Cenozoic Exhumation of the Danba Antiform, Eastern Tibet: Evidence from Low-Temperature Thermochronology

Cenozoic exhumation of the Danba antiform, eastern Tibet: Evidence from low-temperature thermochronology

Xi-Bin Tan1, Yuan-Hsi Lee2,*, Xi-Wei Xu1, and Kristen L. Cook3,4 1KEY LABORATORY OF ACTIVE TECTONICS AND VOLCANO, INSTITUTE OF GEOLOGY, CHINA EARTHQUAKE ADMINISTRATION, HUAYANI #1, CHAOYANG, BEIJING 100029, CHINA 2DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, NATIONAL CHUNG-CHENG UNIVERSITY, 168 UNIVERSITY ROAD, MIN-HSIUNG TOWNSHIP, CHIA-YI COUNTY 621, TAIWAN 3DEPARTMENT OF GEOSCIENCE, NATIONAL TAIWAN UNIVERSITY, 1, SEC. 4, ROOSEVELT ROAD, TAIPEI 10671, TAIWAN 4GERMAN RESEARCH CENTER FOR GEOSCIENCES, GFZ POTSDAM, TELEGRAFENBERG, 14473 POTSDAM, GERMANY

ABSTRACT

The Danba antiform (DA) exposes the highest grade metamorphic rocks in eastern Tibet. The metamorphic grades characterizing the DA evolve from sillimanite-migmatite grade to greenschist grade over a relatively short distance of ~20 km from core to limb. This metamorphic event indicates an important Mesozoic to Cenozoic doming and exhumation history. However, the Cenozoic history of the antiform is poorly constrained due to a lack of data. Consequently, we used fission track dating on zircon and apatite from 22 samples collected throughout the DA. The zircon fission track (ZFT) data show a transition from Cenozoic non-reset (202 Ma), to partially reset (53–37 Ma), to totally reset (24–8 Ma) ages from the periphery to the core of the DA. The oldest totally reset ZFT ages are ca. 25 Ma and likely indicate the onset of Cenozoic folding of the DA. Compared to the apatite fission track (AFT) ages of ca. 10 Ma in the peripheral region, the youngest AFT ages are younger than 3 Ma in the core of the DA, suggesting that folding could be ongoing. Based on these multithermochronometer data, the cooling rate increases from ~8 °C/m.y. on the periphery to ~12–56 °C/m.y. in the core of the DA since ca. 12 Ma. The DA shares a similar cooling history with the Longmen Shan (LMS) fault belt, implying that the detachment fault of the LMS may extend to the DA, resulting in linked uplift histories. The differential exhumation among the samples in the core of the DA and the surrounding area indicates that both upper crustal deformation and crustal channel flow may have developed simultaneously (mainly since ca. 12 Ma) in the DA.

LITHOSPHERE; v. 9; no. 4; p. 534–544 | Published online 18 May 2017 doi:10.1130/L613.1

INTRODUCTION ramp and flat structures as revealed by balanced metamorphism is Mesozoic in age (ca. 190–160 cross sections, seismic profiles, and earthquake Ma), Huang et al. (2003a) observed ca. 30 Ma India began colliding with Eurasia ca. 50 Ma, data (Xu et al., 2009; Hubbard and Shaw, 2009; Rb-Sr biotite ages in the DA, indicating a late and this collision has since accommodated at Wang et al., 2011). Currently, the regional tec- Oligocene cooling event. Zircon fission track least 1400 km of north-south convergence and tonic questions include, is the crustal shortening (ZFT) ages of ca. 24 Ma (Manai granite) and contributed to the build-up of the Tibetan Plateau limited to the plateau margin, or does it extend to apatite fission track (AFT) ages of 33–4 Ma (Yin and Harrison, 2000; Tapponnier et al., 2001). the internal plateau? What are the relative con- around the DA region also indicate rapid exhu- The Longmen Shan (LMS) is located at the east- tributions of lower crustal flow and upper crustal mation and deformation in the Cenozoic (Xu ern margin of the Tibetan Plateau, forming an shortening to the uplift of the internal plateau and Kamp, 2000; Clark et al., 2005; Wilson abrupt drop in elevation of ~4000 m between (e.g., Tian et al., 2013, 2015)? and Fowler, 2011; Jolivet et al., 2015; Fig. 2). the plateau to the west and the Sichuan basin to The Songpan-Garzê fold belt (SGFB), Because the DA is located in the plateau interior, the east, despite low short-time-scale shortening located in eastern Tibet west of the LMS, is its Cenozoic deformation pattern may provide rates (Chen et al., 2000). Several studies have dominated by highly deformed Triassic flysch new constraints on the mode of crustal deforma- invoked the presence of lower crustal flow to with low to middle greenschist facies metamor- tion during the Cenozoic (Royden et al., 1997; explain the discrepancy between the high topog- phism (Huang et al., 2003a, 2003b; Harrowfield Burchfiel, 2004; Xu et al., 2009; Hubbard and raphy and the low shortening rates (e.g., Royden and Wilson, 2005; Yan et al., 2011; Fig. 1). This Shaw, 2009). The existing low-temperature ther- et al., 1997; Burchfiel, 2004; Clark et al., 2005). contrasts with the Danba antiform (DA), a north- mochronology data, however, are too sparse to The Wenchuan earthquake ruptured two main west-trending structure ~40 km in width and evaluate the detailed Cenozoic exhumation his- faults in the LMS and produced a rupture of 6 m ~90 km in length located on the hanging wall tory of the DA region. In this study we systemi- to locally 8 m of reverse slip (Xu et al., 2009; Shi of the LMS in the interior of the plateau, that cally collected samples across the DA and used et al., 2012). These faults are characterized by exposes high metamorphic grade Neoprotero- ZFT and AFT, combined with previous data, to zoic basement with a complete Barrovian-type constrain the Cenozoic exhumation and defor- metamorphic sequence beneath Triassic fly- mation history in the DA and to discuss the Yuan-Hsi Lee http://orcid.org/0000-0002-1800 -2528 sch strata (Fig. 1) (Huang et al., 2003a, 2003b; exhumation mechanism and tectonic implica- *Corresponding author: [email protected] Weller et al., 2013; Fig. 1). While the high-grade tions in eastern Tibet (Fig. 2).

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 Cenozoic exhumation of the Danba antiform, eastern Tibet | RESEARCH

102 E 100rE 101Er r 103Er

N 40 km

X ianshui

Songpan-Garzê Fold Belt N r Barkam 2 3 he f t aul n Jinchuan e Luhuo m t belt A’ g Manai se lt ’ granite B ral nt t be e l Da C u Daofu Xiaojin

n N n fa r

ba antiDanba 31 sha Chengdu t n e form m n B h segmen Sout Long an Basi

Go 40rN A Q t ilia ng ichu TARIM gh Faul n S NORTH tyn Ta ha ga S Al n N

QA CHINA r Ku s Ya’an 0 nlun S 3 uture . Ku Qingling ha nlun Fa Shan Kangding BH ult n G Jins ha Su QT ture XS r Bangon H F. SGFB an g-Nujiang Suture Mesozoic it N LH 30r e Paleozoic Precambrian H Zangbo Suture Yi I A M A L A Y SOUTH Mesozoic pluton CHINA INDIA Cenozoic pluton 500 km Cenozoic sediments 90rE 100r( A A A’ Mesozoic granite Devonian Eocene to Mesozoic

Triassic NP Mesozoic detachemnt Pt2 Pt1 Paleo.+Prot. Paleoozoic +Protozoic strata

B 350 300 250 200 5 Km Figure 1. (A) Simplified geological map around the Danba area (modified from Xu and Kamp, 2000; Harrowfield and Wilson, 2005). Inset shows the tectonic map of Tibet (modified from Roger et al., 2010). Abbreviations: QA—Qaidam block; BH—Bayan Har block; Yi—Yidun block; SGFB—Songpan- Garzê fold belt; QT—Qiangtang block; LH—Lhasa block. (B) North-south–trending structural profile along the Danba antiform (DA); the profile location is shown in A. The Mesozoic and present detachment faults are shown in the section (Harrowfield and Wilson, 2005; Tan et al., 2014).

LITHOSPHERE | Volume 9 | Number 4 | www.gsapubs.org 535

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 XI-BIN TAN ET AL.

E101°30′ E102°0′

N Axi 114 s of a 10.5 z 23.7 a 8.4 0 20 km th a 7 a 10 Ki-12 a 10.4 e DA z 222.02.0 a 6.4 a 8.9 am 119 Ki-10 rb 91 z 20.6 ab 52 am 43 a 4.2 ua 67 Ki-4 z 337.87.8 C’ Bo-13 MNG a 5.8 z 10.8 Ki-14 a 2.9 z 113.3.44 rb 31 a 3.3 Bo-9 z 202 Lian z 20.8 a 12 Ki-1 rb 24 ′ a 9.7 0 He Ka Bo-11 rb 26 Bo-8 z 12.812.8 Limb z 52.7

N31° a 2.0 Core u F Bo-6 aul z 22.1 t a 3.3 Bo-16 ab 71 P Bo-15 Ki-7 e a 2.2 a 3.5 z 11.5 Limb ripher Danba Ki-9 a 3.8 z 8.2 y GCD ab 55 Db-14 ab 51;52;57 Kc-518 a 1.8 ab 259 z 12.8 ab 6655;; 76;81 Kc-533 rb 2828 ab 85 z 7.7 Db-15 Db-13 rb 30 GZD ab 55;48;94 X z 11.8 a 2.3 am 87;102 ianshuihe a 1.8 Db-17 Db 12 z 6.7 ab 47; - 48;58 z 10.2 ′ a 1.6 a 2.0 f Bo-18 ault z 12.3

N30°40 a 5.1 belt Bo-19 a 2.4-2.9 z 7.8-7.9 am 33.5 a 3.1 ab 4.5-5.5 rb 27.7 z 8.7 C ab 5.5 am 62 am 36.1 ab 10.4 ab 10.1 am 56

Gong ga Kann Complex shan g ra ni a 3.8 ′ te a 9 30°20

N a 85.7 a 69.3

Permian and Cenozoic granite Carboniferous Wilson and Fowler, 2011 Clark et al., 2005 Mesozoic granite Devonian Hallowfield, 2001 Wallis et al., 2003 Triassic Silurian Xu and Kamp, 2000 Huang et al., 2003a Precambrian Samples in this paper Zhang et al., 2004 Zhou et al., 2008 Detachment fault and Pre-Sinian gneiss

Figure 2. Geological map of the Danba antiform (modified from 1:250,000-scale geological maps H47C001003 and 48C001001, Bureau of Geology and Mineral Resources of Sichuan Province, 2004a, 2004b; Jolivet et al., 2015), and thermochronology data. The black dots are the sample locations for this study and the squares are previously published data. The symbols with bold letters and numbers are names of samples in this paper, and below them are the thermochronology method and ages (in Ma). Abbreviations: z—zircon fission track; a—apatite zircon fission track; ab—Ar/Ar biotite; am—Ar/Ar muscovite; ak—Ar/Ar K-feldspar; rb—Rb/Sr biotite; ua—SHRIMP (sensitive high-resolution ion microprobe) U/Pb apatite; GCD—Gongcai dome; GZD—Gezong dome; MNG—Manai granite.

536 www.gsapubs.org | Volume 9 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 Cenozoic exhumation of the Danba antiform, eastern Tibet | RESEARCH

GEOLOGICAL SETTING The second event (M2 and D2) only occurred and exhumation from the core to the limb during in the northern part of the DA with sillimanite- the Cenozoic. However, the limited available The SGFB grade metamorphism and local migmatization data do not provide adequate constraints on the at pressure-temperature (P-T) conditions of 4.8– Cenozoic deformation of the DA, which remains The SGFB is located in eastern Tibet, west 6.3 kbar and 640–725 °C ca. 164 Ma. D3 devel- poorly constrained (Wilson and Fowler, 2011). of the Sichuan basin, and is bounded by the oped mainly with northwest-oriented thrusts and LMS to the east, the Kunlun suture to the north, strike-slip shear zones in a transpressional set- METHODS AND RESULTS and the Jinsha suture to the south (Fig. 1). The ting associated with the deformation of the DA Songpan-Garzê basin deposits consist of late during the India-Eurasia collision. Weller et al. Sampling Strategy Neoproterozoic–Paleozoic and Triassic turbi- (2013) used a pseudosection approach to cal- dite sequences, which are underlain by Neo- culate P-T histories and determined that peak The ZFT and AFT dating methods record proterozoic crystalline basement (e.g., Zhou metamorphic conditions ranged from 5.2 kbar the time at which the rocks cooled through the et al., 2008). During the Triassic Period, the and 580 °C at staurolite grade, to 6.0 kbar and annealing zone, which is ~240 °C for ZFT (Bran- basin was inverted and deformed into a series 670 °C at sillimanite grade, and peak condi- don et al., 1998) and ~110 °C for AFT (Gleadow of northwest-trending tight folds with axial tions were reached ca. 191–184 Ma (equal to and Duddy, 1981; Donelick et al., 2005). For plane cleavage development (i.e., the SGFB), ~26 km burial depth). They identified two defor- radiation damaged zircons, Brandon et al. (1998) which record Barrovian metamorphism and a mation events (D1 and D2): D1 is associated considered the temperature limits of the ZFT par- northwest-trending, southwest-verging regional with the fabric S1, in which all metamorphic tial annealing zone (PAZ) to be at 180–240 °C décollement-fold belt (Huang et al., 2003b; index minerals align, and D2 is associated with for time scales of 107 yr. Harrowfield and Wilson, 2005). This shortening a crenulation cleavage fabric (S2) during low- Previous studies have found a ZFT age of event was accompanied by the intrusion of many temperature (280–400 °C) metamorphism. 202 ± 34 Ma outside the DA (Xu and Kamp, granitoids such as the Manai granite (de Sigoyer In the SGFB, the Jurassic–Cretaceous Peri- 2000) and several biotite Rb-Sr ages of ca. 30 et al., 2014; Fig. 1). ods are usually thought to be a time of little Ma within the antiform (Huang et al., 2003a). While much of the SGFB is covered by the tectonic activity, with no significant heating or The 202 ± 34 Ma ZFT age corresponds to the Triassic Songpan-Garzê flysch, older units are cooling events (Kirby et al., 2002; Wilson and Mesozoic deformation event and indicates that exposed in the DA and the LMS (Fig. 1). In Fowler, 2011; Roger et al., 2010). the amount of Cenozoic cooling is less than the these areas, the pre-Sinian granitic basement During the Cenozoic (primarily since Mio- ZFT partial annealing temperature (~180 °C) shares many similarities with the basement of cene time), the SGFB was reactivated by the outside the antiform, while the Cenozoic cool- the Yangze block (Roger et al., 2010) and is India-Asia collision. Although deformation has ing amount is ~300 °C (closure temperature of exposed in bodies including the Gezong and been recognized along the LMS, the Xianshuihe biotite Rb-Sr) inside the antiform. This suggests Gongcai complexes (Figs. 1 and 2). The granitic strike-slip fault and other active faults (e.g., Xu that, from the outside to the core of the anti- rocks are overlain by a sequence of metamor- et al., 2009; Ren et al., 2013; Fig. 1), the regional form, the ZFT ages decrease from Mesozoic phosed Sinian and Paleozoic marine sedimen- uplift history, and mechanism of deformation non-reset ages to partially reset ages and then to tary rocks, dominated by Sinian dolomitic mar- are debated (Wang et al., 2012; Tian et al., completely reset ages. The oldest Cenozoic reset ble and/or metapelite and Silurian–Devonian 2013). Low-temperature thermochronology data ZFT age indicate the onset of Cenozoic defor- metapelite and paragneiss with minor quartzite, for eastern Tibet indicate accelerated cooling mation. For this purpose, we collected 22 sam- marble, and amphibolites (Chengdu Institute of since the Miocene (Arne et al., 1997; Cook et al., ples widely spread from the core to the periphery Geology and Mineral Resources, 1991; Huang 2013; Xu and Kamp, 2000; Clark et al., 2005; of the DA, and measured 17 ZFT ages and 16 et al., 2003b). The basement and the cover Wilson and Fowler, 2011). In the plateau interior, AFT ages (Fig. 2; Table 1). Compared with ZFT, sequence in the Danba area are separated by cooling histories obtained on Mesozoic granites the AFT system has a lower closure temperature, a southwest-verging ductile shear zone, which from the Songpan-Garzê region show very slow allowing us to use this system to evaluate the is thought to be part of the regional Triassic and regular cooling between ca. 150 and 30 Ma more recent cooling history. All samples were décollement that is also now exposed in the (Xu and Kamp, 2000; Kirby et al., 2002; Huang taken from bedrock outcrops along the bottom LMS (Zhou et al., 2008; Harrowfield and Wil- et al., 2003a; Roger et al., 2004, 2010; Zhou of the valley to reduce the influence of elevation, son, 2005; Roger et al., 2010; Figs. 1 and 2). et al., 2008). In southeastern Tibet, Clark et al. hence the samples have similar elevations and (2005) found slow cooling (<1 °C/m.y.) between can be compared along a cross section. Tectonic Settings ca. 100 and ca. 20–10 Ma and a change to rapid cooling after ca. 13 Ma with initiation of rapid Experimental Methods Based on the mineral assemblage of the DA river incision at 0.25–0.5 mm/yr between 13 rocks, the maximum metamorphic temperatures and 9 Ma. The low-temperature thermochro- The sample preparation and experimental are estimated to have been 710 ± 10 °C in the nology data [fission track and (U-Th)/He] show processes follow the methods of Liu et al. (2000, pre-Sinian granitic basement in the core of the abrupt differences in ages across the faults in 2001). We used grain by grain and mica external DA, decreasing to 470 °C in the Triassic rocks the LMS, suggesting significant uplift along the detector techniques to obtain individual grain in the limb (Huang et al., 2003a, fig. 1 therein). faults (Godard et al., 2009; Tian et al., 2013; Tan ages (Wagner and Van Den Haute, 1992). Zeta Huang et al. (2003b) identified three thermo- et al., 2014, 2015). values (Green, 1985; Hurford and Green, 1983) tectonic events. The first event (M1 and D1) in For the DA, several research groups have for the standard glasses CN-5 (apatite) and SRM- the DA region is characterized by Barrovian measured Cenozoic ages using biotite and mus- 610 (zircon) were 340 ± 12 (1σ) and 27.5 ± 1.0 metamorphism, which peaked at kyanite-grade covite Ar-Ar dating and biotite Rb-Sr dating (1σ), respectively. Neutron irradiation was car- conditions of 5.3–8 kbar (equal to ~22–29 km (Huang et al., 2003a; Zhou et al., 2008; Wallis ried out at the National Tsing Hua University burial depth) and 570–600 °C ca. 210–205 Ma. et al., 2003), which suggest differential cooling Reactor of Taiwan. Errors were calculated using

LITHOSPHERE | Volume 9 | Number 4 | www.gsapubs.org 537

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 XI-BIN TAN ET AL.

the conventional analysis given by Green (1981). TABLE 1. SUMMARY OF THE SAMPLE LOCATIONS, LITHOLOGY, AND AGES In order to confirm the age accuracy, two apa- Sample Long Lat Elevation LithologyMethod and ages tite samples (Ki-1 and Bo-15) were analyzed by (°N) (°E) (m) ZFT AFT Apatite to Zircon, Inc. (www.apatite.com/; A2Z) (±1σ Ma) (±1σ Ma) as well as by the low temperature thermochro- Bo-6 30.997 102.057 2133 metasandstone 22.1 ± 1. 33.3 ± 0.6 Bo-8 31.006 102.085 2142 metasandstone 52.7 ± 3.0 nology laboratory at the University of National Bo-9 31.006 102.070 2137 metasandstone 20.8 ± 1. 4 Chung-Cheng. The ages for Ki-1 are 9.7 ± 0.9 Bo-1131.023 101. 8681985granite 12.8 ± 0.62.0 ± 0.6 Ma in our laboratory and 9.2 +1.28/–1.12 Ma by Bo-1331.027 101. 8372333granite 10.8 ± 0.62.9 ± 0.6 A2Z, and those for Bo-15 are 3.5 ± 0.3 Ma in our Bo-15 30.893 101. 8732014gneiss 3.5 ± 0.3 3.77 +0.55/–0.48* laboratory and 3.77 +0.55/–0.48 Ma by A2Z. The Bo-16 30.874101.871 1860 granite dike2.2 ± 0.3 two samples (Ki-1 and Bo-15) were analyzed Bo-18 30.828 101. 8002060granite 12.3 ± 0.75.1 ± 1. 0 with a Cf-252 source to reveal more horizontal Db-12 30.725 102.023 1790 granite10.2 ± 0.82.0 ± 0.3 confined tracks in the A2Z laboratory (Donelick Db-13 30.861 101. 8511947granite 2.3 ± 0.4 and Miller, 1991). The consistency of these ages Db-14 30.861 101. 8531947granite 1. 8 ± 0.3 Db-15 30.777 101. 7292231granite 11.8 ± 0.61.8 ± 0.3 indicates that our ages are reliable (Table 1). Db-17 30.744 101. 7372466granite 6.7 ± 0.51.6 ± 0.3 Ki-1 31.013 102.325 2326 metasandstone 9.7 ± 0.9 Results 9.2 +1.28/–1.12* Ki-4 31.009 102.0742142metasandstone 37.8 ± 1. 95.8 ± 0.6 Ki-7 30.944 101. 9752076granite 11.5 ± 0.6 The summarized ZFT and AFT ages and Ki-9 30.892 101. 9172000granite 8.2 ± 0.5 sample conditions are shown in Table 1 and Ki-1031. 125101.882 2012 granite20.6 ± 1. 04.2 ± 0.5 Figure 2. The detailed dating results are shown Ki-1231. 175101.873 3190 granite22.0 ± 1. 06.4 ± 0.5 in Tables 2 and 3. In the eastern limb of the Ki-1431.030 101. 8681991granite 13.4 ± 0.73.3 ± 0.5 DA, the ZFT pooled ages are 52.7 ± 3.0 Ma Kc-518 30.807 101. 9411843granite 12.8 ± 0.8 Kc-533 30.791 101. 9651814granite 7. 7 ± 0.6 (Bo-8) and 37.8 ± 1.9 Ma (Ki-4), decreasing 114* 31.153 101. 9032240metasandstone 23.7 ± 1. 37.1 ± 1. 5 to 22.1 ± 1.3 Ma (Bo-6) and 20.8 ± 1.4 Ma 116* 31.0283102.241 2800 metasandstone 202.1 ± 3.912.0 ± 6.6 (Bo-9) (Fig. 2). In the northeastern limb the old- 01-11† 101.9297 31.198 2840 granite8.43 ± 0.87 † est ZFT pooled ages are ca. 24 Ma (114 in Xu 01-12 101.9262 31.195 2580 granite10.0 ± 0.7 01-13† 101.9241 31.19372500granite 10.4 ± 0.7 and Kamp, 2000) to ca. 21 Ma (Ki-10). In the 01-14† 101.9290 31.20423000granite 10.5 ± 1. 1 core, the ZFT ages are younger than 12–10 Ma. 01-15† 101.9320 31.17332100granite 8.87 ± 0.49 Generally, our results combined with previous Note: ZFT is zircon fission track; AFT is apatite fission track. studies show ZFT ages decreasing from ca. 202 *Data are from A2Z (Apatite to Zircon, Inc.; www.apatite.com/) for comparison and test. † Ma, 53 Ma, 37 Ma, 24–20 Ma, to 14–12 Ma, and Data are from Xu and Kamp (2000); Clark et al. (2005). finally to 10–8 Ma from the limb to the core area (Fig. 2). AFT ages are generally ca. 12–10 Ma TABLE 2. ZIRCON FISSION TRACK AGES outside of the DA and decrease to ca. 7–3 Ma Sample Crystal ρs Ns ρi Ni ρDP(χ2) Zircon ages in the antiform limbs and 4–2 Ma in the core (%) (±1σ Ma) area (Table 3; Fig. 2). The AFT ages are younger Bo-6 20 44.67 1650 32.678 1207 11.287.9 22.1 ± 1. 3 than 10 Ma and the track density is generally too Bo-8 22 70.181 2704 19.7 759 10.86<0.1 52.7 ± 3.0 low to measure enough confined track lengths (101–26)* Bo-9 10 36.183 1107 25.898 559 10.924.6 20.8 ± 1. 4 (100+) to evaluate the thermal history. The 2 Bo-111788.167 3284 94.799 3531 10.094.8 12.8 ± 0.6 samples analyzed with Cf-252 by A2Z (Ki-1 Bo-131953.909 1880 60.132 2097 8.4 96.210.8 ± 0.6 and Bo-15) yielded enough track lengths, and Bo-181919.443 958 20.296 1009.427.9 12.3 ± 0.7 the mean length of 15.02–14.51 mm indicates Db-121612.513 388 16.124 500 9.642 90.110.2 ± 0.8 relatively rapid cooling through the apatite PAZ Db-152033.6191550 34.79 1604 8.95 99.911. 8 ± 0.6 Db-171615.646 451 28.379 8188.9515.86.7 ± 0.5 (Wagner and Van den haute, 1992). Ki-4 23 60.203 4594 19.421 1482 8.95 <0.1 37.8 ± 1. 9 (116–30)* DISCUSSION (P1:26.8, 47.9%) (P2:47.9, 52.1%) Ki-7 14 62.117 2308 61.17 2362 8.6 34.011. 5 ± 0.6 Onset of the Cenozoic Cooling Ki-9 19 11.533 800 21.119 1465 10.92 23.58.2 ± 0.5 Ki-10 14 90.793 3860 51.841 2204 8.65.6 20.6 ± 1. 0 The Ar/Ar ages from the DA are widely scat- Ki-12 24 61.697 3832 41.894 2602 10.92 40.3 22.0 ± 1. 0 tered; Zhou et al. (2008) attributed this to slow Ki-14 11 43.454 1554 33.61 1202 7. 56 6.313.4 ± 0.7 Kc-5181813.555 828 13.621 832 9.42 8.22 12.8 ± 0.8 cooling from ca. 166 Ma to ca. 47 Ma. The Rb/ Kc-533 12 6.677 41613.113817 11.04 26.77.7 ± 0.6 Sr biotite ages concentrate between ca. 34 and 114† 20 6.487 494 22.64 1724 1. 24 93.7 23.7 ± 1. 3 24 Ma in the DA; Huang et al. (2003a) related 116† 920.809 1053 7.574 397 1. 26 <0.1 202.1 ± 3.9 this to their D3 deformation and the onset of Note: ρs—density of spontaneous tracks (105tr/cm2); Ns—number of spontaneous tracks; ρi—density of 2 Cenozoic rapid cooling in the DA. Compared to induced tracks (105 tr/cm ); Ni—number of induced tracks; Nd—4789 tracks counted on NBS 610 and 612; P(χ2)—probability that single grain ages are strongly not consistent if P(χ2) <0.1% (Galbraith, 2005). the Ar/Ar and Rb/Sr data, the ZFT data provide *Range of grain ages. Pooled ages are calculated using a ζ-value of 340 ± 12 (1σ) and 27.5 ± 1. 0 (1σ), additional constraints on the onset of Cenozoic respectively. Decomposition of fission track ages was based on BinomFit (Brandon, 1992, 1996). † cooling. In the eastern part of the DA, the ZFT Data from Xu and Kamp (2000).

