wrinkle ridge assemblages on the terrestrial planets the wrinkle ridge assemblage is a group of associated features that have been observed on the moon mars and mercury members of the wrinkle ridge assemblage are classified as either arches or ridges based on morphology and ridges are subdivided into first second or third order ridges on the basis of dimensions the ridge assemblages described in this study are known to occur in mare basalts on the moon and smooth plains units that may be volcanic in origin on mars and mercury the structures that occur in the continental flood basalts of the columbia plateau in the northwestern united states are morphologically and dimensionally similar to features in the ridge assemblages on the terrestrial planets the anticlinal ridges of the columbia plateau and many first order ridges on the moon mars and mercury are interpreted to be folds with reverse or thrust faulting developed as a result of buckling flexure fracture or buckling as a consequence of reverse or thrust faulting fracture flexure estimates of the unit shortening across first order ridges using simple geometric forms to approximate the ridge morphology are lower than those determined for the anticlines of the columbia plateau where displacements are the results of folding and reverse to thrust faulting evidence of layer parallel extension that often accompanies folds developed by fracture flexure mechanisms i e drape or fault bend folds is not found in the anticlinal ridges of the columbia plateau and has only been observed in one ridge on the moon a kinematic model involving buckling followed by reverse to thrust faulting flexure fracture is favored for the origin of the anticlinal ridges of the columbia plateau and many of the first order ridges on the moon mars and mercury

centrifuge modelling of laccolith intrusion abstract laccolith intrusion has been investigated by centrifuge modelling silicone putty representing magma with bingham rheology is intruded by overburden pressure through a circular conduit into a stack of paraffin wax layers which represent sedimentary strata the model intrusions evolve through a sequence of geometric forms similar to those exhibited by natural laccoliths the results are also in accord with published mathematical analyses of laccolith formation the model magma first flows laterally between overburden layers to form a sill as the sill increases in area the cover strata are arched upwards into a sinusoidal bending form subsequently the overburden fails abruptly by kinking above the periphery of the intrusion and the form changes to a cupola separated from surrounding horizontal strata by a sharp inflection in the layering finally strata on the flanks of the cupola undergo a second episode of localized failure the top of the dome becomes flat and the dipping flanks become a monoclinal flexure surrounding this kink form a number of variables influence this evolution increased overburden strength thickness of overburden strata and total thickness of overburden all prolong the bending form to larger intrusion size delay the localized failure which marks the cupola and kink stages and reduce the aspect ratio of the intrusion at all stages effectively bonded overburden interfaces have a similar effect but interfaces with sharply defined yield strengths enhance development of the monoclinal flexure magmatic driving pressure affects the rate of intrusion but not the evolution of its form intrusion rate is also influenced by magma flow properties and probably by the form of the feeder conduit the range of final forms preserved by natural laccoliths probably results from competition between rate of intrusion controlled by driving pressure and conduit geometry and rate of chilling of magma in the laccoliths the model laccoliths form in times that scale to between one and a few tens of years in agreement with available estimates for natural laccoliths

pancakelike domes on venus the shape of seven large domes on the plains of venus with volumes between 100 and 1000 km3 is compared with that of an axisymmetric gravity current spreading over a rigid horizontal surface both the altimetric profiles and the horizontal projection of the line of intersection of domes on the synthetic aperture radar images agree well with the theoretical similarity solution for a newtonian fluid but not with the shape calculated for a rigid plastic rheology nor with that for a static model with a strong skin as a viscous current spreads it generates an isotropic strain rate tensor whose magnitude is independent of radius such a flow can account for the randomly oriented cracks that are uniformly distributed on the surface of the domes the stress induced by the flow in the plains material below is obtained and is probably large enough to produce the short radial cracks in the surface of the plains beyond the domes the viscosity of the domes can be estimated from their thermal time constants if spreading is possible only when the fluid is hot and lies between 1014 and 1017 pa s laboratory experiments show that such viscosities correspond to temperatures of 610 to 700 c in dry rhyolitic magmas these temperatures agree with laboratory measurements of the solidus temperature of wet rhyolite these results show that the development of the domes can be understood using simple fluid dynamical ideas and that the magmas involved can be produced by wet melting at depths below 10 km followed by eruption and degassing

age budget and dynamics of an active salt extrusion in iran the hormuz salt of kuh e namak iran began rising through its phanerozoic cover in jurassic times and had surfaced by cretaceous times in miocene times the still active zagros folds began to develop and the salt is still extruding to feed a massive topographic dome and two surface flows of salt which have previously been called salt glaciers but are here called namakiers two crude but independent estimates for the rate of salt extrusion and loss are shown to balance the salt budget if the current salt dynamics are assumed to be in steady state first to replace the extrusive salt likely to be lost in solution in the annual rainfall the salt must rise at an average velocity of about 11 cm a second the foliation pattern shows that the extruding and partially dissolved salt column spreads under its own weight the maximum height of the salt dome is consistent with a viscous fluid with a viscosity of 2 6 x 1017 poises extruding from its orifice at a rate of almost 17 cm a k both estimates are consistent in indicating that salt can extrude onto the surface 42 85 times faster than the average long term rate at which salt diapirs rise to the surface the structure fabrics textures and deformation mechanisms of the impure halite all change along the path of the extrusive salt from the dome down the length of both namakiers such changes tend to occur when the flowing salt encounters changes in its boundary conditions and the recognition of buried namakiers is discussed in the light of such observations episodes of salt flow at a rate of 0 5 m per day have been measured along the margin of the n namakier after significant rain showers such brief episodes of rapid flow alternate with long periods when the namakier is dry and stationary the shape of the colour bands cropping out on the n namakier indicate that the flow over the surface of impure salt with a mylonitic texture obeys a power law with n 3 although the reported annual rainfall has the potential of dissolving both namakiers in about 2000 years a superimposed thin marine cover may protect static parts of them for as long as 30 000 to 300 000 years

effect of mechanical interaction on the development of strike slip faults with echelon patterns abstract the overlapping geometry of echelon strike slip faults is well known we have quantified this observation by measuring the amount of overlap and separation and plotting them against each other for over 120 examples although there is large scatter the data show a linear trend suggesting that the overlap increases proportionally with separation up to a limiting value we have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity the results show that fault interaction is in fact an important factor in contributing to the overlapping geometry of echelon strike slip faults

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

thrust fault mechanics and origin of a frontal ramp several features of thrust sheets in nonmetamorphic and relatively low grade metamorphic terrains indicate that the modes of faulting included both gradual and stick slip types of motion a first order quantitative analysis is made of the mechanics of this type of faulting in the analysis part of a wedge shaped thrust belt slips forward over a fault area of length l and unit width w producing a strain concentration in subhorizontal strata at the front of the growing thrust where a frontal ramp develops by shear fracturing equations for net slip x and the conditions for ramping are evaluated with limit equilibrium principles and coupled oscillator theory the quantitative analysis takes into account anisotropy resulting from contrasts in competency and facies changes a special case solution for maximum net slip associated with stick slip is lw ki 1 cos ot o 12 1 cos 1t where is the stress drop ki are spring constants of the strata 0 and 1 are natural frequencies of the system and t evaluated from another equation is the time for net slip the spring constants and natural frequencies can be evaluated from seismic and stratigraphic data a preliminary solution for piggy back thrusting indicates that ramp spacing can be systematic and that it depends upon the thickness of the deforming package the elastic parameters of the deforming package the slope of the topographic surface and the stresses along the trailing edge of the package

a possible explanation for foreland thrust propagation a common feature of thin skinned fold and thrust belts is the sequential nature of foreland directed thrust systems as a rule younger thrusts develop in the footwalls of older thrusts the whole sequence propagating towards the foreland in the transport direction as each new younger thrust develops the entire sequence is thickened particularly in the frontal region the compressive toe region can be likened to an advancing wave as the mountainous thrust belt advanced the down surface slope stresses drive thrusts ahead of it much like a surfboard rider in an attempt to investigate the stresses in the frontal regions of thrustsheets a numerical method has been devised from the algorithm given by mctigue and mei 1981 the algorithm yields a quickly computed approximate solution of the gravity and tectonic induced stresses of a two dimensional homogeneous elastic half space with an arbitrarily shaped free surface of small slope a comparison of the numerical method with analytical examples shows excellent agreement the numerical method was devised because it greatly facilitates the stress calculations and frees one from using the restrictive simple topographic profiles necessary to obtain an analytical solution the numerical version of the mctigue and mei algorithm shows that there is a region of increased maximum resolved shear stress directly beneath the toe of the overthrust sheet utilizing the mohr coulomb failure criterion predicted fault lines are computed it is shown that they flatten and become horizontal in some portions of this zone of increased thrust sheets are known to advance upon weak decollement zones if there is a coincidence of increased a weak rock layer and a potential fault line parallel to this weak layer we have in place all the elements necessary to initiate a new thrusting event that is this combination acts as a nucleating center to initiate a new thrusting event therefore thrusts develop in sequence towards the foreland as a consequence of the stress concentrating abilities of the toe of the thrust sheet the gravity and tectonic induced stresses due to the surface topography usually ignored in previous analyses of an advancing thrust sheet play a key role in the nature of shallow foreland thrust propagation

reports venus identification of banded terrain in the mountains of ishtar terra high resolution images obtained with the arecibo radar system at a wavelength of 12 6 centimeters reveal numerous parallel 10 to 20 kilometer wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the ishtar terra region of venus geometric and morphological characteristics suggest that the bands represent earth like tectonic deformational features such as folds and faults

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

crease structures indicators of emplacement rates and surface stress regimes of lava flows crease structures are features commonly found on lava flow surfaces and consist of a fracture with curved walls that extend outward from a linear valley these crease structures are found on flows of nearly all compositions and crystallinities we have mapped the distributions of crease structures on many flows in the western united states and found that 1 axial length is not dependent upon composition and crystallinity 2 adjacent crease structures are generally aligned in an en echelon pattern 3 crease structures located adjacent to flow margins are generally perpendicular to these margins and 4 at mount st helens washington large lobe bisecting crease structures are found on lobes situated on slopes of less than 20 degrees a primary surface feature found on many crease structures is striations striations are sets of long stripes on the walls of the crease structure that extend approximately parallel to the axis of the central valley and they appear to be analogous to those found on the faces of cyclically fractured basalt columns observations of developing crease structures on 6 of the nearly 20 mount st helens dome lobes show that they form throughout the extrusion of flows situated on slopes of less than 20 degrees but only at the very beginning and or end of extrusion of flows on steeper slopes these observations imply that crease structures form when the lava flow is forced to spread laterally either as the flow advances over a flat area or as the down slope movement stagnates near the end of extrusion causing the rate of spreading to exceed the downslope rate of flow this lateral spreading of lava results in the concentration of tensile stress along a line oriented perpendicular to the direction of spreading the cooled crust of the extrusion is therefore torn apart about this line of tensile stress concentration forming a central valley that exposes hot ductile material from the flow interior to the atmosphere the presence of striations on many crease structure walls implies that the emplacement mechanism is similar to that suggested for columnar basalts where each striation is produced by incremental fracturing the stress responsible for crease structure formation is due to both cooling and lateral spreading of lava calculations show that stress due to spreading is three orders of magnitude higher than that due to cooling fracture advance times were calculated using a simple one dimensional conductive cooling model adjusted for the special geometry of the crease structure the model closely approximates measured crease structure emplacement times at mount st helens and gives reasonable estimates for crease structures found on older silicic lava flows we then used this model to calculate emplacement times of entire domes that consist of a single crease structure by dividing the dome volume by the formation time we are able to calculate extrusion rates for these older domes these values which include the first flow rate estimates reported for rhyolitic lavas 0 03 106 m 3 s similar to average flow rates of 0 7 40 m 3 s calculated for mount st helens dacite lobes from detailed topographic maps

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

deformation in the overburden of diapiric evaporite ridges examples from the sverdrup basin canadian arctic archipelago abstract a structural analysis was made to determine the origin of two large anticlines in clastic sedimentary rocks of the tertiary eureka sound fold belt canada each upright open anticline is a chain of second order domes which are cored by evaporite diapirs the strain pattern in the anticlines was determined by using bedding parallel slickensides for the sense and direction of shear and a pressure solution cleavage in sandstone for the orientation of the xy plane of finite strain most slickensides have a major component of dip slip bedding parallel shear of normal sense top side moved down near the crests of anticlines cleavage planes dip at a lower angle and in the same direction as bedding planes but near the crests of synclines they dip at a steeper angle and in the opposite direction to bedding planes the results obtained are incompatible with the strain pattern of folds formed by lateral compression but agree with the strain pattern observed in the overburden of diapiric ridges in laboratory models

active normal faulting and crustal extension a world wide review of fault plane solutions and focal depths for large normal faulting earthquakes on the continents shows that the overwhelming majority of such earthquakes nucleate in the depth range 6 15 km on faults dipping between 30 and 60 in the few cases where levelling or seismic data are good enough these normal faults are shown to be approximately planar from the surface to their nucleation depth at the base of the brittle crust there is evidence that in some cases as a result of the enormous transitory increase in strain rate during large earthquakes rupture continues into the normally ductile lower crust on surfaces with substantially gentler dips these low angle surfaces may be analogous to some of the detachments seen in metamorphic core complexes of the western usa but the nature of the motion on them depends on strain rate as well as on rheological contrasts between the detachments and the blocks on either side such contrasts however are unlikely to introduce substantial curvature to an originally planar shear zone if the observed spread in active normal fault dips is caused by rotation of the faults and the blocks they bound during extension then a maximum value of 1 7 can be accommodated by seismic activity on a single generation of normal faults with continued extension either a new generation of steeper faults cutting the rotated first faults is likely to form or the deformation will continue aseismically on faults dipping at less than 30 large earthquakes have not been observed to nucleate on very low angle 20 normal faults within the continental crust anywhere in the world such faults can of course move aseismically but are unlikely to do so on a large scale within the upper crust in areas where steep normal faults are seismically active thus extensional models that require concentrated simple shear on large sub horizontal faults within the brittle upper crust will also require a spatial separation between aseismic very low angle faulting and seismic highangle faulting since seismogenic high angle faults dominate the topography of extending regions upper crustal very low angle faulting if it occurs on a large scale presumably does so in areas that are relatively fiat

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

tectonics of the thingvellir fissure swarm sw iceland abstract the thingvellir fissure swarm dissects 9000 year old pahoehoe lava and contains about 100 fractures of average orientation n29 3e the average length of fractures is 620 m the minimum being 57 m and the maximum 7 7 km the maximum width and throw on a single fracture are 68 m and 40 m respectively most fractures are vertical at the surface and must be the result of an absolute tensile stress the geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures it is concluded that most fractures attain depths of the order of several hundred meters or less but that the largest faults attain depths of many kilometers comparison with the vogar fissure swarm on the reykjanes peninsula suggests that the thingvellir swarm may have the greater rate of dilation the total maximum postglacial dilation of the vogar swarm is only 15 m whereas the corresponding figure for the thingvellir swarm is about 100 m

linear volcanoes along the pacific cocos plate boundary 9 n to the galapagos triple junction abstract a seabeam based reconnaissance of the 500 km of the east pacific rise crest between 7 n and 2 40 n shows that the axial ridge is segmented by four 4 13 km non transform offsets into an en echelon string of distinctively different linear volcanoes these axial volcanoes are oriented orthogonal to relative plate motion except where their overlapping ends veer 15 toward each other and where small intra volcano offsets of their crestal rift zones create abrupt kinks longitudinal gradients of the crestlines are less than 5 m km except where they plunge at rift zones overlapped ends and where they rise locally to small axial peaks transverse profiles vary from trapezoidal to triangular with a steep shield shaped cross section being most common conventional sounding data indicate that this pattern continues to the 140 km offset siqueiros transform fault system at 8 2 n within this fault system is a short spreadingcenter volcano contained in a rift valley that links two strike slip fault zones immediately to the north is the shallow 9 0 8 3 n axial volcano with unusual relief mapped by a deeply towed instrument package at the southern end of the plate boundary as the rise crest enters the region of the pacific cocos nazca triple junction the axial ridge narrows deepens and acquires a more irregular long profile south of 2 30 n the rise crest has a 15 km wide rift valley that contains multiple volcanic ridges with north south strikes structural hypotheses suggested or supported by these morphologic observations include a point source magma supply to the spreading center from mantle diapirs the along strike continuity of axial magma chambers on fast spreading rises even across small rift zone offsets and the importance of magma intrusion as well as eruption for building the axial ridge hypotheses inconsistent with the new data include magma supply and long distance dispersal from a few widely spaced plumes primary control of the topographic volcanic and tectonic characteristics of the rise crest by distance from transform faults and localization of triple junctions over major mantle upwellings

