from a deformed peri tethyan carbonate platform to a fold and thrust belt an example from the central apennines italy abstract in the central apennines of italy the relationships between the foreland deformation of a peri tethyan carbonate platform lazio abruzzi domain and the subsequent contractional pattern have been clearly documented by means of stratigraphic and structural evidence stratigraphic and structural data point out the occurrence of pre thrusting normal fault systems and allow us to define their relationships with contractional structures miocene normal faults are particularly well documented by thickness and facies variation of foredeep deposits and by structural evidence these faults controlled sedimentation during tortonian early messinian times foredeep stage and produced most of the accommodation space for clastic deposits in their hanging walls on the contrary the subsequent positive inversion appears to be quite limited and generally the hanging wall sequences are not uplifted above the regional clear evidence that normal faults formed first and were later cut and or rotated during thrusting is supported by analyses carried out along the main fault surfaces the restoration of a balanced geological section across the study area based on the inference of an extended foreland affected by later contraction provided a total small shortening value this is lower than the previous estimated by several authors for the same foreland fold and thrust belt

precambrian gneiss and late proterozoic sedimentation in north central british columbia u pb analyses of zircons from gneissic granite in the sifton and deserters ranges flanking the northern rocky mountain trench in north central british columbia indicate crystallization ages of 1 85 b y and 728 m y respectively augen gneiss in the sifton ranges appears to be basement rock associated with a sequence of amphibolite quartzite schist and carbonate thrust southerly over upper proterozoic strata of the ingenika group gneissic granite in the deserters range is nonconformably overlain by pebble conglomerate and pure quartzite in turn overlain by a thick sequence of amphibolite schist phyllite gritty clastic rocks carbonate quartzite and diamictite assigned to the upper proterozoic misinchinka group the deserters data establish a probable maximum age for rocks of the windermere supergroup in the northern rocky mountains the age and structural setting of these gneisses are similar to other gneisses of the omineca crystalline belt of the canadian cordillera

mechanisms for cessation of magmatic resurfacing on venus 1 proposed mechanisms of resurfacing on venus about 0 3 1 gyr ago usually involve either some form of tectonic resurfacing in which venusian lithosphere is recycled mechanically or magmatic resurfacing where essentially immobile lithosphere is covered by lava the focus of this study is mechanisms of magmatic resurfacing in the stagnant lid regime of mantle convection parameterized convection models suggest that cessation of magmatic resurfacing can occur in several ways 1 the mantle temperature drops sufficiently such that mantle rising adiabatically does not cross the solidus 2 the molten layer migrates below the solid melt density inversion at 250 500 km so that no melt can escape 3 sublithospheric small scale convection stops and conductive thickening of the lid suppresses melting in each case inability of magma to penetrate thickened venusian lithosphere may play a role the timing of melt cessation and duration of late stage resurfacing depend on various factors such as mantle rheology degree of mantle depletion and depth of resurfacing the models indicate that the waning stages of resurfacing which result in surface age variations can last 0 1 1 gyr a general trend of recent lithospheric thickening is also predicted some cases exhibit a rapid transition from relatively thin to thick lid as suggested by previous authors

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

the middle cambrian mount roosevelt formation new of northeastern british columbia evidence for rifting and development of the kechika graben system the eastern margin of the kechika graben in the vicinity of muncho and moose lakes northeastern british columbia contains middle cambrian rift related strata herein named the mount roosevelt formation the formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late early cambrian gataga group bonnia olenellus zone and is conformably overlain by a thick unnamed carbonate sequence of middle cambrian age plagiura poliella zone the mount roosevelt formation is subdivided into three members the lowermost member is characterized by ooid bearing siltstone and sandstone interbedded with dolostone limestone and hematitic conglomerate conformably above this the middle member is a thick sequence of polymict cobbly pebble conglomerate the upper member includes karstified dolostone calcareous cemented conglomerate and sandstone and limestone collectively the mount roosevelt formation reflects alluvial fan delta progradation into a transgressive mar

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

flow resistance in ice covered alluvial channels this paper describes the influences of floating level ice covers on flow and bed load transport in alluvial channels and proposes a semi empirical procedure for calculating flow resistance in such channels the procedure can be applied in conjunction with several existing methods for estimating flow resistance in alluvial channels under open water conditions previously cover influence was poorly understood with the consequence that flow resistance calculations for ice covered alluvial channels entailed an inordinate amount of guesswork the cover influences described herein were determined from the results of experiments conducted with a laboratory flume the main findings of the flume experiments are that ice covers increase flow depth decrease bulk flow velocity increase dune length and significantly decrease bed load transport rate additionally it was found that sediment transport relationships developed for open water flow are applicable to ice covered flow provided they are used in conjunctio

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

from extension to shortening tectonic inversion distributed in time and space in the alboran sea western mediterranean 2d seismic reflection data tied to biostratigraphical and log information from wells in the central and south eastern alboran sea have allowed us to constrain the spatial and temporal distribution of rifting and inversion normal faults tilted basement blocks and growth wedges reveal a thinned continental crust that formed in response to nw se extension to the east a secondary sw ne trend of extension affects the transitional crust adjacent to the oceanic algerian basin the maximum thickness of syn rift sediments is 3 5 km and the oldest recorded deposits are serravallian the wnw ese yusuf fault formed a buttress separating and accommodating variable extension between two different tectonic domains the thinned continental crust of alboran and the oceanic spreading of the algerian basin late tortonian to present day nw se africa eurasia plate convergence drove shortening and reactivation of some of the earlier extensional structures as reverse and strike slip faults forming complex compartmentalised sub basins tectonic inversion coexisted with the formation of new faults and folds inversion was partial along the habibas basin and al idrisi fault but complete along the alboran ridge where some sw ne trending faults were perpendicular to the recent nw se plate convergence and were reactivated as thrusts the wnw ese yusuf fault is oblique to the convergence vector and therefore reactivation is mainly expressed as transpressional deformation volcanic rocks intruded along the alboran ridge and yusuf faults during the latest stages of extension formed rheological anisotropies that localised the later inversion

resurfacing history of venus the stratigraphy impact cratering record and geoid of venus indicate that a major change in surface geologic activity has occurred before 800 ma venus was dominated by horizontal movement and tectonic deformation of the crust a brief period of global volcanism subsequently filled in all low lying areas and left much of the surface at a nearly uniform elevation current geologic activity is dominated by large volcanoes and limited horizontal movement of the crust this geologic history may have resulted from mantle cooling causing the lithosphere to become positively buoyant globally

role of basin width variation in tectonic inversion insight from analogue modelling and implications for the tectonic inversion of the abanico basin 32 34 s central andes

venus timing and rates of geologic activity we employ the accumulation of the impact crater population as a geochronometer to examine the observable record of the history of venus 1 b y analyzing age relationships between craters and the different geologic units we show that the earlier suite of units from heavily deformed tesserae through slightly deformed regional plains occurred during a time period an order of magnitude shorter than the subsequent period from the end of emplacement of the wrinkle ridge network until the present these results imply high global rates of endogenic volcanic activity during the first era comparable to that of mid oceanic ridge volcanism of earth and much lower global rates of endogenic activity by two orders of magnitude for the second period

deformation of stratiform znpb barite deposits in the northern canadian cordillera abstract the major stratiform sediment hosted zn pb barite deposits of the northern canadian cordillera occur in the palaeozoic selwyn basin kechika trough they formed in euxinic fine grained clastic dominated extensional basins that developed within the carbonate platform of the passive margin of ancestral north america during the late jurassic to mid cretaceous these basins and the stratiform ore deposits were subjected to low greenschist facies regional metamorphism and fold and thrust deformation as they were incorporated into the western part of the rocky mountains mackenzie mountains foreland fold and thrust belt this paper summarises the results of detailed field and microscopic studies on four of these deposits the tom deposit the howards pass deposit the cirque deposit and the driftpile creek deposit the dominant mineralogy of these deposits is sphalerite galena pyrite and in some cases significant barite is present low greenschist grade metamorphism and deformation is characterised by cataclastic flow and brecciation in coarse grained massive pyritic sulphides and by folding recrystallisation and cleavage development in fine grained sulphides and barite relict primary depositional diagenetic structures are found within the orebodies deformation is characterised by strong pressure solution fabrics in barite in fine grained laminated pyrite layers and in mixed sulphide carbonate beds marked local transposition is found with significant remobilisation of galena barite and pyrite the deformation mechanics of these sulphide barite deposits are illustrated using schematic deformation mechanism maps

sediment transport under ice cover a method for calculating sediment transport in ice covered flows is developed laboratory data on bed form frictional characteristics and sediment transport are obtained for equivalent free surface and covered flows comparison of the data leads to the conclusion that the lower layer in a covered flow can be treated as a free surface flow for the purpose of calculating bed load transport using the flow characteristics calculated from the k e turbulence model and a conventional bed load equation the sediment transport for covered flows can be obtained the method was verified using measured data

tectonic inversion and basement buttressing an example from the central appalachian blue ridge province abstract the structural geometry of the central appalachian blue ridge province is primarily the result of paleozoic contractional deformation however the tye river fault zone in the western blue ridge of central virginia preserves an extensional map pattern these ene wsw striking faults place neoproterozoic to early cambrian volcanic and sedimentary rocks over grenvillian granitic basement detailed mapping cross section restoration and balancing reveals partially inverted faults original half grabens and significantly thicker cover units on hanging walls the orientation and geometry of the tye river fault zone as well as the thickness changes in the cover rocks are inconsistent with extensional reactivation of paleozoic reverse faults during mesozoic rifting the extrusion and deposition of cover rocks was contemporaneous with normal displacement on the tye river fault zone and the formation of rift basins recording 5 crustal extension faults were reactivated as paleozoic contractional structures but an extensional map pattern is generally still preserved regional shortening of 10 20 was accomplished primarily by folding of the cover rocks rather than by reactivation of older faults the basement formed a rigid buttress that controlled the geometry of folding in the cover sequence

a shortened intraplate rift system in the proterozoic mount isa terrane nw queensland australia the leichhardt river fault trough of the mount isa terrane developed a complex extensional architecture between approximately 1800 and 1600 ma forming the underlying template upon which compressional structures were superimposed during the 1590 to 1500 ma isan orogeny basin fill material accumulated during at least five multiphase periods of rifting and associated postrift subsidence forming a stacked succession of unconformity bounded sequences initial e w extension was associated with a massive magmatic event half graben greater than 50 km in width and of alternating asymmetry localized the extrusion of up to 4 km of continental tholeiites thereafter a period of n s extension resulted in southward tapering north tilted half graben in which synrift basaltic and siliciclastic strata accumulated n s extension was followed by regional postrift subsidence and the deposition of a laterally continuous quartzite carbonate package a multiphase period of e w to nw se extension ensued during which time two unconformity bounded sequences accumulated the stratal architectures of these sequences are strongly asymmetric in cross section exhibiting a pronounced rotational thickening toward the east consistent with their deposition in the hanging walls of east dipping tilt blocks between 15 and 40 km in width finally a period of n s extension resulted in the development of e w trending f1 drag synclines in the highest level cover rocks the association of angular unconformities and block bounding faults e w trending synclines and e w striking faults and the unique internal fold geometries of fault blocks suggest that many fault bounded blocks originated as coherent structural entities during rifting and continued to act as such during subsequent shortening

