A kinematic approach for investigating magnetic and strain fabrics from constrictional and flattening domains of shear zones in Sikkim Himalayan fold thrust belt

Abstract

We investigate the relationships among magnetic, mylonitic and strain fabrics from the Pelling-Munsiari thrust (PT), a major shear zone in the Sikkim Himalaya, that records varying mylonitic fabrics and strain geometry along its transport direction. The fabric changes from an SL-tectonite with oblate strain ellipsoids in its main exposures, to a local, transport-parallel L-tectonite with prolate strain ellipsoids, forming a synformal klippen. Magnetic fabric is related to the PT deformation, and is affected by the local folding of the klippen. The bearings of magnetic and stretching mineral lineation vary more than the orientations of the planar fabrics. Irrespective of their strain geometry, most of the fault rocks, including SL mylonites of the structurally overlying Main Central thrust, record oblate susceptibility ellipsoids. The paramagnetic phases tracked an incremental deformation stage that froze as oblate susceptibility ellipsoid in the hinterlandmost fault rocks, and was not realigned by successive higher magnitude, constrictional strain, associated with a local, lateral ramp, followed by flattening strain in the forelandmost exposure during translation of the PT. Additionally, strain partitioning between quartz, tracking the finite strain fabric, and micas primarily contributing to the magnetic fabric explain the deviations in the studied fabrics in the fault rocks.