Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp44a..02h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP44A-02
Mathematics
Logic
1525 Paleomagnetism Applied To Tectonics: Regional, Global, 1527 Paleomagnetism Applied To Geologic Processes, 3060 Subduction Zone Processes (1031, 3613, 8170, 8413), 8157 Plate Motions: Past (3040)
Scientific paper
Paleocene volcanic and sedimentary rocks of the Ghost Rocks, part of the Chugach accretionary complex, may provide important constraints for possible paleogeographic models of plate configurations along the NW margin of North America. The Ghost Rocks, along with other Late Cretaceous to E. Eocene units of southern Alaska, contain anomalous near-trench igneous rocks which decrease in age from NW to SE along this margin and record relative motion of a trench-ridge-trench triple junction with respect to the margin. Thick sections of similar-aged volcanic rocks also exist in coastal Washington and Oregon. Two models have been proposed to explain these anomalous volcanic rocks. Moore et al. (1983) ascribes the progression of ages of the TRT rocks to migration of the Chugach terrane past a Kula-Farallon-NA boundary located off of present- day Oregon. Haeussler et al. (2003) call for the existence of two TRT triple junctions, one located off the present-day Oregon margin, the other of which migrated from NW (Sanak) to SE (Baranof) between Paleocene and Eocene time, which requires an intervening oceanic plate (the Resurrection Plate). Our work has included extensive sampling of both volcanic and sedimentary units of the Ghost Rocks, and detailed structural studies. The geological work indicates that much of the Ghost Rocks, particularly the large section containing volcanics at Alitak Bay, is composed of intact blocks bordered by fault zones, with steep upright to locally overturned bedding. The paleomagnetic results have revealed that the majority of the sedimentary rocks have poorly-behaved magnetizations carried by authigenic sulphide minerals. The majority of the igneous rocks (primarily beautiful exposures of basaltic-andesite pillows) have well-defined magnetizations with unblocking temperatures ranging from 450 to 550 C. The sites from Alitak Bay were corrected for vertical-axis rotations by rotating site-specific bedding strikes to agree with an average regional strike of 250 degrees. Incremental rotations applied to the site-mean directions (a paleomagnetic rotation test) indicate that best clustering occurs at the optimal rotation; thus the Alitak rocks were magnetized prior to rotation. Because the resulting rotation-corrected structure at Alitak Bay is monoclinal, all versions of paleomagnetic fold tests on the Alitak Bay rocks are inconclusive. Volcanics from Kiliuda Bay, using data from Plumley et al 1983 and this study, also have well-defined magnetizations. A regional fold test using the combined rotation-corrected Alitak Bay results and the results from Kiliuda Bay indicates best clustering occurs at 100% untilting, resulting in a combined site mean of D = 162, I = 60, k = 19, a95 = 6, N = 30. While a pre-tilting, and pre-rotation, remagnetization cannot be entirely ruled out, our new data suggest these rocks likely retain their original magnetization. Based on this we conclude that the Ghost Rocks were likely magnetized at a latitude of 41° ±7 N. These data thus support migration of at least a portion of the Chugach terrane, and its TRT-related rocks, from a position off shore present-day Oregon, since Paleocene time.
Gallen S.
Housen Bernard A.
O'Connell K.
Roeske Sarah M.
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