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Geological Society of America Bulletin

doi: 10.1130/B30510.1 published online 8 June 2012;Geological Society of America Bulletin

Chuluun MinjinMatthew J. Heumann, Cari L. Johnson, Laura E. Webb, Joshua P. Taylor, Undarya Jalbaa and southern MongoliaPaleogeographic reconstruction of a late Paleozoic arc collision zone,

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Paleogeographic reconstruction of a late Paleozoic arc collision zone, southern Mongolia

Matthew J. Heumann1,†, Cari L. Johnson2, Laura E. Webb3, Joshua P. Taylor4, Undarya Jalbaa5, and Chuluun Minjin5

1ConocoPhillips Company, Houston, Texas, USA2Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA3Department of Geology, University of Vermont, Burlington, Vermont 05405, USA4Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA5School of Geology and Petroleum Engineering, Mongolian University of Science and Technology, PO Box 520, Ulaanbaatar-46, Mongolia

ABSTRACT

Permian sedimentary sequences exposed in southern Mongolia record fi nal closure of the paleo–Asian Ocean and suturing of the terranes of northern China with the south-ern margin of a vast accretionary complex (the Altaids) in Mongolia. Detrital zircon U-Pb geochronology results presented here are the fi rst of their kind in southern Mon-golia. Geologic mapping, stratigraphic inter-pre tations, and provenance data including U-Pb zircon geochronology suggest that sedi-mentary strata at two localities in southern Mon golia, Bulgan Uul and Nomgon, were once part of the same closing ocean basin. The Upper Permian sedimentary deposits at Bulgan Uul record an upward-shallowing marine succession that is unconformably overlain by Lower Triassic fl uvial and allu-vial strata. The Bulgan Uul marine suc-cession is composed of distal turbidite fan deposits in the lowest portion of the section, with interbedded sandstone and limestone interpreted as shallow-marine deposits at the top of the section . Exposures of Permian-aged distal turbidite units at Nomgon are similar in stratigraphic architecture, sand-stone provenance, and detrital zircon age distributions to those documented at Bulgan Uul. Paleo current measurements, sandstone provenance data, and U-Pb ages from de-trital zircons collected from both study lo-cations document southeastern transport directions for sediment derived from extinct Carboniferous and Ordovician–Silurian arcs of the southern Altaids. Results are con-

sistent with depositional models for remnant ocean basins and indicate diachronous west-to-east closure of the paleo–Asian Ocean (a northern segment of Paleotethys) in the Late Permian. Finally, basin reconstruc-tions place the coeval turbidite deposits at Nomgon to the southeast of Bulgan Uul dur-ing the Late Permian. These correlative tur-bidite successions at Nomgon are currently northeast of Bulgan Uul, offset by ~250 km of left-lateral strike-slip faulting across the East Gobi fault zone.

INTRODUCTION

East-central Eurasia formed during rapid continental growth by accretion throughout the Phanerozoic, culminating with Cenozoic–Holo-cene collision with the Indian plate (Molnar and Tapponnier, 1975; Şengör et al., 1993a, 1993b; Yin and Nie, 1996; Şengör and Natal’in, 1996; Jahn, 1999; Xiao et al., 2004, 2009; Windley et al., 2007). The modern Tien Shan–Yin Shan mountain ranges are formed along one of the oldest and longest boundaries in this tectonic collage (Fig. 1). Interpreted by some as a contin-uous, or at least linked, collision zone, the Tien Shan–Yin Shan suture formed at the end of the Paleozoic during collision between accreted arc terranes of the Altaids to the north and continen-tal blocks (North China and Tarim cratons) to the south (Fig. 1; Johnson et al., 2008). Despite its demonstrated importance as a major intra-plate boundary (Windley et al., 2007), relatively little is known about the detailed formation and evolution of this collision zone. Key questions such as the numbers of arcs and continental fragments that are present, and the location, timing, subduction polarity, and modes of col-lision are heavily debated (Chen et al., 2000;

Xiao et al., 2003, 2004, 2009; Cope et al., 2005; Li, 2006; Johnson et al., 2008; Jian et al., 2010).

This integrated basin analysis study pre sents new data from Permian strata in southern Mon-golia that bear directly on these outstanding questions and provide insight on basin evolu-tion related to late Paleozoic collision and sub-sequent deformation. In particular, new isotopic data, 40Ar/39Ar dating of crosscutting dikes, U-Pb dating of detrital zircons, and fi eld map-ping at Bulgan Uul and Nomgon (Fig. 1) re-veal key temporal, structural, and stratigraphic relationships that address Permian collision between the extinct (?) Paleozoic arcs of the southern Altaids (Lamb and Badarch, 2001; Xiao et al., 2004; Blight et al., 2008, 2010; Wainwright et al., 2011) and the Precambrian terranes of northern China. Basin reconstruc-tion and regional correlations also support the interpretation that Permian rocks in the two fi eld areas were once part of the same basin system and were dismembered by Mesozoic–Cenozoic sinistral displacement across the East Gobi fault zone (Lamb et al., 1999; Yue et al., 2005; Webb and Johnson, 2006; Heumann et al., 2008; Heu-mann, 2010; Taylor, 2010).

GEOLOGIC SETTING

The Altaids (also known as the Central Asian orogenic belt; Kröner et al., 2007) are a vast accretionary complex, formed mainly during the Paleozoic, that now comprise the dominant basement rocks of northwestern China and southwestern Mongolia (Fig. 1; Şengör et al., 1993a, 1993b). The massive tectonic collage represents one of the oldest and largest known orogens in the world. The Altaid orogenic zone formed through the accretion of arcs and micro-continents south of Siberia (Angara craton) and

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GSA Bulletin; Month/Month 2012; v. 1xx; no. X/X; p. 1–21; doi: 10.1130/B30510.1; 17 fi gures; 1 table; Data Repository item 2012213.

†E-mails: [email protected]; [email protected]


Heumann et al.

2 Geological Society of America Bulletin, Month/Month 2012

is composed of many dozens of tectonic units, each separated by a regional structure or suture zone (Şengör and Natal’in, 1996; Badarch et al., 2002; Xiao et al., 2004).

The main tectonic boundary between the Altaids and continental blocks of north China (e.g., Tarim and North China craton) is poorly defi ned because it lies mainly along the under-studied and remote China-Mongolia political border. The main suture zone generally follows the modern Tien Shan and Yin Shan moun-tain ranges (Fig. 1), but it is most commonly known by locally named segments includ-ing Suolon, Suolonshan, Solonker, Junggar-Hegen , and South Mongolia–Hinggan sutures (Şengör et al., 1993b; Wang and Mo, 1995; Badarch et al., 2002; Cope et al., 2005; Shi, 2006; Shi et al., 2006; Li, 2006; Jian et al., 2010). The term Tien Shan–Yin Shan suture is used in this study because it implies a regional continuation of the collision zone.

Unlike more local arc and microcontinent ter-rane boundaries found throughout the Altaids (Şengör and Natal’in, 1996; Lamb and Badarch, 2001; Badarch et al., 2002; Windley et al., 2007),

the Tien Shan–Yin Shan suture zone forms the primary structure separating the majority of the southern Altaids from the Precambrian conti-nental blocks of northern China (e.g., the North China craton and Tarim; Fig. 1; Şengör et al., 1993b; Şengör and Natal’in, 1996; Lamb and Badarch, 2001; Badarch et al., 2002). In this re-gard, the study areas examined here are located along the southernmost Altaid margin of the col-lision zone, at Bulgan Uul and Nomgon in south-ern Mongolia (Fig. 1).

The Tien Shan–Yin Shan suture zone formed during diachronous west-to-east closure of the paleo–Asian Ocean (a trapped segment of north-ern Paleotethys; Dobretsov et al., 1996), begin-ning in the Early Permian along the Junggar Basin and terminating in the Late Permian–Early Triassic in southeastern Mongolia (Carroll et al., 1990, 1995; Amory, 1996; Cope et al., 2005; Johnson et al., 2008; Jian et al., 2010). Tim-ing of terminal ocean basin closure along the suture zone is marked in isolated locations by a transition from marine to terrestrial sedimenta-tion during Permian and Triassic time (Mueller et al., 1991; Carroll et al., 1995; Amory, 1996;

Chen et al., 2000; Cope et al., 2005; Johnson et al., 2008).

The polarity of subduction immediately prior to terminal arc-continent collision is debated. Some investigators favor northward subduction of the North China craton beneath the Altaids, based on the inferred presence of Permian igne-ous suites and ophiolite fragments throughout southern Mongolia and to the east in northern China (Hsü et al., 1991; Robinson et al., 1999; Chen et al., 2000; Kovalenko et al., 2006; Li, 2006). Other investigators have proposed bi-polar subduction beneath the Altaids and the North China craton during at least some phase of ocean basin closure (Xiao et al., 2003). Re-cent U-Pb geochronological and geochemical data, including additional data presented in this study, suggest that many of the igneous suites mapped as Permian in southern Mongolia are components of Carboniferous and older arc systems (accreted prior to the fi nal paleo–Asian Ocean basin closure), and therefore are not part of the widespread igneous suites that may re-fl ect Permian subduction beneath the southern Altaids (Fig. 2; Wang and Liu, 1986; Lamb and Badarch, 2001; Cope et al., 2005; Blight et al., 2008, 2010).

STUDY AREAS

In order to further investigate timing, modes, and sedimentary records of ocean basin closure along part of the Tien Shan–Yin Shan suture, this study focuses on Permian outcrops in two regions of southern Mongolia: Bulgan Uul and Nomgon (Fig. 2). Bulgan Uul and Nomgon are located within an ~800-km-long by 150-km-wide belt of mapped Permian strata in south-eastern Mongolia. Isolated locations (including Bulgan Uul and Nomgon) along this belt of Permian strata have been hypothesized to rep-resent portions of a remnant ocean basin (Fig. 3; Johnson et al., 2008), where strata locally reach thicknesses of ~5 km (Fig. 2; Manankov, 1988; Manankov et al., 2006).

