SUBDUCTION-DRIVEN RECYCLING OF CONTINENTAL MARGIN LITHOSPHERE

Alan Levander, Rice University, Earth, Environmental and Planetary Sciences, Houston, United States, Maximiliano J Bezada, University of Minnesota Twin Cities, Minneapolis, MN, United States, Fenglin Niu, University of Science and Technology of China, School of Earth & Space Sciences, Anhui, China, Imma Palomeras, Rice University, Earth Science Department, Houston, United States, Sally Thurner, Rice University, Houston, TX, United States, Eugene Humphreys, Univ Oregon, Earth Sciences, Eugene, United States, M Meghan Miller, University of Southern California, Los Angeles, CA, United States, Ramon Carbonell, ICTJA-CSIC, Barcelona, Spain, Josep Gallart, Geosciences Barcelona, CSIC, Barcelona, Spain, Michael Schmitz, La Fundación Venezolana de Investigaciones Sismológicas, Caracas, Venezuela and BOLIVAR Working Group and PICASSO Working Group
Abstract:
While subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics, recycling continental lithosphere appears far more complicated and is less well understood. Delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts. Here we describe another process that can lead to the loss of continental lithosphere adjacent to a subduction zone: Subducting oceanic plates can entrain and recycle lithospheric mantle from an adjacent continent and disrupt the continental lithosphere far inland from the subduction zone.

Seismic images from recent dense broadband arrays on opposite sides of the Atlantic show higher than expected volumes of positive anomalies identified as the subducted Atlantic (ATL) slab under northeastern South America (SA), and the Alboran slab beneath the Gibraltar arc region (GA). The positive anomalies lie under and are aligned with the continental margins at depths greater than 200 km. Closer to the surface we find that the continental margin lithospheric mantle is significantly thinner than expected beneath the orogens adjacent to the subduction zones. Thinner than expected lithosphere extends inland as far as the edges of nearby cratonic cores. These observations suggest that subducting oceanic plates viscously entrain and remove continental mantle lithosphere from beneath adjacent continental margins, modulating the surface tectonics and pre-conditioning the margins for further deformation. The latter can include delamination of the entire lithospheric mantle, as around GA, inferred by results from active and passive seismic experiments. Secondary downwellings develop under the continental interior inland from the subduction zone: We image one under SA and one or more in the past were likely under GA. The process of subduction-driven continental margin lithosphere removal reconciles numerous, sometimes mutually exclusive, geodynamic models proposed to explain the complex oceanic-continental tectonics of these two subduction zones.

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