A giant late Precambrian chert-bearing olistostrome discovered in the Bohemian Massif: A record of Ocean Plate Stratigraphy (OPS) disrupted by mass-wasting along an outer trench slope |
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| Institution: | 1. Institute of Geology and Paleontology, Faculty of Science, Charles University, Albertov 6, Prague, 12843, Czech Republic;2. Institute of Geology of the Czech Academy of Sciences, Rozvojová 269, Prague, 16500, Czech Republic;3. Czech Geological Survey, Klárov 3, Prague, 11821, Czech Republic;1. Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Kolkata 700108, India;2. Department of Applied Geology, Faculty of Engineering and Science, Curtin University of Technology Sarawak, CDT 250, Miri 98009, Sarawak, Malaysia;3. Department of Geology, University of Calcutta, Kolkata 700019, India;1. Centre for Lithospheric Research, Czech Geological Survey, Prague, Czech Republic;2. Ecole et Observatoire des Sciences de la Terre, Université de Strasbourg, France;3. Institute of Petrology and Structural Geology, Charles University in Prague, Czech Republic;4. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, China;5. Institute of Geological Sciences, Polish Academy of Sciences, Krakow, Poland;6. Department of Earth Science, University of California, Santa Barbara, CA 93106, USA;1. Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080-3021, USA;2. 652 Muckleshoot Circle, La Conner, Washington 98257, USA;1. Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan;2. Graduate School of Environmental Studies, Tohoku University, Sendai, Miyagi 980-8579, Japan;3. Department of Earth and Planetary Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro, Tokyo 152-8550, Japan;4. Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan;5. State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;6. Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1, Komaba, Meguro-ku, Tokyo 153-8902, Japan;1. Institute of Geology and Mineralogy SB RAS, Koptyuga ave. 3, Novosibirsk 630090, Russia;2. Novosibirsk State University, Pirogova St. 2, Novosibirsk 630090, Russia;3. Earth and Life Science Institute, Tokyo Institute of Technology, E-16, 2-12-1, Ookayama, Meguro, Tokyo 152-8550, Japan;4. Department of Civil Engineering, Gifu University, Gifu 501-1193, Japan;5. University of Tokyo, Meguro, Tokyo 153-8902, Japan |
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| Abstract: | An intriguing example of chert–graywacke olistostrome is exceptionally well preserved within the late Neoproterozoic to early Cambrian Blovice accretionary wedge, Bohemian Massif. The olistostrome exhibits a block-in-matrix fabric defined by chert blocks isolated within the graywacke matrix. The major and trace element composition indicates two distinct types of cherts that formed either in a hydrothermal pelagic or hemipelagic environment supplied with a distal terrigenous material. The former is documented by elevated contents of Fe, Co, Zn, Ni, and Ti whereas the latter by high Al2O3 contents, relatively lower LaN/CeN ratios, and higher Eu/Eu* and Ce/Ce* values. Based on these geochemical data integrated with field observations and detrital zircon U–Pb ages of the host graywackes (determined using laser ablation ICP-MS), a new model for the origin of chert–graywacke association is proposed. The cherts are interpreted as representing pelagic and hemipelagic members of the Ocean Plate Stratigraphy (OPS) that formed in a sedimentary basin, carried on top of a subducting plate towards the trench. While moving over the outer swell (rise), the chert basin was intensely fractured and disrupted into large blocks or slabs. Subsequent motion of the plate brought the blocks onto an outer trench slope where they became gravitationally unstable to slide down and mix in the trench with distal, ca. 580–570 Ma turbidites derived from the overriding plate. Finally, this chert–graywacke olistostrome was covered by younger, ca. 560–547 Ma trench-fill turbidites (devoid of chert blocks) and accreted to the accretionary wedge toe, deformed, buried, and exhumed back to the wedge surface. We propose that such an olistostrome composed of pelagic/hemipelagic chert blocks and terrigenous, arc-derived graywacke matrix represents a rarely documented case of submarine, outer trench slope mass-wasting deposits and may be considered a new type of subduction-related mélanges. We coin the term outer-trench-slope mélange. |
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