Exhumation of the ultrahigh‐pressure Tso Morari unit in eastern Ladakh (NW Himalaya): A case study

Abstract

[1] Exhumation processes of the ultrahigh-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 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. D2 is characterized by blueschist mineral associations, and corresponds to the quasi-isothermal decompression from a depth of 90 km 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 during the Oligocene. 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.

No need for lithospheric extension for exhuming (U)HP rocks by normal faulting

“…Warren et al (2008) addressed the exhumation of eclogites of the Tso Morari dome in the Himalaya and argued for coeval normal and thrust faulting closely linked to UHP exhumation processes. As in the examples from the Eastern Alps and the Aegean Sea given above, there is no evidence for any extensional basins that developed during considerable and fast exhumation of the Tso Morari eclogites (de Sigoyer et al 2004). …”

“…The exhumation of most (ultra)high-pressure ((U)HP) complexes occurs soon after these rocks were metamorphosed following lithospheric convergence and deep underthrusting during early orogenic stages (Reddy et al 1999;Gessner et al 2001;de Sigoyer et al 2004). It appears that the tectonic community has agreed that normal faulting resulting from lithospheric extension is the most important agent for structurally accomplishing the bulk of the exhumation of these rocks from great depths (Platt 1986).…”

Subduction undone

“…This inference was challenged in dramatic fashion by the discovery of 'ultrahigh-pressure' (UHP) mineral assemblages in exposed continental rocks in the western Alps 1 and the Scandinavian Caledonides 2 . Since then, UHP assemblages have been documented in many mountain systems 3 -some of the best examples are found in the Tso Morari region of the northwest Indian Himalaya [4][5][6] (Fig. 1).…”

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