ridge segmentation faulting and crustal thickness in the atlantic ocean the mid atlantic ridge mar between the kane and atlantis fracture zones consists of segments 20 85 km in length1 bull s eye patterns in the mantle bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust here i present a map of normal faults inferred from 5 104 km2of sea beam bathymetry4 along the mar between 28 n and the atlantis fracture zone faults are mapped both on and off ridge using a criterion that distinguishes them from volcanic topography abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults towards ridge segment ends these faults are more widely spaced with larger throws than those at segment centres and often define the boundaries of the gravity bull s eyes these two different faulting styles which probably reflect changes in lithospheric strength are preserved into the rift mountains large throw faults are highly correlated with the mantle bouguer highs suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends

flow patterns of magma in dikes makhtesh ramon israel directions of magma flow were measured in a system of radial dikes in makhtesh ramon israel the flow directions were determined from field observations of segments fingers grooves and groove molds of the dikes the study indicates that the mean axis of magma flow is subhorizontal toward the north in agreement with the direction of divergence of the radial dike system two modes of flow were observed 1 regular bedding parallel flow in the well stratified rock units and 2 irregular meandering flow in the massive rock units it is suggested that corrugated dike walls in well stratified host rocks cause magma channelization and random or self generated restrictions in massive host rocks cause the apparent meanders furthermore the major lithologic boundaries in the host units strongly affect segmentation of the dikes

south polar features on venus similar to those near the north pole venus has no seasons slow rotation and a very massive atmosphere which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets infrared observations by previous missions to venus revealed a bright dipole feature surrounded by a cold collar at its north pole the polar dipole is a double eye feature at the centre of a vast vortex that rotates around the pole and is possibly associated with rapid downwelling the polar cold collar is a wide shallow river of cold air that circulates around the polar vortex one outstanding question has been whether the global circulation was symmetric such that a dipole feature existed at the south pole here we report observations of venus south polar region where we have seen clouds with morphology much like those around the north pole but rotating somewhat faster than the northern dipole the vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper the spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition

the snake range decollement an exhumed mid tertiary ductile brittle transition the snake range decollement srd in east central nevada separates supracrustal rocks extended by normal faulting from ductilely deformed igneous and metamorphic rocks a well known stratigraphy unaffected by earlier faulting permits analysis of both upper and lower plate strain leading to a better understanding of how vastly different rock types and deformational styles are juxtaposed along low angle faults in metamorphic core complexes middle cambrian to permian upper plate rocks are cut by two generations of ne trending east directed normal faults both generations were initiated as high angle 60 planar faults that flattened abruptly into the srd and rotated domino style to low angles yielding a total rotation of bedding of about 80 90 faulting is tertiary in age as 35 m y old volcanic rocks are involved and resulted in about 450 500 extension in a n55w s55e direction the srd developed as a subhorizontal surface 6 7 km deep at the top of the cambrian pioche shale lower plate granitic rocks and their late precambrian cambrian metamorphic country rocks were involved in progressive ductile to brittle extension at low greenschist grade forming a penetrative subhorizontal foliation and n55 70w lineation that increases in intensity eastward and upward toward the srd stretching and thinning in the lower plate is coaxial and comparable in magnitude to upper plate extension and is interpreted as synchronous k ar ages ranging from 20 to 40 m y in the lower plate suggests the n snake range represents a tertiary thermal anomaly we conclude that the srd developed as a ductile brittle transition zone at 6 7 km depth gravity data suggests that the gently domed srd is cut by younger basin and range faults but the geology of adjacent ranges suggests that the srd does not continue more than 60 km in any given direction the lack of stratigraphic omission across the srd rules out large amounts of movement on a surface that originally cut downsection and we suggest that extensional detachment faults such as the srd can be developed locally as boundaries between brittlely extended rocks and underlying ductile extension and intrusion

mid crustal cretaceous roots of cordilleran metamorphic core complexes thermobarometry for cretaceous to mid tertiary plutonism and deformation in the lower plate of whipple and santa catalina metamorphic core complexes shows that both crystalline terranes originated in the middle crust moreover they are characterized by a striking acceleration of tectonic decompression coincident with middle tertiary low angle detachment faulting leading to erosional and tectonic unroofing by mid miocene time depth estimates for emplacement of five intrusive suites within the whipple complex include 1 33 4 km for the peraluminous 89 ma whipple wash plutonic suite 2 29 1 km for the 73 ma axtel quartz diorite 3 16 5 km for mylonitization and synkinematic plutonism at 26 ma 4 6 2 1 9 km for the postkinematic 19 ma war eagle gabbro quartz diorite complex and 5 5 2 2 3 km for a 17 ma postkinematic granodiorite decompression initially occurred at a low rate of 0 3 mm yr from 89 to 26 ma and increased to approximately 2 mm yr during the late oligocene to middle miocene estimated depths for four pluton emplacement or deformational events in the santa catalina mountains include 1 21 1 km for the magmatic epidote bearing 68 ma leatherwood quartz diorite 2 15 3 km for the garnet two mica 47 ma wilderness granite 3 9 3 1 9 km for post wilderness mylonitzation and 4 6 3 2 6 km for the 27 ma catalina monzogranite post laramide decompression estimated at 0 3 mm yr accelerated to 1 3 mm yr prior to the cessation of detachment faulting whereas most batholithic terranes of the north american cordillera are representative of an upper crustal setting core complexes provide as a consequence of their tectonic evolution a petrological and structural view into middle crustal processes

solution to the volume problem in serpentinization kernel pattern consists of a rim of completely serpentinized peridotite surrounding a core of partly serpentinized or unserpentinized peridotite it is found adjacent to shear zones in some serpentinites kernels are commonly rectangular with length width ratios of 1 to 3 the boundary between the completely serpentinized rim and the unserpentinized core is commonly cut at a high angle by cross fractures the cross fractures which contain either picrolite or chrysotile asbestos taper out in the core and widen in the rim they are almost always perpendicular to the core rim boundary and point into the core of the kernel in this sense they are radially distributed in the rim the kernel pattern can be observed in thin section in outcrop and on the megascopic scale of tens of metres cross fractures represent expansion fractures in serpentinite in the rim of the kernel caused by an increase in volume acompanying serpentinization of the peridotite in the core of the kernel as the serpentinization front penetrates into the kernel the serpentinized peridotite expands however the serpentinite in the rim cannot expand and must fracture to accommodate the increase in volume as serpentinization proceeds more expansion occurs extending the fractures farther into the core and requiring the fractures in the existing serpentinite to widen these fractures are subsequently filled by serpentine minerals the resulting pattern consists of a rim of serpentinite cut by serpentine veins that surrounds a core of peridotite measured densities of cross fractures in the rims of kernels are consistent with those expected from the magnitude of the volume change accompanying serpentinization of peridotite

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

crustal evolution of the mid atlantic ridge near the fifteen twenty fracture zone in the last 5 ma author posting american geophysical union 2003 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geochemistry geophysics geosystems 4 2003 1024 doi 10 1029 2002gc000364

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

constraints on deformation conditions and the origin of oceanic detachments the mid atlantic ridge core complex at 15 45 n 1 deformed rocks sampled from a corrugated detachment fault surface near the mid atlantic ridge 15 45 n constrain the conditions of deformation and strain localization samples recovered in situ record deformation restricted to the cold shallow lithosphere greenschist facies with no evidence for significant high temperature deformation either at the fault zone or in the footwall near it high temperature deformation 720 750 c is observed only at two sites and cannot be directly linked to the detachment detachment faulting was coeval with dyke intrusions that cross cut it as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones and by the presence of chill margins in diabase against fault rock basalts are very scarce and restricted to clasts in breccias with no evidence of pillows or extrusive structures gabbros crop out along mass wasted and fault scarps structurally below the detachment footwall rocks show little or no deformation due to strain localization along a narrow shear zone 200 m with fluid flow as required to form talc and amphibole schists after an ultramafic protolith we speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold shallow lithosphere is applicable to other corrugated surfaces along slow and intermediate spreading ridges these observations preclude detachment models rooting in melt rich zones i e atlantis bank southwest indian ridge or recording high temperature deformation we infer that oceanic detachment faults 1 localize strain at t 500 300 c 2 persist during active magmatism and 3 root at shallow rheological boundaries such as a melt rich zone or magma chamber hot detachments or an alteration front cold detachments

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

a mechanism for the venus thermospheric superrotation a physical mechanism for in situ forcing of the venus thermospheric superrotation is developed which is in accord with the bulk of the constraining observations saturation of small to medium scale internal gravity waves in the thermosphere can supply the necessary accelerations of the mean flow if the waves have predominantly large westward phase speeds characteristic of the wind speeds in the upper clouds several scenarios for wave forcing are examined and the lindzen holton parameterization is used to derive wave induced accelerations of the mean flow the results are compared to the vtgcm which uses a similar wave drag parameterization with an ad hoc empirically based asymmetry parameter to model the superrotation

the frictional behavior of serpentinite implications for aseismic creep on shallow crustal faults serpentine is common in many active faults and may be responsible for aseismic creep along segments of these faults to test this we have conducted a series of velocity stepping experiments to determine the frictional velocity dependence of serpentinite we slid initially bare rough surfaces of antigorite serpentinite at room temperature with velocities from 0 0032 to 10 0 m s 1 0 102 to 3 2 105 mm yr and normal stresses from 25 to 125 mpa we find that the velocity dependence of serpentinite undergoes a transition from velocity weakening at fast loading velocities to velocity strengthening at slow velocities and that this change is accompanied by other changes in the constitutive behavior these results suggest that serpentinite should not be the site of instability initiation during sliding at plate velocities but may permit propagation of unstable slip initiated elsewhere

late cretaceous to early tertiary ductile deformation catalina rincon metamorphic core complex southeastern arizona detailed mapping and kinematic analysis have revealed east vergent late cretaceous to early tertiary ductile deformation in the eastern santa catalina mountains within the catalina rincon metamorphic core complex a gently dipping tectonite fabric formed concurrently with greenschist to amphibolite facies metamorphism in precambrian through upper cretaceous sedimentary and plutonic rocks the tectonites contrast in age lineation orientation shear sense and deformational style with younger mid tertiary mylonites that are kinematically coordinated with the detachment fault the late cretaceous to early tertiary deformation may reflect crustal thickening prior to mid tertiary extension of the complex

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

circulation of the venus upper mesosphere lower thermosphere doppler wind measurements from 2001 2009 inferior conjunction sub millimeter co absorption line observations abstract sub millimeter 12 co 346 ghz and 13 co 330 ghz line absorptions formed within the mesospheric to lower thermospheric altitude 70 120 km region of the venus atmosphere have been mapped across the nightside disk of venus during 2001 2009 inferior conjunctions employing the james clerk maxwell telescope jcmt radiative transfer analysis of these thermal line absorptions supports temperature and co mixing profile retrievals as described in a companion paper clancy et al 2012 here we consider the analysis of the sharp line absorption cores of these co spectra in terms of accurate doppler wind profile measurements at 95 115 km altitudes versus local time 8 pm 4 am and latitude 60n 60s these doppler wind measurements support determinations of the nightside zonal and subsolar to antisolar ssas circulation components over a variety of timescales the average behavior fitted from 21 retrieved maps of 12 co doppler winds obtained over hourly daily weekly and interannual intervals indicates stronger average zonal 85 m s retrograde versus ssas 65 m s circulation at the 1 bar pressure 108 110 km altitude level however the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales furthermore the individual doppler wind measurements within each nightside mapping observation generally show significant deviations 20 50 m s averaged over 5000 km horizontal scales from the simple zonal ssas solution with distinct local time and latitudinal characters that are also time variable these large scale residual circulations contribute 30 70 of the observed nightside doppler winds at any given time and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures as coincidentally retrieved co abundance and temperature distributions do not correlate with solution retrograde zonal and ssas winds see companion paper clancy et al 2012 limited comparisons of these nightside submillimeter results with dayside infrared doppler wind measurements suggest distinct dayside versus nightside circulations in terms of zonal winds in particular combined 12 co and 13 co doppler wind mapping observations obtained since 2004 indicate that the average zonal and ssas wind components increase by 50 100 between altitudes of 100 and 115 km if gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere alexander m j 1992 geophys res lett 19 2207 2210 such waves deposit substantial momentum i e break in the lower nightside thermosphere

