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.