HR: 1340h
AN: OS23A-1060    [Abstracts]
TI: Analysis of High-Resolution OBS/H and MCS Data Across the Termination of a BSR in Lima Basin, Peru Margin – Velocity Structure and Paleo-BSRs
AU: Garcia, P G
EM: pablo.garcia@pet.hw.ac.uk
AF: Heriot-Watt U, Riccarton, Edinburgh, EH14 4AS, United Kingdom
AU: Pecher, I A
EM: ingo.pecher@pet.hw.ac.uk
AF: Heriot-Watt U, Riccarton, Edinburgh, EH14 4AS, United Kingdom
AU: * Kukowski, N
EM: nina@gfz-potsdam.de
AF: GFZ, Telegrafenberg, Potsdam, 14473, Germany
AU: Huebscher, C
EM: christian.huebscher@zmaw.de
AF: U of Hamburg, Bundesstr 55, Hamburg, 20146, Germany
AU: Clift, P D
EM: pclift@abdn.ac.uk
AF: U of Aberdeen, Meston Bldg, Aberdeen, AB24 3UE, United Kingdom
AU: Ruppel, C D
EM: cruppel@usgs.gov
AF: USGS, 384 Woods Hole Rd, Woods Hole, MA 02543, United States
AU: Bialas, J
EM: jbialas@ifm-geomar.de
AF: IfM-Geomar, Wischhofstr 1-3, Kiel, 24148, Germany
AB: Bottom simulating reflections (BSRs) in Lima Basin are confined to areas where the sub-surface structure is predicted to focus fluid flow. Most BSRs seem to be caused by free gas at the base of gas hydrate stability (BGHS). An increase of fluid flow and hence, methane flux, across the BGHS is thought to "trigger" formation of BSRs once methane concentration exceeds its solubility in pore water. Enhanced methane flux should also lead to elevated gas hydrate concentration above the BGHS. A BSR in the north of the basin coincides with outcropping layers, which often enhance fluid flow. During the GEOPECO campaign of the R/V Sonne in 2000, we deployed nine ocean bottom seismometers/hydrophones (OBS/Hs) at 1.85-km spacing across the termination of this BSR for a high-resolution OBS/H and short-streamer multichannel seismic transect. In order to investigate whether high gas hydrate concentrations are present in the hydrate zone, we compared velocities above the BSR to those in the same hydrate-free layer further landward (sequence L5 in an earlier classification). The OBS/H records were repositioned to sealevel in the τ-p domain followed by semblance analysis for determination of P-wave velocity (Vp). Vp in sequence L5 above the BSR appears lower than that at the hydrate-free site. This finding indicates that highly concentrated gas hydrates, if present at all, are confined to thin layers that do not significantly affect Vp of the entire interval. We have also identified at least two paleo-BSRs above the present BSR level. These paleo-BSRs most likely mark previous levels of the BGHS. Changes of the pressure/temperature regime must have happened sufficiently rapidly in order to "freeze" BSRs. We propose that the most likely mechanism for such rapid changes is sudden exhumation after mass wasting. The presence of at least two paleo-BSRs suggests that mass wasting has occurred repeatedly.
DE: 3004 Gas and hydrate systems
DE: 3025 Marine seismics (0935, 7294)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3070 Submarine landslides
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting