identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

geometry and evolution of a fault bend fold mount bertha anticline mount bertha anticline is a major thrust associated fold in the rocky mountains of western canada well displayed in a 900 m thick middle paleozoic carbonate sequence that forms the core of the fold this carbonate sequence may be divided into three mechanical stratigraphic units of roughly similar thicknesses a lower massive dolostone a middle layered dolostone and an upper micritic limestone substantial variations in structural geometry occur along strike these variations correlate with changes in the thickness of the lower dolostone unit in the core of the fold that result from an oblique hanging wall ramp a mode i fault bend fold develops along most of the exposed strike length of mount bertha anticline this configuration corresponds to regions where a full thickness of the lower dolostone unit occurs in the core of the fold where the lower dolostone unit is thin or absent in the hanging wall of the mount bertha thrust the configuration of mount bertha anticline is dominated by second order fault propagation and detachment folds developed in the upper two mechanical stratigraphic units along the length of the mount bertha structure the limestone unit has been progressively attenuated and or faulted in the lower forelimb region and overridden by the advancing thrust sheet the fold geometry of the central part of mount bertha anticline compares favorably with geometric models of fault bend fold structures that allow forelimb thinning except for parts of the fold affected by the tectonic removal of rock from the lower forelimb existing geometric models of fault bend folds do not consider this process a companion fold within the hanging wall of the mount bertha thrust cranswick anticline is interpreted to be a mode i fault bend fold that has been significantly modified by second order fault propagation folding during the waning phases of movement along the mount bertha thrust

analogue models of folds above a wrench fault abstract this paper describes the experimental deformation of models made with sheets of paraffin wax simulating a bedded cover resting on a basement wrench fault during the experiments en echelon folds appear in the cover as a result of early fault motion folds first appear at heterogeneities in the bedding and with axes at about 45 to the trace of the wrench fault further fault displacement causes a bulk rotation of fold axes towards parallelism with the basement wrench fault and a resulting curvature of fold axes at larger fault displacement folding affects an area which tends to quickly stabilize in width since folding weakens the sheared cover and subsequent deformation is concentrated in it axial surfaces of folds are initially upright then tend to become inclined with an external vergence forming a fan centered on the basement wrench fault deeper layer deformation close to the basement involves fold reorientations that are greater than in the upper layers therefore down a given vertical line there is no continuity between surface and deep structures the geometry and orientation of folds appearing at later stages of wrenching is controlled by the geometry and orientation of already extant folds

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

style of atlassic tectonic deformation and geodynamic evolution of the southern tethyan margin tunisia the structure of the southern atlas fold and thrust belt of tunisia was investigated using new geologic mapping seismic reflection data together with the construction of a balanced cross section the structural architecture of the tunisian foreland consists in a mixed tectonic style with deep seated basement faults shallower decollements within sedimentary cover and salt diapirism the restoration of the cross section shows a surface shortening of 8 1 km similar to 7 3 sequential restoration of the balanced cross section has been permitted to decipher the structure and evolution of the southern tethyan margin of tunisia structural geometry and orientation of the pre existing permian to cretaceous extensional structures controlled subsequent contractional deformation within the sedimentary cover during the shortening this inherited tectonic framework controlled the development of ene trending thrust related anticlines such as the orbata and chemsi structures nw trending lateral ramp such as the gafsa and fejej faults and diapir structures as the beidha anticline the inversion of the margin can be correlated with the onset of the convergence between africa and eurasia the first phase of the orbata thrusting growth might occur in late cretaceous continuous inversion of the margin occurred during the eocene and mainly during the late miocene to present day this study underlines the predominant role of inherited basement structures acquired during the evolution of the southern tethyan margin and their influence on the geometry of the atlassic thrust belt this structural analysis provides new perspectives for future hydrocarbon exploration in this poorly explored region c 2015 elsevier ltd all rights reserved

propagation of a lithospheric tear fault step through the western boundary of the calabrian accretionary wedge offshore eastern sicily southern italy the rollback of a segmented slab of oceanic lithosphere is typically accompanied by vertical lithospheric tear fault s along the lateral slab edge s and by strike slip movement in the upper plate defined as a step fault subduction tear edge propagator the neogene evolution of the central mediterranean is dominated by the interaction between the slow africa eurasia convergence and the se ward rollback of the ionian slab that leads to the back arc opening of the tyrrhenian sea here we present post stack time migrated and pre stack depth migrated archimede 1997 multichannel seismic lines that were acquired offshore eastern sicily at the foot of the malta escarpment first we identify the recent deformation along the lateral ramp of the calabrian accretionary wedge towards the east the calabrian wedge is formed by the accretion of the post evaporitic sediments above a decollement at the base of the messinian evaporites at the latitude of syracuse 50 km east of the malta escarpment a major n150 degrees e trending crustal scale and vertical fault slices through the entire accretionary wedge this fault cuts by several kilometers through the pre evaporitic messinian sediments and into the basement the vertical offset along this vertical fault decreases from north to south and the fault is no longer observed on the seismic lines 50 km se of the alfeo seamount a previously published moho depth isocontour map offshore sicily and the recent gps data combined with the presence of strike slip movements ne onshore sicily allow us to identify this 200 km long crustal scale fault as the surface expression of a step fault the presence of syntectonic pleistocene sediments on top this crustal scale fault suggests a recent lithospheric vertical movement of the step fault in response to the rollback of the ionian slab and to the se ward advance of the calabria peloritan block

morphotectonic characterization of major bathymetric lineaments in gulf of cadiz africa iberia plate boundary insights from analogue modelling experiments abstract new high resolution multi beam bathymetry data allowed the recognition of several bathymetric lineaments ca 100 km long trending wnw ese in seafloor sediments of the gulf of cadiz offshore sw iberia the interpretation of multi channel mcs profiles crosscutting these lineaments showed that they are controlled by underlying deep seated faults which have endured a polyphase reactivation history to get insights on the recent tectonic evolution of these structures we performed two sets of analogue modelling experiments assuming 1 right lateral strike slip basement faulting and coupled passive shearing affecting an overlying soft cover and 2 low angle transpressive deformation along a narrow shear band overlying the fault our results show a good correlation between the experimentally obtained structural patterns and the natural morphotectonic lineaments allowing the use of some of the observed natural features as strain gauges based on this we conclude that the study lineaments correspond to the bathymetric expression of ongoing dextral wrenching reactivation of wnw ese pre existing faults and we estimate the age of this tectonic reactivation as being ca 1 8 ma i e form late pliocene to present day these characteristics agree with the most recent kinematic models derived from geodetic observations indicating that present day convergence between nubia and iberia is subparallel to the newly identified lineaments and occurs at a 4 mm yr rate

halokinesis and structural evolution of the major features in eastern and southern tunisian atlas abstract the eastern and southern tunisian atlas is marked by the presence of three major faults the n s axis fault the nw se gafsa fault and the e w northern chott fault seismic lines crossing these structures show the existence of significant halokinetics that began in the early jurassic the aim of this paper is to study and better understand the triassic salt movements which are mainly linked to periods when the major basement faults were reactivated resulting in extension compression transtension or and transpression analysis of the seismic data allows us to conclude that the triassic salt migration started in the early jurassic and continued during the late jurassic and the early cretaceous although these periods are characterized by an extensional tectonic regime the synsedimentary halokinesis is accompanied by thickening and thinning of the overlying series salt remobilization was accentuated during the compressional deformation which occurred from the late tertiary to quaternary and resulted in local diapiric extrusions along the major structural features

mantle dynamics in the mediterranean the mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt lithospheric dynamics are affected by slab rollback and collision of two large slowly moving plates forcing fragments of continental and oceanic lithosphere to interact this paper reviews the rich and growing set of constraints from geological reconstructions geodetic data and crustal and upper mantle heterogeneity imaged by structural seismology we proceed to discuss a conceptual and quantitative framework for the causes of surface deformation exploring existing and newly developed tectonic and numerical geodynamic models we illustrate the role of mantle convection on surface geology a coherent picture emerges which can be outlined by two almost symmetric upper mantle convection cells the downwellings are found in the center of the mediterranean and are associated with the descent of the tyrrhenian and the hellenic slabs during plate convergence these slabs migrated backward with respect to the eurasian upper plate inducing a return flow of the asthenosphere from the backarc regions towards the subduction zones this flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath anatolia and eastern iberia this convection system provides an explanation for the general pattern of seismic anisotropy in the mediterranean first order anatolia and adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as anatolia and eastern iberia more generally the mediterranean is an illustration of how upper mantle small scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the tethyan collision

tectonic evolution of the northern african margin in tunisia from paleostress data and sedimentary record abstract a reconstruction of the tectonic evolution of the northern african margin in tunisia since the late permian combining paleostress tectonic stratigraphic and sedimentary approaches allows the characterization of several major periods corresponding to consistent stress patterns the extension lasting from the late permian to the middle triassic is contemporaneous of the rifting related to the break up of pangea during liassic times regional extensional tectonics originated the dislocation of the initial continental platform in northern tunisia the evolution of the liassic ne sw rifting led during dogger times to the north african passive continental margin whereas in southern tunisia a n s extension associated with e w trending subsiding basins lasted from the jurassic until the early cretaceous after an upper aptian early albian transpressional event ne sw to ene wsw trending extensions prevailed during late cretaceous in relationship with the general tectonic evolution of the northeastern african plate the inversions started in the late maastrichtian paleocene in northern tunisia probably as a consequence of the africa eurasia convergence two major nw se trending compressions occurred in the late eocene and in the middle late miocene alternating with extensional periods in the eocene oligocene early middle miocene and pliocene the latter compressional event led to the complete inversion of the basins of the northwestern african plate originating the maghrebide chain such a study supported by a high density of paleostress data and including complementary structural and stratigraphic approaches provides a reliable way of determining the regional tectonic evolution

analogue modelling of intraplate strike slip tectonics a review and new experimental results abstract intraplate strike slip zones commonly display intricate 3 d geometries with rapid changes in structural style along strike and with depth strike slip deformation typically results in complex vertical and horizontal sections that can be difficult to interpret coherently physical modelling of strike slip fault systems is a powerful and graphic tool to help in providing a unified picture of the evolution of strike slip zones with considerable spatial and temporal detail a large number of experimental studies have investigated different aspects of strike slip tectonics using materials such as dry sand wet clay or silicone the choice of analogue material and experimental design exerts a strong control on the structures that form in the model here we present a review of different experimental setups used to investigate intraplate strike slip tectonics from the classical riedel experiment to more sophisticated setups using brittle and viscous analogue materials we review our current understanding from models of distributed shear transtension transpression pull apart basins formed in releasing stepovers and popups formed in restraining stepovers in addition we present the results of two new experimental series that investigated 1 the effect of crustal weak zones on strike slip fault zone segmentation and 2 strike slip and transpressional reactivation of extensional basins

extensional collapse related to compressional uplift in the alpine chain off northern tunisia central mediterranean abstract geophysical and geological marine data recently collected allow to outline the structure of the basement involved fold thrust belt developed during the late cenozoic between sardinia and tunisia along the africa europe plate boundary by integrating these with inland data it is possible to document step by step the progression of crustal thickening from north to south and the collapse of the first uplifted northern units while collision was still going on the geodynamic setting suggests that coupled extensional collapse and fold thrust propagation were driven by island arc drifting above a southward retreating subduction zone

