hypocentre determination offshore of eastern taiwan using the maximum intersection method summary the maximum intersection maxi method which derives from the master station method msm determines within a 3 d velocity model the absolute hypocentral location based on observed arrival times first the spatial node that better satisfies the arrival time differences computed at all station pairs plus or minus an error tolerance value in seconds is defined as the preliminary hypocentral solution pred second because pred depends neither on the estimate of origin time nor on the residual root mean square rms residual outliers are objectively detected and cleaned out from the original data set without any iterative process or weighting third a statistical minimization residual rms is conducted in a small domain around the pred node which results in a unique final solution the maxi method is applied to the determination of earthquake hypocentres with the proper station correction terms in the southernmost extremity of the ryukyu subduction zone where several dense seismic clusters occur near the seismogenic plate interface the location of earthquakes recorded at both the taiwanese and japanese networks is obtained for about a thousand events between 1992 and 1997 the process uses a detailed 3 d velocity model based on multiple geophysical data sources obtained in the junction area between subduction and collision east of taiwan the earthquake clustering and the significant drop in residual statistics 1 20 0 80 and 0 35 s for taiwanese catalogue msm and maxim solutions respectively indicate the accuracy of the method which can be used to routinely determine absolute hypocentre location based on observed arrival times

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

automatic reconstruction of fault networks from seismicity catalogs three dimensional optimal anisotropic dynamic clustering 1 we propose a new pattern recognition method that is able to reconstruct the three dimensional structure of the active part of a fault network using the spatial location of earthquakes the method is a generalization of the so called dynamic clustering or k means method that partitions a set of data points into clusters using a global minimization criterion of the variance of the hypocenters locations about their center of mass the new method improves on the original k means method by taking into account the full spatial covariance tensor of each cluster in order to partition the data set into fault like anisotropic clusters given a catalog of seismic events the output is the optimal set of plane segments that fits the spatial structure of the data each plane segment is fully characterized by its location size and orientation the main tunable parameter is the accuracy of the earthquake locations which fixes the resolution i e the residual variance of the fit the resolution determines the number of fault segments needed to describe the earthquake catalog the better the resolution the finer the structure of the reconstructed fault segments the algorithm successfully reconstructs the fault segments of synthetic earthquake catalogs applied to the real catalog constituted of a subset of the aftershock sequence of the 28 june 1992 landers earthquake in southern california the reconstructed plane segments fully agree with faults already known on geological maps or with blind faults that appear quite obvious in longer term catalogs future improvements of the method are discussed as well as its potential use in the multiscale study of the inner structure of fault zones

testing self organized criticality in the crust using entropy a regionalized study of the cmt global earthquake catalogue 1 we test the notion of self organized criticality soc and the proximity to the critical point in the brittle crust if the system were strictly critical we would expect an infinite correlation length with minimal temporal or spatial predictability an alternative view is that the earth is in a self organized subcritical sosc state with a finite and systematically fluctuating correlation length this would imply a system that is sufficiently near critical to maintain power law scaling relations over a finite scale range but can intermittently reach criticality in the form of a single large earthquake when the correlation length becomes effectively infinite over the scale of the observed region here we address the question of proximity to criticality from the viewpoint of statistical physics by describing a regionalized study equivalent to the ensemble approach in thermodynamics flinn engdahl regionalization of global seismicity is used to calculate the expectation of the logarithm of energy lne and entropy s from centroid moment tensor cmt data for different seismic regions we compare a phase diagram for s and lne from the data with an analytical statistical mechanical solution and find that they are in good agreement the analysis shows systematic spatial heterogeneity in entropy that is associated with the tectonic deformation style oceanic ridges are seen to be low entropy relatively ordered and subduction zones have higher entropy less ordered with collision zones scattered at the two extremes statistically resolvable phase variations in the system as a whole point toward it being better described as subcritical in the spatial ensemble

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

stages of late palaeozoic deformation and intrusive activity in the western part of the north patagonian massif southern argentina and their geotectonic implications analyses of structures in the western part of the north patagonian massif southern argentina suggest a polyphase evolution accompanied by continuous intrusive activity the first two deformations d 1 d 2 and metamorphism affected the upper palaeozoic partly possibly older cushamen formation clastic succession and different intrusive rocks a second group of intrusions emplaced after the second deformational episode d 2 in many places contain angular xenoliths of the foliated country rocks indicating high intrusive levels with brittle fracturing of the crust deformation of these magmatic rocks presumably began during the final stage of cooling and continued under solid state conditions it probably coincided with the third deformational event d 3 in the country rocks based on published u pb zircon ages of deformed granitoids the d 2 deformation and younger event along with the regional metamorphism are likely to be permian in age an onset of the deformational and magmatic history during carboniferous times however cannot be excluded the estimated w e to ne sw compression during the d 2 deformation also affecting the first group of intrusive rocks can be related to subduction beneath the western patagonia margin or an advanced stage of collisional tectonics within extra andean patagonia the younger n s to ne sw compression might have been an effect of oblique subduction in the west and or continuing collision related deformation as a cause for its deviating orientation younger block rotations during strike slip faulting cannot be excluded the previous d 2 event presumably also had an effect on compression at the northern patagonia margin that was interpreted as result of patagonia s late palaeozoic collision with the southwestern gondwana margin with the recently proposed carboniferous subduction and collision south of the north patagonian massif the entire scenario might suggest that patagonia consists of two different pieces that were amalgamated with southwestern gondwana during late palaeozoic times

the spatial and temporal distribution of pipe formation offshore namibia a group of nearly 400 pipe structures from the continental slope of northern namibia are analysed for their spatial and temporal distribution the pipes most likely formed as a result of highly focused fluid venting and understanding the factors controlling their distribution in space and time is key to their genesis we analysed their spatio temporal distribution using an arbitrary chronostratigraphic timescale from which it is concluded that the pipes did not form at the same time pipe formation is shown to be intermittent and persistent with 2 29 pipes forming in each of the 20 arbitrary time intervals that are considered to span the neogene period the spatial distribution of these pipes is clustered to dispersed spatial statistics conducted on the distribution of pipe formation timings have shown that two statistically significant groups of pipes exist within the population 1 in the north and west and 2 in the south with the former occurring prior to the latter locally pipe formation is sporadic with clusters and outliers occurring during the same time period a conceptual model is proposed whereby pipe formation in specific locations is the result of localised breaching of the seals for isolated pressure cells which are locally independent yet broadly controlled an inferred basinal fluid source is thought to determine the broader patterns of pipe formation and the focus of this fluid source shifts from north to south with time at a local scale multiple local factors interact producing a sporadic pipe formation distribution through a prolonged period of highly focused fluid migration once formed the pipes continued to focus fluids intermittently leading in some cases to later pockmark formation

diffusion models for corona formation in metagabbros from the western grenville province canada metagabbro bodies in sw grenville province display a variety of disequilibrium corona textures between spinel clouded plagioclase and primary olivine or opaque oxide textural evidence favours a single stage subsolidus origin for the olivine coronas and diffusive mass transfer is believed to have been the rate controlling process irreversible thermodynamics have been used to model two different garnet symplectite bearing corona sequences in terms of steady state diffusion in the models the flux of each component is related to the chemical potential gradients of all diffusing species by the onsager or l coefficients for diffusion these coefficients are analogous to experimentally determined diffusion coefficients d but relate the flux of components to chemical potential rather than concentration gradients

geochemical evidence for lacustrine microbial blooms in the vast permian main karoo parana falkland islands and huab basins of southwestern gondwana early permian black pyritic shale units i e the whitehill irati and huab formations and black rock member in southwestern gondwana basins have elevated 13c values of total organic carbon up to 17 pdb hydrogen indices between 300 and 1000 mg co2 g c and oxygen indices 30 mg co2 g c these data suggest a manifestation of a microbial bloom event values for 13c between 26 and 22 pdb hydrogen 100 mg co2 g c and oxygen indices of total organic carbon above and below these rock units are typical for palaeozoic terrestrial type iii organic matter carbon 13 isotope stratigraphy concur with palaeontological evidence that these formations are stratigraphical equivalents and therefore that the microbial blooms covered a vast area 5 million km2 possibly earth s most expansive microbial bloom the palaeoenvironment of the rock units preceding and including the black shales was most likely lacustrine with fresh to brackish water as shown by stable isotope c o and s evidence from organic matter carbonates and pyrite the implication is that a broad tectonic belt which formed as a result of compressional tectonics during the late carboniferous to early permian from the palaeo west must have been sufficiently well developed to cut off any contact with seawater earlier than predicted late permian from previous stratigraphic and sedimentological studies stable isotope data supported by mineralogical evidence indicate that conditions particularly in the upper portions of the lacustrine oil shales were brackish and locally may have been evaporitic we envisage that the microbial blooms possibly botryococcus peaked in the upper half of the black shale units conditions rapidly became unfavourable for microbial blooms across the entire basin in the overlying formations manifested by a change from oil shales type i organic matter to organic poor type iii shales this may possibly have been a consequence of tectonic activity and interrelated causes stable isotope data of carbonates in the overlying sediments indicate that in the palaeo north the basin became evaporitic drying up and in the palaeo southern part of the basin high latitude altitude water flooded the basin and so brought to a close the microbial blooms

distribution and origin of hybrid beds in sand rich submarine fans of the tanqua depocentre karoo basin south africa abstract this study documents the stratigraphic and palaeogeographic distribution of hybrid event beds that comprise both debris flow cohesive and turbidity current non cohesive deposits this is the first study of such beds in a submarine fan system to combine outcrop and research borehole control and uses a dataset from the skoorsteenberg formation of the tanqua depocentre in the karoo basin south africa three types of 0 1 1 0 m thick hybrid beds are observed which have a basal weakly graded fine grained sandstone turbidite division overlain by a division of variable composition that can comprise 1 poorly sorted carbonaceous rich material supported by a mud rich and micaceous sand matrix 2 poorly sorted mudstone clasts in a mud rich sand silt matrix or 3 gravel grade rounded mudstone clasts in a well sorted mud poor sandstone matrix these upper divisions are interpreted respectively as 1 the deposit of a debris flow most likely derived from shelf edge collapse 2 the deposit of a debris flow most likely developed through flow transformation from turbidity current that eroded a muddy substrate and 3 from a turbidity current with mudstone clasts transported towards the rear of the flow all three hybrid bed types are found concentrated at the fringes of lobes that were deposited during fan initiation and growth the basinward stepping of successive lobes means that the hybrid beds are concentrated at the base of stratigraphic successions in medial and distal fan settings hybrid beds are absent in proximal fan positions and rare and thin in landward stepping lobes deposited during fan retreat this distribution is interpreted to reflect the enhanced amounts of erosion and availability of mud along the transport route during early lowstands of sea level therefore hybrid beds can be used to indicate a fan fringe setting infer lobe stacking patterns and have a sequence stratigraphic significance

patterns of co seismic strain computed from southern california focal mechanisms summary geometrical properties of an earthquake population can be described by summation of seismic potency tensors that provide a strain based description of earthquake focal mechanisms we apply this method to 170 000 potency tensors for 0 ml 5 southern california earthquakes recorded between january 1984 and june 2003 we compare summed tensors for populations defined by faulting region and earthquake magnitude to investigate the relation between earthquake characteristics tectonic domains and fault related length scales we investigate spatial scales ranging from 1 700 km and use the results to identify systematic differences between seismic behaviour for different faults and different regions our results show features that are indicative of both scale invariant and scale dependent processes on the largest scale the overall potency tensor summation for southern california 0 ml 5 earthquakes over 20 yr corresponds closely to a double couple dc mechanism with slip direction parallel to relative plate motion the summed tensors and derived quantities for the different regions show clear persistent variations that are related to the dominant tectonic regime of each region significant differences between the non dc components of the summed tensors which we relate to fault heterogeneity indicate systematic differences in deformation associated with earthquake populations from different fault zones or different magnitude ranges we find an increase of heterogeneity for populations of smaller earthquakes and for regions where faulting deviates strongly from the overall sense of deformation even when corrected for quality the results imply an overall organization of earthquake characteristics into domains that are controlled to first order by geometrical properties of the largest faults and the plate motion smaller scale characteristics are related to local variations in the orientation complexity and size of faults

active detachment of taiwan illuminated by small earthquakes and its control of first order topography we use 110 000 small earthquakes to locate and map active faults in three dimensions within the taiwan arc continent collision the structure is dominated by a nearly horizontal band of small earthquakes at 10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt with other illuminated fault zones abutting the detachment zone the zone steepens below eastern taiwan to 30 90 and reaches depths of 30 60 km the three dimensional shape of the detachment zone in relation to topography allows a new test of critical taper wedge mechanics and suggests that the reversal of topographic slope across taiwan is controlled by the shape of the detachment

lack of continuity of the san andreas fault in southern california three dimensional fault models and earthquake scenarios 1 the 1200 km long san andreas fault loses its apparent continuity in southern california near san gorgonio pass allen 1957 which raises significant questions given the dominant role of this fault in active california tectonics what is the fundamental three dimensional 3 d geometry and kinematic behavior of the san andreas fault system in this complex region is a throughgoing if complex san andreas rupture from the mojave desert to the coachella valley possible we have explored the issue of 3 d continuity by mapping over 60 faults in this region to depths of 15 20 km from hypocenter locations and focal mechanisms we were able to constrain the 3 d geometry of the san andreas fault zone saf near san gorgonio pass from the 3 d geometry of the fault network surrounding it the most likely configuration is for the san andreas fault to merge into the shallow dipping san gorgonio pass thrust northwest of indio we concluded that there is no direct continuity at present but rather a network of faults and the only kind of rupture possible for the saf in this region is a complex rupture involving both strike slip and reverse faulting gps measurements also suggest that despite the fact that large motions must have occurred in the past based on offset geologic markers only minor motion is occurring today in this area applying our findings about the fault geometry we explored several simple earthquake scenarios to determine the most favorable conditions for a throughgoing rupture of the san andreas fault system from the mojave desert to the coachella valley

tectonic control on fluvial styles the balfour formation of the karoo basin south africa abstract the balfour formation represents a fully fluvial succession of late late permian earliest triassic age which accumulated in the foredeep of the karoo basin during the overfilled phase of the foreland system the lack of a coeval marine environment within the limits of the preserved karoo basin provides an opportunity to study the stratigraphic cyclicity developed during a time when accommodation was solely controlled by tectonics the balfour stratigraphy is composed of a succession of six third order fluvial depositional sequences separated by subaerial unconformities they formed in isolation from eustatic influences with a timing controlled by orogenic cycles of loading and unloading sediment accumulation took place during stages of flexural subsidence whereas the bounding surfaces are related to stages of isostatic uplift the vertical profile of all sequences displays an overall fining upward trend related to the gradual decrease in topographic slope during orogenic loading at the same time an upward change in fluvial styles can be observed within each sequence from initial higher to final lower energy systems the actual fluvial styles in each location depend on paleoslope gradients and the position of the stratigraphic section relative to the orogenic front proximal sequences show transitions from braided to meandering systems whereas more distal sequences show changes from sand bed to fine grained meandering systems the average duration of the balfour stratigraphic cycles was 0 66 my i e six cycles during 4 my no climatic fluctuations are recorded during this time with the long term climatic background represented by temperate to humid conditions

a novel approach to double spike pb pb dating of carbonate rocks examples from neoproterozoic sequences in southern africa application of the 207 pb 204 pb double spiking technique to correct for mass fractionation led to marked improvement in the quality of pb isotope data and resulting ages obtained on low grade metamorphosed neoproterozoic carbonate rocks from the pan african gariep and saldania belts in southern namibia and south africa carbonate rock powders were dissolved using 1 n acetic acid residue fractions were separated from the supernatant carbonate fractions and analysed separately marbles from the lower pickelhaube formation gariep belt yielded distinctly different 207 pbr 206 pb isochron ages of 728 32 mswds 1 5 and 545 13 ma mswds 7 4 for the carbonate and residue fractions respectively the former is interpreted as dating early diagenesis whereas the latter is in excellent agreement with the timing of regional metamorphism in contrast both the carbonate and residue fractions from the bloeddrif member holgat formation gariep belt and from the kombuis member matjies river formation saldania belt yielded indistinguishable 207 pbr 206 pb isochron ages the isotope data from both fractions combined define an isochron age for the bloeddrif member marbles of

diversity of the woody vegetation of gondwanan southern africa abstract the flora of gondwanan southern africa is represented in the rock record by micro fossils macro fossils and petrified woods all these types of fossils are seldom preserved together in any one particular facies because of taphonomic and preservational biases in order to obtain as accurate a picture as possible of the woody vegetation both the fossil woods and other macroplant fossils such as leaf impressions fructifications and cuticle of woody plants have been correlated this was done for each formation in the karoo supergroup in order to illustrate the changes in diversity of woody vegetation over time sediments of the karoo supergroup represent the terrestrial fossil record of the period upper carboniferous to the lower cretaceous when africa finally separated from south america in the upper carboniferous to lower permian dwyka formation there are at least five described genera of woods from south africa and namibia early to middle permian woods ecca group are a little more diverse with six genera representing the glossopterids cordaitaleans and possibly other seed fern groups late permian to early triassic beaufort group woods show very little change in diversity in spite of the major floral and biotic turnover evident from the rest of the fossil record although the late triassic molteno formation macro flora has been shown to be an example of explosive diversification the generally poorly preserved woods do not reflect this lower jurassic fossils clarens formation are also poorly preserved but have araucarian characteristics early cretaceous woods represent the araucariaceae cheirolepidiaceae and podocarpaceae with a number of species the diversity of the woods has not changed as much as the rest of the floral components in southern africa from the late carboniferous to the early cretaceous possible reasons for this apparent stasis are the conservative nature of wood functional restrictions limitations of suitable conditions for petrifications and the fact that very little research has been done on southern african woods

investigation of fault mechanisms during geothermal reservoir stimulation experiments in the cooper basin australia abstract subsequent to an initial reservoir stimulation in 2003 a second long term fluid injection experiment was conducted in the cooper basin australia in 2005 to further enhance and enlarge the existent geothermal reservoir approximately 25 000 m 3 of water were injected into the granitic crust at 4250 m depth thereby inducing about 16 000 seismic events which were recorded by a local eight station network event magnitudes calibrated by recordings of the permanent network of geosciences australia are in the range between m l 1 2 and 2 9 with seismic moments between 10 9 and 10 13 n m hypocenter locations could be determined for 8886 events the spatial hypocenter distribution indicates that the seismicity occurred on the same subhorizontal layer structure identified for the initial reservoir stimulation which is further extended in the course of restimulation early seismicity is located near the outer rim of the previous zone of seismic activity and subsequently migrates both toward and away from the injection well the immediate vicinity of the injection well remains seismically quiet during restimulation these effects can be explained by a simple kaiser effect model where the induced seismicity is controlled by the in situ fluid pressure increase compound fault plane solutions based on p phase polarities indicate a similar fault mechanism for nearly all events the dominating fault mechanism is consistent with the regional stress field acting on a larger scale fracture zone with an orientation as outlined by the hypocenter distribution a small number of events with reverse mechanisms might indicate the existence of conjugated fractures locally intersecting the main fracture zone cumulative slip determined by mapping the slip contribution of individual events onto the fracture zone indicates that most fracture patches in the reservoir slipped repeatedly accumulating up to several centimeters of shear displacement

contact metamorphic devolatilization of shales in the karoo basin south africa and the effects of multiple sill intrusions abstract quantification of fluid generation during contact metamorphism of shale is important for the understanding of metamorphic processes fluid flow in sedimentary basins and perturbations of the global carbon cycle in this study we provide geochemical and numerical analyses from the organic rich ecca group in the karoo basin south africa which was affected by contact metamorphism from multiple sill intrusions in the early jurassic organic matter was efficiently converted to hydrocarbons during contact metamorphism and complete loss of organic carbon in the innermost aureole is common mineral dehydration reactions are evident from the occurrence of metamorphic minerals like biotite and loss of the clay fraction towards the intrusive contact we have developed a numerical model in order to quantify fluid production from both inorganic and organic reactions during contact metamorphism the modelling results are constrained by data from two case studies in the karoo basin in order to obtain reliable estimates of the carbon loss from metamorphism of shale we show that single thin 15 m thick sills have a gas production potential of several gigatons of methane ch4 if emplaced over a 1000 km2 area furthermore the vertical spacing between simultaneously emplaced sills has an important influence on the gas generation potential when two sills are emplaced with a vertical spacing of 7 times the intrusion thickness the total ch4 generation is up to 35 more than for two separate sills data and modelling from five sills emplaced within the ecca group show hydrocarbon generation throughout the organic rich section with total carbon loss next to the sills this has implications for the fluid production and metamorphism in volcanic basins where multiple sills are common

seismicity of the hengill area sw iceland details revealed by catalog relocation and collapsing seismicity of the hengill area sw iceland details revealed by catalog relocation and collapsing

tuffs tectonism and glacially related sea level changes carboniferous permian southern namibia abstract the carboniferous permian glacigene dwyka group in southern africa has been widely regarded as amagmatic only in the higher parts of the permian and subsequent younger strata has evidence of magmatism been recognised particularly in the form of pyroclastic fallout ash beds the source of these tuffs has been suggested to be patagonia and west antarctica forming part of an extensive volcanic arc which lay some 1500 to 2000 km to the south and west in their pre break up gondwana positions rhyolitic dacitic and basaltic andesitic tuff beds within the glaciomarine sediments of the dwyka group in namibia reveal new evidence for an early onset of proximal bimodal volcanic activity in southern africa contemporaneous tectonism is recorded by type 1 unconformities and systematic thickness changes across nw se trending extensional normal faults we suggest that this tectonomagmatic period marks initial extensional events in southern namibia and the carboniferous permian volcanic trend appears to coincide with the position of the eventual namibian continental margin significantly the stratigraphic positions of the tuff beds show a distinct association with enclosing marine transgressive depositional sequences most of them coincide with sharply defined flooding surfaces within relatively thick shaly successions reflecting their multiple transgressive nature widening the study context to the latest early permian in southern africa the main karoo basin of southern africa exhibits a similar coincidence of marine transgressions and extrusion of volcanics the relationship between tuff beds and transgressive depositional sequences is not only the result of enhanced preservational potential during transgression but of a genetic coupling between magmatism extensional tectonics and basin subsidence the latter accelerating a rise in relative sea level this interaction is significant not only for understanding potential controls on relative sea level change but also for understanding the early geodynamic evolution of the southern south atlantic continental margin

denudation history of the bocaina plateau serra do mar southeastern brazil relationships to gondwana breakup and passive margin development abstract the bocaina plateau which is situated on the eastern flank of the continental rift of southeastern brazil is the highest part of the serra do mar topographic relief in this area is suggested to be closely related to its complex tectono magmatic evolution since the breakup of western gondwana and opening of the south atlantic ocean apatite fission track ages and track length distributions from 27 basement outcrops were determined to assess these hypotheses and reconstruct the denudation history of the bocaina plateau the ages range between 303 32 and 46 5 ma and are significantly younger than the stratigraphic ages mean track lengths vary from 13 44 1 51 to 11 1 1 48 m with standard deviations between 1 16 and 1 83 m contrasting ages within a single plateau and similar ages at different altitudes indicate a complex regional tectonothermal evolution the thermal histories inferred from these data imply three periods of accelerated cooling related to the early cretaceous continental breakup early cretaceous alkaline magmatism and the paleogene evolution of the continental rift of southeastern brazil the oldest fission track ages 200 ma were obtained in the serra do mar region suggesting that these areas were a long lived source of sediments for the parana bauru and santos basins

a method for determining significant structures in a cloud of earthquakes the interpretation of a cloud of earthquake hypocenters in terms of causative structures is not a simple task locations are subject to uncertainties which will not be the same for every earthquake the data should therefore not be interpreted simply by inspection which is difficult in the case of three dimensional data anyway instead we propose using the location uncertainties as a guide in processing the data earthquake locations are moved inside their uncertainty or confidence ellipsoids until a simplified picture of the earthquake cloud is obtained which can then be interpreted in terms of some simplified structure such as faults the aim of the approach is to give the simplest possible structure that is consistent with all the location and confidence ellipsoid data the method is applied to three synthetic sets of data these illustrate the potential and limitations of the method application to a real earthquake data set from rabaul caldera in papua new guinea gives an image of the caldera ring fault that suggests departures from the simple ring fault structure previously assumed sensitivity analysis on the rabaul data shows that the method is not unduly sensitive to the assumptions that have to be made in applying it

pattern identification in the dynamic fingerprint of seismically active zones abstract the aim of this paper is to present a methodology designed to characterize seismically active areas from the point of view of their dynamic fingerprints which rely on space time aspects of seismicity patterns according to this methodology earthquake events are represented as points in phase space which leads to an events thread connecting the points in their temporal sequence a multi scale analysis procedure is applied to the events thread and scaling exponents are identified the proposed framework considers projections of the events thread corresponding to different spatial orientations for each orientation the events thread is characterized from the point of view of signal persistence the resulting persistence diagrams portray anisotropy aspects of the dynamic fingerprint the identification of anisotropic properties of the dynamic fingerprint provides a new perspective on seismically active zones complementing the information from geological and geophysical studies the paper describes the proposed methodological framework and presents applications focusing on two different regions the aleutian arc and hawaii

crustal shear wave splitting from local earthquakes in the hengill triple junction southwest iceland the hengill region in sw iceland is an unstable ridge ridge transform triple junction between an active and a waning segment of the mid atlantic spreading center and a transform that is transgressing southward the triple junction contains active and extinct spreading segments and a widespread geothermal area we evaluated shear wave birefringence for locally recorded upper crustal earthquakes using an array of 30 three component digital seismographs fast polarization directions are mostly ne to nne subparallel to the spreading axis and probably caused by fissures and microcracks related to spreading however there is significant variability in throughout the array the lag from fast to slow s is not proportional to earthquake depth ray length being scattered at all depths the average wave speed difference between qs1 and qs2 in the upper 2 5 km of the crust is 2 5 our results suggest considerable heterogeneity or strong s scattering

