on the detectability of crustal magma chambers using the magnetotelluric method we utilized resistivity models of silicic magma chambers to explore effects that layering can have on three dimensional 3 d magnetotelluric mt responses model simulations show that mt detection of magma may depend strongly on its one dimensional host the 3 d responses of a model juvenile magma body which is connected electrically to deeper less resistive crust of regional extent are very subdued however intermediate and mature magmatic systems leaving magma no longer in contact with lower less resistive crust may be detectable with the mt method further the release and ascent of volatiles from crystallizing melt may lead to fracturing above the chamber thereby establishing electrical contact of the magma with shallow conductive crust and amplifying its mt response

the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

processed and enhanced gravity and magnetic images over the vredefort structure and their interpretation abstract cryptoexplosion structures are polygonal features which have experienced violent disruption during their formation the vredefort structure is one of the largest known cryptoexplosion structures and the geology is well exposed over its northern half the origin of the violent disruption experienced by the vredefort structure is controversial and many aspects of its history are under intensive study since most of the southern half of the vredefort structure is blanketed by a cover of karoo permian strata geophysical data are essential in any attempt to map the pre karoo geology data sets of the regional gravity and magnetic fields over the vredefort structure have been processed and enhanced with the aid of an image processor the results are presented as a series of images each one enhancing particular attributes of the data sets an interpretation of the geophysical images is presented focusing on the regional geometry and setting of the structure striking features of the geophysical images include a simple concentric geometry and polygonal shape which reflect the distribution of dense and magnetic lithologies within and surrounding the structure the kinematics of the development of a polygonal uplift are discussed inward movement of about seven collar sectors separated by radial faults and uplift of the central sections of each of the sectors may account for the gross polygonal shape pull apart voids formed at kinks in the radial faults might account for the positions of several alkali stocks within the stratified collar of the vredefort structure the enhanced image of the gravity field highlights a se trending gravity low which extends beyond the southeastern quadrant of the vredefort structure this feature forms part of a fundamental nw se trending intracratonic lineament the vredefort axis

influence of offset weak zones on the development of rift basins activation and abandonment during continental extension and breakup 1 we use numerical modelling to investigate reactivation of inherited offset weak zones owz in continental crust and mantle weak zones mwz to form offset rift basins during continental rifting and breakup offset rift basins are basins that are set off offset from the main rift locus of breakup weak zones embedded in a stiff layer are preferentially and rapidly reactivated whereas the same zones are either ignored or slowly reactivated when embedded in pliable layers here stiff implies a nonlinear flow law with a high stress exponent n 10 000 a plastic material and pliable means a low stress exponent n 2 5 as in ductile power law creep of rocks whether offset rift basins form during rifting of a composite lithosphere i e comprising stiff and pliable layers depends on the competition between necking instabilities that develop at the weak zones in the stiff layers and the coupling between the stiff and pliable layers stiff cratonic lithosphere results in early localization of the deformation at the mwz rapid necking and breakup without developing offset rift basins in contrast warm pliable lithosphere develops significant offset basins and has protracted rifting because the mwz is now embedded in a pliable layer we also investigate the influence of owz dip sedimentation and the sensitivity of reactivation to the distance from owz to the mwz and to the size of the mwz a tectonic rifting styles diagram is used to show that the model results agree with natural examples

the magnetotelluric response over 2d media with resistivity frequency dispersion1 the authors investigate the magnetotelluric response of two dimensional bodies characterized by the presence of low frequency dispersion phenomena of the electrical parameters the cole cole dispersion model is assumed to represent the frequency dependence of the impedivity complex function defined as the inverse of stoyer s admittivity complex parameter to simulate real geological situations they consider three structural models representing a sedimentary basin a geothermal system and a magma chamber assumed to be partially or totally dispersive from a detailed study of the frequency and space behaviors of the magnetotelluric parameters taking known non dispersive results as reference they outline the main peculiarities of the local distortion effects caused by the presence of dispersion in the target media finally they discuss the interpretive errors which can be made by neglecting the dispersion phenomena the apparent dispersion function which was defined in a previous paper to describe similar effects in the one dimensional case is again used as a reliable indicator of location shape and spatial extent of the dispersive bodies the general result of this study is a marked improvement in the resolution power of the magnetotelluric method

