INTERACTION BETWEEN STRIKE-SLIP FAULTING AND MAGMATISM ALONG THE SAN MIGUEL SEGMENT, EL SALVADOR FAULT ZONE

The El Salvador fault zone (ESFZ) is an active, EW-trending, right-lateral strike-slip fault zone that is oriented parallel to the Central American magmatic arc. The ESFZ accommodates movement of a forearc sliver, which moves ~11 mm/yr relative to Caribbean plate in the back-arc region. North of the city of San Miguel, El Salvador, a number of felsic magmatic bodies are located adjacent to the inferred trace of the ESFZ.

A felsic igneous body is exposed in a series of quarry cuts intruding an ignimbrite sequence. Ar/Ar dating of this unit suggests a lower Pleistocene emplacement age. There are two separate outcrops: 1) The western outcrop is a NS quarry wall and the margins of the intrusion are exposed; and 2) The eastern outcrop is a quarry with many faces, is located about ~300 m E of the western outcrop, and consists exclusively of rhyolite. The western outcrop of the body shows the strongest evidence for interaction with the ESFZ. The majority of the intrusion consists of banded rhyolite with a steeply dipping foliation that defines a sigmoidal pattern in map view. The N contact contains ~1.5 m of obsidian. The S end consists of a fault zone, including brecciated intrusion (banded rhyolite and obsidian) bounded by a cataclastic zone. Immediately south of the intrusion are heavily faulted, fine grained, diatomaceous sediments overlying a conglomerate; this sequence is interpreted as a small-scale, pull-apart basin. AMS (Anisotropy of Magnetic Susceptibility) analyses reveal mostly shallowly plunging lineations and strongest fabric at the center of the intrusion. Microstructures record dextral sense of shear. In contrast, the eastern outcrop shows no evident interaction with the ESFZ. AMS analyses show moderately plunging lineations, and weakest fabric is found in the center of the outcrop. Microstructures show dextral shear.

Our conclusion, based largely on the field relations observed in the western outcrop, is that the felsic body was syntectonically intruded into the ESFZ and localized deformation while it cooled. After cooling, deformation localized on the S margin of the intrusion and a pull-apart structure formed there. The field area indicates that strike-slip motion has occurred along the axis of the Central American volcanic arc since lower Pleistocene time, with a complex interplay between magmatism and tectonism.