Fig. 8
(A) View of the field of pit craters from the west showing the morphology of pit craters and rimmed pockmarks. V1 and V2 are volcanic cones related to magma source feeding the induced sill intrusions beneath the pit crater field (dashed lines). (B) Two-stage model for explaining the dimensions and shape of the pit craters and rimmed pockmarks: Stage 1—Formation of giant-blow-out craters by hot, buoyant hydrothermal fluids forming V-shaped chimneys. Stage 2—Release of thermogenic methane from the sill-induced reservoir migrating through the former conduit. As increasing the gas overpressure from the sill-induced reservoir then upward flow of gas is triggering through the breaching opal A-CT transition to subseafloor. As decreasing the gas overpressure in the conduit, then the pit crater collapses forcing to the shallow gas to migrate to seafloor forming rings and chains of `standard´ pockmarks distributed radially around the pit crater as observed on seafloor topography in (A). This mechanism would explain the successive reactivations of the pit craters caused by episodes of collapse, sedimentary infilling and overpressure peaks as observed in seismic images from 0.9 Ma to present (Fig. 5).
