foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

no flat wadati benioff zone in the central and southern central andes abstract the wadati benioff zone wbz is an approximate plane defined by earthquakes hypocentres observed in convergent plate boundaries and that usually dips at angles greater than 30 in some areas of the andes where there are gaps in volcanic activity and where heat flow is abnormally low this plane in most studies has nearly horizontal dip at a depth of about 75 100 km and it has been associated to flat subduction of the oceanic lithosphere this situation has been taken as the present day analogue of the laramide orogeny of western north america for which a flat slab episode has been proposed in the past years in this work the observed low heat flow in areas of the andes is assumed to be due to low radiogenic heat generation in geologically old and allochthonous terranes constituting large regions of western south america on the basis of geotherms obtained for areas of ecuador peru chile and argentina and of rheological results describing the partition between brittle and ductile regimes the seismic activity observed both in the lower crust and at depths of about 75 100 km is thoroughly explained at these depths earthquakes occur within the subcontinental upper mantle and then there is no flat wbz associated to subduction of the oceanic lithosphere there is evidence from recent seismological observations that the real wbz lies not horizontally and deeper in the tectonosphere

crustal shortening at the sierra pie de palo sierras pampeanas argentina near surface basement folding and thrusting the sierras pampeanas consist of basement uplifts that are mainly controlled by w verging crustal thrusts one of these the sierra pie de palo is controlled by a blind thrust whose kinematics is debated we analysed the reverse faults located along the eastern side of the range and show that these are characterized by small displacements few hundreds of metres of the neogene and quaternary deposits controlled by the inherited basement metamorphic foliation and shear zones at the sierra scale the inherited foliation pattern appears already folded before the cenozoic shortening forming a series of antiforms and synforms with various wavelengths we propose that this structure is amplified during the plio quaternary shortening resulting in a large asymmetrical basement cored anticline where foliation parallel slip can act as folding mechanism the overall kinematics is consistent with an e verging fault propagation fold this proposed kinematics is an alternative to the frequently suggested w verging fault bend fold model finally we propose a new alternative and speculative crustal and lithospheric structure for this region consisting of two e verging basement thrusts coeval to w verging pampean thrusts both rooting below the precordillera above an eclogitized lower crust

coseismic displacements and holocene slip rates for two active thrust faults at the mountain front of the andean precordillera 33 s 1 during the last few hundred years several destructive earthquakes occurred along the eastern margin of the andean precordillera where gps data reveal a shortening rate of 4 5 mm a we use fault scarp profiles and age determinations of deformed terraces t1 t4 to infer coseismic displacements and quantify slip rates for the penas and cal thrust faults near mendoza city scarps on the lowest terrace level t1 reveal vertical offsets of 0 8 1 0 m for both faults which are interpreted as coseismic displacements during the last earthquake together with the fault dip these offsets indicate that both faults are capable of producing magnitude mw 6 9 earthquakes which is corroborated by a magnitude ms 7 0 event on the cal fault that destroyed mendoza in 1861 at the penas thrust fault terrace t2 has an age of 3 3 ka and is offset by 1 9 m whereas the 12 ka old terrace t3 is displaced by 11 m combined with the fault dip of 25 the age and offset of terrace t3 define a shortening rate of 2 0 mm a on the penas fault i e about half of the present day shortening at the eastern margin of the precordillera at the cal fault terraces t2 to t4 have ages of 0 8 ka osl 3 9 ka 14c and 12 ka 10be and are vertically offset by 2 6 3 6 and 7 0 m respectively which implies that slip on the fault has recently accelerated hence the cal fault poses a serious seismic hazard to the one million inhabitants of mendoza

late quaternary evolution of the la cantera fault system central precordillera argentina a morphotectonic and paleoseismic analysis abstract the la cantera fault system lcfs is the most active quaternary structure in the central precordillera of san juan in central western argentina the system extends for 47 km along the intermountain valley that separates the sierra de la cantera and la invernada north of the san juan river the average fault trend is 20 it dips at angles varying between 15 and 30 w in the northern section to approximately 40 w in the central section and up to 60 w in the southern section the fault affects holocene to recent alluvium deposits in the western piedmont of the sierra de la cantera and is defined by a series of landforms found in compressive tectonic environments including simple and compound counterslope fault scarps staircased alluvial terraces sag ponds flexural scarps aligned springs broom shaped drainage patterns river diversions beheaded channels changes in incision depths sinuosity and a river gradient along channels trench investigations indicated that at least three events occurred in the past 1 1 10 1 ky the topographic profiles of the selected channels and interfluves cutting across the northern and central trace of the fault were analyzed using a stonex vector gps differential system to establish the relationship between the topography and slope of the rivers this morphometric analysis of scarps indicates that active tectonics have played an essential role in controlling the drainage pattern in the piedmont leading the rivers to adjust to these slope variations based on the analyzed geomorphologic stratigraphic and structural characteristics the lcfs is considered to be a relevant seismogenic source in the intraplate portion of southern south america with a recurrence interval of at least 2000 500 years for moderate magnitude earthquakes during the last 11 000 years

geochronology of the pre andean metamorphism and magmatism in the andean cordillera between latitudes 30 and 36 s abstract in the 30 36 s section of the andean cordillera the pre andean basement is composed by metamorphic igneous and sedimentary rocks of precambrian paleozoic and lower triassic ages they crop out in the morphostructural units named from east to west sierras pampeanas precordillera cordillera frontal and bloque de san rafael in argentina and cordillera de la costa in chile isotopic age determinations of the metamorphic rocks indicate a migration of the metamorphic events from east to west such as precambrian to lower paleozoic ages 900 m y and 650 400 m y in the sierras pampeanas lower paleozoic ages about 500 m y in the precordillera and cordillera frontal and upper paleozoic to lower triassic ages 340 210 m y in the cordillera de la costa these metamorphic belts contain a large variety of mafic and ultramafic rocks the distribution of the pre andean granitoids show no apparent chronological zonation however the plutonic activity persisted until the lower carboniferous in the sierras pampeanas to the lower triassic in the cordillera frontal and precordillera associated with an important vulcanism which also develops in the bloque de san rafael and until the jurassic in the cordillera de la costa

a possible explanation for foreland thrust propagation a common feature of thin skinned fold and thrust belts is the sequential nature of foreland directed thrust systems as a rule younger thrusts develop in the footwalls of older thrusts the whole sequence propagating towards the foreland in the transport direction as each new younger thrust develops the entire sequence is thickened particularly in the frontal region the compressive toe region can be likened to an advancing wave as the mountainous thrust belt advanced the down surface slope stresses drive thrusts ahead of it much like a surfboard rider in an attempt to investigate the stresses in the frontal regions of thrustsheets a numerical method has been devised from the algorithm given by mctigue and mei 1981 the algorithm yields a quickly computed approximate solution of the gravity and tectonic induced stresses of a two dimensional homogeneous elastic half space with an arbitrarily shaped free surface of small slope a comparison of the numerical method with analytical examples shows excellent agreement the numerical method was devised because it greatly facilitates the stress calculations and frees one from using the restrictive simple topographic profiles necessary to obtain an analytical solution the numerical version of the mctigue and mei algorithm shows that there is a region of increased maximum resolved shear stress directly beneath the toe of the overthrust sheet utilizing the mohr coulomb failure criterion predicted fault lines are computed it is shown that they flatten and become horizontal in some portions of this zone of increased thrust sheets are known to advance upon weak decollement zones if there is a coincidence of increased a weak rock layer and a potential fault line parallel to this weak layer we have in place all the elements necessary to initiate a new thrusting event that is this combination acts as a nucleating center to initiate a new thrusting event therefore thrusts develop in sequence towards the foreland as a consequence of the stress concentrating abilities of the toe of the thrust sheet the gravity and tectonic induced stresses due to the surface topography usually ignored in previous analyses of an advancing thrust sheet play a key role in the nature of shallow foreland thrust propagation

