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Timing of late Cenozoic volcanic and tectonic events along the western margin of the North American plate WARREN BARRASH Geoscience Research Consultants; and Department of Geology, University of Idaho, Moscow, Idaho 83843 RAMESH VENKATAK.RISHNAN* Geophotography and Remote Sensing Center, University of Idaho, Moscow, Idaho 83843 ABSTRACT TECTONIC SETTING PRIOR TO 16 + 1 M.Y. B.P. A number of well- to relatively well-dated significant volcanic Subduction of the Farallon plate dominated the volcanic and and tectonic events along the western margin of the North Ameri- tectonic setting of western North America during early to middle can plate began, ended, or increased intensity at 16 ± I m.y. B.P., Tertiary time. Subduction-related volcanism migrated over much of 10 ± 2 m.y. B.P., and 5 ± 1 m.y. B.P. Continental events at 16 ± 1 the western United States, including areas as far inland as western m.y. B.P. were related to the upwelling of an elongated mantle Montana, Wyoming, and Colorado. A shallow dip to the subduct- diapir, which created a widespread thermal disturbance east of the ing plate appears to be required by the far inland position of arc subducting Farallon plate. Oceanic events at 16 ± 1 m.y. B.P. were volcanism (Lipman and others, 1972; Coney and Reynolds, 1977). largely related to the approach of the East Pacific Rise toward the Contact of the East Pacific Rise with the North American plate North American plate. Continental and oceanic events at 10 ±2 margin at about 30 m.y. B.P. initiated a northward and southward m.y. B.P. were related to the clockwise rotation of the Pacific plate propagating transform-fault boundary between the Pacific and spreading direction and to the increased rate of relative motion North American plates (Atwater, 1970; Atwaterand Molnar, 1973). between the Pacific and North American plates. Most events at A hiatus of volcanic activity occurred between about 19 and 17 m.y. 5 ± 1 m.y. B.P. were related to the opening of the Gulf of California B.P. throughout the Great Basin (McKee and others, 1970; McK.ee, and to the oceanic plate reorganizations of which the gulf-opening 1971) and between approximately 20 and 16 m.y. B.P. in the Pacific was a part. Different styles of response reflect differences between Northwest (McBirney, 1978). Clockwise rotation and/or northward intraplate volcanic and tectonic subdomains. Pulses of volcanic transport of microplate blocks along the western edge of North activity do not seem to be directly or solely related to changes of America occurred extensively during early to middle Tertiary time, plate motion. Increased volcanic activity at arcs and some intra- with some rotation along the California coast occurring into at least plate volcanic settings may be indicators or precursors of significant late Miocene and Pliocene time (as summarized by Beck, 1980). tectonic transition intervals. If so, we may presently be in the midst West of the North American plate margin, complex plate motions of such a transition interval. and spreading ridge reorganizations occurred during early to mid- dle Tertiary time. The Farallon plate became progressively frag- INTRODUCTION mented, and the remnant plates progressively adjusted to new subduction and geometrical constraints. At about 20 m.y. B.P., Many significant late Cenozoic volcanic and tectonic events both the northern and southern Farallon-remnant plates were piv- along the western margin of the North American plate began, oting, and their ridges became oriented parallel to the Cascade and ended, or increased intensity at 16 ± I m.y. B.P., 10 ± 2 m.y. B.P., Middle American trenches, respectively (Menard, 1978). and 5 ± 1 m.y. B.P. Pulses of arc magmatism in the Cascade Moun- tains and Central America (and elsewhere around the Pacific rim) EVENTS AT 16 ± 1 M.Y. B.P. achieved maximum production at these times (McBirney and oth- ers, 1974; Kennett and others, 1977). We infer genetic relationships Significant events at 16 ± 1 m.y. B.P. (Fig. 2) can be grouped from nearly synchronous oceanic and continental events, although into oceanic-plate reorganization, volcanic activity, and structural the nature of some relationships remains obscure. We present brief deformation categories. descriptions of well- to relatively well-dated significant volcanic and Oceanic-plate reorganization occurred south of the San tectonic events which occurred at the above-listed times. We then Andreas fault. Throughout the 16 ± 1 m.y. B.P. to 10 ± 2 m.y. B.P. attempt to determine the most fundamental tectonic control for period, the Guadalupe plate (that is, southern remnant of the Faral- each time and to explain the corresponding array of volcanic and lon plate) continued pivoting in a counterclockwise direction tectonic events as