HR: 1345h
AN: T22C-0929
TI: A Quantitative Analysis of Flexural Faulting in the Cocos Plate at the Middle America Trench from Nicaragua to Costa Rica
AU: * Kelly, R K
EM: rkelly@whoi.edu
AF: Woods Hole Oceanographic Institution, MS22, Woods Hole, MA 02543 United States
AU: McIntosh, K D
EM: kirk@ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd Bldg 600, Austin, TX 78759 United States
AU: Silver, E A
EM: esilver@emerald.ucsc.edu
AF: Department of Earth and Marine Sciences, University of California, Santa Cruz, CA 95064 United States
AU: Goff, J A
EM: goff@ig.utexas.edu
AF: University of Texas Institute for Geophysics, 4412 Spicewood Springs Rd Bldg 600, Austin, TX 78759 United States
AU: Ranero, C R
EM: cranero@geomar.de
AF: GEOMAR Research Institute, Wischhofstrasse 1-3, Kiel, Germany
AU: von Huene, R
EM: rhuene@geomar.de
AF: GEOMAR Research Institute, Wischhofstrasse 1-3, Kiel, Germany
AB: Flexural faulting in the Cocos plate offshore Nicaragua and Costa Rica seaward of the Middle America Trench was quantitatively analyzed by correlating a stochastic statistical analysis of Cocos plate hydrosweep bathymetry data with a measurement of basement fault relief in MCS data. This work was conducted in order to examine the potential relationship between oceanic basement structure and the efficiency of sediment subduction as signified by geochemical anomalies in the Central American Volcanic Arc. RMS height, characteristic width, and the azimuth of the hydrosweep bathymetry surface topography are shown to be an accurate indicator of basement fault size and distribution as identified in the MCS data. Using hydrosweep bathymetry as a proxy for basement fault structure provides a broader data coverage of the Cocos plate than MCS data alone. The characteristic width and azimuth of the surface topography demonstrate that where the Cocos plate is East Pacific Rise crust, the flexural faults appear to be reactivations of abyssal hill fabric. When the Cocos plate becomes Cocos-Nazca Spreading Center crust off the Nicoya Peninsula, the flexure of the subducting plate breaks new faults. The MCS data image a fairly similar sediment thickness as the Cocos plate approaches the trench (400-500 m) but a decrease in basement fault throw from Nicaragua to Costa Rica. High basement fault relief ($\sim$500 m) is present in the Cocos plate off Nicaragua but low relief basement faults ($<$300 m) exist off Costa Rica. The regional scale over which this transition is observed in the MCS data is narrowed in focus by the stochastic modeling of the bathymetry. This transition in fault size, as illustrated by a sharp decrease in the RMS height of the surface relief, is spatially coincident with the boundary between Nicaragua and Costa Rica. During subduction, the flexural response of the downgoing slab can result in large scale extensional faulting in the oceanic crust that generates horsts and grabens, or half-grabens as in the case of Costa Rica and Nicaragua, at the trench. If the thickness of the subducting sediment column is on the same order of magnitude or less, the grabens can act as traps for the downgoing sediment, preserving the entire sediment column for subduction, including the uppermost units. Offscraping or underplating may be more prevalent when graben formation on the downgoing plate is minimal or small in comparison with the overlying sediment thickness. The Central American Volcanic Arc exhibits a wide range of $^{10}$Be concentrations, from background to highly enriched. As $^{10}$Be is only present in the shallowest sediments on a subducting plate, the high $^{10}$Be concentrations observed in the Central American arc volcanics are a clear indication that the youngest, uppermost sediments on the downgoing plate are transported to the volcanic roots. Young sediment incorporation into the volcanics seems to be strongest for Nicaragua but minimal to nonexistent for Costa Rica. It is possible that variation in graben relief along strike of the trench is partially if not largely responsible for this change in arc volcanic $^{10}$Be signatures.
DE: 3022 Marine sediments--processes and transport
DE: 3025 Marine seismics (0935)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology and bottom photography
DE: 8150 Plate boundary--general (3040)
SC: T
MN: 2001 AGU Fall Meeting