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Feb 26, 2014 - Basin; QT, Qiangtang terrane; SB, Sichuan Basin; SGT, Songpan-Ganze terrane; YA, ... tion of the Qaidam Basin, the Sichuan Basin (west-.
Article Volume 15, Number 2 26 February 2014 doi: 10.1002/2013GC004951 ISSN: 1525-2027

Postorogenic rigid behavior of the eastern Songpan-Ganze terrane: Insights from low-temperature thermochronology and implications for intracontinental deformation in central Asia Yuntao Tian School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia ([email protected]) Now at: Department of Earth Sciences, University College London, London WC1E 6BT, UK

Barry P. Kohn School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia

Shengbiao Hu State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China ([email protected])

Andrew J. W. Gleadow School of Earth Sciences, University of Melbourne, Melbourne, Victoria, Australia

[1] The Songpan-Ganze terrane (SGT), formed by Early Mesozoic closure of the Paleo-Tethys Ocean, occupies a large area of the central-eastern Tibetan Plateau. Late Mesozoic and Cenozoic strike-slip deformation has been identified in the surrounding terranes and faults (e.g., western Qinling, Altyn Tagh, and Kunlun faults); however, the coeval evolution of the SGT has not been well explored. We report apatite fission track and apatite and zircon (U-Th)/He data from a >7 km deep borehole and outcrop samples covering an area of >150 3 150 km in the eastern SGT. Thermal history modeling suggests a distinct phase of Late Jurassic-Early Cretaceous (150–100 Ma) cooling, followed by a prolonged stage of slow cooling, for all samples despite of their differences in depositional age (Mid-Late Triassic time) and locality within a large area. The ubiquitous Late Jurassic-Early Cretaceous cooling implies little differential deformation in the eastern SGT and is best explained by regional rock uplift resulting from the transpressional strain field created by the contemporaneous Lhasa-Qiangtang collision to the south. Projecting the contemporaneous deformation surrounding the SGT onto an Early Cretaceous paleogeographic terrane reconstruction results in a new tectonic model. The model relates the LhasaQiangtang collision to tens to hundreds of kilometers of shearing along the Altyn Tagh and Kunlun faults, which transferred strain into central Asia (e.g., Qinling-Dabie orogen). Results of this study suggest a rigid behavior for the eastern SGT and highlight the important role of crustal strength discontinuities in accommodating and transferring crustal deformation. Components: 12,068 words, 9 figures, 4 tables. Keywords: postorogenic evolution; low-temperature thermochronology; Tibetan Plateau; Songpan-Ganze terrane; LhasaQiangtang collision.

© 2013. American Geophysical Union. All Rights Reserved.

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Index Terms: 8110 Continental tectonics: general: Tectonophysics; 8111 Continental tectonics: strike-slip and transform: Tectonophysics; 8175 Tectonics and landscape evolution: Tectonophysics; 0905 Continental structures: Exploration Geophysics; 9320 Asia: Geographic Location. Received 19 July 2013; Revised 12 December 2013; Accepted 17 December 2013; Published 26 February 2014. Tian Y., B. P. Kohn, S. Hu, and A. J. W. Gleadow (2014), Postorogenic rigid behavior of the eastern Songpan-Ganze terrane: Insights from low-temperature thermochronology and implications for intracontinental deformation in central Asia, Geochem. Geophys. Geosyst., 15, 453–474, doi:10.1002/2013GC004951.

1. Introduction [2] The Tibetan Plateau (TP) forms a natural laboratory for the study of continental tectonics and consists of several terranes or blocks, e.g., Lhasa terrane, Qiangtang terrane, and Songpan-Ganze terrane (SGT) (Figure 1a), which were accreted onto the Asian continent in Mesozoic and Cenozoic times [e.g., Chang et al., 1986; Yin and Harrison, 2000]. The most recent two accretion events are the Jurassic-Early Cretaceous LhasaQiangtang collision along the Bangong suture [e.g., Kapp et al., 2003; K. Zhang et al., 2004] and the Early Cenozoic Indo-Asia collision along the Indus-Yalu suture [e.g., Molnar and Tapponnier, 1975; Ding et al., 2005]. Previous studies suggested that the tectonic influences of these two collisions may have extended for hundreds to thousands of kilometers into the interior of the central Asia continent [e.g., Vincent and Allen, 1999; Ratschbacher et al., 2003; Hu et al., 2006; Vassallo et al., 2007]. However, the possible mechanisms for this transfer remain elusive. [3] One of the most prominent features of the Mesozoic-Cenozoic evolution of the Asian continent is the development of large strike-slip fault zones. For example, the Altyn Tagh and Kunlun faults in the northwestern and northern TP have accommodated large amounts of Mesozoic and Cenozoic sinistral shearing (Figures 1 and 2) [e.g., Arnaud et al., 2003; Molnar and Dayem, 2010]. Therefore, one may speculate on the role these major fault zones played in transferring the deformation associated with the Lhasa-Qiangtang and Indo-Asia collisions into central Asia. One requirement for this mechanism is that the intervening terranes, e.g., the SGT, must have acted as a rigid block, without accommodating significant crustal shortening, so as to render the bounding faults (e.g., Altyn Tagh and Kunlun faults) as relatively weak structures in order to transfer the deformation [e.g., Dayem et al., 2009a, 2009b].

[4] To validate the mechanism introduced above, this study focuses on the postorogenic evolution of the eastern SGT. We present the first batch of lowtemperature thermochronological data derived from apatite fission track (AFT), and apatite and zircon (U-Th)/He (AHe and ZHe) analyses of both surface and deep (>7 km) borehole samples for the interior part of the SGT. These data are interpreted by inverse thermal history modeling, which shows a common post-Late Jurassic cooling history, implying a coherent evolution for the area, without any detectable differential deformation between sites. Considering the regional tectonic framework, our results suggest that the interior of the eastern SGT has been acting as a rigid block during Late Jurassic-Cenozoic time and support a new model highlighting the role of strike-slip faulting along crustal strength discontinuities in transforming the effects of Lhasa-Qiangtang and Indo-Asia collisions into northern areas in central Asia continent.

