Characteristic Remanent Magnetization Directions Research Articles

Paleomagnetic and Geochronologic Results of Latest Cretaceous Lava Flows From the Lhasa Terrane and Their Tectonic Implications

AbstractTo position the Asian southern margin before the India‐Asia collision, paleomagnetic and geochronologic studies were performed on the Dianzhong Formation lava flows from the Shiquanhe area of the westernmost Lhasa terrane (LT). Zircon U‐Pb analyses dated the lava flows to ~69.5 ± 2.5 Ma. The characteristic remanent magnetization directions contain antipodal polarities and pass fold tests, implying that they are primary magnetizations; this interpretation is supported by rock‐magnetic analyses and petrographic observations. Forty‐four site‐mean directions were divided into 17 statistically independent direction groups. The group‐mean direction after tilt correction is Ds = 43.3°, Is = 30.3°, k = 28.0, α95 = 6.9°. The corresponding paleopole at 47.8°N, 181.4°E (A95 = 6.4°) yields a paleolatitude of 16.6° ± 6.4°N for the Shiquanhe area of westernmost Tibet (32.34°N, 80.12°E). Consistent paleolatitudes for the southern margin of the LT calculated from the western and central part of the LT indicate that the leading edge of the LT was aligned relatively W‐E. When compared with the reference pole at 70 Ma for Eurasia, this new paleopole suggests that crustal shortening between the Shiquanhe area and stable Asia was 1,500 ± 800 km. This is supported by the crustal shortening (600–1,000 km) absorbed by Cenozoic thrust and fold belts within this area, indicating that the magnitude of crustal shortening within Asia north of the India‐Asia suture zone was similar in the central and western part of the plateau.

Paleomagnetic age and tectonic constraints on the genesis of the giant Jinding Zn-Pb deposit, Yunnan, China

The Jinding Zn-Pb deposit is located in the Lanping-Simao basin, Yunnan, China. Paleomagnetic results are reported from the Beichang, Jiayashan, and Nanchang mineralization zones that are hosted in the Lower Cretaceous Jinxing and Upper Paleocene Yunlong Formations. The timing of formation of the Zn-Pb mineralization has not been previously defined but is geologically constrained to be younger than Upper Paleocene. Paleomagnetic analyses of 645 specimens from 42 sites using mostly thermal and then alternating field step demagnetization isolated a stable characteristic remanent magnetization (ChRM) for 29 mineralized sites. Step demagnetization and rock magnetic analyses of Zn and Pb concentrates and tailings show that the main remanence carriers are pyrrhotite and magnetite or low Ti titanomagnetite inclusions in sphalerite and galena. Randomly oriented ChRM directions for 18 clast specimens collected from site 40 provide a conglomerate test that shows that the clasts have not been remagnetized by a metamorphic event. A paleomagnetic fold test on the mineralized specimens is negative, showing that the mineralization is epigenetic because its ChRM substantially postdates the major thrust folding event that formed the dome structure of the Jinding deposit. Using extensive regional geologic and paleomagnetic data from other studies with the Jinding paleomagnetic data yields a mean age of 23 ± 3 Ma for the mineralization.

Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision

AbstractThe characteristic remanent magnetization (ChRM) isolated from Paleogene carbonate rocks of the Zongpu Formation in Gamba (28.3°N, 88.5°E) of southern Tibet has previously been interpreted to be primary. These data are pertinent for estimating the width of Greater India and dating the initiation of India‐Asia collision. We have reanalyzed the published ChRM directions and completed thorough rock magnetic tests and petrographic observations on specimens collected throughout the previously investigated sections. Negative nonparametric fold tests demonstrate that the ChRM has a synfolding or postfolding origin. Rock magnetic analyses reveal that the dominant magnetic carrier is magnetite. “Wasp‐waisted” hysteresis loops, suppressed Verwey transitions, high frequency‐dependent in‐phase magnetic susceptibility, and evidence that >70% of the ferrimagnetic material is superparamagnetic at room temperature are consistent with the rock‐magnetic fingerprint of remagnetized carbonate rocks. Scanning electron microscopy observations and energy‐dispersive X‐ray spectrometry analysis confirm that magnetite grains are authigenic. In summary, the carbonate rocks of the Zongpu Formation in Gamba have been chemically remagnetized. Thus, the early Paleogene latitude of the Tibetan Himalaya and size of Greater India have yet to be determined and the initiation of collision cannot yet be precisely dated by paleomagnetism. If collision began at 59 ± 1 Ma at ~19°N, as suggested by sedimentary records and paleomagnetic data from the Lhasa terrane, then a huge Greater India, as large as ~3500–3800 km, is required in the early Paleogene. This size, in sharp contrast to the few hundred kilometers estimated for the Early Cretaceous, implies an ever greater need for extension within Greater India during the Cretaceous.

Paleogeography of the Congo/São Francisco craton at 1.5 Ga: Expanding the core of Nuna supercontinent

The Congo/São Francisco (C/SF) craton, one of the largest cratons in Proterozoic paleogeography, has been lacking reliable paleomagnetic data for the supercontinent Nuna interval (ca. 1600–1300Ma). Here we provide a new paleomagnetic key pole for this craton from recently dated mafic dykes in the Curaçá (1506.7±6.9Ma) region of Brazil. The characteristic remanent magnetization (ChRM) direction D=070.6°, I=54.0° (k=22.1 and a95=13.1°) corresponds with a paleomagnetic pole at 10.1°N, 009.6°E (K=15.6, A95=15.8°), which places C/SF craton in moderate paleolatitudes at the time of remanence acquisition. Primary nature of the paleomagnetic remanence is supported by a baked-contact test. A similar ChRM direction was obtained for four Mesoproterozoic mafic intrusions in Chapada Diamantina region. The new pole, only from Curaçá, for C/SF allows us to reconstruct the extended core of the supercontinent Nuna at 1.5Ga. Based on coeval 1.5Ga and 1.38Ga magmatism in Baltica, Siberia and C/SF, we favor the position where Southwest Congo is reconstructed against present South-Southeast (S-SE) Baltica. We explore two alternative 1.5Ga reconstructions of Nuna’s core. In both of them Baltica and Laurentia are shown in the well-defined NENA (Northern Europe North America) fit, together with Siberia in a tight fit to northern Laurentia. In reconstruction option A, more traditional fit of Amazonia with Baltica is shown, modified from the geologically based SAMBA (South AMerica BAltica) model to accommodate paleomagnetic data. In this option, however, West Africa must be extricated from SAMBA because C/SF has taken its place. For reconstruction option B, Amazonia is shifted to lie adjacent to NE Laurentia and West Baltica. In both options SW Congo is reconstructed against S-SE Baltica, but in option B there is a tighter fit between them, and there is a better match with our new paleomagnetic data for C/SF. In either option, separation of C/SF from Baltica and Siberia probably occurred at 1.38Ga, the age of pronounced mafic magmatism throughout this sector of Nuna.

Preliminary magnetostratigraphic results from the late Miocene Maragheh Formation, NW Iran

Maragheh in northwestern of Iran is a world famous Miocene fossil-bearing area. The area has yielded classical late Miocene Turolian age fauna that has been collected and studied sporadically over the last 150 years. However, the precise correlation of these sediments to the Global Time Scale (GTS) has remained ambiguous. To address this, 115 levels along an approximately 27-m-thick interval were collected from the middle Maragheh Formation at Dareh Gorg (Gort Daresi) section. Characteristic remanent magnetization directions obtained by alternating field demagnetization produce a polarity pattern that is supported by thermal demagnetization on a set of sister specimens. Three polarity intervals were recognised, the middle part of the section at around 15–21 m showing reversed polarity, bounded by normal polarities above and below. Based on the palaeontological constraints and recent K-Ar age determinations from the Maragheh Fm, three correlations to the geomagnetic polarity time scale appear likely. According to these correlations, recently discovered hominoid locality is correlated to C3Br.1n, C4n.1n, or to C4n.2n. For a unique correlation, however, additional palaeomagnetic data is required from the upper and lower parts of the section.

