PROGRESSIVE MELANGE EXHUMATION ALONG A PRE-ANDEAN TRANSPRESSIONAL FAULT SYSTEM, CORDILLERA DE LA COSTA, CHILE (26°S – 42°S)

Mélanges occur as discontinuous mappable units along an extensive, North-South trending, steeply dipping zone of distributed shear along the coast of central Chile. Large mélange zones, from North to South, at Chañaral (26° S), Los Vilos (32°S), Pichilemu (36°S), and Chiloé Island (42.5°S) contain protolith variations, but are consistent in exhibiting cross cutting fabric features indicating progressive transition from earlier ductile to more brittle deformation. In the Infiernillo mélange near Pichilemu, where this transition is well preserved, Permian to early Triassic, sub-horizontal schistosity planes of the Western Series schist (WS) are disrupted and uplifted along high angle, N-S to NNE-SSW trending brittle-ductile shears . Exotic rock types, such as mafic blueschist, were incorporated within the mélange during diapiric piercement and exhumation of subjacent portions of the subduction complex. Small scale deformational features in mylonite and cataclasite zones within the mélange matrix indicate active lateral shear during exhumation from depths exceeding 12 Km [SAF geotherm].

Linkage between the mélange zones is inferred from extensive margin parallel alignment of late shear planes crosscutting low angle schistosity, margin parallel, sub-horizontal stretching lineations, and regional N-S truncation of horizontal to NW-SE striking regional fabric along the zone. Regional folding of schistosity in the WS in the southern portion of this zone indicates widespread NE-SW crustal shortening favoring dextral displacement along the N-S trending zone.

Mélange development and exhumation followed Permian to early Triassic greenschist metamorphism of the WS, and preceded the deposition of unconformably overlying, fossiliferous, late Triassic, rift deposits in the north. The length and extreme linearity of this zone of lateral movement, coincident with a general hiatus of regional arc magmatism, is consistent with large scale dextral transpression, or possible transform movement, during highly oblique convergence along the Pre-Andean margin. This may have weakened and uplifted the crust facilitating later fault localization, gravitational collapse, and subcrustal erosion during convergence along the subsequent Andean continental margin.