The origin of the Upper Palaeozoic Chañaral mélange of N Chile

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Abstract
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.