A comparison of the deep structure along nine transects of the west margin of North America shows many important similarities and differences. Common tectonic elements identified along these transects include (1) actively subducting oceanic crust, (2) accreted oceanic/arc (or oceanic-like) lithosphere of Mesozoic through Cenozoic ages, (3) Cenozoic accretionary prisms, (4) Mesozoic accretionary prisms, (5) backstops to the Mesozoic prisms, and (6) undivided lower crust. Not all of these six elements are present along all transects. In this study, nine transects are plotted at the same scale and vertical exaggeration (V.E. 1:1), using the above scheme for identifying tectonic elements. Four of the transects cross subduction-zones, and five cross a transform-fault (San Andreas fault).
The four subduction-zone transects contain actively subducting oceanic crust, Cenozoic accretionary prisms, and bodies of basaltic rocks accreted in the early Cenozoic, including remnants of a large, oceanic plateau or hot-spot plume head in the Oregon and Vancouver Island transects. A body of Eocene basalt (Yakutat terrane) is currently subducting in southern Alaska, where it is riding atop Pacific oceanic crust, creating a giant asperity, or impediment to subduction. Most of the subduction-zone transects also contain Mesozoic accretionary prisms. Two of them, across Vancouver Island and southern Alaska, also contain thick, tectonically underplated bodies of late Mesozoic/early Cenozoic oceanic lithosphere, interpreted as fragments of the extinct Kula or Resurrection plates.
Most of the five transform-fault transects (all located in California) show, in their upper crusts, (1) tectonic wedging of a Mesozoic accretionary prism into a backstop, which includes Mesozoic/early Cenozoic forearc rocks and Mesozoic ophiolitic/arc basement rocks, and (2) shuffling of the former subduction margin of California by strike-slip faulting. In their lower crusts, these transects appear to show evidence of migration of the Mendocino triple junction northward (seen in rocks east of the San Andreas fault) and migration of the Rivera triple junction southward (seen in rocks west of the San Andreas fault). In northern California, lower-crustal rocks east of the San Andreas fault have measured velocities and thicknesses characteristic of oceanic crust and contain patches of high reflectivity. These rocks may represent basaltic rocks magmatically underplated in the wake of the migration of the Mendocino triple junction, or they may represent stalled, subducted fragments of the Farallon/Gorda plate. However, the latter alternative does not fit the accepted “slabless window” model for the migration of the triple junction. This lower-crustal layer and the Moho are offset or deformed at the San Andreas and Maacama faults. In central California, a similar lower-crustal layer is observed west of the San Andreas fault. West of the continental slope, this layer consists of Pacific oceanic crust, but beneath the continent it may represent either Pacific oceanic crust, stalled, subducted fragments (microplates) of the Farallon plate (left behind as the Rivera triple junction jumped southward), or basaltic rocks magmatically underplated during subduction of the Pacific/Farallon ridge or during breakup of the subducted Farallon plate.
The transect in southern California is only partly representative of regional structure, as the structure here is three-dimensional. In the upper crust of the San Gabriel Mountains, one observes a Mesozoic prism in thrust contact with an overlying carapace of crystalline basement rocks. In the middle crust, at the base of the Mesozoic prism, a bright reflective zone is interpreted as a possible “master” decollement that can be traced from an active fold-and-thrust belt in the northern Los Angeles basin northward to the San Andreas fault. Crustal thickening beneath the San Gabriel Mountains is consistent with downwelling of lithospheric mantle beneath the Transverse Ranges that appears to be driving the compression across the Transverse Ranges and Los Angeles basin.
In summary, transects of the west margin of North America record a complex geologic and tectonic history, but a few recurring themes can be traced among generally neighboring transects.