Carlos Braga, PhD

The active spreading centers that occur between the Juan de Fuca Plate on the western coast of North America and the Pacific Plate host at least tens of seafloor massive sulfide occurrences, including some of the largest known deposits in the ocean (e.g., the 9 Mt Bent Hill deposit at Middle Valley). The sizes and distribution of the deposits, as well as their compositions, are strongly influenced by tectonic and magmatic controls, such as spreading rate and proximity to mantle hot spots. Detailed geological mapping is providing a regional framework for understanding some of these differences.

In this study we present a new 1:1,000,000 scale geologic map of the Juan de Fuca plate. The map covers an area of 890,000 km², extending from the shelf to the offshore abyssal region. It includes the entire Juan de Fuca plate, the Explorer microplate, parts of the Pacific Plate and the Cascadia Margin, the Sovanco and Blanco transforms, Axial Seamount, and a segment of the Mendocino fracture zone. 

Mapping was carried out in ArcGIS Pro using remote predictive mapping techniques adopted by geological surveys on land and other planets. Ship-based multibeam, side-scan sonar, acoustic backscatter, multiparameter geophysical, and groundtruthing datasets constrained the geologic interpretations. The mapped units were then extrapolated to areas with less data by comparison with regional geophysical datasets. Feature classification was guided by an internally consistent legend including structures, lithostratigraphic units, and volcano formations (subdivided into cones, shields, domes, axial volcanoes, ridges and flow fields).

More than 6,500 lineaments, >1,000 polygons of the different geological formations were mapped, as well as >2,700 volcanic features. Further investigation of the mapped formations and structures provide insights into different metal sources along the Gorda, Juan de Fuca, and Explorer ridges, including N-MORB, E-MORB, and sediment-influenced hydrothermal systems, with implications for understanding the variable compositions of volcanogenic massive sulfide deposits on land.