Sergei Medvedev

Indentation of a continent with a built-in thickness change: experiment and nature

Orogens oblique to the direction of plate convergence are currently attributed to obliquity between the margins of one or both of the sutured continents to their direction convergence. We use a single analogue experiment and natural examples to illustrate a potential additional factor: variations in strength of the indented continent at a high angle to the convergence direction. The wavelengths of structures in laterally shortened lithosphere depend on the strength of the most competent layers. Lateral variations in crustal thickness must therefore lead to structures oblique to any applied lateral compression.An analogue experiment was performed to explore this phenomenon. A two-layer ‘indented continent’ was modelled by a brittle upper crust of sand above a lower crust of high-viscosity polymer floating on a single layer of low-viscosity syrup representing the mantle. The well-known strike-slip structures allowing lateral escape to distant weak boundaries were hindered by lateral boundaries in front of the indenter. This allowed us to focus on the effects of a thickness change built into the ‘indented continent’ along a zone parallel to the direction in which a vertical rigid wall advancing at a steady rate represented the indenter. Vertical escape led to an ‘orogenic belt’ oblique to the advancing wall; this obliquity influences subsequent lateral escape. Model scaling and interpretations are based on Extended Thin Sheet Approximation (ETSA) and standard theories of faulting.Four sectors of the Alpine–Himalayan orogen (Iran, Tunisia, the Eastern Alps and the Himalaya) are oblique to the continental convergence direction, and we point to thickness changes at high angles to the suture that may account for this geometry. As crustal thicknesses north of oblique sectors of the Himalayas are not yet known, we speculate on them.We infer from the main difference between our experiment and all our examples chosen from nature that vertical orogenic escape was oblique to our model suture but can be parallel to natural sutures.

Salt extrusion at Kuh-e-Jahani, Iran, from June 1994 to November 1997

Abstract Kuh-e-Jahani is one of the largest extrusions of salt currently active in the Zagros mountains. Salt rises from about 4 km below sea level to nearly 1.5 km, above, where, unable to support its own weight, it spreads over the surrounding scenery in a process responsible for present and past allochthonous salt sheets elsewhere. We report vertical movements and apparent horizontal displacements of 43 markers dispersed over this mountain of salt for 4.5 years in three consecutive intervals, the first of 18 months and two others of 12 months. The geometry and inferred flow rate of the salt changed between surveys emphasizing that the gravity spreading is not steady. Our field readings of the dimensions and velocities of the salt at Kuh-e-Jahani are used to tune a simple numerical model and constrain the viscosity of the salt to between 1016 and 1017 Pa s−1, its rate of surface dissolution to 2–3 cm a−1, and its rate of rise out of its orifice at 2–3 m a−1 for c. 55 ka. These results imply that vigorous extrusion of salt at Kuh-e-Jahani is probably close to evacuating its deep source and that this mountain will soon begin to waste as salt dissolution overtakes extrusion. This progress report is warranted because our results have significant implications for sophisticated engineering in salt.

4D Arctic: A Glimpse into the Structure and Evolution of the Arctic in the Light of New Geophysical Maps, Plate Tectonics and Tomographic Models

Knowledge about the Arctic tectonic structure has changed in the last decade as a large number of new datasets have been collected and systematized. Here, we review the most updated, publicly available Circum-Arctic digital compilations of magnetic and gravity data together with new models of the Arctic’s crust. Available tomographic models have also been scrutinized and evaluated for their potential to reveal the deeper structure of the Arctic region. Although the age and opening mechanisms of the Amerasia Basin are still difficult to establish in detail, interpreted subducted slabs that reside in the High Arctic’s lower mantle point to one or two episodes of subduction that consumed crust of possibly Late Cretaceous–Jurassic age. The origin of major igneous activity during the Cretaceous in the central Arctic (the Alpha–Mendeleev Ridge) and in the proximity of rifted margins (the so-called High Arctic Large Igneous Province—HALIP) is still debated. Models of global plate circuits and the connection with the deep mantle are used here to re-evaluate a possible link between Arctic volcanism and mantle plumes.

Spontaneous dissipation of elastic energy by self-localizing thermal runaway

Thermal runaway instability induced by material softening due to shear heating represents a potential mechanism for mechanical failure of viscoelastic solids. In this work we present a model based on a continuum formulation of a viscoelastic material with Arrhenius dependence of viscosity on temperature and investigate the behavior of the thermal runaway phenomenon by analytical and numerical methods. Approximate analytical descriptions of the problem reveal that onset of thermal runaway instability is controlled by only two dimensionless combinations of physical parameters. Numerical simulations of the model independently verify these analytical results and allow a quantitative examination of the complete time evolutions of the shear stress and the spatial distributions of temperature and displacement during runaway instability. Thus we find that thermal runaway processes may well develop under nonadiabatic conditions. Moreover, nonadiabaticity of the unstable runaway mode leads to continuous and extreme localization of the strain and temperature profiles in space, demonstrating that the thermal runaway process can cause shear banding. Examples of time evolutions of the spatial distribution of the shear displacement between the interior of the shear band and the essentially nondeforming material outside are presented. Finally, a simple relation between evolution of shear stress, displacement, shear-band width, and temperature rise during runaway instability is given.

Controls on the Deformation of the Central and Southern Andes (10–35° S): Insight from Thin-Sheet Numerical Modeling

What mechanisms and conditions formed the Central Andean orocline and the neighboring Altiplano Plateau? Why does deformation decrease going from the central to the Southern Andes? To answer these questions, we present a new thin-sheet model that incorporates three key features of subduction orogenesis: (1) significant temporal and spatial changes in the strength of the continental lithosphere in the upper plate; (2) variable interplate coupling along a weak subduction channel with effectively anisotropic mechanical properties; and (3) channeled flow of partially molten lower crust in the thickened upper plate. Application of this model to the present kinematic situation between the Nazca and South American Plates indicates that the deformed Andean lithosphere is significantly weaker than the undeformed South American foreland, and that channel flow of partially melted lower crust smoothes topographic relief. This channel flow is, therefore, inferred to control intra-orogenic topography and is primarily responsible for the development of the Andean Plateau since the Miocene. A parameter study shows that the decrease in shortening rates from the central to the Southern Andes can be attributed to the weakening of the orogenic Andean lithosphere and to along-strike variations in interplate coupling within the subduction zone. The current rates of deformation are reproduced in the model if: the Andean lithosphere is assumed to be 5–15 times weaker than the lithosphere of the Brazilian shield; interplate coupling is assumed to be relatively weak, such that the subduction zone in the vicinity of the Central Andes is some 10–20 times weaker than the Andean lithosphere; and coupling itself decreases laterally by some 2–5 times going from the central to the Southern Andes.

Computer Simulation of Sedimentary Cover Evolution

A computer program has been developed for the mathematical simulation of the mechanical evolution of a sedimentary basin. It is designed for use in numerical experiments to study the impact of different velocity patterns at the base of the basin on the dynamics of the surface and inner boundaries, as well as for use in reconstructing the formation and evolution of existing basins. A layered basin is considered and the 3D modeling task is reduced to solving of 2D equations describing inner and outer boundaries of basin evolution.