Resolving Crustal Vertical-Axis Rotation Rates since the Eocene in the Western United States

Rates of crustal rotation about a vertical axis can be described with instantaneous, contemporary geodetic data collected through global positioning systems and/or with time-averaged geologic data through paleomagnetic analysis. Evaluation of how these data sets compare, which data provide the more realistic results, and how the data are incorporated into a tectonic framework has notable implications for earth dynamics at a range of temporal and spatial scales. Better understanding as to the rates of processes associated with crustal rotations can help distinguish between episodic and continuous, and rigid and non rigid deformations. Regions in the western United States where contemporary and geologic vertical-axis rotation data produce similar results include data from the Columbia River Basalts suggesting a steady, continuous rotational component of tectonic deformation since the mid-Miocene. On the other hand, 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. In order to resolve these and other enigmatic rotation rates that took place in the western United States since the middle Eocene (ca. 40 Ma), we compile, in a GIS-based format all paleomagnetic data bearing on vertical axis rotations following the cessation of crustal shortening in the Cordillera. This database includes the following characteristics: 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. The existing data bearing on vertical-axis rotations in the western US is impressive in scope, yet estimates of rotation rates are typically of high uncertainty, as they 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 such approximations from paleomagnetic methods require information on the cessation of rotation to have more confidence in their results.