Collaborative Research: The Asthenosphere and Mantle Dynamics

William Marsh Rice University, Houston TX

Investigators

Abstract

The majority of changes that occur on our planet can be tracked back to one of two energy sources: 1) Energy from the sun and 2) Energy from the Earth's interior. How interior energy shapes the Earth's surface links directly to understanding the dynamics of plate tectonics. Plate tectonics stands as one of the three major scientific revolutions of the 20th century, along with quantum mechanics and relativity. As a theory it remains a kinematic one in that it describes the motion of plates at the Earth's surface but does not connect the motions directly to the forces that drive the motion. Extending plate tectonics theory from a kinematic one to a dynamic one has been a something of a grand challenge in the geological and geophysical communities. From the inception of plate tectonics as a kinematic theory, it has been suspected that the dynamics of plate tectonics is connected to the existence of a low viscosity zone below plates - the asthenosphere. Mapping, quantifying, and seeking to bring conceptual understanding to several of the factors that tie into this long suspected connection is the theme of this work. More specifically, over the next one year cycle, the team will: A) Test the hypothesis that changes in asthenosphere structure can drive rapid changes in plate motion of the type recently identified to have occurred on the Earth 50-100 Million years ago and B) Explore the interaction between plate tectonics and the cycling of volatiles between surface reservoirs and the Earth's deep interior (e.g., deep water cycling can effect the nature of the asthenosphere through its role on rock melting). The investigators will explore the effects of a low viscosity asthenosphere on mantle dynamics. Numerical mantle convection experiments will be used to map the combined effects of a low-viscosity asthenosphere and plate boundary failure on maintaining a plate-tectonic mode of mantle convection. A specific goal will be to identify conditions that allow for rapid changes in plate motions, as have recently been inferred for the Earth's Cretaceous. Numerical work will be augmented with theoretical scaling analysis. The analysis will provide insight into energetic balances between the resistance to plate motion associated with dissipation at plate margins, within the asthenosphere, and in the bulk mantle. This will allow the PIs to link dynamic modeling to instantaneous flow studies that isolate the balance of dissipation between plate processes and the bulk mantle. That link will bring added observational constraints to bear on their results and predictions. The analysis will also be extended to model deep volatile cycling (water and carbon) within the Earth and volatile exchange from the surface to the interior of our planet over geologic time. The key feedback this introduces, relative to the role of the asthenosphere on maintaining plate tectonics, comes from the ability of volatiles to effect melting in the upper mantle, which can significantly alter the structure of the asthenosphere.