Jurassic Apparent Polar Wander Research Articles

Jurassic fast polar shift rejected by a new high-quality paleomagnetic pole from southwest Greenland

A selective compilation of paleomagnetic data from North America indicates that a vast amount of rapid polar motion occurred in Late Jurassic time. The over 30° polar shift that accumulated during a relatively short time interval (~160–145 Ma) suggests an episode of fast true polar wander (TPW) and was referred to as the Jurassic “monster polar shift” by some workers. However, this rapid TPW event is not supported by paleomagnetic data on a global scale. Here, we scrutinize the Jurassic apparent polar wander path (APWP) by virtue of a new paleomagnetic and 40Ar/39Ar geochronology study of Mesozoic coast-parallel dykes exposed in southwest Greenland. Combined with existing geochronological data, our results show that the dykes were emplaced during a prolonged period centered at 147.6 ± 3.4 Ma (2σ). A primary nature of the characteristic remanent magnetization is supported by multiple positive baked contact tests and a reversal test. The paleomagnetic pole calculated from 40 site-mean paleomagnetic directions is located at Plat = 69.3°S, Plong = 5.0°E (A95 = 4.6°), or at Plat = 73.9°S and Plong = 0.4°E when reconstructed to North America. Our new high-quality paleomagnetic pole and an updated global APWP do not support the fast Jurassic polar shift but instead indicate steady polar motion with moderate rates of about 0.7°/Myr. The new pole effectively reduces the mismatch between the APWPs for Laurentia and Europe. Our critical reassessment of the monster polar shift indicates that it may be an artifact of paleomagnetic and geochronological data that were previously used to argue for its existence.

Tracking the Late Jurassic apparent (or true) polar shift in U‐Pb‐dated kimberlites from cratonic North America (Superior Province of Canada)

AbstractDifferent versions of a composite apparent polar wander (APW) path of variably selected global poles assembled and averaged in North American coordinates using plate reconstructions show either a smooth progression or a large (∼30°) gap in mean paleopoles in the Late Jurassic, between about 160 and 145 Ma. In an effort to further examine this issue, we sampled accessible outcrops/subcrops of kimberlites associated with high‐precision U‐Pb perovskite ages in the Timiskaming area of Ontario, Canada. The 154.9 ± 1.1 Ma Peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5° of the reverse polarity magnetization of the 157.5 ± 1.2 Ma Triple B kimberlite. The combined ∼156 Ma Triple B and Peddie pole (75.5°N, 189.5°E, A95 = 2.8°) lies about midway between igneous poles from North America nearest in age (169 Ma Moat volcanics and the 146 Ma Ithaca kimberlites), showing that the polar motion was at a relatively steady yet rapid (∼1.5°/Myr) pace. A similar large rapid polar swing has been recognized in the Middle to Late Jurassic APW path for Adria‐Africa and Iran‐Eurasia, suggesting a major mass redistribution. One possibility is that slab breakoff and subduction reversal along the western margin of the Americas triggered an episode of true polar wander.

The role of true polar wander on the Jurassic palaeoclimate

From the Late Carboniferous until the Middle Jurassic, continents were assembled in a quasi-rigid supercontinent called Pangea. The first palaeomagnetic data of South America indicated that the continent remained stationary in similar present-day latitudes during most of the Mesozoic and even the Palaeozoic. However, new palaeomagnetic data suggest that such a scenario is not likely, at least for the Jurassic. In order to test the stationary versus the dynamic-continent model, we studied the Jurassic apparent polar wander paths of the major continents, that is, Eurasia, Africa and North America that all in all show the same shape and chronology of the tracks with respect to those from South America. We thus present a master path that could be useful for the Jurassic Pangea. One of the most remarkable features observed in the path is the change in pole positions at ~197 Ma (Early Jurassic), which denotes the cessation of the counter-clockwise rotation of Pangea and commencement of a clockwise rotation that brought about changes in palaeolatitude and orientation until the end of the Early Jurassic (185 Ma). Here, we analyse a number of phenomena that could have triggered the polar shift between 197 and 185 Ma and conclude that true polar wander is the most likely. In order to do this, we used Morgan’s (Tectonophysics 94:123–139, 1983) grid of hotspots and performed “absolute” palaeogeographical reconstructions of Pangea for the Late Triassic and Jurassic. The palaeolatitudes changes that we observe from our palaeomagnetic data are very well sustained by diverse palaeoclimatic proxies derived from geological and palaeoecological data at this time of both the southern and northern hemispheres.

