spatial and temporal deformation along the northern san jacinto fault southern california implications for slip rates the san timoteo badlands is an area of uplift and erosional dissection that has formed as a result of late quaternary uplift along a restraining bend in the san jacinto fault of the san andreas fault system in southern california this bend currently is located in a region where late quaternary deposits and associated surfaces have formed in lower san timoteo canyon we have used morphometric analysis of these surfaces in conjunction with computer modeling of deformational patterns along the san jacinto fault to reconstruct spatial and temporal variations in uplift along the bend morphometric techniques used include envelope subenvelope map ping a gradient length index along channels and denudation values age control is determined using a combination of thermoluminescence tl and near infrared op tical simulation luminescence dating irosl and correlation of soil development indices these approaches are combined with an elastic half space model used to determine the deformation associated with the fault bend the region of modeled uplift has a similar distribution as that determined by morphometric techniques luminescence dates and soil correlation age estimates generally agree based on soil development surfaces within the study area were stabilized at approximately 300 700 ka for q3 43 67 ka for q2 and 27 5 67 ka for q1 luminescence ages both tl and irosl for the formation of the younger two surfaces are 58 to 94 ka for q2 and 37 to 62 ka for q1 ages reported to 1r uncertainty periods of uplift were determined for the surfaces in the study area resulting in approximate uplift rates of 0 34 to 0 84 m ka for the past 100 ka and 0 13 to 1 00 m ka for the past 66 ka comparison of these rates of uplift to those generated by the model support a higher rate of lateral slip along the san jacinto fault than commonly assumed greater than 20 mm yr as compared to 8 12 mm yr commonly cited this higher slip rate supports the proposal that a greater amount of slip has transferred from the san andreas fault to the san jacinto fault than generally held the san jacinto fault may have accommodated a significant portion of the plate boundary slip during the past 100 ka

present day strain accumulation and slip rates associated with southern san andreas and eastern california shear zone faults 1 we present the first results from a dense network of 36 campaign and 46 continuous gps stations located in the eastern transverse ranges province etr a transition zone between the southernmost san andreas fault ssaf and eastern california shear zone ecsz we analyzed the campaign data together with available data from continuous gps stations for the period 1994 2009 we used the gps velocity estimates to constrain elastic block models to investigate fault loading rates representing four hypotheses characterized by different fault block geometries fault block scenarios include blocks bounded by the east striking left lateral pinto mountain blue cut and chiriaco faults of the etr blocks bounded by a right lateral north northwest striking structure the landers mojave earthquake line that cuts obliquely across the etr and mapped mojave desert faults and combinations of these end member hypotheses each model implies significantly different active fault geometries block rotation rates and slip rates for etr and ecsz structures all models suggest that ssaf slip rate varies appreciably along strike generally consistent with rates derived from tectonic geomorphology and paleoseismology with a maximum of 23 mm yr right lateral along the southernmost coachella valley strand decreasing systematically to 10 mm yr right lateral through the san gorgonio pass region slip rate estimates for the san jacinto fault are 12 mm yr for all models tested all four models fit the data equally well in a statistical sense qualitative comparison among models and consideration of geologic slip rates and other independent data reveals strengths and weaknesses of each model

brazilian structural provinces an introduction abstract the territory of brazil coincides almost entirely with the south american platform the crystalline core of the continent its basement is composed of ancient metamorphic and igneous rocks and it has not suffered any tectonic regeneration since the beginning of the phanerozoic sedimentary rocks with almost horizontal bedding cover this crystalline basement this latter shows ages as old as early precambrian although ages between 500 and 1000 m y are conspicuously frequent the cratonic areas became consolidated more than 1700 m y ago whereas the fold belts formed essentially between 500 and 1700 m y ago the sedimentary cover accumulated from the early silurian in three large intracratonic basins until the platform became completely stabilized a late jurassic to early cretaceous reactivation caused by the break up of the ancient gondwana continent created another basin sequence chiefly along the atlantic continental margin based on the nature of the crystalline basement rocks and the sedimentary cover ten structural provinces can be distinguished 1 rio branco province in the north of the country occupied by the guyana shield and still only poorly known with an important fold belt developed during the so called transamazonian cycle 2000 200m y with high grade metamorphic rocks and only slight influence of later events 2 tapajos province in central brazil corresponding to the amazonas or guaporecraton showing chiefly precambrian crystalline rocks and behaving as an cratonic area during the phanerozoic 3 sa o francisco province located on the atlantic shield with its basement covered by rocks of different ages chiefly affected by the brasiliano cycle between 1000 and 500 m y and constituting another cratonic area 4 tocantins province between the amazonas and sa o francisco cratons with the oldest rocks in its centre ages over 2600 m y and at the eastern and western borders metamorphic sequences of various fold belts and almost no phanerozoic deposits 5 mantiqueira province located along the southern part of the atlantic coast affected chiefly by the brasiliano folding cycle 6 borborema province in the northeast brazilian fold belt affected by the brasiliano cycle in a very complex way and with important faulted zones it was reactivated in phanerozoic times when also sedimentary covers accumulated 7 amazonas province represented by the amazonas sedimentary basin syneclise and subdivided into four parts by three important arcs 8 parnaiba province coinciding with the piauf maranha o syneclise and filled with a rather thick sedimentary sequence 9 paranaprovince the sedimentary basin of southern brazil in which the well known late paleozoic glaciation features are found and which possesses an extensive cover of basaltic rocks of late jurassic to early cretaceous age 10 coastal province and continental margin the youngest structural unit developed during the separation of the continent and represented by rift valleys and coastal basins filled with mesozoic cenozoic deposits of various kinds

gold deposits of the carajas mineral province deposit types and metallogenesis the carajas mineral province is located in the south eastern part of the amazon craton and is divided into the southern and northern tectonic blocks which are referred to as the rio maria granitoid greenstone terrane and the itacaiunas shear belt respectively the geological evolution of this province occurred almost entirely in the archean with no tectonism from 2 4 to 1 9 ga and then an episode of very extensive intraplate granitic magmatism distinct types of gold deposits most associated with archean metavolcano sedimentary sequences including greenstone belts as old as 2 9 ga comprise 1 fe oxide poor cu au 2 fe oxide cu au u ree 3 shear zone related lode gold and 4 sedimentary rock hosted au pge the shear zone related deposits are more typical of the rio maria granitoid greenstone terrane whereas the other types are more abundant in the itacaiunas shear belt the first type represented by the serra verde deposit shows most characteristics of volcanogenic massive sulfide vms deposits except for the dominance of chalcopyrite over pyrite and or pyrrhotite some deposits of the second type igarape salobo and igarape bahia are also syngenetic but their ore mineralogy and paucity of pyrite and or pyrrhotite are inconsistent with or at least unusual for classic vms deposits other fe oxide cu au u ree deposits such as the cristalino and sossego occur as epigenetic stockwork mineralization related to archean granitoid stocks most shear zone related lode gold deposits are classified as orogenic gold deposits e g sapucaia babacu and tucuma that cut greenstone belt rocks the cumaru deposit is also hosted by a shear zone but may have a genetic association with a granitoid intrusion the aguas claras deposit although also hosted in a shear zone and perhaps intrusion related differs from the orogenic gold deposits by its extensional tectonic setting lack of co2 in the ore fluids and voluminous chalcopyrite the sedimentary rock hosted serra pelada au pge deposit is structurally controlled and occurs in an archean clastic chemical sedimentary rock sequence

