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Karakoram and Afghanistan |
Dipartimento di Geologia, Paleontologia e Geofisica Universitá di Padova, Via Giotto 1, 35137 Padova, Italy
Two new gravimetric profiles have been obtained in the Karakoram as part of the activities of the Ev-K2-CNR Project. One profile, established in 1988, starts from Bazar Dara (China), north of the Aghil range, and covers the portion of the Shaksgam valley between the confluence of the Urdok-Gasherbrum glaciers to the SE and, along the Shaksgam river, Tek ri-Sughet Jangal to the NW. The second profile, carried out in 1990, starts from Skardu and ends in Gilgit via the Biafo Hispar glaciers. During both expeditions a number of astrogeodetic stations were established for the measurement of the deflection of the vertical, using the satellites of the Global Positioning System (GPS) to determine geodetic reference coordinates. Two astronomic stations were established in 1988 and fourteen in 1990. The new data, when added to those collected by Marussi and co-workers from 1954 to 1979, improve the coverage north of the Main Karakoram Thrust, where the Bouguer anomalies attain their largest negative values. For all available data Bouguer and Airy isostatic corrections have been computed using the ETOPO5 height grid. A mean crust density of 2.85 g cm–3, a density contrast of 0.3 g cm–3 and a normal thickness T = 30 km have been assumed, so that the surface of isostatic compensation locally best fits the Moho. Two profiles have been selected, one continuing northward of the Nanga Parbat — Haramosh massif the deep seismic sounding profiles done in the 1970s. The other is such that it crosses the Indus Suture Zone, the Main Karakoram Thrust and the Karakoram Fault nearly at right angles. In both profiles, from Pakistan to China, the horizontal gradients of the Bouguer anomalies are in the interval –1.5 to –2 mgal km–1, comparable to gradients found in the region between the Indo-Gangetic Plains and the Himalaya. A marked steepening of the Moho is implied here with increased detail. Isostatic anomalies underneath the Karakoram are found to be more negative than those computed in the earlier work of Marussi, who made different assumptions of density and density contrast. Clearly the new data do not uniquely constrain the local structure of the lithosphere but may be of help in better understanding non-negligible deviations from isostatic equilibrium.
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  A. Caporali The gravity field of the Karakoram Mountain Range and surrounding areas Geological Society, London, Special Publications, 2000; 170: 7 - 23. [Abstract] [PDF]
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