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2019 Spring Technical Meeting ST. JOHN'S, NEWFOUNDLAND AND LABRADOR The annual Spring Technical Meeting was held on February 18 and 19, 2019, in the Johnson GEO CENTRE on scenic Signal [...]

The Cantung tungsten-copper (W-Cu) skarn orebodies are hosted by Proterozoic and Lower Cambrian metasedimentary rocks next to the Cretaceous 'Mine Stock' monzogranite. Paleomagnetic analyses of 283 specimens from the Open Pit scheelitechalcopyrite orebody (17 sites) and from adjacent host rocks including the aplite dikes (11 sites) isolated a stable characteristic remanent magnetization (ChRM), mostly by alternating field and then thermal step demagnetization. The step demagnetization results along with rock magnetic analyses of the W concentrate show that the main remanence carriers are single- or pseudosingle-domain pyrrhotite, titanomagnetite, and (or) magnetite. There is no statistically significant difference at 95% confidence between the site mean ChRM directions for the W-Cu ore, the host rock, or the aplite dikes populations. This result indicates that the intrusion of the Mine Stock is coeval with the genesis of the scheelite skarn ore and with dike emplacement to give an overall mean ChRM direction of declination 342.9°, inclination 82.0° (N =22 sites, radius of cone of 95% confidence [α.sub.95] = 4.2°, precision parameter k = 54.7) that defines a paleopole at 76.2°N latitude, 212.2°E longitude (radius of cone of 95% confidence [A.sub.95] = 8.1°). This paleopole is concordant with the coeval 98 Ma North American paleopole at 92% confidence, which provides strong evidence that the eastern Selwyn Basin has been an autochthonous part of North America since the mid- Cretaceous. Les gisements de skarns a tungstene-cuivre (W-Cu) de Cantung sont encaisses dans des roches metasedimentaires datant du Proterozoique et du Cambrien inferieur, a proximite du stock intrusif de monzogranite << Mine Stock >> datant du Cretace. Des analyses paleomagnetiques sur 283 echantillons provenant du gisement nomme << Open Pit >> de scheelitechalcopyrite (17 sites) et des roches encaissantes adjacentes, incluant des dykes d'aplite (11 sites), ont permis d'isoler une aimantation remanente caracteristique stable (ChRM), principalement par alternance des champs puis par desaimantation thermique par etapes. Les resultats de la desaimantation par etapes, jumeles aux analyses magnetiques du concentre W, montrent que les principaux porteurs de la remanence sont une pyrrhotite, une titano-magnetite et/ou une magnetite, a domaine unique ou pseudo-unique. Il n'y a pas de difference statistique significative, au niveau de confiance de 95 %, entre les directions moyennes de la ChRM pour le minerai W-Cu, la roche encaissante ou les populations de dykes d'aplite. Selon ce resultat, l'intrusion du<>est contemporaine de la genese du minerai de skarns de scheelite et de la mise en place des dykes pour donner une direction moyenne generale de declinaison de la ChRM de 342,9 °, d'inclinaison de 82,0 ° (N = 22 sites, rayon du cone de confiance a 95 % [α.sub.95] = 4,2 °, parametre de precision k = 54,7) definissant un paleopole a une latitude de 76,2 °N et a une longitude de 212,2 °E (rayon du cone de confiance a 95 % [A.sub.95] = 8,1 °). Ce paleopole concorde avec le paleopole nord-americain contemporain de 98 Ma avec un niveau de confiance de 92 %, ce qui fournit de fortes indications que le bassin Selwyn oriental forme une partie autochtone de l'Amerique du Nord depuis le Cretace moyen. [Traduit par la Redaction], Introduction The 98-95 Ma Tungsten plutonic suite in the eastern Selwyn Basin forms an ~ 300 km long northwest-trending belt of plutons in the western Northwest Territories (NWT) and eastern [...]

We report paleomagnetic data from a new section of the ~1.09 Ga Lake Shore Traps exposed on Silver Island (10 flows) and on the adjacent mainland (two flows) along the northwestern coastline of the Keweenaw Peninsula in Michigan. We also present new data from nine additional lava flows, sampled from the tip of the peninsula previously studied by Diehl and Haig in 1994. Samples from all these lava flows yield well-defined characteristic magnetization directions upon thermal demagnetization. After structural tilt correction, the directions from Silver Island (site-mean declination, D = 276.9°; site-mean inclination, I = 44.4°; 95% radius of confidence for site mean, [α.sub.95] = 2.6°; number of samples, N =10) and mainland (D = 298.7°, I = 36.0°, [α.sub.95] = 10.1°, N =2) flows are close to the directions from equivalent lava flows from the upper (D = 277.8°, I = 41.0°, [α.sub.95] = 2.3°, N = 17) and lower (D = 300.0°, I = 34.9°, [α.sub.95] = 2.3°, N = 10) sections of the middle Lake Shore Traps exposed at the eastern tip of the Peninsula, respectively. Testing the paleomagnetic directions for serial correlation shows that some of the sequential lava flows on Silver Island and from the middle Lake Shore Traps at the tip of the Peninsula record the same vector of the geomagnetic field. Combining these correlated directions yielded new mean directions for Silver Island (D = 277.2°, I = 44.1°, [α.sub.95] = 3.1°, N = 8), and the upper (D = 277.0°, I = 40.4°, [α.sub.95] = 3.7°, N = 10) and lower (D = 298.6°, I = 33.3°, [α.sub.95] = 4.3°, N = 5) middle Lake Shore Traps at the tip of the Peninsula. The statistical similarity of paleomagnetic directions obtained from these two locations with significantly different structural trends supports the conclusions of prior studies that the curvature of the Midcontinent Rift is primary. The new paleomagnetic pole for the Lake Shore Traps is located at 23.1°N, 186.4°E (95% confidence for the paleomagnetic pole, [A.sub.95] = 4.0°; N = 31) and merits a nearly perfect six-point classification on the paleomagnetic reliability scale. Nous presentons des donnees paleomagnetiques d'une nouvelle section des trapps de Lake Shore, ~1,09 Ga, qui affleurent sur l'ile Silver (dix coulees) et sur le continent adjacent (deux coulees) le long de la cote nord-ouest de la peninsule de Keweenaw, au Michigan. Nous presentons aussi de nouvelles donnees provenant de neuf autres coulees de lave, echantillonnees a la pointe de la peninsule, anterieurement etudiee par Diehl et Haig en 1994. Des echantillons de toutes ces coulees de lave ont donne des directions de magnetisation caracteristiques et bien definies lors de la demagnetisation thermique. Apres avoir ete corrigees pour l'inclinaison, les directions des coulees de l'ile Silver (declinaison moyenne au site, D = 276.9°; inclinaison moyenne au site, I = 44.4°; rayon de confiance de 95 % pour la moyenne du site, [α.sub.95] = 2.6°; nombre d'echantillons, N = 10) et du continent (D = 298.7°, I = 36.0°, [α.sub.95] = 10.1°, N = 2) se rapprochent des directions de coulees de lave equivalentes provenant respectivement de la section superieure (D = 277.8°, I = 41.0°, [α.sub.95] = 2.3°, N = 17) et inferieure (D = 300.0°, I = 34.9°, [α.sub.95] = 2.3°, N = 10) des trapps centraux de Lake Shore qui affleurent a la pointe est de la peninsule. Des analyses des directions paleomagnetiques pour des correlations de serie montrent que quelques coulees de lave sequentielles sur l'ile Silver et des trapps centraux de Lake Shore, affleurant a la pointe de la peninsule, enregistrent le meme vecteur de champ geomagnetique. La combinaison de ces directions correlees a donne de nouvelles directions moyennes pour l'ile Silver (D = 277.2°, I = 44.1°, [α.sub.95] = 3.1°, N = 8) et pour le centre superieur (D = 277.0°, I = 40.4°, [α.sub.95] = 3.7°, N = 10) et inferieur (D = 298.6°, I =33.3°, [α.sub.95] = 4.3°, N = 5) des trapps de Lake Shore, al la pointe de la peninsule. La similitude statistique des directions paleomagnetiques obtenues de ces deux endroits, pourtant a tendances structurales tres differentes, supporte les conclusions d'etudes anterieures stipulant que la courbature de la distension medio-continentale est primaire. Le nouveau pole paleomagnetique pour les trapps de Lake Shore est situe a 23,1°N, 186,4°E (rayon de confiance de 95 % pour le pole paleomagnetique, [A.sub.95] = 4.0°, N = 31) et il se merite une classification presque parfaite de six points sur l'echelle de fiabilite paleomagnetique. [Traduit par la Redaction], Introduction The Lake Shore Traps (LST) is a sequence of thin lava flows interbedded within the Copper Harbor Formation (CHF) which crop out on the Keweenaw Peninsula in Michigan (Lane [...]

Palaeomagnetic studies (1-5) of Palaeoproterozoic to Cretaceous rocks propose a suite of large and relatively rapid (tens of degrees over 10 to 100 million years) excursions of the rotation pole [...]

Tilting of crystalline basement rocks associated with folding strain at uppermost crustal levels is difficult to recognize if basement rocks are devoid of traceable marker planes. Here we use the spatial variation in strike of Paleoproterozoic mafic dyke segments complemented by compiled paleomagnetic data to identify tilting in Archean basement rocks associated with kilometre-scale folds of the eastern Sudbury Basin, Ontario. Spatial analysis of the strike of dyke segments is consistent with generation of the NE lobe and a newly identified anticline, referred to as the West Bay Anticline, in the layered Sudbury Igneous Complex (SIC). This anticline accounts better for the structural characteristics of the eastern Sudbury Basin than a previously proposed anticline with west-plunging hinge line. The West Bay Anticline is characterized by abrupt plan-view thickness variations in the lower SIC and curved faults displaying significant strike separations of SIC contacts. These structural characteristics are consistent with folding strain imparted to the SIC and adjacent Archean rocks during formation of the West Bay Anticline. Sublayer embayments and associated quartz diorite dykes likely served as zones of mechanical weaknesses, at which the higher-order folds localized. Unfolding magnitudes of the NE lobe based on primary paleomagnetic remanence directions are significantly smaller than inferred magnitudes that are based on the assumption that the basal SIC contact was initially planar. Thus, the basal SIC contact in the NE lobe likely had a trough-like geometry at the time of remanence acquisition. We advocate a scenario for the formation of the NE lobe, in which the trough geometry of the SIC is primary rather than a consequence of tilting prior to solidification of, and remanence acquisition in, the SIC. Finally, we caution the interpretation of photo lineaments in eroded basement terranes purely as a consequence of faulting. Le basculement des roches cristallines du socle associe a la deformation de plissement aux niveaux superieurs de la croute est difficile a reconnaitre si les roches du socle n'ont pas de plans marqueurs tracables. Nous utilisons ici la variation spatiale de direction de segments de dykes mafiques datant du Paleoproterozoique a laquelle nous ajoutons des donnees paleomagnetiques afin d' identifier le basculement dans les roches archeennes du socle associe a des plis d'ordre kilometrique dans la partie est du bassin de Sudbury, Ontario. Une analyse spatiale de la direction des segments de dykes concorde avec la generation du lobe NE et un anticlinal nouvellement identifie, nomme l'anticlinal de West Bay, dans le complexe ignee lite de Sudbury (SIC). Cet anticlinal tient mieux compte des caracteristiques structurales de la partie est du bassin de Sudbury qu'un anticlinal propose anterieurement qui aurait une charniere plongeant vers l'ouest. L'anticlinal de West Bay est caracterise par des variations abruptes d'epaisseur, vues en plan, dans le SIC inferieur et des failles courbees montrant des separations importantes des contacts du SIC selon la direction. Ces caracteristiques structurales concordent avec la deformation par plissement imposee au SIC et aux roches archeennes adjacentes durant la formation de l'anticlinal de West Bay. Des echancrures en sous-couche et des dykes associes de quartz diorite ont sans doute servi de zones de faiblesse mecanique, ou se sont localises les plis d'ordre superieur. Les grandeurs depliees du lobe NE, basees sur les directions primaires paleomagnetiques de remanence, sont significativement inferieures aux grandeurs inferees, lesquelles sont basees sur l'hypothese que le contact basal du SIC etait initialement planaire. Ainsi, le contact basal du SIC dans le lobe NE avait probablement une forme de fosse au moment de l'acquisition de la remanence. Pour la formation du lobe NE, nous preconisons un scenario selon lequel la geometrie de la fosse du SIC est primaire, plutot que la consequence d'un basculement avant la solidification du SIC et l'acquisition de sa remanence. Finalement, nous emettons une mise en garde quant a l'interpretation des lineaments sur photos dans les terranes erodes du socle comme etant uniquement le resultat de failles. [Traduit par la Redaction], Introduction Understanding mechanisms of non-cylindrical, basement-involved folding on the regional scale remains a challenging topic to structural analysts studying deformed basement terranes (e.g., Erslev 1986; Kley et al. 1996; Cristallini [...]

The NE-trending Malley dyke swarm, dated herein at 2231 ± 2 Ma (U-Pb baddeleyite), extends from the central Slave craton to the vicinity of the Kilohigok basin, and may continue farther to the northeast as the geochemically similar Brichta dyke swarm, having been offset sinistrally along the prominent Bathurst fault. It carries a characteristic high unblocking temperature paleomagnetic component of single polarity directed up SE (mean direction: D = 138.3°, I = -53.8°), with corresponding paleopole at 50.8°S, 50.0°W. Lower unblocking temperature components, in some cases directed down SE, similar to ca 1.75 Ga post-Hudsonian overprints, are easily removed using combined alternating field (AF) thermal demagnetization, but difficult to remove using AF cleaning alone. The characteristic remanence has not been demonstrated primary, but is significantly older than 2.03 Ga, the age of Lac de Gras dykes, based on a baked contact test at a Lac de Gras--Malley dyke intersection. In addition, an E- to ESE-trending dyke carries a down WNW remanence, typical of 2.19 Ga Dogrib dykes near Yellowknife, suggesting that regional overprinting has not affected the study area since Dogrib emplacement, and that the Malley remanence was acquired at or shortly after Malley emplacement. Comparing Malley and Lac de Gras paleopoles with the 2.22-2.00 Ga Superior craton apparent polar wander path indicates that the two cratons were (i) not in their present relative orientation at 2.23 or 2.03 Ga, and (ii) likely not drifting in close proximity to one another as parts of a single (super)continent throughout the 2.23-2.03 Ga interval. L'essaim de dykes Malley, a tendance nord-est, est date a 2231 ± 2 Ma (U-Pb sur baddeleyite); il s'etend du craton central des Esclaves jusqu'aux environs du bassin de Kilohigok. Il pourrait s'etendre plus loin vers le nord-est en tant que l'essaim de dykes Brichta, dont la geochimie est semblable et qui a ete decale vers la gauche le long de l'importante faille Bathurst. Cet essaim Malley comporte une composante paleomagnetique de temperature de deblocage elevee caracteristique, a polarite simple dirigee vers le haut au sud-est (D = 138,3°, I = -53,8°), avec un paleopole correspondant a 50,8° Sud, 50,0° Ouest. Les composantes de temperature de deblocage inferieure, dans certains cas dirigees vers le bas au sud-est, semblables aux surimpressions post-hudsoniennes a environ 1,75 Ga, sont facilement enlevees en utilisant une demagnetisation thermique / a champ alterne, mais elles sont difficiles a enlever en utilisant uniquement une demagnetisation a champ alterne. La remanence caracteristique n'a pas ete demontree mais elle est significativement plus ancienne que 2,03 Ga, l'age des dykes Lac de Gras, selon des tests de contact a une intersection des dykes Lac de Gras-Malley. De plus, un dyke a tendance E a ESE comporte une remanence ONO vers le bas, typique des dykes Dogrib (2,19 Ga) a proximite de Yellowknife, suggerant que la surimpression regionale n' ait pas touche le secteur a l'etude depuis la mise en place des dykes Dogrib et que la remanence Malley ait ete acquise en meme temps ou peu apres la mise en place des dykes Malley. Une comparaison des paleopoles Malley et Lac de Gras avec la derive polaire apparente du craton du Superieur (2,22-2,00 Ga) indique que les deux cratons (i)n' avaient pas leur orientation relative actuelle il y a 2,23 ou 2,03 Ga et (ii) ne derivaient probablement pas pres l'un de l'autre en tant que partie d'un seul (super)continent durant tout l'intervalle 2,23-2,03 Ga. [Traduit par la Redaction], Introduction Thirty to forty Archean cratons or cratonic fragments are scattered widely on all present-day continents. Given that many have Paleoproterozoic rifted margins, it is likely that they were once [...]

The Partridge Island stratigraphic section at the Bay of Fundy, Maritime Canada, reveals a continental sedimentary succession with the end-Triassic mass extinction level closely followed by basalts of the Central Atlantic Magmatic Province (CAMP). New Paleomagnetic data show that a short reverse magnetic polarity chron, correlative to E23r of the Newark Geomagnetic Polarity Time Scale (GPTS), is present below the extinction event. Organic carbon isotope data and basalt geochemistry further indicate that the onset of CAMP emplacement in the Bay of Fundy was roughly synchronous with emplacement in the Newark basin, but slightly postdates the oldest CAMP volcanism in Morocco by ~20 ka. These results confirm the potential for long-distance CAMP correlations based on geochemical trace elements, indicate substantiate provincialism of latest Triassic palynoflora, and suggest a very concise period (≫ 100 ka) of CAMP emplacement in the northern Atlantic region. La section stratigraphique de l'ile Partridge dans la baie de Fundy, Maritimes, Canada, revele une succession sedimentaire continentale dans laquelle le niveau d'extinction massive de la fin du Trias est suivi de pres par des basaltes de la Province magmatique centre atlantique (CAMP). De nouvelles donnees paleomagnetiques montrent qu'un chrone de breve inversion de polarite magnetique, correle a E23r de l'echelle de temps geomagnetique de polarite de Newark, est present sous l' evenement d' extinction. Des donnees d' isotopes du carbone organique et la geochimie des basaltes indiquent de plus que le debut de la mise en place de la CAMP dans la baie de Fundy a ete passablement contemporain avec la mise en place du bassin Newark, mais cette mise en place a eu lieu peu de temps apres le plus ancien volcanisme CAMP au Maroc, soit environ 20 ka. Ces resultats confirment le potentiel de correler la CAMP sur de longues distances, en se basant sur des elements traces geochimiques; ils indiquent et valident le provincialisme de la plus recente palynoflore du Trias et suggerent une tres courte periode (≫ 100 ka) pour la mise en place de la CAMP dans la region de l'Atlantique Nord. [Traduit par la Redaction], Introduction The end-Triassic mass extinction (~201.4 Ma Schoene et al. 2010) is considered to be one of the 'big five' extinction events in geological history (Raup and Sepkoski 1982; Sepkoski [...]

[1] We report a combined study of anisotropy of low field magnetic susceptibility (AMS) and paleomagnetism from 16 sites in a sedimentary sequence of Eocene-early Oligocene red beds in southern Peru. Incipient tectonic strain is recorded during the early stages of deformation. Nonhorizontal magnetic lineation in geographic coordinate suggests either noncylindrical folding and/or interference of two phases of compressive deformation and tectonic rotation. Applying the classic tilt correction results in significant dispersion in paleomagnetic declinations and apparent clockwise and counterclockwise relative tectonic rotations. A dispersion in the orientation of the magnetic lineation also arises from a simple classic tilt correction inducing apparent local rotation in paleostress determination. The magnetic lineation is a good proxy to detect a complex history of folding when the finite strain is not large enough to reset the magnetic fabric acquired during the early stages of deformation and when detailed geological field mapping is not available or not possible. In the present study, a double correction rotating first the lineation to the horizontal reduces significantly the dispersion of the paleomagnetic data with respect to conventional tilt correction (Fisher parameter k increases from 14 to 35). The interest of this double correction must obviously be evaluated for each study according to the complexity of the folding and the intensity of the deformation. Assuming a mean age of 40 Ma for the sedimentary sequence, no significant rotation (-4.5[degrees] [+ or -] 8.4) is observed in this area of the Peruvian Andes. Citation: Roperch, P., V. Carlotto, and A. Chauvin (2010), Using anisotropy of magnetic susceptibility to better constrain the tilt correction in paleomagnetism: A case study from southern Peru, Tectonics, 29, TC6005, doi: 10.1029/2009TC002639.

Paleomagnetism provides independent paleolatitude constraints on the India-Asia convergence. However, implied Cenozoic latitudinal convergence within Asia (thousands of km) largely exceeds geologic estimates of tectonic shortening (hundreds of km). This discrepancy may result from a notoriously low bias in paleomagnetically determined Cenozoic paleolatitudes in Asia. We provide here new paleomagnetic data from Cenozoic Mongolian volcanic rocks and from Chinese Paleogene sediments corrected from the depositional bias of inclination shallowing. These results combined with similar Asian data sets, confirm that paleolatitudes are still 5-10[degrees] lower than predicted by the paleomagnetic Apparent Polar Wander Path (APWP) for Asia between 50 and 20 Ma. Inclination-shallowing being excluded from the selected data sets, we investigate the likeliness of other proposed mechanisms for this discrepancy: (1) more southerly positions of Asia than expected by the APWP (due to APWP inaccuracies rather than Eurasian non-rigidity), or (2) non-dipolar geomagnetic field contributions. Fully explaining this discrepancy by only one of these mechanisms would imply either unrealistically large (>10[degrees]) APWP inaccuracies, or unrealistically large octupolar field contributions (up to 16%). A combination of these mechanisms is found more likely to have produced the observed latitudinal discrepancy, but their respective contributions cannot be quantified given the still relatively low amount and poor quality of Cenozoic paleomagnetic data from stable cratons of Asia, India, and Europe. By allowing for reasonable time-dependant non-dipolar contributions and a slight ( doi: 10.1029/2008TC002437.

A range of mafic to intermediate extrusive and intrusive igneous rocks from the central part of King George Island were sampled for isotopic and palaeomagnetic studies. Single-grain U--Pb dating of zircon from basalts to trachytes from the upper part of the section gave Eocene ages (53.0 [+ or -] 0.7 to 47.8 [+ or -] 0.5 Ma), consistent with a whole-rock [sup.40]Ar-[sup.39]Ar age of 52.7 [+ or -] 0.6 Ma. These ages correspond to the first stages of the opening of the Drake Passage. A Late Cretaceous [sup.40]Ar-[sup.39]Ar age (75.4 [+ or -] 0.9 Ma) was obtained only from one basalt sample from the bottom part of the section (Uchatka Point Formation). These new age determinations change substantially the existing stratigraphic chart of King George Island. Palaeomagnetic poles show a marked departure from the East Antarctic apparent polar wander path, reflecting anticlockwise rotation of the rocks after the Early Eocene. The mean age of zircon grains from a basaltic lava flow overlying the tillite at Herve Cove (48.9 [+ or -] 0.7 Ma) and the normal polarity magnetization of surrounding basalts allow correlation of the tillite with the upper part of the C22 polarity chron (49.4-48.6 Ma). The consistency in age, crystal morphology, and U and Th contents strongly suggests that the zircon records the magmatic event. However, derivation of the zircon from an older magma and a slightly younger age for the tillite cannot be totally excluded. doi: 10.1144/0016-76492009-177.

[sup.40]Ar/[sup.30]Ar dating of 15 lava flows indicates that Volcan de Santa Maria grew episodically to 8 [km.sup.3] in size at an average rate of 0.12 [km.sup.3]/ka between 103 and 35 ka. The composite cone grew in four phases, including two periods of intense activity at ca. 72 ka and ca. 35 ka, during which 1.5 [km.sup.3] and 3 [km.sup.3] of basaltic to andesitic magmas were erupted. There is no evidence of further volcanism after ca. 35 ka until the great dacitic eruption in 1902. The average eruptive rate is 0.16 [km.sup.3]/ka, if products of the 1902 eruption and subsequent Santiaguito dome are included. Whereas the Mono Lake excursion is not clearly recorded at Volcan de Santa Maria, as had been inferred from earlier studies, virtual geomagnetic poles (VGPs) of the 35 ka cone-forming lavas exhibit high-amplitude paleosecular variation that may correspond in time to the Mono Lake excursion. Two older packages of lava flows are each associated with a distinctive cluster of VGPs, which supports the [sup.40]Ar/[sup.39]Ar age model and the conclusion that cone building was episodic. During the final 60% of cone growth, lavas evolved from basaltic to andesitic (51%-57% Si[O.sub.2]) with time, but with a regression to slightly less evolved compositions during the onset of the final cone-building phase. Despite the relatively small volume of Santa Maria, cone-growth processes and geochemical evolution through time mirror observations at other currently active volcanoes along the Central American volcanic arc, and may prove useful as an analogy in assessing long-term hazards posed by other predominantly basaltic-andesitic composite volcanoes. 10.1130/B26569.1

Paleomagnetic measurements and U-Pb geochronology of alkalic igneous rocks intruding the Ediacaran Cambridge Argillite and the Lower Cambrian Weymouth Formation north of Boston at Nahant, Massachusetts yield a well-constrained paleopole. The oldest intrusive phases are a syenodiorite sill and a granite dike with [sup.206]Pb/[sup.238]U crystallization ages of 490.19 [+ or -] 0.90 Ma and 489.03 [+ or -] 0.82 Ma, respectively. This sequence along with undated mafic sills penetrating both the argillite and Weymouth strata are cut by 488.53 [+ or -] 0.81 Ma gabbro locally associated with 488.48 [+ or -] 0.79 Ma syenite forming a steep-sided intrusive plug. Ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable A magnetic component with a mean direction of D = 278.5[degrees], I = -76.7[degrees] ([A.sub.95] = 3.9[degrees], N = 10). This is considered to be the primary magnetization direction because it passes both fold and reversals tests. The corresponding paleopole at 320[degrees]E, 34[degrees]N ([A.sub.95] = 7.2[degrees], N = 10) places the Southeastern New England Avalon Zone at a southerly paleolatitude of 65[degrees] slightly removed from the African margin of Gondwana and suggests that bimodal Nahant magmatism is linked with latest Cambrian--Early Ordovician rifting of West Avalonia to form the Rheic Ocean. Stable B and C magnetic components in Nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track Late Silurian through Carboniferous segments of the North American apparent polar wander path, indicating docking of West Avalonia by mid-Paleozoic time. doi: 10.1130/B26581.1

Determining the kinematic history and mechanics of curved fold-and-thrust belts is fundamental to understanding the tectonic evolution of mountain systems. To better understand the development of a classic curved fold-and-thrust belt, we completed an integrated paleomagnetic and strain study of the Wyoming salient. Paleomagnetic data are reported here from 154 sites collected from red beds of the Triassic Ankareh Formation in the salient and nine sites collected from the relatively stable foreland. Red beds display three components with distinctly different magnetic behaviors: (1) a near-primary Triassic magnetization carried by hematite that is stable up to 680[degrees]C (Tr component, 91 sites); (2) a Cretaceous chemical remagnetization carried partly by magnetite (K component, 32 sites); and (3) a recent viscous magnetization that is mostly removed by 350[degrees]C. Site mean vectors for the Tr and K components show a high degree of scatter from expected Triassic and Cretaceous reference directions, suggesting significant tilt and rotation subsequent to magnetization acquisition. Restoration of tilt and folding for individual site means results in well-clustered shallow and moderate inclinations for the Tr and K components, respectively, and in variable declinations related to systematic vertical-axis rotations. Statistical analysis of declinations for both components indicates that ~75% of present-day salient curvature resulted from secondary rotation, and ~25 % of primary curvature was likely related to sedimentary basin architecture. Analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets (~80%) compared to the frontal thrust sheets (~65%), suggesting that rotations were concentrated near the leading edge of the propagating fold-and-thrust wedge, with only minor additional rotation of internal sheets. Transfer zones, oblique ramps, and more deformed overturned fold limbs display locally more complex patterns, which can be understood through careful structural analysis. When combined with internal strain data and regional structural relations, paleomagnetic data support a kinematic model of a progressive arc with curved thrust-slip paths and differential shortening that rotated early layer-parallel shortening fabrics and produced minor strike-parallel extension. This kinematic history likely reflects a combination of processes, including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient, presence of a weak basal detachment and faultzone weakening that favored lower taper, and buttressing by Laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient. doi: 10.1130/B26483.1

The Tianshan Range is one of the longest and highest mountain belts in Central Asia, stretching east-west for ~2500 km. Uplift of this late Paleozoic orogenic belt resulted from intracontinentai deformation caused by the collision of the Indian and Eurasian plates during the Cenozoic era. To constrain the timing of Cenozoic tectonic deformation of the Tianshan Range, we analyzed the magnetostratigraphy of 3780-m-thick Neogene deposits from the Kuqa foreland basin of southern Tianshan. The geometry measurements and magnetic fabric data show that syntectonic growth strata began to accumulate at ~6.5 Ma ago, indicating that crustal shortening initiated in the latest Miocene. The increase in sedimentation rate and an abrupt change of the magnetic fabric parameters also occurred at 6.5 Ma, accompanied by onset of syntectonic growth strata. The latest Miocene crustal shortening is a significant tectonic event in the foreland basin of the southern Tianshan Range in response to the India-Eurasia collision. doi: 10.1130/G30278A.1

Paleomagnetic and geochemical data obtained from six wells in southwestern Manitoba indicate that the Lower Amaranth redbeds were deposited earlier than Jurassic or Triassic, the most commonly cited depositional ages for this formation in the Williston Basin. The magnetization is carried primarily by detrital specular and pigmentary hematite and occasionally magnetite. Inclination-only analysis of paleomagnetic data (83 specimens from 60 plugs) indicates two possible depositional magnetization ages: Devonian-Pennsylvanian (D, found in very few samples) or Carboniferous to Permian, as suggested by the inclination and the polarity of the most predominant magnetization (B). An isolated magnetization (C could be a mid-Jurassic to Neogene localized fluid flow remagnetization event. The oxygen and carbon isotope values of dolomite (-6.45% ‰ to 0.30% ‰ Vienna PeeDee Belemnite (VPDB) [δ.sup.18]O, -1.57% ‰ to 5.44% ‰ VPDB [δ.sup.13]C; n = 18) indicate that dolomitisation could have occurred anytime between Carboniferous and Jurassic. However, the distribution of these values is a function of the three types of dolomite present in the Lower Amaranth strata: detrital, cement, and matrix replacive and reflects both the primary values and diagenetic overprints. Detrital and cement dolomite show depleted values in both oxygen and carbon isotopes, most likely inherited from the original values of the detrital dolomite, with superimposed effects of recrystallization. The matrix replacive dolomite has no detrital content, and its oxygen isotope values are similar to the expected values for primary or early diagenetic dolomite from Carboniferous to Jurassic times. Mixtures of detrital and replacive matrix dolomite give intermediate oxygen and carbon isotope values. Les donnees paleomagnetiques et geochimiques obtenues de six puits dans le sud-ouest du Manitoba indiquent que les lits rouges de la partie inferieure de la Formation d'Amaranth ont ete deposes avant le Jurassique ou le Trias, les ages le plus frequemment cites pour la deposition de cette formation du bassin Williston. La magnetisation se trouve principalement dans l'hematite detritique speculaire et pigmentaire et occasionnellement dans la magnetite. Une analyse de l'inclinaison magnetique seulement des donnees paleomagnetiques (83 echantillons, 60 carottes) indique deux ages possibles de magnetisation de deposition : Devonien -- Pennsylvanien (D, trouve dans tres peu d'echantillons) ou Carbonifere a Permien, tel que suggere par l'inclinaison et la polarite de la magnetisation la plus predominante (B). Une magnetisation isolee (C pourrait etre due a un evenement de remagnetisation par un ecoulement fluide localise (Jurassique moyen a Neogene). Les valeurs des isotopes de l'oxygene et du carbone de la dolomie (-6,45 % ‰ a 0,30 % ‰ VPDB [δ.sup.18]O, -1,57 % ‰ a 5,44 % ‰ VPDB [δ.sup.13]C; n = 18) indiquent que la dolomitisation aurait pu se produire n'importe quand entre le Carbonifere et le Jurassique. Toutefois, la distribution de ces valeurs est fonction des trois types de dolomite presents dans les couches de l'Amaranth inferieur : detritique, de cementation et en remplacement de la matrice. La distribution reflete aussi les valeurs primaires et les surimpressions diagenetiques. La dolomite detritique et de cementation montre des valeurs appauvries en isotopes d'oxygene et de carbone, probablement provenant des valeurs originales de la dolomite detritique, avec des effets superposes de recristallisation. La dolomite de remplacement de la matrice n'a pas de contenu detritique et ses valeurs en isotopes d'oxygene sont semblables a celles attendues pour la dolomite primaire ou diagenetique precoce des epoques du Carbonifere au Jurassique. Des melanges de dolomite detritique et de remplacement de la matrice donnent des valeurs intermediaires pour les isotopes d'oxygene et de carbone. [Traduit par la Redaction], Introduction The Amaranth redbed sediments of the Williston Basin in Manitoba, Canada, represent an interesting geological problem as the depositional age of the Lower Amaranth Member has been placed between [...]

U-Pb baddeleyite ages and paleomagnetic poles obtained for dykes on Devon Island and Ellesmere Island in the Canadian Arctic and the Thule region of Greenland show that they are associated with the Franklin magmatic event. This study is the only one devoted to Franklin igneous rocks where a primary paleomagnetic remanente and U-Pb age have been obtained from the same rocks. Ages from this study range from 721 to 712 Ma, but paleomagnetic directional data show no clear age progression. The paleomagnetic poles from each of the two regional subsets are significantly different at the 95% confidence level from paleomagnetic results previously published for the Franklin event in the CanadianShield. The difference in the pole locations can be accounted for, to first approximation, by a simple model of early Cenozoic block rotations among the North American plate, Greenland, and a hypothesized ancient microplate comprising Ellesmere, Devon, Cornwallis, and perhaps Somerset islands. A new grand-mean paleopole for the Franklin event, including restoration of Greenland and the proposed 'Ellesmere microplate' to North America, is located at (8.4°N, 163.8°E, [A.sub.95] = 2.8°, N = 78 sites) and is a key pole for Neoproterozoic supercontinent reconstructions. Des ages U-Pb, determines sur de la baddeleyite, et des poles paleomagnetiques obtenus de dykes des files de Devon et d'Ellesmere, dans fArctique canadien, et de la region de Thule au Groenland, montrent que ces dykes sont associes a fevenement magmatique de Franklin. La presente etude est la seine consacree aux roches ignees de Franklin ou une remanente paleomagnetique primaire et des ages U-Pb ont ete obtenus des memes roches. Les ages dans cette etude varient de 721 a 712 Ma, mais les donnees sur les directions paleomagnetiques ne montrent aucune progression Claire de l'age. Les poles paleomagnetiques de chacun des deux sous-ensembles regionaux different de maniere significative, au niveau de confdance 95 %, des resultats paleomagnetiques publies anterieurement pour l'evenement de Franklin dans le Bouclier canadien. La difference d'emplacement des poles peut etre expliquee, en une premiere approximation, par un modele simple de rotations de blocs, au Cenozoique precoce, entre la plaque Nord-americaine, le Groenland et une ancienne plaque hypothetique qui comprenait les files d'Ellesmere, de Devon, de Cornwallis et peut-etre de Somerset. Un nouveau paleopole de grande moyenne pour l'evenement Franklin, incluant la restoration du Groenland et de la << microplaque d'Ellesmere >> proposee a l'Amerique du Nord, est situe a (8,4[degre]N, 163,8[degre]E, [A.sub.95] = 2,8[degre], N = 78 sites) et constitue le pole cle pour les reconstructions du super continent au Neoproterozoique. [Traduit par la Redaction], Introduction Precisely dated rocks with well-defined paleomagnetic poles are scarce in the Proterozoic record, yet are critical for reconstructing the past positions of continents and correlating what are today the [...]

Pegmatite dikes in the Peninsular Ranges batholith of southwestern California have produced spectacular crystals of semiprecious and precious minerals for over a century. Aside from their economic importance, these dikes straddle a major tectonic boundary and were used to test hypotheses related to the timing and development of this composite batholith. Paleomagnetic analysis of 252 specimens from 20 sites (12 and 8 sites in the western and eastern zones of the batholith, respectively, from 11 mines in five dike districts) isolated a stable characteristic remanent magnetization direction at 19 sites. The site mean directions for the western and eastern zones are statistically indistinguishable at 95% confidence, supporting petrologic and geochemical arguments that the dikes of the two zones are coeval and cogenetic. After correction for the Neogene opening of the Gulf of California, the paleopole for all 19 site mean directions is indistinguishable from the 94 Ma reference paleopole for North America and supports hypotheses that (1) the dikes are genetically related to intrusion of the La Posta- type plutons; (2) the batholith was already assembled beside the northwestern coastline of Mexico at 94 Ma; (3) ENE- side-up tilting of fault blocks in the batholith's western zone ended by ~94 Ma; and (4) the far-sided and clockwise-rotated discordant paleopoles found commonly in Late Cretaceous and younger sedimentary rocks of the batholith's region are mostly the result of inclination-flattening of the remanence and (or) remagnetization by fluid flow, creating a secondary remanence, excluding Neogene tectonic rotations. Les dykes de pegmatite dans le batholite des Peninsular Ranges du sud-ouest de la Californie produisent des cristaux spectaculaires de mineraux precieux et semi-precieux depuis plus d'un siecle. En plus de leur importance economique, ces dykes chevauchent une frontiere tectonique majeure et ils ont ete utilises pour verifier des hypotheses quant au moment et au developpement de ce batholite composite. Des analyses paleomagnetiques de 252 echantillons preleves sur 20 sites (respectivement 12 sites dans la partie ouest du batholite et 8 sites dans la partie est, de 11 mines dans 5 districts de dykes) ont isole une direction de magnetization caracteristique remanente stable a 19 sites. Les directions moyennes pour les zones ouest et est sont statistiquement indiscernables avec un niveau de confiance de 95 %, supportant les arguments petrologiques et geochimiques que les dykes des deux zones sont contemporains et cogenetiques. Apres avoir corrige pour f ouverture du golfe de la Californie au Neogene, le paleopole pour les 19 directions moyennes est indiscernable du paleopole de reference de 94 Ma pour l'Amerique du Nord et supporte les hypotheses que : (1) les dykes sont genetiquement relies a fintrnsion des plutons de type La Posta; (2) le batholite etait deja en place le long de la cote nord-ouest du Mexique il y a 94 Ma; (3) le basculement des blocs de faille, le cote ENE vers le haut, dans la zone ouest du batholite s'est termine vers 94 Ma et (4) les paleopoles discordants, eloignes et a rotation dans le sens horaire, frequemment trouves dans les roches sedimentaires du Cretace tardif et plus jeunes de la region du batholite decoulent surtout de faplatissement de finclinaison de la remanente et (ou) de la remagnetisation par un ecoulement de fluides creant une remanente secondaire, excluant les rotations tectoniques au Neogene. [Traduit par la Redaction], Introduction The granitic pegmatite dikes of San Diego County in southwestern California have been famous for producing both rough gem material and superb crystal specimens for more than a century [...]

To test whether the Tethyan Himalaya were part of the northern margin of India in the early Palaeozoic we have produced the first primary palaeomagnetic data (bedding-corrected declination 267.5[degrees], inclination 63.0[degrees], [[alpha].sub.95] = 10[degrees]; pole latitude 20.2[degrees]N, longitude 28.6[degrees]E) from low metamorphic grade Ordovician red beds in the Tethyan Himalaya (Shian Formation). The palaeomagnetic data are of excellent quality, and a statistically positive fold test combined with a comparison with late Cambrian--Ordovician Gondwana poles suggests a primary hematite-bearing magnetization, acquired between 470 and 500 Ma. This is in excellent agreement with stratigraphic, faunal and provenance age estimates, and the palaeomagnetic data demonstrate that the Tethyan Himalaya must have been located in proximity to the Indian craton during early Ordovician times, and are therefore consistent with a continuous margin at that time. The Shian Formation pole overlaps with 470-500 Ma Gondwana poles, but ah even better fit can be obtained by invoking a post-Ordovician clockwise rotation of 13[degrees][+ or -] 4[degrees]. Such a rotation is similar in both sense and magnitude to clockwise rotations recorded in primary Triassic sequences as well as Palaeogene palaeomagnetic overprint data from the Tethyan Himalaya: rotations of the Tethyan Himalaya compared with cratonic India are thus probably all of post late Eocene age. Triassic and Early Ordovician data do not imply any crustal shortening between Tethyan Himalaya and cratonic India. However, in the Early Ordovician, India was rotated 90[degrees] compared with its present orientation, and any enlargement of India would not be detected by palaeomagnetic data.

A well-preserved 11.8-million-years-old lower face attributed to the seminal taxon Dryopithecus fontani (Primates, Hominidae) from the Catalan site ACM/C3-Ae of the Hostalets de Pierola area (Valles-Penedes Basin, Catalonia, NE Spain) is described. The new data indicate that D. fontani is distinct at the genus level from Late Miocene European taxa previously attributed to Dryopithecus, which are here reassigned to Hispanopithecus. The new facial specimen also suggests that D. fontani and the Middle Miocene Pierolapithecus catalaunicus are not synonymous. Anatomical and morphometric analyses further indicate that the new specimen shows a combination of lower facial features--hitherto unknown in Miocene hominoids--that resembles the facial pattern of Gorilla, thus providing the first nondental evidence of gorilla-like lower facial morphology in the fossil record. Considering the current evidence, the gorilla-like facial pattern of D. fontani is inferred to be derived relative to previously known stem hominids, and might indicate that this taxon is either an early member of the Homininae or, alternatively, a stem hominid convergent with the lower facial pattern of Gorilla. The biogeographic implications of both alternatives are discussed. This new finding in the Hostalets de Pierola section reinforces the importance of this area for understanding the elusive question of the Middle Miocene origin and early radiation of great apes. KEY WORDS fossil hominoids; great apes; Dryopithecus; Hispanopithecus; Pierolapithecus; Gorilla; Europe; Middle Miocene; magnetostratigraphy

Lac de Gras diabase dykes trend north to NNE across the central Slave Province of the Canadian Shield. U-Pb baddeleyite ages of 2023 ± 2 and 2027 ± 4 Ma are interpreted as dyke emplacement ages. These ages are similar to that of the Booth River igneous complex, exposed along the margins of Kilohigok Basin near the northern end of the dyke swarm. Ten paleomagnetic sites (from four to six dykes) yield a mean paleopole at 11.8°N, 92.1°W (dm = 8.4°, dp = 6.0°). A positive baked contact test where a Lac de Gras dyke crosscuts a NE-trending Malley dyke demonstrates that this pole is primary. It represents the first key Paleoproterozoic pole from the Slave Province and, hence, the first Paleoproterozoic Slave pole suitable for reconstructing paleocontinents. Although a direct comparison is not available with precisely dated paleopoles of identical age from other Archean cratons, a comparison is made with a sequence of precisely dated poles from Superior Province dyke swarms, including those 40-50 million years older and 25 million years younger. It yields two options depending on the relative magnetic polarity assumed for data from the two cratons. The two cratons were either at similar latitudes, but not in their present relative orientations, when the swarms were emplaced, or separated in latitude by ~40°-60°. In either case, they may have drifted separately or formed part of a single (super)continent that subsequently broke up with the two cratons drifting separately to attain their present configuration. Additional key paleopoles are required to distinguish between these interpretations. Les dykes de diabase du Lac de Gras ont une direction N a NNE a travers le centre de la province des Esclaves du Bouclier canadien. Les ages U-Pb, sur de la baddeleyite, de 2023 ± 2 et de 2027 ± 4 Ma, sont interpretes comme l'age de la mise en place des dykes. Ces ages sont semblables a ceux du complexe igne de Booth River, lequel affleure le long des bordures du bassin Kilohigok a proximite de l'extremite nord de l'essaim de dykes. Dix sites paleomagnetiques (de quatre a six dykes) ont donne un paleopole moyen a 11,8° N, 92,1° W (dm = 8,4°, dp = 6,0°). Un test positif du contact, la ou le dyke de Lac de Gras recoupe un dyke Malley, de direction NE, demontre que ce pole est primaire. Il represente le premier pole cle paleoproterozoique de la province des Esclaves et donc le premier pole paleoproterozoique de la province des Esclaves qui convient pour la reconstruction des paleocontinents. Bien qu'il ne soit pas possible de le comparer directement avec des paleopoles d'age identique dates avec precision d'autres cratons archeens, il est compare a une sequence de poles dates avec precision d'un essaim de dykes de la province du Superieur, incluant ceux 40-50 Ma plus vieux et ceux 25 Ma plus jeunes. Cette comparaison donne deux options selon la polarite magnetique relative assumee pour les donnees des deux cratons. Les deux cratons etaient soit a des latitudes semblables, mais n'avaient pas leur orientation relative actuelle, lorsque les essaims ont ete mis en place, soit separes par une difference de latitude d'environ 40°-60°. Dans les deux cas, ils pourraient avoir derive separement ou forme une partie d'un (super)continent qui se serait disloque par la suite; les deux cratons auraient alors derive separement pour atteindre leur configuration actuelle. D'autres pale opoles cles sont requis pour trancher entre ces interpretations. [Traduit par la Redaction], Introduction Laurentia (Fig. 1; Hoffman 1988) is composed of several Archean cratonic blocks or provinces separated by late Paleoproterozoic orogenic belts (e.g., Trans-Hudson Orogen) that are interpreted by most geologists [...]

The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is key to the formation of the Eurasian composite continent during Paleozoic time, but the views on this complicated process remain disparate and sometimes controversial. A study of three volcanic formations of mid-Silurian, Lower to Middle Devonian, and Middle Devonian age from the southwestern boundary of the Chingiz Range (NE Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A single-polarity characteristic component in mid-Silurian andesites yields a positive intraformational conglomerate test, whereas dual-polarity prefolding components are isolated from the two Devonian collections. Post-folding, reversed-polarity overprint directions have also been isolated and are likely of Permo-Triassic age. These new data can be evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow us to establish with confidence the polarity of each result, and hence to determine the hemisphere in which the area was located at a given time. We conclude that NE Kazakhstan was steadily moving northward, albeit with variable velocity, crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz Range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Middle Permian time. A comparison of the Chingiz paleolatitudes with those of Siberia shows, insofar as the sparse data allow, similarities between the northward motion of the Chingiz unit and that of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt. Keywords: paleomagnetism, volcanic arc, orocline, Middle Paleozoic, Central Asia, Siberia, paleogeography.

The Quaternary Bandas del Sur Formation in the south of Tenerife comprises a complex sequence of pyroclastic rocks and lavas. In contrast to the NW- and NE-Rift zone on Tenerife, the S-Rift zone comprises a number of characteristics with respect to the morphological features, eruption cyclicity and the geochemistry of the volcanic deposits. Various flank eruptions of the Las Canadas volcano associated with basaltic lavas and the formation of cinder cones within the Bandas del Sur are important volcanic units for understanding the explosive volcanic cycles during the Pleistocene on Tenerife. A number of palaeomagnetic studies, as well as major and trace element geochemistry and two radio-isotope dates (K-Ar), have been carried out on prominent cinder cones, in order to discover their stratigraphic position. Combining our results with previous K-Ar data, the cones and lavas can be subdivided into three stratigraphic units. The first unit contains cinder cones with reverse magnetization and Y/Nb ratios between 0.37 and 0.41. Cinder cones which belong to the second unit show normal magnetization and Y/Nb ratios of < 0.35. The third unit comprises cinder cones with normal magnetization and Y/Nb ratios of about 0.47. The first two units were constructed between c. 0.948-0.779 Ma and 0.323-0.300 Ma. These units define volcanic cycles ending in violent Plinian eruptions. The third and youngest unit possibly marks the beginning of a further volcanic cycle that started c. 0.095 Ma ago. Keywords: cinder cones, Bandas del Sur, Tenerife, Canary Islands, palaeomagnetism, geochemistry.

The north-south-trending Clarence Head dyke swarm, located on Devon and Ellesmere Islands in the Canadian High Arctic, has a trend orthogonal to that of the Neoproterozoic Franklin swarm that surrounds it. The Clarence Head dykes are dated by the U-Pb method on baddeleyite to between 716 ± 1 and 713 ± 1 Ma, ages apparently younger than, but within the published age range of, the Franklin dykes. Alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages, but a comparison of the U-Pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the Clarence Head dykes are indeed a distinctly younger event within the period of Franklin magmatism. The Clarence Head dykes represent a large swarm tangential to, and cogenetic with, a giant radiating dyke swarm ~800 km from the indicated source. The preferred mechanism for the emplacement of the Clarence Head dykes is the exploitation of concentric zones of extension around a depleting and collapsing plume source. While the paleomagnetism of most Clarence Head dykes agrees with that of the Franklin dykes, two dykes have anomalous remanence directions, interpreted to be a chemical remanent magnetization carried by pyrrhotite. The pyrrhotite was likely deposited from fluids mobilized southward from the Devonian Ellesmerian Orogeny to the north that used the interiors of the dykes as conduits and precipitated pyrrhotite en route. L'essaim de dykes Clarence Head, a tendance nord-sud, situe sur les iles Devon et Ellesmere dans le Grand Nord canadien, a une tendance orthogonale a l'essaim Franklin (Neoproterozoique) qui l'entoure. Les dykes Clarence Head ont ete dates par methode U-Pb sur de la baddeleyite et ils seraient ages entre 716 ± 1 et 713 ± 1 Ma, des ages apparemment plus jeunes que la plage d'age des dykes Franklin, mais a l'interieur des dates publiees pour ces derniers. Le recul alpha dans la baddeleyite pourrait constituer une explication possible pour les differences d'age, mais une comparaison des ages U-Pb sur des grains de taille semblable des deux essaims suggere que les distances de recul dans la baddeleyite soient inferieures a celles dans le zircon et que les dykes de Clarence Head constituent en effet un evenement plus jeune a l'interieur de la periode de magnetisme Franklin. Les dykes Clarence Head representent un vaste essaim qui est coge netique avec, et tangent a, un gigantesque essaim de dykes qui rayonne sur une distance d'environ 800 km de la source indiquee. Le mecanisme preferentiel pour la mise en place des dykes Clarence Head est l'exploitation de zones concentriques d'extension autour d'un panache source qui s'appauvrit et qui s'effondre. Bien que le paleomagnetisme de la plupart des dykes Clarence Head concorde avec celui des dykes Franklin, deux dykes ont des directions remanentes anomales, interpre tees comme une magnetisation chimique remanente portee par la pyrrhotite. La pyrrhotite a probablement ete depose e a partir de fluides mobilises vers le sud a partir de l'orogene ellesmerien (Devonien) au nord; ces fluides auraient utilise l'interieur des dykes comme conduits et auraient precipite la pyrrhotite en chemin. [Traduit par la Redaction], Introduction Giant mafic dyke swarms are common worldwide (Ernst et al. 1995), and many have radiating patterns of emplacement that appear to indicate their source at the focus, often considered [...]

New magnetostratigraphic data from seven Western Ghats sections in the Deccan Volcanic Province are presented. These are combined with an established geochemically defined stratigraphy, and volcanological logs, to provide a correlated, chronological eruptive framework. We identify two magnetic polarities in five of the sections, and these are assigned to chrons 29r and 29n. Importantly, the reversal boundary represents an identifiable isochronous surface within the volcanic pile. This surface occurs at different elevations, as does the altitude of the geochemically defined formation boundary (i.e. Ambenali-Mahabaleshwar Fms), which defines a second isochronous surface. Inspection reveals significant differences in the number and thickness of lava units preserved between these two surfaces. This indicates that there was significant local topography (c. 80 m) across Deccan Volcanic Province lava fields during their development; an interpretation consistent with topographies observed across modern and historical examples (e.g. Hawaii, Iceland). These data also indicate that the geochemical stratigraphies of continental flood basalt provinces can mask local and sub-regional detail in lava stacking patterns when applied at smaller spatial scales (

Paleomagnetic and rock magnetic measurements for 27 sites in the tonalitic ca. 108 Ma Alpine and ca. 100 Ma Ramona complexes in the suspect Yuma terrane or western zone of the Peninsular Ranges Batholith near San Diego show that single- and pseudosingle-domain magnetite carries their characteristic remanence. The Alpine and Ramona paleopoles agree with six published paleopoles from Cretaceous igneous rocks in the Yuma terrane, but all eight paleopoles are discordant for North America. Al-in-hornblende geothermobarometric depth determinations at 16 sites, plus published mapping and depth determinations, suggest that the batholith is divided into blocks by orogen-parallel faults or by orthogonal fault sets with large vertical displacements. The paleomagnetic data show that the fault blocks in the terrane underwent, on average, ENE-side-up lilting of ~10[degrees] [+ or -] 3[degrees] about an orogen-parallel axis between ca. 125 Ma and ca. 89 [+ or -] 7 Ma, but they have not been translated northward as a suspect terrane by [greater than or equal to] 500 km, excluding Neogene opening of the Gulf of California. The tilting is attributed to torque from ENE shallow underthrusting of the Farallon plate beneath the WSW overthrusting North American plate, where subsequent extension has caused differential uplift of the fault blocks. Nonsignificant dextral displacement of ~4[degrees] [+ or -] 1[degrees] (440 [+ or -] 110 km) between the eastern and western zones of the batholith suggests that this boundary represents an intra-arc fault. The large paleopole discordance found in younger sedimentary strata of Baja California is attributed mainly to compaction shallowing of the remanence and/or postdeposittonal remagnetization. Keywords: Cretaceous, geobarometry, geothermometry, paleomagnetism, Peninsular Ranges Batholith, tectonics.

Significant fold-axis-parallel slip is accommodated in the folded strata of the Grayback monocline, northeastern Front Range, Colorado, without visible large strike-slip displacement on the fold surface. In many cases, oblique-slip deformation is partitioned; fold-axis-normal slip is accommodated within folds, and fold-axis-parallel slip is resolved onto adjacent strike-slip faults. Unlike partitioning strike-parallel slip onto adjacent strike-slip faults, fold-axis-parallel slip has deformed the forelimb of the Grayback monocline. Mean compressive paleostress orientations in the forelimb are deflected 15[degrees]-37[degrees] clockwise from the regional paleostress orientation of the northeastern Front Range. Paleomagnetic directions from the Permian Ingleside Formation in the forelimb are rotated 16[degrees]-42[degrees] clockwise about a bedding-normal axis relative to the North American Permian reference direction. The paleostress and paleomagnetic rotations increase with the bedding dip angle and decrease along strike toward the fold tip. These measurements allow for 50-120 m of fold-axis-parallel slip within the forelimb, depending on the kinematics of strike-slip shear. This resolved horizontal slip is nearly equal in magnitude to the ~180 m vertical throw across the fold. For 200 m of oblique-slip displacement (120 m of strike slip and 180 m of reverse slip), the true shortening direction across the fold is N90[degrees]E, indistinguishable from the regionally inferred direction of N90[degrees]E and quite different from the S53[degrees]E fold-normal direction. Recognition of this deformational style means that significant amounts of strike slip can be accommodated within folds without axis-parallel surficial faulting. Keywords: oblique slip, folds, Laramide, vertical-axis rotations, Rocky Mountain foreland.

The surface uplift history of the Tibetan Plateau and Himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during Cenozoic time, its influence on monsoon intensity, and its reflection of the dynamics of continental plateaus. Models of plateau growth vary in time, from pre-India-Asia collision (e.g., [approximately equal to] 100 Ma ago) to gradual uplift after the India-Asia collision (e.g., [approximately equal to] 55 Ma ago) and to more recent abrupt uplift ( climate | tectonics | magnetostratigraphy | Hoh Xil Basin | Cenozoic

Abstract: The age of the Sooke Formation on the southern coast of Vancouver Island, British Columbia, Canada, has long been controversial. Prior paleomagnetic studies have produced a puzzling counterclockwise tectonic [...]

[1] New paleomagnetic data from Permian red beds and Middle Jurassic limestones from the Tarim Basin pose a paradox. Their declinations are similar to Upper Carboniferous to Neogene rocks collected from the same sections, and their inclinations parallel present values. When assuming that lower than expected inclinations in continental sedimentary rocks arise from inclination shallowing effects, then the paleolatitudes of all Upper Carboniferous to Present rocks from Tarim are indistinguishable. Local vertical axis block rotations occuring in the last 20 million years explain why declinations vary at different localities in the basin. Our Middle Jurassic data positions Tarim 23.6 [+ or -] 8.4[degrees] farther south than that predicted from the coeval reference pole for Eurasia; however, no geologic argument exists to support the closure of a large ocean basin between Tarim and Siberia since the Middle Jurassic. Thus the paradox: are the rocks from Tarim totally overprinted, or is the middle Mesozoic part of the reference Eurasian apparent polar wander path erroneous? Several lines of evidence suggest the Tarim rocks are not remagnetized. We conclude that Tarim has experienced little of no apparent polar wander since the Carboniferous. Moreover, our Middle Mesozoic reconstruction of Eurasia using the new Middle Jurassic pole from Tarim results in a more geologically compatible solution for the eastern Asian blocks over previous reconstructions. Citation: Gilder, S. A., J. Gomez, Y. Chen, and J.-P. Cogne (2008), A new paleogeographic configuration of the Eurasian landmass resolves a paleomagnetic paradox of the Tarim Basin (China), Tectonics, 27, TC 1012, doi: 10.1029/2007TC002155.

The plutons of Cairnsmoor of Fleet (392 [+ or -] Ma) and Criffel-Dalbeattie (397 [+ or -] 2 Ma, both mineral isochron ages) comprise two of four major post-tectonic granitic complexes emplaced into the Southern Uplands, an Ordovician--Silurian back-arc and foreland basin complex formed at the northern margin of the Iapetus Suture. To expand the palaeomagnetic record of the Southern Uplands we have studied palaeomagnetism and magnetic fabrics in traverses spanning contacts of these intrusions with host mudrocks. A uniform anisotropy of magnetic susceptibility (AMS) fabric across the Caimsmoor of Fleet contact has been enhanced by recrystallization into hornfels near the contact and records a late Acadian regional stress operative during, or soon after, emplacement of the pluton in Middle Devonian times. Magnetization during slow cooling recorded a dual polarity ('A') remanence in granite and hornfels with mean direction D/I = 92/-2[degrees] ([[alpha].sub.95] = 6.5[degrees]) yielding a palaeopole (Q = 6) at 2[degrees] N, 265[degrees] E linked to cooling at c. 392 Ma. Subsidiary magnetizations are overprints imparted during Variscan tectonism ('B', D/I = 194/6[degrees]) and Jurassic rifting within the adjoining Irish Sea Basin ('C', c. 160-140 Ma, D/I = 172/-52[degrees]). The Criffel-Dalbeattie pluton has more complex AMS fabrics recording both deformation and emplacement effects. Hematite of secondary hydrothermal origin is a significant feature of the rock magnetic record in the aureole, which is otherwise dominated by paramagnetism. The granodiorite is more strongly magnetized than the country rocks, accounting for a positive aeromagnetic anomaly. A fairly dispersed dual polarity remanence (mean D/I = 115/55[degrees], [[alpha].sub.95] = 18[degrees]) in granodiorite and late tectonic porphyrite dykes is probably the oldest magnetization preserved in this pluton because it correlates with an excursion of Britain into southerly palaeolatitudes at c. 410 Ma and indicates an Early Devonian emplacement age. The palaeofield at c. 397 Ma, the currently accepted isotopic age, is recorded by a minority overprinted remanence (mean D/I = 272/2[degrees], [[alpha].sub.95] = 12[degrees]) similar to the record in the Cairnsmoor of Fleet pluton and granites from the adjoining Lake District terrane. Granite complexes of the Southern Uplands Block collectively record regional rotation and excursion of Britain into southerly latitudes between c. 410 and 390 Ma. Comparable SilurianDevonian palaeomagnetic poles identify common apparent polar wander (APW) in paratectonic and orthotectonic terranes from the Variscan Front in the south to the Laurentian foreland in the north following climactic Acadian deformation. APW between 430 and 390 Ma embracing the (post-closure) history of the Caledonian orogen is a loop executed at rates much higher than typical rates of plate motion and appears to record a component of true polar wander. The ~ 110[degrees] arc length is identical to polar shift identified between mid-Silurian and Lower--Middle Devonian poles from Gondwana. The two paths superimpose to show that the western margin of Gondwana was in proximity to the SE margin of Laurentia during Acadian deformation in Early--Middle Devonian times and remote from the Caledonides; the residual Rheic Ocean subsequently closed by a combination of pivotal and left lateral strike-slip motions. Keywords: palaeomagnetism, Devonian, Caledonian, granites, Gondwana, palaeogeography.

A palaeomagnetic study and age determinations have been performed on Ediacaran basalts from the northwestern Ukraine. Whole-rock [40.sup.Ar]/[39.sup.Ar] age determination revealed plateau ages at 590-560 Ma and 393 Ma, the latter probably reflecting a resetting of the radiometric system. Palaeomagnetic poles have been calculated from five basalt flows, two of which (A poles) are considered reliable with ages that range from 580 to 560 Ma. Tentative poles (B poles), calculated from most probably primary magnetizations, have ages estimated at 580-545 Ma. Secondary magnetizations, possibly of late Ediacaran or Devonian age, have also been isolated (C poles). Based on the new poles, Baltica drifted together with Laurentia from an equatorial position at c. 750 Ma to occupy high southern latitude positions at c. 580 Ma. Baltica during that time period was joined to Laurentia in a similar relative position to that at 750 Ma. The two shields then split up from each other and from c. 550 Ma Baltica drifted at moderately high latitudes and rotated some 180[degrees] during the final opening of the Iapetus ocean. This reconstruction suggests that during the Ediacaran glaciation Baltica occupied high-latitude positions, which contradicts the high-obliquity model to explain low-latitude Neoproterozoic glaciations.

New paleomagnetic data from six different sections in the Bighorn Basin are combined with previously published results to construct a basin-wide magnetostratigraphic framework. Geomagnetic polarity reversals between chrons C26r, C26n, C25r, C25n, C24r, and C24n have been identified among multiple stratigraphic sections in different parts of the basin. Using the new magnetostratigraphic framework, paleontological, paleobotanical, and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world. These new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as Biohorizon B, which is slightly below the chron C24r-C24n boundary, close to the position of the Elmo isotope excursion in marine records. Backstripping analysis using these new magnetostratigraphic data helps define the time-transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin. Keywords: magnetostratigraphy, Paleocene, Eocene, Bighorn Basin, mammals, plants.

Abstract: The 2.21 Ga Nipissing diabase sills intrude the Huronian Supergroup of the Southern Province. Coeval Senneterre dykes in the adjacent portion of the Superior Province are potential feeders for [...]

Exposures of gabbros and mantle-derived peridotites at slow-spreading oceanic ridges have been attributed to extension on long-lived, low-angle detachment faults, similar to those described in continental metamorphic core complexes. In continental settings, such detachments have been interpreted as having originated and remained active at shallow dips. Alternatively, currently shallow dipping fault surfaces may have originated at moderate to steep dips and been flattened by subsequent flexure and isostatic uplift. While the latter interpretation would be more consistent with Andersonian faulting theory, it predicts large footwall tilts that have not been observed in continental detachment faults. Here we use the magnetization of oceanic gabbro and peridotite samples exposed near the Fifteen-Twenty Fracture Zone on the Mid-Atlantic Ridge to demonstrate that substantial footwall rotations have occurred. Widespread rotations ranging from 50[degrees] to 80[degrees] indicate that original fault orientations dipped steeply toward the spreading axis. Keywords: Ocean Drilling Program, peridotite, gabbro, mid-ocean ridge, paleomagnetism, detachment fault.

An integrated structural and paleomagnetic study was carried out on Neogene-Quaternary elastic sedimentary sequences exposed in north Calabria to define the brittle postorogenic evolution of the inner sector of the Calabrian arc. The structural, mineralogical, and magnetic fabric data presented in this paper suggest that the post-orogenic basins of north Calabria originated and developed under an extensional tectonic regime that has been active since the middle Miocene. Results were used to define the spatial and temporal evolution of the extensional fault systems in the framework of extensional processes active in the backarc region of the southeastward-migrating Apennine subduction system. Our data indicate that regional backarc extension operated in the brittle crust, and the data trace a continuous evolution from low- to high-angle extensional faulting. Paleomagnetic data show that this continuous extensional process was accompanied by complex interactions among fault-bounded crustal blocks. In fact, the extensional domain is subdivided into different structural compartments, separated by the adjustment in the fault strike imposed by the regional rotation pattern. Keywords: extension tectonics, Calabria, Italy, paleomagnetism, magnetic susceptibility, anisotropy, neutron diffraction analysis.

We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5-6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50[degrees] shifts in paleomagnetic direction are coincident with equally abrupt shifts in [[delta].sup.13]C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fields, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent's changing position relative to Earth's spin axis. Keywords: polar wandering, paleomagnetism, Svalbard, Neoproterozoic, carbon cycle, paleogeography.

Understanding the marine magnetic anomaly record is critical for constructing realistic geodynamo models of global geomagnetic field, polarity reversal mechanisms, and long-term geomagnetic field behavior. One of the least understood portions of the marine magnetic anomaly record is also the oldest part of the record, the Jurassic quiet zone (JQZ), where anomalies become weak and difficult to correlate. The reason for the existence of the JQZ is unclear. It has been suggested that the JQZ is a true polarity superchron, similar to the Cretaceous normal superchron. Continental magnetostratigraphic studies have suggested that the JQZ is a period of rapid polarity reversal, of low field intensity, or both. We show results of a deep-tow survey of Pacific Jurassic crust that confirms the existence of magnetic anomalies within the JQZ. We tie Ocean Drilling Program Hole 801C (167.4 Ma) into the record and show that seafloor-spreading magnetic anomalies are present around the hole and extend to 170 Ma crust. We find a rise in reversal rate with increasing age with reversal rates over 10 rev/m.y, at 160 Ma and at 167 Ma. Anomaly amplitudes decrease in the record from 155 Ma until 162 Ma, where low-amplitude anomalies are difficult to correlate. Prior to 167 Ma, anomalies regain amplitude and remain strong until the end of our record at 170 Ma. The JQZ thus appears to be a combination of low-amplitude magnetic anomalies combined with rapid field fluctuations, which could be due to either intensity or polarity changes. Keywords: Jurassic, magnetostratigraphy, marine magnetic anomalies, geomagnetism, polarity reversals.

[1] We report paleomagnetic results for 131 sites from the modern forearc of northern Chile (25[degrees]S and 28[degrees]S). Remanent magnetization in volcanic and intrusive rocks is mostly primary, while a secondary magnetization is observed in sedimentary rocks. Comparison of locality-mean directions with expected paleomagnetic directions indicates vertical axis rotations from -7.3[degrees] [+ or -] 21.6[degrees] counterclockwise to 52.7[degrees] [+ or -] 17[degrees] clockwise. Jurassic to Early Cretaceous rocks from the Coastal Cordillera and Cretaceous to Paleocene rocks from the Central Depression show similar magnitude (>30[degrees]) clockwise rotations, while more variable rotations occur in Mesozoic to Eocene rocks of the Precordillera. Clockwise rotations in Mesozoic and Paleogene rocks occur in the Chilean Frontal Cordillera south of 27[degrees]30'S. Paleomagnetic results in three large Miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the Precordillera and Pre-Andean Depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 Ma (early Miocene) and likely during and after the 'Incaic' tectonic event, which affected large tracts of the central Andes. The postulated onset of rotations in the north Chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the Eastern Cordillera in Bolivia and northwestern Argentina. Rotation of the Chilean forearc, enhancement of the curvature of the central Andes, and the formation of the Bolivian Orocline seem to be, for the most part, closely linked to the evolution of the Eocene-Oligocene tectonics of the Eastern Cordillera. Citation: Arriagada, C., P. Roperch, C. Mpodozis, and R. Fernandez (2006), Paleomagnetism and tectonics of the southern Atacama Desert (25-28[degrees]S), northern Chile, Tectonics, 25, TC4001, doi:10.1029/2005TC001923.

Abstract: This Lithoprobe-funded paleomagnetic study of the Early Proterozoic Wintering Lake granitoid body supports tectonic models that suggest continental accretion of the Trans-Hudson Orogen with the Superior Craton occurred at [...]

We report a magnetostratigraphic and rock magnetic study of the Yaha section, located on the southern flank of the central Tian Shan mountains, Asia. Our results show a two-fold increase in sedimentation rate as well as marked changes in rock magnetic characteristics ca. 11 Ma. After 11 Ma, sedimentation rate remained remarkably constant until at least 5.2 Ma. These findings are consistent with sedimentary records from other sections surrounding the Tian Shan. We conclude that uplift and erosion of the Tian Shan accelerated ca. 11 Ma, long after the onset of the collision between India and Asia, and that the range rapidly evolved toward a steady-state geometry via a balance between tectonic and erosion processes. Keywords: Tian Shan, uplift, erosion, magnetostratigraphy, Tarim.

Basin and Range normal faulting in west-central Idaho reactivates preexisting Cretaceous structures, resulting in a series of normal fault--bound basins. The most intense extensional deformation is concentrated in the Late Cretaceous western Idaho shear zone, resulting in the development of the Long Valley basin. The area affected by the western Idaho shear zone displays two orientations of steep faults: one set of normal faults strikes north-south and is parallel to fabrics within the western Idaho shear zone; the other set strikes east-west and accommodates components of both normal and strike-slip movement. Areas within the Idaho Batholith that do not have strong Cretaceous fabrics (i.e., outside of the western Idaho shear zone) are characterized by a strong preferred north-northeast orientation of faults. From this preferred orientation we infer that the maximum infinitesimal stretch is oriented at 110/290[degrees] in this part of the Idaho Batholith. Gravity inversion indicates that the north end of Long Valley is an asymmetric basin about one kilometer deep, with the largest basin-bounding normal fault on the west side of the Long Valley. Paleomagnetic analysis of the Columbia River basalts indicates that the north-south elongate fault blocks within the western Idaho shear zone have not rotated. One block, located just to the west of the western Idaho shear zone, may have rotated counterclockwise. The lack of rotation of north-south oriented fault blocks, in combination with the fault orientations of the Idaho Batholith, indicate that the regional neotectonic deformation within and east of the western Idaho shear zone is characterized by dextral transtension with a divergence vector oriented 130/310[degrees]. The extensional reactivation of the western Idaho shear zone demonstrates the effect of material anisotropy at local and regional scales. On a local scale, the mylonitic foliation of the western Idaho shear zone is reactivated as normal faults, even though the regional flow field is oblique to the foliation. On a regional scale, the possible counterclockwise fault block rotation recorded west of the western Idaho shear zone is inconsistent with dextral transtension, suggesting that extensional deformation has reactivated the western edge of the arc-craton boundary as a kinematic domain boundary. We conclude that the preservation of initial features in vertical shear zones and/ or plate boundaries is unlikely, due the tendency for well-developed, subvertical fabrics to reactivate, particularly in extension. Keywords: reactivation, western Idaho shear zone, anisotropy, paleomagnetism, neotectonics, transtension.

Global positioning system (GPS) data from the central Andes record vertical axis rotations that are consistently counterclockwise in Peru and Bolivia north of the bend in the mountain belt, and clockwise to the south in southern Bolivia, Argentina, and Chile. These geologically instantaneous rotations have the same sense as rotations that have accrued over millions of years and are recorded by paleomagnetic and geologic indicators. The change in sign of the rotation at both decadal and million-year time scales occurs across the axis of topographic symmetry that defines the Bolivian orocline. When extrapolated to a common time interval, the magnitudes of rotation from geologic features and from GPS are surprisingly similar, given that a significant part of the instantaneous deformation field is probably elastic and due to interseismic locking of the plate boundary. Some of the interseismic deformation field must reflect permanent deformation, and/or some of the current elastic deformation will be converted to upper-plate permanent deformation over time rather than be recovered by elastic rebound during interplate earthquakes. We suggest that the spatial patterns of the elastic and the permanent modes of bending are similar because they are driven by the same stress field. Keywords: central Andes, orocline, rotation, global positioning system, paleomagnetism.

In 2000, D.L. Wagner and colleagues hypothesized that the middle Miocene Yellowstone hotspot volcanism thought to have produced the great expanses of Columbia River and Oregon Plateau Basalts also gave rise to the Lovejoy Basalt of California. Paleomagnetic directions of lava flows of the Lovejoy Basalt in isolated localities scattered more than 200 km across northeastern and central California show that they were erupted rapidly and that some of them traveled great distances. Most of the paleomagnetic directions form a tight cluster distinct from the Miocene mean field direction for the region, indicating eruption within a relatively short time span compared to geomagnetic secular variation--that is, within a few hundred to a few thousand years. Directional correlations demonstrate that some flows traveled at least 75 km and likely as much as 200 km. These findings support the hypothesis that the Lovejoy flows are flood basalts that compose a large southwestward extension of Yellowstone hotspot volcanism. Keywords: flood basalts, Sierra Nevada, paleomagnetism, correlation, Columbia River Basalt.

Although Earth is widely believed to have undergone a series of extreme low-latitude snowball glaciations during the Neoproterozoic (ca. 1000-543 Ma), only one reliable paleomagnetic result, from Elatina, South Australia, places glacial rocks close to the equator. We report new paleomagnetic data from the Neoproterozoic Huqf Supergroup of Oman that pass fold and reversal tests and yield a paleopole at 52.3[degrees]S, 074.4[degrees]E (N = 25 sites; [[alpha].sub.95] = 7.3[degrees]). This paleopole places the Muscat region of Oman at a latitude of 13[degrees] in the late Neoproterozoic and provides the first direct evidence that both glacial and overlying cap carbonate units were deposited in the tropics. The presence of glacial-interglacial cyclicity within the Huqf Supergroup indicates that areas close to the equator may have been largely free of ice at the time of deposition, a result that is inconsistent with the classic snowball Earth model. Our result precludes the possibility that contrasting lithologies mark a phase of rapid plate motion and provides the first evidence for low-latitude glaciation in Arabia. A series of magnetic reversals in the Fiq tillite and the overlying Hadash dolomite, in northern and central Oman, correlates well with a similar sequence in the Mirbat Formation in southern Oman and indicates that recovery from glacial conditions took place over long time scales (possibly >[10.sup.5]-[10.sup.6] yr). Keywords: Oman, Neoproterozoic, paleomagnetism, glaciation, Marinoan, Arabia.

[1] The Sierra de las Estancias is a metamorphic core complex that developed during the late Oligocene to early Miocene extensional collapse of the Betic Cordillera. Structural and paleomagnetic analysis shows that the northern half of the complex has been strongly modified by rotation about an inclined axis, requiring reassessment of the kinematics of the extensional phase of deformation. The extensional tectonic event produced two major detachment faults, associated mylonitic foliation and lineation, and pervasive sets of shear bands and brittle normal faults with a top-to-NE shear sense. An older foliation was largely transposed and reoriented during this event. The regional foliation now defines a broad antiform with a steep north dipping limb; and the trend of shear-related lineations swings from NNE to ENE from the southern gently dipping limb into the north dipping limb. The remanent magnetization in a suite of near-vertical early Miocene mafic dikes intruded into the north dipping limb has a declination of 243[degrees] and an inclination of -27[degrees]. Assuming that the dikes were originally vertical, a rotation of approximately 50[degrees] about an inclined SSW plunging axis preserves the near-vertical orientation of the dikes and brings the remanence into agreement with the expected orientation for an early Miocene age. Correction for this rotation brings the foliation and lineation in the surrounding schists into orientations subparallel to those on the present southern limb of the antiform, and the dikes into a vertical E-W orientation. The corrected orientation of the shear-related lineation (NNE), normal faults, and dikes suggests that the original direction of extension during the late orogenic extensional event in the Internal Betic Cordillera may therefore have been around NNE/SSW, rather than E/W as currently accepted. The observed net tectonic rotation can be decomposed into a series of geologically plausible rotations involving dominostyle rotation due to slip on the late brittle normal faults, tilting about the regional fold axis related to thrust-ramping onto the Iberian margin during Miocene collision, and a 30[degrees] clockwise rotation about a vertical axis related to oblique convergence with the Iberian margin. INDEX TERMS: 8020 Structural Geology: Mechanics; 8120 Tectonophysics: Dynamics of lithosphere and mantle--general; 8110 Tectonophysics: Continental tectonics--general (0905); 8199 Tectonophysics: General or miscellaneous; KEYWORDS: core complex, paleomagnetism, structure, Betic Cordillera.