African Apparent Polar Wander Path Research Articles

Multimethod palaeointensity results from a rapidly emitted upper Miocene lava flow sequence in São Vicente (Cape Verde): new data for the African record

SUMMARY We present new radiometric ages and full-vector palaeomagnetic data from São Vicente Island, Cape Verde. Samples were obtained from a sequence of 11 flows. A K-Ar age determination was performed on the lowermost flow of the sequence, yielding an upper Miocene age of 6.07 ± 0.16 (2σ) Ma. Considering the normal polarity observed in all flows, the age of the sequence could be narrowed down to an interval between 6.03 and 6.23 Ma. The mean palaeomagnetic direction of the studied sequence was compared with the expected direction, calculated from the African Apparent Polar Wander Path (5 Ma). While inclinations approximately agree, the palaeodeclination appears westwardly deviated by 26.5° ± 2.9°. Moreover, nearly no scatter of virtual geomagnetic poles around their mean value is observed and its comparison with the scatter observed in palaeomagnetic directions obtained from geomagnetic field model SHA.DIF.14k suggests that the sequence was very rapidly emplaced in only a few centuries. Thus, the obtained anomalous palaeomagnetic direction seems to reflect the effect of non-averaged secular variation, although a local vertical-axis counter-clockwise rotation of the sequence, cannot be entirely ruled out. Thellier–Coe palaeointensity experiments on 66 specimens yielded 22 successful determinations and multispecimen (MS) palaeointensity experiments on 11 samples yielded 8 successful determinations. In the upper part of the sequence, Thellier–Coe determinations fail due to the presence of multidomain (MD) grains, but the MS method yields successful results in several of these flows, demonstrating that this method can provide reliable palaeointensity determinations on samples with a higher MD grain fraction. A mean palaeointensity BSV = (18.5 ± 1.9) µT is obtained from the joint analysis of the results from both methods. This value lies clearly below the present-day value BP = 33.1 μT. The virtual axial dipole moment (VADM = 4.3 × 1022 Am2), however, cannot be considered anomalous if compared to available upper Miocene–Pliocene data and represents a new high-reliability contribution to the very limited African palaeointensity database.

Paleomagnetic evidence for counterclockwise rotation of the Dofan magmatic segment, Main Ethiopian Rift

Twenty-six paleomagnetic sites in basalt and trachyte flows and ignimbrite deposits sampled in the Dofan magmatic segment, Main Ethiopian Rift (MER). From each site, 6 to 8 core samples were collected. The samples were then cut into 200 standard specimens and their Natural Remanent Magnetization (NRM) directions were measured using a JR6A spinner magnetometer. Most specimens were subjected to stepwise alternating field (AF) and at least one specimen per site to thermal (TH) demagnetization. The directional analysis of these individual specimens revealed either one or two components of NRM. Where two components are present, the first is isolated below a temperature of 300 °C or AF field below 20 mT; the second is isolated above those steps and mostly defined straight lines directed towards the origin and are interpreted as the Characteristic Remanent Magnetization (ChRM) acquired during cooling. Rock magnetic experiments on representative specimens indicate that the dominant magnetic minerals are titanium poor titanomagnetite and in few cases titanohematites. The overall mean directions calculated for the 23 sites of Dofan is Dec = 354.1°, Inc. = +11.6° (N = 23, K = 35.1, α95 = 5.2°). When these values are compared with the 1.5 Ma expected mean geomagnetic dipole reference field directions Dec = 1.0°, Inc. = +16.4° (N = 32, K = 105.6, α95 = 2.3°), obtained from African Apparent Polar Wander Path Curve; a difference in declination ΔD = −6.9° ± 4.7° and inclination ΔI = +4.8° ± 5.5° are determined. The declination difference is interpreted as a very slight counterclockwise rotation about vertical axis of the Dofan magmatic segment and the result is consistent with previous paleomagnetic reports and analogue modeling in Fentale magmatic segment.

