geochemical characterization and origin of granitoids from the south bohemian batholith in lower austria major and 31 minor elements have been determined in 39 large samples of variscan granitoids from 6 plutons or intrusions from the south bohemian batholith rastenberg weinsberg mauthausen schrems eisgarn and gebharts the granitoids are mainly granites but also diorites tonalites trondhjemites granodiorites average concentrations of ba th u la ce pb nd sr and k in the weinsberg mauthausen and schrems granites exceed those in average felsic i and s type granites by factors ranging between 2 1 and 1 3 the granites melts formed at waterundersaturated conditions and intruded at 10 to 15 km depth during late tectonic and post tectonic phases of the variscan orogeny about 330 to 300 ma ago hydrothermal or low temperature alteration is excluded for the majority of samples from a study of oxygen isotopes the thickness of the plutons is estimated at about 6 km from heat balance constraints by analogy with experimental partial melting three different sources of the granitoids can be identified and chemically characterized 1 the trondhjemites tonalites and diorites in the early rastenberg pluton are products of 15 to 40 melting respectively of a mafic partly amphibolitic lower crust redwitzites from the west bohemian massif which are comparable in age partly resemble the rastenberg rocks the mafic sources of the rastenberg granitoids and redwitzites are crustally contaminated as reflected in their sr nd isotopes 2 the very large syn tectonic weinsberg pluton was formed from about 30 partial melting of a tonalitic lower crust at 800 to 850 c its low proportion of ca 10 restite has a ferrodioritic composition the post tectonic fine grained mauthausen and schrems granites which tend to a granodioritic mode are very low in restite and are also products of melting of a tonalitic source 3 the youngest leuco granite the eisgarn pluton high in si p li rb cs u 87sr 86sr and low in ca sr ba reflects a pelitic source the change from mafic to tonalitic to pelitic source composition for the granitoid sequence may indicate that the depth of melt formation decreased with time the concentration of heavy rare earth elements decreased from weinsberg to eisgarn granites which indicates an increasing proportion of garnet in the source the orogenic heat conformable with a heat flow of about 100 mwm 2 was provided by mafic intrusions an alternative would be a drastic increase of the crustal thickness which cannot be recognized by barometry of the associated metamorphic rocks exposed metamorphic country rocks occur in higher amphibolite facies indicating about 5 kbar pressure mafic intrusions contain gabbros kleinzwettl or have formed quartz diorites gebharts the latter being contaminated by granitic melts from partial melting of the wall rocks mash process largescale contamination by crustal materials can be observed in 18o and in sr nd isotopes the major mafic activity was probably caused by depression of solidus temperatures in the mantle wedge above a subduction zone where water was available from dehydration of subducted ocean crust this water initiated partial melting of ultramafic rocks and metasomatism in the uppermost mantle above the level of melting the water also mobilized highly incompatible elements ba th u la ce pb nd sr and k from the uppermost mantle and transported them into the lower crust indicators of a nearby subduction or collision zone of late variscan age in addition to the specific association of granitoidal rocks are abundant upper mantle tectonites an alternate or additional source of metasomatic fluids may have been dehydration of lower crustal rocks during variscan high grade metamorphism

u pb zircon and sm nd model ages of high grade moldanubian metasediments bohemian massif czechoslovakia we report single grain u pb ion microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the moldanubian complex bohemian massif one of the largest crystalline complexes of the hercynian foldbelt in europe these are complemented by whole rock sm nd model ages the metasediments are of upper amphibolite to granulite grade and come from the bory massif in moravia nw of brno sample aa 1 and from the varied group aa 2 and monotonous group aa 3 in the ceske budejovice region of sw bohemia

role of initial stress rotations in rupture dynamics and ground motion a case study with implications for the wenchuan earthquake 1 motivated by observations in the 2008 mw 7 9 wenchuan earthquake we study effects of systematic changes in the principal stress orientation along the fault strike on rupture dynamics and ground motion using a 3 d finite element method based on anderson s theory of faulting we set up the initial stress field with rotations in stress orientations along strike for a dynamic rupture model of a shallow dipping fault we find that initial stress rotations along strike can cause dramatic changes in rupture speed and produce distinct patterns in slip distribution and peak ground motion when a mismatch unfavorable for faulting between fault geometry and initial stress orientations is encountered rupture can spontaneously stop some first order features in the wenchuan event may be partially caused by rotations in the principal stress orientation along strike such as the rupture arrest at the northeast end and two severe destruction zones in the observed seismic intensity distribution these results may have important implications for assessing seismic hazards imposed by faults with changes in the initial stress field along strike worldwide

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

submerged sand ridges on the western continental shelf off bombay india evidence for late pleistocene holocene sea level changes abstract the morphology distribution and probable origin of submerged sand ridges on the western continental shelf off bombay india have been studied using echosounding side scan sonar and high resolution seismic reflection techniques supplemented by seabed sampling 7 8 rows of well defined broad symmetrical submerged sand ridges were found on the middle and the outer shelf 100 200 km offshore and between 75 and 100 m water depth the ridges being more prominent between 85 and 90 m these ridges are 0 5 to 10 km wide 1 5 to 18 m high several tens of km long with a few kilometers spacing and a trend of nnw sse parallel to the shelf edge the inter ridge areas are characterised by long narrow palaeo lagoonal like depressions some rounded smooth ridges appear to be moribund while others have undergone reworking as indicated by superimposed bedforms sand waves and reefs ridges also occurred on the inner shelf where they were either eroded during the holocene transgression or covered by recent sediments the sand ridges originated as dunes which were submerged and reformed into very large sand bars further drowning and lithification favoured the development of organic growth and sand wave formation which must have taken place during a period of stillstand or slow sea level rise in the early to mid holocene the superimposed bedforms reflect the prevailing flow regime on the shelf

control of estuarine sediment dynamics by interactions between currents and waves at several scales abstract the effect of interactions between continuous tidal currents and intermittent waves processes on sediment dynamics and transport is addressed by presenting detailed field measurements of waves boundary layer currents and suspended sediment from an estuarine channel and an adjacent intertidal sandflat in manukau harbour new zealand the aim is to determine in what ways it is necessary to couple waves and currents in numerical models and thereby put limits on the fundamental structure of process based estuarine sediment transport models waves were important on the intertidal flat turbidity switched on and off with the appearance and disappearance of waves wave groups dominated entrainment of bed sediment a wave current boundary layer model explained measured bed shear stress and hydraulic roughness and the measured near bed time averaged suspended sediment concentration was mostly well predicted by a pure wave model both the waves themselves and wave related processes varied markedly over the tidal cycle the variation in the former principally changes in wave height was related to changes in fetch caused by the harbour wide emergence and submergence of intertidal regions the variation in the latter was related to changes in water depth relative to the wavelength of the waves which controlled the penetration to the bed of wave orbital currents in addition to the variation over the tidal cycle in the intensity of wave processes there was also a change in kind which occurred with the arrival at the measurement site of the turbid fringe which is the narrow highly turbid edge of the estuarine water body the relationship between suspended sediment concentration and wave orbital velocity in the turbid fringe was radically different to the relationship in the estuarine water body proper which suggests a change in dynamics perhaps related to breaking waves a hybrid modelling approach is required i e one that treats discrete events but resolves tidal cycle scale variation within the event there is a need to resolve the variation in the wave train over the tidal cycle and the penetration to the bed of wave orbital motions both of which could only be done adequately within an estuary tidal model in contrast to the situation on the intertidal flat where waves intermittently entrained sediment sediment transport in the channel was continuous driven by tidal currents to predict sediment flux in the channel we need to know the upstream sediment transporting capacity of the flow including that contributed by waves the character of the bed sediment and the sediment settling time scale these factors confounded even the simplest notion of flood and ebb dominance which frequently has been applied to understand estuarine morphodynamics

k feldspar muscovite andalusite quartz brine phase equilibria an experimental study at 25 to 60 mpa and 400 to 550 c abstract felsic magmas may evolve one or more water or chlorine rich fluid phases which can transport heat and solutes into associated hydrothermal systems and can contribute to alteration and ore deposition to understand the role of a high salinity aqueous phase in the magmatic hydrothermal environment the composition of a subcritical vapor undersaturated high salinity liquid phase brine in equilibrium with k feldspar muscovite quartz and muscovite andalusite quartz was determined for pressures and temperatures ranging from 25 mpa and 400 c to 60 mpa and 550 c with total cl nacl kcl hcl concentrations ranging from 3 42 to 8 56 moles of solute kg solution values of log 10 kcl hcl have been obtained for the equilibria 1 5 k feldspar hcl 0 5 muscovite 3 quartz kcland muscovite hcl 1 5 andalusite 1 5 quartz 1 5 h 2 o kcl for the k feldspar muscovite quartz brine equilibrium log 10 kcl hcl 1 6 0 1 0 81 0 06 0 54 0 04 and 0 42 0 08 at 25 mpa and 400 c 40 mpa and 450 c 50 mpa and 500 c and 60 mpa and 550 c pressures and temperatures of the experiments respectively for the muscovite andalusite quartz brine equilibrium log 10 kcl hcl 0 63 0 1 0 063 0 06 0 17 0 05 and 0 25 0 08 at the pressures and temperatures of the experiments respectively comparison of our results with previous studies conducted at higher pressures and with lower salinity aqueous phases show that the mineral stability fields in the k feldspar muscovite andalusite quartz system shift to lower kcl hcl values with increasing salinity and decreasing pressure