Ages of the deposits near Bulgan Uul and Nomgon are based on preserved Permian fl ora and fauna (Zaitsev et al., 1973; Durante, 1976; Vakrameyev et al., 1986; Pavlova et al., 1991; Zinniker and Badarch, 1997; Uranbileg, 2003; Aristov, 2005; Manankov et al., 2006; Johnson et al., 2008). In general, Permian strata of south-ern Mongolia are broadly assigned to the Lugyn Gol (or Lugingol) zone, a general term for the highly deformed, “fl ysch-like” units found near the China-Mongolia border. In the following sections, we retain “Formation” names based on local mapping, although these are not stan-dardized for southern Mongolia (Fig. 4). Strati-graphic nomenclature and formal age divisions

NUBU

NO

QS

0 500 1000 km

100°E

50°N

100°E

30°N30°N

40°N40°N

110°E

110°E 120°E

120°E90°E80°E

50°N

90°E80°E

Tibetan Plateau

Tarim Basin

South China block

North China craton

Altaids/CAOBJunggar Basin EGFZ

OrdosAlxa

Sichuan

Qinling-Dabie orogenHimalaya

Qaidam

Tien Shan

Lake Baikal

Mongolia

Eurasia/Siberia

Siberian craton

India

Tien Shan-Yin Shan suture

Fig. 2

ATF

Mongol-Okho

tsk suture

UUO

Okhoo

Figure 1. Digital elevation model and major tectonic elements of northern India, China, and Mongolia. The Mongolia political boundary is outlined by a narrow black line. Heavy black lines correspond to the Himalayan thrust front (shown with thrust symbols), regional-scale faults, fault zones (dashed where inferred), and sutures. Dashed box indicates the location of Figure 2. Circled stars indicate the approximate location of Beijing and Ulaanbaatar. ATF—Altyn Tagh fault; BU—Bulgan Uul; CAOB—Central Asian orogenic belt; EGFZ—East Gobi fault zone; NO—Nomgon; NU—Noyan Uul; QS—Qilian Shan.



Geological Society of America Bulletin, Month/Month 2012 3

of Permian strata in southern Mongolia are not yet standardized or fully correlated to the inter-national time scale (Shen et al., 2006; Shi, 2006; Manankov et al., 2006), and thus it is diffi cult to provide stage or even epoch assignments for many of the intervals studied. For the purposes

of this study, we retain the use of Upper/Late Permian (Cisuralian, 299–270.6 Ma) versus Lower/Early Permian (Guadalupian–Lopingian, 270.6–253 Ma) (cf. Gradstein and Ogg, 2004), because this terminology is still widely used in the regional literature and provides the best

means for suggesting meaningful correlations (Johnson et al., 2008).

A generalized representation of a remnant ocean basin is shown in Figure 3 (Graham et al., 1975), and this is used as a framework for under-standing Permian strata of southern Mongolia .

112°0′E

112°0′E

110°0′E

110°0′E

108°0′E

108°0′E

106°0′E

106°0′E

44°0′N

44°0′N

42°0′N

42°0′N

kilometers0 25 50 100

Undifferentiated-mainly Upper Cretaceous-Holocene strata

Paleogene strata

Upper Jurassic-Lower Cretaceous rift strata

Triassic-Jurassic granite

Permian sedimentary and volcanic rocksUndifferentiated - mainly Paleozoic volcanic and sedimentary sequences

Carboniferous-Cambrianintrusive suitesMapped as Precambrian(undifferentiated), includes Mesozoic tectonites of the EGFZNE-trending Cenozoic faults of theEGFZ (dashed where inferred)

UR

BU

HB

China-Mongolia border

TH

OH Fig. 5

Fig. 6

NO

Figure 2. Simplifi ed geologic map of southeastern Mongolia modifi ed after Webb and Johnson (2006), Tomurtogoo (1999), and Lamb et al. (1999). The China-Mongolia political boundary is outlined in gray. BU—Bulgan Uul; EGFZ—East Gobi fault zone; HB—Han Bogd; NO—Nomgon; OH—Onch Hayhran; TH—Tavan Har; UR—Urgun. Approximate locations of Figures 5 and 6 are outlined in black dashed boxes.

Uplifted suture

Remnant oceanbasin

A B Predicted chronostratigraphyA

A′

A′A

T1

T2

Generalized stratigraphy

Longitudinal sediment

transport

Collidingmicrocontinent

North

Unconformity

Clastic wedge – nonmarine deposits. Deposition marks final ocean basin closure.

Delta and marginal marine deposits – sourced by sediment shed from the growing orogenic uplift.

Proximal shelf and slope deposits – includes turbidites.

Distal marine turbidites.

Figure 3. Simplifi ed representation of the remnant ocean basin model (after Graham et al., 1975). (A) Map view of a northward-colliding microcontinent (i.e., the North China craton) and diachronous closure of the remnant ocean basin. Arcuate arrows illustrate the principal sediment dispersal pathways, longitudinal to the closing ocean basin. (B) Predicted chronostratigraphy for a section (A-A′) collected parallel to the longitudinal sediment dispersal pathway. “T” indicates nonspecifi c (relative) time intervals and the predicted stratigraphic relationships. Overall stratigraphic rela-tionships illustrate a transition from distal-marine to terrestrial sedimentation.


Heumann et al.


Permian sedimentary units in southern Mon-golia transition regionally from terrestrial to marine depositional facies from west to east for a given chronostratigraphic interval, and also grade from marine to nonmarine upward in a vertical succession (Graham et al., 1975; Inger-

soll et al., 1995; Johnson et al., 2008; Figs. 3 and 4). In southern Mongolia, the Permian units record ocean basin closure and fi nal collision between accreted arc systems and northern China (Carroll et al., 1990, 1995; Amory, 1996; Johnson et al., 2008).

In addition to likely recording Permian col-lision, the two fi eld areas were proposed to expose originally contiguous basin deposits that have been offset by later (Mesozoic and Cenozoic) sinistral movements across the East Gobi fault zone (Lamb et al., 1999; Webb and Johnson, 2006; Johnson et al., 2008; Heumann et al., 2008; Heumann, 2010). Thus, further investigation of Permian strata at Nomgon and Bulgan Uul, specifi cally, testing whether they may have originally been deposited in the same basin, will help to address the magnitude of later (Mesozoic) offset along the East Gobi fault zone by establishing cross-fault offset markers (i.e., “piercing points”). U-Pb dating of late Paleozoic igneous intrusive suites at Han Bogd and Urgun (Fig. 2) was also com-pleted to further characterize igneous con-stituents of the collision zone and investigate whether these intrusions represent Permian-age active arc systems. In summary, this study seeks to better characterize the Permian de-posits located at Bulgan Uul, place them in re-gional tectonic and paleogeographic context, and test the hypothesis that Bulgan Uul and Nomgon were once part of the same basin sys-tem that was subsequently offset by strike-slip faulting.

METHODS AND ANALYTICAL PROCEDURES

The study areas (Nomgon and Bulgan Uul) were mapped, including fi eld verifi cation of satellite imagery interpretations, of existing geologic maps (e.g., Tomurtogoo, 1999), and of unpublished government and academic maps. Initial mapping focused on lithologic identification of units and identifying key structural relationships such as formation boundaries, faults, and folds. Complex defor-mation and greenschist-facies metamorphism are common in Paleozoic sedimentary and volcanic successions of southern Mongolia; nevertheless, several stratigraphic sections were measured in relatively unmetamor-phosed strata at Bulgan Uul. Lithologic and sedimentologic characterization was carried out for each stratigraphic section in order to create stratigraphic correlations and interpret depositional environments and paleogeog-raphy. Paleocurrent indicators were measured and corrected for structural dip.

To characterize modal sandstone provenance, medium-grained sandstone samples were point-counted following a modifi ed Gazzi-Dickinson method (Dickinson et al., 1983; Ingersoll et al., 1984). Modal parameters were normalized and then plotted on ternary diagrams using tectonic discrimination fi elds as outlined in Dickinson

Uncon-formity

Uncon-formity

Uncon-formity

(?)

Uncon-formity

(?)

Uncon-formity

(?)

Ear

ly P

erm

ian

Ear

lyC

isur

alia

nLa

te P

erm

ian

Low

erG

uada

lupi

anLo

ping

ian

250

255

260

265

270

275

280

285

290

295

Per

mia

n

Age

(M

a)

245

Tria

ssic

Carboniferousand older

245 ± 1.8 Ma (Fig. 12).

Argalant Formation, >100 m thick, red beds and volcanic rocks.**

Agui Uul and Bulgan Uul Formations, > 3000 m thick.Shallow-marine carbonate and clastics.**

Argalant Formation, >100 m thick, red beds and volcanic rocks.**

Lugyn Gol Formation (P2lg), >500 m thick. Distal turbidite deposits (Figs. 5, 7 and 8).

Tavan Har (P1th) and Har ErdeneFormations, >300 m thick. Shallow-marine clastics (Figs. 5, 7 and 8).

P2ou1 and P2ou2 Members, >500 m thick. Distal turbidite deposits(Figs. 6, 9 and 11).

Halh Uul Formation (P2hu),>500 mthick. Marine clastics and volcanics(Figs. 6, 9 and 11).

P2ou3 Member, >500 m thick. Shallow-marine clastics and carbonates (Figs. 6, 10 and 11).

Unnamed formation (T1), >500 m thick. Fluvial and alluvial deposits(Figs. 6, 10 and 11).

Onch Uul Formation

Nonmarine Upper Permian – EarlyTriassic units not exposed**

Unconformable and fault contactwith Carboniferous and older.** Unconformable and fault contact

with Devonian and older (?).**

Bulgan Uul and south-central Mongolia

Nomgon and southeastMongolia

Geologic time scale and standard

chronostratigraphy

** Units not directly observed in this study(see Johnson et al., 2008).

Span

of m

axim

um de

posit

ional

ages

(cf. F

ig. 14

)

Figure 4. Chronostratigraphic summary of the Bulgan Uul and Nomgon study areas. Radio-isotopic age is from this study and presented in Figure 12. Formation names and thicknesses are after Burenkhuu et al. (1995), Tomurtogoo (1999), and Johnson et al. (2008, and refer-ences therein) and correspond to units presented in Figures 5 and 6. Lithologic patterns are defi ned in detailed stratigraphic columns in Figures 7, 9, and 10. Photos of specifi c sections appear in Figures 8 and 11. Time scale is from Gradstein and Ogg (2004), although we have retained the use of Early versus Late Permian Epochs as discussed in the text. Age assign ments are approximate; however, in some cases, detrital zircon results presented here provide a better maximum depositional age limit than previous biostratigraphic studies.




(1985). Modal point-counting data are listed in Table DR1 in the GSA Data Repository.1

Sandstone provenance was further charac-terized by U-Pb detrital zircon geochronology. Sixteen sandstone samples (locations in Figs. 5 and 6) were processed for detrital zircon analyses using laser-ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) at the Arizona LaserChron Center at the University of Arizona. Zircons were separated at Syracuse University and Brigham Young Uni-versity and followed procedures outlined by the Arizona LaserChron Center (www.geo.arizona.edu/alc) and methods of Fedo et al. (2003). Crushing and grinding were followed by Wilfl ey table and heavy liquid mineral separation. The Frantz magnetic barrier separator was used mini-mally during the separation process to reduce possible bias in grain selection (Heaman and Parrish, 1991). Purifi ed zircon separates were then incorporated into 1-in.-diameter (2.54 cm) epoxy mounts with fragments of Sri Lanka zircon standard (563.5 ± 3.2 Ma [2σ]; Gehrels et al., 2008), polished to reveal zircon interiors, and imaged using backscattered-electron (BSE) microscopy or cathodoluminescence (CL). In-strument description and operating conditions are outlined in Gehrels et al. (2006) and are described in the GSA Data Repository material (see footnote 1), in addition to methods of age determination and data reduction. Data fi ltering and exclusion from age calculations or display was conducted as follows. Analyses that were >30% discordant or >5% reverse discordant were excluded from consideration, where dis-cordance was based on comparison of 206Pb/238U and 206Pb/207Pb ages. However, because of the great uncertainties in 206Pb/207Pb ages for zircons younger than 500 Ma, the discordance fi lter is not applicable and was not used. The data were also fi ltered for analyses that yielded a >15% uncertainty in 206Pb/238U ages or analyses that displayed >10% change in isotopic ratios during a 12 s measurement. Such samples were inter-preted to be variable in age (or compromised by fractures, inclusions, or a domain with variable Pb loss), and were excluded from further con-sideration. For each analysis reported in the data repository Table DR2 (see footnote 1), errors calculated from the measurement of 206Pb/238U, 206Pb/207Pb, and 206Pb/204Pb are reported at the 1σ level. Plots of U/Th versus age (Ma) are in-cluded in the data repository as Figures DR1A and DR1B (see footnote 1).

Postdepositional (crosscutting) intermediate- to mafi c-composition dikes were identifi ed at the uppermost stratigraphic section measured at Bulgan Uul (labeled “E” in Fig. 6). Samples were collected for 40Ar/39Ar geochronologic analysis to provide a minimum age for deposi-tion at Bulgan Uul. The 40Ar/39Ar step-heating analysis of an amphibole separate from one of the dikes was performed at the Syracuse Univer-sity Noble Gas Isotopic Research Laboratory. Sample preparation and analytical procedures followed those outlined in Webb et al. (2010) and are summarized in the GSA Data Repository (see footnote 1). Step-heating experiment results are presented in Table DR3 (see footnote 1).

Igneous suites mapped as Permian (Tomur-togoo, 1999) were targeted for U-Pb zircon geo-chronology to investigate whether they might refl ect arc activity during closure of the paleo–

Asian Ocean. Felsic components of the Han Bogd pluton and an unnamed granitic unit at Urgun were collected, in addition to a nearby Carboniferous tonalite-quartz diorite for compari-son (Fig. 2). Mineral separates were processed and prepared in much the same manner as the detrital samples outlined previously. However, the Franz magnetic barrier separator was used extensively, and the purifi ed zircon separates were handpicked for the most euhedral and in-clusion-free zircons prior to being incorporated into epoxy mounts with the Sri Lanka zircon standard (563.5 ± 3.2 Ma [2σ]; Gehrels et al., 2008). CL and BSE images were used to inves-tigate the complexity of grains sampled at Han Bogd and Urgun, examples of which are shown in sections to follow. Instrument description and operating conditions are outlined in Gehrels et al. (2008) and are described in the data repository, in

1GSA Data Repository item 2012213, sandstone provenance, U-Pb detrital 14 zircon geochronol-ogy, 40Ar/39Ar thermochronology, and U-Pb igneous zircon geochronology, is available at http://www.geosociety.org/pubs/ft2012.htm or by request to [email protected].

0 5 10kilometers

107°30′E 108°00′E 108°30′E

107°30′E 108°00′E 108°30′E

43°00′N

42°45′N

42°30′N

43°00′N

42°45′N

42°30′N

Fault

Study site

Measured strat.sectionSandstone provenancesampleDetrital zirconsample

Johnson et al. (2008)

A

B

A

Undifferentiated - mainly Upper Cretaceous-Holocene strata

Lower Cretaceous strata

Triassic igneous suites

Permian igneous suites

Upper Permian strata – Mainly Lugyn Gol Fm.

Lower Permian strata –marine depositsCarboniferous igneous suites and undifferentiated strata

Devonian igneous suitesand undifferentiated strata

Precambrian units – includes Mesoproterozoiccarbonate klippe and high-grademetamorphic rocks.

Figure 5. General geologic map of the Nomgon study area. Map units correspond to our observations and studies by Amory (1996) and Johnson et al. (2008; measured section shown). Faults shown were identifi ed by Burenkhuu et al. (1995) and Tomurtogoo (1999). The China-Mongolia political border is identifi ed with a bold black line.


Heumann et al.


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105°

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5°15

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105°

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addition to the methods of age determination and data reduction (see footnote 1). Similar to data fi ltering for the detrital zircon samples, analy-ses of the plutonic zircons that yielded a >15% uncertainty in 206Pb/238U ages or analyses that displayed >10% change in isotopic ratios during a 12 s measurement were excluded from consid-eration. However, because 206Pb/238U ages of the igneous suite samples are commonly younger than 500 Ma, a discordance fi lter of 1.5% (both normal and reverse) was used, where discor-dance was based on comparison of 206Pb/238U and 207Pb/235Pb ages. Analyses that yielded iso topic data of acceptable precision and discordance are shown in Table DR4 (see footnote 1).

RESULTS

Nomgon Stratigraphy

The Permian geology of the Nomgon study area has been summarized by Amory (1996) and Johnson et al. (2008, and references therein). Outcrops in the region are dominated by Upper Permian fl ysch (turbidites) of the Lugyn Gol Formation (P

2lg), which lies unconformably

on, or in fault contact with, Carboniferous and older metasedimentary, volcanic, and intrusive rocks (Figs. 4 and 5; Burenkhuu et al., 1995; Tomur togoo, 1999). The Lugyn Gol Formation at Nomgon is interpreted to be a shallow-marine succession that rapidly grades upward into ma-rine turbidite sequences with a minimum thick-ness of ~800 m (Fig. 4; Johnson et al., 2008). Dip-corrected paleocurrent data (Fig. 7) indicate transport directions toward the east-southeast. Exposed beds throughout much of Nomgon are highly deformed (isoclinal folding, exten-sive faulting, and well-defi ned cleavage; Taylor, 2010) and exhibit extensive quartz and calcite veining throughout. Deformation likely re-fl ects Late Permian–Early Triassic compression caused by North China craton collision, and overprinting by an Early Jurassic thrusting event (Burenkhuu et al., 1995; Zheng et al., 1996; Amory, 1996; Tomurtogoo, 1999; Heumann et al., 2008; Taylor, 2010). Regionally, Meso-proterozoic carbonate klippen lie in horizontal thrust-fault contact over Permian deposits in isolated areas (Fig. 5; Zheng et al., 1996; John-son et al., 2001; Ruzhentsev, 2001).

Two stratigraphic sections measured at Nomgon (Figs. 5 and 7) were selected for com-parison to sections presented in Johnson et al. (2008) and to provide sedimentologic context for detrital zircon analyses. The fi rst strati-graphic section is from the Lower Permian Tavan Har Formation (P

1th), (Fig. 4 and “A” in

Fig. 5). A composite 159-m-thick section was measured by correlating distinctive strata across

several faults (Fig. 7A). The stratigraphic section fi nes upward from sandstone and pebble-cobble conglomerate at the base to interbedded sand-stone and shale at the top. Most sandstone units are tabular with sharp bases, normally graded, and continuous along strike for several tens of meters (Fig. 8A). Asymmetric ripple marks are

present near the tops of some beds, but the sec-tion is pervasively deformed, and these were not measured as paleocurrent indicators. No root marks, mud cracks, or other evidence of sub-aerial exposure were found, and the units do not show evidence of signifi cant erosional scouring or formation of channels.

20

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eSilts

tone

Med.Sand

stone

Pebble

Cobble

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140

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eSilts

tone

Med.Sand

stone

Pebble

Cobble

Upper Permian unidirec-tional and bidirectional flow indicators (n = 31, circle = 32%)

Thic

knes

s (m

)

Thic

knes

s (m

)

Legend (All Sections)CoverMudstone-siltstoneSandstoneGravel-pebble conglomerateCobble-boulder conglomerateLimestone or dolomitePlanar laminaePlanar cross-beds

Carbonate nodulesMud chips and rip-up clastsTrough cross-beds

Detrital zircon sample

Sandstone provenance sample

08-NO-06A

08-NO-10C

08-NO-10E

08-NO-10F

Bioturbation (burrows)

Photographs (e.g., Fig. 8A)

8A

8C

8B

A. Lower Permian– Tavan Har Formation (P1th)

B. Upper Permian – Lugyn Gol Formation (P2 lg)

8A

Figure 7. Measured stratigraphic sections from the Nomgon study area of the Lower Permian Tavan Har Formation (A) (P1th) and Upper Permian Lugyn Gol Formation (B) (P2lg). Key applies to all measured sections presented in this study (Figs. 7, 9, and 10). Paleocurrent measurements were made throughout the Upper Permian section. See Figure 5 for locations.


Heumann et al.


Overlying the Tavan Har Formation, the Upper Permian Lugyn Gol (P

2lg) Formation

was documented along strike and ~10 km west of the sections presented in Johnson et al. (2008) (Fig. 4 and “B” in Fig. 5). Units throughout this study area are crosscut by veins of quartz and calcite, which, along with struc-tural deformation, limit continuity of strati-graphic sections. A representative section was measured along the southern limb of a roughly east-west–trending anticline (Figs. 7B and 8B). The measured section is distinctly fi ner grained than the underlying strata (P

1th) and is domi-

nated by interbedded sandstone-mudstone units that are tabular and continuous for many tens of meters. Sandstones are normally graded with sharp horizontal bases that show little to no erosional relief. Where exposed, the basal units show tool marks and fl ute casts indicating an east-southeast transport direction (Fig. 7). Internal, decimeter-scale bedding is typically characterized by massive and structureless basal sandstone fi ning upward to rippled fi ne sandstone and siltstone, which are overlain by mudstone. The normally graded beds make up full or partial Bouma sequences (Fig. 8C; Bouma, 1962). The section is metamorphosed

to lower greenschist facies, and crenulation cleavage is present in fi ne-grained intervals (Taylor, 2010).

InterpretationThe two sections are interpreted to represent

a proximal to distal marine succession, pos-sibly upward-deepening. Neither succession shows evidence for channelization or subaerial exposure as expected in fl uvial deposits. Cyclic

depo sition of sand and mud, normal grading and Bouma sequences, and continuous tabular bed-ding are most consistent with a turbidite fan, which follows previous interpretations (Amory, 1996; Johnson et al., 2008). The older (P

1th) sec-

tion is notably coarser grained and has thicker beds than the overlying section (P

2lg). Following

Johnson et al. (2008), we interpret this to repre-sent a proximal turbidite fan succession, overlain by more distal turbidite fan deposits (P

2lg).

~20 m

Triassic (?) (T1)

Dike

Klippe – Mesoproterozoic unit

North

~5 km

A B

C D

E F

Figure 8. Photographs of Permian strata at Nomgon, and structural relationships ob-served at Bulgan Uul. (A) Lower Permian at Nomgon. Sand intervals are more com-mon in the Lower Permian than the Upper Permian. Camels in the foreground for scale are ~2.5 m tall. Photo is taken looking north. (B) The Lugyn Gol Formation exposed along the northern limb of a large anticline where it was measured. The beds are thin-ner and muddier compared to the Lower Permian interval. Photo is taken looking west. (C) Upward-fi ning bed in the Lugyn Gol section. Knife is ~10 cm long. (D) Digi-tal elevation model (DEM) and LandSAT image of the carbonate klippe at Bulgan Uul. DEM is vertically exaggerated 5:1. Dashed white lines are the approximate fault sur-face traces. Black ellipse identifi es the loca-tion of the Early Triassic (?) fl uvial/alluvial measured section (Fig. 10). (E) Oncolites along the base of the carbonate klippe. Hammer head is ~25 cm long. Oncolites and stromato lites are the only fossils observed in the Precambrian unit. (F) Exposure along the southeast side of the base of the carbon-ate klippe. Dashed white line is the inferred thrust fault. Structurally below this is the Early Triassic (?) unit, which dips subverti-cally and strikes northeast.




Bulgan Uul Stratigraphy

Permian units at Bulgan Uul are located be-tween correlative nonmarine strata to the west (Noyon Uul) and marine strata to the east (Nomgon) (Figs. 1 and 4; Carroll et al., 1990, 1995; Amory, 1996; Manankov et al., 2006). Geologic mapping and reinterpretation of available geologic maps for the Bulgan Uul study area (Burenkhuu et al., 1995; Tomurto-goo, 1999) reveal the structural complexity of the area, and highlight the Upper Permian and Lower Triassic clastic deposits of this region along the China-Mongolia border (Fig. 6). Pre-dominantly a low-relief expanse, Bulgan Uul is named for a massive carbonate unit (Fig. 6, west of “E” label) that forms an isolated “moun-tain” in the study area (“uul” is Mongolian for mountain). The carbonate unit is reportedly Mesoproterozoic (Riphean?) in age (Burenkhuu et al., 1995; Zheng et al., 1996; Tomurtogoo, 1999; Ruzhentsev, 2001) and lies in sub hori-zontal structural contact upon Late Permian marine and Early Triassic fl uvial and alluvial stratigraphic sequences (Fig. 8D). Internally, the klippe is deformed, brecciated, and dolomitized, typical of other Proterozoic carbonates in the region (Zheng et al., 1996). Oncolites and pos-sible stromatolites were observed near its base, but the unit appears to lack macrofossils (Fig. 8E). In isolated exposures along the northern margin of the range front, the underlying units are highly sheared with foliations oriented sub-parallel to the basal contact of the overlying car-bonate units. Similar prominent range-forming carbonates in the area share the same apparent structural relationship with underlying units (Fig. 6). The Upper Permian–Lower Triassic (?) conglomeratic units exposed along the south-eastern edge of the main Bulgan Uul carbonate range contain no carbonate clasts that would in-dicate its proximity during deposition (Fig. 8F). Therefore, we interpret the massive carbonate assemblage as an allochthon, consistent with re-gional interpretations of klippen throughout the China-Mongolia border region (Figs. 5 and 6; Burenkhuu et al., 1995; Amory, 1996; Tomur-togoo, 1999; Ruzhentsev, 2001; Johnson et al., 2008). The exact timing and structural vergence of thrust emplacement of these klippen are poorly understood and beyond the scope of this study; however, the klippen are thought to be broadly associated with a north-south–directed compression event during the early Mesozoic (Zheng et al., 1996).

In general, metamorphic grade in Permian units decreases from the political border toward the north (locally up to greenschist-facies phyl-lites), but a distinct shear zone delineates the northernmost margin of the Bulgan Uul klippe

(Fig. 6). Outcrops of augen gneiss occur in a band (1 km wide by tens of kilometers in length) striking northeast-southwest with a steeply dip-ping foliation (~55°SE), and locally reach am-phibolite facies in the shear zone center, where garnet and migmatitic textures were observed. Initial 40Ar/39Ar results suggest that shearing oc-curred in the Late Triassic (Webb et al., 2011).

The stratigraphic sections presented in Fig-ures 9 and 10 include Permian through Lower Triassic (?) sedimentary units immediately south of the prominent carbonate klippe (Fig. 6). Folding and faulting are pervasive throughout the succession, but bedding and sedimentary lithologies remain discernible in the fi eld, as presented in the following. Commonly, the sedi-

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eSilts

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stone

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08-BU-06A

08-BU-06C

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eSilts

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08-BU-07A

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08-BU-08D

08-BU-08A

A. Upper Permian– Halh Uul Formation (P2hu)

B. Upper Permian– Onch Uul Formation (P2ou1)

C. Upper Permian– Onch Uul Formation (P2ou2)

11A

11B

Thic

knes

s (m

)

Thic

knes

s (m

)

Thic

knes

s (m

)

Figure 9. Measured stratigraphic sections from the Upper Permian Halh Uul Formation (A) (P2hu) and the lower two members of the Onch Uul Formation (B and C) (P2ou1 and P2ou2). See Figure 6 for locations and Figure 7 for key to lithologies and symbols.


Heumann et al.


mentary succession is crosscut by quartz and calcite veins/dikes, similar to occurrences in Nomgon (Amory, 1996; Johnson et al., 2008). The stratigraphy matches the following units (cf. Fig. 6): the Permian Halh Uul Formation (P

2hu), three members of the Permian Onch

Uul Formation (P2ou1, P

2ou2, and P

2ou3), and an

unnamed Lower Triassic (?) conglomerate unit (T

1) described by Ruzhentsev (2001) (Fig. 4).The depositional age of unnamed conglomer-

atic units that make up the section immediately south-southeast of the carbonate klippe is poorly known. The rocks have been identifi ed as Upper Permian, Lower Triassic, and even Cretaceous in previous investigations (Burenkhuu et al., 1995; Ruzhentsev, 2001). Crosscutting dikes near the top of the section support an upper age limit of Early Triassic (40Ar/39Ar results dis-cussed later herein). Unlike the Upper Permian series, which the fl uvial units unconformably overlie, these strata are unmetamorphosed. We present data from fi ve stratigraphic sections measured for this study (Figs. 9 and 10), each of which is discussed next in order from the old-est Permian unit (P

2hu) to the youngest (Early

Triassic[?], T1).

Late Permian Halh Uul Formation (P2hu)

The Halh Uul Formation is the oldest of the Late Permian units investigated at Bulgan Uul (Fig. 6). The section presented in Figure 9A was measured from a fault contact between the Halh Uul Formation and the lowest member of the Onch Uul Formation (P

2ou1) (labeled “A” and

“B” in Fig. 6). Eighty meters of total section were measured along the most continuous, ac-cessible exposure. The Halh Uul units are highly deformed and have undergone greenschist-facies metamorphism at this location, and there-fore primary sedimentary structures and reliable paleocurrent indicators are absent. The unit is dark-gray/green in color and lacks carbonate beds in the interval measured here, although they have been reported elsewhere (Burenkhuu et al., 1995). Isolated coarse-grained sandstone beds generally have sharp bases, are normally graded, and are laterally continuous for many tens of meters. Bedding dips steeply (>80°E) and strikes generally north-south (Fig. 11A). The measured section fi nes upward overall.

Late Permian Onch Uul Formation (P

2ou1, P

2ou2, P

2ou3)

The Onch Uul Formation is documented by three representative sections; deformation and metamorphism preclude measurement of a single continuous section. The two lowest mem-bers include interbedded sandstone and shale with minor pebbly conglomerate (Figs. 9B and 9C). Beds are generally massive, although

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350

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450

500

550

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Cobble

Upper Permian (P2ou3) bidirec-tional flow indica-tors (n = 12, circle = 42%)

Upper Permian (P2ou3) unidirec-tional flow indica-tors (n = 7, circle = 57%)

Lower Triassic (T1) unidirectional flow indicators (n = 52, circle = 14%)

Lower Triassic (T1) bidirectional flow indicators (n = 23, circle = 26%)

08-BU-18A

08-BU-18F

06-BU-24-18

06-BU-24-2406-BU-24-23

06-BU-24-32

B. Lower Triassic– Unnamed Formation (T1)

A. Upper Permian– Onch Uul Formation (P2ou3)

11C11E

11D

11F

Thic

knes

s (m

)

Thic

knes

s (m

)

0 25 50 75 100

Cla

st ty

pe

Metamorphic (n = 8)Granite (n = 31)Volcanic (n = 54)Sedimentary (n = 115)

Figure 10. Measured stratigraphic sections from the upper member of the Onch Uul For-mation (A) (P2ou3) and the Early Triassic (?) formation (B). Clast-count results are from the upper half of the Early Triassic (?) section. Note that only three carbonate clasts were found among 208 clasts counted. These sections correspond to the fi eld locations identifi ed as “D” and “E” in Figure 6. See Figure 6 for localities and Figure 7 for key to lithologies and symbols.




isolated mud chip intervals and minor ripples/laminations are present (Fig. 11B). One possi-ble planolites-burrowed interval was identifi ed. Sandstone beds have sharp, scoured bases with little erosional relief. Beds are not channelized but rather are continuous and rhythmically bed-ded. The two sections appear generally to fi ne upward into a relatively shale-rich interval at the top of the P

2ou2 section (Figs. 9B and 9C).

