The Jinhe–Qinghe fault—An inactive branch of the Xianshuihe–Xiaojiang fault zone, Eastern Tibet

Tectonophysics 544–545 (2012) 93–102

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The Jinhe–Qinghe fault—An inactive branch of the Xianshuihe–Xiaojiang fault zone, Eastern Tibet Wang Shifeng a,⁎, Jiang Guiguo b, Xu Tiande c, Tian Yuntao d, Zheng Dewen e, Fang Xiaomin a a

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China Geography Resources Sciences, Sichuan Normal University, Chengdu 610101, China Regional Geological Surveying Team, BGEEMRSP, Shuangliu, Sichuan 610213, China d School of Earth Sciences, The University of Melbourne, Victoria, Australia e Institute of Geology, China Seismological Bureau, Beijing 100029, China b c

a r t i c l e

i n f o

Article history: Received 10 November 2011 Received in revised form 30 March 2012 Accepted 2 April 2012 Available online 17 April 2012 Keywords: Xianshuihe–Xiaojiang fault zone Longmen Shan thrust belt Jinhe–Qinghe fault Offset Left-lateral strike slip

a b s t r a c t The Xianshuihe–Xiaojiang fault zone (XFZ) forms a prominent linear NW–SE trending tectonic feature along the southeastern margin of the Tibetan Plateau. The spatial and temporal evolution of the predominantly strike–slip movement along the fault remains unclear, as estimates for the initiation age of its different segments range from 5 to 13 Ma. The Jinhe–Qinghe fault (JqF; the south part of the Longmen Shan thrust belt) provides a key for understanding the tectonic role and deformation history of the Xianshuihe–Xiaojiang fault zone in terms of timing, magnitude of displacement, and spatial–temporal evolution. This paper presents research on the structure and chronology of the JqF. The results of a geometric and kinematic study of the Jinhe–Qinghe fault show it to be a thrust fault with a left-lateral strike slip component; The strike slip offset extends at least 13–18 km to southwest, based on the shortening of its folds and faulting, and based on stream deflection. Ten apatite fission track apparent ages show a pronounced change in age/elevation gradient at ~17± 2 Ma, consistent with an abrupt increase of exhumation due to uplift of the hanging wall of the JqF at that time. Combined with the age of deformed Tertiary strata in the footwall of the JqF and the geometric relationships of the JqF with the Chenghai fault and the XFZ, this indicates that the JqF was active between 17 and 5 Ma. Based on the fault chronology, geometry, and movement, we suggest that the fault was a transpressional structure at the south end of the XFZ during the period from 17 to 5 Ma. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Two end member models have been constructed to interpret the role of faults (e.g. Altyn Tagh fault, East Kunlun fault, Red River fault, Karakoram fault) in the evolution of the Himalayan–Tibetan orogen. The first model interprets the faults as stable long-lived features that initiated during Eocene to Oligocene time along their entire current length, with only minor changes in kinematics (e.g. Leloup et al., 1995; Peltzer and Tapponnier, 1988; Tapponnier et al., 1982). The second model attributes the faults to distributed deformation, and interprets them as having propagated continuously during their relative transient life-spans, with the offsets along the fault being absorbed by transfer structures (e.g. Houseman and England, 1993; Ratschbacher et al., 1996; Wang et al., 1998; Zhang et al., 2004a, 2004b). Until now, the spatial and temporal evolution of the strike–slip movement along most of these faults remains debated. The Xianshuihe–Xiaojiang fault zone (XFZ) forms one of these important active sinistral shear fault zones in eastern Tibet. The NW–SE-trending XFZ extends more than 1200 km, from the Fenghuo Shan thrust belt in northern Tibet to the ⁎ Corresponding author. Tel.: + 86 10 84097049. E-mail address: [email protected] (W. Shifeng). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.04.004

