Detrital zircon and isotopic constraints on the ... - Wiley Online Library

TECTONICS, VOL. 31, TC2016, doi:10.1029/2011TC003013, 2012

Detrital zircon and isotopic constraints on the crustal architecture and tectonic evolution of the northeastern Pamir Alexander C. Robinson,1 Mihai Ducea,2 and Thomas J. Lapen1 Received 30 August 2011; revised 20 January 2012; accepted 6 March 2012; published 26 April 2012.

[1] New detrital zircon and isotopic (Nd and Sr) analyses from the eastern Pamir provide information on the depositional age and tectonic terrane affiliation of regional metamorphic terranes. Our results show the following. First, detrital zircon analyses from metasedimentary units along the Kongur Shan extensional system dominantly yield Triassic maximum depositional ages, with a similar age distribution to the Tibetan Songpan-Ganzi terrane. Further, zircon analyses from quartzofeldspathic gneisses in the core of the Muztaghata massif show the protoliths are Triassic granites. These units are interpreted to be part of the Permian-Triassic Karakul-Mazar arc-accretionary complex terrane. Second, ɛNd(0) compositions of Triassic granites overlap with other metasedimentary units not analyzed for detrital zircons and are also interpreted to be part of the Karakul-Mazar terrane. Third, schists in the Sares-Muztaghata gneiss dome structurally above Triassic orthogneisses yield an Ordovician maximum depositional age with a distinct detrital age distribution similar to the Tibetan Qiangtang terrane and are interpreted to be part of the Central Pamir terrane. Finally, Triassic and Ordovician schists along the Muztaghata massif record an Early Jurassic metamorphic event interpreted to date south-directed subduction of the Karakul-Mazar terrane beneath the Central Pamir during final closure of the Paleo-Tethys. These results, integrated with previously published results and field relations, reveal a complex Mesozoic to Cenozoic interleaving of tectonic terranes in the eastern Pamir with emplacement of the Karakul Mazar terrane both above and below the Kunlun and Central Pamir terranes to the north and south, respectively. Citation: Robinson, A. C., M. Ducea, and T. J. Lapen (2012), Detrital zircon and isotopic constraints on the crustal architecture and tectonic evolution of the northeastern Pamir, Tectonics, 31, TC2016, doi:10.1029/2011TC003013.

1. Introduction [2] One of the most striking aspects in the HimalayanTibetan orogeny is the pronounced northward deflection of tectonic terranes across the Pamir-Karakoram region between the Tibetan Plateau to the east and Afghanistan to the west (Figure 1) [Boulin, 1981; Tapponnier et al., 1981; Burtman and Molnar, 1993]. However, the magnitude and nature of terrane deflection, as well as the continuity of individual terranes across this region, remains unclear. For example, various correlations require large differences in displacement on the Karakoram fault, as well as along-strike differences in the width of individual geologic terranes across the orogenic belt [Searle, 1996; Lacassin et al., 2004; Phillips et al., 2004; Schwab et al., 2004; Robinson, 2009]. Further, it is unclear whether these terranes can be correlated across the entire orogen, or whether certain terranes are not contiguous 1 Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, USA. 2 Department of Geosciences, University of Arizona, Tucson, Arizona, USA.

Copyright 2012 by the American Geophysical Union. 0278-7407/12/2011TC003013

[Burtman and Molnar, 1993; Gaetani, 1997; Yin and Harrison, 2000; Schwab et al., 2004; Burtman, 2010]. [3] Coupled with these issues, it is unclear how similar the crustal structure is between the Pamir-Karakoram and Tibetan Plateau, or whether differences in Mesozoic tectonics and terrane accretion resulted in regional variations in tectonic architecture. For example, studies have shown that much of the Qiangtang terrane in northern Tibet is underlain by metasedimentary rocks which were underplated from the north during the Triassic [Kapp et al., 2000, 2003; Pullen et al., 2008, 2011]. While it has been proposed that the Pamir may also be largely underlain by metasedimentary material [Ducea et al., 2003; Schwab et al., 2004; Hacker et al., 2005], the extent of these similarities is unclear. [4] One problem in resolving these questions has been uncertainties in the age and terrane affiliation of geologic units along the eastern Northern Pamir and within the Central Pamir gneiss domes, which are dominated by metamorphic rocks with poor depositional age control [Robinson et al., 2004; Schwab et al., 2004; Robinson et al., 2007]. In this paper, we address the age and terrane affiliation of lithologies in the northeastern Pamir through (1) detrital zircon geochronology of metasedimentary rocks, (2) zircon geochronology of igneous rocks, and (3) Nd and Sr isotopic compositions of

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Figure 1. (a) Simplified tectonic map of the western end of the Himalayan-Tibetan orogen showing the locations of major suture zones and tectonic terranes. (b) Interpreted terrane correlations between the Tibetan Plateau and Pamir-Karakoram region from Yin and Harrison [2000] and Robinson et al. [2004]. (c) Interpreted terrane correlations from Schwab et al. [2004] and Lacassin et al. [2004]. (d) Interpreted terrane correlations from Burtman and Molnar [1993], Robinson [2009], and this study. metasedimentary and igneous rocks. Our results provide new information on the age and terrane affiliation of different structural units in the region, as well as the tectonic evolution and crustal architecture of the region.

