Processes and Consequences of Deep Subduction

Abstract

Oceanic lithosphere subduction is the phenomenon responsible for some of the most spectacular and powerful expressions of the ceaseless motion of the Earth's tectonic plates. Subduction is manifested at the Earth's surface by a global system of oceanic trenches, the deep abyssal scars in the ocean floor that extend over tens of thousands of kilometers and cut up to 4 km deep into the planet's surface. Subduction zones are also associated with regional gravity anomalies, island arc formation, and a large portion of the world's earthquakes and volcanoes, including the deepest and most energetic earthquakes, which have been observed at depths of 660—700 km.In addition to these dramatic surficial features, the process of subduction affects mineral physics, geochemistry, petrology structural geology, and rock mechanics. The conditions associated with subduction provide a unique natural laboratory in which the entrainment of water and sediment with anomalously cold slabs of subducted oceanic lithosphere creates a dynamic and volatile environment as plates sink and encounter hot ambient mantle, mineralogical phase boundaries, and rheological transitions. Part of the difficulty in unravelling the complexity of these systems lies in understanding the feedback that occurs between phenomena operating at macroscopic and microscopic scales. For example, phase transformation rates affect buoyancy, and therefore subduction rates. This affects the regional thermal structure and thereby the phase transformations. Slab rheology and morphology may similarly be affected by mineral transformations and their kinetics.