modeling the effects of dust radiative forcing on the movement of hurricane helene 2006 the influence of direct dust radiative forcing on the movement and track of hurricane helene 2006 is examined numerically using the weather research and forecasting dust model numerical simulations show that the model generated dust plume modifies the thermal field causing a clockwise turning of the vertical shear surrounding the plume which changes the deep layer steering flow the change in the steering flow modifies helene s moving speed and direction as it transits the plume as helene exits the plume it has a different trajectory than it would have had in the absence of dust radiative forcing consequently the difference in the tracks with and without dust radiative forcing continues to grow with distance from the plume the dust induced changes in temperature and wind together cause helene s modeled storm track to be in closer agreement with the observed track the dust radiative forcing reduces the error in the model s 7 day track forecasts by an average of 27 205 km

u th he thermochronology of the ottawa embayment eastern canada the temperature time history of an ancient intracratonic rift basin abstractthe ottawa embayment is an intracratonic rift basin in eastern north america that preserves a unique and eventful history through deep time we report single grain zircon and apatite u th he and biotite 40ar 39ar data from samples collected along a 250 km transect across the crystalline rift flank and through the sedimentary sequences of the embayment the data sets show significant intrasample age dispersions and we investigate the influence of zircon crystallinity and nonideal crystal shapes on he diffusion through the application of raman spectroscopy and x ray microcomputed tomography ultimately we utilize the relationship between radiation damage and helium diffusion to quantify observed age dispersion we integrate our age data from the basement and overlying sedimentary unit of the embayment with regional geology to utilize multisample inverse modeling to reveal viable thermal histories that we then further evaluate by using the forward modeling method to improve our understanding of t

pre himalayan tectono magmatic imprints in the darjeeling sikkim himalaya dsh constrained by 40 ar 39 ar dating of muscovite abstract the lower lesser himalayan sequence l lhs in darjeeling sikkim himalaya dsh displays intensely deformed low grade meta sedimentary rocks frequently intervened by granite intrusives of varied scales the principal motivation of our present study is to constrain the timing of this granitic event using 40ar 39ar geochronology we dated muscovite from pegmatites emplaced along the earliest fabric in the low grade daling phyllite and obtained 1850 ma ar ar muscovite cooling age which is broadly coeval with crystallization ages of lingtse granite protolith e g 1800 1850 ma u pb zircon ages reported from the l lhs we present here field observations to show the imprints tectonic fabrics of multiple ductile deformation episodes in the lhs terrain the earliest penetrative fabric axial planar to n s trending reclined folds suggest a regional tectonic event in the dsh prior to the active phase of indo asia collision based on the age of granitic bodies and their structural correlation with the earliest fabric we propose that the l lhs as a distinct convergent tectono magmatic belt delineating the northern margin of indian craton in the framework of the 1850 ma columbia supercontinent assembly

comparison of methods to determine q in shallow marine sediments from vertical reflection seismograms four methods for the determination of q in marine sediments are compared two traditional methods i e the risetime and the spectral ratio method and two newly established ones the spectrum modeling and the wavelet modeling method in the latter one q and the reflection time t are determined simultaneously which gives a much better accuracy for t than reading it from the seismogram the risetime and the spectral ratio methods are used for obtaining q directly from the data the principle of the modeling methods is to calculate the effect of absorption and dispersion on a reference wavelet or its spectrum for various values of q and the best fit between the observed and the calculated data leads to the optimum result numerical tests on synthetic data show that a precision of more than 25 for data containing noise or superposed arrivals can hardly be achieved in any case wavelet modeling is the superior method application to data from a vertical reflection profile in the baltic sea yields q in the range of 15 100 for different layers which is to be expected in the sedimentary environment of this area

excess argon in amphiboles from fluid interaction and short intrusion interval at the epizonal marangudzi complex zimbabwe the epizonal marangudzi complex of the karoo associated nuanetsi igneous province in southern zimbabwe was emplaced into precambrian gneisses and consists of a gabbro mass intruded in turn by ring dikes of quartz and nepheline syenites amphibole and biotite from various lithologies have been examined using 40ar 39ar techniques in order to investigate the temporal distribution of intrusions and the possible occurrence of excess ar with one important exception samples give internally concordant incremental release spectra average 40ar 39ar dates range from 177 6 to 181 3 ma three amphiboles give dates which are significantly older than the other samples and contain small yet discernible quantities of excess 40ar excess ar is restricted to amphiboles displaying substantial fe mg zoning which is interpreted as reflecting late magmatic or early sub solidus reaction with fluid phases argon was apparently acquired from the fluid during alteration and ultimately could have been magmatic or could have come from degassing of the country rocks the remaining samples both amphibole and biotite show only a restricted range 1 m y in apparent age and are essentially analytically indistinguishable implying rapid cooling of the complex the average for these analyses 178 1 ma reflects the mean age of emplacement of cogenetic intrusions within a short interval 1 m y in duration this revised age of the marangudzi complex fits well within the temporal framework of karoo province magmatism in the region

geochronology of the porgera gold deposit papua new guinea resolving the effects of excess argon on k ar and age estimates for magmatism and mineralization abstract mesothermal epithermal gold mineralization at porgera in the highlands of papua new guinea png occurs in structurally controlled veins and disseminations which overprint and cross cut a suite of shallow level comagmatic mafic alkaline stocks and dykes the porgera intrusive complex pic and their sedimentary host rocks conventional k ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 ma but four biotite separates are concordant at 6 02 0 29 ma 2 40 ar 39 ar step heating experiments on six of the hornblende separates reveal saddleshaped age spectra which indicate the presence of excess 40ar one of these samples yields a well defined plateau with an apparent age of 5 96 0 25 ma 2 this age is identical to that of the biotites and suggests an age of 6 0 0 3 ma 2 for emplacement of the pic older apparent ages obtained from conventional k ar and 40 ar 39 ar analyses are explained by excess 40ar contamination conventional k ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5 1 and 6 1 ma and indicate that ore deposition occurred within 1 ma of magmatism at porgera evidence for the evolution of a magmatic volatile phase and the presence of excess 40ar both in the intrusives and in hydrothermal fluids associated with the orebody suggest that magmatic fluids may have had some involvement in metallogenesis but the exact nature of this involvement is not yet clear late miocene magmatism and mineralization at porgera are thought to have occurred shortly prior to or during the initiation of continent arc collision and to pre date associated pliocene uplift and foreland deformation in the highlands

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

erosional and depositional history of the atlantic passive margin as recorded in detrital zircon fission track ages and lithic detritus in atlantic coastal plain sediments comparison of fission track ft ages of detrital zircons recovered from atlantic coastal plain sediments to ft ages of zircons from bedrock in source terranes in the appalachians provides a key to understanding the provenance of the sediments and in turn the erosional and depositional history of the atlantic passive margin in appalachian source terranes the oldest zircon fission track zft ages from bedrock in the western appalachians defined for this paper as the appalachian plateau valley and ridge and far western blue ridge are notably older than the oldest ages from bedrock in the eastern appalachians piedmont and main part of the blue ridge the age difference is seen both in zft sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks in the east zft data indicate that the rocks cooled from temperatures high enough to partially or totally reset zft ages during the paleozoic and or mesozoic the majority of the rocks are interpreted to have cooled through the zft closure temperature 235 c at various times during the late paleozoic alleghanian orogeny in contrast most of the rocks sampled in the western appalachians have never been heated to temperatures high enough to totally reset their zft ages reflecting their contrasting thermotectonic histories nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old ft ages in excess of 800 ma zircon grains yielding ft ages this old have not been found in rocks in the piedmont and main part of the blue ridge the zft data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western appalachians was in evidence by no later than the early cretaceous and probably by the late triassic detrital zircon suites from sands collected in the atlantic coastal plain provide a record of detritus eroded from source terranes in the appalachians during the mesozoic and cenozoic in virginia and maryland sands of early cretaceous through late early oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western appalachians very old zircons yielding ft ages 800 ma are only encountered in coastal plain sands of middle early miocene and younger age miocene and younger fluvial deltaic deposits associated with the major mid atlantic coastal plain rivers that now head in the western appalachians the hudson delaware susquehanna potomac james and roanoke contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from paleozoic rocks of the western appalachians these distinctive clasts have not been reported in older coastal plain sediments the zft and lithic detritus data indicate that the drainage divide for one or more east flowing mid atlantic rivers migrated west into the western appalachians and the river s began transporting western appalachian detritus to the atlantic coastal plain sometime between the late early oligocene and middle early miocene by no later than late middle miocene most if not all of the major rivers that now head west of the blue ridge were transporting western appalachian detritus to the coastal plain prior to the drainage divide migrating into the western appalachians the zft data are consistent with the dominant source of atlantic coastal plain sediments being detritus from the piedmont and main part of the blue ridge with possible input from distant volcanic sources the zft data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle atlantic margin offshore basins that peaked in the middle miocene and produced almost 30 percent of the total volume of post rift siliciclastic sediments in the offshore basins began in the early miocene when atlantic river s gained access to the relatively easily eroded paleozoic sedimentary rocks of the western appalachians

amplitude phase and frequency versus offset applications1 in a layered earth the seismic reflection response for incidence at non normal angles is dependent upon the elastic constants and thicknesses of the layers the possibility is investigated of increasing the diagnostic value of seismic data by using the phase and the frequency v offset information in addition to that from the amplitude v offset the combined amplitude phase and frequency versus offset apf vo analysis is carried out through the computation of amplitude phase and frequency indicators based on the analytical description of seismic traces both synthetic and actual data are examined from the analyses of synthetic seismograms it is shown that modifying the velocities and thicknesses of a given target layer by introducing different pore fluids or lithological conditions produces changes in apf vo plots in particular the effects related to interference among reflections and to critical angle phenomena are clearly detected by both the phase and amplitude v offset indicators in terms of phase shifts and amplitude variations the frequency indicator is mainly controlled by the spectrum of the propagating wavelet since the basic synthetic model is derived from an existing well located close to a seismic line some actual cdp gathers are analysed features related to interference and critical angle are again evident in the apf vo plots of the actual data the amplitude indicator appears to be reasonably stable while the phase shows a higher spatial variability and a stronger sensitivity to noise differential interference with offset often occurs in actual layered structures and distorts seismic data significantly therefore avo interpretation and avo inversion procedures must also tackle this problem knowledge of phase and frequency variations v offset may help classical avo interpretation and yield further information for use in inversion techniques

petrogenesis of the magmatic complex at mount ascutney vermont usa the ascutney mountain complex of eastern vermont usa is a composite epizonal pluton of genetically related gabbro to granite intrusives nd isotopic data are reported for mafic rocks granites and nearby country rock the parental mafic magma producing the complex 122 m y ago had 87sr 86sr 0 7039 143nd 144nd 0 512678 nd 3 8 and 18o 6 1 indicating a mantle source with time integrated lithophile element depletion uniform initial radiogenic isotope ratios for granites which are undistinguishable from those for the most primitive gabbro suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in 18o to about 7 8 is the result of fractional crystallization mafic rocks show a large range in initial 143nd 144nd ratio from about 0 51267 to 0 51236 nd 3 7 to 2 5 which is correlated with elevated 87sr 86sr ratios and 18o these data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50 crust the local country rocks include gneiss and schist and assimilation involved representatives of both rock types the isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement

magma sources for mesozoic anorogenic granites of the white mountain magma series new england usa the magma sources for granitic intrusions related to the mesozoic white mountain magma series in northern new england usa are addressed relying principally upon nd isotopes many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts sm nd and rb sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 ma the possibility of crustal sources is further examined with analyses of representative older crust including paleozoic granitoids which serve as probes of the lower crust in the region multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations 1 in end overall mesozoic granite plutons range in end t from 4 2 to 2 3 with most 2 to 0 and in initial 87sr 86sr from 0 7031 to 0 709 the isotopic variations are roughly inversely correlated but are not obviously related to geologic geographic or age differences older igneous and metamorphic crust of the region has much lower nd isotope ratios with the most radiogenic paleozoic granitoid at end 180 ma of 2 8 these data suggest mid proterozoic separation of the crust in central northern new england moreover the bulk of the mesozoic granites cannot be explained as crustal melts but must have large mantle components the ranges of nd and sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation the results indicate production of anorogenic granites from mantle derived mafic magmas

comment on geochronology of calcite filled joints southeast canada insight into late cretaceous deformation of eastern north america jennifer spalding david a schneider and julie brown

teleseismic studies of the lithosphere below the abitibi grenville lithoprobe transect in the past decade the abitibi grenville lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern superior and adjoin ing grenville provinces among the different geophysical methods that have been employed earthquake seismology provides the widest range of information on the deep structures of the upper mantle this paper presents a review of studies both complete and ongoing involving teleseismic datasets that were collected in 1994 and 1996 along the transect a complete shear wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies results suggest a correlation between the two anisotropies supported by xenolith data and favour a lithospheric origin for the seismic anisotropy the two anisotropies are believed to represent the fossilized remnants of archean strain fields in the lithospheric roots of the canadian shield preliminary splitting results for the 1996 experiment suggest that the s wave azimuthal anisotropy may be depth dependent and laterally varying ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area preliminary receiver function results reveal the presence of an s velocity increase at 90 100 km depth which appears to be laterally continuous over 200 km traveltime inversion models indicate the presence of an elongate low velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface e g grenville front preliminary interpretation relates this anomaly to the same process e g fixed mantle plume continental rifting responsible for the emplacement of the monteregian hills igneous province

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

tracking low temperature tectonism of the st lawrence platform and humber zone southern quebec appalachians through apatite and zircon u th he thermochronology the st lawrence platform slp and humber zone hz of the southern quebec appalachians have been the subject of extensive studies to resolve the degree of thermal maturation yet the timing of the thermal maximum is not well understood we have employed apatite ahe and zircon zhe u th he thermochronometry across a network of late cambrian to late ordovician siliciclastic and grenvillian basement outcrops to resolve the thermal history below 200 c single crystal dates from individual samples exhibit age dispersion by as much as 300 million years with a strong positive to negative correlation with increasing effective uranium eu concentration the data in the southwestern portion of the basin allow for thermal maxima of up to 200 c during the late ordovician or early devonian regional burial trends combined with local estimates of paleogeothermal gradients indicate that if sedimentation continued after the late ordovician there was no significant increase in burial in southwestern portions

hydrochronology of a proposed deep geological repository for low and intermediate level nuclear waste in southern ontario from u pb dating of secondary minerals response to silurian and cretaceous events u pb ages have been measured on secondary dolomite and silica cements in cambrian sandstone at the base of an 840 m thick sub horizontally bedded sedimentary sequence beneath the bruce nuclear site

recalibration of the geomagnetic polarity timescale the quest for improved time control to estimate rates of geologic and paleoenvironmental processes is central to the earth sciences until the middle of this century geochronology was based solely on macrofossil evolution providing the largely unmodified definition of geologic stages in the last few decades with impetus from the deep sea drilling project dsdp and the ocean drilling program odp microfossil biostratigraphy has become increasingly important although new radiometric techniques have resulted in improved absolute age precision the direct correlation of absolute ages to biozonations and hence to geologic stage boundaries remains poor this is due to the difficulty of finding materials suitable for radiometric dating which can be directly correlated to biozonations the bridge between biozonations and absolute ages and the interpolation between absolute ages are best accomplished particularly for cenozoic and late mesozoic time through the geomagnetic polarity timescale gpts the template for late jurassic to quaternary geomagnetic polarity is derived from oceanic magnetic anomalies as demonstrated by heirtzler et al 1968 the gpts can be constructed using oceanic magnetic anomaly record s assuming constant seafloor spreading rate s and interpolating between available radiometric ages in the last quadrennium there have been outstanding developments in radiometric dating and astrochronology which have greatly improved the absolute age control of the gpts

low temperature thermal history and landscape development of the eastern adirondack mountains new york constraints from apatite fission track thermochronology and apatite u th he dating the adirondack mountains in northern new york state form an elongate domal exposure of mainly high grade metamor phic tectonites in a mountainous setting with topographic relief of 1 km the origin of the adirondack mountains and this relief has long been enigmatic since the adirondacks presently lie within an intracratonic setting inboard of the north american passive mar gin and far from any active plate boundaries through the application of apatite fi ssion track aft thermochronology and apatite u th he ahe dating within the eastern adirondack mountains this study provides constraints on both the thermal and ero sional effects of mesozoic passage near a hot spot and the timing of relief development aft thermochronology and ahe dating record relatively stable thermal conditions within the eastern adirondacks from the middle jurassic into the early cretaceous during the early cretaceous ca 130 120 ma the region underwent heating as sociated with progressive movement near the great meteor hotspot resulting in the temporary establishment of an elevated geo thermal gradient following regional heat ing cooling rates increased considerably ca 105 95 ma likely due to both thermal doming producing an increase in erosion rate and the relaxation of isotherms after passage near the hotspot the regional distribution of aft ages across the eastern adirondacks reveals no systematic age gradient from core to periph ery which would be expected under con ditions of persistent high relief during the decay of a crustal root over many tens to hun dreds of millions of years instead thermo chronological data suggest that the present relief developed during the late cretaceous cenozoic through plateau dissection during periodic base level changes

evidence for differential unroofing in the adirondack mountains new york state determined by apatite fission track thermochronology abstract apatite fission track ages of 168 83 ma for 39 samples of proterozoic crystalline rocks three samples of cambrian potsdam sandstone and one cretaceous lamprophyre dike from the adirondack mountains in new york state indicate that unroofing in this region occurred from late jurassic through early cretaceous samples from the high peaks section of the adirondack massif yielded the oldest apatite fission track ages 168 135 ma indicating that it was exhumed first unroofing along the northern northwestern and southwestern margins of the adirondacks began slightly later as shown by younger apatite fission track ages 146 114 ma determined for these rocks this delay in exhumation may have resulted from burial of the peripheral regions by sediment shed from the high peaks apatite fission track ages for samples from the southeastern adirondacks are distinctly younger 112 83 ma than those determined for the rest of the adirondack region these younger apatite fission track ages are from a sec

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

age and origin of the cannon point syenite essex county new york southernmost expression of monteregian hills magmatism two syenite sills intrude the local paleozoic strata of eastern new york state and are exposed along the western shore of lake champlain the sills are fine grained alkali feldspar syenites and quartz syenites with phenocrysts of sanidine and albite the two sills are compositionally distinct with crossing rare earth element profiles and different incompatible element ratios which eliminates the possibility of a simple petrogenetic relationship zircon extracted from the upper sill yields a u pb age of 131 1 1 7 ma making the sills the youngest known igneous rocks in new york state this age is similar to that of the earliest intrusions in the monteregian hills of quebec 100 km to the north sr and nd radiogenic isotope ratios are also similar to those observed in some of the syenitic rocks of the eastern monteregian hills the cannon point syenites have compositions typical of a type within plate granitoids they exhibit unusually high ta and nb concentrations resulting in distinct trace eleme

mesozoic hotspot epeirogeny in eastern north america geologic data especially conodont paleotemperatures imply that southeastern canada and new england have been elevated at least 4 km relative to the central appalachian region apatite fission track ages suggest that the uplift and consequent erosion occurred in cretaceous early tertiary time consistent with the formation of a broad swell as north america moved northwestward over the great meteor hotspot