thrust front zone of the precordillera argentina a thick skinned triangle zone the frontal thrust zone of the precordillera thrust belt of western argentina only superficially resembles a classical alberta style triangle zone in this thick skinned triangle zone the thrust front is the result of interaction between the eastward verging thin skinned central precordillera and the westward verging thick skinned eastern precordillera system the western boundary formed by the niquivil thrust plate has been refolded and faulted by thrust faults from the eastern precordillera at several localities the deformation is still active we divide the thick skinned triangle zone into three segments based on the system s activity independent late interfering and collision zones balanced cross sections across a thick skinned triangle zone show a uniform s ortening for the eastern precordillera 17 km the structural geometry suggests multiple reactivation of the main faults the thick skinned triangle zone began to grow after 2 6 ma and is still active episodic and alternating opposing movement is controlled by buttressing of the thin skinned thrust system against the basement structures of the eastern precordillera

pocho volcanic rocks and the melting of depleted continental lithosphere above a shallowly dipping subduction zone in the central andes the late miocene 7 9 to 4 5 ma pocho volcanic field in argentina occurs 700 km east of the chile trench over the modern shallowly dipping andean wadati benioff zone near 32 s latitude in argentina the field is located in the sierra de cordoba which is the easternmost laramide style block faulted range in the sierras pampeanas pampean ranges the arrival of the shallowly dipping slab initiated both volcanism and the uplift of the sierra de cordoba pocho rocks 52 to 68 sio2 feo mgo 2 2 comprise an older 7 5 0 5 ma high k and a younger 5 3 0 7 ma shoshonitic series mineralogic data and fractionation models show that crystallization occurred under hydrous oxidizing conditions which were most extreme in the high k series an unusual pattern of successively lower ree at higher sio2 concentrations can be modeled by sphene apatite and amphibole removal an arc like trace element signature attributed to an arc component is strongest in the younger shoshonitic series an important depleted lower crustal mantle lithospheric source component in both series is indicated by non radiogenic sr and pb isotopic ratios at nd 0 to 2 low rb sr ratios and low u and th concentrations this depleted signature contrasts with the enriched one in potassic back arc central volcanic zone cvz lavas over the steeper subduction zone to the north and is attributed to several processes in the shallow subduction zone first deep crustal mash processes in the nearly normal thickness crust beneath pocho incorporated depleted proterozoic basement components and not complexly mixed structurally thickened crustal components as in the cvz second the association of pocho volcanism with the arrival of the slab allowed little time for modification of the mantle by subduction components third miocene shallowing of the subduction zone beneath the flat slab required thinning of both the astenosphere and the subcontinental lithosphere thus an important subcrustal component could be from blocks removed from the base of the lithosphere to the west and recycled into the asthenosphere similar magmatic sources would have existed during laramide shallow subduction in western north america

present knowledge of the magmatic evolution of the eastern cordillera of peru abstract the studies which have been carried out in the eastern cordillera of peru over the past 20 years prove the existence of at least three orogenic cycles the late precambrian the hercynian and the andean each one accompanied by a more or less abundant magmatism 1 the precambrian leaving aside the prasinites possibly derived from synsedimentary volcanites precambrian magmatism in the huanuco region consists of a meta igneous ultramafic to mafic association serpentinites meta gabbros meta diorites syntectonic meta tonalites and post tectonic dioritic and granitic intrusive bodies 2 the hercynian 550 to 220 m y from the cambrian to the upper devonian the existence of a synsedimentary magmatism is known syntectonic granites were emplaced during the eohercynian phase but the major part of magmatism is of a late permian to early trias age and is characterized by the intrusion of granitoids and by volcanism of calc alkaline tendency it appears that the nepheline syenite of macusani may belong to a terminal episode of this magmatic period 3 during the andean evolution there is considerably less magmatism in the eastern cordillera than during the preceding cycles plutonism which is essentially acid is only well represented in central peru volcanic rocks are more abundant and more widely distributed knowledge of the precambrian is still insufficient to enable us to place the magmatism of this orogenic belt in a geodynamic context it seems that the hercynian magmatism is related to extension episodes affecting a continental crust on the other hand it is probable that the andean magmatism of the eastern cordillera is linked at least indirectly to the presence of an active convergent margin to the west of peru dating from the mesozoic

a high resolution local network study of the nazca plate wadati benioff zone under western argentina this paper presents a precise determination of the fine structure of a wadati benioff zone based on analysis of analog and digital seismic data recorded by a local network in the san juan province of western argentina the network is located above one of the subhorizontal segments of the subducted nazca plate without the complicating effects of a dipping slab on the locations of earthquakes the nearly flat geometry of the subducted plate provides an excellent opportunity to determine the thickness and structure of the wadati benioff zone we found that the depth of the wadati benioff zone beneath the network is 107 5 km for a set of digitally recorded events the distance between the shallowest and the deepest event gives a thickness of 20km with 90 of the data concentrated in a zone 12 km thick the spatial distribution of hypocenters shows no evidence of complex internal structure such as a double wadati benioff zone the local network data also suggest that in the region near 32 s where the dip of the wadati benioff zone changes from flat to steep the nazca plate flexes rather than tears at depths less than at least 125 km the intermediate depth seismicity is separated from an active zone of crustal seismicity by an aseismic region between depths of about 40 and 95 km the upper surface of the subducted plate inferred to be at about 90 km provides an upper limit to the thickness of the south american plate in this region an apparent westward dip of the wadati benioff zone as determined by the local network data can be accounted for by a 6 westward dip of the moho beneath the san juan region a result that is in agreement with regional gravity and topographic data

