structural framework of the gulf of elat aqaba northern red sea the gulf of elat aqaba occupies the southern part of the dead sea rift the rift is considered to be a plate boundary of the transform type partially leaky which connects seafloor spreading in the red sea with the sagros taurus zone of continental collision the deep water in the gulf of elat up to 1850 m provides a rare opportunity to examine the process of continental rifting by marine geophysical techniques the bathymetry alone provides much information about fault patterns in this area the fragmentation of the once continuous arabian african platform is a complicated process it shapes the structure of the gulf which has developed through continuing tectonism primarily consisting of faulting recent geophysical and geological studies of the gulf of elat including bathymetry bottom photographs continuous seismic profiles seismic refraction gravity magnetics heat flow and coring provide new information about the shallow and deep crustal structure of this important segment of the world rift system the shallow structure of the gulf is dominated by three elongated en echelon basins which strike n20 25 e undulations in the floors of the basins produce several distinct deeps these basins are interpreted as pullaparts the new data from the gulf suggest that classical models for the formation of these structures should be modified only one of the longitudinal faults of each depression is a strike slip fault while the other is predominantly a normal fault the situation with the other two sides of the basin which are supposed to be composed of normal faults may also be more complex than previously thought at least in one case one such boundary is composed of a strike slip fault while the other has no significant fault crustal models of the gulf of elat based on gravity data indicate that the basins are rather shallow and do not extend into the lower crust the fill of the basins extends to about 5 km below the seafloor in the northern and southern basins and less in the central basin most significant magnetic anomalies in the gulf of elat extend from land into the sea none of them however extends from one coast to another across the gulf this supports the geological evidence for a shear between the arabia and sinai plates the magnetic field over the southern third of the gulf is rather smooth and markedly different from that of the other parts the nature of the magnetics the crustal structure of the western margin and the heat flow values indicate a thinner crust and different tectonic processes in the southern part of the gulf overall it seems that within the southern gulf of elat a transition occurs between crustal spreading that takes place in the red sea to a rifting without spreading that takes place along the dead sea transform spreading activity propagates from south to north the most active place is the central basin which is being propagated northward into the shallow northern basin

forces associated with mantle plumes this study addresses first order aspects of the physics of continental lithosphere extension above mantle plumes developing a testable model that predicts extension rates from flow parameters radial flow in the head of a plume causes a viscous force and uplift over this head causes a buoyancy force which both drive extension the viscous force is roughly proportional to the plume s volume flux and given my best estimates for flow parameters predominates for the most vigorous plumes extension is opposed by a viscous force beyond the plume head and its rate is thus affected by the dimensions of the plate on which this force acts extension rates are 2 mm yr 1 above yellowstone 1mm yr 1 above baikal and 0 7 mm yr 1 above the east african plume volume fluxes for all these plumes are 50 m3 s 1 the extension rates depend mainly on plate dimensions

normal faulting slip maxima and stress drop variability a geological perspective abstract we present an empirical estimate of maximum slip in continental normal faulting earthquakes and present evidence that stress drop in intraplate extensional environments is dependent on fault maturity a survey of reported slip in historical earthquakes globally and in latest quaternary paleoearthquakes in the western cordillera of the united states indicates maximum vertical displacements as large as 6 6 5 m a difference in the ratio of maximum to mean displacements between data sets of prehistoric and historical earthquakes together with constraints on bias in estimates of mean paleodisplacement suggest that applying a correction factor of 1 4 0 3 to the largest observed displacement along a paleorupture may provide a reasonable estimate of the maximum displacement adjusting the largest paleodisplacements in our regional data set 6 m by a factor of 1 4 yields a possible upper bound vertical displacement for the western cordillera of about 8 4 m although a smaller correction factor may be more appropriate for the longest ruptures because maximum slip is highly localized along strike if such large displacements occur they are extremely rare static stress drop in surface rupturing earthquakes in the western cordillera as represented by maximum reported displacement as a fraction of modeled rupture length appears to be larger on normal faults with low cumulative geologic displacement

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

new seismic data support cenozoic rifting in george vi sound antarctic peninsula summary new multifold seismic reflection and wide angle reflection refraction data across george vi sound antarctic peninsula show the presence of graben and horst structures indicating an extensional origin the data suggest that rocks of an accretionary complex and fore arc basin underlie the sound and are in faulted contact along its eastern boundary with volcanic and plutonic rocks of the associated mesozoic arc of western palmer land a cover of possible syn and postglacial cenozoic deposits drapes the structures the combination of new seismic synthetic aperture radar and previously acquired data suggests subduction related rifting in the sound was segmented with opening in the south predominately by normal extension whilst in the north dextral transtension predominated

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

asymmetry and basin migration in the dead sea rift the dead sea depression sensu stricto forms the deepest continental part of the dead sea rift a transfer which separates the levanthine and arabian plates it is occupied by three distinct sedimentary bodies deposited in basins whose depocenters are displaced northward with time they are the continental red beds of the hazeva formation miocene the bira lido gesher marls and the exceptionally thick rocksalt of the sedom formation pliocene early pleistocene and the successive amora lisan and dead sea evaporites and clastics early pleistocene recent lengthwise and crosswise asymmetries of these sedimentary basins and their respective depocenters are due to leftlateral shear combined with anticlockwise rotation of the arabian eastern plate steeper faulting of the crustal eastern margin than of the western sedimentary margin and modification of depositional pattern by twice filling up of basins by hazeva red beds during late miocene pause of shear and by sedom rocksalt during pliocene marine ingression

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

morphologic dating of fluvial terrace scarps and fault scarps near west yellowstone montana analysis of fault and fluvial terrace scarps in the west yellowstone basin south of hebgen lake montana indicates that during degradation the gradient of the scarp face and the curvatures of the basal concavity and crestal convexity of the scarp face decrease become more rounded with time the pattern of degradation is identical with that predicted by a simple analytical model for degradation by creep in which the change in elevation at a point on a hillslope is proportional to the curvature of the profile at that point the model therefore provides the basis for morphologic dating a technique for determining the age of a scarp by the extent to which its initial morphology has been degraded in order to date a scarp the coefficient of proportionality c must be known the technique is probably not suitable for use on hillslopes degraded by processes other than creep both the potential and the limitations of morphologic dating are demonstrated via measurements of a sequence of terrace scarps along the madison river the calculated relative ages are in agreement with the known relative ages despite the fact that the gradient of the youngest scarp is significantly less than that of the older scarps to calculate an absolute morphologic age c must be derived from analysis of a scarp of known age having the same aspect must be underlain by the same material and must be within the same climatic zone as the scarp to be dated in the west yellowstone basin a value of c 2 00 0 38 10 3 m 2 yr was calculated from a minimum date of 7100 50 yr b p determined by radiocarbon dating for one of the terrace scarps scarps formed by normal faulting in the west yellowstone basin are much more difficult to date than are terrace scarps because of the possible complexity of their initial morphology fault scarps formed during the 1959 yellowstone earthquake commonly consist of an assemblage of two or more smaller scarps or are superimposed on preexisting scarps formed by one or more previous faulting events by using a selective sampling procedure however one can screen out these fault scarps in this way it was possible to determine an age of 2 800 1 100 yr for a pre 1959 fault scarp in the west yellowstone basin because the fault and terrace scarps in the west yellowstone area do not vary significantly in height the morphologic dating technique proposed earlier by nash 1980a cannot be applied to them the substantially modified procedure developed for the present study does not require a wide range of heights for each scarp to be dated it can be performed using a relatively small number of scarp profiles if c is known it is easy to use and when used with considerable care it is reasonably accurate 36 59 in this study it is potentially applicable to scarps in other areas

the anatomy of a pull apart basin seismic reflection observations of the dead sea basin the dead sea basin dsb located along the african arabian plate boundary constitutes a good example of a pull apart basin because of its large dimensions its structural simplicity and its active subsidence a coherent three dimensional picture of the dsb has been constructed on the basis of analysis of seismic stratigraphy together with the interpretation of previously published geological and geophysical data despite the large known vertical offsets across the basin deformation takes place mainly along the transverse and longitudinal faults and the intervening sediments are relatively undeformed and are hardly tilted comparison between e w seismic lines indicate that the basin has widened by the collapse and tilting of arcuate blocks from the western margin but that its original shape is a full graben the southern and central parts of the basin are divided into equidistant segments 20 30 km long by transverse faults activity along these faults commenced only during the pleistocene long after the dead sea strike slip fault system was formed and migrated northward with time a likely scenario for the development of the dsb is one in which the basin grows northward with time by a simultaneous propagation of the southern strand of the dead sea fault and a retardation of the northern strand

internal structure of the dead sea leaky transform rift in relation to plate kinematics abstract the structures along the dead sea transform rift are related to the motions of the sinai and arabia plates which border it and to the irregularities of their boundaries the total slip was 105 km left lateral but the present structures were formed mainly during the last 40 km of slip which probably occurred in the plio pleistocene along the southern half of the transform the strike slip motion takes place on en echelon faults this produces rhomb shaped grabens or pull aparts which are sometimes composite and in which there is local crustal separation thus much of the transform is leaky these structures occur in a morpho tectonic rift valley delimited by normal faults which express a small component of transverse extension along a few segments the shape of the transform is such that lateral motion produces local transverse compression the geometric relations of the structures along the transform define an eulerian pole of relative plate motions at 32 8 n 22 6 e 0 5 the older motion was somewhat different and is described by a pole located about 5 west of the above then the component of transverse extension and crustal separation was much smaller than now while local transverse compression was more important the northern half of the dead sea transform has an irregular shape and the bordering plates did not remain rigid as lateral motion continued here transverse compression is often important

three dimensional modeling of pull apart basins implications for the tectonics of the dead sea basin we model the three dimensional 3 d crustal deformation in a deep pull apart basin as a result of relative plate motion along a transform system and compare the results to the tectonics of the dead sea basin the brittle upper crust is modeled by a boundary element technique as an elastic block broken by two en echelon semi infinite vertical faults the deformation is caused by a horizontal displacement that is imposed everywhere at the bottom of the block except in a stress free shear zone in the vicinity of the fault zone the bottom displacement represents the regional relative plate motion results show that the basin deformation depends critically on the width of the shear zone and on the amount of overlap between basin bounding faults as the width of the shear zone increases the depth of the basin decreases the rotation around a vertical axis near the fault tips decreases and the basin shape the distribution of subsidence normalized by the maximum subsidence becomes broader in contrast two dimensional plane stress modeling predicts a basin shape that is independent of the width of the shear zone our models also predict full graben profiles within the overlapped region between bounding faults and half graben shapes elsewhere increasing overlap also decreases uplift near the fault tips and rotation of blocks within the basin we suggest that the observed structure of the dead sea basin can be described by a 3 d model having a large overlap more than 30 km that probably increased as the basin evolved as a result of a stable shear motion that was distributed laterally over 20 to 40 km

deep drop down basin in the southern dead sea abstract the large dead sea graben is located within the dead sea fault a plate boundary of the transform type the graben is composed of two basins the northern one is occupied by a lake about 300 m deep and has a sedimentary fill of about 6 km the southern dead sea basin however is unusually deep with about 14 km of sedimentary fill the geometry of the southern dead sea basin is anomalous along the entire dead sea fault we suggest that the southern dead sea basin was formed during the first stage of the formation of the dead sea fault when the tips of propagating faults one from the collision front in the north and one from the red sea in the south met in this area the fault tips overlapped and curved towards each other isolating a block of crust and lithosphere that dropped into the mantle geophysical data indicate that the basin is probably bordered on all sides by vertical faults that cut deep into basement the deep part of the southern dead sea basin is not a pull apart but instead a drop down basin the salton trough in california is probably another example of such a basin

the origin of the bir zreir rhomb shaped graben eastern sinai the bir zreir graben a rhomb shaped depression located in eastern sinai is well exposed in three dimensions a nearly horizontal nonconformity between precambrian crystalline basement and overlying phanerozoic sediments contacts of basement plutons and a system of early miocene basaltic dikes permit very precise mapping of the local structures and the determination of vertical and horizontal displacements along faults applying the standard pull apart model to the bir zreir rhomb shaped graben predicts greater vertical separations 3 to 6 times larger than those observed an alternative model based on a rotational movement of the bir zreir block agrees more closely with field evidence it is concluded that the shape and size of the graben was determined by the original fault pattern and that its final shape was changed relatively little by later movements the rotation of the bir zreir block resulted from a combination of sinistral displacement along n s faults a change of their trend to nne and a regional e w extension the model proposed here does not require the presence of large deep gaps along the diagonal faults as in pull aparts

rates of late cenozoic tectonism in the vallecito fish creek basin western imperial valley california the kinetics of continental rifting are recorded in the late neogene sediments of the western imperial valley california the gulf of california opened in its present form about 4 m y ago capturing the colorado river some 5 km of deltaic and fluvial sediments accumulated in the vallecito fish creek area during the interval 4 3 0 9 m y ago initial sedimentation rates were 5 5 mm yr 1 which diminished in an exponential fashion to 0 5 mm yr 1 when isostatic adjustments are accounted for this implies tectonic subsidence rates of 1 5 mm yr 1 decreasing systematically to 0 1 mm yr 1 after 0 9 m y ago right lateral shearing along the elsinore fault zone cut across the basin causing it to be tilted and uplifted at a mean rate of 5 9 mm yr 1 the basin was concurrently rotated as a unit 35 clockwise the dissected pediments and badlands that now characterize the area were formed and eroded in the past 0 9 m y

hydrothermal and tectonic activity in northern yellowstone lake wyoming yellowstone national park is the site of one of the world9s largest calderas the abundance of geothermal and tectonic activity in and around the caldera including historic uplift and subsidence makes it necessary to understand active geologic processes and their associated hazards to that end we here use an extensive grid of high resolution seismic reflection profiles 450 km to document hydrothermal and tectonic features and deposits in northern yellowstone lake sublacustrine geothermal features in northern yellowstone lake include two of the largest known hydrothermal explosion craters mary bay and elliott9s mary bay explosion breccia is distributed uniformly around the crater whereas elliott9s crater breccia has an asymmetric distribution and forms a distinctive 2 km long hummocky lobe on the lake floor hydrothermal vents and low relief domes are abundant on the lake floor their greatest abundance is in and near explosion craters and along linear fissures domed areas on the lake floor that are relatively unbreached by vents are considered the most likely sites of future large hydrothermal explosions four submerged shoreline terraces along the margins of northern yellowstone lake add to the holocene record of postglacial lake level fluctuations attributed to heavy breathing of the yellowstone magma reservoir and associated geothermal system the lake hotel fault cuts through northwestern yellowstone lake and represents part of a 25 km long distributed extensional deformation zone three postglacial ruptures indicate a slip rate of 0 27 to 0 34 mm yr the largest 3 0 m slip and most recent event occurred in the past 2100 yr although high heat flow in the crust limits the rupture area of this fault zone future earthquakes of magnitude 5 3 to 6 5 are possible earthquakes and hydrothermal explosions have probably triggered landslides common features around the lake margins few high resolution seismic reflection surveys have been conducted in lakes in active volcanic areas our data reveal active geothermal features with unprecedented resolution and provide important analogues for recognition of comparable features and potential hazards in other subaqueous geothermal environments

a 12 000 year record of vertical deformation across the yellowstone caldera margin the shorelines of yellowstone lake the 600 ka yellowstone caldera exhibits several signs of unrest the most evident of which is historic ground deformation including both uplift and subsidence we document deformation in the area of the southeastern caldera across 12 000 years using the postglacial shoreline terraces of yellowstone lake raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces with an accuracy of 0 5 m of about 11 recognizable terraces the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns shoreline ages are based on minimum and maximum limiting radiocarbon and obsidian hydration dates each terrace is interpreted as representing an episode of uplift 1 kyr 1 of the caldera interior and subsequent subsidence with little net volume change this cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization and or episodic trapping and release of magmatic fluids evolved during cooling and crystallization in a self sealing reservoir as hypothesized by dzurisin et al 1990 early postglacial shoreline deformation reveals substantial intracaldera subsidence possibly reflecting cooling and loss of trapped hydrothermal fluids extension along north south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin including downwarping and local faulting of hot weak intracaldera crust net deformation over the past 3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim relative to the extracaldera region this uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence the cause of this trend is undetermined but is most likely related to the effects of regional extension and long term cooling within the yellowstone caldera

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

lithospheric strength variations as a control on new plate boundaries examples from the northern red sea region abstract the complex plate boundary between arabia and africa at the northern end of the red sea includes the gulf of suez rift and the gulf of aqaba dead sea transform geologic evidence indicates that during the earliest phase of rifting the red sea propagated nnw towards the mediterranean sea creating the gulf of suez subsequently the majority of the relative movement between the plates shifted eastward to the dead sea transform we propose that an increase in the strength of the lithosphere across the mediterranean continental margin acted as a barrier to the propagation of the rift a new plate boundary the dead sea transform formed along a zone of minimum strength we present an analysis of lithospheric strength variations across the mediterranean continental margin the main factors controlling these variations are the geotherm crustal thickness and composition and sediment thickness the analysis predicts a characteristic strength profile at continental margins which consists of a marked increase in strength seaward of the hinge zone and a strength minimum landward of the hinge zone this strength profile also favors the creation of thin continental slivers such as the levant west of the dead sea transform and the continental promontory containing socotra island at the mouth of the gulf of aden calculations of strength variations based on changes of crustal thickness geotherm and sediment thickness can be extended to other geologic settings as well they can explain the location of rerifting events at intracratonic basins of backarc basins and of major continental strike slip zones

tectonic implications of recent microearthquake data from israel and adjacent areas a recently installed seismograph network in israel enables for the first time a systematic examination of the microearthquake activity and its relation to the regional tectonics the observed microearthquake activity is widespread and not limited to the dead sea rift plate boundary in the rift itself the more seismically active parts are north of the sea of galilee and in the dead sea basin a possible seismic quiescence appears in the jordan valley to the north of the dead sea basin and may be associated with that segment of the plate boundary which broke recently 1927 thus indicating a low strain level inside israel the active nature of the carmel farah fault systems was determined its present level of seismicity appears to be comparable to that of the rift itself indicating the significance of this fault system in the active tectonics of the area the high microseismic activity on these faults and their location in densely populated areas mark them as possibly important sources of seismic hazards diffused seismicity is observed throughout much of israel although the nature of these earthquakes is not yet understood the small magnitude earthquakes near the rift are most probably part of a seismic halo around the plate boundary pronounced seismicity is also observed in the mediterranean sea off the coast of southern lebanon and northern israel this microearthquake activity decreases from north to south and to some extent from east to west whereas near the coast of northern israel and southern lebanon the microearthquake activity is relatively high in the south near the coast of the sinai peninsula it is very low this difference is attributed to a possible combination of a higher strain level in the african plate as it approaches the africa eurasia plate boundary and hinge faulting near the coast

stromatolites in caves of the dead sea fault escarpment implications to latest pleistocene lake levels and tectonic subsidence abstract a varied assemblage of algal stromatolites was encountered in caves along the northern section of the dead sea fault escarpment the caves are situated at the lower part of the escarpment at altitudes 310 to 188 m relative to mean sea level m s l i e ca 110 230 m above the present dead sea level the cave stromatolites are mainly composed of aragonite yielding u th ages of 75 17 ka the altitude mineralogy and ages as well as comparison with previously documented stromatolite outcrops in the area ascribe the cave stromatolites to the aragonite precipitating hypersaline lake lisan the late pleistocene predecessor of the dead sea the stromatolites are used as a lake level gauge based on the algae being reliant upon the light of the upper water layer preservation of the original structure and aragonite mineralogy of the stromatolites suggests a closed system regarding the radioactive elements enabling reliable u th dating a curve of lake lisan levels is constructed based on the stromatolite ages and cave elevations the following points are noted 1 lake levels of 247 m relative to m s l are recorded at 75 72 5 ka 2 relatively high lake levels above 220 m relative to m s l are achieved at 41 5 ka and are still recorded at 17 ka 3 the peak level is 188 m relative to m s l at 35 5 29 5 ka these results indicate lake stands up to 80 m higher than previously accepted for large parts of the lake lisan time span this difference is explained by tectonic subsidence of up to 2 2 m ka within the dead sea depression since the latest pleistocene this subsidence rate is in the same order of magnitude with previously calculated subsidence rates for the dead sea depression begin z b zilberman e 1997 main stages and rate of the relief development in israel geological survey of israel report jerusalem unlike previous lake lisan level estimations the new curve is measured at the relatively stable shoulders of the dead sea depression

oversized u shaped canyon development at the edge of rotating blocks due to wrench faulting on margins of the dead sea graben strike slip fault systems on the western margin of the dead sea active pull apart basin central part of the dead sea transform have been mapped in detail in the vicinity of the ya elim valley in the southeastern judean desert compressional tensional and strike slip shearing features are described and the geomorphology of the valley which changes over a 6 km long segment from a mature meandering valley to a deep and narrow canyon and then to an oversized very deep and very wide u shaped canyon is elucidated using profiles and superimposed cross sections a kinematic model for the formation of the ya elim valley is proposed reflecting dextral shearing on the margins of counterclockwise rotating blocks in the same area subrecent recent spreading of tensional fractures has occurred separating sections of blocks which protrude eastward into the deepest on land depression the large width of shear zones sub perpendicular to the transform may relate to repeated reversals in the sense of rotation of second order rigid block domains caused by rotations of neighbouring first order domains due to earthquake ruptures on individual parallel faults of the transform

dating quaternary fault scarps formed in alluvium using morphologic parameters abstract ages of fault scarps as well as those other types of transport limited slopes can be estimated by comparing their morphology with the morphology of scarps of known age age estimates are derived by fitting the scarp profiles to synthetic profiles generated using a diffusion equation or alternatively by classification using a linear discriminant function the usefulness of morphology derived age estimates depends on the relative importance of non age related morphologic variation data from more than 200 scarp profiles demonstrate that morphologic variation not related to scarp age can introduce significant uncertainties into morphology derived age estimates