Xiaojiang basin-and-range system in southeast Tibet (Allen et al., 1991; He et al., 2008; Wang and Burchfiel, 2000; Wang et al., 1998; Wang et al., 2008a, b; Xu et al., 2003). It consists of numerous branches, with many parallel active or young faults. From NW to SE, the four major branches (Fig. 1) are the Ganzi–Yushu fault (GyF), the Xianshuihe fault (XsF), the Anninghe–Zemuhe fault (AzF), and the Xiaojiang fault (XjF). Initiation of the GzF and the XsF occurred no later than 13 Ma, based on zircon U–Pb, mica 39Ar/40Ar, and apatite fission track dating of a syntectonic granite (Roger et al., 1995; Zhang et al., 2004b; Wang et al., 2009). In contrast, paleomagnetic data from sediments in faultcontrolled basins along the Anninghe–Zemuhe and Xiaojiang faults suggest that the AzF and the XjF became active at ~5 Ma (Yao et al., 2007; Zhu et al., 2008). These different initiation age estimates for different parts of the XFZ raise important questions about the spatial and temporal evolution of the XFZ. Wang et al. (2009) postulated that during the 13–5 Ma period the offset along the XsF transferred to the Jinhe–Qinghe fault (JqF), which formed a southward continuation of the Longmen Shan thrust belt (Burchfiel et al., 1995; Wang et al., 1998). However, there is almost no detailed work on the geometric, kinematic and chronologic character of the JqF. This article reports our progress in attempting to clarify the association between the JqF and the XFZ, and the spatial and temporal evolution the XFZ since 17 ±2 Ma.

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Fig. 1. Topographic map showing the trace of the Xianshuihe–Xiaojiang fault zone (XFZ) within eastern Tibet. Abbreviations: NLM = North Longmen Shan thrust belt; RRF = Red River Fault; YmF = Yuanmou Fault; GyF = Ganzi–Yushu Fault; XsF = Xianshuihe Fault; AzF = Anninghe–Zemuhe Fault; XjF = Xiaojiang Fault; JqF = Jinghe–Qinghe fault; ChF = Chenghai fault; LtF = Litang Fault.

2. Geological background The Longmen Shan thrust belt (LM) became active during early Mesozoic time, as reflected by major changes in depositional patterns along the western margin of the Sichuan Basin (Chen and Wilson, 1996; Meng et al., 2005). The lower and middle Triassic rocks deposited on top of the Yangtze platform are shallow marine to nonmarine, and rapid deposition of terrestrial deposits began in the late Triassic. During the Cenozoic, the LM was reactivated due to the eastward movement of the Tibetan crust (Avouac and Tapponnier, 1993; Burchfiel et al., 1995). In the late Cenozoic, the Xianshuihe fault offset the NE–SW extension of the LM by as much as 60 km, splitting it into the north Longmen Shan thrust belt (NLM) and the south Longmen Shan thrust belt (SLM) (Burchfiel et al., 1995; Wang et al., 1998). Studies of fault kinematics (e.g. Burchfiel et al., 1995; Densmore et al., 2007; Fan et al., 2008) indicate continuing activity of the NLM during the late Cenozoic. However, few studies have investigated the late Cenozoic history of the SLM. The SLM includes both the Jinhe–Qinghe fault (JqF) and the Xiao– Jinghe fault (Figs. 1, 2). In the area of the present study, the JqF consists of northeast trending, east and southeast vergent thrust fault segments which can be traced southeastward into western Yunnan

province, where they are disrupted by the Chenghai fault (ChF), which is part of the transfer structure along the northwest end of the Red River fault system (Fan et al., 2006; Wang et al., 1998). The JqF superimposes the Songpan–Ganzi fold belt (SG) onto late-Triassic to Jurassic terrestrial coal-bearing rocks of the Yangtze platform (YP). Proterozoic metamorphic rocks and Upper Proterozoic and Paleozoic sedimentary rocks form the basement of the Yangtze platform (Sichuan BGMR, 1991; Fig. 2). The main rock types in the study area include Sinian dolomite, Devonian limestone, a Triassic marine flysch unit, late TriassicJurassic terrestrial coal-bearing clastic rocks, Early Tertiary calcareous conglomerate around Ninglang, and late Tertiary clastic rocks in the Yanyuan and Xichang basins. Permian Emeishan basalts and Indosinian to Himalayan episode granites are also present locally. 3. Structural geology of the Jinhe–Qinghe fault system The Jinhe–Qinghe fault system (JqF) appears curvilinear in map view, stretching about 300 km from the Gongga Shan area in the NE, through the Lizhuang, Jinhe, Qinghe and Lizihe areas, and finally disappearing north of Chenghai in the SW (Fig. 2). A branch of the JqF developed near Lizihe village is referred to as the Lizihe–Ninglang fault (LnF). The LnF cuts the hanging wall of the JqF, which is interpreted as a