Magnetostratigraphy of the middle-upper Jurassic sedimentary sequences at Yanshiping, Qiangtang Basin, China

A series of important geological events occurred in the Tibetan Plateau area during the Jurassic, such as the collision of the Lhasa and Qiangtang Terranes, the closure of the Meso-Tethyan Ocean, the opening of the Neo-Tethyan Ocean and the cessation of the mega-monsoon. The similar to 3000 m thick Jurassic sedimentary sequence in the Qiangtang Basin on the central Tibetan Plateau, which is called the Yanshiping (YSP) Group, recorded these geological events. However, the chronology of the sequence is surprisingly poorly constrained. Here, we perform a detailed palaeomagnetic analysis on the similar to 1060 m thick middle and upper portions of the YSP Group (the Xiali and Suowa Formations) in the YSP section of the eastern Qiangtang Basin. Three bivalve zones at stratigraphic intervals of similar to 40-140, 640-800 and 940-1040 m are identified, which yield a Bathonian-Callovian age for the Lower Xiali Fm., a Callovian-Oxfordian age for the Lower Suowa Fm. and an Oxfordian-Kimmeridgian age for the Upper Suowa Fm. A total of 544 oriented palaeomagnetic samples were collected from the section. By combining thermal and alternating field demagnetizations, clear characteristic remanent magnetization (ChRM) directions are isolated for most of the samples. The robust ChRM directions pass fold and reversals tests, which support the primary nature of the ChRMs and yield a palaeopole at 76.8A degrees N/297.2A degrees E (dp = 2.2A degrees, dm = 3.7A degrees). A total of 27 normal and 26 reversed polarity zones were successfully recorded in the section. Combined with fossil age constraints, results suggest that the section is plausibly composed of a Callovian-Early Kimmeridgian age sedimentary sequence.

Early Cretaceous paleomagnetic and geochronologic results from the Tethyan Himalaya: Insights into the Neotethyan paleogeography and the India-Asia collision.

To better understand the Neotethyan paleogeography, a paleomagnetic and geochronological study has been performed on the Early Cretaceous Sangxiu Formation lava flows, which were dated from ~135.1 Ma to ~124.4 Ma, in the Tethyan Himalaya. The tilt-corrected site-mean characteristic remanent magnetization (ChRM) direction for 26 sites is Ds = 296.1°, Is = −65.7°, ks = 51.7, α95 = 4.0°, corresponding to a paleopole at 5.9°S, 308.0°E with A95 = 6.1°. Positive fold and reversal tests prove that the ChRM directions are prefolding primary magnetizations. These results, together with reliable Cretaceous-Paleocene paleomagnetic data observed from the Tethyan Himalaya and the Lhasa terrane, as well as the paleolatitude evolution indicated by the apparent polar wander paths (APWPs) of India, reveal that the Tethyan Himalaya was a part of Greater India during the Early Cretaceous (135.1–124.4 Ma) when the Neotethyan Ocean was up to ~6900 km, it rifted from India sometime after ~130 Ma, and that the India-Asia collision should be a dual-collision process including the first Tethyan Himalaya-Lhasa terrane collision at ~54.9 Ma and the final India-Tethyan Himalaya collision at ~36.7 Ma.

Magnetostratigraphy of the Middle Jurassic Sediments from Kachchh Basin, Western India

The palaeoposition of India after its breakup from Gondwana and its subsequent northward journey during the Mesozoic poses many tectonic riddles. A magnetostratigraphy pattern has been constructed for the Ammonoid rich Middle Jurassic Bathonian succession of Jumara Dome, Kachchh, western India. We present the first magnetostratigraphic results from the Ammonoid bearing exposed Bathonian sediments in Kachchh. The Jumara Dome comprised of mixed carbonate-siliciclastic succession is represented by the Jhurio and Patcham formations and siliciclastic-dominating Chari Formation in the Kachchh Mainland. The Jumara Dome sediments were deposited during sea-level fluctuating, and were interrupted by storms in the shallow marine environment. This study is based on collection of 60 oriented samples from 13 successive levels. Both thermal and AF demagnetization investigations were carried out to isolate the Characteristic Remanent Magnetization (ChRM) directions. Palaeolatitudes were computed using the ChRM directions. Magnetostratigraphic column is prepared and correlated with Geological Time Scale (GTS) of [1] [2], and this column readily matched with GTS at M41 to below and exhibits the dominance of normal polarity in Jumara.

Magnetostratigraphic constraints on the age of the lower Beaufort Group, western Karoo basin, South Africa, and a critical analysis of existing U‐Pb geochronological data

AbstractWe studied three partially overlapping sections with a composite thickness of ∼600 m in the upper Permian fluvial siltstones and fine‐grained sandstones of the Abrahamskraal Formation, the basal unit of the Beaufort Group, in the Karoo Basin of Western Cape Province, South Africa. Paleomagnetic analysis reveals three components of Natural Remanent Magnetization (NRM). Heating to ∼180°C removes a remanent magnetization parallel to the present‐day field, which is interpreted as a viscous overprint. An intermediate unblocking temperature component is removed by heating to 450°C; this direction is always of normal polarity and is identical to a regional overprint imparted during the Early Jurassic emplacement of the Karoo Large Igneous Province. A high‐temperature component isolated above 450°C is of dual polarity and is interpreted as primary on the basis of a positive reversals test. The virtual geomagnetic pole position for the Abrahamskraal Formation computed from the average high‐temperature characteristic remanent magnetization direction is in agreement with the late Permian directions for stable Gondwana and with previous results from the lowermost Abrahamskraal Formation and Waterford Formation at the Ouberg Pass section. The predominantly normal polarity of this magnetization is in agreement with either a middle‐late Lopingian age (ca. 254–256 Ma) or a late Guadalupian age (ca. 262 Ma) according to the global geomagnetic polarity time scale. We integrate these new results with existing magnetostratigraphic, biostratigraphic, and geochronological results from the Karoo Basin, with particular emphasis on the controversy over zircon age data reported from the underlying Ecca Group.

Combined paleomagnetic and geochronological study on Cretaceous strata of the Qiangtang terrane, central Tibet

A combined paleomagnetic and geochronological investigation has been performed on Cretaceous rocks in southern Qiangtang terrane (32.5°N, 84.3°E), near Gerze, central Tibetan Plateau. A total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled. Our new U–Pb geochronologic study of zircons dates the volcanic rocks at 103.8±0.46Ma (Early Cretaceous) while the red beds belong to the Late Cretaceous. Rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers. After removing a low temperature component of viscous magnetic remanence, stable characteristic remanent magnetization (ChRM) was isolated successfully from all the sites by stepwise thermal demagnetization. The tilt-corrected mean direction from the 14 lava sites is D=348.0°, I=47.3°, k=51.0, α95=5.6°, corresponding to a paleopole at 79.3°N, 339.8°E, A95=5.7° and yielding a paleolatitude of 29.3°±5.7°N for the study area. The ChRM directions isolated from the volcanic rocks pass a fold test at 95% confidence, suggesting a primary origin. The volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole (VGP) scatter. The mean inclination from the Late Cretaceous red beds, however, is 13.1° shallower than that of the ~100Ma volcanic rocks. After performing an elongation/inclination analysis on 174 samples of the red beds, a mean inclination of 47.9° with 95% confidence limits between 41.9° and 54.3° is obtained, which is consistent with the mean inclination of the volcanic rocks. The site-mean direction of the Late Cretaceous red beds after tilt-correction and inclination shallowing correction is D=312.6°, I=47.7°, k=109.7, α95=3.0°, N=22 sites, corresponding to a paleopole at 49.2°N, 1.9°E, A95=3.2° (yielding a paleolatitude of 28.7°±3.2°N for the study area). The ChRM of the red beds also passes a fold test at 99% confidence, indicating a primary origin. Comparing the paleolatitude of the Qiangtang terrane with the stable Asia, there is no significant difference between our sampling location in the southern Qiangtang terrane and the stable Asia during ~100Ma and Late Cretaceous. Our results together with the high quality data previously published suggest that an ~550kmN–S convergence between the Qiangtang and Lhasa terranes happened after ~100Ma. Comparison of the mean directions with expected directions from the stable Asia indicates that the Gerze area had experienced a significant counterclockwise rotation after ~100Ma, which is most likely caused by the India–Asia collision.