Tectonic rotations in the Deseado Massif, southern Patagonia, during the breakup of Western Gondwana

Paleomagnetic investigation in the Deseado Massif, southern Patagonia, suggests that Triassic sedimentary rocks carry a latest Triassic to Jurassic remagnetization and that earliest Jurassic plutonic complexes carry a reversed polarity magnetization of thermoremanent origin. Despite uncertainties in the timing of the observed remanence in the Triassic rocks and the lack of paleohorizontal control on the plutonic complexes, comparison of the derived pole positions with the most reliable Late Triassic–Jurassic apparent polar wander paths indicates that the study areas underwent significant clockwise vertical-axis rotation. In contrast, paleomagnetic results from mid-Cretaceous rocks in the region indicate no rotation. The observed crustal rotations in the Deseado Massif are thus bracketed to have occurred between Jurassic and Early Cretaceous times, documenting southern Patagonian deformation during the breakup of Western Gondwana and then enlarging the regional record of clockwise rotations associated with this event. These results suggest a more complex than previously supposed tectonic evolution of this part of South America.

Reply to “A comment on early Jurassic palaeomagnetic study of lower Jurassic marine strata from the Neuquén basin, Argentina: A new Jurassic apparent polar wander path for South America”

Fil: Iglesia Llanos, Maria Paula. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; Argentina

Palaeomagnetic study of Lower Jurassic marine strata from the Neuquén Basin, Argentina: A new Jurassic apparent polar wander path for South America

A thorough palaeomagnetic study in four marine sedimentary sections from the Neuquén Basin in west-central Argentina was carried out. Sections are several hundreds meters-thick and consist of ammonite-bearing beds with intercalated volcanics. Two magnetic components carried by titanomagnetites were identified in the studied sections, one soft corresponding most likely to a present-day remagnetisation and a harder one which is interpreted as the primary magnetisation of the sections, based on optical studies and the results of field tests of palaeomagnetic stability, bearing an Early Jurassic age. As a result, two new palaeomagnetic poles for stable South America are presented: one for the Hettangian–Sinemurian located at 223° E, 51° S, A 95 = 6°, N = 25, and the other for the Pliensbachian–Toarcian located at 67° E, 74° S, A 95 = 5°, N = 52. These and other poles from the literature were used in this study to construct a refined Late Triassic to Jurassic APW path for stable South America, which differs significantly from previous models in showing a cusp between the Sinemurian and the Pliensbachian, indicative of large apparent polar wander. The same feature is observed in other continents of Pangea, like Eurasia. Palaeolatitudes of the Neuquén Basin indicate that South America was subject to considerable N–S movements during the Late Triassic and lowermost Early Jurassic. These latitudinal movements of Pangea are consistent with displacements recorded for marine faunas from South America and Eurasia.

Magnetostratigraphy and biostratigraphy of the Upper Triassic and lowermost Jurassic succession, St. Audrie's Bay, UK

The St. Audrie's Bay section in west Somerset comprises the uppermost Mercia Mudstone Group, the Penarth Group and the basal Lias Group and includes a candidate Global Stratotype Section and Point for the base of the Jurassic. The magnetostratigraphy has been evaluated through 122 m of this section at 147 stratigraphic levels, which range in age from mid-Norian to earliest Hettangian. In red dolomitic mudstones, the remanence is carried predominantly by haematite, whereas in non-red lithologies, it is mostly carried by magnetite. The mean virtual geomagnetic poles fall near the mean Upper Triassic and Lower Jurassic apparent polar wander track and display the start of the northeast-directed track typical of the Jurassic. The magnetostratigraphy comprises nine major magnetozones, five of normal polarity and four reversed, together with several minor magnetozones. In the Mercia Mudstone Group, the 68 m of the Twyning Mudstone Formation examined includes three major normal magnetozones (SA2n, SA3n and SA4n) and the Blue Anchor Formation has predominantly reversed polarity (SA4r) except at its top, in the Williton Member. The Penarth Group and basal Lias Group have predominantly normal polarity (SA5 to SA6n) but short reversals occur within the Westbury Formation and at the base of the Lilstock Formation and the Lias Group (SA5r). This magnetostratigraphy is a good match with that found in the upper part of the Newark Supergroup succession in the eastern USA. The distinctive long reversal (SA4r) in the Blue Anchor Formation is equivalent to Newark Supergroup magnetozone interval E18r to E20r. Magnetozone SA5n, located mainly in the Penarth Group, probably equates with part of E22n and all of E23n in the Newark Supergroup. The reversed magnetozone, SA5r, at the base of the Lias Group, may correspond either with E23r in the Exeter Member (Passaic Formation) in the Newark Supergroup or with an undetected reverse polarity interval within the Newark Basin flood basalts. A change in the composition and diversity of terrestrial microfloras that occurs in the upper part of the Penarth Group at St. Audrie's Bay and elsewhere in the UK, is similar to that interpreted as marking the Triassic–Jurassic boundary in the Newark Supergroup. At St. Audrie's Bay, this change occurs c. 0.6 m below SA5r, within the Rhaetian, whereas in the Newark Supergroup, it occurs c. 20 m above the potentially equivalent E23r. Reconciliation of these disparities requires that either the microfloral changes are not synchronous between these locations or the change in the Newark Supergroup is time-equivalent with late Rhaetian conodont bearing strata. The correlation of marine and nonmarine Upper Triassic magnetostratigraphies is revaluated with the new data from St. Audrie's Bay, indicating the Twyning Mudstone and Blue Anchor formations are mid to late Alaunian (mid-Norian) in age. The Sevatian (late Norian) is represented by the Williton Member and the lower part of the Westbury Formation, but is incomplete because of disconformities at the base of the Williton Member and Penarth Group.

Remagnetization of Jurassic volcanic rocks in the Santa Rita and Patagonia Mountains, Arizona: Implications for North American apparent polar wander

Paleomagnetic poles for the Jurassic Corral Canyon sequence and Glance Conglomerate in southern Arizona have been used to construct apparent polar wander (APW) paths for the North American plate, but they are controversial and conflict with higher‐latitude poles from New England. Lower Jurassic dacites and ash flow tuffs of the Mount Wrightson Formation in the Santa Rita Mountains were initially sampled to provide an additional paleopole for southern Arizona. These rocks, however, have a predominantly reversed‐polarity characteristic magnetization (in situ, I = −47°, D = 154°, α95 = 9°) which is statistically indistinguishable from that for the nearby latest Cretaceous Elephant Head pluton (I = −48°, D = 165°, α95 = 8°). Although magnetizations of both polarities are observed in the ash flow tuffs, they are mostly carried by hematite, and dual polarity components are observed within some specimens. Moreover, widespread mineralization and a K‐Ar age of ∼67 Ma for altered rocks of the Mount Wrightson Formation imply that these rocks were subjected to a prolonged episode (greater than one polarity interval) of low‐temperature alteration and remagnetization. Middle Jurassic ash flow tuffs of the Corral Canyon sequence in the Patagonia Mountains to the south have a similar geologic setting: They are exposed along strike amid mineralized rocks on the eastern flank of a mountain range cored by Paleocene intrusions. Hematite is also the dominant remanence carrier in most of the Corral Canyon sequence, and its predominantly normal‐polarity direction (in situ, I = 51°, D = 326°, α95 = 9°) is indistinguishable from that for the nearby Patagonia Granodiorite (I = 49°, D = 342°, α95 = 8°). Rocks of the Corral Canyon sequence therefore are likely remagnetized as well. Problems also exist with the Glance Conglomerate pole. These rocks are situated within a caldera structure and have been potassium metasomatized. This potassic alteration could have occurred shortly after emplacement or at a later time, postdeformation. The low‐latitude Jurassic APW path for North America and J‐2 cusp therefore are not well supported and may need revision.

