structural and tectonic evolution of the eastern cayman trough caribbean sea from seismic reflection data the eastern cayman trough preserves a record of the late cretaceous to paleogene caribbean history that is largely affected by neogene strike slip tectonics of the current plate boundary we conducted an analysis of seismic data within the eastern cayman trough based upon single and multichannel seismic reflection profiles collected during the seacarib ii cruise in 1987 and the casis cruise in 1992 these data show that the basement of the eastern cayman trough can be divided into four domains from east to west with distinct morphologic and sedimentary character and inferred older to younger ages 1 a province of rifted mesozoic continental crust exhibiting seven parallel horst blocks striking northeast southwest 2 a continent ocean transition bet een provinces 1 and 3 that exhibits seamounts small hills and sedimentary basins 3 an eocene oceanic crust with rough basement but smoother relief than the rifted crust basement trends are roughly north south and oblique to the northwest trend in domain 1 and 4 the northern jamaica slope which forms an east west trending slope with northward dipping strata that flank the three deeper water domains of the cayman trough the domains are interpreted to be the product of the eocene east west opening of the cayman trough as a pull apart basin in a left lateral strike slip setting closure of the 1100 km of eocene and younger oceanic crust of the cayman trough places the fault block province adjacent to the belize margin of central america restoration of normal faults in the fault block province places an additional 190 km of left lateral offset along the cayman trough a neogene phase of transpression has reactivated structures in the four domains along with on land structures described by previous authors in jamaica the proximity of the eastern margin of the cayman trough to petroliferous continental rocks in central america suggests an improved possibility of hydrocarbon potential unfortunately s diment thicknesses of less than 1 km probably are not conducive to hydrocarbon formation

resurfacing history of europa from pole to pole geological mapping abstract we produced geologic maps from two regional mosaics of galileo images across the leading and trailing hemispheres of europa in order to investigate the temporal distribution of units in the visible geologic record five principal terrain types were identified plains bands ridges chaos and crater materials which are interpreted to result from 1 tectonic fracturing and lineament building 2 cryovolcanic reworking of surface units with possible emplacement of sub surface materials and 3 impact cratering the geologic histories of both mapped areas are essentially similar and reflect some common trends tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments the opening of wide bands with infilling of inter plate gaps and the buildup of ridges and ridge complexes along prominent fractures in the ice it also appears that lineaments are narrower and more widely spaced with time the lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation in contrast the degree of cryovolcanic resurfacing appears to increase with time as chaos formation dominates the later parts of the geologic record these trends and the transition from tectonic to cryovolcanic dominated resurfacing could be attributed to the gradual thickening of europa s cryosphere during the visible geologic history that comprises the last 2 or 30 80 myr of europa s history an originally thin brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes the degree of fracturing and plate displacements decreased with time in a thickening shell and lineaments became narrower and more widely spaced formation of chaos regions could have occurred where the thickness threshold for solid state convection was exceeded and can be aided by preferential tidal heating of more ductile ice in a long term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite or cyclic or irregular episodes of tectonic and cryovolcanic activity

getting over continent ocean boundaries the idea of a simple linear boundary between continental and oceanic crust at extended continental margins is widely recognized to be an oversimplification despite this such boundaries continue to be mapped because of their perceived utility in palinspastic and plate kinematic reconstructions to examine whether this perception is justified we review the data and models on which basis continent ocean boundaries are interpreted and map a set of such interpretations worldwide from more than 150 publications the maps show that the location of the continent ocean boundary is rarely consistently estimated within the 10 100 km observational uncertainty that might be expected of the geophysical data used for doing so that this is the case regardless of whether the transition zone behind the boundary is classified as magma rich or magma poor and that the geographical separation of estimates exceeds the width of single study continent ocean transition zones the average of global maximum separations across sets of three or more estimates is large 167 km and mostly a consequence of interpretations published over the last decade we interpret this to indicate an extra component of uncertainty that is related to authors understanding of the range of features that are interpretable at extended continental margins we go on to discuss the implications of this uncertainty for palinspastic and plate kinematic modelling using examples from the literature and from the south atlantic ocean we conclude that a precise continent ocean boundary concept with locational uncertainty defined from the ensembles is of limited value for palinspastic reconstructions because the reconstruction process tends to bunch the ensemble within a region that is i of similar width to the observational uncertainties associated with continent ocean boundary estimates ii narrower than the regions of uncertainty about rotated features implied by the propagation of uncertainties from plate rotation parameters and iii coincident within all the above uncertainties with the more easily mapped continental shelf gravity anomaly secondly we conclude that estimated continent ocean boundaries are of limited use in developing or testing plate kinematic reconstructions because i reconstructions built using them as markers do not within uncertainty limits defined from the ensembles differ greatly from those using more easily determined bathymetric or gravity anomaly contours and ii because it is impossible to segment and date them with useful precision to use as markers of the edges of rigid oceanic lithosphere outside of the constraints of a pre existing plate kinematic model

chaos on europa the characteristics of chaos regions on europa suggest they may be sites of melt through from below they are wide ranging in size location and age the largest are hundreds of kilometers across most are similar to conamara with a matrix reminiscent of frozen slush and often rafts of preexisting crust edges are of two types ramps perhaps the tapering of crustal thickness to zero or cliffs where rafts appear to have broken clear from the shore the small features called lenticulae generally appear to be small chaoses with textured matrix and occasional rafts and many domes may be small chaoses raised by isostatic compensation following refreezing of the crust the extent of chaoses often appears to be limited by ridge systems with the coastline parallel and set back by a distance comparable to the width of the ridge system preexisting ridges often survive as causeways or chains of rafts boundaries of chaoses are apparently not controlled by preexisting cracks consistent with formation by a thermal rather than mechanical process ridges may thicken the crust such that melt through is more likely but not always between ridge systems subsequent cracks and ridges form across preexisting chaoses ranging from fresh cases with few cracks or ridges across them with paths somewhat jagged as they meander among rafts to heavily dissected examples isolated tilted raft like blocks surrounded by densely ridged terrain may be relics of former chaotic terrain thus two fundamental resurfacing processes have alternated over europa s geological history melt through at various places and times forming chaos terrain and tectonic cracking and dilation building densely ridged and banded terrain mapping of chaos features based on morphology at 200 m shows that they correlate albeit imperfectly with dark regions in global 2 km resolution mosaics except dark regions due to ridge margins or craters extrapolating from our mapping of the 5 of europa covered by appropriate images at least 18 of the surface of europa is fresh appearing chaos an additional 4 is slightly modified chaos and much more older chaotic terrain has been overprinted by tectonic structures considerable area has been available globally to accommodate the expansion of crust that occurs along extensional ridges and bands chaos ubiquity suggests that europan geology has been dominated by the effects of having liquid water under a very thin ice shell with chaos regions being widespread indicators of occasional zero shell thickness

gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

strike slip duplexing on jupiter s icy moon europa agenor linea is a 1500 km long 20 30 km wide geologically young zone of deformation on jupiter s icy moon europa on the basis of recent galileo high resolution images we interpret agenor linea as a strike slip zone formed in three stages by a combination of lithospheric separation extension and dextral horizontal shear agenor linea exhibits excellent examples of strike slip duplexes in an icy lithosphere unobscured by vegetation and unaltered by erosion

radiometric and kinematic evidence for middle jurassic strike slip faulting in southern mexico related to the opening of the gulf of mexico one of the least known aspects of the evolution of the gulf of mexico is the nature and location of shear zones along which the relevant continental fragments were displaced the sierra de juarez mylonitic complex located in southern mexico is a polyorogenic north northwest trending structure here we report u pb mylonitization dates of 165 20 ma for igneous zircon from the syntectonic san felipe granite and an integrated 40 ar 39 ar age of 169 3 1 7 ma from synkinematic muscovite both of which indicate a middle jurassic age for the strike slip event along the sierra de juarez mylonitic complex this event therefore occurred during the opening of the gulf of mexico and we propose that the shear zone was kinematically related to the southeast displacement of the yucatan block

geological history of the tyre region of europa a regional perspective on europan surface features and ice thickness galileo images of the tyre macula region of europa at regional 170 m pixel and local 40 m pixel scales allow mapping and understanding of surface processes and landforms ridged plains doublet and complex ridges shallow pits domes chaos areas impact structures tilted blocks and massifs and young fracture systems indicate a complex history of surface deformation on europa regional and local morphologies of the tyre region of europa suggest that an impactor penetrated through several kilometers of water ice to a mobile layer below the surface morphology was initially dominated by formation of ridged plains followed by development of ridge bands and doublet ridges with chaos and fracture formation dominating the latter part of the geologic history of the tyre region two distinct types of chaos have been identified which along with upwarped dome materials appear to represent a continuum of features domes platy chaos knobby chaos resulting from increasing degrees of surface disruption associated with local lithospheric heating and thinning local and regional stratigraphic relationships block heights and the morphology of the tyre impact structure suggest the presence of low viscosity ice or liquid water beneath a thin several kilometers surface ice shell at the time of the impact the very low impact crater density on the surface of europa suggests that this thin shell has either formed or been thoroughly resurfaced in the very recent past

jurassic evolution of the gulf of mexico salt basin we present a new hypothesis for the jurassic plate tectonic evolution of the gulf of mexico basin and discuss how this evolution influenced jurassic salt tectonics four interpretations some based on new data constrain the hypothesis first the limit of normal oceanic crust coincides with a landward dipping basement ramp near the seaward end of the salt basin which has been mapped on seismic data second the deep salt in the deep water gulf of mexico can be separated into provinces on the basis of position with respect to this ramp third paleodepths in the postsalt sequence indicate that salt filled the gulf of mexico salt basin to near sea level fourth seismic data show that postsalt sediments in the central louann and the yucatan salt basins exhibit large magnitudes of late jurassic salt detached extension not balanced by equivalent salt detached shortening in our hypothesis callovian salt was deposited in preexisting crustal depressions on hyperextended continental and transitional crust after salt deposition ended rifting continued for another 7 to 12 m y before sea floor spreading began during this phase of postsalt crustal stretching the salt and its overburden were extended by 100 to 250 km 62 155 mi depending on location sea floor spreading divided the northern gulf of mexico into two segments separated by the northwest trending brazos transform the eastern segment opened from east to west leaving the walker ridge salient in the center of the basin as the final area to break apart in some areas salt flowed seaward onto new oceanic crust first concordantly over the basement as a parautochthonous province then climbing up over stratigraphically younger strata as an allochthonous province

three layered models of ganymede and callisto compositions structures and aspects of evolution abstract three layered structural models are determined for ganymede and callisto each consists of a rock core a mixed ice rock lower mantle and a pure ice upper mantle this structure results from differentiation subsequent to accretional melting attention is given to evaluating various candidates for the rock component and three alternatives representing various degrees of silicate hydration and oxidation are modeled and incorporated structures are calculated on the basis of a 250 k isotherm which is a reasonablev approximation to the gentle adiabats expected to occur in icy satellites differentiation of an ice rock satellite generally involves an increase in radius and the three layered approach allows this process to be examined in some detail it is determined that satellite expansion is most significant early in the process and much less so as differentiation proceeds to completion if tectonics are due to global expansion distinguishing on this basis between a completely differentiated satellite and one that is only partially differentiated is difficult the postaccretional global expansion of ganymede which may have left a tectonic record was probably limited to 1 in radius in agreement with observed limits useful quantities such as silicate mass and volume fraction uncompressed density j2 c22 binding energy and surface heat flow are also determined nonhydrostatic contributions to j2 and c22 are estimated and shown to be nonnegligible encounters with jupiter orbiting spacecraft are unlikely to determine callisto s degree of central condensation we conclude by calculating the relative likelihood of postaccretional melting caused by radiogenic heating three layered satellites have generally hotter interiors because additional thermal boundary layers divide the separately convecting upper and lower mantles inhibiting heat transport ganymedes and callistos that are less than about 1 3 differentiated by mass should experience a second episode of melting as these boundary layers are either above the level of the water ice minimum melting temperature or intersect the melting curve at deeper levels runaway differentiation to at least a depth corresponding to a pressure in the ice v stability field is likely the main point here is that if satellite tectonics are tied to differentiation by melting or its aftermath as in the instability following ocean closure of kirk and stevenson moderate or small amounts of accretional differentiation are unlikely to explain an absence of tectonics as in callisto because extensive differentiation ultimately occurs

polar wander and surface convergence of europa s ice shell evidence from a survey of strike slip displacement two global issues regarding europa are addressed by a survey of strike slip faults first a common type of terrain that appears to represent convergent sites of surface removal which may help compensate for substantial widespread dilation along tectonic bands elsewhere thus helping resolve the problem of conserving global surface area is identified second evidence for polar wander may provide the first confirmation of that theoretically predicted phenomenon these results among others come from an extensive survey of strike slip faults over the portion of the surface where galileo images at 200 m pixel resolution were obtained for regional mapping purposes the images cover two broad swaths that run from the far north to the far south one in the leading hemisphere and the other in the trailing hemisphere among the faults that have been mapped are a fault 170 km long with a strike slip offset of 83 km the greatest yet identified on europa and a quasi circular strike slip fault that surrounds a 500 km wide plate which has undergone rotation as a rigid unit reconstruction of specific examples of strike slip reveals sites of lateral convergence because europa is unique in many ways these sites are not similar to compression features on other bodies which may explain why they had previously been difficult to identify the distribution of strike slip in both hemispheres when compared with predictions of the theory of tidal walking provides evidence for polar wander the crust of europa appears to have slid as a single unit relative to the spin axis such that the site on the crust that was previously at the north rotational pole has wandered probably during the last few million years to a location currently in the leading hemisphere about 30 away from the spin axis such polar wander probably also explains symmetry patterns in the distribution of chaotic terrain pits and uplift features

latest paleozoic early mesozoic structures in the central oaxaca terrane of southern mexico deformation near a triple junction abstract paleozoic rocks in the oaxaca terrane of southern mexico occur as two outliers rio salinas and santiago ixtaltepec unconformably overlying the 1 ga oaxaca complex they consist of the upper cambrian lower ordovician tinu formation mississippian santiago formation lower middle pennsylvanian ixtaltepec formation and the unfossiliferous yododene formation of presumed permian or younger age these paleozoic rocks have been deformed by several sets of structures three moderately westerly dipping bedding parallel shear zones displaying dextral kinematic indicators c s fabrics curvilinear isoclinal folds en echelon boudinage en echelon veins and strained amygdales occur along the boundaries between the tinu and santiago formations between the santiago and ixtaltepec formations and within the ixtaltepec formation in the santiago ixtaltepec outlier the bedding and these bedding parallel shear zones are deformed by n s upright asymmetric subhorizontal folds accompanied by a slaty or spaced cleavage that are in turn deformed by several sets of kink bands subhorizontal and steeply dipping e w nw se and ne sw in the northern part of the santiago ixtaltepec outlier the stratigraphy bedding parallel shear zones and slaty cleavage are displaced by nw se normal faults all of which are truncated by the angular unconformity at the base of the cretaceous which brackets their age as post early permian and pre cretaceous geometric correlation of the major n s folds with e vergent thrusting of the oaxacan complex over the juarez terrane suggests that they are older than middle jurassic no age constraints are available for the kink and chevron folds however most may be related to laramide structures in the overlying cretaceous rocks unfolding the major structures in the paleozoic rocks reorients the bedding parallel shear zones to subhorizontal detachment shear zones or faults with a top to the north sense of displacement they may be related to either gravity sliding following amalgamation of pangea extensional rifting of the yucatan peninsula from southern laurentia or intra arc rifting associated with a permo triassic arc within southern mexico the major n s fold structures and e vergent thrusting of the oaxacan complex over the juarez terrane may represent a response to 1 convergent subduction of the kula and or farallon plates beneath western mexico 2 migration of a gulf of mexico yucatan south america triple junction

spectral analysis of groove spacing on ganymede grooves on ganymede are interpreted as extensional tectonic features whose regular spacing is an indicator of the thickness of a brittle surface layer or lithosphere at the time of deformation we have performed a statistical analysis of groove spacing on ganymede taking fourier transforms of 157 photometric profiles across sets of grooves and examining the resultant power spectra for peaks representing the dominant topographic periodicities the distribution of groove spacing on ganymede is approximately lognormal with a minimum of 3 5 km a maximum of 17 km and a mean of 8 4 km there is no correlation of groove spacing with latitude or longitude the most striking feature of the geographic distribution is that while groove spacing tends to be quite constant within a given groove set it can vary substantially from one groove set to another in a geographic region an exception is the uruk sulcus region with a mean groove spacing of 6 5 km and very little variability we examine several models that relate the observed groove spacing to lithospheric thicknesses the most important conclusion is that lithospheric thickness varied significantly from one groove set to the next while the grooves were being formed we examine four possible causes for the heterogeneous distribution variation in time of formation during a period in which the global mean heat flow was changing variation in strain rate from one groove set to the next local excursions in geothermal gradient caused by variations in the vigor of underlying convective upwellings and local excursions in geothermal gradient caused by resurfacing events the first three appear viable while the fourth requires very rapid groove formation due to the short cooling times involved

