crustal spreading on venus evidence from topography morphology symmetry and map patterns abstract evidence for numerous features similar to terrestrial divergent plate boundaries and spreading centers are present in the topography and morphology of western aphrodite terra venus head and crumpler 1987 linear discontinuities cutting across the strike of western aphrodite terra cross strike discontinuities or csds were examined in further detail it was found that 1 1 they represent the most prominent regional structural trend in western aphrodite and are readily distinguishable from linear patterns caused by ground tracks in the pioneer venus orbiter data 2 2 individual csds are either deep troughs or steep regional scarps and steps 3 3 the spacing between the more prominent csds ranges from about 500 to 1500 km average spacing is 850 km and 4 4 there is a distinctive structural trend orthogonal to the strike of the csds additional smaller scarps and slopes with characteristics of known csds occur in the arecibo altimetric profiles at spacings of as little as 50 km in agreement with previous suggestions of sotin et al 1989 that other smaller csds with less offset may occur in domains between the more prominent ones previously mapped the detailed characteristics of the csds detailed trough and scarp like form association with ridge offsets linear terminations linear slope traces parallelism great length are all very similar to the characteristics of transform faults and fracture zones on the earth s seafloor and this additional analysis supports the earlier interpretation that csds represent analogs to terrestrial oceanic fracture zones assessment of bilateral symmetry of topography and other elements across western aphrodite terra indicate that 1 1 an accurate prediction of the large and small scale symmetrical elements in both the altimetry data and radar images within each domain on one flank of the highlands may be made by substitution of a mirror image of the opposite flank and 2 2 smaller wavelength symmetrical elements are revealed in the altimetric profiles by subtracting the background regional symmetry and examining the residuals residual altimetry profiles obtained in this manner show that individual altimetry features several hundred kilometers wide and several hundred meters high are frequently located at the same distance from the axis of regional symmetry on the north and the south flanks of the highlands topography and geometric features such as ridge crests and transforms have specific predictable relationships in a crustal spreading environment tests were made in western aphrodite for such organized relationships predicted to occur in association with ridge crest offsets at transform faults and fracture zones it was found that 1 a topographic stepdowns occur across csds in accordance with the general consequences of juxtaposing lithospheres of different relative ages 2 b the correct direction of stepdown down toward older crust and lithosphere occurs relative to that predicted from ridge crest offsets and 3 c the observed amount of stepdown is similar in many cases to that predicted from the observed ridge crest offsets these analyses of cross strike discontinuities topographic and image symmetry across aphrodite and predictions of topography based on ridge crest and fracture zone geometric relationships further support the earlier interpretation that the 7500 km extent of western aphrodite terra represents the site of crustal spreading and displays many of the characteristics of terrestrial divergent plate boundaries

the tectonic frame of the variscan alleghanian orogen in southern europe and northern africa abstract by confronting different geological and geophysical data we attempt to reconstruct the variscan alleghanian orogenic belt with especial emphasis on the links between iberia northwest africa and northeast america the northern iberia transect corresponds to the rifted margin of gondwana inverted during the variscan orogeny and overthrust in its westernmost sector by a pile of allochthonous units some of them with oceanic affinity and witnessing an orogenic suture the southern iberia section reveals two sutures at both boundaries of a terrane closely tied to gondwana namely the ossa morena zone the southern boundary of this zone i e the contact with the south portuguese zone is enhanced by amphibolites with oceanic affinity and probably represents the suture of the rheic ocean the moroccan variscides can be divided into i a western external zone namely the coastal block and the central massif ii an internal zone namely the eastern meseta iii an eastern external zone represented in the anti atlas region and iv the african cratonic foreland since neither ophiolites nor eclogites crop out along this transect of the orogen we consider it to be an incomplete transect lacking the main suture zone rheic ocean stratigraphic and faunal affinities between the moroccan meseta on the one hand and the central iberian western asturian leonese and cantabrian zones of iberia on the other hand suggest that they may well have been part of a common pre orogenic domain at the margin of gondwana on the contrary there are no counterparts in morocco for the ossa morena zone thus the northern iberia palaeogeographic zones would prolong in the moroccan meseta through an eastward arcuate structure while the suture of the rheic ocean would do so offshore morocco in our reconstruction an avalonian spur grand banks indentor which included the caledonian sehoul block in northern morocco and the crust of the south portuguese zone in sw iberia played an important role on the kinematic evolution of the orogen as well as on the final configuration of the suture of the rheic ocean this suture would mould around the avalonian spur being laterally translated to southeastern nova scotia newfoundland

reports venus identification of banded terrain in the mountains of ishtar terra high resolution images obtained with the arecibo radar system at a wavelength of 12 6 centimeters reveal numerous parallel 10 to 20 kilometer wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the ishtar terra region of venus geometric and morphological characteristics suggest that the bands represent earth like tectonic deformational features such as folds and faults

triassic evaporites of iberia sedimentological and palaeogeographical implications for the western neotethys evolution during the middle triassic earliest jurassic abstract this paper deals with the middle triassic to earliest jurassic evaporite sedimentation in the epicontinental eastern iberian platform this deposition occurred under extensional regime during the westwards migration of the neotethys ocean at the start of the pangea break up although attention is focused on the evaporitic episodes recorded in the epicontinental germanic triassic of the platform the evaporite units in the carbonatic alpine triassic are also considered in the epicontinental triassic up to six evaporitic episodes which alternated with three carbonatic episodes and a siliciclastic one occurred between the anisian and the lower hettangian the evaporitic episodes may be divided into two groups the stratigraphic units of the older group have similar characteristics in salinity and facies and were formed in chloride rich evaporitic mudflats in the younger group the salinity and depositional features display considerable differences and the environments evolved from evaporitic mudflats to extensive salterns the different evolutive trends in the evaporites of the two groups suggest major re structuration of the platform after the sedimentation of the siliciclastic units of the middle keuper carnian humide episode progressive marine influence in the assemblage of the six evaporitic episodes is deduced from the changing nature of the evaporites and their host sediment all the evaporite units accumulated as transgressive and highstand systems tracts the close relationship between evaporite units and thick accumulation suggests that the structural control played a major role in the sedimentation in the alpine triassic the presence of evaporites interbedded in the allochthonous carbonate units is documented lateral correlation of the evaporite units of the epicontinental and the alpine types of triassic allows us to reconstruct the facies continuum across the original iberian platform such a west to east transect illustrates the palaeogeography structural evolution and major depositional controls in the platform during the period under study the deduced evaporitic pattern could be applied to other triassic platforms which acted as epeiric seaways for the westward advance of the neotethys

the malaguide los reales nappe an example of crustal thinning related to the emplacement of the ronda peridotites betic cordillera abstract the malaga region in southern spain represents a cross section through the continental crust and subcontinental mantle of the westernmost portion of the internal zone of the betic cordillera recent petrological and structural studies carried out in this region show a link between the malaguide complex and the underlying los reales nappe where the ronda peridotites were incorporated as allochthonous slabs the ronda peridotites are covered by the metamorphic sequence of the los reales nappe which consists of acidic granulites migmatites and schists the tectonometamorphic evolution of these granulite facies rocks involves a first high temperature 800 c high pressure 10 kbar metamorphism replaced by high temperature 750 c low pressure 3 5 kbar conditions this strong thermal event which is also recognized in biotite bearing schists of the los reales nappe and even in the overlying malaguide complex developed during an extensional deformation related to the diapiric stage of the ronda peridotite emplacement stretching lineations from the mylonites related to this extensional event are n 110 e in orientation and kinematic criteria show that low grade metamorphic rocks malaguide complex move down on to higher grade ones los reales nappe from west to east consequently we interpret that the malaguide complex and los reales nappe form a single domain of continental crust which was thinned during the emplacement of the ronda peridotites stretching lineations are subparallel to the trend of the betic cordillera and can be integrated in a model of lithospheric transcurrent thinning

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

unidirectional slip vectors in the western betic cordillera implications for the formation of the gibraltar arc we have investigated the kinematics of deformation in a sector of the gibraltar arc in sw spain by synthesizing lineation data from major faults the variation in structural trends and palaeomagnetically determined vertical axis rotations the kinematic data show that transport directions throughout formation of the arc were consistently wnw regardless of structural trend which varies between 050 and 350 in the study area the data suggest that this part of the gibraltar arc can be explained simply by oblique convergence with a curved indenter taken together with other published data it appears likely that the variation in thrust transport direction within the betic cordillera as a whole may be considerably less than that in the moroccan part of the arc unidirectional convergence combined with the presence of palaeomagnetic rotations implies either eycient overprinting of old rotated lineations and or a convergence rate that is much greater than the rotation rate also required is a low degree of partitioning in the thrust wedge the latter probably increases east of the study area where the structural trend becomes more oblique to the transport direction and major strike slip faults occur

