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The leading edge of the Greater Himalayan Crystalline complex revealed in the NW Indian Himalaya: Implications for the evolution of the Himalayan orogen A. Alexander G. Webb* Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, An Yin University of California–Los Angeles, Los Angeles, California 90095-1567, USA T. Mark Harrison Julien Célérier Research School of Earth Sciences, Australia National University, Canberra, ACT 2601, Australia W. Paul Burgess Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California–Los Angeles, Los Angeles, California 90095-1567, USA ABSTRACT Typically, the South Tibet detachment shear The three Himalayan lithologic units, the Lesser Himalayan Sequence, the Greater Himalayan zone is hundreds of meters thick and exhibits Crystalline complex, and the Tethyan Himalayan Sequence, have a specifi c structural correlation both top-to-the-NE and top-to-the-SW shear- with the Main Central thrust and South Tibet detachment in the central Himalaya. There, the sense indicators (also see Jain et al., 1999). This Main Central thrust places the Greater Himalayan Crystalline complex over the Lesser Hima- contrasts to the top-to-the-SW motion associated layan Sequence, and the South Tibet detachment places the Tethyan Himalayan Sequence over with the Main Central thrust ductile shear zone the Greater Himalayan Crystallines. Although this division has formed the basis for all Hima- below. In general, gneisses are common below layan tectonic models, it fails to explain aspects of the geology of the western Himalaya where and schists are prevalent above the South Tibet the Main Central thrust places the Tethyan Himalayan Sequence directly above the Lesser Hima- detachment. Although garnet is present both above layan Sequence. Our mapping in NW India shows that this relationship results from southward and below the South Tibet detachment, kyanite merging of the Main Central thrust and South Tibet detachment. This fi nding, in conjunction with and/or sillimanite are diagnostic of the South observed alternating shear senses on the South Tibet detachment, is inconsistent with the wedge- Tibet detachment footwall. Following Vannay extrusion and erosion-induced channel-fl ow models (both require only top-to-the-N motion on and Steck (1995) and Wyss et al. (1999), we used the South Tibet detachment) but is consistent with a tectonic-wedging model. graphitic quartzite and discontinuous lenses of calc-silicate schists in the THS as marker beds to Keywords: Himalaya, South Tibet detachment, Main Central thrust, tectonic wedge. trace the South Tibet detachment hanging wall. Intrusive contacts around Cambrian-Ordovician INTRODUCTION GEOLOGY OF THE ROHTANG LA AREA granites in the South Tibet detachment hanging The ~2500-km-long Himalayan orogen is Our fi eld area is located in the Rohtang La wall are undeformed, whereas the same con- widely thought to consist of only three major area northwest of the Kulu Window (Figs. 1 and tacts in the South Tibet detachment footwall are units: the Lesser Himalayan Sequence (LHS; 2). Although the South Tibet detachment can intensely transposed by ductile folding. mainly low-grade Proterozoic metasediments), be traced from Nepal to this area, its westward At Rohtang La, the South Tibet detach- the Greater Himalayan Crystalline complex extension is poorly defi ned (Fig. 2) (Choudhuri ment shear zone preserves ductile shear fabrics (GHC; largely high-grade paragneisses and mig- et al., 1992; Vannay and Grasemann, 1998; Jain including top-to-the-SW S-C fabric, top-to-the- matite), and the Tethyan Himalayan Sequence et al., 1999), which had led to various interpre- NE sigma augen, top-to-the-NE and top-to- (THS; dominantly low-grade late Proterozoic tations including connection with the Zanskar the-SW shear band cleavage, and top-to-the-SW to Eocene shelf sediments) (Heim and Gansser, shear zone (Searle et al., 1999) and termina- folds (Fig. 3). The top-to-the-NE shear fabrics 1939). In the central Himalaya, the LHS and GHC tion in the Rohtang La area (Steck, 2003). We overprint top-to-the-SW shear fabrics (also are separated by the Main Central thrust, and the mapped the position of the South Tibet detach- see Jain et al., 1999). A sharp contact between GHC and THS are separated by the South Tibet ment by tracking its deformation zone, meta- mylonitic augen gneiss below and garnet schist detachment (LeFort, 1996) (Fig. 1). However, in morphic grade changes across the fault, and above is present in the South Tibet detachment the western Himalaya (west of 77°E), the Main marker beds along the fault. shear zone, which we interpret as the South Central thrust places THS rocks directly over LHS metasediments (e.g., Yeats and Lawrence, N