the seismic cycle in southern california precursor or response the seismicity rate m 3 0 in southern california shows two cycles with periods of high activity 90 events year from 1945 1952 and 1969 1992 and lower activity 60 70 events year from 1952 1969 and 1992 present abrupt drops in the seismicity rate occur after the 1952 kern county m7 5 and the 1992 landers m7 3 earthquakes the sudden increase in 1969 does not coincide with any major event but approximates the time needed to reaccumulate the seismic moment released in the 1952 earthquake this temporal correlation with the preceding earthquake suggests that the seismic cycle lower seismicity after a major earthquake and higher seismicity before the next major earthquake should be interpreted as a response to the first earthquake rather than a precursor to the second southern california is now at a rate of seismicity as low as it experienced in the 1950s and 1960s

estimation of anisotropy and anisotropic 3 d prestack depth migration offshore zaire chevron operator teikoku oil and unocal hold the offshore zaire concession west africa recently unocal performed an anisotropic 3 d prestack depth migration in an attempt to optimally image tilted fault blocks in the presalt section depth migration was required because large displacement faults juxtaposed a high velocity massive carbonate and a low velocity marl large compressional wave anisotropy caused by thin layering was inferred for most of the depth section evidence on the nature and magnitude of the anisotropy came from comparing stacking velocities with well velocities analyzing a multioffset vertical seismic profile vsp thin layer modeling in the massive carbonate using well log data and rock physics laboratory velocity measurements on core from the marl the subsurface was characterized as being locally transversely isotropic and the thomsen parameters were used to describe the anisotropy this required that five parameters be specified at each subsurface location to represent the compressional wave 3 d velocity field the observed anisotropy deviated far from the elliptical condition and modeling indicated that isotropic migration would exhibit misfocusing and mispositioning of events and its quality would be dip dependent the anisotropic migration was performed using kirchhoff summation for which the traveltimes were determined using a finite difference scheme interpreted horizons in the migrated depth section accurately tied well depths

coseismic fault slip associated with the 1992 m w 6 1 joshua tree california earthquake implications for the joshua tree landers earthquake sequence coseismic surface deformation associated with the mw 6 1 april 23 1992 joshua tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from global positioning system and trilateration measurements the rms signal to noise ratio for these inferred displacements is 1 8 with near fault displacement estimates exceeding 40 mm in order to determine the long wavelength distribution of slip over the plane of rupture a tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right lateral slip and zero surface slip constraints the resulting slip distribution yields a geodetic moment estimate of 1 7 1018 n m with corresponding maximum slip around 0 8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations from empirical green s function analyses a rupture duration of 5 s is obtained which implies a rupture radius of 6 8 km most of the inferred slip lies to the north of the hypocenter consistent with northward rupture propagation stress drop estimates are in the range of 2 4 mpa in addition predicted coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0 1 mpa in contrast predicted stress changes are near zero at the hypocenter of the mw 7 3 june 28 1992 landers earthquake which nucleated about 20 km beyond the northernmost edge of the joshua tree rupture based on aftershock migrations and the predicted static stress field we speculate that redistribution of joshua tree induced stress perturbations played a role in the spatio temporal development of the earthquake sequence culminating in the landers event

seismotectonics of the easternmost transverse ranges california relevance for seismic potential of the southern san andreas fault earthquake locations depths and focal mechanisms from the southern california regional network 1977 1985 are used to identify the orientation and sense of slip of active subsurface faults in the easternmost transverse ranges etr the etr are separated from the salton trough province by the southernmost strands of the san andreas fault saf much of the seismicity in the etr is concentrated well northeast of the saf at relatively shallow depths under the little san bernardino mountains many of these earthquakes reflect slip on steeply dipping left lateral faults striking northeast to east at relatively high angles to the adjacent saf focal mechanisms in the etr show predominantly strike slip normal or oblique normal faulting and share common near horizontal t axes striking wnw p axes range from near vertical to near horizontal and strike mostly nne in contrast reverse and strike slip focal mechanisms that exhibit persistent north trending near horizontal p axes characterize the san gorgonio pass area immediately to the west these different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the saf this boundary which in the coachella valley may be the mission creek fault is also distinguished by abrupt changes in 1 rate and depth of seismic activity 2 topography 3 quaternary vertical deformation 4 strikes and dips of major branches of the saf and 5 seismic velocities in the crust and upper mantle the preponderance of secondary normal faulting in the etr versus secondary reverse faulting in the san gorgonio pass region suggests that fault normal stress is much less across the saf adjacent to the etr if a friction law where strength is proportional to normal stress applies to the saf then a smaller tectonic shear stress would be required tor slip in large earthquakes along the salton trough segment in this case the southernmost saf may have shorter repeat times smaller average displacements and lower moment release rates than earthquakes that rupture through the major restraining bend in san gorgonio pass

seismic activity on neighbouring faults as a long term precursor to large earthquakes in the san francisco bay area activity in moderate size earthquakes accelerated in the several decades before the large california earthquakes of 1989 1906 and 1868 this type of precursor seems to require the presence of several major faults in close enough proximity to one another that moderate size shocks are selectively triggered on surrounding faults during the latter stages of the cycle of strain buildup to large earthquakes it may be possible to use quantitative aspects of similar seismic precursors to make predictions of large earthquakes on timescales of a few years to one decade

variable rates of late quaternary strike slip on the san jacinto fault zone southern california three strike slip displacements of strata with known approximate ages have been measured at two locations on the san jacinto fault zone minimum horizontal offset between 5 7 and 8 6 km in no more than 0 73 m y northeast of anza indicates 8 12 mm yr average slip rate since late pleistocene time two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the coyote creek fault in western imperial valley horizontal slip of 1 7 m has been calculated for the youngest sediment of lake cahuilla since its deposition 275 510 yr b p the corresponding slip rate is 2 8 5 0 mm yr right lateral offset of 10 9 m measured on a buried stream channel older than 5060 yr b p but younger than 6820 yr b p yields average slip rates for the intermediate time periods 400 to 6000 yr b p of 1 2 mm yr the average rates of slip for these three time intervals suggest a relatively quiescent period for the san jacinto fault zone from about 4000 b c to about 1600 a d to the extent that long term variations in seismic activity of major strike slip faults elsewhere are known fluctuations in slip rate for the san jacinto fault would not appear to be abnormal if the san jacinto and adjacent segments of the san andreas fault alternately assume dominant roles in absorbing motion between the pacific and american plates perhaps even more recently than 400 yr ago the san andreas fault south of the transverse ranges expressed most of the motion but has since become relatively inactive

