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Abstract:
Among the recently evolving transform faults, the Blanco transform fault system (BTFS) in the northwest off the coast of Oregon, is strongly segmented and resembles other segmented oceanic transform systems, such as the Siqueiros in the East Pacific Rise, which developed from a pre-existing transform fault that was subjected to several extensional events due to a known shift in spreading direction. However, plate tectonic reconstructions suggested that the BTFS developed from at least two large ridge offsets rather than a single transform fault, emerging from a series of ridge propagation events after the plate reorientation at ~5 Ma.We investigated the seismotectonic behavior and tectonic evolution of the BTFS using ocean-bottom seismometer data from the Blanco Transform OBS Experiment (2012–2013) as well as high-resolution multibeam bathymetry, aeromagnetic, and gravity datasets. Interestingly, we observed no evidence for the existence of either transform faults or fracture zones around the BTFS before 2 Ma, indicating that there were no pre-existing transform faults before the formation of the BTFS. Therefore, we propose that the BTFS developed from two broad transfer zones rather than pre-existing transform faults. Moreover, local seismicity shows significant along-strike variations, revealing different modes of slip in the eastern and western BTFS. Seismic slip vectors suggest that the western BTFS is an immature transform fault system since its reorganization is still taking place, but the eastern BTFS accommodating the plate motion is mature. The BTFS acts as a natural laboratory to understand the mechanisms controlling the development of oceanic transform plate boundaries.
