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°, α 95 = 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.
Abstract The present work aims at understanding the tectonic evolution of the Jebilet massif, Morocco, during the Late Paleozoic as constrained by structural, metamorphic, and geochronological studies. From Late Devonian to Early Carboniferous, bordering faults controlled the opening of the Jebilet intracontinental basin (D 0 stage) as shown by sedimentary infill. This episode was accompanied by a magmatic activity, newly dated between 358 ± 7 Ma and 336 ± 4 Ma. The first record of the Variscan event affected the Jebilet by the Late Visean‐Namurian and is represented by allochthonous superficial nappes emplaced at shallow depth in a moderately lithified sedimentary succession. D 1 also developed regional‐scale recumbent folds trending E‐W that may suggest N‐S crustal shortening not generating crustal thickening nor contributing to metamorphism. The main Variscan D 2 episode consists of a progressive evolution from bulk coaxial deformation to noncoaxial dextral transpression consistent with NW‐SE horizontal shortening, resulting in a moderate thickening. This episode was accompanied by HT‐LP metamorphism and syntectonic intrusions controlled by an inherited thermal anomaly in relation with the intracontinental rifting stage (D 0 ). Based on previous age determinations from syntectonic leucogranite and metamorphic rocks, D 2 is dated between 310 and 280 Ma. The tectono‐metamorphic evolution of the Jebilet massif can be correlated with a plate‐tectonic scenario evolving from, first a Late Devonian‐Early Carboniferous basin formation during stretching of the north‐Gondwana margin and initiation of the Paleotethys Ocean, and, second, to a Late Carboniferous‐Early Permian ocean closure (Rheic or Paleotethys Oceans depending of scenarios) that resulted in the final Variscan‐Alleghanian tectonics.
ABSTRACT The continental material of the Saih Hatat window has been affected by a Late Cretaceous, obduction‐related, HP‐LT metamorphism below the Oman ophiolite. A high‐rate exhumation process is witnessed by the Maastrichtian‐Palaeocene onlap onto the blueschist‐facies rocks. Drastic metamorphic omissions are documented between the lowest, eclogitic units and the overlying, blueschist‐facies ones. Widespread late‐metamorphic shear structures point to a top‐to‐the‐NNE detachment, in opposition to the sense of the Late Cretaceous obduction. The inversion of the shearing sense occurred under similar, low temperature conditions in both the blueschist and eclogite‐facies units. Admitting that these HP‐LT metamorphic rocks formed progressively at various depth in the subducting Arabian margin, a two‐stage extensional mechanism of exhumation is suggested: (i) early uplift of the eclogitic rocks up to the blueschists depth by ductile thinning or squeezing of a ‘blind extensional allochthon'; (ii) exhumation of the whole HP‐LT metamorphic core complex by inversion of the obduction sole‐thrust and isostatic rebound of the lower plate.
Located at an intermediate position in the External Rif nappe pile, the Temsamane units (northern Morocco) are characterized by an abnormally intense metamorphism and a penetrative ductile deformation. We present new metamorphic data showing that, in spite of their external position in the Rif, part of the Temsamane units underwent medium-pressure low-temperature (MP–LT) metamorphism (at c . 7–9 kbar and 330–430 °C), possibly during the Oligocene. Structural data show that the exhumation of these units, during Middle to Late Miocene times, was characterized by an intense approximately east–west stretching and by top-to-the-west shear senses. We tentatively propose two possible origins for the MP–LT Temsamane units: (1) an internal origin related to the subduction and the HP–LT event recorded in the Internal Rif (Alboran Domain), or (2) an external origin, implying a second subduction system within the External Rif, parallel to and almost contemporaneous with that of the Alboran Domain. This tectonometamorphic evolution of the Temsamane units is set within the context of the External Rif evolution. At a larger scale, we show that the exhumation history of the Temsamane units, which strongly resembles that documented in the core of the internal Betics, is compatible with the westward slab retreat occurring during the Middle to Late Miocene in the Betic–Rif region.
Abstract The origin of the Anti‐Atlas relief is one of the currently debated issues of Moroccan geology. To constrain the post‐Variscan evolution of the Central Anti‐Atlas, we collected nine samples from the Precambrian basement of the Bou Azzer‐El Graara inlier for zircon and apatite fission‐track thermochronology. Zircon ages cluster between 340 ± 20 and 306 ± 20 Ma, whereas apatite ages range from 171 ± 7 Ma to 133 ± 5 Ma. Zircon ages reflect the thermal effect of the Variscan orogeny (tectonic thickening of the ca . 7 km‐thick Paleozoic series), likely enhanced by fluid advection. Apatite ages record a complex Mesozoic–Cenozoic exhumation history. Track length modelling yields evidence that, (i) the Precambrian basement was still buried at ca . 5 km depth by Permian times, (ii) the Central Anti‐Atlas was subjected to (erosional) exhumation during the Triassic‐Early Cretaceous, then buried beneath ca . 1.5 km‐thick Cretaceous‐Paleogene deposits, (iii) final exhumation took place during the Neogene, contemporaneously with that of the High Atlas.
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