some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on venus models of coupled crustal deformation and mantle convection on venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine a previous model which assumed a relatively weak basalt flow law more appropriate for hydrated crust is compared to a similar model that assumes a strong basalt flow law more appropriate for dry crust a principal difference is the timescale for significant crustal thickening of the order of 108 years for the weak basalt model and 109 years for the strong basalt model these results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near surface weakness of the type associated with plate margins on earth the lithospheric overturn time which sets the timescale required for convective thickening of crust scales dominantiy with the rayleigh number defined by the average viscosity of the lithosphere theoretical scalings as well as numerical models suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103 such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present day earth the large values inferred for the thickness of the mechanical lithosphere on venus if correct would thus tend to favor timescales of the order of 109 years if one assumes relatively low convective mantle heat loss over an equivalent time this in turn would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows if instead one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust

sequence of tectonic deformation in the history of venus evidence from global stratigraphic relationships analysis of local and regional stratigraphic relationships has permitted the assessment of the nature of tectonic structures and their distribution throughout the observed history of venus spanning the past several hundreds of millions of years we find that shortening characteristic of intensely deformed tessera terrain gave way to widespread distributed fracturing and extension within the tessera and early post tessera volcanic plains this phase was followed by distributed deformation of the widespread younger volcanic plains involving compression to form broad ridge belts and closely following and sometimes simultaneous extension to form fracture belts emplacement of the most areally extensive regional volcanic plains exposed today was followed by widely distributed compression forming wrinkle ridges on the plains surfaces focused extensional deformation localized linear rift systems dominated the latest stages these major temporal trends appear well established from a stratigraphic point of view and provide guidelines and constraints on models for the geologic history of venus

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

evidence of a strong paleoearthquake in 9 1 ka cal bp interpreted from mass transport deposits western quebec northeastern ontario canada this paper reviews and expands upon stratigraphic and chronologic evidence supporting the interpretation of a strong early postglacial paleoearthquake in western quebec northeastern ontario canada event horizon mapping at dasserat dufresnoy and duparquet lakes quebec coring at chassignolle and malartic lakes quebec analysis of varve exposures near both the twin falls and frederick house dams ontario and review of published and unpublished logs of varve exposures reveal that mass transport deposits formed in or are inferred to have formed in varve year vyr 1483 relative to the timiskaming varve series and about 9 1 ka cal bp are present at numerous locations within the study area the spatial extent of the vyr 1483 mass transport deposit signature covers at least 14 600 km2 and is best explained by shaking from a paleoearthquake the magnitude of the event is estimated to be a minimum of m 7 3 which is indicative of a strong paleoearthquake the vyr 1483 paleoearthquake is one of 11 events of m 5 occurring within a 425 varve year period between 9 4 and 8 95 ka cal bp and is likely part of a period of elevated early postglacial seismicity associated with rapid crustal unloading the absence of an equivalent signature within the younger portion of the glaciolacustrine or lacustrine deposits in local lakes implies that this paleoearthquake is the strongest to occur within the study area since about 9 1 ka cal bp the highly precise correlation of widely spaced mass transport and disturbed deposits provided by varves is essential to the paleoearthquake interpretation

evidence from seismites for glacio isostatically induced crustal faulting in front of an advancing land ice mass rugen island sw baltic sea abstract it is hitherto commonly thought that earthquakes triggered by geologically rapid changes in the pressure caused by the fluctuating extent of thick land ice masses occur exclusively during ice retreat our study of deformed sediment layers within an undeformed glaciolacustrine and glaciofluvial succession exposed in a coastal cliff on rugen island south western baltic sea challenges this widespread idea based on their structural and deformational features the layers under investigation are interpreted as seismites which formed as a result of seismically induced liquefaction the stratigraphic context aided by optically stimulated luminescence osl dating indicate that the documented seismites must have formed shortly before the front of the pleistocene scandinavian ice sheet reached the study area during the last glacial maximum thus during a stage of ice advance this implies that the flexural isostatic response of the earth s crust as a consequence of the ice load during ice advance was accompanied by earthquakes probably due to local re activation of pre existing faults the crustal response to an increasing ice load was consequently less gradual than previous studies about glaciation related tectonics suggest

structural evolution of western fortuna tessera venus western fortuna tessera ishtar terra records polyphase deformation structures include ribbons forming long narrow steep sided troughs that reflect surface layer extension n trending folds related to maxwell montes deformation short wide nw trending graben that reflect late local extension and within the southwestern map area gentle e trending warps which postdate ribbon structures flood lava flows postdate deformation ribbon structures change orientation radially from nne to wnw these observations caution against the correlation of fortuna tessera with tessera elsewhere on venus and against the proposal that tessera forms a global stratigraphic layer

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

evidence of late glacial paleoseismicity from submarine landslide deposits within lac dasserat northwestern quebec canada abstract an integrated seismo and chronostratigraphic investigation at lac dasserat northwestern quebec identified 74 separate failures within eight event horizons horizons e and b and h and g have strong or moderately strong multi landslide signatures respectively composed of 11 23 failures while horizons f d c and a have minor landslide signatures consisting of a single or pair of deposit s cores collected at six sites recovered glacial lake ojibway varve deposits that are interbedded with the event horizons the correlation of the varves to the regional timiskaming varve series allowed varve ages or ranges of varve ages to be determined for the event horizons horizons h g e and b are interpreted to be evidence of paleoearthquakes with differing levels of interpretative confidence based on the relative strength of the multi landslide signatures the correlation to other disturbed deposits of similar age in the region and the lack or possibility of alternative aseismic mechanisms the four interpreted paleoearthquakes occurred between 9770 200 and 8470 200 cal yr bp when glacial lake ojibway was impounded behind the laurentide ice sheet during deglaciation they probably represent an elevated period of seismicity at deglaciation that was driven by crustal unloading

active faults and paleoseismicity in fennoscandia especially sweden primary structures and secondary effects abstract fennoscandia today a region of low to moderately low seismicity was at the time of deglaciation with rates of uplift on the order of tens of centimetres per year a region of very high seismicity and active tectonics this is evident both from primary fault structures and from secondary sedimentary and hard rock effects in the region around the epicentral areas the map of active faults in fennoscandia includes numerous structures previously not recognised despite this the recording of active faults and paleoseismic events is still in its initial phase much more data will surely accumulate in the near future

grabens basin tectonics and the maximum total expansion of the moon the presence of grabens and wrinkle ridges indicates that stresses sufficient to deform or fault crustal rocks have existed for at least part of lunar history these structures have been explained as due to global expansion and contraction basin subsidence or both even if the extension implied by all lunar grabens were due to global expansion the total radius increase permissible after terminal bombardment would be much less than that predicted by the most conservative models of lunar thermal evolution the obvious areal association of most grabens and wrinkle ridges with impact basins suggests that a genetic relationship exists also but it is necessary to develop either kinematic or dynamic models to determine if basin subsidence alone can account for these structures kinematic models are the most useful because fewer assumptions regarding material properties are needed than for dynamic models for both types the grabens and wrinkle ridges provide the data that must be predicted by the models considerations of geometry kinematics and mechanics suggest that the simple grabens on the moon are reliable indicators of crustal extension two end member kinematic models of postimpact basin modification are derived one assuming that subsidence is accomplished entirely by an increase in radius of curvature of the basin floor the other assuming that subsidence is accomplished without any change in radius of curvature of the basin floor any combination of the two also would be possible both models predict enough extension to account completely for basin related grabens and thus they indicate that virtually all of the extension implied by lunar grabens can be provided by modest amounts of basin subsidence in addition both models also predict total radial and hoop shortening of the basin floor of about the correct magnitude to explain wrinkle ridges if these features are structures resulting from compression consequently almost all extension and shortening of the lunar crust implied by grabens and wrinkle ridges can be explained by basin subsidence alone significant global expansion after 3 9 b y is unlikely and significant global contraction is unnecessary

major earthquake at the pleistocene holocene transition in lake vattern southern sweden lake vattern sweden is within a graben that formed through rifting along the boundary between two precambrian terrains geophysical mapping and geological coring show that substantial tectonic movements along the lake vattern graben occurred at the very onset of the holocene this is evident from deformation structures in the soft sediment accumulated on the lake floor our interpretation of these structures suggests as much as 13 m of vertical tectonic displacements along sections of a 80 km long fault system if these large displacements are from one tectonic event lake vattern must have had an earthquake with seismic moment magnitudes to 7 5 in addition our geophysical mapping shows large landslides along sections of the steep lake shores pollen analysis of sediment infillings of some of the most prominent sediment deformation structures places this major seismic event at the younger dryas preboreal transition ca 11 5 ka we suggest that this event is mainly related to the rapid release of ice sheet load following the deglaciation this paleoseismic event in lake vattern ranks among the larger known intraplate tectonic events in scandinavia and attests to the significance of glacio isostatic unloading

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

hot spot evolution and the global tectonics of venus the global tectonics of venus may be dominated by plumes rising from the mantle and impinging on the lithosphere giving rise to hot spots global sea floor spreading does not take place but direct convective coupling of mantle flow fields to the lithosphere leads to regional scale deformation and may allow lithospheric transport on a limited scale a hot spot evolutionary sequence comprises i a broad domal uplift resulting from a rising mantle plume ii massive partial melting in the plume head and generation of a thickened crust or crustal plateau iii collapse of dynamic topography and iv creep spreading of the crustal plateau crust on venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea floor spreading

genesis of a giant gravity induced depression gravifossum in the enkoping esker s sweden abstract a structure in the enkoping esker in southern sweden appears to result from a series of deformations that followed essentially loading because of an unstable density gradient diamicton on top of water saturated sands the more or less symmetrical structure which is in fact a structure within a graben structure is some 12 m wide at its top 2 5 m wide at its visible base and some 13 m high it shows a variety of both synthetic and antithetic faults in addition numerous folds are present so that a complex structure results both the structural characteristics and the shape resemble those of the much smaller gravifossums that have been described from subrecent lagoonal silty deposits in the netherlands it is the first time that a gravifossum has been described from glacigenic deposits and consequently also the first time that such a structure has been found in esker deposits

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

in absurdum long term predictions and nuclear waste handling abstract careful use of scientific knowledge allows predictions into the future when the time units for our predictions increases to hundreds of thousands of years we have extended our abilities in absurdum this is true for long term safety statements regarding the handling of our high level nuclear waste the fennoscandian shield has been claimed to offer exceptionally stable bedrock conditions over immense time periods we only need to go back to the last deglacial phase some 10 000 years ago to have a totally different situation from that of today at that time sweden was characterized by exceptionally high seismic activity both in amplitude and frequency these conditions like a number of related phenomena will be repeated at future ice ages in such an environment there can be no safe repository in the bedrock in the absence of true long term safety we can only recommend the utilization of the dry rock deposit drd method where the waste is stored in the bedrock under dry conditions under constant control and monitoring accessible for maintenance and possible future methods of rendering the waste harmless and even removal

paleoseismology the application of multiple parameters in four case studies in sweden abstract many different primary and secondary effects of an earthquake can be used in order to reconstruct and determine a paleoseismic event in this case we advocate the application of multiple criteria four case studies in sweden are examined two late glacial and two late holocene events special accounts are given of bedrock fracturing away from the primary fault liquefaction with multiple phases and structureless sand layers tsunami distribution of seabed turbidites dating by varves methane venting tectonics and magnetic grain rotation the multitude of observational data related to all four cases are combined into simple tables where the environmental and spatial effects can be explicitly assessed in terms of documentation intensity and magnitude

normal faulting in the upper continental crust observations from regions of active extension abstract observations of present day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins the most obvious manifestation of active extension is normal faulting in earthquakes earthquake foci are mostly confined to the upper seismogenic continental crust whose thickness imposes a scale on the observed deformation large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30 60 dips significantly gentler than 20 have not been observed in fault plane solutions of large earthquakes large seismographic normal faults are approximately planar in cross section and cut through the base of the upper seismogenic layer rotating about a horizontal axis as they move a reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound the domino model such faults are rarely continuous for more than 15 20 km but commonly change strike or step in an en echelon fashion this segmentation may occur on a scale controlled by the thickness of the seismogenic layer and is an important influence on sedimentation and drainage these observations are common to large seismogenic normal faults from a variety of tectonic settings and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension which can vary widely in nature and magnitude small earthquakes which move faults that are small compared with the seismogenic layer thickness show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults in some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology faults in the sedimentary cover may be strongly curved in cross section requiring their hangingwalls to deform internally estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation estimates made in this way are often very non unique and prone to large errors we postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis

paleoearthquake deformations recorded by magnetic variables abstract faults and related deformational structures give evidence of earthquakes in the past the ground shaking at high magnitude seismic events usually generates liquefaction the higher seismic magnitude the larger area of liquefaction we can now demonstrate that it is possible to discriminate between plastic and liquefied deformations with magnetic methods besides it is possible to record internal crypto deformations not visibly observed this offers new means of recording past earthquakes fine particles carrying the magnetic remanence are shown to move rotate and re align with respect to the prevailing geomagnetic field direction or in response to the applied shaking forces we call this seismomagnetization this implies that paleoseismics may generate excursion like paleomagnetic signals despite the absence of visible deformation structures

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

enhanced rock slope failure following ice sheet deglaciation timing and causes this research was supported by nerc cosmogenic isotope analysis facility grant number 9046 0308

palaeoseismicity and de geer moraines abstract de geer moraines are small moraine ridges formed partly in crevasses at the base of glacier ice there may be several different origins of the crevasses one possibility proposed by gerard de geer who first studied these ridges was seismic activity in connection with the isostatic rebound during the deglaciation the present study shows a close connection between occurrences of de geer moraines in fennoscandia and seismic activity or fresh looking fault lines and other features indicating neotectonic movements this supports de geer s theory but does not exclude formation in crevasses formed in other ways there is no real proof that the tectonic movements took place in the quaternary but the close connection between the moraines and tectonic features is evidence pointing in this direction if this is correct it has important implications for the stability within cratons of precambrian crystalline bedrock

response of faults to climate driven changes in ice and water volumes on earth s surface numerical models including one or more faults in a rheologically stratified lithosphere show that climate induced variations in ice and water volumes on earth s surface considerably affect the slip evolution of both thrust and normal faults in general the slip rate and hence the seismicity of a fault decreases during loading and increases during unloading here we present several case studies to show that a postglacial slip rate increase occurred on faults worldwide in regions where ice caps and lakes decayed at the end of the last glaciation of note is that the postglacial amplification of seismicity was not restricted to the areas beneath the large laurentide and fennoscandian ice sheets but also occurred in regions affected by smaller ice caps or lakes e g the basin and range province our results do not only have important consequences for the interpretation of palaeoseismological records from faults in these regions but also for the evaluation of the future seismicity in regions currently affected by deglaciation like greenland and antarctica shrinkage of the modern ice sheets owing to global warming may ultimately lead to an increase in earthquake frequency in these regions

lateglacial rock slope failures in north west ireland age causes and implications nine postglacial quartzite rock slope failures rsfs in north west ireland were dated using cosmogenic 10be weighted mean rsf ages range from 17 7 0 9 to 12 5 0 7 ka or 16 6 0 7 to 11 7 0 5 ka depending on assumed 10be production rate all dated rsfs occurred within 5000 years following ice sheet deglaciation at 17 4ka 16 3 ka and all but two occurred within 2000 years after deglaciation the timing of rsfs rules out glacial debuttressing permafrost degradation and enhanced deglacial cleft water pressures as triggers of failure in most cases we infer that paraglacial stress release and associated fracture propagation were critical in reducing rock masses to critical stability although earthquakes caused by lateglacial glacio isostatic rebound and or release of stored tectonic stresses may have triggered failure in some or all cases in conjunction with data from related studies our results imply that most undated rsfs outside the limit of younger dryas glaciation in the british isles are of lateglacial age and that numerous lateglacial rsfs occurred inside these limits with subsequent removal of debris by glaciers they support the view that paraglacial rsf activity in tectonically stable intraplate terrains was concentrated within a few millennia following deglaciation

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

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

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

paleoseismicity the swedish case abstract sweden is part of an old cratonic area it was repeatedly glaciated during the quaternary after the 20 ka glaciation maximum it has experienced a glacial isostatic uplift of over 800 m numerous paleoseismic indices have been recorded giving evidence of frequent high magnitude earthquakes from the time at around and just after the deglaciation late holocene archaeoseismic fracturing is also reported these observations violate all claims of a possible safe long term repository for high level nuclear waste some 500 m down in the bedrock

slip rate variations on faults during glacial loading and post glacial unloading implications for the viscosity structure of the lithosphere abstract many active faults in extensional and contractional tectonic settings experienced a slip rate increase after the last glacial period when the volume of nearby glaciers and lakes decreased this post glacial slip rate increase is caused by transient stresses that are generated by the unloading induced rebound and superimposed on the tectonic background stress field as the latter is different for normal and thrust faults the response to loading and unloading should depend on the fault type here we use finite element models including a fault in rheologically layered lithosphere to explore the conditions under which both normal and thrust faults experience a post glacial slip rate increase the results show that a post glacial slip rate increase occurs on normal faults if the lower crust is stronger than the lithospheric mantle whereas thrust faults accelerate if the lower crust is weaker than the lithospheric mantle these findings imply that the response of faults to mass fluctuations on the earth9s surface may provide constraints on the rheological stratification of the lithosphere we use our results to make predictions on the viscosity structure of the basin and range province and northern scandinavia where palaeoseismological data document a pronounced increase in seismicity as a result of post glacial unloading and rebound

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

fluidization of buried mass wasting deposits in lake sediments and its relevance for paleoseismology results from a reflection seismic study of lakes villarrica and calafquen south central chile abstract a dense grid of very high resolution seismic profiles on lake villarrica provides a quasi 3d view on intercalated lenses of low amplitude reflections which are connected by acoustic wipe out patches and fractures to an underlying voluminous mass wasting deposit the lenses are interpreted as being created by earthquake triggered liquefaction in this buried mass wasting deposit and subsequent sediment fluidization and extrusion at the paleo lake bottom these sediment volcanoes are mapped in detail they have a rather uniform circular geometry and show a linear relationship between apparent width and maximum thickness on a seismic section the largest sediment volcanoes are up to 80 m wide and 1 9 m thick their slope angles designate a syn to post depositional sagging of most sediment volcanoes sediment volcano detection and mapping from nearby lake calafquen further strengthen the revealed geometrical relationships locally some of the sediment fluid escape structures extend to a higher position in the stratigraphy which points to a polyphase escape process associated with multiple multi century spaced strong earthquakes thickness and morphology of the source layer seem to exert a dominant control in the production of sediment fluid extrusions this study shows that reflection seismic profiling allowed recognizing 4 different seismic events in the studied stratigraphic interval which are evidenced by mass wasting deposits and or fluidization features

rock slope failure following late pleistocene deglaciation on tectonically stable mountainous terrain the ages of 31 postglacial rock slope failures rsfs in scotland and nw ireland derived from 89 cosmogenic isotope exposure ages are employed to analyse the temporal pattern of failure and its relationship to the timing of deglaciation rates of glacio isostatic crustal uplift and periods of rapid climate change rsf ages span almost the whole period since ice sheet retreat from 18 2 1 2 ka to 1 7 0 2 ka or from 17 1 1 0 ka to 1 5 0 1 ka depending on the production rate used in 10 be age calculation but catastrophic failure of rock slopes was w4 6 times more frequent prior to w11 7 ka than during the holocene 95 of dated rsfs at sites deglaciated during retreat of the last ice sheet occurred within w5400 years after deglaciation with peak rsf activity 1600e1700 years after deglaciation this time lag is inferred to represent 1 stress release initiated by deglacial unloading leading to 2 time dependent rock mass strength degradation through progressive failure plane development and ultimately 3 to either spontaneous kinematic release or failure triggered by some extrinsic mechanism by contrast 11 dated rsfs at sites reoccupied by glacier ice during the younger dryas stade yds of w12 9e11 7 ka exhibit no clear temporal pattern suggesting that glacial reoccupance during the yds was ineffective in preconditioning a renewed cycle of enhanced rsf activity comparison of timing of individual rsfs with that of deglaciation and rapid warming events at w14 7 ka and w11 7 ka suggests that glacial debuttressing enhanced joint waterpressuresduringdeglaciationandthawofpermafrosticeinrockjointscouldhavetriggeredfailurein only a small number of cases conversely the timing of maximum rsf activity following ice sheet deglaciation corresponds broadly with maximum rates of glacio isostatic crustal uplift suggesting that the two are linked by enhanced seismic activity a seismic failure trigger is consistent with full slope failure at all sites where failure planes are clearly defined our results indicate that numerous rsfs must have occurred in areas that were reoccupied by glacier ice during the yds but have not been identified because runout debris was removed by yds glaciers more generally they provide the first reliably dated body of evidence to support the view that retreat of the last ice sheets in tectonically stable mountainous terrain initiated a period of enhanced rock slope failure due to deglacial unloading and probably lateglacial seismic activity implying that most undated rsfs in such areas are probably of lateglacial or veryearly holocene age they also demonstrate however that a low frequency of rsf activity extended throughout the holocene

future trends in paleoseismology integrated study of the seismic landscape as a vital tool in seismic hazard analyses this paper forms the introduction to this special issue of tectonophysics devoted to selected scientific research presented during events sponsored by the inqua subcommission on paleoseismicity in the past few years in this note we summarize the contents of the contributed papers and use the issues they raise to review the state of the art in paleoseismology from a quaternary geology perspective in our opinion the evolution of paleoseismological studies in the past decade clearly demonstrates that in order to properly understand the seismic potential of a region and to assess the associated hazards broad based multidisciplinary studies are necessary to take full advantage from the geological evidence of past earthquakes a major challenge in future paleoseismic research is to build detailed empirical relations between various categories of coseismic effects in the natural environment and earthquake magnitude intensity these relations should be compiled in a way that is fully representative of the wide variety of natural environments on earth in terms of climatic settings quaternary tectonic evolution rheological parameters of the seismogenic crust and stress environment for instance available data indicate that between earthquake magnitude and surface faulting parameters different scaling laws exist and they are a function of the local geodynamic setting including style of faulting typical focal depths heat flow in this regard we discuss in some detail the concept of seismic landscape which provides the necessary background for developing paleoseismological research strategies the large amount of paleoseismological data collected in recent years shows that each earthquake source creates a signature on the geology and the geomorphology of an area that is unequivocally related with the order of magnitude of its earthquake potential this signature is defined as the seismic landscape of the area e g serva l vittori e ferreli l michetti a m 1997 geology and seismic hazard in grellet b mohammadioun b hays w eds proceedings of the second france united states workshop on earthquake hazard assessment in intraplate regions central and eastern united states and western europe october 16 1995 nice france 20 24 ouest editions nantes france michetti a m hancock p l 1997 paleoseismology understanding past earthquakes using quaternary geology

the origin of tensile fracture lineaments abstract fracture controlled lineaments commonly seen where brittle basement is exposed at the earth s surface are generally restricted to a small number of sets with angles of 45 90 between sets the length frequency distribution of lineaments in each set follows a truncated poisson function such lineaments usually show almost no shearing offset suggesting a tensile origin a simple mechanical model of tensile fracturing is used to explain the spacing directions and length of lineaments as well as their depth frequency distribution results suggest that the penetration depth of tensile fractures which produce lineaments at the earth s surface is directly related to their length and that the fracture density is inversely proportional to fracture depth finally the angles between lineament sets may be controlled by the ratio of strength of unfractured rock to that of pre existing fractures which might heal with time the most likely source of tension is tectonic uplift fractures due to typical uplifts of 0 5 1 km over distances of 10 100 km may penetrate as brittle fractures to several kilometres into the crust perhaps to the depth at which seismic activity ceases

