relationship of mineralogy to depositional environments in the non marine tertiary mudstones of the southwestern ebro basin spain abstract during the middle late eocene oligocene and miocene a non marine succession about 5000 m thick was deposited in the western ebro basin in the northern and southern margins of the basin conglomerate deposits of alluvial fan origin were deposited towards the centre of the basin there is a lateral change to flood plain sandstones and mudstones and finally carbonate and evaporitic lacustrine deposits two types of saline lakes where evaporitec precipitation occurred can be distinguished central saline lakes of high salinity and marginal saline lakes located near the southern margin with lower salinity than the central ones x ray diffraction tem sem and edx analysis were carried out on mudstones from central and southern deposits the clay minerals identified were illite chlorite kaolinite mg rich smectite palygorskite and mixed layers illite smectite and chlorite smectite apart from clays carbonates calcite dolomite and magnesite and other silicates quartz plagioclase k feldspar and analcime were recognised the palygorskite mg rich smectite analcime dolomite and magnesite are of diagenetic origin the other minerals are detrital and derived from the surrounding mountain chains the clay mineral assemblage of each environment is related to the diverse various source areas iberian range and pyrenees and their compositional variation through time as well as to diagenetic processes on the basis of clay mineral distribution the detrital clay mineral assemblages are preserved in the alluvial fans the flood plain without saline influence and the central saline lakes in the saline flood plains and marginal saline lakes diagenetic palygorskite and mg rich smectites were formed in the marginal saline lakes the formation of magnesium clays prevailed but in the central saline lakes where no clay mineral diagenesis is observed the magnesium precipitated as dolomite and magnesite

spring related carbonate rocks mg clays and associated minerals in pliocene deposits of the amargosa desert nevada and california the amargosa desert of nevada california is a spring fed structural basin containing large amounts of pliocene carbonate rocks and mg clays deposited in playas marshland ponds and flood plains pliocene basin fill deposits dated at 2 4 to 3 2 ma are subdivided into three lithofacies designated tpa tld and tpl tpa is chiefly limestone and was deposited in marshland and ponds tld consists chiefly of limestone and montmorillonite claystone that were deposited in varied environments including ponds and flood plains tpl is almost entirely mg clays limestone and dolomite which were deposited in playas and associated marshland the largest area of playa sediments termed east playa is in the east central part of the basin limestone and dolomite seepage mounds and caliche breccia musses were formed along zones of ground water leakage within east playa chemically precipitated mg smectite and sepiolite form large deposits in tpl sepiolite was formed in water of low salinity whereas mg smectite was formed in water of higher salinity authigenic celadonitic illite and k feldspar are commonly associated with mg smectite and are very likely reaction products of montmorillonite in saline alkaline water silicic vitric ash layers were locally altered to phillipsite and k feldspar in saline alkaline water and to montmorillonite as a result of leaching by water of low salinity carbonate rocks are locally silicified along seepage zones carbonate minerals of the pliocene deposits have stable isotopic compositions that vary with their lithofacies tpa limestones deposited in or near areas of spring discharge have low 18o values averaging 16 6 tld limestones recrystallized in the vadose zone have higher 18o values averaging 18 1 carbonate minerals of tpl are the most 18o rich with calcite averaging 20 3 and dolomite 24 3 reflecting evaporative concentration in playas much of the dolomite is of replacement origin yet is in approximate isotopic equilibrium with the calcite it replaces relatively pure sepiolite has lower 18o values than do mg smectite clays which fits with the lower salinity inferred for sepiolite precipitation sepiolite and mg smectite disseminated in limestone fall outside the oxygen isotopic limits of the relatively pure clays possibly because of diagenetic recrystallization the pliocene climate was wetter than that of the present and springs were more widespread in the amargosa basin this dolomite may have formed in water isotopically and chemically about the same as that of the calcite and dolomitization may have been chiefly a result of the differing crystallization kinetics of calcite and dolomite and of fluctuating water chemistry in a playa environment the saline alkaline water of east playa resulted from evaporation of a mixture of alkali bicarbonate runoff from the volcanic terrane to the north with ca mg bicarbonate water discharged in springs from the ash meadows ground water system whose recharge area is principally in paleozoic carbonate rocks to the east and northeast seepage along fault zones in east playa was initially of ash meadows type low in silica and changed to siliceous water derived from volcanic rocks resulting in silicification of carbonate deposits caliche breccias along the western margin of the basin record a similar change in the composition of ground water seepage which may be attributed to a period of decreased precipitation about 2 5 ma which reduced recharge from nearby carbonate rocks relative to that from more distant volcanic rocks to the north and northwest

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

a model of early calcite cementation in alluvial fans evidence from the burdigalian sandstones and limestones of the valles penedes half graben ne spain abstract the valles penedes half graben developed during a neogene extensive period as part of the catalan coastal ranges in the northwestern edge of the valencia trough the neogene deposits of the valles penedes half graben consist of from base to top three lithostratigraphic complexes i a lower continental complex of aquitanian early langhian age ii a continental to marine complex with reefal carbonate platforms of langhian age and iii an upper continental complex of middle serravallian tortonian age this study focuses on the calcite cements of the lower continental complex consisting of red beds mudstones sandstones and conglomerates and lacustrine limestones deposited in alluvial fan environments the studied materials are cemented by calcite precipitated from meteoric waters that circulated through the sediments during the early diagenesis at shallow burial depths the calcite cement was studied by means of petrographic cathodoluminescence microprobe and 18o and 13c isotopic analysis the petrographic and geochemical results show that these cements vary according to the sedimentary environment i sandstones deposited in proximal alluvial fan environments cemented by calcite with low contents of mg fe mn sr and na 13c values of 7 4 pdb and 18o values from 6 5 to 6 2 pdb ii sandstones sedimented in medium to distal alluvial fan environments cemented by mn rich and mg fe sr and na poor calcite with 13c values from 7 9 to 6 9 pdb and 18o values from 10 5 to 8 6 pdb and iii lacustrine limestones of distal alluvial fan environments cemented by fe rich calcite with variable contents of mn and low contents of mg sr and na the distribution of these calcite cements allows us to propose a model of fluid circulation and early calcite cementation within an alluvial fan indicating spatial and temporal variations in chemical composition of meteoric water during migration from proximal to distal alluvial fan environments residence time of waters eh ph conditions and the degree of sediment water interactions influenced by the mineral composition of the host sediments are the most important parameters controlling variations in calcite cement composition

palaeoenvironmental and diagenetic control of the mineralogy of upper cretaceous lower tertiary deposits of the central palaeo andean basin of bolivia potosi area abstract a study involving mainly x ray diffraction of the mineralogy of upper cretaceous and lower tertiary deposits aroifilla chaunaca el molino santa lucia and cayara formations of the andean basin in the bolivian eastern cordillera is presented this work refines environmental reconstructions based on previous sedimentological studies in the potosi basin the results including the clay mineralogy show that the sediments were deposited in a lacustrine system which suffered large variations of water level and groundwater influence the siliciclastic sediments are characterized either by abundant feldspars or analcime suggesting a perennial volcanoclastic influence tuff layers in the santa lucia formation support this interpretation the great diversity of clay minerals identified reflects both detrital influences and diagenetic overprinting although diagenetic changes are common in this sedimentary series it is still possible to identify the nature of detrital supply illite and illite smectite mixed layer clays constitute the background clay sedimentation and reflect predominantly arid climate this had already been deduced from sedimentogical and geochemical investigations however detrital kaolinite occurs in a 40 m thick interval of the el molino formation which probably embraces the k t boundary and reflects more humid conditions further work is necessary to locate more precisely this boundary in the absence of accurate biostratigraphical markers authigenic clays are also recognized in the sandstones in the basal part of the cayara formation kaolinite developed authigenically in the tuff layers of the santa lucia formation well crystallized authigenic smectite characterizes the clay fraction in addition chlorite replaces smectite in limestones beds of the el molino formation which were deposited in evaporitic environments the clay assemblages including illite and chlorite at base of the stratigraphic column aroifilla and chaunaca formations probably results from the replacement of smectitic minerals either by chlorite or illite where depth of burial reached more than 2000 m

mechanisms of uplift preceding rifting abstract mareschal j c 1983 mechanisms of uplift preceding rifting in p morgan and b h baker editors processes of continental rifting tectonophysics 94 51 66 the feasibility of three mechanisms that have been proposed to account for the uplifts and heat flow anomalies which might precede rifting is examined the conductive heating following an increase in the heat flux at the base of the lithosphere could result in large scale uplifts 1000 m for 15 mw m2 change in heat flux on a time scale of 108 years a pulse of additional heat flow could induce a more rapid uplift but not the surface heat flow anomaly which is always delayed the advection of heat by magma intrusion into the lithosphere could also produce the uplift and heat flow anomaly if magmas are injected at a steady rate an uplift of the order of 1000 m can be induced in about 50 106 years but it requires the rate of magma injection to be equivalent to 0 3 of the total volume of the lithosphere per 106 years a short intense episode of magma invasion could explain a rapid uplift and change in surface heat flow but it requires the replacement of about 30 of the total volume of the lithosphere the complete replacement of the lithosphere by the rise of a diapir of lighter and hotter asthenospheric material has been modelled numerically the model indicates that the rise of the asthenospheric diapir can take place in a very short time 3 106 years and produce the surface uplift and the crustal thinning observed in rift systems provided that following heating the lower lithosphere viscosity is reduced to 1021 pa s

authigenic trioctahedral smectites controlling ph alkalinity silica and magnesium concentrations in alkaline lakes abstract the mineralogical composition of salts and the chemical composition of brines of lake yoa ounianga el kebir northern chad both reveal the neoformation of stevensite a trioctahedral smectite accompanied by aragonite the coprecipitation of these two minerals clearly controls the evolution of ph alkalinity and ca mg and silica concentrations in alkaline brines the geochemical evolution characterized by a strong depletion of ca and mg in solution and an increase in alkalinity and ph is confirmed as the dominant process governing the formation of soda alkaline lakes