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

field investigations of the threshold of grain motion by ocean waves and currents abstract instrumented tripods were deployed on three occasions at continental shelf sites twice in the united states december 1978 and march 1979 and once in australia december 1979 a total of 37 days of data were collected data included measurements of current speed and direction 100 cm off the seabed mean bottom pressure and pressure fluctuations water turbidity with nephelometer or transmissometer and photographs of the seabed bottom sediment samples were also collected prior to each deployment these data were analyzed to estimate the vector averaged velocity u100 tides gravity waves near bottom sediment concentration bed configuration and bottom sediment texture during the sampling period sediment resuspension occurred frequently as a result of oscillatory currents due to surface gravity waves the field data have been used to evaluate several existing relationships for predicting the threshold of grain motion under oscillatory flow conditions the first method is an evaluation of the shields 1936 entrainment function for unidirectional flows in which the boundary shear stress is computed using the wave friction factor of jonsson 1966 the second is an equation presented by komar and miller 1973 1975 that is based on the laboratory investigations of bagnold 1946 and manohar 1955 the third method computes the boundary shear stress as a nonlinear combination of stresses due to waves and currents as proposed by grant and madsen 1979 and further developed in this paper the results show that the shields diagram in which the shields entrainment function for unidirectional currents is plotted with respect to a grain reynolds number adequately predicts the threshold of grain motion on the continental shelf although the results of all three computational methods were within the scatter of results of the laboratory studies upon which the predictive relationships were based the wave current model of grant and madsen is preferred because it includes the influence of currents in the computation of boundary shear stress

late orogenic extension in mountain belts insights from the basin and range and the late paleozoic variscan belt from two examples of orogenic domains some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized the basin and range province and the variscan belt in the french massif central have both suffered important compressional orogenic crustal thickening and the results of late orogenic processes can be observed in the field both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed at the scale of the orogenic domains the most prominent tectonic features are the metamorphic core complexes mcc which expose deformed rocks from the middle crust generally affected by high temperature low to medium pressure metamorphism partial melting and widespread granite emplacement in these mcc large scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations they show a complete evolution from early deep seated ductile deformation generally achieved under high temperature low to medium pressure metamorphism to a late shallow brittle stage characterized by cataclastic deformation the late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits two main geometries of mcc are defined that are characterized by differing geometry and kinematics of low angle shear zones in the first case two low angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical mcc often one system is dominant over the other the second geometry characterizes asymmetrical mcc bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain strain distribution within the lower crust is less well constrained by field observation however analogies between cocorp and ecors deep seismic reflection profiles give important constraints on crustal structure wide zones of highly subhorizontally layered lower crust and a flat high amplitude reflection moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension a kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains a weak hot upper mantle allows large scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation heterogeneous strain is accommodated by low angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust combined pure and simple shear occurs along localized shear zones whereas at the scale of the whole lithosphere deformation nearly corresponds to a vertical pure shear such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics thus as much in the andean type west american cordilleran belt as in the collision type variscan belt late orogenic processes produced similar extensional features

indentation as an extrusion mechanism of lower crustal rocks insight from analogue and numerical modelling application to the eastern bohemian massif abstract recent petrological structural and geochronological studies of the eastern margin of the bohemian massif czech republic suggest a conceptual geodynamical model to explain exhumation of lower crustal 20 kbar 800 c felsic rocks the model involves indentation of a weak orogenic lower crust by an adjacent rigid mantle lithosphere resulting in crustal scale buckling of the weak orogenic lower middle crust interface followed by extrusion of a ductile nappe over the rigid promontory the hypothesis has been investigated using both analogue and numerical models analogue experiments using a three layer sand silicone setup were carried out in rennes laboratory france results show that the most important features of the conceptual model can be reproduced extrusion of lowermost silicone over the indenter and flow of horizontal viscous channel underneath a rigid lid above the actively progressing promontory furthermore experimental results show that a plateau develops above the channelling lower crust two sets of sandbox scale numerical simulations were performed the first set of experiments is designed to study the influence of viscosity stratification within the crust on the extrusion process a second set of experiments were performed in order to quantify the influence of the viscosity and the geometry of the indentor non dimensional scaling laws were derived to predict the maximum extrusion rates associated with the indentation mechanism such laws enable the computation vertical extrusion rates that are in good agreement with natural exhumation rates inferred from petrological data finally we discuss the potential positive feedback of rayleigh taylor instability on vertical extrusion for the case of eastern bohemian massif

rhythmic topography in a mesotidal low wave energy environment abstract the intertidal zone in craig bay extends 1200 m offshore making it the broadest non estuarine tidal flat on the east coast of vancouver island british columbia canada superimposed on the low gradient foreshore 1 360 are up to fourteen low amplitude 0 6 m ridges with an average wavelength of 86 3 m these are permanent fixtures of the intertidal zone exhibiting considerable stability in both form and position they are generally symmetrical in cross section and in no instance is a landward slip face evident the ridges have developed in medium to fine sand atop a coarse deltaic platform the thickness of this surficial sand unit is often equal only to the height of the ridges and the gravel sustrate is frequently exposed in the depressions the morphology sediments and hydraulic environment of this rhythmic foreshore were examined to provide insight into the factors controlling ridge development and maintenance although the maximum tidal range exceeds 5 0 m tidal currents are of limited importance only in the major runnels are velocities sufficient to initiate sediment transport waves within the strait of georgia are locally generated h1 3 3 0 m t1 3 7 0 s and appear to the the prime agent responsible for the formation and maintenance of this variant of ridge and runnel morphology the dominant bedform across the intertidal zone is the wave formed ripple although current ripples may locally predominate small scale cross stratification which is associated with these bedforms is the most common primary physical sedimentary structure in the surficial sand unit however a large proportion of the cores were structureless this is attributed to bioturbation by sand dollars dendraster excentricus and to a lesser extent to air entrapment and water escape

pervasive horizontal fabric and rapid vertical extrusion lateral overturning and margin sub parallel flow of deep crustal migmatites northeastern bohemian massif abstract in the west sudetes northeastern bohemia massif geochronometry provides evidence for repeated episodes of rapid cooling that contrasts sharply with an absence of structural evidence for significant tectonic exhumation by crustal extension instead high grade assemblages of the orlica snieznik complex have a regional sub horizontal foliation and sub horizontal lineations that trend parallel to narrow sub vertical shear zones containing exhumed high pressure assemblages mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility ams data from amphibolite facies to migmatitic meta sedimentary and meta igneous rocks reveal remarkably consistent average lineations that plunge shallowly to the ssw on both steep and sub horizontal nne trending planar fabrics the dominant ssw nne fabric orientation is parallel to the bohemia brunia suture which marks a major boundary along the eastern margin of the massif the shape of the ams ellipsoid is predominantly oblate revealing flattened fabrics with only local prolate ellipsoids we envisage that the continental brunian indentor operated as a rigid backstop and allowed the migmatized lower crustal orogenic root to be exhumed along the bohemian margin shortly following terminal arc collision sub vertical extrusion of the orogenic root was arrested in the mid crust where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks upon reaching the crustal high strength lid the exhumed ductile mass of continental material laterally spread sub parallel to the margin underwent subsequent supra barrovian metamorphism and quickly cooled the application of ams techniques to high grade metamorphic rocks in concert with macroscopic structural observations is a powerful approach for resolving the deformation history of a terrane where visible rock fabrics can be tenuous

field observations of sand mixing depths on steep beaches abstract a series of field studies were carried out on three portuguese beaches garra o faro and culatra to assess sand mixing depths under a range of wave heights 0 34 0 80 m wave periods 5 1 7 0 s and mean grain sizes 0 26 and 0 38 mm all the studied beaches had a reflective profile with a steep upper foreshore tan of 0 10 0 14 and a more gentle low tide terrace in all experiments plunging waves were breaking on the beach face the study has identified a linear correlation between significant wave height at breaking h b and average sand mixing depth z m whereby z m 0 27h b the empirical relationship is ten times larger than a previous one proposed by other authors working on gentle slope dissipative beaches and confirms previous findings on similar reflective beaches carried out in the usa it was also confirmed that z m is related to wave period but does not seem to be a function of mean sand size

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

longshore drift estimation using fluorescent tracers new insights from an experiment at comporta beach portugal abstract a fluorescent sand tracer experiment was performed at comporta beach portugal with the aim of acquiring longshore sediment transport data on a reflective beach the optimization of field and laboratory tracer procedures and the improvement of the conceptual model used to support tracer data interpretation the field experiment was performed on a mesotidal reflective beach face in low energetic conditions significant wave height between 0 4 and 0 5 m two different colour tracers orange and blue were injected at low tide and sampled in the two subsequent low tides using a high resolution 3d grid extending 450 m alongshore and 30 m cross shore marked sand was detected using an automatic digital image processing system developed in the scope of the present experiment results for the two colour tracers show a remarkable coherence with high recovery rates attesting data validity sand tracer displayed a high advection velocity but with distinct vertical distribution patterns in the two tides in the first tide there was a clear decrease in tracer advection velocity with depth while in the second tide the tracer exhibited an almost uniform vertical velocity distribution this differing behaviour suggests that in the first tide the tracer had not reached equilibrium within the transport system pointing to a considerable time lag between injection and complete mixing this issue has important implications for the interpretation of tracer data indicating that short term tracer experiments tend to overestimate transport rates in this work therefore longshore estimates were based on tracer results obtained during the second tide the estimated total longshore transport rate at comporta beach was 2 10 3 m3 s more than four times larger than predicted using standard empirical longshore formulas this discrepancy which results from the unusually large active moving layer observed during the experiment confirms the idea that most common longshore transport equations under estimate total sediment transport in plunging surging waves

a deep sea thermohaline flow path in the northwest pacific abstract a recent reevaluation of the bathymetry of the northwest pacific accompanied by an analysis of reflection profiler records and deep sea drilling sites provide geological evidence for a 2000 km long north south flow path east of the emperor seamounts the geological evidence is similar to that associated elsewhere with thermohaline currents as there are no present day physical oceanographic observations of a north south flowing thermohaline current i conclude that the inferred bottom current is either presently inactive or has not been observed by the methods of investigation used to determine its presence

magma homogenization during anatexis ascent and or emplacement constraints from the variscan weinsberg granites recent studies have shown that melts and residues may not equilibrate during anatexis and uncertainty exists about the scale on which magmas can be homogenized this study of elemental and isotopic homogeneity of the south bohemian weinsberg granites 5000 km2 identifies three voluminous relatively homogeneous magma batches each batch has different 87sr 86srinit 0 7080 0 7093 and 0 7106 but all equilibrated at 327 329 ma very similar to the time of monazite crystallization the data cannot entirely prove melt residue equilibration during anatexis however elemental and isotopic compositions imply magma generation by partial melting of heterogeneous south bohemian crust and chemical differentiation subsequent to sr isotope equilibration assuming relatively rapid ascent and solidification rates magma homogenization must have occurred mostly just after partial melting during melt segregation and accumulation in the deeper crust with slow prograde heating models of rapid crustal heating and instantaneous melt extraction are incompatible with the data

