THE ELKHORN MOUNTAINS REVISITED - NEW DATA FOR THE LATE CRETACEOUS PALEOMAGNETIC FIELD OF NORTH-AMERICA

The Late Cretaceous portion of North America's apparent polar wander (APW) path is poorly known due to a dearth of suitable post-85-Ma reference poles. A paleomagnetic re-investigation of Elkhorn Mountains Volcanics (80 Ma) was initiated to fill this void and to determine the degree to which the onset of an apparent rapid episode of APW during this time interval correlates with known Late Cretaceous tectonic events. A total of 249 paleomagnetic samples (30 sites) were collected from three separate areas within the Elkhorn Mountains. Using both alternating field and thermal demagnetization, a characteristic direction of magnetization was isolate at 15 sites (a95 < 15-degrees) which passes a fold test at the 95% confidence level. The mean pole calculated from site virtual geomagnetic poles is located at 80.3-degrees-N, 189.5-degrees-E (N = 15; A95 = 9.6-degrees; k = 16.8). Interestingly, the Elkhorn Mountains pole is not concordant with the Cretaceous stillstand poles but is nearly coincident with the Paleocene reference pole and the recently published pole for the Adel Mountains dated at 76 Ma. These new data suggest the onset of APW which brought the pole from its Cretaceous position to near its early Tertiary position occurred prior to 80 Ma and the transition was exceedingly rapid. The onset of this rapid APW now appears to coincide more closely in time with many of the significant Late Cretaceous tectonic events, e.g., the Laramide orogeny. However, since the Elkhorn Mountains Volcanics are floored by a major east-directed thrust and have undergone crustal scale deformation, the data should be viewed with caution until more results are available for this time interval.