PT Unknown AU Zhu, J TI Crustal structure and subduction erosion in the central Costa Rica subduction zone PY 2010 PU Christian-Albrechts-Universität zu Kiel WP https://macau.uni-kiel.de/receive/diss_mods_00005409 LA en DE Subduction zone processes; continental margins; crustal structure; Subduktionsprozesse; Kontinentalrand; Krustenstruktur AB On the Pacific margin off central Costa Rica, an anomalous lens-shaped zone is located between the overriding plate and the subducting oceanic lithosphere approximately 25 km landward of the deformation front. This feature was previously recognized in reflection seismic data when it was termed ‘megalens’. Its origin and seismic velocity structure, however, could not unambiguously be derived from earlier studies. Therefore during RV SONNE cruise SO163 in 2002, seismic wide-angle data were acquired using closely spaced ocean bottom hydrophones and seismometers along two parallel strike lines and two parallel dip lines above the ‘megalens’, intersecting on the middle slope. In this study, the P-wave velocities and structure of the subducting oceanic Cocos Plate and overriding Caribbean Plate are determined from modeling of the wide-angle seismic data of cruise SO163. In addition, coincident reflection seismic data are analyzed and incorporated into the forward modeling and tomographic inversion of the refraction data. Based on the results of the velocity modeling, synthetic seismograms are calculated for an amplitude analysis of the refraction data to determine seismic attenuation variations across the seaward extent of the margin. The margin wedge is defined by high seismic velocities (4.3-6.1 km/s) identified within a wedge-shaped body covered by a slope sediment drape. It is divided into two layers with different velocity gradients. Seismic velocities of the upper margin wedge vary from 4.0 to 4.3 km/s near the trench to 4.0-5.1 km/s close to the shoreline. The lower margin wedge is clearly constrained by decreasing velocities towards the trench and terminates beneath the middle slope at the location of the ‘megalens’. Seismic velocities of the ‘megalens’ are lower (3.8-4.3 km/s) relative to the margin wedge, implying that the ‘megalens’ represents hybrid material composed of subducted sediment and eroded fragments from the base of the upper plate. Upward-migrating overpressured fluids weaken the base of the margin wedge through hydrofracturing, V thus causing material transfer from the upper plate to the lower plate. Results from amplitude modeling confirm that the ‘megalens’ observed off central Costa Rica is bound by a low velocity zone documenting fluid drainage from the plate boundary to the upper plate. Seismic attenuation of the central Costa Rica margin wedge is determined from amplitude analysis of the wide-angle seismic data. Travel time and amplitude modeling is applied to ocean bottom hydrophones along two trench-parallel profiles, located 30 km (P21 & P22) and 35 km (P18) landward of the deformation front northeast of Quepos Plateau. Tomographic inversion images a progressively thinning margin wedge from the coast to the lower slope at the trench. A 1-1.5 km thick décollement zone with seismic velocities of 3.5-4.5 km/s is sandwiched between the margin wedge basement and the subducting Cocos plate. For strike line P21, amplitude modeling indicates a Qp value of 50-150 for the upper margin wedge with velocities of 3.9-4.9 km/s. Along strike line P18, Qp values of 50-150 are determined with velocities of 4.3-5.0 km/s in the upper margin wedge, increasing to 5.1-5.4 km/s in the lower margin wedge. Quantitative amplitude decay curves support the observed upper plate Qp. In conjunction with earlier results from offshore Nicoya Peninsula, our study documents laterally landward decreasing attenuation across the margin wedge, consistent with a change in lithology from the sediment-dominated frontal prism to the igneous composition of the forearc middle prism. PI Kiel ER