Exhumation of the ultra high-pressure Tso Morari unit in eastern Ladakh (NW Himalaya): a case study.

摘要

Exhumation processes of the ultra-high pressure (UHP) Tso Morari dome (NW-Himalaya) are investigated using structural, petrological and geochronological data. The UHP Tso Morari unit is bounded by the low-grade metamorphic Indus Suture Zone to the NE and Mata unit to the SW. Three deformation phases (D1, D2 and D3) are observed. Only D3 is common to the UHP unit and the surrounding units. In the UHP unit, the first deformation phase (D1) produced upright folds, under eclogitic conditions (> 20 kbar; 580 ± 60 °C). D1 is overprinted by D2 structures related to a NW-SE trending open anticline. This phase is characterized by blueschist mineral associations, and corresponds to the quasi-isothermal decompression from a depth of 90 km (eclogitic conditions) up to 30-40 km. The final exhumation phase of the Tso Morari unit is dominated by tectonic denudation and erosion (D3), associated with a slight temperature increase. Radiochronological analyses indicate that the UHP exhumation process began during the Eocene. Exhumation was fast during D1-D2 and slowed down through D3 in Oligocene time. The change in the deformation style from D1-D2 to D3 in the Tso Morari unit coincides with changes in the exhumation rates and in the metamorphic conditions. These changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge, to the vertical uplift of the Tso Morari unit across the upper crust.