paleoproterozoic high sulfidation mineralization in the tapajos gold province amazonian craton brazil geology mineralogy alunite argon age and stable isotope constraints the brazilian tapajos gold province contains the first evidence of high sulfidation gold mineralization in the amazonian craton the mineralization appears to be in large nested calderas the tapajos parima or ventuari tapajos geological province consists of a metamorphic igneous and sedimentary sequence formed during a 2 10 to 1 87 ga ocean continent orogeny the high sulfidation mineralization with magmatic hydrothermal alunite is related to hydrothermal breccias hosted in a rhyolitic volcanic ring complex that contains granitic stocks ranging in age from 1 89 to 1 87 ga cone shaped hydrothermal breccias which flare upward contain vuggy silica and have an overlying brecciated cap of massive silica the deposits are located in the uppermost part of a ring structure volcanic cone drill cores of one of the hydrothermal breccias contain alunite natroalunite pyrophyllite andalusite quartz rutile diaspore woodhouseite svanbergite kaolinite and pyrite along with inclusions of enargite luzonite chalcopyrite bornite and covellite the siliceous core of this alteration center is surrounded by advanced argillic and argillic alteration zones that grade outward into large areas of propylitically altered rocks with sericitic alteration assemblages at depth several occurrences and generations of alunite are observed alunite is disseminated in the advanced argillic haloes that envelop massive and vuggy silica or that underlie the brecciated silica cap coarse grained alunite also occurs in branching veins and locally is partly replaced by a later generation of fine grained alunite silicified hydrothermal breccias associated with the alunite contain an estimated reserve of 30 tonnes of gold in rock that grades up to 4 5 g t 1 au seven alunite samples gave 40ar 39ar ages of 1 869 to 1 846 ga with various degrees of apparent minor ar loss stable isotopic data require a magmatic hydrothermal origin for the alunite typical for high sulfidation mineralization the 34s values of most samples of alunite range from 14 0 to 36 9 sulfur isotopic alunite pyrite and oxygen isotopic alunite so4 oh temperatures range from 130 to 420 c the dh2o and 18oh2o values for alunite forming hydrothermal fluids suggest a predominance of magmatic water with a small meteoric contribution a rare sample of supergene alunite has a 34s value of 4 1 and an 40ar 39ar age of 51 3 0 1 ma other than local foliation in the volcanic rocks and recrystallization of alunite near faults the mineralization and associated alteration appears to have been remarkably undisturbed by later metamorphism and by supergene alteration the au mineralization was preserved because of burial by sediments and tuffs in taphrogenic basins that probably developed shortly after mineralization and were probably first exhumed at about 60 ma because high sulfidation mineralization forms at relatively shallow crustal levels the discoveries in tapajos province provide new perspectives for mineral exploration for the amazonian and perhaps for other precambrian cratons

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

the importance of earthquake induced landslides to long term slope erosion and slope failure hazards in seismically active regions abstract this paper describes a general method for determining the amount of earthquake induced landsliding that occurs in a seismically active region over time this determination can be used as a quantitative measure of the long term hazard from seismically triggered landslides as well as a measure of the importance of this process to regional slope erosion rates and landscape evolution the method uses data from historical earthquakes to relate total volume of landslide material dislodged by an earthquake to the magnitude m and seismic moment m0 of the earthquake from worldwide data a linear regression relation between landslide volume v and m0 is determined as v m0 1018 9 0 13 where v is measured in m3 and m0 is in dyn cm to determine the amount of earthquake generated landsliding over time this relation is combined with data on seismic moment release for a particular region which may be derived from either earthquake history or fault slip data the form of the m0 v relation allows the rate of production of earthquake induced landslides over time to be determined from total rate of seismic moment release without regard to the distribution of individual events thus simplifying and generalizing the determination application of the method to twelve seismically active regions with areas ranging from 13 275 to 2 308 000 km2 shows that erosion rates from earthquake induced landslides vary significantly from region to region of the regions studied the highest rates were determined for the island of hawaii new zealand western new guinea and the san francisco bay region of california significantly lower rates were determined for iran tibet the sierra nevada great basin region of california and central japan for the time period from 715 ad to the present intermediate rates were determined for peru southern california onshore california turkey and central japan for the time period from 1586 ad to the present to determine the relative long term importance of seismically triggered landslides these erosion rates are compared to erosion rates calculated for other slope processes and to rates calculated from fluvial sediment discharge comparisons with other slope processes indicate that earthquake induced landslides are the predominant agents of slope erosion on the island of hawaii in the san francisco bay region and in western new guinea for hawaii the san francisco bay region and sierra nevada great basin region of california the erosion rates calculated for earthquake induced landslides also exceed the regional erosion rates calculated from fluvial sediment discharge

geology geochemistry and pb pb zircon geochronology of the paleoproterozoic magmatism of vila riozinho tapajos gold province amazonian craton brazil abstract pb pb zircon geochronologic data define two different periods of intense igneous activity in the vila riozinho region tapajos gold province tgp south central amazonian craton at 2 00 1 97 ga the vila riozinho volcanic sequence 2000 4 ma 1998 3 ma and the older sao jorge granite 1981 2 ma 1983 8 ma were formed at 1 89 1 87 ga the younger sao jorge 1891 3 ma jardim do ouro 1880 3 ma and maloquinha 1880 9 ma granites and the moraes almeida volcanic sequence 1890 6 ma 1881 4 ma 1875 4 ma were emplaced similar age intervals are registered throughout the tgp that is thus a little younger than the paleoproterozoic terranes of the maroni itacaiunas province in the guiana shield geochemical and geochronologic data demonstrate that the sao jorge pluton is composed of two different granitoids the older and younger sao jorge granites these as well as the jardim do ouro granite are i type and magnetite series the sao jorge granites are high k calc alkaline and show significant geochemical variation the jardim do ouro granite is more iron rich and less oxidized the vila riozinho formation is calc alkaline to shoshonitic and geochemically similar to the sao jorge granites the maloquinha granite and the moraes almeida formation differ from the other studied rocks in petrographic and geochemical characteristics have aluminous a type affinities and were probably derived by low temperature crustal melting the volcanic sequences of the studied area formerly included into the iriri group of the uatuma supergroup are divided into the vila riozinho and moraes almeida sequences the presence of these two sequences in the same tectonic domain demonstrates the heterogeneity of the uatuma supergroup the tgp registers accretionary processes related to the formation of the atlantica supercontinent at 2 00 ga this was followed at 1 88 ga by an intracontinental taphrogenic event that lasted throughout the mesoproterozoic the tectonic setting of the tgp was thus transitional between a subduction related magmatic arc and a stable continental block undergoing extension

intermediate term pre earthquake phenomena in california 1975 1986 and preliminary forecast of seismicity for the next decade intermediate term observations preceding earthquakes of magnitude 5 7 or greater in california from 1975 through 1986 suggest that 1 the sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years 2 these larger earthquakes tend to occur towards the ends of creeping fault segments 3 one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area and 4 marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area a common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep and suggests that surface fault creep is a sensitive indicator of changes in stress the preceding critieria are used to construct a preliminary forecast of the likely locations of significant earthquakes over the next decade

new curvilinear scheme for elastic wave propagation in presence of curved topography we study a new curvilinear scheme for wave propagation modelling in presence of topography the discrete scheme takes advantage of recent developments in areoacoustics our new scheme relies on the conventional grid coupled with optimized filters to remove numerical noise in case of strong material heterogeneity we used non centred stencils for free surface implementation and optimized the explicit runge kutta scheme for the time differencing we performed a complete theoretical stability and dispersion analysis of the discrete scheme finally we illustrate the numerical accuracy of the new scheme by intensive experiments

