Late Cretaceous paleomagnetism: Tectonic, paleoclimatic and geomagnetic implications

Analyses which seek to determine the intensity of the Earth's past magnetic field are often hindered by sample alteration during the heating cycles required by established methods. The study of magnetic inclusions within plagioclase crystals, rather than bulk rock samples, is one approach to avoid alteration and to obtain reliable paleointensity data. Tests using plagioclase crystals from an historic lava erupted in a known magnetic field demonstrate the viability of the method. The method has been further applied to a study of a Cretaceous lava flow (113–115 million years ago) and compared to results from whole rock samples. The single crystals yielded data which are less affected by alteration and suggest higher Cretaceous paleointensities than their whole rock counterpart data.; The Cretaceous has also been postulated as a time of true polar wander, which is the rotation of the entire solid Earth with respect to the spin axis. However, true polar wander determinations rely on high quality paleomagnetic data distributed globally; these data are particularly difficult to obtain for oceanic plates. An older technique that uses seamount magnetic anomaly modeling to constrain oceanic plate motion and true polar wander is shown to be inconsistent with global data. A new high latitude pole position for the Late Cretaceous Pacific plate based on paleomagnetic analyses of basalt samples recovered by ocean drilling implies fast latitudinal plate motion ($17.9+7.9-10.0$ cm/yr) between 95 and 81 million years ago. This motion can be accommodated by normal plate tectonic driving forces, perhaps assisted by decreases in drag forces associated with mantle plume volcanism. True polar wander appears unnecessary to explain the available data.; The new Late Cretaceous pole position also allows for a reinvestigation of latitudinal temperature gradients used to constrain past climate. The new data support the notion of a more equable Cretaceous climate, but also suggest that tropical temperatures may have been underestimated in some studies.