Quantitative Interpretation of Geomagnetic Induction Response Across the Thrust Zones of the Himalaya along the Ganga-Yamuna Valley.

Abstract

Geomagnetic variations, recorded through a two-phase magnetovariational study carried out along the Ganga-Yamuna valley of the Garhwal Himalaya, northwest India, are reduced to a set of induction arrows spanning a period range of 12-128 minutes. The spatial behaviour of induction response indicates that the Main Frontal Thrust is a major electrical discontinuity with enhanced conductivity to the south, beneath the Indo-Gangetic plains. Simple two-dimensional (2-D) geoelectrical models with geophysical constraints on the thickness and resistivity of the sedimentary sequences indicate that the induced currents in the Indo-Gangetic plains contribute little to the observed induction response.A full 2-D electrical resistivity model which reproduces the observed electromagnetic response, essentially requires a highly conducting layer at mid-crustal depth beneath the Indo-Gangetic Plains, becoming less conducting on underthrusting beneath the frontal Himalayan belt. This layer coincides with the brittle-to-ductile transition zone along which lie the foci of most moderate earthquakes.

Crustal structure in the Siwalik Himalayas using magnetotelluric studies

“…From the Geomagnetic depth sounding (GDS) studies, Reddy and Arora (1993) have reported a high conductivity layer embedded in the crust at depths varying from 10-15 km below the Himalayan collision region. They have noted that the real induction vectors have maximum length near the MFT and become smaller towards the MBT on the NE and the Indo-Gangetic plains on the SW, indicating a large conductance for this conductive layer below MFT.…”

An Integral Sign Conductor in the Frontal Himalaya Region and Its Tectonic Interpretation

“…The northward directed arrows at the northern sites of the UP-West Nepal profile suggests the presence of a conducting zone beneath the higher Himalaya (Pandey and Arora, 1997). The THC and some other anomalies whose induction response were found to be compatible with 2-D structure, have already been modelled using 2-D formulation earlier (Arora, 1990;Reddy and Arora, 1993 sufficient to explain 3-D conductivity distribution in the region. As the 3-D modelling is unwieldy, thin-sheet approximation (3-D) of variable conductance is considered suitable to provide unified picture of near surface 3-D conductivity distribution.…”

“…3 and 4 indicates that the region west of ISC has more complex electrical conductivity distribution than that of the east. On the west, prominent induction anomalies include two high conducting zones parallel to the strike of the Himalaya, which correspond to the two hump-shaped anomalies in 2-D model developed by Reddy and Arora (1993). These two conducting zones together with a localized and narrow conducting zone coincident with the GLHSB (Reddy and Arora, 1992a, b) collectively seen as furrowed conducting zones running approximately parallel to the MFT and MCT (Fig.…”

“…The comparison of the observed and calculated pattern is carried out by directly superposing the observed arrow pattern (without arrow heads) on calculated arrow (with arrow heads) pattern. For better qualitative comparison, the computed responses of 3-D thin sheet as well as 2-D (Reddy and Arora, 1993) models are compared with the observed response along a profile AA' (Fig. 2) and presented in Fig.…”

“…The initial assignment of conductance to each cell was based on the 2-D electrical resistivity model developed by (Reddy and Arora, 1993) and lithological character of the rock types exposed at the surface. Care was also exercised that conductances assigned were such that the stipulated thin-sheet conditions (Weaver, 1982) satisfied in each cell of the anomalous domain.…”