Evidence for Early (> 44 Ma) Himalayan Crustal Thickening, Tethyan Himalaya, southeastern Tibet

Earth and Planetary Science Letters 274 (2008) 14–23

Contents lists available at ScienceDirect

Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / e p s l

Evidence for Early (N44 Ma) Himalayan Crustal Thickening, Tethyan Himalaya, southeastern Tibet Amos B. Aikman a, T. Mark Harrison a,b,⁎, Ding Lin c a b c

Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 900029, China

A R T I C L E

I N F O

Article history: Received 11 March 2008 Received in revised form 18 June 2008 Accepted 20 June 2008 Available online 4 July 2008 Editor: R.W. Carlson Keywords: Tethyan Himalaya Tibet U–Pb zircon Dala granitoids

A B S T R A C T Comprehensive understanding of Himalayan orogenesis is limited in part by a poor knowledge of the Eohimalayan episode; a phase of tectonic activity predating the better understood Neohimalayan crustal thickening associated with coaxial deformation along the Main Central thrust. The Cambo–Ordovician to Tertiary metasedimentary sequences outcropping within the Tethyan Himalaya are the structurally highest units of the Himalayan Fold and Thrust Belt; they and are inferred to have been the first to have accreted to Asia. A compilation of data from transects along the length of the main Himalayan arc shows that the Tethyan sequences have experienced at least five deformation events, although the timing of these episodes is poorly constrained. Emplacement of a series of undeformed granitoid bodies following the second of these events, which accounts for the majority of crustal thickening in the Tethyan Himalayan units, is constrained by U–Pb zircon dating to be older than 44.1 ± 1.2 Ma. Thus, significant crustal thickening had occurred along the length of the proto-Himalayan arc by the mid-Eocene, or within 10 to 20 myr of the initiation of Himalayan orogenesis. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Stretching for over 2000 km from Pakistan to Burma, the Himalayan range is a small but significant portion of the Alpine– Himalayan chain, the world's largest active orogenic belt. Over a century of research has furnished a good understanding of the firstorder geology of this region (Suess, 1875; Argand, 1924; Gansser, 1964; LeFort, 1975, 1996; Yin and Harrison, 2000). For example, the distribution of most Himalayan lithotectonic units and their interrelationships are generally agreed (LeFort, 1975, 1996; Yin, 2006). However, the vast majority of studies to date have focused on the frontal parts of the range, in particular the central and western Himalaya. These areas are dominated by structures pertaining to the Neohimalayan (Miocene to Recent) episode, which may represent less than half of the orogen's history. Relatively little is known about the either the Eohimalayan episode or the geological evolution of the eastern Himalaya. These and other factors limit the degree to which evolutionary models proposed for the Himalaya (e.g., LeFort, 1975; England et al., 1985) are comprehensive or valid. The Tethyan Himalaya comprise the structurally highest units of the Himalayan fold and thrust belt (HFTB) (LeFort, 1975). As the earliest orogen-wide units to be accreted to Asia, they are uniquely placed to retain information regarding the formative stage of Himalayan

orogenesis. Several investigations have constrained aspects of the structural history of the Tethyan Himalaya, through field studies and balanced cross section reconstruction (e.g. Ratschbacher et al., 1994; Vannay and Steck, 1995; Godin et al., 1999; Wiesmayr and Grasemann, 2002; Godin, 2003; Murphy and Yin, 2003); but a unifying structural model is lacking. In part this reflects geopolitical barriers in the Himalayan border regions which have restricted access and continue to present a significant obstacle to realizing detailed regional correlations. Furthermore, the generally low-grade nature of regional metamorphism of the Tethyan Himalaya results in few robustly datable products, thus making assessment of the timing of deformation difficult. In this paper, we summarize the deformation history of the Tethyan metasedimentary sequence and present geochronological results for a coeval group of post-tectonic granitoids in southeastern Tibet, indicating their emplacement during the mid-Eocene. From this we conclude that the majority of crustal thickening in the Tethyan sequence is Eohimalayan and thus occurred during the first 10–20 myr of the Himalayan collision — much earlier than has so far been widely acknowledged. 2. Geologic setting 2.1. Background

⁎ Corresponding author. E-mail address: [email protected] (T.M. Harrison). 0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2008.06.038

The HFTB is divided into four litho-tectonic units thought to represent Indian foreland sequences, separated by regional movement

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

zones that roughly follow the trend of the main Himalayan Arc (Fig. 1; LeFort, 1975, 1996; Yin, 2006). From south to north in increasing structural height they are: the Siwalik molasse incorporated into the hangingwall of the Main Frontal Thrust (MFT); the Lesser Himalayan Series (LHS), a sequence of dominantly siliciclastic rocks separated from the underlying Siwalik molasse by the south vergent Main Boundary Thrust (MBT); the Greater Himalayan Crystallines (GHC), a high grade metasedimentary sequence that has been exhumed between the reverse sense Main Central Thrust (MCT) and normal sense South Tibetan Detachment (STD); and the Tethyan Himalayan Sequences (THS) (Fig. 1) (Gansser, 1964; LeFort, 1975, 1996). The MCT and STD (Fig. 1) experienced their main phase of slip around 24 to 18 Ma, but there is local evidence that activity on both shear zones continued well into the Miocene (see Yin, 2006). The timing of slip on the MBT is poorly constrained, with suggestions that it initiated at either ∼11 Ma (Meigs et al., 1995), or b5 Ma (DeCelles et al., 1998). The active MFT formed during the Quaternary (Larson et al., 1999). Most workers agree that Himalayan deformation began in the Late Cretaceous (Yin and Harrison, 2000, and refs. therein) or by ca. 51 Ma (Zhu et al., 2005), which has been interpreted to mark the onset continental collision between India and Eurasia. An alternative is that India was still too far removed from Eurasia at those times, and that early Himalayan orogenesis may have begun whilst the cratonic regions were still separated by a significant expanse of oceanic and/or transitional crust (Aitchison et al., 2007, and refs. therein). 2.2. The Tethyan Himalaya The Tethyan Himalayan Sequences (THS) extend for more than 1500 km along the Himalayan Arc from Pakistan almost reaching the eastern syntaxis (Fig. 1; Booth et al., 2004; Craw et al., 2005; Zeitler, 2006). For nearly all of this length, the THS are bounded to the north

15

by the south-dipping Great Counter Thrust (GCT) and to the south by the north-dipping STD (Fig. 1). The THS comprise a deformed package of predominantly low-grade Paleoproterozoic to Eocene metasediments thought to have been deposited along the northern edge of the Indian continent (Brookfield, 1993; Liu and Ensele, 1994; Pan et al., 2004). The majority of these units comprise passive margin deposits, although the latest Cretaceous to Paleocene sequence may record the obduction of ophiolitic material (Allègre et al., 1984; Burg et al., 1987; Searle et al., 1987; Willems et al., 1996; Gnos et al., 1997; Makovsky et al., 1999; Aitchison et al., 2000; Ding et al., 2005). A prominent northdipping reflector, interpreted as the northward projection of the STD, appears to form the present-day basal detachment juxtaposing THS against the underlying GHC (Makovsky and Klemperer, 1996). Most workers agree that deformation of the Tethyan Himalaya has led to dissection of the Indian foreland sequences into a predominantly north-dipping regional fold and thrust belt, which accommodated between 30% and 70% horizontal shortening (Fig. 1; transect 1, Vannay and Steck, 1995; transect 2, Wiesmayr and Grasemann, 2002; transect 3; Murphy and Yin, 2003; transect 4, Godin et al., 1999, Godin, 2003; transect 5, Ratschbacher et al., 1992, 1994; this study, Aikman, 2007). The principal phase of Tertiary shortening in southern Tibet and northwest India led to development of widespread south-vergent contractional structures, which are overprinted first by north-directed reverse-slip on the GCT, and subsequently by E–W extension forming N–S trending graben (Armijo et al., 1986; Ratschbacher et al., 1992, 1994; Vannay and Steck, 1995; Wiesmayr and Grasemann, 2002). In parts of northwest India all of these deformations locally overprint E–W trending isoclinal folds that may have formed by re-activation of pre-existing lineaments during the early stages of Himalayan collision (Vannay and Steck, 1995). Palinspastic reconstruction of the THS in southwest Tibet (transect 3, Fig. 1; Murphy and Yin, 2003) indicates that this region experienced

Fig. 1. Regional geological map of the main Himalayan arc (after Yin, 2006). Red lines show the locations of published cross sections through parts of the THS, northwest India (1, Vannay and Steck, 1995; 2, Wiesmayr and Grasemann, 2002), (3) southwest Tibet (Murphy and Yin, 2003), (4) northern Nepal (Godin, 2003), (5, 6) central southern Tibet (Ratschbacher et al., 1992, 1994). The red box shows the location of Fig. 2.

16

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

a similar structural evolution to that documented in central southern Tibet and northwest India, with about 40% horizontal shortening within the THS and 21% within suture-related fault systems. Their reconstruction indicates that the earliest deformation occurred in the late Cretaceous to Paleocene, followed by formation of a complex system of south-directed thrusts, including activity on the Gangdese Thrust (GT) from 30 to 23 Ma (Harrison et al., 2000; Murphy and Yin, 2003). All of these deformations pre-date north-directed slip on the GCT (Harrison et al., 2000; Murphy and Yin, 2003). In northern Nepal, locally developed early isoclinal structures are also overprinted by a major contractional phase that pre-dates both north-directed slip on the STD and E–W extension, however close to the STD the dominant style of folding is north-vergent (Godin et al., 1999; Godin, 2003). 2.3. Eastern Tethyan Himalaya The eastern THS (90–94°E) comprises a deformed, broadly southward-younging, Triassic to Cretaceous sequence of predominantly low-grade metasediments (Liu and Ensele, 1994; Pan et al., 2004; Aikman, 2007). Depositional ages are based primarily on sparse paleontological evidence (Liu and Ensele, 1994; Garzanti, 1999). Northern parts of the section are made up of a thick clastic-dominated sequence, comprising turbidites, fine-grained sandstones and slate (Fig. 2). These units were laid down during the Triassic in association with rifting of the Neotethys Ocean (Liu and Ensele, 1994; Garzanti, 1999). The central THS is primarily composed of shallow water carbonates and siliciclastics deposited as part of the Indian passive margin sequence during the Jurassic phase of drift sedimentation, although some Cretaceous units are found outcropping in structural depressions (Fig. 2). Southern parts of the section are composed almost entirely of Cretaceous marine clastics and carbonate platform deposits formed shortly prior to final closure of the Neotethys Ocean (Fig. 2). The contacts between the main lithotectonic units of the eastern THS are thought to have been depositional prior to Himalayan orogenesis (Liu and Ensele, 1994; Garzanti, 1999); however, present day contacts have in some cases been modified by Tertiary deformation (e.g., Lhunze Thrust, see below). Metamorphic grade is lower greenschist facies in northern parts of the section, decreasing to subgreenschist facies southwards. Our field observations along a roughly N–S oriented transect at ∼ 92° E (Fig. 1) indicate that the structure of the eastern THS is similar to that documented elsewhere in southern Tibet and NW India. In the central THS (N28°43.853′, E91°37.553′), the Cretaceous and Jurassic sequences are deformed together into a series of macroscopic, upright, open folds (Aikman, 2007). The regional extent and geometry of these structures is inferred from outcrop patterns and satellite imagery (ASTER, Google Earth), correlated with field observations. Northwards towards the Triassic strata, the axial spacing between folds decreases and their geometries become south-vergent, marking a progressive tightening of this fold generation (Fig. 2). The contact between the northernmost Jurassic/Cretaceous and southernmost Triassic units was not observed in the field due to access restrictions or poor exposure. However, outcrop patterns suggest that it truncates folded Jurassic/Cretaceous strata, and hence the contact is interpreted to be tectonic. This structure, the Lhunze Fault, is likely comparable to the N-dipping Gyirong-Kangmar Thrust, ∼200 km along strike to the west (Chen et al., 1990; Lee et al., 2000), although we cannot exclude normal-sense displacement on a S-dipping fault plane. The timing of activity on the Lhunze Fault is required to post-date formation of the E-W trending folds (see below). In the hanging wall of the Lhunze Fault (N28°30.421' E092°16.332'), Triassic silciclastic sediments are folded into mesoscopic, E–W trend-

