Date of Award

12-2013

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Peter O. Koons

Second Committee Member

Chris Gerbi

Third Committee Member

Phaedra Upton

Abstract

The coast of Alaska is defined by a subduction boundary to the north, a strike-slip fault (Fairweather fault) to the east, and the Transition fault to the southwest that separates Pacific oceanic crust from thickened continental/oceanic crust. Seismic profiles and kinematic models of the tectonics imply that the Transition fault is currently active, however, a relative lack of data makes it difficult to constrain the style of deformation taken up by the Transition fault. Using three-dimensional mechanical modeling of the tectonics and dynamic ice sheet modeling of the LGM (Last Glacial Maximum) ice extent, I test the sensitivity of the Transition fault to an approximate LGM load using interpretations of velocity data and structure to constrain the crustal thicknesses and the relative strength profiles of the models. I propose two scenarios that could partition strain across the fault; a Yakutat block underlain by weak lower crust or LGM glacial loading. Results indicate that due to the rheology, strain is only partitioned across the fault when the strength of the entire lower crust of the Yakutat block is reduced 7-10 times relative to the North American plate. However, this strength reduction is considerably more than indicated by velocity and seismicity data. Emerging data indicate that the low velocities in the Yakutat block are due to increased porosity in coal bearing layers. I find that for all cases the strain gradient across the Transition fault increases with a LGM load.

Using the latest information on the rheology of the Yakutat block, I improve on the earlier models by incorporating the high porosity layer, adding modem topography, and a modelled LGM load. The strain after glacial unloading matches well with the epicentres of the 1899 earthquakes, a partial response to Little Ice Age unloading. As a comparison, a LGM simulation is run for New Zealand. The model shows that even at the valley glacier scale the system is sensitive to small changes in loading. Both models suggest that the spatial and temporal distribution of glaciers influences the distribution of strain and that observations of the tectonic response to these loads can be used to constrain the rheology of a region.

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