geology and tectonics of the betic zone se spain abstract the southern margin of the iberian continent comprises a complex geological history which can be traced back into the proterozoic when a gneissic basement was formed a cover of nevado filabride rocks was deposited onto this proterozoic basement during the palaeozoic the permian intrusion of the bedar granite suggests that the palaeozoic sedimentary rocks now exposed in the betic cordilleras have been affected by the hercynian orogeny similar to that of palaeozoic rocks in the adjoining ossa morena zone the hercynian basement was probably covered by alpujarride carbonates in the mesozoic when rifting opened tethys and intruded 140 ma old basic dyke swarms the palaeozoic basement of the betic cordilleras comprises a suture of late cretaceous subduction 80 85 ma b p this can be inferred from hp lt metamorphic facies obducted in coeval thrust slices of nevado filabride rocks the nevado filabride nappes include mesozoic carbonates transformed into marbles macael the suture was probably annnealed after the closure of tethys the mesozoic boundary between africa and europe and the rifting in tethys ceased during the palaeogene when the atlas mountains were formed 36 ma b p subsequently relative movements between africa and europe in the neogene were accommodated by strike slip faults these define the iberian microplate moving eastward by atlantic spreading and possibly subducting under italy neogene deformation of iberia is most intensive in southeastern spain due to emplacement of a mantle diapir under the alboran area this occurred early in the neogene aquitanian and caused nappe spreading involving alpujarride and malaguide rocks the subsequent neogene history has now been studied in great detail and is reviewed here it involves clear examples of calc alkaline volcanism most probably not related to subduction seismic motion along strike slip faults within the iberian microplate and syntectonic deposition in intermontane neogene basins

superposition of extensional detachments during the neogene in the internal zones of the betic cordilleras in the internal zones of the betic cordilleras extensional structures have developed from the upper oligocene to the present day they are contemporaneous with compressional structures folds and thrusts in the external zones from the upper oligocene to the aquitanian extension occurred in the malaguide alpujarride detachment and related structures show varying kinematics in different sectors younger deformations with a top to the n sense of movement have affected nevado filabride ductile shear zones alpujarride ductile and brittle shear zones and malaguide rocks normal faults at least from the late burdigalian up to the lower tortonian displacements have occurred in the alpujarride nevado filabride detachment deformations have been generally non coaxial with a top to the w sense of movement stretching lineation trends in the nevado filabride rocks curve from e to w suggesting a progressive variation of the ductile shear zone kinematics related to the alpujarride nevado filabride detachment between the aquitanian and lower tortonian stages deformations from the lower tortonian to the present day are normal faults formed in extensional settings in the upper part of the crust and folds and strike slip faults which indicate n s to nnw sse shortening directions and e w to ene wsw extension directions

the crustal domains of the alboran basin western mediterranean the alboran basin in the westernmost mediterranean hosts the orogenic boundary between the iberian and african plates although numerous geophysical studies of crustal structure onshore iberia have been carried out during the last decade the crustal structure of the alboran basin has comparatively been poorly studied we analyze crustal scale images of a grid of new and reprocessed multichannel seismic profiles showing the tectonic structure and variations in the reflective character of the crust of the basin the nature of the distinct domains has been ground truthed using available basement samples from drilling and dredging our results reveal four different crustal types domains of the alboran basin a a thin continental crust underneath the west alboran and malaga basins which transitions to b a magmatic arc crust in the central part of the alboran sea and the east alboran basin c the north african continental crust containing the pytheas and habibas sub basins and d the oceanic crust in the transition towards the algero balearic basin the alboran basin crust is configured in a fore arc basin west alboran and malaga basins a magmatic arc central and east alboran and a back arc system in the easternmost part of the east alboran basin and mainly algero balearic basin the north african continental crust is influenced by arc related magmatism along its edge and was probably affected by strike slip tectonics during westward migration of the miocene subduction system the distribution of active tectonic structures in the current compressional setting generally corresponds to boundaries between domains possibly representing inherited lithospheric scale weak structures

geomorphology and neogene tectonic evolution of the palomares continental margin western mediterranean abstract the palomares continental margin is located in the southeastern part of spain the margin main structure was formed during miocene times and it is currently part of the wide deformation zone characterizing the region between the iberian and african plates where no well defined plate boundary occurs the convergence between these two plates is here accommodated by several structures including the left lateral strike slip palomares fault the region is characterized by sparse low to moderate magnitude mw

