HR: 08:50h
AN: U51B-04    [Abstracts]
TI: Synorogenic Extensional Tectonics in the Forearc, Arc and Southwest Altiplano of Southern Peru
AU: * Sempere, T
EM: sempere@lmtg.obs-mip.fr
AF: IRD - LMTG, 14 av. E. Belin, Toulouse, 31400, France
AU: Jacay, J
EM: j_jacay@yahoo.com
AF: Universidad San Marcos, av. Venezuela, Lima, Peru
AB: There is increasing evidence that paradigms, as in many fields of science, deeply influence interpretations and even observations of the actual geology of the Andes, to the point that some same areas have be mapped in dramatically different ways by geologists who favored distinct models. The belief that the Central Andes originated by tectonic shortening has commonly biased cartography in this orogen, for instance by forcing high-angle or poorly-exposed faults to be mapped as reverse faults and thrusts. Extensional structures have often been overlooked, because they were thought to be irrelevant in the investigation of orogenic issues. However, observations and models from a variety of undoubtedly extensional settings in Europe and Africa have recently shown that some structural geometries previously thought to be typical of contractional processes, as in the Central Andes, in fact also occur in extensional contexts, in particular where normal faults were initiated as flexure-forming blind faults. Traditional mapping in the Central Andes has therefore to be re-evaluated. Identification and correction of such biases result in major revisions of structural mapping in southwestern Peru. The forearc, arc, and SW Altiplano of southern Peru in fact appear to have been dominated by extension and transcurrence since ~30 Ma, in contrast with the NE Altiplano, Eastern Cordillera, and sub-Andean belt, where shortening has been indeed significant. These two contrasting orogenic domains are separated by the SFUACC fault system, which corresponds to a major lithospheric boundary. Basins SW of the SFUACC formed in extension and along transcurrent faults. At least one low-angle extensional detachment, placing near-vertical Miocene conglomerates over a Cretaceous unit, occurs just west of Lake Titicaca. Other detachments occur in the forearc. Significant transcurrent faulting, including transpressional deformation, developed along specific structures over southern Peru. SW of the SFUACC, undisputable reverse faults are rare, but are common along the lower slope of the Pacific Andean escarpment, suggesting incipient oceanward gravitational collapse of the Western Cordillera. We find that extension has accompanied the Andean orogeny SW of the SFUACC, and therefore question the currently dominant paradigm.
DE: 8038 Regional crustal structure
DE: 8109 Continental tectonics: extensional (0905)
DE: 8110 Continental tectonics: general (0905)
DE: 9360 South America
DE: 9604 Cenozoic
SC: Union [U]
MN: 2007 Joint Assembly