Brittle modification of Triassic architecture in eastern Tibet: implications for the construction of the Cenozoic plateau

Brittle modification of Triassic architecture in eastern Tibet: implications for the construction of the Cenozoic plateau

Journal of Asian Earth Sciences 27 (2006) 341–357 www.elsevier.com/locate/jaes Brittle modification of Triassic architecture in eastern Tibet: implic...

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Journal of Asian Earth Sciences 27 (2006) 341–357 www.elsevier.com/locate/jaes

Brittle modification of Triassic architecture in eastern Tibet: implications for the construction of the Cenozoic plateau Christopher J.L. Wilson*, Mat J. Harrowfield, Anthony J. Reid School of Earth Sciences, The University of Melbourne, Melbourne, Vic. 3010, Australia Received 12 December 2004; accepted 10 April 2005

Abstract In the Songpan-Ganzi Fold Belt, east Tibetan Plateau, post-Triassic deformation is localised within northwest-trending sinistral and southwest-trending dextral brittle fault zones. Between these zones, large tracts of the fold belt preserving Late Triassic (Indosinian) architecture have been transported effectively intact. This transport was almost exclusively strike-slip and did not accommodate significant differential exhumation or uplift. Right-dihedra paleostress reconstructions from fault-plane kinematic data indicate a rotation of the principal compressive stress, from NE–SW-trending in the north to a dominant E–W trend throughout the eastern Songpan-Ganzi fold belt; exceptions occur adjacent to competent Proterozoic basement complexes and other rigid bodies. These stress solutions are consistent with a compressive stress regime that radiates about the Eastern Himalayan Syntaxis. A secondary N–S compressive paleostress, identified in southern regions, may record late southwards transportation of the Songpan-Ganzi fold belt relative to the Eastern Himalayan Syntaxis. The lack of pervasive Cenozoic-Recent upper crustal shortening in this region suggests that thickening of the east Tibetan crust was predominantly accommodated by viscous deformation at depth. We envisage decoupling of the lower and upper crust and partitioning of an E–W compressive stress between: (1) A laterally mobile thin-skinned veneer; and (2) A homogeneously thickened tectonic basement. q 2005 Published by Elsevier Ltd. Keywords: Brittle deformation; Cenozoic tectonics; Fault analysis; Kinematics; Songpan-Ganzi fold belt; Tibetan Plateau

1. Introduction The Cenozoic Indo-Asian collision induced numerous lithospheric-scale strike-slip fault zones and major orogenic thrust systems within the Tibetan Plateau (Yin et al., 1999; Tapponnier et al., 1990). Palaeogene thrusting and sedimentation in central Tibet (Wang et al., 2002), Cenozoic faulting and thrusting in southern Tibet (Ratschbacher et al., 1992; 1994) and Tertiary metamorphism around the Eastern Himalayan Syntaxis (Ding et al., 2001) are all records of ongoing modification of the region in response to the collision. A marked Miocene increase in denudation rate about the eastern margin of the Tibetan Plateau is thought to reflect surface uplift in response to crustal thickening (Kirby et al., 2000; Clark et al., 2000; 2004). Reactivation of inherited Mesozoic terrane boundaries allowed * Corresponding author. Tel.:C61 3 8344 6538; fax: C61 3 8344 7761. E-mail address: [email protected] (C.J.L. Wilson).

1367-9120/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.jseaes.2005.04.004

north-eastward growth of the plateau (Tapponnier et al., 2001), E–W extension of central Tibet (Yin and Harrison, 2000) and accelerated strike-slip transport to the south and east (Shen et al., 2001). In the eastern Tibetan Plateau, IndoAsian collision resulted in reactivation of thrusts in the Longmen Shan (Arne et al., 1997) and brittle strike-slip faulting of the Songpan-Ganzi Fold Belt (Wang et al., 1998; Wang and Burchfiel, 2000). The tectonic architecture of eastern Tibet, comprising the Lhasa, Qiangtang and South China blocks, the SongpanGanzi Fold Belt and the Yidun Arc (Fig. 1a), reflects Triassic accretion (Sengo¨r, 1984; Burchfiel et al., 1995; Harrowfield and Wilson, 2005; Reid et al., 2005a). Although granite plutons were emplaced into the central regions of eastern Tibet during the Cretaceous (Reid et al., in press-b), few structures across the region can be unambiguously attributed to post-Triassic but pre-Cenozoic tectonism. A history of episodic pre-Cenozoic thrusting is recorded by thermochronology (Arne et al., 1997) and in foreland basin sedimentation (Chen et al., 1995; Li et al., 2003) within the Longmen Shan, at the eastern margin of the Tibetan Plateau. This reactivation has been tentatively

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Fig. 1. Tectonic elements across the eastern Tibetan Plateau and surrounding regions. (a) Map showing: (1) the Longmen Shan Thrust Nappe Belt; (2) the Danba Antiform; (3) the Luding Basement Complex; (4) the Peng-Guan Basement Complex; and (5) the Miocene Konga Shan Granite that intrudes the Xianshuihe Fault Zone. WSFB, Western Sichuan Foreland Basin. (b) Major brittle fault zones of the eastern Tibetan Plateau; 3000 m contour shows plateau margin, triangles indicate spot heights above plateau average (w4500 m). BF, Batang Fault. Shaded areas show the location of the four areas, eastern (A), northern (B), central (C) and southern (D), corresponding to the sections shown in Figs. 7 and 8. (c) Geologic map of the eastern study area, showing distribution of brittle faults, Late Triassic form surface, Indosinian and Konga Shan granites and the distribution of basement rocks. Modified from SBGMR (1981).

correlated with the Lhasa collision (Arne et al., 1997), and pre-Cenozoic deformation is reported from the Lhasa Block to the west (Murphy et al., 1997; Ratschbacher et al., 1992). Nevertheless, most workers in the region agree that

Jurassic–Cretaceous tectonism did not significantly modify the Triassic architecture of the eastern Tibetan Plateau (e.g. Burchfiel et al., 1995; Harrowfield and Wilson, 2005; Reid et al., 2005a).

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The most prominent post-Triassic structures that crosscut this architecture are large-scale brittle Cenozoic strike-slip fault zones (Fig. 1b). These can be generalised into a sinistral (left-lateral) NW-trending set and a dextral (rightlateral) SW-trending set (Fig. 1b). The NW-trending set includes the Kunlun, Ganzi, Litang, Zhongdian and Xianshuihe faults, all of which remain seismically active (Allen et al., 1991; Huang and Dong, 1999; Li et al., 1999). The Kunlun Fault has accommodated a minimum of 75 km of sinistral offset since the Cenozoic (Chen et al., 1994), with an average Quaternary slip rate of 10–20 mm/yr (Kidd and Molnar, 1988; Van Der Woerd et al., 1998). The Ganzi Fault shows a total displacement of around 100 km (Wang and Burchfiel, 2000), while the Litang and Zhongdian faults show comparatively small sinistral offsets of w10 km (Wang and Burchfiel, 2000) and w10–25 km (Wang et al., 1998). The sinistral Xianshuihe fault zone has an average Holocene slip rate of 15G5 mm/yr (Allen et al., 1991) and an estimated total offset of w65 km (Burchfiel et al., 1995). The Xianshuihe and Kunlun fault zones are separated in the east by the NE-trending Longmen Shan Thrust Nappe Belt (Fig. 1b). Of the inferior and conjugate NE-trending fault set (Fig. 1b), the principal Batang Fault has caused apparent right-lateral river offsets of w18–30 km (Wang and Burchfiel, 2000). Focal mechanism solutions from seismic activity on the fault in 1989 identify local complexity; three of the five suggest sinistral reverse faulting, one indicates dextral strike-slip and another oblique dextral thrusting (Liu and Chen, 1999). This network of brittle faults and thrusts encloses a large portion of the southeast Songpan-Ganzi Fold Belt (Chen et al., 1994), including high-grade Late Triassic metamorphic complexes and late- syn- to post-orogenic Mesozoic plutons (Fig. 1c). Whilst Cenozoic shortening and exhumation within the Longmen Shan Thrust Nappe Belt has been documented (Chen et al., 1995; Chen and Wilson, 1996; Burchfiel et al., 1995; Arne et al., 1997), field studies have identified little significant post-Triassic shortening of the wider Songpan-Ganzi Fold Belt (Fig. 2a). Kirby et al. (2002) and Clark et al. (2004)) suggest that the drainage pattern throughout most of the Songpan-Ganzi Fold Belt is not severely disrupted by these faults but is more related to Miocene (?) to Recent uplift of the plateau margin in the Longmen Shan. Similarly apatite fission track data identify no differential uplift across the Xianshui He Fault, where the fault trace is linear and crosscuts only pervasively structured Triassic strata (Xu and Kamp, 2000). Field-based investigations of brittle deformation in the eastern Tibetan Plateau have been relatively few in number (e.g. Chen et al., 1987; Ratschbacher et al., 1996; Wang and Burchfiel, 1997; 2000; Wang et al., 1998), however, the data they provide is of prime importance in helping to constrain models for the tectonic evolution of the region. This paper presents the results of an investigation of the central-eastern Tibetan Plateau with the aim of providing a description of the effects of post-Triassic and particularly

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Cenozoic-Recent deformation in the region. What follows in this paper is a description of post-Triassic deformation in the Songpan-Ganzi Fold Belt and attempts to relate such deformation to construction of the Tibetan Plateau.

2. Post-Triassic deformation of the eastern Tibetan Plateau In this section we describe localised post-Triassic faults observed at outcrop-scale in the eastern, northern, central and southern reaches of the eastern Songpan-Ganzi Fold Belt (areas A–D, respectively, in Fig. 1b). However, due to the lack of appropriate magmatism and sedimentation, the age of most of these faults cannot be directly determined. By necessity, our arguments depend on a collection of sparse fault orientation and slip direction data, that we consider to broadly reflect Cenozoic-Recent deformation. 2.1. The eastern region: Danba to Tien Wan Between Daofu and Tien Wan, the Xianshuihe Fault Zone is intruded by the Konga Shan Granite (Fig. 1c), forming a massif some 100 km long and 15 km wide that occupies a sinistral jog between two fault segments. Roger et al. (1995) described an en-echelon fault array within the Konga Shan Granite and dated syn-kinematic migmatites at 12.8G1.4 Ma (U/Pb zircon). This age provides a minimum time constraint on the initiation of sinistral transport on the Xianshuihe Fault Zone. North of the Konga Shan Massif, the surface expression of the Xianshuihe Fault Zone is a zone of graphitic gouge that may be traced for over 200 km between the townships of Barmie and Ganzi (Fig. 1). This gouge zone, locally up to a kilometre in width, crosscuts all ductile structures within the Triassic metasediments and crops out as metallic-grey ridges (Fig. 2a and b). Recent movements on the fault are evidenced by slumped conglomerate horizons within the gouge zones, stranded many metres above the current drainage system (Y in Fig. 2c). North of Kangding, the main body of the Xianshuihe Fault Zone lies within the eastern margin of the Konga Shan Massif, where it is manifested as both ductile synkinematic fabrics with sub-horizontal transport lineations and later brittle gouge zones. Typical gouge zones north of Kangding are several metres in width, sub-vertical and contain a spectrum of internal textures, ranging from semi-ductile foliations to fine-grained cataclasites. Slickenfibre development within these gouge zones is subhorizontal and parallel to ductile transport lineations within the margins of the massif. South of Kangding, the main trunk of the Xianshuihe Fault Zone traces the western margin of the Luding Basement Complex (Fig. 1). At Tien Wan, this basement complex has been serpentinised and pervasively dissected by mesoscale brittle faulting. Transport-normal, dilational

