André Michard

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.

Structure and emplacement of the Alpine-type peridotites from Beni Bousera, Rif, Morocco: A polyphase tectonic interpretation

Abstract The Beni Bousera peridotite antiform, in the southern branch of the Gibraltar orocline exhibits a lherzolitic core, containing thin pyroxenite layers surrounded by harzburgites, then by dunites and garnet-bearing dunite. In the peripheral areas, the amount of pyroxenite greatly increases by the development of garnet-pyroxenite veins. These data indicate a gradient of partial-melting increasing upward. We conclude that a reversed geothermal gradient was temporarily established in the upper part of the peridotite, just under its migmatitic roof (kinzigite aureole, then sillimanite-gneiss). Study of ductile, penetrative structures and discussion of the later “cold” structures show that the foliation of the tectonites was originally flat, with a NW-SE-trending stretching lineation. Orthopyroxene crystals are used as tectonic markers. “Isostrain zones” indicate that simple shear and finite strain increase upward. We conclude that a ductile shear zone acted between peridotite and gneiss. Strain heating along this shear zone probably has been responsible for partial melting of the upper part of the peridotite. In our model, the peridotite emplacement into the mid-continental crust began during crustal extension. Then, as comparison with the Ronda massif suggests, an intracrustal thrusting stage was followed by high-temperature and relatively low-pressure metamorphism and by intrusion of acidic dikes. The uplift of the mantle slab continued by compressive upthrusting and isostatic doming. This complex, polyphase evolution probably extended beyond the Alpine orogeny, as suggested by Kornprobst.

Neogene Tectonic Evolution of the Southwestern Alboran Basin as Inferred from Seismic Data Off Morocco

The southwestern part of the western Mediterranean Alboran Basin, including part of the Alboran ridge (Xaouen Bank), was investigated through the analysis of 28 intersecting multichannel seismic lines. The seismic stratigraphy is tied to the Amoco well El-Jebha 1. Five seismic units or subunits are described from the Quaternary to the middle (and lower?) Miocene. The acoustic basement is interpreted to be mainly Paleozoic and Triassic metamorphic rocks of the Alboran Domain nappes, and, in places, middle Miocene-Messinian calc-alkalic volcanics. In the depocenters, the thickness of the sedimentary infill (mostly clays and turbidites) exceeds 9 km. Normal faults of middle Miocene-Tortonian age are broadly parallel to the coast, and dip either seaward or landward. They were mostly inverted during pre- and post-Messinian episodes of compression, which formed a set of en echelon, north-verging faulted folds in the Alboran ridge area, in relation with sinistral movement along the offshore projection of the Jebha fault. After Pliocene subsidence, a final episode of compression reactivated the earlier folds and pushed the Alboran ridge onto the Moroccan slope. The complex structural history suggests many structural and stratigraphic potential hydrocarbon traps. A high-resolution seismic survey could lead to the definition of new exploration plays.

Geometry and structural evolution of ultra-high-pressure and high-pressure rocks from the Dora-Maira massif, Western Alps, Italy

Abstract The crystalline nappes of the Dora-Maira massif, Western Alps, essentially made of continental material from the upper crust, show petrological relics of an ultra-high-pressure (UHP) to high-pressure (HP, ‘cold’ eclogite) Eoalpine metamorphism. They also display relics of UHP-HP structures, preserved in boudins and/or within large UHP porphyroclasts, in a retrograde, greenschist-facies regional deformation fabric. The greenschist-facies overprint has the character of a shallow-dipping mylonitic foliation ( S m ), bearing a penetrative stretching lineation ( L m ) which roughly parallels the axes of coeval, isoclinal folds. Shear sense markers indicate a W-verging overthrust mechanism. The UHP and HP relic structures arc of variable nature. The coexistence of equant and inequant, either symmetric or asymmetric fabrics, indicates that the deformation at UHP-HP conditions was strongly heterogeneous and partitioned. This is also supported by the local preservation of Hercynian, magmatic fabrics. The UHP and HP deformation involved, at least locally, rotational components, although less intensive than during the latter retrograde stage. The regional structural evolution is envisaged as follows: (i) the Eoalpine subducted crust was subdivided into lenticular bodies surrounded by UHP-HP shear zones. The main part of the exhumation processes remains unconstrained due to the sparseness and late rotation of the UHP-HP structural relics; conflicting models are possible depending on the interpretation of the early sense of movement (normal vs reverse) along the faults that limit the lens-shaped units; and (ii) the late, heterogeneous, regional greenschist deformation can be attributed to the Eocene collapse of the Alpine orogenic wedge.

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.

Tectonic model for the evolution of the western Alps

Both European and Adriatic continental margins of the Alpine Piemont ocean were deformed and metamorphosed in the eclogite facies at ∼100 Ma, prior to the main stage of continental collision in the Eocene. We propose that the precollision (Eoalpine) high-pressure metamorphism in the internal crystalline nappes of the western Alps (Dora Maira, Gran Paradiso, and Monte Rosa) was a result of the subduction of the European continental margin below the lithosphere of the Piemont ocean. Concurrent oceanic subduction and tectonic erosion on the Adriatic side resulted in the Eoalpine high-pressure metamor- phism of the Sesia-Dent-Blanche system. Our model is consistent with the commonly accepted paleogeography of the western Alps, wherein the locus of the oceanic suture is given by the present structural position of the oceanic units. The precollisional setting that we propose for the western Alps is comparable to the tectonic configuration around the Gulf of Oman, where continent-continent collision has not yet occurred.

Late thermal evolution of the Oman Mountains subophiolitic windows: Apatite fission-track thermochronology

In the southern Oman Mountains, high-pressure metamorphic rocks of continental origin crop out beneath the Cretaceous Samail ophiolitic nappe. We collected 14 samples from 15 to 1200 m above sea level and at various structural levels that show apatite fission-track ages between 40 ± 7 and 55 ± 5 Ma. The mean confined track lengths vary from 13.08 to 13.70 µm. Fission-track data optimization shows that the subophiolitic units cooled slowly below 60 °C at ca. 53 Ma, then remained at low temperature until ca. 19 Ma, i.e., during the weak postorogenic subsidence of the obduction belt. The subophiolitic basement was reheated to ∼70–80 °C at ca. 7–4 Ma, before final cooling. The reheating event is correlated with the late Miocene–Pliocene (Zagros) compressional phase. Post-Oligocene heating up to 80 °C is also documented by stable isotope study of the post-nappe Eocene-Oligocene onlap. Both the Paleocene and the Pliocene-Quaternary denudation rates are close to 0.3 mm/yr, and compatible with isostatically assisted erosion processes.

Devonian extension of the Pan-African crust north of the West African Craton, and its bearing on the Variscan foreland deformation; evidence from eastern Anti-Atlas (Morocco)

Abstract The Anti-Atlas belt belongs to the northern fringe of the West African craton, moderately deformed during the Variscan orogeny south of the Meseta Block. Field-based investigations into the stratigraphy and structure of the Palaeozoic cover have been performed in the eastern part of Anti-Atlas, with emphasis on the Devonian terranes. The Pan-African basement, which crops out in the Ougnat massif, was fragmented into a mosaic of tilted blocks during a sequence of extensional faulting events that occurred from Cambrian to (mostly) Late Devonian times. The Devonian normal fault pattern indicates a multi-directional extension, with a dominant northward direction. The Variscan compression resulted in the inversion of the palaeofaults as strike-slip–reverse faults, the kinematics of which points to a NE-trending regional direction of shortening, probably Permian in age. The occurrence of the Late Devonian palaeofault array accounts for the thick-skinned style of the (eastern) Anti-Atlas belt. The Devonian paleogeography of the Anti-Atlas can be correlated with that of the Meseta, but the lack of any Late Devonian compressional event in the Anti-Atlas shows that the two domains were not mechanically coupled at that time.

New constraints on the bending of the Gibraltar Arc from palaeomagnetism of the Ronda peridotites (Betic Cordilleras, Spain)

Abstract The study of 210 cores (15 sites) from the Ronda peridotites (Sierra Bermeja and Sierra Alpujata, Alpujarride nappe complex, Betic zone), and from the granites intruding these peridotites and their country rocks shows the occurrence of two stable antipodal directions of magnetization (D = 46°, I = 47°, α95 = 6.6). The reverse polarity high-temperature component, only found in peridotites, is carried by hematite, while the normal polarity intermediate-temperature component is carried by magnetite in the peridotites, and by sulphides in the granites. Negative fold tests point to a late magnetization. The acquisition of remanence is attributed to the post-metamorphic cooling of the Alpujarrides, bracketed between 23 and 18 Ma by isotopic and stratigraphic data. Structural data and the homogeneity of the in situ mean palaeomagnetic directions preclude significant tilting of the massifs after their magnetization. The observed declination is interpreted as the result of a post-metamorphic, 46° ± 8° clockwise rotation of the Ronda massifs around a vertical axis. These results are compared with those from the Beni Bousera peridotites (southern branch of the Gibraltar Arc). In the latter massif, a c. 74° ± 11° anticlockwise rotation has been documented, and dated from the time of cooling of the peridotite unit. Therefore the opposite rotations of the Spanish and Moroccan massifs occurred rapidly during Early Miocene. A tectonic model involving extensional collapse with preferential displacement towards the Atlantic free-margin is favoured.