Michel Prévot

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.

Intensity of the Earth's magnetic field : evidence for a Mesozoic dipole low

The variation of the strength of the Earths magnetic field throughout geological time is known only poorly. Large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks. Even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units, fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations. These uncertainties, combined with well-known experimental difficulties, can explain the large scatter in paleointensity data reported in most previous review papers. Because experimental difficulties are more rigourously dealt with using the Thellier method, which also offers some opportunities to minimize some fundamental uncertainties, we carried out a compilation of all Triassic and younger paleointensity determinations made using this method. This selected data set indicates the occurrence of large, long-term changes of the geomagnetic dipole strength with time. The average dipole strength seems to have been approximately constant since Late Cretaceous time, but a dipole strength only one third of the Cenozoic value prevailed during most of Mesozoic time, following a period of larger strength ending possibly during Early Jurassic times. The Mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle. The terrestrial dynamo seems therefore to exhibit different long-term stable states, which are probably controlled by the thermal structure in the lowermost mantle.

Evidence suggesting extremely rapid field variation during a geomagnetic reversal

Abstract Large, systematic variations in direction of high-temperature remanence as a function of vertical position occur in a basalt flow from the Miocene volcanic sequence at Steens Mountain, Oregon, that has provided a detailed record of a geomagnetic reversal. These may have been caused by an impulsive change in the transitional field as the flow cooled during the reversal. If this is true, the short time required for the flow to cool by conduction and acquire its thermoremanent magnetization implies astonishingly high rates of change of the geomagnetic field: at least 3° and 300 gammas per day.

Magnetostratigraphy and paleontology of Aït Kandoula basin (High Atlas, Morocco) and the African-European late Miocene terrestrial fauna exchanges

Abstract A magnetostratigraphic study has been carried out on a middle Miocene to upper Pliocene lacustrine sedimentary deposit in the central part of the Ait Kandoula basin, which contains micromammal faunas and is situated in the southern High Atlas (Morocco). In total, 113 samples were subjected to paleomagnetic analysis: 60 out of the 113 studied samples representing 52 different stratigraphic levels yielded a paleomagnetic direction and at least the polarity could be recognized in 42 specimens. Eleven specimens were submitted to AF demagnetization. The mean direction for normal-polarity samples was D = 349.4, I = 50.7 (N = 36, α95 = 4.5, k = 27) and for reversed polarity samples D = 191.2, I = −53.4 (N = 16, α95 = 12.32, k = 9). These results yielded a polarity sequence which we interpret as spanning from Chron C5n.2n to the beginning of Chron C3n.4n. This interpretation relies on biostratigraphic data previously proposed for a part of the continental fauna found in the basin. This result is in agreement with the 40Ar/39Ar dating previously carried out on a volcanic ash layer, which provided an age of 5.9 ± 0.5 Ma [1] and which is shown here to be reversely magnetized. This layer is correlated here with the reverse polarity zone corresponding to Chron C3r. Biostratigraphic studies on the same section have shown that the micromammal levels extend here only from middle Vallesian to upper Turolian (upper Miocene). Four localities have yielded western European species of micromammals, indicating trans-Mediterranean terrestrial faunal exchanges between these two continents during the late Miocene. The European murid rodent Occitanomys is recorded for the first time in North Africa in level 8 of the Afoud section, an age younger than 5.32 Ma being assigned to this level by the present study. Level 1 of the same section yields the lagomorph Prolagus cf. michauxi, with an age of 6.1 Ma. The magnetostratigraphic data suggest therefore that the beginning of terrestrial faunal exchanges between North Africa and Western Europe took place in subchron C3An.1n, at about 6.1 Ma, some 0.4 Myr before the beginning of the Messinian salinity crisis.

Evidence for a 20° tilting of the Earth’s rotation axis 110 million years ago

True polar wander (TPW), the shift of the Earths rotation axis with respect to the entire globe, is most probably due to mass redistribution in the Earths mantle as a result of convection. Using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks, we find that TPW has been clearly intermittent over the last 200 Ma with two long periods of strict standstill from the present to 80 Ma and from approximately 150 to 200 Ma. A single period of shifting is observed, between 80 and about 150 Ma ago. This period culminates around 110 Ma ago in an 20° abrupt tilting during which an angular speed exceeding 5°/Ma (0.5m/yr) may have been reached. Assuming that the time-averaged geomagnetic field is axial, our results indicate that the changes in the position of the rotation axis, and therefore in the inertia tensor of the Earth are intermittent. We suggest that a major reorganization of the mass distribution in the Earths mantle occurred in the Lower Cretaceous. This event, concomitant with plume hyperactivity at the Earths surface and probable drastic changes at the core/mantle boundary attested by the inhibition of geomagnetic reversals, suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core/mantle boundary. The astonishingly strict stability of the time-averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle. Given the intermittence of mantle avalanching, we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two-layered convection system within the mantle.