2. Geologic Setting 2.1. Regional Tectonic Divisions [5] It has long been recognized that the Himalayan-Tibetan orogen consists of distinct tectonic terranes that evolved along, or were accreted to, the southern margin of Asia during the late Paleozoic–Mesozoic prior to the collision with India [Sengor and Natal’in, 1996; Yin and Harrison, 2000; Gehrels et al., 2011]. The Pamir-Karakoram region has generally been divided into four terranes: (1) the Northern Pamir, bound by the Main Pamir thrust in the north and the Tanymas suture in the south; (2) the Central Pamir, bound by the Tanymas suture to the north and the Rushan-Pshart zone to the south; (3) the Southern Pamir-Karakoram terrane bound by the Rushan-Pshart zone to the north and the Shyok suture to the south; and (4) the Kohistan-Ladakh terrane bound by the Shyok suture to the north and the Indus suture to the south (Figure 1a) [Burtman and Molnar, 1993]. [6] The Northern Pamir have been interpreted as a composite Paleozoic arc terrane correlative to the North and South Kunlun Terranes of the Western Kunlun Shan of northwestern Tibet [Tapponnier et al., 1981; Boulin, 1988; Burtman and Molnar, 1993; Yin and Harrison, 2000] based on regional geologic maps which depict the region as Proterozoic to Early Paleozoic lithologies (Figure 1b) [e.g., Liu, 1988; Yin and Bian, 1992; Xinjiang Bureau of Geology and Mineral Resources, 1993; Pan et al., 2004]. Alternatively,

Schwab et al. [2004] interpreted much of the eastern Northern Pamir to be part of the Permian-Triassic Karakul-Mazar terrane (Figure 1c), an accretionary complex correlative to the Triassic Songpan-Ganzi terrane of northern Tibet. [7] The Central Pamir terrane have been interpreted in several ways: (1) as a separate terrane with no direct correlative body in Tibet (Figure 1d) [Burtman and Molnar, 1993; Robinson, 2009] with a variation that the Central Pamir is a partially rifted portion of the Southern Pamir terrane [Burtman, 2010]; (2) as correlative to the Songpan-Ganzi terrane (Figure 1b) [Yin and Harrison, 2000; Robinson et al., 2004]; and (3) as directly equivalent to the Qiangtang terrane of Tibet (Figure 1c) [Schwab et al., 2004; Valli et al., 2008] based in part on the presence of detachment fault bounded gneiss domes interpreted to correlate to core complexes in the Qiangtang anticlinorium [Kapp et al., 2000]. [8] The Southern Pamir-Karakoram terrane are generally interpreted to be continuous, although they may be separated by a suture zone or region of highly attenuated continental crust from Paleozoic rifting (the Tirich Mir fault zone [Zanchi et al., 2000]). Some workers have correlated the Southern Pamir-Karakoram with the Lhasa terrane of Tibet based on distribution of coeval magmatic belts (Figure 1c) [Schwab et al., 2004]. However, based on limited offset along the northern Karakoram fault (150–160 km [Robinson, 2009]) we correlate the Southern Pamir-Karakoram terrane to the Qiangtang terrane of Tibet (Figure 1d). 2.2. Structural Terranes of the Northeast Pamir [9] The most prominent structural feature in the northeastern Pamir is the active Kongur Shan extensional system which extends for 250 km along the eastern margin of the

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Figure 2. Simplified geologic map of the eastern Pamir showing previous interpretations of the lithologic ages, major Cenozoic structures, and ɛNd(0) values for samples at the southern end of the Tashkorgan valley [after Yin and Bian, 1992; Strecker et al., 1995; Sobel and Dumitru, 1997; Schwab et al., 2004; Robinson et al., 2007]. Pamir (Figures 2 and 3) [Brunel et al., 1994; Robinson and Yin, 2004; Robinson et al., 2007]. Both the hanging wall and footwall are cut by multiple pre-extensional faults which juxtapose structural terranes with different lithologies, metamorphic grade, and metamorphic histories (Figure 3) [Robinson et al., 2004, 2007], which we summarize below. For ease of reference we assign numbers to these structurally bounded terranes (I, II, III, etc.). [10] The Baoziya thrust fault lies in the hanging wall of the Kongur Shan extensional system, strikes N-S, and dips moderately (45 ) to the West (Figure 3) [Pan, 1992; Robinson et al., 2004]. The thrust fault juxtaposes upper amphibolite facies schists and quartzites intruded by Triassic granites (terrane I) over greenschist facies metagraywacke (terrane II) [Robinson et al., 2004]. Terrane I preserves a well-defined Late Triassic (225–200 Ma) Buchan metamorphic sequence, overlapping in age with regional Late Triassic granites [Robinson et al., 2004]. [11] The northern footwall of the Kongur Shan extensional system exposes the regionally extensive top-north to topnorthwest Shala Tala thrust fault (Figure 3), which juxtaposes amphibolite facies schists and quartzites (terrane III) over low-grade greywacke (terrane IVa) [Robinson et al., 2004]. Syn-kinematic metamorphism and subsequent cooling in the hanging wall is dated as middle Cretaceous (125–100 Ma) [Robinson et al., 2004] although thrusting may have begun as early as 144 Ma [Arnaud et al., 1993]. [12] The steeply east-dipping right-slip Ghez fault bounds the eastern margin of the Kongur Shan massif and juxtaposes amphibolite facies schists and mylonitic granites to the

east (terrane Va) (age of metamorphism: 9 Ma) against lower greenschist facies metagraywacke (terrane IVb) [Brunel et al., 1994; Robinson et al., 2004]. Recent studies interpret the fault to have been rotated from a sub-horizontal orientation (top-south sense of shear) by rollover in the footwall of the Kongur Shan normal fault [Robinson et al., 2007, 2010]. While the Ghez fault has been interpreted to continue to the north, the trace is not well constrained (Figure 3). [13] The Shen-ti fault lies to the northwest and south of the Muztaghata massif and bounds the metamorphic core of the Sares-Muztaghata gneiss dome (Figures 2 and 3) [Pashkov and Dmitriyev, 1982; Peykre et al., 1982; Schwab et al., 2004; Robinson et al., 2007]. To the north the Shen-ti fault strikes east-west, dipping moderately to the north, and truncates the Baoziya thrust fault before being truncated by the Kongur Shan fault. To the south, the Shen-ti fault strikes east-west and dips moderately to the south, and merges to the east of the Kongur Shan fault with the north-south striking Kuke fault [Sobel et al., 2011]. The footwall of the Shen-ti fault consists of upper amphibolite to granulite facies metasediments and metavolcanics intruded by Miocene granites (terrane Vc), which lie structurally above quartzofeldspathic gneisses intruded by Triassic granites (terrane Vb) (Figure 3). The southern hanging wall (terrane VI) consists of greenschist facies metagraywacke and metavolcanics. Early Jurassic and Miocene metamorphic events are recognized in the dome, with exhumation of the dome occurring in the Late Miocene [Robinson et al., 2007; Yang et al., 2010].

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Figure 3. Tectonic map of the Kongur Shan extensional system showing previously interpreted ages of units, major structures, structural terranes discussed in the text, and sample locations for zircon U-Pb analyses and isotopic analyses. KSES: Kongur Shan Extensional System (modified from Robinson et al. [2007]). [14] The east-west striking, south dipping Torbashi fault lies immediately south of the southern Shen-Ti fault juxtaposing amphibolite facies migmatitic schists (terrane VII) above terrane VI. High grade metamorphism is dated as Late Triassic (220 Ma) [Yang et al., 2010]. Kinematic

indicators in the hanging wall and footwall show topsouthwest to top-west sense of shear with cooling ages recording a poorly defined Cretaceous exhumation event interpreted to constrain the timing of thrusting [Robinson et al., 2007].

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Figure 4. Cumulative 206Pb/238U age probability plots and weighted mean ages of Triassic orthogneisses from the Muztaghata massif. 2.3. Previous Age Assignments [15] Previous protolith age assignments from regional geologic maps generally depict the eastern Pamir to be dominantly Proterozoic to Paleozoic (Figure 3) [Liu, 1988; Pan, 1992; Yin and Bian, 1992; Xinjiang Bureau of Geology and Mineral Resources, 1993]. Alternatively, Schwab et al. [2004] proposed that the eastern half of the Northern Pamir is PermianTriassic in age. Further, Schwab et al. [2004] interpreted the cores of the Central Pamir gneiss domes (terrane Vc) to representing subducted and underplated Carboniferous-Triassic metasediments (rather than Proterozoic to Early Paleozoic basement). However, zircon results from Yang et al. [2010] yield an Ordovician maximum depositional age for metasediments in the footwall of the Shen-ti fault. Finally, zircon results from Yang et al. [2010] and Zhang et al. [2007a] yield a Late Permian-Early Triassic maximum depositional age for the hanging wall of the Torbashi thrust (terrane VII). These various interpretations and new ages highlight the uncertainty in the age and terrane affiliation of medium to high grade metamorphic rocks in the region.

3. U-Pb Zircon Geochronology 3.1. Methods [16] Zircon grains from igneous and metasedimentary samples were separated using conventional techniques, mounted in epoxy, and polished to expose grain interiors. U-Th-Pb analyses were performed on two instruments: (1) a Micromass Isoprobe multicollector inductively coupled plasma-mass spectrometer (MC-ICP-MS) at the University of Arizona Laserchron Center and (2) a Varian quadrupole inductively coupled-mass spectrometer (Q-ICP-MS) at the University of Houston. Laser ablation at the Arizona Laserchron Center was conducted with an Eximer laser with a 25 mm beam diameter at 8 Hz. Instrument fractionation of U/Pb and 206Pb/207Pb during analyses was calibrated by analyzing fragments from a Sri Lanka zircon standard with an age of 564 4 Ma (2s) [Gehrels

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et al., 2006] in between sets of 5 consecutive analyses of unknown sample zircons. Common Pb was corrected for using measured 204Pb and assuming an initial Pb composition from the model of Stacey and Kramers [1975]. Laser ablation analyses at the University of Houston were conducted with two different lasers: a CETAC LSX-213 Nd:YAG laser used with a 50 to 20 mm beam diameter at 15 Hz, and a Photon Machine Anlayte.193 excimer laser used with a 20 mm beam diameter at 10 Hz. Instrument fractionation of U/Pb and 206Pb/207Pb during analyses was calibrated by analyzing fragments from zircon standard FC5z (1096.2 1 Ma (2s) [Shaulis et al., 2010]) in between 5 consecutive unknown analyses. Analyses of zircon standards Peixe (564 4 Ma [Chang et al., 2006]) and Stettin (1565 8 Ma [van Wyck, 1995]) were also conducted after every 10 unknown analyses to monitor the calibration. Analyses were not corrected for common Pb due to interference from Hg (see Shaulis et al. [2010] for discussion). Isotopic ratios are provided in the auxiliary material (Table S1).1 All uncertainties are reported at the 2s level. [17] Age cumulative probability plots were created using Isoplot 3.0 [Ludwig, 2003]. For detrital cumulative probability plots, analyses with a discordance of >30% are not included, and for all plots analyses with U/Pb ages >1.0 Ga are reported as 206Pb/207Pb ages while ages 10 (analyses 08, 19, 50, and 107; Table S1). We interpret the Paleozoic and older analyses as detrital which yield a Middle to Late Ordovician maximum depositional age of 459 6 Ma defined by a cluster of four ages that overlap within uncertainty.

