variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

the structure evolution and symmetry of the magma poor rifted margins of the north and central atlantic a synthesis abstract magma poor rifted margins consistently show extreme crustal thinning accompanied by normal faulting the serpentinization of the mantle beneath crust thinned to less than 8 2 km and the unroofing of a broad zone of mantle within the continent ocean transition accompanied by the development of detachment and other large offset faults these observations are the logical result of the progressive extension of cool lithosphere away from thermal anomalies such as plumes although the paucity of magmatism may be explained by depth dependent extension of the lithosphere and by pre depleted sub crustal lithosphere rifting above initially cool sub lithospheric mantle also may explain the subsidence deficit observed at some margins synrift subsidence is buffered by the simultaneous influx of warmer oceanic asthenosphere as it extends thins and cools ductile creeping layers in the mid and deep crust become progressively more brittle resulting in increased coupling between the upper and lower crust and eventually the embrittlement of the entire crust faults cutting from the surface across the moho bringing water into the mantle and causing its serpentinization increased coupling and the development of serpentine detachments predict the development of late stage asymmetry once the entire crust is brittle such detachments are imaged on some margins and inferred on others analysis of the crustal structure across conjugate margins shows that these are approximately symmetric until this late stage when they become markedly asymmetric similar analyses show that depth dependent stretching of the crust is insufficient to explain the discrepancy between the amount of the visible extension along faults and the amount of crustal thinning instead this extension discrepancy may be related to the complex evolution of brittle deformation through multiple phases and styles of faulting related to the changes in the rheological character and strength of the lithosphere as it is thinned complex polyphase faulting continues after complete crustal separation resulting in the exhumation of broad expanses of peridotitic basement the top of which is everywhere marked by an exhumed slip surface similar to the corrugated surface observed at mid ocean ridges the similarity in processes between mantle unroofing and seafloor spreading makes the distinction between the cot and true oceanic crust difficult and possibly moot

lower crustal earthquakes caused by magma movement beneath askja volcano on the north iceland rift the lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity the icelandic mid atlantic rift is characterized by high heat flow abundant magmatism generating up to 25 30 km thick crust and seismicity within the upper 8 km of the crust in a 20 seismometer survey in july august 2006 within the northern rift zone around the askja volcano we recorded 1700 upper crustal earthquakes cutting off at 7 8 km depth marking the brittle ductile boundary unexpectedly we discovered 100 small magnitude ml 1 5 earthquakes occurring in swarms mostly at 14 26 km depth within the otherwise aseismic lower crust and beneath the completely aseismic middle crust a repeat survey during july august 2007 yielded more than twice as many lower crustal events geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath askja we interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding askja they represent bursts of magma motion opening dykes over distances of a few meters facilitated by the extensional setting of the active rift zone

lower crustal intrusion on the north atlantic continental margin near the faroe islands in the north atlantic the majority of basalt is intruded into the continent ocean transition rather than extruded onto the surface this melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 melt as has often been assumed when continents break apart the rifting is sometimes accompanied by the production of large volumes of molten rock1 2 3 the total melt volume however is uncertain because only part of it has erupted at the surface furthermore the cause of the magmatism is still disputed specifically whether or not it is due to increased mantle temperatures we recorded deep penetration normal incidence and wide angle seismic profiles across the faroe and hatton bank volcanic margins in the northeast atlantic here we show that near the faroe islands for every 1 km along strike 360 400 km3 of basalt is extruded while 540 600 km3 is intruded into the continent ocean transition we find that lower crustal intrusions are focused mainly into a narrow zone 50 km wide on the transition although extruded basalts flow more than 100 km from the rift seismic profiles show that the melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 per cent melt as has often been assumed evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures with no requirement for either significant active small scale mantle convection under the rift or the presence of fertile mantle at the time of continental break up as has previously been suggested for the north atlantic ocean4 5 6

