Geological and geophysical characteristics of the Tuzo Wilson Seamounts: implications for plate geometry in the vicinity of the Pacific – North America – Explorer triple junction

SeaMARC II imagery, SEABEAM bathymetry, seismic reflection profiles, and gravity and magnetic data are used to establish the tectonic significance of the Tuzo Wilson Seamounts, two submarine volcanic edifices located southwest of the southern end of the Queen Charlotte transform fault. SeaMARC II imagery reveals a parallel transform fault, an extension of the Revere–Dellwood Fault, bordering the southwest end of the Dell wood Knolls and terminating at the southwest end of the Tuzo Wilson Seamounts. This transform-fault system links spreading at the north end of Explorer Ridge to extension at the Tuzo Wilson Seamounts. An inactive continuation of this transform 50 km to the northwest of Tuzo Wilson Seamounts is inferred from seismic profiles. Between Dellwood Knolls and Tuzo Wilson Seamounts, this transform fault has offset Pleistocene (ca. 10 000 a) sea-bed features in a right-lateral sense by 250 m and has offset part of the Dellwood Knolls volcanic edifice by 6–8 km. Numerous normal faults at the Tuzo Wilson Seamounts and Dellwood Knolls are roughly orthogonal to the Queen Charlotte and Revere–Dellwood transforms and indicate rifting in an extensional jog between the transforms. Seismic profiles reveal sediments and basement back-tilted northwest and southeast away from the Tuzo Wilson Seamounts, also consistent with extension. Acoustic imagery indicates that the Tuzo Wilson Seamounts are surrounded by basalt flows that are largely free of sediment cover and thus postdate recent rapid sedimentation (< 10 000 a). In contrast, few of the flows around Dellwood Knolls are free of sediment. Basalts from the Tuzo Wilson Seamounts have high magnetizations (average 35 A/m) and are free of manganese encrustation. Tuzo Wilson Seamounts have a + 1000 nT magnetic anomaly, which can be modelled with normal, high-intensity (up to 40 A/m) magnetization and with geometry and depth matching the topography of the seamounts and surficial basalt flows. Their small, positive free-air gravity is largely accounted for by their topography; no appreciable local density contrast exists below the surrounding sea floor.The Tuzo Wilson Seamounts and Dellwood Knolls are separate sites of sea-floor spreading, although the partition of spreading between them is indeterminate. The 50 km inactive continuation of the Revere–Dellwood transform requires that a total of at least 100 km of sea floor has been created at the Tuzo Wilson and Dellwood spreading centres, probably within the last 2.5 Ma. The sea floor between the Tuzo Wilson Seamounts and Dellwood Knolls either is a separate microplate or is under going distributed strain. The triple junction of the Pacific, North America, and Explorer plates is not a discrete point; rather it occupies the strained and seismically active region between the northern Explorer Ridge and the Tuzo Wilson Seamounts.