tectonic evolution of a part of the tethyside orogenic collage the kargi massif northern turkey the central part of the rhodope pontide fragment one of the major tectonic units in turkey provides critical data for evaluating the cimmeride and alpide evolution of the mediterranean tethysides tectonic events that affected the central part of the rhodope pontide fragment since the end of the paleozoic generated east west trending belts with the event of every episode redeforming and partly obliterating the structures of previous episodes this evolution may be conveniently described in terms of three major episodes 1 two different realms of pre dogger oceanic rocks are present in the area the northern realm coincided with main branch of paleo tethys that was being actively destroyed by south dipping subduction the southern realm the karakaya ocean a back arc basin related to this subduction began opening by rifting of a retroarc carbonate platform during the permo triassic to the west a continental domain with sparse magmatism seperated the two oceanic areas toward the east the two oceans become united by the wedging out of the continental domain these two pre dogger oceans closed during the lias and their remnants were emplaced between the southern margin of laurasia and the fragments of the cimmerian continent 2 the second episode partly overlapped the first with rifting south of the cimmerian continent fragment during the lias this rifting was followed by a transgression which covered the ruins of the cimmeride orogenic belt by the malm this rifting concurrently led to the development of the northern branch of the neo tethys and a south facing atlantic type continental margin a southerly thickening sedimentary prism developed on this margin during the lias to early cretaceous interval 3 the floor of the northern branch of neo tethys began to be consumed along the north dipping subduction zone beneath the previosly constructed continental margin this convergent margin generated a magmatic arc to the north and to the south a subduction accretion complex present width of which approaches 80 km to the south of the subduction accretion complex yet another convergent margin is indicated by the presence of the remnants of an ensimatic arc the northern branch of the neo tethys was totally consumed in the late cretaceous to eocene interval

how does venus lose heat the tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior on the earth about 70 of the heat flow through the mantle is attributed to the subduction of cold lithosphere in order to understand the tectonic and volcanic processes on venus it is necessary to understand how heat is transported through its mantle in this paper three alternative end member hypotheses are considered the first is the steady state loss of heat through the mantle to the surface in analogy to the earth however without plate tectonics and subduction on venus a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination the required plume flux would be equivalent to about 80 plumes with the strength of the hawaiian plume the required delamination flux implies a 50 delamination of the entire venus lithosphere every 10 m y neither appears possible so that it is concluded that venus cannot transport heat through its mantle to its surface on a steady state basis the second hypothesis is that there has been a strong upward concentration of the heat producing elements into the crust of venus the heat generated is then lost by conduction surface measurements of the concentrations of the heat producing elements place constraints on this model if everything is favorable this hypothesis might be marginally acceptable but it is considered to be highly unlikely the third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence the near random distribution of craters suggests that the last subduction event occurred about 500 ma this model implies a thick thermal lithosphere 300 km at the present time which is consistent with a variety of surface observations lava lakes on the earth are considered as analogies to plate tectonics they also exhibit episodic subduction events

tectonic and kinematic study of a strike slip zone along the southern margin of central ovda regio venus geodynamical implications for crustal plateaux formation and evolution the tectonic system of the southern margin of central ovda regio a crustal plateau which straddles venus equator has been interpreted as a dextral strike slip array on the basis of evidence clearly identifiable as are riedel fracture patterns of different scales en echelon folds and brittle strike slip faults this transcurrent regime developed two main shear belts inner and outer on respectively thicker and thinner crust whose minimum dextral displacement has been estimated in 30 50 km since the up or downwelling models for plateau formation cannot easily explain tectonic shears of this magnitude along their margins an alternative hypothesis has been built which stands on the proposed collisional belt which could form ovda northern border king et al 1998 lunar planet sci conf 29 abstract 1209 tuckwell and ghail 2002 lunar planet sci conf 33 abstract 1566 within this framework the shear would represent a transcollisional transcurrent zone similar to the strike slip zones produced in the foreland of the himalayas tibet collision front eastern ovda would be an independent area of thickened crust pushed to the sse by the northern collision with the deformation concentrated at its margins and experiencing a shear strain on its southern margin none of the data however either supports nor helps to discard theoretical subduction events as a cause of the collision on the contrary image relationships could be interpreted as evidence that the main shear deformation took place during the last global resurfacing event on the planet

submarine fan and slope apron deposition in a cretaceous forearc basin the gursoku formation kavak samsun n turkey abstract the late cretaceous gursoku formation represents the proximal fill of the sinop samsun forearc basin that was probably initiated by extension during the early cretaceous the succession records sedimentation in two contrasting depositional systems a slope apron flanking a faulted basin margin and coarse grained submarine fans the slope apron deposits consist of thinly bedded turbiditic sandstones and mudstones interbedded with non channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides the submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non cohesive debris flows the occurrence of both slope apron and submarine fan depositional systems in the gursoku formation may indicates that the region was a tectonically active basin margin during the late cretaceous

structural analysis of a sampling of tesserae implications for venus geodynamics abstract understanding the nature and timing of tessera formation is fundamental to venus tectonic and geodynamic models tesserae are commonly considered to exhibit complex deformation histories to represent the oldest global stratigraphic unit to have formed during a global phase of tesserization and to require weak lithosphere for their formation although these characterizations of tesserae are gaining widespread mention in the literature they are essentially unsupported by an assemblage of data yet they figure prominently in the way we frame and address questions about venus tectonics thus they are quickly becoming accepted paradigms despite a lack of scientific foundation open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of venus we examined tessera terrain in ishtar terra crustal plateaus and as inliers within the plains using high resolution magellan radar imagery we describe several types of tessera terrain that record a wide range of structural histories fold and s c terrains are found only in ishtar terra ribbon lava flow and basin and dome terrains reside within the interior of crustal plateaus whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus inliers are divisible into fracture dominated and graben dominated tesserae although some inliers host early contractional fabrics the range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit tessera deformation records local to regional surface strain patterns and reflects near surface rheology at the time of deformation progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation tesserae likely formed in several tectonic environments including 1 as a result of subsurface flow in ishtar terra 2 as sequences of surface layer extension and contraction in crustal plateaus 3 as flooded crustal plateaus and 4 as densely fractured surface layers fractured as a result of corona and chasma formation

segmented african lithosphere beneath the anatolian region inferred from teleseismic p wave tomography summary lithospheric deformation throughout anatolia a part of the alpine himalayan orogenic belt is controlled mainly by collision related tectonic escape of the anatolian plate and subduction roll back along the aegean subduction zone we study the deeper lithosphere and mantle structure of anatolia using teleseismic finite frequency p wave traveltime tomography we use data from several temporary and permanent seismic networks deployed in the region approximately 34 000 p wave relative traveltime residuals measured in multiple frequency bands are inverted using approximate finite frequency sensitivity kernels our tomograms reveal segmented fast seismic anomalies beneath anatolia that corresponds to the subducted portion of the african lithosphere along the cyprean and the aegean trenches we identify these anomalies as the subducted aegean and the cyprus slabs that are separated from each other by a gap as wide as 300 km beneath western anatolia this gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere the eastern termination of the subducting african lithosphere is located near the transition from central anatolia to the eastern anatolian plateau or arabian eurasian collision front that is underlain by large volumes of hot underplating asthenosphere marked by slow velocity perturbations our tomograms also show fast velocity perturbations at shallow depths beneath northwestern anatolia that sharply terminates towards the south at the north anatolian fault zone nafz the associated velocity contrast across the nafz persists down to a depth of 100 150 km hence our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary overall the resolved upper mantle structure of anatolia is directly related with the geology and tectonic features observed at the surface of the anatolian plate and suggest that the segmented nature of the subducted african lithosphere plays an important role in the evolution of anatolia and distribution of its tectonic provinces

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

lithospheric scale folding numerical modelling and application to the himalayan syntaxes we describe the eastern and western himalayan syntaxes which are large scale antiforms situated at geodynamically similar locations and the metamorphic evolution of which is coeval in the india asia collisional history to understand the mechanical plausibility of the structural interpretation we present two dimensional finite element modelling of lithospheric folding the models reveal the coeval development of adjacent synformal basins analogous to the peshawar and kashmir basins on both sides of the western syntaxis similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large scale continental shortening and an efficient mountain building process

folding of oceanic lithosphere folding of the lithosphere just south of the bay of bengal appears as 1 undulations in acoustic basement topography and 2 as linear geoid undulations in the seasat altimeter data from the seasat data we find that the east west trending folds have wavelengths ranging from 130 to 250 km and clustering about 190 km the horizontal gravity disturbances due to the folds range in amplitude from 15 to 50 mgal elastic models of oceanic lithosphere have in the past been used to demonstrate the implausibility of lithosphere buckling or folding in response to compression these elastic models typically predict that compressive stresses of about 5 gpa are required to buckle oceanic lithosphere with an age comparable to that of the northeastern indian ocean 40 70 ma these stresses exceed the strength of lithospheric rock we use an elastic plastic model to show that oceanic lithosphere of this age should have a net compressive strength equal to about 12 of the elastic buckling stress we further demonstrate that loads approaching the net compressive strength cause the lithosphere to fold with a wavelength about 200 km i e the wavelength observed from seasat our results reinforce earlier speculation that this folding may be related to the himalayan orogeny

mesoscale strike slip faults and damage zones at marsalforn gozo island malta well exposed strike slip faults in limestones in the north western part of gozo show damage zones that can be grouped into three categories based on their location along faults tip damage linking damage and distributed damage the predominant fracture types within damage zones include extension fractures and secondary faults tip damage zones usually show wedge shaped patterns formed by antithetic faults and extension fractures commonly accompanied by block rotation several fractures are combined at linking damage zones typically with the concentration of a high intensity of fractures structures in distributed damage zones are typically similar to the classical riedel shear pattern evolutionary and 3d models are proposed in terms of the geometries of damage zones for small displacement strike slip fault zones different evolutionary routes depend on fault tip modes and locations

determining heights and slopes of fault scarps and other surfaces on venus using magellan stereo radar this report presents relationships and techniques for determining the heights and slopes of discretely dipping surfaces such as normal fault scarps on the surface of venus from measurements of their widths in magellan stereo synthetic aperture radar sar images these surfaces are clearly recognizable as distinct bands of increased or decreased radar backscatter relative to flat lying areas in the 120 to 280 m resolution magellan sar images but are not generally imaged in the 10 km resolution magellan altimetry our methods take into account radar distortion effects and allow one to determine whether a slope is foreshortened laid over elongated or in radar shadow the techniques make use of graphs constructed for the magellan incidence angle profiles so that investigators can determine the local height and slope of an individual surface in a straightforward manner but the techniques are applicable to any stereo radar data set if the radar incidence angles are known additionally the techniques can be used to improve digital elevation models constructed using radar stereoscopy in areas that have steep local terrain where stereoscopic fusion may be impossible due to the effects of layover or radar shadow using these techniques we show that there are normal fault scarps on venus with heights in the 700 m range that have remarkable topographic slopes of close to 60 something unheard of on earth and which suggests a high effective cohesive strength of the venusian crust

tectonic evolution of the high himalaya in upper lahul nw himalaya india the upper lahul region in the nw himalaya is located in the transition zone between the high himalayan crystalline hhc to the sw and the tethyan zone sedimentary series to the ne the tectonic evolution of these domains during the himalayan orogeny is the consequence of a succession of five deformation events an early dl phase corresponds to synmetamorphic ne verging folding this deformation created the tandi syncline which consists of permian to jurassic tethyan metasediments cropping out in the core of a large scale synformal fold within the hhc paragneiss this tectonic event is interpreted as related to a ne directed nappe stacking shikar beh nappe probably during the late eocene to the early oligocene a subsequent d2a phase caused sw verging folding in the hhc this deformation is interpreted as contemporaneous with late oligocene to early miocene sw directed thrusting along the main central thrust in the tethyan zone a d2b phase is marked by a decollement thrust a system of reverse faults and gentle folds associated with sw directed tectonic movements this deformation is related to an imbricate structure characteristic of a shallow structural level and developed in the frontal part of a nappe affecting the tethyan zone units of se zanskar nyimaling tsarap nappe a later d3 phase generated the chandra dextral shear zone cdsz a large scale ductile dextral strike slip shear zone located in the transition zone between the hhc and the tethyan himalaya the cdsz most likely represents a part of a system of early miocene extensional and or dextral strike slip shear zones observed at the hhc tethyan zone contact along the entire himalaya a final d4 phase induced large scale doming and ne verging back folding

mechanical model for subduction collision tectonics of alpine type compressional orogens alpine type orogens are characterized by three distinct convergent tectonic phases subduction with deformation that has primarily single vergence a transition from subduction to collision and continental collision with double vergence although the cenozoic history of the european alps has additional complexities a mechanical explanation for these three phases would provide the necessary crustal scale framework in which to develop an understanding of the smaller scale processes we present results from a simple numerical model which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen the development and exhumation of a subduction complex suture zone and basement nappe stack piemont suture penninic nappes the presence of a crustal scale back fold and thrust insubric line and uplift of basement on the pro european side of the orogen external basement massifs may be explained as a simple consequence of changing dynamics during the transition from subduction to collision

evidence for large scale strike slip faulting on venus radar images obtained from the magellan mapping mission reveal that much of the surface of venus has undergone tectonic deformation though normal and thrust faults are common there has been little evidence of discrete large offset strike slip faults on venus we present a clear example of large scale strike slip faulting in lavinia planitia venus our observations of the spatial and temporal relationships between the molpadia linea deformation belt and an associated fracture system indicate that the fractures were formed as the result of strike slip faulting localized along the belt global tectonism on venus is dominated by vertical deformation we attribute this example of horizontal deformation to a reorientation of the regional stress field associated with mantle downwelling

paleozoic and mesozoic stratigraphy of the peshawar basin pakistan correlations and implications the most complete paleozoic sequence described from pakistan is exposed in bedrock inliers and in ranges fringing the eastern peshawar basin interbedded quartzite and argillite of the precambrian and cambrian tanawal formation is overlain unconformably by the cambrian ambar formation the misri banda quartzite unconformably overlies the ambar and contains ordovician cruziana ichnofossils new conodont discoveries restrict the ages of overlying formations as follows panjpir formation llandoverian to pridolian nowshera formation lochkovian to frasnian and jafar kandao formation kinderhookian to westphalian the karapa greenschist consisting of metamorphosed lava flows separates the jafar kandao from upper triassic carnian marbles of the kashala formation the upper triassic and jurassic nikanai ghar formation forms the top of the section correlatives to the peshawar basin stratigraphy are present locally in the sherwan synclinorium of hazara and in the khyber pass region the sequence contrasts markedly with the paleozoic and mesozoic section exposed south of the khairabad thrust in the attock cherat range this thrust and its northeastern continuation in hazara north of abbottabad thus form the boundary in pakistan between the lesser himalayan and tethyan himalayan sections a function performed by the main central thrust mct in the central himalaya of india and nepal the newly dated carboniferous to triassic horizons provide the first firm age constraints on the protoliths of the high grade swat metasediments the dating of the metasediments has in turn provided age constraints on pre himalayan tectonism and associated intrusions two major tectonic episodes during the late cambrian and carboniferous produced positive areas north of the peshawar basin that provided coarse detritus to the misri banda quartzite and jafar kandao formation

structural evolution and asymmetric uplift of the nanga parbat syntaxis pakistan himalya the nanga parbat syntaxis in the nw himalaya is a still growing crustal scale north trending antiformal structure in the core of which indian plate gneisses have been uplifted from beneath the overthrust rocks of the kohistan island arc isotopic and fission track geochronology show that uplift rates within the syntaxis have increased to present day rates of 6 mm yr uplift has been accommodated by a combination of initial northwest verging thrusting on the western margin of the syntaxis followed by crustal scale folding within the syntaxis and latterly by dextral reverse faulting on the western margin this thrusting folding and faulting is the effect of deformation at the north western lateral tips of the main himalayan thrusts where they interfere with the south southeast verging thrusts of the northwest himalaya

an alternative kinematic interpretation of thetis boundary shear zone venus evidence for strike slip ductile duplexes 1 on venus a 1000 km long 50 to 200 km wide anastomosing shear zone system thetis boundary shear zone tbsz separates the eastern ovda and northwestern thetis regiones previous workers mapped a part of this shear zone and interpreted it as a sinistral strike slip shear zone containing an extensional jog in the present study it is reexamined for a detailed structural and kinematic analysis and an alternative kinematic interpretation is provided the tbsz is divided into western central and eastern segments the western segment is narrow and is oriented in a ene wsw direction along its northern boundary the preexisting folds of tessera are cut by the shear zone and the folds are dragged into parallelism with the shear zone in response to dextral strike slip motion both the western and central segments contain a pair of conjugate deformation bands sigmoidal folds and linear ridges which appear similar to s c like structures of large scale ductile shear zones the acute angle between the s and c bands suggests dextral sense of displacement the eastern segment forms the northeastern restraining bend of the tbsz where the shear zone is characterized by a wide branching fan like anastomosing shear zone system with well developed contractional strike slip ductile duplexes the internal fabric defined by the sigmoidal folds consistently shows dextral sense of displacement therefore the tbsz is a dextral strike slip shear zone showing s c like structures and contractional strike slip duplexes similar to the continental scale ductile shear zones on earth

exhumation during crustal folding in the namche barwa syntaxis geological observation in the eastern end of the himalaya shows that the asia india suture is folded metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating establish cooling exhumation of rocks from c 30 km depth within the last 4 myr we argue that exhumation is caused by 10 mm yr 1 erosion coeval with crustal scale folding

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

sedimentary basins and crustal thickening abstract we consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust well known examples are foreland basins others are ramp basins and buckle basins we have reproduced various styles of compressional basins in experiments properly scaled for gravity a multilayered model lithosphere with brittle and ductile layers floats on a model asthenosphere a computer driven piston provides shortening and thickening synchronous with erosion and sedimentation after a first stage of lithospheric buckling thrust faults appear mainly at inflection points slip on an isolated reverse fault is accompanied by flexure footwall flexure results in a foreland basin and becomes accentuated by sedimentation hangingwall flexure is less marked but may become accentuated by erosion motion on a fault leads to hangingwall collapse at the surface either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin simultaneous slip produces a symmetric ramp basin whereas alternating slip results in a butterfly shaped basin with superposed foredeeps some well developed ramp basins become pushed down until bounding faults meet at the surface and the basin disappears from view at this stage the basin depth is equivalent to 15 km or more slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline between two distinct foredeeps in central asia and in western europe cenozoic crustal thickening is due to continental collision for central asia western china kyrgyzstan uzbekistan tajikistan we have compiled a regional structure contour map on the base of the tertiary as well as 4 regional sections foreland basins and ramp basins are numerous and associated with cenozoic thrusts large basins tarim junggar fergana tajik occur around and between mountain ranges but smaller basins issyk kul naryn occur within them in western europe the alps and pyrenees are surrounded by foreland basins ramp basins or intermediate styles in the andes and its foreland neogene thrusts and compressional basins are due to subduction of oceanic lithosphere in colombia they account for much of the cordillera oriental in nw argentina for the altiplano in west central argentina for the sierras pampeanas compressional basins are also common in other areas of older crustal thickening

catastrophic resurfacing and episodic subduction on venus abstract there is clear evidence that a catastrophic resurfacing event occurred on venus in the relatively recent past a primary data source for this resurfacing event is the spatial distribution of impact craters in this paper we apply the pair correlation technique to the observed crater distribution and find that the result is identical to that for a random distribution in order to test the sensitivity of the technique we also apply it to the spatial distribution of coronae on venus for the coronae we find substantial deviations from a random distribution one explanation for the catastrophic resurfacing is the episodic subduction hypothesis we model episodic subduction using a thermal boundary layer stability analysis we find that episodic subduction events with intervals of 500 to 700 myr can transport only 15 25 of the radiogenic heat produced within the planet we suggest that the remainder of the heat must be lost to the surface during a period of vigorous tectonic activity following the subduction event but prior to the subsequent stabilization of a global lithosphere

ten turbidite myths abstract during the past 50 years the turbidite paradigm has promoted many myths related to deep water turbidite deposition john e sanders 1926 1999 a pioneering process sedimentologist first uncovered many of these turbidite myths this paper provides a reality check by undoing 10 of these turbidite myths myth no 1 turbidity currents are non turbulent flows with multiple sediment support mechanisms reality turbidity currents are turbulent flows in which turbulence is the principal sediment support mechanism myth no 2 turbidites are deposits of debris flows grain flows fluidized flows and turbidity currents reality turbidites are the exclusive deposits of turbidity currents myth no 3 turbidity currents are high velocity flows and therefore they elude documentation reality turbidity currents operate under a wide range of velocity conditions myth no 4 high density turbidity currents are true turbidity currents reality ph h kuenen 1950 introduced the concept of turbidity currents of high density based on experimental debris flows not turbidity currents high density turbidity currents are sandy debris flows myth no 5 slurry flows are high density turbidity currents reality slurry flows are debris flows myth no 6 flute structures are indicative of turbidite deposition reality flute structures are indicative only of flow erosion not deposition myth no 7 normal grading is a product of multiple depositional events reality normal grading is the product of a single depositional event myth no 8 cross bedding is a product of turbidity currents reality cross bedding is a product of traction deposition from bottom currents myth no 9 turbidite facies models are useful tools for interpreting deposits of turbidity currents reality a reexamination of the annot sandstone in se france which served as the basis for developing the first turbidite facies model suggests a complex depositional origin by plastic flows and bottom currents myth no 10 turbidite facies can be interpreted using seismic facies and geometries reality individual turbidity current depositional events commonly centimeters to decimeters in thickness cannot be resolved in seismic data all turbidite myths promote falsehood and should be abandoned

boudinage in multilayered rocks under layer normal compression a theoretical analysis abstract this paper presents a dynamic analysis of boudinage in multilayers of alternate brittle and ductile layers under layer normal compression based on the mode of fracturing of individual brittle layers boudinage is classified into three types tensile fracture boudinage type 1 shear fracture boudinage type 2a and extensional shear fracture boudinage type 2b the layer thickness ratio t r t b t d and the strength ratio f t 2 e between the brittle and the ductile units are the principal physical factors determining the type of boudinage type 1 boudinage develops rectangular boudins and occurs when t r is low f is high 0 8 in contrast type 2a boudinage takes place when t r is high 8 5 or f is low a r of all the types of boudins is inversely proportional to layer thickness ratio t r however type 1 and type 2 boudins have contrasting aspect ratios which are generally greater and less than 1 respectively

formation and suppression of strike slip fault systems strike slip faults are a defining feature of plate tectonics yet many aspects of their development and evolution remain unresolved for intact materials and or regions a standard sequence of shear development is predicted from physical models and field studies commencing with the formation of riedel shears and culminating with the development of a throughgoing fault however for materials and or regions that contain crustal heterogeneities normal and or thrust faults joints etc that predate shear deformation kinematic evolution of strike slip faulting is poorly constrained we present a new plane stress finite strain physical analog model developed to investigate primary deformation zone evolution in simple shear pure strike slip fault systems in which faults or joints are present before shear initiation experimental results suggest that preexisting mechanical discontinuities faults and or joints have a marked effect on the geometry of such systems causing deflection lateral distribution and suppression of shears a lower limit is placed on shear offset necessary to produce a throughgoing fault in systems containing preexisting structures fault zone development observed in these experiments provides new insight for kinematic interpretation of structural data from strike slip fault zones on earth venus and other terrestrial bodies

seismic imaging of mantle transition zone discontinuities beneath the northern red sea and adjacent areas summary the dramatic asymmetry in terms of surface elevation cenozoic volcanisms and earthquake activity across the red sea is an enigmatic issue in global tectonics partially due to the unavailability of broad band seismic data on the african plate adjacent to the red sea here we report the first comprehensive image of the mantle transition zone mtz discontinuities using data from the egyptian national seismic network and compare the resulting depths of the 410 and 660 km discontinuities with those observed on the arabian side our results show that when a standard earth model is used for time to depth conversion the resulting depth of thediscontinuities increases systematicallytowards theaxisoftheafro arabiandome aad from both the west and east relative to the westernmost area the maximum depression of the 410 km discontinuity is about 30km and that of the 660 km discontinuity is about 45km the observed systematic variations can best be explained by a model involving a hydrated mtz and an upper mantle low velocity zone beneath the aad models invoking one or more mantle plumes originated from the mtz or the lower mantle beneath the study area are not consistent with the observations

pulsating continents on venus an explanation for crustal plateaus and tessera terrains we propose that tessera terrains on venus represent continental crust that does not participate in the periodic recycling of the lithosphere through global subduction events we have studied the force balance on the boundary of a continental area that survives a global subduction event using an analytical model in the proposed model the ratio between the crustal and lithospheric mantle thicknesses controls the force balance if the crust thickness is less than 2 5 of the lithospheric mantle thickness the continental area will be compressed but if the crustal thickness is higher than 2 5 of the lithospheric mantle thickness the continental area will spread out and collapse consequently if the lithospheric mantle beneath a continental region is delaminated during a global subduction event the continent will collapse generating tessera inliers dominated by extensional tectonics but if a significant portion of lithospheric mantle remains then the continental area will be compressed generating a plateau by crustal shortening the observed plateau heights can be explained by this model a 2 km height plateau can be generated by a lithospheric mantle thickness of 40 km while a 4 km height plateau can be generated by a 90 km thick lithospheric mantle we have modelled this crustal thickening of a continental area by tectonic contraction using a thin viscous sheet approach with a newtonian viscosity for the crust the force from a hot mantle elevated during a global subduction event is enough to build up a plateau by compression in 50 ma using a viscosity for the continental crust of 1021 pa s and 200 ma for 5 1021 pa s during this compressional stage concentric fold and thrust belts are generated in the plateau continent erasing any impact craters that were present the subsequent stabilization of a new crust and lithosphere in the surrounding mantle changes the force balance allowing a moderate gravitational collapse of the plateau continent accommodated by radial grabens the pulsating continent model links for the first time the generation of crustal plateaus and the origin of the volcanic plains predicting the observed equivalent effective crater density for both terrains

effects of crustal heterogeneity on the morphology of chasmata venus 1 the ix chel kuanja and vir ava chasmata ikvc are a 7000 km long collection of discontinuous topographic troughs and deformation suites collectively they account for approximately 12 5 of the 55 000 linear km of chasmata on venus however morphologically the ikvc differs significantly from the majority 75 of venusian chasmata because it lacks corona features coronae represent notable volumes of material both erupted onto the surface and intruded into the crust together with chasmata coronae may also be important indicators of venus s interior convective processes as well as significant contributors to venus s heat loss although ikvc lacks coronae detailed geologic and structural mapping along its length highlights previously unrecognized structural and volcanic relationships indicative of local uplift collapse and volcanism we call pseudocoronae in light of this mapping previous corona formation models theoretical and numerical are evaluated and an amended hypothesis diapir stagnation is presented for pseudocorona formation the diapir stagnation model proposes that variations of the crustal thickness perturb corona formation and thus affect their resultant surface morphology chasmata are accompanied by corona development when forming in regions of relatively thin or normal plains type crust 20 km but lack corona features when influenced by crustal heterogeneities mechanical and chemical at depth associated with thick crustal plateau roots 35 80 km e g ovda thetis and phoebe regios beneath crustal plateaus diapiric rise is inhibited by 1 the presence of a low density crustal keel that creates a deep seated density trap and 2 a composition dependent mechanically weak crust mantle boundary layer diapir stagnation can be characterized or accompanied by minimal broad doming of the surface formation of radial and concentric extension structures lateral dike propagation and collapse all of which may be accompanied by limited volume surface eruptions of the lowest density melt phases through time the crust mantle boundary layer may repeatedly lift and subside through conductive heating and viscous flow which may allow more voluminous melt migration and eruption however true coronae fail to develop

numerical models of complex diapirs abstract numerically modelled diapirs that rise into overburdens with viscous rheology produce a large variety of shapes this work uses the finite element method to study the development of diapirs that rise towards a surface on which a diapir induced topography creeps flat or disperses erodes at different rates slow erosion leads to diapirs with mushroom shapes moderate erosion rate to wine glass diapirs and fast erosion to beer glass and column shaped diapirs the introduction of a low viscosity layer at the top of the overburden causes diapirs to develop into structures resembling a napoleon hat these spread lateral sheets

reinterpretation of depositional processes in a classic flysch sequence pennsylvanian jackfork group ouachita mountains arkansas and oklahoma discussion the pennsylvanian jackfork group in the ouachita mountains of arkansas and oklahoma has conventionally been interpreted by many workers including us as a classic flysch sequence dominated by turbidites in a submarine fan setting however normal size grading and bouma sequences indicative of turbidite deposition are essentially absent in these sandstone beds they appear massive i e structureless in outcrop but when slabbed reveal diagnostic internal features these beds exhibit sharp and irregular upper bedding contacts inverse size grading floating mudstone clasts a planar clast fabric lateral pinch out geometries moderate to high detrital matrix up to 25 sigmoidal deformation duplex structures and contorted layers all these features indicate sand mplacement by debris flows mass flows and slumps mud matrix in these sandstones was sufficient to provide cohesive strength to the flow discrete units of current ripples and horizontal laminae have been interpreted to represent traction processes associated with bottom current reworking the dominance of sandy debris flow and slump deposits nearly 70 at degray spillway section and bottom current reworked deposits 40 at kiamichi mountain section and the lack of turbidites in the jackfork group have led us to propose a slope setting our rejection of a submarine fan setting has important implications for predicting sand body geometry and continuity because deposits of fluidal turbidity currents in fans are laterally more continuous than those of plastic debris flows and slumps on slopes a turbidite dominated fan model would predict an outer fan environment with laterally continuous sheetlike sandstones for the jackfork group in southern oklahoma and western arkansas whereas a debris flow slump model would predict predominantly a slope environment with disconne ted sandstone bodies for the same area

geological evolution of venus rises plains plumes and plateaus crustal plateaus and volcanic rises major physiographic features on venus both formed over mantle plumes crustal plateaus were produced by large degrees of plume melting beneath thin lithosphere the oldest tectonic features in crustal plateaus are ribbon like troughs indicating early uplift and tensile stretching their shallow depths suggest that surface temperature there was higher in the past widespread volcanic plains derived from the broad upwellings of internally heated mantle convection were continually erupted during the time of crustal plateau formation discrete volcanic rises younger than crustal plateaus formed over a thicker lithosphere with less plume partial melting the presence of only one transitional form indicates that the lithosphere thickened rapidly thermal and magmatic models show that the formation of these major features can be tied to an expected change in mantle convective style about 1 billion years ago

the magnetostratigraphy fission track dating and stratigraphic evolution of the peshawar intermontane basin northern pakistan the peshawar basin is situated along the junction of the northern margin of the indo gangetic foredeep and the southern margin of the hindu rush himalayan ranges during the late cenozoic southward encroachment of tectonic disruption into the foredeep terminated molasse deposition and delineated the peshawar intermontane basin through uplift of the attock range along the southern margin of the basin in this study magnetostratigraphy fission track dates on volcanic ashes and stratigraphic data are used to define the chronologic and stratigraphic evolution of the basin following a late miocene to pliocene interval of folding and thrusting of the preexisting molasse sediments deposition began in the peshawar basin by at least 2 8 m y ago subsequently 300 m of basin filling sediments accumulated at rates averaging 15 cm 1 000 yr northward progradation of alluvial fan deposits from the uplifted basin margin had begun by 2 6 m y ago and continued at an inferred rate of 2 cm yr proximal to the attock range in the south alluvial fan facies persisted until 0 6 m y ago contemporaneous sediments closer to the basin center demonstrate the presence of rapid facies transitions to extensive floodplain and shallow lacustrine depositional environments widespread intermontane basin sedimentation was terminated by accelerated uplift of the attock range after 0 6 m y ago subsequently during the brunhes chron catastrophic floods have periodically inundated the peshawar basin continuing tectonic deformation of the basin is indicated by uplifted flood deposits offset terraces and modern seismicity

numerical modeling of intraplate deformation simple mechanical models of continental collision we study a model of continental collision in which one of the continents acts as a rigid die indenting the other plate which flows as an incompressible viscoplastic medium we consider two extreme cases of plane deformation 1 plane strain which corresponds to an infinitely thick lithospheric plate and 2 plane stress corresponding to a very thin plate deformation of the lithosphere a thick plate should be intermediate between those extremes we found that the flow in the plane strain case is quite similar to that obtained by slip line theory the plane stress results are quite different since in this case most of the plate shortening is taken up by the thickening of the lithosphere we also explored the role of boundary conditions on the flow in particular the role of the side walls containing the flow of the lithosphere in the case of a free lateral boundary the main feature is a flow of matter toward this free wall and a s like pattern for the horizontal stress field for a rigid wall on the other hand the plane strain and the plane stress results are quite different in the first case there is a large return flow on the sides of the punch the material being extruded along the only free surface available in the plane stress case the return flow disappears and the material displaced by the penetration of the die tends to thicken the plate the role of a nonlinear constitutive relation is studied for power law creep as the power of the flow limit increases the flow retains its general features but the deformation localizes creating sharper contrasts between high and low strain rate areas in plane stress the effect of nonlinearity is to increase the contrasts in vertical motion available data for asia are discussed in the light of the new results

magellan and comparative planetology magellan data have been widely used to provide explanations for the tectonics and volcanism on venus however it is also important to compare our understanding of these processes on venus with the earth moon and mars one essential question concerning the behavior of these bodies is how does the heat get out internal heat generation and planetary cooling are the energy sources responsible for planetary tectonics and volcanism on the earth the plates of plate tectonics are the thermal boundary layers of mantle convection cells the subduction of these cold plates at ocean trenches contributes about 75 of the mantle heat flux it is now quite clear that active plate tectonics is not occurring on venus the surface of venus is a single rigid plate as is the case for the moon mars and mercury prior to magellan the general consensus was that venus was in a near uniformatarian state with conductive heat losses through the lithosphere balancing internal heat generation and secular cooling of the planet however crater studies from magellan images provide direct evidence for a catastrophic or episodic evolution of venus one explanation is that venus died tectonically 500 m y ago plate tectonics occurred during the early evolution of the planet but ceased as the radioactive heat production decreased an alternative explanation is that venus experiences episodic global subduction events two major questions concerning the earth remain largely unanswered particularly when compared with venus 1 why does the earth have continents 2 why does the earth have plate tectonics a major difference between the earth and venus is the presence of liquid water oceans during much of the evolution of the earth it has been hypothesized that water is an essential ingredient for the silicic volcanism that produces continental rocks on the earth and the absence of well defined continental masses on venus supports this hypothesis it has long been recognized that plate tectonics cannot evolve in time without internal deformation of plates plate evolution in time results in geometrical incompatibilities that require overlaps or gaps on the earth these incompatibilities lead to broad zones of continental deformation i e the western united states and china the absence of plate tectonics on venus suggests that rheologically soft continents are necessary for plate tectonics a reasonable conclusion is that the earth is the remarkable planet in terms of tectonics and volcanism not venus

coarse grained submarine fan and slope apron deposits in a cretaceous back arc basin antarctica the cretaceous of west james ross island antarctica represents the proximal fill of a late mesozoic back arc basin that was probably initiated by oblique extension during the early development of the weddell sea the succession records sedimentation in two contrasting depositional systems a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse grained submarine fans slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones mudstone association interspersed with non channelized chaotic boulder beds intraformational slump sheets and isolated exotic blocks representing a spectrum of mass flow processes from debris flow to submarine gliding localized sand rich sequences sandstone breccia association represent sandy debris lobes at the mouths of active slope chutes the submarine fan sediments conglomerate association are typified by coarse conglomerates and pebbly sandstones interpreted as the deposits of high density turbidity currents and non cohesive debris flows three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse grained submarine fans from major fan channels through ephemeral marginal channels or terraces to levee or interchannel environments the occurrence of both slope apron and submarine fan depositional systems during the early and mid cretaceous is attributed to localized input of coarse arc derived sediment along a tectonically active basin margin periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons slope apron sedimentation was controlled largely by intrabasinal tectonics local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults differential subsidence across the basin margin anchored the basin slope for at least 20 myr and precluded basinward progradation of shallow marine environments

global tectonic evolution of venus from exogenic to endogenic over time and implications for early earth processes venus provides a rich arena in which to stretch one s tectonic imagination with respect to non plate tectonic processes of heat transfer on an earth like planet venus is similar to earth in density size inferred composition and heat budget however venus lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non plate tectonomagmatic processes in this paper i explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time divided respectively into temporal eras impactors played a prominent role in the ancient era characterized by thin global lithosphere the artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume the fracture zone complex era marked by broad zones of tectonomagmatic activity witnessed coupled spreading and underthrusting since arrested these three tectonic regimes provide possible analogue models for terrestrial archaean craton formation continent formation without plate tectonics and mechanisms underlying the emergence of plate tectonics a bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of archaean crust and recycling of significant archaean crust to the mantle this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

correction to a thin viscous sheet model for continental deformation summary the governing equation derived by england mckenzie for the deformation of a thin viscous shell contains an error we give the correct derivation here and correct those figures where the change makes a visible difference the correct results differ quantitatively from those of england mckenzie but their conclusions are not affected

the bouma sequence and the turbidite mind set abstract conventionally the bouma sequence bouma a h 1962 sedimentology of some flysch deposits a graphic approach to facies interpretation elsevier amsterdam 168 pp composed of ta tb tc td and te divisions is interpreted to be the product of a turbidity current however recent core and outcrop studies show that the complete and partial bouma sequences can also be interpreted to be deposits formed by processes other than turbidity currents such as sandy debris flows and bottom current reworking many published examples of turbidites most of them hydrocarbon bearing sands in the north sea the norwegian sea offshore nigeria offshore gabon gulf of mexico and the ouachita mountains are being reinterpreted by the present author as dominantly deposits of sandy debris flows and bottom current reworking with only a minor percentage of true turbidites i e deposits of turbidity currents with fluidal or newtonian rheology in which sediment is suspended by fluid turbulence this reinterpretation is based on detailed description of 21 000 ft 6402 m of conventional cores and 1200 ft 365 m of outcrop sections the predominance of interpreted turbidites in these areas by other workers can be attributed to the following 1 loose applications of turbidity current concepts without regard for fluid rheology flow state and sediment support mechanism that result in a category of turbidity currents that includes debris flows and bottom currents 2 field description of deep water sands using the bouma sequence an interpretive model that invariably leads to a model driven turbidite interpretation 3 the prevailing turbidite mind set that subconsciously forces one to routinely interpret most deep water sands as some kind of turbidites 4 the use of our inability to interpret transport mechanism from the depositional record as an excuse for assuming deep water sands as deposits of turbidity currents 5 the flawed concept of high density turbidity currents that allows room for interpreting debris flow deposits as turbidites 6 the flawed comparison of subaerial river currents fluid gravity flows dominated by bed load transport with subaqueous turbidity currents sediment gravity flows dominated by suspended load transport that results in misinterpreting ungraded or parallel stratified deep sea deposits as turbidites and 7 the attraction to use obsolete submarine fan models with channels and lobes that require a turbidite interpretation although the turbidite paradigm is alive and well for now the turbidites themselves are becoming an endangered facies

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

tessera terrain and crustal plateaus venus many workers assume that tessera terrain marked by multiple tectonic lineaments and exposed in crustal plateaus comprises a global onion skin on venus a growing body of structural mechanical magmatic gravitational topographic and geologic evidence indicates that tesserae record the local interaction of individual deep mantle plumes with an ancient globally thin venusian lithosphere resulting in local regions of thickened crust

shortening of analogue models of the continental lithosphere new hypothesis for the formation of the tibetan plateau initial stages of compression produce periodic buckling of models analogous to a four layer continental lithosphere with further shortening amplification of the buckles occurs by thrust faulting at inflection points in both brittle layers of the upper crust and upper mantle the rescaled results suggest that a continental lithosphere under compression will buckle with a first order wavelength of about 200 km and a second order wavelength of 20 30 km continued shortening results in the amplification of undulations troughs sink and become compressional basins while deformation keeps going on major thrusts within the upper mantle thus giving rise to an irregular topography of the moho fifty percent shortening results in a 60 km thick continental crust we argue that this model history may explain the elevation and deep crustal structure of the tibetan plateau

system of tectonic features common to earth mars and venus investigations of landforms on the terrestrial planets have revealed a system of tectonic features consisting of long narrow regularly spaced folds and or thrust faults referred to as wrinkle ridges and conjugate sets of cross trending strike slip faults these are observed in the yakima fold belt of the columbia plateau earth the ridged plains of the tharsis province mars and the lowland plains of lavinia planitia venus the wrinkle ridges and strike slip faults reflect a relatively small amount of crustal shortening in these regions of distributed deformation the observed geometric relations between the structures are consistent with those predicted by the coulomb anderson model although the tectonic settings of the provinces studied on the three planets are very different the crustal materials appear to have deformed in a similar manner

the chronology of intermontane basin development in the northwestern himalaya and the evolution of the northwest syntaxis abstract the northwest syntaxis delineates a complex zone where the northwesterly trending himalayan ranges meet the northeasterly trending hindu kush and indus kohistan ranges the southern margin of the hindu kush himalayan collisional belt is delineated by a series of imbricate thrusts that transect the northern edge of the modern indo gangetic foredeep the kashmir and peshawar basins are embedded in this still developing thrust belt and are symmetrically oriented about the northwest syntaxis consideration of chronologic stratigraphic structural and geophysical data from the syntaxial zone permits the construction of a model for intermontane basin development and the evolution of the northwest syntaxis during the late cenozoic the formation of the kashmir and peshawar basins results from the transfer from the north of the locus of thrusting and uplift to the southern margins of the basins during the pliocene the morphotectonic emergence of the ancestral pir panjal and attock ranges along the southern margins of the two basins coincides with changes in the patterns of sedimentation and deformation both within the basins and in the bounding foredeep to the south contrasting styles of tectonic deformation on opposite sides of the syntaxis are interpreted as a response to differences in the strength of sediment basement coupling across the syntaxis

measurements of transience and downward fluid flow near episodic methane gas vents hydrate ridge cascadia aqueous flux measurements within cold seep regions on northern hydrate ridge cascadia indicate a high degree of variability with extended periods of downflow and reversals of flow direction over periods of weeks to months local episodic venting of free methane gas was also observed the instruments recorded similar changes in hydrologic flow patterns both on and off clam fields the magnitude of the flow rates decreasing away from the clam field the coincidence of episodic gas venting with nearby highly variable aqueous fluid flow suggests that they may be coupled we propose that these observations are consistent with the action of a gas driven pump that operates somewhat like a geyser the hypothesis of gas driven pumping of seawater through northern hydrate ridge has important ramifications for the mass fluxes through this region

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

the active tectonics of the eastern himalayan syntaxis and surrounding regions source parameters of 53 moderate sized earthquakes obtained from the joint inversion of regional and teleseismic distance long period body waves provide the data set for an analysis of the style of deformation and kinematics in the region of the eastern himalayan syntaxis focal mechanisms of eastern himalayan events show oblique thrust consistent with the n ne directed movement of the indian plate as it underthrusts a boundary that strikes at an oblique angle to the direction of convergence earthquakes near the sagaing fault show strike slip mechanisms with right lateral slip earthquakes on its northern splays however indicate predominant thrusting evidence that the dextral motion on the sagaing fault which accommodates a portion of the lateral motion between india and southeast asia terminates in a zone of thrust faulting at the eastern himalayan syntaxis remaining motion between india and southeast asia is accommodated in a zone of distributed shear in east burma and yunnan manifested by strike slip and oblique normal faulting east west extension crustal thinning and clockwise rotation of crustal blocks we determined strain rates throughout the region with a moment tensor summation using 25 years modern and 85 years modern and historic of earthquake data we matched the observed strains with a fifth order polynomial function and from this we determined both the velocity field and rotations with respect to a specified region velocities calculated relative to south china stationary show that the entire area extending from 20 n 36 n within deforming asia yunnan western sichuan and east tibet constitutes a distributed dextral shear zone with clockwise rotations up to 1 7 m y maximum in the region of the eastern syntaxis proper integrated strains across this zone relative to south china stationary show 38 mm yr 12mm yr of north directed motion at the himalaya remaining plate motion relative to south china fixed must be taken up by the underthrusting of india beneath the lesser himalaya strike slip motion on the sagaing fault and intraplate ne directed shortening within ne india as well as ne directed shortening within the eastern syntaxis proper 10 mm yr 2 mm yr of relative right lateral motion between india and southeast asia is absorbed in the region between the sagaing and red river faults 94 e 100 e it is the clockwise vorticity relative to south china associated with the deformation in yunnan east tibet and western sichuan that provides the relative north directed motion of 38 12 mm yr at the himalaya not all of the deformation is accommodated in right lateral shear between india and south china and between east tibet and south china velocity gradients exist that are parallel to the trend of the shear zone relative to a point within western sichuan 32 n 100 e the velocity field shows that the yunnan crust is moving s se at rates of 8 10 mm yr relative to south china there is no eastward expulsion of crustal material beyond the eastern margin of the tibetan plateau

slab temperature and thickness from seismic tomography delay times of compressional body wave phases from both teleseismic and local events are used to invert for a high resolution p wave velocity model in the tonga subduction zone using only null space components to modify the minimumnorm solution an enhanced model is obtained which has been biased toward the theoretical solution but has the same data misfit as the minimum norm solution the final image shows a very narrow and continuous slab with maximum velocity anomalies of the order of 6 7 many of the gaps within the slab as well as artifacts around the slab which were present in the minimum norm solution are absent in the biased image

magma emplacement in transpression the santa olalla igneous complex ossa morena zone sw iberia the santa olalla igneous complex a late variscan group of intrusions located in the ossa morena zone sw iberia has been the focus of a gravity and structural study the structure outlined by the foliation map is complex showing two different structural domains one characterized by vertical and the other by horizontal magmatic foliations the vertical fabrics are restricted to the ne half of the complex which is in direct contact with a variscan sinistral strike slip fault cherneca fault whereas the horizontal fabrics are developed in the sw half of the complex which is characterized by a horizontal tabular geometry gravity modeling indicates that the deeper floor of the plutons is closely related to the ne margin and the cherneca fault an emplacement and structural evolution model for this igneous complex is proposed following these structural and gravity results 1 magma ascent was favored by releasing bends in the trace of the cherneca fault 2 when magma reached the present level it intruded to the sw with a horizontal sheet geometry generating the subhorizontal foliation domain 3 after emplacement the ne half of the complex suffered the external tectonic stress field provoked by sinistral motion along the cherneca fault subsequently generating the subvertical magmatic foliation domain

geologic constraints on crustal plateau surface histories venus the lava pond and bolide impact hypotheses 1 venusian crustal plateau formation is hotly debated crustal plateaus are large 1500 2500 km quasi circular 0 5 to 4 km high plateaus that host distinctive tectonic fabrics called ribbon tessera debate centers on plateau support and fabric formation detailed geologic mapping of an 360 000 km2 region eastern ovda regio provides critical clues for plateau evolution ribbon tessera fabrics record broadly synchronous layer contraction extension and flooding as an initially strong thin layer 100 m developed across ovda as the layer underwent ductile shortening and brittle extension subsurface lava flooded structural lows with progressive deformation the layer thickened leading to formation of progressively longer wavelength structures early formed shorter wavelength structures and lava filled valleys were carried piggyback on longer wavelength folds this history indicates an extremely large strength contrast that is inconsistent with a conductive geotherm reflecting instead the presence of a near surface molten layer i propose the lava pond hypothesis ribbon tessera fabrics represent progressively solidified scum of a huge lava pond leading to crustal plateau formation i further propose that individual lava ponds formed as a result of large bolide 20 30 km diameter impact with ancient thin lithosphere causing massive partial melting in the upper mantle melt rose buoyantly to form huge 1500 2000 km diameter lava ponds completely resurfacing the local area in the upper mantle depleted residuum would be compositionally more buoyant and stronger than adjacent undepleted mantle isostatic adjustment resulted in pond uplift and plateau formation subsequent secular cooling could freeze a plateau in place or mantle convection could strip away the residuum root leading to plateau collapse

structural evolution of lavinia planitia venus implications for the tectonics of the lowland plains this work shows the results of a detailed structural analysis of the deformation belts of lavinia planitia ridge belts and graben and groove belts can be observed at the studied area while wrinkle ridges and large individual grooves predominate in the smooth plains transcurrent components of displacement are commonly observed and transpression and transtension zones are the rule rather than the exception at most of the studied belts along strike azimuth changes of deformation belts are accommodated by internal variations in the predominance of contractional transcurrent or extensional structures the material of the surrounding plains embays most of these deformation belts the kinematic analysis of this complex network of tectonic structures suggests a broadly synchronous activity of contractional transcurrent and extensional structures the maximum horizontal shortening axis determined in this work describes a steady semicircular pattern centered at alpha regio this deformation continued although with subdued activity after embayment of the deformation belts by the material of the plains future study of the tectonic evolution of the lowland plains should take into account the importance of the coeval history of neighboring uplands and lowlands

tectonic controls on the uplift of the nanga parbat massif pakistan himalayas cenozoic mountain belts like the himalayas contain regions that experienced rapid uplift in comparison with the areas around them1 whether this uplift occurred by buoyant rebound of previously thickened crust2 or during the crustal thickening process itself is uncertain3 4 here we show however that the 7 mm yr 1 recent uplift5 of the nanga parbat massif pakistan himalayas was accomplished during nw directed thrusting located along its western margin this 3 km wide liachar thrust zone operated from amphibolite facies to the present topographic surface and accommodates substantial crustal shortening the adjacent indus river valley itself cut by seismogenic faulting completely transects the massif showing that erosion keeps pace with geologically rapid thrust uplift6

inversion for anisotropy from non double couple components of moment tensors 1 an inversion method for retrieving seismic anisotropy from non double couple components of seismic moment tensors is presented the method requires a set of highly accurate moment tensors of earthquakes that occurred in a homogeneous anisotropic focal area on differently oriented faults in contrast to standard methods retrieving anisotropy from travel times or from shear wave splitting which yield an overall anisotropy averaged along a whole ray path the presented method yields a local value of anisotropy just in the focal area the method is robust being able to retrieve the orientation as well as strength of anisotropy even for low anisotropy symmetries as for orthorhombic symmetry the method can utilize the moment tensors constrained to have zero trace but using unconstrained moment tensors is advantageous the method is applied to retrieving anisotropy in the tonga subduction zone using moment tensors of deep focus earthquakes reported in the harvard centroid moment tensor catalog the inversion is complemented by tests on synthetic data to assess its stability and the accuracy of the results the inversion indicates that the subduction zone is anisotropic with orthorhombic symmetry the orientation of the intra slab anisotropy is defined by axes azimuth dip a1 320 54 a2 121 38 and a3 223 81 the errors in the azimuth and dip are about 5 the first and second axes lie along the downdip motion of the slab and along the normal to the slab respectively the strength of the p s1 and s2 anisotropy is of 7 3 1 5 13 4 2 5 and 12 6 3 5 respectively the errors of anisotropy strength are only rough estimates which reflect random but not systematic errors in the moment tensors used in the inversion the values for anisotropy strength in the slab are remarkably higher than those observed in the surrounding mantle the symmetry axes of anisotropy coincide with the principal stress directions in the slab this manifests a primary impact of stress on anisotropy formation the retrieved anisotropy in the slab can serve as an additional constraint on its structure and mineralogical composition

evidences for a noachian hesperian orogeny in mars a number of tectonic structures have been located at the thaumasia plateau daedalia planum and aonia terra mars they include isolated folds with axial traces up to 200 km long trains of tightly folded structures tens of km long and thrusts their size and geometry are similar to those on earth and the direction of compression seems to have varied amply with time suggesting a complex tectonic evolution crater counts on the deformed terrain point to noachian to early hesperian ages on the basis of the geometry and geological relations of these structures we propose that they form part of an old martian orogen the thaumasia aonia orogen which embraced not only the thaumasia plateau but areas of daedalia planum aonia terra and nereidum montes as well a regional coherent layering is previous to the deformation and could represent the trace of even older stresses on the martian lithosphere

tessera terrain ribbon fabrics on venus reviewed could they be dyke swarms abstract endogenic and exogenic models have been developed over the past 25 years to explain the development of extensive pervasive high aspect ratio troughs and graben like structures ribbons and short to long wavelength folds in tessera terrains commonly associated with crustal plateaus on venus many of the models are complex and appear to be both internally contradictory and inconsistent with well established geological principles they are based on what amount to assumptions regarding i the secular evolution of crustal thickness ii crustal mechanical homogeneity and isotropy iii the mechanics of ribbon formation and iv the relative timing of ribbons in the structural geological sequence of tessera terrains on the basis of both theoretical and observational considerations this contribution will suggest that coupling the formation of tessera ribbons to the emplacement of dyke swarms relatively late in the structural development of tessera terrains may provide a resolution for the apparent inconsistencies in those published models

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

the namche barwa syntaxis evidence for exhumation related to compressional crustal folding abstract geological observation in the eastern end of the himalayas shows that the asia india suture is folded and continues southward into india and burma where the continuation of the transhimalayan plutonic complex can be identified metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating are consistent with the geological interpretation in addition they establish cooling exhumation of rocks from c 30 km depth within the last 4 ma rapid exhumation of deep crustal levels in mountain systems is generally related to extension subsequent to thermal softening of a thickened continental lithosphere however in the eastern himalayas we argue that exhumation is caused by c 10 mm yr 1 erosion coeval with crustal scale folding the general history of this syntaxis resembles the evolution of the western himalayas syntaxis in pakistan

crustal thickening versus lateral expulsion in the indian asian continental collision since the beginning of the continental collision between india and asia there has been about 2500 km of convergence and the northward movement of india has been accommodated by major internal deformation of the asian lithosphere the crustal thickening in and around the tibetan plateau is clearly a direct consequence of this collision but there is considerable debate as to whether a large fraction of the indentation has been accommodated by eastward motion of the lithospheric blocks of southeastern asia and southern china numerical experiments described here test this hypothesis for a range of indentation geometries and rheological models of the lithosphere we employ a thin viscous sheet model of the lithosphere with a depth averaged nonlinear viscous rheology described by a stress strain rate exponent n and including gravitational buoyancy forces scaled by the dimensionless argand number ar the eastern boundary for the collision region is described as a lithostatic boundary the precollision normal stress is determined by static balance and that constant stress is applied throughout the collision the experiments show that during collision the eastern boundary is smoothly displaced to the east at a rate about 1 4 of the indentation rate with only minor variation due to geometry or rheology crustal thickening rates in the region of the plateau are reduced by between 10 and 25 of the corresponding rates determined for similar experiments with a rigid eastern boundary to generalize from the results of these experiments the total north south shortening strain produced by the collision is partitioned between crustal thickening and eastward displacement in the ratio of at least 3 1 and more probably 4 1

ribbon terrain formation southwestern fortuna tessera venus implications for lithosphere evolution abstract the term tessera has been used to describe regions of deformed venusian crust exhibiting two or more intersecting sets of structural elements however tessera includes terrains formed by a variety of spatially and temporally discrete tectonic processes tessera fabric characterizes highland plateau structure and thus understanding the nature of this deformation is critical to understanding the mode of highland plateau formation many tessera fabrics include ribbons folds and late graben in this paper we refine the geometry of ribbons through geologic mapping and radargrammetric analysis of type ribbon structures at southwestern fortuna tessera we extend our findings to ribbon fabrics at thetis regio any model of ribbon formation must account for the following constraints on ribbon geometry a ribbon forming lineaments exhibit sharp contrasts relative to adjacent materials b ribbon bounding lineaments form a distinct pattern alternating between radar dark and radar bright which represent trough walls oriented away from and toward the satellite respectively c ribbons form long narrow troughs that alternate with parallel narrow ridges ridges and troughs display extreme length width aspect ratios d trough walls are near vertical e troughs are shallow with consistent shallow depth along individual troughs and in adjacent troughs f in some cases e g southwest fortuna tessera trough walls are parallel and matched and would exhibit a close fit if the trough was closed in these cases trough walls merge laterally forming v shaped terminations trough floors are smooth and flat lacking small scale interior lineaments g in other cases e g thetis regio ribbons display a d but trough walls are defined by a series of subparallel lineaments including local interior lineaments and trough floors ramp up to join trough walls displaying parallel rather than v shaped terminations trough walls would not display a close fit if closed we propose two member types of ribbons tensile fracture ribbons and shear fracture ribbons tensile fracture ribbons display features a f and formed by the opening of tensile fractures of a thin brittle layer above a ductile substrate they require a near fracture free shallow crust and very shallow depth to the brittle ductile transition bdt the presence of ribbon structures within highland plateaus favors an upwelling model for highland plateau formation in which crustal thickening results from magmatic underplating related to a mantle upwelling or mantle plume in order for the plume to be able to anneal mechanically the crust as required by ribbon formation the lithosphere would likely have to be quite thin these implications are consistent with highland plateaus as an ancient signature of mantle plumes on thin lithosphere whereas volcanic rises which are presently thermally supported reflect thick lithosphere phoebe regio represents a transitional lithospheric thickness

venus records a rich early history the distribution and character of venus s impact craters led to the widely accepted idea that venus underwent global catastrophic resurfacing ca 500 ma and thus venus records only a short history encompassing surface evolution since postulated catastrophic resurfacing ribbon tessera terrain rtt a structurally distinctive unit represents some of venus s old est surfaces and is widely accepted as forming prior to postulated global catastrophic resur facing we constructed a global geologic map of rtt unit exposures and structural trends using national aeronautics and space administration nasa magellan data map relations illustrate that rtt displays planet scale patterns that together with altimetry record a rich geologic history that predates proposed global catastrophic resurfacing

tibetan and indian lithospheres in the upper mantle beneath tibet evidence from broadband surface wave dispersion broadband seismic experiments over the last two decades have produced dense data coverage across tibet yet the mechanism of the india asia lithospheric convergence beneath it remains a puzzle with even its basic features debated and with very different end member models advocated today we measured highly accurate rayleigh and love wave phase velocity curves in broad period ranges up to 5 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds to thousands of interstation measurements made with cross correlation and waveform inversion methods robust shear velocity profiles were then determined by extensive series of nonlinear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships our results reveal strong heterogeneity in the upper mantle beneath tibet very large high velocity anomalies in the upper mantle are consistent with the presence of underthrust beneath southwestern tibet and subducted beneath central and eastern tibet indian lithosphere the corresponding thermal anomalies match those estimated for subducted indian lithosphere in contrast to the indian lithosphere tibetan lithosphere and asthenosphere display low to normal shear speeds tibetan lithosphere is thus warm and thin radial anisotropy in the upper mantle is weak in central and strong in northeastern tibet possibly reflecting asthenospheric flow above the subducting indian lithospheric slab

new evidences for the formation of and for petroleum exploration in the fold thrust zones of the central black sea basin of turkey the central black sea basin of turkey is filled by more than 9 km 6 mi of upper triassic to holocene sedimentary and volcanic rocks the basin has a complex history having evolved from a rift basin to an arc basin and finally having become a retroarc foreland basin the upper triassic lower jurassic akgol and lower cretaceous caglayan formations have a poor to good hydrocarbon source rock potential and the middle eocene kusuri formation has a limited hydrocarbon source rock potential the basin has oil and gas seeps many large structures associated with extensional and compressional tectonics which could be traps for hydrocarbon accumulations exist fifteen onshore and three offshore exploration wells were drilled in the central black sea basin but none of them had commercial quantities of hydrocarbons the assessment of these drilling results suggests that many wells were drilled near the ekinveren erikli and ballifaki thrusts where structures are complex and oil and gas seeps are common many wells were not drilled deep enough to test the potential carbonate and clastic reservoirs of the inalti and caglayan formations because these intervals are locally buried by as much as 5 km 3 mi of sedimentary and volcanic rocks no wells have tested prospective structures in the north and east where the prospective inalti and caglayan formations are not as deeply buried untested hydrocarbons may exist in this area

structural and kinematic analysis of eastern ovda regio venus implications for crustal plateau formation crustal plateaus on venus constitute one of that planet s most intriguing tectonic features they host so called tessera terrain or deformed crust exhibiting two or more sets of intersecting tectonic lineaments which comprises 8 of venus surface major questions surrounding crustal plateaus and tessera terrain include 1 what is the nature of crustal plateau deformation what structures are present and what are their spatial and temporal relations 2 what is the global distribution of tessera terrain does it form a global layer under the regional plains cropping out in crustal plateaus or is it confined to crustal plateaus and 3 what are the implications of 1 and 2 for understanding the processes responsible for formation and deformation of crustal plateaus structural and kinematic analyses reveal that deformation at ovda regio venus largest crustal plateau is characterized by linear shallow steep sided troughs ribbons long arcuate long wavelength folds short mid wavelength folds and wide complex graben temporal relations between structures lead to a three phase regional strain history consisting of 1 layer normal flattening of a thin competent membrane resulting in radially oriented distributed extension ribbon formation 2 modest contraction of a thicker layer concentric folding at plateau margins interference folding in the plateau interior and 3 limited extension localized at fold crests graben formation wavelength analysis of ribbons and folds places limits on the thickness of the competent layer during each stage of deformation and reflects an early shallow brittle ductile transition bdt that deepened with time ovda s deformation topography and gravity are all spatially correlated strongly suggesting a genetic relationship between surface deformation and processes causing crustal thickening and plateau uplift thus tessera terrain at ovda regio is likely confined to the topographically high plateau and does not extend beneath the surrounding regional plains similar patterns persist at other crustal plateaus indicating that tessera terrain is in general confined to crustal plateaus and does not form a globally continuous layer ovda s structures their spatial and temporal relations the resulting strain history and the behavior of the bdt through time support a model of mantle plume upwelling in a thin lithospheric regime for crustal plateau formation and contradict previous downwelling models similarities between structural and kinematic trends at ovda regio and other crustal plateaus lend support to this idea c 1999 academic press

style and sequence of extensional structures in tessera terrain venus recent studies have focused on the question of the stratigraphic sequence and thus the stages of tessera formation specifically if tessera are formed by contractional deformation followed by extensional deformation or vice versa a major question centers on the interpretation of specific lineaments within tesserae as graben bounded by faults 60 or alternatively open tension fractures dipping 90 we document and assess the origin of extensional structures in tesserae at several locations on venus noting the morphology continuity along strike parallelism of walls stratigraphic position and interaction with other structures and variability due to radar viewing geometry in each study area our analyses demonstrate that 1 the extensional structures have variable widths interior subparallel lineaments and ramp terminations 2 ridges and lineaments are continuous across the troughs where the floors of many of these structures contain the lowered sections of preexisting structures and 3 intratessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures we conclude that the morphology of these extensional structures is consistent with an origin as graben not open tension fractures and that these graben postdate the ridges in each study area both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick to further test the tension fracture model we examine the conditions of a venus that could produce tension fractures of the dimension 1 km width of extensional structures found in tessera terrain and find that thermal gradients of a minimum of 400 to 1500 k km 1 heat flows of 800 to 3000 mw m 2 are required for a range of diabase rheologies and strain rates thought typical of venus during tessera formation such a thermal structure would favor partial melting at depths 1 km dike propagation from this region of shallow melting within the tensile stress field would produce vast quantities of volcanism mitigating against the preservation of the closely associated tension fractures this volcanism is not observed both the amplitude and sign of changes in surface temperature induced by atmospheric warming due to massive outpourings of lava are not consistent with the hotspot model on the basis of our analysis of tesserae we conclude that the ridges formed first in response to large scale contraction of the crust and that the graben formed contemporaneously and largely following this phase as the thickened crust relaxed in a manner to what is predicted and observed for plateau regions on earth such as tibet and the altiplano

intermontane basin development in the past 4 myr in the north west himalaya the through the combined use of magnetic polarity stratigraphy with fission track dating of volcanic ashes a new chronology spanning 4 myr has been developed for the intermontane basin of kashmir in the northwestern himalaya

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

periodic instabilities during compression or extension of the lithosphere 1 deformation modes from an analytical perturbation method we argue that the plastic rheology of the lithosphere rather than its recoverable elastic properties is responsible for the systematic development of periodic instabilities during compression or extension we use a linear perturbation model with analytical solutions to calculate the instability modes for various rheologies the growth of such periodic instabilities is enhanced by the highly nonlinear stress strain rheologies encountered in the brittle layers of the lithosphere on the contrary ductile layers deforming according to high temperature creep flow laws tend to inhibit these instabilities for oceanic domains we assume that the only brittle layer is the upper part of the lithosphere in compression the only valid instability is a buckling whose wavelength is around 4 times the thickness of the brittle layer calculated wavelengths and growth rates are consistent with observations available for the indian ocean for continental domains a reasonable assumption is the existence of two plastic layers one in the upper crust the other in the upper mantle for moho temperatures between 450 c and 600 c in compression and extension two instabilities develop a long wavelength instability involving the whole lithosphere coupling mode and a short wavelength instability involving the crust and controlled by the upper brittle layer intrinsic crustal mode in compression the coupling mode is a whole lithosphere buckling with a wavelength about 4 times the thickness of the active lithosphere the two plastic layers plus the intermediate ductile layer in extension the coupling mode is a boudinage of opposite phase in the two plastic layers and a folding of the intermediate ductile layer the intrinsic crustal mode is a crustal buckling in compression crustal boudinage in extension neither deflects the moho the intrinsic crustal mode is favored by an increase in thermal gradient and by a decrease in strength of the ductile lower crust

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

a 400 km scale strike slip zone near the boundary of thetis regio venus we present a structural analysis of an area of venus surface centred on 3 s 116 e the area is dominated by a wide zone of deformation striking 050 n distinguished from the terrain on either side of it by markedly different structural and morphological characteristics the deformation zone contains two principal fault sets defined by form position and orientation members of the first set are present along the length of the deformation zone they are discontinuous consistently right stepping and are interpreted as riedel shears in a sinistral strike slip regime members of the second fault set are interpreted as normal faults and are often seen in pairs forming extensional grabens these faults are most prominent in the central region of the deformation zone and coincide with a deflection in strike of both the first fault set and the boundaries of the deformation zone a detailed kinematic analysis of all fault orientations and the boundaries of the deformation zone fit the predictions of transtensional theory supporting the hypothesis that this sinistral strike slip zone contains an extensional jog this interpretation is further supported by analysis of the interaction between faults belonging to the two sets variation in intercept geometry kinematic linking of the two fault sets and rotation of fault strike between the two fault sets all strongly suggest simultaneous development of these structures and therefore a common tectonic origin a driving mechanism for such large scale horizontal deformation on venus has yet to be established