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Fig. 2. Geological map of the South Longmen Shan thrust belt (SLM) in the study area between the Lizhuang and Yongningping areas. Modified after Sichuan BGMR (1991). U–Th–He sample locations in Ouimet et al (2010) shown as blue square. Abbreviations: LnF = Lizihe–Ninglang Fault; XpF = Xiangfang–Pingchuan Fault; Yalong F. = Yalong fault; Jinhe F. = Jinhe fault.

transfer structure along the Jinhe–Qinghe fault system. This branch bends its trace from west to northwest, passing through Zhanhe town, and finally disappearing north of Ninglang town in the NW (Fig. 2). The Jinhe–Qinghe fault system consists of three branches: the Yalong fault, the Jinhe fault, and the LnF. The Jinhe fault is left-stepping with the Yalong fault south of Lizhuang village. Both the Jinhe and the LnF branches have been displaced by the Chenghai fault (ChF) to the southwest, and the Yalong fault has been offset by the XFZ to the northeast (Figs. 1, 2). In the following section we provide detailed descriptions of geometric and kinematic characteristics at different locations along the fault zone. 3.1. Lizhuang area The NE trending Yalong fault can be traced for about 80 km on the surface. At its NE end, the Yalong fault bounds the southeast side of the Gongga Shan range and is disrupted by the active Xianshuihe fault, which cuts through the northeast side of the Gongga Shan range (Fig. 1) and gradually disappears ~20 km south of Lizhuang village. The NNE striking linear Gongga Shan range, as the hanging wall of the Yalong fault, is one of the highest mountain ranges at the eastern edge of the Tibetan Plateau. The elevation difference between the footwall and hanging wall diminishes south of the Gongga Shan area. The Yalong fault zone forms a >200 m high cliff near Lizhuang, conspicuous because of the different colors of the different rock types of the hanging wall and footwall (Fig. 3a). The hanging wall consists of white Sinian shallow marine limestone with purple laterite, and the footwall consists of gray to brown Triassic slate and granite. The fault dips northwest at a low angle (b40°). We found drag folds along the fault zone next to the footwall (Fig. 3b). The attitude of the upper limb of the fold is 280°∠40° (dip 280° with dip angle 40°) and lower limb is 70°∠35°, which indicates thrust movement of the hanging wall. The thrust sense is consistent with the attitude of the strata (100°∠85°, Fig. 3c), which also show the regional principal stress is almost W–E directed. The rocks display cleavage with an orientation generally around 250–280°∠30–40° (Fig. 3c), which indicates a strike–slip component. An Indosinian episode granite in the fault zone experienced a combination of brittle to ductile deformation. Slickensides and steps on secondary faults in the granite show

that the thrust fault has a left-lateral strike slip component (Fig. 3d). The footwall strata are strongly deformed by several-kilometer-scale asymmetric folds with a 330°∠15° attitude of the fold axes—almost parallel to the orientation of the fault zone, which shows the shortening effect of the thrust fault. The channel of the Yalong River, which flows through the study area from north to south, is characterized by a pronounced bend of about 74 ± 2 km where it passes the Xiao–Jinhe Fault, and an 18 ± 1 km left-lateral deflection of the channel along the Yalong fault (Fig. 4). Since the rapid incision of the Yalong river and the activity of the Yalong fault are almost synchronous since 17–15 Ma (Ouimet et al., 2010; this paper), we interpret the big bend of the Yalong river along the Yalong fault as the result of thrusting along the Yalong fault, and the 18 ± 1 km stream deflection as the effect of the sinistral strike–slip component of the Yalong and Xiaojinhe faults. 3.2. Jinhe area The Jinhe segment of the JqF steps leftwards with the Yalong fault around the south of Lizhuang, where its orientation changes from a NW–SE to a N–S trend. A more than 200 m high fault scarp displays Sinian dolomite overlying late Triassic coal-bearing clastic rocks. The fault zone is a hundreds of meters thick brittle shear zone with a low to moderate dip toward the W to NW. Near Jinhe town, Sinian dolomite also appears in the footwall of the fault. Drag folding along the fault formed a plunging anticline (Fig. 3e); the orientation of one limb is 0°∠50°, the other is 190°∠35°. Its axis lies almost perpendicular to the strike of the Jinhe segment, indicative of a strike–slip component of movement along the Jinhe segment of the JqF. However, several-kilometer-scale asymmetric folds (with the 290°∠15–30° orientation) of Triassic strata developed in the footwall, trending almost parallel to the fault zone, attest to the predominance of thrusting along the fault. Northwest of the main fault zone, a secondary fault thrusting basalt over the Devonian limestone can also be seen. There is a curved structure named the Xiangfang–Pingchuan fault (XpF), almost parallel to the N–S striking Jinhe fault at west side. Permian basalt forms the footwall of the XpF, while Triassic sandstone and late Tertiary Yanyuan basin sediments are exposed on the hanging wall of the fault. East of Xiangfang village, the linear fault scarp