2. Geographic and Topographic Setting [5] The SGT occupies an area of 1–2 3 106 km2 in the central-eastern part of the TP. The study area is located in the interior of the eastern SGT and surrounded to the north by the western Qinling and to the east by the Min Shan and Longmen Shan (LMS; Figure 1). The SGT terrane continues to the south and west of the study area, but with different geomorphic features (see below). [6] The topography and river channel geometry of the study area differ markedly from that in the surrounding areas (Figure 1). The mean elevation of the study area is 3200–3600 m, with few peaks exceeding 3700 m, characterizing a landscape with topographic relief less than 500 m (Figures 1 and 3). However, the surrounding areas, to the west, north, and south, are characterized by ranges with higher peaks (as shown by maximum elevations), deeper valleys (as shown by the minimum 454

102°E

(b) Ye

104°E

36°N

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A’ Linxia

Dingxi

Wes ter Zeku

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ling f

ault

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Kunlu

n fau

lt

Western Qinling 34°N

Zoige

Eastern Tibetan Plateau

Jiuzhi

fault

Longnan

75°E

95°E

35°N

ATF QB KSZ (KF)

Figs. 1b, 2

JSZ SGT

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Elevation (m): 200 - 850 850 - 1450 1450 - 1950 1950 - 2400 2400 - 2900 2900 - 3350 3350 - 3700 3700 - 4050 4050 - 4450 4450 - 6250

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Topographic relief 1

100 200 300 400 Distance along the A-A’ swath (km)

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Topographic elevation & relief (km)

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Strike-slip faults Thrusts Outcrop sample Borehole Towns

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Min Shan

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100 200 300 400 Distance along the B-B’ swath (km)

Figure 1. Morphology of the eastern-northern TP. (a) Digital elevation model image of the TP and surrounding regions, showing the major sutures and fault zones, and the location of the study area, marked by the black square. Also plotted are localities of the Hongcan-1 borehole and outcrop samples. (b) Topography Shuttle Radar Topography Mission (SRTM), major rivers, and major active structures in the eastern-northern TP. Active faults shown are modified after Tapponnier and Molnar [1977] and Taylor and Yin [2009]. (c, d) Topographic swathes along transects A-A0 and B-B0 are shown in Figure 1b. Maximum, mean, and minimum elevations and topographic relief are calculated at a 10 km circle window. Note the relatively lowertopographic relief of the study area, compared with that in surrounding areas. Abbreviations: ATF, Altyn Tagh fault; BSZ, Bangong suture zone; ISZ, Indus suture zone; JSZ, Jinsha suture zone; KF, Kunlun fault; KSZ, Kunlun suture zone; LCF, Longmu Co fault; LSZ, Litang suture zone; LT, Lhasa terrane; QB, Qaidam Basin; QT, Qiangtang terrane; SB, Sichuan Basin; SGT, Songpan-Ganze terrane; YA, Yidun Arc.

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100°E 102°E

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104°E 36°N

Linxia

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Zeku

Lintan

34°N

Zoige Jiuzhi

Legend: Pliocene-Quaternary

Longnan

Fig. 3

Neogene red beds

Hongyuan

Paleogene red beds Cretaceous red beds

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Jurassic red beds Triassic Songpan-Ganze flysch

32°N

Mesozoic Sichuan Basin Paleozoic marine sediments Cenozoic plutons Mesozoic plutons Paleozoic plutons Precambrian basements Strike-slip faults Thrusts Outcrop sample Borehole Towns

Xi

an

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fa

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Figure 2. Generalized geology map of the eastern-northern TP, modified after GBGMR [1989], Qinghai Bureau of Geology and Mineral Resources [1991], and SBGMR [1991]. Also plotted are localities of the Hongcan-1 borehole and outcrop samples. Note the widespread Cenozoic deposits in the northern TP, and the lack of post-Triassic deposits in the SGT, except for local Pliocene-Quaternary sediments. For structural nomenclatures see Figure 1b.

elevations), and thus higher topographic relief (Figures 1c and 1d). The first big bend of the Yellow River, changes in flow direction from NEE to NWW, occurs within the study area (Figures 1a and 2). River channels meander within the study area, whereas those in surrounding areas incise deeply.

3. Geological Setting [7] Tectonically, the SGT is situated at the junction of the Qaidam Basin, the Sichuan Basin (western Yangtze block), and the Qiangtang block and is bounded by the Late Paleozoic-Triassic Kunlun and western Qinling sutures to the north, by the

Late Triassic intracontinental Longmen Shan foldthrust belt to the east, and by the Triassic JinshaLitang suture to the south (Figure 1b). Detailed review of the evolution of the SGT can be sourced from Chang [2000] and Roger et al. [2011]. Below, briefly described are the geological features of the SGT, the western Qinling (which now forms the northeastern TP), and the Longmen Shan (eastern TP margin), as well as the Hongcan-1 borehole, from which several samples were collected.

3.1. SGT [8] More than 80% of the SGT is covered by thick Triassic flysch, which was mainly sourced from 456

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Topography legend (m)

River channels Sample location Hongcan-1 borehole

2400

3300

3700

Example:

12.9 ± 0.1 (102) HS22 123 ± 8 110 ± 8

0.3

Frequency

MTL ± 1se (n) Sample No. AFT age AHe age Length distribution

4500

0.2 0.1 0 0

4

8

12

16

Length (μm) Hongcan-1 borehole

12.8 ± 0.2 (90) HS28 106 ± 5 0.2 45 ± 4

12.1 ± 0.2 (97) HS23 113 ± 7

0.3

Frequency

Frequency

0.3

0.1 0

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0

4

8

12

16

0

12.8 ± 0.2 (114) HS27 123 ± 3 77 ± 8

Frequency

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4

8

12

16

Length (μm)

Length (μm)

12.8 ± 0.1 (104) HS25 120 ± 4 64 ± 6

13.1± 0.1 (148) HS26 112 ± 4

12.3 ± 0.2 (90) HS20 113 ± 7

12.4 ± 0.2 (103) HS21 112 ± 5

12.2 ± 0.2 (94) HS24 118 ± 7

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4

8

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16

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12

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Length (μm)

0

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Length (μm)

