The Beni Bousera marbles, record of a Triassic-Early Jurassic hyperextended margin in the Alpujarrides-Sebtides units (Rif belt, Morocco)?

The timing and process of exhumation of the subcontinental peridotites of the Gibraltar Arc (Ronda, Beni Bousera) have been repeatedly discussed in the last decades. Here we report on high-grade marbles that crop out around the central and southeastern parts of the Beni Bousera antiform of northern Rif. Instead of being mere intercalations in the granulitic envelope (kinzigites) of the peridotites, as currently admitted, they are localized between the kinzigites and the gneisses of the overlying Filali Unit. The marbles occur in the form of minor, dismembered units in a ~30 to 300 m-thick Filali-Beni Bousera ductile shear zone (FBBSZ). They display silicate-rich dolomitic marbles, sandy-conglomeratic calcareous marbles and thinly bedded marble with interleaved phyllites, which demonstrates their sedimentary origin. A stratigraphic or tectonic unconformable contact onto the kinzigites can be locally observed. Pebbles or detrital grains include K-feldspar, quartz, and zircon. Prograde metamorphic minerals are forsterite, Mg-Al-spinel, geikielite, phlogopite, scapolite, diopside, and titanite, which characterize a peak HT-LP metamorphism close to 700-750°C, 4-7 kbar, comparable to that of the overlying Filali gneisses and of the late migmatitic stage of the kinzigites. Second-order structures within the FBBSZ are northwestward ductile thrusts, which determine kinzigite horses thrust over the marbles. Within the latter, NNE-trending folds are conspicuous. The mylonitic structures are crosscut by late, northward dipping normal faults. Varied correlations with comparable settings in the other West Mediterranean Alpine belts are discussed. We propose to correlate the Beni Bousera marbles with the Triassic carbonates deposited over the crustal units of the Alpujarrides-Sebtides. The Triassic protoliths may have been deposited onto the kinzigites or carried as allochthons over a detachment during the Early Jurassic in the frame of the hyper-extension of the Alboran Domain continental crust, as observed in the Adria and Europe inverted margins of the Western Alps. In either of these hypotheses, the currently prevailing paradigm of “hot” exhumation of the Rif–Betic peridotites during the Alpine orogeny would be reconsidered.

The structuration of the External Rif (Rif belt: Northern Morocco). An insights from paleo-thermal and structural analyses

<p>Belonging to the Maghrebides system, the Rif belt (Northern Morocco) suffered an important Cenozoic Alpine compressional deformation as a consequence of the closure of the Maghrebian Tethys and the westward translation and docking of the Alboran Domain onto the African margin during the Late Burdigalian. The Mesozoic North African Margin is still partially preserved in the Eastern Rif (e.g., Senhadja Jurassic-Cretaceous unit) and inverted in its Central portion (North of the Nekor Fault Zone) due to the high shortening in this area. It is in agreement with sub-surface data suggesting that the thickest crust along the chain is located in the central Rif (Izzaren Area, External Rif), and can be interpreted as a deep-rooted crustal imbrication.</p><p>This contribution aims to characterize the role of the structural inheritance of the rifted North African margin in the development and the propagation of the Rif belt by the combination of paleothermal and structural data collected along a NE-SW regional transect (between Chefchaouen and Ouezzane provinces), focusing mainly on the external zones (namely, Intrarif, Mesorif and Prerif) sampling the deformed domains originally developed along the North African paleo-passive margin. A new paleo-thermal dataset of vitrinite reflectance (Ro%), micro-Raman spectroscopy on organic matter and XRD on clayey fraction of sediments displays levels of thermal maturity between early and deep diagenetic conditions (Ro% from 0.49% to 1.15%). The highest thermal maturity values along the section are concentrated in the Lower to middle Cretaceous Loukkos Intrarifain sub-unit that is structurally squeezed between Tangier Intrarif Upper Cretaceous sub-unit and the Mesorif “Izzaren Duplex”. It attests for an important amount of shortening leading to the development of an imbricate fan of thrusts.</p><p>The geometry of the “Izzaren Duplex”, limited at surface by two first-order thrust faults, is controlled by pre-existing tectonic structures, probably inherited by the former architecture of the North African paleomargin. Moreover, the Chattian-Middle Miocene siliciclastic succession filling the Zoumi basin is in a stratigraphic continuity with the Izzaren Upper Jurassic-Upper Cretaceous substratum, sheding new light on its geodynamic meaning. This observation is supported by the homogeneity of deformation and the absence of thermal jump between the Mesozoic and Cenozoic successions, attesting for an active compressive deformation in the area between the Late Serravalian and Late Tortonian.</p><p>In conclusion, the combination of paleo-thermal and structural analysis allowed to reconstruct robust tectono-thermal model in order to propose an accurate reconstruction of the structural evolution and a new geological restoration of the Rif belt with respect to the geometry of the rifted paleo-margin.</p>

