do transcurrent faults guide volcano growth the case of nw bicol volcanic arc luzon philippines this paper shows that although volcanoes can be aligned along transcurrent faults their growth can in reality be controlled by secondary structures that have a different orientation and kinematics assessing the relationships between local structures and volcano growth can be very useful for the evaluation of volcanic hazards and for economic purposes such as geothermal and hydrogeological exploration in the north western bicol volcanic arc luzon philippines the quaternary labo and caayunan volcanoes are aligned with the nw striking transcurrent philippine fault system they are exceptional examples of composite volcanoes mostly comprising lava domes where regional tectonic structures and hydraulic radial fracturing have alternated to control volcano growth field data and analogue modelling demonstrate that apical depressions affecting the domes reflect the geometry of magma feeding fractures these data together with alignments of domes and hot springs and other structural information suggest that magma rose at depth along the nw striking transcurrent faults but that in the uppermost crust magma rose along ne striking fractures parallel to the greatest principal stress

late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

estimates of modern arc parallel strain rates in fore arcs deflections of slip vectors of interplate thrust earthquakes from expected directions are used to estimate arc parallel strain rates within the overriding plates at the world9s major convergent plate margins arc parallel extension strain rates between 10 8 yr and 10 7 yr and significant at 2 standard deviations are observed in the fore arcs of the aegean aleutian mariana sumatran southern kuriles new hebrides scotia and southern central american lat 8 to 12 n subduction zones and the himalayas northern chile lat 17 to 31 s and central america lat 11 to 18 n show arc parallel compression available geologic and geodetic estimates of fore arc slip and strain rates agree within a factor of two with slip vector estimates arc parallel strain in fore arcs is rapid enough to produce geologically significant effects such as unroofing of high grade metamorphic rocks and disruption of transported fore arc terranes fore arcs deform even where convergence is perpendicular to curved margins demonstrating that head on subduction can produce a three dimensional strain field

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

wave reflection on natural beaches an equilibrium beach profile model waves are the most energetic phenomena that control beach morphology the beach profile mostly depends on the way in which the incident wave energy distributes along the profile dissipation and reflection being the main mechanisms while the dissipation phenomena have been widely studied the effect of wave reflection on the beach profile has attracted much less attention and is still poorly known in order to evaluate its importance a new equilibrium profile model that includes reflection is proposed the model is based on a two section profile scheme largely corresponding to the surf and shoaling dominated zones of the beach profile the obtained formulations are represented by the expression of two terms one of the terms accounts for the dissipation effect and coincides with the dean profile the other term integrates the reflection process the model and its coefficients have been calibrated using measured profiles along the spanish coast the validation shows a significant improvement of the fitting parameters with respect to the most popular equilibrium profiles model moreover additional empirical expressions that relate morphology and hydrodynamic in the equilibrium profile model are also presented in this study as a novel contribution to this topic

quaternary transfer faulting in the taiwan foothills evidence from a multisource approach abstract the major structures of the western foothills of taiwan mainly consist of nne ssw trending folds and imbricated west vergent thrust systems the additional occurrence of n140 e striking oblique structures was revealed through a multisource approach involving a digital elevation model dem a study of drainage network anomalies aerial photographs side looking airborne radar slar images and spot p and landsat images these structures are described from north to south based on new field analyses including stratigraphy and tectonics studies they are also compared to seismic data and geodetic reconstruction in order to evaluate their present day activity these n140 e major morphostructures are interpreted as left lateral transfer fault zones either inherited from the eurasian passive margin and or newly formed in the cover in response to the presence of basement highs within the foreland basin peikang and kuanyin highs the sanyi and the chishan transfer fault zones display a high seismic activity the distribution of earthquakes and the related focal mechanisms confirm the left lateral movement along n140 e directions the chiayi chishan and fengshan fault zones act presently as transfer fault zones as indicated by gps data the associated n70 e to n100 e trending faults result from the reactivation of normal faults of the eurasian passive margin as right lateral strike slip faults in the foothills during the plio quaternary collision in taiwan we conclude that multisource and multiscale geomorphic studies combined with tectonic analysis in the field yield a significant contribution to the understanding of the structural and kinematic development of the western foothills at the front of the taiwan collision belt

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

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

the configuration of equilibrium beach profile in south china abstract to understand the evolution and stability of beaches prediction of the equilibrium beach profile ebp is theoretically and practically important in the present paper a new equation h ae bx c is developed to predict the change in beach profile for sections above the water level and the adjacent nearshore portions moreover fractal analysis is applied to predict types of ebp for the first time using the field data collected from liao zuikou and nanwan beaches south china three types of ebp termed upward concave ebp u ebp downward concave ebp d ebp and medium characteristic ebp m ebp are given for the studied region of south china

contrasting nature of deformation along an intra arc shear zone the liquine ofqui fault zone southern chilean andes abstract the liquine ofquifault zone lofz cuts the patagonian batholith and the modern volcanic arc of southern chile for ca 1000 km the rock fabric along three transects of the lofz combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike in the liquine transect 39 s a 1 km wide northeast striking subvertical mylonitic zone shows north plunging stretching lineations this mylonitic zone has been juxtaposed by high angle reverse faulting against a nearly undeformed miocene granitoid metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement deformation pre dates a 100 2 ma undeformed porphyritic dike hornblende 40ar 39ar mean age in the reloncavi transect 41 42 s deformation in cretaceous and miocene plutons is predominantly brittle kinematic analysis of two fault populations yields compressional and dextral strike slip stress regimes interpreted as late miocene in age in the hornopiren transect 42 43 s a 4 km wide mylonitic zone developed in plutons and wallrocks shows subhorizontal stretching lineations and dextral displacement a single fault population overprinting the mylonites supports a dextral strike slip stress regime available u pb k ar and 40ar 39ar dates on deformed cenozoic plutons and wallrock range from 9 to 13 ma on hornblende and from 6 to 3 ma on biotite microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies similar to the biotite ar closure temperature subparallel magmatic and solid state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons dextral oblique subduction of the nazca plate beneath western south america has driven long term intra arc deformation at the southern andes plate boundary zone ridge collision in turn has favored dextral displacement at the leading edge of the continent since the pliocene

postmagmatic cooling and late cenozoic denudation of the north patagonian batholith in the los lagos region of chile 41 42 15 s zircon and apatite fission track ft thermochronology was applied to investigate the history of cooling and denudation of the southern andes between 41 and 42 15 s in relation to the late cenozoic activity of the liquine ofqui fault zone lofz and the northward migration of the chile triple junction ctj fifty six zircon and 51 apatite ft ages plus 37 apatite confined track length distributions were obtained mainly from plutonic rocks of the north patagonian batholith npb in the main andean cordillera apatite ft ages and track lengths indicate a stage of rapid cooling at 5 3 ma along both sides of the lofz whereas older miocene ages with monotonous cooling histories were obtained further away from the fault zircon ft ages range from cretaceous to pliocene with marked differences observed along and across the lofz three different types of temperature time histories characterise the post magmatic cooling of the npb in the region deep intrusions with moderate and steady cooling rates intrusions in the upper crust with very slow cooling rates following a stage of initial rapid cooling and rapidly cooled and exhumed shallow intrusions the latter with younger ages towards the fault zone the most prominent denudation episode along the lofz is late miocene to pliocene coeval with plate tectonic reconstructions for the arrival and subduction of the chile rise beneath the taitao peninsula

drainage network analysis and tectonics of mindanao southern philippines abstract mindanao southern philippines is the site of an active arc continent collision which began in the late pliocene the principal tectonic features are faults and blind folds and duplexes which are not easily detectable in this island on which 75 of the surface is covered by pleistocene and holocene sediments and volcanics a detailed study of the drainage pattern and its discontinuities in the agusan davao and the cotabato basins when compared with field data radar data and subsurface structures observed on seismic documents enables a correlation of distant seismic lines and features existing on the basin edges the drainage network analysis based on description and interpretation of drainage anomalies and frequency allows us to distinguish 1 subhomogenous drainage areas which correspond to areas bearing consistent geological characteristics and 2 drainage anomalies the results have been checked in the field and compared with the tectonic features in the mountainous areas the extension of these structures could then be traced confidently further towards the flat lowlands we point out the presence of two distinct anomaly directions that intersect in eastern mindanao 1 numerous nnw anomalies in the intra arc basin of agusan davao and both pacific and central cordilleras appear as young features in the morphology and are due to strike slip faults of the arc continent collision some of the faults have proven to be reactivated middle miocene faults related to the initial rifting of the agusan davao basin 2 n s anomaly directions are transpressive ridges due to the collision except one anomaly which corresponds to the trace of the philippine fault zone and connects with the collisional features in contrast the cotabato basin in western mindanao lies between two terranes of different nature in the cotabato basin area the nw and nnw anomaly directions are also present and similarly correspond to strike slip faults and thrust related folds respectively pull apart basins occur in places where sagging is evidenced an additional set of anomalies trends ene and corresponds to the active extensional field below the quaternary sediments some of the en echelon features corresponding to blind flat and ramps structures were identified these observations provide reference examples for the evolution of basins generated within the compressive and strike slip setting of accreted belts

longshore sediment transport on non planar beaches abstract for a concave up 2 3 power bruun beach profile the following two energetics based sediment transport models are developed 1 a bagnold type model and 2 a combined wave current stress model the stress model is calibrated with the bagnold model using observed transport rates on planar beaches the sediment transport profiles for the two models are in agreement within the surf zone for the planar beach case but the stress model is also applied seaward of the breaker line where the bagnold model is not a mean swash transport of sand is predicted by the bagnold model for a 1 2 power least squares approximation to total depth including setup setdown on a bruun beach profile the total longshore transport of sand is determined for each transport model as a function of the turbulent lateral mixing strength the total sand transport is found to be less on a concave up beach profile than for the corresponding planar beach case

the southern andes between 36 and 40 s latitude seismicity and average seismic velocities abstract the project issa 2000 integrated seismological experiment in the southern andes consists of a temporary seismological network and a seismic refraction profile a network of 62 seismological stations was deployed across the southern andes at 38 s three hundred thirty three local seismic events were observed in a 3 month period p and s arrival times of a subset of high quality data were inverted simultaneously for 1 d velocity structure hypocentral coordinates and station delays seismic refraction data along a transect at 39 s provide further constraints on the crustal structure low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere the continental moho is not clearly observed in this region average velocities of the crust beneath the arc are higher than those beneath the forearc crustal thickness is about 40 km crustal seismicity concentrates in the forearc region along the bio bio and gastre fault zones the area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the benioff zone in the southern andes a maximum of seismicity at 60 km depth may be caused by dehydration embrittlement

new methodology for describing the equilibrium beach profile applied to the valencia s beaches abstract mathematical models used for the understanding of coastal seabed morphology play a key role in beach nourishment projects these projects have become the fundamental strategy for coastal maintenance during the last few years accordingly the accuracy of these models is vital to optimize the costs of coastal regeneration projects planning of such interventions requires methodologies that do not generate uncertainties in their interpretation a study and comparison of mathematical simulation models of the coastline is carried out in this paper as well as elements that are part of the model that are a source of uncertainty the equilibrium profile ep and the offshore limit corresponding to the depth of closure doc have been analyzed taking into account different timescale ranges the results have thus been compared using data sets from three different periods which are identified as present past and future accuracy in data collection for the beach profiles and the definition of the median grain size calculation using collected samples are the two main factors that have been taken into account in this paper these data can generate high uncertainties and can produce a lack of accuracy in nourishment projects together they can generate excessive costs due to possible excess or shortage of sand used for the nourishment the main goal of this paper is the development of a new methodology to increase the accuracy of the existing equilibrium beach profile models providing an improvement to the inputs used in such models and in the fitting of the formulae used to obtain seabed shape this new methodology has been applied and tested on valencia s beaches

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

fractal analysis of shoreline patterns for crenulate bay beaches southern china shoreline patterns of 34 crenulate bay beaches in southern china were investigated the differentiated criteria for log spiral shoreline curves offered by silvester am soc civ eng proc 96 1970 275 coastal engineering elsevier amsterdam 1974 338 and hsu et al j waterw port coast eng 115 1989 285 were used to determine the equilibrium morphology of each beach results show that the discriminated principles of the equilibrium log spiral curve were not suitable for all the beaches based on the theory of the log spiral stable curve and beach morphology and the dynamic action setting in southern china fractal analysis was adopted to classify the patterns in the shorelines it shows that shorelines include three modes dynamic balance of accretion static equilibrium and the dynamic balance of erosion

space geodetic estimation of the nazca south america euler vector abstract gps data from four sites in the nazca plate easter island galapagos robinson crusoe and san felix islands and from five sites in the stable core of the south american plate enabled us to estimate the euler vector of the nazca plate with respect to south america the observed velocities of easter island 6 6 cm yr at 102 3 galapagos 5 1 cm yr at 90 0 robinson crusoe 6 6 cm yr at 80 1 and san felix 6 0 cm yr at 82 1 are significantly slower than the global plate model nuvel 1a velocites for those four sites our estimated euler pole is located at 48 8 n 91 7 w with a rate of 0 59 m y the nuvel 1a and earlier space geodetic studies give rotation rates that are 20 faster

seismological aspects of the 1988 1989 lonquimay chile volcanic eruption abstract more than 1600 earthquakes were recorded in a four month period on portable seismographic stations installed in the lonquimay area immediately after the initiation of the volcanic eruption of december 25 1988 hypocentral parameters were calculated on a subset of 150 events showing clear p and s arrivals seismic activity was restricted to a north south elongated region that included the main crater of the lonquimay volcano the new navidad crater lies along the eastern margin of the area of seismicity most of the hypocenters are located at depths shallower than 6 km and none have depths exceeding 10 km the largest earthquakes of the sequence took place on dec 28 m 4 7 jan 12 13 m 4 8 and 5 0 and feb 24 m 5 3 the number of events per day varied it peaked on dec 27 with more than 260 events and more than 200 events were recorded on jan 13 both of which corresponded to the days of large earthquakes the relation of large to small events given by the b value of the magnitude frequency distribution log n a bm is typical of volcanic sequences b 1 7 and much larger than the equivalent for tectonic environments the rate of decay of the number of aftershocks of each large earthquake is proportional to t p with p increasing from 0 63 to 1 08 as the eruption progressed in time initial low values of p indicate a slow stress release in the region larger absolute values of p are comparable with tectonic environments and might be indicators of the last stages of the eruption process the total seismic energy release is 9 6 10 19 ergs about five orders of magnitude less than the estimated thermal energy the spatial distribution of earthquakes suggests that the sources of stress change associated with the eruption are located under and possibly to the south of the main crater of the lonquimay volcano and that the evacuation channels of magma occupied a zone of weakness in the region of the new crater

wave setup setdown and longshore current on non planar beaches abstract wave induced setup setdown and longshore currents are examined theoretically for non planar concave up beaches a beach profile in which the still water depth is proportional to the horizontal distance offshore raised to the 2 3 power is examined in detail the total mean water depth which includes the sum of the still water depth for this 2 3 power beach profile plus the wave induced setup setdown may be approximated shoreward of the breaker line in a best least squares sense by a profile that is proportional to the horizontal distance offshore raised to the 1 2 power the longshore current profile for this non planar beach is found to differ significantly from that predicted analytically for a planar beach the longshore current velocity does not vanish at the shoreline as in the planar beach case in addition the peak velocities and the total longshore transport of water are found to be less than for the corresponding planar beach cases for a given concave up beach profile the influence of the lateral mixing increases as the wave height decreases

lidar based detection of the post typhoon recovery of a meso macro tidal beach in the beibu gulf china abstract because of global sea level rising with frequent storm activities most beaches in the world experience widespread erosion which poses a significant hazard to beach management the morphodynamic process of the evolution of a beach in response to storm activities is of increasing concern using a terrestrial laser scanner the topography of yintan beach was continuously observed for 33 days with a 25 cm spatial resolution digital elevation model dem of that covered an area of 15 ha before and after typhoon rammasun the short term beach recovery was explored using a comprehensive approach which included grey relational analysis terrain analysis gaussian fitting gamma fitting and delft3d wave simulation methods the results show that the yintan beach recovery process can be divided into three stages with two transition points stage one began at the end of rammasun which indicated a weakly stable state with an average daily net erosion of 588 m 3 a mean beach slope that ranged from 0 96 to 1 28 and a landform with no obvious alterations after the vast accretion of 6874 m 3 of first transition stage two which was characterized by oscillations occurred in the region with an average daily net erosion of 396 m 3 and a conspicuous beach slope reconstruction in the range of 0 94 2 16 during this stage sandbar reconstruction played a key role the second transition event arose at the end of stage two with a vast accretion of 5762 m 3 afterwards the daily net accretion of 200 m 3 and the beach slope range of 0 93 1 74 dominated the region during stage three and the beach surface became similar to that of the pre rammasun within the entire observation period intense beach elevation changes that ranged from 15 cm to 6 cm and 6 cm to 15 cm mainly occurred in the sandbar and foreshore zones respectively additionally slight elevation changes ranging from 4 cm to 4 cm were densely distributed in the backshore and dunes and moderate elevation changes ranging from 6 cm to 4 cm and 4 cm to 6 cm emerged along the entire beachface it was concluded that the spatial distribution of the bottom shear stress that was induced by wave action was responsible for the reconstruction of the foreshore and sandbar after rammasun whereas the short term beach recovery process was affected by the beach states in various wave breaking conditions

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

stress permutations three dimensional distinct element analysis accounts for a common phenomenon in brittle tectonics 1 using three dimensional 3 d distinct element modeling we explored a variety of simulations to characterize and interpret the stress permutations in brittle tectonics stress inversions of fault slip data or earthquake focal mechanisms often revealed such permutations the main aim of our study is to produce simple mechanically consistent 3 d models that account for these switches between the principal stress axes 1 2 or 2 3 even with simple boundary conditions the stress changes induced by variations in rheology are large enough to modify the local tectonic behavior and to produce permutations of principal stress axes rather than simple directional changes of stress axes which exist but often remain limited the relative variations in principal stress magnitudes are the major cause of permutations 1 2 and 2 3 in nature permutations being left apart the orientations of axes often remain tightly clustered in our experiments we adopted a ratio 2 3 1 3 of 0 5 which makes permutations difficult low favors 2 3 permutations high favors 1 2 permutations and we explored a variety of tectonic situations involving compression extension and strike slip our experiments indicate that the major causes of stress permutations are the heterogeneity of the brittle deformation e g intact rock massifs between heavily faulted deformation zones and the anisotropy of the mechanical properties that results from fracturing and faulting which concur to modify the mechanical balance inside the analyzed volume and to produce stress permutations our models show that contrasts and anisotropy in rock properties favor stress permutations of major importance is the existence of relatively resistant zones at tips of the deformed zones acting as channels where stress concentrates and switches occur because in nature such zones move in time and space it is not surprising that stress permutations are so pervasive

paleomagnetism of the north patagonian batholith southern chile an exercise in shape analysis abstract over 100 sites in granitic country rock mafic inclusions within granitic rock and mafic dikes were sampled in the northernmost 550 km of the patagonian batholith after magnetic cleaning some sites were rejected as magnetically unstable and others were combined because they were contiguous and carried similar directions hence probably representing duplicate samplings of the same direction of the geomagnetic field a total of 42 well determined directions 29 from granitic country rock and inclusions and 13 from dikes survived to be included in the interpretation most of these had a normal magnetic polarity probably indicating that a disproportionate fraction of the north patagonan batholith formed during the mid cretaceous long normal interval tectonic interpretation of paleomagnetic directions from intrusive rocks often is difficult because paleohorizontal indicators in such rocks are usually lacking rather than struggle against this limitation we yielded to it and elected to concentrate on the shapes of our various data sets the method used is simple and straightforward involving bingham statistics and small circle fitting from this analysis it appears that the main structural feature within the north patagonian batholith npb the liquine ofqui fault zone is a site of crustal detachment a sliver of south america west of the fault is detached from the rest of the continent and has moved relatively northward because the main strands of the liquine ofqui fault are curved and concave to the west northward movement was accompanied by counterclockwise rotation east of the fault the npb appears to have been the locus of distributed shear mainly dextral the principal period of deformation of the batholith appears to have been mid or late cenozoic and may have been initiated by the arrival of the south america nazca farallon phoenix aluk triple junction

morphology of pyroclastic cones and tectonics the relationships between morphology and spatial distribution of 1315 quaternary pyroclastic cones and coeval faulting of the volcanic substrate are analyzed in the following regions with different structural settings tepic rift mexico ethiopian rift mexican volcanic belt canary archipelago and mount etna field data and analog experiments of tephra cone emplacement and collapse enable the definition of a number of parameters which can be used to infer the geometry of the fracture feeding the magma to a pyroclastic cone the strike of the feeding plane is directly related to 1 the elongation of cone base and crater 2 the location of depressions on the crater rim and 3 the alignment of pyroclastic cones in relation to a given vent spacing in addition the strike and dip of faults affect the direction of cone breaching these relationships are valid for volcanic substrate topographic surfaces with an inclination of less than 9 and are especially sensitive to fault escarpment and cone height lava and cone density and fault orientation with respect to the dip of the volcanic substrate topography relations 1 and 2 become more pronounced for regions undergoing extensional tectonics where edifices also have a larger dimension whereas breaching in the direction of the fault dip is more widespread in regions under extension breaching along the fault strike as well as the coincidence between fault strike and vent alignment are more frequent in regions with transcurrent or transtensional tectonics

deformation partitioning in flat subduction setting case of the andean foreland of western argentina 28 s 33 s 1 the andean foreland of western argentina 28 s 33 s corresponds to back arc deformations associated with the ongoing flat subduction of the nazca plate beneath the south american lithosphere in this paper short and long term tectonic regimes for this andean region are investigated and compared to nuvel 1 and the new gps derived velocity field for the andes mountains from this comparison it appears that 1 the slight convergence obliquity at the plate boundary is accommodated by the subduction zone itself precluding any deformation partitioning at lithospheric scale 2 clockwise rotation of 1 and p axes within the andean foreland of western argentina suggests that plio quaternary deformation partitioning may occur in the upper plate at crustal scale and 3 comparison of seismic rates of shallow deformations determined from moderate to large earthquakes localized between 22 s and 36 s shows that the amount of shortening is about the same 2 4 cm yr 1 between the forearcs of the 30 dipping slab segments and the back arc of the flat segment all together those observations are in close agreement with the idea that interplate coupling in flat subduction settings may play a key role in the present day transfer of plate boundary forces consequently the andean back arc of western argentina might be regarded as an obliquely converging foreland where plio quaternary deformations are partitioned between strike slip and thrust motions that are localized in the east verging thin skinned argentine precordillera and west verging thick skinned pampean ranges respectively

shorerise and bar berm profiles on ocean beaches a beach equilibrium model is developed that treats the outer shorerise portion of the profile independently from that of the inner bar berm portion the two portions are matched at the breakpoint bar the partitioning of the profile in this way is consistent with the different forcing modes on either side of the breakpoint this formulation utilizes beach profile data not previously available it is shown that both portions of the profile are well fitted by curves of the form h axm where h is positive downward and x is the positive offshore coordinate surprisingly the value of m 0 4 is nearly the same for shorerise and bar berm and does not change significantly with seasonal beach changes summer winter the principal difference between seasonal profiles is that in winter higher waves the breakpoint bar is deeper and farther offshore while the berm crest is displaced landward thus the changes in seasonal equilibria are manifested by simple self similar displacements of the bar berm and shorerise curves as a consequence of changes in surf zone width and o 1 variations in the factor a

kinematic constraints on intra arc shear and strain partitioning in the southern andes between 38 s and 42 s latitude 1 on the basis of structural field work fault kinematic analysis and the analysis of digital imagery we describe the northern part of the southern andean intra arc liquine ofqui fault zone lofz as an sc like fault zone system accommodating part of the nazca south american plate convergence obliquity kinematic modeling suggests that the lofz accommodated 124 24 21 km of dextral displacement between 40 s and 42 s and 67 13 11 km between 38 s and 40 s since the pliocene associated vertical axis rotations are 31 4 clockwise and 9 1 counterclockwise along synthetic and antithetic faults respectively mean pliocene to recent shear rates along the lofz decrease northward from 32 6 mm yr to 13 3 mm yr compatible with partitioning of half of the convergence obliquity into the intra arc zone north of 40 s and complete partitioning to the south the displacement gradient along the intra arc zone results in margin parallel shortening of the fore arc

equilibrium beach profiles under breaking and non breaking waves abstract simple theoretical models to determine the equilibrium profile shape under breaking and non breaking waves are presented for the case of breaking waves it is assumed that the seaward transport in the undertow is locally balanced by a net vertical sedimentation so that no bottom changes occur at equilibrium the parameterization of the water and sediment flux in the surf zone yields a power curve for the equilibrium profile with a power of 2 3 which is in agreement with previous field investigations on surf zone profile shapes three different models were developed to derive the profile shape under non breaking waves namely 1 a variational formulation where the wave energy dissipation in the bottom boundary layer is minimized over the part of the profile affected by non breaking waves 2 an integration of a small scale sediment transport formula over a wave period where the slope conditions that yield zero net transport determine equilibrium and 3 a conceptual formulation of mechanisms for onshore and offshore sediment transport where a balance between the mechanisms defines equilibrium conditions all three models produced equilibrium profile shapes of power type with the power typically in the range 0 15 0 30 comparison with field data supported the results obtained indicating different powers for the equilibrium profile shape under breaking and non breaking waves

paleomagnetism of the patagonian plateau basalts southern chile and argentina a total of 505 paleomagnetic samples were collected from 65 sites volcanic flows of the patagonian plateau basalts at four locations in southern chile and argentina k ar analyses indicate that 38 flows from two locations form a late cretaceous group 64 79 ma while the remaining 27 flows are eocene in age 42 56 ma progressive demagnetization indicates that a characteristic remanent magnetization chrm has been successfully isolated from 59 flows rock magnetic properties and analysis of chrm directions within and between sites allow the secure inference that the chrm is a thermoremanent magnetization acquired during original cooling the eocene flows yield 15 independent samplings of the paleomagnetic field however these data fail the reversals test and probably do not adequately average geomagnetic secular variation the late cretaceous flows yield 18 independent samplings of the paleomagnetic field six of normal polarity and 12 of reversed polarity these data pass the reversals test and have angular dispersion of virtual geomagnetic poles vgps consistent with adequate sampling of geomagnetic secular variation the mean of these 18 vgps yields a late cretaceous paleomagnetic pole for south america latitude 78 7 s longitude 358 4 e a95 6 3 k 31 6 s 14 5 this pole is consistent with similar age poles from north america and africa when the atlantic is reconstructed to 70 ma contrary to previous indications the south american apparent polar wander path does not reach the present rotation axis until after late cretaceous time

characterising the relaxation distance of nearshore submarine morphology a southern baltic sea case study abstract a method for defining the length of coast with alongshore uniformity of nearshore submarine morphology is required in many shore normal nearshore profile modelling approaches here we introduce a new variable the relaxation distance to describe the alongshore spacing between two adjacent shore normal profiles characterised by distinct morphologies within this distance alongshore morphological changes are too small to distinguish so the nearshore submarine morphology is regarded as uniform along the coast we apply a semi variogram approach which expresses decreased spatial correlation of the nearshore submarine morphology with alongshore distance to quantify relaxation distances we apply this morphological variable to analyse 225 measured shore normal submarine profiles at 500 m intervals in three distinct sites of the tideless wave dominated southern baltic coast swina eba and hel the relaxation distances of these three sites are approximately 4500 m 3000 m and 2200 m of the three sites swina has the smoothest nearshore submarine morphology and the weakest wave energy the relaxation distances of different quasi straight segments at each site appear similar but those at different sites decrease with enlarged morphological perturbations which are residuals resulting from detrending the shore normal profiles the standard deviations of these morphological perturbations are correlated with the incidence wave angles and wave energy we also found that large perturbations such as longshore bars and channels have a significant impact on the estimation of underwater volume using shore normal profile trends therefore relaxation distance can indicate morphological smoothness the length of shore parallel uniform nearshore submarine morphology and the impact of local processes on submarine morphology

do storms cause long term beach erosion along the u s east barrier coast abstract in a few hours or days scores of meters of beach width can be lost due to a severe storm however newly available shoreline data from the u s east coast show that beaches recover after storms to positions consistent with their long term 100 yr trend even the largest storms such as the ash wednesday storm of 1962 considered to be the most damaging in the twentieth century appear to have had little effect on the long term trend the gradual recession of beaches along the u s east coast is mainly controlled by other factors such as sea level rise and variations of sediment supply therefore it follows that barrier beaches in a coastal plain setting would not experience long term erosion in response to storm impact if the sea were to stop rising and sediment supply did not change

connections between the stratospheric quasi biennial oscillation and tropospheric circulation over asia in northern autumn 1 we investigate the variations in tropospheric circulation over asia associated with the stratospheric quasi biennial oscillation qbo during northern hemisphere nh autumn september november using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis and ncep department of energy doe reanalysis ii data sets for a 25 year period 1980 2004 the anomaly fields in this study are expressed as the easterly years minus the westerly years of the qbo the zonal wind distribution at 200 hpa indicates easterly anomalies at low latitudes from india through taiwan statistically significant westerly anomalies at midlatitudes including the tibetan plateau to northern china and significant easterly anomalies in the high latitude eurasia north of 50 n a momentum balance analysis indicates that these anomalies over asia are attributable to atmospheric waves and residual mean meridional flow significant northward and descending anomalies are observed in the lower stratosphere over asia with detectable signals in the troposphere around tibet the adiabatic heating due to descending motions in the region centering on india and bangladesh is consistent with the distribution of rainfall anomalies the formation of zonal wind anomalies over south asia seems to be associated with not only wave activities but also adiabatic heating and thermal advection we suggest that the qbo may be linked to tropospheric circulation over asia through wave activities and convective activities in autumn and the wave activities in the low latitudes associated with the qbo seem to be connected with the rainfall distribution over the asian monsoon region

beach cusps as self organized patterns computer simulations of flow and sediment transport in the swash zone on a beach demonstrate that a model that couples local flow acceleration and alongshore surface gradient is sufficient to produce uniformly spaced beach cusps the characteristics of the simulated cusps and the conditions under which they form are in reasonable agreement with observations of natural cusps the self organization mechanism in the model is incompatible with an accepted model in which standing alongshore waves drive the regular pattern of erosion and deposition that gives rise to beach cusps because the models make similar predictions it is concluded that currently available observational data are insufficient for discrimination between them

formation of two section cross shore profile under joint influence of random short waves and groups of long waves abstract we show that beach profiles may develop a two section almost equilibrium structure under joint impact of short wind waves and groups of long waves with duration of a few minutes and properties that are independent of wind waves the upper section of the profile is convex and follows the power law h x4 3 at small depths and in the swash zone wind waves may cause either erosion or accretion of this section but groups of long waves effectively redistribute the sediment over the convex section and practically do not alter the sediment volume the ability of short groups of highly energetic waves to regularly build up such profiles from those created by random sea waves is a principally new feature of wave coastal interaction that may have extensive consequences on the estimates of the wave induced coastal hazards

are s c structures duplexes and conjugate shear zones different manifestations of the same scale invariant phenomenon abstract by measuring s spacing c spacing and the s c angle in deformed rocks this paper investigates the geometry of previously published examples of s c and s c like structures on a scale range between micrometres and several hundred kilometres the results indicate that common s c fabrics of thin section hand specimen and outcrop scale and conjugate fault mylonite zones of map scale define a simple function c spacing 2 s spacing which depicts a scale invariant geometry over ten orders of magnitude logarithmic plots of cumulative frequency suggest that the s c fractal set d 0 13 is restricted to the scale range between 600 800 m and 1 km where genuine s c structures characterized by antithetic shear on the s planes can be formed below 600 800 m grain scale processes seem to influence the development of s c structures above the upper limit 1 km only s c like structures with duplex kinematics synthetic shear on s planes occur the s c and s c c fractals are envisaged as self similar structures where the foliations work as both s or c planes depending on which scale is considered

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

on the nature of buttressing in margin parallel strike slip fault systems margin parallel strike slip faults commonly develop along convergent plate margins where an oceanic plate subducts obliquely beneath a continental plate such faults detach a sliver of continental crust from the leading edge of the overriding plate fault motion then causes the sliver to move parallel to the continental margin in the direction of the transverse component of relative plate motion however few slivers move as rapidly as predicted by simple analyses of the forces acting on them this discrepancy must be due principally to the buttress effect resistance to displacement of the sliver arising from a space problem at its leading edge more simply a sliver can move only if it has somewhere to go buttresses can be geometrical as when the continental margin changes trend abruptly or they can arise from changes in the physical properties of the overriding plate or in other factors such as subduction angle and angle of obliquity that help determine whether sliver motion will occur a strongly buttressed sliver will move only insofar as the buttress can be overcome obvious ways in which a buttress can be overcome are by thickening the crust or widening the sliver examples of both are known crustal thickening entails elevation increase sliver motion is opposed by both gravity and friction widening the sliver is a more complex process that results in areas of extension and compression forming within the sliver as well as a characteristic pattern of overlapping curved fault segments this latter response may be the cause of curvature in many strike slip fault systems

investigation of active faults in morocco using morphometric methods and drainage pattern analysis a survey has been done of recently active faults in morocco in addition to field observations morphometric methods were applied and an analysis was made of the drainage pattern in the morphometric analysis a map of vertical deformation was established from the elevations of summit level surfaces this enables the recognition of neotectonic discontinuities and lithological contrasts the actual discrimination of different rock types can only be achieved by comparison with field observations the analysis of drainage patterns including their perturbations and anomalies can also reveal the presence of features due to neotectonics structural control or lithological variation certain faults can be detected which correspond to structures known from field geology or remote sensing ne sw and nw se oriented sets some new faults are observed which belong to these sets as well as others which define a new n s trending set the present analysis leads to a distinction between ancient faults and currently active faults the results of this study are shown on a map of recently active faults where the respective vertical displacements are also indicated

extended equilibrium beach profiles abstract the idea of an equilibrium beach profile has been a useful concept in both theoretical and practical coastal engineering studies in essence the subject has thus far evolved to a choice of shape functions which are described by a small set of parameters additional efforts have attempted to relate these parameters to more fundamental quantities such as average grain size of the bottom sediment wave environment geometric aspects of the beach and the like this approach has a long history and can rightly claim a certain level of success however the usual collections of shape functions are well known to have difficulties very near the shoreline let alone on the nearshore portion of the beach this study extends the equilibrium profile up to and somewhat past the shoreline three shape functions are used in conjunction with a taylor expansion for the nearshore and above water portions of the profile a nonlinear fitting technique is applied to estimate the model s best parameter values moreover reduced versions of the proposed models can be employed for prediction based only on parameters related to sediment characteristics or wave conditions and geometry of the visible beach the approach is compared with data from the east coast of australia the east coast of north america and the south shore of the mediterranean sea

principles of drainage basin analysis from multisource data application to the structural analysis of the zaire basin abstract classifications which attempt to combine descriptive and genetic aspects of types of drainage basins must take into account the application of new techniques satellite imagery high precision topographic maps and digital terrain models in addition the use of new concepts such as neotectonics and drainage basin history will lead to a reassessment of classification schemes after a review of current classifications we propose a genetic scheme that is based on the development of drainage networks and which includes integration of local and regional scale perturbations state of the art methods of drainage basin analysis are applied to the zaire basin to provide new information on deep structures the influence of basin and block tectonics is revealed in the context of major late proterozoic shear faults which have been rejuvenated up to the present day this is demonstrated from an analysis of the drainage network in the light of geological and geophysical observations

extension and basin formation in the southern andes caused by increased convergence rate a mid cenozoic trigger for the andes the southern andes between 33 and 45 s latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin which itself was characterized by volcanic activity the basins are filled with thick up to 3000 m accumulations of interbedded sedimentary and volcanic strata of late oligocene early miocene age we interpret that these basins developed during a phase of moderate extension within the plate margin system triggered by an increased rate of convergence of the farallon nazca and south american plates between 28 and 26 ma this history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling although extensional basins of this age are only well described in the southern andes the convergence history and volcanic chronology are similar farther north in the central andes 18 33 s leading to the speculation that extension may have characterized the late oligocene early miocene interval regionally we hypothesize that this extension was a necessary condition to subsequent building of the modern andes mountains

active tectonics of the andes nearly 90 mm a 1 of relative plate convergence is absorbed in the andean plate boundary zone the pattern of active tectonics shows remarkable variations in the way in which the plate slip vector is partitioned into displacement and strain and the ways in which compatibility between different segments is solved along any traverse across the plate boundary zone the sum of relative velocities between points must equal the relative plate motion we have developed a kinematic synthesis of displacement and strain partitioning in the andes from 47 s to 5 n relevant for the last 5 ma based upon 1 relative plate motion deduced from oceanic circuits giving a roughly constant azimuth between 075 and 080 2 moment tensor solutions for over 120 crustal earthquakes since 1960 3 structural studies of deformed plio pleistocene rocks 4 topographic geomorphic studies 5 palaeomagnetic data and 6 geodetic data we recognize four neotectonic zones with subzones and boundary transfer zones that are partitioned in different ways these zones are not coincident with the classic zones defined by the presence or absence of a volcanic chain or differences in finite displacements and strains and tectonic form the long term segmentation and finite evolution of the andes may not occur in constantly defined segments in space and time in segment 1 47 39 s the slip vector is partitioned into roughly orthogonal benioff zone slip with large magnitude large slip surface earthquakes and both distributed dextral shear giving clockwise rotations of up to 50 and dextral slip in the curved liquine ofqui fault system giving 5 10 of anticlockwise fore arc rotation in segment 2 39 20 s the slip vector is partitioned into benioff zone slip roughly parallel with the slip vector andean crustal shortening and a very small component of dextral slip including that on the atacama fault system between 39 and 34 s a cross strike dextral transfer which deflects the chile trench and the volcanic arc absorbs the shortening contrast between segments 1 and 2 in segment 3 20 6 s the slip vector is partitioned into roughly orthogonal benioff zone slip crustal shortening sinistral trench parallel faulting and northeast southwest extension compatibility between segments 2 and 3 is maintained by the sinistral east southeast trending cochabamba shear zone and north trending dextral faults in segment 4 6 s to 5 n the slip vector is partitioned into roughly orthogonal benioff zone slip and dextral strike slip faulting in the fore arc and volcanic chain

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region