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 pn 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 pn velocity between 7 5 and 8 2 km s 1 have been found

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

magnetic constraints on the thermal evolution of a collapsing orogen summary the thermal evolution of an orogen undergoing late stage extension was investigated using rock magnetic properties of a suite of mafic dyke rocks affected by greenschist facies metamorphism in the internal zones of the betic cordillera southern spain the natural remanent magnetization nrm in the dykes intruded into the lowest geological unit is made up of up to three components the lowest temperature component lt is in the direction of the present day magnetic field and is believed to be a chemical remanent magnetization crm or viscous remanent magnetization vrm acquired in the recent magnetic field the intermediate temperature it component unblocked between 200 c and 450 c is thought to be largely a thermoviscous overprint acquired during metamorphism this component is carried by either primary or authogenic sulfides and low unblocking temperature magnetite the component of magnetization with the highest blocking temperature ht is isolated above 450 c and is interpreted as the primary component of remanent magnetization it is most likely that this component is carried by magnetite that resides in the plagioclase and has been shielded from the metamorphism which transformed most of the original magnetite to metamorphic amphibole chlorite and biotite thermal demagnetization of these dykes separates the it overprint from the ht primary remanence at a sharp junction occurring at 450 c for single domain grains this translates to a peak palaeotemperature in the natural sample of approximately 370 c which is close to the estimated temperature experienced by the greenschist facies country rocks 400 c in contrast results obtained from a dyke that is intruded into an overlying weakly metamorphosed geological unit indicates that temperatures only reached about 175 c in this unit these results are consistent with temperatures deduced from geological constraints and they imply that between 4 and 6 km of section has been removed between the two units during late orogenic extension these new data are then combined with previously published ar ar wholerock geochronology and information on vertical axis rotations to put constraints on the thermotectonic evolution of this collapsing orogen

inherited palaeozoic and mesozoic rb sr isotopic signatures in neogene calc alkaline volcanics alboran volcanic province se spain

major fracture zones of morocco the south atlas and the transalboran fault systems within the fracturai pattern of morocco 35 45 and 70 90 directions are predominant most of the faults originated already during the upper palaeozoic and were repeatedly reactivated later on two of the large fracture systems are considered here the south atlas fault system saf is composed of different faults with directions varying between 45 90 they were not functionally connected to the southern hinge of the high atlas until its miocene uplift today it seems to be inactive on the whole both the tectonic data and the facies distribution of mesozoic strata contradict clearly the interpretation that the saf is a huge wrench fault or even the mesozoic plate margin of africa the sw striking transalboran fault system taf is still active it crosses morocco from melilla to agadir showing again singular faults which are unified to extended lineaments only in a few areas the intervals between the faults are bridged by linear arrangements of earthquake hypocenters sinistral massflow within the deeper parts of the lithosphere seems to be compensated at the surface by movements along pre existing faults for the moroccan segment of the taf a sinsitral displacement is testified at least since the oligocene while the northeastern segments from the rif to spain did not even originate until the pliocene this would mean that sinistral shear was transmitted from africa into the accreted alboran and iberia blocks after the miocene collision

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

volatiles associated with alkaline igneous rift activity fluid inclusions in the ilimaussaq intrusion and the gardar granitic complexes south greenland abstract fluid inclusions in minerals from si oversaturated and si undersaturated alkalic igneous complexes belonging to the precambrian gardar rift province in south greenland contain distinctly different carbon bearing gases granites and quartz syenites are characterized by co2 and only minor ch4 whereas agpaitic nepheline syenites are dominated by hydrocarbons essentially ch4 and contain only insignificant amounts of co2 and or co in the alkaline ilimaussaq intrusion the hydrocarbon rich gases may well reflect equilibration with graphite at magmatic temperatures and at oxygen fugacities approaching those defined by the synthetic mw buffer reaction in general formation of hydrocarbon gases in agpaitic nepheline syenites is considered to reflect the fulfillment of three basic conditions 1 low oxygen fugacities 2 a wide temperature range of crystallization and a low temperature solidus and 3 retention of volatiles during crystallization the characteristic carbon bearing vapour associated with continental alkaline igneous activity is however considered to be co2

