Gravity constraints on the crustal structure of Central America


Gravity data from Central America and the adjacent offshore regions were analyzed in conjunction with seismic reflection/refraction models, seismicity studies, geologic mapping, and well data to determine a gravity-based crustal structure for the region. Analysis of the gravity data included the construction of a Bouguer and three isostatic residual gravity-anomaly maps, and 2-D gravity models across Guatemala/Belize, Nicaragua/Honduras, and Panama, respectively. The isostatic residual-gravity anomaly map that emphasizes anomalies caused by crustal and upper mantle sources was used to correlate gravity anomalies with known geologic/tectonic features and to emphasize possible previously unknown geologic features in the upper crust. These include granitic rocks in central Guatemala that are related to the Chiapas Massif in southeastern Mexico, basement uplifts under the carbonate platform sediments in northern Guatemala, and thin or dense sediments in the El Salvador depression. In Nicaragua and Honduras, the Nicaragua depression thickens toward the Costa Rican border, and the Mosquitia Basin is seen to consist of a series of depositional centers, possibly pull-apart basins. In Panama, Quaternary volcanic rocks are shown to occur close to the Choco Block boundary in the Gulf of Mexico. However, based on isostatic residual-gravity maxima over mafic igneous material in northern Panama, the boundary between the Chorotega and Choco Blocks may extend 150 km west of its present position. The 2-D gravity models indicate that the crustal thickness of the Maya and Chortis Blocks is approximately 36-38 km and approximately 22 km under Panama. However, the density of the upper mantle is higher under Panama than it is under Guatemala and Nicaragua. To model a large-amplitude gravity maximum along the Middle America subduction zone, a combination of an ophiolitic complex and a steeply dipping Cocos Plate was required. Isostatic residual-gravity anomalies indicate that the ophiolitic complex is not continuous along the Middle America trench but occurs in discontinuous steps. To explain a regional gravity maximum over the Nicaraguan volcanic belt, a deep (>5 km) mafic (?) body was required; however, its exact position cannot be determined from gravity modeling alone. Gravity modeling indicates that subduction does not occur between the Nazca and Caribbean Plates; however, this solution is not unique. If the boundary between the two plates is a transform boundary, this boundary must be dipping at a steep angle. To model a large amplitude gravity maximum and minimum along the northern coast of Panama, a subducting plate (Caribbean) was required, with the Caribbean upper mantle being denser than that beneath Panama.

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AAPG Memoir