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Jurassic to Neogene Quantitative Crustal Thickness Estimates In 10–13 Oct. GSA Connects 2021 VOL. 31, NO. 6 | JUNE 2021 Jurassic to Neogene Quantitative Crustal Thickness Estimates in Southern Tibet Structural and Thermal Evolution of the Himalayan Thrust Belt in Midwestern Nepal By P.G. DeCelles, B. Carrapa, T.P. Ojha, G.E. Gehrels, and D. Collins Spanning eight kilometers of topographic relief, the Himalayan fold-thrust belt in Nepal has accommo- dated more than 700 km of Cenozoic convergence between the Indian subcontinent and Asia. Rapid Special Paper 547 tectonic shortening and erosion in a monsoonal By P.G. DeCelles et al. climate have exhumed greenschist to upper amphi- bolite facies rocks along with unmeta morphosed rocks, including a 5–6-km-thick Ceno zoic foreland basin sequence. This Special Paper presents new geochronology, multisystem thermochro- Structural andThermal Evolution of the HimalayanThrust Belt in Midwestern Nepal Structural and Thermal Evolution of nology, structural geology, and geological mapping of an approximately 37,000 km2 region in midwestern and western Nepal. This the Himalayan Thrust Belt in Midwestern Nepal work informs enduring Himalayan debates, including how and where to map the Main Central thrust, the geometry of the seismi- cally active basal Himalayan detachment, processes of tectonic shortening in the context of postcollisional India-Asia con- vergence, and long-term geodynamics of the orogenic wedge. SPE547, 77 p. + insert ISBN 9780813725475 list price $42.00 member price $30.00 P E R R I B C E M E • M $ GSA Special Paper 547 E • • • • • • • • • • C M 30 I By P.G. DeCelles, B. Carrapa, T.P. Ojha, G.E. Gehrels, and D. Collins E R M P B E R GSA BOOKS } https://rock.geosociety.org/store/ toll-free 1.800.472.1988 | +1.303.357.1000, option 3 | [email protected] JUNE 2021 | VOLUME 31, NUMBER 6 SCIENCE 4 Jurassic to Neogene Quantitative Crustal Thickness Estimates in Southern Tibet Kurt E. Sundell et al. GSA TODAY (ISSN 1052-5173 USPS 0456-530) prints news and information for more than 22,000 GSA member readers and subscribing libraries, with 11 monthly issues (March- Cover: Southwest facing view of Dobzebo (elevation 6436 m), near April is a combined issue). GSA TODAY is published by The Geological Society of America® Inc. (GSA) with offices at Zazê Township ( ), Ngamring County, Shigatse, Tibet. Dobzebo 3300 Penrose Place, Boulder, Colorado, USA, and a mail- owes its relief to Neogene extensional deformation following the attainment of extreme crustal ing address of P.O. Box 9140, Boulder, CO 80301-9140, USA. thickness and high elevation. Photo by Aislin Reynolds, 2019. See related article, p. 4–10. 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Managing Editor: Kristen “Kea” Giles, [email protected], GSA NEWS [email protected] Graphics Production: Emily Levine, [email protected] 30 Call for GSA Committee Service 34 Groundwork: Seeing What Advertising Manager: Ann Crawford, You Know: How Researchers’ +1-800-472-1988 ext. 1053; +1-303-357-1053; 31 GSA Council Extends Bigger Backgrounds Have Shaped the Fax: +1-303-357-1070; [email protected] Discounts on Publication Fees Mima Mound Controversy GSA Online: www.geosociety.org for Members GSA TODAY: www.geosociety.org/gsatoday 36 Groundwork: Recruiting to Printed in the USA using pure soy inks. 31 Scientists in Parks Geosciences through Campus Partnerships 32 Geoscience Jobs & Opportunities 38 GSA Foundation Update 32 GSA Member Community, Powered by You Jurassic to Neogene Quantitative Crustal Thickness Estimates in Southern Tibet Kurt E. Sundell*, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, [email protected]; Andrew K. Laskowski, Dept. of Earth Sciences, Montana State University, Bozeman, Montana 59717, USA; Paul A. Kapp, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA; Mihai N. Ducea, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, and Faculty of Geology and Geophysics, University of Bucharest, 010041, Bucharest, Romania; James B. Chapman, Dept. of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA ABSTRACT 2009), and up to ~85 km (Wittlinger et al., proto-plateau (Kapp et al., 2007; Lai et al., Recent empirical calibrations of Sr/Y and 2004; Xu et al., 2015). The Tibetan Plateau 2019). Alternatively, Late Cretaceous to La/Yb from intermediate igneous rocks as formed from the sequential accretion of con- Paleogene shortening may have been punc- proxies of crustal thickness yield discrepan- tinental fragments and island arc terranes tuated by a 90–70 Ma phase of extension cies when applied to high ratios from thick beginning during the Paleozoic and culmi- that led to the rifting of a southern portion crust. We recalibrated Sr/Y and La/Yb as nated with the Cenozoic collision between of the Gangdese arc and opening of a back- proxies of crustal thickness and applied India and Asia (Argand, 1922; Yin and arc ocean basin (Kapp and DeCelles, 2019). them to the Gangdese Mountains in south- Harrison, 2000; Kapp and DeCelles, 2019). These represent two competing end-member ern Tibet. Crustal thickness at 180–170 Ma The India-Asia collision is largely thought to hypotheses for the Mesozoic tectonic evo- decreased from 36 to 30 km, consistent with have commenced between 60 and 50 Ma lution of southern Tibet that are testable Jurassic backarc extension and ophiolite (e.g., Rowley, 1996; Hu et al., 2016); how- by answering the question: Was the crust formation along the southern Asian margin ever, some raise the possibility for later col- in southern Tibet thickening or thinning during Neo-Tethys slab rollback. Available lisional onset (e.g., Aitchison et al., 2007; van between 90 and 70 Ma? data preclude detailed estimates between Hinsbergen et al., 2012). Despite ongoing Contrasting hypotheses about the Cenozoic 170 and 100 Ma and tentatively suggest ~north-south convergence, the northern tectonic evolution of southern Tibet are test- ~55 km thick crust at ca. 135 Ma. Crustal Himalaya and Tibetan Plateau interior are able by quantifying changes in crustal thick- thinning between 90 and 65 Ma is consis- undergoing east-west extension, expressed ness through time. In particular, the Paleocene tent with a phase of Neo-Tethys slab roll- as an array of approximately north-trending tectonic evolution before, during, and after back that rifted a portion of the southern rifts that extend from the axis of the high the collision between India and Asia was Gangdese arc (the Xigaze arc) from the Himalayas to the Bangong Suture Zone dependent on initial crustal thickness, and in southern Asian margin. Following the con- (Molnar and Tapponnier, 1978; Taylor and part controlled the development of the mod- tinental collision between India and Asia, Yin, 2009) (Fig. 1). ern Himalayan-Tibetan Plateau. Building on crustal thickness increased by ~40 km at The Mesozoic tectonic evolution of the the hypothesis tests for the Late Cretaceous, ~1.3 mm/a between 60 and 30 Ma to near southern Asian margin placed critical ini- if the crust of the southern Asian margin was modern crustal thickness, before the onset tial conditions for the Cenozoic evolution of thickened before or during the Paleocene, of Miocene east-west extension. Sustained the Tibetan Plateau.
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