Foreland thrusting and slab formation in the Pamir

The western and northern sectors of the northward convex Pamir arc are underlain by a steep Benioff zone dipping east to south, traced by earthquakes to depths of 250 km in the southwest and 150 km in the northeast. This slab has been interpreted to indicate intracontinental subduction. However, the convergence accommodated in thrust belts around the western and northern Pamir margins seems to fall short of the values required to produce the observed slab lengths. Delamination models in which the slab only consists of Asian mantle lithosphere avoid that problem but predict shallow asthenosphere beneath the Pamir, conflicting with geophysical evidence. This contradiction is resolved in a forced delamination scenario (Kufner et al. 2016) where indenting/underplating Indian lithosphere forces down and immediately replaces the delaminating Asian lithosphere. In this scenario the formation of the slab would be largely accommodated by south-directed thrust imbrication at crustal level, unrelated to substantial north-vergent thrusting in the Pamir.

Based on published and our own analyses of foreland thrusting we propose that the formation of the slab does to some extent reflect shortening in the Pamir thrust belts. Thin-skinned shortening in the Tajik basin and the External Pamir further north and east decreases northeastward from 150 to 75 and 30(?) km. The slab lengths show a similar trend. Interpreted mimimum shortening values correspond to 60-50 (20?) percent of the slab length on the same transect. With crustal and lithospheric thicknesses taken from seismological data, 70 km of shortening on a translithospheric thrust fault are sufficient to subduct mafic lower crust to asthenospheric depth and probably induce eclogite formation. Rather than the comparison with slab lengths alone, which may be biased by low estimates of shortening, geometrical relations call for additional slab delamination and rollback towards the foreland. The sedimentary cover stacked in the thin-skinned belts restores to at least tens of km of across-strike (N-S) width, underlain by a subhorizontal to gently dipping basal décollement. Basement-involving faults on the internal borders of the thin-skinned belts such as the Darvaz fault and Main Pamir thrust (MPT) must merge with or flatten into this décollement and thus cannot directly connect to the present-day updip end of the slab via a steeply dipping fault. We hypothesize that the Pamir slab was initiated by a translithospheric thrust fault (MPT and equivalents) around 20 Ma and owes at least half of its length to displacement on these faults and imbrication of the sedimentary cover in their footwalls. Delamination and rollback lengthened the slab and displaced it north- and westward. Mantle lithosphere, not necessarily of Indian affinity, contemporaneously moved in from the southeast, preventing the opening of a lithospheric gap and upwelling of asthenosphere.

 

Reference:

Kufner, S. K. et al. (2016). Deep India meets deep Asia: Lithospheric indentation, delamination and break-off under Pamir and Hindu Kush (Central Asia). Earth and Planetary Science Letters, 435, 171-184.