Thermochronological constraints on the late Cenozoic morphotectonic ...

Thermochronological constraints on the late Cenozoic morphotectonic evolution of the Min Shan, the eastern margin of the Tibetan Plateau

Yuntao Tian 1,2, Rui Li 1,2, Yuan Tang 1,2, Xiao Xu 1,2, Yuejun Wang 1,2, Peizhen Zhang 1,2 1

Guangdong Provincial Key Lab of Geodynamics and Geohazards, School of Earth Sciences

and Engineering, Sun Yat-sen University, Guangzhou 510275, China 2

School of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou 510275,

China

Abstract Strain distribution inferred from rock exhumation history could provide significant insights into the geodynamic models proposed for explaining late Cenozoic outward growth of the Tibetan Plateau. In this work, we present a new thermochronological dataset to constrain the exhumation history of the eastern margin of the Tibetan Plateau (i.e. the Min Shan and adjacent areas) and the long-term dip-slip rates of boundary faults, including the Huya fault in the plateau margin and the Minjiang fault in the hinterland. The dataset shows evident age diachroneity between different sides of the faults, with dramatically younger ages in their hanging walls than footwalls, suggesting differential exhumation across the faults. Ageelevation plots and inverse thermal history modelling for a vertical profile dataset from the plateau margin (west of the Huya fault) indicate the differential exhumation started at late Miocene time (~10 Ma), synchronous with the timing at other sites of the eastern Tibetan Plateau. The magnitude of the differential exhumation is constrained as >0.6 km/m.y. and ~0.2 km/m.y. across the Huya and Minjiang faults, as estimated from age-elevation relationships and one-dimensional modelling of exhumation, providing unique constraints for the dip-slip rates along the faults. These two N-S striking faults, together with the Tazang fault (NWW-striking and left-lateral slipping) to the north and Longmen Shan faults (a set of NE-striking reverse faults with right-lateral components) to the south, forms a large-scale fault system to accommodate the late Cenozoic northeastward upper crustal shortening along a deep-seated hinterland-ward dipping detachment. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1029/2017TC004868 © 2018 American Geophysical Union. All rights reserved.

Keywords:

Continental

tectonics,

Upper

crustal

extrusion,

Rock

exhumation,

Thermochronology, Earth’s surface processes

Key points: 

Enhanced exhumation initiated at late Miocene (~10 Ma) time in Min Shan.



Long-term vertical slip along Huya and Minjiang faults occurs at a rate of >0.6 km/m.y. and ~0.2 km/m.y.



Deformation in the region is controlled by upper crustal shortening along a deepseated hinterland-ward dipping detachment.

1. Introduction The Tibetan Plateau, including >80% of the Earth’s surface with elevation >4 km above sea level [Fielding, 1996], results from a series of continental accretions and collisions during Mesozoic and Cenozoic time [e.g., Powell and Conaghan, 1975; Chang et al., 1986; Yin, 2010]. Development of the plateau is thought to have strongly influenced Eurasian geodynamics (e.g., stress distribution) [e.g., Molnar and Tapponnier, 1978; Zhang et al., 2004], climate systems (e.g., formation of the Indian and southeast Asian monsoon) [e.g., An et al., 2001; Dupont-Nivet et al., 2007], and oceanic sediment budget and chemistry [e.g., Raymo and Ruddiman, 1992; P. Zhang et al., 2001]. One of the most remarkable physiographic features of the plateau, revealed by high-resolution digital elevation data, is the presence of highly-incised river gorges in the plateau surrounding margins [Fielding, 1996; Liu-Zeng et al., 2008]. However, debates continue as to (1) the formation time of the highelevation and high-relief landscapes (Fig. 1); (2) the spatial pattern of rock exhumation in response to the plateau growth; and (3) the geodynamics underpinning the exhumation, surface uplift and deformation, as summarized below based on previous publications in the southeastern and eastern Tibetan Plateau. Different initiation times of the high-elevation and high-relief landscape in the southeastern and eastern Tibetan Plateau have been determined using paleoaltimetry, thermochronology and cosmogenic approaches. First, many studies used exhumation history determined from thermochronology data as an index for surface uplift, and suggested that growth of the high-relief topography initiated at the late Miocene (5-12 Ma)

© 2018 American Geophysical Union. All rights reserved.

[Kirby et al., 2002; Clark et al., 2005b; Ouimet et al., 2010; Jolivet et al., 2015; Tan et al., 2017], mid Miocene (15-22 Ma)

[Tian et al.,

2014b], or at variable times in different sectors [Tian et al., 2014b; Shen et al., 2016; Yang et al., 2016; Wang et al., 2017]. It is worth noting that the multi-method thermochronology data from vertical profiles in the central Longen Shan [Wang et al., 2012], Jiulong Shan [Zhang et al., 2016], the First Bend of the Yangtze River [Shen et al., 2016] and Three River region [Liu et al., 2017] suggested an Oligocene – early Miocene (~35-15 Ma) phase of exhumation, prior to the late Miocene phase. However, it remains unknown whether this earlier phase of exhumation was a regional or local event. Second, paleoaltimetry studies suggested that high elevations (~3 km) may have been gained at Eocene time in the southeastern Tibetan Plateau [Hoke et al., 2014; Li et al., 2015; Tang et al., 2017] and before late Miocene time in the eastern Tibetan Plateau [Xu et al., 2016]. Furthermore, cosmogenic dating of cave sediments at the first bend of the Yangtze River suggested a scenario where incision (~1 km) occurred during 18-9 Ma and ceased thereafter [McPhillips et al., 2016]. Debates concerning the spatial exhumation pattern is highly related to the geodynamic models proposed for explaining the intra-continental deformation in the southeastern and eastern Tibetan Plateau. First, a group of studies suggested late Miocene uniform exhumation in both time and space [e.g., Clark et al., 2005b; Ouimet et al., 2010; Tian et al., 2015], which has been interpreted as resulting from either lower crustal channel flow [Clark et al., 2005b; Ouimet et al., 2010] or upper crustal extrusion along a detachment fault at depth [Tian et al., 2015]. The channel flow model infers that thickening and flow of ductile lower crustal material away from the central Tibetan Plateau would pump up topographic relief in marginal areas by inflating its lower crust [e.g., Royden et al., 1997; Clark et al., 2005a; Royden et al., 2008]. The other model proposes that blocks in the central Tibetan Plateau has extruded eastwards [e.g., Tapponnier et al., 2001; Hubbard and Shaw, 2009; Wang et al., 2011]. In this second model, upper crustal shortening occurs above a gently dipping middle crustal detachment that links with steep reverse faults at plateau margins, forming a large-scale listric fault extending from plateau margin to plateau interior [Tian et al., 2013; Tian et al., 2015]. However, another group of studies highlighted that the exhumation in the southeastern Tibetan Plateau is strongly non-uniform in both time and space, indicating differential rock uplift and faulting among crustal blocks [e.g., Tian et al., 2014b; Shen et al., 2016; Yang et al., 2016; Wang et al., 2017].


To test these controversies, this study focuses on the rock exhumation history of the tectonically active and deeply-incised eastern margin of the Tibetan Plateau (Min Shan), which has been struck by several M. 6.5-7.2 earthquakes, during 1976 and 2017 (Fig. 1). Thirteen new apatite fission-track (AFT) and apatite and zircon (U-Th)/He (AHe and ZHe) data from the Min Shan are reported, covering a spatial distance of ~80 km and including a vertical profile spanning an elevation interval of ~4 km (Fig. 2 and Table 1). The results provide a thermochronological constraint on the exhumation history of the study area and the differential exhumation across the major structures defining the plateau margin, having significant implications to the regional late Miocene geodynamics.

2. Topographic and geological setting The Min Shan forms the highly-elevated northern part of the eastern margin of the Tibetan Plateau, where the landscape is highly dissected by ongoing incision of tributaries of the Min and Fu rivers flowing towards the south and southeast (Figs. 1 and 2). Using a 90-mresolution SRTM digital elevation model (DEM), we calculated the mean elevation and topographic relief along a swath across the study area using a 10-km moving window (Fig. 2). The topographic swath shows that the elevation is maximum (~4.5 km) in the central part of the Min Shan, and decreases gently towards the plateau interior (~3.8 km) and rapidly towards the margin (~1.4 km). However, the topographic relief shows a reversed pattern, decreasing rapidly from the central Min Shan towards the plateau interior (from ~ 2 km to 0.7 km/m.y, suggesting a differential exhumation rate of >0.6 km/m.y. across the fault. Such a result is consistent with the exhumation rate derived from the age-elevation profile (Fig. 5). Similarly, the differential exhumation rate across the Minjiang fault can be estimated as ~0.2 km/m.y. by comparing the values derived from the late Miocene AHe and AFT ages at different sides of the fault (Fig. 8).

6. Discussion 6.1. Late Miocene enhanced exhumation The total late Miocene enhanced exhumation is estimated as ~7 ± 2 km based on the new timing (~10 Ma) and rate (0.7 ± 0.2 km/m.y.) derived from both age-elevation plot and inverse thermal history modelling. The timing of the late Miocene phase of enhanced exhumation in the study area is consistent with those reported from adjacent areas, such as the Western Qinling [Enkelmann et al., 2006], the Longmen Shan plateau margin [Kirby et al., 2002; Godard et al., 2009; Wang et al., 2012; Cook et al., 2013], plateau hinterland areas [Tian et al., 2015], and regions to further south [Clark et al., 2005b; Ouimet et al., 2010; Zhang et al., 2017]. Further, the magnitude of exhumation is similar to the late Miocene exhumation in the Longmen Shan hinterland [Cook et al., 2013; Tian et al., 2013; Tan et al., 2017], but evidently higher than the Longmen Shan front (~4 km) [Arne et al., 1997; Godard et al. 2009; Wang et al., 2012], the plateau interior (~4 km) [Clark et al., 2005b; Ouimet et al., 2010; Tian et al., 2015; Shen et al., 2016] and areas surrounding the plateau (1-2 km) [Enkelmann et al., 2006; Tian et al., 2012]. This indicates that the late Miocene enhanced exhumation has uniformly developed in the eastern part of the Tibetan Plateau, but with significant variance in magnitude from plateau interior to surrounding margins. Such a synchronous phase of exhumation indicates a regional late Miocene surface uplift and has important implications for the geodynamics in the eastern Tibetan Plateau (see section 5.3). Previous studies reported an Oligocene – early Miocene phase of enhanced exhumation in the central segment of the Longmen Shan [Wang et al., 2012], Jiulong Shan [Zhang et al., 2016] and sites further to the south [Shen et al., 2016; Liu et al., 2017]. But this phase of exhumation is not found in the Min Shan area, even though we have used a similar combination of thermochronology methods as those previous studies. As shown in the ageelevation plot, the elevation difference between the late Miocene and late Cretaceous ZHe © 2018 American Geophysical Union. All rights reserved.

ages is as small as ~1000 m (Fig. 5c). This indicates that the maximum Oligocene-early Miocene exhumation should be less than ~1000 m, significantly lower than the ~7  2 km late Miocene exhumation. Therefore, it is inferred that the Oligocene-early Miocene phase of rapid exhumation probably did not occur in the study area and was not as regional as the late Miocene enhanced exhumation. 6.2. Fault kinematics The spatial exhumation pattern across the Min Shan, characterized by higher rates in the hanging walls than footwalls of the Huya and Minjiang faults, indicates both faults have a significant east-vergent component of dip-slipping, consistent with previous geological mapping [Chen et al., 1994; Kirby et al., 2000] and short-term GPS observation [Shen et al., 2009]. These two main faults merge into a detachment at a depth of ~20 km (Fig. 9a), as shown by deep seismic reflection profiles [Xu et al., 2017], forming a large-scale listric fault extending from the Min Shan to the plateau interior, similar to that in the Longmen Shan plateau margin [Tian et al., 2013; Tian et al., 2015; Feng et al., 2016]. If the topography in the Min Shan region has been constant during late Miocene time, the differential exhumation rates across the faults (>0.6 km/m.y. across the Huya fault and ~0.2 km/m.y. across the Minjiang fault) quantified above, provide constraints for the long-term dip-slip rates of the faults. These results are consistent with short-term GPS determinations (~0.8-1.0 mm/y.r, i.e., km/m.y.) [Shen et al., 2009]. Combining previously published results, a late Miocene structural architecture for the region can be formulated. Deformation in the region is probably controlled by a large network of faults, including the Huya and Minjiang faults introduced above, the left-lateral strike-slip Tazang fault to the north and the oblique Longmen Shan fault (reverse fault with a right-lateral slip component) to the south (Figs. 1 and 3). Strike-slipping along the Tazang fault (the eastern extension of the Kunlun fault) may trace back to late Cenozoic time, as it cuts basins dated as late Miocene [Jolivet et al., 2003; Fu and Awata, 2007; Ren et al., 2013]. Deformation along the Longmen Shan fault can also trace back to late Miocene and even earlier, as shown by previous structural and thermochronology studies [Wang et al., 2012; Tian et al., 2013; Tian et al., 2016]. Late Cenozoic strike-slipping along the Tazang and Longmen Shan faults has transmitted the northeastward movement of the eastern Tibetan Plateau, nearly orthogonal to the Huya and Minjiang faults. In summary, deformation along


