the mid norwegian margin a discussion of crustal lineaments mafic intrusions and remnants of the caledonian root by 3d density modelling and structural interpretation the high density lower crustal body lcb on the mid norwegian margin is almost universally interpreted to represent magmatically underplated material added to the crust during early tertiary opening of the ne atlantic the thickness of the lcb is uneven and its distribution along the margin is sharply limited by margin perpendicular lineaments three dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments bivrost jan mayen gleipne etc in the crustal architecture with and without the lcb the bivrost lineament separating the voring and lofoten margin segments is clearly expressed and so is arguably a lineament in the sw voring margin notably most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from late caledonian orogenic collapse an alternative interpretation of the lcb is thus that it represents high grade metamorphic rocks remnant from the caledonian root determining the nature of the lcb has profound effects for the volume estimate of magmatic rocks in the north atlantic igneous province and consequently also for the degree of crustal thinning and heat flow

gravity anomalies and associated tectonic features over the indian peninsular shield and adjoining ocean basins abstract a combined gravity map over the indian peninsular shield ips and adjoining oceans brings out well the inter relationships between the older tectonic features of the continent and the adjoining younger oceanic features the nw se ne sw and n s precambrian trends of the ips are reflected in the structural trends of the arabian sea and the bay of bengal suggesting their probable reactivation the simple bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins which is attributed to isostatic compensation due to variations in the crustal thickness a crustal density model computed along a profile across this region suggests a thick crust of 35 40 km under the continent which reduces to 22 20 24 km under the bay of bengal with thick sediments of 8 10 km underlain by crustal layers of density 2720 and 2900 2840 kg m3 large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the bay of bengal up to 150 200 km from the east coast the crustal thickness under the laxmi ridge and east of it in the arabian sea is 20 and 14 km respectively with 5 6 km thick tertiary and mesozoic sediments separated by a thin layer of deccan trap crustal layers of densities 2750 and 2950 kg m3 underlie sediments the crustal density model in this part of the arabian sea east of laxmi ridge and the structural trends similar to the indian peninsular shield suggest a continent ocean transitional crust cotc the cotc may represent down dropped and submerged parts of the indian crust evolved at the time of break up along the west coast of india and passage of reunion hotspot over india during late cretaceous the crustal model under this part also shows an underplated lower crust and a low density upper mantle extending over the continent across the west coast of india which appears to be related to the deccan volcanism the crustal thickness under the western arabian sea west of the laxmi ridge reduces to 8 9 km with crustal layers of densities 2650 and 2870 kg m3 representing an oceanic crust

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

inversion of field data in fault tectonics to obtain the regional stress iii a new rapid direct inversion method by analytical means summary a new method for determining the reduced stress tensor with four degrees of freedom the orientations of the three principal stress axes as well as the ratio of principal stress differences using fault slip data or focal mechanisms of earthquakes is presented from a computational point of view the inversion of fault slip data is made in a direct way by purely analytical means as a result the determination process is extremely fast and adaptable on small microcomputers from a physical point of view the method aims at simultaneously i minimizing the angles between theoretical shear stress and actual slip vector and ii having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown the double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear stria angle

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

relations between fluid inclusion trails and regional stress field a tool for fluid chronology an example of an intragranitic uranium ore deposit northwest massif central france abstract episyenites within the uranium ore bearing le bernardan granite northwest french massif central were formed through the dissolution of quartz by hydrothermal solutions these solutions may now be found fossilized as fluid inclusions f i the f i form rectilinear trails along microcracks which trapped the percolating fluids simultaneous study of the orientations of the trails and the chemistry of the inclusions shows that 1 1 the trails may be grouped according to direction each related to successive regional stress fields 2 2 the fluids of each group are compositionally distinct marking different stages of the hydrothermal event this example shows that the geometric relations between secondary f i trails and regional stress fields may be used to establish the chronology of both during a region s geological evolution

gravity isostasy and andean crustal shortening between latitudes 30 and 35 s we started the work by analyzing an e w crustal section at 33 s latitude which extends from the western chilean trench crosses the andean belt and goes inland into argentina a two layer gravity seismic model was prepared here it provided a maximum moho depth of 62 km beneath the andean axis this model is consistent with a one layer standard crustal model with normal thickness t 33 km and densities of 2 9 g cm3 and 3 3 g cm3 for the crust and the upper mantle respectively then we analyzed the andean isostatic behaviour on four andean gravimetric sections at latitudes of 30 32 33 and 35 s based on one layer simple models these results show antiroots near the chilean trench and crustal thickenings beneath the andean belt which locate the moho discontinuity at depths of 70 km at 30 s 65 km at 32 and 33 s and 57 km at 35 s from the crustal gravimetric models the following provisional charts for the sector were prepared and compared 1 topography 2 bouguer gravity 3 isostatic corrections 4 moho contours these charts are consistent with each other and they point out that the compensation of the andean belt trends approximately to isostasy in the airy concept other results also confirm this 1 the remarkable decrease of the isostatic anomalies in each of the analyzed sections as regards the bouguer anomalies and 2 the relationships between the exceeding areas and the corresponding crustal root areas which are consistent with the airy numerical relations as to the perfect isostatic equilibrium thus the bouguer anomalies are mainly controlled by the m discontinuity finally shortenings in the andean sector were fixed based on the crustal thicknesses found from gravity inversion these are 150 km at 30 s decreasing southwards and reaching only 90 km at 35 s these shortenings explain the cenozoic andean uplift without requiring other mechanisms such as magmatic intrusions

upper mantle s velocity structure of central and western south america we have used broadband seismograms recorded at portable and permanent seismic stations in south america to image the three dimensional s velocity structure beneath central and western south america we have interactively fitted the waveforms of s and surface wave trains in 500 seismograms from 90 earthquakes and 60 different stations the constraints on upper mantle s velocity structure provided by the waveform fits have been combined with independent estimates of depth to the mohorovicic discontinuity yielding model sa99 the s velocity structure of sa99 reveals a highly heterogeneous upper mantle with a maximum peak to peak difference of 1 km s above 100 km the heterogeneity seems to be correlated with tectonic and geologic domains sa99 shows high lithospheric s velocities for the precambrian cratons the amazon basin is also underlain by high s velocities while three other intraplatform basins are underlain by low s velocities at various depths sa99 shows extremely low velocities in the mantle wedge beneath the volcanically active andes we explain these low s velocities as originating from realistic amounts of volatiles released by the subducting nazca crust and resulting amounts of partial melt within the peridotitic mantle wedge without requiring elevated temperatures while sa99 could be consistent with diffuse influx for corner flow in the mantle wedge it does not support models that require influx of subcontinental upper mantle into the mantle wedge through a laterally uniform low viscosity channel

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

thickening the altiplano crust by gravity driven crustal channel flow 1 in the central andes crustal thickness is not well correlated to upper crustal shortening only little shortening is documented in the upper crust of the 60 65 km thick altiplano plateau whose thickening and uplift were delayed with respect to earlier and greater thickening in the adjacent western and eastern cordilleras because crustal thickness variations induce horizontal stress gradients and cause crustal flow a thickness dependent channel flow is modeled here and applied to the central andes in situ thickening is assumed for both cordilleras while the altiplano crustal thickening is generated by lateral flow from these overthickened adjacent domains a 8 1018 pa s viscosity channel is predicted for crustal thicknesses exceeding 45 50 km to match the estimated topographic evolution of the central andes thickening in the cordilleras was sufficient to generate a flow of 6 109 m3 per unit length toward the initially 30 35 km thick altiplano

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

deciphering deformation in ultramafic intrusions via magnetic fabric ams and palaeomagnetic studies savannah ni pge camp nw australia abstract knowledge of the internal architecture of mafic ultramafic intrusions is critical to efficient targeting of magmatic ni pge mineralisation however the visual homogeneity of such rocks in drill core coupled with poor outcrop in many areas can make their structure difficult to decipher using standard techniques in this study we use measurements of anisotropy of magnetic susceptibility ams and remanent magnetisation to examine the structural history of four adjacent temporally equivalent ultramafic intrusions from the east kimberley province nw australia whilst temporally equivalent the intrusions have contrasting shape including bladed dyke sill and stock geometries and are hosted in rocks of contrasting rheology including mafic granulite and meta sedimentary migmatite early palaeomagnetic signatures present at savannah and savannah north originally had shallow nne to ene oriented inclinations consistent with acquisition at low palaeolatitudes at ca 1800 ma however these primary magnetisations were subsequently rotated during the ca 1810 ma mount stafford event and or ca 1400 1200 ma yampi orogeny demonstrating that significant deformation occurred post emplacement magmatic fabrics observed in ams data for savannah savannah north and dave hill all show evidence of ne or sw rotation during the same events however whilst there are similarities in the implied shortening directions across all four intrusions they each preserve a unique deformation style which is attributed to the unique shape of each intrusion coupled with the contrasting rheology of their respective host rocks each intrusion preserves a common dominant palaeomagnetic signature which is consistent with a high grade metamorphic overprint at ca 1200 ma during the latter stages of the yampi orogeny the lack of subsequent rotation of this dominant palaeomagnetic signature demonstrates that the majority of deformation pre dates the yampi event this study demonstrates that even where tectonic conditions are similar that intrusion shape and host rock rheology exert strong control on their deformation style

package fluids 1 computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties the computer package fluids contains five sets of computer programs written in c for the calculation of fluid properties both model fluids and data from fluid inclusions can be analysed in the programs bulk density molar volume and composition of fluid inclusions are calculated with the program bulk which uses microthermometric data micro raman spectroscopic data cation ratios and volume fraction estimations as input the program offers a broad variety of equations of state which are selected according to the fluid system or the temperature and pressure conditions of interest the isochores that correspond to the output or test v x properties of fluid inclusions are calculated by the program isoc in calculating the isochores this program takes account of the compressibility and expansion of selected host minerals the program test allows different equations of state to be compared directly with experimental data thereby facilitating selection of the most adequate model in the programs bulk and isoc loner is a group of programs that handle individual equations of state eos for all fluid systems presented in the programs bulk and isoc aqso is a group of programs that allows purely empirical modelling of electrolyte bearing aqueous solutions henry s law for dilute gas bearing aqueous solutions and osmotic coefficients for dilute electrolyte bearing aqueous solutions the programs are available on the university of leoben web site at http www unileoben ac at buero62 minpet ronald

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3

are fluid inclusion planes useful in structural geology abstract quantification of deformation or stresses in structural geology is often difficult due to the uncertainity of the paleodepth or fluid pressure the best records of fluid percolation are paleofluids trapped as fluid inclusions in healed microcracks of the rock forming minerals fluid inclusion planes fip are mode i cracks which form in sets with a predominant orientation perpendicular to the least principle stress 3 however the repeated microfracturing and healing of the rock forming minerals yield complex superimposed patterns of healed microcracks such patterns are often difficult to interpret due to the lack of suitable chronological criteria these problems have been recently documented and solved by coupling deformation studies detailed examination at all scales of the relationships between trapped fluids and their host structures and studies of fluid inclusions this paper summarizes recent advancements in fip analysis and suggests further ways for research

fluid percolations in a fault zone a study of fluid inclusion planes in the st sylvestre granite northwest massif central france abstract a method for relating fluid flow evolution direction of fluid percolation to the bulk brittle network microfracture system at each state in a sequence of deformational events is presented fluids percolating through cracks are generally trapped as fluid inclusion planes alignment of fluid inclusions the method involves the use of fluid inclusions not only as a tool for the establishment of the physico chemical conditions under which fluids were entrapped but also as a microstructural marker of the geometry of permeability the study shows that through the determination of the paleopressures for each stage of fluid migration the chronology of the cracks and p t conditions prevailing during each crack formation can be established this methodology has focussed on a granitic example from the french massif central but may be applied to other geological environments

the liquine ofqui fault zone a long lived intra arc fault system in southern chile abstract plate reconstructions for the cenozoic document relatively steady right oblique subduction of the farallon nazca plate beneath the chilean continental margin the kinematic signature recorded along the intra arc liquine ofqui fault zone lofz a major feature of the southern andes may help to understand the way in which the nazca south america slip vector has been partitioned into strain and displacement along and across the continental margin the lofz consists of two nne trending right stepping straight lineaments a strike slip duplex at the right step and curved features which splay off the straight lineaments toward the northwest the lofz runs mostly through heterogeneously deformed cenozoic plutonic rock of the north patagonian batholith and patchy metamorphic wall rock early cenozoic volcano sedimentary rocks and dyke swarms found in close spatial association with the strike slip duplex are believed to have developed in strike slip related basins quaternary volcanoes are aligned parallel to the lofz both ductile and brittle kinematic indicators within centimeter to meter wide high strain zones document late cenozoic dextral shear deformation contrasting left lateral deformation recorded on older and wider mylonitic zones suggests that the lofz may be a long lived shear zone that accommodated continental scale deformation arising from the farallon nazca south america plate convergence a block rotation pattern as indicated by paleomagnetic data is consistent with the geometry and cenozoic kinematics of the lofz

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

geodynamic evolution of the central dinarides abstract the central part of the dinarides unlike the northwestern and southeastern dinarides shows a consistent pattern in the distribution of the large tectonostratigraphic units from southwest apulia to northeast the following five main units originating in different parts of the mesozoic tethys can be distinguished 1 adriatic dinaridic carbonate platform formations 2 carbonate clastic units flysch bosniaque of the passive continental margin 3 ophiolite bearing formations of the dinaridic ophiolite zone also containing radiolarites greywackes and shales including melange formations and overstepped late jurassic to cretaceous cover sequences 4 active continental margin units represented by upper cretaceous palaeogene trench sediments with blueschists tectonized ophiolite melange alpine medium pressure metamorphic rocks originated from upper cretaceous palaeogene sediments and alpine synkinematic granitoids 5 a nappe consisting of allochthonous palaeozoic triassic formations which is thrust onto the ophiolites and genetically related sedimentary formations the frontal parts of this nappe directly overlie the northeastern margin of the adriatic dinaridic carbonate platform i e the external dinarides the evolution of the central dinarides was controlled by a sequence of tectonic events which took place within the alpine cycle 1 rifting processes lasting some 40 50 ma which started in the late permian and ended in the norian 2 opening of the oceanic dinaridic tethys commenced in the late triassic and persisted for some 70 80 ma until the end of the jurassic 3 subduction processes and emplacement of ophiolites started in late jurassic early cretaceous times during the closing phase of the dinaridic ocean or dinaridic tethys the palaeozoic triassic nappe was emplaced on the ophiolites the thrusting was accompanied by alpine metamorphism 120 100 ma 4 the main alpine deformational event and medium pressure metamorphism with synkinematic granite plutonism took place in late eocene early oligocene times when the ophiolite and palaeozoic triassic nappes were emplaced on the adriatic dinaridic carbonate platform which at the same time was deformed resulting in the development of the external dinarides fold and thrust belt

synthetic fluid inclusions vi quantitative evaluation of the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure the decrepitation behaviour of fluid inclusions in quartz at one atmosphere confining pressure has been evaluated using pure h2o synthetic inclusions formed by healing fractures in natural quartz three different modes of non elastic deformation referred to as stretching leakage or partial decrepitation and total decrepitation have been observed the internal pressure required to initiate non elastic deformation is inversely related to inclusion size according to the equation internal pressure kbar 4 26 d 0 423 where d is the inclusion diameter in microns regularly shaped inclusions require a higher internal pressure to initiate non elastic deformation than do irregularly shaped inclusions of similar size heating inclusions through the quartz inversion results in mechanical instability in the quartz crystal and leads to mass decrepitation of inclusions owing to structural mismatches generated by pressure gradients in the quartz around each inclusion long term heating experiments 2 years suggest that the internal pressure required to initiate non elastic deformation does not decrease significantly with time and indicates that short lived thermal fluctuations in natural systems should not alter the inclusion density and homogenization temperature inclusions that do exhibit decreased density higher homogenization temperature are however always accompanied by a change in shape from irregular to that of a negative crystal observations of this study are consistent with elasticity theory related to fracture generation and propagation around inclusions in minerals these results indicate that an inclusion will not be influenced by a neighbouring inclusion or other defect in the host phase as long as the distance between the two is 2 4 diameters of the larger of the two inclusions

interaction between the nazca and south american plates and formation of the altiplano puna plateau review of a flexural analysis along the andean margin 15 34 s abstract the results of a two dimensional flexural analysis applied to the andean margin which is based on the correlation between topography and bouguer anomaly are here reviewed in order to characterize rigidity variations across and along the forearc arc transition of the central andes and to understand the role of the forearc in the formation of the altiplano plateau the forearc has maximum rigidities between 15 and 23 s forearc rigidity decreases gradually southward and sharply toward the plateau the main orogen elevations higher than 3000 m is very weak along the entire central andes a semi quantitative interpretation of these trends based on the relationship between flexural rigidity and the thermo mechanically and compositionally controlled strength of the lithosphere allows the following conclusions to be made 1 across strike rigidity variations are dominated by the thermal structure derived from the subduction process 2 the forearc constitutes a strong cold and rigid geotectonic element 3 southward weakening of the forearc is directly related to the decreasing thermal age of the subducted slab 4 very low rigidities along the main orogen are caused by the existence of a thick quartz rich crust with a low strain rate to heat flow ratio 5 the strength of the plateau lithosphere is localized in an upper crustal layer whose base at 15 km could be correlated with a p to s seismic wave converter trac1 of yuan et al 2000 yuan x sobolev s kind r oncken o et al 2000 subduction and collision processes in the central andes constrained by converted seismic phases nature v 408 21 28 diciembre p 958 961 6 the forearc plateau rigidity boundary corresponds to a zone of changing thermal conditions eastward increasing crustal thickness and felsic component in the crust and low strain rate deformation which correlates with a west verging structural system at the surface these conclusions suggest that the rigid forearc acts as a pseudo indenter against the weak plateau and allows the accumulation of ductile crustal material that moves westward from the eastern foreland this pseudo indenter is geometrically represented by a crustal scale triangular zone rooted at trac1 this model allows the integration of existing contradictory ideas on the dynamics of forearc plateau interaction that are related to the relative importance of upper crustal compressive structures and lower crustal accumulation below the forearc

neotectonics in the foothills of the southernmost central andes 37 38 s evidence of strike slip displacement along the antinir copahue fault zone 1 the antinir copahue fault zone acfz is the eastern orogenic front of the andes between 38 and 37 s it is formed by an east vergent fan of high angle dextral transpressive and transtensive faults which invert a paleogene intra arc rift system in an out of sequence order with respect to the cretaceous to miocene fold and thrust belt 3 1 1 7 ma volcanic rocks are folded and fractured through this belt and recent indicators of fault activity in unconsolidated deposits suggest an ongoing deformation in spite of the absence of substantial shallow seismicity associated with the orogenic front neotectonic studies show the existence of active faults in the present mountain front the low shallow seismicity could be linked to the high volumes of retroarc derived volcanic rocks erupted through this fault system during pliocene and quaternary times this thermally weakened basement accommodates the strain of the antinir copahue fault zone absorbing the present convergence between the south america and nazca plates

causes of scaling on bush hammered heritage ashlars a case study plaza mayor of madrid spain masons have traditionally used granite anisotropy to cut and lay the stone scaling a common type of granite decay is observed worldwide this study explored the relationship between weathering and cut planes in heritage ashlars specifically in the stone on madrid s plaza mayor whose construction dates back to 1590 the 71 rectangular granite columns that support its porticoes are oriented towards the four cardinal points all 71 have one exposed side that faces the square one protected side facing inward and two semi protected sides perpendicular to the other two the sides of the columns are also oriented to the points of the compass this study aimed to identify the prevailing orientation of scaling if any in the granite ashlars and to determine how this process is affected by climate microclimate orientation use hewing and exfoliation microcracks all four sides of the 71 columns were mapped 284 in all to analyse scaling height distribution and orientation the findings showed that the microcracks are vertically oriented and decline in density and length with depth from the surface scaling was observed on the lower ashlars in the columns to a maximum depth of 3 mm determining the direction of exfoliation microcracks is imperative to understanding decay mechanisms in granite ashlars and sculptures and that information must be taken into consideration when applying conservation treatments

mesozoic plate tectonic reconstruction of the carpathian region abstract palaeomagnetic palaeobiogeographic and structural comparisons of different parts of the alpine carpathian region suggest that four terranes comprise this area the alcapa tisza dacia and adria terranes these terranes are composed of different mesozoic continental and oceanic fragments that were each assembled during a complex late jurassic cretaceous palaeogene history palaeomagnetic and tectonic data suggest that the carpathians are built up by two major oroclinal bends the alcapa bend has the meliata oceanic unit correlated with the dinaric vardar ophiolite in its core it is composed of the western carpathians eastern alps and southern alcapa units transdanubian range bukk this terrane finds its continuation in the high karst margin of the dinarides further elements of the alcapa terrane are thought to be derived from collided microcontinents czorsztyn in the n and a carbonate unit tisza in the se the tisza dacia bend has the vardar oceanic unit in its core it is composed of the bihor and getic microcontinents this terrane finds its continuation in the serbo macedonian massif of the balkans the bihor getic microcontinent originally laid east of the western carpathians and filled the present carpathian embayment in the late palaeozoic early mesozoic the vardar ocean occupied an intermediate position between the western carpathian austroalpine transdanubian high karst margin and the bihor getic serbo macedonian microcontinent the vardar and pindos oceans were opened in the heart of the mediterranean adriatic microcontinent in the late permian middle triassic vardar subducted by the end of jurassic causing the bihor getic serbo macedonian microcontinent to collide with the internal dinaric western carpathian margin an external penninic vahic ocean tract began opening in the early jurassic separating the austroalpine western carpathian microcontinent and its fauna from the european shelf further east the severin ceahlau magura also began opening in the early jurassic but final separation of the bihor getic ribbon and its fauna from the european shelf did not take place until the late middle jurassic the alcapa and the tisza dacia were bending during the albian maastrichtian the two oroclinal bends were finally opposed and pushed into the gates of the carpathian embayment during the palaeogene and neogene at that time the main n s shortening in distant alpine and hellenic sectors was linked by a broader right lateral shear zone along the former vardar suture

seismic investigations of the o higgins seamount group and juan fernandez ridge aseismic ridge emplacement and lithosphere hydration the o higgins seamount group is a cluster of volcanic domes located 120 km west of the central chilean trench on the crest of the juan fernandez ridge this aseismic hot spot track is subducting under south america triggering a belt of intraslab earthquake hypocenters extending about 700 km inland the juan fernandez ridge marks the southern boundary of a shallow subduction segment subduction of oceanic basement relief has been suggested as a cause for the flat slab segments characterizing the andean trench system the juan fernandez ridge however shows only moderate crustal thickening inadequate to cause significant buoyancy in 2001 wide angle seismic data were collected along two perpendicular profiles crossing the o higgins group we present tomographic images of the volcanic edifices and adjacent outer rise trench environment which indicate a magmatic origin of the seamounts dominated by extrusive processes high resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting a pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases reduced uppermost mantle velocities are solely found underneath the juan fernandez ridge and may indicate mineral alterations enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction which prevails to the north after the southward migration of the juan fernandez ridge

stress variability in the parnaiba basin brazil during cretaceous rifting the cretaceous section of the parnaiba basin designated as grajau basin represents an intracontinental half graben formed during the early cretaceous due to the separation between the south american and african continents during the final dispersal of western gondwana here through a synergetic approach between fluid inclusion planes fips outcropping geologic structures borehole breakouts and remote sensing data we elucidate in different scales the main structural features and their kinematic indicators normal faults strike mainly nne ssw and wnw ese while deformation bands and extension joints trend to nw se and ne sw and fips to ne sw and wnw ese in addition normal fault generated scarps border geomorphological units and constitute dense zones of deformation bands and fluidization microthermometric fip analyses suggest this fracturing event occurred at shallow basin levels at temperatures below 50 c furthermore joints bands and fips present mutually cross cutting relationships thus indicating contemporaneity the numerical inversions applied to striated faults non striated faults joints deformation bands and fips suggest the occurrence of an extensional event characterized by variable direction of extension 3 trending from nw se to wnw ese or ne se to nne ssw this event probably started in the albian during the final gondwana fragmentation stages the quasi perpendicular 3 trend could be caused by one or all of the following phenomena a stress ratio r values obtained 0 4 indicate the possibility of radial extension 2 3 perhaps in response to the deformation partitioning due to kinematic simple shearing constraints along the equatorial atlantic transform margin b along the wnw oriented release normal faults the local extension nne ssw switch positions in relation to the regional extension wnw ese by rotating 90 c influence of the pre existing structures on the regional stress field thus the spatial and temporal relationships between cretaceous fault activity stress field and the development of the geomorphological features in the grajau basin contribute to understanding of the brazilian equatorial margin geodynamics

interannual variation of the east asian cold surge activity abstract the occurrence frequency of the east asian cold surge exhibits an interannual variation in concert with the el nino southern oscillation enso cycle that is the cold surge occurs more less frequently during warm cold enso winters because the cold surge high low dipoles are coupled with the upper level synoptic short waves any mechanism modulating the activity of these waves would affect the cold surge activity the streamfunction budget analysis in the short wave regime indicates that the development of the cold surge short wave train over east asia and the northwest pacific is modulated by the north pacific enso short wave train due to the coupling between the upper level cold surge short wave train and the surface cold surge dipole it is inferred from this streamfunction budget analysis that the interannual variation of the cold surge occurrence frequency is a result of this modulation

seismic imaging of a convergent continental margin and plateau in the central andes andean continental research project 1996 ancorp 96 1 a 400 km long seismic reflection profile andean continental research project 1996 ancorp 96 and integrated geophysical experiments wide angle seismology passive seismology gravity and magnetotelluric depth sounding across the central andes 21 s observed subduction of the nazca plate under the south american continent an east dipping reflector nazca reflector is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km we interpret parts of the nazca reflector as a fluid trap located at the front of recent hydration and shearing of the mantle the fluids being supplied by dehydration of the oceanic plate patches of bright quebrada blanca bright spot to more diffuse reflectivity underlie the plateau domain at 15 30 km depth this reflectivity is associated with a low velocity zone p to s wave conversions the upper limits of high conductivity and high vp vs ratios and to the occurrence of neogene volcanic rocks at surface we interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during neogene shortening and plateau growth the imaging properties of the continental moho beneath the andes indicate a broad transitional character of the crust mantle boundary owing to active processes like hydration of mantle rocks in the cooler parts of the plate margin system magmatic underplating and intraplating under and into the lowermost crust mechanical instability at moho etc hence all first order features appear to be related to fluid assisted processes in a subduction setting

coincident anomalies of seismic attenuation and electrical resistivity beneath the southern bolivian altiplano plateau 1 reassessment of local earthquake data from the ancorp seismological network allowed the calculation of 3d attenuation qp tomographic images of crust and upper mantle beneath the southern bolivian altiplano around 21 s the images reveal a low qp middle and lower crust and a moderate qp upper mantle beneath the southern altiplano beneath the recent magmatic arc qp is not significantly decreased at this latitude the distribution of crustal qp coincides with the variation of electrical resistivity thus limiting the possible mechanisms causing the anomalies our findings support the hypothesis that partial melts in middle and lower crust beneath the altiplano are present on a large scale we see no evidence for a shallow asthenosphere beneath the southern altiplano

the andean orogenic period and its major structures in central and northern peru the andean orogenic period began in the albian and consisted of at least 6 discrete compressional phases which are albian to late miocene in age deformation migrated progressively toward the amazonian foredeep the major andean structures in central and northern peru are related to the eocene incaic early miocene quechua 1 and late miocene quechua 3 phases the maranon thrust and fold belt mtfb which bounds the western cordillera to the ne rose in the eocene it was reactivated partly by the quechua 1 phase which also folded the altiplano farther e and possibly part of the eastern cordillera the sub andean thrust and fold belt stfb is related to the quechua 3 phase in both mtfb and stfb the imbricate thrusts merge at depth in large decollement faults gravity sliding does not play a dominant role for these faults have to be rooted westward in the pre andean basement the corresponding crustal shortening played a significant part in the creation of the sialic root of the andes

microfracturing and regional stress field a study of the preferred orientations of fluid inclusion planes in a granite from the massif central france abstract a study of the deformation of a granitic massif indicates a relationship between palaeostress fields and the geometry of microfractures as defined by fluid inclusion trails that is healed microfractures a statistical study of the distribution of the trails leads to the following conclusions trails exhibit several distinct preferred orientations on the scale of a grain which may be observed in many samples the orientations of the trails are similar to those of micro and mesoscale fractures in the granite and the dominant trail direction is parallel to the main direction of regional shortening thus fluid inclusion trails are mode i cracks which can be used as excellent markers of palaeostress fields

fluid inclusion petrography abstract a procedure of fluid inclusion studies is proposed with emphasis on the criteria of selecting fluid inclusions for detailed microthermometry and spectroscopic analysis an overview of descriptive and genetic classifications of fluid inclusions in single crystals and in massive rocks is given with the intention of further differentiating the commonly used terms primary and secondary fluid inclusions some principles of fluid inclusion modification are explained cathodoluminescence cl studies of quartz with the optical high power cl microscope and the electron microprobe provided with a cl detector are an important help in fluid petrography cl textures are subdivided in primary growth textures and a wide variety of secondary microtextures which are in part induced by fluid inclusions the latter is grouped in textures indicative of local lower crystal order increasing defect structures and microtextures indicative of local quartz healing reduction of the defect structures microtextures showing the genetic relationship between fluid inclusions and the host mineral provide information about the possible post entrapment changes of fluid inclusions and therewith testify their geological relevance

the northern patagonia somuncura plateau basalts a product of slab induced shallow asthenospheric upwelling the meseta de somuncura forms the largest basaltic plateau 20 000 km2 of southern argentina extra andean domain most of these tholeiitic to alkaline rocks were extruded at 25 ma late oligocene the absence of rifting thinning processes plume activity or slab window phenomena leaves only one major possibility for the generation of somuncura asthenospheric oib like corner flow leading to a transient thermal anomaly above the subducting plate it is suggested herein that the intake of hot asthenosphere was forced into a favourable topography concave up of the subducting plate when a major plate reorganization event farallon to nazca was taking place in late oligocene to early miocene time the fast and vigorous intake of asthenosphere would have been induced by slab roll back leading to decoupling of the subducting plate the somuncura volcanic episode can be regarded as a marker of the passage from the extremely oblique subduction of farallon to the birth of the nazca plate and roughly perpendicular convergence between south america and nazca

tectonic and chemical implications of cathodoluminescent microstructures in quartz parry sound domain ontario canada cathodoluminescence cl imaging of quartz in orthogneiss of the parry sound domain grenville province ontario canada reveals four dark microstructures medium dark grains mantles sinuous lines and straight lines the parry sound domain experienced granulite facies deformation and metamorphism followed by variable degrees of amphibolite facies retrogression associated with the development of retrograde shear zones we integrate our observations of cl visible quartz microstructures with structures observable with optical and scanning transmission electron microscopy analyses of water species concentration and trace element distribution in quartz as well as with microstructures in plagioclase to determine their characteristics and elucidate their timing and mechanisms of formation relative to the deformational history medium dark grains are primarily located in and near kilometer scale shear zones at the margins of the parry sound domain have notably darker cores than most samples in the interio

deep seismic refraction and gravity crustal model and tectonic deformation in tocantins province central brazil abstract interpretation of seismic refraction data in the central sector of tocantins province central brazil has produced a seismic crustal model with well defined upper intermediate and lower crust layers having smooth velocity gradient in each layer the depths to moho vary from 32 to 43 km and mean crustal p velocity varies from 6 3 km s beneath goias magmatic arc on the western side to 6 4 km s below goias massif in the central portion and the foreland fold and thrust belt on the eastern side the behaviour of the lower crust layer allows an improved understanding of regional gravimetric features of the central and northern sectors of tocantins province and suggests subduction of the amazon plate in central brazil in the southeastern sector the refraction experiment resulted in the detection of a thinner crust 38 km below brasilia fold belt and a thicker crust 41 km below parana basin and sao francisco craton 42 km the upper crust beneath parana basin is around 20 km thick whereas it is less than 10 km thick below the craton these results bring new insights into the geological history of the central and southeastern sectors of tocantins province gravimetric measurements in the central sector of tocantins province delineate a high and a low anomaly separated by a steep gradient with a ne direction the axis of the gradient seems to bend still further to ne in the northern sector of that province whereas the gravimetric high continues northwards defining a separation between them this suggests that those features belong to different tectonic processes that occurred during tocantins province orogenesis the gravimetric model which incorporates seismically resolved structure beneath tocantins province better matches the observed gravimetric data although tectonic movements have only been monitored with high precision gps for short time interval 1999 2001 the results suggest observable deformations the main seismicity of central brazil the goias tocantins seismic belt seems to be spatially associated with the large gravimetric high anomaly and with the observed tectonic deformation

attenuation tomography in the western central andes a detailed insight into the structure of a magmatic arc high quality data from 1498 local earthquakes recorded by the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 and ancorp 96 andean continental research project 1996 temporary seismological networks allowed the detailed determination of the three dimensional 3 d attenuation structure qp 1 beneath the recent magmatic arc in the western central andes 20 to 24 s assuming a frequency independent qp 1 in a frequency band between 1 and 30 hz whole path attenuation t was estimated from the amplitude spectra of the p waves using spectral ratios and a spectral inversion technique the damped least squares inversion tomography of the data reveals a complex attenuation structure crust and mantle of the forearc and subducting slab are generally characterized by low attenuation qp 1000 crust and mantle beneath the magmatic arc show elevated attenuation the strongest anomaly of extremely low qp is found in the crust between 22 and 23 s beneath the recent volcanic arc qp 100 n s variations can be observed the western flank of the crustal attenuation anomaly follows the curved course of the volcanic front north of 21 s the attenuation is less developed in the northern part of the study area the low qp zone penetrates in the forearc mantle down to the subducting slab in the south a deeper zone of high attenuation is resolved between 23 and 24 s directly above the subducting slab low qp in the mantle correlates with earthquake clusters the strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts the attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction related hydration processes in the mantle wedge from slab derived fluids

