THE EVOLUTION OF THE SOUTHERN PASSIVE MARGIN OF THE HIMALAYAN TETHYS: MODEL IMPLICATIONS AND CONSTRAINTS ‐ I

Abstract

To outline lthe process of formation of the Tethys Ocean and its developemt from teh Late Carboniferous (Hercynian) epeirogenic phase until teh end of the Cretaceous, (when this ocean started vanishing), an earlier proposed model, involving lithospheric uplift, stretching and developmetn f anastomosing normal faults, which was suggested to explain the thermotectonic evolution of the lKashmir basin, is extended to cover the region from Kashmir to Kempa Dzong (NE Silkkim).Basement subsidence curves show an overall exponential trend in agreement with teh common behaviour of passive margins. The anomalour Late Jurassic ‐ early Creataceous uplift of the marigin is difficult to explain wiht the available data; however, it seems likely to be related to the very restricted ocean‐floor spreading phase in early Late Creataceous time. The well preserved continental margin suggests a width of <500 km for the Tethys Ocean.The model and the inferred limited width of the Tethys Ocean have several implications for the evolution of the himalayas. For example: (1) the Main Central Thrust and Main Boundary Thrust are reactivated normal listric faults, which reversed their direction fo move, emt during the closing phase of the Tenthys Ocean; (2) the gravity high along the entire Higher Himalayas is due to the presence of transitional‐zone basic material, intruded during the rift phase; and (3) the limited width fo the Tethys Ocena does not support alleged large‐scale subduction along this belt.Finally, a few date (known and model‐derived) and conceptual constraints are pointed out. These must be considered in the evolultion of the Himalayas in general, and before in voing subduction in the Himalayas in particular. They include: the time required for passive margins to change to fracture boundaries; the north ward movement of India indicated by paleomagnetic data; th sinistral movement between norhtern and soutehrn continents; the p0osition of the Ladakh batholity within the Indus‐Tsansgpo Suture Zone; the 235+‐13Ma†age of a section fo this batholith; the occurrence of radiolarian chert within underformed beds; the suggested involvement of the Earth's rotational forces in plate‐tectonic driving mechanisms; and the possibility of many changes in teh spin velopcity of the Earth during the lifetime of Tethys.