HR: 16:20h
AN: GP34A-01 INVITED     [Abstracts]
TI: An Anisotropy-based Inclination Shallowing Correction: Paleomagnetism and Rock Magnetism of the Shepody Fm., New Brunswick and Nova Scotia, Canada
AU: Bilardello, D P
EM: dabc@lehigh.edu
AF: Lehigh University, Dept. of EES 31 Williams Drive, Bethlehem, PA 18015, United States
AU: * Kodama, K P
EM: kpk0@lehigh.edu
AF: Lehigh University, Dept. of EES 31 Williams Drive, Bethlehem, PA 18015, United States
AB: The North American apparent polar wander path (APWP) for the upper Paleozoic is dominated by red beds, which have been reported to suffer from deposition- or compaction- induced magnetic inclination shallowing. A series of different type anisotropy measurements have been performed on the Upper Mississippian Shepody Formation red beds of Maritime Canada. These measurements have been used in conjunction with standard paleomagnetic measurements to apply an anisotropy-based inclination correction. Paleomagnetic measurements on the Shepody Fm. revealed the presence of a primary component carried by hematite and a secondary, syn-folding component, carried by maghemite. Anisotropy of magnetic susceptibility (AMS) was used in conjunction with chemical demagnetization to calculate the anisotropy of the characteristic remanence-carrying grains. The resulting fabric is strongly bedding parallel with bedding perpendicular minimum axes. It has been interpreted as a primary depositional/ compactional fabric and was successfully used for an inclination shallowing correction: mean inclinations for the formation were corrected from 17.7° to 29.1°. The magnetic mineralogy of the Shepody Fm., as revealed by demagnetization intensity curves, isothermal remanence magnetization (IRM) acquisition experiments and Lowrie tests, is complex and composed of maghemite, hematite and goethite. In order to develop a less time-intensive technique for isolating the contribution of hematite to the magnetic fabric, a new protocol was devised. It consists of measuring the anisotropy by imparting IRM's in high fields (5 T) to activate the hematite, and following each magnetization step with a thermal demagnetization step at 120° and an alternating field demagnetization (AF) step at 100 mT to eliminate the contributions of goethite and maghemite, respectively. The fabric obtained through this procedure is similar to the AMS fabric; however, the orientation of the principal axes shows a slight imbrication that could indicate deposition from a current. A magnetic fabric was also measured for the secondary magnetization component carried by maghemite using anisotropy of anhysteretic remanence (AAR). Since the remanence was acquired at 70 percent unfolding the fabric was analyzed both in stratigraphic and 70 percent unfolded coordinates. In stratigraphic coordinates the fabric mimics the primary fabric determined with high field (hf) IRM anisotropy. At 70 percent unfolding the fabric is no longer discernible. This result suggests that secondary magnetic minerals can inherit a primary-appearing magnetic fabric. An inclination correction for the remanence carried by the primary hematite particles was performed using the hf-IRM anisotropy and an individual particle anisotropy value of a=1.34 estimated from a fit to the theoretical correction curves. It gave a result similar to the AMS-based correction with mean corrected inclinations of 28.8°. These values are consistent with results obtained from two other Lower Carboniferous rocks from North America and imply an approximately 6° increase in co-latitude for the average Lower Carboniferous paleopole.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly