Geology of the northern end of Juan de Fuca Ridge and adjacent continental slope

Bathymetry, seismic reflection profiles, magnetic data, and dredged basalt indicate that two centres of spreading have existed near the northern end of Juan de Fuca Ridge in the last 0.7 m.y. (Brunhes Geomagnetic Polarity Epoch). A short-lived eastern branch functioned only in the early part of the Brunhes Epoch. More prolonged and recent activity has occurred on the western branch, and has involved extrusion of basalt, rifting, and uplift. The anomalous relief of the northern end of Juan de Fuca Ridge compared to the ridge further south is the result of a tectonic event which occurred 100,000350,000 years ago. Prior to this, turbidites were deposited over most of the northern part of the ridge, which consisted of a series of basement ridges and valleys of subdued relief trending NNE parallel to the magnetic anomalies. During the event, the igneous basement together with the overlying turbidites was faulted, tilted, and uplifted along the primary structural trends. The basement relief was increased in places to more than 2 km. Subsequent turbidite deposition has been confined to valleys. The Sovanco Fracture Zone between Juan de Fuca and Explorer Ridges also suffered uplift. The tectonic disturbance may have resulted from an increase in spreading rate or the change in location of the axis of spreading on Juan de Fuca Ridge, or compressional interaction between Explorer sub-plate to the north and Sovanco Fracture Zone and the northern end of Juan de Fuca Ridge. Sovanco Fracture Zone, consisting of a WNW-trending basement ridge (Sovanco Ridge) with complex associated magnetic anomalies, may have originated as a strike-slip fault or fault-zone trending east-west about 5 m.y. ago. Location of present transform movement within the fracture zone is equivocal, but apparently trends NW. A specimen of basalt from the eastern end of Sovanco Ridge yielded a fission track age of <309,000 years. Basalts dredged from 13 locations on Juan de Fuca Ridge, Sovanco Ridge, and Heck and Heckle Seamount Chains on the western flank of Juan de Fuca Ridge are low-potassium tholeiites of the ocean ridge type. Twenty-seven representative specimens of fresh basalt, as well as 15 samples of glass selvages and altered basalt, were analyzed for 9 major elements by an atomic absorption technique set up by the author. The analyses are closely similar, showing no well-developed differentiation trend. They straddle the boundary between the olivine-normative and quartz-normative fields, and the basalts may have been derived by segregation from the mantle at a depth of about 15 km. A specimen from NE of the eastern spreading centre yielded a fission track age of 0.95 ± 0.53 m.y. which is in reasonable agreement with the age of 0.7-0.9 m.y. predicted by the magnetic anomaly time scale. Increasing thicknesses of Fe-Mn coatings on dredged basalts indicate that the two NW-trending seamount chains increase in age away from the crest of Juan de Fuca Ridge. The seamounts originated and grew to virtually their present sizes while s t i l l at or near the ridge crest. Reflection profiles across the continental slope indicate that i t has formed by folding, reverse faulting, and uplift of strata of Cascadia Basin. Internal deformation has caused these sediments to behave as acoustical basement in the structures beneath the slope but their sedimentary nature has been confirmed by dredging. The presence of magnetic anomalies 5-8 m.y. old beneath the lower and middle continental slope indicates subduction. The magnetic basement dips below the slope at an angle of more than 1 0°.