spectral ip studies of finnish ore prospects spectral induced polarization ip field measurements both in the frequency domain and in the time domain were made at six early proterozoic sulfide oxide and graphitic gneiss deposits in finland core samples were also measured the textures of the mineralizations were studied from thin and polished sections deposits with large differences in texture such as graphitic gneiss and coarse grained disseminated sulfide can be separated on the basis of their phasespectra time constants a good correlation was found between the observed grain size in thin sections and the grain size calculated from the apparent field survey phase spectra in the case of homogeneous disseminated textures the measured frequency domain phase spectra and phase spectra calculated from the time domain cole cole parameters are very similar in a comparable frequency band for both techniques the phase maximum lies beyond the lowest measured frequency the possibilities to expand the frequency band to levels low enough to rea

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

implications of selected subsurface data on the structural form and evolution of some basins in the northern basin and range province nevada and utah

causes and consequences of variations in faulting style at the mid atlantic ridge both volcanism and faulting contribute to the rugged topography that is created at the mid atlantic ridge mar and preserved off axis in atlantic abyssal hill terrain distinguishing volcanic from fault generated topography is essential to understanding the variations in these processes and how these variations are affected by the three dimensional pattern of mantle upwelling ridge segmentation and offsets here we describe a new quantitative method for identifying fault generated topography in swath bathymetry data by measuring topographic curvature the curvature method can distinguish large normal faults from volcanic features whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes the combination of curvature and slope information allows inward and outward facing fault faces to be mapped we apply the method to sea beam data collected along the mar between 28 and 29 30 n the fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework long linear small throw faults occur toward segment centers while shorter larger throw curved faults occur toward ends these variations reflect those of active faults within the axial valley we investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the mar in the first model only one fault is active at a time on each side of the rift valley each fault grows while migrating away from the volcanic center due to dike injection extension across the fault causes a flexural rotation of nearby inactive faults the amount of stress necessary to displace the fault increases as the fault grows when reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley this model yields a realistic terracelike morphology from the rift valley floor into the rift mountains the relief is caused by the net rotation accumulated in the lithosphere from the active faults e g 10 reached 20 km from the active fault fault spacing is controlled by lithospheric thickness fault angle and the ratio of amagmatic to magmatic extension we hypothesize that this mechanism may be dominant toward ridge segment offsets an alternative model considers multiple active faults each fault relieves stresses as it grows and inhibits the growth of nearby faults causing a characteristic fault spacing such fault interactions would occur in a region of necking instability involving deformation over an extended area this mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults we hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints causing the homogeneous abyssal hill fabric in these regions

evolution of lithospheric strength after thrusting modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened cratonic lithosphere but after 10 to 20 m y it becomes weaker than cratonic lithosphere reaching a minimum of 40 to 70 of cratonic strength 30 to 100 m y after thrusting this may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases relative weakening results from 1 elevation of the geotherm caused by burial of radioisotope enriched surface rocks and 2 replacement of strong mantle rocks by weak crustal rocks in the lithospheric column low denudation rates enhance weakening by allowing radioisotope enriched rocks to remain buried longer our modeling indicates that 1 magmatism after thrusting is favored by high concentrations of heat producing elements in the lower plate and by slow denudation rates and 2 factors that affect denudation rate such as climate and the development of external drainage can significantly influence evolution of a thrust belt

the shallow structure of the martian lithosphere in the vicinity of the ridged plains wrinkle ridges in the smooth plains of the coprates and lunae planum regions of mars form linear or concentric patterns with a regular spacing of 25 50 km the authors test the hypothesis that the periodic development of deformation was a consequence of unstable horizontal compression that occurred prior to probable ridge related faulting of a strength stratified lithosphere which consists of a mechanically strong surface plains unit that successively overlies a weak megaregolith and a strong lithospheric basement results show that a range of models with both rigid and deformable megaregolith basement interface conditions yield solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials in models that incorporate viscous and plastic rheologies uniform and exponentially varying vertical strength distributions and the presence or absence of interfacial slip at the base of the surface plains unit the ridge spacing is primarily controlled by the power law exponent of the lithosphere the megaregolith plains unit thickness ratio and the plains unit megaregolith strength contrast deformable basement models with a viscous rheology 1 le n approx le 3 can explain the ridge spacing if the megaregolith was thicker than and approximately 1 3 orders of magnitude weaker than themore plains unit similar models with a perfectly plastic n yields infinity rheology require plains unit thicknesses greater than 5 km which exceeds most observational estimates ridge spacing in the combined viscous and plastic models can be consistent with either a dry water rich or ice rich megaregolith at the time of ridge formation less

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

the geomorphology and evolution of hecate chasma venus abstract hecate chasma is a discontinuous trough and fracture system extending from atla regio through asteria regio a detailed examination of hecate chasma using magellan image and altimetry data reveals morphologic features such as normal faults including graben compressional ridges a variety of volcanic edifices and coronae there is no evidence for a systematic age progression among coronae in chains thus a single stationary thermal anomaly underlying a moving lithosphere is an unlikely origin for the chains a comparison of the observed features with two possible models of evolution subduction delamination or lithospheric extension with limited rifting shows some similarities between the predicted morphologies of each model and hecate chasma the dominance of features related to upwelling and extensional tectonism strongly favor an extensional origin for this zone the corona chain may have been formed by upwellings related to rayleigh taylor instabilities similar to those observed at terrestrial mid ocean ridges but in an environment of limited spreading

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

the importance of a small wall deformation in the three dimensional acoustic logging results summary a variety of seismic techniques use fluid filled boreholes to host detectors and seismic sources the complexity of the wavefield generated depends on the position of the receivers and the source frequency excited type of formation and type of source in the present paper the boundary element method is used to study the wavefield elicited by monopole or dipole sources within a fluid filled cylindrical cavity drilled through an unbounded elastic formation this model is used to assess the effects of the existence of a small localized wall deformation on the response such a deformation may occur through the mechanical action of the drill string rock failure adjacent to a drilled borehole plastic deformation and washing out of the borehole in soft or poorly consolidated rocks in addition the effect of a source receiver tool placed in an off centre and or tilted position inside the fluid filled borehole on the propagation of both axisymmetric and non axisymmetric wave modes are studied

a shear experiment over the natih field in oman pilot seismic and borehole data an experimental multicomponent three dimensional 3d seismic survey has been carried out over the natih field in oman this paper describes the small scale two dimensional experiment carried out beforehand and how the results obtained from this pilot were used to assess the feasibility of a nine component three dimensional 9c3d operation as well as to determine the field parameters for the field scale 3d survey it also describes the two vsps and a wireline shear log acquired in conjunction with the pilot experiment and the importance of such borehole data for establishing the correct time to depth relationship for the seismic data and for providing an independent check on the seismic interpretation the observation of cusps in the offset vsp indicated the strong anisotropy of the fiqa shales overlying the natih reservoir

deep crustal structure of the conjugate margins of the sw south china sea from wide angle refraction seismic data abstract the south china sea is the largest marginal basin of se asia yet its mechanism of formation is still debated a 1000 km long wide angle refraction seismic profile was recently acquired along the conjugate margins of the sw sub basin of the south china sea over the longest extended continental crust a joint reflection and refraction seismic travel time inversion is performed to derive a 2 d velocity model of the crustal structure and upper mantle based on this new tomographic model northern and southern margins are genetically linked since they share common structural characteristics most of the continental crust deforms in a brittle manner two scales of deformation are imaged and correlate well with seismic reflection observations small scale normal faults grabens horsts and rotated faults blocks are often associated with a tilt of the velocity isocontours affecting the upper crust the mid crust shows high lateral velocity variation defining low velocity bodies bounded by large scale normal faults recognized in seismic reflection profiles major sedimentary basins are located above low velocity bodies interpreted as hanging wall blocks along the northern margin spacing between these velocity bodies decreases from 90 to 45 km as the total crust thins toward the continent ocean transition the continent ocean transitions are narrow and slightly asymmetric 60 km on the northern side and no more than 30 km on the southern side indicating little space for significant hyper stretched crust although we have no direct indication for mantle exhumation shallow high velocities are observed at the continent ocean transition the moho interface remains rather flat over the extended domain and remains undisturbed by the large scale normal faults the main decollement is thus within the ductile lower crust

an interpretive analysis of magnetotelluric data a simple analysis to determine the dimensionality of magnetotelluric resistivity variations to separate the near surface resistivity variation from the variations at depth and to find the relative resistivity variations with depth is described parameters derived from the magnetotelluric impedance tensor are mapped in pseudosection and planar view for two dimensional 2 d model data and for survey data from montana these data illustrate procedures that can qualitatively map complex resistivity structures in the lateral distance frequency domain or equivalently the lateral distance scaled vertical distance domain phase dependent parameters have better vertical resolution than parameters related to scalar apparent resistivity but the latter allow estimates of the resistivities encountered phase parameters for the montana data provide a semiquantitatively accurate cross section of a geologically known anticlinal structure this suggests a means of subsurface structural mapping with magnetotelluric data the use of these parameters with sufficiently dense and accurate data provides a first order indication of structure for subsequent quantitative modeling

simulation and interpretation of sonic waveforms in high angle and horizontal wells abstractwe have developed a new method to process and interpret sonic waveforms acquired in high angle ha and horizontal hz wells in the vicinity of layer boundaries numerical simulations are examined for ha hz fluid filled borehole models in which sonic tools with monopole and dipole sources operate across a horizontal bed boundary in hard and soft formations simulations are performed with a 3d time domain finite difference method assuming a sonic logging tool in which each receiver station consists of eight azimuthal receivers instead of conventional processing for monopole and dipole waveforms which takes the average and difference respectively of waveforms acquired with azimuthal receivers we analyze the waveforms acquired individually by each azimuthal receiver for dipole sources we refer to the dispersive signals recorded by individual receivers as pseudo flexural waves and we first process them with a weighted spectral semblance method to obtain the frequency dispersion curves we t

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

folding in regions of extension summary many extension zones have been subjected to folding and shortening in a direction perpendicular to the stretching such deformation can be accounted for by the extension of a thin superficial elastic layer overlying a substrate that has small elastic moduli or that deforms in a viscous regime laboratory experiments are used to document the wavelength and amplitude of the folds for a range of geometrical configurations folding is observed even for very small amounts of extension less than 1 per cent with characteristics that are consistent with finite amplitude scaling laws because of the intrinsically 3 d nature of the deformation field the size of the region affected by folding and the direction of the fold axes depend on the orientation of the extension with respect to the rigid blocks that bound the deforming region for regions of extension where the elastic thickness is about 10 km as in the basin and range province for example it is predicted that folding occurs with wavelengths in a 20 40 km range such that it induces little deformation in the lower crust and maintains a flat moho discontinuity these predictions are consistent with the observations the characteristics of faulting that is associated with such deformation are discussed

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

basin and range large scale tectonics constraints from gravity and reflection seismology two geophysical informations constrain the mechanical state of the lithosphere in the basin and range province cocorp reflection seismic structure across nevada shows a rather smooth reflector at about 30 km depth which can be interpreted as indicative of a rather flat moho on the other hand the bouguer map exhibits two marked troughs in the same region and filtering techniques reveal a predominant 200 km wavelength of the deep compensating masses of the topography this paper attributes these deep heterogeneities to lateral thermal variations created by boudinage type instabilities of the stretched mantle lithosphere

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

depth dependent earthquake focal mechanism orientation evidence for a weak zone in the lower crust the traction free boundary condition across the earth s surface provides an opportunity for studying the relationship between stress orientation and earthquake focal mechanisms because it requires alignment of principal stress axes with vertical and horizontal orientations a survey of earthquake focal mechanisms in northern california shows that their principal axes are also closely aligned with the vertical and the horizontal in the upper few kilometers of earth s crust thus the signature of the free surface boundary condition on stress appears in focal mechanism orientations as well the focal mechanism alignment can also be characterized by the relative magnitude of the off diagonal elements mxz and myz of the seismic moment tensor we find significant and systematic depth variations in the horizontal moment tensor element ms which relates to the shear traction acting on a horizontal plane for the special case of perfect alignment between principal stress and focal mechanism axes values of ms near earth s surface are small but increase with depth to a maximum between 5 and 8 km at greater depths there is a gradual decrease which suggests decreasing horizontal shear traction toward the base of the seismogenic zone we interpret this tendency of axes to become oriented near the base of the seismogenic zone and its expression in ms as the signature of a weak zone in the lower crust if correct this observation would have important implications for the mechanics of lithospheric deformation

climate changes control offshore crustal structure at south china sea continental margin a b s t r a c t rifted continental lithosphere subsides as a consequence of combined crustal thinning and mantle lithosphere cooling yet basins on some continental margins experience anomalous subsidence events that postdate active extension deep basins on the northern margin of the south china sea notably the baiyun sag show basement subsidence accelerating after 21 ma postdating extension by several million years we combine geophysical observations and numerical forward modeling to show that loading of the offshore basins by increased sediment flux caused by faster onshore erosion following early miocene monsoon intensification is a viable trigger for ductile flow after the cessation of active extension this illustrates that offshore basin dynamics at continental margins with weak crust can be controlled by onshore surface processes in a newly recognized form of climate tectonic coupling

multipole acoustic waveforms in fluid filled boreholes in biaxially stressed formations a finite difference method a finite difference method is developed to simulate elastic wave propagation in a borehole surrounded by a biaxially stressed solid formation the linear elastic formation is altered by such tectonic stresses that cause significant changes in the characteristics of wave propagation in a borehole the 2 5 dimensional problem addressed in this work concerns the three dimensional wave propagation in a medium inhomogeneous in two spatial coordinates transverse to the borehole axis a second order finite difference method is developed to solve the partial differential equations arising from a published acoustoelastic model for borehole acoustic wave propagation in prestressed formations the algorithm is validated and applied to model both the borehole flexural and axisymmetric stoneley waves the computed waveforms are processed by a variation of prony s algorithm that yields dispersion curves for flexural waves polarized both parallel and perpendicular to the stress direction the flexural dispersion crossover in a uniaxially stressed formation is quantitatively confirmed the stoneley dispersion in the presence of such stresses exhibits approximately a uniform shift toward lower slownesses over the entire bandwidth of interest this implies that azimuthal averaging of formation stiffnesses approximately yields the same effective stoneley stiffness at different radial positions

quasi cylindrical 2 5d wave modeling for large scale seismic surveys 1 the most common form of seismic exploration remains a nearly linear survey with data acquisition lines including the source and receivers the interpretation of amplitude and waveform information for such linear acquisition requires consideration of 3d seismic wavefields in many scenarios the structure is approximately 2d but still modeling is needed for point sources in recent years 2 5d modeling methods have been developed for the simulation of 3d seismic wavefields in media varying in two dimensions although the computer memory requirements are only slightly larger than those for 2d computations the computation times are too long for ready application to actual surveys to overcome this problem we propose a new approach for modeling 2 5d seismic wavefields using a quasi cylindrical representation we show an implementation of this approach using the finite difference method and demonstrate the validity and efficiency of the technique with numerical examples including a profile across a realistic model of subduction zone structure

experimental evidence of resistivity frequency dispersion in magnetotellurics in the newberry oregon snake river plain idaho and campi flegrei italy volcano geothermal areas abstract we show that magnetotelluric measurements carried out in volcano geothermal areas can be significantly affected by the resistivity frequency dispersion phenomenon the polarization effects are recognized by comparing magnetotelluric and dc geoelectrical deep soundings performed in the same station indeed the conventional separate analysis of the two data sets sometimes gives unconformable results especially as it regards the resistivity and thickness of the conductive layers ascribable to the geothermal fluid filled rocks these apparent discrepancies can be overcome by admitting the occurrence of intense induced polarization effects inside the rocks having extensive alterations due to hydrothermal paragenesis the basic theory of the induced polarization effects over magnetotelluric measurements is at first briefly recalled then three impressive field examples taken from previous experiments performed in the newberry volcano snake river plain and campi flegrei geothermal areas are presented and discussed

