effect of the altiplano puna plateau south america on the regional intraplate stresses abstract a previous compilation of stress data for the sub andean region of northwest argentina is complemented with eight additional earthquake focal mechanisms seven events 5 3 5 5 mb had their mechanisms determined by inversion of teleseismic body waves short period p waves and long period sh waves all events had predominantly reverse faulting mechanisms five had almost pure dip slip motion and two had a component of oblique motion the other mechanism was a composite fault plane solution of the hualfin catamarca series of july 1986 maximum 4 mb determined with data from a local temporary network and the permanent inpres regional network the local data shows that the hualfin events occurred on a single fault plane oriented nw se with predominantly strike slip motion the stress data in the sub andean region suggests a radial pattern around the altiplano puna plateau with compressional stresses tending to be perpendicular to the eastern front of the plateau this indicates that local spreading stresses due to lateral density variations crustal thickening or lithospheric thinning of the plateau may be comparable or even exceed the plate wide regional stresses at the southeastern border of the puna plateau where a stress rotation of about 30 may be detected preliminary order of magnitude calculations indicate that the local plateau induced compression can be about 1 2 times greater than the regional horizontal deviatoric stress the present predominance of the local stress over the regional stress is consistent with the decrease of the nazca south america convergence rate observed since about 10 20 ma

tectonic evolution of south america during the late proterozoic abstract this paper discusses the evolution and the late proterozoic assembly of the south american continent i e the west gondwana under the light of global tectonics the northern portion of the continent the amazonian craton is a pre brasiliano domain it may be a fragment derived from laurentia this domain behaved as a large continental plate which was accreted to the central and southeastern portions of the continent brasiliano domain the brasiliano domain had a more complex history and composition during the brasiliano cycle middle proterozoic to early paleozoic in the brasiliano domain several late proterozoic continental plates have been identified sao luiz west africa congo kasai sao francisco rio de la plata arequipa antofalla which acted as forelands to the brasiliano orogenic belts that surround and amalgamate these plates smaller fragments of pre brasiliano continental lithosphere are common in the interior of the brasiliano orogenic belts and played diversified roles microplates microcontinents structural highs some of them have acted as true backlands hinterlands flanked by arc magmatism two principal types of brasiliano orogenic belts are recognized between the continental plates a marginal orogenies to the late proterozoic plates with pelitic carbonatic rocks and discrete volcanism which were thrust by collision onto the margin of the plates and which seem to represent final products of previous passive margin sequences b distal orogenics internal belts within the continental masses sometimes a branching system of orogenics that contain varied clastic and minor carbonate sedimentary rocks accompanied by important bimodal volcanism and calc alkaline plutonism for these orogenies a greater variety of basinal scenarios and tectonic settings may be visualized the amalgamation of the continent during the late proterozoic i e the formation of the brasiliano structures indicates the existence of practically synchronous multilateral stresses within lithospheric plates this can be produced by complementary adjustment of the continental plates as a response to major global events elsewhere analogous to modern continental collisions two large and important tectonic zones may be candidates to such a scenario the hijaz mozambique zone site of connection of east and west gondwana and the transbrasiliano lineament which cuts across south america and extends to north africa following the zone of connection between pre brasiliano and brasiliano domains other small ocean openings and closings inside the various brasiliano areas dom feliciano gariep for instance were minor events and complementary to the other two major tectonic zones mentioned above

teleseismic p wave tomography beneath the pantanal parana and chaco parana basins se south america delimiting lithospheric blocks of the sw gondwana assemblage