neogene tectonic evolution of the southwestern alboran basin as inferred from seismic data off morocco the southwestern part of the western mediterranean alboran basin including part of the alboran ridge xaouen bank was investigated through the analysis of 28 intersecting multichannel seismic lines the seismic stratigraphy is tied to the amoco well el jebha 1 five seismic units or subunits are described from the quaternary to the middle and lower miocene the acoustic basement is interpreted to be mainly paleozoic and triassic metamorphic rocks of the alboran domain nappes and in places middle miocene messinian calc alkalic volcanics in the depocenters the thickness of the sedimentary infill mostly clays and turbidites exceeds 9 km normal faults of middle miocene tortonian age are broadly parallel to the coast and dip either seaward or landward they were mostly inverted during pre and post messinian episodes of compression which formed a set of en echelon north verging faulted folds in the alboran ridge area in relation with sinistral movement along the offshore projection of the jebha fault after pliocene subsidence a final episode of compression reactivated the earlier folds and pushed the alboran ridge onto the moroccan slope the complex structural history suggests many structural and stratigraphic potential hydrocarbon traps a high resolution seismic survey could lead to the definition of new exploration plays

arc related high k magmatism in the ceuta peninsula internal rif spain discovery and consequences we document the occurrence of micro diorite magmatic sills with magmatic enclaves in the ceuta peninsula within metapelites from the lower sebtides units internal rif all magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high k calc alkaline to shoshonitic island arc magmatism moreover these rocks are significantly affected by secondary metamorphic transformations under greenschist to amphibolite facies conditions regionally dated at c 21 ma geometric relationships between the sills and the main regional foliation developed under intermediate pressure granulite facies conditions at c 28 ma demonstrate that the sills emplaced during the late stage of this main tectonic event new u pb in situ analyses of monazite performed on the micro diorite sills provide an age of 20 64 0 19 ma coherent with this chronological framework and interpreted as the age of greenschist facies re equilibration the discovery of pre miocene high k calc alkaline to shoshonitic arc related magmatism is clearly consistent with the subduction context proposed for the alboran basin evolution according to geophysical investigations in this framework the lower sebtides units could be considered as part of the upper plate of the subduction system while the upper sebtides must be regarded as the lower subducted plate

post collision neogene volcanism of the eastern rif morocco magmatic evolution through time abstract neogene volcanism in the eastern rif morocco comprises a series of calc alkaline potassic calc alkaline shoshonitic and alkaline volcanic rocks according to new stratigraphical along with new and previous chronological and geochemical data the orogenic volcanism was successively 1 calc alkaline basaltic andesites and andesites 13 1 to 12 5 ma rhyolites 9 8 ma 2 k calc alkaline basaltic andesitic to rhyolitic lavas and granodiorites 9 0 to 6 6 ma and 3 shoshonitic absarokites shoshonites latites trachytes 7 0 to 5 4 ma the later pliocene volcanism was basaltic and alkaline 5 6 to 1 5 ma the calc alkaline and k calc alkaline series exhibit lower k2o 0 7 5 3 wt nb 8 19 ppm contents and higher 87 sr 86 sr 0 70773 0 71016 than the shoshonitic series k2o 2 4 7 2 wt nb 21 38 ppm 87 sr 86 sr 0 70404 0 70778 pliocene alkaline basalts have a sodic tendency na2o k2o 1 7 3 5 high nb content up to 52 ppm and low 87 sr 86 sr ratio 0 70360 0 70413 the variations through time of k2o nb and sr isotopic ratio reflect different mantle sources i calc alkaline potassic calc alkaline and shoshonitic series are derived from a mantle source modified by older subduction ii alkaline basalts are derived mainly from an enriched mantle source through time incompatible elements such as nb increased while 87 sr 86 sr decreased suggesting a decreasing influence of metasomatized mantle inherited subduction such evolution is related to the post collision regimes operating in this area and could be linked to the succession of extensional compressional and strike slip fault tectonics

insights into extensional events in the betic cordilleras southern spain new fission track and u pb shrimp analyses abstract a combination of the u pb shrimp and zircon fission track analyses has been used to constrain the age and evolution of the extensional tectonics in the betic cordilleras southern spain the studied rocks are dolerite dykes intruded in the malaguide complex and in the underlying los reales nappe and their country rocks the dykes are grouped in two nearly perpendicular systems characterized by n s strikes in the los reales nappe and e w strikes in the malaguide complex two periods of extension have been identified the age of the older event ranging from 33 to 25 ma is constrained by the intrusion of the dykes 33 ma and the joint cooling of the los reales malaguide domain from 25 ma onwards this event is associated with the structural emplacement of the malaguide complex over the los reales nappe along a brittle ductile extensional detachment and is tentatively ascribed to the tectonic collapse of the orogenic belt a minimum time span of 10 m y is established between the onset of the extensional collapse and the widespread aquitanian ht lp metamorphism in the present geographic frame the kinematics of this event is characterized by a change from an e w extension at lower levels los reales nappe to a n s extension in upper levels malaguide complex a younger extensional event of miocene age 18 ma has been deduced based on the fission track analysis within the calizas alabeadas member of the malaguide complex

