Neotectonic deformation within an extensional stepover in El Salvador magmatic arc, Central America: Implication for the interaction of arc magmatism and deformation

Abstract

Dominantly westward movement of the El Salvador forearc at rates of 11mm/yr is accommodated by a series of E-W to WNW oriented, dextral, strike-slip fault zones herein referred to as the El Salvador Fault System (ESFS). The geometry of the ESFS defines a series of extensional step-overs. Along the arc, basaltic volcanism in the stepovers is associated with NNW-oriented normal faults, whereas rhyolitic volcanism is associated with strike-slip fault zones of the ESFS.On the ESFS, the San Salvador Extensional Stepover (SSES) is bound to the south by the San Vicente fault zone, where the rhyolitic Ilopango caldera is located. In the SSES, tephras from Ilopango –the Tierra Blanca (TB) sequence– track long-term elongation. Older TB units (TB5–8) contain abundant normal faults; lying unconformably above these older TB units, younger TB members (TBJ, TB2–4) are generally unfaulted. Analyses of faults in TB5–8 indicate NE- to ENE-oriented elongation in the SSES. Deformation occurred between deposition of the TB4 and TB5 units, during quiescence of the Ilopango eruptive center. Using this temporal constraint, minimum elongation rates of 3.50×10−15s−1, 2.06×10−14s−1 and 4.42×10−14s−1 were calculated for three traverses. From regional geodetic data and fault kinematics throughout El Salvador, we interpret the SSES as part of a series of pull-apart structures along the arc axis. The calculated paleostress orientations are consistent with a pull-apart geometry resulting from forearc movement.The extensional deformation occurs during a ~50k.y. lull in rhyolitic activity, suggesting an interplay between magmatism and deformation within the arc. During significant rhyolitic volcanic activity, only minor elongation is observed in the SSES, despite ongoing translation of the Salvadoran forearc. We speculate that rhyolitic magmatism along upper crustal faults may facilitate strike-slip movement on the ESFS, rather than distributing deformation throughout the extensional stepover.