The interaction of deformation and volcanism in the plutonic record: Do volcanic conduits exploit geological structures?

Abstract:

We utilize four magmatic arc systems to investigate the relationship between tectonic deformation and silicic volcanism. We compare the active El Salvador magmatic arc and the Late Cretaceous Sierra Nevada batholith (USA), which exhibit similar structural styles of granitic emplacement. We also compare the active Laguna del Maule (LdM) volcanic field and Late Miocene Huemul pluton in the Southern Andes, which occurred in the same tectonic setting.

The active El Salvador and fossil Sierra Nevada arcs show a similar en echelon geometry of dextral faults/shear zones, indicating motion of the forearc relative to the backarc. In El Salvador, andesitic/basaltic magmatism occurs within pull-apart regions and major rhyolitic calderas (Ilopango, Coatepeque) occur directly on regional dextral faults. In the Sierra Nevada, fine-grained porphyritic intrusions (Johnson Granite, Silver Pass) contain miarolitic cavities, implying rapid pressure release, suggesting that fabrics resulted from eruptive events. Similar to the El Salvador arc, the Sierran porphyries are associated with regional scale faults/shear zones that were active during emplacement.

In the Southern Andes, the shallow (<5 km), high-silica Huemul pluton has an elongate shape parallel to the trend of the arc suggesting emplacement along a regional structure. Pluton fabrics are magmatic in origin, but they record tectonic deformation during emplacement. Magmatic lineations are subvertical where they are associated with miarolitic cavities, suggesting rhyolite extraction from a reservoir. At the LdM rhyolitic volcanic field, active regional normal-dextral fault bisects the volcanic field. InSAR, seismic tomography and microgravity surveys have located the reservoir at 2-7 km depth, below or adjacent to the fault trace. Given that the trace of both faults is sub-parallel to the trend of the arc, the LdM is a possible current analog of the Huemul pluton. Further, regional tectonic deformation is the common mechanism that facilitates rhyolitic magmatism in this region of the southern Andes. From these four systems, we hypothesize that major rhyolitic eruptions may require the presence of fault/shear zones, which facilitate ascent of viscous magma in the upper crust.