approximately 40 km wide and 120 km long, and Badarch et al., 2002; Lehmann et al., 2010; Taylor et al., 2013). These forms a broad low relief area north of the EGFZ. In contrast, the amalgamated terranes are bound by the Siberian craton to the north, Zuunbayan subbasin is more narrow (∼20 km) and elongate along the Mongol-Okhotsk suture (Zorin et al., 2002; Van der Voo et al., (∼100 km). Prost (2004) estimates total depth to Paleozoic basement 2015; Johnson, 2015), and by the North China block to the south of up to 4000 m for the deepest parts of the basins. The Unegt- (Fig. 1). There is debate regarding nature of the southern boundary Zuunbayan subbasin pair provide excellent exposures of sedimentary with the North China block, including the number of terranes and and volcanic rocks deposited during Late Jurassic-Early Cretaceous timing of the associated suture zones. Many publications interpret the rifting (Graham et al., 2001; Johnson and Graham, 2004a,b), correlated boundary between the Mongolian terranes and the North China block in here to subsurface borehole and seismic reflection data (Johnson, Inner Mongolia and southern Mongolia as the late Paleozoic Solonker 2004). suture zone (e.g., Zhang et al., 1984, 2014; Chen et al., 2009; Jian et al., 2010; Xiao et al., 2003, 2015; Lin et al., 2014). In contrast, other studies 3. Methods favor an earlier suture prior to the Late Devonian (Xu et al., 2013; Zhao et al., 2016), and that sedimentary and magmatic rocks interpreted by Two dimensional (2-D) seismic reflection data are currently avail- others as evidence for the Solonker suture zone instead formed in an able only from the Unegt and Zuunbayan subbasins (Fig. 3). A Soviet intracontinental setting (Zhao et al., 2016; Luo et al., 2016). Our own exploration survey (∼800 km, early 1990s) and a private industry studies, which include geochronologic constraints, support a time- survey (∼2400 km, 1998–1999) provide sufficient areal coverage of transgressive closure of a Permian marine basin between the Mongolian both subbasins that structures can be correlated to previously-mapped terranes and North China, with the marine-to-non-marine transition regional features with high confidence (Lamb et al., 1999; Johnson occurring in southernmost Mongolia by the end of the Early Triassic et al., 2001; Johnson, 2004; Webb and Johnson, 2006). All 2-D seismic (Johnson et al., 2008; Heumann et al., 2012). Regardless of which data presented here are post-stack migrated, time seismic profiles first model is favored, the EGFZ and associated Mesozoic-Cenozoic strata of published by Johnson (2002). Accurate velocity data are not readily the East Gobi Basin are superimposed on the various Paleozoic terranes, available and therefore seismic lines have not been depth converted. in a distinct northeast-southwest trending fault-bounded structural The available migration file shifts the data downward approximately corridor (Fig. 2). 400 ms at the surface, and interpretations have not been corrected for The term ‘phase’ is used throughout this study to describe a subset of this shift. interpreted deformation and/or basin subsidence events which reflect a Digitized well-logs available for this study (Johnson, 2002) include specific geologic time frame and tectonic setting (e.g., rifting in the Late 76 of the total penetrations for the East Gobi Basin. The majority of Jurassic-Early Cretaceous). Groups or ‘generations’ of faults can be these are from the Zuunbayan subbasin where the main hydrocarbon- linked to these phases. Previous investigators identified at least four producing fields are located. Only six are within the Unegt subbasin and distinct tectono-stratigraphic packages deposited during phases of only eight are continuously logged to the top of the Paleozoic basement. deformation responsible for post-assembly (Mesozoic-Cenozoic) basin The main interval for hydrocarbon production in the East Gobi Basin is development and the EGFZ’s current architecture. These events include: the Lower Cretaceous synrift section and this is commonly the only (1) sinistral ductile shear in the Late Triassic (Webb and Johnson, 2006; logged interval (Johnson and Graham, 2004a; Wang et al., 2014). Webb et al., 2010); (2) Late Jurassic- Early Cretaceous rifting (Graham Palinspastic reconstructions are not currently possible for the Unegt et al., 2001); (3) mid-Cretaceous transpression and basin inversion and Zuunbayan subbasins due to structural complexity and lack of a

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Fig. 2. Simplified geologic map of southeastern Mongolia modified from Webb and Johnson (2006) and Tomurtogoo (1999). The Unegt and Zuunbayan subbasins are outlined in narrow dashed lined. Documented large-scale faults within the EGFZ are show in bold black lines (dashed where inferred). The China-Mongolia political boundary is outlined in red. Abbreviations for localities are as follows: BU – Bulgan Uul, HH – Har Hotol, NO – Nomgon, OH – Onch Hayrhan, TH – Tavan Har, TS – Tsagan Subarga, UB – Unegt subbasin; UK – Ulgay Khid, UR – Urgun, ZB – Zuunbayan subbasin. Approximate location of Fig. 3 is outlined by the dashed box. regional velocity model for depth conversions. The subsurface data are separate seismic profiles. The triangulated fault mesh was exported in used here specifically to determine relative life-spans and ages of faults. AutoCAD format and imported into ESRI’s ArcMap geospatial software Faults were interpreted using Halliburton-Landmark’s SeisWorks® 2-D for outcrop correlations. First-order observations of fault geometries Seismic Interpretation software and IHS’s Kingdom suite following were made from the seismic data, and a youngest age of activity was standard interpretation techniques. Automated triangulation operations assigned to a fault segment based upon crosscutting, overlap, and were used to interpolate fault surfaces between fault segments in stratigraphic relationships.

Fig. 3. Detailed geologic map for the study area based on field observation (this study) and adapted from Webb et al. (2010), Graham et al. (2001), Tomurtogoo (1999), Lamb et al. (1999) and Borzakovskiy et al. (1982). Map units have been grouped according to discussion points in the text; Paleozoic rocks and intrusive suites, Late Triassic shear zone, Lower - Middle Jurassic sedimentary rocks, Upper Jurassic - Lower Cretaceous sedimentary rocks, and Upper Cretaceous Quaternary sediments. Generalized basin outlines are show in dashed- lines for the Unegt and Zuunbayan subbasins. Abbreviations for localities are as follows: NZB – North Zuunbayan Fault, UR – Urgun, HH – Har Hotol, TH – Tavan Har, TS – Tsagan Subarga, UK – Ulgay Khid, ZB – Zuunbayan Fault.

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Fig. 4. (A) Simplified tectonostratigraphic summary for the study area. Blackened intervals correspond to mapped unconformities with estimated duration. Radiogenic age constraints are from volcanic ashes and basalts by Johnson and Graham (2004a), Graham et al. (2001) and metamorphic tectonites by Webb et al. (2010). Geologic time scale is from Gradstein et al. (2012). (B) Uninterpreted and interpreted seismic cross-section (shown in two-way travel time); inset map shows approximate location. See tectonostratigraphic column for fill patterns; faults are shown in bold black lines and horizons mapped within major stratigraphic packages are delineated by thin black lines. Examples of phases of deformation discussed in the text are identified with large black arrows. Early Jurassic foreland deposits are absent in this portion of the subbasin, and Phase 4 extension is poorly developed here. The mid-Cretaceous basin inversion event (Phase 5) is well illustrated in folding of the synrift strata and reactivated basement faults. Inset map shows location of seismic profile; TH - Tavan Har.

Fig. 5. Uninterpreted and interpreted seismic cross-section (shown in two-way travel time); inset map shows approximate location. Simplified tectonostratigraphic summary for the study area is included from Fig. 4A; for discussion see Fig. 4A caption. Faults are shown in bold black lines and horizons mapped within major stratigraphic packages are delineated by thin black lines. Examples of phases of deformation discussed in the text are identified with large black arrows. Paleozoic basement which reaches the surface in the center of this seismic profile has been correlated to the Late Triassic shear zone (Phase 2; Webb et al., 2010). Inset map shows location of seismic profile; TH - Tavan Har.

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Fig. 6. Uninterpreted and interpreted seismic cross-section (shown in two-way travel time); inset map shows approximate location. Simplified tectonostratigraphic summary for the study area is included from Fig. 4A; for discussion see Fig. 4A caption. Faults are shown in bold black lines and horizons mapped within major stratigraphic packages are delineated by thin black lines. Examples of phases of deformation discussed in the text are identified with large black arrows. Foreland basin-style deposits form a northward thickening wedge and are internally deformed within the Unegt subbasin. These deposits are absent within the Zuunbayan subbasin. Synrift strata within the Zuunbayan subbasin are apparently three to four times thicker than in the Unegt subbasin in this survey line. Faults associated with Phase 1 beneath the Unegt subbasin are also identified here. Inset map shows location of seismic profile; TH - Tavan Har.

Fig. 7. Uninterpreted and interpreted seismic cross-section (shown in two-way travel time); inset map shows approximate location. Simplified tectonostratigraphic summary for the study area is included from Fig. 4A; for discussion see Fig. 4A caption. Faults are shown in bold black lines and horizons mapped within major stratigraphic packages are delineated by thin black lines. Examples of phases of deformation discussed in the text are identified with large black arrows. An increase in internal folding and faulting is well illustrated in the Phase 3 deposits of the Unegt subbasin on this survey line. Synrift strata within the Zuunbayan subbasin are substantially thicker than the Unegt subbasin in this survey line. Reactivation of faults associated with previous deformation phases is apparent along the northern margin of the Unegt subbasin. Inset map shows location of seismic profile; TH - Tavan Har.

Four seismic profiles were chosen for this publication (Figs. 4–7; deformation. Fault traces have been simplified for display, and only original publication and data release in Johnson (2002)). These profiles major basin-forming faults are included (i.e., faults that can be are all oriented roughly NW-SE, i.e., dip sections across the Cretaceous correlated along strike across multiple seismic profiles). As will be rift basin, and they step sequentially from southwest to northeast across discussed, displaying these deformation phases together in map view the EGFZ. The profiles highlight the key structural and temporal highlights notable differences in the distribution and orientation of relationships for each phase of deformation. A fault generation map fault generations across the EGFZ, and helps illustrate kinematic (Fig. 8) was also created to represent the five brittle phases of relationships and reactivation patterns between them.

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Fig. 8. Generalized geological and fault generation map produced from interpretation of seismic surveys and the correlation of outcrop evidence. Fault generations have been coded by line weight and pattern, corresponding to deformation phase. Faults shown here are assigned normal, reverse, and/or strike-slip sense of displacement depending upon the last documented phase of deformation. Note that the Paleogene strike-slip phase (dashed) is defined by apparent normal and reverse sense offset along individual segments, as is common for the expression of strike-slip faults in 2-D seismic data. Sense of shearing across strike-slip faults was determined in the field from cross-cutting relationships and where indicators were available. Locations of the interpreted subsurface faults correspond to where the faults ‘top-out’ within the seismic profiles. Note the differences in deformation character across the EGFZ (Unegt versus Zuunbayan subbasins). See Figs. 3 and 4 for location and key to map patterns.

To supplement the subsurface interpretations, geologic transects Badarch, 1997, 2001; Tomurtogoo, 1999; Blight et al., 2010). These and area mapping were conducted in locations along the Unegt and arc assemblages are indistinguishable in the seismic dataset and have Zuunbayan subbasin margins, particularly where mapped subsurface not been penetrated by boreholes, so the Paleozoic units are largely features appeared to crop out (e.g., Ulgay Khid, Nomgon, Tsagan undifferentiated in our subsurface interpretations. Subarga, Tavan Har, Har Hotol, Urgun; Fig. 2). Fault plane orientation Faults that moved during this earliest phase of deformation are and kinematic indicators for sense of slip were collected where possible mainly northwest-dipping thrusts. The seismic data presented here only (Taylor et al., 2009), in addition to the age and type of rocks involved in extend to 4 s depth (TWTT) and the quality/resolution of imaging faulting. The correlation of structures observed in outcrop to those rapidly deteriorates with depth. Therefore, décollement surfaces and interpreted in seismic profiles was most successful around Tavan Har, boundaries between fault blocks at depth are largely inferred in the the structural and basement high between the Unegt and Zuunbayan interpreted seismic cross sections. There is little or no offset of the subbasins (Fig. 2). Additional field transects targeted linear features Mesozoic and younger strata that overlap these structures (Fig. 6). The observed in SRTM LandSAT imagery along strike (to the northeast and oldest strata deposited unconformably over Phase 1 structures are Early southwest) of Tavan Har. Several faults crosscut Paleogene or younger Jurassic prerift sequences; Triassic strata are absent in much of the strata and alluvium, and palynology samples were collected for age and study area (Figs. 2 and 4; Tomurtogoo, 1999). Several subsequent environmental determination of these units. deformation phases for the EGFZ exploit the faults formed during Phase 1 deformation, mainly the basin-bounding faults. Phase 1 thrust faults are common beneath the Unegt subbasin, north 4. Results of the EGFZ (Fig. 3), but the structures are notably absent beneath the Zuunbayan subbasin (Fig. 7). The fault generation map best highlights 4.1. Phase 1 this relationship (Fig. 8), as the most continuous of the interpreted Phase 1 faults are located beneath the Unegt subbasin. In part, this The oldest stage of deformation is defined by faults that cut mainly contrast is a result of deeper burial of the Paleozoic sections beneath Devonian, Carboniferous and Permian basement rocks of the East Gobi thick Mesozoic rift sections in the Zuunbayan subbasin (and thus more Basin (Figs. 6 and 7). Devonian and Carboniferous units include difficulty in imaging), but it may also reflect later strike-slip faulting greenschist-grade sedimentary successions, volcanic rocks, and some and juxtaposition of different basement blocks (see Phases 2, 5, 6). intrusions (Fig. 2; Badarch et al., 2002). Silurian and Ordovician Field transects at Har Hotol, Ulgay Khid and Tsagan Subarga (Fig. 3) sedimentary units have been reported in isolated outcrops but are in Paleozoic igneous and sedimentary rocks, including locally dated poorly represented in the study area (Yanshin, 1989; Lamb and

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Fig. 9. Representative field photographs of each deformation phase as observed in outcrop. (A) Carboniferous thrust fault exposed along the northern margin of Ulgay Khid (Fig. 3). Backpack is ∼50 cm tall. (B) Exposed Late Triassic mylonite at Tavan Har. Sense of shear (sinistral) was identified in the field, as annotated by the white arrows. Foliation = 354/71(Dip direction/dip); lineation = 266/06. Pencil for scale is ∼15 cm in length. (C) Type section of the Khamarkhoovor Formation at Har Hotol. Meter-scale thrust fault bisecting the outcrop is interpreted as Phase 3 deformation. Large cobbles and boulders in the unit are derived from nearby Paleozoic volcanic and sedimentary units. Hammer for scale. (D) Synrift (Phase 4) section observed along the southern margin of a basalt ridge (foreground) at Har Hotol. The basalt ridge has been dated as 126 ± 1 Ma (Graham et al., 2001). The synrift section dips steeply to the south, whereas Upper Cretaceous postrift deposits are subhorizontal (background). Approximate orientation of dipping beds is shown by dashed black lines. The middle Cretaceous unconformity is marked by the white dashed line. (E) Postrift section unconformably above synrift deposits at the northeastern edge of Tavan Har (Fig. 3). Approximate dip orientations for the synrift and postrift sequences are shown with dashed black lines. White dashed line denotes the middle Cretaceous unconformity. (F) Fault gouge associated with Phase 6 deformation observed south of Ulgay Khid. Gouge zone was identified in deformed Oligocene-Miocene strata based on palynology (these rocks were formerly mapped as Upper Cretaceous). Outcrop is ∼1.5 m tall.

Carboniferous arc units near Har Hotol (Blight et al., 2008, 2010), also the northern margin of the Unegt subbasin in a wedge-shaped, north- show localized evidence for faulting corresponding to this earliest phase ward-thickening section (Fig. 6). The seismic dataset shows that these (Fig. 9A). Faults located in these areas proved difficult to map over deposits are generally absent or notably thin in the Zuunbayan distances beyond 10’s of meters due to poor exposures, but exposed subbasin, although the formation is mapped in small outcrops to the fault planes strike northeast and do not displace overlying Mesozoic and south of the EGFZ (e.g., Nomgon, Fig. 3). younger strata (Fig. 3). Regional maps (Borzakovsky, 1982) suggest The seismic cross-sections lack evidence for syn-depositional defor- that related thrust structures are present to the south of the EGFZ (i.e., mation of this prerift assemblage. Deformation recorded in the prerift Nomgon area, Fig. 2). deposits partially exploits the lower nonconformity with Paleozoic basement as a basal décollement, and ramp-flat fault geometries and 4.2. Phase 2 broad folds are present within the prerift unit (Figs. 5 and 6). Comparison of the three seismic lines (Figs. 4, 6 and 7) demonstrates The Unegt and Zuunbayan subbasins are separated by an exposed the lack of inferred Early Jurassic strata in the Zuunbayan subbasin, and basement high at Tavan Har (Fig. 3). Webb et al. (2010) identified the illustrates increasing strain (broad into tight folds) and increased horst as an exhumed sinistral shear zone and dated the main phase of internal faulting, as well as basement-involved faulting to the north- ductile deformation via 40Ar/39Ar geochronology as Late Triassic (ca. west, as compared to the Unegt subbasin. These observations argue for 225–210 Ma). Seismic profiles cross the northeastern margin of the reactivation of existing faults in the Paleozoic basement, presumably exposed basement block at Tavan Har study area, but the deep the southwest-striking, northwest-dipping structures identified in Phase subsurface structure is poorly resolved (Fig. 5) due to acquisition and 1. Major fault planes interpreted to accommodate Phase 3 deformation processing problems (e.g., edge-effects), as well as difficulty in imaging were correlated throughout the seismic survey northeast of Har Hotol, steeply dipping foliation. The shear zone is also heavily overprinted by where they occur along a subparallel strike to the mapped Phase 1 fault later brittle deformation (Webb and Johnson, 2006; Taylor et al., observed in the Unegt subbasin (Fig. 8). Additionally, the seismic cross- 2008). sections in Figs. 6 and 7 illustrate an unconformable relationship The exposed shear zone at Tavan Har is part of a zone of tectonites between Early Jurassic prerift strata and the synrift strata deposited that can be traced roughly 250 km along the EGFZ to Ulgay Khid, during Phase 4 (i.e., late Mesozoic rift phase). Tsagan Subarga and Urgun (Fig. 3). As at Tavan Har, along-strike Low-angle faults with several meters of apparent reverse offset of outcrop exposures of the shear zone have been previously mapped as sedimentary layers are exposed in the prerift megasequence at Har Precambrian crystalline basement (Tomurtogoo, 1999). Webb et al. Hotol (Fig. 9C). The faults are interpreted to correspond with the (2010) report Late Triassic 40Ar/39Ar ages (225–210 Ma) from shear deformation observed in the subsurface, which is consistent with the zone samples collected at Tsagan Subarga, similar to Tavan Har. increasing degree of faulting and folding of the unit from southwest to Differing cooling rates inferred from the 40Ar/39Ar data suggest northeast through the seismic cross sections (Figs. 5, 7 and 8). The different uplift histories for the two basement blocks during Late prerift megasequence includes discontinuous outcrops mapped as Jurassic-Early Cretaceous extension (Webb et al., 2010). The ductile Lower Jurassic Khamarkhoovor Formation by Graham et al. (2001). shear zone is dominated by steeply dipping northeast-striking foliation All documented occurrences of the prerift megasequence in the study and lineation that plunges to the WSW or ENE (Fig. 9B; Webb and area strike northeast with variable dips, although exposures are limited. Johnson, 2006). Foliation measurements at Urgun also dip steeply and The rocks comprising the prerift megasequence include thick (meter strike northeast and lineation plunges WSW. The shear sense is scale), fining upward conglomerate sequences with grain sizes ranging consistently sinistral based on sigma and delta clasts, and S-C fabrics from sand matrix to well-rounded cobble and boulder-sized clasts identified in the field, and microstructural analysis (Taylor et al., 2009; (Fig. 9C). Locally, the conglomerates are interbedded with thinly Webb et al., 2010). The exposed shear zone corresponds to a well- bedded (decimeter scale) sandstone channels, consistent with alluvial- defined linear magnetic high (Maus et al., 2009; Guy et al., 2014) which fluvial environments as interpreted by previous investigators continues to the northeast and southwest for a possible total length of at (Jerzykiewicz and Russell, 1991; Yamamoto et al., 1993; Graham least 1000 km, suggesting the shear zone is an even larger regional et al., 2001). The Khamarkhavoor Formation has been interpreted by feature than its current outcrop extent. Furthermore, the regional previous investigators (Graham et al., 2001; Johnson, 2004)asa extent of these consistently-oriented metamorphic fabrics argues possible foreland basin deposit, coeval with an extensive foreland basin against major subsequent rotation of the shear zone. succession exposed ∼1000 km to the west at Noyon Uul (Fig. 1; Hendrix et al., 1996, 2001). Although deposits of the prerift megase- 4.3. Phase 3 quence occur in isolated exposures, it has been suggested that these units were once part of the same regional basin system (Graham et al., A thick (maximum ∼2 s TWTT) assemblage of folded and faulted 2001). reflectors is a prominent feature in the subsurface dataset along the The upper age limit for deposition of the prerift megasequence northeastern margin of the Unegt subbasin between 0.5 and 2.5 s TWTT remains controversial. Minimum ages of the Khamakhoovor Formation (Fig. 6). The reflectors correspond to the Early-Middle Jurassic prerift have been suggested as young as Callovian (i.e., late-Middle Jurassic; megasequence described in Johnson (2004) and Graham et al. (2001) Graham et al., 2001) and as old as late Early Jurassic (Jerzykiewicz and (Fig. 4). Locally named the Khamarkhoovor Formation (Jerzykiewicz Russell, 1991). In all cases, investigators agree that the formation is and Russell, 1991; Yamamoto et al., 1993), these are dominantly marked by a significant regional unconformity (Yamamoto et al., 1993; alluvial-fluvial strata and are mainly observed in the subsurface along Graham et al., 2001; Johnson, 2004). The assemblage was deposited

8 M.J. Heumann et al. Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx unconformably on Paleozoic basement and post-dates the final thrust- 4.5. Phase 5 ing associated in Phase 1. Thus the lower (oldest) age limit for deposition must postdate the Late Triassic ductile shearing (Phase 2 A regional unconformity truncates the synrift and older assemblages deformation; ∼210 Ma) and also predate Late Jurassic-Early Cretac- of the East Gobi Basin, above which a Late Cretaceous-Paleogene eous extension (Phase 4). Additionally, apatite fission track dating postrift or overlap megasequence was deposited (Fig. 9E; Graham et al., suggests that cooling below ∼150 °C was not achieved at Tavan Har 2001; Johnson, 2004). The synrift strata were inverted during a period until the Middle Jurassic (Taylor et al., 2009). This observation is of regional shortening or transpression along the EGFZ. Regional scale consistent with relatively slow cooling during the Early Jurassic as reverse and thrust faults of this phase (post-unconformity) have not indicated by Webb et al. (2010). Therefore, the timing of Phase 3 been identified in the field, nor in the subsurface (Fig. 8), so it is likely deformation and development of the regional unconformity likely that contraction during this phase included an oblique component. occurred during the later Early Jurassic as a minimum age. Folds and faults beneath the regional unconformity are oriented in geometries inconsistent with thermal subsidence or progressive rotation during normal faulting; instead the shallowest synrift units dip steeply 4.4. Phase 4 basinward, whereas the deepest synrift units are reverse-faulted internally for space accommodation (Figs. 6 and 7). Within the Unegt The Late Jurassic through Early Cretaceous is a well-documented subbasin, anticlinal features were formed during this phase (Fig. 1), but period of extension in the study area (Graham et al., 2001; Johnson, at Tavan Har the overlap megasequence onlaps the basement high 2004), in addition to much of East Asia (Dong et al., 2015). Late (Fig. 5). Drag folds indicative of reverse sense of slip across inferred Mesozoic rifting in southeastern Mongolia is contemporaneous with normal faults during Phase 4 are present at the margins of the extension throughout central and eastern China, and the formation of Zuunbayan subbasin (Figs. 6 and 7). We infer basin inversion was large rift basins that contain significant petroleum reserves (Watson accomplished mainly by folding and reverse-sense reactivation of et al., 1987; Traynor and Sladen, 1995; Wei et al., 2010; Graham et al., synrift normal faults. 2012; Lin et al., 2013) as well as uranium deposits (Bonnetti et al., Phase 5 deformation was likely brief, lasting perhaps ∼1–5 million 2016). The synrift deposits are further subdivided into several se- years during Cenomanian or Turonian time (Graham et al., 2001; quences, each reflecting a different period of extension marked by the Johnson, 2004). Similarly, multiple basin inversion events beginning at occurrence of a conglomerate unit in outcrop or other evidence of a ∼95 Ma have been documented in Songliao and Erlian Basins in China renewed basin subsidence and deposition phase (Fig. 9D; Graham et al., (Li et al., 1997; Lin et al., 1997, 2001; Feng et al., 2010; Wei et al., 2001; Johnson, 2004). In this study, the synrift megasequence is left 2010). The magnitude of total shortening across the Unegt and undifferentiated because the overall deformational character observed Zuunbayan subbasins during the basin inversion event is uncertain during this phase is consistently extensional. The synrift units lie but probably relatively minor overall (crude, line-length restoration unconformably on the prerift megasequence and locally on Paleozoic across several inverted structures suggests ∼–3%), and most pro- basement (Fig. 7). Interbedded bimodal volcanic units provide absolute nounced along the main structures of the EGFZ (Fig. 6). Thermal age constraints on the timing of extension from at least 155 to 121 Ma maturation modeling by Johnson (2004) suggested up to 1700 meters (Fig. 4; Graham et al., 2001; Johnson and Graham, 2004a). of eroded synrift strata in one well at the Zuunbayan oilfield. Therefore, Slip along the high-angle basin bounding faults of the Zuunbayan although lateral shortening was minor, significant inversion may have subbasin (Phase 4 deformation) is primarily dip-slip and subparallel to occurred along basin bounding faults. steeply dipping shear zone fabrics developed during the Late Triassic This period marks a significant change in deformation style (Webb and Johnson, 2006). Thus, the high-angle fabrics associated with observed in the subsurface dataset. Only the major basin-bounding the Late Triassic shear zone (Phase 2) were exploited during extension faults (in particular, along Tavan Har) remained active following the by basin-bounding faults as observed in the field (Webb et al., 2010) inversion event and unconformable deposition of the postrift sequences and in the subsurface 2D seismic dataset. Extension resulted in horst (Figs. 8 and 9E). Otherwise, minor antithetic and synthetic faults locally and graben architecture throughout the study area. Deposition of strata offset the unconformity (Fig. 7). is contemporaneous with normal faulting along main basin bounding faults, as illustrated by the packages of growth strata formed along 4.6. Phase 6 subbasin margins (Figs. 4, 6 and 6). The Unegt subbasin occupies a poorly developed half graben to the north of the horst block at Tavan The final and youngest phase of deformation observed along the Har (i.e., the EGFZ), and the Zuunbayan subbasin occupies a well- EGFZ corresponds to Cenozoic strike-slip faulting and minor basin developed graben system to the south (Figs. 7 and 8). The Unegt deposition. The seismic cross sections shown in this study indicate that subbasin lacks a significant basin-bounding fault system and extension the mid-Cretaceous unconformity (Phase 4) is locally deformed, as are across the area is therefore relatively minor. Preserved ‘acoustic overlying Upper Cretaceous and Paleogene strata (Fig. 9F). Faults of thickness’ of synrift fi ll in the Zuunbayan subbasin is more than double this phase are typically reactivated basin-bounding faults of Phases 4 that of the Unegt (∼2.25 s. vs ∼1 s TWTT; Fig. 7). Thickness of and 5. These relationships are best illustrated in Figs. 7 and 8, where exposed synrift strata based on outcrop studies is ∼2–3 km in the the overlying strata are folded and faulted along the EGFZ (specifically, Zuunbayan subbasin (Graham et al., 2001). The difference in thickness the North Zuunbayan fault near Tavan Har). The faults corresponding of depositional packages across the EGFZ may partly indicate of to Phase 6 deformation juxtapose distinct packages of basin fill along differential exhumation during Phase 5 (basin inversion). However, steeply dipping faults, with sudden changes in the apparent amount and there appears to be a fundamental structural control favoring deep sense of offset shown on 2-D seismic sections (cf. Figs. 4, 6 and 7). Such basin development in the Zuunbayan subbasin, and this primarily relationships are common in the subsurface expressions of strike-slip reflects reactivation of structures inherited from previous deformation fault zones in other regions (Zolnai, 1991; Webb and Johnson, 2006). phases. More specifically, slip along the high-angle basin-bounding The generation of faults corresponding to Phase 6 deformation pre- faults of the Zuunbayan subbasin is primarily dip-slip and sub-parallel sented here commonly reaches the surface, are northeast striking and to steeply dipping shear zone fabrics developed during the Late Triassic can be mapped for several kilometers within the seismic data set. (Phase 3). This further argues for reactivation of the shear zone during Surface expressions of Phase 6 faults include multiple zones of fault late Mesozoic extension (Webb et al., 2010). gouge reaching tens of meters in width (Figs. 3 and 9F). Gypsum and related minerals were found in several locations growing along striated fault planes in Paleogene or younger strata. Several faults identified

