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Baja British Columbia Hypothesis 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.
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