Back to: Workshop to Integrate Subduction Factory and Seismogenic Zone Studies in Central America

J. Stephens,

High-resolution (20-250 Hz) multichannel seismic reflection data, totaling approximately 4620 line km, were collected in November-December 2004 (cruise EW04-12) on the inner shelf to slope in the Sandino forearc basin, offshore Nicaragua and Costa Rica. Approximate age constraints are provided by industry well data. The ultimate goal of the project is to identify and distinguish the Neogene stratigraphic signatures of both global sea-level change and local tectonism.

The Mesozoic basement is uplifted to the southeast as it nears the Cocos Ridge and becomes subaerially exposed on the Nicoya Peninsula. Immediately northwest of the peninsula, only a thin (approximately 0.2- 0.4 s) layer of Neogene sediment overlies a prominent unconformity truncating basement. Approximately 50 km farther to the northwest, buried clinoforms that prograde northwestward, nearly perpendicular to the present shelf break, may record episodes of this uplift in the south. Similarly oriented clinoforms are also located still farther to the northwest and are associated with along-strike structural segmentation of the forearc basin.

In general, the basin thickens to the northwest, though some areas are locally uplifted. The northwestern area of the survey images a thick (> 4 s) forearc basin, where a series of angular unconformities truncate heavily faulted Cenozoic strata. Dip profiles show strata with steep basinward dips truncated by an angular unconformity near the seafloor at their landward ends. Continuing basinward, these strata are deformed by a series of approximately trench-parallel folds. Near the shelf edge, the sub-horizontal strata become clinoformal and the section is commonly cut by landward dipping normal faults. Beneath the slope, the faulted top of basement (margin wedge) is resolved, overlain by approximately 1 s of slope sediment. Faulting occurs within slope sediments and there is evidence of both buried and surficial slope failure. In addition, the slope is incised by both modern and buried canyons. A prominent bottom-simulating reflector (BSR), indicative of the presence of a hydrate layer, occurs on the slope throughout the survey area.

Work is underway to construct a regional stratigraphic interpretation. Isopach maps will be developed and used to develop a chronology for basin evolution and timing of major tectonic events.

Poster Session