NicLakes

Principal Investigators: Kirk McIntosh and Paul Mann

Funding agencies: NSF EAR (award # 0440143) and the Jackson School of Geosciences
Start Date: June 15, 2005
Expires: May 31, 2009 (Estimated)

Abstract
Oblique subduction along the Pacific margin of Central America causes the forearc terrane to move slowly northwest, parallel to the margin and the volcanic arc. This motion, relative to the interior of the Caribbean plate, is accommodated by faulting in the low-lying Nicaragua depression, but because of very active volcanism and large lakes that cover much of this area the tectonic nature of this deformation zone is poorly known. We propose a lake-based geophysical mapping program (seismic, sidescan sonar, swath bathymetry) to determine the pattern of active faults that accommodate the forearc sliver and a coring program to date fault activity and investigate the recent volcanic history. Because the deformation may influence volcanic activity and possibly composition, the combined program may help identify such a relationship in this area. Cores will also be used to estimate eruption volumes, and investigate volcanic composition changes through time. Cores will also be used to establish paleo-climate time series, which can be tied to other Central American records and to Pacific and Caribbean marine records.

This proposal describes an integrated study involving scientists from UTIG (McIntosh and Mann), UTDOGS (Gardner), and Department of Geography and Environment (Dull) whose diverse scientific interests overlap significantly in the active processes of the Nicaragua depression. This study represents a logical extension of previous individual projects by this group, and it will also set the stage for further collaborative projects in the future.

Project Summary
Upper plate deformation at convergent margins frequently involves extensional basin formation and arc parallel shearing, with slab rollback and oblique convergence as generally accepted driving mechanisms. Geodetic, earthquake, and geologic structure analyses all indicate that oblique convergence is commonly partitioned into an arc-normal component, accommodated by interactions of the subducting plate and the forearc, and an arc-parallel component, which may be absorbed by deformation in the forearc, backarc, or concentrated along the arc. Two of the major of arc-parallel deformation modes associated with oblique convergence are crustal block rotation and forearc detachment by strike-slip faulting along the volcanic arc--the forearc sliver model. While the forearc sliver model is widely supported in the Sumatran and Kuril arcs, interpretations of the Aleutian arc have supported the block rotational model. The Nicaraguan depression is a prominent Quaternary morphologic and structural depression aligned with the belt of active Central American volcanoes and extending approximately 600 km from the northern Gulf of Fonseca in El Salvador and northern Nicaragua to the Caribbean Sea in Costa Rica. Here, both block rotation and forearc sliver models have been proposed to accommodate dextral forearc translation driven by oblique convergence. To test these two hypotheses, we plan to exploit the existence of two large, shallow lakes (Nicaragua and Managua), which cover a combined area of ~9000 km2 in Nicaragua where the depression encompasses some of the largest, most active, and most intensively faulted volcanoes of the entire Central American volcanic arc. We propose to collect roughly 1000 km of high-resolution seismic data using a portable "uniboom" seismic reflection system mounted on passenger ferries. This work will map the active fault patterns in the lakes and allow us to distinguish between the proposed deformation models. The block rotation model requires a series of NE striking, left-lateral faults crossing the lakes. The Tiscapa fault, responsible for the destructive 1972, M6.2 earthquake in Managua, is one of several known left-lateral faults that project into the lakes; we will map its extent into Lake Managua and acquire data to detect the presence and extent of similar faults in both lakes. The forearc sliver model suggests that a major right-lateral fault system is aligned with the active volcanic chain and that extensional, pull-apart basins will develop between right-stepping arc segments. Thus we will investigate the presence of an intra-arc right-lateral fault and also focus on three major right-step-over areas in the two lakes.

In addition to testing deformation models related to oblique subduction, we also expect learn about development of the Nicaragua depression. Some models suggest that the depression formed largely in Plio-Pleistocene times by arc-normal extension associated with slab rollback. However, if the forearc sliver model applies here, then pull-apart basin development at right-step-overs along the strike slip system may also be an important and ongoing factor in formation of the Nicaragua depression. Our data acquisition will approach both the NE and SW margins of the lakes to investigate the presence and orientation of normal faults. Although the Mateare fault, south and west of Managua, is considered the SW boundary of the depression, we may be able to map associated faults in both lakes.

We consider the Nicaragua depression to be an excellent location to investigate these widely proposed models of upper plate deformation. Geologic, geodetic, and earthquake studies have made good use of the exposed land areas here, and the water-covered areas allow efficient seismic data acquisition over a wide area. The combined data sets should lead to true interdisciplinary progress.

Broader Impacts. Strike-slip earthquakes generated by a right-lateral fault zone along the arc or on a series of left-lateral, across arc faults each pose a major seismic hazard for the populated areas of the Nicaraguan depression; the capital city and economic hub of Managua has been completely destroyed twice in the last 70 years by strike-slip events and has a rapidly growing, present-day population of over one million inhabitants. Any work that can better define the mode of active deformation will potentially help with seismic hazard mitigation efforts. This work will form all or part of a Master's project for a UT graduate student. This will be a collaborative project between UT and scientists at INETER, comparable to the geologic survey of Nicaragua. Finally, this study of the Nicaraguan depression component of the Central American volcanic arc would complement other geologic, geophysical, and geodetic programs supported by the NSF-Margins for the Subduction Factory initiative in Central America.

Publications
Funk, Justin, 2007, Cenozoic Tectonics of the Nicaraguan Depression, Nicaragua, and Median Trough, El Salvador, Based on Seismic Reflection Profiling and Remote Sensing Data, University of Texas at Austin, Masters Thesis, p. 188.