accurate source depths and focal mechanisms of shallow earthquakes in western south america and in the new hebrides island arc synthetic seismograms are matched to long period p waveforms in order to obtain accurate depths of shallow earthquakes with known focal mechanisms accurate depths are obtained for a large sample of moderate sized 6 ms 7 events which produced relatively simple p waveforms and do not require intensive analysis the estimated error in the determination of depth is about 5 km or less for most of the events considered the procedure is applied to two suites of events one in south america 83 events and one in the new hebrides 61 events in these two areas of contrasting styles of subduction the accurate depths provide new information on the bending of descending plates near trenches the seismicity and tectonics of the upper plate and the geometry and structure of the zone of contact between the upper and descending plates depths of suboceanic earthquakes which occurred near the trench and which have either tensional and compressional horizontal stress axes agree well with chapple and forsyth s 1979 preferred model of a bending elastic perfectly plastic plate however an unusually deep event occurred beneath the fiji plateau in an area of young ocean floor supposed to have been formed by sea floor spreading processes only since late miocene time the calculated depth of the event 48 km places it below the lithosphere asthenosphere boundary derived from thermal models vertical cross sections through the shallow parts of the subduction zones show that most of the earthquakes with thrust faulting focal mechanisms can be interpreted to be interplate events located in a thin 10 km thick curved zone of contact dipping arcward or landward from the trench axis however a few events with focal mechanisms similar to the interplate events do not fit on the inferred thin zone of contact these events would be accommodated by an interplate zone 15 km thick in both subduction zones interplate events occur at depths between about 15 and 50 km however the downdip width of the south american interplate zone is 2 to 3 times larger than that in the new hebrides most of the intraplate events near the interplate boundary are located in the descending plate in the upper plate seismicity is more pronounced in the back arc regions of the two subduction zones thus our results support the generality of a relatively aseismic zone in the upper plate located trenchward of the magmatic arc depths of shallow earthquakes in the eastern andes and sub andean regions of south america range between about 9 and 36 km indicating that both the upper and middle crust are seismically active about 75 of the events occurred in the upper 25 km furthermore the focal mechanisms indicate that from ecuador to argentina the crust of the eastern side of the andean cordilleran is in a state of east west compression oriented nearly parallel to the direction of convergence between the nazca and south american plates the depths the orientations of the earthquake nodal planes and the close relationships to quaternary tectonics imply basement involved crustal shortening in much of the eastern andes and sub andean zones of peru and argentina

planktonic percentage of foraminiferal fauna in surface sediments of the arabian sea indian ocean and a regional model for paleodepth determination abstract the percentage of planktonic specimens in foraminiferal assemblages were determined in 126 surface sediment samples depth range 13 to 1050 m from the arabian sea along the western continental margin of india the results obtained are in agreement with previous studies in other areas that planktonic percentage increases with depth the resultant pattern is compared with results from the atlantic margin of the northeastern united states gulf of mexico timor sea and red sea comparisons reveal that in spite of diversity between various areas the over all pattern indicate a positive correlation between planktonic percentage and depth based on the above relationship a regional model has been constructed for the western continental margin of india in the arabian sea region to determine paleodepth from the planktonic percentage of fossil foraminiferal fauna regional differences in salinity patterns could be one of the main reasons for deviation of the regional model for the arabian sea with other regional models from pacific and atlantic regions and also with the global model

crustal structure of the south central andes cordillera and backarc region from regional waveform modelling summary we investigate the crustal structure in the andes cordillera and its backarc region using regional broadband waveforms from crustal earthquakes we consider seismic waveforms recorded at regional distances by the chile argentina geophysical experiment charge during 2000 2002 and utilize previous seismic moment tensor inversion results for each single station earthquake pair we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation cordillera and the accreted terranes in the backarc region our investigation indicates that synthetic seismograms for our earthquake station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters we performed a grid search around crustal thickness p wave seismic velocity vp and p t os wave seismic velocity ratio vp vs fixing mantle parameters we evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components we identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s our results indicate along strike variations in the crustal structure for the north south high cordillera with higher p wave velocity and thickness in the northern segment north of 33 s and persistently high vp vs ratio 1 85 in both segments this is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region in contrast the results for the current volcanic arc south of 33 s agree with a warmer crust consistent with partial melt related to quaternary volcanism presumably of an intermediate to mafic composition a distinctive feature in the backarc region is the marked contrast between the seismic properties of the cuyania and pampia terranes that correlates with their heterogeneous crustal composition the cuyania terrane composed of mafic ultramafic rocks exhibits high vp high vp vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern sierras pampeanas associated with low vp and low vp vs these differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper 30 km in this active compressional region in particular the seismic properties of the cuyania terrane which shows evidence for a high vp and high vp vs crust may be reflecting the complex tectonic evolution history of this terrane including accretion rifting and re accretion processes since the palaeozoic that promote a high level of crustal seismicity in the upper 30 km enhanced by the flat slab subduction in the segment between 31 s and 32 s another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the cuyania terrane between 30 s and 34 s

geodetic evidence for conjugate faulting during the 1988 tennant creek australia earthquake sequence summary repeat levelling measurements and detailed topographic profiles from the epicentral area of the 1988 january 22 tennant creek australia earthquakes are used to constrain the geometry of faulting associated with three m 6 earthquakes the observed elevation changes are modelled assuming elastic deformation and uniform slip on several faults the vertical deformation data are poorly fit by a single fault model and require at least three distinct faults in the preferred model two faults on either end of the zone of surface rupture have similar orientations but the central fault has an orientation conjugate to the flanking faults this interpretation is consistent with the identification of the fault planes with well defined dipping zones of aftershock hypocentres determined with data from portable seismograph arrays it is also consistent with the sense of surficial deformation documented by 75 topographic profiles across the scarps however a fourth fault associated with possible conjugate faulting in the central fault segment at the time of the second main shock is not required by the levelling data

crustal scale pure shear foreland deformation of western argentina 1 new analyses of teleseismic body waves from moderate earthquakes in western argentina demonstrate that active shortening of the andean foreland occurs on reverse faults extending to 40 50 km depth existing crustal scale models of foreland deformation invoke thin skinned fault geometries which root into an east dipping mid crustal decollement whereas thin skinned thrust sheets dominate shallow crustal structure seismological and geological data illustrate that planar reverse faults and pure shear deformation involving more than 75 of the crust characterizes this thick skinned structural province

uplift of the central andean plateau and bending of the bolivian orocline the topography of the central andes can be considered the primary tectonic signal of late cenozoic mountain building in an arid region where the effects of uplift and magmatism are little obscured by denudation the spatial coverage of the topographic signal is more complete than that for sparsely sampled geological and geophysical data a color coded image of digitized topography between 12 s and 37 s highlights the altiplano puna one of the world s most remarkable plateaus and reveals important physiographic clues about the formation of that major feature the topographic data combined with information on structure magmatism seismicity and paleomagnetism support a simple kinematical model for the late cenozoic evolution of the central andes the model does not require collisional effects or enormous volumes of intrusive additions to the crust but instead calls upon plausible amounts of crustal shortening and lithospheric thinning the model interrelates andean uplift a changing geometry of the subducted nazca plate and a changing outline in map view of the leading edge of the south american plate crustal shortening has accommodated convergence between the chilean peruvian forearc and the south american foreland the altiplano puna plateau can be constructed by a combination of crustal shortening and thickening and lithospheric thinning above a shallow dipping 20 30 subducted plate the seawardly concave bend of western south america the bolivian orocline was enhanced but not completely produced by an along strike variation in the amount of late cenozoic shortening maximum shortening in bolivia both produced the widest part of the plateau and increased the seaward concavity of the bolivian orocline the along strike variations of shortening are hypothesized to result from corresponding along strike variations in the width of a weakened zone in the overriding plate weakening occurs above the wedge of asthenosphere located between the subducted and overriding plates hence the width of the zone of weakening depends upon the dip of the subducted plate two types of shortening are recognized 1 a widespread basin and range laramide like shortening that characterizes modem activity in the sierras pampeanas and late miocene deformation of the altiplano puna and 2 on the eastern side of the cordilleras and plateau an east verging foreland fold thrust belt in which the underthrust foreland compresses and thickens the ductile lower crust and produces a plateau uplift of the upper crust the second type of shortening can be applied to plio quaternary deformations throughout the central andes but with a substantial narrowing of the region of plateau uplift in peru and south of 28 s a proposed monoclinal flexure of the upper crust on the western side of the plateau uplift explains the remarkably simple and regular morphology of the main western slope of the central andes the monocline is located above the tip of the asthenospheric wedge between the converging plates it is postulated to occur above the western limit of lower crustal thickening in the regions of horizontal subduction the monocline can be associated with a late miocene asthenospheric wedge tip

subsurface characterization of quaternary scarps and their possible connection to main structures of the western margin of precordillera san juan argentina abstract the el tigre fault provides a significant example of pliocene quaternary tectonic activity at the west of precordillera receiving considerable attention in the last years however longitudinal thrusts of the nearest sierra del tigre and a minor piedmont deformation belt identified in its western piedmont are still poorly understood the present work comprised a detailed map of the geomorphological features of tectonic origin and quaternary alluvial fan terraces located between the sierra del tigre and the el tigre fault several topographic and geoelectrical profiles across minor fault and fold scarps were surveyed to define the displacement scarps the geological structure geometries at subsurface and their relationship with main structures geoelectrical models allowed us to recognize three geoelectrical levels a 200 300 m correlated to pleistocene alluvial fan terraces q4 and q3 b ranging from 50 to 300 450 m under level a and correlated to the neogene substratum rodeo formation c

neogene structures overprinting palaeozoic thrust systems in the andean precordillera at 30 s latitude field studies along a 30 s latitude transect of the precordillera in nw argentina indicate that neogene to recent deformation caused high angle faulting that uplifted and breached a palaeozoic east verging thrust stack palaeozoic structures crop out in the higher linear ranges and intramontane neogene basins occur in the intervening valleys the palaeozoic rocks include three tectonic units with contrasting structural styles and differences in stratigraphy the western allochthon includes clastic ordovician rocks metamorphosed to lower greenschist facies and folded by large scale asymmetric east verging folds with a well developed axial planar cleavage this unit overthrusts a siluro devonian flysch type succession of the central imbricate thrust system and associated fault propagation folds in which the thrusts merge into a basal thrust beneath ordovician carbonates middle to upper carboniferous sediments unconformably overlie structures of this imbricate system in the frontal unit minor thrust faults are interpreted to explain the low angle unconformity within palaeozoic rocks neogene deformation causes only a minor overprint of the palaeozoic structure and large scale folding in the frontal unit appears to be mostly of neogene age the new data indicate that neogene intracontinental shortening may be smaller than previously proposed providing new perspectives on the present orogenic architecture along this andean section

seismotectonics of thin and thick skinned deformation in the andean foreland from local network data evidence for a seismogenic lower crust local network data from san juan argentina provides new information about crustal seismicity in the andean foreland above a horizontal segment of the subducted nazca plate the authors find two areas of foreland seismicity one associated with the sierras pampeanas basement uplifts and the other beneath but not within the precordillera foreland fold thrust belt the precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation in both areas events are concentrated between 15 and 35 km depth and have volumetric rather than planar fault like distributions the depth distribution is usually deep for intraplate earthquakes and suggests a brittle ductile transition near 30 35 km it also suggests the upper plate may have a geotherm similar to stable cratonic regions rather than a steeper one that may be expected due to a lithospheric thinning implied by the location of the subducted plate this can be understood as a transient effect of a neogene flattening of the dip of the subducted plate where a rapid thinning of the upper plate was followed with underplating by the cool subducted plate leaving the upper plate geothermmore unchanged while producing a thinned mechanically weaker lithosphere finally a spatial correlation of upper plate seismicity and structures with subducted plate seismicity raises the possibility that a major lithospheric structure cutting across the strike of the foreland near 31 degree s may be related to subduction of the juan fernandez ridge less

holocene shortening rates of an andean front thrust southern precordillera argentina abstract a significant part of the quaternary shortening between the chilean trench and the relative stable interior of the south american plate at the pampean flat slab 27 33 s has been accommodated at the eastern foothills of the andes and mainly within a narrow neotectonic belt along the eastern side of the argentine precordillera tectonic geodesy results point out that this area is being shortened at a 2 4 mm a rate whereas shortening rates estimated over longer time periods 1 20 ma suggest values ranging from 1 mm a to 16 mm a geomorphic and geologic evidence indicate that the east directed las higueras thrust system is one of the main structures that has accommodated quaternary deformation at this section of the andean orogenic front 32 05 32 35 s an outcrop exhibiting the thrust propagation into fluvial sediments allows the holocene shortening rates at the northern end of this structure to be estimated based on retrodeformation of holocene strata and radiocarbon dating of two charcoal bearing beds estimated shortening rates yielded mean values of 1 90 0 28 mm a for the last 4495 143 cal yr bp and 1 53 0 26 for the last 8245 48 cal yr bp these results pose some uncertainties due to the incompleteness of the exposed deformation at the hanging wall however they correspond to a key timescale which helps to bridge the gap between rates derived from the short term gps data and the long term permanent deformation rates obtained through geologic studies although the estimated rates suggest that slip on the thrust could have accelerated during the last 4 ka more data are necessary to reliably address this key issue

mo mineralized porphyries are relatively hydrous and differentiated insights from the permian triassic granitic complex in the baituyingzi mo cu district eastern inner mongolia ne china mo cu mineralization in the baituyingzi district of eastern inner mongolia occurs within a granitic complex this paper presents and discusses zircon u pb ages and whole rock geochemical and sr nd pb isotopic data from the granitic complex as potential indicators for porphyry mo fertility the u pb ages indicate that five units of the granitic complex were emplaced between 265 2 0 7 and 246 5 1 0 ma constrained by crosscutting dikes mo cu mineralization was probably related to the baituyingzi monzogranite porphyry dated at 248 2 0 64 ma the intrusions belong to high k calc alkaline to shoshonitic series that are characterized by highly fractionated rare earth element ree patterns and strong enrichments of large ion lithophile elements relative to high field strength elements apart from the 246 ma dike that shows negative end t values 14 9 to 13 1 the intrusions have end t values ranging from 3 9 to 1 0 relatively young depleted mantle model ages 811 to 1183 ma 206pb 204pb of 18 137 18 335 and 207pb 204pb of 15 591 15 625 which are consistent with a juvenile lower crustal origin among the intrusions the 248 ma porphyry and the 246 ma dike show adakite like characteristics e g sr y 44 9 185 and listric shaped ree patterns that indicate amphibole fractionation and a hydrous magma source however the porphyry exhibits a higher differentiation index 81 4 91 5 and a steeper ree profile e g lan ybn 25 6 87 0 than those of the 246 ma dike which suggests that it is highly differentiated we propose that the complex was generated by the partial melting of juvenile mafic lower crust containing minor old crustal relicts that was triggered by collision between the north china craton and siberian craton as indicated by the th nb th yb ba th and ba la ratios of the intrusions the crust may have been derived from the melting of the fertile mantle wedge that was metasomatized by various amounts of slab derived fluids or melts due to earlier subduction and was heterogeneous in terms of water cu mo and s contents and possible oxidation state the fertility of the porphyry was likely associated with the addition of earlier subduction related slab melts to the magma source leading to a hydrous and possible high oxidation state and the long time 20 ma collision tectonic setting in which it formed resulting in a highly differentiated state

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

depth dependent seismic scattering attenuation in the nuevo cuyo region southern central andes 1 in the present work we separated intrinsic from scattering attenuation coefficients both for the crust and the upper mantle in the tectonically highly active areas of the southern central andes nuevo cuyo region analyzing two groups of earthquakes well separated in depth this region is characterized by the presence of flat subduction we apply mltwa multiple lapse time window analysis coda normalization and q coda techniques to measure the scattering and intrinsic attenuation coefficient and the total q for s waves we find that intrinsic attenuation does not decrease with depth whereas scattering attenuation is higher in the crust than in the upper mantle and that intrinsic attenuation predominates over scattering attenuation we interpret this observation in terms of the release of water and other fluids into the overlying lithosphere due to the dynamics of the subduction process in agreement with most of the prevalent geodynamic models

patterns of historical earthquake rupture in the iranian plateau the iranian plateau accommodates the 35 mrn yr convergence rate be tween the eurasian and arabian plates by strike slip and reverse faults with relatively low slip rates in a zone 1000 km across although these faults have only locally been the subject of paleoseismological studies a rich historical and archeological record spans several thousand years long enough to establish recurrence intervals of 1000 to 5000 yr on individual fault segments several clusters of earthquakes provide evidence of interaction among reverse and strike slip faults probably due to adjacent faults being loaded by individual earthquakes the dasht e bayaz sequence of 1936 to 1997 includes earthquakes on left lateral right lateral and reverse faults the neyshabur sequence of four earthquakes between 1209 and 1405 respected the seg ment boundary between the neyshabur and binalud reverse fault systems the two pairs of earthquakes may have ruptured different faults in each segment similar to the 1971 and 1994 san fernando california earthquakes the 1978 tabas reverse fault earthquake was preceded by the 1968 ferdows earthquake part of the dasht e bayaz sequence the north tabriz fault system ruptured from southeast to north west in three earthquakes from 1721 to 1786 a previous cluster may have struck this region in 855 to 958 the mosha fault north of tehran ruptured in three earthquakes in 958 1665 and 1830 five large earthquakes struck the tehran region from 743 to 1177 but only two that large have struck the area since 1177 other earthquakes occurred in pairs in the talesh mountains near the caspian sea 1863 1896 the iran turkey border 1840 1843 and the nayband gowk fault system both 1981 other historical events did not occur as parts of sequences the historic seismic moment release in iran accounts for only a small part of the plate convergence rate which may be due to aseismic slip or to the iranian historical record long as it is being too short to sample long term deformation across the plateau no historic earthquakes of m 8 have struck iran however several long straight strike slip faults doruneh west neh east neh and nayband have not sustained large historical earthquakes raising the possibility that these long faults could produce earthquakes of m 8 thereby removing at least part of the apparent slip deficit an increased understanding of iran s seismic hazard could be obtained by an extensive paleoseismology program and space geodetic arrays supplementing the abundant historical and archaeological record

timing and nature of alluvial fan and strath terrace formation in the eastern precordillera of argentina abstract sixty eight 10 be terrestrial cosmogenic nuclide tcn surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper arid san juan region of the argentine precordillera this region is tectonically active and numerous fault scarps traverse quaternary landforms the three study sites marquesado strath complex loma negra alluvial fan and carpinteria strath complex reveal a history of alluvial fan and strath terrace development over the past 225 ka the marquesado complex q3 m surface dates to 17 3 ka whereas the loma negra q1 ln q2 ln q3 ln q4 ln and q5 ln surfaces date to 24 3 ka 48 2 ka 65 13 ka 105 21 ka and 181 29 ka respectively the carpinteria complex comprises eight surfaces that have been dated and include the q1 c 23 3 ka q2 c 5 5 ka q3a c 25 12 ka q3b c 29 15 ka q4 c 61 12 ka q5 c 98 18 ka q6 c 93 18 ka and q7 c 212 37 ka 10 be tcn depth profile data for the loma negra alluvial fan complex and carpinteria strath terrace complex as well as osl ages on some carpinteria deposits aid in refining surface ages for comparison with local and global climate proxies and additionally offer insights into inheritance and erosion rate values for tcns 10 10 4 10 be atoms g of sio 2 and 5 m ma 1 respectively comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the carpinteria and loma negra areas with only younger alluvial fan surfaces preserved both to the north and south these data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region

lateral velocity variations in the andean foreland in argentina determined with the jhd method abstract lateral velocity variations in the andean foreland near san juan province argentina were detected by analyzing the station corrections computed as part of the joint location of local earthquake data the events recorded by a portable network panda have intermediate and shallow depths about 100 km and less than 40 km respectively the jhd technique was applied to intermediate depth events and to two subsets of shallow events that concentrate on two distinct geologic units precordillera and pampean ranges the computed station corrections have a strong dependence on the data processed analysis of the different patterns of corrections shows that the corrections are consistent with the presence of large scale lateral velocity variations bounded by major fault systems based on these corrections a qualitative velocity model is proposed that has relatively high velocities between the eastern precordillera and a westerly dipping plane on the eastern edge of pie de palo one of the mountain blocks of the pampean ranges velocities are lower immediately east of this plane and between the central and eastern precordillera this model has important tectonic implications because it suggests that there is a major boundary between the central and eastern precordillera and that the pampean basement whose extent is unknown underlies the eastern precordillera the existence of large velocity variations across the precordillera is consistent with a westward shift of almost 10 km of the relocated intermediate depth events and with the removal of an apparent westward dip opposite to the direction of subduction observed in the wadati benioff zone as determined from single event location the analysis of the intermediate depth events also exemplifies the deleterious effect on the jhd results introduced by the cancellation of small singular values in the computation of the station corrections

geophysical study of the valle fertil lineament between 28 45 s and 31 30 s boundary between the cuyania and pampia terranes abstract a gravimetric and magnetometric study was carried out in the north eastern portion of the cuyania terrane and adjacent pampia terrane gravimetric models permitted to interpret the occurrence of dense materials at the suture zone between the latter terranes magnetometric models led to propose the existence of different susceptibilities on either side of the suture the curie temperature point depth representing the lower boundary of the magnetised crust was found to be located at 25 km consistent with the lower limit of the brittle crust delineated by seismic data this unusually thick portion of the crust is thought to release stress producing significant seismicity moho depths determined from seismic studies near western sierras pampeanas are significantly greater than those obtained from gravimetric crustal models considering mass and gravity changes originated by the flat slab nazca plate along cuyania and western pampia terranes it is possible to reconcile moho thickness obtained either by seismic or by gravity data thus topography and crustal thickness are controlled not only by erosion and shortening but by upper mantle heterogeneities produced by a the oceanic subducted nazca plate with normal slope also including asthenospheric materials between both continental and oceanic lithospheres b flat slab subducted nazca plate as shown in this work without significant asthenospheric materials between both lithospheres these changes influence the relationship between topographic altitudes and crustal thickness in different ways differing from the simple airy system relationship and modifying the crustal scale shortening calculation these changes are significantly enlarged in the study area future changes in nazca plate slope will produce changes in the isostatic balance

