Mireille Perrin

The age of parana flood volcanism, rifting of gondwanaland, and the jurassic-cretaceous boundary.

The Paran�-Etendeka flood volcanic event produced ∼1.5 x 106 cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New 40Ar/39Ar data combined with earlier paleomagnetic results indicate that Paran� flood volcanism in southern Brazil began at 133 � 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paran� flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.

Is this magnetic fabric normal? A review and case studies in volcanic formations

Abstract When studying weakly anisotropic rocks non-standard relationships between magnetic and expected petrofabric axes are sometimes observed, in particular the so-called inverse or intermediate fabrics. A review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics, often encountered in magnetite-rich volcanic rocks, are due to abnormal preferred orientations or distributions of magnetite grains. Through case studies in dikes (from Skye Island, Yemen and the Iberian Peninsula), and magma and ash flows (from the Ethiopian traps and southern Corsica) the various possible origins of such fabrics are examined. It is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed, leading to non-unique interpretations.

Increasing the efficiency of paleointensity analyses by selection of samples using first-order reversal curve diagrams

The global paleointensity database is restricted by the high failure rate of paleointensity analyses. Excluding thermal alteration, failure is usually caused by the presence of multidomain grains and interactions among grains, two properties that can be identified using first-order reversal curve (FORC) diagrams. We measured FORC diagrams on sister samples of about 200 samples that had been used for Thellier paleointensity determinations and determined criteria to discriminate samples that gave acceptable paleointensity results from those that did not. The three most discriminating criteria are the vertical spread of the FORC distribution (indicative of interactions), expressed as the full width at half maximum (FWHM), the spread of the FORC distribution along the Hc = 0 axis (width), and the bulk coercivity Hc (both indicative of domain state). Setting thresholds at 132 mT for the width of the distribution and 29 mT for the FWHM maximizes the number of unsuccessful rejected samples. Using an additional threshold of Hc = 5.4 mT results in rejection of 32% of unsuccessful samples. Seven samples that barely satisfy the paleointensity selection criteria would also be rejected using these selection criteria. Most of the samples that fail the paleointensity experiment without being detected by our selection criteria have ideal noninteracting single-domain magnetic properties but fail because of the thermal alteration that results from repeated heating. Being able to eliminate at least one third of unsuccessful samples using our FORC diagram-based prescreening procedure should provide a significant improvement in efficiency of paleointensity measurements.

Paleomagnetism, paleointensity and geochronology of Miocene basalts and baked sediments from Velay Oriental, French Massif Central

Paleomagnetic study of Miocene basalts and baked sediments from Velay Oriental, France, yields a paleomagnetic pole with coordinates 84.1°N, 171.2°E (A95 = 8.6°, K = 29, N = 11). The pole is supported by a positive reversal test, and it corresponds well with the apparent polar wander path for Eurasia for 10 Ma. Paleosecular variation, estimated as the angular standard deviation of the VGP distribution 14.9° +6.5°/−3.5°, is close to expected, suggesting that the paleomagnetic pole represents a time-averaged field. Moreover, five new 40Ar/39Ar plateau ages, falling between 9.20 and 13.55 Ma, were obtained. Thellier paleointensity experiments were carried out on both basalts and baked sediments. Almost all basalts are chemically unstable during the laboratory heatings and of the 36 preselected samples only three yield usable results. In contrast, the baked sediments are ideal for Thellier experiments with 11 out of 13 samples giving reliable paleointensity estimates. Rock magnetic experiments show that the main ferromagnetic minerals in the baked sediments are fine-grained magnetite and hematite with a large proportion of superparamagnetic grains. In addition to the standard partial thermoremanent magnetization (pTRM) check, used in Thellier experiments to detect chemical/crystalline alteration, we introduce a second “pTRM tail” check designed to test the basic prerequisite of the method, namely, the equality of unblocking and blocking temperatures. For the baked sediments, unblocking and blocking temperatures are found to be identical, which indicates that the remanence is carried by single-domain grains.

Paleointensity of the Earth's magnetic field in Early Cretaceous time: The Paraná Basalt, Brazil

Paleointensity experiments were carried out with the Coe and Thellier methods on Early Cretaceous basalt (133 My old) from the Parana basin in southern Brazil. Paleointensity estimates could be obtained from only six lava flows: three of normal and three of reversed polarity, of the 71 sampled. Moreover, the quality of the determinations is fairly low, with q ratio usually less than 10, mainly as a result of chemical and/or physical changes of the magnetic oxides during heating. This study strongly underscores the difficulty of finding natural rocks suitable for paleointensity experiments even so the directional results previously obtained were good. The most reliable result give an intensity of 28 ± 1 μT, while the others range from 20.8 μT to 37.7 μT. The corresponding virtual dipole moments (VDMs) range from 4.7 x 10 22 to 7.9 x 10 22 Am 2 . This dispersion is attributed mainly to inaccurate estimates of the ancient field rather than to secular variation. Although the number of cooling units for which paleointensity estimates could be obtained is limited, the results nevertheless indicate that the early Cretaceous VDM was significantly lower than that for the recent field but greater than has been reported for this portion of the Mesozoic Dipole Low period.

Paleomagnetism of the Lesotho basalt, southern Africa

Paleomagnetic study of four sections from Middle Jurassic Karoo lava, located in Lesotho and South Africa, yielded a new paleomagnetic pole for Africa with coordinates 71.6°N, 273.5°E (A95 = 3.7°, K = 33, N = 47). Paleosecular variation, estimated as the angular standard deviation of VGPs distribution is 14.1° (+ 2.4°− 1.8°), which is considerably lower than expected for this time interval and latitudinal band. Preliminary paleointensity results indicate a low field intensity during the acquisition of the magnetization, this new result tends to support a possible correlation between secular variation and dipole intensity. When the lower to middle Jurassic paleomagnetic data set for Africa is considered, a difference of 13.9 ± 8.7° of arc is found to exist between the mean Northwest and South African poles. Furthermore, while the Northwest African pole is in very good agreement with the lower to middle Jurassic North American pole, whichever set of rotation parameters is used to close the North Atlantic ocean, the South African pole falls some 12–15° of arc apart. As intraplate deformation models, developed to achieve tighter fit of West Gondwana prior to the opening of the South Atlantic, fail to account fully for the difference between Northwest and South African poles, the pre-Cretaceous Apparent Polar Wander Path for Africa has to be considered with caution.

Rotation of the Oman ophiolite and initial location of the ridge in the hotspot reference frame

Abstract The volcanic units of the Oman ophiolite were paleomagnetically studied in order to better constrain the large rotation of the nappe during emplacement suggested by previous paleomagnetic results. The volcanic sequence, going from pillow basalts extruded at the mid-oceanic ridge up to submarine lavas extruded during the oceanic thrusting, before the end of the obduction, displays components of magnetization which were acquired at various stages during the movement of the nappe on the sea floor. Paleomagnetic Euler Pole analysis shows that the movement can be modeled by a single, large rotation of about 145–150° around an Euler pole located close to the nappe (less than 200 km away). Combining plate tectonic concepts, a mantle-anchored hotspot reference frame and paleomagnetic data, we were able to determine the initial paleolatitude (around 10°N), the initial paleolongitude (around 45°E) as well as the paleorientation (NNW-SSE to N-S) of the ridge in hotspot coordinates. The linear velocity of the nappe between the period of accretion at the ridge and the end of the obduction onto the Arabian peninsula would have been slightly less than 2 cm per year.

