Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle.     

Thermal structure of the Alboran Domain in the Rif (northern Morocco) and the Western Betics (southern Spain). Constraints from Raman spectroscopy of carbonaceous material

In the Rif (northern Morocco) and the Western Betics (southern Spain), the Alboran Domain forms a complex stack of metamorphic nappes including mantle peridotites (Beni Bousera and Ronda). We present in this paper new temperature data obtained in the Alboran Domain based on Raman spectroscopy of carbonaceous material (RSCM thermometry). In the lower metamorphic nappes of the Alboran Domain (lower Sebtides–Alpujárrides) temperature ranges from > 640 °C at the base of the metapelitic sequence to 500 °C at the top. The relationships between field isotherms and nappe structure show that peak temperatures were reached during strong ductile thinning of these nappes whereas they partly postdate this main episode in the Rif. In the upper nappes of the Alboran Domain (Ghomarides–Maláguides), generally supposed to be only weakly metamorphosed, temperatures range from ～500 °C at their base down to < 330 °C at the top. This temperature gradient is consistent with progressive Cenozoic resetting of K–Ar and ⁴⁰Ar–³⁹Ar ages. These nappes were thus affected by a significant thermal metamorphism, and the available age data in the underlying Sebtides–Alpujárrides show that this metamorphism is related to the metamorphic evolution of the whole Alboran Domain during the Late Oligocene–Early Miocene. Such thermal structure and metamorphic evolution can be explained by generalized extension in the whole Alboran Domain crustal sequence. At a larger scale, the present thermal structure of the Alboran Domain is roughly spatially consistent around the Beni Bousera peridotites in the Rif, but much more affected by late brittle tectonics around the Ronda peridotites in the Western Betics. Therefore, on the basis of the observed thermal structure, the metamorphic evolution of the Alboran Domain can be interpreted as the result of the ascent of hot mantle units contemporaneous with thinning of the whole lithosphere during an Oligo‐Miocene extensional event. The resulting structure has however been dismembered by late brittle tectonics in the Western Betics.     

Structural evolution of the southern Rif Cordillera (Morocco): tectonics and synsedimentary fault processes

This paper attempts to investigate the tectonics of the southern Rif Cordillera. Hydrogeological and oil well data, together with interpretation of seismic reflection lines help to characterize the architecture of the Rharb–Mamora Basin located in the frontal region of the Gibraltar Arc. The facies map constructed from the drilling data exhibits four main types of Pliocene facies: (i) conglomerates; (ii) limestones; (iii) sandstones and sands more or less rich with shelly remains; (iv) clays. The lateral variation of deposits is accompanied by thickening, which can reach a few tens of metres. Thickening of layers and lithofacies variation indicate synsedimentary faulting processes. Two major fault zones have been identified: Kenitra–Sidi Slimane Fault Zone (K2SFZ) and Rabat–Kenitra Fault Zone (RKFZ). In the western coastal area, the geometrical configuration suggests a partition into horsts and grabens in the southern part, and a system of three geological units in the northern part. The analysis and interpretation of the gravity data reveal an important gravity anomaly, referred to as the Kenitra Gravity Anomaly. It corresponds to the Hercynian faults deduced by the seismic reflection line interpretation: K2SFZ and RKFZ. From Larache to El Jadida cities, the Kenitra area represents the hinge between the positive and negative gravity values, with a major negative anomaly in the eastern part of Kenitra. Copyright © 2004 John Wiley & Sons, Ltd.     

The Early Pliocene reflooding in the Western Mediterranean: New insights from the rias of the Internal Rif,Morocco

New field and micropalaeontological investigations have been conducted in four of the Pliocene rias of the internal zone of the Rif in Morocco, on the southern margin of the Alboran Sea. We found that marine sediments outcropping in these rias were deposited mainly during the Early Zanclean, between 5.04 and 3.8 Ma. After a transgressive episode that led to the deposition of terrestrial to marine conglomerates, dark clays deposited first at shallow palaeo-depths and then at bathyal palaeo-depths. The rias were then infilled with a shallowing upward succession comprising marine clays and sandstone and, locally, terrestrial sediments that indicate their final emersion. No Gilbert-deltas were observed in these rias. The presence of transgressive deposits at the bottom of the rias, also identified in other basins of the external zone of the Rif, and the absence of Gilbert deltas question a catastrophic reflooding after the Messinian Salinity Crisis in this area.     