Geophysical and magneto‐structural study of the Maâdna structure (Talemzane, Algeria): Insights on its age and origin

AbstractThe Maâdna structure is located approximately 400 km south of Algiers (33°19′ N, 4°19′ E) and emplaced in Upper‐Cretaceous to Eocene limestones. Although accepted as an impact crater on the basis of alleged observations of shock‐diagnostic features such as planar deformation features (PDFs) in quartz grains, previous works were limited and further studies are desirable to ascertain the structure formation process and its age. For this purpose, the crater was investigated using a multidisciplinary approach including field observations, detailed cartography of the different geological and structural units, geophysical surveys, anisotropy of magnetic susceptibility, paleomagnetism, and petrography of the collected samples. We found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies, ones that might indicate the occurrence of a causative material which is at least in part identical. Geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface. Using soil magnetic susceptibility measurements, the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material. Paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite. A primary magnetization is inferred from a stable remanence with both normal and reverse directions, carried by these two minerals. Although this is supposed to be a chemical remagnetization, its normal polarity nature is considered to be a Pliocene component, subsequent to the crater formation. The pole falls onto the Miocene‐Pliocene part of the African Apparent Polar Wander Path (APWP). Consequently, we estimate the formation of the Maâdna crater to have occurred during the time period extending from the Late Miocene to the Early Pliocene. Unfortunately, our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones, nor shocked minerals, were found. A dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis, which can still be considered as plausible. Only a drilling may provide a definite answer.

Strong clockwise block rotation of the Ali-Sabieh/Aïsha Block: evidence for opening of the Afar Depression by a ‘saloon-door’ mechanism

AbstractPalaeomagnetic investigations were carried out on tilted Jessoma sandstones in the Aïsha block. The average natural remanent magnetization intensity was 0.026A m−1 with good behaviour on demagnetization both by thermal and alternating fields. The normalized magnetization intensity decay curves indicated that fine-grained, Ti-poor titanomagnetites were the carriers of magnetization. A well-defined single-component magnetization with a poorly defined secondary component was found. The latter was removed at temperatures below 300°C or at 10 mT. After these steps, a straight-line segment directed towards the origin was identified, which was interpreted as the characteristic remanent magnetization. Site mean direction coordinates of Decg=227.8°, Incg=–33.9° (n=7, α95=2.8°, K=478) (in situ) and Decs=225.6°, Incs=0.4° (n=7, α95=2.8°, K=478) (tilt-corrected) were obtained. Corresponding mean virtual geomagnetic pole coordinates of Long=117.5° E, Latg=43.3° N (A95=3.2°, n=7) (in situ) and Lons=143.5° E, Lats=43.3° N (A95=2.8°, n=7) (tilt-corrected) were obtained. The corrected in situ mean pole position was compared with the African apparent polar wander path curve and a deposition age for the sandstone of 60±5 Ma was determined, which is consistent with previous reports. A vertical axis rotation (R±ΔR) of the Aïsha block of 29.1±4.06° was obtained, which is equivalent to an inferred 30° counterclockwise rotation of the Danakil block, in agreement with a ‘saloon-door’ mechanism for the opening of the Afar Depression.

Magnetostratigraphy, nannofossil stratigraphy and apparent polar wander for Adria-Africa in the Jurassic–Cretaceous boundary interval