538 www.gsapubs.org | Volume 9 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 Cenozoic exhumation of the Danba antiform, eastern Tibet | RESEARCH

TABLE 3. APATITE FISSION TRACK AGES space across the DA. In the core of the DA, the Sample Crystal ρS Ns ρi Ni ρDP(χ2) Apatite age biotite Rb-Sr ages are between 34 and 24 Ma, (±1σ Ma) close to the onset of the Cenozoic cooling, while Bo-0642 0.255 39 5.712874 4.22 38.373.3 ± 0.6 the ZFT ages are younger than 12 Ma, indicat- Bo-1141 0.086 14 3.094 505 4.22 39.972.0 ± 0.6 ing a slow average cooling rate of ~3 °C/m.y. Bo-1340 0.174 27 4.386 681 4.22 86.292.9 ± 0.6 between ca. 27 Ma and 12 Ma. The cooling rate Bo-1540 0.777 16816.522 3574 4.22 6.95 3.5 ± 0.3 then increases to ~12–25 °C/m.y. between 12 Bo-1641 0.28 56 9.561 1910 4.29 97.512.2 ± 0.3 Bo-1840 0.152 30 2.205 436 4.22 50.365.1 ± 1. 0 Ma and 3 Ma and then ~29–63 °C/m.y. from Db-1240 0.172 40 5.363 1248 3.5 95.622.0 ± 0.3 ca. 3 Ma to present, which indicates a strong Db-1341 0.258 44 6.993 1191 3.5 89.342.3 ± 0.4 acceleration since the middle Miocene (Fig. 4). Db-1442 0.341 55 11.711 1889 3.5 82.161.8 ± 0.3 In the limb of the DA, the cooling rate is Db-1540 0.193 39 6.233 1260 3.37 39.611.8 ± 0.3 Db-1750 0.228 26 8.189 932 3.37 98.7 1. 6 ± 0.3 ~8 °C/m.y. between ca. 24 Ma (~240 °C) and ca. Ki-1 40 1. 153 277 7.795 1872 3.75 97.919.7 ± 0.9 7 Ma (~110 °C), according to the ZFT and AFT Ki-4 40 0.688 1407.995 1628 3.89 86.455.8 ± 0.6 ages. However, considering that the timing of Ki-1040 0.482 1217.877 1976 3.89 47.814.2 ± 0.5 a change in uplift and cooling rate in the limb Ki-1235 2.816 643 28.904 6600 3.75 68.946.4 ± 0.5 area is consistent with the timing in the core Ki-1440 0.308 71 6.895 1591 4.27 73.063.3 ± 0.5 114* 20 0.068 23 2.427 8161.45 99.3 7. 1 ± 1. 5 area, we suggest that the cooling history in the 116* 11 0.136 7 2.858 1471.45 97.8 12.0 ± 6.6 limb area should have pattern similar to that in 01-11† 21 0.179 106 4.841 2863 4.004 8.43 ± 0.87 the core, yielding a rate of ~19 °C/m.y. from 12 01-12† 24 0.641 272 14.52 6161 4.002 10.0 ± 0.7 Ma to present in the limb area (Fig. 4). In the 01-13† 18 0.764 303 16.731 6639 4.006 10.4 ± 0.7 01-14† 21 0.153 96 3.311 2083 4.007 10.5 ± 1. 1 periphery area, the cooling rate is ~8 °C/m.y. 01-15† 22 0.957 470 24.57412063 4.003 8.87 ± 0.49 since ca. 10 Ma according to the AFT ages; this is also consistent with the timing of acceleration Note: ρs—density of spontaneous tracks (105tr/cm2); Ns—number of spontaneous tracks; ρi—density of induced tracks (105 tr/cm2); Ni—number of induced tracks; Nd—4789 tracks counted on NBS 610 and 612; in the core (Fig. 4). P(χ2)—probability that single grain ages are strongly not consistent if P(χ2) < 0.1% (Galbraith, 2005). *Data from Xu and Kamp (2000). †Data from Clark et al. (2005). Mesozoic to Cenozoic Folding of the DA

AFT and ZFT ages progressively increase ages increase progressively from 12–8 Ma in to ca. 25 Ma (between ca. 27 and 24 Ma) (Fig. 4). from the core to the margin of the DA (Fig. 5). the core to 24–20 Ma at the limb, and to ca. This is consistent with a biotite Rb-Sr age of This is consistent with a folding structure, which 202 Ma to the northeast of the DA (Fig. 2). This ca. 26 Ma (Huang et al., 2003a) found near the results in higher exhumation and cooling rates in Mesozoic age, obtained far from the DA, cor- boundary of the total reset ZFT zone (Fig. 2). the core and lower rates in the margin area. On responds to a domain that has recorded a meta- the southwestern limb of the DA there is a small morphic temperature of ~300–400 °C (Wang Accelerated Cooling of the DA Since the dome with Precambrian granitoid exposed in its et al., 2013), higher than the ZFT closure tem- Middle Miocene core (Figs. 2 and 5). The AFT and ZFT ages here perature (~240 °C) (Brandon et al., 1998), sug- are as young as the ages in the core area of the gesting that at this location only a Late Triassic The use of multiple low-temperature ther- DA, suggesting that this small dome has been deformation event is recorded by the ZFT data. mochronometers with different closure tempera- active simultaneously with the DA since the late This implies that the post–Late Triassic exhu- tures (~300 °C biotite Rb-Sr, ~240 °C ZFT, and Miocene. The AFT ages are younger than 3 Ma, mation amount should be less than the burial ~110 °C AFT) and ages affords an opportunity suggesting that the folding maybe still active. depth of the ZFT partial annealing zone (PAZ) to constrain the pattern of Cenozoic cooling The total amount of cooling and exhumation (with temperatures of ~180–240 °C; Brandon et (Fig. 4). The cooling rate varies in time and since the peak Mesozoic metamorphism has been al., 1998), and indicates a non-reset post–Late Triassic zone on the periphery of the DA. In the northeastern margin of the DA, the ZFT pooled ages are widely distributed and decrease from 53 Ma to 37 Ma, indicating that these samples were Bo-8 Ki-4 located within the PAZ for a long period of time (Galbraith, 2005). In order to estimate the old- 68.2 51.1 est Cenozoic reset age we use BinomFit (Bran- don, 1992) to decompose the grain age spectrum, yielding younger group ages of ca. 36 Ma and 35.7 26.8 27 Ma (for Bo-8 and Ki-4, respectively) (Fig. 3; Table 1). The oldest reset zircon ages are ca. 24 Ma in the northeastern limbs (sample 114 from Xu and Kamp, 2000). Although the 27 Ma group age is a maximum for the onset of cooling, the close proximity between sample Ki-4 and the Figure 3. Decomposed grain ages from samples Bo-9 and Ki-4 located near the base of 24 Ma completely reset sample suggests that the the total rest zone, using BinomFit (Brandon, 1992, 1996). FT—fission track; SE—standard onset of Cenozoic cooling could be constrained error; RSE—relative standard error.

