an outsider s review of the astronomical theory of the climate is the eccentricity driven insolation the main driver of the ice ages abstract although the astronomical theory of the climate also known as the milankovitch theory explains a number of key features of the paleoclimate time series especially frequency and phase distribution in terms of orbital influence on the waxing and waning of ice sheets it still faces several important unresolved challenges in this review we discuss five main challenges we believe the theory must resolve in order to survive 1 the large amplitude of the 100 ky ice age cycles 2 the switch from 41 ky to 100 ky long glaciations around 0 9 ma bp 3 the absence of significant spectral power at 413 ky over the past 1 2 ma 4 the variation in glacial cycle duration in the last 800 ky and 5 the presence of non orbital spectral peaks in the climate record but in spite of these problems we conclude that the milankovitch theory may still be a viable one at least in the original sense of hays et al science 194 1976 1121 that orbital changes somehow influenced the ice ages many investigators ourselves included appear to see the impact of orbital forcing in the data clear evidence of astronomical forcing as well as evidence which suggests that the climate system responds nonlinearly to all milankovitch orbital frequencies

on the possible influence of extraterrestrial volatiles on earth s climate and the origin of the oceans abstract a consideration of observational and circumstantial evidence suggests that earth may be subject to high influx rates 1011 1012 kg yr of extraterrestrial sourced volatile elements carbon hydrogen oxygen nitrogen derived from comets or other primitive solar system material the total extraterrestrial influx rate may be four to five orders of magnitude greater than previously thought large enough to account for today s total near surface inventories of water and carbon the possibility of high rates of extraterrestrial volatile accretion suggests a new climatic paradigm wherein earth s surface temperature is influenced by conflicting internal and external processes a variable influx of volatile elements tends to warm the earth while terrestrial processes cool the planet by absorbing these gasses at a more uniform rate variations in extraterrestrial influx rates may explain the variation of sea level and mean global temperature over geologic time as well as some types of climate change the occurrence of the pleistocene ice ages and the asymmetry of the phanerozoic climate record sudden warmings slow coolings the extraterrestrial influx rate may also act as the pacemaker of terrestrial evolution at times leading to mass extinctions through climatic shifts induced by changes in accretion rates with concomitant disruptions of the carbon and nitrogen cycles life on earth may be balanced precariously between cosmic processes which deliver an intermittent stream of life sustaining volatiles from the outer solar system or beyond and biological and tectonic processes which remove these same volatiles from the atmosphere by sequestering water and carbon in the crust and mantle

extraterrestrial 3he in seafloor sediments evidence for correlated 100 kyr periodicity in the accretion rate of interplanetary dust orbital parameters and quaternary climate we have determined the helium abundance and isotopic composition of seafloor carbonate sediments from the flanks of the ontong java plateau western equatorial pacific ocean odp site 806 these results provide a two million year record of the burial flux of extraterrestrial 3he which we believe is a proxy for the terrestrial accretion rate of interplanetary dust particles the 3he burial flux prior to 700 ka was relatively low 0 5 pcc cm 2 kyr 1 but from 700 ka to the present the burial flux gradually increased to a value of 1 0 pcc cm 2 kyr 1 100 kyr periodicity in the 3he burial flux is apparent over the last 700 kyr and correlates with the oxygen isotope record of global climate with high 3he burial fluxes associated with interglacial periods this periodicity and phase are consistent with previous 3he measurements in north atlantic sediments although 100 kyr periodicity in 3he burial flux is in agreement with recent predictions of the accretion rate of interplanetary dust based on a model of the orbital evolution of asteroidal debris the measurements and predictions differ by one half cycle in phase nevertheless our observations suggest the terrestrial accretion rate of interplanetary dust is controlled by orbital eccentricity and or inclination relative to the solar system invariable plane such control is a necessary but not sufficient condition for the hypothesis of muller and macdonald 1995 that variations in extraterrestrial dust accretion modulates terrestrial climate with a 100 kyr period we also identify several brief 25 kyr intervals of strongly enhanced 3he burial possibly related to random and transient fluctuations in the accretion rate of asteroidal or cometary dust particles

petrological and geochemical constraints on the origin of adakites in the garibaldi volcanic complex southwestern british columbia canada the garibaldi volcanic complex gvc is located in southwestern british columbia and comprises two related but distinct volcanic fields the garibaldi lake and the mount garibaldi volcanic fields the rocks of the gvc range from basalt to rhyolite and analyses of samples from both fields distinguish these as adakites the gvc magmas have high sr y mg and al2o3 low k2o na2o and fractionated rare earth element compositions models of adakite genesis fall into two main groups slab melting and non slab melting adakites generated by slab melting commonly occur from young subducting crust 25 ma and are felsic partial melts of the subducting slab that interact with the mantle wedge during ascent non slab melting models vary widely and include basalt fractionation assimilation fractional crystallization processes and partial melting of mafic lower crust data from the gvc are too limited to fully elucidate the mechanisms of adakite genesis however the petrographical and geochemical characteristics of the gvc rocks in this study do not refute an origin by slab partial melts variations in trace elements that reflect non adakitic values e g low la low cr are likely the result of magma mixing at shallow depths within the magma reservoirs of each center for which there is mineralogical and textural evidence the adakite rocks of the gvc share geochemical traits akin to both low sio2 adakite lsa and high sio2 adakite hsa groups though additional data are needed to investigate whether lsa or hsa type dominates within the gvc and by extension which should be the preferred model of adakite genesis

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

fluid flow and alkali transport within a metamorphic terrane western north carolina abstract this paper presents preliminary results from study of a metasedimentary terrane near the great smoky mountains of north carolina grade of metamorphism ranges from garnet to staurolite kyanite zone the stratigraphic section is dominated by metasandstone but contains thick pelitic horizons over the central portion anakeesta formation of the studied interval metasandstone beds contain numerous isolated calcite concretions these react to form leucocratic calc silicate rock at variable grade of metamorphism metasandstones themselves are unreactive study of calc silicate reaction permits the following statements regarding fluid flow and alkali transport within the studied terrane within unreacted concretions the typical mineral assemblage is quartz oligoclase muscovite biotite calcite reaction between micas and calcite proceeded through several stages forming as prograde minerals andesine bytownite clinozoisite zoisite garnet hornblende and chlorite reaction occurred in the presence of a fluid phase containing a h2o co2 ratio of 9 and resulted in massive expulsion of na2o k2o and co2 since postulated reactions would have produced on average a fluid phase whose h2o co2 ratio is only one flow of water rich fluid through the concretions accompanied reaction rough estimate suggests a volumetric ratio of external fluid to rock of at least 1 1 the metamorphic grade at which concretions undergo reaction is controlled by stratigraphic position of the host metasandstone relative to metashale within the anakeesta formation and overlying strata extensive reaction occurs near the pelitic staurolite and kyanite isograds within massive metasandstones of the underlying strata reaction is delayed to the middle staurolite kyanite zone 5 km upgrade some reaction occurs within the upper garnet zone but this is restricted to certain thin metasandstone beds interlayered with metashale study of dehydration reactions within metashale demonstrates that calc silicate reaction was coupled to production of h2o rich fluids within pelitic strata transport of h2o rich fluids beyond outcrop scale was by advection diffusion was limited to outcrop distance on most outcrops all isolated concretions show the same degree of reaction but at larger scale separate metasandstone packets bounded by metashale show differing degrees of reaction also all rocks underwent some degree of penetrative deformation during time of reaction the suggestion is made that advective flow was in fact channelized into fractures within metasandstone and that diffusion between fractures promoted reaction within outcrop size volumes of rock overall direction of fluid transport was in part upsection and in part channelized within metasandstone strata downsection flow was limited for the most part alkalis liberated by calc silicate reaction appear to have left the terrane metasandstones do not appear to constitute a sink for alkalis and no sink of any sort was found for sodium potassium metasomatism of pelitic rock did occur and is marked by conversion of chlorite to biotite but such k deposition is limited to sections within which metashale and calc silicate rock are within outcrop distance no sink is evident for most liberated potassium it is within conjecture that the liberated elements caused alkali metasomatism elsewhere within the metamorphic belt but such an event has yet to be demonstrated

signal analysis of cyclicity in maastrichtian pelagic chalks from the danish north sea abstract low field bulk magnetic susceptibility has been determined on maastrichtian chalk samples from a drill core from the dan field in the danish north sea fast fourier transformations fft have been used to detect possible cycles in the magnetic susceptibility data power spectra from the complete section and from sub sections of the magnetic susceptibility reveal two cyclicities of ca 0 4 cycles m and ca 1 7 cycles m which are present on a 90 confidence level signal analysis of natural gamma ray wire line log data supports these findings sedimentation rate estimates place the cycles in the milankovitch frequency band artificial time series are used to study the applicability of the fft to identifying cyclicity in chalks expected geological distortions e g hiati and sedimentation rate variations are introduced into the time series to investigate the response of the frequency spectra different methods of handling missing data intervals are also examined

evolution of the murphy synclinorium southern appalachian blue ridge usa abstract the western blue ridge allochthon of the southern appalachians is dominated by the 180 km long murphy synclinorium paired with anticlinoria to the northwest these are first generation northwest overturned doubly plunging large amplitude and wavelength 10 km isoclinal folds contemporaneous with peak neo acadian orogeny visian 335 345 ma regional metamorphism the synclinorium folds a regional unconformity separating neoproterozoic rift and lower paleozoic drift sequences from a younger successor basin sequence strain analysis of metaconglomerates from lithologic groups above and below the unconformity indicates coaxial low to moderate oblate to nearly plane strain in both groups the synclinorium evolved via nnw sse crustal shortening 32 combined with orthogonal nne ssw sub horizontal flow stretching 35 45 sub parallel to the developing fold axes differences in metamorphic grade and paleodepth 10 17 km of the exposed synclinorium had essentially no effect on strain magnitudes retrodeformation of the embedded regional unconformity reveals a very broad synclinal warping of the rift and drift facies units predating superposition of the murphy synclinorium suggesting tectonic inheritance in the latter structure s origin the earlier mild deformation is post early cambrian and may represent the only vestige of the dynamic effects of the middle ordovician taconic orogeny to be found in this region

fluid inclusion constraints on the uplift history of the metamorphosed massive sulphide deposits at ducktown tennessee standard petrographic microthermometric and raman spectroscopic analyses of fluid inclusions from the metamorphosed massive sulphide deposits at ducktown tennessee indicate that fluids with a wide range of compositions in the c o h n s salt system were involved in the syn to post metamorphic history of these deposits primary fluid inclusions from peak metamorphic clinopyroxene contain low salinity h2o ch4 fluids and calcite quartz and pyrrhotite daughter crystals many of these inclusions exhibit morphologies resembling those produced in laboratory experiments in which confining pressures significantly exceed the internal pressures of the inclusions secondary inclusions in metamorphic quartz from veins pods and host matrix record a complex uplift history involving a variety of fluids in the c o h n salt system early fluids were generated by local devolatilization reactions while later fluids were derived externally isochores calculated for secondary inclusions in addition to the chronology of trapping and morphological features of primary and secondary fluid inclusions suggest an uplift path which was concave toward the temperature axis over the p t range 6 3 kbar and 550 225 c immiscible h2o ch4 n2 nacl fluids were trapped under lithostatic to hydrostatic pressure conditions at 3 0 5 kbar and 215 20 c entrapment occurred during alleghanian thrusting and the fluids may have been derived by tectonically driven expulsion of pore fluids and thermal maturation of organic material in lower plate sedimentary rocks which are thought to underlie the deposits episodic fracturing and concomitant pressure decreases in upper plate rocks which host the ore bodies would have allowed these fluids to move upward and become immiscible post alleghanian uplift appears to have been temperature convex uplift rates of 0 10 0 05 mm year 1 from middle ordovician to middle silurian late devonian and 0 07 0 12 mm year 1 from middle silurian late devonian to late permian are suggested by our uplift path and available geochronological data

sphalerite geobarometry in metamorphic terranes an appraisal with implications for metamorphic pressure in the otago schist sphalerite geobarometry has long been known to give poor results when applied to regional metamorphic terranes application of the sphalerite geobarometer to three low to medium grade sulphide deposits the moke creek and waitahuna deposits otago and the goose cove deposit newfoundland yields pressures up to 9 kbar which appear to be too high when compared with other geological data textural and mineralogical relationships suggest that the goose cove and possibly the moke creek deposits lacked the required equilibrium assemblage pyrite hexagonal pyrrhotite sphalerite during peak metamorphic conditions rendering the geobarometer inapplicable in addition all three deposits show evidence of re equilibration at t 300 c which has resulted in decreased fes contents and high apparent pressures analyses of sphalerites from very low grade metachert from south georgia island which contains the assemblage sphalerite pyrite monoclinic pyrrhotite chalcopyrite confirm that low temperature equilibration of this assemblage results in approximately 10 11 mol fes in sphalerite comparison of these results with published descriptions of other deposits suggests that lack of the appropriate assemblage and retrograde re equilibration of sphalerite probably account for most anomalously high pressure estimates erratic compositions of sphalerites containing chalcopyrite inclusions may result from replacement of high temperature intermediate solid solution by chalcopyrite during cooling strain may enhance retrograde re equilibration of sphalerite by grain size reduction or dislocation assisted diffusion and or nucleation re evaluation of the data from moke creek suggests that the sulphides experienced pervasive greenschist facies re equilibration at pressures of about 4 5 kbar with late stage mobilization at about 2 8 kbar and thus sphalerite compositions are not likely to reflect blueschist facies conditions pressure estimates based on sphalerite geobarometry should take into account at what stage in the history of a metamorphic terrane the sphalerite composition equilibrated

conditions of dalradian metamorphism in the buchan area ne scotland p t conditions in the buchan dalradian have been estimated using published equilibria garnet biotite corrected according to hodges spear 1982 and garnethornblende temperatures are in good agreement recording 430 15 c 490 15 c and 510 20 c at the cordierite andalusite and staurolite isograds and 550 40 c in the upper staurolite zone garnet clinopyroxene plagioclase quartz and garnet plagioclase epidote quartz fluid with ah 2 o 1 record a consistent minimum pressure of 1 7kbar for the andalusite isograd though the uncertainties in the equilibria indicate this could be 2 kbar higher in the staurolite zone the latter geobarometer is in agreement with pressures of 2 3 5 kbar derived from garnet plagioclase andalusite quartz a calcite dolomite temperature of 537 c from marbles at the portsoy kyanite andalusite isograd implies pressures near 4 5 kbar the p t array defined by these data lies at lower pressures and temperatures than that deduced from pelite petrogenetic grids the possibility of reduced ah 2 o is considered unlikely as the calc silicate assemblages are indicative of h 2 o rich fluids and it is suggested that the discrepancy may result from inadequate garnet activity formulations inferred overburden estimates are consistent with recent overburden estimates for the newer gabbro aureole implying that little or no erosion had occurred by the time of emplacement of the basic intrusions the overburden estimates are combined with the stratigraphic thickness data of sutton watson 1955 to estimate differential erosion uplift rates in the area

simultaneous presence of orbital inclination and eccentricity in proxy climate records from ocean drilling program site 806 ocean drilling program site 806 in the western pacific shows evidence of a remarkably constant average sedimentation rate this feature allows us to analyze ancient climate proxies without the need for orbital tuning a standard procedure in prior work but one that can lead to biased results spectral analysis of stable oxygen isotope ratios at this site a proxy for global ice volume shows a single narrow peak with a period 100 k y a result that supports our model which links glacial cycles to variations in the inclination of the earth s orbit in contrast spectral analysis of the coarse component fraction of the sediment primarily foraminifera shows a structure characteristic of standard milankovitch theory with a triplet of peaks with periods near those expected from the earth s eccentricity 95 125 and 400 k y bispectral analysis confirms these linkages but suggests that orbital inclination also plays some role in the coarse fraction from the clear presence of both signals in different proxies at the same site we conclude that although eccentricity affected the local climate it is orbital inclination that drove the variations in the global ice volume for the past million years

