ANOMALOUS CRUSTAL DEFORMATION IN NORTHEASTERN BAJA AND MEXICALI VALLEY

Perhaps not surprisingly, early geodetic observations in the southern Salton Trough, including northern Baja and Mexicali Valley, aimed at improving our understanding of the distribution of right-lateral shear associated with ongoing Pacific-North America relative plate motion. This shear, which is the most obvious characteristic of the strain field, can be largely understood in terms of strain accumulation on major Quaternary strike slip fault zones in southern California and northern Baja. However, there are important instances where the observed deformation field appears to systematically disagree with expectations based on simple fault models for plate boundary zone deformation. In northeastern Baja, for example, crustal velocities derived from GPS measurements exhibit anomalous strain in the direction N50E, basically perpendicular to the direction of relative plate motion. Of particular interest is a region of N50E extension which appears to coincide with the location of the northernmost portion of the Gulf Extensional Province. Adjacent to this belt of extension lies a region of N50E compression. The boundary between these belts, though not yet well defined, lies conspicuously close to the trace of the right-oblique Laguna Salada normal fault. These signals may reflect clockwise block rotation in the northernmost Gulf [e.g., Nagy and Stock, 2000] which can result in plate boundary perpendicular motions, but they may also indicate deformations driven by forces other than tractions from the sides associated with relative plate motions.