Timing of Late Cenozoic Volcanic and Tectonic Events Along the Western Margin of the North American Plate

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 middle 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 others, 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 others, 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

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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

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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. SNAKE RIVER PLAIN 13. SAN ANDREAS — 6. N. BASIN & RANGE TRANSVERSE RANGES 7. SIERRA NEVADA 14. S. BASIN & RANGE 8. SAN ANDREAS 15. GULF COAST

130 20 / 1°/

Aleutian ridge a volcanic and plutonic episode started at about relationship (Anderson and others, 1972; Wright, 1976). However, 16-15 m.y. B.P. and ended at 10 m.y. B.P. (Scholl and others, 1976; the location of the fault zones was controlled by the presence of DeLong and others, 1978). In both the Cascade and Central Amer- crustal transition zones between the Basin and Range province and ican arcs, strong peaks in volcanic production occurred in the (1) the Colorado Plateau and (2) the Mojave block. Strike slip was a period of 16-14 m.y. B.P. (McBirney and others, 1974; Kennett and means to accommodate different amounts of extension (rather than others, 1977). a response to compression) across the transition zones (Anderson, Three areas of intraplate calc-alkalic volcanism in central to 1973; Wright, 1976). Similarly, Davis and Burchfiel (1973) noted northeastern Oregon were active during the period 17-12 m.y. B.P. that the left-lateral strike-slip Garlock fault separates crustal blocks These areas define a N45° E linear trend that is oblique to the north- of contrasting extensional responses, and they interpreted the Gar- trending, subduction-related Cascade arc but is located in the tran- lock fault to have originated with the start of Basin and Range sition zone or crustal boundary between the Columbia Plateau.and tectonism (that is, 16 ± 1 m.y. B.P.). Also, an east-trending, left- the Basin and Range province (Robyn, 1979). lateral, strike-slip fault zone accommodated approximately 90 km In addition to the widespread extensional tectonism across of crustal extension between the Transverse Ranges area and a north-northwest-trending normal faults and dikes, major strike-slip southern region of extensional tectonism (Yeats, 1968; Yerkes and zones became active at-16 ± 1 m.y. B.P. Between 40 and 72 km of Campbell, 1971). Jahns ( 1973) estimated that this fault zone became right-lateral offset (Ross and Longwell, 1964; Stewart and others, active soon after 20 m.y. B.P. (perhaps at 16 ± 1 m.y. B.P.?). 1968; Fleck, 1970) occurred across the N55°W-trending Las Vegas Tectonic rotation at the margin of the Pacific Northwest has shear zone between 17 m.y. B.P. (Ekren and others, 1968) and 10.7 been variously interpreted to have been completed at about 16 ± I m.y. B.P. (Fleck, 1970). The northeast-trending Lake Mead fault m.y. B.P. or to have been renewed at about 20 m.y. B.P. Hammond zone was active between 15 and 10.7 m.y. B.P. (Anderson, 1971; (1979) presented a model for the evolution of the Cascade Moun- Anderson and others, 1972; Bohannon, 1979), during which time tains in which continental crust rifted and rotated westward, creat- both dip slip and approximately 60-65 km of strike slip occurred ing the Cascade volcanic arc and entraining the Columbia Arc (Anderson, 1973; Bohannon, 1979). The complementary senses of (Taubeneck, 1966). In this model, continental crust migrated west- offset and orientations of the shear zones suggest a shear-couple ward in discrete blocks that were separated by northwest-trending,

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Figure 2. Generalized map of volcanic and tectonic events along the western margin of North America at 16 ± 1 m.y. B.P. Descriptions and references in. text.

right-lateral, strike-slip faults; rotation and migration were com- about 10 m.y. B.P., the Pacific plate was spreading in an east-west pleted about 17 m.y. B.P. Beck and Plumley (1980) speculated that direction relative to the Farallon-remnant plates; since 10 m.y. B.P., the rate of clockwise tectonic rotation of the Oregon Coast Range the spreading has been in a northwest-southeast direction (Vine, microplate may have increased dramatically prior to arrival at its 1966; Pitman and Hayes, 1968). Couch and Heinrichs placed the present location at about 15 m.y. B.P. In contrast, Magill and Cox time of change in Pacific spreading direction within the 12-8 m.y. (1981) concluded that a block consisting of the Oregon Coast B.P. period (in Moen, 1971). Suppe (1970) noted that the direction Range, the Cascade Mountains, and the Klamath Mountains has of movement of the Pacific plate relative to the Mexican mainland rotated approximately 27° in a clockwise direction and that the changed by 25° during the two-stage history of the San Andreas Sierra Nevada block has rotated less than ~ 15° in a counterclock- fault. At 10 m.y. B.P., the relative motion between the Pacific and wise direction during the last 25 m.y. B.P., and probably since 20 North American plates, as measured by slip across the San Andreas m.y. B.P. They associated these rotations with extension in the fault, increased significantly (Atwater and Molnar, 1973). However, Basin and Range province but did not place an absolute date on the much of the plate-margin dextral shear probably was accommo- start of the rotations. dated across the San Gregario-Hosgri fault, which was active con- currently with the San Andreas fault at this time (Graham and EVENTS AT 10 ± 2 M.Y. B.P. Dickinson, 1978). South of the San Andreas fault, changes that began at 16 ± 1 A change in tectonic regime occurred at 10 ± 2 m.y. B.P. along m.y. B.P. culminated at 10 ± 2 m.y. B.P. as the Guadalupe-Pacific the western margin of the North American plate (Fig. 3). Prior to ridge approached the North American plate margin in the Baja

17 V iü-OQ; IP 11 y / ^10 13 -y / GUADALUPE PLATE: / vA5

PACIFIC PLATE 20

11. Great Basin uplift, extension, and volcanism 12. Colorado Plateau 13. Start slip, Las Vegas and Lake Mead shear zones 14. San Andreas fault 15. Start slip, Garlock fault 16. Start slip, south of Transverse Ranges 17. Central America arc volcanic pulse 18. Guadalupe plate fragmentation, pivoting 19. Ridge reorganization between Orozco and Rivera F.Z. 20. E - W Pacific plate spreading

130.'X . -L 20 11L 120

Figure 2. (Continued).

California latitudes and a new pulse of spreading reorganization At 10 ± 2 m.y. B.P., significant events occurred in two bimodal began. As mentioned above, the three-ridge system off Baja Cali- volcanic and tectonic associations, in the intraplate calc-alkalic fornia reorganized into a single-ridge system soon after 12 m.y. associations in eastern Oregon, and in the three arc-trench associa- B.P., and a triple junction between the Pacific, North American, tions. Bimodal volcanism in the Snake River Plain had been migrat- and Cocos (southern remains of the Guadalupe plate) plates formed ing southeastward since its inception at 16 ± 1 m.y. B.P.; at about by 10 m.y. B.P. (Chase and others, 1970; Menard, 1978; Batiza and 10 m.y. B.P., the migration path rotated about 65° counterclock- Chase, 1981). The Guadalupe plate ceased independent existence at wise to a northeastward direction, and the rate of volcanic activity 10 m.y. B.P. and, correspondingly, subduction ceased at this time increased (Armstrong and Leeman, 1971; Armstrong and others, beneath Baja California (Chase and others, 1970; Menard, 1978). 1975). In a somewhat similar manner, between 11 and 10 m.y. B.P., Along the southern ridge of the triple junction, a change in spread- volcanism in southeastern Oregon began migrating westward along ing direction started at 10 m.y. B.P.; a 15° to 20° rotation to a more two subparallel, N75° W-trending belts which are now marked by northwest-southeast spreading direction was accomplished during silicic domes (Walker, 1974; MacLeod and others, 1976). Intraplate the 10-8 m.y. B.P. period (Chase and others, 1970; Larson, 1972). calc-alkalic activity ceased in the Strawberry volcanic field about 12 Farther south, ridge reorganization occurred just north of the Clip- m.y. B.P. and may have ceased at that time in the two other eastern perton fracture zone at about 12 m.y. B.P. (Herron, 1972; Menard, Oregon volcanic fields of similar petrologic-tectonic association 1978). Just north of the equator, a segment of the East Pacific Rise (Robyn, 1979). jumped about 7° eastward at 10 m.y. B.P., and the Galapagos rift Along the Aleutian ridge, the episode of arc magmatism, which zone formed between the Cocos and Nazca plates (Herron, 1972, began about 16 m.y. B.P., ceased activity at 10 m.y. B.P. (Scholl and Fig. 3). others, 1976). A pulse of arc magmatism occurred between 11 and 8

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1. Stop Aleutian arc magmatism 2. Start N - S compression, Columbia Plateau 3. Stop Strawberry • volcanism 10+2 m.y.B.P. 4. Snake River Plain change migration direction 5. Start migration, S.E. Oregon volcanism 6. Cascade arc volcanic pulse 500 1 Mi 7. Stress orientation change, Great Basin 8. Uplift, southern portion Colorado Plateau 500 Km

170 160 -40 150 i. i. \ 140

Figure 3. Generalized map of volcanic and tectonic events along the western margin of North America a 10 ± 2 m.y. B.P. Descriptions and references in text.

m.y. B.P. in the Cascade and Central American arcs (McBirney and and others (in prep.) have noted changes in structural orientations others, 1974; Kennett and others, 1977). in the Columbia Plateau that indicate a 20° to 30° clockwise change At 10 ±2 m.y. B.P., the regional orientation of maximum in stress configuration occurred between 12 and 8 m.y. B.P. horizontal compressive stress in western North America changed As mentioned above, the rate of displacement across the San from approximately N20°W to N30°W to approximately north- Andreas fault increased markedly at about 10 m.y. B.P. Geologic south to north-northeast-south-southwest; this change was of the evidence for this change has been recognized by numerous workers same sense (clockwise) and magnitude (about 25° to 45°) as the and has bred the concept of a two-stage history of middle to late change in spreading direction of the North Pacific. Smith and oth- Cenozoic movement across the fault (Dickinson and others, 1972; ers (1976) noted that present structural trends are at 20° to 25° Huffman, 1972; Crowell, 1975). However, recent work by Page and angles to the mid-Miocene north-northwest extensional trends of others (1979) suggested that the second stage of movement started the northern Great Basin. Zoback and Thompson (1978) cited at 15 m.y. B.P. Further work may resolve this apparent conflict in clockwise changes of about 45° in the orientation of Basin and timing. Silver (1978) correlated the synchronous development of Range structures between 14 and 6 m.y. B.P. Eaton and others large shelf basins off the California coast between the Mendocino (1978) and Eaton (1979) recognized a late Cenozoic change in the fracture zone and about 34° N latitude at approximately 10 m.y. orientation of extensional structures in the western Cordillera from B.P. with the change in direction of movement between the Pacific approximately north-northwest to north; they suggested that this and North American plates at this time. change, in orientation probably began between 9 and 7 m.y. B.P. Strike-slip movement ceased at 10 ± 2 m.y. B.P. across three of Hammond (1979) noted that the regional stress field changed the four major shear zones that became active at 16 ± 1 m.y. B.P. between 12 and 5 m.y. B.P. east of the Cascade Mountains. Barrash Strike-slip faulting south of the Transverse Ranges ceased at 10

Figure 3. (Continued).

m.y. B.P., and piecemeal attachment of continental border terranes changes north of the Mendocino fracture zone, and ridge jumps to the Pacific plate probably ceased at this time (Jahns, 1973). south of the Rivera fracture zone (Fig. 4). Between 5 and 4 m.y. Similarly, lateral transport across the Las Vegas and Lake Mead B.P., the Pacific-North American plate boundary jumped east- fault zones ceased at or soon after 11 m.y. B.P. (Ekren and others, ward, and sea-floor spreading opened the Gulf of California (Lar- 1968; Fleck, 1970; Anderson, 1973). Uplift of the southern portion son and others, 1968; Sclater and others, 1971). Preceding this of the Colorado Plateau began between 10 and 8 m.y. B.P., shortly jump, at about 6 m.y. B.P., the spreading rate across the Gorda after lateral slip ceased in the Las Vegas-Lake Mead region (McKee ridge slowed from 5 to 2-3 cm/yr; at this time, subduction of sedi- and Anderson, 1971). ments stopped, and accretion of sediments against the North Amer- Significant changes in style, orientation, and intensity of ican plate margin began (Silver, 1971a). At 5 m.y. B.P., the Pacific deformation in the Columbia Plateau occurred at 10 + 2 m.y. B.P. plate began overthrusting the Gorda plate across the Mendocino Strong north-south compression started in the John Day, Oregon fracture zone, and the Gorda plate began bending internally (Silver, region at 10 m.y. B.P. (Robyn, 1979). Uplift and major anticlinal 1971 b). Between 5 and 4 m.y. B.P., the Juan de Fuca-North Amer- folding with predominantly east-west fold axes started between 12 ica convergence rate slowed from 4.5 to 3.5 cm/yr (Riddihough, and 8 m.y. B.P. (Shannon & Wilson, 1977; Swanson and Wright, 1977). Similarly, between 5 and 4 m.y. B.P., the Explorer-North 1978, Bentley and others, 1980). America convergence rate slowed from 4 to 1.9 cm/yr, and between 4 and 3 m.y. B.P., the Explorer plate started rotating in a clockwise EVENTS AT 5+1 M.Y. B.P. direction (Riddihough, 1977). Between 5 and 3 m.y. B.P., the Queen Charlotte fault-Explorer ridge-Cascade trench triple junction Oceanic plate events at 5 ± 1 m.y. B.P. include the opening of began migrating northwestward (Riddihough, 1977). the Gulf of California, spreading-rate and spreading-direction South of the Gulf of California, the portion of the East Pacific

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»¿»A'*" 1. Start Aleutian arc magmatism 2. Northern B.C. rifting, volcanism 3. Stop folding, Columbia Plateau 4. Snake River Plain volcanic pulse 50 5. Change migration direction and 5±1 m.y.B.P. rate, S. Oregon volcanism 6. Cascade arc volcanic pluse 500 Mi 7. Uplift, Sierra Nevada J 1 8. San Andreas fault, increase slip rate 500 Km

160 170 i. .40 150\ 14 0 \ ^30

Figure 4. Generalized map of volcanic and tectonic events along the western margin of North America at 5 ± 1 m.y. B.P. Descriptions and references in text.

spreading ridge between the Rivera and Orozco fracture zones ish Columbia, alkaline to peralkaline volcanism associated with jumped 4° eastward from the Mathematicians seamounts to the north-trending normal faults started at 6 m.y. B.P.; the volcanic- East Pacific Rise at about 5 m.y. B.P. (Sclater and others, 1971). tectonic association in this region may be indicative of incipient The Cocos plate, as a whole, continued counterclockwise pivoting crustal rifting (Souther and Symons, 1974; Souther, 1977). at this time (Menard, 1978). The San Gregario-Hosgri fault was largely abandoned at Bimodal volcanism in the Snake River Plain continued migrat- about 6 m.y. B.P. and strike slip at the Pacific-North American ing northeastward, but an increase in activity occurred at about 6 plate margin became concentrated along the more inland San m.y. B.P. (Armstrong, 1975, 1978). At about 5 m.y. B.P., the west- Andreas fault (Graham and Dickinson, 1978). Slip rate across the ward migration of silicic volcanism across southern Oregon slowed San Andreas fault increased from 4 to 5.5 cm/yr at about 4.5 m.y. its rate of migration and changed its direction of migration to a B.P. (Atwater and Molnar, 1973). Severe north-south compression more northerly course (MacLeod and others, 1976). Pulses in vol- started in the Transverse Ranges about 5 m.y. B.P. (Jahns, 1973; canism occurred at the three North American magmatic arcs. After Crowell, 1975). About 4± 1 m.y. B.P., subsidence accelerated to a hiatus in activity since 10 m.y. B.P., magmatism renewed at 6 m.y. extremely high rates in basins clustered about the junction of the B.P. in a more northerly position in the Aleutian arc (Scholl and San Andreas and Garlock faults (Yeats, 1978). Hay (1976) used others, 1976). Increases in volcanic production occurred in the Cas- geometric arguments and time constraints within the 9-4 m.y. B.P. cade and Central American arcs at about 5 ± 1 m.y. B.P. (McBirney period (Christensen, 1966) to infer that the latest major pulse of and others, 1974; Kennett and others, 1977). In north-central Brit- uplift of the Sierra Nevada block began about 4.5 m.y. B.P. Hay

110 9. N - S compression, Transverse Ranges; basining, Garlock - San Andreas junction 10. Open Gulf of California 11. Central America arc volcanic pulse 12. Ridge jump from Mathematicians seamounts 13. Pacific plate overthrusting Gorda plate 14. Internal bending, Gorda plate 15. Juan de Fuca plate 16. Clockwise rotation, Explorer plate 17. NW migration of triple junction

130 \ ¿20 m n 120

Figure 4. (Continued).

noted that the elevation of the block is greatest in the south and that central British Columbia and northern Mexico. Uplift and exten- it decreases progressively northward. He suggested a genetic rela- sional tectonism were consequences of elevated crustal tempera- tionship between northward migration of the Mendocino triple tures. The north-northwest orientation of extensional structures junction, opening of the Gulf of California, and uplift of the Sierra resulted from thermal-related tectonic activity within a pre-existing Nevada block. About 5 ± 1 m.y. B.P., anticlinal folding and asso- north-northwest-oriented regional stress configuration (Fisher, ciated deformation ceased or decreased markedly in the Columbia 1967; Zoback and Thompson, 1978; Eaton, 1979). Plateau (Shannon & Wilson, 1973, 1977; Myers and Price, 1979). Spreading rates across the Mid-Atlantic Ridge (Pitman and Talwani, 1972) and the East Pacific Rise (Sclater and others, 1971; DISCUSSION AND INTERPRETATION Atwater and Molnar, 1973) did not change noticeably at 16 ± 1 m.y. B.P., and so the widespread volcanic-tectonic episode which began Mantle Upwelling and Events at 16 ± 1 m.y. B.P. at this time appears to have been driven by forces unrelated to plate-motion changes. However, the dip of the subducting Farallon The above-described continental events at 16 ± 1 m.y. B.P. plate had been steepening progressively in middle Tertiary time were related to a crustal thermal disturbance which had an axis prior to 16 ± 1 m.y. B.P. (Noble, 1972; Coney and Reynolds, 1977), approximately parallel to the western North American plate mar- and the subducting plate may have ruptured at or soon before gin. Except for the region of metamorphic and plutonic rocks in 16 ± 1 m.y. B.P. (Snyder and others, 1976; Zoback and Thompson, northern Washington and southern British Columbia, this thermal 1978). The rise of an extensive north-trending elongated mantle disturbance produced volcanism (unrelated to subduction) between plume can explain the nearly synchronous inception of volcanic and

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tectonic activity parallel to the western margin of the North Ameri- which largely counterbalanced extension in this region. Westward can plate at this time (Armstrong and others, 1969; Scholz and transport of the Sierra Nevada block by left-lateral slip across the others, 1976). Garlock fault appears to have continued at this time. Geophysical properties of the Great Basin region which are The 25° to 45° clockwise rotation of the regional stress config- consistent with an elongated mantle upwelling include: (1) thin uration as observed in the Columbia Plateau and the Great Basin crust, (2) low upper-mantle and crustal seismic velocities, (3) severe was of the same sense and magnitude as the change in Pacific plate

Pn-wave attenuation, (4) high heat flow, (5) regional uplift, and (6) spreading direction. The causes of the change in migration direction extensional tectonism (Thompson and Burke, 1974; Eaton and oth- of the Snake River Plain volcanism and the start of migration of the ers, 1978; Stewart, 1978). Characteristics consistent with lateral southern Oregon bimodal volcanism are poorly understood, but the spreading of the mantle upwelling beneath the Great Basin include synchronous time relationship with other major tectonic events bilateral symmetry of topography and Bouguer gravity, and the suggests similar underlying causes. Also, the west-directed migra- outward migration of normal faulting and basaltic volcanism tion of the southern Oregon volcanism and the over-all east- (Noble, 1972; Best and Hamblin, 1978; Eaton and others, 1978). directed migration of the Snake River Plain volcanism are of the North of the Columbia Arc, uplift and block faulting did not same sense as the symmetrically outward migrations of Great Basin occur and volcanic activity was restricted to flood-basalt extrusion. basaltic volcanism (Christiansen and McK.ee, 1978). The significance of this different response north of the Columbia The pulse of spreading ridge reorganizations south of the San Arc is not well understood, but the absence of volcanic events asso- Andreas fault at 10 ± 2 m.y. B.P. was due partly to the change in ciated with extension to the north of the subducting Farallon plate Pacific plate spreading direction and partly to the approach of the corresponds well with the model of mantle upwelling at the edge of, East Pacific Rise close to the North American plate margin (Chase or within the rupture window of, the subducting slab (Fig. 2). In the and others, 1970). southern portion of the Great Basin, strike slip across major shear zones, which began at 16 ± I m.y. B.P., accommodated the exten- Gulf of California Opening and Events at 5 ± 1 m.y. B.P. sion produced by the crustal thermal disturbance. More precise dating of the tectonic rotation history of the Oregon Coast Range- Most events at 5 ± 1 m.y. B.P. were fundamentally related to Cascade Mountains-Klamath Mountains block and the Sierra the opening of the Gulf of California at about 4.5 m.y. B.P. and to Nevada block will determine whether these blocks began, or ended, the oceanic plate reorganizations of which the gulf-opening was a post-Oligocene rotation at 16 ± 1 m.y. B.P. part. A parallel series of north-directed tectonic events occurred Continued pivoting of the Guadalupe plate is indicative of its offshore and on land and were restricted to a relatively narrow continued unstable geometrical configuration relative to the North width on either side of the North American plate edge. The north- American plate margin and its subducting slab. As the narrow sliver directed offshore events were: (1) northward overthrusting of the of the Guadalupe plate came close to and approached a parallel Gorda plate by the Pacific plate; (2) internal bending of the Gorda orientation with the North American plate off Baja California, the plate; (3) decrease in convergence rates of the Gorda, Juan de Fuca, rest of the Guadalupe plate became progressively independent of and Explorer plates (due to "pinching"?); (4) clockwise rotation of the northern sliver (Menard, 1978). The occurrence of a pulse of the Explorer plate; and (5) northwestward migration of the Queen Guadalupe plate reorganizations at 16 ± 1 m.y. B.P. suggests a pos- Charlotte fault-Explorer ridge-Cascade trench triple junction. On sible link with the mantle upwelling to the north. land, the north-directed tectonic events were: (1) abandonment of the San Gregorio-Hosgri fault for the San Andreas fault; (2) Changes in Pacific Plate Motion and Events at 10 ± 2 m.y. B.P. increased slip rate across the San Andreas fault; (3) severe compression in the Transverse Ranges; (4) extreme rates of subsidence in Continental and oceanic events at 10 ± 2 m.y. B.P. were fun- basins at the junction of the San Andreas and Garlock faults; (5) damentally related to the change in Pacific plate spreading direction increased rate of uplift of the Sierra Nevada block, with the south and the increase in velocity of relative movement between the lifted more than the north; and (6) northward deflection and Pacific and North American plates. Some of the above-described decreased rate of migration of southern Oregon volcanism. Also on events involve changes in structural style which can be loosely land, but without an obvious directional component, was the cessa- explained by an altered stress configuration. These events are: (1) tion or marked decrease of compressional folding in the Columbia increased slip rate across the San Andreas fault, (2) formation of Plateau. Rift-type volcano-tectonic events that started at 6 m.y. basins off the coast of California, (3) cessation of strike-slip motion B.P. in northern British Columbia were probably a response to across the Las Vegas shear zone, Lake Mead fault zone, and the right-lateral movement on northwest-trending faults at the conti- southern edge of the Transverse Ranges, (4) uplift of the southern nental margin (Souther and Symons, 1974). The connection between portion of the Colorado Plateau, and (5) inception of relatively the rifting and plate reorganizations to the south was less direct strong north-south-oriented compression in the Columbia Plateau. than for the above-listed events. The marked slowdown of extension south of latitude 37°N in the It should be noted that several of the events at 5 ± 1 m.y. B.P. Great Basin (Best and Hamblin, 1978), as exemplified by the cessa- preceded the opening of the Gulf of California. The Gorda ridge tion of strike slip across the Las Vegas and Lake Mead fault zones spreading rate decreased at about 6 m.y. B.P.; overthrusting of the and the uplift of the southern portion of the Colorado Plateau, Gorda plate by the Pacific plate and internal bending of the Gorda suggest that the change in stress configuration at 10 ± 2 m.y. B.P. plate began about 5 m.y. B.P. Apparently the eastward ridge jump caused an approximately east-directed component of compression that opened the Gulf of California was coupled to Pacific plate

behavior to the north and did not cause these events but was a later can arcs at 16 ± 1 m.y. B.P., 10 ± 2 m.y. B.P., and 5 ± 1 m.y. B.P. part of a reorganizational process.- were approximately synchronous with pulses in arcs around the Pacific rim (McBirney and others, 1974; Kennett and others, 1977) Oceanic Plate-Fragment Behavior and with pulses in Snake River Plain volcanism (Armstrong, 1975, 1978). Large-scale synchroneity of volcanic pulses suggests a con- The eastward ridge jumps, such as those which opened the Gulf trolling mechanism of interplate-wide influence and/or a sensitive of California and abandoned the Mathematicians seamounts, may responsiveness to incremental changes in tectonic environment. be qualitative support for models of plate-driving forces which Perhaps arc magmatism and some forms of intraplate volcanism assign the largest force magnitudes to the pull of subducting slabs are sensitive indicators or precursors of significant tectonic events (Forsyth and Uyeda, 1975, Chappie and Tullis, 1977). The or transition intervals and. are not directly or solely related to rates Farallon-remnant' plates south of the San Andreas fault had a high of subduction. If this is true, we may be in the midst of a tectonic and steadily increasing ratio of subducting boundary to plate area transition interval, as the current strong pulses of intraplate volcan- in late Cenozoic time. We infer that the spreading ridges jumped ism at Yellowstone (Armstrong, 1975, 1978) and arc volcanism in eastward in response to the strong pull of the subducting slabs. the Cascades, Central America, and around the Pacific rim are less The synchroneity of continental volcanic and tectonic events than 2 m.y. old (Kennett and Thunnel, 1975; Kennett and others, with pulses of reorganization of the southern Farallon-remnant 1977). Jackson and others (1975) came to a somewhat similar con- plates suggests genetic linkage. We speculate that altered stress clusion by using the geometry and age dates of magmatic loci along conditions in the mantle stimulated the Guadalupe plate reorgani- the Hawaii-Emperor chain to interpret the magnitude, direction, zations at 16 ± 1 m.y. B.P. However, the subducting slabs of the and chronology of stress field Changes in the central Pacific. southern and northern Farallon-remnant plates probably were not mechanically coupled (Menard, 1978). At this time, the Guadalupe ACKNOWLEDGMENTS plate was still in an unstable configuration relative to its subducting slab and the North American plate margin. The mantle upwelling to Reviewed by C. G. Chase, P. W. Lipman, A. R. McBirney, E. the north may have "perturbed" the subducting Guadalupe slab and H. McKee, and E. A. Silver. thereby stimulated the pulse of readjustment activity at 16 ± 1 m.y. B.P. At 10 ± 2 m.y. B.P., the pulse of Cocos plate reorganizations REFERENCES CITED was stimulated by the changes in Pacific plate motions, as were the continental events at this time. At 5 ± 1 m.y. B.P., the continental Anderson, R. 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