A Moderate Translation Alternative to the Baja British Columbia Hypothesis

Rob e r t F.Butler and George E. Ge h re l s , km. The resulting paleogeography has Luyendyk, 1979; Beck et al., 1986). This De p a r tment of Geosciences, the appealing feature of an Andean-like, history suggests that apparent conflicts Un i ve r s i t y of Ar i zo n a , Tuc s o n , AZ 85721, continuous subduction-related magmatic between paleomagnetism and geology USA ,b u t l e r @ g e o . a r i zo n a . e d u ar c along the Cordilleran margin during can lead to significant insights that Cr etaceous time. neither discipline alone could have de l i v e re d . Kenneth P.Kod a m a , De p a r tment of IN T R O D U C T I O N The Baja B.C. controversy is focused Ea r th and Envi ro n m e n t al Sciences, One may quibble over the details, but on the magnitude of post–mid- Lehigh Univer s i t y, Be t h l e h e m , PA 18015- the general picture on paleomagnetism Cr etaceous northward transport of 3 1 8 8 ,U SA is sufficiently compelling that it is much segments of the North American m o re reasonable to accept it than to Cordillera. Did southeast Alaska, d i s re g a rd it. we s t e r n British Columbia, and the North AB S T R AC T —H.H. Hess, 1962 Cascades experience northward The Baja British Columbia (Baja B.C.) It is clear that much of the North transport during the last 100 m.y., hypothesis derives primarily from American Cordillera is composed of limited to 500–1000 km, as interpret e d observed discordant paleomagnetic terranes of oceanic affinity that were fr om the geologic record of inboard di r ections of Insular and Intermo n t a n e added to and shuffled along the strike-slip faults (Price and Charmi c h a e l , terranes interpreted to indicate continental margin during Mesozoic and 1986)? Or was this region, in mid- po s t – m i d - C r etaceous northward Cenozoic time (Coney et al., 1980). Cr etaceous time, situated adjacent to the transport by up to 4000 km with res p e c t Paleomagnetism contributed to the continental margin in the position now to the continental interior. Recent realization that some terranes occupied by Baja California (Cowan et paleomagnetic results from near Prince experienced large-scale latitudinal al., 1997)? The Baja B.C. hypothesis Rupert, British Columbia, document that motion prior to or following accretion to remains controversial and was the focus discordant directions in plutonic roc k s the continental margin (Hillhouse, of a recent Penrose Conferen c e of this area are primarily due to local 1977). But Cordilleran geologists now (Mahoney et al., 2000). The primary de f o r mation rather than transport from a find themselves struggling with the Baja motivation for the Baja B.C. hypothesis Cr etaceous location along the southwest British Columbia (Baja B.C.) hypothesis, comes from discordant paleomagnetic ma r gin of North America. Expanded which many perceive as a fundamental di r ections interpreted to favor post–mid- paleomagnetic studies of Cret a c e o u s conflict between paleomagnetic data Cr etaceous northward transport up to rocks at Duke Island and MacColl Ridge, and geologic observations. We view the 4000 km (Beck, 1980; Irving et al., 1996; Alaska, have led to much smaller co n t r oversy as growing pains in Ward et al., 1997). However, the estimates of latitudinal motion for attempts to decipher the tectonic magnitudes of northward transport Insular terranes than prev i o u s l y evolution of an orogen where in t e r p r eted from paleomagnetic data in concluded. Recognition of likely supracrustal rocks have largely been Cr etaceous rocks range from ~500 km chemical remagnetization of Cret a c e o u s removed and those remaining are (V andall, 1993) to ~4000 km (Panuska, sequences in the Tyaughton and complexly deformed. The remark by 1985). So the paleomagnetic data do not Methow basins, along with effects of Hess (1962) reminds us of the apparen t pr ovide a unified and simple signal to compaction shallowing on marine conflict between paleomagnetism and in t e r p r et. In addition to northward sediments of the Nanaimo Grou p , conventional geological wisdom transport, part of the paleomagnetic indicate that paleomagnetic data from regarding continental drift (Irving, 1988). discordance must result from geologic these rocks may not req u i r e the In his celebrated article, Hess laid out de f o r mations such as faulting or folding magnitude of northward transport concepts of plate tectonics resolving that of crustal panels. But between transport suggested by the Baja B.C. ap p a r ent conflict. A more rec e n t and structural disturbance, which is the paleogeography. Instead, the example of perceived conflict between signal and which is the noise? paleomagnetic observations are paleomagnetism and geology res o l v e d In this paper, we offer ideas and consistent with a Cret a c e o u s to the benefit of both disciplines is observations that permit understanding paleogeography that limits post–mid- vertical-axis rotations of parts of the of paleomagnetic observations from Cr etaceous northward motion to ~1000 Cordilleran margin (Kamerling and Insular and Intermontane terranes (Fig. 1)

4 JUNE 2001, GSA TOD AY with much less northward transport than EXAMPLES OF TI L TING AN D suggested by the Baja B.C. model. Our FOLDING FROM THE PRINCE ap p r oach is conservative and we are RU P E R T AR E A sometimes pushing the confidence limits The Quottoon plutonic complex was on paleomagnetic observations toward emplaced into eastern parts of the Coast lower displacement estimates. The shear zone (Fig. 3), considered by some resulting Cretaceous paleogeography workers to have accommodated ~2000 avoids many of the geological conflicts km of strike-slip motion between the of the Baja B.C. hypothesis but still Insular terranes and Intermo n t a n e req u i r es ~1000 km post–mid-Cret a c e o u s terranes (Hollister and Andron i c o s , northward motion for large segments of l997). Paleomagnetic data were obtained the North American Cordillera. fr om more than 200 sites distributed fr om the Skeena River to Willard Inlet. CHANGES IN LATITUDE U-Pb crystallization ages range from 72.3 OR CHANGES IN ATT I T U D E ? to 55.5 Ma (Klepeis et al., 1998). K-Ar The majority of discordant ho r nblende dates indicate Eocene paleomagnetic results have been cooling and magnetization after the obtained from intrusive igneous roc k s pr oposed northward motion of Baja for which interpretation is complicated B.C. However, locally, the Quottoon by lack of control on paleohorizontal at plutonic complex yields paleomagnetic the time of magnetization. The general di r ections that are shallow and rot a t e d pa t t e r n of shallow inclinations and clockwise, similar to plutons within Baja clockwise-deflected declinations B.C. to the west. Eocene extension of observed in the Cretaceous plutonic the Coast Mountains tilted crustal panels rocks can be explained either by that are bounded by northwest striking Figure 1. Generalized geological map of northward transport coupled with Coast Mountains and terranes of northern east-side-down normal faults and clockwise vertical-axis rotation or by Cordillera. Prince Rupert area is outlined by northeast-striking transfer faults (Fig. 3; systematic northeast-side-up tilt during box. Adapted from Gehrels and Kapp Butler et al., 2001b). Based on a tilting uplift. A “translation versus tilt” (1998). Geologic terranes: A—Alexander; domino model, ~30% extension can co n t r oversy has ensued (Butler et al., C—Cache Creek; K—Kootenay; MP— pr oduce the 40° maximum tilts. 1989; Ague and Brandon, 1997). A Methow-Pasayten basins; NC—high grade We collected paleomagnetic sites central issue is the plausibility of rocks of North Cascades; Q—Quesnel; S— along a northeast-southwest transect plutonic rocks experiencing tilt of Stikine; SJNC—San Juan–North Cascades ac r oss the northern part of the Ecstall su f ficient magnitude and correc t thrust belt; SM—Slide Mountain; TB— pluton (Butler et al., 2000). geometry to explain the discordant Tyaughton Basin; YT—Yukon-Tanana. Ge o b a r ometry and U-Pb analyses paleomagnetic direc t i o n s . Paleomagnetic studies: DI—Duke Island; MB—Methow Basin; MR—MacColl Ridge; indicate 91 Ma crystallization at a depth A bias towards shallower inclinations 40 39 MS—Mount Stuart; NG—Nanaimo Group; of ~25 km. Ar / Ar dates on results from the steep expected direc t i o n SB—Spences Bridge; SP—Spuzzum pluton; ho r nblende from the western margi n in Cretaceous time when the SPC—Silverquick–Powell Creek. indicate cooling and magnetization at 84 paleomagnetic pole was located in Ma while hornblende from the central no r t h e r n Alaska, at its closest approa c h flattening of observed inclination part yields a cooling age of 76 Ma. to the Cordillera. The steep expected indicates ~2500 km of northward Crawford et al. (1987) interpret the di r ection results in high probability that motion while the discordant declination Prince Rupert shear zone as a west- a deflection will result in a shallower indicates 58° of clockwise vertical-axis di r ected thrust with the Ecstall pluton in rather than steeper direction (Fig. 2A). rotation (Fig. 2B). Alternatively, the the upper plate (Fig. 3). For the central Because the expected declination is expected direction can be deflected to part of the pluton, paleomagnetic aligned with the structural grain of the the observed direction by ~30° di r ections are concordant with the Insular terranes and Coast Mountains, northeast-side-up tilt of the pluton about expected Cretaceous direction while tilting of crustal panels about axes a horizontal axis with azimuth ≈330° observed directions from the western subparallel to the structural grain will (Fig. 2C). This northwest-southeast axis ma r gin are discordant by >70°. always produce shallower observed is subparallel to the structural grain of Observed directions are distributed in c l i n a t i o n s . the Coast Mountains and consistent with along a small circle with subhorizontal End-member translation and tilt st r u c t u r es that could tilt panels of crust. axis at ~340° azimuth. These direc t i o n s in t e r p r etations of the discordant Ba r ometry of metamorphic assemblages cannot record northward flight of Baja paleomagnetic direction from the su r r ounding the Spuzzum pluton B.C. because this would req u i r e ~7000 Spuzzum pluton of southern British indicates that this panel of crust tilted km of transport in ~8 m.y. at a velocity Columbia are illustrated in Figure 2. On northeast-side-up by 33° about an axis of ~1 m/yr! Our tectonic interpret a t i o n the assumption that present horizontal with azimuth of 332° (Brown and of the paleomagnetic, geochron o l o g i c , ap p r oximates paleohorizontal, the 17° Bu rm e s t e r , 1991). and barometric data from the Ecstall

GSA TOD AY, JUNE 2001 5 Figure 2. A: Equal-area projection showing expected Cretaceous direction at location of Spuzzum pluton in south- ern British Columbia. Directions with inclina- tions shallower than expected direction occur within blue area; direc- tions with inclinations steeper than expected direction occur within red area. Arrows indicate deflections from expected direction resulting from 20° tilt of a pluton in directions indicated. Only a southeast-side-up tilt can yield an inclination steeper than expected direction; all other tilt directions yield shallower inclinations. B: Translation plus rotation interpretation of observed (red square) and expected (green circle) paleomagnetic directions for Spuzzum pluton of southern British Columbia (Irving et al., 1985). Observed inclination is 17° shallower than expected inclination and is interpreted to indicate ~2500 km of northward translation from a Cretaceous location adjacent to southwestern North America. Observed declination is clockwise of the expected declination and is interpreted to indicate 58° of clockwise vertical-axis rotation. C: Tilt interpreta- tion showing deflection of expected direction (green circle) to observed direction (red square) by 30° northeast-side-up tilt of Spuzzum plu- ton about a horizontal axis with azimuth ≈ 330°. pluton involves thrust displacement above the upward convex fold test and indicate that cumulate layering was highly Prince Rupert shear zone (Fig. 3). We conclude that local contorted at the time of magnetization. Observations of folding with ~1000 km of northward transport produced the cumulate layering in other ultramafic intrusions indicate that paleomagnetic discordance in the Ecstall pluton. These layering can depart