REVISITING THE SEVEN DEVILS-WRANGELLIA CONNECTION: PALEOGEOGRAPHY OF TRIASSIC ROCKS IN WESTERN IDAHO

Wrangellia, first characterized by Jones (1977), is a volcanic terrane that is now scattered from Vancouver Island to south-central Alaska. The Seven Devils arc terrane is thought to have at one time been a part of Wrangellia. Lines of evidence in support of this include stratigraphy, paleontology, and paleomagnetic data. However, previous paleomagnetic studies (Hillhouse et al. 1982) have uncertainties associated with rotation corrections from Jurassic plutons in the Blue Mountains (Wilson and Cox, 1980).

In this study new paleomagnetic data from the the Seven Devils Group, updated rotation estimates for the Blue Mountains Province (Housen and Dorsey, 2005) and updated apparent polar wander data for the Triassic of North America will be used to compare the late Triassic paleolatitude of the Seven Devils terrane to Triassic paleolatitudes of the Nikolai greenstone in Alaska and the Karmutsen Formation on Vancouver Island and to evaluate the results of Hillhouse et al (1982).

Specimens from 22 sites in Seven Devils Group have been demagnetized and measured. Nineteen of the sites fail the McElhinny fold test and are interpreted to have been remagnetized. The in-situ mean direction for these sites (D = 41.8°, I = 56.2°, α95 = 11.1 and κ =10.12) is similar to the Hillhouse et al. (1982) group three direction, which was interpreted as a magnetic overprint. Three sites from this study yield directions similar to Hillhouse et al. group one directions, which was interpreted as unremagnetized. Several types of incremental fold tests show that 8 sites, 5 from Hillhouse et al. (1982) and three from this study, have the best-clustering of directions at 100% untilting and thus are interpreted to pass the fold test. The mean of tilt-corrected directions for these 8 (D = 88.2°, I = -28.4°, α95 = 14.4 and κ = 15.81) yields a late Triassic paleolatitude of 15° north or south. Because magnetic inclination is shallow the hemisphere of origin for the Seven Devils arc is ambiguous. The northern option when compared to the Triassic paleolatitude for the present location of Seven Devils on North America requires no translation. The southern option requires 31° northward translation since the Norian. Continuing research will consist of attempting to characterize the magnetic mineralogy and identify magnetic carriers via major and minor element xrf and hysteresis loop analysis.