evaluation of models for the formation of chaotic terrain on europa chaotic terrain on europa is interpreted to be the result of the breakup of brittle surface materials over a mobile substrate two end member models have been proposed for the mobile substrate a liquid water ocean or solid warm ductile ice direct contact between the brittle surface ice and an underlying ocean might be accomplished by melting through europa s ice shell but plausible sources of thermal energy are insufficient to locally melt through the ice shell we calculate that a liquid or partially liquid substrate is probably necessary for tilted blocks to be observed challenging a solid state model we propose that a model in which chaos areas on europa formed over bodies of melt or partial melt within the ice shell may be the best match to observational and thermal constraints such bodies of melt within the ice shell may be triggered by warm diapirs of relatively clean ice that partially melted an overlying layer of lower melting temperature such as ice contaminated with salts

evidence for separation across a gray band on europa abstract thynia linea a gray band on europa is found to be a 25 km wide and 900 km long region of lithospheric separation which has been infilled by relatively dark material roughly a dozen older features and the cuspate segments that form its outline appear to have been displaced across its width displacement azimuths indicate a best fit pole of opening near 79 s 200 w however displacement magnitude decreases toward either end of the gray band indicating that it is more akin to a tear in a nonrigid europan lithosphere opening was in response to nw se directed tensile stress in accord with the stress predictions of nonsynchronous rotation observations of thynia linea are consistent with a laterally mobile brittle lithosphere decoupled from ductile or liquid material below as previously suggested to account for opening of wedge shaped bands in europa s antijovian region lithospheric separation and contemporaneous emplacement of new material offers a possible volcano tectonic scenario for resurfacing europa if this process is ongoing resurfacing can be accomplished on a time scale consistent with the satellite s surface age if one gray band or zone of wedge shaped bands forms every 10 3 10 4 yr and becomes unrecognizable with age this would imply that features on europa brighten with age as through continuous deposition of frost onto the surface

evidence for a weakly stratified europan ocean sustained by seafloor heat flux we interpret chaos type features on the surface of europa as melt through structures formed by rotationally confined steady and or episodic oceanic plumes that rise to the base of the ice shell from magmatically heated regions of the seafloor smaller lenticular features in the vicinity of chaos type regions might be formed by baroclinically unstable vortices that spin off the main convective plume or by persistent heating from localized hydrothermal venting sites the ocean is assumed to be weakly stratified because of turbulent convection generated by heating from below and cooling from above seafloor heating maintained by tidal dissipation in the rocky interior generates an estimated global heat flux of 8 7 1012 w and limits the mean ice thickness to 2 5 km for seafloor heat sources with radii r that are less than the ocean s deformation radius rd nd n is the brunt vaisala frequency d is the water depth and is the coriolis parameter the diameters of chaos type regions are expected to diminish from o 100 km within equatorial regions to o 10 km at high latitudes assuming spatially uniform water depth and density structure where r rd the scale of the source region determines the scale of the melt through features provided there is sufficient time before refreezing ice rafts in large melt through regions are imbedded in episodes of preferentially anticyclonic circulation corresponding to clockwise counterclockwise motions in the northern southern hemisphere we calculate that 1021 j were required to melt the ice in the 100 km diameter conamara chaos region and that for a steady localized heat flux f 1011 w 1 of the global heat flux it took 1000 years for the initial melt through to occur assuming that ice raft displacements in conamara chaos occurred during a major melt through event maximum current speeds in the region were o 10 cm s 1 and refreezing occurred within 20 hours a lack of well defined ice drift in other major melt through regions suggests that these regions formed through episodes of melting and refreezing that modified the existing structure but left little time for the establishment of organized advective motion

deep crustal structure in the eastern gulf of mexico we use air gun data recorded by ocean bottom seismometers to constrain the velocity structure along gulf of mexico basin opening line 4 a profile extending from the northwestern florida peninsula across the florida escarpment to the central gulf of mexico moderately thinned continental crust with a moho depth of 32 33 km average sediment thickness of 6 km and an average crustal thickness of 27 km is interpreted on the northeast end of the profile offshore florida thinned and intruded continental crust is identified over a horizontal distance of 225 km where the crustal layer thins from 25 km to 6 7 km mean seismic velocities of the crust in this region increase from 6 55 km s to 6 95 km s from northeast to southwest and are evidence for increased magmatic input as rifting developed oceanic crust with an average thickness of 5 6 5 7 km is observed over a distance of 175 km on the southwest end of the profile with an extinct spreading ridge with an axial valley morphology imaged on a coincident seismic reflection profile anomalously high upper oceanic crust velocities of 6 0 6 7 km s are interpreted as massive basalt flows and could reflect increased temperatures during emplacement integrating well seismic reflection and our seismic refraction data allow us to estimate a full spreading rate of 2 2 cm yr for seafloor spreading along the profile this indicates that oceanic crust was emplaced at a slow spreading center

distribution of strike slip faults on europa study of four different regions on europa imaged by the galileo spacecraft during its first 15 orbits has revealed 117 strike slip faults europa appears to form preferentially right lateral faults in the southern hemisphere and left lateral faults in the northern hemisphere this observation is consistent with a model where diurnal tides due to orbital eccentricity drive strike slip motion through a process of walking in which faults open and close out of phase with alternating right and left lateral shear lineaments that record both left and right lateral motion e g agave linea may record the accommodation of compression in nearby chaotic zones nearly all identified strike slip faults were associated with double ridges or bands and few were detected along ridgeless cracks thus the depth of cracks without ridges does not appear to have penetrated to the low viscosity decoupling layer required for diurnal displacement but cracks that have developed ridges do extend down to such a level this result supports a model for ridge formation that requires cracks to penetrate to a decoupling layer such as a liquid water ocean

late cenozoic uplift of the eastern united states revealed by fluvial sequences of the susquehanna and ohio systems coupling between surface processes and lower crustal flow abstract fluvial and karstic data sets indicating late cenozoic surface uplift in the eastern united states are modelled for the first time using a technique incorporating coupling between surface processes and flow in the lower continental crust which has been extensively used for modelling similar data sets elsewhere in the world notably in europe distinct phases of lower crustal flow forcing starting in the early middle miocene late pliocene and late early pleistocene are evident as observed elsewhere and relate to combinations of cyclic surface loading by sea level variations and ice loads and to variations in regional erosion rates as elsewhere however the detailed uplift histories inferred are rather different from those indicated in many other regions notably europe primarily because of different properties of the crust in particular in the late proterozoic phanerozoic crust of the appalachians the mobile lower crustal layer seems to be relatively thick causing a prolonged uplift response for each phase of lower crustal flow forcing whereas in the early proterozoic crust of the yavapai crustal province farther west underlain by a thick basal mafic layer this mobile layer is much thinner leading to a very different response consisting of abrupt alternations of uplift and subsidence as is also observed in other regions of early proterozoic crust another important difference relative to western and central europe is the much smaller number of terraces in the eastern us rivers that have been studied the general applicability of this type of physics based modelling technique is thus confirmed

cryomagmatic mechanisms for the formation of rhadamanthys linea triple band margins and other low albedo features on europa we investigate cryomagmatic mechanisms for the formation of the diffuse low albedo margins of triple bands and haloes associated with elliptical lenticulae on europa one mechanism involves explosive venting of a gas particle spray derived from a subsurface body of volatile bearing water assuming the dark features represent cryoclastic deposits erupted from a medial or central vent modeling the dynamics of the eruptive plumes indicates that eruption velocities of 30 to 250 m s 1 and volatile contents of 0 1 to 20 wt are required to produce deposits having the observed dimensions calculated heights for ballistic plumes range from 1 to 25 km as an alternative mechanism we consider conductive heating of lithospheric ice by a subsurface heat source e g a warm ice diapir or cryomagmatic intrusion which promotes enhanced sublimation of surface ice producing local enrichment of nonice material an analytical model describing a constant temperature infinite heat source places lower bounds on the time scales required to produce a detectable feature a numerical treatment of more realistic geometries and boundary conditions allows cooling of the heat source to be modeled we find that the thermal model satisfactorily explains features with the dimensions of minor lineaments and lenticulae haloes a few kilometers over time intervals of 104 to 105 years the formation of a sublimation lag deposit proceeds rapidly once the ice temperature exceeds a threshold of 130 k however for triple bands measuring up to 25 km in width additional heat sources such as repeated or multiple intrusions are necessary to reproduce the observed dimensions this is consistent with the observation that well developed triple bands typically have multiple medial ridge sets each of which may represent a discrete tabular heat source although neither the volcanic nor intrusive thermal model can be ruled out at this stage the latter is a more satisfactory explanation for some characteristics of the low albedo features analyzed in this study

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

the role of extensional instability in creating ganymede grooved terrain insights from galileo high resolution stereo imaging galileo stereo images covering about 1500 km of uruk sulcus on ganymede have revealed two scales of ridges 1 large scale ridges and troughs spaced 6 km apart corresponding to the grooves seen in voyager images and 2 small scale ridges spaced hundreds of meters apart superimposed on the large scale ridges we interpret the small scale ridges to be the result of tilt block normal faulting of the surface brittle layer while the large scale ridges may be due to necking of the brittle layer over a ductile substrate the geometry of the tilt blocks revealed in galileo images leads to a minimum estimation of 51 to 58 extensional strain in the area the strain estimate when incorporated into a model for the formation of grooved terrain by necking of a brittle layer undergoing extension leads us to estimate a thermal gradient of 20 k km and a strain rate of 10 14 s 1 during groove formation

multispectral terrain analysis of europa from galileo images abstract galileo s solid state imaging camera recorded six images at wavelengths from 0 41 to 0 99 m of europa s trailing hemisphere 1 6 km pixel resolution during the g1 orbit 1st orbit target ganymede of the nominal mission we have photometrically corrected these data and extracted spectra representing europa s diverse geologic terrains the goals of the analysis of these spectra are 1 to determine whether europa s geologic units differ spectrally from one another 2 to determine the number of color components necessary to explain europa s dark material spectral behavior and 3 to examine how europan dark materials may change in color with time our data indicate that europan dark spots lineaments and triple band side materials represent a single dark and reddish endmember component we see no evidence for more than one dark endmember however the dark endmember seen in these g1 data is probably not a pure exposure of europan dark material mottled terrains brighter lineaments and aging triple bands can be modeled by a mixture of bright plains materials and the dark component exposures of europan dark materials may thus be brightening with time eventually blending into the surrounding bright plains

convective instability in europa s floating ice shell models of the tidally heated floating ice shell proposed for the jovian satellite europa generally find shell thicknesses less than 30 km past parameterized convection models indicated that such shells are stable against convective overturn which otherwise ostensibly leads to freezing of the ocean underneath here i apply the temperature dependent viscosity convection scaling developed by solomatov and coworkers to the europan ice shell the temperature dependent properties of ice are linearized about 260 k as any convective interior should be close to this temperature with the colder ice forming an essentially passive stagnant lid ice shells 10 km thick are found to be unstable to convection at their base for melting point viscosities of 1013 pa s as linearized by tidal stresses if the ice deforms by superplastic creep but such low viscosities require small grain sizes 1 mm this requirement may be met if grain sizes observed in terrestrial polar glaciers can be strain rate scaled to europa regardless convection at the base of the ice shell if initiated may not freeze the ocean because of tidal heating a stagnant lid regime ice shell is much more dissipative than a conductive shell of the same thickness such a shell should thin not thicken and the potential exists for further thermal instabilities and runaways

heat flow in the western abyssal plain of the gulf of mexico implications for thermal evolution of the old oceanic lithosphere the seafloor depth of an oceanic basin reflects the average temperature of the lithosphere thus the western abyssal plain of the gulf of mexico which has tectonically subsided much 1 km deeper than other basins of comparable ages late jurassic should be underlain by an anomalously cold lithosphere in order to examine this hypothesis we made suites of high accuracy heat flow measurements at 10 sites along a line connecting deep sea drilling project dsdp sites 90 and 91 in the sigsbee abyssal plain the new heat flow sites were initially surveyed by 3 5 khz echo sounding 4 channel seismic reflection seismic refraction with eight ocean bottom seismometers and nine piston cores we occupied a total of 48 heat flow stations along the seismic survey line 3 to 6 at each site including 28 where we measured in situ thermal conductivities over the practical depth interval 4 m of the new multioutrigger bow heat flow probe we determined the heat flow associated with the lithosphere by correcting the values measured at the seafloor 41 to 45 mw m2 for 1 the thermal effect of the sedimentation and 2 the additional heat from the radioactive elements within the sediments the sedimentation history required for the first was reconstructed at each heat flow site based on ages and thicknesses of the major seismic stratigraphical sequences age data from the dsdp cores 3 5 khz subbottom reflectors and correlation of turbidite units found in the piston cores radiogenic heat production was measured for 55 sediment samples from four dsdp holes in the gulf whose age ranged from present to early cretaceous 0 83 w m3 on the average this provided the correction for the second the effects of these two secondary factors approximately cancel one another the lithospheric heat flow under the abyssal plain thus estimated ranges from 40 to 47 mw m2 these heat flow values are among the lowest in the mesozoic ocean basins where highly reliable data 45 to 55 mw m2 have been reported therefore the lithosphere under the gulf seems indeed colder than that under other old ocean basins however it is not as cold as expected from the large tectonic subsidence the inconsistency between the depth and heat flow may imply an anomaly in the regional thermal isostasy

europa stratigraphy and geological history of the anti jovian region from galileo e14 solid state imaging data a tidal bulge and associated deformation are predicted to occur in europa s anti jovian region voyager data showed polygonal low albedo bands in this region it was demonstrated that when the low albedo material comprising these bands was removed the margins could be reconstructed suggesting that fracturing and lateral separation has occurred higher resolution galileo images of a 300 000 km2 portion of the anti jovian region permit recognition and mapping of a variety of features and documentation of the geological history we identify background ridged plains several distinctive types of bands and ridges lenticulae and regions of chaotic terrain these images are at relatively low incidence angles thus albedo variations predominate over topography this presents a challenge to morphologically based mapping but does not preclude development of a regional stratigraphy stratigraphic relationships show that dark bands crosscut the bright ridged plains and are themselves disrupted by lenticulae and chaos suggesting a geological history beginning with background ridged plains followed by a period dominated by band formation with the most recent history predominantly characterized by formation of lenticulae and chaos the paucity of impact craters in this region indicates that the entire sequence of units is geologically recent

the search for current geologic activity on europa observational evidence and theoretical arguments suggest that jupiter s satellite europa could be geologically active and possess an ocean of liquid water beneath its surface at the present time we have searched for evidence of current geologic activity on europa in the form of active plumes venting material above the surface and by comparison of voyager and galileo images to look for any changes on the surface so far we have observed no plumes and have detected no definitive changes the lack of observed activity allows us to estimate a maximum steady state surface alteration rate of 1 km2 y 1 in the regions analyzed assuming alterations will cover contiguous areas of at least 4 km2 over a period of 20 years assuming this as a constant globally uniform resurfacing rate leads to a minimum average surface age of 30 million years we also suggest that the lack of obvious circular albedo patterns on the surface due to plumes coupled with the presence of bright rayed craters such as pwyll and the predicted sputtering erosion rate implies that no large scale plume activity has taken place over at least the last few thousand years we thus conclude that if europa s surface is currently active any changes must be relatively small in spatial scale or episodic in nature rather than continuous to detect potential small scale surface changes we need high resolution comparisons between the galileo data and future europa orbiter images

folds on europa implications for crustal cycling and accommodation of extension regional scale undulations with associated small scale secondary structures are inferred to be folds on jupiter s moon europa formation is consistent with stresses from tidal deformation potentially triggering compressional instability of a region of locally high thermal gradient folds may compensate for extension elsewhere on europa and then relax away over time

astypalaea linea a large scale strike slip fault on europa abstract astypalaea linea is an 810 km strike slip fault located near the south pole of europa in length it rivals the san andreas fault in california and it is the largest strike slip fault yet known on europa the fault was discovered using voyager 2 images based upon the presence of familiar strike slip features including linearity pull aparts and possible braids and upon the offset of multiple piercing points fault displacement is 42 km right lateral in the southern and central parts and probably throughout pull aparts present along the fault trace probably are gaps in the lithosphere bounded by vertical cracks and which opened due to fault motion and filled with material from below crosscutting relationships suggest the fault to be of intermediate relative age the fault may have initiated as a crack due to tension from combined diurnal tides and nonsynchronous rotation according to the tectonic model of r greenberg et al 1998a icarus 135 64 78 under the influence of varying diurnal tides strike slip offset may have occurred through a process called walking which depends upon an inelastic lithospheric response to displacement alternatively fault displacement may have been driven by currents in the theorized europan ocean which may have created simple shear structures such as braids the discovery of astypalaea linea extends the geographical range of lateral motion on europa such motion requires the presence of a decoupling zone of ductile ice or liquid water a sufficiently rigid lithosphere and a mechanism to consume surface area

brine mobilization during lithospheric heating on europa implications for formation of chaos terrain lenticula texture and color variations the background lineated plains on europa are locally highly modified and destroyed in regions known as chaos and lenticulae produced there are 1 isolated fragments and polygons of background material which rotate and translate 2 matrix which fills in the areas between the fragments and polygons and 3 surface discolorations using observations and constraints from high resolution galileo images we find that a model for the formation of these terrains which involves mobilization and migration of brines and a possible percolation phase transition as the europan lithosphere is warmed can readily explain the vast majority of their characteristics in addition the presence of melt fractions of a few percent in the adjacent ice framework may enhance the creep rate and the accompanying deformation rates the characteristics and distribution of lenticulae suggest that among the strong candidates for heat sources for brine migration and ice mobilization processes is diapirism linked to solid state convection in a layer underlying a brittle lid and possibly overlying a liquid water layer

conamara chaos region europa reconstruction of mobile polygonal ice blocks new galileo images of europa reveal regions of chaotic terrain in otherwise highly lineated background plains examination of conamara chaos shows that 59 of the region is composed of fine textured matrix material lying at low elevations and formed by destruction of lineated plains while the remainder consists of 139 fragmented polygons of linear textured background plains using through trending linear features we reconstruct the original positions of chaos polygons and find that significant lateral translation and rotation have occurred 78 of the polygons have undergone horizontal translations with most moving between 1 and 5 km and 81 have rotated average rotation of 11 movement of polygons appears to be inward from the chaos margins and clockwise in the center while polygon rotation is evenly clockwise and counterclockwise chaos formation in this region thus involves destruction of over half of the pre existing terrain and mobilization translation and rotation of the remaining polygons implying elevated near surface temperatures and a highly mobile substrate over lateral scales of 100 km

eruption of lava flows on europa theory and application to thrace macula europa on the basis of its spectral characteristics and its apparently young surface age is likely to be at least partially resurfaced by liquid water volcanic processes however convincing evidence for lava flow morphology in voyager data has not previously been found and the lower density of liquid water relative to an overlying solid water ice crust suggests that extrusive volcanism could be uncommon we examine a candidate lava flow like feature thrace macula and compare its characteristics with theoretical predictions for the ascent and eruption of water magma on europa outlining a range of conditions under which water ice magma could plausibly be erupted to the surface thrace macula 45 s 171 w is a dark lobate feature extending nnw to sse its northern part is centered in a 140 km diameter arcuate to circular lineament and within this zone are radial and concentric lineaments and three dark lobes similar in appearance to flow lobes extending away from a central region for distances of 40 50 and 160 km the 160 km lobe appears to follow local topographic features producing lobate margins at subdued structures broadening locally parallel to structural trends and terminating against a gray band models for the present structure and state of the predominantly h2o exterior of europa range from solid water ice to a thin ice layer overlying a liquid water layer and these conditions may even vary as a function of space and time ascent and eruption of h2o magma through a less dense water ice crust means that sustained effusive eruptions are very difficult to produce if a globally continuous liquid water or salt water layer exists e g crawford and stevenson 1988 kargel 1991 for flows as extensive as the thrace macula feature we thus require magma source regions sufficiently localized that excess pressure can be built up in the source region given the elastic properties of ice at these source depths the most likely scenario is intrusion of silicate magma into the base of an ice crust and melting and mobilization of water magma with this range of magma source settings we model plausible surface vent geometries for a water lava flow interpretation of thrace macula modeling calculations show that flow lobes with the above geometries and thicknesses on the order of 10 m could be formed in water eruptions on europa involving discharge rates of 2 105 m3 s through dikes with widths in the range 5 10 m these discharge rates about 3 times larger than those of terrestrial flood basalts should not be unusual for water eruptions on europa under these conditions and suggest that water volcanic resurfacing could be common on the satellite if source regions meeting these criteria were readily available the lack of abundant similar dark flows could be due to brightening with time or might imply that conditions for their formation grounded or nearly solid ice crust only evolved in the latter part of europan history

geologic mapping of the northern leading hemisphere of europa from galileo solid state imaging data the northern leading hemisphere of europa was imaged at regional mapping resolution 230 m pixel by the galileo spacecraft ssi camera we produced geologic maps from a regional scale mosaic and a high resolution inset of this region twelve geologic units were sufficient to produce correlative geologic maps at both regional and local scales stratigraphic relationships indicate four major episodes in the geologic history of this area the first episode includes background plains formation and modification followed by a second period of extensive lineament formation the third episode included extensive chaotic disruption of the surface at low and middle latitudes the final episode includes the formation of new sets of ridges and bands especially at high latitudes the low crater density indicates that all these episodes in europa s geologic history are geologically recent the latitudinal distribution of chaos areas broadly matches that of areas of crustal thinning from models of tidal heating we show that stress directions rotated clockwise with time which is consistent with predictions from global stress models involving tidal deformation and nonsynchronous rotation of europa s crust on the basis of the change in lineament orientation with time the reconstructed longitudinal positions of the studied area indicate that europa s crust completed a full rotation relative to the tidally deformed interior

pits and uplifts on europa abstract a survey of depression and uplift features on europa based on galileo regional mapping images shows that these features come in a wide range of sizes with numbers increasing greatly with decreasing size down to the limits of resolution size distributions are similar in the northern leading and southern trailing hemispheres where they are distinctly different from the southern leading and northern trailing hemispheres suggesting an oblique antipodal symmetry pattern similar to that of chaotic and tectonic terrain this pattern is suggestive of polar wander uplifts are usually polygonal or irregular in shape and rarely are cracked patches of chaotic terrain which we had surveyed earlier are not included in the current study because their topography is generally unclear and because there is no a priori known genetic linkage with the pits and uplifts these results contradict generalizations based on the earlier pits spots and domes psd taxonomy most of the type examples for psds were simply patches of chaotic terrain selected from a limited portion of their full size range the use of the term lenticula to collectively describe psds is inconsistent with the iau definition of lenticula a small dark spot seen at low resolution pits and uplifts do not correlate with lenticulae although chaos often does properties of psds that have been widely cited as primary evidence for convective upwelling in thick ice e g that uplifts are generally dome shaped and often cracked that pits and domes are regularly spaced that there is a typical diameter of 10 km were premature and not supported by subsequent data most pits and uplifts are less than 10 km across so if they formed by diapirism or convective upwelling the sources must have been very shallow less than 5 km deep how they actually formed remains unknown

tectonics and water on europa the tectonics of europa one of jupiter s moons are complex this satellite probably hosts a subsurface water ocean but the thickness of the outer ice crust is poorly constrained and the episodic presence of liquid water at the surface is debated we argue that some surface features of europa are formed by soft ice that is heated by viscous dissipation of tidal motion along faults and do not depend on a shallow ocean our model suggests that transient pockets of liquid water or brine could form at shallow depths in the crust

brine volcanism and the interior structures of asteroids and icy satellites abstract cryovolcanism is among the foremost processes responsible for modifying the surfaces of icy satellites volcanic brine petrogenesis in ammonia deficient satellites should mainly involve eutectic melting in relevant salt water systems carbonaceous chondrites provide useful insights into the compositions of salts and aqueously altered rock in icy satellites and asteroids c1 chondrites contain about one fifth by mass of salts in various states of hydration many aspects of the petrogenesis and physical volcanology of icy satellite brines should be well described in the system h2o mgso4 na2so4 minor components include sulfates of k ni mn and ca chondrites also contain abundant carbonates but these are probably not very important in brine magmatism due to their low solubilities under expected conditions chlorides are also unimportant under most circumstances because of the low cosmic abundance ratio of cl s soluble salts may have profound effects on the geology and structure of icy satellites and asteroids in some models late episodes of water volcanism are facilitated by high buoyant forces due to the relatively high densities of sulfate rich mantle and crustal layers in other models early hypersaline brine volcanism quickly yields to plutonic magmatism due to low crustal densities europa probably has a layered crust composed of anhydrous mg na sulfates near the base and a frozen or partially molten eutectic mixture of ice and hydrated mg and na sulfates near the surface ganymede may have a crust about 300 km thick composed of a 10 1 ratio of ice mirabilite and a mantle 500 km thick composed of 50 ice phases plus 50 hydrated mg and na sulfates

does europa have a subsurface ocean evaluation of the geological evidence it has been proposed that jupiter s satellite europa currently possesses a global subsurface ocean of liquid water galileo gravity data verify that the satellite is differentiated into an outer h2o layer about 100 km thick but cannot determine the current physical state of this layer liquid or solid here we summarize the geological evidence regarding an extant subsurface ocean concentrating on galileo imaging data we describe and assess nine pertinent lines of geological evidence impact morphologies lenticulae cryovolcanic features pull apart bands chaos ridges surface frosts topography and global tectonics an internal ocean would be a simple and comprehensive explanation for a broad range of observations however we cannot rule out the possibility that all of the surface morphologies could be due to processes in warm soft ice with only localized or partial melting two different models of impact flux imply very different surface ages for europa the model favored here indicates an average age of 50 myr searches for evidence of current geological activity on europa such as plumes or surface changes have yielded negative results to date the current existence of a global subsurface ocean while attractive in explaining the observations remains inconclusive future geophysical measurements are essential to determine conclusively whether or not there is a liquid water ocean within europa today

quaternary history of axel heiberg island bordering nansen sound northwest territories emphasizing the last glacial maximum the landscape of northeastern axel heiberg island records an erosional history since the neogene whereas the glacial and sea level history relate to late wisconsinan glaciation during the last glacial maximum northeastern axel heiberg island was covered by glaciers emanating from local and regional sources local ice caps expanded from highlands on axel heiberg island across the coastal slope a trunk glacier flowed northward out of eureka and flat sounds into nansen sound and reached northernmost axel heiberg island radiocarbon ages on marine shells suggest that the last glacial maximum was attained along the coast after 29 ka bp and extensive deglaciation was taking place by 10 3 ka bp the postglacial marine limit rises continuously from 78 m above sea level near the entrance to nansen sound to 158 m above sea level at the mouth of eureka sound showing greatest glacioisostatic loading to the south

mechanistic solutions to the opening of the gulf of mexico two mechanistic models which are unlike the traditional plate tectonic landfill models used for most proposed pangea reconstructions of the yucatan block relate the mesozoic opening of the gulf of mexico directly to the movement of the north and south american plates 1 a previous piggyback model in which yucatan moves with south america out of the western gulf and 2 a new edge driven model in which the motion of the yucatan block is caused by forces applied to its margins by the movement of the north and south american plates in the second model yucatan moves out of the northern gulf of mexico as a gear or roller bearing on the basis of magnetic edge anomalies around the gulf this edge driven model predicts that from the bathonian to tithonian 170 to 150 ma yucatan was rotated 60 counterclockwise as a rigid block between north and south america with rift propagation and extension occurring simultaneously in the gulf of mexico and yucatan basin

temperatures on europa from galileo photopolarimeter radiometer nighttime thermal anomalies galileo observations of europa s thermal emission show low latitude diurnal brightness temperatures in the range of 86 to 132 kelvin nighttime temperatures form an unexpected pattern with high temperatures on the bright ejecta blanket of the crater pwyll and an equatorial minimum in temperatures after sunset uncorrelated with surface albedo or geology the nighttime anomalies may be due to regional thermal inertia variations of an unknown origin which are equivalent to a two to threefold variation in thermal conductivity or to endogenic heat fluxes locally reaching 1 watt per square meter endogenic heat flow at this high level although consistent with some geological evidence is theoretically unlikely