New Quantitative Tectonic Models for Southern Alaska

Crustal deformation studies using GPS have made important contributions to our knowledge of the tectonics of Alaska. Since 1995, we have determined precise GPS velocities for more than 350 sites throughout Alaska. We use a subset of these sites to study permanent deformation of the overriding North American plate, in particular the motion on the strike-slip Denali and Fairweather faults and the deformation of interior Alaska. Velocities from almost 100 GPS sites help us to determine how the Pacific-North American plate boundary deformation is distributed and which structures other than the Alaska-Aleutian megathrust are important in accommodating the relative motion of the plates. Based on the GPS velocities, we have constructed new quantitative tectonic models for Alaska. Our models are based on, and a considerable improvement to the model of Lahr and Plafker [1980]. The fundamental difference between our proposed models and theirs is that we use measured slip rates rather than assumed rates or guesses. We thus present the first truly quantitative tectonic models for the deformation of the overriding plate in Alaska, although several important parts of the models need additional data to constrain them. Using dislocation modeling techniques, we estimate slip rates for the McKinley segment of the Denali fault and the Fairweather fault to be approximately 9 mm/yr and 46 mm/yr, respectively. We present three models, all of which involve the Yakutat block, Fairweather block (called the St. Elias block by Lahr and Plafker [1980]), and the southern Alaska block (called the Wrangell block by Lahr and Plafker [1980]). The western boundary to the Southern Alaska block is the most speculative, and the nature and location of this boundary are the only differences between our three proposed models. For each crustal block we determine an Euler pole and angular rotation rate and calculate the slip rates across the boundaries between the blocks. The models provide a first step towards a coherent framework for understanding the tectonics of a large part of Alaska. However, with no GPS data in western Alaska, along with low seismicity and little geological information in this area, it is hard to put constraints on this region of the models. The three models proposed are certainly simplifications of reality, but armed with these quantititative models we have a starting point for further investigations. Reality probably includes some aspects of each of the three end-member models. J. C. Lahr and G. Plafker, Holocene Pacific-North American Plate interaction in southern Alaska; Implications for the Yakataga seismic gap, Geology, 483-486, 1980.