crustal structure of the northern nova scotia rifted continental margin eastern canada 1 the nova scotia continental margin off eastern canada marks a transition from a volcanic to a nonvolcanic style of rifting the northern nonvolcanic segment of the margin was studied by a 490 km long refraction seismic line with dense air gun shots coincident with previous deep reflection profiles a p wave velocity model was developed from forward and inverse modeling of the wide angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles the continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5 7 6 9 km s crustal thinning down to 3 km occurs in a 180 km wide zone and the sediment cover in this area is up to 15 km thick farther seaward a 150 km wide transition zone is observed with a 5 km thick lower layer 7 2 7 6 km s interpreted as partially serpentinized mantle at the landward end this layer is overlain by highly altered continental crust 5 4 km s extending up to the seaward limit of the jurassic salt province farther seaward the upper layer is interpreted as exhumed and highly serpentinized mantle 5 1 km s separated from the lower layer by subhorizontal reflectivity which probably represents a serpentinization front oceanic crustal thickness is 4 km with layer 2 velocities of 4 6 5 0 km s layer 3 velocities of 6 4 6 55 km s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile this reduced magma production might be related to the proximity of the newfoundland transform margin

hatton bank northwest u k continental margin structure summary the continent ocean transition near hatton bank was studied using a dense grid of single ship and two ship multichannel seismic mcs profiles extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin at the landward shallowest end the dipping reflectors overlie continental crust while at the seaward end they are formed above oceanic crust beneath the central and lower part of the margin is a mid crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence beneath this mid crustal layer and above a well defined seismic moho which rises from 27 km continental end to 15 km oceanic end across the margin the present lower crust comprises a 10 15 km thick lens of material with a seismic velocity of 7 3 to 7 4 kds we interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas a considerable quantity of new material has been added to the crust under the rifted margin the present moho is a new boundary formed during creation of the margin and cannot therefore be used to determine the amount of thinning

crustal structure variations along the nw african continental margin a comparison of new and existing models from wide angle and reflection seismic data abstract deep seismic data represent a key to understand the geometry and mechanism of continental rifting the passive continental margin of nw africa is one of the oldest on earth formed during the upper triassic lower liassic rifting of the central atlantic ocean over 200 ma we present new and existing wide angle and reflection seismic data from four study regions along the margin located in the south offshore dakhla on the central continental margin offshore safi in the northern moroccan salt basin and in the gulf of cadiz the thickness of unthinned continental crust decreases from 36 km in the north to about 27 km in the south crustal thinning takes place over a region of 150 km in the north and only 70 km in the south the north moroccan basin is underlain by highly thinned continental crust of only 6 8 km thickness the ocean continent transition zone shows a variable width between 40 and 70 km and is characterized by seismic velocities in between those of typical oceanic and thinned continental crust the neighbouring oceanic crust is characterized by a thickness of 7 8 km along the complete margin relatively high velocities of up to 7 5 km s have been imaged between magnetic anomalies s1 and m25 and are probably related to changes in the spreading velocities at the time of the kimmeridgian tithonian plate reorganization volcanic activity seems to be mostly confined to the region next to the canary islands and is thus not related to the initial opening of the ocean which was associated to only weak volcanism comparison with the conjugate margin off nova scotia shows comparable continental crustal structures but 2 3 km thinner oceanic crust on the american side than on the african margin

a deep seismic reflection profile across the nova scotia continental margin offshore eastern canada results from two deep seismic reflection lines are presented when combined these lines span the rifted continental margin off nova scotia from crust unaltered by rifting to the ocean basin these data provide crustal and upper mantle reflection geometry to depths of over 50 km and elucidate the rifting process on this margin which occurred during the mesozoic breakup of pangaea the continental crust below the continental shelf and slope becomes progressively thinner toward the ocean continent boundary in the upper crust normal faults accommodated mesozoic extension and these flatten and terminate at 5 6 s two way time in the lower crust and upper mantle mesozoic rifting may be reflected in dipping events which are interpreted to be normal faults all mesozoic extensional faulting could be controlled by the preexisting fabric of the crust which in this region would be related to appalachian compression within the meguma terrane below the continental rise there is some evidence for magmatic un

forward potential field modeling of the appalachian orogen in the gaspe peninsula quebec canada implications for the extent of rift magmatism and the geometry of the taconian orogenic wedge the nature and extent of potential source bodies associated with prominent regional scale gravity and magnetic anomalies in the northern quebec appalachians are investigated through forward modeling potential field anomalies are interpreted to be the geophysical expression of basalt flows extruded during the neoproterozoic to cambrian rifting of laurentia prior to opening of iapetus geophysi cal modeling provides substantial new constraints on seismic and structural cross sections and the map distribution of volcanism this study indicates that the volcanism was significantly more widespread than previously recognized basaltic rocks form a sheet up to 4 kilometers thick transported on the main taconic decollement most of this sheet is deeply buried and overlain by a thick pile of paleozoic sediments later tectonic modifications by backthrusting and hinterland directed normal faulting as well as by acadian strike slip faulting substantially changed the geometry of the transported volcanic rocks and the structural style both across and along the strike of the orogen the extension of the gravity anomaly under the st lawrence river also suggests that the ordovician deformation front is 20 to 30 kilometers further to the north than previously interpreted

listric faults imaged in oceanic crust deep seismic reflection profiling has provided exceptional images of listric faulting in oceanic basement of jurassic age off the coast of nova scotia the faults dip toward the mid atlantic ridge and sole near the base of the crust at a common depth of about 4 km presumably the brittle ductile transition at the time of deformation at least five rotated fault blocks with dips ranging from 0 at the surface to 45 at the base are observed along a 36 km flow line segment suggesting a stretching factor of 1 5 2 5 the faults can be traced upward with small displacement in the overlying sedimentary strata to approximately the middle cretaceous strata implying continued movement or consolidation for about 70 m y after crustal formation

