Interaction of surface erosion and sequential thrust progression: implications on exhumation processes

This paper investigates the evolution of thrust wedges with concomitant surface erosion, and its bearing on the exhumation processes in orogenic belts. We performed sandbox experiments, simulating syn-orogenic erosion on forelandward sloping surfaces (~4°). Experiments show that the erosion process has a significant control on the progression of frontal thrusts. In case of no-erosion condition, wedges with high basal friction develop frontal thrusts with strongly increasing spacing. In contrast, for the same basal friction the thrusts show uniform spacing as the wedge development involves concomitant surface erosion. On the other hand, the erosion promotes reactivation of hinterland thrusts in wedges with low basal friction. We show that erosion-assisted thrust reactivation is the principal mechanism for exhumation of deeper level materials in orogens. Efficiency of this mechanism is largely controlled by basal friction. The exhumation of deeper level materials is limited, and occurs within a narrow, sub-vertical zone in the extreme hinterland when the basal friction is high (μb = 0.46). In contrast, the process is quite effective in wedges with low basal friction (μb =0.36), resulting in exhumation along gently dipping foreland-vergent thrusts as well as along thrusts, subsequently rotated into steep attitude. The zone of exhumation also shifts in the foreland direction in the course of horizontal movement. Consequently, deeper level materials cover a large area of the elevated part of the wedge

Numerical modeling of flow patterns around subducting slabs in a viscoelastic medium and its implications in the lithospheric stress analysis

This paper presents results obtained from numerical model experiments to show different patterns of mantle flow produced by lithospheric movement in subduction zones. Using finite element models, based on Maxwell rheology (relaxation time ~ 1011S), we performed three types of experiments: Type 1, Type 2 and Type 3. In Type 1 experiments, the lithospheric slab subducts into the mantle by translational movement, maintaining a constant subduction angle. The experimental results show that the flow perturbations occur in the form of vortices in the mantle wedge, irrespective of subduction rate and angle. The mantle wedge vortex is coupled with another vortex below the subducting plate, which tends to be more conspicuous with decreasing subduction rate. Type 2 experiments take into account a flexural deformation of the plate, and reveal its effect on the flow patterns. The flexural motion induces a flow in the form of spiral pattern at the slab edge. Density-controlled lithospheric flexural motion produces a secondary flow convergence zone beneath the overriding plate. In many convergent zones the subducting lithospheric plate undergoes detachment, and moves down into the mantle freely. Type 3 experiments demonstrate flow perturbations resulting from such slab detachments. Using three-dimensional models we analyze lithospheric stresses in convergent zone, and map the belts of horizontal compression and tension as a function of subduction angle

Development of High-Performance Detector Technology for UV and IR Applications

Sensing and imaging for ultraviolet (UV) and infrared (IR) bands has many applications for NASA, defense, and commercial systems. Recent work has involved developing UV avalanche photodiode (UVAPD) arrays with high gain for high resolution imaging. Various GaN/AlGaN p-i-n UV-APDs have been fabricated from epitaxial structures grown by metalorganic chemical vapor deposition (MOCVD) on GaN substrates with avalanche gains greater than 5105, and high responsivities. Similarly, the IR spectral band is useful for measuring ocean temperatures, atmospheric aerosols, forest fires, etc. We are also developing room temperature operating graphene-enhanced PbSe midwave infrared (MWIR) detectors and focal plane arrays (FPAs). These compact and low-cost MWIR sensors can benefit various NASA remote sensing applications. Here we present recent results from these high performance UV- and IR-band detector and FPA technologies

Mode of development of sigmoidal en echelon fractures

The solution of stress distribution for a multicrack system and model experiments confirm that en echelon cracks mutually interact with each other during their growth. Such a mechanical interaction deviates the crack-tip stress axes orientations from that of the bulk stress field and leads to a continuous change in propagation direction of tension cracks, initially at a right angle to the bulk tension direction. The sigmoidal shape of en echelon fractures evolve through rotation and crack length increments with changing orientations. The theoretical analysis shows that the instantaneous fracture-tip stress orientation is a function of initial crack spacing, orientation of crack array with respect to the principal axes of far-field stress

Impact of Decelerating India‐Asia Convergence on the Crustal Flow Kinematics in Tibet: An Insight From Scaled Laboratory Modeling

The factors controlling the spatiotemporally varying deformation patterns in Tibet, a prolonged period (∼50 to 19 ± 3 Ma) of NNE-SSW shortening, accompanied by eastward flow and orogen-parallel extension in a later stage (19 ± 3 Ma to present-day), are still poorly constrained. Using viscous models, we performed scaled laboratory experiments with steady and unsteady state collision kinematics to address this issue. Our model Tibet under steady-state collision, irrespective of high (5.5 cm/yr) or low (3.5 cm/yr) indentation rates fails to produce the present-day crustal velocity fields and the deformation patterns, reported from GPS observations. An unsteady-state collision with decelerating convergence rates (5.5–3.5 cm/yr) is found to be a necessary condition for the initiation of eastward flow and ESE-WNW extensional deformations. The model results also suggest that the mechanical resistance offered by the rigid Tarim block resulted in crustal uplift at faster rates in western Tibet, setting a west to east topographic gradient, existing till present-day. This topographic gradient eventually polarized the gravity-controlled flow in the east direction when the convergence velocity decelerated to ∼3.5 cm/yr at around 19 ± 3 Ma. Our model shows the present-day eastward flow in central Tibet follows nearly a Poiseuille type velocity profile, bounded by the Himalaya in the south and the Tarim basin in northern Tibet. This flow kinematics allows us to explain the preferential locations of crustal-scale dextral and sinistral faults in southern and northern Tibet, respectively. Finally, the present-day model crustal-flow velocity, strain-rates, and topographic variations are validated with GPS and geological field data

High pressure-temperature proton migration in P-3 brucite [Mg(OH)2]: Implication for electrical conductivity in deep mantle

Hydrous minerals contribute largely to the transport and distribution of water into the mantle of earth to regulate the process of deep-water cycle. Brucite is one of the simplest layered dense hydrous mineral belonging to MgO-SiO2-H2O ternary system, which contains significant amount of water in the form of OH- groups, spanning a wide range of pressure stability. Simultaneously, the pressure (p) and temperature (T) induced mobility of protons within the layered structure of brucite is crucial for consequences on electrical conductivity of the mantle. Using ab initio molecular dynamics (AIMD) simulations, we investigate the diffusion of H in high-pressure trigonal P-3 polymorph of brucite in a combined p-T range of 10-85 GPa and 1250-2000K, relevant to the mantle of earth. The AIMD simulations reveal an unusual pressure-dependence of the proton migration in brucite characterized by maximum H-diffusion in the pressure range of 72-76 GPa along different isotherms. We predict that in the P-3 brucite the H mobility is onset only when a critical hydrostatic pressure is attained. The onset pressure is observed to drop with increasing temperature. The H-diffusion in brucite phase at elevated p-T takes place in such a manner that the process results in the amorphization of the H-sublattice, without disturbing the Mg- and O-sublattices. This selective amorphization yields a pool of highly mobile protons causing a subsequent increment in the electrical conductivity in P-3 brucite. Our calculated values of conductivity are compared with ex-situ geophysical magnetic satellite data indicating that brucite can be present in larger quantities in the lower mantle than previously observed. This hydroxide phase can occur as segregated patches between the dominant constituents e.g., silicates and oxides of the lower mantle and thus can explain the origin of high electrical conductivity therein.Comment: Preliminary draft, 6 figures, presented in Goldschimdt 2023 Conference (Lyon, France), comments are welcom