Exhumation history and faulting activity of the southern segment of the Longmen Shan, eastern Tibet

Journal of Asian Earth Sciences 81 (2014) 91–104

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Exhumation history and faulting activity of the southern segment of the Longmen Shan, eastern Tibet Xi-Bin Tan a, Yuan-Hsi Lee b,⇑, Wen-Yu Chen b, Kristen L. Cook c,d, Xi-Wei Xu a a

Institute of Geology, China Earthquake Administration, Beijing 100029, China Department of Earth and Environmental Sciences, National Chung-Cheng University, Taiwan, ROC c Department of Geosciences, National Taiwan University, Taiwan, ROC d German Research Center for Geosciences GFZ, Potsdam, Germany b

a r t i c l e

i n f o

Article history: Received 13 June 2013 Received in revised form 22 November 2013 Accepted 2 December 2013 Available online 12 December 2013 Keywords: Eastern Tibet Longmen Shan Exhumation history Low temperature thermochronology Fission track

a b s t r a c t The Longmen Shan (LMS), which constitutes the eastern border of the Tibetan Plateau, is about 400 km in length and characterized by a steep topographic transition from the Sichuan Basin to the plateau. The 2008 Mw7.9 Wenchuan earthquake and 2013 Mw6.6 Lushan earthquake were associated with the central to northern segments and southern segment of the LMS fault belt, respectively. In this paper, zircon and apatite fission track (ZFT and AFT, respectively) dating in combination with previously published low temperature thermochronology studies are used to constrain both the exhumation history and fault activity along the LMS, with a special focus on the southern segment. In the southern segment of the LMS, the ZFT ages in the hanging wall of the Wulong-Yanjing fault 10–14 Ma, increasing to ca. 30 Ma to the northwest of the faults and to 100–200 Ma in the plateau region. The AFT ages are 3–5 Ma at the mountain front and increase to 8–26 Ma in the plateau. We show that these age distributions are controlled by fault geometry. Two stages of rapid exhumation were identified using apatite fission track length modeling and the age distributions in the southern segment of the LMS. The first stage is from ca. 30 Ma and the second stage is from 3–5 Ma to present. In contrast with the middle segment of the LMS, the Cenozoic exhumation rate is higher in the southern segment of the LMS, which may be due to the influence of the collision between the India and Eurasia plates and/or different faulting mechanisms in the different segments. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The Longmen Shan (LMS) constitutes the eastern boundary of the Tibetan Plateau (Fig. 1a). It is about 400 km in length and is characterized by a steep topographic transition from 600 m (the Sichuan Basin) to about 5000 m (the Tibet plateau), in about 50 km. In contrast to the more substantial shortening rate of the Himalayas, the shortening rate across the LMS is less than 3 mm/ yr based on GPS (Global Positioning System) and 10 years of geodesy data collected prior to the May 12th 2008 Wenchuan earthquake, Sichuan (King et al., 1997; Chen et al., 2000; Gan et al., 2007). The 2008 Wenchuan earthquake (Mw7.9) was one of the most devastating earthquakes in China’s history, resulting in over 80,000 fatalities, extreme economic loss, and leaving more than 1.5 million people homeless (Stone, 2008). Research has shown that two major faults were active during the Wenchuan earthquake, the Beichuan-Yingxiu fault and Jiangyou-Guanxian fault, which are located in the central to northern segments of the LMS ⇑ Corresponding author. Tel.: +886 52720411x66212; fax: +886 52720807. E-mail address: [email protected] (Y.-H. Lee). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.12.002

(Xu et al., 2009). The rupture of the Wenchuan earthquake did not extend to the southern segment of the LMS. However, the displacement rate (from GPS measurements) in the southern segment of the LMS appears to be similar or even greater than that in the north (Chen et al., 2000) (Fig. 1a). Recently, a Mw6.6 earthquake occurred at the southern tip of the LMS, resulting in 196 fatalities and extensive damage to property and infrastructure (Xu et al., 2013). To better assess the potential for disastrous earthquakes in this region, it is important to understand the area’s Cenozoic exhumation history and fault activity. Cenozoic deformation in the LMS region has been produced by the eastward growth of the Tibetan Plateau, caused by the collision of the India and Eurasian plates starting at 50 Ma (Yin and Harrison, 2000). The collision has progressively propagated north and eastward (Yin and Harrison, 2000; Tapponnier et al., 2001). Previous studies have shown that the central segment of the LMS experienced rapid exhumation starting at 12–5 Ma (Kirby et al., 2002; Godard et al., 2009); furthermore, Wang et al. (2012) found an earlier exhumation event at 30–25 Ma. Topographic profiles show that the southern and central segments of the LMS form a sharp, steep boundary with the Sichuan Basin, while topography

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Fig. 1. (a) Topography and GPS velocities in eastern Tibet. The GPS velocity is relative to the southern China block (Gan et al., 2007). Inset map shows the tectonic setting of the LMS area (Roger et al., 2010). (b) Topographic profiles across the southern, central, and northern segments of the LMS. (c) Structural profile of the central segment of the LMS (Hubbard and Shaw, 2009; Hubbard et al., 2010). (d) Aftershocks, slip distribution, and inferred fault geometry for the Wenchuan earthquake, from Wang et al. (2011).

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in the northern segment is less steep, with a more gradual transition from the basin to the plateau (Fig. 1b). These findings suggest that the southern to central segments of the LMS are potentially more active than the northern segment. Far more thermochronology studies have been done in the central segment (Godard et al., 2009; Wang et al., 2012; Li et al., 2012) than in the southern segment of the LMS, which has remained poorly constrained. To better understand the exhumation history of the LMS, this study uses zircon and apatite fission track analyses combined with previous data to constrain the Cenozoic exhumation history and fault activity of the southern segment of the LMS. Our data also allow us to evaluate variations in exhumation history along strike of the LMS.