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

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

a mechanism for construction of volcanic rifted margins during continental breakup research into the breakup of continents and formation of ocean basins has advanced in recent years with deep marine seismic data providing much of the impetus for new concepts attention has been focused on nonvolcanic magma poor margins meanwhile a satisfactory explanation for volcanic magmatic margins with their distinctive seaward dipping reflectors sdrs has been missing sdrs are composed of subaerial lava flows of tholeiitic basalt erupted during the transition from continental rifting to seafloor spreading when the crust is both stretched and constructed from asthenosphere derived magma here we present remarkable seismic images down to 40 km offshore east greenland showing that a newly described type of extensional fault plays an integral part in the ascent of magma and the building of transitional crust two such faults define an axial horst block supported by rising magma where the plates separate lava collects in seaward growing half graben on either side of the horst while new gabbroic crust is accreted along the flanks of the magma chamber in dilational shear zones beneath where the faults detach ultimately rising asthenosphere reaches the base of the magma chamber causing the horst to split thereby initiating seafloor spreading the model we describe shows how extension and magmatism interact at the boundaries of rifting plates

seaward dipping reflectors and the continent ocean boundary at passive continental margins abstract the definition of the continent ocean boundary at passive continental margins has proved to be an elusive task even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors dipping seaward to form a wedge shaped structure their general characteristics and velocity structure suggest that they may be largely volcanic in nature but there is no agreed upon model for their origin nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins

new curvilinear scheme for elastic wave propagation in presence of curved topography we study a new curvilinear scheme for wave propagation modelling in presence of topography the discrete scheme takes advantage of recent developments in areoacoustics our new scheme relies on the conventional grid coupled with optimized filters to remove numerical noise in case of strong material heterogeneity we used non centred stencils for free surface implementation and optimized the explicit runge kutta scheme for the time differencing we performed a complete theoretical stability and dispersion analysis of the discrete scheme finally we illustrate the numerical accuracy of the new scheme by intensive experiments

crustal control of ridge segmentation inferred from observations of the reykjanes ridge large amplitude variations in topography and inferred crustal thickness along the axes of mid ocean ridges often referred to as segmentation1 are mainly observed at slow spreading ridges2 4 this observation has led to the suggestion that mantle processes give rise to segmentation only when spreading rates are low5 6 here we make the alternative proposal that the development of segmentation is controlled by the temperature of the crust segmentation cannot develop when the lower crust is hot enough to undergo rapid ductile flow thermal models predict that thick crust at a slow spreading ridge may be as hot as normal thickness crust along fast spreading ridges we accordingly test our hypothesis at a slow spreading ridge characterized by thick crust the reykjanes ridge topography and gravity data along the reykjanes ridge axis indeed show an absence of segmentation suggesting that the thermal state of the crust rather than any mantle process controls the development of this structure

seismic constraints on shallow crustal emplacement processes at the fast spreading east pacific rise we present the results of nine on bottom seismic refraction experiments carried out over young east pacific rise crust the experiments are unusual in that both the source and receiver are located within a few meters of the seafloor allowing high resolution determinations of shallow crustal structure three experiments were located within the axial summit caldera asc over zero age crust the seismic structure at these three locations is fundamentally the same with a thin 60 m surficial low velocity 2 5 km s layer a 100 to 150 m thick transition zone with velocities increasing by 2 5 km s and a layer with velocities of 5 km s at a depth beneath the seafloor of 130 190 m the surficial low velocity layer and transition zone are defined as seismic layer 2a and the 5 km s layer is defined as the top of layer 2b both the surficial low velocity layer and the transition zone double in thickness within 1 km of the rise axis we model layer 2a as the extrusive sequence and transition zone and the 2a 2b boundary as the top of the sheeted dikes the primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from 150 m to 300 m within 1 km of the asc the thickening of the extrusive layer is interpreted to be due to lava that either overflows the asc walls is emplaced through eruptions outside of the asc or travels laterally from the asc through conduits the most probable cause for the thickening of the transition zone is sill emplacement outside of the asc either from magma that does not reach the surface in an off axis eruption or magma that is transported laterally during the drainage process creating the asc we suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2a

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

the e greenland rifted continental margin an examination of the coastal flexure the structure of the e greenland coastal flexure a major structure developed during continental break up is interpreted on the basis of field observations an earlier model which involves normal inland dipping antithetic faulting and seaward rotation of fault blocks is confirmed and amplified this model requires that regional dips are very much less than dips measured on bedding planes thus leading to considerably reduced thicknesses for the tertiary volcanic succession in e greenland detailed mapping confirms the validity of these thickness estimates and leads to the conclusion that the volcanic extrusion rate was similar to that of present day iceland thus obviating the need for special mechanisms of magma generation comparable structures occur in other regions of continental break up and voluminous basalt discharge and it is suggested that the flexure developed by collapse of the attenuated crust subsequent to the massive extrusion of plateau basalts

magmatic processes at slow spreading ridges implications of the ramesses experiment at 57 45 n on the mid atlantic ridge summary this paper is the first in a series of three this issue which present the results of the ramesses study reykjanes axial melt experiment structural synthesis from electromagnetics and seismics ramesses was an integrated geophysical study which was carefully targeted on a magmatically active axial volcanic ridge avr segment of the reykjanes ridge centred on 57 45aen it consisted of three major components wide angle seismic profiles along and across the avr using ocean bottom seismometers together with coincident seismic reflection profiles controlled source electromagnetic sounding csem and magnetotelluric sounding mt supplementary data sets included swath bathymetry gravity and magnetics analyses of the major components of the experiment show clearly that the sub axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids the avr is underlain by a significant crustal magma body at a depth of 2 5 km below the sea surface the magma body is characterized by low seismic velocities constrained by the wide angle seismic data a seismic reflection from its upper surface and a region of anomalously low electrical resistivity constrained by the csem data it includes a thin ribbon like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone which flanks and underlies the melt lens ramesses is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge the result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion but are short lived rather than steady state features the magma body can exist for only a small proportion of a tectono magmatic cycle which controls crustal accretion and has a period of at least 20 000 years these findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle and of its delivery into the crust beneath slow spreading mid ocean ridges

tectonic configuration of the western arabian continental margin southern red sea the young continental margin of the western arabian peninsula is uplifted 3 5 to 4 km and is well exposed rift related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18 n and its intensity increases gradually from east to west extension is negligible near the crest of the arabian escarpment but it reaches a value of 8 to 10 in the western asir a highly dissected mountainous region west of the escarpment there is an abrupt increase in extensional deformation in the foothills and pediment west of the asir about 40 km inland of the shoreline where rocks in the upper plate of a system of low angle normal faults with west dips are extended by 60 to 110 the faults were active 23 to 29 ma ago and the uplift occurred after 25 ma ago tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 ma old are concentrated in the foothills and pediment as well the chemistry of the dikes suggests 1 fractionation at 10 to 20 kbar 2 a rapid rise through the upper mantle and lower crust and 3 differentiation and cooling at 1 atm to 5 kbar structural relations between dikes faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval but that most of the extension preceded the expansion a tectonic reconstruction of pre red sea afro arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide steep moho slopes probably developed during rift formation as indicated by published gravity data two published seismic interpretations and the surface geology

crust generated above the iceland mantle plume from continental rift to oceanic spreading center since the north atlantic continental breakup in the early tertiary the process of rifting above a mantle plume has produced large thicknesses of igneous crust we report results of an integrated offshore onshore seismic study of the crust and upper mantle along a transect of the aseismic faroe iceland ridge between the continental fragment beneath the faroe islands and the present day spreading center in northeast iceland normal incidence seismic data provide an image of the uppermost crust which is complemented by a velocity model from streamer refraction analyses these data together image four sedimentary basins up to 400 m deep along the crest of the ridge the streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data the compressional to shear wave velocity ratio in the crust of the faroe iceland ridge is 1 77 0 02 similar to that of eastern iceland the moho beneath the faroe iceland ridge lies at a depth of 25 30 km crust of 25 30 km thickness requires an upper mantle potential temperature elevated 200 250 c above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if as is likely there is a component of active convection in the underlying mantle plume core

major tectonic rotation along an oceanic transform zone northern iceland evidence from field and paleomagnetic investigations abstract we provide structural and paleomagnetic evidence for major distributed rotational shear adjacent to and including the active husavik flatey transform fault in the southern part of the tjornes fracture zone of north iceland in the flateyjarskagi peninsula immediately south of the fault dike and bedding trends gradually change over about 11 km in approaching the transform fault main trace in map view this pattern is seen as a progressive clockwise curvature of structural elements that equals or in some cases exceeds 90 we present paleomagnetic analyses that verify large clockwise rotations of the same order as the curvature of structural elements observed we compare our field and paleomagnetic observations with results from careful analog modeling that demonstrate how regional curvature of structures results from distributed strain that dominates the early stages of transform shear deformation prior to the appearance of a transform fault system comprised of discrete shears our paleomagnetic results were obtained from one relatively undisturbed reference site and three sites of deformed rocks in flateyjarskagi and two in tjornes peninsula these results indicate large vertical axis rotations and the progressive deflection of dike trends throughout northern flateyjarskagi also serves as a separate quantitative estimate of vertical axis rotation the clockwise vertical axis rotation estimates of 71 97 from dikes are less than those deduced from paleomagnetism but the order of magnitude is correct and we attribute the differences as mainly reflecting imperfect bed attitude adjustments used in analyses the problem is typical for deformed rocks from high latitude sites with steeply inclined paleomagnetic vectors we consider our field measurements of dikes as more accurate indicators of true tectonic rotation this regional deformation of miocene rocks occurred early in the evolution of the tfz transform probably beginning ca 7 ma when an older rift zone present in western north iceland jumped to its present nvz location in northeast iceland the early deformations involved shear strain distributed over a region about 20 km broad and produced the curved structures this phase was followed by the sequential development of more narrowly distributed discrete shears generating the initial husavik flatey transform fault

axial melt lenses at oceanic ridges a case study in the oman ophiolite abstract the oman ophiolite a fossil fast spreading oceanic ridge had a melt lens perched on top of the magma chamber wherein the gabbro unit was crystallizing in ophiolites this melt lens is reduced to a horizon that has been identified in the field the gabbros that subsided from the lens horizon and drifted out of the magma chamber while developing a steep magmatic foliation provide insights into the active melt lens processes a gabbro sampled right below the horizon where the lens closed has virtually not subsided and it offers a direct glance at the floor of the lens below the deeper the gabbros are sampled the more they have subsided before drifting out of the magma chamber and the closer they derive from the center of the lens floor cross sections in these gabbros record successive events occurring within the lens in particular anorthosite layers and lenses that are interlayered within the foliated gabbros record the intrusion of a new melt where plagioclase is the first mineral to crystallize on liquidus detailed mapping of anorthosite swarms suggest that melt intrusions are massive lasting less than 4 yr and that their frequency is 100 yr their large volume controls the stability of melt lenses a new melt surge induces rapid and large inflation of the lens melt lens deflation follows that is due to the ongoing process of crustal accretion both in the gabbro unit by continuous subsidence of a thick gabbroic mush from the lens floor and in the lid above the lens roof by sheeted dike intrusions feeding basalt extrusions at the higher frequency of 10 yr

on the lithospheric seismic structure of reykjanes ridge at 62 5 n a long reversed refraction line on the southeast flank of reykjanes ridge subparallel to the axis is modeled with synthetic seismograms these are calculated using the reflectivity method the data have been recorded by two ocean bottom seismometers during the reykjanes ridge iceland seismic program 1977 rrisp77 experiment the observed amplitudes are characterized by a low at around 80 km range and a pronounced maximum at around 100 km distance here the apparent velocity increases from about 8 km s to at least 8 6 km s in both directions the modeling demonstrates that most of the observed features can be described by a laterally homogeneous structure except for layer 2 which thickens markedly toward iceland the structure of the lower crust seems to be similar to that of the bay of islands ophiolite newfoundland there is an indication of a low velocity zone at the base of the crust which may be caused by partial serpentinization of the olivine component of the gabbros at that depth the moho seems to be modeled best by a discontinuity about 10 km below the water surface in the uppermost mantle there is a very slight velocity increase from 8 to 8 25 km s it is underlain by a transition zone to velocities as high as 8 6 8 9 km s at about 20 km depth this could possibly be explained by velocity anisotropy perhaps together with a petrological boundary the direction of maximum velocity of the deep anisotropic layer should be parallel to the ridge axis this could be caused by a pipe flow from the iceland mantle plume which is possibly indicated by the shear wave structure at greater depth

the iceland plate boundary zone propagating rifts migrating transforms and rift parallel strike slip faults unlike most of the mid atlantic ridge the north america eurasia plate boundary in iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures accordingly geologic processes in iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid ocean ridge system combining existing and new data from across iceland provides an integrated view of this active mostly subaerial plate boundary the broad iceland plate boundary zone includes segmented rift zones linked by transform fault zones rift propagation and transform fault migration away from the iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading related structures rift propagation results in block rotations that are accommodated by widespread rift parallel strike slip faulting the geometry and kinematics of faulting in iceland may have implications for spreading processes elsewhere on the mid ocean ridge system where rift propagation and transform migration occur

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

motion in the north iceland volcanic rift zone accommodated by bookshelf faulting along mid ocean ridges the extending crust is segmented1 on length scales of 10 1 000 km where rift segments are offset from one another motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis where segments overlap non transform offsets with a variety of geometries2 accommodate shear motions here we use micro seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north iceland between the rift segments we identify a series of faults that are aligned sub parallel to the orientation of the main rift these faults slip through left lateral strike slip motion yet movement between the overlapping rift segments is through right lateral motion together these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf faulting mechanism named after its resemblance to a tilting row of books on a shelf3 the faults probably reactivated existing crustal weaknesses such as dyke intrusions that were originally oriented parallel to the main rift and have since rotated about 15 clockwise reactivation of pre existing rift parallel weaknesses contrasts with typical mid ocean ridge transform faults and is an important illustration of a non transform offset accommodating shear motion between overlapping rift segments

asymmetric plume ridge interaction around iceland the kolbeinsey ridge iceland seismic experiment we present the results of a seismic refraction experiment that constrains crustal structure and thickness along 225 km of the kolbeinsey ridge and tjornes fracture zone and thus quantifies the influence of the iceland hot spot on melt flux at the spreading center north of iceland north of the iceland shelf crustal thickness is relatively constant over 75 km 9 4 0 2 km along the southern portion of the kolbeinsey ridge on the iceland shelf crustal thickness increases from 9 5 0 1 km to 12 1 0 4 km over 90 km gravity inversion indicates a residual crustal gravity anomaly that decreases by about 30 40 mgal toward iceland we infer that the variations in crustal thickness and gravity are accompanied by mantle temperature changes of 40 to 50 c at similar distances from the iceland hot spot crustal thickness along the kolbeinsey ridge is 2 2 5 km less than at the reykjanes ridge consistent with the asymmetry in plume ridge interaction that has been inferred from the axial depth and geochemistry of these ridges average lower crustal velocities are also higher along the kolbeinsey ridge consistent with a lower degree of active upwelling than along the reykjanes ridge topography and crustal thickness patterns at the spreading centers around iceland are consistent with isostatic support for normal crustal and mantle densities however we infer that the lower crust beneath central iceland is considerably denser than that beneath the adjacent ridges crustal thickness and geochemical patterns suggest that deep melting is spatially limited and asymmetric about iceland while shallow melting is enhanced over a broad region this asymmetry may be due to a mantle plume that is tilted from south to north in the upper mantle and preferentially melts deeper enriched material beneath the reykjanes ridge

cooling of the lower oceanic crust thermal models of mid ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust these models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast spreading ridges the rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge with 50 70 kw of hydrothermal cooling for every 1 m of ridge axis at crustal ages of 0 1 0 4 ma these timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges

ductile deformation and the origin of layered gabbro in ophiolites layered gabbros in ophiolites have been commonly interpreted in terms of crystallization beneath oceanic spreading centers in magma chambers up to 30 km wide and 3 6 km deep although large steady state magma chambers provide a possible explanation for the limited diversity of mid ocean ridge basalts their existence is not supported by geophysical observations sinton and detrick 1992 recently reviewed seismic data from oceanic spreading centers and concluded that there is little evidence for steady state magma chambers beneath slow spreading ridges and that fast spreading ridges appear to be underlain by small 1 4 km wide magma chambers perched on large 8 km wide zones of low velocity material which are interpreted to consist of partially molten crystal cumulates in this paper we compare the geology of the gabbroic rocks of the samail ophiolite which is thought to have formed by fast spreading with results of a finite element analysis of the sinton detrick model for fast spreading centers our results demonstrate that flow similar to that postulated by sleep 1975 within a thick zone of cumulates during crustal extension could produce orientations of foliation and layering approximating those observed in the samail gabbros as a consequence of this deformation 1 layers deposited on the floor of the magma chamber will be deformed and rotated into upwardly concave shapes that dip toward the spreading center 2 the gross fabric defined by these orientations may greatly exceed the dimensions of the deforming regime let alone the magma chamber 3 feeder dikes that penetrate the cumulate pile near the spreading axis will be rotated into parallel with the layering but those that invade farther from the ridge will be less deformed and retain crosscutting relationships and 4 total deviatoric strain should increase down section toward the moho so that primary magmatic structures and textures will be modified or obliterated in the lowermost gabbros upward migration of melts through the subsiding and deforming crystal mush may damp out cryptic varition in the gabbros and help maintain the magma chamber in a compositional steady state

along axis structure and crustal construction processes of spreading segments in iceland implications for magmatic rifts magmatic rift systems are composed of discrete spreading segments defined by morphologic structural and volcanic features that vary systematically along strike in iceland structural features mapped in the glaciated and exhumed miocene age upper crust correlate with analogous features in the seismically and volcanically active neovolcanic zone integrating information from both the active rift zones and ancient crust provides a three dimensional perspective of crustal structure and the volcanic and tectonic processes that construct crust along spreading segments crustal exposures in the skagi region of northern iceland reveal significant along strike variations in geologic structure the upper crust at exhumed magmatic centers segment centers is characterized by a variety of intrusive rocks high temperature hydrothermal alteration and geologic evidence for kilometer scale subsidence in contrast the upper crust along segment limbs which extend along strike from magmatic centers is characterized by thick sections of gently dipping lava flows cut by varying proportions of subvertical dikes this structure implies relatively minor upper crustal subsidence and lateral dike intrusion the differing modes of subsidence beneath segment centers and segment limbs require along axis mass redistribution in the underlying upper middle and lower crust during crustal construction this along axis material transport is accomplished through lateral dike intrusion in the upper crust and by along axis flow of magmatic to high temperature solid state gabbroic material in the middle and lower crust these processes inferred from outcrop evidence in skagi are consistent with processes inferred to be important during active rifting in iceland and at analogous magmatic oceanic and continental rifts

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

the krafla fissure swarm iceland and its formation by rifting events fissure swarms at divergent plate boundaries are activated in rifting events during which intense fracturing occurs in the fissure swarm accompanied by intrusion of magma to form dikes that sometimes lead to eruptions to study the evolution of fissure swarms and the behaviour of rifting events detailed mapping was carried out on fractures and eruptive fissures within the krafla fissure swarm kfs fracture densities of dated lava flows ranging from 10 000 years bp to 30 years old were studied and the fracture pattern was compared with data on the historical m vatn rifting episode 1724 1729 and the instrumentally recorded krafla rifting episode 1975 1984 additionally the interaction of transform faults and fissure swarms was studied by analysing the influence of the husavik transform faults on the kfs during the historical rifting episodes eruptions on the fissure swarm occurred within 7 km from the krafla central volcano although faults and fractures were formed or activated at up to 60 70 km distance this is consistent with earlier rifting patterns as holocene eruptive fissures within the kfs are most common closer to the central volcano most fractures within the central krafla caldera are parallel to the overall orientation of the fissure swarm this suggests that the regional stress field is governing in the krafla central volcano while the local stress field of the volcano is generally weak a sudden widening of the graben in the northern kfs and a local maximum of fracture density at the junction of the kfs and the extrapolation of the husavik transform fault zone indicates possible buried continuation of the husavik transform fault zone which extends to the kfs eruptive fissures are found farther away from the krafla central volcano in the southern kfs than in the northern kfs this is either due to an additional magma source in the southern kfs the hei arspor ur volcanic system or caused by the husavik transform faults transferring some of the plate extension in the northern part fracture density within particular lava flow fields increases with field age indicating that repeated rifting events have occurred in the fissure swarm during the last 10 000 years bp the fracture density in the kfs is also generally higher closer to the krafla central volcano than at the ends of the fissure swarm this suggests that rifting events are more common in the parts of the fissure swarm closer to the krafla central volcano

crustal drift in iceland summary there are indications that tertiary and post tertiary volcanism in iceland has been confined to the present main belt of active volcanism the geological structure of the tertiary lava piles in the east and west of iceland and results of geophysical studies may be explained by invoking an appreciable amount of crustal drift since the beginning of the tertiary the rocks in the extreme east and west of the island may have been carried apart by 400 km or more the crustal drift is believed to result mainly from crustal extension through the injection of dykes the structure of iceland and possibly that of other flood basalt areas is believed to be closely related to the world wide rift system

geologic structure of the uppermost oceanic crust created at fast to intermediate rate spreading centers abstract geological investigations of major fault scarps tectonic windows and dsdp odp drill holes provide direct views of the uppermost oceanic crust generated at fast to intermediate rate spreading centers these areas reveal a consistent upper crustal structural geometry with basaltic lava flows defining a pattern of downward increasing inward dip toward the spreading center at which they formed and dikes in the lavas and underlying sheeted dike complex showing a similar degree of outward dip widespread fracturing faulting and hydrothermal metamorphism accompanied magmatic construction these geological relationships can be interpreted in terms of dramatic asymmetrical subaxial subsidence of upper crustal rock units that diminishes across the very narrow few kilometers wide zone of lava accumulation and dike intrusion at the ridge axis this type of crustal structure is in accord with some existing models of spreading but augments these idealized views with more realistic geological

sea floor spreading in the presence of the iceland plume the structure of the reykjanes ridge at 61 40 n abstract we present results from a combined normal incidence and wide angle seismic survey of the reykjanes ridge at 61 40 n in the north atlantic the seismic data show that the crust is 8 4 10 0 km thick on the ridge axis we attribute this thicker than normal oceanic crust to a high degree of melting caused by hot mantle from the iceland plume beneath the speading centre layer 2a of the oceanic crust is significantly thicker at 0 ma than in crust of age 4 ma our results combined with other seismic data from the north atlantic show that the crustal thickness decreases away from the plume centre to normal oceanic crustal thickness at a distance of approximately 1000 km

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

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

seismic structure beneath the reykjanes peninsula southwest iceland inferred from array derived rayleigh wave dispersion abstract the aim is to obtain a site specific s wave structure beneath the reykjanes peninsula southwest iceland nine broadband stations of the reykjanet network are used to find rayleigh wave phase velocity dispersion in a relatively wide range of periods from 3 to 50 s the records analyzed were made in the years 2013 to 2015 and concern fourteen selected earthquakes whose epicentral distances range from tens of kilometers to almost ten thousand kilometers our approach to retrieving rayleigh phase velocity dispersion involves two partly independent methods allowing for array apertures larger than a wavelength 1 the zero crossing point method and 2 the phase plane method the two methods used here work with seismograms decomposed into quasi harmonic components and implicitly assume a single plane wave propagation the good match between the dispersion curves obtained by means of the two methods indicates that the assumption has been reasonably fulfilled the rayleigh wave phase velocity dispersion data are inverted into a horizontally layered isotropic s wave velocity model of the earth s crust and uppermost mantle by a modified method of the single parameter variation at shallow depths the derived model is rather similar to some previous models that were derived predominantly from the arrival times of body waves at depths exceeding about 20 km the dispersion data require low s wave velocities indicating a noticeable low velocity zone

crustal accretion of thick mafic crust in iceland implications for volcanic rifted margins rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins vrms this mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward seismic velocities crustal drilling and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions the lower crust of these regions is not exposed but is inferred from seismic velocities vp 6 5 km sec and petrological constraints to be gabbroic to ultramafic in composition limited access to crustal sections generated along vrms have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading active processes in iceland provide a glimpse of subaerial spreading with the creati

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

south arch volcanic field newly identified young lava flows on the sea floor south of the hawaiian ridge several young lava fields were imaged by gloria sidescan sonar along the hawaiian arch south of hawaii the largest 35 by 50 km across includes a central area characterized by high sonar backscatter and composed of several flow lobes radiating from a vent area reflection profiling and sea floor photography indicate that the central lobes are flat sheet flowsbounded by pillowed margins thin surface sediment and thin palagonite rinds on lava surfaces suggest ages of 1 10 ka vents are localized along the arch crest near bases of cretaceous seamounts two dredged flows are basanite and alkalic basalt broadly similar to rejuvenated stage and some pre shield alkalic lavas on the hawaiian ridge arch volcanism representsperipheral leakage of melt from the hawaiian hot spot over much larger areas than previously recognized

structural comparison of archetypal atlantic rifted margins a review of observations and concepts abstract in this study we compare three pairs of conjugate rifted margins that are often referred to as archetypes of rift systems despite numerous differences a remarkable first order structural similarity appears between the magma poor iberia newfoundland the magma rich mid norway east greenland and the sediment rich angola brazil rifted margins typical is the seaward arrangement of distinct domains proximal necking distal and outer that each present specific comparable structural characteristics our study also suggests that magma rich systems may go through a stage of hyper extension indicating that extreme crustal thinning does not preclude a magmatic breakup in this contribution we clarify the definition of a number of terms introduced recently in rifted margin studies we review the major features constitutive of these key referenced systems discuss their similarities and differences and examine how the related deformation modes develop in the life cycle of a rift that goes to seafloor spreading we conclude that the distinct domains observed in the margin architecture represent distinct stages in the evolution of rifted margin independently of their later evolution into magma poor or magma rich environments

characterization of humic and fulvic acids isolated from baltic sea sediments using 13c and 1h nuclear magnetic resonance spectra abstract the 13 c and 1 h nmr spectra of marine humic and fulvic acids isolated from baltic sea sediments were obtained using the pulse fourier transform mode and cross polarization magic angle spinning techniques aliphatic structures were found to constitute a much larger fraction of the humic acids than previously expected the fulvic acids in contrast to the humic acids contain significant amounts of polysaccharide like substances the aromatic content of the humic and fulvic acids was found to be lower than expected suggesting that some of the humic substances in these five basins of the baltic sea are of marine origin