Widespread Jurassic extension and rhyolitic volcanism in southern South America were manifested in the southern Andes by the development of a deep-marine volcano-tectonic rift basin extending for some 1,000 km parallel to the continental margin. This basin was contemporaneous with other narrow rift basins that formed within Gondwana during the initial stages of supercontinent fragmentation. A Cretaceous marginal basin in the southern Andes opened as continued extension led to spreading within the preexisting rhyolitic rift, or proto-marginal basin.

Rapid subsidence and inundation of eroded basement immediately preceded deposition of several kilometers of rhyolitic lavas and pyroclastic rocks in deep-marine environments in the Late Jurassic proto-marginal basin. The volcanic rocks record the effects of subaqueous quenching, interaction of uprising magma with thick sections of wet sediment, and large-scale phreatomagmatic eruptions. In contrast, coeval extensionrelated rhyolites elsewhere in southern South America form a typical subaerial ignimbrite field. These contrasting styles of Jurassic volcanism resulted from differential subsidence along the continental margin, which appears to reflect the response of relatively young Gondwanide accretionary basement to extension associated with supercontinent breakup.