Alastair M. John

My current research interest is the exhumation of high-pressure (HP-LT) metamorphic rocks, with particular emphasis on structural analysis and fluid inclusion studies. Out of necessity I have chosen two field areas for my PhD studies: (1) the Caribbean margin in Venezuela focused on the Villa de Cura (VdC) blueschists, and (2) the Pacific margin of the United States focused on the Santa Catalina Island Schist (SCIS) in California .

The Villa de Cura blueschists form a coherent slab of exhumed HP-LT metamorphic rocks in north-central Venezuela . These blueschists record peak metamorphic ages around 90 Ma and are thought to have formed during the subduction of the proto-Caribbean plate beneath the Farallon plate. From plate-tectonic reconstructions it has been inferred that this happened at longitudes comparable to those of central-America at present. Transpressive plate motions have been inferred to provide the forces required to transport and uplift these rocks to their current position onto the South-American plate.

The Santa Catalina Island Schist is thought to have undergone peak metamorphism during the later stages of subduction of the Farallon plate under the North American plate (around 100 Ma). Exhumation is though to have occurred due to the transition from the aforementioned destructive plate boundary to the present transpressive plate boundary between the Pacific and North American plates. The reconstruction for the deformation - and therefore uplift and exhumation - history for this unit may be additionally complicated by block rotations within the Southern California Borderland region.

To better understand the uplift and exhumation of the VdC and SCIS HP-LT metamorphic rocks I am mapping the structural geology of both areas. In this way I will be able to discern the main deformational styles and structures and be able to relate these to existing plate tectonic reconstructions and/or allow me to formulate new plate tectonic models. Additional tools used in both areas include micro-structural analysis of thin sections and fluid inclusion studies. When combined with published geochemical and geothermobarometric data I will be provide a more complete understanding of how the VdC and SCIS HP-LT where returned to the surface after having been subducted to depths of up to 40 km. To better quantify the rotations that the SCIS may have undergone I may do some paleomagnetic studies in addition to the above. Ar/Ar radiometric dating will be done on samples from the VdC to be able to test the hypothesis that the deformation of these rocks is younger in the east than in the west. If this hypothesis proves correct is has great

importance to the understanding of how large bodies of HP-LT rocks can be exhumed in transpressive margins, as well as implications for the temporal evolution of the Caribbean - South America plate margin.