2. Geological setting 2.1. Mesozoic to Cenozoic Tectonics of the Longmen Shan The LMS fault belt is located on the boundary between the Songpan-Garze fold belt (SGFB) and the rigid South China Block (Figs. 1 and 2). The SGFB has undergone at least two major orogenic events since Mesozoic time (Zhou et al., 2008; Roger et al., 2010; Yan et al., 2011). In the Late Triassic to Jurassic period, an orogeny resulting from convergence between the North China, South China (Enkin et al., 1992; Yan et al., 2011) and Qiangtang blocks (Roger et al., 2003, 2010) caused the pervasive deformation of a thick flysch sequence (the Triassic Xikang Group, or SongpanGarze flysch). The isoclinally folded flysch was then intruded by Mesozoic granites that remain undeformed, constraining the timing of this phase of deformation (Fig. 2). In the Longmen Shan, this deformation event involved southeastward directed thrusting that emplaced an allochthonous sequence of Proterozoic and Paleozoic rocks above autochthonous Proterozoic and Paleozoic units associated with the Yangtze Craton (Burchfiel et al., 1995). In the Danba area, exhumation has removed the flysch and much of the underlying Paleozoic strata (Zhou et al., 2008), exposing basement core complexes (Fig. 2) and high grade rocks that record Mesozoic metamorphism (Weller et al., 2013). Like the Danba area, the LMS has experienced greater exhumation than the rest of the SGFB, and the basement core complexes, such as the Pengguan Massif and the Baoxing Massif, are exposed (Figs. 2 and 3) (Xu and Kamp (2000)). From the Jurassic to Cretaceous, the long term cooling rate of the Mesozoic granites within the SGFB was very slow, indicating an absence of any major tectonic events (Burchfiel et al., 1995, 2008; Roger et al., 2003, 2010; Harrowfield and Wilson, 2005; Reid et al., 2005; Wilson et al., 2006; Xu and Kamp, 2000). Cenozoic deformation in the Longmen Shan region results from the collision between the India and Eurasia plates since ca. 50 Ma, with deformation progressively propagating north and eastward (Yin and Harrison, 2000; Tapponnier et al., 2001). Thermochronology studies of exhumation have been conducted for the area using different methods. These methods have given different descriptions of the timing of Cenozoic exhumation in eastern Tibet. According to apatite fission-track modeling by Arne et al. (1997), the cooling rate of the northern LMS was slow through the Mesozoic and Early Tertiary but began to increase from the Miocene (ca. 20 Ma) to the present. Xu and Kamp (2000) considered initial exhumation to be at ca. 20 Ma on the western side of the LMS based on zircon fission-track dating along the Xianshuihe fault, which is an active sinistral strike slip fault to the west of the LMS (Figs. 1 and 2). Kirby et al. (2002) used (U–Th)/He dating of zircon and apatite and suggested that the margin of the plateau started rapid exhumation no earlier than 5–12 Ma. Based on Rb– Sr biotite ages from the Danba area that cluster at ca. 34–24 Ma;

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Huang et al. (2003) suggested that uplift of the Tibetan Plateau (including the Songpan-Garze fold belt) occurred predominantly in the last ca. 30 Ma. Godard et al. (2009) studied the late Cenozoic evolution of the central LMS using (U–Th)/He dating and age-elevation profiling (Fig. 2) and concluded that the major phase of exhumation started at 8–11 Ma, with an average rate of exhumation of 0.65 mm/yr, and a total amount of exhumation of less than 6–8 km. Wang et al. (2012) studied the central segment of the LMS and found two pulses of rapid exhumation: one beginning 30–25 million years ago and a second 10–15 million years ago that continues to the present stage of uplift. 2.2. Surface geology of the southern segment of the Longmen Shan The southern segment of the LMS contains six major faults (F1– F6), and a fold and thrust belt that has developed in the western Sichuan Basin (Figs. 2 and 3) (Burchfiel et al., 1995). In the fold and thrust belt, the Mesozoic sedimentary units of the Sichuan Basin are deformed. In some localities, the sedimentary succession extends up to Eocene/Oligocene strata, which are deformed in concert with the underlying Mesozoic units. The presence of the deformed Cenozoic strata indicates that the fold and thrust deformation in the Sichuan Basin occurred post Oligocene (Burchfiel et al., 1995). Hubbard and Shaw (2009) calculated shortening of up to 25.8 km in the fold and thrust belt east of F1. The Shuangshi-Dachuan fault (F1) is a mountain front fault with Triassic sandstone in the hanging wall and Jurassic to Cretaceous conglomerate units in the footwall. Triassic strata and a klippen belt are found between F1 and F2 (Fig. 3). Between F2 and F3, Upper Silurian to Triassic strata unconformably overlie the eastern limb of the folded Proterozoic basement (Baoxing massif). This thin and incomplete section of Paleozoic rocks is characteristic of an autochthonous sequence with strata of low metamorphic grade. These strata belong to the western margin of the Yangtze carton (Burchfiel et al., 1995; Jia et al., 2006). The Baoxing massif has been folded such that the eastern limb of the massif generally dips steeply eastward and overturns to become subhorizontal in the central part before dipping gently at its western limb. This results in deeper strata being exhumed in the central to western limb of the massif (Burchfiel et al., 1995, 2008). The Baoxing massif is cut by the west dipping F3 on its northwestern side. To the northwest of F3, Triassic strata to the Proterozoic basement are of allochthonic origin. They have experienced ductile deformation and have a higher grade of metamorphism than the footwall rocks, which are not metamorphosed and belong to the autochthon (Burchfiel et al., 1995). Biotite appears in the Ordovician to Devonian metamorphic rock on the hanging wall of F3, which indicates that metamorphic temperatures reached 400 °C (Bucher and Frey, 2002). Based on stratigraphic relationships and field observations of the fault zone, Burchfiel et al. (1995) considered that F3 was initially an eastward thrust fault in the Mesozoic before later becoming a normal fault. Between F4 (Wulong-Yanjing fault) and F5, Paleozoic strata lie unconformably over Neoproterozoic strata. On the northwestern side of F4, Neoproterozoic strata are thrust over Paleozoic strata. F5 repeats Neoproterozoic strata on the hanging wall, and shows similar characteristics to F4. The hanging wall of F6 consists of a Paleozoic sequence with strata younging to the northwest up to the contact with the Triassic Songpan-Garze flysch. 2.3. Deep structure of the Longmen Shan The surface of the LMS is characterized by a steep topographic transition and multiple north-east trending range-parallel faults (Fig. 1), but the LMS is also a major boundary at depth; for instance, it marks a steep transition in crustal thickness (Zhang et al., 2009).

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Fig. 2. Simplified geological map of the LMS and surrounding regions (modify from Sichuan Geological Survey Bureau, 2001). The red numbers are new zircon and apatite fission track ages and sample numbers from the central and northern segments; only sample numbers are shown for the southern segment. The green numbers are previously published fission track ages. Red boxes show the locations of the zircon (U–Th)/He ages by Godard et al. (2009). The yellow overlay shows the zone of completely reset ZFT ages. Red lines are the surface rupture of the 2008 Wenchuan earthquake. Blue lines are major faults in the southern segment of the LMS. Abbreviation: P – Pengguan Massif; B – Baoxing Massif; XSHF – Xianshuihe Fault; MJF – Minjiang Fault; HYF – Huya Fault. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The LMS fault belt is comprised of several major faults, including the Jiangyou-Guanxian, Beichuan-Yingxiu, and Wenchuan-Maoxian faults in the central segment (Xu et al., 2009; Hubbard and Shaw, 2009; Li et al., 2010; Hubbard et al., 2010) (Figs. 1 and 2). The 2008 Wenchuan earthquake occurred along the BeichuanYingxiu fault and Jiangyou-Guanxian fault, which merge at depth according to the interpretations of seismic profiles in the central segment of the LMS (Lu et al., 2010). Other studies have also suggested that the several parallel faults merge at depths of ca. 15–20 km to a soft layer based on P-wave velocity data (Wang

et al., 2003), magnetotelluric data (Wang et al., 2008), surface wave tomographic imaging, and data from both a passive seismological array and an active-source seismic experiment (Zhu, 2008) across the LMS. Using balanced cross-section analysis, Hubbard and Shaw (2009) considered the 2008 Wenchuan earthquake to have occurred along a ramp and flat structure at a depth of about 16– 20 km (Fig. 1c). Yu et al. (2010) constrain the fault geometry of different segments based on the locations and mechanisms of aftershocks. Using geodetic and earthquake data, Wang et al. (2011) also suggest that the Wenchuan earthquake fault has a

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Fig. 3. Geological map, structural profile, and ZFT and AFT ages in the southern segment of the LMS. Red fault lines indicate recently active faults. The black line shows the location of the structural profile. AFT ages are shown in red, ZFT ages in black, and ages from Arne et al. (1997) in green. Geology is modified from the 1:500,000 Geology map of Sichuan Province. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ramp and flat geometry and that the epicenter was near the boundary of the ramp and flat. Their paper shows much more slip occurring at the ramp fault (Fig. 1d). Based on a ramp-flat fault geometry and field observations from previous studies, we construct a structural profile of the southern segment of the LMS (Fig. 3). From F1 to F6 all of the faults connect to the deeper detachment (depth of 15–20 km). F1 and F2 are mountain front faults, which share the same plane at a depth of about 10 km. The Baoxing massif has been folded and deeper rocks are exposed at the core of the fold between F2 and F3. F3 and F4 are splay faults and share the same fault plane at depth; however, we find that significant Cenozoic deformation only occurs on F4, as shown in Section 5. 3. Sampling strategy and methods In order to constrain the exhumation history and the faulting activity of the southern segment of the LMS, we collected samples

from the frontal thrust area (850 m elevation) to the inner plateau (about 4000 m elevation), passing through F1 (Shuangshi-Dachuan fault), F2, F3, F4 (Wulong-Yanjing fault), F5 and F6 (Fig. 2). Additionally, samples were collected from the central and the northern segments of the LMS for comparison. In order to minimize the influence of sample elevation, all samples, except some in the inner plateau, were collected in valley bottoms. The collected samples were analyzed and the results were combined with data from previous studies to discuss exhumation along the LMS. In some samples, both zircon and apatite fission track ages were used to constrain exhumation history. Sample preparation and experimental method were as per Liu et al. (2000, 2001). At least two pieces of standard glass, NBS SRM-610 or SRM-612, calibrated against the fission-track age standard Fish Canyon Tuff (Naeser et al., 1981), were wrapped tightly and irradiated with samples. Grain-by-grain and mica external detector techniques were adopted to obtain individual grain ages (Wagner and Van Den Haute, 1992). The zeta values (Green,

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1985; Hurford and Green, 1983) for the standard glasses SRM-612 and SRM-610 were 340 ± 12 (1r) and 27.5 ± 1.0 (1r), respectively.

