rift flank uplift at the gulf of california no requirement for asthenospheric upwelling continental rifts are commonly flanked by zones of high elevation but the cause of uplift remains controversial proposed uplift mechanisms include active and induced asthenospheric upwelling and isostatically driven lithospheric flexure discrimination between these hypotheses requires close constraint of the timing of rift flank uplift and crustal extension here we focus on the well preserved neogene gulf of california rift the western rift margin is characterized by a prominent east facing kilometer scale escarpment which bounds a west tilted topographically asymmetric rift flank we exploit west draining canyons incised into the rift flank to constrain the timing of uplift to between ca 5 6 and 3 2 ma using 40ar 39ar dating of lavas which show cut and fill relationships to the canyons rift flank uplift closely followed the onset of slip on the principal fault of the loreto rift segment at ca 8 6 ma the age of which we obtain from apatite u th he and fission track thermochronologic analysis of rift escarpment exhumation uplift was therefore coeval with lithospheric rupture and the onset of oceanic spreading between ca 6 and 3 ma but post dates a proposed asthenospheric upwelling event by 8 10 ma the timing of uplift is inconsistent with either active or induced upwelling as uplift mechanisms and we conclude that rift flank uplift was driven by the flexural response to lithospheric unloading

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

floor subsidence and rebound of large venus craters the topography and geology of large craters on venus reveal no evidence for floor rebound or relaxation rather the floors have subsided depressions of the floor centers relative to the margins are evident in topography and floor faulting is interpreted as contractional failure of the likely processes responsible only subsolidus thermal contraction applies to craters that both have and have not been infilled by lavas assuming volcanism occurred within a few tens of millions of years after the impact thermal subsidence satisfies the measured floor depressions for reasonable scaling of impact energies and temperature distributions in the lithosphere further the predicted stresses are generally consistent with observed floor fracturing we constrain the impact heat deposited in the lithosphere to be less than roughly 5 1023 j for diameters of 100 km the absence of perceptible floor subsidence at craters this size on the moon and icy satellites is readily explained by the scaling dependence of impact energy not only on transient crater diameter but also on gravity and target density the unfractured melt sheets of three large young bright floored craters imply sufficient lithospheric rigidity to support the crater cavities an elastic flexural rebound model restricts the elastic plate thickness to at least 10 15 km for the three craters corresponding to maximum geotherms of 20 30 k km 1 structural evidence for early rebound in older dark floored craters may have been buried by subsequent volcanism

properties of geothermal resources in kebilli region southwestern tunisia the kebilli region is located in the southwestern part of tunisia and is characterized by the presence of deep and shallow geothermal systems continental intercalary and complex terminal chemical and isotopic contents are used to classify the type and determine the origin of thermal water an evaluation of reservoir temperature and a possible geothermal fluid mixing are also carried out both continental intercalary deep aquifer and complex terminal shallow aquifer are of na ca cl so4 mixed water type the use of different geothermometers and the computation of saturation indexes for different solid phases suggests that the thermal reservoir temperature of the continental intercalary is between 92 and 105 c while the fluid temperature from the shallow complex terminal aquifer ranges from 50 to 75 c also the isotopic data indicates the old origin of all groundwater of southwestern tunisia mixing effects characterizing the continental intercalary and the complex terminal aquifers were identified using 2h and 18o relationship it appears that the upward movement of thermal water from the deep aquifer to shallow ones is probably due to the abundant fractures in the research area

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

the geomorphology of rhea implications for geologic history and surface processes analyses of landforms on rhea are used to define three physiographic provinces cratered terrain 1 undifferentiated cratered terrain 1 lineated cratered terrain 2 for which stratigraphic relationships are derived and examined in relation to major impact basins and tectonic features the formation of multiringed basins may have caused or at least controlled the locations of major resurfacing and mantling events the diameters of central peaks relative to impact crater diameters are significantly larger than those within craters on the moon and mercury rim segments of polygonal craters have preferential orientations both crater and noncrater lineaments show regional orientations that do not fit current global or regional stress models superposition relationships of provinces and associated features imply the following postaccretional sequence for the geological evolution of rhea s surface cratered terrain 1 undifferentiated formed first then linear troughs and chains of coalescing pits developed extensively on cratered terrain 1 lineated a resurfacing event emplaced cratered terrain 2 on which small craters scarps and ridges formed small grooves on cratered terrain 1 undifferentiated are tentatively assigned a young age based on their susceptibility to impact erosion this geologic history is generally consistent with theoretical models for rhea s evolution

mass movement and landform degradation on the icy galilean satellites results of the galileo nominal mission the galileo mission has revealed remarkable evidence of mass movement and landform degradation on the icy galilean satellites of jupiter weakening of surface materials coupled with mass movement reduces the topographic relief of landforms by moving surface materials down slope throughout the galileo orbiter nominal mission we have studied all known forms of mass movement and landform degradation of the icy galilean satellites of which callisto by far displays the most degraded surface callisto exhibits discrete mass movements that are larger and apparently more common than seen elsewhere most degradation on ganymede appears consistent with sliding or slumping impact erosion and regolith evolution sliding or slumping is also observed at very small 100 m scale on europa sputter ablation while probably playing some role in the evolution of ganymede s and callisto s debris layers appears to be less important than other processes sputter ablation might play a significant role on europa only if that satellite s surface is significantly older than 108 years far older than crater statistics indicate impact erosion and regolith formation on europa are probably minimal as implied by the low density of small craters there impact erosion and regolith formation may be important on the dark terrains of ganymede though some surfaces on this satellite may be modified by sublimation degradation while impact erosion and regolith formation are expected to operate with the same vigor on callisto as on ganymede most of the areas examined at high resolution on callisto have an appearance that implies that some additional process is at work most likely sublimation driven landform modification and mass wasting the extent of surface degradation ascribed to sublimation on the outer two galilean satellites implies that an ice more volatile than h2o is probably involved

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

avalanche slope angles in low gravity environments from active martian sand dunes 1 the properties of granular material have an important effect on surface landforms and processes recently it has been suggested that material properties called dynamic and static angle of repose vary with gravitational acceleration which would have a significant effect on many planetary surface processes such as crater collapse and gully formation in order to test that hypothesis we measured lee slopes of active aeolian sand dunes on mars using the high resolution imaging experiment hirise dtms digital terrain model we examined dune fields in nili patera herschel crater and gale crater our measurements showed that the dynamic angles of repose for the sands in these areas are 33 34 in the first region and 30 31 in the other two these results fall within the 30 to 35 window for the dynamic angles of repose for terrestrial dunes with similar flow depths and grain properties and thus show that this angle does not significantly vary with decreasing gravity

friction of ice on ice 1 new measurements have been made of the friction coefficient of freshwater polycrystalline ice sliding slowly 5 10 8 to 1 10 3 m s 1 upon itself at temperatures from 98 to 263 k under low normal stresses 98 kpa sliding obeys coulomb s law the shear stress is directly proportional to the normal stress across the interface while cohesion offers little contribution to frictional resistance the coefficient of kinetic friction of smooth surfaces varies from k 0 15 to 0 76 and at elevated temperatures 223 k exhibits both velocity strengthening at lower velocities 10 5 to 10 4 m s 1 and velocity weakening at higher velocities strengthening and weakening are attributed to creep deformation of asperities and localized melting respectively at intermediate temperatures of 173 and 133 k the kinetic coefficient appears to not exhibit significant dependence upon velocity however at the low temperature of 98 k the coefficient of kinetic friction exhibits moderate velocity strengthening at both the lowest and the highest velocities but velocity independence over the range of intermediate velocities no effect was detected of either grain size or texture over the range of roughness 0 4 10 6 m ra 12 10 6 m a moderate effect was detected where k ra0 08 slide hold slide experiments revealed that the coefficient of static friction increases by an amount that scales logarithmically with holding time implications of the results are discussed in relation to shearing across tiger stripe faults within the icy crust of saturn s enceladus sliding of the arctic sea ice cover and brittle compressive failure of cold ice

mass wasting and ground collapse in terrains of volatile rich deposits as a solar system wide geological process the pre galileo view the polar terrains of mars are covered in many places with irregular pits and retreating scarps as are some of the surfaces of the outer planet satellites these features are interpreted by us as diagnostic of exogenic degradation due to the loss of a volatile rock forming matrix or cement in this study we propose that sublimation degradation is a plausible solar system wide geological process candidate examples have been identified on mars io and triton and possibly europa and ganymede we envision this process as having two end member expressions pits and scarps for which we hypothesize two end member mechanisms massive localized lenses and areally extensive basal layers in this study we focus on the role this process may play on the surfaces of the galilean satellites our principle modeling results are that for these satellites h2s co2 and nh3are the only viable candidate volatiles for sublimation degradation of landforms in light of galilean satellite cosmochemistry for io s polar regions only h2s and then only from slopes that face the sun and have thin lags is volatile enough to cause the observed sublimation induced erosion at those latitudes so2is not a viable candidate as an agent of erosion especially for these polar landforms in the case of europa only co2and h2s are viable candidates given surface age constraints both species could be efficient eroders in nonpolar regions h2s could generate erosion within the polar regions if the deposition and erosion conditions were essentially identical as those we invoked for io s polar regions for ganymede and callisto nh3might be an agent of erosion in equatorial terrains of great age the sublimation of co2and h2s is much more robust than nh3 the much slower rate of sublimation degradation from nh3might be detectable by galileo and used as a compositional indicator

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

integrated geophysical and geological study and petroleum appraisal of cretaceous plays in the western gulf of gabes tunisia abstract an integrated study of available seismic and calibrated wells has been conducted in order to ascertain the structural development and petroleum potential of the cretaceous formations of the western gulf of gabes this study has resulted in an understanding of the controls of deep seated tethyan tectonic lineaments by analysis of the cretaceous deposits distribution three main unconformities have been identified in this area unconformity u1 between the jurassic and cretaceous series unconformity u2 separating early from late cretaceous and known as the austrian unconformity and the major unconformity u3 separating cretaceous from tertiary series the seismic analysis and interpretation have confirmed the existence of several features dominated by an ne sw extensive tectonic regime evidenced by deep listric faults asymmetric horst and graben and tilted blocks structures indeed the structural mapping of these unconformities displays the presence of dominant nw se fault system n140 to n160 bounding a large number of moderate sized basins a strong inversion event related to the unconformity u3 can be demonstrated by the mapping of the unconformities consequence of the succession of several tectonic manifestations during the cretaceous and post cretaceous periods these tectonic events have resulted in the development of structural and stratigraphic traps further to the porosity and permeability enhancement of cretaceous reservoirs

convection and lithospheric strength in dione an icy satellite of saturn abstract finite element numerical simulations for the structure of convection cells within a self gravitating boussinesq fluid sphere with variable viscosity are used to determine temperature distribution stream functions velocities and intensities of the stresses within dione an icy satellite of saturn a non newtonian i e stress dependent viscosity is considered but the numerical simulation according to u r christensen 1984a geophys j r astron soc 77 343 384 is carried out knowing that the properties of power law convection can be closely simulated by newtonian flow with a reduced value of the activation enthalpy the initial temperature profile has been determined taking into account the amount of energy trapped as heat during the accumulation process and the other energy source is the radioactivity of the long lived isotopes the obtained results seem to show a long duration endogenic activity in agreement with the geological observations the intensities of the stress are able to fracture the icy crust both in extensional and in compressional regimes the thickness of the mechanical lithosphere evolves in a way determined by the complex interplay of the lithospheric temperature gradient and the strain rate

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

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

a cross section of the lithosphere along the european geotraverse southern segment from the alps to tunisia abstract the european geotraverse southern segment egt s is a 200 300 km wide strip from the alps to tunisia which crosses the youngest tectonic provinces of europe where continued alpine compression is accompanied by recent extension e g in the ligurian sea and sicily channel the geophysical research along the egt s has been nearly continuous both on land and sea from 1982 to 1987 with deep seismic sounding dss experiments and collection of heat flow gravity magnetic seismological and seismotectonic data the new dss and seismic reflection data testify to the asymmetry of the alpine edifice the deepest part of the crust mantle boundary is found beneath the internal and external crystalline massifs where the european moho disappears after a steep flexure in the seismic reflection data the continuation of the moho discontinuity into the southern alpine domain is however evident in the seismic refraction data seismic evidence and heat flow data indicate incipient rifting in the ligurian sea which is underlain by a mantle with an anomalously low pn velocity of only 7 5 km s the palaeozoic crust of the sardinia corsica block exhibits a bowl shape with a rather undifferentiated internal structure average p wave velocity of 6 3 6 4 km s crustal thickness increases from 20 km at its margins to more than 30 km beneath the centre of the islands a thinned crust with thrusted blocks in its upper part characterizes the sardinia channel finally the preliminary dss results from tunisia show an abrupt deepening of the moho from 20 to approximately 35 km in the tell area very low average crustal velocity which requires a velocity inversion within the lower crust and subcrustal reflectors revealing a structured upper mantle

gravity constraints on the structure of the northern margin of tunisia implications on the nature of the northern african plate boundary summary bouguer gravity data were analysed to determine the general crustal and upper mantle structure in northern tunisia residual gravity anomalies were determined by removing the gravitational effect of crustal thickness variations imaged by regional seismic experiments residual gravity anomalies contain short wavelength anomalies superimposed on a long wavelength component that decreases in amplitude northward towards the tunisian coastline an edge enhancement analysis e g enhanced analytic signals of the short wavelength anomalies suggests a previously unknown east west trending gravity anomaly south of 37 n with source depths of between 3 and 7 km modelling of residual and bouguer gravity anomalies indicate that there are two possible solutions for the residual gravity decrease in northern tunisia 1 thickening of cenozoic and mesozoic sediments north of a strike slip fault or 2 a crustal and uppermantle low density zone interpreted as being crustal material of the remnant subducted african plate the latter result is favoured based on seismic tomographic images of the mediterranean region which implies that subducting material exists under the african coast geological interpretations suggesting that the tell atlas may be a thrust wedge accreted by underplating of the african continental crust and seismic refraction models indicating a thinning of sediments in northern tunisia the east west trending gravity anomalies south of 37 n correspond to an important structural feature that may be related to either a structural boundary e g a transform fault or subduction beneath the african plate

voyager 2 in the uranian system imaging science results voyager 2 images of the southern hemisphere of uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere the alignment of the cloud bands is similar to that of bands on jupiter and saturn but the zonal winds are nearly opposite at mid latitudes 70 degrees to 27 degrees where winds were measured the atmosphere rotates faster than the magnetic field however the rotation rate of the atmosphere decreases toward the equator so that the two probably corotate at about 20 degrees voyager images confirm the extremely low albedo of the ring particles high phase angle images reveal on the order of 10 2 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings as well as a broad diffuse low optical depth ring just inside the main rings system nine of the newly discovered small satellites 40 to 165 kilometers in diameter orbit between the rings and miranda the tenth is within the ring system two of these small objects may gravitationally confine the e ring oberon and umbriel have heavily cratered surfaces resembling the ancient cratered highlands of earth s moon although umbriel is almost completely covered with uniform dark material which perhaps indicates some ongoing process titania and ariel show crater populations different from those on oberon and umbriel these were probably generated by collisions with debris confined to their orbits titania and ariel also show many extensional fault systems ariel shows strong evidence for the presence of extrusive material about halfof miranda s surface is relatively bland old cratered terrain the remainder comprises three large regions of younger terrain each rectangular to ovoid in plan that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials perhaps suggesting some exotic composition

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

geophysical constraints on the location and nature of the north saharan flexure in southern tunisia gravity data integrated with seismic refraction reflection data well data and geological investigations were used to determine the location of the paleogeographic boundary between the precambrian saharan domain and the younger tunisian atlas domain this boundary north saharan flexure or nsf has not been as clearly defined as it has been to the west in algeria and morocco the gravity data analysis which included the construction of complete bouguer and residual gravity anomaly maps revealed that the atlasic domain is characterized by relative negative gravity anomalies and numerous linear gravity trends implying a thick and deformed sediment cover the saharan domain is characterized by relatively positive gravity anomalies with few gravity trends implying a thin and relatively undeformed sediment cover an edge enhancement analysis of the residual gravity anomalies revealed that the nsf is characterized by a series of discontinuous east and northwest trending linear anomalies south of 34 n that are not related to the well known faults within the gafsa and accident de medenine regions based on the continuity of the amplitudes of seismic reflection data and the trends of the residual gravity anomalies the nsf is not an abrupt discontinuity but a series of step faults dipping toward the atlasic domain to obtain a more quantitative representation of the southern edge of tunisian atlas a regional gravity model constrained by two wells and seismic reflection refraction data was constructed along a north south trending profile which confirms the presence of thicker sediments north of the nsf our analysis shows that the nsf has controlled the depositional environment of the sedimentary rocks within the region since at least triassic time and has acted as a barrier to atlasic deformation south of the nsf the nsf is considered an important tectonic feature that has controlled the paleogeographic evolution of the southern margin of the tethys ocean and it continues to be active today based on seismicity hazard studies

sublimation driven erosion on hyperion topographic analysis and landform simulation model tests abstract the unique appearance of hyperion can be explained in part by the loss to space of ballistic ejecta during impact events as was proposed by thomas et al thomas p c et al 2007a icarus 190 573 584 we conclude that such loss is a partial explanation accounting for the lack of appreciable intercrater plains on a saturation cratered surface in order to create the smooth surfaces and the reticulate honeycomb pattern of narrow divides between old craters appreciable subsequent modification of crater morphology must occur through mass wasting processes accompanied by sublimation probably facilitated by the loss of co 2 as a component of the relief supporting matrix of the bedrock during early stages of crater degradation steep crenulate bedrock slopes occupy the upper crater walls with abrupt transitions downslope onto smooth slopes near the angle of repose mantled by mass wasting debris as can be seen within young craters long continued mass wasting eventually results in slopes totally mantled with particulate debris this mass wasting effectively destroys small craters at least in part accounting for the paucity of sub kilometer craters on hyperion surface temperatures measured by cassini cirs range from 58 k to 127 k and imply a surface thermal inertia of 11 2 j m 2 k 1 s 1 2 and bolometric albedo ranging from 0 05 to 0 33 resulting h 2 o sublimation rates are only tens of cm per billion years for most of the surface so the evolution of the observed landforms is likely to require sublimation of more volatile species such as co 2

structure paleogeographic inheritance and deformation history of the southern atlas foreland fold and thrust belt of tunisia 1 structural analysis of the southern tunisian atlas was carried out using field observation seismic interpretation and cross section balancing it shows a mix of thick skinned and thin skinned tectonics with lateral variations in regional structural geometry and amounts of shortening controlled by nw se oblique ramps and tear faults it confirms the role of the late triassic early jurassic rifting inheritance in the structuring of the active foreland fold and thrust belt of the southern tunisian atlas in particular in the development of nw se oblique structures such as the gafsa fault the late triassic early jurassic structural pattern is characterized by a family of first order nw se trending normal faults dipping to the east and by second order e w trending normal faults limiting a complex system of grabens and horsts these faults have been inverted during two contractional tectonic events the first event occurred between the middle turonian and the late maastrichtian and can be correlated with the onset of the convergence between africa and eurasia the second event corresponding to the principal shortening tectonic event in the southern atlas started in the serravalian tortonian and is still active during the neogene the southern atlas foreland fold and thrust belt propagated on the evaporitic decollement level infilling the late triassic early jurassic rift the major eocene atlas event described in hinterland domains and in eastern tunisia did not deform significantly the southern tunisian atlas which corresponded in this period to a backbulge broad depozone

an oceanic origin for the increase of atmospheric radiocarbon during the younger dryas 1 variations in carbon 14 to carbon 12 ratio in the atmosphere 14catm provide a powerful diagnostic for elucidating the timing and nature of geophysical and anthropological change the atlantic marine archive suggests a rapid 14catm increase of 50 at the onset of the younger dryas yd cold reversal 12 9 11 7 kyr bp which has not yet been satisfactorily explained in terms of magnitude or causal mechanism as either a change in ocean ventilation or production rate using earth system model simulations and comparison of marine based radiocarbon records from different ocean basins we demonstrate that the yd 14catm increase is smaller than suggested by the marine archive this is due to changes in reservoir age predominantly caused by reduced ocean ventilation

geophysical implications of the long wavelength topography of rhea 1 we use limb profiles to investigate the long wavelength topography and topographic variance spectrum of rhea one dimensional variance spectra show a break in slope at a wavelength of 300 km a similar effect is seen on the moon and may be a signature of an elastic lithosphere having a thickness te 10 km the implied heat flux is 15 mw m 2 much higher than can be explained by radiogenic heating we use the 1 d spectral behavior to constrain our solution for the long wavelength global topography of rhea the degree 3 topography is large enough if uncompensated to contaminate estimates of the degree 2 gravity using existing flyby data current models of rhea internal structures which rely on these degree 2 estimates may thus be inaccurate illustrating the need to acquire further rhea gravity data

implications from ithaca chasma for the thermal and orbital history of tethys 1 flexural modeling of ithaca chasma on tethys suggests that the elastic thickness and the surface heat flux were 5 7 km and 18 30 mw m2 respectively when the feature formed 4 gyr b p tidal heating is a plausible heat source suggesting that tethys eccentricity was higher in the past depending on tethys internal structure eccentricities in the range 0 001 to 0 02 are sufficient to have generated the inferred heat flux because the eccentricity damping timescale is short 20 myr it is unlikely that this eccentricity was primordial more likely tethys eccentricity increased during passage through a 3 2 resonance with dione in equilibrium this resonance produces 2 5 mw m2 suggesting that non equilibrium or periodic heating was important similar to the case for enceladus at present

resurfacing saturn s satellites models of partial differentiation and expansion abstract the faulting seen on the surfaces of saturn s icy moons may have been caused either by external events such as large impacts or internal stresses caused by the expansion of the moons as long lived radionuclides produced internal heating and phase changes we estimate the stress as a function of radius expansion is 44 r r kbar the extensional stress needed for fracture is probably something less than 40 bar so extensional fracture is likely to occur when r r is greater than one part in a thousand the radius change for these moons can be calculated analytically given suitable assumptions in addition detailed time dependent computer models of the thermal and physical evolution of tethys dione rhea and iapetus were carried out from these calculations we conclude that the most reasonable cause for rifting on dione and rhea is the refreezing of an ammonia water eutectic melt inside these moons roughly two billion years after their formation while the rift on tethys was caused by a large impact and little rifting should be expected on iapetus

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

relations between normal fault geometry tilting and vertical motions in extensional terrains an example from the southern gulf of suez abstract earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern gulf of suez are approximately planar with dips of 30 40 from the surface to around 10 km depth these faults and the blocks they bound appear to rotate about a horizontal axis as they move causing tilting this tilting is seen both in young vertical movements of the coastline such as raised beaches and marshlands and in the distribution of middle miocene marine rocks which are uplifted to elevations of 400 500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens the absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw tooth topography on a regional subsidence caused by crustal and lithospheric thinning the observations required for this simple model are the present day fault dip the amount of tilting and the width of the rotating blocks the virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents

brine mobilization during lithospheric heating on europa implications for formation of chaos terrain lenticula texture and color variations the background lineated plains on europa are locally highly modified and destroyed in regions known as chaos and lenticulae produced there are 1 isolated fragments and polygons of background material which rotate and translate 2 matrix which fills in the areas between the fragments and polygons and 3 surface discolorations using observations and constraints from high resolution galileo images we find that a model for the formation of these terrains which involves mobilization and migration of brines and a possible percolation phase transition as the europan lithosphere is warmed can readily explain the vast majority of their characteristics in addition the presence of melt fractions of a few percent in the adjacent ice framework may enhance the creep rate and the accompanying deformation rates the characteristics and distribution of lenticulae suggest that among the strong candidates for heat sources for brine migration and ice mobilization processes is diapirism linked to solid state convection in a layer underlying a brittle lid and possibly overlying a liquid water layer

subsurface mesozoic basins in the central atlas of tunisia tectonics sequence deposit distribution and hydrocarbon potential the mesozoic and cenozoic outcrops of the central and meridional atlas of tunisia have been studied by structural biostratigraphic and sedimentologic approaches we extend these studies to the subsurface employing seismic and well data addressing structures sequence stratigraphy and hydrocarbon potential the basin structures are related to strike slip fault systems associated with triassic evaporites mesozoic sedimentary sections in the areas of the gafsa sidi ali ben oun sidi aich and majoura blocks were studied by an integrated seismic sequence stratigraphic and tectonic approach calibrated to wells and surface geologic controls we identified tectonic corridors where upper triassic evaporites intruded younger mesozoic rocks beginning in the jurassic the organization of the tectonic blocks is characterized by the distribution of subsiding basin zones of graben and rim synclines and resistant areas of platforms triassic intrusions induced by the strike slip movements formed salt pillows and domes that accentuate the border elevations between the rim syncline depocenters and the platforms mesozoic sequence deposits are distributed along the flanks of the basin platform borders according to lowstand prograding downlap systems tracts overlapped by aggrading and retrograding onlap and toplap deposits the time space distribution of the systems tract deposits indicates the possibility of formation of triassic jurassic and cretaceous carbonate and sandstone prograding turbidites reefs structural and stratigraphic pinch outs and unconformities in the flanks of salt domes and basins the geometry and distribution of reservoir and seal systems tracts permit the consideration of new triassic jurassic and cretaceous combined stratigraphic and structural hydrocarbon traps around the basins and the platforms this begin page 886 is a new tectonic model of the central atlas of tunisia that shows new potential objectives and traps

the geology of dione abstract dione is one of the more geologically complex of saturn s satellites several geologic units have been identified including ancient heavily cratered terrain two plains units cratered plains and lightly cratered plains lobate deposits crater rim deposits and bright wispy material the only structural features observed are a series of troughs which cross portions of the surface and subtle northeast and northwest trending lineaments the troughs are associated with volcanic deposits and are interpreted to be the vents through which material was erupted correlations exist between telescopically observed albedo patterns and the distribution of geologic units

neotectonics of coastal jeffara southern tunisia state of the art abstract helped by the studies and results of previous researchers we herein study the neotectonic of the coastal jeffara with the input of numerous 2d reflection seismic profiles onshore combined with digital elevation model analyses issued from srtm and field works acquired and available data were then integrated within a gis geodatabase where jerba zarzis and jorf appear to be part of a n s pull apart basin within a nw se transtensive right lateral major fault zone our structural geologic and geomorphologic analyses confirm and prove the presence of nnw sse right lateral en echelon tension gashes nw se aligned salt diapirs numerous active folds offsets en echelon folds and so on they are associated with this major right lateral nw se transtensive major coastal jeffara fault zone that affect the holocene and the villafranchian deposits we therefore confirm herein a new structural geodynamic jeffara model due to the post lower cretaceous northward migration of northern african to the eurasian plates this nw se transtensive fault zone is interpreted as a part of the southern branch of the eastward sahel block extrusion toward the free mediterranean sea boundary therefore this geodynamic movement may explain the presence offshore of small elongated nw se n s and ne sw transtensive basins and grabens with petroleum interest to conclude at the regional scale the structural geomorphologic approach combined with both field work and 2d reflection seismic profile analyses appear to be an excellent tool to prove and confirm the nw se right lateral transtensive extrusion fault zone of the coastal jeffara

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

petroleum geology of east central tunisia the tectonic features of tunisia are complex and include folds all types of faults evaporite diapirs and the saharan flexure which separates a stable paleozoic province on the south from a subsidence zone of mesozoic and cenozoic strata of the offshore ashtart and tripolitania basins the remainder of the offshore region is mostly stable shelf of the pelagian craton which also extends onshore the present study focused on this shelf where two carbonate intervals contain proven hydrocarbon reservoirs the metlaoui formation of earliest eocene ypresian age and the zebbag formation of late cretaceous turonian age regionally well defined belts of metlaoui carbonates trend northwest on the northeast are open marine deep water micrites and marls with abundant planktonic foraminifers thick bars of nummulitid packstone grainstone were deposited in shallow water at an angle to the paleoshelf the reservoir is confined largely to the bars and visible effective porosity is best developed in those areas among the foraminifers filled with sand size nummulitid debris where secondary solution enlargement has occurred this lithology tested oil in two recent wildcat wells and is a commercial reservoir at sidi el itayem and ashtart fields distribution of zebbag carbonates is more complex a northwest trending platform was bounded on three sides by deep water where shale and micrite with planktonic foraminifers were deposited predominately back reef and lagoonal foraminifer rudist wackestones and packstones occur in narrow belts apparently controlled at least locally by block faulting they tested oil in two recent discoveries the most significant porosity is interparticle generally enlarged by solution in foraminifer packstones but intraparticle voids in foraminifers and rudists commonly contribute to the porosity analyses of surface and subsurface samples identified the bahloul basal turonian and bou dabbous ypresian formations as source rocks fluorescence spectra of several oils were compared with extracts from these samples and indicate the bahloul to be the source of oils in recent metlaoui and zebbag discoveries

shallow normal fault slopes on saturnian icy satellites fault dips are a function of the coefficient of internal friction i of the lithospheric material laboratory deformation experiments of h2o ice at conditions applicable to icy bodies yield 0 i 0 55 such that normal faults dip between 45 and 59 we tested the hypothesis that normal faults on icy bodies reflect these values by using digital elevation models to examine geometries of large extensional systems on three saturnian satellites analyzed faults within ithaca chasma on tethys and avaiki chasmata on rhea all exhibit shallower than predicted topographic slopes across the fault scarp which we term fault slopes a scarp of padua chasmata within dione s wispy terrain also has a shallow fault slope although three others that make up palatine chasmata exhibit steeper slopes as predicted we infer that viscous relaxation is the most viable explanation for these shallow fault slopes and we model the potential role of viscous relaxation in creating shallow fault slopes our modeling results support formation of these normal faults with steep dips consistent with deformation experiments followed by their relaxation due to lithospheric heating events related to radionuclide decay the steepest fault slopes in this terrain yield 0 i 0 73 for dione s lithospheric ice which overlaps the dip range predicted from experiments results of this work suggest that viscous relaxation substantially affected fault slopes on tethys rhea and dione by implication these processes may have also affected fault geometries on other icy satellites

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

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