Palaeomagnetic Rotations and Fault Kinematics in the Rif Arc of Morocco
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Journal of the Geological Society, London, Vol. 150, 1993, pp. 707-718, 8 figs. Printed in Northern Ireland Palaeomagnetic rotations and fault kinematics in the Rif Arc of Morocco E. S. PLATZMAN 1'2 J. P. PLATT 1 & P. OLIVIER 3 1Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK 2Formerly at Institut fur Geophysik, ETH-Honggerberg, CH-8093, Zurich, Switzerland 3 Universit~ Paul Sabatier et CNRS, Laboratoire de P#trophysique et Tectonique, 38 Rue des Trente-Six Ponts, 31400, Toulouse, France Abstract: Palaeomagnetic and structural investigations in the Rif arc of Morocco indicate that there have been large rotations associated with a pattern of outwardly directed thrusting. Rock magnetic experiments in conjunction with thermal and alternating field demagnetization demonstrate that eight sites in Jurassic and Cretaceous limestones along the Internal/External boundary have a stable remanent magnetization. This is, in most cases, rotated anticlockwise by as much as 100° from the expected Mesozoic declination but in the Tetuan area there are large clockwise rotations. Kinematic indicators from fault surfaces indicate the following. (1) In the eastern Rif there has been pre- dominantly south-directed thrusting, partly overprinted by extensional and sinistral strike-slip faults. (2) At the eastern end of the N 70°E striking Jebha Fault zone there is a pattern of dominant sinistral NE-trending and subsidiary dextral SE-trending strike-slip faults, overprinted by normal faults while at the western end deformation consists largely of south directed thrusting. (3) In the northern section of the chain, where the structural trends are dominantly N-S, thrusting is directed W to NW. It is suggested that the data are best explained by differential motion and rotation of thrust sheets during outwardly directed thrusting around the arc. The Rif mountains in Morocco form a critical part of the Africa and Iberia. In this paper results from eight successful tightly arcuate Alpine orogenic system that extends from the palaeomagnetic sites and kinematic data from 16 locations Betic Cordillera of Southern Spain across the straits of around the Internal/External zone boundary are presented Gibraltar, to the Rif and Tell mountains of North Africa as a contribution towards an understanding of the origin of (Fig. 1). The Rif lies in the centre of the arc, and an analysis the Betic-Rif arc. of the kinematics of its formation is therefore crucial in understanding the arcuate structure as a whole. The Internal zones of the Betic/Rif system were Geological and tectonic setting of the Rif deformed and metamorphosed as a result of Africa/Iberia The Rif can be divided into three main zones (Fig 1): an convergence in Late Cretaceous (?) and Early Tertiary time. Internal Zone which forms part of the Alboran Domain, an This region, which probably extends under much of the intermediate Flysch Zone and an External Zone (Durand Alboran Sea, can be distinguished by its palaeogeography Delga et al. 1962; Suter 1980a, b). The Internal Zone and deformational history as the Alboran Domain. Most includes three nappe complexes, the Sebtides, Ghomarides, workers now agree that the geometry of the Betic-Rif arc is and the Dorsale Calcaire. a result of the interactions between the Alboran domain and The Sebtides are homologous with the Alpujarrides of the continental margins of Africa and Iberia in Neogene the Betic Cordillera and like them comprise Palaeozoic to time. These interactions have been interpreted as reflecting Triassic schist and phyllite and Triassic carbonate rocks the westward motion of an independent Alboran microplate metamorphosed mainly in the greenschist facies. At the base (Andrieux et al. 1971; Leblanc & Olivier 1984; Bouillin et al. of this unit, however, is the Beni Bousera peridotite massif, 1986), or the post-collisional extensional collapse of the which is surrounded by metamorphic rocks up to granulite Alboran Domain (Garcia-Duefias & Martinez 1988; Platt & facies. This massif is closely analogous to the Ronda Vissers 1989). peridotite complex in the Betics. Several phases of Palaeomagnetic data from the external zones of the Betic synmetamorphic ductile deformation are documented in the Cordillera (Ogg et al. 1984, 1988; Osete et al. 1988; Steiner peridotite and surrounding metamorphic rocks (Kornprobst et al. 1987; Platzman 1992; Platzman & Lowrie 1992; 1974; Reuber et al. 1982). A strong NW-SE stretching Allerton et al. in press) suggest that the rocks of this region lineation associated with NW-directed shear in the have undergone fairly systematic clockwise rotations. These high-grade metamorphic rocks may be related to the data are generally compatible with the structure of the emplacement history of the whole complex (Reuber et al. region, which suggests foreland