pull apart emplacement of the margeride granitic complex french massif central implications for the late evolution of the variscan orogen a microstructural magnetic fabric and gravity study is performed on the carboniferous margeride granitic complex that crops out in the central part of the variscan french massif central this complex consists of three facies namely a main porphyritic monzogranite a two mica granite and late leucogranite dykes and stocks in spite of local variations the magnetic lineation mainly trends nw se with a shallow plunge throughout the complex the magnetic foliation pattern is more complex with various strikes and generally a moderate dip this fabric pattern complies with the late orogenic nw se regional extensional deformation already recognised in this part of the variscan belt new gravity measurements complete available data and are used to build up a gravity map and a cross section with which four gravity minima are identified three gravity minima are interpreted as extensional fractures consistent with a nw se maximum stretching the last gravity low corresponds to an extensional jog between the two westernmost fractures the structural and gravity features of the complex are used to propose a feeding and emplacement model controlled by the regional late orogenic extensional tectonics the margeride complex is interpreted as a kilometre scale laccolith like pluton emplaced in a transtensional setting controlled by a nw se opening direction such a model strengthens the relationships between pluton emplacement and late orogenic collapse of the variscan belt

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

prograde and retrograde metamorphism of hematitebearing basic schists in the sanbagawa belt in central shikoku the mineral assemblages of hematite bearing basic schists in intermediate high pressure metamorphism are temperature dependent for assemblages with excess hematite albite muscovite and quartz the paragenetic relations can be dealt with in terms of a four component system without omitting or grouping major components in the sanbagawa belt in central shikoku the dominant amphibole in the hematite bearing basic schists changes from winchite via crossite and barroisite to hornblende the stability of amphibole is described chemographically within a pseudoternary system with another excess phase epidote many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low t p amphibole around the prograde amphibole the examination of amphibole zoning makes it possible to draw a retrograde p t trajectory which passes on the lower pressure side of the prograde one

microstructures of deformed grains in the augen gneisses of southern menderes massif western turkey and their tectonic significance a detailed fabric and microstructural analysis of the granitic mylonites was carried out on the southern side of bes parmak mountain north of selimiye milas the mylonitic augen gneisses have a blastomylonitic texture characterized by large retort shape porphyroclasts or augen of feldspars around which a more ductile medium to fine grained matrix of muscovite biotite quartz and feldspar is deflected feldspars behave in both plastic and brittle fashion because size reduction occurs through grain boundary migration and or subgrain rotation and also through fracturing typical core and mantle structure characterized by a large feldspar core surrounded by a mantle of fine recrystallized grains is very characteristic the majority of plagioclase twins obey the albite twin law however the association with pericline law twinning suggests that many of the twins are mechanical evidence of strain such as deformation twins bent or curved twins undulatory extinction deformation bands and kink bands occur characteristically in plagioclase myrmekite is ubiquitous at k feldspar grain boundaries most notably on the long sides of inequant grains parallel to the s foliation direction which invariably face the maximum finite shortening direction deformation of quartz in mylonitic augen gneisses commonly results in the development of core and mantle structure and type 4 quartz ribbons of elongated preferably oriented newly recrystallized quartz aggregates suggesting a primary dynamic recrystallization undulatory extinction deformation bands and lamellae are the strain related features associated with quartz porphyroclasts micas especially biotite undergo internal deformation by bend gliding and kinking most of the micas are completely attenuated and aligned such that their 001 planes are subparallel or parallel to the margins of quartz ribbons and define the foliation in the rock these microstructures of feldspars quartz and mica in the mylonitic augen gneisses in this part of the southern menderes massif are broadly consistent with fabric development under upper greenschist to lower amphibolite facies conditions rather than almandine amphibolite facies as was previously believed this supports the previous contention of the authors that the protoliths of augen gneisses are younger granitoids and do not represent an exposed precambrian pan african basement in the menderes massif

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

conceptual and numerical models of ring fault formation abstract most ring faults of collapse calderas are primarily shear fractures the initiation and development of which depends on the state of stress in the host rock the state of stress in a volcano is controlled by the loading conditions such as magma chamber geometry and pressure but also by the mechanical properties of its rock units and structures such as existing contacts faults and joints ring fault formation is thus essentially a problem in rock physics field observations show that some ring faults are dip slip whereas others are partly faults shear fractures and partly ring dykes extension fractures although slip on existing ring faults is much more common in basaltic edifices shield volcanoes than in true composite volcanoes in both types of volcanoes most caldera unrest periods do not result in ring fault slip here i present new conceptual and numerical models of caldera formation in volcanoes with shallow spherical circular or oblate ellipsoidal sill like magma chambers in the layered models the host rock above the chamber is composed of 30 comparatively thin layers with stiffnesses young s moduli alternating between 1 gpa and 100 gpa the chamber itself is located in a single thick layer the crustal segment hosting the chamber is either 20 km or 40 km wide but has a constant thickness of 20 km the loading conditions considered are 1 a crustal segment subject to 5 mpa tension 2 crustal segment subject to excess magmatic pressure of 10 mpa at the bottom doming of the volcanic field containing the chamber 3 a combination of tension and doming and 4 chamber subject to underpressure negative excess pressure of 5 mpa the main results are as follows 1 excess pressure and underpressure in a chamber normally favour dyke injection rather than ring fault formation 2 for doming or tension a spherical magma chamber favours dyke injection except when the layer hosting the chamber is very soft 10 gpa or one with recent dyke injections in which case the surface stress field favours ring fault formation 3 for a sill like chamber in a 20 km wide crustal segment a ring fault can be generated by either tension or tension and doming for a 40 km wide segment doming alone is sufficient to generate a ring fault 4 since individual layers in a volcano may develop different local stresses stress field homogenisation through all the layers between the chamber and the surface is a necessary condition for ring fault formation 5 because the mechanical properties of the layers that constitute basaltic edifices are more uniform than those that constitute true composite volcanoes it follows that stress field homogenisation and thus ring fault formation or slip is more commonly reached in basaltic edifices than in composite volcanoes 6 both for basaltic edifices and composite volcanoes the stress fields most likely to initiate ring faults are those generated around sill like chambers subject to tension doming or both

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

recurrent fault activity in the grimsel test site gts switzerland revealed by rb sr k ar and tritium isotope techniques fault gouge and mylonite samples of prominent faults au 126 m and au 96 m occur at the grimsel test site underground research laboratory in the aar massif fractions the phengite green biotite plagioclase and k feldspar of the penetratively deformed host grimsel granodiorite form a rb sr isochron of 19 3 9 ma the deformation has a similar k ar age the rb sr data from the phengite and albitic plagioclase of the gouges and the mylonite samples suggest an earlier isotopic exchange 26 ma years ago the fault gouge and mylonite seems to have been formed during an early hydrothermal tectonic pulse of the alpine metamorphism however there may be small amounts of inherited 87 sr and a mixing line cannot be excluded the apparent k ar ages of the 18 o 2 h values from percolating waters in a parallel fault confirm the existence of recent fluid activities from

emplacement in an extensional setting of the mont lozere borne granitic complex se france inferred from comprehensive ams structural and gravity studies the emplacement mode and setting of the late hercynian mont lozere borne granitic complex french massif central which consists of several plutons is investigated structural and anisotropy of magnetic susceptibility ams studies have been carried out to characterize the internal fabrics of the granitic plutons throughout the pont de montvert borne pluton an e w trending magnetic lineation is well developed in the host rock and the thermal aureole a conspicuous e w trending lineation is interpreted as an evidence of a late orogenic extensional event to the east of the pluton the ams fabric is characterized by values of anisotropy degree p around 4 5 with a prolate ellipsoid and subsolidus structures whereas to the west the p parameter is weaker with an oblate ellipsoid and purely magmatic microstructures a gravity investigation allows determination of the 3d shape of the pluton the western part of the granitic complex is thicker than the eastern one and is interpreted as the feeder zone this suggests an eastward spreading of the magma the consistency between regional stretching and directions of ams lineations in the pluton and the shape of the complex deduced from gravity strongly argues that the emplacement mode of the complex was influenced by the regional extensional tectonic setting during the collapse of the hercynian belt