LITHOSPHERE | Volume 9 | Number 4 | www.gsapubs.org 539

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 XI-BIN TAN ET AL.

a 40 Exhumation rate: ɗ/M 0.35 mm/yr ~19

80 /M a ɗ

63 9 AFT 120 ~2

160 a ഒ Periphery /M emperature ȭ 200 T 5

12-2 T 240 ~ r K ZF Limb r-A a

A 280 ~2-3!ɗ/M r Bt 320 Core Exhumation rate: Ar-A

Ms 0.8 mm/yr 360 Ar-Ar

400

440

Biotite Rb-Sr age (Huang et al., 2003a)

210 200 60 55 50 45 40 35 30 25 20 15 10 5 0 Age (Ma)

Figure 4. Cenozoic cooling histories of the Danba antiform (DA). Different colored lines show the cooling paths based on the thermochronology data. The orange and yellow shading show the core and limb areas, respectively, and the gray color shows the periphery of the DA. The thick dashed lines indicate cooling histories at the exhumation rates of 0.35 mm/yr and 0.8 mm/yr, which fit the periphery and core of the DA, respectively. The cooling histories are modeled by the TERRA software (www.terrasoftware.com/), with the following conditions: surface temperature 10 °C; basal temperature gradient 25 °C/km, which is slightly smaller than the average geothermal gradient of ~27 °C/km at the Songpan-Aba region (Chen et al., 2013); maximum model depth 30 km; diffusivity 1.3636 × 10−6m2/s. Inset figures show the track length spectrum and thermal modeling results. ZFT—zircon fission track; AFT—apatite zircon fission track; Ms—muscovite; Bt—biotite.

constrained by the metamorphic isograds in the 110 °C isotherms can be defined by the ZFT and activity took place after 12 Ma in the DA. The Danba region (Huang et al., 2003b; Weller et al., AFT ages (Fig. 6C). The 240 °C isotherm is con- Cenozoic folding reactivated the preexisting 2013; Robert et al., 2010; Fig. 6A). To evaluate strained by the ca. 12 Ma ZFT ages (Fig. 6C), and Mesozoic fold, pointing out the importance of the Cenozoic folding history, we constructed in east of the DA, we use the AFT ages of 12–10 inherited structures in partitioning deformation cooling isotherm maps at 30 Ma (Fig. 6B) and Ma to constrain the 110 °C isotherm (Fig. 6C). in the Tibetan Plateau. 12 Ma (Fig. 6C). The Rb-Sr biotite ages are ca. With these isotherms, we made a profile of cool- Because an increase in exhumation rate will 30 Ma in the DA and the oldest reset ZFT age ing temperature (D-D′ in Fig. 6A) across the DA cause the advection of heat toward the surface, is ca. 25 Ma in the northeastern limb. Based on at different times to quantitatively constrain the affecting the geothermal gradient and there- these data, we roughly constrain the 300 °C iso- cooling and exhumation history from Mesozoic fore the calculation of erosion rates (Reiners therm in the core and the 240 °C isotherm in the to Cenozoic time (Fig. 6D). and Brandon, 2006), we used the TERRA soft- limb to ca. 30 Ma (Fig. 6B). East of the DA, the As shown in Figure 6D, the temperature dif- ware (www.terrasoftware.com/; Ehlers et al., AFT ages (110 °C) are 12–10 Ma, and 2 zircon ferences between the core and the outside of 2005) to model the cooling rate since 12 Ma (U-Th)/He ages (180 °C) in the eastern side of the DA in the Mesozoic, and ca. 30 Ma and with various rates of steady-state erosion. The the SGFB are ca. 86–55 Ma (Kirby et al., 2002). ca. 12 Ma are ~300 °C, 165 °C, and 140 °C, results show that the observed cooling rates are These thermochronometry data reveal that cool- respectively. These temperature differences are best fit when the erosion rates are ~0.35 mm/yr ing from 180 °C to 110 °C took place from 55 to interpreted to correspond to the amount of dif- and ~0.8 mm/yr in periphery and core of DA, 12 Ma, yielding a cooling rate of 0.9–1.2 °C/m.y. ferential uplift due to folding, indicating that respectively (Fig. 4). The topographic variation From this rate, we estimate that the periphery more than half of the differential uplift occurred is rather small across the DA, indicating that of the DA had been cooled to ~135 °C at 30 during the Cenozoic. Between 30 Ma and 12 the erosion rate nearly balances with the uplift Ma. Figure 6B shows the 300 °C, 240 °C, and Ma the relative temperature difference is small, rate. The 0.45 mm/yr difference in erosion rate 135 °C isotherms in the core, limb, and outside indicating that folding was not significant during across the DA suggests differential uplift of 5.4 of the DA at 30 Ma. At 12 Ma, the 240 °C and this time and that the major Cenozoic folding km since 12 Ma (Fig. 6D).

540 www.gsapubs.org | Volume 9 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 Cenozoic exhumation of the Danba antiform, eastern Tibet | RESEARCH

A 120

100

) ault

Ma 80 F

(

e he i

g u A 60 sh

40 Xian

( 30

e M Abrupt change Rb/ Sr Biotit e

Ag A e D ? th ng 20 i of m l do Axis mal S 10 ZFT

AFT

0 60 40 20 0 20 40 60 Horizontal distance (km)

ZFT ages in this paper Muscovite Ar/Ar ages, from Wallis et al (2003) AFT ages in this paper and Huang et al (200 3a) Biotit e Ar/A r ages,from Zhang et al (2004), ZFT ages from Xu and Kamp (2000) Wallis et al (2003), Zhou et al (2008) AFT ages from Xu and Kamp (20 00), Cla rk et Bio tite Rb/Sr ages, from Huang et al (2003a) al (2005), and Wilson and Fowler (201 1)

B ) 5 B B’ km (

n 4 Topographic profile o

ti a 3 Elev Xianshuihe 2 fault P&C C’ m) 4 C k ( P&C + + T + + + P&C S + + + D T 0 + D D + S ation + + + + + v + + + + e + + l + Neoproterozoi+ + + + c E Triassic + + Triassic -4 crystalline flysch basement flysch

0 20 40 60 80 100 120 Horizontal distance (km) T: Triassic P&C: Precambrian and Cambrian D: Devonian S: Silurian

Figure 5. (A) Plot of thermochronology ages versus distance along a transect perpendicular to the axis of the Danba antiform (DA), shown in Figure 2. ZFT—zircon fission track; AFT—apatite zircon fission track. (B) Topographic profile and geological cross section across the antiform; see Figure 1 for location of B-B′ and Figure 2 for location of C-C′. The simplified geological cross section is revised from Harrowfield and Wilson (2005) and 1:250,000-scale geological maps H47C001003 and 48C001001 (Bureau of Geology and Mineral Resources of Sichuan Province, 2004, 2004b).

LITHOSPHERE | Volume 9 | Number 4 | www.gsapubs.org 541

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 XI-BIN TAN ET AL.