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Fig. 2. Field exposures of post-Triassic faults within the eastern and northern regions (Fig. 1b) of the eastern Tibetan Plateau. (a) Ridges of gouge mark the Xianshuihe Fault Zone, just south of Barmie; (b) Large raft of Triassic sandstone (X) entrained in the Xianshuihe fault zone, just north of Barmie; (c) Slumped conglomerate horizons (Y) reflect recent movements on the Xianshuihe fault zone, south of Barmie; (d) Stepped slickenfibre and quartz growth on a thrust surface within the Xianshuihe fault, east of Tien Wan; (e) South-east dipping low-angle brittle faults (solid lines) within Paleozoic limestones, west of Dege. Dashed lines indicate Triassic form surface; tree is w2 m high; (f) Field sketch of probable post-Triassic duplex structure, west of Jomda, showing bedding surfaces (thin lines) truncated by faults (thick lines). Rare epidote–chlorite growth suggests that these structures formed at low metamorphic grade; (g) Largescale open folds and (h) duplex structures within Jurassic sandstones of the Chamdo Basin.

jogs within fault planes, typically around 10 cm wide and up to 15 mm across, host euhedral quartz cavity growth and characteristically stepped layered slickenfibres (Fig. 2d). In contrast to the brittle faulting described further north, mesoscale fault planes occur in all orientations within the fault zone and generally preserve more than one generation of non-coaxial slickenfibre growth, many of which indicate steeply inclined transport. East of Danba (Fig. 1c) northwest-trending gouge zones, many metres in width, are associated with sub-horizontal fault-plane slickenfibre growth. Stepped slickenlines are

consistent with sinistral strike-slip transport. Offset magnitudes on these faults are impossible to estimate in outcrop, however, the 1: 200,000 Danba Sheet (SBGMR, 1981) displays horizontal offsets of up to 5 km on such structures, of which there are perhaps 10 examples within the Danba Antiform (Fig. 1c). Similarly, the 1: 200,000 Xiaojin Sheet (SBGMR, 1981) displays several large examples of dextral faults that appear to be conjugate to the NW-trending sets. One such fault intersects the western margin of the Rilonguan Granite (Fig. 1c), with an approximate horizontal offset of 3 km (Fig. 1c). A similar fault, with 2–3 km of

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horizontal offset, terminates against the western margin of the Ma Nai Granite (Fig. 1c). Faults locally crosscut ductile Late Triassic (Indosinian Orogeny) structures within the Danba Antiform (Fig. 1a) and the Indosinian high-strain zone described by (Harrowfield and Wilson, 2005) adjacent to the eastern flank of the Danba Antiform. In general, post-Triassic deformation is manifested as in-sequence faulting and slickenfibre growth on bedding planes with the reactivation of Indosinian-aged surfaces. On the basis of superposition, orientation, nature of slickenlines and kinematic consistency, the majority of brittle faults within the eastern region can be correlated with late Cenozoic-Recent strike-slip movement and are discrete from the Indosinian-aged thrusts (Harrowfield and Wilson, 2005). 2.2. The northern region: Ganzi to Jomda Numerous large fault-bound blocks were observed along the Ganzi Fault Zone (see Fig. 3 in Wang and Burchfiel, 2000) with brittle deformation intensity decreasing dramatically away from major fault segments. To the west, around Dege, low-angle brittle faults truncate

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pre-existing ductile fabrics (Fig. 2e). In contrast to faulting around Ganzi, diffuse faulting around Dege was identified over a wide area and not apparently associated with a single major fault zone. Further west around Jomda in the Qiangtang Block, Upper Triassic rocks were displaced by small-scale east-directed thrusts and duplex structures (Fig. 2f), that overprint west-directed ductile Triassic thrusts (Reid et al., 2005a). 2.3. The central region: Litang to Markham North and south of Litang, mafic volcanic rocks and large blocks of brecciated limestone characterise the melange of the Triassic Ganzi-Litang Suture (Fig. 3). South of Litang, the melange contains an abundance of gouge zones, E-directed thrusting and intensive brittle fracturing. This is typified in section Y–Y 0 (Fig. 3) where the western end is characterised by sheared mafic volcanogenic rocks that preserve a chlorite-defined stretching lineation that plunges shallowly to the south, with kinematic indicators showing dextral, W-to-NE asymmetry (Fig. 3a). The eastern end of this section shows an identical chlorite-defined stretching lineation that plunges steeply to the west and is associated

Fig. 3. Schematic geological cross-section (Y–Y 0 ) south of Litang, illustrating Cenozoic structure. Inset shows location of section and structural data are plotted on lower hemisphere equal area projections. Mineral stretching lineation (ML) orientations change from shallowly south-plunging in the east (b) to steeply west-plunging in the west (a). This change is concomitant with a shallowing of the C plane and slight shallowing of the S plane within SoC shear fabrics (Passchier and Trouw, 1996) in volcanogenic rocks (inset boxes). Bedding (S0) and slaty cleavage (S1) planes within sandstone and shale units at the eastern end of the section show upright folding typical of the Songpan-Ganzi Fold Belt or Yidun Arc (Reid et al., 2005a; Burchfiel et al., 1995). Shales to the west of the river are folded by steeply plunging non-cylindrical folds, presumably related to the deformation of the volcanogenic rocks to the west. These outcrops are interpreted to reflect Cenozoic dextral transpression associated with brittle faulting in the area.

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with W-over-E thrusting (Fig. 3b). Thus there is a change from dextral strike-slip offset, to east-vergent thrusting from west to east across the section, which is interpreted to reflect overall dextral transpression. These brittle structures overprint the predominantly east-dipping Triassic structural grain observed around Litang and are consistent with the sense of movement across strands of the Litang fault to the north and south. These observations support suggestions by both Ratschbacher et al. (1996) and Wang and Burchfiel (2000) that some of the melange that defines the GanziLitang Suture may indeed be Cenozoic in age. Intense cataclastic deformation was observed within Permo-Triassic metasediments along the Jinsha River near Batang, as previously reported by Ratschbacher et al. (1996). Post-Triassic structures were also observed within red-bed sandstones of the Chamdo Basin near the town of Markham, including large west-vergent folds (Fig. 2g) and localised thrusting associated with flexural slip (Fig. 2h). Folding of similarly aged red beds within the Qiangtang Block to the north-west has been attributed to the Lhasa collision (Chang, 1997; Wang et al., 2002), however, deformation in the laterally equivalent Mesozoic Chuxiong basins (Fig. 1b) is Cenozoic in age (Leloup et al., 1995). 2.4. The southern region: Deqin to Benzilan Sporadic brittle faults are common in this southern area and to the north of Benzilan there is a major postTriassic ductile shear zone (location A in Fig. 4). In this outcrop, chlorite-epidote-green amphibole schists strike 1608 and show evidence of intense shearing, parallel to a prominent sub-horizontal stretching lineation. Shear sense indicators, including C–C 0 fabrics (Passchier and Trouw, 1996) and en echelon quartz–calcite tension veins, consistently record a sinistral sense of shear (Fig. 5). Within the shear zone, boudinage of competent marble

layers, foliation boudinage, isoclinal folds and composite planar fabrics indicate non-coaxial shear (Fig. 5). To the north of the shear zone, Paleozoic mica–schists and intercalated marbles strike 3308, dip steeply to the west and show a steep down-dip stretching lineation. To the south, similar lithologies strike between 320 and 3608, are generally west-dipping and preserve a moderately steep, north to south plunging stretching lineation (Fig. 3). This shear zone overprints earlier synmetamorphic structures, suggesting it is post-Triassic. Along the Jinsha River, an 8–10 m wide outcrop of strongly deformed conglomerate (Fig. 6a) preserves a prominent shallow south-plunging elongation lineation. The orientation of this lineation is in marked contrast to the steep stretching lineation within slates to the east and west of the outcrop that is associated with the Indosinian deformation (Harrowfield and Wilson, 2005), this suggests that the deformation within the conglomerate is post-Triassic. Measurement of deformed clasts (Fig. 6b) suggests that the strain lies within the field of apparent sinistral constriction (Fig. 6c). A Cenozoic age for the deformation observed in this conglomerate is suggested, and given the location and identical kinematics of the shear zones identified here, it may be part of a network of shear zones. In the vicinity of Benzilan (Fig. 4), brittle faults along with a number of graphitic gouge zones were observed, which are inferred to be have resulted from movement on the Zhongdian fault (Figs. 1b and 4). In addition, the eastern flank of the early Triassic Baimaxue Shan granite preserves a pseudotachylite, gouge-like fabric, and asymmetric kinematic indicators that show an E-over-W sense of shear. Graphitic gouge zones were also observed within Permian aged mica–schists on the western side of the Baimaxue Shan Pass (Fig. 4) that can likewise be correlated with post-Triassic deformation.

Fig. 4. Simplified geological map between the townships of Benzilan and Deqin, showing the Jinsha and Lancang (Mekong) rivers and key localities described in the text. A, ductile shear zone (see Fig. 5) at 288 20.164 0 N, 998 13.948 0 E; B, strained conglomerate (see Fig. 6) at 288 17.647 0 N, 998 16.714 00 E. U/Pb zircon age for the Baimaxue Shan granodiorite is from Reid et al. (in press). Location A is conglomerate Location.

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Fig. 5. Field sketches, photographs and map of post-Triassic ductile shear zone along the bank of the Jinsha River, north of Benzilan (Fig. 4 location A): (a) Field sketch, showing isolated isoclinal fold hinges and asymmetric foliation boudinage that suggests sinistral kinematics. Dark bands correspond to green marble layers within a chlorite–epidote-green amphibole schist. Boudin necks are filled with quartzCcalcite; (b) Calc-silicate pod (outlined in white) entrained within chloritic schist. Numerous calc-silicate pods or boudins occur in linear arrays within the chlorite schist; (c) Isolated boudins of calc-silicate (dark) and foliation boudinage within chloritic schist; (d) Photomicrograph of chlorite schist showing quartz recrystallisation within pyrite pressure shadows, and dynamically recrystallised quartzCchlorite matrix; field of view w2 cm; (e) Structural map of region adjacent to shear zone outcrop. Stereonet shows dip of shear zone (great circles) and orientation of lineation (dots). Indosinian foliation within the schists is oblique to the trend of the shear zone.

Fig. 6. Strained conglomerate north of Benzilan (Fig. 4 location B) (a) XZ and YZ sections in hand specimen; showing strong elongation parallel to principal stretching lineation; (b) Rf/f plot (Passchier and Trouw, 1996) from measurement of deformed clasts in the XZ section gives Rfmax of 22.5, Rfmin of 3.0, and values for the initial (Ri) and final (Rs) shape of the XZ strain ellipse of 2.7 and 8.2, respectively; (c) Flinn diagram derived from the Rf/f analysis, where aZX/Y, bZY/Z, ZZ1, Ri, restored initial ratio of strain ellipse and Rs, value of strain ellipse.