9 M.J. Heumann et al. Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx outside the seismic survey boundaries at Ulgay Khid and near Urgun Johnson et al., 2008), although regionally the broader suture zone has (Fig. 3) are also consistent with a northeast trending left-lateral strike- evidence for collision ranging from pre-Late Devonian (Xu et al., 2013; slip fault zone. Zhao et al., 2016) to Middle or Late Triassic (Chen et al., 2009; Li et al., Strata cut by Phase 6 faults range from Upper Cretaceous through 2014). Although Permian units do not crop out in the study area Paleogene, although regional maps imply (perhaps incorrectly) only immediately surrounding the subsurface dataset, they are present to the minor Cenozoic deposition. The youngest units are typically poorly southwest at localities such as Bulgan Uul and Nomgon (Fig. 2). At lithified, poorly sorted sandy to conglomeratic successions deposited in these localities, Permian strata are deformed by reverse faulting and alluvial environments. Clasts within the units appear to be locally tight folding associated with penetrative cleavage, dissimilar to the derived (mainly volcanic lithic fragments). These strata thicken away deformation observed in the Unegt and Zuunbayan subbasins (Johnson from the bounding fault systems (Figs. 4–7) and may represent a period et al., 2008; Heumann et al., 2012). Thus the arc accretion phase can be of post-inversion thermal subsidence. distinguished from final closure of the remnant ocean basin in southern A key observation is that the youngest strata involved in faulting are Mongolia. quite distinct from the well-lithified, generally finer grained, fossil- bearing and volcaniclastic Cretaceous units of the synrift sequences. 5.1.2. Phase 2: Late Triassic shear zone formation One successful palynological sample was recovered from deformed red- Outcrops of the Late Triassic sinistral shear zone can be mapped beds that are currently mapped as Upper Cretaceous (postrift), near a along trend of the EGFZ for ∼250 km in outcrop (Fig. 3; Lamb et al., Phase 6 fault zone at Ulgay Khid (Fig. 3). Sample 04UK114 contained 1999; Webb et al., 2010), and possibly more than 1000 km based on pollen forms of Bombacaceae, Chenopodiaceae, and Betulaceae. These subsurface magnetic data (Maus et al., 2009). The main phase of shear three families are derived from temperate-tropical trees and angios- zone activity occurred between ∼225 and 210 Ma, as determined by perms that are most common in the Oligocene-Miocene of China and 40Ar/39Ar thermochronology by Webb et al. (2010) and Lamb et al. Mongolia (Song et al., 2004). They were largely extinct in the region by (1999). The age of ductile deformation thus postdates Late Permian the end of the Paleogene (Wang et al., 1990). These new data suggest (and older) arc-continent collision along the Solonker suture and some of the mapped ‘Upper Cretaceous’ basin fill is in fact at least late occurred in an intracontinental setting (Johnson et al., 2008). Estimates Paleogene (likely Oligocene-Miocene) in age, and therefore there may for left-lateral displacement during Phase 2 are ∼–250 km based on be more of a sedimentary record of this deformation phase than has piercing point studies along the EGFZ by Heumann et al. (2014). been recognized. Recorded seismicity for southeastern Mongolia since Such magnitudes of displacement and the continuity of the shear 1958 (beginning of recording as per the Advanced National Seismic zone suggest regional-scale processes rather than just local drivers. System (ANSS) Worldwide Earthquake Catalog) is sporadic and minor Southeast of Tavan Har, proximal to the Solonker suture zone (Fig. 1), relative to other major fault systems in the region (Baljinnyam et al., Davis et al. (2004) have documented high strain NNE-SSW extension 1993; Adiya et al., 2003; Cunningham et al., 2003), supporting this as a along a low-angle detachment fault in the Late Triassic. E-W Late Paleogene phase of deformation. Apatite fission track studies (Li et al., Triassic extension has also been documented in N-NE striking faults 2016) also suggest muted post-Late Cretaceous exhumation in the along the northwest portion of Ordos basin (Ritts et al., 2009). Sinistral region. shear along NE striking faults is documented in north-central China (Zhou et al., 2012; Zhang et al., 2013; Zhao et al., 2015), and in 5. Discussion northwest China (Wang et al., 2010). These regional extensional features may have been kinematically linked with the EGFZ shear zone, The results of subsurface and outcrop interpretation presented here along a regional scale left-lateral fault zone or series of fault zones, the summarize nearly 270 million years of deformation and basin evolution brittle component of which has since been eroded (cf., Zhao et al., recorded in Paleozoic through Cenozoic rocks of southern Mongolia 2015). (Fig. 3). The six distinct phases of deformation shed light on modes of Timing and modes of closure of the Mongol-Okhotsk ocean is highly intracontinental deformation. In this example of a long-lived intracon- controversial, but in the vicinity of northern Mongolia it likely occurred tinental deformation zone, local and regional tectonics couple with during the Late Triassic, probably suturing from west to east (Zhao dynamic boundary conditions to drive polyphase deformation and et al., 1990; Zorin et al., 2002; Johnson, 2015; Van der Voo et al., basin evolution. The main deformation phases and possible tectonic 2015). Timing of final Mongol-Okhotsk collision in the vicinity of driving forces are summarized in Fig. 10. Furthermore, we argue that northeastern Mongolia remains a point of controversy between Late preexisting fabrics favor the localization of successive deformation Triassic (Zonenshain et al., 1990), Middle-Late Jurassic (Zhao et al., phases along the EGFZ. Multiphase, localized deformation occurring 1990; Zorin et al., 2002; van der Meer et al., 2010), and Early under dynamic boundary conditions suggests that deep-rooted crustal Cretaceous (Van der Voo et al., 1999, 2015). In any case, collision fabrics are the main control on where and how deformation is was apparently ongoing along its western portion (in Mongolia, Fig. 1) manifested along the EGFZ. during Phase 2. Also possibly influencing boundary conditions at this time is terminal collision between the North and South China blocks, 5.1. Deformation phases and tectonic driving forces resulting in the Qinling-Dabie orogen between ∼240 and 220 Ma (Fig. 10; Ames et al., 1993, 1996; Hacker et al., 2000; Pullen et al., 5.1.1. Phase 1: Paleozoic arc accretion and basement structural 2008), and perhaps ongoing deformation along the northern margin of development the North China Block (Tian et al., 2015). Therefore we infer that Structures identified as Phase 1 deformation correspond to the late oblique movement between two rigid bodies (Siberia and the combined Paleozoic period of arc accretion and rapid growth of the Altaids North China Block-South China Block) was focused along a narrow (Fig. 10; Şengör et al., 1993; Windley et al., 2007; Guy et al., 2014). region (i.e., the EGFZ shear zone) of recently accreted arc systems. Where observed in the subsurface and in outcrop, mainly in the Unegt subbasin, Phase 1 faults dip north and deform mainly Ordovician 5.1.3. Phase 3: Early Jurassic foreland basin through Carboniferous rocks. The structural fabric in these units likely The Early Jurassic prerift megasequence forms a north-thickening originated during subduction of arc terranes in the Devonian and wedge (Fig. 6) along the northern margin of the Unegt subbasin. Carboniferous (Lamb and Badarch, 1997; Taylor et al., 2013). The Available outcrops of this succession are poorly preserved (Fig. 3) but terminal Late Permian collision in southern Mongolia was likely where studied in the Unegt subbasin, the prerift megasequence is southward-directed with partial subduction of the Altaids beneath the nonmarine and likely Early Jurassic in depositional age (e.g., the North China Block (Fig. 10; Cope et al., 2005; Zhang et al., 2007; fluvial-lacustrine Khamarkhoovor Formation; Jerzykiewicz and

10 M.J. Heumann et al. Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx

Fig. 10. Summary chart of inferred tectonic driving forces and plate boundary conditions related to the poly-phase deformation history of the EGFZ. (A) Paleozoic through Middle Jurassic. (B) Middle Jurassic through Recent.

Russell, 1991; Yamamoto et al., 1993). The prerift megasequence lies directed shortening (Fig. 6), suggests the prerift succession may have unconformably between Paleozoic basement rocks and late Mesozoic been deposited in a foreland basin setting. rift successions, and thus it post-dates the Permian ocean closure in Phase 3 deformation is consistent with regional contractile tectonics southernmost Mongolia and predates rifting. The prerift megasequence in the Early Jurassic (Yin and Nie, 1996) and contemporaneous with N- also lacks extensive bimodal volcanics and obvious normal faults that S shortening during the Early Jurassic to the southwest of the study area characterize the later rift succession. These observations, along with the near the Beishan and Noyon Uul (Fig. 1; Zheng et al., 1996; Hendrix observed northward thickening wedge-shape and evidence for south- et al., 1996; Webb et al., 1999; Dumitru and Hendrix, 2001; Liu et al.,

11 M.J. Heumann et al. Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx

2012). Also southwest of the study area is the Yagan-Onch Hayrhan Basin inversion is recorded in other rift basins of the China- metamorphic core complex (Fig. 2), where top-to-the-southwest sense Mongolia border region (e.g. Songliao, Erlian basins; Lin et al., 2001; of shear in ductile fabrics are likely early Mesozoic in age (they are Feng et al., 2010; Wei et al., 2010), but was apparently brief (∼5 My; overprinted by the Early Cretaceous detachment fault fabrics dated by Graham et al., 2001; Song et al., 2014). Some discrepancy exists in the Webb et al. (1999)). Field observations from Johnson et al. (2001) literature for exact timing of the initiation for regional contraction, with suggest these overprinted fabrics are coeval with Meso-Neoproterozoic the youngest age estimates as late Turonian (Feng et al., 2010) and the carbonate klippe emplacement in the Beishan orogen (Zheng et al., oldest as Cenomanian (Graham et al., 2001). Regardless, this deforma- 1996). Apparently localized deposition of prerift strata along the tion phase was contemporaneous with shortening across much of northern margin of the Unegt subbasin, and southward propagation eastern Asia (Fig. 10; Yin, 2010). Mesozoic plate reconstructions for of basement thrusting (Fig. 8), suggest that Phase 3 deformation the western Pacific margin suggest a shift from dextral strike-slip reactivated a select subset of steeply-dipping Paleozoic faults north of motion to sinistral strike-slip motion in the middle Cretaceous the Unegt subbasin. (∼100–80 Ma; Osozawa, 1994; Otoh and Yanai, 1996), and a shift in During Early Jurassic time, regional shortening was likely driven by convergence direction as a result of increased spreading rates at the continuing closure of the Mongol-Okhotsk Ocean to the north (Zorin Kula Ridge (Engebretson et al., 1984; Hourigan and Akinin, 2004). The et al., 1993; Zorin et al., 2002; Van der Voo et al., 1999, 2015; van der shift along the Pacific margin was likely punctuated with periods of Meer et al., 2010), and collision of the Qiantang block to the southwest high angle subduction of the Pacific Plate, and structural inversion (Fig. 10; Dewey, 1988; Hendrix et al., 1992; Matte et al., 1996). These recorded throughout the Late Cretaceous and Paleogene in other distant collisional events may have influenced plate boundary condi- Mesozoic rift basins (e.g., Songliao Basin; Feng et al., 2010; Song tions at this time, thus driving intracontinental deformation along the et al., 2015). Additionally, some investigators suggest that collisions of EGFZ. allochthonous terranes influenced deformation along the eastern mar- gin of Asia during this time frame (Faure and Natal’in, 1992; Kodama 5.1.4. Phase 4: Late Jurassic-Early Cretaceous extension and Takeda, 2002; Yang, 2013). Late Mesozoic extension is well documented across most of east- central Asia (e.g., Meng et al., 2003; Ritts et al., 2010; Graham et al., 5.1.6. Phase 6: Paleogene strike-slip faulting 2012). Modes and strain magnitudes for Late Jurassic through Early Phase 6 deformation along the EGFZ has only recently been Cretaceous extension vary. A metamorphic core complex at Yagan- characterized (Webb and Johnson, 2006; Heumann et al., 2008, Onch Hayrhan to the southwest (Fig. 2) records high strain extension in 2009; Taylor et al., 2009). Left-lateral strike-slip faulting and associated the Early Cretaceous (Zheng et al., 1991; Webb et al., 1999; Johnson shortening and extension features are observed along large zones of et al., 2001). This timing is compatible with lower magnitude extension fault gouge that cross-cut deformed (in places) Paleogene alluvium in the Unegt and Zuunbayan subbasins between at least 155 and (Fig. 9F). The lower age limit for initiation of strike-slip faulting is 121 Ma (Fig. 2; Graham et al., 2001). In the Zuunbayan subbasin, main poorly known. However, strike-slip faults observed in the subsurface basin bounding faults are reactivated basement structures from Phase 1 ( Figs. 6 and 7) clearly cross-cut the middle Cretaceous unconformity, and faults that exploit steep fabrics in tectonites from Phase 2. implying faulting may have initiated as early as post ∼95 Ma, although However, faults generated during Phase 4 deformation within the potential tectonic driving mechanisms for strike-slip activity at Unegt subbasin strike north-south and therefore are oblique to the ∼95–100 Ma are lacking (Fig. 10). This study documents unconsoli- North Zuunbayan fault and to those faults of the Zuunbayan subbasin dated alluvium (distinctly different from Upper Cretaceous strata in the (cf. Prost, 2004). Differential extension north and south of Tavan Har region) that is also cross-cut by northeast striking left-lateral strike-slip (EGFZ) is likely accommodated by north-south-striking normal faults faults. The single palynology result presented by this study suggests that (Fig. 9; Johnson et al., 2001). some of this alluvium is Oligocene-Miocene in age. Earthquake data and Although late Mesozoic-Tertiary extensional basins are well known a lack of neotectonic features in both field and remote-sensing (e.g., across the region (Watson et al., 1987; Davis et al., 1996; Darby et al., LandSAT) observations suggest the EGFZ is mostly inactive today 2004; Feng et al., 2010), a single mechanism causing their formation (Adiya et al., 2003; cf. Li et al., 2016). Therefore, although the remains elusive. Proposed extensional causes include backarc extension maximum age range for strike-slip activity across the EGFZ during (Watson et al., 1987), gravitational collapse of over-thickened crust Phase 6 is Late Cretaceous through Miocene, we tentatively favor late (Johnson et al., 2001; Wei et al., 2010), and breakoff of a north-dipping Oligocene through early Miocene as most probable. This favored timing slab causing mantle lithospheric stretching and upwelling (Meng et al., is also easier to explain by possible drivers (Fig. 10), as discussed below. 2003; Feng et al., 2010). Regional extension may reflect a combination The magnitude of left-lateral strike-slip displacement across the of both near-field and far-field forces (Fig. 10). Gravitational collapse EGFZ is likely ∼100 km, from piercing point studies by Heumann et al. resulting in high-strain extension likely followed Jurassic shortening in (2014), with a proposed approximate age of Late Oligocene-Early the Beishan region (Zheng et al., 1996; Webb et al., 1999). Over- Miocene. Post-Cretaceous displacement magnitude across the EGFZ thickening of the crust was potentially enhanced by Early Jurassic contrasts with total displacement across the Altyn Tagh fault presented thrusting observed in the subsurface of our study area (Phase 3 by Yue et al. (2001, 2005) of ∼400–350 km, although the timing for deformation). displacement across each system is similar, beginning in the Late Oligocene. Interpretation by Yue and Liou (1999) suggested that slip 5.1.5. Phase 5: Middle Cretaceous regional shortening and basin inversion across a linked Altyn Tagh-East Mongolia (i.e., the EGFZ) fault system A regional basin inversion event (Phase 5) resulted in deformation essentially ceased in the mid-Miocene, replaced by a second stage of and partial erosion of the synrift succession (Fig. 5). One-dimensional Altyn Tagh movement and significant crustal thickening in the Qilian basin models constructed by Johnson (2004) suggested 1200–1700 m Shan. The discrepancy in magnitudes of displacement across the Altyn of synrift strata were eroded in places, although no specific magnitude Tagh Fault and EGFZ likely results from coeval displacement occurring of shortening has been measured in Phase 5. The differences in across multiple strike-slip faults systems throughout southwestern thickness of preserved synrift strata within the Unegt and Zuunbayan Mongolia and northern China (e.g., the Tost and Onon faults and subbasins arguably resulted from more pronounced inversion in the east-striking faults in the Alxa region (Fig. 1; Lamb et al., 1999; Darby Unegt. However, we argue that the contrast in thicknesses of synrift et al., 2005; Zhang et al., 2009). Therefore, large-magnitude displace- strata is also related to differential extension and basin subsidence ment across a single fault zone (the Altyn Tagh) was perhaps trans- during Phase 4 rifting, governed by inherited structural controls from ferred to multiple structures in horsetail geometry with smaller the previous deformation phases (Fig. 8). individual offset magnitudes during the late Oligocene. Several east-