faulting and erosion in the argentine precordillera during changes in subduction regime reconciling bedrock cooling and detrital records abstract the argentine precordillera is an archetypal retroarc fold and thrust belt that records tectonics associated with changing subduction regimes the interactions between exhumation and faulting in the precordillera were investigated using apatite and zircon u th sm he and apatite fission track thermochronometry from the precordillera and adjacent geologic domains inverse modeling of thermal histories constrains eastward in sequence rock cooling associated with deformation and erosion from 18 to 2 ma across the central precordillera tracking thrusting during this time the youngest ahe ages 5 2 ma and highest erosion rates are located in the eastern and western extremities of the precordillera and indicate that recent denudation is concentrated at its structural boundaries moreover synchronous rapid pliocene cooling of the frontal cordillera eastern precordillera and sierra del valle fertil was coeval with initiation of basement involved faulting in the foreland detrital zircon u pb geochronology from the ca 16 8 1 ma bermejo foreland basin strata suggests fluvial connectivity westward beyond the frontal cordillera to the main cordillera and coast range followed by an important shift in sediment provenance at ca 10 ma at this time we suggest that a substantial decrease in permo triassic igneous sources in the frontal cordillera and concurrent increase in recycled zircons signatures of paleozoic strata are best explained by uplift and erosion of the precordillera during widening of the thrust belt bedrock thermochronology and modeling indicate a 2 6 myr lag time between faulting related cooling in the hinterland and the detrital record of deformation in the foreland basin suggesting that for tectonically active semi arid settings bedrock cooling may be more sensitive to onset of faulting we suggest that high erosion rates in the frontal cordillera and eastern precordillera are associated with increased interplate coupling during shallowing of the subducting nazca plate that may concentrate stress along weak structural boundaries of the precordillera

analysis of neotectonic structures in the eastern precordillera of argentina in relation to seismic hazard by the application of integrated geophysical methods abstract the city of san juan in the central western region of argentina has been the target of very destructive superficial earthquakes some of which have not been associated to a clear structural source up to this date the city is constantly growing outside the valley where it is located towards the area of eastern precordillera which is currently having an increased socio cultural activity thus this study is focused on increasing the geological knowledge of the latter by studying the eastern flank of sierra chica de zonda eastern precordillera whose proved neotectonic activity represents a geohazard on the basis of the general geological setting the neotectonic structures in the study area are related to a major active synclinal folding located just under the western sector of the san juan city geophysical potential methods gravimetric and magnetometric surveys were used to recognize contacts by contrast of density and magnetic susceptibility in order to reduce the ambiguity of these methods the gravi magnetometric results were constrained by using seismic and electrical tomographies these contacts where geophysical properties abruptly change were interpreted as faults despite many of them not having a superficial expression the latter being of great importance to asses the seismic hazard of the study area

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

geometry and brittle deformation of the subducting nazca plate central chile and argentina summary we use data from the chile argentina geophysical experiment charge broad band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the south american subduction zone between 30 s and 36 s this region contains a zone of flat slab subduction where the subducting nazca plate flattens at a depth of 100 km and extends 300 km eastward before continuing its descent into the mantle we use a grid search multiple event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the wadati benioff zone these contours reflect slab geometries from previous studies of intermediate depth seismicity in this region with some small but important deviations our hypocentres indicate that the shallowest portion of the flat slab is a

a comparison of historical and paleoseismicity in a newly formed fault zone and a mature fault zone north canterbury new zealand the timing of large holocene prehistoric earthquakes is determined by dated surface ruptures and landslides at the edge of the australia pacific plate boundary zone in north canterbury new zealand collectively these data indicate two large m 7 earthquakes during the last circa 2500 years within a newly formed zone of hybrid strike slip and thrust faulting herein described as the porter s pass to amberley fault zone ppafz two earlier events during the holocene are also recognized but the data prior to 2500 years are presumed to be incomplete a return period of 1300 2000 years between large earthquakes in the ppafz is consistent with a late holocene slip rate of 3 4 mm yr if each displacement is in the range 4 8 m historical seismicity in the ppafz is characterized by frequent small and moderate magnitude earthquakes and a seismicity rate that is identical to a region surrounding the structurally mature hope fault of the marlborough fault system farther north this is despite an order of magnitude difference in slip rate between the respective fault zones and considerable differences in the recurrence rate of large earthquakes the magnitude frequency distribution in the hope fault region is in accord with the characteristic earthquake model whereas the rate of large earthquakes in the ppafz is approximated but over predicted by the gutenberg richter model the comparison of these two fault zones demonstrates the importance of the structural maturity of the fault zone in relation to seismicity rates inferred from recent historical and paleoseismic data

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

crustal deformation of the andean foreland at 31 30 s argentina constrained by magnetotelluric survey abstract twenty five new long period magnetotelluric sites near 31 5 s were collected in a west east profile this profile and the previous one aligned with and adjacent to the eastern end have been merged to form a single profile of more than 700 km long extending from the precordillera to the chaco pampean plain the geotectonic scenario is characterized by a modern flat subduction zone of the nazca plate located at a depth of around 120 km and clearly defined by the distribution of earthquake hypocenters recorded by local and regional networks a bulge shape at 68 5 w with an anomalous dip to the west is observed within this segment the smooth slab deformation might result from the restriction on eastward motion due to the presence of an electrically resistive zone the magnetotelluric model shows that this thick zone of increased resistivity is found from shallow crustal levels to upper mantle depths the bulge geometry allows hot fluids and volatiles to rise from the deeper asthenospheric wedge and reach the lower crust reducing its viscosity and letting it flow the zones of low resistivity in the lower crust show spatial correlation with the areas of foreland deformation from precordillera to the sierras pampeanas and may also suggest a ductile regime shear zones reactivated by cenozoic faulting must necessarily have their roots in the levels of the ductile lower crust associated to conductive channels the zone where the lower crust is closer to the surface coincides with the areas of greatest structural relief and erosion the interface between the folded ductile lower crust and the brittle upper crust might act as the main level of decollement of the bordering structures between the precordillera sierra de pie de palo and the sierras pampeanas in addition the geometry of the interface might be conditioning the vergence of those structures