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

on entropy and clustering in earthquake hypocentre distributions summary the degree of clustering or disorder within earthquake distributions may be measured using the concept of entropy a method for calculating the entropy of any 3 d point set e g earthquake foci is presented this makes use of voronoi cells convex polyhedra representing nearest neighbour regions to measure point density in three dimensions an estimate of event density can be determined directly from the size of voronoi cells normalizations are introduced to the definition of entropy that allow data sets containing different numbers of events and occupying different volumes to be compared quantitatively for example earthquake catalogues from different tectonic regimes our results show a clear correlation between earthquake entropy and tectonic regime the most ordered are the mid ocean ridges followed by the subduction zones and finally intraplate seismicity we show how entropy may be used to quantify the simplification of earthquake distributions for example due to relocation procedures a recently published algorithm called the collapsing method is used as an example of a technique that reduces entropy while respecting data fit modifications to this method are made that reduce artefacts and use additional temporal information in the earthquake distribution these methods are applied to a global catalogue of 85 000 events and a local catalogue from the sil network in iceland containing 43 300 events the entropy of both catalogues is reduced results from the hengill region within the sil network show lineations whose orientations agree with independent studies using relative location techniques and surface faulting

geodynamic significance of the trm segment in the east african rift w tanzania active tectonics and paleostress in the ufipa plateau and rukwa basin abstract the tanganyika rukwa malawi trm rift segment in western tanzania is a key sector for understanding the opening dynamics of the east african rift system ears in an oblique opening model it is considered as a dextral transfer fault zone that accommodates the general opening of the ears in an nw se direction in an orthogonal opening model it accommodates pure dip slip normal faulting with extension orthogonal to the rift segments and a general e w extension for the entire ears the central part of the trm rift segment is well exposed in the ufipa plateau and rukwa basin within the paleoproterozoic ubende belt it is also one of the most seismically active regions of the ears we investigated the active tectonic architecture and paleostress evolution of the ufipa plateau and adjacent rukwa basin and in order to define their geodynamic role in the development of the ears and highlight their pre rift brittle tectonic history the active fault architecture fault kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control the rift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the trm segment two pre rift brittle events are evidenced an older brittle thrusting is related to the interaction between the bangweulu block and the tanzanian craton during the late pan african early paleozoic it was followed by a transpressional inversion during the early mesozoic this inversion stage is the best expressed in the field and caused dextral strike slip faulting along the fault systems that now control the major rift structures it has been erroneously interpreted as related to the late cenozoic ears which instead is characterized by pure normal faulting our third and last stress stage

post glacial permian stratigraphy and geography of southern and central africa boundary conditions for climatic modelling abstract the post glacial stratigraphy of southern and central africa comprises mudrocks with subordinate sandstones and coal measures belonging to the ecca group overlain by fossiliferous mudstones and sandstones of the beaufort group madumabisa mudstone of south central africa preserved in the karoo congo and extreme western margin of the paranabasins the kalahari zambezi and east africa malagasy basin complexes and the nyasa basin and range complex the palaeogeography of the region was controlled by the stable elevated kalahari and congo cratons a ne trending weak zone between the cratons site of the kalahari zambezi malagasy basin complexes a nnw trending weak zone between the cratons and the mozambique mobile belt site of the nyasa basin and range complex and an e w mobile belt along the southern margin of the continent site of the foreland karoo basin the post glacial artinskian landscape which was inherited from the permo carboniferous glaciation consisted of a rugged interior plateau and shallow seas and seaways in the south and west the early late permian landscape was still dominated by an uneven coastline in the south and freshwater lakes in the congo basin and along the kalahari zambezi malagasy drainage systems a marine incursion occurred in the malagasy basin relief on the interior plateau was steadily denuded by kazanian times large lakes covered the karoo and paranabasins whereas along the coastal region all shallow seas were destroyed by an orogen freshwater lakes in the interior had reached their maximum extent the permo triassic landscape shows an increase in the interior relief due to renewed graben formation with lakes becoming smaller and more desiccated the surface area of southwestern gondwana covered by water reached a maximum of 34 during the early late permian and decreased to about 6 at the permo triassic boundary climatic zones shifted southeastwards as southwestern gondwana drifted into lower latitudes during the permian the ice margin started to retreat in central africa during the sakmarian but the marine ice sheet in the karoo basin collapsed only in the artinskian deglaciation was followed by a cool wet climate suitable for the formation of peatlands in central and south central africa this climate was followed by temperate to warm seasonal conditions which became progressively wetter with less seasonal fluctuation towards the east malagasy basin and drier towards the west these conditions lasted until the end of the kazanian in the karoo basin the cool wet climate of the artinskian became more temperate and also lasted until the end of the kazanian when warm semi arid conditions expanded over the entire southern and central africa the geographic controlling factors for permian climatic change were the interplay of latitudinal setting and continental evolution formation of orographic barriers and loss of coastal seas

late cenozoic evolution of the outer continental shelf at the head of the cape canyon south africa abstract cenozoic sedimentary successions have a restricted distribution both onshore and offshore along the passive western margin of southern africa and much controversy and uncertainty remains on the geological evolution of this margin siliciclastic authigenic and biogenic sediments varying in age from cretaceous to holocene recovered from a number of vibrocores collected between 200 and 500 m water depth at the head of the cape canyon provide the basis for an integrated bio sr isotopic and lithostratigraphic study the study area is underlain by unconsolidated middle cretaceous interbedded woody quartzose sand and kaolinitic mud a large regional erosional hiatus separates the upper oligocene to lower miocene carbonate sediments from these cretaceous sediments the upper oligocene to lower miocene 18 1 27 2 ma carbonate succession records third order sea level fluctuations superimposed on an overall marine transgression the interpreted late oligocene depositional environments imply increased bottom water activity over time and the peak in phosphogenesis at 23 6 26 1 ma indicates the onset of warm water upwelling calcareous deposition ended at around 18 ma when downwarping of the continental shelf associated with uplift of the continental interior resulted in the seaward tilt of the carbonate succession to greater water depths increased terrigenous input is recorded as inner shelf prograding quartzose delta front deposits following uplift of the interior uplift combined with a long term marine regression resulted in the widespread erosion of middle to upper miocene successions generally recovered only as reworked glauconite carbonate fluorapatite cfa mixed mineral pellets above the erosional hiatus glauconite rich sediments indicate overall low accumulation rates and multiple reworking events of organic rich clayey sediments during the middle to late miocene early pliocene onset of cold water upwelling at least intermittently by 15 ma established hydrographic conditions similar to the benguela upwelling system with a coast parallel mudbelt present on the middle shelf a middle pliocene marine regression reworked 15 8 9 1 ma inner shelf quartzose delta front sediments across the outer shelf late pliocene early pleistocene uplift of the continental interior resulted in the fault controlled incision of the head of the cape canyon along the seaward flank of the buoyant agulhas columbine basement block uplift increased quaternary terrigenous sedimentation on the shelf where the facies distribution was clearly controlled by canyon head bathymetry high frequency glacio eustatic sea level fluctuations resulted in late quaternary erosion and slumping across the outer shelf and the continued downcutting of the cape canyon lowstand shelly gravels are common including the last glacial maximum shell lag which grades into a faecal pellet rich holocene mud drape over much of the study area the late cenozoic evolution of the western margin recorded in the complex condensed sediment record of the study area is characterised by low sediment accumulation rates and multiple reworking events the integrated stratigraphic interpretation emphasises the importance of eustasy tectonic activity fluvial sediment input palaeoceanographic and climatic changes

mantle structure beneath indonesia inferred from high resolution tomographic imaging we investigated mantle structure beneath the indonesian region by means of tomographic inversions of traveltime residuals of direct p and the surface reflected depth phases pp and pwp the hypocentres and phase data used in the inversions were derived from the reprocessing of data reported to international data centres which include data from the australian skippy project we used more than 6 0 10 data from more than 50 000 globally distributed earthquakes recorded at a subset of nearly 3000 seismological stations worldwide the use of depth phases improved the sampling of the upper mantle structure in particular beneath the back arc regions the mapping of distant aspherical mantle structure into the study region was minimized by combining a high resolution regional with a low resolution global inversion the inversion was linearized around the ak135 reference velocity model and a combination of minimum norm and gradient damping was applied to constrain the solution inversion of different subsets of the data and test inversions of synthetic data indicate that resolution is sufficient to substantiate the conclusions pertinent to the complex large scale structure of the subducted slab beneath the indonesian archipelago the tomographic images confirm previous evidence for regional slab penetration into the upper part of the lower mantle but suggest a more complex trajectory of mantle flow beneath indonesia the implied lateral variation in the shape of the upper mantle slab is generally in good agreement with inferences from seismicity the data reveal a seismic anomaly beneath the sunda arc which is detected to at least 1500 km in depth and forms the eastern end of a deep anomaly associated with the past subduction of lithosphere of the mesozoic tethys ocean below java the lithospheric slab is probably continuous from the earth s surface to the lower mantle but beneath sumatra the deep slab seems to be detached from the seismogenic slab the high resolution images suggest that the subducted slab is deflected in the transition zone beneath the banda arc and that the descending slabs form a spoon shaped structure north of the banda sea we detect the two opposing subducted slabs of the molucca sea plate this is consistent with previous studies but our images suggest that the westward subduction has a significantly steeper dip than the eastward subduction slab structure beneath the banda and molucca seas is explained in the context of the continent arc collision between australia and the southeast indonesian island arc and the arc arc collision between sangihe and halmahera inversion of traveltime residuals of direct s phases a novelty for this region largely confirms the inferences from the p wave data even though less detail is resolved

fractal analysis applied to characteristic segments of the san andreas fault fractal theory is applied in a quantitative analysis of the san andreas fault saf geometry the method which directly measures the increase in total fault length with a decrease in ruler size gives the fractal dimension d and scaling properties for the chosen length band 0 5 1000 km a physical interpretation of d is that it measures the irregularity of the fault trace in the selected band the fault is subdivided into six segments of distinctive seismic behavior a main fault trace which shows either maximum coseismic slip or creeping was selected for analysis with three alternative branches examined for the saf system south of san bernardino branches of the fault trace are not considered in this work fractal dimensions calculated for the different segments range from 1 0008 to 1 0191 and are different from 1 0 to the 95 confidence interval these small changes reflect the overall smoothness of the main fault trace significant variations in d among segments indicate heterogeneities in the fault smoothness along strike d also changes significantly from the short length band to the long length band where the demarcation point ranges from 1 to 2 km the short length band has larger d values a slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting this indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view

improved location procedures at the international seismological centre summary the international seismological centre isc is a non governmental non profit organization with the primary mission of producing the definitive account of the earth s seismicity the isc bulletin covers some 50 yr 1960 2011 of seismicity the recent years have seen a dramatic increase both in the number of reported events and especially in the number of reported phases owing to the ever increasing number of stations worldwide similar ray paths will produce correlated traveltime prediction errors due to unmodelled heterogeneities in the earth resulting in underestimated location uncertainties and for unfavourable network geometries location bias hence the denser and more unbalanced the global seismic station coverage becomes the less defensible is the assumption that is the observations are independent which is made by most location algorithms to address this challenge we have developed a new location algorithm for the isc that accounts for correlated error structure and uses all iaspei standard phases with a valid ak135 traveltime prediction to obtain more accurate event locations in this paper we describe the new isc locator and present validation tests by relocating the ground truth events in the iaspei reference event list as well as by relocating the entire isc bulletin we show that the new isc location algorithm provides small but consistent location improvements considerable improvements in depth determination and significantly more accurate formal uncertainty estimates we demonstrate that the new algorithm through the use of later phases and testing for depth resolution considerably clusters event locations more tightly thus providing an improved view of the seismicity of the earth

when did the falklands rotate abstract similarities in the styles and relative timings of tectonic events in the outeniqua basin south africa and the north falkland basin suggest that basin formation in both regions may have preceded rotation of the falklands microplate contrary to previous models for the break up of gondwana which suggest jurassic rotation the data implies valanginian rotation contemporaneous with the first recorded motion on the agulhas falkland fracture zone and south atlantic rifting the data also suggests that the formation of the falkland plateau basin may also be a cretaceous event as opposed to the previously assumed jurassic age such a model is consistent with new offshore seismic evidence while the inconclusive nature of the supportive evidence for jurassic rotation does not exclude later rotation as a possibility

oblique collision at about 1 1 ga along the southern margin of the kaapvaal continent south east africa the 1 1 ga natal metamorphic province nmp lies at the heart of a world wide system of grenville age mobile belts which welded early continental fragments into the mesoproterozoic supercontinent of rodinia structural analysis of the three tectonostratigraphic terranes in natal reveals a kinematic history characterized by prolonged ne sw plate convergence manifested as early thrust tectonics and later pervasive sinistral transcurrent shearing consequently superimposed on the natal tectonostratigraphic terranes is a kinematic subdivision into tectonic domains which are characterized by shallow south west dipping foliations south west plunging stretching lineations and north east verging recumbent folds and by younger domains with subvertical shear fabrics subhorizontal to oblique lineations and folding about near vertical axes microtextural and petrographic analyses suggest that the later shearing took place under high temperature conditions of at least 500 c the recorded kinematic indicators suggest that early subhorizontal compressional tectonics gave rise to tectonic thickening of the crust progressively followed by oblique transcurrent shearing within a transpressional regime the shearing event in the southern arc related terranes was associated with the widespread emplacement of late kinematic rapakivi granite charnockite plutons with a type granite geochemical characteristics this orogenic event took place around 1100 ma during prolonged ne sw collisional convergence along the southern margin of the stable archean foreland which lay to the north

an integrated geophysical study of the beattie magnetic anomaly south africa abstract the source of the beattie magnetic anomaly bma still remains unclear with several competing hypotheses here we add a piece to the puzzle by investigating available potential field data over the anomaly filtered magnetic data show the bma as part of a group of linear magnetic anomalies as the linear anomaly north of the bma is associated with exposed supracrustals migmatites and shear zones within the natal thrust terranes we assume a similar source for the bma this source geometry constrained by seismic and mt data fits potential field data over the bma and other magnetic linear anomalies in the south central and south western karoo in these models the bodies deepen from 5 km towards the south with horizontal extents of 20 60 km and vertical extents of 10 15 km densities range from 2800 to 2940 kg m 3 and magnetic susceptibilities from 10 to 100 10 3 si these magnetic susceptibilities are higher than field values from supracrustal rocks 10 60 10 3 si but could be due to the fact that no remanent magnetisation was included in the model the lithologies associated with the different linear anomalies vary as is evident from varying anomaly amplitudes the strong signal of the bma is linked to high magnetic susceptibility granulite facies supracrustals 10 50 10 3 si as seen in the antarctic where the mobile belt continued during gondwana times