90-9 Top down vs bottoms up: We’ve looked at Alaska from both sides now

Abstract:

In a GSA Today article in 2019 a group of us called on a need to integrate investigations based on mantle tomography (bottom up) and surface geology (top down) into a coherent framework to solve fundamental issues in tectonics.  In that spirit we have integrated the geologic record of the northern Cordillera together with Earthscope imaging to evaluate the history of the margin from early Eocene to the present day with a focus on the role of the Yakutat terrane in the evolution of the margin.  This analysis also allows for a reassessment of the plate tectonic record of the margin by constraining allowable plate geometries through use of pinning points like ridge-trench interactions, strike-slip fault system timing, and reanalysis of the Pacific plate magnetic anomalies.  Our initial results include: 1) Mantle imaging indicates the flat slab region overlies a mantle scale lithospheric fold.   Kinematic modeling of the imaged geometry indicates the flat slat originated prior to at least 25 Ma to as early as 40 Ma.   We correlate the western transition from the flat slab to normal subduction with the likely location of a slab window during Kula times.  We suggest one factor in the origin of the flat slab may be anchoring of eastern side of the slab window in the transition zone interacting with the large plate motion change as Kula merged with the Pacific plate.  2) Based on our plate reconstructions we conclude that the Transition fault originated as a sinistral Kula-Farallon transform that developed just prior to the demise of the Kula plate and evolved into a Pacific-Farallon transform that formed a mirror-image Mendocino type triple junction that migrated northward along the Queen Charlotte transform from at least 41Ma to ~25Ma. 3) Although an additional north Pacific plate is allowable, our analysis indicates a Kula-Farallon-Pacific system can explain all major Cenozoic tectonic events along the margin.

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