EXTENSION IN THE PUNA-ALTIPLANO PLATEAU SINCE 1-2 MA: LITHOSPHERIC DELAMINATION, LOWER CRUSTAL FLOW OR GRAVITATIONAL COLLAPSE?

Extensional and strike-slip faults are found throughout the entire Altiplano-Puna Plateau, overprinting or reactivating older thrust and reverse faults. These younger faults are active, cutting Quaternary sediments and volcanic units. The timing of initiation is less certain. On the southern margin of the Puna Plateau, in northwest Argentina, active normal and strike-slip faults overprint earlier NW-SE shortening structures which affect Quaternary unit, thereby confining the age of initiation to Quaternary time. These relationships, noted elsewhere by other researchers, can be seen all along the topographically and hydrologically defined margin of the Puna Plateau: north of the Fiambala Basin, further east in the Punta Negra region, and further east on the west side of the El Cajon basin. Elsewhere, the timing of initiation of extensional and strike-slip faulting is less clear, but is probably of Quaternary age as well.

The recent kinematic shift from shortening to extension in the Altiplano-Puna plateau could be the result of several different mechanisms, each of which would produce distinct spatial and temporal deformation patterns. (1) If extension results from delamination of the mantle lithosphere beneath the Puna Plateau, extension should be confined to this region. (1) If it is a result of flow of weak lower crust from the central plateau to the edges, extension should have successively migrated toward the margins of the orogen and should currently only be active there. (3) If extension reflects gravitational collapse of the orogen, aided by a weak lower crust and a long-term slowing of plate convergence rates, extension should have initiated simultaneously across the entire plateau. Though data are sparse, the preliminary pattern favors the third interpretation. Young, disorganized extension in the Andes today may be the precursor to more extensive extension such as observed on the Tibetan Plateau today.