the palos verdes anticlinorium along the los angeles california coast implications for underlying thrust faulting 1 the palos verdes anticlinorium pva is a 70 km wide nw trending pliocene quaternary transpressional high that separates the onshore los angeles basin from the offshore san pedro basin the pva southwest limb underlies a 70 km long sw facing seafloor escarpment this escarpment is separated into two segments by a 6 km right step it overlies two levels of ne dipping low angle thrust faults these faults project beneath los angeles toward known sw verging blind thrust faults and may be the same regional faults these conclusions are based on a detailed geometric representation constructed from a regional analysis of 5000 km of multichannel seismic reflection profiles logs and paleontology from 19 wells published seafloor geology and multibeam bathymetric data patterns of sedimentation and erosion indicate that folding of pva initiated during pliocene time and propagated southward with folding of part of the southern pva occurring during the quaternary structural relief continues to grow with regional long term rock uplift of the pva crest and ongoing subsidence in the adjacent basins the low angle ne dipping faults beneath the pva must be late quaternary active in order to maintain hanging wall rock uplift above subsiding footwalls structural geometry requires that the underlying faults have slipped at 1 1 1 6 mm yr over quaternary time

prospects for larger or more frequent earthquakes in the los angeles metropolitan region far too few moderate earthquakes have occurred within the los angeles california metropolitan region during the 200 year long historic period to account for observed strain accumulation indicating that the historic era represents either a lull between clusters of moderate earthquakes or part of a centuries long interseismic period between much larger moment magnitude m w 7 2 to 7 6 events geologic slip rates and relations between moment magnitude average coseismic slip and rupture area show that either of these hypotheses is possible but that the latter is the more plausible of the two the average time between m w 7 2 to 7 6 earthquakes from a combination of six fault systems within the metropolitan area was estimated to be about 140 years

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

structural evolution of the northern los angeles basin california based on growth strata subsurface structure contour maps and cross sections show that the northern los angeles basin is underlain by a south facing monocline complicated by secondary faults and folds the monocline and associated structures form a structural high the northern los angeles shelf which marks the northern margin of the los angeles central trough analysis of growth strata sediments deposited during structural growth shows that during the miocene the predominant structural style was extension growth strata show that at approximately the beginning of the pliocene 5 ma contraction reactivated the miocene normal faults as reverse faults and formed the monocline its secondary structures and the los angeles central trough lensing out of earliest pliocene delmontian stage growth sandstones onto the flanks of the monocline and secondary structures the south salt lake the east beverly hills and the las cienegas anticlines indicates that these contractional structures all began to form at this time all of the secondary structures became largely inactive prior to the deposition of upper pico member of early pleistocene age 1 2 ma however thick accumulations of growth strata of the upper pico member of the fernando formation within the los angeles central trough attest to continued monoclinal folding after the secondary structures became largely inactive the growth strata record both vertical and horizontal components of structural growth of the monocline so that the dip of the blind fault zone causing the monocline the los angeles fault can be calculated in the east beverly hills area the dip of the los angeles fault is 61 and at las cienegas the dip is 62 these are maximum values based on the assumption that the growth strata bed lengths record all shortening the pliocene pleistocene average fault slip rates for the los angeles fault are 1 1 1 3 mm yr in the east beverly hills and 1 3 1 5 mm yr at las cienegas the resulting pliocene pleistocene horizontal convergence rates are 0 5 0 6 mm yr and 0 6 0 7 mm yr respectively the pliocene pleistocene growth strata show increased dip with age indicating that the monocline grew by progressive limb rotation rather than by kink band migration therefore fault bend and fault propagation fold models based on kink band migration are inadmissible solutions to explain the growth of the monocline we suggest a basement involved shear zone to explain the geometry of the northern los angeles shelf

the 1930 santa monica and the 1979 malibu california earthquakes the malibu earthquake m l 5 0 that occurred 1 january 1979 in santa monica bay was located at 33 56 9 n and 118 41 3 w and at a depth of 10 km the focal mechanism of the main shock as determined from first motions recorded by short period high gain seismic stations in southern california shows reverse faulting with one plane dipping 55 to 60 to the north striking 275 to 290 and the second plane dipping 30 to 38 to the south and striking 100 to 120 more than 400 aftershocks were recorded during 1979 focal mechanisms for 20 aftershocks of m 3 0 show similar reverse mechanisms as the main shock the main shock and aftershocks m 2 5 were relocated using a local velocity model and a set of corresponding station delays the aftershocks define a volumetric hypocentral zone 4 km 6 km west trending with depths ranging from 6 to 11 km this 1979 malibu main shock aftershock sequence probably occurred on the eastern end of the anacapa dume fault the santa monica earthquake m l 5 2 that occurred 31 august 1930 was relocated using station delays calculated from the 1979 date set the relocated 1930 epicenter is located near the western end of the santa monica fault the 1979 and 1930 main shocks can be interpreted to define a 8 to 10 km north south sidestep between the anacapa dume and santa monica faults possible segmentation of these two fault systems suggests that they are unlikely to rupture simultaneously in one large earthquake

magnetic fabrics and fluid flow directions in hydrothermal systems a case study in the chaillac ba f fe deposits france this study presents a possible use of anisotropy of magnetic susceptibility ams to describe the mineralizing process in hydrothermal systems ba f fe rich deposits within the chaillac basin are on the southern border of the paris basin in these deposits hydrothermal textures and tectonic structures have been described in veins sinters and sandstone cemented by hydrothermal goethite 278 oriented cores from 24 sites have been collected in these formations in addition a lateritic duricrust superimposed on the hydrothermal formation has been sampled rock magnetic investigations show that the principal magnetic carrier is goethite for the hydrothermal mineralization and for the laterite level the ams measurements show distinguishable behaviors in the different mineralogical and geological contexts the k1 magnetic lineation maximum axis is strongly inclined for the vertical veins for the horizontally mineralized sinters the magnetic lineation is almost horizontal with an azimuth similar to the sedimentary flow direction the ams of goethite rich sandstone close to the veins shows strongly inclined k1 as they are probably influenced by the vertical veins however when the distance from the vein is larger than 1 m the ams presents rather horizontal k1 directions parallel to the sedimentary flow the laterite has a foliation dominance of ams with vertically well grouped k3 axes and scattered k1 and k2 axes field structural observations suggest that the ore deposit is mainly controlled by ew extension tectonics associated with ns trending normal faults combining the ams results on the deposit with vein textures and field data a model is proposed in which ams results are interpreted in terms of hydrothermal fluid flow this work opens a new investigation field to constrain hydrodynamic models using the ams method textural study combined with efficient ams fabric measurements should be used for systematic investigation to trace flow direction in fissures and in sand porosity

seismic reflection and geomorphic characterization of the onshore palos verdes fault zone los angeles california abstract two high resolution 25 to 110 hz bandwidth seismic reflection profiles acquired along roads crossing the palos verdes fault zone los angeles basin california strongly suggest that some geomorphic lineaments seen on aerial photographs coincide with steeply dipping faults at roughly the midpoint of the palos verdes peninsula the palos verdes fault zone consists of at least three major strands one of the northern strands which has the most apparent vertical displacement is interpreted to dip 55 sw down to 200 m steepening at greater depth other strands appear to dip from 50 to 90 just 3 km to the southeast however the fault zone is composed of at least five strands and is roughly 1 5 km wide apparently younger strands are located northeast of older less active strands across the fault zone recent right lateral offset on the northeastern fault strands is inferred from a deflection of an ancestral los angeles river channel approximately 1 8 km west of the los angeles harbor the channel incises deposits dated to be as young as 120 to 80 ka and is deflected approximately 300 m this deflection if attributed solely to slip on the palos verdes fault zone suggests an average horizontal slip rate between 2 5 and 3 8 mm yr because very little vertical displacement is detectable in the seismic data acquired near the channel offset that also shows little vertical displacement we infer that recent onshore movement in the palos verdes fault zone on the eastern palos verdes peninsula has been predominantly strike slip

methanogenic calcite 13c depleted bivalve shells and gas hydrate from a mud volcano offshore southern california methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern california in 800 m of water bivalves authigenic calcite and methane hydrate were recovered in a 2 1 m piston core aragonite shells of two bivalve species are unusually depleted in 13c to 19 13c the most 13c depleted shells of marine macrofauna yet discovered carbon isotopes for both living and dead specimens indicate that they used in part carbon derived from anaerobically oxidized methane to construct their shells the 13c values are highly variable but most are within the range 12 to 19 this variability may be diagnostic for identifying cold seep hydrate systems in the geologic record authigenic calcite is abundant in the cores down to 1 5 m subbottom the top of the methane hydrate the calcite is depleted in 13c 13c 46 to 58 indicating that carbon produced by anaerobically oxidized methane is the main source of the calcite methane sources include a geologic hydrocarbon reservoir from miocene source rocks and biogenic and thermogenic degradation of organic matter in basin sediments oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water under the influence of gas hydrate dissociation and strong methane flux high metal content in the mud volcano sediment indicates leaching of basement rocks by fluid circulating along an underlying fault which also allows for a high flux of fossil methane

measuring vertical tectonic motion at the intersection of the santa cruz catalina ridge and northern channel islands platform california continental borderland using submerged paleoshorelines we used submerged paleoshorelines as strain markers to investigate holocene and late pleistocene vertical tectonic movement at the intersection of the offshore santa cruz catalina ridge with the southern boundary of the western transverse ranges within the california continental borderland past submerged shoreline positions were identified using high resolution multibeam bathymetry side scan sonar submersible observations and the presence of intertidal and subtidal invertebrate fossils numerous accelerator mass spectrometry ams 14 c ages of shells from these paleoshorelines were found to be between 27 000 yr radiocarbon rc and 11 500 yr before present indicative of shoreline colonization during and following the last glacial maximum lgm establishing these paleoshorelines as a usable datum for measuring vertical change since this time removal of the nontectonic component of vertical change using an ice volume equivalent eustatic sea level compilation indicates between 20 and 45 m of uplift of the eastern part of the northern channel islands block since the lgm lowstand resulting in an uplift rate of 1 50 0 59 mm yr over the last 23 k y this rate closely matches uplift predicted by published slip rates for the channel islands thrust which underlies the northern channel islands platform results from post lgm shoreline features on pilgrim banks are somewhat more ambiguous submarine paleoshoreline uplift together with the extensive upper crustal fold thrust style of deformation illustrates the transpressional interaction of the borderland and the western transverse ranges blocks where the santa cruz catalina ridge and northern channel islands intersect

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

the serra pelada au pge deposit serra dos carajas para state brazil geological and geochemical indications for a composite mineralising process the serra pelada au pge rich deposit is located in the serra dos carajas a leading mining area in brazil this region is characterised by a complex geological and structural framework and is affected by deep lateritisation which has lasted for more than 70 ma the serra pelada deposit is emplaced in a late archean low grade metasedimentary sequence rio fresco aguas claras formation which is host to other gold deposits in the region igarape bahia aguas claras the rio fresco aguas claras sequence was deposited in tectonic basins developed on archean basement and au bearing greenstone terranes which were intruded by pge rich layered mafic complexes e g luanga the serra pelada mineralisation is located along a regional complex system of strike slip faults cinzento carajas systems which were active during the late archean to early proterozoic the mineralisation appears to be concentrated along a faulted hinge zone of a fold ore zone rock facies are dominated by low grade ferruginous to carbonaceous metasiltstones and minor sandstones locally brecciated and cemented by quartz sulphide stockwork supergene alteration led to partial to total transformation into friable aggregates of kaolinite fe oxide hydroxides silica and secondary phosphate sulphates even at depths exceeding 200 m precious metals are exceptionally enriched with up to more than 1 000 ppm au pge in some peculiar ferruginous graphitic zones locally called hidrotermalito geochemistry shows complex patterns of major and trace elements particularly rare earth elements ree in mineralised vs nonmineralised samples these patterns are interpreted in terms of variable degree of superposition of hydrothermal and supergene alteration precious metals show progressive increase from samples with hydrothermal imprint to samples with supergene imprint the geological evolution of the carajas region and the characteristics of mineralisation at serra pelada may suggest a composite mineralising process hydrothermal activity by fluids likely originated from granitoids was followed by supergene alteration during long lasting lateritisation to develop extreme precious metal enrichments in a geological context probably already anomalous for au and pge

paleoseismology and geomorphology of the raymond fault los angeles county california the raymond fault extends for 20 km in a broad convex to the south arc across the densely populated san gabriel valley northeast of los angeles although south facing scarps characterize much of the fault geomorphic and seismological evidence indicate a predominance of left lateral motion an apparent 3 4 km left lateral offset of a crystalline basement ridge at the east end of the fault may approximate total slip on the fault data from one of our paleoseismologic trenches in combination with published data indicate that the most recent raymond fault surface rupture occurred 1 2 ka estimates of fault plane area suggest that rupture of the entire raymond fault could generate an earthquake of m w 6 7 the similar kinematics of the raymond fault and the hollywood fault along strike to the west together with a pronounced fault parallel gravity lineament and sparse geomorphic evidence suggests that there may be a through going mechanical connection between the two faults and that they could conceivably rupture together in large earthquakes published data however indicate that the hollywood fault ruptured to the surface most recently 6 9 ka indicating that the most recent raymond fault surface rupture apparently did not extend westward onto the hollywood fault data from another of our paleoseismologic trenches yielded evidence for at least five latest pleistocene earthquakes including at least four surface ruptures that occurred during a brief 10 000 year long period between 31 5 and 41 5 ka the 3300 year long average recurrence interval for these events is much shorter than the interval suggested for the past 40 000 years by the frequency of paleoearthquakes recognized in previous trenches thus either the 31 5 ka to 41 5 ka events represent a temporal cluster and the recurrence interval for the fault is highly irregular or at least half of all raymond fault earthquakes that have occurred since 31 5 ka have not yet been recognized

geology of the continental margin beneath santa monica bay southern california from seismic reflection data we interpret seismic reflection data which were collected in santa monica bay using a 70 in 3 generator injector air gun to show the geologic structure of the continental shelf and slope and of the deep water santa monica and san pedro basins the goal of this research is to investigate the earthquake hazard posed to urban areas by offshore faults these data reveal that northwest of the palos verdes peninsula the palos verdes fault neither offsets the seafloor nor cuts through an undeformed sediment apron that postdates the last sea level rise other evidence indicates that this fault extends northwest beneath the shelf in the deep subsurface however other major faults in the study area such as the dume and san pedro basin faults were active recently as indicated by an arched seafloor and offset shallow sediment rocks under the lower continental slope are deformed to differing degrees on opposite sides of santa monica canyon northwest of this canyon the continental slope is underlain by a little deformed sediment apron the main structures that deform this apron are two lower slope anticlines that extend toward point dume and are cored by faults showing reverse or thrust separation southeast of santa monica canyon lower slope rocks are deformed by a complex arrangement of strike slip normal and reverse faults the san pedro escarpment rises abruptly along the southeast side of santa monica canyon reverse faults and folds underpinning this escarpment steepen progressively southeastward locally they form flower structures and cut downward into basement rocks these faults merge downward with the san pedro basin fault zone which is nearly vertical and strike slip the escarpment and its attendant structures diverge from this strike slip fault zone and extend for 60 km along the margin separating the continental shelf from the deep water basins the deep water santa monica basin has large extent but is filled with only a thin less than 1 5 km section of what are probably post miocene rocks and sediment extrapolating ages obtained from ocean drilling program site 1015 indicates that this sedimentary cover is quaternary possibly no older than 600 ka folds and faults along the base of the san pedro escarpment began to form during 8 13 ka ago refraction velocity data show that high velocity rocks probably the catalina schist or miocene volcanic rocks underlie the sedimentary section the san pedro basin developed along a strike slip fault widens to the southeast and is deformed by faults having apparent reverse separation and by folds near redondo canyon and the palos verdes peninsula

