Abstract
The Juan de Fuca lithospheric plate lies between a series of spreading segments offset by dextral transform faults on the west, and a zone of subduction along the continental slope between Cape Mendocino and the Scott Islands on the east. At its northern end spreading commenced in the Dellwood Seamount area less than 2 m.y. ago. Normal faults which cut the sediment and volcanic basement in the valley between Dellwood Knolls, and high heat flow here and in adjacent Revere-Dellwood fracture zone, suggest the valley is a median valley. However, basalt north of the valley is younger and less chemically differentiated than that on the south, suggesting that the northern Dellwood Knolls may be a spreading center. The Dellwood spreading segment and the Revere-Dellwood fault conn ct the Explorer Ridge to the Queen Charlotte fault. The Queen Charlotte fault zone has an east-west width of 100 km from the Queen Charlotte Islands to the Queen Charlotte trough. Its southern end is near Dellwood Knolls. The Explorer Ridge is less than 6 m.y. old and contains several discontinuous median valleys characterized by high heat flow and fresh basalt. Turbidites and coal-bearing strata 300 m thick dip northeast from Paul Revere Ridge into Winona basin, which contains a deformed sedimentary sequence at least 6 km thick. The fault separating Explorer and Dellwood Ridges widens on the north where it constitutes Revere-Dellwood fracture zone. Explorer Ridge is connected to Juan de Fuca ridge by the Sovanco fault, identified by its topographic and magnetic character, but lacking a lear seismologic expression. The continental slope west of Queen Charlotte Islands is steep in its upper and lower reaches, but complex with dammed sediment sequences in the middle. On the south, the continental slope has faulted and crumpled strata suggesting slow contemporaneous or recently ceased subduction. Magnetic anomalies indicate that Juan de Fuca, Explorer, and Dellwood Ridges formed by apparent left-lateral transcurrent offsets of an older meridional ridge. End_of_Article - Last_Page 430------------
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