Active megadetachment beneath the western United States

Geodetic data, interpreted in light of seismic imaging, seismicity, xenolith studies, and the late Quaternary geologic history of the northern Great Basin, suggest that a subcontinental-scale extensional detachment is localized near the Moho. To first order, seismic yielding in the upper crust at any given latitude in this region occurs via an M7 earthquake every 100 years. Here we develop the hypothesis that since 1996, the region has undergone a cycle of strain accumulation and release similar to “slow slip events” observed on subduction megathrusts, but yielding occurred on a subhorizontal surface 5–10 times larger in the slip direction, and at temperatures >800°C. Net slip was variable, ranging from 5 to 10 mm over most of the region. Strain energy with moment magnitude equivalent to an M7 earthquake was released along this “megadetachment,” primarily between 2000.0 and 2005.5. Slip initiated in late 1998 to mid-1999 in northeastern Nevada and is best expressed in late 2003 during a magma injection event at Moho depth beneath the Sierra Nevada, accompanied by more rapid eastward relative displacement across the entire region. The event ended in the east at 2004.0 and in the remainder of the network at about 2005.5. Strain energy thus appears to have been transmitted from the Cordilleran interior toward the plate boundary, from high gravitational potential to low, via yielding on the megadetachment. The size and kinematic function of the proposed structure, in light of various proxies for lithospheric thickness, imply that the subcrustal lithosphere beneath Nevada is a strong, thin plate, even though it resides in a high heat flow tectonic regime. A strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern Great Basin, deformation microstructures and low water content in dunite xenoliths in young lavas in central Nevada, and high-temperature microstructures in analog surface exposures of deformed lower crust. Large-scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere, as inferred by such proxies as low P wave velocity and mafic magmatism

1-D Modelling rock compaction in sedimentary basins using a visco-elastic rheology

International audienceRecent experimental studies on unlithified sediments suggest that the compaction is in part viscous. At the basin scale and on a geological time scale, processes such as pressure solution can be approached by creep deformation, subjected to a viscous rheology. In the present work, we have studied porosity reduction and fluid pressure development resulting from the visco-elastic compaction of a sedimentary basin during its formation. Model equations include continuity equations, Darcy's law and a visco-elastic rheology law which relates the strain rate to the effective stress and to the rate of change of this effective stress. Under the assumption that permeability is a power law function of porosity, the equations become essentially non-linear. Model results illustrate how the decrease of porosity starts at the base of the basin and spreads upward with increasing time. In a wide range of input parameter values calculations indicate a zone of almost linear increasing of pore pressure just below the basin surface, a transition zone of rapidly increasing fluid pressure with a large pressure gradient and a zone of lithostatic fluid pressure below. This is consistent with the general features of zoning of fluid pressure distribution in overpressured areas but zones with high pressure also correspond to low porosity at depth. The relative thickness of the zones depends on time, subsidence velocity and the physical parameters of sediments which can be combined in order to define a characteristic compaction length and a characteristic compaction time. In the upper zone, the decrease of porosity results in a boundary layer; within this layer, the porosity decreases from its initial value down to its minimum value. The deeper zone appears when the time of formation of the given depth basin exceeds the characteristic compaction time and the thickness of the basin is in order of compaction length. Zones of fluid overpressure may also develop due to the spatial variations of the physical properties of the sediments

Evolution of gas hydrates accumulation in zones of submarine mud volcanoes

The article examines the processes of evolution of gas hydrate accumulations, related to submarine mud volcanoes. A mathematical model and the results of numerical modeling of the accumulation of gas hydrates in the seabed in the deep structures of underwater mud volcanoes are presented.&#x0D; Numerical analysis of the influence held feeder layer depth and pressure therein to the evolution of gas hydrate saturation confined to deep water mud volcanoes were performed. Modeling quantitatively showed that hydrate saturation in areas of underwater mud volcanoes is not constant and its evolution depends on the geophysical properties of the bottom medium (temperature gradient, porosity, permeability, physical properties of sediments) and the depth of the supply reservoir and pressure in it, and the rate of hydrate accumulation in tens and hundreds times the rate of hydrate accumulation in the sedimentary basins of passive continental margin.</jats:p

Evolution of gas hydrates accumulation in zones of submarine mud volcanoes

The article examines the processes of evolution of gas hydrate accumulations, related to submarine mud volcanoes. A mathematical model and the results of numerical modeling of the accumulation of gas hydrates in the seabed in the deep structures of underwater mud volcanoes are presented.&#x0D; Numerical analysis of the influence held feeder layer depth and pressure therein to the evolution of gas hydrate saturation confined to deep water mud volcanoes were performed. Modeling quantitatively showed that hydrate saturation in areas of underwater mud volcanoes is not constant and its evolution depends on the geophysical properties of the bottom medium (temperature gradient, porosity, permeability, physical properties of sediments) and the depth of the supply reservoir and pressure in it, and the rate of hydrate accumulation in tens and hundreds times the rate of hydrate accumulation in the sedimentary basins of passive continental margin.</jats:p