Abstract
In this study we present microstructural data from hydrothermal veins in the sedimentary cover and the igneous basement recovered from Hole U1414A, Integrated Ocean Drilling Program (IODP) Expedition 344 (Costa Rica Seismogenesis Project), to constrain deformation mechanism operating in the subducting Cocos Plate. Cathodoluminescence studies, mechanical e‐twin piezometry and electron backscatter diffraction (EBSD) analyses of carbonate veins were used to give insights into the deformation conditions and to help to understand the tectonic deformation history of the Cocos Plate offshore Costa Rica. Analyses of microstructures in the sedimentary rocks and in the basalt of the igneous basement reveal brittle deformation, as well as crystal‐plastic deformation of the host rock and the vein material. Cathodoluminescence images showed that in the basalt fluid flow and related precipitation occurred over several episodes. The differential stresses, obtained from two different piezometers using the same parameter (twin density), indicate various mean differential stresses of 49 ± 11 and 69 ± 30 MPa and EBSD mapping of calcite veins reveals low‐angle subgrain boundaries. Deformation temperatures are restricted to the range from 170°C to 220°C, due to the characteristics of the existing twins and the lack of high‐temperature intracrystalline deformation mechanisms (>220°C). The obtained results suggest that deformation occurred over a period associated with changes of ambient temperatures, occurrence of fluids and hydrofracturing, induced differential stresses due to the bending of the plate at the trench, and related seismic activity.
Highlights
Integrated Ocean Drilling Program (IODP) Expedition 344 as part of the Costa Rica Seismogenesis Project (CRISP) focused on sampling the lithology of the incoming and overriding plates, characterizing recent and paleo fluids, and measuring the ambient temperatures and stresses that lead to a transition from stable to unstable slip along the Cocos Plate-Caribbean Plate boundary [Harris et al, 2013a]
In this study we present microstructural data from hydrothermal veins in the sedimentary cover and the igneous basement recovered from Hole U1414A, Integrated Ocean Drilling Program (IODP) Expedition 344 (Costa Rica Seismogenesis Project), to constrain deformation mechanism operating in the subducting Cocos Plate
Veins are heterogeneously distributed within the lithified sedimentary rocks of Unit III and the oceanic basement
Summary
Integrated Ocean Drilling Program (IODP) Expedition 344 as part of the Costa Rica Seismogenesis Project (CRISP) focused on sampling the lithology of the incoming and overriding plates, characterizing recent and paleo fluids, and measuring the ambient temperatures and stresses that lead to a transition from stable to unstable slip along the Cocos Plate-Caribbean Plate boundary [Harris et al, 2013a]. Previous investigations showed that the Cocos Plate offshore Costa Rica has a complex tectonic history [Meschede et al, 1998; Barckhausen et al, 2001; Brandstätter et al, 2016]. The purpose of this paper is to constrain deformation mechanisms and required conditions in the upper crust of the Cocos Plate offshore Osa Peninsula and to get a better understanding of the deformation history at Site 344-U1414 by the investigation of mineralized veins. The formation and composition of veins and deformation mechanisms operative in the vein material are important to obtain information about stress conditions, fluid sources and circulations, and chemical alteration processes [Janssen et al, 1998; Gratier et al, 2003; Herwegh et al, 2005].
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