Jason D. Chaytor

Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone

We relate the late Holocene northern San Andreas fault (NSAF) paleo- seismic history developed using marine sediment cores along the northern California continental margin to a similar dataset of cores collected along the Cascadia margin, including channels from Barclay Canyon off Vancouver Island to just north of Mon- terey Bay. Stratigraphic correlation and evidence of synchronous triggering imply earthquake origin, and both temporal records are compatible with onshore paleoseis- mic data. In order to make comparisons between the temporal earthquake records from the NSAF and Cascadia, we refine correlations of southern Cascadia great earth- quakes, including the land paleoseismic record. Along the NSAF during the last ∼2800 yr, 15 turbidites, including one likely from the great 1906 earthquake, establish an average repeat time of ∼200 yr, similar to the onshore value of ∼240 yr. The combined land and marine paleoseismic record from the southern Cascadia subduction zone includes a similar number of events during the same period. While the average recurrence interval for full-margin Cascadia events is ∼520 yr, the southern Cascadia margin has a repeat time of ∼220 yr, similar to that of the NSAF. Thirteen of the 15 NSAF events were preceded by Cascadia events by ∼0-80 yr, averaging 25-45 yr (as compared to ∼80-400 yr by which Cascadia events follow the NSAF). Based on the temporal association, we model the coseismic and cumulative post- seismic deformation from great Cascadia megathrust events and compute related stress changes along the NSAF in order to test the possibility that Cascadia earth- quakes triggered the penultimate, and perhaps other, NSAF events. The Coulomb fail- ure stress (CFS) resulting from viscous deformation related to a Cascadia earthquake over ∼60 yr does not contribute significantly to the total CFS on the NSAF. However, the coseismic deformation increases CFS on the northern San Andreas fault (NSAF )b y up to about 9 bars offshore of Point Delgada, most likely enough to trigger that fault to fail in north-to-south propagating ruptures.

Active deformation of the Gorda plate: Constraining deformation models with new geophysical data

The Gorda plate, the southernmost fragment of the larger Juan de Fuca plate system, is an example of a nonrigidly deforming tectonic accommodation zone or buffer plate, absorbing deformation and allowing the surrounding larger plates to act in a more rigid fashion. Here we present a new structural analysis of the plate based on full-plate bathy- metric coverage, augmented by seismic reflection data and earthquake moment tensors and locations. We interpret internal deformation of the Gorda plate as an asymmetrical flexural-slip buckle with a vertical axis, utilizing reactivation of spreading-ridge fabric normal faults as strike-slip faults. Newly formed second-generation faults crosscutting the structural grain overprint the reactivated structures. The spreading fabric faults finally begin a second phase of extension as the plate approaches the subduction zone. This model, based on fault constraints, has allowed investigation of ridge-plate-subduction interac- tions, and suggests that spreading-rate variations along the Gorda Ridge may be con- trolled by internal deformation of the plate rather than the reverse, as previously hypothesized.

Measuring vertical tectonic motion at the intersection of the Santa Cruz-Catalina Ridge and Northern Channel Islands platform, California Continental Borderland, using submerged paleoshorelines

We used submerged paleoshorelines as strain markers to investigate Holocene and late Pleistocene vertical tectonic movement at the intersection of the offshore Santa Cruz–Catalina Ridge with the southern boundary of the Western Transverse Ranges, within the California Continental Borderland. Past submerged shoreline positions were identified using high-resolution multibeam bathymetry, side-scan sonar, submersible observations, and the presence of intertidal and subtidal invertebrate fossils. Numerous accelerator mass spectrometry (AMS) 14 C ages of shells from these paleoshorelines were found to be between ~27,000 yr radiocarbon (RC) and 11,500 yr before present, indicative of shoreline colonization during and following the Last Glacial Maximum (LGM), establishing these paleoshorelines as a usable datum for measuring vertical change since this time. Removal of the nontectonic component of vertical change using an ice-volume-equivalent eustatic sea-level compilation indicates between 20 and 45 m of uplift of the eastern part of the Northern Channel Islands block since the LGM lowstand, resulting in an uplift rate of 1.50 ± 0.59 mm/yr over the last 23 k.y. This rate closely matches uplift predicted by published slip rates for the Channel Islands thrust, which underlies the Northern Channel Islands platform. Results from post-LGM shoreline features on Pilgrim Banks are somewhat more ambiguous. Submarine paleoshoreline uplift, together with the extensive upper-crustal fold-thrust style of deformation, illustrates the transpressional interaction of the Borderland and the Western Transverse Ranges blocks where the Santa Cruz–Catalina Ridge and northern Channel Islands intersect.