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Fig. 3. Structure features of the JqF in study area: a) the more than 200 m-high fault scarp shows the thrust components of the JqF, the hanging wall consists of Sinian limestone and footwall consists of Triassic slate; b) the drag fold in the footwall indicates the thrust character of the JqF; c) the almost vertical strata and the cleavage developed on it; d) steps and slickenside show the left-lateral component of the Yalong segment, e) a drag fold in the footwall of Jinhe segment showing the strike slip components of the JqF; f) the system leftlateral deflection of stream display the strike slip component of the XpF; g) the linear fault scarp and triangular fault facet around Xiangfang village display the normal fault character of the XpF; h) the attitude of the fault hinge of the inclined drag fold in the fault zone around Shuhe town indicating the southeastward thrusting with left-lateral strike slip. For the location of Fig. 3, see the sites of towns mentioned in Fig. 2.

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Fig. 5. Cross-section demonstrating that the JqF interrupted by the now active XpF and AzF, see the A–A/ in Fig. 2 for location.

faults, and thus appear associated with the thrusting. The strata on the hanging wall of the JqF usually align with the monocline strata dipping west or northwest. Southwestwards the strata became folded with their fold axes perpendicular to the strike of the fault, early Paleozoic strata form the core, and Devonian limestone forms the limbs of the fold. 3.4. Lizihe area Several kilometers-scale folds developed on the hanging wall of the JqF between Qinghe and Lizihe, over a distance of about 50 km. Their axes are perpendicular to the strike of the JqF, and early Paleozoic strata form the core of the anticlines. The core of the synclines consists of Permian basalt, and Devonian strata make up the limbs of the folds (Fig. 6). The fold axes are roughly parallel to the LnF north of Lizihe. Correlating the strata on both sides of the fault (labeled F2 in Fig. 6a) suggests a minimum of 2 km left-lateral offset along the JqF. A shortening amount of 11 km is obtained by calculating the folds in the 50 km distance between Qinghe and Lizihe, using a 35° dip angle for the fault limbs (Fig. 6b). This implies a total of at least 13 km (2+ 11 km) of shortening absorbed by the fault and folds of the JqF. The 13 km offset shows the left-lateral component of the JqF. Fig. 4. Eighteen kilometers left-lateral stream deflection along the Yalong segment showing the sinistral strike–slip component of the JqF.

and triangular fault facets display normal faulting characteristics. Moreover, the alluvial fans at the foot of the triangular fault facets bend in a left-lateral sense, indicating that dip and strike–slip movements of the XpF are still active (Fig. 3f, g). The XpF gets difficult to trace at its northeastern end near Lizhuang village. Around Xiangfang village, the fault splits into two branches. One of them appears to cut the JqF southward and connect with the Yuanmou fault (YmF) (Figs. 1, 2), a branch of the XFZ that has been active for 5 Ma (Wang et al., 1998; Zhu et al., 2008). Another branch bends into an east– west strike which controlled the evolution of the Yanyuan basin (Li et al., 2001). The high-angle active XpF and AzF cut the low-angle JqF; the relationship of the AzF, Jinhe fault and the XpF are shown on the E–W direction cross-section (Fig. 5). 3.3. Shuhe area The fault strike changes from N–S to NE–SW in the 50 km between Jinhe and Shuhe, then turns to an almost east–west direction in the Qinghe area. Along the fault trace, a hundreds of meters high fault scarp exposes Devonian limestones thrust over Triassic-Jurassic coal-bearing clastic rocks. The attitude of the upper limb of a drag fold (Fig. 3h) in the brittle shear zone next to the footwall is 290°∠70° (dip 290° with dip angle 70°), and that of the lower limb is 310°∠60°, which indicates upward movement of the hanging wall. Moreover, the attitude of the fold hinge is 210°∠33°, which attests to a predominantly thrust motion with a left-lateral strike slip component. In regional large-scale folds developed in the footwall of the JqF, the fold axes are approximately parallel to the adjacent