Existing correlations of Early Cretaceous nannofossil events to polarity zones in Italian pelagic limestones are the basis for correlation of polarity chrons to geologic stages, and hence calibration of the Early Cretaceous geomagnetic polarity timescale. Here, we present correlations of nannofossil events to polarity chrons for the Late Jurassic and Jurassic/Cretaceous boundary interval from the Southern Alps, making the case for revisions of current geologic timescales in this interval. The Jurassic/Cretaceous and Kimmeridgian/Tithonian boundaries can be usefully defined at the onsets of polarity chrons CM18R and CM22R, respectively. The Oxfordian/Kimmeridgian boundary can be assigned to CM25. An apparent polar wander path (APWP) for the Southern Alps, derived from, and time-calibrated by bio-magnetostratigraphic data, is similar to, and provides a proxy for, the African APWP in the absence of adequate African data of this age. The Southern Alpine APWP is characterized by a cusp (hairpin) at ∼ 30°N/270°E at a time close to the Oxfordian/Kimmeridgian boundary (polarity chron CM25, ∼ 154 Ma), rapid APWP from high latitude (> 60°N) in the preceding middle Jurassic (Bajocian–Oxfordian) interval coincident with the onset of Central Atlantic/Ligurian oceanic rifting, and a return to high latitudes during the Cretaceous. Early Jurassic poles at high latitudes (> 60°N) are evident for Sinemurian to Toarcian–Aalenian time (175–195 Ma). An APWP for this time interval with this level of temporal calibration cannot be constructed using data from Africa itself, or from other Atlantic-bordering continents by rotation to African coordinates.

Rotations in Lebanon inferred from new palaeomagnetic data and implications for the evolution of the Dead Sea Transform system

AbstractA study carried out on widespread sites of the Aptian–Albian formations in Lebanon led to two palaeomagnetic directions corresponding to the primary magnetization (N=37 sites, D=307.1°, I=23.7°, k=18 and α95=5.5° after tilt correction and to a post-folding remagnetization (N=18 sites, D=346.3°, I=49.2°, k=108 and α95=3.2° before tilt correction). Comparison of these data with previous palaeomagnetic results for the Jurassic age in Lebanon and expected directions from African apparent polar wander path yields evidence of three different counter-clockwise regional rotations, of the order of 33° before Aptian deposition, of 11° during Late Miocene times, and of 18° since Miocene period. The two last rotations are related to the relative displacement of the African and Arabian plates. A model is proposed for the evolution of this particular Middle East area, in which the Dead Sea Transform shows a strong deviation relative to its main north–south orientation.

Paleomagnetism of Aptian–Albian sections from the Northern Apennines (Italy): Implications for the 150–100 Ma apparent polar wander of Adria and Africa

We computed a new 120 to 100 Ma high-resolution Apparent Polar Wander Path (APWP) for Adria from magnetostratigraphic studies in the Northern Apennines (Italy). We analyzed 274 cores from three Early Aptian–Late Albian sections of the Marne a Fucoidi Fm., belonging to the sedimentary succession deposited above the Adria passive margin during Meso-Cenozoic time. To solve the problem of tectonic rotations induced during the Apennine orogenesis, we computed relative rotations between sections and realigned them into a common declination reference frame (Bosso, Italy). These new data were added to the previous 150–125 Ma segment computed from the same region [Satolli, S., Besse, J., Speranza, F., Calamita, F., 2007. New 125–150 Ma high-resolution Apparent Polar Wander Path for Adria from magnetostratigraphic sections in Umbria-Marche (Northern Apennines, Italy): Timing and duration of the global Jurassic–Cretaceous hairpin turn. Earth Planet. Sci. Lett. 257, 329–342. doi:10.1016/j.epsl.2007.03.009.] to get a very high-resolution 100 to 150 Ma APWP for Adria. The comparison of our APWP with the African APWP segment of Besse and Courtillot computed with 10 Myr sliding window [Besse, J., Courtillot, V., 2002. Apparent and true polar wander and the geometry of the geomagnetic field over the last 200 Myr. J. Geophys. Res. 107. doi:10.1029/2000JB000050.] leads to the following conclusions: 1) our segment (after a 25° clockwise rotation of the Bosso reference frame) is in very good agreement with the African APWP, as shown by a very similar time evolution of inclinations and declinations, most often statistically indistinct; 2) this agreement vindicates our previous model of structural unit rotation for older periods [Satolli, S., Besse, J., Speranza, F., Calamita, F., 2007. New 125–150 Ma high-resolution Apparent Polar Wander Path for Adria from magnetostratigraphic sections in Umbria-Marche (Northern Apennines, Italy): Timing and duration of the global Jurassic–Cretaceous hairpin turn. Earth Planet. Sci. Lett. 257, 329–342. doi:10.1016/j.epsl.2007.03.009.] 3) the concept of “synthetic” APWP construction is strongly validated; 4) the good agreement found strengthens the fact that Adria has been a promontory of Africa at the lithospheric level during Late Jurassic–Early Cretaceous (150–100 Ma), affected by tectonic rotations linked to the Apennines orogenic phases during Neogene; 5) our data evidence a possible ca. 105 Ma southward motion of Adria undetected from classical African APWPs. Finally, we have not found evidence for reversals interrupting the long normal Cretaceous superchron in our sections.

