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 Arc of

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

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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

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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. 1986). Numbers correspond to localities as indicated in Table 1.

(1987, 1992) and Frizon de Lamotte et al. (1991) consider Internal and the External zones, and this facilitates that thrusting was towards the W-WSW, but this conclusion comparison with studies in the Penibetic rocks of the Betic is based only on considerations of thrust geometry, and not Cordillera, which also lie along this boundary (Platzman & on kinematic data. Lowrie 1992). The Internal Zone can be divided into two segments: the A generalized stratigraphic section of the external N-S-trending Northern Internal Rif, and the E-W-trending Dorsale was described by Didon et al. (1973). The entire Bokoya massif, which are separated by the N70E striking sequence of the External Dorsale has a maximum left-lateral Jebha Fault zone (Olivier 1982, 1984). The Jebha stratigraphic thickness of 1800 m. The basal unit is a thick Fault does not extend into the External Zone of the Rif, and sequence of dark grey Triassic dolostone. This passes appears to have acted as a major lateral ramp or transfer upwards into an alternating sequence of dolostone, fault during the emplacement of the Internal Zone (Frizon limestone and marl (Rhaetian), and then into massive de Lamotte et al. 1991). Another major fault, also limestone that characterizes the lower Lias. In the later left-lateral, is the Nekor fault (Andrieux 1971; Leblanc & stages of the Lias the limestone becomes progressively Olivier 1984) which cuts the external zones 40 km east of AI better bedded, contains abundant silica nodules, and is Hoseima (Figs 1 & 2). Both of these faults have been locally either brecciated or reddish and nodular. Strat- interpreted as fundamental features of the Rif serving to igraphically above the Lias is a poorly developed condensed accommodate large-scale westward motion of the Alboran sequence of Middle Jurassic filament-rich limestone, and Domain relative to Africa (Olivier 1984; Leblanc & Olivier Upper Jurassic ammonitico rosso facies red nodular 1984; Wildi 1983; Frizon de Lamotte et al. 1991). limestone and red to green radiolarian chert. A thin sequence of Cretaceous to Eocene marls is locally developed. The mid-Jurassic and younger rocks have a very Nature of this study irregular distribution and are largely absent in the more This study has been focused largely on the rocks of the internal units of the Dorsale. Dorsale Calcaire which provide a fairly continuous belt of Structurally, the Dorsale consists of a number of thrust rocks potentially suitable for palaeomagnetic work that can slices, each of which shows considerable internal deforma- be traced around the arc. The Dorsale also has the tion commonly involving fairly pervasive small-scale advantage that it lies along the boundary between the fracturing. The massive carbonate rocks of this belt do not

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readily develop cleavage or systematic sets of small-scale Isothermal remanent magnetization (IRM) experiments, folds that would allow correlation with comparable performed on representative specimens (~-20), were used to structures in the Sebtides (Kornprobst 1974) or the slaty characterize the properties of the magnetic carriers and rocks of the External Zone (Andrieux 1971; Frizon de thereby to determine the magnetic mineralogy of the Lamotte 1987). Slip-vectors have been obtained from close sediments. Acquisition experiments were conducted at room to the major faults within and on the boundaries of the temperature using an electromagnet with a maximum field Dorsale, and from the palaeomagnetic sample sites. As strength of 1.1 T. As a final step the samples were placed in discussed below, kinematic data from faults provide a field of 0.05 T along an axis normal to the previously straightforward structural markers that can be discussed in defined axis. Subsequent thermal demagnetization of the conjunction with the palaeomagnetic data. two component IRM enabled the contribution of both the high and the low coercivity component of magnetization to be evaluated. Palaeomagnetic and rockmagnetic methods In this study ten to fourteen cores were collected per site Palaeomagnetic results from six to fourteen beds spread over an area dependent on the specific lithology. Samples were collected using a Palaeomagnetic samples were collected from 15 sites portable drill except at location 2 where four oriented hand extending from Jebel Mousa on the Straits of Gibraltar to samples were taken, and oriented with a Brunton compass the town of Al Hoseima which lies on the eastern extremity and an orienting platform. The pelagic limestones sampled of the Internal Rif mountains. Most sites were in carbonate in this study were weakly magnetized and so did not rocks of the External Dorsale, but two were in late significantly affect these orienting procedures. Standard Cretaceous pelagic limestones within the Flysch Nappes. No cylindrical cores of 2.5 cm diameter and 10 cm length were previous palaeomagnetic work has been published from this later drilled and cut in the laboratory into 2.25 cm length area. samples. Generally one sample per core was demagnetized. Of the fifteen sites sampled (Table 1) only eight yielded In the laboratory the samples were carefully analyzed to a stable ChRM (characteristic component of remanent isolate the stable characteristic component of magnetization magnetization). The other sites were rejected because they (ChRM), to establish the magnetic properties of the were either too weakly magnetized (<1 × 10 -5 A m-t), had sediments and to characterize the minerals responsible for unstable magnetizations, showed significant mineral altera- the magnetization of the rock. tion as a result of heating during thermal demagnetization To isolate the primary component of magnetization procedures or were remagnetized in the present earth's standard stepwise thermal and alternating field demag- magnetic field. These unusable sites are represented by filled netization procedures were used. During the thermal circles on Fig. 2. demagnetization procedure each sample was demagnetized The eight successful sites all came from the northern with a minimum of ten steps based on a scheme designed section of the Rif between the Jebha fault zone and the after a detailed pilot demagnetization study. Measurement Jebel Mousa. Natural remanent magnetization (NRM) of the natural remanent magnetization (NRM) of the intensities of individual samples, even in these sites, were samples was done on a three axis cryogenic magnetometer generally weak ranging from 1 × 10 2 A m ~ to below the (Goree & Fuller 1976). On a set of representative samples resolution of the cryogenic magnetometer. Thermal bulk susceptibility was measured between each thermal demagnetization of the NRM often yielded both a low demagnetization step to monitor possible changes in temperature component, influenced by the present Earth's magnetic mineralogy that can occur as a result of heating magnetic field, and a higher temperature component and dewatering. (ChRM), which was generally isolated above 350-400°C Alternating field (AF) demagnetization, used sometimes where the magnetic vector usually decreased towards the in conjunction with thermal procedures, was used primarily origin of the Zijderveld plot (Fig. 3). Unblocking on samples which underwent mineralogical changes as a temperature spectra were distributed but many samples result of increased temperatures, or whose intensity of show a significant loss of intensity above 350-400 °C. magnetization was on the order of the noise level of the Isothermal remanent magnetization (IRM) studies in cryogenic magnetometer, but which contained a significant conjunction with thermal demagnetization of a two quantity of low coercivity minerals. In the later case, this component IRM indicate that the magnetization is carried procedure was employed with the hope of removing the low by a variety of minerals. Some sites such as B J02 (Fig. 4a) coercivity component of magnetization without dropping the have a high coercivity, high blocking temperature mineral total intensity of magnetization below the sensitivity level of (hematite) as the primary magnetic carrier. Other sites such the magnetometer. as J J01 (Fig. 4b) indicate that the primary carrier is a low Data obtained from the demagnetization experiments coercivity mineral that begins to saturate at applied fields of were plotted on Zijderveld vector diagrams (Zijderveld <0.1 T and has a blocking temperature of around 575 °C. 1967; Dunlop 1979). Palaeomagnetic directions could then These characteristics strongly suggest magnetite. In the be determined by using such techniques as linear regression remainder of the Jurassic sites both magnetite and hematite (Kirschvink 1980) . In rare cases where sample intensities are present. Secondary goethite is also present in these were approaching the limit of resolution of the cryogenic rocks, but does this not contribute to the ChRM which was magnetometer and vectors showed a stable direction but no isolated above the inherent blocking temperature of this straight line segment on the vector diagram stable end mineral. points were determined. Site means and statistical To constrain the age of the magnetization several criteria parameters were determined using Fischer statistics (Fischer can be used. At two sites the Jurassic carbonate rocks carry 1953). reversed as well as normal polarity ChRM, which is

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Table 1. Remanent magnetization parameters for sites used in this study (1-8) and sites not used (9-15)

loc SITE n Age Lat °N/Long °W DEC(h) INC(h) a'95 k DEC(a ) INC(~) ct95 k Av. dip°

1 BC01 4 EC 35.1 r5.2 330.9 54.5 18.8 24.8 299.9 35.3 12.8 52.2 37.0 2 BJ01 5 LJ 35.1 t5.2 252.2 68.1 17.6 19.8 255.9 32.5 17.9 19.3 37.0 3 BJ02 7 EJ 35.1 r5.1 193.5 77.0 7.2 72.0 246.1 43.5 10.9 31.7 26.0 4 EJ01 5 EJ 35.7 t5.4 67.1 35.1 13.5 33.3 222.6 43.9 13.5 33.3 50.0* 5 JJ01 6 LI 35.8 t5.4 185.3 71.4 9.0 56.5 198.4 40.8 8.6 61.5 34.0 6 TC01 9 LC 35.5 t5.5 227.0 56.5 7.9 43.3 22.8 44.6 7.5 48.1 73.0* 7 TJ01 11 EJ 35.5/5.4 154.0 75.2 23.3 7.7 99.1 40.4 19.0 11.1 28.0 8 TJ02 7 EJ 35.5/5.4 108.3 46.9 20.9 5.7 90.5 44.3 14.1 11.5 37.0

Comments 9 BL01 11 EJ 35.1/5.2 352.3 42.4 9.0 26.5 7.9 34.7 present field 10 HC01 LC 35.1/4.1 2.2 50.0 31.0 present field 11 HL01 EJ 35.2•4.0 low intensity 12 HL02 EJ 35.1/4.1 low intensity 13 HL03 EJ 35.1/4.1 low intensity 14 HL04 EJ 35.1/4.1 low intensity 15 JL01 EJ 35.1/4.6 unstable-

loc is locality corresponding to Fig. 3; site is site name; n is number of samples used; E J, early Jurassic; L J, late Jurassic; EC, early Cretaceous; LC, late Cretaceous; DEC, mean declination; INC, mean inclination; subscripts b and a indicated before and after tectonic tilt correction; dip is the average dip of the bedding; * indicates overturned bedding.

commonly regarded as evidence that the magnetization is and thrusting occurred in a limited time span between the primary. It was not possible, however, to perform a fold test Late Oligocene and Mid-Miocene. The possibility that the in the Rif because of the lack of appropriate exposures of limestones were remagnetized after deformation can be folded rocks suitable for palaeomagnetic analysis. The most eliminated on the grounds that samples magnetized in a likely ages for magnetization are those of sedimentation (i.e. post-Miocene field should exhibit approximately present day a primary age) or of deformation. In the Dorsale, folding declinations and inclinations before tilt correction. If

200 E, -Z N. -Z

Z 500~ E,H

4O0

L i i i S,H

W, +Z S, +Z

M/Mo B J02. 3AA CC M/Mo EJOI. O3B CC 1.8 l.l o = - - r~l

8.5 0.5

g.o i I I I -- T [°C] o. 0 n n ~ ~ ' ~TC°C] lBe 21W 311B 411~

Fig. 3. Typical vector diagrams in geographic coordinates and corresponding unblocking temperature spectra for two late Jurassic carbonates samples undergoing thermal demagnetization. Dots represent vector end-points projected on the horizontal plane; crosses are projections on the vertical N-S plane. Curves show that the natural remanent magnetization is composed of two major components.

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-Z z 1

300

-Z 50O

6OO

J ~z +Z M/Mm M/Mm 1.0 ( 1,0

BJ02.3AB SC -~ JJ01.07A SC

0.5

050.0 = . _ -1 T(°C)

0.0 ...... ~T(oC) loo 200 300 400 500 600 100 ~00 300 400 500 600 8. b.

Fig. 4 Isothermal remanent magnetization (IRM) acquisition curves, and vector diagrams showing the thermal demagnetization behavior of a two component IRM for samples from sites (a) B J02 which shows a predominance of a high coercivity, high blocking temperature mineral, and (b) J J01, which shows a predominance of a low coercivity mineral that is unbiocked at temperatures around 575 °C.

remagnetization occurred during the Alpine deformational located in the Beni Ider flysch nappe. The beds at this site event the rotations recorded by these samples would be less were overturned and very steeply dipping. than the total rotation. Therefore, the values determined Where bedding is steep or overturned, a potentially large should represent a minimum rotation. The measured source of error arises from the assumption that the tilting inclinations (after correction for tectonic tilt) cluster around occurred about the present line of strike. The usual the inclinations expected for Jurassic and Cretaceous rocks procedure in palaeomagnetic analysis is to remove this from stable Africa (Westphal et al. 1986), although the component of rotation, and to treat the remaining Mesozoic polar wander curve for Africa is poorly defined. component (a rotation about the normal to bedding) as a After tectonic tilt correction the declination of the declination anomaly. If, however, the component of magnetic vector at a majority of the successful sites appears rotation about the normal to bedding occurred during or to have been rotated in an anticlockwise direction with after tilting, the true tilt axis would not be the present respect to both stable Africa and stable Iberia (Fig. 2). strike-line, and the declination anomaly determined using These sites (Table 1) include the Upper Jurassic from conventional correction procedures would be incorrect. In location 5 (Jebel Mousa) and location 2 (), the the absence of structural information on the true tilt axis for Lower to Middle Jurassic from location 3 (Aoran) and sites EJ01 and TC01 (locations 4 & 6), the significance of location 4 (Suk Tleta) and the Cretaceous from location 1 these data remains uncertain. (Bab Taza). The sites are rotated by varying amounts. They range from 128 ° at location 5 (Jebel Mousa) to 41 ° at location 1 (Bab Taza) from the reference direction for stable Tectonic implications of the palaeomagnetic data Africa. The rocks sampled have experienced large and variable The sites located in the Tetuan area (locations 6, 7 & 8) rotations about vertical axes, a process that is likely to be show significant clockwise rotations. The directions obtained accompanied by significant deformation. These rotations at the two Jurassic sites (locations 7 & 8), however, have postdate the age of the rocks (Early Jurassic to Late relatively large circles of confidence, ac95 = 14.1 ° and 19.0 ° Cretaceous), and almost certainly occurred during Tertiary respectively. The Cretaceous site near Tetuan (location 6) is orogeny in the Betic-Rif arc. The palaeomagnetic data are

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difficult to reconcile with any of the models that have been with the WSW-trending Internal Zone of the Betic proposed to explain the geometric development of the arc, Cordillera, and has rotated by perhaps 70 ° in an however. Models involving a westwardly moving Alboran anticlockwise sense during the Neogene. As well as microplate (Andrieux et al. 1971) predict that the providing an attractive explanation for the predominantly N-S-trending section of the arc from Gibraltar through the anticlockwise rotations in the region, it also helps explains Northern Rif should be a zone of roughly orthogonal the opening of the west Alboran basin, the basement floor convergence. In the absence of a strike-parallel component of which has subsided more than 7km during the Miocene of motion, large rotations would not be expected. The (Watts et al. in press). This hypothesis conflicts with the extensional collapse model as outlined by Platt & Vissers palaeogeographic reconstructions for the region (Durand- (1989) also implies that the western end of the Alboran Delga 1980; Thurow & Kuhnt 1986; Ricou et al. 1986), domain, including the northern Internal Rif, has been which suggest firstly that the Dorsale of the Northern Rif is displaced westwards onto the Moroccan margin. Neither continuous with the E-W-trending Bokoya massif and the model adequately explains the fact that there are significant Dorsale of the Algerian Kabilies and of Sicily, and secondly rotations in both clockwise and anticlockwise senses. that the flysch basin developed south of the palaeomargin (1) One possibility is that the observed rotations are a represented by the Dorsale. However, large-scale anticlock- consequence of motion along a number of discrete wise block rotation of the Northern Internal Rif does not strike-slip zones that cut obliquely across the Rif. The explain the clockwise rotations in the Tetuan area. sinistral Jebha fault zone, which bounds the Northern (3) The variable pattern of rotation may result from Internal Rif on its southern margin, is an obvious candidate. independent rotations of individual thrust sheets during Small-scale rotation in a zone of distributed shear could convergence between the Internal Rif and the possibly quite explain the large anticlockwise rotations along this trend. irregular Moroccan margin. There is, however, no geological evidence for a major (4) Some or all of the deformation may have occurred dextral strike-slip fault in the Tetuan area nor for sinistral during the Late Oligocene deformation of the internal Rif, shear through the Straits of Gibraltar, which would be before its emplacement onto the Flysch nappes and the required to explain the rotations in these areas. If there has External Zone. In this case the rotations would be unrelated been any movement across the straits it has been <20 km of to those observed in the External Zone of southern Spain dextral motion, as shown by the offset of the Jebel Mousa and at the flysch site (TC01) collected west of Tetuan. from the probably correlative rock of Gibraltar. (2) Another possibility is that the entire Northern Internal Rif rotated anticlockwise as a coherent block during Structural analysis: aims and methods the Miocene extensional event that created the Alboran One of the main aims of structural analysis in a thrust belt is basin (Fig. 5). The Northern Internal Rif would then to determine the directions of relative displacement of the originally have been part of a more linear belt continuous thrust sheets that make up the system. This is particularly

0 o /. Iberian marum /. ::::...

,...... --. ,"7" ......

\ ...... basin ...... :: i ~, L ...... ~I~ xx~ " X L .;_ ...... ~00~ X ~ , x (/f : :Rif ~ Extending Alboran: :: :".1 sI -,, N :::::~,, domain ...... iiil-i 2! .}! ::::::::::::::2 x ~.-.~-~-/ African margin

Burdigalian Present

Fig. 5. Schematic diagram illustrating the anticlockwise rotation of the Northern Internal Rif block as a consequence of the extensional event that created the western Alboran basin (modified from Platt & Vissers 1989). Regional configuration during the Burdigalian is shown with coastlines around Gibraltar and Tetuan for reference. WAB is West Alboran Basin.

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important in a tightly arcuate system such as the Betic-Rif Hoseima, however, the basal shear zone of the Ghomaride arc, where the geometry of the thrust belt is not readily nappe shows an intensive pattern of conjugate low-angle explicable in terms of the relative motions of the main normal faults with ENE-WSW displacement. This contact, bounding plates (Africa and Iberia). Kinematic data are which clearly formed as a thrust (it places Devonian rocks essential to test hypotheses such as those involving a above Lias), may therefore have been reactivated by late separate Alboran microplate, or extensional collapse of the extensional motion. Extensional reactivation of thrusts has Alboran domain. Structural analysis, however, only been widely recognized in the Betic Cordillera, in the provides information on relative displacement directions, Internal Zones and along the Internal/External Zone and cannot determine rotations. Only a combination of boundary (Garcia-Duefias & Martinez 1988; Platt & Vissers palaeomagnetic and structural methods has any hope of fully 1989), and this provides evidence of a comparable constraining the patterns of motion in a deforming region. It phenomenon in the Rif. An apparently similar pattern of is noteworthy that in a region of large finite strains and faulting is developed on the basal contact of the Internal rotations, inversion of kinematic data for stress is not Dorsale at , but the shear sense could not be appropriate (Marrett & Almendinger, 1990; Twiss et al. determined at this locality. 1991). At the Rouadi drill-site in the External Dorsale dextral Relatively few data on thrusting directions have been displacement was found on N-S-trending faults that may be been published so far from around the Betic-Rif arc. Frizon related to a major lateral ramp structure, and there is minor de Lamotte et al. (1991) have summarized available data, E-directed normal faulting. At two sites, Tizi Ali at the and made the case for a dominant westerly direction of base of the Dorsale, and Oued Mashtek in the flysch, there thrusting around the system, related to the westerly motion is evidence for sinistral motion on WSW-trending steep of the Alboran microplate. The data from the Rif, however, faults. The dominant deformation at Oued Mashtek, are largely based on thrust geometry, which is not a reliable however, was south-directed displacements on gently criterion for transport direction. None of the data comes south-dipping surfaces that are interpreted as Riedel from the Internal/External Zone boundary, which is the fractures related to south-directed decollement thrusts. In most important single tectonic boundary in the entire summary, the deformation in the Bokoya consists of system. south-directed thrusting within all units (though not The slip-vector orientations presented here were necessarily simultaneously), overprinted by E-W extension measured from frictional wear grooves, solution grooves and and WSW-trending sinistral strike-slip faulting. The relative fibre lineations on fault surfaces. Sense of slip was timing of the last two events is not clear. In the absence of determined from steps on fibre-lineations and from the palaeomagnetic data the possibility that the faults and orientation of gouge fabrics and Riedel fractures associated kinematic indicators have been rotated cannot be assessed. with these faults. These data are most easily collected from marls and thin-bedded limestone: the massive limestone that dominates the stratigraphy of the Dorsale tends to produce structureless breccia in fault zones, with little useful Jebha-Chrafat kinematic data. Most of the data come from along or close The WSW-trending linear zone from Jebha to Chrafat is a to major faults whose orientation can be determined from key tectonic element in the Rif, and data were collected at regional maps. The data have been used to determine the several sites along it (Fig. 7). At Jebha itself, on the coast, sense and direction of slip on the major faults, and these there is a well developed system of both NW-trending determinations are shown in Figs 6 and 7. Each equal area dextral and WSW-trending sinistral strike-slip faults, the plot shows a mean fault plane and the lineations which are sinistral set clearly being dominant. There are also associated with that set of surfaces. Not all the data numerous small normal faults displacing down to the ENE. collected were clearly related to the major faults. Where the The evidence in this region that the Jebha zone is a major data fall clearly into discrete sets of faults with distinct slip sinistral fault seems clearcut. Further west, around Asifan vectors, the mean plane and slip vector for each set has been and Chrafat, however, the basal contacts of the Dorsale are determined and presented on the maps and equal area plots. moderately dipping thrusts with south-directed kinematic The Dorsale has been affected by significant strike-slip indicators (Fig. 7). No evidence was found for sinistral and normal faulting, which postdate the thrusts where they faulting, and it appears that the strike-slip fault either dies occur (though they may have been coeval with thrusting in out to the west or diverges from the boundary of the more external parts of the Rif). Some of the kinematic data Internal Rif. Further north, at Bab Taza, a conjugate clearly reflect these later deformation episodes. pattern of minor strike-slip faults indicates E-W shortening.

Structural results Tetuan Bokoya massif Attempts to collect kinematic data from several areas Structural data for the Bokoya massif in the southern Rif between Chrafat and Tetuan were hindered by difficulties of are presented in Fig. 6. Data were collected from most of access, poor exposure and unsuitable rock-types. Around the major tectonic boundaries in the region and these show Tetuan (Fig. 7), the Tetuan and Telegraph Road sites give evidence for a complex pattern of motions. The basal clear evidence for W- to NW-directed thrusting in the contact of the Ghomaride nappe north of Rouadi (labelled Dorsale. The Telegraph Road and Yarrhite sites also show Rouadi on Fig. 6) shows south-directed thrusting, and this is evidence for sinistral faulting on a NW trend. Their also seen within the Dorsale Units at Mrabet and at the orientation suggests that these faults may be lateral ramps or basal contact of the external Dorsale at Tefensa. At A1 transfer faults associated with the NW-directed thrusts.

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ROUADt • AJOIR

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Fig. 6, Equal area projections of structural data (lineations, mean lineation, mean slip surface) obtained from the structural sites in the Bokoya. Great circles represent mean fault surfaces. N is normal fault, T is thrust fault, filled square is mean lineation, dot is lineation. Open square indicates dextral motion for strike-slip faults. At the Ajdir and Al Hoseima sites closed circles and open squares indicate slip vectors for conjugate sets of normal faults. Diamonds indicate paleomagnetic sites. Map and cross section B-B' based on 1/50 000 Rouadi sheet (Service G6ologique du Maroc 1987) and 1/500 000 structural map of the Rif (Suter 1980b). Key, see Fig. 7.

the southern margin of the Northern Rif around Structural summary and discussion Chrafat, and W- to NW-directed thrusting and The results of the structural study are summarized in Fig. 8, shortening in the northern Rif. which shows the direction and type of slip vector determined (2) Localized extensional faulting in an ENE-WSW at each site. Thrust faults are represented by a solid arrow directed within the Internal Rif. whose head is located at the site. Strike-slip motion is (3) Sinistral strike-slip on WSW-trending faults in the indicated by a plane along which slip occurred and a sense Bokoya and by Jebha. The relative timing of events of motion. Normal faults are represented by a directional 2 & 3 is not clear. vector and the strike of the plane along which strike The evidence for south-directed thrusting around Chrafat occurred. If it is assumed that the kinematic data have not and Asifan raises questions about the nature and role of the been rotated, and can be interpreted at face value, they Jebha Fault. Leblanc & Olivier (1984) interpret this fault as suggest the following. a major strike-slip boundary active during the emplacement (1) S-directed thrusting in the Bokoya massif and along of the Northern Rif. This implies that around Chrafat its

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Fig. 7. Equal area projections of structural data (lineations, mean linea- • ",o - %. i + + tion, mean slip surface) obtained from the structural sites in the Northern Rif. Symbols and Key as in Fig. 6. Map and 0 10 km L i i cross section A-A' based on 1/500 000 , ASIFAN/CHRAFAT structural map of the Rif (Suter 1980b).

