We combine geodetic, structural, and paleomagnetic data from El Salvador with Global Positioning System (GPS) data from southern Honduras and Nicaragua to describe the motions of the Salvadoran and Nicaraguan forearcs and determine the location and style of faulting across the Gulf of Fonseca offset of the volcanic arcs of eastern El Salvador and western Nicaragua. Finite-element modeling of GPS measurements at 35 sites in El Salvador, southern Honduras, and Nicaragua indicates that the Nicaraguan and Salvadoran forearcs both move west to northwest, parallel to their respective trenches, at 15 ± 2 mm yr−1 (95% limit) in a Caribbean plate reference frame. The similar motions of the two forearcs, despite an ∼20°–25° difference in the obliquity of subduction beneath them and absence of any significant convergence obliquity offshore from El Salvador, are consistent with a recent hypothesis that the Nicaraguan forearc pushes the Salvadoran forearc to the northwest, possibly driven by northwestward lateral escape of the Central America forearc from its collision zone with the Cocos Ridge offshore from Costa Rica. The Gulf of Fonseca and adjacent eastern El Salvador form an ∼60-km-wide extensional zone with E-W elongation, determined by diffuse seismicity, GPS velocities, and numerous young, N-S–striking normal faults mapped with a 10 m digital elevation model (DEM), structural measurements, and Lidar (light detection and ranging). Strike-slip earthquakes in the Fonseca pull-apart structure and evidence for modest (∼10°) vertical-axis fault block rotations from paleomagnetic measurements at 33 sites in the Fonseca pull-apart structure both indicate that extension may be accompanied by bookshelf faulting.

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