seismic measurements in the southern dead sea abstract a seismic refraction profile and several seismic cdp reflection lines were recently occupied in the southwestern part of the dead sea the seismic data which are of good quality give a clear picture of the structure of the area the western flank of the rift comprises a series of step faults downthrown to the east with a total throw of some 7 km at which depth the cretaceous base of the post cretaceous fill is located on the east west lines the base of the fill dips to the east while on the north south lines this complex dips to the south with a change in direction of dip being evident in the southern portion of this profile the post cretaceous sediments reach a maximum thickness of 7 km but may be even thicker eastward near the main eastern rift fault these sediments are gently folded possibly due to differential compaction and are dislocated by small magnitude adjustment faults lateral transition from bedded layers of salt in the graben fill to a diapir is clearly seen

active faulting in the dead sea rift abstract manifestations of late quaternary and holocene faulting were studied in a 500 km long segment of the dead sea transform rift most prominent are left slip faults whose characteristic physiographic features are recognizable along most of the studied segment where these faults bend or are stepped to the left rhomb shaped grabens or pull aparts are produced forming depressions in the reverse situation compressional features such as pressure ridges domes and folds form positive topographic features such structures are combined on a variety of scales ranging from a few hundred meters long to tens of kilometers normal faults sub parallel to the left slip faults produce a trough like valley along much of the dead sea transform but are most prominent along the margins of the large rhomb grabens e g the dead sea trough they apparently record a small component of transverse extension generally their motion is slow young slip did not occur along some segments during the last few 10 4 y elsewhere throws of 10 20 m at least occurred in this period the dead sea transform is seismically active the instrumental and historic records indicate a seismic slip rate of 0 15 0 35 cm y during the last 1000 1500 y while estimates of the average pliocene pleistocene rate are 0 7 1 0 cm y either much creep takes place or the slip rate varies over periods of a few 10 3 y

statistical relations among earthquake magnitude surface rupture length and surface fault displacement abstract in order to refine correlations of surface wave magnitude fault rupture length at the ground surface and fault displacement at the surface by including the uncertainties in these variables the existing data were critically reviewed and a new data base was compiled earthquake magnitudes were redetermined as necessary to make them as consistent as possible with the gutenberg methods and results which make up much of the data base measurement errors were estimated for the three variables for 58 moderate to large shallow focus earthquakes regression analyses were then made utilizing the estimated measurement errors the regression analysis demonstrates that the relations among the variables magnitude length and displacement are stochastic in nature the stochastic variance introduced in part by incomplete surface expression of seismogenic faulting variation in shear modulus and regional factors dominates the estimated measurement errors thus it is appropriate to use ordinary least squares for the regression models rather than regression models based upon an underlying deterministic relation in which the variance results primarily from measurement errors significant differences exist in correlations of certain combinations of length displacement and magnitude when events are grouped by fault type or by region including attenuation regions delineated by evernden and others estimates of the magnitude and the standard deviation of the magnitude of a prehistoric or future earthquake associated with a fault can be made by correlating m s with the logarithms of rupture length fault displacement or the product of length and displacement fault rupture area could be reliably estimated for about 20 of the events in the data set regression of m s on rupture area did not result in a marked improvement over regressions that did not involve rupture area because no subduction zone earthquakes are included in this study the reported results do not apply to such zones

contemporary vertical surface displacements in yellowstone national park relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout yellowstone national park ynp by comparing geodetic leveling measurements made in 1923 second order 1936 third order 1941 third order 1955 third order 1960 first order and 1975 1977 first order the 1923 1975 data indicate uplift greater than 400 mm 7 7 mm yr of an area approximately 15 20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600 000 year old yellowstone caldera the maximum 1923 1975 vertical displacement in this area is 726 mm 14 0 mm yr the existence of a well defined and untilted yellowstone lake terrace implies that the 1923 1975 average vertical velocities cannot have been maintained for more than a few hundred years the 1955 1977 data from central ynp indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm yr a similar bias is present in the 1923 1975 data but it is less pronounced the 1923 1960 and 1960 1975 data from northwestern ynp and a 200 mm depression in the 1923 1975 data near norris junction may be interpreted in terms of deformation associated with the 1959 hebgen lake earthquake and the 1975 yellowstone park earthquake a composite profile of the 1936 1977 and 1941 1977 average vertical velocity data which skirts the edge of the yellowstone caldera in northeastern ynp is most easily explained as the result of random leveling error and or bench mark instabilities the apparent youth of the 1923 1975 yellowstone uplift and its close association with the yellowstone caldera suggest that the most likely cause of the uplift is a recent not more than 500 years ago influx of molten material into the upper crust beneath the caldera

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

magnetic field over the dead sea and vicinity abstract a detailed magnetic survey was conducted in the dead sea a large sedimentary basin within the dead sea rift the magnetic anomaly map shows two distinct provinces in the north the anomaly field trends north south and decreases towards the east with a gradient of 10 to 20 nt km in the south the field trends east west the magnetic field is generally smooth and most of the magnetic anomalies have long wavelengths and low amplitudes however a few short wavelength anomalies exist some of which have high amplitudes they may indicate the presence of small basaltic bodies buried at shallow depths a comparison between the magnetic anomaly map of the dead sea and aeromagnetic maps over the land areas to the west and east show that the magnetic anomalies extend uninterrupted from the land area to the west into the basin this configuration suggests that faulting on the western margin of the basin is basically normal in the east the magnetic contours are discontinous across the eastern margin of the basin which suggests that the eastern fault is a strike slip fault a very active fault of unknown nature borders the basin in the south this is associated with magnetic anomalies seismic activity and possibly hydrothermal activity there is no indication of faulting on the northern margin of the lake

effect of height and orientation microclimate on geomorphic degradation rates and processes late glacial terrace scarps in central idaho terrace scarps can serve as a nearly ideal natural laboratory for the study of the evolution of slopes this paper examines the effects of scarp size height and orientation microclimate by keeping constant variables such as age lithology and regional climate if a scarp degrades as a closed system and downslope movement is directly proportional to surface gradient the evolution of the scarp is modeled by the diffusion equation for a group of scarps of same age and known starting angle the diffusion equation model predicts the relation between maximum scarp angle and scarp height h late pleistocene terrace scarps now as steep as 33 25 as well as measured angles of repose for sand and gravel require a starting angle as steep as 33 5 for latest pleistocene idaho and utah scarps as h increases is gentler more degraded than modeled by the diffusion equation with a constant rate coefficient the degradation rate coefficient c increases tenfold with scarp height it should not change with scarp height if downslope movement is solely determined by surface gradient to the first power soil wash appears to be responsible for this departure from the diffusion equation model for transport rate by soil wash is a function of scarp size height south facing scarps are less vegetated and more degraded than north facing scarps for scarps 2 m high the degradation rate c on s facing scarps is 2 times that on n facing scarps for 10 m scarps it is 5 times the observed dependence of the rate coefficient c on scarp height can be removed by normalizing c to values for west facing scarps of the same height the residual c values calculated by this method correlate well with differences in incident solar radiation resulting from the different scarp orientations and maximum gradients this correlation demonstrates the importance of orientation on slope processes and their rates through the differences in freeze thaw cycles soil moisture and vegetative cover scarp morphology may be used to estimate age if one accounts for the effects of climate and for scarp height orientation and lithology for example using the dated bonneville shoreline scarps for calibration and comparing only scarps of equal height we estimate the drum mountains fault scarps to be 9 000 yr old this age is about twice that produced by previous diffusion equation calculations that have not accounted for the height as we have here but it is the same as independent geologic estimates of their age

mechanical aspects of pull apart basins and push up swells with applications to the dead sea transform abstract pull apart basins and push up swells are common features at steps along strike slip faults the structure of these features is examined here for two types of rheology perfect brittle and perfect ductile the stress fields derived for the step area predict that fault patterns mode of deformation and vertical displacement are different for the two rheologies these predictions are illustrated for basins and swells developed in laboratory experiments with modelling materials the structure of basins and swells along the dead sea transform is discussed here the pull apart basins display an intensively faulted southern end and a gently dipping flexure at the northern end a consistent dissimilarity appears between the structures of the eastern and western boundary faults of the basins according to the present interpretation these dissimilarities reflect a relatively ductile crust in arabia and a relatively brittle crust in sinai israel

neotectonics of the owen fracture zone nw indian ocean structural evolution of an oceanic strike slip plate boundary the owen fracture zone is a 800 km long fault system that accommodates the dextral strike slip motion between india and arabia plates because of slow pelagic sedimentation rates that preserve the seafloor expression of the fault since the early pliocene the fault is clearly observed on bathymetric data it is made up of a series of fault segments separated by releasing and restraining bends including a major pull apart basin at latitude 20 n some distal turbiditic channels from the indus deep sea fan overlap the fault system and are disturbed by its activity thus providing landmarks to date successive stages of fault activity and structural evolution of the owen fracture zone from pliocene to present we determine the durability of relay structures and the timing of their evolution along the principal displacement zone from their inception to their extinction we observe subsidence migration in the 20 n basin and alternate activation of fault splays in the vicinity of the qalhat seamount the present day owen fracture zone is the latest stage of structural evolution of the 20 myr old strike slip fault system buried under indus turbiditic deposits whose activity started at the eastern foot of the owen ridge when the gulf of aden opened the evolution of the owen fracture zone since 3 6 myr reflects a steady state plate motion between arabia and india such as inferred by kinematics for the last 20 myr period the structural evolution of the owen fracture zone since 20 myr including fault segments propagation and migration pull apart basin opening and extinction seems to be characterized by a progressive reorganisation of the fault system and does not require any major kinematics change

late quaternary lacustrine deposits of the dead sea basin high resolution sequence stratigraphy from downhole logging data abstract sequence architecture and depositional sequences of the quaternary lacustrine succession deposited in the northern dead sea sub basin were examined using logging data collected during the 2010 2011 icdp campaign methods borrowed from sequence stratigraphy techniques were used to investigate the characteristics of sediments deposited in the central part of the northern lake high resolution wire logging data combined with a detailed lithological description of the icdp 5017 1 a deep borehole were used to examine depositional systems and related processes controlling their formation analysis of sedimentary stacking patterns and stratal surfaces within the late pleistocene holocene lacustrine succession revealed 10 depositional sequences it was possible to identify key stratal boundaries and to discern between three sedimentary stacking patterns interpreted here as representing lowstand systems tracts lst transgressive systems tracts tst and highstand systems tracts hst examined together they may be interpreted in terms of relative lake level changes on the basis of the stratigraphic analysis complemented with new age dating this article presents a record of the sediment accumulation pattern and a relative lake level curve reconstructed for the last ca 225 ka results show that stratigraphic units and depositional and erosional surfaces examined in the deep 5017 1 a borehole can be correlated to the proximal area of the basin this means that changes in relative lake levels were generally synchronous and uniform across the dead sea basin the creation of accommodation space in the northern dead sea was found to generally be in phase with paleoclimatic modulating lake levels and not due to tectonics

origin and deformation of holocene shoreline terraces yellowstone lake wyoming geodetic surveys within the yellowstone caldera have documented active uplift that is most likely caused by magmatic processes in the upper crust along the northeast shore of yellowstone lake maximum relative uplift rates are 10 mm yr for the period 1923 1975 however information on deformation prior to historic instrumental records has been lacking in this study closely spaced data on elevations of postglacial shoreline terraces around the north end of yellowstone lake reveal complex tilting though most holocene deformation is probably magma related the pattern of shoreline tilting deviates significantly from the historic pattern of roughly symmetric inflation of the caldera along the northeast shore where tilt directions of historic and shoreline deformation are similar differential uplift of a 2500 yr old terrace is roughly 10 m this gives a maximum uplift rate of 4 mm yr these unique holocene terraces may exist due to episodic deformation because vertical movements affecting the lake outlet directly control lake level

scarp degraded by linear diffusion inverse solution for age under the assumption that landforms unaffected by drainage channels are degraded according to the linear diffusion equation a procedure is developed to invert a scarp profile to find its diffusion age diffusion age having dimension length 2 is the product of diffusivity times chronological age the second moment of scarp slope grows linearly with age this fact together with an assumption about initial scarp shape allows the inverse determination of diffusion age age found assuming a vertical initial scarp is termed apparent age any nonvertical initial scarp profile has a nonzero initial apparent age true diffusion age differs from apparent age by a fraction of scarp offset squared the inverse procedure applied to synthetic data yields the following rules of thumb evidence of initial scarp shape has been lost when apparent age reaches twice its initial value if a scarp is formed by two events the inversion gives their offset weighted mean age with an error that is a fraction of offset squared a scarp that appears to have been formed by one event may have been formed by two with an interval between them as large as apparent age after scarps of two fault traces have diffused to appear as one the error in inferred age may be as large as half the apparent age variation of apparent age along strike would indicate multiple fault traces the simplicity of scarp profile measurement and this inversion makes profile analysis attractive if linearity of the flow law time for a free face to be reduced to the angle of repose and variation of diffusivity with climate and material could be established profile analysis would become a reliable dating technique

effect of far field slope on morphologic dating of scarplike landforms the principal finding of this paper is that the far field slope has a first order effect on model age determinations of scarplike landforms in weakly consolidated terrains observationally this can be demonstrated in two ways using the lake bonneville and lahontan shoreline scarps as separate and combined data sets use of the reduced scarp slope tan theta sub s minus b where theta sub s is the maximum scarp angle and b is the far field or fan slope instead of tan theta sub s alone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope offset plot for both the separate and combined data sets theoretically the argument can be put even more strongly at least within the range of linear and nonlinear diffusion models that the authors consider here together with a mathematical transformation of the empirical approach of r c bucknam and r e anderson when one correctly takes into account the far field slope one will basically get the same age determination no matter which of these models one uses conversely without accounting properly for the effect of far field slope one is virtuallymore guaranteed to get an erroneous age determination no matter which model is used less

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

a mid crustal contractional stepover zone in a major strike slip system north cascades washington abstract the segmentation of strike slip faults at the earth s surface is well known but the geometry of these faults in the mid crust is poorly understood as are the intervening stepover zones a mid crustal example of a segmented strike slip fault is present within the 500 km long ross lake fault system where the nw striking twisp river fault zone and gabriel peak tectonic belt occur as overlapping left stepping dextral strike slip shear zones the contractional stepover between these structures is occupied by the black peak batholith which is riddled by ductile shear zones that are typically 0 5 1 5 m thick and are restricted to the stepover gently to moderately nw dipping reverse slip ductile shear zones in the center of the stepover strike at a high angle to the twisp river fault zone and gabriel peak tectonic belt and lie in an appropriate position to transfer most of the displacement between the strike slip segments the orientation of the reverse slip ductile shear zones relative to the strike slip segments is broadly similar to that of folds and thrusts in upper crustal stepovers the stepover width is relatively large however in comparison to many active stepovers emplacement of the black peak batholith across an originally narrower stepover or bend in the fault system may have created a rigid buttress and subsequent deformation jumped to the mechanically weak southwest margin of the body because plutons are commonly intruded into strike slip zones such partitioning of later displacements may be widespread

structural geometry and evolution of the dead sea jordan rift system as deduced from new subsurface data abstract analysis of deep drilling data and seismic reflection surveys obtained in recent years permit establishment of a tectonic model of the dead sea jordan rift system that is in part conformable with some of the many often controversial theories on this subject but which also introduces new information negating some concepts that are widely accepted in the literature the dead sea jordan rift system is not a tensional graben between two parallel sets of faults but instead can be best defined as a sinistral transform connecting an incipient oceanic ridge the red sea with an upthrusted collision zone the taurus range the shear is caused by the opening of the red sea and the motion of the arabian sub plate away from the african plate the shearing occurs along a largely south north trending slightly arcuate une consisting of a series of en echelon left stepping left lateral strike slip master faults of varying lengths these faults characteristically die out to the north by bending outward northeast and the movement is taken up by the strike slip faults to the left i e to the west where overlap of the two faults occurs very narrow and very deep grabens have evolved in places the width being equal to the depth the narrowness of these grabens is dictated by the closeness of the overlapping segments of the successive strike slip faults this feature points to the probability that the en echelon strike slip faults have a common root zone at great depth these grabens are not typical leaky pull aparts in that they have no gravimetric magnetic or heat flow anomalies nor are they closed on all four sides by large faults their northern ends are characterized by gradually rising graben floors without major diagonal cross faults the dead sea jordan rift system sensu stricto extends over a distance of 420 km from the northern shore of the gulf of elat to the northern margin of the hula valley southward it is connected with the red sea along the gulf of elat transition zone which has elements of strike slip as well as tensional opening a northern transition zone exists between the hula valley and the taurus arc where diminishing strike slip movement is compensated by several underthrust belts the time of the beginning of the movement along the rift can be established as pre middle miocene because mid miocene sediments overlie an upper cretaceous graben floor rifting has continued intermittently to the present volcanism in the northern fifth of the rift is genetically associated with the seismically active tensional golan hauran jebel drouz wadi sirhan tectonic trend which has a north west southeast orientation and not with the north south trending dead sea jordan rift