displacement should be transferred onto WSW-directed rotated back to their original orientation before their thrust surfaces along the western margin of the Rif. Neither significance can be considered. the south-directed thrusting around Chrafat nor the The questions discussed above about the Jebha fault NW-directed thrusting further north near Tetuan are zone must be reconsidered in this light. If the anticlockwise obviously consistent with this kinematic picture. These rotations indicated by the palaeomagnetic data occurred questions need to be reconsidered in the light of the after thrusting then palaeomagnetic and kinematic vectors palaeomagnetically determined rotations. must be restored to their original positions. This can be accomplished by rotating the palaeomagnetic declinations through an angle of 50-80 ° in a clockwise direction until Structural and palaeomagnetic data: an integrated they are coincident with the expected directions for stable discussion Africa. The kinematic indicators, corrected in this way, then Taking the structural and the palaeomagnetic data together, indicate a direction of thrusting that is just south of west, the effect of the rotations on the kinematic data as well as approximately parallel to the trend of the Jebha fault zone. the contribution of block rotations to the general kinematic This therefore reopens the possibility that the Jebha fault picture can be considered. At least some of the kinematic zone functioned as a major lateral ramp, transferring motion data recorded in the Rif may have been rotated from their onto WSW-directed thrusts at the front of the northern original orientations. If the kinematic data predate the Internal Rif. In this case, the most likely explanation for the palaeomagnetically determined rotations then they must be anticlockwise rotations in the Asifan/Chrafat area is that

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Fig. 8. Summary map of structural data obtained in the Moroccan Rif. Numbers identify the sites: (1) AI Hoseima, (2) Ajdir, (3) Tefensa, (4) Oued Mashtek, (5) Rouadi, (6) Mrabet, (7) Rouadi drill site, (8) Tizi Ali, (9) Jebha, (10) Asifan, (11) Asifan/Chrafat, (12) Chrafat, (13) Bab Taza, (14) Tetuan, (15) Yarrhite, (16) Telegraph road.

they are a local response to distributed sinistral shear along (1) There have been large and variable rotations in both the Jebha fault zone. senses in the Rif, the majority being anticlockwise. These The possibility that the NW-directed thrust vectors in the rotations were most probably associated with Tertiary Tetuan area have been rotated clockwise should be thrusting. considered. If this rotation is corrected, the thrusting (2) Currently observed thrust vectors vary from south in directions in that area were towards the SW, which is the Bokoya massif and along the southern margin of the consistent with the corrected vectors from around Northern Rif around Chrafat, to NW in the Tetuan area. Asifan/Chrafat. Thrusting was locally followed by extension and by sinistral It should be emphasized that there is no direct evidence strike-slip faulting in the Bokoya and at Jebha. for the timing of the rotations, and in the case of those in (3) If the rotations largely predate thrusting, the thrust the Tetuan and Jebel Mousa areas, there is no obvious vectors show a roughly radial pattern of motion around the mechanism for producing them. The most likely explanation arc. If they largely postdate thrusting, the vectors may is that all the observed rotations were produced during the originally have had a more consistent WSW trend. deformation and emplacement of the Internal Rif, and that (4) The large variations in amount and sense of rotation the variations in rotations reflect irregularities in motion suggest that the rotations were at least partly a result of the during thrusting. If this is true then the data become difficult differential motion of individual thrust sheets, in which case to analyze. The kinematic indicators would be only partially the thrust vectors should be only partly corrected for rotated and their real direction would be intermediate rotation. between the observed and 'corrected' directions. In view of the consistency of the south-directed thrust directions Supported by the Swiss National Science Foundation Grant observed at eight locations along the southern boundary of 20-5044.86 and NERC grant GR7125. We thank W. Wildi, W. the Internal Rif, it is suggested that the observed kinematic Lowrie and F. Heller, for help and advice during this study, and D. indicators formed at a relatively late stage in the thrusting Frizon de Lamotte and J.R.AIi for their constructive comments on history, and have not been greatly rotated. the manuscript.

Conclusions References The structural and palaeomagnetic data presented in this AZI~MA, J., FOUCAULT, A., FOURCADE, E., GARCIA-HERNANDEZ, M., paper allow the following conclusions to be drawn. GONZALEZ-DONoSO, J. M., LINARES, A., LINARES, D., LOPEZ-GARRIDO,

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Received 13 April 1992; revised typescript accepted 26 November 1992.

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