Refine
Has Fulltext
- no (14)
Document Type
- Article (14)
Language
- English (14)
Is part of the Bibliography
- yes (14) (remove)
Keywords
- India-Asia collision (4)
- paleomagnetism (3)
- remagnetization (3)
- Cenozoic (2)
- Central Asia (2)
- Pamir (2)
- inclination shallowing (2)
- Asian monsoon (1)
- Cenozoic basins (1)
- Eastern Tibetan Plateau (1)
Institute
The timing and mechanisms of the Cretaceous sea incursions into Central Asia are still poorly constrained. We provide a new chronostratigraphic framework based on biostratigraphy and magnetostratigraphy together with detailed paleoenvironmental analyses of Cretaceous records of the proto-Paratethys Sea fluctuations in the Tajik and Tarim basins. The Early Cretaceous marine incursion in the western Tajik Basin was followed by major marine incursions during the Cenomanian (ca. 100 Ma) and Santonian (ca. 86 Ma) that reached far into the eastern Tajik and Tarim basins. These marine incursions were separated by a Turonian-Coniacian (ca. 92-86 Ma) regression. Basin-wide tectonic subsidence analyses imply that the Early Cretaceous sea incursion into the Tajik Basin was related to increased Pamir tectonism. We find that thrusting along the northern edge of the Pamir at ca. 130-90 Ma resulted in increased subsidence in a retro-arc basin setting. This tectonic event and coeval eustatic highstand resulted in the maximum observed geographic extent of the sea during the Cenomanian (ca. 100 Ma). The following Turonian-Coniacian (ca. 92-86 Ma) major regression, driven by eustasy, coincides with a sharp slowdown in tectonic subsidence during the late orogenic unloading period with limited thrusting. The Santonian (ca. 86 Ma) major sea incursion was likely controlled by eustasy as evidenced by the coeval fluctuations in the west Siberian Basin. An early Maastrichtian cooling (ca. 71-70 Ma), potentially connected to global Late Cretaceous trends, is inferred from the replacement of mollusk-rich limestones by bryozoan- and echinoderm-rich limestones.
The northward indentation of the Pamir salient into the Tarim basin at the western syntaxis of the India-Asia collision zone is the focus of controversial models linking lithospheric to surface and atmospheric processes. Here we report on tectonic events recorded in the most complete and best-dated sedimentary sequences from the western Tarim basin flanking the eastern Pamir (the Aertashi section), based on sedimentologic, provenance, and magnetostratigraphic analyses. Increased tectonic subsidence and a shift from marine to continental fluvio-deltaic deposition at 41Ma indicate that far-field deformation from the south started to affect the Tarim region. A sediment accumulation hiatus from 24.3 to 21.6Ma followed by deposition of proximal conglomerates is linked to fault propagation into the Tarim basin. From 21.6 to 15.0Ma, increasing accumulation rates of fining upward clastics is interpreted as the expression of a major dextral transtensional system linking the Kunlun to the Tian Shan ahead of the northward Pamir indentation. At 15.0Ma, the appearance of North Pamir-sourced conglomerates followed at 11Ma by Central Pamir-sourced volcanics coincides with a shift to E-W compression, clockwise vertical-axis rotations and the onset of growth strata associated with the activation of the local east vergent Qimugen thrust wedge. Together, this enables us to interpret that Pamir indentation into Tarim had started by 24.3Ma, reached the study location by 15.0Ma and had passed it by 11Ma, providing kinematic constraints on proposed tectonic models involving intracontinental subduction and delamination.
The establishment and evolution of the Asian monsoons and arid interior have been linked to uplift of the Tibetan Plateau, retreat of the inland proto-Paratethys Sea and global cooling during the Cenozoic. However, the respective role of these driving mechanisms remains poorly constrained. This is partly due to a lack of continental records covering the key Eocene epoch marked by the onset of Tibetan Plateau uplift, proto-Paratethys Sea incursions and long-term global cooling. In this study, we reconstruct paleoenvironments in the Xining Basin, NE Tibet, to show a long-term drying of the Asian continental interior from the early Eocene to the Oligocene. Superimposed on this trend are three alternations between arid mudflat and wetter saline lake intervals, which are interpreted to reflect atmospheric moisture fluctuations in the basin. We date these fluctuations using magnetostratigraphy and the radiometric age of an intercalated tuff layer. The first saline lake interval is tentatively constrained to the late Paleocene-early Eocene. The other two are firmly dated between similar to 46 Ma (top magnetochron C21n) and similar to 41 Ma (base C18r) and between similar to 40 Ma (base C18n) and similar to 37 Ma (top C17n). Remarkably, these phases correlate in time with highstands of the proto-Paratethys Sea. This strongly suggests that these sea incursions enhanced westerly moisture supply as far inland as the Xining Basin. We conclude that the proto-Paratethys Sea constituted a key driver of Asian climate and should be considered in model and proxy interpretations. (C) 2019 Elsevier B.V. All rights reserved.
Paleogeographic reconstructions of terranes can greatly benefit from the provenance analysis of sediments. A series of Cenozoic basins provide key sedimentary archives for investigating the growth of the Tibetan Plateau, yet the provenance of the sediments in these basins has never been constrained robustly. Here we report sedimentary petrological and detrital zircon geochronological data from the Paleocene-Eocene Nangqian-Xialaxiu and Gongjue basins. Sandstone detrital modes and zircon morphology suggest that the samples collected in these two basins were sourced from recycled orogen. Detrital zircon geochronology indicates that sediments in the Nangqian-Xialaxiu Basin are characterized by two distinct age populations at 220-280 Ma and 405-445 Ma. In contrast, three predominant age populations of 207-256 Ma, 423-445 Ma, and 1851-1868 Ma, and two subordinate age populations of similar to 50 Ma and similar to 2500 Ma, are recognized in the Gongjue Basin. Comparison with detrital zircon ages from the surrounding terranes suggests that sediments in the Nangqian-Xialaxiu Basin come from the neighboring thrust belts, whereas sediments from the Gongjue Basin are predominantly derived from the distant Songpan-Ganzi Terrane with minor contribution from the surrounding areas. A three-stage Cenozoic evolution of the eastern Tibetan Plateau is proposed. During the Paleocene, the Nangqian-Xialaxiu Basin appeared as a set of small intermontane sub-basins and received plentiful sediments from the neighboring mountain belts; during the Eocene, the Gongjue Basin kept a relatively low altitude and was a depression at the edge of a proto-Plateau; since the Oligocene, the Tibetan Plateau further uplifted and the marginal Gongjue Basin was involved in the Tibetan interior orogeny, indicating the eastward propagation of the Tibetan Plateau.
Carbonate rocks, widely used for paleomagnetically quantifying the drift history of the Gondwana derived continental blocks of the Tibetan Plateau and evolution of the Paleo/Meso/Neo-Tethys Oceans, are prone to pervasive remagnetization. Identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests. Here we apply comprehensive paleomagnetic, rock magnetic, and petrographic studies to upper Triassic limestones in the eastern Qiangtang block. Our results reveal that detrital/biogenic magnetite, which may carry the primary natural remanent magnetization (NRM), is rarely preserved in these rocks. In contrast, authigenic magnetite and hematite pseudomorphs after pyrite, and monoclinic pyrrhotite record three episodes of remagnetization. The earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite, probably related to the collision between the northeastern Tibetan Plateau and the Qiangtang block after closure of the Paleo-Tethys Ocean in the Late Triassic. The second remagnetization, residing in hematite and minor goethite, which is the further subsurface oxidation product of pyrite/magnetite, is possibly related to the development of the localized Cenozoic basins soon after India-Asia collision in the Paleocene. The youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite, respectively. Our analyses suggest that a high supply of organic carbon during carbonate deposition, prevailing sulfate reducing conditions during early diagenesis, and widespread orogenic fluid migration related to crustal shortening during later diagenesis, have altered the primary remanence of the shallow-water Tethyan carbonate rocks of the Tibetan Plateau. We emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions. Future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the NRM. (C) 2019 Elsevier B.V. All rights reserved.
The Gongjue basin from the eastern Qiangtang terrane is located in the transition region where the regional structural lineation curves from east-west-oriented in Tibet to north-south-oriented in Yunnan. In this study, we sampled the red beds in the basin from the lower Gongjue to upper Ranmugou formations for the first time covering the entire stratigraphic profile. The stratigraphic ages are bracketed within 53-43Ma by new detrital zircon U-Pb ages constraining the maximum deposition age to 52.51.5Ma. Rock magnetic and petrographic studies indicate that detrital magnetite and hematite are the magnetic carriers. Positive reversals and fold tests demonstrate that the characteristic remanent magnetization has a primary origin. The Gongjue and Ranmugou formations yield mean characteristic remanent magnetization directions of D-s/I-s=31.0 degrees/21.3 degrees and D-s/I-s=15.9 degrees/22.0 degrees, respectively. The magnetic inclination of these characteristic remanent magnetizations is significantly shallowed compared to the expected inclination for the locality. However, the elongation/inclination correction method does not provide a meaningful correction, likely because of syn-depositional rotation. Rotations relative to the Eurasian apparent polar wander path occurred in three stages: Stage I, 33.33.4 degrees clockwise rotation during the deposition of the Gongjue and lower Ranmugou formations; Stage II, 26.93.7 degrees counterclockwise rotation during deposition of the lower and middle Ranmugou formation; and Stage III, 17.73.3 degrees clockwise rotation after 43Ma. The complex rotation history recorded in the basin is possibly linked to sinistral shear along the Qiangtang block during India indentation into Asia and the early stage of the extrusion of the northwestern Indochina blocks away from eastern Tibet.
Asian climate patterns, characterised by highly seasonal monsoons and continentality, are thought to originate in the Eocene epoch (56 to 34 million years ago - Ma) in response to global climate, Tibetan Plateau uplift and the disappearance of the giant Proto-Paratethys sea formerly extending over Eurasia. The influence of this sea on Asian climate has hitherto not been constrained by proxy records despite being recognised as a major driver by climate models. We report here strongly seasonal records preserved in annual lamina of Eocene oysters from the Proto-Paratethys with sedimentological and numerical data showing that monsoons were not dampened by the sea and that aridification was modulated by westerly moisture sourced from the sea. Hot and arid summers despite the presence of the sea suggest a strong anticyclonic zone at Central Asian latitudes and an orographic effect from the emerging Tibetan Plateau. Westerly moisture precipitating during cold and wetter winters appear to have decreased in two steps. First in response to the late Eocene (34-37 Ma) sea retreat; second by the orogeny of the Tian Shan and Pamir ranges shielding the westerlies after 25 Ma. Paleogene sea retreat and Neogene westerly shielding thus provide two successive mechanisms forcing coeval Asian desertification and biotic crises.
The Pamirs represent the indented westward continuation of the northern margin of the Tibetan Plateau, dividing the Tarim and Tajik basins. Their evolution may be a key factor influencing aridification of the Asian interior, yet the tectonics of the Pamir Salient are poorly understood. We present a provenance study of the Aertashi section, a Paleogene to late Neogene clastic succession deposited in the Tarim basin to the north of the NW margin of Tibet (the West Kunlun) and to the east of the Pamirs. Our detrital zircon U-Pb ages coupled with zircon fission track, bulk rock Sm-Nd, and petrography data document changes in contributing source terranes during the Oligocene to Miocene, which can be correlated to regional tectonics. We propose a model for the evolution of the Pamir and West Kunlun (WKL), in which the WKL formed topography since at least similar to 200 Ma. By similar to 25 Ma, movement along the Pamir-bounding faults such as the Kashgar-Yecheng Transfer System had commenced, marking the onset of Pamir indentation into the Tarim-Tajik basin. This is coincident with basinward expansion of the northern WKL margin, which changed the palaeodrainage pattern within the Kunlun, progressively cutting off the more southerly WKL sources from the Tarim basin. An abrupt change in the provenance and facies of sediments at Aertashi has a maximum age of 14 Ma; this change records when the Pamir indenter had propagated sufficiently far north that the North Pamir was now located proximal to the Aertashi region.
Paleomagnetic dating of the India-Asia collision hinges on determining the Paleogene latitude of the Lhasa terrane (southern Tibet). Reported latitudes range from 5 degrees N to 30 degrees N, however, leading to contrasting paleogeographic interpretations. Here we report new data from the Eocene Linzizong volcanic rocks in the Nanmulin Basin, which previously yielded data suggesting a low paleolatitude (similar to 10 degrees N). New zircon U-Pb dates indicate an age of similar to 52Ma. Negative fold tests, however, demonstrate that the isolated characteristic remanent magnetizations, with notably varying inclinations, are not primary. Rock magnetic analyses, end-member modeling of isothermal remanent magnetization acquisition curves, and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low-temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously. Previously reported paleomagnetic data from the Nanmulin Basin implying low paleolatitude should thus not be used to estimate the time and latitude of the India-Asia collision. We show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization. We tentatively propose a new method to recover a primary remanence with inclination of 38.1 degrees (35.7 degrees, 40.5 degrees) (95% significance) and a secondary remanence with inclination of 42.9 degrees (41.5 degrees,44.4 degrees) (95% significance). The paleolatitude defined by the modeled primary remanence21 degrees N (19.8 degrees N, 23.1 degrees N)is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper Linzizong Group corrected for inclination shallowing. The start of the Tibetan Himalaya-Asia collision was situated at similar to 20 degrees N and took place by similar to 50Ma.
Several solutions have been proposed to explain the long-standing kinematic observation that postcollisional upper crustal shortening within the Himalaya and Asia is much less than the magnitude of India-Asia convergence. Here we implement these hypotheses in global plate reconstructions and test paleolatitudes predicted by the global apparent polar wander path against independent, and the most robust paleomagnetic data. Our tests demonstrate that (1) reconstructed 600-750km postcollisional intra-Asian shortening is a minimum value; (2) a 52Ma collision age is only consistent with paleomagnetic data if intra-Asian shortening was 900km; a 56-58Ma collision age requires greater intra-Asian shortening; (3) collision ages of 34 or 65Ma incorrectly predict Late Cretaceous and Paleogene paleolatitudes of the Tibetan Himalaya (TH); and (4) Cretaceous counterclockwise rotation of India cannot explain the paleolatitudinal divergence between the TH and India. All hypotheses, regardless of collision age, require major Cretaceous extension within Greater India.