The uppermost member of the Onch Uul For-mation (P

2ou3) was measured to the southeast of

the carbonate klippe along the northern margin of the Permian formations (“D” in Fig. 6). The measured section (Fig. 10A) is 600 m thick, but poorly exposed for portions of the uppermost section. The observed section markedly dif-fers from the underlying members of the Onch Uul Formation. This section shows little sign of metamorphism except for dark shale units that exhibit pencil cleavage. Sandstone beds throughout the section are brown and fi ne up-

ward from cobble or pebble grain size at the base to fi ne sand. These units are broadly len-ticular (pinch-outs over 20+ m laterally) with erosional bases, and they are trough cross-bedded internally (Fig. 11C), but poorly chan-nelized. Trough cross-beds indicate a northwest to southeast paleofl ow direction (Fig. 11D).The upper section is commonly covered, but where exposed, brown shale units are present, along with some poorly developed carbonate

nodules/concretions. There is no evidence of long-term subaerial exposure (root traces, etc.) or the existence of major fl uvial systems in this section; thus, the nodules may be diagenetic (concretions) rather than representing paleosols. The top of this measured section contains the only occurrences of carbonate beds in any of the Permian sections measured at Bulgan Uul, and it terminates at a ridge-forming carbonate interval above which folding and faulting likely

~20m

A B

C D

E F

Figure 11. Photographs of the Upper Permian–Early Triassic sections at Bulgan Uul. (A) The Halh Uul and lowest mem-ber of the Onch Uul formation in southern Bulgan Uul. Black dashed lines indicate the attitude of bedding in the Onch Uul member; Halh Uul member beds are in the foreground. The dashed white line indi-cates a fault (likely thrust) that placed the two units in contact at this location (A and B in Fig. 6). Photo is taken looking to the southeast. (B) Middle member of the Onch Uul Formation. Geologists for scale at the base of the outcrop (C in Fig. 6). Photo is taken looking to the west. (C) Exposure of the upper member of the Onch Uul For-mation (D in Fig. 6). Exposures of this unit southeast of the klippe are poor, and often large intervals are obscured by cover. Back-pack (circled) is ~50 cm tall. Photo is taken looking to the west. (D) Trough cross-beds near the middle of the measured section in the upper member of the Onch Uul Forma-tion (Fig. 10A). Black dashed lines indicate interpreted base of troughs. Photo is taken looking to the northwest. (E) Large (meter-scale) fl uvial channels in the lower half of the Early Triassic (?) measured section im-mediately southeast of the klippe (Figs. 6 and 10B). White line is ~5 m in length. Photo is taken looking west. (F) Debris fl ows in the upper half (alluvial) of the Early Triassic (?) measured section immediately southeast of the klippe at Bulgan Uul (Fig. 10B; E in Fig. 6). Hammer is ~42 cm in length. Photo is taken looking to the east.


Heumann et al.


repeat the section. Abundant crinoid fossils dis-tinguish these carbonates from nonfossiliferous, massive Proterozoic carbonate klippe to the northwest (Fig. 6).

Unnamed Early Triassic (?) Section (T1)

Structurally below the Bulgan Uul carbon-ate klippe, there is a well-exposed conglomer-ate section, identifi ed by Ruzhentsev (2001) as uppermost Permian or Lower Triassic. The uppermost portion of the section, exposed near the large carbonate klippe, is crosscut by post-depositional dikes of intermediate composition (Fig. 12A). An amphibole separated from one of these dikes (labeled “E” in Fig. 6) yielded a 40Ar/39Ar plateau age of 245.0 ± 1.8 Ma (Figs. 12B and 12C; analytical results for the step-heating experiment are shown in Table DR3 [see footnote 1]). This is the minimum age for deposition of the Lower Triassic section at Bul-gan Uul. The dikes do not crosscut the overlying carbonate klippe, and this relation supports a post–Early Triassic age for thrust emplacement of the carbonate allochthon.

The Lower Triassic (?) sedimentary unit is distinct from all the other strata in the area, and it represents a fundamental shift in the depositional system at Bulgan Uul. Although the unit is deformed, it is unmetamorphosed.

The section consists of red beds ranging from cobble-conglomerate to sandstone and mud-stone. Cobble and pebble clasts are predomi-nantly intraformational sandstone or mudstone with minor amounts of volcanic, metamorphic, and plutonic clasts (Fig. 10B). Sandstones are channelized with meter-scale relief and are pervasively trough cross-bedded (Fig. 11E). Depositional units typically fi ne upward, and mudstones contain carbonate nodules and faint root traces. Abundant paleocurrent indicators indicate southeast-directed transport (Fig. 10B).

Overall, the section coarsens upward, with a dramatic but structurally conformable change at about meter level 300 (Fig. 10B). Above this, mudstone intervals are rare, and the section includes both clast- and matrix-supported con-glomerates. Matrix-supported units are poorly sorted. These conglomerates are poorly chan-nelized but typically have sharp scoured bases with scour marks that indicate southeast trans-port (Fig. 11F).

InterpretationSedimentary units at Bulgan Uul display a

wide range of depositional environments. The degree of metamorphism in the lowermost units (P

2hu and P

2ou1–2) precludes detailed sedimen-

tologic interpretation, but these units resemble

the regional Lugyn Gol “fl ysch” deposits, par-ticularly those at Nomgon. Cyclic sandstone-mudstone beds are upward fi ning and represent partial to fully preserved Bouma sequences in-cluding fl ute casts at their bases and ripples in fi ne sand near the top, overlain by mudstone. Beds are tabular and continuous for many hun-dreds of meters, and are highly deformed. There is no evidence of subaerial exposure during deposition. These units are interpreted to be marine turbidites. It is diffi cult to assign more specifi c environments, but we consider them to be part of a large subaqueous fan system. Sub-marine fans are typical of closing ocean basins and east-southeast transport is consistent with west-to-east closure along this suture zone as proposed by Johnson et al. (2008).

Member P2ou3 is distinctly less metamor-

phosed than the underlying members of the Onch Uul Formation, and it also contains the only carbonate units found in the Permian sections. It may lie unconformably on the older sections based on its lesser degree of metamorphism, and it probably represents a different formation entirely. These strata are likely shallow-marine deposits representing a mixed siliciclastic-car-bonate shoreline system.

The thick Lower Triassic (?) red-bed sequence (T

1) southeast of the Bulgan Uul klippe is also

Dike

0

100

200

300

400

500

0.0 0.2 0.4 0.6 0.8 1.0

Age

(Ma)

box heights are 2σ

36Ar

/40Ar

06BU03-2 amphibolePlateau age = 245.0 ± 1.8 Ma(2σ, including J-error of 0.1%

MSWD +0.12, probability = 0.095Includes 90% of the 39Ar

Age = 250 ± 20 MaInitial 40Ar/36Ar = 266 ± 42

MSWD = 7.8

0.000

0.001

0.002

0.003

0.000 0.004 0.008 0.012 0.016

data-point error crosses are1σ

Cumulative 39Ar fraction39Ar/40Ar

A

B C

Figure 12. Minimum age of the Early Triassic (?) fl uvial/alluvial units. (A) Dike of intermediate composition crosscuts the upper alluvial portion of the strati-graphic section presented in Figure 10B. Hammer is ~33 cm long. (B–C) Thermochronology from amphibole in the cross-cutting dike yielded a plateau age of 245 ± 1.8 Ma and an iso-topic correlation age of 250 ± 20 Ma. (Data table and ana-lytical methods are presented in the GSA Data Repository [see text footnote 1].) Sample loca-tion corresponds to E in Fig-ure 6. MSWD—mean square of weighted deviates.




distinct from the underlying Permian sections. Fining-upward conglomerate, sandstone, and mudstone intervals that are commonly scoured at the base and pervasively trough cross-bedded in the lower part of the section are interpreted as high-energy (braided?) fl uvial channels. Pedo-genic carbonate nodules in the mudstones likely represent overbank and interdistributary de-posits. This succession coarsens upwards over >500 m. The upper interval contains clast- and matrix-supported cobble conglom erates that are commonly lenticular over >20 m laterally but are poorly channelized and poorly sorted. These are interpreted as interbedded debris fl ows and sheet-fl ood deposits. The transition thus likely represents progradation of an allu-vial fan toward the south-southeast. The unit lacks carbonate clasts, supporting deposition prior to emplacement of the carbonate klippe. The crosscutting dike dated at ca. 245 Ma is consistent with latest Permian–Early Triassic age for this unit.

Sandstone Provenance

The modal point-count data are presented in Figure 13 (complete data sets in Table DR1 [see footnote 1]). Figures 7, 9, and 10 show the stratigraphic positions of samples. Several of the Nomgon samples are not tied to measured sections but are located on Figure 5. Low-grade metamorphism may shift modal composition toward greater quartz and feldspar content and decrease lithic fragment abundances, as a re-sult of metamorphic reactions (e.g., feldspar + H

2O → mica and clays) and the physical break-

down of unstable lithic fragments into their constituent modes (e.g., Qt in Figs. 13A and 13B). Similar results have been discussed for Permian samples at Nomgon by Amory (1996) and Johnson et al. (2008). This effect is a partic-ular concern for the Nomgon samples and the older Bulgan Uul sections (P

2hu and P

2ou1–2)

that have been affected by greenschist-facies metamorphism.

Five discrimination diagrams were plotted to defi ne petrofacies and possible source ar-eas (Figs. 13A–13E). Individual samples from Bulgan Uul and Nomgon show a large variance (e.g., Bulgan Uul Qt modes range from 37% to 74%) but in general are rich in lithics and quartz relative to feldspar; petrofacies are al-most entirely lithic arenite (Okada, 1971). They plot in the “recycled orogens” fi eld on a QtFL diagram and at the boundary between arc and recycled provenance fi elds on QmFLt. Lithic fragments are predominately volcanic, with minor metamorphic and sedimentary rock frag-ments as well. Plots of QpLvLsm and QmPK show means for Bulgan Uul and Nomgon that

plot within the “mixed orogenic sands” and “continental block provenance” fi elds. Despite the relatively small sample suite and concerns about metamorphic overprinting, the results are notable in that Bulgan and Nomgon show simi-lar means, and these mean values are close to expected values associated with arc systems, but with fewer feldspar grains. The “recycled oro-gens” fi eld corresponds to various tectonic set-

tings, including subduction complexes, backarc thrust belts, and suture zones (Dickinson, 1985). No active (Permian) arc signature is seen here, but the sands were likely derived from older (Carboniferous–Devonian) arc systems that ex-perienced more recycling. The results are con-sistent with derivation from extinct arc systems involved in collision, as predicted for a closing ocean basin (Fig. 3).