the alboran sea western mediterranean revisited with a view from the moroccan margin this paper presents a new depth to acoustic basement map based on a grid of seismic profiles in the western alboran basin the basement is more than 10 km deep an examination of a seismic profile of this region suggests that late oligocene early miocene 5 km thick synrift formation has been accumulated in this basin this west alboran basin has been formed in an extensional setting but in a fore arc position south of the balearic islands then westwards transported behind a subduction zone

preliminary palaeomagnetic results on oligocene early miocene mafic dykes from southern spain abstract a preliminary palaeomagnetic study has been carried out on oligocene early miocene mafic dykes intruding the malaguide and alpujarride complexes of the internal betic zone southern spain the aims of the study were to ascertain if relative perpendicular orientations ne sw and nw se shown by different dyke families are of primary or secondary origin and to find out if palaeomagnetic rotations can be detected in that area ten sites with dykes showing both orientations were sampled in addition to palaeomagnetic analysis susceptibility versus temperature curves of several representative samples were measured and ore microscopic studies were carried out palaeomagnetic behaviour of samples was characterised by low intensities of magnetisation and alterations occurring during the demagnetisation procedure and often also by overlapping of palaeomagnetic components besides an initial viscous component two different palaeomagnetic components named m and h were recognised component m is carried by pyrrothite and magnetite and seems to record a primary palaeomagnetic direction component h is difficult to isolate and seems to be related to the presence of magnetite and hematite nine of ten studied sites offer consistent palaeomagnetic results although in two cases statistical parameters are relatively weak strong clockwise rotations approximately 90 140 are observed these rotations are in accordance with other block rotations observed in the internal and external zones of the central and western betics although their magnitudes are much stronger on the other hand both dyke families show similar rotations independently of their original direction thus their relative perpendicular orientations seem to be of primary intrusive origin

seismic evidence for the presence of jurassic oceanic crust in the central gulf of cadiz sw iberian margin abstract we investigate the crustal structure of the sw iberian margin along a 340 km long refraction and wide angle reflection seismic profile crossing from the central gulf of cadiz to the variscan continental margin in the algarve southern portugal the seismic velocity and crustal geometry model obtained by joint refraction and reflection travel time inversion reveal three distinct crustal domains the 28 30 km thick variscan crust in the north a 60 km wide transition zone offshore where the crust abruptly thins 20 km and finally a 7 km thick and 150 km wide crustal section that appears to be oceanic in nature the oceanic crust is overlain by a 1 3 km thick section of mesozoic to eocene sediments with an additional 3 4 km of low velocity unconsolidated sediments on top belonging to the miocene age gulf of cadiz imbricated wedge the sharp transition between continental and oceanic crust is best explained by an initial rifting setting as a transform margin during the early jurassic that followed the continental break up in the central atlantic the narrow oceanic basin would have formed during an oblique rifting and seafloor spreading episode between iberia and africa that started shortly thereafter bajocian and lasted up to the initiation of oceanic spreading in the north atlantic at the tithonian late jurassic earliest cretaceous the velocity model displays four wide prominent south dipping low velocity anomalies which seem to be related with the presence of crustal scale faults previously identified in the area some of which could well be extensional faults generated during this rifting episode we propose that this oceanic plate segment is the last remnant of an oceanic corridor that once connected the alpine tethys with the atlantic ocean so it is in turn one of the oldest oceanic crustal fragments currently preserved on earth the presence of oceanic crust in the central gulf of cadiz is consistent with geodynamic models suggesting the existence of a narrow westward retreating oceanic slab beneath the gibraltar arc alboran basin system

