Cretaceous to Cenozoic evolution of the northern Lhasa Terrane and the Early Paleogene development of peneplains at Nam Co,Tibetan Plateau |
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| Institution: | 1. State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences and Resources, Research Center for Tibetan Plateau Geology, China University of Geosciences (Beijing), Beijing 100083, China;2. Department of Earth and Ocean Sciences, University of Waikato, Hamilton 3240, New Zealand;3. Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;3. Department of Earth Sciences, University of St Andrews, North Street, St Andrews KY16 9AL, UK;4. Centre for Exploration Targeting, School of Earth and Environment, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia;5. Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China;1. State Key Laboratory of Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China;2. Department of Earth and Environmental Sciences, Università di Milano-Bicocca, Milano 20126, Italy;1. Department of Geological Sciences, Albion College, Albion, MI 49224, USA;2. Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA;3. Structural Geology Group, School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 10083, China;4. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;5. Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China;3. Department of Geosciences, National Taiwan University, Taipei 106, China;4. Chengdu Institute of Geology and Mineral Resources, Chengdu 610082, China;1. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China;2. Department of Earth and Ocean Science, University of Waikato, Hamilton 2001, New Zealand |
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| Abstract: | Highly elevated and well-preserved peneplains are characteristic geomorphic features of the Tibetan plateau in the northern Lhasa Terrane, north–northwest of Nam Co. The peneplains were carved in granitoids and in their metasedimentary host formations. We use multi-method geochronology (zircon U–Pb and U–Th]/He dating and apatite fission track and U–Th]/He dating) to constrain the post-emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas. LA-ICP-MS U–Pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 Ma and 85 Ma, and a third group records Paleocene volcanic activity (63–58 Ma) in the Nam Co area. The low-temperature thermochronometers indicate common age groups for the entire Nam Co area: zircon (U–Th)/He ages cluster around 75 Ma, apatite fission track ages around 60 Ma and apatite (U–Th)/He ages around 50 Ma. Modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest Cretaceous to earliest Paleogene time. By Middle Eocene time the relief was already flat, documented by a thin alluvial sediment sequence covering a part of the planated area. The present-day horst and graben structure of the peneplains is a Late Cenozoic feature triggered by E–W extension of the Tibetan Plateau. The new thermochronological data precisely bracket the age of the planation to Early Eocene, i.e. between ca. 55 and 45 Ma. The erosional base level can be deduced from the presence of Early Cretaceous zircon grains in Eocene strata of Bengal Basin. The sediment generated during exhumation of the Nam Co area was transported by an Early Cenozoic river system into the ocean, suggesting that planation occurred at low elevation. |
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| Keywords: | Thermochronology U–Pb Geomorphology Peneplain Tibetan Plateau |
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