Immature transform plate boundaries in the Northeast Pacific: Constraints from ocean bottom seismology

Plate tectonics defines oceanic transform faults as long-lived tectonics features. In the Pacific Ocean their traces, called fracture zones, can easily be identified as several thousands of kilometer-long features in bathymetric and gravity field data. However, today none of the fracture zones in the North Pacific are directly linked to any mid-ocean ridge-transform fault. This feature is related to the subduction of the Farallon spreading center and a major change in the direction of plate motion several millions of years ago. Consequently, ridge segmentation is adjusting to a new tectonic framework. The Blanco transform fault system (BTFS) in the northwest off the coast of Oregon is one of the newly evolving transform faults. It is highly segmented and shows strong similarities with other segmented oceanic transform systems, such as the Siqueiros in the East Pacific Rise, which developed from a pre-existing transform fault subjected to a series of extensional events due to a documented change 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 used one year of ocean-bottom-seismometer data from the Blanco Transform OBS Experiment (2012-2013) and high-resolution multibeam bathymetry, aeromagnetic, and gravity datasets to study the seismotectonic behavior and tectonic evolution of the BTFS. Interestingly, all available datasets provide no evidence for the existence of either transform faults or fracture zones around the BTFS before 2 Ma, supporting that there were no pre-existing transform faults before the initiation of the BTFS. Therefore, we suggest the BTFS developed from two broad transfer zones instead of pre-existing transform faults. We present seismicity and focal mechanisms for stronger manually-picked events.  Furthermore, the seismic data were picked with a phase picker learned with a large OBS training dataset. The resulting seismicity of ~8,000 events reveals the present-day deformation of the fault system with very high spatial resolution, and supports substantial along-strike variations, indicating different slip modes in the eastern and western BTFS. Seismic slip vectors suggest that the eastern BTFS is a mature transform fault system accommodating the plate motion. At the same time, the western BTFS is immature as its re-organization is still active. The BTFS acts as a natural laboratory to yield processes governing the development of transform faults away from continental rift zones.