the p t record of synchronous magmatism metamorphism and deformation at petrel cove southern adelaide fold belt porphyroblastic schists in the thermal aureole of the victor harbor granite at petrel cove in the southern adelaide fold belt south australia preserve a record of sequential cordierite andalusite staurolite fibrolite chlorite and muscovite growth along with biotite plagioclase quartz ilmenite during progressive deformation a p t pseudo section appropriate to biotite saturated assemblages in kfmash shows that the sequence of mineral reactions records increasing pressure of at least 1 kbar from c 3 to c 4 kbar during cooling from around 580 c heating at pressures below c 3 kbar is inferred for growth of early formed cordierite porphyroblasts and is attributed in part to the thermal effects of granite emplacement while the pressure increase is attributed to tectonic burial accruing from ongoing deformation the anticlockwise p t path is consistent with convergent deformation being focussed as a consequence of heating as to be expected for a lithospheric rheology that is strongly temperature dependent

when and where the aftershock activity was depressed contrasting decay patterns of the proximate large earthquakes in southern california 1 seismic quiescence has attracted attention as a possible precursor to a large earthquake however sensitive detection of quiescence requires accurate modeling of normal aftershock activity we apply the epidemic type aftershock sequence etas model that is a natural extension of the modified omori formula for aftershock decay allowing further clusters secondary aftershocks within an aftershock sequence the hector mine aftershock activity has been normal relative to the decay predicted by the etas model during the 14 months of available data in contrast although the aftershock sequence of the 1992 landers earthquake m 7 3 including the 1992 big bear earthquake m 6 4 and its aftershocks fits very well to the etas up until about 6 months after the main shock the activity showed clear lowering relative to the modeled rate relative quiescence and lasted nearly 7 years leading up to the hector mine earthquake m 7 1 in 1999 specifically the relative quiescence occurred only in the shallow aftershock activity down to depths of 5 6 km the sequence of deeper events showed clear normal aftershock activity well fitted to the etas throughout the whole period we argue several physical explanations for these results among them we strongly suspect aseismic slips within the hector mine rupture source that could inhibit the crustal relaxation process within shadow zones of the coulomb s failure stress change furthermore the aftershock activity of the 1992 joshua tree earthquake m 6 1 sharply lowered in the same day of the main shock which can be explained by a similar scenario

long term behaviour of australian stable continental region scr faults abstract australia boasts arguably the richest late neogene to quaternary faulting record in stable continental region scr crust anywhere in the world variation in fault scarp length vertical displacement proximity to other faults and relationship to topography permits division of the continent according to fault character six onshore neotectonic domains are recognised with an additional offshore domain proposed by analogy with the eastern united states each domain relates to a distinct underlying crustal type and architecture broadly considered to represent cratonic non cratonic and extended environments in general greater topographic expression associated with faults occurring in extended crust relative to non extended crust suggests a higher rate of seismic activity in the former setting consistent with observations worldwide using the same reasoning non cratonic crust might be expected to have a higher rate of seismic activity than cratonic crust this distinction together with the variation in fault character between domains should be recognised in attempts to identify analogous systems worldwide a common characteristic of morphogenic earthquake occurrence in australia appears to be temporal clustering periods of earthquake activity comprising a finite number of large events are separated by much longer periods of seismic quiescence at the scale of a single fault and of proximal faults in several instances there is evidence for deformation at scales of several hundred kilometres switching on and off over the last several million years what is not clear from the limited palaeoseismological data available is whether successive active periods are comparable in terms of slip number of events magnitude of events etc regardless this apparent bimodal recurrence behaviour poses challenges for probabilistic seismic hazard assessment psha these rely on the simplifying assumption that large earthquake recurrence for long return periods is not random i e poissonian presently our ability to incorporate such time dependent models is limited by available data

time relationships between volcanism plutonism alteration mineralization in cu stratabound ore deposits from the michilla mining district northern chile a 40 ar 39 ar geochronological approach the michilla mining district comprises one of the most important stratabound and breccia style copper deposits of the coastal cordillera of northern chile hosted by the middle jurassic volcanic rocks of the la negra formation 40ar 39ar analyses carried out on igneous and alteration minerals from volcanic and plutonic rocks in the district allow a chronological sequence of several magmatic and alteration events of the district to be established the first event was the extrusion of a thick lava series of the la negra formation dated at 159 9 1 0 ma 2 from the upper part of the series a contemporaneous intrusion is dated at 159 6 1 1 ma and later intrusive events are dated at 145 5 2 8 and 137 4 1 1 ma respectively analyzed alteration minerals such as adularia sericite and actinolite apparently give valid 40ar 39ar plateau and miniplateau ages they indicate the occurrence of several alteration events at ca 160 163 154 157 143 148 and 135 137 ma the first alteration event being partly contemporaneous with volcanic and plutonic rocks was probably produced in a high thermal gradient environment the later events may be related either to a regional low grade hydrothermal alteration metamorphism process or to plutonic intrusions the cu mineralization of the michilla district is robustly bracketed between 163 6 1 9 and 137 4 1 1 ma corresponding to dating of actinolite coexisting with early stage chalcocite and a postmineralization barren dyke respectively more precisely the association of small intrusives a dated stock from the michilla district with cu mineralization in the region strongly suggests that the main michilla ore deposit is related to a magmatic hydrothermal event that occurred between 157 4 3 6 and 163 5 1 9 ma contemporaneous or shortly after the extrusion of the volcanic sequence this age is in agreement with the re os age of 159 16 ma obtained from the mineralization itself trista aguilera et al miner depos 41 99 105 2006

phase relationships in buchan facies series pelitic assemblages calculations with application to andalusite staurolite parageneses in the mount lofty ranges south australia low pressure medium to high temperature buchan type regional metamorphism of pelitic rocks in the mount lofty ranges south australia is defined by the development of biotite staurolite andalusite fibrolite prismatic sillimanite and migmatite zones k feldspar makes its first appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovite concentrating particularly on well developed andalusitestaurolite parageneses in general the spatial distribution and mineral chemical variation of these assemblages accord with the predictions of petrogenetic grids and p t and t xfe pseudo sections constructed from the internally consistent data set of holland and powell 1990 in the system kfmash assuming a h2o 1 although analysed white mica compositions are systematically more aluminous than predicted importantly the stability ranges of most critical assemblages predicted by these grids and pseudo sections coincide closely with p t estimates calculated using the data set of holland and powell 1990 from the mount lofty ranges assemblages with the exception of mn in garnet and zn in one staurolite cordierite muscovite assemblage non kfmash components do not significantly appear to have affected the stability ranges of the observed assemblages an apparent local reversal in isograd zonation in which andalusite first appears down grade of staurolite suggests a metamorphic field gradient concave towards the temperature axis and together with evidence for essentially isobaric heating of individual rocks is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels

a chemostratigraphic overview of the late cryogenian interglacial sequence in the adelaide fold thrust belt south australia within the neoproterozoic sequence of the adelaide fold thrust belt south australia two of the most severe ice ages in earth history sturtian and marinoan are recorded in the glacigenic rocks which mark the base and top of the 4 5 km thick umberatana group the focus of the present chemostratigraphic study rock units from three drillholes and ten measured outcrop sections located in the southern and central flinders ranges and on the stuart shelf were sampled for isotopic analysis of their co existing calcite 13ccal dolomite 13cdol and kerogen 13corg strontium and sulphur isotopic analyses were also run on selected samples the sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen the resulting data when integrated with the lithostratigraphy of the umberatana group yield one of the most complete records of secular variation in the c isotope signature of the late cryogenian ocean between 750 and 680 ma its composite 13ccarb curve begins with a sharp positive excursion 3 to 1 5 in the dolomitic tindelpina shale which caps sturtian glacial diamictites wilyerpa formation then follows a steady climb through the shoaling upward tapley hill formation reaching 4 in the brighton limestone and culminating in a plateau of 13c enrichment within the platformal carbonates of the etina formation 8 to 10 the upper part of the temporal trend shows a decline to 6 to 7 in the stromatolite reef facies of the enorama shale and a dramatic drop to 8 to 9 5 followed by a recovery to 3 in the youngest carbonate unit trezona formation beneath the marinoan glacigenic elatina formation the latter is capped by a thin dolostone the nuccaleena formation which records a consistent up section decrease in 13c 1 to 3 superimposed on this overall pattern of c isotopic variation are second and third order fluctuations attributable to eustatic sea level change pyritic sulphur in the lower tapley hill formation 34s 9 to 40 is distinctly heavier than that in the underlying wilyerpa formation 4 to 20 minimum 87sr 86sr values obtained from the brighton limestone 0 7071 and etina formation 0 7076 are consistent with their late cryogenian age this sturtian to marinoan interglacial succession is more complete than equivalent sequences elsewhere and thus provides a useful reference for global chemostratigraphic correlation notable features of its c isotope curve are the strong post wilyerpa positive excursion also seen in key sections from svalbard and western mongolia the etina plateau of 13c enrichment which is remarkably similar to maxima in the terminal proterozoic c isotope records of western mongolia north western canada and brazil and the spectacular depletion of 13c in the lower trezona relative to similar carbonate facies in the underlying etina formation and enorama shale a preglacial offset in 13ccarb values of such proportions 14 is almost unprecedented except perhaps for that recently documented in the upper ombaatjie formation beneath tillites of the ghaub formation otavi group namibia it represents a key piece of evidence in the case presented by the proponents of a neoproterozoic snowball earth

