Statistical separation of tectonic and inflation-driven components of deformation on silicic reservoirs, Laguna del Maule volcanic field, Chile

Abstract

The reconstruction of the structural history of inflating silicic systems is challenging because their faulting record encompasses tectonic and inflation-driven deformation, and separating the two can rarely be accomplished. Here, we present and utilize a statistical methodology to differentiate tectonic from inflation-driven deformation in the Laguna del Maule (LdM), a post-glacial rhyolitic volcanic field (Southern Volcanic Zone, Chile, latitude 36° S). LdM is cut by the Troncoso fault, a major normal fault that strikes NE and dips to the NW. The Troncoso hanging wall contains abundant, young (< 23 ka) NE-striking normal faults, whereas the footwall is largely unfaulted, with few NNW-striking faults. Activity within the shallow (<8 km) LdM reservoir has caused two inflation events, both centered in different areas of the footwall: 1) a geodetic-based (InSAR) inflation, acting since 2007; and 2) a shoreline-based inflation, evidenced by warping of a 9.4 ka high-stand shoreline. To separate tectonic and inflation-driven faults in the Troncoso hanging wall, we perform a statistical analysis of fault orientation to compare them to the elongation direction predicted by tectonics. The orientation of normal faults is consistent with NW-SE tectonic elongation recorded outside the inflating area. To evaluate if either inflation event has reactivated these tectonic faults, we perform a statistical analysis of orientation-and-heave (horizontal offset on a fault): we calculate the maximum elongation direction (the accumulated horizontal offset) and compare it to elongation predicted by tectonics, shoreline-based inflation and geodetic-based inflation. The maximum elongation direction is sub-parallel to elongation predicted by shoreline-based inflation and is statistically different from tectonic elongation. We interpret a first stage of tectonic extension (>19–9.4 ka) where normal faults are developed on the Troncoso hanging wall. During shoreline-based inflation (<9.4 ka), faults on the hanging wall are reactivated, suppressing uplift; the largely unfaulted footwall uplifts instead. The differential uplift is accommodated by slip on the Troncoso fault. The current, geodetic based uplift, nucleates few faults with large offsets, and potentially reactivates the Troncoso fault. From the structural history, we suggest that the structural architecture is associated with surface volcanism: deformation of the Troncoso hanging wall appears to facilitate repeated, small volume, post-glacial rhyodacitic and andesitic eruptions, whereas the largely unfaulted Troncoso footwall seems to favor rhyolite accumulation and eruption during the late Holocene.