late precambrian crustal accretion rates in northeast africa and arabia apparent late precambrian growth rates for continental crust in the arabian nubian craton are implausibly high for simple arcaccretion models if correct these rates imply that some special non uniformitarian process occurred to rapidly produce continental crust assuming that the dominant mechanism for continental growth is by arc magmatism roughly 100 of global island arc production at current rates would have had to accrete in this region from 900 to 600 ma one solution to this paradox is that large amounts of preexisting sialic basement have so far gone undetected and should not be included in the estimate for crustal growth during late precambrian time published radiometric data indicate that older continental material makes up the eastern and western margins of the exposed shield we interpret these and other data to suggest that the larger part of the arabian nubian craton including the platform obscured by phanerozoic cover east and west of the exposed shield is older continental crust in this case a growth rate approximately 20 of the modern global rate is required and nonuniformitarian crustal growth is not necessary to explain late precambrian accretion of the arabian nubian craton

eulerian and lagrangian statistics in the bluelink numerical model and aviso altimetry validation of model eddy kinetics 1 we compare eulerian and lagrangian timescales velocity autocorrelations and spectra from 14 years of hindcast ocean model and satellite velocities with direct observations our main aim is to determine how well the model bluelink version bran 2 1 and satellite derived currents archiving validation and interpretation of satellite oceanographic data aviso simulate mesoscale eddy variability which is important in mixing heat salinity and momentum and in controlling dispersion ocean eulerian observations are from topex poseidon t p crossover velocities and lagrangian observations are from global drifter program gdp surface drifters eulerian spectra from bluelink are similar to eulerian spectra from crossover velocities over the range of common frequencies the 10 day sampling from t p limits the comparison to low frequencies and eulerian timescales from bluelink are slightly shorter than from the observations part of this difference may be an effect of the t p sampling rate in contrast lagrangian spectra from bluelink are considerably steeper than spectra from gdp drifters and consequently lagrangian timescales from bluelink are longer than from the observations both eulerian and lagrangian spectra from aviso are steeper than from observations so that both eulerian and lagrangian aviso timescales are longer than observed the real ocean is an ocean where the eulerian lagrangian relationship described by middleton 1985 is followed the tasman sea averaged ratio of lagrangian to eulerian timescales is about 0 1 zonally and 0 15 meridionally however the eulerian lagrangian relationship described by middleton 1985 is not obeyed by bluelink and we believe that this behavior in bluelink reflects a loss of short wavelength eddies in the model

accretion of southern alaska abstract paleomagnetic data from southern alaska indicate that the wrangellia and peninsular terranes collided with central alaska probably by 65 ma ago and certainly no later than 55 ma ago the accretion of these terranes to the mainland was followed by the arrival of the ghost rocks volcanic assemblage at the southern margin of kodiak island poleward movement of these terranes can be explained by rapid motion of the kula oceanic plate mainly from 85 to 43 ma ago according to recent reconstructions derived from the hot spot reference frame after accretion much of southwestern alaska underwent a counterclockwise rotation of about 50 as indicated by paleomagnetic poles from volcanic rocks of late cretaceous and early tertiary age compression between north america and asia during opening of the north atlantic 68 44 ma ago may account for the rotation

mesozoic paleomagnetism and northward translation of the baja california peninsula paleomagnetic data from cretaceous igneous and sedimentary rocks of the peninsular ranges and continental borderland of the baja california peninsula yield an average pole position of 83 n 344 e a95 3 4 suggesting that the peninsula as a whole has moved northward 11 and rotated 32 clockwise with respect to the north american craton since cretaceous time only a small portion of this movement is due to the neogene opening of the gulf of california results from stratified rocks giving both positive and negative fold tests are included in the average cretaceous pole the rocks that fail fold tests have been deformed and remagnetized by nearby intrusives that are only slightly younger unconnected directions from these sites closely match corrected directions from sites where the fold tests are positive oceanic rocks were also sampled in the continental borderland from upper triassic and upper jurassic tectonostratigraphic terranes which are apparently allochthonous with respect to the western baja peninsula and each other these oceanic rocks show magnetizations indicating low paleolatitudes although the hemisphere of origin is presently unknown the cretaceous rocks of the baja california peninsula may have originally formed adjacent to the now truncated continental margin along the northern middle america trench northward transport of the peninsula probably occurred by transcurrent faulting associated with oblique subduction of the farallon plate beneath western north america between late cretaceous and late miocene time

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

paleomagnetic evidence for spatially distributed post miocene rotation of western washington and oregon anomalous paleomagnetic directions have been determined for 17 sites in the frenchmans springs member of the wanapum basalt formation columbia river basalt group these sites are located in the ginkgo flows from near vantage washington to portland oregon a distance of approximately 300 km the average paleomagnetic direction for six of these sites centered around vantage is d 147 i 41 95 4 5 the expected miocene field direction is d 355 i 65 at some localities there are two distinct ginkgo flows in direct stratigraphic succession with statistically identical anomalous directions their anomalous paleomagnetic direction makes these flows a valuable marker horizon in the columbia river basalt group the nondipole field direction of the ginkgo flows correlates well with available results from the miocene cape foulweather basalts of oregon this correlation strongly supports the hypothesis that these coastal basalts of oregon are the distal ends of columbia plateau derived basalt flows the spatial distribution of these anomalous field directions suggests about 14 of clockwise rotation between vantage and portland combining these data with data from the oregon coast basalts allows a maximum declination difference of about 35 the increase in declination can be best explained by clockwise rotation about nearby vertical axes increasing to the southwest across the columbia plateau and oregon coast

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

the importance of late and post orogenic crustal growth in the early proterozoic evidence from smnd isotopic studies of igneous rocks in the makkovik province canada abstract sm nd isotopic studies suggest that large tracts of 1900 1700 ma old crust in laurentia baltica is of juvenile origin this crust has generally been ascribed to arc magmatism sustained over long periods and most conceptual models for crustal growth emphasize this process the late to post orogenic granitoid rocks that areally dominate many ancient orogenic belts are commonly viewed as anatectic derivatives of the earlier arc type crust however in regions of short crustal residence the time resolution of isotopic tracers such as the sm nd system does not permit discrimination between this model and continued growth of the crust during post orogenic magmatism the relative contributions of recycled crust and new mantle derived material in late to post orogenic magmas can only be assessed where they also transect much older crustal blocks the nd isotopic signatures of 1800 1720 ma igneous suites in the makkovik province define such a boundary between an archean craton and a juvenile proterozoic domain in the juvenile domain the nd signatures of most igneous suites are equivocal initial nd 0 to 2 and they could be anatectic derivatives of slightly older orthogneisses however addition of new mantle derived material is documented by a type granites with initial nd up to 4 in the cratonic domain temporally and compositionally equivalent igneous suites mostly have initial nd of 7 to 3 significantly above local archean basement which has nd of 15 at 1800 ma conservative calculations suggest that most of these suites contain more than 50 new mantle derived material these results suggest significant crustal growth via late stage magmatism and direct interaction of mantle derived magmas and lower crustal rocks in contrast 1650 ma igneous suites lack clear systematic variation in nd and are interpreted as representing crustal growth via later distal arc type magmatism with crust mantle mixing mainly facilitated by sediment subduction crustal growth in this area thus involved significant addition of new material through late to post orogenic magmatism this process may partially explain the unusually high growth rates calculated for some segments of proterozoic crust which approach or exceed modern global arc addition rates this stage 3 crustal growth may have been proportionally more important in early earth history due to higher ambient mantle temperatures

tomographic inversion for the three dimensional seismic velocity structure of the aswan region egypt the thurber iterative simultaneous inversion program is used to determine the three dimensional p wave velocity structure in the aswan seismic region of egypt the tomographic inversion presented in this study is based on 1131 p phase observations at 13 stations from 89 local earthquakes all of which occurred within the kalabsha fault zone the assumed initial velocity model is that deduced from local explosion experiments the results indicate that the aswan region is characterized by a heterogeneous crust consisting of a shallow low velocity zone and a deeper high velocity anomaly seismic velocity structure within the shallow part demonstrates that the inferred change in velocity exists primarily across the east west trending kalabsha fault scarp whereas the high velocity zone is located south of this fault two well resolved low velocity zones appear within the upper 6 km of the crust the first coincides with a graben structure located between the kalabsha and seiyal faults and the second exists between the n s kurkur fault and the main axis of lake aswan both low velocity zones occupy an area of approximately 30 40 km located along the western bank of the lake the most significant result of this study is that the location of the deeper high velocity anomaly coincides with the concentration of seismic activity in the lower crustal layer

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

cretaceous magnetizations in northwestern nevada and tectonic implications paleozoic and mesozoic arc related rocks of the black rock desert region of northwestern nevada have acquired a very stable postfolding secondary magnetization probably in late cretaceous time the remagnetization which appears to be of regional extent probably accompanied emplacement of granitic plutons in a northeast trending belt across northwestern nevada near the end of mesozoic time the region has experienced no appreciable tectonic rotation since that time

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

pan african magmatism in the wadi el imra district central eastern desert egypt geochemistry and tectonic environment the late proterozoic rocks of the wadi el imra district of the central eastern desert egypt comprise a metagabbro diorite complex oldest and older and younger granitoid complexes trace element characteristics show that the basic and granitoid rocks are of calc alkaline affinity but some of the younger granitoids grade into alkaline granites the o nd values of the the basic and granitoid rocks range from 8 3 to 4 9 and nd model ages are younger or rather similar to crystallization ages these data suggest a predominant depleted mantle derivation indicating formation by subduction in an oceanic setting ree patterns of the granitoid rocks suggest different modes of petrogenesis this coupled with the occurrence of amphibolite xenoliths in the older granitoids may indicate that some of these rocks represent partial melts from basic rocks that have been amphibolitized at deep crustal levels of the oceanic arc

magnetostratigraphy of displaced upper cretaceous strata in southern california a magnetostratigraphic study of upper cretaceous marine strata from the santa ana mountains in southern california has identified a campanian reversed magnetozone this reversed interval corresponding to marine magnetic anomaly 33 34 chron 33r of campanian age can be correlated with a campanian reversed magnetozone that has been reported from strata of the great valley sequence in central california the late cretaceous paleolatitude of the santa ana mountains is estimated from this study to be 26 6 n this is significantly different from the region9s expected cretaceous paleolatitude of 43 8 n as part of the north american stable craton and indicates that this region part of the peninsular ranges terrane was 1900 km farther south in cretaceous time relative to the stable craton

paleomagnetism and the motion of large and small plates the focus of paleomagnetic research during past decades has varied greatly the first generation in paleomagnetism might be defined as beginning with the pioneering studies aimed at discovering whether the geomagnetic field has been a reliable reference frame for measuring and timing continental displacements an enormous effort followed in the form of individual paleomagnetic studies of cratonic rocks culminating with the construction of apw apparent polar wander paths for the continents which could be used to test paleogeographic reconstructions coupled with parallel research in marine geophysics and geology the refined apwp which result from this effort have helped to develop a reasonable first order history of cenozoic mesozoic paleogeography for the major plates

u pb zircon ages for the gneissic terrane west of the nile southern egypt the first u pb zircon ages are reported for the gneissic bedrock inliers previously interpreted as part of the nile craton the inliers crop out in the egyptian western desert east of the uweinat area and west of the eastern desert multi and single grain zircon analyses of granitoid gneiss and migmatite from gebel um shagir aswan and another locality approximately 160 km south west of aswan yield simple discordia with near modern day pb loss trajectories and the following neoproterozoic crystallization ages 626 4 3 634 4 and 741 3 ma in contrast multi and single grain u pb analyses zircon and sphene from an anorthositic gabbro at gebel kamil 22 46 n 26 21 e and an anorthosite at gebel el asr 22 46 n 31 10 e yield archean and paleoproterozoic emplacement ages the former yield a crystallization age of 2 67 ga and a metamorphic age of 2 0 ga the latter a metamorphic age of 0 69 ga and an inheritance age of 1 9 2 1 ga because high grade gneiss and migmatite of neoproterozoic paleoproterozoic and archean age crop out west of the nile pre neoproterozoic crust should no longer be identified by its metamorphic grade by contrast mapping the anorthosite and related rocks might provide first order estimates for the extension of pre neoproterozoic crust in north east africa it is suggested that archean and paleoproterozoic crust of the uweinat and congo craton are contiguous because these u pb zircon data show no evidence for a neoproterozoic thermal overprint in the gebel kamil area and there is no pronounced neoproterozoic magmatic activity south of the uweinat inlier and north of the congo craton

cretaceous and early tertiary paleomagnetic results from the antarctic peninsula paleomagnetic results are presented for early tertiary and cretaceous igneous and volcanic units from the northern antarctic peninsula when combined with previously published data they permit the construction of a polar wander path that is very similar to one derived for the east antarctic craton by rotating tertiary and cretaceous poles from india and australia into the antarctic frame of reference when considered in this context the paleomagnetic data from the orville coast which was previously interpreted as evidence in favor of oroclinal bending or relative rotation of the southern antarctic peninsula with respect to the northern part may be considered as an in situ resulte the implications of the new data are as follows 1 the antarctic peninsula down to the orville coast has been a part of the antarctic plate since 100 m y ago 2 movement of the antarctic peninsula required to avoid overlap in reconstructions of gondwanaland must have occurred before 100 m y ago

late proterozoic evolution of the northern part of the hamisana zone northeast sudan constraints on pan african accretionary tectonics new structural data from the red sea hills northeast sudan add constraints to accretionary tectonic processes that acted in the arabian nubian shield during the late proterozoic geometric constraints provided by field data are used together with landsat thematic mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages in contrast to interpretations reported elsewhere these studies indicate that the hamisana zone a prominent structural feature in the northern red sea hills post dated terrane accretion it is characterized by east west crustal shortening and by steep folds and thrust faults calc alkaline magmatism was contemporaneous with and outlasted main phase deformation thus deformation occurred in an intra arc setting thrust faults were reactivated after east west contraction compromising their usefulness in delineating 9sutures9 or as piercing points across older faults pan african accretionary processes may have been analogous to phanerozoic ophiolite and island arc accretion in the western north american cordillera where penetrative deformation occurred in response to periodic intra plate shortening events rather than an ultimate collision of unrelated crustal fragments such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long lived convergent margin setting consistent with the long standing observation that pan african magmatism became more evolved with time this view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening in contrast to models where crustal shortening results directly from collision of a volcanic arc microcontinent or other buoyant features

geochemistry mineral chemistry and petrogenesis of a neoproterozoic dyke swarm in the north eastern desert egypt dyke swarms traverse neoproterozoic rocks in the hawashiya region in the extreme northern part of the eastern desert of egypt they are a suite of basaltic andesite and andesite mafic dykes and dacitic and rhyolitic felsic dykes the mafic dyke suite is more abundant in the younger granites 577 6 ma than in the older granitoids 614 ma in which the felsic dykes are the most common the dyke swarms trend predominantly ne sw and the felsic dyke suite is older than the mafic dyke suite both dyke suites are calc alkaline alkaline dykes are rare and are relatively poor in tio 2 and nb but enriched in the incompatible elements and hfse the felsic dyke suite is enriched in ree and is strongly lree fractionated relative to the mafic dyke suite although the hawashiya dykes were emplaced at the end of the neoproterozoic era in an extensional tectonic setting they have geochemical characteristics that are consistent with a subduction related regime these chemical signatures were inherited from the lithospheric rocks that produced their host hawashiya granitoids the felsic dyke suite magma may be derived from crustal rocks essential source component by partial melting the mafic dyke suite magma was generated from a lithospheric mantle and has undergone fractional crystallization of plagioclase amphibole clinopyroxene and magnetite as documented by major and trace elements fractionation modelling