4. Results 4.1. Zircon fission track ages Fig. 4 shows the ZFT ages and age spectra, and the central ages are given in Table 1. Sample Bx110906 from an Eocene unit on the footwall of F1 gave an age of 199.3 ± 11.4 Ma. Between F1 (Shuangshi-Dachuan fault) and F2, two zircon fission track (ZFT) ages of ca. 107.6 ± 7.3 Ma (BX-16) and 129.2 ± 8.8 Ma (BX110907) were obtained. Between F2 and F3, ZFT ages vary from 14.2 Ma to 56.5 Ma. We obtain one age of 80.6 ± 8.5 Ma (BO-2) between F3 and F4 (Wulong-Yanjing fault). Along the strike of F4, ZFT ages are consistent at ca. 10–14 Ma near the fault; however, farther northwest of F4, ZFT ages are ca. 25–29 Ma. Between F5 and F6, sample BX-9 has age of 14.1 ± 0.9 Ma. On the hanging wall of the F6, we obtain a ZFT age of ca. 31 Ma (Gong 85) but ages increase to ca. 71–195 Ma to the northwest of F6 (Figs. 3 and 4) (Table 1). In the southernmost segment of F4, three ZFT ages were obtained ranging from ca. 82–136 Ma on the hanging wall. One ZFT age was obtained for the Pengguan massif at 76.4 ± 5.7 Ma (GD-1). One ZFT age of 10 Ma (WCH-1) was obtained for the hanging wall of the Wenchuan-Maoxian fault (Fig. 2). 4.2. Apatite fission track ages Fig. 5 shows the AFT ages and age spectra. Apatite fission track (AFT) dating yielded ages from ca. 4 to 5 Ma between F2 and F4 (Wulong-Yanjing fault), and 3–4 Ma (with one sample 1.9 ± 1.4 Ma from a sample size of only 3 grains) between F4 and F6. In the hanging wall of the F6 and to the northwest, the AFT ages are ca. 11–26 Ma (Figs. 3 and 4). All of these ages indicate complete resetting during Cenozoic deformation. To make comparisons with other studies of exhumation history along the LMS, samples from the central to northern segments of the LMS were collected. In the Pengguan massif, our AFT age is ca. 6.3 ± 0.7 Ma (GD-1), which is similar to the ca. 4.8–6.5 Ma of Arne et al. (1997). On the hanging wall of the Wenchuan-Maoxian fault, one sample gives an age of 2.1 ± 0.2 Ma (WCH-1). In the northern segment of the LMS, we obtain an age of 25.8 ± 2 Ma (PW-12) on the hanging wall of the Beichuan-Yingxiu fault, which is similar to the 33 Ma of Arne et al. (1997). On the hanging wall of the Guanxian-Jiangyou fault, we get ages of 67.3 ± 4.6 Ma (JY-1) and 83.9 ± 9.1 (JY-2) Ma (Fig. 2, Table 1).

5. Discussion 5.1. Timing of rapid Cenozoic exhumation We measured 102 track lengths in the sample BX-3 and used the HeFty computer program (Ketcham, 2005) to model the thermal history of this sample (Fig. 6). The modeled history suggests two stages of rapid cooling, the first stage from ca. 30 Ma and the second stage, with a much higher cooling rate, from ca. 3– 5 Ma to the present. The timing of the first stage of rapid exhumation is consistent with the oldest reset ages of zircon fission tracks, providing further support for a stage of rapid exhumation initiated at ca. 30 Ma. Cook et al. (2013) measured zircon (U–Th)/He in the Baoxing Massif and obtained ages less than 17 Ma, indicating that initial Cenozoic exhumation must be older than 17 Ma, consistent with our results. The central segment of the LMS also experienced a first stage of rapid exhumation starting from 25 to 30 Ma (Wang

et al., 2012), indicating similar timing for this phase between the southern and central segments of the LMS. The second stage of rapid exhumation began at 3–5 Ma. This is consistent with the 3–5 Ma AFT ages we obtained between F2 and F6. Wang et al. (2012) show that the second stage of rapid exhumation began at ca. 10–15 Ma in the central segment of the LMS, which is earlier than in the southern segment. The fact that the timing of the second stage of rapid exhumation is different along strike may result from changes in structural style from the south to the north. During the 2008 Wenchuan earthquake the rupture did not extend to the southern segment of the LMS. The fold and thrust belt in the southern segment of the LMS is wider than that in the north, with multiple folds and faults developed within the Sichuan basin southeast of the mountain front. The style of faulting in these foreland structures differs from that in mountain areas, indicating a difference in fault geometry and activity. The modeled cooling rate of the second stage of exhumation is much higher than that of the first stage. A comparison of the ZFT and the AFT ages also shows that the second stage of rapid cooling was much faster, lending support to the modeled rates. Assuming a geothermal gradient of 30 °C/km, mean surface temperature of 15 °C, and closure temperatures for ZFT and AFT of 240 °C (Brandon et al., 1998) and 110 °C (Gleadow and Dubby, 1981), respectively, we calculate the exhumation rate of the southern segment of the LMS (Fig. 6b). On the hanging walls of F4 and F5, the exhumation rate near the faults was ca. 0.8 mm/yr between 10–14 Ma and ca. 5 Ma; it increased to 0.7–1.2 mm/yr from ca. 5 Ma to the present. Farther northwest into the hanging walls, the exhumation rate was lower, at ca. 0.2 mm/yr between 11–14 Ma and ca. 5 Ma. In the plateau area, the exhumation rate was ca. 0.12–0.3 mm/yr after ca. 10–27 Ma. The cooling histories and the above estimates of exhumation rates in different areas relative to the faults are consistent with the exhumation rate becoming higher after ca. 5 Ma (Fig. 6c). Wang et al. (2009) suggested that activity of the Xianshiuhe fault, located to the western side of the LMS (Fig. 1), could be divided into two stages: from 13 to 5 Ma and from 5 Ma to the present. During the latter stage, from 5 Ma to the present, the fault zone passed through the Yushu, Ganzi, and Gongga Shan areas, offset the LMS fault belt and reached the Kunming area (Wang et al., 2009). The timing of the rapid exhumation from 3–5 Ma to present in the southern segment of the LMS could be related to this second stage of activity of the Xianshuihe fault. 5.2. Controls on the distribution of zircon fission track ages Research has shown that in convergent orogenic settings, consideration of lateral motion of material relative to the regional thermal and kinematic framework is important in interpreting thermochronological data (Batt and Brandon, 2002; Huerta and Rodgers, 2006; Lock and Willett, 2008). Huerta and Rodgers (2006) present a kinematic thermal model of a simple fold bend fault system that can be used to determine the timing of initial thrusting, duration of thrusting, and the thrust rate using low temperature thermochronology. They found a pattern of nonlinear fission track (FT) ages with anomalously young ages from the leading edge to the central part of the system for hanging-wall rocks whose initial temperature (burial depth) was much higher than the closure temperature of the FT system. The ages do not directly indicate the timing of initial thrusting; however, the model suggests that ages measured on the hanging wall of the fault at a burial depth just slightly deeper than the partial annealing zone of the thermochronometer, can be used to ascertain the initial timing, duration, and rate of thrusting. As discussed in Section 2.3, The LMS has a ramp and flat fault geometry (Xu et al., 2009; Hubbard and Shaw, 2009, 2010; Li

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Fig. 4. Age histograms and radial plots for the ZFT samples. For each sample, the pooled age and the number of grains counted are indicated on the histogram.

et al., 2010; Wang et al., 2012). Fig. 7 shows the structural profile, degree of ZFT resetting, and ZFT and AFT age distribution along the southern segment of the LMS. We find two ZFT total annealing

zones and three partial annealing zones in the southern segment of the LMS. The reset zircon ages fall into two groups: 10–14 Ma and 25–31 Ma. The AFT ages are separated by F6 into two groups:

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Fig. 4 (continued)

greater than 10 Ma northwest of F6 and 3–5 Ma southeast of the fault. We consider the variation in the ZFT ages to be controlled by the ramp and flat fault geometry during thrusting. Fig. 8 shows a schematic model for the relationship between the distribution of ZFT ages and the fault geometry and lateral motion along the fault; it is comparable to the age distribution observed from F5 to the inner plateau. The red-dotted line is the 240 °C isothermal line, which shows a small thermal disturbance due to faulting (Fig. 8). In Fig. 8, point series A to D are used to represent movement over time and help elucidate the mechanism contributing to the variation in ZFT ages. The model has a listric fault geometry and the depth of the fault plane is deeper than the ZFT closure temperature. Point D1 is shallower than the partial annealing zone; therefore point D4 retains non-reset ages. Point D4 is analogous to the plateau region, where ZFT ages are 202–213 Ma, indicating limited Cenozoic exhumation. Point C1 is located in the partial annealing zone of ZFT (ca. 180–260 °C, Brandon et al., 1998) indicating that zircons will have partially reset ZFT ages at point C4. This point is analogous to the region northwest of F6, where ZFT ages range from 76 to 195 Ma. Point B1 is located deeper than the 240 °C iso-

therm. While the fault is active, rocks pass through the ZFT closure temperature (240 °C) at B2 and zircons will start to accumulate fission tracks. The surface sample at B4 therefore records the passage of time from B2 to B4. Northwest of F4 and F5, ZFT ages are ca. 25– 30 Ma and are completely reset; these ages are analogous to the B2 age. Point A1 is located well below than the closure temperature and just above the fault plane. While the fault is active, fission tracks for the point A series do not accumulate until the A series moves above point A3. The sample at A4 therefore records the time between A3 and A4. Comparing the ages of B4 and A4, B4 will have much older ages because it started to accumulate fission tracks earlier and has a longer movement path than A4. Because the A series moves primarily along the ramp, this series will have higher vertical displacement and exhumation rates and younger ages. Point A4 is analogous to the ca. 10–14 Ma ZFT ages found in the hanging walls of F4 and F5 (Fig. 3). This model explains why we find the youngest ages of 10–14 Ma near the fault and older ages of 25–30 Ma farther into the hanging wall. Consequently, we consider the 25–30 Ma samples to have been located close to the depth of the ZFT closure isotherm, suggesting that thrusting began at 25–30 Ma. This is consistent with the thermal modeling result of

Table 1 Zircon and apatite fission track ages. Latitude

Longitude

Elevation

Strata age

Lithology

Crystal

RhoS (105)

Ns

RhoI (105)

Ni

P(v2)

Nd

RhoD

Pooled age

Zircon BO-1 BO-2 BO-3 BX-5 BX-9 BX-11 BX-13 BX-14 BX-16 BX110906 BX110907 BX110913 GD-1 GD-2 GD-3 GD-6 GD-10 GD-11 DB-1 DB-2 WCH-1 Gong-41 Gong-78 Gong-83 Gong-85 Gong-90 Gong-91 Gong-92 Gong-95 Kc0510* Kc0755* Kc0513* Kc0756* Kc0757*

30.4136 30.4066 30.5547 30.8313 30.6423 30.6031 30.5289 30.4477 30.2317 30.2063 30.2248 30.4191 31.0614 31.0628 30.8824 30.8881 30.9106 30.9774 30.0403 30.2154 31.3274 30.0403 30.7880 30.3143 30.6887 30.5572 30.5256 30.5120 30.4093 30.5059 30.3720 30.5580 30.4190 30.5787

102.7520 102.7700 102.8994 102.7326 102.7896 120.8679 102.9130 102.8585 102.8385 102.8609 102.8396 102.7268 103.4056 103.3383 102.9861 102.9724 102.8942 102.8635 102.1759 102.2020 103.3987 102.1759 102.7292 102.7855 102.7513 102.8868 102.9152 102.9151 102.8360 102.8395 102.8210 102.8927 102.7620 102.8788

1604 2286 1557 2798 1955 2154 1416 1254 804 1204 770 1378 1219 1451 2690 3460 4389 3608 1430 1679 1766 1488 2444 956 2133 1513 1462 1366 1066 2045 1233 2352 1180 1575

Silurian Silurian Proterozoic Triassic Proterozoic Proterozoic Proterozoic Proterozoic Triassic Eocene Triassic Silurian Proterozoic Proterozoic Triassic Triassic Triassic Triassic Proterozoic Proterozoic Proterozoic Proterozoic Triassic Proterozoic Devonian Proterozoic Proterozoic Proterozoic Proterozoic Proterozoic Proterozoic Proterozoic Proterozoic Proterozoic

Schist Schist Schist Sandstone Metaquartz Granite Granite Granite Sandstone Sandstone Sandstone Sandstone Granite Granite Sandstone Sandstone Sandstone Sandstone Granodiorite Granodiorite Granite Granodiorite Sandstone Granodiorite Sandstone Gneiss Granite Granite Granite Granite Granite Granite Granite Granite

20 15 12 20 20 13 19 8 20 17 10 15 11 6 4 6 6 8 14 14 11 20 19 10 9 22 23 16 13 13 21 20 19 17

12.492 91.922 0.034 120.05 22.462 22.492 15.845 119.7 91.814 367.76 180.76 34.4 73.802 57.207 67.17 49.531 138.49 72.535 100.94 72.841 18.473 107.25 310.83 65.923 67.798 13.845 23.229 19.137 48.048 18.434 20.735 7.766 10.58 21.471

465 1164 372 1762 728 434 1036 318 3283 13,278 3006 3048 1900 434 460 814 1072 915 3520 2060 391 4722 11,398 968 1168 552 2172 1230 2300 266 1362 200 904 810

12.30 16.98 0.03 13.49 21.94 10.832 5.89 33.16 10.26 23.99 22.07 18.05 11.23 10.28 10.95 7.606 19.64 13.32 8.46 286 17.67 13.63 20.73 22.54 22.46 16.08 9.51 7.69 9.59 16.15 15.67 7.426 10.17 25.45

459 215 359 198 711 209 385 88 366 866 367 1599 289 434 75 125 152 168 295 286 374 600 760 331 387 641 889 494 459 233 1029 191 869 960

3.36 0 21.9 0.72 0.02 33.3 8.89 99.3 6.71 0 0.19 0.07 12.6 37.6 0 0.01 16.8 20.2 0 7.14 4.42 44.3 0 99.4 22.9 0.08 8.36 12.4 0 10 4.08 100 75.2 10.2

4798 4798 4798 4798 4798 4798 4798 4798 4798 8875 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798

12.3 10.95 9.32 8.9 10 8.9 7.5 8.8 8.8 9.65 11.65 10.95 8.5 8.5 8.5 8.5 8.5 8.6 8.2 8.2 7.39 8.24 9.65 8.24 7.5 8.1 7.5 7.5 8.2 8.5 7.8 8.1 7.5 11.65

13.2 ± 1.0 80.6 ± 8.5 13.2 ± 1.1 108.0 ± 9.1 14.1 ± 0.9 25.4 ± 2.4 27.7 ± 2.0 43.6 ± 5.5 107.6 ± 7.3 199.3 ± 11.4 129.2 ± 8.8 28.5 ± 1.4 76.4 ± 5.7 64.7 ± 8.4 71.3 ± 9.3 75.7 ± 7.8 81.9 ± 7.8 63.3 ± 5.9 135.5 ± 9.7 82.2 ± 6.1 10.6 ± 0.9 90.2 ± 5.2 195 ± 11.6 33.0 ± 2.5 31.6 ± 2.2 9.6 ± 0.7 25.1 ± 1.4 25.6 ± 1.7 56.3 ± 3.6 13.6 ± 1.3 14.2 ± 0.8 11.9 ± 1.3 10.9 ± 0.7 13.4 ± 0.7

Sample

Latitude

Longitude

Elevation

Formation

Lithology

Crystal

RhoS (105)

(Ns)

RhoI (105)

(Ni)

P(v2)

(Nd)

RhoD

Track length

Pooled age

Apatite BX-1 BX-2 BX-3 BX-6 BX-10 BX-11 BX-13 BX-14 BX-15 GD-1 GD-9 GD-12 Gong-78 Gong-83 Gong-90 Gong-92

30.8776 30.8721 30.8404 30.8121 30.6363 30.6031 30.5289 30.4477 30.3413 31.0614 30.9113 30.9640 30.7880 30.3143 30.5572 30.5120

102.6386 102.6816 102.7041 102.7293 102.8415 102.8679 102.9130 102.8585 102.7921 103.4056 102.9129 102.6505 102.7292 102.7855 102.8869 102.9150

3519 4056 3394 2485 2203 2268 1416 1254 976 1219 4355 2728 2513 956 1513 1366

Triassic Triassic Triassic Triassic Proterozoic Proterozoic Proterozoic Proterozoic Proterozoic Proterozoic Triassic Triassic Triassic Proterozoic Proterozoic Proterozoic

Sandstone Sandstone Sandstone Sandstone Granite Granite Granite Granite Granite Granite Sandstone Sandstone Sandstone Granodiorite Granite Granite

23 20 27 12 3 25 18 17 16 36 25 15 30 39 40 35

1.585 1.732 3.936 1.164 0.09 0.045 0.134 1.443 0.435 0.824 1.469 1.883 0.8 0.544 0.572 0.321

341 104 336 80 2 12 22 165 46 731 60 60 332 139 110 70

4.136 2.92 10.694 6.943 2.964 1.088 2.706 18.229 10.219 6.927 9.669 8.964 2.666 8.63 6.942 3.86

890 259 913 477 66 287 464 2084 1080 731 395 277 1106 2009 1335 842

99.5 77.7 11.9 0 81.5 83.67 92.92 2.47 16.1 100 99.6 100 45.41 29.97 56.6 99.9

4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798 4798

3.96 3.96 3.69 3.75 3.75 3.96 3.75 3.75 3.75 3.14 3.69 3.14 3.5 3.6 2.8 2.83

11.6 ± 0.9(2) – 10.9 ± 1.7(102) – – – – 12.6 ± 0(1) – – – – – – – –

25.7 ± 1.9 27.0 ± 3.0 23.5 ± 2 10.7 ± 1.3 1.9 ± 1.4 2.9 ± 0.8 3.8 ± 0.8 5 ± 0.4 2.7 ± 0.4 6.3 ± 0.7 8.1 ± 1.1 11.6 ± 1.7 16.8 ± 1 4.4 ± 0.4 3.9 ± 0.4 4 ± 0.5 99

(continued on next page)

X.-B. Tan et al. / Journal of Asian Earth Sciences 81 (2014) 91–104

Number

102.8797 102.8360 104.7375 104.7323 102.8772 103.3987 104.7135 30.4614 30.4093 31.8559 31.8648 30.4645 31.3273 32.3859 Gong-94 Gong-95 JY-1 JY-2 Kc0511 WCH-1 PW-12

RhoS density of spontaneous tracks [105 tr/cm2]; Ns: number of spontaneous tracks; RhoI: density of induced tracks [105 tr/cm2]; Ni: number of induced tracks, Nd = 4789 tracks counted on NBS 610 and 612; P(v2): probability that single grain ages are consistent and belong to the same population, test is passed if P(v2) > 5% (Galbraith, 1981). Pooled ages are calculated using a f-value of 340 ± 12 (1r) and 27.5 ± 1.0 (1r), respectively. The samples with * symbol have been published by Cook et al. (2013).

5 ± 0.5 5.3 ± 0.5 67.3 ± 4.6 83.3 ± 9.1 5.4 ± 0.7 2.1 ± 0.2 24.4 ± 1.9 2.83 2.8 4.25 4.25 3.38 3.96 3.05

Pooled age (Nd)

4798 4798 4798 4798 4798 4798 4798 88.8 95.4 82.32 49.93 99.85 0.09 3.05 894 1047 658 180 409 2827 1257 13.69 6.643 4.339 7.021 5.768 3.463 1.771 91 117 616 209 38 94 600 1.394 0.472 4.062 8.152 0.536 0.115 0.845 10 40 43 9 32 24 34 Granite Granite Sandstone Sandstone Granite Granite Sandstone Proterozoic Proterozoic Triassic Triassic Proterozoic Proterozoic Silurian

P(v2) (Ni) RhoI (105) (Ns) RhoS (105) Crystal Lithology Formation Elevation Longitude Latitude Sample

Table 1 (continued)

1464 1066 584 595 1222 1782 1121

RhoD

– – 9.6 ± 1.7(4) 11.6 ± 0.3(2) – – 13.89 ± 0.3(2)

X.-B. Tan et al. / Journal of Asian Earth Sciences 81 (2014) 91–104

Track length

100

the sample BX-3 and can be interpreted as the first stage of rapid exhumation. 5.3. Cenozoic fault activity in the southern segment of the Longmen Shan Based on stratigraphic relationships and field observations of shear sense indicators, Burchfiel et al., 1995 considered F3 to have first been a thrust fault and later a normal fault. On the hanging wall of F3, the ZFT age distribution of sample BO-2 (81 ± 8.5 Ma) ranges from 13 to 229 Ma indicating partial annealing in the Cenozoic. In contrast, the ZFT ages are totally reset on the footwall of F3, indicating a lack of thrust offset in the Cenozoic. It can be inferred from this that the timing of the thrusting on this fault is Mesozoic (Fig. 3). Based on the age differences between the hanging walls and footwalls of each of the faults (Fig. 7), the mountain front faults (F1 and F2), F4, and F5 have been the major active faults during the Cenozoic. The oldest reset ages are ca. 25–31 Ma on the hanging walls of F2, F4, and F5 indicating that all of these faults were active from ca. 25–31 Ma. In contrast, AFT ages are similar across these faults, at ca. 3–5 Ma from F2 to F6, indicating a small degree of offset across F3, F4, F5 and F6 after 3–5 Ma. These ZFT and AFT ages indicate that F4 and F5 probably became active at 25–31 Ma and stopped before ca. 3–5 Ma. Although the oldest reset ZFT ages are similar on the hanging walls of F2, F4, and F5, the zone of reset ZFT ages is much larger on the hanging walls of F4 and F5. Additionally, younger ages of 10–14 Ma are found near these faults, suggesting larger slip amounts for F4 and F5 than for the mountain front faults (F1 and F2) (Figs. 3 and 7). The mountain front faults (F1 and F2) separate reset and partially reset AFT ages. This result is indicative of large amounts of slip being consumed at the mountain front faults after 3–5 Ma (Figs. 2 and 3). In the Tianquan region east of F1, the folding of a conformable succession of Triassic to Eocene/Oligocene strata (Fig. 3) indicates that the formation of this fold and thrust belt occurred post Oligocene (Burchfiel et al., 1995). Our data suggest that the mountain front faults and the fold and thrust belt in the Sichuan Basin may be active areas with the potential to produce a future large earthquake. We previously proposed that this area had a high potential for seismic hazard (Lee et al., 2012) and unfortunately a large Mw6.6 earthquake occurred on 20th April, 2013. The focal mechanism was thrusting and the epicenter was located beneath the mountain front fault at 12.3 km depth (Xu et al., 2013). Surface deformation was not observed, indicating activity on a blind fault system (Xu et al., 2013). This blind fault could connect to the shallow detachment in the Sichaun Basin (Fig. 3). Considering the short seismic gap of ca. 40 km between the 2008 Mw7.9 Wenchuan earthquake and 2013 Mw6.6 earthquake, elastic strain energy has still not been completely released and therefore danger remains in this area. In the central segment of the LMS, the ZFT age of the sample WCH-1 (10.6 ± 0.9 Ma) on the hanging wall of the WenchuanMaoxian fault is totally reset; by contrast, the ZFT ages are partially reset on the hanging wall of the Beichuan-Yingxiu fault (Fig. 2). This indicates that the amount of exhumation at the hanging wall of the Wenchuan-Maoxian fault is larger than of the BeichuanYingxiu fault. The ZFT ages are similar on the hanging wall of the Wenchuan-Maoxian fault and Wulong-Yanjing fault (F4) indicating that they have similar characteristics and could be connected. 5.4. Variation in Cenozoic exhumation along strike of the Longmen Shan Our data and Wang et al. (2012) indicate that the first stage of rapid exhumation took place at ca. 25–30 Ma in both the southern

