middle jurassic rhyolite volcanism of eastern graham land antarctic peninsula age correlations and stratigraphic relationships silicic volcanism at c 168 ma has been identified previously on the antarctic peninsula and the mapple formation which includes those volcanic rocks has been defined and documented from one area of the east coast of graham land based on age and geochemical criteria correlations have been made to the extensive chon aike province of south america which has been demonstrated to be one of the largest silicic volcanic provinces in the world rhyolitic and intermediate composition volcanic successions from six separate localities on the east coast of the antarctic peninsula are described here and are confirmed as correlatives of the mapple formation based on newly acquired geochronology and field observations they are dominantly rhyolitic crystal tuffs and or ignimbrites with ages in the interval 162 168 ma overlapping with the age of the mapple formation 167 171 ma at the type locality andesitic agglomerates are also described which are included in the same event and demonstrate the occurrence of rare intermediate volcanism which is also seen in the chon aike province a new group the graham land volcanic group is defined here and criteria are established which allow the separation of some volcanic successions out of the previously defined antarctic peninsula volcanic group which takes no account of tectonic setting eruption age or geochemistry

isotopic and fluid inclusion constraints on the formation of polymetallic vein deposits in the central argentinian patagonia the lead isotope compositions of galena and the fluid inclusion systematics of nine barite bearing polymetallic au ag pb zn deposits of the central argentinian patagonia chubut and rio negro provinces have been investigated to constrain the compositions and sources of the mineralizing fluids most of the deposits occur as veins with less common wall rock disseminations and or stockworks and are low sulfidation epithermal deposits hosted in jurassic volcanic rocks fluid inclusion homogenization temperatures th from quartz and sphalerite from the deposits fall within the range of 100 300 c with the highest measured average temperatures for the most eastern deposits mina angela 298 c canadon bagual 343 c the salinities of the hydrothermal fluids at all deposits were low to moderate 10 4 equiv wt nacl three groups of ore deposits can be defined on the basis of 206pb 204pb ratios for galena and these show a general decrease from west to east from 18 506 to 18 000 the central argentinian patagonia deposits have distinctly less radiogenic lead isotope compositions than similar deposits from peru and chile except for the porphyry copper deposits of central and southern peru galena from the mina angela deposit is characterized by very low radiogenic lead isotope compositions 18 000 206pb 204pb 18 037 and 38 03 208pb 204pb 38 09 and reflects interaction with precambrian basement the geographic trend in lead isotope compositions of both galena and whole rocks indicates a crustal contribution which increases eastwards also reflected in the strontium neodymium isotope systematics of the host lavas finally due to the lack of precise age determinations for the central patagonian polymetallic deposits a potential link with andean porphyry copper systems remains an open question

petrogenesis of volcanic rocks that host the world class ag pb navidad district north patagonian massif comparison with the jurassic chon aike volcanic province of patagonia argentina abstract we present the first study of the volcanic rocks of the canadon asfalto formation that host the navidad world class ag pb epithermal district located in the north patagonian massif patagonia argentina these volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of gondwana and represent the mafic to intermediate counterparts of the mainly silicic jurassic chon aike volcanic province lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation la icpms u pb ages of zircon crystals from the volcanic units yielded middle jurassic ages of 173 9 1 9 ma and 170 8 3 ma in the navidad district volcanic rocks of the canadon asfalto formation show arc like signatures including high k basaltic andesite to high k dacite compositions rb ba and th enrichment relative to the less mobile hfs elements nb ta enrichment in light rare earth elements lree y ti depletion and high zr contents these characteristics could be explained by assimilation of crustal rocks in the jurassic magmas which is also supported by the presence of zircon xenocrysts with permian and middle upper triassic ages 281 3 ma 246 5 218 1 and 201 3 ma and quartz xenocrysts recognized in these volcanic units furthermore sr and nd isotope compositions suggest a contribution of crustal components in these middle jurassic magmas high k basaltic andesite has initial 87sr 86sr ratios of 0 70416 0 70658 and nd t values of 5 3 and 4 high k dacite and andesite have initial 87sr 86sr compositions of 0 70584 0 70601 and nd t values of 4 1 and 3 2 the range of pb isotope values 206pb 204pb 18 28 18 37 207pb 204pb 15 61 15 62 and 208pb 204pb 38 26 38 43 of navidad volcanic rocks and ore minerals suggest mixing pb sources with contributions of mantle and crust 206pb 204pb isotopic ratios of jurassic volcanic rocks of the chon aike volcanic province and sulfides of associated epithermal deposits increase with time from the volcanic event v1 188 178 ma to volcanic events v2 172 162 ma and v3 157 153 ma reflecting variations in the radiogenic pb source as volcanism was migrating towards the proto pacific margin of gondwana

avo and devonian reef exploration difficulties and possibilities although avo has proven useful in hydrocarbon exploration in shallow gas sands there are few examples of its application to carbonate exploration this paper presents a conjecture that avo can help differentiate between shale and porous dolomite in devonian reefs a critical problem in canada where oil production from nisku reefs in the devonian formation accounts for a large part of industry activity devonian formations have generated 60 of canadian oil production and 25 of gas production

petrogenesis of the swaziland and northern natal rhyolites of the lebombo rifted volcanic margin south east africa thejozini and mbuluzi rhyolites and oribi beds of the southern lebombo monocline southeastern africa have geochemical characteristics that indicate they were derived by partial melting of a mixture of high ti zr and low ti zr sabie river basalt formation types compositional variations within the different rhyolite types can largely be explained by subsequent fractional crystallization the srand nd isotope composition of the rhyolites is unique amongst gondwana silicic large igneous provinces having end values close to bulk earth 0 94 to 0 35 and low but more variable initial sr sr ratios 0 7034 0 7080 quartz phenocryst do values indicate that the rhyolite magmas had do values between 5 3 and 6 7o consistent with derivation from a basaltic protolith with do values between 4 8 and 6 2o the low do rhyolites 56 0o come from the same stratigraphic horizon and are overlain and underlain by rhyolites with more normal do magma values these low do rhyolites cannot have been produced by fractional crystallization or partial melting of mantle derived basaltic material the rhyolites have low water contents making it unlikely that the low do values are the result of post emplacement alteration modification of the source by fluid rock interaction at elevated temperatures is the most plausible mechanism for lowering the do magma value it is proposed that the low do rhyolites were derived by melting of earlier altered rhyolite in calderas situated to the east which were not preserved after gondwana break up

avo as an exploration tool gulf of mexico case studies and examples over the past decade amplitude variation with offset avo has evolved from a relatively new technology to increasingly mature applications particularly successful results have been achieved both offshore and onshore in the us gulf coast careful processing detailed petrophysical modeling from complete log suites and avo analyses closely calibrated to measured properties of target reservoir sands have demonstrated effectiveness however routine application of avo has not been without problems and misapplication has led to disappointing results

some avo failures and what we think we have learned amplitude versus offset avo analysis has had a dramatic impact on exploration in the texas gulf coast it has opened new trends in mature areas as well as lowering drilling risk so that even some small one well targets are more economically attractive from 1987 91 an estimated 300 billion ft 3 gas were found in the eocene yegua and cook mountain trends in stratigraphic traps by using avo analysis see seismic hydrocarbon indicators lower risks by o welper et al oil gas journal november 4 1991

comparison of constrained sparse spike and stochastic inversion for porosity prediction at kristin field kristin field is a recently discovered gas and condensate field offshore norway see figure 1 of dolberg et al reservoir sandstones are tidally influenced laterally continuous shallow marine deposits of jurassic age figure 1 a diagenetic overprint during progressive burial to depths of 5 km creates a patchwork of reservoir quality making 3 d mapping of these properties challenging at the start of reservoir modeling two exploration wells had penetrated the reservoir one main focus was to predict lateral variations in reservoir quality porosity and permeability during the project two additional appraisal wells were drilled and used for reservoir characterization refinement four potential reservoirs exist at kristin field figure 1 only the garn and ile formations are proven to be hydrocarbon bearing for this study porosity prediction techniques are applied only to the ile formation total thickness of ile is close to 50 ms twt 80 100 m with a depth of approximately 4 1 s 4600 m

short paper low grade metamorphism of the welsh basin lower palaeozoic succession an example of diastathermal metamorphism the lower palaeozoic succession of the welsh basin has suffered the effects of low grade metamorphism which shows features that do not accord with classical models of regional or burial metamorphism textural evidence demonstrates the development of greenschist facies metamorphism prior to deformation and cleavage development while mineralogical characters indicate a low pressure facies series in order to accommodate these features it is suggested that the area has suffered diastathermal metamorphism in which an enhanced thermal flux develops early in the tectonic cycle in response to an extensional setting temperatures remained relatively high during final closure associated with deformation of the basin sediments and consequent cleavage development such a model adequately resolves previously conflicting interpretations of burial and deformation related metamorphism

inversion and interpretation of a 3d seismic data set from the ouachita mountains oklahoma acoustic impedance inversion ai and simultaneous angle dependent inversion sadi of a 3d seismic data set characterize reservoirs of mississippian morrowan age in the triangle zone of the frontal ouachita mountains oklahoma acoustic impedance of the near angle seismic data images the 3d spatial distributions of wapanucka limestone and cromwell sandstone lame parameters density and and sections are derived from the p wave and s wave impedance ip and is sections produced by the sadi lithology is identified from the gamma logs and the vp vs and are interpreted in terms of a hydrocarbon distribution pattern the ip is used to identify high ip regions that are consistent with high sand shale ratio the estimated impedances and derived lame parameter sections are consistent with the interpretation that parts of the wapanucka limestone and cromwell sandstone contain potential gas reservoirs in fault bounded compartments the cromwell sandstone contains the main inferred

mineralogy and heavy metal content in sediments from rio gato carelmapu and cucao southern chile in southern chile with a mild and rainy climate high levels of heavy metals have been detected in many gold placer deposits many of the contaminants pose risks to human life and consequently damage the trofic chain in this environment the study zones selected correspond to the handicraft gold bearing sands of rio gato los muermos carelmapu and cucao these are all located in the x region of los lagos in chile the type of methodology used in the characterization of the associated mineralization consists of testing each representative sample with a grain size distribution statistical parameter analysis and a mineralogical analysis using a binocular magnifying glass a petrographic microscope xrd and sem edx the chemical composition was defined by means of x ray fluorescence and micro chemical analysis the results of the study about sediments in southern chile are presented the major concentrations of heavy minerals are generally located in areas of dynamic river energy in the samples more than 70 of the heavy minerals were distributed among grain sizes corresponding to thin sand with good grain selection meshes of 0 25 0 06 mm the main mineral phases present in the analysed samples were gold zircon olivine ilmenite hornblende hypersthene hematite garnet chromite chlorite augite amphibolitic epidote etc the main heavy metals found as a result of the study were mercury lead cadmium chrome tellurium indium zinc cobalt copper platinum gold etc and as well some less common elements like cerium praseodymium gadolinium neodymium samarium and lanthanum this research work is only a starting point for carrying out a risk probability mapping of heavy metals and the mineralogy of the cucao carelmapu and rio gato gold bearing sands

compositionally diverse magmas erupted close together in space and time within a karoo flood basalt crater complex geochemical data and mapping from a karoo flood basalt crater complex reveals new information about the ascent and eruption of magma batches during the earliest phases of flood basalt volcanism flood basalt eruptions at sterkspruit south africa began with emplacement of thin lava flows before abruptly switching to explosive phreatomagmatic and magmatic activity that formed a nest of craters spatter and tuff rings and cones that collectively comprise a crater complex 40 km2 filled by 9 18 km3 of volcaniclastic debris rising magma flux rates combined with reduced access of magma to external water led to effusion of thick karoo flood basalts burying the crater complex beneath the 1 5 km thick lesotho lava pile geochemical data is consistent with flood basalt effusion from local dikes and some lava flows likely shared or re occupied vent sites active during explosive eruptions at sterkspruit flood basalt magmas involved in sterkspruit eruptions were chemically heterogenous this study documents the rapid perhaps simultaneous eruption of three chemically distinct basaltic magmas which cannot be simply related to one another from one vent site within the sterkspruit crater complex stratigraphic and map relationships indicate that eruption of the same three magma types took place from closely spaced vents over a short time during formation of the bulk of the crater complex two magma types recognized there have not been recognized in the karoo province before the variable composition of flood basalts at sterkspruit argues that magma batches in flood basalt fields may be small 0 5 1 km3 and not simply related to one another this implies in turn that heterogeneities in the magma source region may be close to each other in time and space and that eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation in flood basalt fields

the cerro bayo district chilean patagonia late jurassic to cretaceous magmatism and protracted history of epithermal ag au mineralization the cerro bayo low sulfidation epithermal district is located in the aysen region chilean patagonia at the western tip of the deseado massif epithermal silver gold province mineralization is hosted in the mainly rhyolitic fragmental successions of the jurassic ibanez formation new biotite 40ar 39ar and zircon u pb ages together with published ages constrain the eruption of this unit to between 158 and 144 ma rhyolitic domes and dikes including cerro bayo proper aligned on the prominent n s striking cerro bayo fault and intruding rocks of the ibanez formation were dated by u pb on zircon at 146 5 0 2 and 146 3 0 2 ma dacitic domes of 83 0 0 2 and 82 6 0 2 ma u pb on zircon are present in the western part of the district at laguna verde silver and gold mineralization is hosted by steeply dipping dominantly n to nw and subordinate w striking quartz veins and to a lesser extent in breccias which on the basis of new adularia 40ar 39ar ages were emplaced in three main episodes 1 mallines ca 144 to 142 ma 2 bahia jara and brillantes ca 137 to 124 ma and 3 laguna verde 114 to 111 ma overall spanning 33 m y most ag au has been produced from veins emplaced in the latter two episodes veins at mallines and bahia jara are spatially related to the cerro bayo fault the oldest episode of mineralization in the district is similar in age to the youngest epithermal deposits in the deseado massif whereas the economically most important veins are age equivalent to skarn and polymetallic vein mineralization in the patagonian andes of southern chile the protracted history of epithermal processes at cerro bayo records the evolution from continental scale extension related to the gondwana breakup to the establishment of andean type arc and back arc environments the largely extensional tectonics throughout this period resulted in similar epithermal mineralization styles emplaced episodically over a large time interval

la peligrosa caldera 47 15 s 71 40 w a key event during the jurassic ignimbrite flare up in southern patagonia argentina pyroclastic and lava vent facies from the late jurassic el quemado complex are described at the southern lake ghio in the cordillera patagonica austral based on the comprehensive study of lithology and structures the reconstruction of the volcanic architecture has been carried out four ignimbrites and one rhyolitic lava unit affected by oblique slip normal faults have been recognized the evolution of la peligrosa caldera has been modeled in three different stages 1 initial collapse consisting of a precursory downsag subsidence related to a dilatational zone which controlled the location of the caldera 2 main collapse with the emplacement of large volume crystal rich ignimbrites and megabreccias under a progressive subsidence controlled by a pull apart structure related to a transtensional regime and 3 post collapse in which lava flows and associated domes were emplaced under an oblique extensional regime the caldera records a remarkable change from transtension to oblique extension which may represent an important variation in regional deformation conditions during jurassic times la peligrosa caldera may be considered a key event to understand the eruptive mechanisms of the flare up volcanism in the chon aike silicic province 2013 elsevier b v

layers and bright spots seismic amplitudes are affected by both geometric and lithologic features of reflecting layers observation of bright spots is a function of not only pore fluid saturations but also of layer thickness and surrounding rock type the numerical model developed uses a finite sum of reflected and mode converted waves to evaluate the application of wave analysis in determining pore fluid types amplitude analysis alone is not sufficient to define realistically pore fluid type but when amplitude analysis is combined with phase and mode converted shear wave behavior it is possible to differentiate between pore fluid types various configurations which lead to bright spots including coal and limestone layers are evaluated an algorithm is developed based on the numerical examples which can differentiate gas zones from other bright spot data thickness of the plane layer is an important factor in defining the overall reflectivity due to interference of the reflected wave components gas saturation of a lay

seismic impedance inversion and interpretation of a gas carbonate reservoir in the alberta foothills western canada acoustic and simultaneous elastic impedance inversions of a 2d land seismic data set are performed to characterize a carbonate reservoir of mississippian age in the turner valley formation in the rocky mountain foothills of western canada the inversions produce p wave and s wave impedance sections ip and is respectively from which lame parameter density and sections are derived the ip data provide a separation between the clastics and carbonates the data provide an estimate of porosity distribution within the dolomitized limestone target deviations from baseline curves for water saturated carbonates of versus porosity versus porosity and is versus ip are interpreted as indicators of gas potential these indicators all provide similar spatial patterns of areas of high gas potential and are consistent with the gas occurence observed in a well

three dimensional geometry and kinematics of the gale buckeye thrust system ouachita fold and thrust belt latimer and pittsburg counties oklahoma the gale buckeye thrust system consists of a series of blind thrusts and associated folds near the boundary between the ouachita fold and thrust belt and the arkoma basin in oklahoma a dense grid of high resolution seismic reflection data augmented by data from exploration and production wells provides a unique opportunity to analyze the three dimensional geometry and kinematics of these structures in detail results from our interpretation provide new insight into the extent style and sequence of thrusts within the gale buckeye thrust system and contrast with previous models for the region specifically we recognize two previously unmapped transverse structures in the gale buckeye thrust system a sharp left stepping zone of oblique ramps and a tear fault within the gale thrust sheet these transverse structures are spatially coincident with similar structural trends in lower paleozoic strata beneath the thrust system as well as outcropping structural trends on the surface this interpretation extends thrusts in the gale buckeye system to minimum strike lengths of 7 9 mi 11 15 km and fault length to displacement ratios of about 10 1 furthermore interpreted structural relationships suggest that the gale and buckeye thrusts formed in a backward stepping manner a new interpretation for faults in this system measurement of differential transport angles from our interpretations of the gale buckeye thrust system indicates that along strike displacement variations of the gale and buckeye thrusts are consistent with thrusts from other nonmetamorphic fold and thrust belts using the differential transport model we infer that the tear fault in the gale thrust sheet indicates a region that became segmented to accommodate excessive along strike differences in displacement in addition we use the model to explain errors in previous interpretations of the gale buckeye thrust system that resulted in the drilling of dry holes integrating realistic differential transport angles with our interpretations for extent style and sequence of thrusting provides new strategies for hydrocarbon exploration hroughout the region

documenting the sand shale crossover all types of velocity data i e surface seismic and well log or sonic log data indicate that velocity in shallow unconsolidated sands is lower than in associated shales while velocity in older sands in more consolidated rock sequences is higher than in their associated shales as the low velocity sands compact to become the high velocity member in the sand shale sequence sand and shale velocities must become equal or crossover at some age and depth of burial if the same type of velocity data is used consistently this crossover occurs at the same depth and age of rocks in the geologic sequence as would be indicated by other types of velocity data even though the different kinds of velocities would not agree elsewhere if all rocks in the province are older and compacted this crossover of sand and shale velocities may not be observed or if observed may be very near the surface

sensitivity of reflection seismic data to oil column height in high porosity sandstones tuning is the effect of interference between the reflections from the top and bottom of a thin layer on the amplitude of the composite reflection for a homogeneous sandstone reservoir containing an oil column overlying brine interference between the reflection from the top reservoir and the oil water contact is a function of the height of the oil column if the properties of the sandstone do not vary across the oil water contact the ss ps and sp reflection coefficients from the oil water contact are small in comparison to the pp reflection coefficient this allows analytic expressions for the effective pp and ps reflection coefficients from the reservoir to be derived that include all p wave multiples within the oil column for a given source receiver offset the component of the wavevector inside the oil column normal to the interface is larger for the pppp reflection than for the ppps reflection due to the asymmetry in the raypath for the ppps reflection the ppps reflection is therefore useful for determining oil column heights larger than that discriminated by the pppp reflection especially when used at wider offsets a convenient classification of the avo response of hydrocarbon bearing sandstone reservoirs overlain by shale is the scheme of rutherford and williams class 1 sands have higher acoustic impedance for normal incidence than the overlying shale class 2 sands have nearly the same acoustic impedance as the shale and class 3 sands have lower acoustic impedance synthetic shot gathers calculated for these three classes as a function of oil column height show that a combination of the pppp and the ppps amplitudes can be plotted as a tuning trajectory which can be used to determine the oil column height this method is most sensitive for reservoirs that belong to avo classes 1 and 2 and therefore may be useful in avo analysis of class 1 and 2 reservoirs where the traditional avo indicators developed for class 3 reservoirs do not work very well these results demonstrate the usefulness of shear waves recorded in the marine environment at wide offsets

implications of thin layers for amplitude variation with offset avo studies amplitude variation with offset avo or amplitude variation with angle ava analyses of seismic reflection data are becoming increasingly popular in the exploration industry ostrander 1984 pichin and mitchell 1991 mazzotti and mirri 1991 and also in scientific studies of the earth s crust juhlin 1990 in the exploration industry avo analyses are particularly suitable for the detection and mapping of gas zones since reservoirs often consist of shale with high poisson s ratio high vp vs overlying gas bearing sands with low poisson s ratio low vp vs if the gas sand has lower impedance than the overlying shale the magnitude of the reflection coefficient will increase with increasing angle of incidence or offset other combinations of rock types will also show a similar increase in magnitude such as shale over hard limestone but the sign of the reflection coefficient will be positive in most of these cases therefore if the polarity of the reflection can be determined to be negative and

the relationship between acoustic properties and the petrographic character of carbonate rocks laboratory studies of the detailed relationships between acoustic properties and the petrographic character of brine and air saturated carbonate rocks with a wide range of facies porosities lithologies and rock fabrics indicate that porosity is the major factor influencing both p and s wave impedance and velocity primary lithology and secondary mineralogy have only a small influence on impedance and velocity combined use of p and s wave velocity data discriminates porosity changes from lithologic changes all other variables including pore fluid type and petrographic fabric have no significant influence on velocities laboratory measurements of p wave velocity under simulated in situ conditions reproduce well log velocity values reliably laboratory porosity velocity trends agree with the time average equation when the correct matrix velocities are used rock property results were used to interpret porosity lithology variations for an inverted seismic section from the williston basin where well

three dimensional seismic interpretation from the triangle zone of the frontal ouachita mountains and arkoma basin pittsburg county oklahoma analysis of 28 5 km 2 of three dimensional 3 d seismic reflection data from the triangle zone of the ouachita fold and thrust belt and the foreland arkoma basin reveals structural details not recognized previously in conventional two dimensional 2 d seismic data the data indicate that the frontal kiowa syncline in the arkoma basin has been passively uplifted by blind thrusting at the morrowan wapanucka limestone level and that smaller wavelength folds are produced by thrusting at shallower levels in the atoka formation faulting at deeper levels in the hunton and arbuckle groups has been traditionally interpreted as normal but our analysis of this data set indicates that in this area normal faults were reactivated during the ouachita orogeny as reverse faults and the changes in fault separation can be followed along strike these faults show the same trend as the overlying thrusts and are normal or have minor inversion where the overlying thrusts have small displacement these faults have been completely inverted where the overlying thrusts have more displacement suggesting a genetic relation between the wapanucka thrusts and the inversion of the hunton and arbuckle faults four reflections were chosen for analysis one reflection in the lower atoka formation two reflections repeated in the wapanucka limestone and a fourth reflection in the hunton group all of these surfaces exhibit the same geometry with the fold axes plunging to the southwest variations in bearing and plunge of fold axes in the wapanucka limestone can be directly correlated to changes in displacement and ramp height along strike the similarity between surface geometries suggests that the last deformation took place at deeper levels in the hunton and arbuckle groups and folded the overlying thrusts reactivation of atokan normal faults at deeper levels in the arkoma basin and ouachita subthrust play may be more widespread than previously recognized

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

geologic and metallogenic significance of the isotopic composition of lead in galenas of the chilean andes eighty galena samples from different mineral deposits within the chilean andes from the mesozoic and cenozoic eras were analyzed for lead isotopes the results show a restricted dispersion between 18 65 to 18 28 15 68 to 15 56 and 38 72 to 38 11 on 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios respectively galenas from deposits hosted by pre jurassic to jurassic rocks are relatively enriched in radiogenic pb reflecting the involvement of old crustal pb galenas from lower cretaceous hosted deposits are less radiogenic and may reflect a mantlelike component those from tertiary rocks are isotopically more variable reflecting the influence of a more evolved crust there is no obvious correlation between the isotopic composition of the galena leads and the age of the hydrothermal event that gave rise to the particular mineral deposit when such ages can be determined from independent geologic evidence it is concluded that the galena leads were derived from a mixture of different lead reservoirs as defined in zartman and doe 1981 the correlation of isotopic composition with age of host rocks suggests either that the concentration of lead to form galena was more or less contemporaneous with the formation of the host rock or that the galena lead was essentially derived from the country rocks and inherited their original isotopic signature other workers have reported an overall similarity between the isotopic composition of galena leads and whole rock leads from the chilean andes however the isotopic compositions of galena leads from the argentinian andes show different characteristics from those of the chilean andes but the data are insufficient to allow firm conclusions

wireline shear and avo modeling application to avo investigations of the tertiary u k central north sea numerous examples exist in the literature of the successes and failures of the application of amplitude variation with offset avo gas effect theory in hydrocarbon exploration the acquisition of wireline and vsp shear data is considered critical information for accurate calibration quantification modeling and successful application of avo analysis techniques in a particular geologic environment wireline compressional wireline dipole shear well vertical seismic profiling vsp and surface seismic data are used to investigate the avo behavior of the major lithologic boundaries and hydrocarbon bearing zones within the study area lithologic boundaries produce significant avo anomalies that may be easily confused with class iii avo gas effect in addition the principal economic target oil bearing sands does not appear to produce avo gas effect anomalies although a class iii avo gas effect response is observed when as little as 2 m of free gas is present at the top of the reservoir confirmatio

aspects of vertical seismic resolution it is proposed that the vertical resolving power of a seismic signal is controlled by three aspects the width of the central lobe the side lobe ratio and the side tail oscillations a comparative study of zero phase signals covering the same frequency range shows that improvement of any one of these aspects inevitably leads to deterioration of one of the other aspects an analytical simulation model is proposed of zero phase signals free from side tail oscillations in which both the width of the central lobe and the side lobe ratio are adjustable analysis of the spectra of this model shows that while the high frequency content of the spectrum is essential for obtaining a small width of the central lobe the low frequency content of the spectrum plays an essential part in causing a low value of the side lobe ratio

rock lithology and porosity determination from shear and compressional wave velocity data examined in this study are previously published laboratory shear s and compressional p wave velocity measurements on water saturated sandstone calcareous sandstone dolomite and limestone cores as well as laboratory porosity measurements on the sandstone and limestone cores sandstone and limestone porosities range from 092 to 299 and from 006 to 229 respectively differential pressure was varied from 500 to 6000 psi corresponding to approximate burial depths from 290 to 3460 m respectively sandstone limestone and dolomite are effectively separated by poisson s ratio or equivalently by the ratio of p to s wave velocity separation of sandstone and limestone appears to result from the difference in of the matrix material namely quartz 056 and calcite 316 respectively an empirical function 1 v a b was fit by regression analysis to sandstone and limestone velocity vs and vp versus porosity values at each differential pressure in this equation a and b are const

tectonic evolution of the north patagonian andes 41 44 s through recognition of syntectonic strata abstract the north patagonian fold thrust belt 41 44 s is characterized by a low topography reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone this particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment here new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages evaluating the prevailing crustal regime for the different mountain domains through time growth strata and progressive unconformities controlled by extensional or compressive structures were recognized in volcanic and sedimentary rocks from the cordilleran to the extra andean domain these data were used to construct a balanced cross section whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology our results indicate two main compressive stages interrupted by an extensional relaxation period the first contractional stage in the mid cretaceous inverted jurassic lower cretaceous half graben systems reactivating the western canadon asfalto rift border 500 km away from the trench at a time of arc foreland expansion for this stage available thermochronological data reveal forearc cooling episodes and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement widespread synextensional volcanism is recognized throughout the paleogene during plate reorganization retroarc paleocene eocene flare up activity is interpreted as product of a slab rollback and fore to retroarc oligocene slab asthenospheric derived products as an expression of enhanced extension the second stage of mountain growth occurred in miocene time associated with nazca plate subduction reaching nearly the same amplitude than the first compressive stage extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation

characterization of elastic properties of near surface and subsurface deepwater hydrate bearing sediments abstracthydrate bearing sands and shallow nodular hydrate are potential energy resources and geohazards and they both need to be better understood and identified therefore it is useful to develop methodologies for modeling and simulating elastic constants of these hydrate bearing sediments a gas hydrate rock physics model based on the effective medium theory was successfully applied to dry rock water saturated rock and hydrate bearing rock the model was used to investigate the seismic interpretation capability of hydrate bearing sediments in the gulf of mexico by computing elastic constants also known as seismic attributes in terms of seismic interpretation including the normal incident reflectivity ni poisson s ratio pr p wave velocity vp s wave velocity vs and density the study of the model was concerned with the formation of gas hydrate and therefore hydrate bearing sediments were divided into hydrate bearing sands hydrate bearing sands with free gas in the pore space and sh

lp norm deconvolution1 the purpose of deconvolution is to retrieve the reflectivity from seismic data to do this requires an estimate of the seismic wavelet which in some techniques is estimated simultaneously with the reflectivity and in others is assumed known the most popular deconvolution technique is inverse filtering it has the property that the deconvolved reflectivity is band limited band limitation implies that reflectors are not sharply resolved which can lead to serious interpretation problems in detailed delineation to overcome the adverse effects of band limitation various alternatives for inverse filtering have been proposed one class of alternatives is l p norm deconvolution l 1 norm deconvolution being the best known of this class we show that for an exact convolutional forward model and statistically independent reflectivity and additive noise the maximum likelihood estimate of the reflectivity can be obtained by l p norm deconvolution for a range of multivariate probability density functions of the reflectivity and the noise the l norm corresponds to a uniform distribution the l 2 norm to a gaussian distribution the l 1 norm to an exponential distribution and the l 0 norm to a variable that is sparsely distributed for instance if we assume sparse and spiky reflectivity and gaussian noise with zero mean the l p norm deconvolution problem is solved best by minimizing the l 0 norm of the reflectivity and the l 2 norm of the noise however the l 0 norm is difficult to implement in an algorithm from a practical point of view the frequency domain mixed norm method that minimizes the l 1 norm of the reflectivity and the l 2 norm of the noise is the best alternative l p norm deconvolution can be stated in both time and frequency domain we show that both approaches are only equivalent for the case when the noise is minimized with the l 2 norm finally some l p norm deconvolution methods are compared on synthetic and field data for the practical examples the wide range of possible l p norm deconvolution methods is narrowed down to three methods with p 1 and or 2 given the assumptions of sparsely distributed reflectivity and gaussian noise we conclude that the mixed l 1 norm reflectivity l 2 norm noise performs best however the problems inherent to single trace deconvolution techniques for example the problem of generating spurious events remain for practical application a greater problem is that only the main well separated events are properly resolved

a 100 m y long window onto mass flow processes in the patagonian mesozoic subduction zone diego de almagro island chile diego de almagro island was formed by the subduction and accretion of several seafloor derived tectonic slices with very heterogeneous ages and pressure temperature time p t t paths the highest element of the pile the lazaro unit evidences subduction in the high p granulite field 1 3 gpa 750 c at ca 163 ma below it a thin tectonic sliver the garnet amphibolite unit preserves eclogite facies remnants 570 c and 1 7 gpa formed at ca 131 ma in situ u pb zircon rim ages peak assemblages were nearly fully amphibolitized during decompression down to 1 2 gpa and 600 c at 125 120 ma rb sr multimineral dating the underlying blueschist unit has 50 m y younger metamorphic ages and exhibits slightly cooler peak burial conditions 520 c 1 7 gpa ca 80 ma in situ white mica ar ar ages and multimineral rb sr dating and is devoid of amphibolitization the mylonites from the sinistral strike slip seno arcabuz shear zone bounding diego de almagro island to the east also exhibit amphibolite facies 620 c and 0 9 gpa deformation at ca 117 ma multimineral rb sr ages in situ white mica ar ar dating and multimineral rb sr dating of low t mylonites 450 c along the base of the lazaro unit reveal partial resetting of high t assemblages during tectonic displacement between 115 and 72 ma and exhumation of the slice stack detrital zircon u th pb ages indicate that the material accreted on diego de almagro island has been mostly recycled from a permian triassic accretionary wedge madre de dios accretionary complex exposed along the subduction buttress geological and geochronological constraints suggest that the rocks of the seno arcabuz shear zone and the lazaro unit were tectonically eroded from the buttress while the underlying garnet amphibolite and blueschist units instead derive from the subducted oceanic basin with increasingly younger maximum depositional ages the very long residence time of the rocks 90 m y for the lazaro unit along the hanging wall of the subduction interface recorded long term cooling along the patagonian subduction zone during the mesozoic diego de almagro island therefore represents a unique window onto long term tectonic processes such as subduction interface down stepping tectonic erosion and episodic underplating near the base of an accretionary wedge 40 50 km

trace based and geostatistical inversion of 3 d seismic data for thin sand delineation an application in san jorge basin argentina argentina s san jorge basin which straddles the southern provinces of chubut and santa cruz in the heart of patagonia has been producing hydrocarbons since 1907 the region currently accounts for 32 of the country s production the existence and early geologic evolution of the basin is due to the same rift process responsible for the opening of the atlantic ocean in early jurassic times extensive direct faulting and local erosion accompanied the rift evolution thereby facilitating the accumulation of chiefly nonmarine terrigenous sediments well into the early cretaceous at this point andean tectonism became a major source of pyroclastic concentration in the sedimentary column it was also responsible for pervasive batholitic intrusions clastic deposition in the hydrocarbon producing zone is characterized by thick shale laminations of lacustrine and floodplain origin interspersed with much thinner and laterally sparse sand bodies the hydrocarbon reservoirs

rare earth element 87 sr 86 sr and 143 nd 144 nd compositions of cenozoic orogenic dacites from baja california northwestern mexico and adjacent west texas evidence for the predominance of a subcrustal component dacitic lavas and ignimbrites were examined from seven localities that span the entire 700 km width of the mid to late cenozoic magmatic arc of northwestern mexico and adjacent west texas these rocks have remarkably similar ree patterns that are parallel in the heavy ree and have modest negative eu anomalies samples from three localities including baja california the sierra madre occidental and the chihuahuan basin and range have initial 87sr 86sr between 0 7044 and 0 7050 and nd near 0 0 1 0 these dacites are isotopically similar to associated basalts and they show no systematic isotopic variation that is correlated with age or composition of the basement there is no evidence that magmas parental to these dacites interacted significantly with continental crust samples form three other localites in the basin and range vary in initial 87sr 86sr from 0 7051 to 0 7070 and nd from about 1 to 2 the composition of these rocks reflects contamination of the parental magmas by relatively small amounts of precambrian crust collectively the dacites of this study show much less isotopic variation than do mesozoic granitoids farmer and depaolo 1983 and late cenozoic olivine tholeiites hart 1985 from similar transects of the western united states the distinctive source region for the magmas parental to the mexican dacites was relatively uniform isotopically but it was enriched in lil and hfs elements beneath the eastern basin and range

tectonic setting of primitive magmas in volcanic arcs an example from the antarctic peninsula abstract primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment most examples are either associated with high rates of arc parallel extension or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression in two cases the vanuatu and solomon islands arcs such arc compression is being caused by collision of seamounts in the antarctic peninsula primitive mafic dykes were emplaced perpendicular to the continental arc ar ar and k ar data suggest intrusion of the dykes at c 126 106 ma possibly during mid cretaceous regional compression of the arc the dykes form two compositional groups one group has low la n yb n ratios 0 31 0 49 lower nb yb and higher th nb than n morb age corrected nd values of 7 3 to 7 9 and are interpreted as melts of subduction modified sub arc asthenosphere the other has high la n yb n ratios 3 86 8 92 higher nb yb and th nb than n morb age corrected nd values of 2 8 to 3 4 and are interpreted as melts of sub arc lithosphere the absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

