A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)

New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (dp = 5.4, dm = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the ⁴⁰Ar/³⁹Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.     

河北省唐山地区丰台—野鸡坨断裂第四纪活动性——来自14C和磁性地层年代学的证据

丰台—野鸡坨断裂为唐山地区主要断裂之一，西侧为鸦鸿桥凹陷，东侧为唐山凸起，断裂两侧第四系厚度之差巨大。本文依据该断层两侧钻孔对其第四纪以来活动性进行初步的探讨。通过对丰台—野鸡坨断裂上下两盘PZK14和PZK20孔磁性地层学研究，并结合钻孔岩石地层，及浅部光释光和¹⁴C测年结果，建立第四纪地层格架。结果表明：两孔底部“泥包砾”为新近纪沉积；PZK14孔下更新统底界埋深为387 m，中更新统底界埋深为114 m，上更新统底界埋深为71 m，全新统底界埋深为6 m；PZK20孔下更新统底界埋深为155 m，中更新统底界埋深为73 m，上更新统底界为36 m，无全新世地层。丰台—野鸡坨断裂活动在早更新世时表现为逐渐增强的特点，活动速率由早期的5.4 cm/ka增加到13.9 cm/ka。中更新世断裂活动基本处于停滞状态，活动速率为1.0 cm/ka。晚更新世以后，断裂重新活动，且更加剧烈，活动速率达到了54.5 cm/ka。     

Magnetostratigraphic evidence from the Cold Creek bar for onset of ice-age cataclysmic floods in eastern Washington during the Early Pleistocene

This study provides a detailed magnetostratigraphy of sediments composing the Cold Creek cataclysmic flood bar in the Pasco Basin, Washington. Our interpretation suggests onset of Missoula floods or similar events prior to 1.1 myr, later than previously suggested by Bjornstad et al. [Bjornstad, B.N., Fecht, K.R., Pluhar, C.J., 2001. Long history of pre-Wisconsin, Ice Age cataclysmic floods: evidence from southeastern Washington State. Journal of Geology 109 (6), 695-713]. Nonetheless these data suggest that Channeled Scabland features formed over a much longer timespan than commonly cited, that continental ice sheets of the early Pleistocene reached as far south as those of the late Pleistocene, and that similar physiography existed in eastern Washington and perhaps Montana to both generate and route Missoula-flood-like events. This study adds paleomagnetic polarity results from 213 new samples of silts and sands derived from nine new drill cores penetrating the Cold Creek cataclysmic flood bar to our previous database of 53 samples from four boreholes, resulting in a much more robust and detailed magnetostratigraphy. Rock magnetic studies on these sediments show pure magnetite to be the predominant remanence-carrying magnetic mineral, ruling out widespread remagnetization by secondary mineralization. The magnetostratigraphy at eastern Cold Creek bar is characterized by a normal polarity interval bracketed by reversed polarities. Equating the normal zone with the Jaramillo subchron (0.99-1.07 myr) affords the simplest correlation to the magnetic polarity timescale. Western Cold Creek bar was likely deposited during the Brunhes chron (0-0.78 myr) since it exhibits mainly normal polarities with only two thin reversed-polarity horizons that we interpret as magnetic excursions during the Brunhes.     

Paleoproterozoic (2155–1970 Ma) evolution of the Guiana Shield (Transamazonian event) in the light of new paleomagnetic data from French Guiana

We present a comprehensive paleomagnetic study on Paleoproterozoic (2173–2060 Ma) plutonic and metamorphic rocks from French Guiana, representative of the full range of the main Transamazonian tectonothermal steps. Twenty-seven groups of directions and poles were obtained from combination of 102 sites (613 samples) based on age constraint, similar lithology and/or geographical proximity. Paleomagnetic results show variations between rocks of different ages which are supposed to be characteristic of magnetizations acquired during uplift and cooling of successive plutonic pulses and metamorphic phases. This is also reinforced by positive field tests (baked contact and reversal tests). Recent U/Pb and Pb/Pb on zircon and complementary ⁴⁰Ar/³⁹Ar on amphibole and biotite allow questioning the problem of magnetic ages relative to rock formation ages. Estimated magnetic ages, based on amphibole dating as a proxy, enable us to construct a Guiana Shield apparent polar wander path for the 2155–1970 Ma period. It is also possible to present paleolatidudinal evolution and continental drift rates related to specific Transamazonian tectonic regimes.French Guiana and probably the Guiana Shield were located at the Equator from ca. 2155 to 2130 Ma during the Meso-Rhyacian D1 magmatic accretion phase, related to subduction of Eorhyacian oceanic crust. After closure of the Eorhyacian Ocean and collision of West African and Amazonian plates, the Guiana Shield moved. The first evolution towards 60° latitude, occurs after 2080 Ma, during the Neorhyacian D2a post collisional sinistral transcurrent phase. During the Late Rhyacian D2b phase, up to 2050 Ma, the Guiana Shield reaches the pole and starts to move to lower latitudes on an opposite meridian. By the Orosirian D2c phase, from ca. 2050 to 1970 Ma, the Guiana Shield reaches the Equator.Based on the amphibole ⁴⁰Ar/³⁹Ar dates, we estimate the continental drift between 12 and 16 cm/y for the Meso to Late Rhyacian period followed by a lower rate between 9 and 14 cm/y up to Orosirian time. This study highlights rock ages and magnetic ages are prerequisite to any continental reconstruction especially when it is shown continental drift is important for a 100–200 Ma time period. Our results confirm the possibility of APWP construction on Paleoproterozoic plutonic rocks but suggest improvement will rely on the combination with multidisciplinary approaches such as structural geology and multi-method radiometric dating.     

Multiple magnetization events in the Red River carbonates, Williston Basin, Canada: Evidence for fluid-flow?

Samples collected from the Upper Ordovician Red River carbonates in a well at the centre of the Williston Basin revealed two paleomagnetic components with different inclinations, 60.3 ± 3.9° (k = 70.7, N = 12) and 20.4 ± 3.3° (k = 141.2, N = 8), but similar declination values in individual specimens. Inclination-only analysis indicates two possible scenarios for the age of these two magnetizations: in scenario (a) the timing of magnetization happened sometime between Late Ordovician to Devonian; and in scenario (b) there are two different remagnetizations, one that overlaps Pennsylvanian to Permian time while the other can have either a Late Jurassic or a Tertiary age. Whereas dolomitization and some isotopic data tend to support scenario (a), previous paleomagnetic data from the Williston Basin and from younger units in the same well, the tectonic evolution of the basin, and the hydrocarbon maturation pattern in the Red River carbonates all favour chemical remagnetization(s) driven by orogenic fluids during the Alleghenian and Laramide orogenies.     

The growth, collapse and quiescence of Teno volcano, Tenerife: new constraints from paleomagnetic data

Tenerife basically consists of three Miocene shield volcanoes, the Anaga, the Teno and Central shield, as well as the Pliocene Cañadas volcano. The temporal evolution and structural significance of each volcano with respect to the history of Tenerife is still a matter of debate. We present paleomagnetic results in order to enhance the view of the volcanic history of the Teno volcano by means of magnetostratigraphy. It is found that the initial subaerial phase shows reverse magnetizations throughout. After two major sector collapses, dominantly normally magnetized lavas extruded. Comparisons of observed magnetic polarities with the geomagnetic polarity timescale show that these volcanic activities occurred within 0.4 Myr between 6.3 and 5.9 Ma. Significantly younger flows, ～ 5.3 Myr old according to their radiometric age, revealed again normal polarity throughout. The absence of inversely magnetized lavas in-between the two normal periods indicates a volcanic hiatus or erosional phase. The evolutionary sequence and the estimated high production rates for the initial building phase are similar as would be expected for a hotspot volcano. The average geomagnetic field for 6.0 ± 0.2 Ma is close to an axial dipole field showing a slight far-sided/right-handed effect. The field strength, determined by Thellier-type intensity determinations, corresponds to a virtual axial dipole moment of 4.9 × 10²² A m². This value is approximately half of the present day field strength, but similar to values obtained for the mid-Miocene. It also corresponds to the proposed tertiary low-field level of the geomagnetic dipole moment.     

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α₉₅=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.     

Paleomagnetic dating of Alleghanian orogenesis and mineralisation in the Mascot–Jefferson City zinc district of East Tennessee, USA

The Mascot–Jefferson City (M-JC) Mississippi Valley-type (MVT) deposits are in the Valley and Ridge province of the Appalachian orogen in East Tennessee. They have been a major source of zinc for the USA but their age is uncertain and thus their genesis controversial. About 10 specimens from each of 37 sites have been analysed paleomagnetically using alternating field and thermal step demagnetisation methods and saturation isothermal remanence methods. The sites sample limestones, dolostones, breccia clasts and sphalerite–dolomite MVT mineralisation from mines in the Lower Ordovician Kingsport and Mascot formations of the Knox Group. The characteristic remanent magnetisation (ChRM) is carried by magnetite in the limestones, by both magnetite and pyrrhotite in the dolostones and by pyrrhotite preferentially to magnetite in the mineralisation. Mineralized sites have a more intense ChRM than non-mineralised, indicating that the mineralising and magnetisation event are coeval. Paleomagnetic breccia tests on clasts at the three sites are negative, indicating that their ChRM is post-depositional remagnetisation, and a paleomagnetic fold test is negative, indicating that the ChRM is a remagnetisation, and a post-dates peak Alleghanian deformation. The unit mean ChRM direction for the: (a) limestones gives a paleopole at 129°E, 12°N (d_p=18°, d_m=26°, N=3), indicating diagenesis formed a secondary chemical remanent magnetisation during the Late Ordovician–Early Silurian; (b) dolomitic limestones and dolostone host rocks gives a paleopole at 125.3°E, 31.9°N (d_p=5.3°, d_m=9.4°, N=7), recording regional dolomitisation at 334±14 Ma (1σ); and (c) MVT mineralisation gives a paleopole at 128.7°E, 34.0°N (d_p=2.4°, d_m=4.4°, N=25), showing that it acquired its primary chemical remanence at 316±8 Ma (1σ). The mineralisation is interpreted to have formed from hydrothermal fluid flow, either gravity or tectonically driven, after peak Alleghanian deformation in eastern Tennessee with regional dolomitisation of the host rocks occurring as part of a continuum during the 20 Ma prior to and during peak deformation.     

四川江油-北川地区下石炭统灰岩的古地磁研究

对采于四川北部江油 北川地区 (31.4°N ,10 4 .3°E)下石炭统 39个采点的 334块灰岩标本进行了较为系统的古地磁学研究 ,结果表明 ,标本中剩磁方向具有明显的双分量特征 ,低温 (10 0～ 30 0℃ )分量在地理坐标系下与现代地磁场方向基本一致 ,且不能通过褶皱检验 ,应是现代地磁场的重磁化 ,中温 (30 0～ 4 80℃ )反极性分量在地层坐标系中具有高负倾角的特征 ,其形成可能与早侏罗世岩层的褶皱弯曲有关 .     

Paleozoic terranes of eastern Australia and the drift history of Gondwana

Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps.