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

Abstract

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.

Late extension of a passive margin coeval with subduction of the adjacent slab: The Western Alps and Maghrebides files

“…Conversely, and in line with Chalouan et al (2008), we adopt here the "Swiss hypothesis" (Trümpy, 1973;Wildi et al, 1977;Nold et al, 1981) that considers the Dorsale as detached from the Alpujarrides-Sebtides on the Middle Triassic pelitic beds. This is consistent with the thinning of the Alpujarride-Sebtide crust and early exhumation of the Ronda-Beni Bousera peridotites underneath (Farah et al, 2021). The pre-orogenic structure of the Alboran Domain passive margin is discussed in Michard et al (2023) and shown consistently hereafter.…”

Geotechnical Prospects and Electrical Tomography to Study Slope Instability in the Rif Alboran Sea Shoreline on The Mediterranean By-Road (Northern Morocco)

“…Subsequently, the Sebtides units consist of metamorphic rocks of kinzigites and gneisses surrounded by peridotite materials. Consequently, these deposits are overlain by the Quaternary materials deposits, affected by metamorphism tectonic (Farah et al 2021 andMarrone et al 2021). Therefore, several major contacts, characteristic of the stacking nappes in the internal zones, which crosse-section the Ghomarides and Sebtides units.…”