tropical cyclone movement and surrounding flow relationships abstract this paper presents results of a comprehensive study of the relationship between the movement of tropical cyclones and the large scale circulation which surrounds them cyclones have been stratified by direction and speed of movement latitude intensity change and size as determined by the radius of the outermost closed surface isobar in three ocean basins the northwest pacific the west atlantic and the australian south pacific region twenty one different stratifications are available in the northwest pacific 13 in the west atlantic and 6 in the australian south pacific area cyclone movement and surrounding flow relationships were studied at different pressure levels and a variety of radii pressure weighted layer averages were also analyzed in search of such relationships results show an important relationship between surrounding large scale flow and tropical cyclone movement for all stratifications the winds in the mid troposphere 500 700 mb at 5 7 latitude radius from the cyclone

the cretaceous peri atlantic alkaline pulse paap deep mantle plume origin or shallow lithospheric break up abstract although the early jurassic initial phases of atlantic rifting were marked by the tholeiitic basalt flows and dikes of the central atlantic magmatic province camp the cretaceous era was characterised by widespread alkaline igneous activity on widely separated peri atlantic continental margins nearly half the peri atlantic alkaline rocks dated between 250 to 50 ma fall within the 125 80 ma range this period of intense alkaline activity during the mid cretaceous formed the peri atlantic alkaline pulse paap the mechanisms for the opening of the atlantic ocean as well as for the camp and later paap magmatic events have been attributed either to deep mantle plumes or to a combination of tensional forces lithospheric rifting and structural controls we describe the paap event for the first time and test the plume and structural models the paap displays evidence of strong structural control and appears to be related to reactivation of pre existing lithospheric structures throughout the peri atlantic zone early stages of continental separation and abrupt changes in plate motion were the most likely mechanisms for the reactivation of favourably oriented pre existing structures mid cretaceous pulses in alkaline magmatic activity were closely associated with major atlantic tectonic events the opening of the south atlantic the change in intensity and direction of both seafloor spreading and plate motion and the consequent fluctuation in intra plate stresses were responsible for two spikes in peri atlantic alkaline magmatism at 125 ma and 85 80 ma the general lack of evidence for a deep hotspot associated with the peri atlantic zone seems incompatible with a mantle plume origin on the other hand the spatial distribution of the paap along passive continental margins suggests an ideal setting for the development of edge driven convection we propose that the periodic reactivation of deep seated pre existing zones of weakness during the major stages of atlantic tectonic evolution combined with coeval asthenospheric upwelling due to edge driven convection and continental insulation flow enhanced the ascent of alkaline magmas this mechanism of shallow small scale upwelling during periodic structural reactivation provides a more likely explanation for the paap than the commonly proposed deep mantle plume scenario

north america during the lower cretaceous new palaeomagnetic constraints from intrusions in new england summary six late aptian and albian intrusions in maine new hampshire and vermont dated between 113 and 100 ma were sampled for palaeomagnetic analysis the majority of samples unblock between 550 c and 580 c and curie temperatures fall in a discrete interval between 556 c and 580 c this indicates that the dominant remanence carrier is a trm and is carried by titanomagnetite which crystallized from the magma or was produced by deuteric oxidation alteration during post magmatic cooling titanomagnetite occurs as discrete cl c3 grains as exsolution lamellae in the silicates and as fine reaction rims on the silicates the palaeomagnetic results are combined with additional data from four barremian and early aptian intrusions in southern maine to produce nine reliable virtual geomagnetic poles these possess a higher latitude than most published poles for the lower cretaceous ar4o ar3 ages on biotite and hornblende from these intrusions demonstrate three intrusive pulses at approximately 120 ma 111 ma and 100 ma constraining the ages of the calculated vgps the corresponding palaeopoles are 72 3 on 198 2 e a 3 3 74 5 on 195 2 e a 3 8 and 76 6 on 167 5 e a 5 3 based on new palaeomagnetic data reported here significant changes to the north american mesozoic apw path are necessary these changes constrain the palaeoposition of the north american plate in the lower cretaceous

cooling pattern and mineralization history of the saint sylvestre and western marche leucogranite pluton french massif central i 40ar 39ar isotopic constraints abstract the saint sylvestre western marche leucogranite complex lc is a composite pluton emplaced 324 m y ago in the variscan continental crust of the northwestern limousin french massif central petrographic structural and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for ar retention in muscovite part i and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40 50 m y part ii a suite of thirty seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40 ar 39 ar technique muscovite is characterized by well defined plateau ages ranging between 314 and 301 ma while biotite gives less well behaved age spectra with apparent ages varying between 320 and 300 ma the muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton field petrographic gravimetric and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional post cooling fault network cutting through the laccolith when restored to their approximate pre faulting geometry the age variations are shown to correlate positively with sample elevation with a top to base difference as large as 10 14 m y over a mean intrusion thickness of 3 km this pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened variscan crust during the westphalian and is confirmed by two dimensional thermal modelling part ii the data show that 40 ar 39 ar mica ages can postdate the time of crystallization of the host pluton by up to 23 ma depending on the level sampled at the surface they warn against interpreting k ar mica ages from similar low relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure cooling mechanism and post cooling faulting of the pluton in turn provided that the pre faulting geometry of the intrusion is properly assessed the vertical cooling pattern can reveal fine scale variations in the thermal structure of old eroded intrusions that may be extremely useful for recovering quantitative information on the mechanisms and rates of cooling in ancient orogens

diagnosing forecast errors in tropical cyclone motion abstractthis paper reports on the development of a diagnostic approach that can be used to examine the sources of numerical model forecast error that contribute to degraded tropical cyclone tc motion forecasts tropical cyclone motion forecasts depend upon skillful prediction of the environment wind field and by extension the synoptic scale weather systems nearby the tc while previous research suggests that the deep layer mean dlm steering flow typically approximates the actual tc motion it is shown that the motion of even mature tcs can depart from the dlm steering flow an optimal environmental steering flow is defined which varies the vertical extent of the steering layer and the radius over which tc vorticity and divergence are removed errors in predicted tc motion are quantified using a diagnostic equation that accounts for not only differences in the synoptic scale flow but also differences in the depth and radius used to define the steering flow differences in the latter two parameters a

impact of processing on the amplitude versus offset response of a marine seismic data set the analysis of prestack reflections may have a high diagnostic potential in the evaluation of the petrophysical characteristics of subsurface targets however the recovery of reliable seismic responses especially when examining amplitude versus offset avo variations is of the utmost importance and is strictly dependent on the acquisition and processing steps in order to evaluate the impact of different processing sequences the authors examine the avo responses of three seismic events from a marine data set borehole data indicate that these events are related to a lignitic sand a gas sand and a cineritic bed the avo analysis is focused mainly on the reflections from the gas sand in particular they compare the results of a standard processing sequence with results from a surface consistent approach and with results from a processing sequence tailored to this specific case a decreasing avo trend of the gas sand reflection results from the analyses of data that have undergone the standard and the surface consistent processing sequences a detailed study shows that the receiver array attenuation neglected in previous studies plays a major role in attenuating the far offset reflections taking into account the above factors they apply a third processing sequence whose impact on the avomore trend is evaluated step by step and whose results are compared with the previously applied sequences this new sequence leads to better agreement between the avos predicted from borehole data and those measured on surface seismic data less

high resolution 40 ar 39 ar chronology of multiple intrusion igneous complexes the mount brome complex of the monteregian province of southern quebec canada consits of several major intrusions ranging compositionally from gabbro to syenite the relative ages of these intrusives have been investigated with high resolution 40ar 39ar analyses including a specially designed irradiation configuration to cancel the effects of fluence gradients small yet distinct apparent age differences are observed while a number of analytical and geological factors could be proposed to explain the small variations evaluation of these suggests the age differences reflect those in emplacement times the gabbro and nepheline diorite were emplaced within a short span 123 1 ma ago generally more evolved lithologies biotite monzodiorite pulaskite nordmarkite appear to have been emplaced within a restricted interval 1 4 0 3 ma later whole rock rb sr systematics do not give acceptable isochrons because of significant scatter interpreted to reflect initial 87sr 86sr heterogeneities resulting from crustal contamination considering the variations in initial ratio the rb sr data are consistent with the 40ar 39ar age

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

monteregian hotspot track a long lived mantle plume the monteregian hills the younger white mountains and the new england seamounts constitute a well defined hotspot track the evolution of the plume through time is obtained from the marine track the buoyancy flux of the plume when it made the new england seamounts was about 2 mg s 1 which would make it the seventh or eighth most active plume today the hotspot declined in activity after it made nashville seamount the buoyancy flux when it made corner seamounts and today is less than 0 5 mg s 1 the changes in the flux of the marine hotspot limit how far the marine flux can be extrapolated to the west onto the continental track and what can be learned in general about vulnerability of the continental lithosphere to hotspots the lack of an obvious track west of montreal may be due either to failure of the plume to penetrate the canadian shield to the lack of recognizable intrusions or to strengthening of the plume when it approached montreal

diagenesis and anchimetamorphism in an overthrust belt external domain of the taconian orogen southern canadian applachians ii paleogeothermal gradients derived from maturation of different types of organic matter abstract 3hermal maturation patterns have been established for the external domain of taconian orogen of the southern quebec appalachians between quebec city and the u s border reflectance of three types of organic matter om measured in the same sample shows that graptolite reflectance r g is higher than pyrobitumen reflectance r b which in turn is higher than reflectance of amorphous organic matter r a r g r b and r a are converted to vitrinite equivalent reflectance from which maximum paleotemperatures were calculated inverted diagenesis is preserved in northwest part of the chaudiere nappe to the southeast original burial diagenetic grade was upgraded by later synorogenic heating in the st lawrence lowlands paleogeothermal gradients at maximum burial were 33 37 c km a gradual southwestward increase in maturation level suggests increasing post taconian burial depth from 4 6 to 6 7 km indicating an increasing thickness of eroded rocks from northeast to southwest along logan s line one of the most important contributions of this study is the proof that the acadian orogeny has affected the successor basin of the taconian orogeny in the st lawrence lowlands the thermal effect of the mid cretaceous intrusion of the monteregian hills on their country rocks is limited to one diameter or less of the intrusive body as shown by the narrow

paleomagnetism of 122 ma plutons in new england and the mid cretaceous paleomagnetic field in north america true polar wander or large scale differential mantle motion a paleomagnetic study of cretaceous white mountains plutonic complexes in new hampshire and vermont yields high unblocking temperature dual polarity magnetizations in different types of igneous rocks the resulting pole position for three plutons 71 9 n 187 4 e a95 6 9 age 122 5 ma agrees with previously published mid cretaceous poles for north america which together give a mid cretaceous standstill reference pole slightly revised from globerman and irving 1988 at 71 2 n 194 1 e a95 3 7 n 5 studies we argue on the basis of the wide geographic distribution of these studies the variety in tectonic settings and rock types positive reversal tests and an overall reversal pattern consistent with geomagnetic polarity time scales that this mean pole represents the north american mid cretaceous reference field for nominally 36 m y 124 to 88 ma the standstill pole limits to within 4 the motion of the north american plate relative to the earth s spin axis during the same mid cretaceous interval the new england hotspot track 124 ma monteregian hills 122 5 ma cretaceous white mountains and 103 to 84 ma new england seamounts requires 11 4 of north poleward motion of north america in direct conflict with the paleomagnetic standstill a similar 13 discrepancy is independently demonstrated between the spin axis and the tristan da cunha hotspot track on the african plate during the mid cretaceous interval the hotspot spin axis discrepancies ended by 90 ma when it is shown that both atlantic hotspots agree with north american and african dipole paleolatitudes and present day locations nondipole fields are an unlikely explanation of the uniform motion of these two widely separated hotspots with respect to the spin axis leaving as possible interpretations true polar wander and large scale but differential mantle motion the southerly motion of the mid cretaceous louisville hotspot relative to the spin axis is ostensively at odds with what would be predicted under the true polar wander interpretation and points to differential mantle kinematics the motions of the three widely separated mid cretaceous hotspots with respect to the spin axis may be related to the recently proposed increase in global oceanic lithosphere production rates which gave rise to the mid cretaceous superplume

mesozoic east north american alkaline magmatism part 1 evolution of monteregian lamprophyres quebec canada abstract the cretaceous monteregian alkaline igneous province is composed of shallowly emplaced plutonic complexes and cogenetic dykes and sills the mafic dykes are principally camptonites monchiquites and alnoites and sample the magmas that fed the plutons the three principal dyke suites have different incompatible trace element ratios y ce zr nb and are not interrelated by closed system fractional crystallization zonation of pyroxene grains imply a polybaric crystallization history indicating that magmas ascended as mafic crystal charged mushes intrasuite chemical variations among monchiquites are broadly consistent with segregation of the suspended phenocrysts from interstitial syenitic melts partial resorption of suspended amphibole megacrysts assimilation of syenitic material 5 8 and exsolution of carbonatitic melt are required to explain the variations of ba rb k 2 o th la and tio 2 chemical variations among mafic camptonites are acceptably modelled by fractionation of pyroxene and olivine only implying that their amphibole rich character is the result of postfractionation crystallization under volatile rich conditions high zr nb camptonites and rb rich ac monchiquites could not unequivocably be linked to the main lineages and may represent distinct sources or have been affected by crustal contaminants

