Petrologic and geochronologic constraints on the thermal and structural evolution of paleo-subduction interfaces

Abstract

In subduction zones, the interface between the downgoing slab and overriding plate controls a number of important dynamic processes. Both the rheologic behavior and thermal structure of the interface influence fluid release and transport, melt generation, seismic phenomena and viscous coupling between the downgoing and overriding plates. In this dissertation I present field, petrologic and geochronologic observations of paleo-subduction interface rocks from the Catalina Schist (Santa Catalina Island, CA) and Rio San Juan Complex (Dominican Republic). Methods including trace element thermometry, elastic barometry and Sm-Nd garnet geochronology are used to place constraints on spatial and temporal scales of deformation and to understand long-term changes in thermal structure. The applicability of new thermobarometric methods to reconstruct the pressure-temperature evolution of a sample are also assessed.

Mélange, a block-in-matrix structure, is an important constituent of the subduction interface. Blocks within mélange can be mechanically and metasomatically mixed over several kilometers along the subduction interface, likely as a result of rheologic contrasts in the matrix. In the Catalina Schist amphibolite-facies mélange zone, blocks record up to 7 million years of variation in peak metamorphic age, placing constraints on the timescale over which the zone developed prior to underplating and rapid cooling of the system. This mixing process fundamentally changes the composition of the interface and may be an important driver of seismic phenomena including episodic tremor and slip.

Changes in the thermal structure of the subduction interface can be constrained by thermobarometry of exhumed metamorphic rocks and the timescales of those changes can be constrained by geochronology. Here, new age constraints on the timing of amphibolite-facies metamorphism of the Catalina Schist are presented as well as the first evidence of an earlier eclogite-facies metamorphic event that is approximately coeval with the earliest records of subduction in related exhumed terranes. Finally, trace element and elastic thermobarometers are compared to major element thermobarometry and phase equilibria modeling in order to assess the utility of trace element and elastic thermobarometry to reconstructing the metamorphic history of a sample using an eclogite from the Rio San Juan Complex as a type-example.