THE ROLE OF MARGINAL BASIN INVERSION IN CORDILLERAN OROGENESIS

Tectonic uplift of the Andean Cordillera was initiated in the mid-Cretaceous with inversion of a composite marginal basin along 7500 km of the continental margin of South America, from Peru to Tierra del Fuego and the North Scotia Ridge. In the southernmost Andes, from 50-56 degrees S, the quasi-oceanic floor of this basin is preserved in the obducted ophiolitic rocks of the Rocas Verdes (Green Rocks) basin. The basin beneath Bransfield Strait, 61-64 degrees S, separating the South Shetland Islands from the Antarctic Peninsula, may constitute a modern analog for the Rocas Verdes basin.

Multi-channel reflection seismic data from the Bransfield basin reveal an asymmetric structural architecture characterized by steeply-dipping normal faults flanking the South Shetlands island arc and gently dipping listric normal faults along the Antarctic Peninsula margin. This architecture is remarkably similar to that deduced from field structural studies of the Rocas Verdes basin. Notably, the oceanward-dipping, low angle normal faults along the Antarctic Peninsula margin constitute ideally oriented surfaces for reactivation as thrust faults, leading to obduction of the basin floor rocks during any ensuing compressional event.

Seismic refraction studies of Bransfield basin indicate along-strike northeastward thinning of the crust associated with southwestward rift propagation, comparable to that indicated for the Rocas Verdes basin by the results of U-Pb zircon dating of the ophiolitic rocks. Cross-strike segmentation of Bransfield basin also mirrors the isolated ophiolitic complexes of the inverted Rocas Basin in South America and on South Georgia Island.

The results of our geophysical studies of Bransfield basin reveal close similarities in structure and evolution with the Cretaceous Rocas Verdes basin, and therefore encourage the next step of forward modeling in an effort to help elucidate fundamental cordilleran orogenic processes.