The Relative Contribution of Accretion, Shear, and Extension to Cenozoic Tectonic Rotation in the Pacific Northwest

The relative contribution of accretion, shear, and extension to Cenozoic tectonic rotation in the Pacific Northwest

RAY E. WELLS U.S. Geological Survey, 345 Middlefleld Road, Menlo Park, California 94025 PAUL L. HELLER Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071

ABSTRACT tonic models have been suggested to explain the relationships between rotated terranes. Some of regionally extensive rotations, which approach these constraints have been neglected in earlier Large Cenozoic clockwise rotations de- 80° in the Coast Range of western Oregon and reconstructions, and we hope that by consider- fined by paleomagnetic data are an estab- Washington. The models consist of varying ing them, we can quantify the importance of the lished fact in the Pacific Northwest, and many combinations of three end-member rotation various mechanisms to the observed rotations tectonic models have been proposed to ex- mechanisms: (1) microplate rotation caused dur- and better constrain our palinspastic reconstruc- plain them, including (1) rotation of accreted ing docking of allochthonous Coast Range mar- tions. Finally, by correctly partitioning rotations oceanic microplates during docking, (2) dex- ginal terranes, (2) rotation induced by dextral among the various tectonic mechanisms, we can tral shear between North America and shear between the continent and oceanic plates limit the scale of events inboard of the rotated northward-moving oceanic plates to the west, to the west, and (3) rotation of terranes in front terranes and can make independent estimates of and (3) microplate rotation in front of an of differential continental extension in the Basin Basin and Range extension for several intervals expanding Basin and Range province. Strati- and Range region. Real-world examples for all of Tertiary time (DNAG time scale of Palmer, graphic onlap relations and local structure of these models can be found in the global tec- 1983). indicate that microplate rotation during dock- tonic scheme, but not all of them necessarily ing was not a major contributor to the ob- apply to rotations in the Pacific Northwest. PALEOMAGNETIC RESULTS AND served rotations. Coast Range structures, The combination of large rotations and the THEIR RELIABILITY Basin and Range extension, and paleomag- oceanic character of the Coast Range basalt netic data from middle Miocene (15 Ma) basement (Snavely and others, 1968; Snavely The paleomagnetic data require clockwise ro- Coast Range rocks indicate that dextral shear and MacLeod, 1974) led a number of workers tation of as much as 80° for lower and middle is responsible for at least 40% of the post-15 to suggest that much of the rotation could be the Eocene oceanic basalt forming the accreted Ma rotation of the Coast Range and that result of microplate collision (for example, basement of the Oregon and Washington Coast Basin and Range extension is responsible for Simpson and Cox, 1977; Duncan, 1982). Some Range (RB, SV, WH, BH in Fig. 1, which in- the remainder. Reconstructions based on ex- workers have suggested that the rotation is cludes relevant references to this discussion). In trapolation of this ratio back to 37 and 50 Ma primarily the result of continental extension in- the Oregon Coast Range, the amount of rotation are consistent with reconstructions based on board of the rotated region (Simpson and Cox, decreases in successively younger rocks at a rate paleomagnetic and stratigraphic relations in 1977; Hammond, 1979), whereas others have of a little more than 1° per million years (Beck older rocks and suggest that dextral shear has, suggested that a combination of rotations during and Plumley, 1980). The youngest rocks for been a significant contributor to rotation dur- collision and subsequent Basin-Range extension which rotations have been observed are middle ing most of Tertiary time. Changes in the is necessary to explain the rotation (Magill and Miocene flood basalts in western Washington dextral-shear rotation rate over the past 50 others, 1981). Still others have preferred rota- and Oregon. They are part of the Columbia m.y. correlate directly with changes in the ve- tion of crustal blocks of unspecified dimensions River Basalt Group and are rotated clockwise locity of the Farallon plate parallel to the in a dextral shear couple that is driven by 15° to 25°, when compared to the same flows coast and provide a strong argument for northward-moving oceanic plates to the west on the Columbia Plateau. oblique subduction as the driving mechanism. (Beck, 1980; Sheriff, 1984; Reidel and others, In general, volcanic rocks east of the Coast Continental reconstructions incorporating 1984). A few workers have attempted to de- Range are rotated much less than are contempo- shear may provide constraints on the rate of scribe the actual structural geometry that may be raneous volcanic rocks in the Coast Range; extension in the northernmost Basin and accommodating shear rotations (Wells and Coe, overall, the pattern is one of increasing rotation Range region and suggest 17% extension 1985). to the west for a given age unit and decreasing since 15 Ma, 39% since 37 Ma, and 72% since Our purpose in this paper is to consider the rotation northward for Coast Range rocks (Fig. 50 Ma near latitude 42°N. paleomagnetic data in the light of established 2). For example, middle Eocene (50-45 Ma) geologic relationships, some based on new map- volcanic rocks in the Republic graben of north- INTRODUCTION ping, that constrain the applicable tectonic mod- eastern Washington (SP) are rotated about 25°, els. Important factors are the timing of the and the Clarno Formation (47-35 Ma) of cen- Paleomagnetic evidence for large Cenozoic rotation, the age and scale of deformation in tral Oregon (CF) is rotated about 16°, both less clockwise rotations in the Pacific Northwest is both the Coast Range and the northern Basin than half the rotation of time-equivalent Coast abundant and convincing (Fig. 1). Many tec- and Range, and the critical stratigraphic onlap Range rocks (WH, TV, EI of Fig. 1). Western

Geological Society of America Bulletin, v. 100, p. 325-338, 9 figs., 1 table, March 1988.

325 326 WELLS AND HELLER

Cascade Range lavas of Oligocene and early sequence, the Ohanapecosh Formation of Bates in contemporaneous Oligocene sills (01) of the Miocene age (WC3, WC2, WC1, OF), which and others (1981) of Washington (OF), which is Oregon coast. onlap marine rocks of the Coast Range, are ro- also the most deformed unit. There is again a Pacific Northwest paleomagnetic studies do tated 14° to 36°, with the amount of rotation suggestion of increasing rotation toward the not exhibit, in general, statistically significant decreasing to the south into California. The coast, if one compares western Cascade rota- flattening of magnetic inclinations indicative of largest Cascade rotations occur in the oldest tions (Beck and others, 1986) to the 48° rotation northward transport of terranes, although per-

E XP LAN AT ION

Early Tertiary oceanic basalt and * * *— Thrust fault—Sawteeth on upper overlying marine sedimentary rocks plate

Cenozoic volcanic rocks T High-angle fault—Bar and ball on downthrown side Mesozoic plutonic rocks — Fault—Arrows show realative movement Mesozoic and Paleozoic metamorphic rocks 1 Cenozoic fold axes

Figure 1. (a) Geologic provinces of the Pacific Northwest, modified from King and Beikman (1974); SAF, SCF, BFZ, and WF represent San Andreas fault, Straight Creek fault, Brothers fault zone, and Wasatch fault, respectively, (b) Tectonic rotations of rock units, in degrees, slightly modified from compilations by Grommg and others (1986), Magill and others (1981), Bates and others (1981), and original sources; shaded sectors indicate 95% confidence limits on rotations. WASHINGTON: BP = Eocene volcanic rocks at Bremerton-Port Ludlow (Beck and Engebretson, 1982); BH = Eocene Crescent Formation, Black Hills (Globerman and others, 1982); WH = Eocene Crescent Formation, Willapa Hills (Wells and Coe, 1985); GV = upper Eocene Goble Volcanics (Beck and Burr, 1979; Wells and Coe, 1985); PO = Miocene Pomona Member, Saddle Mountains Basalt (Magill and others, 1982); OF = upper Eocene and Oligocene Ohanapecosh Formation of Bates and others (1981); SG = Miocene Snoqualmie and Grotto batholiths (Beske and others, 1973); SP = Eocene Sanpoil Volcanics (Fox and Beck, 1985). OREGON: TV = Eocene Tillamook Volcanics (Magill and others, 1981); EI = Eocene intrusions (Beck and Plumley, 1980); OI = Oligocene intrusions (Beck and Plumley, 1980); SV = Eocene Siletz River Volcanics (Simpson and Cox, 1977); TF = Eocene Tyee Formation (Simpson and Cox, 1977); RB = Paleocene basalt at Roseburg (Wells and others, 1985); YB = upper Eocene Yachats Basalt (Simpson and Cox, 1977); WC1, WC2, WC3 = Oligocene and Miocene volcanic rocks of the western Cascade Range (Magill and Cox, 1980; Beck and others, 1986); CF = Eocene and Oligocene Clarno Formation (Gromme and others, 1986); CB = Miocene Columbia River Basalt Group and Steens Basalt (SB) (Mankinen and others, 1987); MP = Upper Jurassic or Lower Cretaceous plutons (Wilson and Cox, 1980). CALIFORNIA: HF = Upper Cretaceous Hornbrook Formation (Mankinen and Irwin, 1982); SN = Upper Cretaceous Sierra Nevada batholith (Frei and others, 1984). MONTANA: MA = Paleocene and Eocene Montana alkalic province (Diehl and others, 1983). CENOZOIC TECTONIC ROTATION IN PACIFIC NORTHWEST 327

Figure 2. Tectonic rotation in Tertiary rocks of the Pacific Northwest, grouped by age. Unit key as in Figure 1, with the addition of G = Ginkgo flows of the Miocene Wanapum Basalt of the Columbia River Basalt Group (Sheriff, 1984) and POl, P02 = intracanyon Miocene Pomona Member of the Saddle Mountains Basalt of the Columbia River Basalt Group (R. E. Wells, R. W. Simpson, and M. H. Beeson, unpub. data).

sistent slight flattening in most of the Coast extension—that make up most tectonic models ridge may cause rotation of the small plate Range basalts led Beck (1984) to suggest that for the Pacific Northwest (Fig. 3). caught between the ridge and trench. Magill and 300 to 400 km of northward movement of the (1) Rotation during microplate collision. In others (1981) suggested that an Oregon Coast Coast Ranges had occurred. The implications of the first model, elongate microplates or terranes Range microplate, bounded by trenches both this will be considered in a later section. rotate during collision against the continental inboard and outboard, rotated into the continent Large tectonic rotations about vertical axes margin (Fig. 3a). Simpson and Cox (1977) and during subduction of its leading edge. Micro- are a virtual certainty, however, given the excel- Duncan (1982) suggested that large buoyant, plate collision may also be responsible for oro- lent paleomagnetic data base. Diverse rock types oceanic aseismic ridges may have rotated into clinal bending of the accreted oceanic basement representing 50 m.y. of Tertiary time have been the continent during collision about 50 m.y. ago around the Olympic Mountains in Washington sampled over a wide geographic area with con- to form the basalt basement of the Oregon and (Beck and Engebretson, 1982). sistent results. Geomagnetic field or rock mag- Washington Coast Range. Simpson and Cox (2) Simple shear rotation. In the second netic causes for the eastward declinations are (1977) suggested that oblique subduction of a model, clockwise rotation occurs for tectonic highly unlikely. Most of the paleomagnetic stud- small oceanic plate bounded by a spreading blocks caught in a dextral-shear couple between ies meet stringent field and laboratory criteria for reliability, including (1) a characteristic magnetization that has been isolated by thermal or alternating-field removal of secondary com- ponents, (2) the presence of both normal and reversed polarities which are antipolar, and (3) characteristic directions that pass the fold test. Finally, the Tertiary reference poles for stable North America are well defined (Diehl and others, 1983), and they show no evidence for unusual field behavior.

TECTONIC MODELS FOR ROTATION

In this section, we consider the three end- Figure 3. Models for rotation of terranes along the western North America continental member rotation mechanisms—microplate ac- margin, (a) Oblique collision rotation; (b) ball-bearing model; (c) Riedel shear rotation; cretion, dextral shear, and intracontinental (d) asymmetric intra-continental extension; NAM = North American plate, OP = oceanic plate. 328 WELLS AND HELLER

North America and northward-moving oceanic 49° 122° plates to the west (Beck, 1976, 1980), as if the blocks were caught like ball bearings between EXPLANATION transcurrent faults along the plate boundary zone. This process is illustrated schematically in Figure 3b. The size and actual shape of the rotat- Quaternary alluvium ing blocks are unspecified, although the lack of a penetrative tectonic fabric in the gently deformed rocks and the consistency of paleomagnetic results over intermediate distances (1 to 103 m) indicate that the tectonic processes respon- Oligocene and younger sible for rotation are operating on intermediate volcanic rocks to large scales. Where carefully studied, the blocks are in part deforming discontinuously along complex internal fault systems that may be partly inherited from previous tectonic epi- Tertiary marine forearc sodes along the active convergent margin (Wells sedimentary rocks and Coe, 1985). Blocks deforming by dextral shear may break into elongate slices bounded by secondary sinistral shears (R') that accommodate the rotational strain, somewhat analogous Eocene arc and forearc to the opening and closing of shuttered blinds volcanic rocks (Fig. 3c and Freund, 1974). Luyendyk and others (1980), following the structural analysis of Freund (1974), explained large clockwise rota- < \ \ I ; i ; tions in the western Transverse Ranges of south- .\'< ; 11 / « Eocene Tyee Formation 1 ern California as the result of shear rotation of 'V'V'")/ elongate, fault-bounded crustal slices adjacent to the San Andreas fault system. Rotations have also been reported in tectonic contexts where simple shear is a secondary effect. In the eastern Paleocene-Eocene flysch Mediterranean along the Dead Sea rift, Ron and others (1984) have demonstrated both clockwise and counterclockwise rotations associated with sinistral and dextral slip domains, respectively, in a broad area undergoing pure-shear Paleocene-Eocene pillow shortening. 1 r A < basalt basement 4 J ^ Wells and Coe (1985) suggested that similar 7 -J L- mechanisms operating on a smaller scale may explain rotations in western Washington and « \\% Pre-Tertiary rocks Oregon. They have shown that in southwest » * o » // Washington, an early episode of Eocene differential rotation of thrust-bounded blocks occurred in a dextral transpressive regime and was Anticline overprinted by a post-Eocene episode of Riedel shear rotation. Sheriff and Bentley (1980), Sheriff (1984), Reidel and others (1984), and Wells and others (1983b) have suggested, based on paleomagnetic evidence from the Columbia Syncline River Basalt Group, that westward-increasing dextral shear has produced rotations in the Pa- cific Northwest since at least middle Miocene immillili Unconformity time. The cause of the shear couple is probably long-term oblique subduction of the Farallon plate and its present-day remnants beneath the continental margin (Beck, 1980; Engebretson Figure 4. Geologic constraints limiting amount of microplate rotation of Oregon- and others, 1985; Wells and others, 1984). Washington Coast Range during collision with North America: (1) onlap of the highly rotated (3) Microplate rotation caused by inboard Eocene Tyee Formation across suture with pre-Tertiary Klamath Mountains, (2) correlation of extension. In the third model, rotation of conti- the rotated Eocene Tillamook Volcanics across the Columbia River syncline into less rotated, nental margin terranes occurs in response to post-accretion forearc volcanic rocks in Washington, (3) differential rotation of accretion- inboard continental extension in the Basin and related folds in pillow-basalt basement rocks of southwest Washington. CENOZOIC TECTONIC ROTATION IN PACIFIC NORTHWEST 329

Range region (Fig. 3d). Simpson and Cox Eocene Flournoy Formation of Baldwin (1974) leomagnetic and structural studies in southwest (1977) preferred a model of continental rifting and middle Eocene Tyee Formation, which Washington show that folded, fault-bounded similar to that of Hamilton and Myers (1966), in grade northward into a thick, sandy, forearc blocks of the oceanic basalt basement are ro- which western Oregon and Washington rotated basin turbidite sequence (Chan and Dott, 1983; tated varying amounts, ranging from 20° to 70° westward about a northern pivot, whereas the Heller and Dickinson, 1985). The Tyee turbi- clockwise (Wells and Coe, 1985). These middle Blue Mountains of Oregon rotated northwest- dites blanket the subaerial oceanic island basalts Eocene pillow basalts of the Crescent Formation ward about a pivot near the Idaho batholith in of the Siletz River Volcanics, which must have were folded against the continental margin prior response to Tertiary Basin and Range extension. been submerged during partial subduction and to a late middle Eocene erosional event, and the Magill and others (1981), Frei and others accretion along the old trench. average azimuth of fold axes in each rotated (1984), Heller (1983), and Gromme and others Nearly coincident in time with the Tyee- block correlates directly with the amount of (1986) have modified this model and attempted Roseburg unconformity is a marked change in block rotation as determined from paleomag- to quantify it for the Pacific Northwest. Magill sandstone composition from the lithic sand- netic studies. This implies that accretion-related and others (1981) and Frei and others (1984) stones of the Roseburg Formation, which were folding occurred prior to differential block rota- proposed a two-stage model in which oceanic derived from a local Klamath Mountains source, tion. Because the smallest block rotations in the microplate accretion causes about 45° rotation, to micaceous, arkosic petrofacies of the Tyee middle Eocene pillow basalt are equal to rota- whereas post-30 Ma extension is responsible for Formation, which were probably derived from tions of post-collisional volcanic rocks above the the remaining 27°. Heller (1983) and Gromme Jurassic and Cretaceous plutonic rocks far to the unconformity, it is implied that no rotation of and others (1986) have suggested that nearly all east (Heller and Ryberg, 1983; Heller and oth- the pillow basalt occurred prior to the folding. of the clockwise rotation can be explained by ers, 1985). These two facts indicate that block rotation of long-term extension behind a pivoting coastal These major changes are interpreted to mark the Coast Range basement postdates folding microplate that began at about 50 Ma. the end of collisional accretion and the begin- against the continental margin and that the en- Certainly, diverse mechanisms may be re- ning of forearc basin sedimentation in the south- tire rotation postdates any accretion (Wells and sponsible for the tectonic rotation, and it is ern Coast Range. The Tyee Formation is Coe, 1985). critical for our continental reconstructions to therefore considered a post-collisional deposit. determine the relative importance of each at Because the Tyee Formation has rotated nearly Dextral Shear Rotation various times in the rotation history. as much as the underlying volcanic basement (Fig. lb), rotation must have occurred after the There is abundant geologic and paleomag- GEOLOGIC CONSTRAINTS ON end of collision. netic evidence for post-accretion dextral shear ROTATION MECHANISMS (2) Correlation of Eocene volcanic rocks acting in the Pacific Northwest during Cenozoic across the Columbia River. In northwest time. Essentially all of the paleomagnetic rota- Collision Rotation of Microplates Oregon, late middle and late Eocene basalts of tions are clockwise, and differential rotations the Tillamook Volcanics are rotated 45° (TV, across large areas, such as along the lower Co- Of the three mechanisms discussed in the pre- Fig. lb). Some workers have attributed a large lumbia River west of Portland, cannot geomet- vious section, it appears that microplate rotation part of this clockwise rotation to docking of an rically be related to subsequent Basin and Range during accretion has been the least significant Oregon Coast Range microplate (Magill and extension (Fig. 1). contributor to Cenozoic rotation in the Pacific Cox, 1980; Magill and others, 1981; Frei and Paleomagnetic Evidence. Some of the best Northwest. There are three lines of evidence others, 1984). The less-rotated Washington evidence for dextral shear rotation comes from to support this contention (discussion keyed to Coast Range to the north was not considered studies on flows of the Miocene Columbia River Fig. 4). part of this microplate (Beck and Burr, 1979; Basalt Group across Oregon and Washington. (1) Sedimentary basin history in the southern Magill and others, 1981). Recent geologic map- In particular, the Pomona Member of the Saddle Oregon Coast Range during and following ac- ping on both sides of the Columbia River Mountains Basalt and the Ginkgo flows of the cretion. In the southern Coast Range, major (Wells, 1981; Wells and others, 1983a) has led Wanapum Basalt are physically, chemically, and changes in the stratigraphy and structure of the us to conclude that nearly the entire stratigraphic paleomagnetically distinctive and can be traced rotated sedimentary rocks document a transition section is correlative across the Columbia River hundreds of kilometres from their sources near from subduction zone to forearc basin setting between Oregon and Washington and that the the Idaho border to the Pacific Ocean (Swanson accompanying accretion of the Coast Range rotated Tillamook Volcanics has its exact coun- and others, 1979; Choiniere and Swanson, oceanic basalt basement at about 50 Ma (Heller terpart in the much less rotated (-23°) Goble 1979; Magill and others, 1982; Wells and oth- and Ryberg, 1983; Baldwin, 1974). Deep-water Volcanics of Livingston (1966) in Washington ers, 1983a, 1983b; Reidel and others, 1984; submarine fan deposits of the rotated, lower Eo- (Wells, 1985; R. E. Wells and others, unpub. Sheriff, 1984). These flows are excellent re- cene Roseburg Formation of Baldwin (1974) data). The Goble Volcanics are in an unequivo- corders of the magnetic field, and they show a overlie basalt basement, and both are locally in- cal forearc setting; they lie on top of coal-bearing progressive westward increase in clockwise rota- tensely folded and faulted against pre-Tertiary sandstones that interfinger to the east with Cas- tion from the eastern Columbia Plateau to the basement of the Klamath Mountains to the cade arc-derived lavas (Wells, 1981). Both the coast, with significant differential rotation ob- south. The depositional and structural style of Goble Volcanics of Livingston and the correla- served over distances of less than 10 km (Fig. 5 the Roseburg Formation suggests that it formed tive Tillamook Volcanics thus must have been and Table 1). Rotation of coastal sites averages in or near an active trench (Heller and Ryberg, erupted after collision was complete. Any rota- about 22° for 15 Ma flows of the Grande Ronde 1983). At the south end of the Roseburg basin, tion of the Tillamook Volcanics must have oc- and Wanapum Basalts and 16° for the 12 Ma the pre-Tertiary basement and the folded sedi- curred subsequently and must be unrelated to Pomona Member when compared to stable mentary rocks are overlain with angular uncon- the actual accretion event. plateau sites (Magill and others, 1982; Sheriff, formity by much less deformed (but also (3) Differential rotation of fold axes in oce- 1984). Approximately 6° to 11° of the rotation rotated) shelf and deltaic deposits of the middle anic basement of southwest Washington. Pa- in 12 to 15 Ma flows, or about 40% of the total, 330 WELLS AND HELLER

SADDLE MTS. BASALT Figure 5. (a) Magnetic declination of se- Pomona flow lected flows of the Miocene Columbia River ' t 12Ma Basalt Group, plotted on an east-west profile from the Columbia Plateau to the coast. Flow data for the Pomona Member of the Saddle

Mountains Basalt from Magill and others 100 (1982) and R. E. Wells, R. W. Simpson, and M. H. Beeson (unpub. data); Ginkgo flows from SherifT (1984); Sand Hollow (?) flow 100 from Wells and others (1983, and unpub. data). The approximately 10° of rotation WANAPUM BASALT Frenchman Springs across the Coast Range appears to be about Member 40% of the total post-15 Ma clockwise rota- 160 Ginkgo flows tion across the entire region (see Table 1 for 15Ma complete data). c O (b) Idealized curves of rotation plotted o against distance from the Coast for different 140 rotation models: (1) dextral shear distributed across entire region, (2) rigid-plate rotation in front of an expanding Basin and Range, WANAPUM BASALT (3) combination of dextral-shear and rigid- Frenchman Springs plate rotations, (4) dextral shear terminates 10 Member Sand Hollow (?) I low at low-strength region in arc, (5) combina- 15Ma tion of dextral-shear and rigid-plate rotation with weak zone in the arc, (6) one of several possible line fits to Miocene paleomagnetic Coast Cas«* icade „ . -, Coast Cas data in Figure 5a, normalized to a common Range cade I Arc^Columbia Plateau Range eastern Plateau declination; reference direc- Arc Columbia Plateau 100 200 300 400 500 100 200 300 400 500 tions in Reidel and others (1984), Sheriff W Kilometers E W Kilometers E (1984), and Rietman (1966); four outliers have been removed for clarity. The data require dextral shear inboard for at least 200 km and permit a rigid-plate rotation.

occurs within the Coast Range between Port- gence of the down-going slab, as well as on the pected fault geometries in areas undergoing dis- land, Oregon, and the coast (Fig. 5). thickness and thermal structure of the overlying continuous deformation during simple shear and The westward increase in rotation has been plate. The active magmatic arc may be a low- illustrated the quantitative relationship between interpreted as a continuous function in support strength region and thus an effective inboard rotation and fault orientation, offset, and spacing of a distributed dextral shear mechanism (Sher- boundary for convergence-related rotations, but (Fig. 3c). Field examples of this relationship iff, 1984; Wells and others, 1983b) and as a step the paleomagnetic data are inconclusive. Some have been provided by Ron and others (1984), function (with a break at the Cascade arc) in rotations east of the Cascade arc (Fig. 5) occur in Wells and Coe (1985), and Hornafius (1985), support of extension behind a rigid coastal plate the structurally complex Yakima fold and thrust who have shown that paleomagnetically deter- (Magill and others, 1982). Ideally, one could province, which complicates interpretation of the mined rotations match calculated values deter- make fine distinctions among various combina- data in that area (see Reidel and others, 1984). mined from local fault geometries. In the tions of dextral shear and rigid plate rotations In older volcanic sequences, it is harder to densely vegetated Washington and Oregon (Fig. 5b), but the data are not yet sufficiently document progressive rotation because of uncer- Coast Range, fault offset and spacing are diffi- detailed. Taken as a whole, the overriding char- tainties in the ages of units, due to the lack of cult to determine, and only rough estimates of acteristic is a monotonic westward increase in critical radiometric ages, and uncertainties in shear rotation can be made from the structure. rotation throughout a region extending at least comparing the large data sets required to aver- On the basis of mapped fault spacing, Wells and 200 km inland from the coast. This is strong age out short-term secular variation of the mag- Coe (1985) suggested that shear rotation may circumstantial evidence for dextral shear, dis- netic field. Nonetheless, there is a suggestion of account for more than 35% of the total post- 2 4 tributed on some intermediate scale (10 -10 increasing rotation to the west in pre-Miocene Eocene rotation in southwest Washington and m) acting on the forearc and arc regions. We rocks as well (Fig. 2 and introductory discus- less than 50% of post-Eocene rotation along the believe that the driving force is related in part to sion), but more work is required to confirm it. central Oregon coast. increased coupling of the coastal region to the Structural Evidence. Many workers have Best Estimate of Shear Rotation. In sum- northeasterly oblique subduction of the Juan de examined the geometric consequence of simple mary, the paleomagnetic data suggest that Fuca plate beneath it. Just how far inland such shear in both laboratory and geologic environ- dextral shear along the convergent margin has driving forces could be effective is uncertain; it ments (see Ron and others, 1984, for a recent been an important factor in tectonic rotations for would depend on the age and rate of conver- discussion). Freund (1974) summarized the ex- the past 15 m.y. and probably longer. The pa- CENOZOIC TECTONIC ROTATION IN PACIFIC NORTHWEST 331

TABLE 1. PALEOMAGNETISM AND TECTONIC ROTATION OF DISTINCTIVE FLOWS OF THE MIOCENE COLUMBIA RIVER BASALT GROUP, OREGON AND WASHINGTON

Unit Coast Range Cascade Range (Portland) Columbia Plateau Relative rotations Ratios of rotations

Dec Inc a95 (N) Dec Inc a,5 (N) Dec Inc a95 (N) (Coast-Port) (Coast-Plat.) (Coast-Port./Coast-Plat.)

Saddle Mts. Basalt

Pomona Member 205.3 -49.7 2.4 (7) 199.1 -47.1 1.6 (1) 189.8 -50.9 1.8 (14) 6.2 ± 3.4 15.5 ± 3.7 0.40 (12 Ma)

Wanapum Basalt

Basalt of Sand Hollow 012.5 58.5 2.0 (2) 005.0 60.0 3.0 (1) 354.3 60.6 3.9 (5) 7.5 ± 5.5 18.2 ± 6.9 0.41 (14.5-15.5 Ma) Basalt of Ginkgo 171.1 40.5 6.7 (4) 160.6 39.8 9.9 (2) 146.0 41.0 9.9 (6) 10.5 1 12.1 25.1 ± 9.4 0.42 (14.5-15.5 Ma)

Note-, age of units from Swanson and others (1979) and Beeson and others (1985). Dec and Inc are average magnetic declination and inclination in degrees, calculated from a number of sites (N) in each region. is the radius of the 95% cone of confidence about the mean direction in degrees; rotation uncertainties calculated by method of Demarest( 1983). Ratio of rotation is the proportion of the total rotation occurring within the Coast Range, or the minimum simple shear component. Sources of data: Rietman (1966), Choinete and Swanson (1979), Magill and others (1982), Wells and others (1983b, and unpub. data), Reidei and others (1984), Sheriff (1984), and Beeson and others (1985).

leomagnetic data for 15 Ma are suggestive but cause Basin and Range extension decreases canic plutons, low-angle normal faulting, and not detailed enough to define quantitatively the northward toward the Blue Mountains in locally high strain rates, perhaps indicating rigid plate component of rotation due to Basin Oregon (Walker, 1977; Lawrence, 1976), rota- thinner crust and high heat-flow regimes and Range extension. We can only suggest that tion is a natural consequence of extension, given (Zoback and others, 1981; Eaton, 1982). Later dextral shear contributes a minimum of 40% of that western Oregon and southwestern Wash- Basin and Range extension was characterized by the rotation, that is, that part occurring within ington behave as a rigid plate in front of the block faulting forming regularly spaced basins the Coast Range between Portland and the coast extending orogen (Simpson and Cox, 1977; and associated with fundamentally basaltic vol- which cannot be geometrically explained by in- Magill and others, 1981). The amount of rigid canism (Zoback and others, 1981; Eaton, 1982; board Basin-Range extension. This value is in plate rotation depends on the amount of exten- Christiansen and Lipman, 1972). The time of reasonable accord with the simple shear compo- sion to the east, and estimates of extension transition from middle Tertiary extension to the nent determined by structural analysis of fault- across the Basin and Range province vary present-day style of Basin-Range extension ing and suggests that simple shear may be widely, depending on the locale, age of the ex- varies from place to place, although in general, responsible for about 40% of post-15 Ma rota- tended terrane, and geometric assumptions of the later block faulting was well underway by tion in western Oregon and Washington. the models used in the calculations. 10 Ma in the northern Basin and Range (Zoback It is important to note that dextral shear can- Hamilton and Myers (1966) calculated and others, 1981). not account for all of the observed rotation (as 8%-18% extension across the north-central Basin An independent estimate of the amount of much as 80°) in the Oregon Coast Range be- and Range by summing displacements across extension in the northern Basin and Range has cause large shear rotations would have disrupted the 25 major grabens but suggested that total been derived from paleomagnetic data in the critical fades relationships in the Tyee basin. Tertiary extension could approach 100% if addi- Pacific Northwest by assuming that the rotated The widespread, well-constrained northward pa- tions to the crust by abundant volcanism were region behaved as a rigid block (Magill and oth- leocurrents in the rotated Tyee Formation are taken into account. More recent estimates of the ers, 1981,1982). We now know, however, that consistent with east-west-trending lithofacies magnitude of Basin and Range extension gener- the Coast Range has not behaved as a com- boundaries that indicate deepening of the basin ally range between these two figures, with the pletely rigid block, and so it is time to make new to the north (Snavely and others, 1964; Lovell; lower values of 5%-30% calculated for young reconstructions of the Pacific Northwest and re- 1969; Heller and Dickinson, 1985). Microplate Basin and Range block faulting (Thompson, evaluate the amount of northern Basin and rotation in front of an expanding Basin and 1959; Thompson and Burke, 1974; Zoback and Range extension calculated from paleomagnetic Range region thus could be responsible for a Thompson, 1978; Stewart, 1971, 1978, 1980) models. significant fraction of the rotation. and higher values of 64%-100% calculated for On the basis of the paleomagnetism and geol- regions of high stratal tilts and associated low- ogy of adjacent regions in the Pacific Northwest, Basin and Range Extension as a Cause angle detachment faults (Proffett, 1977; Davis our best constraints on the geometry of the of Rotation and Burchfiel, 1973; Wernicke and Burchfiel, northern Basin and Range occur at 15, 37, and 1982), although evidence for extreme local ex- 50 Ma, which are probably also important times Irving (1964) recognized that early paleo- tension in excess of several hundred percent has in its polyphase extensional history. We begin magnetic results of Cox (1957) from the Siletz been reported (Miller and others, 1983). our analysis of extension in the northern Basin River Volcanics of the Oregon Coast Range The discrepancy in these estimates may be and Range at 15 Ma, following the eruption of supported Carey's (1958) model of oroclinal explained in part by the progressive change in great flood basalts in the Pacific Northwest. bending of the western Cordillera by a combina- the nature of northern Basin and Range exten- These widespread basalt sheets and their linear tion of northwest-directed dextral shear along sion during Tertiary time (Zoback and others, vent systems provide useful strain markers in the continental margin and east-west extension 1981). An early extensional phase was contem- both the northern Basin and Range and in the in the Cordilleran interior. Hamilton and Myers poraneous in part with calc-alkaline volcanism surrounding rotated regions of the Pacific (1966) also noted the connection between Cor- and the ignimbrite flare-up in the Basin and Northwest. dilleran extension during Tertiary time and Range between 34 and 17 Ma (Coney, 1979). It Post-15 Ma Extension. In our earlier discus- clockwise rotation in the Pacific Northwest. Be- was characterized by emplacement of subvol- sion of paleomagnetic results from the Columbia 332 WELLS AND HELLER

River Basalt Group, we argued that dextral construction are discussed in a later section; curred following emplacement of the volcanic shear is responsible for at least 40% (about 10°) here, we wish to compare our results with other rocks. Analysis of gravity and fault-slip data in of the total post-15 Ma rotation of the Coast estimates of post-15 Ma extension in the north- the Dixie Valley region of north-central Nevada Range. Thus, microplate rotation in front of an ern Basin and Range (Fig. 6). led Thompson and Burke (1974) to suggest 10% expanding Basin and Range could have contrib- Zoback and Thompson (1978) and Zoback extension for that region in the past 15 m.y. uted a maximum of 12° to the rotation. If we (1979) have examined the geometry of the Stewart (1980), using the method of Morton use the "best fit" Coast Range Euler pole of 14-17 Ma Northern Nevada rift, where range- and Black (1975), in which tilt of range blocks is Magill and others (1981) to rotate this micro- bounding faults that have significant left oblique proportional to extension, calculated 20%-30% plate as a rigid block in front of an expanding slip have offset mafic dikes. Gravity and aero- extension for the entire Great Basin. A different Basin and Range province, the resulting post-15 magnetic data provided constraints on the verti- approach was taken by Lachenbruch and Sass Ma extension in the northern Basin and Range is cal and horizontal displacement, and Zoback (1978), who argued that energy and mass- about 17% at latitude 42° N. Details of this re- estimated that about 20% extension had oc- balance considerations in a volcanically active

440

4£.

40°

Figure 6. Comparison of post-15 Ma extension in the northern Basin and Range calculated from Pacific Northwest paleomagnetic data (17%, in circle) with that calculated from various extensional domains in the northern Basin and Range (shown in boxes). DV = Dixie Valley (Thomp- son and Burke, 1974), GSL = Great Salt Lake (this paper), KF = Klamath Falls (this paper), Northern Nevada rift (Zoback and Thompson, 1978; Zoback, 1979), OP-WSRP = Owyhee Plateau-Western Snake River Plain (this paper), SB = Steens Basalt domain (this paper), RR = Raft River Grouse Creek-Albion Mountains, HR = East Humboldt-Ruby Range, SR = Snake Range. Bold lines represent normal faults; fine-lined areas represent metamorphic core complexes. (See text discussion.) Rotation of Willamette plate of Magill and others (1982) about pivot point "P" was used to calculate overall post-15 Ma extension, assuming 40% of coastal rotation was due to shear (see Table 1 and text). CENOZOIC TECTONIC ROTATION IN PACIFIC NORTHWEST 333

Basin and Range would allow 1l%-25% exten- geometric analysis of tilt blocks (Donath, 1962; quite different from the previous three domains. sion in the past 17 m.y. These calculated values Gettings and Blank, 1974; Travis, 1977). The It is bounded on the east by Paleozoic rocks compare favorably to our paleomagnetically conjugate fault pattern, with both angular and of the Wasatch Mountains, which form the derived estimate of 17% extension in the past large scalloped re-entrants, and the low stratal physiographic eastern margin of the Basin and 15 m.y. tilts argue for a mix of nonrelational, steep Range, and consists of north-trending tilted As a first-order check on our paleomagnetic planar faults and possibly listric faults similar to blocks of Paleozoic rocks rising from a sea of result, we estimated post-15 Ma extension for those described by Anderson and others (1983) Quaternary alluvium (Zoback, 1983). Direct some other localities in the northernmost Basin in north-central Nevada. We calculated post-15 calculation of post-15 Ma extension is impossi- and Range, directly behind the rotating block. Ma extension in the Steens domain by summing ble because of the paucity of exposed Tertiary We have subdivided the area into four regions, maximum displacements on all faults shown at volcanic and sedimentary rocks. One can only each characterized by relatively uniform struc- 1:250,000-scale mapping along a traverse nor- estimate from gravity and seismic reflection data ture and volcanic history: the Steens, Klamath, mal to the range fronts and by trying various a minimum total Tertiary extension from Salt Owyhee Plateau-Western Snake River Plain, fault geometries. Planar faults dipping 60° give Lake City to the Nevada line (Zoback, 1983; and Great Salt Lake domains (Fig. 6). Some of about 6% extension, and listric faults, about 10%, Smith and Bruhn, 1984). The seismic reflection these domains, like the Steens domain, have had given the observed gentle dip of beds and steep records analyzed by Smith and Bruhn (1984) relatively simple histories since 15 Ma; others, faults near the surface. A similar value of show considerable listric faulting with basin fills like the Great Salt Lake domain and adjacent 6%-10% extension can be derived from the tilt- of about 2 km. Their correlation of Paleozoic areas of northern Nevada, have had complicated block model of Morton and Black (1975). These strata across the low-angle faults allows a min- and controversial histories of extension (Wer- values suggest that about 30 km of west- imum of 50 km extension on their section J-J' nicke and others, 1987). We emphasize that in northwest extension took place in post-15 Ma between Crawford and the Silver Islands, but if the absence of estimates of extension by local time across the present 325 km width of the some of the low-angle faults were previously experts, our estimates of post-15 Ma extension Steens domain. thrusts as they propose, the extension could be greater. Gravity data compiled and analyzed by in these complex areas is little more than an The Klamath Falls and Owyhee Plateau- Zoback (1983) indicate that the Wasatch fault educated guess. Snake River Plain domains are similar to the has 3 to 4 km displacement and that depth of The Steens domain, named for the wide- Steens domain in that they also consist of gently fills in 6 major basins averages about 2 km. The spread 15-16 Ma Steens Basalt in southeastern dipping plateau basalts and rhyolites which have greatest vertical offset exceeds 5 km on the west Oregon, is underlain by voluminous middle been dissected by many steeply dipping normal side of the Oquirrh Mountains. Using an aver- Miocene plateau basalts and ash-flow tuffs clus- faults (Fig. 6). They are different from the Steens age of 3 to 4 km vertical offset across the 6 tering in age around 14-17 Ma (Walker, 1963; domain in that they contain abundant volcanic basins gives 27%-40% total extension for a steep Walker and Repenning, 1965, 1966; Watkins rocks younger than 15 Ma (McKee and others, listric-fault model, which can be extrapolated and Baksi, 1974; Rytuba and McKee, 1984; 1983; Ekren and others, 1984). Gentle stratal through similar basins to the Nevada line. These McKee and others, 1983), with younger vol- tilts and steeply dipping faults (Donath, 1962; values may be minima for Tertiary time, consid- canic rocks occurring locally. This area com- Walker, 1963; Peterson and Mclntyre, 1970; ering the large magnitude of Tertiary extension prises a large block-faulted structural domain in Baldwin, 1981; Ekren and others, 1981, 1984) reported from the nearby Snake Range, Ruby- which we can calculate the magnitude of again result in estimates of 7% to 10% extension East Humboldt Range, and Raft River-Albion- post-15 Ma extension. The Steens domain is in each region on the basis of fault geometry, Grouse Creek Mountains (Miller and others, about 325 km wide and contains 7 major block- although the abundance of young volcanic rocks 1983; Snoke and Lush, 1984; Armstrong, 1982; faulted ranges and intervening basins which are almost certainly indicates significant post-15 Compton, 1983; Todd, 1983). For example, intricately sliced by hundreds of high-angle Ma addition to the crust. Seismic refraction and Miller and others (1983) considered the north- normal faults. Major range-bounding faults with gravity studies yield crustal thicknesses of 30-40 ern Snake Range to have undergone in excess of 1 to 4 km of normal displacement trend north- km in nearby areas (Hill and Pakiser, 1967; several hundred percent extension by emplace- northeast and are complemented by west- Thiruvathukal and others, 1970; Fuis and ment of middle Tertiary plutons, ductile thin- northwest faults of much smaller displacement, others, 1987). A more reasonable estimate of ning of the middle crust, and failure of the generally less than 300 m, that form en echelon post-15 Ma extension in the Klamath Falls and overlying brittle Paleozoic upper crust by super- swarms such as the Brothers fault zone (Walker, Owyhee Plateau-Western Snake River Plain imposed rotational normal faults. But with a few 1977; Lawrence, 1976), Orientation of major domains might be based on the mass and energy notable exceptions (Todd, 1983; Compton, range-bounding faults is consistent with the di- balance models of Lachenbruch and Sass (1978) 1983; Wernicke and others, 1987), most of this rection of post-10 Ma west-northwest extension for extension in a volcanically active Basin and intense extensional episode in the northern Basin in the rest of the Basin and Range as indicated Range. Their estimate of 11% to 25% extension and Range predates our 15 Ma window. An by Zoback and others (1981), although the pat- in the past 17 m.y. is greater than the observed accurate estimate of the proportion of the total tern was developed by at least 16 Ma in Oregon, fault pattern would indicate, but it would be extension in the Great Salt Lake domain that has on the basis of the orientation of feeder dikes for consistent with addition of young crust and its occurred in the past 15 m.y. is probably not the Steens Basalt which are parallel to range- blanketing of older structures. This would result possible at present. We have liberally estimated bounding faults and the onlap of middle Mio- in extension of 7 to 16 km across the 80 km that three-fourths or about 47 km of the total cene units onto the already tilted Steens Basalt width of the Klamath Falls domain and 25 to 50 average extension calculated from the geophysi- (Walker, 1977; Wells, 1979). Stratal tilts in the km extension across the 250-km-wide Owyhee cal data may have occurred in the past 15 m.y. Steens Basalt are generally less than 5°. Depth of Plateau-Western Snake River area. fill in major basins ranges from 0.5 to 1.5 km, on The Great Salt Lake region between longi- Summing across the 4 domains gives a total the basis of seismic refraction, gravity, and tudes 114° and 111° W. in northwestern Utah is post-15 Ma extension of somewhere between 334 WELLS AND HELLER

Figure 7. Continental reconstructions at IS, 37, and SO Ma, based on paleomagnetic and geologic constraints. Rotation due to dextral shear (in boxes) was subtracted from western Oregon and Washington rotations before restoration of crustal blocks about Euler pole, P (modified from Frei and others, 1984). Resultant block rotations are shown in figure, along with amount of northern Basin and Range extension required to produce them. Component of Farallon plate motion parallel to continental margin shown in circle in kilometres per million years (Engebret- son and others, 1985). Block patterns of Coast Range, Klamath Mountains, Blue Mountains, Sierra Nevada, and Idaho batholith follow those of Figure 1.

97 and 143 km for an average extension of strained by the paleomagnetically determined from an expanding Basin and Range province. 11%-21% at about latitude 42° N. This much mean rotations shown in Figure 1 and do not These "residual" rigid-plate reconstructions of extension is equivalent to a 9° to 15° rigid-plate violate the cited geologic relationships discussed the Willamette plate are then compared to pa- rotation of western Oregon and Washington in the previous sections. leomagnetic and geologic constraints from sur- using the microplate rotation pole defined by Our method of reconstruction uses the same rounding areas to test for consistency. Magill and others (1981, 1982) and Frei and building blocks used by Frei and others (1984) others (1984). It is, on average, about 54% of the and Magill and others (1982) with some addi- 15 Ma Reconstruction total rotation (22°) observed in 15 Ma coastal tions and modifications. Because we attribute flows of the Columbia River Basalt Group. Our significant rotation to deformation caused by Our earlier calculations indicated that a min- admittedly rough estimates of post-15 Ma shear, we cannot assume truly rigid blocks in imum of 10° post-15 Ma rotation in the Coast extension thus tend to support our analysis of our models, although it is obvious from fades Range is due to distributed dextral shear. Sub- the Miocene paleomagnetic data which suggest relationships and through-going structures that tracting this amount from the total allows about 17% extension in the northernmost Basin and gross structural coherence is maintained. Most 12° rotation of the Willamette plate from its 15 Range and a 60:40 partitioning of Coast Range of the Cenozoic rotations in Oregon and Wash- Ma position (Fig. 7) to be caused by Basin- rotations between rigid-block and dextral-shear ington are contained in a region west of the High Range extension. This agrees very well with the mechanisms. Cascade Range, south of the Olympic-Wallowa 9° to 15° of rotation for the plate calculated lineament, and north of the southern half of the from post-15 Ma Basin-Range extension, and it TECTONIC RECONSTRUCTIONS Klamath Mountains (Fig. 1), which is essentially suggests that about 113 km of extension oc- the Willamette plate of Magill and others curred behind the rotating plate. This amount of As a result of our analysis, we can produce a (1982). We have used the same pole of rotation extension is less than that implied for regions to reasonably well constrained tectonic reconstruc- suggested by Magill and others (1981) and Frei the south in Nevada (Wernicke and others, tion for the Pacific Northwest at 15 Ma (Fig. 7). and others (1984) to restore a modified Willa- 1987) and may reflect the geometric necessity of We also have made reconstructions at 37 and 50 mette plate to past positions after having first northward-decreasing extension at the northern Ma by extrapolating our calculated ratio of subtracted the 40% shear component of rotation termination of the Basin and Range (Lawrence, shear rotation to extension rotation over the 50 indicated by our analysis. Small-scale shear 1976; Magill and others, 1982). No paleomag- m.y. period and by knowing that microplate ac- mechanisms cannot be shown on these recon- netically detectable regional rotation occurred in cretion was not a major contributor to post-50 structions, except in so far as they limit the the Oregon Basin and Range or the eastern Co- Ma rotations. The reconstructions are con- amount of allowable rigid-plate rotation away lumbia Plateau (Mankinen and others, 1987). In CENOZOIC TECTONIC ROTATION IN PACIFIC NORTHWEST 335 lavas of the western Columbia Plateau, east-west sulting in 200-250 km of post-37 Ma Basin- constrains back rotation of the Blue Mountains folds record about 5%-10% north-south shorten- Range extension at about latitude 42° N., or block, which has rotated 60° clockwise since the ing (Davis, 1983), and significant clockwise ro- about 33%-45% extension. Late Jurassic and (or) Early Cretaceous (Wilson tations, probably due to superimposed dextral Oroclinal bending of the Klamath Mountains and Cox, 1980; Hillhouse and others, 1982). shear, are common along the anticlinal crests and the southern Cascades at about latitude The Coast Range basement and the attached (Reidel and others, 1984). The Blue Mountains 42.5° N. is required to accommodate smaller northern Klamath Mountains block thus can be block has been rotated 5° counterclockwise to observed rotations (12°-14°) of Upper Creta- rotated 70° eastward, almost as far as Idaho if account for the north-south shortening. Post-15 ceous strata of the Great Valley sequence and one assumes a purely rigid-plate rotation. There Ma east-west fold axes in northwest Oregon and Oligocene Cascade volcanic rocks to the south, are, however, some problems with such a recon- southwest Washington may reflect rotation of which lap onto the southeast flank of the older struction. (1) Pre-Tertiary terranes overlap to the Willamette plate around its pivot point, Klamath basement (Mankinen and Irwin, 1982; some extent in eastern Oregon, implying that causing compression as the area impinged Beck and others, 1986). At the south end of the unrealistic local extensions have occurred, and against the pre-Tertiary buttress of Vancouver Cascade Range, a flexible boundary is necessary (2) the northern Klamath Mountains would lie Island. At the south end of the Coast Range, the to accommodate the difference in rotation be- east of the essentially unrotated Sierra Nevada Klamath Mountains must have also rotated between the southern Cascade Range and the block, the pre-extension position of which is cause of the onlap of the Eocene sedimentary Sierra Nevada-Great Valley block, which is constrained by the paleomagnetic data and by sequence and the Oligocene western Cascade much less rotated (6° ± 8° clockwise, Frei and the Colorado Plateau to the southeast (Wer- volcanic rocks. others, 1984; Frei, 1986). We follow the recon- nicke and others, 1982; Bogen and Schweickert, structions of Frei and others (1984) and Frei 1985; Frei, 1986). 37 Ma Reconstruction (1986) for the position of the Sierra Nevada We can solve these problems if we can sub- with respect to the southern Klamath Mountains tract a 40% dextral shear component (about 28°) Reconstructions for the close of Eocene time and the Colorado Plateau, and we agree that from the 70° rotations of the Coast Range must account for observed large clockwise rota- some of the misfit at block boundaries is proba- basement before restoring the Coast Range to its tions averaging about 50° in upper Eocene and bly the result of internal deformation within the pre-extension position. This method of recon- lower Oligocene rocks of the Oregon Coast blocks. struction aligns the northern Klamath Moun- Range, whereas correlative volcanic rocks of the tains with the unrotated Sierra Nevada and Clarno Formation in central Oregon are rotated 50 Ma Reconstruction reduces the crowding of pre-Tertiary terranes in only 16° (Gramme and others, 1986). Large ro- southeastern Oregon. It also implies that consid- tation (>10°) during docking of an Oregon Our 50 Ma reconstruction differs considera- erable Tertiary oroclinal bending and dextral Coast Range microplate, as we have seen, is not bly from the earlier models of Magill and Cox shear have occurred in outboard terranes of the supported by the geology, which requires sutur- (1980), Magill and others (1981), and Frei and northern Klamath Mountains. In this recon- ing of the Coast Range to the continent prior to others (1984) because we believe that the geo- struction, the Coast Range suture with the the bulk of the rotation. Large rigid-plate rota- logic evidence favors only small rotation during northern Klamath Mountains trends N30°W, as tion purely in response to post-37 Ma continen- docking of the Oregon Coast Range basement. do accretion-related folds in Coast Range tal extension also meets with difficulty because Onlap of the rotated (67°) turbidites of the Tyee pillow-basalt basement (Wells and others, when the Coast Range is backed up against the Formation over the suture between the folded 1985). This suggests that the early Tertiary con- Blue Mountain uplift in central Oregon, the rela- deep-ocean turbidite-pillow-basalt sequence tinental margin in Oregon also trended N30°W, tively unrotated Clarno Formation is in the way near Roseburg and the pre-Tertiary Klamath more or less on line with the relatively unrotated of a complete restoration (Wells and Coe, basement firmly ties the south end of the Coast Sierra Nevada and Great Valley regions of 1985). Heller (1983) and Gromme and others Range to the continent by about 50 Ma (Bald- California. (1986) have avoided the problem either by al- win, 1974; Simpson and Cox, 1977; Magill and Early dextral shear is suggested in the Oregon lowing the Coast Range to bend and translate others, 1981; Heller and Ryberg, 1983). Similar- Coast Range by a correlation between the trends northward around the Blue Mountain buttress ity of stratigraphy suggests that the northern end of early folds in the pillow-basalt basement and during extension or by allowing post-37 Ma of the Oregon Coast Range and the southern their tectonic rotation, although the structures ac- east-west extension within the Blue Mountain end of the Washington Coast Range have had a commodating local rotation have not been block itself. Alternatively, Wells and Coe (1985) similar history for the past 45 m.y., if not longer mapped into the monotonous sedimentary rocks have suggested that the remainder of Coast (Wells, 1985, and unpub. data). Younger rocks of the Tyee basin. Most of the Tyee Formation is Range rotation left unexplained in a rigid-plate cover the suture in southwest Washington, but only gently folded, and major faulting has not model is probably the result of dextral shear previous analysis shows that all rotation post- yet been recognized, but where detailed map- along the coast. We prefer the latter model be- dates folding of the Coast Range basement ping is available, small to moderate faults are cause subtraction of our previously estimated against the continental margin sometime prior to numerous (Snavely and others, 1976). The con- 40% shear component from the 50° average about 45 Ma (Wells and Coe, 1985). Our re- sistent relationship between north-trending pa- Coast Range rotations since 37 Ma results in a construction, then, has both the Oregon and leocurrent directions and east-west facies Willamette plate reconstruction up against the Washington Coast Range basement against the boundaries in the Tyee Formation argues Blue Mountain block that has no overlap with continental margin by about 50 Ma and farther against a great deal of local deformation accom- the Clarno Formation (Fig. 7). In this model, east than at present. The reconstruction is for a panying shear rotation but is probably compati- rigid Willamette plate rotation in response to time prior to deposition of the Clarno Forma- ble with approximately 28° that we estimate inboard extension would be about 27°-30°, re- tion, and so the small Clarno rotation no longer could have occurred. Our 50 Ma reconstruction 336 WELLS AND HELLER

