modeling of deep water renewal through cold convective plumes abstract in only a few regions in the world ocean surface water is transformed into deep water by open ocean convection in this paper small areas of vertical flow or plumes are studied using a steady state model integrated in streamline coordinates we use an entrainment hypothesis based on knowledge about in mixing of water across the shear zone from an area of low turbulence into an area of high turbulence this results in a model using conservation equations for momentum heat and salt and a nonlinear equation of state effects of earth rotation are included the depth reached by surface water when moving through stationary plumes is predicted and found strongly dependent on the properties in the surface layer this layer has to be colder but only slightly denser than the water just below it to be a source for steady state plumes the thermobaric effect the dependency of compressibility on the temperature plays a major role for the plume penetration depth in weakly stratified cold water like in the greenland sea deep convection is found to be possible and efficient through steady state plumes initial excess density of less than 5 10 3 kg m 3 caused by low temperature can give convection deeper than 1500 m through plumes with horizontal scales of a few hundred meters and vertical velocities of order a few tens of centimeters per second

crustal versus mantle core complexes deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures metamorphic core complexes oceanic core complexes and magma poor passive margins using available analogue and numerical models and their comparison with natural examples the present paper reviews the mechanical processes involved in these different types of extensional setting three main aspects are considered i the primary role of lithosphere rheology ii the lithosphere scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii the initiation and development of detachment zones crustal core complexes develop in continental lithospheres whose moho temperature is higher than 750 c with upper crust dominated strength profiles contrary to what is commonly believed it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface in models convex upward detachments result from a rolling hinge process mantle core complexes develop in either the oceanic lithosphere at slow and ultra slow spreading ridges or in continental lithospheres whose initial moho temperature is lower than 750 c with sub moho mantle dominated strength profiles it is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment except if strong strain localization could occur in the lithosphere mantle distributed crustal extension by upper crust faulting above a decollement along the ductile crust increases toward the rift axis up to crustal breakup mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis during increasing extension

40ar 39ar dating of the manson impact structure a cretaceous tertiary boundary crater candidate the mineralogy of shocked mineral and lithic grains in the cretaceous tertiary k t boundary claystone worldwide is most consistent with a bolide impact on a continent both the concentrations and sizes of these shocked grains are greatest in the western interior of north america these data suggest that the manson impact structure in north central iowa is a viable candidate for the k t boundary impact event argon 40 argon 39 age spectrum dating of shocked microcline from the crystalline central uplift of the manson impact structure indicates that there was severe argon 40 loss at 65 7 1 0 million years ago an age that is indistinguishable from that of the k t boundary within the limits of analytical precision

convection in small basins abstract convection in the world s oceans often occurs in small semienclosed basins where bottom slopes and nearby continental shelf breaks are commonplace the evolution of convectively generated heat anomalies in such settings is studied using quasigeostrophic finite difference and point vortex models the displayed behaviors divide essentially into two categories whole fluid column convection in which bottom slope effects are felt immediately and partial fluid column convection in which the topographic effects can be delayed in both cases topography significantly modifies the evolution of convective patches from that occurring over flat bottoms vertical walls induce strong self propagation mechanisms that accelerate alongslope heat transport while the continental shelf slope is repulsive and rejects lower layer anticyclones these anomalies are then stranded being too far offshore to interact with the shelf break and having lost their heton partner in the interaction weaker deep ocean topo

large scale spreading of deep waters in the western mediterranean sea by submesoscale coherent eddies abstract two large scale free drifting isobaric floats experiments sofargos marine science and technology programme phase 2 mast2 and mass transfer and ecosystem response mater mast3 undertaken in 1994 95 in the northwestern mediterranean sea and in 1997 98 in the algerian basin respectively have revealed for the first time that western mediterranean deep water newly formed by deep convection in the gulf of lion the so called medoc site can be advected several hundreds of kilometers away from the formation area by anticyclonic submesoscale coherent vortices scvs this behavior implies that scvs participate actively in the large scale thermohaline circulation and deep ventilation of the western mediterranean sea these scvs are characterized by small radius 5 km very low potential vorticity high aspect ratio 0 1 and extended lifetime 0 5 yr

a pv approach for dense water formation along fronts application to the northwestern mediterranean the mechanisms of dense water formation 29 0 kg m 3 at work in the baroclinic cyclonic gyre of the north western mediterranean basin are investigated through a pv budget pv potential vorticity the pv budget is diagnosed from an eddy resolving 1 36 ocean simulation driven in surface by hourly air sea fluxes provided by a non hydrostatic atmospheric model at 2 5km resolution the pv budget is controlled by the diabatic frictional and advective pv fluxes around the gyre the surface diabatic pv flux dominates the pv destruction except along the northern branch of the north current where the surface frictional pv flux is strongly negative in this region the bathymetry stabilizes the front and maintains the current northerly in the same direction as the dominant northerly wind this configuration leads to optimal wind current interactions and explains the preponderance of frictional pv destruction on diabatic pv destruction this mechanical forcing drives a cross front ageostrophic circulation which subducts surface low pv waters destroyed by wind on the dense side of the front and obducts high pv waters from the pycnocline on the light side of the front the horizontal pv advections associated with the geostrophic cyclonic gyre and turbulent entrainment at the pycnocline also contribute to the pv refueling in the frontal region the surface non advective pv flux involves energy exchanges down to 1400 w m 2 in the frontal zone this flux is 3 5 times stronger than atmospheric buoyancy flux these energy exchanges quantify the coupling effects between the surface atmospheric forcing with the oceanic frontal structures at submesoscale this article is protected by copyright all rights reserved

localized convection in rotating stratified fluid we study the convective overturning of a rotating stratified fluid in the laboratory convection is induced from the surface of a salt stratified fluid by the introduction of salty fluid over a circular area the external parameters are buoyancy forcing of strength b0 applied over a circular area of radius r the rotation rate as measured by f ambient stratification n and the depth h the experiments are motivated by physical scaling arguments which attempt to predict the length and velocity scales of the convective chimney as it adjusts under gravity and rotation and breaks up through baroclinic instability the scales of interest include the number size and typical speeds of the fragments of the broken chimney the final depth of penetration of the convective mixed layer and the total volume of convectively produced water these scales are tested against the laboratory experiments and found to be appropriate in this idealized problem we have found the depth of penetration depends only on the size and strength of the forcing and the ambient stratification encountered by the convection event it does not depend explicitly on rotation the implications of the work to deep water formation in the labrador sea and elsewhere are discussed finally the study has relevance to the role and representation of baroclinic eddies in large scale circulation of the ocean

mechanics of extensional wedges the extensional wedges that overlie low angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold and thrust belt but with an opposite sense of shear on the basal fault such wedges are common in zones of crustal extension such as the basin and range province of the north american cordillera the north sea and the u s gulf coast this paper examines the mechanics of these upper crustal extensional wedges using a modification of the compressional coulomb critical taper model developed for fold and thrust belts and accretionary wedges if a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline a critically tapered extensional wedge is one that is on the verge of coulomb failure everywhere a wedge whose taper is narrower may be slid stably down the same incline without internal deformation a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical after testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges we apply the theory to a pair of active extensional wedges in the brazos area offshore texas both wedges are underlain by well imaged basal detachment faults dipping at 17 o down to depths of 10 km one wedge exhibits relatively little internal deformation whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical regional values for the coefficients of basal and internal friction in excess of 0 3 together with vertical cohesion gradients in excess of 1 kpa m are consistent with the observed pore pressures and the geometry of the wedges as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults such values are comparable to values inferred from mechanical and thermal models of compressional wedges and they are consistent with laboratory rock mechanics data

shock induced growth and metastability of stishovite and coesite in lithic clasts from suevite of the ries impact crater germany the microtextures of stishovite and coesite in shocked non porous lithic clasts from suevite of the ries impact structure were studied in transmitted light and under the scanning electron microscope both high pressure silica phases were identified in situ by laser raman spectroscopy they formed from silica melt as well as by solid state transformation in weakly shocked rocks stage i fine grained stishovite 1 8 m occurs in thin pseudotachylite veins of quartz rich rocks where it obviously nucleated from high pressure frictional melts generally no stishovite was found in planar deformation features pdfs within grains of rock forming quartz the single exception is a highly shocked quartz grain trapped between a pseudotachylite vein and a large ilmenite grain in which stishovite occurs within two sets of lamellae it is assumed that in this case the small stishovite grains formed by the interplay of conductive heating and shock reverberation in strongly shocked rocks stages ib iii above 30 gpa grains of former quartz typically contain abundant and variably sized stishovite 6 m embedded within a dense amorphous silica phase in the interstices between pdfs the formation of transparent diaplectic glass in adjacent domains results from the breakdown of stishovite and the transformation of the dense amorphous phase and pdfs to diaplectic glass in the solid state coesite formed during unloading occurs in two textural varieties granular micrometre sized coesite occurs embedded in silica melt glass along former fractures and grain boundaries these former high pressure melt pockets are surrounded by diaplectic glass or by domains consisting of microcrystalline coesite and earlier formed stishovite the latter is mostly replaced by amorphous silica

dense water formation in the north western mediterranean area during hymex sop2 in 1 36 ocean simulations sensitivity to initial conditions the north western mediterranean sea is a key location where intense air sea exchanges occur in autumn and winter the succession of strong mistral and tramontane situations leading to significant evaporation and ocean heat loss is well known as the controlling factor in the dense water formation dwf with deep convection episodes during hymex sop2 1 february to 15 march 2013 several platforms sampled the area in order to document dwf and air sea exchanges this study investigates the ability of the nemo wmed36 ocean model 1 36 resolution driven in surface by the hourly air sea fluxes from the arome wmed forecasts 2 5 km resolution to represent dwf during hymex sop2 and focuses on the sensitivity to initial conditions after a short evaluation of the atmospheric forcing the high resolution oceanic simulations using three different data sets as initial and boundary conditions are compared to observations collected during the field campaign it evidences that using regional model outputs may lead to unrealistic thermohaline characteristics for the intermediate and deep waters which degrade the simulated new dense water formed using ocean analyses built from observations permits to obtain more realistic characteristics of the western mediterranean dense water however a low stratification favors an overestimation of the convective area and of the dwf rate the dwf chronology is also impacted nevertheless in every run sop2 is characterized by the production of water denser than 29 11 kg m 3 with a peak during the strong mistral event of 23 25 february followed by a period of restratification before a last event of bottom convection on 13 15 march

