Tethyan subducted slabs under India

Tomographic imaging of the mantle under Tibet, India and the adjacent India n Ocean reveals several zones of relatively high P-wave velocities at vario us depths. Under the Hindu Kush region in northeastern Afghanistan and sout hern Tajikistan, a regional northward-dipping slab is seen in the entire up per 600 km of the mantle and is apparently still attached to the lithospher e of the Indian plate. Under northern Pakistan this same slab shows a roll- over structure with the deeper portion overturned and dipping southward, as can also be seen in the distribution of earthquake hypocenters. Farther ea st-southeast (e.g., in the vicinity of Nepal), a well-resolved anomaly belo w 450 km depth is connected to the slab under the Hindu Kush, but seems to be separated from the lithosphere above 350 km. These upper-mantle anomalie s are interpreted as the remnants of delaminated sub-continental lithospher e that went down when Greater India continued to converge northward with As ia after similar to 45 Ma. The deeper high-velocity anomalies under the Ind ian sub-continent appear clearly separated from the shallower ones as well as from each other, and are inferred to represent remnants of oceanic litho spheric slabs that have sunk into the lower mantle and were subsequently ov erridden by the Indian plate. They occur at depths between 1000 and 2300 km and occasionally descend down to the core-mantle boundary The anomalies fo rm three parallel WNW-ESE striking zones. We interpret the two southern zon es as remnants of oceanic lithosphere that was subducted when the Neo-Tethy s Ocean closed between India and Tibet in the Cretaceous and earliest Terti ary. The northern deep-mantle zone under northern Afghanistan, the Himalaya s and the Lhasa block in southern Tibet may represent the last-subducted re mnant of the Paleo-Tethys Ocean, which is thought to have closed before the Hauterivian stage of the Early Cretaceous. The middle zone continues south eastward as a rather straight high-velocity zone towards Sumatra, where it becomes convex southward and parallel to the subduction zone under the Sund a are. Comparison of this straight middle zone near India with the shallowe r (upper 600-1000 km) northern zone, which displays a cusp-like shape near the Yunnan (SW China) Syntaxis of the eastern Himalayas, supports the notio n that the shallow northern zone represents later subduction than the deepe r middle zone. The suggestion of a counterclockwise rotation (>20 degrees) of the Indian plate during Tertiary indentation of Asia is supported by the se features. The present-day latitudes of 5 degrees-35 degrees N of the dee p slabs under India and adjacent areas correspond to the approximate paleol atitudes of the Cretaceous subduction zones. The slab remnants in the middl e mantle occur therefore near the ancient locations where they started thei r downward journey, which implies that lateral movements in the deeper mant le were not large. (C) 1999 Elsevier Science B.V. All rights reserved.