The North American--Pacific plate boundary is probably the best studied transform margin in the world. My review of recent studies of this margin is ordered geographically from north to south to better illustrate the broad similarities in crustal structure found along it. The similarities in crustal structure suggest that the underlying tectonic processes are identical along the margin. These processes include the under thrusting of oceanic lithosphere prior to the onset of the transform fault motion, imbrication of subducting oceanic lithosphere, magmatic underplating or intrusion of the lower crust during passage of the intersection of three different plates (at triple junctions), simultaneous transform and extensional motion (known as transtension) during the mid-Miocene forming extensional grabens along much if not all the Pacific margin of North America and since the Pliocene, ongoing simultaneous transform and compressional motion (often called transpression).
A reconnaissance grid of 6-second seismic reflection lines on the margin of British Columbia in the vicinity of the Queen Charlotte fault imaged a complex network of sub basins that formed adjacent to the transform Pacific/North American plate boundary in response to Neogene transtension [ Rohr and Dietrich, 1992]. Results from inversion of off-line wide-angle recordings of these lines reveal the three-dimensional geometry of the sedimentary basins in Hecate Strait [ Hole et al., 1992]. As found elsewhere along this margin, a broad zone of transtensional deformation tectonics predominated in the Miocene and transpression predominated in the Pliocene [ Rohr and Dietrich, 1992]. Yuan et al. [1992] and Spence and Asudeh [1993] reported refraction evidence for a prominent velocity contrast at a depth of 15 to 20 km beneath Hecate Strait in the Queen Charlotte Basin, with velocities as high as 6.9 km/s beneath the interface. A similar midcrustal reflector and velocity discontinuity were found beneath San Francisco Bay as described below. Two hypotheses for this higher velocity material include (1) slabs of oceanic crust emplaced during an earlier episode of subduction and (2) mafic intrusions associated with Tertiary volcanism. Wide-angle seismic data provide evidence for a thin, 18-km thick crust beneath Queen Charlotte Sound, supporting the interpretation of crustal extension inferred from the seismic reflection data [ Yuan et al., 1992]. Spence and Asudeh [1993] provide wide-angle evidence for thinning of the continental crust toward the Queen Charlotte fault, which they relate to the strike-slip faulting on that structure.
The San Francisco Bay Area Seismic Imaging eXperiment (BASIX) in
northern California was designed to image the crustal structure and search
for possible gently-dipping (sub-horizontal) detachment surfaces in the
broad North American--Pacific plate boundary. The experiment capitalized
on existing waterways and utilized marine air gun profiling and onshore
recording to help evaluate seismic hazards in an urban environment. A
major finding from BASIX is the detection of a regional, sub-horizontal,
midcrustal reflector beneath San Francisco Bay at a depth of 15 km that
lies below the seismogenic upper crust [ McCarthy et al., 1993;
Brocher et al., 1994]. Velocities increase across the reflection from
6.0 km/s to 
7.0 km/s. BASIX also found evidence for a slab
of mafic crust beneath the continental slope and shelf at the latitude of
San Francisco, similar to that reported by Page and Brocher [1993]
and Brocher et al. [in press] in the vicinity of the Loma Prieta
earthquake near Santa Cruz. Models for the presence of the high velocity
lower crustal layer beneath San Francisco Bay include (1) a slab of
modified oceanic crust which was under thrust during the subduction regime
or (2) a magmatically underplated or intruded layer resulting from the
northward passage of the Mendocino triple junction.
Seismic reflection and wide-angle studies off central California in the vicinity of Morro Bay provided evidence for a slab of imbricated oceanic crust beneath the shelf [ Clark et al., 1991; Ewing and Talwani, 1991; McIntosh et al., 1991; Meltzer and Levander, 1991; Tréhu, 1991; Miller et al., 1992; Henrys et al., 1993; Howie et al., 1993; Levander and Lafond, 1993]. The common image point processing of wide-angle data by Levander and Lafond [1993], involving the processing of air gun signals recorded by a dense onshore array of receivers, is particularly exciting, because it produced a reflection image of the imbricated oceanic slab beneath the continental shelf of central California, where vertical incidence seismic reflection profiling yielded poor results. The methodology relies upon redundancy of raypaths similar to standard multichannel reflection data processing methods (produced by the high density of air gun shots and large numbers of onshore receivers) and the larger amplitude reflections as predicted by ray theory at wide-angles as the critical angle of incidence is approached or exceeded. This work demonstrates the value of dense land recording of marine air gun shots in resolving the fine structure of the lower crust.
Farther south, in the California Borderlands, Nicholson et al. [1992] identified major normal faults of Miocene age on seismic reflection profiles, some of which were reactivated in the Pliocene with reverse (or oblique-reverse) motion, folding the Miocene basin sediments. In the lower crust, they found a major near-vertical discontinuity that corresponds to the projection of the Morro Fracture Zone under the continental margin. This structure defines the southern limit of the partially subducted Monterey micro plate, and suggests that subducted oceanic crust extends underneath the offshore Santa Maria Basin as far south as near Pt. Arguello and as far east as the coast. Henry et al. [1993], however, imaged the southern edge of the Monterey micro plate as having a significant compressional component likely due to late stage oblique compression (tranpression) between microplates.