98 Neotectonics (Posters): Active Deformation of Plate Boundaries and Interiors

Monday, 10 October 2011
9:00 AM - 6:00 PM
Hall C

FAULTING WITHIN THE SAN SALVADOR PULL APART, PASTEL OUTCROP, EL SALVADOR

Paper number: 98-4
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The El Salvador fault system is a series of en echelon, active, right-lateral strike-slip faults. These faults record movement parallel to the volcanic arc, and separate the fore-arc sliver (moving 11 mm/yr westward) from a stable back-arc located on the Caribbean plate. Recent work indicates that the El Salvador fault system contains multiple pull-apart step-overs, including the San Salvador pull-apart segment, which explains the abundance of normal faults along the trend of the volcanic arc. We have documented normal faulting at a six-tier quarry, known locally as El Pastel, located ~35 km east of San Salvador and adjacent to the Pan American highway. This site is located on the western end of the San Salvador pull-apart segment. Here, the faults occur in recent volcanic deposits, including tuffs, breccias, and lava flows. The entire outcrop is capped by an unfaulted ash fall deposit unit (Tierra Blanca Joven).

The dominant feature of the outcrop is a superbly exposed, 165-striking normal fault. The stratigraphy north of the normal fault contains no common units with the stratigraphy on its south side. Based on this constraint, we estimate that the fault must record >30 m of slip. A recent, vertical cut in the adjacent footwall exposes discrete faults and deformation bands, with deformation decreasing away from the major fault. Deformation bands only occur in dominantly tuffaceous units (ignimbrite and coarse pumice fall) and become faults (with steeper dips) as they enter more welded units. Both faults and deformation bands facilitate groundwater flow, as demonstrated by an illuminating pattern of iron staining.

A series of other normal faults, with mapable offsets and strikes between 120-170, occur in both the hanging wall and footwall of the major fault. The footwall (north) side contains more faults and includes horst, graben, and half-graben structures. Bulk elongation for the entire El Pastel outcrop is quantified using field measurements and fault-scaling relationships.