Autunian age constrained by fold tests for paleomagnetic data from the Mezarif and Abadla basins (Algeria)

A paleomagnetic study has been conducted on a formation dated as Autunian in the Nekheila area (31.4°N, 1.5°W) in the Mezarif basin. ChRM was thermally isolated in 117 samples from seven sites. This ChRM ( D = 131.8°, I = 15.7°, k = 196, α 95 = 3.8° after dip correction; corresponding pole 29.3°S, 56.4°E) is very similar to that obtained in the neighboring Abadla basin from a formation of the same age. Fold tests associated with progressive unfolding applied to the full merged data from the dated formations of these two basins clearly indicate that the magnetization acquisition predates the deformation, which is attributed to the last phase of the late-Hercynian. The magnetization in these basins is therefore primary or acquired just after deposition. For the African Apparent Polar Wander Path, the age of the paleomagnetic poles of the Autunian part is now confirmed by paleomagnetic test.

Nouveau pôle paléomagnétique Stephanien inférieur pour le craton saharien (formation de Merkala, bassin de Tindouf, Algérie): New Lower Stephanian palaeomagnetic pole for the Sabaran craton (Merkala formation, Tindouf basin, Algeria)

A palaeomagnetic study carried out in the Lower Stephanian Merkala formation (Tindouf basin) pointed out two juxtaposed neighbouring components of the magnetization. The oldest one allowed a new Stephanian pole located at 32.4°S and 56.6°E ( K= 399, A 9.5 = 2.3°) to be obtained. The other one results from the superimposition of this old component on a Permian remagnetization. The new Stephanian pole, associated with the previous data from the Saharan craton, allows the Stephano-Autunian segment of the African apparent polar wander path to be specified.

Paleomagnetism of the Lower Permian redbeds of the Abadla Basin (Algeria)

One of the reference Permian paleomagnetic poles for the African Apparent Polar Wander Path (APWP) was determined by (Morel et al., 1981, Earth Planet. Sci. Lett. 55, 65–74) in the upper unit of the 2000 m thick Abadla redbeds formation (31.2°N, 2.7°W). Unfortunately, this upper unit remains undated, although its age was presumed to be Autunian, whereas microflora fossils characteristic of the Autunian were discovered in the lower unit of the same formation. The paleomagnetic data ( N=11, D=130.1°, I=13.0°, k=138, α 95=3.6° and pole: 29.1°S, 57.8°E, A 95=2.0°) obtained from the better dated Autunian levels must take the place of the previous Abadla pole for the Autunian part of the African APWP. This new pole is very close to the Morel et al.'s pole determined from the unfossiliferous upper unit of the Abadla formation, giving an Autunian age for this upper unit. Consequently, the subsidence of the Abadla basin and the deposition of the whole succession occurred during a period not longer than 20–25 Myr.

Paleomagnetism of latest Anisian (Middle Triassic) sections of the Prezzo Limestone and the Buchenstein Formation, Southern Alps, Italy

A paleomagnetic study was carried out at six stratigraphic sections (309 specimens) in the latest Anisian (Middle Triassic) Prezzo Limestone and the overlying Buchenstein Formation. These units outcrop over a wide area in the western Southern Alps, although most of the sampled sections are in the vicinity of the Late Eocene-Early Oligocene Adamello batholith. Three sites suffered a complete remagnetization induced by the Adamello, whereas a characteristic component with a positive fold test has been isolated at the three other sites. The mean pole of the characteristic component (Lat. 63.2°N, Long. 229.3°E, N = 3, A 95 = 8° , k = 236) is in agreement with the Triassic portion of the West Gondwana apparent polar wander path (APWP), supporting the use of paleopoles from well-dated rocks in the Southern Alps as useful proxies for the African APWP. The characteristic component, where isolated in the sampled sections, is of normal polarity only, corresponding to the latest Anisian on the basis of well-defined ammonoid and conodont biostratigraphy, but the present results suggest that there are good opportunities for extending Middle Triassic magnetostratigraphy in these Southern Alps rock units. The mean pole of the Adamello-induced component (Lat. 74.5°N, Long. 172.1°E, N = 4, A 95 = 7.6° , K = 145) lies close to the Early Tertiary portion of the APWP for stable Europe. The post-folding, Adamello-induced directions confirm that the 30–42 Ma Adamello batholith intruded after Alpine deformation and that no further deformation apparently occurred in post-emplacement times.

Palaeomagnetism and Opaque Mineral Oxides of Some Basalt From West Central Sinai, Egypt

SUMMARY A stable remanence was isolated from two intrusive occurrences of volcanic rocks collected from west central Sinai, Egypt. Tertiary basalts at Wadi Nukhul have a predominantly reverse magnetization with mean direction D= 173.5°, I=-60.7° (K= 46.6, α95= 8.2°, dyke and sill) and a palaeopole at 76°S, 193°E. Coarse primary magnetite (100–500 μm) with a Curie temperature between 540° and 580 °C is the principal magnetic mineral. A Mesozoic basaltic intrusion at Gebel Farsh El Azraq exhibits a predominantly normal magnetization carried mainly by fine-grained (50–300 μm) titanomagnetite. Its mean direction is D= 139°, 7 = 44.6° (K= 5.9, α95= 24.8°, N= 8 sites) with a palaeopole at 28.6°S and 179°E. These results, together with other results from west central Sinai have been compared with the apparent polar wander path (APWP) for Africa given by Irving & Irving (1982). the Mesozoic poles are not compatible with poles of similar age for Africa, reflecting a local effect of tectonic rotations and/or geomagnetic secular variation. In contrast, the Egyptian Early Tertiary poles compare favourably with the relevant part of the African APWP for the period later than 40 Myr, in good agreement with the K-Ar ages of 22–25 Myr obtained for these intrusives.

Geological and geophysical parameters of mid-plate volcanism

Volcanism occurs mainly at oceanic ridges and subduetion zones, but also takes place within continental plates. Mid-plate Cenozoic volcanism is widespread in Africa, where it is alkaline in composition and characteristically associated with uplift and faulting. There is a clear time correlation between the pause at ca. 45 Ma in the African apparent polar wander path and the outbreak of volcanism at ca. 35 Ma, suggesting that the slowing or coming to rest of the plate may be an important antecedent to the outbreak of volcanism. Moreover, the Cenozoic volcanism is almost completely restricted spatially to non-cratonic areas. Cratons in Africa are characterized geologically by tectonic stability for at least the past 1200 M,and geophysically by low elevations, heat flow less than 45 mW m-2, lithospheric thicknesses greater than 200 km higher than average seismic velocities. In contrast, non- cratonic regions have undergone tectono-thermal events within the past 1200 Ma, stand higher, have greater heat flow and lower lithospheric thicknesses and seismic velocities.

Geological and Geophysical Parameters of Mid-Plate Volcanism [and Discussion

Although volcanism occurs mainly at oceanic ridges and subduction zones, it also takes place within continental plates. Mid-plate Cainozoic volcanism is widespread in Africa where it is alkaline in composition, is characteristically associated with uplift and faulting, and is almost completely restricted to non-cratonic areas. Cratons in Africa are characterized by low elevations, heatflow less than 45 mW m -2 , lithospheric thicknesses greater than 200 km and higher than average seismic velocities. In contrast, non-cratonic regions stand higher and have greater heatflow and lower lithospheric thicknesses and seismic velocities. There is a clear time correlation between the pause at ca . 45 Ma in the African apparent polar wander path and the outbreak of volcanism at ca . 35 Ma. Accepting that sub-lithospheric heat input is regionally variable, geophysical modelling indicates that the spatial and temporal distribution of uplift-associated mid-plate volcanism can be almost entirely explained in terms of plate thickness and velocity. We examine, quantitatively, conductive models that determine the thermal disturbances in the lithosphere explicitly in terms of perturbation strength, plate thickness and plate velocity. For example, for a plate over 200 km thick, a relatively modest movement (> 2 cm a -1 ) will suppress the upward propagation of most sublithospheric thermal anomalies, thereby precluding mid-plate volcanism. Where such thermal anomalies are confined to the base of the lithosphere, uplift alone, without surface volcanism, would result. The thinning of the lithosphere predicted by the theoretical models can be accomplished by deep, strong thermal perturbations or by upward migrating perturbations of lesser strength. The latter is consistent with configurations for the East African-Ethiopian lithothermal systems derived from geophysical and geochemical data. Mesozoic volcanism of Gondwanaland was more widespread and voluminous, and erupted through both cratons and non-cratons, thereby suggesting that the causative thermal anomalies were much more vigorous than those in the Gainozoic.

Precambrian geodynamics — a palaeomagnetic view

The problem of the origin of the major Precambrian orogenic belts presently located within continents is examined using palaeomagnetic data for Africa, Australia and North America. For the period 1900–2300 m.y. data from the West African and Kaapvaal eratons form a coherent set with poles of similar age from each craton falling consistently on a combined apparent polar wander path constructed for this time interval. For the interval 700–1100 m.y. data from the Kalahari and Congo cratons likewise form a coherent set. The consistency of the data strongly suggests that these cratons were not previously widely separated and then converged to form the Pan African and older orogenic belts of Africa. New isotopic information drastically alters the shape of the African apparent polar wander path in the time interval 1050–1250 m.y. The evidence for a Precambrian supercontinent based on the matching of this section of the path with the Logan Loop of North America is no longer tenable. Data from Australia for the time-interval 1100–2500 m.y. also form a coherent set irrespective of the craton from which they were derived. This consistency again suggests that the Ophthalmian, Musgrave and Albany—Frazer belts did not arise from the convergency of widely separated cratons. The extensive data from North America likewise form a coherent set in the time-interval 1400–2600 m.y. using data from the Superior, Churchill, Nain, Bear and Slave Provinces and from the Bear-Tooth uplift of Wyoming and Montana. The Hudsonian orogeny cannot therefore be the result of plate convergence in agreement with that found for the belts on other continents. Results from the Grenville Province are shown to have been derived during a period of post-orogenic uplift and cooling and can be related to thermochrons deduced from K—Ar dating of Grenville rocks. The apparent polar wander path in the interval 700–1100 m.y. derived from these results shows an extension of the Logan Loop into a Grenville Loop. Independent confirmation of this extension is seen in preliminary results reported from the Grand Canyon sequence. Palaeomagnetic data preclude plate-tectonic models involving the convergence of previously widely separated cratons to explain all of the Precambrian orogenic belts examined. Uncertainties and inherent errors associated with the data do not, however, exclude models involving the opening and closing of small (500–1000 km) intercratonic oceans. The requirement, however, is that these oceans must always open and close so as to return the cratons to their same relative positions. Precambrian tectonic mechanisms thus cannot have been the same as present-day plate tectonics. Plate tectonics as seen in the Phanerozoic may therefore have evolved gradually from a more primitive form operating during the Precambrian.