NicStrat

Principal Investigator: Craig Fulthorpe
Co-Principal Investigator: Kirk McIntosh

Funding agency: NSF OCE, award # 0241179
Start Date: October 1, 2003
Expires: September 30, 2006 (Estimated)

Forearc basin sediments are influenced by both tectonic events associated with plate convergence and global sea-level (eustatic) fluctuations. However, sequence stratigraphic investigators have tended to avoid convergent margins because of the perception that tectonic forcing would overwhelm the eustatic effect. Recent seismic investigation of the offshore Eel River Basin, northern California, suggests that this is not the case and supports the hypothesis that unconformities generated by tectonic uplift are localized near structures and distinguishable from the more regional sequences that are of eustatic origin.

This project, therefore, has two overriding objectives: 1) Test the hypothesis that the stratigraphic records of eustatic and tectonic processes can be identified and distinguished on high-resolution, multichannel seismic (MCS) data from the Sandino forearc basin, on the Pacific margin of Nicaragua. These data will be used to interpret unconformities and sequences across and along the margin to document their geometries and extent. Subsequent IODP drilling will date interpreted sequences for correlation with coeval sequences on other margins and with the deep-sea oxygen isotope proxy for sea-level change. 2) Evaluate the record of tectonic events in the form of local unconformities and/or tectonically controlled variations in the geometries and stacking patterns of regional eustatic sequences. These will be correlated with known and proposed tectonic episodes in the region, including changes in plate motion, and uplift and subsidence associated with hypothesized tectonic erosion and accretion.

High-resolution MCS data were collected from the inner shelf to the slope, in the Sandino forearc basin, offshore Costa Rica, Nicaragua, Honduras and El Salvador by the University of Texas Institute for Geophysics (UTIG; Figure 1). The survey (R/V Maurice Ewing cruise EW04-12; 21 November - 22 December 2004) was designed to image Neogene depositional sequences indicative of sea-level change and stratal geometries that may reflect local tectonic history. Benefiting from good weather and the excellent performance of the seismic system, we exceeded our pre-cruise goal for line-km acquisition by covering ~4620 km of our planned profile grid. (This includes distance covered when the seismic system was shut down or powered down owing to proximity of marine mammals or turtles, as well as minor equipment problems.)

The seismic data are now being processed. We hope to complete processing by the fall of 2005. Preliminary processing indicates that we can expect excellent results. Shelf profiles penetrate 2-3 s. two-way traveltime and image anticlines, with which local unconformities may be associated, and regional depositional sequences, all of which can be mapped (Figure 2). Ultra-high-resolution chirp profiles of the upper ~20 m complement the MCS data. Slope profiles document faults, possible current-deposited bedforms, and a bottom-simulating reflector (BSR), indicative of the presence of a hydrate layer. The survey also extends to the Gulf of Fonseca, which provides an opening through which to conduct a marine survey across the line of the volcanic arc. It allows us to image the boundary zone between the volcanic arc and the Nicaraguan Depression, a backarc basin that extends the length of Nicaragua and into El Salvador. We also speculate that the gulf may have provided an opening for sediment from the interior to reach the Sandino Basin offshore, where the bulk of our survey is located.