The GOCE observations for detecting the full extent of the crust to mantle section brought to the surface in the Kohistan Arc, Himalaya

The aim of this work is to use the GOCE observations in a geologically unique part of the Himalayan orogen, which is the Kohistan arc. Here the entire section of the crust from the mantle-crust transition, to the volcanic deposits in the upper crust are exposed. The presence of high density mafic and ultramafic rocks at shallow levels presumably generate a positive gravity signal sensed by GOCE. The Kohistan arc is at the limit of resolution of the GOCE gravity field, due to its relatively small dimensions, as it is 180km wide and 800km long. The GOCE data give the first comprehensive gravity field over this complex, as terrestrial gravity data exist, but do not cover the entire regional extent. The study considers two different scales of the problem: the first, on a more regional scale, is used to determine the crustal thickness contribution to the GOCE field through the isostatic crustal thickness model and spectral gravity inversion. Here the spectral inversion and the flexural model define the general parameters of the earth crust in terms of minimum wavelength due to the Moho and the flexural rigidity of the different geodynamic regions inside the Himalaya Karakorum complex. We find that the isostatic model explains the long-wavelength Bouguer anomalies very well. The residual field is then used to study the more local density inhomogeneities tied to the Kohistan complex. We start from the geological map and from the gravity residual and model the thickness of the main geological units of the Kohistan Arc Complex. The results show that the main anomalies found, which were initially only associated to the high-density masses in the area, do not actually follow the borders of these outcropping masses and therefore must be also associated to deep heterogeneity and to other submerged bodies. These are in fact eclogites with a 2 km thickness located between the Kohistan unit and the underlying Indian plate; their presence had been proposed by petrologic and geologic considerations, but had not been verified by geophysical modeling. Moreover the residual shows us that the Kohistan arc extends further east than what is seen on the surface.