Defining the Himalayan Main Central Thrust in Nepal - Queen's ...

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$Fayek & Kyser 2000 uraninite fractionations.pdf - Queen's University$

530 M. P. SEARLE ET AL. Himalayan Sequence rocks thrust above unmetamorphosed Lesser Himalayan sedimentary rocks in far eastern Nepal and Sikkim–West Bengal (Fig. 1). The full inverted metamorphic isograd sequence has been mapped here, but unlike the Kathmandu complex, and like the Greater Himalayan Sequence, the isograds are structurally inverted from sillimanite down to biotite–chlorite (Mohan et al. 1989; Dasgupta et al. 2004). The structures and metamorphic P–T conditions clearly show that the Darjeeling klippe is linked to the main Main Central Thrust to the north (Searle & Szulc 2005; Fig. 3). Detrital zircon ages and the Main Central Thrust Detrital zircon U–Pb ages provide maximum depositional age constraints of the metamorphic protolith. Parrish & Hodges (1996) originally proposed that Greater and Lesser Himalayan rocks, divided by the Main Central Thrust, had a significant difference in sedimentary provenance. Zircons from Greater Himalayan Sequence rocks have mainly late Proterozoic and early Palaeozoic ages, whereas zircons from the Lesser Himalaya have late Archaean–early Proterozoic ages. DeCelles et al. (2000) showed that metasedimentary rocks from the Greater Himalayan Sequence gave zircon ages of 800–1700 Ma, whereas quartzites of one unit generally mapped within the Lesser Himalaya Sequence, the Nawakot Group, yielded zircons .1.8 Ga old. This reflects the 1866–1833 Ma depositional age of these units. The upper age limit is given by the Ulleri augen gneiss, which was intruded into the Nawakot quartzite. However, a major mylonite zone corresponding to the Main Central Thrust was mapped beneath the Ulleri augen gneiss in the Annapurna region (Searle & Godin 2003), so these rocks are now included within the Greater Himalayan Sequence. The Phaplu augen gneiss in the Everest profile is a similar age, and at a similar structural position to the Ulleri augen gneiss. The Phaplu gneiss is highly sheared and overlain by staurolite- and sillimanite-grade rocks typical of the Greater Himalayan Sequence, so it has also been included here within the Greater Himalayan Sequence (Jessup et al. 2006; Searle et al. 2006). There seems little doubt that many Lesser Himalayan protoliths, particularly in Nepal, are older than the exposed Greater Himalayan Sequence protoliths. The upper structural levels of the Lesser Himalaya in India (Cambrian Krol and Tal Formations, which overlie Proterozoic rocks) are lateral equivalents to the base of the restored Greater Himalayan Sequence (Steck 2003) It is also widely accepted that the Greater Himalayan Sequence protoliths were similar in age to the lower levels of the Tethyan Himalaya, which range in age from Neoproterozoic to Eocene. Indeed, in the Zanskar Himalaya in India, Searle (1986) and Walker et al. (2001) were able to correlate thick garnet amphibolite units in the Greater Himalayan Sequence with unmetamorphosed Permian Panjal volcanic rocks in Kashmir, and thick high-grade marbles of the Greater Himalayan Sequence with unmetamorphosed Triassic (and possible Jurassic) shelf carbonate units within the Tethyan Himalaya. Nd isotopes and the Main Central Thrust Several workers (e.g. DeCelles et al. 2000; Robinson et al. 2001; Richards et al. 2005) have described the Main Central Thrust as a ‘discrete ductile shear zone separating isotopically different protoliths’. Parrish & Hodges (1996) first proposed that there was no overlap between the ranges of 143 Nd/ 144 Nd ratios between Greater Himalayan Sequence and Lesser Himalaya rocks in the Langtang region. DeCelles et al. (2000) and Robinson et al. (2001) showed that ENd(0) average values from Lesser Himalayan rocks in Nepal are 21.5, whereas the Greater and Tethyan Himalaya zones in Nepal have an average ENd(0) value of 16. They suggested that the Greater Himalayan Sequence was not Indian basement, but rather a terrane that was accreted onto India during the Early Palaeozoic, and that the Main Central Thrust had a large amount of pre-Tertiary displacement. However, there is no evidence of Palaeozoic suture zone rocks (e.g. ophiolites, deep-sea sediments, etc.) anywhere along the Main Central Thrust, and palinspastic reconstructions across the Western Himalaya (Searle 1986; Steck 2003), the Everest profile in Nepal (Searle et al. 2006) and the Sikkim–Bhutan Himalaya (Searle & Szulc 2005) show a continuous sedimentary succession from proximal to distal across the Lesser, Greater and Tethyan Himalaya. With larger Nd isotope datasets, the distinctive differences between Greater Himalayan Sequence and Lesser Himalaya protolith start to vanish. Ahmad et al. (2000) and Richards et al. (2005) recognized a separate zone termed the ‘Outer Lesser Himalaya’ that had relatively young source rocks, similar to the Greater Himalayan Sequence. They concluded that Greater Himalayan Sequence and ‘Outer Lesser Himalaya’ rocks showed a Meso-Palaeo-Proterozoic source, whereas the rest of the Lesser Himalaya showed Late Archaean to Early Proterozoic source rocks. However, Myrow et al. (2003) showed that samples from the base of the Tethyan Himalaya, north of the Greater Himalayan Sequence, have similar detrital zircon age spectra and Nd isotopic data to samples from the Kathmandu klippe and Lesser Himalaya south of the Greater Himalayan Sequence, thus eliminating the need for separate Greater Himalayan Sequence–Lesser Himalaya ‘terranes’. Lesser, Greater and Tethyan Himalaya represent a proximal to distal section across a continuous Indian plate prior to collision with Asia (Searle 1986; Myrow et al. 2003; Steck 2003; Searle et al. 2006). Martin et al. (2005) also correctly recognized that the Lesser–Greater Himalayan distinction was a protolith designation, fixed at the time of deposition. Because the Main Central Thrust is a Tertiary thrust fault that certainly cuts across stratigraphy, detrital zircon ages or Nd isotopes cannot be used to define the location of the Main Central Thrust. Metamorphism, U–Th–Pb monazite ages and the Main Central Thrust Inverted metamorphism along the Main Central Thrust zone is almost certainly related to movement along the Main Central Thrust. Within the inverted metamorphic field gradient, the rocks are highly sheared showing ubiquitous C–S–C9 fabrics and north-plunging lineations that indicate southward transport. Approximately 5–8 km of thickness has been flattened by pure shear to a section 1–2 km thick along the inverted metamorphic isograd zone (Searle & Rex 1989). There are no major metamorphic discontinuities within the inverted metamorphic sequence, suggesting post-metamorphic pure shear flattening. The geometry of the inverted isograds along the Main Central Thrust zone is similar along the entire Himalayan chain between the Zanskar–Kishtwar area in the west (Stephenson et al. 2000, 2001) to Nepal, Sikkim and Bhutan in the east (e.g. Bollinger et al. 2004; Dasgupta et al. 2004; Jessup et al. 2006). Dating of peak metamorphism has relied on Sm–Nd dating of garnet (e.g. Vance & Harris 1999), or U–Pb dating of monazites (eg: Walker et al. 1999; Simpson et al. 2000; Foster et al. 2002), that grew in equilibrium with kyanite or sillimanite. These are the only methods that date minerals with high enough closure tempera-
HIMALAYAN MAIN CENTRAL THRUST, NEPAL 531 tures. 40 Ar/ 39 Ar ages of hornblendes or micas record only a point on the cooling path after peak metamorphism, during exhumation. Within the Greater Himalayan Sequence initiation of garnet growth and burial metamorphism occurred at 44 Ma, with garnet rims growing as late as 29 3 Ma. (Prince et al. 2001; Foster et al. 2002). In the Everest region peak kyanite-grade metamorphism within the Greater Himalayan Sequence has been dated by U–Pb monazite at 32.2 0.4 Ma with later HT–LP sillimanitegrade metamorphism at 22.7 0.2 Ma (Simpson et al. 2000). Most leucogranite ages, interpreted as dating peak sillimanitegrade metamorphism and migmatization, along the Nepalese Himalaya range from c. 24 to 16 Ma (see Searle et al. 1997, 2003, for reviews). Along the base of the Greater Himalayan Sequence in Nepal, Harrison et al. (1997) first reported surprisingly young ages of c. 6 Ma from in situ 208 Pb/ 232 Th dating of monazite inclusions in garnet. These were interpreted as dating metamorphic recrystallization. Catlos et al. (2001) reported ages as young as 3.3 0.1 Ma from the Marysandi Valley and Kohn et al. (2001) reported ages of 9–8 Ma from the Lesser Himalaya (reinterpreted here as the lower levels of the Greater Himalayan Sequence above the Main Central Thrust). Th–Pb monazite ages from the Main Central Thrust zone in Langtang are c. 16Ma (Kohn et al. 2005), and the youngest monazite along the Dudh Kosi transect south of Everest has an age of 10.3 0.8 Ma (Catlos et al. 2002). In Sikkim, in situ Th–Pb monazite ages from the Main Central Thrust zone cluster at c. 22, 15–14 and 12–10 Ma (Catlos et al. 2004). These workers all interpreted the Th–Pb monazite ages as dating metamorphism along the Main Central Thrust, and hence timing of slip. However, Bollinger & Janots (2006) cautioned that some young Himalayan monazites were a retrograde growth product from the breakdown of allanite at low temperatures (,370 8C). In this scenario, the Th–Pb monazite ages only date a retrograde event at low temperature and may have nothing to do with slip along the Main Central Thrust. It seems quite likely that the very young monazite ages along the Main Central Thrust zone may record crystallization from metasomatic hydrothermal fluids rather than a ‘metamorphic event’. The ubiquitous presence of hot springs along the Main Central Thrust zone today testifies to the high level of hydrothermal fluids channelled up along the Main Central Thrust zone, and to the insignificance of shear heating along the thrust. The Main Central Thrust system evolved through time and space. Deep crustal levels show a wide ductile shear zone in kyanite-grade rocks. Higher-level brittle faults show more discrete fracture planes (e.g. the Ramgarh thrust). Rocks with young matrix monazite ages would be expected above the youngest, southernmost thrust planes of the Main Central Thrust. Restoration of the Main Central Thrust system A generalized restoration of the Main Central Thrust system in Nepal is shown in Figure 7. Late Proterozoic rocks extend from the Lesser Himalaya to the Greater Himalayan Sequence and across to the base of the Tethyan Himalaya (Haimanta Group– Cheka Formation). Unmetamorphosed Nawakot Group sedimentary rocks in the Lesser Himalaya pass north into the same protolith age rocks which have been metamorphosed to greenschist–upper amphibolite facies in the Ramgarh thrust sheet (Beyssac et al. 2004), up to kyanite grade in the Kathmandu thrust sheet (Johnson et al. 2001), and finally into high-grade sillimanite gneisses in the Dadeldhura and Greater Himalayan Sequence thrust sheets in the high Himalaya. In the internal parts of the Greater Himalayan Sequence, the sillimanite gneisses (commonly referred to as Greater Himalayan Sequence Formation 1; Colchen et al. 1986) are metamorphosed equivalents of the same late Proterozoic protoliths. As there are no suture zone rocks along the Main Central Thrust and the stratigraphy is continuous across the restored thrusts, there cannot be a Palaeozoic suture zone along the Main Central Thrust (DeCelles et al. 2000; Richards et al. 2005). Instead, the restored Lesser– Greater–Tethyan Himalaya was a contiguous continental section (Searle 1986; Myrow et al. 2003; Steck 2003; Searle et al. 2006). One major implication of the restoration of the Himalaya is that the Main Central Thrust follows a flat for a long distance across strike. This flat follows a rheologically weak horizon along the Neoproterozoic shales. True Indian basement rocks (Archaean–Lower Proterozoic) are never exposed in the Himalaya. Although impossible to determine accurately because of the high degree of ductile strain, minimum crustal shortening estimates from the Himalaya are between 500 and 900 km (Searle 1986; DeCelles et al. 2001; Robinson et al. 2006). Because the across-strike width of the basement and cover (Neoproterozoic–Eocene rocks exposed in the Himalaya) must balance on the restoration, it must be concluded that Indian basement has underthrust northwards beneath the Himalaya north of the Main Central Thrust, and beneath the southern margin of Asia (Lhasa block) by a similar distance across strike. Very large-scale subhorizontal detachments such as the Main Central Thrust therefore can play a major role in the mechanical decoupling of the crust. In the Himalaya, the Main Central Thrust and the South Tibetan Detachment effectively decouple the upper (Tethyan Himalaya), middle (Greater Himalaya) and lower (Indian basement) crust. The Main Central Thrust, like all major thrust faults, changes in style in both the horizontal and vertical planes (Fig. 8), as well as through time. In deep crustal profiles it is a 1–3 km thick ductile shear zone in kyanite-grade metamorphic rocks. U–Th– Pb monazite ages of these rocks range between c. 24 and 18 Ma (for a review, see Godin et al. 2006). At higher structural levels ductile shearing passes up into brittle thrust faulting along a discrete thrust plane. Many of these shallower levels follow flats placing similar age or younger rocks over similar age or older rocks (Fig. 7). In more southerly, outboard profiles such as at the Kathmandu complex, the Main Central Thrust (Mahabharat thrust) ramps up to higher levels where metamorphism in the hanging wall is right-way-up (Fig. 8). Here, a new, younger brittle thrust (Ramgarh thrust) developed in the footwall of the Mahabharat thrust, placing greenschist-grade rocks over unmetamorphosed Lesser Himalayan rocks (Fig. 8). In the Darjeeling area, a late out-of-sequence breakback Main Central Thrust developed behind the Darjeeling klippe (Searle & Szulc 2005). All these ductile shear zones and brittle thrust faults are part of the Main Central Thrust system, which developed over a period .14 Ma (from c. 24 to 10 Ma), from depths equivalent to at least 10–12 kbar (35–40 km) to the surface. Because the inverted metamorphic gradient resulted from post-metamorphic shearing, the observed metamorphic gradient should not be directly compared with the geotherm (Searle & Rex 1989; Bollinger et al. 2004). Conclusions None of the criteria commonly used for mapping the Main Central Thrust are valid in themselves for defining the Main Central Thrust. Lithology, detrital zircon U–Pb ages, and Nd isotope signatures reveal information about provenance and
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