FORMATION AND EVOLUTION OF THE MASIRAH OPHIOLITE CONSTRAINED BY PALEOMAGNETIC STUDY OF VOLCANIC ROCKS

Abstract The extrusive rocks of the lower ophiolitic nappe of Masirah Island (Oman) were paleomagnetically studied. Four mean directions of magnetization were isolated, one at low temperature, the others at high temperature. The low-temperature component, found in most samples, corresponds to a recent viscous and / or chemical remagnetization. The high-temperature component, found at three sites in the Centre area, is interpreted as a possible primary magnetization acquired during extrusion or, more probably, as an early remagnetization related to subsequent hydrothermal alteration and indicates formation of the Masirah ophiolite around a paleolatitude of 40°S, close to the present position of the West Somali basin. A second high-temperature component, found in three other sample sites in the Hakl and in the Thumi areas, is probably a chemical remagnetization related to the emplacement of the upper ophiolite nappe in late Maastrichtian to Paleocene times. Finally a last high-temperature component, found only in the two sites from Naft area, could be a post-tectonic remagnetization acquired in Oligocene or Miocene times or represents differential tilting of pillows and sedimentary blocks. The high-temperature components are usually fairly scattered due to the very complex tectonic history of the area. Nevertheless, the relation between the defined paleopoles and the Indian polar wander is unambiguous and the evolution of the Masirah ophiolite from formation up to the emplacement of the second ophiolitic nappe is clearly related to the northward movement of the Indian plate. This new paleomagnetic study rules out the possibility of a common or related origin for the Masirah and the Semail ophiolites.

Paleointensity of the Earth's magnetic field in the Jurassic: New results from a Thellier study of the Lesotho Basalt, southern Africa

Thellier paleointensity experiments carried out on the Jurassic (180 m.y. old) Karoo basalt, Lesotho, southern Africa, provided paleointensity estimates for 40 samples from 10 distinct lava flows with reversed magnetization. Mean paleointensity was found to be equal to 24.2 ± 11.3 μT (standard deviation), with a minimum of 12 μT and a maximum of 41.3 μT. This corresponds to a mean virtual dipole moment (VDM) of 4.6±2.0×1022 Am2. The between-flow dispersion is ascribed to paleosecular variation, which thus would approach up to 40% of the mean paleointensity, as is observed from the published paleointensity data set covering last 20 m.y. This result yields further evidence for the period of low geomagnetic field in Jurassic to Early Cretaceous time (Mesozoic Dipole Low). A symmetrical intensity versus time pattern is suggested for this time interval, with a minimum VDM of 3×1022 Am2 in Late Jurassic and substantially higher (∼ 4.5×1022 Am2) VDMs in Early to Middle Jurassic and Early Cretaceous, respectively.

Revisiting the Jurassic geomagnetic reversal recorded in the Lesotho Basalt (Southern Africa)

SUMMARY We carried out a detailed and continuous palaeomagnetic sampling of the reversed to normal geomagnetic transition recorded by some 60 consecutive flow units near the base of the Lesotho Basalt (183 ± 1 Ma). After alternating field or thermal cleaning the directions of remanence are generally well clustered within flow units. In contrast, the thermal instability of the samples did not allow us to obtain reliable palaeointensity determinations. The geomagnetic transition is incompletely recorded due to a gap in volcanic activity attested to both by eolian deposits and a large angular distance between the field directions of the flows underlying or overlying these deposits. The transition path is noticeably different from that reported in a pioneering work carried out in 1962. The most transitional virtual geomagnetic poles (VGPs) are observed after the volcanic hiatus. Once continents are replaced in their relative position 180 Myr, the post-hiatus VGPs cluster over Russia. However, two successive rebounds from that cluster are found, with VGPs reaching repeatedly the Eastern Asia coast. Thus, the VGP path is not narrowly constrained in palaeolongitude. The decrease in intensity of magnetization as the field deviates from the normal or reversed direction suggests that the decrease in field magnitude during the reversal reached 80‐90 per cent. We conclude that although the reversal is of a dipole of much weaker moment than that which existed on average during Cenozoic time, the characteristics of the reversing geodynamo seem to be basically similar.