Mendocino Triple Junction

The region onshore from the Mendocino triple junction, where the the Juan de Fuca, Pacific, and North American lithospheric plates meet, is complex tectonically (Atwater, 1970). It lies transitionally between terrains on the south undergoing transform displacements and those to the north undergoing oblique convergence. Moreover, the relative movement between the major plates is carrying the triple junction slowly northwestward relative to the continent on the east, so that through time, transform-belt tectonic features are being overprinted on those formed under a regime of oblique convergence. Investigation of a few patches of Quaternary strata show both folding and faulting. In fact, wrench faulting forming ahead of the arrival of the San Andreas transform system is recognized 120 km north of the triple junction (Kelsey and Cashman, 1983).

Oregon and Washington Coast Ranges

From the Mendocino triple junction, coastal mountain ranges extend on northward from within California through Oregon and Washington. Farther inland in Oregon lies the Willamette Valley and still farther eastward the Cascade Range. Active volcanoes surmount the Cascade Range, including Mount St. Helens (Lipman and Mullineaux, 1981), and are inferred to be the consequence of subduction of the Juan de Fuca plate beneath the continent. The belt of active volcanoes extends from Mount Lassen on the south, onshore to the southeast from the Mendocino triple junction, through Mount Shasta, and on northward through both Oregon and Washington and southernmost British Columbia. Along this trend, active deformation associated with volcanism therefore also requires evaluation, but the volcanic belt lies well to the east of the coastline.

Along the coast, uplifted marine terraces attest to vertical tectonic movements, but active-tectonic investigations are sparse. The coastal ranges, including the Olympic Mountains in Washington, were uplifted beginning in Pliocene time and are still rising (Snavely and Wagner, 1963; Gable and Hatton, 1983). Fault scarps of Quaternary age have been identified on the southeastern part of the Olympic Peninsula (Wilson et al., 1979) and on the seafloor (Snavely et al., 1980). Earthquakes, such as those near Mount Rainier in 1973 and 1974 (Crosson and Frank, 1975; Crosson and Lin, 1975), and geodetic measurements prove recent deformation (Ando and Balazs, 1979). Investigations stimulated by the 1980 eruption of Mount St. Helens are adding much to understanding of the tectonics of the region (e.g., Weaver and Smith, 1983).

In northwestern Washington the tectonic style of active deformation changes gradually from one of oblique convergence to one of transform displacements associated with the Queen Charlotte Fault zone. This zone, which lies off the coast of British Columbia, forms the boundary between the Pacific and North American plates northwest of the Juan de Fuca Rise. The transition zone lies inboard primarily of the fragmented Juan de Fuca plate (Figure 1.1; Clowes et al.; 1983). High river terraces and upland surfaces of Quaternary age attest to active uplift in the Coast Mountains of British Columbia, inferred to be due chiefly to vertical expansion after heating of a thick crustal slab; this active uplift is documented by the unroofing and setting of fission-track dates (Parrish, 1983). Active tectonics in this region appear to be the consequence of isostatic adjustments to a previous history of oblique subduction that has thickened the crustal slab.

Other Active-Tectonic Realms

Several other provinces lie to the east of the coastal belt and locally display active deformation but are not dealt with in detail here (Figure 1.1). They include the Great Valley of California, the Willamette-Puget Sound Lowlands of Oregon and Washington, Sierra Nevada, Cascade Ranges, and Basin and Range province. In fact, from a global viewpoint, the whole of western North America constitutes the deformed margin of the North American plate (Atwater, 1970). The eastern edge of this belt corresponds to the abrupt eastern escarpment of the Rocky Mountains where they meet the Great Plains. The relatively stable craton lies still farther east and underlies the middle of the continent.

DISCUSSION

These brief descriptions of active-tectonic realms along the Pacific Coast emphasize the mobility of the region. Earthquakes, geodetic surveys, other geophysical measurements of several types, geomorphic studies, and geologic observations document irregular ground movement both vertically and horizontally. Many small areas the size of cities have been intensively studied, so that their deformational history is well known, but these areas are scattered and unevenly distributed. In addition, the kind and quality of data documenting active deformation is unsatisfactorily variable.

Documentation of recent deformation from earthquakes and from most geophysical measurements deals with the present and the past few decades only. It does not span time intervals long enough to reveal an understanding of average conditions—the time sampling is



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