Abstract

We use geodetic measurements from very long baseline interferometry to estimate the motion of the Sierra Nevadan microplate, which is composed of the Sierra Nevada and the Great Valley. The motion of the Sierra Nevadan microplate relative to the North American plate is described by a right-handed rotation of 0.61°/m.y. about lat 32°N, long 128°W. This Euler pole lies only 10° southwest of the Sierra Nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis. It further predicts a velocity of the eastern edge of the Sierra Nevada (at 38.0°N, 119.3°W) relative to stable North America of 11 ±1 mm/yr toward N36° ±3°W (quoted uncertainties are plus or minus one standard error), which accounts for about one-fourth of the velocity between the Pacific and North American plates and is ∼25° clockwise of many prior estimates. The velocity nearly parallels the boundary between the Sierra Nevada and the Great Basin, which implies that current motion within the Great Basin results in a rotational, noncoaxial deformation. We use this velocity to estimate how motion is distributed across the broad deforming zone taking up Pacific-North America plate motion. We find that the vector sum of strike slip along the San Andreas fault and motion of the Sierra Nevada relative to stable North America (taken up by deformation within the Great Basin) differs little from the Pacific-North America plate velocity. The difference can be described at 36°N along the San Andreas fault by a vector of 6 mm/yr directed toward N20°W. This vector resolves into components of 5 mm/yr parallel to the fault and 2 mm/yr perpendicular to the fault with 95% confidence intervals of 0 to 10 mm/yr and -1 to +5 mm/yr, respectively. The component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault-perpendicular shortening. We conclude that motion previously inferred to be taken up by deformation other than strike slip along the San Andreas fault or deformation within the Great Basin is much smaller than previously thought.

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