Ridge Basin, Southern California: Why John Crowell Got It Right in the First Place

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

John Crowell's synthesis of Ridge basin is one of the finest examples of the integration of field mapping, structure, sedimentology, stratigraphy and tectonics. His model of conveyor-belt sedimentation, induced by transpressional downwarp and rebound as basement moved through a restraining bend has withstood all tests, and provides insight for interpretation of other transpressional basins. The durability of this model indicates the soundness of John's masterful synthesis of all data sets. The only challenge to John's transpressional model is the transtensional model of May et al. (1993), based primarily on a seismic line through part of Ridge basin. The following evidence negates this transtensional model: 1. Seismic line does not cross surface trace of San Gabriel fault (SGf), so there is no evidence that imaged feature is the SGf. 2. Seismic line is oblique to surface trace of SGf, thus giving apparent dip, rather than true dip of imaged “SGf”. 3. SGf is close to vertical, with straight linear trace for over 100km, and demonstrated slip of approximately 80km. 4. When Quaternary units are unrotated in Saugus area, steeply east-dipping SGf returns to vertical (Yeats and Stitt, 2003). 5. All faults with documented significant offset in Ridge basin are transpressional (either dominantly strike or reverse slip). 6. There are no documented normal-fault-related growth strata (e.g., rollover). 7. There is no associated magmatism, such as basaltic volcanism typical of transtensional basins. 8. Entire 13km section has same low thermal maturity (e.g., uniform vintrinite reflectance), indicating lack of crustal stretching.