Spreading-subsidence and generation of ensialic marginal basins: an example from the early Cretaceous of central Chile

Abstract

Summary During the early Cretaceous, intracontinental rifting, spreading and subsidence led to the formation of an ensialic trough in central Chile. This trough is interpreted as an aborted marginal basin since no oceanic crust was generated. However, mantle-derived material, represented by flood basalts with low initial 87 Sr/ 86 Sr ratio and evolved geochemical characteristics, was deposited in the basin. Rb-Sr dating shows that the lavas suffered burial metamorphism shortly after their extrusion. The flood basalts were extruded during an episode of slow oceanic spreading in the Pacific, and subsequent folding, granitoid intrusion and uplift are correlated with an episode of fast oceanic spreading and plate subduction. The intracontinental events related to these two episodes constitute a cycle, repetition of which might account for the geological evolution of central Chile during the Mesozoic and the Palaeogene. The western margin of South America, during the Cretaceous, was characterized by basin formation with eruption of mantle-derived basalts. Some basins developed into marginal basins proper, with generation of oceanic crust, others were aborted. The type of basin was determined by the rate and volume of upwelling mantle material, and continental crust thickness. Rapid upwelling gave rise to back-arc spreading and marginal basins with primitive basalts and ocean-floor metamorphism or burial metamorphism reflecting steep thermal gradients. Slow upwelling produced ensialic basins with evolved basalts affected by burial metamorphism with less steep thermal gradients, as in central Chile during the early Cretaceous.