Limit of detrit

al m

odes

Qt

Lithicarenite

Feldspathicarenite

QuartzareniteQuartzose

arenite

Qt

QmQp

Qm

FF

L

F Lt

P K

LsmLv

L

Cratoninterior

Mixed

Cratoninterior

Quartzose recycled

Transitionalcontinental

Transitionalcontinental

Basementuplift

Transitionalarc

Transitionalarc

Dissectedarc

Dissectedarc

Recycled orogen

Undissected arc

Undissected arc

Basementuplift

Transitional recycled

Lithic recycled

Subductioncomplex

Arc orogen

Collision suture and fold-thrust belt

Mixed orogenic sands

Circum-Pacificvolcanoplutonicsuites

Continental block provenance

Magmatic arcprovenance

Bulgan Uul sample

Nomgon sample

Bulgan Uul mean (n = 9)

Nomgon mean (n = 7)

One standard deviation

B

C D

E

Key:

A

Figure 13. Normalized ternary plots of sandstone modes from Bul-gan Uul and Nomgon. Modal parameters are summarized in the GSA Data Repository (see text footnote 1). The key corresponds to all plots. (A) Sandstone classifi cation plot after Okada (1971). (B) QtFL plot. (C) QmFLt plot. (D) QpLvLsm plot. (E) QmPK plot. Provenance fi elds in B–E are from Dickinson (1985). The framework components were determined as follows: Qt = Qm + Qp + Cht; Qpt = Qp + Cht; F = P + K; Lst = Ls + Lc; Lt = Lst + Lm + Lv + Qpt; L = Lc + Ls + Lm + Lv; Lsm = Lst + Lm, where Qm—monocrystalline quartz (raw count); Qp—polycrystalline quartz (raw count); Cht—chert (raw count); P—plagioclase (raw count); K—potassium feldspar (raw count); Lv—volcanic rock fragment (raw count); Ls—sedimentary rock fragment, excluding carbonates (raw count); Lc—carbonate rock fragment (raw count); Lm—metamorphic rock fragment (raw count).


Heumann et al.


U-Pb Zircon Geochronology

Detrital Suites from Bulgan Uul and Nomgon Sandstones

U-Pb ages were collected from detrital zir-cons separated from sandstones at Bulgan Uul and Nomgon (Figs. 5, 6, and 7; Table DR2 [see footnote 1]). The data set includes nine samples from Bulgan Uul, all from measured sections, and fi ve Nomgon samples from measured sec-tions, plus two additional samples from Late Permian Lugyn Gol outcrops in the area. Zircon populations of all samples were dominated by rounded to subrounded grains of various colors. A small fraction of the total zircon population analyzed (n = 1582) was subangular and dark amber in color. There was no systematic corre-lation between shape and color of grains to the U-Pb ages presented here.

The measured ages presented are 206Pb/238U ages where ages are younger than 1000 Ma, and 207Pb/206Pb ages where zircons are older than 1000 Ma. The 206Pb/238U ages are used here because they are generally considered more accurate for mean age calculations than the 207Pb/235U system for grains younger than 1 Ga (Dickinson and Gehrels, 2009). For all of the detrital zircons analyzed, ages range from ca. 200 to 2900 Ma (Table DR2 [see footnote 1]). Two samples (08-BU-08D and 08-BU-06A) produced three analyses signifi cantly younger than 250 Ma, but the ages were not reproduced systematically in any other sample, nor are they statistically viable so as to warrant specifi c consideration in this study. Individual samples show a strong bimodal distribution in their rela-tive probability spectra. The younger age peaks for individual samples fall within a range of ca. 275–300 Ma, while the older peaks consistently center around ca. 420 Ma (Fig. 14). Composite age distribution curves for the Bulgan Uul and Nomgon samples emphasize the bimodality in age distributions (Fig. 15).

Youngest single-grain ages from all the sam-ples, except 05-NW-10A, range from ca. 251 to 279 Ma and are listed in Table 1. Sample 05-NW-10A is included in Table 1 but is likely not Permian (see later discussion). The maxi-mum depositional ages for the full data set (ex-cept 05-NW-10A) range from ca. 262 to 295 Ma (Table 1). These maximum depositional ages were calculated using 206Pb/238U ages following statistical methods described by Dickinson and Gehrels (2009). Specifi cally, a weighted mean age was calculated (at the 2σ level) for the young-est cluster of three or more grains to meet the conditions for acceptable precision and discor-dance (see Methods and Analytical Procedures), using the DZ Age Pick program developed in house by the Arizona LaserChron Center (www

.geo.arizona.edu/alc). These estimates provide some of the fi rst isotopic age limits on Permian deposits at Bulgan Uul and Nomgon. Normal-ized probability density plots of detrital age spec-tra appear in stratigraphic order for all samples in Figure 14. The probability spectra show that the majority of zircons present in Upper Permian through Lower Triassic strata at Bulgan Uul are Cambrian–Late Permian in age. Zircons of Pre-cambrian age are present in all of the Bulgan Uul samples in small abundances. U/Th ratios of <5 in the grains analyzed from the Bulgan Uul strati-graphic sections (Fig. DR1A [see footnote 1]) indicate zircons derived from crystallization in igneous bodies (ratios >5 are interpreted as in-dicative of metamorphic sources; Rubatto, 2002; Hoskin and Schaltegger , 2003; Gehrels et al., 2006, and references therein). These zircons were likely derived from Paleozoic arc terranes located throughout southern Mongolia (Tomur-togoo, 1999; Lamb and Badarch , 2001; Badarch et al., 2002; Blight et al., 2008, 2010). Only the youngest (Early Triassic) samples from Bulgan Uul contain zircons exhibiting U/Th ratios in-dicative of metamorphic source terranes (Table DR2; Fig. DR1A [see footnote 1]). The ages of these “metamorphic-sourced” zircons cluster around ca. 910 Ma and likely originated from Proterozoic units of northern China and west of the study area (Gehrels et al., 2003a, 2003b; Yue et al., 2005). The Protero zoic age distributions observed in zircons at Bulgan Uul differ signifi -cantly from those ages observed for Proterozoic-Archean rocks in the North China craton (Darby and Gehrels, 2006).

Age results for all of the Nomgon samples (n = 679) are presented in the GSA Data Re-pository (Table DR3 [see footnote 1]). Similar to the samples from Bulgan Uul, age results from Nomgon show signifi cant contributions of Late Permian–Carboniferous and Silurian–Ordovician zircons. Precambrian ages were obtained for some Nomgon zircons but did not defi ne populations of statistical signifi cance (Table DR2 [see footnote 1]). U/Th ratios for the analyzed Nomgon samples are character-istic of zircons derived from igneous systems, similar to Bulgan Uul samples (Fig. DR1B [see footnote 1]).

A single sample from Nomgon (sample 05-NW-10A), though previously believed to be Early Permian, reveals ages clustering ca. 430 Ma with U/Th ratios >5, implying meta-morphic overprinting (Fig. DR1B; Table DR2 [see footnote 1]). This age (ca. 430 Ma, Silu-rian) corresponds to a poorly constrained period of metamorphism in the region and may also re-sult from the introduction of fl uids locally dur-ing arc formation and/or accretion in the mid- to late Paleozoic (Lamb and Badarch, 1997, 2001).

The probability density plots (and weighted mean ages for maximum depositional age) from Bulgan Uul and Nomgon are quite simi-lar in their bimodal age distributions, ca. 262–295 Ma (Middle Permian–Carboniferous) and ca. 420–500 Ma (Silurian–Ordovician) (Fig. 14; Table 1). These results are consistent with both regions receiving sediment from similar sources, perhaps within a single continuous sediment dispersal system. The composite prob-ability density plot for both study areas further supports this conclusion (Fig. 15). Addition-ally, both of the probability density plots show an increase in the probability of younger zircon populations from the oldest to youngest strati-graphic units. Specifi cally, this relationship is observed in the shift from higher probabilities of detrital zircon ages older than 400 Ma in the lower Bulgan Uul samples versus the dominant age probability peaks younger than 320 Ma in the upper member of the Onch Uul Formation (P

2ou2) (Fig. 14). This age shift may represent

the encroachment of an active igneous arc sys-tem near the depocenter, as discussed below.

Igneous Suites at Han Bogd and UrgunTwo igneous suites were targeted for U-Pb

zircon geochronology in order to investigate the existence of controversial Permian magmatic arcs in southern Mongolia. These include three samples from Han Bogd and a single sample from Urgun (Fig. 2). Both sampling locations are in granitic bodies mapped as Early to Middle Permian in age (Burenkhuu et al., 1995; Tomur-togoo, 1999). Sample 08-HB-02A is a tonalite from a small Carboniferous unit in the center of the main plutonic body at Han Bogd (Fig. 2; Lamb and Cox, 1998; Perello et al., 2001; Blight et al., 2008, 2010). The remaining Han Bogd samples (07-HB-07A and 07-HB-07AA) are from granodiorite exposed along a transect lead-ing from the pluton’s center to its northeastern margin. The single sample collected at Urgun (07-UR-10; Fig. 2) appears to be similar in com-position to the granodiorite sampled at Han Bogd (based on qualitative hand-sample observations). BSE and CL imaging results (Figs. 16A and 16B) show oscillatory zoning, a texture consistent with igneous zircon growth (Corfu et al., 2003).

Individual analytical results and a detailed explanation of data reduction are presented in the GSA Data Repository (Table DR4 [see footnote 1]). Only analyses within 1.5% discor-dance (206Pb/238U and 207Pb/235U ages, exclud-ing error ellipses) were used in calculating the weighted mean age for each sample. Method-ology was summarized earlier, and excluded analyses are identifi ed in the GSA Data Reposi-tory (Table DR4 [see footnote 1]). Individual analyses with 206Pb/238U ages older than 400 Ma




were rejected in the weighted mean calculation because they likely refl ect inheritance of older zircon. Weighted mean age calculations were achieved using the Age Pick program created by the Arizona LaserChron Center (www.geo.arizona.edu/alc).

Age results are ca. 328–330 Ma for three of the samples and younger (ca. 292 Ma) for one of the Han Bogd samples. These results argue for a Carboniferous age, rather than Permian age (Tomurtogoo, 1999), for samples of plutons at Han Bogd and Urgun. Although

an Early Permian age is reported from a single sample (n = 4), this alone does not support major Early Permian arc activity in the study areas (cf. Kovalenko et al., 2006). A single sam-ple from Han Bogd (07-HB-07AA) contained the youngest observed population of 206Pb/238U

Prob

abili

ty d

ensi

ty

Age (Ma)

0

100

200

300

400

500

600

700

800

900

1000

05-NW-10A (n = 92)

(06-NO-02-01 [n = 85])

(08-NO-10C [n = 131])

(08-NO-10F [n = 104])

(08-NO-06A [n = 106])

(05-NO-08A [n = 99])

(05-NW-12A [n = 62])

(08-BU-07A [n = 119])

(08-BU-06A [n = 106])

(08-BU-06C [n = 96])

(08-BU-08D [n = 112])

(08-BU-18A [n = 104])

(08-BU-18F [n = 109])

(06-BU-24-18 [n = 73])

(06-BU-24-24 [n = 84])

(06-BU-24-32 [n = 100])

Nom

gon

sam

ples

Bul

gan

Uul

sam

ples

Base of section

Top of section

Base of section

Top of section

Upper Permian (P2lg)

Upper Permian (P2hu)

Upper Permian (P2ou1)





Early Triassic(?) (T1)



Lower Permian (P1th)





Devonian Unit

Figure 14. Detrital zircon prob-ability density plots for 16 sand-stone samples from this study; N = 9 for Bulgan Uul and N = 7 for Nomgon. Letter “n” indi-cates the number of analyzed zircon grains in each of the 16 samples. The curves represent a sum of the probability dis-tributions of all analyses from a given sample, normalized such that the areas beneath the probability curves are equal for all samples. Individual prob-ability plots are arranged in stratigraphic order and have been truncated at 1000 Ma. Sample locations and intervals are identifi ed in Figures 5, 6, 7, 9, and 10. Data from individual analyses, analytical procedures, and data reduction techniques are outlined in the GSA Data Repository (see text footnote 1).


Heumann et al.


ages (ca. 124–210 Ma; n = 12). U/Th ratios are <3 for these analyses, and we interpret this as a period of localized hydrothermal activity. These analyses were also rejected from the reported age of the sample.

DISCUSSION

Regional stratigraphic correlations, based on detailed study of Bulgan Uul and geochrono-logic data, have important implications for ba-sin evolution along this central part of the Tien Shan–Yin Shan suture. In particular, the results support previous suggestions that Permian and Lower Triassic deposits in southern Mongolia represent the sedimentary record of fi nal ocean closure between the terranes of northern China and the southern part of the Altaids (Johnson et al., 2008). Paleogeographic reconstructions further suggest that Bulgan Uul and Nomgon represent a once-continuous closing ocean ba-sin system that was dismembered or offset by Mesozoic–Cenozoic strike-slip displacement across the East Gobi fault zone (Fig. 17; Lamb et al., 1999; Webb and Johnson, 2006; Taylor et al., 2009; Heumann, 2010).

Paleogeographic Reconstruction of the Bulgan Uul–Nomgon Basin System

We broadly reconstruct the Late Permian–Early Triassic basin system based on the stratigraphic correlations, U-Pb ages, and prov-enance data following the predictions for rem-nant ocean basins described by Graham et al. (1975) (Fig. 3). Note that the reconstructions presented in this study (Fig. 17) place Nomgon to the east/southeast of Bulgan Uul, i.e., prior to strike-slip displacement across the East Gobi

fault zone. During the Early Permian, a closing remnant ocean basin likely occupied the region between a passive margin (which was composed of amalgamated, extinct arcs on mainly oceanic lithosphere) to the north and an encroaching, but still distant, subduction zone and active arc to the south. Stratigraphic analysis at Bulgan Uul indicates a shallowing-upward marine succes-sion of distal turbidites to proximal shoreline

and shallow-water carbonates during the Early to Late Permian. This succession was uncon-formably overlain by nonmarine fl uvial and alluvial deposits in the Early Triassic (pre–245 Ma). In general, this marine-nonmarine transi-tion represents a fl ysch-molasse transition, con-sistent with closure of a remnant ocean basin between two colliding terranes (Graham et al., 1975; Ingersoll et al., 1995, 2003; Miall, 2000).

Age (Ma)

Pro

babi

lity

dens

ity

0 500 1000 1500 2000 2500 3000

Nomgon (N = 7; n = 679)

Bulgan Uul (N = 9; n = 903)

Bulgan Uul and Nomgon Composite Detrital Zircon Age Distribution Curves

Figure 15. Composite probability density plots for all analyses made from Bulgan Uul (N = 9, n = 903) and Nomgon (N = 7, n = 679). The curves represent a sum of the probability distribu-tions of all sample analyses from Bulgan Uul and Nomgon, normalized such that the areas beneath the probability curves are equal for both study areas. The curves have been truncated to 3000 Ma, but all analyses made are included. N = sample number, n = total analyses made.

TABLE 1. DETRITAL ZIRCON DETERMINATIONS FOR MAXIMUM AGE OF DEPOSITION FOR UPPER PERMIAN SEDIMENTARY SEQUENCES, SOUTHERN MONGOLIA

SamplenoitaluclacegalanoitisopedmumixaM—stluserkciPegAZDnorhCresaLanozirAniargelgnistsegnuoY

Age (Ma) ± m.y. (1σ) Age peak (Ma)* n† Weighted mean age (Ma) ± m.y. (2σ)§

4.42.082910825.83.17223-42-UB-608.42.272211728.44.56242-42-UB-600.47.772615721.96.16281-42-UB-607.36.18249724.15.872F81-UB-805.30.072020728.20.252A81-UB-804.49.26242624.25.162D80-UB-806.40.77267728.20.472C60-UB-804.42.97256728.56.752A60-UB-809.41.26261625.915.052A70-UB-809.35.082019724.32.272A21-WN-507.31.282930827.318.062A80-ON-501.43.182018821.43.452A60-ON-808.41.78295828.419.362F01-ON-807.35.982539828.119.472C01-ON-808.39.492357928.914.56210-20-ON-605.54.134169346.46.704A01-WN-50

Note: TE = SQRT (RE*RE + SE*SE), where TE = total error, SE = systematic error, RE = random error, all in % then recalculated for m.y. Systematic error in all cases was 1.2%. Sample name abbreviations are: BU—Bulgan Uul; NO or NW—Nomgon.

*Age peak refers to a derived peak in a cluster of ages determined by the DZ Age Pick program developed in-house by the Arizona LaserChron Center (http://sites.google.com/a/laserchron.org/laserchron/home).

†n equals the number of zircons that make up the cluster and were used in the calculation for a weighted mean age.§The uncertainty in the weighted mean age is calculated by including the systematic error, where individual analyses do not.




By the end of the Permian, diachronous clo-sure of the ocean basin resulted in terminal ba-sin closure at Bulgan Uul and orogenic uplift of the surrounding area. This transition is refl ected in the shallowing-upward turbidite to shallow-marine to nonmarine facies evolution present at Bulgan Uul. Paleocurrents indicate transport toward the southeast through much of this time, which is consistent with a narrow basin clos-ing from west to east. This also implies that the basin deposits at Bulgan Uul may have distal correlatives to the east. Chronostratigraphic relations (Fig. 4) suggest that Nomgon was lo-cated in a distal marine setting at this time, and that the main marine depocenter thus shifted eastward during the Late Permian (Fig. 17).

Sedimentary deposits at Nomgon likely rep-resent the regional continuation of the closing ocean basin. Permian strata at Nomgon are entirely marine, and are mainly turbidite fan deposits, although there may be a slight rela-tive deepening (shallow marine to deep-water fan) from Early to Late Permian. Paleocurrents here are also east/southeast directed. No Lower Triassic strata have been identifi ed at Nomgon, so the fi nal marine-nonmarine transition is un-determined here but must be younger than Late Permian and older than Late Jurassic, which is the earliest age of intracontinental rifting in the area (Graham et al., 2001).

This regional stratigraphic correlation of fa-cies is supported by provenance data indicat-ing common source areas for Bulgan Uul and Nomgon. Modal compositions of sandstone samples indicate a common lithic-rich, recycled-orogen provenance of mixed sands derived from exhumed arc systems that formed the collision orogen (Şengör and Natal’in, 1996; Chen et al., 2000, 2009; Xiao et al., 2003; Cope et al., 2005; Li, 2006; Jian et al., 2010).

The detrital zircon geochronology results presented in this study are the fi rst of their kind for Permian deposits of southern Mongo-lia. The cumulative age probability plot (Fig. 15) suggests that Permian strata in these study areas originated almost entirely from ca. 262–420 Ma igneous terranes. Ordovician through Carboniferous arc-related units have been well documented to the north, west, and east of Bul-gan Uul (Burenkhuu et al., 1995; Lamb and Badarch , 1997, 2001; Tomurtogoo, 1999). Zir-cons of Permian age are present in our samples; however, probability plots suggest very low abundances with respect to the whole population analyzed (Fig. 14). Although Permian zircons are relatively rare, the younger age population (ca. 262–288 Ma) helps to establish maximum depositional ages for the sedimentary units, thus complementing existing biostratigraphic data (Manankov, 1988, 2004; Manankov et al.,

Backscattered electron (BSE) ~70 μm Cathodoluminescence (CL) ~70 μm

Urgun (07-UR-10) - granodiorite

Mean = 328.6 ± 4.1(1.3%) 95% conf.MSWD = 3.2, probability = 0.00

240

280

320

360

400

440

480

0.035

0.045

0.055

0.065

0.075

0.085

0.25 0.35 0.45 0.55 0.65

206 P

b/238 U

207Pb/235U

Han Bogd (07-HB-07A) - granodiorite

Mean = 291.8 ± 4.4 (1.5%) 95% conf.MSWD = 0.91, probability = 0.43

240

280

320

360

400

440

480

0.035

0.045

0.055

0.065

0.075

0.085

0.25 0.35 0.45 0.55 0.65

206 P

b/238 U

207Pb/235U

Han Bogd (07-HB-07AA) - granodiorite

Mean = 328.4 ± 5.3 (1.6%) 95% conf.MSWD = 3.0, probability = 0.00

100140

180220

260300

340380

420

0.015

0.025

0.035

0.045

0.055

0.065

0.075

0.1 0.2 0.3 0.4 0.5 0.6

206 P

b/238 U

207Pb/235U

data-point error ellipses are 68.3% conf.

Han Bogd (08-HB-02A) - tonalite

Mean = 330.6 ± 7.0 (2.1%) 95% conf.MSWD = 27, probability = 0.00240

280

320

360

400440

0.035

0.045

0.055

0.065

0.075

0.26 0.30 0.34 0.38 0.42 0.46 0.50 0.54

206 P

b/238 U

207Pb/235U

data-point error ellipses are 68.3% conf.

data-point error ellipses are 68.3% conf. data-point error ellipses are 68.3% conf.

A B

Figure 16. U-Pb zircon geochronology results from the Han Bogd and Urgun plutonic suites (Fig. 2). (A–B) Backscattered electron (BSE) and cathodoluminescence (CL) images of Han Bogd zircons revealed no resorbed cores. Magmatic zircon growth textures (Corfu et al., 2003) were common throughout. U-Pb concordia diagrams show dates obtained that are younger than 400 Ma (intercept age). Only those analyses that are 1.5% discordant (normal or reverse) were utilized in age calculation (fi lled error ellipses), where discordance is based on comparison of 206Pb/238U and 207Pb/235U ages. Analyses not used in age calculation are identifi ed by unfi lled error ellipses. Reported ages are inferred to indicate crystallization of the granites at Han Bogd and Urgun. The reported uncertainties were determined as the quadratic sum of the weighted mean error plus the total systematic error for the set of analyses. Isoplot 3.00 (Ludwig, 2003) was used to plot U-Pb concordia diagrams from the analyses. MSWD—mean square of weighted deviates.


Heumann et al.


2006; Shen et al., 2006; Shi, 2006; Table 1). We present these newly established ages as the for-mation and member boundaries in the chrono-stratigraphic summary in Figure 4.

Well-defi ned age probability peaks are lack-ing in age spectra from the lowest stratigraphic samples (P

2hu and P

2ou1; Fig. 14). This suggests

that multiple source regions fl anking the margin of the basin contributed zircons deposited at Bul-gan Uul (specifi cally 450–260 Ma; mainly Ordo-vician to Permian). In contrast, time-correlative samples from Nomgon suggest that sediment was derived from a more specifi c source region, because their relative age probability spectra show better-defi ned peaks (Fig. 14). Age prob-ability peaks for samples presented in Figure 14 become increasingly well defi ned up section, which we interpret to mean sediment sources were becoming less “diverse” or perhaps more locally derived in the Late Permian and Early Triassic. This interpretation is consistent with an ocean basin closing from west to east through Permian time, where suturing of the source area to the west favors development of large, inte-grated fl uvial systems draining an amalgamated and exhumed source terrane (Fig. 17).

Tectonic Implications of Zircon U-Pb Age Populations

The polarity of subduction associated with closure of a remnant ocean basin leading up to terminal arc-continent collision is debated. Some investigators favor northward subduction of the North China craton beneath the Altaids, and base their conclusion partly on the presence of Permian igneous suites and ophiolite fragments throughout southern Mongolia and to the east in northern China (Hsü et al., 1991; Burenkhuu et al., 1995; Tomurtogoo, 1999; Robinson et al., 1999; Chen et al., 2000; Li, 2006). In recent stud-ies, some authors have also proposed simultane-ous dual subduction in the region during ocean basin closure (Xiao et al., 2003).

U-Pb zircon geochronology results from this study indicate that samples from two of the larg-est pluton and igneous suites in southern Mon-golia are Carboniferous, not Permian (Fig. 16). Although Permian arc activity in southeastern Mongolia cannot be entirely discounted, the re-sults join a growing number of geochronologic data that argue for more widespread Carbonif-erous rather than Permian intrusive suites in the region (Fig. 2; Lamb and Badarch, 2001; Heu-mann et al., 2008; Blight et al., 2008, 2010). In contrast, plutonic suites along the north margin of the North China block show subduction-related geochemical signatures, and well-established crystallization ages in intrusive suites consis-tent with terminal closure in the Late Permian,

A. Early Permian

B. Late Permian

C. Post-Triassic

Carboniferous arc

Ordovician-Silurian arc

Microcontinent crust

East Gobi fault zone

Plutonic/igneous suites

Shoreline deposits

Ocean basin

Turbidite fan system

Fluvial-deltaic systems

Legend

North

North China craton

Alxa/Yin Shan

EGFZ

North China craton

North China craton

Altaids

Altaids

Bulgan Uul

Nomgon

Altaids

Figure 17. Schematic illustrations of the Bulgan Uul and Nomgon study areas in the Early Permian–Late Permian and post-Triassic. (A) Early Permian. Both Bulgan Uul and Nomgon receive sediment from Carboniferous and Ordovician–Silurian arc terranes located north-northwest of the ocean basin. Bulgan Uul and Nomgon are both within the turbidite fan system during the Early Permian. Nomgon received additional sediment locally from a fl uvial drainage system entering the basin nearby. Sediment dispersal is from north-west to southeast. (B) Late Permian. Northward thrust emplacement of the terranes of northern China and/or the North China craton closes the western end of the Permian basin. The proximity of the ac-tive arc and growing orogen west of the basin creates a topographic high near Bulgan Uul. Collision to the west of Bulgan Uul results in a shift to terrestrial sedimentation at Bulgan Uul, which lasts into the Early Triassic, whereas Nomgon remains in a distal-marine depo-sitional setting and still receives sediment from the northwest. At this time, sediment from northern China terranes enters the Bulgan Uul–Nomgon depositional system. (C) Post-Triassic. The Bulgan Uul–Nomgon depositional system is crosscut by the northeast-striking shear zone and faults of the East Gobi fault zone (EGFZ) beginning in the Late Triassic, followed by Cretaceous–Holocene brittle reacti-vation (Heumann, 2010; Webb et al., 2010). Total sinistral displace-ment across the East Gobi fault zone is ~250 ± 50 km based on the offset basin system described here (Heumann, 2010).




implying that subduction occurred southward beneath the North China craton (Figs. 17A and 17B; Wang and Liu, 1986; Nie et al., 1994; Cope et al., 2005; Jian et al., 2010). Detrital zircon re-sults from Bulgan Uul and Nomgon show that the majority of zircon ages correspond to source terranes with crystallization ages of 300 Ma and older (Figs. 14 and 16).

Small Precambrian age peaks are present in the probability spectra for Bulgan Uul samples (specifi cally the Early Triassic samples, with peaks of ca. 910 Ma), whereas fewer probability peaks for Precambrian ages are present in the Nomgon samples (Fig. 15). Although the Pre-cambrian zircons are a statistically small popu-lation (n = 79 for Bulgan Uul and n = 69 for Nomgon), their presence potentially bears upon a larger debate regarding the oldest age of ter-ranes in the southern Altaids. Regional maps show isolated outcrops of Precambrian meta-morphic rocks thought to represent basement of the “South Gobi microcontinent” (Şengör et al., 1993b; Tomurtogoo, 1999; Wang et al., 2001; Xiao et al., 2003; Chen et al., 2009). Recent work (Webb et al., 1999, 2010; Webb and John-son, 2006; Taylor et al., 2008, 2009) demon-strates that many of these outcrops (e.g., Tavan Har, Onch Hayrhan; Fig. 2) are in fact Mesozoic tectonites with mainly Paleozoic protoliths.

The closest known Precambrian crust is to the south of Bulgan Uul and Nomgon in northern China. Notably, the early Neoproterozoic popu-lations we observed (ca. 910 Ma) are most likely not derived from the North China craton (Darby and Gehrels, 2006). Rather, these zircons may be derived from terranes of northwest China includ-ing the Qilian, Qaidam, and Alxa region (Fig. 1; Sun et al., 1989; Li and Qian, 1995; Zhao et al., 2001; Wang et al., 2001; Gehrels et al., 2003a, 2003b; Yue et al., 2005). In any case, the statisti-cally small populations of Precambrian zircons in these samples cast further doubt on the exis-tence of a major Precambrian micro continental block in southeastern Mon golia (Taylor et al., 2009). Our results suggest that the south Gobi region is underlain by a hetero geneous amalga-mation of accreted early Paleozoic arc terranes (Jahn et al., 2000; Lamb and Badarch, 2001; Kröner et al., 2007).

Mesozoic–Cenozoic Strike-Slip Displacement along the East Gobi Fault Zone

The East Gobi fault zone is a >250-km-long, northeast-trending structural corridor in south-east Mongolia (Fig. 2), with a protracted geo-logic history that includes two main phases of left-lateral strike-slip movement: Triassic duc-tile shearing and post-Cretaceous brittle faulting

(Lamb et al., 1999; Webb and Johnson, 2006; Webb et al., 2010; Taylor, 2010). If Bulgan Uul and Nomgon were linked as a single rem-nant ocean basin during the Late Permian, this implies ~250 km of total offset along the East Gobi fault zone (Fig. 17C). Although this esti-mate comes with large errors (at least ± 50 km) due to uncertainty in locating basin margins, it is the best available estimate on total offset and also demonstrates the usefulness of basin recon-structions as general piercing points (Heumann, 2010). The Carboniferous intrusive suites from Han Bogd and Urgun (Fig. 2) support a simi-lar magnitude of total offset and thus may also represent Paleozoic units offset by ~250 km of strike-slip displacement.

CONCLUSIONS

Stratigraphic correlations, sandstone prov-enance, and U-Pb geochronology results suggest that Bulgan Uul and Nomgon were once compo-nents of the same closing ocean basin system. The regional depositional system occupied the north-ern margin of a remnant ocean basin, north of which extinct Ordovician–Silurian and Carbonif-erous accreted arc terranes made up the southern margin of accreted Paleozoic arc terranes during the Late Permian. The remnant ocean basin was subsequently closed during the Permian by the northward advancement of the North China cra-ton and terranes of northern China, culminating with the diachronous (west-to-east) collision of north China and the southern Altaids along the Tien Shan–Yin Shan suture zone. Coeval turbi-dite fan deposits at Nomgon represent the distal facies equivalents of the marine successions at Bulgan Uul. In the Late Permian, Nomgon re-mained a closing ocean basin, receiving sediment from the orogen to the west, and coeval Bulgan Uul deposits indicate shallowing as that orogen approached. Unconformable deposition of ter-restrial red beds (alluvial-fl uvial strata) in the Early Triassic followed. Stratigraphic and geo-chronologic data presented here bracket the fi nal marine deposition at Bulgan Uul and initial ter-restrial deposition during the Late Permian and Early Triassic, thus constraining terminal closure of the paleo–Asian Ocean in south-central Mon-golia. Finally, this basin system and possibly Carboniferous intrusions of southern Mongolia were offset by strike-slip motion across the East Gobi fault zone. The basin reconstruction implies ~250 km of total sinistral displacement across the East Gobi fault zone.

ACKNOWLEDGMENTS

This work was supported by the National Sci-ence Foundation grant numbers EAR-0537318 to C. Johnson and EAR-0537165 and EAR-0929902

to L. Webb. The research also benefi ted from Ameri-can Chemical Society, Petroleum Research Fund grant 40193-G8 to C.L. Johnson. We thank Ian Semple and Megan Frederick for their help in the fi eld during the 2006–2009 fi eld seasons. We also thank Bolortsetseg Minjin, director of the Institute for the Study of Mon-golian Dinosaurs, Ivanhoe Mines, and Rio Tinto for logistical support while in South Gobi in our 2005, 2006, 2007, and 2008 fi eld seasons. Thorough reviews and editorial comments by John Bartley, Tim Cope, and Lisa Lamb greatly improved this manuscript, and we thank George Gehrels for his advice regarding the U-Pb data. Additional guidance by GSA Bulletin Asso ciate Editor Margaret Thompson helped us im-prove the manuscript further.

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SCIENCE EDITOR: NANCY RIGGS

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