the large fault network, consisting of the Tazang, Minjiang, Huya and the Longmen Shan faults, has accommodated most of the late Miocene deformation in the region. As to the W-E-striking Xueshan fault, located between the Huya and Minjiang faults contrasting deformation histories have been proposed. A group of studies suggested that the fault is a Quaternary active fault [Tang et al., 1993; Chen et al., 1994]. In this case, the Xueshan fault may have been acting as an east-verging reverse fault coordinating the deformation between the Huya and Minjiang faults. However, as pointed out by Kirby et al. [2000], the east end of the fault was intruded by a Mesozoic granite (Figs. 2 and S1), indicating pre-intrusion initiation of the fault. This proposal is also possible considering the multi-phase deformation history of the region, as shown by different deformation trajectories of Paleozoic and Mesozoic strata (see the Introduction section). 6.3. Tectonic model The late Miocene exhumation and structural geometry and kinematics of the Huya and Minjiang faults provides a unique opportunity to test relevant geodynamic models. Previous studies proposed two end-member geodynamic models for explaining Cenozoic growth of the Tibetan Plateau, including the upper crustal extrusion model and the lower crustal channel flow model, as introduced above. The common appearance of the late Miocene phase of exhumation implies that the eastern Tibetan Plateau was formed by a tectonic process that had resulted in synchronous rock and surface uplift. As illustrated in Figure 9, the two models predict simultaneous rock uplift from plateau margin to plateau interior by different manners. For the upper crustal extrusion model, synchronous surface uplift is produced by rock uplift through crustal shortening along a hinterland-ward dipping detachment, with the amount of rock uplift equivalent to the vertical component of fault slip [Tian et al., 2015] (Fig. 9a). However, for the lower crustal channel flow model, rock uplift in plateau margin is a combination of the isostasy and lower crustal extrusion (Fig. 9b), which is facilitated by the positive feedback between enhanced surface exhumation and the extrusion [e.g., Beaumont et al., 2001]. It is worth noting that this lower crustal flow model differs from that argued in Clark et al. [2005a] and Kirby et al. [2000], which proposes that the over-thickened lower crustal materials flow northeastwards into the Western Qinling via the Min Shan, rather than extruding towards the Earth’s surface. Further, these two models predict different deformation pattern from plateau interior to marginal areas. For the upper crustal extrusion model, all plateau margin faults are predicted © 2018 American Geophysical Union. All rights reserved.

to be reverse faults (Fig. 9a). However, the channel flow model requires a marginal reverse fault and a hinterland normal fault for accommodating active extrusion in between (Fig. 9a). Accordingly, if the second model is applicable for the Min Shan region, the Minjiang faults in the hinterland are predicted to be normal faults by the lower crustal flow and extrusion model, inconsistent with the east-vergent reverse faulting shown by the differential exhumation pattern. From this point of view, the upper crustal extrusion model is more applicable for explaining the late Cenozoic deformation in the Min Shan area. Furthermore, the upper crustal extrusion model can easily explain the late Miocene regional strain field in the study area, controlled by the network of faults (Figs. 1 and 3). Northeastward extrusion of the eastern Tibetan Plateau is compatible with not only the leftlateral slipping along the Tazang fault and the right-lateral component of slipping along the Longmen Shan fault, but with also the reverse faulting along the Huya and Minjiang faults. However, the lower crustal flow model would require a separate mechanism to explain the deformation along the faults.

