Subduction of spreading ridges as a factor in the evolution of continental margins

Abstract

The subduction of spreading ridges creates a special geodynamic setting distinguished by the interference of convergent and divergent boundaries between lithospheric plates and their long-term interaction accompanied by the formation of characteristic geological complexes and structures. The available data on subduction of the contemporary Chile Ridge make it possible to reconstruct such settings in the geological past. The subduction of the spreading ridge leads to uplift of the continental margin, cut off the accretionary wedge by means of tectonic erosion, emplacement of a fold-thrust structure and longitudinal strike-slip faults, and creates settings favorable for obduction of the young oceanic lithosphere. A lithospheric window expressed in geological and geophysical features opens beneath the continental margin at the continuation of the ridge axis. The subduction-related volcanic activity ceases above this window, giving way to specific proximal magmatism close to the boundary with the ocean and distal magmatism at a distance from this boundary. The proximal bimodal magmatism was related to the sources of tholeiitic basalts characteristic of the ridge involved in subduction and to the partial melting of its oceanic crust and sediments. The distal basaltic magmatism is a product of melting of the fertile oceanic asthenosphere ascending through the lithospheric window with subsequent transformation of magma in the mantle wedge and the continental crust. The use of the Chilean tectonotype for paleoreconstructions is limited by the diverse settings of ridge subduction. The Paleogene magmatism at the Pacific margin of Alaska, where the kinematics of subduction was close to the Chilean subduction, is similar to the proximal igneous rocks of Chile in composition and zoning, retaining some geological differences. Another aspect of the paleoreconstruction is discussed on the basis of Jurassic and Cretaceous granitoids of the Ekonai Terrane of the Anadyr-Koryak System and terranes of southern Alaska. These localities are known for a special, accretionary type of granitoids in the forearc region related to anatectic magma formation without participation of the plunging ridge. Proceeding from comparison with the Chilean tectonotype, the criteria for the identification of granitoids varying in their origin are considered. The effect of subducting ridges on continental margins changed over geologic time and was subject to the rhythm of supercontinental cycles.