crack fusion dynamics a model for large earthquakes the physical processes of the fusion of small cracks into larger ones are nonlinear in character a study of the nonlinear properties of fusion may lead to an understanding of the instabilities that give rise to clustering of large earthquakes we have investigated the properties of simple versions of fusion processes to see if instabilities culminating in repetitive massive earthquakes are possible we have taken into account such diverse phenomena as the production of aftershocks the rapid extension of large cracks to overwhelm and absorb smaller cracks the influence of anelastic creep induced time delays healing the genesis of juvenile cracks due to plate motions and others a preliminary conclusion is that the time delays introduced by anelastic creep may be responsible for producing catastrophic instabilities characteristic of large earthquakes as well as aftershock sequences however it seems that nonlocal influences i e the spatial diffusion of cracks may play a dominant role in producing episodes of seismicity and clustering

precambrian basement geology of the permian basin region of west texas and eastern new mexico a geophysical perspective because most of the permian basin region of west texas and southern new mexico is covered by phanerozoic rocks other means must be found to examine the precambrian upper crustal geology of the region we have combined geologic information on the precambrian from outcrops and wells with geophysical information from gravity and magnetic surveys in an integrated analysis of the history and structure of basement rocks in the region geophysical anomalies can be related to six precambrian events formation of the early proterozoic outer tectonic belt igneous activity in the southern granite rhyolite province an episode of pre grenville extension the grenville orogeny rifting to form the delaware aulacogen and eocambrian rifting to form the early paleozoic continental mar in two geophysical features were studied in detail the abilene gravity minimum and the central basin platform gravity high the abilene gravity minimum is shown to extend from the delaware basin across north central texas and is interpreted to be caused by a granitic batholith similar in size to the sierra nevada batholith in california and nevada this batholith appears to be related to formation of the southern granite rhyolite province possibly as a continental margin arc batholith because of this interpretation we have located the grenville tectonic front southward from its commonly quoted position closer to the llano uplift middle proterozoic mafic intrusions are found to core the central basin platform and the roosevelt uplift these intrusions formed at about 1 1 ga and are related in time to both the mid continent rift system and the grenville orogeny in texas because these features are likely to be rift related they suggest that the concept of a delaware aulacogen needs to be revised only to the extent that the rifting is proterozoic in age not eocambrian precambrian basement structures and changes in lithology have influenced the structure and stratigraphy in the overlying permian basin and thus have potential exploration significance interpretation of the gravity and magnetic data with geologic information also leads us to suggest the existence of pre ellenburger basins which may be extensive and of potential exploration interest

sublacustrine gravity induced deposits the diversity of external geometries and origins abstract our current knowledge of deep water depositional systems is mainly built of extensive achievements obtained from marine basins although lacustrine basins are important oil gas productive areas of the world gravity induced deposits in these basins are poorly understood in the literature when compared with those documented in marine basins based on new insights from 3d seismic data this paper attempts to investigate the effect of the interaction between gravity induced flows and lake floor topography on the resulting external geometry of gravity induced deposits in the cretaceous qingshankou and nenjiang formations of the songliao basin slb eight different shapes of gravity induced deposits were identified in the slb and which were in turn grouped into four distinct categories according to external shape internal architecture and spatial assembly 1 channel fan complexes 2 fan form complexes 3 strip form complexes and 4 pond form complexes furthermore fan form complexes can be subdivided into four subcategories isolated fan with compressional ridges isolated fan without compressional ridges mother son fans and stacked fans strip form complexes can be subdivided into two subcategories including strip form controlled by gully system and strip form controlled by growth faults sediment transport mechanism density flows or mass transport flows and lake floor topography together influence the pathways runout distances discharge places and internal characteristics of gravity induced deposits which could ultimately influence external geometries of gravity induced deposits in the slb consequently sublacustrine gravity induced deposits does not exhibit classic fan like geometry frequent and powerful hyperpycnal flows associated with flood events are fairly common and develop channel distal fan complexes mtds exhibit various external geometries which are significantly affected by growth faults slope gullies gradient and basin floor irregularities the new depositional model built from the slb is vital to understanding the distribution of gravity flow deposits in a sublacustrine slope

evidence for late middle proterozoic extension in the precambrian basement beneath the permian basin through an integrated interpretation of gravity magnetic seismic reflection and well data we are able to define the geometry lateral extent and tectonic setting of late middle proterozoic mafic rocks the pecos mafic intrusive suite in the basement of the permian basin of west texas and eastern new mexico in this region seismic reflection data are characterized by subhorizontal layered reflectivity within basement that can be tied to compositional variation in a layered mafic intrusion through well cuttings and a synthetic seismogram in places the intrusion forms a sill and the base of the sill is imaged in the reflection data integration of the seismic interpretation with gravity models shows that the pecos mafic intrusive suite is 3 10 km thick and 50 100 km wide the overall structure of the suite is that of a sill with a locally dikelike keel north south trending gravity and magnetic anomalies associated with the pecos mafic intrusive suite suggest that it is composed of four large intrusive centers which cover a minimum area of 9200 km2 making it one seventh the size of the bushveld complex and one of the largest layered mafic intrusions in the world we interpret this massive intrusion to be associated with collisional rifting in the foreland of the grenville orogeny

borehole geophysical log signatures and stratigraphic assessment in a glacial basin southern ontario over the past two decades the geological survey of canada has used a standardized suite of slim hole geophysical tools to log 57 polyvinyl chloride cased boreholes drilled in the glacial sediments of southern ontario this article documents downhole tool responses natural gamma apparent conductivity magnetic susceptibility and seismic velocity in the context of mineralogical characteristics of the region and grain size data from 28 of the 57 boreholes characteristic geophysical properties and or patterns are identified within the units of a regional hydrostratigraphic framework in southern ontario the importance of a calibrated suite of tools is emphasized as stratigraphic units may have variable response from site to site the use of a high sensitivity magnetic susceptibility induction probe is shown to be an important tool in the log suite for lithostratigraphic interpretation and more broadly for provenance studies of source rock across the region ranges of compressional p and shear s

a model for repetitive cycles of large earthquakes the theory of the fusion of small cracks into large ones reproduces certain features also observed in the clustering of earthquake sequences by modifying our earlier model to take into account the stress release associated with the occurrence of large earthquakes we obtain repetitive periodic cycles of large earthquakes a preliminary conclusion is that a combination of the stress release or elastic rebound mechanism plus time delays in the fusion process are sufficient to destabilize the crack populations and ultimately give rise to repetitive episodes of seismicity

the miocene to pleistocene filling of a mature extensional basin in trans pecos texas geomorphic and hydrologic controls on deposition abstract northwest eagle flat basin in trans pecos west texas is a late tertiary quaternary extensional basin in the basin and range province the basin has largely filled with sediment so that bedrock uplands only crop out around its rim movement on normal faults has effectively ceased and the geomorphology and deposition differ from active extensional basins where tectonism is the primary control on basin evolution eagle flat differs from typical extensional basins because progressive changes in basin morphology caused by the aggrading basin fill were the principal controls on sedimentation a low gradient alluvial basin floor expanded to cover the former fault defined basin margins the playa lies on what was upland through the miocene and early pliocene only the upper few meters of basin fill crop out however a suite of 88 cores was drilled in the southern part of the basin nine of the cores and one trench were sampled for paleomagnetic reversal dating correlating the dated cores with interspersed cores allowed us to piece together the basin filling history and explain how the mature features developed the cores record the gradual burial of the southern half of the basin the oldest basin fill strata were cored in the deepest part of the basin 219 m below the surface at 12 ma in the miocene after deposition of 50 m of alluvial fan gravel along the trend of an inferred normal fault the basin floor aggraded and expanded by the 780 ka brunhes matuyama reversal basin floor drainage had reversed and was from the north sediment accumulation ended during the mid pleistocene a fine grained sediment supply that did not decrease and outpaced subsidence was the primary control on basin deposition this caused a progressive loss of relief and drainage basin area as uplands were buried under the aggrading basin sediments after the basin margin faults were buried the shape of the basin and its filling history were little controlled by the original tectonically formed geomorphic elements

mylonites from the towaliga fault zone central georgia products of heterogeneous non coaxial deformation abstract retrograde mylonitic rocks occur in the segment of the towaliga fault southeast of jackson georgia mylonite protoliths are heterogeneous quartz feldspathic gneisses upper amphibolite facies sillimanite zone from the pine mountain and piedmont terranes protoliths contain no distinctive lithological markers that can be used to determine fault displacement shear sense can only be determined using microstructural criteria and type porphyroclast systems shear bands asymmetric microfolded quartz ribbons subgrain fabrics in quartz ribbons the most common porphyroclast system developed in mylonites and ultramylonites is type type systems do not have readily identifiable tails but do produce distinctive asymmetrical pile ups in the matrix such that the angle of intersection between the perturbed zone and the mylonitic foliation points in the shear direction differences in morphology between and type systems can be related to differences in the recrystallization rate deformation rate ratio r reflecting changes in ductility contrast between matrix and porphyroclast that controls the amount of recrystallized material in a porphyroclast mantle that is potentially available to form tails in early stages of mylonitization with low ductility contrasts and large r ratios large mantles occur around porphyroclasts and tailed systems develop in advanced stages of mylonitization with large ductility contrasts and small r ratios either very small or partially or totally missing mantles occur and type systems develop microstructural criteria ubiquitously record a right lateral strike slip motion component to the deformation textural evidence e g unidirectional shear bands consistent vorticity recorded by porphyroclasts and asymmetrical microfolds suggest that the mylonites are the products of non coaxial deformation the local presence of fabrics with coaxial symmetry result from heterogeneity in the deformation the right lateral offset on the towaliga fault most likely occurred during the late paleozoic alleghanian orogeny

tightening the siberian connection to western laurentia newly available geological and geophysical data tighten the proterozoic connection between the rifted margins of the northern siberian craton and western laurentia and permit a siberia laurentia australia troika with northern australia connected to the southern margin of the siberian craton the continental assembly is linked by a prominent 2 0 1 8 ga collisional belt and the ca 1 3 1 0 ga grenville orogen the reconstruction also aligns a 1 5 1 45 ga dike sill swarm that extends from the wyoming province through the belt purcell basin into the northern siberian craton separation of australia and siberia may have occurred by the early neoproterozoic but separation of siberia and laurentia may not have been completed until the early cambrian continental extension began along the zigzag siberia laurentia rift zone in the early mesoproterozoic rift related igneous and sedimentary assemblages dated at ca 1 5 1 38 and 1 2 to 1 0 ga correlate across the reconstructed siberia laurentia rift zone renewed rifting in the neoproterozoic and vendian culminated with seafloor spreading and thermal subsidence of the conjugate rift shoulders correlative archeocyathan reefs endemic olenellid trilobite fauna and exchange of detritus between the cratons imply that the rift may have remained relatively narrow until the atdabanian stage black sulfidic shales buried the archeocyathan reefs on the rapidly subsiding rift margins during the botomian sinsk event

geochemistry and tectonic affinities of a proterozoic bimodal igneous suite west texas diabasic sills from the 1 27 b y old carrizo mountain group of west texas have immobile element abundances similar to those of modern mid ocean ridge basalts morb the presence of thick terrigenous sedimentary rocks and peraluminous rhyolitic ash flow tuffs within the carrizo mountain group preclude the diabases forming in a typical morb environment field petrographic and geochemical evidence suggests that the carrizo mountain group was deposited in an incipient continental margin back arc basin that may have extended northeastward into the texas panhandle region the overlying allamoore formation consists of sedimentary rocks and basalt accumulated within a mature marginal basin behind the tholeiite dominated island arc postulated for the llano area

micro and macroscopic models of rock fracture summary the anelastic deformation of solids is often treated using continuum damage mechanics an alternative approach to the brittle failure of a solid is provided by the discrete fiber bundle model here we show that the continuum damage model can give exactly the same solution for material failure as the fiber bundle model we compare both models with laboratory experiments on the time dependent failure of chipboard and fiberglass the power law scaling obtained in both models and in the experiments is consistent with the power law seismic activation observed prior to some earthquakes

40ar39ar thermochronological constraints on the structural evolution of the mesoproterozoic natal metamorphic province se africa abstract ten 40 ar 39 ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the mesoproterozoic 1 2 1 0 ga natal metamorphic province nmp since the proposal of the sweat hypothesis the nmp has become a crucial area in which to study grenville aged locally termed namaquan accretion in this part of rodinia gondwana unlike many segments of the world s 1 1 ga belts the nmp is particularly suited to such studies since it is devoid of a high temperature pan african 500 ma overprint in this study we have dated hornblendes from within the earliest d1 ne directed thrust dominated structural domains of the belt the d1 domains are characterized by ne directed recumbent fold and thrust nappes and a pervasive generally s to sw dipping metamorphic foliation with down dip stretching lineations the new data constrain the minimum age of this early event which includes obduction of the northernmost tugela terrane onto the kaapvaal craton to 1135 9 ma in the d1 domains of the mzumbe terrane to the south cooling to below 550 c had only been attained by 1005 ma we also dated amphibolites from the later d2 structural domains which are characterized by sub vertical sinistral mylonite belts with sub horizontal to oblique stretching lineations our data show that d2 oblique shearing commenced at 1050 1035 ma in the mzumbe terrane and only terminated at 980 ma the youngest movements also included reactivation of major d1 structures such as the melville thrust mzumbe margate terrane boundary at 990 ma finally indications of minor pan african resetting was detected during the first 5 of the ar ar spectra from mylonites of the lilani matigulu shear zone tugela mzumbe terrane boundary indicating the polyphase nature of this major structure it has been suggested that the d1 event records the early ne sw directed arc continent collision history of the belt whilst the d2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening the new data place important time constraints on the major tectonic events in the nmp which appear comparable in many ways to other parts of the global grenvillian orogen

stretching lineations in transpressional shear zones an example from the sierra nevada batholith california abstract ductile shear zones and associated stretching lineations are generally considered to be the result of two dimensional simple shear deformations with stretching lineations interpreted to rotate into parallelism with the direction of tectonic transport with increasing deformation however stretching lineations perpendicular to the inferred tectonic transport direction are displayed by some shear zones field studies in the sierra nevada batholith have revealed a single shear zone the rosy finch gem lake shear zone that contains both steeply plunging stretching lineations in older metamorphosed sedimentary and granitic rocks and shallowly plunging stretching lineations in syntectonic granitoids dextral sense of shear indicators are found in all these units and deformation occurred simultaneously along the length of the shear zone theoretical strain modeling indicates that stretching lineations can be either horizontal or vertical within transpressional shear zones to investigate the kinematics of three dimensional deformation we examine the role of stretching lineations which form as a result of finite strain including the possibility of partitioning a component of the strike slip movement onto shear bands comparing the field data with the strain modeling we interpret the along strike variation in lineation behavior within the rosy finch gem lake shear zone to be a result of the differences in finite strain recorded by different units the older units showing steeply plunging lineation have recorded more finite strain across a narrower shear zone than the syntectonic granites which record shallowly plunging lineations we conclude that the orientation of the lineation within transpressional shear zones does not necessarily correlate with the transport direction but may reflect along strike variations in finite strain and or strain partitioning

late middle proterozoic hazel formation near van horn trans pecos texas evidence for transpressive deformation in grenvillian basement the latest middle proterozoic hazel formation near van horn in the trans pecos region of west texas is an orogenic elastic wedge that provides insight into paleotectonic evolution of the southern margin of the laurentian craton the hazel formation is 2 5 km thick adjacent to the contemporaneous streeruwitz thrust fault and consists exclusively of terrestrial sedimentary rocks boulder sized clasts in conglomerate were deposited within alluvial fans north of the streeruwitz thrust sheet 0along the southern margin of a basin of unknown geometry and extent the alluvial fan conglomerate interfingers with eolian sedimentary rocks in the northern part of the basin alluvial fan conglomerate megasequences indicate that at least two episodes of faulting accompanied hazel sedimentation alluvial megasequences are overall aggradational within the hazel succession as revealed by clast size variations within the conglomerate systematic variation of pebble composition up through the hazel conglomerate provides an erosional history within the source terrain carbonate and subordinate mafic volcanic rocks derived from the pre orogenic allamoore and tumbledown formations were the principal source off the hazel formation metavolcanic and metasedimentary rocks from the carrizo mountain group in the present tectonic hinterland are notably absent in the hazel conglomerate moreover a rhyolite terrain presently not exposed in the van horn area was a significant component of the hazel source juxtaposition between source terrain and sedimentary basin coupled with aggradation of alluvial fan megasequences along the southern fault bounded basin margin supports a transpressive regime for this orogenic belt rather than a purely convergent margin deformation associated with the hazel orogenic sequence post dated a 1 126 to 1 070 m y old regional felsic igneous event grenville deformation in the llano uplift of central texas predated the same igneous event this implies that the hazel tectonic episode was a second later grenvillian deformation the terms llano orogeny and hazel orogeny are introduced here for the grenville province in texas and may be comparable with the elzevirian and ottawan orogenies within the grenville province of canada

from orogenic collapse to rifting a case study of the northern porcupine basin offshore ireland abstract offshore ireland the north atlantic opening is generally interpreted as successive 10 to 15 myr long rifting events during the mesozoic however their interaction is poorly documented in terms of structural inheritance and fault reactivation from extensive seismic and well data in the northern porcupine basin we show that extension actually evolved over an abnormal period of ca 220 myrs with overlapping tectonic stages that all together reflect the irish atlantic margin construction from the palaeozoic during the carboniferous extension initiated throughout the variscan and caledonian thickened crusts and controlled the deposition of generally slightly tilted continental clastics two subsequent rifting events during the triassic lower jurassic and upper jurassic lower cretaceous controlled shallow to deep marine sedimentation in separated sub basins that progressively widen and connect once extension localised along bounding faults we propose that extensional tectonics first controlled by a general orogenic collapse reflecting an early stage of stretching is then followed by several rifting periods that ultimately evolved toward the hyper thinning of the margin and shaped an aborted rift propagator this evolution is due to pre existing orogenic related structures and boundary conditions variations such a continuum of deformation although pulsed implies varying structural interactions of crustal scale that are often recorded in syn tectonic sediments

grenville age a type and related magmatism in southern laurentia texas and new mexico u s a abstract large grenville age plutons are known to be present in the 1340 1370 ma basement in texas and eastern new mexico but their distribution and compositional ranges are poorly known new u pb zircon dates shrimp rg stanford university on subsurface samples have identified widespread presence of compositionally diverse grenville age 1070 1110 ma plutonic rocks in the texas panhandle core from a 175 m thick sill like gabbroic intrusion yielded a date of 1081 8 3 ma in situ differentiation of this tholeiitic magma led to a 7 fold increase in incompatible element concentrations as ree patterns remained essentially flat trace element data suggest an e morb like source to the south alkali feldspar granite with a type affinities was recovered from drill core from the abilene gravity minimum near albany texas it yielded a date of 1078 23 ma similar to undeformed post orogenic granites in the llano uplift further west an anorthosite xenolith from the eocene three sisters intrusion in el paso texas yielded a date of 1068 30 ma which is within error of the 1110 19 ma age determined for the main stage of the nearby a type red bluff granitic suite xenoliths from potrillo maar volcano in the center of the southern rio grande rift include monzonitic xenoliths with granulitic texture which yielded dates of 1072 ma in contrast potrillo maar xenoliths with igneous textures yielded u pb dates of 27 ma development of the granulitic texture is interpreted to reflect granulite facies metamorphism during tertiary time 26 27 ma on the basis of age zoning and u concentrations in the zircon crystals our results show that grenville age magmatism in the texas and new mexico subsurface was widespread and was coeval with syn and post deformation granites in the llano uplift of central texas the compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were a type these magmatic affinities are thought to be associated with crustal extension rather than with subduction moreover the presence of anorthosite xenoliths in the el paso kilbourne hole region the broadly tholeiitic character of mafic rocks and the a type nature of the granitic rocks are broadly similar to amcg anorthosite mangerite charnockite granite suites present in the grenville province of eastern laurentia

