CHARACTERIZATION OF DEFORMATION MICROSTRUCTURES IN SUBDUCTION ZONE COMPLEXES AND IMPLICATIONS FOR EXHUMATION MECHANISMS

Understanding the rheological behavior of subducted crust at depth is essential to elucidate the geochemical, geophysical and geodynamic evolution of convergent margins. To accomplish this, quantitative analysis of petrofabrics of high-pressure rocks can define deformation mechanisms of high-pressure textures and fabrics that were developed during subduction, exhumation and collision. The Sivrihisar Massif, west-central Turkey, exposes very well preserved blueschist and eclogite facies metasedimentary and metabasaltic rocks that represent exhumed remnants of a passive continental margin sequence of the Neo-Tethyan Ocean. In the NW part of the massif, coherent layers of marble are interlayered with quartzite, calc-schist, and blueschist containing centimeter to meter scale pods of lawsonite eclogite that record metamorphic conditions up to 24 kbar and ~550 ^°C.

In the southeast part of the Sivrihisar Massif, high-pressure low temperature (HP-LT) rocks were partially to completely overprinted by regional metamorphism at lower pressures and a range of temperature, from greenschist to amphibolite facies (sillimanite zone). In a N-S traverse across this part of the massif, blueschist facies assemblages are preserved at the northern end, but are progressively overprinted to the south. Lawsonite has been pseudomorphed by white mica and epidote, garnet in eclogite pods is partially to completely replaced by chlorite, and phengite is rimmed by muscovite. HP marble is characterized by rod-shape calcite crystals (aragonite pseudomorphs) with a very pronounced crystallographic orientation; the strength of the fabric diminishes towards the Barrovian sequence. Quartzite contains relict HP mineral compositions up to the staurolite-in isograd, but evidence for earlier HP conditions in marble is obliterated in the greenschist facies. Some kyanite and chloritoid in the Barrovian sequence may be inherited from the HP metamorphism, as indicated by the presence of kyanite before staurolite and the occurrence of chloritoid in HP quartzite that have not been overprinted.

Quartzite microstructure shows the preservation of a HP texture that was overprinted by Barrovian deformation and recrystallization. The HP texture consists of thoroughly recrystallized grains of quartz and phengite. Argon ages for phengite in quartzite and marble are 88-81 Ma. The Barrovian overprint took place under relatively high flow stress that produced fine recrystallized grain size (50 micron), with evidence of grain boundary migration recrystallization. Micas are recrystallized into very fine grains as well as layers in which bundles of grains display conjugate shear bands; argon ages vary but are typically 62-58 Ma for muscovite at the southern end of the terrain. EBSD pole figures from Barrovian quartzite across this traverse show pronounced constrictional strain girdles in the c-axis (wide-angle small circles about lineation) and a-axis (low angle small circles about lineation).

Relations at the regional to microstructural scales indicate that the Barrovian domain developed in transtension. 3D strain modeling shows that the metamorphic sequence was thinned, resulting in the collapse of isograds. Given the orientation of foliation and lineation, the shape of the finite strain ellipsoid determined from qualitative aspect ratio of tectonites, and the patterns of quartz microfabrics, the Barrovian terrain deformed in an E-W dextral with N-S extension transtension zone, likely during Early Cenozoic time.