deep structure of the u s atlantic continental margin offshore south carolina from coincident ocean bottom and multichannel seismic data we present the results of a combined multichannel seismic reflection mcs and wide angle ocean bottom seismic profile collected in 1988 across the carolina trough on the u s atlantic continental margin inversion of vertical incidence and wide angle travel time data has produced a velocity model of the entire crust across the continent ocean transition the margin consists of three structural elements 1 rifted continental crust comprising 1 4 km of post rift sedimentary rocks overlying a 30 34 km thick subsedimentary crust 2 transitional crust a 70 to 80 km wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10 to 24 km thick subsedimentary crust and 3 oceanic crust comprising 8 km of sedimentary rocks overlying an 8 km thick crystalline crust the boundary between rifted continental and transitional crust marked by the brunswick magnetic anomaly represents an abrupt change in physical properties with strong lateral increases in seismic velocity density and magnetic susceptibility the transitional crust contains mid crustal seaward dipping reflections observed on the mcs section and has seismic velocities of 6 5 6 9 km s in the midcrust and 7 2 7 5 km s in the lower crust modeling of potential field data shows that transitional crust also produces the prominent margin parallel gravity anomaly and the brunswick and east coast magnetic anomalies these observations support the interpretation that the transitional crust was formed by magmatism during continental breakup the prodigious thickness up to 24 km of igneous material rivals that interpreted on continental margins of the north atlantic e g hatton bank and voring plateau which formed in the vicinity of the iceland hotspot these observations when combined with other transects across the margin confirm previous suggestions that the u s atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long lived mantle plume

crustal structure of the central nova scotia margin off eastern canada summary the central nova scotia margin off eastern canada is located at a transition from a volcanic margin in the south to a non volcanic margin in the north in order to study this transition a wide angle refraction seismic line with dense airgun shots was acquired across the central nova scotia margin the 500 km long transect is coincident with previous deep reflection profiles across the lahave platform and extending into the sohm abyssal plain a p wave velocity model was developed from forward and inverse modelling of the wide angle data from 21 ocean bottom seismometers and coincident normal incidence reflection profiles the velocity model shows that the continental crust is divided into three layers with velocities of 5 5 6 9 km s 1 the maximum thickness is 36 km a minor amount 5 km of thinning occurs beneath the outer shelf while the major thinning to a thickness of 8 km occurs over the slope region the seaward limit of the continental crust consists of 5 km thick highly faulted basement there is no evidence for magmatic underplating beneath the continental crust on the contrary a 4 km thick layer of partially serpentinized mantle 7 6 7 95 km s 1 begins beneath the highly faulted continental crust and extends 200 km seawards forming the lower part of the oceancontinent transition zone the upper part of the transition zone consists of the highly faulted continental crust and 4 to 5 km thick initial oceanic crust the continent ocean boundary is moved 50 km farther seawards compared to an earlier interpretation based only on reflection seismic data the oceanic crust in the transition zone consists of layer 2 and a high velocity lower crustal layer layer 2 is 1 3 km thick with velocities of 5 6 6 0 km s 1 the high velocity lower crustal layer is 1 2 km thick with velocities of 7 25 7 4 km s 1 suggesting a composite layer of serpentinized peridotite and gabbroic layer 3 oceanic crust with normal thickness of 5 7 km and more typical layer 3 with velocities of 6 95 7 3 km s 1 is observed at the seaward end of the profile

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

crustal structure of the continental margin of korea in the east sea japan sea from deep seismic sounding data evidence for rifting affected by the hotter than normal mantle abstract despite the various opening models of the southwestern part of the east sea japan sea between the korean peninsula and the japan arc the continental margin of the korean peninsula remains unknown in crustal structure as a result continental rifting and subsequent seafloor spreading processes to explain the opening of the east sea have not been adequately addressed we investigated crustal and sedimentary velocity structures across the korean margin into the adjacent ulleung basin from multichannel seismic mcs reflection and ocean bottom seismometer obs data the ulleung basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal the continental margin documents rapid transition from continental to oceanic crust exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of moho over a distance of about 50 km the crustal model of the margin is characterized by a high velocity up to 7 4 km s lower crustal hvlc layer that is thicker than 10 km under the slope base and pinches out seawards the hvlc layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature the acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption these features suggest that the evolution of the korean margin can be explained by the processes occurring at volcanic rifted margins global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the korean margin and in the ulleung basin

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

diachronous rifting drifting and inversion on the passive margin of central eastern north america an analog for other passive margins integration of new data with existing information indicates that the tectonic development of the passive margin of eastern north america between the carolina trough and scotian basin was considerably more complex than the classic two stage rift drift model first the transition from rifting to drifting was diachronous in the southeastern united states the rift drift transition occurred after the late triassic synrift deposition and before eastern north america magmatism in the earliest jurassic 200 ma in maritime canada the rift drift transition occurred after eastern north america magmatic activity and synrift deposition in the early jurassic and before postrift deposition in the early middle jurassic 185 ma second the deformational regime changed substantially after rifting on both the southern and northern segments of the margin generally northwest southeast postrift shortening replaced northwest southeast synrift extension northeast striking reverse faults formed and many of the rift basin boundary faults had reverse displacements in the southeastern united states the change in the deformational regime occurred in the late triassic early jurassic during the rift drift transition simultaneously diabase sills and dikes many striking nearly perpendicular to the trend of the rift basins intruded the continental crust and a massive wedge of volcanic or volcaniclastic rocks developed near the continent ocean boundary in maritime canada the change in the deformational regime occurred during or after the early jurassic and before or during the early cretaceous that is during the rift drift transition or early stages of sea floor spreading

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

imaging a lithospheric detachment at the continent ocean crustal transition off morocco abstract the sismar seismic survey on the moroccan atlantic margin recorded deep penetration images of the continent ocean boundary this paper focuses on the 3d observation of a landward dipping reflector overlaid by a layered unit the deep part of the reflector plunges towards the continent beneath thinned continental crust whereas its upper part finishes at the top of the basement in a transition zone where likely volcanics dipping reflectors are visible and where a magnetic anomaly named s1 marks the continent ocean boundary location as the landward dipping reflector crosses the moho it is interpreted as a lithospheric detachment with associated volcanic material originating from the asthenospheric mantle observations are used i to build a 2d model to explain the mechanisms of the lithospheric breaking off between moroccan margin and its conjugate canadian margin the landward dipping reflector allowing mantle exhumation ii to propose an ene wnw trend for the initial rifting extension in this part of atlantic during late triassic direction compatible with that of the southern grand banks transform margin

crustal structure beneath orphan basin and implications for nonvolcanic continental rifting a wide angle seismic profile was acquired across the northeast newfoundland margin seaward to orphan knoll the profile is complemented by existing multichannel reflection data two online deep wells and other deep seismic reflection and refraction lines modeling of data from 15 ocean bottom seismometers provides the seismic velocity structure for a significant portion of the 400 km wide stretched continental crust on this margin prerift metasedimentary rocks have a topographic relief of up to 5 km with a velocity of 5 2 km s increasing to 5 9 km s unaltered crystalline crust below the metasedimentary section has a velocity range of 6 1 6 5 km s most of the lower crust landward of orphan knoll has a typical continental lower crustal velocity 6 8 7 0 km s and the lower crust shallows significantly beneath orphan knoll our model shows no evidence for the 5 km thick 7 35 km s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating implying nonvolcanic rifting for the ne newfoundland margin we suggest that continental stretching persisted from 180 ma to chron 34n time 101 ma when the final breakup between canada and western europe took place leaving a 400 km wide zone of thinned continental crust underneath the shelf and deep water area of the orphan basin on the landward side of the basin gravity modeling indicates a zone of very thin 6 8 km crust possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between canada and europe a possible sub moho reflector suggests generally uniform stretching of the crust and upper mantle and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the grand banks

early paleozoic transform margin structure beneath the mississippi coastal plain southeast united states a geophysical transect across the central gulf of mexico coastal plain shows that the early paleozoic continental margin of southern laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late paleozoic ouachita orogeny and formation of the modern gulf of mexico coastal plain the thickness of the crystalline crust decreases abruptly across the margin over a distance of 50 km from 35 km beneath the black warrior foreland basin to 10 km beneath the ouachita fold and thrust belt this abrupt decrease in crustal thickness is similar to modern transform margins but very different from most rifted margins which display much more gradual transitions in crustal thickness the geophysical data indicate an absence of synrift intrusive and volcanic rocks underplated mafic rocks at the base of the crust and abnormally thick oceanic crust adjacent to the margin the lack of these features is also characteristic of modern transform margins combined with transects across the margin farther west the data confirm previous suggestions that the central gulf of mexico coastal plain overlies an 800 km long transform segment of the late proterozoic early paleozoic southern laurentian continental margin that extends continuously from western arkansas to southeast alabama

transform margin of the northern levant eastern mediterranean from formation to reactivation 1 the levant continental margin eastern mediterranean is usually defined as a passive margin that developed between the late paleozoic early mid mesozoic previous studies suggest that the margin is composed of two segments with different structural characteristics the southern segment south of the carmel structure was formed by rifting whereas the origin of the northern segment is not known the present study focuses on the northern segment which is a key for understanding the tectonic history of the eastern mediterranean investigation is carried out offshore on two scales 1 deep crustal observations analysis of seismic refraction and magnetic data and their comparison with other continental margins worldwide and 2 shallow crustal scale observations of recent tectonic deformations based on multichannel and single channel seismic reflection profiles integration of the deep and shallow observations with reevaluation of geological data onland show strong structural resemblance between the northern levant segment and other transform margins a modified scenario is suggested for the formation and development of the levant basin and its margins combining all previous studies into one simple picture where a signature of older structures on more recent features is evident located in the midst of the progressive afro eurasian collision the northern levant presents a unique lifecycle for transform margins worldwide from formation to reactivation

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

magnetic properties of the bay of islands ophiolite suite and implications for the magnetization of oceanic crust rock magnetic properties opaque mineralogy and degree of metamorphism were determined for 101 unoriented samples from the north arm and blow me down massifs of the bay of islands ophiolite complex newfoundland the weathered and metamorphosed extrusive basalt samples have a weak secondary magnetization arising from oxidation and exsolution of ilmenite of unknown origin the initial magnetization of the underlying sheeted dike complex appears to have been destroyed by hydrothermal alteration soon after formation the magnetic intensity of the gabbroic samples increases as the degree of alteration increases with the highly altered upper metagabbros having an average intensity of 3 10 3 emu c3 because magnetization of the metagabbro samples is related to nonpervasive variable alteration these crustal units are unlikely to make a significant contribution to lineated magnetic anomalies a compilation of our results and other studies suggests a model in which oceanic crust magnetization results from an upper extrusive basalt source layer roughly 600 m thick with no contribution from a deeper source layer recognizable from these bay of islands data

lithospheric deformation in the canadian appalachians evidence from shear wave splitting plate scale deformation is expected to impart seismic anisotropic fabrics on the lithosphere determination of the fast shear wave orientation and the delay time between the fast and slow split shear waves t via sks splitting can help place spatial and temporal constraints on lithospheric deformation the canadian appalachians experienced multiple episodes of deformation during the phanerozoic accretionary collisions during the palaeozoic prior to the collision between laurentia and gondwana and rifting related to the mesozoic opening of the north atlantic however the extent to which extensional events have overprinted older orogenic trends is uncertain we address this issue through measurements of seismic anisotropy beneath the canadian appalachians computing shear wave splitting parameters t for new and existing seismic stations in nova scotia and new brunswick average t values of 1 2 s relatively short length scale 100 km splitting parameter variations and a lack of correlation with absolute plate motion direction and mantle flow models demonstrate that fossil lithospheric anisotropic fabrics dominate our results most fast directions parallel appalachian orogenic trends observed at the surface while t values point towards coherent deformation of the crust and mantle lithosphere mesozoic rifting had minimal impact on our study area except locally within the bay of fundy and in southern nova scotia where fast directions are subparallel to the opening direction of mesozoic rifting associated t values of 1 s require an anisotropic layer that spans both the crust and mantle meaning the formation of the bay of fundy was not merely a thin skinned tectonic event

crustal structure of the southeast georgia embayment carolina trough preliminary results of a composite seismic image of a continental suture and a volcanic passive margin new deep penetration multichannel seismic reflection data combined with refraction results and magnetics modeling support a hypothesis that the carolina trough is a mesozoic volcanic passive margin exhibiting a seaward dipping wedge and associated underplating the structure of carolina platform continental crust is consistent with the late paleozoic continental collision that produced the appalachians but imbrication has had no obvious effect on shallower structures produced by mesozoic extension and volcanism the origin of prominent magnetic anomalies crossing the southeast georgia embayment can be explained by processes attending mesozoic separation of africa and north america and is not related to a paleozoic continental suture as previously postulated

segmentation and volcano tectonic characteristics along the sw african continental margin south atlantic as derived from multichannel seismic and potential field data abstract regional seismic reflection and potential field data document the south atlantic s break up history between 39 s and 19 s from the early cretaceous onwards previous maps of distribution of volcanics along the margin showed volcanics along the whole african margin based on extrapolation of data based on previously unpublished marine geophysical data we found the southernmost 460 km long margin segment to be lacking huge volumes of break up related volcanic effusives northwards break up was accompanied by the emplacement of huge volumes of volcanic material prominently featured in seismic sections as huge wedge shaped seaward dipping reflectors sdrs detailed mapping of offsets left and right stepping and variations in structural character of the volcanics reveal the segmentation along and the break up history of the margin several superimposed sdr sequences suggesting episodicity of volcanic emplacement divided by periods of erosion and sedimentation are distinct along southerly lines losing prominence northwards a main outcome of our study is that this passive margin is not continuously of the volcanic type and that the change from a non volcanic to a volcanic margin occurs abruptly we define four distinct first order segments along the 2400 km section of the southwestern african margin covered by our seismic data from south to north these first order segments are magma poor segment i segment ii with enormous sdrs volumes decreasing sdrs volumes in segment iii segment iv again with enormous volcanic output likely influenced by walvis ridge volcanism most important is that there is no systematic increase in the volumes of the effusives towards the tristan da cunha hot spot rather there is an alternating pattern in the sdrs volumes and widths the boundary between the volcanic and magma poor margin segments in the southernmost study area is sharp 10s of km which we propose is reflected in magnetic anomaly data as well we suggest that this variability along the margin is mainly due to a change in stretching rifting character from oblique during the early stages of breakup to conventional seafloor spreading from chron m4 130 ma onwards

mesozoic carbonate platforms and banks of the eastern north american margin abstract the jurassic lower cretaceous carbonate platforms and banks form a discontinuous belt extending over distance of 6000 km from the grand banks up to the bahamas six types of carbonate buildups are recognized and document the variability of depositional paleo oceanographic and tectonic processes on the eastern north american margin the composition of the carbonates closely resembles the recent deposits of the western great bahama bank since oolitic shoals were present near to the shelf edge and skeletal peloid wackestones and biomicrites were deposited in the inner part of the platform coralstromatoporoid and sponge bioherms were only rare constituents of the carbonate banks the thickness of carbonate buildups progressively increases southward along the margin attaining a thickness of more than 5 km on the bahamas the carbonate platforms also become younger southwards which is thought to reflect the northward movement of the north american plate of less than 1 5 cm yr the carbonate platforms were seeded over the continental basement following the taphrogenic period of plate tectonics building of carbonate ramps which characterized the early jurassic has began during the transitional period between continental rifting and early drift the second stage in construction of the carbonate platforms and offshore banks proceeded mainly after separation of the continental plates

symmetric conjugation of continent ocean boundary structures along the norwegian and east greenland margins abstract a characteristic structural feature of the norwegian continental margin that has been described from multichannel seismic data as well as dsdp and odp drilling is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin models for their origin describe them as either products of a late stage intra continental rift phenomenon with the volcanic rocks overlying attenuated continental crust or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the norwegian greenland sea from studies of the norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the east greenland margin was undertaken which used two ship multichannel seismic acquisition as its focus we found that the wedges were not only present on the east greenland margin but that details of their along strike variability are mirrored on both margins together with their spatial relationship to the oldest magnetic lineation pattern the wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation 24 moving south we see that the wedges broaden encroach into the pattern of lineation 24 then develop into two separate wedges the seaward of which is associated with lineation 23 lineations 24 and 23 on both margins are distorted when associated with wedges we believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary and we believe that the most likely mechanism to generate the excess volcanism is the convective partial melting scheme recently proposed by mutter and buck 1986 and mutter et al in press

withdrawn 3d crustal architecture of a magma poor passive margin santos campos and espirito santo basins comparisons with a volcanic passive margin pelotas basin offshore brazil

the transition from a volcanic to a nonvolcanic rifted margin off eastern canada three multichannel seismic reflection profiles were collected on the rifted continental margin southeast of nova scotia eastern canada the profiles cross the east coast magnetic anomaly ecma which parallels much of the margin of eastern north america south of the grand banks and which is usually associated with the transition from continental to oceanic crust studies to the south of the work reported here suggest that the ecma may be related to the emplacement of large thicknesses of late rift stage or early drift stage igneous material which is characterized by seaward dipping reflections in basement and a high velocity lower crustal layer the seismic data show that seaward dipping reflections sdr continue northward into the study area and support the correlation between the sdr unit and the presence of a well developed ecma magnetic modeling confirms this association although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture just north of the study area the ecma diminishes and is no longer well developed the sdr unit also terminates and it is not observed over most of the nova scotian margin if our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north the association of the transition with significant changes in the prerift fabric of the adjacent continental crust in the trend of synrift extensional structures and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced we suggest that these clues are consistent with small scale convection as a mechanism for delivering large melt volumes to crustal depths during rifting

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

integration of cocorp deep reflection and magnetic anomaly analysis in the southeastern united states implications for origin of the brunswick and east coast magnetic anomalies alternative interpretation and reply

the east coast magnetic anomaly a model is proposed for the east coast magnetic anomaly that for the first time incorporates an observed basement feature a normal fault that consistently lies below the landward flank of the anomaly the east coast magnetic anomaly is similar to slope anomalies found over passive continental margins in many parts of the world thus the proposed model can be used to interpret slope anomalies in general

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

contrasting styles of seafloor spreading in the woodlark basin indications of rift induced secondary mantle convection the woodlark basin in the southwest pacific is a young ocean basin which began forming by 6 ma following the rifting of continental and arc lithosphere the n s striking moresby transform divides the oceanic basin into eastern and western parts which have contrasting characteristics seafloor spreading west of moresby transform began after 2 ma and although spreading rates decrease to the west the western basin has faster spreading characteristics than the eastern basin these include 1 500 m shallower seafloor 2 bouguer gravity anomalies that are 30 mgals lower 3 magnetic anomaly and modeled seafloor magnetization amplitudes that are higher 4 a spreading center with an axial high in contrast to the axial valleys of the eastern basin 5 smoother seafloor fabric and 6 exclusively nontransform spreading center offsets in contrast to the eastern basin which has transform faults and fracture zones that extend across most of the basin overall depth contrasts and bouguer anomalies can be matched by end member models of thicker crust 2 km or thinner lithosphere 1 3 in the western basin correlated with these contrasts the surrounding rifted margins abruptly thicken westward of the longitude of moresby transform we examine alternative explanations for these contrasts and propose that rift induced secondary mantle convection driven by thicker western margin lithosphere is most consistent with the observations although rift induced convection has been cited as a cause for the voluminous excess magmatism at some rifted margins the observations in the woodlark basin suggest that this mechanism may significantly affect the morphology structure and geophysical characteristics of young ocean basins in alternate ways which resemble increased spreading rate

abrupt transition from magma starved to magma rich rifting in the eastern black sea the amount of magmatism that accompanies the extension and rupture of the continental lithosphere varies dramatically at rifts and margins around the world based on widely spaced geophysical transects some margins are known to preserve a transition from magmatically robust to magmatically starved rifting along strike but the nature of the transition is unknown wide angle seismic data from the black sea provide the first direct observations of such a transition and show that it is abrupt occurring over only 20 30 km and coincides with a transform fault this abrupt transition cannot be explained solely by gradual along margin variations in mantle properties since these would be expected to result in a smooth transition from magma poor to magma rich rifting over hundreds of kilometers we suggest that the abruptness of the transition results from the development of three dimensional 3 d melt migration due to along strike variations in extension and thus the thickness of the lithosphere at the time of rifting localized magmatic addition attributed to melt focusing has been observed in modern mid ocean ridges and active rift environments but here we show that such processes can also produce abrupt along strike changes from magma poor to magma rich rifting

refining the formation and early evolution of the eastern north american margin new insights from multiscale magnetic anomaly analyses to investigate the oceanic lithosphere formation and early seafloor spreading history of the north atlantic ocean we examine multiscale magnetic anomaly data from the jurassic early cretaceous age eastern north american margin enam between 31 40 n we integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids satellite derived gravity anomaly and satellite derived and shipboard bathymetry data we evaluate these data sets to 1 refine magnetic anomaly correlations throughout the enam and assign updated ages and chron numbers to m0 m25 and eight pre m25 anomalies 2 identify five correlatable magnetic anomalies between the east coast magnetic anomaly ecma and blake spur magnetic anomaly bsma which may document the earliest atlantic seafloor spreading or syn rift magmatism 3 suggest pre existing margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the atlantic jurassic quiet zone jqz 4 suggest that if the bsma source is oceanic crust the bsma may be m series magnetic anomaly m42 168 5 ma 5 examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the bsma to m25 suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ecma to the more linear m25 and 6 observe anomalously high amplitude magnetic anomalies near the hudson fan which may be related to a short lived propagating rift segment that could have helped accommodate the crustal alignment during the early atlantic opening

structural development of the jurassic magnetic quiet zone off morocco and identification of middle jurassic magnetic lineations 1 geophysical investigations carried out during two meteor cruises have revealed weak linear magnetic anomalies in those parts of the jurassic magnetic quiet zone jmqz off morocco that are not affected by cenozoic igneous activity the linear magnetic anomalies are not correlated with variations in relief or structure of oceanic crust using the reversal sequence m25 m41 the anomalies of the jmqz can be modeled with a half spreading rate of 2 2 cm yr extrapolation of the weak lineations back to the slope anomaly s1 results in a breakup age of 170 ma according to this interpretation the crust of the jmqz off morocco has formed during the time 170 155 ma bajocian to oxfordian however much lower spreading rates around 1 cm yr cannot be excluded s1 coincides with a poorly developed seaward dipping reflector sequence sdrs which is most likely its source the sdrs corroborates the earlier claim that s1 marks the ocean continent transition seaward of s1 is a 70 km wide strip of horizontal reflectors that become landward dipping to the west in the uppermost part of the basement it parallels s1 over a distance of 200 km and may indicate excessive magma supply within the first 2 m y of seafloor spreading similar landward dipping reflectors are observed also in the conjugate sohm abyssal plain off nova scotia the average magnetization of the landward dipping reflectors is much lower than that of the sdrs a lower crustal body with a seismic velocity of 7 3 km s which formed during the neogene beneath the eastern part of the essaouira rise has only a weak magnetization 0 5 a m

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

the crustal structure of the nw moroccan continental margin from wide angle and reflection seismic data summary the atlantic margin off morocco with its neighbouring jurassic oceanic crust is one of the oldest on earth it is conjugate to the nova scotia margin of north america the sismar marine seismic survey acquired deep reflection seismic data as well as wide angle seismic profiles in order to image the deep structure of the margin characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins we present results from the combined interpretation of the reflection seismic wide angle seismic and gravity data along a 440 km long profile perpendicular to the margin at 33 34 n extending from nearly normal oceanic crust in the vicinity of coral patch seamount to the coast at el jadida and approximately 130 km inland the shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting the sedimentary basin thickens from 1 5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope multichannel seismic mcs data image basement structures including a few tilted fault blocks and a transition zone to a thin crust a strong discontinuous reflection at 12 s two way travel time twt is interpreted as the moho discontinuity as a result of the good data quality the deep crustal structure depth and velocity field is well constrained through the wide angle seismic modelling the crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile the transitional region has a width of 150 km crustal velocities are lowest at the continental slope probably as a result of faulting and fracturing of the upper crust uppermantle velocities could be well defined from the ocean bottom seismometer obs and land station data throughout the model

northeastern brazilian margin regional tectonic evolution based on integrated analysis of seismic reflection and potential field data and modelling abstract integration of regional seismic reflection and potential field data along the northeastern brazilian margin complemented by crustal scale gravity modelling is used to reveal and illustrate onshore offshore crustal structure correlation the character of the continent ocean boundary and the relationship of crustal structure to regional variation of potential field anomalies the study reveals distinct along margin structural and magmatic changes that are spatially related to a number of conjugate brazil west africa transfer systems governing the margin segmentation and evolution several conceptual tectonic models are invoked to explain the structural evolution of the different margin segments in a conjugate margin context furthermore the constructed transects the observed and modelled moho relief and the potential field anomalies indicate that the reconcavo tucano and jatoba rift system may reflect a polyphase deformation rifting mode associated with a complex time dependent thermal structure of the lithosphere the constructed transects and available seismic reflection profiles indicate that the northern part of the study area lacks major breakup related magmatic activity suggesting a rifted non volcanic margin affinity in contrast the southern part of the study area is characterized by abrupt crustal thinning and evidence for breakup magmatic activity suggesting that this region evolved partially with a rifted volcanic margin affinity and character