7. Conclusions Results of the thermochronological study of this work lead to the following three major conclusions. (1) Enhanced exhumation in the Min Shan area initiated at late Miocene time (~10 Ma), as revealed by age-elevation relationships and the corresponding inverse thermal history modelling. The exhumation is synchronous with surrounding areas, indicating a uniform late Miocene phase of river incision and topographic growth in the eastern Tibetan Plateau. (2) The average long-term dip-slip rates along the Huya and Minjiang faults are estimated as >0.6 km/m.y. and ~0.2 km/m.y. from the differential exhumation across the fault. This rate is consistent with geodetic observations (0.8-1.0 km/m.y.). (3) Spatial thermochronological age distribution suggests that the Huya and Minjiang faults bounding the eastern and western flanks of the Min Shan have been acting as east-vergent reverse faults since late Miocene time. This deformation accommodates the northeastward extrusion of the eastern Tibetan Plateau that has been documented as strike-slipping along the Tazang and Longmen Shan faults to the north and south. Such a late Miocene strain distribution along the Tazang-Minjiang-Huya-Longmen © 2018 American Geophysical Union. All rights reserved.

Shan fault system is compatible with upper crustal extrusion along a deep-seated hinterland-ward dipping detachment.

Acknowledgement Funding for this research was provided by Chinese 1000 Young Talents Program, National Natural Science Foundation of China (NSFC) grants (no. 41772211 and U1701641) and the Guangdong Province Introduced Innovative R&D Team (2016ZT06N331). The authors are grateful to B. Kohn and A. Carter for their assistance in (U-Th)/He analyses, to H. Zhang for his kind invitation for this contribution, to M. Jolivet and an anonymous reviewer for their constructive comments which clarified many points of this paper, and to Editor N. Niemi for his editorial efforts. The data for this paper are included in the manuscript and the associated supporting information [Murakami et al., 1991].


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Table 1. Information of samples reported in this study. Locations Sample No.

Lithology

Elevation (m)

HS15

Devonian Sandstone

HS16

Longitude (oE)

Latitude (oN)

865

32.433

104.531

Mesozoic Granite

867

32.524

104.541

HS17

Mesozoic Granite

1631

32.619

104.560

HS18

Precambrian meta-sandstone

1608

32.617

104.503

HS19

Mesozoic Granite

1688

32.708

104.403

HS81

Silurian sandstone

972

32.466

104.422

HS82

Precambrian meta-sandstone

1143

32.497

104.220

East of the Huya fault

Vertical profile between the Minjiang and Huya fault HS83

Mesozoic Granite

1370

32.518

104.122

HS84

Triassic turbidite

1523

32.571

104.123

HS85

Triassic turbidite

2513

32.751

103.964

HS86

Devonian turbidite

2896

32.778

103.898

HS87

Triassic turbidite

3795

32.744

103.749

HS88

Triassic turbidite

4007

32.739

103.734


Table 2. New apatite fission-track results. For sample information, see Table 1.

Sample No.

No. of grains (n)

Spontaneous tracks No. (n)

Density (106 cm-2)

1

Pooled 238 U (ppm)

Dispersion （%）

2

Central Age (Ma±1SD)

Dpar (μm)

East of the Huya fault HS15

20

150

0.1842

9.0

15

40.8

4.1

1.9 (1.2-2.3)

HS16

27

261

0.1032

3.5

14

60.3

4.3

1.6 (1.2-1.9)

HS17

34

640

0.4634

14.0

12

68.2

4.5

1.4 (1.2-1.8)

HS18

21

132

0.1239

5.9

19

41.8

3.7

1.9 (1.3-2.3)

HS19

25

958

0.7038

18.9

16

56.0

5.7

1.5 (1.2-2.0)

HS82

37

229

0.1901

12.4

31

34.0

3.3

1.9 (1.0-2.5)

Vertical profile between the Minjiang and Huya fault HS83

30

74

0.0865

39.9

17

4.7

0.5

1.5 (1.2-2.0)

HS84

41

66

0.0487

25.9

25

4.5

0.6

2.0 (1.3-2.4)

HS85

53

160

0.0849

32.8

23

5.9

0.5

1.9 (1.3-2.3)

HS86

46

214

0.0990

27.7

21

7.8

1.0

2.1 (1.4-2.4)

HS87

50

2389

1.1344

23.1

15

102.7

4.8

2.0 (1.3-2.4)

HS88

51

2808

1.5997

28.5

17

111.8

5.2

1.9 (1.3-2.3)

1

Pooled uranium content of all grains measured by LA-ICP-MS.

2

Central age calculated using the method of Galbraith [2005], performed using the

RadialPlotter program of Vermeesch [2009].


Table 3. Results of apatite and zircon (U-Th-Sm)/He dating. For sample information, see Table 1.

Sample

Data type

Grain Length (μm)

Grain width (μm)

He (ncc)

Mass (mg)

[eU]

Raw age (Ma)

Corrected Age (Ma)

Error (±1s)

HS87-1

AHe

329.8

111.6

0.206

0.8

15.5

10.5

13.4

0.6

HS87-2

AHe

255.4

107.3

306.5

0.7

14.3

11.9

15.4

0.7

HS87-3

AHe

293.9

8.0

282.9

0.5

16.9

7.6

9.6

0.4

HS87-4

AHe

8.5

8.9

253.9

1.0

10.6

8.4

10.7

0.5

HS87-5

0.71

36.6

13.2

205.8

0.4

39.7

29.5

39.6

1.8

0.0087

0.76

12.3

20.2

627.0

1.6

17.1

8.6

11.0

0.5

0.035

0.0040

0.69

4.8

8.0

521.7

1.7

6.6

9.8

13.6

0.6

92.9

0.054

0.0047

0.71

6.1

7.6

466.1

1.2

7.9

11.2

15.3

0.7

218.2

88.5

0.124

0.0042

0.69

24.4

4.6

441.1

0.2

25.5

9.3

12.7

0.6

ZHe

270.1

97.7

9.727

0.0088

0.79

110.3

64.7

-

0.6

125.5

71.7

90.4

5.6

HS81-2

ZHe

251.2

102.2

17.761

0.0086

0.80

139.2

84.9

-

0.6

159.1

105.6

132.0

8.2

HS81-3

ZHe

140.9

81.6

10.151

0.0025

0.70

286.5

308.1

-

1.1

358.9

91.0

129.3

8.0

HS81-4

ZHe

248.3

100.8

9.442

0.0083

0.80

87.7

45.8

-

0.5

98.5

94.5

118.2

7.3

HS81-5

ZHe

220.3

73.2

6.413

0.0042

0.74

113.6

59.4

-

0.5

127.5

98.1

133.4

8.3

HS84-1

ZHe

156.4

68.6

1.380

0.0023

0.70

1188.6

324.4

-

0.3

1264.9

3.9

5.5

0.3

HS84-2

ZHe

122.7

67.4

0.331

0.0016

0.68

554.9

120.0

-

0.2

583.1

3.0

4.4

0.3

HS84-3

ZHe

145.5

85.9

1.208

0.0028

0.73

789.4

48.7

-

0.1

800.9

4.4

6.0

0.4

HS84-4

ZHe

123.7

66.0

0.820

0.0015

0.67

1222.8

377.7

0.3

1311.6

3.3

5.0

0.3

HS85-1

ZHe

236.4

65.6

7.603

0.0051

0.74

237.2

108.8

0.5

262.8

48.0

65.1

4.0

4

a

Mean FT

U (ppm)

Th (ppm)

Sm (ppm)

