M. Ben Clennell

PHYSICAL PROPERTIES OF MUDS EXTRUDED FROM MUD VOLCANOES: IMPLICATIONS FOR EPISODICITY OF ERUPTIONS AND RELATIONSHIP TO SEISMICITY

Scientific drilling into submarine mud volcanoes on the Mediterranean Ridge accretionary complex has documented episodic eruptive activity over the last 1 to >1.5 million years. Mud extrusion is related to plate convergence between Africa and Eurasia that caused backthrust faulting of accreted strata over the seismically active, rigid backstop of Crete (Greece). The domes consist of mud breccia with up to 65% of polymictic clasts embedded in a clayey matrix dominated by kaolinite, smectite and hallyosite. Laboratory measurements of viscosity, permeability and frictional strength of the clay-rich mud from Napoli Dome shed light on the extrusion dynamics and its relationship to seismicity. Viscosities of 106 Pa-s lead to predictions of ascent velocities up to 60–300 km/a based on Poiseuille’s flow law. Frictional shear strength and permeability were found to have very low values. Friction coefficients (μ) determined during ring shear and direct shear tests are below 0.26. These results point to velocity-strengthening behaviour of both the mud volcano clay and reference mineral standards of smectite, illite, and kaolinite. Permeability of deformed clay-rich matrix measured using a ring shear permeameter, is less than 10−19 m2 at ∼1 MPa normal stress. We propose that the low permeability and strength observed during our tests have two important geological implications. First, these properties allow pore pressure build-up at depth, especially within poorly drained fault zones, accretionary prisms, and mud reservoirs. Fault movement is facilitated by the low intrinsic strength and reduced effective stress of material in the fault zones while the elevated porosity, low viscosity and high internal pressure of the mud promotes subsurface mobilization, leading utimatelylink between seismicity and mud volcanism since the mud and clay reference standards tested all underwent stable sliding when sheared under fixed load-point velocity or stress. We believe that seismogenesis occurs at deeper levels than mud mobilization, but still within a kinematically-linked (and perhaps hydraulically-linked) fault system. Increased mud volcano activity may thereby serve as an earthquake precursor, since seismic faulting at depth may cause stress state perturbations along the fault, which in turn may trigger liquefaction, excess pore pressure transients, and ascent/extrusion.

Generation of amorphous carbon and crystallographic texture during low-temperature subseismic slip in calcite fault gouge

Identification of the nano-scale to micro-scale mechanochemical processes occurring during fault slip is of fundamental importance to understand earthquake nucleation and propagation. Here we explore the micromechanical processes occurring during fault nucleation and slip at subseismic rates (∼3 × 10−6 m s–1) in carbonate rocks. We experimentally sheared calcite-rich travertine blocks at simulated upper crustal conditions, producing a nano-grained fault gouge. Strain in the gouge is accommodated by cataclastic comminution of calcite grains and concurrent crystal-plastic deformation through twinning and dislocation glide, producing a crystallographic preferred orientation (CPO). Continued wear of fine-grained gouge particles results in the mechanical decomposition of calcite and production of amorphous carbon. We show that CPO and the production of amorphous carbon, previously attributed to frictional heating and weakening during seismic slip, can be produced at low temperature during stable slip at subseismic rates without slip weakening.