VARIOUS TYPES OF OLISTOSTROMES IN A CLOSING OCEAN-BASIN, TETHYAN HIMALAYA (CRETACEOUS, TIBET)

The Cretaceous rocks of the central Tethyan Himalayas display differen t types of deep-sea olistostromes and chaotic deposits which are accom panied by turbidites. The deposits were incorporated into an accretion ary prism and more or less deformed. According to the composition and provenance of their clasts, we distinguish four types of olistostromes : (1) P-type, derived from the passive (Indian) continental margin; (2 ) PO-type, derived from both the passive Indian margin and ocean floor (pelagic sediments and oceanic crust); (3) POA-type with clasts from the same sources as the PO-type, but also containing material from the active margin along the Lhasa block; and (4) A- and OA-type, mainly d erived from the active margin (accretionary prism and magmatic are). T he occurrence of these different types of deep-sea chaotic deposits in space and time is closely related to the evolution of the Neo-Tethyan basin from a wide open ocean basin to a narrowing remnant and trench basin prior to collision. Whereas pure P-type olistostromes formed whe n the basin was still wide, predominating A-type deposits characterize a late stage of basin evolution prior to collision. Mixed-type olisto stromes record an intermediate stage of basin evolution and require sp ecific syn-sedimentary tectonic activity.