ALBERT

Description: The Cretaceous and Palaeogene sediments of the basins in Central Asia include the remnants of the easternmost extent of a vast shallow epicontinental sea, which extended across the Eurasian continent before it retreated westwards and eventually isolated as the Paratethys Sea. To improve understanding of its long-term palaeogeographical evolution, we complement the well-constrained chronological framework of the Tarim Basin in China with stratigraphic records of the sea retreat from the Fergana Basin and the Alai Valley Basin in southern Kyrgyzstan and the Afghan–Tajik Basin in SW Tajikistan. By lithostratigraphic analyses and identification of bivalve assemblages, this study establishes for the first time a clear and detailed regional correlation of Palaeogene marine strata across Central Asia, showing that the basins share a similar palaeogeographical evolution characterized by a long-term stepwise retreat punctuated by short-term shallow-marine incursions. Our correlation shows that the last two marine incursions recognized in the Tarim Basin can be traced westwards. The permanent disappearance of the sea from Central Asia probably occurred with limited diachroneity in the late Eocene, before the isolation of the Paratethys Sea, shifting the easternmost margin of the sea hundreds of kilometres westwards and probably significantly reducing moisture supply to the Asian interior.

Description: We recently presented a compilation of paleomagnetic data arguing for Cretaceous extension within Greater India. These data imply that a Tibetan Himalayan (TH) microcontinent rifted away from India, opening an oceanic Greater India Basin (GIB) in its wake. Consequently, we postulated a two-stage India-Asia collision at ∼52 and 25–20 Ma...

Description: Cenozoic convergence between the Indian and Asian plates produced the archetypical continental collision zone comprising the Himalaya mountain belt and the Tibetan Plateau. How and where India–Asia convergence was accommodated after collision at or before 52 Ma remains a long-standing controversy. Since 52 Ma, the two plates have converged up to 3,600 ± 35 km, yet the upper crustal shortening documented from the geological record of Asia and the Himalaya is up to approximately 2,350-km less. Here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic Indian lithosphere within the Himalaya between approximately 50 and 25 Ma. Paleomagnetic data show that this extended continental and oceanic “Greater India” promontory resulted from 2,675 ± 700 km of North–South extension between 120 and 70 Ma, accommodated between the Tibetan Himalaya and cratonic India. We suggest that the approximately 50 Ma “India”–Asia collision was a collision of a Tibetan-Himalayan microcontinent with Asia, followed by subduction of the largely oceanic Greater India Basin along a subduction zone at the location of the Greater Himalaya. The “hard” India–Asia collision with thicker and contiguous Indian continental lithosphere occurred around 25–20 Ma. This hard collision is coincident with far-field deformation in central Asia and rapid exhumation of Greater Himalaya crystalline rocks, and may be linked to intensification of the Asian monsoon system. This two-stage collision between India and Asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography.

Description: 〈span〉The geological history of the Burmese subduction margin, where India obliquely subducts below Indochina, remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader Tibetan-Himalayan orogen. Various scenarios for the evolution of the orogen have been proposed, including a collision of India with Myanmar in the Paleogene, a significant extrusion of Myanmar and Indochina from the India-Asia collision zone, or very little change in paleogeography and subduction regime since the India-Asia collision. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene–Oligocene times to reconstruct the evolution of the Burmese margin during the early stages of the India-Asia collision. We report on sedimentological, geochemical, petrographical, and geochronological data from the Chindwin Basin—the northern part of the Burmese forearc—and integrate these results with previous data from other basins in central Myanmar.Our results show that the Burmese margin acted as a regular Andean-type subduction margin until the late middle Eocene, with a forearc basin that was open to the trench and fed by the denudation of the Andean volcanic arc to the east. We show that the modern tectonic configuration of central Myanmar formed 39–37 million years ago, when the Burmese margin shifted from an Andean-type margin to a hyper-oblique margin. The forearc basin was quickly partitioned into individual pull-apart basins, bounded to the west by a quickly emerged accretionary prism, and to the east by synchronously exhumed basement rocks, including coeval high-grade metamorphics. We interpret this shift as resulting from the onset of strike-slip deformation on the subduction margin leading to the formation of a paleo-sliver plate, with a paleo fault system in the accretionary prism, pull-apart basins in the forearc, and another paleo fault system in the backarc. This evolution implies that hyper-oblique convergence below the Burmese margin is at least twice older than previously thought. Our results reject any India-Asia convergence scenario involving an early Paleogene collision of India with Myanmar. In contrast, our results validate conservative geodynamic models arguing for a close-to-modern pre-collisional paleogeometry for the Indochina Peninsula, and indicate that any post-collisional rotation of Indochina, if it occurred at all, must have been achieved by the late middle Eocene.〈/span〉

Description: A systematic bias towards low palaeomagnetic inclination recorded in clastic sediments, that is, inclination shallowing, has been recognized and studied for decades. Identification, understanding and correction of this inclination shallowing are critical for palaeogeographic reconstructions, particularly those used in climate models and to date collisional events in convergent orogenic systems, such as those surrounding the Neotethys. Here we report palaeomagnetic inclinations from the sedimentary Eocene upper Linzizong Group of Southern Tibet that are ~20° lower than conformable underlying volcanic units. At face value, the palaeomagnetic results from these sedimentary rocks suggest the southern margin of Asia was located ~10°N, which is inconsistent with recent reviews of the palaeolatitude of Southern Tibet. We apply two different correction methods to estimate the magnitude of inclination shallowing independently from the volcanics. The mean inclination is corrected from 20.5° to 40.0° within 95 per cent confidence limits between 33.1° and 49.5° by the elongation/inclination ( E / I ) correction method; an anisotropy-based inclination correction method steepens the mean inclination to 41.3 ± 3.3° after a curve fitting- determined particle anisotropy of 1.39 is applied. These corrected inclinations are statistically indistinguishable from the well-determined 40.3 ± 4.5º mean inclination of the underlying volcanic rocks that provides an independent check on the validity of these correction methods. Our results show that inclination shallowing in sedimentary rocks can be corrected. Careful inspection of stratigraphic variations of rock magnetic properties and remanence anisotropy suggests shallowing was caused mainly by a combination of syn- and post-depositional processes such as particle imbrication and sedimentary compaction that vary in importance throughout the section. Palaeolatitudes calculated from palaeomagnetic directions from Eocene sedimentary rocks of the upper Linzizong Group that have corrected for inclination shallowing are consistent with palaeolatitude history of the Lhasa terrane, and suggest that the India–Asia collision began at ~20°N by 45–55 Ma.

Description: The exhumation history of the central Himalaya is well documented, but lateral variations in exhumation remain poorly constrained. In this study, we identify sediment source areas and examine the late Neogene exhumation history of the eastern Himalaya from the synorogenic sedimentary record of its foreland basin. We present Nd and Hf isotopic data as well as apatite and zircon fission-track analyses from the Miocene–Pliocene Siwalik Group along the recently dated Kameng River section in Arunachal Pradesh, northeastern India. Our isotopic data show that Siwalik Group sediments deposited between 13–7 and 〈2.6 Ma in Arunachal Pradesh were mainly derived from Higher Himalayan source rocks. In contrast, sediments deposited between ca. 7 and 3 Ma have far less negative Nd and Hf values that require involvement of the Gangdese Batholith and Yarlung suture zone source areas via the Brahmaputra River system. Consequently, these sediments should also record incision of the Namche Barwa massif by this river. Source-area exhumation rates of Himalayan-derived sediments, determined from detrital zircon fission-track data, were on the order of 1.8 km/m.y. in the fastest-exhuming areas. These rates are very similar to those calculated for the central Himalaya and have been relatively constant since ca. 13 Ma. Our results do not support the hypothesis of a major change in exhumation rate linked to either local or regional climate change or to Shillong Plateau uplift during the Miocene, as reported elsewhere. The zircon fission-track data further suggest that exhumation of the Namche Barwa massif between 7 and 3 Ma was much slower than the ~10 km/m.y. rate recorded in the recent past. Detrital apatite fission-track data indicate deformation of the Siwaliks due to forward propagation of the frontal thrust since around 1 Ma.

Description: 〈span〉〈div〉Abstract〈/div〉The geological history of the Burmese subduction margin, where India obliquely subducts below Indochina, remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader Tibetan-Himalayan orogen. Various scenarios for the evolution of the orogen have been proposed, including a collision of India with Myanmar in the Paleogene, a significant extrusion of Myanmar and Indochina from the India-Asia collision zone, or very little change in paleogeography and subduction regime since the India-Asia collision. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene–Oligocene times to reconstruct the evolution of the Burmese margin during the early stages of the India-Asia collision. We report on sedimentological, geochemical, petrographical, and geochronological data from the Chindwin Basin—the northern part of the Burmese forearc—and integrate these results with previous data from other basins in central Myanmar.Our results show that the Burmese margin acted as a regular Andean-type subduction margin until the late middle Eocene, with a forearc basin that was open to the trench and fed by the denudation of the Andean volcanic arc to the east. We show that the modern tectonic configuration of central Myanmar formed 39–37 million years ago, when the Burmese margin shifted from an Andean-type margin to a hyper-oblique margin. The forearc basin was quickly partitioned into individual pull-apart basins, bounded to the west by a quickly emerged accretionary prism, and to the east by synchronously exhumed basement rocks, including coeval high-grade metamorphics. We interpret this shift as resulting from the onset of strike-slip deformation on the subduction margin leading to the formation of a paleo-sliver plate, with a paleo fault system in the accretionary prism, pull-apart basins in the forearc, and another paleo fault system in the backarc. This evolution implies that hyper-oblique convergence below the Burmese margin is at least twice older than previously thought. Our results reject any India-Asia convergence scenario involving an early Paleogene collision of India with Myanmar. In contrast, our results validate conservative geodynamic models arguing for a close-to-modern pre-collisional paleogeometry for the Indochina Peninsula, and indicate that any post-collisional rotation of Indochina, if it occurred at all, must have been achieved by the late middle Eocene.〈/span〉

Description: The Cretaceous and Palaeogene sediments of the basins in Central Asia include the remnants of the easternmost extent of a vast shallow epicontinental sea, which extended across the Eurasian continent before it retreated westwards and eventually isolated as the Paratethys Sea. To improve understanding of its long-term palaeogeographical evolution, we complement the well-constrained chronological framework of the Tarim Basin in China with stratigraphic records of the sea retreat from the Fergana Basin and the Alai Valley Basin in southern Kyrgyzstan and the Afghan–Tajik Basin in SW Tajikistan. By lithostratigraphic analyses and identification of bivalve assemblages, this study establishes for the first time a clear and detailed regional correlation of Palaeogene marine strata across Central Asia, showing that the basins share a similar palaeogeographical evolution characterized by a long-term stepwise retreat punctuated by short-term shallow-marine incursions. Our correlation shows that the last two marine incursions recognized in the Tarim Basin can be traced westwards. The permanent disappearance of the sea from Central Asia probably occurred with limited diachroneity in the late Eocene, before the isolation of the Paratethys Sea, shifting the easternmost margin of the sea hundreds of kilometres westwards and probably significantly reducing moisture supply to the Asian interior.