erosion rates and landscape evolution of the lowlands of the upper paraguay river basin brazil from cosmogenic 10be abstract the importance of earth s low sloping areas in regard to global erosion and sediment fluxes has been widely and vigorously debated it is a crucial area of research to elucidate geologically meaningful rates of land surface change and thus the speed of element cycling on earth however there are large portions of earth where erosion rates have not been well or extensively measured for example the tropical lowlands the cuiabana lowlands are an extensive low altitude and low relief dissected metamorphic terrain situated in the upper paraguay river basin central west brazil besides exposures of highly variable dissected metamorphic rocks flat residual lateritic caps related to a late cenozoic planation surface dominate interfluves of the cuiabana lowlands the timescale over which the lowlands evolved and the planation surface developed and the rate at which they have been modified by erosion are poorly known here we present measurements of in situ produced cosmogenic 10 be in outcropping metamorphic bedrock and clastic lateritic caps to quantify rates of erosion of the surface and associated landforms in order to better understand the quaternary landscape evolution of these lowlands overall slow erosion rates mean 10 m ma suggest a stable tectonic environment in these lowlands erosion rates vary widely between different lithologies range 0 57 to 28 3 m ma consistent with differential erosion driving regional landform evolution the lowest erosion rates are associated with the low relief area irregular plains where clastic laterite mean 0 67 m ma and quartzite mean 2 6 m ma crop out whereas the highest erosion rates are associated with dissection of residual hills dominated by metasandstone mean 11 6 m ma and phyllite mean 27 6 m ma these data imply that the cuiabana lowland is comprised of two dominant landform sets with distinct and different dynamics because the planation surface mostly lowlands is lowering and losing mass more slowly than associated residual hills regional relief is decreasing over time and the landscape is not in steady state the extremely slow erosion rates of the clastic laterite are similar to the slowest outcrop erosion rates reported worldwide these slow rates are due to the material s properties and resistance being comprised of quartzite fragments cemented by an iron rich crust and reflecting long term weathering with iron chemical precipitation and ferricrete formation at least since the middle pleistocene the lateritic caprock appears to be a key factor maintaining hilltop summits of the planation surface over long timescales

evolution of calc alkaline to alkaline magmatism through carboniferous convergence to permian transcurrent tectonics western chinese tianshan continuous magmatic activity occurred in the western chinese tianshan central asia from the carboniferous to the permian i e before and after the late carboniferous amalgamation of junggar and the yili blocks zircon u pb la icpms and ar ar data reveal a coincidence in time between regional wrench faulting and granitoid emplacement permian post collisional granitoids crop out within or at the margins of large scale dextral strike slip shear zones some of them show synkinematic fabrics the whole rock geochemical features of the early middle permian granitoids indicate an evolution from high k calc alkaline towards alkaline series in other places of the north tianshan alkaline magmatism occurred together with deep marine sedimentation within elongated troughs controlled by wrench faults therefore in contrast with previous interpretations that forwarded continental rift or mantle plume hypotheses the coexistence of diverse magmatic sources during the same tectonic episode suggests that post collisional lithosphere scale transcurrent shearing tightly controlled the magmatic activity during the transition from convergent margin to intraplate anorogenic processes

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

spatial and temporal uplift history of south america from calibrated drainage analysis a multi disciplinary approach is used to analyze the cenozoic uplift history of south america residual depth anomalies of oceanic crust abutting this continent help to determine the pattern of present day dynamic topography admittance analysis and crustal thickness measurements indicate that the elastic thickness of the borborema and altiplano regions is 10 km with evidence for sub plate support at longer wavelengths a drainage inventory of 1827 river profiles is assembled and used to investigate landscape development linear inverse modeling enables river profiles to be fitted as a function of the spatial and temporal history of regional uplift erosional parameters are calibrated using observations from the borborema plateau and tested against continent wide stratigraphic and thermochronologic constraints our results predict that two phases of regional uplift of the altiplano plateau occurred in neogene times regional uplift of the southern patagonian andes also appears to have occurred in early miocene times the consistency between observed and predicted histories for the borborema altiplano and patagonian plateaux implies that drainage networks record coherent signals that are amenable to simple modeling strategies finally the predicted pattern of incision across the amazon catchment constrains solid sedimentary flux at the foz do amazonas observed and calculated flux estimates match suggesting that erosion and deposition were triggered by regional andean uplift during miocene times

geochemical evidence for the trindade hotspot trace columbia seamount ankaramite abstract the columbia seamount 825 km offshore from brazil at 20 s lies on the east west trace of the trindade hotspot continental and oceanic magmatism believed to have originated with this hotspot is alkalic and sio2 undersaturated and dates from 85 ma in southern brazil to

fate of mantle plume material trapped within a lithospheric catchment with reference to brazil 1 local regions of thin lithosphere act as catchments of hot buoyant plume material unless replenished the trapped plume material cools by convection to the mantle adiabat by several tens of million years in particular currently hot material from the 130 ma parana starting plume head is unlikely to supply 85 ma to recent volcanism on the mainland of brazil and the martin vaz and fernando hot spots rather a plume tail may now underlie southern brazil where tomographic studies detect conduit shaped velocity anomaly through the upper mantle if the tomographic study in fact found a plume tail the track crossed the amazon rift at 85 ma and since then lateral flow along thin regions of the lithosphere from the tail fed widespread feeble volcanism including the flow line hot spots of martin vaz and fernando

continental scale rheological heterogeneities and complex intraplate tectono metamorphic patterns insights from a case study and numerical models abstract continental plates are built over long periods of time through successive extensional and compressional cycles they are therefore rheologically heterogeneous this heterogeneity should significantly influence the mechanical response of the continental lithosphere during collision processes the study of the neoproterozoic borborema shear zone system of northeast brazil highlights a systematic link between marked changes in its tectono metamorphic pattern and the pre existing structure of the plate that is characterized by juxtaposition of continental domains either comprising an old basement palaeo to eoproterozoic or accreted during an extensional event between 1 0 and 0 7 ga in neoproterozoic time when the shear zone system was developed these domains displayed different geotherms and lithospheric thicknesses and therefore contrasted rheological behaviours we use numerical models simulating the mechanical evolution of a continental plate comprising multiple thermally induced rheological heterogeneities submitted to compression to investigate how these heterogeneities may affect strain localization and the distribution of deformation regimes and vertical strain within the plate from the very beginning of the deformation weak and stiff heterogeneities induce strain localization due to a lower initial effective viscosity or to stress concentrations at their tips respectively shear zones propagate from the heterogeneities and finally coalesce forming a network of high strain zones bounding almost underformed blocks within this network shear zones transfer strain between the different heterogeneities and model boundaries the evolution of the system depends essentially on the geometrical distribution of heterogeneities and on their strength contrast relative to the surrounding lithosphere the resulting finite strain field is heterogeneous and displays rapid lateral variations in vertical and or rotational deformation such a heterogeneous strain distribution may induce contrasted magmatic metamorphic and uplift evolutions within an orogenic belt as observed in the borborema shear zone system and other collisional belts

analysis of the south american intraplate stress field the first order south american intraplate stress field was modeled through a finite element analysis to evaluate the relative contribution of plate boundary forces and intraplate stress sources the finite element mesh consisted of 3100 nodes in a network of 5993 equal area triangular elements which provided a spatial resolution of about 1 at the equator an important aspect of our modeling is the inclusion of topographic forces due to the cooling oceanic lithosphere along the mid atlantic ridge e g ridge push the continental margins along the east coast of brazil and argentina and the elevated continental crust e g the andean cordillera predicted intraplate stresses for two representations of the western collisional boundary forces are evaluated pinned collisional boundaries and applied collisional boundary forces constraint for the modeling was provided by information about the orientation of the maximum horizontal compressive stress shmax provided by 217 stress indicators from the world stress map project as well as by shmax magnitude estimates and torque information from previous investigations our modeling results demonstrate that the first order features of the observed stress field can be explained with simple tectonic models which balance the torque acting on the plate either with a fixed western margin or drag forces applied along the base of the plate the predicted intraplate stress field is characterized by a nearly uniform e w shmax orientation throughout most regions of the plate with stress magnitudes generally less than 20 mpa averaged over a 100 km thick lithosphere significant perturbation of this regional stress field occurs in the western part of the plate in response to forces associated with the high topography of the andes although the magnitude of the collisional boundary forces acting along the western margin remains poorly constrained we estimate a plausible upper bound on the force per unit length acting along the peru chile trench to be about 2 5 1012 n m 1 while some of our models are consistent with a driving basal drag to balance the torques acting on the plate the magnitude of the drag torque is small compared to the contribution from other sources of stress such as the ridge push force

geothermal investigations in an area of induced seismic activity northern sao paulo state brazil abstract geothermal investigations were carried out in nuporanga state of sao paulo brazil where occurrence of seismic activity has been found to be closely related to opening of groundwater wells results of macroseismic studies show that seismic activity had its beginning in may 1977 soon after completion of drilling of the colaba well and most of the initial seismic events are located close to it geothermal investigations were initiated in september 1977 on the assumption that fluid movements associated with seismic activities are capable of producing short term time dependent changes in the local thermal regime and in the hope that identification of such time dependent changes would contribute to a better understanding of the nature of local seismicity results of thermal logs in the colaba well reveal the existence of an unusual thermal regime with a constant temperature zone ctz down to 175 m followed by a zone in which temperature rises rapidly tgz in the interval of 180 204 m repeated thermal logs carried out over a period of three months reveal temperature drops of up to 0 8 c taking place in the tgz immediately after periods of intense microseismic activity temperature measurements of pumped water also show changes of lesser magnitude occurring in the ctz closely related to the frequency of seismic events substantial temperature changes related to periods of seismic activity were also observed in two nearby wells in nuporanga the available geothermal and macroseismic data have been used in the development of a simple model of the process that have triggered seismicity in nuporanga according to this model the colaba well acts as a natural siphon drawing water from a perched aquifer at depths of less than 40 m and injecting it to a fault zone at about 175 m the model allows a coherent explanation for the observed correlation between seismicity and absence of pumping during periods in which pumping is suspended the static level of water in the well is high and the pressure exerted by the water column induces a reduction in frictional resistance at the fault zone during pumping the water level falls to its dynamic level and consequently the pressure at the fault zone is also lower such pressure changes are apparently sufficient to trigger microseismic activity in nuporanga the observed temperature drops immediately after tremors could be attributed to the cooling effect associated with the penetration of relatively cold water from upper levels into newly opened fractures also the small but significant rise in temperatures during seismically quiescent periods can be considered as a result of warming up of stationary fluid bodies within fracture zones measures taken on the basis of this model has been successful in switching off seismicity on at least two occasions in view of this success we conclude that geothermal investigations can be of considerable use in understanding the nature of fluid movements associated with near surface earthquakes

rheology of some continental lower crustal rocks abstract to gain insight into the rheology of selected feldspar bearing rocks from the continental lower crust clc three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca 10 strain at confining pressures of 0 8 1 0 gpa temperatures of 600 900 c and constant strain rates of 10 4 10 7 s 1 the most extensive work was done on the mafic pikwitonei and felsic adirondack granulites the pikwitonei mg hornblende plag an70 2 px has only a limited t range of apparent steady state behavior and strain softens at t 800 c at 10 5 10 6s 1 apparently associated with developing shear zones this strain softening is pressure insensitive from pc 0 8 1 0 gpa at t 850 c 10 6s 1 even though the dominant strain accommodation mechanisms change from semibrittle cataclastic to crystal plastic including dynamic recrystallization with increasing pressure plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs kapuskasing plag cpx gt granulite deformed at 800 c and at 850 c 10 6 s 1 has deformed in the steady state at stress levels only slightly greater than the pikwitonei the stillwater microgabbro and adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady state behavior over the entire range of conditions investigated for the adirondack 2 felds 2 px gt qtz granulite quartz 21 appears to control the creep rate at these low strains and steady state mechanical data fit well a creep power law with a 8 10 3 mpa ns 1 qc 243 kj mol and n 3 1 extrapolation of this flow law and of preliminary power law creep relations for the pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone with the lower crust generally being the weaker material in addition the stain softening exhibited by the pikwitonei may bear directly on the development of detachment zones at mid to lower crustal depths during prograde metamorphism

intraplate seismicity in se brazil stress concentration in lithospheric thin spots summary intraplate seismicity has generally poor correlation with surface geological patterns except for major extensional features such as aborted continental rifts which may act as weak zones it is usually difficult to find simple geology based models to explain differences in seismic activity in stable continental regions seismicity in brazil is clearly not uniform and a few areas of higher activity have been identified however the seismic areas show almost no correlation with the main geological provinces which is typical of other intraplate settings a recent uppermantle tomography study in se and central brazil using approximately 8500 p wave and 2000 pkp wave arrivals recorded in 59 sites since 1992 has mapped p wave velocity anomalies from lithospheric depths down to 1300 km in this region higher seismic activity occurs preferentially in areas with low p wave velocities at 150 250 km depth the low p wave velocities are interpreted as shallower asthenosphere in such areas a hotter geotherm will reduce the strength of the lithospheric upper mantle causing most of the intraplate forces to be concentrated in the brittle upper crust the low velocity anomalies coincide with late cretaceous provinces of alkaline intrusions the proposed ponding of the trindade plume head beneath lithospheric thin spots is consistent with our tomography results suggesting that plume effects may have helped to preserve lithosphere asthenosphere topography although other factors are also important the present data show that stress concentrations resulting from lithosphere asthenosphere topography should play an important role in explaining the intraplate seismicity in the brazilian platform

average lithospheric structure of southern africa summary we present a new 1 d earth model for the crust and upper mantle beneath the archaean core of southern africa the crustal structure is constrained by published seismic refraction reflection data and by the modelling of teleseismic receiver function data the mantle structure is constrained by traveltime measurements for p and s waves by waveform inversion of multimode surface waves and by forward modelling of p waveforms for regional earthquakes the 42 km thick crust of southern africa consists of four layers a 2 km thick 5 05 km s surface layer a 5 km thick 6 08 km s upper crust a 20 km thick 6 30 km s l mid crust and a 15 km thick 6 73 km s lower crust below the crust there is an 80 km thick upper mantle lid v and v are 8 09 and 4 62 km s l respectively at the top of the lid and the compressional and shearvelocity gradients through the lid are 0 0008 s l and 0 0013 s respectively below the lid there is a substantial shear wave lvz with the shear wave velocity dropping from 4 72 km s l in the lower part of the lid to 4 32 km s at 250 km depth the shear wave lvz is required to fit both the v measurements and the regional surface waveforms forward modelling of p suggests that no upper mantle p wave lvz exists there is an increase in the p wave velocity gradient at 125 km depth to 0 0015 s and a second increase at 250 km depth to 0 0035 s velocities and densities in the seismic model of the lid agree closely with velocity and density estimates from the geochemical analysis of garnet periodotite nodules from kimberlites of the kaapvaal craton implying that the estimates from kirnberlites are representative of a wider region of the archaean core of southern africa not just of the restricted region beneath which the kimberlites are found a comparison of the southern african seismic model with seismic models for other shield regions where nodule data are not available shows that the nodule results may also be representative of the upper mantle in those regions

signification structurale des anomalies gravimetriques de la partie sud du craton de sao francisco bresil abstract the relationships between the cratonic area and orogenic belts of the southern part of the state of minas gerais brazil are analyzed using data from a recent bouguer anomaly map the interpretation was carried out using inverse anomaly approach and statistical analysis the stable area is characterized by elongate anomalies trending nne ssw which correspond to a higher level of the basement under the upper precambrian sedimentary cover bambui and macaubas groups they can be related to rejuvenated nne ssw trending fractures the gravity trends are either deformed or interrupted over the brasilian fold belt where an important negative linear anomaly is present this anomaly is produced by a thick sequence of folded sedimentary or metasedimentary terrains which can be interrupted as miogeoclinal units the alfenas mobile belt is characterized by a strong positive anomaly probably associated with heavy infracrustal rock complexes granulites charnockites a crustal thickening probably associated with a pre brazilian continental collision can be inferred from a long wavelength linear negative anomaly

some seismological and geometric features of intraplate earthquakes to understand the nature and cause of intraplate earthquakes we carried out a systematic search for common features associated with them various geological geophysical and seismic data were analyzed and geometric and seismic parameters were compared for 29 intraplate events m 4 5 occurring worldwide the results suggest that intraplate earthquakes occur by the reactivation of pre existing zones of weakness in a compressional stress regime which is generally oriented parallel to the absolute direction of plate motion two styles of faulting were observed for type a earthquakes observed in the eastern united states eastern china western europe and west africa deformation occurs by strike slip motion on steeply dipping faults type b events are usually associated with thrust or normal faulting and were observed in eastern canada fennoscandia australia and peninsular india focal depths did not explain the differences between type a and type b earthquakes rather the differences are due to the perturbation of a regional stress field by a local stress field the regional stress field for type a events is primarily due to ridge push forces the method of stress perturbation varied from region to region the earthquakes in the type b category were further divided into three subcategories type b1 occurring in eastern canada and fennoscandia type b2 occurring in australia peninsular india and central brazil and type b3 occurring in the gulf coast of the u s the inferred causes of the perturbing stresses are deglaciation intracontinental resistive forces resulting from the collision of plates and the deposition of a large thickness of sediments respectively type a events were found to be associated with intersecting faults and generally supported the intersection model of talwani 1988 and had analogs in plate boundary events

the intracratonic caraibas itacarambi earthquake of december 09 2007 4 9 mb minas gerais state brazil abstract on december 9 2007 a 4 9 mb earthquake occurred in the middle of the sao francisco craton in a region with no known previous activity larger than 4 mb this event reached intensity vii mm modified mercalli causing the first fatal victim in brazil the activity had started in may 25 2007 with a 3 5 magnitude event and continued for several months motivating the deployment of a local 6 station network a three week seismic quiescence was observed before the mainshock initial absolute hypocenters were calculated with best fitting velocity models and then relative locations were determined with hypodd the aftershock distribution indicates a 3 km long rupture for the mainshock the fault plane solution based on p wave polarities and hypocentral trend indicates a reverse faulting mechanism on a n30 e striking plane dipping about 40 to the se the rupture depth extends from about 0 3 to 1 2 km only despite the shallow depth of the mainshock no surface feature could be correlated with the fault plane aeromagnetic data in the epicentral area show short wavelength lineaments trending nne ssw to ne sw which we interpret as faults and fractures in the craton basement beneath the surface limestone layer we propose that the caraibas itacarambi seismicity is probably associated with reactivation of these basement fractures and faults under the present e w compressional stress field in this region of the south american plate

effects of density contrasts on the orientation of stresses in the lithosphere relation to principal stress directions in the transverse ranges california the influence of stresses arising from horizontal density contrasts on the orientation and relative magnitudes of principal stresses in an otherwise uniform lithospheric stress field is investigated a simple model is constructed in which a local deviatoric stress due to a density anomaly embedded within or just below the lithosphere and a regionally constant deviatoric stress field are each approximated by biaxial tensors the net stress field is obtained from the sum of the two both the relative magnitudes of principal stresses and the magnitude of the angular difference in principal stress direction of the summed tensor compared with that obtained in the absence of buoyancy forces depend on two parameters the first is the ratio where is a measure of the magnitude of the regional deviatoric stress and is the magnitude of the stress arising from buoyancy forces associated with the density anomaly the second parameter is the angle between the trend of the density anomaly and the direction of maximum compressional stress that obtains in the absence of any perturbation by the local buoyancy forces the directions of the principal axes of the total stress field are found to differ by up to 90 from those of the reference stress field the model is applied to the transverse ranges california where the observed 23 difference in orientation of principal horizontal compressive stress compared with the principal compressive stress direction in central california constrains the predicted value of to be approximately 0 4 this is consistent with an independently calculated range of in which is inferred from seismological constraints on the magnitude of density variations underneath the transverse ranges and is inferred from observations of heat flow along the san andreas fault in central california the agreement between the two estimates of supports the hypothesis that the observed differences in horizontal principal stress orientation in california can be explained by the combined influence of a local negative buoyancy force under the transverse ranges and a regional stress field associated with transcurrent deformation within the pacific north american plate boundary zone the observed counterclockwise angular difference in principal horizontal stress direction in the transverse ranges compared with central california implies that the plane of maximum right lateral shear stress is also rotated counterclockwise relative to that in central california this supports the possibility that the big bend in the san andreas fault may be a consequence of the negative buoyancy forces acting in the transverse ranges and not the cause of transverse ranges formation as has often been assumed

statistical trends in the intraplate stress field the world stress map wsm database contains thousands of intraplate stress indicators with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion previous studies which relied almost exclusively on visual inspection of the data established the existence of broad regions of uniform maximum horizontal compressive stress orientation shmax and stress regimes on a regional scale in the present study we present a statistical analysis of the wsm stress indicators with the aim of quantifying trends in both the shmax orientations and stress regimes the analysis was carried out within 5 5 bins which provide a resolution of several hundred kilometers only the 4537 high quality wsm indicators with rating of a to c were used in the analysis we present results for two types of analysis on the information contained within the bins first we evaluate the spatial distribution of the average stress regime normal strike slip or thrust second we apply the rayleigh test a standard statistical method in the analysis of directional data to the distribution of shmax orientations to test the null hypothesis that the orientations are random an important aspect of our study is the quantification of the conclusions drawn from visual inspection of the world stress map our results indicate that broad regions of uniform shmax orientations exist in most continental regions at high confidence levels 90 and 95 and are less robust in the slowest moving continental plates we also quantify the predominance of strike slip and compressional stress regimes in continental regions importantly our analysis provides information about trends in the shmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data for example in western north america and continental australia furthermore we find a strong correlation between average shmax orientations and both the ridge push torque and the absolute plate velocity azimuths our observation that a greater number of shmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force

focal mechanisms of small earthquakes in the southeastern brazilian shield a test of stress models of the south american plate summary focal mechanisms of small earthquakes with magnitudes of about 3 in the se brazilian shield are calculated using s p amplitude ratios low attenuation q p from 400 to 800 in the shield upper crustal layers allowed sharp s arrivals to be recorded up to distances of 100 km besides p wave polarities sh wave first motions were also used to constrain the nodal plane orientations normal and reverse faulting mechanisms with strike slip components were found the inversion of four mechanisms to estimate the stress tensor indicated a strike slip stress regime with roughly e w orientated s 1 and n s s 3 both the orientations and the shape factor q 0 7 of the inverted stress are in excellent agreement with theoretical predictions for that part of brazil from the driving force model of coblentz richardson 1996 good agreement with the nature of the stress as well as its orientation was also found for the model of meijer 1995 both of these theoretical models include spreading stresses along the continent ocean lithospheric transition because the earthquakes are more than 300 km from the continental shelf they should not be avected by the local flexural forces caused by sediment load in the marginal basins the agreement between observed and theoretical stresses then confirms the importance of continental spreading forces in modelling intraplate stresses

u and th zoning in cerro de mercado durango mexico fluorapatite insights regarding the impact of recoil redistribution of radiogenic 4he on u th he thermochronology abstract fluorapatite from cerro de mercado durango mexico has been used in several important diffusion studies aimed at calibrating the closure temperature of the u th he apatite thermochronometer and has become a de facto standard for u th he laboratories nevertheless replicate laser fusion analyses of small crystal fragments of the durango fluorapatite typically yield a range of u th he dates that cannot be explained by reported analytical imprecision alone laser ablation icpms measurements of u and th of a section cut through a representative crystal of durango fluorapatite perpendicular to the c axis reveal the existence of significant zoning in these elements since alpha stopping distances in apatite are comparable to the scale of this zoning the ejection of radiogenic 4he from one zone to another may explain some of the reported fragment to fragment variation in u th he dates two dimensional numerical models of the recoil redistribution of 4he in a crystal with the zoning we observed lead to predicted age variations of several percent for fragments with sizes and aspect ratios typical of those used for laser fusion of durango fluorapatite it is likely that large crystals or large aliquots of small fragments of zoned materials may be acceptable as bulk interlaboratory standards however the current trend is toward the use of smaller and smaller sample sizes including broken crystals for u th he thermochronology our work indicates that heterogeneities in the u and th concentrations of such samples are a serious concern so much so that high precision u th he geochronologic studies should include a concerted effort to characterize the extent of u and th zoning in analyzed crystals

stress perturbation associated with the amazonas and other ancient continental rifts the state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts specifically a dense rift pillow in the lower crust may modify substantially the regional stress field both shallow 2 0 2 6 km depth breakout data and deep 20 45 km depth crustal earthquake focal mechanisms indicate a n to nne maximum horizontal compression in the vicinity of the paleozoic amazonas rift in central brazil this compressive stress direction is nearly perpendicular to the rift structure and represents a 75 rotation relative to a regional e w compressive stress direction in the south american plate elastic two dimensional finite element models of the density structure associated with the amazonas rift as inferred from independent gravity modeling indicate that elastic support of this dense feature would generate horizontal rift normal compressional stresses between 60 and 120 mpa with values of 80 100 mpa probably most representative of the overall structure the observed 75 stress rotation constrains the ratio of the regional horizontal stress difference to the rift normal compressive stress to be between 0 25 and 1 0 suggesting that this rift normal stress may be from 1 to 4 times larger than the regional horizontal stress difference a general expression for the modification of the normalized local horizontal shear stress relative to the regional horizontal shear stress shows that the same ratio of the rift normal compression relative to the regional horizontal stress difference which controls the amount of stress rotation also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress the potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike slip stress regimes in the amazonas rift case because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift normal compression to be between 0 25 and 1 0 our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced for ratios 0 5 or enhanced for ratios 0 5 additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust faulting stress regime in the vicinity of the amazonas rift a rift normal stress associated with the seismically active new madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike slip faults parallel to the axis of the rift although this results in a 20 40 reduction in the local horizontal shear stress within the seismic zone sparse stress data in the vicinity of the seismically quiescent midcontinent rift of the central united states suggest a stress state similar to that of new madrid with the local horizontal shear stress potentially reduced by as much as 60 thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows the modeling and analysis here demonstrate that rift normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

seismicity and stresses in the brazilian passive margin abstract seismicity and stresses along the brazilian continental margin show different patterns between the northeastern and southeastern regions in the northeastern margin earthquakes tend to occur onshore under a strike slip regime with horizontal compression parallel to the northern coast line in the southeastern margin higher seismicity is observed offshore in areas where the continental crust was highly extended during the south atlantic rifting in the mesozoic three new events in the southeast continental shelf are shown to have reverse faulting mechanisms from waveform modeling of short period teleseismic p waves multiple depth phases reflections from the top and bottom of the water layer and double reflections in the water layer could be identified and better constrained the hypocenters to middle and upper crustal depths in south america models of intraplate stresses caused by plate boundary forces and spreading effects due to the continental oceanic crustal transition indicate higher compressional stresses in the se offshore area as compared with the continental area in agreement with the observed higher seismicity and reverse faulting mechanisms the combination of regional stresses local flexural effects from thick sedimentary loads and a presumably weaker crust from mesozoic thinning explains the main patterns of seismicity in the northeastern and southeastern brazilian margins

stress orientations in brazilian sedimentary basins from breakout analysis implications for force models in the south american plate summary regional patterns of crustal stresses in brazil were studied with a detailed breakout analysis performed in 541 wells distributed throughout the country 481 from basins along the continental margin and 60 from intracratonic basins a total of about 591 km of cumulative well length digitally sampled at 0 15 m intervals was analysed most intervals 80 per cent are confined between 400 and 4000 m only wells that deviate less than 10d from the vertical were considered for a given well 1 the mean orientation and the standard deviation of each interval with breakouts were calculated 2 all intervals with mean breakout orientations less than 10d from the orientation of the hole deviation were discarded 3 all intervals with standard deviations less than 12d were selected 4 the selected breakouts were weighted by their interval length to calculate the mean orientation and standard deviation of the whole well and 5 the final results were classified according to the world stress map criteria thus 16 wells were classified as a a total of 7 2 km of breakouts 42 as b 7 1 km 78 as c 7 4 km 205 as d 3 3 km 184 as e 0 7 km and 16 were discarded in most basins the breakout orientations from different wells were usually consistent for qualities a b and c allowing a good estimate of the regional maximum horizontal stress shmax wells classified as quality d showed a large scatter the regional shmax determined from breakouts is generally in good agreement with the available nearby focal mechanisms the main trends of shmax are 1 nw se parallel to the coast along the equatorial marginal basins 2 e w in the alagoas basin 3 nne ssw parallel to the coast in the sergipe and reconcavo basins and 4 nw se in the middle amazon basin in the equatorial and eastern continental margins north of 15ds breakouts and focal mechanism measurements indicate that the shmax orientation is remarkably parallel to the coastline following a 90d bend of the coast in northeastern brazil because the theoretically predicted intraplate stresses in northern brazil trend about wnw to ene e g meijer 1995 coblentz richardson 1996 we interpret our observations as indicating that local sources of stress at the continental margins e g flexural stresses and lateral density contrasts dominate the plate wide stresses and may have been underestimated in theoretical stress models of the south american plate the new stress data 136 a c quality out of a population of 541 wells cover a region of mid plate south america where only 23 data points had been compiled previously in the world stress map database the more detailed observed patterns of the intraplate stress field should be helpful in better constraining future models of the forces driving the south american plate

seismic velocity anomalies beneath se brazil from p and s wave travel time inversions 1 we present the result from teleseismic travel time inversions for p and s wave data mainly recorded at portable broadband stations in se brazil the stations were deployed at 45 sites within an area 1000 1700 km during the years 1992 1999 more than 10 000 relative p and s wave arrival times including core phases were obtained from the waveforms using a new coherence functional these p and s relative phase times are independently inverted for slowness perturbations earthquake relocations and station corrections the final models represent the least amount of structure required to explain the residuals within a defined standard error the robust and consistent features in the velocity anomaly models are interpreted and their resolution is tested with synthetic case inversions we confirm the existence of a cylindrical low velocity anomaly beneath the parana basin which has been interpreted by vandecar et al 1995 as the fossil conduit through which the initial tristan da cunha plume head traveled to generate the parana etendeka flood basalts about 130 ma we now show that this low velocity cylindrical structure seems to be confined to the upper mantle beneath the upper mantle the velocity anomalies show a n s oriented pattern which we interpret as due to the nazca plate subducted slab at lithospheric depths the archean southern part of the sao francisco craton shows high velocities down to 200 300 km all areas with late cretaceous postrift alkaline intrusions are characterized by low velocities at lithospheric depths

passive margin evolution initiation of subduction and the wilson cycle we have constructed finite element models at various stages of passive margin evolution in which we have incorporated the system of forces acting on the margin depth dependent rheological properties and lateral variations across the margin we have studied the interrelations between age dependent forces geometry and rheology to decipher their net effect on the state of stress at passive margins lithospheric flexure induced by sediment loading dominates the state of stress at passive margins this study has shown that if after a short evolution of the margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favourable for transformation into an active margin although much geological evidence is available in support of the key role small ocean basins play in orogeny and ophiolite emplacement evolutionary frameworks of the wilson cycle usually are cast in terms of opening and closing of wide ocean basins we propose a more limited role for large oceans in the wilson cycle concept