palaeogeography facies and nomenclature of the triassic units in the different domains of the betic cordillera s spain abstract this paper reviews and discusses the different facies and nomenclature of the triassic in the betic cordillera s spain emphasis is placed on problems encountered when studying betic basin facies and palaeogeographic domains in triassic times the betic basin was positioned between the iberian and mesomediterranean plates we therefore propose the designations southiberian triassic and mesomediterranean triassic for the triassic rocks that crop out in the external and internal zones respectively of the cordillera although facies types vary substantially several similarities in their main stratigraphic features link the two palaeogeographic domains thus buntsandstein muschelkalk and keuper facies can be currently distinguished in both domains of the external and the internal zones of the betic cordillera most rocks were coastal and shallow marine sediments deposited across an extensive and varied palaeogeography which defines the sephardic domain for some epochs this last domain shows the influence of alpine fauna and even palaeogeographically gives rise to subdomains characteristic of alpine facies that today crop out in some internal zone units this variety of facies makes the study of the triassic in the betic cordillera all the more interesting

on venus impact basins viscous relaxation of topographic relief the question of whether impact basins dating from the time of heavy bombardment of the inner solar system are preserved on the surface of venus is of paramount importance for an understanding of the geological processes that have shaped that planet two types of features have been suggested as candidates for impact basins on venus 1 large quasi circular topographic depressions and 2 large quasi circular regions of low radar backscatter surrounded by terrain of greater radar backscatter and generally displaying little topographic relief as a guide to the expected character of an impact basin on venus we have calculated the viscous relaxation with time of the topographic relief of such a basin under the assumption that the surface temperature of the planet has been at or near its presently high value since the time of basin formation the viscous relaxation model includes the effects of a decrease in effective viscosity with depth and the partial to complete isostatic compensation of initial topography we conclude that an impact basin several hundred kilometers or greater in diameter and 3 b y old or greater in age should have negligible topographic relief at present this conclusion does not exclude the hypothesis that quasi circular features of low radar backscatter but little topographic relief may be relaxed remnants of ancient impact basins large quasi circular depressions on venus such as atalanta planitia however cannot exceed a few hundred million years in age otherwise their initial topographic relief would be excessive such large depressions are thus likely to be geologically young and to have resulted from processes other than impact an additional process must therefore have operated on venus to create large roughly circular areas of lowland terrain one possibility is that these topographic features are analogs on venus to terrestrial platform basins produced by lithospheric extension and thermal subsidence

significant contribution of authigenic carbonate to marine carbon burial carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals an analysis of pore water profiles collected from marine sediments around the globe suggests that the precipitation of authigenic calcium carbonate accounts for around 10 of the carbonate that accumulates in marine sediments globally carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals1 2 the formation of calcium carbonate and its burial in marine sediments accounts for around 80 of the total carbon removed from the earth s surface however the fraction of calcium carbonate that precipitates in the oceans versus that which precipitates authigenically in marine sediments is unclear here we compile measurements of the calcium concentration of pore fluids collected at 672 seafloor sites around the globe to calculate the global flux of calcium within marine sediments we use these data combined with alkalinity measurements of pore fluids to quantify authigenic calcium carbonate precipitation we estimate that the net calcium flux into marine sediments that can be ascribed to authigenic carbonate precipitation amounts to around 1 1012 mol yr 1 as such we estimate that authigenic carbonate precipitation accounts for at least 10 of global carbonate accumulation we show that much of the precipitation occurs along the eastern margins of ocean basins where organic matter delivery to the sea floor is likely to be high we suggest that authigenic calcium carbonate precipitation represents a non negligible component of the global carbon cycle

venusian highlands geoid to topography ratios and their implications abstract using pioneer venus line of sight gravity data and orbit simulation procedures we have estimated apparent depths of isostatic compensation adcs for twelve venusian highland features asteria atla bell beta ovda phoebe tellus thetis and ulfrun regiones and nokomis gula and sappho montes adcs range from 50 km to 270 km half of the values are less than 100 km using these adcs we estimate geoid to topography ratios gtrs for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots swells and plateaus the geoid is estimated in the wavenumber domain from the isostatic formula using the topography and adc for each region in the space domain the gtr is equal to the least squares slope of the linear fit of the geoid to the topography the resulting gtr range is 7 31 m km which is much higher than terrestrial oceanic values 1 to 5 m km the features fall into two distinct groups one with a gtr range of 7 13 m km and one with a range of 19 25 m km the exception is beta regio which has a gtr of 31 m km a model for thermal thinning of a 100 km thick lithosphere fits all values in the lower gtr group to within one standard deviation airy compensation could also be present but cannot fully compensate these features partial dynamic compensation of the lower gtr group in combination with lithospheric mechanisms is also possible but not required to fit the data the upper gtr range 19 25 m km can be fit with an upper mantle constant viscosity convection model the large gtr values are inconsistent with the presence of a low viscosity zone if more than one compensation mechanism is present in the regions in the higher gtr group the gtrs will be underestimated in terms of a dynamic interpretation we thus fit the convection models to the upper end of the gtr range 25 m km rayleigh numbers in the range 10 4 10 6 will produce a gtr of 25 m km when combined with conductive lid thicknesses of 85 150 km the 6 m km range in both of the gtr groups is probably due to varying degrees of crustal and thermal compensation combined with dynamic compensation in the upper gtr group the difference between terrestrial and venusian gtr ranges can be explained largely by the lack of a low viscosity zone on venus

quantitative tests for plate tectonics on venus if plate tectonics exists on venus the rate of topographic dropoff from spreading centers should be about half that on earth due to greater rock fluid density contrast and lower temperature differential between the surface and interior using 1 1 mean elevations 21 000 km of ridge were identified on venus and 33 000 km on earth heights h at distances up to 1000 km to either side of the ridges were analyzed to make least squares determinations of the parameters ho h and n in where s is off crest distance the venus ridges have a much less well defined mode in crest heights ho than the earth ridges and a somewhat smaller exponent n on the average indicating greater concavity fixing n to the theoretical value of 0 5 plate velocities were inferred from the h s using h t from north atlantic and north pacific data on earth these velocity solutions were compared to those from plate rotations which indicate the h t is probably about 40 too high for a global mean the resulting plate creation rates on venus assuming comparable total heat generation and taking into account the density and temperature differences are 0 7 km yr or less and indicate that not more than 15 of venus s energy is delivered to the surface by plate tectonics in contrast to 2 9 km yr and 70 energy delivery on earth

paleomagnetic evidence for rotation of the iberian peninsula and the external betic cordillera southern spain abstract paleomagnetic investigations of jurassic cretaceous and tertiary sites in the external betic cordillera indicate that while the observed remanence vectors have inclinations which are conformable with the expected inclinations for stable iberia declinations deviate significantly these results indicate that 1 the mesozoic limestones of the penibetic have been rotated clockwise with respect to stable iberia and 2 there is a differential rotation between the cretaceous sites dec 61 11 and the jurassic sites dec 38 8 the differential rotation of 23 between the late jurassic and late cretaceous is consistent with the cretaceous anticlockwise rotation of the iberian peninsula with respect to eurasia it follows that during this time period this part of the betic mobile belt waspart of the continental margin of the iberian peninsula a study which was confined to a single unbroken stratigraphic section west of ronda indicates that most of the anticlockwise rotation 20 probably occurred between the cenomanian and the campanian the anticlockwise rotation of these rocks must have been followed by a clockwise rotation of at least 60 to account for the observed clockwise declination anomalies

testing the preservation potential of early diagenetic dolomites as geochemical archives early marine diagenetic dolomite is a rather thermodynamically stable carbonate phase and has potential to act as an archive of marine porewater properties however the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity here the archive potential of early marine dolomites exposed to extreme post depositional processes is tested using various types of analyses including petrography fluid inclusion data stable 13c and 18o isotopes 87sr 86sr ratios and u pb age dating of various dolomite phases in this example a triassic carbonate platform was dissected and overprinted diagenetic temperatures of 50 to 430 c in a strike slip zone in southern spain eight episodes of dolomitization a dolostone cataclasite and late stage meteoric vadose cementation were recognized the following processes were found to be diagenetically relevant i protolith deposition and fabric preservation and marine dolomitization of precursor aragonite and calcite during the middle late triassic ii intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a proto atlantic opening stress regime t ca 250 c during the early middle jurassic iii dolomite cement precipitation during early alpine tectonism rapid burial to ca 15 km and high grade anchizone overprint during alpine tectonic evolution in the early eocene to early miocene iv brecciation of dolostones to cataclasite during the onset of the carboneras fault zone activity during the middle miocene and v late stage regression and subsequent meteoric overprint data shown here document that under favourable conditions early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years evidence for this preservation includes preserved late triassic seawater 13cdic values and primary fluid inclusion data data also indicate that oversimplified statements based on bulk data from other petrographically complex dolomite archives must be considered with caution

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

bilateral topographic symmetry patterns across aphrodite terra venus western aphrodite terra venus is characterized by a series of parallel linear structural discontinuities 2000 4000 km in length and 100 200 km wide which strike at high angles to the general topographic trend of the aphrodite terra highlands the broad characteristics of the cross strike discontinuities csds are similar to both strike slip fault zones and terrestrial oceanic fracture zones in an effort to distinguish between these two hypotheses topographic profiles were taken across aphrodite terra to test for bilateral symmetry of the type associated with thermal boundary layer topography at divergent plate boundaries on earth in addition to a broad bilateral symmetry at a range of angles across aphrodite terra detailed bilateral symmetry is observed within domains between linear discontinuities in directions generally parallel to the strike of the discontinuities in addition within a domain the centers of symmetry of several profiles define a linear rise crest that is oriented normal to the bounding csds and terminates against them furthermore linear rise crests are offset in a right lateral sense along the csds between domains by 200 1200 km if the discontinuities represent strike slip movement then retrodeformation of the axes of symmetry along the csds to form a straight line should form a broadly symmetrical topographic high instead when the topography is retrodeformed the edges of the highlands are characterized by distinctive misfits of topography at the edge of the rise where it intersects the csds the nature and configuration of the csds the characteristics of the topography and topographic patterns including bilateral topographic symmetry generally parallel to csds within domains the existence of linear rise crests defined by the axes of bilateral symmetry within domains and the offsets of these linear rise crests between domains all suggest that western aphrodite terra on venus is similar to terrestrial oceanic divergent plate boundary environments and that the cross strike discontinuities are analogous to oceanic fracture zones rather than strike slip faults

triassic paleomagnetic data from the subbetic and the malaguide complex of the betic cordilleras southeast spain makel g h rondeel h e and vandenberg j 1984 triassic palaeomagnetic data from the subbetic and the malaguide complex of the betic cordilleras southeast spain tectonophysics 101 13i 141 the analysis of triassic paleomagnetic directions from the betic cordilleras in southeast spain indicates a considerable amount of relative displacement of the subbetic of the external zone of this orogen with respect to the iberian block the paleomagnetic pole positions of the malaguide complex of the internal zone of the orogen are closely related to those of the subbetic none of the poles coincide with european african or iberian poles of permo triassic age

the triassic palaeogeographic transition between the alpujarride and malaguide complexes betic rif internal zone s spain n morocco abstract various tectonic units situated between the malaguide and alpujarride complexes in the betic rif internal zone s spain and n morocco present triassic successions sharing features of both complexes the presence of which indicates a transitional domain these units which we term intermediate units are situated preferentially near the external limit of the betic rif internal zone generally adapted to the shape of the arc of gibraltar the palaeogeography of this area is reconstructed on the basis of the general geometric position of these units and their relationships with equivalent domains in the western mediterranean the scarce existing palaeomagnetic data and the directions of tectonic displacement the most problematic reconstructions concern those units on the n border of the betic rif zone as they have significant clockwise rotations up to 200 s and se tectonic displacements and a different geometric distribution from the rest of the units we conclude that in general the transition between the malaguide and alpujarride complexes was gradual the malaguide domain located s of the alpujarride with the intermediate units lying evidently between the two complexes

triassic continental rifting of pangaea direct evidence from the alpujarride carbonates betic cordillera se spain abstract evidence is presented for triassic rift related palaeo structures from the alpujarride complex carbonates of the betic cordillera se spain direct evidence of synsedimentary extensional tectonics is provided by macro to meso scale normal faults overstepped by younger strata most faults define domino like horst and graben structures the faults are associated with an expanded synrift sedimentary prism in which soft sediment deformation gravity flow deposits and unconformities are widely developed syntectonic mafic igneous intrusions also occur the age of this phase of extension is constrained as ladinian carnian c 237 216 ma from a palaeogeographical standpoint the thick triassic carbonates of the betic internal zone together with comparable successions in northern calabria southern apennines occupied a belt with elevated subsidence connecting the neo tethys to the east with the eastern north american intracontinental rift system to the west their carbonate facies intermediate between classical alpine and germanic type triassic facies recorded the main episodes of rifting affecting central pangaea

gravity driven deformation of the crust on venus intense horizontal deformation features are observed on the surface of venus despite the apparent absence of terrestrial style plate tectonics high surface temperature low erosion rate and regions of elevated terrain suggest that gravity spreading may be responsible for some of these features gravity spreading on the earth is known to cause extensive deformation in certain thin skinned tectonic regions where the uppermost layers become detached along a decollement while the high pore pressures and soft sedimentary rocks frequently observed along decollements on the earth are not plausible for venus temperature induced regions of ductile deformation may exist simple viscous models of gravity spreading above a ductile region of the crust are developed to examine the effects of rheology thermal gradient surface slope and crustal thickness on deformation for a reasonable range of parameters the models show that surface slope driven ductile deformation at relatively shallow depths in the crust is possible and may be capable of detaching the uppermost rigid portion of the crust resulting in horizontal translation

paleomagnetic rotations and the kinematics of the gibraltar arc paleomagnetic investigations of a mesozoic limestone sequence around the gibraltar arc show that there have been large systematic rotations about a vertical axis and imply that there must have been a significant component of westward motion within the betic rif orogenic belt rotations of the late jurassic and late cretaceous limestones in the betic cordillera of southern spain are clockwise with the exception of the unrotated sierra gorda whereas the jurassic sites in the rif mountains of morocco are rotated counterclockwise except in the area around tetuan these data are generally consistent with formation of the arc either by a westward moving alboran microplate or by extensional collapse of a collisional mountain chain the former model however fails to recognize the nonrigid nonplate like character of the alboran domain and lacks a definable driving mechanism a model of a collapsing east west elongated ridge that takes into account the obliquity of the convergence along the active margin may provide mechanisms both for the observed rotations and for the large westward component of motion and therefore can explain the geometry of the gibraltar are

palaeomagnetic rotations and fault kinematics in the rif arc of morocco palaeomagnetic and structural investigations in the rif arc of morocco indicate that there have been large rotations associated with a pattern of outwardly directed thrusting rock magnetic experiments in conjunction with thermal and alternating field demagnetization demonstrate that eight sites in jurassic and cretaceous limestones along the internal external boundary have a stable remanent magnetization this is in most cases rotated anticlockwise by as much as 100 from the expected mesozoic declination but in the tetuan area there are large clockwise rotations kinematic indicators from fault surfaces indicate the following 1 in the eastern rif there has been predominantly south directed thrusting partly overprinted by extensional and sinistral strike slip faults 2 at the eastern end of the n 70 e striking jebha fault zone there is a pattern of dominant sinistral ne trending and subsidiary dextral se trending strike slip faults overprinted by normal faults while at the western end deformation consists largely of south directed thrusting 3 in the northern section of the chain where the structural trends are dominantly n s thrusting is directed w to nw it is suggested that the data are best explained by differential motion and rotation of thrust sheets during outwardly directed thrusting around the arc

geologic evolution of the betic cordilleras in the western mediterranean miocene to the present abstract the betic cordilleras were formed during the eo meso and neo alpine evolution of the western mediterranean area during the neo alpine stage the northward subduction of the african plate took place and the algerian provencal basin opened with the creation of oceanic floor associated space problems produced the expulsion and extensive stretching of the internal zones to the west which in turn disrupted and deformed the external zones of the betic and rif cordilleras these occurrences essentially took place in the burdigalian and continued with diminished intensity into the middle miocene following the suturing of the transcurrent contact between the internal and external zones important n60 90 e and nw se faults occurred the betic cordilleras are characterized as having been deformed by regional horizontal shortening compression oriented wnw ese or nw se which gradually rotated to the nnw sse in the late miocene when its most important intramontane neogene basins were formed the alboran basin with a thinned continental crust was formed as a western prolongation of the opening of the algerian provencal basin with its basic features already apparent in the burdigalian the volcanism present in the southeast of the betic cordilleras and in northeast morocco crossing the alboran basin is mainly late miocene in age it is linked to fracturing and exhibits chemistry related to the different thickness of the crust

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

effects of lateral resolution on the identification of volcanotectonic provinces on earth and venus topographic data sets for the continental united states and the north pacific have been degraded and displayed in the same manner as the pioneer venus altimeter data for venus results for the u s data show that the cordillera orogenic zone would be seen as a broad swell with a height of 2 km and a wavelength of about 2500 km individual structural elements within the cordillera are reduced to mild height about 700 m wavelength about 500 km swells that are superimposed on the broad cordillera rise for the pacific data the hawaiian swell the e w trending fractures in the eastern pacific the increase in depth away from the east pacific rise and most trenches can still be discerned the venusian data do not show the integrated trench transform and ridge system evident in the degraded pacific data these features would be discernible in the degraded pacific data even after correcting depths for a the higher surface temperature on venus and b the lack of loading due to ocean water thus a plate tectonic regime similar to earth s does not appear to currently exist on venus unfortunately as shown by the degraded cordillera data the pv altimetry data are not of sufficient quality to discern whether or not the highlands of venus preserve evidence for orogenic events related to an ancient epoch of plate tectonics

palaeomagnetic rotations in the eastern betic cordillera southern spain palaeomagnetic declinations from the external zones of the eastern betic cordillera southern spain and an adjacent area of the internal zone indicate variable and locally very large clockwise rotations the rotations occurred after latest oligocene times and probably before the late miocene this overlaps a period of dextrally oblique convergence during the early to middle miocene along the internal external zone boundary and within the external zone rotations in the external zone are recorded by palaeomagnetic results from 27 sites in upper jurassic ammonitico rosso facies limestones from the subbetic zone and one site of a similar age from the prebetic the sites show either two or three component behaviour the low temperature component is coincident with the present field direction the intermediate temperature component was probably acquired during miocene folding the high temperature component passes fold tests miocene age and a conglomerate test eocene early oligocene age palaeomagnetic declinations from individual tectonic blocks in the subbetic are consistent but indicate large differential rotations between blocks these blocks are underlain by low angle thrust faults which probably accommodated much of the rotation the largest rotations occur on relatively small isolated blocks of jurassic carbonates in a highly deformed triassic evaporite sequence the single site in the prebetic has not rotated significantly relative to stable iberia in malaguide rocks of the sierra espun a in the adjacent internal zone stable palaeomagnetic components were measured at one site in upper miocene sedimentary rocks at three sites in upper oligocene lower miocene red marls at one site in oligocene marls at one site in jurassic limestones and at two sites in permo triassic red beds the palaeomagnetic results suggest that about 60 of clockwise rotation occurred in the latest oligocene earliest miocene and a further 140 of clockwise rotation subsequently one site in late miocene sedimentary rocks yields unrotated declinations which if not representing an overprinted direction indicate that the rotation was complete by the end of the miocene

mechanisms for lithospheric heat transport on venus implications for tectonic style and volcanism the tectonic and volcanic characteristics of the surface of venus are poorly known but these characteristics must be closely related to the mechanism by which venus rids itself of internal heat on the other solid planets and satellites of the solar system lithospheric heat transport is dominated by one of three mechanisms 1 plate recycling 2 lithospheric conduction and 3 hot spot volcanism we evaluate each mechanism as a candidate for the dominant mode of lithospheric heat transfer on venus and we explore the implications of each mechanism for the interpretation of venus surface features despite claims made to the contrary in the literature plate recycling on venus cannot be excluded on the basis of either theoretical arguments or present observations on topography and radar backscatter landforms resulting from plate convergence and divergence on venus would differ substantially from those on the earth because of the high surface temperature and the absence of oceans on venus the lack of free or hydrated water in subducted material the possibility that subduction would more commonly be accompanied by lithospheric delamination and the rapid spreading rates that would be required if plate recycling removes a significant fraction of the internal heat if plate recycling occurs on venus the rolling plains and lowlands provinces would be approximate analogs to terrestrial ocean basins in terms of age igneous rock type and formative process highlands on venus would be roughly analogous to terrestrial continents the hypothesis that lithospheric conduction dominates shallow heat transfer on venus leads to the prediction that the lithosphere is thin if venus has a global heat loss per mass equal to that for earth then temperatures marking the base of the thermal lithosphere on earth would be reached on venus at an average depth of about 40 km unless the mantle convective planform can maintain lithospheric regions of persistently low heat flow or unless the present atmospheric greenhouse on venus is geologically recent then such a lithospheric thickness leads to the conclusion that the topographic features contributing to the 13 km of relief on the planet must be geologically young the hypothesis that hot spot volcanism dominates lithospheric heat transfer on venus leads to the prediction that the surface must be covered with numerous active volcanic sources in particular if a typical venus hot spot has a volcanic flux equal to the average flux for the hawaiian hot spot for the last 40 m y then 104 such hot spots are necessary to remove the venus internal heat by volcanism such a number would produce enough volcanic material to resurface the entire planet to a depth of 1 km every 2 m y few areas of the planet would escape resurfacing for geologically long periods of time we find that none of the mechanisms for lithospheric heat transfer on venus can be excluded as unimportant at present it is likely that as on earth a combination of mechanisms operates on venus the strongest conclusion to emerge from this evaluation is that most of the major topographic features and probably many of the surface geological units on venus are young by comparison with the surfaces of the smaller terrestrial planets

widespread neogene remagnetization in jurassic limestones of the south iberian palaeomargin western betics gibraltar arc abstract palaeomagnetic investigations in upper jurassic carbonate rocks from the western subbetics betic cordillera southern spain testify to a widespread neogene remagnetization progressive thermal and alternating field demagnetization analyses reveal the presence of two stable components of the natural remanent magnetization nrm the nrm is dominated by a pervasive neogene overprint which always shows normal polarity and maximum unblocking temperatures of 450 c throughout the whole region studied the primary jurassic component however can also be isolated in many outcrops this component has low intensity values and maximum unblocking temperatures of about 550 c showing both normal and reversed polarities palaeomagnetic and rock magnetic analyses indicate that both primary jurassic and secondary neogene components are carried by magnetite the incremental fold test performed in all sampling localities demonstrates that the remagnetization occurred during some stage of the neogene deformation of the subbetics being pre syn or post folding in the various folds studied

triassic redbeds in the malaguide complex betic cordillera spain petrography geochemistry and geodynamic implications abstract sandstone petrography and mudstone mineralogy and geochemistry of triassic mudstones and sandstones from continental redbeds of the malaguide complex betic cordillera southern spain provide useful information on provenance palaeoclimate and geodynamics during the early stages of the pangea break up and on their diagenetic evolution the sandstones are quartzarenites to sub litharenites with minor lithic fragments and rare feldspars the mudstone samples show a paas like elemental distribution the samples likely record recycling processes from their metasedimentary basement rocks that significantly affected the weathering indices and monitors cumulative effects including a first cycle of weathering at the source rocks sandstone composition and chemical mineralogical features of mudstones record a provenance derived from continental block and recycled orogen that were weathered under warm and episodically wet climate source areas were located towards the east of the present day malaguide outcrops and were formed by fairly silicic rock types made up mainly of palaezoic metasedimentary rocks similar to those of the paleozoic underlying series with subordinate contributions from magmatic metamorphic sources and a rare supply from mafic metavolcanic rocks clay mineral distribution of mudstones is dominated by illite and illite smectite mixed layer that result from differences in provenance weathering and burial temperature history illite crystallinity values illitization of kaolinite occurrence of typical authigenic minerals and apatite fission track studies coupled with a subsidence analysis of the whole malaguide succession suggest burial depths of at least 4 6 km with temperatures of 140 160 c typical of the burial diagenetic stage and confirm the middle miocene exhumation of the betic internal domain tectonic stack topped by the malaguide complex

ammonium vermiculite in schists from the betic cordillera spain nh 4 bearing phases from pre permian schists of the internal zones of the betic cordillera spain have been characterized by x ray diffraction differential thermal analysis and thermogravimetry infrared spectroscopy and transmission analytical electron microscopy the content and distribution of nh 4 in these phases have been estimated by combining elemental analysis of n with mineral quantification based on x ray diffraction whole rock composition and electron microprobe data the obtained results indicate that the nh 4 content in vermiculite ranges between 0 5 and 1 0 wt leading to terms with nh 4 k in the interlayer the nh 4 content is 4 contents 0 04 wt through the complete sequence we interpret that initially contained in low grade dioctahedral mica nh 4 was incorporated in the structure of the trioctahedral phases formed at increasing metamorphic conditions the entry of nh 4 in the interlayer was favored in chlorite vermiculite mixed layered structures and transitional phases with vacancies in the a site vermiculite and weak cohesion between layers nh 4 appears to favor the formation of intermediate phases instead the direct transformation of chlorite to biotite

electron backscatter diffraction based identification and quantification of diamonds from the rif gneisses spain and morocco economic implications microdiamonds from granulite facies gneisses of the northern rif spain and morocco have been studied by electron backscatter diffraction ebsd diamond identification using other techniques such as raman spectroscopy is hindered by microdiamond size and by the presence of graphite bands in contrast ebsd enabled the following 1 the rapid identification of diamonds and their distribution 2 the estimate of the diamond concentration and 3 the analysis of the crystallographic relationships between diamonds and the surrounding phases ebsd study has revealed that 1 diamonds are concentrated mainly in large mesoperthite and quartz coesite grains 2 the diamond concentration in the rocks is notably higher than that estimated on the basis of diamond concentration in garnet and 3 clear orientation relationships exist among diamonds coesite and quartz these data would not have been revealed using classical methods since most diamonds can be recognized by optical microscopy only when ebsd maps have previously revealed their location the high diamond grades determined in this preliminary study suggest that these deposits could be of economic interest