evidence for rapid regional plains emplacement on venus from the population of volcanically embayed impact craters we have reexamined the population of craters on venus believed to be embayed by lava flows and discuss the reliability of criteria for determining the volcanic embayment state of a crater presenting nonvolcanic processes which may produce similar crater morphologies we conclude that 29 craters have been embayed by lava flows from the exterior and an additional 27 craters are ambiguous cases 3 6 of the total crater population all of the embayed craters are embayed by the stratigraphically youngest lava flows except for five to 11 craters embayed by stratigraphically older regional plains deposits which are typically deformed by wrinkle ridges through geologic mapping of an area covering over 8 of the surface we examined the spatial relationships between wrinkle ridged plains and stratigraphically older plains units and estimate that a large portion of the wrinkle ridged plains are thinner than 500 m which is thin enough for the rims of the largest half of preexisting craters to be preserved there are not enough craters embayed by the wrinkle ridged plains to support a model in which the plains were erupted directly onto an ancient surface given the number of large craters embayed by thin deposits of wrinkle ridged plains we develop a simple statistical model to overestimate the number of craters on the preplains surface this model suggests that the average age of the surface beneath the plains plus the average length of time over which the plains were emplaced was of the order of tens of millions of years the short exposure age for the preplains surface indicates that some event or events must have erased the ancient population of craters from the surface before the emplacement of the wrinkle ridged plains

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

basin inversion by distributed deformation the southern margin of the bristol channel basin england several models of basin inversion described in the literature are tested in a study of triassic and early jurassic strata exposed along the southern margin of the bristol channel basin in somerset england that has been exhumed by 95 of them are still in net extension despite widespread kinematic evidence for reverse reactivation when coupled with the general absence of thin skinned thrusts and the widespread occurrence of regional contractional folds it appears that none of three main inversion models the fault reactivation model the thin skinned model and the buttress model are by themselves applicable we erect a new model of basin inversion the distributed deformation model which consists of three stages of basin inversion stage one involved early partial reactivation of large displacement steep normal faults stage two was dominated by folding wherein fault blocks underwent oblique non coaxial shortening by map scale folding accompanied by formation of outer arc normal faults minor cleavage and neoformed thrusts stage three involved reverse reactivation of outer arc normal faults and activation of oblique and strike slip faults that partitioned deformation into compartments

sequential limb rotation and kink band migration recorded by growth strata almazan basin north spain abstract the geometry of growth strata in the almazan monocline demonstrates that limb rotation and kink band migration can coexist as folding mechanisms the almazan basin showing an overall synclinal geometry was one of the most actively subsiding areas during the tertiary in the inner part of the iberian plate with more than 3500 m of preserved tertiary non marine sediments they are arranged in five tectonosedimentary units tsu a1 to a5 bounded by unconformities at the basin borders the southern basin border is defined by the wnw ese trending almazan monocline that can be followed for 25 km along strike with a maximum structural relief of about 2500 m the maximum changes in thickness on both limbs of the monocline occur in units a2 and a3 which constrains the maximum deformation period to between the middle upper eocene headonian top of a1 and upper oligocene lower miocene agenian base of a4 the detailed geometry of the mesozoic pre growth and the tertiary growth strata of the almazan monocline was determined from the analysis of six seismic reflection sections at high angles or perpendicular with the fold axis the average dip of the monocline changes from 15 in the east to more than 45 in the western sector being replaced westwards by a north verging steeply dipping thrust in some sections growth strata in the dipping limb show rotational offlap onlap syntectonic unconformities with progressively decreasing dips from bottom to top indicating that limb rotation took place during the first stages of folding at the limits of the dipping limb anticline and syncline two kink bands appear in some of the sections the one located on the syncline being more developed within each kink band beds show a constant dip of 30 40 the axial surfaces of each kink band converge upwards in the third and fourth tecto sedimentary units being parallel in the older units the fixed axial surface occurs in a down dip position with respect to the active axial surface the geometry of growth strata indicates that the mechanism of folding followed a sequence of i a first stage of large scale limb rotation wavelength larger than 5 km with minor development of kink bands in some sections and ii a second stage of more localised kink band migration probably favoured by fault propagation in the basement

the role of inherited tectono sedimentary architecture in the development of the central andean mountain belt insights from the cordillera de domeyko abstract the structure of the cordillera de domeyko is dominated by a number of elongated n s trending basement ridges these ridges were uplifted by steep reverse n s faults that deformed the mesozoic cenozoic cover the vergence of the fault system varies along strike conferring an apparent doubly vergent pop up geometry to the axial zone two mesozoic pre compressional extensional events were recorded in the area new structural data presented in this paper indicate that most of the generated n s trending thrusts and related folds were controlled by the inversion of the pre existing mesozoic extensional faults thin skin structures in the mesozoic cenozoic cover are genetically linked to major basement upthrusts which could be interpreted as basement short cuts formed during inversion rather than as uplifted blocks associated with major cenozoic strike slip faults growth strata geometries date the beginning of the andean compressional event which generates the chilean precordillera as far back as 90 ma ago the resulting structural architecture is strongly controlled by inherited pre andean extensional structures the association of porphyry intrusives with major reverse faults suggests that the emplacement of the eocene oligocene porphyry cu mo deposits in northern chile can be explained by an oblique inversion tectonics model the upper eocene lower oligocene giant porphyry copper bodies chuquicamata la escondida el salvador located in the cordillera de domeyko show an adakitic affinity this magma affinity together with structural evidence presented in this work indicates that porphyry emplacement occurred at the end of the basement involved contractional stage that generates the anomalous thickened crust needed to generate these magmas this tectonic evolution is coherent with the existence of a flat slab subducting beneath the central andes 22 26 s during early cenozoic that will also produce the eastward migrating of the compressional regime in the upper plate since late cretaceous

structural style and early stage growth of inversion structures 3d seismic insights from the egersund basin offshore norway abstract high quality three dimensional 3d seismic reflection and borehole data from the egersund basin offshore norway are used to characterise the structural style and determine the timing of growth of inversion related anticlines adjacent to a segmented normal fault system two thick skinned normal faults which offset permian clastics and evaporites delineate the north eastern margin of the basin these faults strike nnw sse have up to 1900 m of displacement and are separated by an ese dipping c 10 km wide relay ramp both of these faults display exclusively normal separation at all structural levels and tip out upwards into the upper part of the lower cretaceous succession at relatively shallow structural levels in the hangingwalls of these faults a series of open low amplitude fault parallel anticlines are developed these anticlines which are asymmetric and verge towards the footwalls of the adjacent faults are interpreted to have formed in response to mild inversion of the egersund basin the amplitude of and apparent shortening associated with the anticlines vary along strike and these variations mimic the along strike variations in throw observed on the adjacent fault segments we suggest that this relationship can be explained by along strike changes in the propensity of the normal faults to reactivate during shortening wider damage zones and lower angles of internal friction coupled with higher pore fluids pressures at the fault centre mean that reactivation is easier at this location than at the fault tips or in the undeformed country rock seismic stratigraphic analysis of growth strata indicate that the folds initiated in the latest turonian to earliest coniacian c 88 6 ma and santonian c 82 6 ma the control on this c 6 myr diachroneity in the initiation of fold growth is not clear but it may be related to strain partitioning during the early stages of shortening anticline growth ceased in the maastrichtian and the inversion event is therefore interpreted to have lasted at least c 20 myr this study indicates that 3d seismic reflection data is a key tool to investigate the role that normal fault segmentation can play in controlling the structural style and timing of inversion in sedimentary basins furthermore our results highlight the impact that this structural style variability may have on the development of structural and stratigraphic hydrocarbon traps in weakly inverted rifts

filling of flexural moats around large volcanoes on venus implications for volcano structure and global magmatic flux the absence of topographic moats and concentric normal faulting around large volcanoes on venus is attributed to filling of the annular flexural depression by lava flows from the central edifice large volcanoes on venus are characterized by prominent approximately conical edifices surrounded by relatively flat flow aprons the surfaces of both of these constructional components consist dominantly of radially oriented flows from analytic plate flexure models we generate a synthetic stratigraphy for venus volcanoes from which we calculate the volume of material filling the flexural moats to the level of the flow apron the total volume of volcano associated extrusive lavas including moat fill can be an order of magnitude greater than the volume of the edifice alone extended to all large volcanoes on venus this procedure yields estimates for magmatic flux on venus comparable to the present terrestrial intraplate extrusive flux but this scenario is nonetheless consistent with observations of areally limited resurfacing since the most recent global resurfacing event the absence of flexure related tectonic features around most large volcanoes by the arguments advanced here is attributable to masking by apron flows and low stresses in the apron moat fill on venus a volcanic edifice and associated moat filling material constitute a single structurally coherent unit large volcanoes on venus thus appear to be structurally distinct from those on earth and mars

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

major dextral transcurrent displacements along the northern rocky mountain trench and related lineaments in north central british columbia the northern rocky mountain trench and a number of other prominent lineaments along and east of the eastern margin of the intermontane belt mark faults along which dextral transcurrent movements have been dominant offsets of shelf to off shelf facies boundaries in lower paleozoic rocks indicate a cumulative displacement of at least 750 km and probably 900 km within the system of faults related to those in the northern rocky mountain and the tintina trenches farther west another system of faults appears to offset plutons and stratigraphic assemblages along the eastern margin of the intermontane belt by as much as 300 km these faults including the kutcho and the pinchi connect in part with the teslin suture zone in yukon territory and probably with the fraser river straight creek fault zone in southern british columbia although dextral transcurrent faulting may have taken place between the middle jurassic and early cenozoic the most convincing evidence points to middle cretaceous and particularly to early cenozoic eocene displacements the eocene movements were temporally related to plutonism volcanism lamprophyre dike emplacement high heat flow sedimentation in grabens and rapid uplift of northwesterly trending elongate ranges climactic episodes of granite emplacement particularly in and near the northern omineca crystalline belt at 100 m y 70 m y and 50 m y ago may have been facilitated by changes from dominantly compressional to dominantly transcurrent and related tensional strain

antler elastics in the canadian cordillera during late devonian through mid mississippian time thick sequences of elastics flooded the canadian cordilleran miogeocline chert conglomerate chert quartz sandstone and pebbly mudstone of the westerly derived earn sequence formed local massive channel deposits up to 200 m thick on submarine fans and in narrow grabens within a broad region of shale deposition extension and local contraction of the miogeocline during rifting or strike slip outboard of present preserved limits can explain 1 the derivation of earn elastics from older miogeoclinal units 2 local alkaline volcanism 3 exhalite mineralization and 4 tremendous thickness variations related to steep syndepositional faults

geodynamic evolution of the proterozoic mount isa terrain the proterozoic mount isa terrain records the effects of four periods of intraplate tectonism the c 1870 ma barramundi orogeny was characterized by a massive felsic magmatic event and global correlations suggest a physical link between australia and laurentia at this time thereafter the terrain underwent an extensional history spanning 200 ma involving repeated episoded of rifting post rift subsidence and associated depositional and magmatic phases this protracted rifting history resulted in a cumulative stratigraphic thickness of up to 25km above attenuated continental crust rifting was interrupted prior to the formation of ocean crust by the compressional isan orogeny 1590 1500 ma the isan orogeny was synchronous with low pressure high temperature metamorphism and widespread metasomatism in the waning stages of shortening the mount isa terrain evolved into a wrench system characterized by an extensive network of strike slip faults the current level of exposure in this terrain provides spectacular examples of superimposed rifts basin inversion and wrench geometries developed at middle to upper crustal levels