Figure 3. 3-D topography, geology, and low-temperature thermochronology data (AFT age, length, and AHe age) of surface samples. Also plotted on the 3-D topography and geology maps are major rivers. Each length frequency plot compiles the nonprojected lengths for each sample with Dpar ranging between 1.5 and 2.0 mm (the low-Dpar group). AFT ages compiled are central ages, whereas AHe ages are weighted means.

the Qinling-Dabie orogen to the northeast and terranes to the north [e.g., Enkelmann et al., 2007; Ding et al., 2013]. By Late Triassic time, the Songpan-Ganze basin had shallowed, as documented by coeval coal-bearing clastic deposits [SBGMR, 1991; Chang, 2000]. [9] In response to strong folding of the flysch during the Late Triassic closure of the Paleo-Tethys Ocean, the basin evolved into a fold belt [e.g., Xu et al., 1992; Roger et al., 2011]. The phase of shortening may have lasted to Early Jurassic time, as suggested by (1) Barrovian metamorphism in the time interval of 204–190 Ma [Huang et al., 2003], (2) extensive synorogenic (220–190 Ma) and postorgenic (188–153 Ma) granitic magmatism throughout the Songpan-Garze terrane [Roger et al., 2004], (3) the youngest Ar/Ar ages (195 Ma) of deformed rocks along the Ganze-Litang suture [Yang et al., 2012], (4) angular unconformable contacts between Triassic flysch and a few

sites of unmetamorphosed Lower Jurassic coalbearing deposits [SBGMR, 1991; Zhou and Graham, 1996; Chang, 2000], and (5) an unconformity between recently identified Jurassic marine silciclasts and Triassic flysch [Ding et al., 2013]. Except for Pliocene-Quaternary glacial and fluvial sediments, Jurassic-Cenozoic deposits are regionally absent from the terrane (Figures 1 and 3). [10] The SGT was intruded by Late TriassicJurassic granitoids [see reviews by Roger et al., 2011]. Metamorphic grade varies across the SGT. In general, the metamorphic overprint is relatively strong along the terrane margins where mudstones were metamorphosed to phyllite but weak within the interior [Chang, 2000; Y. Tang et al., 2012].

3.2. The Western Qinling (Northeastern TP) [11] The western Qinling was formed by the Late Paleozoic-Early Mesozoic closure of a branch of 457

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the Paleo-Tethys Ocean, and the subsequent collision between the Yangtze and Sino-Korean plates [e.g., Meng and Zhang, 2000; Ratschbacher et al., 2003]. Several Cretaceous-Cenozoic basins (e.g., Linxia, Xunhua, Lintan, Guide, and Gonghe basins), which received several kilometers of detritus, developed in the western Qinling [Gansu Bureau of Geology and Mineral Resources (GBGMR), 1989]. Numerous studies on basin structure, magnetostratigraphy, provenance, etc., suggest that (1) Cretaceous basins at different sites were formed by different kinematics, e.g., extension, pull-apart, or contraction [e.g., Horton et al., 2004; Craddock et al., 2012] and (2) Cenozoic basins were formed in response to crustal transpressional deformation [e.g., Fang et al., 2003; Craddock et al., 2011; Lease et al., 2012]. Thermochronological studies on the ranges, bounding the Cenozoic basins, suggest that crustal shortening in the northeastern TP may have initiated in Early Eocene time and lasted to at least the Miocene [Clark et al., 2010; Zheng et al., 2010; Duvall et al., 2011; Lease et al., 2011, 2012].

3.3. The Longmen Shan (Eastern TP Margin) [12] The Longmen Shan was formed by LateCenozoic rejuvenation of a Mesozoic intracontinental fold-thrust belt [e.g., Arne et al., 1997; Kirby et al., 2002; Godard et al., 2009; Wang et al., 2012; Cook et al., 2013; Tian et al., 2013]. Cenozoic crustal shortening along the Longmen Shan has been loosely constrained and highly debated. Earlier studies, based on the slow shortening rate (1–3 mm/yr) shown by interseismic GPS data [e.g., P. Zhang et al., 2004] and the observation of the lack of a frontal Cenozoic foredeep, speculated that little Cenozoic shortening in the Longmen Shan and its foreland had occurred [Burchfiel et al., 1995]. However, high coseismic (e.g., the M. 7.9 12 May 2008 Wenchuan earthquake) shortening and uplift at scales of hundreds to thousands of millimeters are suggested by field measurements [e.g., Liu-Zeng et al., 2009; Xu et al., 2009] and the GPS velocity field [Zhang et al., 2010]. Further, without separating the Late Cenozoic component of shortening from that of earlier phases, the total shortening shows a positive relationship with topography in both the Longmen Shan and southwestern Sichuan Basin, implying the importance of upper crustal shortening in building up the high topography in the LMS [Hubbard and Shaw, 2009]. As quantified by Hubbard et al. [2010], more than 45 km of shortening has occurred over a

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distance of 50 km across the LMS, suggesting a high average shortening percentage (>95%). Structural and thermochornological studies suggest that the Longmen Shan is characterized by several outof-sequence thrusts that merge into a detachment fault at a depth of 20–30 km [Tian et al., 2013]. Thrusting commenced at least since Late Miocene time [Tian et al., 2013], or even earlier since Oligocene time as speculated by Wang et al. [2012]. [13] Between the Longmen Shan and the interior of the SGT (the study area), several oblique thrust faults have developed (Figures 1b and 2). The Longriba fault to the east of the study area (Figure 1b) is suggested to be a dextral strike-slip fault with a component of eastward thrusting [Ren et al., 2013]. Movement along the Minjiang fault is dominated by reverse faulting with dextral slipping, whereas that along the Huya fault, to the east of the Minjiang fault, is dominated by reverse faulting with sinistral slip, as suggested by surface geology and focal mechanism determinations of historical earthquakes [Chen et al., 1994].

