anelastic response of the earth to a dip slip earthquake the deformation of the earth s surface immediately following an earthquake is well modeled by a dislocation in an elastic half space some time after the event however the elastic stresses begin to relax due to flow in the more fluid regions of the upper mantle the asthenosphere this relaxation causes further slow displacements observable at the earth s surface this paper examines the pattern and timing of these postseismic motions for a vertical dip slip fault and a 30 dip thrust fault a variety of rheological models are studied these models encompass variations in asthenosphere thickness mesosphere viscosity fault depth and most important power law flow both newtonian n 1 and non newtonian n 3 maxwell rheologies are considered for the asthenosphere and mesosphere the results for a vertical dip slip fault show that a peripheral upwarp develops within a distance of a few lithosphere thicknesses from the rupture zone on the downthrown side of the fault a comparable downwarp develops on the upthrown side these peripheral warps grow to a maximum height of about 5 of the total fault slip some time after the earthquake event and then collapse at much greater times the vertical motions for a 30 dip thrust fault show that after the earthquake a general uplift occurs in the region near the rupture zone horizontal displacements propagate asymmetrically away from the rupture zone the largest displacements occurring in the overthrust plate the principal result of the comparison of n 1 and n 3 rheological laws is that the basic pattern of uplifts and horizontal motions is very similar the major difference is the time dependence of the development of the pattern in spite of the complex geometrical structure of the subduction zone the displacements ui 1 t for newtonian rheology are nearly the same as ui 3 t for the non newtonian rheology if t t3 thus although the qualitative pattern of the displacements is not diagnostic of power law flow the great difference in time dependence allows a clear discrimination of the two flow laws

interplay between subduction retreat and lateral extrusion tectonics of the western carpathians 1 analysis of brittle microstructures reveals three tertiary deformation events in the western carpathians 1 late paleogene bedding parallel extension 2 oligocene to middle miocene nne ssw compression and ese wnw extension 3 post mid miocene nw se extension these regional deformations applied to crustal blocks in the intra carpathian region resulted from forces which were induced by the coupled plate tectonic processes of subduction retreat beneath the carpathian arc and lateral extrusion from the eastern alps toward the carpathian region material flow from the alps spread toward east and ne guided by sinistral strike slip zones along the ne trending continental margin subduction retreat was terminated by mid miocene soft collision of the inner western carpathians with the european foreland ongoing subduction retreat beneath the eastern carpathians caused post mid miocene back arc extension forming the pannonian basin extension spread into the western carpathians by reactivating the extrusion related strike slip faults as normal faults

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

new insights on the tectonics along the new hebrides subduction zone based on gps results 1 at the new hebrides nh subduction zone ridges born by the subducting australia plate enter the trench and collide with the overriding margin results from gps surveys conducted on both sides of the trench and new bathymetry maps of the nh archipelago bring new light on the complex tectonics of this area convergence vectors present large variations that are not explained by australia pacific a p poles and that define four segments vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors slow convergence i e 30 40 mm yr is found at the central segment facing the d entrecasteaux ridge the southern segment moves faster than a p motion predicts 89 to 124 mm yr relatively to a western north fiji basin wnfb reference the northern and southern segments rotate in opposite directions consistently with the extension observed in the troughs east of both segments both rotations combine in central vanuatu into an eastward translation that bulldozes the central segment into the wnfb at 55 mm yr that model suggests that the motion of the central segment forced by the subduction collision of the d entrecasteaux ridge influences the motion of the adjoining segments the new caledonia archipelago is motionless with respect to the rest of the australia plate despite the incipient interaction between the loyalty ridge and the nh margin southeast of the interaction area convergence is partitioned into a 50 mm yr trench normal component accommodated at the trench and a 90 mm yr trench parallel component close to the a p convergence and presumably accommodated by a transform boundary at the rear of the nh arc

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

variability in flow and temperatures within mantle subduction zones 1 a series of laboratory experiments is used to model three dimensional aspects of flow in subduction zones and the consequent temperature variations in the slab and overlying mantle wedge the effects of longitudinal rollback and slab steepening components of motions are considered along with different thicknesses of the over riding lithosphere the results show that the style of plate sinking influences the evolution of subduction zones both in terms of the speed orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab in the simplest case of longitudinal sinking without rollback motion velocities in the mantle wedge are 30 40 of the downdip plate speed return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds and there is no mass flux around the edges of a slab segment of finite width rollback subduction leads to flow both around and beneath the sinking slab with larger velocities in the wedge up to 150 of the slab speed and flow focused toward the center of the plate segment rollback subduction in which either the trench migrates or the plate steepens with time induces shallow and steep wedge return flow trajectories respectively the thermal evolution of the plate is strongly influenced by sinking style and rate highest temperatures are along the edges of the slab for longitudinal sinking but along the centerline of the slab segment for rollback motion slab surface temperatures when scaled to the mantle are higher than those given by previous two dimensional numerical models and are consistent with recent observational and experimental data on melt compositions

the two stage opening of the western central mediterranean basins a forward modeling test to a new evolutionary model abstract the algero provencal and the tyrrhenian extensional basins developed in two stages in a geodynamic setting characterized by the nearly n s convergence between africa and eurasia the spreading of the provencal basin occurred in the early miocene following a long period of rifting in the western mediterranean area a dramatic eastward shift of the active extensional deformation resulted in the tortonian to quaternary opening of the tyrrhenian basin in a companion paper carminati et al propose that a trench retreat process in a geodynamic setting locked by the continental collisions in the alps and in the betic chain is a viable mechanism for the late oligocene early langhian opening of the western mediterranean a langhian slab detachment episode along the north african margin is likely to have caused the end of the trench retreat along this part of the boundary determining the end of active expansion in the western mediterranean and the beginning of active extension in the tyrrhenian basin the objective of the present paper is to quantitatively test this proposed scenario we calculate by means of a thin shell model the effects of these plate boundary reorganizations on the european stress and strain field we show that the two stage opening of the western and central mediterranean can be explained by the evolution proposed by carminati et al and that in particular the eastward shift of the active extension which produced the termination of the first opening stage and the beginning of the second is likely to have been triggered by the slab detachment episode along the north african margin

toroidal mantle flow around the calabrian slab italy from sks splitting 1 sks wave birefringence is likely caused by ongoing and or fossil mantle flow splitting parameters in the southern tyrrhenian subduction zone define a quite complex pattern of fast directions with large delay times up to 2 7 s fast directions are prevalently trench parallel below the slab and rotate to trench normal at the western edge of the slab depicting a ring around it we interpret these anisotropic patterns also observed in laboratory experiment as toroidal mantle flow induced by rollback subduction

analogue modeling of arc and backarc deformation in the new hebrides arc and north fiji basin in most backarc basins extension is perpendicular to the arc thus individual spreading ridges extend approximately parallel to the arc in the north fiji basin however several ancient and active spreading ridges strike 70 90 to the new hebrides arc these high angle spreading ridges relocated southward during the asymmetric opening of the north fiji basin we have simulated the structural development of the north fiji basin and the new hebrides arc with scaled analogue models and the results have inspired us to come to several tentative conclusions we interpret the orientation of the high angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge where they form close to the hinge relocation of these spreading ridges is most likely related to subduction of the west torres plateau along the new hebrides trench this resulted in localized collision retarded rollback of the subducting slab along the northwest corner of the trench and reduced extension and shearing in the northwest corner of the north fiji basin backarc extension continued in the rest of the north fiji basin owing to continued rollback of the southern part of the subducting slab here active extension was separated from the slightly or nonextending northwest corner by a zone striking at high angle to the new hebrides arc i e the hazel holme extensional zone moreover impingement of the d entrecasteaux ridge into the overriding plate led to local deformation and fragmentation of the arc

counter clockwise rotations in the southern apennines during the pleistocene paleomagnetic evidence from the matera area sediments from late pliocene to early pleistocene age have been sampled at six different localities sections 38 sites in the matera area which is in the southern part of peninsular italy basilicata three of the localities are from the apenninic belt the other three from the bradano unit both of them west of the earlier sampled apulian unit stepwise thermal demagnetisation revealed that usually two important components contribute to the natural remanent magnetisation nrm of these clay sediments a low temperature lt component removed between 100 and 300 320 c and a high temperature ht component removed between 300 320 and 430 450 c the ht component is a characteristic remanent magnetisation chrm for the sediments and has a clear primary pre folding origin in fresh unweathered sediments the lt component is identical in direction to the ht component but there is evidence that moderate weathering replaced a high intensity primary lt component with a low intensity secondary lt magnetisation the ht magnetisation is affected by this weathering process in a decrease of its intensity a counter clockwise deflection in declination is observed for the sites sampled in the apenninic belt the ht directions reveal a 23 counter clockwise rotation relative to the magnetic north dm 336 6 and im 57 2 sites in the bradano unit have no tectonic rotation dm 359 1 and im 56 3 earlier sampled sites from the apulian unit also showed no tectonic rotation dm 359 4 and im 57 0 paleomagnetic data support the hypothesis that the southern apennines rotated in the middle late pleistocene after the major phase of extension in the southern part of the tyrrhenian basin and contemporaneous with a regional compressional phase

global seismicity characteristics of subduction to strike slip transitions there are at least 30 major plate boundary segments worldwide where the plate boundary changes from subduction to strike slip these include six triple junctions and 24 two plate boundaries this study investigates earthquake seismicity in the 24 two plate subduction to strike slip transition ssst regions by utilizing recently published earthquake relocations ternary diagrams of focal mechanisms and moment rate calculations to facilitate cross regional comparisons we categorize the geometry of ssst plate boundaries in terms of 1 their radius of curvature 2 their sense of curvature that is whether they are convex or concave as viewed from the downgoing plate and 3 their tectonic complexity that is the variability of crustal thickness and the segmentation of the plate boundary trace we observe three main trends in ssst regions 1 there is a conspicuous scarcity of strike slip earthquakes along plate boundary segments that plate motion models indicate are strike slip boundaries 2 in these apparent strike slip segments both the rate of occurrence of earthquakes of any kind and the moment release rate are low compared to adjacent subduction segments and 3 there were few observable differences in seismicity between convex and concave boundaries the observation that transform zones exhibit moment rate deficiencies that is have few large magnitude earthquakes in the historical record may have important implications for seismic hazard assessment in ssst regions in particular is motion along these boundaries aseismic with little seismic hazard or is motion expressed in very large magnitude infrequent but potentially devastating earthquakes in at least three such regions new zealand the philippines and the dominican republic paleoseismic evidence and the historical record of seismicity suggest that very large infrequent earthquakes do occur

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

a complex pattern of mantle flow in the lau backarc shear wave splitting analysis of local events recorded on land and on the ocean floor in the tonga arc and lau backarc indicate a complex pattern of azimuthal anisotropy that cannot be explained by mantle flow coupled to the downgoing plate these observations suggest that the direction of mantle flow rotates from convergence parallel in the fiji plateau to north south beneath the lau basin and arc parallel beneath the tonga arc these results correlate with helium isotopes that map mantle flow of the samoan plume into the lau basin through an opening tear in the pacific plate

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

current tectonics of the tonga new hebrides region abstract this paper proposes a present day tectonic map of the tonga new hebrides region the result of combining data from bathymetry satellite derived gravity seismicity magnetic measurements and gps observations focal mechanisms and a large amount of bathymetric data collected during the last decade in the lau and north fiji back arc basins indicate a deformation distributed over numerous spreading ridges rather than diffuse and shear dominated as was previously thought the region is also characterised by a large variation of both consumption rate along the arcs and opening rate along the back arc basins spreading centres geometry and rate of spreading in the lau and north fiji back arc basins and segmentation of the tonga and new hebrides arcs are closely related to and significantly influenced by the subduction of aseismic ridges namely the louisville d entrecasteaux and loyalty ridges where subducting aseismic ridges enter the trenches we observe slower convergence arc transverse strike slip faulting thrusting at the rear of arcs and slow or absent back arc spreading rapid subduction correlates with fast back arc opening sometimes parallel spreading ridges in the back arc domain are required to accommodate the fast motion of convergence

triple junction instability as cause for late cenozoic extension and fragmentation of the western united states regional extension in the western united states began soon after 30 m y b p the time of first interaction between the pacific and north american plates previous explanations for extension include lithospheric active processes and asthenospheric active processes the former are favored because of coincidence of timing with lithospheric interactions relative timing of volcanism and deformation and coincidence with pre existing structures although extension has occurred in part in a back arc setting it cannot be attributed to age of subducted crust or azimuth of subduction because both attributes favor back arc contraction characteristic behavior before 40 m y b p the mendocino triple junction has been unstable since its inception the continental margin was relatively straight before 30 m y b p and it has become more convex westward as the triple junction has migrated northward the continental margin and arc have been anchored to the subducting slab farallon juan de fuca whereas the triple junction must move parallel to the san andreas transform the combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the san andreas transform relative to the coast these effects result from the unstable geometry of the mendocino triple junction lack of a subducted slab slab window beneath the extended lithosphere enables asthenospheric rise but does not cause extension

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

new gps constraints on the africa eurasia plate boundary zone in southern italy 1 southern italy is a key area for understanding the tectonic processes in the africa eurasia collision zone we analyze new gps measurements carried out between 1994 and 2001 the results are presented in terms of time series trajectories and velocities of crustal motion as well as a geodetic strain rate field while central italy corsica sardinia and the tyrrhenian sea move like the eurasian plate the overall motion of the sicily rift zone region matches african plate motion unexpected are the north northwest directed motions of northeastern sicily and the eolian islands most striking on the deformation field are a north south oriented compression along the northern sicilian coast compression between apulia and northwestern greece as well as extension in the sicily rift zone and the interior of sicily

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

paleomagnetic evidence for counter clockwise rotations in the southern apennines fold and thrust belt during the late pliocene and middle pleistocene abstract marine and continental clay silt sediments from late miocene to middle late pleistocene age have been sampled in the southern apennines fold and thrust belt to detect the spatial and temporal aspects of tectonic rotations we sampled different parts of the tectonic system sites from both the internal western and the external eastern part the characteristic remanent magnetisation chrm component for the sediments is generally removed from 320 340 up to 450 480 c a counter clockwise rotation is observed for nearly all sites all data indicate that the main tectonic rotations in the southern apennines fold and thrust belt occurred in the middle pleistocene the late miocene to middle pliocene sediments reveal a 39 counter clockwise rotation which is similar to the rotation of the cretaceous carbonate rocks this indicates at least two phases of counter clockwise rotation a first rotation through approximately 15 during the late pliocene possibly contemporaneous with clockwise rotations in sicily 3 05 ma and a second rotation through at least 15 during the middle pleistocene there was also a third smaller rotation through about 9 since the middle pleistocene the results imply that there were no tectonic rotations in the southern apennines fold and thrust belt during the period of major shortening that took place in the early middle miocene nor during the opening of the southern part of the tyrrhenian basin in the latest pliocene earliest pleistocene our data further support the hypothesis that the late oroclinal rotations in the tyrrhenian arc were confined to distinct phases of regional compression

paleomagnetic evidence for a pleistocene clockwise rotation of the calabro peloritan block southern italy abstract a paleomagnetic study of marine marls of early pliocene age and clays of late pliocene to middle pleistocene age from sites along the ionian coast spartivento basin and catanzaro depression of calabria and from sites along the tyrrhenian coast of both calabria and sicily was carried out to determine the amount and timing of tectonic rotation of the underlying calabro peloritan block the magnetisations of the lower pliocene trubi marls have maximum blocking temperatures indicating the presence of magnetite these lower pliocene sediments as well as the younger upper pliocene lower pleistocene clays of this area reveal consistent clockwise rotations of 10 20 there is evidence for a uniform 15 clockwise rotation of the entire calabro peloritan block during the middle pleistocene this is opposite in sense of rotation to the earlier established 23 counterclockwise rotations for the southern apennines but the timing is the same between 0 7 and 0 2 ma the tectonic rotations occur 1 myr after the major phases of extension in the southeastern part of the tyrrhenian basin but simultaneously with the onset of major uplift of the block

european mediterranean regional centroid moment tensor catalog solutions for years 2001 and 2002 abstract we present regional centroid moment tensor rcmt solutions for 168 moderate magnitude earthquakes that occurred in the european mediterranean region during 2001 and 2002 events with moment magnitudes as low as 4 0 were successfully analyzed although this low threshold is only achievable in regions with the best azimuthal coverage and with stations at a few hundred kilometers distance earthquakes with focal depths from 10 to more than 200 km were analyzed comparison with standard harvard cmts when available shows good agreement the solutions shown in this paper represent an addition to the european mediterranean rcmt catalog that we are maintaining the rcmt catalog now spans 6 years from 1997 to 2002 and contains 420 solutions we also outline the main seismic sequences that occurred in the european mediterranean region during 2001 2002

aseismic continuation of the lesser antilles slab beneath continental south america 1 we present results of travel time inversions of teleseismic p and s waves recorded at the secasa92 southeast caribbean south america 1992 temporary broadband array in northeastern venezuela and trinidad the inversions reveal the unusual structure of the southern termination of the lesser antilles subduction zone a minimum 2 relatively high velocity anomaly trends wsw from the seismically defined lesser antilles slab beneath and nw of the paria peninsula to a point below the venezuelan serrania del interior well south of the caribbean coast resolution tests utilizing actual ray geometries and densities of the source data indicate that the regional scale structure beneath the study area is reasonably well resolved thus a detached and detaching subducted south american slab appears to lie beneath continental south america we infer that oceanic south american lithosphere has been overridden to a significant degree by continental south america the detached slab now lying beneath continental south america was driven into its current position after detaching from the former eastward striking mesozoic ocean continent passive margin as this margin entered the subduction zone because oceanic and continental south america are still attached without apparent relative motion between them along the atlantic passive margin southeast of our study region the slab must be the actively moving element during continental overriding thus the slab and its surrounding mantle both caribbean and south america beneath northeastern south america are mobile and have moved ese relative to the stable guyana shield craton

seismological evidence for tearing of the pacific plate at the northern termination of the tonga subduction zone we present seismological evidence for tearing of the pacific plate at the northern termination of the tonga subduction zone seismicity cross sections indicate 1 abrupt termination of intermediate depth seismicity along an east west trending zone coincident with the northern termination of the subduction zone and 2 shoaling of the subducted slab at its northern termination earthquake distribution and source mechanism determinations indicate progressive downwarping and tearing of the pacific plate as it enters the northernmost segment of the tonga subduction zone source mechanisms for 57 events located along a narrow belt of high seismicity in northernmost tonga indicate dip slip faulting along near vertical planes oriented 285 coinciding with the observed direction of plate convergence precisely determined depths for 21 events 18 57 km suggest that the plate is downwarped 15 km prior to initiation of tearing depths of the 7 april 1995 m s 8 event and aftershocks suggest tearing of the plate may extend through the entire thickness of the oceanic lithosphere

mantle flow mechanisms for the large scale subsidence of continental interiors evidence in the geological record shows that continental interiors periodically undergo enigmatic episodes of large scale subsidence we propose that mantle flow associated with the descent of cold plumes and slabs and their interaction with the endothermic phase change at 660 km depth may provide a plausible mechanism for these epeirogenic events simulations of mantle convection that incorporate the thermodynamic effects of the phase change and both depth and temperature dependent viscosity are used to model the descent of plumes and slabs through the 660 km boundary we find that the plume slab flow scenarios are capable of supporting topographic deflections of amplitudes of 1 km and horizontal wavelengths of 1000 km that persist over time scales of 100 150 m y

evidence for a contribution from two mantle plumes to island arc lavas from northern tonga lavas from the islands of tafahi and niuatoputapu at the northern end of the active tonga kermadec arc in the southwest pacific were erupted at a convergent plate margin yet they can be shown to contain a contribution from two different mantle plumes high concentrations of nb relative to other high field strength elements in these lavas compared to other tonga lavas reflect an ocean island basalt component in the mantle wedge derived from the nearby samoa mantle plume pb isotope compositions indicate that most of the pb in these lavas is derived from the oceanic crust of the plume generated louisville seamount chain which is being subducted beneath the tonga arc these two plume components were thus introduced into the arc lavas in very different ways and provide insight into upper mantle dynamics and magma generation processes occurring in an active arc back arc system

palinspastic restoration and paleogeographic reconstruction of the peri tyrrhenian area during the neogene abstract this paper presents the results of research carried out on the paleogeographic and geodynamic post collisional evolution of the peri tyrrhenian area from the southern alps to sicily during this post collisional evolution a back arc area thrust belt foredeep system developed in which the following tectono sedimentary zones can be distinguished from the external to the internal ones the foreland zone connected to the foredeep through a slope generally controlled by normal faults the foredeep an assymmetric depression with strong subsidence related to the compressional activity of the thrust zone the chain whose outward migration from the late oligocene onward was discontinuus with important stages of crisis partially connected to the mechanical response of different crustal thicknesses existing in the foreland the discontinuous satellite basins developed within the thrust belt the back arc basins developed in the inner part of the chain where the superposition of the extensional tectonics over previous compressional domains causes the fragmentation of the thrust belt leading to its destruction according to this model seven palinspatic maps are discussed regarding the following time intervals late oligocene 25 24 m y burdigalian 22 20 m y langhian 17 16 m y serravallin 15 13 m y tortonian 10 9 m y late messinian 5 5 5 3 m y and early pliocene 5 3 4 6 m y during five of these time intervals significant geodynamic modifications occurred in the whole peri tyrrhenian area leading to paleogeographic changes in any case the geodynamic evolution of the whole area seems clearly controlled by the activity of important bundles of tectonic lines transversal to the chain which in the same time interval in the outermost part of the thrust belt separated areas with different values of shortening and in tthe most internal zones separated segments having different amount of extension these tectonic lines certainly have a crustal significance and have probably acted along previous formed discontinuities