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Thrupp, G.A., 1987

The paleomagnetism of Paleogene lava flows on the Alaska Peninsula, and the tectonics of southern Alaska

Bibliographic Reference

Thrupp, G.A., 1987, The paleomagnetism of Paleogene lava flows on the Alaska Peninsula, and the tectonics of southern Alaska: University of California, Santa Cruz, Ph.D. dissertation, 248 p.


Although the exotic nature of portions of southern Alaska are well documented, the displacement histories of the far-traveled terranes are largely unknown. The Peninsular terrane, which consists chiefly of the Alaska Peninsula, occupies a key tectonic position. Paleomagnetic data indicate that geologic provinces on opposite sides of the Peninsula terrane were separated by approximately 25 degrees of latitude in Paleocene time. Geologic evidence, however, suggests that by Paleocene time the Peninsular terrane was linked to both of the bordering terranes. This paradox challenges the status of the Peninsula terrane as a coherent tectonic entity. Numerous sequences of superposed lava flows on the Alaska Peninsula retain collectively a paleomagnetic record of paleonorth and paleolatitude at the time of the Paleogene volcanic activity. Most of the sequences have been radiometrically dated. Twelve sequences of superposed Middle Eocene - Early Oligocene lava flows (a total of 94 flows) span a large portion of the Peninsular terrane from southwest of Chignik to north of Lake Clark. The mean paleomagnetic result indicates 8 +/- 8o of southward displacement and 11 +/- 76o of counterclockwise azimuthal rotation relative to North America (uncertainties at 95% confidence). The paleomagnetism of 30 Paleocene lava flows just north of Lake Clark indicates 9 +/- 11o of northward displacement, and 55 +/- 28o of counterclockwise azimuthal rotation. At a high confidence level both indications of latitudinal displacement are negligible. Thus, the principal conclusion of this study is that the Peninsular terrane in early Tertiary time was within a few degrees of its present latitudinal position with respect to North America. Also at a high level of confidence, the results indicate that at least 30o of counterclockwise rotation, of at least the Lake Clark region, occurred between the early Paleocene and early Oligocene. Similar indications of rotation by other paleomagnetic studies in western Alaska suggest that the counterclockwise rotation is of regional extent. At a highly speculative level of confidence, the paleomagnetic data suggest that hundreds of kilometers of northward displacement, of at least the Lake Clark region, took place in Paleocene to Eocene time. Cumulatively large, intra-terrane displacements during late Cretaceous to early Eocene time may be sufficient to account for the translation of the allochthonous early Tertiary tectonic elements to essentially their present positions in southern Alaska.

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