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3. Fault data analysis Paleostress reconstructions from reliable fault orientation and slickenline measurements were determined using the right dihedra method (Angelier, 1994; Angelier and Mechler, 1977); this is essentially a 3D reconstruction of the quadrants that have boundaries normal to both the fault plane and transport lineation (Oncken, 1988; Miller and Wilson, 2004). Unlike the analysis of Ratschbacher et al. (1996) who assign confidence levels to each slip-sense datum and assign deviatoric-stress tensors to individual sites we rejected unreliable measurements in the field and areally averaged our measurements to obtain the regional continuity of the compressive stress over a given region. Superimposition of right dihedra solutions progressively refines the area of the common compressional field for a population of faults. If superimposed solutions do not overlap and no common s1 is inferred, the faults were probably active under different stress fields and/or at different times (Miller and Wilson, 2004). Coeval fault populations may also be identified by plotting fault-plane kinematic solutions using the orientation, transport lineation and sense of movement of individual faults; these effectively show 2D projections of the hangingwall transport vector (Miller and Wilson, 2004). 3.1. Eastern region Across the Songpan-Ganzi Fold Belt, compression stress solutions (Fig. 7) vary locally between ENE-trending (subsets A1–A4, A7, A8, A10) and SE-trending orientations (subsets A2 A5, A9). Faulting is predominantly manifested as sinistral strike-slip (e.g. A1–A2, A9) with consistent fault-plane kinematic solutions that indicate a simple orthogonal relationship; this probably reflects the relatively simple geometry of this faulting and a consistently orientated far-field stress. Thus, strike-slip faulting in the eastern region appears to have been driven by a broadly E– W oriented compressive stress field. Such an interpretation is consistent with that described from seismic data (Chang, 1997), that inferred from structural trends (Burchfiel et al., 1995) within the Longmen Shan and the regional crustal velocity field (Lave´ et al., 1996; Holt, 2000). There is also a component of north–south compression with local strike-slip and reverse movements (subset A6) that occurs where there is a bend in the Ganzi Fault. Adjacent to the Luding Basement Complex paleostress and fault-plane kinematic solutions are considerably less systematic and preserve both reverse and dextral strike slip movements. Superimposed right-dihedra reconstructions identify both E–W and N–S compressive solutions (subsets A11–A13) that could in fact reflect a wide spectrum of stress orientations. Likewise, fault-plane kinematic solutions show considerable scatter, with no simple relationship between extensional and compressional structures. This breakdown of paleostress reconstructions is correlated with

pervasive brittle faulting adjacent to large Proterozoic basement complexes. In outcrop, fault planes of highly variable orientation typically preserve a spectrum of episodic slickenfibre growth, much of which indicates steep transport. In addition, identically oriented fault surfaces within a single outcrop commonly preserve very different slickenfibre orientations and thus record very different transport histories. It seems probable that much of this geometric complexity arises from the relative competency of these basement blocks and their ability to behave as stress-guides. On a regional scale, such an observation is consistent with the style of basement-guided crustal rotation and stress-field evolution (Wang and Burchfiel, 1997). 3.2. Northern section Subset B1 (Fig. 8a), near Daofu, resolves east–west compression. This area is dominated by steeply dipping, NW-trending faults that show identical sinistral kinematics to the Xianshuihe Fault Zone further south (Fig. 7 subset A1). As the greater part of the deformation is strike-slip and the dihedra solutions indicate a sub-horizontal s1, the minimum compressive stress, s3, is most likely subhorizontal in this region. Further north, around the township of Ganzi, subsets B2 and B3 show less coherent stress solutions from a variety of fault orientations and kinematics (Fig. 8a). Subset B2 shows NE- and NW-trending palaeostress solutions, and three coarse fault populations: S-side-down normal faults, S-over-N reverse faults and NW-trending sinistral strike-slip faults. This suggests either a temporal difference between the dip-slip faults and the strike-slip faults or, more probably, complex strain heterogeneity arising from the competency contrast of mafic igneous rocks, limestone blocks, and Triassic sediments. Such competency contrasts may have locally resulted in block shuffling within the Ganzi Fault Zone. Subset B3 shows dominantly NE-trending compression that is virtually identical to the paleostress solutions from other subsets to the west (B4, B5, B6). Faults in subset B3 show dextral normal and sinistral strike-slip kinematics along dominantly NW-trending faults that parallel the Ganzi Fault. This suggests that movement of the Ganzi Fault is also a result of NE-trending compression. Minor W-over-E thrusting observed in subset B3 suggests that the Ganzi Fault is locally transpressional. Northeast of Dege, three subsets, B4, B5 and B6, show NE-trending compression solutions, across a spectrum of fault orientations and kinematics (Fig. 8a). Subset B4 is dominated by NW-trending W-over-E reverse faults, while subsets B5 and B6 show populations of NE-trending sinistral and dextral strike-slip faults, respectively. NEtrending strike-slip faults appear to be conjugate to the larger NW-trending faults further east, both of which can be reconciled with NE-trending sub-horizontal compression. That similarly oriented faults record both strike-slip and reverse movement suggests that s3 varied locally between

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Fig. 7. Geographic subsets of paleostress data in the eastern region. Stereonets coincide with areas where the data was collected. Shown are 2D fault-plane transport solutions for both extensional (open arrows) and compressional (solid arrows) faults and superimposed 3D right-dihedra paleostress reconstructions (compressional fields shaded). Reconstructions indicate a broadly E–W compressive stress, with fault-plane kinematic solutions tending towards a simple orthogonal relationship. Closer to the margins of the fold belt, however, both paleostress and fault-plane kinematic solutions become increasingly complex (e.g. A11 and A12) where a second compressional field (grey) is identified.

sub-horizontal and sub-vertical. In the vicinity of Jomda, subsets B7 and B8 both yield E–W compression solutions from NW-trending W-over-E reverse and sinistral strikeslip faults (Fig. 8a). Subset B7 also shows a component of NE-trending compression, similar to that observed in subsets to the east (B4 to B6). 3.3. Central region The subset centred on Litang (C1; Fig. 8b) yields two sets of right dihedra solutions that indicate compression trending towards w130 and w508. Individual faults trend both NE and NW, and show a predominance of sinistral

kinematics, consistent with the nearby Litang Fault (Fig. 8b). There is some variability in the data, with more N–S trending faults seemingly indicating a steep s 1, consistent with sub-horizontal extension. This variability of both kinematics and paleostress solutions is interpreted to reflect local competency contrasts between massive limestone blocks and weaker shale/ volcanogenic rocks, similar to the complexities identified near Ganzi in the north. Subset C2, centred on the town of Yidun (Fig. 8b), shows E–W compression consistent with Cenozoic stress tensors determined by Ratschbacher et al. (1996). Complex paleostress solutions from subset C3 show

