crustal lineaments distribution of lower crustal intrusives and structural evolution of the voring margin ne atlantic new insight from wide angle seismic models abstract five lineaments on the volcanic voring margin ne atlantic have been identified in crustal scale models derived from ocean bottom seismograph obs data it is suggested that the voring basin can be divided in four compartments bounded by the jan mayen fracture zone lineament a new lineament defined from this study the gleipne lineament the surt lineament and the bivrost lineament the nw se trending jan mayen gleipne and bivrost lineaments probably represent old zones of weakness controlling the onset of the early eocene seafloor spreading whereas the surt and new lineaments rotated ca 30 symmetrically from the azimuth of the gleipne lineament may represent adjustment features related to the early cretaceous early tertiary rifting the longest landward extent of a lower crustal high velocity body assumed to represent intrusions related to the last phase of rifting is found between the new lineament and the gleipne lineament where the body extends across the helland hansen arch northeastwards in the voring basin the landward limit of the body steps gradually seawards closely related to the interpreted lineaments northeast of the gleipne lineament the body terminates close to the fles fault complex north of the surt lineament it extends across the nyk high and northeast of the bivrost lineament the intrusions terminate around the voring escarpment evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin the active component is evident through the decrease in magmatism with increased distance from the icelandic plume and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement the increased thickness of the continental crust on the seaward side of the voring escarpment the upwarping of moho and thinning of the lower crustal high velocity layer in the western part of the voring basin as well as a strong shallowing of the moho observed in parts of the area between the jan mayen fracture zone lineament and the new lineament can be explained by lithospheric delamination models

cambrian depositional history of the zanskar valley region of the indian himalaya tectonic implications a well preserved cambrian section in the zanskar valley of northern india has previously been interpreted to record the transition from a passive to an active tectonic margin related to cambrian ordovician orogenesis this interpretation has been used to support the tectonostratigraphic interpretation of other successions across the tethyan himalaya our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these cambrian deposits no definitive record of impending cambrian ordovician orogenesis is recorded in these late middle cambrian rocks a critical transition from an 125 m thick stromatolite bearing carbonate deposit the karsha formation into shale and sandstone of the kurgiakh formation was interpreted to represent tectonically induced drowning of a carbonate platform siliciclastic strata of the kurgiakh formation were thought to record deep water flysch deposition in a tectonically active foreland basin next to an arc trench system this interpretation was based on sandstone beds with classic bouma sequences we show that these event beds in the kurgiakh instead contain hummocky cross stratification quasi planar lamination and combined flow ripple stratification all of which reflect deposition in shallow marine storm influenced environments thus although the karsha carbonate platform may have been drowned it did not culminate in deep sea flysch deposition and this in turn eliminates a major line of evidence linking kurgiakh deposition to the onset of cambrian ordovician orogenesis other aspects of cambrian ordovician deposits of northern india also shed doubt on the proposed link between kurgiakh sedimentation and the cambrian ordovician orogenic event first our improved biostratigraphic database suggests that the transition from the karsha carbonate to the kurgiakh formation may have predated the main phase of cambrian ordovician orogenesis as recorded by overlying ordovician molasse by as much as 20 30 my second published data from the ordovician molasse indicate northward paleocurrents which are parallel to those recorded by siliciclastic deposits of the parahio formation below the karsha and thus are at odds with standard models of foreland basin development for the cambrian ordovician event our sedimentological analysis of depositional cycles of the parahio formation indicates that these strata record storm influenced environments from offshore marine to shoreface to fluvial settings this is at odds with previous paleoenvironmental interpretations that ranged from deep sea flysch to intertidal deposits paleocurrent data for marine and fluvial facies of the parahio formation in both zanskar and the spiti valley to the south indicate northeast sediment transport this supports the view that the parahio and overlying carbonate of the karsha formation record the ancient northern passive margin of india during the cambrian and that these strata may be distal equivalents of the younger cambrian deposits of the lesser himalaya

cenozoic and late mesozoic stratigraphy and subsidence history of the norwegian margin the stratigraphy of the passive continental margin off norway is mapped utilizing both deep sea drilling project dsdp data at five sites on the outer margin and multichannel seismic mcs data collected by lamont doherty geological observatory distinct units in the acoustic stratigraphy are tied to biostratigraphic and lithostratigraphic units in the drill data and mapped throughout most of the margin stratigraphy of the pre lower eocene section not encountered in drilling is inferred from depositional patterns and correlation with the continental shelf sequences the stratigraphy of which has been developed independently from detailed surveys the margin can be divided into three physiographic provinces two broad shelf and slope regions separated by the jan mayen fracture zone and the steep narrow shelf and slope off the lofoten islands each experienced a separate cenozoic subsidence history the voring plateau is partly underlain by oceanic crust it subsided nearly 1 000 m less than expected from thermal contraction but it did so following the development of a relatively deep water environment in the paleocene north of the voring plateau the margin may have subsided a few hundred metres more than thermal studies had predicted there is no evidence for a phase of extensional deformation associated with the inception of spreading the youngest faulted level occurs at the late jurassic level preceding sea floor spreading by nearly 100 ma and deformation in the cretaceous section is characterized by large uplift structures that may be of halokinetic origin the physiographic provinces as well as the cenozoic subsidence patterns along the margin appear to be related to the style and extent of mesozoic basin development and tectonics along the margin

a crustal study off lofoten n norway by use of 3 component ocean bottom seismographs abstract twenty four 3 component ocean bottom seismographs obs were used in august 1988 in a combined seismic refraction and reflection study off lofoten n norway the purpose of the experiment was to map the crustal structure from the continental shelf to the oceanic crust off lofoten the very high data quality is demonstrated by the strong p wave and shear wave reflections as well as converted waves from the moho observed on the continental shelf these arrivals are observed continuously from near vertical to wide angle incidence very high seismic sea floor velocities in this area 3 1 km s to 5 1 km s indicates absence or very thin sequences of mesozoic sediments the 5 1 km s refractor coincides with the base cretaceous reflection interpreted from the multichannel reflection data the crystalline continental crust is here divided in layers with velocities of 6 0 km s 6 4 km s and 6 8 km s respectively on the seaward side of the escarpment tertiary sediments varying in thickness from 1 0 1 5 km are situated on top of a 1 5 2 0 km thick layer of flood basalt containing seaward dipping reflectors a layer with velocity of 6 7 km s is observed above the lower crust which in this area is found to have a velocity as high as 7 3 km s these high velocities indicate that the crust in this area is of oceanic origin or alternatively the high velocity layer in the lower crust might represent a magmatic body underplating highly thinned and intruded continental crust seven obss were deployed in the area that was covered by landward flowing basalt deposited during the early eocene breakup between norway and greenland the survey was partly performed to investigate whether this method can be used to map structures below the basalt which is impenetrable with conventional seismic reflection techniques the obs data contain considerable information about structures below the flood basalt pre opening sediments up to 4 0 km thick are indicated below the 1 0 2 5 km thick landward flowing basalt the velocities of the crystalline portion of the crust are found to be similar to those observed under the continental shelf 6 0 6 8 km s which implies that the crust east of the escarpment is of continental origin the crystalline crust is strongly thinned in this area showing a minimum thickness of about 6 km the depth to the moho increases from about 15 km in the western part of the area to about 26 km on the continental shelf the success of the obs survey indicates that such measurements can become an important tool in investigations on passive volcanic margins and potentially in other areas where highly reflective surfaces make the reflection technique inefficient

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

melanosclerites from the late ordovician strata of the shiala formation indian gondwana abstract melanosclerites organic walled microfossils of problematic affinity are recorded herein from the lower to middle part of the shiala formation of late ordovician age and is situated below ordovician silurian boundary established previously by acritarchs data the assemblages recovered yielded the melanosclerites taxa melanoporella clava melanoporella bulla melanoporella polonica melanoporella sp 1 melanoporella sp 2 and melanosteus sp the ordovician silurian sedimentary sequence discussed herein was a part of the gondwanan palaeocontinent situated at 25 to 30 s the melanosclerite assemblage recovered herein from the shiala formation is broadly comparable with the ojlemyr cherts of sweden situated at high palaeolatitude and also of late ordovician age melanosclerites have not previously been studied in detail from indian gondwana and published records on this fossil group are rare in general the present findings improve our knowledge on this group

the rio grande fracture zone in the western south atlantic and its tectonic implications abstract a broad zone of linear mappable basement structures is observed north and northeast of the rio grande rise in the south atlantic ocean these structures lie along the same flow line as the sa o paulo ridge the florianopolis high and onshore lineaments suggesting that they all comprise the same tectonic trend the rio grande fracture zone the morphology developed along this fracture zone during the early opening of the south atlantic ocean formed a barrier to open ocean circulation during the aptian and allowed the formation of extensive evaporite deposits to the north of it

tectonic evolution of the southwest indian ocean since the mid cretaceous plate motions and stability of the pole of antarctica africa for at least 80 myr summary the southwest indian ridge is the boundary where seafloor is being created between the african and antarctic plates east of the newly recognized du toit fracture zone at 27 e it trends north easterly and is cut by a series of six deep near north south clefts marking transform faults this series starts with the dual prince edward fracture zone near 35 e and includes discovery i1 also dual at 42 e indomed at 46 e and gallieni at 52 e terminating with the spectacular atlantis i1 and melville fracture zones at 57 30 e and 60 30 e respectively the trend of these fracture zones is compatible with an instantaneous pole of relative motion for antarctica africa at 8 4 n 42 4 w between 35 e and 55 e the development of the southwest indian ridge has separated the elevated madagascar ridge from the discontinuous crozet plateau that was contiguous to it prior to eocene time in this entire sector the ocean floor overall is elevated with respect to ocean floor of equivalent age elsewhere between these features anomalies 0 22 0 53 ma have been identified north and 13 26 35 60 ma south of the ridge axis these anomalies and 0 34 0 80 ma located just to the west in the mozambique basin lie at the same distance from the ridge axis as a similar sequence east of the bouvet triple junction the similarity in distance from the ridge axis of these anomaly sequences is evidence that africa and east antarctica have been separated by a single plate boundary from 80ma to the present the pole representing the motion of these two plates at the present has not moved in totality far from the position it occupied in the late cretaceous 80 ma furthermore calculation of intermediate finite rotation poles for four intervals within this span reveals only minor 15 excursions from the present pole the spatial reconstruction of the southwest indian ridge at 80ma anomaly 34 places the ridge midway between africa and eastern antarctica and lying at right angles to the north trending mozambique ridge the separation of antarctica from africa following initial opening in the jurassic may have been as straightforward as it appears to have been from 80 ma to the present

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

breakup between australia and antarctica a brief review in the light of new data abstract the arguments justifying the revised timing of breakup between australia and antarctica cande and mutter 1982 and the reconstruction of broken ridge and kerguelen plateau mutter and cande 1983 are reviewed and considered with respect to new subsidence data the age of breakup was revised from anomaly 22 time 55 my b p to anomaly 34 time 85 my b p the rough topography of the diamantina zone can be attributed to very slow spreading 5 mm yr beginning between the times of anomaly 34 and anomaly 19 the reconstruction of broken ridge and kerguelen plateau at anomaly 34 time shows overlap of these two features but the overlap problem is nearly resolved by anomaly 18 time 42 my b p normal seafloor spreading rates 22 mm yr commenced at anomaly 19 time 43 my b p subsidence patterns calculated from biostratigraphic data from wells drilled along australia s southern margin are interpreted as more consistent with the revised age of australia antarctic breakup subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally controlled subsidence during the drift phase the timing of the rift to drift transition is believed to coincide with the age of breakup 60 to 110 my b p in addition the subsidence curves indicate a west to east propagation of breakup along the southern margin magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis

the tethyan himalaya palaeogeographical and tectonic constraints from ordovician palaeomagnetic data abstract to test whether the tethyan himalaya were part of the northern margin of india in the early palaeozoic we have produced the first primary palaeomagnetic data bedding corrected declination 267 5 inclination 63 0 95 10 pole latitude 20 2 n longitude 28 6 e from low metamorphic grade ordovician red beds in the tethyan himalaya shian formation the palaeomagnetic data are of excellent quality and a statistically positive fold test combined with a comparison with late cambrian ordovician gondwana poles suggests a primary hematite bearing magnetization acquired between 470 and 500 ma this is in excellent agreement with stratigraphic faunal and provenance age estimates and the palaeomagnetic data demonstrate that the tethyan himalaya must have been located in proximity to the indian craton during early ordovician times and are therefore consistent with a continuous margin at that time the shian formation pole overlaps with 470 500 ma gondwana poles but an even better fit can be obtained by invoking a post ordovician clockwise rotation of 13 4 such a rotation is similar in both sense and magnitude to clockwise rotations recorded in primary triassic sequences as well as palaeogene palaeomagnetic overprint data from the tethyan himalaya rotations of the tethyan himalaya compared with cratonic india are thus probably all of post late eocene age triassic and early ordovician data do not imply any crustal shortening between tethyan himalaya and cratonic india however in the early ordovician india was rotated 90 compared with its present orientation and any enlargement of india would not be detected by palaeomagnetic data

recent advances in modeling swash zone dynamics influence of surf swash interaction on nearshore hydrodynamics and morphodynamics the role of the swash zone in influencing the whole nearshore dynamics is reviewed with a focus on the interaction between surf and swash zone processes local and global hydromorphodynamic phenomena are discussed in detail and a description of the overall swash zone operation is given the effects of swash zone boundary conditions are highlighted together with the importance of surf zone boundary conditions major emphasis is placed on illustrating the interactions of various hydrodynamic modes which in turn control the swash and surf zone morphology finally methods to account for swash zone processes in coastal models with different temporal and spatial resolutions are proposed

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

the early palaeozoic geography of europe in the early ordovician three main continents are recognized by faunas in europe laurentia northwest of the closed suture of the iapetus ocean which was tropical baltica north of the general area of the trans european suture zone which was temperate and gondwana south of the tesz which was high latitude as the ordovician progressed various terranes separated and drifted away from the gondwana supercontinent at different times namely avalonia which was probably originally part of gondwana near south america iberia armorica perunica bohemia and various alpine fragments each terrane developed progressively different faunas as time went by avalonia collided with baltica near the end of ordovician confirmed by faunas tectonics and palaeomagnetism and subsequently avalonia baltica collided with laurentia to form laurussia from the mid silurian to the early devonian the ibero armorica perunica and alpine terranes did not join the european collage until the devonian the modern sites of the terrane boundaries bear little direct relationships to the original palaeogeographical boundaries in the early palaeozoic

deep seismic and geochemical constraints on the nature of rift induced magmatism during breakup of the north atlantic abstract the deep crustal structure of the norwegian and east greenland conjugate passive margins has been investigated with two ship multichannel reflection and refraction techniques in the region from the iceland faeroe ridge to the greenland fracture zone the data demonstrate the presence of thick 15 20 km igneous crust that was emplaced along the conjugate margins during the initiation of seafloor spreading seaward dipping units comprise the uppermost 3 6 km of the thick crust the thickness of igneous crust diminishes seaward with oceanic layer 2 approaching normal thickness nearer to the margin than does oceanic layer 3 wide angle reflection techniques used in conjunction with expanded spread profiling reveal deep moho reflections beneath 20 km thick oceanic crust that continue seaward into the ocean basin margins that have experienced unusually prolific magmatic activity during breakup have been termed volcanic to distinguish them from non volcanic margins that have evolved into accreting plate boundaries without excessive volcanism the origin of volcanic margins cannot be determined from seismically derived crustal thicknesses alone however thermal and chemical properties of the mantle source may be revealed in the compositions of magmatic products emplaced within volcanic passive margins a combined investigation of crustal structure and associated basalt composition could better resolve processes that result in volcanic passive margins on the voring plateau of the norwegian margin seismic lines coincide with dsdp leg 38 drill sites whole rock major element abundances in the freshest basalts are compatible with moderate extents of partial melting of mantle much like that believed to underlie modern mid ocean ridge systems and would be expected to result in the emplacement of oceanic crust about 9 2 km thick this is about the thickness of oceanic crust located seaward of the dipping basaltic units off the hatton more and voring plateau margins and 10 km south of iceland on the reykjanes ridge these results together with regional heat flow depth gravity and geoid anomalies suggest that the mantle beneath this region has experienced a moderate long lived temperature increase compared to mantle beneath normal ocean basins the much greater thicknesses of igneous crust adjacent to the margins do not appear to reflect an unusually hot flux associated with their construction since this would lead to much greater extents of partial melting than are inferred from the basalt geochemistry evidently the difference between volcanic and non volcanic margins is not simply proximity to a hot spot instead we suggest that the magmatism is closely related to the continental rifting phase tectonism resulting in an abrupt transition from continental to oceanic lithosphere may induce small scale convection that results in the circulation of a greater amount of mantle material into the pressure temperature field where partial melting begins than would occur through passive upwelling alone initial crustal thicknesses are enhanced as a result of the convective partial melting crustal thickness is expected to diminish as this secondary convection slows but the degree of partial melting of the source should not change dramatically

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

marginal faunal belts and their structural implications with examples from the lower palaeozoic fossils provide an independent criterion for testing structural hypotheses generated from field evidence or geochemistry as well as functioning in their traditional role in the dating of rocks or determining biogeographic realms on a global scale the identification of exterior oceanic biofacies may be of use in discriminating the course of former continental margins the isograptid graptolite biofacies early to middle ordovician can be distinguished from contemporary epicratonic graptolite biofacies its worldwide occurrence corresponds with continent margins reconstructed from other criteria hence a new isograptid record from irian jaya e new guinea although based on a small sample suggests that there may have been an ordovician oceanic site there on a continental scale the tectonic juxtaposition of different biofacies or of different stratigraphical packages contributes to an understanding of tectonics and can serve to quantify displacement the accretionary prism hypothesis for the southern uplands of scotland is supported and contrasted with the rearrangement of biofacies in the cambro ordovician cow head group of western newfoundland which reveals not only the original shelf margin slope topography now vanished but also provides an estimate of the movement involved in the emplacement of one taconic allochthon on a more local scale faunal evidence is used in assessing the original position of 9suspect9 marginal areas using the simplest possible model this shows for example that anglesey was not greatly separated from gondwana in the early ordovician but strike slip movement cannot be disproved local changes in relative sea level are determinable from sediments and faunas in marginal belts these may differ from the global eustatic curve and when this happens it is a measure of local tectonic activity particular sea level curves may serve to define structurally coherent units within mobile belts new sea level curves are contrasted for a segment of n wales north of the bala fault with that for central sw wales arenig llandovery the s welsh area mostly responded passively to the eustatic curve which can adequately account for shifts in facies and faunas while n wales had a curve often at odds with eustasy and reflecting its structural history

structure of a volcanic continental margin derived from ocean bottom seismographic data the northern voring margin off norway this paper presents a crustal model derived from an ocean bottom seismograph obs study along the northern voring margin off norway the profile was acquired to map the crustal structure in the northernmost part of the voring basin and to link crustal models of the lofoten and central voring basin obtained by previous obs studies the voring margin as well as the lofoten margin to the north was created by continental breakup between norway and greenland in late paleocene early eocene the rifting and continental breakup process were accompanied by intense extrusive and intrusive magmatic activities the obs data provide the whole crustal structure along the northern voring margin in the area where the deep crustal structure cannot be resolved by conventional multichannel reflection data due to sill intrusions in the sedimentary sequence the shallow part of the crustal model is characterized by up to 10 km thick sediments a sequence of flood basalts and sill intrusions the p wave velocities in the flood basalts and sill intrusions are estimated to 5 0 km s and 4 7 5 8 km s respectively the model indicates an abrupt thickening of the upper crystalline crust from approx 3 km in the ne to about 10 km towards the se with velocities of 6 0 6 2 km s the lower crustal velocities are not well resolved due to lack of clear refraction arrivals from the lower crust however the observed amplitude versus offsets are best explained by a model with a change in lower crustal velocities from 6 8 to 7 2 km s beneath the bivrost lineament the modelling infers the presence of a lower crustal reflector beneath the lineament which represents the landward continuation of the bivrost lineament reflection arrivals from the moho reveal a moho depth of 23 km in the middle of the profile and 18 20 km in the northeastern part of the profile a 370 km long crustal section from the central part of the voring basin to the lofoten margin obtained by the results of this study and previous obs studies shows a simple thinned continental crust on the lofoten margin and a high velocity lower crust underlying an upper crust of varying thickness in the voring basin the transition between these structures is situated beneath the bivrost lineament in the lower crust and beneath the basement high about 40 km south of the lineament in the upper crust

the karakorum block in central asia from ordovician to cretaceous abstract the karakorum block records a predominantly marine ordovician to cretaceous sedimentary history six major sedimentary cycles are recognised the oldest sediments early ordovician transgress over a crystalline basement the 1 km thick ordovician silurian mostly shaly succession contains rare carbonate intercalations in the early devonian a wide peritidal platform spread over the craton sedimentation rates were low 10 20 m ma and upper devonian probable lower carboniferous carbonates and clastics rest unconformably on the early devonian carbonates no evidence has been found for either late carboniferous or glaciogenic deposits sedimentation rates increased during the permian up to 50 m ma with thicknesses between 1 and 2 5 km three steps are identified 1 a huge alluvial to marginal marine terrigenous prism aggraded during the asselian sakmarian 2 the artinskian murgabian is characterised by local emergence and erosion linked to extension with block rotations 3 greater differentiation occurred from late permian to middle triassic when a peritidal carbonate flat developed in the southwest facing a deeper basin to the northeast carbonate sediments prevailed throughout the carnian norian the permo triassic evolution is interpreted as the passive margin stage of the karakorum block which previously belonged to the perigondwanian fringe when like mega lhasa it drifted northward on the tethyan transit plate mega lhasa is considered as a collage of blocks possibly separated by thinned crust or short lived seas with ocean crust quartzo lithic sandstones with grains of mafic volcanics and serpentinite overlain with gentle unconformity by pliensbachian toarcian red sandstones with sedimentary and metasedimentary clasts record the eo cimmerian deformation in the karakorum this orogenic episode was over by the aalenian when a shallow water carbonate ramp aggraded onto the previously emergent area the pre barremian upper jurassic and lower cretaceous are poorly documented in post barremian times the sedimentary succession of n karakorum was severely deformed in huge thrust sheets including slabs of crystalline basement a coarse conglomerate of mid cretaceous age sealed the thrust sheet edifice finally isolated campanian pelagic mudstones are recorded anticlockwise rotation of mega lhasa followed the late triassic docking of the iranian spur using the latter as pivot the rotation was completed during the middle jurassic when se pamir shaksgam and karakorum joined the qiangtang more closely assembling mega lhasa as it approached the asian margin

a quantitative study on the use of converted waves for sub basalt imaging the idea of imaging beneath a high velocity layer using converted waves has been popular since 1990 because these wave types have their maximum amplitudes at mid to far offsets the search for pure p waves at the highly multiple contaminated near offsets can be avoided for the atlantic margin with buried thin layered basalts our quantitative study shows that the initial single layered approach is not viable even in an unrealistic ideal geological setting the amplitude of the symmetrical psp mode is far too weak to be recognized on towed streamer data furthermore in the far offset window where locally converted waves have their strongest amplitudes there is a multitude of other reflections refractions and interbedded multiples which have similar moveouts and often higher amplitudes without the removal of these events a reliable image of the subsurface cannot be produced we show that even if this problem were solved it would be far easier to use the p wave reflection from beneath the basalt at near offsets our study shows that this wave type is by far the strongest response a borehole derived model using a thin layered basalt sequence reveals that the strongest locally converted wave has an asymmetrical path and is 10 times weaker all our results indicate that the pure p modes provide the best chance of imaging sub basalt sedimentary interfaces

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

a review of lower and middle palaeozoic biostratigraphy in west peninsular malaysia and southern thailand in its context within the sibumasu terrane abstract fossils from the cambrian to devonian rocks of southern thailand the langkawi islands mainland kedah perlis north perak and central west peninsular malaysia are listed and reviewed and their stratigraphy and correlation reassessed the hitherto anomalous record of the trilobite dalmanitina from malaysia is reviewed and found to be of latest ordovician hirnantian age rather than lower silurian age as previously reported and is considered a probable synonym of the widespread mucronaspis mucronata a new stratigraphical nomenclature is erected for part of the langkawi mainland kedah and perlis area successions in which the term setul limestone which stretched from the ordovician to the devonian is abandoned and replaced by the middle ordovician kaki bukit limestone the late ordovician and early silurian tanjong dendang formation the silurian mempelam limestone and the early devonian timah tasoh formation all underlying the paraconformity with the late devonian langgun red beds there was a single depositional basin in the generally shallow water and cratonic areas of southern thailand langkawi and mainland kedah and perlis in contrast to the deeper water basin of north perak only silurian rocks are dated with certainty within another basin in central west malaysia near kuala lumpur which were also cratonic and shallow water although to the east in west pahang there are basal devonian deeper water sediments with graptolites the area is reviewed in its position within the sibumasu terrane which in the palaeozoic also included central and northern thailand burma myanmar and southwest china part of yunnan province

crustal structure of the northern part of the voring basin mid norway margin from wide angle seismic and gravity data abstract three regional ocean bottom seismograph obs profiles with a total of fifteen recovered obss were acquired in the northern part of the voring basin mid norway margin in 1996 the data have been modelled by inversion and forward modelling of the obs vertical components p waves the velocity is found to increase downwards within the sedimentary layers due to increasing depth of burial confining pressure within the two deepest sedimentary layers there seems to be an increase of 0 5 1 km s in velocity northwestwards this increase is most likely caused by high velocity sill intrusions in the sedimentary rocks emplaced during the rift episode leading to early eocene opening of the ne atlantic the presence of significant amounts of sills seems to terminate close to the nyk high along profile 5 dip profile and close to a dome northwest of the shelf edge along profile 6 dip profile the velocity in the top of this dome is anomalously high 3 4 km s at only 1 km depth beneath the seafloor suggesting that the high consists of volcanic rocks or heavily intruded sedimentary rocks the relatively low velocities derived from within the upper crystalline crust 5 9 6 2 km s confirms that the crust is of continental origin within the lower crust there seems to be a clear increase in velocity northwestwards from about 7 0 km s to about 7 4 km s suggesting that the amount of high velocity intrusions underplating in the lower crust decreases landwards the modelling of the strike profile profile 7 suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile the model for profile 7 ties closely southwestwards to the models derived from obs data acquired in 1992 and northeastwards to lofoten obs data acquired in 1988 the models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey and that the transition zone is related to the surt lineament the distribution of magmatic rocks thus seems to correlate strongly with pre breakup structures in addition to the p wave modelling gravity modelling has been performed along all profiles the gravity modelling confirmed all main aspects of the obs models and provided constraints on the crustal structures towards the ends of the profiles

the separation of madagascar and africa identification of a sequence of east west trending magnetic anomalies of mesozoic age in the western somali basin helps define the position of madagascar in the gondwana reconstruction the anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the jurassic magnetic quiet zone the motion of madagascar relative to africa was from the north and began in the middle jurassic about the same time as the initial breakup of gondwanaland sea floor spreading ceased when madagascar assumed its present position in the early cretaceous

cambrian stratigraphy and depositional history of the northern indian himalaya spiti valley north central india recent work on himalayan tectonics indicates that prior to the cenozoic collision of india and asia an enigmatic cambrian ordovician event may have strongly infl uenced the regional geology of the himalaya stratigraphic and sedimentological analyses of well preserved cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the himalaya the parahio formation defi ned herein of the parahio valley spiti region in the tethyan himalaya of india is the best biostratigraphically resolved section of cambrian strata in the entire himalaya this formation consists of 1350 m of dominantly siliciclastic deltaic deposits the formation ranges from uppermost lower cambrian lungwangmiaoan stage to middle middle cambrian hsuchuangian stage representing a time span of 5 10 m y it contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross bedded fl uvial facies many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits the cycles are interpreted to have resulted from delta lobe switching based on a lack of systematic stratigraphic changes in cycle or facies thicknesses this paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep sea fl ysch to shallow marine tidalites in addition our paleoenvironmental analysis and paleocurrent data suggest that the uppermost lower to middle cambrian deposits of the lesser and tethyan himalaya are parts of the same ancient northward prograding fl uvial deltaic depositional system of the paleo tethys margin of india an angular unconformity with overlying ordovician conglomeratic rocks has considerable local relief with meter scale scours and a valley fi ll 100 m thick the scours have northeast southwest orientations which parallel both the paleocurrents in the underlying parahio formation as well as published paleocurrent readings from the coarse red beds of the overlying ordovician strata the cambrian ordovician unconformity is of regional extent and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in spiti is 15 m y our sedimentological analysis and associated paleocurrent data from the parahio formation along with additional data from units both above and below the unconformity indicate that published models portraying foreland basin development at this time with southward directed thrusting are problematic an alternate possibility that uplift took place south of the tethyan himalaya is also problematic because no published stratigraphic or structural evidence exists for such an uplift to the south for either the greater or the lesser himalaya lithotectonic zones

the lower palaeozoic of oman and its context in the evolution of a gondwanan continental margin the lower palaeozoic haima group of the sultanate of oman comprises a sequence dominated by siliciclastic rocks up to several kilometres thick and broadly divisible into two groups the sequence is known in outline from subsurface exploration but recent field work on outcrops in the isolated huqf area in east central oman has allowed a more detailed environmental interpretation of the succession at the base coarse continental deposits of the lower haima group rest unconformably on precambrian lower cambrian huqf group sediments mainly carbonates and evaporites the overlying upper haima group comprises from base to top the aeolian dominated amin formation the non marine coastal plain to shallow marine intertidal subtidal sediments of the andam formation the non marine to marginally marine ghudun formation and the complex cyclical deep to shallow water deposits of the safiq formation correlation of the haima group of oman with the lower palaeozoic of the arabian peninsula north africa and iran allows the evolution of the gondwanan margin to be modelled a widespread lower cambrian sequence of coarse alluvium resting on peneplained upper precambrian infracambrian huqf group strata represents the final stage of uplift and basin fill associated with the late proterozoic suturing of arabia and adjacent plates which resulted in the formation of the gondwanan continent the overlying thick sequence of continental fluvial and aeolian sediments was deposited in a series of stable intracratonic basins across the gondwanan landmass in the mid and late cambrian an overall sea level rise led to the repeated development of shallow marine shelf carbonates across the margin the subsequent replacement of this marginal carbonate sequence with a thick marginal to non marine sand dominated sequence may reflect the interplay of several events the drift of gondwana towards more southerly latitudes a phase of strong tectonic rejuvenation during the latest cambrian early ordovician and or a eustatic fall in sea level organic rich marine mudrocks were deposited over much of the gondwanan platform during a series of marked eustatic sea level rises during the mid late ordovician and early silurian in oman no evidence has yet been found for the significant late ordovician glacial event recorded from north africa and saudi arabia

late ordovician glaciation in southern turkey abstract we present a new survey of several palaeozoic sections in both the taurus range and the border folds that documents typical glacial features including a glacial pavement and striated dropstones halevikdere formation and demonstrates the former presence of an ice sheet in southern and south eastern turkey evidence for the late ashgill hirnantian age of this episode is provided by macro and microfossils found within the glacial formation the extension of ice related deposits into the northernmost part of the arabian platform mardin region implies a much wider distribution of the ordovician ice sheet than was previously believed and strongly suggests that southern turkey lay close to egypt during the lower palaeozoic

ordovician paleomagnetism of eastern yunnan china three magnetic components have been isolated in ordovician formations of the yangtze paraplatform south china block two of these daqing a and hongshiya b components yield paleopoles that conform to the carboniferous to triassic segment of the apparent polar wander path for south china and are therefore interpreted as remagnetizations the third component declination inclination 301 degree 66 degree n 5 sites k 21 4 alpha sub 95 17 degree passes the fold test and is interpreted as primary late early ordovician the paleopole at 39 degree s 236 degree e and the paleolatitude of 48 degree s support an ordovician position of south china adjacent to gondwana

review of trilobite biofacies distributions at the cambrian ordovician boundary detailed analysis of trilobite assemblages at the cambrian tremadoc transition reveals the presence of four possibly five biogeographic provinces baltic north china southeast china north american and possibly siberian one or more discrete biofacies characterize the transition in each province when plotted palaeogeographically these appear to be largely temperature controlled interdigitation of provincialized trilobite biofacies is recognized by the representation of dominant types of trilobite olenaceans ceratopygaceans remopleuridaceans dikelocephalaceans and leio stegiaceans in particular on key boundary sections these sections at nochixtlan mexico batyrbay southern kazakhstan broken skull river district of mackenzie canada and jiangshan chang shan western zhejiang china permit the correlation of biogeographic provinces globally as a result of the application of trilobite biostratigraphy relatively few problems prevent the maintenance of a cambrian ordovician boundary stratotype section in its traditional baltic biogeographic setting

crustal structure of the outer voring plateau offshore norway from ocean bottom seismic and gravity data four ocean bottom seismograph obs profiles acquired across the continent ocean transition zone of the outer voring plateau have been modeled by inversion and forward modeling of the obs vertical component traces p waves the thickness of the sedimentary layers deposited since the early eocene continental breakup varies from up to 2 2 km above the basaltic inner flows if to a minimum of 0 2 km about 20 km seaward of the voring escarpment ve with a velocity of 1 6 2 55 km s the thickness of the uppermost layer of basalt varies from 0 5 to 4 5 km and its velocity ranges from 3 4 to 5 km s the modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend 40 km seaward of the ve farther seaward another layer of extrusives intrusives is found beneath the uppermost layer of basalt the lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent ocean transition extends over a 30 50 km wide zone in between strike profile 3 oceanic 6 6 7 35 km s and strike profile 4 continental 6 1 7 0 km s an up to 7 km thick lower crustal high velocity body 7 1 7 4 km s is interpreted in terms of intrusions in the lower crust beneath and landward of the continent ocean transition the total thickness of the normal oceanic crust is estimated to be 7 9 km the anomalously thick oceanic crust reaches a maximum thickness of about 25 km and the crustal thickness is estimated to be 17 22 km beneath the if gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low

a modern assessment of ordovician chitinozoans from the shelve and caradoc areas shropshire and their significance for correlation new chitinozoan data are presented from the classical section along the onny river in the type caradoc area and from the deeper water sections in the shelve area including the former british candidate gssp for the base of the upper ordovician series the rich and well preserved chitinozoan fauna of the onny river has been a standard for 40 years but new data revise some of the identifications the assemblages are now attributed to biozones that are more readily applicable for international correlation the main part of the section can be interpreted as belonging to the originally baltoscandian spinachitina cervicornis biozone although this is uncertain in the lower part within this biozone the fungochitina actonica subzone has been defined the onny formation at the top of the section is equated with the acanthochitina latebrosa ancyrochitina onniensis biozone contrary to earlier reports acanthochitina barbata is absent the lower wood brook and spy wood brook section from the shelve inlier yielded a great number of moderately to well preserved chitinozoans but a low diversity assemblage their ranges have been neatly positioned against the well known graptolite stratigraphy in the area a local eisenackitina rhenana biozone has been recognized allowing us to suggest some international correlations

characterizing the mid faeroe ridge using seismic velocity measurements the character of the basement and overlying sedimentary sequences in the vicinity of the mid faeroe ridge is investigated through a synthesis of seismic velocity measurements a finite difference tomographic algorithm was applied to a wide aperture seismic profile acquired across the center of the faeroe basin to produce a velocity model free from a priori artifacts a series of parameterizations and smoothing constraints were run to assess the uniqueness of the final model uncertainties in the velocity structure were estimated at 5 comparison of the in situ velocity of the mid faeroe ridge with deterministic lithology velocity data extracted from nearby boreholes strongly implies that tertiary to cretaceous shale and sandstone sequences extend to depths of 4 5 km the occurrence of pre cretaceous sedimentary lithologies at 4 5 8 0 km depth is supported by in situ velocities of 4 0 4 5 km s which are best explained by well consolidated sediments and are incompatible with likely constituents of the basement evidence for a major structural discordance across the southeastern flank of the mid faeroe ridge is provided by a prominent fault whose location demarcates a lateral velocity variation of 0 5 km s a rapid increase of the in situ velocity at 8 km depth is strongly suggestive of a transition into the basement whose velocity 5 5 km s is comparable with high pressure laboratory measurements of basalts from the faeroe islands this basaltic basement in the center of the faeroe basin lies above a moho depth of 18 3 km which suggests cumulative stretching factors of about 3 0 and implies substantial igneous activity we conclude that in the region of the mid faeroe ridge the prerift lewisian basement has been highly attenuated pervasively intruded and possibly wholly replaced by basaltic material

the evolution of the rio grande rise in the southwest atlantic ocean abstract the rio grande rise a major aseismic rise in the western south atlantic ocean is composed of two portions with distinct morphologies and geological histories the western portion of the rise wrgr is a large elliptical bulge with its crest at a mean depth of about 2000 m numerous guyots protrude from its main platform the crust underlying this portion of the rise is older than 80 m y b p and was formed at a spreading center probably situated close to the sea level a widespread volcanic event affected this region during the eocene and generated several volcanic islands located at the center of the present wrgr this eocene volcanism is responsible for the unusual high elevation of this region and for the generation of the observed guyots much less is known about the eastern portion of the rio grande rise ergr it has a north south trend parallel to the present south atlantic spreading center is bounded by fracture zones is in a conjugate position to part of the walvis ridge in the eastern south atlantic and may represent an abandoned spreading center

lower palaeozoic stratigraphy and palaeontology karadere zirze area pontus mountains northern turkey lower palaeozoic rocks in the karadere zirze area east of safranbolu pontides northern turkey range from early ordovician to silurian overlying the probably tremadoc bakacak formation are aydos formation quartzites followed conformably by the karadere formation dated as early arenig to early llanvirn by means of graptolites which are assigned to seventeen genera and include three new forms eoglyptograptus bouceki prolasiograptus haplus praecursor and undulograptus mui late arenig trilobites from the karadere formation include bergamia cyclopyge dionidella leioshumardia and seleneceme in the limestone member of the overlying ketencikdere formation uncommon trilobites suggest only a mid to late ordovician age but con odonts with colour alteration index 5 6 indicate the amorphognathus tvaerensis biozone early caradoc macrofossils are rare in the siltstone member but conodonts from the middle of the unit suggest the highest subzone of the a tvaerensis biozone the youngest visible strata are on acritarch evidence at least as high as caradoc but the ashgill is not confirmed and the contact with overlying silurian rocks is unexposed the findikli formation comprises a lower member black argillites with llandovery graptolites and acritarchs and an upper member grey shales with late wenlock graptolites overlain unconformably by devonian rocks the succession differs significantly from contemporaneous deposits in southern turkey and its affinities lie with western europe including the welsh basin

paleomagnetic results from the silurian of the yangtze paraplatform gpdyke n d huang k xu g zhang w y and kent d v 1987 paleomagnetic results from the silurian of the yangtze paraplatform in d v kent and m km editors laurasian paleoma etism and tectonics teetonophysics 139 123 132 samples have been collected from sites from three localities located in sichuan and in yunnan provinces the samples were taken from limestones and red siltstones covering most of the silurian of the south china block the results from the limestone sites were disappointing and all the sites that yielded successful results were from red siltstones characteristic high temperature components were successfully isolated from fifteen of these sites the sites from xiushan county sichuan 28 48o n 108 99 0 e successfully pass the fold test at the 99 level indicating that the magnetization was acquired before folding in the mesozoic the direction obtained from this locality is directed toward the east with shallow positive inclinations d 85 6 o i 12 1 cuss 5 33 the results obtained from a single red bed locality from qijiang county sichuan 28 58 n 106 85 e yield a similar direction the results from the upper silurian of yunnan 25 5 n 103 73o e were obtained from only two sites and the isolated directions were towards the west with shallow inclinations this direction is opposed to the characteristic directions obtained from sichuan and are interpreted as a reversed silurian direction the paleomagnetic pole position determined from all sites given equal weight n 15 falls at 4 9 n lat 194 7 e long with ass 5 61 o

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

an oceanwide view of palaeogene plate tectonic events abstract the tectonic events taking place during the palaeogene are discussed ocean by ocean three predominant periods of tectonic changes are determined separating periods of continuous stable seafloor spreading these periods of change occur at the palaeocene eocene boundary 56 53 m y at the eocene oligocene boundary 42 35 m y and during the early middle eocene 35 30 m y during the course of writing this review it became very evident how much marine surveying and data analysis took place during the early 1970s this decade showed great leaps forward in our understanding of seafloor anatomy and tectonics

evolution of the lofoten vesteralen margin inferred from gravity and crustal modeling 1 the lofoten vesteralen continental margin in norway shows pronounced elongated positive and negative gravity anomalies the source of these anomalies cannot be explained by the flat bathymetry but can be explained by the crustal density anomalies that are observed we propose a boudinage mechanism to explain the generation of these density anomalies and respective crustal layer undulations free air gravity anomalies have been derived using ers 1 and geosat altimetry data and have been validated by comparison with shipborne data structural models of the margin are available from extensive seismic surveys which have been used to carry out a gravity inversion to cross validate the observed gravity anomalies with seismic data the crustal structure data reveal two wavelengths of crustal undulations the first is related to crustal blocks separated by listric faults and the second wavelength is related to an undulated moho the undulations were likely generated during the opening of the north atlantic 56 myr ago and were particularly formed during the transition phase between rifting and spreading the dominating wavelengths of the undulations are 46 and 109 km respectively to explain the evolution of crustal undulations in an extensional stress regime we use a two dimensional finite element model a viscoelastoplastic power law rheology is used to study the growth of instabilities developing in a three layer lithosphere prescribed random vertical undulations show wavelength dependent growth under extension and the dominating wavelengths are 18 65 and 129 km respectively these results are in good agreement with the gravity observations and provide a strong indication that the proposed boudinage mechanism influences the evolution of the passive margin segment

weathering flux and co2 consumption determined from palaeosol sequences across the eocene oligocene transition abstract rates of base cation depletion are determined for late eocene and early oligocene alluvial palaeosols pacific northwest of north america from pedogenic mass balance geochemistry and time estimates of palaeosol sequences pedogenic strain volume change and mass transfer of base cations silica and other pertinent elements are calculated from the geochemical differences between palaeosol horizons and parent material parent material compositions are determined by identification of the least altered alluvial strata in the sequence time spans of sequences of palaeosols are estimated from 40ar 39ar age dates of interbedded tuff beds and from time of formation estimates of palaeosols based on their degree of development palaeosol sequences from the mid latitude setting of central oregon document a significant decrease in the degree of weathering of individual palaeosols across the eocene oligocene transition as the climate became cooler and drier this decrease in the degree of weathering is accompanied by an increase in the accumulation rate of alluvial soils as indicated by sedimentation rates of stratigraphic packages according to these calculations and estimates the weathering flux of base cations in the john day basin measured in grams per 1000 years per square centimetre increased across the eocene oligocene transition these results when attributed to climate change indicate that weathering flux at this local increased as climate changed from warm humid late eocene conditions to cooler drier oligocene conditions furthermore the extrapolation of these weathering flux results to other mid latitude settings with similar climate change history may help to explain the severe climate change of the eocene oligocene boundary it is proposed that in some mid latitude settings greater quantities of less weathered silicate material were exposed during the early oligocene than during the late eocene thus causing an increase in flux of weathered product increased weathering flux during this time in certain regions of the mid latitudes may have increased both co2 consumption and bicarbonate transport to the ocean and contributed to atmospheric pco2 drawdown and global cooling

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

further paleomagnetic results from the silurian of the yangtze block and their implications abstract silurian redbeds were sampled at two localities in northeastern guizhou and yunnan provinces of china for paleomagnetic study progressive thermal demagnetization revealed multiple magnetic components from these samples a low unblocking temperature component a of recent origin a post folding middle to high unblocking temperature component b probably acquired during late mesozoic time and a pre folding high unblocking temperature component c two statistically distinguishable groups of directions were further recognized from the c component with one c1 having more easterly declination than the other c2 and both having a of shallow inclination the paleomagnetic pole from the c1 magnetization coincides with a pre folding pole reported from the coeval rocks in neighboring southern sichuan province its relation to the apparent polar wander path apwp for the yangtze block yb suggests that the c1 magnetization was probably acquired during the silurian by the same argument the c2 magnetization was probably acquired sometime between the silurian and carboniferous these results reconfirm the equatorial position for the yb during mid paleozoic time and clockwise rotation of the yb since at least the mid cambrian