High Unblocking Temperature Component Research Articles

Magnetostratigraphy across the Triassic-Jurassic boundary in the main Karoo Basin

The end-Triassic mass extinction and the transition and explosive diversification of fauna over the Triassic-Jurassic boundary is poorly understood and poorly represented in the rock record of the Southern Hemisphere. This is despite the rich diversity in both body and trace fossils of Triassic-Jurassic age in southern Africa, which is not found in coeval Northern Hemisphere localities. We report here the first palaeomagnetic polarity zonation of the Upper Triassic-Lower Jurassic continental red bed succession (Elliot Formation; Stormberg Group) in southern Africa. The results from 10 partially overlapping sections, with a composite thickness of ~280m, provide a magnetic polarity chronology of the main Karoo Basin in South Africa and Lesotho. Palaeomagnetic analyses reveal that heating samples to between 150°C and ~300°C removes the secondary, moderately inclined (~48°) normal-polarity component of remanent magnetization. This component overlaps with the present-day field and is comparable to the overprint direction expected from Lower Jurassic Karoo dolerite intrusions. In contrast, a likely primary, high unblocking temperature component, of dual polarity, consistently is of steeper inclination (~63°). This characteristic remanence passes the reversals test, except where means are based on small sample populations. There are only two resulting polarity zones for the ~200m thick lower Elliot Formation (LEF) with potential for a thin 3rd magnetozone in the uppermost part. The upper Elliot Formation (UEF), in contrast, which was sampled over a thickness of ~ 80 m, has five polarity zones. The failure of the reversal test for the UEF and combined Elliot Formation (LEF+UEF) indicates that the normal polarity samples may be biased by a younger overprint of either the Jurassic normal polarity of the Karoo Large Igneous Province or present day field. The separate poles calculated for the four sites in the LEF and ten sites in the UEF overlap with the Late Triassic and Early to Middle Jurassic Gondwana poles, respectively. The combined Elliot Formation and UEF pole positions are better constrained than the LEF and therefore considered more reliable. Overall the LEF shows considerable overlap with the Late Triassic Apparent Polar Wander Paths (APWP) poles.

Signals from the ancient geodynamo: A paleomagnetic field test on the Jack Hills metaconglomerate

The oldest history of Earth's magnetic field cannot be directly read from extant bulk rocks because of subsequent metamorphism at temperatures close to or exceeding the Curie temperature of common magnetic minerals. The Jack Hills metasediments of Western Australia, which have seen lower peak metamorphic temperatures, contain zircons as old as ∼4.4 billion-yr-old. To assess whether these sediments can retain an ancient signal of the geodynamo, we present a paleomagnetic conglomerate test on a cobble-bearing Jack Hills unit. Thermal demagnetization reveals a distinct magnetic component with high unblocking temperatures between ∼550 and 580°C that passes the conglomerate test, indicating magnetization prior to deposition of the conglomerate. This result, together with rock magnetic data, indicates that the high unblocking temperature component is carried by magnetite which records magnetization in an ambient field, and the simplest explanation is that a dynamo was present. Existing geochronological data imply that the clasts could contain mixtures of minerals extending to ages only slightly older than the maximum depositional age at 3.05 billion-yr-ago. However, the positive conglomerate test reported here indicates that the Jack Hills metasediments have the potential to record Paleorchean to Hadean magnetic fields, on a clast or sub-clast mineral scale.

Palaeogeographic implications of differential inclination shallowing in permo-carboniferous sediments from the donets basin, Ukraine

We present new palaeomagnetic data from Upper Carboniferous and Lower Permian grey and red sediments from the Donets Basin, Ukraine, part of the Palaeozoic East European Platform. Detailed demagnetization of these units reveals two ancient components of magnetization: component “B”, which is carried by magnetite and pigmentary haematite, and a high unblocking temperature component “C”, present only in the red beds, carried by detrital haematite. The “B” and “C” components both pass fold tests indicating a primary or a near primary-origin for magnetizations. The “C” component, however, yields palaeolatiudes that are consistently lower (by up to ∼ 12° of latitude or ∼ 1330 km) than those derived from the “B” component, and we argue that this is due to significant inclination-shallowing of the “C”-component. A comparison with European reference palaeomagnetic data reveals that the reference data also span a large spread of palaeolatitudes for this time, and we argue that unrecognized shallowing may have crept into the reference data when based on sedimentary units. A more rigorous approach to selecting reference palaeomagnetic data may well be key to resolving palaeogeographic controversies at this time.

Evidence for a 3.45‐billion‐year‐old magnetic remanence: Hints of an ancient geodynamo from conglomerates of South Africa

Paleomagnetic and rock magnetic analyses of ∼3445‐million‐year‐old dacite conglomerate clasts and parent body rocks from the Barberton greenstone belt, South Africa, define two dominant components of magnetization. One component, unblocked at low temperature, is an overprint acquired ∼180 million years ago. The other component is unblocked at high temperatures and passes a conglomerate test, indicating that this component is older than the depositional age of the conglomerate (∼3416 Ma). The high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes, Archean overprinting, and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations. Alternatively, this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo, including magnetization by an external field related to solar wind interaction with the atmosphere. Such exotic mechanisms can be tested with the acquisition of paleointensity data. While more scattered than paleomagnetic data recording the more recent geomagnetic field, the high unblocking temperature component in the dacite parent body shows some consistency, and the simplest explanation of the data is that they reflect a geodynamo ∼3445 million years ago.

Anomalously shallow inclination in middle-northern part of the South China block: palaeomagnetic study of Late Cretaceous red beds from Yichang area

SUMMARY We present new palaeomagnetic results from Late Cretaceous red beds of the Paomagang Formation collected at 42 sites in the Yichang area (30.7°N, 111.7°E), middle–northern part of the South China Block. A high unblocking temperature component around 680°C was isolated from 26 sites by stepwise thermal demagnetization. Fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the Paomagang and Wangdian areas, respectively. Normal and reversed polarity sequence found in the Wangdian area passed a reversal test at the 95 per cent confidence limit. The tilt-corrected mean direction for Yichang area is D= 3.8°, I= 23.3° (α95= 5.1°, N= 26), corresponding to a palaeopole at 71.5°N, 280.0°E with A95= 4.3°. Comparison with the expected inclination from the 80 Ma Eurasian APWP pole indicates a inclination flattening of 26.3°± 6.4°. Middle Cretaceous to Cenozoic NW–SE extension tectonics within the SCB contributes only 2.0° to the inclination flattening. While the massive samples of the Paomagang Formation show a moderate degree of anisotropy (1.017 < Pj < 1.086) and steeper inclination (I= 29.3°), the laminated samples, which form more than 75 per cent of the studied samples, show a higher degree of anisotropy (1.052 < Pj < 1.227) and shallower inclination value (I= 22.7°). A good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the Yichang area.

Paleomagnetism of mid-Cretaceous red beds in west-central Kyushu Island, southwest Japan: paleoposition of Cretaceous sedimentary basins along the eastern margin of Asia

