A PALEOMAGNETIC FRAMEWORK FOR THE PALEOGEOGRAPHY OF NORTH AMERICA AND CORDILLERAN TERRANES DURING CRETACEOUS TIME

The time-averaged nature of the geomagnetic field allows for the use of paleomagnetism to determine the locations of tectonic units relative to the spin axis. For the Baja-BC hypothesis the timing and displacement of terranes depends on both the paleomagnetic data from North America as well as from accreted terranes. 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) show only small changes in pole position for most of Cretaceous time. This period of very slow apparent polar wander is referred to as the Cretaceous polar still-stand. Reanalysis of NA still-stand poles, with updated geochronology for the igneous units refines its geometry and duration. Mean poles for paleomagnetic sites from individual geographic areas or rock units are combined into age groups. A total of 27 locality means were defined, with ages spanning from 130 to 85 Ma. The APW path connecting these mean poles can be described best as a single cluster, and indicates that motion of NA was mainly or entirely toroidal during that time- with the Euler pole for NA motion coinciding with the mean pole for the still-stand (71.8 N, 192.7 E, A₉₅=2.4, N=27). The still-stand starts between 145 and 130 Ma, and ends between 85 and 80 Ma.

For terranes comprising Baja BC, robust data from 90 Ma volcanic rocks (Wynne et al, 1995, Enkin et al, 2003 and 2006) indicate locations ~2500 km south wrt to NA. The 105 Ma volcanics of the Spences Bridge (Irving et al, 1995; Haskin et al, 2003) have lesser (~1000 km) displacement wrt NA. Collectively, these results can be interpreted en-bloc- with Baja BC experiencing southward motion wrt NA between 105 and 90 Ma followed by northward motion after 90 Ma- or with separate displacement histories for more inboard terranes- Intermontane, and outboard terranes- Coast Plutonic belt/Insular (after Irving et al 1996, Cowan et al 1997). For the latter part of the Cretaceous and the Paleocene, studies of volcanic rocks of the Carmacks (Yukon-Tanana), Ghost Rocks (Chugach), and McColl Ridge (Wrangellia) all indicate large and significant displacements (1500-2000 km) of these terranes wrt NA- highlighting the need to better understand the role of margin-parallel displacement in this orogenic belt.