Crustal Shortenning and tectonics of the NW Himalaya from GPS measurements
Needless to say, the 2500km long Himalayan convergence arc is much more complicated, and needs more wide coverage of GPS geodetic measurements to bring out the along-the-arc variations, as well as more details of the convergence geometry and mechanism. We carried out GPS measurements covering a major portion of the western Himalayan sector between 75°E to 80°E. This covers the entire Kangra reentrant and DehraDun reentrant. Out of the four great earthquakes (1897, 1905, 1934 and 1950) that have struck the Himalayan foothills over the last 100 years, the 1905 Kangra event (Middlemiss, 1910, Molnar, 1987) has produced nearly 100 km long rupture zone in HP and possibly another smaller rupture in Dehra Dun region. The region between 1905 Kangra and 1934 Bihar events, the region under present study, has been identified as a seismic gap where no major earthquakes have occurred during last 200-300 years (Bilham et al 1995, Khattri, 1987,Seeber et al, 1981, Yeats et al 1992).
An important step in using GPS geodetic techniques for India-Tibet convergence studies came from the Nepal GPS geodetic campaigns. Bilham et al, (1997) and Kristine et al (1999) used six years of GPS data in Nepal Himalayas to constrain maximum surface contraction rate to 18±2 mm/yr with a corresponding slip rate of nearly 20mm/yr. They also reported an eastward motion between Lhasa and Nepal at nearly 11 mm/yr, thus confirming that lateral extrusion is indeed taking place to accommodate the indenting rigid Indian plate through strike-slip faulting (Molnar and Tapponier, 1995). Burgmann et al,( 1999) proposed a segmented fault model for the Nepal Himalaya suggesting along-the-arc variation in the convergence process. The fault models for the east and west Nepal dipping at different angles (3-8°), shows locking at different depths (15-25 km). At least 500 km long stretch of the fault system with an width of nearly 140km were said to be locked, accumulating 6-15m of potential slip, which would eventually be released through a future great earthquake.
Isoseismals of 1905 Kangra event (Middlemiss, 1910) indicate two zones of strong ground shaking, one with maximum intensity=X around Kangra region, and another with max. intensity of VII around DehraDun region, leaving the in between Simla sector relatively unharmed. The preferential rupturing during the 1905 Kangra earthquake, reported upliftment of Dehra Dun region (Gehlaut and Chandra,1997), the sinuous nature of the Main Boundary thrust, varying thickness of the Sub-Himalayan ranges and the very existence of the Kangra and Dehra Dun re-entrants, intervened by a lateral basement ramp (Powers et al, 1998), presence of evaporites in some parts of HP, transverse alignment of MBT-PT in Kulu-Bilaspur region and several transverse lineaments picked up in satellite imagery perhaps suggests segmentation of Himalayan frontal belt into different blocks. It would be interesting to know if such segmentation exists and if exist, how different blocks respond to the stress developed due to the Indian plate movement and how the built-up energy is distributed between aseismic creep and seismic events. In the region covering both the Kangra and Dehra Dun re-entrants, we established two permanent stations and nearly 50 episodic GPS stations (Fig.2). In this paper, we have presented results from repeat GPS measurements covering both the region, including sites in the Siwaliks, lesser Himalaya as well as Higher Himalaya, since 1996 till date. Through model inversion of the GPS data, we have explained the distribution of velocity vectors through a three-segment dislocation model, representing the three adjacent Himalayan segments, viz. The Kangra reentrant, the Simla block, and the DehraDun reentrant. These three blocks are detached from one another by strike slip movement, and have different configuration of the detachment surface.
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