forward modeling of the effects of mixed volatile reaction volume diffusion and formation of submicroscopic exsolution lamellae on calcite dolomite thermometry this paper reports the results of several thousand analyses of the mg content of calcite from 31 samples from the ubehebe peak contact aureole death valley california all data reported are from metamorphic calcite formed during mixed volatile mineral reactions in which dolomite remained in the rock the mg content generally increases toward the intrusive contact and bend with increasing temperature but it varies strongly indeed probability distributions for each sample are near gaussian possess a relatively small skewness 1 72 to 3 32 and a variance that is a multiple of the estimated measuring uncertainty these findings complicate direct application of the mg content in calcite for use as an accurate thermometer the purpose of the study presented in the second part of the paper is to explore the significance of these systematic variations of mg composition of calcite to aid the interpretation of contact metamorphic temperatures recorded in carbonates we developed forward models to evaluate the effect of growth zoning volume diffusion and the formation of submicroscopic exsolution lamellae 1 m on the measured mg distribution in individual calcite crystals and compared the modeling results to the field data modeled mg distributions were transformed into histograms by taking into account intersection probabilities and random microprobe analyses modeling results reveal that the original prograde mg zoning in calcite crystal will be reset if the calcite crystal is assumed to grow slowly along a prograde path original low mg compositions can only be preserved if the entire grain forms over a small temperature interval as can be expected for infiltration driven mineral reactions it is shown that all three mechanisms combined give an adequate model for the mg content data we demonstrate that mg distributions in calcite grains of the ubehebe peak contact aureole are the consequence of rapid crystal growth in combination with diffusion and exsolution

evaluation of the volatilization and infiltration effects on the stable isotopic and mineralogical variations in the carbonate rocks adjacent to the cretaceous muamsa granite south korea the muamsa granite in the hwanggangri mineralized zone south korea intruded the cambro ordovician samtaesan formation which dominantly consists of calcitic marble and dolomitic marble with minor intercalations of pelitic rock and amphibolite intrusion of the granite caused an isotopic depletion of 13c and 18o in the carbonate rocks calcitic carbonates show systematic depletion from unrecrystallized limestone through calcitic marble to skarn it was initiated with volatilization accompanying calcsilicate forming reactions and then with increased permeability h2o rich siliceous fluids of igneous origin actively infiltrated the system promoting fluid rock interaction for the skarn fluid infiltration yielded fluid rock ratios as high as 4 2 in open system and caused increased 18o depletion rates compared to the volatilization stage more than 70 of the isotopic depletion in the calcitic marble is attributed to the infiltration process with such strong infiltration mineral assemblages such as phlogopite in the absence of k feldspar phlogopite diopside and wollastonite are commonly observed in the marble the dolomitic marble was comparatively less depleted in heavy isotopes compared to the calcitic marble and formed under a wide range of xco2 and low fluid rock ratios 1 0 active decarbonation in dolomitic marble was restricted by silica poor dolomite protolith which caused limited fluid infiltration and resulted in high xco2 mineral assemblages represented by dolomite calcite diopside forsterite and more 13c depleted nature compared to dolomite calcite tremolite forsterite assemblage

sediment focusing creates 100 ka cycles in interplanetary dust accumulation on the ontong java plateau the accumulation of extraterrestrial 3 he a tracer for interplanetary dust particles idps in sediments from the ontong java plateau ojp western equatorial pacific ocean has been shown previously to exhibit a regular cyclicity during the late pleistocene with a period of v100 ka those results have been interpreted to reflect periodic variability in the global accretion of idps that in turn has been linked to changes in the inclination of earth s orbit with respect to the invariable plane of the solar system here we show that the accumulation in ojp sediments of authigenic 230 th produced by radioactive decay of 234 u in seawater exhibits a 100 ka cyclicity similar in phase and amplitude to that evident in the 3 he record we interpret the similar patterns of 230 th and 3 he accumulation to reflect a common origin within the ocean climate system comparing spatial and temporal patterns of sediment accumulation against regional patterns of biological productivity and against the well established pattern of caco3 dissolution in the deep pacific ocean leads to the further conclusion that a common 100 ka cycle in accumulation of biogenic authigenic and extraterrestrial constituents in ojp sediments reflects the influence of climate related changes in sediment focusing rather than changes in the rate of production or supply of sedimentary constituents 7 2002 elsevier science b v all rights reserved

the late cenozoic uplift climate change paradox the geologic evidence for worldwide uplift of mountain ranges in the neogene is ambiguous estimates of paleoelevation vary according to whether they are based on the characteristics of fossil floras on the masses and grain sizes of eroded sediments or on calculations of increased thickness of the lithosphere as a result of faulting detrital erosion rates can be increased both by increased relief in the drainage basin and by a change to more seasonal rainfall patterns the geologic record provides no clear answer to the question whether uplift caused the climatic deterioration of the neogene or whether the changing climate affected the erosion system in such a way as to create an illusion of uplift we suggest that the spread of c4 plants in the late miocene may have altered both the erosion and climate systems these changes are responsible for the apparent contradictions between data supporting uplift and those supporting high elevations in the past

bracketing ages for the formation of the ring creek lava flow mount garibaldi volcanic field southwestern british columbia the ring creek lava flow extending from opal cone generally is considered to be the youngest volcanic feature in the southern section of the mount garibaldi volcanic field southwestern british columbia an in situ stump dated 10 650 70 bp beta 43865 obtained from a raised delta partially overridden by the lava flow indicates that the lava was extruded after ca 10 7 ka at the skookum creek and mamquam river confluence sediments eroded from the ring creek lava flow form an alluvial fan that contains charcoal dated 9360 160 bp beta 38914 which indicates that the extrusion probably occurred before ca 9 3 ka thus these radiocarbon ages bracket the age of the ring creek lava flow and indicate that volcanic activity in the southern section of the mount garibaldi volcanic field has been dormant since at least ca 9 3 ka

geology and petrology of quaternary volcanic rocks garibaldi lake area southwestern british columbia summary recent geochemical studies of convergent margin volcanoes have been strongly influenced by the concept that calc alkaline andesites are primary magmas formed by partial melting of wet mantle peridotite above subducted oceanic crust fountain 1979 rogers and ragland 1980 suzuki and shiraki 1980 partial melting of the peridotite is thought to occur through access of volatiles released by dehydration of the descending oceanic crust mcbirney 1969 best 1975 or of volatile rich silicate melts generated by partial fusion of the crustal portion of the subducted plate ringwood 1975 such an interpretation is critically dependent upon the assumption that the composition of melts erupted at the surface has been little modified by crystal fractionation this paper examines the petrogenesis of pleistocene holocene andesites from the garibaldi lake area of southwestern british columbia on the light of observed mineralogical and chemical variations these andesites provide evidence that crystal fractionation processes have significantly modified magma compositions evolution of british columbia9s continental margin during the past 10 m y has been dominated by a complex pattern of plate interactions riddihough 1977 keen and hyndman 1979 the present plate tectonic setting of southwestern british columbia is depicted in figure 1 the queen charlotte fault system which parallels the edge of the continental shelf from southeastern alaska to a poorly defined ridge trench fault triple junction just north of vancouver island marks the locus of right lateral transcurrent motion between the pacific and north american plates en echelon spreading axes juan de fuca ridge explorer ridge dellwood and j tuzo wilson knolls offset by short transform segments form the juan de fuca pacific and explorer pacific plate boundaries the explorer plate detached from the juan de fuca plate as the spreading ridge system rotated to become more nearly orthogonal to the queen charlotte fault has been shown to be moving independently during the past 3 m y riddihough 1977 left lateral motion between the juan de fuca and explorer plates occurs across the nootka tranform fault system which extends north easterly from the northern end of the juan de fuca ridge to nootka island off the west coast of vancouver island keen and hyndman 1979 south of the nootaka fault the boundary between the juan de fuca and north american plates is a zone of convergence seismic data show that the oceanic lithosphere dips at 5 toward the margin and reaches depths of 4 to 5 km before disappearing under vancouver island riddihough 1979 the absence of a clearly defined eastward dipping seismic zone and the lack of a major bathymetric trench at the foot of the continental slope have contributed to uncertainty as to whether subduction has taken place recently riddihough and hyndman 1979 ando and balazs 1979 and riddihough 1979 however have reviewed relevant geological geodetic and geophysical data and concluded that the juan de fuca plate is now continuously underthrusting western north america the plate motion is largely aseismic but a few benioff type events at about 50 to 60 km beneath southern georgia strait keen and hyndman 1979 provide some weak constraints on the present position of the subducted plate whether the explorer plate is now being subducted beneath the continent is less certain riddihough 1977 suggested the possibility that the oceanic lithosphere east of dellwood knolls has become locked to the north american plate

sedimentary cyclicity in the marine pliocene pleistocene of the wanganui basin new zealand sequence stratigraphic motifs characteristic of the past 2 5 m y earth s climatic history since 2 5 ma has been controlled by milankovitch variations in the planetary orbit comprising alternate periods of glaciation and interglaciation with a dominant frequency of 41 000 yr concomitantly eustatic sea level has fluctuated 70 to 130 m causing rapid transgressions and regressions of the shoreline across the world s continental shelves the resulting sedimentary record is cyclothemic each cyclothem corresponding to a single climate and sea level cycle the wanganui basin new zealand contains a 2 km thick almost complete composite record since isotope stage 100 ca 2 5 ma in the form of 47 superposed cyclothems of shelf origin each cyclothem corresponds to an unconformity bound stratigraphic sequence and typically contains a transgressive systems tract sometimes a mid cycle shell bed a highstand systems tract and sometimes a regressive systems tract no advantage accrues from using transgressive regressive units rather than cyclothems and or sequences in description of the succession six basic sequence motifs represent deposition in locations between the shoreline and offshore shelf i e the hawera birdgrove turakina seafield castlecliff and rangitikei motifs a seventh the nukumaru motif which includes dominant coquina limestone represents deposition in shallow water areas of reduced terrigenous sediment on the flank of the basin the sequence motifs represented in any section change systematically in sympathy with basin scale changes in subsidence and sediment supply in contrast with the 41000 year length of individual glacioeustatic sequences these basin wide tectonic cycles have a periodicity of many hundreds of thousands to a few million years i e that of third or fourth order sequences of the exxon type this coupled with the restriction of strongly cyclothemic sediments to geological periods of known glacio eustasy permiancarboniferous pliocene pleistocene suggests that tectonic subsidence cycles rather than glacio eustasy are the driving forces behind the development of the third and fourth order unconformity bound sequences that are reported to occur throughout the stratigraphic record

