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

crustal structure of the labrador sea conjugate margin and implications for the formation of nonvolcanic continental margins wide angle seismic studies have determined the detailed velocity structure along a 350 km long profile across the labrador margin combination of this model with a previously published cross section for the southwestern greenland margin constitutes the first combined conjugate margin study based on seismic velocity structure the results indicate three distinct zones across the labrador margin similar to the structure of the conjugate greenland margin zone 1 represents 27 to 30 km thick continental crust thinning gradually seaward over 100 km distance farther seaward zone 2 is 70 80 km wide characterized by a distinct lower crust 4 5 km thick in which velocity increases with depth from 6 4 to 7 7 km s interpretation for this lower crustal block favors an origin by serpentinized peridotite rather than by magmatic under plating zone 3 represents two layered normal oceanic crust the cross sections from both margins are reconstructed to an early drift stage at chron 27 this demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of chrons 31 and 33 on both margins zone 1 shows a marked asymmetry with a gradual thinning of continental crust off labrador contrasted with a rapid thinning off greenland the abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are both probably related to a very slow rate of continental rifting which produced little if any melt

depth age and dynamic topography of oceanic lithosphere beneath heavily sedimented atlantic margins our understanding of the thermal structure of oceanic lithosphere is based upon observations of water loaded subsidence and plate age these observations are predominantly from areas with sparse sedimentary cover which are unperturbed by crustal thickening and by regional epeirogeny few such measurements are available from old oceanic lithosphere where sediment thickness is large in order to address this omission we have analyzed a legacy dataset of seismic reflection and wide angle profiles from the margins of the atlantic ocean our aims are twofold first we determine the isostatically corrected age depth variation of old oceanic lithosphere adjacent to continental margins and compare it with existing trends secondly we isolate residual depth anomalies and compare them with dynamic topographic predictions made from long wavelength gravity anomalies within the sedimentary pile we have used a suite of coincident seismic reflection and wide angle measurements to create a method for converting two way travel time into depth and sediment density we also correct for changes in oceanic crustal thickness on the oldest portions of the atlantic ocean the corrected age depth variation is consistent with results from the pacific ocean which suggests that the oceanic plate has a thickness of 90 95 km positive and negative deviations from the global age depth trend are common and can be as large as 1 km they mostly but not always correlate with long wavelength gravity anomalies if the admittance is 30 mgal km 1 we propose that these deviations are primarily generated by convective circulation within the upper mantle

ti distribution in quartz across a heterogeneous shear zone within a granodiorite the effect of deformation mechanism and strain on ti resetting abstract the study of a heterogeneous ductile shear zone that developed at 500 c and 0 2 gpa during post magmatic cooling of a granodiorite has allowed the effect of strain and recrystallization on ti re equilibration of quartz to be assessed understanding this effect is critical for applying ti in quartz thermobarometry to mylonites differently strained quartz across the shear zone shows a heterogeneous distribution of ti concentrations ti measured by secondary ion mass spectrometry sims ranging between 2 and 45 ppm quartz cathodoluminescence cl is proven by spectral analysis to be correlated with ti allowing cl images to be calibrated as ti maps using sims measurements coarse grained weakly deformed domains consist of magmatic quartz extensively recrystallized by grain boundary migration gbm and mostly 65 75 area contain 20 38 ppm ti resetting to lower ti occurred locally i in haloes surrounding titanite and biotite inclusions ti as low as 6 ppm ii along grain boundaries and iii towards the interface of quartz domains with other mineral domains with increasing strain quartz underwent progressive grain size reduction and developed a bimodal microstructure with elongate grains 100 s m long surrounded by mantles of new grains 10 30 m in size recrystallized by subgrain rotation sgr dynamic recrystallization by sgr associated with prism slip became increasingly dominant over gbm as strain increased towards the shear zone core significant resetting of ti in quartz only occurred in high strain domains at shear strain probably 10 in the shear zone core where fine recrystallization amounts to 50 60 by area and coarser cores are strongly sub structured these domains are not compositionally homogeneous and still show a range of ti mostly between 2 and 10 ppm in all strain facies of the shear zone quartz filled pressure shadows associated with feldspar show an almost constant ti of 2 ppm the pristine ti content of the magmatic quartz mylonitized in the shear zone core is therefore significantly reset and converges asymptotically towards the equilibrium 2 ppm ti shown by new quartz precipitated in pressure shadows it is inferred that extensive recrystallization by sgr and repeated cycles of dislocation creep and rearrangement provided fluid access to quartz grain interiors promoting chemical buffering and leading to partial re equilibration to low ti these observations imply limitations on the use of the ti in quartz thermobarometry to constrain ambient conditions of ductile deformation

the correction for sediment loading on the seafloor available measurements of sediment velocity and density indicate that a factor of 600 m s 1 of two way sediment reflection time will correct seafloor depth to yield basement depth in the absence of sediment loading this simple linear relation is derived from the results of six deep drill holes in abyssal sediments of the north and south atlantic oceans the relation predicts the sediment correction with an accuracy of about 50 m regardless of the lithology sampled down to a depth of 1 5 s of reflection time the cost wells on the u s continental shelf and empirical relations among velocity density porosity and depth provide estimates of the deeper behavior of abyssal sediments and suggest that the 600 m s 1 factor continues to provide a satisfactory sediment correction to a depth of 3 s

small basins in the scotia sea the eocene drake passage gateway after isostatic correction for their sedimentary fill the depths of two small oceanic basins in the southern scotia sea suggest that both started to open in mid to late eocene times models of the short magnetic anomaly profiles across the basins provide support for these determinations the location of the basins adjacent to the present day drake passage marine gateway and the timing of continental stretching leading up to their opening during the ypresian early eocene onset of global cooling mean that their importance is potentially far greater than their small size implies extension in the region of the two basins would have opened drake passage to shallow or intermediate depth oceanic circulation between the pacific and atlantic oceans for the first time this coincided with a reorganisation of vertical mixing patterns in the global ocean a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments and the onset of a long decline in atmospheric carbon dioxide concentrations cenozoic global cooling may therefore have begun as a result of the shallow opening of drake passage

lithospheric structure of the labrador sea from constrained 3 d gravity inversion regional inversions of free air gravity data constrained by bathymetric and sediment thickness information were undertaken over the labrador sea and its margins to generate 3 d density anomaly models to investigate broad scale crustal structural variations across the extinct spreading centre benchmarked against independent seismic moho depth constraints a density anomaly isosurface within the inverted volumes was selected as a moho proxy and regional maps of moho structure were developed inversions using two different sources for depth to basement constraints revealed similar moho structures with a depth to moho of 12 km beneath the labrador sea which deepens to 20 km and greater towards davis strait and beneath the offshore extension of the grenville province density anomaly slices through the models corresponding to seismic lines show good agreement between the inverted moho proxy and the seismic moho with the only exceptions occurring where a high velocity lower crustal zone or underplate has been modelled from wide angle reflection refraction profiling studies the inverted depth to moho estimates were combined with depth to basement constraints to investigate crustal thickness both for the full crust and for individual crustal layers revealing that the crust of the labrador sea is generally 5 10 km thick but thickens to 20 25 km towards davis strait and beneath the offshore extension of the grenville province not taking into account high density underplates or anomalously high density lower crust sediment and crustal thickness variations were investigated to compute stretching factors across the labrador sea and to identify zones which deviate from local isostatic compensation assuming both an initial unstretched crustal thickness of 35 km and using a variable unstretched crustal thickness model much of the labrador sea has experienced 70 90 per cent thinning the derived values suggest that embrittlement of the entire crust and serpentinization of the upper mantle are likely to have occurred for large portions of the central and southern labrador sea inboard of known oceanic crust isostatic considerations reveal sediment excess and deficiency on the labrador and greenland margins respectively possibly reflecting fundamental rheological asymmetry with the greenland margin appearing weaker than the labrador margin a strong gradient from sediment deficiency to excess exclusive to the labrador margin may reveal the southwestern limit of a regional graben bounding listric detachment or of a zone of distributed faults and detachments

gridded isopach maps from the south pacific and their use in interpreting the sedimentation history of the west antarctic continental margin large scale maps of sediment thicknesses are essential prerequisites for the next generation of high resolution paleobathymetric reconstructions in addition maps of sediment thicknesses on continental margins are useful for studies of terrigenous sediment supply and sediment distribution on the ocean floor based on a compilation of more than 10000 km of single and multichannel seismic reflection data and correlations with ocean drilling sites we model isopach grids of the southern pacific margin of west antarctica following recent seismic stratigraphic models we differentiate between pre glacial and glacially transported sediments the subsequent modelling of sediment thickness grids allows us to compare the pre glacial and glacial sedimentary development in the study area based on available drillhole dates we estimate the onset of accumulation of glacially transported sediments to have occurred at about 10 ma based on approximate sediment deposition rates the thickest glacial sediment accumulations occur in front of major glacial drainage outlets i e marguerite trough on the western antarctic peninsula margin belgica trough in the bellingshausen sea and the trough off pine island bay in the amundsen sea sedimentation rates between 140 and 170 m m y are calculated for these sites

development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse grained quartz veins 1 the microstructure and crystallographic preferred orientation cpo of quartz were quantified in 17 samples of natural monomineralic tabular veins the veins opened and were deformed up to shear strain 15 in a small temperature window about 25 c above 500 c as established by ti in quartz thermometry the veins filled a set of fractures within the adamello tonalite southern alps italy and localized homogeneous simple shear during postmagmatic cooling the local square millimeter scale and bulk square centimeter cpo were investigated by computer integrated polarization microscopy cip and x ray texture goniometry weakly deformed veins wdv 1 consist of millimeter to centimeter sized crystals with a strong cpo showing a c axis girdle slightly inclined mostly with the shear sense to the foliation xy plane and a strong maximum close to the lineation x moderately deformed veins mdv 2 3 consist of elongated nonrecrystallized ribbon grains and most have a cpo showing a strong y maximum of c axes some with weak extension into a yz girdle strongly deformed veins sdv 4 to 15 are pervasively to completely recrystallized to fine 34 40 m grain size aggregates with a strong cpo similar to that of mdv the slip systems during plastic deformation were dominantly prism a with subordinate rhomb and basal a slip recrystallization occurred rather abruptly for 3 4 in contrast to dislocation creep experiments in quartz and other minerals a steady state recrystallized fabric is achieved at early stages of deformation 4 as there is no evidence with increasing strain of strengthening of the cpo of rotation of the fabric skeleton or of change in grain size wdv represent weakly deformed relicts of veins with an initial cpo believed to have developed during crystal growth but unsuitably oriented for prism a slip during subsequent shear mdv and sdv appear to derive from veins different from wdv where the vein crystals grew with orientation favorable for prism a slip the relationship between the initial growth cpo and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism c and prism a slip with the temperature of growth causing growth of crystals well oriented for slip the initial cpo of veins from which quartz mylonites are commonly derived plays a critical role in the fabric evolution the strong growth and strain induced cpos of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal a slip would have been dominant

upper mantle structure beneath the azores hotspot from finite frequency seismic tomography the azores archipelago is a classic expression of a hotspot that has interacted with a mid ocean ridge avariety of geochemical studies and geophysical observations point to the influence of some sort of mantle plume on the formation of the azores plateau and the adjacent mid atlantic ridge mar but the mantle structure beneath the azores has not heretofore been imaged tomographically at a sufficiently fine resolution to resolve a narrow plume we present a p wave velocity model of the mantle beneath the hotspot derived from teleseismic body waves recorded by six broadband seismic stations on the azores islands including the global seismographic network station cmla three dimensional travel time sensitivity kernels are used to account for the effects of wavefront healing and scattering of seismic waves an inversion of 228 p wave travel time delays reveal a lowvelocity anomaly in the shallow mantle less than 200 km deep along the azores archipelago and beneath the azores plateau centered approximately at 38 5 n 28 5 w the low velocity anomaly extends northeastward and downward to connect to a plume like column of low velocity material from 250 km to at least 400 km depth centered northeast of terceira these results are consistent with other geophysical and geochemical observations and provide seismic evidence for a model of plume ridge interaction in which the plume conduit is deflected to the southwest in the shallow mantle by asthenospheric flow and plate motion and contributes hot mantle material and excess melt to the mar south of the azores

the relationship between depth age and gravity in the oceans we reassess the applicability of the thermal plate cooling model to the subsidence of the north pacific atlantic and north indian ocean basins we use a new numerical plate model in which the thermophysical parameters of the lithosphere vary with temperature according to the results of laboratory experiments and the ridge temperature structure is consistent with the thickness of the oceanic crust we first attempt to exclude thickened crust from our data set and then to exclude swells and downwellings by masking regions of the data that remains that have significant gravity anomalies when there exists a clear regional correlation between intermediate wavelength gravity and topography we find that the average variation of depth with age is consistent with conventional half space models until about 90 myr thereafter the departure from the half space cooling curve is more rapid than predicted using simple conductive plate cooling models the depth age curves in the pacific and atlantic show 250 m of temporary shallowing between the ages of 90 130 myr a result consistent with the outcome of experiments on the initiation of small scale boundary layer convection the results do not change significantly if the estimated component of the gravity arising from plate cooling is subtracted prior to calculation of the correlation between gravity and topography a 90 km thick conductive plate is nevertheless a reasonable model for the average temperature structure of the oldest part of the pacific ocean lithosphere in the pacific the broad topographic undulations associated with the line island swell the hawaiian swell and surrounding basins have correlated gravity anomalies and an admittance of approximately 30 mgal km 1 and are likely to result from convective circulation in the upper mantle in the northeast atlantic the intermediate wavelength admittance over the cape verde swell is similar in the northwest atlantic over the bermuda swell it is slightly larger but not as well constrained

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

three dimensional gravity and magnetic modelling of the irish sector of the ne atlantic margin abstract a new 3d lithospheric model has been constructed using high resolution gravity data from the irish national seabed survey the sedimentary component of the model incorporated density variations due to laterally varying overcompaction associated with cenozoic denudation after optimisation based on gravity inversion regional crustal thickness variations were defined which are in reasonable agreement with the results of wide angle seismic experiments high crustal extension factors 5 characterise the deeper parts of the rockall and porcupine basins and in places the model indicates extreme stretching 10 beneath these basins this could be because of instability in the gravity inversion although other recent investigations have independently suggested similarly high extension factors in contrast the hatton basin is characterised by an apparent extension factor of about 2 the modelling resolves a pattern of ne to nne trending local mesozoic basins on the margins of the rockall trough helping to delineate structures that were previously only sparsely sampled by seismic surveys it appears possible that rifts with similar trends underlie the volcanic rocks which obscure the deeper parts of the hatton basin the linear trends of the basins to the south and east of ireland are interpreted to have been inherited from a basement fabric that was initially established during the late precambrian assembly of this basement and subsequently subjected to caledonian and variscan reactivation magnetic modelling indicates that the variations in the thickness of the crystalline crust predicted by the gravity models can explain the regional magnetic anomaly patterns over the rockall and porcupine basins but that significant additional magnetic material probably igneous rocks of both palaeogene and cretaceous ages is required to explain the anomalies in the hatton basin region the magnetic signature of the rockall basin is distinctly different to that over the basement of similar apparent thickness formed during mid cretaceous c34n opening of the ocean basin to the south this is an impediment to hypotheses that invoke mid cretaceous sea floor spreading rather than intracontinental rifting to explain the development of the basin the exception is in the extreme south of the basin where the volcanism associated with the barra volcanic ridges combined with indications of relatively strong lithosphere could be evidence of incipient ocean opening

structure and origin of the j anomaly ridge western north atlantic ocean the j anomaly ridge is a structural ridge or step in oceanic basement that extends southwest from the eastern end of the grand banks it lies beneath the j magnetic anomaly at the young end m 4 to m 0 of the m series magnetic anomalies its structural counterpart beneath the j anomaly in the eastern atlantic is the madeira tore rise but this feature has been overprinted by post middle cretaceous deformation and volcanism in order to study the origin and evolution of the j anomaly ridge madeira tore rise system we obtained seismic refraction and multichannel reflection profiles across the j anomaly ridge near 39 n latitude the western ridge flank consists of a series of crustal blocks downdropped along west dipping normal faults but the eastern slope to younger crust is gentle and relatively unfaulted the western flank also is subparallel to seafloor isochrons becoming younger to the south anomalously smooth basement caps the ridge crest and it locally exhibits internal eastward dipping reflectors similar in configuration to those within subaerially emplaced basalt flows on iceland when isostatically corrected for sediment load the northern part of the j anomaly ridge has basement depths about 1400 m shallower than in our study area and deep sea drilling has shown that the northern ridge was subaerially exposed during the middle cretaceous we suggest that most of the system originated under subaerial conditions at the time of late stage rifting between the adjacent grand banks and iberia the excess magma required to form the ridge may have been vented from a mantle plume beneath the grand banks iberia rift zone and channelled southward beneath the rift axis of the abutting mid atlantic ridge resulting edifice building volcanism constructed the ridge system between anomalies m 4 and m 0 moving southward along the ridge axis at about 50 mm yr about m 0 time when true drift began between iberia and the grand banks this southward venting rapidly declined the results were rapid return of the spreading axis to normal elevations division of the ridge system into the separate j anomaly ridge and madeira tore rise and unusually fast subsidence of at least parts of these ridges to depths that presently are near normal this proposed origin and evolutionary sequence for the j anomaly ridge madeira tore rise system closely matches events of uplift and unconformity development on the adjacent grand banks

the elastic properties of the lithosphere beneath scotian basin abstract to assess the possibility that the north atlantic ocean may subduct at scotian basin east of canada we investigate the present compensation state of this deep basin a fourier domain analysis of the bathymetry depth to basement and observed gravity anomalies over the oceanic area east of nova scotia indicates that the basin is not isostatically compensated moreover the analysis emphasizes that in addition to the sediments density perturbations exist beneath the basin the load produced by the sediments and these density perturbations must have been supported by the lithosphere we simulate the flexure of the lithosphere under this load by that of a thin elastic plate overlying an inviscid interior it is shown that a plate with a uniform rigidity does not adequately represent the lithosphere beneath the basin as well as the oceanic lithosphere far from the basin rather the rigidity of the lithosphere directly beneath the basin is about one to two orders of magnitude smaller than elsewhere we relate this weakening to the thermal blanketing effects of the thick sediments and the fact that the lithosphere has a temperature dependent rheology we suggest that this weak zone would have a controlling effect on the reactivation of normal faults at the hinge zone of the basin that were formed during the break up of africa and north america and were locked in the early stages after the break up the weak zone would facilitate reactivation of the faults if tensional stresses were produced by possible reorientation of the spreading direction of the north atlantic ocean in the future the reactivation of the faults would create a free boundary condition at the hinge zone allowing further bending of the lithosphere beneath the basin and juxtaposition of this lithosphere to the mantle beneath the continent this may provide a favorable situation for initiation of slow subduction due to subsequent compressional forces

microstructural and crystal fabric evolution during shear zone formation abstract the microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo feldspathic mylonitic shear zone torridon nw scotland have been investigated using sem electron channelling our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history microstructural variation is particularly pronounced at low shear strains significant intragranular deformation occurs via a domino faulting style process whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures a sudden reduction in grain size defines the transition to medium shear strains but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin grains within these bands have somewhat steeper preferred dimensional orientations these domains persist into the high shear strain mylonitic region where they are oriented subparallel to the shear zone margin and consist of sub 20 m grains the microstructures suggest that the principal deformation mechanism was intracrystalline plasticity with contributions from grain size reduction via dynamic recrystallization grain boundary migration and grain boundary sliding crystal fabrics measured from the shear zone vary with position depending on the shear strain involved and are consistent with the operation of several crystal slip systems e g prism basal rhomb and acute rhomb planes in a consistent direction probably parallel to a and or m they also reveal the presence of dauphine twinning and suggest that this may be a significant process in quartz deformation a single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed rather the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation the mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution the stability of some microstructures to higher shear strains the exploitation of others at lower shear strains and a continuously evolving crystal fabric mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient

the thermal structure of the central nova scotia slope eastern canada seafloor heat flow and thermal maturation models the thermal history and maturation potential of the central scotian slope is constrained using a combination of 47 recently acquired seafloor heat flow measurements two dimensional 2d seismic reflection data available well data simple lithospheric rift models and thermal and petroleum systems modelling consistent heat flow values of 41 46 mw m 2 were measured seaward of the salt diapiric province and across the slope away from the influence of salt structures significant but highly variable increases in heat flow were measured for stations overlying salt diapiric structures reaching values upwards of 72 mw m 2 simple models of conductive heat transfer with static salt geometries constrained from reflection profiles indicate that two of the four models fit the data whereas two indicate much higher values suggestive of additional convective effects dynamic 2d thermal models were developed to incorporate the effects of lithospheric rifting crustal stretching and radiogenic heat production in t

structure of the se greenland margin from seismic reflection and refraction data implications for nascent spreading center subsidence and asymmetric crustal accretion during north atlantic opening 1 seismic reflection and refraction data from the se greenland margin provide a detailed view of a volcanic rifted margin from archean continental crust to near to average oceanic crust over a spatial scale of 400 km the sigma iii transect located 600 km south of the greenland iceland ridge and the presumed track of the iceland hot spot shows that the continent ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup initially 18 3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8 10 km thick crust changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system which was close to sea level for at least 1 m y of subaerial spreading despite a reduction in productivity from 17 to 13 5 km thick crust over this time interval this could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system the thermal anomaly must be exhaustible consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial to shallow marine and finally to deep marine extrusion in 2 m y is shown by the morphological changes in addition comparison to the conjugate hatton bank shows a clear asymmetry in the early accretion history of north atlantic oceanic crust nearly double the volume of material was emplaced on the greenland margin compared to hatton bank and may indicate east directed ridge migration during initial opening

crustal structure of edoras bank continental margin and mantle thermal anomalies beneath the north atlantic we discuss the structure of the continent ocean transition of the edoras bank area of the ne atlantic margin and the influence of the iceland mantle plume during continental breakup edoras bank lies close to the present day limit of influence of the iceland plume sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well developed over much of the margin they form aerially extensive dipping sheets with individual reflectors imaged on margin parallel profiles as laterally continuous and subhorizontal features we interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup a crustal velocity model for the margin was developed by travel time modeling of wide angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities the crustal thickness increases to the se reaching 17 km beneath the continental fragment of edoras bank the continent ocean transition is dominated in the lower crust by a lens up to 8 km thick of high velocity material 7 2 7 5 km s 1 which exhibits a very low velocity gradient it extends over a 60 km wide region beneath the seaward dipping reflectors the high velocities and densities of the lower crust are probably caused by igneous rock with a high mgo content that underplated the margin or heavily intruded pre existing crustal material during continental breakup both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by deep sea drilling project dsdp boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup comparison of the structure and subsidence history of the edoras bank margin with other areas of the nw european margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the iceland mantle plume during and following continental breakup we find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle 150 c hotter than normal lay beneath the 2000 km long rift extending from the lofoten margin off norway in the north to the edoras bank margin in the south over a period of 5 10 m y following the onset of rifting the mantle thermal anomaly decayed rapidly to only 50 c or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism

plastic anisotropy and fabric evolution in sheared and recrystallized quartz single crystals 1 the effect of a lattice preferred orientation on the flow strength of quartz aggregates dynamically recrystallized from single crystals of synthetic quartz was investigated using general shear experiments in a griggs apparatus experiments were conducted at shear strains up to 5 at a temperature of 900 c confining pressure of 1 5 gpa and shear strain rate of 10 5 s 1 three starting orientations of crystal were used to activate three slip systems basal a prism c and prism a although slip induced rotation of the crystal axes in the first two orientations led to the activation of additional slip systems for crystals with higher water contents basal a and prism a orientations are relatively weak and prism c orientations are stronger all three initial crystal orientations undergo dynamic recrystallization with increasing shear strain although the strain required for 100 recrystallization varies 2 for prism c slip 3 8 for basal a slip and 5 for prism a slip for all three starting orientations distinct domains of recrystallized grains develop with c axes parallel to y of the strain ellipsoid ymax replacing recrystallized grains of other orientations the ymax domains increase in size with increasing strain in addition strain markers show that strain is highly localized within the ymax domains indicating geometrical softening of up to an order of magnitude in effective viscosity

crustal structure of the madeira tore rise eastern north atlantic results of a dobs wide angle and normal incidence seismic experiment in the josephine seamount region summary during november december 1988 an extensive geophysical data set was collected over the josephine seamount which is located at the northeasterly end of the madeira tore rise in the eastern north atlantic the josephine seamount lies at the intersection of the madeira tore rise and azores gibraltar fracture zone the latter representing the africa eurasia plate boundary in this part of the eastern north atlantic from this data set a 275 km long explosive refraction line has been modelled together with wide angle airgun profiles seismic reflection and gravity data the velocity depth model shows that the crust either side of the seamount is typically oceanic in character however beneath the seamount there exists a region of anomalously high velocity and crustal thickening to a depth of about 17 18 km gravity modelling also suggests that the josephine seamount is compensated by a crustal root and that the josephine seamount madeira tore rise system is in local isostatic equilibrium calculations of the flexural rigidity and effective elastic thickness of the lithosphere in this region suggest that the madeira tore rise formed contemporaneously with the lithosphere on which it lies this age of crustal loading is consistent with the proposal that the madeira tore rise is an aseismic ridge which formed at or near the mid atlantic ridge

heat flow and depth versus age for the mesozoic northwest atlantic ocean results from the sohm abyssal plain and implications for the bermuda rise abstract heat flow in the sohm abyssal plain is measured to be 53 mw m 2 at an age of 163 ma this is 25 higher than predicted by conductive cooling models even though the sediment corrected basement depth of 6 5 km at this location is normal for its age an analysis of existing heat flow depth and geoid anomalies in the northwest atlantic shows that there is little correlation between heat flow and depth throughout the entire region depth and geoid are clearly related to the bermuda swell while the associated heat flow anomaly once adjusted for variations with age is limited to 5 mw m 2 and only decays to the south this means that the bermuda swell is probably not caused by extensive thermal reheating within the lithosphere but instead by dynamic uplift at its lower boundary due to the convective upwelling of a mantle plume the regionally high heat flow in the northwest atlantic may be a thermal remanent of previous plumes which passed beneath this region early in its history therefore depth and heat flow anomalies from this region cannot be used to provide constraints on steady state parameters of the lithosphere such as the presence or absence of a long term boundary layer at its base

oceanic spreading center hotspot interactions constraints from along isochron bathymetric and gravity anomalies we analyzed bathymetric and gravity anomalies along present and paleoaxes of oceanic spreading centers influenced by the iceland azores galapagos tristan and easter hotspots residual bathymetry up to 4 7 km and mantle bouguer gravity up to 340 km anomalies are maximum at on axis hotspots and decrease with increasing ridge hotspot separation distance d until becoming insignificant at d 500 km along isochron widths of bathymetric anomalies up to 2700 km depend inversely on paleo spreading rate reflecting the extent to which plume material will flow along axis before being swept away by the spreading lithosphere flux balance arguments suggest that the five hotspots feed material to ridges with comparable fluxes of 2 2 106 km3 m y assuming that the amplitudes of these geophysical anomalies reflect temperature dependent crustal thickness and mantle density variations we suggest that ridge temperature anomalies are maximum 150 225 c when plumes are ridge centered and decrease with increasing ridge hotspot distance due to cooling of the ridgeward migrating plume material

the structure and origin of the azores biscay rise north east atlantic ocean summary the azores biscay rise is a roughly linear north east south west trending feature rising 1500 3000 m above its surroundings which extends from about 45 n 15 30 w towards the azores its south western termination is near 40 30 n 21 30 w about halfway along its length the rise intersects the wnw trending king s trough in 1978 a set of bathymetric magnetic gravity gloria and seismic reflection and refraction data were obtained in the vicinity of the rise together with earlier data these observations suggest that 1 there has been no substantial post emplacement tectonic activity with the possible exception of the construction of some volcanic seamounts at the south western end of the rise and 2 the rise is underlain by a low velocity low density lower crust and is in isostatic equilibrium the rise can be convincingly shown to be the eastern half of a pair of ridges formed by abnormal crustal generation at the mid atlantic ridge crest between the times of anomalies 33 and 24 76 56ma ago the western counterpart of the rise includes gauss and milne seamounts in the newfoundland basin magnetic anomaly 3 1 passes uninterruptedly across the rise and therefore hypotheses that the northern part of the rise was the site of a cenozoic transform fault or subduction zone are not supported by our data it is speculated that king s trough was linked to the north spanish trough by an early cenozoic east west transform fault across the northern iberia abyssal plain this plate boundary became inactive about the middle of the oligocene epoch

miocene to pleistocene transatlantic dispersal of ceratoconcha coral dwelling barnacles and north atlantic island biogeography coral dwelling pyrgomatid barnacles subfamily ceratoconchinae were widely dispersed throughout the paratethys and mediterranean seas as well as the atlantic ocean during the neogene but today are limited to the western atlantic herein the paleobiogeographic origin and dispersal of the genus ceratoconcha is studied based on a combination of field taxonomic and literature studies the first confirmed appearances of ceratoconcha occur in lower miocene strata burdigalian with two closely related species on both sides of the atlantic in western france and florida fossils from the miocene of lanzarote in the canary islands and pleistocene of maio in the cape verde islands extend the known geographical and temporal range of the ceratoconcha barnacles in the eastern atlantic during the neogene dispersal of marine taxa was a two way process due to tectonic changes both influencing oceanic circulation and appearance and disappearance of oceanic islands during the early miocene gyre formation was weak and the atlantic ocean mid latitudes were warmer than today this resulted in increased hurricane activity and the expansion of hermatypic coral hosts farther north in the north atlantic normal ocean circulation transported barnacle larvae from east to west but currents generated by hurricanes may have transported them in the opposite direction towards the margins of the northeastern atlantic islands in between abetted barnacle contact and dispersal the temporal range for ceratoconcha is extended considerably in the eastern atlantic from the early pliocene to the pleistocene the hermatypic host corals of ceratoconcha suffered a severe decline in the eastern atlantic and the mediterranean after the miocene corals were present during the pliocene and pleistocene in the cape verde islands this suggests that the southernmost oceanic islands acted as a tropical refuge for host corals and their likely barnacle symbionts

crustal trace of a hot convective sheet the iceland plume has played an influential role in the evolution of the north atlantic ocean and margins over the past 60 m y it is believed that this plume formed at the conjunction of a tetrad of hot subvertical convective sheets the impingement of these hot sheets at the base of the lithospheric lid caused decompressional melting generating substantial quantities of high temperature magma that were injected into the cold overlying lid over the next 10 m y these sheets partly coalesced to form a crudely axisymmetric plume head here we analyze the lithospheric fingerprint of one of these hot convective sheets by forward and inverse modeling of densely sampled wide angle seismic data in conjunction with gravity observations we determined the three dimensional shape of magmatic underplating trapped within the lithosphere the injection of this melt into the lithosphere generated substantial permanent and minor transient uplift of earth s surface predicted and measured amounts of consequent denudation and sedimentation agree within error temporal variations in the patterns of deposition and oceanic circulation adjacent to the convective sheet show its evolution through time and space our results suggest that this linear sheet has probably been directly and indirectly responsible for cyclical events over 60 m y these events have 0 5 1 and 4 6 m y periodicities the existence of which may help to elucidate the dynamic behavior of convective sheets during and after impingement thus in particular circumstances surficial geological processes yield an indirect record of mantle convection and melt generation processes

orientation analysis of localized shear deformation in quartz fibres at the brittle ductile transition abstract the crystallographic preferred orientation of fibrous quartz veins deformed under subgreenschist facies conditions along a temperature gradient from 270 to 370 c has been investigated to explore the active mechanism during deformation of the quartz fibres the complete crystallographic orientation of individual recrystallized grains and subgrains was determined by using a combination of cip computer integrated polarization microscopy ebsd electron back scatter diffraction and additional specially developed image analysis techniques it was found that the quartz fibres deformed along crystallographically controlled shear bands from inverse pole figures of normals to the shear band boundaries it is evident that these bands occur along the positive and negative rhomb planes of quartz it is suggested that they are initiated as fractures along these planes inside the shear bands small grains broken off from the host grain show passive rotation of the lattice consistent with the shear sense of the shear bands continued deformation and increasing temperature lead to different recrystallization microstructures inside and outside the shear bands and to a partial obliteration of the sharp orientation relationships

crustal thickness anomalies in the north atlantic ocean basin from gravity analysis gravity derived crustal thickness models were calculated for the north atlantic ocean between 76 n and the chain fracture zone and calibrated using seismically determined crustal thickness about 7 of the ocean crust is 7 km thick and is interpreted to have been affected by excess magmatism thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis by far the most prominent thick crust anomaly is associated with iceland and adjacent areas which accounts for 57 of total crustal volume in excess of 7 km much smaller anomalies include the azores 8 cape verde islands 6 canary islands 5 madeira 4 and new england great meteor seamount chain 2 all of which appear to be associated with hot spots hot spot related crustal thickening is largely intermittent suggesting that melt production is episodic on time scales of tens of millions of years thickened crust shows both symmetrical and asymmetrical patterns about the mid atlantic ridge mar axis reflecting whether melt anomalies were or were not centered on the mar axis respectively thickened crust at the bermuda and cape verde rises appears to have been formed by isolated melt anomalies over periods of only 20 25 myr crustal thickness anomalies on the african plate generally are larger than those on the north american plate this most likely results from slower absolute plate speed of the african plate over relatively fixed hot spots

iceland the farallon slab and dynamic topography of the north atlantic upwelling or downwelling flow in earth s mantle is thought to elevate or depress earth s surface on a continental scale direct observation of this dynamic topography on the seafloor however has remained elusive because it is obscured by isostatically supported topography caused by near surface density variations we calculate the nonisostatic topography of the north atlantic by correcting seafloor depths for lithospheric cooling and sediment loading and find that seafloor west of the mid atlantic ridge is an average of 0 5 km deeper than it is to the east we are able to reproduce this basic observation in a model of mantle flow driven by tomographically inferred mantle densities this model shows that the farallon slab currently in the lower mantle beneath the east coast of north america induces downwelling flow that deepens the western north atlantic relative to the east our model also predicts dynamic support of observed topographic highs near iceland and the azores but suggests that the icelandic high is due to local upper mantle upwelling while the azores high is part of a plate scale lower mantle upwelling to the south an anomalously deep area off the coast of nova scotia may be associated with the downwelling component of edge driven convection at the continental boundary thus several of the seafloor s topographic features can only be understood in terms of dynamic support from flow in earth s mantle

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

40ar 39ar ages and paleomagnetism of sao miguel lavas azores we present new 40 ar 39 ar ages and paleomagnetic data for sao miguel island azores paleomagnetic samples were obtained for 34 flows and one dike successful mean paleomagnetic directions were obtained for 28 of these 35 sites 40 ar 39 ar age determinations on 12 flows from the nordeste complex were attempted successfully ages obtained are between 0 78 ma and 0 88 ma in contrast to published k ar ages of 1 ma to 4 ma our radiometric ages are consistent with the reverse polarity paleomagnetic field directions and indicate that the entire exposed part of the nordeste complex is of a late matuyama age the duration of volcanism across sao miguel is significantly less than previously believed which has important implications for regional melt generation processes and temporal sampling of the geomagnetic field observed stable isotope and trace element trends across the island can be explained at least in part by communication between different magma source regions at depth the 40 ar 39 ar ages indicate that our normal polarity paleomagnetic data sample at least 0 1 myr 0 0 1 ma and up to 0 78 myr 0 0 78 ma of paleosecular variation and our reverse polarity data sample approximately 0 1 myr 0 78 0 88 ma of paleosecular variation our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long term geomagnetic field behavior negative inclination anomalies are observed for both the normal and reverse polarity time averaged field within the data uncertainties normal and reverse polarity field directions are antipodal but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction 1998 elsevier science b v all rights reserved

electron backscatter diffraction analysis to determine the mechanisms that operated during dynamic recrystallisation of quartz rich rocks abstract determination of the controlling nucleation and recrystallisation mechanisms from a samples microstructure are essential for understanding how the microstructure formed and evolved through time the aim of our research was to apply a quantified analytical approach to the identification of the controlling nucleation recrystallisation and microstructural modification mechanisms we used electron backscatter diffraction to quantify the microstructures of naturally deformed quartz rich rocks which were deformed at various temperature and pressure conditions our results show that ratios of the recrystallised grain size to the subgrain size with values less than 1 0 5 0 7 in the data presented here suggest bulge nucleation whereas ratios of 1 suggest subgrain rotation nucleation other supporting evidence for subgrain rotation nucleation is an increase in misorientation from the centre of an original protolith parent grain to the edge all samples show evidence for modification of the microstructure due to grain boundary sliding including increased misorientation angles between grains and movement of recrystallised grains between parent grains by systematically analysing sample microstructures it is possible to separate out evidence to determine the controlling nucleation and recrystallisation mechanisms as well as being able to identify microstructure modification mechanisms using microstructural quantification via ebsd allows a systematic methodology to analyse samples from any location from an objective viewpoint

numerical modelling of the growth dynamics of a simple silicic lava dome 1 two dimensional axisymmetrical computational models have been developed using finite element techniques to represent the rheological stiffening due to degassing induced crystallisation and the effects of a brittle carapace within a simple lava dome the dome is modelled numerically considering the effects of temperature pressure and rate of extrusion using these state variables the bulk viscosity is derived from empirical equations and the bulk rheology approximated using a solidification viscosity dividing the dome into solid dominated and fluid dominated domains elastoplastic equations for the solid component and fluid mechanical equations for the fluid domain are solved simultaneously allowing continuous stress across the fluid solid interface to ensure the two components move in unison we apply this model a hookean solid carapace model and a purely fluid lava dome model to the case of the october 1980 lava dome at mount st helens the results show that dynamic growth models involving a solid carapace are required to simulate this lava dome

fabric development in experimentally sheared quartzites abstract a fine grained quartzite was deformed under a combination of simple shear and axial compression to varying amounts of shear strain 1 3 to 2 9 the experiments were conducted at 800 c 1500 mpa and an axial strain rate of 10 6 s in the dislocation creep regime shear zone boundaries were oriented at 45 or 30 to the compressior axis the progressive development of deformation microstructures including planar fabrics and c axis preferred orientations was studied as a function of increasing shear strain elongate original grains define a foliation s which rotates from 45 to 0 to the shear zone boundary with increasing strain producing a sigmoidal foliation in a single shear zone a second foliation invariant in orientation is defined by elongate recrystallized grains which lie at about 30 40 to the shear zone boundaries no c surfaces parallel to the shear zone boundaries were observed however extensional crenulation cleavages c defined by parallel zones of recrystallized grains as well as impurity and melt segregations were observed even at low strain lying at approximately 30 to the shear zone boundaries and these could easily be mistaken for c surfaces c axis preferred orientations of original grains define symmetric type i crossed circle girdles at low strain 2 with the greater concentration of c axes in the direction of shear these features are very similar to those observed in natural ductile shear zones

dislocation generation slip systems and dynamic recrystallization in experimentally deformed plagioclase single crystals abstract three samples of gem quality plagioclase crystals of an60 were experimentally deformed at 900 c 1 gpa confining pressure and strain rates of 7 5 8 7 10 7 s 1 the starting material is effectively dislocation free so that all observed defects were introduced during the experiments two samples were shortened normal to one of the principal slip planes 010 corresponding to a hard orientation and one sample was deformed with a schmid factor of 0 45 for the principal slip system 001 010 corresponding to a soft orientation several slip systems were activated in the soft sample dislocations of the 001 010 and 110 001 system are about equally abundant whereas 110 111 and 101 in 131 to 242 are less common in the soft sample plastic deformation is pervasive and deformation bands are abundant in the hard samples the plastic deformation is concentrated in rims along the sample boundaries deformation bands and shear fractures are common twinning occurs in close association with fracturing and the processes are clearly interrelated glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation grain boundaries of tiny defect free grains in healed fracture zones have migrated subsequent to fracturing these grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization nucleation via small fragments can explain a non host controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi brittle to plastic transition

variation in microseism power and direction of approach in northeast greenland abstract previous work on background noise at seismic stations in greenland has shown minimum seismic noise in the winter months for frequencies around 1 hz and maximum seismic noise in the winter months for periods around 6 sec we have analyzed microseism data from three new digital seismic stations installed during the summer of 1991 in northeast greenland at nord daneborg and scoresbysund we determined seasonal and station to station variations in spectral power density between august and december in the frequency band between 10 sec periods and 5 hz frequencies these variations are in agreement with previous studies at periods of 1 and 6 sec during the summer months all three stations recorded a minimum for the average spectral power density in the microseism band between 10 and 5 sec periods from about 3 sec periods to at least 5 hz frequencies the average spectral power density is at a maximum during the summer at all three stations conversely the winter months have a maximum in spectral power density between 10 and 5 sec periods and a minimum between about 3 sec periods and at least 5 hz frequencies at all stations station to station average spectral power density comparisons show that nord and daneborg are roughly comparable over most of the frequency band between 10 sec periods and at least 5 hz frequencies scoresbysund has a systematically higher spectral power density between 8 sec periods and at least 5 hz frequencies overall nord had the lowest background seismic noise at some frequencies approaching the values of a low noise model we determined average direction of approaches in the 8 to 4 sec period band for each station during the months of august and november these determinations agreed with previous studies the predominant average direction of approaches were southwest for nord south for daneborg and southeast for scoresbysund although the microseism amplitude is larger and the direction of approach scatter is smaller during the winter months at all three stations the direction of approach mean is apparently independent of season a large number of storms develop around iceland and typically track northeast giving rise to large amplitude microseisms at scoresbysund but relatively small amplitude microseisms at daneborg and no microseism activity at nord this complete lack of microseism energy at nord and to a lesser degree daneborg from known frequent microseism sources in the greenland sea is shown for one 5 day period in august 1991 other studies have shown that thick sediments in the atlantic ocean9s continental margins are responsible for the absence of short period surface waves from mid ocean ridge earthquakes that have paths traversing such continental margins thick sediments act to attenuate scatter and disperse short period surface waves indirect evidence indicates that the northeast greenland shelf has thick and variable sediment layers because the paths of surface waves to nord and to a lesser extent daneborg originating from typical storms in the greenland sea have long path lengths traversing the northeast greenland shelf we conclude that this is the likely explanation for the lack of southeast directions from nord and to a lesser degree daneborg in the observed microseism direction of approaches

high temperature fracturing and ductile deformation during cooling of a pluton the lake edison granodiorite sierra nevada batholith california abstract in the bear creek area of the sierra nevada batholith california the high temperature postmagmatic deformation structures of the lake edison granodiorite include steeply dipping orthogneiss foliations joints and ductile shear zones that nucleated on joints and leucocratic dykes exploitation of segmented joints resulted in sharply bounded thin shear zones and in large slip gradients near the shear zone tips causing the deformation of the host rock at contractional domains the orthogneiss foliation intensifies towards the contact with the younger mono creek granite and locally defines the dextral rosy finch shear zone rfsz a major kilometre wide zone crosscutting the pluton contacts joints predominantly strike at n70 90 e over most of the lake edison pluton and are exploited as sinistral shear zones both within and outside the rfsz in a narrow 250 m thick zone at the contact with the younger mono creek granite within the rfsz the lake edison granodiorite includes different sets of dextral and sinistral shear zones joints the latter corresponding to the set that dominates over the rest of the lake edison pluton these shear zones joints potentially fit with a composite y r r shear fracture pattern associated with the rfsz or with a pattern consisting of y r shear fractures and rotated t mode i extensional fractures the mineral assemblage of shear zones and the microstructure and texture of quartz mylonites indicate that ductile deformation occurred above 500 c joints and ductile shearing alternated and developed coevally the existing kinematic models do not fully capture the structural complexity of the area or the spatial distribution of the deformation and magmatic structures future models should account more completely for the character of ductile and brittle deformation as these plutons were emplaced and cooled

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

eurasia africa plate boundary affected by a south atlantic asthenospheric channel in the gulf of cadiz region the gulf of cadiz has been affected by a long and complex geodynamic evolution the lithospheric structure is poorly understood in this region and it also shows a diffuse seismicity spanning over a broad area the canary archipelago has been extensively studied nevertheless there are fundamental topics that are still under debate no studies have addressed or suggested the possibility of a plausible geodynamic connection between both remarkable locations in this study we integrate total tectonic subsidence tts curie point depth cpd bouguer gravity anomaly and seismic information tts shows the existence of a basement bulge in the area of the canary archipelago that extends to the north and in the area of madeira island which extends in a more subtle way to the north too likewise the cpd reaches the shallowest values in the same location at the canary archipelago these two aspects suggest a cause effect relationship between tts and cpd at this specific area gravity data and cpd show a linear feature which links the nw of the canary archipelago and the gulf of cadiz the data we manage in this work show remarkable clues as a the absence of a similar signal in the tts b the fact that cpd it is rather constant along this track c cpd amplitude also almost doubles the values obtained on canary archipelago and d the existence of a clear correlation between seismogenic depths and cpd which points to the existence of a correlation between seismicity and the thermal architecture of the lithosphere all of these evidences support the presence of a lithospheric thinning between canary islands and gulf of cadiz area and in turn the presence of an asthenospheric channel which feeds and alter locally the eurasia africa plate boundary

local shear zone pattern and bulk deformation in the gran paradiso metagranite nw italian alps the gran paradiso nappe of the northwestern alps mostly consists of augen gneisses derived from the alpine deformation of permian granitoids the regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top to west sense of shear the granitoid protolith is preserved in the kilometre scale low strain domain of the piantonetto valley and mainly consists of a porphyritic metagranite including joints leucocratic dykes and biotite rich schlieren in this low strain domain the alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite the shear zones mostly dip towards s se in a shallow shear zones1 to steep inclination shear zones2 the shear zones show typical features that can be explained by reactivation of pre existing joints and planar compositional heterogeneities palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening the kinematics of individual shear zones depends on the orientation of the original heterogeneities acting as nucleation planes and by partitioning of strain components at the kilometre scale with concentration of the flattening component to the piantonetto low strain domain the strain geometry and the kinematics of individual shear zones within piantonetto are not directly connected to the top to west sense of tectonic transport observed elsewhere in the gran paradiso nappe however the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the gran paradiso massif we conclude that the shear zone network of the piantonetto valley is representative of the incipient stages of ductile deformation of a granite nappe even if its architecture is determined by the arrangement of pre existing structural and compositional heterogeneities aspects of the large scale bulk strain can be derived from this local shear zone pattern