3.5. The Lizihe–Ninglang thrust fault In the area ~30 km west of Lizihe village, the LnF trends east–west and dips northward. Toward Zhanhe town, the LnF curves and assumes a northwesterly trend, and dips toward the northeast. East of Zhanhe town, the hanging wall consists mainly of Devonian strata and the footwall of Triassic clastic rocks. West of Zhanhe town, Devonian limestone are thrust onto the Oligocene to early Miocene conglomerate. Fig. 7a and b shows the Devonian strata thrust onto the Oligocene to early Miocene strata, with the faults and fold axes in the Cenozoic strata paralleling the thrust fault (50°∠35°) near Ninglang town. The thrust fault system is completely disrupted by the ChF. The linear fault scarp and triangular fault facets show the ChF to be an active normal-strike–slip fault with altitude 230°∠60° (Fig. 7c). Stream deflections of tens to several hundreds of meters and “wine-glass” type valleys attest to recent fault activity (Fig. 7d). The strata involved in the faults and the geometric relationships between the LnF and the ChF indicate that the LnF had been active after the early Miocene, then ceased its movement when it was cut off by the ChF which initiated its movement from about 5 Ma to present as indicated by apatite fission track ages measured along the RedRiver fault zone (e.g. Leloup et al., 1993; Xiang et al., 2007) (Fig. 7). As there is only 2 km of the 18 km offset left after accounting for faulting and folding along the JqF between Lizhuang and Lizihe area, we interpret the LnF to be a transfer fault absorbing part of left-lateral offset along the JqF. 4. Geochronology constraints on regional tectonic events In order to constrain the timing of thrust and strike–slip movement across the southern part of the LM, we sampled the Mesozoic granite exposed along the hanging wall of the JqF from Mianning to

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Fig. 6. a. Geological map showing the JqF character in the Lizihe area, modified from Sichuan BGMR (1991). Folds and faults developed in the hanging wall of the JqF, a 2 km offset of the strata two sides of F2 indicates the left-lateral strike slip of the JqF. Fig. 6b. About 11 km shortening amount is achieved in a 50 km long balanced cross-section.

Shimian (sample sites see stars in Fig. 2) for apatite fission track analysis, performed in the laboratory of Institute of Geology, China Seismological Bureau. Ten samples were successfully analyzed (Table 1, Fig. 8).

Four apatite fission track ages (samples APF 1, 2, 3, 4) range from 33.9± 4.1 Ma to 20.2 ± 2.9 Ma (from elevations of 2434 m to 2210 m). Track lengths range from 13.2 to 13.6 μm, displaying a unimodal length

Fig. 7. a) The triangular fault facet shows that the activity of the ChF cuts the LnF; 7b ) the imbricate branches of the LnF around Ninglang town; Fig. 7c) the linear fault triangle facets show the normal fault component of Chenghai fault (ChF); Fig. 7d) the “wine-glass” type valley and left-lateral deflection of stream show the strike–slip component of the ChF; Fig. 7e) cross-section showing that the LnF interrupted by the now active ChF, see the C–C/ in Fig. 2 for location.

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earlier episode of exhumation before 13 Ma, another interpretation is that the 17 ± 2 Ma just reflects the different thermo-chronological evolution history of the South as compared to the North Longmen Shan.

distribution (Fig. 8). The trend of the FT ages is consistent with a normal age vs. elevation pattern (i.e., the higher the sample locality, the older it should be). From ~1890 m to 920 m, six apatite fission track ages (APF 6, 9, 10, 11, 12, 13) range from 17.7 Ma to 8.2 Ma, with apatite fission track lengths from 12.3 to 12.6 μm (Fig. 8, Table 1). FT apparent ages on the vertical transect increase with elevation and show a distinct change in the apparent exhumation rate at ~1890 m. Below 1890 m elevation, analyses define a steep age/elevation gradient with an increase from 13.2 to 16.6 Ma. Above 1890 m, analyses define a shallow age/elevation gradient, with ages increasing from 16.6 to 33.9 Ma and a pronounced change in age/elevation gradient at ~ 17 ± 2 Ma (Fig. 9). Since all samples were collected in the same Indosinian granite batholith on the hanging wall of the JqF, the data seems convincing. However, since the samples were collected over a distance about 80 km (Fig. 2), additional analyses are necessary for confirmation. Notwithstanding this, the data is in good agreement with several distinctive topographic features of the study area. The elevation of the break-in-slope point, where an extensive erosion surface is developed in the landscape, is about 1900 m. Relict mountains with U-type valleys are developed above the break-in-slope point, while steep mountains with V-type valleys are characteristic below 1900 m. In a compressional tectonic setting, an abrupt increase in apparent exhumation rate on an age/elevation plot typically signals accelerated erosion most likely related to the upward motion of the hanging wall over the footwall (e.g., Fitzgerald et al., 1995; Reiners and Brandon, 2006; Wagner and Reimer, 1972). Thus, we attribute the age of ~17± 2 Ma to thrusting along the JqF. Ouimet et al. (2010) obtained 15 Ma apatite U–Th–He ages in the Yalong river transect where is very close to the JqF (Fig. 2). In conjunction with our new data this implies an episode of uplift and exhumation event at 17–15 Ma in the south Longmen Shan area. Additionally, Tian et al. (2012) also documented a 16 ± 4 Ma thermal– chronological event in the north Longmen Shan area. The 17 ± 2 Ma age from a block of South Longmen is within the 22–8 Ma range of chronological data obtained from the Gongga Shan area (Xu and Kamp, 2000), but is different from the 13 Ma got from the North Longmen Shan area (Kirby et al., 2002; Clark et al., 2005). One interpretation accounting for this difference is that the 17± 2 Ma age represents an

