A SYNOPSIS OF PALEOMAGNETIC DATA FROM CRETACEOUS ROCKS OF NORTH AMERICA AND CORDILLERAN TERRANES

Following pioneering work by Beck and Nosen (1972), paleolatitudes and displacements of terranes relative to the Cordilleran margin of NA for Cretaceous-Paleocene time have posed important questions and unresolved problems. Paleomagnetic compilations (Beck and Housen, 2003; Enkin, 2006, Torsvik et al 2008) show the Cordilleran margin of NA experienced little change in latitude throughout most of the Cretaceous, and southward motion during latest Cretaceous to Eocene time. Reanalysis of NA data, with updated geochronology, finds very little (less than 10°) change in pole position for rocks spanning 130 to 85 Ma. Lack of APWP indicates that motion of NA was 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). This provides a very robust reference for Cretaceous rocks from Cordilleran terranes.

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. 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). New results (Callebert et al, this meeting) from ~90 Ma sedimentary rocks, corrected for inclination error, place the Blue Mtns ~800 km south of their present location- closely matching positions indicated by new U-Pb and Hf isotopic studies of detrital zircons from these same strata (Surpless et al, this meeting). Proposed linkages between the Blue Mtns and the Intermontane terranes may argue against some of the en-bloc tectonic models. 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 to North America- highlighting the need to better understand the role of margin-parallel displacement in this orogenic belt.