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

the middle permian hongshanliang manto type copper deposit in the east tianshan constraints from geology geochronology fluid inclusions and h o s isotopes abstract the hongshanliang copper deposit hosted in the tuff of the lower carboniferous yamansu formation is a typical copper deposit in the aqishan yamansu belt east tianshan based on crosscutting relationships of veins textural relationships and mineral assemblages five alteration mineralization stages at hongshanliang have been established chlorite sulfide stage stage i quartz pyrite stage stage ii quartz polymetallic sulfide stage stage iii late veins stage iv and supergene process stage v stage i has mineral assemblage of chlorite pyrite chalcopyrite quartz sericite and is commonly characterized by euhedral pyrite with silicates e g chlorite quartz and sericite and or chalcopyrite as its pressure shadow indicating an early mineralization event with apparent deformation stage ii is characterized by quartz pyrite chalcopyrite or locally quartz gypsum anhydrite pyrite veins cutting stage i chlorite pyrite the main mineralization stage stage iii at hongshanliang has typical mineral assemblages of quartz chalcopyrite chlorite calcite sericite quartz chalcopyrite sphalerite galena pyrite and quartz sphalerite galena chlorite with veins veinlets disseminations and local massive ore types similar to manto type copper deposits regarding mineralization and ore structures detailed fluid inclusion study shows temperature of fluids decreased from stage ii 307 484 c through stage iii peak at 160 180 c consistent with quartz chlorite oxygen isotope geothermometer of 190 c to stage iv 129 169 c with corresponding salinities of 2 7 26 2 wt nacl equiv peak at 8 wt 2 1 12 3 wt nacl equiv peak at 8 wt and 0 5 7 7 wt nacl equiv peaks at 4 6 wt respectively indicating an evolved hydrothermal ore forming system such fluid evolution can also be supported by h o isotopes during water rock reaction from stage ii 18ofluid 6 3 8 3 and dfluid 77 to 64 through stage iii 18ofluid 1 7 to 7 0 and dfluid 82 to 58 to stage iv 18ofluid 2 5 to 2 1 and dfluid 77 to 73 with a magmatic hydrothermal origin furthermore sulfur isotopes suggest that fluids of the two mineralization stages stage i and iii are predominantly magmatic hydrothermal with other influx contribution into the ore forming system i e minor early carboniferous seawater in stage i 34sfluid varying from 3 5 to 6 1 with peak at 0 and minor organic materials in stage iii 34sfluid varying from 6 5 to 4 1 with peak at 0 1 and detection of ch4 and c2h6 from fluid inclusions in combination of alteration paragenesis nature and source of ore forming fluids and comparison with other typical deposits we proposed that the hongshanliang copper deposit underwent two mineralization events with the main mineralization similar to manto type copper deposits 40ar 39ar dating of sericite from a massive chalcopyrite ore indicates the hongshanliang main mineralization formed at 269 0 0 4 ma generally coeval with regional k feldspar granite emplacement ca 272 ma integrating the regional tectonic setting magmatism and metallogenesis of the aqishan yamansu belt and the east tianshan we suggest that the aqishan yamansu belt has potential of mineral prospecting for the middle permian manto type copper deposits

physical modelling of deformation in the tasman orogenic zone abstract structural vergence within the western subprovince of the lachlan fold belt is towards the hinterland rather than the foreland in contrast to many well known orogenic belts high angle reverse faults and upright folds verge eastwards away from the australian craton towards the inferred centre of orogenic and magmatic activity we designed a series of analogue models to test the anomalous vergence in the western lachlan fold belt particularly the interaction of a stable australian craton with tasman line geometry interacting with weaker oceanic or transitional lithospheric material we found consistently that vergence direction in the models was towards the hinterland not the foreland as in the western lachlan fold belt irrespective of the way the model was deformed strength gradients between the oceanic and cratonic lithosphere control the deformation patterns an important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere

precise u pb zircon dating of a post d2 meta dolerite constraints for rapid tectonic development of the southern adelaide fold belt during the cambrian numerous morb like mafic dykes plugs and sills intruded the kanmantoo group and older meta sediments of the southern adelaide fold belt from pre d1 to post d2 during the delamerian orogeny a post d2 meta dolerite yields a precise u pb zircon age of 510 2 ma this age combined with a published shrimp zircon age of 526 4 ma reveals a rapid tectonic evolution of the fold belt during the cambrian events including the thick kanmantoo group9s deposition major mafic magmatism low p high t regional metamorphism and multiple deformations all took place from about 526 to 510 ma this rapid evolution supports the suggestion that the heat source for the metamorphism originated from continental extension associated with mantle upwelling and mafic magmatism

early palaeozoic foreland thrusting and basin reactivation at the palaeo pacific margin of the southeastern australian precambrian craton a reappraisal of the structural evolution of the southern adelaide fold thrust belt abstract regional and detailed structural mapping kinematic analysis and balancing and restoration of cross sections has lead to a re interpretation of the adelaide fold belt in south australia in this belt sedimentary rocks of the late proterozoic adelaidean and early cambrian normanville and kanmantoo sequences were deposited during at least two episodes of subsidence related to major crustal attenuation contraction and crustal thickening accompanied by granitoid intrusions are related to the cambro ordovician delamerian orogeny during this event both basins were reactivated inverted and the sedimentary rocks are now incorporated in a wnw verging foreland fold and thrust belt at the margin of the proterozoic southeast australian craton maximal shortening of the orogen was around 55 within the adelaidean basin shortening is dominantly accommodated by major mylonitic shear zones and reverse faults in the cambrian kanmantoo basin several thrusts are demonstrably reactivated growth faults across which thickness changes of cambrian sedimentary rocks are revealed by balancing and restoration of cross sections owing to the steep easterly dips of these faults in the western part of the kanmantoo basin lateral shortening of up to 58 is largely accommodated by intense folding and fold axial planar flattening strain further east strain magnitudes wane and the overall shortening of around 30 is accommodated by both discrete thrust zones and regional folds the basin reactivation along the australian part of gondwana s palaeo pacific craton margin reflects the rapid transition from passive to active tectonism which possibly is a consequence of the disintegration of the formerly conjugate margins of gondwana and laurentia

on two characteristic frequencies of acceleration spectra patch corner frequency and fmax abstract we examine two issues related to the radiation of tectonic earthquake sources i the complexity of the shape of observed source spectra and ii the effect of recording site characteristics on the cut off frequency f max of acceleration spectra beyond which spectral amplitudes diminish sharply we compute the far field spectra of the 1979 imperial valley and the 1984 morgan hill earthquakes from the slip functions that were inferred from detailed inversion studies of strong motion and teleseismic data of these two california strike slip events these spectra along with data from the 1983 central japan sea earthquake sequence main shock and aftershocks demonstrate the existence of the so called patch corner frequency which is related to the scale length of heterogeneities of the fault plane such a frequency is a distinct feature of the source spectrum of the specific barrier model of papageorgiou and aki by analyzing accelerograms of the san fernando earthquake of 1971 we found that f max determined from records at soil stations is nearly the same as that determined from records at hard rock stations

mechanisms of continental growth in extensional arcs an example from the andean plate boundary zone during the late triassic jurassic and early cretaceous an extensional magmatic arc was formed in the andean margin of northern chile plutons emplaced at ramps within a hinterland propagating extensional duplex were fed by dikes that transferred magma through the lower crust from a reservoir in the mantle phases of volcanism separated phases of plutonism and our model demonstrates that plutonism was favored only when upper plate extensional fault systems were active when they were not dikes cut across inactive faults and magma rose directly to the surface during volcanic phases of arc growth

3 d seismic tomography of the adelaide fold belt summary a lithospheric tomography experiment was conducted in the adelaide fold belt of south australia to elucidate the 3 d velocity structure down to depths of several hundred kilometres in a region of major geological and tectonic significance the data comprised arrival times for compressional waves from 148 teleseismic events recorded on a temporary array of 40 digital seismographs spanning an area of 200 300 km a new irregular shortest path ray tracer and non linear inversion scheme were used to map the crust and upper mantle structure synthetic 3 d imaging tests were first performed to validate the procedure and establish resolution limits for the particular earthquake distribution and recording geometry wave speed anomalies of up to 4 per cent relative to the iasp91 layered earth model were found the boundary between a prominent velocity high and a prominent velocity low in the shallow crustal depth slices seems to coincide with the synclinal axis of adelaidean and early palaeozoic sediments other velocity features at shallow depths correlate with gravity highs and lows and compare favourably with structure revealed from explosion seismic profiles recorded in the area at depths greater than 60 km the tomographic images reveal isolated velocity highs enclosed by regions of lower velocity within the fold belt these features of dimensions tens of kilometres could indicate the existence of fragments of old mantle broken off from the thick subcontinental lithosphere beneath the gawler craton and scattered in the rodinian break up rift zone

40ar 39ar dating of volcanism and subsequent very low grade metamorphism in a subsiding basin example of the cretaceous lava series from central chile abstract 40ar 39ar geochronological method is applied to date both emplacement and subsequent very low grade metamorphism of a thick lava series the veta negra formation from the cordon de chacana area in the coastal range of central chile on the same lava series and even on the same rock sample it was possible to measure apparently valid ages of both the emplacement of lava flows by dating transparent plagioclase and the very low grade metamorphic event by dating sericite and or adularia sericite was dated on step heated single grains of strongly altered plagioclases the radiogenic argon from remaining fresh plagioclase appearing as negligible on most part of the age spectrum the concordance between one plateau age of 118 7 0 6 ma displayed by fresh plagioclase and a previously measured plateau age on plagioclase from one lava flow from a region due west from santiago bustamante hill aguirre l feraud g morata d vergara m robinson d 1999 time interval between volcanism and burial metamorphism and rate of basin subsidence in a cretaceous andean extensional setting tectonophysics 313 433 447 may indicate that the volcanic activity of the veta negra formation was synchronous along the 80 km long latitudinal segment studied ages of 96 8 0 2 and 97 0 1 6 ma displayed by adularia and sericite respectively are slightly older than ages of 93 94 ma given by adularia from the bustamante hill these data allow to define a period of about 22 ma between volcanism and metamorphism because of the contemporaneity of some metamorphic minerals and cretaceous plutons of the region the very low grade metamorphic event may be the result of the additional effect of burial and a regional thermal event nevertheless a significant effect of burial relatively to the contribution of high thermal gradient event s seems revealed by an apparent age gradient on very low grade minerals with the stratigraphic depth