deformation and metamorphic history of the western blue ridge eastern blue ridge terrane boundary southern appalachian orogen 1 the western eastern blue ridge boundary involved formation by premetamorphic thrusting of an oceanic fragment accretionary wedge island arc upon laurentian basement and metasedimentary cover detailed outcrop scale mapping structural and petrographic analysis reveal both terranes experienced the same deformation at the same metamorphic grade early f1 and f2 isoclinal folds are overprinted by f3 open to tight folds f3 transposes s2 to an axial planar cleavage s3 the youngest penetrative feature prekinematic to synkinematic relative to s2 barrovian metamorphism attained kyanite grade in the western blue ridge wbr upper amphibolite facies in the eastern blue ridge ebr and anatexis in both terranes postkinematic m2b is characterized by appearance of fibrolite throughout the kyanite zone coarsening to granular sillimanite and identification of a kyanite sillimanite isograd in the wbr d3 strain was accommodated by dynamic recrystallization of quartz neocrystallization folding and basal slip of micas pressure solution of plagioclase folding of kyanite and fracturing of garnet scattered lower greenschist facies m4 static retrograde metamorphism resulted in replacement of kyanite and garnet by sericite and chlorite respectively although the juxtaposition of two lithologically distinct terranes implies a fault relationship polymetamorphism and deformation obliterated any microfabrics mineral assemblages or structures that might have developed during premetamorphic thrusting the boundary cannot be an extension of a continent scale dextral strike slip fault system inferred to have developed along laurentian margin in the devonian

ore mineralogy of the applachian caledonian stratabound sulfide deposits abstract the diverse ore mineralogies of more than 100 stratabound sulfide deposits that occur along the length of the appalachian caledonian orogen have been tabulated the minerals reflect the varying cu zn pb ratios that have been used as a means to separate the ore deposits into six distinct types following the igcp project 60 guidelines the ore deposits each containing at least one million metric tons of ore are typical of massive sulfide deposits throughout the world and provide a basis for contrasting and comparing this orogen with others differing grades of metamorphism have significantly modified the ore mineral grain sizes and textures but do not seem to have effectively influenced the number or variety of mineral species present

orbital frequencies in radiolarian assemblages of the central indian ocean implications on the indian summer monsoon radiolarian distribution in surface sediments and its relationships with overlying surface oceanography provided impetus for down core faunal variation related to changes in monsoon intensity due to the earth s orbital eccentricity in the geologic past in present study a high resolution t 4 75 ka radiolarian distribution for the last 485 ka in a core aas 2 3 from the central indian ocean basin revealed orbital and sub orbital cycles relationships between temporal distribution of 1 high salinity pyloniids and didymocyrtis 2 low salinity spongodiscids and spongaster 3 transitional euchitoniids and 4 southern anthocyrtidium assemblages and the orbital forcing etp i e the normalized sum of the earth s eccentricity tilt and precession and the may july insolation were analyzed spectral analyses revealed significant cycles of pyloniids 400 126 95 54 41 31 23 19 17 and 15 ka spongaster 95 41 29 23 19 17 and 15 ka euchitoniids 126 95 51 31 21 18 and 17 ka and anthocyrtidium 400 69 41 31 23 19 and 15 ka cross spectral analysis between pyloniids and etp suggested coherent pyloniid cycles lagging etp by 9 ka at 100 ka eccentricity while both were in phase 2 ka at 41 ka tilt and 23 ka precession cycles coherent spongaster cycles led etp by 38 ka at 100 ka eccentricity 15 ka at 41 ka tilt 5 ka at 23 ka precession cycles coherent anthocyrtidium cycles led etp by 14 ka at 100 ka eccentricity 10 ka at 31 ka tilt and were in phase at 41 ka tilt as well as 23 and 19 ka precession cycles similarly a radiolarian monsoon index defined as the normalized sum of pyloniids didymocyrtis spongodiscids spongaster euchitoniids and anthocyrtidium pdssea led etp at 54 and 23 ka cycles while both were nearly in phase at 41 31 and 15 ka cycles coherent pdssea cycles led insolation during may july at 8 s core site at 23 ka precession lagged 100 54 and 29 ka cycles and were in phase at 41 ka tilt and 15 ka sub orbital cycles the results suggest that radiolarian assemblages exhibit proxy monsoon cycles due to the earth s orbital eccentricity at 400 126 and 95 ka tilt at 54 41 and 31 ka next to precession at 23 19 and 17 ka as well as insolation forcings at the core site during the late quaternary

is the spectral signature of the 100 kyr glacial cycle consistent with a milankovitch origin spectral analysis techniques traditionally applied to proxy climate records have involved the use of specialized windowing functions because of the finite length of the record available these have the effect of artificially broadening the spectral peaks such that closely spaced peaks may not be fully resolved alternatively use of a simple boxcar window following muller and macdonald 1997b will maximize the spectral frequency resolution this is achieved at the expense of potential truncation artifacts manifested as spurious peaks in the spectra although in the spectral analyses presented here such additional peaks are not significant in addition simple methods like this are not able to place any confidence intervals on the results global ice volume proxy records obtained

rare earth element concentrations in quaternary volcanic rocks of southwestern british columbia a suite of hy normative hawaiites ne normative mugearite and calc alkaline andesitic rocks from the garibaldi lake area exhibits fractionated slightly concave upward ree patterns cen ybn 4 5 15 heavy ree contents about 5 10 times the chondritic abundances and no eu anomalies it is unlikely that the ree patterns provide information concerning partial melting conditions beneath southwestern british columbia because they have probably been modified substantially by upper crustal processes including crustal contamination and or crystal fractionation the ree contents of the garibaldi lake lavas are not incompatible with previous interpretations that 1 the hawaiites have undergone considerable fractionation of olivine plagioclase and clinopyroxene and 2 the individual andesitic suites were derived from separate batches of chemically distinct magma that evolved along different high level crystallization trends in general however the andesites are characterized by lower light ree contents t

the variation of the southwest monsoon from the high resolution pollen record in heqing basin yunnan province china for the last 2 78 ma abstract a high resolution pollen record has been obtained from the heqing basin of yunnan province china the 737 72 m long continuous lacustrine sequence covers the last 2 78 ma according to the results of ams 14 c and magnetostratigraphy dating seven pollen ecological groups several high percentage pollen taxa and two palynological diversity indices d total and h total are combined to reconstruct synthetically the palaeoclimatic evolution in the heqing region of yunnan province according to the palaeoclimatic changes eight major variations of the southwest monsoon occurred during the periods 2 780 2 729 ma 2 608 1 447 ma and 0 342 0 118 ma the climate was predominantly relative warm and dry indicating a rather weak southwest monsoon between 2 729 and 2 608 ma the climate was mostly humid with increased rainfall seasonality mirroring a relative strong southwest monsoon from 1 447 to 0 342 ma the southwest monsoon gradually strengthened and its intensity achieved its maximum strength within wet phases between 0 991 and 0 342 ma the southwest monsoon during the period 0 118 ma 11 83 ka bp fluctuated greatly with a pattern of weak strong weak the spectral analysis of mesic conifer pollen percentages revealed that the climate and the southwest monsoon in the heqing region have been dominated by the 100 ka eccentricity period and also influenced by the other major orbital and sub orbital frequencies including 400 ka eccentricity obliquity 41 and 29 ka periods 23 ka precession and 10 ka

glacial cycles and astronomical forcing narrow spectral features in ocean sediment records offer strong evidence that the cycles of glaciation were driven by astronomical forces two million years ago the cycles match the 41 000 year period of earth s obliquity this supports the croll milankovitch theory which attributes the cycles to variations in insolation but for the past million years the spectrum is dominated by a single 100 000 year feature and is a poor match to the predictions of insolation models the spectrum can be accounted for by a theory that derives the cycles of glaciation from variations in the inclination of earth s orbital plane

morphology of the explorer juan de fuca slab edge in northern cascadia imaging plate capture at a ridge trench transform triple junction the explorer plate is a young oceanic microplate that accommodates relative motion between the pacific juan de fuca and north america plates near northern vancouver island canada the northern limit of explorer plate juan de fuca subduction and the fate of the slab in northern cascadia are poorly understood we use passive teleseismic recordings from an array of polaris broadband seismic stations to image crustal and upper mantle structure beneath northern vancouver island into the interior of british columbia a clear signature of subducted material extends northeast from the brooks peninsula at crustal levels beneath georgia strait and the mainland deep into the mantle to 300 km depth complexity in slab morphology results from juan de fuca ridge subduction and toroidal flow around the slab edge in agreement with geophysical and geological data we propose a tectonic model for the explorer plate in which its separation from the juan de fuca plate is caused by the thermomechanical erosion of the slab edge and slab thinning at shallow levels both of which slow convergence with north america and lead eventually to plate capture

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

eruptive history and k ar geochronology of the late cenozoic garibaldi volcanic belt southwestern british columbia the cenozoic garibaldi belt comprises 6 volcanic fields spaced at irregular intervals along an axis extending 240 km north northwest from the head of howe sound to the bridge river area 2 additional fields the franklin glacier and silvert lie 140 and 190 km west of the north end of the main volcanic belt the volcanoes erupted lavas ranging in composition from augite olivine basalt through hypersthene andesite hornblende andesite and hornblende biotite andesite to biotite rhyodacite many of the volcanic complexes are characterized by geomorphic features which indicate complex interactions between volcanism and the pleistocene ice sheets but preglacial and postglacial phases are also present whole rock samples from 18 volcanic complexes have been dated by the k ar method most of the results are internally consistent with stratigraphic relationships and with limited 14 c and paleomaguetic data they suggest that volcanic activity was episodic most of the analyzed andesitic and dacitic lavas were erupted in the intervals 2 3 to 1 7 ma and 1 1 ma to present in the northern part and in the intervals 1 4 to 1 0 ma and 0 7 ma to present in the southern part of the belt basaltic volcanism occurred only during the past 0 15 m y except in the salal glacier area where hawaiite and alkali olivine basalt which are perhaps an edge effect related to subduction of the juan de fuca plate were erupted as early as 0 97 ma the timing of garibaldi belt volcanism provides information bearing on the distribution of pre wisconsin glaciers in southwestern british columbia and constrains interpretations of late cenozoic changes in explorer juan de fuca north american plate configuration along the continental margin