a study of the hydrothermal alteration in paleoproterozoic volcanic centers sao felix do xingu region amazonian craton brazil using short wave infrared spectroscopy abstract hypogene hydrothermal minerals have been identified by short wave infrared spectroscopy in hydrothermally altered rocks from the sobreiro and santa rosa formations which belong to a paleoproterozoic volcano plutonic system in amazonian craton three clay minerals are spectrally recognized montmorillonite kaolinite and illite the integration of these data with those available in the literature including gold occurrences suggests that those rocks are hydrothermal products of both volcanic thermal sources and later crustal intrusions as evidenced by variable styles of propylitic sericitic potassic and intermediate argillic alteration the influence of meteoric fluids is emphasized this low cost exploratory technique which can be applied to hand samples seems to be promising in the separation of hydrothermally altered volcano plutonic centers in regions submitted to severe weathering conditions in addition to aid elaborating models for prospecting mineral deposits

great earthquakes and great asperities san andreas fault southern california the big bend region of the southern san andreas fault consists of two great asperities that rupture infrequently in great earthquakes the eastern knot near san gorgonio pass which has not ruptured historically in a large event is the main locus of plate motion appears to break in great events every few hundred years and is more advanced in the cycle of strain accumulation than the western knot the large size of these asperities results in the properties of these earthquakes being nearly invariant over many cycles of great shocks an unusual sequence of moderate size shocks within the eastern knot from 1940 to 1948 is an example of the type of precursory phenomenon that might precede a large earthquake

crustal strain near the big bend of the san andreas fault analysis of the los padres tehachapi trilateration networks california in the region of the los padres tehachapi geodetic network the san andreas fault saf changes its orientation by over 30 degree from n 40 degree w close to that predicted by plate motion for a transform boundary to n 73 degree w the strain orientation near the saf is consistent with right lateral shear along the fault with maximum shear rate of 0 38 plus minus 0 01 mu rad yr at n 63 degree w in contrast away from the saf the strain orientations on both sides of the fault are consistent with the plate motion direction with maximum shear rate of 0 19 plus minus 0 01 mu rad yr at n 44 degree w the strain rate does not drop off rapidly away from the fault and thus the area is fit by either a broad shear zone below the saf or a single fault with a relatively deep locking depth the fit to the line length data is poor for locking depth d less than 25 km for d of 25 km a buried slip rate of 30 plus minus 6 mm yr is estimated the authors also estimated buried slip for models that include the garlock and big pine faults in addition to the saf slip rates on other faults are poorly constrained bymore the los padres tehachapi network the best fitting garlock fault model had computed left lateral slip rate of 11 plus minus 2 mm yr below 10 km buried left lateral slip of 15 plus minus 6 mm yr on the big pine fault within the western transverse ranges provides significant reduction in line length residuals however deformation there may be more complicated than a single vertical fault a subhorizontal detachment on the southern side of the saf cannot be well constrained by these data the authors investigated the location of the saf and found that a vertical fault below the surface trace fits the data much better than either a dipping fault zone located south of the surface trace less

the san andreas fault system through the transverse ranges as illuminated by earthquakes the pattern of seismogenic faulting in the san gorgonio pass san bernardino basin area of the san andreas fault saf zone is mapped from about 7000 quality selected focal mechanisms 1981 nov 92 they are derived from phase data of the southern california network by a relocation procedure based on location dependent station corrections and by a grid search procedure about 3 4 of the mechanisms have been interpreted as planes of rupture or slip planes that delineate many distinct faults one of these faults is right lateral dips steeply northeast and is continuous trough the san gorgonio pass area we interpret it as the main branch of the saf a very large earthquake seems possible on this portion of the saf because of its continuity and because of the unusually deep reach of the seismicity 23 km southwest of this fault a volume of diffuse and persistent seismicity has a sharp downward cut off which may reflect a basal detachment dipping 20 northeast and intersecting the saf along the deepest seismicity northeast of the saf the floor of the seismicity is shallower by as much as 10 km stress characteristics are derived from slip planes in 10 selected subregions the regime is transpressional near the constricting bend of the saf at san gorgonio pass it is dominated by horizontal extension in the san bernardino basin area and it is again transpressional in the eastern san gorgonio mountains north of the cucamonga thrust the broad features of this zonation may be related to the intersection of the saf with the pinto mountain fault and with the san jacinto fault a change of fault kinematics in the yucaipa cluster coincides with the 1992 landers and big bear main shocks and may be a manifestation of static stress change

source mechanisms of three moderate california earthquakes of july 1986 abstract we studied the source processes of three moderate size earthquakes in california the m s 6 0 event of july 8 1986 near palm springs the m s 5 8 shock of july 13 1986 offshore of oceanside and the m s 6 2 event of july 21 1986 in the chalfant valley on the east side of the sierra nevada the centroidal source parameters are estimated by inversion of long period p and sh waves recorded at teleseismic distances by seismographs of worldwide networks a more detailed description of the source is then obtained by analyzing broadband seismograms the north palm springs earthquake had a simple rupture process most of the seismic moment was released in the first 3 sec in a single step the strike of the source fault plane follows the average trend of the banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks the faulting mechanism of the main shock is right lateral strike slip with a sizable reverse component that is consistent with uplift north of the banning fault the earthquake offshore of oceanside had a slightly more complex rupture process the far field source time function consists of two pulses that represent the release of most of the moment in the first 3 sec the source mechanism indicates reverse faulting with the compressional axis trending northeast almost perpendicular to the trend of mapped faults offshore the event does not appear to be associated with major mapped faults in the area but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike slip fault systems and the idea of a weak san andreas fault system the rupture process of the chalfant valley earthquake was yet more complicated the source time function indicates that in a 4 sec time window three asperities were broken the source mechanism indicates right lateral strike slip faulting with a small normal faulting component on a fault that dips approximately 50 southwest earthquake activity suggests that presently the deformation in the northern part of the owens valley which includes the chalfant valley is dominated by conjugate strike slip faults which strike obliquely to the main range bounding normal faults the waveforms indicate the presence of a low velocity zone at a depth of about 20 km in the crust in the chalfant valley a synthesis of information from waveform modeling first motion fault plane solutions and aftershock distributions indicates that the nucleation and or termination of rupture for the north palm springs and the chalfant valley events were controlled by preexisting fault geometry

morphotectonics of the iberian peninsula a morphotectonic interpretation of the relief of the iberian peninsula ip is presented based on an analysis of its gross forms several geological geophysical and geomorphological methods were used in order to build up a morphostructural sketch three main categories were established using the approach of rantsman 1979 territorial units 1 megablock 9 macroblocks 34 mesoblocks 1 374 blocks and 2 523 microblocks morphostructural alignments 2 first 8 second 20 third and 1 fourth rank and 43 knots between morphostructural alignments second fourth rank the main seismic activity is concentrated on the first and second rank lineaments and some important epicenters are located near the lineament intersections the origin of the earthquakes in the vicinity of such knots can be explained by the forcing pushing of macroblocks westwards the existence of earthquakes along the lineaments may be explained by tension from the present study it appears that earthquake occurrence in the ip is due principally to stress concentrations around morphotectonic zones a seismotectonic interpretation of the ip is also presented on this map three zones are distinguished a b c each of them with a different active level and dimensions

a new understanding of the provinces of the amazon craton based on integration of field mapping and u pb and sm nd geochronology abstract new conventional and sensitive high resolution ion microprobe zircon u pb dating has led to a new understanding of the subdivision and evolution of the amazon craton during precambrian time with major improvements and changes made to the previous rb sr based model the interpretation of u pb and sm nd isotopic data identifies eight main precambrian tectonic provinces in the craton with ages ranging from 3 1 to 0 99 ga some of the provinces were generated by accretional arc related processes carajas transamazonic tapajos parima and rondonia juruena and others by recycling of continental crust central amazon rio negro and sunsas the exposed archean crust is restricted to the east carajas and south amapa in brazil and north imataca in venezuela of the craton indicating that the amazon craton is largely a proterozoic crust the carajas imataca 3 10 2 53 ga and transamazonian 2 25 2 00 ga provinces are composed predominantly of granite greenstone terranes the tapajos parima 2 10 1 87 ga and rondonia juruena 1 75 1 47 ga provinces represent new crust added as orogenic belts while the rio negro 1 86 1 52 ga and sunsas 1 33 0 99 ga provinces originated mainly by magmatic tectonic recycling of the above two orogenic belts the only zone with a prominent northeast trend is the poorly known k mudku shear belt characterized by a 1 20 ga shear zone which deforms the rocks of at least three different provinces rio negro tapajos parima and transamazonic the central amazon province comprises mostly orosirian volcano plutonic rocks uatuma magmatism and is a terrane in which the exposed crustal structure and deformation are pluton related the sm nd tdm model ages and nd suggest that the central amazon province was generated by the partial melting of archean continental crust carajas province perhaps related to underplating that began at the end of the tapajos parima orogeny 1 88 1 86 ga

seismic behavior of the southern san andreas fault zone in the northern coachella valley california comparison of the 1948 and 1986 earthquake sequences moderate earthquakes in 1948 and 1986 that occurred along the southern san andreas fault zone are examined to help understand the subsurface geometry and seismic behavior of active fault segments in the northern coachella valley the 1986 north palm springs earthquake occurred between the banning and mission creek traces of the san andreas fault zone based on data from both portable and permanent stations aftershocks of the 1986 event define a nearly planar surface that strikes about n60 to 70 w is about 15 km long and dips northeast from near the banning surface trace at about 45 to 50 relocation of the mainshock using revised station corrections and a local velocity model inverted from the aftershock arrival times indicates a location at 34 n00 26 116 w36 34 and a focal depth of 10 4 km the first motion focal mechanism for the mainshock indicates pure right slip on a plane dipping northeast at 40 to 45 with a strike of n60 w previous moment tensor solutions e g hartzell 1989 exhibit significant oblique reverse slip on a plane with a more westerly strike and a seismic moment of 1 4 to 1 7 1018 n m mw 6 1 actual and synthetic wood anderson recordings indicate a local magnitude of ml 6 0 the 1948 desert hot springs earthquake produced waveforms similar to 1986 although amplitudes for 1948 are typically 20 to 30 larger the 1948 mainshock is relocated at 33 n55 2 116 w28 9 with a focal depth of 12 km a revised magnitude of ml 6 3 and an estimated seismic moment of 2 to 3 1018 n m based on arrival time information from close 20 km portable stations deployed in 1948 to 1949 the 1948 aftershock distribution is about 15 km long 8 to 10 km wide and abuts the 1986 aftershock sequence to the southeast along the banning fault the 1948 aftershock zone is consistent with a focal mechanism that exhibits predominantly right slip motion rake 169 on a plane that strikes n55 w and dips more steeply northeast at 60 to 70 in 1986 the horizontal extent of the rupture may have been controlled by secondary cross faults and possible changes in fault geometry while the down dip extent may have been controlled by the presence at depth of a regional detachment in 1948 the rupture likely had a steeper dip than in 1986 and extended southeast as far as the northern indio hills where the surface trace of the banning fault undergoes an approximate 7 change in strike these results indicate that the banning fault is nonvertical is likely segmented according to fault dip as well as fault strike and has been the primary locus of recent moderate sized earthquake activity in the northern coachella valley

transpression along the southern san andreas fault durmid hill california transpression is expressed at durmid hill in the development and subsequent rotation of en echelon folds and ongoing vertical uplift the transpression related strain field can be quantified by an analysis of 1 orientation of rotated fold axes which display an angle of about 20 between the san andreas fault and the fold axial traces 2 shortening perpendicular to the fold axes 40 60 3 extension parallel to the fold axes 58 68 measured using a boudinaged ash bed and 4 uplift determined from an antecedent stream 1 1 1 7 mm yr the topographic expression of durmid hill and the exposed stratigraphy the observed deformation may be approximated by a homogeneous transpressional deformation this model suggests that an estimated 9 14 of the total displacement predicted by plate motion models across the san andreas fault has been accommodated by the durmid hill deformation zone adjacent to the fault this off fault deformation is not directly coupled to coseismic or interseismic movement on the fault itself

reverse time migration of offset vertical seismic profiling data using the excitation time imaging condition to apply reverse time migration to prestack finite offset data from variable velocity media the standard time zero imaging condition must be generalized because each point in the image space has a different image time or times this generalization is the excitation time imaging condition in which each point is imaged at the one way traveltime from the source to that point reverse time migration with the excitation time imaging condition consists of three elements 1 computation of the imaging condition 2 extrapolation of the recorder wave field and 3 application of the imaging condition computation of the imaging condition for each point in the image is done by ray tracing from the source point this is equivalent to extrapolation of the source wave field through the medium extrapolation of the recorded wave field is done by an acoustic finite difference algorithm imaging is performed at each step of the finite difference extrapolation by extracting from the propagating wave field the

reverse time migration migration of stacked or zero offset sections is based on deriving the wave amplitude in space from wave field observations at the surface conventionally this calculation has been carried out through a depth extrapolation we examine the alternative of carrying out the migration through a reverse time extrapolation this approach may offer improvements over existing migration methods especially in cases of steeply dipping structures with strong velocity contrasts this migration method is tested using appropriate synthetic data sets

landsliding during a m 7 7 earthquake influence of geology and topography the 1929 m 7 7 murchison earthquake in northwestern south island new zealand triggered at least 1850 landslides larger than 0 25 ha within a 1200 km 2 45 25 km area close to the epicenter the area surveyed in detail totals 25 of the area in which the earthquake initiated landslides about 2 5 10 8 m 3 of debris was transported by landslides in the area surveyed equivalent to an average debris yield of 210 000 m 3 km 2 landsliding was most common on well bedded and jointed calcareous tertiary mudstones and fine sandstones and was particularly concentrated on scarp slopes in areas of both gentle and moderate to steep dip on moderately to steeply dipping mudstones scarp slopes facing away from the origin of the seismic waves were particularly prone to landsliding large dip slope landslides were common on thick bedded tertiary sandstones landslides on granitic rocks were generally smaller shallower and composed of finer grained debris than those of sedimentary rocks

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

complexities of the san andreas fault near san gorgonio pass implications for large earthquakes 1 geologic relationships and patterns of crustal seismicity constrain the three dimensional geometry of the active portions of san andreas fault zone near san gorgonio pass southern california within a 20 km wide contractional stepover between two segments of the fault zone the san bernardino and coachella valley segments folds and dextral reverse and dextral normal faults form an east west belt of active structures the dominant active structure within the stepover is the san gorgonio pass garnet hill faults a dextral reverse fault system that dips moderately northward within the hanging wall block of the san gorgonio pass garnet hill fault system are subsidiary active dextral and dextral normal faults these faults relate in complex but understandable ways to the strike slip faults that bound the stepover the pattern of crustal seismicity beneath these structures includes a 5 8 km high east west striking step in the base of crustal seismicity which corresponds to the downdip limit of rupture of the 1986 north palm springs earthquake we infer that this step has been produced by slip on the linked san gorgonio pass garnet hill coachella valley banning sgp gh cvb fault this association enables us to construct a structure contour map of the fault plane the large step in the base of seismicity downdip from the sgp gh cvb fault system probably reflects a several kilometers offset of the midcrustal brittle plastic transition u th he thermochronometry supports our interpretation that this south under north thickening of the crust has created the region s 3 km of topographic relief we conclude that future large earthquakes generated along the san andreas fault in this region will have a multiplicity of mostly specifiable sources having dimensions of 1 20 km two tasks in seismic hazard evaluation may now be attempted with greater confidence first the construction of synthetic seismograms that make useful predictions of ground shaking and second theoretical investigations of the role of this complexity in retarding the propagation of future seismic ruptures

well preserved late paleoproterozoic volcanic centers in the sao felix do xingu region amazonian craton brazil abstract the amazonian craton in the sao felix do xingu city southeast region of the para state north of brazil hosts exceptionally well preserved paleoproterozoic bimodal magmatic units grouped in the sobreiro and santa rosa formations these formations are correlated to the uatuma magmatic event which is largely distributed in the amazonian craton occupying more than 1 500 000 km 2 geological mapping and petrographical observations reveal distinct spectra of volcanic facies in both formations the basal calc alkaline sobreiro formation is composed mainly of andesitic and dacitic lava flows and associated volcaniclastic facies of autoclastic origin with subordinate pyroclastic flow deposits this formation shows inferred eruption style that is similar to those in flood basalt provinces with rare scutulum type lava shields the upper a type santa rosa formation was generated by multicyclic explosive and effusive episodes predominantly associated with large fissures and is materialized by voluminous ignimbrites with subordinated ash fall tuff crystal tuff lapilli tuff co ignimbritic breccias rhyolitic dikes and domes and associated granitic porphyries and equigranular granitic intrusions ignimbrite and rhyolite dikes reveal conspicuous vertical flow pattern pointing to a fissure controlled eruption similar to sierra madre occidental ignimbrite province the proposed evolutionary model for the sao felix do xingu units differs from those of other occurrences related to the uatuma magmatic event in the amazonian craton characterized by predominance of a type volcanism and contemporaneous granites

archean crustal sources for paleoproterozoic tin mineralized granites in the carajas province sse para brazil pb pb geochronology and nd isotope geochemistry whole rock and k feldspar pb single zircon pb evaporation and nd whole rock isotopic data are presented for granites of the velho guilherme intrusive suite and the volcanic rocks of the uatumagroup in the carajas mineral province southern para brazil pb pb zircon ages of 18679 4 ma 18629 16 ma and 18669 3 ma for the antonio vicente mocambo and rio xingu granite massifs respectively show that the granites were emplaced at 1870 ma pb pb whole rock reference age of the uatumavolcanic rocks 18759 158 ma is comparable within the limits of the error 1870 ma thus represents the time of an important magmatic event in the eastern part of the amazonian craton nd model ages of the granites 3 0 3 2 ga and volcanic rocks 2 9 3 1 ga and their strongly negative initial ond values indicate that the magmas were derived from mesoarchean rocks with a long crustal residence time a mantle contribution cannot be completely ruled out however 2002 elsevier science b v all rights reserved

investigation of earthquake triggering for deep seated landslides taranaki new zealand because large earthquakes can trigger landslides the landslide record has potential use for establishing the palaeoseismicity of a region however before this potential can be realized other possible landslide triggering mechanisms must be investigated in southeastern taranaki new zealand over 100 deep seated complex rock slumps occur in a region of steeply dissected gently dipping pliocene marine sediments three age populations can be detected by using relative and radiometric dating methods the youngest group of landslides displays a consistent depth of soil development lacks an integrated drainage network dams catchments to produce lakes and has well defined mass movement features this group covers an area of 1015 km2 yields radiocarbon dates of around 1300 bp and is clearly distinguishable from the two older groups of landslides the synchronicity of landslide formation over a wide area before new zealand was populated rules out human factors and slope undercutting as triggering agents comparison with modern analogues also rules out rainstorm as a triggering agent principally on the basis of depth of movement the distribution of landslides and the amount of failed surface is shown to be closely related to the location of active fault traces the increase in landsliding towards the fault cannot be explained by variation in lithology or structure limiting equilibrium analysis of one landslide further points to the need for earthquake shaking to induce movement together the lines of evidence investigated provide a strong case for earthquake triggering of prehistoric landslides in this region

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

paleoproterozoic andesitic volcanism in the southern amazonian craton the sobreiro formation new insights from lithofacies analysis of the volcaniclastic sequences abstract the sobreiro formation sf records one excellent and well preserved example of subaerial precambrian ca 1 88 ga volcanism on earth it is located in the sao felix do xingu region sfx in the eastern part of para state southern amazonian craton northern of brazil the high k calc alkaline composition of the sobreiro rocks indicates that this formation likely generated in an ocean continent convergent margin this paper documents the architecture of a series of basaltic andesite to andesite and minor dacite lava flows and associated volcaniclastic rocks these rocks are divided into primary and secondary lithotypes depending if they resulted from a direct volcanic activity pyroclastic or reworked processes and massive to stratified depending on the different transport and emplacement mechanisms primary lithofacies is formed of pyroclastic flow and surge deposits volcanic breccias and welded ignimbrites secondary volcaniclastic lithofacies consist of reworked debris mass flows hyperconcentrated flows and stream floods are interpreted as fluvial alluvial deposits representing periods of stream and river reworking and re establishment after an eruptive phase or an edifice failure event these different lithofacies record the volcanic history of the sobreiro formation a complex volcanic environment is thought to have existed with emission of large lava flows and explosive eruptions the modern volcanological approach used here can serve as a model for the evolution of precambrian volcano sedimentary basins our approach sheds new light on the different processes operating on volcanic edifices and to constrain the depositional environment and thus geodynamic setting of precambrian continental volcanic belts

estimation of interval anisotropy parameters using velocity independent layer stripping moveout analysis of long spread p wave data is widely used toestimatethekeytime processingparameter inlayeredtransversely isotropic media with a vertical symmetry axis vti inversion for interval values however suffers from instability caused by the trade off between the effective moveout parameters and by subsequent error amplification during dix type layer stripping we propose an alternative approach to nonhyperbolic moveout inversion based on the velocity independent layerstripping vils method of dewangan and tsvankin also we developthe3dversionofvilsandapplyittointervalparameter estimation in orthorhombic media using wide azimuth longspreaddata iftheoverburdenislaterallyhomogeneousandhasa horizontal symmetry plane vils produces the exact interval traveltime offset function in the target layer without knowledge of the velocity field hence dix type differentiation of moveout parameters used in existing techniques is replaced by the much morestablelayerstrippingofreflectiontraveltimes theinterval traveltimes are then inverted for the moveout parameters using the single layer nonhyperbolic moveout equation the superior accuracy and stability of the algorithm are illustrated on raytraced synthetic data for typical vti and orthorhombic models even small correlated noise in reflection traveltimes causes substantial distortions in the interval values computed by conventional dix type differentiation in contrast the output ofvils is insensitive to mild correlated traveltime errors the algorithm is also tested on wide azimuth p wave reflection data recorded above a fractured reservoir at rulison field in colorado the interval moveout parameters estimated by vils in the shale layer above the reservoir are more plausible and less influenced by noisethanthoseobtainedbythedix typemethod