viscous relaxation of impact crater relief on venus constraints on crustal thickness and thermal gradient because of the high surface temperature on venus and the strong temperature dependence of strain rate in silicates solid state creep may be an important mechanism for the reduction of topographic relief on that planet venera 15 16 radar altimetric profiles of impact craters 30 140 km in diameter however indicate crater depths of several hundred meters suggesting that substantial relaxation has not occurred for these features on the basis of models for the viscous relaxation of impact crater topography we derive constraints on the thickness of the crust and the mean lithospheric thermal gradient beneath the craters from the mean and variance of observed crater depth as a function of crater diameter on venus and from estimates of the initial depths of fresh impact craters on venus obtained by scaling from lunar observations we formulate a statistical test for the maximum amount of viscous relaxation that is consistent with the observations at a specified confidence level we develop a general formulation for gravity driven flow in a linearly viscous fluid incorporating the densities and temperature dependent effective viscosities of distinct crust and mantle layers the statistical comparison of predicted and observed depths yields linked upper limits to crustal thickness h and thermal gradient dt dz since dt dz can be estimated from global heat loss arguments or thermal models an upper bound to h may be derived the preserved relief of the largest craters constrain h to be less than or equal to 10 20 km because the craters with measured depths appear to be representative of a larger population of impact structures in terms of regional elevation geological unit and degree of preservation of morphological detail extrapolation of this result to lowlands and rolling plains regions on a global basis is not unreasonable such an extrapolation together with isostatic considerations yields an upper limit to the crustal volume of venus of 1010 km3 this value is an order of magnitude less than the time integrated volume of crust produced on earth and implies either that the average rate of crustal generation has been much smaller on venus than on earth or that some form of crustal recycling has occurred on venus

tectonic effects of climate change on venus venusian plains regions are commonly crossed by small strain deformation features such as wrinkle ridges polygonal terrains and gridded terrains of these polygonal terrains are observed to have a relatively uniform spacing and are widely distributed on plains that formed during global resurfacing models of venusian climate that assume resurfacing occurred through massive volcanic events suggest that surface temperatures could have dramatically changed over nearly a billion years propagating thermal stresses into the surface and potentially causing small strain features we investigate this hypothesis by approximating the temperatures predicted by climate models as a step function and employ a fixed plate and strength envelope model to predict the resulting depth of failure and amount of strain our calculations indicate that strains due to temperature changes of 50 100 k which are favored for volcanic resurfacing events of 1 10 km thickness are consistent with the observed spacing of polygonal and gridded terrains as well as some wrinkle ridges the global nature of the climate change event is consistent with the global distribution and uniformity of such features and implies that such terrains may be a global stratigraphic marker

multipole acoustic waveforms in nonaxisymmetric boreholes and formations using a 2 5 dimensional velocity stress finite difference vs fd technique waveforms are calculated for multipole excitation of azimuthally nonsymmetric boreholes and formations axial uniformity of the borehole cross section and formation is assumed the vs fd waveforms agree with previous transform methods for circular boreholes and the modes found for elliptical boreholes agree with those derived by a boundary integral technique borehole ellipticity substantially reduces monopole compressional headwave amplitudes in fast formations low frequency dipole waveforms in elliptical boreholes show marked sensitivity to source dipole orientation in fast formations but small sensitivity in slow formations waveforms from dipole receivers perpendicular to the source dipole may be comparable in amplitude to those recorded by parallel dipole receivers dipole waveforms from an exaggerated keyhole shaped borehole are quite similar to those from an elliptical borehole of similar aspect ratio in a slow formation finally the behavior of early compressional headwaves arising from a nearby faster bed parallel to the borehole axis is qualitatively similar to that for a point source above a simple planar interface

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

finite element models of continental extension abstract numerical models of the initial deformation of extending continental lithosphere have been computed to investigate the control of pre existing thermal and mechanical heterogeneities on the style of deformation the finite element technique was used to calculate deformation with a viscoelastic plastic model for the lithosphere initial heterogeneities tested included a variety of thermal anomalies anomalous fracturing in the upper crust and anomalous crustal thickening deformation is strongly localized by thermal mechanical or compositional heterogeneities the style of initial deformation is controlled by the depth and width of the heterogeneities shallow weakening of the lithosphere causes broad uparching of the lithosphere in the early stages of deformation due to elastic bending stresses caused by non uniform extensional deformation deeper weakening of the lithosphere has the opposite effect the width of the zone of necking is closely controlled by the width of the initial heterogeneity with a minimum width controlled by the layer structure of the model the continuum mechanics formulation of the models prevent the reproduction of fault details in the models but general features of extensional style are indicated thermal heterogeneities are most effective in controlling deformational style broad zones of high strain are predicted by wide thermal anomalies penetrating up to the moho or shallower brittle low strain extension is predicted by models with deeper narrower thermal anomalies deep penetrating low angle faults are not required to reproduce the broad lithospheric deformational styles observed in lithospheric extension

tectonic overpressure in competent mafic layers and the development of isolated eclogites variation in the state of stress during heterogeneous deformation should be reflected in variation in the effective pressure of metamorphic reactions whether this is mean stress or the normal stress acting across the reacting interface the magnitude of this pressure variation will determine whether it is discernible in the preserved metamorphic mineral assemblages of heterogeneously deformed rocks the magnitude of the mean stress difference across a non slipping interface between two materials with viscosity ratio c 20 1 is effectively equal to the maximum shear stress for flow in the more viscous material progressive shortening of the interface results in a higher mean stress in the more competent material whereas extension results in a lower mean stress for high p low t eclogite facies conditions current experimental data indicate that clinopyroxene and garnet rich layers of eclogite should be very strong and that pressure differences of up to 800 mpa 8 kbar between competent layer and weaker matrix may be possible such high values can be obtained in widely separated competent layers for values of bulk stress in the overall multilayer that are much lower by a factor approaching the viscosity ratio extrusion of material between more rigid plates which has been proposed as a regional mechanism of lateral continental escape for both the alps and the himalayas should also be accompanied by a lateral gradient in effective pressure otherwise extrusion could not occur maximum mean stresses with magnitudes that are many times the maximum shear stress required for plastic flow should develop for deformation zones that are long relative to their width e g around 20 times for a width to thickness ratio of 10 tectonic overpressure in progressively shortened competent layers particularly in regions of extrusion between more rigid plates might help explain the occurrence of isolated layers and pods of low t eclogite c 550 c the dramatic weakening of clinopyroxene in the power law creep field precludes the development of significant overpressures in eclogite layers

tutorial interpretation of magnetotelluric measurements over an electrically dispersive one dimensional earth frequency dispersion of electromagnetic parameters of earth materials has been widely documented in recent years it is claimed that magnetotellurics mt may be significantly affected by dispersion the paper studies the mt plane wave interpretative problem for a one dimensional earth characterized by the presence of dispersive layers the theoretical properties of the mt field under the dispersion hypothesis andthe main features of the dispersion phenomenon are synthetically reviewed the examination of previously published mt curve responses over some models of dispersive earth section shows that ambiguity can arise when interpreting mt data with no other source of information thus it may be almost impossible to distinguish between the response of a dispersive section and an equally probable dispersion free section the dispersion magnetotelluric dmt method is proposed as a means to resolve the ambiguity the dmt method is based on the execution at the same site of an mt sounding and of an always dispersion free dc geoelectric deep sounding the latter technique can be used to compute a synthetic dispersion free mt wave impedance to be compared with the measured mt wave impedance the apparent dispersion function is introduced as a measure of divergence between the two wave impedances

geodetic strain of greece in the interval 1892 1992 a first order triangulation of greece was carried out in the 1890s reoccupation using global positioning system receivers of 46 of the 93 original markers yielded estimates of the deformation of the region over the intervening interval broad regions have similar geodetic strain over the 100 year time span strain north of the gulf of korinthos is predominantly north south extension though with a significant east west component the central peloponnisos is relatively stable whereas the gulfs of the southern peloponnisos are all characterized by uniaxial east west extension the seismic expression of strain for the entire region calculated from the seismic moment tensors of earthquakes of m s 5 8 during the past 100 years accounts for only 20 50 of the geodetically determined strain at a scale of 50 100 km the fraction of the strain that is expressed seismically varies much more than this range in particular whereas seismic strain in the eastern gulf of korinthos over the past 100 years is commensurate with the geodetic strain there is rapid extension across the western gulf of korinthos 0 3 strain yr 1 with negligible seismic strain for the 100 year period prior to 1992 the egion earthquake of june 1995 in the western gulf of korinthos released only a small proportion 20 of the elastic strain that had accumulated in that region the observed distribution of displacements can be explained by the relative rotation of two plates with a broad accommodation zone between them but it is equally consistent with the deformation that would be expected of a sheet of fluid moving toward a low pressure boundary at the hellenic trench a simple calculation implies that if the region does behave as a fluid then its effective viscosity is 10 22 10 23 pa s such viscosities are consistent with the deformation of a lithosphere obeying a rheological law similar to that obtained for olivine in the laboratory

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

tessera deformation and the contemporaneous thermal state of the plateau highlands venus abstract geodynamic modeling of tessera deformation at the ancient venusian plateau highlands implies lithospheric thermal gradients exceeding 17 k km 1 regularly spaced contractional ridges in the highlands manifest the unstable growth of perturbations at a dominant wavelength of 20 km controlled by a mechanical layer of relatively uniform thickness we model the deformation as the shortening of a plastic layer over a viscous substrate a layer thickness of less than 6 km is consistent with the observed ridge spacings assuming an anhydrous diabase crustal rheology and a minimum strain rate of 10 16 s 1 the geotherm was at least 17 k km 1 higher strain rates consequently higher thermal gradients are more probable we rule out the possibility that the plateau highlands were locally anomalously hot at the time ridges formed in the tessera that the highlands are dominantly composed of rock more felsic than diabase or that the crust was weak because it was hydrated therefore the average global lithospheric heat flux when the highlands tesserae were deforming was considerably higher than it has been since

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

lithospheric necking a dynamic model for rift morphology abstract rifting is examined in terms of the growth of a necking instability in a lithosphere consisting of a strong plastic or viscous surface layer of uniform strength overlying a weaker viscous substrate in which strength is either uniform or decreases exponentially with depth as the lithosphere extends deformation localizes about a small imposed initial perturbation in the strong layer thickness for a narrow perturbation the resulting surface topography consists of a central depression and uplifted flanks the layer thins beneath the central depression the width of the rift zone is related to the dominant wavelength of the necking instability which in turn is controlled by the layer thickness and the mechanical properties of the lithosphere for an initial thickness perturbation with a width less than the dominant wavelength deformation concentrates into a zone comparable to the dominant wavelength if the initial perturbation is wider than the dominant wavelength then the width of the zone of deformation is controlled by the width of the initial perturbation deformation concentrates in the region of enhanced thinning and develops periodically at the dominant wavelength a surface layer with limiting plastic stress exponent n behavior produces a rift like structure with a width typical of continental rifts for a strong layer thickness consistent with various estimates of the maximum depth of brittle deformation in the continental lithosphere the width of the rift is essentially independent of the layer substrate strength ratio for a power law viscous surface layer n 3 the dominant wavelength varies with layer substrate strength ratio to the one third power and is always larger than for a plastic surface layer of the same thickness the great widths of rift zones on venus may be explained by unstable extension of a strong viscous surface layer

extension of continental lithosphere a model for two scales of basin and range deformation we develop a model for deformation in an extending continental lithosphere that is stratified in density and strength assuming a rheology consistent with seismic focal depths and experimental flow laws the model demonstrates that necking instabilities at two wavelengths will arise due to the presence of a strong upper crust and upper mantle separated by a weak lower crust the magnitudes of the instabilities are directly related to strength contrasts within the lithosphere while the dominant wavelengths of necking are controlled mainly by the thicknesses of the strong layers the results are applied to the basin and range province of the western united states where two scales of deformation can be recognized one corresponding to the spacing of ranges and the other to the width of tilt domains a bouguer gravity anomaly and associated regional topography with a wavelength comparable to the width of tilt domains has also been recognized for plausible density and strength stratifications our results show that the horizontal scale of short wavelength necking is consistent with the spacing of individual basins and ranges while that of the longer wavelength necking is consistent with the width of tilt domains we thus suggest that basin and range deformation may be controlled by two scales of extensional instability extension in the weak lower crust in this model is laterally displaced from regions of upper crustal extension the resultant horizontal shearing in the lower crust may be a mechanism for the initiation of low angle extensional detachments