Magnetic polarity stratigraphy of the Siwalik sequence in Nurpur area, NW Himalaya, India

Magnetic polarity stratigraphy studies were carried out on Jabbar Khad section of Nurpur area, Kangra District, Himachal Pradesh, India. Oriented block samples were collected at 202 sites with 3–5 samples per site from a 2350-m thick continuous sedimentary sequence consisting of sandstone–mudstone alternating beds. Specimens were subjected to thermal demagnetization (100–700∘C) after measuring their natural remanent magnetization (NRM). Palaeolatitudes of virtual geomagnetic pole (VGP) from each site were calculated from the acquired characteristic remanent magnetization (ChRM) directions. The observed remanence carrier could be haematite. Magnetic polarity scale (MPS) was erected. This MPS is correlated with the standard geomagnetic polarity time scale (GPTS) of Lourens et al. (2004). The MPS readily matched from chron C2An.1n to chron C4n.2n, therefore assigning an age 8.20–2.60 Ma to the middle and upper Siwalik sections at Nurpur. The rate of sedimentation in middle Siwalik is 29 mm/yr and upper Siwalik is 59 mm/yr.

Paleomagnetic results from the Early Cretaceous Lakang Formation lavas: Constraints on the paleolatitude of the Tethyan Himalaya and the India–Asia collision

To better constrain the Early Cretaceous paleogeographic position of the Tethyan Himalaya and the India–Asia collision process, a paleomagnetic study was performed on the Lakang Formation lava flows in the Cuona area in the southeastern Tethyan Himalaya. Stepwise thermal and alternating field demagnetizations successfully isolated reliable characteristic remanent magnetization (ChRM) directions that include antipodal dual polarities and pass positive fold tests at the 99% confidence level and reversal tests at 95% confidence level, indicating prefolding primary magnetizations. The distribution patterns of ChRM directions from the Lakang Formation lava flows are consistent with young lava flows at similar latitudes, suggesting that secular variation has likely been averaged out. The tilt-corrected site-mean direction for 31 sites is D=261.6°, I=−68.5° with α95=3.6°, which provides a paleopole at 26.8°S, 315.2°E (A95=5.7°), corresponding to a paleolatitude of 52.2°±5.7°S for the study area. Comparison of the paleolatitude observed from the Lakang Formation lava flows with that expected from the apparent polar wander paths of India at 130 Ma show a paleolatitude difference of ∼2.1° (∼230 km), indicating that neither a great north-south continental crustal shortening occurred between the Indian craton and the Tethyan Himalaya after 130 Ma, nor that a wide ocean separated them at that time. Comparison with reliable Cretaceous–Paleocene paleomagnetic results observed from the Tethyan Himalaya and the Lhasa terrane indicates that the latitudinal width of the Neo-Tethyan Ocean could have been up to ∼7000 km at 134–130 Ma and an extension should have existed between the Indian craton and the Tethyan Himalaya during the Late Cretaceous and Paleocene. Furthermore, reliable paleomagnetic results suggest that the India–Asia collision was a dual-collision process, consisting of a first collision of the Tethyan Himalaya with the Lhasa terrane (Asia) at 54.9±2.3 Ma and a final continent–continent collision of the Indian craton with the Tethyan Himalaya at 40.0±3.3 Ma.

Timing of clockwise rotation of Southwest Japan: constraints from new middle Miocene paleomagnetic results

Southwest Japan rotated clockwise during the late stage of the opening of the Japan Sea, although the exact timing of the rotation is controversial. A recent biostratigraphic study has revealed that the Miocene Oidawara Formation in eastern Southwest Japan was deposited just before 15 Ma; consequently, its paleomagnetic direction may help constrain the timing of rotation. For this purpose, we collected fine felsic tuffs and siltstones at 71 stratigraphic sites (horizons) in the Oidawara Formation. An analysis of alternating field and thermal demagnetization results yielded characteristic remanent magnetization (ChRM) directions for 177 samples. Approximately 80 % (142) of the samples exhibit reverse polarity ChRM directions that are thought to be paleofield directions of reverse polarity Chron C5Br. Normal polarity ChRM directions in 35 samples include primary paleofield records as well as records of secondary magnetization. The data suggest that a short normal polarity interval (microchron or cryptochron) at ~15.8 Ma is present within the dominantly reverse polarity interval of Chron C5Br. Reliable site-mean directions for 19 sites yield a tilt-corrected formation-mean direction of D = 10.5°, I = 41.1°, α 95 = 7.0°, and k = 23.9, indicating virtually no rotation with respect to a reference paleomagnetic direction for the Asian continent. A rotation versus age plot for Southwest Japan indicates that the clockwise rotation started after 17.5 Ma and ceased largely before 15.8 Ma, yielding a rotation rate of ~23°/Myr.

