Mega-fold interference patterns in the Beishan orogen (NW China) created by change in plate configuration during Permo-Triassic termination of the Altaids |
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| Institution: | 1. Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MLR, Xi''an Center of Geological Survey, CGS, Xi''an 710054, China;2. Research Center for Orogenic Geology, Xi''an Center of Geological Survey, Geological Survey of China, Xi''an 710054, China;3. Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, Australia;1. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;3. Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, IN 47405, USA |
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| Abstract: | Kilometer-size fold interference patterns in the Beishan Orogenic Collage (BOC) in the southernmost Altaids formed by fold superimposition in fossiliferous Permian sedimentary rocks. First-phase (F1), upright and almost north-trending folds, were refolded by E- to ENE-trending F2 folds, whose axial planes and axes are vertical or subvertical. From east to west there is a regional change in style of interference patterns from lobate–cuspate-, to crescent- to mushroom-shape. This variation is accompanied by a westward decrease in the F2 interlimb angle and related to a higher percentage of coarse-grained clastic rocks, suggesting a dependence of the F2 deformation on lithology. Axial planar slaty cleavages are well developed in F1 and poorly developed in F2 folds. The superposed folds mainly underwent flexural-slip and flexural flow folding to give rise to the lobate–cuspate pattern, and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids. The two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter-arc basin during the final amalgamation of the BOC in the latest Permian to early to mid-Triassic. The two folding events bracketed between a maximum detrital zircon age of <273 Ma, and the youngest age of an intruded dyke at 219.0 ± 1.2 Ma suggest rapid plate reconfiguration related to final amalgamation of the Altaids orogen. |
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| Keywords: | Fold superimposition Interference pattern Beishan Orogenic Collage Altaids |
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