global eocene tectonic unrest possible causes and effects around the north american plate abstract many of our planet s crises were the result of sudden changes in plate tectonic configuration or catastrophic outbursts of volcanism caused by mantle plume impingement at the base of the lithosphere at the paleocene eocene boundary and in the early eocene several mantle plumes continental collision and mid ocean ridge subduction triggered a series of changes in seafloor spreading dynamics we have constructed a detailed global model of oceanic lithosphere age and spreading rates for the 60 to 35 ma interval we revise evidence for changes in seafloor spreading direction in the north atlantic arctic and ne pacific oceans at least two periods of spreading rate highs which are separated by sharp value decrease occurred along the entire eastern north american plate boundary from c25 to c18 time c 57 to 40 ma the collision and incipient subduction of the early eocene siletzia oceanic lip may have caused the sharp decrease in spreading rate at c23 time in the labrador sea and north of charlie gibbs fracture zone the post c23 rapid farallon slab break off and subsequent upper mantle flow upwelling may have led to further variations in north atlantic spreading rates at c22 21 time eastward pacific subduction may have resumed at c 43 ma as indicated by a steady ne pacific seafloor spreading regime which resumed at or shortly after c21 the north atlantic realm shows a delayed response to tectonic events west of north america with an increase in spreading rate south of charlie gibbs fracture zone from c20 to c18 time followed by a steady decrease until the oligocene north american late paleocene early eocene kimberlite magma that erupted more than 1000 km from its western plate boundary constitutes additional evidence that tectonic stresses due to changes in the mantle lithosphere interactions may have affected the entire plate and therefore also its eastern boundaries

a geophysical study of the hatton bank volcanic margin a summary of the results from a combined seismic gravity and magnetic experiment abstract this paper summarizes the results of a geophysical study of the hatton bank volcanic margin in the north atlantic normal incidence and synthetic aperture profiles saps show seaward dipping reflectors interpreted as subaerial basalt flows on the continental slope and in the adjacent ocean basin velocity depth profiles derived from expanding spread profiles esps shot along strike show elevated velocities in the lower crust on the continental slope the high velocities are interpreted as evidence for igneous underplating or intrusion gravity modelling shows high densities in the lower crust supporting a model of igneous accretion an ocean bottom seismometer obs and multichannel refraction line shot perpendicularly to strike through the midpoints of the esps further constrained the velocity structure across the margin the velocity model was refined as each dataset was processed clearly showing that multiple datasets were required in order to constrain the velocity structure tightly in particular esps were found to produce misleading results in places where the assumption that the sub surface structure is one dimensional inherent in the esp technique was not valid the final velocity model is used to infer the thickness of igneous material accreted across the hatton margin which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent iceland basin as the oceanic crust thins its seismic velocity decreases suggesting a decrease in percentage partial melting these observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures

crustal structure beneath the faroe islands and the faroe iceland ridge abstract we have mapped the transition from the continental faroe block the faroe islands and surrounding shelf to the thickened oceanic crust of the faroe iceland ridge in the north atlantic using the results of a detailed sea to land seismic profile with wide angle to normal incidence recordings of explosive and airgun shots fired at sea along the faroe iceland ridge interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30 3 km thick oceanic crust with a gradual transition to ancient continental crust from 100 to 40 km northwest of the faroe islands close to the shelf edge this confirms that the crust beneath the faroe islands which may be up to 46 km thick comprises continental material in agreement with previous seismic and geochemical results results suggest that the upper 5 2 0 7 km of the faroe crust consists of tertiary basalts generated during continental breakup overlying the continental crust beneath the lower crust where seismic constraint is poor may exhibit high seismic velocities 7 1 7 6 km s 1 which we attribute to underplating or intrusion by mafic melts during continental breakup in the early tertiary

extremely thin crust in the indian ocean possibly resulting from plume ridge interaction the thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle it is 5 8 km for a crust formed at slow and fast spreading centres and 2 4 km at ultra slow spreading centres away from hotspots and mantle anomalies the crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces here we present results for the crust generated at the fast wharton spreading centre 55 58 ma ago using seismic reflection and refraction data we find that the crust over a 200 km segment of the wharton basin is only 3 5 4 5 km thick the thinnest crust ever observed in a fast spreading environment a thin crust could be produced by the presence of depleted and or cold mantle numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond because of a strongly temperature dependent viscosity of lithospheric material the hotter therefore less viscous bottom of the lithosphere can be mobilized by an impinging plume if sampled by a spreading centre the locally cold and depleted mantle should result in low production of melt we suggest that the observed thin crust in the wharton basin is likely to have been formed by the interaction between the kerguelen mantle plume and the wharton spreading centre 55 ma ago

seismic evidence for plume derived volcanism during formation of the continental margin in southern davis strait and northern labrador sea the crustal structure in the southern davis strait and the adjacent ocean continent transition zone in ne labrador sea was determined along a 185 km long refraction wide angle reflection seismic transect to study the impact of the iceland mantle plume to this region a p wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by ocean bottom seismographs a coincident industry multichannel reflection seismic profile was used to guide the modelling as reflectivity could be identified down to moho the model displays a marked lateral change of velocity structure the sedimentary cover velocities 1 8 3 9 km s 1 is up to 4 km thick in the north and thins to 1 km in the south the segment of the line within southern davis strait is interpreted to be of continental character with a two layered 13 km thick crust with p wave velocities of 5 6 5 8 and 6 4 6 7 km s 1 in the upper and lower crust respectively the crust is underlain by a 2 to 4 km thick high velocity layer 7 5 km s 1 this layer we interpret as underplated material related to the iceland plume the southern segment of the line in labrador sea displays a 2 km thick layer with a velocity of 4 5 km s 1 this layer can be correlated to a well about 100 km to the west of the line where palaeocene basalts and interbedded sediments were drilled underneath is a 12 km thick crust with a 2 km thick upper layer 5 8 6 6 km s 1 and a 10 km thick lower layer 6 8 7 2 km s 1 this crust is interpreted to be of oceanic character s wave modelling yields a poisson s ratio of 0 28 for the lower crust compatible with a gabbroic composition the igneous crust is 5 km thicker than normal oceanic crust we suggest that the increased magma production was created by buoyancy driving flow

shear zones and magma ascent a model based on a review of the tertiary magmatism in the alps 1 the alpine oligocene plutons are spatially and temporally associated with the activity of the periadriatic fault system pfs an orogen parallel crustal scale transpressive mylonitic belt excellent three dimensional exposure combined with a wealth of structural seismic petrological geochronological geochemical and paleomagnetic data collected over the last decades help to constrain the relationships between deformation ascent and emplacement of the plutons magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the periadriatic fault system which was used as ascent pathway to cover vertical lengths of 20 to 40 km therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth ascent of the melts is controlled by the mylonitic foliation of the pfs which forms the only steep anisotropy continuously traversing the entire alpine crust in contrast the flow direction is not influenced by the specific kinematics of the faults final emplacement of the plutons occurred by extrusion from the periadriatic fault system into the adjacent country rocks the transition from ascent to final emplacement is favored by partitioning of transpressive deformation

development of the australian antarctic depth anomaly the oceanic australian antarctic discordance aad contains two unusual features 1 n s trending anomalously deep bathymetries and 2 rough basement morphologies in young 45 spreading obliquities

quantifying transient mantle convective uplift an example from the faroe shetland basin the icelandic mantle plume generated maximum uplift in early paleogene times the faroe shetland basin which fringes the north atlantic margin of europe was close to the center of early paleogene icelandic plume activity three dimensional seismic reflection data from the faroe shetland basin reveal sedimentary geometries that allow a phase of transient uplift to be accurately reconstructed and quantified close to the paleocene eocene boundary circa 56 ma rapid uplift resulted in fluvial incision into marine sediments this unconformity was buried by nonmarine sediments recording the decay of transient uplift relief on the unconformity was 550 m constraining the minimum amount of surface uplift some 60 m of this uplift can be attributed to the isostatic response to erosional unloading tectonic uplift of over 490 m peaked and decayed within 3 ma rates of water loaded tectonic subsidence following peak uplift are several times greater than maximum expected postrift subsidence rates the amplitude and duration of this transient effect is best explained by a mantle convective phenomenon we suggest that a region of hot plume material flowed laterally beneath the lithosphere producing transient uplift which decayed when plume material was advected farther away our analysis suggests that under certain circumstances stratigraphic records can yield valuable quantitative information about aspects of mantle convective circulation

the role of dynamic recrystallization in the development of lattice preferred orientations in experimentally deformed quartz aggregates abstract experiments on coaxially deformed quartz aggregates show that when dynamic recrystallization involves grain size reduction the lattice preferred orientation lpo of the recrystallized grains depends on the recrystallization mechanism at low temperatures and fast strain rates strain induced grain boundary migration recrystallization favors growth of grains oriented poorly for slip on the basal and prism planes resulting in an lpo of c axes parallel to the 1 direction at higher temperatures or slower strain rates progressive subgrain rotation produces recrystallized grains which inherit the small circle girdle lpo of their deformed host grains and which continue to re orient by slip at even higher temperatures and slower strain rates progressive subgrain rotation is accompanied by rapid grain boundary migration which does not favor grains of any particular orientation thus the lpo is again similar to that of non recrystallized grains in contrast when dynamic recrystallization involves grain growth driven by surface energy reduction a strong lpo of c axes parallel to 1 develops during grain growth but then evolves with increasing strain toward a small circle girdle the correlation of lpos with recrystallization mechanisms has important implications for the assumption of homogeneous strain in theoretical models of lpos and for interpreting natural lpos in recrystallized rocks

