Palaeomagnetic Data Research Articles

New constraints on the evolution of the Gibraltar Arc from palaeomagnetic data of the Ceuta and Beni Bousera peridotites (Rif, northern Africa)

The Betic Cordillera and the Moroccan Rif together form one of the smallest and tightest orogenic arcs on Earth and almost completely close the Mediterranean to the west. For the explanation of the geodynamic evolution of the mountain belt, palaeomagnetic data that generally found clockwise block rotations in the Iberian and anticlockwise rotations in the Moroccan part of the mountain belt, have played a key role in recent works. This palaeomagnetic study has found new constraints on the rotations and timing of the peridotitic bodies outcropping in the key position at the westernmost margin of the mountain belt, in Ceuta and Beni Bousera (Rif, northern Africa).Detailed thermal demagnetization of 115 individually oriented samples from 14 sites was combined with rock magnetic and scanning electron microscopic experiments to analyze the magnetic mineralogy responsible for the remanences and the mechanisms and relative times of their acquisition. In Ceuta, up to three magnetic components, and in Beni Bousera, up to two magnetic components have been found, that are all to be interpreted as chemical remanent magnetizations (CRM). The data suggests the following succession of geodynamic events affecting the peridotites until recent times: (1) after their exhumation and subsequent cooling about 20Ma ago, they recorded a characteristic remanent magnetization of both normal and reversed polarities, carried by (pseudo-)single-domain magnetite grains; (2) after their dismembering, the Ceuta peridotites were tilted southward by 22–34° about a horizontal or tilted axis (up to plunge 50°) with an azimuth of 72–145° and the Beni Bousera peridotites were rotated anticlockwise by 72.3±12.1° about a vertical axis and (3) both recorded another magnetic signal of normal polarity only, carried by multi-domain magnetite grains; and finally (4) the Ceuta peridotites rotated anticlockwise by 19.7±5.9° about a vertical axis.This study provides the first palaeomagnetic data for the Ceuta peridotites that, with their tilt and recent small net rotation, had a distinct geodynamic evolution from the large net rotations about vertical axes in Beni Bousera and Ronda (Betic Cordillera). Moreover, earlier palaemagnetic data for Beni Bousera is improved, as mixed polarities have been found in the older of the remanences for the first time, and its interpretation as a CRM changes the rotation timing that was proposed previously. The sequence of events exposed in this work are important constraints that need to be incorporated in any geodynamic model of the evolution of the Betic–Rifean mountain belt.

Non-Dipole and Regional Effects on the Geomagnetic Dipole Moment Estimation

The study of the temporal evolution of the dipole moment variations is a forefront research topic in Earth sciences. It constrains geodynamo simulations and is used to correct cosmogenic isotope production, which is evidence of past solar activity, and it is used to study possible correlations between the geomagnetic field and the climate. In this work, we have analysed the main error sources in the geomagnetic dipole moment computation from palaeomagnetic data: the influence of the non-dipole terms in the average approach, the inhomogeneous distribution of the current palaeomagnetic database, and the averaging procedure used to obtain the evolution of the dipole moment. To evaluate and quantify these effects, we have used synthetic data from a global model based on instrumental and satellite data, the International Geomagnetic Reference Field: 11th generation. Results indicate that the non-dipole terms contribute on a global scale of <6 % in the averaged dipole moment, whereas the regional non-dipole contribution can show deviations of up to 35 % in some regions such as Oceania, and different temporal trends with respect to the global dipole moment evolution in other ones, such as Europe and Asia. A regional weighting scheme seems the best option to mitigate these effects in the dipole moment average approach. But when directional and intensity palaeomagnetic information is available on a global scale, and in spite of the inhomogeneity of the database, global modelling presents more reliable values of the geomagnetic dipole moment.

Widespread Cretaceous secondary magnetization in the High Atlas (Morocco). A common origin for the Cretaceous remagnetizations in the western Tethys?

In this paper, we present the first palaeomagnetic data (51 sites) for Mesozoic (Lower–Middle Jurassic) sediments of the Moroccan Central High Atlas and address the study of a widespread remagnetization. The remagnetization is characterized by a very stable component with systematic normal polarity, carried by magnetite. The relationship between the magnetic properties and location within the basin suggests that the acquisition of the remagnetization is controlled by basin geometry. Fold-tests indicate that the overprint acquisition is syn-folding in some structures but clearly predates the Tertiary compressional stage. Using the small circle intersection method we have calculated the remagnetization direction ( D = 336.4°, I = 29.2°). Comparison with the global apparent polar wander path indicates that the remagnetization was acquired during the Late Cretaceous (probably Cenomanian). Considering both the basinal confinement of remagnetization and the connection with other remagnetization events in the western Tethys, we propose a scenario explaining widespread remagnetizations in the region, concerning both basin-scale conditions mainly related to sediment thickness and a regional-scale thermal event acting as catalyst of remagnetizations in those sedimentary basins that satisfy the basin-scale conditions.

NEW DATA ABOUT MATUYAMA-BRUNHES BOUNDARY IN ROXOLANY SECTION

From new palaeomagnetic data of 55-meter key section of Quaternary loess-soil sediments on the Dniester estuary near Roxsolany village the position of Matuyama-Brunhes boundary have been determined. Earlier this section had been studied by different research groups [Третяк, 1983; Третяк и др., 1987; Третяк и др., 1989; Tsatskin et al., 1998; Tsatskin et al., 2001; Dodonov et al., 2006; Gendler et al., 2006] and the position and ages of individual soil horizons caused much discussion. The primary component of natural remanent magnetization and their polarity was identifying both in loess and soil horizons due to detail measurements on the modern equipments. Position of Matuyama-Brunhes boundary has been identified at a depth of 46.6 m between Martonosha and Lubny soil horizons. This is agree with modern knowledge of Ukrainian scientists about Quaternary records in south Ukraine and is not consistent with the results which identified this boundary at a depth of 34 m in loesses above the level of РК7.

Triassic tectonics and mineral systems in the Qinling Orogen, central China

The Qinling Orogen in central China preserves the records of a complex geological and tectonic history, and also carries abundant mineral resources. Previous investigations on the Qinling Orogen have identified that the final transition from the Palaeo‐Tethys Ocean to a continental orogen occurred in the Triassic, although the timing of the oceanic closure and the tectonic affinity of the Triassic Qinling belt have been poorly constrained. In this contribution, we compile the palaeomagnetic data and geological evidence related to sedimentation, magmatism and mineral systems. The palaeomagnetic data show that the Palaeo‐Tethys Ocean in the Qinling Orogen gradually closed westward during the Triassic, terminating in the beginning of the Jurassic. Triassic strata are widely developed in the Qinling Orogen and conformably overlie the Permian strata. They are composed of carbonates and siliciclastic rocks that were strongly deformed and weakly metamorphosed during the Late Triassic to Late Jurassic. The Triassic magmatic rocks are dominated by I‐type granitoids, with subordinate S‐type and alkaline granites and minor carbonatite dykes, with a clear northward transition in lithology and geochemical features from the Mian‐Lue Suture. Different types of Triassic mineral systems have been reported from this region, including sedimentary‐hosted epizonogenic hydrothermal Hg–Sb, Pb–Zn ± Ag, Au deposits, metamorphic hydrothermal lodes of Au, Ag–Pb–Zn, Mo–Au and Mo, magmatic hydrothermal systems including porphyry Mo, breccia pipe Au, and carbonatite‐hosted Mo deposits. We infer that the Triassic Qinling Orogen witnessed synchronous oceanic plate subduction in the west and continental linkage in the east, with a transition from oceanic basin to continental orogen through gradual subduction‐related consumption of the oceanic plate. The Palaeo‐Tethys Ocean closed in a westward scissor‐like fashion in the Triassic and continental collision and extensive crustal shortening in the Jurassic, following which post‐collisional extension occurred in the Early Cretaceous. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Correcting for inclination shallowing of early Carboniferous sedimentary rocks from Kyrgyzstan—indication of stable subtropical position of the North Tianshan Zone in the mid-late Palaeozoic

SUMMARY High-quality palaeomagnetic data for the early Carboniferous of Central Asia are scarce and the palaeogeographic evolution of this area prior to final amalgamation of the region east of the Ural mountains is still rather obscure. Here, we present palaeomagnetic data for early Carboniferous deposits from two areas in the Kyrgyz North Tianshan (NTS). Detailed rockmagnetic analysis indicates the presence of magnetite and haematite as magnetic carriers in these red sediments. In the Kazakh basin section (KEL), we identify a high-temperature component (HTC) of magnetization during stepwise thermal demagnetization at temperatures of up to ∼680 ◦ C yielding a site mean direction of D = 176.2 ◦ , I =− 36.4 ◦ , k = 57.4 and α95 = 8.9 ◦ after tilt correction. Two HTCs of magnetization were identified in samples from the Sonkul Basin (DUN) with maximum blocking temperatures of ∼600 ◦ C (magnetite) and ∼680 ◦ C (haematite). The magnetite component was also identified with alternating field demagnetization. The resulting site mean directions for these two components identified in 16 and 14 sites, respectively, are D = 149.3 ◦ , I =− 50.3 ◦ , k = 73.6 and α95 = 4.3 ◦ for the magnetite and D = 139.6 ◦ , I =− 35.1 ◦ , k = 71.6 and α95 = 4.7 ◦ for the haematite component. All three mean directions show a significant increase of the precision parameter k after tilt correction indicating acquisition of the high-temperature magnetization prior to the main folding event in the Jurassic. We explain the difference of the two components of DUN by a process of inclination bias due to compaction to which the platy haematite particles are more susceptible. Applying the elongation-inclination (E/I) method to directional data from over 100 individual samples from location DUN results in a negligible correction for the magnetite component (<5 ◦ ), whereas the inclination of the haematite component corrects from −35.0 ◦ to −50.3 ◦ (f = 0.6, error interval −41.4 ◦ to −57.9 ◦ ), which is then equal to the uncorrected magnetite inclination. The small number of samples from section KEL does not allow application of the E/I technique and inclination correction based on high field anisotropy of isothermal remanent magnetization was applied, yielding a corrected inclination of −75.2 ◦ ± 4 ◦ . Assuming comparable degrees of compaction for both study areas and applying the flattening factor obtained in DUN on samples from KEL, however, would result in comparable inclinations. The identification of inclination shallowing at both sections indicates that the age of magnetization is close to the deposition age. Assuming a reversed polarity of the directions from both areas results in palaeolatitudes of ∼30 ◦ N for section DUN and ∼60 ◦ N for the anisotropy-based correction of section KEL. The large difference, however, is geologically very unlikely. The inclination of the magnetite component of DUN (unaffected by inclination shallowing) favours a palaeoposition of ∼30 ◦ N. This is supported by the inclination shallowing corrected haematite component of DUN yielding a comparable inclination. Therefore, our results indicate that the NTS domain was situated at ∼30 ◦ Ni n the early Carboniferous. Furthermore, the NTS zone was probably not connected to Baltica or Siberia prior to the late Palaeozoic.

40Ar/39Ar geochronology and the paleoposition of Christmas Island (Australia), Northeast Indian Ocean

The Christmas Island Seamount Province is an extensive zone of volcanism in the Northeast Indian Ocean, consisting of numerous submerged seamounts and flat-topped guyots. Within this region lies two subaerial island groups, Christmas Island, and the Cocos Keeling archipelago. Christmas Island has experienced multiple episodes of volcanism that are exposed sporadically along its coastline. Here, we dated these volcanics using 40Ar/39Ar geochronology and analysed them for paleomagnetism. The oldest exposed volcanism occurred in the Eocene between 43 and 37Ma. This is followed by a time gap of ~33million years, before the eruption of a younger episode of Pliocene age (4.32±0.17Ma). It has, however, been suggested by previous workers that there is a much older Late Cretaceous event beneath the limestone which is unexposed. In addition, this study conducted the first paleomagnetic analysis of samples from Christmas Island to determine its paleoposition and the paleomagnetic polarity of the sampled sites. Two normal and two reversal magnetic events have been recorded, that agree with the geomagnetic reversal timescales. Late Eocene (38–39Ma) palaeomagnetic data suggest a palaeolatitude of −43.5°−11.2°+9.0°, which is further south than palaeolatitudes (around 30° S) derived from existing plate reconstruction models for the Australian plate. However, the Late Eocene palaeomagnetic data are limited (only two sites) and secular variation may not have been averaged out. During the Pliocene (ca. 4Ma) we estimate a palaeolatitude of approximately 13° S. The presence of the Late Eocene ages at Christmas Island correlates well with the cessation of spreading of the Wharton Ridge (~43Ma), the initiation of spreading along the South East Indian Ridge, and the transit of Christmas Island over a broad low velocity zone in the upper mantle. This suggests that changes in stress regimes following the tectonic reorganisation of the region (prior to~43Ma) may have allowed deeper-origin mantle melts to rise. Similarly, changes in the plate's stress regime at the flexural bulge of the Sunda–Java subduction zone may be implicated in renewed melting at ~4Ma, suggesting that tectonic stresses have exerted a first-order effect on the timing and emplacement of volcanism at Christmas Island.

Pliocene to Pleistocene vertical movements in the forearc of the Lesser Antilles subduction: insights from chronostratigraphy of shallow-water carbonate platforms (Guadeloupe archipelago)

An integrated stratigraphic study was conducted on the shallow water carbonate platforms of the Guadeloupe archipelago to refine the tectonic evolution of the Lesser Antilles forearc. The carbonate platforms are now dated to the Zanclean–Calabrian interval, and their demise occurred between 1.5 and 1.07 Ma. The precise chronostratigraphy allows dating of the main extensional tectonic events since the late Miocene. An initial episode occurred during the late Miocene, related to the reactivation of inherited N130°E-trending shear zones, and led to the emergence of most parts of the forearc. Subsequently, Zanclean to early Piacenzian carbonate platforms developed in association with a general subsidence of the forearc. During the late Piacenzan, a second extensional episode occurred. At this time La Désirade underwent major uplift and emergence whereas most of the forearc remained submerged. Prior to 1.07 Ma, a third north–south extensional episode occurred and led to the final demise of the carbonate platforms. Thus the forearc was characterized by general subsidence since the early Pliocene interrupted by three main extensional episodes and related differential uplifts. This suggests that the Lesser Antilles subduction is probably erosive north of latitude 15°N since c . 5 Ma, related to aseismic ridge subduction. Supplementary data: Planktonic foraminiferal assemblages, calcareous nannofossil taxa associations, 40 Ar/ 39 Ar geochronological data and palaeomagnetic data are available at www.geolsoc.org.uk/SUP18724 .

New palaeomagnetic data from metamorphosed carbonates of Western Oscar II Land, Western Spitsbergen

* corresponding author Abstract: Palaeomagnetic−petrographic−structural analyses of Proterozoic-Lower Palaeozoic metamorphosed carbonates from 12 locations within Oscar II Land (Western Spitsbergen) have been carried out to determine their usefulness in palaeogeographic reconstructions for Caledonian time. Structural analyses confirm that metacarbonates record several stages of deformation: D1, D2 ductile phases to Caledonian metamorphism and a D3 brittle phase to Late Cretaceous-Paleogene evolution of the West Spitsbergen Fold Belt. The latter is represented by thrust faults, localized folds with strain slip cleavages and late extensional collapse. Petrographic investigations reveal that Caledonian greenschist facies metamorphism was characterized by the high activity of H2O−CO2−rich fluids which pro− moted extensive recrystallization and within−rock spatial reorganization of sampled meta− carbonates. Microscopic, SEM and microprobe analyses exclude the existence of any pri− mary pre−metamorphic ferromagnetic minerals (primary−related to sedimentation and or early diagenesis) and point to metamorphic 4C superstructure (Fe7S8) as the main ferromagnetic carrier in investigated rocks. This is confirmed by the three−component iso− thermal remanent magnetization (IRM) procedures and the results of thermal demagnetiza− tions. In 12 sites a total number of 72 independently oriented palaeomagnetic samples were collected from which 181 specimens were drilled and thermally demagnetized. Sampled metacarbonates are weakly magnetized (NRM <0.2mA/m). The statistically significant palaeomagnetic results were achieved only from 1 of 12 investigated sites. In one site situ− ated in the Western overturned limb of the Holmesletfjellet Syncline intermediate unblock− ing temperatures - pyrrhotite related component WTSJ5M superimposed on the S1 Cal− edonian schistosity was recognized (D = 100.7, I = −21.495% = 5.5, = 58.23). Coinci− dence of WTSJ5M with Silurian-Devonian sector of the Baltica reference path after unfold− ing of the syncline by the angle of 130 suggests synfolding origin of this direction. Further, this suggests that Holmesletfjellet Syncline originated as an open fold and has been trans− formed into an overturned syncline during the Late Caledonian shortening or in the Late Cretaceous-Palaeogene time.

Early Permian 90° clockwise rotation of the Maures–Estérel–Corsica–Sardinia block confirmed by new palaeomagnetic data and followed by a Triassic 60° clockwise rotation

Abstract Palaeomagnetic investigations of the Corso-Sardinian block and Maures–Estérel show that there has been a change in their magnetic orientation during the Late Carboniferous–Early Permian period (305–280 Ma). This trend is interpreted in terms of a large-scale 90° clockwise rotation of the southern branch of the Variscan belt that matches the successive change in shortening directions revealed by structural geology. The evidence is based on existing structural studies of the fabrics of syntectonically emplaced granitoids partly based on the anisotropy of magnetic susceptibility, combined with a large database of isotopic ages. The chronological match between the palaeomagnetic and tectonic datasets is interpreted here as a result of large-scale dextral wrench movements in the lithosphere between the Gondwana and Laurussia supercontinents. This wrench deformation is regarded as a sequel to the dextral rotation of the northern branch of the Variscan belt during 330–315 Ma which terminated in frontal collision with Avalonia. The continuation of movement in the southern Variscan realm was due to shearing along the southern margin of the Avalonian block. An additional clockwise rotation is inferred to have taken place during the Triassic period. The age of this motion remains to be determined. Supplementary material: Palaeomagnetic and geochronological data from the Maures–Estérel, Corsica–Sardinia block presented in Figure 7 and discussed in the text are available at http://www.geolsoc.org.uk/SUP18742 .

A palaeomagnetic and 40Ar/39Ar study of mafic dykes in southern Sweden: A new Early Neoproterozoic key-pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops

We present the results of palaeomagnetic and 40Ar/39Ar studies of the Proterozoic mafic dykes in the Norrköping and Falun areas of the southern Sweden. The primary remanence of two 939±3Ma dykes is supported by the rigorous baked contact test. The remanence direction of two other dykes, one of which was previously U–Pb dated at 946±1Ma is close to the reverse direction of 939Ma dykes. Using these results together with previously published 935±5Ma palaeomagnetic data from the Göteborg-Slussen mafic dykes and some dykes from the Falun area we calculated the mean 946–935Ma palaeopole for Baltica (0.9°S, 240.7°E, A95=6.7), which can be qualified as the key pole. Using this pole together with other date we conclude that the Grenville and Sveconorwegian loops of Laurentian and Baltican Apparent Polar Wander Paths are temporary displaced by 100–150m.y. We propose new palaeogeographic reconstructions of Baltica and Laurentia at ca. 940Ma and ca. 850Ma. We also present two new Mesoproterozoic non-key poles from 1410Ma and 1595Ma dykes.

Macquarie Arc and the Lachlan Orocline hypothesis: Magnetic analysis and development of geologically constrained forward model of lithospheric magnetisation

Potential field analysis, with focus on magnetic interpretation is ideally suited to geological problems where the targets are deep, totally undercover, or when there is little or no outcrop available for mapping or sampling. In these scenarios understanding the location, orientation and relationships between deep geological structures is the key to developing a clear picture of an areas tectonic history. In this project, we conduct a comprehensive magnetic analysis of the Lachlan Orocline and Macquarie Arc, both located in the Lachlan Orogen, eastern Australia to test the potential relative rotation of this block to the surrounding units. These results are compared with palaeomagnetic data from the region and a series of geologically constrained crustal-scale forward models of lithospheric magnetisation for the project area, with focus on the large-scale structural components of the Lachlan Orocline. The forward model method treats each geological unit present in both the Macquarie Arc itself and the surrounding areas as independent parameterised stratigraphic units. Using a Geographical Information System (GIS) approach, known geological, structural and physical properties are used to produce a series of vertically integrated value grids. These grids are then used as input to model the regional magnetisation in the global scale magnetic field forward model.

Evidence of late Gelasian dispersal of African fauna at Coste San Giacomo (Anagni Basin, central Italy): Early Pleistocene environments and the background of early human occupation in Europe

Since the late 70s, the Early Pleistocene (Gelasian) site of Coste San Giacomo (Anagni Basin, central Italy) has been known amongst palaeontologists for its diverse vertebrate fauna. During the last 5 years, new excavations and the drilling of a 46-m-deep core have provided novel pieces of information. Palaeomagnetic data, pollen and small vertebrates analyses are presented here for the first time and combined with the updated list of the large vertebrates and ostracod analysis in a multidisciplinary perspective. Large and small mammals, pollen and ostracod analyses have allowed an integrated palaeoenvironmental reconstruction of the sedimentary sequence, depicting the evolution of the alluvial plain in the surrounding landscape. Moreover, magnetostratigraphy, pollen and small mammal biochronological data have confirmed the position of the Coste San Giacomo Faunal Unit, focusing the possible age of the mammal assemblage around 2.1 Ma, in a reversed phase before the base of the Olduvai chron. In particular, the occurrence of the large vole Mimomys pliocaenicus has important biochronological significance. The Coste San Giacomo site offers a unique opportunity to investigate the faunal and environmental changes that occurred in Mediterranean Europe during the Early Pleistocene, coinciding with major climatic changes at a global scale. The occurrence of taxa such as Hippopotamus sp. in the assemblage provides evidence of early dispersal events of African taxa prior to the early Homo diffusion into Europe.

