Magnetic fabrics indicating Late Quaternary seismicity in the Himalayan foothills

Abstract

The anisotropy of magnetic susceptibility (AMS) study was performed on soft sediment samples from a trenched fault zone across the Himalayan frontal thrust (HFT), western Himalaya. AMS orientation of K min axes in the trench sediments is consistent with lateral shortening revealed by geometry of deformed regional structures and recent earthquakes. Well-defined vertical magnetic foliation parallel to the flexure cleavage in which a vertical magnetic lineation is developed, high anisotropy, and triaxial ellipsoids suggest large overprinting of earthquake-related fabrics. The AMS data suggest a gradual variation from layer parallel shortening (LPS) at a distance from the fault trace to a simple shear fabric close to the fault trace. An abrupt change in the shortening direction (K min ) from NE-SW to E-W suggests a juxtaposition of pre-existing layer parallel shortening fabric, and bendingrelated flexure associated with an earthquake. Hence the orientation pattern of magnetic susceptibility axes helps in identifying co-seismic structures in Late Holocene surface sediments.

Faulting Processes Unveiled by Magnetic Properties of Fault Rocks

“…In the Dead Sea Basin (Israel), one of the most active tectonic areas in the Middle East, for instance, host sediments appear to be characterized by oblate AMS ellipsoids with vertical axes of the minimum magnetic susceptibility ( κ min ) that indicate a normal sedimentary magnetic fabric, whereas the liquefied sediments are characterized by triaxial fabrics with subvertical κ int axes and subhorizontal κ max axes parallel to the dike strike (Levi et al, 2006a). Similar AMS fabrics have been reported by Jayangondaperumal et al (2010) in the Himalayan frontal thrust (in the western Himalaya), by Lakshmi et al (2017) in the Dauki fault (Shillong Plateau, India), and by Cho et al (2017) in the Dadaepo Basin (SE Korea). These contrasting magnetic fabrics have provided crucial evidence for seismically triggered fluidization of clastic materials.…”

Near-Surface Expression of Early to Late Holocene Displacement along the Northeastern Himalayan Frontal Thrust at Marbang Korong Creek, Arunachal Pradesh, India

“…The dipping beds of unit 2 are thus interpreted to represent the dip panel of a fold related to displacement on an underlying thrust fault that extends to near the surface and the normal fault strands near the scarp crest to reflect extension from bending that has produced the dip panel (e.g. Kumar et al, 2006;Jayangondaperumal et al, 2010). The fine sands of units 3 and 4 are interpreted to be sediments that accumulated as flat-lying beds (growth strata) along the scarp from periodic flooding of adjacent creeks and rivers.…”