Structures of the continental margin of Central America from northern Nicaragua to northern Panama Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6969z410k

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  • Gravity and seismic data obtained by the geophysical group at Oregon State University on the R/V YAQUINA during 1969, 1971, and 1973 plus other available data over the area just west of Nicaragua and Costa Rica indicate the tectonic complexity of the region. Gravity measurements show negative free-air anomalies over the Middle America Trench and the continental shelf of Nicaragua with values as low as -120 mgal and -90 mgal respectively. An outer shell gravity high of +35 mgal occurs between the lows. A large positive anomaly attaining values as high as +110 mgal is associated with the Nicoya Complex, a late Mesozoic assemblage, on the Nicoya Peninsula, Costa Rica. Continuous seismic reflection records acquired along the continental shelf of Nicaragua and Costa Rica indicate a large sediment-filled basin off the coast of Nicaragua coincident with the large negative gravity anomaly on the shelf. Free-air gravity anomalies and seismic reflection profiles suggest an offshore continuation of the Nicoya Complex. Two crustal and subcrustal cross sections of the continental margin of Nicaragua show a large anticlinal structure near the surface of the outer shelf. This structure and an underlying layer are postulated to be marine sediments and basalts similar to those of the Nicoya Complex. The cross sections suggest a ruptured oceanic layer beneath the trench. Imbricate thrusting of oceanic crust along the continental slope is postulated as the mechanism of formation of the continental slope and shelf, and hence the Nicoya Complex of Costa Rica. This model has to be modified for southern Costa Rica where the Cocos Ridge terminates the Middle America Trench. The present elevation of the Nicoya Complex onshore may be attributed to imbricate thrusting and concurrent uplift of the shelf by the Cocos Ridge during subduction.
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