a new approach to the relief of great britain ii a classification of rocks based on relative resistance to denudation abstract we have assembled a digital database for great britain containing square by square data for the 1 km squares of the national grid on altitude and geology age and lithology as well as other variables it is used here to classify the 71 most common rocks of great britain i e all outcrops over 500 km2 in terms of the degree to which they form high ground the empirical classification utilised a large number of measures eventually 19 to allow for the varying location and spatial pattern coastal vs inland large continuous outcrops vs scattered patches of these outcrops and several different combinations of variables produced very similar sequences when the rocks were arranged in rank order a six fold classification provides a basis for further work which will seek to remove the influence of varying resistance to denudation from the pattern of relief for rocks of similar age rank order is determined by lithology but for all rocks age is a stronger influence than lithology it is shown that variations in the resistance of rocks to erosion account for a large part of the local relative relief of britain the pattern of mean altitude predicted from river distance values using regressions for each rock resistance class simulates remarkably closely the overall pattern of british relief

serial ruptures of the san andreas fault carrizo plain california revealed by three dimensional excavations 1 it is poorly known if fault slip repeats regularly through many earthquake cycles well documented measurements of successive slips rarely span more than three earthquake cycles in this paper we present evidence of six sequential offsets across the san andreas fault at a site in the carrizo plain using stream channels as piercing lines we opened a latticework of trenches across the offset channels on both sides of the fault to expose their subsurface stratigraphy we can correlate the channels across the fault on the basis of their elevations shapes stratigraphy and ages the three dimensional excavations allow us to locate accurately the offset channel pairs and to determine the amounts of motion for each pair we find that the dextral slips associated with the six events in the last millennium are from oldest to youngest 5 4 0 6 8 0 0 5 1 4 0 5 5 2 0 6 7 6 0 4 and 7 9 0 1 m in this series three and possibly four of the six offset values are between 7 and 8 m the common occurrence of 7 8 m offsets suggests remarkably regular but not strictly uniform slip behavior age constraints for these events at our site combined with previous paleoseismic investigations within a few kilometers allow a construction of offset history and a preliminary evaluation of slip and time predictable models the average slip rate over the span of the past five events between a d 1210 and a d 1857 has been 34 mm yr not resolvably different from the previously determined late holocene slip rate and the modern geodetic strain accumulation rate we find that the slip predictable model is a better fit than the time predictable model in general earthquake slip is positively correlated with the time interval preceding the event smaller offsets coincide with shorter prior intervals and larger offset with longer prior intervals

minor and trace element partitioning between immiscible ocelli matrix pairs from lamprophyre dikes and sills monteregian hills petrographic province quebec many lamprophyre dike and sill rocks in the monteregian hills petrographic province of southwestern quebec contain felsic segregations ocelli which have been interpreted as globules of immiscible liquid philpotts 1976 ocelli and matrix material were separated from a number of these rocks and analyzed for major and trace elements the major element data when plotted on a greig diagram outline a field of possible silicate liquid immiscibility at higher alumina alkali content than that previously mapped in iron rich experimental systems the trace element data support a liquid immiscibility hypothesis for the formation of these ocelli since high charge density cations are preferentially concentrated in the matrix mafic material a result which is consistent with theoretical and experimental studies

geochronological constraints on paleoarchean thrust nappe and neoarchean accretionary tectonics in southern west greenland abstract major regional deformation and metamorphic events in the godthabsfjord region southern west greenland occurred at 3650 and 2820 2720 ma e g precambrian res 78 1996 1 new geochronological constraints u pb zircon sensitive high resolution ion microprobe shrimp and thermal ionisation mass spectrometry tims have been obtained from a stack of mylonitic crystalline thrust nappes in the footwall of the western part of the paleoarchean 3 8 3 7 ga isua greenstone belt isukasia a mylonitic tonalite sheet interpreted to have intruded synkinematically with respect to mylonitisation yields a magmatic crystallisation age of 3640 3 ma a cross cutting pegmatite and a post kinematic tonalite pluton yield magmatic crystallisation ages of 2948 8 and 2991 2 ma respectively accordingly we interpret the thrust nappe stack to have formed during the paleoarchean 3640 ma making it the oldest example known on earth the similarity of this structural regime to that of modern mountain belts suggests that paleoarchean and modern continental crust were comparable in terms of mechanical strength and constitution southern west greenland has been interpreted in terms of neoarchean accretion comparable with modern plate tectonics e g earth planet sci lett 142 1996 353 isukasia lies just east of a purported neoarchean accretionary boundary between the akia terrane to the northwest and the akulleq terrane to the southeast the akia terrane was previously considered to overthrust the akulleq terrane at 2820 2720 ma our geochronological and geological data indicate i that the two terranes as presently defined were stitched at 2991 2 ma and ii that thrusting across the boundary was directed toward the akia terrane therefore we suggest that the akia akulleq interface was not a fundamental tectonic structure during the neoarchean and we question its identification as an accretionary boundary

plagiogranite magmatism in the mesozoic island arc structure of the pekulney ridge chukotka peninsula ne russia tectonic setting geochemistry and petrogenesis of low k granitoid magmatism in the fore arc of late jurassic early cretaceous pekulney zolotogorskaya fossil arc are addressed using a case study consisting in two types of neocomian plagiogranite complexes from the eastern pekulney ridge central chukotka these plagiogranites outcrop as small dyke like bodies and are confined to a zone of strong deformation related to an olistostrome development at the boundary between allochthonous middle jurassic early cretaceous oceanic volcanics and cherts and parautochthonous late jurassic early cretaceous island arc complexes the geochemical features of plagiogranites hfse depletion and lile enrichment nb and ta negative anomalies in ocean ridge granite org normalized plots and rb vs nb y and zr vs nb zr n correlations indicate that these rocks are likely to have originated in suprasubduction zone setting the parameters of tectonic structure and the morphology of the pekulney fossil arc segment in neocomian time structural position of plagiogranites and their ree patterns militate in favour of the following models for plagiogranite origin 1 migration of tholeiite basalt melts from mantle wedge into the fore arc of the island arc and their subsequent fractionation 2 partial melting of metabasite crustal source in the fore arc and subsequent migration of felsic melts geochemical modelling shows that one type of plagiogranites could have originated by 10 15 partial melting of metabasite source in a metabasite gabbroic complex of pre late mesozoic basement of the island arc pile and that the other type could be generated by progressive fractional crystallization of low k tholeiite from the late jurassic early cretaceous island arc complex

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

an application of synthetic seismicity in earthquake statistics the middle america trench this paper demonstrates how synthetic seismicity calculations which are based on the concept of fault segmentation and incorporate the physics of faulting through static dislocation theory can improve earthquake recurrence statistics and hone the probabilities of hazard compared to forecasts constructed from a handful of earthquake recurrence intervals forecasts constructed from synthetic seismicity are more robust in that they embody regional seismicity information over several units of magnitude they can extrapolate seismicity to higher magnitudes than have actually been observed and they are formulated from a catalog which can be extended as long as needed to be statistically significant synthetic seismicity models can also be used to judge the stability of common rate estimates and the appropriateness of idealizations to the earthquake cycle i find that estimates of fault slip rate are unbiased regardless of sampling duration while estimates of earthquake recurrence time are strongly biased recurrence intervals estimated from seismicity samples less than about 10 times the actual recurrence interval will almost certainly be too short for the middle america trench mat it would take 200 and 400 years of monitoring to constrain slip and recurrence rates to 10 events m 6 have as much as a 60 probability of recurrence within 5 years due to the clustering of small earthquakes in foreshocks and aftershocks this probability drops to less than 15 for m 7 events increasing gap time generally increases conditional probability of earthquake occurrence but the effect is weak for the mat the spread parameters of the best fitting lognormal or weibull distributions 0 75 are much larger than the 0 21 intrinsic spread proposed in the nishenko buland hypothesis stress interaction between fault segments disrupts time or slip predictability and causes earthquake recurrence to be far more aperiodic than has been suggested

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

upgrading a synthetic seismicity model for more realistic fault ruptures a quasi static synthetic seismicity model for multiple interacting faults of any orientation is upgraded to include a more realistic rupture model in the original model entire ruptures covering many fault patches occur instantaneously and rupture propagation is ignored now time is introduced into the rupture process via a patch interaction delay in proportion to distance shear wave velocity also to try to mimic observed rise times for real earthquakes a simple time dependent friction law is used comparison of results for the old and new models for a simple case of two strike slip faults show that clustering of large events is somewhat reduced in the new model it remains to be seen if this conclusion will be verified in more complex fault networks the new model takes about twice as much computation time but the results may now be used to calculate synthetic seismograms

upper mantle processes beneath the 2 7 ga abitibi belt canada a trace element perspective abstract mantle processes associated with volcanism in the abitibi belt remain poorly understood despite numerous detailed studies of its volcano sedimentary successions this paper employs log plots of nb y and zr y to distinguish the sources of well defined abitibi volcanic suites and to clarify processes in the upper mantle during greenstone belt development mafic rocks of the southern volcanic zone in the abitibi belt define two main trends on log plots of nb y versus zr y trends for the kidd munro assemblage and malartic block correspond to mixtures of strongly deleted plume asthenosphere and adakitic melts the shared trace element systematics of these volcanic successions confirm they are parts of a single authochthonous supracrustal assemblage that extended across the southern abitibi belt the younger kamiskotia assemblage and blake river group correspond to crustal extension episodes within an abitibi island arc trace element plots establish that they were derived from mantle plume related asthenosphere and not the late archean morb source upper mantle associated with island arc assemblages elsewhere in the southern superior province the data suggests widespread lateral flow of mantle plume asthenosphere occurred in the vicinity of the abitibi arc mantle mixing with adakitic melts continued between 2716 and 2698 ma the data are not consistent with wet komatiite models that generate ultramafic volcanic rocks from typical upper mantle following hydration of a mantle wedge chemical mixing trends for mafic magma sources indicate melt hybridization of the abitibi mantle wedge the trends require a slab melt origin for adakitic magmas rather than internal differentiation of underplated mafic crust comparisons with other 2 7 ga greenstone belts suggest that mantle plume interaction with archean subduction zones as proposed for the abitibi belt can be distinguished in the nb zr y systematics of associated mafic volcanic rocks

geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites central anatolian crystalline complex turkey plagiogranites associated with the sarikaraman ophiolite of the central anatolian crystalline complex turkey closely resemble other plagiogranites from supra subduction zone type ophiolites of neotethys the ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite ca 10 by volume than is usually associated with such bodies the sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex the plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber both the deep seated trondhjemites and the volcanic rhyolites constitute the sarikaraman plagiogranite suite geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites which are characterised by morb normalized low hfs element contents with small negative nb ta anomalies and variably enhanced lil element abundances unlike other plagiogranites however the sarikaraman suite is not characterized by consistently low k2o contents a feature that reflects the variable mobilization of the lil elements under lower greenschist facies conditions the ree are uniformly enriched relative to the basic components of the complex but have similar normalized patterns exhibiting mild light ree depletion in terms of their origin the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization 70 85 of clinopyroxene feldspar amphibole or partial melting 5 15 batch melting of a gabbroic source material although only the first process can produce most of the range of the plagiogranite compositions as a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole

morphological and morphometric approach to the study of the structural arrangement of northeastern abruzzo central italy abstract the morphotectonic research at the dipartimento di scienze della terra of the universita di roma la sapienza aims to define the role of tectonics and neotectonics in the morphological evolution of central italy in this context the relationships between morphology and tectonics have been investigated in an area of northern abruzzo extending from the apennine chain to the adriatic sea morphological and morphometric analyses carried out show that the geomorphological evolution of the study area was strongly affected by tectonics especially recently the most recent extensional tectonics intersect the previous compressive structures overthrust reverse faults folds strike slip faults determining the structural arrangement characterized by uplifted and downthrown blocks morphological analysis shows that extentional tectonics occurred with the following alignments nw se nnw sse ne sw ene wsw n s and e w the spatial variability of amplitude of relief values corroborated the observations obtained from previous analyses in particular the existence of different blocks differentially uplifted along the alignments mentioned above finally data obtained through the statistical analysis of preferred stream orientations confirm the previous considerations showing that the same tectonic directions markedly influenced the drainage network development streams of lower order preferentially follow the n s and e w directions which suggests that these tectonic trends were active until very recent times at least up to the late pleistocene the results obtained testify both to the effectiveness of the methodological approach and the likelihood of the proposed morphotectonic arrangement of the study area

tectonic predesign of mass movements with examples from the chinese himalaya abstract the genesis of landslides has traditionally been ascribed essentially to exogenic nontectonic causes thus it has generally been assumed that progressive erosion oversteepens slopes until they become unstable and collapse by forming slides the specific triggers in each case would be exogenic influences such as for instance an extreme weather period this paper shows that in practically all cases of mass movements endogenic processes are also of fundamental importance in the first place a landscape responds as a complex open system existing in a quasi stationary state at the edge of chaos in which an endogenic input and exogenic outflow of mass and energy occur this quasi stationary state can become unstable resulting in landslides in the second place a direct predesign is present because of endogenic causes viz by a former or the present geotectonic stress field thus some common notable contentions regarding the origin of landslides are shown to be false a better understanding of the mechanisms for landslide genesis is illuminated using examples from the chinese mountain ranges at the east end of the himalayas in these particular cases the orientations of mass movements are directly controlled predesigned by the present day local tectonic stress

a simulation based approach to forecasting the next great san francisco earthquake in 1906 the great san francisco earthquake and fire destroyed much of the city as we approach the 100 year anniversary of that event a critical concern is the hazard posed by another such earthquake in this article we examine the assumptions presently used to compute the probability of occurrence of these earthquakes we also present the results of a numerical simulation of interacting faults on the san andreas system called virtual california this simulation can be used to compute the times locations and magnitudes of simulated earthquakes on the san andreas fault in the vicinity of san francisco of particular importance are results for the statistical distribution of recurrence times between great earthquakes results that are difficult or impossible to obtain from a purely field based approach

holocene paleoseismicity temporal clustering and probabilities of future large m 7 earthquakes on the wasatch fault zone utah the chronology of m 7 paleoearthquakes on the central five segments of the wasatch fault zone wfz is one of the best dated in the world and contains 16 earthquakes in the past 5600 years with an average repeat time of 350 years repeat times for individual segments vary by a factor of 2 and range from about 1200 to 2600 years four of the central five segments ruptured between 620 30 and 1230 60 calendar years b p the remaining segment brigham city segment has not ruptured in the past 2120 100 years comparison of the wfz space time diagram of paleoearthquakes with synthetic paleoseismic histories indicates that the observed temporal clusters and gaps have about an equal probability depending on model assumptions of reflecting random coincidence as opposed to intersegment contagion regional seismicity suggests that for exposure times of 50 and 100 years the probability for an earthquake of m 7 anywhere within the wasatch front region based on a poisson model is 0 16 and 0 30 respectively a fault specific wfz model predicts 50 and 100 year probabilities for an m 7 earthquake on the wfz itself based on a poisson model as 0 13 and 0 25 respectively in contrast segment specific earthquake probabilities that assume quasi periodic recurrence behavior on the weber provo and nephi segments are less 0 01 0 07 in 100 years than the regional or fault specific estimates 0 25 0 30 in 100 years due to the short elapsed times compared to average recurrence intervals on those segments the brigham city and salt lake city segments however have time dependent probabilities that approach or exceed the regional and fault specific probabilities for the salt lake city segment these elevated probabilities are due to the elapsed time being approximately equal to the average late holocene recurrence time for the brigham city segment the elapsed time is significantly longer than the segment specific late holocene recurrence time

comment on a vestige of earth s oldest ophiolite the claim by furnes et al reports 23 march 2007 p 1704 that greenland metavolcanic rocks require paleoarchean sea floor spreading is incompatible with their own data the purported sheeted dikes have the composition of pyroxenitic komatiite and could not have fed the adjacent ferroandesitic pillow lavas neither type has ophiolitic analogs and both are likely ensialic

a viscoelastic earthquake simulator with application to the san francisco bay region earthquake simulation on synthetic fault networks carries great potential for characterizing the statistical patterns of earthquake occurrence i present an earthquake simulator based on elastic dislocation theory it accounts for the effects of interseismic tectonic loading static stress steps at the time of earthquakes and postearthquake stress readjustment through viscoelastic relaxation of the lower crust and mantle earthquake rupture initiation and termination are determined with a coulomb failure stress criterion and the static cascade model the simulator is applied to interacting multifault systems one a synthetic two fault network and the other a fault network representative of the san francisco bay region the faults are discretized both along strike and along dip and can accommodate both strike slip and dip slip stress and seismicity functions are evaluated over 30 000 yr trial time periods resulting in a detailed statistical characterization of the fault systems seismicity functions such as the coefficient of variation and a and b values exhibit systematic patterns with respect to simple model parameters this suggests that reliable estimation of the controlling parameters of an earthquake simulator is a prerequisite to the interpretation of its output in terms of seismic hazard

evolution of the stress field in the northern aegean sea greece summary the evolution of the stress field in the area of the northern aegean sea during the 20th century has been studied the area is dominated by dextral strike slip faulting and is characterized by frequent strong earthquakes coulomb stress changes dcff were calculated assuming that earthquakes can be modelled as static dislocations in an elastic half space and taking into account both the coseismic slip in large mi7 0 earthquakes and the slow tectonic stress build up along the major fault segments the stress change calculations were performed for strike slip faults of strike dip and rake appropriate to the large events we evaluate whether these stress changes brought a given large earthquake closer to or farther from failure it was found that each of the large events occurred in regions of increased calculated coulomb stress moreover the majority of smaller events for which reliable fault plane solutions are available were also located in areas of positive dcff by extending the calculations to 2020 and assuming that no additional large mi7 0 earthquake occurs between 1999 and 2020 possible sites of future large earthquakes are identified

stratigraphic and geochemical evidence for the depositional environment of the early archaean isua supracrustal belt southern west greenland the highly deformed c 3800 ma isua supracrustal belt is a fragment of a more extensive early archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic amftsoq gneisses in the period 3800 3600 ma the supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 ma in the late archaean and locally at c 1800 ma layered sequences of rock of sedimentary and probable volcanic origin form over 50 of the belt bodies of high mg al basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of amitsoq gneisses the layered rocks which are 1 km thick are divided into two sequences that are in faulted contact with each other the way up of these sequences has been determined from facing directions of locally preserved graded layering in felsic metasediments at several localities the overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments clastic sedimentation took place against a background of chemical sedimentation shown by interlayers of banded iron formation metachert and calc silicate rocks throughout the sequences the felsic rocks locally preserve graded bedding and possible conglomerate structures indicating deposition from turbidite flows and possibly as debris flows nodules in the felsic rocks contain structures interpreted as fiamme there is an irregular enrichment in k2o na2o in many of the felsic rocks at constant sio2 and al2o3 content interpreted as owing to alteration of original andesitic to dacitic volcanic rocks banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking possibly under shallow water conditions lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source

structural controls on the orientation of erosion gullies in mid western new south wales australia abstract the role of geology in determining the drainage network of gullying has been evaluated by quantitative methods of analysis at gully erosion sites in new south wales field evidence suggests that soil cracks are sites of preferential flow and erosion at both the surface and subsurface soil fabric and rock structure and weathering studies were undertaken to establish the inheritance of soil cracks from the underlying parent material gully orientation was correlated with the strike of joints and bedding planes in a range of rock materials including sedimentary intrusive and extrusive igneous and thermal and regional metamorphic rocks these units vary not only in terms of mineralogy but also in degree of weathering however all are steeply dipping structures usually exceeding 60 correlations exist between gully orientation and joints in the underlying material but the strength of the relationship varies according to the parent material in the ordovician hornfels where no correlation between joints and gully orientation occurred the evidence showed that gullying was controlled by faulting along the hornfels granite boundary these relationships support the view that throughflow processes are dominant in the development of gullies since convergence of runoff would not be expected to align with subsurface soil discontinuities

holocene earthquake records from the cascadia subduction zone and northern san andreas fault based on precise dating of offshore turbidites abstract we present preliminary evidence for a 10 000 year earthquake record from two major fault systems based on sediment cores collected along the continental margins of western north america new stratigraphic evidence from cascadia demonstrates that 13 earthquakes ruptured the entire margin from vancouver island to at least the california border since the eruption of the mazama ash 7700 years ago the 13 events above this prominent stratigraphic marker have an average repeat time of 600 years and the youngest event 300 years ago coincides with the coastal record we also extend the record of past earthquakes to the base of the holocene at least 9800 years ago during which 18 events correlate along the same region the sequence of holocene events in cascadia appears to contain a repeating pattern of events a tantalizing first look at what may be the long term behavior of a major fault system the northern california margin cores show a cyclic record of turbidite beds that may represent holoce

a synthetic seismicity model for southern california cycles probabilities and hazard the absence of a long historical catalog of observed seismicity with which to constrain earthquake recurrence behaviors is a fundamental stumbling block to earthquake prediction in california conceding that this limitation is not likely to relax in the foreseeable future alternative approaches must be sought to extend the catalog artificially in this article i evaluate the long term behaviors of earthquakes on a map like set of southern california faults through computer simulations that incorporate the physics of earthquake stress transfer and are constrained by excellent but restricted bodies of geological and seismological data i find that model seismicity fluctuates on both short decades and long centuries timescales but that it possesses a well defined mean and standard deviation seismicity fluctuations correlate across different magnitudes and the long term cycles of smaller events seem to lead cycles of larger events short period seismicity fluctuations do not exhibit this tendency and short term changes in low magnitude m5 seismicity are not likely to be an effective predictor of future large events at least for the region as a whole as do real faults the model faults produce characteristic and power law quakes in variable ratios with diverse periodic and nonperiodic behaviors generally larger events tend to occur quasi periodically and smaller ones tend to cluster however only for a few earthquake classes and certain locations is recurrence notably non poissonian an important use of synthetic seismicity is in the construction of earthquake hazard maps because it firmly grounds previously ad hoc assumptions regarding frequency magnitude distributions multiple segment failure statistics and rupture extents while satisfying a spectrum of geological constraints such as fault slip rate segment recurrence interval and slip per event with its depth of temporal and spatial coverage synthetic seismicity also provides a means to investigate the time dependence of seismic hazard because hazard likelihood is a concatenation of the recurrence statistics from many seismic sources in only about 40 50 of the regions near the major faults do sequences of 0 1 g or 0 2 g exceedances differ from poissonian

tectonic setting for the genesis of oceanic plagiogranites evidence from a paleo spreading structure in the oman ophiolite abstract in the oman ophiolite detailed structural and lithological mapping of the 60 45 km maqsad area has revealed several plagiogranite intrusions ranging from metre scale dykes to hectometre scale bodies these plagiogranites are spatially related to kilometre scale mafic plutons displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick isotope data demonstrate the mantle origin of these pegmatites and plagiogranites nd ranges from 6 3 to 9 9 and has an average value 26 samples of 8 1 within the field of present day morbs and similar to the average nd 7 8 of the oman ophiolite primary igneous sequence these intrusions are not restricted to a given structural level their paleovertical extent exceeds 5 6 km from 2 km below the moho up to the base of the sheeted dyke complex their modal composition evolves up section from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites diorites and quartz diorites at crustal level petrography whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt plagiogranites although more abundant at upper crustal level are not restricted to this horizon plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth from highly calcic 70 95 an in the mantle section intermediate and highly scattered in the lower crust 10 85 an to quite sodic at upper crustal level 5 20 an highly calcic plagioclases in quartz bearing rocks is a puzzling character inconsistent with fractional crystallization even if high degrees of fractional crystallization of a hydrated basaltic melt remain the simplest way to account for the chemistry of the upper crustal plagiogranites processes such as fluid induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites mantle and crustal structures in the maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting when a mantle diapir soaked with basaltic melt intruded a cool hydrothermally altered lithosphere they were eventually transposed in an off axis setting as the ascension of the diapir progressed up to moho level it is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid this is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time

comment on a vestige of earth s oldest ophiolite furnes et al reports 23 march 2007 p 1704 reported the identification of an ophiolite sequence within the approximately 3 8 billion year old isua supracrustal belt however they did not acknowledge that the belt contains supracrustal rocks and mafic dikes of different ages nor did they demonstrate that the proposed components of the ophiolite are coeval

a multidisciplinary approach to seismic hazard in southern california abstract a serious obstacle facing seismic hazard assessment in southern california has been the characterization of earthquake potential in areas far from known major faults where historical seismicity and paleoseismic data are sparse this article attempts to fill the voids in earthquake statistics by generating master model maps of seismic hazard that blend information from geology paleoseismology space geodesy observational seismology and synthetic seismicity the current model suggests that about 40 of the seismic moment release in southern california could occur in widely scattered areas away from the principal faults as a result over a 30 yr period nearly all of the region from the pacific ocean to 50 km east of the san andreas fault has a greater than 50 50 chance of experiencing moderate shaking of 0 1 g or greater and about a 1 in 20 chance of suffering levels exceeding 0 3 g for most of the residents of southern california thelion9s share of hazard from moderate earthquake shaking over a 30 yr period derives from smaller closer more frequent earthquakes in the magnitude range 5 m 7 rather than from large san andreas ruptures whatever their likelihood

a synthetic seismicity model for the middle america trench this paper develops a new iterative technique based on the concept of fault segmentation and computed using two dimensional static dislocation theory to build models of seismicity and fault interaction which are physically acceptable and geometrically and kinematically correct the technique is applied in two steps to seismicity observed at the middle america trench the first step constructs generic models which randomly draw segment strengths and lengths from a two dimensional probability distribution the second step constructs predictive models in which segment lengths and strengths are adjusted to mimic the actual geography and timing of large historical earthquakes both types of models not only reproduce the statistics of seismicity over five units of magnitude but also duplicate other aspects including foreshock and aftershock sequences migration of foci and the capacity to produce both characteristic when an isolated segment fails and noncharacteristic earthquakes when several adjacent or nearby segments fail in concert over a period of about 150 years the complex interaction of fault segments and the nonlinear failure conditions conspire to transform an apparently deterministic model into a chaotic one in that small changes in initial conditions ultimately lead to very different patterns of events at least for the middle america trench it seems doubtful that successful predictive models of earthquakes could span a much longer duration

slip accumulation and lateral propagation of active normal faults in afar we investigate fault growth in afar where normal fault systems are known to be currently growing fast and most are propagating to the northwest using digital elevation models we have examined the cumulative slip distribution along 255 faults with lengths ranging from 0 3 to 60 km faults exhibiting the elliptical or bell shaped slip profiles predicted by simple linear elastic fracture mechanics or elastic plastic theories are rare most slip profiles are roughly linear for more than half of their length with overall slopes always 0 035 for the dominant population of nw striking faults and fault systems longer than 2 km the slip profiles are asymmetric with slip being maximum near the eastern ends of the profiles where it drops abruptly to zero whereas slip decreases roughly linearly and tapers in the direction of overall aden rift propagation at a more detailed level most faults appear to be composed of distinct shorter subfaults or segments whose slip profiles while different from one to the next combine to produce the roughly linear overall slip decrease along the entire fault on a larger scale faults cluster into kinematically coupled systems along which the slip on any scale individual fault or fault system complements that of its neighbors so that the total slip of the whole system is roughly linearly related to its length with an average slope again 0 035 we discuss the origin of these quasilinear asymmetric profiles in terms of initiation points where slip starts and barriers where fault propagation is arrested in the absence of a barrier slip apparently extends with a roughly linear profile tapered in the direction of fault propagation

synthetic seismicity models for the wellington region new zealand implications for the temporal distribution of large events our previous synthetic seismicity model for multiple interacting faults in a three dimensional half space has been extended and applied to the wellington region new zealand generating long catalogs of earthquakes for studying the effects of elastic interactions on the temporal distribution of large events the region is one of oblique plate convergence with the interface between the subducting and overlying plates at an average depth of 22 km faults included besides the subduction thrust are segments of the four major arc parallel strike slip faults overlying the plate interface we have used 40 different models with geometric and mechanical parameters chosen at random within reasonable ranges to generate catalogs of 200 000 years duration each for comparison each model was rerun with the elastic interactions suppressed considering events of magnitude 7 2 or more characteristic events in the sense that they rupture most of a fault plane the number of short 250 years the relative numbers are again mostly higher experiments with simple models indicate that this pattern requires both interfault enhancement and inhibition of large events mutual enhancement occurs most often between the subduction thrust and the overlying strike slip faults and between the two segments of the wellington fault that almost join end to end mutual inhibition mostly occurs between the subparallel strike slip faults

earliest part of earth s stratigraphic record a reappraisal of the 3 7 ga isua greenland supracrustal sequence investigations of the isua supracrustal rocks in west greenland allow us to identify the protoliths and alteration history for most isua rocks the protoliths consisted of alternating basalt and banded iron formation this sequence was invaded by dunitic sills metamorphosed and later intruded by felsic gneisses pervasive carbonation and k metasomatism produced a sequence of lithologies mimicking those found in modern platform deposits however the protoliths could have originated in a purely oceanic environment with no sialic detrital components the isua sequence probably consists of several tectonic panels some of which are cut by 3810 ma felsic intrusive rocks

the statistical relationship between unconfined compressive strengths and the frequency distributions of slope gradients a case study in northern hungary this paper focuses on the question of whether there is a deterministic connection between the slope gradient and unconfined compression strength ucs as a lithological factor and on describing the nature of this connection moreover the authors determined the sensitive statistical parameters in the statistical surface analyses the surface analyses were carried out in an area extending over 1500 km 2 in northern hungary and containing parts of the uplifted palaeozoic and mesozoic basement and the semi consolidated material of the palaeogene and neogene molasse sediments the 67 geological formations of the area were grouped into 10 petrophysical categories characterised by unconfined compressive strength as a petrophysical parameter the geological database was the digitalized geological map of north hungarian region 1 100000 the digital topographic database was based on 10 m contour lines of 1 50000 maps the digital elevation model was generated by kriging interpolation three topographic models were created with resolutions of 25 25 50 50 and 100 100 m per pixels evident correlation can be shown between the ucs and the relative frequency of the slope gradient the adequate regression procedure is the power regression for low slope category values while logarithmic regression is applicable at high slope angles based on the characteristic of the relationship and the value of r 2 slope category intervals can be identified the relative frequency of which is proven to be determined by the ucs these intervals are found to be between 4 10 10 16 16 22 22 44 and over 44 using the determination equations of slope gradient between 4 10 and over 44 the ucs of the bedrock can be calculated approximately as the average value of the two calculated results so the quotient of the frequency of these two category intervals can be regarded as an important morphometric index for a given bedrock

the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland the mesoarchean ca 3075 ma ivisaartoq greenstone belt contains well preserved primary magmatic structures such as pillow lavas volcanic breccias and clinopyroxene cumulate layers picrites despite the isoclinal folding and amphibolite facies metamorphism the belt also includes variably deformed gabbroic to dioritic dykes and sills actinolite schists and serpentinites the ivisaartoq rocks underwent at least two stages of post magmatic metamorphic alteration including seafloor hydrothermal alteration and syn to post tectonic calc silicate metasomatism between 3075 and 2961 ma these alteration processes resulted in the mobilization of many major and trace elements the trace element characteristics of the least altered rocks are consistent with a supra subduction zone geodynamic setting and shallow mantle sources on the basis of geological similarities between the ivisaartoq greenstone belt and phanerozoic forearc ophiolites and intra oceanic island arcs we suggest that the ivisaartoq greenstone belt represents a relic of dismembered mesoarchean supra subduction zone oceanic crust this crust might originally have been composed of a lower layer of leucogabbros and anorthosites and an upper layer of pillow lavas picritic flows gabbroic to dioritic dykes and sills and dunitic to wehrlitic sills the sm nd and u pb isotope systems have been disturbed in strongly altered actinolite schists in addition the u pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration gabbros and diorites have the least disturbed pb isotopic compositions in contrast the sm nd isotope system appears to have remained relatively undisturbed in picrites pillow lavas gabbros and diorites as a group picrites have more depleted initial nd isotopic signatures end 4 23 to 4 97 than pillow lavas gabbros and diorites end 0 30 to 3 04 consistent with a variably depleted heterogeneous mantle source in some areas gabbros include up to 15 cm long white inclusions xenoliths these inclusions are composed primarily 90 of ca rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas the anorthositic cumulates have significantly higher initial end 4 8 to 6 0 values than the surrounding gabbroic matrix 2 3 to 2 8 consistent with different mantle sources for the two rock types

evidence for mesoarchean 3 2ga rifting of the pilbara craton the missing link in an early precambrian wilson cycle abstract a sequence of c 3 2 ga low grade sedimentary rocks bimodal volcanic rocks and subvolcanic layered mafic ultramafic sills and dykes occur in greenstone belts that flank the margins of the 3 51 3 24 ga east pilbara terrane of the pilbara craton western australia u pb shrimp zircon data and sm nd model age data suggest that these rocks represent juvenile addition of material to the margins of the craton during a period of major extension accompanied by the local emplacement of granitic rocks this extensional event is also represented in adjacent terranes by thick basalt flows with c 3 2 ga zircon and sm nd model ages and by granitic rocks based on the available age data geochemistry of basaltic rocks and the distribution of this event in space and time within the craton we interpret the c 3 2 ga rocks to reflect widespread rifting of the protocraton and associated thinning of the lithosphere this event resulted in separation of the flanking kurrana and karratha terranes from their parent east pilbara terrane which represents the ancient nucleus of the craton these terranes were then reassembled during subsequent accretion at 3 07 2 905 ga combined the data suggest the preservation of a mesoarchean wilson cycle in the pilbara craton

irregular earthquake cycle along the southern tianshan front aksu area china 1 a long deformation record 16 5 ka showing nonsteady earthquake recurrence is established for the aksu thrust tianshan china on the basis of cosmogenic 10be dating of faulted surfaces topographic leveling across the aksu fault scarp indicates up to 10 m uplift across a moraine abandoned 16 5 ka and similar uplift of 6 m across two inset surfaces yielding ages of 12 5 and 5 ka successively smaller uplifts of terraces younger than 5 ka indicate that three or more major earthquakes occurred during this period these data show that the aksu thrust fault was quiet for at least 7500 years and active in the last 5000 years and probably in the interval 16 5 12 5 ka the seismic cycle along the aksu thrust fault over the last 16 500 years was thus strongly irregular showing long quiescence and clustering which is a challenge for paleoseismology and hazard assessment this irregularity also makes it more difficult to estimate long term shortening rates nevertheless we are able to constrain a minimum total shortening rate 7 mm yr across the southern tianshan front for the last 12 500 years that is about a third of the total geodetic rate for the western tianshan 20 mm yr

liquid immiscibility in the archean greenstone belt of piumhi minais gerais brazil abstract liquid immiscibility has been used for explaining several modern situations either in effusive or in intrusive rocks liquid immiscibility is also proposed for explaining the variolitic structures in the archean rocks of the abitibi greenstone belt ontario the volcanic sequence of the lower portion of the greenstone belt of piumhi minas gerais brazil shows the occurrence of variolitic structure that may have been formed by liquid immiscibility this possibility is supported by the following points a the contact between varioles and matrix is very sharp b spinifex texture occurs both in the varioles and in the matrix and the single crystal may cross the varioles c the varioles and the matrix show contrasting compositions the latter has the composition of a basaltic komatiite and the former varies form that of an alkali basalt to andesite and d a test by a thermodynamic model from literature shows that although with some doubt the free energy of the possible parent liquid is higher than that of the two unmixed liquids

a new approach to the relief of great britain i the machine readable database abstract we describe a simple if fairly large database of relief and geology for great britain and the surrounding shelf based on the kilometre squares of the national grid the area covered is 560 000 km 2 319 490 km 2 classified as sea and 240 510 km 2 as land squares which included the coastline were classed as land the database affords a new approach to establishing the importance of rock resistance to erosion as a factor affecting the form of the present land surface the legacy of older tectonic movements and the role of neotectonics it can be used to test existing hypotheses or propose new ones which can then be tested against other evidence initial data on land shelf contrasts are presented the current coastal zone is found to be a sector of relatively abrupt transition from shelf depths between 60 and 70 m to land elevations exceeding an average of over 100 m even within a few kilometres of the coast this transition zone is attributed to the isostatic buoyancy of a land area steadily unloaded by denudation and attacked by the transgressing sea as water levels rise after each glacial stage the database is a novel and powerful way to tackle some fundamental aspects of the evolution of the relief of great britain and further papers will explore issues such as relative rock resistance and evidence for neotectonic movement

dacitic ocelli in mafic lavas 3 8 3 7 ga isua greenstone belt west greenland geochemical evidence for partial melting of oceanic crust and magma mixing abstract mafic volcanic rocks in the 3 8 3 7 ga isua greenstone belt southern west greenland contain randomly distributed 1 to 10 centimeters long white spheroidal structures in this study these structures are called ocelli in the western part of the belt ocelli bearing lavas are enclosed in basaltic to picritic flows mgo 9 21 wt with a subduction zone geochemical signature the ocelli are composed predominantly of polycrystalline na plagioclase and quartz with minor hornblende and biotite whereas the surrounding amphibolite matrix basaltic host is composed mainly of hornblende ca plagioclase and quartz the ocelli are devoid of radial or concentric internal structure and display all stages of coalescence contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational compositionally the ocelli are calc alkaline dacites sio2 62 9 72 0 wt mgo 0 60 3 50 wt ni 58 143 ppm cr 250 510 ppm whereas the surrounding matrix is tholeiitic basalt sio2 46 6 50 6 wt mgo 8 70 12 30 wt ni 119 175 ppm cr 330 600 ppm in terms of major element composition the isua ocelli closely resemble plagiogranites in phanerozoic supra subduction zone ophiolites field and petrographic observations and geochemical data sio2 54 2 60 7 wt mgo 3 95 7 72 wt ni 127 158 ppm cr 500 570 ppm on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts on a chondrite normalized diagram the basaltic host is characterized by variably depleted lree patterns la smcn 0 30 0 94 gd ybcn 1 03 1 45 whereas the dacitic ocelli display lree enriched patterns la smcn 1 30 2 60 gd ybcn 1 32 2 58 the strongly depleted ree patterns in the basaltic host are attributed to lree loss during carbonate alteration partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host the geochemical characteristic of the ocelli cannot be explained by post magmatic alteration slab melting fractional crystallization of tholeiitic melts or liquid immiscibility we suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust high mg volcanic rocks that fell into the magma chamber suggesting magma crust interaction in the early earth formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early archean

seismic gap hypothesis ten years after the seismic gap hypothesis states that earthquake hazard increases with time since the last large earthquake on certain faults or plate boundaries one of the earliest and clearest applications of the seismic gap theory to earthquake forecasting was by mccann et al 1979 who postulated zones of high medium and low seismic potential around the pacific rim in the 10 years since there have been over 40 large m 7 0 earthquakes enough to test statistically the earlier forecast we also analyze another forecast of long term earthquake risk that by kelleher et al 1973 the hypothesis of increased earthquake potential after a long quiet period can be rejected with a large confidence the data suggest that contrary to these forecasts places of recent earthquake activity have larger than usual seismic hazard whereas the segments of the circum pacific belt with no large earthquakes in recent decades have remained relatively quiet the clustering of earthquake times does not contradict the plate tectonic model which constrains only the long term average slip rate not the regularity of earthquakes

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

irregular recurrence of large earthquakes an analysis of historic and paleoseismic catalogs assessment of a region s seismic hazard requires knowledge of the repeat times and the regularity of earthquake recurrence for most areas the earthquake catalog is too short to determine recurrence parameters reliably using a large set of events for many regions however it should be possible to constrain the average characteristics of earthquake recurrence behavior this study compiles a set of the most complete historic and paleoseismic series of m 7 earthquakes from the middle american trench alaska chile japan and the san andreas fault the 52 recurrence series show a large average aperiodicity that is variations in recurrence time are larger than 40 of the average recurrence interval aperiodicity may decrease with increasing magnitude neither the use of different magnitude thresholds the exclusion of foreshocks and aftershocks the type of statistics normal or lognormal used nor the choice of segmentation results in a significantly smaller aperiodicity the irregular recurrence behavior exhibited by the data implies that earthquakes are more unpredictable than often assumed in hazard analyses

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

morphometric analysis and its relation to tectonics in macaronesia abstract for an investigation of the relation between the morphology and the tectonics in the macaronesian islands i e the geographical group of middle atlantic islands between 10 n and 40 n orientation studies have been made of steeply dipping joints and where possible of the trends of valleys and ridges a coincidence of the preferred directions of these features indicates a probable genetic relation between them the results are 1 joint strikes stream directions and ridge trends where available generally agree with each other on each individual island indicating a common genetic tectonic origin of these features thus the main result of this study is that the morphology of all macaronesian islands although surficially of volcanic origin has been significantly codesigend by tectonic processes 2 the direction data for corresponding geomorphic features are essentially the same across each individual archipelago certainly with regard to the joints somewhat less so with regard to the links and ridges 3 the canary and the madeira islands archipelago yield the same values for the direction maxima of joints and streams 4 the compression 1 axis indicated by the fault plane solution of the canary earthquake of 9 may 1989 lines up with one of the bisectrices and not with one of the strike maxima of the surrounding joints this indicates that the joint directions may be indicative of the shear lines rather than of the principal stress directions of the neotectonic stress field in the area 5 there is a steady anticlockwise rotation of corresponding morphotectonic directions as one moves from africa westward from the canary islands through madeira and the cape verde islands to the azores this may be due to a general rotation of the area whilst drifting n africa has been subject to an anticlockwise rotation pole west of madeira owing to the drag caused by the eastward spreading atlantic ocean floor 6 the mid atlantic ridge seems to have no influence on the orientation of the joints streams ridges on the islands in the plate tectonic model this lining up of joint orientations may be the consequence of the mirror reflection of the displacements away from the ridge on its two sides 7 the situation at the triple junction of the eurasian african and american plates is not clear additional microplates azorean iberian might even be present the azores might i have arisen around the triple junction from basaltic lava upwelling from a mantle hot spot to be carried away as islands on the moving plates or ii they may represent a hot spot trace on moving plates since trachytic andesitic lava flows have been found on the islands iii a tectonic control model has been proposed in the literature without recurrence to basaltic plates the results of our morphometric studies would be able to conform to all three models

a power law function for earthquake interarrival time and magnitude the question of time dependent seismic hazard models is still an open one while most hazard studies assume stationarity of seismicity there has been some debate on the relative merits of poissonian and non poissonian recurrence models and opinions about the viability of the seismic gap hypothesis also vary previous attempts to treat seismic hazard as time dependent have however concentrated on large earthquakes which do not always control the hazard at a site in this study earthquake interarrival times are studied for several regions in japan and greece it is found that a lognormal distribution provides a good model and that seismicity can be represented by the equation ln iat a b m c where ln iat is the log interarrival time of earthquakes exceeding magnitude m and a b and c are regional constants this power law is clearly related to the normal gutenberg richter magnitude frequency law but actually contains more information this law provides a basis for time dependent seismic hazard analysis in which the whole earthquake catalog is used rather than just the largest events a question still remains as to whether c the standard deviation is significantly dependent on magnitude manuscript received 10 january 2000

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

the predictability of large earthquakes evidence from an analogue model of earthquake rupture the recurrence behaviour of large earthquakes in several tectonic settings has been explained by simple models of stress accumulation and release which assume that the fault stress state is solely a function of the far field tectonic strain rate however the limited dataset of large event recurrence intervals has been a major obstacle to the verification of these and other models we present the results from a simple analogue model of earthquake rupture and stick slip which displays power law frequency size statistics and involves many cycles of large events we show that despite the macroscopic homogeneity of the model large events do not conform to simple deterministic time or slip predictable patterns however when the recurrence intervals for large events are divided by the median recurrence interval the normalized data are composed of two distinct lognormally distributed populations one population is characterized by events which are strongly clustered in time with relatively short recurrence intervals and low moment release the other by events which are weakly clustered in time with median sized recurrence intervals it is suggested that the long term recurrence behaviour of large earthquakes whilst being non deterministic may be modelled by a well defined statistical distribution of recurrence intervals

stress variability and brittle tectonic structures the problem of stress variability both in space and time during brittle deformation of rocks is faced in this note several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed a genetic partitioning of the stress tensor is proposed showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific genetic components the genetic components discussed in this note are the gravitational the tectonic the fluid the thermal and the diagenetic one each genetic component is a function of several parameters but for all of them the possible important role played by the variable time is emphasised according to numerous researches based on theoretical and experimental grounds brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small and large scale spatial variations as well as about short and long term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event the relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches

how regularly do earthquakes recur a synthetic seismicity model for the san andreas fault we have been attempting to improve estimates of long term earthquake recurrence probabilities along the san andreas fault by means of synthetic seismicity calculations the calculations are based on the concept of fault segmentation and incorporate the physics of static dislocation theory forecasts constructed from synthetic seismicity are robust in that they embody regional seismicity information over several units of magnitude they tie together in a physical manner a spectrum of fault segment features such as length strength characteristic magnitude mean repeat time and slip rate they can reasonably account for fault segment interactions and they are formulated from a catalog which can be extended as long as needed to be statistically significant we contend earthquake recurrence is more aperiodic than previously thought and as a result probabilities of major san andreas earthquakes should be revised downward

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

contrasting geochemical patterns in the 3 7 3 8 ga pillow basalt cores and rims isua greenstone belt southwest greenland implications for postmagmatic alteration processes abstract pillow basalts from the early archean 3 7 to 3 8 ga isua greenstone belt west greenland are characterized by well preserved rims and concentric core structures the pillow rims and cores have different mineral assemblages and chemical and isotopic compositions the rims have systematically higher contents of fe2o3 mgo mno k2o rb ba ga y and transition metals than the cores in contrast the cores possess higher concentrations of sio2 na2o p2o5 sr pb u nb and the light rare earth elements rees than the rims these compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration variations of many major and trace element concentrations between the rims and cores of the isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration al2o3 tio2 th zr and the heavy rees display similar values in both rims and cores suggesting that these elements were relatively immobile during postemplacement alteration in addition the rims and cores have distinctive sm nd and rb sr isotopic compositions in that the rims are characterized by higher 143nd 144nd and 87sr 86sr ratios than the cores the pillow basalts yield 2569 170 ma and 1604 170 ma errorchron ages on 143nd 144nd vs 147sm 144nd and 87sr 86sr vs 87rb 86sr diagrams respectively the sm nd errorchron age may correspond within errors to a late archean tectonothermal metamorphic event recorded in the region the sm nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of nd relative to sm during late archean metamorphism although the rb sr errorchron age overlaps with the timing of an early to mid proterozoic tectonothermal metamorphic event recorded in the region because of a considerably large mean square of weighted deviates value and scatter in 86sr 87sr and 87rb 86sr ratios this age may not have a precise geological significance the 1 6 ga rb sr errorchron is likely to have resulted from the loss of radiogenic 87sr collectively the sm nd and rb sr data obtained from the 3 7 3 8 ga isua pillow basalt rims and cores are consistent with disturbances of the sm nd and rb sr systems by tectonothermal metamorphic events long after their eruption in contrast to the sm nd and rb sr systems the lu hf system appears to be largely undisturbed by metamorphism five core samples and three rim samples yield a 3935 350 ma age within error of the approximate age of eruption 3 7 to 3 8 ga two rim samples that have gained lu give an age of 1707 140 ma within error of the rb sr errorchron age initial 176hf 177hf ratios of the undisturbed samples at 3 75 ga lie within 1 e unit of the chondritic value suggesting no long term depletion in the mantle source of the basalts

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

constraints on fault slip rates of the southern california plate boundary from gps velocity and stress inversions summary we use global positioning system gps velocities and stress orientations inferred from seismicity to invert for the distribution of slip on faults in the southern california plate boundary region of particular interest is how long term slip rates are partitioned between the indio segment of the san andreas fault saf the san jacinto fault sjf and the san bernardino segment of the saf we use two new sets of constraints to address this problem the first is geodetic velocities from the southern california earthquake center s scec crustal motion map version 3 by shen et al which includes significantly more data than previous models the second is a regional model of stress field orientations at seismogenic depths as determined from earthquake focal mechanisms while gps data have been used in similar studies before this is the first application of stress field observations to this problem we construct a simplified model of the southern california fault system and estimate the interseismic surface velocities using a backslip approach with purely elastic strain accumulation following meade et al in addition we model the stress orientations at seismogenic depths assuming that crustal stress results from the loading of active faults the geodetically derived stressing rates are found to be aligned with the stress orientations from seismicity we therefore proceed to invert simultaneously gps and stress observations for slip rates of the faults in our network we find that the regional patterns of crustal deformation as imaged by both data sets can be explained by our model and that joint inversions lead to better constrained slip rates in our preferred model the sjf accommodates 15 mm yr 1 and the indio segment of the saf 23 mm yr 1 of right lateral motion accompanied by a low slip rate on the san bernardino segment of the saf anomalous fault segments such as around the 1992 m w 7 3 landers surface rupture can be detected there observed stresses deviate strongly from the long term loading as predicted by our simple model evaluation of model misfits together with information from palaeoseismology may provide further insights into the time dependence of strain accumulation along the san andreas system

surface joint systems tectonic stresses and geomorphology a reconciliation of conflicting observations this note seeks to reconcile the widespread small scale fractographic observations indicating that surface joints are extension fractures with the frequently observed occurrence of such joints as what appear to be nearly orthogonal conjugate sets their strikes fitting the shear lines of the neotectonic stress field it is suggested that the local appearance of joints as extension fractures may have nothing to do with their orientation in the large scale neotectonic stress field inasmuch as extrapolations from a local scale to plate tectonic dimensions are quite speculative thus the local extension characteristics of the joint surfaces may be acquired at the latest stage of their genesis as a result of the corresponding rock faces becoming exposed but the geometrically orientational attributes may be conditioned by the shear in the surrounding large scale neotectonic stress field a new possible mechanism for reconciling the conflicting local and large scale observations is suggested

metamorphic history suggested by garnet growth chronologies in the isua greenstone belt west greenland abstract compositional maps for garnets from the five tectonic domains of the isua greenstone belt in west greenland are used to identify episodes of garnet growth during the metamorphism of the greenstone belt three distinct episodes of garnet growth are recorded in domains iii v two in domain ii and one in domain i a wide variety of garnet textures are observed including growth zoning post growth homogenisation replacement rim resorption and oscillatory zoning there is no simple correlation of garnet growth events among domains indicating that at least some events are related to localised metasomatic processes in the greenstone belt however the relative growth chronology combined with the available geochronology indicates that the observed metamorphic processes span both the early and late archaean

recognizable primary volcanic and sedimentary features in a low strain domain of the highly deformed oldest known 3 7 3 8 gyr greenstone belt isua west greenland a low strain domain has been identified in the metamorphosed mostly highly deformed volcanic and sedimentary rocks of the early archaean isua supracrustal belt this domain contains well preserved volcanic and sedimentary features including basaltic pillow lavas pillow breccia heterogeneous volcanic breccia amygdules in metabasalt and polymict conglomerate dominated by recrystallized chert and volcanic clasts the low strain domain is bounded by highly deformed rocks mostly derived from basalt chert and banded iron formation these discoveries demonstrate that some primary features have escaped the pervasive metasomatism dominant in other parts of the belt and furthermore strengthen the characterization of the isua supracrustals as a typical greenstone belt

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

a new approach to the relief of great britain iii derivation of the contribution of neotectonic movements and exceptional regional denudation to the present relief abstract this paper builds on the contribution of relative rock resistance to contrasts in the relief of britain to establish the contribution of other factors these include the slope of the major rivers towards the sea itself in part a function of the size of material eroded from the headwater uplands and thus of their lithology and the steepness of their valleyside slopes the pattern of denudational unloading and thus of isostatic uplift is mapped and compared with actual mean altitude it is shown that the sum of predicted isostatic uplift and local base level is closely related to the current mean altitude of our uplands the predicted mean height based on the six equations for each rock resistance class relating mean altitude to river distance can also be compared with actual mean elevation this shows positive departures which are most readily explained by neotectonic uplift notable in the scottish highlands the alston block of the northern pennines and fforest fawr in south wales negative departures may be due to relative neotectonic subsidence as in buchan where there is coincidence with positive gravity anomalies however most negative departures can be linked with severe glacial erosion especially in such areas as westernmost scotland the lake district the vale of belvoir and the wash fens basin where evidence for deep glacial erosion is already strong regional height differences related to the major structural regions of britain are calculated they make a smaller contribution to differences in average elevation and thus to the overall relief of britain

a 100 year average recurrence interval for the san andreas fault at wrightwood california evidence for five large earthquakes during the past five centuries along the san andreas fault zone 70 kilometers northeast of los angeles california indicates that the average recurrence interval and the temporal variability are significantly smaller than previously thought rapid sedimentation during the past 5000 years in a 150 meter wide structural depression has produced a greater than 21 meter thick sequence of debris flow and stream deposits interbedded with more than 50 datable peat layers fault scarps colluvial wedges fissure infills upward termination of ruptures and tilted and folded deposits above listric faults provide evidence for large earthquakes that occurred in a d 1857 1812 and about 1700 1610 and 1470

late holocene rupture of the northern san andreas fault and possible stress linkage to the cascadia subduction zone we relate the late holocene northern san andreas fault nsaf paleo seismic history developed using marine sediment cores along the northern california continental margin to a similar dataset of cores collected along the cascadia margin including channels from barclay canyon off vancouver island to just north of mon terey bay stratigraphic correlation and evidence of synchronous triggering imply earthquake origin and both temporal records are compatible with onshore paleoseis mic data in order to make comparisons between the temporal earthquake records from the nsaf and cascadia we refine correlations of southern cascadia great earth quakes including the land paleoseismic record along the nsaf during the last 2800 yr 15 turbidites including one likely from the great 1906 earthquake establish an average repeat time of 200 yr similar to the onshore value of 240 yr the combined land and marine paleoseismic record from the southern cascadia subduction zone includes a similar number of events during the same period while the average recurrence interval for full margin cascadia events is 520 yr the southern cascadia margin has a repeat time of 220 yr similar to that of the nsaf thirteen of the 15 nsaf events were preceded by cascadia events by 0 80 yr averaging 25 45 yr as compared to 80 400 yr by which cascadia events follow the nsaf based on the temporal association we model the coseismic and cumulative post seismic deformation from great cascadia megathrust events and compute related stress changes along the nsaf in order to test the possibility that cascadia earth quakes triggered the penultimate and perhaps other nsaf events the coulomb fail ure stress cfs resulting from viscous deformation related to a cascadia earthquake over 60 yr does not contribute significantly to the total cfs on the nsaf however the coseismic deformation increases cfs on the northern san andreas fault nsaf b y up to about 9 bars offshore of point delgada most likely enough to trigger that fault to fail in north to south propagating ruptures

potential earthquake triggering in a complex fault network the northern south island new zealand summary a synthetic seismicity computer model of multiple interacting faults has been used to investigate potential earthquake triggering in the alpine marlborough buller fault network northern south island new zealand of particular interest is whether large characteristic earthquakes on the central 300 km long segment of the alpine fault average recurrence time of 240 yr might trigger events to the north and northeast possible temporal clustering of other large events and stress shadows the synthetic seismicity model generates long homogeneous catalogues that sample a wide range of possible stress interactions and stress states allowing statistical analysis the catalogues used here include 1500 central alpine fault events of m 7 8 and approximately 2 000 000 other events of magnitudes down to 4 8 these other events occur on four additional segments of the alpine fault on 30 segments of the major marlborough and buller faults and on 4650 randomly distributed smaller faults the synthetic seismicity model is of the quasi static type governed by the coulomb failure criterion with extensions to better account for rupture propagation and fault healing true rate and state friction and viscoelastic relaxation are not yet included mechanical properties are adjusted so that most events rupture an entire fault segment i e the faults behave characteristically the regional b value of 1 thus arises mainly from the fault size distribution the driving mechanism and fault properties are iteratively adjusted so that the resulting long term slip rates single event displacements and recurrence times match the observed real world values the major results are as follows i large alpine fault earthquakes m 7 8 on the central segment have recurrence times with a broad distribution from 81 to 601 yr average 234 yr ii the regional moment release rate is constant over periods of 1000 yr but is quite variable on scales of a few hundred years iii large events on the central alpine fault will usually propagate at least part way along strike to the northeast past the junctures with the marlborough faults that splay off to the east iv thirty seven per cent of such large alpine fault events lafes will also instantaneously trigger significant activity m 6 5 on one or another of the other major faults here instantaneous means with a 0 to approximately 3 yr delay v characteristic events initiating on other major faults also have a high chance of triggering activity elsewhere vi following the short period of increased risk of triggered events a stress shadow effect predominates probably reflecting the mutually inhibitory nature of parallel strike slip faults and the need for a constant long term moment release rate

oxygen isotope and chemical studies on the origin of large plagiogranite bodies in northern oman and their relationship to the overlying massive sulphide deposits the extraordinarily well preserved and well exposed semail ophiolite of northern oman hosts several large plagiogranite intrusions in proximity to economic copper sulphide deposits of the lasail mining district a progression of isotopic chemical and mineralogical transformations observed within the plagiogranites and high level gabbros hlg and a comparison of these effects with those in the lowermost dykes of the immediately overlying sheeted dyke complex sdc tracks the evolution of hydrothermal fluids and the alteration of overlying dykes and pillow lavas during discharge of these fluids on the sea floor the largest hydrothermal alteration aureoles and the greatest extent of metamorphic veins and metasomatic replacement features are found adjacent to the largest high level plagiogranite bodies beneath and adjacent to the major ore bodies in northern oman the ubiquitous presence of metamorphic actinolitic hornblende sodic plagioclase epidote and titanite in metabasalts within the high temperature alteration zones points to the most likely mineralogical and structural controls on the development and evolution of the hydrothermal fluids depleted cu contents of the adjacent crustal rocks and cu enrichments above the plagiogranite intrusions demonstrate the redistribution of heavy metals adjacent to the complexes field relationships implicate the formation of both the epidosites and plagiogranites in the genesis of the ore deposits an important process inferred from the field and geochemical data is the assimilation of previously hydrothermally altered basaltic and gabbroic country rocks by stoping into the magma chambers developed near the sdc gabbro horizon in the ophiolite we suggest that this process of combined assimilation fractional crystallization together with replenishment and recharge by injection and quenching of basaltic magma pillows into these plagiogranite magma chambers i e rafc plays a major role in the development of these composite intrusions

protoliths of the 3 8 3 7 ga isua greenstone belt west greenland abstract the isua greenstone belt fig 1 contains the oldest known relatively well preserved metavolcanic and metasedimentary rocks on earth the rocks are all deformed and many were substantially altered by metasomatism but both the deformation and metasomatism were heterogeneous transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated likewise different kinds and different episodes of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths new geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents by this means the primary nature of the schists that make up most of the isua greenstone belt was reinterpreted and a new map that better portrays the primary nature of the rocks has been produced the previously mapped stratigraphy was found to be of little value in understanding the geology stratigraphic units were defined by different and diverse criteria such as current composition structure metamorphic texture and inferred protoliths much of this stratigraphy represents a misinterpretation of the primary nature of the rocks the new work indicates that most of the isua greenstone belt consists of fault bounded rock packages mainly derived from basaltic and high mg basaltic pillow lava and pillow lava breccia chert bif and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks a previously mapped extensive unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite

the origin of early archean banded iron formations and of continental crust isua southern west greenland abstract a long standing geological debate is whether the fe component of precambrian banded iron formations bifs was derived from continental sources or had its origin in hydrothermal alteration of the oceanic crust recent ree and nd and fe isotope studies of iron rich layers in bifs have provided strong supporting evidence for the hydrothermal origin of the fe component pillow basalts from the 3800 3700 ma isua greenstone belt west greenland underwent significant enrichment of fe up to 21 wt fe2o3 and some chalcophile elements e g pb 5 4 7 2 ppm prior to a late archean tectonothermal metamorphic event recorded in the region published and new pb pb isotopic data from the isua pillow basalts and the spatially associated bifs suggest that the enrichment of fe and pb and deposition of the bifs took place between 3800 and 3600 ma similar pb isotopic compositions in pillow basalts and bifs are consistent with a common source of pb for both supracrustal rocks iron and pb were probably scavenged from the oceanic crust and were re deposited on the ocean floor by hydrothermal fluids suggesting that high temperature hydrothermal alteration of early archean oceanic crust played an important role in the deposition of the isua bifs geochemical lithological and structural data from the isua greenstone belt are all collectively consistent with a convergent margin geodynamic setting the isua bifs are spatially and temporally associated with boninitic and island arc picritic pillow basalts given the observations that cenozoic boninites and picrites tend to form in an intra oceanic arc forearc setting we suggest that the isua bif boninite and bif picrite associations were also deposited in a similar geodynamic setting the original pillow basalt bif stratigraphy was dismembered and transposed throughout the belt during early to late archean tectonothermal events the high temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of fe necessary to contribute to the deposition of the isua bifs the hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early archean arc forearc region the ridge subduction model can also explain the origin of the contemporaneous tonalite trondhjemite granodiorite ttg intrusions in the isua region partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced ttg melts the diapirically upwelling ttg melts formed the early archean continental crust and the complementary eclogitic residues contributed to the sub continental lithospheric mantle

implications of melting for stabilisation of the lithosphere and heat loss in the archaean abstract the increased depth and volume of melting induced in a higher temperature archaean mantle controls the stability of the lithosphere heat loss rates and the thickness of the oceanic crust the relationship between density distributions in oceanic lithosphere and the depth of melting at spreading centres is investigated by calculating the mineral proportions and densities of residual mantle depleted by extraction of melt fractions the density changes related to compositional gradients are comparable to those produced by thermal effects for lithosphere formed from a mantle which is 200 c or more hotter than modern upper mantle if archaean continental crust formed initially above oceanic lithosphere the compositional density gradients may be sufficient to preserve a thick archaean continental lithosphere within which the archaean age diamonds are preserved the amount of heat advected by melts at mid ocean ridges today is small but heat advected by melting becomes proportionally more important as higher mantle temperatures lead to a greater volume of melt and as the rate of production of oceanic plates increases archaean tectonics could have been dominated by spreading rates 2 3 times greater than now and with mantle temperatures between ca 1600 c and 1800 c at the depth of the solidus mid ocean ridge melting would produce a relatively thick but light refractory lithosphere on which continents could form protected from copious volcanism and high mantle temperatures

morphotectonic evidence from lateral propagation of an active frontal fold pakuashan anticline foothills of taiwan abstract the pakuashan anticline is uniquely suited for study of the forward and lateral growth of fault related folds the pakuashan ridge development arises from the late quaternary uplift of the most external thrust zone of the western foothills of taiwan from kaoshiung to taichung recent and active westward thrusting occurs at the front of the foothills the pakuashan anticline trending n 150 e in the northern part to n 000 in the southern part has been active throughout the quaternary period this activity is marked by geological structures tectonic geomorphology and seismicity a multisource and multiscale approach to study of the continental collision setting has been undertaken to combine tectonics sedimentology and geomorphology studies of fracture patterns allow identification of two main features of stress orientations a wnw ese compression direction and e w and n s extension directions quantitative geomorphic parameters have been used to define the morphotectonic evolution and to infer tectonic style along the mountain front geomorphic evidence provides significant information on the processes that govern lateral propagation of an active anticline quaternary terraces are uplifted tilted and folded over the pakuashan ridge drainage systems in areas of active compression give information on the thrust zone structures and their development steep drainage and high local relief indicate that the pakuashan anticline forms a well defined zone of high uplift especially in the southern part the two main controls on drainage in that area are rock strength in the hanging wall and propagation of the deformation towards the south

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

rupture lengths and temporal history of significant earthquakes on the offshore and north coast segments of the northern san andreas fault based on turbidite stratigraphy piston gravity and jumbo kasten cores were collected from channel and canyon systems draining the northern california continentalmargintoinvestigatetherecordofperiodicholoceneturbiditesforpossibleconnectiontolargemagnitudeearthquakeson the adjacent northern san andreas fault poorly known channel systems were mapped with multibeam sonar to define pathways and channel confluences cores sampled all major and many minor channel systems extending from cape mendocino to just north of monterey bay sampling both along and across channels was done and particular attention was paid to channel confluences as these areas afford opportunities to test for synchronous triggering of turbidity currents while at sea all cores were scanned using a geotek multisensor core logger mscl which collects high resolution photography p wave velocity gamma ray density and magnetic susceptibility data from the unsplit cores lithology was logged visually and cores were later imaged with x radiography we use 14 c ages relative dating tests at channel confluences and stratigraphic correlation using physical properties to determine

stress transfer in earthquakes hazard estimation and ensemble forecasting inferences from numerical simulations abstract observations indicate that earthquake faults occur in topologically complex multi scale networks driven by plate tectonic forces we present realistic numerical simulations involving data mining pattern recognition theoretical analyses and ensemble forecasting techniques to understand how the observable space time earthquake patterns are related to the fundamentally inaccessible and unobservable dynamics numerical simulations can also help us to understand how the different scales involved in earthquake physics interact and influence the resulting dynamics our simulations indicate that elastic interactions stress transfer combined with the nonlinearity in the frictional failure threshold law lead to the self organization of the statistical dynamics producing 1 statistical distributions for magnitudes and frequencies of earthquakes that have characteristics similar to those possessed by the gutenberg richter magnitude frequency distributions observed in nature and 2 clear examples of stress transfer among fault activity described by stress shadows in which an earthquake on one group of faults reduces the coulomb failure stress on other faults thereby delaying activity on those faults in this paper we describe the current state of modeling and simulation efforts for virtual california a model for all the major active strike slip faults in california noting that the working group on california earthquake probabilities wgcep uses statistical distributions to produce earthquake forecast probabilities we demonstrate that virtual california provides a powerful tool for testing the applicability and reliability of the wgcep statistical methods furthermore we show how the simulations can be used to develop statistical earthquake forecasting techniques that are complementary to the methods used by the wgcep but improve upon those methods in a number of important ways in doing so we distinguish between the official forecasts of the wgcep and the research quality forecasts that we discuss here finally we provide a brief discussion of future problems and issues related to the development of ensemble earthquake hazard estimation and forecasting techniques

a physical model for earthquakes 2 application to southern california the purpose of this paper is to apply ideas developed in a previous paper to the construction of a detailed model for earthquake dynamics in southern california the basis upon which the approach is formulated is that earthquakes are perturbations on or more specifically fluctuations about the long term motions of the plates this concept is made mathematically precise by means of a fluctuation hypothesis which states that all physical quantities associated with earthquakes can be expressed as integral expansions in a fluctuating quantity called the offset phase while in general the frictional stick slip properties of the complex interacting faults should properly come out of the underlying physics a simplification is made here and a simple spatially varying friction law is assumed together with the complex geometry of the major active faults an assumed spatially varying earth rheology the average rates of long term offsets on all the major faults and the friction coefficients one can generate synthetic earthquake histories for comparison to the real data the result is a set of slip time histories for all of the major faults which are similar to data obtained by geologic trenching studies however the patterns of earthquake occurrence are evidently chaotic in the technical sense meaning that although there is an element of periodicity to the events the patterns shift change and evolve with time time scales for pattern evolution seem to be of the order of a thousand years for average recurrance intervals of a hundred years or so in the synthetic slip histories for the model faults a series of events on the big bend is identified which resembles the pattern of events leading up to the 1857 fort tejon earthquake one of the events is similar enough to the 1857 event that it is taken as a model of that earthquake the horizontal surface deformation 130 years after the model event is calculated for comparison to the data observed by various methods during the past decade in southern california for example horizontal surface rates of strain and velocities are calculated and compared to u s geological survey trilateration data to very long baseline interferometry line length changes and to laser ranging data on the san andreas fault experiment baseline with a few exceptions agreement is good