ing, south-vergent folds. The style and orientation of these structures is very similar to that observed in the northernmost Jurassic/ Cretaceous units outcropping along strike to the west. Detailed structural observations were collected along a short transect (∼30 km, marked as a solid red line on (Fig. 2) from Chegatsu Pass (N28°50.716' E91°38.480') northwards down a valley towards the village of Chunjaye (N29°1.727' E091°40.898'). Southern parts of the section are characterized by turbidites deformed into a series of meso- to macroscopic E–W trending, south vergent folds, with axial planes dipping at ∼ 55° to the north (Aikman, 2007). Northwards, the spacing between fold axes is reduced, associated with a further tightening of folds and more intense axial planar foliation development. North of Chunjaye towards the Indus-Tsangpo Suture, a progressive rotation of the dominant foliation in the Triassic metasediments (which is axial planar to isoclinal folds at Chunjaye) is observed. This fabric eventually becomes south-dipping, sub-parallel to the GCT adjacent to the suture; a relationship which suggests that the axial planar foliation has been re-oriented by motion on this structure (Aikman, 2007). Further proof of this overprinting relationship is provided by rootless isoclinal folds preserved in low strain zones adjacent to the GCT. This geometry is interpreted to post-date isoclinal folding, and is attributed to rotation and reactivation of existing fabrics during formation of major faultbend fold structure associated with motion on the GCT (Aikman, 2007). Collectively, the structures documented in the THS metasediments of the eastern Himalaya are interpreted to record three regional deformation events. The first comprises N–S shortening associated with formation of a series of E–W trending, shallow W-plunging, upright or south-vergent folds. These structures are constrained to be Tertiary in age by stratigraphic arguments, and represent the majority of deformation related to Himalayan orogenesis. The second phase of deformation is characterised by N–S shortening associated with displacement along north- and south-directed thrusts that overprint south-vergent folds. The Lhunze Fault, outcropping in the central Tethyan Himalaya, is considered analogous to the north-dipping Gyirong–Kangmar Thrust, which has been mapped ∼200 km along strike to the west and is thought to have accommodated ∼2 km vertical displacement during the Miocene (Quigley et al., 2006). The south-dipping GCT, adjacent to the Indus Tsangpo Suture, accommodated a minimum of 12 km top-to-the-north displacement between 19 and 15 Ma (Quidelleur et al., 1997). Structural observations conducted in this study do not constrain the relative timing of movement on the GCT and Lhunze Fault. The third phase of deformation (local D3) is represented by roughly E–W extension associated with formation of a major NNE–SSW trending rift. This structure is similar to other N–S trending rifts found at various locations along the Himalayan Arc and throughout the Tibetan Plateau, and likely formed during the Pliocene (Armijo et al., 1986). 2.4. Location of the eastern ITS In the central Himalaya, the southern margin of Eurasia adjacent to the ITS is marked by Xigaze group sediments, which are thought to have originated as forearc deposits to the south-facing Gangdese arc prior to closure of the Neotethys (Liu and Ensele, 1994; Pan et al., 2004). In the eastern Himalaya Xigaze units are absent and calcalkaline plutonics of the Gangdese batholith have been thrust over components of the ITS (Harrison et al., 1992; Yin et al., 1994, 1999; Harrison et al., 2000). Based on sparse paleontological evidence and their present day outcrop position, rocks directly south of the surface expression of the ITS have traditionally been assigned to the Triassic successions of the THS forming part of the continental margin deposits of northern India (Liu and Ensele, 1994; Pan et al., 2004).

Fig. 2. Overview regional geological map of the eastern Tethyan Himalaya (after Harrison et al., 2000; Pan et al., 2004; Aikman, 2007). Inset shows structural data from the transect marked in red.

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

17

18

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

However a few investigators have proposed that these rocks represent either remnants of the Xigaze group or Indus-Tsangpo melange, thereby placing the true location of the ITS up to 120 km further south (Yin, 2006; Aitchison et al., 2007). Reconnaissance mapping at ∼ 92° E has provided no confirming evidence that rocks of the northern THS represent components of a melange. Indeed the structural continuity of the THS section argues against this hypothesis. Nevertheless, the possibility that they represent parts of the Xigaze forearc cannot be immediately discounted. Widespread calc-alkaline magamatism in the Gangdese arc and deposition of the Xigaze forearc sediments are thought to have largely occurred during the Cretaceous and Paleocene (Debon et al., 1986; Dürr, 1996), at which time the northern margin of India was undergoing a relatively tectonically stable phase of drift sedimentation (Liu and Ensele, 1994; Garzanti, 1999). Detrital zircons were separated from six samples collected from the Triassic units (Fig. 2) (Pan et al., 2004) and analysed by U–Pb ion microprobe dating using methods described in Aikman (2007) to test for the presence of a Cretaceous sedimentary source. Two hundred and fifty zircons from the six samples, exhibiting a broad spectrum of morphologies and cathodoluminesence zonation patterns, yielded U–Pb ages from ∼ 200 to 3000 Ma (Fig. 3; Supplementary Data Table 1). While Cambrian and older source rocks are common in parts of the Himalaya and northern India, the source of younger Paleozoic grains is unclear (Aikman, 2007; Paul Kapp, pers. comm.). The absence of zircons younger than ∼ 200 Ma is inconsistent with either the Xigaze group or IndusTsangpo melange and we conclude that there is currently no evidence to dispute the Chinese stratigraphy (Liu and Ensele, 1994; Pan et al., 2004). 3. Regional structural evolution 3.1. Structural correlation across the THS The earliest potentially correlative deformations are found in the THS of northwest India and northern Nepal, represented by broadly E– W trending (NW–SE in the western Himalaya) isoclinal folds associated with a penetrative axial planar schistosity. In northwest India, these structures are present throughout the stratigraphic section, whereas in northern Nepal, they have only been documented in Paleozoic and older strata (Godin, 2003). Although this observation could be interpreted as indicating that structures in northern Nepal have pre-Himalayan formative ages, workers in both regions have ascribed them to the Eohimalayan episode, possibly associated with reactivation of the pre-existing structural grain. They are assigned to the first phase of regional deformation (D1). The apparent absence of similar structures in central and eastern Tibet may be due to the lack of detailed mapping in these areas. The dominant structures found in the THS metasediments of northwest India and central and eastern Tibet comprise a system of E– W trending, predominantly south-vergent folds and thrusts. The style of folding in all of these areas is typically open to close (although sometimes disharmonic), associated with a spaced axial planar cleavage in competent units and sporadically developed N–S oriented stretching lineation. Most workers independently ascribe these as the principal Tertiary thickening structures in their respective study areas. The similarity of structural style, orientation, fabric development and position in the overall local deformation scheme, suggest that these events are correlative, representing a regional phase of Tertiary shortening responsible for major crustal thickening throughout the THS (D2). Some workers consider that the reported prevalence of northvergent folds in northern Nepal provides evidence that the structural evolution of this area differs from that described elsewhere. These structures were originally attributed to gravitational sliding associated with Miocene movement on the STD (Burchfield et al., 1992), but

Fig. 3. Age-probability plot showing the U–Pb ages of approximately 250 detrital zircons from six samples of the THS metasediments outcropping in the eastern Himalaya. Note that there is no evidence of a Cretaceous sedimentary source.