high pressure micro inclusions and development of an inverted metamorphic gradient in the santiago schists ordenes complex nw iberian massif spain evidence of subduction and syncollisional decompression the santiago schists are located in the basal unit of the ordenes complex one of the allochthonous complexes outcropping in the inner part of the hercynian belt in the north west of the iberian peninsula their tectonothermal evolution is characterized by the development of an eo hercynian metamorphic episode c 374 ma of high p low to intermediate t the mineral assemblage of the high p episode is preserved as a very thin si s1 foliation included in albite porphyroblasts being composed of albite garnet i white mica 1 chlorite 1 epidote quartz rutile ilmenite the equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 10 c and 14 7 0 7 kbar probably minimum pressure the later evolution syn d2 of the schists defines a decompressive and slightly prograde p t path which reached its thermal peak at c 525 10 c and 7 kbar decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient so that the zones of garnet ii biotite with an upper subzone with chloritoid and staurolite developed from bottom to top of the formation the estimated p t path for the santiago schists suggests that the basal unit probably a fragment of the gondwana continental margin was uplifted immediately after its subduction at the beginning of the hercynian orogeny it also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension probably related to the continued subduction of the gondwana continental margin and the contemporaneous development of compensatory extension above it the inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin when it came into contact with the upper parts of the schists along a detachment probably of extensional character the general metamorphic evolution of the santiago schists with the development of high p assemblages with garnet prior to decompressive and prograde parageneses with biotite is unusual in the context of the european hercynian belt and shows a close similarity to the tectonothermal evolution of several high p low to intermediate t circum pacific belts

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

petrology structure and geotectonic implications of glaucophane bearing eclogites and related rocks from the malpica tuy mt unit galicia northwest spain abstract the malpica tuy mt unit is a narrow band of basement in galicia northwest spain composed of mica schists paragneisses and calc alkaline to peralkaline orthogneisses with minor amounts of metamorphosed basic rocks garnet omphacite glaucophane barroisite rutile phengite paragonite kyanite zoisite developed in eclogites s s and eclogitic impure sandstones garnet phengite rutile zoisite in the calc alkaline orthogeneisses and garnet phengite rutile chloritoid kyanite in the metasediments biotite breakdown in the orthogneisses and related hornfelses produced garnet with development of coronitic textures plagioclase porphyroblasts with aligned inclusions of rutile and phengite are a common feature of many paragneisses the minimum p t conditions for the equilibrium of garnet omphacite parageneses at the peak of the metamorphism were 600 c 12 2 kbar the mineral chemistry of the orthogneisses and metasediments is compatible with these conditions furthermore the structural data also suggest an in situ origin for the eclogites i e the same as the metamorphism of the enclosing rocks the crystalline area surrounding the mt unit is mainly composed of metasediments and anatectic granites the t at the peak of the metamorphism was higher than 700 c the p being of the order of 5 kbar the two domains represent different parts of the hercynian foldbelt tectonically juxtaposed the juxtaposition is the result of complex tangential tectonics three main phases of compressive deformation having been recognized a first stage of subduction of a thinned crust the materials of the mt unit and subsequent obduction development of hydrated minerals in eclogitic rocks with emplacement of these materials over an already metamorphosed crustal area is suggested the mt unit would represent a remnant of a hercynian thrust nappe

3d analysis of an ordovician igneous ensemble a complex magmatic structure hidden in a polydeformed allochthonous variscan unit the basal units of the allochthonous complexes of nw iberia represent a fragment of the external edge of gondwana subducted and subsequently exhumed during the variscan collision the structural analysis carried out in orthogneissic massifs of the southern part of one of these the malpica tui unit reveals the generation of recumbent folds and associated axial planar foliation during their exhumation these folds nucleated in irregular igneous bodies that were initially deformed during the subduction event and show east to southeast vergence down plunge projection of surface data and a series of regularly spaced cross sections have been used to build 3d models of the two main bodies of orthogneiss of calc alkaline and peralkaline compositions respectively the first is presently a lens shaped body folded in a recumbent syncline whereas the peralkaline gneiss also affected by a train of asymmetric recumbent folds in the south exhibits in the north a fold like structure which is interpreted as inherited from its primary geometry that of a ring dike pluton

lower paleozoic extensional tectonics in the limit between the west asturian leonese and central iberian zones of the variscan fold belt in nw spain the limit between the west asturian leonese and central iberian zones in the nw of the iberian peninsula is described in the area of caurel truchas provinces of orense and leon from the distribution of the sedimentary formations and the variscan structures it is inferred that the transition between the two zones was a fault system which was active during at least the ordovician and lower silurian the faults were supposedly normal though they had probably some strike slip component and gave rise to a half graben in which a syn rift sequence was deposited the latter underwent inversion at the onset of the variscan tectonism producing a ramp anticline syncline pair that forms the more important folds in the area the varying geometry of these folds is interpreted partly as being due to the existence of previous en echelon folds related to the strike slip component of the faults and partly as a result of the variable intensity of the shear strain undergone during the first variscan phase

tectonic evolution of the careon ophiolite northwest spain a remnant of oceanic lithosphere in the variscan belt abstract analysis of the careon unit in the ordenes complex northwest iberian massif has supplied relevant data concerning the existence of a paleozoic oceanic lithosphere probably related to the rheic realm and the early subduction related events that were obscured along much of the variscan belt by subsequent collision tectonics the ophiolite consists of serpentinized harzburgite and dunite in the lower section and a crustal section made up of coarse grained and pegmatitic gabbros an early devonian zircon age documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10

granite emplacement in orogenic compressional conditions the la alberca bejar granitic area spanish central system variscan iberian belt abstract the la alberca bejar granitic area comprises a set of intrusions of upper carboniferous to lower permian age located in the hinterland of the variscan belt of western spain gravity data anisotropy of magnetic susceptibility and microstructural studies carried out in the granitoid and its country rocks indicate that the granitoids of this region have tabular and inverted drop shapes and show sub horizontal magmatic fabrics and sw ne lineations the granitoid bodies were intruded episodically with single intrusive pulses recorded by deformation in the country rocks all intrusive events took place during d 3 compressive stage in the core of large d 1 anticlines and are not related to structures in the country rock a syn orogenic emplacement model for granite intrusion in such conditions is proposed the proposed model implies the activity of coeval emplacement mechanisms and room generation

magnetic fabrics in the cabanas granite ne brazil interplay between emplacement and regional fabrics in a dextral transpressive regime the late neoproterozoic cabanas granite northeastern brazil is a tabular body that was emplaced south of the dextral east pernambuco shear zone system anisotropy of magnetic susceptibility ams measurements reveal that the cabanas granite has fabric patterns more complex than usually reported in plutons intruded in similar settings most structures have ew to ene trending magnetic lineations consistent with the overall regional structural direction in the north these structures parallel the mylonitic fabric encountered in shear zones that bound or cross cut the pluton in the central and southern portions of the pluton magnetic foliation trajectories the overall banded disposition of the iso susceptibility lines and shape parameters of the ams ellipsoids suggest that the cabanas granite was subjected to ene trending folding along with its underlying and enclosing country rocks it is concluded that emplacement took place during regional dextral transpression a secondary but well developed family of structures consists of n to nw plunging lineations which is interpreted to represent the trace of the vertical conduits that fed the pluton this study emphasizes the utility of the ams to unravel both the internal structure of low anisotropy plutons and the interplay between emplacement and regional straining during magma crystallization

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

careon ophiolite nw spain suprasubduction zone setting for the youngest rheic ocean floor the careon ophiolite galicia nw iberian massif shows lithological and geochemical features suggestive of an origin in a suprasubduction zone setting as with other devonian ophiolites in the european variscan belt it was generated within a contracting rheic ocean this setting and the general absence of large silurian devonian volcanic arcs on both of the rheic ocean margins strongly suggest that this ocean was closed by intraoceanic subduction directed to the north this subduction removed the older normal n mid oceanic ridge basalt morb oceanic lithosphere and gave rise to a limited volume of new suprasubduction zone oceanic lithosphere the careon ophiolite is a key element in understanding the evolution of the rheic ocean which was the main oceanic domain that closed during the paleozoic convergence of gondwana and laurussia preceding the assembly of pangea

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

40ar 39ar mineral age constraints for the tectonothermal evolution of the sambagawa metamorphic belt central shikoku japan a cretaceous accretionary prism abstract the sambagawa metamorphic belt exposed in central shikoku japan is tectonostratigraphically represented by the oboke and structurally overlying besshi nappe complexes oboke units consist of metamorphic rocks largely derived from clastic protoliths sandstone and conglomerate whereas besshi units are dominated by rocks with oceanic protolith affinities shale chert and greenstone whole rock schist samples from low grade oboke units record intermediate temperature 40 ar 39 ar plateau ages of 70 77 ma which are interpreted to closely date attainment of maximum metamorphic conditions whole rock schist samples from besshi units chlorite zone record plateau ages of 85 94 ma suggesting an earlier metamorphic climax within this structural level amphibole within the albite biotite zone in besshi records 36 ar 40 ar vs 39 ar 40 ar plateau isotope correlation ages of 84 87 ma which are interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon a slightly older isotope correlation age 94 ma is recorded by amphibole from the higher grade oligioclase biotite zone contrasts in amphibole cooling ages are interpreted as reflecting thrust related metamorphic inversion and resultant earlier cooling of higher grade structural units muscovite from all structural levels of the besshi nappe complex records 40 ar 39 ar plateau ages of 76 89 ma these are interpreted to date post metamorphic cooling through appropriate closure temperatures following structural assembly of the besshi nappe complex protoliths of the besshi units originated at approx 130 ma and attained maximum metamorphic conditions during entrainment in an accretionary complex at approx 90 100 ma higher grade sequences developed at deeper levels along the hanging wall of the accretionary complex during subsequent structural uplift to shallower crustal levels the besshi nappe complex was tectonically emplaced over oboke units

the catastrophic geomorphological processes in humid tropical africa a case study of the recent landslide disasters in cameroon abstract a retrospective analysis of data from the last three decades clearly indicates an upward trend in the number of landslides and mudflows in cameroon their impact on human beings environment and properties has increased considerably for example since june 2001 several cases have been recorded among which two resulted in significant human losses twenty three deaths in june 2001 in limbe town twenty five others in july 2003 at magha and its surroundings in the bamboutos mountains these last two events were really spectacular and may be sounding the alarm of a geomorphological crisis in the region according to each case several tens or hundreds of landslides of different types and magnitudes occurred within a few minutes or hours extending over an area of several tens of square kilometres the reasons for this catastrophic evolution in the geomorphological processes in cameroon are natural and anthropogenic among others the natural factors are the changing climatic conditions with the occurrence of very exceptional rainfalls and the uneven topography of the affected zones for example with very steep slopes as in the magha zone of which area about fifty percent is above twenty degrees the anthropogenic factors are also very relevant with the increasing human pressure on the environment characterised by the extension and or intensification of the agro pastoral activities in the rural areas and the anarchical settlement on the sloping urban zones

the vila de cruces ophiolite a remnant of the early rheic ocean in the variscan suture of galicia northwest iberian massif abstract the vila de cruces ophiolite is one of the ophiolitic units involved in the variscan suture of the northwest iberian massif this ophiolite consists of a tectonically repeated succession of greenschist facies volcanic rocks common alternations of metasediments of pelitic or siliceous character and scarce orthogneisses metagabbros and serpentinites the protolith age of a granitic orthogneiss that intruded the mafic rocks is dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 497 pm 4 end document ma u pb in