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

salt related structures in the gulf of mexico a field guide for geophysicists in finding economic quantities of oil and gas below a salt sheet the mahogany discovery well in ship shoal south addition offshore louisiana dramatically boosted exploration interest in the gulf of mexico s subsalt play in shallow water interest was initially aroused by the earlier deep water discovery below a salt sheet forming part of mickey mouse a cluster of pancake shaped salt sheets in mississippi canyon block 211 a major technologic obstacle in this play has been the difficulty of imaging the base of salt sheets and the subsalt structures targeted for exploration some recent articles have showcased the spectacular fidelity of 3 d seismic data sets collected on closely spaced survey lines and processed by techniques such as 3 d prestack depth migration and 3 d dip moveout followed by 3 d turning ray kirchhoff migration however at present the cost of this technology limits its use to specific rigorously selected targets

reverse time migration of multiples for subsalt imaging some hydrocarbon reservoirs are trapped beneath salt bodies where seismic imaging is greatly challenged due to poor illumination multiple reflections have different propagation wave paths from primary reflections and thus can be used to complement the illuminations where primary reflections from beneath the salt are not acquired consequently migration of multiples can sometimes provide better subsalt images compared to conventional migration which uses primary reflections only in this paper we propose to modify conventional reverse time migration so that multiples can be used as constructive reflection energy for subsalt imaging this new approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data instead of primary reflections in the reverse time migration process multiples recorded on the surface are extrapolated backward in time to each depth level and the observed data with both primaries and mult

impact craters and craterform structures in fennoscandia abstract sixty two nearly circular topographic morphological or geophysical structures in fennoscandia and estonia were studied to discover their origin the structures were divided into five classes fifteen were caused by meteorite impacts class a and a four of them lappajarvi janisjarvi mien and dellen contain large volumes of impact melt the age of the recognized impact craters varies from pre historic 3500 b c to 700 ma and their diameters from 0 04 km to 55 km the majority of the other structures are probable class b or possible class c craters for which there is not yet sufficient proof of impact origin there is increasing evidence that some of the large arcuate morphological or geophysical structures class e represent the deeply eroded scars of very old craters but owing to the lack of identified shock metamorphic features or impact generated rocks e g allogenic breccias they cannot so far be classified as impact craters so far no craterform structure or ejecta layer of archaean or early proterozoic age has been found in fennoscandia although statistically remnants of ancient cratering events should be found in the fennoscandian shield new ways of searching for these structures are proposed the impact cratering rate in fennoscandia is 2 4 10 14 km 2a 1 n 12 the very small craters are omitted which corresponds to about two events per 100 ma for the last 700 ma this is a minimum estimate and is higher than the global cratering rate

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

dirty salt velocity inversion the road to a clearer subsalt image for deep water gulf of mexico accurate salt geometry is criticaltosubsaltimaging thisrequiresthedefinitionofboth externalandinternal saltgeometries inrecentyears external salt geometry i e boundaries between allochthonous salt and background sediment has improved a great deal due to advances in acquisition velocity model building and migration algorithms but when it comes to defining internal salt geometry i e intrasalt inclusions or dirty salt no efficient method has yet been developed in common industry practices intrasalt inclusions and thus their velocity anomalies are generally ignored during the model building stages however as external salt geometries reach higher levels of accuracy it becomes more important to consider the once ignored effects of dirty salt we have developed a reflectivity based approach for dirty salt velocity inversion this method takes true amplitude reverse time migration stack volumes as input then estimates the dirty salt velocity based on reflectivity under a 1d assumption results from a 2d synthetic data set and a real 3d wide azimuth data set demonstrated that the reflectivity inversion scheme significantly improves the subsalt image for certain areas in general we believe that this method produces a better salt model than the traditional clean salt velocity approach

controls on the composition of ni cu sulfide deposits as illustrated by those at noril sk siberia the ni cu pge ore deposits of the noril sk region have developed in feeder channels for permo triassic continental flood basalt volcanism particularly within 100 to 300 m thick elongate stratified intrusions that merge with an extensive system of peripheral sills through which the magma is thought to have continued to surface the mineralization is centered within two ore junctions the noril sk and talnakh ore junctions principal ore types include massive ore which occurs typically below or at the lowest margin of the intrusions disseminated ore in picritic and taxitic gabbrodolerite which constitute the lower part of the intrusions and copper ore which either forms an impregnation in hornfelsic country rock adjacent to massive ore or occupies the matrix of breccia zones that have developed at the front and upper contact of the intrusions although showing a close spatial relationship to the intrusions the massive ore particularly that underlying the northwest talnakh intrusion appears to have been injected separately over 500 samples have been analyzed for ni cu s pt pd rh ru ir os and au by nis fire assay acid dissolution and neutron activation the massive ore of the kharayelakh orebody beneath the northwest talnakh intrusion has fractionated as injection proceeded from east to west with the fractionated liquid moving progressively farther west this liquid has also soaked into the surrounding hornfels to produce the cu rich mantle and above the western part of the kharayelakh orebody where the cu enrichment is most pronounced has also affected the lower part of disseminated ore zones within the intrusion study of the disseminated ore has revealed that this has not fractionated to a great degree and that adequate sampling gives an average composition close to that of the primary sulfide liquid disseminated ore from different parts of the talnakh ore junction are characterized by very different metal tenors with rh ranging from 4 000 to 10 000 ppb the composition of the disseminated ore is modeled as the result of initial low tenor sulfides becoming trapped in the ore bearing intrusions and then interacting with successive surges of new magma passing along the feeder channels represented by their host intrusions on the way to the surface the different tenors of the different ore junctions and parts of ore junctions are the consequence of the effective number of times the magma passing through a particular channel exceeds the amount of sulfide within the channel higher ratios leading to higher tenors

advances in velocity model building technology for subsalt imaging seismic imaging of hydrocarbon accumulations below salt is a formidable challenge because complexly shaped salt bodies severely distort wavefronts or scatter seismic energy we have highlighted some recent advances in building a velocity model for subsalt imaging there are three main stages suprasalt velocity determination salt model definition and subsalt velocity update volumetric high resolution tomography that incorporates high velocity contrast boundaries is used to derive a good sediment velocity model before building a salt model to facilitate integration of interpretation and depth processing beam based interactive imaging is used to refine the salt geometry for subsalt velocity update either subsalt tomography or subsalt scan based techniques can be used depending on the quality of subsalt reflections there are concepts and techniques for attaining subsalt images suitable for hydrocarbon exploration beneath complexly shaped salt bodies

deep towed gravity surveys in the southern california continental borderland we surveyed two sites in the southern california continental borderland with a newly developed instrument a towed deep ocean gravimeter a gravity sensor that can be towed a few tens of meters above the sea floor during its development phase we used the instrument to survey two regions off the coast of southern california the first was along two tracks in the san diego trough the second was over a seamount like feature named emery knoll results of the trough survey reveal a basin with a geometry consistent with seismic data we observed no significant density contrast across the san diego trough fault in the near surface sediments the survey of emery knoll shed light on the question of the origin of this structure modeling the knoll to determine its bulk density suggested a nearly uniform structure surrounded by sedimentary basins with a more massive central intrusive body derived densities of 2850 kg m3 for the knoll and 3050 kg m3 for the central intrusion assume that no deep unmodeled sources e

sea beam survey of an active strike slip fault the san clemente fault in the california continental borderland the san clemente fault located in the california continental borderland is an active northwest trending right lateral wrench fault sea beam data are used to map the major tectonic landforms associated with active submarine faulting in detail unavailable using conventional echo sounding or seismic reflection data in the area between north san clemente basin and fortymile bank the major late cenozoic faults are delineated by alignments of numerous tectonic landforms including scarps linear trenches benches and sags character and spatial patterns of these landforms are consistent with dextral wrench faulting although vertical offsets may be substantial locally the main trace of the san clemente fault cuts a straight path directly across the rugged topography of the region evidence of a steeply dipping fault surface basins or sags located at each right step in the en echelon pattern of faults are manifestations of pull apart basin development in a right slip fault zone seismic reflection profiles show offset reflectors and a graben in late quaternary turbidites of the navy fan where the fault zone follows a more northerly trend modern tectonic activity along the san clemente fault zone is demonstrated by numerous earthquakes with epicenters located along the fault s trend the average strike of the san clemente fault is parallel to the predicted pacific north american relative plate motion vector at this location therefore we conclude that the san clemente fault zone is a part of the broad pacific north american transform plate boundary and that the southern california region may be considered as a broad shear zone

origin of the mount pinatubo climactic eruption cloud implications for volcanic hazards and atmospheric impacts volcanic ash clouds can be fed by an upward directed eruption column plinian column or by elutriation from extensive pyroclastic flows coignimbrite cloud there is considerable uncertainty about which mechanism is dominant in large scale eruptions here we analyze in a novel way a comprehensive grain size database for pyroclastic deposits we demonstrate that the mount pinatubo climactic eruption deposits were substantially derived from coignimbrite clouds and not only by a plinian cloud as generally thought coignimbrite ash fall deposits are much richer in breathable 10 mm ash 5 25 wt than pure plinian ash at most distances from the source volcano we also show that coignimbrite ash clouds as at pinatubo are expected to be more water rich than plinian clouds leading to removal of more hcl prior to stratospheric injection thereby reducing their atmospheric impact

superwide angle one way wave propagator and its application in imaging steep salt flanks one way wave propagators have been used extensively in forward and inverse problems the advantages of one way wave propagators are the fast speed of computation and the large savings in computer resources however regular one way wave propagators have inherent angle limitation to within 90 of the preferred direction e g z direction we propose to overcome the limitation by a superwide angle one way method which combines and interpolates two orthogonally propagated one way wavefields in the 2d case a weight function determined by the propagation angle is employed to combine the two one way wavefields in the current implementation we use the wavefield gradient to estimate the propagation angle numerical experiments show that the proposed method has accurate superwide angle 90 propagation and can model turning waves well by applying the superwide angle one way wave propagator on several synthetic model data sets we can demonstrate the validity of the method in imaging steep and overhanging

high resolution mapping of two large scale transpressional fault zones in the california continental borderland santa cruz catalina ridge and ferrelo faults new mapping of two active transpressional fault zones in the california continental borderland the santa cruz catalina ridge fault and the ferrelo fault was carried out to characterize their geometries using over 4500 line km of new multibeam bathymetry data collected in 2010 combined with existing data faults identified from seafloor morphology were verified in the subsurface using existing seismic reflection data including single channel and multichannel seismic profiles compiled over the past three decades the two fault systems are parallel and are capable of large lateral offsets and reverse slip during earthquakes the geometry of the fault systems shows evidence of multiple segments that could experience throughgoing rupture over distances exceeding 100 km published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures historical and recent focal mechanisms obtained from first motion and moment tensor studies confirm regional strain partitioning dominated by right slip on major throughgoing faults with reverse oblique mechanisms on adjacent structures transpression on west and northwest trending structures persists as far as 270 km south of the transverse ranges extension persists in the southern borderland a logjam model describes the tectonic evolution of crustal blocks bounded by strike slip and reverse faults which are restrained from northwest displacement by the transverse ranges and the southern san andreas fault big bend because of their potential for dip slip rupture the faults may also be capable of generating local tsunamis that would impact southern california coastlines including populated regions in the channel islands

a detailed image of the continent borderland transition beneath long beach california new crustal images beneath long beach california show the region of the inner borderland to continent transition the cross sections are obtained from stacked autocorrelations of virtual sources generated from oil industry data recorded in the city of long beach ca they show that the moho is dipping at 65 and obliquely truncates an 10 km thick flat lying lower crustal fabric the moho appears to be fault controlled and an integral part of the extrusion of the catalina schist that underlays the inner borderland the basement interface has significant offsets of up to 2 km none of which correspond to the mapped trace of the newport inglewood fault

u pb zircon geochronology and tempo of the end permian mass extinction the mass extinction at the end of the permian was the most profound in the history of life fundamental to understanding its cause is determining the tempo and duration of the extinction uranium lead zircon data from late permian and early triassic rocks from south china place the permian triassic boundary at 251 4 0 3 million years ago biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the changhsingian pulse of the end permian extinction corresponding to the disappearance of about 85 percent of marine species lasted less than 1 million years at meishan a negative excursion in 13 c at the boundary had a duration of 165 000 years or less suggesting a catastrophic addition of light carbon

stratigraphy structure and volcanology of the se deccan continental flood basalt province implications for eruptive extent and volumes the deccan volcanic province is one of the world9s largest continental flood basalt provinces and derives additional importance because its eruptions 64 67 ma straddle the cretaceous tertiary boundary to better assess the environmental impact of deccan volcanism and its possible effect upon cretaceous tertiary boundary biota it is necessary to document the stratigraphy chronology and volume of the eruptions new chemostratigraphical data permit mapping of the se deccan these data strengthen the likelihood that the rajahmundry traps of eastern india were originally fed by long distance flows and are an extension of the main deccan volcanic province an east west cross section reveals a depression or moat around the se periphery of the deccan volcanic province this provided a site in which shallow lakes initially formed and along which later lava eruptions became channelled and confined published palaeomagnetic data indicate that the lavas of the se deccan were erupted during chron 29r coeval with the cretaceous tertiary boundary and the chemostratigraphic data place the associated lake sediments i e lameta group beneath and within lavas of the wai subgroup finally these new map data are combined with previous work to provide a quantitative estimate for the original deccan volcanic province eruptive volume of c 1 3 10 6 km 3

assimilation of sulfate and carbonaceous rocks experimental study thermodynamic modeling and application to the noril sk talnakh region russia most of the intrusions in the noril sk talnakh region siberia are hosted in thick sedimentary sequences including abundant evaporitic and terrigenous sedimentary rocks three mafic ultramafic intrusions in this region contain unusually thick massive sulfide deposits which represent one of the world s largest economic concentrations of ni cu and pge the interaction of siberian magmas with sulfate and organic matter rich sedimentary rocks has been proposed as a possible mechanism for the origin of these exceptional sulfide deposits but the interaction process and the reaction paths have never been fully investigated here we clarify by both experimental petrology and thermodynamic modeling how sulfate and organic matter assimilation occur in mafic ultramafic magmas affecting magma composition crystallization and sulfide saturation interaction experiments were conducted at conditions relevant to the emplacement of noril sk type intrusions 1200 c 80 mpa to simulate the assimilation of sulfate and or organic compounds by ultramafic magmas we used a picrite from noril sk1 intrusion and coal and anhydrite from the area as starting materials the experimental results show that the incorporation of anhydrite into the magma occurs by chemical dissolution in the melt which increases the magma s sulfur content but suppresses sulfide saturation and reduces olivine crystallization extreme assimilation leads to sulfate saturation in the magma and high dissolved sulfur contents of 0 9 0 1 wt s conversely coal assimilation promotes sulfide segregation and magma crystallization while decreasing the dissolved h2o content of the melt and increasing the amount of coexisting fluid phase we also employed gas melt thermodynamic calculations to quantify the effect of these assimilations on the redox conditions and the s content of the magma and investigate the role of temperature pressure and initial gas content of the magma in the assimilation process we quantify how sulfate assimilation strongly oxidizes the magma and increases its s content both effects are intensified by increasing pressure from 50 to 100 mpa in this study decreasing temperature from 1350 to 1200 c in this study and decreasing amounts of fluid phase initially coexisting with the magma from 2 to 0 wt the interaction with organic matter ch in this study induces a strong reduction of the magma even for extremely low degrees of assimilation few tenths of wt and the dehydration of the melt we therefore suggest that in the noril sk talnakh district 1 additional s was supplied to mantle derived magmas by the assimilation of evaporitic rocks and was transported during magma ascent in the form of dissolved oxidized s 2 a substantial reduction of the magma inducing sulfide segregation and important crystallization then occurred due to the interaction with carbonaceous sediments this mechanism can potentially produce massive sulfide deposits by important sulfate assimilation locally higher than 3 wt caso4 and minor organic matter assimilation few tenths of wt ch however if one of the two steps does not occur or the assimilation in 1 is not large enough disseminated sub economic or no sulfide deposits are produced we conclude that exceptional conditions favoring substantial assimilation of sediments are needed to generate exceptional ore deposits like those of the noril sk talnakh district

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

siderophile and chalcophile metals as tracers of the evolution of the siberian trap in the noril sk region russia abstract in this study cu ni and platinum group elements pge were determined in a sequence of basaltic and picritic lavas from the siberian trap in the noril sk area of russia to constrain genetic relationships between the basalts and the petrogenesis of ni cu pge sulfide deposits associated with the talnakh and noril sk intrusions in the most primitive basalts 8 19 wt mgo of the tuklonsky tk suite pt and pd concentrations range from 4 13 ppb increasing with decreasing mgo content whereas ir contents decrease with mgo from 0 8 0 05 ppb the contrasting behavior of these elements which all have very high sulfide silicate partition coefficients as well as the primitive mantle like ratios of cu y and pd y suggests that these magmas were not sulfide saturated the high pge abundances imply that their parental magmas were also not sulfide saturated during partial melting in the mantle due to sulfide segregation the overlying basalts of the nadezhdinsky nd series are low in cu and ni 52 and 38 ppm respectively highly depleted in all pge and have very low cu y pd y and pd cu ratios however in stratigraphically higher levels cu ni and pge concentrations increase systematically through the morongovsky mr suite to reach a concentration plateau in the uppermost mokulaevsky mk suite pt 8 ppb pd 9 ppb ir 0 12 ppb rh 0 4 ppb at the same time ratios such as cu y increase and approach primitive mantle values however ratios involving pge such as pd y remain low suggesting the removal of small amounts of sulfide 0 01 0 03 the compositional variations in the basalts and the sulfide liquids can be quantitatively described by fractional segregation of a sulfide liquid in an open or closed system magma chamber the latter model suggests that the basalts represent the eruption products of a zoned magma chamber in which light magma with crustal components contaminated overlies less contaminated denser magma crustal contamination caused sulfide saturation and the resulting sulfide liquids settled through a magma column and accumulated at the bottom of the chamber in this model the sulfide liquid is not in equilibrium with the whole magma mass and sulfide segregation is compared with the zone refining process of metallurgy the sulfides become more enriched as they move through the magma and although the magma left behind is depleted in pge cu and ni their concentrations also increase with depth eventually the magma chamber is emptied from the top to the bottom producing the flood basalt sequence and the associated intrusions and ore deposits in the open system model sulfide saturation was initially caused by assimilation of crustal material by the tuklonsky magma continuous and simultaneous replenishment assimilation and crystallization processes formed the lower nd lavas the concurrent removal of 0 5 1 sulfide strongly depleted these magmas in chalcophile and siderophile metals due to the continuous replenishment of the magma chamber with uncontaminated pge rich magma succeeding lavas mr mk show diminishing signs of crustal contamination and become less sulfide saturated as indicated by the increasing ni cu and pge abundances during the evolution of the chamber the magma remained sulfur saturated and sulfides accumulated at the base the composition of the sulfide ores could be regarded as a mixture consisting of low ni cu and pge sulfides derived with a low silicate sulfide ratio 100 from the tk nd magma and high ni cu and pge sulfides formed with a high ratio 10 000 from the mr mk magma

sulfur and chlorine in late cretaceous deccan magmas and eruptive gas release large volume p hoehoe lava flows erupted 67 to 65 million years ago forming the deccan traps india the impact of these flood basalt eruptions on the global atmosphere and the coeval end cretaceous mass extinction has been uncertain to assess the potential gas release from this volcanism we measured sulfur and chlorine concentrations in rare glass inclusions inside crystals and on glassy selvages preserved within lavas concentrations range from 1400 parts per million of s and 900 parts per million of cl in inclusions down to a few hundred parts per million in the lava these data indicate that eruptions of deccan lavas could have released at most 0 103 weight of s yielding up to 5 4 teragrams of so2 per cubic kilometer of lava a more conservative estimate is 0 07 weight of s and 0 04 weight of cl yielding 3 5 teragrams of so2 and 1 teragram of hcl for every cubic kilometer of lava erupted the flows were very large in volume and these results imply that huge amounts of s and cl gases were released the environmental impact from even individual eruptions during past flood basalt activity was probably severe

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

sulfur bearing magmatic accessory minerals almost all magmas contain sulfide or sulfate bearing phases in most natural samples the sulfur bearing phase is a sulfide which typically is pyrrhotite fe1 x s or pyrite fes2 although chalcopyrite cufes2 pentlandite fe ni 9s8 sphalerite zns or molybdenite mos2 may be present as well sulfate minerals are rare at magmatic conditions and anhydrite caso4 is the most common other magmatic so4 bearing minerals include the sodalite group minerals hauyne simplified formula na ca 4 8 al6si6 o s 24 so4 cl 1 2 scapolite minerals silvialite ca na 4al6si6o24 so4 co3 and s bearing apatite ca5 po4 3 f cl oh barite baso4 has been mentioned in rare cases marchev 1991 sulfur bearing minerals usually constitute a negligible fraction of the mineral assemblage in magmatic rocks and thus can be classified as accessory minerals the crystallization of sulfide sulfate and s bearing minerals strongly depends on melt composition temperature and pressure and the s speciation in melt which in turn is strongly dependent on the prevailing oxygen fugacity baker and moretti 2011 this volume wilke et al 2011 this volume irrespectively of the low abundance of s bearing minerals the evolution of sulfur in magmas may be evaluated from the occurrence of sulfides and sulfates these minerals are critical for estimating the activity of various sulfur bearing species in magmas and can be used to constrain the oxygen fugacity and the s concentration in the melt e g pre eruptive sulfur concentration in melts the presence of either sulfide or sulfate in silicate melt indicates that the predominant dissolved sulfur species in the melt are s2 or s6 respectively typically one of these species is dominant but there are conditions where s2

modeling the distribution of the volcanic aerosol cloud from the 1783 1784 laki eruption 1 we conducted simulations of the atmospheric transformation and transport of the emissions of the 1783 1784 laki basaltic flood lava eruption 64 10 n 17 15 w using the nasa goddard institute for space studies modele climate model coupled to a sulfur cycle chemistry model the model simulations successfully reproduced the aerosol clouds of the 1912 katmai and 1991 mount pinatubo eruptions giving us confidence in the laki simulations simulations of the laki eruption produce peak zonal mean sulfate so4 concentrations of over 70 ppbv during august and into september 1783 in the upper troposphere and lower stratosphere at high latitudes while the majority of the sulfate aerosol was removed during the fall and early winter a significant aerosol perturbation remained into 1784 the peak so2 gas loading was just over 37 megatons mt in late june with the sulfate loading peaking in late august 1783 at 60 mt over the average of 3 runs this yielded a peak sulfate aerosol 75 h2so4 25 h2o loading of over 80 mt with the total aerosol produced during the entire eruption being about 165 mt the resulting sulfate deposition compares well with ice cores taken across greenland the top of atmosphere net radiative forcing peaks at 27 w m2 over the high latitudes during late summer 1783 and produces a global mean forcing of 4 w m2 the model results confirm that northern hemisphere high latitude volcanic eruptions produce aerosols that remain mostly confined north of 30 n latitude

modeling the impact of wide azimuth acquisition on subsalt imaging wide azimuth towed streamer wats acquisition improves the subsalt seismic image by suppressing multiples improves the results of 3d surface related multiple elimination srme processing and provides more uniform seismic illumination of subsalt targets a simple model shows that the additional suppression of multiples in the case of wats acquisition is the result of a natural weighting of the traces going into the stack due to the areal nature of the acquisition this simple model also shows that the extent of the additional multiple suppression is strongly dependent on the acquisition effort a sparse acquisition effort will result in little additional multiple suppression the use of 3d srme processing is shown to be more accurate in predicting multiples given input data with multiple azimuths compared to making similar predictions from narrow azimuth data three dimensional srme has the potential to reduce the residual multiples to the same extent as wats acquisition with a higher acquisition eff

when a cosmic impact strikes the sea bed most bolides that collide with the earth hit the sea limited knowledge about marine target impacts hampers predictions about their perilousness this study presents geological features that are particular to craters formed at sea the features are most likely a result of the influence of the target on the cratering process marine target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed when the crater diameter is large compared to the water depth the crater resembles its counterparts that are formed on land craters formed in deeper water are concentric and often lack melt sheets and rim walls but have deposits and radial gullies formed by the resurge of the sea impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater

subsalt 3d vsp imaging at deimos field in the deepwater gulf of mexico at deimos field in the deepwater gulf of mexico surface seismic imaging cannot fully image sedimentary structure near the steeply dipping base of salt 3d vertical seismic profile vsp imaging was introduced to the deepwater gom to improve seismic imaging of complex structural and stratigraphic features ray et al 2003 hornby et al 2006 and recently has been used for imaging blind spots in surface seismic coverage hornby et al 2005 hornby et al 2007 with this method 3d surveys are conducted using a surface source vessel and downhole geophones here we investigate the use of 3d vsp imaging to complement the surface results with additional coverage beyond that of the surface seismic adjacent to the complex salt body

paleomagnetism and neogene tectonics of the northern channel islands california geologic observations and previous paleomagnetic studies have suggested that the western transverse ranges arrived at their anomalous east west orientation by clockwise tectonic rotation a paleomagnetic study of the northern channel islands was undertaken in order to test the extent of rotated areas and to develop constraints on tectonic models concerning the formation of the southern california borderland the islands of anacapa santa cruz santa rosa and san miguel form an east west trending chain and are considered part of the transverse range physiographic province oligocene through miocene volcanic and intrusive rocks occur on these islands and were sampled for this paleomagnetic study eocene sandstones were sampled on san miguel island and eocene sandstones and miocene siltstones were sampled on santa cruz island paleomagnetic results from igneous units are characterized by declinations deflected clockwise by 69 to 81 from expected directions and inclinations which are too shallow by 10 to 25 normal and reversely magnetized units yield antipodal mean directions the mean result is concordant to the directions obtained from individual units which passed fold and baked contact tests these results suggest that the northern channel islands have been tectonically rotated into place since early miocene time as the outer borderland area translated northwestward within a large shear zone between the pacific and north american plates the data from the northern channel islands when averaged with the data from the santa monica mountains yield a result of i 36 1 5 1 d 72 6 6 3 for the southern part of the western transverse ranges 52 units 405 samples the shallow paleomagnetic inclinations suggest a northward latitude translation of 14 0 3 7 or 3 9 for the western transverse ranges however most plate tectonic reconstructions suggest only 4 3 of northward translation the discrepancy may be due to initial dips of the volcanic flows which would cause an artificial shallowing of inclination when the structural dip of the units was corrected to an assumed original horizontal other causes for the shallow inclinations could be the effects of an offset dipole field or nondipole field components or motions of small tectonic plates between the north american and pacific plates as the farallan plate broke up and was subducted

crater features diagnostic of oblique impacts the size and position of the central peak using magellan data we investigated two crater characteristics that have been cited as diagnostic of oblique impacts an uprange offset of the central peak in complex craters and an increasing central peak diameter relative to crater diameter with decreasing impact angle we find that the offset distribution is random and very similar to that for high angle impacts and that there is no correlation between central peak diameter and impact angle accordingly these two crater characteristics cannot be used to infer the impact angle or direction

the scaling of complex craters the empirical relation between the transient crater diameter dg and final crater diameter dr of complex craters and basins is estimated using cumulative terrace widths central uplift diameters continuous ejecta radii and transient crater reconstructions determined from lunar and terrestrial impact structures the ratio dg dr is a power law function of dr decreasing uniformly from unity at the diameter of the simple complex crater morphology transition to about 0 5 for large multiring basins like imbrium on the moon the empirical constants in the dg dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater and hence the extent of collapse is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock the continuity of the dg dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences

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

observations on the origins and ascent mechanisms of granitic magmas this review presents a summary of most current beliefs about the origins of granitic magmas together with calculations of some important properties of granitic melts and magmas turning attention to magma ascent mechanisms diapirism and dyking represent two competing end member models the differences in processes and products between the two are fundamental and the implications of the differences are profound for the evolution of the continental crust the weight of evidence favours dyke ascent for the majority of granitic magmas and modelling shows that dyke ascent is both rapid and thermally effcient furthermore the structural and petrological features expected in crust through which granitic diapirs have passed have not been discovered in nature granites in the upper crust commonly have forms and structures indicating that diapirism could not have been the emplacement mechanism in contrast granitic dykes are common suggesting that it is reasonable to consider the ascent of granitic magmas through fracture systems many granitic batholiths are tabular and some have been fed by large dykes the rate limit in granitic plutonism is not magma ascent but either the rate of melting and segregation near the magma source or the rate of deformation at the emplacement site

microplate capture rotation of the western transverse ranges and initiation of the san andreas transform as a low angle fault system tectonic rotation of the western transverse ranges block is explained by capture of the partially subducted monterey microplate by the pacific plate at about anomaly 6 time ca 20 ma as pacific monterey spreading slowed and eventually ceased the slip vector along the gently northeast dipping subduction interface beneath the california margin changed from slightly oblique subduction to transtensional dextral transform motion this change in slip vector and a shift of pacific plate motion eastward along the already subducted monterey plate interface imply that the san andreas transform began as a system of low angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension this basal shear produced the rotated western transverse ranges this model helps explain the timing of initial rotation and basin formation the sudden appearance of widely distributed transform motion well inland of the margin in early miocene time why the western transverse ranges uniquely rotated as a large coherent crustal block and several fundamental structural characteristics of central and southern california the model also provides major constraints on the amount of pacific north america strike slip motion the position through time of offshore oceanic plates relative to onshore geology and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system

