Onset Of Continental Collision Research Articles

Cretaceous and Paleogene boundary strata in southern Tibet and their implication for the India‐Eurasia collision

Recent stratigraphic studies in southern Tibet provide new information about the timing of the initial collision between the India and Eurasia continental blocks. The stratigraphic and paleontological evidence document dramatic changes in sedimentary facies and microfauna content across the Cretaceous-Paleogene (K/Pg) boundary. In the Zhongba and Gamba areas in southern Tibet, the K/Pg boundary is marked by a major disconformity, separating platform carbonates from overlying terrigenous conglomerates and sandstones. The stratigraphy of the boundary sequences has recently been improved with the recognition of three foraminiferal assemblages. They are: Maastrichtian Orbitoides-Omphalocyclus, Danian Rotalia-Smoutina-Lockhartia and Thanetian Miscellanea-Daviesina microfaunal assemblages. The K/Pg boundary at the Gamba area is placed between the Orbitoides-Omphalocyclus and Rotalia-Smoutina-Lockhartia faunas. In Tingri, Cretaceous Globotruncana and tertiary Globigerina-Globorotalia microfauna demark the position of the K/Pg boundary. The occurrence of terrigenous sandstones and boulder-size conglomerates in the early Paleocene is compelling evidence for tectonic uplift and emergence of the southern margin of the Lhasa block and of the northern margin of the Indian plate. Therefore, supported by biostratigraphic evidence, we argue that the uplift is caused by the onset of continental collision during the earliest Danian. Progressing plate collision resulted in deformation and fragmentation of the Paleocene carbonate platform and deposition of limestone breccias, which we consider as further evidence for tectonic compression as a result of early continental collision during the Thanetian, earlier than indicated by previous studies in the Himalayas. It is the change in the sedimentary facies and depositional environment that provides the earliest evidence and dating of the initiation of the collision process. From studies of sedimentary strata in southern Tibet, the collision of the India and Lhasa continental blocks was initiated at ∼K/Pg boundary time (∼65Ma). If that is the case, than the major lithofacies changes at the K/Pg boundary observed in the western Tethys, mostly referred to as intrinsic to the eustatic sea level change, has been driven by continental convergence and collision of the Indian and Eurasian plates.

Magmatic constraints on geodynamic models of subduction in the East Carpathians, Romania

The East Carpathian volcanic arc is the youngest region of calc-alkaline magmatic activity in Eastern Europe. A general age progression of the onset and cessation of magmatic activity occurs along the East Carpathian arc from older volcanic structures (ca. 12 Ma) in the NW to the youngest (<1 Ma) in the SE. Magmatism continued into the Plio-Pleistocene, significantly later than the end of basin closure and the onset of continental collision along the Inner Carpathian arc that is thought to have taken place during the Miocene (9–5 Ma). Migration of magmatic activity from NW to SE along the arc may be explained by a corresponding migration of the magma-generating zone at mantle depths. Major and trace element characteristics of the erupted products are typical of subduction-related magmas and suggest an input of fluids from a dehydrating subducting slab into their mantle source region. Subduction of a narrow oceanic basin is considered to be the most probable cause of the East Carpathian magmatism and its migration. As thick continental crust began to enter the northern part of the trench at around 9 Ma, slab breakoff began although subduction of the detached slab continued at depth. As breakoff progressed from north to south, a rupture or tear propagated along the slab, causing termination of volcanism as the slab sank out of the magma-generation zone. Breakoff of the slab occurred at progressively shallower levels, southward along the arc, causing the volume of erupted arc magmas to diminish. Some unusual geological features at the southern end of the volcanic arc (e.g. contemporaneous eruption of alkaline and calc-alkaline magmas; extreme enrichment in K and other large ion lithophile elements in the arc magmas) may be accounted for by asthenospheric mantle upwelling into the void left behind by the broken slab and increased efficiency of dehydration of the remnants of the slab under the higher thermal regime.

Subduction of Asian lithospheric mantle beneath Tibet inferred from models of continental collision

THE relative northward motion of the Indian subcontinent that followed the onset of continental collision with Asia has produced extensive deformation of the Earth's crust, giving rise to the world's highest mountains in the Himalayan chain and the world's largest high-elevation region, the Tibetan plateau. The formation of the broad mountain belt implies that, contrary to the original tenets of plate tectonics, the lithospheric plates have experienced widespread deformation far from the plate boundary1. Several models have been proposed2–6 to explain the manner in which this post-collisional deformation is distributed within the continental lithosphere of the Indian and Asian plates. Here we propose an alternative model in which subduction of the Asian lithospheric mantle develops following the collision of India. Our model is supported by numerical calculations of crustal deformation and thickening, and is consistent with available geological and geophysical data7–9. This picture suggests that lithospheric mantle is not deformed along with the crust, and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed.