Hugues Feinberg

How does the Alpine belt end between Spain and Morocco

The Betic-Rif arcuate mountain belt (southern Spain, northern Morocco) has been interpreted as a symmetrical collisional orogen, partly collapsed through convective removal of its lithospheric mantle root, or else as resulting of the African plate subduction beneath Iberia, with further extension due either to slab break-off or to slab retreat. In both cases, the Betic-Rif orogen would show little continuity with the western Alps. However, it can be recognized in this belt a composite orocline which includes a deformed, exotic terrane, i.e. the Alboran Terrane, thrust through oceanic/transitional crust-floored units onto two distinct plates, i.e. the Iberian and African plates. During the Jurassic-Early Cretaceous, the yet undeformed Alboran Terrane was part of a larger, Alkapeca microcontinent bounded by two arms of the Tethyan-African oceanic domain, alike the Sesia-Margna Austroalpine block further to the northeast. Blueschist- and eclogite-facies metamorphism affected the Alkapeka northern margin and adjacent oceanic crust during the Late Cretaceous-Eocene interval. This testifies the occurrence of a SE-dipping subduction zone which is regarded as the SW projection of the western Alps subduction zone. During the late Eocene-Oligocene, the Alkapeca-Iberia collision triggered back-thrust tectonics, then NW-dipping subduction of the African margin beneath the Alboran Terrane. This Maghrebian-Apenninic subduction resulted in the Mediterranean basin opening, and drifting of the deformed Alkapeca fragments through slab roll back process and back-arc extension, as reported in several publications. In the Gibraltar area, the western tip of the Apenninic-Maghrebian subduction merges with that of the Alpine-Betic subduction zone, and their Neogene roll back resulted in the Alboran Terrane collage astride the Azores-Gibraltar transpressive plate boundary. Therefore, the Betic-Rif belt appears as an asymmetrical, subduction/collision orogen formed through a protracted evolution straightfully related to the Alpine-Apenninic mountain building.

The Rif mountain building (Morocco); a new tectonic scenario

The building of the Alpine Rif belt (southern limb of the Betic-Rif orocline) is restored, mostly based on the Tertiary stratigraphic and metamorphic data set. The Betic-Rif Internal zones derive from an exotic Alboran Terrane partly involved in a S-dipping Betic subduction during the Late Cretaceous ?-Eocene. Incipient collision of the terrane against Iberia triggered back-thrust tectonics south of the Internal mountain belt during the latest Eocene-Oligocene. A N-dipping Maghrebian subduction developed from that time up to Middle Miocene, responsible for the rifting of the internal Alboran Terrane. Docking of the extending Alboran Terrane onto the North African margin occurred during the Neogene through the closure of the Maghrebian Flysch oceanic trough, with southwestward growth of the external accretionary prism, and foredeep subsidence. Subduction zone westward roll back associated with delamination of the dense lithosphere seem to account for the Betic-Rif late orogenic evolution.

Paleomagnetic study of Sicily: consequences for the deformation of Italian and African margins over the last 100 million years

Abstract Paleomagnetic analysis of Cenozoic sediments from the Iblean platform (12 sites, 100 samples) has provided four new poles. These data are combined with those available from volcanic series to yield a new apparent polar wander path for “stable” Sicily since the Upper Cretaceous. This curve, when compared to a revised curve for Africa (using a selection of available data points), demonstrates a 15° rotation of Sicily with respect to Africa during the Plio-Pleistocene. This rotation accounts for the extensional tectonic regime that prevails in the Pelagian Sea and which is evidenced by “en relais” rifts (Malta, Pantelleria, Linosa), associated with alkaline volcanism of Pliocene and Quaternary age. Paleomagnetic directions from various areas of the Italian Peninsula also indicate anticlockwise rotations with diverse amplitudes. The chronology of these rotations is consistent with a deformation of the western Italian margin during progressive suturation from north to south of the Corsican-Sardinian block since the end of the Oligocene.

A serpentinite ridge in a collisional paleomargin setting: the Beni Malek massif, External Rif, Morocco

Abstract In the Eastern Rif Belt of northern Morocco the Beni Malek serpentinised peridotite massif is included in a low-grade metasedimentary tectonic pile, which was thrust southward onto the Atlas foreland, and which belongs to the external zones of the Maghrebides belt. We describe serpentinite clasts within calcareous deposits that are transgressive onto the ultramafic (lherzolitic) massif and detrital serpentinite layers that occur within greenschist formations. Reworking of the serpentinite clasts occurred at the beginning of the Late Jurassic-Early Cretaceous accumulation of the overlying, marginal sedimentary prism. We connect the ultramafic outcrop with a regional, NE-SW trending magnetic anomaly (the Temsamane anomaly), and conclude that the source of the serpentinite clasts extended roughly parallel to the local, NE-SW trend of the North African margin. By comparison with the submarine serpentinite ridge of the Galician margin, we suggest that the Beni Malek ultramafics were part of a similar ridge of serpentinised mantle rocks exhumed by the Tethyan rifting off northwestern Africa. The extensional emplacement of this ridge is correlated with the early stage of uplift of the Ronda-Beni Bousera lherzolite massifs under the Alboran thinned continental block, and with ocean-floor spreading on both sides(?) of the Alboran block. The seismic Nekor fault parallels the Temsamane anomaly to the southeast and is interpreted as a former extensional fault of the North African paleomargin that was reactivated as a sinistral wrench fault during the Miocene by the Iberian-African collision, which caused the obduction of the Beni Malek massif itself.

Paleomagnetism in the Tajikistan: continental shortening of European margin in the Pamirs during Indian Eurasian collision

Abstract A stable and well defined magnetization withD = 350°,I = 55.5° andα95 = 4°, carried by hematite, is isolated in the red beds of the Tajik basin. The fold test is positive (99% probability) and this magnetization predates the Late Eocene-Early Oligocene folding phase in the Tajik basin. The coordinates of the VGP are: 82°N, 323°E,dp = 4,dm = 6 with a paleolatitude of Dushanbe (38°38N, 68°51E) of35.6 ± 5°. A high blocking temperature component with weak intensity and reverse polarity has been demonstrated in the site of Ragoon, but it has not been isolated. This component predates the Long Cretaceous Normal Period and is compatible with the older stratigraphic age attributed to the formation (Valanginian); these arguments indicate that this component is likely of primary origin. The other component, of higher intensity and clearly isolated in all the sites, is a pre-folding remagnetization. The process of remagnetization is attributed to the tectonic events, such as fluids migration and slight heating during a few million years, which took place just before the first Late Eocene-Early Oligocene phase of folding An anticlockwise rotation of 26° encountered by the region of Dushanbe is in good agreement with the indentation model of collision leading to anticlockwise rotation West of the Indian wedge and clockwise rotation to the east. A comparison of the paleolatitude of Dushanbe, computed from the direction of magnetization, and the paleolatitude expected from the apparent polar wander path for Eurasia, gives an estimate of the post-collision continental shortening north of the Tajik basin. This shortening did not exceed a few hundred kilometers and was probably between 100 and 300 km. An estimate of the roles of the different classical mechanisms of crustal shortening is discussed and a paleogeographic reconstruction giving the location of the southern margin of Eurasia at 45 Ma is proposed. The paleolatitude deduced from the prefolding remagnetization of the Red Beds of the Tajik basin is then compared with the results obtained by Soviet workers in Tertiary sediments of the same area, revealing strong contradictions in interpretation. This contradiction emphasizes the problem of the rigidity of European plate.

Pliocene-Pleistocene evolution of the Tyrrhenian arc: paleomagnetic determination of uplift and rotational deformation

Abstract The Jaramillo subchron and the Brunhes/Matuyama transition have been identified in the pelagic Pleistocene marls located on the Calabrian side of the Messina Straits (Italy). Polarity of the characteristic magnetization has been deduced from an analysis of multicomponent magnetization. This magnetostratigraphic age determination combined with the paleobathymetry of the Quaternary marls implies uplift rates of fault bounded blocks of 0.7–1.0 m/1000 yr. A positive fold test on three sites from the “Trubi” Formation (4.3 Ma) shows a clockwise rotation of 36°. An antipodal test on Pliocene-Pleistocene micritic limestones (0.9–2.0 Ma) also indicates a rotation of 14° with the same trend. Both results suggest a rotation rate of about 9°/Ma. These results are in agreement with a tectonic mechanism involving simultaneous rotation and uplifts. A discussion of the possible mechanisms indicates that a compression phase oriented ENE-WSW could have played a dominant role.

Paleomagnetism and magnetostratigraphy of the traps from Western Taimyr (northern Siberia) and the Permo-Triassic crisis

Abstract The Siberian traps were emplaced near the Permo-Triassic boundary and are among the largest continental igneous provinces. A 1750 m thick section of traps from the western Taimyr (72.9°N, 84°E) has been sampled. This section is equivalent to the lower and middle part of the total Siberian traps section. Thirty-one oriented hand samples were studied and demagnetised by alternating fields and heating cycles. A stable characteristic component passes the fold test and is thought to be primary. The mean direction is D = 84 °, I = 75 ° with α 95 = 10 ° ( N = 29) and the corresponding pole is 59°N, 150°E. The bottom part (660 m) is of reversed polarity, the middle part (900 m) is normal and the uppermost level (150 m) is mixed. Correlations with reference Permo-Triassic magnetostratigraphic timescales are discussed. The most probable correlation indicates an age quite close to the Permian/Triassic boundary and a duration for the volcanism of less than 1 m.y. The data are therefore consistent with the hypothesis that this volcanism triggered the Permian/Triassic crisis.

Devonian-Carboniferous paleopoles for Africa: Consequences for Hercynian geodynamics

Abstract Paleomagnetic analysis of marine formations in the northwestern Sahara (Algeria) yields the following results: three sites in the lower Devonian Orthoceras-bearing limestones of Beni-Abbes (29.67° N, 2.15° W) indicate a mean direction of remanence: Dm = 149.6°; Im = 1.6°; α95 = 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°, α95 = 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°, α95 = 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 A2 dominating model, and define mean values for shortening rate and plate velocity comparable to the present ones (3 cm/yr).

Supra-ophiolitic formations from the Thessaloniki nappe (Greece), and associated magmatism : An intra-oceanic subduction predates the Vardar obduction

Abstract The Vardar ophiolites from Chalkidiki (Thessaloniki nappe) show autochthonous cover formations. These formations include reefal limestones of Kimmeridgian-Tithonian age, and clastic deposits with spilite, andesite and granodiorite elements, suggesting an intra-oceanic island arc setting. The earliest continental clasts are found in post-Tithonian layers. A granodioritic intrusion associated with granitoid dykes crosscutting the ophiolite corresponds to deep parts of the arc. The Chaikidiki arc, 150–140 My old, can be followed northward for at least 150 km (Guevgueli). The Vardarian obduction would have occurred during the Early Cretaceous owing to the arc-continent collision.

In-situ magnetostratigraphy: interpretation of magnetic logging in sediments

Abstract The interest of magnetic logging in different geological settings: oceanic crust, sedimentary basins and deep continental crust is discussed. In all these cases, a good estimate of both the magnetic susceptibility and the remanence of the surrounding rocks could yield invaluable information about the geological formations penetrated in boreholes. A calculation of all the components of the magnetic field produced in a borehole by horizontal magnetized layers is presented. The variations of these components are calculated along the axis and off axis through simple geological formations and particularly through a zone of transition of polarity of the earths magnetic field. Both susceptibility and remanence are involved in the interpretation of the field variations recorded when a magnetometer is passed through a borehole drilled in sediments; the separation of these variations is envisaged using magnetic field and susceptibility logs recorded simultaneously. We present an example of magnetic logging in sediments by the measurement of total field intensity. The results are compared with a model calculated from laboratory measurements of the natural remanent magnetization and magnetic susceptibility of cores.