propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

birth of a volcanic margin off argentina south atlantic 1 based on a dense grid of multichannel reflection seismic lines we study the evolution of a volcanic rifted margin segment off argentina the segment under study is located between 44 s and 41 s we describe in detail the facies of the extrusive basaltic complexes across the margin these include single and multiple inner seaward dipping reflectors sdrs with varying architecture along the margin outer highs and outer sdr wedges a detailed interpretation of a transitional zone between the inner and outer sdr wedges is presented multiple inner sdrs are concentrated at the southern part of the margin segment while in the north only one steeply dipping wedge is present the dip of the inner sdr wedges changes along the margin and we propose subsidence by loading as main cause the transitional zone between the inner and outer sdr wedges becomes narrower toward the north by correlating with magnetic data we conclude that the emplacement of the 30 100 km wide multiple inner sdrs occurred episodically we propose an injection center which migrated in a seaward direction with proceeding extension as the origin for the multiple inner sdrs a scissor like opening of the margin segment resulted in different amounts of stretching along strike of the margin segment and is likely the cause for the varying distribution of the extrusive basaltic complexes the varying extension rates within the margin segment contribute to decreasing volumes of melts generated in a northward direction within this rift segment

three dimensional dynamical model of continental rift propagation and margin plateau formation we investigate the dynamics of rift propagation through continental lithosphere and the resulting differential extension along passive margins using a full three dimensional finite element implementation of the yield stress envelope model in its initial configuration the three dimensional model represents a 1000 km by 1000 km area of continental lithosphere an initial weak trend representing a suture zone or other preexisting weakness within the continental lithosphere extends 500 km from one model edge to the center of the model and terminates the model region beyond the weak trend is uniform model conditions are varied by changing the thickness and composition of the crust the geotherm and extension rate the strength of the initial weakness and the dimensions of the preweakened and uniform regions for each model breakup nucleates at a point on the weak trend in response to an applied constant extension rate the break propagates rapidly along the weak trend and then slows as it crosses the uniform region this produces a locked zone in which breakup is delayed resulting in an increase in the width of the model margin analogous to offsets associated with margin plateaus the total extension required to tear across the 1000 km length of the model is 210 km for 40 km thick crust 400 km for 45 km thick crust and 710 km for 50 km thick crust smaller increases in total extension occur for elevated geotherms and reduced extension rates this indicates that prior extension estimates based on subsidence which exceed 600 km for margin plateaus can be explained by rift propagation in three dimensions differential extension along the model margin is linearly related to the length of the locked zone the slope of the line which we call is a function of the rheological properties of the locked zone and reflects the extent to which continental lithosphere within the locked zone resists rift propagation model calculations for crustal thickness from 35 km to 50 km and compositions from quartz diorite to granite predict a range in from 0 2 to 1 3 we compare values computed from our model to those estimated from the subsidence of bonavista platform and rockall plateau conjugate margins in the north atlantic basin and georges bank and blake plateau and their conjugate margins in the central atlantic basin these results indicate that the differential extension associated with these locked zones which ranges from 170 km to 550 km can be predicted in terms of their initial crustal thickness and their length along the margins

styles of lithospheric extension controlled by underplated mafic bodies abstract the role of underplated mafic bodies upmb in the localization of deformation is examined using a two dimensional thermo mechanical finite element model rheological heterogeneity brought about by the upmb is linked with the following two main physical effects the upmb material is assumed to have 1 an anomalous high temperature and 2 a mafic crustal rock composition that is intrinsically weaker than the mantle the thermal effect will disappear rather quickly but the rock composition can have an effect at any stage of extension we show that the upmb has a strong influence on the style of lithospheric extension which depends on the thermal condition of the uppermost mantle at the time when the upmb is emplaced since the strength contrast between the weakened and non weakened regions is the most important factor the upmb works more efficiently on the localization of deformation for a colder uppermost mantle dependence of the localization of deformation on the temperature and thickness of the upmb i e more significant localization for the upmb with higher temperature and greater thickness also depends on the thermal condition of the uppermost mantle however the width of the upmb has a strong influence on the localization for any thermal condition of the uppermost mantle a greater amount of thinning is distributed into a narrower weakened region such a model behaviour implies that various styles of lithospheric extension including inward or outward migration of deformation and asymmetric extension can be simply obtained by considering the emplacement of the upmb which also plays an important role in controlling the onset of continental break up

detailed mineralogy and petrology of manganese oxyhydroxide deposits of the imini district morocco manganese ore in the cenomanian turonian dolostone of the imini district south of the high atlas morocco displays a high mn content due to the occurrence of pyrolusite cryptomelane hollandite sensu stricto coronadite romanechite and lithiophorite the orebodies occur mainly as three stratabound layers along the 25 km long ore belt following a wsw ene direction using scanning electron microscopy sem eds coupled with x ray diffraction xrd we here refine the mineralogy and petrology of mn oxides and oxyhydroxides in order to constrain the paragenetic sequence and define the main processes of ore formation deposition and concentration of mn oxyhydroxides follow a two step sequence after dolomitization 1 replacement of dolomite fabric dolomite micrite and dolomite rhombs by hollandite group minerals leading to a textural conservation and then 2 neoformation of collomorphous aggregates in an opened and brecciated system these observations are consistent with a multistage evolution including multiple reworking and brecciation pyrolusite is the main mn oxide observed in the imini district during both early and late stages the superficial environment provides supergene conditions suitable for the deposition of only mn oxyhydroxides this supergene environment and the occurrence of multiple bands of hollandite group minerals indicate an external migration allochthonous supply of mn and associated elements contemporaneous to an in situ chemical and mechanical dissolution of the host dolostone the lithological heterogeneities of dolostone and its chemical environment compared to less permeable surrounding rocks may have provided the conditions to concentrate mn oxides and oxyhydroxides

relative displacement between eurasia and north america prior to the formation of oceanic crust in the north atlantic abstract the relative positions of north america and eurasia prior to the opening of the north atlantic can be reconstructed using three independent data sets ocean floor magnetic anomalies and fracture zones the shape of the shelf line at the continental margins of north america and eurasia and the apparent polar wander paths apwps of these two continents together with geological constraints we here examine the internal consistency of these reconstructions discuss the tectonic implications of the inconsistencies and show how all three data sets can be combined to obtain a best fit reconstruction pole in our analysis we find that the combined errors of the new north american and european apwps are smaller for 290 240 ma than at earlier or later times therefore this interval was used rotation poles derived from magnetic anomalies from the north atlantic do not bring the apwps into coincidence a rotation pole located at 88 n 215 e with rotation angle 42 superimposes the two apwps and is reasonably consistent with the geology of the continental margins in the north atlantic the difference between the rotation pole based on apwps and that based on magnetic anomalies probably reflects relative motion of the two cratons between the early triassic and the late cretaceous cratonal germany moving about 800 km to the east relative to north america and norway moving about 500 km northeast relative to greenland this motion is expressed in extension in the north sea and the continental shelves on the other side of the globe in the vicinity of siberia and alaska our reconstruction places cratonal north america farther away from cratonal eurasia than do reconstructions based on north atlantic magnetic anomalies this greater separation is consistent with the presence in the late paleozoic and early mesozoic of a sea way in siberia located in the gap between eurasia and north america the magnitude of this gap is uncertain because of mesozoic deformation in alaska and siberia

total tectonic subsidence a parameter for distinguishing crust type at the u s atlantic continental margin total tectonic subsidence tts is the difference between the prerifting continental crust elevation and the present sediment unloaded basement depth in a sedimentary basin tts is calculated using observations of bathymetry and basement depth and assumptions about sediment density and the loading response of the lithosphere i use the extensional model for sedimentary basin formation to interpret the tts of a passive continental margin that model predicts that the tts on an old margin should define the major crust type boundaries and indicate the lateral distribution of extension within the extended continental crust tts analysis of the u s atlantic continental margin indicates that the boundary between extended continental and oceanic crusts lies along the east coast magnetic anomaly ecma to the north of the blake spur fracture zone bsfz and along the blake escarpment to the south there is no tts anomaly associated with the blake spur magnetic anomaly which seems to be underlain by normal oceanic crust the ecma appears to be associated with a tts minimum but i attribute that to artificially shallow estimates of depth to magnetic basement under the ecma tts estimates of extended continental crust thickness agree well with gravity models except where the problem with magnetic basement depth estimates interferes the amount of crust extension prior to seafloor spreading varies along the strike of the margin the most abrupt change is at the bsfz across the bsfz the amount of extension varies by 160 km but the volume of continental material does not vary the extension variation may be caused by lateral changes in the gross rheology of the continental lithosphere in the case of the bsfz it would be the suture of more ductile pre late paleozoic african craton with more brittle north american craton that could cause the abrupt change

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

paleomagnetism and the evolution of pangea we present new apparent polar wandering apw paths for the latest carboniferous to middle jurassic of europe northern asia north america and gondwana the paths have been constructed by ordering the paleopoles into a time sequence and calculating running averages globally this statistical analysis cannot be carried back before the latest carboniferous because the paleomagnetic record becomes inadequate the polar paths have all been rotated along with the land masses from which they have been observed into the classical configuration of wegener pangea a as quantified by later workers the mean paleopoles agree in the early jurassic but disagree in the latest carboniferous permian and early triassic this result is insensitive to reasonable choices made regarding the arrangement of the continents within gondwana it is true either for results from gondwana as a whole or from africa and south america alone this disagreement in apw paths suggests but does not prove that pangea a lasted for only a short interval of time in the early to middle jurassic 190 170 m y and perhaps the latest triassic 200 m y alternatively we may say either that the paleomagnetic record is biassed by sampling inadequacies or by systematically incorrect age assignments or that the earth s field has been non dipolar we regard these explanations as less likely although they cannot be discounted the polar paths can be reconciled by rotating gondwana anticlockwise by about 35 relative to laurasia placing africa beneath europe and south america beneath eastern north america this configuration is called pangea b pangea b could be a valid reconstruction for the latest carboniferous and early permian approximately 290 260 m y pangea b provides a statistically satisfactory resolution of the paleomagnetic problem it also provides a symmetrical arrangement of the continents bordering the appalachian hercynian fold belt for the late paleozoic which pangea a does not it provides a satisfactory framework for the early evolution of the gulf of mexico and it provides an explanation of the absence of geological evidence for a tethys ocean in the paleozoic the transition between b and a cannot be accurately reconstructed from the paleomagnetic data because the resolution of the record is as yet inadequate but it presumably occurred by a 3500 km minimum dextral megashear between laurasia and gondwana during the late permian and triassic approximately 250 200 m y without the formation of ocean between them and was accompanied by lesser motions between north america and europe the sense of megashear is the same as that invoked by arthaud and matte 1977 on geological grounds but the magnitude and timing are different our interpretation favors the idea that continental drift is a continuous process not confined to jurassic and later times but extending backwards into the triassic and permian we suggest that pangea was not an immobile configuration as envisaged by wegener and most subsequent workers but that it evolved more or less continuously

the structure of the rockall trough from seismic refraction and wide angle reflection measurements there are few detailed descriptions of the deep crustal structure of failed cratonic rifts and passive margins despite their import ance in constraining the variety of structural models invoked to account for their common structural symmetry or gross half graben form1 3 such deep crustal structure profiles also have considerable value in defining the continent ocean boundary con straining plate reconstructions and determining the overall setting of sedimentary basin development the rockall trough in the north atlantic ocean is a sinuous elongate bathymetric depression 200 km wide and more than 2 000 m deep lying between rockall plateau and the british isles and the structure nature and age of its underlying crust are disputed in the absence of deep crustal structure profiles the presence of oceanic crust has been assumed because the trough has to be accommodated in reconstructions of the north atlantic although stretched continental crust is equally plausible the nature of the crust is important for the precise reconstruction of the late jurassic early cretaceous plate history and setting of sedimentary basins of the north atlantic4 8 the nature of the crust beneath the trough as well as the structure thickness and composition of the sedimentary cover is also relevant to oil exploration here we report the first observations of deep crustal structure across the rockall trough and its margins from seismic refraction and wide angle reflection profiles our results show that the northern rockall trough is underlain by highly stretched continental crust and not by oceanic crust as previously postulated

new cocorp profiling in the southeastern united states part ii brunswick and east coast magnetic anomalies opening of the north central atlantic ocean new cocorp profiles on the coastal plain of georgia and northern florida support the hypothesis that the brunswick anomaly marks a late paleozoic suture they do not support the alternate view that this anomaly is caused by a mesozoic rift basin the trend of the brunswick anomaly relative to the appalachian gravity gradient indicates that in westernmost georgia and adjacent alabama african basement suwannee terrane is in proximity to autochthonous north american basement grenville farther east one or more paleozoic accreted terranes intervene between the north american and african sides of the orogen offshore the brunswick anomaly closely parallels the east coast magnetic anomaly this relationship implies that the east coast magnetic anomaly marks not only the present continental oceanic crustal transition but also the northward continuation of the late paleozoic suture between north america and africa transitional crust beneath the carolina trough baltimore canyon trough and correlative parts of the african atlantic margin is thus likely to have formed within a preexisting suture zone the dramatic change in character of the u s atlantic margin southward from the carolina trough to the blake plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

kinematic evolution of the southern north atlantic implications for the formation of hyperextended rift systems we focus on the southern north atlantic rifted margins to investigate the partitioning and propagation of deformation in hyperextended rift systems using plate kinematic modeling the kinematic evolution of this area is well determined by oceanic magnetic anomalies after the cretaceous normal polarity superchron however the rift and early seafloor spreading evolution 200 83 ma remains highly disputed due to contentious interpretations of the j magnetic anomaly on the iberia newfoundland conjugate margins recent studies highlight that the j anomaly is probably polygenic related to polyphased magmatic events and therefore does not correspond to an isochron we present a new palinspastic restoration without using the j magnetic anomaly as the chron m0 we combine 3 d gravity inversion results with local structural stratigraphic and geochronological constraints on the rift deformation history the restoration of the southern north atlantic itself is not the primary aim of the study but rather is used as a method to investigate the spatiotemporal evolution of hyperextended rift systems we include continental microblocks that enable the partitioning of the deformation between different rift segments which is of particular importance for the evolution of the iberia eurasia plate boundary our modeling highlights the following 1 the segmentation of the iberia newfoundland rift system during continental crust thinning 2 the northward v shape propagation of mantle exhumation and seafloor spreading 3 the complex partitioning of deformation along the iberia eurasia plate boundary and 4 a three plate propagation model which implies transtension

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

dynamics of continental rift propagation the end member modes abstract an important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation in regions of rift propagation specifically transition zones from continental rifting to seafloor spreading it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones the focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere more specifically this study explores how modeled rift behavior depends on the value of rheological parameters of the crust a three dimensional finite element model is used to simulate lithosphere deformation in an extensional regime the chosen approach emphasizes understanding the tectonic forces involved in rift propagation dependent on plate strength two end member modes are distinguished the stalled rift phase is characterized by absence of rift propagation for a certain amount of time extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone which requires a buildup of shear stresses near the rift tip and significant intra plate deformation this stage represents a situation in which a rift meets a locked zone localized deformation changes to distributed deformation in the locked zone and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend in the alternative rift propagation mode rift propagation is a continuous process when the initial crust is weak the extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses model predictions address aspects of previously unexplained rift evolution in the laptev sea and its contrast with the tectonic evolution of for example the gulf of aden and woodlark basin

wide and narrow margins of the brazilian south atlantic two contrasting end members of passive continental margins here referred to as wide and narrow margins are present in the south atlantic although some margin segments are intermediate between these two styles narrow margins are characterized by a large bounding fault 4 km throw near the shelf edge a sharp transition zone 10 20 km wide from normal thickness to substantially thinned continental crust or oceanic crust where the total width of extended continental crust is usually less than 100 km wide continental margins are characterized by a broad continental shelf and a wide zone 50 600 km of thinned crust with an even distribution of faults wide margins usually have a more gentle continental shelf and slope narrow narrow wide wide and wide narrow conjugate margin pairs exist in the south atlantic and are suggested to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting basement structure and strain rate appear to have exerted little control on the style of margin extension although basement architecture appears to control the trend of the initial rifts the positions of fracture zones which separate narrow and wide segments and the rate of lateral rift propagation

implications of continental crust extension for plate reconstruction an example from the gulf of mexico the prerift reconstruction of continental plates bounded by rifted margins requires the closure of relative motion accomplished both by seafloor spreading and by the extension of continental crust during the rift phase of continental breakup continental extension is not expected to be pervasive throughout the plate but to be confined to a zone of up to several hundred kilometers in width the direction of particle motion in this zone is expected to be parallel to flow lines followed by the rigid portions of the plates the best fit between rifted continental margins is then described by the rotation angle about an euler pole which best closes both the oceanic crust and the extension within the continental crust along small circles about the euler pole as an example application of these concepts the pattern of late mesozoic crust extension within the gulf of mexico basin is used to constrain the location of the euler pole and angle of rotation of the yucatan block with respect to north america both crust type and the degree of extension within the transitional crust surrounding the basin are estimated on a point by point basis from bathymetry and basement depth the root mean square rms misfit in the apparent total closure of both oceanic crust and extension within the continental crust is computed for a range of possible poles of opening a contour map of misfit versus pole location reveals a global minimum rms misfit of 47 km for a 45 counterclockwise opening of the gulf of mexico basin about an euler pole at 25 n 79 w an elongate trend of poles for which the misfit is less than 100 km extends from the bahama islands eastward across the atlantic and through north africa the reconstruction of the gulf of mexico for the best fit pole places the palinspastically restored yucatan block in contact with north america along the texas and louisiana shelf the resulting restored southern margin of north america fits against the the northern margin of south america in its prerift position to within the uncertainty in the north america africa south america plate circuit we conclude that the prerift placement of the yucatan block between north and south america along the texas louisiana shelf is consistent with both the pattern of extension within the gulf of mexico basin and seafloor spreading history of the central atlantic basin

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

komatiites and picrites evidence that the plume source is depleted high temperature magmas such as picrites and komatiites are candidates for the material in the hot axial region of mantle plumes in current plume theories these magmas should have an enriched ocean island basalt oib type chemistry high mgo magmas however are often more like depleted mid ocean ridge basalts morb in trace element and isotopic chemistry this is a paradox for deep mantle plume or primitive mantle theories but is expected if hotspots and morb share a common depleted source and the transition to oib chemistry involves shallow level cooling crystal fractionation and contamination ancient komatiites appear to have come from the same deep depleted reservoir as modern picrites and morb

continental breakup and the onset of ultraslow seafloor spreading off flemish cap on the newfoundland rifted margin prestack depth migrated seismic reflection data collected off flemish cap on the newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system within continental crust there is no clear evidence for detachment surfaces analogous to the s reflection off the conjugate galicia bank margin demonstrating a first order asymmetry in final rift development anomalously thin 3 4 km magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized this indicates that initial accretion of the oceanic crust was in a magma limited setting similar to present day ultraslow spreading environments seaward oceanic crust thins to 1 3 km and exhibits an unusual highly reflective layering we propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep marine sheet flows to form this layering subsequent seafloor spreading formed normal 6 km thick oceanic crust this interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow spreading system was established

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

deep seismic reflection characteristics of the continental crust four types of seismic reflection fabrics appear to characterize the continental crust 1 a cratonic fabric dominated by diffractions and dipping reflections but lacking a pronounced reflection moho 2 regions of thin skinned foreland deformation with abundant shallow but sparse deep reflections 3 a unidirectional fabric that dips toward the hinterland and may represent a crustal scale ramp zone and 4 zones of highly layered lower crust and high amplitude reflection moho the first three types are commonly observed in transects across single mountain belts but the fourth appears to correspond to regions of elevated lower crustal temperatures such as those encountered in rift provinces and perhaps elsewhere

patterns of continental extension along the conjugate margins of the central and north atlantic oceans and labrador sea the anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading we estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and north atlantic oceans and labrador sea from the total tectonic subsidence of the margins in the central atlantic basin the integrated continental extension varies from 225 km for the break between the southern appalachian segment of the north american margin and africa to over 600 km for the break between the blake plateau region of north america and the cape verde terrace region of africa the original width of the extended zone varies from 240 to 770 km in the north atlantic and labrador basins the integrated extension varies from 60 km for the break between northern labrador and greenland to over 600 km for the break between newfoundland and europe the original width of the extended zones in these basins varies from 100 km to over 1460 km in general continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain as a result seafloor spreading tends to begin first along segments which follow the structural grain extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain failure parallel to the structural grain exhibits greater variability from margin to margin than failure across the grain this phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain we conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain

the black lake shear zone a boundary between terranes in the adirondack lowlands grenville province abstract the black lake shear zone has been proposed as a potentially important tectonic boundary between the frontenac and adirondack lowlands terranes based on geologic and isotopic evidence we present new structural geochronologic and anisotropy of magnetic susceptibility ams results from the black lake area that constrain the location kinematics and timing of deformation in the region both structural and ams results document a northeast trending zone of high strain between black lake and the saint lawrence river within this zone strain is heterogeneous and includes a strongly developed sub vertical northeast trending foliation less commonly two orthogonal lineation sets are present one that is sub horizontal and another that is down dip the foliation and granitic dikes are locally folded forming tight to isoclinal northeast trending folds kinematic indicators show thrust deformation in areas of down dip lineation but are mixed in areas of sub horizontal lineation with a slight majority suggesting left lateral shear these results suggest major northwest directed shortening accommodated by flattening and ductile thrust faults possibly with a component of sub horizontal shear the two lineations may have formed during discrete episodes or synchronously during transpression u pb shrimp geochronology on igneous titanite from deformed dikes in combination with previous geochronology show that significant deformation occurred from ca 1170 1100 ma this deformation likely occurred during the late stages of the shawinigan orogeny and may record accretion of the adirondacks to the margin of laurentia during terrane assembly