A paleomagnetic study was carried out on the mid-Cretaceous sedimentary strata in west-central Kyushu Island, southwest Japan, to elucidate the origin of sedimentary basins along the Asian continental margin in the Cretaceous. We collected paleomagnetic samples from a total of 34 sites of the mid-Cretaceous Goshonoura Group, shallow-marine clastic deposits in west-central Kyushu, and characteristic remanent magnetizations were recognized from 18 horizons of red beds. Thermal demagnetization has revealed that the red beds contain three magnetization components, with low (<240°C), intermediate (240–480°C), and high (480–680°C) unblocking temperatures. The low unblocking temperature component is present-field viscous magnetization, and the intermediate one is interpreted as chemical remanent magnetization carried by maghemite that was presumably formed by post-folding, partial oxidation of detrital magnetite. Rock magnetic and petrographic studies suggest that the high unblocking temperature component resides largely in hematite (martite and pigmentary hematite) and partly in maghemite. Because of the positive fold test, this high temperature component can be regarded as primary, detrital remanent magnetization. The tilt-corrected mean direction of the high temperature component is Dec=65°, Inc=63° with α 95=5°, which yields a paleomagnetic pole at 39°N, 186°E and A 95=8°. A combination of this pole with those of the Late Cretaceous rocks in southwest Japan defines an apparent polar wander path (APWP), which is featured by a cusp between the Late Cretaceous and the Paleogene. A comparison of this APWP with the coeval paleomagnetic pole from northeast Asia suggests an approximately 50° post-Cretaceous clockwise rotation and 18±8° southward drift with respect to northeast Asia. The southward transport of the Cretaceous basin suggests that the proto-Japanese arc originated north of its present position. We propose that the coast-parallel translation of this landmass was caused by dextral motion of strike-slip faults, which previous geodynamic models interpreted to be sinistral through the Mesozoic. The change in strike-slip motion may have resulted from Mesozoic collision and penetration of exotic terranes, such as the Okhotsk microcontinent, with the northeastern part of Asia.

Timing of lateritization on the Guiana Shield: synthesis of paleomagnetic results from French Guiana and Suriname

A paleomagnetic study was carried out on representative duricrust paleosurfaces of the Guiana Shield in French Guiana (Cayenne region and Kaw Mountain) and Suriname (Moengo and Bakhuis Mountains). The age of weathering of ferruginous and bauxitic duricrusts was estimated from comparison of paleomagnetic poles determined from these duricrust horizons with the South American apparent polar wander path (APWP). In most of the studied profiles, hematite is the dominant magnetic carrier, as indicated by unblocking temperatures that exceed 580°C for about 20–60% of the natural remanent magnetization (NRM). Magnetite and goethite may also be present and contribute to the overall NRM, but contribute little or nothing to the characteristic remanent magnetization, which is estimated from principal component analysis of the medium to high unblocking temperature component. Paleomagnetic directions from the duricrust profiles generally cluster well (dispersion parameters >30) and give paleomagnetic poles that fall between the Cenozoic and the present-day portion of the APWP. Recent paleomagnetic directions are usually detected in the uppermost part of the duricrust profiles which may have sustained some recent reworking. In contrast, the middle part of the duricrust, of massive facies, usually shows ancient directions. The bauxitic surfaces of Bakhuis Mountains at 400 m elevation, Kaw Mountain (upper surface, 300 m elevation) and probably the Cayenne Unit 1 (75–200 m elevation) can be attributed to the ‘Sul Americano’ lateritization cycle of Paleocene–Eocene age, with respectively average relative ages of 60, 50 and 40 Ma. The bauxitic paleosurface of Kaw Mountain (lower surface, 220 m), the coastal Moengo deposit (40 m) and the ferruginous surface of Cayenne Unit 2 (20–50 m) may be related to the ‘Late Velhas’ lateritization cycle of Miocene age with relative average ages of 13 and 10 Ma. The Fe-duricrust of Cayenne Unit 3, close to sea level and occupying most of the coastal plain, is geomorphologically the youngest duricrust paleosurface, but also seems related to a Late Miocene lateritization event (5–13 Ma). The Oligocene appears to represent a major break in the development of weathering profiles, with erosion as the predominant process which stripped 50–80 m from the landscape between the two main lateritization phases of Paleocene–Eocene and Miocene.

Further paleomagnetic results from the Silurian of the Yangtze Block and their implications

Silurian redbeds were sampled at two localities in northeastern Guizhou and Yunnan provinces of China for paleomagnetic study. Progressive thermal demagnetization revealed multiple magnetic components from these samples: a low unblocking temperature component (A) of recent origin, a post-folding middle to high unblocking temperature component (B) probably acquired during late Mesozoic time, and a pre-folding high unblocking temperature component (C). Two statistically distinguishable groups of directions were further recognized from the C component with one (C1) having more easterly declination than the other (C2) and both having a of shallow inclination. The paleomagnetic pole from the C1 magnetization coincides with a pre-folding pole reported from the coeval rocks in neighboring southern Sichuan Province; its relation to the apparent polar wander path (APWP) for the Yangtze Block (YB) suggests that the C1 magnetization was probably acquired during the Silurian. By the same argument, the C2 magnetization was probably acquired sometime between the Silurian and Carboniferous. These results reconfirm the equatorial position for the YB during mid-Paleozoic time and clockwise rotation of the YB since at least the mid-Cambrian.

Miocene clockwise rotation of southwest Japan and formation of curvature of the Median Tectonic Line: Paleomagnetic implications

The origin of the curvature of the Median Tectonic Line (MTL) in southwest Japan is still controversial. In order to investigate its formation, we carried out a paleomagnetic study in the Nohi area, north of the curved part of the MTL in the eastern part of southwest Japan. More than 420 paleomagnetic samples were collected at 35 sites from Cretaceous to Paleogene welded tuffs in the Kasagatake Rhyolites, Oamamiyama Group, and Nohi Rhyolite. Characteristic directions with high unblocking temperature component above 560°C were isolated from 32 sites. Paleomagnetic directions of the Kasagatake Rhyolites and Oamamiyama Group (55–66 Ma) have a clockwise deflected declination (D = 51.7°, I = 52.6°, α95 = 8.6°) which is consistent with the characteristic Paleogene paleomagnetic direction of the central part of southwest Japan. A similarly deflected declination is also observed in the peripheral part of the Nohi Rhyolite region, far away from the Atera fault system. Paleomagnetic data indicate that the Nohi area experienced a clockwise rotation through more than 45° with respect to the Asian continent as a part of southwest Japan but that the amount of rotation of the Nohi area is 23° smaller than that of the central part of southwest Japan. A new model is developed to explain the tectonic rotation. At about 15 Ma during the latest stage of the clockwise rotation of southwest Japan, the eastern end moved southward by more than 60 km along a left‐lateral strike‐slip fault which comprised the present north‐south segment of the MTL and the Akaishi Tectonic Line. We conclude that the curvature of the MTL is a composite of the 15 Ma strike‐slip fault system and a small bend of the MTL associated with the small differential rotation of the Nohi area with respect to the central part of southwest Japan.

PALEOMAGNETISM OF THE DJEBEL REOUINA NAMURIAN FORMATION (TINDOUF BASIN, ALGERIA)

The paleomagnetic study of the Namurian of Reouina (28.9°N, 08.0°W) revealed the existence of two magnetization components, either juxtaposed or superimposed, besides a viscous component. The high blocking temperature component, carried by hematite, has a mean direction defined by D = 126.9° and I = 10.8°. It provides a Namurian paleomagnetic pole located at 28.4°S and 56.9°E (K = 642, A95=1.7°). The second component is carried at least in part, by grains with blocking temperatures lower than 550°C. Though well defined, it consists of two superimposed components, the high unblocking temperature component with a likely Permian overprint.

Palaeomagnetic and chronological study of Miocene welded tuffs in the northern part of Central Japan: tectonic implications for the latest stage of arc formation of Japan

Kinematical adjustment of tectonic blocks has occurred in the latest stage of arc formation of Japan within central Japan. Miocene welded tuffs have been sampled for chronological and palaeomagnetic studies from the northern part of central Japan. Zircon fission-track dating confirms Middle to Late Miocene ages (13.7-5.0 Ma) for the Yatakezawa, Shiobara, Kawaji, Kinugawa, Katashinagawa, Okkai and Ashio Formations. Characteristic directions with high unblocking temperature component above 560°C are isolated from 36 sites. Palaeomagnetic directions after tilt correction provide a Miocene characteristic palaeomagnetic direction of the northern part of central Japan (36.8°N, 139.3°E): D = 7.3°, I = 50.3°, α 95 = 6.6°. Presence of normal and reversed polarities, and a fairly long time average exclude the effect of geomagnetic field secular variation. Comparison with the Miocene palaeomagnetic poles of the neighbouring blocks indicates that the northern part of central Japan and northeast Japan behaved as a unified block since 15 Ma, whereas the Kanto Mountain region moved kinematically as an independent block from the northern block comprised of central Japan and northeast Japan at least between 16 Ma and 12 Ma. The boundary between the northern part of central Japan and the Kanto Mountain was in a convergent regime with dextral strike slip. A southward force exerted from the northern part of central Japan, as well as a northward force by the Izu-Bonin ridge, induced the clockwise rotation of the Kanto Mountain region. The northern part of central Japan had welded completely with the Kanto Mountain region by 6 Ma.

