middepth flow near hydrothermal venting sites on the southern juan de fuca ridge long term moored current measurements around the southern end of the juan de fuca ridge show that flow parallels the ridge and may be intensified by its presence the mean flow is northward on the west side and southward on the east thus plumes emanating from the ridge probably are advected along the ridge before flowing away from the ridge other topographic features may be responsible for westward or eastward turning along the west side of the ridge we hypothesize a regional flow pattern with southward flow north of axial seamount northward flow south of axial and a convergence somewhere south of axial resulting in westward flow along the east side the flow may be southward everywhere above the ridge crest the long term mean flow is weaker and more variable and it includes tidal inertial and a dominant broadband 4 day oscillation this 4 day energy is maximum in the rift valley where the inertial is least decreases upward and away from the ridge and is more energetic in winter this oscillation is predominately a clockwise rotating feature which along with the presence of inertial oscillations suggests that it may be storm induced from the sea surface these observations along with those by cannon and pashinski 1990 suggest northward propagation flow variations across the ridge lasting one to a few months have mean speeds of 1 3 cm s and are capable of transporting plumes off axis to the west or east into the northerly or southerly flows respectively

nature of the continent ocean transition on the non volcanic rifted margin of the central great australian bight a region of 50 120km width defines the continent ocean transition cot in the central great australian bight it is characterized by a thin apron of post break up sediments overlying complexly deformed sediments and intruded crust bounded landward by a basement ridge complex and oceanward by rough oceanic basement recently acquired deep reflection and refraction seismic data have significantly enhanced understanding of the cot and basement ridge modelled gravity and magnetic data and features interpreted from seismic data are consistent with aspects of extensional and break up models proposed for the west iberia margin many of the features and relationships observed beneath the outer margin of the central great australian bight can be explained by extension within a lithosphere scale pure shear environment involving four layers brittle upper crust and upper mantle and ductile lower crust and lower lithospheric mantle the cot is interpreted to be underlain by extended continental lithosphere thus the continentocean boundary is unequivocally defined between oceanic crust and the cot and appears to be associated with sea floor spreading magnetic anomaly 33 indicating that break up and sea floor spreading did not commence until c 83 ma early carnpanian time later than the currently accepted 95ma age the major part of the basement ridge complex is probably a combination of serpentinized peridotites and mafic intrusions or extrusions derived by mantle upwelling and limited partial melting the magmatic products of this process probably cooled during chron 34 producing a distinctive magnetic anomaly but one that does not relate to break up and sea floor spreading

simple shear conjugate rift margins of the argentine precordillera and the ouachita embayment of laurentia the argentine precordillera was rifted from the ouachita embayment of laurentia during cambrian time the ouachita rifted margin along the texas promontory of laurentia exhibits a narrow zone of transitional crust a lack of synrift rocks and a thin passive margin succession indicating slow and limited postrift subsidence in contrast data from the western margin of the precordillera suggest more extensive synrift sediment accumulations and document a thick passive margin succession indicating more rapid and relatively greater postrift subsidence than along the ouachita margin of laurentia passive margin deposition began in latest early cambrian time around the precordillera but it did not begin before latest middle cambrian time around the ouachita embayment the contrasts in structure and stratigraphy are best explained by an asymmetric rift system in the context of simple shear low angle detachment models for continental rifting differences in subsidence rates and inferred crustal structure suggest that the rifted margin of the ouachita embayment represents an upper plate configuration whereas the conjugate margin on the western precordillera represents a lower plate configuration

the carolina terrane in northwestern south carolina u s a late precambrian cambrian deformation and metamorphism in a peri gondwanan oceanic arc the carolina terrane comprises an exotic volcanic island arc in the hinterland of the southern appalachian orogen the western portions of the carolina terrane consist of zoned mafic ultramafic plutonic complexes intruded into a pile of basalts and basaltic andesites this sequence of rocks has been interpreted to represent an episode of intra arc rifting prior to regional metamorphism and deformation new u pb zircon ages from the mean crossroads complex in northwestern south carolina along the central piedmont suture confirm relative ages inferred from detailed mapping two foliated metadiorites yield u pb dates of 579 4 and 571 16 ma interpreted to be crystallization ages a foliated metaquartz diorite yields a u pb date of 538 5 ma interpreted to be a crystallization age an undeformed unmetamorpbosed diorite intruding these metamorphosed zoned complex intrusive rocks yields an age of 535 ma hence we believe that intra arc rifting and regional metamorphism and foliation development both occurred between 580 and 535 ma while petrographic and ar ar studies support subsequent middle to late paleozoic regional metamorphic overprint s or at least static recrystallization and or uplift through hornblende biotite muscovite blocking temperatures for ar the 535 ma undeformed unmetamorphosed intrusion suggests late precambrian early cambrian regional metamorphism and deformation was the event responsible for the regional metamorphic fabric in this area of the piedmont these observations contradict the idea that this metamorphism and fabric development are related to presumed early paleozoic accretion of the carolina arc to laurentia as well as correlations with middle ordovician fabric elements in the eastern piedmont instead this fabric is interpreted to record a changing plate kinematics e g migration of triple junction or change in relative motion vectors during semicontinuous diachronous development of an island arc on the gondwanide margin

analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

control of barrier island shape by inlet sediment bypassing east frisian islands west germany abstract a study of the east frisian islands has shown that the plan form of these islands can be explained by processes of inlet sediment bypassing this island chain is located on a high wave energy high tide range shoreline where the average deep water significant wave height exceeds 1 0 m and the spring tidal range varies from 2 7 m at juist to 2 9 m at wangerooge an abundant sediment supply and a strong eastward component of wave power 4 4 103 w m 1 have caused a persistent eastward growth of the barrier islands the eastward extension of the barriers has been accommodated more by inlet narrowing than by inlet migration it is estimated from morphological evidence that a minimum of 2 7 105 m3 of sand is delivered to the inlets each year via the easterly longshore transport system much of this sand ultimately bypasses the inlets in the form of large migrating swash bars the location where the swash bars attach to the beach is controlled by the amount of overlap of the ebb tidal delta along the downdrift inlet shoreline the configuration of the ebbtidal delta in turn is a function of inlet size and position of the main ebb channel the swash bar welding process has caused preferential beach nourishment and historical shoreline progradation along the east frisian islands this process has produced barrier islands with humpbacked bulbous updrift and bulbous downdrift shapes the model of barrier island development presented in this paper not only explains well the configuration of the german barriers but also the morphology of barriers along many other mixed energy coasts

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

hydrodynamics and sedimentation in wave dominated coastal environmentscontrol of barrier island shape by inlet sediment bypassing east frisian islands west germany a study of the east frisian islands has shown that the plan form of these islands can be explained by processes of inlet sediment bypassing this island chain is located on a high wave energy high tide range shoreline where the average deep water significant wave height exceeds 1 0 m and the spring tidal range varies from 2 7 m at juist to 2 9 m at wangerooge an abundant sediment supply and a strong eastward component of wave power 4 4 103 w m 1 have caused a persistent eastward growth of the barrier islands the eastward extension of the barriers has been accommodated more by inlet narrowing than by inlet migration it is estimated from morphological evidence that a minimum of 2 7 105 m3 of sand is delivered to the inlets each year via the easterly longshore transport system much of this sand ultimately bypasses the inlets in the form of large migrating swash bars the location where the swash bars attach to the beach is controlled by the amount of overlap of the ebb tidal delta along the downdrift inlet shoreline the configuration of the ebbtidal delta in turn is a function of inlet size and position of the main ebb channel the swash bar welding process has caused preferential beach nourishment and historical shoreline progradation along the east frisian islands this process has produced barrier islands with humpbacked bulbous updrift and bulbous downdrift shapes the model of barrier island development presented in this paper not only explains well the configuration of the german barriers but also the morphology of barriers along many other mixed energy coasts

deep in the heart of dixie pre alleghanian eclogite and hp granulite metamorphism in the carolina terrane south carolina usa the central part of the carolina terrane in western south carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the carolina slate belt to the se and amphibolite facies metavolcanic and metaplutonic rocks of the charlotte belt to the nw this region termed the silverstreet domain is characterized by penetratively deformed felsic gneisses granitic gneisses and amphibolites mineral assemblages and textures suggest that these rocks formed under high pressure metamorphic conditions ranging from eclogite facies through high p granulite to upper amphibolite facies mafic rocks occur as amphibolite dykes as metre scale blocks of coarse grained garnet clinopyroxene amphibolite in felsic gneiss and as residual boulders in deeply weathered felsic gneiss inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies con ditions all samples have been partially or wholly retrograded to amphibolite assemblages we infer the following p t t history 1 eclogite facies p t conditions at 1 4 gpa 650 730 c 2 high p granulite facies p t conditions at 1 2 1 5 gpa 700 800 c 3 retrograde amphibolite facies p t conditions at 0 9 1 2 gpa and 720 660 c this metamorphic evolution must predate intrusion of the 415 ma newberry granite and must postdate formation of the charlotte belt and slate belt arcs 620 to 550 ma comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis eclogite and high p granulite facies metamorphism in the silverstreet domain may coincide with a 570 535 ma event documented in the western charlotte belt or to a late ordovician early silurian event the occurrence of these high p assemblages within the carolina terrane implies that prior to this event the western carolina terrane charlotte belt and the eastern carolina terrane carolina slate belt formed separate terranes the collisional event represented by these high pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to laurentia

the 18 6 yr nodal cycle and its impact on tidal sedimentation abstract the 18 6 yr nodal cycle modulates tidal amplitudes and currents and consequently sedimentation in tide influenced sedimentary environments data are presented which show that such effects are obvious along the coast of the dutch barrier islands and in the sedimentary fill of abandoned channels

relations between sill intrusions and bedding parallel extensional shear zones in the mid continent rift system of the lake superior region abstract the mid continent rift system is a continuous u shaped feature extending from kansas north through the lake superior region and then south through michigan the logan sills on the northwest side of the lake were intruded during the rifting stage 1109 ma deformation features in the country rock adjacent to the logan sills indicate two episodes of shear strain the first consists of minor thrusts and mesoscopic shear bands both suggesting top to the south southwest movement parallel to the bedding the second rotates the previously formed shear bands in the opposite sense top to the north northeast and is localized at the basal contact between the sills and the country rock we attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding parallel shear zones that are the surface expression of a south dipping detachment plane the detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting in this scenario the first episode of shear strain might be related to bedding parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south the polarity of both events is supported by small scale kinematic indicators such as veins fiber growths and foliation

a 6 year time series of hydrothermal plumes over the cleft segment of the juan de fuca ridge the causal links between magmatic tectonic processes and hydrothermal venting along the mid ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity i report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the cleft segment of the juan de fuca ridge hydrothermal temperature and light attenuation c anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991 intensity of and c in the chronic plume overlying the north cleft vent field showed more variability between years than within a given year both and c were exceptionally high in 1986 fell to moderate levels during 1988 to 1990 and declined to minimum values in 1991 the chronic plume over the south cleft vent field varied similarly superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height event plumes at the north cleft field ranged from the 1986 megaplume with a rise height exceeding 1000 m to smaller events in 1987 and 1989 with rise heights of 400 m chronic hydrothermal heat 660 277 1 mw and mass 204 86 1 g s 1 fluxes from the north cleft field were determined by assuming these fluxes were equivalent to the long term net advection of a mean cross section of the vertically integrated and c in the plume this 6 year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly though not consistently between 1986 and 1991

the butterfly delta system of qiongzhou strait morphology seismic stratigraphy and sedimentation abstract analysis of high resolution sub bottom profiles topographical data and sediment samples reveals that the tidal delta system of qiongzhou strait is morphologically a butterfly delta system consisting of western mouth tidal delta wmtd and eastern mouth tidal delta emtd wmtd covers an area of about 11 000 km 2 and is more than two times larger than emtd 4000 km 2 wmtd is fan shaped with a tributary branch extending southwards along hainan island while emtd is typically fan shaped seismic architectures of wmtd and emtd are both characterized by a thick progradational unit overlying a thin transgression unit both wmtd and emtd start to develop at about 8 7 cal ka bp when qiongzhou strait completely opened and have the same three sedimentary facies tidal delta top tidal delta slope and tidal delta toe the formation of wmtd and emtd is attributed to qiongzhou strait s hydrodynamic regime which is dominated by the strong east west reciprocal tidal currents and influenced by the year round westward mean flow modified by the wind driven seasonal currents

on the dynamics of current jets trapped to the flanks of mid ocean ridges 1 time mean abyssal current observations over the flanks of the east pacific rise epr at 9 10 n and at the juan de fuca ridge at 45 n document the occurrence of paired along ridge current jets that are trapped to the ridge flanks and sheared across the ridge in an anticyclonic sense a coincident feature where local hydrothermal discharge effects are not in play is the upward bowing of isopycnals over ridge crests and isopycnals plunging into ridge flanks it would be tempting to explain the jets primarily as geostrophic responses to the doming plunging isopycnal distribution though that should lead to the question as to how the isopycnal perturbations originate a numerical model of time dependent flow on a cross ridge x z transect forced in a way to be consistent with a yearlong hourly sampled record of currents measured at the epr ridge crest is used to investigate some of the underlying physics it will be shown that the jets can arise from oscillatory flows via eddy momentum eddy heat and eddy salt fluxes that ultimately cause the isopycnals to dome over the ridge as the probable offspring of velocity velocity and velocity density correlations that depend upon oscillatory motion the jets are likely examples of stratified topographic flow rectification an ancillary feature is a slight yearlong averaged downward current o 0 1 0 5 mm s over the epr ridge crest that crosses the time mean upward bowing isopycnals in a counter intuitive vertical direction

gravimetric determination of the continental oceanic boundary of the argentine continental margin from 36 s to 50 s fil arecco maria alejandra universidad de buenos aires facultad de ingenieria argentina

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

mean hydrographic and velocity sections near pipe organ vent at juan de fuca ridge a composite cross ridge hydrographic and velocity section near the pipe organ vent site on the juan de fuca ridge is presented the hydrographic section is composed of surveys obtained from 1990 to 1994 the potential temperature field shows pronounced downward sloping of the isotherms below 1 9 centered at about 150 m above the bottom on either side of the ridge crest the horizontal scale is about 2 km comparable to the local rossby deformation radius the maximum potential temperature anomaly is greater than 0 03 c significant light attenuation anomalies are also present the mean along ridge velocity section is constructed from the records of 85 current meters the mean flow is southward at and to the east of the vent and northward to the west largest velocities occur at the depth of the neutrally buoyant plume the cross ridge shear is thus anti cyclonic with a characteristic value of 0 05f geostrophic velocity fields obtained from the density section differ significantly from the field produced from the current meter data an estimate of the net mean flux of anomalous heat gives 780 400 mw to the south

variation in ebb delta morphologies along the west and east frisian islands the netherlands and germany abstract ebb delta morphologies in the west and east frisian islands differ mainly in terms of the direction and the degree of asymmetry of the ebb deltas in the east frisian islands the ebb tidal deltas and their main ebb channels are asymmetrically directed downdrift to the east in the west frisian islands they are asymmetrically directed updrift to the west except for a few of the small ebb tidal deltas which are asymmetrically directed downdrift earlier theories for the east frisian islands considered wind driven waves coming dominantly from the west to be the main cause of the downdrift asymmetry of the ebb tidal deltas theories for the west frisian islands related the updrift directed ebb tidal deltas to tidal currents none of these theories can explain the morphological variation in the ebb tidal deltas for both the west and east frisian islands observation of the ebb delta morphologies of the west and east frisian islands in relation to tidal and wave regimes shows that the above mentioned difference in the ebb delta morphologies is due to the varying importance of the tidal prism and the wave induced longshore drift for the different inlets the difference is especially pronounced between the large and the small ebb tidal deltas because their tidal prisms are different the inlets with a large tidal prism have large ebb tidal deltas and they are dominated by tidal currents interaction of marine shore parallel tidal currents with onshore offshore inlet tidal currents which is enhanced by the ebb delta shoals causes the large ebb tidal deltas and their main ebb channels to be asymmetrically directed updrift to the west the inlets with a small tidal prism have small ebb tidal deltas and they are dominated by waves eastward moving deep water wave and wave induced longshore drift drive the small ebb tidal deltas asymmetrically downdrift to the east the general model proposed not only explains the varying morphologies of the ebb tidal deltas of the west and east frisian islands but is also applicable to other ebb tidal deltas

confirmation of the carolina slate belt as an exotic terrane an assemblage of middle cambrian atlantic faunal province trilobites has been found in the rocks of the carolina slate belt near batesburg south carolina geologic and paleomagnetic data suggest that the carolina slate belt and the adjacent charlotte belt constitute an exotic terrane that was accreted to north america in early to middle paleozoic time

sand transport patterns in the ebb tidal delta off texel inlet wadden sea the netherlands abstract sand transport patterns in the ebb tidal delta of texel inlet were studied using current and wave measurements calculations of sediment transport rate bedform orientations dips of cross beds and the morphological evolution a qualitative picture of sand transport patterns was formed sand supplied from the north sea and the adjacent coast is transported in and out through several neighboring flood and ebb dominated channels in the southwestern ebb delta along the seaward margin of the ebb tidal delta of texel inlet sand is transported in both directions parallel to the coast by tidal currents but predominantly northwards and landwards some sand is desposited where a weak rotational tidal current pattern is produced north of the inlet by the interruption of tidal currents parallel to the coast by the inlet waves transport the sand further onshore over the swash platform some of it returns to the main circulation of the inlet through the molengat sand transport through the inlet is flood dominated it is revealed that not only the interaction of tidal currents and waves but also the interaction between the tidal currents in the north sea and through the inlet greatly influence the system

structural characteristics of the northern okinawa trough and adjacent areas from regional seismic reflection data geologic and tectonic implications abstract analysis of regional multi channel seismic data from the northern okinawa trough and adjacent shelf provides some important constraints on the structural development of the area the sedimentary strata in the northern east china sea shelf basin separated from the okinawa trough by the taiwan sinzi belt are affected by the miocene compressional tectonism and truncated by the late miocene unconformity in contrast those in the western margin of the northern okinawa trough are cut by numerous normal faults and the late miocene horizon forms a conformable surface this suggests that the taiwan sinzi belt acted as a buttress for the northern okinawa trough against the compressional tectonism our data also reveal the ho basin in the western margin of the northern okinawa trough previously known only from proprietary industry data the longwan ridge lying between the ho basin and the northern okinawa trough may be the youngest of the relict arcs or buried ridges in the area the abrupt along strike change in the fault polarity near the northern margin of the depocenter of the ho basin suggests a nw trending left lateral transfer zone or fault the western boundary of the northern okinawa trough is characterized by the gently deepening seafloor whereas that of the southern okinawa trough is marked by steep border faults suggesting more rapid subsidence in the south this together with the well developed symmetric axial faults in the south and the greater width and the lack of well developed axial zone in the north may suggest focused rifting extension in the southern okinawa trough and diffuse rifting extension in the northern okinawa trough we postulate that the diffuse rifting extension in the northern okinawa trough is due to the tectonic perturbation caused by the convergence or subduction of the topographically high and buoyant amami plateau at the northern ryukyu arc

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

the ellsworth terrane coastal maine geochronology geochemistry and nd pb isotopic composition implications for the rifting of ganderia the ellsworth terrane is one of a number of fault bounded blocks that occur along the eastern margin of ganderia the westernmost of the peri gondwanan domains in the northern appalachians that were accreted to laurentia in the paleozoic geologic relations detrital zircon ages and basalt geochemistry suggest that the ellsworth terrane is part of ganderia and not an exotic terrane in the penobscot bay area of coastal maine the ellsworth terrane is dominantly composed of bimodal basalt rhyolite volcanic sequences of the ellsworth schist and unconformably overlying castine volcanics we use new u pb zircon geochronology geochemistry and nd and pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the ellsworth terrane and its relationship with ganderia u pb zircon geochronology for rhyolites indicates that both the ellsworth schist 508 6 0 8 ma and overlying castine volcanics 503 5 2 5 ma are middle cambrian in age two tholeiitic basalt types are recognized type tb 1 basalt present as pillowed and massive lava fl ows and as sills in both units has depleted la and ce la nd n 0 53 0 87 values fl at heavy rare earth element ree values and no positive th or negative ta anomalies on primitive mantle normalized diagrams in contrast type tb 2 basalt present only in the castine volcanics has slightly enriched lree la yb n 1 42 2 92 values and no th or ta anomalies both basalt types have strongly positive e nd 500 values tb 1 7 9 8 6 tb 2 5 6 7 0 and relatively enriched pb isotopic compositions 206

dip analysis as a tool for estimating regional kinematics in extensional terranes abstract in basin analyses that rely almost solely on seismic reflection data to configure the underlying structure it has become common practice to use extensional fault geometries to infer kinematic histories this procedure relies on the existence of fault geometries predicted by ideal orthogonal extension and assumes the structure is well constrained in areas where data are sparse or of equivocal quality this often leads to circular analysis that is ideal orthogonal extension is assumed to have been operative then under constrained fault geometries are mapped according to model predictions fault geometries so derived are subsequently used as evidence that orthogonal extension was operative and that the direction of extension is determinable we develop an alternative tool for inferring kinematics which is independent of fault geometries the method relies on the empirical evidence that the dip of the hangingwall s is systematic and predictable in extensional systems even when fault geometries are non ideal although not without significant limitations dip analysis can provide additional constraints for structural and kinematic interpretations especially where seismic reflection data are the main source of information and dip domains define discrete crustal blocks

on subinertial oscillations trapped by the juan de fuca ridge northeast pacific spectra of currents measured along the 2100 m deep juan de fuca ridge in the northeast pacific have prominent tidal inertial and weather band 3 7 day period spectral peaks the weather band peak in particular has a number of interesting features it is strongest in close lateral proximity to the ridge and strongest near ridge depth intensification near the ridge is characteristic of trapped motion spectral peak intensities vary seasonally with largest amplitudes occurring in autumn and winter seasonal variation suggests that surface weather is forcing flow at depth together trapping and seasonality indicate opportunistic amplification of oscillatory motion at the ridge at all frequencies forcing topography and stratification together shape current and hydrographic distributions near the ridge effects of those interactions for subinertial motion are quantified here using a primitive equation numerical model forcing period 1 10 days and friction are the principal dependencies examined results show that flow over the juan de fuca ridge can be amplified by factors of 3 4 for diurnal and up to a factor of 7 for weather band frequencies amplification is ridge trapped with effects extending many hundreds of meters upward and extending 5 10 km laterally for 5 day period flow the strength and area of amplification increase with increasing period over the 1 to 10 day period band oscillatory weather band flow leads to vertical velocities on the order of 0 3 cm s 1 on ridge flanks which in turn cause periodic temperature t and salinity s variations with amplitudes of as much as 0 05 c and 0 01 practical salinity units respectively the vertical motion and consequent vertical displacement 100 m lead to isotherms that plunge below the crest alternately each side of the ridge a distribution observed in conductivity temperature depth transect data near the ridge crest cross ridge baroclinic pressure gradients caused by cyclically plunging isopycnals are in near geostrophic balance with the coriolis force associated with along ridge flow along ridge current amplification is therefore closely tied to isopycnal movement up and down ridge flanks since weather band oscillations lead to much larger cross ridge baroclinic pressure gradients than do diurnal motions of the same amplitude weather band forcing causes greater along ridge current amplification results vary strongly with forcing period and direction of forcing with respect to the ridge but depend only moderately on friction and turbulent mixing coefficients

interactions between tidal flows and ooid shoals northern bahamas abstract oolitic strata are common through the geologic record ooids generally form in high energy environments but it remains unclear how ooids remain in such systems and form geomorphic features such as bars and shoals integrating remote sensing hydrodynamic bathymetric granulometric and field observations of modern tidal systems in the bahamas provides insight into this fundamental question oolitic tidal sands in the northern abacos display a geomorphic pattern in which bedrock islands restrict and focus tidal flow down a main channel within the channel a shallow shoal separates an ebb dominated subchannel from a flood dominated subchannel tidal velocities in these subchannels can exceed 1 m s enough to transport the oolitic sediments hydrodynamics and bathymetry in these subchannels produce a net circular hydrodynamic pattern around the shoal the spin cycle allowing the sands to remain in motion without being transported out of the ooid factory this general pattern is apparent in several other ooid shoal complexes this concept provides integrated insights into the physical influences impacting the formation suspension transport and deposition of ooids and the resulting geomorphic forms these results represent first steps toward developing more comprehensive and predictive analogs of spatial heterogeneity in ancient tidally dominated oolitic shoals

surface sediments and sequence models in the ebb tidal delta of texel inlet wadden sea the netherlands abstract grain size and primary sedimentary structures in the ebb tidal delta of texel inlet are described with the emphasis on the relations with tidal and wave processes grain size distribution curves represent two major types wave dominated very well sorted sand with dominantly saltation population and tide dominated less well sorted with relatively large suspension and traction populations areal distribution of mean grain size sorting and different distribution curves is closely related to sediment transport patterns the deep inlet channel and the offshore area serve as source areas for the ebb tidal delta and have coarse relatively poorly sorted sediments deposition of very well sorted fine sand occurs on the shallow ebb delta shoal the size and sorting of the sands are the result of selective transport and wave processes sedimentary structures formed by waves and tidal currents are distinctly distributed over the shallow ebb delta shoal the channels and the adjacent inner shelf the areal distribution of mud drapes in the ebb tidal delta is a combined effect of the local tidal current pattern and the related locally high concentrations of fine grained suspended sediment alternation of tidal current dominance during and immediately after storms with wave dominance during fair weather conditions results in the formation of storm deposits at specific locations in the ebb tidal delta i e seaward of the main ebb channel on the basis of this study sequential models are proposed for the migrational inlet channel the progradational ebb delta lobe and the landward migrating ebb delta shoal the transgressive sequence formed after abandonment of the ebb delta lobe

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

a forced baroclinic vortex around a hydrothermal plume a steady frictional flow around a hydrothermal plume is described which is forced by the entrainment of water into the buoyant part of the plume as it rises above its source the circulation is anticyclonic clockwise in the northern hemisphere near the equilibrium density level or spreading level of the plume and cyclonic below this level the strength of this flow is adequate to trap anomalous properties from the hydrothermal fluid near the source in the presence of weak background flow some observational support for such a flow is presented

application of the critical coulomb wedge theory to hyper extended magma poor rifted margins the critical coulomb wedge theory ccwt has been extensively used in compressional tectonics to resolve the shape of orogenic or accretionary prisms while it is less applied to extensional and gravitational wedges despite the fact that it can be described by the same equation in particular the hyper extended domain at magma poor rifted margins forming the oceanward termination of extended continental crust satisfies the three main requirements of the ccwt 1 it presents a wedge shape 2 the rocks forming the wedge are completely brittle frictional and 3 the base of the wedge corresponds to a low friction decollement however hyper extended margins present a fully frictional behaviour only for a very thin crust therefore this study is limited to the termination of hyper extended continental crust which deforms in the latest stage of continental rifting in this paper we define a method to measure the surface slope and the basal deep of this wedge that we apply to 17 hyper extended magma poor rifted margins in order to compare the results to the values predicted by the ccwt because conjugate pairs of hyper extended magma poor rifted margins are commonly asymmetric due to detachment faulting the wedges in the upper and lower plate margins corresponding respectively to the hanging wall and footwall of the detachment system are different while the stress field in the upper plate wedge corresponds to a tectonic extensional wedge the one in the lower plate matches that of a gravity extensional wedge using typical frictional properties of phyllosilicates e g clays and serpentine the shape of the hyper extended wedges can be resolved by the ccwt using consistent fluid overpressures our results show that all lower plate margins are gravitationally stable and therefore have a close to critical shape whereas the tectonic extensional wedges at upper plate margins are critical sub or sup critical due to the detachment initial angle and the duration of the tectonic activity in this paper we discuss the geometry and structural evolution of the most distal parts of hyper extended continental margins and the formation of extensional allochthons during hyper extension using the ccwt theory

dynamics of continental rift propagation the end member modes abstract an important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation in regions of rift propagation specifically transition zones from continental rifting to seafloor spreading it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones the focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere more specifically this study explores how modeled rift behavior depends on the value of rheological parameters of the crust a three dimensional finite element model is used to simulate lithosphere deformation in an extensional regime the chosen approach emphasizes understanding the tectonic forces involved in rift propagation dependent on plate strength two end member modes are distinguished the stalled rift phase is characterized by absence of rift propagation for a certain amount of time extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone which requires a buildup of shear stresses near the rift tip and significant intra plate deformation this stage represents a situation in which a rift meets a locked zone localized deformation changes to distributed deformation in the locked zone and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend in the alternative rift propagation mode rift propagation is a continuous process when the initial crust is weak the extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses model predictions address aspects of previously unexplained rift evolution in the laptev sea and its contrast with the tectonic evolution of for example the gulf of aden and woodlark basin

sand dynamics at the mouth of a rock bound tide dominated estuary abstract the lowermost kennebec river is a relatively narrow deep bedrock cut estuary situated along the highly indented central coast of maine a large tidal prism and strong tidal currents control bedload sediment transport at the estuary mouth riverine sediment is introduced to the estuary during spring freshets when freshwater discharge may increase by an order of magnitude during the same time some of the existing estuarine sediment is exported to the nearshore a barrier adjacent to the estuary is the product of an abundant riverine sand supply and numerous bedrock ridges which anchor this sand deposit a variety of sedimentologic geomorphic and hydraulic data collected over a 14 year period document a clockwise sand circulation cell that involves the exchange of bedload among the entrance channel to the estuary adjacent beaches nearshore and offshore region tidal currents transport sand from the estuary through a subsidiary channel bordered on both sides by bedrock islands to an offshore subtidal bar northeast storm waves breaking along this outer bar move sand westward and landward toward the beach leading to the development of large bar complexes the welding of a bar complex to the western end of hunnewell beach occurs every 6 9 years and adds 2 300 000 m3 of sand to the shoreline sediment transport calculations along the outer bar are consistent with this volume of sand the sediment gyre is completed as beach sands are moved eastward along the beach and dumped into the estuarine channel this movement of sand is produced by wave action and tidal currents that enter the estuary peripherally during the flooding tide during the ebb cycle strong tidal currents flowing seaward through the subsidiary channel advect water from along the beach which enhances sand transport into the estuary thirteen years of beach profile surveys demonstrate a good correspondence between bar welding events and short term 1 4 years accretionary shoreline changes however the volumetric changes of hunnewell beach cannot be accounted for by bar welding alone the apparent movement of sand between hunnewell beach and other sand is further evidenced by the 135 year record of historical shoreline changes that reveal up to 200 m of shoreline excursions over a 26 year period these data suggest that erosional depositional trends along hunnewell beach are complex and the result of the exchange of sand among the adjacent beaches nearshore zone fox and wood island tombolos kennebec river channel and offshore region the effects of major spring floods and strong bedrock influences produce patterns of sedimentation and a distribution of sand bodies at the mouth of kennebec river that differ markedly from accepted estuarine models that are based on the relative importance of wave and tidal energy this study demonstrates the need to consider structural controls when using these models

holocene pleistocene interface and three dimensional geometry of the ebb delta complex texel inlet the netherlands abstract the holocene geological history in the area of texel inlet and its ebb tidal delta in particular the holocene pleistocene interface is studied using shallow marine seismic records and borehole data it is shown that the ebb delta sand body erosionally unconformably lies on deposits of the upper pleistocene i e from the south to the north weichselian eemian and saalian the sand body is up to 30 m thick in the central part of the ebb tidal delta and radially pinches out seaward the sand body covers an area of 20 km in a n s direction and 10 km in the seaward direction in general the ebb delta sand body has a downward convex lower surface and an upward convex upper surface the total volume of the ebb delta sand body is about 1187 106 m3 the delta lobes of the sand body contain low angle foresets 0 8 1 5 which are recognized on the seismic records the origin and growth of the ebb tidal delta is shown by means of the characteristics of the erosional surface on top of the pleistocene the lower boundary of the ebb tidal delta and the geometry of the ebb delta sand body

deep ocean circulation and transport where the east pacific rise at 9 10 n meets the lamont seamount chain 1 we report the first 3 d numerical model study of abyssal ocean circulation and transport over the steep topography of the east pacific rise epr and adjoining lamont seamount chain in the eastern tropical pacific we begin by comparing results of hydrodynamical model calculations with observations of currents hydrography and sf6 tracer dispersion taken during larval dispersal on the deep east pacific rise ladder field expeditions in 2006 2007 model results are then used to extend observations in time and space regional patterns are pronounced in their temporal variability at m2 tidal and subinertial periods mean velocities show ridge trapped current jets flowing poleward west and equatorward east of the ridge with time varying magnitudes weekly average maximum of 10 cm s 1 that make the jets important features with regard to ridge originating particle larval transport isotherms bow upward over the ridge and plunge downward into seamount flanks below ridge crest depth the passage p1 between the epr and the first lamont seamount to the west is a choke point for northward flux at ridge crest depths and below weekly averaged velocities show times of anticyclonic flow around the lamont seamount chain as a whole and anticyclonic flow around individual seamounts results show that during the ladder tracer experiment sf6 reached p1 from the south in the western flank jet a short lived change in regional flow direction just at the time of sf6 arrival at p1 started the transport of sf6 to the west on a course south of the seamounts as field observations suggest approximately 20 days later a longer lasting shift in regional flow from west to sse returned a small fraction of the tracer to the epr ridge crest

symmetry vs asymmetry of a hyper thinned rift example of the mauleon basin western pyrenees france abstract the aim of this study is to unravel the tectono sedimentary evolution of a hyper thinned rift based on the example of the mauleon basin a basin filled by thick synrift deposits the integrated study combines field data detailed geological mapping and seismic interpretation the field study focuses on the iberian margin of the mauleon basin seismic interpretation and well calibration along a n s transect of the mauleon basin enable imaging the transition with the northern conjugate margin the synrift records are very different on either side of the basin the southern margin is composed of a proximal turbiditic s l siliciclastic system whereas the northern margin is characterized by a carbonate system extending from the platform to the basin we recognize the mauleon rift as an apparent symmetric hyper thinned rift related to a southward dipping albian detachment and a northward dipping cenomanian one two stages of continental crustal thinning are inferred to explain the development of the mauleon basin first a barremian to earliest albian ductile pure shear thinning phase responsible for the lower crustal thinning and the formation of a symmetric sag basin second an albian cenomanian simple shear thinning phase responsible for the onset of the southward dipping saint palais detachment faulting and for evolution to an asymmetric basin the iberian margin appears as an upper plate and the european one as a lower plate during albian time at early cenomanian time the basin was affected by structural changes of the margins resulting from shift in detachment direction interpreted as flip flop detachment tectonics

variations in mean currents affecting hydrothermal plumes on the juan de fuca ridge a summary of current meter observations near the juan de fuca ridge shows new features of topographic steering of flow that determine the fate of plumes from hydrothermal vents an anticyclonic circulation exists over the ridge with north and south mean flows along the west and east flanks speeds are up to 2 3 cm s 1 near the ridge and decrease to zero about 25 km from the crest contributions from hydrothermal forcing and tidal rectification may generate the along ridge flows average anomalous heat fluxes associated with the north and south flows were estimated at 1400 and 2600 mw respectively and they are larger than previous estimates based on single point current measurements variability of flow occurs on several time scales and space scales daily average currents over a 4 day period can rotate over the entire ridge with sufficient cross axis speeds to move plumes away from the vents into the stronger north south flows over the flanks currents along the west side of north cleft diverge with near ridge flow 10 km from the ridge turning eastward around the north end of the segment and offshore flow turning westward farther west in winter off axis currents can be several times as large as the long term mean and could advect plume water relatively long distances significant cross axis flow occurs on the north side of axial volcano that may be the result of interaction between the general circulation and topography generating eddy circulations around or near axial or the entire axial cobb massif

estimated likelihood of observing a large earthquake on a continental low angle normal fault and implications for low angle normal fault activity the lack of observed continental earthquakes that clearly occurred on low angle normal faults lanfs may indicate that these structures are not seismically active or that these earthquakes are simply rare events to address this we compile all potentially active continental lanfs 24 in total and calculate the likelihood of observing a significant earthquake on them over periods of 1 100 years this probability depends on several factors including the frequency magnitude distribution for either a characteristic or gutenberg richter distribution we calculate a probability of about 0 5 that an earthquake greater than m6 5 large enough to avoid ambiguity in dip angle will be observed on any lanf in a period of 35 years which is the current length of the global centroid moment tensor catalog we then use bayes theorem to illustrate how the absence of observed significant lanf seismicity over the catalog period moderately decreases the likelihood that the structures generate large earthquakes

vertical and temporal vorticity observations at juan de fuca ridge hydrothermal signatures moored current measurements made from april 1994 to june 1995 reveal the vertical and temporal structure of the vorticity over the northern segment of the juan de fuca ridge at a depth of 2100m near the thermal and particle maximum associated with the neutrally buoyant hydrothermal plume the relative vorticity is anticyclonic with a mean value of nearly 0 09f with variability of the same magnitude this arises from cross ridge shear in the horizontal currents at this level in the vertical the anticyclonic shear has a peak value at plume level decreasing in magnitude both upward and downward from depths of 2100 2150m as the bottom is approached the transverse shear changes sign with cyclonic values of up to 0 09f below 2250m depth this vorticity signature is suggestive of dynamics of a line source of buoyant fluid entering the plume at its neutral density horizon and not due to individual point sources

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

4 d evolution of rift systems insights from scaled physical models the four dimensional 4 d evolution of brittle fault systems in orthogonal oblique and offset rift systems has been simulated by scaled sandbox models using dry cohesionless fine grained quartz sand extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model the results of these analog model studies are compared with natural examples of rift fault systems rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction segmentation of the rift margin increases with increase in obliquity of the rift axis resulting in a consequent increase in displacement on intrarift fault systems offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone along strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones high relief extension parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults the second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low relief oblique slip zone or grabens these typically are found in highly oblique rift systems 45 degrees and above right steps in offset rift models

biogeographical significance of cambrian trilobites from the carolina slate belt abundant middle middle cambrian trilobites of the ptychagnostus atavus interval zone are described from the upper asbill pond formation informal name near batesburg south carolina the fauna containing at least nine species has strong affinities with cool water faunas of both armorica and baltica but little affinity with warm water shelf faunas of laurentia this is compatible with previous conclusions that the carolina slate belt is an exotic terrane that was accreted to north america during the early or middle paleozoic further evaluation however introduces an alternative possibility that the fauna could have lived in deep cool water environments along the periphery of laurentia and subsequent displacement need not necessarily have been great combined faunal and stratigraphic components of the carolina slate belt do not precisely match those of any known region but may be closest to those of bohemia although a southpolar location in proximity to armorica baltica and gondwana seems likely a definitive placement in a reconstruction of the cambrian world is not yet possible included in the trilobite fauna are the agnostoids hypagnostus mammillatus h parvifrons peronopsis fallax ptychagnostus sp and tomagnostus fissus the polymeroids include agraulos sp paradoxides cf polonicus paradoxides sp and skreiaspis sp

sediment connectivity a framework for analyzing coastal sediment transport pathways connectivity provides a framework for analyzing coastal sediment transport pathways building on conceptual advances in graph theory from other scientific disciplines connectivity schematizes sedi