Three-Dimensional Models of Faulting During Oblique Continental Extension

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).A newly developed three dimensional numerical modeling code that can deal with elasto-visco-plastic deformation will be used to study the controls on faulting during oblique continental extension. This project is a two-phase study that focuses on the controls on patterns of tectonic extension of continental lithosphere. The first phase of the project deals with general questions about the physics of fault development and builds directly on previous work on two dimensional extensional faulting: (1) determination of fault weakening parameters that are needed to match normal fault population statistics for natural and analog studies: size frequency distribution and length offset ratios of model and observed faults would be considered; (2) the effect of elasticity on the pattern of faulting, which has bearing on which numerical methods most efficiently simulate faulting during lithospheric extension. In the second phase of the project, three natural type examples of oblique rifting will be modeled, each of which extended with different initial conditions of thermal and crustal thickness: (1) a region where the crust was likely cold at the time of rifting in order to assess the degree of pre-heating of the rift zone that is needed to explain the along-axis segmentation and across-axis structure of faulting in the Gulf of Suez; (2) a region of moderately thick, warm crust, where highly oblique extension was apparently accommodated by two distinct geometries of transtension to determine what is needed to produce the change from strain partitioning to transtension seen in the Gulf of California; (3) extension of thick, hot crust that may have led to the formation of metamorphic core complexes in places like the Colorado River Extensional Corridor to evaluate if buoyant pluton emplacement is needed for the formation of domal cores of metamorphic core complexes. All the models would consider extreme amounts of oblique extension of areas with thin lithosphere and one suite of models would include a buoyant mid-crustal layer.The breakup of continents controls the shape of the Earth's surface and directly affects regional and global climate. Besides curiosity about geography and climate there are economic reasons to study the how breakup, or rifting, occurs. Much of the world's hydrocarbon reserves were formed and preserved in the geologic structures of rifted continental margins. The research team has recently advanced the hypothesis that continental breakup occurs only under one of two special circumstances: when injection of magmatic dikes heats and weakens the lithosphere or when thickening the crust during mountain building also warms and weakens the lithosphere. The goal of this project is to investigate how these very different processes may produce the great variety of structures observed at rifted continental margins. To study the development of rift structures advanced numerical models, similar to those used by engineers to design structures that do not fail under reasonable loads will be employed. These models differ from most engineering models in that how multiple failure or slip events on faults combine to produce structures on a very large spatial and temporal scale are considered. Previously, some of these structures have been modeled in two dimensions. Now, the combination of better numerical methods and bigger computer clusters permits examination of how these structures develop in three dimensions. This is particularly important since some of the structures are intrinsically three dimensional. By trying to reproduce the observed patterns of faults, crustal thickness, and topography the strength of faults may be constrained and aid in understanding earthquake behavior.