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

2021 ◽  
Author(s):  
André Michard ◽  
Omar Saddiqi ◽  
Ahmed Chalouan ◽  
Christian Chopin ◽  
Michel Corsini ◽  
...  
Keyword(s):  
Western Alps ◽  
Early Jurassic ◽  
Normal Faults ◽  
High Grade ◽  
Ductile Shear Zone ◽  
Alpine Orogeny ◽  
Prevailing Paradigm ◽  
Rif Belt ◽  
Unconformable Contact ◽  
Order Structures

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.

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

2021 ◽  
Author(s):  
Aboubaker Farah ◽  
André Michard ◽  
Omar Saddiqi ◽  
Ahmed Chalouan ◽  
Christian Chopin ◽  
...  
Keyword(s):  
Mineral Assemblage ◽  
Field Data ◽  
Regional Scale ◽  
Normal Faults ◽  
The West ◽  
Northern Margin ◽  
Rif Belt ◽  
New Interpretation ◽  
Unconformable Contact ◽  
Tectonic Origin

<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>

scholarly journals Metasediments Covering Ophiolites in the HP Internal Belt of the Western Alps: Review of Tectono-Stratigraphic Successions and Constraints for the Alpine Evolution

Minerals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 411
Author(s):  
Paola Tartarotti ◽  
Silvana Martin ◽  
Andrea Festa ◽  
Gianni Balestro
Keyword(s):  
High Pressure ◽  
Western Alps ◽  
Metamorphic Evolution ◽  
Alpine Orogeny ◽  
Sedimentary Evolution ◽  
Hp Metamorphism ◽  
Tethys Ocean ◽  
Depositional Setting ◽  
Oceanic Rocks ◽  
Outer Band

Ophiolites of the Alpine belt derive from the closure of the Mesozoic Tethys Ocean that was interposed between the palaeo-Europe and palaeo-Adria continental plates. The Alpine orogeny has intensely reworked the oceanic rocks into metaophiolites with various metamorphic imprints. In the Western Alps, metaophiolites and continental-derived units are distributed within two paired bands: An inner band where Alpine subduction-related high-pressure (HP) metamorphism is preserved, and an outer band where blueschist to greenschist facies recrystallisation due to the decompression path prevails. The metaophiolites of the inner band are hugely important not just because they provide records of the prograde tectonic and metamorphic evolution of the Western Alps, but also because they retain the signature of the intra-oceanic tectono-sedimentary evolution. Lithostratigraphic and petrographic criteria applied to metasediments associated with HP metaophiolites reveal the occurrence of distinct tectono-stratigraphic successions including quartzites with marbles, chaotic rock units, and layered calc schists. These successions, although sliced, deformed, and superposed in complex ways during the orogenic stage, preserve remnants of their primary depositional setting constraining the pre-orogenic evolution of the Jurassic Tethys Ocean.

How does the Alpine belt end between Spain and Morocco ?

2002 ◽  
Vol 173 (1) ◽  
pp. 3-15 ◽  
Author(s):  
André Michard ◽  
Ahmed Chalouan ◽  
Hugues Feinberg ◽  
Bruno Goffé ◽  
Raymond Montigny
Keyword(s):  
Subduction Zone ◽  
Plate Boundary ◽  
Western Alps ◽  
Late Eocene ◽  
African Plate ◽  
Northern Margin ◽  
Thrust Tectonics ◽  
Rif Belt ◽  
Transitional Crust ◽  
Roll Back

Abstract 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.

Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans−North China orogen

2020 ◽  
Author(s):  
Lingchao He ◽  
Jian Zhang ◽  
Guochun Zhao ◽  
Changqing Yin ◽  
Jiahui Qian ◽  
...  
Keyword(s):  
Shear Zone ◽  
North China ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Slip Systems ◽  
High Grade ◽  
Ductile Shear Zone ◽  
Crustal Rocks ◽  
Ductile Shear ◽  
Lower Crustal

In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans−North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650−700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans−North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans−North China orogen at 1.9−1.8 Ga.

scholarly journals A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U–Pb geochronology of Permian magmatism

2020 ◽  
Vol 113 (1) ◽  
Author(s):  
Michel Ballèvre ◽  
Audrey Camonin ◽  
Paola Manzotti ◽  
Marc Poujol
Keyword(s):  
Early Stage ◽  
Western Alps ◽  
Geochemical Data ◽  
Alkaline Magmatism ◽  
Low Grade ◽  
Calc Alkaline ◽  
Alpine Orogeny ◽  
History Of ◽  
External Part ◽  
The One

Abstract The Briançonnais Domain (Western Alps) represented the thinned continental margin facing the Piemonte-Liguria Ocean, later shortened during the Alpine orogeny. In the external part of the External Briançonnais Domain (Zone Houillère), the Palaeozoic basement displays microdioritic intrusions into Carboniferous sediments and andesitic volcanics resting on top of the Carboniferous sediments. These magmatic rocks are analysed at two well-known localities (Guil volcanics and Combarine sill). Geochemical data show that the two occurrences belong to the same calc-alkaline association. LA-ICP-MS U–Pb ages have been obtained for the Guil volcanics (zircon: 291.3 ± 2.0 Ma and apatite: 287.5 ± 2.6 Ma), and the Combarine sill (zircon: 295.9 ± 2.6 Ma and apatite: 288.0 ± 4.5 Ma). These ages show that the calc-alkaline magmatism is of Early Permian age. During Alpine orogeny, a low-grade metamorphism, best recorded by lawsonite-bearing veins in the Guil andesites, took place at about 0.4 GPa, 350 °C in the External Briançonnais and Alpine metamorphism was not able to reset the U–Pb system in apatite. The Late Palaeozoic history of the Zone Houillère is identical to the one recorded in the Pinerolo Unit, located further East in the Dora-Maira Massif, and having experienced a garnet-blueschist metamorphism during the Alpine orogeny. The comparison of these two units allows for a better understanding of the link between the Palaeozoic basements, mostly subducted during the Alpine convergence, and their Mesozoic covers, generally detached at an early stage of the convergence history.

