INTERACTION OF DEFORMATION AND MAGMATISM ALONG THE EL SALVADOR VOLCANIC ARC

The magmatic arc in El Salvador results from the subduction of the Cocos plate underneath the Caribbean plate along the Middle America trench. Despite a nearly orthogonal angle of convergence (with a convergence rate of ~77 mm/yr), El Salvador contains a fore-arc sliver that moves 11 mm/yr westward relative to a stable back-arc. Previous work has inferred the presence of an El Salvador fault zone that is oriented EW to ESE-WNW (parallel to the trend of the magmatic arc) and records dextral, strike-slip motion. Regional scale mapping, however, indicates that no single strike-slip fault continues for more than ~100 km. Rather, normal faults are the dominant faulting type in El Salvador and generally are oriented at a high angle to the volcanic arc.

We propose that the magmatic arc fault zone includes multiple, en echelon pull-apart step-overs. Abundant normal faults are found in the areas of overlap of the strike-slip faults. All magmatic centers – both basaltic andesite volcanoes (San Miguel, San Vincente, San Salvador, Santa Ana) and rhyolitic calderas (Ilopango, Coatepeque) – are located within these stepovers, as are many of the major population centers (San Salvador, San Miguel).

In a detailed study of faulting in the San Salvador step-over, faulting seems to occur in pulses that occur during periods of magmatic quiescence. In this locality, a series of eight ashes (Tierra Blanca sequence - TB) are erupted from the Ilopango rhyolitic center. A prominent unconformity occurs between the 4th (TB4 fall pumice) and 5th (TB5) ignimbrite layer. All ignimbrite units below the unconformity are affected equally by normal faulting, while ignimbrite units above the unconformity are largely unfaulted. These observations, combined with dating of the ash beds, allow us to quantify strain rates that occur in these pull-apart stepovers. The occurrence of intense faulting during a period that lacks ignimbrite deposition suggests that periods of active felsic magmatism do not correlate with times of deformation. This inference suggests that felsic magmatism accommodates deformation within the step-overs although the mechanism is unclear.