Recent structural and geodetic data define the Guatemala City graben region as the continental triple junction between the North American plate, Caribbean plate, and a forearc sliver. We present a minor fault analysis, geochronological and geochemical analyses, and newly updated GPS velocities in western Guatemala, west of the Guatemala City graben, to characterize the magnitude and timing of extensional deformation in this poorly understood area. Elongations estimated from fault data are parallel (~east-west) and perpendicular to the Polochic-Motagua fault system to the north, similar to geodetically-measured active deformation observed east of the Guatemala City graben. Four new 40Ar/39Ar dates and correlation of tephra deposits suggests that faulting was active during the Pliocene, but ceased eastward towards the Guatemala City graben over time. From west to east, fault cessation occurred before the deposition of the Los Chocoyos ash (84 ka) and E tephra (51 ka). Faulting just west of the Guatemala City graben appears to be active, where a major fault cuts the most recent Amatitlan tephras. Based on this data, we propose a time-progressive strain model for deformation related to North America-Caribbean plate interactions, whereby distributed elongation of the westernmost Caribbean plate occurred during the Pliocene but localized mostly within the Guatemala City graben and nearby faults during the Quaternary. Our model supports that: 1) The Guatemala City graben is effectively the western limit of the Caribbean plate; and 2) Western Guatemala, which used to be the trailing edge of the Caribbean plate, has been transferred to the forearc region.

The rhyolite-producing Laguna del Maule volcanic field (LdMVF) records magma-induced surface inflation rates of ~ 25 cm/year since 2007. During the Holocene, ~60 meters of cumulative surface uplift is recorded by paleoshorelines of the Laguna del Maule, located on the southeast edge of the LdMVF (Chile-Argentina border) near the Barrancas volcanic complex. Rhyolites from the Barrancas complex erupted over ~14 ka including some of the youngest (1.4 ± 0.6 ka) lava flows in the field. New gravity data collected on the Barrancas complex reveals a Bouguer low (-6 mGal, Barrancas anomaly) that is distinct from the pronounced gravity low (-19 mGal; Lake anomaly) associated with present-day deformation and magma intrusion to the north. Three-dimensional inversion of the Barrancas anomaly indicates the presence of a magma body with a maximum density contrast of -250 kg/m3 centered at a depth of ~ 3 km below surface. Comparison of model densities with measured densities from nearby silicic plutons suggest that a magma body, containing < 30 % melt phase and a low volatile content, exists beneath the Barrancas complex. The Barrancas and Lake gravity lows represent magma in different physical states, associated with past and present-day storage beneath the LdMVF. The gravity model mirrors existing geochemical observations which independently indicate that at least two distinct rhyolites were generated and stored as discrete magma bodies within the broader LdMVF. Small temperature changes of these discrete bodies could reverse crystallization and viscous lock-up and propel magma toward a crystal-poor eruptible state.