4. Nd and Sr Isotopes 4.1. Methods [28] Isotopic ratios of 87Sr/86Sr, 143Nd/144Nd, and trace element concentrations of Rb, Sr, Sm, and Nd were measured

4.2. Results [30] Initial Nd and Sr isotopic compositions of lithologies in the Eastern Pamir show a significant spread in values, with several important clusters (Tables 1a and 1b and Figures 7 and 8). Triassic orthogneisses in the core of the Kongur Shan and Muztaghata gneiss domes yield ɛNd(0) values from 5.46 to 9.23 and 87Sr/86Sr values from 0.7120 to 0.7249, overlapping with other Triassic granites in the Northern Pamir (Figure 7) [Schwab et al., 2004]. Analyses from metasedimentary units from several structural terranes (terranes II, Va, and VI) yield overlapping ɛNd(0) and 87 Sr/86Sr values (Figures 7 and 10), with values as low as 4.53 (sample 9-1-99-1 from terrane Va) which gives a Late Triassic Nd model age. Two samples from Triassic schists in the hanging wall of the Shala Tala fault (terrane III) yield slightly lower ɛNd(0) values of 13.30 and 13.52 (87Sr/86Sr values of 0.7298 and 0.7307). Devonian flysch east of the Ghez fault (terrane IVb), and Ordovician schists from the southern Muztagata gneiss dome (terrane Vc) yield similar ɛNd(0) values of 13.2 to 19.5. Permian slates from the southern end of the Tashkorgan valley yield significantly more negative ɛNd(0) values of 27 and 87Sr/86Sr values of 0.86 (Figure 2). [31] The overlap in isotopic compositions between the Triassic orthogneisses and metasediments from terranes II, Va, and VI suggest a genetic relationship. We interpret our results to show the metasedimentary units from terranes II and VI (for which we don’t have detrital zircon data) are Permian to Triassic in age (as our detrital zircon results show for terrane Va), and were largely derived from the regional Permian-Triassic arc. For terrane VI, this interpretation is

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Table 1a. Isotopic Ratios Sr (ppm)

Rb (ppm)

Sm Nd (ppm) (ppm)

0.113799 3.330963

9.336 58.263

Terrane II 5.049 26.055 1.064 5.744

0.327568 9.594469

0.718115 0.724922

0.117152 0.111943

Terrane III 38 57.608′ 74 43.952′ 0.956282 54.092 2.252 12.143 38 54.352′ 74 52.425′ 0.970232 168.527 2.170 9.122

2.756021 2.795972

0.730714 0.729777

Terrane IVb 4-30-00-5b Graywacke 38 44.428′ 75 46.507′ 2.747367 134.837 0.764 4.302 7.938598 0.757415 8-12-03-4b Graywacke 38 53.788′ 75 11.126′ 3.680808 177.089 1.681 8.537 10.648448 0.769626 8-19-03-3 Graywacke 38 44.111′ 75 15.076′ 1.349319 51.599 2.745 13.885 3.890498 0.735289

Sample

Lithology

6-14-00-2 6-14-00-5

Schist Schist

38 21.333′ 38 25.216′

5-30-00-2 6-4-00-3

Schist Schist

9-1-99-1 6-12-00-5a 6-18-00-4 7-06-00-5a 8-18-03-5 8-18-03-6 8-19-03-4 9-3-03-4a

Schist Orthogneiss Orthogneiss Orthogneiss Granite Schist Granite Orthogneiss

N Latitude E Longitude

38 38 38 37 38 38 38 38

42.241′ 24.254′ 21.670′ 54.039′ 45.061′ 45.025′ 42.967′ 12.117′

74 2.153′ 74 4.349′

75 2.438′ 75 14.530′ 75 13.050′ 75 24.410′ 74 11.445′ 75 8.291′ 75 15.353′ 75 4.087′

Terrane Va + b 0.393033 89.533 3.477 13.243 1.574934 167.066 7.459 40.052 1.157639 59.950 2.977 13.299 0.544188 84.181 2.354 13.252 0.187458 29.778 2.828 15.242 0.215227 23.275 2.549 13.003 0.713685 178.157 3.886 21.445 0.892844 108.210 5.697 31.973

87

Rb/86Sr

1.130784 4.536051 3.333088 1.565512 0.539740 0.619618 2.053591 2.570554

Std Err (%)

ɛNd(0)

0.512190 0.512198

0.0013 0.0013

8.74 8.58

0.112201 0.143801

0.511945 0.511956

0.0012 0.0016

13.52 13.30

0.107443 0.119129 0.119606

0.511640 0.511768 0.511858

0.0011 0.0020 0.0015

19.47 16.97 15.22

0.713130 0.724084 0.720736 0.712016 0.720932 0.719636 0.714469 0.720214

0.158722 0.112586 0.135430 0.107403 0.112171 0.118599 0.109631 0.107712

0.512406 0.512346 0.512177 0.512358 0.512298 0.512233 0.512377 0.512290

0.0015 0.0008 0.0018 0.0014 0.0018 0.0012 0.0014 0.0014

4.53 5.70 8.99 5.46 6.63 7.90 5.09 6.79

0.119174 0.109142

0.511687 0.511958

0.0017 0.0016

18.55 13.26

87

Sr/86Sra

147

Sm/144Nd

143

Nd/144Ndb

9-4-03-1 9-6-03-1

Schist Schist

Terrane Vc 37 56.563′ 75 11.890′ 4.857918 316.690 1.775 9.011 14.046862 0.764561 37 57.390′ 75 16.126′ 1.548241 106.500 4.532 25.122 4.465844 0.739406

4-28-00-6c

Schist

37 51.880′ 75 18.620′ 1.051214

0.723290

0.114953

0.512165

0.0015

9.23

6-26-00-1b 6-26-00-14

Slate Slate

Qiangtang-Tianshuihai 37 8.512′ 75 28.942′ 7.174342 183.334 7.213 42.590 20.927435 0.855008 37 15.751′ 75 28.084′ 7.549492 176.258 5.697 27.845 21.818033 0.858368

0.102354 0.123686

0.511275 0.511231

0.0014 0.0016

26.59 27.45

64.715

Terrane VI 2.395 12.593

3.027424

a

Sr isotopes normalized to 86Sr/88Sr = 0.1194. Nd isotopes normalized to 146Nd/144Nd = 0.7219.

b

supported by the presence of a Late Triassic metarhyolite [Zhang et al., 2007a].

5. Discussion [32] First order results from our zircon U-Pb geochronology and Nd-Sr isotopic analyses regarding the age of metamorphic units in the northeast Pamir include the following. [33] 1. Protoliths of quartzofeldspathic gneisses in the core of the Muztaghata and Kongur Shan gneiss domes are Early to Middle Triassic granites rather than Proterozoic to Early Paleozoic as have previously been interpreted. Further, Triassic maximum depositional ages from detrital zircon analyses and ɛNd(0) values of metasedimentary units show that many of the units in the northeast Pamir previously interpreted as early Paleozoic in age are Triassic (terranes I, II, III, IVa, Va, and VI; Figure 8). [34] 2. Detrital zircon analyses from metasediments which lie structurally above Triassic orthogneisses in the SaresMuztaghata gneiss dome yield Ordovician maximum depositional ages [Yang et al., 2010; this study], and a distinct detrital zircon signature with a strong Neoproterozoic to Ordovician population. [35] 3. Zircon analyses from Ordovician and Triassic schists from the Sares-Muztaghata gneiss dome both show a clear Jurassic metamorphic signature (Figure 6) [Yang et al., 2010; this study]. This is clearest in 8-31-30-1 which has a distinct population of ages with high U/Th ratios from 160–200 Ma (peak at 190 Ma) which overlap with high

U/Th ratio ages of 165–195 Ma from 6 to 22-00-1 as well as high U/Th ages from Yang et al. [2010]. 5.1. Tectonic Terrane Assignments in the Northeast Pamir and Structural Implications [36] The widespread presence of Triassic metasedimentary rocks that (1) are intruded by Triassic granites, (2) experienced metamorphism coeval with intrusion, and (3) have ɛNd(0) values which overlap with those from Triassic granites indicate that much of the northeastern Pamir were part of an arc-accretionary prism system along the southern margin of Asia. Further, recently published detrital zircon results from the hanging wall of the Torbashi fault (terrane VII) yield a maximum depositional age of 253 2 Ma with a metamorphic overprint at 220 Ma, similar to terrane I [Yang et al., 2010]. We interpret the above to show that Table 1b. Results of Standard Runs

NRbAAA (n = 10) Sr987 (n = 15) nSmb (n = 5) LaJolla Nd (n = 15)

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Mean Results of Standard Runs

Procedural Blanks (From 5 Determinations)

Rb/87Rb = 2.61199 20 Sr/86Sr = 0.710265 7 148 Sm/147Sm = 0.74890 21 148 Sm/152Sm = 0.42110 6 142 Nd/144Nd = 1.14184 2 143 Nd/144Nd = 5118491 2 145 Nd/144Nd = 0.348390 2 150 Nd/144Nd = 0.23638 2

Rb: 10 pg Sr: 150 pg Sm: 2.7 pg

85

87

Nd: 5.5 pg

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Figure 7. An ɛNd(0) versus 87Sr/86Sr plot of igneous and metasedimentary samples from the eastern Pamir, and Karakul-Mazar terrane granites from Schwab et al. [2004] (stars). structural terranes I, II, III, Va+b, VI, and VII are all part of the Permian-Triassic Karakul-Mazar terrane (Figures 1c, 1d, and 8) as suggested by Schwab et al. [2004]. This interpretation is further supported by a strong similarity between our detrital zircon results to the Songpan-Ganzi terrane in northeastern Tibet with prominent Early Paleozoic and Permian-Triassic peaks (Figure 9) [Weislogel et al., 2006, 2010; Gehrels et al., 2011]. One outlier is the hanging wall of the Shala Tala thrust which yields ɛNd(0) values between Triassic granites and Paleozoic Kunlun terrane sediments (Figure 7 and 8), suggesting terrane III had a stronger component of detritus from, and may have been proximal to, the Paleozoic Kunlun terranes to the north. [37] Our detrital zircon analyses and those of Bershaw et al. [2012] from greywacke in the footwall of the Shala tala thrust fault (terrane IVa) also yield Triassic maximum depositional ages rather than Paleozoic as previously interpreted. This was unexpected given the similarity of the rocks to a thick succession of Devonian flysch east of the Ghez shear zone (terrane IVb) (e.g., similar color, grain size, and grain composition). An important field observation are deformed cobble conglomerates near to the sampling site which contain clasts of what are interpreted to be Devonian flysch (Figure 10). This suggests the Triassic rocks in the footwall of the Shala Tala fault were in part sourced from, and deposited proximal to, exposed Devonian flysch. Therefore, we tentatively suggest terrane IVa is part of the Western Kunlun Terrane (Figure 8). [38] Results along the northern flank of the Kongur Shan massif from this study and Bershaw et al. [2012] reveal complicated structural relationships which have not been previously recognized. While our detrital zircon and ɛNd(0) values from rocks within the Kongur Shan massif (terrane Va, Figure 8) indicate they are part of the Karakul-Mazar terrane, structurally higher garnet-kyanite schists exposed north of the massif (terrane Vb, Figure 8) yield a Late Silurian-Early Devonian maximum depositional age (411 15 Ma) [Bershaw et al., 2012] showing they are part of the Kunlun terrane (Figure 8). This relationship requires the

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contact between the units to be tectonic in nature which we interpret to be a thrust juxtaposing Kunlun terrane above Karakul-Mazar terrane. We interpret the contact to be the continuation of the right-slip Ghez fault exposed along the eastern margin of the Kongur Shan massif [Brunel et al., 1994; Robinson et al., 2004, 2007] which is folded around the northern margin of the Kongur Shan massif (Figure 8). While this redefines the previously interpreted trace of the fault [Robinson et al., 2007], it is consistent with (1) the abrupt truncation of a 3–4 km thick Triassic sill exposed along the northeastern portion of the massif, (2) the northplunging antiformal structure of the Kongur Shan gneiss dome [Brunel et al., 1994; Robinson et al., 2004], and (3) the interpretation that the Ghez fault has been rotated from a sub-horizontal pre-extensional orientation [Robinson et al., 2007, 2010]. [39] Upper amphibolite facies schists along the western flank of the Muztaghata massif (terrane Vc, Figures 3 and 8) [Schwab et al., 2004; Robinson et al., 2007] yield an Ordovician maximum depositional age, consistent with a maximum depositional age of 480 Ma from the same unit further south from Yang et al. [2010] (Figure 8). An important aspect is that the detrital age signature is distinct from the Kunlun and Karakul-Mazar terranes with a strong Late Proterozoic-Early Paleozoic peak from 700–450 Ma (Figures 5g and 9b). This peak is similar to the Qiangtang terrane of northern Tibet which has a similar Late ProterozoicEarly Paleozoic peak from 700–500 Ma [Gehrels et al., 2011; Pullen et al., 2011]. Further, while the peak from 450 to 500 Ma is not prevalent in Qiangtang terrane detrital ages, it overlaps with the age of Cambrian-Ordovician crystalline basement beneath the southern Qiangtang terrane [Pullen et al., 2011]. We interpret the high grade metasediments of terrane Vc to be correlative to the Qiangtang terrane of Tibet, and represent structurally deep portions of the Central Pamir terrane. These results are consistent with the interpretation that the Central Pamir rifted off the larger Southern Pamir-Karakoram terrane to form the Rushan Ocean in the Permian [Burtman, 2010]. Further, it shows the contact between the schists and Triassic orthogneisses is a tectonic in nature, and accommodated emplacement of the Central Pamir terrane over the Karakul-Mazar terrane. [40] At the southern end of the Tashkorgan valley, Permian slates yield ɛNd(0)values of 27 (Figures 2 and 7), significantly lower than other analyses from the Pamir from this study, Schwab et al. [2004], and Carboniferous shales from the western Qiangtang terrane (which have minimum ɛNd(0)values of 18 [Zhang et al., 2007b]). However, as they are clearly not part of the Karakul-Mazar terrane, we interpret the southern end of the Tashkorgan valley to be part of Central or Southern Pamir to the west. Thus, the southern exposure of the Torbashi thrust fault places Karakul-Mazar terrane above the Central or Southern Pamir terrane (Figure 2) [Robinson et al., 2007]. 5.2. Tectonic Evolution of the Northeastern Pamir [41] Our reassessment of the age and terrane affiliation of regional units provide new insights on the Mesozoic through Recent tectonic evolution of the northeastern Pamir which build upon the framework established by previous studies [Robinson et al., 2004, 2007].

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Figure 8. Geologic map of the northeast Pamir along the Kongur Shan extensional system showing revised age assignments for the various structural panels (from Figure 3), regional tectonic terrane correlations, and structural boundaries based on new zircon and Nd and Sr isotopic data. KSES: Kongur Shan Extensional System. Letters: a, Robinson et al. [2004]; b, Bershaw et al. [2012]; c, Yang et al. [2010], and d, Zhang et al. [2007a].