Chromian jadeite, phengite, pumpellyite, and lawsonite in a high–pressure metamorphosed gabbro from the French Alps

1980 ◽  
Vol 43 (332) ◽  
pp. 979-984 ◽  
Author(s):  
C. Mevel ◽  
J. R. Kienast
Keyword(s):  
High Pressure ◽  
Low Temperature ◽  
Western Alps ◽  
Alpine Orogeny ◽  
French Alps ◽  
Cr Substitution

SummarySmall ophiolitic bodies are enclosed in the calcschists of the Piemont zone (western Alps). They have been metamorphosed in the blueschist facies during the alpine orogeny. One of them, the Roche Noire massif, includes gabbro breccias. The magmatic mineralogy of the gabbro was plagioclase + clinopyroxene + minor chromite. There was no chemical homogenization during metamorphism because of the lack of penetrative deformation and on the site of previous chromites, chromium-rich high-pressure and low-temperature minerals (jadeite, phengite, pumpellyite, and lawsonite) were formed. The Al ⇌ Cr substitution does not affect other P- and T-dependent substitutions.

Seismic deformation in the Western Alps : new insights from high resolution seismotectonic analysis

2020 ◽  
Author(s):  
Marguerite Mathey ◽  
Christian Sue ◽  
Bertrand Potin ◽  
Colin Pagani ◽  
Thomas Bodin ◽  
...  
Keyword(s):  
Interpolation Method ◽  
Western Alps ◽  
Focal Mechanisms ◽  
P Wave ◽  
Normal Faults ◽  
Stress And Strain ◽  
Strain Fields ◽  
3D Velocity Model ◽  
Moderate Seismicity ◽  
Seismic Deformation

<p>In the Western Alpine arc, GNSS measurements indicate that the far field convergence responsible for the Oligo-Miocene continental collision is now over. However, seismicity and slow deformation are still active. Former collisional tectonic features, such as the Penninic Front, are nowadays reactivated as normal faults. Indeed, geodetic and seismotectonic studies show that the inner part of the chain is undergoing transtensional deformation, although local compression is observed in the foothills at the periphery of the arc. Due to the low to moderate seismicity of the Western Alps, the stress and strain fields remain partly elusive.</p><p>The aim of the present study is to quantitatively assess the current seismic stress and strain fields within the Western Alps, from a probabilistic standpoint. We used a new set of more than 30,000 Alpine earthquakes recorded by the dense local Sismalp seismic network since 1989. We first computed well-constrained focal mechanisms (f.m.) for more than 2,000 events with Ml ranging from 0.5 to 4.9 based on first motion (P-wave) polarity. This is the first time that such a huge focal mechanism dataset can be analyzed in the Alps. Corresponding events have been localized using a 3D velocity model (B. Potin, 2016). The global distribution of P and T axes dips confirms a vast majority of dextral transtensive focal mechanisms in the overall Alpine realm. We interpolated these results based on a Bayesian interpolation method, providing a probabilistic 2D map of the styles of seismic deformation in the Western Alps. Compression is robustly retrieved only in the Pô plain where seismicity depth differs from the shallow seismicity of the Western Alps. Extension is localized at the center of the belt. Importantly, extension is clustered instead of continuous along the belt. We then summed seismic moment tensors in homogeneous volumes of crust, to obtain seismic strain rates directly comparable to geodetic ones. Last, we inverted f.m. together in specific areas to obtain principal stress directions. A major outcome is the orientation of the extension, which is surprisingly oblique to the arc, rather than normal, as commonly thought.</p><p>These results bring new insights on the geodynamic processes driving the seismotectonic activity of the Western Alps, such as the relative contributions of crustal tectonics and deep processes.</p>

Sur la presence de galets de laves rhyolitiques dans les conglomerats du delta miocene de Voreppe (Isere)

1966 ◽  
Vol S7-VIII (2) ◽  
pp. 281-284 ◽  
Author(s):  
Jacqueline Bocquet ◽  
Robert Michel
Keyword(s):  
Volcanic Rocks ◽  
Western Alps ◽  
Magmatic Evolution ◽  
Alpine Orogeny ◽  
Lower Tertiary ◽  
Important Link

Abstract The Miocene of the Grenoble region (France) is composed almost entirely of conglomerates of deltaic origin. Among the pebbles are volcanic rocks of a rhyolitic nature, which have been studied microscopically and chemically. These rhyolitic pebbles may be the remains of a lower Tertiary volcanic episode with a center located in the ultra-Dauphinois flysch zone, now obliterated by erosion. For the western Alps, lower Tertiary volcanism would be considered an important link in the magmatic evolution connected with the Alpine orogeny.

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