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Fig. 8. Geographic subsets of paleostress data within the (a) northern, (b) central and (c) southern regions; see Fig. 1b for location. Shown are 2D fault-plane transport solutions for both extensional (open arrows) and compressional (solid arrows) faults and superimposed 3D right-dihedra paleostress reconstructions (compressional fields shaded). The darkest shading within each subset indicates the greatest number of superposed solutions in each subset. BF, Batang Fault; DSF, Deqin-Shigasi Fault; GF, Ganzi Fault.

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generally E–W to NW–SE compressive s1 orientations (Fig. 8b). Probable conjugate populations of sinistral NW-trending faults and dextral NE-trending faults are observed in this subset, mirroring the large-scale conjugate geometries of the Litang and Batang faults (Fig. 1b). Subset C4 shows generally east–west sub-horizontal compression, with a minor component of northeast-trending compression (Fig. 8b). Faults in this subset are generally either NE- or NW-trending, and show a variation between sinistral transtension and dextral transpression across probable conjugate arrays (Fig. 8b). Ratschbacher et al. (1996) described similar variability in the orientation and kinematics of brittle faults from this area. To the west of subset C4, compressive stress solutions are dominated by E–W orientations, however, subsets C6, C7 and C8 also record a component of N–S compression. This apparent divergence in compression orientation may reflect anisotropic deformation, or a timing difference between faults that developed due to N–S or E–W compression. Similar N–S-trending dihedra solutions from subset C9, near the town of Zhongza, suggest that such compression operated over a wider area than any individual subset. Alternatively, N–S solutions may simply reflect localised block shuffling that is not representative of the regional far-field stress field. In general, it appears that Cenozoic deformation across the central region can be considered to be the result of largely E–W compression and to be dominated by conjugate strike-slip faults at both local and regional scales. 3.4. Southern region In the east of the southern region, subsets D1 and D2 show dominantly N–S-trending compression, consistent with conjugate NW-trending dextral and NE-trending sinistral faults (Fig. 8c). In the region between the towns of Derong and Deqin, subsets D3, D4 and D5 yield E–W to NE–SW-trending compressive solutions. Faulting in this region is dominated by sinistral transpression on generally E- to SE-trending faults, with a conjugate set of NE-trending faults showing dextral transpression. The sinistral kinematics of SE-trending faults are identical to those of the Zhongdian Fault Zone (Figs. 4, 8c), and consistent with NE–SW compression. Subset D6 shows a complex array of faults with both N–S and E–W-trending dihedra solutions (Fig. 8c). NW-trending faults in this subset generally show sinistral kinematics, mimicking the Zhongdian Fault. Further west, subsets D7, D8 and D9 show predominantly E–W compression, with abundant of W-over-E reverse faults in addition to NEtrending dextral strike-slip faults. Subsets D8 and D9 both reveal a component of N–S compression. Brittle deformation of the southern region was thus dominated by an E–W compression, with minor local N–S shortening.

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4. Discussion 4.1. Cenozoic stress patterns and the crustal evolution of eastern Tibet Cenozoic faults formed diachronously across eastern Tibet. While larger structures such as the Xianshuihe and Kunlun faults have been active since at least w13 Ma (Roger et al., 1995) and w3–7 Ma (Guilbert et al., 1996; Yin and Harrison, 2000) respectively, smaller structures such as the Ganzi and Litang faults were only initiated at around 2–4 Ma (Wang and Burchfiel, 2000). We interpret this trend to reflect increasingly penetrative faulting with progressive strain accumulation and crustal block rotations. These Cenozoic faults, at all scales, record broadly NE- and E-trending far field compressional stress—a conclusion that is also supported by data from Ratschbacher et al. (1996). Deviation from these orientations occurs in the south– western Songpan-Ganzi Fold Belt, where N–S-trending compression solutions were identified (Fig. 9). The paleostress reconstructions presented here seem consistent with the rotation of maximum compressive stress about the Eastern Himalayan Syntaxis inferred from seismic studies (Holt, 2000; Holt et al., 1991), GPS measurements (Chen et al., 2000), kinematic reconstructions (Avouac and Tapponnier, 1993), and geodynamic models (e.g. Dewey et al., 1989; Shen et al., 2001) and paleostress analyses (Ratschbacher et al., 1996). In the northern fold belt, paleostress solutions are NE-trending (Fig. 8a), consistent with this region being located to the north of the Eastern Himalayan Syntaxis (Ratschbacher et al., 1996). In contrast, the eastern, central and southern regions show both E–W and N–S compression (Fig. 9), consistent with these regions being close to the syntaxis. It is suggested that the N–S trending stress field may have post-dated the E–W-trending stress field and developed as a result of northward migration of the Eastern Himalayan Syntaxis (Ratschbacher et al., 1996). A similar change of compression orientation with time has been documented in the Yulong Shan (Lacassin et al., 1996). The Eocene-aged Yulong Shan de´collement, produced initially by NE to NNW compression, was subsequently reoriented at w17 Ma by E–W compressive stress (Lacassin et al., 1996). Taking these stress orientations and applying Huchon et al.’s (1994) model of the changing stress distribution for a point that moves relative to the northward migrating syntaxis, it may be surmised that the Yulong Shan (Fig. 1b) lay to the northeast of the Eastern Himalayan Syntaxis in the Eocene and had reached the same latitude as the syntaxis by 17 Ma. That the Yulong Shan de´collement developed as a sub-horizontal shear zone, and not the sinistral strike-slip structure that might be predicted from the orientation of the compressive stress field in the model (Huchon et al., 1994), demonstrates that deformation kinematics are locality dependent upon the orientation of

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Fig. 9. Interpretation of Cenozoic-Recent stress field and associated structures across the eastern Tibetan Plateau: (a) Interpretation of the northern region. Note rotation of large crustal block causing minor internal deformation around Jomda. Whereas, the upper crust in the northeastern region began to deform through large scale strike slip faults; (b) Interpretation of central region. The Batang and Litang faults define a conjugate pair, resulting from E–W compression. Localised N–S compression observed in west of section; (c) Kinematic interpretation of southern region. Crustal blocks defined by the Zhongdian and DeqinShigasi faults appear to rotate clockwise, and local N–S compression is resolved near the intersection of these faults. N–S compression in the central and southern sections may reflect local strain anisotropies, block shuffling, or temporally distinct faulting in this region.

the s2 and s3 stress tensors as pointed out by Ratschbacher et al. (1996). 4.2. Heterogeneity of Cenozoic deformation in the Songpan-Ganzi Fold Belt Cenozoic deformation within the eastern Songpan-Ganzi Fold Belt is manifested as brittle strike-slip faults, predominantly concentrated along the Xianshuihe, Kunlun, Ganzi and Litang fault zones. This was locally accompanied by Cenozoic igneous intrusions, preserving fabrics that are

related to an E–W compressive regime at mid- to uppercrustal levels (Roger et al., 1995). There is no post-Triassic folding and thrusting that might be consistent with the style of upper-crustal shortening identified to the north adjacent to the Kunlun Fault (Tapponnier et al., 1990; Meyer et al., 1998) and to the west in the Hoh Xil region (Wang et al., 2002), nor the style of oblique, upper-crustal faulting and thickening identified south of the Jinsha Jiang-Red River Fault Zone (Wang and Burchfiel, 1997; Lacassin et al., 1996). Reactivation of the Longmen Shan Thrust Nappe Belt certainly accommodated differential uplift between

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the Songpan-Ganzi Fold Belt and the South China Block (Arne et al., 1997). However, limited Cenozoic shortening cannot account for the 3000Cm elevation and thickened crust of the plateau margin (Fig. 1b). Similarly, development of this elevation did not result in significant accommodation generation in the Sichuan Basin, as the Cenozoic sediments there are still minor (!200 m thick) in comparison to the Mesozoic deposits that they overlie (Chen and Wilson, 1996; Li et al., 2003). In the northern fold belt, an E–W compressive stress could effectively have been partitioned between dextral transpressive shortening in the Longmen Shan Thrust Nappe Belt (Burchfiel et al., 1995) and sinistral strike-slip displacement on the Kunlun Fault. We suggest that these two systems were mechanically conjugate (Fig. 10) and that coupled strike-slip offset on the Kunlun Fault and SEdirected shortening within the Longmen Shan Thrust Nappe Belt displaced a large rigid tract of the fold belt to the southeast (Fig. 10). Coupling of these two structures is consistent with the E–W paleostress and localised differential uplift of the Min Shan at their intersection (Chen et al., 1994; Kirby et al., 2000). The amount and timing of Cenozoic-Recent shortening that occurred within the Longmen Shan Thrust Nappe Belt is hard to estimate. This largely reflects the difficulty of isolating Cenozoic deformation from the protracted Triassic–Cretaceous reactivation of this margin and the ambiguous relationship of Cenozic deformation to current surface elevations (Burchfiel et al., 1995; Chen et al., 1995; Arne et al., 1997). We consider that SE-directed shortening within the Longmen Shan absorbed differential strain between the South China Block and a large displaced fragment of the Songpan-Ganzi Fold Belt (Fig. 11). Reconciling the inferred eastward displacement of this terrane fragment with the w65 km sinistral offset on the Xianshuihe Fault Zone requires that the tract of fold belt lying southwest of the Xianshuihe Fault (the Yadjiang terrane) has been displaced even further southeastward; the absolute displacement of the Yadjiang Terrane is inferred to be much greater than the 65 km of relative offset across the Xianshuihe Fault (Fig. 10). Such an interpretation is consistent with the regional synthesis of Ratschbacher et al. (1996) and Wang and Burchfiel (2000), who proposed that both the absolute southeastward displacement of such fragments and the extent to which these tracts of crust have been rotated clockwise increases systematically southward toward the Eastern Himalayan Syntaxis (Fig. 1b). 4.3. Construction of the eastern Tibetan Plateau Thermal history studies suggest that the presently exposed surface of the eastern Tibetan Plateau had cooled below w200 8C prior to the Eocene collision of India with Asia (Xu and Kamp, 2000; Reid et al., in press-a) thus, its rheology must have lain well within the