paleostress analysis of atlantic crustal extension in the quebec appalachians abstract a paleostress analysis using normal fault slip data and jurassic dike trends has been conducted in the quebec and new england appalachians stress tensor analysis of fault slip by a numerical inversion method has distinguished homogeneous and directionally consistent e w and nw se oriented extensional stress regimes in the quebec appalachians and st lawrence lowlands the directions of extension compare well with the mean direction of extensional stress inferred from the jurassic dolerite dike trend in new england both extensions are interpreted as the result of rift related stress associated with the separation of north america and africa early in the late triassic early jurassic fragmentation of pangea the paleostress field configuration is assumed to be related to the direction of divergent plate motion the n s and ne sw late proterozoic faults that bordered the quebec reentrant and taconian acadian structures of the quebec appalachians appear to have played a significant role in the dis

age geochemical and isotopic variations in volcanic rocks from the coastal range of taiwan implications for magma generation in the northern luzon arc abstract this paper reports the first systematic analysis of age and geochemical variations in volcanic rocks from the coastal range of taiwan the northern luzon arc the rocks recovered from four main volcanoes vary from low k tholeiitic to medium k calc alkaline basalts to dacites the rocks are typical of arc magmatic products exhibiting enrichment in the large ion lithophile elements and depletion in the high field strength elements our new 40ar 39ar age data constrain the youngest eruption time in each of the four volcanoes i e from north to south at 7 2 ma yuemei 4 2 ma chimei 6 2 ma chengkuang ao and 8 5 ma tuluanshan respectively these data indicate that volcanism in the northern luzon arc did not cease progressively from north to south as previously alleged the high and broadly uniform nd isotope ratios end 10 1 to 8 8 and trace element characteristics of the rocks suggest a principal magma source from the depleted mantle wedge their overall geochemical variations are ascribed to magma chamber processes the effects of magmatic differentiation and crustal contamination differ among each volcano most likely owing to the discrepancy of residence time in individual magma chambers consequently we propose a binary mixing model for the magma generation that involves arc magmas sourced from the depleted mantle wedge and up to 5 crustal contamination with a continental fragment split off from the eurasian margin

enclaves from the a type granite of the megantic complex white mountain magma series clues to granite magmagenesis the subsolvus biotite hornblende granite at megantic contains mafic to leucocratic enclaves the groundmass of leucocratic enclaves is microgranitic that of mafic enclaves is meladioritic apatite is commonly acicular in the enclaves suggesting quenching of mafic magmas commingled with the granitic host mafic enclaves contain mafic clots hornblende cores biotite rims some with relic cores of vial cr rich diopsidic augite similar to phenocrysts in associated basaltic dykes the clots are interpreted to be pseudomorphs after mafic phenocrysts the enclaves also contain granitic microxenoliths xenocrysts of quartz with hornblende reaction rims partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts the xenocrysts record hybridization between a mafic enclave magma and a granitic magma and or rock enclaves define mixing lines on many variation diagrams however rb sr ba zr nb y ree zn pb and f do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles the data suggest that the granite is anatectic heat being provided by crystallization of associated basalts mixing calculations suggest that as little as 5 hybridization with basalt or hawaiite can account for the sr isotopic heterogeneity of the granites

quantitative seismic pre stack analysis of potential gas hydrate resources in the makran accretionary prism offshore iran abstract gas hydrates are classified as a major unconventional resource and assumed to be a future potential energy source high pressures low temperature stability conditions of the hydrates restrict their occurrence to the permafrost and deep sea regions from the continental slop to the abyssal in these regions where the well information is most often absent or sparse the amplitude variation with angle ava is a common method of hydrate free gas assessment in this method various linear reflection coefficient equations are fitted to a series of data points to compute some pre stack attributes these attributes have the advantage of being an interface property rather than a layer property which makes them an easy to use characterization tool in lack of well data in recent years rock physics templates nomograms have been presented to estimate the hydrate free gas saturations solely based on these interface properties ava inversion approach was applied to the seismic data in the makran accretionary prism offshore iran to estimate the bottom simulating reflector bsr vicinity hydrate and gas saturations this inversion process included four pre stack attributes of the intercept gradient normal incidence ni and poisson reflectivity pr and was limited to the bsr estimated bsr vicinity saturations were ranged from 7 8 5 for hydrate and 5 6 5 for free gas at three representative locations accuracy of each attribute was analyzed by a synthetic modeling in this modeling the effective medium theory emt was considered to calculate the physical properties of hydrate and gas bearing sediments lithological properties and the acquisition geometry were set to the corresponding values of the field data results of the synthetic ava inversion showed that if the hydrate saturation does not exceed 30 the error of the estimation for pre stack attributes would be in acceptable range the expected estimation errors for actual attributes were derived from this synthetic modeling

saharan dust and the african easterly jet african easterly wave system structure location and energetics the radiative effects of saharan mineral dust smd aerosols on the structure location and energetics of the african easterly jet african easterly wave aej aew system are examined for july september 2006 experiments are conducted with and without smd using the weather research and forecasting wrf model which is radiatively coupled to an interactive dust model the smd modified heating field shifts the aej northward upward and westward and enhances its zonal asymmetry these smd induced changes to the aej are manifest in the aews the northern and southern tracks of the aews shift northward like the aej and the zonal scale of the aews expands and their westward propagation increases the smd also strengthens the energetics of the aej aew system the domain and temporally averaged baroclinic energy conversion which is an order of magnitude larger than the barotropic conversion increases by a factor of 2 5 the eddy kinetic energy and generation of available potential energy increase by factors of 1 5 and 2 7 respectively the implications of an smd modified aej aew system for west african precipitation and tropical cyclogenesis in the eastern atlantic ocean are discussed

a short interval of jurassic continental flood basalt volcanism in antarctica as demonstrated by 40ar39ar geochronology a continental flood basalt province the ferrar group kirkpatrick basalt and ferrar dolerite crops out along 3000 km of the transantarctic mountains in antarctica and is temporally related to the break up of gondwanaland although a wide range of dates between 90 and 193 ma have been published for the kirkpatrick basalt it is now recognized that the young dates reflect non ideal behavior of ar in the matrix in order to refine the geochronology feldspar separates have been analyzed by the 40ar39ar incremental heating method the main objectives are to constrain the duration of extrusive activity and the timing of volcanism along the outcrop belt basalt samples have been studied from the three principal outcrop areas yielding the following apparent ages central transantarctic mountains 176 8 0 5 ma south victoria land 176 4 0 4 ma north victoria land 176 6 0 7 ma ages from different stratigraphic levels within each area and from the three different areas are not analytically distinct the data imply that the eruptive activity which produced the kirkpatrick basalt occurred within a short interval of less than about 1 m y at 176 6 1 8 ma over an area which included more than 1200 km of the transantarctic mountains the jurassic volcanism in antarctica represents a short episode of magmatism comparable in duration with other well dated continental flood basalt provinces the linearly extensive outcrop of the ferrar province and the rapid eruption of the lavas suggests that lithospheric stretching exerted a major control on magmatism the poorly constrained age of the bajocian bathonian boundary makes the previously suggested connection between ferrar volcanism and an extinction event at that boundary uncertain

range equation weights for avo duren 1991 presented a detailed development of the seismic range equation showing how to calculate a wavelet s voltage amplitude spectrum and phase spectrum and how to use the resulting wavelets in forming model seismic gathers these same wavelets can then be used to process model and seismic gathers via prestack deconvolution to reveal a target s amplitude behavior with offset

petrogenesis of cambro ordovician subduction related granitic magmas of the quebec appalachians canada abstract in the quebec appalachians cambro ordovician subduction related granitic rocks are found as intrusive bodies aci in the ascot complex volcanic arc and as eroded fragments mgc in the adjacent magog group sedimentary basin major and trace element geochemistry indicates that aci and mgc rocks are typical peraluminous granitoids both suites are lree enriched with minor negative eu anomalies aci granitoids displaying higher mree and hree abundances than mgc ree and trace element data indicate that aci and mgc granitoids are chemically similar but differences in their respective fractionation trends and trace element ratios demonstrate a significant petrological decoupling 143 nd 144 nd are higher in the mgc granitoids than in the aci both suites display negative nd ratios 10 to 3 implying limited juvenile material input sm nd isotopes from both suites suggest that they originated from the anatectic partial melting of a compositionally zoned grenville like continental crust

the distribution of igneous rocks beneath mont megantic the easternmost monteregian as revealed by gravity at the surface mont megantic the easternmost and most imposing of the monteregian hills is composed of 77 5 leucogranite and syenite and 22 5 gabbro nonetheless the intrusion is characterized by a major positive bouguer anomaly of 20 mgal a computed three dimensional gravity adumbration is consonant with the intrusion having the form of a vertical prismatic column with a flat base at a depth of 10 km the base appears to coincide with the unconformity atop precambrian basement rocks gabbro composes 96 of the adumbration leucogranite and syenite make up the remainder and are confined to an uppermost layer no more than about 1 km thick the subvolcanic texture of the granite and presence of miarolitic cavities at megantic show the shallow level of the intrusion the present surface perhaps having been no deeper than 1 or 2 km this is the same as the depth of emplacement elucidated at mont royal at the opposite end of the monteregian chain which in turn points to uniform uplift and erosion sin

brittle fault evolution of the montreal area st lawrence lowlands canada rift related structural inheritance and tectonism approached by palaeostress analysis the montreal area belongs to the st lawrence lowlands a cambrian early ordovician passive margin of the iapetus ocean later covered by appalachian middle to upper ordovician foreland deposits a structural and palaeostress analysis has been carried out in order to reconstruct its tectonic evolution the structural map has been revised with new data palaeostresses are reconstructed based on inversion of fault slip data and these results are independently corroborated by the microstructural study of calcite mechanical twinning field relationships are used to establish the relative chronology of fractures and to deduce the motion on regional faults the reconstructed structural and tectonic evolution brings to light some relationships between structural inheritance and tectonic events that have affected the area since early palaeozoic times an early nw se extension is responsible for n040 trending faults along the northern border of the st lawrence lowlands and for n090 and n120 trending faults cross cutting the montreal area this extension is followed by wnw ese and nnw compressions which have induced reverse motion on pre existing faults and generated strike slip conjugate faults subsequent ne sw and nnw sse directed extensions have reactivated previous faults with normal to strike slip motions a late ne sw compression is recorded in the monteregian plutons compressions in wnw ese and nnw directions are consistent with appalachian collisional tectonism but n040 and n090 trending faults cross cut appalachian folds and foreland deposits although the early nw se extension is consistent with the collapse of the iapetan margin in early palaeozoic times most of the present geometry of the st lawrence lowlands could be attributed to mesozoic tectonism recorded as nearly n s directed extensional events

tectonic model for kimberlite emplacement in the appalachian plateau of pennsylvania the two known kimberlite intrusions in the appalachian plateau of pennsylvania are spatially distributed along the extension of the rome trough at cross structural lineament intersections it is inferred that the down to the east basement faults associated with the rome trough in pennsylvania were reactivated by tensional forces during the reopening of the atlantic ocean during the jurassic cross structural lineaments defined by gravity magnetic structural and landsat data appear to be expressions of basement fractures areas of crustal weakness where these fractures intersect fault zones caused by rifting are likely sites of intrusions this model of kimberlite intrusion can also be used to explain the location of the known new york kimberlites

the accuracy of estimating q from seismic data a major aim of seismic interpretation is the inference of petrophysical properties of reservoir rocks because the inversion from seismic to petrophysical characteristics is far from unique this task requires a range of seismic parameters prominent among which are seismic velocity impedance and poisson s ratio the inclusion of seismic absorption in this list could add desirable complementary information for example absorption may be more sensitive to clay content than seismic velocity klimento and mccann 1990 however seismic absorption is difficult to measure particularly over depth intervals as short as most reservoir intervals

a barremian aptian early cretaceous north american paleomagnetic reference pole four intrusive complexes from southern maine containing rocks ranging in composition from olivine gabbro to granodiorite were sampled to determine an early cretaceous reference pole for the north american plate sampling included the lebanon diorite the tatnic complex alfred complex and the cape neddick complex dated radiometrically by the 40ar 39ar or k ar method on biotite between 125 and 120 ma the majority of samples have unblocking temperatures between 550 c and 580 c and curie temperatures between 550 c and 570 c indicating that the dominant remanence carrier is titanomagnetite which crystallized from the magma or was produced by deuteric oxidation alteration during post magmatic cooling titanomagnetite occurs as discrete c1 c3 grains as exsolution or oxidation exsolution lamellae in the silicates as a product of magmatic reactions forming magnetite ilmenite symplectites with pyroxene or olivine or as ultrafine dust hemo ilmenite is present as a minor component in the lebanon diorite and in this case the remanence direction carried by the hemo ilmenite is indistinguishable from the titanomagnetite detailed petrographic observations and rockmagnetic experiments demonstrate that despite the coarse average grain size of the oxides in these rocks which might lead one to expect unstable magnetic remanence the magnetic remanence in most samples is extremely stable and caused by single domain or pseudo single domain behavior in magnetite containing fine oxidation exsolution lamellae of ilmenite the tatnic and alfred intrusions are of normal polarity d 333 i 62 pole 70 5 n 210 3 e a95 3 7 n 24 sites the cape neddick and lebanon intrusions are of reversed polarity d 161 i 57 pole 73 7 n 185 7 e a95 5 0 n 17 sites together these intrusions yield a mean paleomagnetic pole at 72 2 n 198 9 e a95 3 3 n 41 sites the well characterized reversed polarity recorded in the precisely dated cape neddick complex may represent the best age for the magnetic polarity chron m0r the last reversal before the onset of the cretaceous normal superchron at the barremian aptian stage boundary

alleghanian paleostress reconstruction in the northern appalachians intraplate deformation between laurentia and gondwana a numerical paleostress tensor analysis using striated fault planes has been conducted in the quebec reentrant of the northern appalachians to characterize the stress field of late paleozoic deformations three directions of maximum compressional stress axes s1 have been found and correlated to 1 an early north northwest south southeast compression 2 a north northeast south southwest compression and 3 a late west northwest east southeast compression fault populations associated with these stress regimes are present in all tectonic zones of the quebec and northern new brunswick appalachians directions of 1 axes determined in the northern appalachians resemble in orientation and in relative chronology layer parallel shortening fabrics and joint patterns found in the appalachian foreland of the central appalachians the paleostress regimes are interpreted as the record of intraplate deformation related to the indentation of gondwana into laurentia during the late paleozoic alleghanian orogeny

a plume generated segment of the rifted margin of laurentia southern canadian appalachians seen through a completed wilson cycle the southern part of the canadian appalachians and the adjacent parts of the craton show a range of distinctive features pertaining to the evolution of the rift and rift drift transition stages of a segment of the continental margin of laurentia despite the effects of a completed wilson cycle several lines of evidence point to an initial lithospheric rupture in the form of a three pronged rift possibly induced by a rising mantle plume the earliest synrift igneous products to penetrate the crust in the area are represented by a swarm of tholeiitic diabase dykes intruded at ca 590 ma the swarm is more than 700 km long and radiates from the inferred location of the rrr triple junction dyke emplacement and the initiation of rifting were probably synchronous approximately 35 m y later a rapid outburst of volcanism occurred at the triple junction probably as a harbinger of sea floor spreading the volcanic products are predominantly basalts of mildly alkaline to transitional character but with a minor comenditic component the voluminous nature of the volcanic outburst and the compositional characteristics of its products indicate higher than normal asthenospheric temperatures coupled with thinning of the lithosphere suggesting continued mantle plume activity the volcanic sequence is overlain by rift facies elastics that are locally interrupted and augmented by a deltaic deposit representing the delta of a river that drained the failed arm into the nascent lapetus ocean the rift facies elastics and the associated deltaic deposits are overlain by rocks of marine origin signalling the beginning of the drifting stage possibly at ca 550 ma

the iugs systematics of igneous rocks in order to create a sustainable classification of igneous rocks which all geologists might use an international body was set up by the iugs the iugs subcommission on the systematics of igneous rocks in the course of creating the classification the subcommission has established ten principles for its construction and for defining an appropriate nomenclature the principles are 1 use descriptive attributes 2 use actual properties 3 ensure suitability for all geologists 4 use current terminology 5 define boundaries of rock species 6 keep it simple to apply 7 follow natural relations 8 use modal mineralogy 9 if mode not feasible use chemistry 10 follow terminology of other iugs advisory bodies these principles and their rationale have not previously been enunciated the classification separates and individually classifies the pyroclastic carbonatitic melititic lamprophyric and charnockitic rocks before entering the main qapf classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz q alkali feldspar a and plagioclase p or of alkali feldspar a plagioclase p and feldspathoids f rocks with mafic content 90 have their own classification if the mineral mode cannot be determined as is often the case for volcanic rocks then a chemical classification of total alkalis versus silica tas is used the nomenclature for these classifications necessitates only 297 rock names out of the c 1500 that exist

paleomagnetic calibration of milankovitch cyclicity in lower cretaceous sediments abstract bedded pelagic limestones of early to mid cretaceous age may record cyclic climate variability driven by changes in the earth s orbit milankovitch cycles if properly calibrated the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy the lower cretaceous maiolica formation of northern and central italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper m series marine anomalies the magnetic reversals divide measured sections into 0 5 2 myr increments in which sedimentation rate assuming continous deposition can be measured to 15 25 accuracy paleomagnetically estimated sedimentation rates over a number of polarity zones establish with one exception a narrow range in periods of the carbonate cycles carbonate couplets have an estimated mean period of 23 5 kyr and prominent modulations in bedding thickness bundling have a mean period of 117 kyr kent and gradstein 1985 time scale the close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing comparison of measured sections from different locations shows that bed to bed correlations are possible regionally cosistent with the milankovitch hypothesis episodes of barremian black shale deposition occur in the troughs of the short eccentricity cycle as roughly one half of the carbonate marl couplet or about 10 kyr duration it is possible to estimate both the timing relative to the top of reversed chron m 0 and duration of a period of unusual organic carbon rich deposition selli horizon in the early part of the cretaceous long normal magnetic polarity interval by tracing the sedimentary cycles up from the barremian strata

the interpretation of 40ar 39ar apparent age spectra produced by mixing application of the method of asymptotes and limits abstract a method is presented for the analysis and interpretation of apparent age spectra produced during 40 ar 39 ar step heating experiments application of this method is particularly relevant to complex apparent age spectra produced from minerals taken from exhumed metamorphic tectonites particularly when these rocks once resided in an ancient argon partial retention zone such rocks may have been subject to localised deformation and or recrystallisation during orogenesis yet microstructures relict of earlier episodes of deformation and or metamorphism may retain relatively old apparent ages as a result complex apparent age spectra can be produced during a conventional step heating experiment and these age spectra often cannot be simply interpreted the recognition and the interpretation of a plateau in such spectra is sometimes uncertain and in any case the definition of a plateau is then a concept of limited theoretical validity to avoid these difficulties an alternative strategy is presented based on a theory of mixing gas from different microstructural microchemical reservoirs this method relies on the definition of asymptotes and limits in sequences of apparent ages in apparent age spectra obtained from step heating experiments frequently measured ages fmas in individual datasets can then be recognized using statistical analysis the significance of fmas must be independently assessed