A CASE FOR PUNCTUATED VERTICAL DEFORMATION ALONG CONTINENTAL STRIKE-SLIP FAULTS, BASED ON OBSERVATIONS FROM THE SOUTHERN SAN ANDREAS FAULT (Invited Presentation)

Continental transform faults typically contain numerous geometric complexities and deviations from plate-motion parallelism, leading to routine occurrence of vertical deformation. This is illustrated by the proximal topography of strike-slip faults, which are characterized by near ubiquitous distributions of mountains and basins. While topographic expressions give the impression of pervasive, continuous transpression or transtension, the general degree to which vertical deformation is spatially isolated or temporally punctuated is unclear. Based on typical slip rates and frequency of fault bends, as well as the observed history of strand reorganization in case studies, the timescale at which convergence or divergence may be encountered alongside of a strike-slip fault is likely on the order of 1 Ma. In contrast, the longevity of uplifted topography or basins may be considerably longer. Nascent uplifts may exhibit transient topography for several million years and linger even longer, depending on local climate and lithology, making topography a sluggish marker of vertical deformation. Basins may continue to experience deposition and only reveal a detailed history once inverted. Other techniques geared towards quantifying long-term vertical deformation, including average exhumation history based on thermochronometry or the occurrence of undated brittle structures, are similarly imprecise at documenting punctuated deformation. This inspires a hypothesis that vertical deformation along strike-slip faults is highly localized and punctuated, and that much of the evidence for pervasive, continuous deformation is relict. This hypothesis is supported by histories of deformation along the southern San Andreas fault system, including portions of the San Gabriel, San Bernardino, and San Jacinto Mountains, all of which bear evidence for punctuated deformation. It is also corroborated by prior work on basin inversion and cross-cutting brittle structures. Implications of punctuated deformation are that fault-zone complexity may be more important for controlling vertical deformation than far-field obliquity to plate motion and that snap-shots of topography or structure may not reflect active vertical deformation.