Metamorphism and deformation in subduction zones

at the slab-mantle interface

 

 

 

The rheology of subducted crust strongly influences the geophysical and geochemical evolution of convergent plate boundaries, including the interaction of deformation, metamorphism, and fluid-rock reaction during subduction and exhumation, and the rates of processes such as exhumation of high-pressure rocks. We have been investigating the mechanisms of large-magnitude transport (exhumation) of high-pressure rocks in subduction zones by characterizing the kinematics of deformation from the map scale to the grain-scale using syn-kinematic high-pressure index minerals such as lawsonite and glaucophane in metamorphosed oceanic crust (Teyssier et al., 2010 - Geology) as well as rheologically important minerals in interlayered metasedimentary rocks (quartz in quartzite, calcite in marble; Seaton et al., 2009 – Tectonophysics; Seaton et al., 2014 – Geodinamica Acta).

 

The focus of this investigation is the Sivrihisar Massif, Turkey: an exhumed subduction complex with pervasive, syn-kinematic high-pressure assemblages preserved in metasedimentary (marble, quartzite) and metabasaltic (eclogite, blueschist) rocks (Davis & Whitney, 2006; Whitney & Davis, 2006, 2008). This area is particularly interesting because it is one of the few places in the world where lawsonite eclogite has been exhumed to the Earth's surface (although lawsonite eclogite is predicted from phase equilibria and experimental studies to be common in all but the hottest subduction zones).

 

Photograph of a thin section of lawsonite blueschist (Sivrihisar, Turkey). The white crystals are lawsonite.

 

Because the rocks are so well preserved, despite being exhumed from ~80 km depth in a subduction zone, the minerals and fabrics are valuable chemical and physical archives of processes involving (re)recrystallization, mineral-fluid interaction, and deformation. The structural position of the lawsonite-bearing rocks, along with other evidence, indicates that the rocks were deformed and metamorphosed at or near the interface with the overlying mantle wedge.

 

In addition, the massif contains a range of mineral assemblages and structures that record the subduction-to-collision transition: from pristine high-pressure rocks and (micro)structures, a transitional zone of partial overprint, and a complete transition to lower-pressure/higher-temperature metamorphism (Whitney et al., 2011). The Sivrihisar Massif is a very interesting place.

 

 

 

This research was supported by NSF grant EAR-0711263 to Whitney and Teyssier.

 

In addition to Whitney and Teyssier, other members of the research team are Nick Seaton (research scientist), PhD student Katherine Fornash, and colleagues in Turkey and Australia (Daniela Rubatto, Laure Martin, Laure Gauthiez-Putallaz). Previous work in this region was done by former graduate student Peter Davis (PhD 2008 - now Assistant Professor, Pacific Lutheran University).