Deformation mechanisms within an erosive plate margin

The Costa Rica Seismogenesis Project (CRISP) was designed to elucidate the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. CRISP is located at the only known seismogenic zone at an erosional margin within reach of scientific drilling. With a low sediment supply, fast convergence rate, abundant seismicity, subduction erosion, and a change in subducting plate relief along strike, CRISP offers excellent opportunities to better understand earthquake nucleation, rupture propagation, and the mechanisms of deformation along the updip sections of a convergent plate margin. This project aims to test various hypotheses related to the transition from aseismic to seismic behaviour along erosive plate boundaries, that are mainly related to the activity of fluids within the subduction zone: 1) The architecture of the subduction megathrust evolves down dip and the transition from stable to unstable slip corresponds to the transition from a fluid-rich, broad fault zone to a thinner and drier fault. Geological, physical and structural characteristics of material in the subduction channel influence fault mechanics and the transition from stable to unstable slip. 2) Fluid advection affects the localization of faulting and locking of erosional plate boundaries. Fluid chemistry, P-T conditions and residence time affect the state of eroded material through upper-plate basement alteration, diagenesis and low-grade metamorphism. Variations in material/fluid properties and distribution affect fault propagation. This study will focus on constraining the boundary conditions of lithology and fluid flow that control the deformation mechanisms in the seismogenic and aseismic zone along the IODP Expedition 344 drilling transect.

The better understanding of processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones was the reason to found the Costa Rica Seismogenesis Project (CRISP). The area offshore Costa Rica is characterized by an erosional convergent margin, low sediment supply from the upper plate, and abundant plate interface seismicity. During two IODP (Integrated Ocean Drilling Program) Expeditions, 334 and 344, sedimentary and igneous rock material from this area were successfully recovered. In the course of this study, rocks and the hosted veins from Hole 344-U1414A, located on the northern flank of the Cocos Ridge, were analyzed by structural, petrological and geochemical methods. The aim was to get a better understanding of the nature of the sediments and igneous rocks of the upper oceanic crust at the erosive Cocos PlateCaribbean Plate boundary and to evaluate fluid/rock interaction linked with the tectonic evolution of the incoming Cocos Plate, which concern primary tasks of CRISP. Microthermometric analyses of fluid inclusions hosted by carbonate and quartz veins, the oxygen isotopic composition and strontium isotope ratios of carbonate veins suggest seawater as main fluid source, which was later on modified into a hot, hydrothermal fluid. Melt inclusion analysis suggest that the altered basalt provided CO2-rich fluid. The carbon isotopic compositions of the veins are close to values for seawater (0 ) or lower (-3 ). The oxygen isotopic compositions indicate elevated formation temperatures (> 30 C), particularly for the veins in the sedimentary rocks (70 92 C). The elemental compositions of the veins in the basalt and in the sedimentary rocks differ, notably in their rare earth elements and yttrium pattern and in their Mg/Ca ratios, which is mainly the result of fluid-rock interaction. The 87Sr/86Sr ratios reveal various fluid sources for the veins in the basalt, whereas the veins in the sedimentary rocks obtained coherent 87Sr/86Sr ratios, however showing a more primitive source. Microstructural analysis and calcite piezometry on hydrothermal veins showed that a variation of deformation mechanisms affected the rocks at Site 344-U1414. Brittle deformation in the sedimentary rocks and in the basalt, such as vein formation, microcracks in phenocrysts, and crack-seal fabrics, are ascribed to thermal contraction, hydrofracturing, and seismic activity. The increase of bending-related intraplate differential stresses caused crystal-plastic deformation of the vein filling minerals, such as calcite twins and subgrains. Veins in the igneous basement and lithification of the sediments are the result of off-axis low- temperature interaction of seawater with the Cocos Ridge basalt. Intraplate magmatism caused interaction of the rocks with a hot fluid resulted in vein formation in the sedimentary rocks and modification and new formation of veins in the basalt.