Description: Time-dependent, or 4D, microgravity changes observed at the Laguna del Maule volcanic field, Chile, since 2013, indicate significant (1.5 x 10 11 kg), ongoing mass injection. Mass injection is focused along the Troncoso fault, and subparallel structures beneath the lake, at 1.5 - 2 km depth, and is best modelled by a vertical rectangular prism source. The low density change (156 to 307 kg/m 3 ) and limited depth extent, suggest a mechanism of hydrothermal fluid intrusion into existing voids, or voids created by the substantial uplift, rather than deeper sourced dyke intrusion of rhyolite or basalt magma. Although the gravity changes are broadly spatially coincident with ongoing surface deformation, existing models that explain the deformation, are deeper sourced, and can not explain the gravity changes. To account for this discrepancy and the correspondence in time of the deformation and gravity changes, we explore a coupled magma-tectonic interaction mechanism that allows for shallow mass addition, facilitated by deeper magma injection. Computing the strain and mean, normal and Coulomb stress changes, on northeast trending faults, caused by the opening of a sill at 5 km depth, shows an increase in strain, mean and normal stresses along these faults, coincident with the areas of mass addition. Seismic swarms in mid 2012, to the west and southwest of the mass intrusion area may be responsible for dynamically increasing permeability on the Troncoso fault, promoting influx of hydrothermal fluids, which in turn causes larger gravity changes in the 2013 to 2014 interval, compared to the subsequent intervals.