GEODYNAMICS, DEFORMATION AND MATHEMATICAL ANALYSIS OF METAMORPHIC BELTS, NW HIMALAYA

Abstract

Intra-continental collision between the Indian- and the Eurasian Plates since ~ 55 Ma remobilized the Indian crust and resulted in the exhumation of different parts of the mountain in various episodes. The Higher Himalayan Shear Zone (HHSZ), a part of the Himalayan Metamorphic Belt, was deformed and exhumed as a delayed response to the post-collisional crustal shortening. The ductile extensional shearing at the top of the HHSZ took place simultaneously with the ductile compressional shearing at the lower boundary of this shear zone. Based on structural geology, mathematical analyses and analogue modelling, models of exhumation of the HHSZ are proposed taking the Zanskar- and the Sutlej sections of the shear zone in the NW Indian Himalaya as the study areas. Additionally, flanking structures in micro-scales from the Sutlej section of the HHSZ are studied and their compatibility with the proposed exhumation model is addressed. Thin-section studies of the rocks of the Zanskar Shear Zone (ZSZ) reveal (a) an initial top-to-SW sense of ductile shearing; (b) subsequent top-to-NE sense of ductile shear coeval with the ongoing top-to-SW sense of ductile shearing; (c) top-to-NE sense of ductile synthetic secondary shearing; (d) brittle-ductile extension; (e) top-to-SW sense of brittle shear; (f) northeasterly steeply dipping brittle faults; and (g) brittle extension. A two-phase model of exhumation of the HHSZ is proposed. The first phase represented top-to-SW sense of non-coaxial shearing same as the Couette flow during the Neo- Himalayan Period. The second phase was guided by combined top-to-SW sense of simple n shear and the channel flow during the Middle Miocene Period. This simulated a thin ZSZ characterized by a top-to-NE sense of ductile shearing. Variation in the ratio of relative velocity of the boundaries of the HHSZ to the pressure gradient explains the variable thickness of the ZSZ. Fieldwork and micro-structural studies of the HHSZ in the Sutlej section reveal (a) initial top-to-SW sense of ductile shearing; (b) late stage top-to-NE sense of ductile shearing in two zones- the 'Himalayan Detachment-1' (HD1) and the 'Himalayan Detachment-2' (HD2); (c) uniform top-to-SW sense of brittle shearing; and (d) brittle-ductile extension. A three-phase model of exhumation of the HHSZ is proposed. The first phase represented top-to-SW sense of non-coaxial shearing, similar to what is proposed for the Zanskar section of the HHSZ, during the Neo-Himalayan Period. The second phase was guided by combined top-to-SW sense of simple shear and channel flow in two pulses. One of the pulses took place throughout the HHSZ during the Middle Miocene Period; the other pulse was restricted within the lower sub-channel. Thin HD1 and HD2 were produced during the respective pulsed flows. The third phase of exhumation is idealized by brittle slip of markers in a top-to-SW sense. The channel flow component had differentially acted in various sections of the HHSZ. For example, while a single pulse of channel flow is deciphered from the Zanskar section; two distinct spatially and temporally separated pulses acted in the Sutlej section. The models of exhumation of the HHSZ for both these sections predict that (i) the base of in the ZSZ (or the HD1) exhumed with highest rate; and (ii) the ZSZ (or the HD1) might be missing in certain sections of the HHSZ even if channel flow was active. The exhumation mechanism of the HHSZ is studied with 10 analogue models of channel flow initiating from a horizontal channel and extrusion through a linked inclined channel. The inclined channel is the model HHSZ and is of parallel, gently diverging-up and strongly diverging-up geometries in different considerations. In these experiments, Polydimethylsiloxane (PDMS) is used as the model material and only geometric similarity is maintained with the prototype. Six flow zones are deciphered in the two channels. The flow zone-4, formed nearly at the middle of the inclined channel, reveals a single zone of ductile extensional shearing similar to that in the South Tibetan Detachment System (or the HD1). This indicates that, in certain sections of the HHSZ, exhumation of the HHSZ took place by channel flow in two distinct pulses and gave rise to two ductile extensional shear zones, even when non-parallel geometry of the walls of the inclined channel is considered. A blind ductile secondary thrust, formed in zone-4, rotates while moving up and finally crops to the surface. In Sutlej section of the HHSZ, this thrust may be correlated with the Chaura Thrust with the recorded activation at least 13 Ma after the ongoing extrusion of the HHSZ by channel flow mechanism 18 Ma ago. The analogue models generate intrafolial folds within the ductile extensional shear zone, which is similar to the field observations from the detachments from different sections of the HHSZ. Ductile sheared rocks of the HHSZ in Sutlej section outside the HDland the HD2, in IV micro-scale, reveal 'microflanking structures' (MFS) defined by the nucleated minerals (the crosscutting elements- CEs), and deflected cleavages and grain margins (the host fabric elements- HEs). Depending on whether the drag of HEs across the CE is different or same, the MFS are grouped into the Type-(i) and the Type-(ii) varieties, respectively. The Type-(ii) MFS indicates directional growth of the CE. The Type-(i) MFS indicates non-coaxial shearing, where the external HEs bounding the CEs act as the C-plains, and that the CE minerals undergo crystal-plastic deformation. Salient morphological variations in the MFS are: (1) variable intensity and senses of drag along the single- and the opposite CE margins; (2) HE defined only at one of the sides of the CE; and (3) presence of thin hazy zone at the HE-CE contact. The HEs are dragged even in absence of rheological softening at the CE boundaries. The Type-(i) MFS from the HHSZ reveals top-to-SW sense of shearing, which matches with other shear sense indicators. This also supports a channel flow model of exhumation of the studied shear zone. Rheological possibilities, except for a weaker CE mineral within a stronger host grain, have been encountered in the present study, and are represented in a graph of its constitutive equation.