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

irregular recurrence of large earthquakes along the san andreas fault evidence from trees old trees growing along the san andreas fault near wrightwood california record in their annual ring width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813 paleoseismic data and historical information indicate that this event was the san juan capistrano earthquake of 8 december 1812 with a magnitude of 7 5 the discovery that at least 12 kilometers of the mojave segment of the san andreas fault ruptured in 1812 only 44 years before the great january 1857 rupture demonstrates that intervals between large earthquakes on this part of the fault are highly variable this variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of san andreas fault dynamics

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

influence of fault bends on ruptures data on slip along the san andreas fault have been examined to understand the influence of a bend in the fault system on the extent of slip release a correlation of slip along the fault in the great 1906 and 1857 earthquakes with fault geometry shows that maximum slip occurred near a major change in the fault strike and may be a function of this change the data appear to suggest that in general it takes about 10 years of strain accumulation to negotiate a 1 change in fault strike assuming a slip rate of about 3 cm yr seismic hazard along the san andreas fault may therefore also be a function of fault geometry

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

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

vertical deformation 1952 1987 in the salton trough california from water level recordings analysis of 36 years of water level data from three sites on the shore of the salton sea reveals a history of vertical deformation in the salton trough data from two sites allow estimation of tilt along a 36 km nw se baseline between 1952 and 1970 since 1970 the addition of a 13 km wnw ese baseline allows the full tilt vector to be estimated we find down to the southeast tilting at an average rate of about 0 01 0 01 rad yr from the early 1960s to 1987 an anomalous period of more rapid 0 2 0 04 rad yr tilting occurs between the mid 1950s and early 1960s but this coincides with a period from which we have been unable to examine all the original data a previous analysis of the 1952 1978 data suggested a precursory increase in tilt rate prior to the 1968 borrego mountain earthquake and a tilt reversal in 1972 we show that there was no significant tilt change prior to the 1968 earthquake and that the apparent 1972 reversal resulted from uncorrected offsets in the water level observations tilts derived from water level and from reliable first order leveling surveys show limited agreement but the leveling data contain evidence for nonuniform tilting recent analyses of leveling data by others using different techniques show 20 30 cm uplift and subsidence events in the salton trough between 1968 and 1978 our data constrain the relative elevations of the three water level sites to be stable within 1 2 cm during these proposed events

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

palaeoseismology in steep terrain the big bend of the san andreas fault transverse ranges california transpressional tectonics are typically associated with restraining bends on major active strike slip faults resulting in uplift and steep terrain this produces dynamic erosional and depositional conditions and difficulties for established lines of palaeoseismological investigation consequently in these areas data are lacking to determine tectonic behaviour and future hazard potential along these important fault segments the big bend of the san andreas fault in the transverse ranges of southern california exemplifies these problems however landslides probably seismically triggered are widespread in the rugged terrain of the big bend fluvial reworking of these deposits rapidly produces geomorphic planes and lines that are markers for subsequent fault slip the most useful are offset and abandoned stream channels for these are relatively high precision markers for identifying individual faulting events palaeoseismological studies from the central big bend involving 14c ages of charcoal fragments from trench exposures illustrate these points and indicate that the past three faulting events including the great 1857 earthquake were relatively similar in scale each producing offsets of about 7 7 5 m the mean recurrence interval is 140 220 years the pre 1857 event here may be the 1812 event documented south of the big bend or an event which took place probably between 1630 and 1690 ground breakage in both events extended south of the big bend unlike the 1857 event where rupture was skewed to the north the preceding faulting event ruptured both to the north and south of the big bend and probably occurred between 1465 and 1495 all these events centred on the big bend and may be typical for this fault segment suggesting that models of uniform long term slip rates may not be applicable to the south central san andreas a slip rate estimate of 34 51 mm a 1 for the central big bend although uncertain may also imply higher slip in the big bend and highlights difficulties in correlating slip rates between sites with different tectonic settings slip rates on the san andreas may increase within the broad compressional tectonics zone of the big bend compared to the north and south where the plate boundary is a relatively linear and sub parallel series of dominantly strike slip faults partitioning slip within the big bend is inherently uncertain and insufficient suitably comparable data are available to sustain a uniform slip model although such models are a common working assumption

paleoseismic evidence of clustered earthquakes on the san andreas fault in the carrizo plain california exposures we have excavated across the san andreas fault contradict the hypothesis that part of the fault in the carrizo plain is unusually strong and experiences relatively infrequent rupture the exposures record evidence of at least seven surface rupturing earthquakes which have been approximately dated by accelerated mass spectrometry radiocarbon analysis of detrital charcoal and buried in situ plants five large earthquakes have occurred since 1218 a d the most recent earthquake event a was the 1857 fort tejon earthquake which we have associated with 6 6 10 m of dextral slip along the main fault trace the penultimate earthquake event b most likely occurred within the period a d 1405 1510 slip from either events b and c combined or from event b alone totals 7 11 m three earthquakes events c d and e occurred in a temporal cluster prior to event b and after approximately a d 1218 the average recurrence interval within this cluster is 73 116 years depending on assumptions events f and g occurred after 200 years b c a depositional hiatus between events e and f may hide evidence of additional earthquakes events b and d within the carrizo cluster of a d 1218 1510 may correlate with events t a d 1329 1363 and v a d 1465 1495 at pallett creek on the mojave segment of the fault this suggests two fault ruptures similar in length to that of 1857 events c and e apparently did not rupture the mojave section which suggests that the carrizo segment has ruptured independently or in combination with segments to the north irregular repeat times of large earthquakes suggest a pattern of clustered events at the end of seismic supercycles

geophysical evidence for wedging in the san gorgonio pass structural knot southern san andreas fault zone southern california geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the san gorgonio pass region this picture serves as the basis for inferring past fault movements within the san andreas system which are fundamental to understanding the tectonic evolution of the san gorgonio pass region interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of san gorgonio pass beneath basement rocks of the southeast san bernardino mountains subtle long wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of san gorgonio pass and south under peninsular ranges basement and has a southern edge that is roughly parallel to but 5 6 km south of the surface trace of the banning fault this deep magnetic body is composed either of upper plate rocks of san gabriel mountains basement or rocks of san bernardino mountains basement or both we suggest that transpression across the san gorgonio pass region drove a wedge of peninsular ranges basement and its overlying sedimentary cover northward into the san bernardino mountains during the neogene offsetting the banning fault at shallow depth average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data seismicity suggests a deeper detachment surface beneath the deep magnetic body this interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross sectional view given the multilayered nature of deformation it is unlikely that the san andreas fault will rupture cleanly through the complex structures in san gorgonio pass

three dimensional simulation of the near fault ground motion for the 1995 hyogo ken nanbu kobe japan earthquake abstract the 17 january 1995 hyogo ken nanbu earthquake is a typical example showing that the ground motions along basin edge faults can be very destructive in this study we simulate the near fault ground motion from this earthquake based on a kinematic fault model and a simplified 3d velocity structure of the kobe area the kinematic earthquake rupture and the wave propagation are modeled using a 3d finite difference method fdm our simulation identifies the basin edge effect as an important factor that influenced the ground motion amplification pattern in the kobe area we found that the coupling of the source directivity and basin edge effects causes impulsive ground motions with extremely high amplitude at periods greater than 1 sec and in a narrow zone offset less than 1 km from the basin edge the combination of these effects acted to create a fairly continuous band of amplification that extends about 30 km in an elongated zone parallel to the basin edge boundary in some areas localized site effects might have been as important as the abovementioned effects but they cannot explain the continuity of the extended east west zone of damage

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

spatial and temporal zoning of hydrothermal alteration and mineralization in the sossego iron oxide copper gold deposit carajas mineral province brazil paragenesis and stable isotope constraints the sossego iron oxide copper gold deposit 245 mt 1 1 cu 0 28 g t au in the carajas mineral province of brazil consists of two major groups of orebodies pista sequeirinho baiano and sossego curral with distinct alteration assemblages that are separated from each other by a major high angle fault the deposit is located along a regional wnw ese striking shear zone that defines the contact between metavolcano sedimentary units of the 2 76 ga itacaiunas supergroup and tonalitic to trondhjemitic gneisses and migmatites of the 2 8 ga xingu complex the deposit is hosted by granite granophyric granite gabbro and felsic metavolcanic rocks the pista sequeirinho baiano orebodies have undergone regional sodic albite hematite alteration and later sodic calcic actinolite rich alteration associated with the formation of massive magnetite apatite bodies both these alteration assemblages display ductile to ductile brittle fabrics they are cut by spatially restricted zones of potassic biotite and potassium feldspar alteration that grades outward to chlorite rich assemblages the sossego curral orebodies contain weakly developed early albitic alteration and very poorly developed subsequent calcic sodic alteration these orebodies contain well developed potassic alteration assemblages that were formed during brittle deformation that resulted in the formation of breccia bodies breccia matrix commonly displays coarse mineral infill suggestive of growth into open space sulfides in both groups of deposits were precipitated first with potassic alteration and more importantly with a later assemblage of calcite quartz epidote chlorite in the sequeirinho orebodies sulfides range from undeformed to deformed sulfides in the sossego curral orebodies are undeformed very late weakly mineralized hydrolytic alteration is present in the sossego currral orebodies the sulfide assemblage is dominated by chalcopyrite with subsidiary siegenite and millerite pyrrhotite and pyrite are minor constituents of ore in the sequerinho orebodies while pyrite is relatively abundant in the sossego curral bodies oxygen isotope partitioning between mineral pairs constrains temperatures in the deposit spatially and through time in the sequeirinho orebody the early sodic calcic alteration stage was characterized by temperatures exceeding 500 c and delta text 18 text o text h text 2 text o values for the alteration fluid of 6 9 0 9 temperature declines outward and upward from the zone of most intense alteration paragenetically later copper gold mineralization displays markedly lower temperatures 300 c and was characterized by the introduction of 18o depleted hydrothermal fluids 1 8 3 4 the calculated dh2o and delta text 18 text o text h text 2 text o values suggest that the fluids that formed the early calcic sodic alteration assemblage were of formational metamorphic or magmatic origin the decrease of delta text 18 text o text h text 2 text o values through time may reflect influx of surficially derived waters during later alteration and mineralization events influx of such fluids could be related to episodic fluid overpressure resulting in dilution and cooling of the metalliferous fluid causing deposition of metals transported as metal chloride complexes

changes in state of stress on the southern san andreas fault resulting from the california earthquake sequence of april to june 1992 the april to june 1992 landers earthquake sequence in southern california modified the state of stress along nearby segments of the san andreas fault causing a 50 kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near san gorgonio pass the decrease in compressive normal stress may also have reduced fluid pressures along that fault segment as pressures are reequilibrated by diffusion that fault segment should move closer to failure with time that fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years

growth of wheeler ridge anticline california geomorphic evidence for fault bend folding behaviour during earthquakes abstract unique landforms which we term fold growth terraces and alluvial fan ridges are interpreted to be produced by kink band migration of the front limb of wheeler ridge anticline an active fault bend fold in the southern san joaquin valley based on a folding related model these features form by onlap of alluvium across an active axial surface along the tip of a thrust wedge structure alluvial fan ridges record the sense of slip on the underlying blind thrust which is composed of both thrust and left lateral components our model predicts formation of fold growth terraces by upward propagation of an active axial surface through onlapped alluvium during a folding event to produce a prism of translated but otherwise undeformed sediment alternative hypotheses for the terraces include flexural slip faulting solifluction and erosive processes the width of fold limb between adjacent terraces is equivalent to fault displacement at depth which we interpret to be related to slip caused by earthquakes on the underlying blind thrust earthquake magnitudes derived from the terraces are much larger than expected for the length of the active fold and thrust belt and may indicate a kinematic link with the san andreas fault that drives shortening in the region

lateral offsets and revised dates of large prehistoric earthquakes at pallett creek southern california recent excavation and new radiocarbon dates of sediments at pallett creek are the basis for new conclusions regarding the late holocene history of the san andreas fault systematic dissection of a 50 m long 15 m wide 5 m deep volume of earth centered on the fault enables documentation in three dimensions of fault patterns lateral offsets and vertical deformation associated with large earthquakes of the past the excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 a d with an average recurrence interval of about 145 years prehistoric slip events that occurred in 1720 50 1550 70 1350 50 1080 65 and 845 75 a d have lateral offsets that are comparable to those of the most recent great earthquake of 1857 thus all of these events represent earthquakes of large magnitude the lateral offsets of two other events in 935 85 and 1015 100 a d are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events the new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years but suggest a monotonic decrease in individual intervals to below this range during the past 900 years on the basis of these data the probability of a large earthquake with surficial fault rupture at this site is between 0 2 and 5 during 1984 and 7 and 60 by the year 2000

earthquakes of the holocene geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events trenching and geomorphic analysis to define the timing and size of past earthquakes and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock this research is now being referred to as paleoseismology seismic geology and earthquake geology since the mid 1970s it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time hanks 1985 allen 1986 the present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquakes

paleoproterozoic volcanic centers of the sao felix do xingu region amazonian craton brazil hydrothermal alteration and metallogenetic potential abstract geological petrographic scanning electron microscopy and x ray diffraction studies revealed hydrothermalized lithotypes evidenced by overprinted zones of potassic propylitic sericitic and intermediate argillic alterations types with pervasive and fracture controlled styles in paleoproterozoic volcano plutonic units of the sao felix do xingu region amazonian craton northern brazil the sobreiro formation presents propylitic epidote chlorite carbonate clinozoisite sericite quartz albite hematite pyrite sericitic sericite quartz carbonate and potassic potassic feldspar hematite alterations the prehnite pumpellyite pair that is common in geothermal fields also occurs in this unit the santa rosa formation shows mainly potassic biotite microcline magnetite sericitic sericite quartz carbonate chlorite gold and intermediate argillic montmorillonite kaolinite halloysite illite alterations these findings strongly suggest the involvement of magma sourced and meteoric fluids and draw attention to the metallogenetic potential of these volcanic units for paleoproterozoic epithermal and rare and base metal porphyry type mineralizations similar to those already identified in other portions of the amazonian craton