HR: 0830h
AN: GP11C-0274    [PDF]
TI: Paleomagnetic Evidence for Neogene Tilting of Crustal Panels in Southeastern Alaska
AU: * Butzer, C M
EM: cbutzer@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East 4th Street, Tucson, AZ 85721 United States
AU: Butler, R F
EM: butler@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East 4th Street, Tucson, AZ 85721 United States
AU: Gehrels, G E
EM: ggehrels@geo.arizona.edu
AF: Department of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East 4th Street, Tucson, AZ 85721 United States
AU: Davidson, C
AF: Department of Geology, Carleton College, Northfield, MN 55057 United States
AU: Crawford, M L
AF: Department of Geology, Bryn Mawr College, Bryn Mawr, PA 19010 United States
AB: Paleomagnetic samples were collected from Tertiary (mostly Oligocene) plutons and dikes along a transect from west of Prince of Wales Island to the mainland near Wrangell, Alaska. Thermal demagnetization and principal component analysis were used to determine the characteristic remanent magnetization (ChRM). Site-mean ChRM directions with 95% confidence limit ($\alpha$95) $<$ $15\deg$ were acquired from 19 normal-polarity sites and nine reversed-polarity sites located on the western part of the transect. These directions pass the reversals test with class C designation. The observed mean paleomagnetic direction is: I= $72.5\deg$; D = $47.0\deg$; $\alpha$95 = $5.1\deg$. The expected Oligocene direction is: I = $73.3\deg$ $\pm$ $2.5\deg$ and D = $350.0\deg$ $\pm$ $7.8\deg$. The observed concordant inclination and discordant declination indicate either: (1) $57.0\deg$ $\pm$ $15.1\deg$ clockwise vertical-axis rotation or, (2) ESE-side-up tilt by $17\deg$ with azimuth of tilt axis = $18\deg$. Several lines of geologic evidence favor the crustal tilt interpretation. Rocks in this region ranging in age from Proterozoic to Tertiary form elongate crustal panels consistently oriented NW-SE. The continental margin has experienced strike-slip to transtensional Tertiary deformation with dominant structural grain oriented NW-SE. Thus large Neogene vertical-axis rotation is unlikely to have affected this area. Additionally, evidence from metamorphic grade of Alexander terrane rocks on Prince of Wales Island and crustal levels within the igneous complex of Zarembo and Etolin Islands suggest SE-side-up tilting. Paleomagnetic analyses of Tertiary dikes from the central and eastern parts of the transect are underway and should determine the amount and direction of Neogene crustal tilt across the full width of the continental margin.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting