COMPARISON OF DEEP-STRUCTURE ALONG 3 TRANSECTS OF THE WESTERN NORTH-AMERICAN CONTINENTAL-MARGIN

Similarities in geology and potential field data that have in the past been noted among the regions of southern Alaska, southern Vancouver I sland, and central California are now seen to be accompanied by simila rities in deep crustal structure. A number of tectonic elements have b een identified in the deep structure along transects in these three re gions, although not all elements are present along each transect. Thes e elements are (A) an actively subducting oceanic plate and (B) an ove rriding continental plate that consists of (1) a Cenozoic accretionary prism, (2) a Mesozoic accretionary prism, (3) a backstop to the Mesoz oic prism, (4) a tectonically underplated body of oceanic rocks, and ( 5) a crustal root. The Mesozoic prism is in some cases an underthrust body (type 2a) but in other cases forms the principal component of a l andward verging tectonic wedge (type 2b). The tectonically underplated body of oceanic rocks extends landward from the fault contact between the Cenozoic and Mesozoic prisms to a point beneath the backstop. The crustal root lies beneath the backstop and landward of the underplate d body. All of these elements are interpreted to be present along the Alaskan and Vancouver Island transects. In Alaska the underplated body is interpreted to be fragments of the Kula plate; the same may be tru e at Vancouver Island. These two transects appear to differ in that, i n Alaska, the Mesozoic prism, in one interpretation, is the principal component of a tectonic wedge (type 2b), whereas at Vancouver Island, it is an underthrust body (type 2a). Along the central California tran sect, active subduction is no longer taking place, and the San Andreas fault has removed the Cenozoic prism from this region of the North Am erican plate. On the North American plate (i.e., east of the San Andre as fault), the Mesozoic prism, interpreted as the main component of a tectonic wedge (type 2b), and the backstop to the Mesozoic prism are p resent. There is, however, no clear evidence of tectonically underplat ed oceanic rocks, and the crust is thin (no root). In both Alaska and Vancouver Island, the Mesozoic prisms above the underplated bodies exp erienced low-pressure/high-temperature metamorphism at about the time of tectonic underplating; no such metamorphism is currently exposed in California. The metamorphism may have been caused by the underplating of young, hot oceanic crust, or, alternatively, by subduction of an o ceanic ridge. The presence of a tectonic wedge (type 2b) in Alaska and California and the absence of such a wedge at Vancouver Island could arise either from the fact that in the former two locations the Mesozo ic prisms were more voluminous, owing to either more rapid trench sedi mentation or more rapid convergence, or to the possibility that at the latter location the Mesozoic prism was juxtaposed with the backstop p rimarily by strike-slip faulting.