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

Justin Funk, Kirk McIntosh, Paul Mann, Jason Stephens, Sabine Wulf
Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin

Lakes Nicaragua and Managua are the two largest lakes in Central America and cover a combined area of ~9800 km2 of the presently active Central America volcanic arc (CAVA). As part of the NSF Subduction Factory program, we acquired ~1900 km of shallow geophysical data over Lakes Nicaragua and Managua in May, 2006, to establish their late Quaternary structural and stratigraphic history and to better constrain tectonic models for the opening of the Nicaraguan depression. We took a multidisciplinary approach by integrating these data with earthquake relocations, focal mechanisms, NASA Shuttle Radar Topography Mission (SRTM) data, aeromagnetic data, GPS vectors, onland geology, and two previous seismic profiles across the Gulf of Fonseca to investigate upper crustal deformation resulting from forearc sliver transport and/or slab rollback of the Cocos plate. Both lakes and the Gulf of Fonseca form three isolated sub-basins within the Nicaragua Depression, an elongate, asymmetrical, 40-100-km-wide depression extending the length of Nicaragua and into the Gulf of Fonseca and eastern El Salvador. Across-strike profiles of the Nicaragua Depression using shallow seismic profiles and SRTM topographic data indicate a consistently asymmetrical basin controlled by northeast-dipping faults along its southwestern edge. Prominent northeast-dipping surficial faults have been imaged in the Gulf of Fonseca and south of Lake Managua (Mateare fault zone). In general, surficial faults are well exposed in the absence of volcanoes but absent or subtle adjacent to clusters of stratovolcanoes. We used an aeromagnetic data set to better constrain the subsurface location of other segments of the north-east dipping normal or oblique-slip fault that imparts the prominent asymmetry to the Nicaraguan depression. The fault along the southwestern coast of Lake Nicaragua was not imaged in this study but can be seen as a prominent discontinuity on the aeromagnetic map. Faults imaged in Lakes Managua and Nicaragua mainly strike parallel to the long axis of the valley and are interpreted as antithetic normal faults formed during motion along the larger basin-edge fault to the southwest. One major transverse (east-west) fault was mapped across Lake Nicaragua. This fault is left-lateral and formed a large clastic wedge that we have dated as Holocene in age. Our new observations can be used to examine previous models for the opening of the Nicaraguan depression which are based either on earthquake data or surficial mapping studies. These models can be divided into two main groups: 1) basin opening along normal faults parallel to the trend of the depression; transverse faults at right angles to the trend of the valley are thought to function as transform or transverse faults during opening; the mechanism of opening is attributed to slab rollback; and 2) basin opening along transtensional strike-slip faulting parallel to the trend of the depression; strike-slip tectonics is related to the northwestward motion of a forearc sliver known from GPS studies. Our studies show that slab rollback is the likely cause of the asymmetrical half-graben basin shape. Strike-slip faulting likely plays a role in modifying these extensional fault structures.

Poster Session