Geodynamics of the Gibraltar Arc and the Alboran Sea region

Located at the Westernmost tip of the Mediterranean sea, the Gibraltar Arc is a very complex zone. The Betics in Spain and the Rif belt in Morocco surround the Alboran sea characterized by a thinned continental crust. The geodynamic evolution of this region results from the convergence of African and Iberian margins since the Late Cretaceous. It is controlled both by plate convergence and mantle dynamics, which significantly impact on morphology, sedimentary environments, tectonics, metamorphism and magmatism. We present here the contents of the special issue on the Gibraltar Arc and nearby regions, following the workshop organized at the University Abdelmalek Essaadi of Tetouan in Morocco from 27 to 28 October, 2011. The goal of this international workshop was to have an overview of the actual advance in research concerning the Rif and Betics chains, the Alboran basin, and their influence on the Iberian and African forelands.     

Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula

Travel times of 11,612 Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocity and anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions. Pn phases are routinely identified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imaging of variations of Pn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of 8.0 km/s. The peninsular area covered by the Iberian massif is characterized by high Pn velocity, as expected in tectonically stable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsula have undergone a great number of recent tectonic events and are characterized by a pronouncedly low Pn velocity, as is common in areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal root might be underlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sharp variation in the uppermost mantle velocities that coincides with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente whereas high velocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity found is almost parallel to the Africa–Eurasia plate convergence vector (northwest–southeast) whereas to the north, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east–west in its central part and north–south in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that the Neogene extension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastest Pn velocity and major Hercynian tectonic trends cannot be directly established.     

Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field

We present a new set of brittle microtectonic measurements carried out in the Pliocene and Quaternary rocks outcropping in several key sectors of the western Betic and Rif orogen, the so-called Gibraltar orogenic arc. This data set, along with available earthquake focal mechanisms and borehole breakouts, allowed us to compile the Pliocene and Quaternary stress map of this area. This map provides new constraints for tectonic models and the present-day tectonic activity of the proposed active eastward subduction of oceanic lithosphere beneath the Gibraltar Arc and roll-back. The horizontal maximum compressive stress (S_Hmax) is NW-SE in the Betic Orogen and N-S/NNW-SSE in the southern Rif Cordillera. There is a significant consistency between S_Hmax and the displacement field deduced from GPS measurements with respect to the African plate: both appear to reflect the NW-SE convergence between the African and the European plates that is perturbed in the Rif. We propose that part of the eastern Rif behaves as a quasi-rigid block welded to the stable African plate. This block is bounded by important faults that localized most of the deformation disturbing the stress and surface displacement field. Pliocene to Quaternary N-S to NW-SE Africa-Europe plate convergence seem to be associated to the reorganization of the remnant Early Miocene subduction system in a continental–continental collision framework. Three-dimensional reconstruction of available seismic tomography plotted against the intermediate seismicity shows that only part of the old subduction system, whose orientation ranges from N20°E to N100°E, remains active: the portion ranging from N30°E to N40°E, orthogonal to the regional convergence.     

Geodetic constraints on active tectonics of the Western Mediterranean: Implications for the kinematics and dynamics of the Nubia-Eurasia plate boundary zone

We present GPS observations in Morocco and adjacent areas of Spain from 15 continuous (CGPS) and 31 survey-mode (SGPS) sites extending from the stable part of the Nubian plate to central Spain. We determine a robust velocity field for the W Mediterranean that we use to constrain models for the Iberia-Nubia plate boundary. South of the High Atlas Mountain system, GPS motions are consistent with Nubia plate motions from prior geodetic studies. We constrain shortening in the Atlas system to <1.5 mm/yr, 95% confidence level. North of the Atlas Mountains, the GPS velocities indicate Nubia motion with respect to Eurasia, but also a component of motion normal to the direction of Nubia-Eurasia motion, consisting of southward translation of the Rif Mountains in N Morocco at rates exceeding 5 mm/yr. This southward motion appears to be directly related to Miocene opening of the Alboran Sea. The Betic Mountain system north of the Alboran Sea is characterized by WNW motion with respect to Eurasia at ～1–2 mm/yr, paralleling Nubia-Eurasia relative motion. In addition, sites located in the Betics north of the southerly moving Rif Mountains also indicate a component of southerly motion with respect to Eurasia. We interpret this as indicating that deformation associated with Nubia-Eurasia plate motion extends into the southern Betics, but also that the Betic system may be affected by the same processes that are causing southward motion of the Rif Mountains south of the Alboran Sea. Kinematic modeling indicates that plate boundary geometries that include a boundary through the Straits of Gibraltar are most compatible with the component of motion in the direction of relative plate motion, but that two additional blocks (Alboran-Rif block, Betic Mountain block), independent of both Nubia and Eurasia are needed to account for the motions of the Rif and Betic Mountains normal to the direction of relative plate motion. We speculate that the southward motions of the Alboran-Rif and Betic blocks may be related to mantle flow, possibly induced by southward rollback of the subducted Nubian plate beneath the Alboran Sea and Rif Mountains.     

Main features of the deep structure by local earthquake tomography and active tectonics: case of Rif Mountain (morocco) and Betic Cordillera (Spain)

Within the Spain and Moroccan networks, a large volume of seismic data has been collected and used for investigating the lithosphere in the Betic–Rif Cordillera. The present study has two main goals: (1) Use the most actual seismological data from recent earthquakes in the Betic–Rif arc for investigating the lithosphere through the application of seismic local tomography techniques. (2) Define the possible structural blocks and explain the GPS velocities perturbation in this region. The resolution tests results indicate that the calculated images gave a close true structure for the studied regions from 5- to 60-km depth. The resulting tomographic image shows that the presence of two upper crust body (velocity 6.5 km/s) at 3- to 13-km depth between Iberian Betic and Moroccan Rif in the western and in the middle of Alboran Sea also shows the low velocity favoring the presence of melt in the base of these two bodies. The crustal bodies forms tectonic blocks in the Central Rif and in the Central Betic Cordillera.     

基于P波三重震相研究青藏高原上地幔速度结构及其动力学意义

青藏高原因其复杂的结构和演化历史,一直都是研究大陆碰撞、构造运动及其动力学的热点区域。本文采用三重震相波形拟合技术,基于中国地震观测台网和大型流动台阵记录到的某地震P波垂向记录,获得了包括拉萨、南羌塘和松潘甘孜地块在内的青藏高原上地幔P波速度结构。结果表明:①拉萨和南羌塘地块下方地幔过渡带存在高速异常,推测是俯冲的印度板片滞留体,过渡带底部的板片残余温度较低,使得660-km相变滞后约3～8km。而松潘甘孜地块下方过渡带同样存在高速异常,可能是欧亚岩石圈发生拆沉进入地幔过渡带所致。这说明印度板块俯冲作用的影响已经到达地幔过渡带,其俯冲前缘位于班公怒江缝合带附近。②从拉萨、南羌塘到松潘甘孜地块,200km之上的地幔岩石圈高速盖层速度由南向北逐渐减小,松潘甘孜地块则出现盖层缺失。推测受小规模地幔对流或者热不稳定性的影响,在南羌塘和松潘甘孜地块,增厚的欧亚岩石圈发生拆沉作用,岩石圈被减薄和弱化,造成羌塘地块上地幔低速和松潘甘孜地块上地幔高速盖层缺失。拆沉的冷的欧亚岩石圈可能部分停留在410-km上方,使得410-km抬升约10km,部分沉入地幔过渡带,表现为松潘甘孜地块地幔过渡带中存在高速异常。低温造成660-km下沉约8km,导致地幔过渡带增厚。      

Local earthquakes seismic tomography in the Betic Cordillera (southern Spain)

A crustal tomographic image, from the surface down to 35 km depth beneath the Betic Cordillera (southern Spain), is obtained using data on local earthquakes recorded at stations from the National and Andalusian Seismic Networks. The velocity structure and the hypocentre locations are derived from the inversion of P first arrival times, using an iterative simultaneous inversion method. The reliability of the results is assessed using different control parameters. The inverted velocity field in the uppermost layers shows a significant lateral variability which reflects most of the large-scale geological features of the Betic Cordillera. Well determined local surface anomalies allow to constrain the location and geometry of the most prominent Neogene sedimentary basins. The upper crust is well resolved throughout the whole region, and is characterized by relatively high velocities in the Internal Betics and in the South Iberian Massif and lower velocities within the External Betics. A relatively well constrained event cluster displays a NNE–SSW trend, and outlines the contact zone between the Internal and the External domains. The middle and lower crustal levels show reliable results beneath the central part of the Betic Cordillera. High averaged velocities are obtained within the South Iberian and the Alboran domains, in contrast to a relatively low velocity anomaly which characterizes the boundary between them. These findings support the hypothesis of the lack of well differentiated crustal levels below the contact zone, while crustal layering is better defined beneath the Alboran and the Iberian domains.     

Recent Tectonic Structures in a Transect of the Central Betic Cordillera

—The Betic Cordillera has undergone recent Alpine deformations related to the Eurasian-African plate interaction boundary. Most of the present-day relief has been built up since Tortonian times, and is related to the development of folds and faults that are overprinted on older deformations, and some of the faults may be considered as out-of-sequence. The combination of geophysical and geological data makes it possible to determine the main features of the recent tectonic structures, or those recently active, in its central transect. The main fault is a crustal detachment that separates a footwall constituted by the Iberian Massif and a hanging wall formed by the rocks of the Betic Cordillera. While the footwall is practically undeformed, the hanging wall has been folded and faulted. The folds are mainly E-W to NE-SW and have larger sizes and higher related relieves towards the South. The reverse faults are mainly concentrated in the northern mountain front. However, normal faults affect the southern part of the Cordillera and are associated with the development of large asymmetrical basins such as the Granada Depression. In this setting, the slip along the crustal detachment is variable and should increase southwards. The model of the recent tectonics in the central transect of the Cordillera is compatible with the presence of an active subduction in the Alboran Sea, and contrasts notably with the setting of the eastern Betic Cordillera, mainly deformed by transcurrent faults.     

Neogene tectonic evolution of the Gibraltar Arc: New paleomagnetic constrains from the Betic chain

New paleomagnetic results from Neogene sedimentary sequences from the Betic chain (Spain) are here presented. Sedimentary basins located in different areas were selected in order to obtain paleomagnetic data from structural domains that experienced different tectonic evolution during the Neogene. Whereas no rotations have been evidenced in the Late Tortonian sediments in the Guadalquivir foreland basin, clockwise vertical axis rotations have been measured in sedimentary basins located in the central part of the Betics: the Aquitanian to Messinian sediments in the Alcalà la Real basin and the Tortonian and Messinian sediments in the Granada basin. Moreover, counterclockwise vertical axis rotations, associated to left lateral strike-slip faults have been locally measured from sedimetary basins in the eastern Betics: the Middle Miocene to Lower Pliocene sites from the Lorca and Vera basins and, locally, the Tortonian units of the Huercal-Overa basin. Our results show that, conversely from what was believed up to now, paleomagnetic rotations continued in the Betics after Late Miocene, enhancing the role of vertical axis rotations in the recent tectonic evolution of the Gibraltar Arc.     

Diverse protolith ages for the Mindoro and Romblon Metamorphics (Philippines): Evidence from single zircon U–Pb dating

Within the north‐eastern part of the Palawan Continental Terrane, which forms the south‐western part of the Philippine archipelago, several metamorphic complexes are exposed that are considered to be rifted parts of the Asian margin in South‐East China. The protolith age(s) and correlations of these complexes are contentious. The largest metamorphic complex of the Palawan Continental Terrane comprises the Mindoro Metamorphics. The north‐eastern part of this metamorphic complex has recently been found to be composed of protoliths of Late Carboniferous to Late Permian protolith age. However, meta‐sediments exposed at the westernmost tip and close to the southern boundary of the exposure of the Mindoro Metamorphics contain detrital zircons and with U–Pb ages, determined by LA–ICP–MS, in the range 22–56 Ma. In addition, zircons as young as 112 Ma were found in a sample of the Romblon Metamorphics in Tablas. As the youngest detrital zircons provide an upper age limit for the time of deposition in meta‐sediments, these results suggest that the Mindoro and Romblon Metamorphics comprise protoliths of variable age: Late Carboniferous to Late Permian in NE Mindoro; Eocene or later in NW Mindoro; Miocene at the southern margin of the Mindoro metamorphics; and Cretaceous or later on Tablas. The presence of non‐metamorphic sediments of Late Eocene to Early Oligocene age in Mindoro (Lasala Formation), which are older than the youngest metasediments, suggests that metamorphism of the young meta‐sediments of Mindoro is the result of the collision of the Palawan Continental terrane with the Philippine Mobile Belt in Late Miocene. Similarities of the age spectra of zircons from the Eocene to Miocene metamorphics with the Eocene to Early Miocene Lasala Formation suggest that the protoliths of the young metamorphics may be equivalents of the Lasala Formation or were recycled from the Lasala Formation.     

Late Miocene to present-day exhumation and uplift of the Internal Zone of the Rif chain: Insights from low temperature thermochronometry and basin analysis

Located on the margin of the west Alboran basin, the Gibraltar Arc (Betic-Rif mountain belt) displays post-Pliocene vertical movements evidenced by uplifted marine sedimentary basins and marine terraces. Quantification of vertical movements is an important clue to understand the origin of present-day relief generation in the Betic-Rif mountain chain together with the causes of the Messinian Salinity Crisis. In this paper, we present the results of a pluridisciplinary study combining an analysis of low temperature thermochronology and Pliocene basins evolution to constrain the exhumation history and surface uplift of internals units of the Rif belt (Northern Morocco). The mean (U-Th)/He apatite ages obtained from 11 samples are comprised between 14.1 and 17.8 Ma and display a wide dispersion, which could be explained by a great variability of apatite chemistries in the analyzed samples. No correlations between altitude and age have been found along altitudinal profile suggesting a rapid exhumation during this period. Thermal modeling using our (U-Th)/He apatite ages and geochronological data previously obtained in the same area (⁴⁰Ar/³⁹Ar and K/Ar data on biotite, zircon and apatite fission track) allow us to propose a cooling history. The rocks suffered a rapid cooling at 60–100 °C/Ma between 22.5 and 19 Ma, then cooled to temperatures around 40 °C between 19 and 18 Ma. They were re-heated at around 110 °C between 18 and 15 Ma then rapidly cooled and exhumed to reach the surface temperature at around 13 Ma. The re-heating could be related to a renewal in thrusting and burying of the inner zones. Between 15 and 13 Ma the cooling resumed at a rate of 50 °C/Ma indicating an exhumation rate of 0.8 mm/y considering an average 40 °C/km geothermal gradient. This exhumation may be linked to the extension in the Alboran Sea. Otherwise biostratigraphic and sedimentological analysis of Pliocene basins of the internal Rif provided informations on the more recent events and vertical movements. Pliocene deposits of the Rifian coast represent the passive infilling of palaeo-rias between 5.33 and 3.8 Ma. The whole coastal area was uplifted at slow average rates (0.01–0.03 mm/y) in relation with a northeastward tilting of 0.2–0.3° since the Lower-Pliocene. A late Pliocene to present extensional tectonics associated to uplift has been identified all along the coastal ranges of the Internal Zone of the Rif chain. This extension was coeval with the major late Pliocene to Pleistocene extensional episode of the Alboran Sea and appears to be still active nowadays. No significant late Messinian uplift was evidenced, thus calling into question the geodynamic models relating the closure of the marine gateways and the MSC to slab roll back.     

Crustal structure beneath Spain from deep seismic sounding experiments

Major tectonic units of Spain have been investigated by deep seismic sounding experiments since 1974 to determine crustal structures and to delineate their differences. These areas are the central part of the Hercynian Meseta, and the Alpine chains: the Betic Cordillera in the south, including the Balearic promontory and the Alboran Sea, and the Pyrenees in the north.The main features of the crust and the upper mantle along a NNE-SSW cross-section from the Pyrenees to the Alboran Sea are described.The crust under the Meseta is typical of Hercynian areas found elsewhere in Europe, with an average thickness of 31 km, whereas the two Alpine regions are characterized by very large lateral inhomogeneities, such as rapid thickening of the crust to 50 and 40 km under the Pyrenees and the Betics, respectively. The deep-reaching E-W-trending North Pyrenean fault has a throw of 10–15 km at the base of the crust. A P_n velocity of 8.1 km s^?1 is found under the entire Iberian Peninsula.In the Alboran Sea, strongly varying thicknesses of sediments, shallow variable depths to the Moho (～ 13 km under the Alboran ridge), and strong variations of P_n velocity between 7.5 and 8.2 km s^?1 have been found.     

Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: An illite crystallinity and Mite bo, lattice spacing study

Abstract Illite crystallinity (IC) and illite b, lattice spacing were measured across the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan. For the IC survey, 103 samples of argillaceous rocks were analyzed from the central area and the western area of the belt. Values of IC (Kubler Index) vary between 0.28 and 0.71 Δ°2θ and indicate diagenetic and anchizone metamorphism respectively. The IC distribution reveals two contrasting patterns of thermal maturity. The Hanazono Formation, exposed in the northern area of the belt, generally dips north, but IC values increase systematically from 0.28 Δ°2θ in the north to 0.54 Δ°2θ in the south and indicate an inverted thermal structure. Values in other formations vary widely in the southern area of the belt ranging between 0.45 and 0.71 Δ°2θ, but the values do not show any systematic change from north to south and on average remain almost constant. Illite b_o, lattice spacing values for 56 samples vary between 9.006 and 9.041 Å corresponding to intermediate pressure conditions of the metamorphic facies. These values, combined with paleotemperatures estimated from IC, indicate paleogeothermal gradients of 22 and 31°C/km for the northern and southern areas of the belt, respectively. The inverted thermal structure of the Hanazono Formation, together with a lower paleogeothermal gradient, possibly is a result of the subduction of a relatively cold oceanic plate during the Late Cretaceous. The higher geothermal gradient could be a product of late thermal overprinting caused by the later subduction of a comparatively younger and hotter oceanic plate during the Eocene.     

Differential exhumation of tectonic units and ultrahigh-pressure metamorphic rocks in the Dabie Mountains, China

A model involving buoyancy, wedging and thermal doming is postulated to explain the differential exhumation of ultrahigh-pressure (UHP) metamorphic rocks in the Dabie Mountains, China, with an emphasis on the exhumation of the UHP rocks from the base of the crust to the upper crust by opposite wedging of the North China Block (NCB). The Yangtze Block was subducted northward under the NCB and Northern Dabie microblock, forming UHP metamorphic rocks in the Triassic (240–220 Ma). After delamination of the subduction wedge, the UHP rocks were exhumed rapidly to the base of the crust by buoyancy (220–200 Ma). Subsequently, when the left-lateral Tan–Lu transform fault began to be activated, continuous north–south compression and uplifting of the orogen forced the NCB to be subducted southward under the Dabie Orogen (`opposite subduction'). Opposite subduction and wedging of the North China continental crust is responsible for the rapid exhumation of the UHP and South Dabie Block units during the Early Jurassic, at ca 200–180 Ma at a rate of ∼ 3.0 mm/year. The UHP eclogite suffered retrograde metamorphism to greenschist facies. Rapid exhumation of the North Dabie Block (NDB) occurred during 135–120 Ma because of thermal doming and granitoid formation during extension of continental margin of the Eurasia. Amphibolite facies rocks from NDB suffered retrograde metamorphism to greenschist facies. Different unit(s) and terrane(s) were welded together by granites and the wedging ceased. Since 120–110 Ma, slow uplift of the entire Dabie terrane is caused by gravitational equilibrium.     

Source mechanism studies of earthquakes in the Ibero-Maghrebian region and their tectonic implications

Seismicity of the Ibero-Maghrebian region includes the occurrence of shallow, intermediate depth, and very deep earthquakes. This is a very rare occurrence for a region not associated to an active subduction zone. Detailed studies of the source mechanism of these three types of earthquakes have been made possible through the collaboration with Prof. Madariaga. They give important information about the complex tectonic of the region. Shallow earthquakes at the west and east ends of the region have predominant reverse faulting with NW-SE trending horizontal pressure axes. The center part is the most tectonically complex. At the Strait of Gibraltar, there is a change on focal mechanisms from reverse faulting to strike-slip motion in northern Morocco, conserving the horizontal compression on NW-SE direction. In the Alboran Sea, mechanisms are of normal faulting with E-W trending horizontal tension axes, and in south Spain, mechanisms are of mixed solutions. The intermediate depth earthquakes (40–130 km) are located at both sides of the Strait of Gibraltar, at the western part distributed in E-W direction. The most important concentration, however, is located at the east of Gibraltar in a N-S trending thin vertical body and has different mechanisms. The very deep earthquakes (650 km) are concentrated at a small volume, and their mechanism corresponds to N-S vertical planes or horizontal ones. A tectonic model for the region is presented to explain the shallow, intermediate, and deep earthquakes.     

Active faulting offshore SE Spain (Alboran Sea): Implications for earthquake hazard assessment in the Southern Iberian Margin

The southern margin of the Iberian Peninsula hosts the convergent boundary between the European and African Plates. The area is characterised by low to moderate magnitude shallow earthquakes, although large historical events have also occurred. In order to determine the possible sources of these events, we recently acquired swath-bathymetry, TOBI sidescan sonar and high-resolution seismic data on the Almería Margin (Eastern Alboran Sea). The new dataset reveals the offshore continuation of the NE–SW trending Carboneras Fault, a master fault in the Eastern Betic Shear Zone, and its associated structures (N150 and NS faults). These structures are active since they cut the Late Quaternary sedimentary units. The submarine Carboneras Fault zone is 100 km long, 5–10 km wide, and is divided into two N045 and N060 segments separated by an underlapping restraining stepover. Geomorphic features typically found in subaerial strike-slip faults, such as deflected drainage, water gaps, shutter ridges, pressure ridges and “en echelon” folds suggest a strike-slip motion combined with a vertical component along the submarine Carboneras Fault. Considering the NNW–SSE regional shortening axis, a left-lateral movement is deduced for the Carboneras Fault, whereas right-lateral and normal components are suggested for the associated N150 and NS faults, respectively. The offshore portion of this fault is at least twice as long as its onshore portion and together they constitute one of the longest structures in the southeastern Iberian Margin. Despite the fact that present day seismicity in the Almería margin seems to be associated with the N150 to NS faults, the Carboneras Fault is a potential source of large magnitude (M_w ∼7.2) events. Hence, the Carboneras Fault zone could pose a significant earthquake and tsunami hazard to the coasts of Spain and North Africa, and should therefore be considered in any hazard re-evaluation.     

Tectonic setting of porphyry Cu–Au, Mo and related mineralization associated with contrasted Neogene magmatism in the Western Sulawesi Arc

The Neogene Western Sulawesi Arc, from the south going northwards, can be divided into three magmatic provinces of K alkaline–shoshonitic (AK-SH), high-K calc-alkaline (KCA), and low-K–normal calc-alkaline (TH-CA) affinity, referred to, respectively, as South, Central and North Sulawesi. The origin of this magmatism in terms of subduction and collision processes is contentious. Four widely spaced Cu–Au porphyry, and one Mo porphyry district(s) occur along the Western Sulawesi Arc, with the North Sulawesi province being the most mineralized. This porphyry mineralization is part of a regional belt that extends north into the Philippines and possibly south to the Sunda Arc. In western Sulawesi, common features that define the porphyry belt are obscure because the porphyry districts cannot be simply related, either in terms of their magmatic affinity, nature of basement, or tectonic setting. Nevertheless, it can be suggested that the generation of porphyry Mo systems requires involvement of continental crust in terms of magma source, while Au-rich porphyry systems are independent of the nature of the crust, and are derived from a mantle source.     

Mountain Front Development by Folding and Crustal Thickening in the Internal Zone of the Betic Cordillera-Alboran Sea Boundary

The boundary between the Alboran Sea and Betic Cordillera is a good example of a fold related mountain front in the Internal Zone of an alpine mountain range. Since the late Miocene, NNW-SSE convergence between the Eurasian and African plates has produced shortening and related orthogonal extension. To improve the characterisation of the geometry of the deep structure in the region and to establish the recent tectonic evolution of the mountain front, well logs and newly acquired geophysical data (multichannel reflection seismic and gravimetric surveys) have been interpreted and integrated with available surface data. The most marked tectonic structure corresponds to large antiforms and synforms of ENE-WSW trend which are related to mountain ranges and basins, respectively. The fold belt continues toward the northern continental shelf of the Alboran Sea. The fold vergence is generally northwards and its amplitude decreases progressively towards SSE, until disappearring in a sharp boundary where the reflectors are undeformed. The deep geometry suggests that fold growth started during upper Tortonian times and continued its activity up to Pliocene or even Quaternary times. The NNW-SSE compression produces crustal thickening and a regional and progressive southwards emersion. The location of main present-day deformation fronts in the Internal Zones contrasts with classical models where the deformation progresses towards the frontal part of External Zones of cordilleras. In addition, this fold-related deformation mountain front has features different from fault related fronts, as it does not show a sharp boundary, and folds that determine rectilinear mountain boundaries decrease progressively in amplitude or in wavelength up to undeformed areas.     

Sur un modele explicatif de l'arc de Gibraltar

Many authors have explained the Gibraltar arc in various manners. It is our purpose to set up an explanation which takes into account the recent geological discoveries about the Geology of this region, and the concept of Plate Tectonics. Stratigraphic, metamorphic and structural arguments support the fundamental opposition between internal and external zones in the Betic-Rif mountain system. Internal zones, clearly showing an arcuate structure, were built before Miocene. External zones, on the contrary, were folded mainly during Middle Miocene. Both zones have been involved in important shortening (with strike-slip faulting) just before Messinian (late Miocene). Taking into account the later deformations, we can assume that the internal zones constituted, at the beginning of the Miocene, the sub-plate of Alboran, which separated, eastwards from the Azores transform-fault, the European and African plates. According to this model, we can suppose the Alboran sub-plate to be fixed, whereas the European and African plates move eastwards. So, tectonic structures oblique to the direction of drift, like folds and thrusts along a transcurrent-fault, appear along the north-western and south-western margins of the Alboran sub-plate; along its western margin, N-S structures form, thrusting towards the west. Our geometrical model is able to account for peculiar and unexplained structures of this region. Because of its simplicity, we are conscious of the limits of our explanation, but it seems to us to be a valuable working hypothesis, which needs further geophysical and geological tests.     

New constraints on the evolution of the Gibraltar Arc from palaeomagnetic data of the Ceuta and Beni Bousera peridotites (Rif,northern Africa)

The Betic Cordillera and the Moroccan Rif together form one of the smallest and tightest orogenic arcs on Earth and almost completely close the Mediterranean to the west. For the explanation of the geodynamic evolution of the mountain belt, palaeomagnetic data that generally found clockwise block rotations in the Iberian and anticlockwise rotations in the Moroccan part of the mountain belt, have played a key role in recent works. This palaeomagnetic study has found new constraints on the rotations and timing of the peridotitic bodies outcropping in the key position at the westernmost margin of the mountain belt, in Ceuta and Beni Bousera (Rif, northern Africa).Detailed thermal demagnetization of 115 individually oriented samples from 14 sites was combined with rock magnetic and scanning electron microscopic experiments to analyze the magnetic mineralogy responsible for the remanences and the mechanisms and relative times of their acquisition. In Ceuta, up to three magnetic components, and in Beni Bousera, up to two magnetic components have been found, that are all to be interpreted as chemical remanent magnetizations (CRM). The data suggests the following succession of geodynamic events affecting the peridotites until recent times: (1) after their exhumation and subsequent cooling about 20 Ma ago, they recorded a characteristic remanent magnetization of both normal and reversed polarities, carried by (pseudo-)single-domain magnetite grains; (2) after their dismembering, the Ceuta peridotites were tilted southward by 22–34° about a horizontal or tilted axis (up to plunge 50°) with an azimuth of 72–145° and the Beni Bousera peridotites were rotated anticlockwise by 72.3 ± 12.1° about a vertical axis and (3) both recorded another magnetic signal of normal polarity only, carried by multi-domain magnetite grains; and finally (4) the Ceuta peridotites rotated anticlockwise by 19.7 ± 5.9° about a vertical axis.This study provides the first palaeomagnetic data for the Ceuta peridotites that, with their tilt and recent small net rotation, had a distinct geodynamic evolution from the large net rotations about vertical axes in Beni Bousera and Ronda (Betic Cordillera). Moreover, earlier palaemagnetic data for Beni Bousera is improved, as mixed polarities have been found in the older of the remanences for the first time, and its interpretation as a CRM changes the rotation timing that was proposed previously. The sequence of events exposed in this work are important constraints that need to be incorporated in any geodynamic model of the evolution of the Betic–Rifean mountain belt.