Rock magnetism applied to tectonic and environmental problems
With the disciplines of rock magnetism in common, three projects in different spatial and temporal scales were carried out for tectonic and environmental applications. Paleomagnetic samples collected from the Northumberland Formation on Hornby Island, British Columbia were analyzed for their anisotropy of anhysteretic remanence (AAR) to identify and correct for any inclination shallowing by depositional-compactional processes. A revised correction equation was used to account for the effects of either triaxial magnetic fabrics or small angular deviations of principal axes from the bedding plane. The average inclination (I = 60.5Â°) was increased by 9.6Â° after the compaction correction placing Hornby Island and the Insular Superterrane at a paleolatitude of 41Â°N. This paleolatitude agrees with the proposed position of the Nanaimo Group constrained by the distribution of rudists, and fits a position predicted by a plate model. Environmental magnetic studies were carried out for annually laminated sediments from Lake Ely, northeastern PA. The presence of the magnetotactic bacteria in the water column, the low temperature test for the magnetosomes in lake sediment, and the direct observation of magnetic extract under the TEM all suggest biogenic magneties are the main magnetic minerals in the Lake Ely sediment. Magnetic concentration in the sediments records a disturbance of watershed by European settelment. The variation of magnetic signal in Lake Ely is probably controlled by the change of biogenic productivity, on which precipiation and the fall mixing of water might have a great impact. Carbonate concretions from Gambier Bay in the Gravina belt, southeastern Alaska, were sampled to examine the relationship between the remanence directions and the magnetic fabrics. A fold test applied to the site mean directions and the orientation of magnetic fabrics demonstrated the best clustering of the magnetic directions at 30% unfolding of the directions, yielding the mean direction of Dec = 348.8Â°, Inc = 65.9, k = 98.1, alpha95 = 6.0Â°. The geological constraints on remanence acquisition suggest that the paleomagnetic direction of the concretion samples was probably a secondary chemical magnetization caused by 110--90 Ma plutonic activities in the Gravina belt, which would suggest ca. 2000 km displacement of the Gravina belt.
Minerals Data and Information Rescue in Alaska (MDIRA)