HOW DOES THE PERMANENT RECORD (PALEOMAGNETIC DATA) OF CRUSTAL VERTICAL AXIS ROTATIONS IN THE WESTERN US CORDILLERA COMPARE WITH THE GROWING (BUT SHORT) GEODETIC RECORD?

The acquisition of Global Positioning System (GPS) geodetic data for well over a decade has allowed estimates of contemporary, instantaneous vertical-axis rotation rates of continental lithosphere in deforming zones. How these estimates compare with those provided by paleomagnetic data, which integrate rotation over some interval of geologic time, is of considerable interest because of the ultimate ability to better understand processes behind crustal rotations. We compile, in a GIS-based format, all paleomagnetic data from the western US that provide quantitative information on the magnitude and sense of vertical axis rotation of crustal elements, approximately since the cessation of crustal shortening related to Sevier-style fold/thrust belt and/or Laramide-style basement-involved deformation. Thus, we assess the paleomagnetic data base bearing on vertical axis rotations in the western US since the early Eocene (ca. 50 Ma). Our compiled base includes the following: formation name (if any), rock type, age of rocks, sense/magnitude of rotation estimate, error associated with rotation estimate, timing of rotation, estimate of area affected by rotation, (approximate) rotation rate (if possible), and quality of determination. Although the existing paleomagnetic data bearing on vertical-axis rotations in the western US is impressive in scope, estimating an accurate rotation rate from individual studies is not simple. Typically, these estimates are compromised by a lack of adequate geologic constraints. The lowest rotation rate is easy to estimate if the age of the rocks (characteristic magnetization) is known, but is this estimate from paleomagnetic methods close to realistic? For example, early Oligocene intrusive and volcaniclastic rocks of the Cerillos Hills area, southern Espanola Basin, New Mexico may show only a modest magnitude vertical axis rotation (Harlan and Geissman, 2009, Lithosphere), yet, on the basis of independent paleomagnetic data from younger volcanic rocks nearby, the rotation may have taken place since the late Pliocene, and thus the rate of rotation may be very high. On the other hand, in some cases the paleomagnetic and geodetic observations are quite comparable (e.g., mid-Miocene Columbia River basalts).