Paleomagnetism of Silver Island, Keweenaw Peninsula, Michigan: Additional Support (?) for the Primary Curvature of the MCR
Abstract
Silver Island lies between Eagle Harbor and Copper Harbor on the NW coastline of the Keweenaw Peninsula, Michigan. The island consists of a series of basaltic lava flows which dip to the north-northwest (356°) at an angle of approximately 38°. These flows constitute part of the Lake Shore Traps (LST), a series of interbedded lava flows within the Copper Harbor Conglomerate (CHC). The LST represent the youngest eruptive material associated with the 1.1 Ga Mid-Continent Rift (MCR). The most recent paleomagnetic study (now 15 years old) on the Lakeshore Traps (LST) defined three distinct directional clusters. Each cluster of directions corresponded to a different stratigraphic package of the LST within the CHC and all have extremely low between-site dispersion (s ∼ 4°) suggesting rather rapid emplacement of the LST packages. Consequently, each cluster has its own unique direction of magnetization. Since the lower two LST packages crop out along the coast line of the Peninsula with different structural trends, an opportunity was presented to test the conclusions of Hnat et al. (2006) that the curvature of the MCR was primary. To that end nine lava flows were sampled on Silver Island and their mean direction compared to the equivalent mean from lava flows sampled by Diehl and Haig (1994) from the tip of the Keweenaw Peninsula (upper lava flows of the Middle LST) which have an entirely different structural trend. Characteristic directions of magnetization of the Silver Island lava flows were isolated either using alternating field, thermal or a combination of both. The mean direction of magnetization for the nine sites is: D = 277.7°, I = 46.9°, α95 = 3.0°, k = 292.5. The mean direction recalculated from the Diehl and Haig study is: D = 277.8°, I = 40.6°, α95 = 2.9°, k = 315.7. Although the declinations of the two means are identical, interestingly the two means are statistically distinct at the 95% confidence level. Fold tests were inconclusive. Nevertheless, the nearly identical declinations from two locations with different structural trends suggest the curvature of the MCR on the Keweenaw Peninsula is primary. Differences in inclinations at this writing remain unresolved.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSMGP11E..01D
- Keywords:
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- 1525 Paleomagnetism applied to tectonics: regional;
- global