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Figure 9. U/Pb cumulative age probability plots comparing results from this study to detrital populations of terranes in northern Tibet [Gehrels et al., 2011]. (a) Detrital zircon populations from Triassic KarakulMazar terrane samples from this study and the Songpan Ganzi terrane. (b) Detrital populations from sample 8-31-03-1 and the northern and southern Qiangtang terrane. 5.2.1. Permian-Triassic [42] During the Permian to Triassic the Karakul-Mazar terrane developed as a broad accretionary prism during northward subduction of the Paleo-Tethys ocean into which arc magmatism migrated resulting in regional high grade metamorphism (Figure 11a) [Youngun and Hsu, 1994; Xiao et al., 2002a, 2002b; Robinson et al., 2004]. Our detrital zircon results show the accretionary complex received a strong component of material from the Paleozoic Kunlun terranes, although ɛNd(0) values suggest arc material may have dominated sediment input. Current map distances and structural relationships suggest a minimum terrane width of 100 km. 5.2.2. Jurassic [43] During the Early Jurassic, collision between the Central Pamir and Karakul-Mazar terranes resulted in closure of the Paleo-Tethys Ocean (Figure 11b). While Triassic magmatism is not documented in the Central Pamir, southward subduction beneath the Central Pamir must have initiated at some point (immediately prior to final closure?) as field relations clearly document subduction of the KarakulMazar terrane beneath the Central Pamir. The timing of collision and subduction is documented by Early Jurassic metamorphism of both the underthrust Karakul-Mazar terrane as well as the overriding Central Pamir terrane [Yang et al., 2010; this study]. This is similar to the tectonic evolution of northern Tibet where underplating of Triassic mélange beneath the Qiangtang terrane occurred along the Jinsha suture to the north during closure of the Paleo-Tethys Ocean [Kapp et al., 2000, 2003; Pullen et al., 2008, 2011]. However, we see no evidence for the extensive underplating of flysch that occurred throughout the Triassic in northern Tibet [Pullen et al., 2008].

[44] Early Jurassic closure of the Paleo-Tethys and underthrusting of the Karakul-Mazar terrane appears to have been roughly coeval with closure of the small ocean basin between the Central and Southern Pamir, the RushanShuanghu Basin, along the south dipping Rushan-Pshart suture [Schwab et al., 2004]. As both the Central and Southern Pamir are correlative to the Qiangtang terrane based on detrital zircon ages (this study) and low displacement on the northern Karakoram fault [Robinson, 2009], this suture zone likely terminates to the east as has been previously suggested [Burtman, 2010].

Figure 10. Field photo showing Triassic interbedded conglomerate and sandstone in terrane IVa. Conglomerate clasts consist of sandstone and limestone, the former of which is interpreted to be Devonian flysch exposed east of Kongur Shan.

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Figure 11. Model for the Mesozoic tectonic evolution of the Pamir. (a) Triassic: Northward subduction of the Paleo-Tethys ocean beneath the southern margin of Asia and development of a wide accretionary complex and volcanic arc (the Karakul-Mazar terrane), with possible Late Triassic initiation of southdirected subduction beneath the Central Pamir. (b) Early Jurassic: During final closure of the PaleoTethys, the Karakul-Mazar terrane is subducted beneath the Central Pamir resulting in metamorphism in both the subducting terrane and the overriding plate. To the south, south-directed subduction beneath the Southern Pamir/Karakoram terrane leads to the closure of the small Rushan Ocean. (c and d) Cretaceous: Subduction beneath the southern margin of the Southern Pamir/Karakoram results in development of an arc system to the south coeval with north directed thrusting and crustal thickening within the Karakul-Mazar terrane along the Shala Tala thrust and south directed thrusting of the Karakul-Mazar terrane over the Central Pamir along the Torbashi thrust. 5.2.3. Cretaceous [45] During the Cretaceous, shortening in the Northern Pamir resulted in burial and prograde metamorphism in the Karakul-Mazar terrane (terrane III [Robinson et al., 2004]), and north-directed thrusting of the Karakul-Mazar terrane (terranes I, II and III) over the Kunlun terranes along the Shala Tala fault (Figures 11c and 11d). Based on current map relations, Cretaceous thrusting resulted in the KarakulMazar terrane largely overriding the Kunlun terranes along the northern edge of the Pamir, nearly to the current trace of the Main Pamir Thrust in places, resulting in the decrease in Kunlun terrane map width (Figure 1c). Coeval development of Cretaceous non-marine deposits in the Tarim basin along the northern margin of the Pamir with detrital zircon age populations broadly similar to the Karakul-Mazar terrane [Sobel, 1999; Bershaw et al., 2012] indicate they were sourced from these thrust sheets. [46] To the south, the west-southwest directed Torbashi thrust fault places Karakul-Mazar terrane over the Central or

South Pamir terrane, similar to the structural relationship along the Tanymas fault further west (Figure 2) [Burtman and Molnar, 1993]. Based on the interpreted Cretaceous age of the Torbashi thrust [Robinson et al., 2007], south directed emplacement of the Karakul-Mazar terrane over the Central Pamir was likely coeval with Cretaceous shortening to the north (Figures 11c and 11d). [47] Widespread Cretaceous shortening in the northern Pamir overlaps with the development of an extensive Cretaceous magmatic arc within the Southern Pamir-Karakoram terrane and regional metamorphism within the Karakoram terrane [Debon et al., 1987; Searle et al., 1990; Crawford and Searle, 1992; Hildebrand et al., 2001; Schwab et al., 2004]. We suggest that crustal shortening and thickening in the northern Pamir was related to retroarc deformation north of this Cretaceous arc. Finally, while arc magmatism and crustal shortening in the Pamir is broadly coeval with magmatism and shortening within the Lhasa and southern Qiangtang terranes of Tibet [e.g., DeCelles et al., 2007;

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Figure 12. Interpreted crustal structure beneath the Karakoram and Pamir along 74 N showing complex interleaving of terranes. RPZ: Rushan-Pshart Zone; CPGD: Central Pamir Gneiss Domes; TF: Tanymas Fault; GF: Ghez Fault; STT: Shala-Tala Fault; MPT: Main Pamir Thrust. Kapp et al., 2007; Leier et al., 2007], magmatism, deformation, and crustal thickening at the western end of the orogen appear to be displaced further northward, involving Qiangtang and Songpan-Ganzi equivalent terranes. 5.2.4. Cenozoic [48] Robinson et al. [2007] proposed a model in which the Kongur Shan and Muztaghata massifs were part of the footwall of a regional sub-horizontal decollement which accommodated north-directed crustal underthrusting and crustal thickening. This model predicts: 1) Significant northward translation (i.e., 100 km) of the footwall (Kongur Shan and Muztaghata gneiss domes), 2) North-directed underthrusting at the northern end of the exposed Cenozoic metamorphic terrane, and 3) Prograde metamorphism generally restricted the footwall of the decollement. [49] Our results show the first prediction is unlikely, as the Sares-Muztaghata dome consists of Central Pamir basement structurally above Karakul-Mazar terrane with no evidence for significant horizontal displacement prior to exhumation in the Miocene. In regards to the latter two predictions, detrital zircon results [Bershaw et al., 2012; this study] clearly document underthrusting of the Karakul-Mazar terrane beneath the Kunlun terranes (Figure 8). However, documented Late Miocene prograde metamorphism at Kongur Shan is located in the hanging wall of the thrust fault rather than just the footwall. Thus, while north-directed underthrusting at Kongur Shan appears to have occurred during the Cenozoic, the extent and relationship between underthrusting and Late Miocene metamorphism remains unclear. 5.3. Crustal Architecture of the Pamir [50] The results from this study point to extensive interleaving of tectonic terranes within the Pamir as a result of protracted tectonism during the Mesozoic and Cenozoic

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(Figure 12). This includes (1) subduction of the KarakulMazar terrane beneath the Central Pamir during Jurassic closure of the Paleo-Tethys, (2) Cretaceous thrusting of the Karakul-Mazar terrane over the Kunlun terranes to the north (Shala Tala thrust) and likely over the Central and Southern Pamir to the south (Torbashi and Tanymas thrusts), and (3) Miocene underthrusting of the Karakul-Mazar terrane beneath the Kunlun (Ghez fault). Thus, the Karakul-Mazar terrane has been emplaced both structurally above and below the terranes to its north and south (Figure 12). Further, lower crustal xenoliths in the Central Pamir are interpreted to be sourced from tectonically underthrust portions of the Southern Pamir-Karakoram terrane indicating Cenozoic interleaving in the deep crust [Ducea et al., 2003; Hacker et al., 2005]. [51] While similar crustal interleaving is interpreted within the northern Tibetan Plateau, several differences point to important along-strike variations in the tectonic evolution and present tectonic architecture of the Himalayan-Tibetan orogen. First, while the Qiangtang terrane was underplated by Triassic mélange (Songpan-Ganzi terrane) from the north throughout the Triassic prior to final closure of the PaleoTethys [Kapp et al., 2000, 2003; Pullen et al., 2008], the extent and timing of underplating of the Karakul-Mazar terrane appears to be much more limited in the Pamir. Second, unlike the Pamir, extensive tectonic emplacement of the Songpan-Ganzi structurally above the Qiangtang and Eastern Kunlun terranes does not appear to have occurred in Tibet. Further, as mentioned above, extensive Cretaceous shortening is shifted to more northerly terranes in the Pamir than in Tibet, indicating significant along-strike differences in the Mesozoic tectonic evolution of the Tibetan orogen.

6. Conclusions [52] Our results from detrital zircon analyses and Nd and Sr isotopic analyses of metamorphic rocks along the northeastern Pamir, integrated with results from previous studies, document the following relationships: [53] (1) The northern Pamir, including quartzofeldspathic gneisses in the core of the Muztaghata massif, is dominated by Triassic igneous and metasedimentary lithologies rather than Proterozoic to Early Paleozoic lithologies as have been previously interpreted. [54] (2) Our new age assignments, overlap of isotopic values between metasediments and Triassic granites, and similarities to the detrital age signature of the SongpanGanzi terrane of northern Tibet [Gehrels et al., 2011] show that much of the eastern Northern Pamir are part of the Late Permian-Triassic Karakul-Mazar arc-accretionary complex terrane as suggested by Schwab et al. [2004]. [55] (3) High grade metasedimentary rocks in the SaresMuztaghata gneiss dome which lie structurally above Triassic orthogneisses yield a Late Ordovician maximum depositional age and a distinct detrital age signature similar to the northern Qiangtang terrane of northern Tibet [Gehrels et al., 2011]. These are interpreted to be basement of the Central Pamir terrane and are correlative to the Qiangtang terrane. This requires the contact between the metasediments and Triassic orthogneisses to be a terrane boundary juxtaposing the Central Pamir terrane above the Karakul-Mazar terrane.

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[56] (4) Ordovician and Late Triassic schists along the Muztaghata massif underwent Early Jurassic metamorphism interpreted to date the timing of underthrusting of the Karakul-Mazar terrane beneath the Central Pamir during final closure of the Paleo-Tethys Ocean. [57] (5) The current crustal architecture of the Pamir consists of extensive interleaving of various tectonic terranes which has occurred during protracted Mesozoic to Cenozoic tectonism with emplacement of the Karakul Mazar terrane both above and below the Kunlun and Central Pamir terranes to the north and south respectively. [58] Acknowledgments. This research was supported by a grant from the U.S. National Science Foundation Tectonics Program (EAR 0911589), a University of Houston New Faculty Grant, and a University of Houston Small Grant to Robinson. We thank John Bershaw for enlightening discussions regarding the geology of the Kongur Shan massif and Alex Pullen and Barry Shaulis for assistance in collecting the zircon data. We also thank Ed Sobel and two anonymous reviewers whose comments helped improve the clarity and presentation of the ideas in this paper.

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