inversion tectonics on the continental margin east of newfoundland industrial multichannel seismic data show a series of elongate sedimentary basins beneath the outer continental shelf east of newfoundland the basin sediments range in age from carboniferous to early cretaceous regional subsidence since the late cretaceous buried these basins under cenozoic sediments the underlying basement is composed of precambrian and paleozoic rocks of the appalachian orogen seismic data show that the flanks of the basins are primarily erosional surfaces that record uplift followed by subsidence in places total displacement exceeds 10 km the development of these basins can be reasonably explained in terms of recurrent vertical displacements inversion tectonics it does not appear that these movements involved significant crustal extension the central segment of the series of basins coincides with a pronounced positive shelf edge gravity anomaly it reflects an underlying dense body presumably related to basin formation if this is a typical shelf edge gravity anomaly it suggests that similar anomalies elsewhere may also indicate inversion tectonics

simulating coseismic deformation of quartz in the middle crust and fabric evolution during postseismic stress relaxation an experimental study abstract non steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake driven episodic deformation in the middle crust three types of experiments were carried out using a modified griggs type solid medium deformation apparatus all three start with high stress deformation at a temperature of 400 c and a constant strain rate of 10 4 s 1 type a some are followed by annealing in the stability field of quartz for 14 15 h at zero nominal differential stress and temperatures of 800 1000 c type a b or by annealing for 15 h at 900 c and at a residual stress type a c the quartz samples reveal a very high strength 2 gpa at a few percent of permanent strain the microstructures after short term high stress deformation type a record localized brittle and plastic deformation statisc annealing type a b results in recrystallisation restricted to the highly damaged zones the new grains aligned in strings and without crystallographic preferred orientation indicate nucleation and growth annealing at non hydrostatic conditions type a c results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation in this case however the recrystallised zone is several grain diameters wide the grains are elongate and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation quartz microstructures identical to those produced in type a b experiments are observed in cores recovered from long valley exploratory well in the quaternary long valley caldera california with considerable seismic activity the experiments demonstrate the behaviour of quartz at coseismic loading type a and subsequent static annealing type a b or creep at decaying stress type a c in the middle crust the experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks

crystallographic orientation of microcracks in quartz and inferred deformation processes a study on gneisses from the german continental deep drilling project ktb abstract this study was carried out on four gneiss samples from the german continental deep drilling project ktb taken at depths between 556 and 8633 m the crystallographic orientation of microcracks in quartz was determined by a combination of the electron channelling pattern method ecp and u stage microscopy the distinct preferred crystallographic orientations of various crack generations point to different processes of crack initiation and propagation which partly seem to be depth dependent for older healed cracks a crystalplastic initiation due to dislocation pile up and related lattice distortion is indicated by cracking normal to prominent slip directions which are a at lower or c at comparatively higher temperatures in contrast younger open cracks preferentially formed on crystallographic planes with low surface energy rhombs and prisms which is interpreted in terms of pure elastic crack mechanisms because of the thermoelastic anisotropy of quartz internal stresses resulting from thermal contraction during cooling may be the main driving force for the initiation or further propagation of cracks parallel to the c axis crack propagation preferentially affects grains with crystal lattices and operating crack mechanisms being in a direction appropriate to the applied external tectonic stresses this selective cracking explains why cracks also show constant orientations with respect to geographic directions up to regional dimensions

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

the 40ar 39ar age dating of the madeira archipelago and hotspot track eastern north atlantic the 40ar 39ar ages for 35 volcanic rocks and 14c ages for two charcoal samples from the madeira archipelago and ampere seamount eastern north atlantic are presented the volcanic evolution of madeira can be divided into a voluminous shield stage 4 6 0 7 ma and a subsequent low volume posterosional stage 70 myr old hotspot which formed porto santo island 11 1 14 3 ma seine ampere 31 ma corral patch and ormond 65 67 ma feraud et al 1982 1986 seamounts and the serra de monchique 70 72 ma mcintyre and berger 1982 complex in southern portugal age and spatial relationships result in a calculated absolute african plate motion above the hotspot of 1 2 cm yr around a rotation pole located at 43 36 n 24 33 w

crystallographic control and texture inheritance during mylonitization of coarse grained quartz veins abstract quartz veins within rieserferner pluton underwent deformation during post magmatic cooling at temperature around 450 c different crystallographic orientations of cm sized quartz vein crystals conditioned the evolution of microstructures and crystallographic preferred orientations cpo during vein parallel simple shear up to high shear strains 10 for glide in ribbons with c axis sub parallel to the shear zone vorticity y axis ribbons with c axis orthogonal to y xz type ribbons were stronger and hardened more quickly they show lower aspect ratios and fine grain size 10 20 m recrystallization along sets of microshear zones szs exploiting crystallographic planes distortion of xz type ribbons and recrystallization preferentially exploited the slip systems with misorientation axis close to y new grains of szs initiated by subgrain rotation recrystallization sgr and thereupon achieved high angle misorientations by a concurrent process of heterogeneous rigid grain rotation around y associated with the confined shear within the sz dauphine twinning occurred pervasively but did not play a dominant role on sz nucleation recrystallization became widespread at 2 and pervasive at 10 ultramylonitic quartz veins are fine grained 10 m similar to new grains of sz and show a cpo banding resulting in a bulk c axis cpo with a y maximum as part of a single girdle about orthogonal to the foliation and orientations at the pole figure periphery at moderate to high angle to the foliation this bulk cpo derives from steady state sgr associated with preferential activity in the different cpo bands of slip systems generating subgrain boundaries with misorientation axes close to y the cpo of individual recrystallized bands is largely inherited from the original crystallographic orientation of the ribbons and therefore vein crystals from which they derived high strain and pervasive recrystallization were not enough to reset the initial crystallographic heterogeneity and this cpo memory is explained by the dominance of sgr this contrast with experimental observation of a rapid erasure of a pristine cpo by cannibalism from grains with the most favourably oriented slip system under dominant grain boundary migration recrystallization

evidence for age and evolution of corner seamounts and great meteor seamount chain from multibeam bathymetry the comer seamounts in the western north atlantic and great meteor seamount chain in the eastern north atlantic are thought to progress in age from late cretaceous through late cenozoic they both presumably formed by volcanism above the new england hotspot when first the north american plate and then the mid atlantic ridge axis and african plate moved over the hotspot high resolution multibeam bathymetry of the seamounts shows geomorphic features such as guyots terraces and a base level plateau cruiser plateau that we interpret to have formed at sea level we have backtracked these features to sea level along the north atlantic crustal age depth curve in order to estimate their ages the derived age pattern of volcanism indicates formation of the comer seamounts at ca 80 ma to 76 ma with migration of the mid atlantic ridge plate boundary over the hotspot and formation of the cruiser plateau about 76 ma seamount ages suggest that subsequent volcanism on the african plate moved first northward in the late cretaceous to early cenozoic plato tyro and atlantis seamount groups then southward to great meteor seamount in the late cenozoic recurrent volcanism appears to have occurred at some seamounts up to 20 30 m y after their initial passage over the hotspot it would thus appear that intralithospheric conduits can link the hotspot to old seamounts several hundred kilometers away

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

the southern margin of the rockall plateau stratigraphy tertiary volcanism and plate tectonic evolution the geological evolution of the southwest rockall plateau in the area of edoras bank has been clarified using seismic reflection gravity and magnetic data four principal reflectors are observed within the tertiary sedimentary sequence i late miocene ii latest early miocene iii late eocene iv earliest eocene a period of pronounced sediment drift accumulation marks the interval i ii reflector iii the top of a sediment wedge prograding southward from edoras bank marks a change from terrigenous to pelagic sedimentation that is probably related to subsidence of the rockall plateau following the separation of greenland from eurasia in the earliest eocene reflector iv marks the top of a wedge shaped seismically transparent layer that also thins southward away from edoras bank on the basis of its seismic attributes and magnetic signature this layer is interpreted as a volcanic sheet formed as part of the north atlantic tertiary volcanic province during rifting of greenland from eurasia the recognition of voluminous volcanic rocks south of edoras bank extends the known area of the tertiary volcanic province several hundred kilometres to the south gravity anomaly modelling and continental reconstructions suggest that the region south of edoras bank is underlain by thinned continental crust a four stage geological evolution for this region is indicated i initial rifting associated with the separation of labrador from greenland in the late cretaceous is characterized by enhanced crustal thinning and subsidence in the region of a rift triple junction ii passive subsidence and accumulation of late cretaceous and earliest tertiary sediments followed the initiation of seafloor spreading in the labrador sea iii blanketing of the area by palaeocene volcanic rocks masked pre existing magnetic lineations providing an explanation for some of the problems in earlier interpretations based mainly on magnetic data iv post volcanic sedimentation continuing to the present day

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

determining continuous basins across conjugate margins the east orphan porcupine and galicia interior basins of the southern north atlantic ocean abstract the east orphan porcupine and galicia interior basins are several of the most promising basins for hydrocarbon exploration along the rifted continental margins of the southern north atlantic ocean despite having formed at similar geological times the basins exhibit fundamental differences in symmetry crustal thickness sedimentary cover thickness and amount of extension interpretation of seismic reflection and well data was integrated with published 3d grids of depth to basement and moho proxy depth to interpret and restore select seismic reflection lines publicly available kinematic evolution models were also integrated to evaluate and compare the restored seismic reflection lines in a more global regional context interpretation of five seismo stratigraphic units and three tectonostratigraphic megasequences along the east orphan porcupine and galicia interior basins reveals similar seismic character for each unit and comparable tectonic history the structural restoration of the selected lines indicates that evolution sedimentary cover thickness faulting style crustal structure and kinematic evolution of the east orphan and porcupine basins differ significantly a variable and asymmetrical crustal structure is found in the east orphan basin contrasting with the elongated and symmetric porcupine and galicia interior basins rift domain maps of the three basins reveal that they are each underlain by hyperextended crust with possible exhumed mantle in the centre of the porcupine basin based on a holistic analysis of the results obtained the linkage between the east orphan and the porcupine basins seems implausible but rather a contemporaneous relationship is interpreted moreover a potential connection between the galicia interior basin and the porcupine basin during the early to late jurassic is proposed such scenarios imply an oblique and synchronous rifting evolution around the bay of biscay triple junction of the southern north atlantic

texture development of recrystallised quartz polycrystals unravelled by orientation and misorientation characteristics abstract the development of microstructures and textures i e crystallographic preferred orientations during recrystallisation of naturally deformed quartz polycrystals has been studied via electron diffraction techniques in the scanning electron microscope in the investigated sample series of quartz rich rocks originating from different deformation regimes the microstructural and textural changes in quartz have been significantly influenced by dynamic recrystallisation based on microstructural observations paired with orientation and misorientation analyses down to the scale of grains and subgrains criteria could be established which characterise the dominant recrystallisation process and its influence on texture development it is shown that the texture development during dynamic recrystallisation is controlled by a differential activation of slip systems in grains of soft and hard orientations the analyses provide further evidence that specific grain orientations are preferred during crystal plastic deformation recrystallisation and grain growth the influence of twinning after the dauphine law was also investigated observations of a progressive reduction in the population of dauphine twin boundaries during recrystallisation and a penetrative deformation in both hosts and twins indicate a generation prior to deformation and recrystallisation a mechanical origin for twinning and possible influence on texture development was therefore discarded

regional anomalies of sediment thickness basement depth and isostatic crustal thickness in the north atlantic ocean abstract we calculate the anomalous basement topography for the north atlantic ocean from 30 to 70 n latitude and from 0 to 70 w longitude at a resolution of roughly 6 6 km using grids of total sediment thickness and observed and predicted sea floor bathymetry to correct for the effects of isostatic sediment loading and lithospheric age plotting this residual topography for various plate reconstructions during opening of the north atlantic we delineate consistent patterns of basement highs related to variations in hotspot related volcanism in addition to iceland and the azores we recognize three centers of excess volcanism at the mid atlantic ridge the milne seamounts and azores biscay rise 75 40 ma the southeast newfoundland ridge and madeira tore rise 130 110 ma and the east and west thulean rises 60 50 ma the duration of volcanic activity ranges from 8 to 10 m y thulean rises to 60 m y iceland and thus it appears that both long and short lived hotspots coexist even in relatively close proximity in contrast during the period 110 60 ma we observe little excess volcanism during either continental breakup or seafloor spreading we estimate isostatic crustal thickness from the anomalous basement depths after first removing dynamic effects created by mantle flow maximum thicknesses of volcanic features from 30 km beneath the greenland iceland faeroe ridge to 15 km beneath the azores biscay rise are broadly consistent with seismic data and predictions of decompression melting widths of volcanic features indicate that thickening primarily occurs within 100 200 km of hotspots except along continental margins that rifted at the time of the hotspot activity i e east greenland and the hatton rockall bank we observe conjugate structures south of greenland and edoras bank where excess volcanism appears to have extended beyond the margin proper and into oceanic crust similar conjugate features appear in the labrador sea south of davis strait finally we identify anomalous oceanic regions adjacent to some continental margins where unusually low values of predicted crustal thickness suggest either additional variations in plate properties or non isostatic effects within the mantle

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

the hatton basin and continental margin crustal structure from wide angle seismic and gravity data results from a wide angle seismic and gravity study between the rockall bank and the iceland basin in the north atlantic are presented crustal and sedimentary structures are resolved in the hatton basin and across the hatton continental margin hcm east of magnetic anomaly 24 the structure of the oceanic crust west of the anomaly is also determined gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide angle seismic data are poor a two layer sedimentary sequence is present both in the hatton basin and across the continental margin the lower layer with p wave velocity of about 4 km s is interpreted as pre eocene synrift sediments and is up to 3 5 km thick a younger and thinner 1 2 5 km postrift sequence with a velocity of about 2 km s defines a strong velocity contrast which suggests an erosional unconformity surface the sedimentary structure is distinctly different from that in the rockall trough where a third intermediate layer vp 3 km s occurs the three layer crust characterized by two intracrustal reflections pip1 and pip2 varies from 30 km thick under the rockall bank to about 15 km below the hatton basin where it is stretched by a factor of 2 relative to onshore ireland the crust is thinnest below the hatton bank where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit below the hcm a region of thick lower crust with anomalously high velocity vp 7 2 km s is resolved by the seismic and gravity data it is connected west of anomaly 24 to a region of oceanic crust which is thicker than in the iceland basin these relationships between the thick lower crust below the hcm and the oceanic crust in the iceland basin are interpreted as evidence for magmatic underplating consistent with previous models for the hcm the inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 early eocene time

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

spatial and temporal patterns of cenozoic dynamic topography around australia 1 despite its importance the spatial and temporal pattern of dynamic topography generated by mantle convective circulation is poorly known we present accurate estimates of dynamic topography from oceanic basins and continental margins surrounding australia our starting point is measurement of residual depth anomalies on the oldest oceanic floor adjacent to the continental shelf these anomalies were determined from a combined dataset of 200 seismic reflection and wide angle images of well sedimented oceanic crust they have amplitudes of between 1 km and 0 5 km and their spatial variation is broadly consistent with long wavelength free air gravity and shallow seismic tomographic anomalies along the northwest shelf a regional depth anomaly of 300 to 700 m intersects the adjacent continental shelf the temporal evolution of this anomaly was determined by analyzing the stratigraphic architecture of an extensive carbonate platform which fringes the shelf and records a dramatic switch from progradation to aggradation during neogene times three dimensional seismic mapping calibrated by boreholes was used to calculate water loaded subsidence histories at rollover points of clinoforms along the shelf at 9 3 ma the rate of subsidence increases from 5 to up 75 m myr 1 generating a subsidence anomaly of 300 to 700 m the amplitude of this anomaly varies along the shelf and cannot be generated by glacio eustatic sea level variation instead we propose that a regional subsidence episode which affects both the proximal shelf and the distal oceanic basin was generated by convective drawdown by combining our results with other published estimates of uplift and subsidence a map of australia which shows the spatial and temporal pattern of dynamic topography is presented most but not all of australia s epeirogeny can be attributed to rapid northward motion of the australian plate over a pre existing pattern of convective circulation

why allanite may swindle about its true age allanite from the tertiary rieserferner pluton austrian alps is texturally and chemically heterogeneous continuous covariation trends reflect coupled substitution of ca al vs fe ree th in allanite whereas systematic variations in la nd demonstrate the increasingly stronger depletion of lree in the melt during its crystallization allanite samples corrected for 206pbexcess from two rocks scatter in the concordia diagram and define discordias from 31 8 0 4 ma and 32 2 0 4 ma to ca 540 ma the apparent inheritance does not originate from the inclusion of older allanite or a high phase such as monazite xenotime or zircon but from the incorporation of radiogenic pb originating from a precursor since allanite requires a high enrichment of th u and lree it may form at the expense of a th lree rich precursor in metamorphic rocks or where such a phase had dissolved in melts likely precursors acquire with time radiogenic pb isotopic compositions this pb if incorporated in the product mineral may give the illusion of inheritance the allanite samples from the rieserferner pluton show a tendency from high th ucalc 30 50 in samples with an alpine age to low th ucalc 4 12 in samples with distinct inheritance this relation between extent of inheritance and apparent th ucalc indicates a lower th ucalc for a possible precursor falling into the th u range commonly encountered for monazite precursor monazite would have originated from assimilated palaeozoic rocks and give rise to localized enrichments of th and lree in the melt thus eventually enabling the growth of allanite

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

king s trough reactivated pseudo fault of a propagating rift summary a compilation of magnetic data in the western north atlantic shows that seafloor spreading magnetic anomalies 26 to 18 are dislocated across a pseudo fault with a direction mirroring that of king s trough it is interpreted that this feature and king s trough were formed by northward propagation of the rift initiated by a hotspot that created the azores biscay rise the rift propagation stopped when king s trough became part of the plate boundary between eurasia and iberia at chron 17 most of the present day structure of king s trough was created by extensional motion across this plate boundary from chron 17 to 9 6c

low temperature intracrystalline deformation microstructures in quartz a review of numerous genetic interpretations of the individual low temperature intracrystalline deformation microstructures in quartz shows that there is no consensus concerning their formation mechanisms therefore we introduce a new purely descriptive terminology for the three categories of intracrystalline deformation microstructures formed in the low quartz stability field fine extinction bands feb wide extinction bands web and localised extinction bands leb the localised extinction bands are further subdivided into blocky bleb straight sleb and granular gleb morphological types a detailed polarised light microscopy study of vein quartz from the low grade metamorphic high ardenne slate belt belgium further reveals a series of particular geometric relationships between these newly defined intracrystalline deformation microstructures these geometric relationships are largely unrecognised or underemphasised in the literature and need to be taken into account in any future genetic interpretation based on our observations and a critical assessment of the current genetic models we argue that the interpretation of the pertinent microstructures in terms of ambient conditions and deformation history should be made with care as long as the genesis of these microstructures is not better confined