Paleomagnetism of the Brushy Basin Member of the Morrison Formation: Implications for Jurassic apparent polar wander

The paleomagnetism of the ∼147 Ma (Tithonian) Brushy Basin Member of the Morrison Formation was analyzed to obtain a Late Jurassic paleomagnetic pole for North America. A total of 200 samples were collected from 25 sedimentary horizons (sites) at Norwood Hill in southwest Colorado. At Montezuma Creek in southeast Utah, 184 samples were collected from 26 sites. Detailed thermal demagnetization (up to nine temperature steps between 600°C and 680°C) and principal component analysis were required to confidently isolate characteristic remanent magnetization (ChRM) directions carried by hematite. Demagnetization behavior for many horizons is erratic and does not allow isolation of a high unblocking‐temperature ChRM. Data selection criteria required sample ChRM directions to be defined by three or more thennal demagnetization steps and maximum angular deviations of sample ChRM directions to be ≤20°. Eight sites from the Norwood Hill location and 10 sites from the Montezuma Creek location passed these criteria. The 18 site‐mean virtual geomagnetic poles yield a paleomagnetic pole position from the Brushy Basin Member of 68.3°N, 156.2°E (A95 = 4.8°, K = 53). This pole position is within 2° of the paleomagnetic pole which Steiner and Helsley (1975a) reported for the “upper” Morrison Formation at Norwood Hill, Colorado. A second paleomagnetic pole was calculated after excluding sites with site‐mean α95 > 20° and sites with fewer than three samples that passed the above selection criteria. This additional editing did not significantly change the paleomagnetic pole position at the 95% confidence level. Along with other paleomagnetic poles from the continental interior the paleomagnetic data from the Brushy Basin Member of the Morrison Formation are interpreted to indicate that the Late Jurassic part of the North American apparent polar wander path progresses from a late Middle Jurassic (∼160 Ma) position at ∼60°N, 135°E toward the mid‐Cretaceous pole position at 72°N, 191°E.

North American Jurassic apparent polar wander: the anwser from other continents?

Jurassic paleomagnetic data from North America, which come essentially from the NE and SW USA, outline two contradictory apparent polar wander paths (APWPs). Following Besse and Courtillot (J. Geophys. Res., 96: 4029–4050, 1991), we derive synthetic APWPs for North America based on data from other Atlantic-bordering continents without resorting to the North American data themselves. We discuss variations in the data bases and in the kinematics used for the reconstructions, and find that a set of some 12 predicted APWPs is well constrained to lie within a band of 5° half-width along 75°N latitude. The main uncertainties occur along the track (i.e. longitude) but not perendicular to it (i.e. latitude). We conclude that, as has been suggested before, most of the Jurassic North American data show the effect of either partial remagnetization in a recent field or tectonic rotation, and that, until more reliable data become available, the synthetic transferred paths from other Atlantic-bordering continents offer more reliable estimates of Jurassic North American paleogeography and polar wander.

New early Jurassic paleopole from France and Jurassic apparent polar wander

A new early Jurassic paleopole for stable Europe obtained from a doleritic dyke system in Brittany, France is given at latitude 61°N and longitude 79°E ( K = 36 and A 95 = 10.2° ) based on seven equivalent sites and more than a hundred samples demagnetized by alternating field and thermal treatments ( D = 41°, I = 62°, k = 66, α 95 = 7.5° ). Radiometric datings provide ages between 190 and 200 Ma. This result represents the first indication obtained from European igneous data for a very westerly cusp in the Apparent Polar Wander Path around 200 Ma. The European data also tend to support the hypothesis of a higher latitude track for the Jurassic APWP as already proposed from North American data.

North American Jurassic APW: The current dilemma

Geologists are in a quandary over the correct interpretation of paleomagnetic data for Jurassic rocks of the North American plate. Conflicting reference paleopoles and alternate methods of constructing apparent polar wander (APW) paths have led to a controversy regarding the configuration of Jurassic APW for cratonic North America. These differences have been recently disputed at meetings (see Eos, Spring Meeting Supplement, April 7, 1992, p. 94) and in an exchange of letters in the Journal of Geophysical Research‐Solid Earth [Butler et al., 1992; Van Fossen and Kent, 1992a]. At stake is important information concerning the Jurassic paleogeography of North America and the whole of Pangea, as well as the nature and driving mechanism of plate movements. In addition, cratonic paleopoles provide a reference frame for measuring relative displacements of tectonostratigraphic terranes.

Slow apparent polar wander for North America in the Late Triassic and large Colorado Plateau rotation

Several recent analyses of North American paleomagnetic data suggest fast apparent polar wander (APW) (∼0.75°/m.y.) during the Late Triassic and a modest amount (∼5°) of Colorado Plateau clockwise rotation. Paleomagnetic poles from the lower (Carnian), middle (Norian), and upper (Hettangian) stratigraphic intervals of the Newark Basin, however, indicate very slow APW over the Late Triassic and provide an alternative interpretation for plateau rotation. The middle Newark pole is supported by positive fold and reversal tests, precluding remagnetization, and agrees well with the pole reported from the Norian Upper Shale Member of the Chinle Formation from east central New Mexico as well as the 214 Ma Manicouagan pole from Quebec. These poles provide a well‐defined mean Norian reference pole for cratonic North America at 57.4°N 91.0°E A95=3.8°. Paleomagnetic poles from the Chinle Formation on the Colorado Plateau (Owl Rock Member, Church Rock Member, and our new result from the upper Chinle in Utah) are also well grouped, consistent with slow APW over the Norian, but give a mean pole position (57.7°N 65.6°E A95=2.5°) that differs significantly from the Norian pole for cratonic North America. The North American Norian poles can be closely reconciled by a 13.5° ± 3.5° correction for accumulated post‐Triassic clockwise rotation of the Colorado Plateau associated with Laramide deformation and Cenozoic opening of the Rio Grande Rift. This estimate of Colorado Plateau rotation is consistent with a systematic discrepancy between plateau and cratonic poles for the Early Triassic, whereas available late Paleozoic and Jurassic poles are judged not to provide definitive constraints on plateau rotation. A revised Triassic and Early Jurassic APW path for North America shows that the virtual standstill in the Norian, the last 15 m.y. of the Triassic, was preceded and followed by intervals of fast (∼1°/m.y.) angular plate velocity.

Comment [on “High‐latitude paleomagnetic poles from Middle Jurassic plutons and moat volcanics in New England and the controversy regarding Jurassic apparent polar wander for North America” by Mickey C. Van Fossen and Dennis V. Kent

Journal of Geophysical Research: Solid EarthVolume 97, Issue B2 p. 1801-1802 CommentariesFree Access Comment [on “High-latitude paleomagnetic poles from Middle Jurassic plutons and moat volcanics in New England and the controversy regarding Jurassic apparent polar wander for North America” by Mickey C. Van Fossen and Dennis V. Kent] Robert F. Butler, Robert F. ButlerSearch for more papers by this authorSteven R. May, Steven R. MaySearch for more papers by this authorDavid R. Bazard, David R. BazardSearch for more papers by this author Robert F. Butler, Robert F. ButlerSearch for more papers by this authorSteven R. May, Steven R. MaySearch for more papers by this authorDavid R. Bazard, David R. BazardSearch for more papers by this author First published: 10 February 1992 https://doi.org/10.1029/91JB02745Citations: 19AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Halvorsen, E., A paleomagnetic pole position of Late Jurassic/Early Cretaceous dolerites from Hinlopenstretet, Svalbard, and its tectonic implications, Earth Planet. Sci. Lett., 94, 398– 408, 1989. Kluth, C. F., R. F. Butler, L. E. Harding, M. Shafiqullah, P. E. Damon, Paleomagnetism of Late Jurassic rocks in the northern Canelo Hills, southeastern Arizona, J. Geophys. Res., 87, 707– 7086, 1982. May, S. R., R. F. Butler, North American Jurassic Apparent polar wander: Implications for plate motions, paleogeography, and Cordilleran tectonics, J. Geophys. Res., 91, 11,519– 11,544, 1986. May, S. R., R. F. Butler, M. Shafiqullah, P. E. Damon, Paleomagnetism of Jurassic volcanic rocks in the Patagonia Mountains, southeastern Arizona: Implications for the North American 170 Ma reference pole, J. Geophys. Res., 91, 11,545– 11,555, 1986. Van Fossen, M. C., D. V. Kent, High-latitude paleomagnetic poles from Middle Jurassic plutons and Moat Volcanics in New England and the controversy regarding Jurassic apparent polar wander for North America, J. Geophys. Res., 95, 17,503– 17,516, 1990. Citing Literature Volume97, IssueB210 February 1992Pages 1801-1802 ReferencesRelatedInformation

Reply [to “Comment on “High‐latitude paleomagnetic poles from Middle Jurassic plutons and moat volcanics in New England and the controversy regarding Jurassic apparent polar wander for North America” by Mickey C. Van Fossen and Dennis V. Kent”

Reply [to “Comment on “High‐latitude paleomagnetic poles from Middle Jurassic plutons and moat volcanics in New England and the controversy regarding Jurassic apparent polar wander for North America” by Mickey C. Van Fossen and Dennis V. Kent”

Tectonic implications of a remagnetization event in the Newark Basin

The Newark basin red beds contain a secondary magnetization (the B component) acquired during the Middle Jurassic after the 5°–20° basin‐wide northwesterly dip was imparted to the strata of the basin and after most, if not all, of the limb rotation in the Jacksonwald syncline. The B component magnetization was most likely related to the same hydrothermal event which evidently remagnetized many of the igneous intrusions in the basin and reset their K/Ar systems at 175 Ma. The remagnetization of the red beds occurred over a few million years and was approximately coincident with the transition from continental rifting to seafloor spreading in the adjacent North Atlantic. The B component magnetization direction yields a paleomagnetic pole at 74°N, 96°E (K = 63, A95 = 2.6°, N = 50 sites) after structural correction for ⅓ of the Jacksonwald folding and none of the regional tilt. This pole supports recent evidence for a high‐latitude model of Jurassic apparent polar wander for North America.

Concordant paleolatitudes from ophiolite sequences in the northern California Coast Ranges, U.S.A.

Paleomagnetic data have been obtained from two ophiolite sequences in the northern California Coast Ranges: from Mount Diablo in the San Francisco Bay area and from Potter Valley, north of Clear Lake. The ophiolite exposed at Mount Diablo is part of the late Middle to Late Jurassic Coast Range ophiolite, and that exposed near Potter Valley is Late Jurassic to perhaps Early Cretaceous in age and occurs within the Franciscan assemblage. Data from the sheeted-dike complex at Mount Diablo show these rocks to be strongly overprinted, probably following uplift and erosion of the ophiolite. Samples whose primary remanent magnetization seems to be recovered yield a mean paleomagnetic pole at 30.7°N, 159.5°E with α 95 = 5.6°. A comparison of this pole with the Jurassic apparent polar wander path for North America indicates that the ophiolite has rotated 45° ± 7° counterclockwise relative to the craton and has not been latitudinally displaced. The diabase and pillow basalt in Potter Valley have not been strongly overprinted and data from those rocks yield a paleomagnetic pole at 79.0°N, 61.5°E with α 95 = 6.4°. This result indicates that the ophiolite at Potter Valley has rotated approximately 29° ± 8° clockwise, and has undergone little or no latitudinal displacement. Because of the predominantly northeastward transport of oceanic plates converging with the western margin of North America since middle Mesozoic time, the absence of appreciable northward displacement of either ophiolite fragment indicates that both formed close to the continental margin.

High‐latitude paleomagnetic poles from Middle Jurassic Plutons and moat volcanics in New England and the controversy regarding Jurassic Apparent Polar Wander for North America

A paleomagnetic study of Middle Jurassic plutonic and volcanic rocks in New England (White Mountains Magma Series) yields high‐latitude pole positions for North America. High unblocking temperature, moderate to high coercivity magnetizations of normal polarity have been isolated in three plutons (White Mountains batholith, Mount Monadnock, and the Belknap Mountains; mean age ∼169 Ma), but the mean pole (88.4°N, 82.1°E, A95 = 6.1°) is not distinguishable from the geographic axis and therefore the hypothesis that the plutons have been contaminated by recent field overprints can not be rejected. However, a dual polarity, high unblocking temperature, and high coercivity magnetization isolated from the Moat volcanics (169 Ma, Rb‐Sr age) was apparently acquired soon after caldera collapse and tilting, at about the time of intrusion and cooling of the Conway granite (reported ages K‐Ar biotite, 168 Ma; zircon fission track, 163 Ma). The Moat volcanics pole position (78.7°N, 90.3°E, dp = 7.1°, dm = 10.2°) calculated using the mean magnetization direction of reversed polarity (the Cr component) falls at high latitude but is distinguishable from the spin axis. Moreover, published Middle Jurassic paleomagnetic poles from Gondwana (Africa, Australia, and East Antarctica) transferred to the North American reference frame also suggest a high‐latitude Middle Jurassic pole position for North America, in agreement with the Moat volcanics pole. The new evidence for a Middle Jurassic loop to high latitudes in the North American apparent polar wander path conflicts by 15°–20° with some key published Jurassic reference poles (e.g., the Newark Trend N2 and the Corral Canyon poles) used to constrain current paleomagnetic Euler pole (PEP) apparent polar wander paths for the Jurassic. We suggest that a plausible explanation for the discrepancy is that the N2 and Corral Canyon magnetizations are in fact secondary and were acquired after tilting. The hypothesis that the North American apparent polar wander path ventured to high latitude in the Middle Jurassic requires further testing, however the results of this study already suggest that the path may be more complicated than that proposed by recently published PEP studies.

North American Jurassic Apparent Polar Wander: Implications for plate motion, paleogeography and Cordilleran tectonics

Eight paleomagnetic poles are considered to be reliable Jurassic reference poles for cratonic North America. These poles form a consistent chronological progression defining two arcuate tracks of apparent polar wander (APW) from Sinemurian through Tithonian time (203–145 Ma). Combined with reliable Triassic and Cretaceous reference poles, the resulting path is well modeled by paleomagnetic Euler pole (PEP) analysis and is significantly different from previous APW compilations. These differences reflect differences in original data sets, modes of analysis, and geologic time scales and translate into substantial and important differences in paleolatitude estimates for cratonic North America. PEP analysis reveals two cusps, or changes in the direction of APW: one in the Late Triassic to Early Jurassic (Jl) and one in the Late Jurassic (J2). The Jl cusp represents the change in North American absolute plate motion associated with rifting of the central Atlantic and Gulf of Mexico, while the J2 cusp correlates temporally with the marine magnetic anomaly M21 plate reorganization and to various North American intraplate tectonomagmatic events (e.g., Nevadan Orogeny). Analysis of pole progression along the Jl to J2 and J2 to Cretaceous APW tracks indicates constant angular plate velocity of 0.6°–0.7°/m.y. from 203 to 150 Ma followed by significantly higher velocity from 150 to 130? Ma. Late Triassic‐Jurassic reference poles indicate more southerly paleolatitudes for cratonic North America than have previous compilations requiring modification of displacement scenarios for suspect terranes along the western Cordillera.

Reversal pattern and apparent polar wander for the Late Jurassic

A paleomagnetic study of the Upper Jurassic (Kimmeridgian-Tithonian) Morrison Formation near Norwood, Colorado, indicates the existence of thirteen polarity intervals. The seven reversed intervals occupy much more of the section than the six normal intervals, suggesting that during this time the dominant polarity was reversed. Pole positions were computed for each portion of the section where directions were tightly grouped. The pole positions form two separate groups directly related to the stratigraphic position in the section of the samples from which they were computed. The two mean pole positions for the Morrison Formation, 142.2°E, 61.4°N (dp = 4.0°, dm = 6.5°) and 161.8°E, 67.5°N (dp = 3.5°, dm = 5.0°), define a path which includes the Cretaceous pole positions for North America. The data indicate that the Jurassic apparent polar wander curve for North America is approximately a line of latitude (present-day coordinates) connecting published Triassic and Cretaceous pole positions. The data disagree with the commonly held view that the Jurassic portion of the North American apparent polar wander curve includes the present axial dipole.