Crustal Shortenning and tectonics of the NW Himalaya from GPS measurements
Paramesh Banerjee
Wadia Institute of Himalayan Geology, Dehra Dun, India
Abstract
Data from two permanent GPS station and more than 25 campaign mode GPS stations in the NW Himalayan region, were processed. The region under study covers the seismic gap area between the 1905 Kangra earthquake and 1934 Bihar earthquake. GPS sites distributed over the Higher Himalaya as well as in the Lesser Himalaya show crustal shortening is taking place at nearly 15-20mm/yr rate within a narrow zone of 100-150 km north of the Himalayan Frontal Fault. Model inversion of the GPS data shows that nearly 50km length of the frontal thrust system is locked with the advancing India plate along the detachment surface, at depths of 6-10km. Kangra, Simla and DehraDun region along the NW Himalayan front show lateral variation in configuration and geometry of the decollement surface. Both the Kangra and Dehra Dun blocks offer greater resistence to the convergence process and are accumulating strain through the locked portion of the detachment surface, which will eventually be released through future earthquakes. The in-between Nahan block slides relatively smoothly over the detachment surface, and shows only 10 km length of locking, thus have less potential for big earthquakes.
Introduction
The topography, geologic structure and the earthquakes of the Himalaya are a consequence of the collision and subsequent convergence of the northward moving Indian plate with the Eurasian plate. Since late Cretaceous time, an estimated 2000-3000 km of convergence is believed to have taken place (Molnar and Tapponier, 1977). Current rate of the Indian plate has been estimated to be around 50mm/yr (Demetes et al, 1994; Freymuller et al, 1996; Larsen et al, 1999). There is a broad agreement that at least 25-30% of the total convergence is being accommodated within the Himalayan arc. Most of the estimations of crustal shortening happening within the Himalaya are from various geological and seismic evidences (Leathers 1987, Baker et al, 1988, Molnar and Deng, 1984, Armijo et al, 1986, Molnar and Lyon Caen 1989, Avouvac et all, 1998) range between 8-18mm/yr (Powers et al, 1998). Recent GPS measurements in Nepal Himalaya (Bilham et al, 1997, Larsen et al, 1999, Burgmann et all, 1999) have produced a rate of 18mm/yr of surface contraction taking place across the Nepal Himalaya. Our GPS measurement (discussed in this paper) from the western Himalayan region have brought out contraction rate of 14-21mm/yr between Higher Himalayan sites and Bangalore (South India), and a lower rate of 7-14mm/yr between these sites and Delhi, the northern tip of the Indian shield (Fig.1).
Fig.1 Surface relief map of the Indian subcontinent
showing velocity vectors (relative to IISC, Bangalore) of three permanent GPS
stations aloong the Himalayan arc (Nadi, WIH2, and NAGA). Also shown the IGS
sites used as fiducial stations for the current GPS data processing.
Representative sites of Higher Himalaya and Lesser Himalaya shows the amount of
shortening happening along the Himalayan arc in the study area.
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