the early andean magmatic province eamp 40ar 39ar dating on mesozoic volcanic and plutonic rocks from the coastal cordillera northern chile abstract the early andean magmatic province eamp consists of about 150 000 km 3 of volcanic and plutonic units in the coastal cordillera of northern chile and southern peru and represents a major magmatic mesozoic event in the world for which the precise age of the thick volcanic series was unknown thirty 40 ar 39 ar analyses were carried out on primary mineral phases of volcanic and plutonic rocks from northern chile 18 30 24 s reliable plateau and mini plateau ages were obtained on plagioclase amphibole and biotite from volcanic and plutonic rocks despite widespread strong alteration degree in the arica tocopilla and antofagasta 700 km apart regions the ages obtained on lava flows constrain the volcanic activity between 164 and 150 ma and no n s migration of volcanism is observed the uppermost lava flows of the volcanic sequence at the type locality of the la negra formation extruded at ca 153 150 ma suggesting the end of the volcanic activity of the arc at that time the oldest volcanic activity occurred probably at ca 175 170 ma in the iquique area although no plateau age could be obtained the plutonic bodies of the same regions were dated between ca 160 and 142 ma indicating that they were partly contemporaneous with the volcanic activity at least one volcanic pulse around 160 ma is evidenced over the entire investigated reach of the eamp according to the ages found in arica tocopilla michilla and mantos blancos regions the episodic emplacement of huge amounts of subduction related volcanism is observed throughout the whole andean history and particularly during the jurassic southern peru northern chile and southern argentina these events probably correspond to periodic extensional geodynamic episodes as a consequence of particular subduction conditions such as change of obliquity of the convergence change in the subduction angle slab roll back effect or lower convergence rate that remain to be precisely defined

new isotopic constraints on the age of the halls reward metamorphics north queensland australia delamerian metamorphic ages and grenville detrital zircons abstract the halls reward metamorphics in north queensland australia consist mainly of mica schist and have previously been regarded as a part of the proterozoic craton and separated from the paleozoic orogenic zone on its eastern margin by the tasman tectonic line however our rb sr dating by whole rock muscovite isochron from the halls reward metamorphics shows ca 500 ma k ar dating of muscovite in the quartz mica schist also shows the same age furthermore u th pb chemical dating chime gives ca 510 ma crystallization age for monazite these results suggest that the halls reward metamorphics is not the part of the proterozoic craton but belongs to the early paleozoic delamerian orogenic belt extending the length of the eastern gondwana margin the location of the tasman tectonic line should therefore be west of the halls reward metamorphics u th pb chemical dating of detrital zircon from quartz mica schists also shows an age distribution of peak at ca 1000 ma with some older components the presence of 1000 ma grenvillian detrital zircon from both geological units is consistent with the close proximity of grenville age source suggested by the ausmex continental reconstruction

geophysical differences in the lithosphere between phanerozoic and precambrian australia abstract cannikin atomic bomb recordings indicate that there are differences in travel times from the aleutian islands test site to phanerozoic and precambrian provinces in australia of up to 1 1 s explosion seismic studies in central and southeastern australia enable travel time corrections for crustal and upper mantle structure to be made to recordings of such teleseismic events structure in the upper 60 km can account for at most about 0 2 s of the residual difference but attempts to constrain the remaining residual time to the region above the lehmann discontinuity at about 200 km depth are difficult to reconcile with explosion seismic models regional differences in seismic velocity structure between phanerozoic and precambrian australia therefore appear to exist at depths greater than 200 km electrical conductivities within the mantle have been investigated using two methods long period electromagnetic depth sounding using magnetometer arrays demonstrates that conductivities increase at about 200 km under phanerozoic australia but not until about 500 km depth under precambrian australia shorter period magnetotelluric measurements can only resolve shallower structures these too indicate a similar trend but with sub crustal conductivities increasing at less than 100 km under phanerozoic australia magma at these depths and shallower may be the source for cainozoic volcanism in eastern australia under precambrian central and northern australia magnetotelluric investigations indicate that pronounced conductivity increases do not occur until depths of 150 200 km are reached oceanic magnetic observations indicate that the australian lithospheric plate as a whole is separating from antarctica at a rate of about 7 cm yr the seismic and conductivity structures under the continental region of this plate indicate that lateral inhomogeneities possibly extend to depths as great as 500 km and are probably caused by the passage of eastern australia over a hot spot hawaiian studies indicate that hot spots are not local features but result from large scale disturbances in the mantle conductivity increases commencing in the depth range 100 250 km may give an indication of uppermost zones within which the palaeozoic lithospherc has been substantially modified resulting in elevated surface heat flow volcanism and seismic travel time anomalies

magnitude frequency relation for small earthquakes a clue to the origin of ƒmax of large earthquakes in order to test the hyothesis that the max of large earthquakes is due to the source effect and is causally related to the corner frequency of small earthquakes when it becomes constant below magnitude about 3 we constructed the frequency magnitude relation for earthquakes with magnitude range from 1 3 to 4 recorded at a borehole seismograph station operated by the university of southern california at baldwin hills in the middle of the newport inglewood fault a procedure was developed for an accurate estimation of moment magnitude from the measurement of coda amplitude as a function of lapse time the cumulative number of earthquakes per unit time period and area was estimated by finding the detection limit for ts p for each magnitude we found a clear depature of the observed log frequency magnitude relation for m 3 the observed frequency of earthquakes with m 0 5 is almost 10 times less than that expected from the extrapolation of empirical relation for earthquakes with magnitudes larger than 3 we conclude that the observed frequency magnitude relation departs from self similarity for earthquakes with magnitude smaller than about 3 this departure coincides with the widely observed departure of the moment corner frequency relation from self similarity these observations are consistent with the interpretation of max of large earthquakes in terms of the slip weakening instability model in which the critical slip may be a few tenths of a meter to a few meters and the cohesive zone size may be a few hundred meters to a few kilometers

mechanisms of seismic quiescences in the past decade there have been major advances in understanding the seismic cycle in terms of the recognition of characteristic patterns of seismicity over the entire tectonic loading cycle the most distinctive types of patterns are seismic quiescences of which three types can be recognized post seismic quiescence which occurs in the region of the rupture zone of an earthquake and persists for a substantial fraction of the recurrence time following the earthquake intermediate term quiescences which appear over a similar region and persist for several years prior to large plate rupturing earthquakes andshort term quiescences which are pronounced lulls in premonitory swarms that occur in the hypocentral region hours or days before an earthquake although the frequency with which intermediate term and short term quiescences precede earthquakes is not known and the statistical significance of some of the former has been challenged there is a need if this phenomena is to be considered a possibly real precursor to consider physical mechanisms that may be responsible for them the characteristic features of these quiescences are reviewed and possible mechanisms for their cause are discussed post seismic quiescence can be readily explained by any simple model of the tectonic loading cycle as due to the regional effect of the stress drop of the previous principal earthquake the other types of quiescence require significant modification to any such simple model of the possibilities considered only two seem viable in predicting the observed phenomena dilatancy hardening and slip weakening intermediate term quiescences typically occur over a region equal to or several times the size of the rupture zone of the later earthquake and exhibit a relationship between the quiescence duration and size of the earthquake they thus involve regional hardening or stress relaxation and agree with the predictions of the dilatancy diffusion theory short term quiescences on the other hand are more likely explained by fault zone dilatancy hardening and or slip weakening within a small nucleation zone because seismicity is a locally relaxing process seismicity should follow a behaviour known in rock mechanics as the kaiser effect in which only a very slight increase in strength due to dilatancy hardening or decrease in stress due to slip weakening is required to cause quiescence this is in contrast to other precursory phenomena predicted by dilatancy which require large dilatant strains and complete dilatancy hardening

considerations about the plate tectonic model volcanism and continental crust in the southern part of the central andes abstract mesozoic and cenozoic volcanism in the central andes occurs near the boundary of convergent oceanic and continental lithospheric plates the rocks show a chemical and temporal zonation with respect to the present chile peru trench it is inferred that the geometry of the subduction zone has not changed in the last 190 m y the continental crust during jurassic time in northern chile may have been between 15 and 27 km thick

re os isotope systematics for the lince estefania deposit constraints on the timing and source of copper mineralization in a stratabound copper deposit coastal cordillera of northern chile the lince estefania stratabound copper deposit in the michilla district is one of the most important deposits in the coastal cordillera of northern chile and is one of the most representative of this type of deposit chalcocite and bornite characterize the main stage of hypogene copper sulfide mineralization rhenium and osmium isotopes are used here to constrain the age of hypogene mineralization and the source of osmium contained in these ore minerals a re os isochron yielded an age of 160 16 ma 2 with an associated initial 187os 188os ratio of 1 06 0 09 mean square of weighted deviates 1 8 this age is consistent with available geochronological data from volcanic rocks that host the mineralization and associated alteration phases the high initial 187os 188os ratio indicates a lower crustal component for the source of os and by inference the cu sulfides that contain this os late hematite occurs as an isolated phase or more commonly is associated with the chalcocite bornite and supergene chalcocite covellite associations analyses performed on pure hematite indicate a disturbance of the re os system and hence this mineral phase is not useful as a re os geochronometer

multi methods combined analysis of future earthquake potential prior to an earthquake natural seismicity is correlated across multiple spatial and temporal scales many studies have indicated that an earthquake is hard to accurately predict by a single time dependent precursory method in this study we attempt to combine four earthquake prediction methods i e the pattern informatics pi load unload response ratio lurr state vector sv and accelerating moment release amr to estimate future earthquake potential the pi technique is founded on the premise that the change in the seismicity rate is a proxy for the change in the underlying stress we first use the pi method to quantify localized changes surrounding the epicenters of large earthquakes to objectively quantify the anomalous areas hot spots of the upcoming events next we delineate the seismic hazard regions by integrating with regional active fault zones and small earthquake activities then we further evaluate the earthquake potential in the seismic hazard regions using the lurr sv and amr methods retrospective tests of this new approach on the large earthquakes m 6 5 which have occurred in western china over the last 3 years show that the lurr and sv time series usually climb to an anomalously high peak months to years prior to occurrence of a large earthquake and the asymptote time t c predicted by the amr method correspond to the time of the actual events the results may suggest that the multi methods combined approach can be a useful tool to provide stronger constraints on forecasts of the time and location of future large events

numerical modelling of space and time variations of seismic activity before major earthquakes summary the spatial and temporal variations of seismic activity before large earthquakes including precursory swarm earthquakes clustering pre seismic quiescence in the epicentral area doughnut patterns and foreshocks are numerically modelled by quasi static and dynamical rupture processes on three dimensional heterogeneous faults with various types of non uniform strength distributions the main physical parameters that could yield these precursory changes of seismicity patterns are the form of distributions of static and sliding frictional strengths and their spatial variations over the fault and also the rate of increase of tectonic stress for weakly heterogeneous faults a large scale main shock rupturing the entire fault surface occurs without any precursory indications if the fault has moderately to heavily non uniform strengths on a small scale gradually increasing activity tends to occur over a long period prior to a main event with no swarms or quiescence for extremely heterogeneous faults including a number of small to moderate size asperities with medium to high strengths a typical sequence appears with swarms quiescence foreshocks and a main shock event the temporal behaviour of these events seems to depend primarily on the form of distribution of the strengths if the fault includes a large size asperity with medium to high strengths in its central part a doughnut pattern of seismicity repeatedly appears with high activity and quiescence in the surrounding and central asperity regions if a belt like barrier region with extremely high strengths lies in the central part of the fault dividing the entire region into two sections large scale shocks take place successively in the adjacent unruptured regions at slightly different times and a main shock event rupturing the entire fault occurs after a long period of quiescence these results from numerical simulations could provide possible explanations of observed seismicity patterns prior to major earthquakes

seismicity simulation with a mass spring model and a displacement hardening softening friction law dieterich simulated aftershocks numerically using a one dimensional mass spring model with a time dependent friction law but an important precursory phenomenon called quiescence cannot be produced by this model unless as mikumo and miyatake showed with a three dimensional continuum model a somewhat arbitrary bimodal distribution of frictional strength is assumed here we used the friction law proposed by stuart which is a displacement hardening softening model and simulated the quiescence by varying the parameters of the friction law in our mass spring model we found a variety of seismicity patterns when we choose extremely large critical displacement we get a recurrent sequence of creep followed by mainshock without small earthquakes but when we choose a critical displacement in the same order of magnitude as the slip weakening critical displacement estimated by papageorgiou and aki from strong motion data we get a normal seismicity pattern including quiescence before large events this simple model points to a promising approach for the interpretation of the rupture process during an earthquake by the same physical model

spatial and temporal correlation between coda q 1 and seismicity and its physical mechanism from the analysis of local earthquake coda waves recorded by the benioff short period vertical seismograph at riverside california we found a surprisingly large and systematic temporal variation in coda q 1 in the southern california during the 55 year period from 1933 to 1987 comparison of the coda q 1 variation with the b value obtained for earthquakes with m 3 occurred in the area within 180 km from riverside corresponding to the area for which q 1 is estimated by the coda method led to an extraordinary result that coda q 1 correlates positively with b value with the correlation coefficient of 0 79 the significance of correlation cannot be rejected at the confidence level of 0 974 since the data used for measuring q 1 namely the coda part of seismograms and the data used for measuring b namely the seismicity catalog are totally independent the observed correlation supports strongly the physical reality of temporal change in both coda q 1 and b value the correlation between coda q 1 and b value has been reported for several other cases but the correlation was positive in some cases and negative in others our current preferred model for explaining the observed correlation is the following aseismic creep is going on from time to time over various sizes of cracks in the ductile part and in some cases in the brittle part of lithosphere at and near the plate boundary creep activities tend to increase the crack density and coda q 1 observed in the region if the creek occurs over distributed cracks with a certain predominant scale length spatial stress concentration may enhance seismicity for earthquakes with a similar size if this characteristic size is in the lower part of the magnitude range for which the b value is estimated the enhanced seismicity will increase b on the other hand if it is in the upper part of the magnitude range it will decrease b thus we can explain the observed strong correlation between coda q 1 and b value for both positive and negative cases the existence of such a characteristic crack size in a seismic region has been supported by various observations on earthquake scaling

seismic trapped modes in the oroville and san andreas fault zones three component borehole seismic profiling of the recently active oroville california normal fault and microearthquake event recording with a near fault three component borehole seismometer on the san andreas fault at parkfield california have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals these wave trains are interpreted as fault zone trapped seismic modes parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers as shallow as 4 kilometers and extend 12 kilometers along the fault on either side of the recording station selected oroville and parkfield wave forms are modeled as the fundamental and first higher trapped sh modes of a narrow low velocity layer at the fault modeling results suggest that the oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second whereas at parkfield the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1 1 to 1 8 kilometers per second these low velocity layers are probably the rupture planes on which earthquakes occur

seismicity and geodetic anomalies in a wide area preceding the niigata ken chuetsu earthquake of 23 october 2004 central japan 1 the ordinary short term occurrence rate of earthquakes in a region is predicted using the epidemic type aftershock sequence etas model of triggering seismicity any anomalous seismic activity such as quiescence and activation can be quantified by identifying significant deviation from the predicted rate such anomalies are revealed to have occurred during the three year period leading up to the 2004 chuetsu earthquake of m6 8 mw6 6 central japan the quiescence and activation which took place in the zone of negative and positive increments of the coulomb failure stress respectively were possibly caused by aseismic slip on or near the focal fault plane such slip is further supported by transient crustal movement around the rupture source time series records of the baseline distances between permanent gps stations deviated from the predicted trends with the deviations mostly consistent with the coseismic horizontal displacements of the stations due to the chuetsu earthquake the cumulated preceding slip is estimated to be roughly equivalent to mw6

mesozoic magmatic and tectonic events within the andean plate boundary zone 26 27 30 s north chile constraints from 40 ar 39 ar mineral ages in north chile jurassic cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences new 40 ar 39 ar mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes c 202 188 ma c 160 153 ma c 142 138 ma c 130 127 ma and c 106 103 ma conjugate dike sets were emplaced immediately before periods of plutonism and each distinct episode of magmatism prior to c 127 ma was located successively eastward inboard from the subduction boundary features interpreted to result from regional extension within the magmatic arc steeply dipping ductile mylonitic shear zones were developed in wall rocks along western contacts of late jurassic and early cretaceous plutons steeply plunging mineral stretching lineations and a variety of kinematic indicators record an east side down extension along eastern faults of the atacama fault system steeply dipping ductile mylonitic shear zones display subhorizontal mineral stretching lin

tectonic development and metallogeny of the kanmantoo trough in south australia abstract the kanmantoo trough is the sedimentary and tectonic link between the adelaide geosyncline composed of late proterozoic sediments and volcano sedimentary sequences of the tasman fold belt system in eastern australia cambrian flyschoid sediments of the kanmantoo trough were likely deposited on a marginal shelf reflecting tectonic activity to the west and a continental margin to the east the sediments may extend eastwards under the murray basin but the tectonic significance of the padthaway ridge and of mafic volcanics northeast of that region are poorly understood copper mineralisation is scattered throughout the kanmantoo trough and is commonly associated with the intersection of northwest trending fracture zones and pyritic sedimentary horizons

the progressive development of a macroscopic upright fold pair during five near orthogonal foliation producing events complex microstructures versus a simple macrostructure abstract a macroscopic anticline in the adelaide fold belt with a profile similar to first deformation folds with an axial plane slaty cleavage elsewhere in the orogen folds bedding schistosity and a differentiated crenulation cleavage all these structures are readily visible in outcrop and a young crenulation cleavage has locally developed parallel to the axial plane of the fold yet foliations preserved as inclusion trails in andalusite garnet and staurolite porphyroblasts reveal a history of five foliations that formed successively in steep shallow steep shallow and finally steep orientations without producing a refold geometry in bedding at the macroscopic scale the predominantly dextral looking north asymmetry of curvature of overprinting foliations on both limbs of this fold tightly constrains any model for its development the anticline formed as one hinge of an open fold couplet with a sub vertical axial plane slaty cleavage during the first deformation it was progressively tightened during each subsequent event without producing a refold geometry in bedding steep axial plane foliations were overprinted by shallow differentiated crenulated cleavages that transected the fold couplet these shallow foliations were then folded during the next deformation producing another steeply dipping foliation as the fold couplet tightened thus simple macroscopic folds in orogens with a multideformational history may appear to have formed as late structures because they fold differentiated crenulation cleavages and a bedding parallel schistosity but actually formed very early during orogenesis and were then tightened by a succession of near orthogonal foliation producing deformations such deformation histories provide a simple explanation for the lack of large scale refold geometries in many fold belts with a complex history of deformation small scale structures are not always a direct reflection of larger scale geometries in fold belts

detailed 40ar 39ar dating of geologic events associated with the mantos blancos copper deposit northern chile the 40ar 39ar geochronological method was applied to date magmatic and hydrothermal alteration events in the mantos blancos mining district in the coastal cordillera of northern chile allowing the distinction of two separate mineralization events the late jurassic mantos blancos orebody hosted in jurassic volcanic rocks is a magmatic hydrothermal breccia style cu deposit two superimposed mineralization events have been recently proposed the first event is accompanied by a phyllic hydrothermal alteration affecting a rhyolitic dome the second mineralization event is related to the intrusion of bimodal stocks and sills inside the deposit because of the superposition of several magmatic and hydrothermal events the obtained 40ar 39ar age data are complex however with a careful interpretation of the age spectra it is possible to detect complex histories of successive emplacement alteration mineralization and thermal resetting the extrusion of jurassic basic to intermediate volcanic rocks of the la negra formation is dated at 156 3 1 4 ma 2 using plagioclase from an andesitic lava flow the first mineralization event and associated phyllic alteration affecting the rhyolitic dome occurred around 155 156 ma a younger bimodal intrusive event supposed to be equivalent to the bimodal stock and sill system inside the deposit is probably responsible for the second mineralization event dated at ca 142 ma other low temperature alteration events have been dated on sericitized plagioclase at ca 145 146 125 and 101 ma this is the first time that two distinct mineralization events have been documented from radiometric data for a copper deposit in the metallogenic belt of the coastal cordillera of northern chile

interaction between deformation and magma extraction in migmatites examples from kangaroo island south australia migmatite terranes are structurally complex because of strong rheological contrast between layers with different melt contents and because of magma migration leading to irregular volume changes migmatite deformation is intimately linked with magma extraction and the origin of granitoids we investigate here the relationships between evolving deformation and magma extraction in migmatites formed during the ca 500 ma delamerian orogeny exposed on kangaroo island south australia here several phases of deformation occurred in the presence of melt during an early upright noncylindrical folding event magma was channeled toward the hinge zones of antiforms funnel shaped networks of leucosomes formed a root zone that linked up toward a central axial planar channel forming the main magma extraction paths during folding extraction was associated with fold limb collapse and antiformal hinge disruption by magma accumulation and transfer during a later deformation phase melt rich diatexites were deformed and schollen blocks or rafts of source rock were disaggregated into smaller blocks and schlieren and these were deformed into asymmetric sigmoidal shapes indicative of dextral shearing flow during flow magma accumulated preferentially along shear planes indicating a dilatational component during shearing transtension and in strain shadows of schollen at the waning stages of deformation magma extraction from these diatexites gave rise to n trending steeply dipping funnel shaped channels not associated with any externally imposed deformational feature the funnel shape of these structures indicates the direction of magma flow structures developed during this phase are comparable with those formed during dewatering of soft sediments despite a high degree of complexity magma migration and extraction features record distinct responses to the evolving deformation that can be used to understand deformation and the nature and direction of magma extraction the oldest and youngest magmatic rocks from migmatites were dated by u pb shrimp sensitive high resolution ion microprobe dating techniques both reveal a continuous age spread between ca 495 and 465 ma the age range is interpreted to indicate the duration of anatexis order of 30 m y with the older ages marking onset of monazite growth as the system approached peak metamorphism and younger ages representing final growth close to the solidus during cooling

physical models of earthquake instability and precursory processes this paper selectively reviews physical models of earthquake instability in these models instability arises as a result of interaction of a fault constitutive relation with deformation of the surrounding material that occurs in response to remote tectonic loading in contrast to kinematic models in which the fault slip is imposed it is calculated in physical models and consequently these models are essential for understanding precursory processes some kind of weakening behavior for the fault constitutive relation is required to produce an instability analogous to an earthquake two commonly employed idealizations discussed here are rate independent slip weakening and rate state dependent friction when these constitutive models are employed on surfaces embedded in elastic half spaces or layers possibly coupled to a viscoelastic substrate the results are capable of simulating realistically some aspects of earthquake occurrence common to all models is the prediction that earthquake instability is preceded by precursory slip which produces a departure of surface strain rate from the background level near the epicenter of a moderate to large earthquake the magnitude of this departure appears to be well within the range of current geodetic measurement accuracy and its duration is of the order of months to years however details depend on a variety of factors including the modelling of the constitutive relation near peak stress coupling of elastic crust to the asthenosphere and coupling of deformation with pore fluid diffusion

geophysical testing of balanced cross sections of fold thrust belts with potential field data an example from the fleurieu arc of the delamerian orogen south australia abstract the delamerian south australia and ross antarctica orogens once formed a continuous chain along the paleo pacific margin of gondwana correlations between the two are based on many characteristics including style of shortening and deformation constraining structural features in one of these orogens impacts on interpretations of tectonic processes for the other balanced cross sections have provided crucial information on the internal architecture of the delamerian orogen south australia and form a key piece of evidence in some widely circulated models for the deformational style these sections have not been constrained by geophysical investigations which can provide significant information about the subsurface geometry we present magnetic forward models for two balanced cross sections of the fleurieu arc in the delamerian orogen the forward models require a larger volume of magnetic basement at shallower depths than predicted by balanced sections requiring a revision of currently proposed shortening mechanisms the shortening mechanism inferred from the models requires less imbrication of upper crustal units and more folding above thrusts that involve both basement and cover sequences this style of shortening compares favorably with published observations from the ross orogen in antarctica

model for intermediate term precursory clustering of earthquakes a causal sequence of an earthquake swarm extended quiescence foreshocks and main shock can be understood in terms of an aggregation model of crack growth and ultimate fusion on a multiplanar earthquake fault system the rheological model is one of accelerated crack growth under conditions of slip weakening coupled with a slow recovery of static friction after fracture once a slip feature has been generated that spans a large part of the available geometry a stress shadow is cast on other neighboring cracks that strongly retards and inhibits crack growth thus initiating the quiescent phase abruptly the success of the model depends on the assumption that nonlinear rheology regulates the rate of slip and that earthquake faults are not simply connected surfaces but instead have many strands

interactions between the landers and hector mine california earthquakes from space geodesy boundary element modeling and time dependent friction the m w 7 1 hector mine earthquake happened 7 years after the 1992 m w 7 3 landers earthquake within a system of right lateral strike slip faults comprising the eastern california shear zone because estimates of recurrence intervals of m 7 earthquakes on these faults range anywhere from 1500 to 50 000 years the close temporal spacing of these two earthquakes suggests that there is some interaction between the two current models of simple coulomb static stress interactions between the earthquake ruptures do not predict an obvious cause and effect relationship the interaction between the computed normal and right lateral shear stress reductions induced by the landers earthquake at the hector mine hypocenter is unclear we use a combination of space geodesy boundary element modeling and computer modeling of time dependent fault friction to investigate the interaction between the two earthquakes we compare stress changes on the hector mine rupture plane induced by the landers earthquake with the detailed slip distribution inferred using a combination of gps and insar data the slip distributions of both earthquakes are also used to infer the magnitude of the shear stress drop on each earthquake rupture and the orientation of the remote background stress consistent with these two recent events for each earthquake the azimuth of the remote maximum compressive stress was approximately 17 6 and the magnitude of the shear stress drop was 8 1 mpa on the landers rupture and 10 2 mpa on the hector mine rupture using a simple spring and slider model as a proxy for the rate and state frictional response of the hector mine faults to landers earthquake imposed stress steps we find that while a decrease in normal stress at the hector mine hypocenter would by itself have caused the hector mine earthquake to nucleate immediately the simultaneous decrease in shear stress may have caused a delay in the peak shear loading consideration of an acceptable range of rate and state friction parameters secular strain rates and crustal stiffnesses leads us to conclude that this delay could have been 0 to 40 years

3 d simulation of earthquake generation cycles and evolution of fault constitutitve properties the earthquake generation cycle consists of tectonic loading quasi static rupture nucleation dynamic rupture propagation and stop and subsequent stress redistribution and fault restrengthening from a macroscopic point of view the entire process of earthquake generation cycles should be consistently described by a coupled nonlinear system of a slip response function a fault constitutive law and a driving force on the basis of such a general idea we constructed a realistic 3 d simulation model for earthquake generation cycles at a transcurrent plate boundary by combining the viscoelastic slip response function derived for a two layered elastic viscoelastic structure model the slip and time dependent fault constitutive law that has an inherent mechanism of fault restrengthening and the steady relative plate motion as a driving force into a single closed system with this model we numerically simulated the earthquake generation cycles repeated in a seismogenic region on a plate interface and examined space time changes in shear stress slip deficits and fault constitutive properties during one complete cycle in detail the occurrence of unstable dynamic slip brings about decrease both in fault strength and shear stress to a constant residual level after the arrest of dynamic slip the breakdown strength drop p of fault is restored rapidly and the process of stress accumulation resumes in the seismogenic region on the other hand the restoration of the critical weakening displacement d c proceeds gradually with time through the interseismic period the restoration of d c can be regarded as the macroscopic manifestation of the microscopic recovery process of fractal fault surface structure through numerical simulation with a multi segmented fault model we examined the effects of viscoelastic faultto fault interaction the effect of transient viscoelastic stress transfer through the asthenosphere is significant as well as the direct effect of elastic stress transfer and it possibly explains the time lag of the sequential occurrence of large events along a plate boundary

effects of physical fault properties on frictional instabilities produced on simulated faults laboratory studies of large scale simulated faults show that physical properties of the fault specifically normal stress and fault roughness significantly influence the unstable shear failure behavior of the fault in addition the experiments provide insights into important length scaling effects that are useful for assessing concepts such as critical crack length or rupture nucleation dimension stick slip shear failures have been generated along a 2 m long simulated fault in a block of sierra white granite experiments were conducted at normal stresses between 0 6 and 4 mpa with two different prepared roughnesses of the simulated fault a smooth fault with a profilometer measured roughness of 0 2 m and a rough fault with a measured roughness of 80 m high speed records of shear strains and slip velocities clearly show both the propagation of the stick slip failures and details of the local deformations and motions from which slip weakening of the fault at the onset of sliding can be resolved both the dynamic stress drops of the stick slip events and the apparent fracture energies of the events determined from the records of the fault slip weakening increase with increasing normal stress although critical slip weakening displacements are insensitive to normal stress they increase with increased fault roughness such that the apparent fracture energies are also larger on the rough fault than on the smooth fault slip velocities in these experiments are influenced by roughness and are proportional to stress drop although the onset of slip is sharply defined the termination of slip is more gradual without a clear stopping phase measured rupture velocities on the rough fault are lower than those on the smooth fault when a length scale based on rupture nucleation dimensions directly proportional to critical displacements is introduced the data suggest that at fault lengths close to the nucleation lengths rupture velocity and slip velocity increase with increasing fault length

structure and origin of the fleurieu and nackara arcs in the adelaide fold thrust belt south australia salient and recess development in the delamerian orogen abstract the regional map pattern of structural trends in the early paleozoic adelaide fold thrust belt of south australia resembles an upright letter s in general structures in the fold thrust belt verge toward the gawler craton to the west so the northern curve nackara arc of the s is a salient whereas the southern curve fleurieu arc is a recess the map view axial trace of the nackara arc coincides with the thickest portion of the adelaidean late proterozoic basin and the map view axial trace of the fleurieu arc coincides with the intersection between the adelaidean and kanmantoo cambrian basins our structural analysis of the adelaide fold thrust belt demonstrates that the cross sectional structural geometry of the nackara arc contrasts markedly with that of the fleurieu arc in the nackara arc the belt consists of open detachment folds that probably formed above an evaporite hosted decollement whereas in the fleurieu arc the belt consists of an imbricate fan of basement involved thrusts further the foreland edge of the nackara arc coincides with a blind ramp whose strike parallels fold axes of the arc whereas the foreland edge of the fleurieu arc coincides with emergent thrusts that cut obliquely across earlier formed folds of the arc in the transition region between the two arcs the fold thrust belt consists of an intensely sheared basal duplex overlain by open folds structural contrasts between the nackara and fleurieu arcs indicate that the two curves did not form in the same way we present a model for development of these curves based on a comparison of their structural features to those of curves in other fold thrust belts and based on previous studies of curve formation using sandbox analogs we suggest that curvature of the nackara arc reflects the control of basin stratigraphy on the width of a fold thrust belt while curvature of the fleurieu arc reflects oroclinal bending of the fold thrust belt when it impinged on the southeastern corner of the gawler craton our model explains the structural contrast between the two arcs and illustrates correlations between fold thrust belt curve formation and pre deformational basin geometry

sedimentary thickness variations and deformation intensity during basin inversion in the flinders ranges south australia abstract the central and northern parts of the adelaide fold belt in the flinders ranges south australia consist of a sequence of neo proterozoic cambrian sediments overlying a meso proterozoic basement complex both of which were mildly deformed in an intracratonic setting during the 500 ma delamerian orogeny the fold belt lies within a prominent heat flow anomaly average heat flows of 90 mwm 2 reflecting extraordinary enrichments in heat producing elements in the meso proterozoic basement suggesting that anomalous thermal regimes may have been significant in localising delamerian deformation however spatial variations in deformation intensity correlate more closely with variations in the thickness of the sedimentary sequence than with observed variations in heat flow suggesting that the thickness of the sedimentary blanket plays a crucial role in localising delamerian deformation during basin inversion we use simple numerical models of lithospheric strength to investigate the potential role of sedimentary thickness variations on the distribution and style of deformation focussing on the impact of a variable thickness sediment pile deposited above a radioactive basement we show that for thermal parameters appropriate to the flinders ranges moho temperatures may vary by 25 30 c for every additional kilometre of sediment for a brace goetze lithospheric rheology controlled by a combination of temperature dependent creep processes and frictional sliding the observed variations in thickness of the sedimentary pile are sufficient to cause dramatic reductions in the vertically integrated strength of the lithosphere by many orders of magnitude thereby providing a plausible explanation for observed correlation between sediment thickness and deformation intensity during basin inversion

mechanical consequences of granite emplacement during high t low p metamorphism and the origin of anticlockwise pt paths high temperature t low pressure p metamorphic belts preserve evidence of metamorphism under extremely perturbed and hence transient thermal regimes pt paths are commonly anticlockwise that is the maximumt was attained prior to or at the same time as the maximump with cooling at near constant or increasingp and in many terrains peak metamorphic temperatures prevailed during convergent deformation using a simple coupled thermal mechanical model that assumes a strongly temperature dependent rheology for the continental lithosphere we show that the coincidence of metamorphism with convergent deformation in high t low p terrains may reflect the thermal weakening attendant with rapid advective movement of heat within the lithosphere via granite magma ascent the model shows that the magnitude of the thermal weakening effect is sensitive to the critical temperature for granite segregation tm crit the depth of emplacement of the magmas and most importantly the granite forming mechanism during the prograde cycle material points attain maximum temperatures during a transient 3 ma high strain rate pulse withe up to 10 14 s 1 at levels near the site of granite emplacement the attainment of maximum pressures follows peak temperatures with rapid cooling paths that are close to isobaric or involve slight compression

seismicity simulation with a rate and state dependent friction law the dynamic motions and stabilities of a single degree of freedom elastic system controlled by different friction laws are compared the system consists of a sliding block connected to an elastic spring driven at a constant velocity the friction laws are a laboratory inferred friction law called the rate and state dependent friction law proposed by dieterich and ruina and a simple friction law described by dynamic and static frictions we further extend the solution to a one dimensional mass spring model which is an analog of a fault controlled by the rate and state dependent friction law this model predicts non uniform slip and stress drop along the rupture length of a heterogeneous fault this result is very different from some earlier modelings based on the simple friction law and a slip weakening friction law in those earlier modelings the stress and slip functions become smoother with time along the length of the fault rupture owing to the interactions between fault segments during slip because of this smoothing process the number of small events will decrease with time and the universilly observed stationary magnitude frequency relation cannot be explained the interaction between a fault segment and its neighboring segments can be measured when the post slip stress on this segment is compared with the stress on an identical segment represented by a block in this modeling without neighboring segments if the post slip stress of the former is much higher than that of the latter strong interaction exists if the two are close only weak interaction exists the interaction is determined by the relative motion between fault segments and the time duration of interaction our new modeling with the rate and state dependent friction law appears to show no such smoothing effect and provides a physical mechanism for the roughening process in the difference between the fault strength and stress that is necessary to explain the observed stationary magnitude frequency relation the noninstantaneous healing predicted by the rate and state dependent friction law may be repsonsible for the recurring nonuniform slip and stress drop and may be explained by the reduction of interaction among fault segments due to the low frictional strength during the fault stopping the very low friction during slip stopping allows much longer times than does the higher friction due to instantaneous healing for the fault segments to adjust their motions from an upper limit slip velocity to almost rest according to newton s second law a process with fixed masses and constant velocity changes involves low forces and weak interactions if it is accomplished in a long time period and vice versa our modeling also indicates that the existence of strong patches with higher effective stress on a fault is needed for the occurrence of major events the creeping section of a fault such as the one along the san andreas fault in central california on the other hand can be simulated with the rate and state dependent friction law by certain model parameters which however must not include strong patches in this case small earthquakes and aseismic creep relieve the accumulating strain without any large events

formation and reactivation of the cambrian kanmantoo trough se australia implications for early palaeozoic tectonics at eastern gondwana s plate margin the kanmantoo group is a thick and largely metamorphosed sedimentary succession that filled an isolated arcuate cambrian basin kanmantoo trough which formed within continental gondwana and now lies on the southern margin of the present australian continent kanmantoo group sediments unconformably overlie neoproterozoic and older cambrian rocks we consider that the geometry of the southern part of this trough was influenced by strike slip movement along an intra continental tear fault to the north the basin changes to a style dominated by orthogonal extension and eventually tapers and dies out normal to the tear fault balanced sections show that the kinematic style and strain distribution developed during early palaeozoic inversion was controlled by the specific architecture of the kanmantoo trough early palaeozoic tear faulting could have linked contrasting subduction polarities along the then contiguous palaeo pacific margin of gondwana the kanmantoo trough is considered to have formed at a passive margin related to east directed subduction in what is now the australian continent in contrast west directed subduction formed an active margin at contiguous parts of current antarctica kanmantoo group sediments were derived from the south by erosion of a grenvillean province mixed with sediments eroded from the emergent active margin of gondwana the inception localization and sedimentation in the kanmantoo trough reflects a complex interaction of tectonic processes along the encroaching ross delamerian orogen

the mid crustal horizon under the eromanga basin eastern australia abstract long line explosion seismic recordings indicate that a major mid crustal p wave velocity increase exists under the eromanga basin which contrasts the crust of this region from its neighbouring tectonic provinces the velocity increase is shown to be a regional feature and to correspond to the upper boundary of a zone of discontinuous seismic reflections characteristic of the lower crust the velocity increase of 0 35 0 6 km s occurs over a small depth range at an average depth of 24 km this velocity gradient produces conspicuous wide angle reflections at distances greater than 60 km from the shot point the seismic data present the clearest evidence to date of the existence of a major mid crustal horizon in continental australia a feature not prominent in other cratonic provinces which have been investigated in detail the velocity in the lower crust below the horizon is 6 7 7 0 km s the horizon represents a boundary between the upper and lower crust which could imply a different tectonic history for these two zones

the mantos blancos copper deposit an upper jurassic breccia style hydrothermal system in the coastal range of northern chile the upper jurassic mantos blancos copper deposit 500 mt at 1 0 cu located in the coastal range of northern chile displays two superimposed hydrothermal events an older phyllic alteration probably related to felsic magmatic hydrothermal brecciation at 155 ma and younger 141 142 ma potassic propylitic and sodic alterations coeval with dioritic and granodioritic stocks and sills and dioritic dikes main ore formation is genetically related to the second hydrothermal event and consists of hydrothermal breccias disseminations and stockwork style mineralization associated with sodic alteration hypogene sulfide assemblages show distinctive vertical and lateral zoning centered on magmatic and hydrothermal breccia bodies which constitute the feeders to mineralization a barren pyrite root zone is overlain by pyrite chalcopyrite and followed upwards and laterally by chalcopyrite digenite or chalcopyrite bornite the assemblage digenite supergene chalcocite characterizes the central portions of high grade mineralization in the breccia bodies fluid inclusions show evidence of boiling during the potassic and sodic alteration events which occurred at temperatures around 450 460 c and 350 410 c and salinities between 3 53 and 13 45 wt nacl eq respectively the hydrothermal events occurred during episodic decompression due to fluid overpressuring hydrofracturing and sharp changes from lithostatic to hydrostatic conditions sulfur isotope results of hypogene sulfide minerals fall in a narrow range around 0 per mil suggesting a dominance of magmatic sulfur carbon and oxygen isotopic data of calcites from propylitic alteration suggest a mantle derived carbon and oxygen isotope fractionation due to low temperature alteration

two categories of earthquake precursors physical and tectonic and their roles in intermediate term earthquake prediction i suggest that earthquake precursors can be divided into two major categories physical and tectonic i define physical precursor to be a direct or indirect indication of initiation or progression of an irreversible rupture generating physical process within the preparation zone of a forthcoming earthquake tectonic precursor is defined as a manifestation of tectonic movement which takes place outside the preparation zone of an impending earthquake as a link in a chain of particular local tectonism in each individual area preceding the earthquake most intermediate term short term and immediate precursors of various disciplines within the source regions of main shocks are considered physical ones some precursory crustal deformations around the source regions are however possibly tectonic precursors because they may be caused by episodic plate motions or resultant block movements in the neighboring regions of the fault segments that will break a possible example of this phenomena is the anomalous crustal uplift in the izu peninsula japan before the 1978 izu oshima earthquake ofm s 6 8 some precursory changes in seismicity patterns in wide areas surrounding source regions also seem to be tectonic precursors because they were probably caused by the particular tectonic setting of each region a typical example is a so called doughnut pattern before the 1923 kanto japan earthquake ofm s 8 2 although most studies on earthquake precursors so far seem to regard implicitly all precursory phenomena observed as physical ones the two categories should be distinguished carefully when statistical analysis or physical modeling is carried out based on reported precursory phenomena in active plate boundary zones where a practical strategy for earthquake prediction may well be different from that in intraplate regions tectonic precursors can be powerful additional tools for intermediate term earthquake prediction

synthetic seismicity models of multiple interacting faults a synthetic seismicity model for multiple interacting faults of any strike and dip in a three dimensional elastic half space has been developed the ultimate purpose is to determine how hazard is modified by elastic interactions of events in complex fault systems and to examine the synthetic seismicity for unusual behavior before large events that might be observable in the real world each fault is subdivided into some number of equal sized patches and a coefficient of static friction random within a specified range is assigned to each the patch static strength is then the product of friction and differential normal stress stress both shear and normal accumulates due to some predefined tectonic driving force until one or more patches fail and slip instantaneously the slip can be in any direction within the fault plane and its magnitude is determined by a specified fractional stress drop this initial slip induces shear stress and strength changes on all other patches and faults and perhaps results in other patches failing patches can slip more than once once having failed their strength is reduced resulting in an overshoot that approximates the effect of dynamic stress enhancement an event is over when all patches are stable although present computational constraints do not allow models with more than about 1500 patches or a large number of faults experiments with simple models of two parallel strike slip faults 25 km length 15 km depth 694 m x 714 m patch size driven at different rates and separated by 10 or 15 km give some indicative results a range of friction between 0 1 and 0 2 with a stress drop of 10 about 3 mpa or 30 bars produces a b value 500 000 events close to 1 the models generate a distinct class of large characteristic events that rupture the full fault plane the distribution of interevent times for the background seismicity is very similar to that for a poisson process while the characteristic events are quasi periodic nearly half the larger events directly induce some activity on the other fault the effect of a characteristic event on the occurrence of another characteristic event on the opposite fault is small and depends on the separation at 10 km distance other events are retarded while at 15 km distance they are advanced at 10 km separation the probability of a pair of characteristic events within 1 year is reduced to 0 3 from 1 7 for the case with no interaction consideration of interactions between slip on the faults and the driving mechanism as well as higher stress drops and more overshoot would increase the magnitude of the interaction effects

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

influence of basin architecture on the style of inversion and fold thrust belt tectonics the southern adelaide fold thrust belt south australia abstract the southern adelaide fold thrust belt involves neoproterozoic adelaidean strata deposited during protracted rift and sag phases and an overlying cambrian kanmantoo sequence represented by a thick wedge of turbidites rapidly deposited in an eastwards deepening basin deeply incised into the underlying rocks the belt was deformed during the delamerian orogeny developed at the southeastern margin of the australian craton during early palaeozoic nw directed displacement during this shortening event the margin of the kanmantoo basin was reactivated and formed a significant strain guide that controlled key features of the fold thrust belt strain integrated and restored sections reveal contrasting geometric and kinematic styles along the strike of the belt a transpressional zone is developed at the steep basin margin in the south oriented oblique to the structural grain the central parts of the belt are dominated by strong buttressing growth fault reactivation and basement involved footwall shortcut thrusting in the northern part of the belt shortening is accommodated by homogeneous folding above a master de collement these variations in regional structural style and internal strain partitioning reflect significant variations in the original kanmantoo basin margin geometry rheological parameters control the partitioning of strain associated with pervasive fabrics for example cleavage and transposed bedding folding is accompanied by negative line length changes shortening in the northern part of the belt in its thrusted central part pelites commonly show positive line length changes psammites were least affected by line length changes and thus provide key beds for section balancing pervasive strain makes a full restoration of cross sections impossible however incorporating regional strain data assists establishment of balanced and restorable sections which provide a powerful tool in understanding both pre deformational basin geometry and its shortening

a geochemical traverse across the north chilean andes evidence for crust generation from the mantle wedge major and trace element and sr and nd isotope analyses are presented on 186 0 ma magmatic rocks along an east west traverse across north chile at 22 s esr ranges from 25 to 100 and end from 6 to 9 but the low end and high esr values are in rocks generated in the last 15 ma it is argued that previous discussions of petrogenesis in north chile have been hampered because the changes in magma chemistry in this area of unusually thick crust reflect not one but two processes one results in a progressive shift of esr from 25 to 20 and end from 6 to 6 in jurassic to recent rocks which is accompanied by increasing ta sm and sr and decreasing th ta the second is largely confined to the younger rocks and it is characterised by esr increasing up to 100 with increasing sio2 and decreasing sr and it results in relatively shallow trends on an end esr diagram the preferred interpretation is that trend 1 is due to the mobilisation of old late proterozoic mantle lithosphere as magmatism migrated eastwards and that trend 2 is due to crustal melting and contamination with differentiation in this area of thickened continental crust it follows that the mantle wedge is the principal site of crust generation and it is argued that 20 of the sr in the recent northern chile rocks is derived from the subducted ocean crust

assessing the role of bitumen in the formation of stratabound cu ag deposits insights from the lorena deposit las luces district northern chile abstract stratabound cu ag deposits located in north central chile have been grouped into two ns parallel belts according to the age of their volcanic and volcanoclastic host rocks deposits from the cretaceous belt are characterized by an unusual assemblage of cu sulfides with migrated hydrocarbons whereas the presence of organic matter has not yet been recognized in deposits from the jurassic belt here we report the first evidence of pyrobitumen in a jurassic stratabound cu ag deposit and evaluate its role in ore formation we present a micro analytical study of pyrobitumen associated with cu fe sulfides in the lorena deposit located in northern chile our approach involved a combination of surface and drill core sample characterization fe sem observations empa and xrf mapping of pyrobitumen in addition we determined the speciation of sulfur in pyrobitumen by using synchrotron xanes two main events of hypogene mineralization were identified at lorena deposit i e a pre ore and a main cu ore stage the pre ore stage is characterized by globularly shaped pyrobitumen and minor pyrite and a widespread albite alteration of the andesite host rock pre ore pyrite was replaced by chalcopyrite bornite and chalcocite providing evidence for a late cu inception in the main cu ore stage chalcocite is often associated with sharp edged angular pyrobitumen most likely indicating remobilization or a later pulse of hydrocarbons empa data show that pyrobitumen at lorena is enriched in fe 0 02 1 74 wt cu 0 02 0 87 wt s 0 01 0 43 wt and cl 0 01 0 4 wt other elements detected include v co ni cr sb as ag and au both ag and au were only detected in a few spots with au reaching up to 600 ppm desktop and synchrotron xrf mapping shows that s is homogeneously distributed within pyrobitumen with lower s concentrations observed at grain rims on the other hand empa wds x ray maps show that cu precipitated on pyrobitumen grain surfaces filling fractures and forming veinlets in addition textures showing the presence of micro to nano sized pyrobitumen inclusions within both pre ore pyrite and main ore chalcocite grains point unequivocally to the interaction of hydrothermal fluids with hydrocarbons during the formation of this deposit the high s concentrations measured in pyrobitumen and the presence of organosulfur compounds detected by xanes can be attributed to an organic source the pre ore assemblage is interpreted as formed from a petroleum aqueous fluid connate waters mixture that migrated from the sedimentary source rock into the jurassic host rocks where pyrite and pyrobitumen precipitated in the andesite late cu bearing hydrothermal fluids interacted with this pyrobitumen pyrite assemblage to form cu sulfides either by replacement of pyrite or precipitation we conclude that some of the s contained in pyrobitumen could have been released during interaction with the cu bearing hydrothermal fluid contributing at least partially to the s budget required for sulfide mineralization in the main ore stage our results provide evidence supporting pyrobitumen fluid interaction as a key process in the formation of the lorena deposit and possibly other stratabound cu ag deposits in chile

basement cover interaction in the adelaide foldbelt south australia the development of an arcuate foldbelt abstract the upper proterozoic to cambrian aged sedimentary and volcanic rocks comprising the adelaide foldbelt were deformed and in places metamorphosed during the cambro ordovician delamerian orogeny tectonic fabrics developed in the central portion of the foldbelt mt lofty ranges demonstrate westward transport during the orogeny the sigmoidal shape outlined by the kangaroo is mt lofty ranges olary portion of the foldbelt is interpreted to have been the result of dextral wrench faulting in the lower to mid proterozoic basement thus cover rocks overlying such basement wrench fault zones would have suffered a transpressional stress regime giving rise to the observed fold axis oriented at an oblique angle to the thrust boundary in the northern portion of the foldbelt northern flinders ranges wrench faulting is interpreted to have accommodated considerable basement shortening which initiated a basement cover decollement and resulted in thrust bound pop up structures in the cover