a mechanism for episodic subduction on venus we propose a mechanism previously developed as a hypothetical cause of the initiation of subduction in the earth s mantle to describe a situation where such subduction may occur transiently at irregular intervals of time it has been suggested that tectonics on venus may be described by such a scenario in our model a subduction event is followed by resumption of high rayleigh number mantle convection below a stagnant lithosphere which thickens due to conductive cooling as it thickens differential buoyancy causes large lithospheric stresses which eventually lead to plastic failure in the upper portions of the lithosphere this plastic zone thickens faster than the lithosphere so that at some critical time it reaches the base of the lithosphere at this point the effective lithosphere viscosity decreases to that of the underlying mantle and subduction can occur we suggest that this is mechanistically consistent with the postulated venusian tectonic style

constraints on the thermal evolution of venus inferred from magellan data one interpretation of the magellan data suggests that the cratering record on venus was erased by a global resurfacing event or events the latest ending about 500 m y ago in this global resurfacing model the resurfacing was followed by minor volcanism and tectonism that has been concentrated primarily in the equatorial highland regions characterized by extensive fracture belts and rifts a thermal evolution model of venus that can explain these observations is one in which a deformable lithosphere capable of being incorporated in mantle circulations provides an almost stress free condition at the surface mantle convection with an almost stress free boundary at the surface cools the interior more efficiently rapid cooling decreases the rayleigh number of mantle convection below a transition value required for oscillatory convection and the vigor of convection diminishes as the mantle changes to a quasi steady circulation after about 500 m y ago

episodic large scale overturn of two layer mantles in terrestrial planets it is usually assumed that the upper and lower mantles of a chemically stratified planet are arranged so that the upper mantle is chemically less dense and that these layers convect separately possible buoyant overturn of the two mantle layers has not previously been considered such overturn would initially occur when thermal expansion of a chemically denser lower mantle more than offsets the compositional density difference between the layers reversing the relative sense of buoyancy once overturn has occurred the chemically denser but thermally less dense upper mantle cools more efficiently than the lower mantle and loses its relative thermal buoyancy if mixing is slow this leads to repeated overturns that result in thermal histories that differ radically from those obtained without this large scale overturning thermal evolution calculations for a two layer mantle over a wide range of parameter space show that large scale overturn occurs cyclically with a well defined period this period depends most strongly on the viscosity of the lower mantle to which it is approximately proportional geologically interesting overturn periods on the order of 107 to 109 years result for lower mantle viscosities of 1022 to 1024 pa s for the earth and venus and 1021 to 1023 pa s for mars the mantles of mercury and the moon are too thin to permit two layer convection and therefore the model is not appropriate for them overturn cannot occur on earth or venus if the compositional density difference between the layers exceeds about 4 or on mars if it exceeds about 2 large scale mantle overturn could have significant tectonic consequences such as the initiation of a new plate tectonic cycle on the earth or a major resurfacing event on mars or venus such episodic events in the evolution of a planet are not easily explained by whole mantle thermal convection

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

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

insights into the dynamics of planetary interiors obtained through the study of global distribution of volcanoes ii tectonic implications from venus abstract the distribution of volcanic features is ultimately controlled by processes taking place beneath the surface of a planet for this reason characterization of volcano distribution at a global scale can be used to obtain insights concerning dynamic aspects of planetary interiors until present studies of this type commonly have focused on volcanic features of a specific type e g large volcanoes in venus or hot spot volcanism on earth or have concentrated on relatively small regions i e vent distribution within individual volcanic fields but no comparison of extensive databases has been made by using the same tools in both planets in this work the description of the distribution of volcanic features observed over the entire surface of venus is made using the same tool used for earth and is applied to an extensive database the analysis is based on density contours obtained with the fisher kernel as a result several groupings of volcanoes are identified refining the already documented concentration of volcanoes on the bat zone in particular some doughnut like patterns are observed that might be related to the action of mantle plumes the occurrence of such features on earth as well as the existence of a uniform distribution of background volcanism on both planets suggests similarities on their geodynamic behavior that had not been identified previously

modes of reverse reactivation of domino type normal faults experimental and theoretical approach experiments with sand cement models show that reverse reactivation of a set of parallel faults bounding domino blocks takes place in two principal modes in mode i reactivation the domino blocks undergo counter rotation during contraction while normal faults are reactivated as reverse faults mode ii reactivation occurs by fault block translation we present a theoretical analysis of the two modes which considers forces and their rotational moments in a domino block and show that low angle normal faults are commonly reactivated in mode ii by contrast steep faults are reactivated in mode i furthermore fault reactivation depends on the initial fault spacing a large initial fault spacing opposes fault reactivation especially in mode i instead the system deforms along new thrusts dipping opposite to the normal faults we generate reactivation parameters for the conditions of the two modes as functions of the ratio r between fault spacing and effective thickness of fault horizon the fault dip the coefficient of friction and the ratio of principal stresses sr during late contraction