Independently dated paleomagnetic secular variation records from the Tibetan Plateau

Magnetostratigraphy has been serving as a valuable tool for dating and confirming chronologies of lacustrine sediments in many parts of the world. Suitable paleomagnetic records on the Tibetan Plateau (TP) and adjacent areas are, however, extremely scarce. Here, we derive paleomagnetic records from independently radiocarbon-dated sediments from two lakes separated by 250 km on the southern central TP, Tangra Yumco and Taro Co. Studied through alternating field demagnetization of u-channel samples, characteristic remanent magnetization (ChRM) directions document similar inclination patterns in multiple sediment cores for the past 4000 years. Comparisons to an existing record from Nam Co, a lake 350 km east of Tangra Yumco, a varve-dated record from the Makran Accretionary Wedge, records from Lakes Issyk-Kul and Baikal, and a stack record from East Asia reveal many similarities in inclination. This regional similarity demonstrates the high potential of inclination to compare records over the Tibetan Plateau and eventually date other Tibetan records stratigraphically. PSV similarities over such a large area (>3000 km) suggest a large-scale core dynamic origin rather than small scale processes like drift of the non-dipole field often associated with PSV records.

Posterior archaeomagnetic dating: An example from the Early Medieval site Thunau am Kamp, Austria

The Early Medieval valley settlement of Thunau am Kamp in Lower Austria has been under archaeological excavation for 10years. The site was occupied during the 9th and 10th centuries AD according to potsherds, which seem to indicate two phases of activity: in the older phase ovens were placed in the corners of houses while during the younger phase they are found in the middle of the wall. The present study has been conducted in order to increase the archaeomagnetic database and fill the temporal gap around 900 AD. For this purpose 14 ovens have been sampled for their paleaomagnetic signals. Laboratory treatment generally confirmed that the baked clay has preserved stable directions. Apart from one exception, all the mean characteristic remanent magnetisation directions are concentrated on the Early Medieval part of the directional archaeomagnetic reference curve of Austria at about 900 AD. Using this curve archaeomagnetic dating provides ages between 800 and 1100 AD, which are in agreement with the archaeological dating. Together with the archaeological age estimates and stratigraphic information the new data have been included into the database of the Austrian curve and it has been recalculated using a new version of RenCurve. The new data confine the curve and its error band considerably in the time interval 800 to 1100 AD. This calibration process also provides probability density distributions for each included structure, which allows for posterior dating and refines temporal errors considerably. Because such dating includes archaeological information it is not an independent age estimate but is a combination of all available dating methods.

New palaeomagnetic results from the Oslo Graben, a Permian Superchron lava province

We have performed an extended palaeomagnetic study of the Oslo Graben volcanics, compared to the study of half a century ago by van Everdingen, using modern techniques and a four times larger amount of sites, plus additional rock magnetic experiments. We conclude that the average direction (D = 204.0, I = −37.9, k = 46.9, α95 = 2.0) and associated palaeomagnetic pole (λ = 48.3, ϕ = 155.5, K = 52.2, A95 = 1.9) of the Krokskogen and Vestfold volcanics together are statistically identical to those of the earlier study. This gives confidence in the fact that older palaeomagnetic studies can be reliable and robust, even though methods have improved. Our larger number of samples, and better age constraints, enable us to separate the data into two major intervals: the younger, on average, Krokskogen area and the older Vestfold area. The results show firstly that palaeolatitudes are slightly higher than predicted by the latest apparent polar wander path (APWP) for Eurasia by Torsvik et al. These data support an early Permian Pangaea A configuration and do not necessitate a Pangaea B configuration. The larger data set also allows us to assess the distribution of the characteristic remanent magnetization directions of the Oslo Graben in terms of geomagnetic field behaviour, which were acquired during a long period of dominantly single polarity the Permo-Carboniferous Reversed Superchron (PCRS). The distributions show a significantly lower virtual geomagnetic pole (VGP) scatter at the observed (low) latitudes than expected from a compilation from lavas of the last 5 Myr. The data do however show excellent agreement with the scatter observed both during the Cretaceous Normal Superchron and the PCRS. A comparison of the directional distributions in terms of elongation is less discriminating, since the large errors in all cases allow a fit to the predicted elongation/inclination behaviour of the TK03.GAD model.