role of strike slip faults in the betic rifian orogeny abstract a new model for the betic rifian orogeny of the western mediterranean spain and north africa is proposed in which four strike slip faults play an important role the faults are not of the same age two faults the left lateral jebha fault to the south in morocco and principally in the mediterranean sea and the right lateral north betic fault southern spain to the north define the boundaries of the alboran block betic and rifian internal zones final movement along these faults was during the burdigalian time two other faults the left lateral nekor fault north africa to the south of the jebha fault and the right lateral crevillente fault somewhat to the north of the north betic fault define a larger alboran block including part of the betic and rifian external zones that was present during the tortonian the following sequence of events is proposed 1 a during the eocene and oligocene the african and european plates converged in a n s sense causing the breakup and overthrusting of the betic rifian and kabyle internal zones and then the movement towards the wsw of the alboran block by slip along the jebha and north betic faults 2 b by the end of burdigalian time movement along the jebha and north betic faults ceased 3 c with continued n s convergence the nekor and crevillente faults which bound a larger alboran block were formed during the mid and late miocene the arc of gibraltar the zone lying between the four major faults seems to be a result of wsw motion of a crustal block being thrust over external zones the model proposed adds to the earlier idea that tectogenesis proceeds from the interior to the exterior of an erogenic belt in the betic rifian orogeny major strike slip fracture zones shifted to the exterior of the orogenic belt as the orogeny progressed in order to relieve the stress caused by locking of the more internal faults

post collisional volcanism in a sinking slab setting crustal anatectic origin of pyroxene andesite magma caldear volcanic group neogene alboran volcanic province southeastern spain abstract caldear volcanic group cvg a stratigraphically well defined calc alkaline rock complex within sa de gata in the eastern part of the alpine betic mountain chain s spain consists of three distinct formations hernandez pyroxene andesites bujo hornblende bearing pyroxene andesites and viuda hornblende bearing pyroxene dacites rhyolites the letter rock formation may have developed through crystal fractionation of mainly plagioclase and pyroxenes however there is no direct relation between two formations cvg has a domainal structure with a northeastern domain where hernandez formation is overlain by bujo formation while viuda formation is absent and a southwestern domain where viuda formation forms the only fractionate after hernandez formation hernandez parent magma is thought generated through crustal anatexis by dehydration melting of a predominantly amphibolitic source rock complex which was formed by metamorphism from c 500 ma volcano sedimentary parent material the domainal structure of cvg is explained by compositional variation within this protogenetic complex single crystal u pb ages of c 500 ma to 1800 ma for inherited zircon support the presence of clastic material of proterozoic derivation within the original volcano sedimentary complex regional study of syn collisional rock formations alpine nappe complexes indicate that the collisional tectonic stage in the betic rif orogenic belt took place rather early 25 30 ma and was followed by a stage of rapid regional rock uplift fast cooling c 500 c my and extensional tectonics in the period 22 17 ma this later tectonic stage was set into motion by slab break off which set the stage for a high temperature regime in the overlying lithosphere providing the framework for the crustal melting and magma production responsible for the calc alkaline rocks of alboran volcanic province miocene zircon with ages ranging from c 17 to 11 ma indicate a rather protracted magmatic development prior to eruption at c 11 ma post collisional character of caldear volcanic group thus seems well established

fluid evolution wallrock alteration and ore mineralization associated with the rodalquilar epithermal gold deposit in southeast spain at rodalquilar gold mineralization is found in late tertiary volcanic rocks of the sierra del cabo de gata and is related to a caldera collapse radial and concentric faults were preferred sites for gold deposition hydrothermal activity produced a specific alteration zoning around gold bearing vein structures grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone advanced argillic alteration with silica pyrophyllite alunite and kaolinite extends down to several hundred m indicating a hypogene origin high grade gold mineralization in vein structures is confined to the near surface part of the advanced argillic alteration fine grained gold is associated with hematite jarosite limonite or silica at a depth of about 120 m the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite covellite bornite enargite and tennantite two types of fluids from different sources were involved in the hydrothermal system overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system subsequent hydrothermal processes were characterized by the influx of low salinity solutions of probable marine origin and by interactions between both fluids deep reaching advanced argillic alteration formed from high salinity fluids with 20 30 equiv wt nacl at about 225 c near surface gold precipitation and silification are related to fluids with temperatures of about 175 c and 3 4 equiv wt nacl gold was transported as au hs 2 and precipitation resulted from boiling with a concomitant decrease in temperature pressure and ph and an increase in fo2 all features of the rodalquilar gold deposit reveal a close relationship to acid sulfate type epithermal gold mineralization

crustal types and tertiary tectonic evolution of the alboran sea western mediterranean multichannel seismic reflection images across the transition between the east alboran and the algero balearic basins show how crustal thickness decreases from about 5 s two way traveltime twtt 15 km thick in the west east alboran basin to 2 s twtt typical of oceanic crust 6 km thick in the east algero balearic basin we have differentiated three different crustal domains in this transition mainly on the basis of crustal thickness and seismic signature boundaries between the three crustal domains are transitional and lack evidence for major faults tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic suggesting a strong relationship between magmatism and crustal structure stratigraphic correlation of lithoseismic units with sedimentary units of southeastern betic basins indicates that sediments onlap igneous basement approximately at 12 ma in the eastern area and at 8 ma in the western area linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent from west to east thin continental crust modified by arc magmatism magmatic arc crust and oceanic crust middle to late miocene arc and oceanic crust formation in the east alboran and algero balearic basins respectively occurred during westward migration of the gibraltar accretionary wedge and shortening in the betic rif foreland basins arc magmatism and associated backarc oceanic crust formation were related to early to middle miocene subduction and rollback of the flysch trough oceanic basement subduction of this narrow slab beneath the alboran basin was coeval with collision of the alboran domain with the iberian and african passive margins and subsequent subcontinental lithosphere edge delamination along the betic rif margins

magmatic evolution of the alboran region the role of subduction in forming the western mediterranean and causing the messinian salinity crisis the magmatic evolution of the alboran region westernmost mediterranean contains important clues for improving our understanding of the origin of mediterranean style back arc basins and the desiccation of the mediterranean sea in the messinian we use new laser 40ar 39ar age and geochemical major and trace element and o sr nd pb isotope data from igneous rocks from southern spain the alboran sea and northern morocco to reconstruct the magmatic evolution of the westernmost mediterranean since the eocene lower oligocene dikes near malaga 33 6 0 6 ma and middle to upper miocene volcanic rocks from the alboran sea area 6 57 0 04 to 11 8 0 4 ma can be subdivided into two groups 1 lree depleted relative to n morb primarily tholeiitic series and 2 lree enriched primarily calc alkaline series volcanic rocks both groups are generally enriched in fluid mobile elements e g rb th u k and pb relative to fluid immobile elements e g nb ta lree the lree depleted group has 143nd 144nd 0 5128 0 5130 isotope ratios similar to atlantic morb but higher 87sr 86sr 0 7046 0 7100 in contrast the lree enriched group has less radiogenic nd 0 5121 0 5126 and tend to more radiogenic sr 0 7066 0 7205 isotopic composition pb isotope ratios are surprisingly uniform and have compositions similar to marine sediments analyses of mineral separates show that mafic melts with relatively low 18o 5 6 7 2 had high 87sr 86sr 0 7048 0 7088 7 4 10 6 14 1 and 8 4 40 0 49 3 modeling of the trace elements and sr nd pb o isotopic compositions provides compelling evidence for the contamination of the mantle source with hydrous fluids melts which can be explained through subduction of oceanic lithosphere beneath the alboran basin but not through detachment delamination of lithospheric mantle we present a geodynamic model that reconstructs the late eocene to quaternary evolution of the western mediterranean through westward roll back of subducted tethys oceanic lithosphere slab roll back resulted in a large scale reorganization of the western mediterranean paleogeography causing the closure of marine gateways linking the atlantic ocean to the mediterranean sea isolation of the mediterranean sea led to its desiccation causing the messinian salinity crisis

the betic segment of the lithospheric trans alboran shear zone during the late miocene abstract the betic sinistral transcurrent shear zone betic cordilleras crosses the southeastern part of spain the study of the geodynamics of this area is approached from tectonic sedimentary magmatic and geophysical data large strike slip movements tectono sedimentary processes and magmatic events are seen at the boundary between two different lithospheric slabs they are located in a thinned compartment with a high geothermal gradient the large scale movements vertical and horizontal and the intensity of magmatic activity are the expression of the mechanical behaviour of this hot and thin crust all the geological events from the late miocene to recent are integrated in a crustal scale cross section which shows that the betic sinistral transcurrent shear zone widely affects the lithosphere it appears that subduction zones are not the only geodynamic context in which calc alkaline magmatism occurs

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

new latitude based nannofossil zonations for the campanian maastrichtian of the south atlantic ocean and their paleoceanographic implications the global evolution of calcareous nannofossils during the campanian maastrichtian interval is not fully understood and nannofossil zonations for the central and south atlantic ocean are yet to be constructed here we test the applicability of previous calcareous nannofossil zonations developed for other ocean basins for biostratigraphy in the south atlantic ocean and refine campanian maastrichtian biostratigraphy in the region we document calcareous nannofossil distributions at deep sea drilling project dsdp and ocean drilling program odp sites from the high mid and low latitudes in the south atlantic ocean dsdp sites 354 356 511 525a and 530a and odp sites 690c 700b 661a and 1258a combining nannofossil occurrences with magnetostratigraphic schemes previously constructed for some of the sites we propose three new zonations tied to the global magnetostratigraphic chrons with distinct bio horizons selected at different latitudinal regions in this study we present detailed definitions of new south atlantic ocean low latitude sal south atlantic mid latitude sam and south atlantic high latitude sah zonations we identified significant diachronism of some species between latitudes throughout the campanian and maastrichtian these species diachronism mainly between zonal markers from low and mid latitudes during the late maastrichtian was probably related to migration patterns due to fundamental paleoceanographic changes

petrological and structural evolution of high grade metamorphic rocks from the floor of the alboran sea basin western mediterranean the metamorphic basement beneath the alboran sea a neogene introduction extensional basin drilled during ocean drilling program odp late orogenic extension of previously thickened conleg 161 site 976 consists of high grade pelitic schist overlying tinental lithosphere is a widely documented phenomenon migmatitic pelitic gneiss inferred assemblages in the high grade e g dewey 1988 malavieille 1993 ruppel 1995 schist evolved from garnet staurolite biotite muscovite but it is unclear whether the thermal evolution of the plagioclase assemblage 1 to biotite sillimanite k feldspar lithosphere in this situation differs significantly from that plagioclase garnet assemblage 2 to andalusite biotite associated with extension in non orogenic settings if k feldspar plagioclase assemblage 3 in the gneiss which extension is triggered by removal of part of the lower shows abundant migmatitic textures the present assemblage is biotite lithosphere e g england houseman 1989 there may andalusite sillimanite muscovite cordierite kbe a detectable rise in temperature during exhumation feldspar plagioclase garnet phase relations and thermoof rocks platt england 1994 whereas extension barometric calculations suggest that the high grade schist experienced caused by far field forces is likely to produce isothermal decompression accompanied by heating from 500 c at 10 5 kbar decompression paths followed by cooling ruppel et al through 600 30 c at 6 7 kbar assemblage 1 and thence 1988 platt et al 1999 decompression accompanied by to 650 700 c at 3 4 kbar assemblage 2 followed by cooling heating in extensional settings has been demonstrated in through 500 600 c at 2 kbar or less assemblage 3 peak the seward peninsula in alaska patrick lieberman temperature and melting occurred under low pressure conditions 1988 the cordillera darwin in the southern andes the widespread disequilibrium and overstepping of metamorphic kohn et al 1993 the tauern window in the alps reactions suggests that decompression was rapid the p t evolution selverstone et al 1984 the aegean buick holland is consistent with metamorphism in a late orogenic extensional 1989 the high himalaya inger harris 1992 and basin and suggests the existence of an external source of heat the betic cordillera van der wal vissers 1993 an probably indicating the complete removal of lithospheric mantle interesting related question is the significance of crustal beneath the extending region melting in extensional settings in principle partial melt

an early miocene arc tholeiitic magmatic dike event from the alboran sea evidence for precollisional subduction and back arc crustal extension in the westernmost mediterranean a discussion is made on the general significance of an early miocene 22 23 ma k ar dating basaltic dike swarm that outcrops within large areas of the central and western betic zone of the betic cordilleras southern spain this dike swarm is thought to represent the remains of one of the earliest magmatic episodes within the the neogene volcanic province that is associated with the late orogenic evolution of the alboran region and undoubtedly offers the best preserved igneous material related to this early stage that is known at present the dike rocks hypabyssal equivalents of andesitic basalts and basaltic andesites have both major and trace element abundance patterns that are akin to those of arc tholeiitic suites the field relationships and areal distribution of the dike swarm suggest on the other hand that a major e w trending rifting belt could have been developing within the alboran block in oligo miocene times previous to its collision with adjacent continental borderlands in the early middle miocene