Single-domain chemical, thermochemical and thermal remanences in a basaltic rock

SUMMARY Tiny basaltic samples containing finely grained titanomagnetite with Curie temperature less than 100°C were heated in air in weak field (25–100μT) at temperatures between 400°C and 560°C for times as long as 32 hr. Oxyexsolution of titanomagnetite resulted in the crystallization of interacting single domain particles with Curie point close to 540°C and the concomitant development of one of two types chemical remanence, depending upon thermal treatment: isothermal chemical remanence (CRM) or thermochemical remanence (TCRM), the latter acquired under the combined effects of chemical change and temperature decrease. CRM and TCRM acquired under various conditions were subjected to Thellier-type experiments. All these treatments were carried out using a vibrating sample thermomagnetometer allowing the continuous recording of magnetization and a very precise temperature control. All CRM–TRM and TCRM–TRM plots were found to be linear over almost the entire TRM blocking temperature range, whether pTRM checks are positive or not. An apparent strength of the acquisition field of CRM or TCRM could thus be obtained and divided by the actually applied field to obtain a ratio R, which is representative of the CRM/TRM or TCRM/TRM ratios over most of the unblocking/blocking temperature spectrum. For CRM, it is found that R is less than 1 and increases rapidly with acquisition temperature (0.36 ± 0.07 at 400°C, 0.66 ± 0.02 at 450°C, and 0.90 ± 0.02 at 500°C), in qualitative agreement with expressions derived from the theory of non-interacting single domain grains. Thus, very large underestimate of geomagnetic field palaeostrength can occur when a natural CRM is not recognized as such and is believed to be a natural TRM. Palaeointensity data obtained from geological material prone to the development of secondary minerals, as for example baked contacts and volcanic glasses have, therefore, to be considered with caution. In an attempt to mimic deuteric oxyexsolution in cooling magma, TCRM was imparted from 560°C to 400°C at a cooling rate of 0.1°C/min. In contrast to the CRM case, R is found to slightly exceed 1 (1.11 ± 0.05). According to theoretical considerations, this finding suggests that in practice TCRM approximates to TRM.

40Ar/39Ar dating of a tephra layer in the Pliocene Senèze maar lacustrine sequence (French Massif Central): constraint on the age of the Réunion–Matuyama transition and implications on paleoenvironmental archives

Abstract The sedimentary sequence cored in the Seneze maar (Velay, France) is very favourable for magnetostratigraphy of the Late Pliocene and for studying the paleoenvironmental changes that occurred during that period. The magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the Matuyama reverse period, except for the upper part where a short normal polarity episode was recorded. A tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2.10±0.01 Ma by the 40 Ar/ 39 Ar method. Using this chronological marker, it can be inferred that the normal polarity episode recorded in the Seneze sequence corresponds to the Reunion event and that the age of the normal–reverse transition closing this event is ca. 2.09 Ma. These chronological constraints also permit to (1) demonstrate that the ‘Villafranchian’ mammal fauna found in the Seneze maar is younger than 2.09 Ma and (2) correlate the climatic events recorded in the Seneze sequence to the marine δ 18 O records. This suggests that the Seneze pollen sequence (5–120 m depth) ranges from isotopic stage 85 to 76.

A Statistical Model of the Fluctuations in the Geomagnetic Field from Paleosecular Variation to Reversal

The statistical characteristics of the local magnetic field of Earth during paleosecular variation, excursions, and reversals are described on the basis of a database that gathers the cleaned mean direction and average remanent intensity of 2741 lava flows that have erupted over the last 20 million years. A model consisting of a normally distributed axial dipole component plus an independent isotropic set of vectors with a Maxwellian distribution that simulates secular variation fits the range of geomagnetic fluctuations, in terms of both direction and intensity. This result suggests that the magnitude of secular variation vectors is independent of the magnitude of Earths axial dipole moment and that the amplitude of secular variation is unchanged during reversals.

Geomagnetic paleosecular variation recorded in Plio-Pleistocene volcanic rocks from Possession Island (Crozet Archipelago, southern Indian Ocean)

Possession Island, in the Crozet Archipelago, consists of volcanic units erupted mainly between ~5 and 0.5 Ma. A paleomagnetic sampling was carried out along several sections distributed near the northern, eastern, and southeastern coasts. A total of 45 independent flows were sampled (320 samples). For each flow a precisely defined characteristic remanence direction was usually isolated after a careful progressive cleaning in alternating fields. However, particularly complex remanence behavior is often observed. The magnetostratigraphy of the lava pile is quite simple, with reversed rocks in the lower part and normal units in the upper part of two sections. A third section is of normal polarity throughout its whole thickness, including three excursional directions. We did not find any intermediate directions between the normal and reverse magnetozones. Thus we have no evidence for the recording of the Matuyama-Brunhes transition expected from a previous study (Watkins, 1972). The amplitude of paleosecular variation, estimated from between-flow dispersion from the field of an axial dipole, is 11.8 with 95% confidence limits between 9.3 deg and 14.0 deg. This value is consistent with the general anisotropic statistical model for paleosecular variation of Constable and Johnson, 1999.

Transitional geomagnetic impulse hypothesis: Geomagnetic fact or rock-magnetic artifact?

A striking feature of the Steens Mountain (Oregon) geomagnetic polarity reversal is the two (maybe three) extremely rapid field directional changes (6 degrees per day) proposed to account for unusual behavior in direction of remanent magnetization in a single lava flow. Each of these very fast field changes, or impulses, is associated with a large directional gap (some 90°) in the record. In order to check the spatial reproducibility of the paleomagnetic signal over distances up to several kilometers, we have carried out a paleomagnetic investigation of two new sections (B and F) in the Steens summit region which cover the second and the third directional gap. The main result is the description of two new directions, which are located between the pre second and post second impulse directions. These findings weigh against the hypothesis that the geomagnetic field cause the unusual intraflow fluctuations, which now appears to be more ad hoc as an explanation of the paleomagnetic data. However, the alternative baking hypothesis remains also ad hoc since we have to assume variable rock magnetic properties that we have not yet been able to detect within the flows at the original section Steens A and D 1.5 km to the north. In addition, new results for 22 transitional and normal lava flows in section B are presented that correlate well with earlier results from section A.