5. Discussion 5.1. Summary of the geometric and kinematic characteristics of the Jinhe–Qinghe fault system The JqF's more than 200-m-high cliff maintains a consistent topographic character along the fault trace from the Lizhuang area to south of the Lizihe area. The fault is a low-to-moderate west to northwest dipping thrust fault, as deduced from secondary structures exposed along the fault trace. The hanging wall consists mainly of Devonian limestone, but includes some early Paleozoic (and even Proterozoic) strata. The Triassic Songpan–Ganzi flysch rocks mostly lie 15–30 km away from the fault zone in the hanging wall, which exposes the thrust fault in the basement and overlying rocks of the Yangtze platform as part of the thrust sheet. Late Triassic-early Jurassic coal-bearing clastic rocks compose the main part of the footwall, whereas Oligocene-Miocene strata are present in the Ninglang area. The fold axes in the footwall always parallel the fault strike, showing that the shortening by the thrust movement affected a broad area of the footwall. In addition to the thrust component of the fault zone, accompanying structures in the fault zone such as slickensides, steps, and drag folds with axes perpendicular to the fault also demonstrate a left-lateral strike–slip component. Moreover, the Paleozoic strata in the hanging wall expose 2 km of left lateral offset near Lizihe. We estimate the offset along the JqF to at least 11 km, as calculated from the shortening accommodated by folding over the 50 km between Qinghe to Lizihe, and perhaps as much as 18 km, based on the Yalong River deflection. As opposed to the study of the JqF (part of SLM), the NLM shifted its movement to a thrust fault with a right-lateral component during the Wenchuan earthquake in 2008, with at least 6 m offset (Liu-zeng et al., 2009; Xu et al., 2009). Moreover, field measurements show that the right-lateral offset of the NLM has accumulated several hundreds of meters to as much as perhaps 17 km of offset (Densmore et al.,

Table 1 Apatite fission track ages around the Mianning–Shimian area. Sites of the samples see the star symbols in Fig. 2. Sample no.

Altitude (m)

Nc

APF-1

2434

26

APF-2

2410

27

APF-3

2309

27

APF-4

2210

27

APF-6

920

16

APF-9

1276

20

APF-10

917

23

APF-11

1892

23

APF-12

1898

25

APF-13

1887

24

ρd(Nd) (× 106 cm− 2)

ρs(Ns) (×105 cm− 2)

ρi(Ni) (×106 cm− 2)

0.969 (2422) 0.959 (2398) 0.950 (2374) 0.940 (2350) 0.921 (2302) 0.892 (2230) 0.882 (2206) 0.873 (2182) 0.863 (2158) 0.854 (2134)

1.000 (165) 0.836 (127) 0.626 (112) 0.480 (83) 0.389 (28) 0.653 (79) 0.668 (125) 0.523 (101) 1.451 (280) 1.673 (286)

0.502 (828) 0.455 (692) 0.405 (724) 0.393 (680) 0.479 (345) 0.627 (759) 0.127 (2365) 0.453 (875) 1.328 (2563) 1.496 (2559)

P(x2) %

r

6.5

48.3

5.9

Fission track

Mean track

Age (Ma ± 1σ)

Length (μm ± σ) (Nj)