101rಿE 101rಿE 101rಿE 101rಿE +Bt A 0 4 8 B 0 4 8 N N km km N ~135 N ಿ D’ z24 D’ ಿ z22 r Ser-Chl ab 52 r am119 0 z 21 ua 67 ɗ 3 300400 ɗˢ 30 670-710 ɗˢ am43 +St +Bt ~2 z202 +Sil +Mig z13 4 a 12 0 ɗ rb 31 z13 a 10 rb24 z11 470 640-650 ɗˢ z20 z38 ɗˢ z53 z22 +Sil 5 ab71 70 z12 z8

585-590 ɗˢ ɗˢ Danba ab55-81 Danba +Grt +Grt +Ky ab259 z 12 Ser-Chl 580-590ć ab52;52;57 ab85 +Bt +St z12 rb28 rb 30 ab94 D D ab55;ab 48; z6 ab47; am 87;am 102 48;58

X X SH SH F F N ಿ N ಿ r r ca.30 Ma 30 30

101rಿE 101 E rಿ 800 C D 0 4 8 N 0 4 8 Metamorphic a 9 Temperature km a 8-11 km N

z22 D’ ಿ a 6 z24 a 7 0 r 600 z21 3 a 4 ~110 ɗ / ć

z11 0 ɗ z202 e a 3 z13 z38 a 10 30 z20 a 12 ur a 6 t ~ z13 z53 a ɗˢ z22 er 300-400

p 400 a 3 m ~300 ɗ z12 Te z8 ~240 ɗ a 2 a 4 Danba 5 ɗ a 2 ) z12 6 ~135 ɗ 200 a 5 z13 ~1 40 ɗ ɗ ~2 4 km 30 Ma z12 z8 5.

D a 2 ~140 (~ z7 12 Ma a 2 z10 Present (~10 ȭ) ~110 ɗ 0 D D’

X SH F N

ಿ Precambrian Mesozoic Cenozoic Paleozoic Triassic complex granite granite ca. 12 Ma 0 r 3

Figure 6. (A) Metamorphic map of the Danba antiform (DA) showing the metamorphic isograds (dotted lines; revised from Huang et al., 2003b). Abbre- viations: Bt—biotite; Grt—garnet; St—staurolite; Ky—kyanite; Sil—sillimanite; Mig—migmatite; Ser—sericite; Chl—chlorite; XSHF—Xianshuihe fault. (B) Estimated isotherms at 30 Ma, based on the thermochronology data. The black star is an outlier sample. (C) The isotherms at 12 Ma. (D) Profiles of temperature at peak metamorphic grade, ca. 30 Ma, ca. 12 Ma, and the present along line D-D′.

542 www.gsapubs.org | Volume 9 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 Cenozoic exhumation of the Danba antiform, eastern Tibet | RESEARCH

Tectonic Implications of DA Evolution magnetotelluric and P and S wave studies) sug- Bai, D., et al., 2010, Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging: Nature gest that crustal channel flow may have devel- Geoscience, v. 3, p. 358–362, doi:10.1038/ngeo830. Recent evidence suggests that the onset oped on the hanging wall of the LMS fault (Bai Brandon, M.T., 1992, Decomposition of fission-track grain- of uplift of the eastern margin of the Tibetan et al., 2010; Liu et al., 2014). We compared age distributions: American Journal of Science, v. 292, p. 535–564, doi:10.2475/ajs.292.8.535. Plateau is early-middle Cenozoic (30–25 Ma) the difference of cooling rates across the DA Brandon, M.T., 1996, Probability density plot for fission- (Wang et al., 2012; Li et al., 2012; Tan et al., (Fig. 6D) and estimated the uplift due to fold- track grain-age samples: Radiation Measurements, v. 26, 2014), instead of late Cenozoic, as previously ing to be ~5.4 km, which cannot explain all of p. 663–676, doi:10.1016/S1350-4487(97)82880-6. Brandon, M.T., Roden-Tice, M., and Garver, J.I., 1998, Late Ce- proposed (Kirby et al., 2002; Clark et al., 2005; the measured exhumation (~11 km, assuming a nozoic exhumation of the Cascadia accretionary wedge Godard et al., 2009). A late Oligocene cooling 25 °C geothermal gradient) since 12 Ma in the in the Olympic Mountains, northwest Washington State: event in the DA was also proposed (Huang et al., core of DA. In addition, on the periphery of the Geological Society of America Bulletin, v. 110, p. 985– 1009, doi:10.1130/0016-7606(1998)110<0985:LCEOTC>2 2003a). Our new thermochronology data show DA the exhumation rate increased after ca. 12 .3.CO;2. that an onset of Cenozoic exhumation ca. 25 Ma Ma (based on the 12–10 Ma AFT ages). Because Burchfiel, B.C., 2004, 2003 Presidential address: New technol- ogy, new geological challenges: GSA Today, v. 14, p. 4–9, is consistent with the Oligocene cooling event the detachment fault of the LMS is nearly flat doi:10.1130/1052-5173(2004)014<4:PANNGC>2.0.CO;2. in the DA and with the onset of uplift in the in the plateau region of the SGFB, it should Bureau of Geology and Mineral Resources of Sichuan Prov- east margin of the plateau. Moreover, our study not contribute to the exhumation here. There- ince, 2004a, Geological map H47C001003: Beijing, Geo- logical Publishing House, scale 1:250,000. shows that the cooling rate in the DA remained fore the rest of the regional exhumation, which Bureau of Geology and Mineral Resources of Sichuan Prov- relatively low from ca. 25 Ma until ca. 12 Ma is widely distributed in eastern Tibet (Clark et ince, 2004b, Geological map 48C001001: Beijing, Geo- (Fig. 4). The acceleration of cooling and dif- al., 2005; Tian et al., 2015), could be caused logical Publishing House, scale 1:250,000. Chen, H., Wu, Y., and Xiao, Q., 2013, Thermal regime and pa- ferential uplift ca. 12 Ma is consistent with the by crustal channel flow (Royden et al., 1997). leogeothermal gradient evolution of Mesozoic–Ceno- timing of both the second phase of rapid exhu- Our observations indicate that both upper crustal zoic sedimentary basins in the Tibetan plateau, China: China University of Geosciences Earth Science Journal, mation at 15–10 Ma in the LMS (Godard et al., deformation and crustal channel flow may have v. 38, p. 541–552. 2009; Wang et al., 2012; Tan et al., 2014) and the developed simultaneously in the Danba area. Chen, Z., Burchfiel, B.C., Liu, Y., King, R.W., Royden, L.H., Tang, initiation of rapid river incision between 13 and W., Wang, E., Zhao, J., and Zhang, X., 2000, Global po- sitioning system measurements from eastern Tibet and 9 Ma in eastern Tibet (Clark et al., 2005; Tian CONCLUSION their implications for India/Eurasia intercontinental et al., 2015), all of which suggest that the Oli- deformation: Journal of Geophysical Research, v. 105, gocene and middle Miocene events are regional We use low-temperature thermochronology p. 16,215–16,227, doi:10.1029/2000JB900092. Chengdu Institute of Geology and Mineral Resources, 1991, tectonic events in eastern Tibet. data to constrain the cooling and deformation Geological map of Qinghai-Xizang (Tibet) Plateau and Our thermochronology data provide evi- history across the DA. The ZFT data include adjacent areas and explanation: Chengdu Geological dence for Cenozoic folding in the DA, indicating Cenozoic non-reset, partially reset, and totally Publishing House, scale 1:1,500,000. Clark, M.K., House, M.A., Royden, L.H., Whipple, K.X., Burch- that crustal shortening was not limited to the pla- reset ages from the periphery to the core of the fiel, B.C., Zhang, X., and Tang, W., 2005, Late Cenozoic teau margin, but extended to the interior of the DA, and the oldest Cenozoic ZFT ages of ca. 25 uplift of southeastern Tibet: Geology, v. 33, p. 525–528, doi:10.1130/G21265.1. plateau. Using balanced cross-section analysis, Ma indicate the onset of the Cenozoic cooling Cook, K.L., Royden, L.H., Burchfiel, B.C., Lee, Y.H., and Tan, X., Hubbard and Shaw (2009) concluded that the event. However the cooling rate is rather low 2013, Constraints on Cenozoic tectonics in the southwest- 2008 Wenchuan earthquake occurred along a from 25 Ma to 12 Ma, then abruptly increased ern Longmen Shan from low-temperature thermochronology: Lithosphere, v. 5, p. 393-406, doi:10 .1130/L263.1. ramp and flat structure at a depth of ~16–20 km, from ca. 12 Ma. The AFT ages are as young as de Sigoyer, J., Vanderhaeghe, O., Duchene, S., and Billerot, consistent with earthquake and geodetic data. post–3 Ma in the antiform core, suggesting that A., 2014, Generation and emplacement of Triassic gran- The low-temperature thermochronology data the folding may be ongoing. itoids within the Songpan Ganze accretionary-orogenic wedge in a context of slab retreat accommodated by of Tan et al. (2014) also suggest a ramp and Both the LMS and the DA show similar exhu- tear faulting, Eastern Tibetan plateau, China: Journal flat structure under the southern LMS, as well mation histories since the Oligocene, suggesting of Asian Earth Sciences, v. 88, p. 192–216, doi:10.1016 /j.jseaes.2014.01.010. as an onset of Cenozoic exhumation of southern that the two regions may be related and that the Donelick, R.A., and Miller, D.S., 1991, Enhanced TINT fission segment of LMS at 30–20 Ma and one major detachment fault of the LMS could extend to the track densities in low spontaneous track density apatites fault with rapid exhumation since 15–10 Ma. DA and explain the Cenozoic folding. However, using 252Cf derived fission fragment tracks: A model and experimental observations: Nuclear Tracks and Radia- Because the DA is located on the hanging wall upper crustal folding cannot explain all of the tion Measurements, v. 18, p. 301–307, doi:10 .1016/1359 of the southern segment of the LMS fault, we Cenozoic exhumation in the region, suggesting -0189(91)90022-A. argue that the detachment fault of the LMS may that crustal channel flow may have developed Donelick, R.A., O’Sullivan, P.B., and Ketcham, R.A., 2005, Ap- atite fission-track analysis: Reviews in Mineralogy and have extended north to the Danba area (Fig. 1B). simultaneously to enhance the exhumation. Geochemistry, v. 58, p. 49–94, doi:10 .2138 /rmg .2005 .58 .3. In both the LMS and the DA, Mesozoic detach- Ehlers, T.A., et al., 2005, Computational tools for low-tem- ment faults from different depths with top-to-the ACKNOWLEDGMENTS perature thermochronometer interpretation: Reviews in Mineralogy and Geochemistry, v. 58, p. 589–622, doi: This project was fully supported by the Special Projects for Basic south shearing are now exposed at the surface 10.2138/rmg.2005.58.22. Research Work of the Institute of Geology, China Earthquake Galbraith, R.F., 2005, Statistics for fission track analysis: Boca (Huang et al., 2003b; Harrowfield and Wilson Administration (IGCEA1518) and the National Science Council, Raton, Florida, Chapman & Hall/CRC, 211 p., doi:10.1201 Taiwan, ROC, under grant NSC 100-2119-M-94-002. We thank 2005), indicating that the Mesozoic folding of /9781420034929. Xin-Mei Tu, Wen-Lin Tsai, Cheng-Yang Xu, Shao-Jun Wang, Gleadow, A.J.W., and Duddy, I.R., 1981, A natural long-term the DA was likely related to shortening in the Chong Xu, Kang Li, and Qi Yao for their help with field work, track annealing experiment for apatite: Nuclear Tracks LMS area and could have been associated with figure editing, and experiments. We appreciate the editorial and Radiation Measurements, v. 5, p. 169–174, doi:10 effort of A.B. Weil; three anonymous reviews provided detailed antiformal stacking of thrust duplexes at depth .1016/0191-278X(81)90039-1. and constructive comments that improved the manuscript. or detachment folding. A similar deformation Godard, V., Pik, R., Lave, J., Cattin, R., Tibari, B., Sigoyer, J., mechanism may have occurred during the Ceno- Pubellier, M., and Zhu, J., 2009, Late Cenozoic evolution REFERENCES CITED of the central Longmen Shan, eastern Tibet: Insight from zoic, causing the LMS and the DA to share simi- Arne, D., Worley, B., Wilson, C., Chen, S.F., Foster, D., Luo, Z.L., (U-Th)/He thermochronometry: Tectonics, v. 28, TC5009, lar exhumation histories (Fig. 1B). Liu, S.G., and Dirks, P., 1997, Differential exhumation in doi:10.1029/2008TC002407. response to episodic thrusting along the eastern margin Green, P.F., 1981, ‘Track-in-track’ length measurements in an- Instead of crustal shortening in the shal- of the Tibetan Plateau: Tectonophysics, v. 280, p. 239–256, nealed apatites: Nuclear Tracks, v. 5, p. 121–128, doi:10 low crust, recent geophysical data (including doi:10.1016/S0040-1951(97)00040-1. .1016/0191-278X(81)90034-2.

LITHOSPHERE | Volume 9 | Number 4 | www.gsapubs.org 543

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021 XI-BIN TAN ET AL.

Green, P.F., 1985, Comparison of zeta calibration baselines Earth and Planetary Science Letters, v. 186, p. 45–56, doi: Wallis, S., Tsujimori, T., Aoya, M., Kawakami, T., Terada, K., Su- for fission-track dating of apatite, zircon and sphene: 10.1016/S0012-821X(01)00232-1. zuki, K., and Hyodo, H., 2003, Cenozoic and Mesozoic Chemical Geology, v. 58, p. 1–22, doi:10.1016/0168-9622 Reiners, P.W., and Brandon, P.K., 2006, Using thermochronol- metamorphism in the Longmenshanorogen: Implications (85)90023-5. ogy to understand orogenic erosion: Annual Review of for geodynamic models of eastern Tibet: Geology, v. 31, Harrowfield, M.J., 2001, The Tectonic Evolution of the Song- Earth and Planetary Sciences, v. 34, p. 419–466, doi:10 p. 745–748, doi:10 .1130 /G19562 .1. pan-Garze Fold Belt, Southwest China [Ph.D. thesis]: Mel- .1146/annurev.earth.34.031405.125202. Wang, E., Kirby, E., Furlong, K.P., van Soest, M., Xu, G., Shi, X., bourne, The University of Melbourne, 271 p. Ren, J., Xu, X., Yeats, R.S., Zhang, S., Ding, R., and Gong, Z., Kamp, P.J.J., and Hodges, K.V., 2012, Two-phase growth of Harrowfield, M.J., and Wilson, C.J.L., 2005, Indosinian defor- 2013, Holocene paleoearthquakes of the Maoergai fault, high topography in eastern Tibet during the Cenozoic: Na- mation of the Songpan Garzê Fold Belt, northeast Tibetan eastern Tibet: Tectonophysics, v. 590, p. 121–135, doi:10 ture Geoscience, v. 5, p. 640–645, doi:10.1038 /NGEO1538. Plateau: Journal of Structural Geology, v. 27, p. 101–117, .1016/j.tecto.2013.01.017. Wang, H., Rahn, M., Zhou, J., and Tao, X., 2013, Tectonother- doi:10.1016/j.jsg.2004.06.010. Robert, A., Pubellier, M., de Sigoyer, J., Vergne, J., Lahfid, mal evolution of the Triassic flysch in the Songpan-Garzê Huang, M., Maas, R., Buick, I.S., and Williams, I.S., 2003a, A., Cattin, R., Findling, N., and Zhu, J., 2010, Structural orogen, eastern Tibetan plateau: Tectonophysics, v. 608, Crustal response to continental collisions between and thermal characters of the Longmen Shan (Sichuan, p. 505–516, doi:10.1016/j.tecto.2013.08.036. the Tibet, Indian, South China and North China Blocks: China): Tectonophysics, v. 491, p. 165–173, doi:10.1016/j Wang, Q., Qiao, X., Lan, Q., Freymueller, J., Yang, S., Xu, C., Geochronological constraints from the Songpan-Garze .tecto.2010.03.018. Yang, Y., You, X., Tan, K., and Chen, G., 2011, Rupture of orogenic belt, western China: Journal of Metamorphic Roger, F., Malavieille, J., Leloup, P.H., Calassou, S., and Xu, deep faults in the 2008 Wenchuan earthquake and uplift Geology, v. 21, p. 223–240, doi:10.1046/j.1525-1314.2003 Z., 2004, Timing of granite emplacement and cooling in of the Longmen Shan: Nature Geoscience, v. 4, p. 634– .00438.x. the Songpan-Garze fold belt (eastern Tibetan plateau) 640, doi:10.1038/NGEO1210. Huang, M.H., Buick, I.S., and Hou, L.W., 2003b, Tectonometa- with tectonic implication: Journal of Asian Earth Sciences, Weller, O., Waters, S.O., Rayner, D.N., Searle, M., Chung, S.-L., morphic evolution of the eastern Tibet Plateau: Evidence v. 22, p. 465–481, doi:10.1016/S1367-9120(03)00089-0. Palin, R., Lee, Y.-H., and Xu, X., 2013, Quantifying Barro- from the central Songpan-Garzê Orogenic Belt, western Roger, F., Jolivet, M., and Malavieille, J., 2010, The tectonic vian metamorphism in the Danba structural culmination China: Journal of Petrology, v. 44, p. 255–278, doi:10 .1093 evolution of the Songpan-Garzê (north Tibet) and adja- of eastern Tibet: Journal of Metamorphic Geology, v. 31, /petrology/44.2.255. cent areas from Proterozoic to present: A synthesis: Jour- p. 909–935, doi:10.1111/jmg.12050. Hubbard, J., and Shaw, J.H., 2009, Uplift of the Longmen nal of Asian Earth Sciences, v. 39, p. 254–269, doi:10 .1016 Wilson, C.J.L., and Fowler, W.P., 2011, Denudational response Shan and Tibetan plateau, and the 2008 Wenchuan /j.jseaes.2010.03.008. to surface uplift in east Tibet: Evidence from apatite fis- (M=7.9) earthquake: Nature, v. 458, p. 194–197, doi:10 Royden, L.H., Burchfiel, B.C., King, R.W., Wang, E., Chen, Z., sion-track thermochronology: Geological Society of Amer- .1038/nature07837. Shen, F., and Liu, Y., 1997, Surface deformation and lower ica Bulletin, v. 123, p. 1966–1987, doi:10 .1130 /B30331 .1. Hurford, A.J., and Green, P.F., 1983, The zeta calibration of crustal flow in eastern Tibet: Science, v. 276, p. 788–790, Xu, G., and Kamp, P.J.J., 2000, Tectonic and denudation ad- fission track dating: Isotope Geoscience, v. 1, p. 285–317, doi:10.1126/science.276.5313.788. jacent to the Xianshuihe fault, eastern Tibetan Plateau: doi:10.1016/S0009-2541(83)80026-6. Shi, F., He, H.L., and Wei, Z.Y., 2012, Coseismic horizontal Constraints from fission track thermochronology: Jour- Jolivet, M., Roger, F., Xu, Z.Q., Paquette, J.L., and Cao, H., shortening associated with the 2008 Wenchuan earth- nal of Geophysical Research, v. 105, p. 19,231–19,251, doi: 2015, Mesozoic–Cenozoic evolution of the Danba dome quake along the Bashahe segment from high resolution 10.1029/2000JB900159. (Songpan Garzê, east Tibet) as inferred from LA-ICPMS satellite images: Journal of Asian Earth Sciences, v. 50, Xu, X., Wen, X., Yu, G., Chen, G., Klinger, Y., Hubbard, J., and Shaw, U-Pb and fission-track data: Journal of Asian Earth Sci- p. 164–170, doi:10.1016/j.jseaes.2012.01.001. J., 2009, Coseismic reverse- and oblique-slip surface faulting ences, v. 102, p. 180–204, doi:10 .1016 /j .jseaes .2015 .02 .009. Tan, X.B., Lee, Y.H., Chen, W.Y., Cook, K.L., and Xu, X.W., 2014, generated by the 2008 Mw 7.9 Wenchuan earthquake, China: Kirby, E., Reiners, P.W., Krol, M.A., Whipple, K.X., Hodges, K.V., Exhumation history and faulting activity of the southern Geology, v. 37, p. 515–518, doi:10 .1130 /G25462A .1. Farley, K.A., Tang, W., and Chen, Z., 2002, Late Cenozoic segment of the Longmen Shan, eastern Tibet: Journal Yan, D.P., Zhou, M.F., Li, S.B., and Wei, G.Q., 2011, Structural evolution of the eastern margin of the Tibetan Plateau: In- of Asian Earth Sciences, v. 81, p. 91–104, doi:10.1016/j and geochronological constraints on the Mesozoic-Ceno- ferences from 40Ar/39Ar and (U-Th)/He thermochronology: .jseaes.2013.12.002. zoic tectonic evolution of the Longmen Shan thrust belt, Tectonics, v. 21, p. 1-1–6-17, doi:10.1029/2000TC001246. Tan, X.B., Xu, X.W., Lee, Y.H., Yuan, R.M., Yu, G.H., and Xu, eastern Tibetan Plateau: Tectonics, v. 30, TC6005, doi:10 Li, Z., Liu, S., Chen, H., Deng, B., Hou, M., Wu, W., and Cao, C., 2015, Differential late Cenozoic vertical motions of .1029/2011TC002867. J., 2012, Spatial variation in Meso-Cenozoic exhumation the Beichuan-Yingxiu fault and the Jiangyou-Guanxian Yin, A., and Harrison, T.M., 2000, Geologic evolution of the history of the Longmen Shan thrust belt (eastern Tibetan fault in the central Longmenshan range and their tec- Himalayan-Tibetan orogen: Annual Review of Earth and Plateau) and the adjacent western Sichuan basin: Con- tonic implications: Chinese Journal of Geophysics, v. 58, Planetary Sciences, v. 28, p. 211–280, doi:10 .1146 /annurev straints from fission track thermochronology: Journal p. 143–152 (in Chinese with English abstract). .earth.28.1.211. of Asian Earth Sciences, v. 47, p. 185–203, doi:10.1016/j Tapponnier, P., Xu, Z.Q., Roger, F., Meyer, B., Arnaud, N., Witt Zhang, Y., Chen, W., and Yang, N., 2004, 40Ar/39Ar dating of .jseaes.2011.10.016. linger, G., and Yang, J.S., 2001, Oblique stepwise rise and shear deformation of the Xianshuihe fault zone in west Liu, Q.Y., van der Hilst, R.D., Li, Y., Yao, H., Chen, J., Guo, B., growth of the Tibet Plateau: Science, v. 294, p. 1671–1677, Sichuan and its tectonic significance: Science in China Qi, S., Wang, J., Huang, H., and Li, S., 2014, Eastward ex- doi:10.1126/science.105978. Ser. D Earth Sciences, v. 47, no. 9, p. 794-803. pansion of the Tibetan Plateau by crustal flow and strain Tian, Y., Kohn, B.P., Gleadow, A.J.W., and Hu, S., 2013, Con- Zhou, M.F., Yan, D.P., Asconcelos, P.M., Li, J.W., and Hu, R.Z., partitioning across faults: Nature Geoscience, p. 361–365, structing the Longmen Shan eastern Tibetan Plateau 2008, Structural and geochronological constraints on the doi:10.1038/ngeo2130. margin: Insights from low-temperature thermochronol- tectono-thermal evolution of the Danbadomal terrane, east- Liu, T.K., Chen, Y.G., Chen, W.S., and Jiang, S.H., 2000, Rates ogy: Tectonics, v. 32, p. 576–592, doi:10 .1002/tect.20043. ern margin of the Tibetan plateau: Journal of Asian Earth of cooling and denudation of the early Penglai orogeny, Tian, Y., Kohn, B.P., Hu, S., and Gleadow, A.J.W., 2015, Syn- Sciences, v. 33, p. 414–427, doi:10 .1016 /j .jseaes .2008 .03 .003. Taiwan, as assessed by fission-track constraints: Tecto- chronous fluvial response to surface uplift in the eastern nophysics, v. 320, p. 69–82, doi:10.1016/S0040-1951(00) Tibetan Plateau: Implications for crustal dynamics: River MANUSCRIPT RECEIVED 30 AUGUST 2016 00028-7. incision history, eastern Tibet: Geophysical Research Let- REVISED MANUSCRIPT RECEIVED 4 MARCH 2017 Liu, T.K., Hsieh, S., Chen, Y.G., and Chen, W.S., 2001, Thermo- ters, v. 42, p. 29–35, doi:10.1002/2014GL062383. MANUSCRIPT ACCEPTED 30 MARCH 2017 kinematic evolution of the Taiwan oblique-collision Wagner, G., and Van den Haute, P., 1992, Fission track dating: mountain belt as revealed by zircon fission track dating: Dordrecht, Kluwer, 285 p. Printed in the USA

544 www.gsapubs.org | Volume 9 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/9/4/534/2316037/534.pdf by guest on 30 September 2021