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Journal of the Geological Society

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The origin of the Upper Palaeozoic Chañaral mélange of N Chile

School of Geography and Geology, College of St Paul and St Mary, Cheltenham GL50 2RH, UK

The fragmented and mixed strata of the Chañaral melange extend for some 220 km from N to S in the coastal ranges of N Chile (26°20' to 28°20'S). The mélange was produced in an accretionary wedge resulting from the NE subduction of the ancestral Pacific oceanic plate beneath the Gondwana-land margin during Carboniferous times. The mélange consists essentially of blocks of sandstone in a pelitic matrix. These sediments were deposited as deep-sea basin-plain turbidites prior to their fragmentation. Other blocks in the mélange include mafic volcanic rocks, with the geochemical characteristics of oceanic island basalts, and rare shallow-marine limestones.

Structures in the mélange suggest an origin by two distinct deformation processes involving unlithified sediments. Initial boudinage and break-up of the strata was apparently accomplished by intrastratal movement resulting from imbricate thrusting within the accretionary wedge. The second process produced cross-cutting zones of breccia which are interpreted as fluid escape conduits resulting from the high pore pressures produced during underthrusting.

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