Exhumation history of a shear zone constrained by microstructural and fluid inclusion techniques: an example from the Satluj valley, NW Himalaya, India
Introduction
The Himalayan mountain range has formed by collision and continued convergence of the Indian plate with the Eurasian plate since late Paleocene or early Eocene time. This continued convergence is being accommodated along the foreland propagating thrust zones, namely the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT) from north to south (Fig. 1 inset). These thrusts structurally divide the Himalaya into the Higher Himalaya, the Lesser Himalaya and the Outer Himalaya with the MCT, MBT and HFT as their base thrusts, respectively. The MCT is equivalent to the Vaikrita Thrust (MC(V)T; Valdiya, 1980) and ∼10 km thick highly deformed footwall zone called the MCT zone with the Jutogh (Munsiari) Thrust (J(M)T) at its base in the NW Himalaya. The MC(V)T was active at ∼26 to 23 Ma (Hubbard and Harrison, 1989) at mid-crustal level (>25 km) and has continued until ∼7 Ma in the MCT zone (Harrison et al., 1997). The MBT was formed at shallower level, ∼10 to 12 km, around 10 Ma (Meigs et al., 1995). The HFT is the active thrust system that separates the Outer Himalaya from the flat alluvial plains. Several out-of-sequence thrusts in the hanging wall of the MBT have been mapped in Garhwal and Nepal (Srivastava and Mitra, 1994, Schelling, 1992) and the occurrence of intermediate intensity earthquakes and microseismicity (Thakur et al., 2000 and references therein) in the region suggests that strain is also being accommodated by the thrusts in the hinterland of the active deformation front of Himalaya. The Jakhri Thrust Zone (JTZ) is one such out-of-sequence thrust in the Lesser Himalayan zone and is best exposed in the Satluj valley of NW Himalaya (Fig. 1). Fission track data on apatite and zircon from the hanging wall of the JTZ suggest that it has been active during the past 4.5 myr (Jain et al., 2000), which is younger than the age of MBT (Meigs et al., 1995). The partial annealing temperatures of fission tracks in zircon and apatite are significantly lower than the peak metamorphic temperatures in the hanging wall of JTZ (i.e. ∼610 °C in the amphibolite facies) (Vannay et al., 1999). Therefore the data of Jain et al. (2000) probably dates the very late stages of activity along the JTZ.
In the present study, we investigate the exhumation history of the JTZ using microstructure and fluid inclusion studies. Fluid inclusion analyses have been conducted only on the footwall quartzite as it preserves the deformation history after peak deformation (and metamorphism). Microthermometry has been conducted only on fluid inclusion trails that show a genetic relationship with microstructures. The occurrence of fluid inclusion trails (2D analogue of fluid inclusion plane) suggests that the rocks may behave in a brittle manner even under conditions of ductile deformation due to strain incompatibility and sliding between neighboring grains (Boullier, 1999). Many fluid inclusion studies (Pêcher, 1981, Pêcher et al., 1985, Lespinasse and Pêcher, 1986, Cathelineau et al., 1990, Boullier et al., 1991, Boullier, 1999) have demonstrated that fluid inclusion trails are related to specific deformation events and therefore are useful in tectonic studies. Systematic correlation of structural fabric with the fluid inclusion trails has yielded useful information regarding fault kinematics and upper crustal response to mid-crustal deformation (Selverstone et al., 1995). Sauniac and Touret, 1983, Craw, 1990, Boullier et al., 1991, Sachan et al., 2001 have correlated fluid inclusions with exhumation history of gneissic rocks of the Higher Himalaya. However, a major problem with fluid inclusions in these studies is the uncertainty due to leakage, recrystallization and the time of fluid entrapment.
Section snippets
Geological setting
The Satluj river provides a natural cross-section through the Kulu-Rampur window in the Lesser Himalaya (Fig. 1) where the Lesser Himalayan Crystalline nappes viz. Chail and Jutogh (Munsiari) nappes tectonically overlie the parautochthonous Rampur Group and Wangtu Gneissic Complex (WGC). The Rampur Group consists mainly of quartzite and penecontemporaneous volcanics/metavolcanic rocks (Fig. 2). The metavolcanic rocks have yielded an 1800±13 Ma U–Pb Zircon crystallization age (Miller et al., 2000
Structures in the Jakhri Thrust Zone
Two sets of structures with different deformation patterns and timing have been observed in the hanging wall of the JTZ. The ductile shearing with top-to-SW shearing marks the thrust related fabric, which is followed by a ductile and brittle–ductile apparent tensional regime with top-to-NE shearing. The top-to-SW ductile shearing related to the thrusting is documented in the form of σ-type asymmetric strain shadows around feldspar porphyroclasts (Fig. 4a) with preferred alignment of micas, S–C
Fluid inclusion study
The fluid inclusion plane (FIP) represents the fluids entrapped in the healed microcracks formed during deformation. The orientations of these FIP have been used as reliable paleostress indicators (Lespinasse and Pêcher, 1986, Boullier et al., 1991, Selverstone et al., 1995). Their orientation patterns within a rock may correspond to different phases of deformation the rock has suffered. With this background, the fluid inclusion trails (2D analogue of FIP) in the X–Z section (parallel to
Evolution of the microstructure in the JTZ
Microstructures in a shear zone reflect the kinematics involved in their evolution. The hanging wall of the JTZ displays a mylonitic fabric with dominant southwestward shear. Exhumation of the hanging wall along the JTZ will also affect the footwall with progressively decreasing intensity of deformation away from the thrust. The progression in microstructures can be observed in the form of dynamic recrystallization, shape-preferred alignment of mineral grains and grain size in the JTZ. The
Conclusions
The field evidence across the active JTZ show a mismatch in the macroscopic fold pattern, but the stretching lineation and kinematic indicators suggest a common top-to-SW shear sense. Contrary to previous interpretations, the JTZ is a SW propagating thrust which crosscuts the folded Jutogh (Munsiari) nappe and therefore represents a subsequent structure. The quartz recrystallization microstructures, aided by fluid inclusion microthermometry, suggest lower greenschist facies conditions during
Acknowledgements
The manuscript is greatly improved by the constructive comments and suggestions of Prof J.-P. Burg, Drs B. Grasemann, D. Craw and M. Murphy (journal's referee) on the earlier version of the manuscript. AKP acknowledges the financial support from CSIR, India in the form of research fellowship (Grant no. 9/420(10)/94-EMR.I). The HKS thanks Prof R. J. Bodnar for providing facility to carry out Fluid inclusion microthermometry at Virginia Polytechnic Institute, Virginia, USA.
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