magmatic evolution of the austral patagonian andes abstract new geological evidence and geochronological data obtained within the patagonian cordillera between lat 47 and 49 s allow the redefinition of an andean region with different and characteristic geological evolution compared to the adjacent segments an acidic middle jurassic volcanism covered a metamorphic basement formed by low grade metasediments and metabasites of upper paleozoic age dike swarms of basic alkaline rocks were emplaced since the cretaceous in a marginal basin developed on the jurassic sialic crust the outcrop density of such bodies seems to indicate an attenuation of the sialic crust towards the south where oceanic crust of upper jurassic lower cretaceous age floored the marginal basin the cretaceous volcanic activity is well developed in the adjacent sectors but almost absent in the studied segment several magmatic pulses lead to the final emplacement of the patagonian batholith which yields its peak activity around 98 m y and presents typical geochemical variations the upper cretaceous and cenozoic magmatism are mainly exposed to the east of the cordillera and are represented by a series of alkali plateau basalts different pulses of activity have been recorded during eocene upper miocene pliocene and even during quaternary times associated with deep crustal fractures and probable upper mantle origin as evidenced by the low 87sr 86sr ratios and the ultramafic and mafic xenoliths of the lavas granitic stocks in the andean domain are post orogenic in relation to a compressive tectonic phase during miocene times the upper cenozoic calc alkaline volcanism presents a gap in this segment of the andean belt probably related with the post miocene migration of the chile rise

modulus decomposition of compressional and shear velocities in sand bodies the advent of borehole shear slowness measurements in sonically slow formations has lead to breakthroughs in the subsurface profiling of geological bodies in sand bodies compressional and shear velocities depend predictably on porosity mineralogy grain contacts and fluid saturation an interpretation is best performed by decomposing the velocities into moduli that are intrinsic measures of the rock frame and pore fluid compressibilities careful experiments on pure materials i e pure quartz sandstones demonstrate two simplifying constitutive relationships first the bulk and shear frame moduli are simple functions of the porosity a comparison of the measured shear frame modulus to the prediction for the pure material distinguishes sand from shale second the ratio of the bulk and shear frame moduli is a constant 0 9 independent of the porosity the measured velocities are directly inverted to yield the bulk modulus of the pore fluid the fluid saturation effects are so dramatic at high porosit

cretaceous slab segmentation in southwestern gondwana the mesozoic austral basin of patagonia in southwestern gondwana experienced a major tectonic segmentation during aptian times sometime between 121 and 118 ma aptian the northern part of the austral basin known as the aisen basin or rio mayo embayment was inverted with the sediments overlain by calc alkaline subaerial volcanic rocks of aptian to maastrichtian age in the southern segment of the austral basin known as the magallanes basin predominantly marine sediments accumulated until cenozoic times in a back arc position relative to a magmatic arc located to the west the subduction related n s trending volcanic chains of both segments were geographically displaced during aptian to late cretaceous times in the aisen segment north of 49 50 s the volcanic chain was located further east than the coeval arc in the magallanes segment a transform fault connected the trenches of both segments with the aisen segment dipping at a shallower angle than the magallanes segment

towards a comprehensive classification of igneous rocks and magmas abstract the iugs subcommission on the systematics of igneous rocks has recently published an excellent book on the classification of these rocks this event has shifted the vexed question of classification towards the top of the agenda in igneous petrology over the years the subcommission has used many different criteria to establish the positions of the boundaries between the various common igneous rocks it now has to adopt a holistic approach and develop a comprehensive coherent classification that is purged of all the minor anomalies that arise between the various classifications that it has approved it is appreciated that the subcommission s classification was never intended to have any genetic implications however it is suggested that an ideal classification should he presented in such a way that it is able to group rocks into an order that directs attention to petrogenetic relationships between individual rocks and larger groups of rocks unfortunately many of the subcommission s definitions are earth chauvinistic for example igneous rocks are defined as being those rocks that solidified from a molten state either within or on the surface of the earth nowhere in the book is it acknowledged that during the past 20 years while the subcommission has been framing its many recommendations a whole new science of planetary petrology has subsumed classical petrology in any new edition of the book the subcommission should acknowledge that rocks are essentially the solid materials of which planets natural satellites and other broadly similar cosmic bodies are made the subcommission should also explicitly recognise that igneous rocks can be divided into either a main sequence of essentially common rocks or a number of supplementary clans of special rocks that evolved outside the main sequence it is hoped that in the near future the subcommission will rescind its recommendation that the tas classification should be regarded as an adjunct to its more traditional qapf modal classification the qapf and tas classifications are regarded as being of equal validity with the tas classification being of more practical value in the classification of the common volcanic rocks and the various magmas conjured up in petrogenetic discussions a new comprehensive hierarchical classification of igneous rocks is introduced and the petrographic character and systematic position of the various rocks and clans that make up this classification are reviewed

the influence of strain lithology and stratigraphical depth on white mica illite crystallinity in mudrocks from the vicinity of the corris slate belt wales implications for the timing of metamorphism in the welsh basin low and very low grade metamorphism in a mudrock dominated succession ranging from upper cambrian to wenlock is characterized by the white mica illite crystallinity technique kubler indices indicate that grade ranges from late diagenetic to low epizonal the prograde reaction most easily observed by xrd study is the progressive ordering of k na micas diagnostic 2m 1 polytypic reflections become recognizable at 0 4 2 and strengthen as grade advances grade generally correlates with thickness of overburden in the 9 km thick succession however strain exercises additionally a profound influence on grade and the distribution of strain was related to decollement resulting from inhomogeneities and ductility contrasts within the succession strain energy is seen as an additional and important variable in the development of white mica crystallinity in slates the distribution of metamorphic grade implies that the lower palaeozoic succession was augmented by several kilometres of lower devonian old red sandstone strata at the metamorphic culmination the culmination was syntectonic and represents an acadian rather than an early diastathermal event subsequent dextral movement on the bala fault displaced the regional isocryst pattern by c 4 km retrogressive smectite appears to be localized to the vicinity of the bala fault and is probably of variscan age

integration of hydraulically induced microseismic event locations with active seismic attributes a north texas barnett shale case study abstracthydraulic fractures are delineated by induced microseismic event distributions and typically propagate perpendicular to the regional minimum stress direction however at a smaller scale varying mineralogical composition and existing fault and fracture networks can influence developing fracture networks we integrated microseismic event locations with seismic attributes from multichannel seismic reflection data including inversion results for impedance and lame parameters and seismic curvature attributes we found that microseismic event locations consistently correlate to zones of low seismic impedance and low and values describing characteristic material properties of fracture prone zones within the north texas lower and upper barnett shale additionally event locations showed a weak correlation with anticlinal structures as defined by volumetric curvature attributes we suggest that the low impedance low and zones were related to the boundary between calcite filled fractures a

the south patagonian batholith 150 my of granite magmatism on a plate margin a new database of 70 u pb zircon ages mostly determined by shrimp indicates that the south patagonian batholith resulted from the amalgamation of subduction related plutons from the late jurassic to the neogene construction of the batholith began with a voluminous previously undetected late jurassic bimodal body mainly composed of leucogranite with some gabbro emplaced along its present eastern margin within a restricted time span 157 to 145 ma this episode is at least in part coeval with voluminous rhyolitic ignimbrites of the tobifera formation deposited in the deep rocas verdes basin east of the batholith this was the last of several southwestward migrating silicic volcanic episodes in patagonia that commenced in an early jurassic extensional tectonic regime the quasi oceanic mafic floor of the basin was also contemporaneous with this late jurassic batholithic event as indicated by mutually cross cutting field relationships changes in subduction parameters then triggered the generation of earliest cretaceous plutons cretaceous 1 144 137 ma west of the late jurassic ones a westward shift that culminated at 136 127 ma cretaceous 2 along the present western margin of the batholith most mid to late cretaceous cretaceous 3 126 75 ma and paleogene 67 40 ma granitoids are represented by geographically restricted plutons mainly emplaced between the previously established margins of the batholith and mostly in the far south no associated volcanic rocks of similar age are known at present in this area during the final neogene stage of plutonism 25 15 ma a recurrence of coeval volcanism is recognized within and east of the batholith typical endt values for the granitoids vary from strongly negative 5 in the late jurassic to progressively higher values for cretaceous 1 4 cretaceous 2 0 7 cretaceous 3 2 and the paleogene 5 followed by lower and more variable ones in the neogene 1 to 5 these variations may reflect different modes of pluton emplacement large crustal magma chambers developed in the early stages late jurassic to cretaceous 1 leading to widespread emplacement of plutons with a crustal signature whereas the cretaceous 2 cretaceous 3 and palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher endt this does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed

palaeomagnetism and 40ar 39ar dating from lower jurassic rocks in gastre central patagonia further data to explore tectonomagmatic events associated with the break up of gondwana new 40 ar 39 ar data indicate ages of c 185 ma for the lonco trapial volcanic field in gastre north central patagonia implying that this andesitic unit is roughly coeval with the marifil silicic province that crops out in the eastern part of northern patagonia these volcanic fields are therefore roughly coeval with the karoo ferrar large igneous province further contributing to the huge 185 180 ma magmatic outpouring in southern gondwana the comparison of palaeomagnetic results from the unit with early jurassic reference palaeopoles suggests that gastre records a small anticlockwise finite rotation with respect to stable south america this contrasts sharply with the large clockwise rotation recorded by roughly coeval dolerites from the falkland malvinas islands and argues against a direct relationship between the rotation of the islands and the tectonic activity in gastre the results support previous suggestions that the gastre fault system is not the locus of major strike slip displacement activity in the jurassic as some workers have suggested taken as a whole palaeomagnetic data from patagonia suggest a more complex than previously thought pattern of distributed and variable deformation during the early stages of gondwana breakup the crustal block boundaries in the region remain cryptic supplementary material 40 ar 39 ar analytical procedures and expanded data are available at http www geolsoc org uk sup18522

granitoid pluton formation by spreading of continental crust the wiley glacier complex northwest palmer land antarctica abstract the emplacement mechanism geometry and isotope geochemistry of plutons of the wiley glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the antarctic peninsula magmatic arc in early cretaceous times the suggested mechanism is analogous to basalt dyke injection during sea floor spreading during normal dextral shear the burns bluff pluton a sheeted moderately east dipping syn magmatically sheared tonalite granodiorite intruded syn magmatically sheared quartz diorite of the creswick gap pluton and 140 5 ma hornblende gabbro u pb dating of zircon and ar ar dating of hornblende and biotite suggest that both granite s l plutons were emplaced between 145 and 140 ma but that extensional shearing was active from the time of emplacement until ca 127 ma the burns bluff pluton is chilled at its margin and grades through mylonitised porphyritic tonalite granodiorite sheets and tonalite granodiorite sheets with minor chilling to a kilometre scale body of coarse grained hypidiomorphic tonalite granodiorite co magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves these dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal dextral shear pluton marginal porphyritic and hypidiomorphic tonalite granodiorite forms large fault hosted sheets emplaced progressively under extension with minor dextral shear kinematic indicators from pluton marginal granite s l dykes suggest that early in pluton accretion intrusive sheets cooled rapidly with simple shear prior to full crystallisation changing to ductile simple shear during cooling kinematic indicators towards the pluton core suggest that as the pluton grew and cooled more slowly emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma cross cutting relationships with the coeval syn extensional creswick gap pluton suggest that the burns bluff pluton was emplaced in a steeper second generation shear structure like those in normal fault systems this suggests that the wiley glacier complex was emplaced above the base of the brittle ductile transition zone 15 18 km depth the burns bluff pluton has nd and sr isotope values that range from mantle dominated nd141 3 8 87 sr 86 sr 141 0 70468 to more crustally influenced nd141 1 7 87 sr 86 sr 141 0 70652 this range probably represents different degrees of mixing between mantle derived magma and lower crustal partial melts generated in the garnet stability zone 40 km depth addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s l and mafic modified underplated magma to mid crustal levels along extensional shear zones as the arc spread were the primary mechanisms of crustal growth

the role of crustal and mantle sources in the genesis of granitoids of the antarctic peninsula and adjacent crustal blocks abstract magmatic rocks from the antarctic peninsula show marked variations in isotope composition which reflect changes in the geodynamic evolution of the peninsula through time most antarctic peninsula granitoids formed as a result of subduction they fall on well defined trends on plots of nd 207 pb 204 pb and 18 o against 87 sr 86 sr i between a component derived from subduction modified mantle or juvenile basaltic underplate ndi 6 207 pb 204 pb 15 61 18 o 5 5 87 sr 86 sr nd 7 207 pb 204 pb 15 67 18 o 10 87 sr 86 sr 0 709 a small group of granitoids emplaced before or during gondwana break up plot on distinct trends towards high 87 sr 86 sr i compositions reflecting mixing between melts derived from proterozoic lower crust and melts of middle upper crustal rocks ndi 9 207 pb 204 pb 15 64 18 o 10 87 sr 86 sr 0 726 with little or no input of new material derived from the mantle or from juvenile basaltic underplate these granitoids are thought to have formed as a result of crustal attenuation during the initial rifting phase of gondwana break up similar trends are shown by data from granitoids of the adjacent crustal blocks of west antarctica the isotope data suggest that an enriched ferrar karoo type lithosphere was not involved in the genesis of granitoids of the antarctic peninsula or of the ellsworth whitmore mountains crustal block

the jurassic cretaceous volcanism in the septentrional patagonian andes abstract an extensive jurassic cretaceous volcanism was recognized in the section of the patagonian cordillera situated between the 43 and 47 s parallels field observations and k ar geochronological determinations allowed to obtain a complete picture of the events that occured in the region the volcanic activity started with andesitic pyroclastics with some dacitic intercalations in the malm and continued until the late jurassic tithonian neocomian marine sediments in some areas point out a partial regional subsidence in the early cretaceous the volcanic activity was mainly andesitic with some interbedded dacites in the upper part of the sequence and intercalations of pyroclastic beds and clastic sediments the calc alkaline volcanics are associated to granitic and granodioritic plutons and together characterize a volcanic arc on a continental margin

porosity identification using amplitude variations with offset examples from south sumatra abstract this paper describes the use of a relatively new exploration technique in the modeling and interpretation of seismic data the technique holds promise for identification of primary lithology and of local lithological facies variations its underlying principle is that any single reflection within a cdp gather will show an amplitude variation between near and far offset traces given certain physical properties of rocks across the reflecting boundary stacking velocity information above the reflector and the cdp geometry it is possible to model the observed amplitude variation such modeling can result in useful information on lithology two examples will be shown of the application of this technique in the detection of porosity in miocene reefal buildups in south sumatra

significance of 40ar 39ar encapsulation ages of metapelites from late palaeozoic metamorphic complexes of aysen chile the ages obtained by the 40 ar 39 ar encapsulation technique retention and total gas ages on 2 m fractions of five metapelites from the eastern andean metamorphic complex and two from the chonos metamorphic complex allow discussion of the latest recorded metamorphic event in each zone the kubler index ki of illite muscovite principal component of the metapelites varies between 0 15 and 0 45 2 indicating regional variation from diagenetic to epizonal metamorphic grade the 40 ar 39 ar encapsulation analyses reveal 39 ar loss varying between 21 and 25 which shows a limited positive correlation with ki values the obtained retention and total gas metapelite ages reflect distinct metamorphic conditions retention ages most probably indicate burial or regional metamorphic events without plutonic influence in the southern eastern andean metamorphic complex total gas ages reflect contact metamorphic ages for metapelites close to intrusions in the northern and southern eastern andean metamorphic complex and in the chonos metamorphic complex the thermal overprinting of metapelites occurred in early cretaceous times at 130 ma and 145 ma and is related to the contact metamorphism of an emplacement pulse of the north patagonian batholith total gas metapelite ages obtained from the western belt of the chonos metamorphic complex suggest a thermal event related to a distinct pulse of the north patagonian batholith

avo analysis in high impedance sandstone reservoirs avo analysis is now established as an effective hydrocarbon detection tool in many exploration provinces in most areas in which it is widely used the impedance of the reservoir rocks is less than or equal to the impedance of the surrounding rocks the avo response of low impedance reservoirs is an increase in amplitude with offset methodologies for detecting and analyzing these types of avo anomalies have been developed and have resulted in greatly increased drilling success ratios for some formations such as the yegua formation of onshore texas

integrated geological and geophysical interpretation case study and lame rock parameter extractions using avo analysis on the blackfoot 3c 3d seismic data southern alberta canada blackfoot field southeast of calgary alberta canada has produced oil and gas from a glauconitic compound incised valley system the glauconitic compound incised valley has three cycles of incision and valley fill lower lithic and upper incised valleys the upper and lower incised valleys are the main reservoirs the geophysical interpretation of compressional pp seismic data resulted in the definition of the compound valley extent and in the mapping of the upper and lower incised valleys a stratigraphic well log template was built using the most significant lithological information and well logs to integrate both geological and geophysical interpretations the well log cross sections and corresponding depth converted seismic were superimposed furthermore a detailed geological facies interpretation of the upper and lower incised valleys was undertaken and incorporated a good correlation was found between the interpreted geological facies and the seismic data response information about the natu

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

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

geology of the epithermal ag au huevos verdes vein system and san jose district deseado massif patagonia argentina the san jose district is located in the northwest part of the deseado massif and hosts a number of epithermal ag au quartz veins of intermediate sulfidation style including the huevos verdes vein system veins are hosted by andesitic rocks of the bajo pobre formation and locally by rhyodacitic pyroclastic rocks of the chon aike formation new 40ar 39ar constraints on the age of host rocks and mineralization define late jurassic ages of 151 3 0 7 ma to 144 7 0 1 ma for volcanic rocks of the bajo pobre formation and of 147 6 1 1 ma for the chon aike formation illite ages of the huevos verdes vein system of 140 8 0 2 and 140 5 0 3 ma are 4 m y younger than the volcanic host rock unit these age dates are among the youngest reported for jurassic volcanism in the deseado massif and correlate well with the regional context of magmatic and hydrothermal activity the huevos verdes vein system has a strike length of 2 000 m with several ore shoots along strike the vein consists of a pre ore stage and three main ore stages early barren quartz and chalcedony are followed by a mottled quartz stage of coarse saccharoidal quartz with irregular streaks and discontinuous bands of sulfide rich material the banded quartz sulfide stage consists of sulfide rich bands alternating with bands of quartz and bands of chlorite illite late stage sulfide rich veinlets are associated with kaolinite gangue ore minerals are argentite and electrum together with pyrite sphalerite galena chalcopyrite minor bornite covellite and ruby silver wall rock alteration is characterized by narrow 220 c kaolinite occurring with the late sulfide rich veinlet stage indicates ph 315 whereas strike directions of 315 are predicted with an induced dextral strike slip movement the components of the structural model appear to be present on a regional scale and are not restricted to the san jose district

synchronous very low grade metamorphism contraction and inversion in the central part of the welsh lower palaeozoic basin a metamorphic map white mica crystallinity based on 663 data points has been generated for the recently published british geological survey 1 50 000 geological sheets 178 llanilar and 179 rhayader epizone upper anchizone lower anchizone and sub anchizone are delineated metamorphic cross sections are drawn and superimposed on structural cross sections assumptions made in order to draw metamorphic cross sections are 1 the anchizone spans 100 c 2 the metamorphic field gradient was 36 ckm 1 3 metamorphic surfaces were initially generated parallel to the contemporary ground surface restoration of the sections indicates that at the time of metamorphism a contractional phase had begun because reverse movement on earlier extensional faults development of back thrusts footwall short cuts and limited growth of precursor folds had taken place metamorphism continued locally during later stages of the contractional phase as a strain related phenomenon finally tensional faulting followed the cessation of folding and metamorphism

amplitude versus offset variations in gas sands seismic reflections from gas sands exhibit a wide range of amplitude versus offset avo characteristics the two factors that most strongly determine the avo behavior of a gas sand reflection are the normal incidence reflection coefficient r0 and the contrast in poisson s ratio at the reflector of these two factors r0 is the least constrained based on their avo characteristics gas sand reflectors can be grouped into three classes defined in terms of r0 at the top of the gas sand class 1 gas sands have higher impedance than the encasing shale with relatively large positive values for r0 class 2 gas sands have nearly the same impedance as the encasing shale and are characterized by values of r0 near zero class 3 sands have lower impedance than the encasing shale with negative large magnitude values for r0 each of these sand classes has a distinct avo characteristic an example of a gas sand from each of the three classes is presented in the paper the class 1 example involves a hartshorn channel s

early tectonic extension between the agulhas bank and the falkland plateau due to the rotation of the lafonia microplate abstract along the southeast african continental margin a deep sedimentary basin and marginal ridge bordering the agulhas fracture zone formed coevally with south american counterparts underlying the falkland plateau extension in both the southern outeniqua and the falkland basin is related to large angle 100 rotation of the lafonia falkland islands block in middle late jurassic times during attempted propagation of the proto indian ocean across the active circum pacific margin of gondwanaland in the early cretaceous the jurassic microplate boundaries were partly reactivated and partly transected by a right lateral shear along the 1200 km long falkland agulhas transform fault preceded by substantial vorticity of the surrounding crustal blocks this boundary segment separating the south american and african plates was probably generated by complex geodynamic interactions between oceanic plate subduction continental to back arc rift propagation and the impingement of deep mantle plumes on the lithosphere

tectonic events reflected by palaeocurrents zircon geochronology and palaeobotany in the sierra baguales of chilean patagonia abstract the sierra baguales situated north of the torres del paine national park in the magallanes region of southern chile shows a well exposed stratigraphic sequence ranging from the late cretaceous to late pliocene which presents a unique opportunity to study the evolution of sedimentological styles and trends palaeoclimate changes and tectonic events during this period the depositional environment changed from a continental slope and shelf during the cenomanian campanian tres pasos formation to deltaic between the campanian maastrichtian dorotea formation and estuarine in the lutetian bartonian man aike formation during the rupelian a continental environment with meandering rivers and overbank marshes was established rio leona formation this area was flooded in the early burdigalian estancia 25 de mayo formation during the patagonian transgression but emerged again during the late burdigalian santa cruz formation measured palaeocurrent directions in this mesozoic cenozoic succession indicate source areas situated between the northeast and east southeast during the late cretaceous east southeast during the middle eocene and southwest during the early oligocene to early miocene this is confirmed by detrital zircon age populations in the different units which can be linked to probable sources of similar ages in these areas the east southeastern provenance is here identified as the antarctic peninsula or its northeastern extension which is postulated to have been attached to fuegian patagonia during the eocene the southwestern and western sources were exhumed during gradual uplift of the southern patagonian andes coinciding with a change from marine to continental conditions in the magallanes austral basin as well as a decrease in mean annual temperature and precipitation indicated by fossil leaves in the rio leona formation the rain shadow to the east of the andes thus started to develop here during the late eocene early oligocene 34 ma long before the quechua phase of andean tectonics 19 18 ma that is generally invoked for its evolution at lower latitudes

the effect of oil under in situ conditions on the seismic properties of rocks direct hydrocarbon indicators dhis on seismic sections are commonly thought to be diagnostic only of gas however oil sands can also generate dhis such as bright spots and flat events since oils under in situ conditions can contain large amounts of solution gas this dissolved gas substantially decreases the velocity of sound and the density of the oils as compared to measurements of these properties at surface conditions hydrocarbon indicators caused by oil sands are investigated by first measuring the elastic properties of an oil as a function of gas oil ratio next calculating the elastic properties of additional oil compositions under in situ conditions using standard pressure volume temperature pvt measurements and then calculating the compressional velocity in oil saturated rocks for several typical oils using gaasmann s equation the potential for seismic anomalies caused by oil saturated rocks is higher than thought because the properties of oil under reservoir conditions can differ signif

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

late paleozoic to jurassic silicic magmatism at the gondwana margin analogy to the middle proterozoic in north america a vast region of upper paleozoic to middle jurassic 300 150 ma silicic magmatic rocks that erupted inboard of the gondwana margin is a possible phanerozoic analogue to the extensive middle proterozoic 1500 1350 ma silicic magmatic province that underlies much of the southern mid continent of north america like the north american rocks the gondwana silicic magmas appear to be melts of crust that formed about 200 300 m y earlier in the north american case this older crust formed and was accreted to the continent during a major period of crustal formation 1700 1900 ma whereas in the gondwana case the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the paleozoic in both cases basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older and previously melted crust the similarities between the north american and gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas ponded at the base of the preheated crust caused extensive crustal melting that inhibited upward passage of the basalts in both cases silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes and before breakup of the supercontinent by analogy with the polar wander curves for gondwana the granite rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles

reservoir characterization of the flag sandstone barrow sub basin using an integrated multiparameter seismic avo inversion technique editor s note this material was presented at the 17th aseg geophysical conference and exhibition a series of oil and gas fields have been discovered since 1979 in the early cretaceous flag sandstone in the barrow sub basin of western australia s carnarvon basin the flag sandstone is a lowstand complex of gravity flow sandstones deposited beyond the shelf edge of the barrow delta figure 1 shows a stratigraphic chart the permeable flag sandstone forms a consistent high quality reservoir that is capped by the impermeable muderong shale in the area immediately adjacent to and south of varanus island the main trapping mechanism is four way dip closed anticlines formed by drape compaction structural closure alone does not appear sufficient for oil emplacement as a limited number of prominent closed structures have been drilled and found water wet the known oil fields are nearing depletion and the goal is to properly locate additional wells to ensure complete development of the area figure 1 stratigraphic chart for carnarvon basin showing the early cretaceous flag sandstone and overlying muderong shale barrow sub basin the identification of the closed anticlinal structures on seismic data is complicated due to a poor p impedance contrast between the flag sandstone and the overlying muderong shale however a strong v p v s contrast exists between the sandstone and shale resulting in reasonably consistent avo class ii effects a far angle stack with an angle range of 30 45 has been used successfully to map the structure on the top of the flag sandstone the difficulty with this mapping technique is that it is subject to tuning effects and assumes a consistent avo response however some wells in the area demonstrate an avo response that does not match a simple model thus a more robust technique was required that could model the true avo response provide an accurate pick

back arc extension in the southern andes a review and critical reappraisal the interpretation that the mafic rocas verdes green rocks complex of the southern andes represents part of the uplifted floor of a late jurassic to early cretaceous back arc basin has proved particularly useful in understanding the geological evolution of the southern andes the north scotia ridge and the antarctic peninsula clear field evidence of the back arc setting of the 9rocas verdes gabbro sheeted dyke pillow lava ophiolitic assemblages has encouraged fruitful petrological and geochemical comparison with mid ocean ridge and marginal basin basalts other onshore ophiolite complexes and archaean greenstone belts uncertainty still surrounds estimates of the original width and depth of the basin as well as the proportion of new mafic crust compared with relict sialic crust in the basin floor these questions are unresolved owing mainly to the considerable lower cretaceous turbiditic basin infill and the effects of mid cretaceous compressional deformation while the field relations clearly indicate that the rocas verdes basin is not an older piece of ocean floor trapped behind a volcanic arc it is not yet clear whether the basin is directly subduction related or falls in the category of back arc leaky transforms like the proto gulf of california or apparent rip off features like the andaman sea

v p v s and lithology published laboratory investigations suggest an association exists between the ratio of seismic compressional and shear wave velocities vp vs and sedimentary rock lithology comparisons of some theoretical models with these laboratory studies suggest that crack or pore geometry has a stronger effect on observed vp vs values than elastic constants of the minerals comprising the matrix further it can be inferred that the observed association between lithology and vp vs is a result of an association between lithology and distribution of pore and crack shapes direct observation of crack shapes for a variety of lithologies is a next step in strengthening these inferences the present study reviews the empirical relations between vp vs and lithology and examines two published theoretical models of cracked elastic media the models suggest that seismic velocities of sandstones may be controlled by cracks and pores with aspect ratios in the range of 10 1 to 1 dolomite in the range of 10 2 to 10 1 and dens

nature and composition of the lower continental crust in central spain and the granulite granite linkage inferences from granulitic xenoliths xenolith bearing alkaline ultrabasic dykes were intruded into the of the hercynian basement of the spanish central system in early mesozoic times the suite of lower crustal xenoliths in the dykes divided into three groups felsic peraluminous granulites metapelitic granulites and charnockitic granulites the felsic granulites form 95 of the total volume of the xenoliths whereas the charnockitic and metapelitic granulites are much less abundant 0 01 5 respectively thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850 950 c 7 11 kbar thus the xenoliths represent lower continental crustal material superimposed on this high t high p assemblage is a high t low p paragenesis represented mainly by kelyphitic coronas reflecting re equilibration during transport in the clearly restitic mineral assemblages with up to 50 garnet and 37 sillimanite major and trace element modelling supports the idea that the late hercynian peraluminous granites of central spain represent liquids in equilibrium with restitic material of similar composition to the studied lower crustal xenoliths 87sr 86sr and end of the felsic xenoliths calculated at an average hercynian age of 300 ma are in the range 0 706 0 712 and 1 4 to 8 2 respectively these values match the isotopic composition of the outcropping late hercynian granites the sr isotopic composition of the xenoliths is lower than that of the outcropping mid crustal lithologies orthogneisses pelites a major contribution from the lower crust to the source of hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis the felsic nature of the lower continental crust in central spain contrasts with the more mafic lower crustal composition estimated in other european hercynian areas suggesting a non underplated crust in this region of the hercynian orogenic belt

production of jurassic rhyolite by anatexis of the lower crust of patagonia abstract the mid jurassic marifil and chon aike volcanic rocks of eastern patagonia are part of one of the largest silicic igneous provinces known rb sr geochronology indicates eruptive ages of 175 190 ma for the marifil complex mostly toarcian aalenian the majority of the rocks are isotopically uniform with initial 87sr 86sr 0 7067 0 0003 and endt 4 2 primary magmas of andesitic composition were generated by partial melting of mafic grenvillian lower crust indentified by depleted mantle model ages of 1150 1600 ma lower crustal pyroxene granulite xenoliths can be modelled as residual jurassic source although they may alternatively be co genetic cumulates the dacite rhyolite suite formed by crystal liquid fractionation processes from the primary andesites involving multistage crystallization and re melting during magma ascent with the later stages of evolution being explicable by fractional crystallization of plagioclase amphibole and accessory minerals the province represents large scale lower crustal reworking associated with the unique tectonic and igneous environment of gondwana break up

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

the appalachian ouachita connection paleozoic orogenic belt at the southern margin of north america late paleozoic orogenic belt structures exposed in the appalachian mountains of alabama and in the ouachita mountains of arkansas plunge from opposite directions beneath postorogenic mesozoic cenozoic strata in the mississippi embayment of the gulf coastal plain figure 1 the paleozoic stratigraphic sequence and details of structural style in the ouachita outcrops contrast strongly with those in the appalachian outcrops furthermore straight line projection of structural strike from the outcrops does not lead to a simple connection of structures beneath the coastal plain concealed by the coastal plain cover are answers to questions such as the following are the orogenic belts continuous discontinuous offset or intersecting how are along strike changes in stratigraphy and structural style expressed what is the sequence of tectonic evolution early attempts to interpret the subsurface relationships between the appalachian and ouachita structures were of necessity based on the first few scattered wells that were drilled through coastal plain strata into paleozoic rocks subsequent studies have enjoyed the availability of progressively more numerous deep wells and a growing accumulation of geophysical data various stages in the evolution of thought were outlined by king 1950 1961 1975 and by thomas 1973 1976 this review summarizes available subsurface data in a pre mesozoic paleogeologic

fundamentals of seismic rock physics during the past 50 years or so tremendous progress has been made in studying physical properties of rocks and minerals in relation to seismic exploration and earthquake seismology during this period many theories have been developed and many experiments have been carried out some of these theories and experimental results have played important roles in advancing earth sciences and exploration technologies this tutorial paper attempts to summarize some of these results

jurassic extensional tectono magmatism and associated mineralization of the el faldeo polymetallic district chilean patagonia geochemical and isotopic evidence of crustal contribution the el faldeo is the southernmost andean mineral district and consists of paleozoic polymetamorphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the ibanez formation in addition two epizonal intrusive units and related subvolcanic porphyries sills and dikes which intrude both the metamorphic basement and the ibanez formation are recognized the quebrada colorada granodiorite qcg and the cerro esmeralda tonalite cet the polymetallic mineralization was formed in four stages and includes pyrite arsenopyrite gold silver sphalerite galena and chalcopyrite hosted in felsic tuffs of the ibanez formation hydrothermal breccias and epizonal intrusions field observations and geochronological data u pb ar ar and fission track on cet rocks suggest 1 the development of subsiding pull apart basins and coeval formation of spaces for magmatic emplacement and mineralization in an extensional tectonic regime in mid jurassic time 2 a tectonic exhumation of the rocks of the district probably triggered by the chile ridge trench interactions that started in late miocene time sr nd isotopic data from cet samples and the pb isotope characteristics of galenas from the second and third mineralization stages are indicative of crustal contribution to the origin of the magmas and galena mineralization rocks of the paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas

a case study of stratigraphic interpretation using shear and compressional seismic data this paper describes the use of surface recorded compressional and horizontal shear wave seismic data to detect lateral changes in the physical properties of a clastic unit shear and compressional wave transit times were measured across a selected interval from cdp stacked sections derived from data collected along coincident shear and compressional seismic lines at each surface position the ratio of the shear to compressional transit time across the target horizon is calculated it is shown that lateral variations in this ratio coupled with the behavior of the individual transit time curves can be used to infer changes in the physical properties of a formation the horizon selected for this case study was the lower pennsylvanian morrow formation which produces gas from channel sand bodies at the empire abo field new mexico a detailed geologic section of the producing horizon was mapped along a seismic line oriented so that it crossed productive and nonproductive regions of the field shear and comp

40ar 39ar dating of the jurassic volcanic province of patagonia migrating magmatism related to gondwana break up and subduction abstract the mesozoic large igneous province lip of patagonia southern south america which is one of the largest silicic provinces on earth has been investigated by the 40ar 39ar method twenty seven ages considered as valid including twenty plateau ages show that the volcanic activity ranging from 187 to 144 ma occurred between and contemporaneously with the initial break up of gondwana starting with the karoo antarctic tasmanian kat flood basalt province in the east and a subduction in the west the data display a regular decreasing of ages from the ene 187 ma to the wsw 144 ma along about 650 km apparently related to the tectonic structure in half grabens oriented nnw sse the good fitting of this trend with the opening of the rocas verdes sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the ssw in a continental disruption behind the magmatic arc the observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction the spreading and thermal effect of the kat plume may have an additional effect and also could account for the unusually large volume of magma

transform normal extension and asymmetric basins an alternative to pull apart models continental transforms are commonly associated with relatively small scale pull apart basins however much larger scale basins which are not consistent with the pull apart model commonly exist adjacent to transforms structures associated with several basins along the dead sea rift and el pilar fault illustrate this these basins are remarkably asymmetric bounded by essentially linear transform segments on one side and subparallel normal faults on the other suggesting simultaneous strike slip motion and transform normal extension this observation which is incompatible with classical faulting theory can be explained along divergent plate boundaries if the transform fault is much weaker than the adjacent crust analysis of the orientation of the horizontal principal stress direction near a weak transform fault embedded in an otherwise strong crust shows that convergent and divergent plate motion results in a stress field characterized by nearly fault normal compression and extension respectively structural styles along the san andreas dead sea and el pilar transform systems show that predictable changes between these two states have occurred through time along strike of the transforms