X.-B. Tan et al. / Journal of Asian Earth Sciences 81 (2014) 91–104

101

Fig. 5. Age histograms and radial plots for the AFT samples. For each sample, the pooled age and the number of grains counted are indicated on the histogram.

and central LMS; however, the degree of Cenozoic exhumation is different along strike (Fig. 2). In the central segment, reset ZFT ages are found only on the hanging wall of the Wenchuan-Maoxian

fault. In contrast, completely reset ZFT samples are found extending from Danba southeast to the Baoxing massif (Fig. 2). Apatite ages are ca. 3–5 Ma on the hanging wall of the mountain front

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Fig. 6. (a) The results of thermal modeling for the sample BX-3. Two stages of rapid exhumation were observed. The black line is the best fit model and the blue line is the weighted mean path. Both show that there were two rapid exhumation stages. The first stage at ca. 30 Ma and the second stage at ca. 3–5 Ma. (b) Track length distribution of the sample BX-3. The green line shows the best fit curve from the thermal modeling. (c) Exhumation history of the southern segment of the LMS. The exhumation rate is lower in the plateau area and higher near the faults F4 and F5. After 3–5 Ma the exhumation increases, consistent with the thermal modeling result. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

faults in the southern segment, 4.8–6.4 Ma in the Pengguan massif in the middle segment, and 26–33 Ma on the hanging wall of the northern segment of the Beichuan-Yingxiu fault. Since AFT ages increase from southwest to the northeast, there must be a decrease in the exhumation rate from southwest to northeast. The influence of the collision between the India and Eurasia plates propagates from south to north, which potentially results in a decrease in the exhumation rate from southwest to northeast. The 2008 Wenchuan earthquake was associated with more thrusting in the central segment of the LMS and more right-lateral strikeslip faulting in the northern segment (Xu et al., 2009). Strike-slip faulting with higher horizontal slip and lower vertical displacement could result in a lower exhumation rate in the northern segment. Compared to the central and northern segments of the

LMS, a foreland fold and thrust belt is well developed in the southern segment; in this belt, more displacement is being consumed and a different structural style exists than in the mountain area, indicating a potential difference in fault geometry. Different subsurface structures may have the potential to influence the amount of exhumation.

6. Conclusion In the southern segment of the LMS, our fission track data indicate two stages of rapid exhumation, with the first stage at ca. 30 Ma and the second stage from ca. 3–5 Ma. The timing of the second stage of rapid exhumation is consistent with the second stage

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103

Fig. 7. Structural profile, degree of ZFT resetting, and age distribution along the southern segment of the LMS. There are two total annealing zones and three partial annealing zones in the southern segment of the LMS. There are two groupings for reset ZFT ages, 10–14 Ma and 25–31 Ma. To the south of F6, the apatite fission track ages are ca. 3– 5 Ma, indicating that activity of the F4 and F5 is reduced after 3–5 Ma. After 3–5 Ma, most of the displacement was consumed by the F2 or the blind fault. To the north and south of the F6, the apatite fission track ages are greater than 10 Ma and 3–5 Ma, respectively. For strata located in the ramp area of F2, the apatite fission track ages are 3– 5 Ma and have a higher exhumation rate than strata located above the fault flat.

Reset ZFT ages vary from 10 to 14 Ma on the hanging wall of faults F4 and F5 to ca. 25–31 Ma at distance from these faults. AFT ages are 3–5 Ma between F2 and F6 and increase to 8–26 Ma in the plateau area. The uniform AFT ages between F2 and F6, and the contrast with unreset ages southeast of F1 and F2 indicate that the mountain front faults and the fold and thrust belt have been the most active areas in the southern segment of the LMS from ca. 3–5 Ma to present. Along the strike of the LMS, the amount of exhumation decreases from the southwest to northeast, which may be related to differences in fault geometry and thrust belt architecture. Acknowledgements

Fig. 8. Schematic model to interpret the ZFT ages. On the hanging wall of F4 and F5, the ZFT ages are 10–14 Ma; ages change to ca. 25–31 Ma at a distance from the faults before becoming partially reset to unreset ages in the plateau area. The age distribution can be explained by strata moving along the listric fault geometry. See the text for a description of the particle movement paths and their relationship to the expected ZFT ages.

of activity of the Xianshuihe fault and may be related. The exhumation rate in the second stage was higher than in the first, implying an increased slip rate along the LMS.

This project has been fully supported by the National Science Council, Taiwan, ROC, under Grant NSC 100-2119-M-94-002 and National Natural Science Foundation of China (M40821160550). The authors want to thanks their colleagues from the institute of Geology, China Earthquake Administration, China who offered significant help with field work. References Arne, D., Worley, B., Wilson, C., Chen, S.F., Foster, D., Luo, Z.L., Liu, S.G., Dirks, P., 1997. Differential exhumation in response to episodic thrusting along the

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