a dynamic upper atmosphere of venus as revealed by virtis on venus express esa s venus express probe has been in orbit since april 2006 eight research papers in this issue present new results from the mission covering the atmosphere polar features interactions with the solar wind and the controversial matter of venusian lightning hakan svedham et al open the section with a review of the similarities and mostly differences between venus and its twin the earth andrew ingersoll considers the latest results and also how the project teams plan to make the most of the probe s remaining six years of life observations of infrared emission from co2 o2 and no established that photochemical and dynamic activity controls the structure of the upper atmosphere of venus but were unable to identify the altitude of the emission but it is reported here that day side co2 emission extends from 90 120 km altitude with a peak at 115 km night side o2 emission peaks at 96 km and is visible over the range 95 100 km the upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space molecular emissions from the upper atmosphere 90 120 km altitude of venus can be used to investigate the energetics and to trace the circulation of this hitherto little studied region previous spacecraft1 and ground based2 3 4 observations of infrared emission from co2 o2 and no have established that photochemical and dynamic activity controls the structure of the upper atmosphere of venus these data however have left unresolved the precise altitude of the emission1 owing to a lack of data and of an adequate observing geometry5 6 here we report measurements of day side co2 non local thermodynamic equilibrium emission at 4 3 m extending from 90 to 120 km altitude and of night side o2 emission extending from 95 to 100 km the co2 emission peak occurs at 115 km and varies with solar zenith angle over a range of 10 km this confirms previous modelling7 and permits the beginning of a systematic study of the variability of the emission the o2 peak emission happens at 96 km 1 km which is consistent with three body recombination of oxygen atoms transported from the day side by a global thermospheric sub solar to anti solar circulation as previously predicted8

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

submersible study of an oceanic megamullion in the central north atlantic recently discovered megamullions on the seafloor have been interpreted to be the exhumed footwalls of long lived detachment faults operating near the ends of spreading segments in slow spreading crust we conducted five submersible dives on one of these features just east of the rift valley in the mid atlantic ridge at 26 35 n and obtained visual rock sample gravity and heat flow data along a transect from the breakaway zone where the fault is interpreted to have first nucleated in 2 0 2 2 ma crust westward to near the termination 0 7 ma our observations are consistent with the detachment fault hypothesis and show the following features in the breakaway zone faulted and steeply backtilted basaltic blocks suggest rotation above a deeper shear zone the youngest normal faults in this sequence are interpreted to have evolved into the long lived detachment fault in younger crust the interpreted detachment surface rises as monotonously flat seafloor in a pair of broad gently sloping domes that formed simultaneously along isochrons and are now thinly covered by sediment the detachment surface is locally littered with basaltic debris that may have been clipped from the hanging wall the domes coincide with a gravity high that continues along isochrons within the spreading segment modeling of on bottom gravity measurements and recovery of serpentinites imply that mantle rises steeply and is exposed within 7 km west of the breakaway but that rocks with intermediate densities prevail farther west within 5 km of the termination small volcanic cones appear on the detachment surface indicating melt input into the footwall we interpret the megamullion to have developed during a phase of limited magmatism in the spreading segment with mantle being exhumed by the detachment fault 0 5 m y after its initiation increasing magmatism may eventually have weakened the lithosphere and facilitated propagation of a rift that terminated slip on the detachment fault progressively between 1 3 m y and 0 7 m y identifiable but low amplitude magnetic anomalies over the megamullion indicate that it incorporates a magmatic component we infer that much of the footwall is composed of variably serpentinized peridotite intruded by plutons and dikes

geological investigation of a lineated massif at the kane transform fault implications for oceanic core complexes recently available new perspectives on the morphology of the sea floor near the mid atlantic ridge reveal the widespread development of dome like massifs with lineated upper surfaces in form and dimensions these features resemble continental metamorphic core complexes developed in regions of large magnitude extension and thus may be oceanic core complexes excellent examples of these features occur south of the kane transform fault where high resolution geological data from side looking sonar deeply towed cameras and manned submersibles are available the geological data from two different massifs representing somewhat different types of crustal structure provide important constraints on the development of these oceanic core complexes inferences from these direct geological investigations of oceanic core complexes are a first step toward understanding the processes by which these features form in slow spread crust specific relationships in these massifs and nearby areas suggest that these structures have formed over significant lengths of ridge axis tens of kilometres and are not necessarily confined to any specific setting with respect to morphologic segmentation patterns in some cases they are developed in thick gabbroic crust but elsewhere have formed in crust composed serpentinite riddled with gabbroic intrusions ductile stretching and thinning of these sections is recorded in extensive mylonites and shear zones syntectonic gabbroic intrusions are widespread and contradict the notion that the massifs develop during intervals of amagmatic spreading cataclastic fault zones in gabbros and serpentinites developed during uplift cooling and hydration of the stretched crust and may have been localized as major detachment faults sparse diabase dikes and basaltic lavas are the latest magmatic additions to these areas numerous joints and steeply dipping small offset faults are the youngest observed structures across part of the top surface of one massif small scale faults appear to bound elongated ridges of fractured basement rocks creating the lineated terrain thus the extensive lineated surfaces nearby need not reflect prolonged displacement along a continuous detachment fault surface geological relationships developed in the dome like massifs along the kane transform fault hint at a complex array of processes involved in the evolution of these features and raise new questions regarding their origin and analogies with continental core complexes