origin of periodically spaced wrinkle ridges on the tharsis plateau of mars ridged plains are a major geologic unit on mars and are a predominant unit on the tharsis plateau a region that has undergone uplift extension and extensive shield and flood volcanism these units probably flood volcanic in origin are characterized by landforms classed as wrinkle ridges wrinkle ridges are interpreted to be folds resulting from buckling followed by reverse to thrust faulting flexure fracture a prominent characteristic of many of the wrinkle ridges on mars particularly those in the tharsis ridge system is the periodic nature of their spacing the periodic spacing has been evaluated in six major provinces on the tharsis plateau with regions further divided into domains based largely on variation in ridge orientation the average spacing of the tharsis ridges based on 2934 measurements is 30 km in an effort to account for the periodic nature of the wrinkle ridges the ridged plains material is modeled as both a single member and a multilayer with frictionless contacts that has buckled at a critical wavelength of folding free slip between layers is assumed based on the possible existence of mechanically weak interbeds in the ridged plains sequence separating groups of flows the near surface mechanical structure in the area of ridged plains is approximated by a strong layer or layers overlying a weak megaregolith of finite thickness overlying a strong basement buckling of the ridged plains is assumed to be decoupled from the basement by the weak megaregolith decoupling is enhanced if the megaregolith were water or ice rich at the time of deformation the rheologic behavior of the ridged plains and megaregolith is approximated by a linear elastic and linear viscous material the models are examined for a range in 1 the strength contrast between the ridged plains material and the underlying megaregolith 2 thickness of the ridged plains material 3 thickness of the megaregolith and 4 number of layers the elastic model can explain the observed ridge spacing only if the ridged plains are relatively thick 2400 m with multiple layers n 1 and relatively high contrasts in young s modulus e e0 100 the high e e0 required is possible only if the megaregolith was water rich at the time of deformation over the same range in values of the parameters viscous buckling is much less restricted the viscous model can explain the observed ridge spacing over a wide range in ridged plains thickness 250 m to several kilometers megaregolith thickness number of layers n 1 to 8 and contrasts in viscosity 0 10 in addition viscous buckling is viable if the megaregolith were dry water rich or ice rich at the time of deformation the absence of folds with a cross sectional geometry in the shape of a sinusoid anticline syncline pairs may be the result of initial deformation of the ridged plains material into low amplitude folds in infinitesimal strain followed by plastic yielding in the cores of the anticlines in finite strain initial elastic or viscous buckling coupled with plastic yielding confined to the hinge area followed by the development of reverse to thrust faulting could account for the asymmetric fold geometry of many of the wrinkle ridges

mechanics of active salt diapirism abstract an active diapir forcefully intrudes its overburden driven by diapir pressure that overcomes the resistance of the overburden strength possible causes for the driving pressure are differential loading of the source layer and a density contrast with the overburden possible resisting forces derive from the mass of the roof block and resistance to the faulting and folding that accommodate the intrusion for a typical density contrast between salt and a sedimentary overburden a simple force balance demonstrates that the diapir height must be more than two thirds to three quarters the thickness of the surrounding overburden to initiate substantial active diapirism narrow diapirs must be even taller physical and numerical modeling show that typical structures of active diapirism are a central crestal graben flanked by relatively unstrained flaps that rotate upward and outward curved reverse faults can separate the flaps from the regional overburden normal faults in the crestal graben propagate downward with new faults created farther outward as the roof arches over the rising diapir thin wide roofs develop multiple grabens separating a relatively flat roof from the rotating flaps in cases leading to emergence the piercing diapir evolves from a pointed crest to a progressively rounded and widening crest as the flaps rotate outward the numerical models show that overburden flexure contributes to early formation of active diapir structures but piercement does not continue to emergence unless the diapir is very wide or tall relative to the overburden thickness active diapirism becomes progressively more difficult for diapir crests ranging from a rectangle a round cornered rectangle a semicircle to a triangle with a pointed crest

voyager 2 in the uranian system imaging science results voyager 2 images of the southern hemisphere of uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere the alignment of the cloud bands is similar to that of bands on jupiter and saturn but the zonal winds are nearly opposite at mid latitudes 70 degrees to 27 degrees where winds were measured the atmosphere rotates faster than the magnetic field however the rotation rate of the atmosphere decreases toward the equator so that the two probably corotate at about 20 degrees voyager images confirm the extremely low albedo of the ring particles high phase angle images reveal on the order of 10 2 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings as well as a broad diffuse low optical depth ring just inside the main rings system nine of the newly discovered small satellites 40 to 165 kilometers in diameter orbit between the rings and miranda the tenth is within the ring system two of these small objects may gravitationally confine the e ring oberon and umbriel have heavily cratered surfaces resembling the ancient cratered highlands of earth s moon although umbriel is almost completely covered with uniform dark material which perhaps indicates some ongoing process titania and ariel show crater populations different from those on oberon and umbriel these were probably generated by collisions with debris confined to their orbits titania and ariel also show many extensional fault systems ariel shows strong evidence for the presence of extrusive material about halfof miranda s surface is relatively bland old cratered terrain the remainder comprises three large regions of younger terrain each rectangular to ovoid in plan that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials perhaps suggesting some exotic composition