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brittle deformation regime. Low temperature and 40 Ar/39Ar thermochronology (Arne et al., 1997; Kirby et al., 2002; Reid et al., in press-a) also suggests that high relief of the plateau is younger still (!12 Ma). Many models of Cenozoic tectonism in eastern Tibet have sought to correlate penetrative upper-crustal shortening with the generation of plateau topography (Tapponnier et al., 1990; Gaudemer et al., 1995; Meyer et al., 1998). However, the interpretation of post-Triassic modification presented above and the thermal regime preclude, at least within the crustal levels currently exposed, any significant Cenozoic shortening. Reconciling mid-lower crustal thickening with strikeslip faulting of the fold belt must require mechanical decoupling of the upper and lower crust; effectively describing pervasive thickening of a tectonic basement beneath a detached, laterally mobile upper crustal veneer (Fig. 11). Such an interpretation is consistent with the explanation for the support of the topography of the margin of the fold belt (Kirby et al., 2000; 2002; Clark et al., 2004). The conspicuous absence of differential exhumation within the thin-skinned lower Triassic sedimentary sequence, even across major anisotropies such as the Xianshuihe Fault Zone (Xu and Kamp, 2000), suggests basement thickening was highly isotropic and was probably accommodated well below the brittle/ductile transition (O20 km depth; Fig. 11). Regionally, much of the tectonic character of the eastern Tibetan Plateau seems consistent with the basement thickening models of Dewey et al. (1989) and Royden et al. (1997). The Cenozoic faulting of the region has displayed a remarkable sensitivity to pre-Himalayan terrane boundaries and crustal geometries. Indeed, a majority of the active strike-slip fault zones in central Asia, including the Jinsha Jiang and Huaiyan faults (Fig. 1a), are located on the boundaries of ex-Gondwanan terrane fragments and Mesozoic mobile belts and continue to mark major lithospheric discontinuities and separate heterogeneous basement topology at depth (Wittlinger et al., 1996). It thus seems probable that penetrative thickening at viscous crustal levels was also constrained by pre-existing anisotropies (Royden et al., 1997). It is inferred that a pervasively deformed and thickened lower-crustal basement supports the elevation of the eastern margin of the Tibetan Plateau (cf. Clark and Royden, 2000). However, such homogeneous shortening would have necessitated a far greater areal contraction of the terrane at depth than is reflected by faulting of the Songpan-Ganzi Fold Belt. Faulting and eastward displacement of the fold belt veneer is therefore interpreted to have accommodated the differential areal change between this detached thinskinned veneer and its pervasively shortened tectonic basement (Fig. 11). Our model highlights the role played by the South China Block in the Cenozoic-Recent evolution of northeast Tibet. The abrupt 3000Cm increase of

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Fig. 10. Eastward displacement of the southeast Songpan-Ganzi Fold Belt (SGFB). Reactivation of the Longmen Shan Thrust Nappe Belt (LSTNB), synthetic to left-lateral slip on the Kunlun Fault, accommodated eastward displacement of a large, rigid tract of the fold belt. Sinistral offset of the Xianshuihe Fault Zone suggests that displacement of the Yadjiang Terrane was greater than the Kunlun Fault (a minimum of 75C65 km).

Fig. 11. Conceptual model of the Cenozoic-Recent crustal dynamics of the Songpan-Ganzi Fold Belt. The elevation of the thin-skinned fold belt veneer is supported at depth by a homogeneously thickened, viscous lower crust. Basement thickening is inferred to reflect shortening and internal deformation, rather than viscous lithospheric flow. Eastward displacement of the Songpan-Ganzi Fold Belt is thought to have accommodated the necessary differential areal change between the detached rigid veneer and the shortened tectonic basement. Differential strain between the fold belt and the South China Block was absorbed at shallow levels by reactivation of the Longmen Shan Thrust Nappe Belt. At deeper levels, the east-directed mantle velocity field of northeast Tibet (Holt, 2000) is presumed to terminate against the western margin of the South China Block.

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elevation (Fig. 1b) of the eastern plateau margin strongly suggests that the western boundary of the South China Block marks the eastward limit of distributed viscous deformation in Asia and is supported by the GPS observations of Zhang et al. (2004). Furthermore, there is no evidence to suggest that there is significant distributed deformation or elevated topography being propagated into the South China Block as India indents Asia. We therefore infer that the South China Block was the rigid backstop against which deeper levels of the Songpan-Ganzi Fold Belt were shortened under a dominantly E–W compressive stress. This implies that the inferred E-directed mantle flow of northern Tibet (Holt, 2000) decays dramatically or even terminates against the western margin of the South China Block (Fig. 11).

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(Zhang et al., 2004). The velocity field is inferred to terminate against the western margin of the South China Block, resulting in distributed thickening of the deeper Tibetan lithosphere.

Acknowledgements Fieldwork for this study was supported by ARC grant A1002030Z. The help of Liu Shun in the field and the support of Professors Wang Chengshan, Liu Shugen and staff of the Chengdu University of Technology is gratefully acknowledged. Constructive reviews on an early version of this paper by Clark Burchfiel, Lothar Ratschbacher and Alex Densmore are greatly appreciated.

5. Conclusions

References

1. Post-Triassic modification of the Songpan-Ganzi Fold Belt did not accommodate significant upper-crustal thickening or differential exhumation. Strike-slip faulting laterally transported large tracts of the fold belt to the east and southeast, effectively intact. 2. A broadly E–W compressive stress regime probably drove Cenozoic-Recent deformation at all crustal levels of the fold belt. Fault analyses resolve a clockwise rotation of compressive stress from NE-trending in the northern regions to E–W in the eastern, central and southern region. This stress regime was locally disrupted by competency contrasts between fragments of Proterozoic basement within the sedimentary pile. 3. Fault development across the eastern Tibetan Plateau was diachronous, with localised uplift zones at the eastern terminations of sinistral NW-striking faults. These observations suggest that there was a clockwise rotation of upper crustal material during the Cenozoic that has become increasingly penetrative since 2–4 Ma. 4. The geometry and kinematics of upper-crustal faults are probably controlled by pre-existing anisotropies and inherited terrane geometries to a far greater extent than many regional models currently consider. Care should be exercised when inferring lithospheric dynamics from the upper-crustal faulting. 5. The elevation of the present-day Tibetan Plateau and Songpan-Ganzi Fold Belt reflects significant homogeneous crustal thickening at depth. This requires mechanical decoupling of the upper crust from a more isotropically thickened viscous basement. Southeastward displacement of the upper-crustal veneer and exhumation of the Longmen Shan Thrust Nappe Belt accommodated the differential areal change between upper crust and viscous basement. 6. The E-directed upper-mantle velocity field of eastern Tibet is not indicative of the field for east Asia as a whole

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