USING GRAVITY MEASUREMENTS FROM SANTA CRUZ ISLAND, GALAPAGOS TO INVESTIGATE DIFFERENCES IN MAGMATIC PLUMBING SYSTEMS BETWEEN EASTERN AND WESTERN GALAPAGOS ISLANDS

Volcanoes located on the western islands of the Galápagos Archipelago exhibit large calderas with diameters up to several kilometers and floors up to hundreds of meters deep. Formation of these deep calderas is associated with a shallow magma chamber, the presence of which is supported by eruptive fissure patterns, deformation measurements, modeling of InSAR data, and Bouguer Anomaly (BA) highs. In contrast, volcanoes located on the eastern Galápagos Islands do not currently form calderas or show evidence of previous caldera formation. The morphological differences between western and eastern islands indicate a dichotomy in magmatic plumbing systems that is likely governed by either a definitive shift in volcanic processes or by processes that evolve over time in Galápagos volcanoes. This study uses gravity measurements and a series of damped linear inversions to investigate subsurface density variations across Santa Cruz Island and to constrain the magmatic plumbing system of this eastern island. In the summer of 2015, we conducted a ~28 hour gravity survey across Santa Cruz Island that included 65 measurements at 1 km intervals along 4 transects. The resulting BA has a total range of ~20 mGals. Much of the variation within the BA can be correlated with surface expressions of monogenetic volcanic cones and numerous observed normal faults along the northern coast of Santa Cruz. One ~10 mGal relative high along the northern flank of the Santa Cruz volcano has no observed surface expression, but is consistent with a model of a vertically oriented, rectangular prism of anomalously dense material that parallels the crest of Santa Cruz Volcano. We discuss this relative BA high in the context of the magmatic plumbing system of Santa Cruz volcano and its implications for volcanic processes across the Galápagos Archipelago.