California’s Changing Paleogeography and Paleotectonics, Before and After John Crowell’s Definitive Work on Strike-Slip Systems

Ingersoll, Raymond V.¹ (1) University of California, Los Angeles, CA

When John Crowell first visited Ridge basin in 1939, it was the start of almost seventy years of field observations and insights about Ridge basin and strike-slip systems. The magnitude of strike-slip offset along the San Andreas system was unrecognized, and the significance of transform faults to plate tectonics wasn't imagined. The Garlock and San Andreas faults were known as “rifts.” Small strike-slip offsets were assumed, unless proven incontrovertibly otherwise. Within this framework, John began careful mapping and observations of Ridge basin, and expanded to the entire southern San Andreas fault system. Crowell (1952) demonstrated at least 25-40 km of right slip on the San Gabriel fault. Hill and Dibblee (1953) suggested hundreds of kilometers of lateral offset along several major faults of California. John then documented 260 km of right slip on the combined San Gabriel and San Andreas faults, since increased to 320 km. When Wilson (1965) proposed that transform faults were plate boundaries, along which hundreds of kilometers of displacement could occur, John was one of the first to recognize the significance for California geology. The plate-tectonic revolution led to syntheses by Atwater (1970) of oceanic-plate interactions with North America, Luyendyk (1991) of vertical-axis rotations, and Nicholson et al. (1994) of microplate capture. These breakthroughs led to radical modification of concepts of California paleotectonics and paleogeography. John Crowell's careful and insightful work has contributed greatly to our understanding of these dynamic systems.