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

compressional and transpressional stress pattern for pliocene and quaternary brittle deformation in fore arc and intra arc zones andes of central and southern chile kinematic analysis of fault slip data for stress determination was carried out on late miocene to quaternary rocks from the fore arc and intra arc regions of the chilean andes between 33 and 46 south latitudes studies of neogene and quaternary infilling the central depression as well as plutonic rocks of the north patagonian batholith along the liquine ofqui fault zone have revealed various compressional and or transpressional states of stress in the pliocene the maximum compressional stress 1 was generally oriented east west during the quaternary the deformation was partitioned into two coeval distinctive states of stress in the fore arc zone the state of stress was compressional with 1 oriented in a n s to nne ssw direction in the intra arc zone the state of stress was transpressional with 1 striking ne sw along the coast in one site 37 30 s the quaternary strain deformation is extensional with an e w direction which can be explained by a co seismic crustal bending readjustment

three dimensional density model of the nazca plate and the andean continental margin 1 we forward modeled the bouguer anomaly in a region encompassing the pacific ocean 85 w and the andean margin 60 w between northern peru 5 s and patagonia 45 s the three dimensional density model that reproduces the gravity field is a continental scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic nazca plate subducted slab and continental margin with a minimum number of bodies we predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure temperature conditions appropriate for the andean setting a database of independent geophysical information constrains the geometry of the top of the subducted slab locally the moho of the oceanic and continental crusts and indirectly the lithosphere asthenosphere boundary underneath the continental plate other boundaries notably the intracrustal density discontinuity separating upper from lower crust below the continent were not constrained and their geometry is the result of fitting the observed and calculated bouguer anomaly during forward modeling this contribution presents the model to the andean geoscientific community and contains some tools like a sensitivity analysis that helps potential users of the model to interpret its results we describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena e g modification of oceanic plate structure by hot spots shape of the subducted slab thermal structure of the continental lithosphere compensation mechanism and formation of orogenic relieve causes of andean segmentation

seismic gravity and petrological evidence for partial melt beneath the thickened central andean crust 21 23 s on the basis of seismic refraction investigations and gravimetric data we have modelled the crustal structure of the southern central andes 21 23 s a pronounced variation in crustal parameters is seen in n s and w e crossing seismic profiles over the entire andean orogene characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc strongly reduced seismic velocities and a bouguer minimum of 450 mgal anomalously low velocities of 5 9 6 0 km s in the deeper crust of the western cordillera and altiplano regions lead to an over compensation of the bouguer minima resulting in values of crustal densities higher than estimates based purely on seismic velocity measurements in an attempt to reconcile these differences the behavior of crystalline rocks based on published laboratory data was studied under varying pressure and temperature conditions up to the range of partial melting if the temperature is increased above the melting point a rapid decrease in seismic velocity is accompanied by a slow decrease in density for the central andes a good fit of the observed and calculated bouguer anomalies is obtained if the densities of the rocks from the low velocity zone lvz beneath the western cordillera and the altiplano are varied model calculations lead to a velocity density relation for partial molten rocks that allows the melt proportions of rocks to be estimated model calculations indicate that 15 20 vol of basaltic to andesitic melt at depth is necessary to explain the lvz and bouguer anomaly beneath the arc and parts of the backarc high heat flow values 100 mw m2 support the idea that large areas of the deeper andean crust are strongly weakened by the presence of partially molten rocks resulting in reduced seismic velocities with the western cordillera the active volcanic arc of the andean mountain range acting as a ductile buffer between the two more rigid crustal blocks of the forearc and backarc regions

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

evidence for exhumation of a granite intrusion in a regional extensional stress regime based on coupled microstructural and fluid inclusion plane studies an example from the velence mts hungary abstract a new approach the fluid inclusion plane technique coupled with fluid inclusion microthermometry and field measurements have been applied to demonstrate the exhumation and relative vertical displacements of an allochthonous permian granite intrusion velence mts which was situated in the middle triassic at the western end of the neotethys rift system the pressure and the temperature conditions during the permian fluid flow 350 c and 2 kbar in the granite were considerably higher than during the triassic fluid flow it is suggested that the exhumation of the velence mts along with the regional fluid flow can be connected to the passive pre rift phase of the northern adriatic block during the middle late triassic the variations in homogenization temperatures of triassic regional fluid inclusion assemblages between the blocks of the granite as well as variations in the fluid inclusion plane orientations suggest a post triassic vertical segmentation and relative block rotation of the granite

a 3 d geodynamic model of lateral crustal flow during andean mountain building although the andes are believed to have resulted mainly from crustal shortening the shortening history remains debated and appears to require lateral along strike crustal flow three dimensional viscous flow modeling shows that within geological uncertainties the andes may have been produced by either neogene shortening alone or with significant pre neogene shortening these scenarios require major along strike crustal flow and predict significantly different histories of uplift and crustal motion

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting