Plate boundary deformation between the Pacific and North America in the Explorer region

One of the consequences of plate tectonics is that a spreading ridge will eventually approach a subduction zone. The problem whether the possible break-up of the approaching ridge will lead to the development of independent micro-plates, or not, is still unresolved. Some 4 million years ago the interaction between the Juan de Fuca Ridge and the Cascadia subduction zone resulted in ridge fragmentation in the Explorer region. There are two proposed post-Miocene kinematic models: one that proposes the presence of a micro-plate and the other that treats the region as a transform deformation zone, or so-called pseudo-plate. We use earthquake strain rates derived from 74 events since 1948 to estimate a long-term velocity field for the region. By comparing this result with the predicted velocity fields for both models we try to discriminate between the two. The earthquake strain rates indicate the presence of a transform deformation zone between the North American (NAM) and Pacific (PAC) plates. The velocity field derived from the inversion of the earthquake strain rates indicates that seismic activity takes up 50š 30% .1¦/ of the PAC–NAM relative motion (NUVEL-1A; DeMets et al., 1994. Effect of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions. Geophys. Res. Lett. 21, 2191–2194.) within the seismogenic layer of this zone and is indistinguishable in direction from the NUVEL-1A PAC–NAM model. The presence of this Explorer transform zone is consistent with the strain rate and velocity field for the ‘pseudo-plate model’ and indicates that seismicity defines a (new) plate boundary zone between the Pacific and North American plates. Earthquake-derived strain rates are low along the Nootka Transform, which accommodates relative motion between the Juan de Fuca and North American plates. The cause for the absence of significant seismic slip along this transform is unclear and may be closely linked to the fact that the adjacent Cascadia subduction zone is locked. The ‘micro-plate model’, which we reject, predicts SW–NE convergence in the eastern Explorer region and this is inconsistent with the earthquake strain rates as well as with surface fault observations.  1998 Elsevier Science B.V. All rights reserved.