chronology of neogene sedimentation stratigraphy and tectonism in the campo de talampaya region la rioja province argentina abstract chronostratigraphic paleoenvironmental and tectonic analysis of a 2320 m thick neogene continental section was conducted along the southern limit of the campo de talampaya magnetic polarity stratigraphy and fission track ages were used to develop a relatively precise geochronology the strata record a period of mixed evaporitic and terrigenous elastic sedimentation 20 6 1 ma that persisted in the distal bermejo basin during eastward migration of the precordillera fold thrust belt a critical transition of depositional environments and a change from western to eastern source areas occurred between 12 9 and 12 3 ma the saline mud flat environment of the rio manero formation derived from a western source and characterized by fine grained sandstones mudstones and evaporites yielded to a short period of playa lake deposition this was soon overwhelmed by deposition of the desencuentro formation composed of sandstones and mudstones with an eastern source area and deposited in a sand flat mud flat environment we propose that this stratigraphic transition resulted from initiation of uplift of the sierra de velasco a range of the sierras pampeanas located 65 km to the east northeast of the section basement clasts probably derived from the sierra de velasco are found in conglomeratic lenses beginning in the lower part of the desencuentro formation the modern campo de talampaya basin formed after deposition of the top of the alto de san nicolas paleomagnetic section 6 1 ma when west verging reverse faults uplifted the sierra morada and sierra de los tarjados all strata in the section were tilted eastward during these culminating uplift events

mass movement in cordon de las osamentas de la flecha river basin san juan argentina abstract the de la flecha river basin is located to the southwest of san juan province at around 31 50 s 68 57 w in this area a total of twenty debris rock slides in the cordon de las osamentas were mapped and studied the mode of failure of each was determined to be a rotational slide the areal extent of the smallest observable landslide body is approximately 0 58 km2 and the largest is 367 76 km2 the slides were detected through high resolution satellite imagery interpretation digital elevation models and field surveys by identifying both the landslide detachment zones and landslide bodies based on the geographic information system gis the slides have varying controlling factors including lithology and slope which seem to be the main controlling factors in causing slope instability the spatial relationships among the landslides and the distribution of seismic epicentres historical earthquakes and quaternary faults maradona las osamentas papagayos faults la chilca and el mocho faults were analysed these gravitational movements were triggered by seismic shocks associated with active faults

inferring a thrust related earthquake history from secondary faulting a long rupture record of la laja fault san juan argentina abstract trenches excavated across the surface rupture of the 15 january 1944 m w 7 san juan argentina earthquake show evidence for repeated rupture of la laja fault in the late pleistocene and holocene the 1944 rupture extended for about 7 km striking n35 e and dipping 42 e parallel to neogene bedding with a maximum east side up vertical displacement of 30 cm we interpret nine discrete colluvial wedges each capped by a soil as evidence for nine surface ruptures in the past 32 35 ka our topographic profile of the t3 terrace surface projected across the fault suggests these nine events collectively produced 10 3 m of dip displacement we used radiocarbon and optically stimulated luminescence dating to define the timing of past events yielding an average recurrence interval of 3 9 0 3 ka the average estimate for displacement per event is about 1 1 m compared with the 0 5 m of dip slip that occurred in 1944 using a fault related fold model that infers 28 m of shortening based on the axial surface migration for the t 3 terrace along with the 35 ka age of the terrace yields a shortening rate of 0 8 mm yr by the 1944 source thrust fault which is about 20 of the global positioning system constrained total shortening rate across the precordillera using this shortening rate and an assumed fault dip of 30 implies a fault slip rate of about 1 1 mm yr which combined with the average recurrence interval implies 4 m of average slip per event on the causative fault at depth online material unit descriptions from the trenches stepwise reconstruction from trench 1 and annotated photographs of the la laja region

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

basement seismicity beneath the andean precordillera thin skinned thrust belt and implications for crustal and lithospheric behavior data from a digitally recording seismic network in san juan argentina provide the first images of crustal scale basement faults beneath the precordillera this seismicity is near the boundary between the precordillera a thin skinned thrust belt and the sierras pampeanas a region of thick skinned basement deformation two seismically active tectonic provinces of the andean foreland the seismicity data support models for this region in which crustal thickening rather than magmatic addition or thermal uplift plays the dominant mountain building role the precordillera seismicity occurs in three segments distributed north to south the southern segment is an area of diffuse activity extending across the precordillera and eastward into the sierras pampeanas that shows no patterns in map or cross section the northern and central segments have well defined dipping planes that define crustal scale faults extending from 5 to 35 km depth it is clear from the relative fault geometries that the overlying precordillera is not simply related to the basement activity the seismicity here may result from reactivation of an ancient suture between the precordillera and pampeanas terranes or be occurring in basement of unknown affinity west of the suture the seismicity provides the first constraints on basement fault geometries and we present models integrating this information with the surface geology these basement faults may have been responsible for the 1944 ms 7 4 earthquake that destroyed the city of san juan the imaging of these faults suggests that seismic risk estimates for san juan made on the basis of surface geologic studies may be too low

regional relative tectonic activity of structures in the pampean flat slab segment of argentina from 30 to 32 s abstract the pampean flat slab segment of the central andes in argentina is a broad active deformation zone subject to both plate boundary related and intraplate deformation to the west plate boundary related deformation is dominated by thin skinned fold and thrusts of the precordillera while to the east intraplate deformation involves thick skinned reverse faults of the sierras pampeanas gps data and absolute dating of displaced geomorphic features suggest that most of the active permanent deformation occurs between these two regions at the andean orogenic front and that there is a west to east trend of decreasing shortening rates however gps stations and sites with geomorphic slip rate measurements are currently too sparse and cover too short a time period to make reliable inferences about the longer term distribution and relative rates of deformation mountain range scale geomorphic indices on the other hand reflect tectonic activity since at least the pleistocene in this study we measured geomorphic indices hypsometric integral and curves basin elongation ratio basin volume to area ratio valley floor width to height ratio mountain front sinuosity and normalized steepness indices of rivers from 10 different n s striking range bounding faults that span both the precordillera and sierras pampeanas regions from west to east we use these data to assess the regional relative tectonic activity of major quaternary deformation features in the pampean segment of the central andes even when variations in climate and geology are considered mean values for each geomorphic index suggests uplift rates for the 10 mountain range bounding faults display local trends that are closely associated with the structural style and tectonic evolution of these structures although vertical uplift rates vary little horizontal shortening rates are higher for the precordillera fold and thrusts in the west than the thick skinned reverse faults of the sierras pampeanas in the east due to shallower fault dips for precordilleran structures the similarity with decadal slip rate gradients from gps studies could possibly indicate that the stress field has been constant since at least the pleistocene but further slip rate studies over broader time intervals in the late quaternary are necessary to confirm this variations observed in relative uplift rates along each fault point to sites that warrant further detailed study of slip rates and evaluation of associated seismic hazards

active basement uplift of sierra pie de palo northwestern argentina rates and inception from10be cosmogenic nuclide concentrations quaternary tectonic and denudation rates are investigated for an actively growing basement anticline the sierra pie de palo range which belongs to the andean foreland of northwestern argentina 28 s 33 s in this study a detailed morphometric analysis of the topography is combined with in situ produced cosmogenic10be concentrations measured in 1 surface boulders abandoned on alluvial terraces affected by fault activity along the north bounding fault and growth of the basement fold along the southeastern border 2 bedrock outcrops corresponding to an exhumed and folded regional erosion surface and 3 fluvial sediments sampled at the outlets of several watersheds along the eastern and northern borders of the range incision and uplift rates have been estimated at approximately 0 5 and 1 mm yr when integrated on holocene and pleistocene time scales in close agreement with both long term structural and basin evolution data and short term gps derived velocity field analyses cosmogenic derived denudation and uplift rates combined with geomorphic characteristics of watersheds and river channels allows estimating the onset of the uplift at 4 6 ma followed by a more recent period of topographic rejuvenation at roughly 1 2 ma probably synchronous with steepening of the eastern and northern flanks of the anticline

distribution of landslides along the andean active orogenic front argentinean precordillera 31 33 s abstract as one of the main environmental effects of earthquakes the distribution of landslides could reflect regional neotectonic activity to that end we carried out an inventory of landslides along the precordillera range 31 33 s comprising the most seismically active region of argentina main quaternary deformation is concentrated in this western central part of the country associated with active faults linked to intense shallow seismic activity

crustal seismicity in the andean precordillera of argentina using seismic broadband data abstract in this study we analyze 100 crustal precordilleran earthquakes recorded in 2008 and 2009 by 52 broadband seismic stations from the siembra and esp two temporary experiments deployed in the pampean flat slab region between the andean cordillera and the sierras pampeanas in the argentine andean backarc region in order to determine more accurate hypocenters focal mechanisms and regional stress orientations we relocated 100 earthquakes using the jhd technique and a local velocity model the focal depths of our relocated events vary between 6 and 50 km we estimated local magnitudes between 0 4 ml 5 3 and moment magnitudes between 1 3 mw 5 3 focal mechanisms were determined from new hypocenter relocations and first motion p wave polarities our solutions exhibit a majority of the earthquakes with reverse faulting mechanism regional stress tensor from the inversion of p and t axis orientations shows a maximum stress axis 1 almost horizontal with a strike of 85 and a minimum stress axis 3 almost vertical we correlate this small to moderate magnitude seismicity with the presence of large basement structures beneath the iglesia calingasta basin in the west and the eastern precordillera in the east the nucleation of deep earthquakes beneath the iglesia basin could be related to the presence of a major ramp accommodating the crustal shortening between the frontal cordillera and the precordillera the crustal seismicity beneath the precordillera seems to correlate with west dipping structures rooting deep into the cuyania basement suggesting a thick skinned basement deformation system beneath the precordillera and its shallow thin skinned fold and thrust belt

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

crustal wedging triggering recent deformation in the andean thrust front between 31 s and 33 s sierras pampeanas precordillera interaction 1 we document a new model of crustal structure of the andean front in argentina where numerous historic earthquakes destroyed the cities of mendoza in 1861 ms 7 and san juan in 1944 mw 7 0 the cerro salinas anticline is formed above the west directed cerro salinas thrust an east facing monocline with an amplitude of about 2 km folds the cerro salinas thrust and overlying neogene succession this monocline is formed above a blind crustal thrust in the basement its dip of 14 west is inferred from fold geometry this thick skinned east directed blind thrust and the thin skinned west directed cerro salinas thrust define a tectonic wedge the wedge tip occurs at a depth of 5 4 km growth of the monocline after 8 5 ma is revealed on multichannel seismic msc profile 31017 repsol ypf rates of cerro salinas thrust displacement are of the order of 1 mm yr whereas vertical uplift of 0 45 mm yr results from the combined displacement on the cerro salinas thrust and growth of east facing monocline the lateral extent of the east directed crustal blind ramp corresponds with the along strike extent of the eastern precordillera when combined with the low displacement rate a long earthquake recurrence interval is implied smaller magnitude earthquakes however indicate that segments of the blind thrust ramps ruptured in historic events if all the segments of the blind thrust ruptured together the fault area is 7000 km 2 and could produce a mw 7 7 earthquake the crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region

seismicity and tectonics in jujuy province northwestern argentina the portable array for numerical digital analysis panda network a digitally recorded seismic array operated for nine months in jujuy province of northwestern argentina the network was deployed along the eastern edge of the altiplano puna plateau in a major n s thrust belt that is transitional in style between the thin skinned deformation of the bolivian foreland to the north and basement involved deformation of the pampean region to the south teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front no crustal seismicity was detected beneath the puna plateau to the west peak seismicity levels beneath the foreland occurred between 20 and 25 km depth a sharp decrease in seismicity was observed below 25 km an estimate of 42 km for the thickness of the jujuy foreland crust was inferred from wide angle moho reflections observed on the digital seismograms the highest concentration of crustal seismicity was located beneath sierra de zapla a broad anticline immediately east of san salvador de jujuy many of the earthquakes in the 20 25 km depth range have a shallow west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74 closely paralleling both the current nazca south america convergence direction and the shortening direction derived from regional quaternary fault slip data we interpret the earthquakes as occurring on planes of weakness first produced during cretaceous rifting and later reactivated by neogene compressive stresses crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment along which significant late cenozoic e w shortening has occurred

seismic hazard reappraisal from combined structural geology geomorphology and cosmic ray exposure dating analyses the eastern precordillera thrust system nw argentina because earthquakes on large active thrust or reverse faults are not always accompanied with surface rupture paleoseismological estimation of their associated seismic hazard is a difficult task to improve the seismic hazard assessments in the andean foreland of western argentina san juan province this paper proposes a novel approach that combines structural geology geomorphology and exposure age dating the eastern precordillera of san juan is probably one of the most active zones of thrust tectonics in the world we concentrated on one major regional active reverse structure the 145 km long villic um pedernal thrust where this methodology allows one to 1 constrain the quaternary stress regime by inversion of geologically determined slip vectors on minor or major fault planes 2 analyse the geometry and the geomorphic characteristics of the villic um pedernal thrust and 3 estimate uplift and shortening rates through determination of in situ produced 1 0 be cosmic ray exposure cre ages of abandoned and uplifted alluvial terraces from a structural point of view the villic um pedernal thrust can be subdivided into three thrust portions constituting major structural segments separated by oblique n40 e trending fault branches along the three segments inversion of fault slip data shows that the development of the eastern precordillera between 31 s and 32 s latitude is dominated by a pure compressive reverse faulting stress regime characterized by a n110 10 e trending compressional stress axis 1 a geomorphic study realized along the 18 km long las tapias fault segment combined with cre ages shows that the minimum shortening rate calculated over the previous 20 kyr is at least of the order of 1 mm yr 1 an earthquake moment tensor sum has also been used to calculate a regional shortening rate caused by seismic deformation this analysis of the focal solutions available for the last 23 yr shows that the seismic contribution may be three times greater than the shortening rate we determined for the las tapias fault i e 3 mm yr 1 suggesting that the san juan region may have experienced a seismic crisis during the 20th century moreover the ramp that controls the development of the eastern precordillera appears to be one of the main seismic sources in the san juan area particularly the 65 km long villic um las tapias segment a first order evaluation of the seismic hazard parameters shows that this thrust segment can produce a maximum earthquake characterized by a moment magnitude of 7 3 0 1 and a recurrence interval of 2 4 1 5 kyr this part of the villic um pedernal ramp may have ruptured during the ms 7 4 1944 san juan earthquake producing very few surface ruptures and only distributed flexural slip deformation on to the neogene foreland bedding planes between the eastern precordillera and pie de palo