elements of an integrated system. An illustration (Menard, 1978). At about 17 m.y. B.P., fragmentation of the Guad- (Fig. 1) of the present distribution of stress within tectonic subdo- alupe plate began and proceeded until about 12 m.y. B.P., when the mains of western North America is included for comparison with Cocos plate separated completely from the Guadalupe plate previous tectonic regimes that are discussed below. (Menard, 1978), and a triple junction was formed (Chase and oth- ers, 1970). West of Baja California, reorganization events that con- •Present address: Department of Geophysical Sciences, Old Dominion tributed to this separation included: (1) at 17 m.y. B.P., a change in University, Norfolk, Virginia 23508. spreading half-rate from 4.3 to 3.0 cm/yr with a slight change in Geological Society of America Bulletin, v. 93, p. 977-989, 4 figs., October 977 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/10/977/3444639/i0016-7606-93-10-977.pdf by guest on 25 September 2021 978 BARRASH AND VENK.ATAK.RISHNAN Figure 1. Generalized map of present stress distribution in western North America (after Lambert and Vanicek, 1979; Nakamura and Uyeda, 1980; Zoback and Zoback, 1980). Granite plutons Tectonic province boundary Thrust Regional compression A Trench Spreading ridge ^—• Regional extension Transform fault Transcurrent fault 0 500 Mi. • i 1 Other faults 0 500 o Triple junction spreading direction and (2) between 17 and 15 m.y. B.P., creation of Widespread basaltic and bimodal volcanism began erupting in a three-ridge system which lasted until a short time after 12 m.y. the Great Basin region at 16 ± 1 m.y. B.P. synchronously with B.P. (Chase and others, 1970). A major reorganization between the regional uplift and north-northwest-trending extensional deforma- Pacific and Guadalupe plates also occurred along the spreading tion (McKee, 1971; Stewart, 1971; Noble, 1972). Three bimodal ridge between the Rivera and Orozco fracture zones during the associations are particularly noteworthy. Bimodal volcanism, period of 15-10 m.y. B.P. (Menard, 1978). including the extrusion of numerous silicic flow domes, was distrib- Volcanic activity of at least four different petrologic-tectonic uted throughout southeastern Oregon between 17 and 11 m.y. B.P. associations started or reached high intensity at 16 ± 1 m.y. B.P. (MacLeod and others, 1976). Western Snake River Plain bimodal Starting at 16 ± 1 m.y. B.P., plateau-forming basalts issued from volcanism and concurrent northwest-trending downwarping began fissure systems in the Pacific Northwest. The Columbia River about 16-15 m.y. B.P. in southwestern Idaho, and migrated south- Basalts were the most voluminous outpourings; these basalts eastward until about 10 m.y. B.P. (Armstrong and Leeman, 1971; erupted from north-northwest-trending dike swarms (Thayer, 1957; Armstrong and others, 1975). Bimodal volcanism with ages of 16.3- Waters, 1961) mostly between about 17 and 13.5 m.y. B.P. (McK.ee 13.8 m.y. B.P. erupted along the north-northwest-trending North- and others, 1977, 1981). Concurrent fissure-fed volcanism produced ern Nevada rift (Cortez rift); this rift overlies a prominent linear the Steens Basalt (15.2-14.9 m.y. B.P.) in southeastern Oregon magnetic anomaly that is believed to be a buried feeder system (Watkins and Baksi, 1974) and basalts in the Owyhee Plateau start- (Stewart and others, 1975; Zoback and Thompson, 1978). Palin- ing at approximately 16.6 m.y. B.P. in southwestern Idaho (Pansze, spastic realignment of the north-northwest-trending Chief Joseph 1975) and ending at 13.6-13.4 m.y. B.P. in southeastern Oregon dike swarm, the northwest-trending western Snake River Plain, and (Watkins and Baksi, 1974). West of the Cascade Mountains, basalts the north-northwest-trending Northern Nevada rift suggests that with a geochemical affinity to the Columbia River Basalts appear to these features were parts of a complex, 700-km-long rift system have erupted from north-trending fissures in western Washington which was initiated about 17 m.y. B.P. (Zoback and Thompson, and Oregon (Snavely and others, 1973). In central British Colum- 1978). bia, north-trending fissures fed plateau basalts during the period of At 16 ± 1 m.y. B.P., calc-alkalic arc volcanism began or peaked approximately 16 ± 1 to 10 ± 2 m.y. B.P. (Souther, 1970, 1977). activity at the three North American magmatic arcs. Along the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/10/977/3444639/i0016-7606-93-10-977.pdf by guest on 25 September 2021 GENOZOIC VOLCANIC AND TECTONIC EVENTS, NORTH AMERICAN PLATE 979 I li ••— / 1 « TECTONIC PROVINCES 1. N.ALASKA 9. COLORADO PLATEAU 2. S. ALASKA 10. RIO GRANDE RIFT 3. PACIFIC NORTHWEST 11. S. GREAT PLAINS 4. N. ROCKY MOUNTAINS 12. MID CONTINENT 5.

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