3.4. The Hongcan-1 Borehole [14] The Hongcan-1 borehole is the first deep borehole that has been drilled in the eastern TP [Ma et al., 2006; Liu et al., 2013]. It penetrates the upper middle Triassic flysch strata down to >7 km beneath the surface. Stratigraphic and structural studies on the surface geology and borehole records suggest that the flysch has been strongly and repeatedly folded and faulted (Figure 4a). As mentioned previously, the folding and shortening of the Triassic flysch occurred in Late TriassicEarly Jurassic time. [15] Down borehole, vitrinite reflectance (Ro) increases from 2% to 6% (Figure 4b), and illite crystallinity decreases from 0.2 (D2h) to 0.8 (D2h) (Figure 4c) [Zhao, 2008; Y. Tang et al., 2012]. These paleotemperature indices indicate that the Triassic flysch may have experienced temperatures >150–180 C [e.g., Burnham and Sweeney, 1989; Kisch, 1990], which would be high enough to reset the AFT, AHe, and ZHe thermochronology systems [Farley, 2000; Gleadow et al., 2002; Reiners et al., 2004; Guenthner et al., 2013].

4. Sampling, Methods, and Results 4.1. Sampling and Methods [16] Using 1:200 000 geological maps, fieldwork was carried out in the area around the Hongcan-1 458

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Hongcan-1 Borehole

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Illite Crystalliity ( Δ2θ)

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IC sandstone IC mudstone Vitrinite Reflectance

ZHe age AHe age WTM age 2

3

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7 6

Vitrinite Reflectance (%)

Figure 4. Surface and Hongcan-1 borehole geology, and borehole thermochronology and paleotemperature indices (Ro and IC). (left) Surface and borehole geology suggesting that the Triassic flysch has been extensively folded and faulted. The subsurface structures are modified from Liu et al. [2013]. These structures were formed in Late Triassic-Early Jurassic time (see text for details). See Table 1 for explanation of strata. (middle) A compilation of AHe and ZHe ages from the Hongcan-1 borehole. WTM age, weighted mean age. (right) Down borehole data; the Ro increases from 2.5% at the near-surface depth to 5% at a depth of 3–4.5 km, where the invariant values indicate Ro may have become saturated. The IC decreases from 0.4 to 0.8 at the near-surface depth to 0.25 at a depth of 5–6 km, where invariant values indicate that the IC may also have also become saturated. Ro and IC data sourced from Zhao [2008] and Y. Tang et al. [2012], respectively.

Table 1. Sample Information Longitude, Latitude, and Elevation/Depth Sample Number Outcrop samples HS20 HS21 HS22 HS23 HS24 HS25 HS26 HS27 HS28 Hongcan-1 borehole samples HC1-9 HC1-10 HC1-1 HC1-2 HC1-5 HC1-6 HC1-8

Lithology and Formation

( E)

( N)

(m)

Sandstone-Mid Triassic Zagashan formation (T2zg) Sandstone-Mid Triassic Zagashan formation (T2zg) Sandstone-Late Triassic Zagunao formation (T3z) Sandstone-Mid Triassic Zagashan formation (T2zg) Sandstone-Latest Triassic Zhuwo formation (T3zh) Sandstone-Late Triassic Zagunao formation (T3z) Sandstone-Latest Triassic Zhuwo formation (T3zh) Sandstone-Latest Triassic Zhuwo formation (T3zh) Sandstone-Late Triassic Zagunao formation (T3z)

103.24 103.08 102.95 102.96 102.95 102.88 102.71 102.65 102.56

33.26 33.59 33.82 33.73 33.64 33.57 33.52 33.46 33.21

3634 3497 3478 3466 3540 3516 3501 3505 3465

Sandstone-Latest Triassic Zhuwo formation (T3zh) Sandstone-Late Triassic Zagunao formation (T3z) Sandstone-Late Triassic Zagunao formation (T3z) Sandstone-Late Triassic Zagunao formation (T3z) Sandstone-Mid Triassic Zagashan formation (T2zg) Sandstone-Mid Triassic Zagashan formation (T2zg) Sandstone-Mid Triassic Zagashan formation (T2zg)

2101 2339 2468.6 21951.2 23844.7 24347.9 27012.6

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4.2. Results

(a)

4.2.1. Dpar Dependence of AFT Data AFT age (Ma)

200

100

HS24 HS25 HS26

0 1

1.5

2

2.5

3

3.5

Dpar (μm) 20

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Length (μm)

15

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HS25 HS26 5 1

1.5

2

2.5

3

3.5

Dpar (μm)

Figure 5. Plots of (a) AFT age and (b) track length versus Dpar. (a) Single grain ages show a broad positive relationship with Dpar, suggesting the influence of the differential annealing properties of different grains. (b) The gross single lengths show no clear relationship with Dpar, but the mean track length of the low-Dpar population is slightly shorter than that of the high-Dpar population (Table 2).

borehole to study the surface deformation and to collect coarser sandstone samples from different stratigraphic formations for low-temperature thermochronology analyses (Figures 1, 3, and 4). Apatite and zircon concentrates derived using standard crushing, sieving, electromagnetic, and heavy liquid mineral separation techniques were prepared for fission track and (U-Th)/He analyses. In total, 9 outcrop and 7 out of 13 borehole samples yielded suitable apatite and zircon grains for fission track and (U-Th)/He analyses. One borehole sample (HC1-10) yielded a few poor apatite grains on which AHe was analyzed. Sample information for the 16 analyzable samples is summarized in Table 1. For details of the AFT, AHe, and ZHe methodology used see supporting information (S1).1

1 Additional supporting information may be found in the online version of this article.

[17] We noticed considerable Dpar variations between the apatite grains from each sandstone sample. To qualify the potential relationship between Dpar and AFT data, Dpar measurements were made on grains on which each length and/or grain age were determined. Results, obtained from 188 dated grains from samples HS24, HS25, and HS26, show that Dpar values vary from 1.5 to 3.2 mm and show a broad and consistent positive relationship with AFT age (Figure 5a). Although no apparent relationship is shown in the AFT length versus Dpar plot (Figure 5b), mean track length of the population with relatively higher Dpar values is longer than those with lower Dpar values, as shown by samples HS25 and HS26 (Table 2). [18] The positive relationship between Dpar and AFT data invites a grouping of the AFT data [Ketcham, 2005]. Here, where a broad Dpar range was found in each sample, the AFT ages and length data are divided into two populations: one (named as low-Dpar group) with Dpar ranging between 1.5 and 2.0 mm and the other (named as high-Dpar group) with Dpar >2.0 mm. Results of AFT analysis of the different Dpar populations are summarized in Table 2. 4.2.2. Outcrop AFT and AHe Data

[19] Outcrop samples yielded pooled AFT ages between 103 and 126 Ma and central ages between 106 and 123 Ma for the low-Dpar population (Table 2). For the high-Dpar population, in samples HS24, HS25, and HS26 pooled AFT ages range between 131 and 155 Ma with corresponding central ages between 136 and 157 Ma. All these AFT ages are significantly younger than the deposition age of their host strata. AFT length distributions of the low-Dpar populations show dominant peaks at 12–14 mm, with minor tails extending to as short as 5 mm (Figure 3). Also noteworthy is that the Dpar values for the largeDpar group of samples HS24, HS25, and HS26 range widely between 2.0 and 3.2 mm (Table 2). As such, it is very likely that neither the pooled nor the central AFT ages of these groups are suitable for providing constraints on the thermal history of the host sample because these mathematically pooled and weighted ages carry signals, related to not only their thermal history but also the differential thermal annealing properties of grains according to their variable Dpar values [Ketcham, 2005]. Therefore, in this study, only the AFT data of the low-Dpar group with limited Dpar variation 460

22 22 21 17 34 16 47 24 45 22 26 27

Number of Grains (n)

1097 825 582 1629 3146 1664 4088 1768 3011 659 1961 1475

Number (n)

0.98 1.02 0.67 1.73 2.08 2.54 2.15 2.08 1.74 1.24 2.10 1.97

Density (106 cm22) 19.41 15.18 9.99 27.89 33.25 34.83 36.68 22.88 26.65 14.79 29.45 32.47

103.8 6 6.7 109.0 6 4.4 126.1 6 7.1 119.2 6 6.7 117.2 6 6.8 154.5 6 9.2 110.6 6 4.0 149.8 6 7.4 110.0 6 3.5 130.9 6 7.8 122.6 6 2.7 102.7 6 4.9

Pooled 238Ub Pooled Agec (ppm) (Ma 6 1SD)

Age Results

12 33 44 59 56 14 44 30 34 39 16 34

p(v2)d (%) 19 8 18 18 29 18 16 17 14 19 0 17

113.0 6 6.6 112.3 6 4.8 123.2 6 8.1 113.6 6 6.9 118.2 6 6.7 157.1 6 8.9 119.8 6 3.9 155.0 6 7.2 112.0 6 3.5 136.2 6 8.5 122.9 6 3.0 105.6 6 5.1

Dispersion Central Agee (%) (Ma 6 1SD) 1.8 1.8 1.4 1.6 1.8 1.5 1.5 1.3 1.8 1.7 1.7

12.8 6 0.1 13.3 6 0.2 13.1 6 0.1 13.3 6 0.3 12.8 6 0.2 12.8 6 0.2

c

b

SD (mm)

12.3 6 0.2 12.4 6 0.2 12.9 6 0.1 12.1 6 0.2 12.2 6 0.2

Mean (mm 6 SE)

13.9 6 0.1 14.3 6 0.2 14.1 6 0.1 14.3 6 0.2 14.0 6 0.1 13.9 6 0.1

13.6 6 0.1 13.6 6 0.1 14.0 6 0.1 13.5 6 0.1 13.5 6 0.1

Mean (mm 6 SE)

Projectedg

Track Length and Dpar Results Nonprojectedf

For sample information, see Table 1. Pooled uranium content of all grains measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Pooled AFT ages of all grains. d p value of v2 for (n 2 1) degrees of freedom [Galbraith, 1981]. e Central age calculated using the RadialPlotter program of Vermeesch [2009]. f Lengths measured after 252Cf irradiation. g c axis projected mean track length after Ketcham et al. [2007].

a

HS26 HS27 HS28

HS20 HS21 HS22 HS23 HS24 HS25

Sample Number

Spontaneous Tracks

Table 2. Apatite Fission Track Resultsa

1.1 1.1 0.9 1.2 1.1 1.1

1.2 1.3 1.0 1.1 1.3

SD (mm)

104 45 148 27 114 90

90 103 102 97 94

Number (n)

1.6–2.0 1.6–2.0 1.6–2.0 1.6–2.0 1.5–2.0 2.0–3.2 1.4–2.0 2.0–3.2 1.4–2.0 2.0–2.8 1.5–2.0 1.6–2.0

Dpar (mm)

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461

1 1 1 1 1 1 1 1 1 1 1 1 1 1 7 6 7

0.9072 0.2295 1.4051 0.2546 0.1211 0.2175 1.9610 0.1440 2.0259 0.2130 0.5508 0.5849 0.1881 0.4656 7.2070 3.0930 3.3210

Number 4 of He Grains (n) (ncc)

0.0053 0.0070 0.0020 0.0018 0.0049 0.0043 0.0053 0.0024 0.0045 0.0050 0.0058 0.0047 0.0040 0.0077 0.0114 0.0068 0.0057

Mass (mg)

11.8 0.8 51.1 12.8 1.6 3.4 14.3 1.4 34.3 1.9 7.4 7.4 1.7 9.9 61.8 57.6 48.3

4.8 6.0 4.6 15.1 7.5 3.1 49.5 10.3 33.3 8.3 11.9 32.0 24.9 8.7 51.3 50.4 87.8

U Th (ppm) (ppm) 117.4 35.6 551.1 125.2 26.6 40.0 552.9 199.7 375.3 154.7 194.0 130.0 182.4 41.1 191.9 260.4 265.3

Sm (ppm) 0.41 7.18 0.09 1.18 4.75 0.92 3.47 7.13 0.97 4.27 1.62 4.30 14.92 0.87 0.8 0.9 1.8

Th/U ratio 56.6 60.0 50.2 35.9 52.6 55.7 58.2 52.6 51.2 61.6 61.0 67.5 45.8 67.8 94.9 126.8 59.8

MWARb (mm) 113.1 119.9 100.4 71.7 105.1 111.3 154.9 137.4 206.0 198.6 155.4 135.0 91.6 165.5 224.7 155.0 136.6

Grain Length (mm) 56.6 60.0 50.2 35.9 52.6 55.7 63.5 57.1 61.5 70.5 65.7 67.5 45.8 72.1 100.1 104.6 62.4

Rsc (mm)

For sample information, see Table 1. b Mass-weighted average radius of multiple-grain aliquots. c Radius of a sphere with the equivalent surface-area-to-volume ratio as cylindrical crystals [Meesters and Dunai, 2002]. d a-Ejection correction [Farley et al., 1996] calculated using mass-weighted average radii. e Weighted means at 95% confidence level calculated using Isoplot V3.25 [Ludwig, 1991]. f Effective uranium content, [eU] 5 [U] 1 0.235 3 [Th] [Flowers et al., 2009]. g Excluded in calculating weighted age.

a

HS22 HS22 HS22 HS25 HS25 HS25g HS25g HS25g HS27 HS27 HS27 HS27 HS28 HS28 HC1-10 HC1-10 HC1-10

Sample Number

Table 3. Results of Apatite (U-Th-Sm)/He Datinga

81.0 87.4 76.5 44.2 43.4 73.4 84.3 83.2 61.4 64.8 56.2 50.8 35.0 32.5 69.8 53.1 68.3

Raw Age (Ma) 0.76 0.75 0.71 0.63 0.73 0.74 0.74 0.69 0.71 0.76 0.76 0.76 0.70 0.79 0.83 0.76 0.73

FTd 106.8 115.9 108.2 70.3 59.7 99.8 113.9 120.6 86.5 85.8 74.4 67.2 49.9 41.4 84.0 70.2 93.0

Corrected Age (Ma) 6.6 7.2 6.7 4.4 3.7 6.2 7.1 7.5 5.4 5.3 4.6 4.2 3.1 2.6 5.2 4.4 5.8

Error (61r)

82.0 6 7.5

44.8 6 3.9

76.7 6 8.4

64.1 6 5.6

110.0 6 7.9

Weighted Mean Agee (Ma 6 2r)

12.9 2.2 52.2 16.3 3.4 4.1 25.9 3.8 42.1 3.9 10.2 14.9 7.6 11.9 73.9 69.4 68.9

[eU]f TIAN ET AL.: POSTOROGENIC SONGPAN-GANZE TERRANE

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Table 4. Single Grain Zircon (U-Th)/He Resultsa Sample Number

U (ppm)

Th (ppm)

Th/ U

HC1-9 HC1-9 HC1-9e HC1-1 HC1-1 HC1-2 HC1-2 HC1-2e HC1-5 HC1-5 HC1-5 HC1-6 HC1-6 HC1-6 HC1-8 HC1-8

191.6 103.5 0.54 452.2 100.0 0.22 619.0 221.5 0.36 246.3 48.2 0.20 150.6 117.0 0.78 384.0 207.5 0.54 217.0 138.5 0.64 1321.1 3541.3 2.68 519.4 193.5 0.37 388.3 108.5 0.28 159.3 109.4 0.69 91.2 63.7 0.70 194.2 101.3 0.52 763.2 846.9 1.11 334.2 227.0 0.68 928.1 112.1 0.12

Length (mm)

Average Width (mm)

Mass (mg)

Mean FTb

Corrected Gas (ncc)

Age (Ma)

Error (1r)

306.8 150.4 159.3 326.9 264.0 162.4 159.3 214.2 203.6 240.2 182.1 200.5 210.4 168.0 174.1 129.8

84.5 101.9 73.7 80.1 99.8 82.8 84.0 88.7 89.4 96.7 87.0 130.9 80.2 91.2 79.0 69.2

0.0085 0.0058 0.0029 0.0083 0.0108 0.0036 0.0044 0.0059 0.0066 0.0079 0.0045 0.0096 0.0048 0.0043 0.0037 0.0019

0.77 0.79 0.72 0.77 0.80 0.74 0.75 0.76 0.78 0.79 0.76 0.81 0.75 0.75 0.74 0.70

32.163 53.603 17.602 43.305 39.601 27.843 18.083 100.952 51.832 38.708 10.404 7.833 9.865 45.818 2.616 2.808

142.8 158.5 74.2 164.2 166.6 146.5 134.3 65.5 114.0 96.8 101.0 62.8 76.6 89.5 15.2 12.4

8.8 9.8 4.6 10.2 10.3 9.1 8.3 4.1 7.1 6.0 6.3 3.9 4.7 5.5 0.9 0.8

Weighted Mean Agec (Ma 6 1r) 150.2 6 6.7 165 6 7.5 140.2 6 6.8

102.8 6 7.4 76.3 6 8.5 13.7 6 4.2

[eU]d 215.9 475.8 671.0 257.6 178.1 432.7 249.6 2153.3 564.9 413.8 185.0 106.2 218.0 962.2 387.6 954.4

a

For sample information, see Table 1. FT is the a-ejection correction after Farley et al. [1996]. Weighted means at 95% confidence level calculated using Isoplot V3.25 [Ludwig, 1991]. d Effective Uranium content, [eU] 5 [U] 1 0.235 3 [Th] [Flowers et al., 2009]. e Excluded in calculating weighted age. b c

(1.5–2.0 mm) are used to interpret the geological evolution of the study area. [20] AHe analyses from outcrop samples yield reproducible single grain ages that are younger than their corresponding AFT ages, except for sample HS25 (Table 3). Among the five single grain ages analyzed for HS25, the oldest two are between 114 and 121 Ma, which is older than the pooled AFT age (110.6 6 4.0 Ma), but similar to the central age (119.8 6 3.9 Ma) of the low-Dpar group of this sample. The youngest two between 60 and 70 Ma are here used to calculate the weighted mean age (64.1 6 5.6 Ma) for this sample. The weighted mean ages of other three outcrop samples range between 44.8 6 3.9 Ma for HS28 and 110.0 6 7.9 Ma for HS22. Further, no relationship is observed between AHe age and effective uranium content, [eU], [U] 1 0.235 3 [Th], which is used as a proxy for a-radiation damage [Flowers et al., 2009]. 4.2.3. Borehole AHe and ZHe Data

[21] Samples collected from the Hongcan-1 borehole yield ZHe ages younger than the depositional age (Mid-Late Triassic) of their strata (Table 4). The single grain ages of each sample are reproducible within broad analytical uncertainties, except for two age anomalies for samples HC1-9 and HC1-2 (Table 4). Two out of the three analyses for HC1-9 yielded reproducible ages between 143 and 159 Ma, whereas a third analysis produced a significantly younger age (74.2 6 4.4 Ma). For HC12, two reproducible single grain ages range

between 134 and 147 Ma, whereas a third yields a significantly younger age (65.5 6 4.1 Ma). These two single grains are characterized by higher [eU] content, indicating that a high radiation damage effect on helium diffusion may have been present in these two grains [Guenthner et al., 2013]. Except for these anomalies, the ages of the uppermost sample (HC1-9) overlaps with the following sample (HC1-1). Further down borehole, ZHe ages decrease to 12–15 Ma of the lowermost sample (HC1-8) at a depth of 7.0 km (Figure 4). Sample HC1-10 at a depth of 339 m yields three AHe ages ranging between 70 and 93 Ma and a weighted mean of 82.0 6 7.5 Ma.

5. Data Interpretation by Thermal History Modeling 5.1. Thermal History Modeling Strategy and Geological Constraints [22] Thermal history modeling for both individual and borehole samples was carried out using the a probabilistic approach that uses a Bayesian algorithm with transdimensional Markov chain Monte Carlo sampling to constrain the probability of the inferred time-temperature history (for details see Gallagher [2012]). The multikinetic annealing model was used for modeling projected AFT track lengths [Ketcham et al., 2007], using Dpar as a kinetic parameter. Helium diffusion in apatite is modeled using the diffusion parameters of Farley 463

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[2000]; while helium diffusion in zircon using the diffusion parameters of Reiners et al. [2004]. [23] For thermal history inverse modeling of the AFT and AHe data of individual samples, additional geological constraints were taken into account. These constraints included the followings (Figure 6). (i) A present-day surface temperature of 15 6 10 C. (ii) An initial time-temperature constraint is set at 80–120 C at a time span slightly older than the corresponding AFT age because the presence of relatively short tracks in samples suggests a memory of pre-AFT age thermal history in the AFT data. (iii) Where available, a constraint of 30–80 C was set at the time of the weighted mean AHe age. It is worth noting that these premodeling constraints are always presented with large uncertainties so as to give the modeling protocol sufficient freedom to search for data-constrained thermal histories. [24] For thermal history inverse modeling of borehole AHe and ZHe data, several geological constraints were used (Figure 7) as follows: (1) a box of 170 6 20 Ma at 190 6 30 C, as defined by the oldest ZHe ages of the upper two samples (HC1-9 and HC1-1); (2) present-day temperature for the uppermost sample (HC1-9 at a depth of 101 m) of 15 6 10 C, and (3) a temperature offset between the uppermost and lowermost samples (HC1-8 at a depth of 7013 m) of 220 6 100 C (Figure 6). As for the individual sample modeling, these additional constraints are set with large uncertainties, so as to give the modeling protocol sufficient freedom to search for data-constrained thermal paths. For details about this modeling approach, see Gallagher [2012].

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Figure 7. The results show a distinct Late JurassicCretaceous cooling, followed by a period of long thermal quiescence in Late Cretaceous-Cenozoic time. [27] Comparing the modeling results of surface and borehole samples (Figures 6 and 7), it is evident that a Late Jurassic-Early Cretaceous cooling at temperatures higher more than 50 C, where AHe, AFT, and/or ZHe methods are sensitively, is commonly revealed. However, modeling of surface and borehole samples suggested different Late Cretaceous-Cenozoic thermal histories at relative lower temperatures (10 mm/yr) and has played an important role in accommodating the 1800–2800 km northward indentation of the Indian subcontinent [e.g., Yin and Harrison, 2000; Johnson, 2002; Searle et al., 2011].

6.3. Ambiguous Cenozoic Cooling [34] The weak and ambiguous Cenozoic cooling suggests that overall the study area has only experienced little, if any, denudation during the

Figure 9. (a) Compilation of Late Jurassic-Early Cretaceous tectonic events in northern-central TP. These tectonic events include (1) Lhasa-Qiangtang collision, which is probably diachronous along the Bangong suture; (2) sinistral shearing along the western and northern margins of the SGT (the Lungmu Co, Altyn Tagh, and Kunlun faults); (3) crustal shortening between the Qaidam Basin and the Qilian Shan; (4) crustal extensional along the Lapeiquan fault, west of the Althn Tagh fault; (5) formation of rift and pull-apart basins in Hexi Corridor areas; (6) crustal extension along the eastern margin of the SGT (the Danba anticline and Longmen Shan fault); and (7) shearing and denudation along the Qinling-Dabie orogen. For details see supporting information (S2 and S3). Key to references as follows: [1], [Vincent and Allen, 1999]; [2], [Li, 2001; Yang et al., 2001; W. Tang et al., 2012]; [3], [Liu et al., 2003; Wang et al., 2005]; [4], [Chen et al., 2003]; [5], [Liu et al., 2001]; [6], [Arnaud et al., 2003; Xu et al., 2007]; [7], [Leloup et al., 2012]; [8], [Wu et al., 2011]; [9], [Mock et al., 1999]; [10], [Liu et al., 2005]; [11], [Ratschbacher et al., 2003; Hu et al., 2006; Tian et al., 2012]; [12], [Arne et al., 1997; Xu et al., 2008]; [13], [Xu et al., 2008; Zhou et al., 2008]. The main Cretaceous structures (introduced above), are marked by red lines, whereas minor coeval structures are marked by black lines. Thick white lines are Mesozoic and Cenozoic suture zones in and around the TP. (b) An Early Cretaceous tectonic transect (A-A0 ) from Indian subcontinent to Sino-Korean block (see Figure 9a for location), showing the Lhasa-Qiangtang collision along the Bangong suture, sinistral shearing along the Kunlun suture (Kunlun fault) and subduction of the Neo-Tethys beneath the Lhasa terrane. (c) Projection of major Late Jurassic-Cretaceous tectonic events compiled in Figure 9a onto an Early Cretaceous paleogeographic terrane reconstruction. The paleogeography is adapted from Halim et al. [1998], with modifications including (1) integration of fault or suture zones between terranes or basins and (2) insertion of the Yindun Arc between the Songpan-Ganze and Qiangtang terranes. This synthesis suggests a consistent stress regime between the Lhasa-Qiangtang collision and sinistral shearing to the north along the western and northern margins of the SGT (the Lungmu Co, Altyn Tagh, and Kunlun faults) and indicates that crustal shortening related to the Lhasa-Qiangtang collision may have been transferred into central Asia (e.g., Qinling-Dabie orogen) along these shear zones. Abbreviations: ATF, Altyn Tagh fault; BSZ, Bangong suture zone; DA, Danba Anticline; ISZ, Indus suture zone; JSZ, Jinsha suture zone; KF, Kunlun fault, inherited from an ancient Kunlun suture zone; LCF, Lungmu Co fault; LMS, Longmen Shan; LSZ, Litang suture zone; YA, Yidun Arc. 469

