Browsing by Subject "Cenozoic tectonics"

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Cenozoic tectonics of the Nicaraguan depression, Nicaragua, and median trough, El Salvador, based on seismic reflection profiling and remote sensing data
(2007-08) Funk, Justin O'Leary, 1982-; Mann, Paul, 1956-; McIntosh, Kirk D.
Lakes Nicaragua and Managua are the two largest lakes in Central America and cover a combined area of ~9000 km2 of the presently active Central America volcanic front (CAVF). As part of the Subduction Factory focus area of the U.S. National Science “Margins” program, we acquired ~1925 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 regional models for active tectonics in western Nicaragua and El Salvador. In order to investigate regional, upper crustal deformation resulting from forearc sliver transport and/or slab rollback of the Cocos plate, I took a multidisciplinary approach by integrating these new data with: relocated earthquake epicenters; earthquake focal mechanisms; high-resolution digital topography from the NASA Shuttle Radar Topography Mission (SRTM); published global positioning system (GPS) vectors showing recent plate motions; onland geologic maps showing bedrock geology; previous maps of lake bathymetry and bottom sediment types; a previously unpublished, regional aeromagnetic data set; and multichannel seismic reflection profiles from the Gulf of Fonseca and Pacific Ocean. Both lakes and the Gulf of Fonseca occupy the Nicaraguan depression, which is an elongate, asymmetrical, 40-100-km-wide depression extending 342 km across the length of Nicaragua, across the 57-km-wide Gulf of Fonseca, and 225 km into the neighboring country of El Salvador where it is called the Median trough. Profiles across the Nicaraguan depression using subbottom profiles and high-resolution SRTM topographic data indicate that the basin structure is a highly asymmetrical half-graben bounded to the southwest by northeast-dipping faults with basement shallowing to the northeast. Depth to basement in the lakes area is uncertain due to the lack of deep-penetration multichannel seismic data and exploration wells. Depth to basement in the Gulf of Fonseca is also not known but does contain an overlying asymmetrical wedge of low-velocity sediments based on multichannel seismic velocity models. New multichannel seismic, subbottom profiler, and bottom core data from the 2006 NicLakes study have improved bathymetric, bottom sediment, and recent fault maps for both lakes that can be compared to previous, speculative models for the lake’s geology and tectonics. Previous work using only subaerial observations from the Nicaraguan depression - Median trough have proposed three different models for the formation of the depression and its active, seismogenic transverse faults: 1) Neogene normal faulting parallel to the length of the depression induced by slab rollback of the underlying subducted slab of the Cocos plate beneath Nicaragua and El Salvador; the zone of extension is proposed to have migrated from the northeast to the southwest from Miocene to the present; 2) Neogene to present right-lateral strike-slip faulting parallel to the volcanic front and offset locally by pull-apart basins, or centers of fault-bounded extension 3) Negoene to present right-lateral strike-slip faulting parallel to the volcanic front that is accommodated by left-lateral transverse or “bookshelf faults” at high angle to the volcanic front. The 2006 data set combined with the many preexisting data sets listed above have been integrated into three regional structural cross sections. These sections show the asymmetrical half-graben structure for the Nicaraguan depression-Median trough. The area of greatest subsidence and footwall uplift is in the Lake Nicaragua area; in the southeast; the area of least subsidence/footwall uplift is in the Gulf of Fonseca in the northwest. I interpret this structural pattern as a time-transgressive basin opening with the oldest extension pre-Miocene) starting in the southeast and migrating to the northwest. GPS data indicates that this earlier phase of intra-arc rifting is now being modified by arc parallel shear related to the northwestward transport of the Central American forearc sliver.