photosynthetic community responses to upwelling in mesoscale eddies in the subtropical north atlantic and pacific oceans in this paper we summarise the photo physiological responses of phytoplankton to upwelling of macronutrients in mesoscale eddies in the subtropical north atlantic eddies project sargasso sea and subtropical north pacific e flux project hawaii the observations obtained on two sets of cruises over 2 years occupied six cyclonic eddies and two mode water eddies the photosynthetic physiological parameters were measured using a bench top fluorescence induction and relaxation fire system and a submersible in situ fast repetition rate fluorometer frrf deployed on an undulating towed vehicle both of these instruments were used to provide highly sensitive and well resolved data on community responses the responses are dependent on both the type of eddy and its stage of development our results indicate that while cyclonic eddies in the atlantic and pacific can increase primary photosynthetic production early in their development mode water eddies in the subtropical north atlantic can support patchy blooms of large diatoms for long periods of time more than 3 months

estimates of strain due to brittle faulting sampling of fault populations abstract the geometry of sampling domains is a first order consideration in the characterization of brittle fault populations in most cases descriptions of fault size distributions based on map cross section traverse or borehole data systematically underestimate the number of small faults present in a volume the geometry of sampling domains may be accounted for using an empirical proportionality between fault displacement and trace length estimates of strain in which the sampling geometry is considered suggest that small faults accommodate a significant portion of the total strain due to the brittle faulting process

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

effects of two century land use changes on soil iron crystallinity and accumulation in southeastern piedmont region usa abstract to evaluate effects of land use changes on iron fe crystallinity in mineral soil this study quantified labile short range ordered and relatively crystalline iron pools in upper mineral soils of old hardwood forests cultivated agricultural fields and old field pine forests a common trajectory of land use history in the upland of the southern piedmont usa particularly in comparable old field pine forest but with varying erosion histories we indexed their surface mineral soil iron crystallinity and compared iron cycling accretion in forest floor and mineral soil horizons in a variety of different forest ecosystems our results indicate that i comparable sites with three land uses showed a significantly higher iron crystallinity in cultivated soil than soils supporting old hardwoods or in formerly cultivated soils that have supported 50 years of pine forest regeneration ii five selected pine forest soils showed increasing iron crystallinity with elevated erosion and iii iron accretion in organic and surface mineral soils horizons are ubiquitous across a variety of forests with a higher accretion in non needle materials than needles given the continued soil erosion and more technologically intensive plowing across the piedmont region we attribute that agriculture activities accelerate loss of organic complexed fe in the surface soil and resulted in the crystalline iron oxide at depth exposed to surface also large organic input during forest regrowth for decades reestablished the reactivity and significant accretion of iron oxides in soil profiles suggesting major influence of biological activities in mediating the transformation of soil iron oxides and carbon compounds particularly in organic soil horizon this study suggests that land use changes agriculture and reforestation play a profound role in transforming the iron crystallinity and its interaction with organic matter decomposition in biologically active soil and litter layers it remains unclear how biological activities via animal or microbes in these layers have driven the iron biogeochemical transformation and redistribution in terrestrial forest ecosystems

