Abstract
The presence of offsets, appearing at intervals ranging from 10s to 100s of kilometres, is a distinct characteristic of constructive tectonic plate margins. By comparison, boundaries associated with subduction exhibit uninterrupted continuity. Here, we present global mantle convection calculations that result in a mobile lithosphere featuring dynamically derived plate boundaries exhibiting a contrasting superficial structure which distinguishes convergence and divergence. Implementing a yield-stress that governs the viscosity in the lithosphere, spreading boundaries at the top of a vigorously convecting mantle form as divergent linear segments regularly offset by similar length zones that correlate with a large degree of shear but comparatively minimal divergence. Analogous offset segments do not emerge in the boundaries associated with surface convergence. Comparing the similarity in the morphologies of the model plate margins to the Earth’s plate boundaries demonstrates that transform-like offsets are a result of stress induced weakness in the lithosphere owing to passive rupturing.
Highlights
Plastic yielding in a spherical shell mantle convection model featuring a highly temperature-dependent viscosity, internal and basal heat sources representative of radiogenic mantle heating and secular cooling of the Earth’s core, respectively, and a factor of 30 increase in viscosity in the lower mantle, were found to be sufficient requirements to obtain the distinct structure of oceanic ridge systems characterized by linear spreading segments offset by transform faults
Through systematically varying yield stress profiles across the model lithosphere, we find that convergent and divergent features transition from being arclike, with a length scale comparable to mantle depth (Fig. 1a), to becoming longer, often terminating at triple junctions (Fig. 1b)
Upon reaching a surface yield stress of some threshold value divergent boundaries break into segments that tend to align normal to plate motion and linear offsets form to join these segments
Summary
Another impact on the number of plates and boundary structure obtained could be the absence of model continent cratons, which have been shown to focus stress on their margins Plastic yielding in a spherical shell mantle convection model featuring a highly temperature-dependent viscosity, internal and basal heat sources representative of radiogenic mantle heating and secular cooling of the Earth’s core, respectively, and a factor of 30 increase in viscosity in the lower mantle, were found to be sufficient requirements to obtain the distinct structure of oceanic ridge systems characterized by linear spreading segments offset by transform faults.
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