Godin et al. (1999) showed that they pre-date a mylonitic foliation formed during normal-sense motion on this structure and, Godin (2003) noted a northward progression from north-vergent to upright folds analogous to those observed in central-southern parts of the eastern THS. Moreover, a recent cross-section constructed in this region from a compilation of existing and new data, clearly shows the axial surfaces of upright folds rotated into parallelism with the STD (Crouzet et al., 2007). The apparent northward vergence of these structures in northern Nepal is therefore more likely attributable to local reorientation associated with motion on the STD-MCT fault systems, and/or reactivation of the pre-existing structural grain. This interpretation is considered preferable to the alternative in which north-vergent structures represent an additional deformation episode for which there is as yet no correlative evidence elsewhere along the Himalayan Arc. Southern parts of the THS in NW-India and northern Nepal are cut by a mylonitic foliation analogous to that which is found in the underlying units of the GHC. Most workers agree that this fabric formed during Miocene motion on the MCT-STD fault system, which is thought to have been contemporaneous with reverse-sense slip on the GCT (Quidelleur et al., 1997). These events are collectively ascribed to the third phase of regional deformation (D3). Other similar structures such as the Gyirong–Kangmar Thrust and Lhunze Fault outcropping within the THS are also assigned to this phase, although their exact age is not known. Some workers have also noted late-stage spatially restricted deformations, such as minor kink folding in northern Nepal (Godin et al., 1999, Godin, 2003) and doming in NW-India (Vannay and Steck, 1995). These are likely attributable to localised effects associated with assembly of the Lesser-Himalayan duplex, out of sequence motion on the MCT and renewed shortening in the THS following cessation of normal-sense slip on the STD. They are assigned to D4, which may not be correlative across the entire THS. The most recent regionally correlative deformation event so far documented is associated with formation of a series of NNE–SSW trending (arcnormal) rifts; these structures are assigned to D5. 3.2. Timing of Tethyan Himalayan deformation The absence of regional unconformities, thickening strata or voluminous coarse-grained continental clastic sedimentation in the Cretaceous units of the THS (Liu and Ensele, 1994) suggest that, with the exception of D1 whose age is ambiguous, all regional deformation structures are Paleocene or younger. Geometric arguments require the majority of crustal thickening in the THS (D1 and D2) to have occurred prior to Miocene slip on the MCT-STD and GCT fault systems; however

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

difficulties in directly establishing the deformation age of low grade metasediments have limited constraints on the absolute timing of these events. E–W extension associated with the formation of N–S trending graben is generally considered to have begun during the Pliocene (Armijo et al., 1986). Ratschbacher et al. (1994) analyzed two samples of white mica from the central Himalaya by the K–Ar method. Three size fractions from a Triassic quartz phyllite all yielded ages within error of 49 Ma, which they tentatively suggested to be the age of major regional crustal thickening (D2). Three samples of recrystallized illite separated from the axial planar cleavage domains of D2 folds in NW India yielded 40Ar/39Ar total gas ages from 47 to 56 Ma (Wiesmayr and Grasemann, 2002). These data are similarly interpreted to record the approximate timing of major south-vergent shortening in the in the NW Indian THS. Ding et al. (2005) dated 5 zircons from a 44.8 ± 2.6 Ma undeformed leucogranite outcropping within a thrust-bounded unit of pelitic schist adjacent to the Indus Tsangopo Suture in central southern Tibet. Muscovite from the same pelitic schist also yielded a 40 Ar/39Ar plateau age of 41.0 ± 2.0 Ma. Together, they interpret these results as recording crustal anatexis and cooling following the initiation of shortening within the northern Tethyan thrust belt (Ding et al., 2005). Despite these published studies, the view that Eohimalayan deformation was widespread in the THS has struggled to gain acceptance. In part this is due to the lack of a coherent regional deformation scheme, the inability to assess whether mica cooling ages could reflect partial loss of radiogenic 40Ar, and the obscuring effects of subsequent Neohimalayan deformations particularly adjacent to the suture zone. In the following sections we present data from newly discovered Eohimalayan granitoid plutons (the Dala granitoids) outcropping in the central Tethyan Himalaya which places a clear constraint on the timing of significant deformation in the THS. Our data provides unambiguous support of the view that significant crustal thickening occurred within the first ∼ 20 myr of Himalayan orogenesis along the length of the proto Himalayan Arc. 3.3. The Dala granitoids Most granitoids identified within the HFTB along the main Himalayan Arc have so far been assigned to one of two groups — the High Himalayan Leucogranites or the North Himalayan Granites. The High Himalayan Leucogranites (HHL) comprise a suite of Miocene sheet, dyke and laccolithic bodies outcropping along the peaks of the High Himalaya in association with the STD (LeFort, 1996). The North Himalayan Granites (NHG) typically outcrop in the cores of North Himalayan Domes (NHD) (Lee et al., 2000). Most have crystallization ages between 18 and 9 Ma (Harrison et al., 1997), although at least one older body has been found (Zhang et al., 2004). The Dala granitoids comprise a series of undeformed sub-elliptical plutons and dykes outcropping within the deformed upper-Triassic sub-greenschist facies metasediments of the eastern THS, ∼ 70– 100 km south of the ITS (Fig. 2). Access restrictions precluded detailed mapping of individual granitoids, but contacts observed in the field can be traced on satellite imagery to define the largest of these bodies: an elliptical pluton approximately 10 km in longest dimension (Fig. 2). At outcrop scale, the principal regional foliation in the Triassic metasediments is axial planar to E–W trending folds and abuts directly against the Dala contact aureole. In the surrounding psammitic units, this fabric comprises a variably developed spaced cleavage defined by the alignment of chlorite, graphite and iron oxide minerals; in thin section, associated quartz grains show fracturing, abundant deformation lamellae and undulose-extinction patterns characteristic of a lowtemperature deformation regime (Fig. 4a). This fabric is axial planar to regional south-vergent folds that are interpreted to have formed during regional D2 deformation representing the majority of crustal thickening in this area.

19

Microstructural observations indicate that foliated host phyllites in the Dala granitoid contact aureoles have been overgrown by abundant andalusite porphyroblasts (now largely pseudomorphed by white mica) during contact metamorphism (Fig. 4b). Relics of the original fabric are seen as graphite stringers within some porphyroblasts aligned parallel to the principal matrix foliation. In some sections, a second fabric (associated with new-grown biotite, now largely transformed to chlorite) is observed crenulating the axial planar regional foliation. This fabric was not detected in the surrounding metasediments, and its relationship to andalusite porphyroblasts growth is ambiguous. It is attributed to minor localised deformation associated with early emplacement of the granitoid bodies. The Dala granitoids are comprised of quartz, plagioclase and Kfeldspar, with lesser biotite and occasional muscovite (Fig. 4c). Quartz grains in some samples show evidence of grainsize reduction suggestive of minor deformation; however, uniform extinction patterns, foam textures and the absence of deformation lamellae indicate that, unlike in the surrounding metasediments, this minor deformation occurred at sub-magmatic temperatures sufficient to allow near complete strain-recovery. Structural observations from the Dala granitoids and their country rock collectively indicate that emplacement occurred after major south-vergent deformation (D2) accounting for the majority of crustal thickening in the THS metasediments. The presence of andalusite in the contact aureole constrains the depth of emplacement and maximum overburden removed since that time to be ∼ 12 km (e.g., Spear, 1993). 4. Results and discussion Zircons from eight samples of the Dala Granitoids were dated using the U–Pb ion microprobe method to determine their crystallization age, and biotite and K-feldspar separates analyzed by K-Ar and 40Ar/ 39 Ar thermochronology to recover their sub-solidus thermal histories. Samples were chosen from a variety of locations across several granitoid bodies (Fig. 2), and target minerals extracted using conventional crushing, heavy liquid and magnetic separation techniques. Zircon grains were mounted in epoxy along with reference zircon FC1 (1099.0 ± 0.5 Ma; Paces and Miller, 1993), and polished to expose mid-sections using a rotary polisher and diamond paste. Polished mounts were examined using optical and electron microscopy including cathodoluminescence (CL) spectroscopy to check for inclusions, cracks and other imperfections (which were avoided during analysis), and to assess the two-dimensional internal structure of the grains. Grains typically ranged from 100 to 500 µm in length, exhibiting subhedral to euhedral morphologies and CL patterns indicative of restitic cores overgrown by concentrically zoned relatively pristine magmatic rims. Isotopic analyses were conducted using the SHRIMP RG facility at The Australian National University according to established analytical procedures (Williams, 1998; Aikman, 2007). Analytical results were evaluated using a procedure designed to maximize automatic detection of potentially spurious analyses, while reducing potential biasing of the final results through human error induced by qualitative comparison of data points individually (Aikman, 2007). Zircon data reductions were performed using the SQUID Excel macro (Ludwig, 2001) including corrections for fractionation of Pb relative to U and Th, standardizing U–Pb isotopic ratio measurements against FC1 (Paces and Miller, 1993), and absolute U + Th content measurements against SL13 zircon (Claoue-Long et al., 1995). A further correction was also applied to account for matrix effects on inter-element ratios associated with the relatively high U + Th content of some grains (Aikman, 2007). The majority of grains yielded concordant apparent ages ranging from ∼42 to ∼1800 Ma (Fig. 5a). Final U–Pb ages were calculated from the 204Pb-, 207Pb- and 208Pb-corrected 206Pb/238U and the 207Pb/206Pb apparent ages, using an automated procedure designed to obtain a

20

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

Fig. 4. Thin section photomicrographs from the Dala granitoids, their country rock and contact aureole. (a) Psammitic lithologies from the nearby THS metasediments. (b) The andalusite hornfels Dala contact aureole. (c) Dala granitoid.

robust statistical best-estimate and minimize the requirement for qualitative judgments of individual datum (Aikman, 2007; Supplementary Data Table 2). Results indicate that the rim domains of all samples yielded ages that cluster at ∼44 Ma, whereas the core domains range from ∼200 to ∼ 1800 Ma. The distributions of rim ages do not differ significantly between samples at the resolution of the data, and therefore are displayed as a combined age-probability function (Fig. 5b). The best estimate for the timing of crystallisation of the Dala Granitoids is interpreted to be the median and standard error of these data 44.1 ± 1.2 Ma (2σ). Biotites separated from three of these samples (Fig. 2) were analyzed by the K–Ar method using methods described in McDougall and Harrison (1999). Two samples from a large pluton (0405008 and 0405011) yielded ages of 31.5 ± 0.3 Ma and 39.8 ± 0.5 Ma respectively, and a third sample from a dyke, outcropping a few kilometers north of the Lhunze thrust (0405013) yielded an age of, 42.8 ± 0.5 Ma (see Supplementary Data Table 5). K-feldspars separated from the samples 0405008 and 0405011 were analyzed using the step-heating method, including the use of isothermal duplicates designed to allow correc-

tion for excess radiogenic argon (40Ar⁎) derived from decrepitation of fluid inclusions (Harrison et al., 1994). Excluding excess argon released on the first isothermal duplicate of the initial steps, both samples yielded coherent age spectra, in which age increases proportional to cumulative 39Ar fraction over the majority of the gas release (Fig. 6; Supplementary Data Tables 3 and 4) . Data from both K-feldspars were interpreted using multiple diffusion domain (MDD) theory to recover their thermal histories (Lovera et al., 1989). Results indicate that both samples experienced a prolonged post-emplacement isothermal phase followed by rapid cooling at 15 Ma. This cooling appears to have initiated in sample 0405011 slightly earlier than sample 0405008, perhaps due to the obscuring effects of small amount of excess argon in the older sample. Collectively, these data are interpreted as indicating that following crystallization at 44.1 ± 1.2 Ma, the Dala Granitoids cooled to ambient mid-crustal temperatures (∼300 °C) by the Late-Eocene to earlyOligocene, where they remained broadly isothermal until their rapid exhumation at around 15 Ma (Fig. 7), consistent with the timing of slip on the GCT (e.g., Quidelleur et al., 1997).

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

21

Fig. 5. U–Pb concordia and age probability plots for zircons from the Dala granitoids. (a) Concordia plots showing the results of all analyses (i) and analyses of the rim domains (ii) respectively. (b) Age probability plots showing the results of all analyses (i) and analyses of the rim domains (ii) respectively.

The isothermal portion of the thermal history of the Dala granites is particularly intriguing as its form is effectively constrained by the Eocene biotite and Miocene K-feldspar ages. A recent study has suggested, on the bases of U–Pb zircon ages from a migmatite outcropping in the core of the Mabja dome in southern Tibet, that peak metamorphic conditions in this region occured at around 35 Ma, followed by a major phase of ductile deformation (Lee and Whitehouse, 2007). If correct, these results could be interpreted as indicating that while low-grade units of the THS remained relatively stable from the mid Eocene to mid Miocene, deformation was ongoing at lower crustal levels. Alternatively, the 35.0 ± 0.8 Ma age, obtained by Lee and Whitehouse (2007) from a statistical interpretation of approximately half their published data, could reflect the onset of cooling and crystallization following attainment of an earlier metamorphic peak. Mid Miocene exumation of the Dala granitoids was co-incident with exhumation of virtually all North Himalayan Domes so far documented (e.g. Lee et al., 2000; Zhang et al., 2004;

Fig. 6. K-feldspar 40Ar/39Ar age spectra for sample 0405008 (a) and 0405011 (b) from the Dala granitoids showing fits using the multi-diffusion domain model.

Fig. 7. Thermal history of the Dala granitoids based on zircon U–Pb dating and zircon Ti thermometry (Aikman, 2007), biotite K–Ar and K-feldspar 40Ar/39Ar thermochronology. Note that the Dala granitoids cooled rapidly to mid crustal temperatures and remained broadly isothermal until the Miocene.

22

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23

Quigley et al., 2006; and refs. therein), providing strong support for a regional structural control, probably associated with the GCT.

an anonymous reviewer, whose comments greatly improved this manuscript.

5. Conclusions

Appendix A. Supplementary data

The THS metasediments are the structurally highest and inferred earliest accreted units of the Himalayan Fold and Thrust Belt. They represent components of the Indian passive margin sequences, deformed into a predominantly north-dipping fold and thrust belt during the formative stages of Himalayan orogenesis. Data from geotransects at various locations along the length of the main Himalayan arc consistent with five stage deformation history for the Tethyan Himalaya. The first regionally correlative deformation event (D1) led to formation of broadly E–W trending isoclinal structures and a penetrative axial planar schistosity; the style and distribution of these structures may have been influenced by stress localization associated with reactivation of the pre-existing structural grain. The second phase of regional deformation (D2) was associated with formation of a series of E–W trending, predominantly south-vergent folds and thrusts, and a variably developed spaced axial planar foliation; these are the dominant thickening structures affecting the THS metasediments and account for the majority of crustal thickening in the Tethyan Himalaya. Eohimalayan deformations (D1–D2) are cross cut and overprinted by Neohimalayan structures associated with slip on the STD-MCT and GCT fault system (D3) during the Miocene. Subsequent local folding and doming (D4) are attributed to ongoing deformation and assembly of the frontal ranges. The youngest regional deformation so far detected is associated with rifting and development of N–S trending graben structures (D5) during the Pliocene. Suggestions that metasediments assigned a Triassic age within the eastern Himalayan THS are instead either remnants of the forearc sequence or a collisional mélange, are not supported by the absence of detrital zircons in these deposits younger than ∼200 Ma. U–Pb zircon ages indicate that the Eohimalayan Dala granitoids, emplaced into the eastern THS metasediments after regional D2 deformation, crystallized at 44.1 ± 1.2 Ma. This age places a clear lower bound on the timing of the D2 event and provides strong support for the view that significant crustal thickening occurred during the first ∼ 10 to 20 Ma of Himalayan orogenesis along the length of the proto Himalayan arc. Our results are further supported by studies indicating that detritus eroded from the Tethyan sequences was being shed into the nascent foreland basin during the mid Eocene (DeCelles et al., 2004; Najman et al., 2005). Andalusite hornfels contact aureoles adjacent the Dalagranitoids constrains the depth of emplacement and maximum overburden removed since the mid Eocene to ≤3.5 kbars and ≤12 km, respectively. The thermal history of the Dala Granitoids indicates that they remained broadly isothermal from the mid Eocene to mid Miocene, consistent with a period of relative tectonic quiescence, although shortening may have continued at lower crustal levels. Rapid cooling at ∼15 Ma is interpreted to record exhumation associated with Miocene movement on the STD-GCT fault systems, coincident with exhumation of the NHD.

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2008.06.038.

Acknowledgements This work derives largely from research conducted by A.B. Aikman during his postgraduate studies at The Australian National University and was supported by ANU scholarships, grants from the Australian Research Council and National Science Foundation, and grant 40625008 to Ding Lin from the National Natural Science Foundation of China. The authors would like to thank members of the Chinese Academy of Sciences for fieldwork assistance, Trevor Ireland and Peter Holden for help with SHRIMP U–Pb dating, and Jim Dunlap and Xiaodong Zhang for assistance with K–Ar and 40Ar/39Ar thermochronology. Grateful thanks are also extended to Paul Kapp and

References Aikman, A.B., 2007. Tectonics of the eastern Tethyan Himalaya. PhD Thesis, The Australian National University. Aitchison, J.C., Badengzhu, Davis, A.M., Liu, J., Luo, H., Malpas, J.G., McDermid, I.R.C., Wu, H., Ziabrev, S.V., Zhou, M., 2000. Remnants of a Cretaceous intra-oceanic subduction system within the Yarlung–Zangbo suture (southern Tibet). Earth Planet. Sci. Lett. 183, 231–244. Aitchison, J.C., Ali, J.R., Davis, A.M., 2007. When and where did India and Asia collide? J. Geophys. Res. 112, B05423. doi:10.1029/2006JB004706. Allègre, C.J., Courtillot, V., Tapponnier, P., Hirn, A., Mattauer, M., Coulon, C., Jaeger, J.J., Achache, J., Scharer, U., Marcoux, J., Burg, J.P., Girardeau, J., Armijo, R., Gariepy, C., Gopel, C., Li, T., Xiao, X., Chang, C., Li, G., Lin, B., Teng, J., Wang, N., Chen, G., Han, T., Wang, X., Den, W., Sheng, H., Cao, Y., Zhou, J., Qiu, H., Bao, P., Wang, S., Wang, B., Zhou, Y., Xu, R., 1984. Structure and evolution of the Himalaya–Tibet orogenic belt. Nature 307, 17–22. Argand, E., 1924. La tectonique de l'Asie. International Geological Congress Report Session 13/1. Armijo, R., Tapponnier, P., Mercier, J., Han, T., 1986. Quaternary extension in southern Tibet: Field observations and tectonic implications. J. Geophys. Res. 91, 13,803–13,872. Booth, A.L., Zeitler, P.K., Kidd, W.S.F., Wooden, J., Liu, Y.P., Idleman, B., Hren, M., Chamberlain, C.P., 2004. U–Pb zircon constraints on the tectonic evolution of southeastern Tibet, Namche Barwa area. Am. J. Sci. 304, 889–929. Brookfield, M., 1993. The Himalayan passive margin from Precambrian to Cretaceous. Sediment. Geol. 84, 1–35. Burchfield, B.C., Chen, Z., Hodges, K., Liu, Y., Royden, L., Deng, C., Xu, J., 1992. The South Tibetan Detachment System, Himalayan orogen: Extension contemporaneous with and parallel to shortening in a collisional mountain belt. Geol. Soc. Am. Spec. Pap. 269, 1–41. Burg, J.-P., Leyreloup, A., Girardeau, J., Chen, G.-M., 1987. Structure and metamorphism of a tectonically thickened continental crust: the Yalu Tsangpo suture zone (Tibet). Philos. Trans. R. Soc. Lond. Ser. A 321, 67–86. Chen, Z., Liu, Y., Hodges, K., Burchfield, B., Royden, L., Deng, C., 1990. The Kangmar Dome — a metamorphic core complex in southern Xizang (Tibet). Science 250, 1552–1556. Claoue-Long, J., Compston, W., Roberts, J., Fanning, C., 1995. Two Carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and Ar/Ar analysis. In: Berggren, W., Kent, D., Aubrey, M., Hardenbol, J. (Eds.), Geochronology, Time Scales and Global Stratigraphic Correlation. SEPM Special Publication, vol. 54, pp. 3–21. Craw, D., Koons, P.O., Zeitler, P.K., Kidd, W.S.F., 2005. Fluid evolution and thermal structure in the rapidly exhuming gneiss complex of Namche Barwa Gyala Peri, eastern Himalayan syntaxis. J. Metamorph. Geol. 23, 829–845. Crouzet, C., Dunkl, I., Paudel, L., Arkai, P., Rainer, T., Baglogh, K., Appel, E., 2007. Temperature and age constraints on the metamorphism of the Tethyan HImalaya in Central Nepal: A multidisciplinary approach. J. Asian Earth Sci 30 (1), 113–130. Debon, F., LeFort, P., Sheppard, S., Sonet, J., 1986. The four plutonic belts of the Transhimalaya–Himalaya: a chemical, mineralogical , isotopic, and chronological synthesis along a Tibet–Nepal granite section. J. Petrol. 27, 219–250. DeCelles, P.G., Gehrels, G.E., Quade, J., Ojha, T.P., Kapp, P.A., Upreti, B.N., 1998. Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal. Geol. Soc. Amer. Bull. 110, 2–21. DeCelles, P.G., Gehrels, G.E., Najman, Y., Martin, A.J., Carter, A., Garzanti, E., 2004. Detrital geochronology and geochemistry of Cretaceous–Early Miocene strata of Nepal: implications for timing and diachroneity of initial Himalayan orogenesis. Earth Planet. Sci. Lett. 227, 313–330. Ding, L., Kapp, P., Wan, X., 2005. Paleocene–Eocene record of ophiolite obduction and initial India–Asia collision, south central Tibet. Tectonics 24, TC3001. doi:10.1029/ 2004TC001729. Dürr, S., 1996. Provenance of Xigaze fore-arc basin clastic rocks (Cretaceous, south Tibet). Geol. Soc. Amer. Bull. 108, 669–684. England, P., Houseman, G., Sonder, L., 1985. Length scales for continental deformation in converent, divergent, and strike–slip environments: analytical and approximate solutions for a thin viscous sheet model. J. Geophys. Res. 90, 4797–4810. Gansser, A., 1964. The geology of the Himalayas. Wiley Interscience, New York. Garzanti, E., 1999. Stratigraphy and sedimentary history of the Nepal Tethys Himalaya passive margin. J. Asian Earth Sci. 17, 805–827. Gnos, E., Immenhauser, A., Peters, T., 1997. Late Cretaceous/early Tertiary convergence between the Indian and Arabian plates recorded in ophiolites and related sediments. Tectonophysics 271, 1–19. Godin, L., 2003. Structural evolution of the Tethyan sedimentary sequence in the Annapurna area, central Nepal Himalaya. J. Asian Earth Sci. 22, 307–328. Godin, L., Brown, R., Hanmer, S., Parrish, R.R., 1999. Back folds in the core of the Himalayan orogen: An alternative interpretation. Geology 27, 151–154. Harrison, T.M., Wenji, C., Leloup, P.H., Ryerson, F.J., Tapponnier, P., 1992. An early Miocene transition in deformation regime within the red river fault zone, Yunnan, and its significance for Indo–Asian tectonics. J. Geophys. Res. 97, 7159–7182.

A.B. Aikman et al. / Earth and Planetary Science Letters 274 (2008) 14–23 Harrison, T.M., Heizler, M., Lovera, O.M., Chen, W., Grove, M., 1994. A chlorine disinfectant for excess argon released from K-feldspar during step-heating. Earth Planet. Sci. Lett. 123, 95–104. Harrison, T.M., Lovera, O.M., Grove, M., 1997. New insights into the origin of two contrasting Himalayan granite belts. Geology 25, 899–902. Harrison, T.M., Yin, A., Grove, M., Lovera, O.M., Ryerson, F.J., Zhou, X., 2000. The Zedong Window; a record of superposed Tertiary convergence in southeastern Tibet. J. Geophys. Res. 105, 19,211–19,230. Larson, K., Burgmann, R., Bilham, R., Freymueller, J., 1999. Kinematics of the India– Eurasia collsion zone from GPS. J. Geophys. Res. 104, 1077–1093. Lee, J., Whitehouse, M.J., 2007. Onset of mid-crustal extensional flow in southern Tibet: Evidence from U/Pb zircon ages. Geology 35, 45048. Lee, J., Hacker, B.R., Dinklage, W.S., Wang, Y., Gans, P., Calvert, A., Wan, J.L., Chen, W.J., Blythe, A.E., McClelland, W., 2000. Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints. Tectonics 19, 872–895. LeFort, P., 1975. Himalayas, the collided range. Present knowledge of the continental arc. Am. J. Sci. 275A, 1–44. LeFort, P., 1996. Evolution of the Himalaya. In: Yin, A., Harrison, T. (Eds.), The Tectonics of Asia. Cambridge University Press, pp. 95–106. Liu, G., Ensele, G., 1994. Sedimentary history of the Tethyan basin in the Tibetan Himalayas. Geol. Rundsch. 82, 32–61. Lovera, O.M., Richter, F., Harrison, T.M., 1989. 40Ar/39Ar thermochronology for slowly cooled samples having a distribution of diffusion domain sizes. J. Geophys. Res. 94, 17,917–17,936. Ludwig, K., 2001. Squid 1.02: A user manual. Berkeley Geochronological Center Special Publication, 2, 19. Makovsky, Y., Klemperer, S., 1996. Structural elements of the southern Tethyan Himalaya crust from wide-angle seismic data. Tectonics 15, 997–1005. Makovsky, Y., Klemperer, S.L., Ratschbacher, L., Alsdorf, D., 1999. Midcrustal reflector on INDEPTH wide-angle profiles: an ophiolitic slab beneath the India–Asia suture in southern Tibet? Tectonics 18, 793–808. McDougall, I., Harrison, T.M., 1999. Geochronology and Thermochronology by the 40Ar/ 39 Ar Method, 2nd ed. Oxford University Press, New York. 269 pp. Meigs, A., Burbank, D., Beck, R., 1995. Middle–Late Miocene (N 10 Ma) formation of the Main Boundary Thrust in the western Himalaya. Geology 23, 423–426. Murphy, M., Yin, A., 2003. Structural evolution and sequence of thrusting in the Tethyan fold-thrust belt and Indus–Yalu suture zone, southwest Tibet. Geol. Soc. Amer. Bull. 115, 21–34. Najman, Y., Carter, A., Oliver, G., Garzanti, E., 2005. Provenance of Eocene foreland basin sediments, Nepal: Constraints to the timing and diachroneity of early Himalayan orogenesis. Geology 33, 309–312. Paces, J., Miller, J., 1993. U–Pb ages of Duluth complex and related mafic intrusions, NE Minnesota: geochronological insights to physical, petrogenetic, palaeomagnetic, and tectonomagmatic processes associated with the 1 Ga mid-continent rift system. J. Geophys. Res. 98, 13997–14013. Pan, G., Ding, J., Yao, D., and Wang, L., 2004. Geological map of Qinghai-Xiang (Tibet) Plateau and Adjacent Areas (1:1,500,000).

23

Quidelleur, X., Grove, M., Lovera, O.M., Harrison, T.M., Yin, A., 1997. Thermal evolution and slip history of the Renbu–Zedong Thrust, southeastern Tibet. J. Geophys. Res. 102, 2659–2679. Quigley, M., Yu, L.J., Liu, X.H., Wilson, C.J.L., Sandiford, M., Phillips, D., 2006. 40Ar–39Ar thermochronology of the Kampa Dome, southern Tibet: Implications for tectonic evolution of the North Himalayan gneiss domes. Tectonophysics 421, 269–297. Ratschbacher, L., Frisch, W., Chen, C., Pan, G.T., 1992. Deformation and motion along the southern margin of the Lhasa Block (Tibet) prior to and during the India Asia collision. J. Geodyn. 16, 21–54. Ratschbacher, L., Frisch, W., Liu, T., Chen, C., 1994. Distributed deformation in southern and western Tibet during and after the India–Asia collision. J. Geophys. Res. 99, 19,817–19,945. Searle, M.P., Windley, B.F., Coward, M.P., Cooper, D.J.W., Rex, A.J., Li, T., Xiao, X., Jan, M.Q., Thakur, V.C., Kumar, S., 1987. The closing of the Tethys and the tectonics of the Himalaya. Geol. Soc. Amer. Bull. 98, 678–701. Spear, F.S., 1993. Metamoprhic phase equlibria and pressure–temperature–time paths. Mineral. Soc. Am. Monograph.. 799 pp. Suess, E., 1875. Die Entstehung der Alpen. W. Braumüller. Vannay, J., Steck, A., 1995. Tectonic evolution of the High Himalaya in Upper Lahul (NW Himalaya, India). Tectonics 14, 253–263. Wiesmayr, G., Grasemann, B., 2002. Eohimalayan fold and thrust belt: Implications for the geodynamic evolution of the northwest Himalaya (India). Tectonics 21. doi:10.1029/2002TC001363. Williams, I.S., 1998. U–Th–Pb geochronology by Ion Microprobe. In: McKibbenIII III, M.A., W.C.S., Ridley, W.I. (Eds.), Applications of microanalytical techniques to understanding mineralizing processes. Revs. Econ. Geol. 7, 1–35. Willems, H., Zhou, Z., Zhang, B., Grafe, K.U., 1996. Stratigraphy of the Upper Cretaceous and Lower Tertiary Strata in the Tethyan Himalayas of Tibet (Tingri area, China). Geologische Rundschau 85, 723–754. Yin, A., 2006. Cenozoic evolution of the Himalayan Orogen as constrained by alongstrike variations of structural geometry, exhumation history, and foreland sedimentation. Earth Sci. Revs. 76, 1–134. Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan–Tibetan orogen. Ann. Revs. Earth Planet. Sci. 28, 211–280. Yin, A., Harrison, T.M., Ryerson, F.J., Wenji, C., Kidd, W., Copeland, P., 1994. Tertiary structural evolution of the Gangdese thrust system, southeastern Tibet. J. Geophys. Res. 99, 18,175–18,201. Yin, A., Harrison, T.M., Murphy, M.A., Grove, M., Nie, S., Ryerson, F.J., Wang, X., Chen, Z., 1999. Tertiary deformation history of southeastern and southwestern Tibet during the Indo–Asian collision. Geol. Soc. Am. Bull. 111, 1644–1664. Zeitler, P.K., 2006. Geochronological evidence for rates of geodynamic evolution in the Himalayan syntaxes. Geochim. Cosmochim. Acta 70, A732. Zhang, H.F., Harris, N., Parrish, R., Zhang, L., Zhao, Z., 2004. U–Pb ages of Kude and Sajia leucogranites in Saffia Dome from North Himalaya and their geological implications. Chin. Sci. Bull. 49, 2087–2092. Zhu, B., Kidd, W.S.F., Rowley, D.B., Currie, B.S., Shafique, N., 2005. Age of initiation of the India–Asia collision in the east–central Himalaya. J. Geol. 113, 265–285.