Gravity gradients delineate uncharacteristically straight, north-south trending graben faults in the Rio Grande rift west of Questa caldera, and gravity and audiomagnetotelluric (AMT) data show north-south trending low-density and high-resistivity zones from the caldera southward. A gravity inversion technique was used to isolate the gravity anomaly of the caldera and related intrusive rocks from the complicated Bouguer gravity field. The residual gravity anomaly together with AMT data reported in a companion paper by C. L. Long seem to delineate the subcaldera batholith. Geophysical models indicate this to be a north-south trending dikelike or beamlike body about 25 km wide, and >4 km thick, thickness estimate being limited by the penetration depth of the AMT soundings. The north-south trend is not consistent with the regional pattern of early Miocene southwest directed extension and instead may reflect passive control by preexisting north-south strike-slip faults.