tem study of shock metamorphism in quartz from the sedan nuclear test site abstract nuclear explosions represent a useful simulation of meteorite impacts stresses of tens of gigapascals are produced during detonation and stress pulses can last 0 1 ms or longer which is 3 4 orders of magnitude longer than laboratory experiments samples studied here were taken from the ejecta of the sedan nuclear test site and consist of a coarse grained granodiorite containing quartz plagioclase k feldspar cordierite and hornblende shocked quartz grains were selected optically in thin section and studied by tem tem reveals the presence of planar deformation features pdfs which consist of multiple sets of planar amorphous lamellae with the crystallographic orientations 10 12 10 12 and rarely 10 11 some lamellae exhibit a substructure which consist of amorphous sublamellae organized enechelon no high pressure phases were detected along the transformation lamellae some lamellae show evidence of beginning recrystallization which probably occurred during cooling of the hot ejecta the shock induced defects present at the sedan site are very similar to those seen in laboratory shock recovery experiments and also to those present at meteorite impact craters e g meteor crater or ries crater this suggests that shock induced amorphization in quartz is not strongly dependant on shock pulse duration

characterization of convective plumes associated with oceanic deep convection in the northwestern mediterranean from high resolution in situ data collected by gliders numerous gliders have been deployed in the gulf of lions northwestern mediterranean sea and in particular during episodes of open ocean deep convection in the winter 2012 2013 the data collected represents an unprecedented density of in situ observations providing a first in situ statistical and 3d characterization of the important mixing agents of the deep convection phenomenon the so called plumes a methodology based on a glider static flight model was applied to infer the oceanic vertical velocity signal from the glider navigation data we demonstrate that during the active phase of mixing the gliders underwent significant oceanic vertical velocities up to 18 cm s 1 focusing on the data collected by two gliders during the 2012 2013 winter 120 small scale convective downward plumes were detected with a mean radius of 350 m and separated by about 2 km we estimate that the plumes cover 27 of the convection area gliders detected downward velocities with a magnitude larger than that of the upward ones 6 cm s 1 versus 2 cm s 1 on average along track recordings of temperature and salinity as well as biogeochemical properties dissolved oxygen fluorescence turbidity allow a statistical characterization of the water masses properties in the plumes core with respect to the background the average downward signal is of colder 1 8 10 3 c slightly saltier 4 9 10 4 psu and thus denser waters 7 5 10 4 kg m 3 the plunging waters are also on average more fluorescent 2 3 10 2 g l 1 the plumes are associated with a vertical diffusion coefficient of 7 0 m2 s 1 and their vertical velocity variance scales with the ratio of the buoyancy loss over the coriolis parameter to the power 0 86

on the relationship between the water mass pathways and eddy variability in the western mediterranean sea the role of eddies on the formation and spreading of water masses in the western mediterranean sea is studied with an ocean general circulation model the model is forced with interannually variable surface forcing for the years from 1979 to 1999 it is found that the model reproduces the major features of the observed mesoscale variability in the gulf of lions and the large eddies evolution in the algerian basin the seasonal evolution of circulation in the gulf of lions and processes of spreading of newly formed intermediate deep waters in the postconvection period is studied for years 1987 and 1992 the model results are compared with data from observations it is shown that the instability of the transition zone between old and newly formed deep waters which takes place after the violent mixing stages of the deep convection leads to collapse of the mixed patch and formation of mesoscale eddies some of these eddies propagate out of the gulf of lions transporting deep waters into the algerian basin the rest of the mesoscale eddies filled with newly formed deep waters remain in the gulf of lions and tend to merge and enlarge after the cyclonic eddies reach the algerian basin they interact with the intense mesoscale field existing there the energy analysis shows that the winter and spring are seasons of intensified baroclinic instability of the mean flow in the two regions of interest the gulf of lions and algerian basin the kinetic energy is released by baroclinic instability in spring and summer the spring spectra in the two regions have maximums at horizontal scales of about 80 100 km which is the typical scale of the eddies in the model these eddies propagate a cyclonic circuit the resulting eddy induced mass transport in deep and intermediate layers is directed out of the gulf of lions and toward the gibraltar strait equivalently we can argue the intermediate and deep waters conveyor belt of the western mediterranean sea is eddy drive

gravity and aeromagnetic constraints on the structure of the woods mountains volcanic center southeastern california the woods mountain volcanic center is a well exposed mildly alkaline volcanic center that formed during the miocene in southeastern california detailed geologic mapping and geochemical studies have distinguished three major volcanic phases precaldera caldera forming and postcaldera geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava flow eruptions rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude circular gravity and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern california analysis of a bouguer gravity anomaly map reduced to the pole magnetic intensity map and three dimensional gravity and magnetic models indicates that there is a single funnel to bowl shaped caldera approximately 4 km thick and approximately 10 km wide at the surface this model is consistent with other siliceous pyroclastic filled calderas on continental crust except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera

experimental constraints on shock induced microstructures in naturally deformed silicates abstract planar deformation features pdfs in various minerals have long been accepted as evidence of impact induced deformation the uniqueness of this association was challenged in the context of the k t boundary extinction debate after mosaicism and microstructures similar to pdfs were reported from the products of explosive volcanism as a result of this debate a significant volume of new experimental and observational data on the development of shock induced microstructures has become available over the last ten years the results reveal that factors such as pre shock temperature pulse duration and crystallographic orientation of target minerals to the shock wave have a primary influence on how these microstructures develop data from diamond anvil cell and high pressure friction experiments reveal that the same solid state amorphization process that produces shock induced pdfs at low temperatures also occurs at much lower strain rates in static experiments the experimental data indicate that the amorphization process is thermally activated and that the character of the resulting pdfs is a function of the applied strain rate shock induced amorphization occurs along those crystallographic planes that are most readily transformed to the high pressure phase during very short pulse durations and produces pdfs that are visible at the optical scale lower strain rate deformation produces tem scale amorphization with orientations that are more homogeneously distributed throughout the target mineral and produces no optically visible pdfs the data confirm the uniqueness of multiple intersecting sets of optically visible pdfs as a diagnostic indicator of hypervelocity impact the data also support the hypothesis that the amorphization process can occur at a wide range of strain rates and that the limiting pressure for the process is controlled by the phase stability of the target mineral under the applied loading conditions not by the hel the data also suggest that the onset pressure the maximum pressure and the pressure range for producing optically visible pdfs decrease with increasing temperature and decreasing strain rate as the pressure range for optically visible pdf formation decreases it is replaced by homogeneous amorphization as observed in the anvil cell experiments thus the uniqueness of multiple intersecting sets of optically visible pdfs to hypervelocity impact is not due to a unique process but rather to a specific set of loading conditions that produce an optically visible microstructure likewise single sets of microdeformations observed in volcanic rocks are produced by the same process but at different loading conditions that preclude the development of multiple intersecting sets of pdfs the data also indicate that shock mosaicism which occurs above the hel represents a plastic response of the target mineral to loading rates that are too large to be accommodated by crystal plastic mechanisms observational data for some naturally deformed samples from the k t boundary the vredefort dome and volcanic rocks along with the experimental observations are used to constrain the range of conditions under which the natural microstructures form and to understand the differences between microstructures produced by impact volcanic and other natural processes finally some possible mechanisms for producing the microstructures observed in volcanic rocks are proposed

a three dimensional numerical model of deep ventilation in temperate lakes because of the variation in the temperature of maximum density with depth it is known that the upper above 250 m and lower water columns circulate separately in deep temperate lakes while near surface water overturns twice per year the deep waters often mix only partially during a complete annual cycle it is believed that deep convection is triggered by storm surge forcing of some of the relatively cold upper water column downward through its compensation depth so that it becomes unstable and sinks the scale and intensity of the resulting deep water forming plumes are studied numerically a high resolution model based on the nonhydrostatic boussinesq equations is used deep water formation is initiated by applying a statically unstable initial temperature profile over various regions of the domain at depths below 300 m the grid spacing is small enough to resolve individual plumes which carry surface water to the lake bottom the domain of study is also large enough that the geostrophic eady 1949 wave breakup of newly formed deep water can be observed it is found that when the initial instability is applied in midbasin remote from the solid sidewalls relatively little fresh deep water is formed and the fluid quickly reaches a geostrophic and hydrostatic balance vigorous vertical mixing ceases after 4 days and the resulting baroclinic fluid has a well defined and predictable scale in contrast when the initial instability is applied immediately adjacent to a solid boundary vigorous plume motion continues for the duration of the numerical simulations 12 days producing a much greater volume of fresh deep water a parametric study investigates the scale and intensity of this boundary mixing it is found that reduced surface water temperature coriolis acceleration or higher horizontal diffusion coefficients increase the rate of deep water renewal

recent changes in deep water formation and spreading in the eastern mediterranean sea a review abstract observations of the last decade testify that the characteristics of the deep thermohaline circulation in the eastern mediterranean sea have changed thoroughly the source of the most dense waters of the basin has moved from the adriatic to the aegean sea this new source has proved to be very efficient since the estimated formation rate for the period 1989 95 was more than 1 sv about three times more than the dense water formation rate of the adriatic sea these new waters with hydrological characteristics that are warmer and more saline have replaced almost 20 of the older deep waters of the basin and have uplifted the deep isopycnals by about 500 m this major event can be attributed to important meteorological anomalies in the eastern mediterranean and to changes of circulation patterns the extended dry period of 1988 93 and the exceptionally cold winters of 1987 and 1992 93 created favorable conditions for increased formation of dense water in the aegean furthermore changes in the circulation patterns in the intermediate water range levantine intermediate water liw and cretan intermediate water themselves possibly linked to meteorological anomalies appear to have played an important role in the redistribution of salt as a result of an interruption to exchanges between the ionian and levantine basin the salinities in the latter started to rise high salinity waters were diverted into the aegean malanotte rizzoli p manca b b ribera dacala m theocharis a 1998 the eastern mediterranean in the 80s and in the 90s the big transition emerged from the poem bc observational evidence rapport du commission international de la mer medittanee 35 174 175 and the westward transport of liw was reduced an additional effect of the deep water discharge from the aegean and the resulting uplifting of mid depth waters was to lower salinities in the liw layer this effect is most strongly felt in the ionian sea a 3 d primitive equation numerical model for the eastern mediterranean with a 20 km grid size is used to simulate the observed changes and understand the basic mechanisms which caused them under appropriate atmospheric forcing the model successfully reproduces the main characteristics of the transient these results indicate that the observed changes can be at least partially explained as a response of the eastern mediterranean and more specifically of the aegean to atmospheric forcing variability

a methodology for assessing the postconvection mesoscale dynamics in the gulf of lion from composite datasets in order to access the statistical properties of the mesoscale dynamics in the western mediterranean and its associated transport and heat fluxes during the postconvection period the authors have applied data combination methods for analyzing a wide range of in situ measurements collected during the thetis 1 and convhiv experiments ctd and xbt profiles were merged with times series at a fixed or moving point and also with integral time series obtained from acoustic tomography data estimates of temperature and currents within a box of approximately one degree square over a time period of 35 days during the postconvection period were produced during this winter convection has been only partial rarely penetrating deeper than 1200 m the analysis concentrates on the upper 1000 m where most changes occur geostrophy is used as a dynamical constraint relating the parameters the time evolution is controlled by a kalman filter using simple persistence the contribution of the different datasets to the estimation indicates their complementarity in the time and space dimensions hydrography and eulerian measurements provide a major contribution to the estimation of the baroclinic modes tomography data complement the estimate at all horizontal scales float data bring some information on the barotropic mode but the major contribution on this mode comes from the reciprocal tomography data particularly at the largest scales the period analyzed mostly covers the postconvection estimation of the kinetic energy indicates that the barotropic contribution represents 85 of the total energy horizontal advection transfers heat toward the central area at a mean rate of 50 w m 2 compensating for the heat losses through the surface the mesoscale flow field observed is characterized by strongly barotropic coherent vortices with a size o 30 40 km these barotropic eddies are present during all phases of convection

nuclear blast induced nanotextures in quartz and zircon within trinitite the intense heat and pressure resulting from the detonation of the world s first nuclear device in the new mexico desert july 16 1945 severely altered the arkosic sand producing the fused glassy material referred to as trinitite the study of trinitite is key to the development of nuclear forensic techniques that can provide crucial information about a nuclear event such as device composition and radionuclide distribution moreover nuclear blasts are often considered analogs to catastrophic natural events such as meteorite impacts and it is well documented that with increasing impact severity zircon and quartz grains deform systematically in trinitite a sufficient number of primary quartz and zircon grains remain identifiable here a multi technique approach focused ion beam scanning electron microscopy transmission electron microscopy and micro raman spectroscopy is employed to study the micrometer to nanometer scale deformation features in altered grains of zircon and quartz to constrain blast pressure and temperature conditions trinitite zircon grains consistently show an outer halo of fibrous baddeleyite radiating from a relatively unaltered core hrtem images show complex twinning likely originating from an intermediate tetragonal zirconia precursor trinitite quartz grains show various states of melting that appear to vary predictably with depth below the surface of the desert sand grains occurring deeper than 1 5 cm are crystalline with occasional planar fractures at the optical scale at shallower depths a systematic increase in quartz vitrification is observed considered together these data suggest maximal temperatures in excess of 1500 c and pressures of 10 gpa the latter being considerably less than for any natural impact event taken in a broader context the implications of this work extend toward exploiting the use of advanced imaging techniques to improve our understanding of mineral processes in extreme non equilibrium environments at the near atomic scale

localization of deep ocean convection by a mesoscale eddy abstract observations of open ocean deep convection indicate that it is a highly localized phenomenon occurring over areas of tens of kilometers in diameter the cause of this localization has been ascribed to preconditioning the local weakening of the stable density stratification associated with upwardly domed isopycnal surfaces in a surface intensified cyclonic circulation however most numerical and laboratory studies of localized convection have prescribed the localization artificially by confining the surface buoyancy loss to a circular disk in contrast in the numerical simulations described here deep convection forced by horizontally uniform buoyancy loss is localized within a region of initially weaker stratification than its surroundings the preconditioned region is associated with a cold core cyclonic eddy in geostrophic and cyclostrophic balance as in previous studies of disk shaped cooling the localized convection region undergoes baroclinic instability at late times causing the br

mediterranean sea large scale low frequency ocean variability and water mass formation rates from 1987 to 2007 a retrospective analysis abstract we describe a synthesis of the mediterranean sea circulation structure and dynamics from a 23 year long reanalysis of the ocean circulation carried out by adani et al 2011 this mesoscale permitting dynamical reconstruction of past ocean variability in the mediterranean sea allows the study of the time mean circulation and its low frequency decadal components it is found that the time mean circulation is composed of boundary and open ocean intensified jets at the border of cyclonic and anticyclonic gyres the large scale basin circulation is generally characterized in the northern regions by cyclonic gyres and in its southern parts by anticyclonic gyres and eddy dominated flow fields with the exception of the tyrrhenian and the northern ionian sea the time mean tyrrhenian sea circulation is dominated by cyclonic gyres of different intensity and intermittency the northern ionian sea circulation however reverses in sign in two ten year periods the first in 1987 1996 and the second in 1997 2006 which is here called the northern ionian reversal phenomenon this reversal is provoked by the excursion of the atlantic ionian stream from the middle to the northern parts of the basin the decadal variability of other parts of the basin is characterized by changes in strength of the basin scale structures the water mass formation rates and variability are dominated by event like periods where the intermediate and deep waters are formed for 2 3 years at higher rates the largest deep water formation events of the past 23 years occurred separately in the western and eastern mediterranean basin the first coincided with the eastern mediterranean transient roether et al 1996 and the second with the western mediterranean deep water formation event in 2005 2006 smith et al 2008 a new schematic of the basin scale circulation is formulated and commented

lithology and evolution of the crust mantle boundary region in the southwestern basin and range province mantle and crustal xenoliths from volcanic rocks in the southwestern basin and range province and colorado plateau transition zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region seismic transects in this area show a strongly reflective moho of generally low relief which in the area of modern transects consists of a thin zone 2 km thick of short reflectors the upper mantle is transparent and has a pn of 7 8 8 0 km s similar to much of the western united states a lower crustal zone 2 13 km thick has variable internal reflectivity and a relatively low velocity of 6 6 6 8 km s upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths whereas melt infiltration preceded and followed ductile deformation brittle deformation represented by closely spaced joint systems and faults followed ductile deformation and is related to the youngest magmatic episodes these structural characteristics and high uppermost mantle temperature 1000 c may combine to explain the relatively low pn alternating layers of ductily deformed and undeformed peridotites with or without igneous intrusions may contribute to the reflectivity of the moho lower crustal xenoliths are dominantly igneous textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths the crustal xenoliths also commonly are jointed and in addition many show partial melting and have abundant cavities that probably were filled with co2 rich fluids these rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records the mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems partial melts and high temperatures garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late miocene extension felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust the preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes the earliest magmatic events preceded extension and later events accompanied and followed extension

when the sky fell on our heads identification and interpretation of impact products in the sedimentary record research on impact products mechanisms and consequences has benefited greatly in the last ten years from the highly publicized cretaceous tertiary kt boundary debate this debate has led to an increasing awareness among earth scientists of the importance and potential effects of impact events in earth history with more and more researchers realizing the role played by impacts and becoming familiar with the characteristics of impact materials it is likely that in the coming years an increasing number of impact events will be identified in agreement with the earth cratering rate estimations grieve 1984 shoemaker et al 1990 the increasing recognition of impact event is demonstrated by the discovery or identification in recent years of several impact craters some of large diameter chicxulub 180 km in yucatan mjolnir 40 km in the barents sea ames 16 km in oklahoma avak 12 km in alaska pretoria saltpan 1 1 km and kalkkop 640 m both in south africa sediments record impact events in the form of impact glass shocked minerals ir anomalies and perhaps also polymic chaotic breccias and or tsunami deposits identification of impact products particularly in ancient rocks is hampered by poor preservation diagenetic alteration metamorphism misinterpretation or even lack of interest or awareness of their significance impact products represent very short term events most likely geographically localized and stratigraphically concentrated over a thin interval of sediments and often diluted by normal background deposition finding these impact signatures requires very detailed petrographic and geochemical investigations with closely spaced sampling a few cm such detail is not commonly used when studying ancient mesozoic and paleozoic sediments

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

composition of the lower crust in west central arizona from three component seismic data new p and s wave seismic data from the basin and range province colorado plateau transition zone allow a better determination of the composition of the lower crust than would be possible from p wave data alone the data were collected in 1987 by recording large seismic refraction shots into stationary reflection spreads during the us geological survey s pacific to arizona crustal experiment significant seismic observations about the lower crust include 1 strong p wave reflections at its top 2 moderate strength s wave reflections from the entire lower crust 3 an average p wave velocity of 6 6 plus minus 0 15 km s sup minus 1 and 4 an average poisson s ratio of sigma 0 27 plus minus 0 02 using these constraints and physical property measurements from laboratory rock samples the authors conclude that the average composition of the lower crust in this region is intermediate to mafic and that mafic underplating has been minor consequently the cenozoic age mafic and ultramafic xenoliths that are found in the transition zone must have resided in the upper mantle or in a thin transition zone at the base of the crust since these rock have higher p wave velocities and poisson s ratio than observed seismically

the role of preconditioning in the evolution of open ocean deep convection large eddy simulation of open ocean deep convection shows that the evolution of convection appears in a fundamentally different pattern depending on the precondition of the ocean and the magnitude of the surface buoyancy flux as the intensity of the cyclonic gyre in the ocean under the cooling increases the pattern of convection is transformed from distributed convection to localized convection in localized convection the typical pattern of open ocean deep convection appears such as the generation of baroclinic instability and large lateral buoyancy transfer secondary circulation restratification and the breakup of the original cyclonic gyre on the other hand in distributed convection small scale convective plumes appear uniformly over the whole surface similarly to the convective boundary layer without generating the typical features of open ocean deep convection hence an enormous difference in the generated eddy kinetic energies exists between the two cases it is also found that a stronger cooling at the sea surface suppresses the transition from distributed convection to localized convection dimensional analysis provides the parameters to characterize the pattern of convection and the critical condition for the transition is estimated by analyzing the numerical results

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

the k t boundary at beloc haiti compared stratigraphic distributions of the boundary markers at beloc haiti the cretaceous tertiary boundary ktb is characterized by a spherule bed containing glass particles these particles are considered by some authors as remains of tektites resulting from a nearby impact however because of the stratigraphic complexity of the beloc sections the genetic link between the ktb cosmic event and the spherule bed is not obvious in this paper we report new data on shocked quartz and ni rich spinels at haitian ktb sites the detailed stratigraphy of these minerals shows that there is no empty gap in the sedimentary sequence the first and largest shocked quartz is found in the upper part of the spherule layer they are abundant and size graded over the 25 30 cm of carbonate rich sediments overlying this layer the first ni rich spinels which are also rich in cr appear in the carbonate sediments the size grading of the spherules and shocked quartz and the stratigraphical overlapping of their distributions suggests that these two components were derived from the same event although different from typical impact glasses tektites beloc glass particles must be considered as impact derived products the enormous fluence of shocked quartz 104 grains cm2 is consistent with a proximal event the upper part of the sequence is more complex a second distribution of shocked quartz associated with ni rich spinels of different compositions appears in the layer containing the maximum ir concentration we propose that these features which are not easily explained by a sedimentary artifact result from a second collisional event

planar deformation features in shocked quartz a transmission electron microscopy investigation abstract we present a detailed investigation by transmission electron microscopy tem of the fine structure of planar deformation features pdf in quartz grains within which shock metamorphism has been detected optically and which originate from various sites slate islands la malbaie and manicouagan lake canada ries crater germany the toba caldera sumatra the vredefort complex in south africa and the k t boundary at raton basin colorado pdfs appear on tem micrographs as straight and very narrow tem reveals many more pdfs than are optically detected probably because only concentrations of such defects in narrow areas can induce an optically detectable contrast caution should therefore be exercised when applying criteria based on the optical observation of single or multiple sets of pdfs pdfs in samples from slate islands and raton basin exhibit a peculiar morphology a high density of very tiny bubbles precipitated on the dislocation lines comparison with the behavior of wet quartz strongly suggests that these materials experienced lengthy annealing at moderate pressure and temperature

dynamic deformation of quartz and feldspar clues to causes of some natural crises abstract indications of natural catastrophes and crises are preserved in some geologic strutures and in the stratigraphic record emphasizing the dynamic nature of the earth s evolution the crises may result either from exogenic or endogenic energy sources distinguishing between these sources is a complex multidisciplinary problem in the earth sciences shock deformation by impact has been well studied both in shock recovery experiments and in known impactites but only recently has the possibility of generation of shock waves by high temperature internal explosions been seriously considered an optically based reconaissance survey of microstructures in quartz and feldspar in some explosive silicic volcanics suggests that they may have been induced by dynamic deformation at high temperature in some respects the character of the microstructures differs greatly from those induced by room temperature shock whereas in others the differences are subtle results from recent shock recovery experiments on quartzite preheated to 440 and shocked to 28 gpa support this contention for two classic cretaceous tertiary sections at gubbio italy it is shown that shock mosaicism and shock lamellae occur over a 4 m interval bisected by the boundary in a pattern approximately correlatable with an independently established iridium enrichment these data are incompatible with any single event and are most plausibly interpreted as reflecting a period 0 5 my of intense probably episodic volcanism with a culmination at the k t transition multiple dynamic events may also account for macroscopic and microscopic structures developed during the evolution of the vredefort structure south africa

co 2 rich fluid inclusions along planar elements of quartz in basement rocks of the vredefort dome south africa along a nw se profile through the basement core starting below the sedimentary unconformity and ending in the center of the nearly circular structure the constituent quartz grains and their fluid inclusions exhibit the following characteristics

estimating shock pressures based on high pressure minerals in shock induced melt veins of l chondrites here we report the transmission electron microscopy tem observations of the mineral assemblages and textures in shock induced melt veins from seven l chondrites of shock stages ranging from s3 to s6 the mineral assemblages combined with phase equilibrium data are used to constrain the crystallization pressures which can be used to constrain shock pressure in some cases thick melt veins in the tenhaml6 chondrite contain majorite and magnesiowstite in the center and ringwoodite akimotoite vitrified silicate perovskite and majorite in the edge of the vein indicating crystallization pressure of 25 gpa however very thin melt veins 5 30 m wide in tenham contain glass olivine clinopyroxene and ringwoodite suggesting crystallization during transient low pressure excursions as the shock pressure equilibrated to a continuum level melt veins of umbarger include ringwoodite akimotoite and clinopyroxene in the vein matrix and fe2sio4 spinel and stishovite in sio2 feo rich melt indicating a crystallization pressure of 18 gpa the silicate melt veins in roy contain majorite plus ringwoodite indicating pressure of 20 gpa melt veins of ramsdorf and nakhon pathon contain olivine and clinoenstatite indicating pressure of less than 15 gpa melt veins of kunashak and la lande include albite and olivine indicating crystallization at less than 2 5 gpa based upon the assemblages observed crystallization of shock veins can occur before during or after pressure release when the assemblage consists of high pressure minerals and that assemblage is constant across a larger melt vein or pocket the crystallization pressure represents the equilibrium shock pressure

evidence for shear zones in the lower crust offshore britain lower crustal reflections seen on british institutions reflection profiling syndicate birps deep seismic reflection data recorded offshore the united kingdom are commonly short convex up and crosscutting consequently the lower crust should not be described as layered as much of the reflecting structure is not fully resolved and may be quite complex moreover the lower crustal reflective zone lcrz is not a uniformly reflective layer but contains reflective bands that wrap around nonreflective transparent zones because transparent zones probably correspond to a lack of a reflecting fabric understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust although transparent zones may have a variety of explanations e g large shear pods or frozen magma bodies the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip line field theory and by drawing analogies with structures developed in highly tectonised high grade metamorphic terrains this interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms in particular extension may be accommodated by the relative movement of low strain lozenges along intervening zones of simple shear the lower crust may be stretched during extension but it must also transfer deformation between the upper crust and the mantle the observation on birps data of mantle reflections interpreted as mantle shear zones implies that deformation in the mantle is localised however the manue reflections are never colinear with faults in the upper crust requiring at least locally a component of sub horizontal bulk strain within the lower crust which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectivity of the lcrz

a four dimensional analysis of the thermal structure in the gulf of lion the theoretical and experimental tomography in the sea experiment thetis 1 took place in the gulf of lion to observe the evolution of the temperature field and the process of deep convection during the 1991 1992 winter the temperature measurements consist of moored sensors conductivity temperature depth and expendable bathythermograph surveys and acoustic tomography because of this diverse data set and since the field evolves rather fast the analysis uses a unified framework based on estimation theory and implementing a kaiman filter the resolution and the errors associated with the model are systematically estimated temperature is a good tracer of water masses the time evolving three dimensional view of the field resulting from the analysis shows the details of the three classical convection phases preconditioning vigourous convection and relaxation in all phases there is strong spatial nonuniformity with mesoscale activity short timescales and sporadic evidence of advective events surface capping intrusions of levantine intermediate water liw deep convection reaching 1500 m was observed in late february by late april the field had not yet returned to its initial conditions strong deficit of liw comparison with available atmospheric flux data shows that advection acts to delay the occurence of convection and confirms the essential role of buoyancy fluxes for this winter the deep mixing results in an injection of anomalously warm water t 0 03 to a depth of 1500 m compatible with the deep warming previously reported

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

shock metamorphism of quartz at the submarine mjolnir impact crater barents sea shock metamorphosed quartz grains have been discovered in a drill core from the central peak of the late jurassic marine mjolnir structure this finding further corroborates the impact origin of mjolnir the intersected strata represent the upper jurassic hekkingen formation and underlying jurassic and upper triassic formations the appearance orientation and origin of shock features in quartz grains and their stratigraphic distribution within the core units have been studied by optical and transmission electron microscopy the quartz grains contain planar fractures pfs planar deformation features pdfs and mechanical brazil twins the formation of pfs is the predominant shock effect and is attributed to the large impedance differences between the water rich pores and constituent minerals in target sediments this situation may have strengthened tensional extensional and shear movements during shock compression and decompression the combination of various shock effects indicates possible shock pressures between 5 and at least 20 gpa for three core units with a total thickness of 86 m from 74 00 m to 171 09 m core depth crater fill material from the lower part of the core typically shows the least pressures whereas the uppermost part of the allochthonous crater deposits displays the highest pressures the orientations of pfs in studied quartz grains seem to become more diverse as the pressure rises from predominantly 0001 pfs to a combination of 0001 and orientations however the lack of experimental data on porous sedimentary rocks does not allow us to further constrain the shock conditions on the basis of pf orientations

shocked quartz in the cretaceous tertiary boundary clays evidence for a global distribution shocked quartz grains displaying planar features were isolated from cretaceous tertiary boundary clays at five sites in europe a core from the north central pacific ocean and a site in new zealand at all of these sites the planar features in the shocked quartz can be indexed to rational crystallographic planes of the quartz lattice the grains display streaking indicative of shock in x ray diffraction photographs and also show reduced refractive indices these characteristic features of shocked quartz at several sites worldwide confirm that an impact event at the cretaceous tertiary boundary distributed ejecta products in an earth girdling dust cloud as postulated by the alvarez impact hypothesis

structural evolution of the whipple and south mountains shear zones southwestern united states the whipple and south mountains of the southwestern united states have undergone a strikingly similar sequence of deformations in both ranges gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low angle detachment fault kinematic indicators reveal that the mylonitic rocks brittle structures and detachment faults are kinematically coordinated and were all formed by top to the northeast shear the structural evolution of both areas can be explained in terms of major shallow dipping shear zones that accommodated tertiary crustal extension we suggest that detachment faults and associated zones of brecciation cataclasis and seismic slip were originally continuous downdip along the low angle shear zones into mylonitic gneisses formed below or near the ductile brittle transition as the mylonites were drawn out from beneath the brittlely extending upper plate they were progressively uplifted above the ductile brittle transition and were overprinted by successively more brittle structures

a three dimensional numerical study of deep water formation in the northwestern mediterranean sea abstract deep water formation dwf in the northwestern mediterranean sea and the subsequent horizontal circulation are investigated in a rectangular basin with a three dimensional primitive equation model the basin is forced by constant climatological heat and salt fluxes convective motion is parameterized by a simple nonpenetrative convective adjustment process plus richardson number dependent vertical eddy viscosity and diffusivity a homogeneous column of dense water is progressively formed in the forcing area meanders of 40 km wavelength develop at the periphery of the column these features agree with observations energy studies show that the meanders are generated mainly through a baroclinic instability process these meanders and the associated cells of vertical motion contribute to the process of dwf they generate vertical advection while the associated horizontal advection tends to restratify the surface water of the column and thus to inhibit very deep convection just before the end o

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

a late mesoproterozoic 40ar 39ar age for a melt breccia from the keurusselka impact structure finland field investigations in the eroded central uplift of the 30 km keurusselka impact structure finland revealed a thin dark melt vein that intersects the autochthonous shatter cone bearing target rocks near the homestead of kirkkoranta close to the center of the impact structure the petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding 8 10 gpa the mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia 40ar 39ar dating of dark and clast poor whole rock chips yielded five concordant late mesoproterozoic miniplateau ages and one plateau age of 1151 10 ma 11 ma 2 mswd 0 11 p 0 98 considered here as the statistically most robust age for the rock the new 40ar 39ar age is incompatible with 1 88 ga svecofennian tectonism and magmatism in south central finland and probably reflects the keurusselka impact followed by impact induced hydrothermal chloritization of the crater basement in keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration the late mesoproterozoic 40ar 39ar age of 1150 ma should be treated as a minimum age for the impact the new 40ar 39ar results are consistent with paleomagnetic results that suggested a similar age for keurusselka which is shown to be one of the oldest impact structures currently known in europe and worldwide

modeling deep ocean convection large eddy simulation in comparison with laboratory experiments a large eddy simulation model les has been applied to study deep convective processes in a stratified ocean driven by the energetic cooling at the ocean surface closely related to a recent laboratory experiment the numerical experiment deals with the inverted problem of the growth of a convective mixed layer driven by a localized source of bottom heating in a rotating stably stratified fluid in general good agreement is found between numerical and laboratory results after onset of the heating a well mixed layer forms above the heated circular surface although small scale turbulence quantities like rms velocities and length scale can be best described by the nonrotating turbulent velocity and length scales they are also found to differ significantly from a nonrotating control run which indicates that rotation affects but does not control the turbulence due to the horizontal radial temperature gradient between the mixed layer and the ambient fluid a rim current develops around the periphery of the heated surface its near surface maximum can be well described by a simple thermal wind law the strong counterrotating current also observed in the laboratory at greater heights above the surface is found to be mainly driven by surface friction and should not be observed in the ocean as time progresses the rim current becomes unstable eventually generating a field of baroclinic eddies that stop the mixed layer growth by causing some horizontal exchange between the convective layer and its cooler surrounding the wavelength of the instabilities slowly increases with time and is clearly related to the local rossby radius

modeling the deep convection in the northwestern mediterranean sea using an eddy permitting and an eddy resolving model case study of winter 1986 1987 1 in the northwestern mediterranean sea winter 1986 1987 was particularly cold inducing a strong open ocean convection event in order to investigate the impact of numerical models spatial resolution on the convection representation and the effects of deep convection on the northwestern mediterranean circulation we perform two numerical three dimensional simulations eddy permitting versus eddy resolving models are forced at the surface by the era40 atmospheric fluxes with a simple heat flux correction to better mimic the observed value we examine the characteristics of the deep convection mixed layer water masses characteristics convection zone and mesoscale structures and perform temporal analysis of this event in terms of kinetic energy buoyancy equilibrium and deep water dw evolution the convection characteristics are similarly represented on a global scale by both models and are in good agreement with observations except for the size of the convection region however the eddy resolving model better reproduces the mesoscale structures whose role in the dw formation mixing and transport is shown to be essential the boundary circulation and the overturning are enhanced during the convection event sixty six percent of the dw spreading is due to the bleeding effect into the catalan sea during the convection event whereas 33 is due to the mesoscale structures southwestward advection after the event sixty percent of the restratification with respect of the water column initial structure occurs before july 1987 and is due to light water advection afterward restratification is due to the mixing and is not complete before next year convection

an axial view of a metamorphic core complex crustal structure of the whipple and chemehuevi mountains southeastern california a 135 km long nw se trending seismic refraction wide angle reflection profile provides a unique along strike view of the crustal structure of a belt of metamorphic core complexes in southeastern california the whipple chemehuevi and sacramento mountains metamorphic core complexes interpretation of the seismic data was done by two dimensional forward modeling of travel times and amplitudes the final model consists of 1 a thin 1 5 km veneer of upper plate and fractured lower plate rocks velocities of 1 5 5 3 km s 1 overlying a fairly homogeneous basement with velocities of 6 0 km s 1 2 a localized high velocity 6 4 km s 1 body situated directly beneath the whipple mountains 3 a 6 3 6 4 km s 1 middle crust that is thickest beneath the core complexes 4 a 6 65 0 15 km s 1 lower crust 5 crustal thickness of 27 km with a deeper crustal root 3 km beneath the whipple mountains metamorphic core complex and 6 a pn velocity of 8 0 0 10 km s 1 the crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust upper crustal velocities are higher beneath the whipple mountains where velocities increase to 6 4 km s 1 at 5 km depth than beneath the chemehuevi and sacramento mountains in addition midcrustal discontinuities rise 2 5 km beneath the whipple complex compared to the other complexes these observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the whipple mountains compared to rocks in the chemehuevi and sacramento mountains despite the enhanced uplift and extension in the whipple mountains the crust is thicker here 30 km than anywhere else along the colorado river extensional corridor this may be in part a relic of compressional and magmatic thickening during the mesozoic however we suggest that inflation of the crust during tertiary extension was the dominant mechanism both mantle derived magmatism and lateral ductile inflow in the crust are proposed

recent advances in observing the physical oceanography of the western mediterranean sea the mediterranean sea has been investigated intensively since the early nineties using modern techniques and collaborative approaches this overview summarizes some of the resulting advances that were made concerning the physical oceanography of the western mediterranean the water mass formation processes are now much better understood and have been quantified to a large extent the boundary conditions of the system in terms of surface fluxes and strait transports can be determined with improved accuracy thus enabling future investigation of interannual variability the dynamics of the surface and intermediate layers have revealed a variety of eddy and mesoscale processes that are important for the circulation and spreading of water masses the deep circulation is being investigated with lagrangian techniques tracers and floats first results show a large component of the deep water originating from the tyrrhenian sea and intense cyclonic and anticyclonic eddy flows

multiscale observations of deep convection in the northwestern mediterranean sea during winter 2012 2013 using multiple platforms during winter 2012 2013 open ocean deep convection which is a major driver for the thermohaline circulation and ventilation of the ocean occurred in the gulf of lions northwestern mediterranean sea and has been thoroughly documented thanks in particular to the deployment of several gliders argo profiling floats several dedicated ship cruises and a mooring array during a period of about a year thanks to these intense observational efforts we show that deep convection reached the bottom in winter early in february 2013 in a area of maximum 28 3 109 m2 we present new quantitative results with estimates of heat and salt content at the subbasin scale at different time scales on the seasonal scale to a 10 days basis through optimal interpolation techniques and robust estimates of the deep water formation rate of 2 0 0 2 sv we provide an overview of the spatiotemporal coverage that has been reached throughout the seasons this year and we highlight some results based on data analysis and numerical modeling that are presented in this special issue they concern key circulation features for the deep convection and the subsequent bloom such as submesoscale coherent vortices scvs the plumes and symmetric instability at the edge of the deep convection area

an ocean largeheddy simulation model with application to deep convection in the greenland sea a nonhydrostatic boussinesq threehdimensional model the ocean largeheddy simulation model lolemr has been developed to study deep oceanic convection the model uses a subgridhscale parameterization of turbulence developed for largeheddy simulation models and the advection of scalars is accomplished using a monotonic scheme a set of experiments was performed using olem to provide a direct comparison with laboratory results and aircraft measurements of the atmospheric convective boundary layer the results from these experiments are in excellent agreement with laboratory and atmospheric convective boundary layer measurements of the mean profiles of zonal and vertical velocity variance potential temperature variance and heat flux the horizontal wavenumber spectra of zonal and vertical velocity are also in good agreement with laboratory measurements and kolmogorov s theoretical inertial subrange spectrum a set of experiments using a potential temperaturehsalinity profile from the central greenland sea for model initialization was conducted to study the effect of the thermobaric instability and rotation on the structure and evolution of deep oceanic convection the artificial removal of the thermobaric instability suppresses penetrative convection which is responsible for rapid changes in water properties at depths much greater than occurs for convective mixedhlayer deepening the vertical velocity and diameter 0 08 m s 1 and 300 m respectively of the penetrative plumes are in good agreement with observations from the greenland sea a period of strong penetrative convection is followed by a gradual transition to convective mixedhlayer deepening during penetrative convection the values of heat flux are about 2 times greater than convective mixedhlayer deepening in the absence of rotation the evolution of penetrative convection occurs more rapidly and vertical motions are more vigorous the presence of the horizontal component of rotation forces asymmetries in the circulation around a penetrative plume these experiments clearly demonstrate the importance of thermobaric instability and rotation on deep convection to properly model largehscale flows in regions of penetrative convection it is necessary to include these effects in the vertical mixing parameterization

regional gravity studies in southeastern california western arizona and southern nevada bouguer gravity anomaly low pass and band pass maps of southeastern california western arizona and southern nevada are analyzed to determine regional crustal structure based on 100 250 km band pass wavelength filtering of the bouguer anomaly map a low amplitude gravity maximum is associated with the highly extended terrane along the colorado river region of california arizona and nevada two crustal models constructed using bouguer gravity anomaly data and constrained by geological mapping aeromagnetic data and seismic refraction data are consistent with the dome shaped midcrustal lens of probable dioritic composition centered under the whipple mountains proposed by mccarthy et al 1989 although other models are not precluded this midcrustal lens extends from the western edge of the colorado river extensional corridor to east of the buckskin and hualapai mountains in arizona dioritic material is exposed in various mountain ranges in the colorado river extensional corridor and may be part of the source of the gravity maximum that is associated with each range