sulfur isotopic studies of continental flood basalts in the noril sk region implications for the association between lavas and ore bearing intrusions previous studies of both ore and non ore bearing intrusives in the permo triassic flood basalts of the siberian platform in the noril sk area have shown that high grade ni cu platinum group elements pge mineralization is associated with anomalously high 34s values of 8 to 12 in addition several researchers have proposed that observed depletions in the cu ni and pge content of basaltic lavas of the nadezhdinsky nd formation are related to diffusional exchange with and upgrading in metal tenor of sulfides in the volcanic conduit system sulfur isotopic studies of the lavas at noril sk were initiated to determine if interaction with crustally derived sulfur in the conduit system was evident and if the nd lavas in particular were characterized by an anomalous isotopic signature 34s values of the lavas range from 4 5 to 8 7 vienna canon diablo troilite vcdt with s concentrations from 40 to 1373 ppm the majority of 34s values range from 0 to 4 and are similar to those from s poor intrusions in the noril sk area although textural data are not supportive of early sulfide saturation and the presence of immiscible sulfide droplets in the lavas recrystallization may have erased expected mineralogical and textural evidence mineralogical data indicate that hydrothermal alteration of the lavas has occurred but s redistribution has been restricted to localized areas and 34s values have not been affected the relatively low s concentrations of the lavas are thought to be due in large part to degassing of the lavas in the shallow conduit system and during eruption our calculations are consistent with the premise that degassing of basaltic magmas at temperatures in excess of 900 c at qfm leads to only minor 34s depletion of sulfur remaining in the melt and decreases in 34s values of less than 2 at 90 degassing for this reason all lavas with 34s values in excess of 2 require a contribution of 34s enriched country rock sulfur because of the high s content and 34s value 16 20 of evaporites in the country rocks at noril sk contamination of less than 0 5 is required to explain the most 34s enriched lavas the nd lavas have an average 34s of 2 9 but show no difference in s isotopic composition relative to the other lavas suggesting that metal depletion involved only limited s transfer or that exchange between mantle derived s and s of crustal origin buffered 34s values to less than 5 anomalously positive 34s values similar to those of the ore bearing intrusives in the noril sk region are not consistently found in low s rocks either lavas or intrusives although the mechanism for the derivation of sulfide in the ore bearing intrusions remain speculative it is clear that the formation of sulfide ores characterized by high metal tenors proceeded only in the presence of sulfur of crustal origin

age and timing of the permian mass extinctions u pb dating of closed system zircons the age and timing of the permian triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts zircons from nine ash beds within the shangsi and meishan sections china pretreated by annealing followed by partial attack with hydrofluoric acid result in suites of consistent and concordant uranium lead u pb ages eliminating the effects of lead loss the u pb age of the main pulse of the extinction is 252 6 0 2 million years synchronous with the siberian flood volcanism and it occurred within the quoted uncertainty

volcanic geomorphology an overview abstract the review examines the role of geomorphology in analyzing the volcanoes on earth five objectives are stressed first classifications of volcanic landforms should be improved to take care of the complexity in volcanic landform generation as magmatic systems style of eruption and the erupted material all influence the morphology second geomorphology should contribute to the science of volcanology through its capability in reconstructing growth stages in complex volcanoes and also in analyzing the structural factors which contribute to the catastrophic collapse of volcanoes third geomorphology can contribute to physical volcanology by assessing the effects of topography on transport erosion and deposition of volcanogenic flows and identifying the sources and climatic tectonic conditions which govern the emplacement of volcaniclastic deposits fourth volcanic geomorphology a identifies sedimentary facies associations b constructs facies models for dynamic volcano delivery systems and c analyzes the characteristics of sediment gravity flows in order to determine relevant parameters for modelling their behaviour fifth process oriented geomorphology is critical in developing accurate methods for measuring rates of geomorphic processes that shape ephemeral volcanic constructs and for evaluating and comparing geomorphic impacts on disturbed catchments and the related hydrologic response before during and after eruptions this should help to refine parameters for the exponential decay model finally volcanic geomorphology is essential for risk assessment through geomorphic hazard zonation and composite risk zonation such treatments are necessary in order to face the enhanced challenge posed by the combination of natural hazards and the increasing number of people who are at risk around volcanoes

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

magmatic anhydrite sulfide assemblages in the plumbing system of the siberian traps we report the first discovery of magmatic anhydrite sulfide assemblages in a subvolcanic intrusion associated with the siberian traps the 34s values of anhydrite and coexisting sulfide crystals analyzed by ion probe are 18 22 and 9 11 respectively more than 50 of the total sulfur in the intrusion is estimated to derive from marine evaporites in the footwall strata the contaminated magma was highly oxidized and able to dissolve up to one order of magnitude more sulfur than pure mantle derived basaltic magma such contaminated magma if erupted would have released far more so2 into the atmosphere than is generally appreciated

fate of fossil fuel co2 in geologic time 1 a model of the ocean and seafloor carbon cycle is subjected to injection of new co2 pulses of varying sizes to estimate the resident atmospheric fraction over the coming 100 kyr the model is used to separate the processes of air sea equilibrium an ocean temperature feedback caco3 compensation and silicate weathering on the residual anthropogenic pco2 in the atmosphere at 1 10 and 100 kyr the mean lifetime of anthropogenic co2 is dominated by the long tail resulting in a range of 30 35 kyr the long lifetime of fossil fuel carbon release implies that the anthropogenic climate perturbation may have time to interact with ice sheets methane clathrate deposits and glacial interglacial climate dynamics

original size of the vredefort structure implications for the geological evolution of the witwatersrand basin abstract historically there have been a range of diameter estimates for the large deeply eroded vredefort impact structure within the witwatersrand basin south africa here we estimate the diameter of the transient cavity at the present level of erosion as 124 140 km based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted transvaal outliers taking erosion into account 6 km and scaling to original final rim diameter an estimate of close to 300 km for the rim diameter is obtained independent estimates of the final rim diameter based on an empirical relation of central uplift diameter to rim diameter spatial distribution of pseudotachylites and concentric large scale structural patterns give a similar estimate of close to 300 km for the original final rim diameter an impact structure of this size is expected to have had an original multi ring form at this size the vredefort impact structure encompasses the bulk of the witwatersrand basin which appears to owe its preservation to the vredefort impact in addition the vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area which in recent interpretations is believed to be a component in the creation of the world class gold deposits of the witwatersrand basin

understanding subsalt illumination through ray trace modeling part 1 simple 2 d salt models the complex structure and high velocity contrast of salt in the gulf of mexico create a difficult seismic imaging problem 3 d prestack depth migration presdm of seismic data has allowed imaging of reflectors under the salt sheets and of detached bodies of irregular shape but 3 d presdm cannot fill in shadow zones below salt where little energy is reflected additionally amplitude variations caused by salt structures focusing or dispersing seismic energy are usually poorly handled by 3 d presdm ray trace modeling can clarify subsalt imaging problems and the modeling results should be incorporated into exploration and development plans

generalized staining algorithm for seismic modeling and migration abstractthe staining algorithm is introduced to improve the signal to noise ratio s n of poorly illuminated subsurface structures in seismic imaging however the amplitudes of the original and the stained wavefield i e the real and the imaginary wavefields differ by several orders of magnitude and the waveform of the stained wavefield may be greatly distorted we have developed a generalized staining algorithm gsa to achieve amplitude preservation in the stained wavefield the real wavefield and the stained wavefield propagate in the same velocity model a source wavelet is used as the source of the real wavefield however the real wavefield is extracted from the stained area as the source of the stained wavefield the gsa maintains some properties of the original staining algorithm the stained wavefield is synchronized with the real wavefield and it contains only information relevant to the target region by imaging with the stained wavefield we obtain higher s ns in images of target struct

how will we recognize buried impact craters in terrestrial sedimentary basins criteria that are deemed critical for recognition of terrestrial impact craters include features from hand specimens such as shock metamorphic structures although these criteria have been selected because they are unique physical products of bolide impact strict adherence to these criteria means that we cannot positively identify buried astroblemes that are tomographically imaged on three dimensional seismic data but are undrilled because many of earth9s deeply buried structures may never be drilled because of prohibitive cost and lack of commercial incentive a large proportion of as yet undiscovered terrestrial astroblemes including the best preserved examples may remain unclassified and be excluded from impact structure research a framework for the identification of impact structures imaged on three dimensional seismic data is proposed on the basis of geometrical criteria plus additional image quality and preservation criteria that assign a level of confidence to the assessment

modelling massive sulphate aerosol pollution following the large 1783 laki basaltic eruption abstract the climatic impact of volcanic eruptions depends mainly on the amount of sulphur rich gases released into the stratosphere these are rapidly converted to sulphate aerosols which result in cooling of the lower troposphere and warming of the lower stratosphere these gases are retained in the stratosphere and then concentrated by atmospheric processes forming acid fog near the earth s surface with potentially dramatic environmental consequences on plants animals and humans we have modelled such emissions in the case of the unusually large 1783 1784 basaltic fissure eruption of laki iceland using an atmospheric general circulation model results show good agreement with historical observations such as typical time taken by individual pulses to reach continental europe or geographical extent of the deadly haze that covered much of the northern hemisphere the model could be adjusted to predict the climatic consequences of very large eruptions up to the scale of massive flood basalts and test the proposal that these are the main agent in most mass extinctions of life on earth

migration by extrapolation of time dependent boundary values migration of an observed zero offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time this concept is implemented through a finite difference solution of the two dimensional acoustic wave equation all depths are imaged simultaneously at time 0 the imaging condition and all dips right up to vertical are correctly migrated numerical examples illustrate this technique in both constant and variable velocity media

large enigmatic crater structures offshore southern california summary digital mosaics of swath and conventional bathymetry data reveal large distinct near circular crater structures in the inner continental borderland offshore southern california two have maximum crater diameters that exceed 30 km and a third has a crater diameter of approximately 12 km all three features exhibit the morphology of large complex craters raised outer rim ring moat and central uplift yet their exact origin remains a mystery preliminary analyses of available seismic gravity and magnetic data over these structures reveal both similarities and distinct differences in geometry structure and geophysical signature to known impact sites all three crater structures however occur within the catalina terrane a highly extended volcanic and metamorphic province floored by catalina schist basement a likely alternative origin may thus involve explosive volcanism caldera collapse and resurgent magmatism and or possibly plutonism and schist remobilization associated with the catalina terrane no single model for crater formation whether impact caldera or pluton fully accounts for all of the present observations regarding the morphology internal structure and known geology of these near circular features timing of crater formation post dates the initial rifting and rotation of the western transverse ranges and appears to predate major right slip along the san clemente and san diego trough fault systems or approximately 18 to 16 ma regardless of their origin these complex craters represent some of the largest structures of their kind in western north america and provide a unique opportunity to better understand the development of unusual crater structures in a submarine environment

unique cu rich sulphide ores of the southern 2 orebody in the talnakh intrusion noril sk area russia geochemistry mineralogy and conditions of crystallization abstract we present new geochemical and mineralogical data from the southern 2 orebody located in the south western branch of the talnakh intrusion russia and composed of massive and disseminated sulphides the southern 2 orebody consists dominantly of chalcopyrite along with lesser pentlandite and subordinate cubanite bornite and pyrrhotite and differs from cu rich ores of the oktyabr sky deposit due to the absence of moikhukite talnakhite assemblages ores from the southern 2 orebody are highly enriched in copper nickel and platinum group elements pges cu up to 28 12 and 10 58 wt ni up to 6 29 and 2 45 wt and pt pd up to 220 and 173 ppm in the massive and disseminated ores respectively the ratios of cu ni 3 85 7 34 in massive and 2 64 6 58 in disseminated ores the ratios of the platinum to iridium group elements ppge ipge reaches more than 40000 these features indicate an extremely different degree of fractionation of the sulphide melt during the formation of ores the sequence of crystallization of the pge minerals pgms on the basis of textural information was as follows pt pd sn rustenburgite atokite pd cu sn stannopalladinite pt fe cu ni tetraferroplatinum pd ni as majakite pd as palladoarsenide pd pb bi zvyagintsevite polarite and pd5pb3 unnamed phase au cu pd tetra auricupride and auricupride au ag alloys this is consistent with experimental data on pge minerals the pgm assemblage was caused by the evolution of a significantly fractionated sulphide melt that was enriched in cu ni pt pd pb and au and as a result produced minerals with peculiar compositions e g cu rich stannopalladinite pb rich polarite ni rich palladoarsenide which do not occur in other ores furthermore pt fe cu ni alloys are abundant and sperrylite is absent which is a very common phase in all ores of the noril sk area the composition of pentlandite with ni ni fe ratios ranging from 0 48 to 0 68 indicates that it formed between 560 c ni poor to 500 c ni rich and 10 5 to 7 lgfs2 respectively the pgms and au minerals were formed at temperatures ranging from 550 to 225 c the specificity of sulphide with pgms association is due to the evolution of the cu rich and ni rich melt under elevated sulphur fugacity conditions the mechanism of filter pressing which precipitated sulphide drops at the bottom of the chamber and penetration of sulphide melts into the country rocks is supported by the same geochemical composition of disseminated and massive ores

transient and disruption cavity dimensions of complex terrestrial impact structures derived from magnetic data 1 accurate transient and disruption cavity dimensions are critical for estimating the energy release associated with impact transient and disruption cavity size can in principle be inferred from morphometric relationships based on crater diameter however locating the crater rim can be difficult for eroded terrestrial craters and existing morphometric relationships are mostly based on observations of extraterrestrial craters where morphologic features at best provide imprecise constraints on the collapsed disruption cavity margin fortunately magnetic survey data collected over terrestrial impact structures demonstrate that collapsed disruption cavity size can be estimated directly from changes in the magnetic anomaly character a lower bound on this parameter can be defined by the outer limit of short wavelength intense magnetic anomalies produced by impact melt and or suevite deposits an upper bound is given by the inner limit of magnetic anomaly trends associated with the pre impact target rock configuration using published values of crater diameters d and values of collapsed disruption cavity diameters dcdc derived from magnetic data for 19 complex terrestrial impact structures we derive the relationship dcdc 0 49d these data and the possibility of geometrical similarity in crater collapse suggest that this relationship is independent of complex crater size over more than a decade of size variation

compositional variations of olivine and sulfur isotopes in the noril sk and talnakh intrusions siberia implications for ore forming processes in dynamic magma conduits olivine from the noril sk i main and northwestern talnakh ore bearing intrusions in the noril sk region occurs as inclusions discrete grains and poikilitic crystals the original compositions of olivine inclusions have been well preserved by their host oikocrysts from reaction with interstitial silicate and sulfide liquids in contrast the compositions of discrete and poikilitic olivine crystals have been variably modified by reequilibration with trapped silicate and sulfide liquids there are abrupt changes in the contents of nickel in olivine inclusions 500 1 500 ppm between different rock units in all three intrusions which indicates that the different rock units in these intrusions are not related to each other via in situ differentiation of a common parental magma but rather are consistent with separate pulses of magma with variable degrees of differentiation possibly involving olivine crystallization and assimilation of siliceous crustal material in a staging chamber the 34s values of the samples with high sulfur contents 800 ppm sulfur from all three ore bearing intrusions vary between 8 and 14 per mil two sulfur poor samples 600 ppm sulfur from the northwestern talnakh intrusion yield 34s values of 1 5 and 3 6 per mil these values are similar to the values of the sulfur poor rocks from the ore barren intrusions in the region and illustrate the fact that high sulfur contents couple with increasing 34s values such a relationship is consistent with the addition of crustally derived 34s enriched sulfur to the magma of the ore bearing intrusions in contrast to other models of sulfide segregation in closed systems we propose that sulfide segregation occurred in an open system and the magma involved was undersaturated in sulfur and undepleted in chalcophile elements we envision that the 34s enriched sulfur was introduced via circulating fluids that transported sulfur from evaporite in the underlying sedimentary sequence to the magma in a dynamic magma conduit the immiscible sulfide liquids that segregated during sulfur contamination settled down in the wider parts of the conduit as the magma continued to travel successive pulses of new magma entered the conduit reacted with the early sulfide and displaced much of the early magma in the conduit the ore bearing intrusions are thought to have crystallized within the wider parts of the conduits and their voluminous sulfide poor peripheral sills represent some if not all of the magma that flowed out of the conduits

geochemistry and origin of the intrusive hosts of the noril sk talnakhcu ni pge sulfide deposits the world class deposits of cu ni and platinum group elements pge of the noril sk talnakh district in russia are hosted by relatively small complex mafic ultramafic bodies that intrude permian sedimentary rocks and the lowermost suites of the siberian continental flood volcanic sequence the noril sk type ore bearing intrusions immediately overlie and are probably penecontemporaneous with weakly mineralized intrusions of the lower talnakh type which have similar petrological features but contrasting and much more variable nd os and sr isotopic compositions several types of spatially associated intrusions were emplaced after the ore bearing intrusions and the flood volcanic sequence to clarify relations between the various types of igneous bodies flood volcanic rocks noril sk type intrusions lower talnakh type intrusions and later intrusions and to help understand the formation of the ore deposits we undertook a detailed geochemical and nd os and sr isotopic study of eight of the more important intrusions the data indicate that these intrusions formed by the input of a series of magma pulses each of which produced rocks with distinctive petrology and chemical compositions these compositions are interpreted to have formed from mixtures of mantle derived magmas and different types of crustal rock which were encountered and assimilated during passage to the surface some of these variations may be superimposed on heterogeneities inherited from the mantle source the magmas that formed the lower talnakh type intrusions became contaminated with large amounts of crustal rock in a deep staging chamber the chemical and isotopic characteristics of the resultant hybrid magmas are broadly similar to those of basalts of the lower and middle nadezhdinsky lava subsuites nd 1 2 which are located about one third of the way through the overlying lava sequence these magmas became saturated in sulfur during contamination and segregation of sulfides resulted in the depletion of chalcophile elements recorded in the nd 1 2 lavas magmas feeding the ore bearing intrusions had trace element and isotopic compositions similar to those of the basalts in the upper two thirds of the volcanic sequence these magmas underwent lower degrees of contamination in a deep staging chamber and sulfur saturation was not reached these magmas therefore contained a full compliment of chalcophile elements when they encountered anhydrite rich sediments during passage toward the surface assimilation of these sediments led to the segregation of sulfides to form the ores two aspects of our data impose important constraints on the ore forming process the first is the os isotopic composition of the ore sulfides avg os 7 which is similar to that of mantle rocks and very different from those of sulfides that segregate from highly contaminated magma the second is the absence of ni depletion in olivine in rocks from the ore bearing intrusions most notably in the ore rich picritic gabbrodolerites the simplest explanation is that the sulfides segregated from magmas that were not highly contaminated and were transported into the intrusions by magmas different from those that yielded the sulfide if this explanation is correct it imposes some modifications to earlier models of sulfide ore deposition in the conduits of flood basalts

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

staining algorithm for seismic modeling and migration abstractin seismic migration some structures such as those in subsalt shadow zones are not imaged well the signal in these areas may be even weaker than the artifacts elsewhere we evaluated a method to significantly improve the signal to noise ratio s n in poorly illuminated areas of the model we constructed a phantom wavefield an extension of the wavefield to the complex domain the imaginary wavefield was synchronized with the real wavefield but it contained only the events relevant to a target region of the model which was specified using a staining algorithm the real wavefield interacted with the entire model however all structures except for the target were transparent to the imaginary wavefield which is excited only when the real wavefront arrives at the target structure the real and the imaginary source wavefields were crosscorrelated with the regular receiver wavefield the results were revealed in two images the conventional reverse time migration image and an image of the target