dynamical lagrangian remeshing dlr a new algorithm for solving large strain deformation problems and its application to fault propagation folding abstract we present a new algorithm to solve the problem of rock deformation the method is based on a classical lagrangian finite element solver in which the connectivity between the nodes of the numerical mesh is updated at the end of each time or deformation step the method which we term dynamic lagrangian remeshing dlr relies on the theory of delaunay triangulation and produces optimal finite element meshes in the sense that nodes are always connected to their set of nearest neighbour nodes because it is based on a lagrangian method in which the numerical grid is attached to material particles and advected with the deformation the dlr algorithm is ideally suited to track material boundaries and properly represent free surfaces in addition in comparison to classical lagrangian solvers the dlr method is not limited to small strain problems and may be used to model discontinuities such as faults and narrow shear zones we have used the dlr method to study the evolution of a crustal layer undergoing finite shortening driven by a velocity discontinuity along its base the numerical model predicts the formation of a large fold that is progressively cut by a fault originating at the velocity discontinuity and propagating through the crustal layer the model predictions provide support for the fault propagation folding model of suppe 1

closure of the tornquist sea constraints from mona lisa deep seismic reflection data deep seismic reflection profiles west of denmark across the suture between baltica and eastern avalonia reveal weak southward dipping reflectors within the crystalline basement these reflectors are interpreted as thrusts resulting from emplacement of ea

a numerical dynamic model for the norwegian danish basin abstract a 2d numerical dynamic model for the late palaeozoic and mesozoic formation and evolution of the norwegian danish basin ndb is presented this basin developed in the eastern part of the northern permian basin within the system of northwest european sedimentary basins the basin forming processes are modelled in a lithospheric plane strain model along a 400 km profile across the basin layered rheology is used with elasto plastic behaviour in the brittle parts and elasto viscous behaviour at higher temperatures and pressures the mechanical equations of equilibrium and the heat equation are solved by the finite element technique the model contains two tectonic events basin initiation in late carboniferous and early permian times is modelled by combined lithospheric heating and lithospheric extension additional lithospheric extension however less pronounced and taking place over a longer period is introduced during the triassic isostatic effects associated with lithospheric extension combined with thermal contraction and sediment loading accounts for up to 6 km of sediments of which 2 3 km are of triassic age the model shows a generally good agreement with observations in terms of basin geometry sediment layer thicknesses stratigraphy crustal thickness and heat flow in the central parts of the basin the model implies a crustal thinning factor of about 1 5 from 35 km to about 23 km close to that observed in seismic data

changing perspectives in the concept of lago mare in mediterranean late miocene evolution abstract the cenozoic alpine orogeny caused the partition of tethys into several basins during the late neogene the mediterranean attained its final configuration whereas eastwards the paratethys isolated from the world ocean disintegrated progressively into a series of smaller basins as a result an endemic fauna developed in these basins mainly composed of brackish to freshwater faunas indicating an environment affected by changes in water salinity these small basins of the paratethys were named sea lakes by andrusov andrusov d 1890 les dreissenidae fossiles et actuelles d eurasie geol min 25 1 683 in russian subsequently this name was translated into lac mer gignoux m 1936 geologie stratigraphique 2 edition masson paris in the mediterranean isolated from the atlantic at the end of the miocene messinian thick evaporites deposited consisting of a marine lower evaporite unit and an upper evaporite unit mainly of continental origin ruggieri ruggieri g 1962 la serie marine pliocenica e quaternaria della val marecchia atti acad sci lett arti palermo 19 1 169 used the term lago mare to characterize the brackish to fresh water environment which occurred within the mediterranean at the end of the messinian during recent decades numerous scientific investigations concerning the history of the messinian within the mediterranean were devoted to the understanding of conditions prevailing after the deposition of the marine evaporites brackish to freshwater faunas are found in several outcrops and boreholes in the mediterranean both in the uppermost beds of gypsum and inter bedded within the clastic sediments of the upper evaporite unit immediately preceeding the flooding by the marine pliocene waters these faunas because of their similarities with the fauna described in the paratethys were named paratethyan or caspi brackish fauna this leading some authors to imply a migration of these fauna from paratethys to the mediterranean however others refute this hypothesis new data induced some researchers to consider that exchanges existed between the mediterranean and the eastern paratethys and also between the mediterranean and the atlantic ocean at the miocene pliocene transition these investigations now take advantage of the accurate time scales established by authors biostratigraphy cyclostratigraphy magnetostratigraphy allowing good stratigraphic correlations between the mediterranean and the paratethys and precisions on the geodynamic evolution of this area furthermore sediments at the base of the zanclean mpl1 locally containing brackish to fresh water faunas conducted authors to attribute this formation to an infra or pre pliocene and also to a lago mare event thus the lago mare concept drifted from its original meaning and is evolving because of progresses in the understanding of the mediterranean geodynamics and the adjacent areas during the miocene pliocene transition

late miocene to pliocene palaeogeography of the paratethys and its relation to the mediterranean abstract the palaeogeographic evolution of the paratethyan and mediterranean realms are reconstructed with three maps ranging from the late miocene to the middle late pliocene the maps are based on the facial analysis of selected deposits clastic input slope slide process analyses as well as biogeographic data of the planktic benthic and terrestrial biota characteristic fossil assemblages are used for palaeobathymetric and palaeohydrologic interpretations restoration of palaeogeographic connections the palaeogeographic reconstructions are palinspastically restored after dercourt j ricou l e vrielynck b eds 1993 atlas tethys palaeoenvironmental maps gauthier villars paris pp 1 307 14 maps the paleogeographic atlas of northern eurasia 1997 inst tectonics lithospheric plates moscow 26 maps with modifications maps have been prepared for the terminal tortonian early messinian late pannonian early maeotian late messinian pontian salinity crisis time and piacenzian gelasian akchagilian they illustrate the neogene palaeogeographic evolution during and after the attic orogenesis though the emerging mountain system of the alpine foldbelt increasingly separated the paratethys from the mediterranean tethys paratethys connections remained extant and sufficiently effective for limited communication between both basins they governed many features of the cyclic depositional history and biogeographic evolution of the eastern paratethys

