A (VERY) BRIEF HISTORY OF SUBDUCTION: INSIGHTS FROM THERMODYNAMIC AND GEODYNAMIC MODELS

In the absence of direct observation of subduction zone environments, geodynamic models based upon the physical and thermal properties of simplified lithologies have become important tools for understanding mechanical and thermal evolution of these settings. Combining estimated plate burial rates from such models with the P-T paths that they generate allows for calculation of subduction duration. A first-order observation suggests that, for the basaltic upper crust, eclogite-facies conditions are reached in less than 3 million years for most subduction zones. A prominent feature of many model outputs [e.g. 1] is the existence of a sharp thermal gradient, separating a colder fore-arc from a warmer regime dominated by viscous flow of the mantle wedge. Calculations suggest that heating within this speculated zone should be very rapid (>1000°C/Ma), and a transition in thermal regime at such high rates must also result in rapid changes of various properties of the subduction interface. By coupling geodynamic models with constraints from phase equilibria, we explore the potential implications of such short metamorphic durations and rapid changes in P-T regime, focusing on the geochemical, rheological, and petrological properties and processes in subduction zones and subduction-related metamorphic rocks. Particularly profound implications of rapid heating would be seen in terms of: a) dehydration, fluid-rock interaction [2], and elemental mass transfer; b) slab densification, solid and fluid volume changes [3], and deformation; and c) required rates of metamorphism [4], kinetics of mineral nucleation and growth, the development of metamorphic textures and the likelihood of substantial departures from chemical equilibrium. Where preserved, field-based examples of these brief metamorphism-related phenomena are striking within the context of the overall subduction (and exhumation) timescale.

[1] Syracuse et al., 2010, PEPI, v. 183, p. 73-90. [2] John et al., 2012, Nature Geoscience, v. 5, p. 489-492. [3] Abers et al., 2013, EPSL, v. 369-370, p. 178-187. [4] Dragovic et al., 2015, EPSL, v. 413, p. 111-122.