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.