The Karla Tectonic Unit (KTU): A Blueschist Shear Zone Juxtaposed with the Ultra-High Pressure Tso Morari Dome (TMD), Eastern Ladakh, India (Himalaya)

The KTU is a newly recognized 8 sq.km tectonic unit separating the TMD and the Zildat Ophiolitic Melange of the Indus Suture Zone (ISZ) southwest of the villages of Sumdo and Puga Sumdo. Blueschist in the ISZ in both eastern and western Ladakh records the Late Cretaceous subduction of the NeoTethys whereas the KTU blueschist is linked to the subduction of the TMD (Indian Plate) by the Karla Fault, the boundary between the KTU and TMD. The Zildat Fault, a late-stage normal fault, hides the Karla Fault and the KTU except in the study area. The northeastern boundary of the KTU is not exposed but is likely a subduction fault that brought the KTU beneath the ISZ.

From an assumed position of minus 15 km, the vertical downward displacements from the original positions of the TMD and the KTU are estimated to be 115 and 40 km, respectively. By adopting the Leech (2005) subduction model ---lithosphere bends and subduction zones are curved--- and assuming a radius of 350 km for the bend, the subduction displacements can be estimated: 210 km for the subduction of the KTU beneath the ISZ and 85 km for the TMD beneath the KTU. Exhumation of the KTU and the northeastern TMD may have taken place by shear and slip in reverse direction, and by slip along the Zildat Fault.

The TMD was heated during its slow crustal exhumation at mid-crustal depths after an initial rapid ascent from the upper mantle. Petrologic evidence shows that the KTU did not experience the same late-stage heating event: mechanical coupling of the KTU with the TMD along the Karla Fault during initial stages of exhumation of the TMD can explain this lack of heating: the KTU had already been exhumed to the upper crust.