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

ocean continent boundary in the iberia abyssal plain from multichannel seismic data abstract the ocean continent boundary of the west iberia margin is marked by a basement ridge trending n s four segments of this ridge are recognized each of them being progressively offset westward from 40 n to 43 n because the setting and seismic character of the ridge in the iberia abyssal plain are similar to those of the galicia margin ridge which is made of serpentinized peridotite we think that the southern segments of the ridge are also made of the same mantle material the segmentation of the ridge suggests that the northward propagation of the continental break up during the north atlantic opening in early cretaceous times was discontinuous each segment possibly corresponding to a propagation step east of the ridge the basement of the whole iberia abyssal plain consists of highly thinned continental crust locally resting on a seismic reflector that resembles the s reflector previously recognized off galicia by analogy with the galicia margin we propose that the tilted crustal blocks lay on serpentinized peridotite derived from the upper mantle the s reflector corresponding to the contact between crustal rocks and serpentinite

some comments on two layer extensional models for the evolution of sedimentary basins simple extensional models that involve listric faulting in the brittle upper crust and plastic flow in the lower lithosphere have been shown to account for the subsidence history of various continental sedimentary basins and continental shelves in this paper we present structural evidence that extension in the continental lithosphere during rifting is not uniform we review evidence that the rifting process in major intracratonic rift basins is spasmodic rather than continuous that extension and subsidence within the rift during a tectonic phase is generally accompanied by uplift and erosion of the rift flanks that subcrustal heat input during rifting occurs over an area much broader than the rift proper and that there may be broad regional uplift before and or during rifting to explain the uplift of the flanks during rifting and their postrift subsidence we invoke two mechanisms subcrustal thinning by extension below the flanks and tilted major normal faults in the crust between graben and flanks the uplift and erosion of the flanks due to tilted faults probably do not significantly affect estimates of extension from subsidence analysis but uplift due to both sources can in certain cases be incorrectly attributed to regional uplift or doming we present a simplified analysis of the two layer extensional model for the elementary case in which extension is instantaneous and the crust is thinned by a different amount from the subcrustal lithosphere we provide a simple graphical method of data analysis to obtain the parameters that describe thinning of the crust and lithosphere during extension from the subsidence history application of this method to the central part of the pattani trough in the intracratonic gulf of thailand shows that extension and thinning of the lithosphere of a factor of 2 since late eocene can account for much of the subsidence history and the present day heat flow our analysis suggests that thinning of the crust in this area is 20 greater than thinning of the whole lithosphere assuming that none of the lithosphere is removed from the region of extension during rifting we show by simple geometric examples that a process of nonuniform extension implies that much greater thicknesses of sediment can be deposited in the center of a young basin than in the case of uniform extension of both crust and subcrustal lithosphere such an extensional process produces significant uplift of the flanks of the rift and as a result of erosion of the uplifted areas the effective cross sectional area of the basin can be increased by as much as 10 20 depending on the rate of erosion compared to the area that would have been created by a process of uniform extension finally we show that it may ultimately be possible to compute erosion rates from seismic cross sections of basins

rift tectonics on the passive continental margin off galicia spain abstract new migrated multichannel seismic profiles on the starved passive continental margin off galicia spain illustrate the tectonic style of the early cretaceous rifting event which preceded the separation of iberia and north america the main episode of rifting was preceded by other tensional episodes probably of earliest cretaceous and late jurassic age listric faulting associated with rifting was accompanied by large scale mass movements from the crests of the tilted blocks two subaerial erosional surfaces are visible one of late jurassic or earliest cretaceous age was affected by the final faulting episode the other was synchronous with the main phase of rifting and is present only on the upper part of the margin because the central trough was under deep water the significance of a ridge near the ocean continent boundary where altered lherzolite has been dredged and cored is discussed below the tilted blocks which include the basement rocks the seismic horizon s is visible as on the armorican margin its significance is also discussed

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

results from a detailed aeromagnetic survey across the northeast newfoundland margin part ii early opening of the north atlantic between the british isles and newfoundland abstract poles of rotation for the north atlantic have been derived from the results of a new aeromagnetic survey northeast of newfoundland reconstruction of the north atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the atlantic from flemish cap and goban spur to the azores gibraltar fracture zone a group of similar anomalies has also been identified in the bay of biscay north of goban spur and flemish cap these anomalies follow the ocean continent boundary poles of rotation derived for this anomaly show that it forms an isochron 100 110 m y during the long cretaceous normal polarity interval the cause of this anomaly is not definite but it may represent an increase in the magnetization of the crust during a limited time within the cretaceous magnetic quiet zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by raymond and labrecque the north atlantic has also been reconstructed at the time of the initial opening in the region between flemish cap and the charlie gibbs fracture zone using inferred ocean continent boundaries on the west and east sides it has been shown that the entire region could not have saparated at one time but that spreading between the british isles and newfoundland had to progress from south to north consequently when active sea floor spreading was taking place between goban spur and flemish cap about 110 m y the region to the north was still being stretched the calculated amount of stretching as derived from the reconstructions about 25 agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region and with the results of deep seismic studies active spreading in the north started about 100 m y ago

thin crust at the western iberia ocean continent transition and ophiolites western iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments the age of breakup decreases from south to north seismic refraction and reflection profiles and magnetic and gravity data from each segment show a consistent pattern of geophysical observations across the ocean continent transition oct zone which is a few tens of kilometers wide we emphasize here the discovery of thin 2 4 km oceanic crust underlain by 7 6 km s 1 material within the oct the available evidence favors the suggestion that the 7 6 km s 1 layer is serpentinized peridotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup this thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities

evidence for extensional shear zones in the mantle offshore britain and their implications for the extension of the continental lithosphere deep seismic profiles recorded in the seas around the british isles have revealed bands of seismic reflections in the uppermost mantle on the basis of their appearance geophysical characteristics and association with major crustal faults these features are best interpreted as mantle shear zones furthermore in the north sea the mantle reflectors appear to follow an extensional trend and when interpreted as extensional shear zones help explain the regional stratigraphy of the overlying basin the crosscutting relationships between mantle reflections to the west of scotland and a major strike slip fault the great glen fault ggf suggest that the mantle structure postdates the ggf and hence is probably related to subsequent extension furthermore kinematic analysis of lithospheric deformation to the north of scotland is consistent with the structure observed on the seismic data if the flannan mantle reflection is an extensional shear zone reconstruction of the lithosphere prior to permian extension restores the flannan to the edge of the rockall trough a rift probably initiated in the carboniferous thus the flannan fault may have developed during carboniferous rifting it is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle and the role of such structures during lithospheric extension is considered

viscosity and thickness of the asthenosphere detected from the fennoscandian uplift the earth s response to the deglaciation in fennoscandia is modelled using a layered viscous model with an elastic lithosphere the modelled tilting of palaeoshorelines at particular locations peripheral to the former ice load and the pattern of the present rate of uplift consistent with the observations strongly suggest a low viscosity asthenosphere earth models without a low viscosity asthenosphere give significant mismatch to the present rate of uplift and are unable to mimic the observed tilting of palaeoshorelines models with a viscosity of the lower mantle of 1 0 2 0 1021 pa s and a viscosity of the upper mantle of 0 7 1 0 1021 pa s overlain by low viscosity asthenosphere with a thickness ranging from 25 to 100 km fit equally well with the observations it is suggested strongly that the asthenosphere has a thickness of less than 150 km and a viscosity of less than 7 0 1019 pa s

relation between phosphorus fractions and development of soils from galicia nw spain abstract the relationship between soil development and the distribution of p among its fractions in the surface horizons of native soils in galicia nw spain was studied p forms were determined by sequential fractionations and the fractions were regrouped as slow turnover and rapid turnover organic and inorganic forms in soils developed on siliceous rock an accumulation of slow turnover organic p sonic naoh extractable organic p is accompanied by an increase in rapid turnover inorganic p resin extractable p bicarbonate extractable inorganic p and a decrease in slow turnover inorganic p naoh sonic naoh hcl extractable inorganic p that could be due to destruction of inorganic p adsorbing complex in soils developed on carbonate bearing rock slow turnover organic p declined and rapid turnover organic p bicarbonate naoh extractable organic p increased these trends are explained in terms of soil processes

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

gabbro and related rock emplacement beneath rifting continental crust upb geochronological and geochemical constraints for the galicia passive margin spain the thinned continental crust of the west galicia margin is bound by a belt of serpentinized peridotites peridotite ridge lying about 300 km off the coast in the north atlantic ocean from this ridge a gabbro and a chlorite rock were studied in an attempt to substantiate rift related subcontinental magmatism occurring prior to sea floor spreading u pb dating of 13 different zircon fractions yields a precise age of 122 1 0 3 ma 2 for the emplacement of the chlorite rock protolith from which more than 50 of si and alkali calc alkali elements were lost during greenschist facies tectonometamorphism sr and nd isotope signatures suggest that the gabbro and chlorite rock protoliths were derived from mantle sources that were moderately depleted in lile relative to a chondritic reservoir no evidence for the presence of continental material in the magma source regions can be observed from the new zircon age of 122 1 0 3 ma and earlier determined39ar40ar age of 122 0 0 6 ma for amphibole from the same locality it can be documented that magma formation solidification and unroofing of the mantle rocks occurred during a short period of time of about 3 4 ma which means that the peridotite ridge detached from the continent and rose to the surface immediately after or even coevally with mantle melting

the formation of passive margins constraints from the crustal structure and segmentation of the deep galicia margin spain the crustal structure of the mesozoic deep galicia margin and adjacent ocean continent boundary ocb was investigated by seismic reflection including pre stack depth migration and attenuation of seismic waves with time the seismic data were calibrated using numerous geological samples recovered by drilling and or by diving with submersible the n s trending margin and ocb are divided in two distinct segments by ne sw synrift transverse faults locally reactivated and inverted by cenozoic tectonics the transverse faulting and ocb segmentation result from crustal stretching probably in a ne sw direction during the rifting stage of the margin in early cretaceous times the cenozoic tectonics are related to iberia eurasia convergence in palaeogene times pyrenean event in both segments of the deep margin the seismic crust is made of four horizontal layers 1 two sedimentary layers corresponding to post and syn rift sequences where velocity ranges from 1 9 to 3 5 km s 1 and where the q factor is low the two sedimentary layers being separated by a strong reflector marking the break up unconformity 2 a faulted layer where velocity ranges from 4 0 to 5 2 km s 1 and where the q factor is high this layer corresponds to the margin tilted blocks where continental basement and lithified pre rift sediments were sampled 3 the lower seismic crust where the velocity 7 km s 1 and more and the q factor are the highest this layer probably made of partly serpentinized peridotite is roofed by a strong s s seismic reflector and resting on a scattering poorly reflective moho a composite model based both on analogue modelling of lithosphere stretching and on available structural data accounts for the present structure of the margin and ocb stretching and thinning of the lithosphere are accommodated by boudinage of the brittle levels upper crust and uppermost mantle and by simple shear in the ductile levels lower crust and upper lithospheric mantle two main conjugate shear zones may account for the observations and seismic data one sz1 located in the lower ductile continental crust is synthetic to the tilting sense of the margin crustal blocks another sz2 located in the ductile mantle accounts for the deformation of mantle terranes and their final unroofing and exposure at the continental rift axis now the ocb the s s reflector is interpreted as the seismic signature of the tectonic contact between crustal terranes and mantle rocks partly transformed into serpentinite by syn rift hydrothermal activity it is probably related to both shear zones sz1 and sz2 the seismic moho is lower within the lithosphere at the fresh serpentinized peridotite boundary

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

development of the north viking graben inferences from laboratory modelling abstract the north viking graben is a classic sedimentary basin in the north sea that has been thoroughly studied over the last decade despite the amount and quality of seismic and well data now available the processes of extension remain unclear and the relationship between sedimentation and deformation is still a matter of debate we present the results of small scale laboratory modelling the models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale the experiments were designed to study the effects of a the degree of coupling between brittle and ductile layers b the obliquity of extension and c the reactivation of inherited crustal scale structures the results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales as well as the relationship between sedimentation and deformation it is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive a model involving nearly pure shear on the lithospheric scale is used to explain the development of an asymmetrical basin at shallow level in addition this model accounts for the nearly horizontal stratification of lower cretaceous sediments the basal cretaceous unconformity which caps many of the normal faults and the localization of deformation to the west of the graben during the lower cretaceous

mantle exhumation at passive margins abstract laboratory models of lithosphere necking have been used to study the modes of passive margin formation and related mantle exhumation at continent ocean boundary four layer models were constructed with sand and silicone putty to represent the brittle and ductile layers respectively of both crust and mantle it is shown that necking of the whole lithosphere model is nearly symmetrical pure shear but that asymmetrical structures simple shear develop internally due to heterogeneous boudinage and or faulting of brittle layers relative movements between brittle layers is accommodated by layer parallel shear in the ductile layers defining a set of conjugate shear zones with increasing stretching lower crust and mantle shear zones become juxtaposed as the lower ductile mantle layer rises up through the separation zone of boudins in the brittle layer the experimental results are used to propose a general model of passive margin formation leading to mantle exhumation they are also compared to the situation observed on the west iberia margin the exhumation of lithosphere mantle is the result of a bulk pure shear at lithospheric scale shearing of exhumed mantle rocks does not correspond to detachment faults cross cutting the whole lithosphere at the onset of rifting but results from heterogeneous stretching and boudinage of the high strength sub moho mantle

how preexisting weaknesses control the style of continental breakup continental breakup is a highly variable process differences occur in the relative timing and extent of volcanism uplift and graben formation as well as in the mode and amount of continental extension before breakup we propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends continental lithosphere is viewed as a composite material composed of two strong layers one in the upper mantle and one in the middle crust finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust this leads to early melt segregation and volcanism margin uplift during the late stages of the extension process and relatively narrow symmetrical extended margins in contrast failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle this produces initial graben formation cooling in the lower crust and upper mantle and broad asymmetrical extended margins volcanism only occurs late in the process failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup

nature of the s reflector beneath the galicia banks rifted margin preliminary results from prestack depth migration beneath the tilted fault blocks of the galicia rifted margin a bright reflection termed s has been interpreted as the master detachment fault controlling the extension of the margin however on time migrated sections s appears to be locally offset by the steep faults of the upper section we investigate the true geometry of s and its relation to the overlying faults through iterative prestack depth migration we conclude that it is not significantly offset by the faults which instead appear to stop abruptly at s thus we interpret s as the master detachment and consider its role during extension

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

iapetus suture located beneath the north sea by birps deep seismic reflection profiling the iapetus suture one of the more fundamental geologic boundaries of the british isles has been located beneath the north sea by deep seismic reflection profiling carried out by the british institutions reflection profiling syndicate birps reflection profiling and magnetotelluric sounding have previously been used to infer the position of the iapetus suture beneath the irish sea and the southern uplands but the present study provides the first direct evidence for the location of the iapetus suture east of britain a comparison of the seismic reflection profiles over the iapetus suture east and west of britain shows variations in the dip of the suture which steepens to the northeast beneath the north sea the comparison also reveals unexpected changes in lower crustal reflectivity along the strike of the suture and suggests that there may be no straightforward relation in this area between the crustal reflectivity and the crustal age

ultramafic blocks from the ocean floor southwest of australia abstract samples dredged from the ocean floor near the junction of the naturaliste fracture zone and the diamantina zone 300 km southwest of cape leeuwin western australia contain 3 to 10 cm blocks of cr spinel lherzolites and al spinel olivine clinopyroxenites both assemblages show textural evidence for deformation and recrystallization with extensive development of kaersutite in one clinopyroxenite the blocks are enclosed by clay and fe mn oxide rich fragmental material which also contains a diverse detrital mineral suite and microfossils diopsides in the lherzolites are na and cr rich with marked similarities to those of lherzolite nodules in alkaline basaltic suites the al and ti rich diopsides of the clinopyroxenites are more magnesium than those of common pyroxene rich nodules the lherzolites are interpreted as upper mantle residues while the clinopyroxenites probably represent partly recrystallized cumulates from high temperature basaltic magmas traversing the lherzolite mantle both are thought to have been derived from an ultramafic body emplaced into the shallow crust near the cretaceous early tertiary continent ocean boundary off australia

deep crustal structure of the rhine graben from dekorp ecors seismic reflection data a summary abstract results of two deep reflection seismic lines crossing the northern and southern part of the rhine graben which were recorded in 1988 by the french ecors and the german dekorp teams indicate that the asymmetry of this rift persists to the base of the crust the brittle ductile transition zone approximately coincides with the top of the lower crust ductile shear zones appear to affect the crust mantle boundary upper crustal extension by faulting accounts for a divergence of the graben flanks by 5 7 km whereas the crustal configuration of the rhine graben suggest an extension value of some 17 km

kinematics of peridotite emplacement during north atlantic continental rifting galicia northwestern spain abstract the western part of the galicia passive continental margin western edge of spain is bordered by serpentinized peridotites over a n s distance of about 125 km these peridotites were emplaced at the end of continental rifting and or at the very beginning of oceanic accretion they have been observed in situ and sampled in five sites with the nautile submersible during the galinaute cruise 1986 some crustal continental rocks were also collected at two sites on the tilted blocks adjacent to the peridotite ridge most of the escarpments covered by the nautile on the peridotite ridge have yielded only ultramafic rocks except at dive 10 where the peridotites are overlain by a sheared chlonte bearing schist and at dive 14 where the ultramafic rocks are covered by basalts all the peridotites are plagioclase bearing harzburgites and iherzolites locally cross cut by rare plagioclase rich veins and dioritic intrusives some pyroxenites and gabbros are associated with the peridotites at dive 14 the plagioclase bearing peridotites are similar in composition to those drilled during odp leg 103 primary structures have not been found except in the northern part of the studied zone where a few samples exhibit magmatic textures elsewhere the peridotites show evidence of a strong mylonitization event which has overprinted a former primary high temperature fabric at these localities boudinage structures and ultramylonitic bands referred to as shear bands occur and the rocks have a disrupted mylonitic texture a few dioritic dykes have also suffered this mylonitization shearing occurred in a rotational regime at high and decreasing temperature 1000 850 c and under a high deviatoric stress 102 mpa i e under lithospheric conditions the structural pattern of the peridotite ridge deduced from measurements on oriented samples is complex the trend of the mylonitic foliation appears nearly constant from south to north n125 n105 but its dip increases 20 45 northward from south to north the stretching lineation pitch varies from subvertical to subhorizontal stretching has occurred along a shear zone acting as a normal fault in a northeasterly direction in the southern part of the studied area to the north it occurred in a northwesterly direction with a strike slip component stretching in the chlorite bearing schist and in the gneissose rocks of the continental basement seems to be unrelated to the peridotite deformation this study confirms the uplift of the galicia peridotites during the continental rifting of the north atlantic ocean in cretaceous times during their ascent the peridotites underwent some melting and high temperature low stress ductile deformation followed by intense mylonitization at decreasing temperature and increasing stress that has overprinted most of the primary textures the geometry of the mylonitic structures is related both to the ascent of the peridotite dome and to the extensive stress strain field prevailing in the continental lithosphere during the rifting episode it is consistent with that expected in the upper part of a dome locally offset by transverse faults in its northern part due to the vicinity of a triple junction point the kinematics of emplacement agree well with an e w opening of the north atlantic ocean