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

crustal shortening root spreading isostasy and the growth of orogenic belts a dimensional analysis scaling laws that describe the growth of mountain belts and the change in their regional topographic profiles with time t are obtained by physical arguments that account for crustal shortening isostasy and creeping flow at the mountain roots an average power law rheology characterized by an exponent n is assumed and different values of n are considered the model predicts the formation of a localized range characterized by a height h and a width a as crust on the sides of a range is compressed the root broadens due to buoyancy driven creep this process in conjunction with isostasy causes h to increase with time as t1 4 t1 3 and a as t3 4 t2 3 as n is varied from 1 newtonian rheology to plastic rheology accordingly the aspect ratio a h increases as t1 2 t1 3 the scaling laws depend on the rheology but rather weakly the aspect ratio increases with age but is nearly independent of the rate of shortening the deformation extends gradually inland with increasing time so that the age of tectonic deformation decreases with distance from the continental border uplift takes place inland from the crest while subsidence occurs at the opposite side the ratio a h of the andes and tibet is consistent with this model

sm nd isotope systematics of high ree accessory minerals and major phases id tims la icp ms and epma data constrain multiple permian triassic pegmatite emplacement in the koralpe eastern alps abstract detailed sm nd mineral id tims empa and la icp ms analysis of meta pegmatite minerals from the koralpe stubalpe area austria involving single crystal separation step wise dissolution and acid leaching techniques yielded i information about the partitioning of ree between accessory and major mineral phases in meta pegmatites and ii age constraints on the long lasting permian triassic event in the austroalpine basement units of the eastern alps internal sm nd mineral isochrons performed on porphyroclast domains from the three localities rostockbach kuppergrund and wildbachgraben including garnet xenotime apatite monazite and feldspar indicate multiple emplacement of pegmatoid melts between 273 2 and 258 3 ma other localities yielded younger ages indicating that magmatic garnet crystallisation persisted from 251 7 to c 230 ma ages calculated between high ree accessories monazite apatite xenotime and the low ree major phases feldspar and garnet are largely consistent zircon however plots clearly off the internal regression line h 2 so 4 leaching experiments on meta pegmatite garnet rich in micron sized xenotime and apatite inclusions locally affected by dynamic recrystallisation yielded information about the behaviour of the sm nd system during strong tectono metamorphic overprinting the data suggest multiple injections of pegmatite melts at distinctly different times during protracted crustal thinning and heating in the permian triassic ky paramorphs after andalusite and micro inclusions of kyanite staurolite and sillimanite in magmatic garnet indicate crystallisation of the garnet at moderately high but variable pressure conditions during subsequent eclogite facies overprinting peak pt c 700 c 2 2 gpa and intense deformation in cretaceous time the magmatic chemical and isotopic signatures of porphyroclasts were largely preserved except for localized within grain micro shear zones and outermost rim domains whereas part of the feldspar rich matrix recrystallised during pervasive mylonitisation along the eclogite to amphibolite facies grade exhumation path

polyphase tectonics and late variscan extension in austria moldanubian zone strudengau area new data suggest syn convergent extrusion and polyphase tectonics followed by late variscan extension in the strudengau area of the southern moldanubian zone in austria the tectonic history can be summarized as follows 1 the oldest ductile event is observed in ht lp metamorphic pelitic gneisses which preserve e dipping foliation planes d1 fabric with nw se trending lineations 2 the overlying gneisses record ht hp conditions with decompression induced anatexis in the central part of the domain these gneisses exhibit n s trending horizontal lineations along steep dipping foliation planes d2 fabric crosscutting the d1 fabric of the pelitic gneisses along the margin these rocks have been strongly mylonitized under amphibolite facies conditions d2 d2 is interpreted as a significant vertical shear zone which juxtaposes the ht lp rocks against the orogenic lower crust 3 lastly the whole area is overprinted by localized shear zones d3 fabric with top to the nw kinematics this newly discovered strudengau shearing event is associated with isoclinal folding that possesses axial planes parallel to the mylonitic foliation and fold axes parallel to the stretching lineations initial mylonitization occurred under greenschist facies representing the latest ductile event of the strudengau area the new geochronological data presented here indicate a narrow time frame c 323 318 ma for the d3 deformation therefore this event is contemporaneous with the intrusion of the granites of the south bohemian batholith 330 310 ma the nearby south bohemian batholith and generally steep dyke swarms in the strudengau area and to the north trend in a ne sw preferred orientation interpreted to be d3 synkinematic magmatism in a regional context the nw se stretching during d3 together with the synkinematic intrusion of dykes is associated with late orogenic extension in the austrian moldanubian zone kinematic data of brittle normal faults and tension gashes are consistent with nw se oriented extension under cooler conditions

