TILT AND NORTHWARD OFFSET OF CORDILLERAN BATHOLITHS RESOLVED USING IGNEOUS BAROMETRY

CONSIDERABLE controversy surrounds the suggestion, based on palaeomagnetic evidence, that large segments of the North American Cordillera travelled long distances parallel to the coast during the latest Cretaceous and early Tertiary periods, well after the amalgamation of exotic terranes. Discordant palaeomagnetic data from mid-Cretaceous plutonic rocks of the Peninsular Ranges batholith of coastal southern California and Baja and the Mount Stuart batholith of the Cascade Range in Washington play a pivotal role in the controversy. The discordant data were originally interpreted to reflect northward transport of greater-than-or-equal-to 1,000 km relative to cratonal North America, after the batholiths cooled through their magnetic blocking temperatures1-5. More recently it has been argued that the discordances arise from local tilting of batholiths, rather than northward offset6,7. Here we present and implement new methods based on hornblende barometry8 for determining the palaeohorizontal in granitic batholiths and correcting palaeomagnetic data for tilting. Our results indicate that the Peninsular Ranges and Mount Stuart batholiths have undergone northward offsets of approximately 1,000 +/-450 and approximately 2,900 +/- 700 km, respectively, and also significant tilting.