a model of propagating brittle failure in heterogeneous media i consider a nonelastodynamic model for the propagation of brittle failure in a heterogeneously stressed medium numerical simulation shows a power law distribution of failure sizes with a slope not far different from the size distribution of seismic events the time history of calculated failure activity is not consistent with observed seismic behavior and some explanations and implications are discussed

strong motion seismology although this is the first review on this topic to appear in a quadrennial report the roots of strong motion seismology extend back to at least 1932 when far sighted engineers in the seismological field survey of the u s coast and geodetic survey installed rugged fieldworthy instruments designed to make on scale recordings of large earthquakes carder 1964 these instruments are called accelerographs for their output closely mimics ground acceleration hudson 1979 the original instruments and their offspring have provided a wealth of information about ground motions of direct use to engineers due in large part to the continued devotion of the seismological field survey now part of the branch of engineering seismology and geology at the u s geological survey the number of recordings has increased substantially particularly in the last two decades and multiple recordings of a few california earthquakes have provided the data necessary to begin to unravel the complexities of the ground motions and to predict these motions on an empirical basis although seismologists used accelerograph records in studies of earthquake energy and ground motion attenuation as long ago as 1942 gutenberg and richter 1942 1956 widespread seismological use of the records began with aki s 1968 analysis of the 1966 parkfield earthquake

late thermal evolution of proterozoic rocks in the northeastern llano uplift central texas abstract the llano uplift of central texas usa contains large exposures of polydeformed grenvillian metaigneous and metasedimentary rocks that were intruded by late to post orogenic granites new u pb and 40 ar 39 ar single mineral ages and rb sr mineral isochrons document the retrograde cooling history that followed grenville age metamorphism in the north central and northeastern llano uplift and explain discrepancies between previous u pb crystallization ages and younger determinations based on rb sr and k ar isotopic systems dating has been carried out in the valley spring gneiss vsg and the post tectonic lone grove pluton lgp in three areas of the llano uplift from west to east vsg rocks in the babyhead area north of llano texas yielding hornblende muscovite and biotite 40 ar 39 ar ages and rb sr mineral isochron ages from 1098 to 1014 ma a sample from the lgp at petrick quarry yielding u pb titanite and hornblende and biotite 40 ar 39 ar ages from 1096 to 1076 ma and vsg samples from the inks lake area in the eastern llano uplift yielding u pb titanite hornblende and biotite 40 ar 39 ar and rb sr mineral isochron ages from 1087 to 1029 ma titanite from the oatman granite to the southwest of these areas yields a u pb age of 1119 6 ma these ages combined with closure temperature estimates document cooling in the llano uplift from 1100 1000 ma at rates between 5 and 14 c ma 1 although rapid uplift following early high pressure metamorphism has important regional thermal effects the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity the low temperature history documented in this study is probably unrelated to uplift rates this supports the suggestion that magmatic heating was an important factor in the late high temperature low pressure metamorphism of the llano region it further suggests that the late pressure and temperature conditions recorded in the rocks were likely not synchronous across the region

outcrop gamma ray logging of siliciclastic turbidites separating the detrital provenance signal from facies in the foreland basin turbidites of the moravo silesian basin czech republic abstract standard and spectral gamma ray grs logs are widely used as indicators of facies in the subsurface in siliciclastics however the gamma ray signal is often influenced by changes in the provenance of the k u and th bearing detrital minerals in this paper we have compared outcrop and laboratory grs with modal and chemical sandstone and mudstone composition and facies in an approximately 2 5 km thick siliciclastic turbidite system of the moravo silesian culm basin lower carboniferous czech republic the aim was to separate the facies signal from the detrital provenance one the siliciclastics have moderately high outcrop gamma ray values 174 api on average and slightly lower laboratory values 127 api both the outcrop and laboratory data show low sensitivity to facies which is demonstrated by the low contrast between the k u and th concentrations in the seven facies types ranging from proximal to distal turbidites markedly higher grs variability is observed between equivalent facies at different stratigraphic levels major carriers of the grs signal include k feldspars muscovite sericite biotite and albite for k zircon apatite monazite and xenotime for u and monazite thorite ree secondary minerals xenotime apatite and zircon for th with the effect of facies filtered out the grs values reveal a stratigraphic variability which coincide with the changes in the sandstone modal composition a shift from the low grade metamorphic and volcano sedimentary provenance to predominantly magmatic sources with ultrapotassic plutonites in the early late visean is associated with a marked increase in u and th concentrations and generally higher sandstone radioactivity compared to mudstones another provenance shift to high grade metamorphic sources with granulites in the latest visean is associated with a rapid decrease in th u and partly k concentrations and an increase in the grs contrast between sandstone and mudstone facies the grs data sensitively reflect the extremely rapid exhumation of mid crustal and deep crustal rocks in the major source area the moldanubian zone of the bohemian massif

characterization and tectonic evolution of a mesoproterozoic island arc in the southern grenville orogen llano uplift central texas abstract integrated field and u pb geochronologic data document the presence and tectonic evolution of a distinct lithotectonic terrane in the southeastern llano uplift here termed the coal creek domain ccd the ccd consists of tonalitic and amphibolitic gneisses foliated gabbroic tonalitic and granodioritic intrusions and the structurally emplaced coal creek serpentinite the oldest rocks in the ccd consist of tonalitic gneisses with protolith crystallization ages of 1326 ma and 1301 ma and pre 1292 ma amphibolitic gneisses all ages are u pb zircon they were intruded by dominantly mafic and intermediate with subordinate amounts of felsic low k plutonic rocks between 1286 ma and 1275 ma high temperature metamorphism and local anatexis that resulted in the formation of gneisses and migmatites occurred between 1301 ma and 1286 ma and is suggested to have occurred at 1292 ma metamorphism at 1256 ma resulted in the growth of zircons in mafic rocks the regional significance of this metamorphic event is unclear but it may be related to dynamothermal metamorphism of the ccd the contemporaneous emplacement of low k gabbro tonalite and granodiorite and their spatial association with serpentinite combined with published geochemical data strongly suggest that the ccd represents an ensimatic arc the coal creek serpentinite is interpreted as a fragment of oceanic crust that formed either basement to the arc or in a related back arc or intra arc setting combined field u pb geochronologic isotopic and geochemical data indicate that the magmatic metamorphic and structural history of the ccd differs from that of coeval rocks elsewhere in the llano uplift the boundary between the ccd and deformed and metamorphosed granitic and supracrustal rocks to the north is a zone of intense mylonitization and polyphase deformation this zone is interpreted as the structural boundary between the arc terrane and units to the north

why does not lithology correlate with gamma ray spikes in the shaley source rocks of the upper jurassic alge member southwestern barents sea abstract prominent high value gamma ray spikes in the maximum flooding surfaces of the middle oxfordian kimmeridgian succession alge member display stratigraphic and regional variations in the southwestern barents sea these variations suggest heterogeneities in the composition of the alge member thus challenging the common perception that this interval is a homogenous succession of organic rich black shale this study explores the overall variation in clay mineral composition source rock quality minerals and macerals core logs and seismic facies of the alge member gamma log fluctuations were found to be independent of these variations uranium precipitates on sea bottom sediments when conditions change from oxic to anoxic fluctuations in sea level may result in such changes and hence guide gamma ray responses sea level fluctuations also lead to a changing distance to the shoreline which again influences sediment types grain sizes and source rock quality thus indirectly sea level appears to be the common denominator which sometimes matches source rock richness with spikes on gamma logs presence and absence of gamma ray spikes are hence an additional tool to reveal the redox conditions in the sw barents sea which has implications for the understanding of the paleogeography or paleobathymetry during deposition

structure of the carrizo mountain group southeastern carrizo mountains west texas a transpressional zone of the grenville orogen 1 the west texas segment of the grenville orogen includes an oblique dextral transpressive zone the orogen exposed near van horn texas displays a transition from the mid amphibolite facies metamorphic core to a foreland fold and thrust belt in this paper detailed structural mapping and analyses presented for key exposures of the metamorphic rocks the mesoproterozoic carrizo mountain group show a southeastward increase in number of deformational phases and intensity of deformation the central portion is dominated by oblique northwest convergence which produced two phases of northwest verging folds and synchronous oblique reverse and dextral shearing the thick parts of pre tectonic mafic plutons which had been emplaced as locally discordant sills control the spatial distribution of structures two dominant phases of folds form type 0 interference patterns essentially coaxial and coplanar i e fold tightening was dominant this central belt also contains en echelon reverse to oblique shear zones and comprises a transfer zone within the orogen between structurally higher and lower major shear zones overall the mapped part of the central carrizo mountains represents localized dextral transpression within a larger region dominated by reverse motion possibly indicating a change through time from orthogonal to oblique contraction further south in the bass canyon region the earlier fold generations are refolded into type 2 mushroom interference patterns these structures show a westward vergence and are kinematically linked to those further north across a zone of disharmonic folding and heterogeneous shear the structure and kinematics of the polyphase deformation and ductile shearing in the metamorphic portion of the orogen is compatible with that seen in the foreland fold thrust belt suggesting that in both foreland and hinterland transpression was a characteristic feature of the west texas grenville orogeny the timing and kinematics of deformation in the van horn exposures contrasts with that seen in the llano uplift but compares favorably with those of the natal belt south africa a relationship with implications for recently proposed reconstructions of rodinia

an improved approach to characterize potash bearing evaporite deposits evidenced in north yorkshire united kingdom traditionally potash mineral deposits have been characterized using downhole geophysical logging in tandem with geochemical analysis of core samples to establish the critical potassium k2o content these techniques have been employed in a recent exploration study of the permian evaporite succession of north yorkshire united kingdom but the characterization of these complex deposits has been led by mineralogical analysis using quantitative x ray diffraction qxrd the novel qxrd approach provides data on k content with the level of confidence needed for reliable reporting of resources and also identifies and quantifies more precisely the nature of the k bearing minerals errors have also been identified when employing traditional geochemical approaches for this deposit which would have resulted in underestimated potash grades qxrd analysis has consistently identified polyhalite k2ca2mg so4 4 2 h2o in the fordon evaporite formation and sylvite kcl in the boulby potash and sneaton potash members as the principal k bearing host minerals in north yorkshire however other k hosts including kalistrontite k2sr so4 2 a first recorded occurrence in the uk and a range of boron bearing minerals have also been detected application of the qxrd led characterization program across the evaporitic basin has helped to produce a descriptive empirical model for the deposits including the polyhalite bearing shelf and basin seams and two newly discovered sylvite bearing bittern salt horizons the pasture beck and gough seams the characterization program has enabled a polyhalite mineral inventory in excess of 2 5 billion metric tons bt to be identified suggesting that this region possesses the world s largest known resource of polyhalite this is an open access article distributed under the terms of the creative commons attribution license http creativecommons org licenses by 3 0 which permits unrestricted use distribution and reproduction in any medium provided that the original work is properly attributed

grenville age polyphase deformation of mid proterozoic basement nw van horn mountains trans pecos texas psammitic pelitic and mafic schists in a basement horst in the nw van horn mountains of west texas preserve evidence of a multiphase synmetamorphic grenville age ductile deformation three phases of progressive isoclinal folding followed by two later less intense nearly coaxial folding phases have been identified the second phase of deformation produced the dominant foliation and foliatipn intersection lineation observed throughout the area this deformation is inferred to have resulted from a regional scale folding possibly associated with nappe formation at or near peak metamorphic conditions mineral assemblages indicate amphibolite facies and fe mg exchange thermometry provide maximum temperatures of 640 pm 50 circ c corresponding to mid amphibolite facies conditions three subsequent fold generations f 3 f 5 formed under progressively lower metamorphic conditions f 3 produced a weakly developed foliation s 3 whereas neither of the two later folding phases fo

the theoretical seismicity of asperity models an application to the coast of oaxaca summary we solve the statistical dynamical equations of an asperity model to obtain both the time dependent seismicity and the stationary seismicity with its associated magnitude frequency relation the parameters of the model are chosen to reproduce the seismicity of the oaxaca region on the western coast of mexico the observed enhancement of the seismicity in the high magnitude range is well reproduced and the calculated b value of about 7 is also acceptable the predicted features of the time dependent seismicity are as follows there is an enhancement of the small scale seismicity immediately following a characteristic earthquake this enhancement approximately doubles the rate of occurrence of small events relative to the stationary values both intermediate and characteristic earthquakes present a gap with a subsequent enhancement of the seismicity superior to long term averages there are strong correlations between the characteristic and intermediate seismicities indicating that the latter have premonitory value

diagenetic modelling an integrated approach using wireline logs and geological data abstract diagenetic effects may be recognized by integrating wireline log measurements with other sources of geplogical data diagenetic features often give subtle changes in the log response that are difficult to interpret owing to these difficulties a forward modelling procedure using computers is being developed to aid interpretation a diagenetic model can be constructed with a knowledge of the controlling processes the model can then be iteratively refined by confirming or denying the existence of modelled features and this is achieved by comparing synthetic logs with actual wireline logs the diagenesis of the rotliegend sandstone of the southern north sea is well understood and there is extensive modern wireline log coverage the rotliegend provides suitable sequences in which to test the systems approach that is being developed and provides illustrative examples

