Topographic development of a compressional mountain range, the western Transverse Ranges of California, USA, resulted from localized uplift along individual structures and regional uplift from deeper shortening

Abstract

Abstract The western Transverse Ranges are a tectonically active mountain belt in southern California (USA) characterized by fast rates of shortening and rock uplift. Large drainages at the western end of this mountain belt, including the Santa Ynez River and its tributaries, transect regional west–northwest-striking reverse faults and folds. We used fluvial strath terraces within the Santa Ynez River watershed as geomorphic markers for measuring Quaternary rock uplift and deformation across these structures. Mapping, surveying, and numerical dating of these strath terraces in both hanging-wall and footwall blocks of the major reverse faults allow us to separate regional uplift from localized uplift along individual structures. Luminescence dates from 18 sites within the Santa Ynez River watershed show that the three prominent terrace levels present throughout the area formed between ca. 85 ka and 95 ka, 55 ka and 75 ka, and 30 ka and 45 ka. All three fluvial terrace straths grade into marine paleo-shore platforms along the coast that formed during sea-level highstands. The fluvial straths were formed as a result of lateral erosion during warm, dry climate intervals when vertical incision was temporarily arrested. Incision of the terraces followed during intervening periods of wet climate. Mapping and valley-long profiles of the terraces document deformation by faults and folds, and we infer minimum rock-uplift rates from the amount of incision below the terrace strath surfaces. Rock-uplift rates range from 0.3 mm/yr to 4.9 mm/yr, with faster rates in the hanging-wall blocks of the major reverse faults and slower rates in the footwall blocks. Rock-uplift rates calculated from strath terraces in the footwall blocks range from 0.3 mm/yr to 1.6 mm/yr, which indicates a regional component of uplift that results from deeper deformation. Higher rates of rock uplift in the hanging-wall blocks (0.5–4.9 mm/yr) are superposed on this regional component. Incremental rock-uplift rates calculated over three time intervals and differences in terrace deformation with age suggest that deformation rates across some structures have decreased over the past 85 k.y. We conclude that topographic growth of the western Transverse Ranges results from a combination of localized uplift along individual structures that varies both spatially and temporally and a more constant regional uplift that likely results from deeper ductile deformation or slip along detachment faults that have been inferred to underlie the area.