Ravi V. S. Kanda

Suction mechanism for iron entrainment into the lower mantle

Perturbations in the Earths rotation rate at decadal time periods strongly favor the existence of dissipative coupling at the Core–Mantle Boundary (CMB). Here, we explored the plausibility of maintaining a conducting layer on the mantle-side of the CMB, which can couple the outer core and mantle through Lorentz torques. We propose a suction mechanism that maintains a porous medium on the mantle side of the CMB, with the interconnected pore-space partly or entirely filled with liquid iron up to a thickness of ∼1 km. The suction arises from the deviatoric stresses supported by the mantle-solid in regions of mantle downwelling. Infiltration of liquid iron occurs by percolation, but is inhibited by the rate of viscous dilation of the solid mantle. Our model enables core-mantle material exchange, and maintains a thin conducting layer that has seismic detection potential. Our model is only marginally satisfactory in explaining the inferred CMB coupling.

An elastic plate model for interseismic deformation in subduction zones

Geodetic observations of interseismic surface deformation in the vicinity of subduction zones are frequently interpreted using simple kinematic elastic dislocation models (EDM). In this theoretical study, we develop a kinematic EDM that simulates plate subduction over the interseismic period (the elastic subducting plate model (ESPM)) having only 2 more degrees of freedom than the well-established back slip model (BSM): an elastic plate thickness and the fraction of flexural stresses due to bending at the trench that are released continuously. Unlike the BSM, in which steady state deformation in both plates is assumed to be negligible, the ESPM includes deformation in the subducting and overriding plates (owing to plate thickness), while still preserving the correct sense of convergence velocity between the subducting and overriding plates, as well as zero net steady state vertical offset between the two plates when integrated over many seismic cycles. The ESPM links elastic plate flexure processes to interseismic deformation and helps clarify under what conditions the BSM is appropriate for fitting interseismic geodetic data at convergent margins. We show that the ESPM is identical to the BSM in the limiting case of zero plate thickness, thereby providing an alternative motivation for the BSM. The ESPM also provides a consistent convention for applying the BSM to any megathrust interface geometry. Even in the case of nonnegligible plate thickness, the deformation field predicted by the ESPM reduces to that of the BSM if stresses related to plate flexure at the trench are released either continuously and completely at shallow depths during the interseismic period or deep in the subduction zone (below ∼100 km). However, if at least a portion of these stresses are not continuously released in the shallow portion of the subduction zone (via seismic or aseismic events), then the predicted surface velocities of these two models can differ significantly at horizontal distances from the trench equivalent to a few times the effective interseismic locking depth.

Thermal footprint of an eroded thrust sheet in the southern Appalachian thrust belt, Alabama, USA

Erosion of the leading hanging-wall cutoffs of thrust sheets commonly obscures the magnitude of thrusting. The Jones Valley thrust fault in the southern Appalachian thrust belt in Alabama, USA, is exposed along a northwest-directed, large-scale frontal ramp, and the leading part of the thrust sheet has been eroded. Previously published and newly collected vitrinite reflectance data from Pennsylvanian coal beds document a distinct, northeast-trending, elongate, oval-shaped thermal anomaly northwest of the trace of the Jones Valley fault. The northwest edge of the thermal anomaly is ~18 km northwest of the fault trace, suggesting the original extent of the eroded thrust sheet. The anomaly ends both northeastward and southwestward along strike at lateral ramps. The southeast edge of the anomaly corresponds to the location of a footwall frontal ramp. A three-dimensional heat conduction model for simultaneous horizontal (two-dimensional) and vertical heat flow in a rectangular thrust sheet is designed to test whether the documented thermal anomaly (%Ro = 1.0–1.6) may reflect the former extent of thrust-sheet cover. The model uses a 3-km-thick thrust sheet with horizontal dimensions of 10 x 30 km, as well as a three-dimensional analytical solution to the heat conduction equation, whereby the thrust sheet cools both laterally and vertically. The model reproduces the magnitude and oval shape of the vitrinite reflectance anomaly at 100–500 k.y. after thrust emplacement. The geothermal gradient reaches a steady state at ~2 m.y., and is never fully reestablished even for long times because of lateral cooling in the hanging wall. Thickness and extent of the thrust sheet from the thermal model are consistent with balanced and restored cross sections of the Jones Valley thrust sheet based on geologic data; a thrust sheet ~3 km thick was emplaced ~18 km onto the foreland over the site of the thermal anomaly. The three-dimensional thermal evolution of both the hanging wall and the footwall is distinct from that predicted from one-dimensional models; a three-dimensional model predicts less heating of the footwall because of horizontal heat loss across bounding ramps.