from horizontal by 10°–20°, even on the examples show that local folding and tilting are the dominant kilometer scale (Irvine et al., 1998). We conclude that use of cause of discordant paleomagnetic directions at least in this such cumulate layering as a proxy for paleohorizontal is not part of the Insular terranes and Coast orog e n . justified and paleomagnetic data from Duke Island cannot be used to infer a Cretaceous paleolatitude for the Insular THE MODERATE TR A N S L A TION ALT E R N ATI V E te r r a n e s . A full synthesis of paleomagnetic data relevant to the Baja Based on paleomagnetic data from only 20 specimens of B.C. controversy is beyond the scope of this paper. sedimentary rock from the Upper Cretaceous MacColl Ridge Nevertheless, we provide alternative interpretations of key Fo r mation in the southern Alaska fragment of the Wra n g e l l i a paleomagnetic studies (Fig. 1) considered by proponents of terrane (Fig. 1), Panuska (1985) reported a paleolatitude of the Baja B.C. paleogeography to req u i r e ~3000 km post–mid- 32°N. This result suggested a dramatic 4000 km of post- Cr etaceous northward transport (Irving et al., 1996; Cowan et Cr etaceous northward transport. However, Trop et al. (1999) al., 1997). Our alternative interpretation is consistent with the resampled the MacColl Ridge Formation, concentrating on suggestion that the combined effects of ~1000 km northward volcanic tuff layers, and arrived at a much-reduced estimate of translation with tilting of crustal panels about axes parallel to displacement. The data from 15 sites (129 samples) yield an the Coast orogen and compaction shallowing of in f e r r ed Late Cretaceous paleolatitude of 53°N ± 8°, indicating paleomagnetic inclination in sedimentary rocks can explain 1650 ± 1100 km of northward displacement. the paleomagnetic observations. Note that we do not rej e c t The paleomagnetic results from the Cenomanian to any paleomagnetic data. Instead, we offer alterna t i v e Campanian Silverquick Conglomerate and the Powell Cree k in t e r p r etations of these important observations. volcanic rocks in the Tyaughton Basin were interpreted by Bogue et al. (1995) reported a paleolatitude based on Wynne et al. (1995) to req u i r e 3000 km of northward paleomagnetic results from the mid-Cretaceous layered translation. The conflict of this interpretation with geological ultramafic intrusion of Duke Island in southeast Alaska. They co r r elations and estimates of offset between the Methow and ar gued that unplunging of fold axes followed by unfolding Tyaughton basins has been described by Monger and Price could res t o r e cumulate layering and paleomagnetic vectors to (1996). In the Silverquick–Powell Creek sequence, there is paleohorizontal. We recently obtained paleomagnetic evidence for authigenic magnetite in the sedimentary roc k s , di r ections from sites where the attitude of cumulate layering suggesting the magnetization could be of secondary chemical was directly measured (Butler et al., 2001a). These data fail a origin. Indeed, the maximum clustering of paleomagnetic

6 JUNE 2001, GSA TOD AY di r ections occurs at 70% unfolding. horizontal during fabric development Cr etaceous volcanic rocks could never Fu r t h e r , considering the evidence for accompanying compaction, thus co n s p i r e to produce the shallow remagnetization of equivalent rocks in shallowing the inclination (Sun and observed paleomagnetic inclinations the Methow Basin (Bazard et al., 1990), Kodama, 1992). The rem a n e n c e fr om the Insular terranes and Coast we conclude that the Silverqu i c k – P o w e l l an i s o t r opy of paleomagnetic samples Mountains. If these processes resulted in Cr eek strata contain a synfolding can be used to correct for inclination both steeper and shallower inclinations remagnetization which cannot be used shallowing caused by compaction with equal probabilities, we would to infer its motion history. Recently, (Kodama, 1997). Kim and Kodama ag r ee with their assessment. But Haskin et al. (2000) obtained (1999) showed that Nanaimo roc k s , compaction of sedimentary rocks will paleomagnetic data from 104 Ma including some of the calcite shallow paleomagnetic inclinations, volcanic rocks that unconforma b l y co n c r etions sampled for paleomag- tilting of the plutonic rocks stron g l y underlie the Silverquick Conglomerate netism, have a magnetic fabric favors shallowed inclinations, and the and Powell Creek volcanics. The characteristic of burial compaction, same bias affects rocks that experience mean paleomagnetic direction is in- indicating ~15° of inclination tilting or folding subsequent to distinguishable from that of the shallowing. Correcting for the effects of rem a g n e t i z a t i o n . co r r elative Spences Bridge Grou p compaction reduces the latitudinal It is worth recalling that early (Irving et al., 1995), and the combined displacement by 1500 km indicating that conclusions of 2500 km northward results provide a strong indication that the Nanaimo rocks were slightly south transport for coastal and Baja Californi a this assemblage was 1100 ± 600 km of the Intermontane terranes in Late we r e based on paleomagnetic data from south at 104 Ma. Cr etaceous time. a half dozen Mesozoic and Cenozoic The paleomagnetic direction from the The above discussion pres e n t e d fo r mations. When about 20 studies had Mount Stuart batholith is discordant, viable alternatives to the Baja B.C. been published, the implied motion again with shallow inclination and paleogeography for interpreting the histories became complex with interna l clockwise declination compared with paleomagnetic observations. In some inconsistencies begging alterna t i v e the expected Cretaceous direction (Beck cases, these “tilt plus moderate explanation. Dickinson and Butler et al., 1981). Butler et al. (1989) argu e d translation” interpretations are well (1998) concluded that Neogene offs e t that southwest dipping sedimentary supported by the geology. In other on the San Andreas fault system is rocks of the Swauk Formation adjacent cases, this explanation is speculative su f ficient to account for the paleo- to the southwestern part of the batholith and req u i r es further study. Housen and magnetic data, when compaction req u i r e post–early Eocene northeast- Beck (1999) argued that tilting of shallowing in sedimentary rocks and side-up tilt which largely explains the Cr etaceous plutonic rocks, compaction tilting of plutonic rocks are taken into discordant paleomagnetism. However, of Cretaceous sedimentary rocks, and account. Might history repeat in the case that interpretation was challenged by synfolding remagnetization of of the Baja B.C. hypothesis once Miller et al. (1990). Ague and Brandon (1996) applied aluminum-in- ho r nblende barometry to infer a tilt of 8° northwest-side-up. Anderson (1997) criticized the pres s u r e estimates of Ague and Brandon (1996), concluding that their pres s u r e estimates are not reliable. That conclusion was, in turn, refuted by Ague and Brandon (1997). Pe r haps the most secure conclusion to reach about the Mount Stuart batholith is that the postmagnetization history of de f o r mation is disputed and complex. Paleomagnetic data from marine sedimentary rocks of the Upper Cr etaceous Nanaimo Group have been in t e r p r eted by Ward et al. (1997) to support the Baja B.C. model. However, the clay content of the Nanaimo sedimentary rocks likely made them Figure 3. Summary of paleomagnetic observations of tilting and folding of plutonic rocks in Prince Rupert area. Diagram at upper right shows along-strike variation in amount of susceptible to inclination shallowing by northeast-side-up tilt that affected Quottoon plutonic complex. Illustration at right is tectonic burial compaction. Laboratory model for tilting of crustal panels containing Quottoon plutonic complex during Eocene experiments with synthetic and natural extension. Folding model for Ecstall pluton is shown by block diagram at left with arrows clay-rich sediments indicate that detrital indicating paleomagnetic directions. Ecstall pluton is shown above west vergent and convex Fe-oxide particles become attached to upward Prince Rupert shear zone. View is toward north. Geologic map is from Hutchison clay particles and rotate toward the (1982) while geologic structures are adapted from Crawford et al. (1987).

GSA TOD AY, JUNE 2001 7 additional paleomagnetic data and a often appealed to the southern option supporting array of geologic for the Kula-Farallon ridge (Engebret s o n observations are acquired ? et al., 1985) so that Baja B.C. could be moved northward on the Kula plate. TOWARDS A MID-CRETACE O U S Instead, the northeast direction of PAL E O G E O G R A P H Y fo r eland shortening south of 49°N Re i n t e r p r etation of paleomagnetic between 75 and 50 Ma and the contrast data from Insular and Intermo n t a n e with Rocky Mountain deformation closer terranes allows for the construction of to the continental margin farther north is a simplified Late Cretaceous paleo- readily understood using the simpler geographic map of western North no r t h e r n option for the Kula-Farallon America that is consistent with both ridge. Indeed, the northern option is geologic relations and with conservative st r ongly favored by seismic tomography estimates of displacement based on of the subducted Kula-Farallon plate paleomagnetism (Fig. 4). The likely boundary, which is imaged under the positions of Cordilleran terranes, mag- Gr eat Lakes region (Bunge and Grand, matic belts, and first-order strike-slip 20 0 0 ) . fault systems at ~90 Ma differ signi- ficantly from the “Northern Option” CO N C L U S I O N S reconstruction of Cowan et al. (1997). The Baja B.C. controversy is primarily We incorporate ~1000 km of northward the result of sparse and piecewise translation of Insular terranes and ~800 application of paleomagnetism and km of northward motion of Inter- supporting geologic and geochemical montane terranes, most of which oc- methods to an orogen which experi- cu r r ed during Late Cretaceous throu g h Figure 4. Generalized paleogeography of enced complex deformation and from western North America at ~90 Ma, based on Eocene time. These transport values are which supracrustal rocks have large l y geologic relations and the reinterpreted generally at the low end of the trans- paleomagnetic data (adapted from Oldow been removed. The above analysis lation estimates based on paleomagnetic et al., 1989; Burchfiel et al., 1992; Plafker suggests that discordant paleomagnetic studies from the Insular and Inter- and Berg 1994; and Nokleberg et al., 1998). di r ections from the Cordilleran margi n montane terranes. An appealing aspect Restorations include 325 km of Basin and of North America are understandable of this reconstruction is the existence of Range province extension, 500 km north- within a paleogeography which limits a continuous, subduction-related mag- ward motion of Salinia, 300 km opening and po s t – m i d - C r etaceous northward motion matic arc along the Cordilleran margi n . offset in Gulf of California, 800 km offset on to ~1000 km. A necessary condition for The inboard strike-slip fault (Fig. 4) is Tintina–Rocky Mountain Trench fault system embracing this smaller amount of in t e r p r eted to include a broad zone of and possible continuations to the south, and latitudinal motion, as opposed to the st r u c t u r es related to the Tin t i n a – R o c k y 200 km offset on Coast shear zone and ~3000 km displacements of the Baja related structures and their possible contin- Mountain trench system in the northern uations. Known and inferred trace of main B.C. hypothesis, is recognition that Cordillera, and their cryptic contin- locus of subduction-related magmatism is crustal panels of the Insular terranes and uations into southern British Columbia represented by red “volcanoes.” Coast orogen have experienced tilting or and northern Washington (Gabrielse, folding around axes subparallel to the 1985). The fault shown between Insular ar e more easily understood. Dickinson continental margin. Dense and Intermontane terranes is interpret e d and Snyder (1978) suggested that onset paleomagnetic sampling along with to have ~200 km of dextral slip. of flat subduction of the Farallon plate U-Pb geochronology, 40 Ar / 39 Ar Ex p r essions of this structure would under North America could explain the th e rm o c h r onology, geobarom e t r i c include the Denali fault in Alaska (north inland migration of magmatism and analyses, and structural geology have of the intersection with the Chatham fo r eland deformation during the established that such tilting and folding Strait fault), the Coast shear zone and Laramide orogeny which commenced at has affected the margin near Prince related structures within the Coast 75 Ma. Geodynamic models confirm that Rupert, British Columbia. The utility of Mountains (Lanphere, 1978; Hollister basal traction from flat slab subduction paleomagnetism in deciphering the and Andronicos, 1997) and the is req u i r ed to account for forel a n d tectonics of midcrustal rocks in Yalakom, Harrison Lake, and rel a t e d de f o r mation far inboard from the extensional settings has been faults in southern British Columbia and continental margin (Bird, 1998). Models documented in the Omineca belt of no r t h e r n Washington (Umhoefer and depending on interactions at the sub- southeast British Columbia (Win g a t e Schiarizza, 1996). duction margin, such as the hit-and-run and Irving, 1994) and in the Basin and If we are relieved of the necessity to collision of Baja B.C. with the south- Range (Livaccari et al., 1995). These move Baja B.C. 3000 km north during we s t e r n margin (Maxson and Tik o ff , successful applications are traceable in the Late Cretaceous and Paleogene, 1996), cannot transmit compres s i o n a l part to tighter constraints on inter- some aspects of plate kinematics and fo r ces sufficiently far inboard. pr etation because large transport of North American continental dynamics Pr oponents of the Baja B.C. model have these areas is not an option. We expect

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Panuska, B.C., 1985, Paleomagnetic evidence for a post-Cretaceous accretion of Wrangellia: Geology, v. 13, p. 880–883. Plafker, G., and Berg, H.C., 1994, Overview of the geology and tectonic evolution of Alaska, in Plafker, G., and Berg, H.C., eds., The Geology of Alaska: Geology of North America: Boulder, Colorado, USA, Geological Society of America, p. 989–1022. Price, R.A., and Charmichael, D.M., 1986, Geometric test for Late Cretaceous– Paleocene intracontinental transform faulting in the Canadian Cordillera: Geology, v. 14, p. 468–471. Sun, W.W., and Kodama, K.P., 1992, Magnetic anisotropy, scanning electron microscopy, and X-ray pole figure goniometry study of inclination shallowing in a compacting clay-rich sediment: Journal of Geophysical Research, v. 97, p. 19,599–19,615. Trop, J.M., Stamatakos, J.A., and Ridgway, K.D., 1999, Determining the Late Cretaceous paleolatitude and accretion history of the allochthonous Wrangellia Terrane: Paleomagnetism, geochronology, and tectonic analysis of the MacColl Ridge Formation, southern Alaska: Eos (Transactions, American Geophysical Union), v. 80, p. F948. Umhoefer, P.J., and Schiarizza, P., 1996, Latest Cretaceous to early Tertiary strike-slip faulting on the southeastern Yalakom fault system, southeastern Coast Belt, British Columbia: Geological Society of America Bulletin, v. 108, p. 768–785. Vandall, T.A., 1993, Cretaceous Coast Belt paleomagnetic data from the Spetch Creek pluton, B.C.: Evidence for the “tilt and moderate displacement” model: Canadian Journal of Earth Science, v. 30, p. 1037–1048. Ward, P.D., Hurtado, J.M., Kirschvink, J.L., and Verosub, K.L., 1997, Measurements of the Cretaceous paleolatitude of Vancouver Island: Consistent with the Baja–British Columbia hypothesis: Science, v. 277, p. 1642–1645. Wingate, M.T.D., and Irving, E., 1994, Extension in high-grade terranes of the southern Omineca Belt, British Columbia: Evidence from paleomagnetism: Tectonics, v. 13, p. 686–711. Wynne, P.J., Irving, E., Maxson, J.A., and Kleinspehn, K.L., 1995, Paleomagnetism of the Upper Cretaceous strata of Mount Tatlow: Evidence for 3000 km of northward displacement of the eastern Coast Belt, British Columbia: Journal of Geophysical Research, v. 100, p. 6073–6091. Manuscript received March 7, 2001; accepted April 9, 2001. ▲

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