Cenozoic northward translation of the Kitakami massif in northeast Japan: paleomagnetic evidence

Paleogene and early Cretaceous welded tuffs have been sampled from eleven sites at the Omoe Peninsula (39.6°N, 142.0°E) in the Kitakami massif of northeast Japan for paleomagnetic study. Characteristic paleomagnetic directions with a high unblocking temperature component above 560°C are isolated: Westerly declinations with shallow inclination are identified after tilt correction in the 62–71 Ma Heizaki volcanics ( D=283.5°, I=9.4°, α 95=8.0°) and in the 114–119 Ma Harachiyama Formation ( D=274.2°, I=−20.4°). The shallow inclination, which was previously reported from early Cretaceous plutonic rocks without tilt correction in the Kitakami massif, has been discovered in the tilt-corrected paleomagnetic directions. The presence of both normal and reversed polarities suggests reliability of this shallow and westerly paleomagnetic direction. The shallow inclination indicates that the Kitakami massif was located at low latitude (5°±4°N) at 71–62 Ma. Taking into account a reference paleomagnetic pole of the Paleocene expected from Sikhote Alin, the Kitakami massif was translated northward over 25° in latitude since 65 Ma. The Kitakami massif can be subjected to latitudinal northward motion more than 24° during the last 70 Ma, when it was transported by push of the moving Pacific plate and reached the Asian continental margin near Sikhote Alin later than 30 Ma. We conclude that the Kitakami massif was incorporated into northeast Japan at some time between 30 and 22 Ma after the Cenozoic northward translation.

Paleomagnetism of Paleozoic strata, Brabant and Ardennes Massifs, Belgium: Implications of prefolding and postfolding Late Carboniferous secondary magnetizations for European apparent polar wander

Paleozoic carbonate rocks of the Brabant Massif, Namur Basin, and Dinant Synclinorium, Belgium, are characterized by two secondary Late Carboniferous magnetizations of uniform reverse polarity: a postfolding low unblocking temperature (300–375°C) component (LT) and a prefolding high unblocking temperature (400–550°C) component (HT). In the Namur Basin, LT magnetizations postdate late Westphalian deformation (∼305 Ma); in the Dinant Synclinorium, HT magnetizations predate early Westphalian deformation (∼316 Ma). LT magnetizations for the combined Brabant Massif and Namur Basin sites give a south paleopole at 42.3°S, 339.6°E (dp = 1.9°, dm = 3.8°), consistent with the European cratonic reference. The LT magnetizations in the Dinant Synclinorium give a paleopole at 36.2°S 321.0°E (dp = 2.4°, dm = 4.7°). The HT magnetization gives paleopoles at 34.1°S, 333.4°E (dp = 2.6°, dm = 5.1°; Brabant and Namur) and at 25.9°S, 321.5°E (dp = 3.5°, dm = 6.8°;, Dinant Synclinorium). Declinations indicate ∼15° clockwise rotation of the Dinant Synclinorium with respect to the Brabant Massif after acquisition of the LT magnetization. Devonian red beds yield near univectorial apparent synfolding magnetizations of distributed unblocking temperature that we interpret as artifacts of the remanence acquisition or deformation processes. The HT paleopole for the Brabant Massif is indistinguishable from other early Late Carboniferous European paleopoles and is similar to poles interpreted to be derived from remagnetizations in the British Isles. Age constraints on the time of remanence acquisition of secondary magetizations allow a refined calibration of the European apparent polar wander path. Inclinations of the LT and HT components indicate about 6–8° (∼700 km) northward motion during the Late Carboniferous.

A paleomagnetic study of Mesozoic sediments from the Junggar and Turfan basins, northwestern China

Mesozoic sedimentary rocks have been collected from the Karamay area (northwestern Junggar block) and in the Turfan basin (northeastern Tarim block) for a paleomagnetic study with the aim of testing the rigid nature of these blocks and to better constrain their paleogeographic relationships within central Asia prior to the collision with India. The results from the Karamay area are disappointing, Lower Cretaceous grey sandstones having apparently been recently remagnetized. This remagnetization is carried by magnetite, and yields highly clustered normal polarity directions. Both a medium (MTC) and a high (HTC) unblocking temperature component are isolated from flat-lying or weakly tilted Jurassic red sandstones. The MTC could be carried by maghemite and the HTC is carried by hematite. The two components are statistically identical, and seem to correspond to an ancient magnetization. However, there is no fold test and comparison with previous data from Junggar and the reference APWP of Siberia does not allow us to decide between a Middle Jurassic primary magnetization and an Upper Jurassic or Cretaceous remagnetization. Stronger constraints are obtained from Upper Jurassic to lower Eocene fine-grained red sandstones from anticlines that have been thrust southwards, in the Turfan area of South Tien Shan. After the elimination of a recent overprint that is probably related to weathering, a high coercivity, high unblocking temperature component carried essentially by hematite is isolated. All directions from the Jurassic to the lower Eocene, which yield positive fold and reversal tests, are statistically indistinguishable. This is compatible with a primary origin for these magnetizations, similar to what has previously been found in Junggar, Tarim and western Tibet. The corresponding Turfan pole is not statistically distinct from that of Tarim for the same period, in accordance with the notion of a large, quasi-rigid Tarim block remaining attached to Eurasia in a rather constant position and at a constant latitude throughout the second half of the Mesozoic. This new datum can therefore be included to derive a new mean pole for the Tarim that is valid for the period from the Middle Jurassic to the end of the Cretaceous: 68.4° N, 224.5° E ( A 95 = 3.1°).

Magnetostratigraphic dating of an Upper Miocene shallow-marine and continental sedimentary succession in northeastern Morocco

A high-resolution magnetostratigraphy has been developed for an Upper Miocene shallow-marine carbonate complex, equivalent basinal marls, diatomites, and lime mudstones, and an overlying succession of marine to continental carbonate and siliciclastic sedimentary rocks in the Melilla Basin, northeastern Morocco. Units of the carbonate complex, from bottom to top, contain a bryozoan/red algal ramp, bioclastic platform, fringing-reef complex and a unit referred to as the Terminal Carbonate Complex (TCC) [1]. Lithologies from these units contain at least three components of magnetic remanence: (1) a low unblocking temperature component that is likely carried by a viscous remanent magnetization, (2) a low-coercivity, intermediate unblocking temperature component that is probably carried by magnetite or magnetite and maghemite, and (3) a high unblocking temperature component that is probably carried by hematite. Lithologies are typically dominated by the low-coercivity component and single-domain to pseudo-single-domain grain behavior. Rock-magnetic studies suggest biogenic magnetite may contribute to the magnetic remanence of the low-coercivity component. The Melilla Basin magnetostratigraphy, independently corroborated with new 40 Ar 39 Ar dating of volcanic ashes and foraminifer biostratigraphy, has been correlated to the recently developed geomagnetic polarity time scale of Shackleton et al. [2]. This correlation suggests deposition of the bioclastic platform began approximately at the Tortonian/Messinian boundary, which is defined as the base of chron 3Ar [3], and continued into chron 3An.2n. The fringing-reefs span an interval that begins in chron 3An.2n until just above the base of chron 3r (lower Gilbert). The overlying TCC and mixed carbonate and siliciclastic succession correlate to within chron 3r. The new magnetostratigraphy is not without minor ambiguities, possibly due to either remagnetization within the TCC or the presence of a short-duration normal subchron in the lower portion of chron 3r. Results of this study have important implications for correlating between Late Miocene shallow-marine carbonate sections and basinal sections within the Mediterranean Basin, for improving the understanding of the history of the Messinian Salinity Crisis, and for deciphering the roles of eustacy, tectonics, and regional processes in the development of shallow-marine carbonate sequences in the Mediterranean region.

