HR: 0830h
AB: In May and June this year we acquired over 2000 km of MCS data offshore the Pacific margin of Nicaragua and northern Costa Rica. We used the R/V Maurice Ewing for this work, employing its 240-channel, 6-km-long streamer, and a modified 20-airgun source array. We nominally recorded for 16 seconds after each shot, but increased the record length to 20 seconds to capture deep reflections beneath the Sandino forearc basin near the Nicaraguan coast. In addition to the MCS data, we acquired about 600 km of OBH data along two dip transects and one strike profile. The cruise had many important goals, which include the following: 1) Document the structure of the subducting Cocos plate as it varies along strike; 2) Observe interaction of the upper and lower plates in the vicinity of the trench and lower slope; 3) Observe the plate boundary position and its morphology into and through the seismogenic zone; 4) Observe the stratigraphy and deformational structures in the Sandino Basin. Based on the preliminary seismic data processing performed on board the ship, we found that the Cocos plate is significantly more faulted entering the trench off southern Nicaragua than it is off the central Nicoya Peninsula (Costa Rica), about 80 km to the SE; the intensity of faulting continues to increase NW along the Nicaragua margin. On all profiles we observe a reflection marking the Cocos/Caribbean plate boundary, which extends from the trench across most or all of the seismic images. We roughly estimate that this reflection can be commonly observed to more than 20 km depth and possibly to as deep as 40 km. Thus in some cases we may image the boundary through the entire seismogenic zone of plate boundary earthquakes. The subset of our profiles that cross the entire forearc shelf show that the Sandino Basin deepens dramatically to the NW, with a sedimentary section at least 10 km thick in places. These preliminary observations will be tested following full processing of the data, which is now in progress. In addition, the OBH data will provide velocity constraints to more accurately estimate the plate boundary geometry.
AN: T11A-09
TI: Preliminary Results of a Nicaragua-Costa Rica MCS/OBH Seismic Survey
AU: * McIntosh, K D
EM: kirk@ig.utexas.edu
AF: Univ of Texas Inst for Geophysics, 4412 Spicewood Sprgs Rd, Bldg 600, Austin, TX 78759 United States
AU: Silver, E A
EM: esilver@es.ucsc.edu
AF: Earth Sciences Department, Univ of California, Santa Cruz, A112 E&MS Bldg , Santa Cruz, CA 95064 United States
AU: Ranero, C R
EM: cranero@geomar.de
AF: GEOMAR, Wischhofstrasse 1-3, Kiel, 24148 Germany
DE: 8105 Continental margins and sedimentary basins
DE: 3025 Marine seismics (0935)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: T
MN: Fall Meeting 2000