crustal structure north of the taiping island itu aba island southern margin of the south china sea abstract based on the multi channel seismic mcs and gravity data offshore north of taiping island itu aba island in the spratly islands nansha islands we revisited the crustal structures in the northern part of the southern margin of the southwest sw sub basin of the south china sea scs the mcs data suggest that the basement structural highs in the southwest margin of the scs are dominated by both fault blocks and volcanic basement structures that probably formed along with the basement faults the gravity modeling results reveal that these volcanic basement structures were probably associated with the high velocity or density lower crust hvdlc suggesting that the southern margin of the sw sub basin serves as an intermediate margin based on the tectonic features in the scs margins including 1 the transition between the magma poor margins of the northwest nw sub basin and intermediate margins of the east and sw sub basins 2 the cessation of the seafloor spreading in the nw sub basin being succeeded by a major ridge jump event which was very likely to reflect an active mantle upwelling event 3 the extensive distribution of the hvdlc beneath in the well investigated northern margin of the east sub basin and as explored in the sw sub basin in this study and 4 basement faulting and volcanism shown in this study that may be related to the hvdlc and hyper extension we proposed a conceptual model to interpret the origin of the hvdlc in the southern margin of the sw sub basin the margin of the scs basin was magma poor at the time it occurred an active mantle upwelling event caused by small scale mantle convection possibly related to the influence of the nascent hainan mantle plume was formed subsequently resulting in a southward ridge jump and the hvdlc beneath the current margins of the east and sw sub basins the east and sw sub basins continued to spread latitudinally dividing their hvdlcs as the margins separated accordingly afterwards the hyper extension in the distal margins may also be responsible for a crust weakening zone wherein the subsequent volcanic events could occur

deep seismic reflection data of edge u s mid atlantic continental margin experiment implications for appalachian sutures and mesozoic rifting and magmatic underplating the edge seismic experiment across the virginia continental margin delineated a paleozoic suture buried appalachian terranes and mesozoic rifting and magmatic events the seismic grid revealed that the mesozoic norfolk rift basin exists only in the northern one third of the previously mapped area the north striking listric border fault of the norfolk basin half graben parallels seismic laminations in the basement the jurassic volcanic wedge pinches out just landward of the baltimore canyon trough hinge zone and downlaps on the hummocky oceanic basement under the continental rise under the continental slope the volcanic wedgereaches depths 9 s 20 km two distinct intracrustal reflections at 4 0 5 0 s and at 7 0 s twit two way traveltime dip southeastward at low angles 15 the moho reflection is disrupted where it is intersected by the 7 0 s reflection northwest of this point the moho dips landward seaward it is horizontal seaward of this point the lower crustal boundary laminations exist in a narrow interval 10 5 11 0 s and are of strong amplitude these changes in the moho and lower crust represent the seaward edge of the grenville age north american crust and the landward edge of jurassic magmatic underplating a northwest dipping reflection observed for the first time on the u s atlantic margin may be the top of the jurassic magmatic underplating layer the northwest dipping reflection truncates the southeast dipping 7 0 s twtt reflection landward projection of the 7 0 s reflection yields a north south trace on the postrift unconformity under the center of lower chesapeake bay this trace is near a basement fault between low grade metamorphic rocks carolina slate avalonia on the east and high grade rocks goochland terrane on the west this fault boundary and the southeastdipping 7 0 s reflection probably represent the taconic suture

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

transition from rifted continental to oceanic crust at the southeastern korean margin in the east sea japan sea 1 the southeastern korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the east sea japan sea two dimensional crustal structure of the southeastern korean margin was computed from ocean bottom seismometer data by tomographic inversion and iterative forward modeling the crustal structure shows the emplacement of high velocity 7 km s lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust the high velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the korean peninsula we suggest that the rifting and subsequent seafloor spreading at the korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature

offshore continuation of meguma terrane southwestern nova scotia eight short drill cores have been examined from the continental shelf southwest of nova scotia four cores recovered granitoid rocks of two types ilmenite bearing granitoid rocks petrographically and geochemically resemble granodiorites of the south mountain batholith and granites of the seal island pluton magnetite bearing granitoid rocks are also peraluminous but have no exact analogues onshore in nova scotia two cores recovered metamorphic rocks in a small area 50 km south of seal island one consits of chlorite muscovite quartz schist geochemically similar to rocks of the halifax formation the second sampled epidote chlorite quartz schist similar to metavolcanic rocks of the white rock formation one further core sampled quartzite and another sampled a metavolcanic rock possibly erratic the regional extent of these lithotypes can be inferred from gravity and aeromagnetic data regional gravity data suggest the presence of a large granite body off southwestern nova scotia in this area magneti

contrasting rifted margin styles south of greenland implications for mantle plume dynamics abstract we present new and reprocessed seismic reflection data from the area where the southeast and southwest greenland margins intersected to form a triple junction south of greenland in the early tertiary during breakup at 56 ma thick igneous crust was accreted along the entire 1300 km long southeast greenland margin from the greenland iceland ridge to and possibly 100 km beyond the triple junction into the labrador sea however highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest greenland margin at the same time thus a transition from a volcanic to a non volcanic margin over only 100 200 km is observed magmatism related to the impact of the iceland plume below the north atlantic around 61 ma is known from central west and southeast greenland the new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction the extent of initial plume related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere plume mantle is channelled to great distances provided that significant melting does not take place melting causes cooling and dehydration of the plume mantle the associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift we further suggest that thick archaean lithosphere blocked direct flow of plume material into the magma starved southwest greenland margin while the plume was free to flow into the central west and east greenland margins the model is consistent with a plume layer that is only moderately hotter 100 200 c than ambient mantle temperature and has a thickness comparable to lithospheric thickness variations 50 100 km lithospheric architecture the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism

integration of cocorp deep reflection and magnetic anomaly analysis in the southeastern united states implications for origin of the brunswick and east coast magnetic anomalies integration of magnetic anomaly analysis with cocorp deep reflection data from the southeastern united states provides three new constraints on the interpretation of the brunswick and east coast magnetic anomalies as well as on the reflection data these are as follows 1 the source of the brunswick anomaly lies within the deep crust this anomaly is not caused by a mesozoic rift basin as proposed by some workers 2 a simple seaward dipping high susceptibility slab model can explain both the brunswick and east coast magnetic anomalies the along strike change in character of the two anomalies results largely from a change in azimuth of the source body 3 beneath the southeastern united states this source body dips south lies immediately on the south flank of the prominent southward dipping reflective zone revealed on cocorp surveys and was previously associated with the alleghanian suture between north america and africa these results imply that a dipping highly magnetized zone in the upper plate of the alleghanian suture is responsible for both the brunswick and east coast magnetic anomalies the high susceptibility material responsible for these anomalies might be mafic lower continental or oceanic crust thrust upward during alleghanian continental collision or mafic igneous material intruded into the upper plate of the suture zone during subsequent mesozoic rifting or both the latter hypothesis implies that the alleghanian suture acted as a zone of weakness a repository which was reactivated to control the site of ultimate atlantic rifting and possibly initial sea floor spreading

breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary a new central pangaea fit type a is proposed for the late ladinian 230 ma together with a plate motions model for the subsequent phases of rifting continental breakup and initial spreading in the central atlantic this model is based on 1 a reinterpretation of the process of formation of the east coast magnetic anomaly along the eastern margin of north america and the corresponding magnetic anomalies at the conjugate margins of northwest africa and the moroccan meseta 2 an analysis of major rifting events in the central atlantic atlas and central mediterranean and 3 a crustal balancing of the stretched margins of north america moroccan meseta and northwest africa the process of fragmentation of central pangaea can be described by three major phases spanning the time interval from the late ladinian 230 ma to the tithonian 147 7 ma during the first phase from the late ladinian 230 ma to the latest rhaetian 200 ma rifting proceeded along the eastern margin of north america the northwest african margin and the high saharan and tunisian atlas determining the formation of a separate moroccan microplate at the interface between gondwana and laurasia during the second phase from the latest rhaetian 200 ma to the late pliensbachian 185 ma oceanic crust started forming between the east coast and blake spur magnetic anomalies whereas the morrocan meseta simply continued to rift away from north america during this time interval the atlas rift reached its maximum extent finally the third phase encompassing the time interval from the late pliensbachian 185 ma to chron m21 147 7 ma was triggered by the northward jump of the main plate boundary connecting the central atlantic with the tethys area therefore as soon as rifting in the atlas zone ceased plate motion started along complex fault systems between morocco and iberia whereas a rift drift transition occurred in the northern segment of the central atlantic between morocco and the conjugate margin of nova scotia the inversion of the atlas rift and the subsequent formation of the atlas mountain belt occurred during the oligocene early miocene time interval in the central atlantic this event was associated with higher spreading rates of the ridge segments north of the atlantis fz an estimate of 170 km of dextral offset of morocco relative to northwest africa in the central atlantic is required by an analysis of marine magnetic anomalies five plate tectonic reconstructions and a computer animation are proposed to illustrate the late triassic and jurassic process of breakup of pangaea in the central atlantic and atlas regions

formation and evolution of the nova scotian rifted margin evidence from deep seismic reflection data a deep marine seismic reflection profile was obtained across the mesozoic rifted continental margin off nova scotia eastern canada this profile crosses the seotian basin one of the deepest basins on the margin of eastern north america and it complements other deep crustal seismic data on this margin the seismic data have been interpreted in conjunction with gravity anomaly and subsidence data they show significant thinning of the continental crust over a zone about 200 km wide the mode of extensional deformation is probably a combination of pure and simple shear there is evidence for simple shear in the crust the continent ocean boundary lies near the seaward edge of synrift salt below the continental rise a 100 km wide zone of very thin approximately 9 km or less continental or transitional crust extends seaward from the outer shelf to this boundary reflectivity of the oceanic crust adjacent to the margin shows evidence of progressive igneous construction perhaps modified by extensional faulting this margin is nonvolcanic and the transition to the volcanic margin off the eastern united states occurs about 500 km southwest of the seismic line the width of the zone of crustal extension is much greater on this nonvolcanic margin segment than it is on the volcanic margin to the south it seems likely that the prerift fabric of the continental lithosphere controls this width a narrow rift may be prone to vigorous asthenospheric convection and therefore to more voluminous volcanism however significantly narrower zones of crustal extension occur on other nonvolcanic margins so factors in addition to rift width such as the rate of rifting may also be important

margin segmentation of baffin bay davis strait eastern canada based on seismic reflection and potential field data abstract the continental margin of baffin island was interpreted from a compilation of seismic reflection and potential field data based on these data it was divided into volcanic and non volcanic segments in the northernmost area coast parallel grabens and continental basement highs are superseded beneath the slope by irregular basement topography in combination with the velocities from refraction data and the lack of a magnetic slope anomaly we propose a non volcanic margin in the central region the seismic data show a limited region with a seaward dipping reflector sequence that may be intercalated with a sedimentary section both north and south of cape dyer extensive volcanic rocks are interpreted at and below seabed consistent with a volcanic margin in the southern region the depth to basement drops rapidly offshore a thick prograding sedimentary wedge produces a high amplitude long wavelength free air gravity anomaly that was previously confused with the continent ocean boundary although some volcanic rocks are mapped in the region this area is adjacent to the non volcanic labrador sea margin and is interpreted to be of similar structure segmentation of the margin by major faults can explain the abrupt transitions from volcanic to non volcanic margin segments the continent ocean boundary for the region is determined based on the potential field and seismic data along the greenland plate in davis strait the continent ocean boundary coincides with the ungava fault zone the continent ocean boundary is used in conjunction with published poles of rotation to evaluate plate reconstructions at chrons 33n and 27n the earlier reconstruction produces an extensive overlap of boundaries in the north the later reconstruction shows a gap in the southern region thus additional information is required to refine the reconstructions

initial micromorphological results from liang bua flores indonesia site formation processes and hominin activities at the type locality of homo floresiensis liang bua a karstic cave located on the island of flores in eastern indonesia is best known for yielding the holotype of the diminutive hominin homo floresiensis from late pleistocene sediments modern human remains have also been recovered from the holocene deposits and abundant archaeological and faunal remains occur throughout the sequence the cave the catchment in which it is located and the gross aggradational phases of the sediment sequence have all been subject to a great deal of scientific scrutiny since the discovery of the holotype of h floresiensis in 2003 a recent program of geoarchaeological research has extended analyses of the site s deposits to the microstratigraphic micromorphological level the stratigraphic sequence in the cave is well defined but complex comprising interstratified sediments of diverse lithologies and polygenetic origins including volcanic tephras fine grained colluvium coarse autogenic limestone gravels speleothems and anthropogenic sediments such as combustion features the sedimentological and chemical heterogeneity suggest that processes of site formation and diagenesis varied markedly through time both laterally and vertically we present initial results from samples collected in 2014 from an excavation area near the rear of the cave which yielded radiocarbon ages from charcoal that fill an important temporal gap in the chrono stratigraphic sequence of previously excavated areas of the site the results indicate marked changes in site environment and hominin activity during the late pleistocene relating primarily to the degree to which the cave was connected to the hydrogeological system and to the varying intensities of use of the cave by hominins importantly we identify anthropogenic signs of fire use at the site between 41 and 24 thousand years ago most likely related to the presence of modern humans

insights from the jan mayen system in the norwegian greenland sea i mapping of a microcontinent summary in this contribution we present a model of microcontinent architecture within a system that involves a complex rifted margin setting and different phases of deformation and continental breakup our case study is the jan mayen microcontinent jmmc located in the central part of the norwegian greenland sea we have revised its basement and sedimentary geometries using modern and vintage seismic reflection profiles and updated potential field data the northern part of the jmmc consists of a 10 15 km thick continental crust body flanked by two major sedimentary basins we propose that the microcontinent is divided into six distinct segments characterized by different basement and sedimentary architectures the shallow stratigraphy is well imaged and has been detailed three distinct sedimentary units have been defined together with pronounced reflectors ja red jo and f related to erosional magmatic thermal or uplift events the continent ocean boundary has been revised together with the mapping of the distinct volcanic events that characterize the evolution of the microcontinent a companion paper aims to further detail the interpretation of the jmmc structure presented here by constraining the conjugated mid norwegian and mid east greenland structures

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

involvement of anomalously as au rich fluids in the mineralization of the heilan gou gold deposit jiaodong china evidence from trace element mapping and in situ sulfur isotope composition abstract the mesozoic 120 ma heilan gou gold deposit located in the jiaodong gold province eastern margin of the north china craton ncc consists of several ultra high grade up to 600 g t auriferous quartz veins in the early cretaceous guojialing granodioritic pluton ore related pyrite crystals from this deposit show complex textures in this study emp nano sims trace element mapping and in situ sulfur isotope analyses were conducted to characterize the textural and compositional features two types of pyrite were identified type i crystals py i hosted in gold quartz pyrite veins are generally homogeneous in chemical composition they contain low concentrations of as based on the distinctive textural and compositional signatures between py iia and py iib it is proposed that anomalously as au rich fluids were injected into hydrothermal systems during the late ore forming stage which may have intensively interacted with certain sedimentary rocks the involvement of these fluids firstly documented in this study can largely explain the occurrence of native gold ore shoots at heilan gou in addition the intimate association between as and au highlights the important roles of as in enrichment of au in the jiaodong gold district

evolution of the eastern margin of korea constraints on the opening of the east sea japan sea abstract we interpreted marine seismic profiles in conjunction with swath bathymetric and magnetic data to investigate rifting to breakup processes at the eastern korean margin that led to the separation of the southwestern japan arc the eastern korean margin is rimmed by fundamental elements of rift architecture comprising a seaward succession of a rift basin and an uplifted rift flank passing into the slope typical of a passive continental margin in the northern part rifting occurred in the korea plateau that is a continental fragment extended and partially segmented from the korean peninsula two distinguished rift basins onnuri and bandal basins in the korea plateau are bounded by major synthetic and smaller antithetic faults creating wide and considerably symmetric profiles the large offset border fault zones of these basins have convex dip slopes and demonstrate a zig zag arrangement along strike in contrast the southern margin is engraved along its length with a single narrow rift basin hupo basin that is an elongated asymmetric half graben analysis of rift fault patterns suggests that rifting at the korean margin was primarily controlled by normal faulting resulting from extension rather than strike slip deformation two extension directions for rifting are recognized the onnuri and hupo basins were rifted in the east west direction the bandal basin in the east west and northwest southeast directions suggesting two rift stages we interpret that the east west direction represents initial rifting at the inner margin while the japan basin widened rifting propagated southeastward repeatedly from the japan basin toward the korean margin but could not penetrate the strong continental lithosphere of the korean shield and changed the direction to the south resulting in east west extension to create the rift basins at the korean margin the northwest southeast direction probably represents the direction of rifting orthogonal to the inferred line of breakup along the base of the slope of the korea plateau after breakup the southwestern japan arc separated in the southeast direction indicating a response to tensional tectonics associated with the subduction of the pacific plate in the northwest direction no significant volcanism was involved in initial rifting in contrast the inception of sea floor spreading documents a pronounced volcanic phase which appears to reflect asthenospheric upwelling as well as rift induced convection particularly in the narrow southern margin we suggest that structural and igneous evolution of the korean margin although it is in a back arc setting can be explained by the processes occurring at the passive continental margin with magmatism influenced by asthenospheric upwelling

deep seismic reflection data from the bay of fundy and gulf of maine tectonic implications for the northern appalachians three deep penetration seismic reflection profiles were collected off southwest nova scotia to determine the crustal structure and geometry beneath the avalon and meguma zones of the appalachian orogen in canada onshore geological features have been traced seawards using new gravity and magnetic anomaly maps the seismic data can also be correlated with the previous united states geological survey profile in the central gulf of maine two seismically distinct lower crustal blocks are identified the avalon and sable lower crustal blocks separated by a major north dipping reflection zone that cuts the entire crust the recognition of the sable block adds a fourth block to the three already identified in the canadian appalachians the sable block is overlain by the meguma zone the avalon zone overlies at least the northern part of the avalon lower crustal block although offshore extension of geological features is not unequivocal it appears that a north dipping reflection zone southwest of nova scotia m