Jurassic invertebrates from the Himalayan Central Gneiss

Poorly preserved scleractinian corals and cephalopod molluscs occur in metamorphosed and foliated limestones in the Himalayas of northwest India. The nautiloid Cenoceras and macrocephalitid ammonites indicate a Middle Jurassic age. The fossils occur within a zone of high grade metamorphic rocks known as the Central Gneiss, previously regarded as an axial massif of Precambrian basement. Most palaeogeographic reconstructions feature this zone as a prominent topographic arch or ‘Himalayan Ridge’ which arose in the late Precambrian or early Palaeozoic and, through periodic rejuvenation, persisted throughout the Phanero-zoic. Many workers believe that the ‘Himalayan Ridge’ constituted an effective barrier to north-south faunal migration because the terrains of sedimentary rock which flank the Central Gneiss contain quite different faunas. The Jurassic fossils provide new stratigraphic evidence that the tectonic events which generated the Central Gneiss are relatively young, and this, together with structural considerations, precludes the emergence of a ‘Himalayan Ridge’ prior to this time. Other evidence shows that the Central Gneiss was generated in the Tertiary, and suggests that the faunal contrast between the Lesser and Tethyan Himalayas reflects a tectonic juxtaposition of dissimilar sedimentary terrains that were previously separated by many hundreds of kilometres.