the elat aqaba dead sea jordan subgraben system abstract the elat dead sea jordan rift zone is divided into distinct subgrabens these subgrabens are separated from one another by pivotal uplifted areas the subgrabens are usually asymmetric with the sense of asymmetry changing from west to east and vice versa along the length of the rift zone most faults are meridional n s trending or submeridional nne ssw trending transverse faulting is extremely rare thus opposing the key stone theory and also the fashionable rhomb chain model our levantine chain of subgrabens are but an infra pleistocene initial stage in the afro arabian rift system

tiran straits the transfer zone between the red sea and the gulf of elat and its tectonic significance abstract a precipitous submarine horst is located between sinai peninsula and tiran island but also between hume deep 1400 m in northernmost red sea and tiran deep 1400 m in the southernmost gulf of elat the horst outcrops above sealevel in places where corals built small reefs trending nne ssw the horst forms a structural transfer zone trending nne ssw and constrained by series of normal faults on both flanks transfer zones were repeatedly encountered to separate between axial grabens along regional rifts in east africa and elsewhere while similar structures were observed in analog models transfer zones between grabens along axial rifts were attributed to oblique extension consequently the occurrence of the tiran transfer zone could indicate that geodynamic regime of oblique extension prevails in the northern red sea and exists also in elat rift tiran horst is structurally associated with the arcuate system of grabens and elongated salt diapirs that were encountered in the bathyal northern red sea which swings from the red sea nw se orientation to the n s trend of elat rift the age of the initiation of the rifting and the contemporaneous ascent of its margin was attributed to late miocene early pliocene as indicated by extensive series of coralline terraces and by non branching fluvial system on tiran island the research did not encounter supporting evidence that the elat rift is a transform fault that branched off the red sea during the early middle miocene nor that tiran deep is a pull apart basin

modes of continental lithospheric extension experimental verification of strain localization processes abstract three modes of continental lithospheric extension narrow rift wide rift and core complex were studied by means of analogue experiments the experimental method is based on models which simulate an idealized lithospheric strength profile and are scaled for gravity various styles of lithospheric extension were induced by systematic variation of brittle upper crustal thicknesses narrow rifting occurs during stable lithospheric extension when the resistance of the brittle upper crust dominates the deformation process wide rifting results from unstable lithospheric extension when the brittle upper crust is thin and ductile flow in the lower crust allows expansion of the deformation zone periodic instabilities with two wavelengths dominate wide rifting core complex mode extension is transitional between unstable and stable lithospheric extension it begins as a distributed zone of unstable extension due to thin upper brittle crust but lower crustal flow induced by the gravity force causes a relocalization of strain into narrow zones transitions between the three modes of extension are characterized in terms of competition between the strength of lithosphere and gravity forces as well as between stable and unstable deformation processes

the rift to break up evolution of the gulf of aden insights from 3d numerical lithospheric scale modelling abstract the gulf of aden provides an ideal setting to study oblique rifting since numerous structural data are available onshore and offshore recent surveys showed that the spatio temporal evolution of the gulf of aden rift system is dominated by three fault orientations displacement orthogonal wsw rift parallel wnw and an intermediate e w trend the oldest parts of the rift that are exposed onshore feature displacement orthogonal and intermediate directions whereas the subsequently active necking zone involves mainly rift parallel faults the final rift phase recorded at the distal margin is characterised by displacement orthogonal and intermediate fault orientations we investigate the evolution of the gulf of aden from rift initiation to break up by means of 3d numerical experiments on lithospheric scale we apply the finite element model slim3d which includes realistic elasto visco plastic rheology and a free surface despite recent advances 3d numerical experiments still require relatively coarse resolution so that individual faults are poorly resolved we address this issue by proposing a simple post processing method that uses the surface stress tensor to evaluate stress regime extensional strike slip compressional and preferred fault azimuth the described method is applicable to any geodynamic model and easy to introduce our model reproduces the observed fault pattern of the gulf of aden and illustrates how multiple fault directions arise from the interaction of local and far field tectonic stresses in an evolving rift system the numerical simulations robustly feature intermediate faults during the initial rift phase followed by rift parallel normal faulting at the rift flanks and strike slip faults in the central part of the rift system upon break up displacement orthogonal as well as intermediate faults occur this study corroborates and extends findings from previous analogue experiments of oblique rifting on lithospheric scale and allows new insights in the timing of fault successions of the gulf of aden and continental rifts in general

oblique slip deformation in extensional terrains a case study of the lakes tanganyika and malawi rift zones the east african rift system ear is the archetypal continental rift and a widely proposed analogue for the early stages of evolution of passive continental margins the three dimensional structure of parts of the ear has been recently elucidated by a multifold seismic mfs survey of lakes tanganyika and malawi project probe analysis of fault geometries displayed on the probe mfs data coupled with the more extensive 28 khz echosounder data has improved understanding of the geometry and kinematics of the linked fault systems that underlie the lakes in particular it has been recognized that profiles across i e at high angles to the rift elongation commonly display fault geometries that are not readily retrodeformable there must therefore be significant displacement out of the plane of what would conventionally be regarded as dip lines that is perpendicular to rift elongation and parallel to the tectonic transport or extension direction careful determination of fault plane dip and the dip of synrift reflectors in the hanging wall demonstrates that the dip directions of the shallowest faults and the steepest hanging wall sediment dips are oriented either nw or se this defines the direction of maximum fault block rotation and therefore the direction of extension or tectonic transport as nw se oblique to the trend of the regional rift axis and to the apparent strike of many of the major border faults in the survey area the border faults and many other faults imaged on the probe mfs data must therefore have significant components of strike slip motion and the tanganyika and malawi rift zones have undergone extension oblique to and not perpendicular to their axes near dip slip normal faults steeply dipping oblique slip faults and subvertical strike slip or transfer faults have all been recognized in a complex linked fault system that accomplishes the extension the overall orientation of the rift lakes and their internal segmentation are influenced by major preexisting subvertical fault zones within the basement complex accommodation zones act principally as transfer zones that allow switches in gross polarity of the border fault system the detailed geometries of the accommodation zones result from the specific relations between juxtaposed half graben and the strike and internal geometry of the influencing basement structure the variations in fault geometry subsidence water depth and basin or rift morphology can be better explained by an oblique slip extensional model influenced by basement structures than by a simple orthogonal extensional model

rift propagation in rotational versus orthogonal extension insights from 4d analogue models abstract in rift settings extension rates often vary along strike due to rotation about a vertical axis or euler pole yet tectonic modelers traditionally apply constant along strike deformation rates here we compare rift development and propagation under traditional orthogonal extension versus rotational extension conditions the set ups involve brittle viscous layering and localize deformation through structural weaknesses seeds our models provide first order insights into the differences in rift development between both boundary conditions orthogonal extension produces a rift basin with constant synchronous along strike features whereas rotational extension induces along strike structural gradients diachronous rift development causing rift propagation and the development of v shaped basins we observe important viscous flow associated with differential pressure gradients in rotational extension we also describe the important effects of strain partitioning between rift axis and model boundaries the quantifying of which is crucial to avoid incorrect model interpretations although our model results are first order only they are in good agreement with various natural examples and previous modeling studies and highlight the importance of considering the third dimension when studying tectonic systems

kinematics of the mw 7 2 12 november 1999 duzce turkey earthquake the november 12 1999 duzce earthquake ruptured a 40 km long fault segment of the north anatolian fault system immediately to the east of the august 17 1999 izmit rupture we use displacements of 32 sites derived from gps measurements immediately before and after the duzce earthquake to estimate the geometry and slip distribution of the coseismic rupture the 51 northward dipping rupture plane the rake of the slip vector average 3 76 0 04 m right lateral 0 76 0 04 m normal slip and the slip distribution inferred from the gps data are consistent with seismic observations and the distribution of surface offsets measured in the field the geodetically determined moment magnitude is mw 7 2 the duzce earthquake had the highest slip to rupture length ratio of any historic earthquake along the north anatolian fault this is consistent with the duzce earthquake being a part of a composite rupture with the preceding izmit event

geometry of propagating continental rifts reconstruction of past plate configurations through palinspastic restoration of continental margins1 4 requires a detailed knowledge of the geometries of horizontal continental extension extension is often restricted to discrete linear zones termed rifts5 6 and rift models that invoke propagating vertical dykes and cracks7 10 are useful for assessing the relative significance and timing of uplift subsidence and extension in the tectonic evolution of rift basins however many models do not adequately account for the observed asymmetry of rifts i describe here the general three dimensional character of young and aborted continental rifts which can be used to derive a structural model for the propagation of rifts in continental lithosphere the rifts become asymmetric as a consequence of the role played by low angle normal faults in the overall rift geometry

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

interactions between propagating rotational rifts and linear rheological heterogeneities insights from three dimensional laboratory experiments the lateral propagation of rifts is a consequence of the relative divergence of lithospheric plates about a pole of rotation modern and ancient examples of rifts are known to overprint pre existing linear anisotropies in the crust and lithosphere such as lithospheric boundaries crustal sutures and thermal anomalies here we investigate how propagating rifts interact with pre existing structures by using three dimensional analogue experiments with rotational extensional boundary conditions and variably oriented linear weak zones in the lithospheric mantle when linear weaknesses are oriented at low angles to the rift axis early strain localization occurs in narrow domains which merge at later stages resulting in continental break up by unzipping strong strain partitioning is observed when the linear heterogeneity is oriented at high angles with respect to the rift axis in these experiments early sub parallel v shaped basins propagate toward the pole of rotation until they are abandoned and strain is transferred entirely to structures developed in the vicinity of the strongly oblique weak lithosphere zone boundary the experimental results are characterized in terms of their evolution patterns of strain localization and surface topography as a function of the lithospheric heterogeneity obliquity angle comparison of the experiments to ancient and modern examples in nature may help to elucidate the common but still poorly understood process of propagating rift lithospheric heterogeneity interaction

glauberite halite association of the zaragoza gypsum formation lower miocene ebro basin ne spain glauberite is the most common mineral in the ancient sodium sulphate deposits in the mediterranean region although its origin primary or diagenetic continues to be a matter of debate a number of glauberite deposits of oligocene miocene age in spain display facies characteristics of sedimentologic significance in particular those in which a glauberite halite association is predominant in this context a log study of four boreholes in the zaragoza gypsum formation lower miocene ebro basin ne spain was carried out two glauberite halite lithofacies associations a and b are distinguished association a is composed of bedded cloudy halite and minor amounts of massive and clastic glauberite association b is made up of laminated to thin bedded clear macrocrystalline massive clastic and contorted lithofacies of glauberite and small amounts of bedded cloudy halite transparent glauberite cemented by clear halite as well as normal graded and reverse graded glauberite textures are common this type of transparent glauberite is interpreted as a primary subaqueous precipitate gypsum thenardite or mirabilite are absent in the two associations the depositional environment is interpreted as a shallow perennial saline lake system in which chloride brines association a and sulphate chloride brines association b are developed the geochemical study of halite crystals bromine contents and fluid inclusion compositions demonstrates that conditions for co precipitation of halite and glauberite or for precipitation of na sulphates mirabilite thenardite were never fulfilled in the saline lake system

the dead sea rift from continental rift to incipient ocean abstract the dead sea rift is a 600 km long approximately 20 km wide geomorphological depression showing a topographic relief of 1 000 to 3 000 m from the axial depression to the mountainous margins in many places the rift is the northern extension of the red sea young oceanic spreading centre and it is built of a series of internal basins the rift is the product of vertical displacements that downfaulted its floor and uplifted its margins but some sinistral strike slip offset affected the rift as well the tectonic evolution of the rift started probably in the pliocene subsequent to the termination of the intensive tectonic activity in the suez rift the transition of the tectonic activity from suez rift to the dead sea rift is due to a probable clockwise jump of the northern extension of the red sea spreading centre the dead sea rift with its internal basins and faulted precipitous escarpments presents an exceptional example for the earliest stages of the evolution of continental break up and of nascent passive continental margins tectonic interpretations of the voluminous geological and geophysical data from the rift and its margins are complicated because the information is equivocal part of the data suggests that the rift is an extensional structure whereas another part indicates that the rift is a product of regional sinistral strike slip faulting however it is suggested that the equivocal data are not necessarily conflicting because the evolution of the rift is probably associated with oblique resultant displacement of combined normal and strike slip faulting the quantitative ratio between the extensional and the lateral offset processes in the dead sea rift is controversial there is ground to presume that large downfaulting of the rift floor and intensive erosion of its uplifted margins removed and obscured critical data that could have provided reliable correlation between the eastern and western flanks nevertheless reconstruction of mesozoic depositional fades zones across the rift apparently indicates that the lateral displacements are approximately 10 km and seismic reflection profiles suggest vertical offsets reaching 7 km the series of secondary internal basins in the rift that seem equivalent to the linear system of axial basins in the northern red sea support the concept that the dead sea is under a tectonic regime that is comparable to the one that affected the red sea in the miocene these findings and the 25 to 50 reduction of crustal thicknesses underneath the rift indicate an incipient oblique oceanic spreading centre

control of the rheological structure of the lithosphere on the inward migration of tectonic activity during continental rifting analogue models are used to analyse the parameters controlling the evolution of extensional deformation in continental rifts models are deformed in a centrifuge and simulate the continental lithosphere floating and extending above a low viscosity asthenosphere model results reproduce the typical evolution of deformation during continental narrow rifting with early activation of large boundary faults and basin subsidence followed by their abandonment and localization of tectonic activity in internal faults near the centre of the rift the experiments document the strong influence exerted by the thickness of both brittle and ductile crustal layers and syn rift sediment accumulation on the evolution of deformation namely on the amount of bulk extension preceding inward fault migration thin upper and or lower crust and absent or low syn rift sedimentation promote a rapid abandonment of boundary faults and a transition to in rift fault development for low amounts of extension conversely thick upper and or lower crust and high syn rift sediment accumulation favour prolonged slip on boundary faults and delayed development of internal faulting the experimental results suggest that the inward migration of faulting during extension of continental lithosphere results from the interplay between the ductile stresses acting at the base of the upper crust and the total resistance of this brittle layer

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

sedimentary processes related to the groundwater flows from the mesozoic carbonate aquifer of the iberian chain in the tertiary ebro basin northeast spain abstract in this work the importance of groundwater in the formation and evolution of evaporitic lacustrine facies in the iberian chain and ebro basin contact spain is studied there are outcrops of geological materials from palaeozoic to quaternary times these materials have been classified into eight hydrostratigraphic units the jurassic carniolas and limestones in the iberian chain hu 4 and 5 are the materials which have the best hydraulic properties for underground water catchment and for forming the regional aquifer the triassic gypsum and marl of hu 3 form the impermeable substratum of the overlying jurassic and cretaceous carbonated aquifers and they are therefore the impermeable base of the aquifers being studied the equipotential line map of the regional aquifer shows the pattern of the groundwater flowing to specific points which mark the springs producing the underground drainage of the iberian chain the volume of water throughout the aquifer is currently evaluated at approximately 250 hm3 year which discharge through springs with high flowrates or through diffuse discharges in the riverbeds and in wetlands saline lakes the water discharged in these springs has a high mineralisation with a dry residue of over 1000 mg l calcium sulphate compositions dominate originating in the presence of soluble anhydrous materials within the lias and keuper formations the current sedimentation in relation to the groundwater flows from the iberian chain can only be found in the areas of diffuse discharge where the evaporitic sedimentation can be observed because of the frequently endorheic character on the right bank of the ebro river more than 60 depressions are known where very mineralised lakes form these are locally referred to as saladas during the miocene the hydrogeological functioning would be similar to the present one leaving the groundwater and the dissolved salts in a non marine basin they would therefore accumulate in large areas of diffuse discharge creating lakes where the evaporites would precipitate the deep regional groundwater flows would ascend to the surface from the overthrust lamina fronts going through thick tertiary alluvial deposits in the ebro basin and therefore the discharges were mainly diffuse

new observations of the shallow seismic structure of young oceanic crust the results are presented of three new experiments carried out on the mid atlantic ridge mar near latitude 23 n using a unique method of studying the seismic structure of the uppermost few hundred meters of the oceanic crust the data were collected using a fixed ocean floor hydrophone receiver and a controllable explosive source that was towed within a few tens of meters of the rugged bottom topography these 1 to 2 km long refraction lines produced for the first time direct observations of the compressional wave velocity structure of the uppermost 200 300 m of the young igneous crust one experiment was carried out over the site of hole 648b of the ocean drilling program on a small volcano within the median valley of the mar the seafloor velocity was observed to be 2 1 km s 1 underlain by an approximately linear velocity gradient of 4 s 1 given that we know the crust at this location consists of fresh basalt lavas with laboratory measured velocities in excess of 5 8 km s 1 porosities at the seafloor of as high as 30 50 are inferred the two other experiments were located over 7 m y old crust near deep sea drilling project site 395 west of the mar separated by only 14 km laterally but by over 1400 m in water depth the experiment positioned in the 100 m thick sediment pond and that located 14 km to the south atop a prominent topographic high produced results that within the data resolution were indistinguishable the velocity of the uppermost basaltic basement was 4 1 km s 1 and the velocity gradient was less than 0 5 s 1 this 2 km s 1 difference in velocity between the two sites is assumed to be a consequence of age related modifications to the physical properties of the crust if the primary change is in only the total porosity then a 15 20 reduction is required to explain these observations this seems unlikely because any such pervasive process would have been previously observed in geophysical and sampling data e g off axis volcanism secondary mineral deposition tectonic compression a combination of many processes acting in unison would be necessary to produce such a decrease in porosity if the geometry of the cracks and voids were modified in just the right way then no change in total porosity is required to explain the velocity difference these on bottom refraction profiles are proven to be practical experiments that can provide precise information on a scale appropriate to many of the ocean floor s geologic features they produce results that may be correlated in a meaningful way with results of ocean drilling and downhole measurements

active deformation and shallow structure of the wagner consag and delfin basins northern gulf of california mexico oblique rifting began synchronously along the length of the gulf of california at 6 ma yet there is no evidence for the existence of oceanic crust or a spreading transform fault system in the northern gulf instead multichannel seismic data show a broad shallow depression 70 200 km marked by active distributed deformation and six 10 km wide segmented basins lacking well defined transform faults we present detailed images of faulting and magmatism based on the high resolution and quality of these data the northern gulf crust contains a dense up to 18 faults in 5 km complex network of mainly oblique normal faults with small offsets dips of 60 80 and strikes of n n30 e faults with seafloor offsets of tens of meters bound the lower and two upper delfin basins these subparallel basins developed along splays from a transtensional zone at the nw end of the ballenas transform fault twelve volcanic knolls were identified and are associated with the strands or horsetails from this zone a structural connection between the two upper delfin basins is evident in the switching of the center of extension along axis sonobuoy refraction data suggest that the basement consists of mixed igneous sedimentary material atypical of mid ocean ridges on the basis of the near surface manifestations of active faulting and magmatism seafloor spreading will likely first occur in the lower delfin basin we suggest the transition to seafloor spreading is delayed by the lack of strain partitioned and focused deformation as a consequence of shear in a broad zone beneath a thick sediment cover

structural analysis of fault populations along the oblique ultra slow spreading knipovich ridge north atlantic ocean 74 30 n 77 50 n abstract the knipovich ridge 73 30 78 40 n is an extreme end member of the mid ocean ridge spreading system both in terms of spreading rate fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems for the ridge as a whole faults in the axial region are short straight isolated not linked into complex fault zones and exhibit length scaling relationships characteristic of young and active fault systems faults are generally oblique to both the ridge axis and the spreading direction and orientations vary systematically with angle between ridge and spreading direction along axis analysis reveals the influence of axial segmentation on fault population characteristics segment centers are dominated by faults perpendicular and sub perpendicular to plate motion with longer characteristic length and generally lower fracture density conversely segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density we infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion while faulting in segment ends is controlled by the mechanics of oblique rifting non transform discontinuities and or accommodation zones the contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra slow spreading rate of this spreading system

intraplate en echelon volcanic ridges in the south pacific west of the easter microplate satellite altimetric data for the south central pacific ocean show linear geoid anomalies extending from pitcairn island 130 w 24 s to the western border of the easter microplate 118 w 26 s previously these anomalies have been interpreted as a submarine volcanic chain more than 1300 km long inclusive of henderson island ducie island and crough seamount and constructed on young 30 ma oceanic lithosphere recent bathymetric data and side scan sonar images gloria from the eastern part of this volcanic chain suggest that the anomalies correspond to a succession of elongate ridges arranged in a right stepping en echelon pattern along a fracture zone at least five volcanic ridges were recognized during the so80a cruise 1992 between 123 w and 118 w at 25 s trending 275 280 parallel to plate motion the ridges range from 70 to 200 km long 20 to 35 km wide and 1100 to 2900 m high their volumes 600 to 1500 km3 are equivalent to those of large intraplate submarine volcanoes from the polynesian hotspots sediment thickness and the degree of alteration of samples increase progressively from east to west suggesting that the chain grew from a volcanic centre located near 118 w 25 s close to the west rift that forms the western border of the easter microplate the ridges are built along a fracture zone of the pacific plate trending n100 110 which shifts the magnetic anomalies more than 300 km to the left the origin of this fracture zone is not well assessed but it strikes in the continuity of the present day orongo transform fault forming the southwestern boundary between the easter and pacific plates east of 114 50 w the crests of the volcanic ridges are oriented n055 075 at angles of 30 50 with respect to the fracture zone the arrangement of both features could be compared to that of right stepping opening cracks volcanic ridges along a sinistral shear zone transform fault an alternative model is based on the orientation of the abyssal hills which is almost perpendicular to that of the ridges especially in the eastern part of the volcanic alignment the origin of the en echelon ridges could also be related to tension and fracturing in the youngest lithosphere along leaky transform faults

nature and origin of sedimentary clasts associated with mud volcanoes in the nile deep sea fan relationships with fluid venting abstract during the last 10 years several marine geological geophysical surveys conducted on the nile deep sea fan have provided much new information about the large fluid seepage phenomenon which characterizes this passive margin segment among the data more than 60 sediment cores have been recovered in this area rock clasts were collected from eight of them as well as during deep sea dives notably in the area of isis osiris and amon mud volcanoes in few areas especially in the western and the north eastern provinces of the nile fan some clasts consist of crust fragments resulting from cementation of muddy sediment by precipitation of high mg calcite these clasts correspond to a simple cementation of the mud by precipitation of high mg calcite as a consequence no obvious textural or faunal difference with the ambient matrix mud can be observed these clasts are made of autochthonous crusts derived from microbial anaerobic oxidation of methane and release of bicarbonate and sulphide in the surrounding pore water these clasts also lack aragonite in most of the other cases menes caldera isis osiris and amon mud volcanoes allochthonous clasts characterized by low mg calcite cement siderite or dolomite mixed with expelled mud outcrop on the seafloor nannofossils have allowed us to date some of these clasts most of the ages range from pliocene to pleistocene one clast is lower cretaceous in age the lithological facies often indicate coastal or deltaic environments some clasts are closely linked to an evaporite rich environment halite gypsum and analcime they all provide useful information on the nature of buried sedimentary series covering the margin their presence on the seafloor is due to the expulsion of a pressurized mixture of sediment water and gas this process is believed to have used pre existing tectonic conduits

paleoenvironmental evolution of the pliocene villarroya lake northern spain a multidisciplinary approach abstract the pliocene villarroya basin fill consists of lacustrine and alluvial deposits over 100 m thick the lacustrine deposits up to 30 m thick comprise three sequences the two lower sequences consist of profundal laminated deposits formed by mostly terrigenous facies that are overlain by charophytic carbonates and varves the uppermost third sequence made up of massive bioturbated mudstones corresponds to a very shallow lacustrine palustrine environment bulk mineralogy comprises carbonates clay minerals quartz and feldspars the carbonates are exogenic biogenic calcite and biogenic aragonite biogenic calcite constitutes ostracod shells and charophyte stem encrustations whereas aragonite forms gastropod shells and in some varve intervals charophyte encrustations the clay minerals are illite kaolinite palygorskite chlorite and paragonite which are inherited from source rocks and smectite and interstratified clays which are probably formed in the soils of the source area variations in clay mineralogy reflect the interplay between the climate tectonic rejuvenation and the paleogeography of the basin the pollen assemblages indicate significant variations in climate a first phase of varying climate was followed by cool temperate and sub humid phases that culminated in a temperate and dry episode which is recorded in the lower part of the second lacustrine sequence this was succeeded by a marked warm and humid episode which may be correlated with the mid pliocene warmth about 3 0 ma and the subsequent cooling phase which is recorded in the upper third sequence in the two lower sequences profundal lacustrine deposits ostracods indicate oligosaline to mesosaline waters with salinity variations and some warm and na cl episodes molluscs are characterized by hydrobiids suggesting a salinity stress in the upper palustrine deposits the ostracods and aquatic mollusc assemblages indicate fresh cold waters faunal records indicate that changes in salinity do not correlate with climate suggesting phases of significant saline inputs to the lake which were recorded in the lower first sequence and in the upper part of the second sequence the lacustrine sequence of villarroya record pronounced variations in climate and in the lacustrine environment however there are no consistent relationships between the climate conditions inferred from the pollen data and the sedimentary record of the environmental changes showing that other factors especially tectonics have exerted an influence on the paleoenvironmental evolution and on the sedimentary record of the pliocene villarroya lake

physical models of rifting and transform faulting due to ridge push in a wedge shaped oceanic lithosphere by scaled physical modelling we have investigated the mechanical response to gravitational forces in an oceanic lithosphere overlying a less dense asthenosphere in the models an upper wedge shaped layer of sand represented an oceanic lithosphere 0 35 ma old with a half spreading velocity of 3 cm yr and a lower layer of polydimethylsiloxane pdms mixed with dense wolframite powder represented the asthenosphere in the models as in nature isostatic compensation resulted in uplift of ridges and subsidence on their flanks the resulting relief was responsible for ridge push we tested two main configurations straight ridges and offset ridges in all the models ridge push was sufficient to cause plate motion underlying advection and symmetrical rifting at the ridge axis there was no need to impose plate motions through external pistons and motors in models of straight ridges the style of normal faults in the axial rift zone depended on the local thickness of the brittle sand layer for thick layers normal faults rafted out from the active zone of rifting creating a fossil topography of tilted blocks between faults dipping toward the ridge in a model of an offset ridge with thin lithosphere at the ridge crest and no embedded weakness ridge push was responsible for a short transform fault linking en echelon rifts in a similar model but with thick lithosphere an oblique rift formed at about 20 to the offset trace we conclude that ridge push was not adequate to create an ideal transform fault in a model of an offset ridge with an embedded thin vertical layer of pure pdms at 90 to the ridge transform motion concentrated along this weak layer and the resulting structural style was very similar to that in nature on the basis of these results we infer that in nature 1 ridge push can indeed drive plate motion and 2 ridge push can drive strike slip motion on transform faults provided that these are weaker than the adjacent oceanic lithosphere and that they form early in the history of spreading

the seismotectonics of the marmara region turkey results from a microseismic experiment the marmara region is an active tectonic zone characterised by the transition between the dextral strike slip regime of the north anatolian fault naf and the extension regime of the aegean sea strong historical earthquakes m 7 and the presence of known seismic gaps imply a high level of seismic hazard a synthesis of recent studies of active tectonics in the region is presented including inland and underwater observations the branching of the naf is explained in terms of increasing influence of the extension historical information and instrumental seismicity are reinterpreted in order to have a critical appraisal of the existence of large seismic gaps in the central and eastern marmara sea focal mechanisms of strong earthquakes are used to obtain orientation and shape factor of the deviator of the stress tensor the resulting tensor is in shear regime s 2 vertical but close to extension r 0 93 with s 1 oriented n145 0 a microseismic experiment with 48 stations distributed around the marmara sea was carried out in october december 1995 a total of 137 microearthquakes were located and 23 of those were selected to obtain focal mechanism solutions the epicentral distribution indicates activity along the system of pull apart basins north of the marmara sea the segment between marmara sea and the saros bay activated in 1912 and the gulf of izmit site of the 1754 earthquake are now silent seismic activity is very linear along the northern branch of the naf but it is more divused on the bursa and iznik branches southeast of the marmara sea the stress tensor obtained from the focal mechanisms of the micro earthquakes is compared to the one inverted from teleseismic data the microseismic stress is compatible with a shear intermediate regime like the one obtained from strong earthquakes though not as well constrained one of the acceptable solutions having the same orientation but diverent shape r 0 5 2000 elsevier science b v all rights reserved

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

the geometry of propagating rifts abstract the kinematics of two different processes are investigated both of which have been described as rift propagation courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin the term localisation is instead used here to describe this process to distinguish it from hey s type of propagation localisation generally leads to rotation of the direction of magnetisation to hey propagation means the extension of a rift into the undeformed plate beyond a transform fault detail surveys of the galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault but by a zone which is undergoing shear the principal deformation is simple shear and the kinematics of this deformation are investigated in some detail the strike of most of the lineations observed in the area can be produced by such deformation the mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth

analogue experiments of propagation of oblique rifts abstract we present the results of analogue models of oblique rifting which were carried out using a isostatically compensated two layered small scale models the experiments show that structures formed in the brittle layer under oblique extension comprise a series of lenticular rifted basins set en echelon along the axial zone of the experiment box in time the basins propagate lengthways to merge and form a unified composite rift however traces of the initial en echelon rifts expressed in deeper axial basins separated by shallower threshold zones are clearly discernible in the mature rifts both the basins and the threshold zones are located within the composite rift so that the rift margins are abrupt and steep along the basins and more diffuse along the threshold zones the thinning in the brittle layer along the rift is enhanced by diapirs that ascend from the ductile layer and penetrate the rift floor model experiments of nearly orthogonal extension showed a single unified rift which differed conspicuously from the composite oblique rifts the results of the experiments suggest that oblique extension forms en echelon rift segments the axes of which are perpendicular to the extension direction separated by threshold zones therefore there is ground to presume that occurrence of a series of alternating basins and threshold zones inside the axial section of a major rift could suggest a regime of oblique tectonic extension

magnetobiochronology of lower miocene ramblian continental sediments from the tudela formation western ebro basin spain abstract the calibration of european lower miocene continental stages is poorly constrained due to the lack of robust magnetostratigraphic data from continental sediments of this age in this paper we present the first magnetostratigraphic results from the mammal fossil bearing continental sediments of the lower middle miocene tudela formation western ebro basin spain the study is based on 331 paleomagnetic sites distributed along a 790 m thick composite section the resulting magnetostratigraphy provides an independent correlation to the geomagnetic polarity time scale and allows accurate dating of previously known and new ramblian upper lower miocene fossil localities of the tudela formation the new age constraints derived for the tudela formation fossil localities provide together with a revaluation of other iberian faunas the first magnetochronology for the ramblian continental stage the lower boundary of the ramblian is placed at the lower part of chron c6r ca 20 4 ma and the boundary between the lower and upper ramblian is located at the middle of chron c6n ca 19 6 ma the upper boundary of the ramblian and hence the base of the aragonian is placed at the upper middle part of chron c5cr 16 8 17 ma our results also provide the first magnetostratigraphically dated mn3 faunas in spain which together with a revaluation of other iberian and european faunas provide a revised calibration of european mammal zone mn3 the lower boundary of zone mn3 is placed at the boundary between chrons c6r and c6n ca 20 1 ma and its upper boundary is located at the upper middle part of chron c5cr 16 8 17 ma

anatomy and kinematics of oblique continental rifting revealed a three dimensional case study of the southeast upper rhine graben germany the upper rhine graben is a north northeast trending small displacement crustal scale rift of tertiary age retrodeformation of its southeastern part demonstrates that it is a product of sinistral oblique rifting early extension was toward 80 later the major stretching axis changed to a 60 direction the modeling results suggest that the eastern main border fault developed first and that faulting later propagated into the evolving graben interior considerable along strike variations in heave throw and displacement are evident displacement partitioning causes warping of the rift floor with a 30 35 km wavelength we consider this to be a characteristic of oblique rifting contact deformation of the wall rocks to the major faults may have caused widespread smaller scale faulting and brecciation and may be the location of later movements close spatial coincidence of the depth projections of some of the faults studied and the hypocenters of recent small earthquakes indicates continuing activity of the fault system apparently three fault segments in the freiburg area are currently active and may be an increased earthquake risk

gloria observations of the propagating rift at 95 50w on the cocos nazca spreading center in this paper we describe a gloria long range sidescan sonar survey of the area around 95 5 w on the cocos nazca rise where a propagating rift has been predicted to occur the sonar images clearly show a wedge of new crust that has been emplaced by propagation into 1 ma old crust the new crust is up to 800 m deeper than the surrounding seafloor although the elevation difference appears to decay with time the pseudofaults which bound this new crust have been observed from the rift tip back to crustal isochron 0 65 ma near the rift tip they consist of scarps several hundred meters high with narrow shallow valleys at their feet the sheared zone between the southern pseudofault and the trace of the failed rift to the south is characterized by abnormally shallow seafloor and nnw sse trending lineaments these may have been produced by the rotation of tectonic fabric formed at the dying rift perhaps accompanied by other deformation the youngest tectonic and volcanic lineaments on both spreading segments strike 273 but on the dying rift short spreading segments are offset en echelon so that the regional trend of the rise axis is 265 older lineaments throughout the area strike 265 the propagating rift is thus replacing an oblique rise axis by one which is orthogonal to the spreading direction and it is suggested that this is occurring in response to a recent change in spreading direction from 355 to 003 variations in the strikes of the pseudofaults indicate that the propagation direction has varied but that the rift has propagated most rapidly when following the 273 direction which we take to be normal to the new spreading direction

deformation during the 12 november 1999 duzce turkey earthquake from gps and insar data only 87 days after the mw 7 5 17 august 1999 izmit earthquake the duzce earthquake ruptured a ca 40 km long adjoining strand of the north anatolian fault naf system to the east we used displacements of 50 global positioning system gps sites together with interferometric synthetic aperture radar insar range change data spanning the event to estimate the geometry and slip distribution of the coseismic rupture postseismic deformation transients from the duzce earth quake and the preceding i zmit event that are included in some of the measurements are corrected for using dislocation models fit to gps data spanning the various time periods nonlinear inversions for fault geometry indicate that the rupture occurred on a ca 54 north dipping oblique normal right lateral fault distributed slip inver sions indicate maximum strike slip near the center of the duzce fault close to the earthquake hypocenter slip magnitude and depth of faulting decrease to the west and east of the hypocenter both gps and insar data suggest that normal slip is restricted to the shallow portion of the rupture the duzce earthquake had the highest slip to rupture length ratio of any historic earthquake along the naf

small scale models of oceanic transform zones previous studies about oceanic transform zones reveal major morphological and structural differences that cannot be explained by simple factors such as spreading rates or offsets in order to determine how kinematic and mechanic parameters control the deformation pattern we perform small scale modeling using sand and silicone putty as analog material of brittle and ductile layers of oceanic lithosphere respectively the three dimensional depth geometry of these layers follows the thermal structure of the ridge transform system the structure of the deformed transform zone is very sensitive to input parameters specially to the offset the obliquity of the deformed zone with the extension direction i e spreading direction in a natural system depends on the offset and on the brittle layer thickness for greater thickness or a shorter offset the transform domain widens and the components of extension become significant for smaller lithosphere thickness or larger offset the transform domain becomes narrow with steeper walls and mainly undergoes strike slip deformation such variability in possible controls on the offset length may help explain the structure complexity observed within the oceanic transform zones a comparison with 28 natural systems indicates the same relationship of the offset with the angle between the principal transform displacement zone ptdz and spreading direction a relationship between offset and width is clearly established for mid atlantic ridge mar transforms as on analog models a similar relationship is observed for the east pacific rise epr transforms but with greater variabilty this difference is interpreted as resulting either from greater complexity of transform zones on epr than on mar making it difficult to estimate some parameters from different mechanical behaviors between slow and fast spreading systems or from higher rheological complexity of natural systems than out model

new evidence of magmatic diapirs in the intermediate crust under the dead sea israel simultaneous inversion of p wave arrivals of 262 local earthquakes recorded in the region of the dead sea was carried out to model the velocity stratification in the underlying crust and uppermost mantle the resulting tomographic model delineates several large magmatic domes rising from the lower crust at depth of approximately 20 km and ascending more than 12 km into the overlying intermediate crust commonly the domal ascent took place along the boundary faults of the dead sea rift the domes are elliptical in shape measuring nearly 5 15 km in diameter and are spaced about 20 km apart earthquake foci are abundant around the diapirs but rare within them the occurrence of these magmatic diapirs under the dead sea implies anomalously high temperatures with metastable stratification in the lower crust and suggests magmatic upwelling

controls of lithospheric structure and plate kinematics on rift architecture and evolution an experimental modeling of the baikal rift 1 analog models investigate the evolution and architecture of the baikal rift in relation to the rheology of the extending lithosphere and rift kinematics the models focus on the development of the narrow deep and asymmetric basins composing lake baikal and reproduce the extension between the strong siberian craton and the weaker sayan baikal belt model results suggest that the presence of a near vertical weak suture separating the cratonic keel from the mobile belt represents the more convenient rheological configuration leading to a narrow rift characterized by prominent vertical motions and deep depressions these depressions are typically asymmetric and model results suggest that this asymmetry is a consequence of lateral variations in lithospheric rheology which is in turn related to both the variation in thickness of the strong mantle and more importantly the variation in the brittle ductile transition depth between the craton and the belt a significant shallowing of the brittle ductile transition in the crust passing from the craton to the belt is required to fit the asymmetric architecture of the baikal basins with a master fault on the craton side and a monocline with no significant faulting on the belt side analysis of the model deformation pattern suggests that the overall architecture of the basins hosting lake baikal is best fitted for a n140 e directed extension similar to the current gps derived motion and compatible with the stress field inferred on the basis of fault and focal mechanism data this kinematics along with the shape of the siberian craton exerted the major control on the plan view fault architecture and its along axis variations

along rift propagation of pleistocene holocene faults from a central volcano abstract the mechanisms of rift propagation are still not fully understood especially at mid oceanic ridges owing to the inherent difficulty in collecting submarine data here we investigate the fault slip profiles of the 60 km long theistareykir rift northern iceland that may suggest the direction of along axis rift propagation this is one of the few places on earth where rifting processes and mid oceanic ridge formation can be studied directly moreover this rift hosts an active central volcano and this enables to fully understand the relations between rift propagation and magma systems we reconstructed the slip profiles of all the 281 main pleistocene holocene faults that compose this n s rift by merging measurements performed in the field collected by unmanned aerial vehicle surveys and derived from digital surface models results indicate that north of the volcano 75 of the asymmetric faults propagated northward the value increases to 82 if the cumulated fault length is considered south of the volcano 47 of the asymmetric faults propagated southward 54 if the cumulated fault length is considered these data point to a dominant mechanism of along axis propagation of the rift outward from the volcano suggesting a genetic link with the underlying magma chamber two possible processes are suggested which can also occur in combination i faults develop following lateral dyke propagation outward from the magma chamber and ii faults nucleate near the volcano as a consequence of the different crustal rock rheology produced by a higher heat flux the rift architecture is complicated by the presence of tectonic zones with different dominant fault dips separated by transversal accommodation zones the latter also play the role of barriers guiding local fault propagation

evolution and structure of the upper rhine graben insights from three dimensional thermomechanical modelling the evolution and geometry of the tertiary upper rhine graben were controlled by a continually changing stress field and the reactivation of pre existing crustal discontinuities a period of wnw ese extension in the late eocene and oligocene was followed by lateral translation from the early miocene onwards this study utilizes 3d finite element techniques to simulate extension and lateral translation on a lithospheric scale brittle and creep behaviour of lithospheric rocks are represented by elastoplasticity and thermally activated power law viscoplasticity respectively contact elements allocated with cohesion and frictional coefficients are used to describe pre existing zones of weakness in the elastic brittle field our results suggest that 1 extension is accommodated along listric border faults to midcrustal depth of 15 16 km beneath pure shear stretching occurs without a need for localized shear zones in lower crust and upper mantle 2 ductile flow at midcrustal depth across the graben accounts for the pronounced halfgraben morphology thereby the shape of the border faults their frictional coefficients and sedimentary loads have profound effects on the rate of ductile flow across the graben 3 horizontal extension of 8 8 5 km and sinistral displacement across the rift of 3 4 km are needed to accommodate the observed sediment thickness

deformation partitioning in a slow spreading ridge undergoing oblique extension mohns ridge norwegian sea although oceanic spreading is often perpendicular to the ridge trends in some cases the angle between these two directions can be significantly less than 90 40 50 this occurs because of either a bend of the ridge trend or a change of the spreading direction we here describe oblique spreading in the mohns ridge resulting in deformation partitioning between the valley walls which are dominantly affected by strike slip displacements and the axial valley which is subject to nearly pure extension the axial valley walls are characterized by en echelon normal faults affecting the walls while the axial valley is affected by parallel faults grouped into oblique sets these fault sets define different structures horst or tilted blocks that are regularly spaced inside the axial valley moreover some ridge segments mainly undergo pure extension whereas others are affected by oblique extension we explain this faulting pattern including the along strike and transverse variations as a consequence of depth variations of the brittle ductile transition

review of the tectonics of the levant rift system the structural significance of oblique continental breakup abstract the levant rift system is an elongated series of structural basins that extends for more than 1000 km from the northern red sea to southern anatolia the system consists of three major segments the jordan rift in the south el gharb kara su rift in the north and the lebanese fault splay in between the rifted parts of this structural system are accompanied by intensively uplifted margins that mirror image the basinal pattern namely the deeper the basin the higher its margins and vice versa uplifts also occur along the fault splay section the jordan rift comprises axial basins that diminish in size from the south northwards and are separated from each other by shallow threshold zones along the axis of the rift where the margins are also subdued the lebanese fault splay consists of five faults that emerge from the northern edge of the jordan rift and trend like a fan between the north and the northeast one of these faults connects the jordan and el gharb kara su rifts the levant rift and its uplifted margins started to develop in the middle late miocene and most of the structural development occurred in the plio pleistocene the levant rift system is characterized by its oblique displacement and evidence for both dip slip and strike slip displacement was measured on its faults earthquakes also indicate that same mixed pattern some of them show strike slip offset and others normal it is generally conceded that the amount of normal offset along the boundary faults of the rift system reaches 8 10 km but the lateral displacement is disputed and offsets ranging from 11 to 107 km were suggested assessment of the available data led us to suggest that the sinistral offset along the levant rift system is approximately 10 20 km the similarity between the vertical and the lateral displacements the basin and threshold structural pattern of the rift model experiments in oblique rifting as well as the significant tectonic resemblance to the red sea and the east african rifts indicate that the levant rift is the product of continental breakup and it is probably an emerging oceanic spreading center

new artificial granular materials for analogue laboratory experiments aluminium and siliceous microspheres abstract laboratory tests have been carried out to determine the physical properties of high sphericity and low density fine grained dry granular materials in particular we tested hollow aluminium microspheres and hollow siliceous microspheres shear tests have been carried out with a casagrande apparatus for different values of normal stress from 12 27 to 491 8 kpa in order to determine the frictional properties of the microspheres diagrams of the shear stress against displacement have been used to describe the strength of the investigated materials at different confining pressures the shear tests show that hollow aluminium and siliceous microspheres successfully replicate the mechanical behaviour of weaker layers within natural sedimentary successions undergoing deformation the use of these artificial granular materials in specifically designed experiments indicated that they can be successfully used for constructing more natural multilayers in the laboratory

reliability of magnetic fabric of weakly deformed mudrocks as a palaeostress indicator in compressive settings abstract in this work we compare the results of anisotropy of magnetic susceptibility ams with palaeostress analysis at local scale the ams data refer to 14 sites from weakly deformed mudrocks lower and middle miocene lacustrine sediments from two areas of the internal part of the ebro foreland basin the bardenas reales and monegros areas nine sites display a magnetic fabric related to a subtle tectonic overprint showing a roughly e w magnetic lineation whereas the rest of sites most of them located at the monegros area show a sedimentary fabric without a well defined magnetic lineation palaeostress data result from fault populations and joint sets exposed in the limestone beds interbedded with the mudrocks at the nearest possible outcrop from the ams site the origin of the magnetic lineation is related to a n s compression coeval with the earliest diagenesis of the studied sediments prior to consolidation the comparison between magnetic fabric and palaeostress data demonstrates the coaxiality between the ams and palaeostress ellipsoids both at local and regional scales thereby validating the reliability of ams as palaeostress indicator in compressive settings

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

coseismic and postseismic fault slip for the 17 august 1999 m 7 5 izmit turkey earthquake we use global positioning system gps observations and elastic half space models to estimate the distribution of coseismic and postseismic slip along the izmit earthquake rupture our results indicate that large coseismic slip reaching 5 7 meters is confined to the upper 10 kilometers of the crust correlates with structurally distinct fault segments and is relatively low near the hypocenter continued surface deformation during the first 75 days after the earthquake indicates an aseismic fault slip of as much as 0 43 meters on and below the coseismic rupture these observations are consistent with a transition from unstable episodic large earthquakes to stable fault creep sliding at the base of the seismogenic zone

a study of the tectonically active marmara region turkey using a global positioning system gps the marmara region is an active tectonic zone characterized by a transition in the dextral strike slip regime of the western part of the north anatolian fault zone the main goal of this paper is to assess the use of global posi tioning system gps data sets collected during five relatively short time intervals during the period december 2000 march 2002 to detect potential crustal deformations to determine if the deformations measured with gps are real or only a data artifact a statistical reliability analysis of the solutions is performed the results indicate that each station has statistically different temporal behavior and significant relative motions this area is consequently still very active with significant deformation patterns although the average magnitude for our estimated displacement rates with re spect to ankr station which represents the rigid motion of the anatolian plate is in the order of 1 1 cm year in the south of the north anatolian fault it increases to 2 3 cm year in the northern part of the area

postseismic deformation near the izmit earthquake 17 august 1999 m 7 5 rupture zone we present and interpret the results of postseismic global positioning system monitoring of the first 298 days following the 17 august 1999 izmit earth quake whereas the data suggest some spatial and temporal complexity in the post seismic motions the overall pattern can be characterized by time dependent relaxa tion functions and suggests exponential decay with an estimated 57 day relaxation time the very rapid deformation during the first few weeks following the mainshock suggests rapid afterslip on and below the coseismic rupture segments the exponen tial decay of the postseismic deformations through the end of the observation period suggests contributions from the lower crustal viscoelastic relaxation

oblique rifting at oceanic ridges relationship between spreading and stretching directions from earthquake focal mechanisms the relationship between spreading and stretching directions is investigated at oblique spreading oceanic ridges using earthquake focal mechanisms the stretching direction at ridge axes corresponds to the direction of the greatest principal strain 31 taken as the mean trend of the seismic t axes of extensional earthquake focal mechanisms it is compared with the spreading direction provided by global plate motion models we find that the stretching direction trends approximately halfway between the spreading direction and the normal to the ridge trend a result in line with analogue experiments of oblique rifting this result is satisfactorily accounted for with an analytical model of oblique rifting for which the direction of 31 is calculated with respect to rifting obliquity for different amounts of stretching using continuum mechanics for low stretching factors typical of incremental seismic deformations 31 obliquity is two times lower than rifting obliquity for higher stretching factors the stretching and spreading directions become parallel 2006 elsevier ltd all rights reserved

seismic images of a magma chamber beneath the lau basin back arc spreading centre we present normal incidence seismic reflection profiles from a back arc spreading centre in the southern lau basin which show a bright mid crustal reflector beneath the ridge axis analysis of wide angle seismic data collected simultaneously with the reflection profiles shows that the reflector coincides with the top of a low seismic velocity zone we interpret the reflector as the top surface of a magma chamber the lau basin is one of only two places where a magma body has been clearly imaged beneath a spreading centre the other being the east pacific rise between 9 and 13 n ref 1 the lau basin reflector is imaged beneath an entire 35 km long ridge segment it is also present beneath both ridges and the overlap basin of a small overlapping spreading centre this is the first observation of a widespread magma body beneath an overlapping spreading centre and indicates that the offset here is not related to a break in continuity of the magma chamber

clay mineral assemblages and analcime formation in a palaeogene fluvial lacustrine sequence maiz gordo formation palaeogen from northwestern argentina abstract the palaeogene maiz gordo formation is one of the main lacustrine events recorded in northwestern argentina it consists of sandstone mudstone and limestone beds 200 m thick deposited in a brackish alkaline lake and braided alluvial systems the maiz gordo lake evolved mainly as a closed system with brief periods as an open one x ray diffraction xrd and scanning electron microscopy sem were used to study samples from seven sites corresponding respectively to proximal intermediate and transitional positions of the fluvial environment and marginal and inner lake environment focusing on the clay mineralogy and analcime formation the basinward zonation of diagenetic minerals identified in the maiz gordo lake was mordenite analcime k feldspar although not a typical zonation of saline alkaline lakes it does indicate an increase in salinity and alkalinity towards the centre in proximal fluvial settings smectite predominates at the base of the sequence with scarce kaolinite towards the top a striking increase in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons towards a warm and humid climate kaolinite content clearly decreases in a basinward direction such a variation is attributable to changes in hydro geochemistry denoting the progressive influence of the brackish and alkaline lake water on interstitial pores sem images of intermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets in littoral and inner lake settings the clay fraction is composed of muscovite sometimes with subordinate smectite analcime occurs in variable amounts in all sedimentary facies in rock pores or filling veins it forms subhedral square to hexagonal or anhedral rounded crystals denoting that they coarsened at low to moderate degrees of supersaturation although the mordenite identified in a fluvial level would have been the precursor of analcime in the maiz gordo basin no textural evidence of analcime formation through replacement of mordenite or other precursor zeolite was found hence it is more probable that analcime formation took place by direct authigenic precipitation or through the reaction between interstitial brines and clay minerals or plagioclase

3d analogue modeling of the south china sea a discussion on breakup pattern the opening of the south china sea is one of the most important cenozoic events in se asia in order to investigate its tectonic evolution three analogue modeling experiments were compared the modeling results suggest that rifting pattern and orientation change of the rift zone were related to initial theological stratification affected by the ductile flow of silicone lower crust and honey asthenosphere the faults became flattened especially close to the breakup area we conjecture that the slope area may bear relatively hotter and thinner lithosphere than the shelf area from the beginning of rifting due to stretching and mantle upwelling associated with this regional extension which in turn lead to the change in initial rheological stratification and therefore the rifting pattern from shelf to slope in the experiments breakup developed first at isolated points which grew and coalesced to become a single spreading area the conjugate boundaries were either all concave or all convex where a rigid massif was located at the divergent boundary stretching history was different in this case the northern and southern areas thinned rapidly and developed into two deep troughs which may finally evolve into spreading centers the shape of the massif controls the orientation and boundary shape of the spreading area it is interpreted that crustal breakup was more viscous in style and the nw sub sea basin developed along the northern trough of the zhongsha xisha massif macclesfield bank c 2008 elsevier ltd all rights reserved

the northern red sea and the dead sea rift abstract the results of geophysical measurements at sea when combined with mesostructural measurements on land demonstrate systematic extensional features in the region of the northern red sea and the dead sea rift rifts and diapirs were encountered in the northern red sea showing an arcuate trend whereas the faulted continental margins at that region are oriented nw se the rifts and the diapirs changed their trend northwards from nnw sse to n s systems of rifts and diapirs are considered to be extensional features therefore the tectonic extensional axis probably changed its orientation accordingly a series of rifts and diapirs was found also in the gulf of elat trending approximately parallel to the coastlines and is interpreted to be the northern extension of the red sea rift and diapir system mesostructural measurements that were carried out in the western flank of the dead sea rift indicated further evidence for structural extension trending e w to ene wsw in addition to measurements of left lateral offset of approximately 10 km all these data when combined with the geophysical measurements of the characteristics of the earth crust underneath the dead sea rift suggest a tectonic regime of oblique extension in the region of the red sea and the dead sea rift this tectonic regime indicates that the red sea spreading center extends northwards along the dead sea rift and the clockwise change of the axial trend imposed a sinistral offset to the tectonic extension thus it is suggested that the dead sea rift is an incipient oblique spreading center

inward migration of faulting during continental rifting effects of pre existing lithospheric structure and extension rate abstract lithospheric scale analogue models are used to analyse the parameters controlling the typical evolution of deformation during continental narrow rifting characterized by early activation of large boundary faults and basin subsidence followed by localization of tectonic activity in internal faults at the rift axis integration of current and previous experiments shows that the evolution of deformation in particular the amount of extension needed for the abandonment of boundary faults and migration of deformation to in rift faults is dependent on at least five boundary conditions i thickness of brittle layers including syn rift sediments ii thickness of ductile layers iii extension rate iv width of the weak zone localizing extension and v rift obliquity with respect to the extension direction an increase in the amount of extension corresponding to the inward migration of faulting i e a longer phase of slip on boundary faults is observed for a an increase in the thickness of both brittle and ductile crustal layers and syn rift sediment accumulation b a decrease in extension rate and width of the weak zone and c a decrease in rift obliquity a unified account of these correlations is presented based on the hypothesis that fault migration occurs when boundary faults can no longer accommodate the imposed bulk extension leading to time space variations of internal strain and strain rate and consequently stress in the ductile layers which overcome the total resistance of brittle layers to thoroughgoing faulting

geodynamics of the middle east domain since the oligocene research summary several analogue models were merged to link the post oligocene closure of the neotethys seaway with the break up of the gulf of aden and the red sea the seaway closure since the early neogene which was a result of northwards subduction of neotethyan lithosphere under south central eurasia generated two styles of kinematic convergence its western branch forms the tethyan subduction under south central eurasia along the hellenic deformation front generating the break up of the aegean sea and the westward migration of anatolia the eastern branch comprises the oblique convergence of arabia with eastern anatolia and iran along the bitlis zagros suture which gradually shifted from subduction to collision because the velocity of plate collision is slower than that of subduction the subducting section of the underthrust slab moves faster than the colliding segment consequently the underthrust arabian slab rotated counterclockwise and developed detachment zones such extension triggered the resumption of the propagation of the carlsberg ridge into the gulf of aden after being stalled for nearly 30 ma it is presumed further that the break up of the red sea in the mid miocene was superimposed on the regional uplift and exhumation of the arabo nubian massif which was caused by a mantle plume in the oligocene a plume that subsequently affected the ethiopian and the east african plateaux in the neogene

surficial structures of the northern red sea axial valley from 23 n to 28 n time and space evolution of neo oceanic structures abstract during recent french cruises in the northern red sea the surficial structures of the axial valley were studied by means of bathymetry magnetics and single channel reflection seismics new compilation maps have been established and comparison of the various data allow different segments of the northern red sea axial valley to the distinguished where only intra evaporitic deformations 24 25 n large volcanic outcrops mabahiss deep 25 30 26 n or several recent isolated volcanic intrusions 26 27 n characterize different stages of evolution this new information allows reconstruction of the evolution of oceanization cells from a simple intra evaporitic deep to a limited oceanic rift zone such as those observed in the central red sea a major characteristic of this evolution also depicted in other young oceanic rifts is a succession of volcanic and tectonic phases the major event in the northern red sea axial valley is a recent episode of voluminous but well localized magmatism it would represent a new stage in the red sea evolution and thus may have some implications on earlier phases

jurassic lake t oo dichi a large alkaline saline lake morrison formation eastern colorado plateau a large alkaline saline lake lake t9oo9dichi9 occupied the entire eastern part of the colorado plateau region during deposition of the brushy basin member of the upper jurassic morrison formation the lake extended from near the site of albuquerque new mexico to near the site of grand junction colorado and occupied a region that encompassed the san juan and ancestral paradox basins making it the largest ancient alkaline saline lake known the lake was typically shallow and frequently evaporated to dryness although the water table was probably never far beneath the surface intermittent streams carried detritus from source areas to the west and southwest far out into the lake basin a reflection of a low depositional gradient and the ephemeral nature of the lake prevailing westerly winds carried silicic volcanic ash to the lake basin from an arc region to the west and southwest a lateral hydrogeochemical gradient characterized by increasing salinity and alkalinity from the margin of the lake to the center developed in pore waters of the lacustrine sediments and resulted in alteration of the ash to a variety of authigenic minerals that define concentric zones within the lake basin the basinward progression of diagenetic mineral zones is smectite clinoptilolite analcime potassium feldspar albite a concomitant lateral zonation of mixed layer illite smectite developed from highly smectitic in the outermost zones to highly illitic in the central zone authigenic albite and illitic mixed layer clay minerals previously thought to require elevated temperatures for formation crystallized during early diagenesis at syndepositional temperatures in the lacus trine sediments of lake t9oo9dichi9 this study presents evidence for a syndepositional origin for both of these minerals these findings have important implications in terms of limiting the use of these minerals as geothermometers recognition of alkaline saline lake deposits in the morrison formation significantly alters interpretations of depositional environments of this formation and it also has important implications for paleoclimatic interpretation late jurassic climate was apparently much more and than had previously been thought in fact sedimentologic evidence suggests that the lake basin was typically dry for extended periods and enjoyed only brief wet intervals this conclusion has important consequences for environmental interpretation of the habitat that was favorable for large herbivorous dinosaurs which thrived in the late jurassic

global positioning system constraints on plate kinematics and dynamics in the eastern mediterranean and caucasus we present and interpret global positioning system gps measurements of crustal motions for the period 1988 1997 at 189 sites extending east west from the caucasus mountains to the adriatic sea and north south from the southern edge of the eurasian plate to the northern edge of the african plate sites on the northern arabian platform move 18 2 mm yr at n25 5 w relative to eurasia less than the nuvel 1a circuit closure rate 25 1 mm yr at n21 7 w preliminary motion estimates 1994 1997 for stations located in egypt on the northeastern part of africa show northward motion at 5 6 2 mm yr also slower than nuvel ia estimates 10 1 mm yr at n2 4 e eastern turkey is characterized by distributed deformation while central turkey is characterized by coherent plate motion internal deformation of 2 mm yr involving westward displacement and counterclockwise rotation of the anatolian plate the anatolian plate is de coupled from eurasia along the right lateral strike slip north anatolian fault naf we derive a best fitting euler vector for anatolia eurasia motion of 30 7 0 8 n 32 6 0 4 e 1 2 0 1 myr the euler vector gives an upper bound for naf slip rate of 24 1 mm yr we determine a preliminary gps arabia anatolia euler vector of 32 9 1 2 n 40 3 1 1 e 0 8 0 2 myr and an upper bound on left lateral slip on the east anatolian fault eaf of 9 1 mm yr the central and southern aegean is characterized by coherent motion internal deformation of 2 mm yr toward the sw at 30 1 mm yr relative to eurasia stations in the se aegean deviate significantly from the overall motion of the southern aegean showing increasing velocities toward the trench and reaching 10 1 mm yr relative to the southern aegean as a whole

re orientation of the extension direction and pure extensional faulting at oblique rift margins comparison between the main ethiopian rift and laboratory experiments in this study we draw on a unique combination of well resolved fault slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending main ethiopian rift east africa these data show that both boundary and internal faults oblique and orthogonal to the plate divergence pd respectively exhibit almost pure dip slip motion and indicate significant local deflection in orientation of the extension direction at rift margins scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep seated pre existing weakness oblique to the direction of pd that is able to cause a local rotation in the orientation of the extension direction at rift margins minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems as supported by existing and new palaeomagnetic data from the rift

periodic instabilities during compression of the lithosphere 2 analogue experiments we have modeled the behavior of the continental and oceanic lithospheres under compression using materials with analogous properties in laboratory experiments to study the development of lithospheric buckling periodic instabilities which are a major deformation process during the compression of the lithosphere have already been described by several authors using an analytical perturbation method at small strains analogue experiments corroborate most of the results obtained by the perturbation method 1 the deformation modes geometrical relationships of interfaces and related wavelengths are mainly dependent on the spatial distribution of the brittle layer s and 2 the amplitude of buckling is an exponential function of the horizontal strain some departure from the perturbation method occurs when there are two instabilities growing concurrently the breakdown of the exponential growth occurs for strains of about 5 and is concomitant with the appearance of thrust faults in experiments including one brittle layer which model the compression of the oceanic lithosphere faults are regularly located at the inflection points of the folds in experiments including two brittle layers which model continental lithosphere faults form more complicated patterns with an asymmetrical deep thrust overlain by a fan shaped symmetrical thrust system in the upper brittle layer such fault geometries give some new highlights on typical compressive geological structures such as those encountered in central asia

the seismic activity of the marmara sea region over the last 2000 years we examined the seismicity of the region of the sea of marmara in northwest turkey over the last 2000 yr using the historical record of the region we found no evidence for truly large earthquakes of a size comparable to those in the north anatolian fault zone in the marmara basin events are smaller in keeping with the known fault segmentation of the basin over the last 2000 yr seismic mo ment release accounted for the known right lateral shear velocity across the marmara region observed by global positioning system measurements its average rate is relatively constant and varies between 1 6 and 2 4 cm yr the long term seismicity in the marmara sea region shows that large earthquakes are less frequent than pre dicted from the 100 yr long instrumental period

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

structure of the rift basins in the central gulf of california kinematic implications for oblique rifting abstract we use structural and seismostratigraphic interpretation of multichannel seismic reflection data to understand the structure and kinematic history of the central gulf of california our analysis reveals that oblique strain in the central gulf formed two tectono sedimentary domains during distinct deformation stages the eastern domain offshore sonora is bounded by the east and west pedro nolasco faults that may constitute the southernmost segments of the tiburon fault system within this domain the dip slip yaqui fault controlled deposition of 3 9 km of sediments in the half graben yaqui basin the western domain offshore baja california is bounded by the guaymas transform fault which controlled the accumulation of 1 45 km of sediments within a half graben that formed the early guaymas basin the tectono sedimentary activity offshore sonoran likely ranges from late miocene pliocene to late pliocene time while activity in the guaymas basin commenced in late pliocene time extinction of the main faults offshore sonora was nearly coeval to the initiation of the guaymas transform fault our results suggest that oblique strain has been accommodated by strain partition since the onset of rifting in the central gulf the guaymas basin is now a nascent spreading center but prior to this it evolved as a half graben controlled by the guaymas transform fault such drastic transition is not constrained but likely occurred during the pleistocene time and must be localized

transfer faults and pull apart model in the rhinegraben from analysis of multisource data from analysis of gravimetric data comparisons of levelings seismic profiling paleostress analysis and seismicity together with remotely sensed and morphological data the rhinegraben may be divided into several rhomb shaped basins separated by ene trending basement highs these data express neotectonic movements and slight spatial differences in the subsidence rate these basins are asymmetrical on one side they are bounded by a large several kilometers of vertical throw normal fault zone on the opposite side there is a megaflexure cut by smaller normal faults shaping tilted blocks since the middle miocene extensional directions are subparallel to the ene trending basement highs that consequently may be interpreted as strike slip fault zones of transfer type across the erstein basement high there is a reversal in the rift asymmetry major bordering fault on the western side to the south on the eastern side to the north when considered in the context of the west european rift the rhinegraben is interpreted in terms of a pull apart depression formed during the horizontal movement of blocks guided by large ene trending transcurrent faults the normal faults that are curved shaped in map view may mark the boundaries between areas where the maximum component 1 of the paleostress field developed in pull apart patterns was changing from horizontal to vertical

how does the initial configuration of oceanic ridge segments affect their interaction insights from analogue models 1 oceanic ridge segments interact forming different types of structures of which transform faults and overlapping spreading centers represent the most common end members this study uses analogue models for testing the role of the initial configuration of the ridges on the geometry and kinematics of the resulting interaction zones the models run in a centrifuge consist of a lighter silicone lds rising within a denser silicone hds below a brittle layer simulating the oceanic lithosphere precut fractures in the brittle layer simulate the initial configuration of oceanic ridges the rise and lateral spreading of the lds induce the propagation widening interaction and linkage of the fractures we varied the offset and overlap between the fractures obtaining 10 distinctive types of interaction variations in fracture overlap and offset define several interaction types increase of fracture overlap leads to interaction zones of lower aspect ratio with fractures propagating at lower angles to the mean extension direction increase of offset leads to the elongation of the interaction zones with the new fractures propagating subparallel to the extension direction the comparison with several examples of natural ridges shows close geometric similarities confirming the existence of predominant types of ridge interaction among these ridges with smaller offsets develop interactions similar to overlapping spreading centers whereas ridges with larger offsets have geometries reminiscent of transform zones the comparison between experimental and natural examples suggests that the observed wide spectrum of ridge interaction types in nature results from the initial configuration of the divergent plate boundary

analogue modeling of faulting pattern ductile deformation and vertical motion in strike slip fault zones the relationship between faulting ductile deformation and vertical displacement in provinces of strike slip faults were analyzed using analogue modeling the described experiment considered a two layer small scale model for the lithosphere set on a low viscosity fluid that allowed free isostatic compensation the development of the faults and their orientation follow generally riedel s model but fault spacing depends on the thickness of the brittle layer and lateral displacement of the brittle layer was associated with series of deep elongated depressions that developed along the trace of the principal strike slip fault the depressions correspond to local thinning of the brittle layer and the amount of thinning is more than 60 in places the underlying ductile layer displayed two types of superimposed deformation namely a series of tight parallel folds occurring on top of elongated domes the folds are attributed to the compressive component of the strike slip displacement and the updoming to the extensional component the elongated domes are located beneath the superficial depressions and the deeper troughs underlie shallow uplifted structures in the deformed band significant vertical motion was observed along faults that are considered as perfectly strike slip faults according to the classical riedel model the thinning of the brittle layer and the deep deformation of the ductile layer are in good agreement with actual examples of pull apart basins and elongated swells in large strike slip zones a new structural pattern developed during strike slip shear is proposed relating the geometrical relationships between faults folds and elongated domes to the stress axes

analog models of oblique rifting in a cold lithosphere new lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution basin segmentation and mantle exhumation occurring during rift localization we present two models one with a preexisting oblique lithospheric weakness model b and another with no weakness zone model a both oblique rifts have an obliquity of about 40 the main results are as follows 1 the fault populations especially during the early stages of deformation are composed of faults that in strike are largely intermediate between rift parallel and perpendicular to displacement this fault population is characteristic of oblique rifts observed in previous studies 2 in later stages faults parallel to the rift become numerous in both models buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift parallel faults 3 during the final stages of extension in model b the crust is deformed by rift parallel faults while in the basins the small scale deformation pattern is composed of displacement normal faults however in model a displacement normal faults tend to accommodate most of the extension controlling its final stages they probably also control the formation of the ocean continent transition any possible mantle exhumation as well as the geometry of oceanic accretion centers these results provide an insight into the possible evolution of the gulf of aden conjugate margins which developed in an oblique context and most probably without any preexisting rift parallel localizing heterogeneity in the lithosphere

the rift like structure and asymmetry of the dead sea fault abstract whereas the dead sea fault is a major continental transform active since ca 13 18 ma ago it has a rift like morphology along its southern part it has been argued that this results from a transtensional component active since 5 ma ago due to a regional plate kinematics change we present the results of 3d laboratory experiments carried out to test this hypothesis and to explore its consequences for the structure and morphology of the dead sea fault we conclude that a two stage tectonic history invoking a first stage of pure strike slip and a second stage marked by the addition of a minor transtensional component is consistent with most of the striking geological and geophysical features of the dead sea fault the structural and morphological asymmetry of the dead sea fault can be explained by a transverse horizontal shear in the ductile lower crust below the transform zone a large scale heating event of the arabian mantle is not required to explain these features of the dead sea fault