MAJOR PLATE BOUNDARY CHANGES RECORDED IN MID-CAMBRIAN PAMPEAN OROGEN, ARGENTINA

An extensive belt of chevron-folded Vendian turbidites in north central Argentina preserves unique evidence for early to mid-Cambrian (Pampean) tectonism along a major segment of the paleo-Gondwana margin. A widely accepted tectonic model considers the turbidites as part of an originally passive margin sequence that developed into an accretionary complex above an east-vergent subduction zone beneath the Rio de la Plata craton, with associated Pampean arc magmatism. However, a brief, intense, static, high T/low P metamorphism of the already folded turbidites peaked at ca. 520 Ma, well after the peak of Pampean arc magmatism. The high-grade meta-turbidites can be traced for 1000km along strike into less deformed, unmetamorphosed equivalents (Puncoviscana Fm.). There is no evidence for passive margin sediments beneath the turbidites and no post-metamorphism collisonal events related to the Pampean orogen. To explain the static and apparently localized mid-Cambrian thermal pulse, a variation of the accepted model considered that Pampean tectonism ended with subduction of a spreading ridge.

We have developed an alternative and expanded model, in which the initial stages of the Pampean orogen involved deposition of turbidites in a series of back arc basins formed by slab roll-back, followed by chevron folding of the turbidites during sediment offscraping - analogous to development of the turbidite-dominated Lachlan orogen in Australia. A roll-back model explains the lack of passive margin sedimentary rocks beneath the turbidite prism, obviates the need for arc rocks of an equivalent age to be nearby, is consistent with a small amount of rift-related lavas in the lowest part of the section, and is consistent with the lack of evidence for continental collision in the region.

Subsequent ridge subduction would have allowed a major segment of the slab to descend, followed by a rapid and dramatic loss of the slab pull effect and cessation of subduction along that segment of the plate boundary. Conversion to a transcurrent/transpressional plate boundary brought Pampean metamorphism to an abrupt end at ca. 510 Ma and paved the way for mid-Ordovician accretion of the Famatinian arc terrane. This model may be applicable to many orogens that involve accretion of successive terranes along transpressional boundaries.