Sedimentary processes and provenance of Quaternary marine formations from the Tangier Peninsula (Northern Rif, Morocco)

This work deals with new lithostratigraphic, sedimentological, petrographic and geochemical data collected from coastal Quaternary formations of the Tangier Peninsula along the Northern Atlantic and Mediterranean coasts in the southern side of the Gibraltar Strait (Morocco).The sedimentological features of the analyzed sections reflect a palaeoenvironment evolution from a submerged beach-type to a high energy littoral depositional system, namely lower shoreface to beachface environment with a regressive trend to thickening- and coarsening-upward sequences. Other successions, located nearby Larache city and in the Sidi Kankouch area, are also characterized by a positive trend to fining- and thinning-upward sequences, reflecting an evolution from lower beachface or upper shoreface to lower shoreface. It is possible that the transgressive to regressive trend inversion could be related to fluctuations of sediment input rate versus accommodation space during the progradation of a coastal palaeoenvironment.The lateral and vertical evolution of the studied marine formations is related to late Quaternary neotectonics, mainly to the last repercussions of isostatic rebound of the External and Flysch Basin Domains, during a period of relatively uniform sea level between 280,000 and 125,000 years B.P.The provenance of arenites of these Quaternary marine formations, studied by means of modal counting and geochemical analysis, seems to be linked mainly to Middle-Upper Miocene and Pliocene terrigenous successions, unconformably resting on various formations of the Neogene accretionary prism. The latter has been built by the stacking of Flysch Nappes and External Units of the Northern Rif Chain.     

Stratigraphy,age and petrography of the Beni Issef successions (External Rif; Morocco): Insights for the evolution of the Maghrebian Chain

In the Beni Issef Massif, nearly 30 km west of Chefchaouen (Morocco), the thickest post-nappe succession within the Rifian sector of the Maghrebian Chain seals the tectonic contact between the Intrarifian External Tanger and Loukkos Units, related to the Rifian External Domain. This succession is very important for the reconstruction of the deformation timing of the Rifian Maghrebids. The age of its base, in fact, is an important constraint for defining an upper boundary to the stacking of both the Intrarifian and Maghrebian Flysch Basin Units, because clasts fed by the Melloussa and Numidian Flysch Nappes are abundant in the conglomerate layers. Field and biostratigraphic analyses pointed out the presence of a Lower Beni Issef Fm, unconformable on the Intrarifian External Tanger and Loukkos Units, and an Upper Beni Issef Fm, unconformable on both the Intrarifian Units and the Lower Beni Issef Fm. The Lower Beni Issef Fm, 150 m thick, consists of lenticular conglomerates with huge blocks in a marly-clayey matrix, followed by marls and minor sandstones. It deposited in a siliciclastic platform, shows a fining upward trend and is affected by metre- to hectometre-sized, locally reversed, folds. Samples collected 45–50 m above the base of the formation resulted not older than Late Tortonian in age, but an older age for the base of the formation cannot be excluded. The Upper Beni Issef Fm, up to 550 m thick, starts with coarse conglomerates followed by medium- to coarse-grained well-bedded sandstones and by grey-blue marls and mudrocks. It indicates deposition in a channelized marine delta, with evolution towards pro-delta pelites, and shows sub-horizontal or gently dipping beds towards the east. Biostratigraphic data indicate a probable Messinian age for this formation. The composition of the arenites of both Lower Beni Issef and Upper Beni Issef Fms is quartzolithic and all samples show a notable content of monocrystalline well-rounded quartz and sedimentary lithic fragments. Detrital modes, all falling in the Quartzose Recycled and Transitional Recycled fields, suggest a provenance from recycling of sedimentary successions, easily recognizable in the Flysch Basin and External Units, mainly the Numidian Nappe sandstones. A Tortonian age of the Lower Beni Issef Fm would agree with the Late Serravallian age of the uppermost beds of the External Tanger Unit and indicate that the most probable age for the stacking of the Intrarifian Units falls in the Late Serravallian-Middle Tortonian time span. The Lower Beni Issef Fm was involved in a compressive tectonic phase testified by north-south striking folds. Later, probably during Messinian, the Upper Beni Issef Fm deposited in a younger intramontane basin, resting on both the Intrarifian Units and the Lower Beni Issef Fm. Successively, the Upper Beni Issef Fm was passively transported piggyback on top of the fold and thrust belt during later tectonic evolution of the Rifian Maghrebids. This tectonic evolution results quite similar to that recognized in the Tellian and Sicilian Maghrebids and also in the southern Apennines.     