paleostress reconstructions and geodynamics of the baikal region central asia part 2 cenozoic rifting abstract investigations on the kinematics of rift opening and the associated stress field present a renewed interest since it has recently been shown that the control of the origin and evolution of sedimentary basins depends to a large extent on the interplay between lithospheric strength and applied stresses it appears that changes of stress field with time are an important factor that either controls or results from the rifting process the object of this paper is to study the changes of fault kinematics and paleostress field with time in the baikal rift system during the cenozoic reduced paleostress tensors were determined by inversion from fault slip data measured in the central part of the rift and its southwestern termination between 1991 and 1995 results show that the stress field varies as well in time as in space two major paleostress stages are determined corresponding broadly to the classical stages of rift evolution late oligocene early pliocene and late pliocene quaternary the first paleostress stage is related to the rift initiation and the second to the major stage of rift development similarities between the recent paleostress field and the present day stress field inverted from focal mechanisms indicate that the second paleostress stage is still active therefore we propose to use proto rift for the late oligocene early pliocene stage and active rift for the late pliocene quaternary stage of rift development during the proto rift stage the stress field was characterized by a compressional to strike slip regime a progressive change from transpression to transtension is suspected for the central part of the rift baikal and barguzin basins during this period in the western termination of the rift sayan massif tunka depression a strongly compressional stress field with oblique thrusting kinematics is well constrained in the late miocene early pliocene interval the active rift stage was initiated by a marked change in fault kinematics and stress regime in the late pliocene in the central part of the rift the stress regime changed into pure extension while in the southwestern extremity it changed into pure strike slip fault kinematics suggests that rifting was initiated by an extrusion mechanism due to the interaction of far field compressional stress on a mechanically heterogeneous crust with the southwards pointing wedge of the siberian craton acting as a passive indentor the cenozoic time space evolution of the stress field is believed to reflect the increasing influence of locally generated buoyancy extensional stresses associated with density anomalies of the lithosphere on intraplate stresses generated by the india eurasia convergence and the west pacific subduction

the role of slab detachment processes in the opening of the western central mediterranean basins some geological and geophysical evidence abstract a review of the geological and geophysical data from the central and western mediterranean region and the present day upper mantle structure derived from tomographic studies are utilized in order to define the oligocene recent geodynamic evolution for the area in line with previous work we suggest that the miocene quaternary opening of the western and central mediterranean basins is the result of back arc extension due to the roll back toward the southeast of a northwestward subducting african slab in a geodynamic setting pinned between the alpine and betic collisional zones we find however that this general pattern is complicated by four different detachment events which occurred beneath the alps early oligocene the betic chain aquitanian northern africa langhian and the apennines late miocene pliocene we show that each of these events determines a major tectonic reorganization within the european plate

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

numerical modeling of fold initiation at thrust ramps abstract the movement of the hangingwall over the footwall at thrust ramps produces a variety of structures found commonly at different scales in fold and thrust belts the nature of the structure depends on the relative rigidity of the hangingwall and footwall friction along the fault fault dip fault displacement and the boundary conditions of the deformation structures include fault bend style folds fault propagation style folds and wedge folds we investigate the initial stages of development of such structures using the finite difference code flac the rock layers are represented as continua with elastic plastic mohr coulomb constitutive relations and the fault and bounding bedding planes are assigned normal and shear stiffnesses and coefficients of friction under conditions with all layers compressed antisymmetric fault propagation style folds develop in both the hangingwall and footwall with rigid footwall and restricted far field slip in the hangingwall a single fault propagation style fold develops with far field displacement of the hangingwall allowed broader antisymmetric wedge folds develop if hangingwall and footwall are deformable and a single fault bend style fold develops if the footwall is rigid all structures become accentuated with increasing slip on the fault where both hangingwall and footwall are deformable the deformation reduces the ramp angle and tends to minimize distortion of the rock adjacent to the fault fault propagation style folds paired wedge folds and fault bend style folds are common in nature and small scale examples can be found in various stages of development continued slip on thrust faults may lead to the mature structures commonly seen in fold and thrust belts

transcurrent fault activity on the dead sea transform in lebanon and its implications for plate tectonics and seismic hazard fault activity within the lebanese transpression zone one of the classic examples of restraining bend development on a continental transform is here investigated using dated geomorphological features the northern part of the yammouneh fault commonly considered to be the principal active strand on this part of the dead sea transform has been inactive for the past 5 ma field observations show that basalts dated as late pliocene hi age apparently offset by the yammouneh fault unconformably overlie it the active transcurrent structure is principally perhaps exclusively the roum fault the lebanese transpressive zone has evolved through tune with migration of fault activity these results confirm the overlapping transform hypothesis for the dead sea system require the active triple junction between the transform and the tethyan collision belt to lie offshore se cyprus and have profound implications for assessing seismic hazard hi the levant

paleomagnetic study of cenozoic sediments from the zaisan basin se kazakhstan and the chuya depression siberian altai tectonic implications for central asia abstract this paper presents new paleomagnetic results on cenozoic rocks from northern central asia eighteen sites were sampled in pliocene to miocene clays and sandy clays of the zaisan basin southeastern kazakhstan and 12 sites in the upper oligocene to pleistocene clays and sandstones of the chuya depression siberian altai thermal demagnetization of isothermal remanent magnetization irm showed that hematite and magnetite are the main ferromagnetic minerals in the deposits of the zaisan basin stepwise thermal demagnetization up to 640 660 c isolated a characteristic chrm component of either normal or reverse polarity at nine sites at two other sites the great circles convergence method yielded a definite direction measurements of the anisotropy of magnetic susceptibility showed that the hematite bearing sediments preserved their depositional fabric these results suggest a primary origin of the chrm and were substantiated by positive fold and reversal tests the mean paleomagnetic direction for the zaisan basin d 9 i 59 k 19 95 11 is close to the expected direction derived from the apw path of eurasia j geophys res 96 1991 4029 and shows that the basin did not rotated relative to stable asia during the tertiary in the upper pliocene pleistocene sandstones of the chuya depression a very stable chrm carried by hematite was found its mean direction d 9 i 46 k 25 95 7 is characterized by declination close to the one excepted for early quaternary whereas inclination is lower in the middle miocene to lower pliocene clays and sandstones a stable chrm of both normal and reverse polarities carried by magnetite was isolated its mean direction d 332 i 63 k 31 95 4 is deviated with respect to the reference direction and implies a neogene 39 8 counterclockwise rotation of the chuya depression relative to stable asia these results and those from the literature suggest that the different amount of rotation found in the two basins is related to a sharp variation in their tectonic style predominantly compressive in the zaisan basin and transpressive in the siberian altai at a larger scale the pattern of vertical axis rotations deduced from paleomagnetic data in northern central asia is consistent with the hypothesis of a large left lateral shear zone running from the pamirs to the baikal heterogeneous rotations however indicate changes in style of faulting along the shear zone and local effect for the domains with the largest rotations

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

the post triassic sedimentary cover of tunisia seismic sequences and structure abstract a series of geological cross sections based on data from the new geological map of tunisia coupled with stratigraphic and structural studies indicate that a folded post triassic sedimentary cover was separated from the basement by a decollement zone located in triassic evaporitic deposits four depositional basins were formed during the triassic and liassic in an extensional regime on a presumably thinned continental crust geological fieldwork studies of metamorphic grade and gravimetric and magnetic surveys indicate that their depocentres shifted with time from basin to basin finally resulting in today s tellian and tunisian troughs to the north gafsa trough to the west of gabes and the tuniso libyan trough to the east subsidence in the latter trough commenced at the end of the cretaceous and is still continuing today there are four well defined seismic stratigraphic sequences comprising mesozoic and cenozoic sedimentary rocks each with distinctive lithoseismic characteristics we display the distribution and structure of these seismic sequences in a series of cross sections in addition we note that the regional tectonic style of tunisia has been greatly influenced by a conjugate strike slip fault system the role of wrench faulting has been well documented in the kasserine and other areas by detailed structural studies

4d analogue modelling of transtensional pull apart basins abstract scaled sandbox models were used to investigate the 4d evolution of pull apart basins formed above underlapping releasing stepovers in both pure strike slip and transtensional basement fault systems serial sectioning and 3d volume reconstruction permitted analysis of the full 3d fault geometries results show that very different pull apart basins are developed in transtension compared to pure strike slip both types of models produced elongate sigmoidal to rhomboidal pull apart systems but the transtensional pull apart basins were significantly wider and uniquely developed a basin margin of en echelon oblique extensional faults dual opposing depocentres formed in the transtensional model whereas a single central depocentre formed in pure strike slip in transtension a distinct narrow graben system formed above the principal displacement zones pdzs cross basin fault systems that linked the offset pdzs formed earlier in the transtensional models sequential model runs to higher pdz displacements allowed the progressive evolution of the fault systems to be evaluated in cross section transtensional pull aparts initiated as asymmetric grabens bounded by planar oblique extensional faults with increasing displacement on the pdzs basin subsidence caused these faults to become concave upwards and lower in dip angle due to fault block collapse towards the interior of the basin in addition strain partitioning caused fault slip to become either predominantly extensional or strike slip the models compare closely with the geometries of natural pull apart basins including the southern dead sea fault system and the vienna basin austria

a 17 ma onset for the post collisional k rich calc alkaline magmatism in the maghrebides evidence from bougaroun northeastern algeria and geodynamic implications bougaroun is the largest pluton 200 km2 in the 1200 km long neogene magmatic belt located along the mediterranean coast of maghreb new u pb dating on zircons and k ar ages on whole rocks and separated minerals document its emplacement at 17 ma within the lesser kabylian basement a continental block that collided with the african margin during the neogene this upper burdigalian intrusion is therefore the oldest presently identified k rich calc alkaline massif in the whole maghrebides magmatic lineament and marks the onset of its activity the bougaroun peraluminous felsic rocks display a very strong crustal imprint associated mafic rocks lree enriched gabbros have preserved the orogenic subduction related geochemical signature of their mantle source older depleted gabbros cropping out at cap bougaroun are devoid of clear subduction related imprint and yielded ar ar hornblende ages of 27 0 3 0 ma and 23 3 3 2 ma we suggest that they are related to the upper oligocene back arc rifted margin and early miocene oceanic crust formation of the nearby jijel basin an extension of the algerian basin developed during the african tethyan slab rollback the fact that the bougaroun pluton intrudes exhumed kabylian lower crustal units mantle slices and flysch nappes indicates that the kabylian margin was already stretched and in a post collisional setting at 17 ma we propose a tectono magmatic model involving an early miocene tethyan slab breakoff combined with delamination of the edges of the african and kabylian continental lithospheres at 17 ma the asthenospheric thermal flux upwelling through the slab tear induced the thermal erosion of the kabylian lithospheric mantle metasomatized during the previous subduction event and triggered its partial melting we attribute the strong trace element and isotopic crustal signature of bougaroun felsic rocks to extensive interactions between ascending mafic melts and the african crust underthrust beneath the kabylie de collo basement

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

tectonics of strike slip restraining and releasing bends restraining and releasing bends are common but enigmatic features of strike slip fault systems occurring in all crustal environments and at regional to microscopic scales of observation regional scale restraining bends are sites of mountain building transpressional deformation and basement exhumation whereas releasing bends are sites of topographic subsidence transtensional deformation basin sedimentation and possible volcanism and economic mineralization because restraining and releasing bends often occur as singular self contained domains of complex deformation they are appealing natural laboratories for earth scientists to study fault processes earthquake seismology active faulting and sedimentation fault and fluid flow relationships links between tectonics and topography tectonic and erosional controls on exhumation and tectonic geomorphology this volume addresses the tectonic complexity and diversity of strike slip restraining and releasing bends with 18 contributions divided into four thematic sections 1 a topical review of fault bends and their global distribution 2 bends sedimentary basins and earthquake hazards 3 restraining bends transpressional deformation and basement controls on development 4 releasing bends transtensional deformation and fluid flow

active oblique ramp faulting in the southern tunisian atlas abstract the gafsa fault is the longest and most active structure of the fold and thrust belt achieving southeastward propagation of the atlas belt of eastern north africa onto the saharan platform the gafsa fault is a 75 km long dextral oblique basement fault ramp that poses a sizable challenge in earthquake hazard assessment because the post paleozoic sedimentary cover is decoupled from its basement above the basement fault in this study we combine seismic lines interpretation tectonic geomorphology and paleoseismological investigations to assess the level of seismic hazard of this fault and evaluate its role in the geodynamic framework of the central mediterranean we show that despite a moderate instrumental and historical seismicity the fault has produced m 6 earthquakes with a return period of ca 500 5000 years during the late quaternary the latest large event having produced a surface rupture on the fault occurred around 8000 yr bp suggesting an m 6 earthquake is overdue on the fault the fault has a minimum reverse component of slip rate of 0 21 0 34 mm yr over the past 50 ka the occurrence of m 7 paleoearthquakes on the fault may be suspected but not established such very strong earthquakes would require transient coseismic linkage of the buried basement fault with the overlying listric fault ramping off the decollement layer the level of seismic hazard may be underestimated on the gafsa fault indeed given the geometry of the basement cover fault system a number of earthquakes generated in the basement would have led to coseismic surface folding instead of to surface rupture the gafsa fault is a major structure accommodating eastward extrusion spreading of the atlas belt onto the saharan and pelagian plateforms above the retreating ionian lithospheric slab

block rotation and termination of the hosgri strike slip fault california from three dimensional map restoration the hosgri fault is located immediately offshore of south central california and is part of the transform boundary between the pacific and north american plates this fault terminates to the southeast into east trending folds and reverse separation faults of the western transverse ranges our new structure contour maps of deformed horizons show a spatial relationship between faulting and folding consistent with right lateral slip restoration of these digital maps quantifies post miocene right lateral slip across the southern hosgri fault to be 3 5 km this slip is absorbed by folding thrust overlap and clockwise vertical axis rotation of elongate blocks between strands of the fault the restored part of a block located to the east has rotated 8 clockwise extrapolating this restored rotation to the 50 km block length produces an estimate of 7 km of dextral shear for a total including the 3 5 km fault slip of 10 5 km of post miocene displacement our three dimensional approach precludes interpretations for reverse slip on the hosgri fault and is not consistent with models for more than 80 km of late cenozoic right lateral fault slip

the structure and rheological evolution of reactivated continental fault zones a review and case study abstract repeated reactivation of structures and reworking of crustal volumes are characteristic though not ubiquitous features of continental deformation reactivated faults and shear zones exposed in the deeply exhumed parts of ancient orogenic belts present opportunities to study processes that influence the mechanical properties of long lived fault zones at different palaeo depths ancient basement fault systems typically comprise heterogeneous superimposed assemblages of fault rocks formed at different times and depths for which down temperature thermal histories are most common several lithological and environmental factors influence the evolution of fault rock fabrics and rheology but most fault shear zone arrays appear to develop as self organized deformation systems once mature the kinematic and mechanical evolution of the system is strongly influenced by the rheological behaviour of the interconnected fault shear zone network a case study from the crustal scale great glen fault zone ggfz scotland reveals a complex evolution of mid to upper crustal deformation textures formed adjacent to the frictional viscous transition fluid influx in the mid crust has led to reaction softening of the rock aggregate as strong pre existing phases such as feldspar are replaced by fine grained strongly aligned aggregates of weak phyllosilicates in addition a grainsize controlled switch to fluid assisted diffusional creep occurs in the highest strain regions of the fault zone it is proposed that this led to a shallowing and narrowing of the frictional viscous transition and to long term overall weakening of the fault zone relative to the surrounding wall rocks cataclasis is particularly important in the deeper part of the frictional regime as it helps to promote retrograde metamorphism and changes in deformation regime by both reducing grainsize and promoting pervasive fluid influx along fault strands due to grain scale dilatancy equivalent processes are likely to occur along many other long lived crustal scale fault zones

eocene simple shear and plio quaternary pure shear folding in the central eastern algerian atlas tertiary folds of the central and eastern part of the algerian atlas were studied in order to assess their kinematics and regimes of deformation folds developed following two main phases of deformation during the eocene and the pliocene the ne sw trending eocene folds show a clockwise rotation of fold axis with depth noncylindrical geometry and a right stepping en echelon configuration early secondary structures inside each fold are also rotated clockwise with respect to the younger folds these data are in agreement with progressive simple shear deformation pliocene folds strike e w and display a cylindrical geometry associated plio quaternary brittle structures show no rotational path these observations are compatible with pure shear style of deformation these results in combination with focal mechanism solutions have important implications for the understanding of the kinematic evolution of the western segment of the african european plate boundary for algeria they imply that transcurrent motion during the paleogene was followed by coaxial shortening with a nnw direction during neogene quaternary times comparison with published data for morocco and tunisia suggests that this transcurrent regime developed from west to east continuing from the middle jurassic through to the miocene

fault development and interaction in distributed strike slip shear zones an experimental approach analogue model experiments using both brittle and viscous materials were performed to investigate the development and interaction of strike slip faults in zones of distributed shear deformation at low strain bulk dextral shear deformation of an initial rectangular model is dominantly accommodated by left stepping en echelon strike slip faults riedel shears r that form in response to the regional bulk stress field push up zones form in the area of interaction between adjacent left stepping riedel shears in cross sections faults bounding push up zones have an arcuate shape or merge at depth adjacent left stepping r shears merge by sideways propagation or link by short synthetic shears that strike subparallel to the bulk shear direction coalescence of en echelon r shears results in major through going faults zones master faults several parallel master faults develop due to the distributed nature of deformation spacing between master faults is related to the thickness of the brittle layers overlying the basal viscous layer master faults control to a large extent the subsequent fault pattern with increasing strain relatively short antithetic and synthetic faults develop mostly between old but still active master faults the orientation and evolution of the new faults indicate local modifications of the stress field in experiments lacking lateral borders closely spaced parallel antithetic faults cross faults define blocks that undergo clockwise rotation about a vertical axis with continuing deformation fault development and fault interaction at different stages of shear strain in our models show similarities with natural examples that have undergone distributed shear

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

analog models of restraining stepovers in strike slip fault systems scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike slip fault systems in rigid basement models were run both with and without synkinematic sedimentation which was added incrementally to cover the growing antiformal structures vertical and horizontal sections of the completed models permit the full three dimensional 3 d structure of the pop ups to be analyzed in detail three representative end member experiments are described 30 underlapping restraining stepovers 90 neutral restraining stepovers and 150 overlapping restraining stepovers the experimental pop ups are typically sigmoidal to lozenge shaped antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults underlapping restraining stepovers typically form elongate lozenge shaped pop ups 90 neutral restraining stepovers produce shorter squat rhomboidal pop ups and overlapping restraining stepovers produce sigmoidal antiformal pop ups trans pop up cross fault systems are characteristic at large displacements on the basement fault system above the offset principal displacement zones the pop ups are commonly small narrow positive flower structures whereas in the stepover region they widen out and become markedly asymmetric this pop up asymmetry switches across the center of the stepover where the pop ups are largely symmetical maximum rotations measured within the central highly uplifted region of the pop ups increase from 7 counterclockwise for the underlapping 30 stepovers to 14 counterclockwise for the neutral 90 stepovers to 16 counterclockwise for the overlapping 150 stepovers in models where no synkinematic sediments were added during deformation the pop up structures are bound by convex flattening upward oblique slip reverse fault systems that link downward to the offsets in the basement fault system in contrast in the begin page 234 experiments where synkinematic sediments were added incrementally during deformation the pop ups are formed by oblique slip reverse faults that steepen upward into the synkinematic strata with the formation of fault propagation growth folds the analog models are compared with natural examples of pop up structures and show strong similarities in structural geometries and stratal architectures these models may provide structural templates for seismic interpretation of complex contractional structures in offset strike slip fault systems

analog modeling of pull apart basins scaled sandbox models have successfully simulated the geometries and progressive evolution of pull apart basins developed in a weak sedimentary cover above right stepping releasing dextral strike slip fault systems in rigid basement synkinematic basin infill was added progressively as the models were deformed vertical and horizontal sectioning of the completed models has allowed the full three dimensional architecture of the pull apart system to be analyzed we present three representative end member experiments 30 underlapping releasing sidestep 90 sharp nonoverlapping releasing sidestep and 150 overlapping releasing sidestep the pull apart basin geometries are typically sigmoidal to rhombic grabens the geometries of which are dependent upon the architecture of the underlying basement fault systems underlapping releasing sidestepping faults offset angles of 30 75 typically form elongate rhomboidal grabens 90 releasing offset sidesteps produce shorter squat rhomboid pull apart basins and overlapping releasing sidesteps 115 150 produce box like grabens with highly kinked basin sidewalls the pull apart basins are bounded by terraced oblique slip extensional sidewall fault systems that link the laterally offset principal displacement zones pdz of the main dextral strike slip faults the sidewall faults show changes in kinematics from dominantly dip slip extension in copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received june 24 1996 revised manuscript received november 20 1996 final acceptance june 10 1997 2fault dynamics project geology department royal holloway university of london egham surrey tw20 oex united kingdom supported by the fault dynamics project sponsored by arco british limited petrobras u k ltd bp exploration conoco u k limited mobil north sea limited and sun oil britain we also acknowledge additional funding from sun oil britain landsat thematic mapper data courtesy of rtz mining and exploration limited south american division jurrian reijs is thanked for field data relating to the salina del fraile pull apart basin chris elders is thanked for reviewing earlier versions of this manuscript and j crowell c morley and k biddle are thanked for constructive reviews of the manuscript the contents of this paper have been

strike slip basins part 1 various types of sedimentary basins that contain a great variety of sedimentary successions and structural attributes form along strike slip faults these basins have commonly been called pull apart basins however we believe that the term strike slip basin is more suitable because pull apart basins are only one of many varieties that may develop along strike slip faults

spatial variation of present day stress field and tectonic regime in tunisia and surroundings from formal inversion of focal mechanisms geodynamic implications for central mediterranean we compiled 123 focal mechanisms from various sources for tunisia and adjacent regions up to sicily to image the current stress field in the maghrebides chain from tunisia to sicily and its foreland stress inversion of all the available data provides a first order stress field with a n150 e horizontal compression shmax and a transpressional tectonic regime but the obtained stress tensor poorly fit to the data set we separated them into regional subsets boxes in function of their geographical proximity kinematic regime homogeneity of kinematic orientations and tectonic setting their respective inversion evidences second and third order spatial variations in tectonic regime and horizontal stress directions the stress field gradually changes from compression in the maghrebides thrust belt to transpression and strike slip in the atlassic and pelagian foreland respectively where preexisting nw se to e w deep faults system are reactivated this spatial variation of the sismotectonic stress field and tectonic regime is consistent with the neotectonic stress field determined by others from fault slip data the major slab transfer edge propagator faults i e north south axis hammamet relay and malte escarpment which laterally delimit the subducting slabs play an active role in second and third order lateral variations of the tectonic regime and stress field orientations over the tunisian sicilian domain the past and current tectonic deformations and kinematics of the central mediterranean are subordinately guided by the plate convergence i e africa eurasia controlled or influenced by lateral slab migration segmentation and by deep dynamics such as lithosphere mantle interaction

geometry and kinematics of the south atlas front algeria and tunisia abstract the south atlas front is the structural boundary between the sahara platform and the atlas mountains it is frequently indicated as a foreland dipping monocline known as the sahara flexure earlier work generally considered that such a cover structure resulted from direct inversion of basement faults this assumption is not applicable to the studied area which is situated close to the algeria tunisia border our study demonstrates a thin skinned style characterised by ramp related folds ten parallel balanced cross sections give an interpretation for the south atlas front from negrine to gafsa about 100 km along strike the geometry at depth of each cross section is defined by forward modelling of each individual anticline the depth to detachment is defined using the geometric characteristics of the fault propagation folds well exposed along the deformation front in all sections the modelling is consistent with basal decollements located within cretaceous stratigraphic levels lateral changes of the tectonic style accommodated by tear faults are observed they seem related to lateral variations in the efficiency of the based decollement faults inducing either a jump of the decollement to another level or the development of en echelon folds the modelling predicts small rotations of cover sheets separated by large scale tear faults

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

deformation styles related to intraplate strike slip fault systems of the saharan tunisian southern atlas north africa new kinematic models the complex geometry and tectonic evolution of the saharan tunisian southern atlas strike slip fault systems north africa have not been investigated comprehensively to date this paper proposes new conceptual models to better understand the structural styles along these intraplate fault zones we use a multidisciplinary approach including field observations geological mapping stress analysis and seismic data to investigate the development and interaction of major strike slip fault segments the regional fault system is dominated by the e w trending biskra alima orbata bouhedma and chotts basement faults that constitute the principal displacement zones pdzs our analyses and interpretations reveal that each pdz has a typical dextral strike slip architecture comprising multi scale fault segments that are linked through extensional and contractional stepovers bends during mesozoic times under ne sw shmin the transtensional fault segments of biskra and alima formed the boundaries of the khanguet sidi neji gafsa ksg basin which developed as a z shaped dextral releasing stepover in cenozoic the ksg basin was inverted in response to the africa eurasia convergence when there was a regional nw se orientation for maximum horizontal stress this inversion occurred mainly along basin sidewalls by reactivation of the pre existing normal oblique faults as right lateral transpressional shears further east the e w dextral strike slip faults of orbata and bouhedma are connected via a ne sw restraining stepover finally all e w dextral strike slip fault segments are associated with ene right stepping en echelon folds and riedel shear patterns of different scales in response to basement wrenching currently most fault zones are still active with deep strike slip earthquakes

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

the origin of intraplate deformation in the atlas system of western and central algeria from jurassic rifting to cenozoic quaternary inversion abstract analysis of petroleum exploration data supplemented by paleostress data enabled discussion of the origin of the deformation in the western and central saharan atlas algeria this intraplate area has recorded the breakup of pangea late triassic the opening of the maghrebian tethys since the dogger and subsequently its closure oligocene to present however the two periods of strong coupling between europe and africa late lutetian and pleistocene which correspond to rapid uplifts of the atlas system and important deformations are not collision related they can be correlated to the beginning and the end of the development of the western mediterranean sea

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

regional variations in the chemical and helium carbon isotope composition of geothermal fluids across tunisia abstract tunisia has numerous thermo mineral springs previous studies have shown that their chemical composition and occurrence are strongly influenced by the regional geology however little work has been done so far to study the isotopic composition of volatiles associated with these geothermal manifestations here we report on the results of an extensive survey of both natural hot springs and production wells across tunisia aimed at investigating the spatial distribution of thermal fluids geochemical characteristics and he c isotopic composition the chemistry of the analyzed samples highlights the heterogeneity of the water mineralization processes in tunisia as a consequence of the complex geological and tectonic setting of the country in terms of chemical composition we are able to conclude however that dissolution of halite and gypsum plays a key control on groundwater chemistry helium and carbon isotope systematics confirm the prevalently crustal origin of the volatiles interacting with the aquifer systems consistent with the absence of any recent magmatism most samples are characterized by crustal type helium 3 he 4 he in the range 0 02 ra 0 4 ra associated with a co 2 predominantly metamorphic in origin with the exception of the saharan platform where the carbon content is low and mostly organically derived in eastern tunisia however a few samples have he c isotope compositions which suggest at least partial mantle derivation of the dissolved gas phase the 3 he 4 he ratio reaches 2 4 ra corresponding to 30 of mantle derived helium at the ain garci site a co 2 rich mineral spring located some 30 km south of the city of zaghouan this mantle signature is consistent with the fact that the pelagian block to which eastern tunisia belongs has been deeply affected by extensional and transtensional tectonics since the opening of the tethys a process which is still ongoing in the sicily channel pantelleria rift as a whole however our results show that the italian mantle gas anomaly only marginally extends to northwestern africa

first order kinematic elements in tunisia and the pelagian block abstract the effects of post collisional deformation in tunisia and the pelagian block are shown to be pervasively distributed over wide areas major structural provinces of the peri mediterranean mountain chains or localized in narrow belts major structural boundaries a redefinition and reinterpretation of the tell and atlas provinces the north south axis and the gafsa tozeur belts in tunisia and the rift zone in the strait of sicily help to understand both the tectonic significance of distinctive structural associations which in the past were thought to be not compatible with the kinematics of the europe africa system in neogene time and their role as first order kinematic elements during the post collisional deformation history of the central mediterranean region

calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor calculating horizontal stress orientations with full or partial knowledge of the tectonic stress tensor

structures and tectonics of tunisia abstract in the tunisian atlas nothing is known about the nature of the acoustic basement and in the saharan cratonic part only a few deep exploration wells have reached the precambrian below a thick unfolded palaeozoic sequence north of the craton a major set of faults w e or wnw ese represents the southern limit of palaeo tethys thick sequences of marine late carboniferous and permian were developed below the jeffara and chott jerid triassic evaporites are thick below the tunisian atlas which has enabled detachment of the cover from the basement and folding into plis de couverture structures large diapiric intrusions are known especially in the northwestern sillon tunisien tunisian trough from east to west there is a progressive variation from the stable platform on the eastern side to the unstable shelf of western tunisia between these a zone of tectonic instability named the north south axis corresponds to a deep fault system that acted as a positive suture characterized by tilted blocks gaps in sedimentation and reduced or condensed sequences in the north the unstable shelf of western tunisia grades to the deep furrows where the sediments now forming the tellian and numidian nappes were deposited these nappes overthrust the autochthonous foreland during the miocene orogeny the main folding took place in the pliocene and early pleistocene recent tectonic activity occurs along the main fault zones

from mantle to crust stretching the mediterranean abstract the origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated this work is based on a compilation of sks fast splitting directions that give an image of flow lines in the mantle around the mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts we find that 1 the two sets of directions are parallel in the three main backarc regions namely the alboran sea the tyrrhenian sea and the aegean sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle suggesting that 2 crustal deformation is mainly driven from below by slab retreat and 3 the lithospheric fabric is reset within a few millions of years in backarc environments

late stage evolution of fault propagation folds principles and example abstract fault propagation folding commonly occurs near the fronts of mountain belts this type of folding may even represent in some thrust systems the major mode of deformation however fault propagation folds are frequently altered by two kinds of late stage evolution breakthrough thrusting or transport on the flat we model the geometric features of these kinds of fold in particular the late stage modification and present the algorithms and equations used in an original program an application based on an example from the atlas mountains of algeria illustrates the main features of the model

modelling of structural complexity in sedimentary basins the role of pre existing faults in thrust tectonics abstract analogue and numerical models have been used to study the role of pre existing faults in compressive regimes from a theoretical point of view reactivation is mainly controlled by fault attitude stress regime and frictional properties of fault planes in scaled down sandbox experiments precut faults are introduced in the homogeneous granular media with a nylon wire which is forced through the sand cake producing a thin planar disturbed zone systematic experiments of thrust inversion with various dips and strikes of such planar discontinuities have been modelled comparison of experimental results with theoretical diagrams indicate that disturbed zones have a friction angle which is 10 20 lower than the homogeneous sand and that the compressive regime in the sandbox has a shape factor close to 0 4 the static analysis of fault reactivation is in accordance with the experimental observations except for pre existing faults dipping at very low angle however numerical modelling using the udec code shows that low angle faults can be reactivated as a result of stress concentration in the lower part of the fault in addition sandbox experiments indicate that in thrust systems reactivation of pre existing faults is not only dependent on their attitude but also on their spacing and location relative to the thrust system

field examples of strike slip fault terminations in mongolia and their tectonic significance deformation at the ends of large intracontinental strike slip faults that do not simply link other major structures often involves rotations about a vertical axis we use earthquake slip vectors surface rupture in earthquakes and geomorphology to examine the ends of three major strike slip faults in mongolia in these places a simple pattern is seen consisting of a thrust fault on one side with a displacement that decreases away from the strike slip fault consistent with local rotational deformation strike slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area for example from dominantly strike slip to dominantly dip slip while still accommodating the overall deformation required by larger scale regional motions such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis which may be detected paleomagnetically the kind of strike slip fault termination described here may have consequences for how large strike slip fxaults evolve and grow and for the variation in displacement along their length

the geomorphologic responses to hinge migration in the fault related folds in the southern tunisian atlas in kinematic models of fault related folds hinges must be mobile during fold growth yet hinge migration has been rarely demonstrated in nature several complementary and independent geomorphological responses are presented here that indicate fold hinge migration in the southern tunisian atlas the deformation of the overlying quaternary pediment suggests recent migration of the forelimb synclinal hinge as predicted by the rafini and mercier model the uplift of the first order transverse drainage network the poor maturation and shallow incision features of limb drainage networks toward the base of the limb slope suggest that new and un eroded material was regularly uplifted from the down slope flat plains as predicted by the fault propagation fold model

structural and geodynamic study in central tunisia using field and geophysical data new structural interpretation of the n s axis and associated atlassic structures a structural and geodynamic evolution of central tunisia is constructed from field studies and geophysical data within the tunisian atlassic domain bouguer gravity and horizontal gradient magnitude hgm maps indicate that computed anomalies are related to subsurface structures and are in accordance with the general structural trends of central tunisia near cherichira fault the strike is ene wsw whereas it is n s along the n s axis a major discontinuity between the pelagian shelf to the east and central atlas mountains to the west e w oriented hgm profiles on the north side of the cherichira fault show moderate hgm variations which correspond to the ne sw fault system of the ousselet bou dabbous this trend is also expressed by the second order vertical derivative map of this area however on the south side of the fault a dominant peak is indicated rather than the anomalies characteristic of the north side of the fault this anomaly is associated with the n s axis discontinuity seismic reflection data are also used to elucidate the structural configuration and style north and south of the cherichira fault the seismic lines demonstrate that faulting and associate halokinesis in ousselet bou dabbous area are not comparable to the paleogeographic fault system of the n s axis southward to the south of cherichira fault the seismic lines show massive triassic evaporates piercing along the major paleogeographic faults associated with thickness variations of mesozoic and cenozoic stratigraphic units and a reverse slip of western structural elements it seems that the n s axis corresponds to a steeply dipping major zone of weakness in the tunisian atlassic mountains and indicates evidence of several stages of activity including the rise of triassic evaporates rise up which outcrop at several localities such as rheouis and mezzouna the loading and upward rise of triassic evaporites along this weak zone are a key factor in the thinning of the mesozoic cenozoic sedimentary cover along this paleogeographic discontinuity these new data indicate the northeast extrapolation of fold and grabens of the central atlas of tunisia to the northeast of kairouan whereas the northern extent of the n s axis is limited to south of the cherichira fault the cherichira fault is ne sw oriented and parallels the ballouta zaghouan fault system which reflects the regional strike of the tellian orogeny and the associated nappes to the north the geometry and kinematics of these tellian features are linked to the tectonic evolution of the northern african margin including mesozoic extension separating the african and european plates to the inversion of these structures since the late cretaceous the n s axis is correlated in its direction and tectonic framework to the strike slip fault system of the african craton which is present to the south in the algerian saharan domain of hoggar