Kinematic, thermal and petrological model of the Himalayas: constraints related to metamorphism within the underthrust Indian crust and topographic elevation
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文摘
The maximum average elevation of the Alpine-Himalayan belt does not exceed 2.5 km from the Alps to the Zagros but abruptly increases to 5 km in the High Himalayas. The limitation of the elevation to 2.5 km can be explained if eclogitization occurs below 55 km from the Alps to the Zagros. The 5-km elevation of the High Himalayas is due to the presence of crustal-like densities to 75 km and this presence requires a mechanism to prevent the eclogitization to occur above 75 km there. Thermal models are constructed assuming a geometry based on the INDEPTH seismic profile, which shows underthrusting of the whole Indian crust below the crustal wedge of the High Himalayas, and possibly as far as the Indus-Tsangpo suture. Assuming a rate of underthrusting of about 15 mm/yr on a 10° dip fault plane, undeformed Indian crust had already reached the Indus-Tsangpo suture at the end of the main phase of thrusting along the Main Central Thrust (MCT) 20 myr ago. Consequently, it is possible to estimate the current isotherms in the Himalayan belt from a single thrust model. The high temperatures and partial melting observed in the Higher Himalayan Crystallines (HHC) imply that similar conditions should occur within the upper crustal wedge. Our models show that these conditions at the base of the upper crustal wedge can be obtained by the conjunction of a thick upper crustal pile (with radioactive heat production 2 × 10−6 W/m3) and a high erosion rate (1 mm/yr). The absence of eclogitization above 75 km within the underthrust Indian lower crust can be explained either if the temperature is higher than 800°C (in the granulite field) or lower than 500-550°C, within the blueschist field. Because the fast rate of underthrusting of the lower crust prevents its heating, the temperature there cannot reach values in excess of 800°C. On the other hand, with reasonable thermal parameters, temperatures of less than 500-550°C may prevail. In this case, the 5-km elevation can be reproduced if the transformation in blueschist is inhibited in the lower crust, presumably because of low water availability. We consequently propose that the combination of a relatively cold underthrust Precambrian crust and of a fast rate of underthrusting prevents eclogitization above 75 km and enables the mountain belt to reach altitudes of 5 km. Thus the main difference between the Zagros and the Himalayas would be the critically slower rate of underthrusting in the Zagros compared to the Himalayas.