the central australian seismic experiment 1985 preliminary results summary the major objective of the central australian seismic experiment is to investigate the structural evolution of the arunta block and the ngalia and amadeus basins a regional north south reflection line of 420 km length from the northern arunta province to the southern part of the amadeus basin was recorded in 1985 the most significant basement features are prominent bands of reflectors from beneath the northern arunta province and the ngalia basin at times of between 4 and 10 s that dip towards the north deep crustal features south of the ngalia basin are less clear except in the redbank zone bands of deep reflectors similar to those observed in the north occur at times of between 5 and 10 s beneath the southern part of the amadeus basin additional seismic profiling included a reflection line of 40 km length recorded across the northern margin of the redbank zone three expanding spread reflection profiles and a tomographic experiment an east west seismic refraction profile of 400 km length was recorded within the arunta block and suggests an average crustal thickness of 55 km

time interval between volcanism and burial metamorphism and rate of basin subsidence in a cretaceous andean extensional setting 40ar 39ar ages were obtained from basaltic flows belonging to a 9 km thick sequence generated in an extensional ensialic setting of an arc back arc basin type during the early cretaceous and presently exposed along the coastal range of central chile the basalts have been affected by very low to low grade burial metamorphism mostly under prehnite pumpellyite facies age values obtained from primary volcanic and secondary metamorphic minerals permit to quantify the time interval between volcanism and burial metamorphism a plateau age of 119 1 2 ma from primary plagioclase represents the best estimation of the age of the volcanism whereas adularia in low variance assemblages contained in amygdules gave a plateau age of 93 1 0 3 ma which is interpreted as the age of the metamorphism considering the p t conditions estimated for this metamorphic event the c 25 ma time interval between volcanic emplacement and prehnite pumpellyite facies metamorphism the rate of basin subsidence in this extensional geodynamic setting would be comprised in the interval 150 180 m ma

fluids migration and dynamics of a blocks and faults system abstract we explore the impact of fluids migrating through a fault network on the dynamics of lithosphere both on slow movements and seismicity for that purpose fluids in the fault zones are incorporated into modelling of blocks and faults systems which takes into account driving forces and the system s geometry simulations have been performed for two dimensional models an idealised brick wall structure and a coarse image of sinai subplate migrating fluids originating in different locations are considered as well as fluids trapped in closed pockets basic features of the modelled and observed seismicity are in good accord as shown by comparison with the earthquake catalog compiled by geophysical institute of israel

rb sr dating of differentiated cleavage from the upper adelaidean metasediments at hallett cove southern adelaide fold belt abstract upper adelaidean psammites from the foreland part of the late proterozoic adelaide fold belt at hallett cove have suffered one major deformation which produced upright folds that plunge gently south axial to these folds is a well developed differentiation cleavage inferred to have resulted from constant volume plane strain deformation by solution transfer and now defined by alternating p phyllosilicate rich and q quartz rich domains geochemical analyses show that cleavage development resulted in a 2 4 fold fractionation of rb sr between the p and q domains thus the age of deformation is datable and isotopic analyses show the p and q domains form a 531 32 ma isochron this age is statistically older than the pervasive deformation and metamorphism in the more interior parts of the fold belt inferred from intrusive ages to have occurred at 485 6 ma and thus suggests that deformation either propagated across the fold belt from external to interior zones or that it occurred episodically the age data allow but do not demand that cleavage formation predated the deposition of the overlying cambrian kanmantoo group sediments

crustal heterogeneity in south australia earthquake evidence summary lateral velocity variations in the south australian crust have been mapped using the relatively new technique of seismic tomography the input data comprised p1 travel times from 1340 local earthquakes recorded on the south australian seismic network during the period 1978 1983 to reduce errors in the earthquake locations focal depths and origin times we used an averaging technique of dividing the area under consideration into a series of overlapping cylindrical cells all earthquakes and seismic stations were assigned to the centre of the nearest cell travel times between all centres were computed by averaging over all relevant cell pairs after first scaling all travel times by a factor equal to the distance between cell centres divided by the actual ray path length only ray paths appropriate to the p1 phase were considered starting with a first guess model based on the number of ray intersections with each cell the travel times were computed along all ray paths and compared with the observed averaged values the errors and residuals were back projected for each ray over all the chords along the ray and the average cell slowness adjusted in an iterative sense until the computed and observed travel times agreed within a specified error tolerance the areas with poor raypath coverage lack of earthquakes and or stations could not be imaged the area covered lies mostly within the adelaide geosyncline where there is believed to be a thick cover of proterozoic sediments the velocity image is of the upper 20 km of the crust so no direct correlation between the surface geology and the crustal velocity is to be expected results indicate a significant variation in crustal velocity from 5 9 to 6 5 km s 1 the velocities correlate remarkably with time terms computed in a separate study of moho refracted arrivals pn low time terms correspond to high crustal velocity and high time terms reflect anomalously low crustal velocity the residual component of the time terms can be explained in terms of variations in depth to the moho throughout the geosyncline of the order of 10 km the prominent features of the tomographic velocity map are a n s trending velocity low over the spencer gulf graben and a ridge of high velocity beneath the flinders ranges the velocity low can be associated with a region of low gravity aligned parallel to the torrens hinge zone which marks the western boundary of the geosyncline the northern end of the high velocity ridge coincides with a region of positive bouguer gravity teleseismic residuals from both earthquakes and nuclear explosions show a similar trend to the pn time terms and indicate lateral variability in crustal thickness and velocity a component of the residual must be attributed to mantle velocity inhomogeneity which extends to depths of 300 km

observations on the tectonic evolution of the southern adelaide fold belt jenkins r j f and sandiford m 1992 observations on the tectonic evolution of the southern adelaide fold belt in cl fergusson and r a glen editors the palaeozoic eastern margin of gondwanaland tectonics of the lachlan fold belt southeastern australia and related orogens tectonophysics 214 27 36 the mount lofty ranges fieurieu peninsula and kangaroo island regions of south australia expose a section across the west and no hwestern margin of the southern adelaide fold belt that was deformed and metamo h ed during the cambro ordovician delamerian orogeny in the external parts of the belt thrust complexes involving basement and a late proterozoic platformal succession show a characteristic asymmetry reflecting west to northwest vergence towards the australian craton in the more internal zones convergent deformation involving large scale upright folding and high t low p metamorphism of mainly cambrian sediments which were in part turbiditic occurred between c 516 and c 490 ma in this part of the belt the distribution of heat and possibly strain reflects intimately the advective movement of magmas within the erogenic belt convergent deformation in the southern adelaide fold belt was immediately preceded by rapid subsidence initiated during the de sition of the upper parts of the no anvilie group and which continued during deposition of the kanmantoo group mafic alkaline volcanism attendant with normanville group deposition is indicative of lithospheric thinning at c 526 ma the adelaide and lachlan fold belts record the spatial and temporal evolution of the southeastern margin of the australian continent through the late proterozoic and the palaeozoic in the southern adelaide fold belt comprising the motet lofty ranges fleurieu peninsulakangaroo island regions of south australia the terminal stages of sedimentation as well as the deformation metamorphism and felsic magmatism during the delamerian orogeny are restricted to the cambrian and earliest ordovician that is at the same time as the formation of the cambrian magmatic arcs and the initial accu

explosion seismic determination of crustal structure beneath the adelaide geosyncline south australia abstract the structure of the earth s crust beneath the adelaide geosyncline south australia has been investigated over the last few decades using both earthquake and explosion seismic techniques these studies have led to the simple average model of a single homogeneous layer having a p wave velocity of 6 32 km s 1 and thickness of 38 km overlying a homogeneous mantle with a velocity of 8 05 km s 1 the only compelling evidence for crustal layering comes from two recent long range refraction profiles recorded along n s and perpendicular e w to the axis of the geosyncline using quarry blast sources head waves and wide angle reflections have been identified as originating from a mid crustal boundary separating an upper crust of velocity 5 94 km s 1 from a lower crust of velocity 6 42 km s 1 at a depth of 10 to 18 km the depth variation to this boundary and the corresponding moho topography were interpreted using an iterative ray tracing technique both discontinuities shallow towards the murray basin which flanks the geosyncline to the south east the crustal model obtained was further investigated by means of a synthetic aperture seismic array experiment carried out on the n s refraction profile at a source offset of 120 km recordings were made on a 21 element unreversed linear profile using digital seismographs spaced at 0 5 km intervals an accurate p 1 velocity of 5 88 km s 1 was obtained secondary p arrivals were studied with the aid of beam formers and tau p processors tentative reflections from within the crust and upper mantle were obtained however the variation in seismic waveform generated by different blasts was a definite drawback of the experiment prominent dipping reflectors and faults at depths of several kilometres have been observed in near normal incidence reflection surveys shot in porterozoic rocks just west of the adelaide geosyncline the reflectors may represent layering within the crystalline basement or they may relate to the rifting stage of basin development

enhanced intraplate seismicity along continental margins some causes and consequences abstract in australia much of the seismic activity is restricted to zones up to several hundred kilometres wide inboard of continental margins at high angle to the trend in maximum horizontal compression shmax intriguingly along the margin side of one such zone a near optimally oriented crustal scale structure the darling fault has no recorded historical seismicity to explain these enigmatic features we explore how the compositional and thermal structuring of continental passive margins may lead to variations in mechanical response to imposed forcing using forward modelling we show how lateral heat flow across such margins can produce increases in moho temperatures by several 10s of degrees relative to the continental interior extending up to 200 km inboard of the ocean continent transition provided the step in thermal lithosphere thickness into the continent is greater than 30 km for smaller lithospheric steps moho temperatures beneath continental margins will be cooler than beneath interiors our mechanical models show that such thermal effects can elevate strain rates along the margin by several factors relative to continental interiors when subject to far field forcing in the shallow seismogenic realm the strain rate maximum is concentrated at around 100 150 km inboard of the ocean to continent lithospheric step providing a plausible explanation for the pattern of seismicity observed in some of australia s marginal seismic zones this interpretation requires that strength changes associated with moho temperature variations of several 10s of degrees are sufficient to override the reactivation potential of near optimally oriented crustal penetrative structures with thermal processes providing an important switch controlling reactivation potential of existing structures it also provides an insight into the mechanism of formation of continental ribbons during rifting of thermally mature margins