orbital forcing as principal driver for fine grained deep marine siliciclastic sedimentation middle eocene ainsa basin spanish pyrenees abstract the relative importance of climate tectonics and or autocyclicity in driving deposition of laminated deep marine fine grained siliciclastic sediments in tectonically active basins remains controversial consequently the recognition of milankovitch periodicities in such successions remains poorly documented this study evaluates the cyclostratigraphy and its likely drivers in the middle eocene ainsa basin spanish pyrenees a basin that is known to have been associated with major syndepositional tectonics this basin was infilled by a thick succession of siliciclastic sediments characterised by an alternation of coarse grained sandbodies interpreted as submarine fans and finer grained packages of interfan fan lateral margin and channel levee overbank sediments this study presents time series analysis of outcrop spectral gamma ray and cumulative sandstone turbidite abundance data from eight fine grained essentially interfan sections located between coarse grained sandbodies submarine fans the 1 4 km of cumulative stratigraphy studied show that milankovitch cyclicity can be recognised throughout the entire stratigraphic evolution of the ainsa basin even though the basin was tectonically active and suggest that the accumulation of the interfan deposits was orbitally controlled orbital parameters most likely paced the cyclic delivery of the fine grained sediments mainly by river and delta supplied turbidity currents from the non marine and shallow marine tremp basin immediately to the east sediment accumulation rates determined from spectral analysis show a gradual decrease throughout the deep marine stratigraphy from 57 to 24 cm kyr this study shows that orbitally induced climate change acted as a principal driver on deep marine terrigenous sediment accumulation within a tectonically active basin high frequency climate variability therefore should be considered when studying controls on siliciclastic sedimentation in tectonically active regions

last gasp of the grenville orogeny thermochronology of the grenville front tectonic zone near killarney ontario we present u pb titanite zircon and 40 ar 39 ar hornblende mica k feldspar data from a transect across the western part of the grenville front tectonic zone gftz near killarney ontario high grade metamorphic assemblages 1450 ma in this part of the gftz pre date the grenvillian orogeny and were primarily exhumed with little or no metamorphic overprinting by grenvillian deformation the titanite and zircon data form a discordant array with an upper intercept of 1454 pm 8 ma and a lower intercept of 978 pm 13 ma these data are interpreted in terms of partial lead loss during a short lived thermal event that increased in intensity from west to east across the transect 40 ar 39 ar data from hornblende indicate cooling through 450 c at 993 979 ma multiple diffusion domain models for the interpretation of discordant k feldspar spectra indicate cooling through 365 340 c at 990 960 ma and muscovite data indicate cooling through 320 c at 930 ma biotite data are not easily

theoretical calculations of the seismicity in oaxaca mexico the time dependent and time independent seismicity of the coast of oaxaca mexico is computed by solving the statistical dynamic equations of an asperity model the observed enhancement of the seismicity in the large magnitude range associated with characteristic earthquakes is well reproduced and the calculated b value of about 0 7 is also acceptable the predicted features of the time dependent seismicity are as follows there is an enhancement of the small scale seismicity immediately following a characteristic earthquake this enhancement approximately doubles the rate of occurrence of small events relative to the stationary values both intermediate and characteristic earthquakes present a gap with a subsequent enhancement of the seismicity superior to long term averages the predicted gap for large events is 17 yrs and the most likely interoccurrence time between characteristic earthquakes is 37 yrs with the most hazardous period being the interval 27 48 yrs after the previous event there is a maximum in the intermediate seismicity which precedes the maximum in characteristic seismicity indicating that these factors are of predictive value as foreshocks

controls on spatial and temporal distribution of precambrian eolianites abstract inversely graded stratification generated by the migration of wind ripples and adhesion structures permit unequivocal identification of precambrian eolianites these criteria in combination with scale of cross beds angle of inclination of foresets geometry of depositional units and associated non eolian facies are used to discriminate between precambrian dune draa dune plinth sand sheet and interdune deposits that formed in inland and coastal settings based on an analysis of published literature fundamental conclusions can be drawn on the spatial and temporal distribution of precambrian eolianites the oldest reported eolianites are from the ca 2 1 ga deweras group in zimbabwe and hurwitz group in canada and numerous examples of eolianites are reported from the 1 8 ga and younger rock record lack of archean and early paleoproterozoic eolianites and their widespread development after 1 8 ga are examined with respect to absence of vegetation crustal growth and tectonic setting relative sea level fluctuations unfavorable atmospheric and or climatic change and non recognition the lack of pre 2 2 ga eolianites may be related to reworking by braided rivers combing across non vegetated floodplains reworking of coastal eolianites during transgression or their non recognition in the early precambrian record the temporal concentration of eolianites at 1 8 ga may best be related to the early stages of breakup and the assembly phases of supercontinents

thermochronology of the grenville orogeny in west texas abstract the carrizo mountain group cmg comprises the westernmost exposures of the metamorphic core of the grenville orogen in texas unlike the correlative llano uplift 500 km to the east the cmg underwent one phase of medium p t metamorphism peak t was attained during d2 in all cmg exposures the peak metamorphic paleotemperatures decrease to the northwest in the overall direction of tectonic transport from staurolite to biotite grade garnet in the southeast carrizo mountains grew isothermally and probably during decompression i e part of a clockwise p t loop at ca 520 c in between biotite grade d1 and d3 events hornblende in this region cooled below closure t during d2 at 1057 6 ma within error of muscovite closure in the northwest carrizo mountains at 1059 2 ma hornblende in higher grade exposures to the southeast cooled through closure t as late as 1002 7 ma muscovite in the garnet grade and higher exposures closed to ar diffusion at 992 4 ma to 981 3 ma biotite and partially recrystallized hornblende from the northwest carrizo mountains cooled through closure in this same time period we interpret these data as signifying 1 ar loss due to peak metamorphism in hornblende southeast carrizo mountains and muscovite northwest carrizo mountains at ca 1059 ma 2 post tmax cooling through 500 c in the higher grade outcrops until ca 1000 ma and 3 cooling of all cmg exposures through 300 c due to uplift along the streeruwitz thrust and erosion from 1000 to 980 ma these ages significantly post date known tectonism in the llano uplift and extend the duration of grenville orogenesis in texas by ca 115 million years the 1000 980 ma time period is a time of thrusting in frontal regions of the grenville province of canada and the northeastern us and shearing in the natal province south africa suggesting ultimate closure of the grenville orogen and supporting recent rodinia reconstructions