12 M.J. Heumann et al. Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx striking faults were documented by Darby et al. (2005) in the western along pre-existing high-angle structural fabrics inherited from Alxa region, where they interpreted ∼70 km of offset magnitude Paleozoic accretion (Phase 1), Late Triassic shearing (Phase 2) and (Fig. 1). along basement uplifts formed during the Early Jurassic (Phase 3). Tectonic driving forces responsible for strike-slip initiation across Subdued extension in the Unegt subbasin (relative to the Zuunbayan) is the EGFZ (Phase 6) are difficult to determine. Pacific Plate velocities partly controlled by the thrust sheet beneath the basin, which is relative to Eurasia decreased at ∼65 Ma by more than 40 mm/yr and inferred to have an orientation unfavorable for reactivation for high- began to increase in the Eocene before reaching modern rate of angle normal faulting (Fig. 6). Faults within the Unegt subbasin show ∼100 mm/yr at ∼20–10 Ma (Northrup et al., 1995; Zhuang et al., minor displacement in comparison to the Phase 4 extensional faults 2010). Collision of the India subcontinent with southern Asia is observed in the Zuunbayan subbasin (Fig. 7). contemporaneous to the period of decreased relative Pacific Plate Middle Cretaceous basin inversion (Phase 5) resulted from regional velocity (Rowley, 1996). In agreement with proposed driving forces contraction and reverse movement along those same basin bounding for initiation for strike-slip activity along the Altyn Tagh Fault (Yin faults exploited during Phase 4. Drag folds present in the synrift strata et al., 2002), it is likely similar forces associated with the collision are against the basin-bounding faults are indicative of a reverse sense of responsible for faulting initiation across the EGFZ. Following a brief displacement (Figus. 6 and 7). Magnitudes of displacement along basin period of left-lateral displacement across the EGFZ, an increase in bounding faults remain undetermined. Potential differences in thick- Pacific Plate velocity and a shift from transtension to transpression ness of the synrift strata between the Unegt and Zuunbayan subbasins along the northeastern Pacific margin during the Miocene removed the may be linked to asymmetrical inversion during Phase 5 (as observed in necessary free surface (Jones et al., 1997) for the continuation of lateral the Songliao basin; Feng et al., 2010). Finally, left-lateral strike-slip extrusion along the EGFZ (Fig. 10; Jolivet et al., 1994; Worrall et al., displacement during the late Oligocene (Phase 6) continued exploita- 1996; Webb and Johnson, 2006). This shift in boundary conditions is tion of basin bounding faults, locally reactivating the high-angle fabrics inferred to cause the end (or significant slowing) of Phase 6 deforma- of the Late Triassic shear zone (Phase 2; Webb and Johnson, 2006). tion in the East Gobi basin, while allowing for deformation observed in The six successive deformation phases presented here encompass western Mongolia to remain active today (Fig. 10; Cunningham et al., disparate far-field tectonic driving forces over some ∼270 million 2003; Howard et al., 2003, 2006). years, and thus we infer that a deep-rooted crustal fabric (possible a terrane boundary or series of arc-system boundaries) favored multi- 5.2. Reactivation of preexisting crustal fabrics phase reactivation of the EGFZ. Such occurrences are documented in other rifted regions (e.g., South Africa, South America; Tommasi and Multiple phases of deformation identified by this study are evident Vauchez, 2001; Paton, 2006), suggesting that the propagation of along a defined structural corridor in southeastern Mongolia (the deformation is not random, but rather controlled by orogenic fabrics EGFZ), and occurred during a long term range of boundary conditions and structure within the lithosphere. and far-field stress regimes (Fig. 10). The correlation of the EGFZ to a linear magnetic anomaly present in the EMAG2 survey (Maus et al., 6. Conclusions 2009), suggests the EGFZ is a significant and well defined feature (∼1000 km long) with regional implications. The recurrence of six Six main deformation phases are linked to the EGFZ, beginning with distinct deformation phases along EGFZ suggests that several deforma- late Paleozoic continental assembly of southeastern Mongolia via arc tion phases may exploit structures that were formed or modified during and terrane accretion, four phases of Mesozoic reactivation including preceding deformation phases. We infer that the focused nature of the Triassic sinistral shear, early Jurassic contraction, Late Jurassic-Early deformation phases described here reflects inherited crustal weaknesses Cretaceous extension, mid-Cretaceous inversion (oblique contraction), that played a major role in defining the long term, polyphase evolution and finally left-lateral strike-slip in the Paleogene. Sedimentary basin of the East Gobi basin and the EGFZ. Such controls likely originated as a response to each of these is recorded in stratigraphic fill as well as deep crustal fabric created during Paleozoic arc-arc and arc-micro- unconformities and cross cutting and overlapping relationships with continent amalgamation (Badarch et al., 2002; Johnson et al., 2008). structures. Despite the obvious complexity associated with this long- Crustal-scale features are indicative of relatively weak crust in south- lived intracontinental deformation zone, these discrete features and east Mongolia, resulting from the heterogeneous oceanic arc terranes of phases are elucidated by a multi-proxy approach that incorporates the Altaids as well as a fluctuating geothermal gradient formed during outcrop to subcrop correlations, fault generation maps, and published terrane accretion and rifting. Faults observed in the Paleozoic basement analytical data. Tectonic drivers for these phases largely reflect far-field beneath the Unegt subbasin therefore are likely restricted to a single and plate boundary effects of the ongoing Mesozoic-Cenozoic growth of terrane (Badarch et al., 2002), helping to explain why similar faults are Eurasia, although the causes of some remain enigmatic. Polyphase apparently absent in the subsurface south of the EGFZ (Fig. 8). In the reactivation of the EGFZ is likely controlled by basement structures and subsurface and in outcrop, faults of Phase 1 are interpreted as bimodal fabrics inherited from previous deformation events, which define the in how they control later phases of deformation. Specifically, these strong northeast fabrics of the EGB. faults provide either a preexisting weakness enabling continued defor- mation (typically at the basin margins), or they remain intact beneath Acknowledgements the basin thus partitioning and localizing strain elsewhere. Steeply dipping foliation in the shear zone (Phase 2) characterized This work was supported by the National Science Foundation – by Webb et al. (2010) and Lamb et al. (1999) strikes southwest, United states grant numbers EAR-0537318 to C. Johnson and EAR- approximately parallel to the strikes of faults characterized as Phase 1 0537165 and EAR-0929902 to L. Webb. The research also benefited and the orientation of the regional linear feature in the magnetic from a Petroleum Research Fund (American Chemical Society) grant anomaly dataset. Therefore, we infer that the shear zone exploited some 40193-G8 to C.L. Johnson. We thank Joshua Taylor, Ian Semple and weakness in the accreted arc system, possibly a terrane boundary Megan Frederick for their help in the field during the 2006–2009 field (Badarch et al., 2002). During Phase 3 deformation (shortening phase seasons. We also thank Bolortsetseg Minjin, director of the Institute for resulting in an Early Jurassic foreland deposits; Fig. 5), we infer that the Study of Mongolian Dinosaurs, Ivanhoe Mines, and Rio Tinto for Phase 1 high-angle basement faults were reactivated in a reverse sense logistical support while in South Gobi in our 2005–2008 field seasons. (south-directed) as basement uplifts along only the northern margin of IHS, Halliburton, and ESRI provided academic licenses of interpretation the Unegt subbasin. software that made this study possible. Thorough reviews and editorial High-angle normal faults of Phase 4 deformation (rifting) formed comments by John Bartley, Michel Faure, and Bradley Ritts greatly

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