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Cenozoic. The maximum Cenozoic cooling is 25–30 C, as estimated from the thermal histories of sample HS20, HS21, HS23, HS24, and HS28 (Figure 6). Assuming a constant geothermal gradient of 25–30 C/km, which is the present gradient [Hu et al., 2000; Xu et al., 2011], no more than 1 km denudation would have occurred. Such subtle Cenozoic cooling and denudation are apparently not compatible with the significantly thickened crust and highly elevated surface of the study area, unless focused uplift occurred in the surrounding areas, preventing the efficient headward river incision from the plateau margins into the interior, where the study area is located. [35] The above interpretation is supported by several lines of evidence. First, the areas to the north and east of the study area are characterized by markedly higher topographic relief, indicating that an extraordinary amount of river incision has been focused along the plateau margins (Figure 1). Second, the widely developed flat-lying glacial and fluvial deposits, which can be dated back to Pliocene time by isotopic and paleopalynologic data [e.g., Sheng and Wang, 2009], suggests that the study area has not experienced significant denudation but rather received hundreds of meters of intermontane glacial and fluvial deposits since at least Pliocene time. Further, younger Cenozoic thermochronological ages reported from the eastern TP margin (the Longmen Shan) [e.g., Kirby et al., 2002; Godard et al., 2009; Wang et al., 2012; Tian et al., 2013] and the northern TP margin (the western Qinling) [Clark et al., 2010; Lease et al., 2011], compared to those of the study area, suggest higher rates of erosion and tectonic uplift along the plateau margin than in the study area. Furthermore, structural studies at the eastern and northern margins suggest a significant amount of Cenozoic crustal shortening, supporting the higher topography and more rapid Cenozoic erosion in that area [e.g., Hubbard et al., 2010; Lease et al., 2012; Tian et al., 2013].

6.4. Tectonic Implications [36] Considering that significant Late JurassicCenozoic crustal deformation has occurred in areas surrounding the eastern SGT, the absence of evident coeval crustal deformation in the interior of the terrane suggests that the terrane has acted as a rigid block and has not undergone any significant crustal deformation during Late Jurassic-Cenozoic time. The rigid behavior of the upper sedimentary SGT crust may indicate that it is probably underlain by a strong lithosphere, inherited from the

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Paleo-Tethys Ocean [e.g., Pullen et al., 2008; Roger et al., 2011; Ding et al., 2013]. [37] The rigid behavior of the eastern SGT is consistent with several lithospheric viscosity and composition estimates and has important implications for the Cenozoic evolution of the northeastern and eastern parts of the TP. First, our results are consistent with the high lithospheric viscosity of the SGT. The lower-crustal viscosity of the SGT is constrained to be as high as 1 3 1019 to 2 3 1021 Pa s by a Bayesian analysis of the geologic and geodetic data [Hilley et al., 2005]. This high viscosity value is consistent with that estimated from the topography of the southeastern-eastern TP margin [Medvedev and Beaumont, 2006] and that from paleolake shorelines of the central-southern TP [England et al., 2013]. Second, the rigid behavior of the SGT is also consistent with the crustal composition, which is characterized by felsic rocks in the upper 40 km and intermediate rocks in 40–70 km as constrained by seismic studies [Liu et al., 2006; Wang et al., 2013; Xu et al., 2013].

7. Conclusions [38] Low-temperature thermochronology data derived from both surface and borehole samples from the interior part of the eastern SGT suggest a common thermal history, characterized by a distinct phase of Late Jurassic-Early Cretaceous cooling, followed by a prolonged stage of slow cooling through to the present day. This ubiquitous postLate Jurassic cooling history found in all samples from different stratigraphic levels suggests a uniform evolution of the study area (without detectable upper crustal deformation). [39] Contemporaneous with the Late JurassicEarly Cretaceous subdued deformation of the interior part of the eastern SGT, the strike-slip faults (e.g., Lungmu Co, Altyn Tagh, and Kunlun faults) bounding the SGT and other terranes, have undergone tens to hundreds of kilometers of sinistral shearing. Projecting these aforementioned events as well as the Lhasa-Qiangtang collision onto an Early Cretaceous paleogeographic terrane reconstruction results in a new tectonic model. The model relates the Lhasa-Qiangtang collision to tens to hundreds of kilometers of shearing along the northern and western margins of the SGT, which may have transferred strain into central Asia (e.g., Qinling-Dabie orogen). This tectonic framework has been reactivated during Cenozoic 470

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tectonism, which has been dominated by the indentation of the Indian subcontinent into the Asian continent. [40] Results of this study support a rigid postorogenic behavior of the eastern SGT and highlight the importance of crustal strength discontinuities (i.e., fault zones in this study), surrounding strong terranes, in accommodating and transporting crustal deformation.

Acknowledgments [41] The University of Melbourne thermochronology laboratory receives infrastructure support under the AuScope Program of NCRIS. S.H. received support from the National Natural Science Foundation of China (NSFC) (41072186). Y.T. received support from IPRS and MIRS scholarships at the University of Melbourne. Y.T. is grateful to Abaz Alimanovic for assistance with (U-Th)/He dating and to Zhonghua Tian and Zhaokun Yan for their assistance during fieldwork. Constructive reviews from anonymous reviewers clarified points of this work. Editorial work of James Tyburczy is gratefully appreciated.

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