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Kinematic Model for the Main Central Thrust in Nepal

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Kinematic Model for the Main Central Thrust in Nepal Kinematic model for the Main Central thrust in Nepal D.M. Robinson* Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA P.G. DeCelles C.N. Garzione Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA O.N. Pearson Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA T.M. Harrison Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia E.J. Catlos School of Geology, Oklahoma State University, Stillwater, Oklahoma 74078, USA ABSTRACT thrust to ;4 kbar near the South Tibetan de- We present a kinematic model for the Himalayan thrust belt that satis®es structural tachment system. In the northern part of the and metamorphic data and explains recently reported late Miocene±Pliocene geochrono- Lesser Himalaya, the rocks have been meta- logic and thermochronologic ages from rocks in the Main Central thrust zone in central morphosed to upper greenschist facies, and Nepal. At its current exposure level, the Main Central thrust juxtaposes a hanging-wall metamorphic isograds in the footwall of the ¯at in Greater Himalayan rocks with a footwall ¯at in Lesser Himalayan rocks of the Main Central thrust are inverted, progressing Ramgarh thrust sheet, which is the roof thrust of a large Lesser Himalayan duplex. Se- from chlorite to garnet in a northward direc- quential emplacement of the Main Central (early Miocene) and Ramgarh (middle Mio- tion (e.g., Harrison et al., 1998). The kine- cene) thrust sheets was followed by insertion of thrust sheets within the Lesser Himalayan matic history of the thrust belt involved a gen- duplex and folding of the Main Central and Ramgarh thrusts during late Miocene± eral southward progression of main phases of Pliocene time. Thorium-lead (Th-Pb) ages of monazite inclusions in garnets from central thrusting from Eocene to present (Fig. 2; Nepal record the timing of coeval, progressive metamorphism of Lesser Himalayan rocks Ratschbacher et al., 1994; DeCelles et al., in the footwall of the Main Central thrust. Although this model does not rule out minor, 2001). late-stage reactivation of the Main Central thrust, major late Miocene reactivation is not required. STRUCTURAL GEOMETRY The Main Central thrust in Nepal is tradi- Keywords: Main Central thrust, Ramgarh thrust, Himalaya, Nepal, Lesser Himalayan duplex. tionally de®ned as a ductile shear zone that is several hundred meters to several kilometers INTRODUCTION regional-scale structural and metamorphic thick with a top-to-the-south sense of shear Spanning the entire length of the Himalaya, characteristics of the Nepalese sector of the (e.g., Brunel, 1986). The concept of the Main the Main Central thrust has accommodated a thrust belt. Central thrust zone was developed because in signi®cant fraction of the total shortening in many places, lower greenschist facies Lesser the orogenic belt (see summaries in Hodges, REGIONAL TECTONIC SETTING Himalayan rocks grade into upper amphibolite 2000; Yin and Harrison, 2000). Despite many The tectonostratigraphy of the Himalayan facies Greater Himalayan rocks without an ob- detailed studies, the kinematic history of the orogenic belt is divided into the Tibetan Him- vious structural break (Arita, 1983; PeÃcher, fault and its relationship to the other structures alaya, Greater Himalaya, Lesser Himalaya, 1989; Vannay and Hodges, 1996). Highly in the Himalaya remain poorly understood. and Subhimalaya zones (Fig. 1; Gansser, strained Lesser Himalayan rocks in the foot- Particularly intriguing are geochronologic and 1964). Separating the zones are major fault wall of the Main Central thrust are commonly thermochronologic data from central and east- systems: the South Tibetan detachment system included in the Main Central thrust zone. The ern Nepal that suggest major reactivation of between the Tibetan and Greater Himalayas, transition zone between unambiguous Lesser the Main Central thrust, with perhaps as much the Main Central thrust between the Greater Himalayan and Greater Himalayan rocks con- as 40 km of slip during late Miocene±Pliocene and Lesser Himalayas, and the Main Bound- tains a variety of metasedimentary rocks, in- time (Harrison et al., 1997, 1998; Catlos et al., ary thrust between the Lesser Himalaya and cluding phyllite, schist (commonly graphitic), 2001, 2002). Although out-of-sequence thrust- Subhimalaya. Other important structures in- marble, and quartzite. Greater Himalayan ing is ubiquitous in thrust belts, the resulting clude the Ramgarh and Main Frontal thrusts rocks in western and central Nepal are divided displacements are generally only a few kilo- and the Lesser Himalayan duplex (Fig. 1). into three units: a lower metapelitic unit, a meters (Boyer, 1992). The Himalayan thrust The Greater Himalaya consists of a 5±20- middle metacarbonate unit, and an upper or- belt is commonly adopted as a paradigm for km-thick assemblage of Late Proterozoic± thogneiss unit (LeFort, 1975). collisional orogens, so the possibility of ex- early Paleozoic metasedimentary and metaig- Compositional layers and foliation in traordinary out-of-sequence slip on the Main neous rocks (PeÃcher, 1989; Schelling, 1992; Greater Himalayan rocks dip 308±608 north- Central thrust is worthy of careful consider- Vannay and Hodges, 1996). The Lesser Him- northeast and generally strike parallel to the ation. In this paper we combine geochrono- alaya in Nepal is composed of an ;10-km- trace of the Main Central thrust (e.g., Frank logic and thermochronologic data from central thick succession of Proterozoic, upper Paleozoic, and Fuchs, 1969; LeFort, 1975; Arita, 1983; Nepal with new structural data from western and Cretaceous±lower Miocene sedimentary Brunel, 1986; Schelling, 1992; Vannay and Nepal to develop a conceptual model that ex- rocks (e.g., DeCelles et al., 2000, 2001). Grasseman, 2001; DeCelles et al., 2001). Al- plains the Main Central thrust within the The peak metamorphic temperatures in the though some layering in Greater Himalayan broader context of the entire thrust belt. Al- Greater Himalaya increase northward in an rocks is probably the result of cleavage trans- though the ®eld and geochronologic data upsection direction from ;550 to ;800 8C position, the threefold lithostratigraphy can be needed to test the model are not yet available (e.g., PeÃcher, 1989; Ganguly et al., 2000; Van- traced in many transects .100 km across from a single region of the Himalaya, we sug- nay and Grasemann, 2001), accompanied by a strike, and no evidence exists to support the gest that the model may explain many of the northward-declining (Harrison et al., 1999; presence of a regional isoclinal fold in these Guillot, 1999) ®eld gradient in recorded peak rocks. Moreover, thermobarometric data (e.g., *Corresponding author. pressure from ;10 kbar near the Main Central Johnson et al., 2001) do not suggest a signif- q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; April 2003; v. 31; no. 4; p. 359±362; 2 ®gures. 359 cause emplacement of thrust sheets within the duplex passively uplifted and tilted the over- lying Greater Himalayan rocks into their steep northward dips. In central and western Nepal (Johnson, 1994; Paudel and Arita, 2000; Robinson, 2001; DeCelles et al., 2001; Pearson, 2002) and northern India (Srivastava and Mitra, 1994), the Lesser Himalayan duplex has a hinterland-dipping, antiformal geometry. The roof thrust is the Ramgarh thrust in western Nepal (DeCelles et al., 2001), and the ¯oor thrust is the Main Himalayan thrust (Hauck et al., 1998). In western Nepal, the growth of the duplex commenced with the emplacement of the Ramgarh thrust sheet along a regional footwall ¯at (Fig. 2C). Thrust sheets of Lesser Himalayan rocks were subsequently excised from the footwall and incorporated into the hanging wall of the ¯oor thrust (Fig. 2D). The total slip was fed from the lower thrust ¯at to the upper ¯at, but individual thrusts in the du- plex experienced only a faction of this total slip. As each new thrust sheet was incorpo- rated into the duplex, all overlying rocks, in- cluding the Greater Himalayan rocks above the Ramgarh sheet, were folded into a broad antiform (Figs. 2D, 2E). Many workers have considered rocks that we map as part of the Ramgarh thrust sheet and the northern limb of the Lesser Himalayan duplex in western, central, and eastern Nepal to be within the Main Central thrust zone (e.g., Paudel and Arita, 2000). Regional strati- graphic, structural, and isotopic studies con- Figure 1. A: Geologic map of Nepal showing major tectonostratigraphic zones, faults, and ®rm that these are Lesser Himalayan rocks crystalline klippen of Greater Himalayan af®nity. Box in western Nepal delineates area (DeCelles et al., 2001; Robinson et al., 2001; mapped by Robinson (2001), and line of cross section is marked by line A±A9. Box in central Nepal shows area sampled by Catlos et al. (2001). B: Generalized cross section from western Pearson, 2002). In western Nepal, garnet-bearing Nepal showing major faults and tectonostratigraphic zones, modi®ed from DeCelles et al. phyllite (equivalent to domains 2 and 3 of (2001). DT is Dadeldura thrust. Catlos et al., 2001) is present in the Ramgarh sheet and in at least two additional thrust sheets within the duplex (Robinson, 2001; icant change in peak pressures right above the tation (Figs. 2B±2E). The regional-scale, ¯at- Pearson, 2002). Main Central thrust across the entire north- on-¯at structural geometry is well preserved south width of exposure in the Greater Hima- in the Arun Valley of eastern Nepal (Schel- layan rocks. Therefore, we interpret the ®rst- ling, 1992), but is obscured in central and IMPLICATIONS OF THE MODEL order, hanging-wall structure of the Main western Nepal because erosion has breached The qualitative kinematic model (Fig. 2) Central thrust to be a regional ¯at in the di- the Greater Himalayan rocks and Lesser Hi- makes several predictions that can be tested rection of tectonic transport.
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