extensional and compressional instabilities in icy satellite lithospheres a simple mechanical model of planetary lithospheres and asthenospheres is combined with the most recent experimental results concerning the brittle and ductile deformation of ice i h to determine the plausibility of forming the grooved terrains on ganymede enceladus and miranda by means of lithospheric instabilities the model assumes a lithosphere of uniform strength atop a substrate whose strength decreases exponentially with depth the thickness of the lithosphere is defined by the intersection of a near surface byerlee s law relationship for brittle failure with a standard exponential ductile flow law at depth for ganymede we find that the relatively high surface gravity warm temperatures render virtually unachievable an instability strong enough to account for the observed topographic relief however a sufficiently strong instability capable of producing features with the proper spacing could develop on a smaller colder body such as enceladus or miranda the requirements in this case would be a strain rate greater than about 10 14 sec 1 and a near surface thermal gradient of a few tens of degrees kelvin per kilometer the most reasonable way to accomplish these requirements is by the intrusive emplacement of a thick layer of warm ice beneath a preexisting lithosphere of about a kilometer or so in thickness accompanied almost contemporaneously by a strain event lasting on the order of 10 5 years for extension or 10 3 years for compression however observed properties of grooved terrains on enceladus and miranda such as nonuniform feature spacing and low crater counts force us to conclude that unstable deformation must be more complicated than this simple model would suggest if it is to explain these features

origin of planetary wrinkle ridges based on the study of terrestrial analogs wrinkle ridges linear asymmetric topographic highs are common physiographic features on the moon mars and mercury we describe here some terrestrial features of similar morphology and scale that have formed under compressional stress systems similar to planetary wrinkle ridges all of the terrestrial analogs are produced by the anticlinal folding of rocks above reverse faults that shallow and typically break the surface characteristics common to both terrestrial and planetary ridges include 1 a linear asymmetric antiformal shape 2 tension cracks and grabens along the hinges of the antiform 3 overlapping en echelon lobes 4 reversals of the sense of asymmetry along strike 5 occurrence in compressional environments and 6 similarity to features produced during compression of various materials in laboratory scale models these similarities strongly suggest that planetary wrinkle ridges result from deformation associated with shallow thrust faults that either break or come close to the surface kinematic models capable of explaining the development of wrinkle ridges include 1 anticlinal folding of rocks over a buried thrust fault that may subsequently propagate to the surface and 2 fault bend folding of upper plate rocks over the surface flattening bend of a thrust fault

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

the mechanical paradox of large overthrusts the mechanical paradox of large overthrusts originates with a fallacious assumption in the conceptual model upon which mechanical analyses of overthrust faulting have been based the notion that the maximum width of a sheet of rock that may be displaced along an overthrust fault can be determined by using the mohr coulomb failure criterion to compare the strength of the rocks with the stress required to overcome the total frictional resistance to slip and the total cohesion or adhesion over the entire fault surface tacitly assumes that slip is initiated and occurs simultaneously over the entire fault surface this unstated and untenable assumption which is incompatible with what is known about real faults is the real explanation for the mechanical paradox a realistic model for overthrust faulting must be compatible with the nature of real overthrust faults and particularly with the way in which displacements including earthquake displacements occur on real faults most large overthrusts are discrete shear surfaces within a coherent mass of rock that is physically continuous around their ends total displacement clearly varies from place to place over these discrete shear surfaces and moreover must be the cumulative result of many relatively small incremental displacements that also varied from place to place over the finite part of the fault surface on which they occurred each increment began as a local shear failure and propagated along the fault as a smeared out somigliana dislocation at a velocity that was small considering the time required for propagation over the total area of the fault during any individual displacement event whether a megathrust earthquake or a creep event only a small part of the fault surface was in relative motion at any one time fault displacements may appear to occur simultaneously across laboratory specimens that are very small relative to the velocity of propagation of displacement because the transit time is so short when the linear scale is expanded over six orders of magnitude to encompass an entire surface of a large overthrust fault however the transit time is much longer because the velocity of propagation is very small relative to the area of the fault surface and the displacement clearly involves only a very small part of the fault surface at any one time the shape and size of overthrust faults result from the propagation of dislocations and this process is controlled by the strength heterogeneity and anisotropy of the rock mass as well as by variations in the regional stress field but not by the frictional resistance to sliding integrated over the entire fault surface

relations between normal fault geometry tilting and vertical motions in extensional terrains an example from the southern gulf of suez abstract earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern gulf of suez are approximately planar with dips of 30 40 from the surface to around 10 km depth these faults and the blocks they bound appear to rotate about a horizontal axis as they move causing tilting this tilting is seen both in young vertical movements of the coastline such as raised beaches and marshlands and in the distribution of middle miocene marine rocks which are uplifted to elevations of 400 500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens the absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw tooth topography on a regional subsidence caused by crustal and lithospheric thinning the observations required for this simple model are the present day fault dip the amount of tilting and the width of the rotating blocks the virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents

gloria survey of the east pacific rise near 3 5 s tectonic and volcanic characteristics of a fast spreading mid ocean rise abstract a long range sidescan sonar gloria survey of an approximately 100 km square area of the east pacific rise crest between 3 s and 4 s extends results obtained in the same area by lonsdale 1977 using deep tow the axial linear volcano was found to be continuous over a distance of 75 km the presence of major inward and outward facing fault scarps was confirmed but the gloria data show several distinct differences between the two fault sets the inward dipping faults are more numerous more closely spaced and longer than the outward dipping ones and their dip slopes backscatter sound more extensively than those of the outward dipping faults moreover most of them appear to be formed within 2 km of the axis whereas the majority of the outward dipping faults begin to develop between 5 and 8 km from the axis these differences suggest that the two sets of faults have different origins the horizontal pattern of inward dipping faults is similar to those observed on other mid ocean rises at all spreading rates though the lengths and separations of the scarps are slightly and their throws considerably less than on slower spreading rises this common horizontal pattern suggests that inward dipping faults on all rises have a common mode of formation regardless of spreading rate horizontal tension is probably the dominant factor but an additional mechanism is needed to explain the polarization of fault dips that occurs in the region 2 8 km from the axis the similarity of major fault spacings on the east pacific rise to those on slower spreading rises suggests that faulting is invariant in space rather than time and that the lithosphere where these faults are formed about 2 km from the spreading axis has a similar small thickness for all spreading rates this is attributed to the regulating effect of hydrothermal circulation and plate cooling

experimental modeling of role of gravity and lateral shortening in zagros mountain belt dynamically scaled analogs of the geologic structures of the zagros mountain belt are used to argue that different parts of the zagros mountains of iran record different combinations of the effects of a gravity driven overturn and a southwest northeast lateral shortening superimposed on the zagros overturn partially scaled material models have been used to simulate the zagros geodynamics which involve layer parallel compression of a 6 to 7 km thick phanerozoic carbonate cover containing a pattern of preshortening diapirs the folds in the zagros form rapidly 1 5 mm yr in a 20 to 30 km wide zone reactive some of the preshortening diapirs and generate new synshortening listric diapirs a third set of postshortening diapirs rises from the hormuz decollement behind the fold thrust front model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs which flatten in the synclines model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs i suggest that any salt pillows in currently diapir free zones of the zagros fold thrust belt may surface as diapirs through the anticlines in the future in the absence of well data seismic and field observations i have used only three map pattern criteria to recognize the timing of diapirs these are locally obscured by extrusive salt sheets preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular they are more common in the anticlines

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

effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies what controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood i propose that for volcanoes at divergent plate boundaries these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them this stress concentration is mainly a function of the applied tensile stress associated with spreading and the aspect height width ratios of the chambers high spreading rates and or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies it is found that for chambers at the same depth in a volcanic zone the one of the highest aspect ratio will normally intrude magma most frequently also if the chambers are of equal aspect ratios the one at the greatest depth will intrude magma most frequently because the extrusion frequency of a volcano is a fraction of its intrusion frequency the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano these results are applied to the divergent plate boundary in iceland where many of the volcanoes appear to be fed by double chambers that is shallow chambers fed by deep seated chambers it is found that except when the aspect ratio of the deep seated chamber is much less than that of the shallow chamber the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep seated chamber it is concluded that whereas the short term i e 103 yrs extrusion frequencies of volcanoes at the plate boundary in iceland may be similar to the dike intrusion frequencies of the source chambers the long term i e 104 yrs extrusion frequencies may be about ten times lower than the intrusion frequencies

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

monoclinal bending of strata over laccolithic intrusions abstract sedimentary strata on top of some laccolithic intrusions are nearly horizontal and little deformed but are bent into steeply dipping monoclinal flexures over the peripheries of these intrusions this form of bending is not explained by previous theories of laccolithic intrusion which predict either horizontal undeformed strata over the center and faulted strata around the periphery or strata bent continuously into a dome however a slight generalization of these theories accomodates the observed form and contains the previous forms as special cases a critical assumption is that the strength of contacts within a multilayered overburden is overcome locally by layer parallel shear if this strength is less than the strength of the layers themselves then layers over the center remain bonded together and display negligible bending whereas layers over the periphery slip over one another and are readily bent into a monoclinal flexure