Radial fracture systems at large volcanoes on Venus offer opportunities to unravel the tectonic and magmatic evolution of such edifices. However, the relatively low spatial resolution (10–20 km) of altimetry data for Venus has hampered previous efforts at quantitative analysis of fault systems. Recently released stereo-derived topography enables kilometer-scale strain estimates to be made at tectonic features. The new data resolve fault valleys (grabens) that cut the summit regions of two large volcanoes, showing them to be deeper than previously assumed. The grabens formed after most edifice building was complete. We demonstrate that inflation of oblate sill-like magma chambers is consistent with the magnitudes and distributions of strain at these volcanoes, and estimate chamber depth, radius, and overpressure.