contemporaneous extension and compression in the northern apennines from earthquake fault plane solutions summary the northern apenninic arc belongs to the deformation zones surrounding the adriatic plate which behaves as a rigid block it is a ne verging fold and thrust belt which developed since the miocene with the migration of an extensional compressional pair previous seismological data are roughly in agreement with this deformation picture although the outer compressional front was not clearly defined by the few available solutions in this work we confirm the existence of two adjacent zones of contemporaneous extension and compression in the northern apennines defining the extent of these two zones and their interim1 deformation better than was previously done this is accomplished through the analysis of 125 new focal mechanisms of earthquakes 2 6 md 4 8 recorded by the national network of the lstituto nazionale di geofisica in the period 1988 1995 the two deformation zones are clearly separated and lie very close to each other with a partial overlapping in the emilian tuscan apennines strikeslip events are scattered in most of the study area thrust faulting earthquakes are located in the internal part of the most recently active thrust front middle upper pleistocene there is a general trend of the compressional p axes of thrust fault and strike slip solutions to be perpendicular to the apenninic direction in the external zone while the tensional axes of the normal fault and strike slip solutions in the belt do not show a uniform orientation however clearly evident is the ene direction of extension in the peri tyrrhenian region consistent with the direction of shmln in the quaternary volcanoes of this region inferred from borehole breakouts and microearthquake fault plane solutions we determined the orientation of the stress tensor in the two main deformation zones by inversion of the 125 fault plane solutions determined in this study we found that the external belt is characterized by ne sw compression ol horizontal oriented n45 e while a normal faulting regime with o3 e w oriented is characteristic of the umbria tuscany back arc region

measurements and analyses of velocity profiles and frazil ice crystal rise velocities during periods of frazil ice formation in rivers the vertical concentration distribution of frazil ice crystals in a stream during the form ation and gro lth of frazil ice was discussed in a prel imi nary way by gosi nk and osterkamp 1981 thi s paper extends and completes the analysis of buoyant rise velocities of frazil ice crystals and applies the results to an interpretation of measured veloc ity profiles in rivers during frazil ice events additional experimental data are also presented two tillle scales are defined the buoyant time scale tb whi ch represents the time requi red for a frazil crystal to rise huoyantly from the river bottom to the water surface and the diffusive time scale to which represents the time required for a frazil crystal to be transported by turbulence through the derth it is shown that the ratio of the time scales tb td defines the nature of the layering processes in particular if tb to l i11 be layered conversely if tb to l turbulent mixing will proceed faster than buoyant 1 ifti ng and the fl ow ii 11 be well mixed this ratio for frazil particles of di arneter 2 illfll or more corresponds to rul e of thumb velocity criteria developed in non ay and canada to di sti ngui sh 1 ayered frazil i ce l later fl o i frol l we11mixed flow the development of this theory depends in large part upon the deterr lination of tb which depends upon the rise velocity of frazil ice crystals a force ba lance node 1 was developed for the ri se velocity of a frazil crystal field observations during frazil ice formation in goldstream creek and in the chatanika river north of fairbanks are reported including a series of measure ents of the rise velocities of frazil ice crystals typical particle size of frazil ice was about 2 n with a rise velocity of about 10 0 mm s l the agreement of measured rise velo cities with the theoretical model is good considering uncertainties in the drag coefficient and in the deterlllination of frazil crystal sizes under field conditions velocity profiles in the chatanika river and in sol dstream creek duri ng frazil formati on suggest that the time scale ratio may serve as a transition criter ioi1 betvlee l layered frazil ice water flow and we11iilixed flo l this ratio was calculated with the rise velocity of frazil ice crystals arbitrarily chosen to be 0 01 m s i i ntrooucti on observations of ice formation in northern rivers during the freeze up period show that three ice flol regimes are common sheet ice floating frazil ice and a well mixed flow frazil ice crystals and water in the last case if the velocity of the river is sufficiently high open water conditions may persist throughout the winter designers of hydraulic structures such as canals water transportation facilities hydroelectric power structures water intake structures etc may require information on which flow regime to expect under a given set of flow conditions and on the trans ition from an ice cover to open water flow thus there is substantial interest in developing criteria which can be used to predict the conditions that lead to each of these three flovl regimes and the presence or absence of an ice cover it could be argued that the criteria for the transition from floating frazil ice to a well mixed flow should be the same as the criteria for the form ation of an ice cover the reason for this is that if the frazil crystals remain mixed with the flow then they cannot accumulate on the surface to form an ice cover however it is clear that other hydrological aspects of rivers enter into criteria on the pres ence or absence of an ice cover these include the sinuosity channel and slope variability bed rough ness and also the mechani cs of fraz il crystal floc pan and floe interactions osterkamp and gosink in press and changes of these parameters during the growth of an ice cover for many locations rule of thumb flow velocities have been determined which define the transitions from well mixed flow to floating frazil ice and the formation of a coherent ice cover for example in norway wllere rivers are often wide shallow with steep slopes and underlain with rocks or boulders the water surface is usually covered with moving frazil slush if the water velocity is 0 5 and 1 2 m s l carstens 1971 on the saint lawrence waterway in canada layered frazil ice flow may be expected at river velocities 0 8 and 1 0 m s i

frazil ice formation in an ice shelf water plume 1 we present a model for the growth of frazil ice crystals and their accumulation as marine ice at the base of antarctic ice shelves the model describes the flow of buoyant water upward along the ice shelf base and includes the differential growth of a range of crystal sizes frazil ice formation starts when the rising plume becomes supercooled initially the majority of crystals have a radius of similar to0 3 mm and concentrations are below 0 1 g l depending on the ice shelf slope which controls the plume speed frazil crystals increase in size and number typically crystals up to 1 0 mm in radius are kept in suspension and concentrations reach a maximum of 0 4 g l the frazil ice in suspension decreases the plume density and thus increases the plume speed larger crystals precipitate upward onto the ice shelf base first with smaller crystals following as the plume slows down in this way marine ice is formed at rates of up to 4 m yr in some places consistent with areas of observed basal accumulation on filchner ronne ice shelf the plume continues below the ice shelf as long as it is buoyant if the plume reaches the ice front its rapid rise produces high supercooling and the ice crystals attain a radius of several millimeters before reaching the surface similar ice crystals have been trawled at depth north of antarctic ice shelves but otherwise no observations exist to verify these first predictions of ice crystal sizes and volumes

water temperature measurements in turbulent streams during periods of frazil ice formation field measurements of water temperatures in two turbulent streams in interior alaska have been made during periods of frazil ice production the measured equilibrium temperature of the water te 0 005 c agrees with the value calculated from the electrical conductivity of the water average cooling rates of the streams during the summer to fall stream cooling period were on the order of several tenths of a degree per day with average surface heat losses of 5 to 18 w m 2 just prior to a frazil ice event the water cooling rates were 1 to 3 mk min 1 with surface heat losses of 47 to 140 w m 2 super cooling at the water surface of a stream prior to and during frazil ice production does not exceed o 2 c as shown by measurements of air water tempera ture profiles and by radiometer measurements water supercooling at the time of frazil ice nucleation was 10 mk these measurements show that frazil ice nuclei in streams must be other ice particles cold organic materials cold soil particles or a combin ation of these that may be introduced into the stream by mass exchange processes at the air water interface the maximum observed supercooling was 6tm 40 mk two measured values for the residual supercooling were 3 and 9 mk frazil growth rates calculated from the observed values of supercooling show that unless the period of residual supercooling is very long most of the frazil ice production during one night of super cooling occurred in the transient thermal period from the time of nucleation to the time that the water became residually supercooled i ntroducti on frazil ice formation in northern rivers causes many problems which are fascinating from scientific and engineering points of view and are of immediate economic importance frazil ice can cause flooding block water supply intakes reduce or halt power generation in hydroelectric facilities interfere with ship and barge transportation and damage or restrict the use of hydraulic structures and facil ities development of rational design methods for alleviating or avoiding these problems is still hindered by deficiencies in the present state of understanding of frazil ice formation a knowledge of the water temperature in turbulent streams during periods of frazil ice formation can be used to increase our understanding of heat losses supercooling nucleation and growth of frazil ice and other processes and parameters associated with its formation however very few in situ measurements of water temperature during periods of frazil ice production have been reported this situation is surprising in view of the fact that frazil ice appears to be the dominant form of ice occurring in turbulent streams in alaska as well as in other northern regions figure 1 illustrates an idealized thermal history of a turbulent stream during periods of frazil ice production these ideal curves are observed in laboratory experiments when heat loss from the water surface is constant carstens 1966 michel 1967 however the variability of heat loss from natural streams leads to more uneven curves osterkamp 1978 and the magnitudes of the various parameters that define the curves are different for curves obtained under laboratory conditions compared to those obtained in streams figure 1 illustrates the fact that the equilibrium temperature of the water te at time te is usually depressed because of the presence of dissolved substances in the water in principle frazil i ce nucl eati on can occur at any time t after the water temperature has been reduced to te nucle ation occurs at time tn and temperature tn the minimum stream temperature tm occurs at time tm at any time t te the magnitude of the supercooling of the water is

frazil dynamics and precipitation in a water column with depth dependent supercooling when seawater becomes supercooled collections of small ice crystals known as frazil ice form and grow a model of frazil ice dynamics is presented that deals explicitly with the buoyant settling of frazil crystals onto an overlying surface this yields further insight into transport associated with the ice pump mechanism whereby ice is melted at depth and transferred to a shallower location as a result of the pressure variation of seawater s freezing temperature the model is applied to a vertical cross section through an ice shelf water plume beneath filchner ronne ice shelf antarctica and helps to elucidate the depth variation in its properties for the first time as well as predicting the precipitation rate of frazil crystals the model predicts that frazil ice should be preferentially located in a narrow layer near the ice shelf base as a result of the maximum supercooling there and an influx of crystals rising under their own buoyancy the deposition of these crystals onto the ice shelf is governed by the balance between crystal rising and turbulent transfer of frazil away from the shelf which is investigated in some detail

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

lateral variations of thermo mechanical properties in the tyrrhenian northern apennine region abstract we present lithospheric strength profiles calculated for a transect running from the island of corsica in the west to the apennine foredeep in the east strength profiles are constructed at locations selected on the basis of the availability of seismic heat flow gravity data and earthquake foci distribution rheological modelling points to the presence of strong lateral variations in the thermo mechanical state of the lithosphere the tyrrhenian tuscany area has a low rigidity in relation with the intense stretching active from the early miocene and with very high heat flow associated with asthenospheric rise on the contrary hinterland corsica and apenninic foreland areas adriatic area show a higher level of lithospheric strength effective elastic thickness estimates confirm this scenario exhibiting a strong reduction from the northern tyrrhenian to the inner apennines and then an abrupt increase in correspondence of the adriatic plate the distribution of earthquake focal depths confirms the presence of a brittle rheology in the first kilometres of the crust in the tuscany area whereas close to the plate boundary much lower thermal gradients partly induced by the westward plunging adriatic plate enable seismic activity to occur at larger depths

segmentation and configuration of subducted lithosphere in italy an important control on thrust belt and foredeep basin evolution the apennine foredeep basin system located on continental crust consists of four roughly parallel basin segments each containing a flexural outer rise and an inner trough that deepens continuously toward the thrust belt flexural modeling of subsidence data across the basin indicates that this foredeep basin results mainly from deep subsurface loads acting on the subducted slab from within the subduction zone rather than from the topographic load of the apennine mountains the geometry of the four basin segments observed within the apennine foredeep is thus inferred to reflect subduction of segmented lithosphere where each lithospheric segment corresponds to a segment of the foredeep basin beneath the basin and in the foreland the lithosphere is continuous segmentation occurs only at depth beneath the thrust belt this pattern olf segmentation of the subducted lithosphere at depth is reflected in the large scale geometry of the outer part of the apennine thrust belt and appears to control it similarly the pattern of segmentation also appears to control the position of the eastern limit of back arc type extension in western italy these relations strongly suggest a genetic relation between subduction zone processes foredeep basin geometry and thrust belt evolution in which the configuration of the subducted lithosphere largely controls the large scale surface deformation observed in the apennine system

frazil ice entrainment of sediment large tank laboratory experiments laboratory experiments that simulate natural ice formation processes and sediment entrainment in shallow water are presented a 10 30 cm s 1 current was forced with impellers in a 20 m long 1 m deep indoor tank turbulence in the flow maintained a suspension of sediments at concentrations of 10 20 mg l 1 at 0 5 m depth low air temperatures 15 c and 5 m s 1 winds resulted in total upward heat fluxes in the range 140 260 w m 2 the cooling produced frazil ice crystals up to 2 cm in diameter with concentrations up to 4 5 g l 1 at 0 5 m depth considerable temporal variability with time scales of 1 min was documented a close to constant portion of the smaller frazil crystals remained in suspension after some hours the larger crystals which made up most of the ice volume accumulated as slush at the surface current measurements were used to calculate the turbulent dissipation rate and estimates of vertical diffusion were derived after 5 8 hours sediment concentrations in the surface slush were normally close to those of the water after 24 hours however they were 2 4 times higher data indicate that sediment entrainment depends on high heat fluxes and correspondingly high frazil ice production rates as well as sufficiently strong turbulence waves do not seem to increase sediment entrainment significantly

a model of marine ice formation within antarctic ice shelf rifts large rifts that open in antarctic ice shelves are known to be filled by snow accumulations ice shelf fragments and sea ice this work demonstrates how these rifts may also be filled from below through their interaction with ocean water by marine ice the model presented here quantifies both the rate of marine ice accumulation at the top of the rift which results from melt driven convection at its sides and the impact of this process on waters occupying the confined environment of the rift the results show that such a system could fill the rift with tens of meters of marine ice in the process the temperature and salinity of the ambient water in the rift evolve through two distinct phases the first is characterized by rapid change that takes the properties of the ambient water to near equilibrium values followed by a second phase of much slower change the melting rate at the lower part of the walls of the rift emerges as the principal factor influencing many aspects of model behavior testing the model against field observations from an antarctic rift showed it to be robust and successful in reproducing the main observed features including the presence of a thick layer of supercooled ocean water inside the rift

the strength of the continental crust detachment zones and the development of plastic instabilities abstract the maximum strength of the continental crust is constrained by the geothermal gradient the lithological make up of the crust and whether or not the byerlee relation holds to the base of the continental crust if this linear byerlee relation holds then maximum shear stress levels sustained at low geothermal gradients 10 20 c km 1 could be as high as 300 mpa in the upper part of the mantle for strike slip regimes and up to 700 mpa in thrust regimes towards the base of the crust however if byerlee s relation breaks down at moderate temperatures and pressures and if such breakdown is associated with the transition from unstable to stable sliding on pre existing faults then the maximum stress levels in the crust being set by the shear stress at the breakdown depth are much lower ca 300 mpa for low geothermal gradients and thrust regimes the lithosphere is quite weak below the crust for geothermal gradients greater than 20 c km 1 so attempts to treat the lithosphere as an elastic slab for time scales for which plastic flow contributes significantly to the deformation should be questioned for a geothermal gradient of 10 c km 1 the entire continental crust and part of the upper mantle behave as an elastic brittle slab at higher geothermal gradients the distribution of strength is variable vertically according to the magnitude of the geothermal gradient and the lithological make up large contrasts in strength occur across rheologically defined boundaries as well as across lithological boundaries at these higher geothermal gradients such contrasts in strength could act as nuclei for detachment zones within the continental crust and upper mantle detachment at or near the moho is to be expected only for low geothermal gradients lateral variations in the geothermal gradient produce shallowly dipping zones of highly contrasting strength throughout the continental crust and uppermost mantle possibly resulting in flat lying thrusts and normal faults which may ramp upwards through the crust instabilities leading to seismic events which occur solely during plastic shearing should be common in thrust terranes where the stresses are relatively high ca 100 200 mpa for geothermal gradients ranging from 10 to 50 c km 1 and for quartz rich felsic mafic and peridotitic rock types thrust terranes are therefore predicted to be characterized by coseismic plastic instabilities on all detachment zones throughout the crust and into the uppermost mantle but embryonic rift zones should be aseismic at depth ca 10 km

extension of an ice shelf water plume model beneath sea ice with application in mcmurdo sound antarctica a one dimensional frazil laden plume model predicts the properties of ice shelf water isw as it evolves beneath sea ice beyond the ice shelf edge an idealized background ocean circulation which moves parallel to the plume imitates forcings other than the plume s own buoyancy the size distribution and concentration of the plume s suspended frazil ice crystals are affected by the background circulation velocity the root mean square tidal velocity the drag coefficient and the efficiency of secondary nucleation consequently these variables are the key physical controls on the survival of supercooled water with distance from the ice shelf which is predicted using several realistic parameter choices starting at 65 m thick the in situ supercooled layer thins to 11 5 and 4 3 m at distances of 50 and 100 km respectively we apply the extended model in mcmurdo sound antarctica along the expected path of the coldest water three late winter oceanographic stations along this path in conjunction with historical data provide initial conditions and evaluation of the simulations near the ice shelf in the western sound the water column consisted entirely of isw and the subice platelet layer thickness exceeded 5 m with platelet crystals dominating the sea ice structure suggesting that isw persisted throughout winter presuming a constant isw flux the model predicts that the plume increases thermodynamic growth of sea ice by approximately 0 1 m yr 1 5 of the average growth rate even as far as 100 km beyond the ice shelf edge

the tectonic regime in italy inferred from borehole breakout data abstract in italy the horizontal stress directions are well constrained in many regions but the tectonic regime is not well known because the stress magnitudes are unknown our intention is to improve the knowledge of crustal stress in italy both at shallow depth and in low seismicity areas therefore we inferred the tectonic regime from the comparison between the depth of breakout occurrence and the physical properties of the rocks in 20 boreholes the critical value of the maximum horizontal stress for which the effective tangential stress at the borehole wall overcomes the rock strength to form breakouts could be computed from rock strength and density comparing the theoretical stress distributions for different tectonic regimes with the depth distribution of breakout occurrence it is possible to infer the tectonic regime that fits best to the breakout depth distribution we investigated boreholes up to 6 km deep located in different tectonic environments over the italian peninsula the po plain the apenninic chain the adriatic foredeep and the tyrrhenian quaternary volcanic region these wells are characterised by breakout data of good quality a b and c according to world stress map quality ranking system the results are in general agreement with the style of faulting derived from earthquake focal mechanisms and other stress indicators our results show a predominance of a normal faulting nf regime in the inner apennines and both normal faulting and strike slip faulting ss style in the surrounding regions possibly also associated with changes in the tectonic regime with depth

the tyrrhenian apennines system structural setting and seismotectogenesis abstract the available geological geophysical and seismological data of the tyrrhenian apennines system are briefly reviewed and synthesized in a number of sketch maps integrating and comparing these data a large scale simple shear kinematic model is proposed for the tyrrhenian apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the adriatic foreland from the late miocene to the present in the frame of this working hypothesis the thinning and stretching of the tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices bounded by low angle west dipping oblique shear zones rotating above a common detachment that penetrates into the mantle lithosphere at any given time the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom the displacement within the hanging wall of the transtensional shear zone is accommodated by high angle normal faults and en echelon grabens the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co axial en echelon sets of folds thrusts and high angle strike slip faults thus the development of the late miocene pliocene pleistocene contractional strain field of the apennines of italy might not be a consequence of complex subduction processes but subordinate to the deformation processes that lead to the progressive opening of the tyrrhenian sea the seismicity of the crustal apennines of italy is analyzed in the frame of the above history of progressive deformation and a seismotectogenetic model is outlined

lithosphere strength inferred from fracture strength of rocks at high confining pressures and temperatures abstract a model is proposed for the strength of the crust or lithosphere in which the peak strength is in the brittle regime but not on the brittle ductile transition the model is based on experimental evidence which suggests that the fracture mechanism changes from ordinary brittle fracture to a high pressure type when the compressive strength equals the frictional strength the model assumes that a high pressure type of fracture is dominant in the earth s lower crust except where very high pore pressures exist it takes account of the scale effect on the strength of rocks the strength of the high pressure regime is estimated as a function of depth or pressure and temperature using experimental results derived at high confining pressures and temperatures in the upper part of the brittle zone where the frictional strength is lower than the compressive strength byerlee s friction law is used in the ductile regime the steady state creep law is used two cases of lithospheric strength dry and wet hydrostatic water pressure respectively are evaluated for thrust strike slip and normal faulting mechanisms with the exception of a wet lithosphere and a normal faulting mechanism the strength between the frictional and creep regimes is approximately constant with depth although the peak strength is in this part

deep crustal roots beneath the northern apennines inferred from teleseismic receiver functions abstract teleseismic waveforms recorded by a regional array crossing the northern italian peninsula and northern corsica are analyzed using the receiver function technique to determine the first order crustal structure the receiver function approach is used to isolate receiver side ps conversions generated at the crust mantle boundary and any major velocity discontinuity beneath the stations we used the time delay between the direct p wave and the ps wave converted at the moho discontinuity to infer crustal thickness beneath the stations the crust mantle boundary is estimated at 25 1 km of depth in northern corsica 20 2 km beneath the elba island and 20 to 24 2 km beneath tuscany in the eastern portion of the array moho depth increases from 28 2 km beneath the adriatic coast to 49 3 km beneath the apennine chain in a distance of about 100 km a double ps conversion produced beneath the val tiberina graben to the west of the apennines corresponds to interfaces as deep as 20 2 and 52 2 km this observation supports the hypothesis of partial overlapping between the shallow tuscan moho and the deeper adriatic moho the westward deepening of the adriatic moho beneath the northern apennines can be explained by lithosphere delamination that has dragged downward the adriatic lower crust the deep crustal root estimated beneath the northern apennines indicates that this portion of the chain is isostatically overcompensated

rheology of the northern apennines lateral variations of lithospheric strength abstract a thermorheological wsw ene profile along the northern apennines is presented the variations in rheology are estimated using two different models the conventional model in which only frictional sliding and power law creep are taken into account and a new model in which in addition to the previous two mechanisms a high pressure brittle fracture mechanism is included upper and lower bounds for the rheology are considered a hard case where the lower crust is mafic granulite and the lithospheric mantle is dry peridotite and a soft case where the lower crust is felsic granulite and the lithospheric mantle is wet peridotite important differences in lithospheric rheological structure are inferred between the western and the eastern sectors of the chain in the former a creme brulee structure generally applies in the latter a jelly sandwich structure applies these rheological models therefore are not alternative but end members of a continuous spectrum of rheological behaviours

extensional basins in the tectonically bimodal central apennines fold thrust belt italy response to corner flow above a subducting slab in retrograde motion the apennines fold thrust belt has developed over the past 20 m y in response to southeastward retrograde migration of the adriatic trench in a region of ongoing subduction but little plate convergence the orogenic belt is characterized by a gently sloping southwestern flank in a state of regional extensional collapse and a steeply sloping northeastern flank in regional contraction in any given area in the central apennines the chronologies of thrusting and subsequent extension provide an estimate of the time required for attainment of high elevation 2 5 3 0 km the time between initial extension and eruption of local ultramafic lavas provides an estimate of the time required to collapse by regional crustal thinning these estimates suggest that the northeastern contractional flank of the range is rising at least twice as fast as the southwestern flank is collapsing we propose that the bimodal state of stress in the apennines is maintained by corner flow in the mantle wedge beneath the crest of the range

modeling the effects of frazil ice crystals on the dynamics and thermodynamics of ice shelf water plumes the seawater that comes into contact with the base of a floating ice shelf is modified as a result of the phase changes that occur melting is prevalent in the deepest parts of the subice cavity and this drives a buoyant flow of ice shelf water isw along the sloping ice shelf base the ascent of the isw toward the surface of the ocean causes supercooling because the freezing point rises with the falling pressure and this induces a change from melting to freezing assuming that seed crystals exist the isw now fulfills the condition for the rapid growth of disc shaped frazil ice crystals which may subsequently settle upward out of suspension under the action of gravity a simple numerical model of these processes has been developed based on the theory of inclined plumes the isw is treated as a turbulent particle laden gravity current ascending a reactive boundary and containing a suspended crystal load which evolves in response to the supercooling of the water and the inverted sedimentation of the crystals the frazil ice has two important effects on the behavior of the isw plume because the generation of crystals through out the plume provides such a large area over which phase changes can occur the conversion of supercooling into ice happens much more readily than is possible through turbulent transfer of heat and mass at the ice shelf base a suspended crystal load reduces the bulk density of the isw so that a growing suspension causes the plume to accelerate while deposition of crystals onto the ice shelf effects a deceleration there are positive feedbacks in that an acceleration of the plume induces more rapid crystal growth while a deceleration allows suspended crystals to settle out more easily

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

cover thrust reactivations related to internal basement involvement during neogene quaternary evolution of the northern apennines up to recently the neogene quaternary evolution of the northern apennines italy has been described by the classic model of a migrating eastward compressive external front with an extensional regime in the back areas connected with the tyrrhenian basin formation however in the last few years new structural data have been collected in the internal marine and continental episutural basins and in the external exposed thrust belt a complex structural evolution has now been reconstructed with coeval main tectonic phases that affect both areas with stress field change four main tectonic phases have been identified since late tortonian times these are dated as messinian late pliocene middle and late pleistocene the thrust belt has a complex evolution detected in a wide external area of the chain after the first emplacement of the main thrust sheets with reactivations and out of sequence thrusting these reactivation phases fit very well with the compressive phases affecting the sediments of the internal basins suggesting a direct relationship within a single evolutive model the increased knowledge of the deep structure of the northern apennines through geophysical and subsurface data acquired in the last few years indicates the basement involvement at least for the internal side of the northern apennines this basement involvement also played an important role in the tectonic evolution of the external sector of the apennines in this paper a new model is proposed that integrates field data geophysical evidence and geodynamic constraints the thrust reactivations and the out of sequence structures of the external area are related to internal crustal thrust activity the deformed sediments of the neogene quaternary basins are dating the thrust activity all the evidences point to a late neogene rejuvenation of the tectonic evolution but some inferences can be drawn on the development of the foredeep

a model for vertical frazil distribution a model is presented for the evolution of frazil over depth and with time in a turbulent flow the net upward migration due to buoyancy of the frazil is opposed by intermittent mixing induced by large energy containing eddies a surface renewal model is used to describe the effects of large eddy mixing parameters that represent an entire water body are obtained by averaging those of discrete water columns using a probability density function these parameters include the concentration profile the surface age and the surface layer thickness a dimensionless surface renewal frequency characterizes the frazil distribution at equilibrium the rate of heat loss from the water surface the surface renewal frequency and the critical surface layer thickness determine whether the frazil will evolve toward a well mixed equilibrium state or a layered state the model provides a physical basis for understanding the transition between these states consistent with existing empirical criteria and field data

a proposal for the kinematic modelling of w dipping subductions possible applications to the tyrrhenian apennines system in w dipping subduction zones there is a general eastward progression of the back arc basin accretionary wedge foredeep complex with the forward progression early stages of the complex are revealed by slices of upper crust and sedimentary cover abandoned to the west left floating above a new section of mantle a major shear zone should form at the new moho separating upper crust slices of earlier accretionary stages and the eastward flowing mantle the mantle wedging at the top of the subduction plane could be responsible for the uplift of the central parts of the belt the retreating of the subduction hinge is interpreted as due to the push generated by the eastward mantle flow detected in the hot spot reference frame the foredeep depth is mainly a function of the radius of curvature of the subduction hinge the frontal wedge is constructed from the stacking of the upper layers of the subducting plate and the syntectonic clastics that fill the foredeep which are progressively involved in thrusting and later by extension in order to preserve volume balance the lithosphere of the eastern plate before subduction has to be the same size as that which has been subducted due to the longer length of the arc with respect to the original length of the linear margin between the two converging plates laterally stretched subducted lithosphere is predicted at depth w dipping subductions usually have a short life probably due to their inherent capability to produce new lateral heterogeneities of the lithosphere the thin back arc which are a key factor in controlling and generating new subductions both e and w dipping this model is applied to the apennines tyrrhenian sea system

recent eruptions of mount adams washington cascades usa the postglacial eruption rate for the mount adams volcanic field is 0 1 km3 k y four to seven times smaller than the average rate for the past 520 k y ten vents have been active since the last main deglaciation 15 ka seven high flank vents at 2100 2600 m and the central summit vent of the 3742 m stratocone produced varied andesites and two peripheral vents at 2100 and 1200 m produced mildly alkalic basalt eruptive ages of most of these units are bracketed with respect to regional tephra layers from mount mazama and mount st helens the basaltic lavas and scoria cones north and south of mount adams and a 13 km long andesitic lava flow on its east flank are of early postglacial age the three most extensive andesitic lava flow complexes were emplaced in the mid holocene 7 4 ka ages of three smaller holocene andesite units are less well constrained a phreatomagmatic ejecta cone and associated andesite lavas that together cap the summit may be of latest pleistocene age but a thin layer of mid holocene tephra appears to have erupted there as well an alpine meadow section on the southeast flank contains 24 locally derived holocene andesitic ash layers intercalated with several silicic tephras from mazama and st helens microprobe analyses of phenocrysts from the ash layers and postglacial lavas suggest a few correlations and refine some age constraints approximately 6 ka a 0 07 km3 debris avalanche from the southwest face of mount adams generated a clay rich debris flow that devastated 30 km2 south of the volcano a gravitationally metastable 2 to 3 km3 reservoir of hydrothermally altered fragmental andesite remains on the ice capped summit and towering 3 km above the surrounding lowlands represents a greater hazard than an eruptive recurrence in the style of the last 15 k y

evaluation of stresses in two geodynamically different areas stable foreland and extensional backarc areas which are geodynamically different have different behaviors both in their thermal regime and seismic activity a stable area has a geotherm which can be considered as standard extensional and compressional areas have respectively high and low temperature gradients the italian region includes different geodynamical areas and all such situations are present we consider the apulian platform as an example of a stable area and the tuscany latium as an example of an extensional area for both of them the present geotherms are calculated taking into account for the tuscany latium its thermal history assuming that each region is subject to a constant strain rate the stresses are calculated as functions of depth and time the rheological behavior is assumed to be linear viscoelastic with viscosity dependent on temperature and elastic parameters dependent on lithology the geothermal profile and the rheological structure of the lithosphere remarkably affect the processes of stress accumulation which control the distribution of seismic activity the abrupt decrease of the temperature gradient at the moho produces considerably higher stress values with respect to the case of uniform gradient thus favoring subcrustal seismicity in the case of a standard temperature gradient subcrustal seismicity is predicted and a gap in seismicity indicating a soft intracrustal layer exists if there is a discontinuity in rheology by contrast in the case of a high temperature gradient subcrustal seismicity is not to be expected even in the presence of a discontinuity in rheology since subcrustal temperatures are already too high to permit a sufficient stress accumluation

frazil deposition under growing sea ice 1 platelet ice may be an important component of antarctic landfast sea ice typically it is found at depth in first year landfast sea ice cover near ice shelves to explain why platelet ice is not commonly observed at shallower depths we consider a new mechanism our hypothesis is that platelet ice eventually appears due to the sudden deposition of frazil ice against the fast ice ocean interface providing randomly oriented nucleation sites for crystal growth brine rejected in plumes from landfast ice generates stirring sufficient to prevent frazil ice from attaching to the interface forcing some of it to remain in suspension until ice growth rate and brine rejection slow to the point that frazil can stick we calculate a brine plume velocity and match this to frazil rise velocity we consider both laminar and turbulent environments we find that brine plume velocities are generally powerful enough to prevent a significant range of frazil sizes from sticking in the case of laminar flow and that in the turbulent case there may be a critical ice thickness at which most remaining circulating frazil suddenly settles

upper mantle domains beneath central southern italy petrological geochemical and geophysical constraints the italian peninsula shows high complexity of the mantle crust system and of the plio quaternary magmatism the lithospheric thickness has remarkable lateral variations from about 110 km to about 30 km intermediate and deep focus earthquakes indicate the presence of a lithospheric slab under the aeolian calabrian area and at the southern end of campania much less extensive intermediate depth seismicity characterizes the roman tuscany region where the existence of a relic slab has been hypothesized the deep seismicity in the southern tyrrhenian sea is associated with active calcalkaline to shoshonitic volcanism in the aeolian arc alkaline potassic volcanism occurs in central italy and potassic lamproitic magmatism coexists with crustal anatectic and various types of hybrid rocks in the tuscany area the parallelism between changing magmatism and variation of the structure of the crust mantle system makes central southern italy a key place where petrological and geophysical data can be used to work out an integrated model of the structure and composition of the upper mantle beneath tuscany the upper mantle has been affected by intensive subduction related metasomatism this caused the formation of phlogopite rich veins that cut through residual spinel harzburgite and dunite these veins possibly partially molten may explain the unusually soft mechanical properties that are detected just below the moho in the roman province the upper mantle is formed by a relatively thin lid the mantle part of the lithosphere and by metasomatic fertile peridotite probably connected with the upraise of an asthenospheric mantle wedge above the apennines subduction zone geochemical data indicate that metasomatism though still related to subduction had different characteristics and age than in tuscany in the eastern sector of the aeolian arc and in the neapolitan area the upper mantle appears to be distinct from the roman and tuscany areas and is probably formed by fertile peridotite contaminated by the presently active subduction of the ionian sea floor the overall picture is that of a mosaic of various mantle domains that have undergone different evolutionary history in terms of both metasomatism and pre metasomatic events the coexistence side by side of these sectors is a key factor that has to be considered by models of the geodynamic evolution of the central mediterranean area

modeling ice ocean interaction in ice shelf crevasses ocean freezing within ice shelf basal crevasses could potentially act as a stabilizing influence on ice shelves however ice ocean interaction and ocean dynamics within these crevasses are as yet poorly understood to this end an idealized 2 d model of an ice shelf basal crevasse has been developed using fluidity a finite element ocean model using an unstructured mesh a simple model of frazil ice formation and deposition has been incorporated into fluidity to better represent the freezing process model results show two different flow regimes dependent on the amount of freezing in the crevasse one driven by freezing at the top of the crevasse and the other by the ingress of meltwater from outside the crevasse in the first freezing at the top of the crevasse leads to the formation of an unstable overturning circulation due to the rejection of dense salty water in the second a buoyant layer is formed along the sides and roof of the crevasse stratifying the water column frazil ice precipitation is found to be the dominant freezing process at the top of the basal crevasse in the freeze driven case with direct freezing being dominant in the melt driven case in both cases melting occurs lower down on the walls of the crevasse due to the strong overturning circulation the freezing in ice shelf crevasses and rifts is found to be highly dependent upon ocean temperature providing a stabilizing influence on ice shelves underlain by cold waters that is not present elsewhere

thermomechanical structure of european continental lithosphere constraints from rheological profiles and eet estimates summary the eet equivalent elastic thickness of the lithosphere is a measure of the integrated lithospheric strength it is directly related to the mechanical thickness and rheology of the crustal and mantle lithosphere we present a comparison of eet estimates and strength profiles based on the extrapolation of rock mechanics data for different parts of the european and eurasian continental lithosphere we discuss the temporal and spatial variations in the mechanical thickness and strength inferred from data for precambrian segments of europe s lithosphere variscan europe and the alpine collision belt this analysis demonstrates important spatial and temporal variations in lithospheric rigidity for orogenic belts and sedimentary basins in eastern and western europe and asian parts of eurasia the eet estimates based on synthetic rheological profiles constrained by newly available geophysical data are consistent with the estimates of eet derived from flexural studies of sedimentary basins forelands and orogenic belts these rheological profiles suggest weakening of the major parts of the european and eurasian continental lithosphere by decoupling of the crustal and upper mantle parts a comparison with the seismicity depth distribution for some selected sites suggests that the intra plate seismicity is essentially restricted to the upper crustal parts of europe s lithosphere providing additional support to the notion of the decoupled lithosphere the presence of intra plate stress fields can explain a significant part of the observed variations in eet estimates within individual thermotectonic age groups a comparison of wavelengths of crustal and lithospheric folding with observations shows these wavelengths to be consistent with estimates of eet inferred from the rheological response of basins and orogens at more moderate levels of intra plate stress

submarine rifting at mid ocean ridges an analytical model for the formation of mid ocean ridges is presented in which the central rift zone is interpreted as a belt of localized extension of width 2w centred on the ridge axis upwelling of the asthenosphere occurs passively below the zone of extension at the rate required to conserve mass at distances greater than w from the ridge axis the lithospheric plates move at a uniform horizontal velocity u0 the model is thermally and mechanically consistent and predicts the surface heat flow and topography across the rift zone plus the vertical thermal structure and the thickness of the lithosphere below the zone of extension measurements of ridge heat flow and topography can be used to constrain estimates of the width of the zone of intrusion this width may be as large as 100 km comparable to the thickness of the lithosphere and that of mantle plumes as proposed in some models of mantle convection a comparison of the thermal structure at a ridge crest as produced passively by horizontal extension and actively by a mantle convection cell shows a qualitative similarity this similarity illustrates the difficulty in distinguishing cause and effect with regard to plate formation and motion

the effects of rotation and ice shelf topography on frazil laden ice shelf water plumes a model of the dynamics and thermodynamics of a plume of meltwater at the base of an ice shelf is presented such ice shelf water plumes may become supercooled and deposit marine ice if they rise because of the pressure decrease in the in situ freezing temperature so the model incorporates both melting and freezing at the ice shelf base and a multiple size class model of frazil ice dynamics and deposition the plume is considered in two horizontal dimensions so the influence of coriolis forces is incorporated for the first time it is found that rotation is extremely influential with simulated plumes flowing in near geostrophy because of the low friction at a smooth ice shelf base as a result an ice shelf water plume will only rise and become supercooled and thus deposit marine ice if it is constrained to flow upslope by topography this result agrees with the observed distribution of marine ice under filchner ronne ice shelf antarctica in addition it is found that the model only produces reasonable marine ice formation rates when an accurate ice shelf draft is used implying that the characteristics of real ice shelf water plumes can only be captured using models with both rotation and a realistic topography

average strain rate in the italian crust inferred from a permanent gps network ii strain rate versus seismicity and structural geology summary high quality weekly solutions for the coordinates of 45 permanent gps stations spanning 1 6 years and covering the alpine mediterranean area are used to estimate a horizontal velocity field aligned with the itrf2000 velocity datum after subtracting model velocities for the eurasian plate we show that the residual velocity field north of the apennines in the adriatic dinaric region describes a nne trending tectonic flow with a negative velocity gradient from 6 mm yr 1 ne of the apennines to virtually zero ne of the dinarids in the calabria region we observe a divergent pattern with nnw orientation which turns to west as one moves north towards tuscany very small residual velocities are observed for stations in the sardinia corsica block spain southern france and north of the alps this regional pattern of velocities defines areas of intraplate strain on the basis of good station coverage and of a strain rate value above a threshold we define five provinces calabria central apennines both in the extensional regime and north adriatic eastern alps and south adriatic under compression we show that the orientations of the principal strain rate axes agree with the stress pattern that is inferred from local seismicity a comparison of the geodetically determined strain rate with the strain rate associated with the seismic moment of earthquakes in the past 30 yr shows that each province may differ considerably from the others in the amount of seismic moment that is accommodated by surface strain for example the umbria seismic sequence of 1997 seems to have released a seismic moment that was not exceptionally large but concentrated over a short time interval so that the equivalent coseismic strain rate was larger than the value observed geodetically on average depending on the assumptions on the seismogenic volume and time span 70 100 per cent of the surface strain seems to be accommodated by the release of seismic moment in the upper italian crust

crustal section based on crop seismic data across the north tyrrhenian northern apennines adriatic sea abstract using deep seismic reflection data from the italian lithospheric exploration project crop in the central mediterranean region a 400 km long section composed of three different profiles crossing the northern tyrrhenian sea crop m 12a profile the northern apennines crop 03 and the adriatic sea crop m 16 is reconstructed and discussed new data allow us to outline a seismically consistent tectono stratigraphic setting for the crust and upper mantle of the northern apennines thrust belt system and its adriatic foreland time space analysis of the deformation of the investigated chain and identification of existing macrostratigraphic crustal intervals and tectonic units allow a reasonably controlled interpretation of the geodynamic evolution and of the main orogenic stages careful seismic reprocessing and application of advanced techniques to key zones of the explored area such as the tuscan archipelago were determinants in obtaining fundamental information for understanding of the complex lithospheric structures and their evolution profile interpretation supports that the northern apennine chain is dominated by a compressive thrust system crustal extension assumed by some authors as the dominating tectonic process for the whole tuscan apennine area represents a subordinate geodynamic event of the last stage tyrrhenian in the early cretaceous late jurassic the paleogeographic framework consisted of the europe and adria plates separated by the alpine tethys ocean during the late cretaceous early eocene adria europe convergence eo alpine stage and subduction beneath the adria plate closed the alpine tethys sea with the tethyan slab being clearly seismically imaged the first apenninic geodynamic stage occurred in the late oligocene early miocene with the opening of the balearic basin which generated a first lithospheric root of the apenninic chain in the tuscan archipelago area this root is represented by adria verging thrust faults that progressively flatten eastward upper parts of the west verging eo alpine thrust blocks were truncated by the east verging thrust faults of the balearic stage a deeper seismic reflector attributed to the top of the asthenosphere forms a mantle high below the elba island from the late miocene to present the corsica basin and western hinterland area were affected by extensional tectonics related to the tyrrhenian opening whereas compressional tectonics continued in the eastern hinterland and mostly on the eastward migrating foreland with development of a second lithospheric root constituted by high angle thrust faults these faults give rise to a huge basement culmination below the main apennines watershed impressive e directed gravity sliding of sedimentary blocks over their sloping basement occur generating the umbria marche shallow seismicity crustal shortening of the apennines system amounts to 170 km 14 km of which are due to the eo alpine stage 71 km to the balearic and 85 km to the tyrrhenian one in the frame of africa europe convergence the tyrrhenian apennines tectonodynamics were mainly conditioned by the mesozoic paleogeography

the crustal structure of the northern apennines central italy an insight by the crop03 seismic line in this paper the crop03 deep seismic reflection profile in the northern apennines is described and re considered in light of new geophysical data and interpretations made available in the last five years particularly from heat flow measurements aeromagnetics tomography active stress determination and passive seismology the crustal structure of the northern apennines is shown to be composed of two distinct domains to the west is the tyrrhenian domain and to the east is the adriatic domain these domains have distinctive geological and geophysical characteristics that exhibit distinct reflectivity patterns at all crustal levels in the tyrrhenian domain the upper oligocene lower miocene compressive structures are no longer recognizable because they are dissected by subsequent extensional tectonic features the seismic profile highlights the strong asymmetry of extensional deformation and the upper crust is affected by a set of six major east dipping low angle normal faults in the adriatic domain compressive tectonics have acted since the middle miocene and the pattern of shallow contractional structures is well preserved the geological interpretation of the seismic data supports a thick skinned style of deformation where the basement is involved in the major thrust sheets the good quality of seismic data also allows for determining the total shortening produced by the contractional structures in the central part of the profile at the border between the tyrrhenian and adriatic domains seismic data shows the presence of an intermediate sector the sector consists of a highly reflective window where the refraction data indicate a local doubling of the crust for about 30 km a scenario is presented that attempts to describe the geodynamics that drove the tectonic evolution of the northern apennines since the upper oligocene

a model for entrainment of sediment into sea ice by aggregation between frazil ice crystals and sediment grains avertical numerical model has been developed that simulates tank experiments of sediment entrainment into sea ice physical processes considered were turbulent vertical diffusion of heat salt sediment frazil ice and their aggregates differential growth of frazil ice crystals secondary nucleation of crystals and aggregation between sediment and ice the model approximated the real size distribution of frazil ice and sediment using five classes of each frazil crystals 25 m to 1 5 cm were modelled as discs with a constant thickness of their diameter each class had a constant rise velocity based on the density of ice and drag forces sediment grains 1 600 m were modelled as constant density spheres with corresponding sinking velocities the vertical diffusion was set constant for experiments based on calculated turbulent rms velocities and dissipation rates from current data the balance between the rise sinking velocities and the constant vertical diffusion is an important feature of the model the efficiency of the modeled entrainment process was estimated through a an aggregation factor values for a are in the range but average values are often close to 0 01 entrainment increases with increasing sediment concentration and turbulence of the water and heat flux to the air

numerical model of frazil ice and suspended sediment concentrations and formation of sediment laden ice in the kara sea a one dimensional vertical numerical model of currents mixing frazil ice concentration and suspended sediment concentration has been developed and applied in the shallow southeastern kara sea the objective of the calculations is to determine whether conditions suitable for turbid ice formation can occur during times of rapid cooling and wind and wave induced sediment resuspension although the model uses a simplistic approach to ice particles and neglects ice sediment interactions the results for low stratification shallow 20 m freeze up conditions indicate that the coconcentrations of frazil ice and suspended sediment in the water column are similar to observed concentrations of sediment in turbid ice this suggests that wave induced sediment resuspension is a viable mechanism for turbid ice formation and enrichment mechanisms proposed to explain the high concentrations of sediment in turbid ice relative to sediment concentrations in underlying water may not be necessary in energetic conditions however salinity stratification found near the ob and yenisey rivers damps mixing between ice laden surface water and sediment laden bottom water and probably limits incorporation of resuspended sediment into turbid ice until prolonged or repeated wind events mix away the stratification sensitivity analyses indicate that shallow 20 m unstratified waters with fine bottom sediment settling speeds of 1 mm s 1 or less and long open water fetches 25 km are ideal conditions for resuspension

crustal shortening and duplication of the moho in the northern apennines a view from seismic refraction data abstract a reappraisal of the dss refraction seismic campaigns of 1978 and 1974 in the northern apennines and the northern tyrrhenian sea after digitization of the original analog data and a new interpretation of crustal structures are presented in this paper the layering of the adria crust consists of a low velocity lower crust topped by a 6 7 km s horizon and an upper crust again formed by a low velocity layer capped by a faster one in the tuscany sector the crust mantle boundary and lower crust show attenuated velocities and a reduced thickness with respect to the adriatic counterpart these differences are due to the extensional tectonics restricted to tuscany and occurred in a high heat flow regime related to an uplifting astenosphere it has been confirmed that a deep thrust is responsible for moho doubling in correspondence with the zero bouguer anomaly line the tuscan moho overlaps the umbro marchean one for a length of about 30 km it is hypothesized that another shallower thrust involves upper crust in the mid tuscany range mt cetona and perugia massifs had their roots in the mantle and that subsequent extensional tectonics obliterated every trace of thrusting in lower crust and mantle rocks no conclusive proof of the existence of a european moho below a tuscan one in the area underneath elba has yet been found revision of dss data excludes the presence of a refractor reflector 60 km deep the same data however are compatible with the existence of a crust mantle boundary at 30 35 km depth

effect of brittle fracture on the rheological structure of the lithosphere and its application in the ordos abstract an empirical formula for the fracture strength of the principal rock type in the lithosphere is obtained based on the experimental data from previous studies in which the effects of the confining pressure size of the rock sample temperature strain rate and the pore pressure are taken into account the empirical formulae for the effects of them are also presented by comparing the frictional strength to the fracture strength calculated using the new empirical formula it is shown that frictional sliding is dominant in the upper crust but brittle fracture is dominant in the lower part of the crust and the lithosphere beneath the crust therefore the fracture mechanism must be taken into account in the study of the rheological structure of the lithosphere the empirical formula for the fracture strength is applied to study the rheological structure of the lithosphere in the ordos block brittle regime in the rheological structure can be divided into two sub regions in one of which brittle fracture and in the other frictional sliding are dominant respectively unlike previous conventional studies in which frictional sliding is assumed to be the only factor the magnitude of the rheological strength of the lithosphere calculated by the empirical formula is also lower than that obtained in previous conventional studies

the main characteristics of the lithosphere asthenosphere system in italy and surrounding regions we present a summary of the available information on rayleigh wave dispersion data for the italian area the dispersion curves can be grouped into two main families according to their shape one with an almost linear trend in the period range from 40 to 150 sec the other with a relatively flat portion in the period range from 40 to 80 sec the inversion of these data shows that there are strong lateral variation in the lithosphere system a very rapid transition in the lithospheric thickness from about 130 km to about 30 km as well as in the s wave velocity of the lithosphere and asthenosphere

limits on rock strength under high confinement abstract understanding of deep earthquake source mechanisms requires knowledge of failure processes active under high confinement under low confinement the compressive strength of rock is well known to be limited by frictional sliding along stress concentrating flaws under higher confinement strength is usually assumed limited by power law creep associated with the movement of dislocations in a review of existing experimental data we find that when the confinement is high enough to suppress frictional sliding rock strength increases as a power law function only up to a critical normalized strain rate within the regime where frictional sliding is suppressed and the normalized strain rate is below the critical rate both globally distributed ductile flow and localized brittle like failure are observed when frictional sliding is suppressed and the normalized strain rate is above the critical rate failure is always localized in a brittle like manner at a stress that is independent of the degree of confinement within the high confinement high strain rate regime the similarity in normalized failure strengths across a variety of rock types and minerals precludes both transformational faulting and dehydration embrittlement as strength limiting mechanisms the magnitude of the normalized failure strength corresponding to the transition to the high confinement high strain rate regime and the observed weak dependence of failure strength on strain rate within this regime are consistent with a localized peierls type strength limiting mechanism at the highest strain rates the normalized strengths approach the theoretical limit for crystalline materials near theoretical strengths have previously been observed only in nano and micro scale regions of materials that are effectively defect free results are summarized in a new deformation mechanism map revealing that when confinement and strain rate are sufficient strengths approaching the theoretical limit can be achieved in cm scale sized samples of rocks rich in defects thus non frictional failure processes must be considered when interpreting rock deformation data collected under high confinement and low temperature further even at higher temperatures the load bearing ability of crustal rocks under high confinement may not be limited by a frictional process under typical geologic strain rates

the role of langmuir circulation in suspension freezing in november 2004 we used an acoustic doppler velocimeter adv to measure the three components of velocity in langmuir circulation lc cells when frazil was forming in a lake lc circulation was indicated by windrows of slush ice on the water surface we also collected samples of lake water ice interstitial water and ice to determine their sediment concentrations the adv record showed rotating currents in the cross wind vertical plane indicative of lc downward directed velocities were large enough to entrain frazil into the water column for 27 of the 97 min observation time at the adv location sediment concentrations in the ice and interstitial water samples were greater than concentrations in water column samples collected in upwelling zones of lc cells we conclude that suspension freezing was forming particle laden ice as frazil trapped below lc convergence zones grew in downwelled supercooled water the rotating roll vortices of langmuir circulation play an important role in forming sediment laden ice

tectonic regime granite emplacement and crustal structure in the inner zone of the northern apennines tuscany italy a new hypothesis abstract geological and geophysical data on the inner part of the northern apennines and the deep 4 km geothermal wells contained therein have been reviewed in order to better characterize the tectonic regime of the region the area is affected by neogene quaternary igneous activity reduced crustal thickness high heat flow and active geothermal systems the available information leads to the following conclusions 1 at present part of the chain including its inner regions are undergoing extensional activity a tectonic situation in southern tuscany dating back to the late tortonian however the simple model of solely extensional tectonics cannot exhaustively account for the phenomena observed in the area as there is ample evidence of compressive events of pliocene and pleistocene ages 2 the geophysical data indicate reduced lithosphere thickness and the uprising of the asthenosphere to shallow crust levels the upper crust has been intruded by many granite bodies and the brittle ductile transition is probably very shallow an integrated geophysical model which accounts for heat flow seismic gravimetric and magnetic data indicates that a regional strong reflector rather than coinciding with the beginning of the ductile crust more likely indicates the presence of deep brines rather than a gneiss core complex and a generalized post collision extensional model cameli et al 1993 we support a more complex geodynamic framework including compressive phases of the internal zone and re activation of the thrust of the apennines as a consequence of crustal thrust emplacements from the late tortonian in the frame of a subducting delaminated lithosphere

extensional shear zones as imaged by reflection seismic lines the larderello geothermal field central italy the larderello geothermal field is located in the inner northern apennines in an area which has been subject to extension since the early miocene the latest extensional episode pliocene present has resulted in the formation of nw trending nedipping listric normal faults whose geometry is controlled down to f3 km by borehole data in this paper we integrate a new interpretation of seismic reflection lines with existing seismic field and borehole data to analyse the relations among listric normal faults the top of the brittle ductile transition and the migration of geothermal fluids in accordance with previous interpretations we consider the strong reflector k horizon marking the top of the reflective mid lower crust and located at a depth of 3 5 km in the geothermal area to represent the top of the brittle ductile transition its reflectivity most probably derives from the presence of overpressured fluids we identify three main nw trending nedipping extensional brittle shear zones showing listric geometry and soling out in the vicinity of the k horizon the latter appears to be dislocated in correspondence of the soling out of the shear zones these shear zones because of the associated intense fracturing represent the most natural channels of upward migration of geothermal fluids from the magmatic sources located below the k horizon we suggest that these two conclusions that listric normal faults root at or near the brittle ductile transition and that they act as preferential upward migration paths for magmatic fluids may be of general validity for geothermal fields located in extensional settings d 2002 elsevier science b v all rights reserved

crustal structure and moho geometry beneath the northern apennines italy 1 we used receiver functions rfs from broad band seismic stations to investigate the crustal structure of the northern apennines italy additionally we use data obtained in this study to provide initial constraints for a map of the moho depth of italy ten stations were deployed along a transect n75 e during the 1994 geomodap project rf analysis shows the presence of lateral variations in the crust we observed patterns of symmetric and anti symmetric converted phases from radial and tangential rfs vs back azimuth these patterns can be explained by the presence of dipping interfaces and or anisotropy within the crust we then inverted rfs following the inversion scheme proposed by sambridge 1999 the results show the presence of s velocity inversions in the lower crust beneath the apennines the upwelling of moho in the tyrrhenian area and its progressive deepening from the tyrrhenian sea toward the adriatic coast

tomographic constraints on the geodynamic evolution of the italian region in this paper we present p wave tomographic images of the mantle beneath italy obtained by inverting 6000 teleseismic p and pkp wave arrival times accurately repicked recorded in the time period 1988 1994 by the stations of the national seismic network of the istituto nazionale di geofisica we pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set the data were inverted with the well established aki christofferson husebye tomographic technique different reference models and residuals computation have been tried to verify the stability of the results the high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the alps and the apennines looking for their lateral and vertical continuity down to 800 km depth the main finding of this study is a continuous high velocity body located between 250 and 670 km depth beneath the entire apenninic system dipping toward the tyrrhenian area which continues upward segmented in two main anomalies in the northern apenninic and the calabrian arcs we interpret this high velocity feature as the subducted oceanic lithosphere between the eurasian and african plates dipping down to the upper lower mantle boundary beneath the tyrrhenian sea the retrieved images of the lithosphere subducting beneath apennines are reliable in terms of thickness about 80 90 km and p wave velocity contrast 2 4 higher than the normal mantle furthermore our tomographic images which focus on the deep geometry and continuity of the velocity structures provide new keys to understanding the geodynamic evolution of the italian region the segmentation of the high velocity slab upward suggests a complex evolution of the arc trench system and the initially continuous subduction of the ionian adriatic plate progressively developed in subordinate arcs probably due to lateral heterogeneity of the subducting lithosphere

the north tyrrhenian northern apennines post collisional system constraints for a geodynamic model the north tyrrhenian northern apennines post collisional system ntas has been analysed on the basis of marine reflection profiles and bibliographic sources with the aim of providing an up to date geotectonic and evolutive framework consistent with structural data the continental rifting began to affect the most internal western side of the system in the middle upper miocene while the most external eastern zones were touched by rifting only in pleistocene times the birth and development of extensive asymmetric basins took place with a progressive speeding up of rifting towards the external sectors a diachronous regionally extended and eastward rejuvenating unconformity recognizable within the neogene quaternary basins marks the passage from the syn rift to the post rift tectonic regime while the first is everywhere dominated by extension the second one produces vertical movements reflection terminations related to this unconformity are interpreted in terms of vertical tectonic component three zones that experienced different post rift vertical movements are thus recognized in the ntas subsidence in the north tyrrhenian deep water region tectonic stability or alternation of moderate subsidence and uplift in the continental shelf and western tuscany strong uplift in eastern tuscany the updated structural data demonstrate that the ntas upper crust is crosscut by both tyrrhenian dipping and apennine dipping low angle detachment planes above which the upper crustal blocks rotated and experienced extensional transport along two opposite directions to the west and to the east respectively the coexistence of opposite dipping crustal surfaces bears two fundamental implications for the ntas geodynamics first it stresses the importance of the anti apennine transfer faults since they separate crustal blocks of opposite extension polarity secondly it testifies that the extension tectonics not only re utilized former crustal thrust planes but also gave origin to new significant e and ne dipping detachment planes plan and crossview geometries of faults together with considerations on the role of the metamorphic core complexes point to an interpretation of the ntas structure that is more consistent with the anastomosing shear model rather than the simple shear or the delamination ones

belt bending driven by lateral bending of subducting lithospheric slab geophysical evidences from the northern apennines italy abstract many of the mountain belts displaying a curved shape are oroclines i e are produced after progressive bending of an originally straight fold and thrust belt the bending process was previously explained as a consequence of several possible events taking place in the crustal orogenic wedge such as occurrence of obstacles non coaxial deformation and movements on wrench faults recent paleomagnetic results from the northern apenninic arc document that this belt is properly an orocline and results from late messinian early pliocene bending of a messinian straight belt foredeep system tomographic images in turn show the presence of a high velocity body interpreted as subducted slab in the upper mantle beneath the northern apennines between 35 and 670 km depth down to 100 km this body displays an arcuate shape which closely mirrors the geological outlines while it appears to be straight and parallel to the messinian pre rotated belt at depth we explore here the possibility that the arcuate shape of the northern apennines is a consequence closely following in time of much deeper processes than previously suggested i e the lateral bending of the subducting adriatic plate

travel time tomography of the european mediterranean mantle down to 1400 km abstract the 3 d p wave velocity structure of the mantle below europe the mediterranean region and a part of asia minor is investigated this study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper mantle structure only here the earth s volume under study encompasses the mantle to a depth of 1400 km and we increase the number of international seismological centre isc data for inversion by a factor of four by taking more years of observation and by including data from teleseismic events the most important departure from the earlier study is that we do not use the jeffreys bullen model as a reference model but an improved radially symmetric velocity model the pm2 model which is appropriate for the european mediterranean mantle our inversion procedure consists of two steps first the radial model pm2 is determined from the isc delay times by a nonlinear trial and error inversion of the data as opposed to the jeffreys bullen model the new reference model has a high velocity lithosphere a low velocity zone and seismic discontinuities at depths of 400 and 670 km next the isc data are corrected for effects related to the change in reference model and inverted for 3 d heterogeneity relative to the pm2 model we follow this two step approach to attain a better linearizable tomographic problem in which ray paths computed in the pm2 model provide a better approximation of the actual ray paths than those computed from the jeffreys bullen model hence the two step scheme leads to a more credible application of fermat s principle in linearizing the tomographic equations inversion results for the 3 d heterogeneity are computed for both the uncorrected isc data and for the pm2 data the data fit obtained in the two step approach is slightly better than in the inversion of isc data using the jeffreys bullen reference model a comparison of the tomographic results demonstrates that the pm2 data inversion is to be preferred to assess the spatial resolution an analysis is given of hit count patterns sampling of the mantle by ray paths and results of sensitivity tests with 3 d synthetic velocity models the spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross section in the well sampled regions of the mantle the spatial resolution for larger scale structure can qualitatively be denoted as reasonable to good and at least sufficient to allow interpretation of larger scale anomalies a comparison is made of the results of this study with independent models of s velocity heterogeneity obtained in a number of investigations and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the mediterranean region in the context of this comparison interpretations of large scale positive anomalies found in the mediterranean upper mantle in terms of subducted lithosphere are given specifically addressed are subduction below southern spain below the western mediterranean and italy and below the aegean in the last region a slab anomaly is mapped down to depths of 800 km

on the conditional frazil ice instability in seawater abstractit has been suggested that the presence of frazil ice can lead to a conditional instability in seawater any frazil forming in the water column reduces the bulk density of a parcel of frazil seawater mixture causing it to rise as a result of the pressure decrease in the freezing point this causes more frazil to form causing the parcel to accelerate and so on this study uses linear stability analysis and a nonhydrostatic ocean model to study this instability the authors find that frazil ice growth caused by the rising of supercooled water is indeed able to generate a buoyancy driven instability even in a gravitationally stable water column the frazil ice mechanism can still generate convection the instability does not operate in the presence of strong density stratification high thermal driving warm water a small initial perturbation high background mixing or the prevalence of large frazil ice crystals in an unstable water column the instability is not necessarily expressed in fra

lagrangian modelling of frazil ice in the ocean a new modelling framework using lagrangian particle tracking has been developed to assess dynamic and thermodynamic effects of underwater frazil ice this frazil ice model treats a lagrangian particle as a bulk cluster of many frazil crystals and calculates the thermodynamic growth of each particle and the corresponding budget of latent heat and fresh water the effective density and viscosity of sea water depend on the mass fraction of underwater frazil ice and hence affect ocean convection an idealized experiment using our model successfully reproduces the formation of underwater frazil ice and its transition to grease ice at the surface because underwater frazil ice does not reduce the atmosphere ocean heat exchange surface heat flux and net sea ice production in the experiment with frazil ice are relatively high compared with the experiment where surface cooling directly leads to columnar growth of a solid ice cover which effectively insulates the heat flux these results suggest that large scale sea ice models which do not take account of the effects of frazil ice might underestimate atmosphere ocean heat exchange particularly at times of active new ice formation