Counterclockwise rotation of Northeast Japan: Paleomagnetic evidence for regional extent and timing of rotation

Early to Late Miocene welded tuffs have been sampled from northeast Honshu Island and the Oshima Peninsula of Hokkaido Island, Japan for paleomagnetic study. Characteristic directions with a high unblocking temperature component above 560°C are isolated from 24 sites. Westerly declinations ( D = −1° ∼ −91°) are identified in welded tuffs older than 17 Ma from these areas. When combined with previous paleomagnetic data from welded tuffs, the Early Miocene paleomagnetic poles derived from northeast Honshu Island and the Oshima Peninsula are statistically identical and indicate that northeast Honshu Island and the Oshima Peninsula were subjected to counterclockwise rotation as a single tectonic block. These results establish that the rotating block of Northeast Japan extends north to the Oshima Peninsula to the latitude 43°N. The dimension of this rotating block is of the order of 500 km. The paleomagnetic data provide a best curve for the rotation process, with the counterclockwise rotation reaching a climax at 15.0 Ma and a net amount of rotation of 46.4°. The 500 km long Northeast Japan block was rotated counterclockwise through more than 45° and accompanied a clockwise rotation of the 600 km long Southwest Japan block at about 15 Ma.

Magnetostratigraphy of the Kavur Tepe section (southwestern Turkey): A magnetic polarity time scale for the Norian

We present the magnetostratigraphy of the late Triassic pelagic Kavur Tepe section from the Taurides (southwestern Turkey). The section investigated is about 30 m thick and consists of fine-grained whitish limestones. The biostratigraphy, based on the conodont zonation, gives an age from the Lacian 2 zone (lower Norian) to the Sevatian 2 zone (upper Norian). The paleomagnetic analyses indicate very favourable behaviour, with two magnetic components which can be isolated both by thermal and AF demagnetization. The magnetization is carried by magnetite and goethite, which are clearly evident in IRM experiments. The first component of normal magnetic polarity has roughly the direction of the present-day field. The high unblocking temperature component shows directions of both polarities. In total, fifteen magnetic polarity intervals are observed within the Kavur Tepe section. The characteristic calculated Fisherian mean direction is particularly tightly grouped: D = 220.2° and I = 32.1°, with K = 40.9, α 95 = 1.7° and N samp = 179 (after bedding correction). Polarity correlations with two other sections in Turkey and in Austria (Northern Calcareous Alps) show that the Kavur Tepe sediments were deposited in the southern hemisphere. By combining the magnetostratigraphic results from the Bolücektasi Tepe [1] and the Kavur Tepe sections, an almost complete Norian magnetic polarity sequence is established, showing 30 magnetic intervals that are well calibrated with the conodont and ammonoid zonations. A magnetic reversal frequency close to about 2.7 reversals/Myr is derived for the Norian. This medium frequency is in agreement with the hypothesis of a long time constant (150–200 Myr) in the changes of the magnetic reversal frequency since the Kiaman reversed superchron.

The paleomagnetism of red beds and basalts of the Hettangian Extrusive Zone, Newark Basin, New Jersey

A high unblocking temperature component of magnetization residing in hematite was isolated in 11 outcrop sites and 4 boreholes from Hettangian sedimentary rocks interbedded with the igneous extrusives of the Newark basin. This normal polarity characteristic magnetization has a mean direction (declination = 6.3°, inclination = 12.9° after tilt correction) which is significantly shallower (by about 15°) than reported magnetization directions from the coeval basalts. Based on our demagnetization analysis of representative basalt samples, we attribute the difference between the sedimentary and basaltic directions to contamination of some of the basalt magnetizations by a steeper overprint. An intermediate unblocking temperature magnetization recovered from the extrusive zone sedimentary rocks, consistent with evidence elsewhere in the Newark basin for a pervasive hydrothermal and remagnetization event at about 175 Ma, is a likely cause of the basalt contamination. The characteristic magnetization from the Hettangian sedimentary rocks yields a paleopole (55.5°N, 94.6°E, dp = 5.2°, dm = 5.4°) which lies on the westbound, precusp portion of current paleomagnetic Euler pole apparent polar wander (APW) paths, in contrast with the Newark (and Hartford) basin basalt reference pole positions (such as the “Newark trend” N1 pole) which lie on the eastbound, postcusp portion of the APW path. A precusp pole position is more compatible with the relative age of the Hettangian extrusive zone pole with respect to the Sinemurian/Pliensbachian poles which define the cusp but implies large changes in the rate of APW in the Late Triassic and Early Jurassic.

Devonian-Carboniferous paleopoles for Africa: Consequences for Hercynian geodynamics

Paleomagnetic analysis of marine formations in the northwestern Sahara (Algeria) yields the following results: three sites in the lower Devonian Orthoceras-bearing limestones of Béni-Abbès (29.67° N, 2.15° W) indicate a mean direction of remanence: D m = 149.6°; I m = 1.6°; α 95 = 6.8°. This typical Permian direction implies a complete remagnetization of the sites. The Famennian griotte limestones of the Ben-Zireg anticline (31.92° N, 1.82° W) show high remanent magnetization intensities. In this structure, which has undergone polyphase tectonism, multicomponent analysis evidences a high unblocking temperature (HTb) component carried by haematite leading to a positive fold test (after partial unfolding) at the 95% confidence level with a mean direction D = 156.4°, I = 47.6°, α 95 = 0.9° and attributed to the Early Carboniferous (355 Ma) with a paleopole at 25.3°S, 21.1°E, dp = 0.8°, dm = 1.2°. An intermediate unblocking temperature (ITb) component of magnetization carried probably by magnetite, also present, satisfies a positive fold test (after tilt correction), with a mean direction D = 154.9°, I = 54.3°, α 95 = 3.7°. This pre-folding direction of magnetization is attributed to the Famennian. It gives a paleopole at 19.2°S, 19.8°E, dp = 3.7°, dm = 5.3°. Comparison with previous results shows a significant S-N crustal shortening of about 2500 km during Carboniferous times accompanied by a clockwise rotation of the Saharan region of about 25°. These new results give information about the tectonic phenomena involved in the collision between Gondwanaland and Laurasia: they argue for a Pangea A 2 dominating model, and define mean values for shortening rate and plate velocity comparable to the present ones (3 cm/yr).

Paleomagnetic results from the southeastern margin of the Ebro Basin (northeastern Spain): evidence for a Tertiary clockwise rotation

Abstract Triassic rocks underlying the Paleogene deposits of the Ebro Basin are exposed along its SE margin. About 200 samples of Middle Triassic and Paleogene rocks from this margin have been demagnetized and analysed. The Middle Triassic red beds contain one or two secondary components of magnetization. The high unblocking temperature component has a mean direction of D = 1° and I = 6°. The Eocene red beds display a very stable magnetization. Nevertheless some samples have a high-temperature component with the present-day field direction. The Eocene mean direction is D = 22° and I = 51°. The paleomagnetic results show a systematic 20° clockwise rotation relative to the data from the Pyrenean and Cantabrian Ranges and the western and SW part of Iberia. To explain these results two hypotheses are presented. One implies the rotation of the NE part of Iberia, whereas the other refers to a rigid block rotation mechanism along the Catalan Coastal Range.