SPATIAL AND TEMPORAL VARIATIONS IN DEFORMATION MECHANISMS AND KINEMATICS OF DOMINANT THRUSTS FROM HINTERLAND OF MOUNTAIN BELTS: A CASE STUDY FROM THE MAIN CENTRAL THRUST (MCT) AND THE PELLING THRUST, SIKKIM HIMALAYAN FOLD THRUST BELT

To examine spatial and temporal variations in deformation mechanisms and kinematic evolution of dominant thrusts from the hinterland of the Himalayan fold thrust belt (FTB), we analyze fault rocks from structurally higher Main Central thrust (MCT) and its immediate footwall, the Pelling thrust (PT), from the eastern Sikkim Himalaya. Both these faults have mineralogically similar quartzo-feldspathic gneissic protolith. True thicknesses of the amphibolite facies MCT and the greenschist facies PT zones are ~1170m and ~938m, respectively. Based on minimum displacements vs. thicknesses of these two fault zones, we infer the MCT and the PT to be strain softened. The average mylonitic foliation, defined by preferential orientations of recrystallized quartz, muscovite, feldspar and micas, trends 41°, 057° in the MCT, and 32°, 020° along the PT zone. Asymmetric feldspar porphyroclasts indicate top-to-the-south shearing in both the zones that is consistent with the regional transport-direction. Based on recrystallized quartz grain-sizes, we estimated temperatures of ~435°-510°C and ~ 405°-425°C for the MCT and the PT zones, respectively.

The MCT protolith, Kanchenjunga gneiss, records ~70% grain-size reduction in quartz and ~55% in feldspar within the MCT zone. The Paro gneiss records ~85% grain-size reduction in quartz and ~70% in feldspar within the PT zone. Quartz dominantly deformed by dislocation creep, and feldspar by microfracturing. Fracturing is ~10% higher in the structurally higher MCT than in the PT zone, while feldspar grains record lower evidence of pressure solution in the MCT (~1-3% of all grains) than in the PT zone (~17%-20%). Thus, the deformation mechanisms vary spatially. Penetrative plastic strain, estimated from relict quartz and feldspar grains, record stronger flattening strain (~k=0.12-0.14) in the MCT zone than in the PT zone (k=0.34-0.36). Based on microstructural data, we infer pressure solution and microfracturing have accommodated additional finite strain. We estimated W_k from two incremental strain markers: (a) oblique-grain-shape fabric and (b) subgrains. The results reveal that the W_k varied spatially and temporally indicating non-steady deformation in both the fault zones.