Geochemistry, Geophysics, Geosystems
© American Geophysical Union
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ISI Journal Citation Reports © Ranking: 2014: 19/79 (Geochemistry & Geophysics)
Online ISSN: 1525-2027
Physical controls of rift interaction identified
Cutting into the surface of every continent, and even the faces of other planets, the valleys formed by lithospheric rifting are a distinctive marker of tectonic activity. Rifts form from a structurally weak segment of the brittle upper crust, when stress-induced fracturing causes lengthening and spreading over hundreds of thousands of years. Rift valleys, known as graben, can extend for thousands of kilometers and spread at rates up to a few centimeters per year. Where multiple individual rift systems come together, tectonic plates can be cut in two or new crust formation regions can arise. Whether separate rifts in close proximity will lead to one of these more consequential outcomes or stagnate as a series of valleys and peaks depends on the physical properties of the rifts, brittle crust, and viscous lower layers of the lithosphere. Using a three-dimensional deformation model, Allken et al. (2012) analyzed how changes in these initial physical variables affect the interactions between rifts. In the model, which simulated evolving rift systems over millions of years through a series of 20,000-year snapshots, the authors varied the number of weak zones that serve as the nuclei for rift formation, the viscosity of the lower layer, how crustal stress weakened over time, and the offset between two elongating grabens. They found three distinct modes of interaction: (1) two grabens can grow into one another forming a single large rift; (2) the grabens can grow toward each other without connecting, forming a yin and yang-style structure; or (3) the grabens can develop independently, creating a highly fractured and undulating topography. The authors found that decreasing the offset, decreasing the lower layer viscosity, and increasing the rate of stress weakening increased the chance of the grabens connecting into a single large rift system.