Crustal attenuation as a tracer for the emplacement of the Beni Bousera ultramafic massif (Betico-Rifean belt)

Abstract

The study of petrology (fi eldwork, petrography, and phase diagram modeling) and structural data of the metapelitic granulites and the southern, high-temperature exposed peridotites in the Beni Bousera massif (northern Morocco), combined with results from previous regional studies of the Alborán, suggest a new emplacement mechanism for the mantle rocks in the Betico-Rifean belt. We document two key metamorphic episodes in the granulites within a temperature window of 710-830 ± 50 °C: (1) An earlier prograde high-pressure period (from 9 ± 1.0 to 12 ± 1.0 kbar) characterized by the assemblage garnet + biotite + kyanite + K-feldspar + rutile. Pressure differences of ~3 kbar are found over a continuous crustal section of ~1.5 km of exposed granulites that indicate a significant crustal attenuation during exhumation of the ultramafi c rocks; and (2) a later postkinematic low-pressure (5 ± 0.8 kbar) symplectic assemblage of cordierite + spinel + plagioclase + sillimanite.At the scale of the entire Betico-Rifean belt, two main contacts are observed as mirror images in both sides of the Alborán Sea:(1) the long axis of the high-temperature ductile contact between granulites and peridotites occurs in the west side of the Beni Bousera and Ronda massifs, coupled with (2) the consistent high-angle, east-dipping normal fault in the east parts of the belt massifs. The integration of the petrologic results with information on the rotation of both contacts reveals ductile deformation in the lower crust related to the emplacement of the ultramafi c rocks in the Betico-Rifean belt along deep-reaching normal faults. The presence of the early hightemperature contact suggests that it was originally a shallow, west-dipping detachment fault developed in a back-arc environment of the east-dipping, retreating subduction zone (current western part of the Gibraltar arc).This scenario is in concordance with the tectonic evolution in western Italy, where anticlockwise Pleistocene rotations associated with northeast-directed thrusting in the Apennines-and coeval with the southeastward motion in the Calabria-Peloritani terrane-were triggered by retreat and rollback of the Adriatic-Ionian slab toward the southeast during the northwest-directed subduction beneath the Calabrian arc.

Factors Affecting the Thickness of Thermal Aureoles

“…Asymmetric aureoles can be explained by incremental emplacement (Figure 10) or by the presence of a fault on one side of the pluton along which the intrusion has been exhumed (Alvarez-Valero et al, 2014). Interestingly, higher temperatures at the roof than at the floor are reported for Skaeregaard mafic intrusion (Bufe et al, 2014) whereas the opposite is true for Manaslu leucogranite intrusion (Guillot et al, 1995).…”

“…Several asymmetric aureoles have been reported (Guillot et al, 1995;Alvarez-Valero et al, 2014;Bufe et al, 2014). Asymmetric aureoles can be explained by incremental emplacement (Figure 10) or by the presence of a fault on one side of the pluton along which the intrusion has been exhumed (Alvarez-Valero et al, 2014).…”

Western Mediterranean Subcontinental Mantle Emplacement by Continental Margin Obduction

“…In addition, the metamorphic and tectonic study of the Nieves sedimentary unit in the footwall of the Ronda peridotite thrust provides independent constraints to a hot emplacement of the Ronda peridotites after the Aquitanian (metamorphosed sediments of Aquitanian ages below the peridotites; Martín‐Algarra, ; Mazzoli et al, ). A fast switch from extension (between 30 and 21 Ma) to thrusting (at 21 Ma) has therefore been proposed to explain the thrusting of the Ronda peridotites onto the Iberian/Moroccan margin (Álvarez‐valero et al, ; Frasca et al, ).…”

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

“…2D). Most authors currently adopt similar views in describing a long-term exhumation of the Ronda-Beni Bousera peridotites (Afiri et al, 2011;Précigout et al, 2013;Álvarez-Valero et al, 2014;Hidas et al, 2015;Gueydan et al, 2015;Gervilla et al, 2019). However, Rossetti et al (2010Rossetti et al ( , 2020 and Melchiorre et al (2017) demonstrated that the Beni Bousera peridotites were exhumed to lower-crust depth during the waning stage of the Hercynian orogeny.…”