one hundred million years of geomagnetic polarity history since 1968 absolute ages have been assigned to the late cretaceous cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic anomaly profile and interpolating between or extrapolating beyond these points assuming constant spreading rates in each interval previously no more than 4 calibration points were used but it is now possible to specify 11 calibration points in addition to the 0 m y datum this improvement is based on magnetostratigraphic studies in italian pelagic limestones these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations absolute ages of calibration points are provided by the best available dates on stage boundaries which are located from the biostratigraphic zonation the greatest changes from previous scales come in the late paleocene early eocene where the new ages are as much as 3 m y younger than in the 1977 scale of labrecque and others and as much as 1 8 m y younger than in the 1980 scale of ness and others

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

the fold test in palaeomagnetism summary the fold test suggested by mcelhinny is shown to be inappropriate more correctly one should determine whether the mean direction of a group of sites from one limb of a fold may be distinguished statistically from the mean direction of a group of sites from another limb details of the application of this test are given with examples it is shown that the test indicates significance under far less severe folding than does the test suggested by mcelhinny

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

paleogene evolution of the kodiak islands alaska consequences of ridge trench interaction in a more southerly latitude the kodiak islands of alaska are principally composed of a late cretaceous and paleogene complex of accreted deep sea rocks an early paleocene magmatic event invaded the accretionary complex with mid ocean ridge basalt morb andesite and granodiorite geochemical evidence suggests both the andesite and granodiorite are mixtures of morb and sediment the volcanic rocks are interlayered with coarse clastic sediments and deformed with the latter the granodioritic intrusions with k ar ages as old as 62 ma postdate emplacement of the early paleocene sedimentary and volcanic rocks interaction of the subduction complex with the kula farallon ridge most simply explains this early paleocene magmatic episode paleomagnetic studies of the volcanic rocks indicate this interaction occurred at 40 3 6 north latitude which is 25 3 9 south of the expected position of the kodiak islands relative to north america the magmatic arc presently northwest of the kodiak accretionary complex was active during the late cretaceous and early paleocene diminished after the ridge trench interaction and resurged from late eocene into miocene time the magmatic resurgence is coeval with the obductive offscraping of eocene to oligocene submarine fan deposits in the kodiak islands the petrology of these fan deposits indicates they are the most likely proximal equivalent for the zodiac fan thereby limiting its relative motion with respect to the kodiak islands possible models accounting for the northward motion of the kodiak accretionary complex include 1 coupling the accretionary complex and possibly related arc to the northward moving kula plate and closing an ocean basin north of the arc in paleogene time 2 emplacing the kodiak accretionary complex at the northward limit of the paleomagnetic data with subsequent northward motion accomplished by closure of a small back arc basin and or intracontinental shortening and 3 transcurrent faulting and northward motion of the kodiak accretionary complex without the presently adjacent arc

hydrogen and oxygen isotope geochemistry of ascension island lavas and granites variation with crystal fractionation and interaction with sea water lavas and pyroclastics on ascension island contain plutonic blocks that include fluid inclusion bearing peralkaline granite 18o 16o ratios f and cl have been analysed on whole rocks and or minerals for lavas and granites and d h ratios and h2o for comenditic obsidians and granites whole rock 18o 16o ratios of fresh alkali basalt hawaiite trachyandesite trachyte and comendite range from 6 0 to 6 9 with 18o tending to increase with increase in sio2 the 18o values of the granites are from 0 0 to 0 3 depleted in 18o relative to the comendites comenditic obsidians have d 80 4 and h2o 0 3 wt while amphiboles from the granites have d 56 2 the o isotope trend of the lavas is consistent with a crystal fractionation model fresh igneous rocks with 18o values greater than 7 involve processes in addition to crystal fractionation of a basaltic magma the d h ratios and cl contents 3 000 ppm of the h2o poor comenditic obsidians represent undegassed primary magmatic values the h isotope compositions and low h2o and cl 167 ppm contents of the granites are consistent with the major degassing loss of 90 of initial h2o of an h2osaturated magma derived from the interaction of sea or possibly meteoric water with the h2o undersaturated comenditic melt it is proposed that associated with caldera subsidence and stoping water was sucked in around the residual magma before the system had time to be sealed up the h2o undersaturated magma consumed this h2o with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks at a pressure of about 1 5 kb the granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites granites from oceanic islands may in general be a result of generating an h2o saturated acid melt by such direct or indirect crustal water magma interaction processes

late palaeozoic palaeomagnetic data for wrangellia resolution of the polarity ambiguity wrangellia and the alaska peninsula terrane are two of the many allocthonous terranes making up southern alaska today palaeomagnetic data from these areas clearly indicate low palaeolatitudes in the early mesozoic because of local and regional rotations the mean declination of the palaeomagnetic vectors is uncertain this in addition to incomplete temporal coverage has led to an ambiguity in the polarity of the vector and hence an ambiguity in whether the palaeolatitudes determined are in the northern or southern hemisphere the data considered here have resolved the ambiguity for wrangellia and by implication for the alaska pennisula terrane and also allow the construction of a highly speculative apparent polar wander curve

crustal origin for the parana rhyolites discussion of description and petrogenesis of the parana rhyolites southern brazil by garland et al 1995 our criticisms of the garland et al 1995 paper are introduction three fold and concern 1 the interpretation of oxygen the south atlantic ocean divides what was once a single isotope data on the rhyolites 2 radiogenic isotope large igneous province into the larger parana flood basalt fingerprinting of potential source material 3 the major province of south america and the smaller etendeka element composition of melts produced from potential flood basalt province of namibia the term flood basalt sources in the lower crust province is in this case something of a misnomer because a substantial thickness of silicic volcanic rocks is preserved on both sides of the atlantic the parana rhyolites can be divided into two main types the chapeco high ti oxygen isotopes type which have a high concentration of high field one of the problems of using oxygen isotopes to solve strength elements and low sr isotope ratios sr sr at problems of magma genesis is that oxygen isotope ratios 130 ma 0 705 0 708 and the palmas low ti type are changed during low temperature alteration processes which has lower concentrations of high field strength it is well known that large shifts in do value can occur elements and elevated sr isotope ratios sr sr at 130 in glassy volcanic rocks such as the parana rhyolites e g ma 0 714 0 727 mantovani et al 1985 bellieni et al taylor 1968 cerling et al 1985 these problems can 1986 peate et al 1992 garland et al 1995 suggested be circumvented by analysing fresh phenocrysts such as that the former were produced by partial melting of pyroxene which are unaffected by the alteration process high ti parana basalts whereas the more voluminous harris et al 1990 used the pyroxene do values to palmas rhyolites were produced by prolonged asestimate the magma do values of the palmas low ti similation accompanying crystal fractionation of low ti type rhyolite of the parana to be between 10 1 and parana basalt magmas previous workers have suggested 10 7 with the equivalent low ti magma type in the that the palmas type rhyolites are derived from melting etendeka having do values between 9 7 and 11 2 of pre mesozoic middle to lower crust piccirillo et al these high values led harris et al 1990 to suggest an 1988 harris et al 1990 and we believe that garland et origin by crustal melting al 1995 have overlooked several important lines of garland et al 1995 acknowledged the high do evidence in rejecting the crustal melting hypothesis in values and went on to state that the high do values this comment we will draw on data from the etendeka were used by harris et al 1990 to invoke an entirely province of namibia which was once part of the same crustal origin but it has been argued elsewhere that post eruptive alteration can lead to an increase in do continental flood basalt province

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

the cenozoic denali fault system and the cretaceous accretionary development of southern alaska the juxtaposition of disparate geologic terranes in southern alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the denali fault system in cenozoic time recent geologic investigations in the healy quadrangle strongly suggest that cenozoic horizontal displacements of such magnitude along the denali fault system do not exist in the healy quadrangle isograds and metamorphic facies boundaries of an early late cretaceous metamorphic belt trend across the cenozoic mckinley strand of the denali system without significant horizontal offsets the present geologic makeup of most of southern alaska is primarily the result of the talkeetna superterrane consisting of the previously assembled peninsular terrane and wrangellia colliding with and subsequently being thrust upon the yukon tanana and nixon fork terranes of the ancient north american continent in about middle cretaceous time the leading edge of the talkeetna superterrane faces a wide complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with jurassic and cretaceous flysch the flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks the postcollisional denali fault system developed in cenozoic time across the already accreted continental margin in eastern alaska along an older cretaceous suture

the cretaceous igneous province of madagascar geochemistry and petrogenesis of lavas and dykes from the central western sector 1999 similarly the buchan 1997 crops out in the area between thecontributions of hotspot usually taken to be plume mailaka limestones and the contact between the permo mantle and non hotspot asthenosphere to ood basalt triassic sedimentary rocks of the morondava basin andmagmas appear to vary greatly from province to province the precambrian basement fig 1b the dykes reach e g kent

early cretaceous paleolatitude of the yukon koyukuk province alaska we report results from a paleomagnetic study of the yukon koyukuk province a key region for reconstructing the cretaceous paleogeography of northern alaska and northeastern asia the province lies between the displaced continental fragment of arctic alaska and the accreted terranes of southern alaska although lower cretaceous volcanogenic rocks of the yukon koyukuk province preserve primary remanent magnetizations the primary component is masked by a pervasive overprint the over printing magnetization which has dual polarity was acquired after a late cretaceous orogeny left the region highly deformed by fitting planes to tilt corrected thermal demagnetization data and solving for the best intersection of the corresponding remagnetization circles we separated the primary component presumably a vestige of the original thermoremanent magnetization from the overprinting component of the natural remanence the primary magnetization corresponds to a mean paleopole of early cretaceous age at 60 2 n 159 e the mean paleolatitude is 69 n with the 95 confidence ellipse ranging from 60 to 75 n this analysis was successful because the tilt corrections of the sampled strata had sufficient variation to ensure a strong intersection of the remagnetization circles we infer that lower cretaceous rocks of the brooks range and the yukon koyukuk province were once separated by a wide ocean basin that was closed by subduction probably during the cretaceous the northern limb of the angayucham terrane which lies between the brooks range and the yukon koyukuk province may contain remnants of the now closed ocean basin

chemical stratigraphy of the parana lavas south america classification of magma types and their spatial distribution a new classification scheme has been developed to assign the lava flows of the parana continental flood basalt province south america into geochemically distinct magma types with six basaltic major and trace element abundances and or ratios by mapping out the spatial distribution of these magma types within the lava sequences it has been possible to determine the internal stratigraphy of the lava pile on a regional scale previous studies on road profiles traversing the well exposed coastal serra geral escarpment of southern brazil are summarised together with results from some new sampled sections more widespread stratigraphical investigations of the parana lavas have been hampered by the lack of sufficient topographic relief and the cover of sedimentary rocks however access to drill core chippings from nine boreholes in the central parana region has provided a unique opportunity to investigate the stratigraphy of the otherwise inaccessible deeper levels of the lava pile and to map out stratigraphic variations in three dimensions the borehole samples have indicated cated a stacking of units of different magma types all overlapping towards the north which suggests that the main locus of magmatism moved northwards with time within the parana basin this migration could be related to the northward propagation of rifting during the initiation of the south atlantic ocean maps of the surface distribution of samples of each magma type show a pattern consistent with the stratigraphy inferred from the boreholes although suggesting that the shift in magmatism may have been towards the northwest on the basis of geochemical similarities between magma types and their inferred stratigraphical relationships it is proposed that the parana can be divided into two principal magmatic centres 1 an older one in the south comprising the gramado esmeralda and urubici magma types and 2 a younger one developed about 750 km to the north formed by the pitanga paranapanema and ribeira magma types

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

seismic image of the cantabrian mountains in the western extension of the pyrenees from integrated escin reflection and refraction data abstract integrated analysis of normal incidence and large aperture seismic reflection data collected in 1992 and 1993 within the spanish escin and complementary projects provide a first complete n s crustal transect across the northern iberian peninsula and continental margin images of the crustal structure of the cantabrian mountains and their transition to the duero basin and to the cantabrian margin are obtained from a a 65 km long vertical reflection profile escin 2 on land b a 200 km long reversed refraction profile and c wide angle recordings of the marine escin 4 profile consistent results between reflectivity pattern and velocity depth distribution reveal important lateral variations in the deep structure the reflective crust imaged in the escin 2 profile changes its attitude from sub horizontal beneath the duero basin to north dipping beyond the mountain front basement thrusts are observed in the upper crust merging into a detachment at 6 s twt and may have triggered the alpine uplift of the range the moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile about 35 km inland modelling of the refraction data laterally extends the seismic image and provides evidence for variscan crustal features beneath the duero basin northwards the velocity in the lower crust decreases and the moho constrained by the wide angle data from profile escin 2 deepens to about 60 km ending abruptly at the shoreline the velocity depth model is constrained along the asturian platform up to the continental slope where the crust mantle boundary is located at 24 km depth this margin moho shows a progressive deepening southwards and extends to the coast where it is found at 30 km depth the present seismic data support an important alpine reworking and thickening of the crust under the cantabrian mountains the onshore offshore transition is marked by an imbrication of two crusts of very different thicknesses this signature offers a strong parallelism with the one previously observed further east across the pyrenees in the ecors seismic profile

origins of large volume rhyolitic volcanism in the antarctic peninsula and patagonia by crustal melting voluminous rhyolitic volcanism along the palaeo pacific margin of gondwana was marked by three principal episodes of magmatism the first of these v1 is essentially coincident with the main episode of karoo ferrar magmatism at 184 ma a younger v2 episode occurred at 168 ma and a third episode v3 occurred in the interval 157 153 ma we evaluate the origin of v1 and v2 rhyolites from the antarctic peninsula using major and trace element and isotopic sr nd o data an isotopically uniform 87sr 86sri 0 707 endi 3 andesite dacite magma was generated as a result of anatexis of grenvillian age hydrous mafic lower crust linked to earlier arc related underplating the lower crustal partial melts would have mixed with fractionated components of the mafic underplate followed by subsequent storage and homogenization early jurassic v1 rocks of the southern antarctic peninsula are interpreted as melts of upper crustal paragneiss which have mixed with the isotopically uniform magma in upper crustal magma chambers the v2 rhyolites are the result of assimilation fractional crystallization of the isotopically uniform magma this occurred in upper crustal magma chambers involving assimilants with similar isotopic composition to that of the magma a continental margin setting was crucial in developing hydrous readily fusible lower crust lower crustal anatexis was in response to mafic underplating associated with the discovery shona bouvet group of plumes thought to be responsible for the karoo magmatic province the progression old to young of volcanism from ne to sw in patagonia and south to north in the antarctic peninsula is consistent with migration away from the mantle plumes towards the proto pacific margin of gondwana during rifting and break up

an interpretation of cretaceous and tertiary biota in polar regions abstract rich fossil floras and vertebrate faunas lived within the arctic and atarctic circles during the cretaceous and paleogene these occurrences seem explicable by the nature of the thermal environments in polar regions and by the physiological responses of plants and animals allied to those that lived there there seems no need to invoke a lower tilt to the earth s axis to provide greater warmth and light in polar regions to account for these fossil records

the bero volcanic complex extension of the parana etendeka igneous province into sw angola abstract an extension of the etendeka parana igneous province into sw angola occurs as minor basalt lavas intrusive gabbro sheets minor mafic dykes and thick sheets and lava flows with minor pyroclastics of quartz latite composition this suite crops out along the eastern margin of the cretaceous namibe basin in sw angola the quartz latites from one locality have been referred to informally as the giraul volcanics but the name giraul has previously been used for cretaceous conglomerates we propose the name bero volcanic complex for this suite of intrusive and extrusive rocks on the basis that the full compositional range of this diverse suite crops out along the rio bero major and trace element compositions and sr nd and pb isotopic compositions indicate that the basalts and gabbros are equivalent to the high ti khumib urubici and pitanga types from the etendeka and parana the basalts underlie the quartz latites which are cut by mafic dykes some of which are compositionally equivalent to the paranapanema lavas in the parana five different geochemical types of high ti quartz latite are recognised amongst the silicic volcanics 3 of which have very close geochemical affinities to the ventura sarusas and khoraseb types of the northern etendeka their relative stratigraphic position in the bero volcanic sequence is the same as in the etendeka sequence and extend significantly the area over which these types were erupted the two remaining types chinguau and high nb are not known from either the etendeka or the parana provinces

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

palaeomagnetism of lower cretaceous tuffs from yukon kuskokwim delta region western alaska during the past decade the prescient arguments1 3 for the allochthoneity of large portions of southern alaska have been corroborated by detailed geological and palaeomagnetic studies in south central alaska4 9 the alaska peninsula10 kodiak island11 12 and the prince william sound area13 fig 1 these investigations have demonstrated sizeable northward displacements for rocks of late palaeozoic mesozoic and early tertiary age in those regions with northward motion at times culminating in collision of the allochthonous terranes against the backstop of nuclear alaska14 15 a fundamental question is which parts of alaska underwent significantly less latitudinal translation relative to the stable north american continent thereby serving as the accretionary nucleus into which the displaced microplates 16 were eventually incorporated17 18 here we present new palaeomagnetic results from tuffs and associated volcaniclastic rocks of early cretaceous age from the yukon kuskokwin delta region in western alaska these rocks were probably overprinted during the cretaceous long normal polarity interval although a remagnetization event as recent as palaeocene cannot be ruled out this overprint direction is not appreciably discordant from the expected late cretaceous direction for cratonal north america the implied absence of appreciable northward displacement for this region is consistent with the general late mesozoic early tertiary tectonic pattern for alaska based on more definitive studies little to no poleward displacement for central alaska though substantially more northward drift for the southern alaska terranes comprising alaska peninsula kodiak island prince william sound area and matunuska valley since late cretaceous to palaeocene time

on the causes of 18 o depletion and 18 o 16 o homogenization during regional metamorphism the east humboldt range core complex nevada previous stable isotope studies at lizzies basin revealed that metasedimentary rocks are 18o depleted relative to protolith values particularly in the lower parts of the section lower zone where the rocks are also isotopically homogeneous on a scale of hundreds of meters quartz 18o 9 0 to 9 6 per mil in contrast metasedimentary rocks at higher levels at lizzies basin upper zone are less 18o depleted and more heterogeneous in 18o in order to understand more fully the isotopic evolution of this terrane a series of detailed meter scale traverses across various metamorphic and igneous lithologies were completed at lizzies basin and at the structurally higher angel lake locality traverses in the lizzies basin lower zone and in the lower parts of angel lake angel lake lower sequence across various silicate lithologies including abundant granitoids reveal similar degrees of homogeneity although the average 18o values are higher at angel lake in contrast traverses which include substantial thicknesses of marble and calc silicate gneiss and very little granitoid have more heterogeneous quartz 18o values 11 9 to 13 4 per mil and also have a higher average 18o 12 9 per mil than observed elsewhere the scale of 18o 16o homogeneity in quartz observed at lizzies basin and angel lake meters to hundreds of meters requires fluid mediated isotope exchange which accompanied tertiary metamorphism there is a correlation between the degree of 18o depletion in metasedimentary rocks 18o 16o homogenization between lithologies and the proportion of granitoids leucogranites in particular within any part of the section and a corresponding anticor relation with the proportion of marble this points to a causal relationship whereby the leucogranites as well as the tertiary hornblende diorite and biotite monzogranite acted as both a relatively low 18o reservoir and a source of fluids to enhance exchange while the marbles hindered isotope deplction and homogenization by acting as relatively high 18o reservoirs and impermeable layers material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments single pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations fluidmediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations the generalized model for the isotopic evolution of the east humboldt range core complex provides and excellent opportunity to establish the main causes and controlling factors of 18o depletion and 18o 16o homogenization during regional metamorphism

protracted felsic magmatic activity associated with the opening of the south atlantic felsic magmatism is often observed in the final stages of the eruption of continental large igneous provinces the occurrence of these felsic extrusive igneous rocks potentially provides key information on the complex interplay of magmatism and tectonics in this paper we investigate the timing of eruption of early cretaceous felsic magmas associated with the parana etendeka flood basalt province using the 40 ar 39 ar technique and interpret the results in terms of the recently proposed petrogenetic histories and dominant stress factors there appears to be a link between lithospheric thinning and a change in the origin of the felsic magmas from open system fractional crystallization to mid lower crustal melting particularly in southern uruguay

paleomagnetism of the paleocene ghost rocks formation prince william terrane alaska the mean paleomagnetic inclination of paleocene volcanics from two regions on kodiak island in the ghost rocks formation suggests that the prince william terrane originated at mid latitudes about 25 south of its expected alaskan latitude in paleocene time the remanent magnetization of these rocks passes both the fold and reversal tests and is well constrained in age the mean declinations of the two regions however differ by approximately 120 suggesting they have rotated with respect to each other perhaps during emplacement we suggest that despite the lack of evidence for a major tertiary suture zone between the prince william terrane and central alaska the prince william and perhaps adjacent terranes may have lain substantially south of their present position in the early tertiary

old model nd ages in namibian pan african rocks the nd isotope analytical technique is a powerful tool for studying many geological processes particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle derived volcanic rocks1 3 and magmatic processes along destructive plate margins4 5 however previous studies of continental crust6 8 have been largely restricted to archaean examples and concerned primarily with dating their time of derivation from the upper mantle in the present study we investigated the nd and sr isotope characteristics of rocks involved in a relatively young 800 450 myr orogenic event such events represent critical stages in the evolution of most of the earth s continental crust new material is added from the mantle and pre existing crust is remobilized by erosion and sedimentation deformation and magmatic activity nd and sr isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre existing crust which were sampled during the orogeny and hence provided the major components of what is now a stable segment of continental crust

paleolatitudes versus time for southern alaska paleomagnetic determinations of the inclination of the ancient geomagnetic field for localities in the alaska peninsula wrangellia chugach and kuskokwim tectonostratigraphic terranes of alaska show systematic northward motion from late cretaceous time until the present prior to this the ambiguity of polarity of the ancient geomagnetic field allow several interpretations however the preferred interpretation involves southerly motion of the terranes from triassic to early jurrassic time and steady northward motion from then up to the present the latitudinal rate is about 6 cm yr

oxygen isotope geochemistry of the silicic volcanic rocks of the etendeka parana province source constraints oxygen isotope ratios of pyroxene phenocrysts in the silicic volcanic rocks from the cretaceous etendeka parana flood basalt province namibia south america are believed to reflect the delta sup 18 o values of the original magmas the authors recognize a high delta sup 18 o value type delta sup 18 o pyroxene approximately 10 per thousand found in the south of both regions and a low delta sup 18 o value type delta sup 18 o pyroxene approximately 6 5 per thousand found in the north other differences between thee two rhyolite types include higher concentrations of incompatible elements and lower initial sup 87 sr sup 86 sr ratios in the low delta sup 18 o value type the authors suggest that the regional distribution of rhyolite types reflects differences in source composition which can best be explained if the sources are lower crustal late proterozoic mobile belt material high delta sup 18 o and archean lower crust low delta sup 18 o

experimental and thermodynamic constraints on the sulphur yield of peralkaline and metaluminous silicic flood eruptions many basaltic flood provinces are characterized by the existence of voluminous amounts of silicic magmas yet the role of the silicic component in sulphur emissions associated with trap activity remains poorly known we have performed experiments and theoretical calculations to address this issue the melt sulphur content and fluid melt partitioning at saturation with either sulphide or sulphate or both have been experimentally determined in three peralkaline rhyolites which are a major component of some flood provinces experiments were performed at 150mpa 800 900 c fo2 in the range nno 2 to nno 3 and under water rich conditions the sulphur content is strongly dependent on the peralkalinity of the melt in addition to fo2 and reaches 1000ppm at nno 1 in the most strongly peralkaline composition at 800 c at all values of fo2 peralkaline melts can carry 5 20 times more sulphur than their metaluminous equivalents mildly peralkaline compositions show little variation in fluid melt sulphur partitioning with changing fo2 ds 270 in the most peralkaline melt ds rises sharply at fo2 nno 1 to values of 500 the partition coefficient increases steadily for sbulk between 1 and 6wt but remains about constant for sbulk between 0 5 and 1wt at bulk sulphur contents lower than 4wt a temperature increase from 800 to 900 c decreases ds by 10 these results along with 1 thermodynamic calculations on the behaviour of sulphur during the crystallization of basalt and partial melting of the crust and 2 recent experimental constraints on sulphur solubility in metaluminous rhyolites show that basalt fractionation can produce rhyolitic magmas having much more sulphur than rhyolites derived from crustal anatexis in particular hot and dry metaluminous silicic magmas produced by melting of dehydrated lower crust are virtually devoid of sulphur in contrast peralkaline rhyolites formed by crystal fractionation of alkali basalt can concentrate up to 90 of the original sulphur content of the parental magmas especially when the basalt is co2 rich on this basis we estimate the amounts of sulphur potentially released to the atmosphere by the silicic component of flood eruptive sequences the peralkaline ethiopian and deccan rhyolites could have produced 10 17 and 10 18 g of s respectively which are comparable amounts to published estimates for the basaltic activity of each province in contrast despite similar erupted volumes the metaluminous parana etendeka silicic eruptives could have injected only 4 6 10 15 g of s in the atmosphere peralkaline flood sequences may thus have greater environmental effects than those of metaluminous affinity in agreement with evidence available from mass extinctions and oceanic anoxic events

2 d seismic modeling of the variscan foreland thrust and fold belt crust in nw spain from escin 1 deep seismic reflection data abstract the foreland thrust and fold belt cantabrian zone of the variscan belt in nw spain and the transition to the hinterland westasturian leonese zone was the location of a seismic experiment in 1991 the seismic reflection profile escin 1 is 140 km long and runs in an e w direction the interpretation and seismic modeling of the main reflective interfaces in the profile were made integrating available geological and geophysical data including surface geology deep seismic reflection data from escin 1 transmission velocities from a borehole refraction and laboratory data the geological and velocity model of the crust was tested obtaining synthetic seismograms and can be correlated with surface geology the velocity model images the main crustal structures interpreted from escin 1 the basal detachment of the foreland thrust and fold belt dips gently from 12 km in the easternmost part of the profile to 16 km in the transition to the hinterland to the west a new interpretation is proposed for the structure above the basal detachment in the eastern end of the profile where the basal detachment at a depth of 12 km is duplicated at 6 5 km by a n dipping alpine thrust that also duplicates part of the basement in the western part two deep reflective bands dip westward and the most conspicuous one can be followed from 16 27 km depth the two bands previously interpreted as crustal ramps join a reflective lower crust that is located between 25 and 29 km and fades westwards a reflective moho interpreted at the base of the reflective lower crust dips and fades in the same direction from 30 34 km

stratigraphy and sedimentology of the mid cretaceous deposits of the yukon koyukuk basin west central alaska the northeast trending yukon koyukuk basin of west central alaska consists of two subbasins the kobuk koyukuk subbasin to the north and east and the lower yukon subbasin to the southwest the subbasins are separated by an arcuate lower cretaceous volcanic pile the hogatza trend which is thought to be an accreted volcanic arc the oldest part of the sedimentary fill of the subbasins consists of valanginian to lower albian volcaniclastic rocks deposited on the flanks of the hogatza trend following subsidence of the hogatza trend mid cretaceous clastic sedimentary strata of mainly albian and cenomanian age and possibly as thick as 8000 m were shed into the basin these deposits were derived from surrounding uplands or borderlands in the seward peninsula to the west the brooks range to the north and the ruby geanticline to the southeast these mid cretaceous basin fill deposits can be divided into four main facies 1 basin margin conglomerate facies chiefly alluvial fan deposits that were transported basinward and rest in part unconformably on the surrounding uplands 2 shelf facies chiefly cross stratified and hummocky cross stratified sandstone deposited by wave generated currents on a shelf that rimmed the basin on its western and northern margins 3 deltaic facies chiefly sandstone and shale deposited in delta plain and delta front environments on a large constructional delta that prograded westward from the eastern basin margin across both subbasins and across the subsided southern part of the hogatza trend and 4 turbidite facies chiefly interbedded sandstone and shale deposited as elongate deep sea fans and related deep sea clastic systems by flows that transported sediment to the axial parts of both subbasins northeastward in the lower yukon subbasin and eastward to southward in the kobuk koyukuk subbasin sedimentation appears to have ended in the santonian followed by uplift folding and faulting of the basin fill less deformed lower tertiary nonmarine volcanic and volcaniclastic rocks unconformably overlie the more highly deformed cretaceous strata

denali fault system of southern alaska an interior strike slip structure responding to dextral and sinistral shear coupling the denali fault system dfs extends for 1200 km from southeast to south central alaska the dfs has been generally regarded as a right lateral strike slip fault along which post late mesozoic offsets of up to 400 km have been suggested the offset history of the dfs is relatively unconstrained particularly at its western end for this study we calculated relative motion vectors at discrete points along the length of the dfs based on the well understood kinematic interaction between the north american pacific and kula plates and the following assumptions 1 the arcuate geometry of the dfs has existed essentially unchanged since the late cretaceous 2 the yukon tanana terrane and other terranes north of the dfs were fixed in situ prior to the accretion of the southern alaskan terranes and 3 tangential and normal relative motion vector components calculated for points along the dfs using the plate model of kelley 1993 describe the plate kinematics of the dfs since the late cretaceous the consequent kinematic model for the dfs predicts that left lateral stresses have acted upon the western end of the dfs for much of its history and conflicting senses of shear exist between the eastern and western ends of the system the offset history of the western end of the denali fault system should be significantly different than the history of the central and eastern sections consequently individual crustal blocks in southeast and southwest alaska may have undergone respectively clockwise and counterclockwise rotations the sense of rotation predicted by our model is in agreement with rotations determined by paleomagnetic studies and provides an alternative model to the alaskan orocline hypotheses

limits to northward drift of the paleocene cantwell formation central alaska volcanic rocks of the paleocene cantwell formation in central alaska apparently originated at a paleolatitude of 83 n 95 9 7 as indicated by paleomagnetic results when compared with the paleocene pole for the north american craton the 95 confidence limits of the results suggest that terranes north of the denali fault have moved no more than 550 km northward relative to the north american craton since paleocene time

crustal signatures in the serra geral flood basalt province southern brazil o and sr isotope evidence strontium and oxygen isotopes in igneous rocks from a 35 000 km 2 region of the serra geral parana continental flood basalt province southern brazil indicate that certain mantle derived magmas that formed during the rifting and breakup of gondwana incorporated continental crust sr o and trace element assimilation fractional crystallization models point to variable amounts and types of warm archean granulitic crust 18 o 11 15 and 87 sr 86 sr i 0 735 having contaminated ascending mafic magmas resulting products include evolved rocks basaltic andesite to rhyolite that have 18 o 11 to 15 and 87 sr 86 sr i 0 714 to 0 729 and basaltic rocks that have 18 o 8 to 10 and 87 sr 86 sr i 0 710

origin of k feldspar megacrysts in rhyolites from the emeishan large igneous province southwest china abstract silicic rocks occur in the uppermost units of the longest volcanic succession 5000 m thick in the binchuan area of the permian emeishan flood basalt province of sw china they are predominantly rhyolites and to a lesser extent trachytes both containing potassium feldspar megacrysts as the dominant phenocryst phase up to approximately 20 mm in size these megacrysts contain domains of albite arranged in vein like networks likely formed by post magmatic alteration crystal size distributions csd suggest that these megacrysts grew in a stable magmatic system consistent with relatively uniform core to rim compositional k 2 o 14 16 wt and isotopic profiles 208 pb 206 pb ratios 2 06 2 08 0 005 both whole rock trace elements and pb isotope ratios of these silicic rocks are similar to the emeishan basalts suggesting a common source for both mafic and felsic units and a limited role of crustal melting in genesis of the felsic units major and trace element models further indicate that these rocks could not have formed exclusively by re melting of old crust or solidified basaltic rock but must have formed through crystal fractionation from the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization k feldspar bearing rhyolites are also observed in the last stages of other large igneous provinces we suggest that they represent final melt fractions and their appearance in the magmatic system coincides with waning of voluminous magmatic activities