lu — reconstruction is that it requires less east-west extension in the northern Basin and Range region (about 40%) and more strike-slip faulting in the western Cordillera. Another interesting point is that the model places the thick Paleocene and Eocene oceanic basalt basement of the Coast Range closer to the presumed early Tertiary lo- cation of the Yellowstone Hotspot, which if active, would have been just off Cape Mendocino in the fixed hotspot reference frame (Engebret- son and others, 1985). Most of the translation could have occurred in the early Tertiary, prior to deposition of the middle Eocene Tyee Formation on the Coast Range basement. Hundreds of kilometres of dextral slip have been accommodated on major northwest-trending faults in British Columbia (Davis and others, 1978; Ewing, 1980; Price Figure 8. Correlation between shear rotation of Coast Range determined from paleomag- and Carmichael, 1986) in Late Cretaceous and netic and geologic data and component of Farallon plate motion parallel to coast determined early Tertiary time, and in northern Washing- from plate reconstructions of Engebretson and others (1985). Deviation from linear relation- ton, dextral slip on the Straight Creek fault seg- ship at 15 Ma may reflect more efficient coupling of progressively younger subducting plate. ment must have occurred prior to emplacement of stitching plutons about 35 m.y. old (Tabor and others, 1984). Any relative motion between the Coast Range and the untranslated Cascade Range must also have occurred before the un- faulted 35-40 Ma onlap of Cascade lavas onto Effect of Northward Translation of Terranes implies that about 72%, or about 335 km of, the Coast Range. extension has occurred in the northernmost Basin and Range area since the middle Eocene Northward transport of western Cordilleran Tyee Formation was deposited. terranes such as the Salinian block (Champion and others, 1984) and parts of the Franciscan Correlation of Rotation Rate with assemblage (Harbert and others, 1984; Tarduno Plate Motions and others, 1985) has occurred during Cenozoic time. The paleomagnetic data from less suspect Our reconstructions indicate that the rate of terranes, however, also suggest a few degrees of dextral shear rotation in the Coast Ranges has poleward displacement (Beck, 1976, 1980, increased over the past 50 m.y. from 0.4° per 1984; Frei and others, 1984; Frei, 1986). Ter- million years between 50 and 37 Ma to 0.7° per ranes that may have been translated northward million years between 15 and 0 Ma. Both the geologically significant (not necessarily paleo- Kula and Farallon plates have had a significant magnetically significant) amounts include the dextral component of motion with respect to the Oregon-Washington Coast Range, the Sierra Pacific Northwest during Cenozoic time (Enge- Nevada, and possibly the Colorado Plateau bretson and others, 1985) and are the likely driv- (Beck, 1984; Frei and others, 1984; Hamilton, ing force for the shear rotations. Although the 1981; Chapin and Cather, 1981). In such a fast, highly oblique motion of the Kula plate is a broad, distended orogen, northward transport most appealing driving force, the increase in the may be accommodated across the entire orogen rate of coastal rotation postdates the demise of rather than entirely on a major, proto-San An- the Kula plate at about 43 Ma. This implies that dreas strike-slip fault. the Farallon plate has been offshore since 50 We have illustrated, as an example, a specula- EARLY Ma. In fact, there is an excellent correlation be- tive early Tertiary reconstruction that accounts TERTIARY tween the cumulative dextral shear rotation over for 100(?) km northward motion of the Colo- the past 50 m.y. and the cumulative Farallon rado Plateau; 300 km, or about half of the plate motion component calculated parallel to post-90 Ma translation possible, in the Sierra the reconstructed continental margin (Fig. 8). Nevada; and 300 km of northward transport of Figure 9. Speculative early Tertiary recon- We suggest that this confirms oblique subduc- the Oregon-Washington Coast Range basement, struction, which includes small northward tion of the Farallon plate as the driving mecha- assuming that the mean paleolatitudes are tec- transport of Coast Range basement, Sierra nism for shear rotation. tonically significant (Fig. 9). One aspect of the Nevada, and Colorado Plateau. CENOZOIC TECTONIC ROTATION IN PACIFIC NORTHWEST 337

Bates, R. B„ Beck, M. E., Jr., 8nd Burmester, R. F., 1981, Tectonic rotations in Idaho: U.S. Geological Survey Professional Paper 1272,76 p. SUMMARY the Cascade range of southern Washington: Geology, v. 9, p. 184-189. Engebretson, D. C., Cox, A., and Gordon, R. G., 1985, Relative motions Beck, M. E., Jr., 1976, Discordant paleomagnetic pole position as evidence of between oceanic and continental plates in the Pacific basin: Geological regional shear in the western Cordillera of North America: American Society of America Special Paper 206, 59 p. Our analysis of Cenozoic tectonic rotations Journal of Science, v. 276, p. 694-712. Ewing, T. E., 1980, Paleogene tectonic evolution of the Pacific Northwest: 1980, Paleomagnetic record of plate-margin tectonic processes along Journal of Geology, v. 88, p. 6019-6038. in the Pacific Northwest has resulted in an the western edge of North America: Journal of Geophysical Research, Fox, K. F., and Beck, M. 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L., 1985, Regional and tectonic significance of Eocene basalts from the Black Hills, Wash- ious structural analyses of rotation mechanisms correlations within the Frenchman Springs Member of the Columbia ington Coast Range: Geological Society of America Bulletin, v. 93, suggests that dextral shear may have contributed River Basalt Group: New insights into the middle Miocene tectonics of p. 1151-1159. northwestern Oregon: Oregon Geology, v. 47, p. 87-96. Grommfc, C. S., Beck, M. E., Jr., Wells, R. E., and Engebretson, D. C., 1986, to rotations throughout Tertiary time. Increasing Beske, S. J., Beck, M. E., Jr., and Noson, L., 1973, Paleomagnetism of the Paleomagnetism of the Tertiary Clamo Formation of central Oregon rotation toward the coast implies that significant Miocene Grotto and Snoqualmie batholiths, central Cascades, Wash- and its significance for the tectonic history of the Pacific Northwest: ington: Journal of Geophysical Research, v. 78, p. 2601-2608. 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S., 1984, Paleomagnetic and Hammond, P. E., 1979, A tectonic model for evolution of the Cascade Range, sion, then our reconstructions of the Pacific geologic data indicating 2500 km of northward displacement for the in Armentrout, J. M., Cole, M. R., and TerBest, H., eds., Cenozoic Northwest may place valuable limits on north- Salinian and related terranes, California: Journal of Geophysical Re- paleogeography of the western United States: Society of Economic search, v. 89, p. 7736-7752. Paleontologists and Mineralogists Pacific Section, Special Publication 3, ern Basin and Range extension at several times Chan, M. A., and Dott, R. H„ Jr., 1983, Shelf and deep sea sedimentation in p. 219-238. Eocene forearc basin, western Oregon—Fan or non-fan?: American Harbert, W. P., McLaughlin, R. J., and Sliter, W. V., 1984, Paleomagnetic and during the Tertiary. Our model predicts 17% Association of Petroleum Geologists Bulletin, v. 67, p. 2100-2116. tectonic interpretation of the Parkhurst Limestone, coastal belt Francis- extension in the northernmost Basin and Range Chapin, C. E., and Cather, S. M., 1981, Eocene tectonics and sedimentation in can, northern California, in Blake, M. C., Jr., ed., Franciscan geology of the Colorado Plateau-Rocky Mountain area: Arizona Geological So- northern California: Society of Economic Paleontologists and Mineral- since 15 Ma, 39% since 37 Ma, and 72% since ciety Digest, v. 14, p. 173-198. ogists, Pacific Section, v. 43, p. 175-183. Choinere, S. A., and Swanson, D. A., 1979, Magnetostratigraphy and correla- Heller, P., 1983, Sedimentary response to Eocene tectonic rotation in western 50 Ma at about latitude 42° N., if we assume no tion of Miocene basalts of the northern Oregon coast and Columbia Oregon [Ph.D. thesis]: Tucson, Arizona, University of Arizona, 321 p. northward transport of outboard blocks. 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