seismicity and late quaternary faulting of the northern basin and range province montana and idaho abstract examination of seismicity and late quaternary faults in montana and idaho north of the snake river plain shows a geographic correspondence between high seismicity and 24 faults that have experienced surface rupture during the late quaternary the lewis and clark zone delineates the northern boundary of this tectonically active extensional region earthquakes greater than magnitude 5 5 and all identified late quaternary faults are confined to the montana idaho portion of the basin and range province south of the lewis and clark zone furthermore all 12 holocene faults are confined to a seismologic zone centennial tectonic belt parallel to the northern flank of the snake river plain in extreme southwestern montana and adjacent idaho fault trends strain data and focal mechanisms suggest that both late quaternary faulting and seismicity in the region are primarily products of two distinct stress regimes 1 overall basin and range extension along a s45 w direction relative to the mid continent and 2 localized effects of the yellowstone hot spot which appears to have the least principle stress axis oriented horizontally at about s4 w the three largest historic earthquakes north of the snake river plain are indicative of three different stress provinces the 1983 borah peak earthquake occurred in the montana idaho basin and range the 1959 hebgen lake earthquake is associated with the yellowstone hot spot area and the 1925 clarkston valley event appears to be associated with the stress field of the united states mid continent

high resolution aeromagnetic mapping of volcanic terrain yellowstone national park high resolution aeromagnetic data acquired over yellowstone national park ynp show contrasting patterns reflecting differences in rock composition types and degree of alteration and crustal structures that mirror the variable geology of the yellowstone plateau the older eocene absaroka volcanic supergroup a series of mostly altered andesitic volcanic and volcaniclastic rocks partially exposed in mountains on the eastern margin of ynp produces high amplitude positive magnetic anomalies strongly contrasting with the less magnetic younger latest cenozoic yellowstone plateau group primarily a series of fresh and variably altered rhyolitic rocks covering most of ynp the yellowstone caldera is the centerpiece of the yellowstone plateau part of its boundary can be identified on the aeromagnetic map as a series of discontinuous negative magnetic anomalies that reflect faults or zones along which extensive hydrothermal alteration is localized the large volume rhyolitic ignimbrite deposits of the 0 63 ma lava creek tuff and the 2 1 ma huckleberry ridge tuff which are prominent lithologies peripheral to the yellowstone caldera produce insignificant magnetic signatures a zone of moderate amplitude positive anomalies coincides with the mapped extent of several post caldera rhyolitic lavas linear magnetic anomalies reflect the rectilinear fault systems characteristic of resurgent domes in the center of the caldera peripheral to the caldera the high resolution aeromagnetic map clearly delineates flow unit boundaries of pre and post caldera basalt flows which occur stratigraphically below the post caldera rhyolitic lavas and are not exposed extensively at the surface all of the hot spring and geyser basins such as norris upper and lower geyser basins west thumb and gibbon are associated with negative magnetic anomalies reflecting hydrothermal alteration that has destroyed the magnetic susceptibility of minerals in the volcanic rocks within yellowstone lake which is mostly within the yellowstone caldera aeromagnetic lows also are associated with known hydrothermal activity in the lake many of the magnetic lows extend beyond the areas of alteration and hot springs suggesting a more extensive currently active or fossil hydrothermal system than is currently mapped steep magnetic gradients suggesting faults or fractures bound the magnetic lows this implies that fractures localize the hot springs magnetic gradient trends reflect the mapped basin and range structural trends of north and northwest as well as northeasterly trends that parallel the regional trend of the snake river plain and the track of the yellowstone hot spot which follow the precambrian structural grain these trends are found both at small scales such as in hydrothermal basins and at more regional fault scales which suggests that the regional stress field and reactivated

constraints on the thermal history of the dead sea graben as revealed by coal ranks in deep boreholes abstract coal ranks have been studied by vitrinite reflectance measurements in three boreholes zuk tamrur 1 and zohar 8 on the judea desert plateau adjacent to the graben margin and amiaz 1 on a down faulted block at the southwest dead sea graben in the plateau boreholes time temperature index tti calculations reveal that the coalification profile probably evolved under a late cretaceous thermal event with a thermal gradient of 35 38 c km followed by gradual decay to the present level c 20 c km by miocene time in amiaz 1 the highest coal rank encountered 0 5 ro is marginal for independent tti calculations however some constraints on thermal history can be inferred stratigraphic and structural relationships between the 0 5 ro isoreflectance in amiaz 1 and the plateau boreholes reveals that most of the coalification measured in amiaz 1 pre dates the formation of the graben and that an additional 3 4 km of miocene holocene graben fill barely affected the coalification cessation of coalification despite increasing burial indicates that the post miocene thermal gradient in the amiaz block could not have exceeded 20 23 c km this finding is consistent with present day heat flow and other geophysical information tectonic models for the evolution of the dead sea graben requiring a high thermal regime are inconsistent with the presented data this raises questions as to the mechanism involved in the formation of rhomb shaped grabens in general on the one hand and for the hypothesis of the leaky nature of the dead sea transform north of the gulf of elat on the other