Phrampus
Benjamin J.
Phrampus
Benjamin J.
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ArticleThe hidden history of the South‐Central Cascadia Subduction Zone Recorded on the Juan de Fuca Plate Offshore Southwest Oregon(American Geophysical Union, 2022-08-18) Tréhu, Anne M. ; Tominaga, Masako ; Lyle, Mitch ; Davenport, Kathy ; Phrampus, Benjamin J. ; Favorito, Jules ; Zhang, Edward ; Lenz, Brandi L. ; Shreedharan, Srisharan ; Yelisetti, SubbaraoNew seismic reflection data collected and processed as part of early career scientist training at sea and in classroom projects fill gaps in seismic coverage of the Cascadia subduction zone and provide new insights into anomalous subduction behavior and mass wasting along the south‐central Cascadia Subduction Zone (CSZ) between 42°20’N and 44°15’N. The data reveal at least six distinct buried horizons of folded and faulted sediments similar to strata recently interpreted to result from in situ deformation induced by the load imposed by a large blocky mass transport deposit known as the 44°N slide. Although our results support prior studies indicating that the south‐central CSZ has experienced large slope instabilities, they indicate that the slides were more frequent but volumetrically smaller than previously thought. Similar strata have not been observed elsewhere beneath the abyssal plain adjacent to the Cascadia subduction zone. The structure of the deformation front along this segment is also indistinct, in contrast to the clear frontal faults outboard of folded trench strata observed immediately to the north and south (and generally throughout the rest of Cascadia). We attribute the anomalous nature of this segment of the margin to past subduction of shallow and rough seafloor, which resulted in greater uplift of the forearc than elsewhere along the margin. A consequence of this postulated history would be the shedding of older, more consolidated blocks onto the Juan de Fuca plate, resulting in the observed distinctive stratigraphy offshore southern Oregon.Key PointsThe number of large blocky slides on the south‐central Cascadia margin has been underestimated, while their volume has been overestimatedBlocky slides of similar scale have not occurred along other segments of the Cascadia subduction zoneThe history of large landslides on the south‐central Cascadia margin is more complex than previously suggested and may be a response to past subduction of high topography
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ArticleWidespread gas hydrate instability on the upper U.S. Beaufort margin(John Wiley & Sons, 2014-12-09) Phrampus, Benjamin J. ; Hornbach, Matthew J. ; Ruppel, Carolyn D. ; Hart, Patrick E.The most climate-sensitive methane hydrate deposits occur on upper continental slopes at depths close to the minimum pressure and maximum temperature for gas hydrate stability. At these water depths, small perturbations in intermediate ocean water temperatures can lead to gas hydrate dissociation. The Arctic Ocean has experienced more dramatic warming than lower latitudes, but observational data have not been used to study the interplay between upper slope gas hydrates and warming ocean waters. Here we use (a) legacy seismic data that constrain upper slope gas hydrate distributions on the U.S. Beaufort Sea margin, (b) Alaskan North Slope borehole data and offshore thermal gradients determined from gas hydrate stability zone thickness to infer regional heat flow, and (c) 1088 direct measurements to characterize multidecadal intermediate ocean warming in the U.S. Beaufort Sea. Combining these data with a three-dimensional thermal model shows that the observed gas hydrate stability zone is too deep by 100 to 250 m. The disparity can be partially attributed to several processes, but the most important is the reequilibration (thinning) of gas hydrates in response to significant (~0.5°C at 2σ certainty) warming of intermediate ocean temperatures over 39 years in a depth range that brackets the upper slope extent of the gas hydrate stability zone. Even in the absence of additional ocean warming, 0.44 to 2.2 Gt of methane could be released from reequilibrating gas hydrates into the sediments underlying an area of ~5–7.5 × 103 km2 on the U.S. Beaufort Sea upper slope during the next century.