tectonic controls of metamorphic field gradients abstract we investigate the effects of horizontal variations in tectonic factors heat production and rate amount and duration of erosion on the magnitudes and gradients of pressure p temperature t and cooling ages observed in mountain belts horizontal gradients of p t and cooling ages are largely controlled by two dimensionless parameters 1 the horizontal length scale over which erosion erosion rate or erosion duration varies and 2 the peclet number pe which describes the relative rates of advection and diffusion of heat in contrast the magnitudes of observed p and t depend strongly on the degree of thickening and original burial depth as well as pe and since horizontal heat conduction can usually be ignored do not depend strongly on p and t gradients generally increase with decreasing and pe for calculations in which either but not both erosion rate or erosion duration is horizontally constant p and t gradients are moderate in magnitude and increase in the direction of increasing total erosion in contrast calculations with horizontally constant total erosion but varying erosion rate and erosion duration show small temperature and pressure gradients pressures and temperatures can increase in the same or in opposite directions depending on the magnitude of calculations with horizontally varying crustal heat production show small but non zero temperature gradients and negligible pressure gradients these results indicate that knowledge of temperature pressure and cooling age gradients in individual mountain belts can constrain fundamental quantities such as magnitude and horizontal gradient of the exhumation rate and the average shear strain rate occurring during exhumation on the basis of their metamorphic field gradients most mountain belts can be divided into one of three groups the first and largest group shows strongly positive p and t gradients precambrian orogenic areas comprise the second group and have small t gradients

noble gases in ocean waters and sediments noble gases are widely used in studies of the basic properties and dynamics of natural systems including the ocean this chapter describes some of the more extensive applications of noble gases mainly helium isotopes to studies of oceanographic problems they include the modern oceanic circulation paleo oceanography hydrothermal and cold brine systems in the deep ocean and ocean atmosphere gas exchange originally oceanic noble gas studies were focused on geochemical problems such as the question of the existence of excess helium in seawater derived from radioactive decay of the elements of the uranium and thorium series in ocean sediments suess and wanke 1965 bieri et al 1966 after the discovery of mantle primordial helium during the 1960s clarke et al 1969 craig and weiss 1971 craig et al 1975 and 3he derived from decay of tritium in the near surface waters of the ocean jenkins and clarke 1976 large programs were developed to exploit helium isotopes frequently in combination with tritium for studies of water mass formation circulation and variability e g jenkins 1987 schlosser et al 1991 more recent studies started to explore the use of helium isotopes to paleoceanographic objectives these studies build upon the discovery of extraterrestrial helium in deep sea sediments in the early 1960s merrihue 1964 this signal is now being systematically examined for its potential as a tool for investigations of sediment accumulation rates on very long time scales millions of years and their correlation with changes in paleoceanography e g farley 1995 farley and patterson 1995 marcantonio et al 1995 1996 2001 soon after the discovery of mantle helium in seawater noble gases were applied to studies of hydrothermal systems originally these studies were focused on the source of the fluids emanating from hydrothermal vents as well as

seismic attribute based characterization of coalbed methane reservoirs an example from the fruitland formation san juan basin new mexico the fruitland formation of the san juan basin is the largest producer of coalbed methane in the world production patterns vary from one well to another throughout the basin reflecting factors such as coal thickness and fracture and cleat density in this study we integrated conventional p wave three dimensional 3 d seismic and well data to investigate geological controls on production from a thick continuous coal seam in the lower part of the fruitland formation our objective was to show the potential of using 3 d seismic data to predict coal thickness as well as the distribution and orientation of subtle structures that may be associated with enhanced permeability zones to do this we first derived a seismic attribute based model that predicts coal thickness we then used curvature attributes derived from seismic horizons to detect subtle structural features that might be associated with zones of enhanced permeability production data show that the best producing wells are associated with seismically definable structural features and thick coal although other factors e g completion practices and coal type affect coalbed methane production our results suggest that conventional 3 d seismic data integrated with wire line logs and production data are useful for characterizing coalbed methane reservoirs

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

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

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

lith5 0 a revised crustal model for canada based on lithoprobe results summary during the last 15 yr numerous seismic refraction experiments have been conducted overthe canadian landmass most as part of the lithoprobe initiative the new results providethe basis for revising existing global models of crustal structure e g crust5 1 mooney et al 1998 which do not incorporate the new lithoprobe data the crustal model discussedhere lith5 0 combines the lithoprobe and crust5 1 data to provide better constraints oncrustal structure in canada the objectives of the study are to 1 compile available new datainto a single consistent database of crustal thickness and velocity including the uppermostmantle pn velocity and 2 to construct a revised 5 5 crustal model of the canadianlandmass in a format that is generally compatible with existing global models the lithoproberesults suggest a 5 5 km average root mean square rms crustal thickness difference betweenlith5 0andcrust5 1 evenwiththeincorporationofthenewdata thegeographiccoveragein north and central canada remains sparse the new model does not explicitly incorporategeological constraints but instead relies on spherical spline interpolation for extrapolatingvalues into regions that lack control to illustrate the implications of this new crustal model we use lith5 0 to correct for the isostatically compensated crustal heterogeneity in order toobtain improved estimates of the dynamic surface topography the latter provides importantconstraints on the density structure of the uppermost mantle