mineralogy petrology and geochemistry of beryllium an introduction and list of beryllium minerals in 1996 in collaboration with lawrence anovitz i edited boron mineralogy petrology and geochemistry volume 33 in the reviews in mineralogy series a book that has been reprinted with addenda in 2002 further addenda and corrections are posted at http www minsocam org where you may also find corrections to this volume many of the same reasons for inviting investigators to contribute to a volume on boron apply equally to a volume on beryllium like b be poses analytical difficulties and it has been neglected in many geochemical mineralogical and petrological studies however with the development of instruments to measure cosmogenic isotopes greater availability and refinement of the ion microprobe and with overall improvement in analytical technology interest in be and its cosmogenic isotopes has increased and more studies are being published thus i decided that it was an appropriate time to invite those actively involved in research on be to contribute to this volume which is intended to be a companion to boron mineralogy petrology and geochemistry note in this chapter individual review papers are referred to by author name s and chapter number the be mineral beryl and its colored variants emerald aquamarine and chrysoberyl golden beryl not the present chrysoberyl were known to the ancients and pliny the elder had noted that many persons considered emerald and beryl to be of the same nature sinkankas 1981 p 20 also dana 1892 weeks and leicester 1968 however not until 1798 was it realized that beryl contained a previously unknown constituent analyses before then yielded only silica alumina lime and minor iron oxide vauquelin 1798 anonymous 1930 weeks and leicester 1968 greenwood and earnshaw 1997 the mineralogist rene just hauy asked nicolas louis vauquelin fig 1 to analyze beryl and emerald in order to

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

a crustal model for northern melville bay baffin bay the interpretation of seismic refraction and gravity data acquired in 2010 gives new insights into the crustal structure of the west greenland coast and the adjacent deep central baffin bay basin underneath melville bay the depth of the moho varies between 26 and 17 km stretched continental crust with a thickness of 25 to 14 km and deep sedimentary basins are present in this area the deep melville bay graben contains an up to 11 km thick infill of consolidated and unconsolidated sediments with velocities of 1 6 to 4 9 km s seaward at the 60 km wide transition between oceanic and stretched continental crust a mount shaped magmatic structure is observed which most likely formed prior to the initial formation of oceanic crust the up to 4 km high magmatic structure is underlain by a 2 km thick and 50 km wide high velocity lower crust more to the west in the oceanic part of the baffin bay basin we identify a two layered 3 5 to 6 km thin igneous oceanic crust with increasing thickness toward the shelf beneath the oceanic crust the depth of the moho ranges between 11 5 and 13 5 km in the western part of the profile oceanic layer 3 is unusually thin 1 5 km a possible explanation for the thin crust is accretion due to slow spreading although the basement is notably smooth compared to the basement of other regions formed by ultra slow spreading the oceanic crust is underlain by partly serpentinized upper mantle with velocities of 7 6 to 7 8 km s

initial transient response of the winter polar stratospheric vortex to idealized equatorial pacific sea surface temperature anomalies in the ncar waccm abstractperpetual winter simulations using the ncar whole atmosphere community climate model waccm are conducted to document the differences of the initial transient response of the boreal winter northern hemisphere stratospheric polar vortex to central cpw and eastern pacific warming epw events idealized patches of positive sea surface temperature sst anomalies are superimposed onto a climatological sst field to mimic canonical cpw and epw forcings a 20 member ensemble was created by varying initial atmospheric conditions for both cpw and epw cases in the ensemble average the vortex weakens under both cpw and epw forcing indicated by a negative zonal mean zonal wind tendency this tendency is mainly tied to changes in the eddy driven mean meridional circulation mmc a negative anomaly in the eddy momentum flux convergence also plays a secondary role in the weakening the vortex response however differs dramatically among individual ensemble members a few ensemble members exhibit initial

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

basin seismic stratigraphy and aspects of prospectivity in the ne baffin bay northwest greenland abstract subsurface mapping in the northeastern part of baffin bay offshore northwest greenland indicates the presence of large basement structures and sedimentary basins we identify eight seismic stratigraphic units that can be genetically related to the regional geology and tectonic development the seismic interpretation includes the top of paleocene to eocene volcanic rocks which cover most parts of the southern and western study area using this horizon as a time marker the seismic units are divided into pre cenozoic and palaeogene to neogene strata based on comparison to equivalent units exposed onshore west greenland and canada and sampled by exploration wells farther south in the disko west offshore nuuk region our interpretation of the faults and tectonic elements suggest that the main extensional faults and basin formation took place mainly during the cretaceous and into the paleocene subsequent compressional tectonics and inversion were accompanied by strike slip faulting and local transtensional faults and flexures this last phase of deformation corresponds to the well documented eurekan orogeny in ellesmere island and northern greenland that developed as a result of northward motion and possible counterclockwise rotation of greenland relative to the north american plate we surmise that these tectonic phases exerted a strong control on basin infill by activation of hinterland sediment sources during cretaceous and palaeogene times the deep basins may include cretaceous and palaeogene organic rich sediments with hydrocarbon source potential we estimate that maturation could have started during the eocene

crustal structure of the ocean continent transition at flemish cap seismic refraction results 1 we conducted a seismic refraction experiment across flemish cap and into the deep basin east of newfoundland canada and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots the continental crust at flemish cap is 30 km thick and is divided into three layers with p wave velocities of 6 0 6 7 km s across the southeast flemish cap margin the continental crust thins over a 90 km wide zone to only 1 2 km the ocean continent boundary is near the base of flemish cap and is marked by a fault between thinned continental crust and 3 km thick crust with velocities of 4 7 7 0 km s interpreted as crust from magma starved oceanic accretion this thin crust continues seaward for 55 km and thins locally to 1 5 km below a sediment cover 1 9 3 1 km s oceanic layer 2 4 7 4 9 km s is 1 5 km thick while layer 3 6 9 km s seems to disappear in the thinnest segment of the oceanic crust at the seawardmost end of the line the crust thickens to 6 km mantle with velocities of 7 6 8 0 km s underlies both the thin continental and thin oceanic crust in an 80 km wide zone a gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth normal mantle velocities of 8 0 km s are observed 6 km below basement there are major differences compared to the conjugate galicia bank margin which has a wide zone of extended continental crust more faulting and prominent detachment faults crust formed by seafloor spreading appears symmetric however with 30 km wide zones of oceanic crust accreted on both margins beginning about 4 5 m y before formation of magnetic anomaly m0 118 ma

tectonic evolution of southern baffin bay and davis strait results from a seismic refraction transect between canada and greenland 1 wide angle reflection refraction seismic data were acquired on a 450 km long transect in southern baffin bay extending from baffin island to greenland dense air gun shots were recorded on 22 ocean bottom seismometers apwave velocity model was developed from forward and inverse modeling of the observed travel times beneath the baffin island shelf a three layered continental crust is observed with velocities of 5 5 to 6 9 km s typical for transform margins there is a sharp transition between continental and oceanic crust off baffin island 7 km thick oceanic crust is interpreted to lie in a major transform fault identified on the gravity map beneath the deep baffin bay basin 9 km thick oceanic crust is encountered but thins to 6 km within an assumed fracture zone the thicker than normal oceanic crust indicates an ample magma supply possibly related to melt extracted from a mantle plume seaward of the greenland continental crust 20 km thick igneous crust 6 3 to 7 3 km s is encountered in a 25 km wide zone interpreted as a leaky transform fault that can be correlated southward through davis strait the igneous crust is bounded by a 20 km wide basin to the west underlain by 4 km thick crust of unknown affinity this structure is probably associated with transform movements a high velocity lower crustal layer 7 1 km s of 8 km thickness is indicated beneath the greenland crust and can be correlated into the adjacent thick igneous crust both the thick igneous and greenland crust are covered by up to 4 km of paleogene volcanics 5 2 to 5 7 km s

the crustal structure of the central mozambique continental margin wide angle seismic gravity and magnetic study in the mozambique channel eastern africa abstract the continental margin of mozambique formed during the initial dispersal of gondwana about 180 ma due to the lack of deep seismic and dense potential field data many details of the timing and geometry of the early breakup in this region remained unknown to date to close this gap a research project mobamasis mozambique basin marine seismic survey with the french research vessel r v marion dufresne ii was conducted in 2007 this paper presents the results of p wave magnetic and 2d gravity modelling along two parallel seismic refraction profiles between 37 and 41 e crossing the mozambique rifted margin the crust shows the characteristics of normal to slightly thickened oceanic crust a lower crustal high velocity body with p wave velocities of 7 0 7 5 km s is observed along both profiles its origin is discussed in the context of upper mantle convection and thermal properties the existing magnetic anomaly identifications have been extended to older ages we postulate that the oldest oceanic crust near the central mozambique continental margin has been formed around m41n 166 ma closer to the coast a pronounced negative magnetic anomaly exists that we interpret to coincide with the continent ocean transition this implies that the position of the continent ocean transition is located significantly closer to the shoreline than proposed before

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

seismic waves velocities and anisotropy in serpentinized peridotites from xigaze ophiolite abundance of serpentine in slow spreading ridge the effect of serpentinization on seismic wave velocity and anisotropy has been analysed in 6 peridotite samples of the xigaze ophiolite having harzburgitic composition and a degree of serpentinization ranging from 3 to 70 we found i p and s wave velocities are linearly correlated with serpentine content ii anisotropy of p and s waves decreases with increasing serpentinization while in fresh peridotites anisotropy is of the order of 6 for 70 of serpentinization it drops to 1 for p waves and to 2 for s waves comparison of published laboratory velocity data in oceanic lithologies like gabbros and dolerites with serpentinized peridotite data shows the difficulty of distinguishing between gabbros and peridotites having a percentage of serpentine between 10 and 40 by seismic waves velocities only this comparison suggests that seismic velocity data are consistent with the existence of partialy serpentinized peridotites up to 40 of serpentinization in the oceanic crust formed at slow spreading ridges in accordance with geological observation

the davis strait crust a transform margin between two oceanic basins the davis strait is located between canada and greenland and connects the labrador sea and the baffin bay basins both basins formed in cretaceous to eocene time and were connected by a transform fault system in the davis strait whether the crust in the central davis strait is oceanic or continental has been disputed this information is needed to understand the evolution of this transform margin during the separation of the north american plate and greenland we here present a 315 km long east west oriented profile that crosses the davis strait and two major transform fault systems the ungava fault complex and the hudson fracture zone by forward modelling of data from 12 ocean bottom seismographs we develop a p wave velocity model we compare thismodel with a density model from ship borne gravity data seismic reflection and magnetic anomaly data support and complement the interpretation most of the crust is covered by basalt flows that indicate extensive volcanism in the davis strait while the upper crust is uniform the middle and lower crust are characterized by higher p wave velocities and densities at the location of the ungava fault complex here p wave velocities of the middle crust are 6 6 km s 1 and of the lower crust are 7 1 km s 1 compared to 6 3 and 6 8 km s 1 outside this area densities are 2850 and 3050 kg m 3 compared to 2800 and 2900 kg m 3 we here interpret a 45 km long section as stretched and intruded crust or as new igneous crust that correlates with oceanic crust in the southern davis strait a high velocity lower crust 6 9 7 3 km s 1 indicates a high content of mafic material this mantle derived material gradually intruded the lower crust of the adjacent continental crust and can be related to the iceland mantle plume with plate kinematic modelling we can demonstrate the importance of two transform fault systems in the davis strait the ungava fault complex with transpression and the hudson fracture zone with pure strike slip motion we show that with recent poles of rotation most of the relative motion between the north american plate and greenland took place along the hudson fracture zone

a new frontier province offshore northwest greenland structure basin development and petroleum potential of the melville bay area in the melville bay area offshore northwest greenland very large structures and sedimentary basins which were predicted many years ago on the basis of magnetic and gravity data have been confirmed by a recent reconnaissance seismic survey with implications that are encouraging for petroleum exploration in the area the melville bay area flanks a small ocean basin in baffin bay that is thought to have formed by oblique sea floor spreading in the eocene there are two major coast parallel basins in the area the inner basin the melville bay graben is essentially a half graben with a maximum thickness of sediments exceeding 13 km a complex fault controlled ridge system separates this basin from the outer kivioq basin in which up to 7 km of sediments have accumulated by analogy with onshore geology in the surrounding areas and well data from the continental shelves off southern west greenland and labrador to the south it is expected that the first phase of rifting and sedimentation took place in the early middle cretaceous while a second phase of rifting took place in the latest cretaceous and early paleocene later compression and inversion affected the northern part of the area leading to the formation of large anticlinal structures copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received january 30 1996 revised manuscript received september 9 1996 final acceptance january 21 1997 2geoarctic consulting ltd 1620 9th street nw calgary alberta t2m 3l4 canada 3nunaoil a s pilestraede 52 dk 1112 copenhagen denmark 4geological survey of denmark and greenland thoravej 8 dk 2400 copenhagen denmark we wish to acknowledge the companies partaking in the kanumas project bp exxon japan national oil company shell statoil and texaco for allowing us the opportunity to publish this paper our thanks also to rene forsberg and simon ekholm national survey and cadastre for processing the gravity data for the paper and to alan menelly consultant for discussions on the structural development of the area airborne gravity data were made available courtesy of john brozena of the u s naval research laboratory

crustal structures of the boreas basin and the knipovich ridge north atlantic the boreas basin is located in the norwegian greenland sea between northeast greenland and svalbard towards the east it is bounded by the ultraslow mid ocean knipovich ridge here we present a 340 km long seismic refraction line acquired during the expedition ark xxiv 3 of research vessel polarstern in 2009 using 18 ocean bottom seismometers it crosses the central boreas basin from the knipovich ridge to the northeast greenland margin thus the line provides the first reliable crustal structure information of this basin in addition the gravity data acquired parallel to the seismic refraction line are used to calculate a 2 5 d gravity model the p wave velocity model shows an unusual 3 km thin oceanic crust with seismic velocities less than 6 3 km s 1 indicating the absence of a significant oceanic layer 3 mantle velocities vary between 7 5 kms 1 in the uppermost mantle and 8 0 km s 1 at approximately 15 km depth the low velocities within the upper mantle may be explained by 13 per cent serpentinisation which is negligible at about 15 km depth furthermore the s wave velocity model shows low vp vs ratios in the mantle indicating a highly serpentinised mantle at shallow depths the gravity model has crustal densities between 2 3 and 2 9 g cm 3 which also point towards the absence of a significant thick oceanic layer 3 the results of our seismic refraction line and other geophysical data indicate that the entire boreas basin opened at ultraslow spreading rates since at least 28 ma no evidence for an extinct spreading ridge in the centre of the boreas basin was found

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

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

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

crustal thickness and earthquake distribution south of the logachev seamount knipovich ridge 1 during rv polarstern cruise ark xxiv 3 2009 a geophysical study along the ultraslow spreading knipovich ridge was conducted the survey located in the rift valley south of the logachev seamount 76 36 n provides a crustal thickness of 4 5 km in the amagmatic parts and 5 7 km underneath the seamount itself the velocity depth function indicates the presence of a thick oceanic layer 2 4 5 km but no indication for a thick oceanic layer 3 the only exception is the area underneath the logachev seamount where velocities higher than 6 km s are detected this indicates a stronger and focussed melt supply underneath the seamount local seismicity was analysed for two days in total 191 quakes were identified in the vicinity of the rift valley with magnitudes up to ml 2 6 at least 48 of them are located in the upper mantle up to 18 km below sea level supporting models predicting a cold mantle or conductive cooling underneath ultraslow ridges to explain the reduced melt supply

gravity inferred crustal thickness of greenland abstract greenland s crust contains clues for understanding the dynamics of the earth s second largest ice sheet as well as the iceland hotspot over the past 100 ma however our knowledge of the sub ice crust is very limited due to the paucity of outcrops and seismic observations particularly in central greenland in order to augment these sparse constraints we modeled terrain and airborne free air gravity data which provide estimates for moho and related crustal thickness these estimates were obtained from the inversion of isostatically corrected terrain effects where the computed gravity effects of the terrain s rock water and ice components were subtracted from the terrain correlated free air anomalies the terrain correlated free air components were estimated from the correlation spectrum between the free air anomalies and the terrain s gravity effects which are assumed to reflect isostatic disturbances of the crustal terrain the results indicated an average crustal thickness of about 37 km for greenland with greater values by 5 10 km in the southern greenland archean blocks and caledonian foldbelt and comparably lower values in northern greenland the thinner crustal thickness estimates in the north are strongly correlated with the faster basal melt regions of the ice sheet and glaciers the crustal thickness map provides constraints for an improved interpretation of the iceland hotspot track in northern greenland at about 100 70 ma

development of the continental margins of the labrador sea a review abstract the labrador sea is a small oceanic basin that developed when the north american and greenland plates separated an initial period of stretching in early cretaceous time formed sedimentary basins now preserved under the continental shelves and around the margins of the oceanic crust the basins subsided thermally during late cretaceous time and a second episode of tectonism took place during latest cretaceous and early paleocene time before the onset of sea floor spreading in mid paleocene time around the northern labrador sea davis strait and in southern baffin bay voluminous picrites and basalts were erupted at and shortly after the commencement of sea floor spreading volcanism occurred again in early eocene time at the same time as sea floor spreading commenced in the northern north atlantic farther southeast along the labrador and southern west greenland margins oceanic crust is separated from continental crust by highly stretched but non magmatic transition zones which developed before sea floor spreading a complex transform zone which developed during sea floor spreading in late paleocene and early eocene time separates continental and oceanic crust along the baffin island margin the greenland and labrador ocean continent transitions are asymmetric across the only available conjugate cross sections however a cross section through the labrador margin farther north resembles the greenland cross section in the conjugate pair more than it does the labrador cross section of this pair consideration of the geological history of the area suggests that the non magmatic transition zones may have formed by slow extension of a few millimetres per year through a period of 53 ma during cretaceous and early paleocene time