China E 0 100 200 km E ° 85° 35° Indus-Tsangpo Sutu Ka 80 N re India rako 30° 1984; Frank et al., 1995; Pogue et al., 1999) ia ru N Ind GCT m F Asia plate GCT THS (Fig. 1). Several scenarios have been advanced Pakistan ault S to explain this different relationship (e.g., Thakur, TD/ZSZ China GHC Nepal 1998; DiPietro and Pogue, 2004; Yin, 2006), but LHS GHC Figure 2. THS STD MCT Q THS uncertainty regarding the position of the South STD E Tibet detachment in many locations in the NW 75° China India Himalaya (cf. Fig. 1 of Searle et al. [1999] MCT LHS ia and plate 1 of Steck [2003]) limits efforts to Ind SH LHS India SH understand its signifi cance. This paper summa- Q PakistanKulu Window Nepal E 5° 0°N E India Q N rizes the results of fi eld work undertaken in the 7 Simla Klippe 3 80° western Himalaya that lead to an interpretation that explains this relationship but that challenges Figure 1. Map of central and western Himalaya compiled from DiPietro and Pogue (2004), Valdiya (1980), Yin (2006), and references for Figure 2. GCT—Great Counter Thrust; GHC— current views of Himalayan thrust tectonics. Greater Himalayan Crystalline complex; LHS—Lesser Himalayan Sequence; MCT—Main Central thrust; Q—Quaternary alluvium; SH—Sub-Himalayan Sequence; STD—South Tibet *E-mail: [email protected]. detachment; THS—Tethyan Himalayan Sequence; ZSZ—Zanskar shear zone. © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, October October 2007; 2007 v. 35; no. 10; p. 955–958; doi: 10.1130/G23931A.1; 4 fi gures; Data Repository item 2007235. 955 Tso 76°30′E 77°E 77°30′E 0 10 20 30 40 50 km Mo Beaumont et al. (2001) in that the latter predicts rar GHC Sub-Himalayan Sequencei (SH) India crust to be subducted and then return to the Mz-Pz THS grC-O Tethyan Himalayan Sequence (THS) surface without transporting Asian rocks to the Cha ndrabh Mesozoic-Paleozoic (Mz-Pz THS) present Himalayan range. This physical process aga River Late Proterozoic (Pt THS) Cambrian-Ordovician granite may be more appropriately characterized as cor- Rohtang La ? o 32°30′N (grC-O) 32 30’N Pt THS ner fl ow (see Cloos, 1982). None of these mod- Greater Himalayan els explain our observed southward merging of Figure 3A. Crystalline complex (GHC) Figure 2. Map of Kulu Lesser Himalayan Window region (after the Main Central thrust and South Tibet detach- Sequence (LHS) ? Frank et al., 1973, 1975; ment, but instead require the Main Central thrust Main Central thrust Sharma, 1977; Vannay and South Tibet detachment to be surface faults. SouthSpi Tibet ti Riv and Steck, 1995; Vannay 78°E detachmenter The tunneling stage of channel fl ow of Beaumont and Grasemann, 1998; 32°N 32°N Overturned Wyss et al., 1999; our own et al. (2001) is compatible with the observed South Tibet Main Central thrust–South Tibet detachment Beas River LHS detachment observations, and addi- grC-O 78°30′N tional references in the branch line geometry, but fails to explain two 76°30′E GSA Data Repository1). SH K key kinematic observations. First, its predicted u Pt THS Over lu GHC W top-to-the-N South Tibet detachment kinemat- -turned in anticline dow ics is inconsistent with the observed alternating River Pt THS Sutlej 31°30′N top-to-the-N and top-to-the-S shear fabrics in Thrust 31o30’N the South Tibet detachment zone in NW India Normal and throughout the Himalaya (e.g., Patel et al., fault Pabbar er 77°E LHS 77°30′E Riv 78°E 78°30′N 1993; Hodges et al., 1996; Grujic et al., 2002; Robinson et al., 2006; this study). Second, chan- nel-fl ow tunneling requires slip on the South Tibet detachment fault. The South Tibet detach- detachment hanging wall pinches out west of the Tibet detachment and Main Central thrust to ment can be traced south of Rohtang La into the well-studied Sutlej River section where Greater vanish at their branch line where the tunnel ter- south-verging overturned Phojal anticline (i.e., Himalayan Crystalline gneiss is exposed contin- minates, which is inconsistent with >100 km of the Phojal Nappe of Frank et al., 1973) along uously between the Main Central thrust and the Main Central thrust slip south of the Main Cen- the west bank of the Beas River (Fig. 3A). The South Tibet detachment (e.g., Vannay and Grase- tral thrust–South Tibet detachment branch line South Tibet detachment is overturned in the mann, 1998) (Fig. 2). Thus, the merging of the as constrained by the distance between the Kulu Phojal anticline (Figs, 3A and 3B) as indicated South Tibet detachment and Main Central thrust Window and the Simla Klippe (Fig. 1). by the folding of (1) the gneiss-schist contact in map view defi nes the tip line of a southward- To reconcile the new observation for the that marks the fault and features top-to-the- tapering GHC wedge (Fig.

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