The Benefits of Using a Consistent Tangent Operator for Viscoelastoplastic Computations in Geodynamics

Strain localization is ubiquitous in geodynamics and occurs at all scales within the lithosphere. How the lithosphere accommodates deformation controls, for example, the structure of orogenic belts and the architecture of rifted margins. Understanding and predicting strain localization is therefore of major importance in geodynamics. While the deeper parts of the lithosphere effectively deform in a viscous manner, shallower levels are characterized by an elastoplastic rheological behavior. Herein we propose a fast and accurate way of solving problems that involve elastoplastic deformations based on the consistent linearization of the time-discretized elastoplastic relation and the finite difference method. The models currently account for the pressure-insensitive Von Mises and the pressure-dependent Drucker-Prager yield criteria. Consistent linearization allows for resolving strain localization at kilometer scale while providing optimal, that is, quadratic convergence of the force residual. We have validated our approach by a qualitative and quantitative comparison with results obtained using an independent code based on the finite element method. We also provide a consistent linearization for a viscoelastoplastic framework, and we demonstrate its ability to deliver exact partitioning between the viscous, the elastic, and the plastic strain components. The results of the study are fully reproducible, and the codes are available as a subset of M2Di MATLAB routines

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

Segmentation and kinematics of the North America-Caribbean plate boundary offshore Hispaniola

We explored the submarine portions of the Enriquillo–Plantain Garden Fault zone (EPGFZ) and the Septentrional–Oriente Fault zone (SOFZ) along the Northern Caribbean plate boundary using high-resolution multibeam echo-sounding and shallow seismic reflection. The bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between Haiti and Jamaica (EPGFZ) and 300 km between the Dominican Republic and Cuba (SOFZ). The primary plate-boundary structures are a series of strike-slip fault segments associated with pressure ridges, restraining bends, step overs and dogleg offsets indicating very active tectonics. Several distinct segments 50–100 km long cut across pre-existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike-slip regime. Along the most recent trace of the SOFZ, we measured a strike-slip offset of 16.5 km, which indicates steady activity for the past ~1.8 Ma if its current GPS-derived motion of 9.8 ± 2 mm a−1 has remained stable during the entire Quaternary.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu

A genetic link between transform and hyper-extended margins

International audienceThe similarity between the geometry of the West African and South American coastlines is among one of the strongest natural observations supporting the plate tectonic paradigm. However, using classical plate tectonic approaches to model these conjugate transform margins results in a high degree of variability in palaeogeographic reconstructions. Using state-of-the-art 3D coupled thermo-mechanical numerical models, we simulate for the first time, crustal deformation at the onset of oceanisation along large offset oblique margins. Our models show that obliquity causes oceanic rift propagation to stall, resulting in an apparent polyphased tectonic evolution, and in some circumstances leads to the formation of hyper-extended margins. As a result, conjugate margins located at the edge of future fracture zones are highly asymmetric from rifting to spreading, with their lengths differing by a factor of 5 to 10, before the the final phase of break-up occurs. Accounting for this discrepancy should ameliorate future palaeogeographic reconstructions

6 Ma subsidence and exhumation of the southeastern San Joaquin Basin, California, in response to mantle lithosphere removal

New thermo-mechanical models of mantle lithosphere removal from beneath the southern Sierra Nevada, California, predict a complex spatio – temporal pattern of vertical surface displacements. We evaluate these models by using (U-Th)/He thermochronometry, together with other paleothermometry estimates, to investigate such topographic transients. We target sediments from the Kern Arch, a fan-shaped uplift located in the southeastern San Joaquin Basin, along the western flank of the southern Sierra. Kern Arch stratigraphy provides a unique record of subsidence and exhumation in a sensitive region immediately adjacent to the delaminating mantle lithosphere at depth. Detrital apatite (U-Th)/He ages from Oligo-Miocene sandstones collected in Kern Arch well cores indicate post-depositional heating to temperatures beyond those corresponding with their present burial depths. When integrated with available geologic and stratigraphic constraints, temperature – time modeling of thermochronometry data suggests partial He loss from apatites at temperatures of 70° – 90°C, followed by exhumation to present burial temperatures of 35° – 60°C since ca. 6 Ma. Assuming a regional late Cenozoic geothermal gradient of 25°C/km, our results imply 1.0 – 1.6 km of rapid (~ 0.4 mm/yr) burial and subsequent exhumation of southeastern San Joaquin sediments in latest Miocene - Quaternary time. Subtle differences in the maximum temperatures achieved in various wells may reflect differing degrees of tectonic subsidence and sedimentation as a function of distance from the range front. Our results are consistent with estimates of surface subsidence and uplift from Sierran delamination models, which predict a minimum of 0.8 km of subsidence in regions presently associated with mantle lithosphere at depth, and a minimum of 0.6 km of surface uplift in regions where delamination has recently occurred. We attribute the marked pulse of tectonic subsidence in the San Joaquin Basin to viscous coupling between the lower crust and a downwelling mass in the delaminating slab. The ensuing episode of denudation is interpreted to result from the northwestward peeling back of the slab and the associated replacement of dense lithosphere with buoyant asthenosphere

Buffet and buffeting control in transonic flow

Communication to : ODAS 2003 Toulouse (France), June 04-06, 2003Available from INIST (FR), Document Supply Service, under shelf-number : 22419, issue : a.2003 n.74 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc