the thermal structure of mantle plumes axisymmetric or triple junction summary a mantle plume is probably a complex 3 d thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below but followed down towards the base of the layer probably consists of a triple junction or quadruple junction of connected hot sheets a relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes these conclusions are suggested by numerical experiments on a 3 d constant viscosity plane layer with stress free boundaries which detail the gradational change in the planform of a convecting layer from the top of the layer to its base the planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer these anomalies are the main sources of positive for hot fluid or negative for cold fluid buoyancy and therefore they drive the convective flow they may appear in cross section as structures with either axial symmetry columns planar symmetry sheets or some complex asymmetric form when convection is driven at least partially by basal heating the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns the hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface near the upper surface the apparent planform of this experiment is analogous to that of mantle convection the cold sheets compared to subduction zones and the hot columns compared to mantle plumes the hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies surface uplift and extensional stress fields however the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume formed by the junction of three or four hot sheets on the base of the layer it seems likely that the commonly occurring triple junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath

the ocean continent boundary off the western continental margin of iberia i crustal structure at 40 30 n summary the western continental margin of the iberian peninsula has the characteristics of a rifted non volcanic margin with half graben and tilted fault blocks seen in several places the ocean continent boundary ocb is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed as at many similar margins worldwide it is particularly useful to locate the ocb off western iberia in order to constrain the pre rift fit of north america to iberia and by implication the shape of the proto bay of biscay the fit is only marginally constrained by sea floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the ocb and it is even possible that all older oceanic crust was created during the cretaceous constant polarity interval the best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure therefore in 1986 a series of seismic refraction lines was shot parallel to and normal to the continental margin these lines enabled us to bracket the location of the ocb a further constraint on the location was obtained by modelling an east west magnetic profile which included the enigmatic j anomaly this anomaly can be explained as either just pre anomaly mo or as part of the cretaceous constant polarity interval depending on whether spreading began about 127 or after 118 myr ago respectively the evidence favours the former explanation lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles this indicated a new fracture zone at 41 15 n which offsets the ocb a few key reflection profiles also suggest that the ocb can be identified by an abrupt landward step down in acoustic basement we conclude that the ocb in the eastern iberia abyssal plain lies between 12 10 and 12 30 w and has a trend just east of north this westerly location is consistent with recent estimates of the location of the ocb off the grand banks but brings into question the proposed location at about 11 w of the ocb in the tagus abyssal plain

hot spot activity and the break up of pangea abstract the mesozoic and cenozoic positions of the continents that formed pangea in the triassic jurassic were derived from paleomagnetic and intraplate volcanic data paleoclimatic observations such as reef and fossil flora distribution and geological observations major hot spots helped to determine the longitudinal position of pangea and to construct a model of plate motion during the pangean break up the position of the northern part of pangea was constrained using iceland and jan mayen hot spots the iceland hot spot was traced from its present day position to greenland in the paleocene to baffin bay in the late cretaceous to the alpha ridge in the early cretaceous to the chukchi borderland in the middle late jurassic to franz joseph land in the early jurassic the yenisei khatanga trough in the middle triassic and finally to west siberia in the late permian early triassic the hot spot activity is expressed by eastern and western greenland volcanics the siberian trap basalts and perhaps by alpha ridge and chukchi borderland volcanics the chukchi borderland volcanics are related to the early stage of the opening of the canadian basin the position of the southern part of pangea was constrained using the bouvet hot spot this hot spot was tracked from its present day position to western antarctica in the early cretaceous late jurassic and to south africa in the early jurassic late triassic this hot spot activity produced the ferrar and karoo volcanics the model of plate motion obtained agrees with other data on intraplate volcanics which are also related to hot spots at the time of the opening of the central atlantic the cape verde and canary island hot spots were located along the ocean s spreading axis the long lasting location of hot spot and associated mantle upwelling plumes could help to explain forces driving the tethys transit plates opening of the ligurian ocean and eastern mediterranean the european hot spots are related to the several mesozoic phases of rifting for example the rhine graben hot spot may have been located at the rifting axis in the north sea several authors have already discussed the contribution of hot spots to the opening of the south atlantic and indian oceans mantle plumes associated with hot spots play an active role in rifting and initial phases of spreading the lithospheric displacement caused by upwelling combined with sometimes remote collisional forces on the other side of continental plate may result in compression and basin inversion

the role of alluvial fans in the mountain fluvial systems of southeast spain implications of climatic change the mountain fluvial systems of southeast spain involve sediment supply from steep mountain slopes into headwater channels alluvial fans often occur where these headwater channels emerge from the mountain areas and may influence the connectivity of the sediment transport system from the mountain source areas to the main lowland drainages critical in this role is whether the alluvial fans are aggrading or dissecting and whether there is a break or continuity in the channel through the fan environment previous work has identified some of the factors influencing the behaviour of the alluvial fans in southeast spain this paper deals with the mountain front alluvial fans in the semi arid areas of murcia and almeria provinces it attempts by mapping the location of alluvial fans then their classification into aggrading or dissecting fans to identify the extent to which the mountain fluvial systems are buffered by aggrading alluvial fans or exhibit channel continuity through the mountain front environment it further considers the implications of climatically induced changes between aggradational and dissectional behaviour on alluvial fans

geochemical correlation of three large volume ignimbrites from the yellowstone hotspot track idaho usa three voluminous rhyolitic ignimbrites have been identified along the southern margin of the central snake river plain as a result of wide scale correlations new volume estimates can be made for these deposits 350 km3 for the steer basin tuff and cougar point tuff xi and 1 000 km3 for cougar point tuff xiii these volumes exclude any associated regional ashfalls and correlation across to the north side of the plain which has yet to be attempted each correlation was achieved using a combination of methods including field logging whole rock and mineral chemistry magnetic polarity oxygen isotope signature and high precision 40ar 39ar geochronology the steer basin tuff cougar point tuff xi and cougar point tuff xiii have deposit characteristics typical of snake river sr type volcanism they are very dense intensely welded and rheomorphic unusually well sorted with scarce pumice and lithic lapilli these features differ significantly from those of deposits from the better known younger eruptions of yellowstone the ignimbrites also exhibit marked depletion in 18o which is known to characterise the sr type rhyolites of the central snake river plain and cumulatively represent 1 700 km3 of low 18o rhyolitic magma feldspar values 2 3 2 9 erupted within 800 000 years our work reduces the total number of ignimbrites recognised in the central snake river plain by 6 improves the link with the ashfall record of yellowstone hotspot volcanism and suggests that more large volume ignimbrites await discovery through detailed correlation work amidst the vast ignimbrite record of volcanism in this bimodal large igneous province

local and regional controls on the lateral emplacement of the ben hiant dolerite intrusion ardnamurchan nw scotland abstract sub volcanic and related satellite intrusions record the delivery storage and accommodation of magma before eruption however when several volcanic centres are in close proximity the relationship between centre and satellites may be ambiguous here we examine the structure of the ben hiant dolerite satellite intrusion discuss its relationship with the ardnamurchan central complex located 2 km to the northwest and explore the possibility of a genetic connection with the next nearest centre the mull central complex 35 km to the southeast structural field observations and anisotropy of magnetic susceptibility ams fabric analyses reveal that the ben hiant dolerite was emplaced as a series of lobes in a sequence of stacked sheets the ams fabric data further indicate that the ben hiant dolerite intruded laterally and may have been sourced from either the ardnamurchan central complex or the mull central complex testing either model and discriminating between these two possible source reservoirs remains an outstanding challenge however the potential implications of the lateral movement of magma over many 10 s of kilometres between upper crustal magmatic centres suggests hidden complexities associated with volcanic plumbing systems that must be tested structurally as well as geochemically

crustal structure of the goban spur rifted continental margin ne atlantic summary we present results that constrain the crustal structure across a typical non volcanic rifted continental margin the goban spur in the ne atlantic traveltime and amplitude modelling of wide angle ocean bottom seismograph profiles shows that the continental crust thins in a seaward direction to about 7 km before it breaks to allow a new oceanic spereading centre to develop the crustal velocity structure the stretching factors and the density distribution inferred from gravity anomalies are consistent with bulk pure shear stretching for the development of the rifted continental margin the seismic velocity structure of the thinned continental crust suggests that thinning was accompanied by only limited igneous intrusion and extrusion in the upper crust the adjacent oceanic crust is abnormally thin 5 4km this is explained most readily by accretion at a very slow spreading centre approx 11 mm yr full rate although additional cooling by conductive heat loss from the parent mantle as it rose slowly beneath the stretched and thinned continental lithosphere together with minor igneous intrusion into and surface flows onto the adjacent continental crust may also have reduced the oceanic igneous thickness immediately adjacent to the continent ocean transition

crustal structure of the goban spur continental margin northeast atlantic from deep seismic reflection profiling deep reflection data across the uk continental margin are used to test theories concerning the origin of a pervasive reflective fabric in the lower continental crust seen on many deep reflection profiles worldwide the origin of the reflections is at present little understood but clearly represents an important crustal process the western approaches margin wam line was designed to determine the behaviour of the lower crustal reflections in crust which has undergone significant extension the wam profile across the continental shelf is characterized by bright continuous lower crustal reflections some of which reach 30 km in length and have minimum reflection coefficients of 0 1 deep water multiples in the area of the continental slope partially obscure reflections from the lower crust but detailed analyses show that reflectors are present in the lower crust on the continental slope adjacent to the continent ocean boundary the lower crust on the wam profile is well imaged and free of noise and multiples this very thin crust exhibits the same reflectivity pattern as is observed on the continental shelf i e the upper half of the crust is blank and the lower half contains sub horizontal reflections the lower crustal reflections adjacent to the continent ocean boundary are less continuous than the reflections seen on the shelf and are interpreted as being the remnants after extension of the lower crustal reflectors observed on the continental shelf the lower crustal reflectors are thus interpreted to pre date the extensional episode that led to the opening of the atlantic the preservation of thinned sections of both transparent upper crust and reflective lower crust adjacent to the continent ocean boundary suggests that extension by pure shear was the mechanism by which the crust was thinned in the goban spur area

mantle plumes and flood basalt stratigraphy in the parana south america geochemical studies of the parana continental flood basalts in brazil have led to the recognition of distinct magma types which have been used to infer the internal stratigraphy of the lava pile the overstepping of stratigraphic units toward the north and a similar compositional change in sills within the underlying parana basin sedimentary rocks imply that the site of magnatism migrated 750 km toward the north during this volcanic event this has important implications for recent plume related models that have argued whether fithospheric rifting must accompany the presence of a mantle plume in order to generate continental flood basalt magmatism it is difficult to ascribe the magnitude and direction of the shift in the locus of the parana magmatism to movement of the brazilian lithosphere relative to the underlying tristan mantle plume instead it is suggested that the observed lava distribution and internal structure of the parana etendeka flood basalt province were imposed by the intracacies of the rifting process during the initial opening of the south atlantic ocean

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

early tertiary magmatism in ne greenland abstract between 73 and 74 n in ne greenland a lower series of quartz tholeiite lavas is overlain by an upper series which varies from quartz tholeiitic to basanitic the lower series basalts are attributed to large melt fractions produced east of the present outcrop where lithospheric thinning permitted the iceland plume to ascend to shallow levels relocation of eruption sites up to 150 km further west preceeded upper series activity in this small degree melts generated principally from garnet lherzolite facies sources were followed by products of more extensive shallower melting which gave rise successively to the upper series lavas a post upper series basaltic dyke swarm and its associated caldera complex palaeomagnetic observations and 40 ar 39 ar dating suggest that the lower series and part of the upper series erupted in c24r at 58 57 ma rejuvenescent activity at c 32 ma marking the termination of volcanism may relate to decompressive mantle melting associated with late regional uplift

the timing of crustal extension and the eruption of continental flood basalts the association between mantle plumes continental flood basalt eruption and the extension and rifting of the crust is well recognized1 3 but there has always been controversy as to whether the plume or the rifting was the initiating factor4 5 was rifting triggered by doming above the thermal plume2 3 6 or was rifting the driving force preceding the associated volcanicity7 8 here i show that recent flow by flow mapping of two continental flood basalt provinces the columbia river basalt group and the deccan basalt group allows one to infer the relative timing of volcanicity and crustal extension in both provinces eruption of the main tholeiitic phase preceded significant extension and crustal thinning this suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume and that the subsequent crustal extension rifting and decompression was in part a consequence of the initiation of that plume

lithospheric thickness as a control on basalt geochemistry variations in the thickness of the lithosphere are likely to influence the trace element compositions of basaits by controlling the distribution of melt and the minerals in the melting assemblage in the asthenosphere examples of systematic trace element variations that appear to be related to the thickness of the lithosphere are found in both oceanic and continental basalts in the oceans lithospheric thickness is closely related to age and its control is manifested by a relation between trace element composition and age of basement ocean crust in atlantic ocean island basalts beneath continents lithospheric thickness is less age sensitive but it varies widely in response to extensional tectonics contemporary hot spots which have been implicated in the generation of ancient flood basalts show variations in the rare earth element content of their associated basalts which apparently reflect changes in lithospheric thickness between the time of flood basalt eruption and the present

the seismic structure of thinned continental crust in the northern bay of biscay summary the stretching and thinning of the continental crust which occurs during the formation of passive continental margins may cause important changes in the velocity structure of such crust further crust attenuated to a few kilometres thickness can be found underlying oceanic water depths this paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness a single seismic refraction line shot over thinned continental crust as part of the north biscay margin transect in 1979 was studied in detail tau p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure this was confirmed when synthetic seismograms were calculated the thinned continental crust p 7 0 exhibits a two gradient structure in the non sedimentary crust with velocities between 5 9 and 7 4 km s l an upper 0 8 s l layer overlies a 0 4 s l layer no layer comparable to oceanic layer 3 was detected the uppermost mantle also contains a low velocity zone

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

factors influencing magmatism during continental breakup new insights from a wide angle seismic experiment across the conjugate seychelles indian margins we present a model of the northern seychelles continental margin derived from controlled source wide angle seismic traveltime inversion and teleseismic receiver functions this margin has been widely cited as a classic example of rifting in association with a continental flood basalt province the deccan traps however we do not find the typical set of geophysical characteristics reported at other margins linked to continental flood basalts such as those of the north atlantic the oceanic crust formed immediately after breakup and throughout the first 3 ma of seafloor spreading is just 5 2 km thick less than half that typically seen at other volcanic margins the continent ocean transition zone is narrow and while two packages of seaward dipping reflectors are imaged within this transition they are weakly developed beneath the thinned continental crust there is an approximately 4 km thick layer of high velocity material 7 5 7 8 km s that we interpret as mafic material intruded and underplating the lower crust however we believe that this underplating most likely happened prior to the breakup overall the observations show that the rifting of india from the seychelles was characterized by modest magmatism the spatial extent of the deccan flood basalt province is therefore smaller than previously thought we speculate that either the lateral flow of deccan related hot material beneath the breakup region was hampered perhaps as the rifted margins did not intersect the center of the deccan source or there was incomplete melt extraction from the wide melting region that formed between the rapidly diverging plates if the latter explanation is correct then the rate of plate separation as indicated by the initial seafloor spreading rate is more important in controlling the volume of magmatism generated during continental rifting than has been previously recognized

conjugate margins of canada and europe results from deep reflection profiling deep seismic reflection profiles have been collected across the conjugate margins of the north atlantic ocean the eastern north atlantic margin is traversed in the goban spur region and the western north atlantic margin is crossed in the vicinity of flemish cap in the grand banks region these seismic profiles allow us to examine the deep structures and mode of extension in crust that was once contiguous the flemish cap and goban spur margins have different structural styles thick and relatively unfaulted crust is present on the flemish cap margin west of the continent ocean boundary whereas the goban spur margin exhibits a zone of extensional faulting and thinned continental crust the restored rift zone displays overall symmetry in which a bridge of thin crust 15 km thick about 60 km wide joins the two crustal blocks of normal continental thickness 28 km the configuration of the moho the geometry and distribution of the lower crustal reflections and the overall symmetry of the rift zone favor pure shear stretching in the lower lithosphere although either pure or simple shear could accommodate crustal extension

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

crustal structure of the celtic sea and western approaches from gravity data and deep seismic profiles constraints on the formation of continental basins a free air anomaly map of the celtic sea and the western approaches is presented including all available marine gravimetric data and bouguer anomalies on land n60 features are predominant in the area they correspond to the n60 so called caledonian grain which played a major role in the variscan orogeny and in the formation of the permo triassic sedimentary basins the fundamental new point which comes out from this gravimetric map is the existence of a closely spaced pattern of n105 features which we regard as late carboniferous transpressional structures in the eastern pan of the celtic sea thrusts are in the same n105 direction west of 6 w longitude however folds and thrusts trend preferentially in a n60 direction following the older grain there the n105 lineaments may be regarded as dextral shear structures with a possible compressive component the hercynian shortening of the celtic sea domain located between the variscan front and the lizard suture resulted from the displacement along the n160 direction of the two adjacent rigid domains north of the variscan front the n105 and n135 features were possibly active in more recent times then both the n60 and n105 trends controlled the formation of permo triassic basins the n105 directions being transfer faults two dimensional gravity modelling of the swat profiles in the celtic sea shows that the base of the reflective lower crust may be associated with the moho discontinuity everywhere in the celtic sea except below the saint george s channel basin the lower crust does not change significantly in depth and thickness beneath the permo triassic basins none of the available models simply satisfies such conditions they all require at least a modification of the geometry of the lower crust after the formation of the celtic sea basins nevertheless if the simple shear model can be ruled out mainly from structural arguments the uniform stretching and to a lesser degree the thermomechanical models seem to be more relevant models however two additional conditions are necessary 1 the preservation of crustal seismic features formed before the creation of basins must be explained and 2 a significant portion of the volume of the stretched lower crust about 40 of this volume for the north celtic sea basin must be added below or perhaps partly above the lower crust by lateral flow of the crust or at the expense of the upper mantle

anomalous heat flow and geoid across the cape verde rise evidence for dynamic support from a thermal plume in the mantle summary we describe results from 200 heat flow measurements across the cape verde rise in the north atlantic they show that the heat flow through normal 125 myr old crust is 45 5 3 4mwm 2 close to the prediction for a lithospheric plate model heat flow increases towards the centre of the rise reaching a maximum of 16 4 mw m 2 above the normal oceanic value the concommitant geoid anomaly at the centre of the rise is 7 6 0 3 m and the depth anomaly is 1900 200 m the anomalous heat flow geoid and bathymetric values are used to constrain a variety of theoretical models of hot spot mechanisms lithospheric reheating models alone whether involving transient or sustained thinning of the lithosphere fail to model the broad heat flow anomaly and the long duration of uplift and volcanism observed on the cape verde rise axisymmetric convection models provide a better fit to the observed data and suggest that the swell is a consequence primarily of dynamic uplift generated by an ascending thermal plume in the underlying mantle uplift caused by expansion of the solid lithosphere is of smaller importance the observed geoid bathymetry and heat flow anomalies are reproduced well using convection models with upper mantle viscosities of 2 4 times 1020 pa s and a coefficient of thermal expansion in the range 2 0 2 5 times 10 5 k 1 the effect of melting in the ascending plume is to buffer the heat flow anomalies in the case of the cape verde rise partial melting is minimal but its effect on other hotspots will be to limit observed heat flow anomalies at the surface to less than 20 25 mw m 2

an alternative emplacement model for the classic ardnamurchan cone sheet swarm nw scotland involving lateral magma supply via regional dykes abstract sub volcanic intrusive networks of which cone sheets are recognised as a major constituent partially control volcano growth and eruption style the accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip parallel magma flow however the emplacement mechanism of cone sheets remains poorly understood the 58 ma cone sheet swarm of ardnamurchan nw scotland offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets through studying magma flow and their importance in volcanic edifice construction structural measurements and anisotropy of magnetic susceptibility ams analyses have constrained a lateral magma flow pattern consistently oriented nw se in the majority of the ardnamurchan cone sheets field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry our observations suggest cone sheet formation may be linked to laterally propagating nw se trending regional dykes sourced from laterally adjacent magmatic systems likely the palaeogene mull central complex which are deflected around the central complex by stress field interference implicitly edifice construction and potential eruption precursors observed at a volcano may instigate or result from magmatic activity in laterally adjacent volcanic systems

the bulawayan supergroup a late archaean passive margin related large igneous province in the zimbabwe craton the four principal lithostratigraphic units koodoovale manjeri reliance and zeederbergs of the widespread late archaean bulawayan supergroup in the zimbabwe craton record the early to middle stage development of a major c 250 000 km 2 volcanosedimentary basin the craton wide c 2 70 ga manjeri unit varies systematically from a tens of metres thick dominantly chemogenic basinal sediment in gradational conformable contact with the underlying several kilometres thick diachronous 2 83 2 71 ga felsic koodoovale unit in the north and west to an up to hundreds of metres thick dominantly siliciclastic retrogradational platformal sediment unconformably overlying protocratonic basement in the south and east the intermittent c 2 70 ga reliance unit consists of the flow fields of several discrete komatiitic sill flow complexes scattered throughout the craton and fed by an interlinked system of mid to upper crustal flow through feeder sills the c 2 70 ga zeederbergs unit comprises a submarine flood basalt sequence of several kilometres thickness that erupted from ene trending parallel mafic dykes preserved in the south and east the basalts form a craton wide blanket to the reliance flow fields and intervening manjeri sediments certain structural stratigraphic features rule out primary construction of the bulawayan supergroup by early lateral accretion as proposed by several previous workers instead evolution of the bulawayan basin can best be explained by vertical accretion involving 1 abundant episodic and bimodal plume related volcanism and 2 extension and rapid subsidence of continental lithosphere many aspects of the bulawayan supergroup provide important insights into archaean tectonics mantle plume activity magma generation and lithospheric plumbing

variations in heat flow across the goban spur and galicia bank continental margins we present the results of 44 new heat flow stations which were taken in 1984 and 1989 in profiles across the goban spur and galicia bank continental margins nw atlantic ocean simple extensional models indicate that the heat flow across both these early cretaceous rifted margins should increase from values of 45 50 mw m2 over oceanic crust to 65 80 mw m2 on the continents the rate of this increase should help to constrain the mechanism simple versus pure shear by which the upper more radiogenic continental crust is thinned measurements across goban spur increase from values of 40 45 mw m2 over oceanic crust to 50 55 mw m2 near the continental shelf they follow the predicted trend for pure shear rifting but only if the value of upper crustal radiogenic heating is low 1 2 mu w m3 otherwise they would require the upper crust to thin more rapidly than the total crustal thickness as with a lower plate simple shear margin measurements across galicia bank show a very different pattern with similar values over oceanic crust but much lower values 30 35 mw m2 nearer land this is difficult to reconcile with any simple single rifting event but is more compatible with an origin as a pure shear or lower plate rather than upper plate margin we also note that oceanic values of heat flow require asthenospheric temperatures 100 degrees c lower than normal for both margins this indicates that the triple junction in existence between these margin segments during the breakup of iberia europe and north america was not the site of a major mantle plume

transition from continental break up to punctiform seafloor spreading how fast symmetric and magmatic 1 we present a comparison between numerical and analogue models focusing on the role of inherited lithospheric structures in influencing the process of continental break up our results highlight that the presence of pre existing anisotropies localizes strain and favors continental break up and formation of a new ocean for a fixed strain rate the pre rift lithosphere configuration influences rift duration melt production and width and symmetry of the continental margin pair model results show a mainly two phase tectonic history from continental extension to oceanization in the first phase extension affects contemporaneously the whole rift structure while in the second phase asthenosphere upwelling occurs into punctiform regularly spaced spots sequentially propagating in an extension orthogonal direction index terms 8109 tectonophysics continental tectonics extensional 0905 8120 tectonophysics dynamics of lithosphere and mantle general 8150 tectonophysics plate boundary general 3040 citation corti g j van wijk m bonini d sokoutis s cloetingh f innocenti and p manetti transition from continental break up to punctiform seafloor spreading how fast symmetric and magmatic geophys res lett 30 12 1604 doi 10 1029 2003gl017374 2003

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

a short interval of jurassic continental flood basalt volcanism in antarctica as demonstrated by 40ar39ar geochronology a continental flood basalt province the ferrar group kirkpatrick basalt and ferrar dolerite crops out along 3000 km of the transantarctic mountains in antarctica and is temporally related to the break up of gondwanaland although a wide range of dates between 90 and 193 ma have been published for the kirkpatrick basalt it is now recognized that the young dates reflect non ideal behavior of ar in the matrix in order to refine the geochronology feldspar separates have been analyzed by the 40ar39ar incremental heating method the main objectives are to constrain the duration of extrusive activity and the timing of volcanism along the outcrop belt basalt samples have been studied from the three principal outcrop areas yielding the following apparent ages central transantarctic mountains 176 8 0 5 ma south victoria land 176 4 0 4 ma north victoria land 176 6 0 7 ma ages from different stratigraphic levels within each area and from the three different areas are not analytically distinct the data imply that the eruptive activity which produced the kirkpatrick basalt occurred within a short interval of less than about 1 m y at 176 6 1 8 ma over an area which included more than 1200 km of the transantarctic mountains the jurassic volcanism in antarctica represents a short episode of magmatism comparable in duration with other well dated continental flood basalt provinces the linearly extensive outcrop of the ferrar province and the rapid eruption of the lavas suggests that lithospheric stretching exerted a major control on magmatism the poorly constrained age of the bajocian bathonian boundary makes the previously suggested connection between ferrar volcanism and an extinction event at that boundary uncertain

was a short lived baffin bay plume active prior to initiation of the present icelandic plume clues from the high mg picrites of west greenland abstract the tertiary volcanism of the north atlantic region provides one of the most extensive and best described magmatic records of interaction between a starting plume and continental lithosphere and the role of plumes in continental break up many authors consider the entire volcanic province from baffin island in the north west to scotland in the south east to be the product of a single proto icelandic starting plume head initially centred beneath east greenland whose initial eruptions were strongly influenced by the pre break up structure of the continental lithosphere we demonstrate that the presence of mesozoic palaeocene extensional sedimentary basins that developed within older n s trending mobile belts influenced the location of the earliest on shore tertiary basalts even taking this into account however the prevalence of high magnesium picrites among volcanics bordering the davis strait in the extreme north west of the province and their much lower abundance in east greenland directly above the supposed plume axis contradict the single plume model best estimates of the most refractory liquid compositions erupted in the disko area imply potential temperatures beneath west greenland of 1540 1600 c such high temperatures are more consistent with the axial region of a separate hot plume beneath west greenland than with the periphery of the icelandic plume head west greenland picrites cannot be attributed to an expanding hot doughnut as they are not present in other peripheral areas likely to be swept by such a doughnut uniformly high feo contents and comparison with mckenzie bickle melting parameterisations suggest the west greenland picrites are the product of 25 melting constrained to depths of 60 90 by a lithospheric lid geographical zonation in incompatible element and radiogenic isotope enrichment holm et al 1993 provides support for a separate somewhat earlier plume centred beneath baffin bay picrites from svartenhuk halvo north are more enriched than equivalent picrites on disko and baffin island south this pattern reminiscent of trends along the reykjanes ridge would be radial to a plume axis beneath baffin bay but not to one centred beneath east greenland

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

geology and geochronology of the line islands geological and geophysical studies along the entire length of the line islands were undertaken in order to test the hot spot model for the origin of this major linear island chain volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls volcanic rocks from the line islands are alkalic basalts and hawaiites in addition a tholeiitic basalt and a phonolite have been recovered from the central part of the line chain microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high tio2 1 0 4 0 wt and al2o3 3 4 9 1 wt and are of alkaline to peralkaline affinities major element compositions of the line islands volcanic rocks are very comparable to hawaiian volcanic rocks trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas the line islands lavas are very much unlike typical mid ocean ridge or fracture zone basalts dating of these rocks by 40ar 39ar k ar and paleontological methods combined with deep sea drilling project data from sites 165 315 and 316 and previously dated dredged rocks provide ages of volcanic events at 20 localities along the chain from 18 n to 9 s a distance of almost 4000 km all of these dates define mid cretaceous to late eocene edifice or ridge building volcanic events eocene volcanic events took place from 15 n to 9 s and late cretaceous events took place from 18 n to 9 s in the southern line islands both cretaceous and eocene events took place on the same edifice or ridge indicating recurrent volcanism at a single locality the irregular distribution of atolls in the chain the fact that late cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern line islands and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1 2 related subsidence path magnetic surveys of 11 seamounts show that four of the seamounts from the central line chain give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of cretaceous seamounts these four poles agree with other paleomagnetic data of middle late eocene early oligocene age from the pacific one of these four seamounts yielded a 40ar 39ar total fusion age of 39 ma because the poles of all four seamounts fall into a tight group we infer that they are probably of middle late eocene age to early oligocene age the other three seamounts as well as one seamont from the line islands analyzed previously all give virtual geomagnetic poles which agree with late cretaceous paleomagnetic data from the pacific of these four seamounts three give late cretaceous 40ar 39ar ages ranging from 71 to 85 ma another in the southern line islands is interpreted on paleontological evidence to be of late cretaceous age the origin of the line islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots the single hot spot model cannot account for all of the volcanic edifices in the line islands although it does explain a general age progression of 9 6 0 4 cm yr from north to south along the chain derived from a number of dated edifices the four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data the petrologic data argue against a mid ocean ridge or transform fault origin proposed by earlier workers the complex volcanic province represented by the line islands remains a challenge to existing models for the origin of midplate volcanism in the pacific an atoll drilling program which could determine edifice building histories paleolatitudes of seamount formation and subsidence rates and patterns in the line islands is needed

the hatton bank continental margin ii deep structure from two ship expanding spread seismic profiles summary the continent ocean transition adjacent to hatton bank was studied using a dense grid of single ship and two ship multichannel seismic profiles the interpretation of the explosive expanding spread profiles esps which were shot as part of this survey are discussed here in detail extensive seaward dipping reflectors are developed in the upper crust across the entire margin these seaward dipping reflectors continue northwards on the faeroes and voring margins where they have been shown to be caused by basaltic lavas as well as on the conjugate margin of east greenland the dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension the esps show clear seismic arrivals out to ranges of 100 km wide angle moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals the determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms the results from the esps show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7 3 7 4 km s this lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the moho the lower crustal lens of high velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas now seen as dipping reflectors on the margin

the hatton bank continental margin iii structure from wide angle obs and multichannel seismic refraction profiles summary in two recent papers normal incidence and expanding spread profiles esps shot across the hatton bank passive continental margin showed seaward dipping reflectors in the upper crust and elevated velocities in the lower crust the reflectors were interpreted as subaerial basalt flows and the high velocities as underplated or intruded igneous material in the lower crust a recent gravity model shows high densities in the lower crust supporting the velocity model in this study we present the forward modelling of two wide angle seismic refraction datasets shot perpendicular to strike and through the esp midpoints the final velocity model across the margin is consistent with all the seismic datasets but differs in some important respects from the esp based model a high velocity volcanic pile is outlined mid way up the continental slope and the inferred velocities in the lower cnst are slightly lower the differences between the two models can be explained by the three dimensional nature of the margin the esps were modelled assuming that the sub sediment structure was invariant along strike the final velocity model is used to obtain an estimate of the total thickness of melt accreted across the margin which is calculated to thicken from zero at hatton bank to about 12 km mid way down the continental slope at the bottom of the continental slope the adjacent igneous section forming the oceanic crust is about 18 km thick and thins rapidly northwestwards to 11 km as the oceanic crust thins its seismic velocity decreases this suggests that it is less mafic and that there is a post rift decrease in the percentage of mantle melting this decrease may be explained by a decrease in asthenospheric temperature and or by a reduction in volatile content

new constraints on the formation of the non volcanic continental galicia flemish cap conjugate margins numerous multichannel seismic data acquired on the starved continental margin off galicia show that the prominent intra crustal s reflector is observed only below the oceanward portion of the continental margin close to the ocean continent boundary superficial extension has been computed from the geometry of the well imaged tilted fault blocks of the entire margin by using a series of parallel seismic lines specifically designed for this purpose the pure shear model clearly accounts for the lithospheric extension of the continentward portion of the continental margin but not for the oceanward portion of the continental margin where the s reflector is present the s reflector is a detachment zone which presents a decreasing offset in the direction of the continent and this detachment zone has been active since the beginning of rifting recent kinematic reconstructions show that the conjugate margin to galicia is the southeastern flemish cap margin from the few seismic profiles available on this canadian margin a major master fault is observed beneath the continental slope and is interpreted as the outcropping portion of a main detachment zone identified on the galicia margin as the s reflector the proposed mechanism of extension for these two conjugate margins is pure shear which affects the whole lithosphere but in the upper part of the crust the rifting mechanism is either pure shear for the continentward portion of the galicia margin or simple shear for the oceanward portion of the galicia and the whole southeastern flemish cap margins where the detachment zone is active i consider that pure shear is the basic lithospheric mechanism and that simple shear is a mechanism which affects only the upper crust