HR: 0830h
AN: T51F-0221    [PDF]
TI: Subduction at an Erosive Margin: Effects of Bathymetry on Fore arc Deformation in Costa Rica
AU: * Sitchler, J C
EM: sitchler@psu.edu
AF: The Pennsylvania State University, Department of Geosciences, 534 Deike Building, University Park, PA 16802 United States
AU: Fisher, D M
EM: fisher@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, 339 Deike Building, University Park, PA 16802 United States
AB: The Fila Costena, an inner fore arc thrust system along the Middle America Trench, shows systematic variations in fault geometry and shortening that reflect subducting seafloor bathymetry. The thrust belt is approximately 20 km wide at the Terraba gorge and Rio Claro areas where a series of faults imbricate the late Tertiary fore arc basin sequence comprised of the Eocene Brito Fm. and Oligocene to early Miocene Terraba Fm. Strata and thrust faults strike WNW-ESE and dip approximately 15-40 degrees NE at the thrust front. Along structural strike to the ESE, the thrust belt abruptly dies at the onland projection of the Panama Fracture Zone. Displacement is greatest on the frontal thrust that separates the outer fore arc, characterized by exposed crystalline basement rocks, from the inner fore arc. Although the Fila Costena is a continuous thrust belt, there are only two regions where the Brito Fm., the basal unit of the fore arc basin sequence, has been exposed through thrusting: 1) the onland projection of the axis of the subducting Cocos Ridge and 2) the flanks of the Herradura block inboard of a major chain of subducting seamounts. There are two possibilities to explain the appearance of limestones in these localities. The basal decollement either steps down laterally into the basal limestones in these regions or the fore arc basin experienced long wavelength folding over subducting features prior to development of a decollement at constant depth. In both cases shortening based on balanced cross sections increases inboard of large subducting asperities. A comparison of cross sections at the Rio Terraba gorge and near Rio Claro shows that total shortening increases from approximately 17km (Rio Terraba) to considerably greater values where the thrust belt nears the onland projection of the Cocos Ridge axis, as indicated by additional thrust sheets that sole into the Brito limestone.
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting