Westward drift of the lithosphere: Not a result of rotational drag

It is shown that any non-zero torque resulting from differences in angular velocity between individual shells in the Earth would be an extremely short transient phenomenon as a consequence of the viscosity of the asthenosphere. Consequently, it cannot be a factor in the origin of the toroidal velocity field of degree one ('westward drift') of the lithosphere

Методика и методология социолингвистических исследований в условиях билингвизма и диглоссии

Lithospheric-scale analogue models are used to analyse the parameters controlling the typical evolution of deformation during continental narrow rifting, characterized by early activation of large boundary faults and basin subsidence, followed by localization of tectonic activity in internal faults at the rift axis. Integration of current and previous experiments shows that the evolution of deformation, in particular the amount of extension needed for the abandonment of boundary faults and migration of deformation to in-rift faults, is dependent on at least five boundary conditions: (i) thickness of brittle layers (including syn-rift sediments); (ii) thickness of ductile layers; (iii) extension rate; (iv) width of the weak zone localizing extension; and (v) rift obliquity with respect to the extension direction. An increase in the amount of extension corresponding to the inward migration of faulting (i.e., a longer phase of slip on boundary faults) is observed for (a) an increase in the thickness of both brittle and ductile crustal layers and syn-rift sediment accumulation, (b) a decrease in extension rate and width of the weak zone, and (c) a decrease in rift obliquity. A unified account of these correlations is presented, based on the hypothesis that fault migration occurs when boundary faults can no longer accommodate the imposed bulk extension, leading to time-space variations of internal strain and strain rate (and consequently stress) in the ductile layers which overcome the total resistance of brittle layers to thoroughgoing faulting

Channel flow, tectonic overpressure, and exhumation of high-pressure rocks in the greater himalayas

The Himalayas are the archetype of continental collision, where a number of long-standing fundamental problems persist in the Greater Himalayan Sequence (GHS): (1) contemporaneous reverse and normal faulting, (2) inversion of metamorphic grade, (3) origin of high-(HP) and ultrahigh-pressure (UHP) rocks, (4) mode of ductile extrusion and exhumation of HP and UHP rocks close to the GHS hanging wall, (5) flow kinematics in the subduction channel, and (6) tectonic overpressure, here defined as TOP  Combining double low line Pĝ•PL where P is total (dynamic) pressure and PL is lithostatic pressure. In this study we couple Himalayan geodynamics to numerical simulations to show how one single model, upward-Tapering channel (UTC) flow, can be used to find a unified explanation for the evidence. The UTC simulates a flat-ramp geometry of the main underthrust faults, as proposed for many sections across the Himalayan continental subduction. Based on the current knowledge of the Himalayan subduction channel geometry and geological/geophysical data, the simulations predict that a UTC can be responsible for high TOP ( > 2). TOP increases exponentially with a decrease in UTC mouth width, and with an increase in underthrusting velocity and channel viscosity. The highest overpressure occurs at depths < ĝ'60 km, which, combined with the flow configuration in the UTC, forces HP and UHP rocks to exhume along the channel's hanging wall, as in the Himalayas. By matching the computed velocities and pressures wi

Correlation between length and offset in strike-slip faults

Strike-slip faults in continental crust are shown to exhibit non-linear positive correlation between length and offset. This correlation can be empirically explained as the combined effect of the changes in the growth rates of offset and length during the time of tectonic activity of a fault

The expansion-undation hypothesis for geotectonic evolution

A review of the available geological, experimental and theoretical evidence shows that the hightemperature creep of a polycrystalline solid such as the mantle is nonlinear and very close to ideal plasticity. Viscous (Newtonian) convection currents cannot therefore be regarded as a realistic geodynamic process. Thermal considerations indicate that the earth has undergone a radial expansion of at least 100 km (but not more than a few hundred km) since the formation of the oldest datable rocks (3.5·109 years). This expansion has been slowing down in time. The structural unit that has been actively expanding is the interior, where heat accumulates, while the outer shell, a few hundred km thick, has been subject to a tensional deviatoric stress field. Consequently, a condition of plastic instability has developed in the outer shell, which has originated elongated tensile features. This process accounts for the formation of some new crust, but not in the amount required by the continental drift hypothesis. Also, plastic convection can take place in the rheosphere (100-400 km of depth), where the ductility is locally increased by thermal and/or chemical factors. However, the convection pattern indicates that this is a local phenomenon unlikely to affect material outside a few mobile belts in the tectonosphere. Continental drift, accordingly, can be brought about only by a decoupling of the lithosphere from its substratum. The simplest way in which horizontal displacements can be accomplished is by the action of the force of gravity (gravity megatectonics). The required slope at the surface of decoupling is very small (less than 1 4 of a degree), and undations at the lithosphere-rheosphere boundary can be originated by a variety of processes. The equality between oceanic and continental heat flow, which is the main obstacle to the continental drift hypothesis, can be reconciled with it if the continents have a thermal blanketing effect on the underlying mantle. Thus, it appears that lithospheric plate drifting is physically possible. This conclusion, however, merely establishes a possibility, and claims that the problems of global tectonics have been "solved" have not yet been substantiated

Tectonic role of oceanic fracture zones

A critical analysis of the geometric properties of oceanic fracture zones leads to the conclusion that their identification with ridge-ridge transform faults is an oversimplification. Models are presented, and examples are given, of processes (asymmetric spreading and differential spreading) which can alter the ridge-fracture zone pattern and extend active shear within lithospheric plates. These processes have a number of interesting tectonic consequences and could offer an explanation for, among other things, belts of intra-plate seismic activity and reversal of slip directions along a strike-slip fault