buried topography of utopia mars persistence of a giant impact depression knobs partially buried craters ring fractures and some mesas permit a qualitative determination of the topography buried beneath younger northern plains materials these features are widely distributed in the utopia area but are absent in a large roughly circular region centered at about 48 degree n 240 degree w this implies the existence of a circular depression about 3 300 km in diameter buried beneath utopia planitia that is here interpreted to represent the central part of a very large impact basin the presence of buried curved massifs around part of this depression and a roughly coincident mascon lend further support present topography areal geology and paleotopography of buried surfaces all point to the persistence of this major depression for almost the entire history of mars

precambrian basement geology of the permian basin region of west texas and eastern new mexico a geophysical perspective because most of the permian basin region of west texas and southern new mexico is covered by phanerozoic rocks other means must be found to examine the precambrian upper crustal geology of the region we have combined geologic information on the precambrian from outcrops and wells with geophysical information from gravity and magnetic surveys in an integrated analysis of the history and structure of basement rocks in the region geophysical anomalies can be related to six precambrian events formation of the early proterozoic outer tectonic belt igneous activity in the southern granite rhyolite province an episode of pre grenville extension the grenville orogeny rifting to form the delaware aulacogen and eocambrian rifting to form the early paleozoic continental mar in two geophysical features were studied in detail the abilene gravity minimum and the central basin platform gravity high the abilene gravity minimum is shown to extend from the delaware basin across north central texas and is interpreted to be caused by a granitic batholith similar in size to the sierra nevada batholith in california and nevada this batholith appears to be related to formation of the southern granite rhyolite province possibly as a continental margin arc batholith because of this interpretation we have located the grenville tectonic front southward from its commonly quoted position closer to the llano uplift middle proterozoic mafic intrusions are found to core the central basin platform and the roosevelt uplift these intrusions formed at about 1 1 ga and are related in time to both the mid continent rift system and the grenville orogeny in texas because these features are likely to be rift related they suggest that the concept of a delaware aulacogen needs to be revised only to the extent that the rifting is proterozoic in age not eocambrian precambrian basement structures and changes in lithology have influenced the structure and stratigraphy in the overlying permian basin and thus have potential exploration significance interpretation of the gravity and magnetic data with geologic information also leads us to suggest the existence of pre ellenburger basins which may be extensive and of potential exploration interest

detailed crustal thickness variations beneath the illinois basin area implications for crustal evolution of the midcontinent we present high resolution imaging results of crustal and upper mantle velocity discontinuities across the illinois basin area using both common conversion point stacking and plane wave migration methods applied to p wave receiver functions from the earthscope oiink ozark illinois indiana and kentucky experiment the images reveal unusually thick crust up to 62 km throughout the central and southeastern illinois basin area a significant moho gradient underlies the nw trending ste genevieve fault zone which delineates the boundary between the illinois basin and ozark dome relatively thinner crust 45 km underlies most of the precambrian highlands surrounding the illinois basin and beneath the rift related structures of the reelfoot rift and the rough creek graben we consider four hypotheses to explain the presence of thick crust under the central and southeastern illinois basin crustal thickening may have been produced 1 prior to its accretion to north america around 1 55 ga and is an inherited characteristic of this crustal province 2 by underthrusting or shortening during proterozoic convergent margin tectonics around 1 55 1 35 ga 3 by late precambrian magmatic underplating at the base of older crust associated with the creation of the eastern granite rhyolite province around 1 3 ga and 4 through crustal relamination during an episode of proterozoic flat slab subduction beneath the illinois basin possibly associated with the grenville orogeny

complex evolution of paleolacustrine systems on mars an example from the holden crater 1 the 150 km wide holden crater lies in an area characterized by high density of valley networks implying conditions conducive to forming of water related environments we undertook geological mapping and a stratigraphic survey in order to probe the evolution of water related landforms and their paleoenvironmental implications our investigations lead us to propose that the holden area was subjected to a wet lacustrine phase of hesperian age and an icy phase during the amazonian deltaic coastal and lacustrine environments occurred during the wet phase some displaying a cyclic depositional pattern presumably related to autogenic processes water was delivered to the basin by the uzboi vallis and by surface runoff channels from a series of drainage basins along the crater walls fan delta geometries and coastal onlap enabled estimation of major water levels two levels of major stand of the water have been recognized possibly reflecting allogenic controls geologic units related to this wet lacustrine phase were subsequently eroded by glacial abrasion and plucking and were disconformably overlain by glacial deposits of amazonian age defining an icy phase these features are consistent with a warm based glacier entering the holden crater through the wide uzboi vallis to form a proglacial lake in the central part of the crater changes in sedimentary units reflect changes of depositional environments probably connected with climatic variation

evidence for late middle proterozoic extension in the precambrian basement beneath the permian basin through an integrated interpretation of gravity magnetic seismic reflection and well data we are able to define the geometry lateral extent and tectonic setting of late middle proterozoic mafic rocks the pecos mafic intrusive suite in the basement of the permian basin of west texas and eastern new mexico in this region seismic reflection data are characterized by subhorizontal layered reflectivity within basement that can be tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram in places the intrusion forms a sill and the base of the sill is imaged in the reflection data integration of the seismic interpretation with gravity models shows that the pecos mafic intrusive suite is 3 10 km thick and 50 100 km wide the overall structure of the suite is that of a sill with a locally dikelike keel north south trending gravity and magnetic anomalies associated with the pecos mafic intrusive suite suggest that it is composed of four large intrusive centers which cover a minimum area of 9200 km2 making it one seventh the size of the bushveld complex and one of the largest layered mafic intrusions in the world we interpret this massive intrusion to be associated with collisional rifting in the foreland of the grenville orogeny

seismic reflections from the basal contacts of batholiths although seismic reflection data in batholithic terranes are scarce datasets available to us from three different regions show surprising similarities all three profiles one across precambrian rocks in texas and two across mesozoic batholiths in california and nevada show strong subhorizontal reflections at depths of 6 10 km these reflections are interpreted to originate from the base of the batholiths and indicate that these batholiths are tabular in shape the large amplitudes of the reflections require a marked contrast in acoustic impedance product of velocity and density between the batholith and the underlying rocks two of the profiles show reflective zones having appreciable thicknesses 0 5 2 5 km while the third profile shows a single continuous reflection the four most favored candidates for sub batholithic reflections are 1 underplated gabbro 2 cumulate layers 3 migmatites and 4 thrust faults cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance

topography within circular grabens implications for polygon origin utopia planitia mars 1 very large fracture bounded polygons occur in acidalia and utopia planitiae mars two models for the origin of these large polygons postulate drape folding of a sedimentary cover material over an uneven buried surface with the strain enhancement needed to explain the large size due to differential compaction throughout the polygonal terrain are circular grabens inferred to overlie buried crater rims drape folding models predict that 1 these circular grabens will bound topographic lows and 2 the surface relief and percent compaction within circular grabens will be proportional to ring diameters relief and compaction within 8 circular grabens have been determined by gridding mola data in arcview all 8 fulfill the first prediction and 7 of them fulfill the second prediction these results support a sedimentary origin for polygonal terrain materials and thus are consistent with the past existence of an ocean in the martian lowland

transitional morphology in west deuteronilus mensae mars implications for modification of the lowland upland boundary abstract west deuteronilus mensae which lies along the lowland upland boundary of mars at 45 latitude 345 longitude exhibits both gradational l a rossbacher 1985 in models in geomorphology woldenberg ed pp 343 372 allen unwin boston and fretted terrain r p sharp 1973 j geophys res 78 4073 4083 boundary types to the west the boundary is gradational and to the east it is defined by fretted terrain the two types of boundaries do not merge with one another within the west deuteronilus mensae region however the gradational boundary materials appear to overlap the fretted terrain by about 500 km contacts between units associated with the gradational boundary and the fretted terrain boundary can both be recognized in the region of overlap the fretted terrain can be identified as much as 300 km north of the gradational boundary in this region lowland units associated with the gradational boundary embay canyons of the fretted terrain in a topographically conformal fashion changes in the fretted terrain across the gradational boundary from uplands to lowlands include a reduction of canyon wall slopes and depths such that the fretted terrain north of the gradational boundary appears mantled but not obscured there are at least two major classes of processes which might explain the lateral overlap 1 erosion of stratified upland terrain and 2 deposition of plains materials onto the sloping upland margin and fretted terrain erosion of stratified upland terrain does not adequately explain the plainward decrease in crater densities across the gradational boundary nor is it consistent with evidence that lowland plains material appears to onlap the sloping upland margin in several places of the possible plains emplacement mechanisms eolian deposition would not produce the sharp apparently topographically conformal gradational unit contacts volcanics plains emplacement would not preserve the complex geometry of the underlying fretted terrain sediment deposition in either a liquid or ice covered sea could produce the draped appearance of the fretted terrain outflow channels along the lowland upland boundary particularly those of the circum chryse and west elysium regions may have flooded the northern lowlands to depths of tens to hundreds of meters the gradational unit contacts may represent the shorelines of such a sea the scale of characteristic morphologies along these contacts may require an unfrozen condition of sufficient duration to allow lacustrine style wave erosion and redistribution of material along the contacts our limited understanding of the martian paleoclimate and h 2 o inventories allows the possibility of clement periods in the past and other geological evidence e g small valley networks and outflow channels strongly suggests an extensive role of liquid water

coastal geomorphology of the martian northern plains the paper considers the question of the formation of the outflow channels and valley networks discovered on the martian northern plains during the mariner 9 mission parker and saunders 1987 and parker et al 1987 1989 data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains it is suggested that of the geological processes capable of producing similar morphologies on earth lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on mars

an ancient rift complex and its relation to contemporary seismicity in the new madrid seismic zone gravity magnetic geologic and seismicity data have been combined in a seismotectonic analysis of the new madrid seismic zone previous studies have presented evidence for several rift zones in this area upper mississippi embayment including the reelfoot rift a late precambrian early paleozoic failed arm which extends n ne from the ancient continental margin in this study we propose that the northern terminus of the reelfoot rift forms a rift complex which appears to correlate well with the seismicity in the area this correlation suggests that faults associated with this rift complex are being reactivated in the contemporary stress field ene compression if this interpretation is valid it represents a seismotectonic model which can be used to predict the extent of future seismicity in the new madrid seismic zone the proposed rift complex also provides a coherent model for the tectonic development of this region of the north american midcontinent

the control of stress history and flaw distribution on the evolution of polygonal fracture networks abstract we use boundary element models of fracture propagation and linkage to investigate the factors controlling the development of two dimensional multi directional polygonal fracture networks characterised by a large number of abutting intersections between fractures the position and orientation of a number of fracture seeds are prescribed in the model which propagate when the applied stress reaches a critical value according to linear elastic fracture mechanics theory the applied boundary condition is a remote isotropic horizontal tension where the stress is increased at a steady rate throughout each model to simulate continued fracture growth realistic polygonal systems are developed with the boundary element model simulations which are comparable with those observed in natural systems such as those found within eocene and oligocene mudrocks in the north sea and on the surface of mars if conditions exist where a small number of fracture seeds propagate and develop significant structures before others then these will dominate the resulting fracture network geometry not only do such early structures represent the largest fractures in the system they also significantly modify the stress field around them preventing some other seeds from developing and influencing the propagation paths of nearby fractures fracture seeding distribution and the rate at which the stress is increased are found to be the most significant parameters affecting the development of fracture network geometry these results suggest that the geometry of evolving fracture networks should be considered not only in terms of the mechanical properties of the deforming material but also in terms of the stress rate driving deformation

oceans on mars an assessment of the observational evidence and possible fate the edges of the northern plains and interpreted to be shorelines remaining from these former standing bodies of water we examine the elevations and geologic relations along these contacts in detail and find little support for their interpretation as shorelines some contacts are clearly of volcanic origin and all have significant variations in elevation better support for the former presence of water over large parts of the northern plains is provided by the vastitas borealis formation vbf most of the post noachian fill within the northern basin is ridged plains of lower hesperian age interpreted to be volcanic in origin overlying the ridged plains is the vbf a thin veneer of material of upper hesperian age the vbf may have been deposited from large floods support for this interpretation is the similarity in age between the outflow channels and the vbf the presence of the vbf at the lower ends of the outflow channels and identification of numerous features in the outcrop areas of the vbf that are suggestive of basal melting of an ice sheet to cover all the area over which the vbf is exposed would require 2 3 10 7 km 3 of water spread over the entire surface of mars this volume is equal to a global layer global equivalent layer or gel 156 m deep we find no support for the larger estimates of ocean volumes that range up to 3 10 8 km 3 and which imply comparable amounts of water per unit area as are currently present on the surface of the earth under present climatic conditions on mars an ocean would freeze in a geologically short time period 10 4 years then would sublimate away at rates strongly dependent on the presence or absence of debris on the ice surface the present vbf is interpreted as a sublimation residue from the ponded outflow channel effluents the fate of a volume of water thought to have been emplaced by the outflow channels 2 3 10 7 km 3 is largely accounted for by the presence of other existing reservoirs on the planet an approximately 20 30 m gel of water is estimated to be in the present polar caps and a 50 m gel may have escaped to space since the hesperian leaving 80 m gel unaccounted for this amount may be partly trapped in other volatile rich deposits on the surface and a significant amount could have reentered the groundwater system by south polar basal melting and been progressively cold trapped at the base of a growing cryosphere on the basis of our assessment of the hesperian aged deposits we predict that testing of the clifford and parker 2001 hypothesis that a noachian aged ocean covered up to one third of the surface of mars will be made very difficult by the enhanced degradation rates in the noachian and subsequent geological events in the northern lowlands index terms 6225 planetology solar system objects mars 4267 oceanography general paleoceanography 5407 planetary sciences atmospheres evolution 5470 planetary sciences surface materials and properties 1635 global change oceans 4203 keywords mars oceans channels shorelines hesperian

evidence for large reservoirs of water mud in utopia and acidalia planitiae on mars abstract utopia and acidalia planitiae show a set of similar morphologic features indicative of the presence of large reservoirs of water mud in these regional topographic lows the most important of these features are 1 impact crater ejecta morphology that progressively changes from ballistic rampart at the basins periphery to strongly degraded pancake like in the central portions of the basins and 2 etched flows interpreted as mudflows that are concentrated in those regions of the central portions of both basins where the pancake like ejecta prevail the vastitas borealis formation vbf in utopia and acidalia planitiae represents the uppermost stratigraphic limit of possible reservoirs in both basins and has practically the same age in these regions 3 57 0 02 ga in utopia planitia and 3 61 0 05 0 08 ga in acidalia planitia the southern contact of the vbf closely follows specific contour lines in utopia planitia 3 6 km 54 m over a distance of 1500 km and acidalia planitia 3 9 km 70 m over the distance of 6000 km the presence of the large water mud reservoirs and the age and topographic configuration of the vbf in utopia and acidalia planitiae are consistent with the existence of an extensive single body of water an ocean in the northern plains of mars

the role of transfer structures on volcanic activity at campi flegrei southern italy abstract the tyrrhenian margin of central italy underwent extension during pliocene and quaternary extension occurred mainly through nw se normal faults bordering a sequence of plio quaternary basins these basins are offset by coeval ne sw faults which show strike slip and normal motions and have been interpreted as transfer faults plio quaternary volcanic activity along the margin occurred along a nw se belt systematically in correspondence with ne sw transverse systems the campi flegrei volcanic district cfvd on the southern tyrrhenian margin consists of an active ne sw volcanic ridge developed along ne sw fractures we performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at campi flegrei field analysis at campi flegrei recognized ne sw and to a lesser extent coeval nw se active fractures analogue experiments have simulated the development of transfer fault systems in brittle extensional domains the experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 60 the mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses it shows how pre existing subvertical fractures require the smallest magmatic pressures to be penetrated for a given magmatic pressure subvertical fractures might be penetrated more deeply tapping more easily primitive magmas these results suggest that the cfvd is located along a ne sw transfer zone connecting nw se regional normal faults volcanic activity along such ne sw trend would be induced by the subvertical dip of the transfer faults the subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along ne sw systems at campi flegrei these considerations find a wider application on the remaining volcanic districts of the margin located in the same overall structural setting

stealth quasi circular depressions sqcds in the northern lowlands of mars 1 while qcds are quasi circular depressions revealed in the mars orbiter laser altimeter mola gridded data set stealth qcds sqcds are those qcds that have no obvious structural representation in visible images of mars it has been suggested that sqcds are surface representations of buried impact craters and on the basis of this assumption size frequency distributions of all qcds in the northern lowlands have been constructed to date the buried lowland floor expanding upon previous studies of stealth qcds around the utopia basin 446 sqcds with diameters ranging from 7 to 115 km have been identified in the northern lowlands of mars the direct relationship of sqcd surface relief to diameter indicates that stealth qcds form due to differential compaction of a cover material over buried impact craters a comparison of sqcd location to the geologic map of the northern lowlands reveals that stealth qcds are only located in geologic units predicted to be capable of differential compaction consistent with models of their formation the slope of the trend of the sqcds surface relief versus diameter has a coefficient of determination r2 approaching 1 when sqcds are evaluated by geologic unit and by regions of inferred comparable cover thickness trend slope analysis also indicates that the lowland cover material is thickest in the scandia region and the center of the borealis and utopia basins and thinnest in isidis planitia and north of arabia terra

convection in a catastrophic flood deposit as the mechanism for the giant polygons on mars a formational mechanism for the large scale polygons that are defined by intersecting fractures in the northern plains of mars is proposed in this work the presented model is based on rayleigh convection of interstitial water driven by an unstable density temperature gradient within a saturated porous medium catastrophic emplacement of water rich sediments into the martian northern plain basins could have provided a thick sedimentary layer in which rayleigh convection could have occurred convection within this active layer is proposed to have differentially thawed an underlying ice rich permafrost layer that had been at the planet s surface before being rapidly buried the resulting morphology of the permafrost subsurface is thought to have resembled the scalloped solid liquid interface morphology produced in terrestrial convective flow visualization studies the exact size and shape of the scallops on the frozen subsurface would have been controlled by the dimensions of the convection cells which are estimated to have had width to depth ratios of between 3 4 and 4 5 subsequent stresses e g gravity and bending stresses would have produced maximum tensile stresses in the overlying sediments preferentially above the subsurface topographic highs as described previously by mcgill and hills 1992 additional desiccation of the sediments would have required contraction of the sediment cover and would have produced large scale polygonal fractures along the preexisting weaknesses located above the raised subsurface topography thus the underlying subsurface geometric morphology would have been translated to the surface and would be represented by the large scale polygonal fractures visible in the viking orbiter and mars global surveyor images of the martian northern plains

chemical evolution of magmas in the proterozoic terrane of the st francois mountains southeastern missouri 2 trace element data the st francois mountains igneous complex of proterozoic age consists of hypabyssal granite plutons intruded into the overlying rhyolites of similar age melting and crystallization models of these rocks suggested by the field petrographic and major element contents are further refined using rb ba sr ree rare earth element th sc and co contents there are two groups of relatively undifferentiated plutons associated with the butler hill caldera and taum sauk area that could have formed by partial melting of lower crustal rocks the plutons associated with the butler hill caldera knoblick and silvermine granites could have formed by about 30 aggregate melting of a quartz diorite graywacke or subgraywacke the plutons associated with the taum sauk area could have formed by about 30 equilibrium melting of a subarkose or arkose more silicic rocks of the butler hill caldera could have formed by fractional crystallization the silvermine granite is internally zoned crystallization of plagioclase biotite hornblende magnetite apatite zircon sphene in a ratio of 0 70 0 10 0 15 0 04 0 005 0 003 0 007 from the least differentiated portion of the silvermine granite could have formed intermediate portions of the silvermine further crystallization of the silvermine requires more biotite ba begins to decrease and less hornblende zircon and sphene hree increase to precipitate and form the most differentiated portions also fractional crystallization of undifferentiated silvermine type magmas could have produced the sequence of butler hill breadtray graniteville granites as residual liquids respectively the early crystallizing mineral assemblage is dominated by plagioclase and biotite while the late crystallizing mineral assemblage is dominated by quartz k feldspar and lesser plagioclase the gneissoid granodiorite in the hawn park area has the mineral assemblage and composition of a cumulate derived from the silvermine granite type differentiation sequence the associated volcanic rocks could not have formed from melts of exactly the same composition of any of the associated intrusive rocks rather the volcanic rocks formed from melts of slightly different chemistry

mud volcanism and morphology of impact craters in utopia planitia on mars evidence for the ancient ocean abstract results of our detailed geological mapping and interpretation of the nature and relative and absolute model ages of units and structures in the sw portion of utopia planitia 20 45 n 100 120 e suggest the following 1 the size frequency distribution sfd of craters that both are buried by materials of the vastitas borealis units vb and superpose its surface indicate that the absolute model ages of terrain predating the emplacement of the vb is 3 7 ga 2 lack of craters that are partly embayed by materials of the vb in the sw portion of utopia planitia implies that the emplacement of the vb was faster than the rate of accumulation of impact craters and is consistent with the geologically short time of emplacement of the vb due to catastrophic release of water from outflow channels e g carr m h 1996 water on mars oxford university press new york p 229 3 the sfd of craters that superpose the surface of the vb indicates an absolute model age of 3 6 3 5 ga the absolute model ages of etched flows which represent the upper stratigraphic limit of the vb are estimated to be 3 5 ga 4 the majority of the larger i e 1 km impact craters show ejecta morphologies rampart and pancake like ejecta that are indicative of the presence of ice water in the target materials the distal portions of the pancake like ejecta are heavily degraded not due to embayment this suggests that these craters formed in targets that contained higher abundances of volatiles 5 the diameter ranges of the craters with either rampart or pancake like ejecta are overlapping from 2 to 60 km craters with pancake like ejecta are concentrated within the central portion of the utopia basin less than 1000 km from the basin center and rampart craters occur at the periphery of the basin this pattern of the crater spatial distribution suggests that materials within the center of utopia planitia contained more ice water 6 etched flows around the central portion of utopia planitia were erupted from beneath of the surface of the vb their morphology and pattern of degradation however are inconsistent with lava and instead indicate formation of the flows due to mud volcanism 7 etched flows are spatially associated with giant polygons and there is evidence that these features populated the center portion of utopia planitia before it was covered by the elysium derived units the outer southern edge of the zone of polygonal troughs and etched flows approximately corresponds to the transition from pancake like ejecta to rampart ejecta this suggest that the outer edge of the zone of the polygons and flows may outline the deeper portions of the large body 2000 km across of water ice that likely existed in the center of utopia planitia in late hesperian

origin of giant martian polygons extensive areas of the martian northern plains in utopia and acidalia planitiae are characterized by polygonal terrane polygonal terrane consists of material cut by complex troughs defining a pattern resembling mudcracks columnar joints or frost wedge polygons on earth however the martian polygons are orders of magnitude larger than these potential earth analogues leading to severe mechanical difficulties for genetic models based on simple analogy arguments plate bending and finite element models indicate that shrinkage of desiccating sediment or cooling volcanics accompanied by differential compaction over buried topography can account for the stresses responsible for polygon troughs as well as the large size of the polygons although trough widths and depths relate primarily to shrinkage the large scale of the polygonal pattern relates to the spacing between topographic elevations on the surface buried beneath polygonal terrane material geological relationships favor a sedimentary origin for polygonal terrane material but our model is not dependent on the specific genesis our analysis also suggests that the polygons must have formed at a geologically rapid rate

deep faulting and structural reactivation beneath the southern illinois basin abstract the investigation of deep fault structure and seismogenesis within stable continental interiors has been hindered by the paucity of detailed subsurface information and by low levels of seismicity outstanding seismotectonic questions for these areas include whether pre existing structures govern the release of seismic energy as earthquakes can reactivation of such structures be recognized and to what extent have precambrian basement structures exerted long lived controls on the development of overlying phanerozoic features the southern portion of the illinois basin provides a premier area in which to study the relation between contemporary seismicity and pre existing structures due to the frequency of seismic events the concentration of available geophysical data and the wealth of borehole information we have integrated the study of this information in order to create a 2 5 dimensional picture of the earth for local seismogenic depths 0 15 km for a study area of moderate 20th century earthquake activity the area is located along the western flanks of two of the major structures within the illinois basin the wabash valley fault system wvfs and the la salle anticlinal belt lsa the results of reprocessing seismic reflection profiles combined with earthquake hypocenter parameters suggest three distinct seismotectonic environments in the upper crust first we have delineated a fault pattern that appears to correspond to the steep nodal plane of a strike slip mechanism event 1974 04 03 mb 4 7 the fault pattern is interpreted to be a deeply buried rift zone or zone of intense normal faulting underpinning a major paleozoic depocenter of the illinois basin fairfield basin second a similar event 1987 06 10 mb 5 2 and its well located aftershocks define a narrow zone of deformation that occurs along and parallel to the frontal thrust of the lsa third the hypocenter of the largest event in the study area 1968 11 09 mb 5 5 may be spatially associated with a prominent zone of dipping middle crustal reflections just west of the wvfs which have been interpreted as a deeply buried blind thrust the proposed correlation of pre existing structures with earthquakes having consistently oriented structural parameters supports the reactivation of old deformation zones by contemporary stresses as previously proposed by earlier workers however the degree to which deformation has propagated upward from precambrian basement into the paleozoic rocks varied significantly even over a small study area the societal value of associating an earthquake with a specific pre existing deformation zone in the seismogenic crust is to improve the assessment of seismic hazard or to assess the integrity of a stratigraphic formation being considered as a target for natural gas storage or carbon sequestration

anorogenic felsic magmatism rift sedimentation and giant proterozoic pb zn deposits the giant lead deposits that developed after 1 8 ga in australia north america and africa are absent in the earlier geologic record these proterozoic lead zinc deposits were all preceded by major outbreaks of anorogenic felsic magmatism that brought significant amounts of lead rich material 30 ppm to the surface of proterozoic cratons erosion of these volcanics apparently transferred major quantities of the lead to rift basins where giant lead deposits were generated all known giant proterozoic sediment hosted lead zinc deposits more than 50 mt of lead metal are explained by a model that is supported by sm nd data relevant to mantle separation ages of volcanics and the metasedimentary source rocks for the lead ores

a cocorp deep reflection profile across the buried reelfoot rift south central united states abstract a cocorp deep reflection survey across the northern mississippi embayment reveals essential features of the late precambrian early paleozoic reelfoot rift stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness the base ranging from 2 0 to a maximum of 3 5 s twt 4 to 8 km depth in contrast the base of phanerozoic the sedimentary section immediately east of the rift is at 1 4 s 2 8 km depth correlation with a basement penetrating well indicates that the base of the upper cretaceous holocene mississippi embayment supergroup base of the cambrian ordovician knox group and the lower upper cambrian bonneterre formation are basin wide seismic stratigraphic markers clastic sedimentary strata of the lamotte formation which immediately overlie precambrian basement show clear angular discordance with the overlying subhorizontal bonneterre formation indicating that the base of the bonneterre marks the transition from active extension to passive subsidence within the reelfoot rift palinspastic restoration of the basement suggests relatively modest extension associated with rift formation 17 the blytheville arch an axial antiformal feature as well as a number of lesser structures indicative of multiple episodes of fault reactivation are also evident on the cocorp profile the crystalline basement beneath the reelfoot rift is relatively devoid of reflections down to 5 0 s 12 15 km depth beyond which it passes gradationally into a complexly reflective diffractive middle and lower crust the lower crust however does not exhibit the distinctly laminated character commonly observed in mesozoic cenozoic extensional provinces the crust mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12 13 s 40 km depth followed by an unreflective zone which in turn is followed by a distinct horizontal reflection at 14 5 s the latter is not observed on existing profiles outside the rift at present it is not possible to determine strictly whether the deep horizon represents moho or alternatively a discrete feature sill within the uppermost mantle however its spatial association with the reelfoot rift suggests that the two are genetically related regional relations together with the existence of dipping fabric in the deep crust beneath the rift suggest the possibility that the reelfoot rift formed along the southern extension of the circa 1 ga grenville front

fossil flat slab subduction beneath the illinois basin usa abstract the illinois basin is one of several well studied intracratonic sedimentary basins within the north american craton whose formational mechanisms and subcrustal structure are not well understood we study the s velocity structure of the upper mantle beneath the illinois basin and its surrounding area through seismic tomography we utilize continental scale waveform data of seismic s and surface waves enhanced by regional earthquakes located near the illinois basin our 3d tomographic model il05 confirms the existence of a slow s velocity structure in the uppermost mantle beneath the illinois basin region this anomalously slow region exists from the base of the crust to depths of 90 km and is slower than the north american cratonic average by about 200 m s this anomalous uppermost mantle beneath the illinois basin is underlain by a faster lithosphere typical of the surrounding craton to depths of 200 km excluding the formation of the reelfoot rift this area of north american has been stable for over 1 0 gy thus we do not expect thermal anomalies from before that time to persist into present day s velocity anomalies and we consider a delamination origin as an explanation of illinois basin subsidence unlikely we cannot rule out that the slow mid lithosphere beneath the illinois basin is caused by an uppermost mantle enriched by a deep but weak plume we attribute the slow mid lithosphere to the presence of either oceanic hydrous crust or a relatively cool mantle wedge with preserved hydrous minerals in the illinois basin s uppermost mantle related to a fossilized flat subduction zone

geophysical constraints on understanding the origin of the illinois basin and its underlying crust abstract interpretation of reprocessed seismic reflection profiles reveals three highly coherent layered unconformity bounded sequences that overlie or are incorporated within the proterozoic granite rhyolite province beneath the paleozoic illinois basin and extend down into middle crustal depths the sequences which are situated in east central illinois and west central indiana are bounded by strong laterally continuous reflectors that are mappable over distances in excess of 200 km and are expressed as broad basinal packages that become areally more restricted with depth normal fault reflector offsets progressively disrupt the sequences with depth along their outer margins we interpret these sequences as being remnants of a proterozoic rhyolitic caldera complex and or rift episode related to the original thermal event that produced the granite rhyolite province the overall thickness and distribution of the sequences mimic closely those of the overlying mt simon late cambrian clastic sediments and indicate that an episode of localized subsidence was underway before deposition of the post cambrian illinois basin stratigraphic succession which is centered farther south over the new madrid rift system i e reelfoot rift and rough creek graben the present configuration of the illinois basin was therefore shaped by the cumulative effects of subsidence in two separate regions the proterozoic caldera complex and or rift in east central illinois and west central indiana and the new madrid rift system to the south filtered isostatic gravity and magnetic intensity data preclude a large mafic igneous component to the crust so that any proterozoic volcanic or rift episode must not have tapped deeply or significantly into the lower crust or upper mantle during the heating event responsible for the granite rhyolite

extensional troughs in the caloris basin of mercury evidence of lateral crustal flow thirty years ago mariner 10 revealed extensional troughs that form giant polygons in the floor material of the caloris impact basin mercury the polygonal troughs occur in the interior of the basin and overprint wrinkle ridges formed in an earlier stage of compression in contrast lunar and martian basins exhibit extensional troughs that are circumferential and confined to basin margins loading by basin filling material can explain the extensional and compressional features seen in deformed lunar and martian basins but not the existence of the caloris polygonal troughs here we suggest that the caloris troughs formed from late stage basin uplift and extension due to lateral flow of a relatively thick crust toward the basin center if such lateral flow occurs the resulting timing location and magnitude of the extensional stresses predicted by our model are consistent with those inferred from the polygonal troughs these results are not strongly dependent on the degree of lateral flow or the assumed crustal rigidity for a dry plagioclase rheology and likely radiogenic heat fluxes the crustal thickness around caloris is 90 140 km similar late stage uplift and extension probably do not occur in basins on the moon and mars because their crusts are too thin to allow analogous lateral flow

possible ancient oceans on mars evidence from mars orbiter laser altimeter data high resolution altimetric data define the detailed topography of the northern lowlands of mars and a range of data is consistent with the hypothesis that a lowland encircling geologic contact represents the ancient shoreline of a large standing body of water present in middle mars history the contact altitude is close to an equipotential line the topography is smoother at all scales below the contact than above it the volume enclosed by this contact is within the range of estimates of available water on mars and a series of extensive terraces parallel the contact in many places

hydrocarbon shows and petroleum source rocks in sediments as old as 1 7 109 years we report the discovery of indigenous live oil in 1 4 gyr old rocks in the mcarthur basin of northern australia previously reported occurrences of indigenous precambrian oil are 1 gyr old1 5 potential petroleum source rocks in the mcarthur basin range in age from 1 4 to 1 7 gyr and were deposited in marine and lacustrine environments in parts of the basin they have been buried sufficiently deeply to have generated hydrocarbons they span the period corresponding to the appearance of eukaryotic organisms and because of their low degree of thermal alteration they provide a valuable resource for the study of primitive biota through their hydrocarbon biomarkers the hydrocarbon composition of the oil is consistent with a derivation from organic matter of prokaryotic origin these results show that exploration of previously ignored mid proterozoic sediments may lead to the discovery of new reserves of oil

tectonics of the jemez lineament in the jemez mountains and rio grande rift the jemez lineament is a ne trending crustal flaw that controlled volcanism and tectonism in the jemez mountains and the rio grande rift zone the fault system associated with the lineament in the rift zone includes from west to east the jemez fault zone southwest of the valles toledo caldera complex a series of ne trending faults on the resurgent dome in the valles caldera a structural discontinuity with a high fracture intensity in the ne jemez mountains and the embudo fault zone in the espanola basin the active western boundary faulting of the espanola basin may have been restricted to the south side of the lineament since the mid miocene the faulting apparently began on the sierrita fault on the east side of the nacimiento mountains in the late oligocene and stepped eastward in the early miocene to the canada de cochiti fault zone at the end of the miocene about 5 ma the active boundary faulting again stepped eastward to the pajarito fault zone on the east side of the jemez mountains the north end of the pajarito fault terminates against the jemez lineament at a point where it changes from a structural discontinuity zone of high fracture intensity on the west to the embudo fault zone on the east major transcurrent movement occurred on the embudo fault zone during the pliocene and has continued at a much slower rate since then the relative sense of displacement changes from right slip on the western part of the fault zone to left slip on the east the kinematics of this faulting probably reflect the combined effects of faster spreading in the espanola basin than the area north of the lineament abiquiu embayment and san luis basin the right step in the rift that juxtaposes the san luis basin against the picuris mountains and counterclockwise rotation of various crustal blocks within the rift zone no strike slip displacements have occurred on the lineament in the central and eastern jemez mountains since at least the mid miocene although movements on the still active jemez fault zone in the western jemez mountains may have a significant strike slip component basaltic volcanism was occurring in the jemez mountains at four discrete vent areas on the lineament between about 15 ma and 10 ma and possibly as late as 7 ma indicating that it was being extended during that time

origin of the martian global dichotomy by crustal thinning in the late noachian or early hesperian the marked dichotomy in topography surface age and crustal thickness between the northern lowland and southern upland of mars has been explained as due to an initially inhomogeneous crust a single mega impact event several overlapping large basin impacts and first order convective overturn of the martian mantle all of the published hypotheses propose that the dichotomy was formed early in martian history before the end of the primordial heavy bombardment a primordial origin is inherent in the initial crustal inhomogeneity hypothesis and required for both impact hypotheses endogenic hypotheses are not so constrained geological data indicate episodes of fracturing and faulting in the late noachian and the early hesperian this fracturing and faulting occurred primarily within the northern lowland and along the boundary between lowland and highland igneous activity also peaked in the late noachian and early hesperian these data suggest a tectonic event near the noachian hesperian boundary characterized by enhanced heat loss and extensive fracturing including formation of the faults that define much of the highland lowland boundary we argue that the major result of this tectonic event was formation of the dichotomy by thinning of the crust above a large convection cell or plume

anorogenic granites magmatic underplating and the origin of intracratonic basins in a non extensional setting abstract we present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating a case study of a deeply eroded continental rift belt in south finland shows close association of anorogenic intrusives and remnants of cratonic basins lower crustal thinning took place during anorogenic magmatism whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies combined analyses of field data petrological constraints and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning this model involves the formation of an incipient crustal dome by asthenospheric upwelling associated with basaltic underplating and migration of induced lower crustal partial melts to upper crustal levels in this model light lower crustal diorite is replaced by a denser mafic layer the consequences for the post magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification subsidence occurs in the absence of crustal extension suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin subsidence curves for this model show an incipient nearly linear fast downwarp related to crystallization of gabbro at the moho followed by slow contraction induced by solid state cooling the predicted subsidence is similar to the pattern of the vertical movement induced by passive extension in the proposed model of continental rifting the effects of extension and underplating largely cancel out concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion the model provides a consistent explanation for a number of key aspects of cratonic basins including their circular shape crustal configuration and evolution and their spatial distribution on continents

the giant polygons of utopia northern martian plains much of utopia planitia is underlain by polygonal terrane most of the troughs defining the polygons are 250 m to 1 km wide typical diameters of polygons are 4 5 km these dimensions are orders of magnitude greater than the corresponding dimensions for familiar polygonal structures on earth consequently the earth structures are not valid mechanical analogues crater and superposition relationships indicate that trough formation was coeval with deposition of the polygonal terrane material based on crater age correlations with elysium volcanism and outflow channel activity either a volcanic or a sedimentary source for this material is possible tensile stresses responsible for the troughs were produced by cooling or dessication shrinkage plus differential compaction over a rough buried surface that is the structurally disrupted remnant of the cratered highlands shrinkage alone will produce polygons orders of magnitude smaller than those present but differential compaction will reflect the scale and pattern of the buried topography and thus control the scale and pattern of the giant polygons of utopia

characteristics and origin of polygonal terrain in southern utopia planitia mars results from mars orbiter laser altimeter and mars orbiter camera data giant polygons on mars with several kilometers in diameter were first observed in mariner 9 and viking orbiter images and their origin formation and evolution have remained enigmatic since that time new data obtained by the mars orbiter laser altimeter mola and the mars orbiter camera moc on board the mars global surveyor spacecraft now permit analysis of topography and morphology of the polygonal terrain in unprecedented detail mola data show that 1 giant polygonal terrain in utopia is located at the lower slopes of a basin structure that is the utopia basin 2 the onset of polygonal terrain in utopia is close to the same elevation in several profiles and lies at about the same elevation as a terrace which was interpreted as a shore line of an ancient body of standing water within the utopia basin proper 3 polygonal terrain occurs over a wide range of elevations on regional slopes of 0 1 4 the depths of the troughs range from 5 m to 115 m averaging about 30 m and tend to be greater toward the center of the utopia basin 5 the mean width of polygonal troughs is of the order of 2 km ranging from 0 5 to 7 5 km data from the mars orbiter camera moc indicate that 1 the troughs are generally broad graben like features with varying morphologies i e terraces and lobateness and dimensions widths and depths 2 trough morphology has been modified by aeolian processes 3 polygonal troughs are weakly interconnected at moc resolutions and there are no smaller polygons observed within the giant polygons 4 circular depressions which are probably related to subsurface collapse sometimes occur at the bottoms of polygonal troughs 5 small scale polygonal terrain on the ejecta blankets of young craters with fluidized ejecta in the investigated area exhibits different characteristics i e smaller size presence of rims greater degree of interconnections than giant polygons in the viking images we see that 1 old impact craters are increasingly buried with younger materials toward the basin center and that 2 a large number of superposed impact craters on polygonal terrain exhibit distinctive ejecta morphologies such as rampart ejecta blankets and single and double lobate ejecta blankets mola and moc observations are consistent with the formation of polygonal terrain in the area of a former standing body of water but polygon size seems too large to be readily accounted for by desiccation or freezing processes alone on the basis of the observed characteristics we propose that the giant polygons are primarily of tectonic origin being caused by uplift of the floor of the utopia basin there are several candidates for the cause of uplift 1 the removal of the load representing former standing bodies of water water thickness estimated for the utopia basin is about 1 km in this scenario removal of this load comparable to loads placed on terrestrial continental lithosphere by the laurentide and scandinavian ice sheets by loss of the water or ice could cause uplift of the floor and polygon formation 2 the freezing and expansion of residual water buried by sediments or in the near subsurface or 3 a combination of these from the discussion of alternative models in the light of the mola and moc data we conclude that formation of giant polygons by tectonic uplift of the basin floor is most likely and deserves further investigation

buried impact craters a topographic analysis of quasi circular depressions utopia basin mars 1 many quasi circular depressions qcds revealed in the mars orbiter laser altimeter data set have no obvious structural representation in visible images of mars it has been suggested that these qcds are surface representations of buried impact craters and on the basis of this assumption cumulative count versus diameter plots of qcds in the northern lowlands have been constructed to date the buried lowland floor if the qcds are in fact buried impact craters their surface relief should scale directly with their diameter this relationship holds true for 115 qcds with diameters ranging from 7 to 100 km within roughly 1500 km of the center of the utopia basin the coefficient of determination r2 of the relationship more closely approaches 1 when qcds in zones of comparable cover thickness are evaluated the surface relief versus diameter relationship of 55 randomly selected pseudo qcds was also determined their poor correlation indicates that the direct relationship of surface relief to diameter of actual qcds is due to differential compaction of a cover material over buried impact craters and is not just a random effect of qcd diameter the slope of the trend of the qcds surface relief versus diameter when divided into zones of comparable cover thickness can be used to determine which zones have thicker or thinner cover

paucity of candidate coastal constructional landforms along proposed shorelines on mars implications for a northern lowlands filling ocean abstract the case for an ocean having once occupied the northern lowlands of mars has largely been based indirectly on the debouching of the outflow channels into the lowlands and directly on erosional features along the margins of the lowlands interpreted to be the result of wave action two global shorelines were previously mapped from albedo variation embayment relationships and scarps interpreted as coastal cliffs however not since the early viking based studies has there been a focused assessment of the presence or absence of coastal constructional landforms such as barrier ridges and spits located on or near the mapped shorelines such constructional landforms are typically found in association with coastal erosional features on earth and therefore warrant a detailed search for their presence on mars all presently available themis vis and moc na images located on or near either of the two shorelines within the chryse planitia arabia terra region 10 to 44 n 300 to 0 e and the isidis planitia region 0 to 30 n 70 to 105 e were examined in search of any features that could reasonably be considered candidate coastal ridges additionally raw mola profiles were used in conjunction with a technique developed from differential global positioning system profiles across terrestrial paleo shorelines to search for coastal ridges throughout these same regions out of 447 themis vis and 735 moc na images examined only four candidates are observed that are plausibly interpreted as coastal ridges no candidate coastal ridges are observed in the mola profiles this overwhelming paucity of candidate features suggests one of five possible scenarios in terms of the existence of standing bodies of water within the martian lowlands 1 no ocean existed up to the level of either of the previously mapped shorelines 2 an ocean existed however wave action the primary agent responsible for construction of coastal landforms was minimal to non existent 3 an ocean existed but sediment input was not significant enough to form coastal deposits 4 an ocean existed but readily froze and over time sublimated and lastly 5 an ocean existed and coastal landforms were constructed but in the intervening time since their formation they have nearly all been eroded away

the utopia basin revisited regional slope and shorelines from mola profiles mola profiles indicate that utopia planitia including the utopia basin has been tilted northward by 0 025 since formation of the basin but no later than mid late hesperian this tilt could be a relic of the endogenic formation of the global dichotomy in late noachian to early hesperian by a process involving subcrustal erosion in the lowland the utopia basin survives as a deep circular depression within utopia planitia along its south flank are several horizontal or near horizontal terraces and benches that may be shorelines of an extensive northern lowland ocean and a more restricted lake within the utopia basin the large size up to 200 km wide of these putative shorelines represents a major geomorphic problem to be solved

formation of double ring circular grabens due to volumetric compaction over buried impact craters implications for thickness and nature of cover material in utopia planitia mars 1 throughout the polygonal terrain of utopia planitia are circular grabens inferred to overlie buried crater rims thirty four of the 37 circular grabens discovered in viking and thermal emission imaging system themis images southwest of the utopia basin 21 36 n 240 265 w have two nested rings while the remaining three have a single ring all 37 circular grabens bound topographic depressions whose surface relief scales directly with diameter suggesting differential compaction of cover material within the polygonal terrain the distance of doubled circular grabens from the center of the utopia basin scales directly with the spacing between the two rings implying a relation between ring spacing and cover thickness numerical models of horizontal shrinkage during volumetric compaction of material over crater rims show two maxima of horizontal normal tension on the surface these maxima where we would expect grabens to form move away from each other with increasing cover thickness the modeled spacings of stress maxima within 1 2 km thick cover are comparable to the graben spacing observed in southwest utopia planitia implying that cover thickness in this polygonal terrain may be 1 2 km thick where cover material is thin the proximity of the two normal stress maxima may only produce a single circular graben accounting for the observation that the three single ringed grabens are most distal to the center of the basin we contend that the correlation of shrinkage model results and observed ring spacing supports a wet sedimentary origin of polygonal terrain materials consistent with the past existence of standing water in the martian lowland

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

rift segment interaction and its relation to hydrocarbon exploration in continental rift systems 1 in both the early stages of continental rifting and the onset of sea floor spreading in the more advanced rifting longitudinally continuous rifting is accomplished by the propagation and linkage of isolated pockets of extension in the early continental rift phase these isolated pockets of deformation are defined as rift segments rift segments initially are distributed in a loose association of in line and en echelon geometry their propagation interaction and linkage into a continuous rift system creates a variety of rift geometries including rift splays gaps offsets passes and large scale transfer zones an understanding of these geometries and their associated fault patterns is critical for effective hydrocarbon exploration in these zones

seismic reflection profiling studies of a buried precambrian rift beneath the wabash valley fault zone sixty eight kilometers of 12 fold seismic reflection data were collected in the wabash river valley of southwestern indiana and southeastern illinois to investigate the configuration of a basement structure inferred from regional gravity and magnetic anomaly data the seismic profiles were also positioned to cross faults of the wabash valley fault system in a number of locations interpretation of the seismic reflection profiles and detailed gravity and magnetic profile data provides evidence for a series of northeasterly trending grabens in the basement these grabens are filled with pre mt simon layered rocks and are overlain by paleozoic sedimentary rocks of the illinois basin beneath the wabash river near grayville illinois an interpreted graben the grayville graben is approximately 15 km wide and contains about 3 km of fill individual boundary faults for the graben cut prominent reflectors within pre mt simon rocks and display offsets of up to 500 m the interpreted configuration of basement

deep reflection seismics in the precambrian of sweden summary a total of 161 km of deep seismic profiles have been shot in the region one profile crosses the protogine zone in sw sweden over most of the profile short weak reflectors are seen the only area with a concentration of reflectors is in the upper two seconds between the two tectonic zones a nearly transparent area east of the protogine zone is interperted as a deep granite intrustion in the siljan impact structure where four profiles were shot the ne part of the structure is dominated by upper crustal high amplitude reflectors possible causes are discussed

analogue modelling of multiphase rift systems abstract analogue models of multiphase rift systems show that structures produced by the first phase of extension exertsignificant control on the geometries of the later phase extensional faults two experimental series series 1 and series 2 used different modelling media subjected to two phases of extension series 1 models used a simple homogeneous sandpack whereas series 2 models used a sand polymer multilayer in each series kinematic scenarios included orthogonal extension followed by oblique extension or vice versa extension directions for oblique rift kinematics varied from 75 to 30 to the axis of rifting in all models where oblique rifting superseded earlier orthogonal rift structures the second deformation phase produced segmented fault systems and reorientation of fault tips into high angles with respect to the extension vector higher angles of obliquity resulted in increased fault segmentation orthogonal extension superposed on earlier oblique rifting leads to coalescence of segmented faults although remnants of the inherited initial rift structure remained even for low angles of obliquity the sand polymer multilayer models produced more complex fault patterns than those in the simple homogeneous sandpack showing the possible complexities that can be generated during polyphase rifting persistent first phase rift faults caused boundary fault segmentation and salients and embayments along the boundary faults during the second deformation phase the natural occurrence of similar structural styles particularly in map view may indicate that previously unrecognised multiphase rifting has occurred in these natural examples

tectonic development of the new madrid rift complex mississippi embayment north america abstract geological and geophysical studies of the new madrid seismic zone have revealed a buried late precambrian rift beneath the upper mississippi embayment area the rift has influenced the tectonics and geologic history of the area since late precambrian time and is presently associated with the contemporary earthquake activity of the new madrid seismic zone the rift formed during late precambrian to earliest cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions the configuration of the buried rift is interpreted from gravity magnetic seismic refraction seismic reflection and stratigraphic studies the increased mass of the crust in the rift zone which is reflected by regional positive gravity anomalies over the upper mississippi embayment area has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex the correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rift structures the slippage is due to reactivation of the structure by the contemporary nearly e w regional compressive stress which is the result of plate motions

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

mars orbiter camera observation of linear and curvilinear features in the hellas basin indications for multiple processes of formation 1 the high resolution of the mars orbiter camera images reveals many detailed landforms on mars surface which include thin dark or bright linear curvilinear features the latter are generally only a few tens of meters wide but can be a couple of kilometers in length some of these features have previously been attributed to the action of dust devils our study of the linear curvilinear features in the hellas basin reveals a variety that can only be explained by the formation from many different processes in order to evaluate different causes for their formation we created a mapping based on a classification scheme where the linear curvilinear features were sorted into three groups and compared to terrestrial analogues features in group 1 are most frequent and contain streaks that appear only to affect the ground surface the best analogue is dust devil tracks the second very rare group appears to penetrate the surface in a way expected of fractures in the third likewise rare group the features have a topographic expression that makes them similar to grooves and or trenches between narrow ridges this group is probably formed by multiple processes possible terrestrial analogues are found in glacial and periglacial environments e g glacial scouring fluted moraine cryoturbation patterns and iceberg plow marks the existence of such landforms in the hellas basin is consistent with previously presented glacial models and has an important implication for the understanding of the past climatic evolution of mars

resurfacing of the martian highlands in the amenthes and tyrrhena region the southern cratered highlands of mars contain a large population of flat floored rimless craters which have previously been interpreted to have formed by aeolian mantling or flood volcanism neither of these geologic processes accurately explains the observed morphology or the crater statistics geologic mapping in the amenthes and tyrrhena region indicates that craters with this morphology occur on undulating intercrater materials near the dichotomy boundary and on more rugged materials farther into the highlands containing numerous ancient valley networks cumulative size frequency distribution curves indicate ages of n 5 790 early to middle noachian and n 5 540 middle noachian for the cratered plateau and cratered highland materials respectively opposite the observed stratigraphic relations for crater diameters 16 km the population of impact craters is consistent with stratigraphy but the population of smaller craters in the region indicates the importance of resultacing superposed fresh craters indicate a resultacing event that ceased at n 5 200 250 late noachian to early hesperian crater counts divided into 5 o latitudinal bins show an increase in the number of craters between the 8 and 50 km diameter range with increasing northerly latitudes suggesting that the resurfacing was not a single event statistical modelling of an erosive event capable of removing the continuous ejecta deposits from the craters eroding them to reduce the apparent diameter and simultaneously burying smaller eroded craters explains both the morphology of the flat floored rimless craters and their population distribution matching the slope of the modelled curves with that of the cratered highland materials suggests that up to 1400 m of erosion may have taken place in the highlands some of this eroded material probably remained locally burying the smaller craters later stripping of this material indicated by exposed layering and inverted topography has exhumed the smaller craters in the cratered plateau material resulting in the apparent age discrepancy determined from crater counts up to 1 0 km of material may have been removed by this later erosive event to explain the low density of ancient valley networks in the cratered plateau materials

major proterozoic basement features of the eastern midcontinent of north america revealed by recent cocorp profiling cocorp profiling in the eastern midcontinent of north america has 1 traced an extensive sequence of precambrian layered rocks beneath southern illinois indiana and western ohio 2 detected a broad zone of east dipping basement reflectors associated with the grenville front beneath western ohio and 3 discovered a wide region of west dipping reflectors penetrating most of the crust beneath eastern ohio the precambrian layered assemblage may be as much as 11 km thick beneath southern illinois extends at least 170 km in an east west direction and contains several strong reflectors that have a lateral continuity of 80 km or more industry seismic data indicate that the layering is extensive in a north south direction as well possible explanations for the layering include the silicic igneous rocks of the ca 1 48 ga eastern granite rhyolite province which are penetrated by basement drill holes throughout the region perhaps intermixed or underlain by mafic igneous or sedimentary rocks the 40 50 km wide zone of strong east dipping 25 30 reflectors beneath west central ohio corresponds to the position of the grenville front as determined from potential field and drill hole data these dipping reflectors in the upper and middle crust are interpreted to result from ductile deformation zones mylonites like those exposed at the grenville front in canada and imaged on the glimpce seismic reflection lines in lake huron both the cocorp and glimpce lines show a remarkably similar reflection geometry despite the more than 500 km separating the two profiles easternmost ohio appears to be underlain by pronounced west dipping 40 reflectors in the middle and lower crust which are also interpreted as marking a region of pervasive ductile deformation 80 km or more in width analogy with similar reflection packages elsewhere suggests that these reflections may mark a major collision zone the west dipping reflectors may be correlative with similar reflectors imaged on another cocorp survey in northern alabama the correlations suggested by these new results though tentative imply that the eastern midcontinent is composed of a relatively simple assemblage of crustal blocks

the restless resurgent campi flegrei nested caldera italy constraints on its evolution and configuration abstract we have collected new stratigraphic data in the campi flegrei caldera through surface geological mapping and study of cores drilled to various depths we integrate these data with geological geochronological and geophysical constraints available in the literature to reconstruct the volcanic and deformational history of the caldera the caldera is a resurgent nested structure resulting from two main collapses related to the campanian ignimbrite 37 ka and the neapolitan yellow tuff 12 ka eruptions while the structure is affected by a broad subsidence ongoing local resurgence and unrest occur inside the younger nested caldera structure although most of the caldera floor consists of multiple structural blocks due to long term deformations magmas were able to erupt only in one sector of the resurgent part the caldera has shown signs of unrest during the past 25 years with short term deformations that have generated a net uplift of 3 5 m the short term deformations are related to the neapolitan yellow tuff caldera resurgence and interpreted as the net result of brittle and ductile components the long term deformations likely represent the sum of the permanent brittle component of each short term deformational event

the possible bearing of the granite of the uph deep drill holes northern illinois on the origin of mississippi valley ore deposits the pb pb th pb and u pb systems of whole rock drill core samples of the granite of uph record at least one postcrystallization event treatment of the data on a concordia plot indicate the timing of this disturbance was probably 260 35 m y ago and probably no older than 400 m y ago the upper intercept age on concordia is 1416 20 m y and the pb pb isochron age for the four least disturbed rocks plus the four potassium feldspars is 1451 19 m y these ages are in close agreement with rb sr whole rock and u pb zircon ages determined by others and should be the age of granite formation considerable pb loss is indicated for many of the shallower samples that have a more red rather than pink color probably to a factor of two to three or more as a result of the disturbance although the granite remains pb rich today 40 80 ppm in some other studies of disturbed u th pb systems the radiogenic lead has been taken up by potassium feldspars however the pb for most samples seems to have been lost from the system for the granite of uph one sample with an unusually high pb content of about 195 ppm has the most radiogenic pb of any sample but normal values of th u this sample has obviously gained radiogenic pb by more than a factor of four and confirms that pb was indeed mobilized in parts of the system within the limits of the uncertainties equal amounts of pb were lost or gained in both the u pb and th pb systems which is compatible with mobilization being restricted to lead there is some tendency however for the more affected samples to have lower th u values and higher u contents a complex model in which u is gained pb lost and 208pb lost preferentially to 207pb and 206pb cannot be ruled out u and th have somewhat different crystallographic locations and decay energies so that 208pb might be in more accessible sites than the other pb isotopes fission track mapping of the samples indicates most of the u today is located in submicroscopic sites along edges and cleavages of biotite and iron oxides so u is readily available for remobilization lead extraction with only minor u or th removal may have been accomplished by chloride rich brines lead isotope data on the granite of uph indicates that some of the pb in the ore in the nearby upper mississippi valley lead zinc district may have been derived from these granites an even better match is made with the ores of southeastern missouri although a granite like that of uph would only be one end member most likely the radiogenic end member of a mixed source curiously similar aged and heavy metal rich granites from southeastern missouri are not a good match for theckn nearby ores basement rocks such as the granite of uph must be reconsidered as an important contributor to the ores of the mississippi valley but it is unlikely that they constitute the sole source

utopia basin mars characterization of topography and morphology and assessment of the origin and evolution of basin internal structure recently obtained mars orbiter laser altimeter mola topography has permitted a new assessment of the morphology structure and history of utopia planitia the new topographic data convincingly demonstrate that the utopia region is an impact basin as originally proposed by mcgill 1989 whose major topographic expression is a circular 1 3 km deep depression 3200 km in diameter utopia basin is the largest easily recognizable impact structure in the northern hemisphere of mars and is the only portion of the northern lowlands that exhibits a distinct large scale impact signature its presence there and its ancient age verify that a significant part of the northern lowlands dates back to the noachian period using slope maps derived from gridded topography we have mapped a series of linear slope anomalies within the basin and have analyzed their origin these features can be classified into two groups one oriented dominantly radial to the basin and the other oriented dominantly circumferential the circumferential features are tens of kilometers wide and hundreds of kilometers long and appear to remain relatively constant with respect to elevation we explore end member hypotheses 1 that the features formed by the direct action of water ice or 2 that they are tectonic wrinkle ridges we interpret the majority of these features to be due to tectonic deformation primarily wrinkle ridge formation modified by later sedimentation associated with the emplacement of the outflow channels and the formation of the vastitas borealis formation on the basis of the characteristics of these features and their association with polygonal terrain thumbprint terrain smoothness of units and evidence of sedimentary deposits we find that our results support earlier hypotheses proposing that a significant volume of water occupied utopia basin during the late hesperian period of outflow channel formation utopia basin differs significantly in depth and degradation state from the deeper and fresher appearing hellas basin although the two are both noachian in age a significant part of this difference seems reasonably interpreted to be the result of the extensive sedimentary and volcanic infilling history of the northern lowlands

fluid expulsion in terrestrial sedimentary basins a process providing potential analogs for giant polygons and mounds in the martian lowlands abstract on earth burial of fine grained sediments in offshore passive margins e g underwater fans and deltas commonly results in fluid expulsion features including large scale polygonal fractures mud volcanoes and pockmarks comparison of resulting offshore polygons and mud volcanoes with giant polygons and high albedo mounds in the chryse acidalia region of mars shows the terrestrial and martian features to be similar in size morphology geologic context and general co occurrence within the same basin these similarities suggest that the process of terrestrial fluid expulsion may provide an analog that could link the giant polygons and mounds in chryse and acidalia to a single process moreover while the terrestrial offshore polygons and mud volcanoes commonly develop in the same basins these features do not necessarily occur in exactly the same locations within those basins as they are independent responses to compaction and dewatering thus the fluid expulsion analog does not require that the martian giant polygons and mounds have identical distributions this is the situation in chryse and acidalia where the giant polygons and mounds are extensively developed and generally have overlapping distributions but where each set of features may occur in places without the other this fluid expulsion analog is enhanced by the fact that giant polygons and mounds in chryse and acidalia co occur in a regional sense and in a geologic setting that is consistent with a fluid expulsion model of formation implications of this analog may impact our view of the role of water in the depositional history of the martian lowlands

spatial distribution of putative water related features in southern acidalia cydonia mensae mars abstract many putative water related features exist in the northern lowlands of mars these features may provide clues to the abundance and timing of water or ice that existed there in the past the cydonia mensae and southern acidalia area was chosen as the study area owing to the abundance of two of these features giant polygons and pitted cones in addition a section of the deuteronilus shoreline is located there the abundance and close proximity of the features makes this area an excellent place to study the spatial relationships between these landforms as well as the morphological characteristics of pitted cones the features were mapped into a gis for spatial analyses the highest densities of pitted cones and giant polygons are adjacent but distinctly separated by a knobby ridge that is surrounded by the deuteronilus putative shoreline pitted cones were measured and examined to determine if a classification by morphology is possible but the results were inconclusive statistical tests on pit to cone diameter ratios and tests of surface temperatures of cone material suggest but do not verify a single cone origin the various shapes sizes and putative ages of pitted cones may be attributed to temporal variation in emplacement and spatial variation in material properties among the possible scenarios put forth for pitted cone genesis on mars two are likely candidates in cydonia mensae 1 the sublimation of a cold based glacier and 2 a buried lens of methane and or co2 clathrates

the utopia isidis overlap possible conduit for mud volcanism on mars abstract the largest areal concentration of pitted cones on mars is located in the southwest section of utopia basin this particular area of pitted cones has been attributed to mud volcanism several factors may have facilitated extensive mud volcanism at this location the concentration of pitted cones is located where utopia basin intersects isidis basin both features are multi ring impact basins on earth seismic investigations have shown that the outer rings of the chicxulub multi ring impact basin extend to the mohorovicic discontinuity moho if this is true on mars as well the fractures could act as conduits for water from utopia planitia the site of a large putative water body it has been shown that methane can be generated at the mantle on earth on mars this possible source of methane could combine with the infiltrated water to generate clathrates while methane is not currently being released at the location of the pitted cones it could have been in the past three locations of methane release have been observed on mars two of which are located on the same outer ring of isidis basin that intersects the pitted cone population the area of utopia basin that contains the large population of pitted cones is adjacent to the highland lowland boundary where extensive deposition would have occurred extensive deposition combined with the potential for methane release may have contributed to the large population of pitted cones in this area of the utopia basin

calderas in the precambrian terrane of the st francois mountains southeastern missouri precambrian igneous rocks in the st francois mountains of southeastern missouri consist of silicic volcanic rocks mostly ash flow tuff which crop out mostly in the western and southwestern part of the region and epizonal granitic plutons which are mostly exposed in the northeastern part of the area these relations suggest that the exposures in the northeast are of a somewhat deeper part of the crust and that rocks there are now exposed because of west southwesterly tilting and beveling by erosion in the eastern st francois mountains the occurrence of a thick rhyolitic ash flow tuff grassy mountain ignimbrite and its distribution relative to a large high silica pluton butler hill granite suggest that the ash flow is the principal eruptive material of a caldera the butler hill caldera and that butler hill granite is the frozen magma chamber that was originally beneath it structural relations indicate that butler hill granite was emplaced into the volcanic rocks of the caldera during resurgent doming four small low silica plutons silvermine stono knoblick and slabtown granites are distributed peripherally to the butler hill granite and are interpreted as ring plutons emplaced along the ring fracture of the caldera or along arcuate intracaldera faults after resurgent doming and emplacement of butler hill granite more than 3 km of silicic volcanic rocks occupy a basinal structure in the western st francois mountains these rocks lie with angular unconformity upon up domed older rocks of the butler hill caldera along an arcuate zone of megabreccia and disturbed structure on the western margin of the older caldera this feature is interpreted as the topographic wall of a second caldera which lies to the west the occurrence of the thick taum sauk rhyolite stratigraphically about 3 km higher than the rocks involved in the megabreccias may indicate that there was yet another caldera formed in this western part of the st francois mountains

primary centers and secondary concentrations of tectonic activity through time in the western hemisphere of mars five main stages of radial and concentric structures formed around tharsis from the noachian through the amazonian as determined by geologic mapping of 24 452 structures within the stratigraphic framework of mars and by testing their radial and concentric orientations tectonic activity peaked in the noachian stage 1 around the largest center claritas an elongate center extending more than 20 in latitude and defined by about half of the total grabens which are concentrated in the syria planum thaumasia and tempe terra regions during the late noachian and early hesperian stage 2 extensional structures formed along the length of present day valles marineris and in thaumasia with a secondary concentration near warrego vallis radial to a region just to the south of the central margin of valles marineris early hesperian stage 3 radial grabens in pavonis syria ulysses and tempe terra and somewhat concentric wrinkle ridges in lunae and solis plana and in thaumasia sirenum memnonia and amazonis are centered northwest of syria with secondary centers at thaumasia tempe terra ulysses fossae and western valles marineris late hesperian early amazonian stage 4 structures around alba patera the northeast trending alignment of tharsis montes and olympus mons appears centered on alba patera stage 5 structures middle late amazonian represent the last pulse of tharsis related activity and are found around the large shield volcanoes and are centered near pavonis mons tectonic activity around tharsis began in the noachian and generally decreased through geologic time to the amazonian statistically significant radial distributions of structures formed during each stage centered at different locations within the higher elevations of tharsis secondary centers of radial structures during many of the stages appear related to previously identified local magmatic centers that formed at different times and locations throughout tharsis

middle proterozoic early palaeozoic evolution of central baltoscandian intracratonic basins evidence for asthenospheric diapirs abstract the three intracratonic sedimentary basins located in central baltoscandinavia namely the bothnian gulf basin the bothnian sea basin and the baltic basin developed in response to middle proterozoic and late proterozoic tectonic events separated in time by about 800 ma only the baltic basin was subsequently affected by caledonian orogenesis and mesozoic rifting crustal extension was minor or did not take place during the proterozoic basin evolution phases however according to the moho topography crustal thinning did take place this was probably a result of subcrustal magmatism on a craton wide scale the ages of granitoids which intruded during the middle proterozoic basin formation generally decrease from east to west this fact combined with the evidence provided by mantle derived flood basalt magmatism points to a moving asthenospheric diapir as the cause for basin development asthenospheric upwelling was probably also responsible for the second late proterozoic basin evolution phase as evidenced by the lack of crustal thinning and extension and the occurrence of tholeiitic intrusions in addition a late proterozoic thermally induced palaeo high located at about the position of the intracratonic basins is compatible with indications from glaciations as the ages of late proterozoic intracratonic basins also decrease from east to west across the craton the location of asthenospheric diapirism during this time interval was also moving for the fennoscandian lithosphere the presence of fundamental lithospheric weakness zones e g terrane boundaries might be an explanation for the formation of two generations of basins originating from asthenospheric upwelling at about the same location in the fennoscandian shield the spacing and size of the proterozoic intracratonic basins suggest that the asthenospheric diapirism was not deep seated therefore sublithospheric convective processes might be the cause for the asthenospheric upwellings such processes are related to rayleigh taylor instabilities in the sublithospheric mantle emplacement of an asthenospheric diapir causes a thermal bulge at the surface of the lithosphere modelling results demonstrate that erosion of the surficial high succeeded by cooling of the lithosphere can explain the accumulation of early palaeozoic sediments in the bothnian sea basin taking into account post ordovician vertical and lateral erosion of the basin fill

the cottage grove fault system illinois basin late paleozoic transpression along a precambrian crustal boundary the cottage grove fault system in southern illinois has long been interpreted as an intracratonic dextral strike slip fault system we investigated its structural geometry and kinematics in detail using 1 outcrop data 2 extensive exposures in underground coal mines 3 abundant borehole data and 4 a network of industry seismic reflection profiles including data reprocessed by us structural contour mapping delineates distinct monoclines broad anticlines and synclines that express paleozoic age deformation associated with strike slip along the fault system as shown on seismic reflection profiles prominent near vertical faults that cut the entire paleozoic section and basement cover contact branch upward into outward splaying high angle reverse faults the master fault sinuous along strike is characterized along its length by an elongate anticline 3 km wide that parallels the southern side of the master fault these features signify that the overall kinematic regime was transpressional due to the absence of suitable piercing points the amount of slip cannot be measured but is constrained at less than 300 m near the ground surface the cottage grove fault system apparently follows a precambrian terrane boundary as suggested by magnetic intensity data the distribution of ultramafic igneous intrusions and patterns of earthquake activity the fault system was primarily active during the alleghanian orogeny of late pennsylvanian and early permian time when ultramafic igneous magma intruded along en echelon tensional fractures

widespread buried precambrian layered sequences in the u s mid continent evidence for large proterozoic depositional basins large regions of the north american mid continent are underlain by precambrian layered rocks buried beneath phanerozoic sedimentary strata south of the wichita mountains published seismic reflection profiles show a precambrian layered assemblage extending for at least 40 km in both the north south and east west directions and industry data show that it may continue 150 km to the southeast seismic reflection data in the illinois region show a precambrian layered assemblage extending 320 km in an east west direction and 200 km in a north south direction in both cases the layered rocks are as much as 12 km thick apparent sequence boundaries onlap downlap within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strat due to faulting the layered sequences correlate with regions of relatively long wavelength and low amplitude magnetic anomalies the extent of this magnetic signature suggests that about 200 000 km 2 of illinois indiana and western ohio about 50 000 km 2 of southernmost oklahoma and north central texas and about 32 000 km 2 of southern missouri and northern arkansas may be underlain by similar precambrian strata drill holes indicate that the top of the mid continent precambrian basement is composed largely of silicic igneous rocks such material may comprise a large part of the layered sequences alternatively these igneous rocks could be intermixed with or underlain by nonvolcanic meta sedimentary strata the strong reflectivity of some layers suggest that minor mafic flows and or sills may also be present analysis of u pb and nd sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1 7 2 0 ga and predate or are contemporaneous with the 1 3 1 5 ga crystallization ages of the granites and rhyolites though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed the possibility tha they contain precambrian hydrocarbons that escaped heating is as yet untested

quantitative investigations of the missouri gravity low a possible expression of a large late precambrian batholith intersecting the new madrid seismic zone analysis of gravity and magnetic anomaly data helps characterize the geometry and physical properties of the source of the missouri gravity low an important cratonic feature of substantial width about 125 km and length 600 km filtered anomaly maps show that this prominent feature extends nw from the reelfoot rift to the midcontinent rift system geologic reasoning and the simultaneous inversion of the gravity and magnetic data lead to an interpretation that the gravity anomaly reflects an upper crustal 11 km thick batholith with either near vertical or outward dipping boundaries considering the modeled characteristics of the batholith structural fabric of missouri and relations of the batholith with plutons and regions of alteration a tectonic model for the formation of the batholith is proposed the model includes a mantle plume that heated the crust during late precambrian and melted portions of lower and middle crust from which the low density granitic rocks forming the batholith were partly derived the batholith called the missouri batholith may be currently related to the release of seismic energy in the new madrid seismic zone earthquake concentrations occur at the intersection of the missouri batholith and the new madrid seismic zone three qualitative mechanical models are suggested to explain this relationship with seismicity

importance of aeolian processes in the origin of the north polar chasmata mars abstract a detailed examination of the location and orientation of sand dunes and other aeolian features within the north polar chasmata indicates that steep scarps strongly influence the direction and intensity of prevailing winds these steep scarps are present at the heads and along the margins of the north polar chasmata topographic profiles of the arcuate head scarps and equator facing wall of chasma boreale reveal unusually steep polar slopes ranging from 6 30 the relatively steep sloped 8 sinuous scarp at the head of two smaller chasmata located west of chasma boreale exhibits an obvious resistant cap forming unit scarp retreat is occurring in places where the cap unit is actively being undercut by descending slope winds low albedo surfaces lacking sand dunes or dust mantles are present at the base of the polar scarps a 100 300 m deep moat located at the base of the scarps corresponds with these surfaces and indicates an area of active aeolian scour from descending katabatic winds small local dust storms observed along the equator facing wall of chasma boreale imply that slope wind velocities in chasma boreale are sufficient to mobilize dust and sand sized particles in the polar layered deposits pld two amphitheater forms located above the cap forming unit of the sinuous scarp and west of chasma boreale may represent an early stage of polar scarp and chasma formation these two forms are developing within a younger section of polar layered materials the unusually steep scarps associated with the polar chasmata have developed where resistant layers are present in the pld offering resistance during the headward erosion and poleward retreat of the scarps steep slopes that formed under these circumstances enhance the flow of down scarp katabatic winds on the basis of these observations we reject the fluvial outflood hypothesis for the origin of the north polar chasmata and embrace a wind erosion model for their long term development in the aeolian model off pole katabatic winds progressively remove materials from the steep slopes below chasmata scarps undermining resistant layers at the tops of scarps and causing retreat by headward erosion assuming a minimum age for the onset of formation of chasma boreale 105 yr results in a maximum volumetric erosion rate of 0 35 km 3 yr 1 removal of this volume of material from the equator facing wall and head scarps of chasma would require a rate for scarp retreat of 0 5 m yr 1

possible extension of the midcontinent rift in west texas and eastern new mexico the midcontinent rift system forms one of the most prominent gravity features in north america the recognized geophysical anomaly extends in an arc from southern oklahoma to lake superior and then into southern michigan the midcontinent rift system was active between 1185 1010 ma as indicated in the lake superior region by age determinations on intrusive igneous rocks we suggest that the period of formation of the midcontinent rift was also a time of extensive igneous activity in texas and new mexico this activity is represented by intrusions beneath the central basin platform texas and new mexico intrusions which crop out at the pajarito mountain in the sacramento mountains new mexico a basaltic debris flow in the franklin mountains texas basalt flows at van horn texas and the crosbyton geophysical anomaly east of lubbock texas these bodies and other bodies located by geophysical anomalies and wells drilled into mafic precambrian rocks may be related to the midcontinent rift system

grid mapping the northern plains of mars geomorphological radar and water equivalent hydrogen results from arcadia plantia a project of mapping ice related landforms was undertaken to understand the role of sub surface ice in the northern plains this work is the first continuous regional mapping from ctx context camera 6 m pixel malin et al 2007 imagery in arcadia planitia along a strip 300 km across stretching from 30 n to 80 n centred on the 170 west line of longitude the distribution and morphotypes of these landforms were used to understand the permafrost cryolithology the mantled and textured signatures occur almost ubiquitously between 35 n and 78 n and have a positive spatial correlation with inferred ice stability based on thermal modelling neutron spectroscopy and radar data the degradational features into the ldm latitude dependent mantle include pits scallops and 100 m polygons and provide supporting evidence for sub surface ice and volatile loss between 35 70 n in arcadia with the mantle between 70 78 n appearing much more intact pitted terrain appears to be much more pervasive in arcadia than in acidalia and utopia suggesting that the arcadia study area had more wide spread near surface sub surface ice and thus was more susceptible to pitting or that the ice was less well buried by sediments correlations with ice stability models suggest that lack of pits north of 65 70 n could indicate a relatively young age 1ma however this could also be explained through regional variations in degradation rates the deposition of the ldm is consistent with an airfall hypothesis however there appears to be substantial evidence for fluvial processes in southern arcadia with older underlying processes being equally dominant with the ldm and degradation thereof in shaping the landscape

33 000 yr k ar dating of the volcano tectonic horst of the isle of ischia gulf of naples the gulf of naples corresponds to a tectonic sink connected with recent volcanic activity in the south campanian area phlegrean fields ischia and vesuvius at its extreme northwest the isle of ischia was early recognized as a volcano tectonic horst by rittmann1 previous data2 concluded that its uplift ended around 300 000 yr and that the green tuff formation of which it is predominantly composed had an age of some 700 000 yr but tectonic and morphological studies together with original tephrostratigraphical observations on the slope of the horst led us to do some new k ar datings these revealed that the 780 m uplift of the horst from the sea level is younger than 33 000 yr the green tuff which was considered to be the oldest volcanic unit in the island is now dated at 56 000 yr its eruption following the dismantlement of a first volcanic complex now dated as not older than 130 000 yr

fault plane determination of the 18 april 2008 mount carmel illinois earthquake by detecting and relocating aftershocks we developed a sliding window cross correlation scc detection technique and applied the technique to continuous waveforms recorded by the cooperative new madrid seismic network stations following the 18 april 2008 illinois earthquake the technique detected more than 120 aftershocks down to m l 1 0 in the 2 week time window following the mainshock which is three times more than the number of aftershocks reported by the seismic network most aftershocks happened within 24 hrs of the mainshock we then relocated all events by the double difference relocation algorithm accurate p and s wave differential arrival times between events were obtained by waveform cross correlation after relocation we used the l1 norm to fit all located events by a plane to determine the mainshock fault plane the best fit plane has a strike of 292 11 and dips 81 7 to the northeast this plane agrees well with the focal mechanism solutions of the mainshock and four largest aftershocks by combining the aftershock locations and focal mechanism solutions we conclude that the 18 april earthquake occurred on a nearly vertical left lateral strike slip fault orienting in the west northwest east southeast direction the fault coincides with the proposed left stepping divide accommodation zone in the la salle deformation belt and indicates reactivation of old deformation zone by contemporary stresses in the midcontinent

intraplate magmatism related to opening of the southern iapetus ocean cambrian wichita igneous province in the southern oklahoma rift zone abstract southern oklahoma and adjacent parts of texas contain an extensive igneous province emplaced during early cambrian rifting within the southern oklahoma rift zone the rift zone was initiated in association with the opening of the southern iapetus ocean during rodinia supercontinent breakup and later became the site of a series of linked uplifts and basins as a result of late paleozoic inversion igneous rocks within the rift are referred to as the wichita province and are present mostly in the subsurface although critical exposures occur in the wichita and arbuckle mountains in southwestern and southern oklahoma wells drilled into basement in the region provide a wealth of information on the distribution and relations of the major igneous units in the upper crust and geophysical data provide important constraints on deeper levels of the rift zone the upper parts of the igneous rift fill comprise the carlton rhyolite group which has an estimated subsurface areal extent of 40 000 km 2 and the related wichita sheet granites which intrude the lower parts of the rhyolite succession these rocks have a type characteristics and were emplaced after intrusion tilting and uplift of a large tholeiitic layered mafic complex smaller bodies of gabbro also intruded the layered complex after it was tilted u pb zircon and 40 ar 39 ar geochronology indicates the felsic rocks and at least some of the mafic units were emplaced in a relatively narrow time frame at 539 530 ma basalts and intermediate lavas are present only in the subsurface our new work shows these lavas to have tholeiitic to mildly alkaline compositions and to be more extensive than previously realized forming thick successions that both underlie and are intercalated with the rhyolites diabase dikes were intruded into older crust during initial opening of the rift and a suite of late diabase intrusions also cuts the rhyolites and granites indicating that mafic magma was supplied to the rift throughout its igneous history geophysical data show that an enormous mass of mafic rock occupies deeper parts of the rift extending to depths of at least 10 km beneath the surface the total volume of igneous rock emplaced within the rift is estimated to be in excess of 250 000 km 3 detailed studies of carlton rhyolite exposures in the wichita mountains complemented by work on drill cuttings from basement wells indicate that lava flows up to 400 m thick make up major parts of the rhyolitic succession the generally limited exposures of these rhyolite lavas in the wichita mountains are interpreted to be remnants of laterally extensive flow units similar to those documented from other a type felsic provinces trace element abundances define three distinct geochemical groups of rhyolite suggesting derivation from three different sources or magma reservoirs flows belonging to the different geochemical groups are intercalated in the wichita mountains exposures which may indicate that laterally extensive flows from separate magma chambers came to rest on top of one another during accumulation of the rhyolitic volcanic pile

aeromagnetic and gravity anomaly constraints on the crustal geology of ohio the u s geological survey and the ohio geological survey recently compiled aero magnetic anomaly maps and revised gravity anomaly data for crustal studies of ohio updated structural and lithologic maps of the precambrian basement surface provide additional constraints to limit interpretational ambiguities of the geopotential anomaly data the combined modeling of five east west gravity and magnetic anomaly profiles supports a generalized crustal model for ohio involving a lower zone of intermediate composition a more felsic and relatively homogeneous middle zone and an upper zone consisting of intrusive and extrusive igneous rock in the western third of the state and medium grade grenville metamorphic rocks to the east modeling of the geopotential field anomalies indicates the presence of large dense lower crustal bodies in western and south central ohio that may be related to a paleo rift system large possibly anorthositic masses in southern ohio post tectonic granitization in northwestern ohio and a volcanic source for igneous rocks in southwestern ohio may also be interpreted from the combined anomaly analysis

reflection seismic investigations of the siljan impact structure sweden reflection seismic measurements were carried out during 1985 in the area of the siljan impact structure of sweden a total profile length of 80 km was shot to get a clearer picture of the subsurface geometry of impact structures and the processes that formed them than could be obtained from surface observations or theoretical modeling studies standard stacked sections have been interpreted in terms of current impact models the transient crater diameter is found to be approximately 22 km considerably smaller than current estimates based on interpretation of surface geology the structural uplift is estimated to be 3 km at the 5 to 8 km level and the material flow necessary to support the uplift is mainly horizontal rather than vertical