0.688

33.9 ± 4.1

60.2

0.528

30.9 ± 4.0

5.3

99.8

0.697

25.8 ± 3.4

5.2

96.5

0.551

20.2 ± 2.9

6.5

99.6

0.682

13.2 ± 2.8

8.8

72.2

0.405

16.3 ± 2.4

17.9

58.3

0.725

8.2 ± 1.0

6.5

90.8

0.618

17.7 ± 2.4

19.2

67.9

0.831

16.6 ± 1.8

21.9

20.3

0.825

16.8 ± 1.8

13.25 ± 0.20 (47) 13.62 ± 0.16 (50) 13.51 ± 0.20 (44) 13.15 ± 0.18 (53) 12.30 ± 0.21 (36) 12.64 ± 0.14 (50) 12.39 ± 0.18 (62) 12.53 ± 0.17 (62) 12.42 ± 0.14 (76) 12.30 ± 0.14 (83)

U concentration (ppm)

Standard deviation (μm) 1.41 1.14 1.34 1.37 1.13 0.90 1.32 1.38 1.28 1.33

Nc = number of apatite crystals analyzed; ρd = induced fission-track density calculated from muscovite external detectors used with SRM612 dosimeter; Nd = total number of fission tracks counted inρd; ρs = spontaneous fission-track density on the internal surfaces of apatite crystals analyzed; Ns = total number of fission tracks counted inρs; ρi = induced fission-track density on the muscovite external detector for crystals analyzed; Ni = total number of fission tracks counted inρi; P(X2) = chi-squared probability that all single-crystal ages represent a single population of ages where degrees of freedom = Nc-1(Galbraith 1981); r = correlation coefficient between Ns and Ni; Nj = number of horizontally confined fission-track lengths measured.

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Fig. 8. Apatite fission-track age histograms and radial plots. Locations of the samples see the star symbols in Fig. 2.

2007; Fan et al., 2008), indicating that right-lateral movement must have begun in Cenozoic time. This show the NLM and SLM have had different tectonic evolution histories since the LM was offset by the movement of the XFZ.

Fig. 9. An apatite fission track age-elevation profile around Mianning–Shimian area in the South Longmen Shan.

5.2. Constraints on the time of latest activity on the JqF The JqF is a part of the Longmen Shan thrust belt, which became active during the early Mesozoic era, as reflected in major changes in depositional patterns along the western margin of the Sichuan Basin. The fault has been reactivated as the boundary fault of the Songpan–Ganzi fold belt since the onset of convergence of the Indian and Eurasian plates. But until now, no detailed studies have been conducted regarding the tectonic episodes of the JqF segment. This study constrains that the latest activity of the JqF was between 17 Ma and 5 Ma, based on the following evidence: (1) Ten apatite fission track apparent ages show a pronounced change in age/elevation gradient at ~ 17 ± 2 Ma. This implies an abrupt increase of exhumation rates around 17 Ma, which can be attributed to uplift of the hanging wall of the JqF during the fault's re-activation. (2) Deformed Oligocene-Early Miocene strata in the footwall of the Lizihe–Ninglang thrust fault in Zhanhe–Ninglang area constrain the age of faulting to be after the middle Miocene (Fig. 7a, b). Moreover, the JqF has been truncated by the Chenghai Fault, the Anninghe–Zemuhe Fault, the Xiangfang–Pingchuan

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Fault, and maybe the Yuanmou Fault (Figs. 1, 5, 7), all of which have been active since 5 Ma (e.g. Leloup et al., 1993; Xiang et al., 2007; Yao et al., 2007; Zhu et al., 2008). It thus seems that the JqF was active during some period between the middle Miocene and 5 Ma. The preliminary results of our ongoing work suggest that the 18–13 km of left-lateral offset accumulated during the 17–5 Ma period of fault activity, based on the fact that the LM is mainly a thrust fault with right-lateral components during the Cenozoic (e.g., Burchfiel et al., 1995; Densmore et al., 2007; Fan et al., 2008; Kirby et al., 2002; Liu-zeng et al., 2009; Xu et al., 2009). We therefore propose that the JqF had a right-lateral component before it was offset by the XFZ, and thus that the left-lateral offset along the JqF must have accumulated between 17 Ma and 5 Ma. 5.3. The relationship between the JqF and the XFZ and its tectonic implications The initiation age of the XFZ segments west of the LM appears to be no younger than 13 Ma (Roger et al., 1995; Zhang et al., 2004b; Wang et al., 2009), and the XFZ segments east of the LM began movement at about 5 Ma (Wang et al., 1998; Yao et al., 2007; Zhu et al., 2008). The different initiation times for the two sides of the LM show that the XFZ did not truncate the LM and reach the Xiaojiang area before 5 Ma. Kinematic studies on the Yushu–Ganzi segment show that they are mainly strike–slip faults, with a total of ~80 km left-lateral offset (Wang and Burchfiel, 2000; Wang et al., 2008a), and that the XFZ offset the LM by ~60 km (Burchfiel et al., 1995; Wang et al., 1998). About 20 km of offset was absorbed by structures west of LM before 5 Ma, and Wang et al. (2009) postulated that the JqF accepted most of that offset. Detailed geometric, kinematic and chronologic data from this study support the hypothesis that, during early stage of the fault activity, the JqF was a transfer structure at the southern end of the XFZ. If this is correct, then the temporal and spatial evolution of the fault can be summarized as follows, based on this and previous studies (Allen et al., 1991; He et al., 2008; Ratschbacher et al., 1996; Wang et al., 1998; Wang and Burchfiel, 2000; Wang et al., 2008a, b; Wang et al., 2009; Fig. 10): (1) The Songpan–Ganzi fold belt between the North Kunlun thrust belt and the Fenghuo Shan–Nangqian fold-and-thrust belt became shortened and thickened, with part of the crust extruding eastward and the LM becoming the eastern boundary fault during north–south shortening before 17 Ma. (2) Due to continued shortening, the XSF split the Songpan–Ganzi fold belt into two parts after 17 Ma, a northern part situated between the Kunlun fault, the NLM and the XFZ, and a southern part between the XFZ, the JqF and the RRF. The crustal

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extrusion resulted in the Songpan–Ganzi Fold belt in the south thrusting onto the Yangtze belt, with at least 13–18 km of offset absorbed along the XFZ. (3) Both parts of the Songpan–Ganzi block experienced continuous southeastward extrusion and rotation around the eastern Himalayan syntaxis, and the strike of the JqF also changed. At about 5 Ma, the strike of the JqF was no longer compatible with continued extrusion of the block, so the JqF ceased movement and became an internal fault of the new block, while the Anninghe–Zemuhe fault was initiated to accommodate further extrusion. The abandonment of the JqF may be the main reason for the pronounced differences between the topographic features on the western and eastern sides of the LM at the SLM compared to those at the NLM. The temporal and spatial evolution of the JqF thus supports the view that distributed deformation is an important characteristic of the convergence between the Indian and Eurasia plates during the Cenozoic.

6. Conclusions Geometric and kinematic studies of the Jinhe–Qinghe fault system show that the fault has acted mainly as a thrust fault, with some leftlateral strike slip component. We estimate the strike slip offset to be at least 13 km, based on observed fault offsets and the amount of shortening in the folds. The most recent tectonic activity along the JqF occurred between 17 and 5 Ma, based on AFT data, the age of deformed Tertiary strata, and geometric relationships with the Chenghai fault and XFZ. The fault chronologic, geometric and kinematic studies suggest that the fault functioned as the southern continuation of the Xianshuihe–Xiaojiang fault system. The activity of the JqF since 17 Ma has been determined by the nature of the activity of the Songpan–Ganzi block. As the boundary fault of the block, the strike of the JqF changed with the block's southeastward extrusion and rotation around the eastern syntaxis. When the strike of the JqF was no longer compatible with the overall kinematic regime in this area, slip along it ceased, and new faults (the Anninghe–Zemuhe and Xiaojiang faults) were initiated to accommodate the continued westward extrusion of crustal blocks.

Acknowledgments This research was funded jointly by the China National Key Project (2011CB403106) and National Natural Science Foundation of China (41172192, 41021001, 40672142). We would like to thank Prof. Liu Ruixun, Dr. Fan Chun for their help in field. Thanks are also given to Dr. Bill Isherwood for his help in revising an early version of this manuscript.

Fig. 10. This three-stage evolutionary model of the Xianshuihe fault system indicates that the continuous rotation of the crustal blocks could have been the main cause for the abandoned of the Jinhe–Qinghe fault and the activity of the Anninghe–Zemuhe fault and the Xiaojiang fault. The Xianshuihe–Xiaojiang fault zone lies within the transitional zone between the central Tibetan Plateau undergoing N–S compression and the southeastern margin of the Plateau experiencing clockwise rotation.

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