equilibrium resurfacing of venus results from new monte carlo modeling and implications for venus surface histories abstract venus impact crater population imposes two observational constraints that must be met by possible model surface histories 1 near random spatial distribution of 975 craters and 2 few obviously modified impact craters catastrophic resurfacing obviously meets these constraints but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2 whereas those with large incremental resurfacing areas meet constraint 2 but not 1 results of monte carlo modeling of equilibrium resurfacing strom et al 1994 is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing however the monte carlo models did not consider intermediate size incremental resurfacing areas nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing we construct three suites of monte carlo experiments that examine incremental resurfacing areas not previously considered 5 1 0 7 and 0 1 and that vary the duration of resurfacing relative to impact crater formation time 1 1 suite a 5 6 suite b and 2 3 suite c we test the model results against the two impact crater constraints several experiments met both constraints the shorter the time period of equilibrium resurfacing or the longer the time of crater formation following the cessation of equilibrium resurfacing the larger the possible areas of incremental resurfacing that satisfy both constraints equilibrium resurfacing is statistically viable for suite a at 0 1 suite b at 0 1 and suite c for 1 0 7 and 0 1 areas of incremental resurfacing

low temperature effects on flow in sand bed streams laboratory experiments were performed in a 40ft long 2ft wide 1ft deep 12 m 0 6 m 0 3 m tilting flume to elucidate the effects of temperature variations on the flow and sedimenttransport charac

evolution of large shield volcanoes on venus 1 we studied the geologic history topographic expression and gravity signature of 29 large venusian shield volcanoes with similar morphologies in magellan synthetic aperture radar imagery while they appear similar in imagery 16 have a domical topographic expression and 13 have a central depression typical dimensions for the central depression are 150 km wide and 500 m deep the central depressions are probably not calderas resulting from collapse of a shallow magma chamber but instead are the result of a corona like sagging of a previously domical volcano the depressions all have some later volcanic filling all but one of the central depression volcanoes have been postdated by geologic features unrelated to the volcano while most of the domical volcanoes are at the top of the stratigraphic column analysis of the gravity signatures in the spatial and spectral domains shows a strong correlation between the absence of postdating features and the presence of dynamic support by an underlying plume we infer that the formation of the central depressions occurred as a result of cessation of dynamic support however there are some domical volcanoes whose geologic histories and gravity signatures also indicate that they are extinct so sagging of the central region apparently does not always occur when dynamic support is removed we suggest that the thickness of the elastic lithosphere may be a factor in determining whether a central depression forms when dynamic support is removed but the gravity data are of insufficient resolution to test this hypothesis with admittance methods

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

global distribution of seamounts from ship track bathymetry data the distribution of submarine volcanoes or seamounts reflects melting within the earth and how the magma generated ascends through the overlying lithosphere globally 60 latitude we use bathymetry data acquired along 39 5 106 km of ship tracks to find 201 055 probable seamounts an order of magnitude more than previous counts across a wider height range 0 1 1 km implying that 24 000 60 remain to be discovered

small volcanic edifices and volcanism in the plains of venus the most widespread terrain type on venus consists of volcanic lowland plains several styles of volcanism are represented in the plains the most extensive volcanic units consist of flood lavas the largest of which have volumes of the order of thousands of cubic kilometers as with terrestrial flood lavas they are inferred to have erupted at high effusion rates they show a range of radar backscatter characteristics indicating different surface textures and ages small edifices on the plains occur mainly in clusters associated with fracture belts the majority are shield volcanoes that may be up to a few tens of kilometers across but are generally 10 km or less in diameter volcanic cones have the same size range volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 km3 these are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava for many domes the flanks were unstable during and afte eruption and suffered gravity sliding that produced steep scalloped outer margins because of the high atmospheric pressures on venus explosive activity is less likely to occur than on earth however n a few plains areas there is evidence of pyroclastic deposits surrounding craters indicating that volatile contents in some of the magmas may be high in comparison to earth the clusters of small volcanic edifices are considered to be analogous to plains volcanism similar to that of the snake river plain of idaho there may also be analogues with terrestrial volcanic clusters associated with mid oceanic ridges

heterogeneous stretching simple shear and basin development abstract the models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric scale shear zone are but two end members of a range of possible extensional models the homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin the shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence it is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy but will often overlap in plan view this will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural stratigraphic and geophysical techniques the model which combines heterogeneous lithospheric stretching and associated simple shear may explain 1 variations in dip of the major detachment zones in the large basins 2 variations in types of strain on or beneath the detachment zones 3 regional uplift of part of a basin to erode the earlier fault blocks and 4 the development of volcanism in basins with only low values of upper crustal extension the zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching or to one side it may be linked to the upper crustal zone by faults which dip consistently in one direction or by extensional systems which change their dominant dip direction with depth pulling out the mid crust as one or more wedges possible examples are discussed from the basin and range province and northwest european continental shelf

geometry of inverted faults and related folds in the monterey formation implications for the structural evolution of the southern santa maria basin california abstract a wide variety of mesoscopic structures observed in the monterey formation of coastal california reveal contrasting styles of deformation among mechanical units and provide a relative chronology of neogene deformation for the southern santa maria basin these structures which include ptygmatically folded veins folded beds of chert inverted normal faults fault propagation folds and axial planar breccia zones document an early extensional phase in the middle late miocene followed by two distinct episodes of contraction between the pliocene and the present miocene normal faults in interbedded carbonates and mudstones were inverted resulting in geometries that include normal faults truncated by bedding plane detachments low angle thrusts and thrust duplexes and normal faults reactivated in reverse fault block geometry drag folds and culmination folds are characteristic features that help identify inverted structures normal fault inversion coincides with the development of early chert folds and related structures higher in the stratigraphie section representing a regional phase of layerparallel contraction the second phase of regional contraction resulted in the development of a fold and thrust belt which in the monterey formation is manifested by detachment and fault related folds aligned parallel to regional fold axes effects of silica diagenesis contribute to the development of mesoscale structures throughout the deformation history of the monterey formation

bank erosion processes in a cool temperate environment orwell lake minnesota orwell lake in west central minnesota is a flood control water management reservoir first impounded in 1953 subsequent concern about erosion of the shoreline and a lack of knowledge of slope erosion processes in this part of the upper midwest prompted this study to identify and quantify the processes there the processes were measured at selected sites between june 1980 and june 1983 erosion of the banks is predominantly by thaw failure and secondarily by rainsplash and sheetwash in winter the upland surface adjacent to the lake freezes to a depth of between 1 and 2 m depending on surface temperature snow cover and distance from exposed banks in late winter soil aggregates released by the sublimation of interstitial ice within the banks begin to accumulate at the base of the slopes often veneering snowbanks as soon as thaw begins slab failure of bank sediment is followed by mudflows and earthflows thaw failure at orwell lake in the spring of 1982 accounted for 89 of the erosion 824 mg the following spring 78 746 mg such slope failure is most intense along north facing banks and considerably less intense on south facing banks where more effective desiccation and sublimation reduce soil moisture content summer rainsplash and sheetwash were responsible for 11 22 of the total erosion amounting to 102 mg in 1981 and 208 mg in 1982 although rainsplash is the most consistent process during summer the occasional storm during which rainwash occurs causes greater total erosion erosion by rain increased in each of the three summers largely because of increased precipitation infrequent massive deep slip failures slumps have occurred at the east end of the lake where a buried clay rich unit is stratigraphically and topographically positioned to favor such failures drought years followed by heavy spring rains probably will result in additional deep slip slope failures

an assessment of the age and tectonic setting of volcanics near the base of the windermere supergroup in northeastern washington implications for latest proterozoic earliest cambrian continental separation mafic metavolcanics of the huckleberry formation in northeastern washington occur near the base of the windermere supergroup a sequence of immature clastic rocks thought to have been deposited dur

morphometry of small shield volcanoes on venus implications for the thickness of regional plains the morphometry of small volcanic shields on venus is used to assess stratigraphic relationships and the thickness of regional volcanic plains comparison of shield width and height in three different regions supports the interpretation that plains with shields form a distinctive stratigraphic unit psh that is subsequently embayed and flooded by younger regional plains with wrinkle ridges pwr leaving lava shield kipukas among the plains near the contact differences in shield morphometry in the two units show that pwr averages less than 150 m in these three embayed areas and where the underlying shields disappear the minimum thickness of pwr is likely to be in excess of 400 m this technique can be applied elsewhere on venus in order to provide improved estimates of the total volume of regional plains

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

venus crater distribution and plains resurfacing models detailed analysis of the distribution of craters on venus using mth nearest neighbor analysis coupled with models based upon surface morphology constraints indicates that the hypothesis of complete spatial randomness csr cannot be rejected but is not a unique model of the observed crater distribution based on morphologic mapping the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0 5t the mean surface age of the planet this four unit plains model along with its derivatives produce test statistics that indicate such models also cannot be rejected further the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis model corresponds to venus is true is lowest for the csr model there is no particular reason to pick a csr model along with its implications for catastrophic resurfacing as a constraint on the evolution of venus and there are geological reasons to choose the multiage models we find that we cannot distinguish statistically among models that have two three or four distinct production ages within the plains however the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons first the binomial probability that such a process could exist within each of the plains units is 0 05 except the smallest and youngest unit pl1 second using a chi squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10 4 indicating confident rejection of the hypothesis thus csr cannot be used as a constraint on models of resurfacing or planetary evolution of venus because of the non uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age geological and geophysical constraints provide our best clues for understanding venus

the geologic history of venus a stratigraphic view on the basis of regional and global stratigraphic analyses we outline the major events in the geologic history of venus determined by photogeological study of surface features because the morphological signatures of terrain emplaced prior to the time of tessera formation are not preserved the stratigraphic record presented comprises only the last 10 20 of the total history of venus the estimated range of the mean crater retention age of the surface from 200 to 1600 million years leads us to describe the timing and duration of different events in terms of fractions of the mean surface age t the beginning of the observed history of venus was characterized by intensive tectonic deformation of global or semi global scale which formed the tessera terrain termination of the compressional stage is estimated to have occurred at about 1 4t while the tensional stage lasted for another 0 1 0 2t after tessera formation several stages of extensive volcanism occurred burying vast areas of tessera and forming what are now observed as regional plains the combined duration of the emplacement of these plains is estimated to be about 0 2 0 3t with an implied average global rate of volcanism of a few cubic kilometers per year regional plains forming materials can be subdivided and are separated from each other and from underlying and overlying units by unconformities these unconformities are formed from oldest to youngest by tessera forming deformation dense fracturing broad ridging and finally wrinkle ridging these tectonic episodes are interpreted to be generally globally synchronous and to represent successive episodes characterized by the dominance of compression then tension then again compression and finally tension the last global scale tectonic episode extensive wrinkle ridging happened at about time t which was very close in time to the emplacement of the most areally abundant plains unit this marked the transition to the present stage of the history of venus which is characterized by a predominance of regional rifting and related volcanism this stage appears to have lasted from about time t to the present making it the longest time duration among the stratigraphic units considered although the resulting tectonic and volcanic features and deposits cover only 10 20 of the surface of venus these observations mean that the general intensity of tectonics and the flux of volcanism a few tenths of a cubic kilometer per year in this latest period were much lower than those in earlier times in summary the morphologically observable part of the history of venus was characterized by two key characteristics that stand in contrast to the comparable period of earth history approximately the phanerozoic when global geodynamic processes were dominated by plate tectonics 1 venus shows no signature of plate tectonics instead its global tectonic environment passed from an initial dominance of compression through tension then again compression and finally tension with the density of deformational structures and the strain rate declining with time 2 in the beginning of this period of time on venus plains forming volcanism occurred at a rate comparable to volcanism at mid ocean ridges but was emplaced in an entirely different style the predominant component of volcanism on earth during this time was the extrusive volcanism at mid oceanic ridges for the last few hundred million years venus has been dominated primarily by rift associated volcanism emplaced at a production rate comparable to or even lower than present intraplate volcanism production rates on earth

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

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

the potential role of non newtonian rheology in the resurfacing of venus several analyses of the impact cratering record on venus have concluded that 80 of venus was resurfaced sometime in the last 300 m y 700 m y the cratering record further indicates that the resurfacing took place over a 30 m y 100 m y interval since this period there has been little large scale tectonic activity on venus this study proposes that such behavior may result from the interaction of phase change induced avalanches with a non newtonian lithosphere the mechanism proposed here suggests that because of non newtonian rheology the venusian lithosphere becomes highly mobile during brief periods when avalanches occur afterwards due to the reduction in stress levels the lithosphere becomes rigid and large scale tectonics ceases

the inverted triassic rift of the marrakech high atlas a reappraisal of basin geometries and faulting histories the high atlas of morocco is an aborted rift developed during the triassic jurassic and moderately inverted during the cenozoic the marrakech high atlas with large exposures of basement and triassic early syn rift deposits is ideal to investigate the geometries of the deepest parts of a rift constituting a good analogue for pre salt domains it allows unraveling geometries and kinematics of the extensional and compressional structures and the influence that they exert over one another a detailed structural study of the main triassic basins and basin margin faults of the marrakech high atlas shows that only a few rift faults were reactivated during the cenozoic compressional stage in contrast to previous interpretations and emphasizes that fault reactivation cannot be taken for granted in inverted rift systems preserved extensional features demonstrate a dominant dip slip opening kinematics with strike slip playing a minor role at variance to models proposing a major strike slip component along the main basin bounding faults including faults belonging to the tizi n test fault zone a new middle triassic paleogeographic reconstruction shows that the marrakech high atlas was a narrow and segmented orthogonal rift sub perpendicular to the main regional extension direction which was nw se in contrast to the central and eastern segments of the atlas rift which developed obliquely this difference in orientation is attributed to the indented ouzellarh precambrian salient part of the west african craton which deflected the general rift trend in the area evidencing the major role of inherited lithospheric anisotropies in rift direction and evolution as for the cenozoic inversion total orogenic shortening is moderate 16 and appears accommodated by basement involved large scale folding and by newly formed shortcut and by pass thrusting with rare left lateral strike slip indicators triassic faults commonly acted as buttresses

transpressional inversion in an extensional transfer zone the saltacaballos fault northern spain abstract this paper deals with an extensional transfer zone and its main structure the saltacaballos fault developed during early cretaceous times and obliquely inverted during the alpine shortening geological mapping and structural analysis were carried out to determine the kinematic history of the deformation the saltacaballos ridge is the relay zone between two major and conjugate normal fault systems the bilbao and the la granja samano fault systems three distinct phases in the structural evolution of this transfer zone can be identified 1 during the initial stages of extensional deformation shallow water marine limestones were deposited on the relay ridge and deep water marls on the flanking depocenters at the same time submarine landslides developed on paleoslope generated either by normal drag or by a fault propagation fold related to the saltacaballos normal fault 2 with increasing displacement normal faults antithetic to the saltacaballos fault were developed probably in response to the collapse of its hanging wall 3 during the subsequent alpine compressional stage these normal faults were reactivated as dextral strike slip faults as a result of transpressional inversion whereas previous extensional fault related folds were tightened at the same time some normal faults were overturned as a consequence of passive rotation in fold limbs

the frequency area distribution of volcanic units on venus implications for planetary resurfacing the areas of volcanic units on venus have been measured on the 1 5000000 geological maps published by nasa usgs these data were used to obtain a frequency area distribution the cumulative frequency area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit 30 106 km2 to the smallest 20 km2 the probability distribution function has been calculated the medium and large volcanic units correlate well with a power law fractal relation for the dependence of frequency on area with a slope of 1 83 there are fewer small units than the expected values provided by the power law relation our measurements cover 21 02 of the planetary surface 3 59 of the study area was found to be tessera terrain and is excluded from this study of volcanism the measurements were restricted to areas where geological maps have been published the analysis was performed on two independent areas of the planet with a complete coverage of published maps in both areas the largest volcanic unit covers a significant portion of the surface 58 75 and 63 64 respectively for the total measured volcanic units excluding tessera these two largest units that could correspond to the same unit or not cover the 61 18 and they are stratigraphically superimposed on older volcanic units which cover 3 37 of the area the remaining area 35 45 is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit s these results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of venus and different criteria for geological mapping despite this fact the presence of these very large units is incompatible with the equilibrium resurfacing models because their generation at different ages would destroy the crater randomness our frequency area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit s followed by a decay of volcanism our data for the frequency area distribution of volcanic units provide new support for catastrophic resurfacing models it is difficult to make our observations compatible with equilibrium steady state resurfacing models

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

ice influences on channel stability insights from missouri s fort peck reach this paper presents insights from a comprehensive study of river ice influences on alluvial channel bathymetry and stability the study entailed unique wintertime fieldwork along the fort peck reach of the missouri river the insights show how ice formation presence and breakup can influence channel stability in several important ways especially when channels must convey substantial water flow during winter ice may hasten the migration of channel bends cause transient scour and sediment deposition during winter and induce cyclic shifts of flow thalweg through sinuous braided subreaches the insights are of direct significance for engineering activities along the missouri s fort peck reach and other alluvial channels subject to frigid winters they also are significant for understanding the frigid winter habitat of aquatic life in such channels

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

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