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

Porous compaction in transient creep regime and implications for melt, petroleum, and CO 2 circulation

International audienceLiquid segregation through a porous medium depends on the ability of the matrix to deform and compact. Earth's materials have a complex rheology, in which the balance between the elastic and viscous contribution to the deformation is time-dependent. In this paper, we propose a Burger-type model to investigate the implications of transient rheology for viscous compaction of a porous material. The model is characterized by three dimensionless parameters: (1) the Deborah number, De, defined as the ratio of an elastic timescale over the compaction timescale, (2) the ratio of the transient and steady viscosities, l m , and (3) the ratio of the transient and steady elastic moduli, l G. For De < 10 À2 the compaction occurs in the classic viscous mode, and solitary waves (magmons) are generated. For larger De, compaction is mainly controlled by l m. For small transient viscosity, compaction occurs in an elastic mode, and shock waves are generated. For increasing l m , two new regimes are observed, first ''shaggy'' shock waves and then ''polytons''. Shaggy shock waves are characterized by the presence of secondary peaks at the wave propagation front. The length scale of the peaks is a decreasing function of l G , and their amplitude decreases along the propagation. In the polytons regime, the peaks tend to detach and mimic the behavior of solitary waves. Polytons and shaggy shock waves are expected both in the mantle and in sedimentary basins. Polytons will require a particular attention as they imply larger extraction velocities and smaller compaction length scales than the usual magmons. Citation: Chauveau, B., and E. Kaminski (2008), Porous compaction in transient creep regime and implications for melt, petroleum, and CO 2 circulation