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The remarkable early to middle Eocene volcanic sequence of the Crescent Formation exposed on the Olympic Peninsula consists predominantly of tholeiitic to minor transitional alkaline basalts with sparse sedimentary interbeds. A composite section measured in the vicinity of the Dosewallips River includes 8.4 km of pillowed to massive submarine basalts overlain by 7.8 km of subaerial flows. An upper limit of about 48 Ma on the age of the Crescent basalts is indicated by faunal assemblages in sediments interbedded with the uppermost flows in the sequence and a circa 50 Ma 40Ar/39Ar age on a leucogabbro from the presumably correlative Bremerton Igneous Complex. Stratigraphically controlled samples collected from throughout the Crescent basalt sequence show that two distinctly different chemical types exist. The lower part of the sequence originated from a relatively depleted mantle course resembling normal (N) to enriched (E)‐MORB. The upper flows have a chemistry resembling E‐MORB to oceanic island tholeiites. This difference could be due to either variable metasomatism of a single source domain, or influx of a separate enriched‐mantle source component during the extrusion of the upper part of the sequence. Paleomagnetic measurements indicate that the Crescent basalts have not been significantly rotated, nor translated northwards since their extrusion. Paleotectonic reconstructions show that formation of the Crescent basalts and the Coast Range volcanic province as a whole coincided with a marked increase in the velocity of oblique convergence of the Kula plate with North America at about 60 Ma. Other geologic, geochemical, and paleomagnetic data are consistent with the interpretation that extrusion occurred in a basin or series of basins formed by a rift system along the continental margin of North America. Rifting might have been initiated by the influence of a hotspot, an increase in the rate of oblique convergence, or the kinematic effects of the Kula‐Farallon ridge as it migrated along the margin. If extrusion is related to the passage of the triple junction, then the Coast Ranges can be considered to be an important tectonic marker for early to middle Eocene plate reconstructions.

Appendix tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C., 20009. Document B84‐012; $2.50. Payment must accompany order. Relative motion poles describing the displacement histories between the Pacific plate and once adjacent oceanic plates (Farallon, Kula, Izanagi I, Izanagi II, and Phoenix) were derived for the late Mesozoic and Cenozoic eras. Because fracture zone and magnetic anomaly data are generally available from the Pacific plate but not from adjacent plates, a new method of analysis for onesided data was required. This analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions. The following are the main conclusions drawn from our analysis: (1) For time intervals of the order of 107 years, termed stages, relative motion poles for plate pairs remained nearly fixed. Between stages, shifts in poles were commonly both large and abrupt. Within stages, rates of plate motion were commonly observed to change markedly, indicating that plates changed speed more frequently than they changed direction. (2) The relative motions of all of the plates analyzed changed at about chron M11 (135 Ma), chron 34 (85 Ma), and chron 25 (56 Ma). (3) During the Early Cretaceous there were five oceanic plates in the Pacific basin rather than the four recognized by previous workers. (4) To determine the number of Farallon plates that existed to the east of the Pacific plate during the time interval from chron 34 (85 Ma) to chron 25 (56 Ma), fracture zones and magnetic anomalies that record Pacific‐Farallon spreading from the northern, central, and southern Pacific plate were analyzed separately and collectively. The analysis shows that a single Pacific‐Farallon relative motion pole and a single rate are consistent with all of the data. (5) Spreading rates along the Pacific‐Kula ridge decreased markedly between chrons 32b and 25 (72–56 Ma), probably in response to the arrival of buoyant Kula lithosphere at a subduction zone northwest of the Bering Sea. (6) Soon after chron 25 (56 Ma), a major reorganization is recorded along the Pacific‐Farallon boundary. The Kula‐Farallon boundary is also thought to have changed during this reorganization. (7) Kula‐Pacific anomalies north of anomaly 25 are modeled by assuming continued Kula‐Pacific spreading after chron 25 (56 Ma). (8) Probably by chron 18 (43 Ma), Pacific‐Kula spreading had ceased and the Kula‐Farallon ridge system had evolved into alignment with the Pacific‐Farallon spreading direction. However, the timing of these events is uncertain because Pacific‐Kula data younger than chron 25 (56 Ma) are sparse.

A refined northeast Pacific plate‐motion model provides a framework for analysis of the Tertiary volcanic and tectonic history of western Oregon and Washington. We examine three possible models for the origin of the allochthonous Paleocene and Eocene oceanic basalt basement of the Coast Range: (1) accretion to the continent of hot spot generated linear seamount chains; (2) accretion of thick oceanic crust and seamounts generated during Farallon‐Kula spreading reorganizations between 61 and 48 Ma; and (3) eruption of basalt during oblique rifting of the continental margin as it overrode an active Yellowstone hot spot on the Kula‐Farallon ridge. The plate model suggests that microplate rotation and accretion of hot spot generated linear aseismic ridges cannot be easily reconciled with rapid northeast motion of the KuIa and Farallon plates and the well‐established paleomagnetic rotations. Following emplacement of the Coast Range basement, changes in the character of forearc, back arc and Cascade arc volcanism correlate with a marked decrease in the rate of Farallon‐North America convergence between 43 and and 28 Ma. This slowdown may be responsible for (1) westward stepping of the volcanic arc front from the Challis axis to a Cascade axis at about 42 Ma; (2) a subsequent episode of increased ash flow tuff volcanism and extension in the Cascade arc between 37 and 18 Ma that correlates with the “ignimbrite flare‐up” in the Basin and Range; and (3) a period of extensional basaltic and alkalic volcanism and intrusion in the Coast Range between 44 and 28 Ma. Reduction of the convergence rate and westward stepping of the flexure in the subducted slab may have reduced the horizontal compressive stress on the continent, allowing increased injection of magma into the crust, development of large, shallow magma chambers, and the outbreak of extensional volcanism over a large area behind the Farallon‐North America subduction zone.