the nature distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the vredefort dome south africa abstract the vredefort dome represents the uplift centre of a well known 2 00 ga old impact structure of unusually large magnitude shock features like shatter cones planar features and high pressure silica polymorphs are common the present study deals with the description mode of occurrence field distribution and post shock metamorphic alteration of coesite and stishovite which were poorly documented up to now these minerals occur as unusually large crystals in the quartzite of the wall rock in contact with very thin pseudotachylite veins in the pseudotachylite itself which can be interpreted as a friction and or a shock recovery melt fine needles of kyanite are ubiquitous it is proposed that these thin pseudotachylites a type formed during the transit of the shock wave they preceded the development of thick pseudotachylite and microbreccia veins b type which formed during the tensional period which immediately followed from comparison with the model of formation of high pressure polymorphs in porous sandstone it is suggested that higher pressure and stress was concentrated along the a type veins at the arrival of the shock front at this time the quartz was transformed into a high pressure phase which was probably poorly crystalline behind the shock front that is during the rarefaction the pressure was progressively released and the polymorphs crystallized from this initial high pressure phase stishovite crystallized first followed by coesite the high pressure conditions may have lasted an unusually long time due to the magnitude of the vredefort impact this long time probably about one second may account for the large size of the coesite and stishovite crystals most of the high pressure silica polymorphs are corroded to a variable degree by secondary quartz and preserved only in a restricted area of the impact structure this alteration is attributed to post shock metamorphism due to the temperature of the rock before impact plus the heat left after shock recovery some of the evidence put forward by authors against the impact model is discussed according to the facts arising from this investigation it is concluded that an origin by impact remains the simplest and most realistic model to explain the origin of the vredefort dome

shock experiments on pre heated α and β quartz ii x ray and tem investigations quartz single crystals pre heated to different temperatures 20 275 540 and 630 c were experimentally shocked to pressures ranging from 20 to 40 gpa with the shock front propagating parallel to 1010 or 0001 shocked quartz pre heated to 630 c is nearly x ray amorphous at 26 gpa while quartz shocked at room temperature still displays several x ray diffraction lines up to 30 gpa all samples that were still crystalline when shocked above 25 gpa exhibit lattice expansion internal strain and reduced crystallite sizes lattice expansion was up to 1 for the constants a o and co and up to 3 for the cell volume v o quartz shocked parallel to 1010 always shows larger lattice constants and is more strained than quartz shocked parallel to 0001 indicating that shock effects are controlled by the structural anisotropy internal strain is anisotropic with minimum strain in the planes belonging to 1012 the dominant shock defect revealed by tem are sets of parallel lamellae consisting of pure sio 2 glass so called planar deformation features pdfs below 25 gpa pdfs are extremely rare and thin 30 nm at pressures above 25 gpa however pdfs occur frequently and are about ten times thicker this qualitative observation of increasing pdf frequency and thickness with increasing pressure is quantified by calculations of the amount of glass using x ray and density data the results of this study substantiate the crucial role of pdfs for the lattice expansion of quartz and the formation of diaplectic glass and lechatelierite lattice expansion is ascribed to the enormous expansion of pdfs during decompression leading to elastic stretching in adjacent crystalline areas the observation that pdfs occur predominantly parallel to 1012 where minimum strain is present supports this hypothesis the increasing amount of pdfs above 25 gpa is interpreted as the cause for the formation of diaplectic glass shock temperature calculations support the assumption that these pdfs consist under compression of superheated melt the heat is possibly sufficient to melt crystalline areas adjacent to the superheated pdfs at enhanced pressures such molten bands coalesce to a total melt which is quenched upon decompression thus diaplectic glass represents a quenched high pressure melt while lechatelierite sio 2 glass with a fluidal texture is interpreted as quenched low pressure melt

distinguishing shocked from tectonically deformed quartz by the use of the sem and chemical etching abstract multiple sets of crystallographically oriented planar deformation features pdfs are generated by high strain rate shock waves at pressures of 12 gpa in naturally shocked quartz samples on surfaces pdfs appear as narrow 50 500 nm lamellae filled with amorphosed quartz diaplectic glass which can be etched with hydrofluoric acid or with hydrothermal alkaline solutions in contrast slow strain rate tectonic deformation pressure produces wider semi linear and widely spaced arrays of dislocation loops that are not glass filled etching samples with hf before examination in a scanning electron microscope sem allows for unambiguous visual distinction between glass filled pdfs and glass free tectonic deformation arrays in quartz this etching also reveals the internal pillaring often characteristic of shock induced pdfs this technique is useful for easily distinguishing between shock and tectonic deformation in quartz but does not replace optical techniques for characterizing the shock features

the s reflector west of galicia the seismic signature of a detachment fault the so called s reflector is a group of bright continuous reflections underlying the landward tilted fault blocks of the west galicia rifted margin s has been interpreted as the brittle ductile transition the lop of an intrusion a detachment fault and the crust mantle boundary to constrain the internal structure of the reflector we have carried out detailed analyses of these reflections we compare the waveforms of the seafloor reflection and its first multiple both to determine the amplitude of the seafloor reflection and to show hat the seafloor is in effect a spike in the reflectivity series so that the seafloor reflection can be used as the far field wavelet including both source and receiver ghosts we compare he waveform of the seafloor and 5 and show that within the resolution of our data s is a reflection from a step increase in acoustic impedance this result is confirmed through complex trace analysis and in particular the determination of the apparent polarity of s and the examination of the instantaneous frequency function s is consistently positive polarity and shows no significant frequency anomaly simple modelling shows that s is very unlikely to be a reflection from a thin layer we thus conclude that s is probably a single steplike interface from the varying frequency content of the data we determine a value for the effective q between s and the seafloor and use this to assess the amplitude loss due to attenuation and scattering we use a comparison between the seafloor and the s reflection to constrain the amplitude of s and estimate a reflection coefficient for s of at least 0 2 in places decreasing landwards by analogy with structures developed in the highly extended regions of the western united states we consider that the most likely interpretation of s is as a sharp west dipping detachment fault separating a granitic upper plate from a higher velocity lower plate locally probably serpentinized mantle

significance of seismic reflections beneath a tilted exposure of deep continental crust tehachapi mountains california the focus of this article is a process whereby lower crustal crystalline and schistose rocks can rise to the surface with the tehachapi mountains in california being the case in point as a prime example of the lower crust these mountains expose cretaceous gneisses that formed 25 30 km down in the sierra nevada batholith and appear to be underlain by the ensimatic rand schist integrated geophysical and geological studies by the calcrust program have produced a cross section through this post mid cretaceous structure and suggest a general model for its development seismic reflection and refraction profiles show that the batholithic rocks dip northward as a tilted slab and extend beneath the southern end of the san joaquin basin s tejon embayment two south dipping reverse faults on the rim of the tejon embayment were discovered in the reflection data and verified in the field the faults have a combined separation of several kilometers and cut through an upper crustal reflection zone that projects to the surface outcrop of the rand schist the upper and lower crusts are separated by a zone of laterally discontinuous reflectors reflections from the lower crust form a wedge the base of which is a nearly flat moho at 33 km regional geological relations and gravity models both suggest that the reflective zone corresponds to the rand schist and the newly recognized faults account for its neogene exposure alternatively the reflective zone maybe part of the gneiss complex suggesting that the schist either lies deeper or is not present under the gneisses if the rand schist underlies the tehachapi mountains and mojave region to their south a model for their evolution can be constructed from regional geological relations it seems that during late cretaceous laramide subduction the protolith of the schist was thrust eastward beneath the mojave along this portion of the cordilleran batholithic belt the subduction was evidently at very low angles the bottom of the batholith was removed and replaced by a thick section of schist fluids from which weakened the overlying batholith this thickened crust collapsed by horizontal flow in the schist and faulting of the upper crust into flat lying slabs when emplacement of the schist ended in latest cretaceous earliest paleocene the underlying mantle rose compensating for the extension and providing material for magmatic underplating in the neogene transpression and rotation of the upper crust along the san andreas and garlock faults resulted in the exposure of the schist

the nature of maskelynite in shocked meteorites not diaplectic glass but a glass quenched from shock induced dense melt at high pressures abstract maskelynite an important constituent of shocked meteorites once thought to be diaplectic plagioclase glass formed by shock induced solid state transformation our systematic investigation of shocked l chondrites and snc meteorites indicates that maskelynite does not contain inherited fractures or cleavage and shock induced fractures we found no evidence for models calling for melting that initiated in pdfs and affected the whole crystals maskelynite grains are smooth and display radiating cracks emerging from their surfaces into neighboring pyroxene this is indicative of shock induced melting and quenching of the dense melt at high pressure thus erasing the inherited and shock induced fractures this was followed by relaxation of the dense plagioclase glass which induced the expansion cracks in pyroxene and olivine enrichment in potassium deviation from stoichiometry degradation of igneous zoning the presence of offshoots of maskelynite in pyroxene the lack of vesiculation in the melt pockets melt veins and molten mesostasis are clear evidence for melting and quenching under high pressure our investigations present unequivocal evidence that maskelynite in meteorites is not diaplectic plagioclase glass formed by solid state transformation but a dense quenched glass the duration of the shock pulse in natural events can be several orders of magnitude longer than in shock experiments since kinetic effects are crucial factors in promoting phase transitions vitrification and melting experimentally induced solid state vitrification of plagioclase produced in dynamic experiments is inadequate for calibration of peak shock pressures in maskelynite bearing natural samples

impact of the spatial distribution of the atmospheric forcing on water mass formation in the mediterranean sea the impact of the atmospheric forcing on the winter ocean convection in the mediterranean sea was studied with a high resolution ocean general circulation model the major areas of focus are the levantine basin the aegean cretan sea the adriatic sea and the gulf of lion two companion simulations differing by the horizontal resolution of the atmospheric forcing were compared the first simulation med16 era40 was forced by air sea fields from era40 which is the ecmwf reanalysis the second simulation med16 ecmwf was forced by the ecmwf analyzed surface fields that have a horizontal resolution twice as high as those of era40 the analysis of the standard deviations of the atmospheric fields shows that increasing the resolution of the atmospheric forcing leads in all regions to a better channeling of the winds by mountains and to the generation of atmospheric mesoscale patterns comparing the companion ocean simulation results with available observations in the adriatic sea and in the gulf of lion shows that med16 ecmwf is more realistic than med16 era40 in the eastern mediterranean although deep water formation occurs in the two experiments the depth reached by the convection is deeper in med16 ecmwf in the gulf of lion deep water formation occurs only in med16 ecmwf this larger sensitivity of the western mediterranean convection to the forcing resolution is investigated by running a set of sensitivity experiments to analyze the impact of different time space resolutions of the forcing on the intense winter convection event in winter 1998 1999 the sensitivity to the forcing appears to be mainly related to the effect of wind channeling by the land orography which can only be reproduced in atmospheric models of sufficient resolution thus well positioned patterns of enhanced wind stress and ocean surface heat loss are able to maintain a vigorous gyre circulation favoring efficient preconditioning of the area at the beginning of winter and to drive realistic buoyancy loss and mixing responsible for strong convection at the end of winter

weathering features in shocked quartz from the ries impact crater germany shocked quartz from the ejecta of the ries impact structure has been investigated by analytical transmission electron microscopy atem quartz grains display numerous planar fractures pfs and planar deformation features pdfs both are partly or fully replaced by a mineral of the kaolinite group likely halloysite its formation involves fluid circulation into the dense fracture networks dissolution and removal of the amorphous phase initially present in pdfs and finally precipitation and crystallization of the kaolinite group mineral from solutions resulting from the chemical alteration of adjacent minerals feldspars and biotite kaolinite group minerals are typical of hydrothermal alteration at low temperature in humid climate and under moderately acid conditions and thus this alteration may not be directly related to the impact event itself however the weathering features were strongly enhanced by the shock generated microstructure in particular by fractures that provided pathways for fluid circulation