a physical explanation of the relation between flank uplifts and the breakup unconformity at rifted continental margins rift flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted passive or atlantic type continental margins but are not predicted as primary features of simple lithospheric stretching models the explanations that have been proposed for their origin remain controversial either because they call upon special circumstances or because they are difficult to test plane strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture the results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases although the amplitudes of these uplifts depend on the model parameter values the relations between and relative signs of these two effects appear to be fundamental the explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models which assume local isostatic equilibrium through out the rifting process such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension which is not true if our explanation is correct

exchange through the bab el mandab during the last 10 years there have been a number of important developments in the understanding of the exchange flow in the bab el mandab new and improved estimates of the fluxes of mass heat and salt through the strait have been made directly from oceanographic measurements and indirectly from air sea flux data hydraulic theory has been applied to explain the seasonal variations in the exchange flow and in particular the three layer summer circulation the effects of wind stress friction and seasonal sea level variability on hydraulic control are addressed in this paper analyses of oceanographic data have failed to provide direct evidence of hydraulic control but they have highlighted the importance of tidal scale variability and differences between real continuously stratified fluids and idealised layer flows often used in hydraulic theory other studies have investigated the coupling between the flow in the strait and the circulation within the red sea

the post triassic evolution of the sorgenfrei tornquist zone results from thermo mechanical modelling abstract the sorgenfrei tornquist zone stz is part of the fennoscandian border zone separating the danish basin from the fennoscandian shield the stz as a structural element is of palaeozoic origin and represents the north westerly segment of the tornquist teisseyre zone which separates the younger west european crust from the older east european platform and extends from the black sea to the eastern north sea area the stz was reactivated in triassic jurassic extension and late cretaceous and paleogene compression this paper investigates the regional geological consequences of the reactivations by quantitative modelling along a profile across the stz in the danish area the numerical model invokes elastic viscous and plastic deformations of the lithosphere as well as surface processes governed by erosion sedimentation and lateral transport under the influence of eustatic sea level variations and regional isostatic compensation surface processes and lithospheric mechanics are coupled through thermal blanketing effects and loading the results in general address the regional geological consequences of the existence of intracontinental zones of structural weakness more specifically the results show that the late cretaceous and paleogene chalk depocentres in the danish basin are a direct consequence of the inversion of the stz and that the stz inversion together with falling sea level in cenozoic time are amongst the principal controlling factors in the geological evolution in the eastern north sea area

evidence for deep water deposition of abyssal mediterranean evaporites during the messinian salinity crisis scientific drilling of the abyssal evaporites beneath the deepest parts of the mediterranean basin gave rise to the idea that the mediterranean sea completely evaporated at the end of the messinian herein we show using new organic geochemical data that those evaporites were deposited beneath a deep water saline basin not in a subaerial saltpan as originally proposed abundant fossil organic lipids were extracted from evaporites in mediterranean deep sea drilling project cores the archaeal lipid distribution and new analyses using the ace salinity proxy and tex86 temperature proxy indicate that surface waters at the time of evaporite deposition had normal marine salinity ranging from 26 to 34 practical salinity units and temperatures of 25 28 c these conditions require a deep water setting with a mixed layer with normal marine salinity and an underlying brine layer at gypsum and halite saturation after correction for isostatic rebound our results indicate maximum drawdown of 2000 m and 2900 m relative to modern sea level in the western and eastern mediterranean basins respectively our results are consistent with previously proposed scenarios for sea level drawdown based on both subaerial and submarine incision and backfilling of the rhone and nile rivers which require messinian sea level drops of 1300 m and 200 m respectively this study provides new evidence for an old debate and also demonstrates the importance of further scientific drilling and sampling of deeper part of the abyssal messinian units

evolution of a carboniferous carbonate hosted sphalerite breccia deposit isle of man a newly discovered extensive sphalerite bearing breccia 7 5 wt zn is hosted in dolomitised carboniferous limestones overlying ordovician silurian metasedimentary rocks on the isle of man although base metal sulphide deposits have been mined historically on the island they are nearly all quartz vein deposits in the metamorphic basement this study investigates the origin of the unusual sphalerite breccia and its relationship to basement hosted deposits through a combination of petrographic cathodoluminescence fluid inclusion stable isotope and hydrogeologic modelling techniques breccia mineralisation comprises four stages marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry in stage i high temperature t h 300 c high salinity 20 45 wt equiv nacl fluid of likely basement origin deposited a discontinuous quartz vein this vein was subsequently dismembered during a major brecciation event stages ii iv are dominated by open space filling sphalerite quartz and dolomite respectively fluid inclusions in these minerals record temperatures of 105 180 c and salinities of 15 20 wt equiv nacl the 34s values of sphalerite 6 5 6 9 vienna canyon diablo troilite are nearly identical to those of ore sulphides from mines in the lower palaeozoic metamorphic rocks the 18o values for quartz and dolomite indicate two main fluid sources in the breccia s hydrothermal system local carboniferous hosted brines 0 5 6 0 vienna standard mean ocean water and basement involved fluids 5 5 11 5 ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot basement derived fluid that interacted with local sedimentary brines

post mid cretaceous eastern north sea evolution inferred from 3d thermo mechanical modelling abstract the late cretaceous cenozoic evolution of the eastern north sea region is investigated by 3d thermo mechanical modelling the model quantifies the integrated effects on basin evolution of large scale lithospheric processes rheology strength heterogeneities tectonics eustasy sedimentation and erosion the evolution of the area is influenced by a number of factors 1 thermal subsidence centred in the central north sea providing accommodation space for thick sediment deposits 2 250 m eustatic fall from the late cretaceous to present which causes exhumation of the north sea basin margins 3 varying sediment supply 4 isostatic adjustments following erosion and sedimentation 5 late cretaceous early cenozoic alpine compressional phases causing tectonic inversion of the sorgenfrei tornquist zone stz and other weak zones the stress field and the lateral variations in lithospheric strength control lithospheric deformation under compression the lithosphere is relatively weak in areas where moho is deep and the upper mantle warm and weak in these areas the lithosphere is thickened during compression producing surface uplift and erosion e g at the ringkobing fyn high and in the southern part of sweden observed late cretaceous early cenozoic shallow water depths at the ringkobing fyn high as well as cenozoic surface uplift in southern sweden the south swedish dome ssd are explained by this mechanism the stz is a prominent crustal structural weakness zone under compression this zone is inverted and its surface uplifted and eroded contemporaneously marginal depositional troughs develop post compressional relaxation causes a regional uplift of this zone the model predicts sediment distributions and paleo water depths in accordance with observations sediment truncation and exhumation at the north sea basin margins are explained by fall in global sea level isostatic adjustments to exhumation and uplift of the inverted stz this underlines the importance of the mechanisms dealt with in this paper for the evolution of intra cratonic sedimentary basins

seismic and gravity modelling of crustal structure in the central graben north sea observations along mona lisa profile 3 abstract high quality wide angle and deep normal incidence seismic data sets recorded during the mona lisa marine and onshore north sea acquisition for lithospheric seismic analysis experiment have allowed detailed crustal investigations in the area of the central graben and south eastern north sea to be conducted an integrated velocity density model based on a joint inversion of traveltimes and gravity data is presented for the crust and uppermost mantle along mona lisa line 3 which crosses the central graben approximately perpendicular to its strike direction this model shows pronounced crustal thinning and associated moho uplift beneath the deepest part of the central graben in the graben area moho is interpreted to be situated at 25 km depth and the crystalline crust is about a factor of 2 thinner than in the surrounding crustal provinces where moho is interpreted to reach depths between 32 and 36 km maximum thickness of the sedimentary column in the central graben is 10 11 km the mass deficit of the thick sediments is compensated by the moho uplift the modelled moho coincides with the base of lower crustal reflectivity observed in the coincident normal incidence seismic section good overall agreement between the integrated velocity density model and the normal incidence data is observed the migrated deep seismic reflection section recorded to 26 s traveltime contains deep west ward and east ward dipping events symmetric about the deepest part of the central graben these dipping upper mantle reflections may be linked to extensional processes that formed the central graben at least one of the upper mantle dipping reflectors may be a reactivated structure formed originally during caledonian collision

on the origin of the southern permian basin central europe abstract a detailed study of the structural and stratigraphic evolution of the southern permian basin during latest carboniferous to early jurassic times supported by quantitative subsidence analyses and forward basin modelling for 25 wells leads us to modify the conventional model for the rotliegend zechstein development of this basin the late permian early jurassic tectonic subsidence curves are typical for a permian to early triassic extensional stage that is followed by thermal subsidence however a purely extensional model is extremely problematic because active faulting during this time is minor and generally hard to document using inverse techniques to model the subsidence curves we quantitatively show that a significant component of late permian and triassic tectonic subsidence can be explained by thermal relaxation of early permian lithospheric thinning and by delayed infilling of paleo topographic depressions that developed during the early permian in this interpretation stephanian autunian wrenching resulted in thermal destabilisation of the lithosphere deep fracturing of the crust disruption and erosion of its sedimentary cover and regional uplift of the area of the future southern permian basin upon termination of wrench tectonics and associated volcanism towards the end of the autunian the southern permian basin began to subside in response to thermal contraction of the lithosphere the evolving basin was isolated from the world oceans and had subsided possibly up to some 700 m below their level at the beginning of upper rotliegend sedimentation after catastrophic flooding of this paleo topographic depression at the beginning of the zechstein changing sea level sedimentation and subsidence rates remained essentially in balance although the effects of triassic rifting overprinted parts of the southern permian basin its overall subsidence pattern persisted well into the jurassic in contrast to the remainder of the southern permian basin permian and triassic crustal extension contributed significantly to the subsidence of the polish trough

post permian evolution of the central north sea a numerical model abstract a dynamic thermomechanical model is presented for the post permian evolution of the central north sea including the central graben permo carboniferous and jurassic thermal events as well as triassic and jurassic extensional phases and late cretaceous shortening are considered the equations of force balance for a continuum and the transient heat equation are solved using the finite element method the lithosphere responds to applied forces by elastic deformation at low deviatoric stress levels fracture at low temperature and pressure conditions and nonlinear viscous flow at high temperature and pressure a first order rheological strain softening is incorporated to simulate a transition from strong to soft material behaviour such as a change from dislocation creep to diffusion creep seismic refraction and reflection data indicate crustal thinning below the central graben by a factor of 2 while the triassic rifting appears to have been associated with a more distributed deformation pattern the upper jurassic rifting is more localised pure shear extensional models have been advocated to explain this however dipping seismic reflectors in the upper mantle indicate that simple shear could be important the strain softening of the present model allows for modelling of strain localisation during the upper jurassic rifting phase and detailed modelling of moho structures the present model accounts for the main structural and stratigraphic observations in the central north sea furthermore the results indicate that in the part of the central north sea investigated in this paper a pure shear regime in triassic changed into a simple shear dominated regime with mantle shear zones developing during late jurassic extension

characteristics and controlling factors of reservoir space of mudstone and shale in es3x in the zhanhua sag abstract zhanhua sag is a widely accepted target zone with huge exploration and development potential for shale oil and shale gas resources many detailed studies have been undertaken around the geochemistry of the lower section of the third member of the shahejie formation es3x while few studies have focused on the reservoir in this study based on the mineralogical features and geochemical characteristics and by using statistical methods the characteristics and controlling factors of reservoir space of mudstone and shale in es3x in the zhanhua sag are explored through field emission scanning electron microscopy fe sem high pressure mercury injection capillary pressure micp and nuclear magnetic resonance nmr techniques three major findings were obtained there are micropores and microfractures in the reservoir space which include intergranular pores clay intercrystal pores pyrite intercrystal pores dissolved pores structural microfractures and bedding microfractures according to the features of pore size distribution psd the pore distribution can be divided into the following three categories 0 50 nm 50 nm 2 m and 2 m the average volumes of these components are 0 01079 ml g 1 0 00361 ml g 1 and 0 00355 ml g 1 respectively thus showing that the pores whose radii are distributed at 0 50 nm form the most important reservoir space though those with the 50 nm 2 m and 2 m radii are also important and cannot be ignored there are different controlling factors when it comes to different scale pores based on statistics and fe sem results the dissolved pores in calcite were determined to be the controlling factor for the 0 50 nm portion the intercrystalline pores in clay and pyrite and intergranular pores between authigenic minerals calcite dolomite and pyrite and clastic minerals calcite and dolomite were determined to be the controlling factors for the 50 nm 2 m portion and the structural microfractures and bedding microfractures were determined to be the main factors for the 2 m portion furthermore it is the brittle minerals content and bedded structure that control the microfractures this study thus clarifies the types and characteristics of reservoir space and identifies pore structure controlling factors of mudstone and shale in es3x in the zhanhua sag this information has important significance for future reservoir evaluations

dynamic hydrocarbon predictions for the northern part of the danish central graben an integrated basin analysis assessment abstract geohistory thermal history source capacity and oil generation have been examined in the north west part of the danish central graben using a one dimensional fluid flow compaction model the burial history suggests that this part of the danish central trough developed through three stages of subsidence a late jurassic differential stage a late cretaceous early tertiary uniform stage and a late miocene quaternary uniform stage pseudo wells drilled on seismic sections in areas without well data are used to improve the spacial distribution of wells the palaeotemperature and palaeoheat flow have been modelled by inversion of vitrinite reflectance data the inversion was carried out on wells with available vitrinite reflectance data and was based on known bottom hole temperature some temperature measurements with depth and the surface temperature the thermal history assessed by inversion of vitrinite reflectance data gives a consistently cooler past the available data s resolution of the thermal history is discussed the modelled maturation history of the upper jurassic shale in terms of vitrinite reflectance suggests that the shale reached maturity some 5 50 ma ago a geochemical study of the upper jurassic shales shows that these shales contain a mixture of type ii and type iii kerogens and have good to excellent source capacity modelling of the hydrocarbon generation data indicates that the peak generation took place some time between 10 ma bp and the present day possible migration paths are determined from modelled excess fluid pressure and four areas of possible accumulation of hydrocarbons are indicated the hydrocarbon potential of the areas are evaluated and an area along the eastern boundary fault between the tail end graben and the ringkobing fyn high is suggested as a new target area for further exploration

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

cretaceous early tertiary inversion tectonism in the danish central trough abstract the inversion of the danish central trough was heralded by a mild precursor during the late hauterivian before rifting had ended the main inversion took place during the late cretaceous it can be subdivided into several sub phases of which those occurring during the turonian santonian mid maastrichtian and post danian early tertiary are the most pronounced minor inversion tectonism occurred more or less continuously between these major events the turonian santonian event had the greatest impact in the northern part of the danish central trough whereas the effects of later events are most pronounced in the south throughout the late cretaceous danian a gradual change from more brittle deformation with faulting to more plastic deformation characterised by folding and flexuring can be seen

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

messinian events new constraints from sedimentological investigations and new 40ar 39ar ages in the melilla nador basin morocco abstract new 40ar 39ar ages obtained both from within a platform and from the adjacent basin allow through the extension of a previous argon dataset the establishment of a chronological framework for the messinian carbonate complex of melilla morocco in the platform prograding bioclastic deposits began around 6 87 0 02 ma whereas the youngest preserved deposits are 5 77 0 04 ma old the southern part of the basin was uplifted and emergent prior to 6 23 0 03 ma the tortonian messinian and the messinian pliocene boundaries have not yet been identified in the melilla region durations of the prograding bioclastic deposits ca 410 ka the prograding porites reefs ca 360 ka and the preserved part of the terminal carbonate complex tcc ca 330 ka can be estimated sedimentological and geochronological investigations do not indicate any major subaerial unconformity nor any major time gap if a major sea level fall did exist during messinian times it developed after 5 77 0 04 ma chronological correlations show that the onset of the sorbas gypsum deposition southern spain considered as the marker of the messinian salinity crisis onset is contemporaneous with aggrading porites reef building of the terminal carbonate complex of melilla a major paleoceanographic change in the marginal areas of the mediterranean with the cessation of upwelling systems could be partly coeval with the onset of the messinian salinity crisis

the messinian salinity crisis past and future of a great challenge for marine sciences forty years after the image of the mediterranean transformed into a giant salty lake was first conceived the fascinating history of the messinian salinity crisis msc still arouses great interest across a large and diverse scientific community early outcrop studies which identified severe palaeoenvironmental changes affecting the circum mediterranean at the end of the miocene were followed by investigations of the marine geology during the 1950s to 1970s these were fundamental to understanding the true scale and importance of the messinian event now after a long period of debate over several entrenched but largely untested hypotheses a unifying stratigraphic framework of msc events has been constructed this scenario is derived mainly from onshore data and observations but incorporates different perspectives for the offshore and provides hypotheses that can be tested by drilling the deep mediterranean basins the msc was an ecological crisis induced by a powerful combination of geodynamic and climatic drivers which had a great impact on the subsequent geological history of the mediterranean area and on the salinity of the global oceans these changed the mediterranean s connections with both the atlantic ocean and the freshwater paratethyan basins causing high amplitude fluctuations in the hydrology of the mediterranean the msc developed in three main stages each of them characterized by different palaeoenvironmental conditions during the first stage evaporites precipitated in shallow sub basins the msc peaked in the second stage when evaporite precipitation shifted to the deepest depocentres and the third stage was characterized by large scale environmental fluctuations in a mediterranean transformed into a brackish water lake the very high resolution timescale available for some late miocene intervals in the mediterranean makes it possible to consider environmental variability on extremely short time scales including in some places annual changes despite this fundamental questions remain some of which could be answered through new cores from the deepest mediterranean basins improvements in seismic imaging and drilling techniques over the last few decades make it possible to plan to core the entire basinal messinian succession for the first time the resulting data would allow us to decipher the causes of this extreme environmental change and its global scale consequences

orbitally forced paleoenvironmental and paleoclimate changes in the late postevaporitic messinian of the central mediterranean basin paleoenvironmental and paleoclimate changes that occurred during the late postevaporitic stage of the mediterranean basin in the messinian foreland domain of the adriatic region offer a new perspective on the relationship between orbital forcing and climate response the magnetic susceptibility record of the fonte dei pulcini a section maiella mountains italy allows us to orbitally tune the record between 5 394 and 5 336 ma and to temporally constrain the paleoenvironmental and paleoclimate changes evidenced by quantitative paleontological palynomorphs ostracods and calcareous nannofossils stable isotope 18 o and 13 c and x ray diffraction xrd analyses the base of the fonte dei pulcini a section is characterized by paratethyan ostracods and dinocysts which point to the late messinian lago mare biofacies loxocorniculina djafarovi zone of the mediterranean messinian stratigraphy from paleontological and geochemical 18 o analyses there is no evidence of a marine incursion in the fonte dei pulcini a section the major changes in terms of paleodepth paleosalinity evaporation versus precipitation aridity versus humidity and reworking processes occurred in the upper part of the fonte dei pulcini a section during the last messinian insolation cycle i cycle 511 512 which is characterized by high amplitude oscillations in contrast the lower part of the fonte dei pulcini a section which was deposited during relatively low amplitude insolation cycles is characterized by more stable environmental conditions comparing summer insolation with the paleoenvironmental changes at the fonte dei pulcini a section we identify delays of several thousands of years between orbital forcing and climate response

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

seismic reflectivity of a finely layered granulite facies ductile shear zone in the southern grenville province quebec abstract lithoprobe reflection profile 54 across the granulite facies morin shear zone msz in the grenville tectonic province quebec canada reveals a west dipping zone of strong reflections the origin of these reflections has been investigated by detailed field mapping laboratory measurements of p wave velocities and densities of representative rock samples from the main lithological units and forward synthetic modeling petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin layer clusters likely resulted from intensive tectonic transposition and metamorphic differentiation lattice preferred orientation lpo induced anisotropy of granulite facies mylonites except for those containing abundant sillimanite biotite or amphibole is very low and hence unlikely to be the primary cause for reflections confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 mpa since composite reflections of thin layer clusters can cause high reflectivity the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin layer mafic clusters within dominantly felsic rocks

heat flow and lithospheric thermal regime in the northeast german basin abstract new values of surface heat flow are reported for 13 deep borehole locations in the northeast german basin negb ranging from 68 to 91 mw m 2 with a mean of 77 3 mw m 2 the values are derived from continuous temperature logs measured thermal conductivity and log derived radiogenic heat production the heat flow values are supposed free of effects from surface palaeoclimatic temperature variations from regional as well as local fluid flow and from thermal refraction in the vicinity of salt structures and thus represent unperturbed crustal heat flow two d numerical lithospheric thermal models are developed for a 500 km section along the dekorp basin 9601 deep seismic line across the basin with a north eastward extension across the tornquist zone a detailed conceptual model of crustal structure and composition thermal conductivity and heat production distribution is developed different boundary conditions for the thickness of thermal lithosphere were used to fit surface heat flow the best fit is achieved with a thickness of thermal lithosphere of about 75 km beneath the negb this estimate is corroborated by seismological studies and somewhat less than typical for stabilized phanerozoic lithosphere modelled moho temperatures in the basin are about 800 c heat flow from the mantle is about 35 to 40 mw m 2 in the southernmost part of the section beneath the harz mountains higher moho temperatures up to 900 to 1000 c are shown while the relatively high level of surface heat flow in the negb obviously is of longer wave length and related to lithosphere thickness changes in crustal structure and composition are responsible for short wave length anomalies

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

the primary lower gypsum in the mediterranean a new facies interpretation for the first stage of the messinian salinity crisis abstract the detailed facies and physical stratigraphic analysis of the primary lower gypsum in the mediterranean indicates a surprising bed by bed correlation at basin scale spain italy hellenic arc and cyprus arc that is tuned to the orbital calibration for the first stage of the messinian salinity crisis from 5 96 to 5 61 ma ago a total of 16 precessionally controlled gypsum cycles were deposited rapidly in less than 350 ka forming sequences up to 300 m thick the lack of subaerial exposure features and the common facies associations and stacking pattern for sections located thousands of kilometers apart in different geological settings indicates a modest depositional depth not extremely shallow selenite deposition occurred only at the bottom of restricted marginal basins less than 200 m deep while no gypsum could precipitate in the deeper euxinic mediterranean portions where only thin and barren shale dolostone couplets formed the lowermost selenite beds pass laterally to dolomite rich limestones interbedded with barren euxinic shales in poorly oxygenated settings indicating that the gypsum sedimentation was diachronous and did not necessarily mark the onset on the messinian salinity crisis evaporite facies sequences ef1 to 8 within individual gypsum beds show small scale subaqueous sedimentary cycles that mimic regressive transgressive cycles a initial evaporite precipitation at relatively low supersaturation produced the massive selenite facies ef3 in a relatively deep setting b continuous evaporation and drawdown by oscillating brine level formed the banded selenite ef4 at the aridity acme of the precessionally controlled cycle c general progressive brine level rise with strong brine flow led to the formation of large selenite supercones branching laterally supercones in spain and branching selenite ef5 in the rest of the mediterranean and d flooding by undersaturated continental water terminated gypsum precipitation with the deposition of argillaceous sediments ef1 northern apennines and or limestone ef2 sicily and spain during the humid phase in the precession climate cycle the stacking pattern and selenite facies associations suggest an overall shallowing upward trend with a basin wide hydrologic change starting from the 6th cycle 5 84 ma which is marked by the appearance of the branching selenite facies supercones in spain and indicates that the brines became current dominated the sr isotope stratigraphy suggests that in the northern apennines the brines were strongly modified by continental waters 87 sr 86 sr 0 708893 to 0 708998 and received direct pulses of atlantic seawater 87 sr 86 sr 0 70900 to 0 709024 only in the upper part of the section in areas away from the mainland such as sicily the continental input was by far less important

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

upper mantle reflectors modelling of seismic wavefield characteristics and tectonic implications summary deep seismic experiments on continental lithosphere generally reveal marked reflectivity from structures in the crust and a significant decrease in reflectivity from the upper mantle however reflected and refracted energy from mantle lithosphere are observed in both near normal incidence and wide angle data the origin of the reflective structures is a matter of debate hypotheses include remnant subduction zones shear zones fluids and seismic anisotropy through analytical and numerical modelling studies including full wavefield modelling we investigate seismic characteristic signatures generated from a variety of geologically plausible models we have found that both upper mantle shear zones of reduced velocity and density and remnant subduction slabs containing high density eclogites may contain sufficient seismic impedance contrasts to normal mantle peridotites to generate near normal incidence reflectivity wide angle energy originates from subduction slabs containing either high or lowvelocity eclogites whereas intermediate velocity eclogites are unlikely to produce significant wide angle phases in general energy of seismic phases originating from upper mantle zones of anomalous seismic velocities and densities is significantly increased if homogeneous zones are replaced by zones of inhomogeneous petrophysical properties resulting from constructive interference maximum wavefield anomalies are generated from sub moho dipping slabs of incomplete transformation of low velocity low density crustal material to high velocity highdensity eclogites localized shear zones generated in mantle peridotite generally do not produce significant wide angle energy only if highly inhomogeneous structures containing material of marked ca 10 per cent velocity and density reduction are present may shear zones be observed in wide angle data analyses of two specific deep seismic data sets mona lisa data from the north sea and babel data from the baltic sea show good agreement between observations and modelling results for dipping remnant subduction slabs containing small scale inhomogeneities associated with incomplete transformation our modelling results improve our possibilities of distinguishing between two often contrasting tectonic interpretations for dipping upper mantle seismic reflectors the remnant subduction and shear zone models

mediterranean outflow pump an alternative mechanism for the lago mare and the end of the messinian salinity crisis the final stage of the messinian salinity crisis msc was characterized by brackish water lago mare conditions in the intermediate and marginal basins of the mediterranean sea the presence of paratethyan former black sea fauna in these deposits has fueled long lasting controversies over the connectivity between the mediterranean and paratethys and contemporary sea level drops in both basins here we use the results of sub precessional climate simulations to calculate the freshwater budget of the mediterranean and paratethys in the messinian we show that during the msc the freshwater budget of paratethys was positive while the mediterranean was negative using these numerical constraints we propose a mediterranean outflow pump as an alternative scenario for the two most dramatic hydrological changes in the msc 1 the halite lago mare transition and 2 the pliocene reestablishment of marine conditions following the maximum msc lowstand during halite formation progressive mediterranean sea level rise resulting from african river runoff and overspill from both the atlantic and paratethys eventually reached the level of the paratethys sill a density contrast at this gateway caused dense mediterranean waters to flow into the paratethys driving a compensatory return flow this pump mechanism significantly enhanced paratethyan inflow to the mediterranean creating suitable conditions for the lago mare fauna to migrate and thrive when the mediterranean sea level finally reached the height of the gibraltar sill mediterranean outflow restarted there and enhanced exchange with the atlantic ocean during this reorganization of the circulation brackish and hypersaline waters were pumped out of the mediterranean and open marine conditions were reestablished without major flooding of the basin at the miocene pliocene boundary

regional velocity depth anomalies north sea chalk a record of overpressure and neogene uplift and erosion a normal velocity depth trend for the upper cretaceous danian chalk group is determined by identifying interval velocity data that represent maximum burial in areas unaffected by overpressuring these data are derived from 845 wells throughout the north sea basin data from pelagic carbonate deposits on a stable plateau constrain the trend for shallow depths positive velocity anomalies relative to the trend are mapped along the western and eastern margins of the north sea basin and reflect regional neogene uplift and erosion of up to 1 km along the present day limit of the chalk a hiatus at the base of the quaternary increases in magnitude away from the basin center where a complete cenozoic succession is found this hiatus is consistent in size with the missing section estimated from chalk velocities when allowance is made for the quaternary reburial of the chalk negative velocity anomalies in the central and southern parts of the basin outline an area within which overpressures in the chalk exceed 10 mpa equivalent to a burial anomaly greater than 1 km relative to the normal trend the chalk pressure system is primarily dependent on overburden properties because retention of overpressure generated by the load of the upper overburden depends on the thickness and sealing quality of the lower overburden therefore the chalk is considered to represent a regional aquitard and the hydrodynamic model of long distance migration within the chalk is rejected the neogene uplift and erosion of the margins of the north sea basin and the rapid late cenozoic subsidence of its center fit into a pattern of late cenozoic vertical movements around the north atlantic

how did the messinian salinity crisis end the cause of the desiccation of the mediterranean sea during the messinian salinity crisis has been widely debated but its re flooding remains poorly investigated interpretations generally involve tectonic collapse of the strait of gibraltar or global sea level rise or even a combination of both the dramatic sea level fall in the mediterranean has induced deep fluvial incision all around the desiccated basin we investigate erosion dynamics related to this base level drop by using the numerical simulator eros we show that intense regressive erosion develops inevitably in the gibraltar area eventually inducing the piracy of the atlantic waters by an eastward flowing stream and the subsequent re flooding of the mediterranean

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

late cretaceous early tertiary tectonic evolution of the central european basin system cebs constraints from numerical modelling abstract the results obtained from a suite of spherical thin sheet finite element model calculations on the late cretaceous early tertiary tectonic evolution of the area of the central european basin system cebs are presented and discussed the calculations incorporate the effects of variations in lithospheric structures and compositions and changes in the imposed boundary conditions the impact of lateral contrasts such as pre existing weak domains is quantified through a thermo mechanical model for the entire cebs the obtained predictions have been constrained by comparisons with more recent observed field data and resemble the present day dynamics and kinematics a trial and error reconstruction of the development through time of the dynamic of the alpine orogeny as well as of the early atlantic extension have been considered in order to investigate the role of boundary forces in the late cretaceous early tertiary tectonic evolution of the cebs the observed evolution of subsidence depocentres and areas of uplift for the cebs during this time span has been reconstructed the results are in agreement with previous studies which have related the large scale late cretaceous early tertiary inversion within the cebs to long wavelength intra plate compressive stresses exerted mainly by the alpine thrust the models show that lateral heterogeneities have to be considered in order to model more local features such as the observed differential response of adjacent areas to the large scale stress regime this indicates that both intra plate tectonic forces and rheological structural contrasts must be taken into account to reconcile the style of intra plate deformation and to model the evolution through time of complex basin systems

three dimensional numerical simulations of crustal scale wrenching using a non linear failure criterion we have developed a three dimensional finite element model to study wrench deformation of the crust regarded as an elasto plastic material obeying murrell s extension of griffith s failure criterion numerical experiments using this model predict that the imposed basal wrenching is accommodated by an array of oblique riedel like shears and y shears parallel to the direction of wrenching the partitioning of deformation between the two types of structure depends on the width of the zone of imposed basal wrenching and the existence of a component of deformation in the x direction normal to the direction of wrenching the riedel shears are arranged in spiral like structures that root into the basal wrench zone in cross section the riedel shears resemble wedge shaped flower structures similar to those often observed in seismic cross sections the polarity of the flower structures is positive or palm tree like in transpression experiments and negative or tulip like in transtension experiments the orientation of the riedel shears throughout the crust obeys mohr s hypothesis for incipient faulting combined with murrell s failure criterion the model also predicts plastic dilatancy inversely proportional to the square root of the confining pressure this result agrees qualitatively with field observations and the results of sand box experiments and quantitatively with direct measurement of dilatancy during high pressure rock deformation experiments

seismic reflectivity of the whipple mountain shear zone in southern california the whipple mountains shear zone of southeastern california comprises part of the whipple mountains metamorphic core complex of the north american cordillera the 3 9 km thick shear zone displays excellent exposures of ductilely deformed mid tertiary mylonitic gneisses as well as kinematically related but younger cataclasites which underlie a major low angle normal fault the whipple detachment fault prominent crustal reflections beginning at 3 4 s were recorded on calcrust seismic profiles southwest of the whipple mountains in order to interpret these seismic reflections and to understand why detachment faults are imaged on some profiles but not on others we collected oriented rock specimens from all major structural units in the whipple mountains and determined p wave velocities for these samples parallel to three principal fabric directions under laboratory induced confining pressures up to 500 mpa using a geologic section through the mylonitic gneisses from a previous investigation we then constructed a detailed acoustic impedance section for the whipple mountain shear zone based upon laboratory results this section was in turn used to compose two dimensional synthetic reflection seismograms for comparison with the calcrust records significant impedance contrasts are enhanced by the opposing fabric orientations between the interlayered but structurally isolated relict domains of nonmylonitized rocks and the surrounding mylonitic gneisses the finite thickness and lateral extent of these rock units cause constructive interference in seismic modeling to create the seismic fabric of strong laterally discontinuous but subparallel reflections as seen in the calcrust profiles southwest of the whipple mountains chloritic breccias beneath the whipple detachment fault wdf have much lower seismic impedance than that of rocks occurring both structurally above and below however the significant thinning of the chloritic breccia zone to the southwest of the whipple mountains may have caused the poor image of the wdf on the calcrust profiles the absence of a detachment reflection is thus not necessarily indicative of the absence of a subsurface fault zone

the deep record of the messinian salinity crisis evidence of a non desiccated mediterranean sea abstract this research is focused on a complete reexamination of the evaporite facies present in all the cores that cut through the topmost deposits of the messinian salinity crisis lying below the floor of the mediterranean sea dsdp legs 13 and 42a odp legs 107 and 161 this review suggests that the uppermost evaporite units in both western and eastern deep mediterranean basins consist mainly of clastic gypsrudite gypsarenite and gypsiltite and fully subaqueous deposits laminar gypsum selenite and cumulate halite that are partially affected by burial anhydritization and tectonic induced recrystallization no unequivocal evidence of shallow water or even supratidal sabkha deposition is in evidence suggesting that at the very last phase of the salinity crisis the mediterranean sea did not experience desiccation but that deposition took place under permanent subaqueous conditions

a new facies model for the upper gypsum of sicily italy chronological and palaeoenvironmental constraints for the messinian salinity crisis in the mediterranean the upper gypsum unit of the caltanissetta basin sicily records the last phase of the messinian salinity crisis comprising the so called lago mare event a new facies analysis study recognizes nine to ten depositional cycles consisting of seven rhythmically interbedded primary gypsum bodies and two to three sandstone bodies separated by marly terrigenous horizons showing laterally persistent vertical organization a basal thin gypsum bed is overlain by a cluster of five thicker gypsum bodies a marly interval containing two distinct sandstone horizons separates this cluster from the overlying uppermost seventh gypsum body the terrigenous arenazzolo formation in turn followed by the lower pliocene trubi formation is considered here to form the uppermost part of the upper gypsum unit the rhythmic alternation in the sandy marls and gypsum sandstone bodies records the response of sediments from shelfal to deltaic systems to precession driven arid wet climate fluctuations causing cyclical changes of both base level and water concentration during wet climate phases at insolation maxima marl and sandstone were deposited in a hypohaline environment as suggested by i the typical lago mare faunal assemblage and ii the negative 18o values during arid phases at insolation minima the reduced meteoric supply recorded by higher 18o values in the carbonate caused the development of a negative hydrological budget leading to evaporite precipitation at a basinal scale the upper gypsum unit unconformably overlies a mainly clastic evaporite unit containing carbonate breccia the so called calcare di base and or clastic gypsum towards the basin centres where the basal contact becomes conformable a primary gypsum cumulate horizon is present this layer is interpreted as a possible lateral equivalent of the halite unit present only in the deepest depocentres based on astronomical calibration of the depositional cycles the upper gypsum unit including the arenazzolo formation spans the interval between 5 33 and 5 53 ma this new age calibration allows the deposition of the halite unit to be dated between 5 6 ma top of the lower evaporites and 5 55 ma base of the upper evaporites corresponding to isotopic stages tg12 and or tg14

paratethyan ostracods in the spanish lago mare more evidence for interbasinal exchange at high mediterranean sea level abstract a gigantic cascade of atlantic waters filling the deep desiccated mediterranean basin at the beginning of the pliocene has commonly been envisaged to end the messinian salinity crisis msc the mediterranean lowstand during its final lago mare phase however has long been subject to major controversy and has recently been seriously questioned again here we present high resolution ostracod distribution patterns of the msc successions in the black sea basin zheleznyi rog section russia and the mediterranean cuevas del almanzora section spain to study the origin and migration history of the lago mare ostracods we conclude that two major phases of faunal migration have taken place in the messinian the first phase corresponds to the maeotian pontian boundary interval 6 1 6 0 ma of the paratethys where mainly pannonian species suddenly invaded the black sea region the second migration event corresponds to the lago mare phase of the mediterranean when first 5 55 5 47 ma some opportunistic taxa species of cyprideis and loxoconcha genera and then 5 40 5 33 ma a more diverse assemblage of paratethyan species occupied the entire mediterranean region the spanish ostracod assemblages show a high percentage of paratethyan pontian ostracods in agreement with previously studied italian lago mare sections the similar palaeoenvironmental changes that developed roughly synchronously in the western and central mediterranean marginal basins provide more evidence for intrabasinal exchange at high water level during the final stage of the lago mare phase this indicates that the mediterranean in the latest messinian was full of water comparable to the present caspian sea and that the zanclean deluge if happened only surged some tens up to few hundred metres into the mediterranean

numerical modelling of strain localisation during extension of the continental lithosphere dipping reflectors in the upper mantle are observed on reflection seismic data and in some cases have been interpreted to represent mantle shear zones it is not clear however how these shear zones develop and under what circumstances previous attempts to model strain localisation by strain softening in the ductile regime have failed to show the development of mantle shear zones we present here results from a numerical model of lithospheric extension the model uses an elasto visco plastic rheology dependent on temperature pressure deviatoric stress and composition strain softening is incorporated into the mechanical model via a simple ad hoc strain viscosity relationship the results demonstrate that strain localisation can dominate lithospheric deformation in an extensional regime we illustrate the effects of varying the parameters which define the strain viscosity relationship on the geometry of lithospheric scale shear zones and the rift basin we also investigate the effects on shear zone development of introducing a variety of pre extension weak zones in the crust and or mantle our results suggest that if dipping reflectors observed on deep seismic data around the world are evidence of the existence of mantle shear zones there is a need to review our conceptual models of strain strength evolution during the development of continental rifts

sr isotope constraints on the mediterranean environment at the end of the messinian salinity crisis probably the most profound event that has affected the mediter ranean is the desiccation which occurred at the end of the miocene1 8 during this period there was a very rapid change from deep open marine conditions to environments of shallow water carbonate and massive evaporite deposition1 10 here we show that ostracod valves found in messinian lago mare lake sea sediments have a uniform but generally lower sr isotopic composition than contemporary sea water thus providing evidence that some of the major mediterranean basins for example balearic and levantine basins formed large lacustrine systems that were probably interconnected towards the end of the messinian salinity crisis1 3 after the major evaporites were deposited the most likely source of fresh water is from continental rivers flowing from the eastern european inland paratethys sea or perhaps the nile river in the lower part of the lago mare some ostracods have sr isotopic compositions identical to contemporary sea water indicating a transient influence of marine derived water sr compo sitions greater than that in contemporary sea water are only found in ostracods from onshore sediments in the sorbas basin4 5 of southeastern spain suggesting that this small basin adjacent to the straits of gibraltar became isolated after the deposition of the major evaporite sequences