fault type predictions from stress distributions on planetary surfaces importance of fault initiation depth faults on planetary surfaces initiate at depth where stresses due to the weight of overburden are important consequently the prediction of fault type from calculated stresses at depth can yield different faults than those predicted using the surface criteria commonly employed in geophysical models for elastic plate flexure models of mascon loading on the moon stresses calculated at the surface predict that the most likely faulting is strike slip for which there is no direct evidence at a distance from the basin center where grabens are found modifying these plate flexure calculations to account for the weak surface layer of ejecta megaregolith 1 3 km thick that overlies the relatively strong lunar lithosphere results in model stresses in the outer layers of the moon tens of bars that are an order of magnitude lower than in the lithosphere hundreds of bars because faults bounding lunar grabens initiate at the mechanical discontinuity between the megaregolith and lithosphere and because the reverse faults likely responsible for wrinkle ridges probably initiate at the basalt basin floor contact 0 5 2 km deep model stresses must be calculated at these depths superposing nonisotropic stresses due to the weight of the overburden at the depths of these mechanical discontinuities with plate flexure model stresses correctly predicts maximum stress differences in an interior zone of expected wrinkle ridges surrounded by a zone of concentric normal faults or grabens and effectively eliminates the strike slip zone in agreement with observations the absence of strike slip faults from almost all planetary and satellite surfaces may be explainable if lithospheric deformation model stresses are calculated at important shallow crustal mechanical discontinuities where the maximum compressive stress is vertical thereby favoring the formation of normal faults if true mechanical discontinuities in the shallow crusts of planets and satellites play an extremely important and hitherto unrecognized role in the formation of structures observed on their surfaces

characterization of venera 15 16 geologic units from pioneer venus reflectivity and roughness data abstract geologic units have been defined for the surface of venus from venera 15 16 image data a characterization of these geologic units is carried out using information on surface properties derived from pioneer venus pv reflectivity and rms slope data the geologic context provided by venera 15 16 units allows additional more specific interpretations of surface radar properties to be made characterization of venera units results in the definition of four groups of venera units i smooth rocky units ii rough rocky units iii rough high dielectric units and iv diffusely scattering units on the basis of correlations of surface morphology to spatial and statistical distributions in rms slope and reflectivity data we test models for the origin of the surface properties of some units we conclude that plains and tectonic units can be contrasted in terms of the average roughness of the surface and that tectonic deformation appears to roughen the surface at 0 5 to 10 m and 5 to 50 cm scales this tectonic weathering process appears to dominate the erosional regime of venus unlike earth or mars production and transport of soils dominates only a small portion 5 of the surface some of the venera units display distinctive spatial and statistical distributions of pv radar data in particular apparent low reflectivity in the tesserae appears to be caused by small 5 50 cm rock fragments on the surface which cause diffuse scattering at pioneer venus wavelenghts analysis of models for the formation of these fragments suggests that they are due to the pervasive deformation undergone by the tesserae finally aspects of this study have been used to extend results of venera image data analysis southward of 30 n lat resulting in a prediction of the distribution of tessera such results can aid in magellan investigations

three dimensional numerical modeling of slip rate variations on normal and thrust fault arrays during ice cap growth and melting 1 changes in the volumes of ice caps considerably alter the stress state of the lithosphere by generating a transient signal that is added to the tectonic background stress field these stress field changes in turn affect crustal deformation and in particular the slip behavior of existing faults here we use three dimensional finite element models to investigate how arrays of normal and thrust faults near a growing and subsequently melting ice cap are influenced in their slip evolution the results show that regardless of fault dip both types of faults experience a decrease in their slip rate during ice cap advance and an increase in their slip rate during ice cap retreat if they are located beneath the ice cap in contrast faults outside the ice cap that are loaded on their footwall or hanging wall only show the opposite pattern their slip rate increases during glacial loading and decreases during subsequent unloading if the load is located along strike of the fault that is at one of its tips the slip behavior of normal and thrust faults is different the normal fault shows a slip rate increase during unloading the thrust fault during loading our results explain the location and timing of deglaciation induced paleoearthquakes in scandinavia and the contrasting slip histories reported from normal faults in the basin and range province which are located at different positions relative to the former yellowstone ice cap more generally our findings imply that a uniform slip behavior of faults in formerly glaciated regions should not be expected

course and origin of the fennoscandian uplift the case for two separate mechanisms the classical glacial isostatic uplift paradigm of fennoscandia is in need of a thorough revision as to its course and origin the total uplift amounts to 830 m not 300 m the centre of uplift started to rise 13 000 bp not 9300 bp the uplift appears to be composed of two separate mechanisms one true glacial isostatic rebound and one linear uplift factor possibly representing a phase boundary adjustment calculations and modelling must treat these two factors independently which has not been the case

geomorphological effects of tsunami run up and backwash abstract geomorphological processes associated with tsunami run up and backwash are highly complex to date most studies have considered the sedimentology of palaeo tsunamis while almost no detailed studies have been undertaken in areas known to have been affected by tsunamis in recent times research on contemporary tsunamis show that coastal landscapes may be greatly altered not only by direct tsunami run up orthogonal to the shoreline but also by episodes of vigorous backwash and by water flow sub parallel to the coastline the combined effect of these processes produces hitherto unrecognised coastal landforms that are dominated by the effects of high magnitude erosion and deposition deposition of large boulders is also associated with severe tsunamis while in adjacent areas run up processes often result in the deposition of continuous and discontinuous sediment sheets in many instances the upper limit of sediment deposition lies well below the upper limit of wave run up which is frequently marked by a well defined zone of stripped vegetation and soil the processes described have considerable implications for models of coastal evolution it may even be the case that the evolution of particular stretches of coastline are dominated by such high magnitude low frequency events

liquefaction and varve deformation as evidence of paleoseismic events and tsunamis the autumn 10 430 bp case in sweden coastal seismic events generate instantaneous changes in relative sea level i e land level vs sea level tsunamis may cause disastrous damage to coasts and coastal habitation liquefaction and deformations of annually varved sediments provide information on paleoseismic events the evidence for a major earthquake and associated tsunami waves in sweden are explored thanks to the varve chronology liquefaction structures and varve deformations caused by this event can be dated at the autumn 10 430 varve years bp the magnitude is estimated at 8 or more on the richter scale the tsunami washed the previously blocked outlet of the baltic free of icebergs and pack ice so that marine water could suddenly invade the entire baltic forming the yoldia sea

deglacial paleoseismicity in sweden the 9663 bp iggesund event abstract the iggeesund event is an example of the high seismicity which occurred in fennoscandia during deglaciation as a function of the high rate of glacial isostatic uplift this event is dated to varve 9663 bp the iggesund hudiksvall area is represented by intensive postglacial fracturing of the bedrock recorded over an area of at least 50 50 km the most remarkable locality is the boda cave system a hill fractured into a field of large detached blocks with a cave system of more than 2 km length in the varve year 9663 bp an extensive turbidite was spread over an area of 210 80 km liquefaction structures are recorded over an area of 80 30 km and can be assigned to the same varve year structures and deposits of a tsunami are recorded over 80 50 km