the orogenic superstructure infrastructure concept revisited quantified and revived the historical superstructure infrastructure concept s i expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels two dimensional thermal mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening phase 1 thermal relaxation and rheological weakening phase 2 and ductile flow at depth phase 3 results predict an upper crustal superstructure dominated by early steep structures separated across a subhorizontal high strain zone from a ductile infrastructure with late gently dipping structures this is consistent with observations from the western superior province these models can account for contrasts in structural style metamorphic grade seismic reflectivity and age between upper and lower crustal levels in contrast to conventional thrust tectonics models the revived s i model shows how young structures can form beneath older ones during progressive convergence thereby encouraging reassessment of standard seismic reflection interpretations

conjugate shear zones in the southern bohemian massif austria implications for variscan and alpine tectonothermal activity abstract mylonitic fabrics developed within conjugate wrench ductile shear and fault systems in the southern bohemian massif display both dextral nw se trending systems and sinistral ne sw trending systems shear senses contrasting temperature conditions of deformation can be observed in the different shear zones temperatures above 650 c were reached in the pfahl shear zone shearing under greenschist facies conditions took place in the rodl and danube shear zones brittle deformation dominated in the vitis and diendorf shear zones 40 ar 39 ar dating of various size fractions of muscovite formed and or deformed during mylonitization yield ages of ca 287 ma the nw se trending system and ca 288 281 ma the ne sw trending system the shear zones are interpreted as a late variscan conjugate system 40 ar 39 ar age spectra of fine grained newly grown sericite or rejuvenation of ages give evidence for post variscan reactivation of the shear zones brittle deformation within the shear zones was probably associated with maintenance of very high fluid pressure during variscan deformation foreland deformation during the alpine orogeny also played a significant role leading not only to the development of the ceske budejovice graben but also to local reactivation of the shear zones at higher crustal levels

crustal influx indentation ductile thinning and gravity redistribution in a continental wedge building a moldanubian mantled gneiss dome with underthrust saxothuringian material european variscan belt 1 the contribution of lateral forces vertical load gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated in the orlica snieznik dome moldanubian zone european variscan belt the polyphase tectono metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium to high grade metamorphic conditions in different levels of the crust it reflects the eastward influx of a saxothuringian type passive margin sequence below a tepla barrandian upper plate the ongoing influx of continental crust creates a thick felsic orogenic root with hp rocks and migmatitic orthogneiss the orogenic wedge is subsequently indented by the eastern brunia microcontinent producing a multiscale folding of the orogenic infrastructure the resulting kilometre scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms these localized vertical exchanges of material and heat are coeval with a larger crustal scale folding of the whole infrastructure generating a general uplift of the dome it is exemplified by increasing metamorphic conditions and younging of 40ar 39ar cooling ages toward the extruded migmatitic subdomes cored by hp rocks the vertical growth of the dome induces exhumation by pure shear dominated ductile thinning laterally evolving to non coaxial detachment faulting while erosion feeds the surrounding sedimentary basins modeling of the bouguer anomaly grid is compatible with crustal scale mass transfers between a dense superstructure and a lighter infrastructure the model implies that the moldanubian orlica snieznik mantled gneiss dome derives from polyphase recycling of saxothuringian material

oceanographic processes and sediment mixing on a sand flat in an enclosed sea manukau harbour new zealand abstract studies of oceanographic and sedimentary processes on intertidal sand flats in an enclosed sea were undertaken to gain a better understanding of the factors controlling the mixing and dispersal of sediment bound contaminants field investigations included a 90 day process experiment during which wind waves tidal currents tides depth of disturbance and sand flat morphology were measured and 27 months of sand flat profile monitoring sediment entrainment by strong spring tidal currents is restricted to the middle and lower regions of the sand flat which are inundated during the peak tidal flows the upper 2 3 cm of sediment is re worked across the middle and upper sand flat by mild storm wave events h s 70 cm which occurred four times during the 90 day experiment numerous ridges and runnels in the upper sand flats are wave formed features and are maintained by the lack of currents of sufficient magnitude to re work the features the ridge and runnel morphology is testimony to large infrequent storm events which re work the sediment to depths of 20 cm such storms are an important mechanism for the release of contaminants and were recorded on 3 occasions in the 27 month profile record

a geophysical model of the variscan orogenic root bohemian massif implications for modern collisional orogens abstract a new model of the structure and composition of the variscan crust in the bohemian massif is proposed based on 3d gravity modelling geological data seismic refraction cel09 and reflection 9hr sections the bohemian massif crust is characterized by a succession of positive and negative anomalies of about 60 80 km wavelength for nearly constant moho depths the south western part of the bohemian massif displays a large negative bouguer anomaly corresponding to high grade rocks granulites and migmatites of the palaeozoic crustal root represented by the moldanubian domain the adjacent neo proterozoic bruno vistulian microcontinent displays an important gravity high reflecting mafic and intermediate medium grade metamorphic and magmatic rocks the deep crustal boundary between the root domain and the bruno vistulian microcontinent is represented by a strong gradient located 50 to 70 km westwards from the surface boundary between these units indicating that the high density basement rocks are covered by a thin sheet of low density granulites and migmatites north west from the moldanubian domain occurs an important gravity high corresponding to the neo proterozoic basement of the tepla barrandian unit limited in the north by southeast dipping reflectors of the tepla suture which is characterized by high density eclogites and ultramafics the footwall of the suture corresponds to low density felsic crust of the saxothuringian basement the reflection and refraction seismics and gravity modelling suggest a complex lithological structure of the moldanubian domain marked by a low density 5 10 km thick lower crustal layer located above the moho a 5 10 km thick heavy mafic layer a 10 km thick mid crustal layer of intermediate density and a locally developed 2 5 km thick low density layer at the surface the low density lower crust correlates well with low p wave velocities in the range 6 0 6 4 km s 1 in the cel09 section this complex geophysical structure and surface geology are interpreted as a result of carboniferous partial overturn of low density lower crust and high and intermediate density crust in the area of central root and by viscous extrusion of low density orogenic lower crust over the high density bruno vistulian continent comparison of these data with geophysical profiling of the andean and the tibetan plateaus suggests that modern orogenic systems reveal comparable deep crustal geophysical pattern based on these similarities we propose that the variscan root represents a deep crustal section of above mentioned plateaus which may have develop by the same orogenic process

multiple longshore sand bars in the upper chesapeake bay abstract in the upper chesapeake bay maryland u s a field surveys were conducted at 18 multiple longshore sand bar sites the multiple bar systems were found in water depths less than approximately 2 m mean sea level and exhibited mild bottom slopes of 0 0052 or less the number of bars composing each system ranged from four to 17 and the spacing between the crests typically increased in the offshore direction ranging from 12 to 70 m bar height also typically increased with distance offshore and ranged from 0 03 to 0 61 m a grain size analysis of crest and trough sediment did not reveal any significant differences and the sediment was categorized as fine sand a review of the literature data indicated that the chesapeake bay multiple bars possessed similar characteristics to those found in gelding bay baltic sea similarities in fetch wave height and tidal range between the two bays may account for this finding the surf scaling parameter indicated that the multiple bar systems were extremely dissipative with regard to wave energy and wave height appeared to be an important factor in controlling bar spacing and bar height a multiple wave break point hypothesis was discussed as a possible mechanism for the formation of chesapeake bay multiple longshore bars and limited observational evidence appeared to support such a mechanism

influence of the geologic and geomorphologic characteristics and of crab burrows on the interrelation between surface water and groundwater in an estuarine coastal wetland the interrelation between surface water and groundwater in intertidal flats is often studied through mathematical models in many cases these models need to be supported by an integral analysis of the geologic geomorphologic hydrologic and biological characteristics of the environment that are to be obtained from field surveys the marshy environment of the river ajo in the samborombon bay wetland argentina is a typical example of an estuarine coastal zone geologic and geomorphologic surveys were carried out as well as measurements of surface water and groundwater level changes as a response of the aquifer to tidal forcing the banks of the river ajo are either scarped with storm flats or mildly sloped with intertidal flats and numerous crab burrows sediments are mainly silty clayey with low permeability and lie over silty sandy layers at the erosion scarps the tidal wave enters the aquifer as a sub horizontal flow through the pore space of the sediments the tidal range in the aquifer depends on the lithological characteristics of the sediments and on the side changes of their hydraulic conductivity the rise of the water table at high water and its subsequent fall are nearly sinusoidal with a period similar to that of the tide at the river at the intertidal flats instead the tidal wave enters the aquifer mainly as a sub vertical flow through the crab burrows as the crab burrows are not interconnected they are not distinct pathways for preferential flow therefore the groundwater flux into the river is very slow during low water and the recovery of the water table takes a long time the tidal influence upon the water table on both kinds of banks affects only a narrow strip of the aquifer not only are the characteristics of the marshy environment of the river ajo representative of most of the samborombon bay wetland they can also be extended to other similar coastal wetlands to help preserve these invaluable environments

silt and sand transport in a deep tidal channel of a large estuary manukau harbour new zealand abstract time series of suspended silt concentration measured by an optical backscatter sensor and suspended sand concentration measured by an acoustic backscatter sensor in a deep channel 14 m msl of a large new zealand estuary manukau harbour are analysed suspended sand concentration varied in phase with the current speed and bed shear stress on a 6 2 h cycle but suspended silt concentration varied on a semidiurnal 12 4 h cycle such that the channel was clear of silt at high tide and maximum silt concentration occurred at low tide threshold for initiation of sand motion was found to be related to local skin friction sand concentration profiles were consistent with settling flux balanced by gradient diffusion with a two layer sediment diffusivity dependent on local friction velocity suspended sand reference concentration was explainable in terms of local skin friction and changes in the channel bed sediment were correlated with the suspended sand load therefore sand suspension in the channel is a local process and the channel bed is the source of the suspended sand load suspended sand flux was found to be a highly nonlinear function of the tidal current speed in the channel in that case tidal current velocity asymmetry is the principal determinant of direction of net sand transport and the ebb flood dominance concept is valid the relative phase of the m 2 and m 4 constituents of the tidal current velocity which quantifies tidal current asymmetry was calculated from a tide model of the estuary and results were mapped the harbour was found to be broadly ebb dominant and therefore self flushing of sand silt suspension in the channel was not related to the local boundary layer dynamics but was explainable in terms of the movement up and down the channel of a horizontal gradient in silt concentration which implies the existence of a turbid water mass that is perched on the surrounding intertidal flats at high tide and that drains into the channel during ebb tide silt concentration in the channel increased during a short storm which corresponded to the passage through the measurement site of a turbid fringe that was formed on the surrounding intertidal flats during the previous high tide under the action of breaking waves wave activity on the surrounding intertidal flats is therefore the principal determinant of suspended silt load in the channel and an approach that treats waves and sediment pathways between intertidal flats and channels is needed for modelling silt transport

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

dating granulite facies structures and the exhumation of lower crust in the moldanubian zone of the bohemian massif deformation of granulite facies rocks in the moldanubian zone of the southern bohemian massif is expressed in two intersecting planar fabrics steeply disposed s1 and flat laying s2 which correspond to two deformation stages d1 and d2 the existing sm nd garnet ages from banded granulite gneisses new u pb zircon data from deformed granite intrusions within the granulite gneisses and the p t and field structural relations constrain the ages and p t conditions of the two deformation phases the early deformation d1 was associated with a hp ht metamorphic stage with a minimum age of ca 354 ma which was followed by a near isothermal decompression a concordant u pb zircon age of 318 1 ma dates the emplacement of intrusions of deformed granite into the granulite gneisses and constrains deformation phase d2 this phase was associated with an lp ht metamorphism dated in the region at ca 340 330 ma the available structural and isotopic data indicate that granulites in the southern bohemian massif were exhumed from lower to middle crust during compression the structural relations and p t t data for the studied granulites are consistent with their exhumation by near vertical extrusion of the softened orogenic root

prediction of average mixing depth of sediment in the surf zone abstract a predictive model for the average depth of mixing of sediment in the surf zone is formulated on the basis of the wave induced shear stress on the bottom the model is calibrated using field data obtained through tracer experiments performed on pacific beaches of japan the result gives z d 81 4 b c where z is the average depth of sediment mixing in the surf zone d is the grain size of the sediment b is the shields parameter at the wave breaking point and c is the critical shields number for oscillatory flow the predicted mixing depth is found to increase in an approximately linear fashion with the breaking wave height for significant breaking wave heights up to about 1 5 m for greater breaking wave heights the rate of increase in the mixing depth decreases with increasing wave height the mixing depth is also a strong function of the wave period in the larger wave height region finally the predicted mixing depth is found to be a weakly increasing function of the sediment grain size

clasts of variscan high grade rocks within upper visean conglomerates constraints on exhumation history from petrology and u pb chronology in this paper u pb zircon monazite and rutile data for crystalline rocks deposited as clasts in the upper visean conglomerates at the eastern margin of the bohemian massif are reported u pb data of spherical zircon from three different granulite clasts yielded a mean age of 339 0 0 7 ma 2 while oval and spherical grains of another granulite pebble define a slightly younger date of 337 1 1 1 ma these ages are interpreted as dating granulite facies metamorphism thermochronology and the derived pressure temperature p t path of the granulite pebbles reflect two stage exhumation of the granulites near to isothermal decompression from at least 44 km to mid crustal depths of around 22 km was followed by a near isobaric cooling stage based on reaction textures and geothermobarometry minimum average exhumation rate corresponds to 2 8 4 3 mm year 1 the extensive medium pressure high temperature overprint on granulite assemblages is dated by u pb in monazite at c 333 ma this thermal event probably has a close link to generation and emplacement of voluminous moldanubian granites including the cordierite granite present in clasts this granite was emplaced at mid crustal levels at 331 3 ma u pb monazite whereas the u pb zircon ages record only a previous magmatic event at c 378 ma eclogites and garnet peridotites normally associated with high pressure granulites are absent in the clasts but exotic subvolcanic and volcanic members of the ultrapotassic igneous rock series durbachites of the bohemian massif have been found in the clasts it is therefore assumed that the clasts deposited in the upper visean conglomerates sampled a structurally higher tectonic unit than the one that corresponds to the present denudation level of the moldanubicum of the bohemian massif the strong medium temperature overprint on granulites dated at c 333 ma is attributed to the relatively small size of the entirely eroded bodies compared with the presently exposed granulites