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

effects of bottom friction on nonlinear equilibration of an oceanic baroclinic jet abstract bottom friction is an important sink of energy in the ocean indeed high resolution ocean models need bottom friction to achieve a satisfactory kinetic energy level at equilibrium however bottom friction has also subtle and discriminating effects on the different energy transfers and therefore on the 3d structure of the flow some of which have to be clarified in this study those effects on an unstable baroclinic jet are reexamined using a primitive equation model as in previous studies using quasigeostrophic models it was found that bottom friction strongly affects the barotropic mode whereas the baroclinic modes are weakly changed the new result is that bottom friction yields a significant space scale selection analysis of the dynamics reveals strong agreement with previous quasigeostrophic studies at the mesoscale in the interior but differences in the eddy field at small scales close to the surface a rationalization of these results is proposed by a comparison with preceding atmosph

an investigation by transmission electron microscopy of planar deformation features in naturally shocked quartz we have investigated by transmission electron microscopy tem the planar deformation features pdf in quartz from various sites where shock metamorphism has been detected optically impact sites the vredefort complex south africa and the k t boundary at raton basin colorado pdfs are straight narrow bands of glass or microtwins or huge densities of dislocations such contrasting microstructures must reflect different shock scenarios in the vredefort complex the post shock thermal history has strongly overprinted the shock induced defects in the raton basin samples very tiny bubbles have precipitated on the dislocations strongly suggesting a lengthy annealing at moderate pressure and temperature this new information should be taken into account in any model of the origin of the k t transition in any case observation of pdfs by optical microscopy alone is not sufficient to unambiguously conclude on their origin

fluid inclusions planar elements and pseudotachylites in the basement rocks of the vredefort structure south africa abstract the basement rocks of the vredefort structure are characterized by high temperature static metamorphism in the course of which a violent shock event occurred quartz in these rocks contains numerous fluid inclusions that consist of almost pure co 2 commonly such fluid inclusions are aligned along planar elements in quartz that were formed during the shock event crystallographic orientations of the planar elements were used to estimate minimum shock pressures of 75 100 30 kbar the fact that fluid inclusions are situated on planar elements indicates that the co 2 gas must have been available immediately after the shock event measurements of the homogenization temperatures of the fluid inclusions in samples from the vredefort basement allow subdivision of the co 2 inclusions into three groups the main group type 1 representing nearly 99 of the co 2 inclusions homogenizes to liquid in the range 26 30 c and therefore has densities of between 0 6 and 0 7 g cm 3 inclusions of this most important group are generally situated on planar elements high density type 2 and low density type 3 inclusions occur isolated in coarse unshocked quartz grains without planar elements in post shock recrystallized quartz grains and in mafic minerals apparently unaffected by the shock event type 2 inclusions with densities of about 0 9 g cm 3 are believed to belong to an early generation which was formed during the granulite facies metamorphism predating the static high temperature vredefort metamorphism low density inclusions of type 3 homogenizing to the gaseous state could be explained as a very late generation of inclusions formed under near surface conditions on the basis of the observation that type 1 inclusions are located on shock induced planar elements the time of entrapment must have been immediately after the shock event and a minimum pressure for co 2 between 2 and 5 kbar can be determined for this time within the entire basement based on independent temperature estimations the observations that a multitude of type 1 inclusions has been found in a minute millimetre sized pseudotachylite vein produced by the shock event is explained by recrystallization of diaplectic glass with this knowledge of microstructures in the vredefort basement rocks it is suggested that the ubiquity of co 2 and the shock event are related to each other

formation mechanisms of planar deformation features in naturally shocked quartz abstract shock waves induce peculiar defects in quartz shock mosaics high pressure polymorphs coesite and stishovite diaplectic glass and planar deformation features pdfs together these features are the indices of shock metamorphism pdfs appear under the optical microscope as straight and narrow defects parallel to the 10 1n rhombohedral planes with n 3 and 2 as most frequent values although n 1 4 and i e basal plane are also found we report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of pdfs in shocked quartz grains from a variety of sites slate islands la malbaie and manson in north america the vredefort complex in south africa the ries crater in germany and the cretaceous tertiary boundary layer at raton basin colorado we distinguish four pdf fine structures 1 bands of dislocations 2 lamellae of mixtures in various proportions of amorphous silica and small crystallites 3 thin brazil twin lamellae 4 short parallel lamellae forming serrated ladder structures the possible formation mechanisms of these pdfs are discussed in the second part of the paper it is suggested that the dislocation bands are probably not original pdf structures but stem from a later recrystallization stage the brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave the other pdf structures which consist of partial amorphization in the 10 1 n planes must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 gpa the growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave

shock metamorphic features in quartz grains from the saarijarvi and soderfjarden impact structures finland shock metamorphic features at the saarijarvi d 2 km and soderfjarden d 6 5 km structures in finland have so far only been studied tentatively although both are considered to be proven impact structures this work presents the first detailed universal stage study of planar deformation features pdfs feather feature lamellae ffl and planar fractures pfs in quartz grains from a polymict impact breccia dike from soderfjarden and from sedimentary crater fill rocks from saarijarvi planar microstructures particularly pdfs are very rare and poorly developed or preserved in saarijarvi whereas in soderfjarden they are much more common and well defined miller bravais indices of the planar microstructures in both saarijarvi and soderfjarden are indicative of relatively low shock pressure but high shear conditions only compatible with an impact origin for these structures although a proterozoic age for saarijarvi cannot be ruled out the observations of shock features throughout the sedimentary crater fill sequence and a brecciated sedimentary dike below the crater floor are more consistent with a lower cambrian or younger impact age

u pb ages of shocked zircon grains link distal k pg boundary sites in spain and italy with the chicxulub impact abstract the u pb ages of shocked zircon crystals from the chicxulub impact crater and cretaceous paleogene k pg boundary sites in haiti the usa and canada and the pattern of decreasing particle size with paleodistance from the crater have been used as evidence of a genetic link between chicxulub and the k pg boundary despite this the inference that the k pg boundary layer formed as a direct consequence of the chicxulub impact has been repeatedly questioned here we present u pb id tims ages and textural evidence of shock metamorphosed zircon grains from the k pg boundary at caravaca spain and petriccio italy that establish a causal connection between the impact and formation of the k pg boundary layer the shocked zircon grains give data that produce a characteristic age pattern which indicates a primary source age of 549 5 5 7 ma and a secondary event at the approximate time of impact at 66 ma the intensity of the shock features is proportional to the degree of isotopic resetting and all textural features and ages are analytically identical to those of previously analyzed zircon from chicxulub and k pg boundary sites in north america caravaca and petriccio were 8000 km from chicxulub at the time of impact and are therefore the farthest k pg sites identified that can be linked to chicxulub through the dating of individual shocked zircon grains we conclude that the combined age data and textural observations provide unambiguous evidence that ejecta from the chicxulub impact formed the global k pg boundary layer these data cannot be explained by the alternative scenario that the chicxulub impact occurred 300 ka prior to the k pg boundary

a transmission electron microscopy tem investigation of opaque phases in shocked chondrites shock defects in the most common silicate minerals of chondrites olivine pyroxenes and feldspars have been investigated in detail but there have been almost no studies of the shock defects in other components like metal and sulfide this probably stems from the fact that these latter phases are opaque in the optical microscope the same reason explains why veins and melt pockets which are constituted of microcrystalline or glassy phases i e isotropic are also poorly documented we have investigated such phases by analytical transmission electron microscopy atem in two shocked chondrites tenham l6 and gaines county h5 we have characterized shock defects in troilite very similar to those occurring in silicates i e a mosaic texture and sets of straight and very narrow 10 nm lamellae of amorphized fes there are many small regions in shocked chondrites that are composed of very fine grained 1 m mixtures of metal and sulfide or of various silicates they must result from local melting followed by a rapid cooling that prevented grain growth we have determined the chemical compositions and the volume proportions of the tiny grains in these veins and melt pockets which has allowed their temperature and pressure t p history to be partially deciphered finally we have observed a dense network of very narrow fractures down to 10 nm in the olivine and enstatite grains these fractures are systematically filled with an amorphous or cryptocrystalline material that stems from the melt pockets and was injected when the fractures were opened by the rarefaction wave this material was then quenched at the contact with the colder crystalline rims

experimental shock deformation in zircon a transmission electron microscopic study abstract in recent years apparently shock induced and thus impact characteristic microdeformations in the form of planar microdeformation features and so called strawberry granular texture have been observed in zircons in rocks from confirmed impact structures and from the k t boundary the nature of the planar microdeformations in this mineral is however still unknown and critical information is needed regarding the shock pressure range in which these deformation effects are produced we experimentally shock deformed two series of thin zircon zrsio 4 target plates cut perpendicular to the c axis at shock pressures of 20 40 and 60 gpa the recovered samples were characterized by optical and scanning electron microscopy in addition one sample series was studied by transmission electron microscopy tem microdeformation effects observed at 20 gpa include pervasive micro cleavage and dislocation patterns plastic deformation is indicated by a high density of straight dislocations in glide configuration the dominant glide systems are 010 micro cleavages induced by shear stresses during the compression stage occur mostly in the 100 planes the large density of dislocations at crack tips shows that plastic deformation was initiated by the micro cracking processs at 40 gpa the sample was partly transformed from the zircon z to a scheelite cawo 4 type s structure planar deformation features pdfs containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics the phase with scheelite structure initiated in the 100 planes of zircon consists of thin 0 1 to several m bands that crosscut the zircon matrix the phase transformation is displacive martensitic and can be related by 100 z 112 s and 001 z s the scheelite structure phase is densely twinned with twins in the 112 plane the 60 gpa sample consists completely of the scheelite structure phase crosscutting and displacing relationships between twins and pdfs demonstrate that pdfs are formed in the zircon structure i e before the phase transformation to the scheelite structure occurred most likely at the shock front crystallographic orientations of optically visible planar features in zircon in comparison with orientations of planar defects at the tem scale suggest that the optically visible features are more likely planar microfractures than pdfs

a possible influence of the north atlantic oscillation on the circulation of the western mediterranean sea a current time series spanning from 1985 to 1996 in the corsica channel shows that seasonal and interannual variabilities are stable components of the circulation of the northern part of the western mediterranean sea for the first time these characteristics are related to the state of the north atlantic oscillation during winter we have found that this large scale control is mainly maintained through the air sea interaction processes which take place in the northern basin these results provide new elements for a consistent explanation of the major features of the basin circulation such as the seasonal increase of the boundary current and deep water formation

calcrust 1985 seismic reflection survey whipple mountains detachment terrane california an overview calcrust is a new consortium of six academic institutions from california engaged in seismic reflection studies of the continental crust calcrust activities are concentrated in the southwestern u s where a diversity of geotectonic styles occur which are fundamental to the evolution of continental margins the good surface geologic control provided by excellent outcrop exposure and more than two decades of intensive mapping adds to the attractiveness of the region for deep crustal investigation current calcrust efforts are directed in large part toward development of a crustal transect from the colorado plateau stable north american craton to the salton trougwgulf of california continental riftinghansform the transect crosses a variety of tectonic elements central to the mesozoic and cenozoic evolution of the continental margin of western north america e g coney 1981 dickinson 1981 the line of transect is shown schematically in fig 1a also shown are the various major tectonic elements to be crossed both compressional and extensional structures are involved calcrust is not only addressing the seismic character and geometry of the individual elements or structures but also relationships between them the first work on the transect began during calcrust s pilot program from 1984 to 1986 it focussed on the detachment fault terrane of the whipple mountains in southeastern california fig 1b the whipple mountains are part of the metamorphic core complex of the north american cordillera davis et al 1979 1980 crittenden et al 1980 well exposed in the whipple mountains and common to the core complexes of arizona and southeastern california is a series of low angle normal or detachment faults of mid tertiary age indicating appreciable crustal extension also exposed in the range are zones of pronounced mylonitic foliation which are seen on a regional basis throughout large parts of the southwestern u s crittenden et al 1980 finally immediately south of the whipple mountains mesozoic thrusts and nappes hamilton 1982 are exposed in the riverside and big maria mountains fig ib two important questions are posed by the presence of these three structural elements detachments mylonites and thrusts in the vicinity of the whipple mountains

experimental reproduction of tectonic deformation lamellae in quartz and comparison to shock induced planar deformation features planar features can develop in quartz during comparatively slow tectonic deformation and during very fast dynamic shock metamorphism despite their very different structural nature tectonically induced deformation lamellae have sometimes been mistaken as shock induced planar deformation features pdfs to understand the formation of deformation lamellae and to address the substantial differences between them and pdfs we have conducted deformation experiments on single crystals of quartz in a griggs type apparatus at a temperature of 800 c a confining pressure of 12 kbar and a strain rate of 0 7 1 1 x 10 6 the deformed samples were analyzed with transmission electron microscopy tem and compared to natural pdfs from the ries crater germany tem revealed that tectonic deformation lamellae are associated with numerous sub parallel curved subgrain walls across which the orientation of the crystal changes slightly the formation of deformation lamellae is due to glide and climb controlled deformation in the exponential creep regime in contrast the pdfs in shocked quartz from the ries are perfectly planar crystallographically controlled features that originally represented amorphous lamellae due to postshock annealing and hydrothermal activity they are recrystallized and decorated with fluid inclusions

ibn batutah a possible simple impact structure in southeastern libya a remote sensing study abstract the ibn batutah feature is a circular structure centered on 21 34 10 n and 20 50 15 e and located in the prefecture of the hamadat ibn batutah plateau southeastern libya it is a singular geological structure formed in nubian sandstone of most likely early cretaceous age remote sensing investigations based on landsat etm dual band l and c and dual polarization hh and hv radar sir c and srtm data reveal a morphological resemblance of the ibn batuta structure to other confirmed terrestrial simple impact structures the structure is in the form of a circular basin 2 5 km in diameter and 25 m deep surrounded by a low relief rim that rises to within 5 m of the surrounding terrains over much of its extent no endogenic geological processes such as magmatism diapirism karst dissolution and glacial or fluvial erosion can decisively explain the formation of the ibn batutah structure within a large area of flat lying mesozoic sandstones thus this paper proposes that the ibn batutah structure represents a simple impact structure verification of this hypothesis will require collection and analyses of rock samples from in and around the structure although the area is currently almost inaccessible for environmental and security reasons

modeling the dispersal of levantine intermediate water and its role in mediterranean deep water formation this paper demonstrates the importance of levantine intermediate water liw in the deep water formation process in the mediterranean using the modular ocean general circulation model at 0 25 resolution 19 vertical levels over the entire mediterranean with an open gibraltar strait liw formation is strongly prescribed in the rhodes gyre region by haney 1971 relaxation while in other regions surface salinity relaxation is much reduced by applying the mixed thermohaline surface boundary conditions isopycnal diagnostics are used to trace water mass movements and volume fluxes are monitored at straits low viscosity and diffusion are used to permit baroclinic eddies to play a role in water mass dispersal the overall water budget is measured by an average flux at gibraltar of 0 8 sv of which 0 7 sv is exchanged with the eastern basin at sicily liw density around 28 95 spreads rapidly after formation throughout the entire levantine due to baroclinic eddies toward the west liw accumulates in the northern and central ionian with some entering the adriatic through otranto and some mixing southward in eddies and exiting to the western mediterranean through sicily liw is converted to deep water in the south adriatic at an average rate of 0 4 sv water exchange through the otranto strait appears to be buoyancy driven with a strong bias to the end of winter march april while at sicily the exchange has a strong symmetric seasonal cycle with maximum transport of 1 1 sv in december indicating the effects of wind driving liw pathways in the west are complex and variable in the tyrrhenian intermediate water becomes uniform on isopycnal surfaces due to eddy stirring west of sardinia two liw boundary currents are formed in the balearic basin one flows northward up the west coast of sardinia and corsica and one westward along the northern african coast the northward current is consistent with observations while the westward current is intermittent for the first 10 years often breaking up into eddies which enter the basin interior some observations of high salinity waters near the african coast may support this interpretation liw retains a subsurface salinity maximum of 38 4 38 5 practical salinity units psu when reaching the northwestern mediterranean contrasting with surface waters fresher than 38 0 psu west mediterranean deep water is formed below 1500 m depth with climatological characteristics when it is mixed and cooled during winter convection in lions gyre

the role of winter intermediate waters in the spring summer circulation of the balearic sea 1 hydrography and inverse box modeling hydrographic transects performed across the balearic sea western mediterranean in june 1993 are used to estimate the absolute geostrophic flow by means of a linear inverse model ensuring conservation of water properties mass salt and heat in closed boxes the computed fluxes are supporting the concept that most of the northern waters that flow into the basin over the spanish peninsula slope 1 1 sv recirculate back to the north in a cyclonic pattern while southern waters that enter through the ibiza channel mainly modified atlantic waters 0 5 sv transit to the north over the balearic slope results point out that winter intermediate waters wiw distributed in several coherent mesoscale structures play an active role in these dynamics a major anticyclonic wiw eddy weddy lying in the southern basin obstructs the water exchange through the ibiza channel and deflects the continental current back to the north a similar though smaller weddy strained by the balearic current into an elongated along slope wiw filament controls the circulation through the mallorca channel all these results are discussed in the light of data collected during past years

near vertical and intermediate offset seismic reflection data from west of the whipple mountains se california during a seismic reflection survey conducted by the california consortium for crustal studies in the basin and range province west of the whipple mountains se california a piggyback experiment was carried out to collect intermediate offset data 12 31 km these data were obtained by recording the vibroseis energy with a second passive recording array deployed twice at fixed positions at opposite ends of the reflection lines the reflection midpoints fall into a 3 km wide and 15 km long region in vidal valley roughly parallel to a segment of one of the near vertical reflection profiles this data set makes three unique contributions to the geophysical study of this region 1 from forward modeling of the observed travel times using ray tracing techniques a shallow layer with velocities ranging from 6 0 to 6 5 km s was found this layer dips to the south from 2 km depth near the whipple mountains to a depth of 5 km in rice valley these depths correspond closely to the westward projection of the whipple detachment fault which is exposed 1 km east of the near vertical profiles in the whipple mountains 2 on the near vertical profile the reflections from the mylonitically deformed lower plate at upper crustal and mid crustal depths are seen to cease underneath a sedimentary basin in vidal valley however the piggyback data which undershoot this basin show that these reflections are continuous beneath the basin thus near surface energy transmission problems were responsible for the apparent lateral termination of the reflections on the near vertical reflection profile 3 the areal distribution of the midpoints allows us to construct a quasi three dimensional image on perpendicular profiles at the cross points we determined the true strike and dip of reflecting horizons this analysis shows that the reflections from the mylonitically deformed lower plate dip to the southwest westward of the whipple mountains and dip to the south southward of the turtle mountains the results of this study support the interpretation of crustal reflectivity in the near vertical reflection profiles to be related to the mid tertiary episode of extension which produced the whipple metamorphic core complex this association geometrically suggests a more regionally distributed mechanism for crustal thinning as compared with single detachment fault models