Recent movements of the Juan de Fuca Plate System

Abstract

Analysis of the magnetic anomalies of the Juan de Fuca plate system allows instantaneous poles of rotation relative to the Pacific plate to be calculated from 7 Ma to the present. By combining these with global solutions for Pacific/America and “absolute” (relative to hot spot) motions, a plate motion sequence can be constructed. This sequence shows that both absolute motions and motions relative to America are characterized by slower velocities where younger and more buoyant material enters the convergence zone: “pivoting subduction.” The resistance provided by the youngest portion of the Juan de Fuca plate apparently resulted in its detachment at 4 Ma as the independent Explorer plate. In relation to the hot spot framework, this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased. After 4 Ma the remainder of the Juan de Fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south. Differences in seismicity and recent uplift between northern and southern Vancouver Island may reflect a distinction in tectonic style between the “normal” subduction of the Juan de Fuca plate to the south and a complex “underplating” occurring as the Explorer plate is overridden by the continent. The history of the Explorer plate may exemplify the conditions under which the self‐driving forces of small subducting plates are overcome by the influence of larger, adjacent plates. The recent rapid migration of the absolute pole of rotation of the Juan de Fuca plate toward the plate suggests that it, too, may be nearing this condition.

Assessment of submarine slope failures off Vancouver Island, British Columbia

“…The Juan de Fuca plate is subducting beneath the North American plate at a rate of ~45 mm/yr (e.g. Riddihough, 1984). Offshore of Vancouver Island the oceanic plate is relatively young (2 to 6 Ma), and therefore warm and buoyant (Davis et al, 1990).…”

Location of the southern edge of the Gorda slab and evidence for an adjacent asthenospheric window: Results from seismic profiling and gravity

“…Features in these density models that are held constant are the density structure of the Franciscan rocks above 20 km depth and the apparent dip of the Gorda plate. In all of these 2.5-D models, the thickness of Gorda lithosphere is based on a trench age of 5.3 Ma and a half spreading rate of 38 mm/yr for the last 5 Myr [Riddihough, 1984] …”

Seismotectonics of the San Andreas Fault System between Point Arena and Cape Mendocino in northern California: Implications for the development and evolution of a young transform

“…The southeast trend of the inferred slab edge between Cape Mendocino and the Bartlett Springs fault area parallels the current direction of plate motion for the Pacificsouthern Juan de Fuca (Gorda) plates. The eastern and deeper trend parallels the plate motion direction at --•3.5 Ma [Riddihough, 1984]. Jachens and Griscom [1983] show that the top of the Gorda slab is at about 7-8 km depth beneath Cape Mendocino and deepens to a depth of 20 km, about 120 km to the southeast near where the Bartlett Springs fault terminates (Figure 1).…”