Graduate Thesis Or Dissertation

 

Heat Flow along the Southern Costa Rica Margin : Insights on the Updip Limit of Seismicity Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/nv9355513

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  • Heat flow analysis of the Costa Rica convergent margin is carried out for seven core sites drilled during Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344 as part of the Costa Rica Seismogenesis Project (CRISP). These expeditions were designed to develop a better understanding of erosional subduction zones. Heat flow measurements were made to improve estimates of the thermal structure and fluid-flow system of this erosive margin. Drilling sites are located on the incoming plate, and the toe, lower, middle, and upper slopes of the margin. Heat flow estimates for each site are determined according to Bullard analysis using thermal conductivity values measured on board the ship and temperature measurements made in situ. Heat flow values are corrected for effects of seafloor bathymetry and sedimentation. Bathymetry corrections are relatively small, < ±6% for all sites. Sedimentation corrections range from 5-10% at the frontal prism and oceanic plate sites and increase to 10-30% at the middle and upper slope sites where rapid sedimentation rates suppresses heat flow. Heat flow on the incoming plate is approximately 160 to 210 mW/m², decreases to 116 mW/m² on the lower slope and then to values of 46-56 mW/m² on the middle and upper slopes. These values agree with previously reported BSR-derived and shallow marine probe measurements and together show a landward decrease in heat flow consistent with the downward advection of the Cocos plate. Thermal models of the shallow subduction zone successfully predict observed values of heat flow and suggest that temperatures on the subduction thrust increase from 2° C at the deformation front to 100° C at a distance of 45 km landward of the deformation front. The updip limit of seismicity, as defined by aftershocks events of ML 1-4 recorded following the 1999 M[subscript w] 6.9 Quepos earthquake and 2002 M[subscript w] 6.4 Osa Earthquake, occurs at cooler temperatures than the 100-150°C typically predicted. I propose that the rough incoming bathymetry of the Cocos Ridge in this sediment-deprived margin enables rupture closer to the surface than at margins with a smooth, heavily sedimented incoming plate.
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