Morpho-sedimentary evolution of the lower Moulouya (North Eastern Morocco) during middle and upper Holocene. Seismicity and neotectonic effects

Analysis of sediments and morpho-structural elements of the last 25 km of the lower Moulouya valley has shown that this river evolved during the middle and upper Holocene in a tectonic context of uplift, at the South-Eastern end of the Kebdana Mountains. This geodynamic context limited the sedimentation and inhibited the formation of a delta on the Mediterranean shore. A brief sedimentary episode allowed about 10 m of sediments to be deposited, forming a well-marked alluvial terrace. The sedimentation appeared before 3,750 years cal BC in connection with the Holocene sea transgression (“Mellahian” period) and a probably more humid period. It developed at an average rate of 0.6 mm.a–1, five kilometres from the shoreline, before stopping just after 395 years cal BC, at the same time as the post- Mellahian regression. The sediments show evidence of syn- and post-sedimentary seismic activity (seismites, diaclases, faults). They also show some traces of the “Mellahian” transgression (beds containing halite and gypsum) which indicate an average rise of 2.0 mm.a–1 since about 3,700 years cal BC, 22 km from the Moulouya mouth. The sediment composition indicates a subarid climate during the Holocene, but does not enable possible climatic variations to be detected.     

Source-area controls on the composition of beach and fluvial sands on the southern side of the Gibraltar Strait and Western Alboran Sea (Flysch Basin, Internal and External, Domains, Northern Rif Chain)

The southern side of Gibraltar and the Western Alboran Sea of the northern Rif coasts and rivers provide a natural field laboratory for sampling modern sand at different scales: small catchment basins (first order) and rivers draining mountain belts (second order). The Rifian chain represents a deformed and uplifted thrust-belt and related forelands composed of Palaeozoic nappes, metamorphic and plutonic basement, and their sedimentary Mesozoic and Cenozoic siliciclastic and carbonate cover, respectively. The present physiography of the Rif Chain is shaped by a rugged mountainous relief drained by different scale catchment basins that supply the nearby coastal and marine deep-sea environments. The analysis of the composition of modern fluvial and beach sands is useful for the interpretation of transported sediments by surface processes from the continent toward coasts and later to deep-water environments.Modern beach and fluvial sands of the southern side of Gibraltar and the Western Alboran Sea display three distinct petrologic littoral provinces, from the east to the west and from the north to the south, respectively, designated as: (i) the Tangier–Bel Younech Littoral Province with 90% of sand derived from erosion of Flysch Nappes (Flysch Basin Domain); (ii) the Bel Younech–Sebta Littoral Province with 64% of sand fed mainly by the metamorphic Units of Upper Sebtides and (iii) the Sebta–Ras Mazari Littoral Province with 74% of sand supplied from the epimetamorphic Palaeozoic Ghomaride Nappes and Alpine cover rather than Mesozoic and Cenozoic sedimentary successions of the “Dorsale Calcaire” Units. Comparison of detrital modes of fluvial and coastal marine environments highlights their dispersal pathways and drainage patterns of actualistic sand petrofacies.     

The Variscan post-collisional volcanism in Late Carboniferous–Permian sequences of Ligurian Alps, Southern Alps and Sardinia (Italy): a synthesis

In the considered wide sector of the West-Mediterranean southern Europe, the collisional phase of the Variscan orogeny during Late Carboniferous and Permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental, alluvial to lacustrine basins originated by wrench- to normal-fault systems. The first volcanic cycle (generally Late Carboniferous-Early Permian in age) is represented by early calc-alkaline andesites and rhyolites, in variable amounts, and by following large volume of rhyolites, and by dacites. Both andesites and rhyolites show K-normal and high-K calc-alkaline character. The origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle–crust interface telescoped within a thickened crust. The melting is considered as the consequence of thermal re-equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt; the ascent of liquids occurred in a (trans-)tensional regime. The second magmatic cycle is represented by alkaline magmatism, and exhibits typical anorogenic features consistent with a rifting regime. This event was no more related with the collapse of the Variscan belt, but rather to the post-Variscan global re-organization of plates that evolved during Late Triassic times to the neo-Tethyan rifting. In both cycles, important differences in timing, areal distribution and outpoured volumes arise.     

Morphological characteristics and medium-term evolution of the beaches between Ceuta and Cabo Negro (Morocco)

This work presents the results of a combined study on the beach morphology and the evolution at short- and medium-term of the littoral between Ceuta and Cabo Negro (Morocco). It is an interesting sector showing a great increase of human occupation and coastal structures. The monitoring program allowed for the reconstruction of the beach morphological behavior and the seasonal changes. The studied beaches presented reflective profiles recording little seasonality, with the most notable morphological changes being strictly related to storms. Surf Similarity and Surf Scaling parameters highlighted the existence of intermediate and reflective beach states, characterized by plunging breakers. Aerial photographs and a satellite image have been geo-referenced and elaborated with GIS tools to reconstruct the short- and medium-term evolution of the littoral and the sediment transport pathways. The littoral showed important erosion at short- and medium-term related to a negative sedimentary budget because of offshore transport. Sand accumulation was recorded close to the main ports, i.e., Marina Smir and Marina Kabila. These port structures constituted impermeable, fixed limits, which divided the studied area into littoral cells. Other free, transit limits were also observed.     

Structural evolution of the Rides Prerifaines (Morocco): structural and seismic interpretation and analogue modelling experiments

The Rides Prerifaines (RP) of Morocco constitute the leading edge of the Rif chain. They involve a Triassic–Palaeocene succession deposited on a peneplained Palaeozoic fold belt and accumulated in basins delimited by NE–SW-trending normal fault systems. A significant hiatus separates an overlying Middle Miocene–Upper Miocene foredeep sequence. The reconstruction of the complex structural evolution of the RP during the later compressive phases that affected the Rif chain since Middle Miocene time has been the aim of this paper. We integrated field structural analyses, seismic line interpretation, and analogue modelling in order to evaluate the control exerted by the Late Triassic–Jurassic normal fault systems onto the later compressive tectonics. The maximum compression direction associated with the first compressive phase is roughly NE–SW to ENE–WSW oriented. During this phase the Mesozoic basin fill was scooped-out from the graben and the main décollement level were the Triassic evaporites. Since Pliocene times the maximum compression direction was oriented roughly N–S. During this phase the RP assumed their present structural setting. The earlier normal faults delimiting the Mesozoic graben were reactivated in a strike–slip mode also involving the Palaeozoic basement. The analogue modelling experiments demonstrated that the basement reactivation promoted salt tectonics and favoured fluid circulation.     

The Plio-Pleistocene evolution of the Southern Middle Atlas Fault Zone (SMAFZ) front of Morocco

The South Middle Atlas front constitutes a northeast-trending shear zone, located north of the Neogene Missour basin and east of the Taza Guercif basin. This paper analyses the Southern Middle Atlas Fault Zone (SMAFZ) deformation since the Pliocene. The set of structures observed suggests that reverse and thrust faulting along the central part of the SMAFZ are combined with left-lateral slip along N–S striking faults of its south-western termination and right-lateral faulting along E–NE striking faults of the east–northeast termination. Thrusts and oblique thrust-related anticlines of the two lateral ramps partly accommodate north-west directed motion of the African plate. The Thrusts probably resulted from rejuvenation of Jurassic normal faults; they were active during the Upper Miocene–Pliocene and the Pleistocene. The geometries of positive inversion structures and buttressing effects are clearly dependent on the geometry and sedimentology of the original basin-controlling fault system and on the presence of a décollement level. Field mapping is integrated with Landsat imagery and a digital elevation model to investigate the morphotectonic evolution of the south-eastern range front of the Middle Atlas. Geomorphological features provide significant information on the processes that govern lateral propagation of active anticlines. Both suggest that the deformation front may have been active since Pliocene.     

New chronological and geochemical constraints on the genesis and geological evolution of Ponza and Palmarola Volcanic Islands (Tyrrhenian Sea, Italy)

A new geochronological and geochemical study of the volcanic rocks of the Ponza and Palmarola Islands, Pontine Archipelago, has been carried out. This archipelago is located along the boundary between the Italian continental shelf and the opening Tyrrhenian basin. It is a key area to study volcanism related to the opening of the Tyrrhenian Sea. Ponza is the oldest felsic magmatic manifestation in the central Tyrrhenian area. Previous studies suggested that Ponza volcanic activity began before 5 Ma. Twenty-five new K–Ar ages constrain the volcanic activity (rhyolitic hyaloclastites and dykes) to the last 4.2 Ma, with two episodes of quiescence between 3.7 and 3.2 Ma and between 2.9 and 1.0 Ma. A new volcanic episode dated at 3.2–2.9 Ma has been identified on the central and southern Ponza, with emplacement of pyroclastic units. At 1.0 Ma, a trachytic episode ended the volcanic activity. The near island of Palmarola exhibits rhyolitic hyaloclastites and domes dated between 1.6 and 1.5 Ma, indicating that the island was entirely built during the Early Pleistocene in a short span of time of ca. 120 ka. Although only 6–8 km apart, the two islands display significantly different geochemical signatures. Ponza rhyolites show major and trace element compositions representative of orogenic magmas of subduction/collision zones: high-K calc-alkaline and metaluminous rhyolites (Agpaitic Index [AI] and Alumina Saturation Index [ASI] <1), high LILE/HFSE (Th/Ta=16–21) and LREE/HFSE ratios (La/Nb>3), and Nb–Ta negative anomalies. In Palmarola, the orogenic character is also present, but much less marked than in Ponza: rhyolites have a peralkaline character (AI>1), lower LILE/HFSE (Th/Ta=11–15), low LREE/HFSE ratios (La/Nb=1–2) close to those of anorogenic lavas, and the Nb–Ta negative anomalies are almost absent. Y/Nb ratios indicate different magmatic sources, one similar to island-arc or active continental margin basalts for Ponza rhyolites, and the others probably involving an OIB type component for Palmarola rhyolites and Ponza trachytes. Palmarola volcanics represent a transitional magmatism: although a preserved collisional geochemical imprint, they show geochemical features approaching those of anorogenic lavas erupted in a within-plate context. The change of magmatism evidenced in this study can be related to the tectonic evolution of the area. Indeed, Hf, Ta and Rb contents suggest that the oldest Pliocene rhyolites of Ponza would emplace in a syn- to late-collisional setting, while the younger Pleistocene rhyolites of Palmarola would be emplaced in a post-collisional setting in which the orogenic character (Th/Ta) decreases and mantle influence (Nb/Ta) increases. Geochemical modeling strongly suggests that the Palmarola rhyolites represent the waning stages of a subduction-related magmatism. The K–Ar datings allow us to estimate precisely the transition of magmatism to last less than 1.3 Ma. The transitional magmas may be the result of the upwelling of asthenospheric mantle inducing melting of a metasomatized lithospheric mantle and the mixing between these two sources. This upwelling could occur during the extension of the Tyrrhenian basin, caused by the slab retreat and steepening, or during a process of slab break-off starting in the Pliocene.     

Stratigraphy and evolution of the trachy-rhyolitic volcanism of the Senafe area (Eastern Eritrean Plateau)

The Senafe area reveals a pile of stratoid volcanic rocks (“Senafe” ignimbrite), of considerable extent and thickness, which are the products of the first volcanic event which took place in this sector, close to the upper margin of the Afar escarpment.The Senafe ignimbrite is composed prevalently of trachyte with differing degrees of alkalinity: trachy-dacite of transitional series, and trachyte s.s. of mildly alkaline series. K/Ar radiometric measurements carried out on three samples give ages ranging between 21 and 23 Ma (Lower Miocene) and show that the Senafe ignimbrite with transitional character is an extension of the Serae rhyolite of the Central Eritrean Plateau, and may also be correlated with the Miocene Alaji rhyolite of the Central Ethiopian Plateau. In contrast, the more alkaline ignimbrite shows good correlations with the trachyte emitted by the Miocene Termaber alkaline central volcanoes of Ethiopia.It is noted that, in the course of the Miocene volcanism in Eritrea, the volumetric ratio between associated basalt and ignimbrite diminishes from west to east, i.e., approaching the Afar escarpment.The stratoid volcanic rocks are injected by thick trachytic and rhyolitic dykes. As radiometric measurements on them could not be performed, their age is unknown, but it is probably more recent than that of the injected ignimbrite, according to Merla and Minucci [Merla, G., Minucci, E., 1938. Missione geologica nel Tigrai. In: La serie dei terreni, vol. 1. Regia Accademia d’Italia, Centro Studi per l’Africa Orientale Italiana, Rome, Italy, pp. 1–362] for similar dykes and domes occurring in the Adwa-Axum area (Tigrai, Ethiopia), not far from Senafe.A section is devoted to the dyke feeders of the Eritrean and Adwa-Axum volcanism.     

Mesozoic and Cenozoic vertical movements in the Atlas system (Algeria, Morocco, Tunisia): An overview

The E-W trending Atlas System of Maghreb consists of weakly shortened, intra-continental fold belts associated with plateau areas (“Mesetas”), extending between the south-westernmost branch of the Mediterranean Alpine Belt (Rif-Tell) and the Sahara Platform. Although the Atlas system has been erected contemporaneously from Morocco to Algeria and Tunisia during the Middle Eocene to Recent, it displays a conspicuous longitudinal asymmetry, with i) Paleozoic outcrops restricted to its western part; ii) highest elevation occurring in the west, both in the Atlas System and its foreland (Anti-Atlas); iii) low elevation corridors (e.g. Hodna) and depressed foreland (Tunisian Chotts and Sahel area) in the east. We analyse the origin of these striking contrasts in relation with i) the Variscan heritage; ii) crustal vertical movements during the Mesozoic; iii) crustal shortening during the Cenozoic and finally, iv) the occurrence of a Miocene-Quaternary hot mantle anomaly in the west. The Maghreb lithosphere was affected by the Variscan orogeny, and thus thickened only in its western part. During the Late Permian-Triassic, a paleo-high formed in the west between the Central Atlantic and Alpine Tethys rift systems, giving birth to the emergent/poorly subsident West Moroccan Arch. During the late Middle Jurassic-Early Cretaceous, Morocco and western Algeria were dominantly emergent whereas rifting lasted on in eastern Algeria and Tunisia. We ascribe the uplift of the western regions to thermal doming, consistent with the Late Jurassic and Barremian gabbroic magmatism observed there. After the widespread transgression of the high stand Cenomanian-Turonian seas, the inversion of the Atlas System began during the Senonian as a consequence of the Africa-Eurasia convergence. Erosion affected three ENE-trending uplifted areas of NW Africa, which we consider as lithospheric anticlines related to the incipient Africa-Europe convergence. In contrast, in eastern Algeria and Tunisia a NW-trending rift system developed contemporaneously (Sirt rifting), normal to the general trend of the Atlas System. The general inversion and orogenesis of the Atlas System occurred during two distinct episodes, Middle-Late Eocene-Oligocene and Late Miocene-Pliocene, respectively, whereas during the intervening period, the Africa-Europe convergence was mainly accommodated in the Rif-Tell system. Inversion tectonics and crustal thickening may account for the moderate uplift of the eastern Atlas System, not for the high elevation of the western mountain ranges (Middle Atlas, High Atlas, Anti-Atlas). In line with previous authors, we ascribe part of the recent uplift of the latter regions to the occurrence of a NE-trending, high-temperature mantle anomaly, here labelled the Moroccan Hot Line (MHL), which is also marked by a strip of late Miocene-Quaternary alkaline magmatism and significant seismicity.     

Southern margin of the Variscan belt: the north-western Gondwana mobile zone (eastern Morocco and Northern Algeria)

Stratigraphic and structural correlations between the Palaeozoic massifs of eastern Morocco and northern Algeria allow three tectonic domains to be distinguished: (1) The cratonic zone, i.e. the West African platform which remained outside the Variscan chain and its peripherical margin (Moroccan Anti-Atlas and Algerian Ougarta); (2) a WSW-ENE trending zone, over 1500 km from Marrakech to Kabylia and Calabria (in their assumed Palaeozoic location). — This zone was characterized during the Late Palaeozoic by a continuous instability indicated by the development of successive turbiditic basins and a major orogeny at the Devonian-Carboniferous boundary; and (3) central and western Morocco, which corresponds to the external zones of the European Hercynides.The Marrakech-Kabylia zone separates the Variscan domain from the stable and undeformed West African craton. During Early Palaeozoic times it began as an extensive or transtensive zone. It has been deformed by the Late Devonian orogeny and by Carboniferous and Permian reactivation. The zone represents the southern limit of the Hercynian chain and is distinguished by its transcurrent regime throughout the Late Palaeozoic. Correspondence to: A. Piqué     

The Selloum Basin: new element of the Middle Liassic paleogeography in the southern Middle Atlas (Morocco)

A comparative study of the Middle Liassic series in the south western part of the Middle Atlas system leads to the definition of three new formations called Fellat 1, 2 and 3. They mainly involve gravity-flow deposits related to the breakup of the regional carbonate platform in a subsiding area, the Selloum Basin. The creation of this basin provides evidence for a rifting stage during the Carixian–Domerian interval, inducing the separation of the Selloum unit with respect to the Middle Atlasic trough and its possible connection with the High Atlasic sedimentary domain.     

岩浆熔融体性质及演化在成矿过程中的作用——以西藏玉龙斑岩铜(钼)矿床为例

玉龙斑岩铜(钼)矿床是亚洲最大的斑岩型铜矿床,包含多种矿化类型,成矿作用复杂。作为典型的斑岩型矿床,岩浆熔融体的性质在玉龙铜矿床的形成过程中起到了至关重要的作用。本文将分别从岩浆熔融体的物理和化学性质出发,解释玉龙矿床为何能成为玉龙成矿带中唯一一个超大型矿床的原因。在熔体演化方面,笔者主要通过对熔体密度、粘度的计算获得有关数据,以了解熔体运移、含矿流体分离的过程,以解释玉龙铜矿床为何能形成如此规模的矿床。     

Geological evolution and structural style of the Palaeozoic Tafilalt sub‐basin,eastern Anti‐Atlas (Morocco,North Africa)

The Tafilalt is one of a number of generally unexplored sub‐basins in the eastern Anti‐Atlas of Morocco, all of which probably underwent a similar tectono‐stratigraphic evolution during the Palaeozoic Era. Analysis of over 1000 km of 2‐D seismic reflection profiles, with the interpretation of ten regional seismic sections and five isopach and isobath maps, suggests a multi‐phase deformation history for the Palaeozoic‐aged Tafilalt sub‐basins. Extensional phases were probably initiated in the Cambrian, followed by uniform thermal subsidence up to at least the end of the Silurian. Major extension and subsidence did not begin prior to Middle/Upper Devonian times. Extensional movements on the major faults bounding the basin to the north and to the south took place in synchronisation with Upper Devonian sedimentation, which provides the thickest part of the sedimentary sequence in the basin. The onset of the compressional phase in Carboniferous times is indicated by reflectors in the Carboniferous sequence progressively onlapping onto the Upper Devonian sequence. This period of compression developed folds and faults in the Upper Palaeozoic‐aged strata, producing a structural style characteristic of thin‐skinned fold and thrust belts. The Late Palaeozoic units are detached over a regional décollement with a northward tectonic vergence. The folds have been formed by the process of fault‐propagation folding related to the thrust imbricates that ramp up‐section from the décollement. Copyright © 2007 John Wiley & Sons, Ltd.