new insights into late pleistocene glacial and postglacial history of northernmost ungava canada from pingualuit crater lake sediments abstract the pingualuit crater was formed by a meteoritic impact ca 1 4 million years ago in northernmost ungava canada due to its geographical position near the center of successive north american ice sheets and its favorable morphometry the pingualuit crater lake water depth 246 m promises to yield a unique continuous sedimentary sequence covering several glacial interglacial cycles in the terrestrial canadian arctic in this paper we suggest the existence of a subglacial lake at least during the last glacial maximum lgm by hydraulic potential modeling using lgm ice surface elevation and bed topography derived from a digital elevation model these results support the hypothesis that the bottom sediments of the crater lake escaped glacial erosion and may contain a long term continental sedimentary sequence we also present the stratigraphy of a 9 m long core retrieved from the deep basin of the lake as well as a multiproxy reconstruction of its deglacial and postglacial history the base of the core is formed by very dense diamicton reflecting basal melt out environments marking the end of subglacial conditions at the coring site the overlying finely laminated silt are related to the onset of proglacial conditions characterized by extremely low lacustrine productivity infra red stimulated luminescence and ams 14 c dating as well as biostratigraphic data indicate sediment mixing between recent e g holocene and much older pre to mid wisconsinan material reworked by glacier activity this process prevents the precise dating of these sediments that we interpret as being deposited just before the final deglaciation of the lake two finer grained and organic rich intervals reflect the inception of lacustrine productivity resulting from the cessation of glacial meltwater inputs and ice free periods the lower organic interval corresponds to the early postglacial period 6850 5750 cal bp and marks the transition between proglacial and postglacial conditions during the holocene thermal maximum while the uppermost organic rich core section represents late holocene sediments 4200 600 cal bp the organic intervals are separated by a basin scale erosive slide occurring around 4200 cal bp and likely related to 1 a seismic event due to the glacio isostatic rebound following the last deglaciation or 2 slope instabilities associated with rapid discharge events of the lake

submarine holocene sedimentary disturbances in the olkiluoto area of the gulf of bothnia baltic sea a case of postglacial palaeoseismicity abstract high resolution echo soundings were used to study the fault activity in the olkiluoto area of the gulf of bothnia southwestern finland echo sounding profiles in holocene submarine sediments show normal faults structures related to gas or groundwater seepage submarine slides and slumps and debris flow type structures the faults and other disturbance structures occur along two bedrock lineaments all faults occur at the same stratigraphical level whose age is estimated at 10 700 calendar years bp 9500 radiocarbon years bp it is suggested that their movement has been triggered by palaeoseismic event s when the late weichselian ice sheet was retreating from the site and bedrock stresses were released through lineaments and fracture zones these are the first observations of postglacial fault movements in quaternary sediments from southern finland but the timing of these movements is contemporaneous with the incidence of postglacial bedrock faulting 9000 10 000 radiocarbon years bp in northern fennoscandia the origin of the gas groundwater seepages remains unclear their reflections in the echo sounding profiles imply that part of the gas is derived from the organic bearing litorina and recent gyttja clays however at least some part of the gas is derived from the bedrock deep gas

the use of varved clay chronology for dating paleoseismic events the erstavik record in the stockholm area south sweden abstract geological structures suggest that the fennoscandian shield was subjected to a higher seismicity at the end of the last glaciation than today this article demonstrates the use of varved clay chronology for dating paleoseismic events it is argued that the deposited annually layered glacial varves were sensitive to past ground movements in the stockholm area the erstavik varved clay chronology suggests four paleoseismic events a first i dating from varve year 10 473 to 10 468 bp a second ii 10 451 to 10 445 bp a third iii 10 429 to 10 425 bp and a fourth iv 10 409 to 10 404 bp in de geer s old 1940 chronology the first i dating corresponds with 1117 to 1112 the second ii with 1095 to 1089 the third iii with 1073 to 1069 and the fourth iv with 1053 to 1048 the most pronounced event was the one at around varve year 10 429 bp varve 1073 in de geer s old chronology the recurrence time of about 20 years suggests a totally different seismic regime at the time of deglaciation than what exists today it coincided with the period of maximum isostatic uplift the complexity of the varved clay response to seismic events is also discussed

eocene paleoseismic record of the green river formation fossil basin wyoming u s a implications of synsedimentary deformation structures in lacustrine carbonate mudstones abstract laterally extensive intervals containing sedimentary deformation features are identified in outcrops of lacustrine deposits of the eocene 52 51 ma green river formation in fossil basin southwestern wyoming fossil basin is the smallest of the paleo lakes in the green river system and is located in the sevier fold and thrust belt which was tectonically active during deposition deformation structures include convolute lamination load structures hybrid brittle ductile features sedimentary dikes microfaults fluid escape features such as cusps and pipes oil shale breccias and mass transport deposits in most cases these structures are hosted by finely laminated variably organic rich carbonate mudstones that accumulated in a low energy profundal lacustrine environment deformed intervals range from a few centimeters to 5 m in thickness and are bound above and below by undeformed strata of the same facies which demonstrates the early post depositional timing of deformation the distribution of these features indicates a primary control exerted by sediment rheology resulting from variation in grain size clay and organic content and degree of lithification which in turn were governed by the depositional environment and early diagenesis based on their morphological attributes lateral extent and recurrence at different stratigraphic levels along with their sedimentary environment and the tectonic setting of the basin the deformation structures are interpreted to be seismites i e the result of stresses induced by syndepositional earthquakes and other potential trigger agents are rejected moreover stratigraphic relationships indicate that these small scale deformation features as well as the mass transport deposits formed on a virtually flat lake floor ruling out instability caused by the gravitational regime syndepositional deformation in fossil basin points to tectonic activity along the nearby absaroka and or hogsback thrusts and related fault systems seismites point to possible tectonic forcing as a mechanism for changes in bathymetry and sedimentation in the green river formation and consequently demonstrate their potential for elucidating the tectonic history of other lacustrine and marine basins

neotectonics and paleoseismicity within the stockholm intracratonal region in sweden abstract a number of sites with fresh faults and fractures have been mapped in detail their postglacial and sometimes late glacial age is confirmed they form simple and similar patterns consistent with stress release due to a combination of acting forces they are associated with the peak rate of glacial isostasy and increased strain rates by two orders of magnitude they often cut straight across bedrock hills between so called weak zones the fault throws correspond to approximately magnitude 7 on the richter scale sedimentary and morphological data confirm the seismo tectonic origin newly discovered large scale network of crossing faults is found to have changed the course of the stockholm esker in a way that reveals at least two major seismo tectonic events this is consistent with paleoseismic events of magnitude 7 or more

twenty years of paleoseismology in italy abstract italy has one of the most complete and historically extensive seismic catalogues in the world due to a unique and uninterrupted flow of written sources that have narrated its seismic history since about the end of the iron age seismic hazard studies have therefore always been mainly based upon this huge mass of data nevertheless the italian catalogue probably lacks many m 6 5 events the seismogenetic structures responsible for which are characterized by recurrence times that are longer than the time span covered by our historical sources for these reasons and as in other countries earthquake data that in italy have been derived from paleoseismological studies should finally become a necessary ingredient in seismic risk assessment indeed over the past 20 years some hundred trenches have been excavated supplying reliable and conclusive data on the recent activities of many faults through to many robust datings of surface fault events these studies have provided the ages of several unknown or poorly known m 6 5 earthquakes here we summarize the state of the art of paleoseismology in italy and present a first catalogue of 56 paleoearthquakes pci that occurred mainly in the past 6 kyr the pci integrates the historical instrumental seismic catalogue and extends it beyond the recurrence time of the seismogenetic faults 2000 1000 yr we feel confident that the use of the pci will enhance future probabilistic seismic hazard assessment and thus contribute to more reliable seismic risk mitigation programs

stagnant lid convection on venus the effect of strongly temperature dependent viscosity on convection in the interior of venus is studied systematically with the help of finite element numerical models for viscosity contrasts satisfying experimental constraints on the rheology of rocks venus is likely to be in the regime of stagnant lid convection this regime is characterized by the formation of a slowly creeping very viscous lid on top of the mantle venusian lithosphere and is in agreement with the tectonic style observed on venus stagnant lid convection explains large geoid to topography ratios on venus by the thermal thinning of a thick lithosphere the thickness of the lithosphere can be as large as 400 550 km for beta regio and 200 400 km on average geoid and topography data and experimental data on the rheology of rocks provide constraints on the viscosity of the mantle 10 20 10 21 pa s the convective stresses in the interior 0 2 0 5 mpa the stresses in the lid 100 200 mpa the velocity in the interior 0 5 3 cm yr 1 and the heat flux beneath the lithosphere 8 16 mw m 2 parameterized convection calculations of thermal history of venus are difficult to reconcile with a thick present day lithosphere however a sufficiently thick lithosphere can be formed if a convective regime with mobile plates was replaced by stagnant lid convection around 0 5 b y ago one of the possible explanations for the cessation of venusian plate tectonics is that during the evolution of venus stresses in the lid dropped below the yield strength of the lithosphere this model predicts a drastic drop in the heat flux thickening of the lithosphere and suppression of melting and is consistent with the hypothesis of cessation of resurfacing on venus around 0 5 b y ago

fault slip versus slope deformations experience from paleoseismic trenches in the region with low slip rate faults and strong pleistocene periglacial mass wasting bohemian massif abstract successions of colluvia and loesses covering the faults in the eastern and central parts of the bohemian massif were exposed in six trenches two trenches at each fault up to 6 m deep and studied in detail in terms of stratigraphy and deformation based on osl and radiocarbon ages these sediments were dated as weichselian early glacial to younger dryas holocene 120 ka to 11 ka on the kosi fault nw se strike the offset strata bring evidence for late pleistocene oblique slip normal and strike slip components on the main fault plane with minimum slip rate in order of 0 1 mm a although the undisrupted topsoil suggests the absence of significant slip in holocene and no clear evidence of paleoseismic events was found this fault structure should be included in seismic hazard assessment conversely on the hluboka and diendorf boskovice faults nw se and nne ssw strikes respectively the tectonic slip is contradicted for the last 15 23 ka based on dating of undeformed strata sealing the fault planes multiple independent evidence suggests that these two faults were not active in late pleistocene at least and their prominent scarps is probably largely due to exhumation by differential denudation examples given document the mechanisms of slope related deformations which may lead to destruction of the records of older faulting and occasionally produce deformation structures resembling the tectonic ones

paleoseismicity and geodynamics in sweden abstract paleoseismicity denotes past pre instrumental earthquakes as recorded by bedrock structures morphological features and sedimentological criteria glacial varves eskers and deltas are excellent records of paleoseismic events data from sweden are presented faults fractures and various types of sedimentary disturbances of late pleistocene and holocene age frequently occur it is concluded that the seismic activity was very intensive in connection with the peak rates of glacial isostatic uplift and that this was a natural effect of the geodynamic processes operating

a review of the origins of subparallel ridges and troughs generalized morphological predictions from terrestrial models the morphologies of ridges and troughs on earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites we consider tension fracturing normal faulting thrust faulting buckle folding strike slip tectonism fissure eruption wall diapirism surface folding of a flow and near surface laccolithic intrusion the landform characteristics and geologic associations predicted of each process are generalized to be independent of planet specific parameters we find that the combinations of landforms derived from each process are unique making each distinguishable in the idealized case morphological parameters most diagnostic of formational process are ridge and trough cross sectional shape termination style and incipient character the characteristics described are useful in constraining the origin of ridge and trough terrains observed in voyager images and will aid in analysis of high resolution images from the galileo and cassini spacecraft

a fluctuating ice front over an esker near ryssjon s sweden as a cause of a giant load cast abstract a well exposed section in an esker near ryssjon s sweden strongly affected by several phases of glacitectonism showed two phenomena that are uncommon and that seem to have a causal relationship the first phenomenon is the occurrence of a load cast consisting predominantly of gravel in deposits formed just inside or outside a tunnel mouth the load cast is over 3 m high and 8 m wide a size that has previously not been described from unconsolidated sediments two nearby somewhat smaller load casts consisting mainly of sand also have dimensions of over 2 m indicating that the formation of huge load casts was apparently a feature related to the special conditions under which loading could develop at this site the second remarkable feature is that not only is a till present in the succession above the load cast thus indicating re advance of the ice but deposits overlying this till are also deformed by glacitectonism proving that at least a second phase of ice re advance must have occurred this situation has not previously been documented it is concluded that the varying differential overburden of sediment and particularly the retreating and re advancing ice in this area were responsible for the formation of the extremely large load casts possibly triggered by seismic shocks related to isostatic adjustment caused by oscillations of the ice front

an interpretation and catalogue of paleoseismicity in sweden abstract paleoseismic data provide a long term record of seismic activity to predict hazards for periods longer than one to a few centuries in sweden the analysis reveals there was a drastic change in dominant seismic mode from a high to super high deglacial mode to a low to moderately low mode in present and late holocene time paleoseismic criteria and characteristics include numerous different sources of information viz primary faults bedrock deformation sedimentary deformation rock and sediment slides liquefaction sorting by shaking tsunamis differing geomorphic expressions disordering and ordering of magnetic particles by applying multiple criteria it was possible to identify 44 paleoseismic events including 23 events of estimated m 6 7 12 events of m 7 8 and 6 events of m 8 varve dating often allows a precision as to a single year in one case even to the season of a year the key for paleoseismic reconstruction and testing is the application of multiple criteria

recent deformation rates on venus constraints on the recent geological evolution of venus may be provided by quantitative estimates of the rates of the principal resurfacing processes volcanism and tectonism this paper focuses on the latter using impact craters as strain indicators the total postimpact tectonic strain lies in the range 0 5 6 5 which defines a recent mean strain rate of 10 18 10 17 s 1 when divided by the mean surface age interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 ma catastrophic resurfacing model if distributed tectonic resurfacing contributed strongly before that time as suggested by the widespread occurrence of tessera as inliers the mean global strain rate must have been at least 10 155 s 1 which is also typical of terrestrial active margins numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years parameterized convection models predict that the mean global thermal gradient decreases by only about 5 k km over this time even with the exponential dependence of viscosity upon temperature the surface strain rate drops by little more than one order of magnitude strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model an alternative uniformitarian resurfacing hypothesis holds that venus is resurfaced in quasi random patches several hundred kilometers in size that occur in response to changing mantle convection patterns under such a model the observed crater strain distribution indicates that about 1 of the planet s surface is tectonically active at any time however this model requires a very weak crustal rheology to achieve surface velocities 100 mm yr appropriate to the required patch size without well developed lateral weak zones venus is essentially a one plate planet but one in which the lithosphere is able to respond to topography produced by mantle convection through faulting and limited horizontal movement the net rate of tectonic activity is logarithmically intermediate between earth and mars about 100 times slower than plate tectonics but up to 100 times faster than planets where tectonic stress arises largely from lithospheric cooling and contraction

the long singular ridges of the halland coastal moraines south western sweden radiocarbon dates of shells from the long singular ridges belonging to the halland coastal moraines indicate that the ridges are younger than 12 400 yr bp whereas according to the existing chronostratigraphy of western sweden the area was ice free before 12 700 yr bp the ridges are interpreted as push moraines and not deposits of a stationary ice margin the chronostratigraphy of western sweden seems to need revision however further study is necessary before it is possible to present a new model

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

comments on the growth of accretionary wedges uplift within accretionary foreland wedges has been explained by the development of duplexes which provides a mechanism for underplating in submarine accretionary wedges with little deformation in the upper part of the wedge we interpret duplex development beneath the costa rica forearc a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge large scale structural geometry of accretionary wedges is compatible with low angle trenchward dipping backstops in centrist to reverse angle backstops used commonly in models of return flow within wedges which have been invoked to explain the presence of high p t metamorphic rocks we suggest that emplacement of high p t metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges

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

duration of tessera deformation on venus the density and distribution of impact craters superposed on the highly deformed tessera terrain on venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains eighty percent of tesserae craters are undeformed no existing craters exhibit evidence of contractional deformation suggesting that the early compressional stage of tessera deformation ended abruptly the small number of craters fractured by late stage tessera extension constrains the duration of this phase to less than 20 of the average crater retention age of the tesserae or approximately 30 60 ma these results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

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

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

effect of ice sheet growth and melting on the slip evolution of thrust faults abstract field investigations suggest that postglacial unloading and rebound led to the formation or re activation of reverse faults even in continental shields like scandinavia here we use finite element models including a thrust fault embedded in a rheologically layered lithosphere to investigate its slip evolution during glacial loading and subsequent postglacial unloading the model results show that the rate of thrusting decreases during the presence of an ice sheet and strongly increases during deglaciation the magnitude of the slip acceleration is primarily controlled by the thickness of the ice sheet the viscosity of the lithospheric layers and the long term shortening rate in contrast the width of the ice sheet the rate of deglaciation or the fault dip have an only minor influence on the slip evolution in all experiments the slip rate variations are caused by changes in the differential stress the modelled deglaciation induced slip acceleration agrees well with the occurrence of large earthquakes soon after the melting of the fennoscandian ice sheet which led to the formation of spectacular fault scarps in particular in the lapland fault province furthermore our model results support the idea that the low level of seismicity in currently glaciated regions like greenland and antarctica is caused by the presence of the ice sheets based on our models we expect that the decay of the greenland and antarctica ice sheets in the course of global warming will ultimately lead to an increase in earthquake frequency in these regions

the baltic ice lake yoldia sea transition abstract the baltic ice lake the yoldia sea the ancylus lake and the littorina sea are the four classical stages in the postglacial evolution of the baltic basin the picture is complicated by what concerns the details of each of these stages between the baltic ice lake and the yoldia sea it is necessary to introduce a separate lake stage an entirely lacustrine phase of 300 years right after the drainage of the baltic ice lake and before the ingression of salt water that by definition signifies the onset of the yoldia sea during this lake stage a fully lacustrine ancylusfauna lived along the coasts of the islands of gotland and oland for historical reasons and mistakes we term this new stage the yoldia lake stage thus avoiding too much reorganization