New kinematic models for Pacific-North America motion from 3 Ma to present, I: Evidence for steady motion and biases in the NUVEL-1A model

We use velocities derived from 2-4.5 years of continuous GPS observations a t 21 sites on the Pacific and North American plates along with a subset of the NUVEL-1A data to examine the steadiness of Pacific-North America motion since 3.16 Ma, the transfer of Baja California to the Pacific plate, and t he magnitude of biases in the NUVEL-1A estimate of Pacific-North America mo tion. We find that Pacific-North America motion has remained steady since 3 .16 Ma, but at rates significantly faster than predicted by NUVEL-1A. In th e vicinity of Baja California, our GPS-derived model and recent seafloor sp reading rates in the southern Gulf of California both indicate that the NUV EL-1A model underestimates Pacific-North America rates by 4+/-2 mm yr(-1). Steady Pacific-North America motion since 3.16 Myr and increasing seafloor spreading rates since 3.58 Myr in the Gulf of California imply that Pacific -North America motion was partitioned between seafloor spreading in the Gul f of California and decelerating slip along faults in or offshore from the Baja peninsula.