Globerman, B.R., 1985, A paleomagnetic and geochemical study of upper Cretaceous to lower Tertiary volcanic rocks from the Bristol Bay region, southwestern Alaska: University of California, Santa Cruz, Ph.D. dissertation, 398 p., illust.
A thick volcanic sequence of latest Cretaceous (68 Ma) age is exposed on several nearshore islands in the northern Bristol Bay region of southwestern Alaska. The Bristol Bay volcanic series comprises the southwestern extension of the Kuskokwim Mountains magmatic belt. Major- and trace-element analyses indicate that the volcanic suite is strongly calcalkaline. Average ratios of Ba/La (42), La/Nb (3.5), and Ba/Ta* (2,300) are characteristic of orogenic andesites associated either with convergent margins or continent-continent collision zones. Paleomagnetic directions were determined for 74 sites, which comprise lava flows and pyroclastic deposits. All sites have normal polarity, although the presence of thick, well-laminated sedimentary interbeds, and the occurrence of at least three geomagnetic field excursions in the Hagemeister section, suggests that the effects of secular variation have been adequately time-averaged. The fold test is positive at 99% confidence, suggesting that the remanence was acquired prior to deformation. The observed mean inclination is not different at 95% confidence from the expected inclination for the North American reference field, suggesting no significant latitudinal displacement since 68 Ma. However, comparison of expected and observed declinations indicates about 46 degrees +/- 21 degrees of counterclockwise rotation since latest Cretaceous time. Because lower Tertiary volcanic suites from other localities in western and central Alaska also show counterclockwise rotation, the inferred rotation of the northern Bristol Bay area is thought to represent a regional phenomenon (e.g., oroclinal bending), rather than a local effect. Regional counterclockwise rotation apparently produced the sharply arcuate pattern of the southern Alaska continental margin and inland orographic features. Eurasia - North America continental convergence between 70 and 50 Ma probably served as the driving mechanism for oroclinal bending. Removal of about 50 degrees of counterclockwise rotation orients the Kuskokwim Mountains magmatic belt such that Kula - North America relative convergence would have been much less oblique in Late Cretaceous to early Tertiary time. The pre-rotation paleogeography of southern Alaska continental margin would have been more suitable for coastwise translation of allochthonous terranes (e.g., Chugach and Prince William terranes) by dextral transcurrent faulting along the continental borderland.
Theses and Dissertations