Palaeomagnetism and magnetic fabrics of the Late Palaeozoic volcanism in the Castejón-Laspaúles basin (Central Pyrenees). Implications for palaeoflow directions and basin configuration

AbstractThe Castejón-Laspaúles basin is one of the South Pyrenean basins of Late Variscan age that were strongly inverted during the Alpine compression (Late Cretaceous–Tertiary). It is mainly composed by Stephanian pyroclastic and volcanic deposits that reach a maximum thickness of ~ 500 m, and are overlain by Permian and Triassic sedimentary units. A palaeomagnetic and magnetic fabrics (AMS) study was carried out in the Stephanian units, where the general absence of flow markers at the outcrop scale and the Alpine inversional structure prevent the straightforward reconstruction of the original volcanic and basinal configuration. Magnetic fabric data are not overprinted by Alpine internal deformation and can be interpreted in terms of primary volcanic and pyroclastic fabrics. The obtained directions coincide in the different sampled units, suggesting a constant source area during the development of the basin, and show the dominance of N–S-trending K1axes that are interpreted to be parallel to flow directions. Palaeomagnetic data indicate the presence of a pre-folding palaeomagnetic component that is rotated clockwise by an average of +37° (±32°) with regards to the Stephanian reference. This rotation probably took place during Alpine thrusting since it is also registered by the overlying Triassic deposits. The whole dataset is interpreted in terms of basin development under sinistral transtension with two main fault sets: deep-rooted E–W-striking faults, probably responsible for magmatic emissions, and shallow-rooted, listric faults of N–S orientation.

The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana

This paper presents combined U/Pb, Th/U and Hf isotope analyses on detrital and magmatic zircon grains together with whole-rock geochemical analyses of two basement and eight sedimentary rock samples from the Namuskluft and the Dreigratberg in southern Namibia (Gariep Belt). The sedimentary sections evolved during the Cryogenian on the SW part of the Kalahari Craton and where therefore deposited in an active rift setting during the break-up of Rodinia. Due to insufficient palaeomagnetic data, the position of the Kalahari Craton within Rodinia is still under discussion. There are possibilities to locate Kalahari along the western side of Australia/Mawsonland (Pisarevski et al. in Proterozoic East Gondwana: supercontinent assembly and break-up, Geological Society, London, 2003; Evans in Ancient Orogens and modern analogues. Geological Society, London, 2009; and others) or together with the Congo-Sao Francisco and Rio de la Plata Cratons (Li et al. in Prec Res 45: 203–2014, 2008; Frimmel et al. in Int J Earth Sci (Geol Rundsch) 100: 323–354, 2011; and others). It is sill unclear which craton rifted away from the Kalahari Craton during the Cryogenian. Although Middle to Upper Cryogenian magmatic activity is known for the SE Kalahari Craton (our working area) (Richtersveld Suite, Rosh Pinah Fm), all the presented samples show no U/Pb zircon ages younger than ca. 1.0 Ga and non-older than 2.06 Ga. The obtained U/Pb ages fit very well to the exposed basement of the Kalahari Craton (1.0–1.4 Ga Namaqua Province, 1.7–2.0 Ga Vioolsdrif Granite Suite and Orange River Group) and allow no correlation with a foreign craton such as the Rio de la Plata or Australia/Mawsonland. Lu–Hf isotopic signatures of detrital zircon point to the recycling of mainly Palaeoproterozoic and to a smaller amount of Archean crust in the source areas. eHf(t) signatures range between −24 and +14.8, which relate to TDM model ages between 1.05 and 3.1 Ga. Only few detrital zircon grains derived from magmas generated from Mesoproterozoic crustal material show more juvenile eHf(t) signatures of +14, +8 to +4 with TDM model ages of 1.05–1.6 Ga. During Neoproterozoic deposition, only old cratonic crust with an inherited continental arc signature was available in the source area clearly demonstrated by Hf isotope composition of detrital zircon and geochemical bulk analysis of sedimentary rocks. The granodiorites of the Palaeoproterozoic basement underlying Namuskluft section are ca. 1.9 Ga old and show eHf(t) signatures of −3 to −5.5 with TDM model ages of 2.4–2.7 Ga. These basement rocks demonstrate the extreme uplift and deep erosion of the underlying Kalahari Craton at its western margin before general subsidence during Cryogenian and Ediacaran time. The sedimentary sequence of the two examined sections (Namuskluft and Dreigratberg) proposes the presence of a basin and an increasing subsidence at the SW part of the Kalahari Craton during the Cryogenian. Therefore, we propose the initial formation of an intra-cratonic sag basin during the Lower Cryogenian that evolved later to a rift basin at the cratonic margin due to increasing crustal tension and rifting together with the opening of the Adamastor Ocean. As the zircons of the sedimentary rocks filling this basin show neither rift-related U/Pb ages nor an exotic craton as a possible source area, the only plausible sedimentary transport direction providing the found U/Pb ages would be from the E or the SE, directly from the heart of the Kalahari Craton. Due to subsidence and ongoing sedimentation from E/SE directions, the rift-related magmatic rocks were simply covered by the input of old intra-cratonic material that explains the absence of Neoproterozoic zircon grains in our samples. The geochemical analyses show the erosion of a continental arc and related sedimentary rocks with an overall felsic provenance. The source area was a deeply eroded and incised magmatic arc that evolved on continental crust, without any evidence for a passive margin. All of this can be explained by the erosion of rocks related to the Namaqua Belt, which represents one of the two major peaks of zircon U–Pb ages in all analysed samples. Therefore, the Namaqua Belt was well exposed during the Cryogenian, available to erosion and apart from the also well-exposed Palaeoproterozoic basement of the Kalahari Craton one potential source area for the sedimentary rocks in the investigated areas.

Overview of the magnetic signatures of the Palaeoproterozoic Rustenburg Layered Suite, Bushveld Complex, South Africa

Aeromagnetic data clearly delineate the mafic rocks of the economically significant Bushveld Igneous Complex. This is mainly due to the abundance of magnetite in the Upper Zone of the Rustenburg Layered Suite of the Bushveld, but strongly remanently magnetised rocks in the Main Zone also contribute significantly in places. In addition to delineating the extent of the magnetic rocks in the complex, the magnetic anomalies also provide information about the dip and depth of these units. The presence of varying degrees of remanent magnetisation in most of the magnetic lithologies of the Rustenburg Layered Suite complicates the interpretation of the data. The combination of available regional and high resolution airborne magnetic data with published palaeomagnetic data reveals characteristic magnetic signatures associated with the different magnetic lithologies in the Rustenburg Layered Suite. As expected, the ferrogabbros of the Upper Zone cause the highest amplitude magnetic anomalies, but in places subtle features within the Main Zone can also be detected. A marker with strong remanent magnetisation located in the Main Zone close to the contact with the Upper Zone is responsible for very high amplitude negative anomalies in the southern parts of both the eastern and western lobes of the Bushveld Complex. Prominent anomalies are not necessarily related to a specific lithology, but can result from the interaction between anomalies caused by differently magnetised bodies.The magnetic data provided substantial information at different levels of detail, ranging from contacts between zones, and layering within zones, to magnetite pipes dykes and faults that can have an impact on mine planning. Finally, simple modelling of the magnetic data supports the concept of continuous mafic rocks between the western and eastern lobes.

La secuencia de terrazas cuaternarias del río Alcanadre (provincia de Huesca): caracterización y consideraciones medioambientales

A very extensive sequence of Quaternary fluvial terraces composed of nine strath levels (from Qt1 to Qt9) has been recognized in the Alcanadre River valley (Ebro basin, Huesca province). New mapping, altimetric, stratigraphic, pedogenic and chronological data have been supplied. A braided fluvial sedimentary environment is deduced. The confinement of the fluvial system increases as a response to drainage incision. Pedogenic carbonate morphology reaches highest development (stage VI) with the age. Palaeomagnetic data indicate that, tentatively, the age of Qt1 is near 1000 ky and the age of Qt2 is around 780 ky. In addition, numerical ages of Qt5, Qt6 and Qt7 are 44 ± 2 ky, 19 ± 2 ky and 10 ± 1 ky respectively by using Optically Stimulated Luminescence (OSL). This Upper Pleistocene chronology matches well with the regional fluvial chronostratigraphy recorded in non-glaciated river valleys. It is correlated to cold periods MIS3, LGM and YD at global scale. In a palaeoenvironmental scenario dominated by periglacial conditions, fluvial aggradation is enhanced by high sediment supply from slopes and a seasonal behaviour of rainfalls.

The palaeomagnetism of glauconitic sediments

The palaeoenvironmental significance of glaucony has long been appreciated, but accurate palaeomagnetic dating of events recorded by glauconitic horizons requires an understanding of how glauconitic sediments acquire a remanent magnetization. Pure glauconitic minerals are paramagnetic, but glauconite grains are large and slow-forming (over periods that can exceed 100kyr), with complex and variable morphologies. It is, thus, possible that small magnetic grains within glaucony particles may carry a significant fraction of the remanence in weakly magnetized sediments. Any remanence carried by glauconitic grains may therefore represent the geomagnetic field at a time significantly later than the time of deposition, or a time-averaged signal over some or all of the formation period. We investigated this problem using weakly magnetic Palaeocene glauconitic siltstones from southern New Zealand. We disaggregated the rock and separated it magnetically into glauconitic and non-glauconitic fractions. Results from stepwise isothermal remanent magnetization (IRM) acquisition, alternating-field demagnetization, temperature dependence of magnetic susceptibility, and stepwise thermal demagnetization of a triaxial IRM were used to demonstrate that the remanent magnetization is carried by single-domain or pseudo-single-domain magnetite in the non-glauconitic sediment fraction, and that the glauconite grains themselves make no contribution to the remanent magnetization. However, accurate measurement of the primary remanence is complicated by a strong viscous overprint and mineral alteration during thermal demagnetization studies. Identification of magnetite as the remanence carrier in sediments within a reducing diagenetic environment gives confidence that the remanence has a depositional origin. Glauconite does not carry a remanence; therefore, its effect is to dilute and weaken the overall magnetization. Furthermore, the use of rock magnetic parameters may be problematic when glauconite concentrations are (as in the studied sediments) orders of magnitude greater than remanence carrier concentrations, because in such cases the glauconite susceptibility can dominate that of the remanence carriers.

Palaeogeography of the Siberian platform during middle Palaeozoic Times (∼450–400 Ma): new palaeomagnetic evidence from the Lena and Nyuya rivers

New reliable palaeomagnetic data from the Siberian platform help in deciphering its palaeogeography during ∼450–400 Ma. Geochronology of late Devonian mafic sills provides time constraints for tectonic deformation along the southern margin of the Siberian Platform and thus a minimum age for the regional magnetic overprint. From a late Ordovician–Silurian sedimentary section of the Nyuya syncline in the southern part of the Siberian platform with the Devonian sills hosted nearby in early Palaeozoic sediments, pre-folding presumably primary magnetization was isolated from the sediments during stepwise thermal cleaning. High unblocking temperatures imply that haematite is the main carrier of magnetization. The sample-mean direction for 37 Ordovician samples from nine sites in stratigraphic coordinates is Ds = 168.5, Is = −5, ks = 22.3 and α 95 = 5.1 and for 77 Silurian samples from six sites is Ds = 193.9, Is = 20.9, ks = 16 and α 95 = 4.2. Another component recorded in both the Silurian and Ordovician samples is pre-folding, with a sample-mean direction of Ds = 204.4, Is = 37.9, ks = 27.2 and α 95 = 2.7 for 104 samples from eight sites. This component was probably formed during a regional remagnetization event, which took place in post-early Silurian time. Putting this secondary component into a framework with available Palaeozoic data and geochronology further constrains its age to be early Devonian.The isolated components yield new late Ordovician, early Silurian and early-middle Devonian palaeomagnetic poles. The revised middle Palaeozoic segment of the apparent polar wander path (APWP) for the Siberian platform provides new palaeogeographic constraints. Our data suggest that in late Ordovician the platform was situated in equatorial latitudes and was rotated 180° with respect to its present position. During middle-late Ordovician time, the platform did not experience any notable latitudinal drift. It started drifting to the north in the late Ordovician, and by the late Silurian it had travelled ∼1500 km northwards and had rotated ∼30° counter-clockwise (CCW). During late Silurian time, the platform continued northward drift and CCW rotation, and by the early Devonian it had drifted ∼1100 km northwards and rotated 10° CCW relative to its Silurian position. After that, the rotation of the platform changed to clockwise (CW), and by the late Devonian had drifted another 1500 km to the north and had rotated ∼60° CW.We evaluated palaeomagnetically viable positions from 450–400 Ma of the three largest Laurasian cratons, Siberia, Baltica and Laurentia, based on the new data and previously published APWPs. Contrary to several published reconstructions, the Siberian platform could not have been situated to the north of the Caledonian suture in mid-Silurian time, but was probably located either at the eastern or the western side of Laurussia. The new data are compatible with an early Devonian position of Siberia similar to the modern Eurasian configuration. They also support the post- early or middle Devonian relative rotation between the Aldan and Angara blocks of the Siberian platform.

Re‐orientation of the extension direction and pure extensional faulting at oblique rift margins: comparison between the Main Ethiopian Rift and laboratory experiments

AbstractIn this study, we draw on a unique combination of well‐resolved fault‐slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending Main Ethiopian Rift, East Africa. These data show that both boundary and internal faults – oblique and orthogonal to the plate divergence (PD) respectively – exhibit almost pure dip‐slip motion, and indicate significant local deflection in orientation of the extension direction at rift margins. Scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep‐seated, pre‐existing weakness – oblique to the direction of PD – that is able to cause a local rotation in the orientation of the extension direction at rift margins. Minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems, as supported by existing and new palaeomagnetic data from the rift.