geodynamic implications of earthquake data in the southern tyrrhenian sea abstract data from short period seismic stations operating in calabria sicily and the aeolian islands have been used to investigate earthquake properties of the southern tyrrhenian lithosphere the results have been analyzed taking into account other geophysical and geological information available in the literature for the same region with the main purpose of contributing to the definition of local geodynamic processes space distribution and focal mechanisms of earthquakes support the hypothesis of southeastward migration of the southern tyrrhenian lithosphere shared by several authors in their tectonic modelling of the area we find that most seismicity appears to be related to two main faults crossing the aeolian island archipelago sisifo and vulcano and propose that these faults are important dislocation structures associated with the lithosphere migration process we also give a detailed picture of the seismic behaviour of the above mentioned fault systems by analyzing the earthquake depth distribution focal mechanisms seismic clustering and strain release some degree of activity was also detected at the eastern boundary of the southern tyrrhenian area calabrian arc in particular our results demonstrate that in this highly stressed zone of contact between different tectonic units tyrrhenian and ionian basins strain is released under seismic form up to depths of the order of 25 km in the crust and not just in the shallowest structures as proposed by other authors

microearthquakes in the tyrrhenian sea as revealed by joint land and sea bottom seismographs abstract five independent ocean bottom seismographs obs were operated from 17 to 28 may 1987 in the southeastern tyrrhenian sea at a depth range of 1000 2800 m these were concomitantly run with nine vesuvian observatory and university of calabria temporary digital seismic stations along the calabrian coast together with fifteen permanent seismic stations in southern italy the latter stations also belong to the university of calabria and to the italian national institute of geophysics about 200 local shocks mainly microearthquakes ml

the deep seismicity of the tyrrhenian sea summary l he study reappraises the deep seismicity of the tyrrhenian sea careful examination of the quality of reported hypocentres shows that the earthquakes define a zone dipping nw about 200 km along strike 50 km thick and reaching a depth of about 500 km the zone is slightly concave to the nw at a depth of 300 km but contrary to many previous reports is not tightly concave nor are there significant spatial gaps in the seismicity which is effectively continuous with depth seismicity is however concentrated in the depth interval 250 300 km where the dip of the seismic zone changes from 70 above 250 km to a more gentle dip of 45 at greater depths seven fault plane solutions are available for the largest earthquakes in this depth interval all of them consistent with a p axis down the dip of the seismic zone and all of them requiring movement on faults out of the plane of the subducting slab two deep earthquakes near naples lie well outside the main zone of activity for one of which a fault plane solution is available that has ap axis not aligned with the dip of the seismic zone the tightly concave slabgeometry favoured by other reports is supported mainly by the location of these events near naples which we think may represent deformation in a separate probably shallower dipping piece of subducted lithosphere the lack of shallow seismicity and particularly of thrust faulting earthquakes at the surface projection of the benioff zone suggests that active subduction has ceased estimates of the convergence rate responsible for subduction in the last 10 myr far exceed the present convergence rate of africa and eurasia suggesting that the subduction was related instead to the stretching and thinning of the crust in the tyrrhenian sea

a 550 km long moho traverse in the tyrrhenian sea from o b s recorded pn waves six obs deployed on a 550 km line in the tyrrhenian sea furnish reversed and overlapping profiles from 43 shots at 12 km intervals head waves recorded from a 8 km s refractor the moho permitted the computation of the crustal thickness which shows this basin to be underlain by an oceanic type of crust at least in two 100 km wide regions variations of pn arrival times mirror strong topographical changes of the moho differences in dips at the extremities towards corsica and calabria are documented

earthquake swarms in the southern apennines chain italy the 1997 seismic sequence in the sannio matese mountains abstract in this paper we analyze a seismic sequence that occurred in the sannio matese mountains southern apennines italy one of the most dangerous seismic areas of the italian peninsula on 19 march 1997 at 23 10 utc a seismic event of md 4 1 hit a zone near the border between the campania and the molise regions swarm type earthquake activity followed the main event and lasted for more than six months with alternation of intense activity and relative quiescence periods evolution of the seismic sequence both in time and space has allowed us to divide it into three stages with rather similar characteristics the seismic energy release process of this sequence exhibits the typical swarm pattern of the minor seismic sequences occurring in the southern apennines and involving relatively minor faults located in the marginal border of the chain such pattern unlike the main shock aftershock model consists of low energy events closely clustered in time and space without a single main shock about 2800 micro earthquakes were recorded from march to september 1997 by the seismic networks of the istituto nazionale di geofisica and of the osservatorio vesuviano among them only 20 events had md 3 0 a selection of 370 earthquakes recorded at a minimum of five stations with at least three s phase readings have been located between latitudes 41 20 and 41 30 and longitudes 14 36 and 14 42 the events are clustered along a nearly vertical plane striking nne ssw and dipping towards the east their hypocentral depths range from 5 to 15 km the focal mechanisms of 67 events show a variety of different solutions even if a prevalence of normal dip slip solutions with a strike component is observed the t axes distribution of all mechanisms show two trends the prevalent one is aligned along a nnw sse direction whereas the other one is along a nne ssw direction the hypocentral distribution the prevalence of focal mechanisms with fault planes striking about ne sw as well as the t axes distribution suggest that earthquakes relative to this sequence took place along nne ssw oriented faults and that the local stress field acting in this sector of the southern apennines is prevalently transtensional and is strongly controlled by local faults heterogeneities

volcanic geomorphology and tectonics of the aeolian archipelago southern italy based on integrated dem data the geomorphological and morphometric analysis of the sea floor topography surrounding the aeolian islands south tyrrhenian sea italy provides insights into the relationships between the volcanological evolution of the islands and their tectonic features we constructed geomorphological maps of the submarine portions of the seven large edifices constituting the islands on the basis of a dem with a 5 m resolution step these maps include constructional and destructional landforms such as submarine volcanic vents located west of lipari and north of alicudi and hummocky surfaces recognised north of lipari and salina the latter landforms together with the occurrence of large scars affecting the main edifices on land suggest that sector collapses affected some islands geomorphological data indicate that the location of subaerial and submarine vents is strongly controlled by local tectonic structures striking wnw ese alicudi filicudi sector nnw sse salina lipari vulcano sector and ne sw panarea stromboli sector the islands can be divided into two groups on the basis of some morphometric parameters a first group with a pancake like shape dp d abrasion platform diameter basal diameter higher than 0 40 and h d total height basal diameter lower than 0 13 and a second group with a conical shape characterised by dp d lower than 0 34 and h d higher than 0 14 these ratios and other morphometric parameters reflect the different volcanological and structural evolution of the aeolian islands the pancake like shaped complexes have been created in addition to their submarine stage by extrusive and highly explosive activity whereas the cone shaped edifices have been characterised by effusive or moderate explosive activity

a physical model for the origin of volcanism of the tyrrhenian margin the case of neapolitan area abstract the onset of volcanism in the neapolitan area and the tensile tectonics of the tyrrhenian margin of the apennine chain have been related to the opening of the tyrrhenian basin which may have resulted in horizontal asthenosphere flows giving rise in turn to crustal distension local mantle upwellings and ensuing volcanism geological and structural data were taken into consideration the existence of a shallow crust mantle discontinuity in the neapolitan area the onset of volcanism in a graben type structure the distribution of volcanism along three main directions these facts are all consistent with processes of crustal deformation associated with hot masses as well as fracture systems along three branches often separated by a 120 angle such fracture mechanism was therefore analyzed in physical terms on the assumption of an elasto plastic behaviour of the crust and with reference to the limit analysis theorems

continental collision and basin opening in southern italy a new plate subduction in the tyrrhenian sea an analysis of the spatial distribution of the seismicity of the southern tyrrhenian sea is presented with the aim of putting constraints on the geometry of the complex benioff zone of this area by relocating the earthquakes that occurred in the period 1985 1990 the data set used in this analysis is based on information from ing isc and neis databases in order to ensure the quality of epi hypocentral locations we based our selection of earthquakes on both the available number of p and s readings at least eight p and two s pickings and the epicentral distances of the stations reporting the event 5 such a selection enabled us to obtain a catalogue of 450 well located events with focal depths of up to 500 km the seismogenetic volumes estimated from the distribution of hypocentres of the whole seismicity analysed were interpreted according to a recent geodynamic model proposed for this area this has led us to the hypothesis of an active nw se oriented subduction of the lithosphere generated and stressed by the opening of the tyrrhenian basin

relationships among crustal structure volcanism and strike slip tectonics in the lipari vulcano volcanic complex aeolian islands southern tyrrhenian sea italy abstract the lipari vulcano volcanic complex lvvc aeolian arc southern tyrrhenian sea italy develops along a strike slip fault system that bisects the aeolian arc the lvvc crustal structure is investigated by applying seismic methods to two data sets p wavefronts generated by local and distant events constrain the shallow velocity structure dss data provide shape and depth of the main crustal discontinuities the obtained velocity model shows that the lvvc shallow structure is characterized by the presence of two sharp lateral discontinuities the northern discontinuity bounds a low velocity basin like structure including the la fossa caldera and mt guardia caldera depressions the southern discontinuity coincides with the southern boundary of a high velocity diffractor located beneath the la fossa cone direct modeling of dss data define three crustal discontinuities showing a complex geometry the intra crustal interfaces and the moho discontinuity show an upheaval beneath the lvvc central sector the estimated crustal velocities are lower than those generally reported for the continental crust this feature is due to the high heat flux and fluid circulation affecting the lvvc area the ratio between the thickness of the upper and lower crust and the presence of the moho upheaval are consistent with that reported for continental areas affected by extensional strain the collected geophysical data combined with volcanological and structural information indicate that a the mt guardia and la fossa caldera depressions represent the surface expression of a single structure whose formation is mainly due to tectonic processes b the lvvc crustal structure and the evolution of the volcanism are consistent with those recognized in zones of transition from arc related to rift volcanism in addition results of a preliminary mechanical analysis based on the available structural data indicate low values of the ratio between magmatic overpressure and tectonic stress this suggests a passive mechanism of mantle upwelling the mantle upwelling is due the post collisional normal strike slip tectonics affecting the aeolian arc

seismotectonics of the calabrian arc abstract relocation of intermediate and deep earthquakes of tyrrhenian sea area through joint hypocenter determination for the period 1962 1979 has allowed a more detailed definition of the geometry of this peculiar benioff zone earthquakes dip along a quasi vertical plane to 250 km depth there is a 50 dip in the 250 340 km depth range and a low dip angle to 480 km depth the structure sketched from the hypocenters is almost continuous but most energy has been released in the 230 340 km depth interval an evaluation of fault plane solutions of intermediate earthquakes in this area indicates predominance of down dip compressions in the central part of the slab at the border strike slip motion occurs independent of depth some earthquakes that occurred at intermediate depth less than 100 km along the ionian margin of calabria show predominance of reverse faulting with the p axis oriented se nw however shallow earthquakes in the calabria sicily region indicate a more complex motion with predominance of normal faulting a possible interpretation of these features according to the available geological history which involves subduction of continental lithosphere is discussed

geochemistry and tectonic setting of lewisian clastic metasediments from the early proterozoic loch maree group of gairloch nw scotland abstract the early proterozoic 2 0 ga loch maree group lmg of the lewisian complex consists of a supracrustal sequence of metasedimentary schists and associated tholeiitic metavolcanic amphibolites within the archaean basement the metasediments comprise three main mapped units the flowerdale schists kerrysdale schists and charlestown schists together with numerous narrower bands the dominant clastic metasediments within the schist units semipelites and psammites are chemically distinguished as a series of greywackes and lithic sandstones with variable lil and transition element abundances fractionated ree patterns and overall compositions similar to paas three chemical groups are recognized whose range of composition largely reflects the mixing between two main source components lewisian gneissose basement with an intermediate acid continental crust composition and local rift related amphibolites with a primitive tholeiitic composition which also incorporates components not preserved within the lmg the geochemistry of the clastic metasediments confirms a previous tectonic model for the lmg as an intracontinental extensional rift receiving clastic detritus from the adjacent uplifted basement as well as from internal volcanogenic sources

subduction zone geometry and stress patterns in the tyrrhenian sea joint hypocenter determination is performed for intermediate and deep earthquakes of the tyrrhenian sea region

the tectonic significance of italian magmatism an alternative view to the popular interpretation the tyrrhenian rift zone has been the site of widespread magmatism since late tortonian times a pronounced asymmetrical distribution reflecting the tectonic structure characterizes italian magmatism sodic basalts occur on the western tyrrhenian flank and transitional morb basalts occur in the tyrhenian sea the eastern flank however is characterized by k alkaline and hk to ultra alkaline e g carbonatites and melilitites rocks major trace elements and isotopic compositions allow two major magmatic lineages to be identified one relating to a non radiogenic basaltic end member and the other to a mantle end member enriched in ca with high lile hfse ratio and high sr isotopic ratios their mantle sources are located within the lithosphere thermal boundary layer tbl and the metasomatized phlogopite carbonate asthenosphere at the base of the tbl respectively the composition and spatial distribution of volcanism and relative mantle sources tend to map the geometry of the lithospheric mantle and to define a pronounced increase in depth from less than 60 km to about 100 km across the boundary between the thinned tyrrhenian lithosphere and the adriatic lithosphere a mechanism of intra continental passive rifting which drives mantle upwelling is sufficient to satisfy the petrological and geochemical constraints and the observed tectonic environment without requiring a subduction plane

a model for the tectonic evolution of south cornwall a stratigraphy for south cornwall is combined with an analysis of the chronology of thrust nappe emplacement the resulting model suggests that northward overthrusting of nappes of flysch and olistostrome ophiolite and continental basement is related to the closure of an oceanic basin in south cornwall involving southward subduction throughout the devonian the subsequent continental collision between the normannian high and part of laurentia occurred in the upper devonian with deformatian continuing until late carboniferous times

chemical profiles in k t boundary section of meghalaya india cometary asteroidal or volcanic abstract the cretaceous tertiary boundary ktb exposed as a 1 5 cm thick limonitic layer in the um sohryngkew river basin in meghalaya is marked by enhanced concentrations of ir co ni os fe zn sb by factors of 4 to 1200 and ree by factors of 1 7 to 5 compared to the cretaceous shales ir concentration is generally 10 in 2 m thick broad band around the ktb abruptly reaching a peak concentration of 12 1 ng g 1 within the limonitic layer the os ir ratio at the ktb is 0 37 but in the adjacent shales it is 1 8 suggesting that the sources of these elements in the peak and the broad band may be different cretaceous planktonic foraminifera and dynocysts start disappearing before the ir rich limonitic layer was deposited but several diminutive cretaceous foraminifera survive after this layer the observations indicate that the extinctions are confined to a short span of time but are probably not as abrupt as anticipated by the asteroidal impact hypothesis the chemical data are discussed in terms of cometary asteroidal and volcanic hypotheses the profiles of ir and os ir can be understood in the frame work of a model of cometary impact in which cometary debris falls onto the earth preceding and succeeding the fall of the cometary nucleus

delay time tomography of the upper mantle below europe the mediterranean and asia minor summary the method of linearized delay time tomography is applied to retrieve the p velocity heterogeneity of the upper mantle in central europe the mediterranean and asia minor the upper mantle is parametrized in slowness cell functions with lateral dimensions of 1 the model parametrization includes event mislocation vectors and station delay time corrections in all 20 000 parameters describe the model delay time data selected from the isc data base 1964 1982 from regional events and from stations up to 90 of epicentral distance are used about 18 000 events and 937 stations contributed nearly 500 000 delay times we discuss the tomographic method the data processing the cell model employed the computation of the ray geometry the use of composite rays in tomography and the cell hit count the inversions are performed with two different iterative row action methods lsqr and sirt results obtained with the lsqr method will be shown only the spatial resolution is investigated with sensitivity analyses using two different synthetic velocity models a spike model and a model with harmonically varying velocity anomalies the spatial resolution is estimated to be high on the order of the cell size in the mantle below central and southeastern europe the mapped velocity heterogeneity exhibits primarily low velocities around a depth of 200 km and just above the transition from upper to lower mantle this indicates a systematic deviation of the reference model jeffreys bullen from the radially averaged earth structure which may violate the basic assumptions underlying the linearized approach a posteriori investigation of the data also indicates the presence of a low velocity zone and the existence of strong velocity depth gradients

effects of sedimentary sorting on neodymium isotopes in deep sea turbidites the study of neodymium model ages of sedimentary rocks has added considerably to our understanding of crustal growth and evolution so far however the possible effects of various sedimentary processes on nd isotope distributions have not been fully evaluated here we present nd isotope data for a series of sand mud pairs in modern deep sea turbidites from different tectonic settings which show that sands and muds commonly have significantly different nd isotope compositions that in only some cases are associated with differences in sm nd ratio differences are as great as 7 ndunits corresponding to nd model ages that differ by up to 0 44 gyr and may be explained by separation or unmixing of components from different sources during sedimentary transport and sorting for one sample heavy mineral fraction ation may be indicated these results have implications for studies of nd model ages in sedimentary rocks and for the interpretation of provenance from sedimentary petrography and geochemistry

dome growth rates eruption frequency and assessment of volcanic hazard insights from new u th dating of the panarea and basiluzzo dome lavas and pyroclastics aeolian islands italy abstract new geochronological data are presented and discussed together with field data in order to reconstruct the geo volcanological evolution of panarea island attention is focused on the discrepancies between geochronological data compared with field evidence field data provide evidence that the panarea dome was continuously built up without a significant break in volcanic activity continuous extrusion of new magma allowed the dome to emerge subaerially subaerial effusive activity followed with the alternation of exogenous and endogenous phases of growth that led to the formation of radial and tangential lava lobes and plugs the younger volcanic products are represented by a thick succession of pyroclastic products discontinuously outcropping all over the island dated here for the first time samples were dated through the u th disequilibrium method which is also able to provide information on the geological processes at the origin of u th fractionation the chronology of the volcanic events at panarea suggests that the volcanic activity developed through two main phases effusive activity mainly characterized the first phase of the dome growth while during the second phase explosive activity prevailed the second phase occurred from 31 7 to 20 2 ka assessing the age of the first phase is more difficult the age of 155 12 ka which is in the same time span obtained from k ar or ar ar methods is not consistent with field evidence furthermore although the alignment of the points on the isochron is very good the initial value of the 230th 232th ratio is very low and does not have a clear meaning our data strongly suggest that the age of 155 12 ka is not realistic two alternative scenarios are proposed implying a revision of the geological and geochronological evolution of the island together with the significance of the marine terraces on panarea

the significance of garnet in the permo carboniferous volcanic rocks of the pyrenees garnet occurs within rhyolitic to dacitic lavas ignimbrites and volcaniclastic sediments of the pyrenean calc alkaline permo carboniferous volcanic rocks on the basis of texture and major and trace element composition the garnet is interpreted as having a phenocrystal as opposed to a xenocrystal or restite origin the pyrenean silicic volcanics are strongly peraluminous consistent with derivation as a partial melt from a sedimentary source they were erupted contemporaneously with the hercynian thermal anomaly which produced regional granulite facies metamorphism and anatectic granites the late hercynian granodiorites were also intruded at about this time geochemical data suggest that the petrogenesis of the volcanic rocks differed from that of the granodiorites ree garnet matrix partition coefficients for garnets from a welded rhyolitic ignimbrite are similar to those reported for other phenocrystal garnets in rhyolites ree abundances imply that the garnets crystallized from a hree depleted melt and this is considered to reflect the effects of garnet in the restite experimental constraints suggest that the garnet crystallized at pressures 4 5kb implying that the rhyolites originated from 14 17km depth and were erupted rapidly from these depths in order for the garnet to survive these figures place constraints on the thermal regime prevailing in the late stages of the hercynian orogeny

evidence of crustal melting events below the island of salina aeolian arc southern italy quartz rich xenoliths hosted in the basaltic andesitic and andesitic lavas of monte dei porri in the island of salina contain fluid and melt inclusions whose study makes it possible to trace back the magma ascent path of this volcano moreover they are a source of information about the anatectic processes that occurred under this island the texture mineralogy and whole rock chemistry of these rocks exhibit the typical characteristics of a residual rock which went through several melt extraction events the chemistry of melt inclusions hosted inside quartz grains sio 2 72 7 al 2 o 3 9 0 13 5 cao 0 2 4 3 k 2 o 3 1 mgo feo t 1 5 supports the hypothesis that a progressive and continuous anatectic processes affected a peraluminous high grade metamorphic rock gneiss or micaschist that constituted the calabro peloritani basement and generated a quartz bearing residue the fluids trapped inside these inclusions are rarely composed of high density co 2 with a homogenization temperature to liquid phase th l within the range 26 2 31 0 c corresponding to densities of 520 to 690 kg m 3 more commonly they are made up of low density co 2 with a homogenization temperature to vapour phase th v within the range 13 2 31 0 c corresponding to density values of 150 to 460 kg m 3 it is proposed that during ascent batches of magma rising from the mantle sampled various fragments of a residual rock from a depleted metamorphic basal layer and carried them to the surface magmas probably stopped inside at least two distinct reservoirs located at different depths 12 7 km and between 1 7 and 4 7 km in which they underwent fractional crystallization the ascent speed of the xenolith bearing magmas from the deeper storage area to the shallower reservoir was fairly high as confirmed by the low number of high density fluid inclusions preserved that show re equilibration features and by the contemporaneous presence of denser cumulitic ol cpx opx xenoliths in the host lava

upper palaeozoic basins of the british isles caledonide inheritance versus hercynian plate margin processes hypotheses to explain the origin and location of upper palaeozoic sedimentary basins are presented group a basins were those old red sandstone basins whose location and subsidence were controlled by caledonide features including active and remnant fore arc basins midland valley and major transcurrent movements orcadian basin newer granite plutonic magmas in the southern uplands subduction complex post dated subduction and resulted from post collision cratonization processes as yet little understood group b basins were those whose initial positions in space were controlled by palaeo plate factors in particular the location of granitoid plutons but whose subsidence was controlled by active hercynian neoplate processes some group a basins were regenerated by this process midland valley a new hypothesis is presented to account for the origin of a carboniferous rift to sag trend similar to that proven in modern continental margin basins such as the north sea the hypothesis is an adaptation of mckenzie9s lithospheric stretching model some of whose major features are well shown in the carboniferous history of the north and central british isles group b processes were controlled by ligurian bretonic plate consumption in the s brittany massif central vosges areas group c basins in the rheno hercynian zone also owe their location and origin to back arc rifting and continental crustal oceanization processes caused by liguro bretonic subduction

the marsili ridge southern tyrrhenian sea italy an island arc volcanic complex emplaced on a relict back arc basin marsili seamount 1 ma southern tyrrhenian sea italy is classically interpreted as the spreading ridge of the about 2 ma old marsili oceanic back arc associated to the southern tyrrhenian sea calabrian arc subduction setting high resolution bathymetric data show that the ridge extends along a nne ssw strike and consists of 4 sectors and 11 major segments seafloor failures and central type activity from an overpressurized sill like reservoir characterize the central sector whereas fissural volcanism and passive magma ascent take places at the edges cones indicative of high effusion rates concentrate in the axial zone whereas flat top shields associated to lower rates are located at the northern tip along axis lateral magma migration due to a local deepening of the elastic brittle thickness of the crust develops from the central sector toward the northern tip the opening of the marsili back arc basin vanished from about 1 ma and the marsili ridge formed by passive magma ascent along pre existing fractures inherited by early spreading activity the values of the long term output rate the ratio between magmatic pressure and tectonic stress and the morphological features of the lava flows and dikes are consistent with those found in volcanic arc subduction settings marsili ridge represents a volcanic arc edifice emplaced on an older relict back arc the formation of the marsili ridge marks the transition from an extensional subduction setting to a compressive one

exceptions to the relationship between plate tectonics and sandstone composition abstract routine use of qfl qplsmlvm and lmlslv plate tectonic provenance diagrams has led to the recognition of error populations for these sandstones the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams the anomalous sandstones can be separated into at least four categories i sandstones deposited during the transition between tectonic regimes may be derived in part from relict source rocks ii the relative enrichment of sandstones in detrital quartz by weathering and or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data iii sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields iv the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams and is an unresolved problem the first two categories represent permanent problems in the use of sandstone composition as an indicator of plate tectonic setting whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate tectonic setting

ray tracing for earthquake location in laterally heterogeneous media we propose a technique for the fast solution of ray tracing in three dimensional laterally heterogeneous media analytical solutions for both ray tracing and paraxial ray tracing for a medium with an arbitrary gradient of the square of slowness are used to develop a systematic procedure to construct exact solutions complex heterogeneous media are divided into tetrahedral finite elements inside which the square of the slowness has a simple linear distribution arbitrarily complex media may be studied by judicious choice of the elements we develop appropriate boundary conditions for interfaces with zeroth or first order velocity discontinuities two point ray tracing is performed by a newton method based on paraxial ray theory with a few iterations the ray trajectories through the source and the observer are calculated with the same overall speed as with ray bending the paraxial method has the additional advantage that it can be used to identify caustics and to separate travel time branches we used the previously described method to replace the ray tracing algorithm of the hypo71 program probably the most commonly used earthquake location program the ray tracing routine may be easily modified to adapt it to other earthquake location environments with the modified hypo71 program we relocated the aftershock sequence of the november 23 1980 irpinia italy earthquake the introduction of a low velocity zone proposed by different authors shifts aftershock locations to the ne this horizontal displacement of hypocenters is not very sensitive to the thickness of the low velocity zone

the confrontation of structural styles and the evolution of a foreland basin in central sw england detailed mapping of the rocks to the north and south of plymouth reveals a sedimentary sequence deformed by a series of folds and thrusts two structural zones exist each with a different style of deformation in the lower to upper devonian rocks of the southern zone slip vectors vergence and facing of d1 folds indicate that the transport direction is to the north west an associated first phase cleavage dips to the south east the northern zone of upper devonian and lower carboniferous strata extends into central devon and east cornwall and the geometry and facing of the early folds in these rocks indicate a transport direction to the south or south east d1 folds generally verge north but are downward facing towards the south a flat lying penetrative slaty cleavage occurs in the mudrocks and also indicates southward facing generally the carboniferous rocks in this area are inverted the two structural zones confront each other at an e w trending line which passes through cargreen 8 km north of plymouth here the deformation is more intense as the cleavage common to the southern zone overprints that of the northern zone resulting in the formation of d2 folds the confrontation is interpreted as a northerly dipping backthrust produced by large scale underthrusting it is proposed that the northward advancing variscan thrust belt progressively underthrust the carboniferous foreland basin flysch deposits which became inverted and backthrust towards the south

the deep seismicity of the tyrrhenian sea the deep seismicity of the tyrrhenian sea is analysed using data from a new instrumental catalogue of the seismicity of the italian area we use algorithms for the determination of absolute and relative hypocentral locations and for the evaluation of the geometry and coherence of the state of stress within the subducting slab the structure of the benioff zone although simpler than previously indicated reveals anomalous traits both in the seismicity distribution and in the stress geometry confirming that standard subduction models cannot be applied in the tyrrhenian region the velocity anomaly and the location of few isolated events indicate that the subducted slabs extend to the north along the apenninic chain approximately to the latitude of the irpinia region in central italy 42 n

the seismicity in the southern tyrrhenian area and its neural forecasting the southern tyrrhenian basin is a region characterized by complex tectonics and large scale geodynamic deformations and its seismicity has been recently recognized as following fractal laws in its five dimensional set magnitude time and space in particular the time occurrences of earthquakes are found to follow a significant fractal clustering within the interval ranging from 16 minutes to 24 hours for longer intervals the time clustering regime was not significantly different from that of a random process an artificial neural network is applied here to the same seismicity data set and further evidence of the nonlinear dynamic process for which the fractal approach failed is obtained the monthly values of magnitude predicted by extrapolation are found to be in fair agreement with the historical data

thermal evolution of a rift basin the tyrrhenian sea existing geological and geophysical data support the view that the opening of the tyrrhenian sea was by rifting of a formerly continuous continental lithosphere which began in upper miocene on the upper sardinian margin and migrated southeastward to the calabria margin heat flow across the tyrrhenian sea shows a pronounced asymmetry from about 50 70 mw m 2 over the sardinian margin to more than 150 mw m 2 over its southeastern margin off southern italy and calabria in this study we use the newly acquired data from ocean drilling program leg 107 heat flow seismic velocity and bathymetric data to constrain a group of models that supposes the tyrrhenian sea opened by detachment of a formerly continuous lithosphere along a southeastern facing low angle normal fault specific models tested include 1 the wernicke model in which thinning and extension of the continental lithosphere are supposed to be accomplished by simple shear along a large scale gently dipping detachment fault zone which cut through the entire lithosphere and 2 the delamination model in which the low angle detachment fault is supposed to cut only the upper and middle crust but to merge subhorizontally into the lower crust below which concurrent pure shear may occur a versatile two dimensional finite element procedure is used to evaluate the tectonic and thermal evolution of these models and the results are compared with observations to test the validity of the models delamination models with large pure shear components in the lower lithosphere and the wernicke model predict heat flow within 1 of the measurements the wernicke model however predicts a basin configuration much too deep in comparison with bathymetry and a subbasinal lithospheric thickness substantially different from rayleigh wave dispersion results the delamination model with a large component of pure shear in the subbasinal lower lithosphere on the other hand predicts satisfactorily both the bathymetry and the lithospheric thickness beneath the basin

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