directed thrusting (Garcia- 1982). Hernandez et al. 1980; Banks & Warburton 1991; Guezou et The Ghomarides are the southern equivalents of the al. 1991) with a dextrally oblique component of motion Malaguides in the Betic Cordillera. This nappe complex is (Platzman et al. 1991). composed of a slightly metamorphosed or unmetamor- Platzman (1992) has suggested that the overall pattern of phosed sequence of Ordovician to Carboniferous sediments palaeomagnetically defined rotations in the Betic-Rif arc overlain by a mainly Triassic and early Jurassic cover. It can be interpreted in terms of the interactions of the structurally overlies the Sebtides in the east, and the extending Alboran domain with the passive margins of Dorsale Calcaire in the west. An Oligocene folding and 707 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/707/4892562/gsjgs.150.4.0707.pdf by guest on 25 September 2021 708 E.S. PLATZMAN ET AL. b IBERIAN FORELAND IBIZA "< ¢/"1 \ / eogene -39° Loja Malaga ~ DORSALE N ~ ~~ooOo o o° o° o° °o° oO~OoO~ o%O° ° °"~'~'r Gibraltar ,c~Study PE.,BET,C I -36 ° ~ !¢~ Area ~ uan 0 150km L • / °~".,;-7~<<3;-:'Yl I I i i I ~,° ,o, o Fig. 1. Simplified geological map of the Betic/Rif orogenic belt. (Base on IGME 1:50 000, 1:200 000, 1:100 0000, Wildi 1983, Azema et al. 1979) JFZ is Jebha fault zone, NFZ is Nekor Fault zone. thrusting event in the Ghomarides is fairly well constrained, post-Aquitanian folding and thrusting to the S and W (plus as late Oligocene-early Miocene conglomerates unconfor- local backthrusting) within the Dorsale and flysch nappes mably overlie a nappe pile whose youngest strata are late which may be related to the emplacement of the Internal Rif Eocene in age (Feinberg et al. 1990). Late W- or over the Flysch and External Zones (Durand Delga et al. SW-vergent folds in the Ghomarides and in the Sebtides 1962; Nold et al. 1981; Wildi et al. 1977; Wildi 1983; Morley may be related to thrusting over the Dorsale (Kornprobst 1988). The Flysch nappes, which span the Straits of 1974). Gibraltar and link the discontinuous External Zones of the The Dorsale forms the highest relief in the internal Rif, Rif and the Betic Cordillera, consist mainly of Early and is composed mainly of carbonate rocks of late Cretaceous to Early Miocene deep marine clastic deposits Triassic-early Jurassic age, but the sequence extends up to (Wildi 1983). the late Oligocene (Fallot 1937; Griffon 1966; Megard 1969; The External Zone of the Rif is a foreland Wildi 1983). In places these sediments are stratigraphically fold-and-thrust belt formed from the Mesozoic and Tertiary continuous with the upper Ghomaride nappes. However, sedimentary cover of the African margin. These rocks because they have been substantially shortened, probably by include mainly clastic sediments, pelagic carbonates, shales several tens of kilometres in an E-W direction in the and Miocene synorogenic (foreland basin) deposits (Wildi Northern Rif (Wildi et al. 1977; Nold et al. 1981), they 1983). Deformation continued through the Miocene in this generally form an imbricate thrust stack beneath the Zone (Andrieux 1971; Frizon de Lamotte 1987; Morley Ghomaride nappes. The formation of this thrust stack was 1988) and in places the rocks have been affected by low in the latest Oligocene-earliest Miocene, as late Oligocene grade metamorphism (Andrieux 1971; Leblanc 1979; Frizon strata in the thrust stack are locally unconformably overlain de Lamotte 1987). The swing in fold trends and thrust traces by the same late Oligocene-early Miocene conglomerates around the arc suggest a change in the shortening direction, that overlie the Ghomarides. The Dorsale Calcaire can be but the only published kinematic data come from the traced into the western part of the Betic Cordillera, but is southeastern part of the belt, where stretching lineations in best developed in the Rif. slaty rocks trend SW (Frizon de Lamotte 1987) and The Internal Zone was thrust as a whole onto the Flysch kinematic data from thrust surfaces indicate SW-NE motion nappes in the Early Miocene: there are several phases of (Favre et al. 1991). In the southern and western Rif, Morley Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/4/707/4892562/gsjgs.150.4.0707.pdf by guest on 25 September 2021 PALAEOMAGNETIC ROTATIONS, RIF ARC 709 I I I Jebel Mousa Expected Declinations E.Jur. L.Jur. L.Cret. 335; 49 ° 326; 35 ° 341. 39 ° N Tetuan 0 20 km .~ I I / o o o :::::2"" ."b ~ . o • ...................... .... , ~ i .OoO )R.SALE i 5,,, 3e,~. AI Hoseima / oo iiiiiiiiiil ::::::::::::::::::::::::::::::::::: o o o , o o o o o ....... 2 o ei0-¥c o 1 ..... 16LYre h NA~PE~IOo o I I "~ i~1 ~.. ~ o" o 1~,~"lP~ c. ~ I I "~.- J I 1~ o _ 35"N 1 , Fig. 2. Palaeodeclinations of ~ite means from the Northern Rif. E. Jur, early Jurassic; L. Jur, late Jurassic; L. Cret, late Cretaceous. Reference declinations calculated from the polar wander path for Africa (Westphal et al.