ANOTHER ANALYSIS OF THE CRETACEOUS APPARENT POLAR WANDER OF NORTH AMERICA AND THE END OF THE POLAR STILL-STAND/SLOW-WALK

Compilations of paleomagnetic from Cretaceous rocks of stable North America (Mankinen, 1978; Van Fossen and Kent, 1992; Besse and Courtillot, 2002, Beck and Housen, 2003; Enkin, 2006, Torsvik et al 2008) all show relatively small changes in pole position for most of Cretaceous time. Despite over 30 years of study, there remain questions regarding the nature and duration of this APWP still-stand, particularly regarding the onset and termination of this feature, and its geodynamic significance. This study will focus on a comparison of North-America derived paleomagnetic data and data derived from other tectonic blocks, and on constraints that may be provided by sedimentary rocks that can help determine when this polar still-stand ended.

Reanalysis of NA data, with updated geochronology for many of the igneous units, is used to examine possible temporal trends in the pole positions. For this analysis, mean poles for sets of paleomagnetic sites from individual geographic areas or rock units are determined from published paleomagnetic studies. These locality means are then combined into age groups. A total of 27 locality means were defined, with ages spanning from 130 to 85 Ma. Mean poles calculated for 10 Ma age ranges (centered on 130, 120, 110, 100, and 90 Ma) are separated by less than 10 degrees- the APW path connecting these mean poles can be described as defining a very small loop, or as a single cluster. This short path, or lack of APW, indicates that motion of NA was mainly or entirely toroidal during that time- with the Euler pole for NA coinciding with the mean pole for the still-stand (71.8 N, 192.7 E, A95=2.4, N=27). Estimates on the end of the polar still-stand vary between Late Cretaceous to Eocene time, and depend on data selection criteria. A comparison between paleopoles determined from Late Cretaceous to Paleocene sedimentary sequences from undisturbed sections in North America can help provide some constraints to address these issues. Poles from sediments of the Hell Creek Fm are far-sided relative to other Late K poles, and fall on a great-circle path connecting the Hell Creek locations and poles from igneous rocks from Montana (Elkhorn and Adel Mtns)- this may support the validity of the Montana volcanics and intrusives to constrain the end of the Cretaceous polar still-stand at 75 to 80 Ma.