Site survey data evaluation IODP Proposal 633
Final Report Abstract
IODP drilling proposal 633 aims to enhance the general understanding of complex forearc dewatering processes of the erosive subduction system off Costa Rica. Major sites of dewatering planned for drilling are mounds, related to mud diapirism/volcanism and precipitation of authigenic carbonates, and a large-scale slide related to the subduction of a seamount. In order to promote the proposal as well as to test the hypothesis of deeply sourced and fault-controlled dewatering sites, new seismic pre-site survey data were acquired for the proposed drill locations at Mound Culebra, Mounds 11 & 12, and at Jaco Scar, which is the site for the selected seamount subduction. A first set of 2-D MCS seismic profiles was acquired at the mounds using the 36-gun, fourstring linear gun array of R/V Marcus Langseth, and a 240-channel streamer with 3000 m of active length. The seismic lines were prestack depth migrated, i.e. the velocity model was iteratively improved using depth focussing analysis and residual moveout correction on common image point gathers. Improvement of the deep imaging involved multiple attenuation and detailed velocity analysis of the lower sedimentary portions and beneath the basement down to the plate boundary. Another set of 2-D profiles was acquired at Jaco Scar, using a 128 channel fragment of the IFM-GEOMAR 3-D P-cable streamer system providing 200 m of active length, and two 520-cinch G-guns as the seismic source. Additionally, R/V Sonne deployed 6 ocean bottom seismometers (OBS) in order to receive complementary velocity information for the uppermost 1-2 kilometers of sediment around the proposed drill locations. For the subducted seamount site at Jaco Scar the new seismic data resolve the sedimentary portions directly underneath the proposed drill locations and thus are complementary to the close-by and virtually-parallel running lower-frequency SO81 records which additionally image the plate interface and the top of the subducted seamount. However, both strike and cross line reflection records image strong side echoes from the adjacent scarp which makes the identification of fault structures spurious. For the mud mound sites our results reveal an upward bending of the bottom simulating reflection (BSR), which implies a thinning of the gas hydrate bearing layer (GHSZ) probably due to channeled fluid flow and associated hydrate dissociation in the area of the fluid conduit. BSR-derived heat flow anomalies underneath the mounds suggest higher fluxes at Mound 12 than beneath close-by Mound 11, which corroborates differences in terms of the mounds' activity and stage of development. The obtained depth models show that both mounds are situated above a prominent basement high, which indicates a higher permeability for the basement than for the sedimentary overburden. The strong relief of the basement imaged in our data demonstrates that these mounds are both related to deep-reaching fault systems. A band of reflections in the lower margin wedge above the plate interface might present an accumulation of subducted lower plate sediments and eroded upper plate fragments. Our data provide new images of the fluid flow system offshore Costa Rica and may help to better understand the interactions of gas hydrate formation and dissociation with the fluid ascent from the deep sources. Moreover, the results indicate that the fracture porosity of deepreaching faults is the dominant factor governing the hydrogeological system at this erosive continental margin. Besides, submission of the new seismic data to the IODP site survey data base resulted in a top ranking for 8 out of 9 proposed drill locations.
Publications
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(2009) Drilling into dewatering sites along the Costa Rica margin (IODP proposal 633): first results from pre-site survey seismic data evaluation. In: 69. Jahrestagung der Deutschen Geophysikalischen Gesellschaft (DGG), 23.03.-26.03, Kiel
Planert, L., Klaeschen, D., Berndt, C., Brückmann, W. und Hensen, C.
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(2010) Buildup and gas hydrate system of mound structures offshore Costa Rica revealed by 2D MCS seismic data. In: 70. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 15.-18.03, Bochum
Planert, L., Klaeschen, D., Berndt, C., Brückmann, W. und Hensen, C.
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(2010) Controls on mound formation and effects of fluid ascent on the gas hydrate system of mound structures offshore Costa Rica. In: AGU Fall Meeting 2010, 13.- 17.12.2010, San Francisco, USA
Planert, L., Klaeschen, D., Berndt, C., Brückmann, W. und Hensen, C.