emplacement of deep crustal and mantle rocks on the west median valley wall of the mark area mar 23 n abstract exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow spreading mid atlantic ridge nine dives of the submersible nautile explored the western axial valley wall in the northern cell of the mark area mid atlantic ridge kane fracture zone where peridotite and gabbro outcrops had been previously reported in order to constrain the structure and determine emplacement mechanisms the ridge transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl locally overlain by metabasalts and metadolerites capped by slightly weathered basalts the morphology is controlled by ridge parallel faults dipping moderately 40 65 to the east transform parallel scarps present in the northernmost dives become rare toward the south brittle deformation along moderately dipping fault scarps produced a dense microcrack network filled with greenschist facies minerals chlorite actinolite epidote quartz locally overprinting high temperature ductile deformation fabrics the small hill located at 23 20 in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl above 3100 mbsl the summit of the hill is composed of pillow basalts and sediments the peridotite outerops display a strong schistosity dipping 20 40 to the east parallel to striated normal fault planes some steeper east facing fault scarps truncate the lower dipping fault surfaces the serpentinites deriving from a harzburgitic protolith are cut by rare dikelets of highly differentiated gabbro these new data combined with previous results of alvin dives are used to draw a generalized geological map of the western axial valley wall this map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the mark area the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets low angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south

ultramafic and gabbroic exposures at the mid atlantic ridge geological mapping in the 15 n region abstract the outcrops of mantle derived ultramafic rocks in the 15 n region are the most extensive yet reported for the mid atlantic ridge north of the fifteen twenty fracture zone these outcrops form a belt at least 20 km long along the west wall of the axial valley and also crop out on the east axial valley wall ultramafic rocks also crop out extensively south of the fifteen twenty fracture zone based on dredging and on a morphological analysis of the bathymetric map we propose that ultramafic outcrops may be common in the crust formed between 14 30 n and 15 50 n during at least the past 2 4 m y moderately dipping fault planes and large expanses of tectonic breccia have been observed during dives on the ultramafic outcrops diving observations also show that the ultramafic rocks are capped in stratigraphic contact by a thin layer of basalt this suggests that these rocks were tectonically emplaced at the axial seafloor or very close to it then uplifted in the footwall of the faults that bound the axial valley the occurrence of ultramafic rocks on both walls of the axial valley may be due to frequent changes of faulting polarity in the axial region instead of one master shear zone there would be a complex array of cross cutting conjugate faults and shear zones that could jump inward in the axial domain as spreading proceeds

abyssal peridotite mylonites implications for grain size sensitive flow and strain localization in the oceanic lithosphere abstract microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation fabrics associated with deformation processes at slow spreading mid ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations the most abundant class are medium to coarse grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere these tectonites give geothermometric temperatures of 755 c or higher interpreted to represent lower temperature limits for diffusive exchange in coarse grained abyssal peridotites during cooling this conclusion is consistent with flow laws for olivine at these temperatures the second class of abyssal peridotites previously largely undescribed for the mid ocean ridge environment include fine grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle plastic transition extends into mantle rocks these mylonites give temperatures of 600 c which we suggest represent a lower temperature limit for plastic deformation reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse grained tectonites based on flow laws for olivine we suggest that deformation in some fine grained mylonites occurred by diffusion creep down to 600 c rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature we conclude that a reduction in grain size by cataclasis or dynamic recrystallization resulted in a transition in deformation mechanisms from dislocation to diffusion creep during uplift and or cooling the observation of these fine grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep weak faults may promote the development of long lived detachments in the upper mantle this inference supports mid ocean ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge axis discontinuities are exhumed along long lived detachment faults

latitudinal structure of the venus o2 infrared airglow a signature of small scale dynamical processes in the upper atmosphere abstract images of the nightside limb of venus have been obtained in the northern hemisphere with the virtis multispectral infrared imager on board venus express between april 2006 and october 2008 we analyze the latitudinal distribution of the o 2 a 1 airglow limb profiles at 1 27 m to characterize its distribution and variability we show that the instantaneous structure of the emission is very different from the statistical global view of an enhanced emission near the equator decreasing in brightness and slightly increasing in altitude toward the poles the peak intensity of the limb profiles varies by a factor up to 50 between the brightest spots and the darkest regions the bright airglow spots correspond to regions of enhanced downward flow of oxygen atoms originating from the dayside considerable variations in brightness and morphology are observed in the altitude latitudinal distribution over a 24 h period analysis of the limb profiles indicates that secondary airglow peaks located at altitudes higher than the mean value of 96 km are observed on about 30 of the latitudinal cuts but they are concentrated in narrow latitude areas extending over a few hundred kilometers most of them occur in transition regions between two altitude regimes in the 50 to 60 n region possibly associated with the drop of the cloud top altitude observed equatorward of the cold collar we interpret these results as an indication that the strength of vertical transport in this mesosphere thermosphere transition region is very variable both in location and time this variability also observed in nadir airglow images and wind measurements is a key characteristic of the mesosphere thermosphere transition region it may be caused by fluctuations of the global day to night circulation generated by gravity waves we show with a one dimensional model that local enhancements of eddy transport is a possibility this variability is currently not accounted for by global circulation models that predict a single stable region of enhanced airglow in the vicinity of the antisolar point

evolving force balance at columbia glacier alaska during its rapid retreat 1 changes in driving and resistive stresses play an essential role in governing the buoyancy forces that are important controls on the speed and irreversibility of tidewater glacier retreats we describe changes in geometry velocity and strain rate and present a top down force balance analysis performed over the lower reach of columbia glacier our analysis uses new measurements and estimates of basal topography and photogrammetric surface velocity measurements made between 1977 and 2001 while assuming depth independent strain sensitivity tests show that the method is robust and insensitive to small changes in the calculation parameters spatial distributions of ice speed show little correspondence with driving stress instead spatial patterns of ice speed exhibit a nonlinear correspondence with basal drag primary resistance to flow comes from basal drag but lateral drag becomes increasingly more important throughout the retreat which may account for observed increases in speed maximum basal drag is always located in a prominent constriction located 12 km upstream from the preretreat terminus once the terminus retreated into deep water off the terminal moraine marking the modern maximum extent the upstream location of this maximum basal drag helped to promote thinning and decrease effective pressure in the lower region by limiting replenishing ice flow from upstream an increase in both ice velocity and calving resulted initiating what appears to be an irreversible retreat

altimetry of the venus cloud tops from the venus express observations 1 simultaneous observations of venus by visible and infrared thermal imaging spectrometer and venus monitoring camera onboard the venus express spacecraft are used to map the cloud top altitude and to relate it to the ultraviolet uv markings the cloud top altitude is retrieved from the depth of co2 absorption band at 1 6 m in low and middle latitudes the cloud top is located at 74 1 km it decreases poleward of 50 and reaches 63 69 km in the polar regions this depression coincides with the eye of the planetary vortex at the same latitude and hour angle cloud top can experience fast variations of about 1 km in tens of hours while larger long term variations of several kilometers have been observed only at high latitudes uv markings correlate with the cloud altimetry however the difference between adjacent uv dark and bright regions does not exceed several hundred meters surprisingly co2 absorption bands are often weaker in the dark uv features indicating that these clouds may be a few hundred meters higher or have a larger scale height than neighboring clouds ultraviolet dark spiral arms which are often seen at about 70 correspond to higher altitudes or to the regions with strong latitudinal gradient of the cloud top altitude cloud altimetry in the polar region reveals the structure that correlates with the thermal emission maps but is invisible in uv images this implies that the uv optically thick polar hood is transparent in the near ir

geologic continuous casting below continental and deep sea detachment faults and at the striated extrusion of sacsayhuaman peru in one common type of industrial continuous casting partially molten metal is extruded from a vessel through a shaped orifice called a mold in which the metal assumes the cross sectional form of the mold as it cools and solidifies continuous casting can be sustained as long as molten metal is supplied and thermal conditions are maintained i propose that a similar process produced parallel sets of grooves in three geologic settings as follows 1 corrugated metamorphic core complexes where mylonitized mid crustal rocks were exhumed by movement along low angle normal faults known as detachment faults 2 corrugated submarine surfaces where ultramafic and mafic rocks were exhumed by normal faulting within oceanic spreading centers and 3 striated magma extrusions exemplified by the famous grooved outcrops at the inca fortress of sacsayhuaman in peru in each case rocks inferred to have overlain the corrugated surface during corrugation genesis molded and shaped a plastic to partially molten rock mass as it was extruded from a moderate to high temperature reservoir

mechanisms for accommodation of miocene extension low angle normal faulting magmatism and secondary breakaway faulting in the southern sacramento mountains southeastern california the colorado river extensional corridor crec accommodated up to 100 crustal extension between 23 and 12 ma the southernmost sacramento mountains core complex lies within this region of extreme extension and exposes a footwall of proterozoic mesozoic and miocene crystalline rocks as well as miocene volcanic and sedimentary rocks in the hanging wall to the regionally developed chemehuevi sacramento detachment fault csdf system new structural u pb zircon ar ar and fission track geochronologic and paleomagnetic studies detail the episodic character of both magmatic and tectonic extension in this region extension in this part of the crec was initiated with tectonic slip along a detachment fault system at a depth between 10 and 15 km magmatic extension at these crustal levels began at 20 19 ma and directly account for 5 18 km of extension 10 20 of total extension in the southern sacramento mountains three discrete magmatic episodes record rotation of the least principal stress direction in the horizontal plane from 55 to 15 over the following 3 myr the three intrusions bear brittle and semibrittle fabrics and show no crystal plastic fabric development the final 3 4 myr of stretching were dominated by amagmatic or tectonic extension along a detachment fault system with extension directions rotating back toward 75 the data are consistent with extremely rapid cooling and uplift of miocene footwall rocks the 19 ma sacram suite was emplaced at a mean pressure of 3 0 kbars and uplifted rapidly to a level in the crust where brittle deformation was manifested by movement on the detachment fault at 16 ma by 14 ma the footwall was exposed at the surface with detritus shed off and deposited in adjacent hanging wall basins

circulation of venusian atmosphere at 90 110 km based on apparent motions of the o2 1 27 μm nightglow from virtis m venus express data the paper is devoted to the investigation of venus mesosphere circulation at 90 110 km altitudes where tracking of the o2 a1 g 1 27 m nightglow is practically the only method of studying the circulation the images of the nightglow were obtained by virtis m on venus express over the course of more than 2 years the resulting global mean velocity vector field covers the nightside between latitudes 75 s 20 n and local time 19 5 h the main observed mode of circulation is two opposite flows from terminators to midnight however the wind speed in the eastward direction from the morning side exceeds the westward evening by 20 30 m s and the streams meet at 22 5 0 5 h the influence of underlying topography was suggested in some cases above mountain regions flows behave as if they encounter an obstacle and wrap around highlands instances of circular motion were discovered encompassing areas of 1 500 4 000 km

late miocene ductile extension and detachment faulting mykonos greece the geometry and style of structural relations at map to microscopic scale on mykonos cyclades aegean sea greece suggests ductile extension and detachment faulting during the late miocene this island is dominated by granitic rocks that have a well developed shallowly dipping mylonitic foliation and a northeast southwest trending mineral elongation lineation kinematic indicators in these rocks exhibit a top to the northeast down dip sense of shear textural relations suggest that conditions during mylonitic deformation were greenschist to amphibolite facies and published k ar geochronology suggests a late miocene age for this deformation event mylonitic rocks lie in the footwall of a low angle fault and fault striae parallel to the elongation lineation suggest kinematic coordination between mylonitic deformation and movement along the fault translation along the fault juxtaposes younger tilted sedimentary rocks of low metamorphic grade upon the once deeper mylonitic rocks the style and geometry of deformational fabrics and map relations on this island are strikingly similar to those observed in metamorphic core complexes of the western united states

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin