<p>Constraining spatial and temporal patterns of topography and exhumation along the Himalayan orogen is a starting point for studies aimed at understanding the development of Asian climate and tectonic evolution. Starting from the pioneering work of Cerveny et al. (1988), many scientists have applied a detrital thermochronologic approach to reveal the Cenozoic exhumation history of the Himalayas. Thermochronologic studies involve analyses of modern river sediments and sedimentary successions either accreted on the southern side of the orogen or accumulated in the Indus and Bengal fans. As datasets have grown and techniques evolved, the available interpretations are often contradictory.</p><p>In this contribution, we analyse previously published detrital-thermochronology datasets in the Himalayan region using the interpretive keys illustrated in Malus&#224; and Fitzgerald (2020). These keys reinforce existing approaches and provide new perspectives for the application of detrital thermochronology to tectonic settings where the geologic evolution is often still debated. Different thermochronologic systems applied to proximal and distal sedimentary successions derived from Himalayan erosion yield a complex exhumation and tectonic history, but a relatively consistent picture for the Cenozoic evolution of India-Eurasia collision emerges. Detrital thermochronology data are supportive of a progressive southward thrust propagation towards the Himalayan foreland, progressively involving new eroding sources. The onset of fast exhumation in the Lesser Himalaya is constrained by different thermochronologic methods and datasets, indicative of onset at ~10 Ma, in line with independent geologic evidence. Coeval fast exhumation is also recorded in detritus derived from the Greater Himalaya. These findings are supportive of a major morphogenic phase of mountain building in the Himalayas at ~10 Ma, prior to the onset of fast exhumation in the Namche Barwa syntaxis.</p><p>Cerveny PF et al (1988). <em>History of uplift and relief of the Himalaya during the past 18 million years: Evidence from fission-track ages of detrital zircons from sandstones of the Siwalik Group</em>. In: New perspectives in basin analysis, Springer.</p><p>Malus&#224; MG, Fitzgerald PG (2020). <em>The geologic interpretation of the detrital thermochronology record within a stratigraphic framework, with examples from the European Alps, Taiwan and the Himalayas.</em> Earth-Science Reviews, https://doi.org/10.1016/j.earscirev.2019.103074</p>
Abstract The present-day topography of Tianshan is the product of repeated phases of Meso-Cenozoic intracontinental deformation and reactivation, whereas the long-term Mesozoic topographic evolution and the timing of the onset of Cenozoic deformation remain debated. New insights into the Meso-Cenozoic geodynamic evolution and related basin-range interactions in the Tianshan were obtained based on new detrital single-grain apatite U-Pb, fission-track, and trace-element provenance data from Mesozoic sedimentary sequences on the northern margin of the Tarim Basin. Detrital apatite U-Pb age data from Early-Middle Triassic clastic rocks show two prominent age populations at 500–390 Ma and 330–260 Ma, with a paucity of ages between 390 and 330 Ma, suggesting that sediment source is predominantly from the northern Tarim and South Tianshan. From the Late Triassic to Early Jurassic, the first appearance of populations in the 390–330 Ma and 260–220 age ranges indicates that the Central Tianshan-Yili Block and Western Kunlun Orogen were source regions for the northern margin of Tarim Basin. In the Cretaceous strata, south-directed paleocurrents combined with the decrease in the 390–330 Ma age population from the Central Tianshan-Yili Block imply that South Tianshan was uplifted and again became the main source region to the Baicheng-Kuqa depression during the Cretaceous. Our new apatite fission-track data from the southern Chinese Tianshan suggest that rapid cooling commenced at c. 30 Ma along the southern margin, and the Early Mesozoic strata exposed on the southern flank of the Tianshan underwent c. 4–5 km of late Cenozoic exhumation during this period. This age is approximately synchronous with the onset of exhumation/deformation not only in the whole Tianshan but also in the interior of the Tibetan Plateau and its margins. It suggests that far-field, N-directed shortening resulting from the India-Asia collision was transmitted to the Tianshan at that time.
<p>The Alpine orogenic belt is the result of the continental collision and convergence&#160;between the Adriatic microplate and European plate&#160;during the Mesozoic.&#160;The Alps orogenic belt has a complex tectonic history and the deformation in and around the Alps are significantly affected by several microplates (e.g., Adriatic and Iberia)&#160;and&#160;blocks,&#160;in particular the Apennines, Betics, Dinarides. The mantle transition zone is delineated by seismic velocity discontinuities around the depths of 410 and 660 km&#160;which are generally interpreted as polymorphic phase changes in the olivine system and garnet-pyroxene system.The subduction depth of the European plate and the origin of the mantle flow behind the plate plays crucial roles for our understanding of regional geodynamic (Zhao et al., 2016; Hua et al.,&#160;2017).&#160;Therefore, we use&#160;receiver function&#160;method to study the seismic features of discontinuities beneath&#160;the Western Alps&#160;to constrain the structure of subducted plate and study the geodynamic origin of the low velocity anomaly behind the subduction zone and its relationship with the high-relief&#160;topography.&#160;</p><p>This study uses data collected from 293 permanent and temporary broadband seismic stations (e.g., CIFALPS). Teleseismic events are selected from 30<sup>o</sup>&#160;to 90<sup>o </sup>epicentral distrance with magnitudes (Mw) between 5.3 and 9.0. Data are carefully checked&#160;by automated&#160;and manual&#160;procedures to to give a total of 24904&#160;receiver functions. Both 1D velocity model of the IASP91 and 3D velocity model of the EU60 (Zhu et al., 2015) are used for time-to-depth migration. The results show that&#160;using 3D velocity model to image the two discontinuities may obtain a more accurate structure image of the mantle transition zone.</p><p>In the northern part of the study area, along the alpine orogenic belt, we find a localized arc-shaped thinning area&#160;with a depressed&#160;410 discontinuity, which is attributed to hot mantle upwellings.&#160;The uplift&#160;is&#160;hardly&#160;seen on the 660 discontinuity, suggesting that&#160;the thermal anomaly is unlikely to be interpreted as a mantle plume. The uplift of the 410-km can be interpreted as the European plate subducting to the depth of the upper transition zone. The depression of the 660-km &#160;is likely attributed&#160;to the remnants from the oceanic mantle lithosphere that detached from the Eurasian plate after closure of the Alpine Tethys.&#160;Our results show&#160;a good agreement&#160;between the thinning area of MTZ and the area of topographic uplift, the mantle upwelling promotes the temperature increase which is conducive to the uplift of topographic.</p><p>Reference</p><p>Zhao L , Paul A , Marco G. Malus&#224;, et al. Continuity of the Alpine slab unraveled by high-resolution P-wave tomography. Journal of Geophysical Research: Solid Earth, 2016, 121.</p><p>Hua, Y., D. Zhao, and Y. Xu (2017), P wave anisotropic tomography of the Alps, J. Geophys. Res. Solid Earth, 122, 4509&#8211;4528, doi:10.1002/2016JB013831.</p><p>Zhu H,Bozdag E and Tromp J.Seismic structure of the European upper mantle based on adjoint tomography.Geophys. J. Int. 2015, 201, 18&#8211;52</p>
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