Implications of shortening in the Himalayan fold‐thrust belt for uplift of the Tibetan Plateau

Abstract

Recent research in the Himalayan fold‐thrust belt provides two new sets of observations that are crucial to understanding the evolution of the Himalayan‐Tibetan orogenic system. First, U‐Pb zircon ages and Sm‐Nd isotopic studies demonstrate that (1) Greater Himalayan medium‐ to high‐grade metasedimentary rocks are much younger than true Indian cratonic basement; and (2) these rocks were tectonically mobilized and consolidated with the northern margin of Gondwana during early Paleozoic orogenic activity. These observations require that Greater Himalayan rocks be treated as supracrustal material in restorations of the Himalayan fold‐thrust belt, rather than as Indian cratonic basement. In turn, this implies the existence of Greater Himalayan lower crust that is not exposed anywhere in the fold‐thrust belt. Second, a regional compilation of shortening estimates along the Himalayan arc from Pakistan to Sikkim reveals that (1) total minimum shortening in the fold‐thrust belt is up to ∼670 km; (2) total shortening is greatest in western Nepal and northern India, near the apex of the Himalayan salient; and (3) the amount of Himalayan shortening is equal to the present width of the Tibetan Plateau measured in an arc‐normal direction north of the Indus‐Yalu suture zone. This information suggests that a slab of Greater Indian lower crust (composed of both Indian cratonic lower crust and Greater Himalayan lower crust) with a north‐south length of ∼700 km may have been inserted beneath the Tibetan crust during the Cenozoic orogeny. We present a modified version of the crustal underthrusting model for Himalayan‐Tibetan orogenesis that integrates surface geological data, recent results of mantle tomographic studies, and broadband seismic studies of the crust and upper mantle beneath the Tibetan Plateau. Previous studies have shown that incremental Mesozoic and early Cenozoic shortening had probably thickened Tibetan crust to ∼45–55 km before the onset of the main Cenozoic orogenic event. Thus, the insertion of a slab of Greater Indian lower crust with maximum thickness of ∼20 km (tapering northward) could explain the Cenozoic uplift of the Tibetan Plateau. The need for Tibetan crust to stretch laterally as the Greater Indian lower crust was inserted may explain the widespread east‐west extension in the southern half of the Plateau. Our reconstruction of the Himalayan fold‐thrust belt suggests that Indian cratonic lower crust, of presumed mafic composition and high strength, should extend approximately halfway across the Tibetan Plateau, to the Banggong suture. From there northward, we predict that the Tibetan Plateau is underlain by more felsic, and therefore weaker, lower crust of Greater Himalayan affinity. Two slab break‐off events are predicted by the model: the first involved Neotethyan oceanic lithosphere that foundered ∼45–35 Ma, and the second consisted of Greater Indian lithosphere (most likely composed of Greater Himalayan material) that delaminated and foundered ∼20–10 Ma. Asthenospheric upwelling associated with th...

Crustal–lithospheric structure and continental extrusion of Tibet

“…At about 18 km depth a horizontal band of dense ophiolite has been imaged beneath the Indus-Yarlung suture zone (Makovsky et al 1999). Because at least 300 km of shortening has occurred across the upper crust Cambrian to Eocene rocks of the Tethyan Himalaya in the Himalaya to the south (Corfield & Searle 2000), and between 500-700 km of shortening across the Lesser-Greater Himalaya (DeCelles et al 2002;Robinson 2008), simple balancing requires a similar amount of underthrusting to the north of Indian lower crust comprising Precambrian granulites of the Indian Shield. As the rocks of the lower crust of India underthrust the Himalaya and southern margin of Tibet they extend to depths of c. 60-80 km and enter the high-pressure granulite or eclogite fields.…”

Geometry of the Pamir‐Hindu Kush intermediate‐depth earthquake zone from local seismic data

“…In such a model, the Indian slab would have had to be lost in order to make space for subduction of Eurasian lithosphere. A major break-off of the subducted Indian lithosphere has been suggested to have occurred 10-25 Ma ago [Maheo et al, 2002;Chemenda et al, 2000;DeCelles et al, 2002].…”

Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia

“…Composite balanced cross-sections across the Himalaya documented approximately 500-900 km of shortening (19,20). These estimates are the sum of (i) shortening in the Tibetan Himalaya (since the Paleogene); (ii) the amount of overlap between the Greater Himalaya and the Lesser Himalaya along the Main Central thrust that was largely established between approximately 25-20 and 15-10 Ma; and (iii) the amount of shortening within the Lesser Himalaya since approximately 15-10 Ma (2,19,20).…”

“…These estimates are the sum of (i) shortening in the Tibetan Himalaya (since the Paleogene); (ii) the amount of overlap between the Greater Himalaya and the Lesser Himalaya along the Main Central thrust that was largely established between approximately 25-20 and 15-10 Ma; and (iii) the amount of shortening within the Lesser Himalaya since approximately 15-10 Ma (2,19,20). The main uncertainty in the Himalayan shortening estimates are associated with the Greater Himalaya.…”