Th/U

0.0102

0.76

12.9

10.8

259.0

0.155

0.0073

0.75

12.3

8.5

118.5

0.164

0.0102

0.77

15.0

340.3

109.8

0.113

0.0102

0.76

AHe

363.8

87.8

0.997

0.0070

HS88-1

AHe

256.5

116.8

0.162

HS88-2

AHe

203.3

88.9

HS88-3

AHe

218.7

HS88-4

AHe

HS81-1

c

-

b

Weight mean (Ma)

Error (±1s)

11.4

0.5

12.8

0.6

115.2

6.5

5.1

0.4


HS85-2

ZHe

262.7

111.1

49.086

0.0104

0.81

358.7

224.2

-

0.6

411.3

88.5

109.6

6.8

HS85-3

ZHe

272.2

126.4

58.495

0.0134

0.83

360.6

262.8

-

0.7

422.3

78.5

94.5

5.9

HS85-4

ZHe

279.5

101.3

24.116

0.0098

0.80

198.3

171.8

-

0.9

238.6

79.0

98.9

6.1

HS86-1

ZHe

204.5

90.8

23.031

0.0053

0.77

409.0

56.2

-

0.1

422.3

83.7

108.2

6.7

HS86-2

ZHe

210.3

92.5

8.792

0.0057

0.78

112.4

20.3

-

0.2

117.2

107.1

137.4

8.5

HS86-3

ZHe

202.9

85.7

9.830

0.0048

0.75

162.7

74.4

-

0.5

180.2

92.4

123.3

7.6

HS86-4

ZHe

228.7

122.5

21.033

0.0097

0.81

123.1

152.5

-

1.2

159.0

110.5

135.7

8.4

HS86-5

ZHe

213.0

99.6

25.867

0.0065

0.78

307.4

386.5

-

1.3

398.3

81.6

104.3

6.5

HS87-1

ZHe

246.2

98.2

86.885

0.0078

0.80

954.1

156.2

-

0.2

990.8

91.0

114.4

7.1

HS87-2

ZHe

235.2

96.5

48.337

0.0071

0.79

270.2

167.4

-

0.6

309.5

176.2

223.9

13.9

HS87-3

ZHe

226.2

84.3

19.211

0.0055

0.76

167.9

126.6

-

0.8

197.7

144.1

188.9

11.7

HS87-4

ZHe

188.4

93.2

44.064

0.0049

0.77

495.0

97.7

-

0.2

518.0

141.2

182.6

11.3

HS88-1

ZHe

307.5

107.2

152.225

0.0123

0.82

666.0

41.0

-

0.1

675.6

148.4

180.7

11.2

HS88-2

ZHe

196.4

88.4

53.523

0.0048

0.76

714.6

473.7

-

0.7

825.9

109.7

144.3

8.9

HS88-3

ZHe

253.4

87.6

83.376

0.0068

0.78

708.4

238.3

-

0.3

764.5

130.5

168.2

10.4

HS88-4

ZHe

194.7

88.6

24.162

0.0048

0.75

249.7

183.8

-

0.7

292.9

140.4

186.4

11.6

HS88-5

ZHe

248.2

107.7

106.522

0.0092

0.81

577.7

266.8

-

0.5

640.4

146.9

181.9

11.3

HS88-6

ZHe

197.9

107.4

47.415

0.0064

0.80

324.6

132.1

-

0.4

355.6

167.6

210.9

13.1

a

α-ejection correction [Farley et al., 1996] calculated using mass-weighted mean radii.

b

Effective uranium content, [eU] = [U] + 0.235 × [Th].

c

Rejected by the weighted mean age calculation using the Isoplot of Ludwig [2012].

84.7

6.4

118.7

6.6

155.0

11.0

174.0

10.9


Figure 1. (a) Landscape of the Tibetan Plateau and surrounding regions, in which the yellow rectangle marks the area of panel b. (b) Topography (SRTM3), major rivers, structures and focal mechanisms of several historic large earthquakes (sourced from Global Centroid Moment Tensor Catalog) in the eastern and southeastern Tibetan Plateau. The black square marks the location of Figure 2.


Figure 2. (a) DEM map of the study area, on which new (circles) and previous (diamonds) data are compiled. Red circles are vertical-profile samples between the Huya and Minjiang faults; whereas black circles were collected from the eastern side of the Huya fault. Thermochronological ages of the samples are compiled in rectangles (see legend for explanation). Numbered previous studies are: (1) Arne et al. [1997], (2) Kirby et al. [2002], (3) Yang et al. [2017]. The dashed line marks the possible northern continuation of the Huya fault. (b) Topographic swath along A-A’ transect, showing maximum, mean and minimum elevations and topographic relief. Also plotted are locations and elevations of previous and new data, with vertical-profile samples marked in red circles. Vertical red lines mark the Huya and Minjang faults.


Figure 3. Generalized geology map of the Min Shan and its vicinity, modified after QBGMR [1991], SBGMR [1991]. The black square marks the location of this work (Fig. S1). The study area shares common borders with the Western Qinling to the north, the Songpan-Ganze terrane to the west and the Sichuan Basin to the southeast.


Figure 4. ZHe age versus eU plot, showing negative correlations for samples HS86, HS87 and HS88. The correlation indicates effects of radiation damage on helium diffusion in zircon crustal [Guenthner et al., 2013].


Figure 5. AHe, AFT and ZHe age-elevation plots for samples located between the Huya and Minjiang faults (a, c) and east of the Huya fault (b). In panels a and c, new AHe data from the top two samples, two AHe and ZHe ages from Kirby et al. [2002] (ref. 1) and two AFT ages from Yang et al. [2017] (ref. 2) are plotted for comparison. Inserted plots in panels a and c are close-up views for areas marked by dashed rectangles. Linear regression of the new AFT data against elevations suggests an apparent exhumation rate of 0.7 ± 0.2 km/m.y (insert in panel a), with one sigma uncertainty marked by the grey area. AFT ages of samples from the eastern side of the Huya fault also show a positive relationship with elevations (b), probably indicating exhumation between 70-40 Ma.


Figure 6. Inverse thermal modeling results (a and b) and comparison between observations and model predictions for the age-elevation profile (c and d). In panel a, black boxes and thick lines mark geological constraints for the uppermost sample. The thermal history of the uppermost sample is plotted in blue, the lowermost sample in red, and intermediate samples in grey. For the uppermost thermal history, thin blue lines depict 95% credible intervals, reflecting the uncertainty in the inferred thermal history alone. For the lowermost thermal history, thin red lines show the 95% credible intervals, reflecting combined uncertainties in the inferred thermal history and temperature offset parameters (see text for details). Panels b and d are close-up views of yellow regions of panels a and c, respectively. Thermal history modeling for combined data from the profile indicates episodic cooling and exhumation of the area, with the latest phase starting at late Miocene time (~10 Ma), as marked by the thick black arrow in panel b.


Figure 7. Projection of low-temperature thermochronology data, compiled in figure 2, onto the A-A’ transect across the Min Shan. Topographic features were calculated using a 10-kmcircle window. For swath location, see figure 2. The thick black lines show the locations of the Huya and Minjiang faults at surface. The pink area envelopes all AFT ages. This plot shows apparent age differences across the Huya and Minjiang faults.


Figure 8. Spatial exhumation pattern across the Min Shan. For locations of the swath, see figure 2. The black lines show the locations of the faults at surface. The pink area envelopes all exhumation rates calculated from AFT ages; whereas grey rectangles mark ranges of rates calculated from young ages (Miocene, except for one Oligocene age east of the Huya fault). This plot shows that average exhumation rates in the study area are strongly controlled by the Huya and Minjiang faults.


Figure 9. Conceptual models for explaining the late Miocene exhumation and uplift in the Min Shan area. (a) Uplift results from upper crustal shortening along a hinterland-ward dipping detachment at a depth of ~20 km. This model predicts uniform uplift in eastern Tibetan Plateau and crustal shortening in the plateau margin (Min Shan), where major boundaries should be reverse faults. (b) Uplift is produced by lower crustal channel extrusion, predicting uniform uplift and a structural framework including a reverse fault in the channel front and a normal fault along the backside. Arrows to the left illustrate distribution of the velocity vectors at different crustal levels. Focal mechanisms compiled in panel (a) are from USGS (https://earthquake.usgs.gov/earthquakes/) and Jones et al. [1984].
