Obliquity favours propagation pulses during continental break-up

<p>V-shaped propagators are ubiquist and the seafloor age map is often sufficient to unravel the first order features of the timing of continental break-up at regional or more global scale. Some propagators show  pulses in the rate of continental break-up propagation highlighted by the geometry of magnetic anomalies. These pulses, which were first introduced by Courtillot (1982) in the Gulf of Aden, represent a major element of plate tectonics. Despite the well documented geological record of these changes of rate, and their implications for plate kinematic reconstructions or the thermal regime of oblique margins, the dynamics of ridge and rift propagation at long/geodynamic timescale remains poorly studied nor understood. To date, despite the large progress made in understanding lithospheric dynamics and continental break-up, no lithospheric scale dynamic models has been able to produce self consistently pulse of ridgepropagation followed by a phase of stagnation. One obvious reason for this lack of dynamic ground stands from the fact that this problem mandates 3D thermo-mechanically coupled simulation approach that is just starting to emerge. In this work we chose to adopt a numerical modelling set-up after Le Pourhiet et al. (2018) to produce V-shaped propagators. Simulations investigate the influence of both kinematic and rheology of the lithosphere on the propagation trend and rate. The tectonic evolution of these margins shows 3 different modes of continental break-up propagation and a major change of deformation regime between phases of propagations and phases of stagnation.</p>

Erosion in southern Tibet shut down at ∼10 Ma due to enhanced rock uplift within the Himalaya

Exhumation of the southern Tibetan plateau margin reflects interplay between surface and lithospheric dynamics within the Himalaya–Tibet orogen. We report thermochronometric data from a 1.2-km elevation transect within granitoids of the eastern Lhasa terrane, southern Tibet, which indicate rapid exhumation exceeding 1 km/Ma from 17–16 to 12–11 Ma followed by very slow exhumation to the present. We hypothesize that these changes in exhumation occurred in response to changes in the loci and rate of rock uplift and the resulting southward shift of the main topographic and drainage divides from within the Lhasa terrane to their current positions within the Himalaya. At ∼17 Ma, steep erosive drainage networks would have flowed across the Himalaya and greater amounts of moisture would have advected into the Lhasa terrane to drive large-scale erosional exhumation. As convergence thickened and widened the Himalaya, the orographic barrier to precipitation in southern Tibet terrane would have strengthened. Previously documented midcrustal duplexing around 10 Ma generated a zone of high rock uplift within the Himalaya. We use numerical simulations as a conceptual tool to highlight how a zone of high rock uplift could have defeated transverse drainage networks, resulting in substantial drainage reorganization. When combined with a strengthening orographic barrier to precipitation, this drainage reorganization would have driven the sharp reduction in exhumation rate we observe in southern Tibet.

Practical analytical solutions for benchmarking of 2-D and 3-D geodynamic Stokes problems with variable viscosity

Geodynamic modeling is often related with challenging computations involving solution of the Stokes and continuity equations under the condition of highly variable viscosity. Based on a new analytical approach we have developed particular analytical solutions for 2-D and 3-D incompressible Stokes flows with both linearly and exponentially variable viscosity. We demonstrate how these particular solutions can be converted into 2-D and 3-D test problems suitable for benchmarking numerical codes aimed at modeling various mantle convection and lithospheric dynamics problems. The Main advantage of this new generalized approach is that a large variety of benchmark solutions can be generated, including relatively complex cases with open model boundaries, non-vertical gravity and variable gradients of the viscosity and density fields, which are not parallel to the Cartesian axes. Examples of respective 2-D and 3-D MatLab codes are provided with this paper.