Paper No. 9
Presentation Time: 4:00 PM
PALEOMAGNETICS AND TECTONIC SIGNIFICANCE OF CRETACEOUS SEDIMENTARY ROCKS IN THE MITCHELL INLIER, BLUE MOUNTAINS, OREGON
The Blue Mountains of eastern Oregon is a collage of terranes that lie immediately west of a large transpressional strike-slip fault zone along which outboard terranes were moved 100s of km in a dextral sense during Cretaceous time (Wyld and Wright, 2001). To evaluate possible translation and rotation of the Blue Mts, we conducted a paleomagnetic study of the Cenomanian Gable Creek Formation, a submarine-fan turbidite sequence that overlies the more complexly deformed Main Mudstone Member of the Hudspeth Formation. A total of 120 samples from 18 sites were demagnetized. All samples displayed two components of remanence; the first-removed component unblocked between 80 and 300 C, while the second-removed component unblocked between 250 and 580 C. The age of the second-removed component is constrained to be mid-Cretaceous by: (1) a positive baked-contact test with an andesite dike of the Eocene Clarno Volcanics, (2) a positive conglomerate test in conglomerate of the Gable Creek Formation, and (3) a positive fold test. Inclination error was tested for using magnetic fabric and shape-distribution of site level directions (Tauxe and Kent, 2004), and was found to be insignificant in these rocks. Using 100% untilting for sites south of the Mitchell fault yields a Fisher mean direction of D=10.8°, I=58.5°, k=127, &alpha95=4.1°, N=11. A paleolatitude of 39.2° +/- 4.5° N is obtained from this direction for the Gable Creek Formation. Compared with the current location of these rocks on North America, and using the recalculated mid Cretaceous NA reference pole from Housen et al. (2003), the expected paleolatitude of the study area is 54.8° N. Comparing the expected (for NA) and observed paleomagnetic results indicates a northward translation of 1700 +/-450 km, and a CW rotation of 37° +/-5° for these rocks since the Late Cretaceous (93 Ma), with respect to North America. Using a 5° inclination error indicates a paleolatitude of 45° N, and would result in 1100 +/-450 km of northward translation wrt North America. Comparison of these results with other Cordilleran units suggests a tectonic affinity between the Blue Mts and Intermontane superterrane of SW Canada (e.g. Mortimer, 1986). Such correlation, if correct, would require revision of existing tectonic models for the North American Cordillera.