Morphology, structure and evolution of California Continental Borderland restraining bends

Abstract Exceptional examples of restraining and releasing bend structures along major strike-slip fault zones are found in the California continental Borderland. Erosion in the deep sea is diminished, thereby preserving the morphology of active oblique fault deformation. Long-lived deposition of turbidites and other marine sediments preserve a high-resolution geological record of fault zone deformation and regional tectonic evolution. Two large restraining bends with varied structural styles are compared to derive a typical morphology of Borderland restraining bends. A 60-km-long, 15° left bend in the dextral San Clemente Fault creates two primary deformation zones. The southeastern uplift involves ‘soft’ turbidite sediments and is expressed as a broad asymmetrical ridge with right-stepping en echelon anticlines and local pull-apart basins at minor releasing stepovers along the fault. The northwest uplift involves more rigid sedimentary and possibly igneous or metamorphic basement rocks creating a steep-sided, narrow and more symmetrical pop-up. The restraining bend terminates in a releasing stepover basin at the NW end, but curves gently into a transtensional releasing bend to the SE. Seismic stratigraphy indicates that the uplift and transpression along this bend occurred within Quaternary times. The 80-km-long, 30–40° left bend in the San Diego Trough–Catalina fault zone creates a large pop-up structure that emerges to form Santa Catalina Island. This ridge of igneous and metamorphic basement rocks has steep flanks and a classic ‘rhomboid’ shape. For both major restraining bends, and most others in the Borderland, the uplift is asymmetrical, with the principal displacement zone lying along one flank of the pop-up. Faults within the pop-up structure are very steep dipping and subvertical for the principal displacement zone. In most cases, a Miocene basin has been structurally inverted by the transpression. Development of major restraining bends offshore of southern California appears to result from reactivation of major transform faults associated with Mid-Miocene oblique rifting during the evolution of the Pacific–North America plate boundary. Seismicity offshore of southern California demonstrates that deformation along these major strike-slip fault systems continues today.

Seamount morphology in the Bowie and Cobb hot spot trails, Gulf of Alaska

Full-coverage multibeam bathymetric mapping of twelve seamounts in the Gulf of Alaska reveals that they are characterized by flat-topped summits (rarely with summit craters) and by terraced, or step-bench, flanks. These summit plateaus contain relict volcanic features (e.g., flow levees, late-stage cones, and collapse craters) and as such must have been constructed by volcanic processes such as lava ponding above a central vent, rather than by erosion above sea level. The terraced flanks are composed of a sequence of stacked lava deltas and cones, probably tube-fed from a central lava pond, a morphology which is suggestive of long-lived, stable central lava sources and low to moderate eruption rates, indicative of significant time spent above a hot spot outlet. Most of these seamounts have summit plateaus surrounded, and cut into, by amphitheater headwall scarps, and flanks that are scarred by debris chutes, but lack visible debris accumulations at their base. We interpret the lack of blocky debris fields as evidence that the slope failures are mainly small-scale debris flows, rather than large-scale flank collapses. However, we cannot rule out the possibility that large flank-collapse blocks from early in the histories of these seamounts are now hidden beneath the thick glacio-fluvial fan deposits that cover the Gulf of Alaska seafloor. These slope failure features become smoother and longer and increase in size and abundance with increasing age of a seamount, suggesting that slope failure processes continue long after volcanic activity ceases.