Abstract
Silicic calderas form during explosive volcanic eruptions when magma withdrawal triggers collapse along bounding faults. The nature of specific interactions between magmatism and tectonism in caldera-forming systems is, however, unclear. Regional stress patterns may control the location and geometry of magma reservoirs, which in turn may control the spatial and temporal development of faults. Here we provide new insight into strike-slip volcano-tectonic relations by analysing Bouguer gravity data from Ilopango caldera, El Salvador, which has a long history of catastrophic explosive eruptions. The observed low gravity beneath the caldera is aligned along the principal horizontal stress orientations of the El Salvador Fault Zone. Data inversion shows that the causative low-density structure extends to ca. 6 km depth, which we interpret as a shallow plumbing system comprising a fractured hydrothermal reservoir overlying a magmatic reservoir with vol% exsolved vapour. Fault-controlled localization of magma constrains potential vent locations for future eruptions.
Highlights
Silicic calderas form during explosive volcanic eruptions when magma withdrawal triggers collapse along bounding faults
Unclear is the extent to which regional tectonic stresses influence the geometry of magma accumulation between large caldera-forming eruptions
We address the role of tectonic stresses on magmatic accumulation during inter-eruptive periods between large caldera-forming eruptions with a detailed analysis of new gravity data from the Ilopango caldera
Summary
Silicic calderas form during explosive volcanic eruptions when magma withdrawal triggers collapse along bounding faults. Unclear is the extent to which regional tectonic stresses influence the geometry of magma accumulation between large caldera-forming eruptions Addressing this question is important for understanding controls on the development of upper crustal magmatic systems, and for forecasting probable locations of future eruptive activity from caldera-forming volcanoes. The El Salvador Fault Zone (ESFZ) is a complex array of dextral strike-slip faults with a dominant E-W strike It forms part of major tectonic lineaments in Central America (Fig. 1) that are associated with large-volume silicic calderas. The low-density structure likely maps a complex shallow plumbing system composed of magmatic and fractured hydrothermal reservoirs with a considerable vapour fraction (\3 vol%) These data suggest that localized extension along the complex ESFZ controls accumulation, ascent and eruption of magma. Fault-controlled localization of magma accumulation and movement through a mechanically weak crust constrains potential vent locations for future eruptions
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