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

upper crustal structure and neogene tectonic development of the california continental borderland multichannel seismic reflection data sonobuoy seismic refraction data and regional geology are used to define the upper crustal structure of the southern california continental borderland and to delineate the characteristics of the main lithotectonic belts of the region the catalina schist belt is separated on its west side from the gently deformed nicolas forearc belt by faults that have steep west dips and pronounced normal separations on its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the peninsular ranges belt at the coastline near oceanside the catalina schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early miocene time in conjunction with about 10 of clockwise rotation of the western transverse ranges belt part of the uplift of the catalina schist could have occurred on the detachment fault but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt a large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western transverse ranges and nicolas forearc belts extension accompanied by probable large amounts of right slip continued in the borderland region during and after middle miocene time the later stage of extension was accompanied by rapid clockwise rotation of the western transverse ranges of at least 90 most of the borderland including the belt of schist that was uplifted in early miocene time was further deformed into numerous basins and ridges during this stage of oblique extension the primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the pacific plate after it had become coupled to the farallon plate system which had previously been subducted beneath the borderland

holocene to pliocene tectonic evolution of the region offshore of the los angeles urban corridor southern california 1 quaternary tectonism in the coastal belt of the los angeles urban corridor is diverse in this paper we report the results of studies of multibeam bathymetry and a network of seismic reflection profiles that have been aimed at deciphering the diverse tectonism and at evaluating the relevance of published explanations of the region s tectonic history rapid uplift subsidence in basins folds and thrusts extensional faulting and strike slip faulting have all been active at one place or another throughout the quaternary period the tectonic strain is reflected in the modern physiography at all scales los angeles la basin has filled from a deep submarine basin to its present condition with sediment impounded behind a large sill formed behind uplifts near the present shoreline newport trough to the south southeast of la basin also accumulated a large volume of sediment but remained at midbathyal depths throughout the period there is little or no evidence of quaternary extensional tectonism in either basin although as much as 6 km of subsidence which mainly occurred by sagging has been recorded in places since the middle miocene the uplifts include folded and thrust faulted terranes in the palos verdes hills and the shelves of santa monica and san pedro bays the uplifted areas have been shortened in a southwest northeast direction by 10 or slightly more and some folds are reflected in the bathymetry two large adjacent midbathyal basins santa monica and san pedro show strong evidence of subsidence and slight west northwest extension 10 during the same time folding was taking place in the uplifts the tectonic boundaries between uplifts and basins are folded normal faulted reverse faulted and strike slip faulted depending on location the rapid quaternary uplift and subsidence along with the filling of la basin have produced a reversal in the regional physiography in the early pliocene la basin was a submarine deep palos verdes and the shelves comprised a northeast basin slope and the present offshore basins and catalina island formed an emergent or shallowly submerged shelf since extensional compressional and lateral strains are all locally in evidence simple notions that this part of southern california underwent a change from miocene transtension to quaternary transpression fail to explain our observations

an elusive blind thrust fault beneath metropolitan los angeles seismic reflection profiles petroleum wells and relocated earthquakes reveal the presence of an active blind thrust fault beneath metropolitan los angeles a segment of this fault likely caused the 1987 whittier narrows magnitude 6 0 earthquake mapped sizes of other fault segments suggest that the system is capable of much larger magnitude 6 5 to 7 and more destructive earthquakes

fluid geochemistry of the serra pelada au pd pt deposit carajas brazil exceptional metal enrichment caused by deep reaching hydrothermal oxidation abstract serra pelada is a high grade hydrothermal au pd pt deposit within the carajas mineral province eastern amazon craton which shows an exceptional metal enrichment at carbon enriched hosted ores 1000 s ppm au pt pd the main orebody is structurally hosted at the hinge of a recumbent syncline consisting of dolomitic metasandstones at the outer part of the fold a layer of carbonaceous metasiltstone in which most orebodies are located and red siltstone in the core of the fold distal hydrothermal alteration comprises spatially separated chlorite carbon reduced and hematite oxidized alteration zones the mineralized zones include silicification and argillic carbon kaolinite and hematite rich alteration zones fluid inclusions hosted by quartz and dolomite crystals from distal alteration zones and silicified portions within the mineralized zone record a wide range of salinity 5 22 wt nacleqv but relatively constant homogenization temperatures 170 25 c pressure and temperature estimations based in the fluid inclusion microthermometry and selenide thermometry constrain the formation of the deposit at 250 270 c and 1 2 1 7 kbar element concentrations of alkali metals pb cs and to a lesser extent ba correlate with salinity whereas redox sensitive elements including u and ce vary widely gold concentrations from 50 to 500 ppb detected in some inclusions are consistent with the highly oxidizing conditions defined by selenide mineralogy the presence of uranium and gold within fluid inclusions of the distal hematite alteration zone confirms the oxidizing metal carrier fluids the occurrence of hydrothermally precipitated carbon implies that a reducing ch4 bearing fluid was also present and the structural relationships suggest that mixing between this reducing fluid and interaction with carbon upgraded rocks with an acidic oxidizing and metal carrying fluids was the main process driving ore metal precipitation the genesis of the serra pelada au pd pt deposit shows similarities with sediment hosted copper deposits and unconformity related u au pge deposits having in common the downflow of highly oxidizing fluids into the hydrothermal domain of the earth s upper crust major differences in metal proportions of these broad classes of ore deposits and other sediment hosted systems in different ore provinces are primarily caused by differences in source rocks available for oxidative leaching and variations in the geometry of the hydrothermal system

reactivation of archaean strike slip fault systems amazon region brazil the itacaiunas belt on the southeastern margin of the amazonian craton comprises two e w trending archaean fault zones carajas and cinzento strike slip systems developed sub parallel to a broad zone of earlier ductile shearing older basement rocks display a pervasive e w trending mylonitic fabric formed at high temperatures during sinistral transpression c 2 8 ga a sequence of younger archaean cover rocks was deformed under low greenschist facies conditions during a second phase of sinistral transpression associated with retrogression and reworking of basement fabrics all these rocks are overlain unconformably by very low grade to unmetamorphosed volcanic and sedimentary rocks of late archaean early proterozoic age the carajas and cinzento strike slip fault systems formed during subsequent regional dextral transtension c 2 6 2 7 ga which down faulted cover sequence rocks into dilational fault jogs later sinistral transpression c 1 9 ga partially inverted these dilation zones producing oblique compressional faults and folds evident mainly in cover rocks renewed fracturing regional emplacement of granitic plutons and swarms of dykes c 1 8 ga were followed by fault controlled deposition of immature sandstones and conglomerates phanerozoic activity is limited although some faults display evidence of minor neotectonic activity the waning influence of the basement architecture and decreasing intensity of later reactivations is consistent with lithosphere scale weakening with a finite life span

structural geometry and slip of the palos verdes fault southern california implications for earthquake hazards abstract the palos verdes fault pvf is an active structure in southern california comprised of several segments that together form a complex fault system the fault has been active since the miocene and is a major regional seismic source using marine petroleum industry seismic data we define the geometry of the fault and offsets of tertiary stratigraphic units that constrain the along strike segmentation and slip rates on the fault system in san pedro bay patterns of deformed stratigraphic units suggest a deep fault geometry that dips moderately to the southwest and was initially formed as a miocene normal fault plio pleistocene transpression resulted in the reactivation of this normal fault and its subsequent inversion with oblique right lateral reverse displacement to the south the lasuen knoll segment dips steeply to the northeast and is not a reactivated miocene age structure but rather formed in middle pliocene time plio pleistocene transpression has linked these and other segments together to form the currently active pvf system the resulting complex fault geometry and slip patterns suggest that the pvf is a highly segmented fault which may rupture in moderate size earthquakes that involve individual segments or large events that are able to propagate across significant geometric discontinuities estimates of moment magnitudes based on empirical relationships with rupture fault area indicate the possibility of m w 6 6 6 9 earthquakes for single segment ruptures and m w 7 1 7 3 multisegment ruptures with recurrence intervals ranging from 181 to 534 yr given the high inferred slip rates on the fault and its proximity to the urban population these events pose significant earthquake hazards to metropolitan los angeles

characteristics and origin of the oak dam east breccia hosted iron oxide cu u au deposit olympic dam region gawler craton south australia oak dam east is a large 560 mt of 41 56 wt fe cu au u bearing massive fe oxide deposit in the northeastern gawler craton south australia it is one of several unmined deposits in the same province as the giant olympic dam deposit and it lies beneath more than 500 m of younger cover rocks the oak dam east breccia complex formed in a near surface environment and was characterized by an early history of skarnlike albite calc silicate magnetite formation from hypersaline 400 500 c fluids large scale brecciation was an early feature and the resulting complex was progressively filled in and replaced by 1 hematite and goethite hematite stage i 2 magnetite apatite quartz actinolite magnetite stage 3 hematite goethite quartz chalcedony pyrite chlorite monazite hematite stage ii and 4 chalcopyrite pitchblende illite hydromus covite florencite carrolite cu u au mineralization stage hematite stage ii assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent nacl equiv that were boiling at 170 to 190 c and had 18o values more negative than 3 7 to 1 7 per mil interpreted as meteoric water the upper 200 m of the breccia complex contains spectacular colloform hematite void fillings interpreted to have replaced hydrothermal goethite the cu u mineralized zones resemble olympic dam ore in terms of geometries being grossly subhorizontal 10 to 70 m thick and crosscutting with respect to the host breccias the mineralization is zoned vertically from pyrite 34s 6 to 0 at depth to shallower chalcopyrite 34s 14 to 7 high grade pitchblende occurs centrally within the chalcopyrite zone e g 10 m of 0 46 cu and 3 700 ppm u3o8 straddling a boundary between k mica illite alteration and underlying fe mg chlorite alteration the deposit is assigned to the 1570 to 1595 ma iron oxide cu au u event in the eastern gawler craton on the basis of its similarity to other cu au u deposits in the province an la icpms pb isotope age for monazite in hematite stage ii was calculated to be 1455 20 ma and possibly as young as 1370 54 ma to explain all isotopic data this is an age of isotopic resetting our preferred model is that cu u mineralization developed within a stratified hydrothermal brine complex analogous to the salton sea geothermal system copper fe and s in the deep brine was sourced from an evolving deep reservoir with local compositional controls on sulfur isotopes including cooling boiling and replacement of previous pyrite chalcopyrite u veins and disseminations formed as a result of mixing with overlying steam heated u rich saline ground water mixing induced cooling and dilution only occurred episodically following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex by contrast high grade uo2 formation occurred as deep upflow waned via reaction of the upper brine with hematite stage ii fe mg chlorite along an interface that descended with time this resulted in an increase of u grades through time caused by leaching and reprecipitation

the palos verdes terraces california bathtub rings from a buried reverse fault uplift of the palos verdes peninsula has long been associated with a northwest trending southwest dipping reverse fault unfortunately the palos verdes hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension orientation or earthquake potential in this paper we investigate the tectonic style and slip rate of the palos verdes hills fault and the uplift history of the palos verdes hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration elevations of 211 terrace remnants constrain a fault model with 3 0 to 3 7 mm yr 1 of oblique dextral reverse slip on a fault dipping 67 at 6 to 12 km depth beneath the peninsula if the rate was constant through time fault inception would have occurred 2 4 3 0 ma we propose that the largest credible earthquakes on the fault have magnitude 6 and could revisit every 2000 years

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

u pb geochronology of archean magmatism and basement reactivation in the carajas area amazon shield brazil abstract the archean carajas subprovince is a metallogenic district bif bauxite cu ni mn au located in the easternmost amazonian craton in brazil the main conclusions from a u pb study of the itacaiunas supergroup and a locality in the xingu complex are the following the xingu basement complex the oldest unit underwent migmatization at 2859 2 ma and also grew new zircon at 2851 4 ma the luanga mafic intrusion is 2763 6 ma old and coeval with the 2759 2 ma felsic volcanics of the grao para group amphibolites from the salobo group contain 2851 ma zircons were metamorphosed 2761 3 ma and intruded by granitic veins ca 2758 ma at the time of grao para magmatism and also grew new zircon at 2732 2742 ma the data indicate that the salobo group is part of the xingu complex and not of the itacaiunas supergroup as heretofore believed the pojuca group possibly coeval with the salobo group was metamorphosed at 2732 3 ma a late archean reactivation event of regional extent is marked by titanite growth at 2581 5 ma in the mirim area intrusion of the old salobo granite at 2573 2 ma new zircon growth in salobo group amphibolites at 2555 4 3 ma and monazite growth in a metamorphosed iron formation at 2551 2 ma in addition k metasomatism associated with shear zones formed titanite in salobo group amphibolites at 2497 5 ma and new titanite at 2519 5 ma in the xingu complex events in the 2581 2497 ma interval are probably associated with the latest activity of a wnw ese sinistral shear system which could also be responsible for the establishment of a transtensional basin filled by the volcano sedimentary sequence of the grao para bahia and rio fresco groups finally undeformed proterozoic granitoid intrusions were dated at 1883 2 ma cigano 1883 5 2 ma musa 1880 2 ma carajas and 1874 2 ma pojuca

compaction control of topography and fault network structure along strike slip faults in sedimentary basins strike slip faults commonly display structurally complex areas of positive or negative topography understanding the development of such areas has important implications for earthquake studies and hydrocarbon exploration previous workers identified the key factors controlling the occurrence of both topographic modes and the related structural styles kinematic and stress boundary conditions are of first order relevance surface mass transport and material properties affect fault network structure experiments demonstrate that dilatancy can generate positive topography even under simple shear boundary conditions here we use physical models with sand to show that the degree of compaction of the deformed rocks alone can determine the type of topography and related surface fault network structure in simple shear settings in our experiments volume changes of 5 are sufficient to generate localized uplift or subsidence we discuss scalability of model volume changes and fault network structure and show that our model fault zones satisfy geometrical similarity with natural flower structures our results imply that compaction may be an important factor in the development of topography and fault network structure along strike slip faults in sedimentary basins

multi stage enrichment processes for large gold bearing ore deposits abstract a review of previous studies of the world s large hydrothermal gold deposits indicates that the largest deposits tend to show complicated parageneses where multiple gold enrichment events and processes have been involved in the deposit generation these observations suggest that multistage processes may even be a requirement for the formation of large deposits in some deposits e g witwatersrand boddington cadia sukhoi log or carlin the different enrichment processes occur millions of years apart in others such as many large porphyry deposits the different stages are much closer in time in many deposits particularly sedimentary hosted deposits early diffuse enrichment occurs within a particular province that is then upgraded by more focused processes e g sukhoi log kalgoorlie the presence of this early diffuse enrichment could explain the tendency for gold deposits to cluster into camps this model has important implications as the presence or absence of multiple gold events could be used to discriminate at the exploration and feasibility stages between small deposits with single stage ore genesis and more complicated deposits with multistage enrichment and the potential for larger gold endowment

late cenozoic tectonics of the northern los angeles fault system california the northern los angeles fault system along the southern range front of the santa monica mountains includes potentially seismogenic faults directly beneath the los angeles metropolitan area for a better assessment of seismic hazards we mapped late cenozoic faults and folds in the northern los angeles basin using an extensive set of oil well and surface geologic data the northern los angeles fault system developed through early to late miocene transrotational and transtensional regimes and a pliocene and quaternary transpressional regime the santa monica san vicente and las cienegas faults are early to late miocene normal faults that were later reactivated as reverse faults suggesting that the orientation of reverse faults is largely controlled by miocene extensional tectonics rather than by the post miocene stress field tectonic inversion occurred at the beginning of pliocene time with the reactivation of miocene normal faults and initiation of reverse faults many pliocene contractile structures became inactive by the middle pleistocene and younger deformation is taken up by new active structures including the west beverly hills lineament and an active strand of the santa monica fault the west beverly hills lineament is the northernmost segment of the newport inglewood fault zone which may have propagated northward to the santa monica mountains in quaternary time the lineament acts as a segment boundary for the active left lateral santa monica hollywood fault system and bounds the hollywood basin to the west uplift of an oxygen isotope substage 5e marine terrace north of the city of santa monica and an assumed dip of 45 for the santa monica mountains thrust fault underlying and uplifting the santa monica mountains suggest that an average dip slip rate for the fault is 1 3 mm yr crustal shortening across the northern los angeles fault system accounts for less than a third of the current rate of shortening between the san gabriel mountains and palos verdes hills based on global positioning system observations

using high resolution multibeam bathymetry to identify seafloor surface rupture along the palos verdes fault complex in offshore southern california recently acquired high resolution multibeam bathymetric data reveal several linear traces that are the surficial expressions of seafloor rupture of holocene faults on the upper continental slope southeast of the palos verdes peninsula high resolution multichannel and boomer seismic reflection profiles show that these linear ruptures are the surficial expressions of holocene faults with vertical to steep dips the most prominent fault on the multibeam bathymetry is about 10 km to the west of the mapped trace of the palos verdes fault and extends for at least 14 km between the shelf edge and the base of the continental slope this fault is informally called the avalon knoll fault for the nearby geographic feature of that name seismic reflection profiles show that the avalon knoll fault is part of a northwest trending complex of faults and anticlinal uplifts that are evident as scarps and bathymetric highs on the multibeam bathymetry this fault complex may extend onshore and contribute to the missing balance of quaternary uplift determined for the palos verdes hills and not accounted for by vertical uplift along the onshore palos verdes fault we investigate the extent of the newly located offshore avalon knoll fault and use this mapped fault length to estimate likely minimum magnitudes for events along this fault

analog models of restraining stepovers in strike slip fault systems scaled sandbox models have successfully simulated the geometries and progressive evolution of antiformal pop up structures developed in a weak sedimentary cover above restraining stepovers in offset sinistral strike slip fault systems in rigid basement models were run both with and without synkinematic sedimentation which was added incrementally to cover the growing antiformal structures vertical and horizontal sections of the completed models permit the full three dimensional 3 d structure of the pop ups to be analyzed in detail three representative end member experiments are described 30 underlapping restraining stepovers 90 neutral restraining stepovers and 150 overlapping restraining stepovers the experimental pop ups are typically sigmoidal to lozenge shaped antiformal structures having geometries that are dependent on both the stepover angle and stepover width in the underlying basement faults underlapping restraining stepovers typically form elongate lozenge shaped pop ups 90 neutral restraining stepovers produce shorter squat rhomboidal pop ups and overlapping restraining stepovers produce sigmoidal antiformal pop ups trans pop up cross fault systems are characteristic at large displacements on the basement fault system above the offset principal displacement zones the pop ups are commonly small narrow positive flower structures whereas in the stepover region they widen out and become markedly asymmetric this pop up asymmetry switches across the center of the stepover where the pop ups are largely symmetical maximum rotations measured within the central highly uplifted region of the pop ups increase from 7 counterclockwise for the underlapping 30 stepovers to 14 counterclockwise for the neutral 90 stepovers to 16 counterclockwise for the overlapping 150 stepovers in models where no synkinematic sediments were added during deformation the pop up structures are bound by convex flattening upward oblique slip reverse fault systems that link downward to the offsets in the basement fault system in contrast in the begin page 234 experiments where synkinematic sediments were added incrementally during deformation the pop ups are formed by oblique slip reverse faults that steepen upward into the synkinematic strata with the formation of fault propagation growth folds the analog models are compared with natural examples of pop up structures and show strong similarities in structural geometries and stratal architectures these models may provide structural templates for seismic interpretation of complex contractional structures in offset strike slip fault systems

contrasting mineralogical geochemical signatures of manganese oxides guides to metallogenesis manganese oxides have a strong sorption capacity for cationic species in the depositional fluid this sorption capacity is reflected in the chemistry of the oxides which is different from that for the oxides precipitated from fresh water seawater and hydrothermal fluids characteristic mineralogical geochemical enrichment and geochemical association defined statistically signatures in conjunction with several diagnostic plots can be employed to distinguish between the following different genetic types of manganese deposits supergene hydrothermal supergene marine terrestrial supergene dubhite and supergene marine hydrothermal sedimentary exhalative sedex dubhites are manganese oxides derived from the weathering of a mineralized sequence i e base and precious ore minerals and gangue hydrothermal deposits characteristically show geochemical enrichments in the assemblage as ba cu li mo pb sb sr v zn and an mn as geochemical association supergene marine deposits show na k ca mg sr and co cu ni geochemical enrichments whereas supergene terrestrial deposits tend to have very high ba contents and an mn ba association dubhites contain high levels of pb and zn show an mn pb base metal association and are composed of significant quantities of base metal bearing manganese oxides bixbyite braunite hausmannite huebnerite jacobsite and pyrochroite occur predominantly only in hydrothermal deposits whereas chalcophanite coronadite crednerite delta mno 2 groutite hollandite lithiophorite manganite nsutite quenselite ramsdellite romanechite todorokite and woodruffite are generally of supergene origin in addition to established plots presently used to identify marine sedimentary exhalative deposits the following diagnostic graphs were devised na vs mg distinguishes deposits of marine and fresh water origin co ni vs as cu mo pb v zn discriminates between hydrothermal and supergene oxides and pb vs zn can be used to recognize dubhites and therefore prospective sequences

sea level and tectonic control of middle to late pleistocene turbidite systems in santa monica basin offshore california small turbidite systems offshore from southern california provide an opportunity to track sediment from river source through the turbidity current initiation process to ultimate deposition and to evaluate the impact of changing sea level and tectonics the santa monica basin is almost a closed system for terrigenous sediment input and is supplied principally from the santa clara river the hueneme fan is supplied directly by the river whereas the smaller mugu and dume fans are nourished by southward longshore drift this study of the late quaternary turbidite fill of the santa monica basin uses a dense grid of high resolution seismic reflection profiles tied to new radiocarbon ages for ocean drilling program odp site 1015 back to 32 ka over the last glacial cycle sedimentation rates in the distal part of santa monica basin averaged 2 3 mm yr 1 with increases at times of extreme relative sea level lowstand coarser grained mid fan lobes prograded into the basin from the hueneme mugu and dume fans at times of rapid sea level fall these pulses of coarse grained sediment resulted from river channel incision and delta cannibalization during the extreme lowstand of the last glacial maximum sediment delivery was concentrated on the hueneme fan with mean depositional rates of up to 13 mm yr 1 on the mid and upper fan during the marine isotope stage mis 2 transgression enhanced rates of sedimentation of 4 mm yr 1 occurred on the mugu and dume fans as a result of distributary switching and southward littoral drift providing nourishment to these fan systems longer term sediment delivery to santa monica basin was controlled by tectonics prior to mis 10 the anacapa ridge blocked the southward discharge of the santa clara river into the santa monica basin the pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents these two factors appear to have been more important than the absolute position of sea level

late quaternary activity and seismic potential of the santa monica fault system los angeles california the santa monica fault is a 40 km long oblique left lateral reverse fault that extends through the densely urbanized northwestern los angeles california area and offshore parallel to the malibu coast the fault exhibits near surface strain partitioning in paleoseismologic trenches and has undergone at least six surface ruptures during the past 50 k y although events may be missing from the older part of the record and the ages of older events are based on poorly constrained soil age estimates at least two and probably three events occurred after the well dated burial of a prominent paleosol ca 16 17 ka the resulting 7 8 k y latest pleistocene holocene average recurrence interval for events large enough to cause surface rupture is much longer than the 1 9 3 3 k y maximum interval we calculate for hypothetical m w 6 9 7 0 earthquakes generated by rupture of the entire santa monica fault the pronounced disparity between the measured and calculated average recurrence intervals suggests that the santa monica fault undergoes infrequent and therefore probably very large earthquakes m w 7 0 if such large earthquakes have occurred we speculate that they may have been generated by simultaneous rupture of the santa monica fault with other faults in the transverse ranges southern boundary fault system such as the hollywood or anacapa dume faults the most recent definite santa monica fault surface rupture occurred between ca 10 and 17 ka although a more recent event probably occurred between ca 1 and 3 ka

escape tectonics in the los angeles metropolitan region and implications for seismic risk recent damaging earthquakes in california including the 1971 san fernando1 1983 coalinga2 1987 whittier narrows3 and 1994 northridge4 events have drawn attention to thrust faults as both potentially hazardous seismic sources and as a mechanism for accommodating shortening in many regions of southern california consequently many geological studies5 6 have concluded that thrust faults in southern california pose the greatest seismic hazard and also account for most of the estimated 5 7 mm yr 1 of contraction across the greater los angeles metropolitan area7 8 indicated by global positioning system geodetic measurements9 our study demonstrates however that less than 50 of the geodetically observed contraction is accommodated on the principal thrust systems across the los angeles region we integrate the most recent geological geodetic and seismological data to assess the spatial distribution of strain across the los angeles metropolitan region we then demonstrate that a significant component of seismic moment release and shortening in this region is accommodated by east west crustal escape extrusion along known strike slip and oblique slip faults

restraining segments and reactivation of the santa monica dume malibu coast fault system offshore los angeles california 1 a regional west striking system of surface and blind faults transects northern metropolitan los angeles separating the santa monica mountains from two deep sedimentary basins the surface faults include the santa monica dume and malibu coast faults the three dimensional 3 d geometries of these faults and deformed dated strata were examined in order to determine how oblique shortening is accommodated how structural relief grows along a mountain front and how block translation is related to block rotation industry seismic reflection well and outcrop data were used to construct digital structure contour maps of several of these faults and three stratigraphic horizons that intersect them these maps swath bathymetry digital elevation models and seismicity were incorporated into a 3 d digital database modeling included new approaches to determining strike slip displacement by separate analyses of shortening and structural relief in a restraining double bend stratigraphic thicknesses indicate miocene extension across the santa monica and dume faults which have been reactivated as a single or linked moderately dipping arcuate fault that accommodates one coherent block motion via left lateral and left reverse displacement modeling indicates 5 km 8 1 km of left lateral displacement on part of the santa monica dume fault and 11 of clockwise rotation of the santa monica mountains during the last 4 1 m y modern displacement rates modeled from gps data are similar to our modeled post 4 ma rates of westward escape and clockwise rotation of the santa monica mountains

slip rate on the san diego trough fault zone inner california borderland and the 1986 oceanside earthquake swarm revisited the san diego trough fault zone sdtfz is part of a 90 km wide zone of faults within the inner california borderland that accommodates motion between the pacific and north american plates along with most faults offshore southern california the slip rate and paleoseismic history of the sdtfz are unknown we pre sent new seismic reflection data that show that the fault zone steps across a 5 km wide stepover to continue for an additional 60 km north of its previously mapped extent the 1986 oceanside earthquake swarm is located within the 20 km long restraining stepover farther north at the latitude of santa catalina island the sdtfz bends 20 to the west and may be linked via a complex zone of folds with the san pedro basin fault zone spbfz in a cooperative program between the u s geological survey usgs and the monterey bay aquarium research institute mbari we measure and date the coseismic offset of a submarine channel that intersects the fault zone near the sdtfz spbfz junction we estimate a horizontal slip rate of about 1 5 0 3 mm yr over the past 12 270 yr

style and rate of holocene slip palos verdes fault southern california recent studies of the palos verdes fault in the los angeles outer harbor characterize the fault s holocene activity the studies included areal and local geophysical surveys as well as drilling sampling and dating of the subsurface materials the studies identify the fault s location in the shallow subsurface document evidence of holocene strike slip quantify dip slip versus sediment age and quantify the net slip where an early holocene paleochannel crosses the fault the slip rate for the palos verdes fault is constrained to between 2 7 and 3 mm yr for the past 7 8 to 8 ka because the new evidence shows that the palos verdes fault has one of the highest slip rates in the los angeles basin the studies provide significant new insight into the regional tectonics and the seismic risk of the los angeles region using this rate of slip and segmentation models for the fault the coastal los angeles region can expect an mw 7 to 7 2 event about every 400 to 900 years from this potential seismic source

crustal structure and tectonics from the los angeles basin to the mojave desert southern california a seismic refraction and low fold reflection survey known as the los angeles region seismic experiment larse was conducted along a transect line 1 extending from seal beach california to the mojave desert crossing the los angeles and san gabriel valley basins and san gabriel mountains the chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of southern california that have been debated for decades and have important implications for earthquake hazard assessment the results or constraints are as follows 1 the maximum depth of the los angeles basin along line 1 is 8 9 km 2 the deep structure of the sierra madre fault zone in the northern san gabriel valley is as follows the duarte branch of the sierra madre fault zone forms a buried 2 5 km high moderately north dipping buttress between the sedimentary and volcanic rocks of the san gabriel valley and the igneous and metamorphic rocks of the san gabriel mountains for deeper structure see following 3 there are active crustal decollements in southern california at middle crustal depths the sierra madre fault zone appears to sole into a master decollement that terminates northward at the san andreas fault and projects southward beneath the san gabriel valley to the puente hills blind thrust fault 4 the dip and depth extent of the san andreas fault along line 1 dips steeply 83 northward and extends to at least the moho 5 the subsurface lateral extent of the pelona schist in southern california is as follows along line 1 the pelona schist underlies much if not all of the san gabriel mountains south of the san andreas fault to middle crustal depths north of the san andreas fault it is apparently not present along the transect

timing and extent of neogene tectonic rotation in the western transverse ranges california paleomagnetic data collected from lower miocene rocks throughout the western transverse ranges province have consistently clockwise deflected magnetic declinations which suggests that the entire province has experienced clockwise tectonic rotation during neogene time paleomagnetic declinations in lower miocene rocks increase westward from 33 5 c 11 in the san gabriel block to 91 7 7 in the western santa ynez range suggesting a westward increase in the net amount of clockwise tectonic rotation paleomagnetic data that constrain the tuning of rotation suggest that the entire western transverse ranges rotated uniformly 50 60 clockwise during the middle miocene this implies a minimum of 200 km of distributed dextral shear along the california coast and offshore during middle miocene time little tectonic rotation appears to have occurred in the western transverse ranges during late miocene time since miocene time continued clockwise rotation in the western santa ynez range 31 3 combined with counterclockwise rotation in the san gabriel block 16 has oroclinally bent the transverse ranges to the west of the san andreas fault the pliocene pleistocene clockwise rotation implies an additional 30 60 km of distributed dextral shear across the southern california coast ranges during post miocene time

direct dating of weathering phenomena by 40ar39ar and k ar analysis of supergene k mn oxides abstract potassium bearing manganese oxides cryptomelane k1 2 mn3 mn4 8 o16 xh2oand hollandite k ba 1 2 mn3 mn4 8 o16 xh2o are often authigenically precipitated in weathering profiles the presence of structural potassium in manganese oxides makes these minerals datable by the k ar and 40 ar 39 ar methods dating of these phases allows us to time the progression of oxidation fronts during weathering and pedogenic processes detailed age resolution of the weathering history allows us to use deep weathering profiles as land based stratigraphic records of past climatic and geomorphic events complementing evidence from the marine sedimentary and paleontological record potential problems in manganese oxide dating such as ar and or k losses excess argon 39ar loss by recoil during neutron irradiation contamination by primary minerals resistant to weathering the presence of multiple generations of manganese oxides and proportionately low radiogenic ar yield due to adsorbed atmospheric gases are addressed the k ar and 40 ar 39 ar analytical results indicate that ar and or k losses excess 40ar and 39ar recoil seem not to pose problems in manganese oxide dating this investigation suggests that fine scale laser probe 40 ar 39 ar technique is most appropriate for dating of weathering phenomena because this technique permits us to identify contaminating phases and the presence of multiple generations of weathering minerals in the inherently complex mineral assemblage characteristic of weathering profiles k ar and 40 ar 39 ar dating of supergene k bearing manganese oxides formed during lateritization of archean and proterozoic bedrocks in the caraj s region amazonia brazil indicates that weathering started before 72 6 ma petrographic electron microscope and electron microprobe investigation reveal multiple generations of manganese oxide precipitation k ar and 40 ar 39 ar laser probe step heating results independently confirm the presence of these various generations of manganese oxides age clusters at 65 69 51 56 40 43 33 35 20 24 12 17 ma and zero age 0 2 0 2 ma suggest episodic precipitation of k mn oxides resulting from changing weathering conditions in the amazon throughout the cenozoic k ar and 40 ar 39 ar dating of supergene cryptomelane from weathering profiles in eastern minas gerais southeastern brazil suggests continuous weathering from 10 to 5 6 ma ago possibly reflecting local climatic conditions due to the proximity with the atlantic ocean

the serra pelada au pd pt deposit carajas brazil geochemistry mineralogy and zoning of hydrothermal alteration the serra pelada au pd pt deposit is located within the carajas mineral province which also hosts the world class iron ore and iron oxide hosted copper gold deposits of the amazon craton in brazil the unusual low temperature hydrothermal mineralization at serra pelada is epigenetic hosted by metasedimentary rocks of the aguas claras formation and structurally localized in the serra pelada overturned syncline the orebodies are controlled by the intersection of subvertical ne trending fault zones with metasiltstones mainly at the syncline s hinge with minor ore occurrences at the upper and lower limb intense tropical weathering over the last 70 m y has completely overprinted the shallow ore in and near the flooded open pit but primary hydrothermal features are preserved in deeper drill core delineating the remaining resource gold platinum and palladium mineralization is associated with intense argillic alteration hematite breccias and silicification with the highest grade ore hosted by brecciated metasiltstones that are highly enriched in amorphous carbon distal alteration zones comprise a reducing and oxidizing alteration front the hydrothermal mineral paragenesis comprises kaolinite quartz sericite amesite mg rich al silicate amorphous carbon hematite monazite rutile pyrite and a complex assemblage of bi ag pb cu co ni pt pd and au bearing chalcogenide s se and arsenide as sb minerals major element changes during hydrothermal alteration include c and mg addition k depletion localized silica loss and silicification with notable introduction of trace elements including light rare earth elements lree bi pb u v cu co ni and as the hydrothermal alteration and element association of the serra pelada deposit show geochemical similarities with unconformity related uranium deposits which may also be enriched in au pd and pt and were formed by mixing of fluids that interacted with a highly oxidized cover sequence and highly reduced rock packages in structures of brittle ductile strain

naturally occurring gold nanoparticles and nanoplates during the weathering of gold deposits exceptionally pure 200 nm diameter nanoparticulate gold plates 6 nm thick are formed the particles display controlled growth of both size and shape and signs of assembly to form belts and sheets the gold is associated and intergrown with minerals formed by evaporation and is interpreted to have been deposited rapidly from saline groundwater during a drying event the size and morphology of the gold nanoparticles and nanoplates are identical to the products of experimentally manufactured gold colloids this represents the fi rst direct observation of colloidal nanoparticulate gold in nature confi rming this as an active mechanism of gold transport during the weathering of gold deposits

the offshore palos verdes fault zone near san pedro southern california high resolution seismic reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the san pedro shelf near los angeles california prominent structures investigated include the wilmington graben the palos verdes fault zone various faults below the west part of the san pedro shelf and slope and the deep water san pedro basin the structure of the palos verdes fault zone changes markedly along strike southeastward across the san pedro shelf and slope under the north part of the shelf this fault zone includes several strands with the main strand dipping west under the slope the main fault strands exhibit normal separation and mostly dip east to the southeast near lasuen knoll the palos verdes fault zone locally is low angle but elsewhere near this knoll the fault dips steeply fresh seafloor scarps near lasuen knoll indicate recent fault movement we explain the observed structural variation along the palos verdes fault zone as the result of changes in strike and fault geometry along a master right lateral strike slip fault at depth complicated movement along this deep fault zone is suggested by the possible wave cut terraces on lasuen knoll which indicate subaerial exposure during the last sea level lowstand and subsequent subsidence of the knoll modeling of aeromagnetic data indicates a large magnetic body under the west part of the san pedro shelf and upper slope we interpret this body to be thick basalt of probable miocene age this basalt mass appears to have affected the pattern of rock deformation perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt west of the palos verdes fault zone other northwest striking faults deform the outer shelf and slope evidence for recent movement along these faults is equivocal because we lack age dates on deformed or offset sediment manuscript received 26 february 2003

petrographic and geochemical evidence for a hypogene origin of granite hosted vein type mn mineralization at the new ross mn deposits lunenburg county nova scotia canada the new ross mn deposits consist of two centers of past mining activity located in the 370 ma south mountain batholith 10 km north of new ross lunenburg county the deposits occur as mn oxide veins and lenses formed along steeply dipping northeast trending faults previous investigators disagree as to whether the most economically important mn oxides pyrolusite and manganite are of supergene or hypogene origin recent diamond drilling and geologic mapping have provided new data that support a hypogene origin the south mountain batholith in the area of the deposits consists mostly of coarse grained megacrystic biotite monzogranite a swarm of small elongate leucomonzogranite plutons both intrude and are in fault contact with the monzogranite to the north and west of the deposits and at depth below one of the mine workings these plutons are elongate parallel to a series of persistent northeast trending topographic linears which appear to represent faults and or shear zones this spatial association suggests that these structures played a role in the emplacement of the plutons brecciation and hydrothermal alteration of the leucomonzogranite plutons indicates that movement and hydrothermal activity continued on the faults after final crystallization of the plutons several types of hydrothermal alteration are well developed in the faults hosting the mn mineralization and along several other northeast trending structures in this region the diamond drilling shows that although the mn mineralization is absent at depth the fault zone and hydrothermal alteration persist to at least 452 m below the mine workings low temperature hematization and argillic alteration is best developed within the main ore bearing zone from surface to a depth of about 150 m with increasing depth the amount of argillic alteration decreases but hematization remains well developed desilicification episyenitization accompanied hematization at several localities in the megacrystic monzogranite and the leucomonzogranite intrusions the desilicification is developed to the point that no primary quartz remains in the rock higher temperature alterations such as silicification greisenization k feldspathization and albitization were caused by emplacement of the leucomonzogranite the silicification occurs as total replacement of the rock and grades through greisenized selvages into unaltered leucomonzogranite or monzogranite textural evidence indicates that precipitation of carbonate minerals occurred prior to during and after crystallization of the mn and fe oxides the activity of phosphate during the alteration sequences is of particular importance below the mine p 2 o 5 enrichment occurs in the higher temperature silicified greisenized k feldspathized and albitized rocks up to 5 20 wt p 2 o 5 and is manifest as abundant secondary fluorapatite phosphate remained present in the hydrothermal fluid at lower temperatures and resulted in crystallization of fluorapatite with mn and fe oxides in the mineralized zones and formation of a 1 2 m wide quartz fluorapatite manjiroite vein 9 95 wt p 2 o 5 samples of manganite and pyrolusite often contain vug fillings and inclusions of rhodochrosite and minor amounts of other hypogene minerals such as knebelite and pinakiolite furthermore these samples also contain anomalous amounts of be 90 ppm w 140 ppm mo 28 ppm pb 3 000 ppm and cu 1 500 ppm which is consistent with their formation from fluids derived from a magmatic source these textural relations and geochemical signatures indicate that the alteration sequence and mn oxides including those that were previously mined could all be part of an evolving hydrothermal system developed in upwelling hydrothermal fluids these processes operated along other northeast trending faults throughout much of this region of the south mountain batholith which suggests a high potential for as yet undiscovered deposits of mn and other granophile elements

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

late cretaceous bonanza style metal enrichment in the serra pelada au pd pt deposit para brazil a 40 ar 39 ar age of 75 6 ma 2 was obtained for aggregates of mn ba oxide intergrown with palladiferous gold and pd pt minerals representative of near surface bonanza au pd pt mineralization that triggered the gold rush to serra pelada in northern brazil the mineralization occurs in goethite rich tectonic breccia and records recurrent fault displacement in the still active cinzento strike slip fault system the goethite rich tectonic breccia is truncated by the bonanza style au pd pt mn ba enrichment with which tabular hematite is spatially associated on a micrometric scale the tabular hematite is coarsely crystalline 5 m in grain size and lines cavity walls in the goethite rich breccia the presence of coarsely crystalline tabular hematite constrains the metalliferous overprint to temperatures in excess of about 100 c we suggest that the bonanza mineralization is a late cretaceous epithermal like fault breccia system that overprinted paleoproterozoic intrusion related au pd pt mineralization preserved at depth

late cenozoic sedimentation in california continental borderland basins as revealed by seismic facies analysis the california continental borderland began to form during late miocene time only the latest miocene and younger sediments are ponded in the contemporary basins upper miocene strata are characterized by widespread diatomaceous deposits that conformably drape all of the underlying topographic irregularities the pliocene quaternary sediments occur as flat lying ponded clastic sequences that fill the contemporary basins a seismic facies analysis has defined facies patterns in the pliocene quaternary basin fill sequences of each basin in the offshore basins patton tanner santa cruz and san nicolas fine grained mass flow and hemipelagic sediments derived from the local insular sources dominate the basin fill sequences santa catalina basin is a transitional basin filled with fine grained turbidites and hemipelagic sediments derived from both continental and local insular sources the nearshore basins santa barbara santa monica and san pedro basins and san diego trough are filled by thick sequences of continental sediments that form prominent deep sea fans the borderland was dominated by biogenic pelagic sedimentation in the late miocene and by detrital sediment gravity flow deposition in the pliocene and quaternary accumulation rates estimated from pliocene quaternary sediment thickness for each basin show that in the offshore basins as well as in san diego trough rates have remained fairly constant since the pliocene in contrast the nearshore basins exhibit an increasing rate of accumulation versus time that is related to the progradation of continent derived sediments outer and intermediate basin accumulation data indicate decreasing contributions with time from offshore highs because of erosion and regional subsidence intermediate basins show an increase in mass wasting deposits slope facies with time which is the result of increasing fine suspension slope deposition and eventual failure

seismicity and active compressional tectonics in santa monica bay southern california during 1973 1986 the seismicity in santa monica bay consisted of an average of four events per year of m l 2 5 in addition to the 1979 malibu m l 5 0 mainshock aftershock sequence the 1979 sequence ruptured a north dipping thrust fault at 10 15 km depth beneath the bay a diffuse distribution of seismicity extends across the bay from the terminus of the palos verdes fault to point dume two thirds of the single event focal mechanisms of 36 events that are located within the bay show mostly thrust or reverse faulting on west to west northwest striking planes a third of the focal mechanisms exhibit strike slip faulting on north to north northwest striking planes a stress inversion of the focal mechanism data shows that the maximum principal stress is oriented n13 e in a horizontal plane and the minimum principal stress is oriented n102 e and plunging 47 hence the tectonics in santa monica bay are dominated by compression with thrust reverse and strike slip faulting the coexistence of thrust and strike slip faults suggests that the bay is not a part of the peninsular ranges terrane but forms a transition zone that accommodates the change from strike slip in the peninsular ranges to the south to reverse faulting in the transverse ranges to the north mapped anticlines and the previously unrecognized thrust faults beneath the bay form two fold and thrust belts that present additional seismic hazards to los angeles and the southern california coastal zone through seismic shaking and possible generation of small tsunamis

critical assessment of geochronological data from the carajas mineral province brazil implications for metallogeny and tectonic evolution abstract the carajas domain in the amazonian craton brazil displays remarkable metallogenetic diversity containing iron oxide copper gold iocg cu au mn fe ni co pge cr and au pge mineral systems it consists of mesoarchean 2 83 3 08 ga basement granitoids and gneisses overlain by the ca 2 73 2 76 ga rift related meta volcanosedimentary units of the neoarchean carajas basin coeval ca 2 76 mafic ultramafic complexes host nickel cobalt laterite vermelho puma onca and magmatic pge cr luanga deposits unconformably overlying this is the aguas claras formation a package of fluvial to shallow marine sedimentary rocks of controversial but probable archean age based on 2 6 2 7 ga cross cutting mafic dikes this hosts manganese deposits azul small polymetallic cu au deposits breves and the serra pelada au pt pd deposit iocg deposits are located along wnw regional shear zones hosted within the itacaiunas supergroup mesoarchean basement and bimodal 2 76 2 73 ga neoarchean granitoids and gabbro geochronology supports the existence of multiple superimposed and episodic hydrothermal and mineral systems multiple mineralization ages are common within single deposits and establishing the age of primary mineralization as opposed to overprinting hydrothermal events is challenging copper mineralization formed as early as ca 2 76 ga coeval with basin formation while structurally controlled iocg deposits formed or were substantially modified during multiple hydrothermal episodes ca 2 72 2 68 ga 2 6 2 45 ga and 1 88 ga for the northern iocg deposits e g salobo and igarape bahia alemao a temporal overlap between 2 57 ga mineralization and the 2 57 ga alkaline old salobo granite is commonly used to infer a genetic link yet this geochronology s reliability is debatable younger ages exist for the old salobo granite 2 55 2 53 ga zircons are metamict and older mineralization ages have been reported suggesting a more complex history the occurrence of ages ca 2 60 2 45 ga is spatially restricted to the north and coincides with an important yet poorly understood tectono thermal event involving reactivation of the northern cinzento shear zone and amphibolite facies metamorphism whether iocg deposits formed during this tectonic event or represent substantially modified or metamorphosed older mineralization or both is contentious older mineralization ca 2 71 ga at the igarape cinzento iocg deposit in the north is coeval with the southern iocg deposits formed ca 2 72 2 68 ga sequeirinho bacaba and bacuri this does not convincingly overlap with the intrusion of widespread 2 76 2 73 ga syntectonic granitoids so a genetic link with magmatic fluids is speculative and basinal brines may play a role due to controversy regarding the timing of compression and basin closure the tectonic setting is debated mineralization either formed during rifting or synchronous with basin inversion but the latter is more supported by geochronology and ore textures in the south paleoproterozoic ca 1 88 ga iocgs the sossego curral orebodies of the sossego deposit and alvo 118 or alternatively overprinting cu au systems formed coeval with craton wide a type 1 88 ga granite magmatism this magmatic hydrothermal system is responsible for a large part of the metallogenetic diversity of carajas domain small generally

hydrothermal overprint on cenozoic sediments in the quadrilatero ferrifero of minas gerais implications for precious metals in cratonic terrains cenozoic sediments deposited in graben like basins in the quadrilatero ferrifero southern sao francisco craton brazil have anomalous contents of hg and sb two localities have been studied the topaz deposit of antonio pereira and the remains of the old opencast workings at cata preta one of the earliest au mines of minas gerais that dates back to about 1690 at cata preta a brecciated wad like rock exhibits geochemical fingerprints that point to fractionation of pt and pd from ag this metal fractionation reflected in the au ingots produced in the region genetically links cata preta to the near surface bonanza ore of serra pelada which formed as a result of a late cretaceous hydrothermal overprint in the amazon craton here we propose that anorogenic hydrothermal activity in cratonic terrains prepared the ground for the enrichment in precious metals that gave rise to two of the most important au rushes in brazilian history

geophysical evidence for the evolution of the california inner continental borderland as a metamorphic core complex we use new seismic and gravity data collected during the 1994 los angeles region seismic experiment larse to discuss the origin of the california inner continental borderland icb as an extended terrain possibly in a metamorphic core complex mode the data provide detailed crustal structure of the borderland and its transition to mainland southern california using tomographic inversion as well as traditional forward ray tracing to model the wide angle seismic data we find little or no sediments low 6 6 km s p wave velocity extending down to the crust mantle boundary and a thin crust 19 to 23 km thick coincident multichannel seismic reflection data show a reflective lower crust under catalina ridge contrary to other parts of coastal california we do not find evidence for an underplated fossil oceanic layer at the base of the crust coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge which represents the transition to the western transverse ranges on the shelf the palos verdes fault merges downward into a landward dipping surface which separates basement from low velocity sediments but interpretation of this surface as a detachment fault is inconclusive the seismic velocity structure is interpreted to represent catalina schist rocks extending from top to bottom of the crust this interpretation is compatible with a model for the origin of the icb as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks the basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until 13 m y ago subduction of the young and hot monterey and arguello microplates under the continental borderland followed by rotation and translation of the western transverse ranges may have provided the necessary thermomechanical conditions for this extension and crustal inflow

triggering of tsunamigenic aftershocks from large strike slip earthquakes analysis of the november 2000 new ireland earthquake sequence 1 the november 2000 new ireland earthquake sequence started with a mw 8 0 left lateral main shock on 16 november and was followed by a series of aftershocks with primarily thrust mechanisms the earthquake sequence was associated with a locally damaging tsunami on the islands of new ireland and nearby new britain bougainville and buka results from numerical tsunami propagation models of the main shock and two of the largest thrust aftershocks mw 7 0 indicate that the largest tsunami was caused by an aftershock located near the southeastern termination of the main shock off the southern tip of new ireland aftershock 1 numerical modeling and tide gauge records at regional and far field distances indicate that the main shock also generated tsunami waves large horizontal displacements associated with the main shock in regions of steep bathymetry accentuated tsunami generation for this event most of the damage on bougainville and buka islands was caused by focusing and amplification of tsunami energy from a ridge wave between the source region and these islands modeling of changes in the coulomb failure stress field caused by the main shock indicate that aftershock 1 was likely triggered by static stress changes provided the fault was on or synthetic to the new britain interplate thrust as specified by the harvard cmt mechanism for other possible focal mechanisms of aftershock 1 and the regional occurrence of thrust aftershocks in general evidence for static stress change triggering is not as clear other triggering mechanisms such as changes in dynamic stress may also have been important the 2000 new ireland earthquake sequence provides evidence that tsunamis caused by thrust aftershocks can be triggered by large strike slip earthquakes similar tectonic regimes that include offshore accommodation structures near large strike slip faults are found in southern california the sea of marmara turkey along the queen charlotte fault in british columbia and near the alpine fault of new zealand results from this study and previous stress modeling studies suggest that the likelihood of local tsunamis in these regions may significantly increase after a great strike slip earthquake

mobility of gold and silver in lateritic weathering profiles some observations from western australia experimental evidence suggests that very acid chloride solutions generated in lateritic profiles by ferrolysis oxidation and hydrolysis of iron are responsible for the dissolution of gold and silver au is subsequently reprecipitated by reduction of the aucl 4 super ion with fe super 2 reduction of the agcl complex does not similarly occur because of the relative redox potentials of fe super 2 fe super 3 and ag agcl thus chemical refinement of gold occurs during lateritization the development of low ph and the redistribution of gold and silver in lateritic weathering profiles appear to be more common over granitic and gneissic basement and may be inhibited by the presence of carbonate in the weathering zone of basic rock sequences modified journal abstract