Quaternary remagnetization of the Neoproterozoic limestone of Negash Synclinorium (Arabian–Nubian Shield, northern Ethiopia): With implications of no paleomagnetic testing for the proposed Snowball Earth events

Eighty-one paleomagnetic cores were collected from 10 locations across a black limestone unit within the core of Negash Synclinorium, northern Ethiopia in order to test the proposed Snowball Earth events for the diamictite unit of the Tambien Group. Cores were cut into two standard paleomagnetic specimens and were subjected to stepwise demagnetizations using both Thermal (TH) and alternating field (AF) techniques. Rock magnetic analyses on representative specimens were done and results revealed goethite, pyrrhotite, titano-magnetite, and titano-hematite to be the major magnetic materials carrying the magnetizations with PSD (pseudo single domain) grain size range. In most cases paleomagnetic directions are defined by a single component of magnetization, where a viscous component is present it is usually removed by heating to a temperature of ∼200°C or an AF of ∼10mT. The high stability component isolated above temperature of 200°C or AF of 15mT, defined straight line trajectories directed towards the origin and considered as the Characteristic Remanent Magnetization Direction (ChRM). The direction of magnetization of the ChRM is determined for samples with stable straight line segments by the best-fit line using the least square technique of Kirschvink (1980). In the cases of overlapping spectra and unblocking temperatures, direction of magnetization is determined by remagnetization circles of Halls (1976, 1978). When site mean ChRM directions are plotted on stereogram, their distribution is relatively clustered in geographic coordinates and the overall mean direction is Decg=358.5°, Incg=16.6° (α95=3.8°, K=162.8, N=10). After a structural restoration to the horizontal is made the directions disperse and fail the fold test of both McElhinny’s and McFadden’s tests and the mean direction is Decs=353.5°, Incs=8.8° (α95=18.9°, K=7.5, N=10). This is interpreted to result from a later remagnetization of the black limestone. All directions are normal in polarity and have mean unrestored paleomagnetic directions comparable to the Quaternary paleomagnetic directions. Virtual Geomagnetic poles (VGP) in the unrestored position is used to calculate overall mean VGP position resulting long=235.7°E, latg=84.5°N (A95=3.0°, N=10). Comparison of the obtained pole with the apparent polar wander path (APWP) curve for Africa of Besse and Courtillot (1991, 2003) and with the 2Ma reference pole of stable Africa (Kidane et al., 2003) is found to be consistent with remagnetizations during the Quaternary period. Hence supporting evidence for the proposed Snowball Earth event of the Sturtian glaciation in the Negash rocks could not, unfortunately, be obtained from paleomagnetism.

New insights into the India–Asia collision process from Cretaceous paleomagnetic and geochronologic results in the Lhasa terrane

To further constrain the India–Asia collisional process, a combined paleomagnetic and geochronologic study has been carried out on the Upper Cretaceous Jingzhushan Formation redbeds and the Lower Cretaceous Dianzhong Formation volcanic rocks dated at ~121–117Ma from the Cuoqin area in the central Lhasa terrane. Stepwise thermal demagnetization successfully isolated reliable characteristic remanent magnetization (ChRM) directions that include dual polarity and pass positive fold tests at 95% and 99% confidence levels, indicating prefolding primary magnetizations. The tilt-corrected site-mean direction for 33 redbed sites is D=316.8°, I=30.2° with α95=5.4°, corresponding to a paleopole at 49.0°N, 344.3°E with A95=5.3°, and the other for 12 volcanic sites is D=350.5°, I=25.5° with α95=7.7°, corresponding to a paleopole at 70.5°N, 292.9°E with A95=7.4°. Our new paleomagnetic results, together with reliable Cretaceous paleomagnetic data obtained from the Lhasa terrane, demonstrate that the southern margin of Asia was located at ~15.1°N during the Cretaceous. Comparison with the apparent polar wander paths (APWP) of India and the Cretaceous–Paleocene paleopoles of the Himalayan terrane suggests that the India–Asia collision was likely a complex process, which consists of the collision of the Lhasa and Himalayan terranes at 54.0±2.1Ma, the longtime subduction of an intra-oceanic basin between the Himalayan terrane and the Indian craton from ~54.0 to ~40.4Ma, and the collision of the Himalayan terrane and the Indian craton at 40.4±4.1Ma. Comparing with the Late Cretaceous average pole of the East Asia APWP reveals that a latitudinal convergence of 780±240km has taken place between the Lhasa terrane and East Asia (the Hexi corridor) since the India–Asia collision; the amount of latitudinal shortening deduced from paleomagnetic data is very consistent with the 600–1000km accommodated by the Cenozoic fold and thrust belts between the Lhasa terrane and Hexi corridor.

Early human settlements in Northern Africa: paleomagnetic evidence from the Ain Hanech Formation (northeastern Algeria)

The question of the earliest hominid settlements in northern Africa has been under debate for a number of years due to the lack of precise chronologies. Here we present new paleomagnetic data that supports an Olduvai Subchron age for the archaeological sites at Ain Hanech and El-Kherba, in northern Algeria. Our study is based on a 22 m-thick magnetostratigraphy of the Ain Hanech Formation, which includes contextualized Oldowan and Acheulian lithic tools. Characteristic remanent magnetization directions were obtained from both thermal and alternating field demagnetization procedures of specimens from twenty five sampled horizons. Our results reveal the presence of the Olduvai Subchron in the upper part of the stratigraphic section, constraining the age of these important archaeological sites in northern Africa.

Magnetostratigraphy of the Fossil-Rich Shungura Formation, southwest Ethiopia

Three hundred eighty-six oriented block samples were collected from the lower 334m of Pliocene and Pleistocene fluvial strata of the Shungura Formation in two field seasons. Paleomagnetic polarity of these was determined on samples from these blocks, using a routine of 8–13 steps for AF demagnetization, and 15–20 steps for thermal demagnetization. Rock magnetic properties revealed titanomagnetite with subordinate maghemite as the dominant magnetic minerals with pseudo-single domain magnetic grain sizes. Directional analyses indicate one or two components of magnetization both in AF and thermal techniques. The first component is mostly removed by 5–10mT AF fields or heating to 300°C. The magnetization component after these steps generally defined straight-line segments directed towards the origin which are interpreted as the Characteristic Remanent Magnetization (ChRM). ChRM directions so determined and averaged at the site level revealed both normal and reversed polarities. The directions of the normal and reversed polarities are respectively Dec=357°, Inc=−0.1 (N=36, α95=3.7°) and Dec=176°, Inc=−3.4 (N=55, α95=3.1°) and are antipodal. A sequence of R4-N3-R3-N2-(R2)-N1-R1 polarities was identified with R2 being an anomalous direction within N2/N1. Published weighted mean 40Ar/39Ar ages on volcanic ash layers in this part of the section with height from the base and ages in parentheses are: Tuff A (34m; 3.60±0.02Ma), Tuff B-α (65m; 3.43±0.01Ma), Tuff B-δ (77m; 3.41±0.01Ma), Tuff in B-10-1 (150m; 2.97±0.01Ma), Tuff C (170m; 2.99±0.09Ma), Tuff D (252m; 2.53±0.01Ma), Tuff D-3-2 (272m; 2.47±0.01Ma), and Tuff F (228m; 2.32±0.01Ma). These direct age results imply that the identified polarity zones correspond to the Gilbert Chron, lower Gauss Chron, Mammoth Subchron, upper Gauss Chron, and lower Matuyama Chron, respectively. No new paleomagnetic data are available for the Usno Formation, but tephrostratigraphic work results in reinterpretation of the stratigraphy suggesting that only the Mammoth Subchron is preserved in the upper part of this formation. In the Shungura, Usno and Koobi Fora formations, the thickness represented by the lower Gauss and Mammoth Subchron is very similar, but the Kaena Subchron is either not recorded in the type area of the Shungura Formation, or only incompletely revealed.