Chagos Bank is a region of thick crust in the central Indian Ocean where a concentration of seismicity indicates that extension is ongoing. We have modeled bathymetry and gravity data to show that the crust-mantle boundary shallows near a seabed trough, consistent with a total extension of ~15 km over a zone ;50 km broad. This estimate is consistent with 5–10 m.y. of deformation at the present rate of relative India-Capricorn motion; the ~15 km of extension is toward the lower limit of the total extension expected from magnetic anomaly reconstructions, but would be accumulated in 1–3 m.y. in a regime having the present-day seismicity. We attribute the localization of seismicity beneath Chagos Bank to a weak rheology caused by its thick crust.