Early exhumation of the Beni Bousera granulites and peridotites at the northern margin of the westernmost Tethys (Rif belt, Morocco); new constraints from overlying marbles

<p>The West Mediterranean Alpine belts of the Rif and its northern counterpart, the Betics, are famous for the subcontinental peridotites exposed in their Internal zones (Alboran Domain), the Beni Bousera (BB) and Ronda massifs, respectively. The Beni Bousera Marbles (BBMs) here described are known for long in the northern Rif, but remained overlooked so far. Since <em>Kornprobst (1974)</em>, these marbles have been considered as simple intercalations within the kinzigites (migmatitic granulites) envelope of the BB peridotite. Based on the integration of field mapping, structural and petrology investigations and supported by SHRIMP U-Th-Pb geochronology, we present a new interpretation of these marbles and infer geodynamic implications at the local and regional scale. The field data show that the BBMs form minor, dismembered units within a ~30 to 300 m-thick mylonitic contact zone between the kinzigites and the overlying gneisses of the Filali Unit (Filali-Beni Bousera Shear Zone, FBBSZ). They display bedding structures marked by more or less siliceous marbles and some mica-rich or conglomeratic beds. The FBBSZ includes secondary ductile thrusts that determine kinzigite horses carried NW-ward over the marbles. Within the latter, NNE-trending folds are conspicuous. Brittle, northward-dipping normal faults crosscut the FBBSZ ductile structures. An unconformable contact, either of stratigraphic or tectonic origin, onto the kinzigites can be locally observed. The petrological investigation allows us to define pebbles and/or detrital grains, including K-feldspar, quartz, garnet, and zircon in these high-grade marbles. Peak mineral assemblage consists of forsterite, Mg-Al-spinel, phlogopite, and geikielite (MgTiO3) in dolomite marbles, phlogopite, scapolite, diopside, and titanite in calcite marbles. This characterizes a peak HT-LP metamorphism at ~700-750°C, 4-8 kbar. The BBMs compare with the Triassic carbonates deposited over the crustal units of the Alpujarrides-Sebtides. The detrital cores of the zircon grains from the BBMs yield two U-Th-Pb age clusters of ~270 Ma and ~340 Ma, distinct from the 290-300 Ma age of the zircon grains from the kinzigites (<em>Rossetti et al., 2020</em>), and supporting a Triassic age of the protoliths; the zircon rims yield ~21 Ma ages. The BBMs protoliths may have been deposited onto the kinzigites or carried later as extensional allochthons over a detachment in the frame of the incipient formation of the Alboran Domain continental margin, which is dated from the late Liassic-Dogger in the “Dorsale calcaire” detached units (<em>Chalouan et al., 2008</em>). Thus, the Beni Bousera mantle rocks would have been exhumed at shallow depth during the early rifting events responsible for the birth of the Maghrebian Tethys, i.e., as early as the Triassic-late Liassic.</p><p><strong>Keywords:</strong> BBMs/ FFBSZ/ HT-LP metamorphism/ SHRIMP U-Th-Pb geochronology / hyperextended margin/ mantle rocks exhumation / Gibraltar Arc</p><p><strong>References </strong>:</p><p>Please use this link for access to the cited references:  https://www.docdroid.net/hPSheTG/references-farah-et-al-2021-vegu-pdf </p><p> </p><p> </p><p> </p>

Lateral variations of pressure-temperature evolution in non-cylindrical orogens and 3-D subduction dynamics: the Betic-Rif Cordillera example

The long-term Pressure-Temperature-time-deformation (P-T-t-d) evolution of the internal zones of orogens results from complex interactions between the subducting lithosphere, the overriding plate and the intervening asthenosphere. 2-D numerical models successfully reproduce natural P-T-t-d paths, but most orogens are non-cylindrical and the situation is far more complex because of 3-D pre-orogenic inheritance and 3-D subduction dynamics. The Mediterranean orogens are intrinsically non-cylindrical because of the complex shape of the Eurasian and African margins before convergence and because subducting slabs changed configuration during retreat, getting narrower through a series of tearing events leading to strongly arcuate finite geometries. The Betic-Rif belt is archetypal of this behavior. A synthesis of the tectonometamorphic evolution of the Internal Zones, also based on recent findings by our group in the framework of the Orogen Project (Alboran domain, including the Alpujárride and Nevado-Filabride complexes) shows the relations in space and time between deformation and P-T evolution. The reinterpretation of the contact between peridotite massifs and Mesozoic sediments as an extensional detachment leads to a discussion of the geodynamic setting and timing of mantle exhumation. We then find that the age of the HP-LT metamorphism is Eocene in all units, based on new 40Ar/39Ar ages in the Alpujarride complex and a discussion of published ages in the Nevado-Filabride complex. A first-order observation is the contrast between the well-preserved Eocene HP-LT blueschists-facies rocks of the eastern Alpujárride complex and the younger HT-LP conditions reaching partial melting recorded in the Western Alpujárride. We propose a model where the large longitudinal variations in the P-T evolution are mainly due to (i) differences in the timing of subduction and exhumation, (ii) the nature of the subducting lithosphere and (iii) a major change in subduction dynamics at ~20 Ma associated with a slab tearing event. The clustering of radiometric ages around 20 Ma results from a regional exhumation episode coeval with slab tearing, westward migration of the trench, back-arc extension and thrusting of the whole orogen onto the African and Iberian margins.

Validating Structural Styles in the Flysch Basin Northern Rif (Morocco) by Means of Thermal Modeling

Vitrinite reflectance and a micro-Raman spectroscopy parameters data set have been acquired on dispersed organic matter of the Maghrebian flysch basin and the Tangiers unit across a NE-SW section in the north-western Rif belt (North Morocco). Thermal maturity shows increasing values from the hinterland to the external unit (from NE to SW). Paleo-thermal indicators show that the internal flysch basin (i.e., the Mauretanian unit) is less mature than the external one, (i.e., the Massylian unit), with Ro% and Ro eq. Raman values ranging from 0.64% to 1.02% (from early mature to late mature stages of hydrocarbon generation). 1D thermal modeling estimates the overburden now totally eroded ranging from 3.1 km to 6.0 km, and has been used as constraint to reconstruct the complete thrust wedge geometry in Miocene times. The reconstructed geometry accounts for high shortening (about 63%) due to the development of an antiformal stack in the frontal part of the wedge made up by the flysch succession. This stacking is interpreted as a consequence of the western translation of the Alboran Domain in the core of the Betic-Rif orogenic system.

Assessment of geosites in northern Morocco: diversity and richness with potential for socioeconomic development

Despite the importance of geoheritage as an interdisciplinary geo-based topic, it is poorly documented as a tool of harmonious socioeconomic development and territorial strategic planning. The promotion of this natural resource in rural areas of the southern Mediterranean may strengthen social resilience and consolidate management strategies if it is conducted according to a responsible and sustainable approach that takes account of local specificities. Here, an exhaustive, multi-criteria and qualitative assessment of the geomorphological sites of the large Alpine Rif belt was established to assess their scientific and additional values based on a widely adopted score ranking method. Later, it was established a restricted list of the most outstanding sites based on qualitative conditions. This list contains 39 sites which may be linked in three thematic geocircuits to serve as gathering points for initiatives and development projects. We anticipate that this may create an economic diversity to revive a social fabric capable of satisfying its needs and expectations, reducing the gap and inequalities between disadvantaged rural areas upstream and industrialized urban areas downstream, and inspiring a benign and balanced development.