colliding cascades model for earthquake prediction a wide set of premonitory seismicity patterns is reproduced on a numerical model of seismicity and their performance in the prediction of major model earthquakes is evaluated seismicity is generated by the colliding cascades model recently developed by the authors the model has a hierarchical structure it describes the interaction of two cascades a direct cascade of loading which is applied to the top largest element and transfers down the hierarchy and an inverse cascade of failures which goes up the hierarchy from the smaller to the larger elements these cascades collide and interact loading leads to failures while failures release and redistribute the loading three basic types of earthquake precursors are considered i the clustering of earthquakes in space and time ii the intensity of earthquake sequences and iii the correlation distance between earthquakes patterns of the first two types are used in intermediate term earthquake prediction algorithms patterns of the third type are found in the colliding cascades model although they were hypothesized previously they have not been validated by observations for each precursor we explore what is called an error diagram showing the total duration of alarms the rate of failures to predict and the rate of false alarms

sequence stratigraphy and orbital forcing in permian rotliegend desert deposits a discussion rotliegend sequence stratigraphy in the southern permian basin suggests climatically forced fluctuations in the level of an expanding playa lake system the associated facies changes are abrupt and define essentially isochronous sequence boundaries spectral analyses of gamma ray logs have indicated that individual sequences display internal cyclicity consistent with orbital milankovitch forcing confirmatory results have been reported from the spectral analysis of field logs from the continental permian brodick beds of the isle of arran these studies claim that since the cycles are orbitally forced they have chronostratigraphic significance however they are based on dubious assumptions concerning the unvarying rate and continuity of deposition of the sequences in question in the case of the southern permian basin the assumption is applied to c 50 m units but the result is an implausible chronostratigraphic model for the brodick beds the assumption is applied to a 675 m succession and there is a crucial ambiguity in the relationship between the lithostratigraphic cycles and the permian orbital cycles the studies do not provide persuasive evidence of the consistent encoding of milankovitch orbital cycles in permian desert deposits

comparison between outcrop spectral gamma ray logging and whole rock geochemistry implications for quantitative reservoir characterisation in continental sequences abstract a high resolution spectral gamma ray sgr log is compared to elemental whole rock geochemical data in a reservoir scale outcrop analogue the otter sandstone formation sherwood sandstone group east devon sgr data compare closely to those generated for point samples by inductively coupled plasma icp techniques with disparities attributable to the different sample volumes icp data reveal separate mineralogical sources for radio elements which enable three lithotypes to be discriminated using th u ratio and k 2 o data once established the discriminants are applied to the sgr log enabling three lithotypes to be dicriminated these conclusions suggests that sgr logs have significant potential for real time lithotype prediction in continental sequences which in turn enable predictions of reservoir quality and increase confidence in the interpretation of sedimentary architecture

a renormalization group approach to the stick slip behavior of faults we treat a fault as an array of asperities with a prescribed statistical distribution of strengths when an asperity fails the stress on the failed asperity is transferred to one or more adjacent asperities for a linear array the stress is transferred to a single adjacent asperity and for a two dimensional array to three adjacent asperities using a renormalization group rg method we investigate the properties of a scale invariant hierarchical model for the stochastic growth of fault breaks through induced failure by stress transfer an extrapolation to arbitrarily large scales shows the existence of a critical applied stress at which the solutions bifurcate at stresses less than the critical stress virtually no asperities fail on a large scale and the fault is locked above the critical stress asperity failure cascades away from the nucleus of failure we interpret this catastrophic failure as an earthquake and it corresponds to the transition from stick to slip behavior on the fault thus the stick slip behavior of most faults can be attributed to the distribution of asperities on the fault we propose our stochastic mechanism as an alternative to the traditional hypotheses for the stick slip behavior of faults a major advantage of our approach is the inclusion of scale invariance thus the observed frequency magnitude relation for seismicity is a natural consequence of our basic hypothesis

a model of foreshock occurrence summary we have constructed a model for the occurrence of foreshocks and foreshock sequences the foreshocks as well as the main shock are assumed to occur because of the abrupt coalescence of pre existing cracks coalescence occurs if the stress intensity factors at the crack tips exceed a certain critical value in the sub critical state each crack undergoes accelerated extension quasi statically due to stress corrosion until it reaches the critical state at which point growth becomes catastrophic the rate of foreshock occurrence increases as the time of the main shock approaches and is described by a power law function of time the exponent in the power law is independent of the distributions of crack sizes spacings and fracture strengths this universality implies that foreshock patterns should be useful identifiers for the prediction of the occurrence times of large earthquakes however we find that there is considerable diversity in the patterns of energy release nevertheless there is some hope that the magnitudes of large earthquakes can be predicted from the magnitudes of the events in the foreshock sequences if one has a priori information describing the distributions of barrier strengths and geometries

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

the influence of initial fold geometry on type 1 and type 2 interference patterns an experimental approach abstract experiments on superposed folding were performed in a plane strain pure shear rig using paraffin wax as an analogue for rocks a series of pre formed cylindrical folds were refolded with the compression direction acting parallel to the initial fold hinge direction the intermediate axis perpendicular to the first fold axial plane and the extension direction parallel to the first fold axial plane and perpendicular to its axis type 1 interference geometry roundness tightness and amplitude were varied to investigate the influence of first fold geometry on the interference patterns experimental folding of an initially planar layer oriented parallel to the first fold envelope provides a reference geometry for comparison the results suggest that hinge roundness has less influence on the interference pattern than other factors close initial folds refold into type 2 interference patterns which become even more pronounced as the tightness increases open folds produce type 1 interference patterns folds with the same interlimb angle and the same roundness produce different interference patterns depending on relative amplitude folds with relatively large amplitude are refolded into type 2 patterns whereas folds with small amplitude give clear dome and basin structures in a second set of experiments the compression direction also acted parallel to the first fold hinge but the orientations of the intermediate and extension axes were interchanged the interference patterns obtained are very similar to those in type 1 interference geometry suggesting that the major factor in determining whether type 1 or type 2 interference patterns develop is the initial fold geometry and not the kinematics of the second deformation

new earthquake rebound theory abstract the model adopted in this paper assumes that a medium is permeated with a continuum field of defects and that a flow of defects describes an advanced state of deformation leading to fracturing the obtained system of equations describes a field of internal stresses and a relative flow velocity of cracks the form of equations shows that before an earthquake a field of internal stresses reaches its maximum and that this moment corresponds to a specific silent gap expressed by the disappearance of the relative velocity field of cracks later on there starts a rebound process with an opposite sense of relative velocities with decrease of defect density joining of cracks and with a main process of earthquake release

late mesoproterozoic deformation of sw amazonia rondonia brazil geochronological and structural evidence for collision with southern laurentia abstract proposed assembly of the rodinia supercontinent in the late mesoproterozoic involved the collision of the amazon craton with some portion of the southern or eastern margin of cratonic north america previously reported paleomagnetic data from the sw amazon craton suggest a paleogeographic link between grenvillian deformation of the sw amazon craton and late mesoproterozoic tectonometamorphism in southern laurentia a structural geochronological and petrological investigation of the western amazon basement rocks rondonia brazil was carried out in order to document evidence of a grenvillian collision connecting the amazon to laurentia integration of 40ar 39ar data and feldspar thermometry data from regionally extensive strike slip mylonitic shear zones ji parana shear zone network indicates that deformation took place at 450 550 c between 1 18 and 1 15 ga an older ca 1 35 ga event found exclusively in less deformed basement rocks is interpreted as recording cooling from an earlier met

eolian deposits in the neoproterozoic big bear group san bernardino mountains california usa abstract strata interpreted to be eolian are recognized in the neoproterozoic big bear group in the san bernardino mountains of southern california usa the strata consist of medium to large scale 30 cm to 6 m cross stratified quartzite considered to be eolian dune deposits and interstratified thinly laminated quartzite that are problematically interpreted as either eolian translatent climbing ripple laminae or as tidal flat deposits high index ripples and adhesion structures considered to be eolian are associated with the thinly laminated and cross stratified strata the eolian strata are in a succession that is characterized by flaser bedding aqueous ripple marks mudcracks and interstratified small scale cross strata that are suggestive of a tidal environment containing local fluvial deposits the eolian strata may have formed in a near shore environment inland of a tidal flat the neoproterozoic big bear group is unusual in the western united states and may represent a remnant of strata that were originally more widespread and part of the hypothetical neoproterozoic supercontinent of rodinia the big bear group perhaps is preserved only in blocks that were downdropped along neoproterozoic extensional faults the eolian deposits of the big bear group may have been deposited during arid conditions that preceded worldwide glacial events in the late neoproterozoic possibly similar pre glacial arid events are recognized in northern mexico northeast washington australia and northwest canada

recent research on the physical aspects of earthquakes abstract recent developments in the field of physical aspects of earthquakes which encompasses in present day terminology the analysis of focal dynamics and of catastrophic effects of earthquakes are reviewed in particular individual sections of this review deal with the earthquake source effects of earthquakes on the ground the geographic and temporal distribution of earthquakes the characterization of seismic risk earthquake prediction and with the artificial release of earthquakes in this instance the review supplements earlier information by the author 1975 by new data published mainly between 1975 and 1984

spectral gamma ray logging of the gres d annot se france an outcrop analogue to geophysical facies mapping and well log correlation of sand rich turbidite reservoirs abstract spectral gamma ray grs logging is a powerful tool in cyclo and sequence stratigraphy of carbonate depositional systems in siliciclastic systems with low chemical maturity the multi component character of sediment complicates the interpretation of such logs this work focuses on understand better the relationships between grs logs and siliciclastic facies stacking patterns using grs measurements combined with lithology modal composition and geochemistry this study is focused on outcrops in the st antonin annot grand coyer chalufy areas of the oligocene gres d annot formation se france the studied siliciclastics reveal moderately high total radioactivity and mean concentrations of k 3 8 u 4 5 ppm and th 13 ppm u and th are predominantly contained within heavy minerals and so tend to be concentrated in mudstones and heterolithic sand mud facies whereas sandstone and conglomerate facies have slightly higher levels of k due to higher contents of k feldspars and micas the abundance of minerals containing radioactive elements and the relatively low compositional contrast between different facies are considered the main reasons why the standard gamma ray measurements provide only a limited proxy for lithology in this case two complete regressive to transgressive cycles were identified based on combined th and th k logs and facies stacking patterns the cycle boundaries are marked by shifts towards coarse grained facies abrupt increase of u and th concentrations and th k ratio within the proximal deposits at the st antonin section in the distal chalufy section the response of the gamma ray proxies to facies shifts is reversed these boundaries interpreted as basal surfaces of forced regression can be correlated with prominent erosional surfaces at the annot section grs logging is a sensitive method capable of indicating changes of genetic depositional units in sand rich turbidite systems however detailed facies analysis mineralogy and geochemistry producing the radioactive signal are necessary to correctly interpret such logs

precise u pb ages for grenvillian and pre grenvillian thrusting of proterozoic and archean metamorphic assemblages in the grenville front tectonic zone canada while the position of the grenville front is placed at a structural feature associated with the onset of high grade rocks when approached from the north the time of first metamorphism in these rocks is in general pre grenvillian and highly variable at killarney amphibolite to granulite grade rocks were metamorphosed lineated and foliated to their present front parallel configuration mainly by 1453 6 ma north of north bay amphibolite grade migmatites have been subjected to a pre 2650 ma metamorphism granulite grade gneisses south of the grenville front near val d or record a 2640 ma metamorphic event south of chibougamau mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 ma in central labrador a 1650 ma foliated granite is present south of the grenville front and at least some of the banding is this old at the atlantic coast granulite facies metamorphism in front parallel gneisses predates a 1790 1662 ma deformation in contrast each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies nw directed thrusting or crustal loading near killarney on georgian bay titanite records an episode of 977 2 ma partial resetting the degree of which increases with distance into the grenville province here grenvillian deformation where present is manifest as feldspar porphyroclastic fabric with strong se plunging lineations near north bay archean pegmatites in the grenville front zone were similarly deformed at 988 10 ma and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 2 ma a 991 4 ma cooling age for titanite in this melt is analytically identical to the zircon growth age hence rapid cooling occurred south of chibougamau new zircons grew in high pressure coronite in response to tectonic loading at 995 6 ma and in central labrador melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 2 ma titanite in the granitic host rock has an analytically identical age of 992 4 ma again indicating rapid cooling this second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 5 ma to 988 5 ma two age mixing line grenville resetting at about 1037 19 ma in the grenville front zone on the labrador coast is indicated but major thrusting is pre 1662 ma u pb isotopic results indicate that the grenville front zone is characterized along its length by partially reset u pb systems high pressure assemblages local melting and rapid cooling crustal loading and shearing with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling are required

grenville magmatism in west texas petrology and geochemistry of the red bluff granitic suite zone of regional extension as suggested by recent geophysical and the red bluff granitic suite rbgs underlies part of the franklin petrological studies mountains in west texas it was emplaced into a middle proterozoic shelf sequence at 1120 35 ma the suite is predominantly metaluminous it consists of early granitic to quartz syenitic sills a main mass of alkali feldspar granite small bodies of alkali feldspar syenite leucogranitic dikes and late stage peralkaline