THE ROOF OF THE WORLD IS PASSIVE: REINTERPRETING THE SOUTH TIBET DETACHMENT AS A MIOCENE PASSIVE ROOF FAULT IN A CRUSTAL-SCALE FAULT-BEND FOLD SYSTEM

The Miocene South Tibet Detachment (STD) has been interpreted to result from crustal thinning due to gravitational collapse of thick Himalayan crust. Because the STD and Main Central Thrust (MCT) were coeval, the Greater Himalayan Crystalline Complex between the two structures has been regarded as an extruding wedge. Although this model explains key relationships in the central Himalaya, it is inconsistent with geology in the western and eastern Himalaya. First, the STD in NW India merges with the MCT and forms a leading-edge rather than trailing-edge branch line. Second, the STD near the eastern Himalayan syntaxis is linked with the Miocene north-dipping Great Counter Thrust (GCT) instead of the MCT. Third, the continuity of geology in the Zanskar region suggests that the north-dipping 130-km-long Zanskar shear zone (=STD) and the south-dipping Warwan-Kilar-Miyar thrust system are parts of the same fault zone which defines the 130 x 60 km Zanskar window. Had the Zanskar shear zone been a Cordilleran-style extensional detachment, it would require the presence of a major breakaway fault accommodating at least 60 km of top-north slip north of the MCT. Because such a structure is lacking, the STD may be alternatively interpreted as a passive-roof fault in a fault-bend-fold system. This system was created during coeval motion along the MCT, STD, and GCT at about 22-9 Ma. In this model, the STD locally exhibits extensional-fault geometry, but it merges in its up-dip direction with the MCT and links with the GCT in its down-dip direction. The passive-roof fault model has the following implications. (1) Crustal thickening rather than thinning occurred in the Himalaya during motion on the STD at ~22-9 Ma. (2) Exposure of high-grade metamorphic rocks in the Himalaya mainly resulted from erosion rather than tectonic denudation. (3) No lower crustal extrusion from Tibet is required, which is consistent with the lack of Gangdese batholith rocks in the Himalaya. (4) The passive roof fault (STD) is present in the Lesser Himalayan crystalline nappes separating the Tethyan strata above from the Greater Himalayan Crystalline Complex below. This model may explain the common observations of continuous metamorphic grades across the STD, thrusts using the STD as their roof, and the late appearance of high-grade metamorphic clasts in the Himalayan foreland basin.