Explorer Ridge Research Articles

Acoustic stratigraphy, structure, and history of quaternary deposition in Cascadia Basin

Seismic reflection profiles over northern Cascadia Basin, Juan de Fuca Ridge, and Juan de Fuca Abyssal Plain exhibit a thick sedimentary sequence, consisting largerly of turbidites, covering basement. Although most of the section is less than 0·7 × 106 years old, two periods of tectonism and four stratigraphic units can be identified. The two lower units once extended from the continental margin across the basin, covering the north end of Juan de Fuca Ridge and the area west to Explorer Ridge as an abyssal plain. The principal dispersal route at this time, Juan de Fuca Channel, trended roughly north-south on the western side of Cascadia Basin. Block faulting on northern Juan de Fuca Ridge and later, broad upwarping of sediments in the western half of the basin altered this dispersal system, isolating Juan de Fuca Abyssal Plain from subsequent turbidite deposition and shifting the primary thalweg of Cascadia Basin to its present position along Vancouver Valley. Nitinat Fan began to develop during the most recent sedimentary regime, perhaps in response to increased turbidity current activity on the continental margin. The present basin morphology can be attributed largely to topographic control of turbidite dispersal and deposition. The topography, in turn, has been modified by tectonic movements on or near northern Juan de Fuca Ridge.

Dellwood Seamount Area, Possible New Spreading Center, and Other Tectonic Features of Pacific Ocean West of British Columbia: 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------------

Late Quaternary tectonics, northern end of Juan de Fuca Ridge (northeast Pacific)

Seismic reflection profiles from the northern end of Juan de Fuca Ridge reveal three axial valleys having a basement relief of as much as 2 sec (two-way travel time). A thick sequence, presumably of turbidites, mainly less than 0.7 m.y. old, covers much of the area. The oldest turbidites form the upper part of the fill of a possible Tertiary trench between the ridge and North America. The second turbidite unit extends beyond the trench and once formed an abyssal plain over most of northern Juan de Fuca Ridge and the area west to Explorer Ridge. Following formation of the plain, vertical movements began that broadly uplifted the crest of Juan de Fuca Ridge, block-faulted its northern end, produced faulting along Sovanco Fracture Zone, and upwarped the basement north of the ridge. Younger turbidites have filled the lowlands created by the vertical movements. The present sea floor topography and seismic activity show evidence of continued movements. Our findings indicate that the basement relief of the ridge crest and fracture zone were formed after the time of creation of the magnetic anomalies associated with sea floor spreading. Although abruptly initiated, these vertical movements have continued in a moderated form to the present. The vertical movements are thought to be associated with horizontal plate motions in this complex area and with the episodic release of strain as the northwest-salient block of the Juan de Fuca Plate is possibly being severed from the remainder of the plate.

Preliminary Analysis of Geophysical Measurements North of Juan de Fuca Ridge

Preliminary analyses of gravity, magnetic, seismic reflection and refraction, dredging, and heat flow measurements off Queen Charlotte Islands and Vancouver Island are presented. Seismic reflection and gravity measurements show the presence of a sedimentary basin at the foot of the continental slope in which the sediments are progressively more intensely deformed from north to south, indicating the interactions between the lithospheric plates. Heat flow values in the northern part of Explorer Ridge, and recovery of fresh basalts with little mineral coating in this region suggest that Explorer Ridge is a presently active spreading segment of East Pacific Rise. Seismic Refraction results in the deep ocean basin west of Queen Charlotte Islands show a marked anisotropic mantle P wave velocity, the direction of maximum velocity being 107° east of north and the maximum change in velocity being about 0.6 km/s.

Deformation of the Sea Floor off the North-west Coast of the United States

A new fracture zone separating the Tufts Abyssal Plain from the mountainous topography of the Explorer and Juan de Fuca ridges has been discovered on the basis of the magnetic anomaly offsets. North-east–south-west compression is the likely cause for the rotation, strike-slip faulting and deformation of the small crustal blocks of the area.