the initiation of subduction by crustal extension at a continental margin summary we investigate how subduction may be triggered by continental crust extension at a continental margin the large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement subduction requires the oceanic plate to get submerged in mantle so that negative buoyancy forces may take over and drive further descent this is promoted by two mechanisms loading by continental crust bends the oceanic plate downwards extension in the continental domain induces crustal thinning which acts to raise mantle above the oceanic plate in this model the width of the continental region undergoing extension is an important control parameter the main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading three governing dimensionless parameters are identified one involves the poorly constrained oceanic plate buoyancy we find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant enabling metamorphic reactions and density increase in the oceanic crust another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong the third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness such that a weak plate and a thick crust do not favour subduction thus the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere it is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed

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

vertical averages of rheology of the continental lithosphere relation to thin sheet parameters abstract we calculate vertically averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional compressional and strike slip regimes we assume that brittle deformation occurs by byerlee s law and that ductile deformation occurs by steady state creep mechanisms under these conditions the vertically averaged rheology may be approximated by a single power law over the range of strain rates from 10 13 to 10 17 s 1 this is true not only when either brittle or ductile behavior dominates the strength of the lithospere but also when the contributions of brittle and ductile behavior are comparable the value of the power law exponent n and the strength of the lithosphere depend on the depth and stress difference at the brittle ductile transition the temperature at the moho and the geothermal gradient in the range of vertically integrated stress between 1012 and 1013 n m 1n reaches a minimum of between 5 and 10 under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior the strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere

temperature and rheological properties of the mantle beneath the north american craton from an analysis of heat flux and seismic data 1 we combine heat flux data and seismic velocity models for the north american lithosphere to derive constraints on thermal conditions and deformation mechanisms in the underlying convecting mantle local heat flux averages that are not affected by shallow crustal heat production contrasts allow calculation of reliable lithospheric geotherms and uncertainty ranges for consistency with the seismic data the mantle potential temperature beneath north america must lie within a 1290 c 1450 c range close to that for the oceanic mantle sampled at mid ocean ridges the heat flux at the base of the lithosphere varies laterally from 11 3 mw m 2 beneath the 250 km thick archean core of the superior province to 15 3 mw m 2 beneath the thinner younger appalachians province it is shown that the most likely cause of such rates of heat supply into the north american continent is small scale convection in an unstable boundary layer beneath the rigid mechanical lithosphere this allows useful constraints on the mantle rheological properties we show that the most likely deformation mechanism is dislocation creep in wet mantle rocks ranges for the mantle temperature water content and rheological parameters could be tightened very significantly once strong constraints are obtained on radiogenic heat production in the lithospheric mantle

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

on the initiation of subduction estimates of shear resistance associated with lithospheric thrusting and convergence represent lower bounds on the force necessary to promote trench formation three environments proposed as preferential sites of incipient subduction are investigated passive continental margins transform faults fracture zones and extinct ridges none of these are predicted to convert into subduction zones simply by the accumulation of local gravitational stresses subduction cannot initiate through the foundering of dense oceanic lithosphere immediately adjacent to passive continental margins the attempted subduction of buoyant material at a mature trench can result in large compressional forces in both subducting and overriding plates this is the only tectonic force sufficient to trigger the nucleation of a new subduction zone the ubiquitous distribution of transform faults and fracture zones combined with the common proximity of these features to mature subduction complexes suggests that they may represent the most likely sites of trench formation if they are even marginally weaker than normal oceanic lithosphere

evolution of middle america and the in situ caribbean plate model james k h 2009 evolution of middle america and the in situ caribbean plate model origin and evolution of the caribbean plate 328 127 138 james k h lorente m a pindell j l eds conference on geology of the area between north and south america with focus on the origin of the caribbean plate siguenza spain may 29 jun 02 2006

cenozoic postrift domal uplift of north atlantic margins an asthenospheric diapirism model postrift domal uplifts are a distinct feature of the northern north atlantic rifted margins uplifted regions southern norway finnmark svalbard northern united kingdom occur with a typical spacing of 900 km the amount of domal uplift is substantial 1 2 km thermochronological geomorphological and seismic stratigraphic data suggest a coincident neogene onset of uplift postdating north atlantic breakup and predating onset of glaciation the norwegian domes are characterized by strongly negative gravity anomalies and reduced mantle p wave and s wave velocities previous models for uplift around the north atlantic e g glacial erosion intraplate stresses secondary convection fail to explain the above characteristics we present an asthenospheric diapir model based on a rayleigh taylor instability wherein a hot icelandic asthenosphere layer meets cold cratonic lithosphere the generated asthenospheric diapirs and associated partial melting offer a viable explanation for updoming application of this model to the south norwegian dome succesfully explains most of the currently available data

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

cenozoic evolution of the eastern black sea a test of depth dependent stretching models abstract subsidence analysis of the eastern black sea basin suggests that the stratigraphy of this deep extensional basin can be explained by a predominantly pure shear stretching history a strain rate inversion method that assumes pure shear extension obtains good fits between observed and predicted stratigraphy a relatively pure shear strain distribution is also obtained when a strain rate inversion algorithm is applied that allows extension to vary with depth without assuming its existence or form the timing of opening of the eastern black sea which occupied a back arc position during the closure of the tethys ocean has also been a subject of intense debate competing theories called for basin opening during the jurassic cretaceous or paleocene eocene our work suggests that extension likely continued into the early cenozoic in agreement with stratigraphic relationships onshore and with estimates for the timing of arc magmatism further basin deepening also appears to have occurred in the last 20 myr this anomalous subsidence event is focused in the northern part of the basin and reaches its peak at 15 10 ma we suggest that this comparatively localized shortening is associated with the northward movement of the arabian plate we also explore the effects of paleowater depth and elastic thickness on the results these parameters are controversial particularly for deep water basins and margins but their estimation is a necessary step in any analysis of the tectonic subsidence record stored in stratigraphy

crustal structure of the arabian plate new constraints from the analysis of teleseismic receiver functions an edited version of this paper was published by elsevier science copyright 2005 elsevier science see also http dx doi org 10 1016 j epsl 2004 12 020 http atlas geo cornell edu saudiarabia publications al damegh 202005 htm

the transition from the continent to the ocean a deeper view on the norwegian margin we present a regional crustal scale 3d structural model of the norwegian continental margin integrating sedimentary and crustal layers from the continental and the oceanic domain the model includes six sedimentary units underlain on the continental side by a thinned crystalline crust and a lower crustal high velocity body in the oceanic domain three crustal layers 2ab 3a and 3b thickened at the continent ocean transition cot are modelled below the post breakup deposits two major rift phases with different rift axes late jurassic early cretaceous and late cretaceous early tertiary have caused post jurassic subsidence and post depositional deformation of the pre cretaceous units the modelled cot suggests that the pre breakup rifting event was related to differential stretching focused at the outer margin and that breakup took place in a base up magmatic process as a continuation of underplating for the earlier rift event stretching was distributed over the entire margin and led to accumulation of up to 12 km of cretaceous deposits the large sediment thickness of the cretaceous units requires deep water conditions and abundant sediment supply and thus coeval offshore subsidence and onshore uplift all layers indicate a sinistral offset along the jan mayen fracture zone and its continentward continuation

lower crustal flow in an extensional setting constraints from the halloran hills region eastern mojave desert california postextensional uplift and tilting of quaternary lake sediments in the halloran hills eastern mojave desert california suggest that regional scale flow within the lower crust is occurring in response to tectonic loads created during upper crustal extension in late miocene time the concept that significant flow within the lower crust may occur for millions of years after the cessation of extension indicates that topographic gradients can be modified by regional scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting an analytic solution developed in this paper combines viscous flow in the lower crust flexural bending within the midcrust and isostasy into an invertible linear differential equation that describes the topographic response to upper crustal extension assuming that there was no topographic relief across the halloran hills region prior to extension inversion of modern topographic data indicates that crustal thinning above the halloran hills detachment increases westward with a listric geometry and that little crustal thinning has occurred east of the western flank of clark mountain in good agreement with the known geometry of the fault and the location of its breakaway zone incorporation of quaternary tilting data indicates that the modern viscosity beneath the halloran hills assuming a 10 km lower crustal channel is a maximum of 1019 pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 ma this corresponds to a temperature increase of at least 75 to 100 c at the moho in our opinion the most likely source of this temperature increase is a regional scale thermal event within the underlying mantle and diffusion of heat upward into the lower crust if correct these results have important implications for the way in which crustal extension is linked to mantle heating within the basin and range province

the denudation history of the onshore continental margin of se brazil inferred from apatite fission track data a suite of 68 new apatite fission track analysis results from the onshore rift margin in southeast brazil are presented some of the results reflect resetting related to the eruption of the parana continental flood basalt 130 ma and four others are directly related to late cretaceous magmatism 80 ma the majority of the results were obtained from precambrian basement rocks and sediments from the parana basin the fission track ages broadly increase inland from 60 90 ma on the coastal plain to 300 ma in the continental hinterland the track length distributions show a characteristic trend being negatively skewed at the coast and showing increasing bimodality inland except for the oldest samples which are more unimodal these trends do not reflect rift related heating and resetting but are the result of protracted denudation since the opening of the south atlantic the results are consistent with more than 3 km of exhumation on the coastal plain but little more than 1 km in the hinterland the topographic morphology in the northern region of the study area is complex and is clearly influenced by structure and bedrock lithology for the region further south model calculations of the isostatic response to the denudation suggest that isostatic rebound makes a minor contribution to the long wavelength topography unless the flexural rigidity is more than 1025 n m in this case the initial elevation of the rift margin would need to be 2 5 km in order to accommodate the amount of denudation inferred from the fission track data however it is possible that the effective flexural rigidity varies across the margin becoming greater in the continental interior alternatively the broad scale wavelength topography may be the result of magmatic underplating related to the parana continental flood basalt event if so estimates of the volume of magma generated would need to be revised upward by as much as a factor of 5

late cenozoic uplift and volcanism on spitsbergen caused by mantle convection regional tectonic uplift heat flow measurements and p t estimates on upper mantle xenoliths associated with neogene volcanic rocks indicate a strongly thinned 50 km thick lithosphere beneath svalbard 650 km north of scandinavia data on neogene volcanic rocks and xenoliths indicate an 50 c temperature anomaly in the underlying asthenosphere together with volatile rich conditions in the upper mantle the svalbard uplift is one of several cenozoic epeirogenic uplifts along the margins of the north atlantic boundary processes occurring where hot spot influenced asthenosphere abuts cold deep continental lithosphere seem to be likely explanations for the uplifts

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

a tensile flexural model for the initiation of subduction we argue that subduction may be initiated at passive continental margins without shortening the lithosphere overcoming the lithosphere s high compressive strength requires special circumstances and these make it difficult to explain the nearly complete recycling of old sea floor instead we present a model that predicts tensile decoupling of the continental and oceanic lithosphere passive rifting and foundering of the seafloor beneath material welling up in the rift this occurs because the lithosphere in a new ocean basin establishes mechanical continuity with the continent at a depth comparable to mid ocean ridges later subsidence at the margin is therefore inhibited by flexure which implies shear stresses that promote fault slip and tensile stresses necessary to balance the component of the plate s weight directed down the margin slope we show that this tension can more than offset ridge push in our model an important additional tension arises from basal shear tractions resisting the plate s motion away from the mid ocean ridge although these tractions cannot be evaluated with confidence slip on a high angle fault decouples the oceanic and continental lithosphere when shear stresses arising from flexure and the applied tension exceed the lithosphere s shear strength under these loads a passive rift then forms allowing a mantle column to rise to the height of mid ocean ridges over 3 km above the old seafloor and flow onto the surface in a gravity current this load flexes the plate downwards which enhances the flow and lets the old oceanic lithosphere founder this model is consistent with the presence of oceanic material in continental forearcs and the youth of ophiolites when they are obducted as both might be explained by rifting a margin and underthrusting the juvenile crust formed there boninites in ophiolite complexes and tectonites at their bases show depleted and hydrated geochemistries consistent with the melting of rifted mantle lithosphere that receives volatiles from foundering oceanic crust

ichnology in oceanic islands case studies from the cape verde archipelago abstract miocene and pleistocene marine deposits in the cape verde archipelago are represented by a series of thin transgressive sandy bioclastic limestones that typically occur between subaerial and or submarine volcanic flows this position within volcanic flows makes them an ideal paleobathymetric case study because the submarine subaerial passage zone above them provides a reliable and independent indicator of paleodepth in terms of paleoichnology the older miocene sediments are characterized by the thalassinoides ichnoassociation whereas pleistocene sediments are characterized by the macaronichnus dactyloidites ichnoassociation the fair weather suite of the proximal cruziana ichnosubfacies is represented by the ichnotaxa dactyloidites bichordites cardioichnus phycodes teichichnus aff taenidium palaeophycus and thalassinoides in addition to the thalassinoides ichnoassociation the macaronichnus dactyloidites ichnoassociation is more complex including representatives of both the proximal cruziana and skolithos ichnofacies only representatives of the skolithos ichnofacies skolithos macaronichnus conichnus and ophiomorpha are represented in the post storm suite so far bioerosive structures were found on the miocene pleistocene unconformity surface of maio island and on basaltic paleocliffs at ponta das bicudas on santiago island they are represented by the ichnoassemblage gastrochaenolites torpedo entobia isp corresponding to the entobia ichnofacies bathymetric calibration deduced from the position of the following passage zone between submarine and sub aerial lava flow allows for confirmation of the proximal cruziana ichnosubfacies and skolithos ichnofacies at a paleodepth between 12 and 18 m depth

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

the erosional and uplift history of ne atlantic passive margins constraints on a passing plume new apatite fission track analyses from nw britain indicate that a maximum of 2 5 km of erosion has occurred there during the cenozoic similar to values for se greenland and the east greenland coast north of scoresby sund the erosion may have been facilitated by magmatic underplating during break up however at kangerdlugssuaq east greenland 4 6 km of erosion is measured since 45 ma lower mid eocene marine sedimentary rocks overlying the lavas on the blosseville coast indicate that magmatic underplating on the central greenland coast substantially post dated flood volcanism and break up behaviour not predicted by simple plume rift models subsidence reconstructions of the hebrides shelf and the east and west greenland coasts show that rapid dynamic uplift was effectively synchronous at 63 ma and preceded volcanism by c 400 m is compatible with a mantle temperature anomaly of c 100 c these data suggest very rapid lateral flow of the impacting iceland plume head the predicted crossing of the plume by the east greenland coast in the mid late eocene would account for post rift magmatic underplating and dynamic support on the greenland but not the european side of the north atlantic basin

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

microplate versus continuum descriptions of active tectonic deformation whether deformation of continents is more accurately described by the motions of a few small rigid plates or by quasi continuous flow has important implications for lithospheric dynamics fault mechanics and earthquake hazard assessment actively deforming regions of the western united states central asia japan and new zealand show features that argue for both styles of movement but new observations are necessary to determine which is most appropriate and at what scale the description applies geologic geodetic seismic and paleomagnetic measurements tend to sample complementary aspects of the deformation field so an integrated observation program can utilize the strengths of each method and overcome their separate spatial or temporal biases provided the total relative motion across each region is known and the distribution of active faults is well mapped determination of fault slip rates can provide potentially decisive constraints reconnaissance geological studies supply useful slip rate estimates but precise values depend upon detailed intensive investigation of individual sites geodetic survey measurements can determine the spatial pattern of contemporary movements and extract slip rate information but the sometimes elusive effects of cyclic elastic strain buildup and relief must be accounted for in relating current movements to the long term deformation pattern earthquake catalogs can be applied to determine seismic strain rates and relative velocities but must be averaged over large regions and are usually limited by the inadequate duration of historical or instrumental seismicity catalogs paleomagnetic determinations of vertical axis rotations provide estimates of block rotation rates but are often locally variable and averaged over many millions of years which of the two descriptions of continental tectonics is more nearly correct depends on the local rheological stratification of the lithosphere especially the strength and thickness of the elastic crust relative to the ductile lithosphere and dynamical models can provide contrasting forecasts of observable features with testable consequences

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

block versus continuum deformation in the western united states abstract the relative role of block versus continuum deformation of continental lithosphere is a current subject of debate continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies in contrast block models are favored by geologic studies of displacement in places like asia a problem in this debate is a lack of data from which unequivocal conclusions may be reached in this paper we apply the techniques of study used in regions such as the alpine himalayan belt to an area with a wealth of instrumental data the western united states by comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60 of the pacific north america motion across the plate boundary in california and nevada has occurred seismically and 40 aseismically the san francisco bay area shows similar partitioning between seismic and aseismic deformation and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth in some cases this deformation can be located on creeping surface faults but elsewhere it is spread over a several kilometer wide zone adjacent to the fault these superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere our results support the dominant role of non continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle our conclusions apply only to the regions where the data are good and even within the western united states i e the basin and range deformation styles remain poorly resolved nonetheless we maintain that block motion is the deformation style of choice for those continental regions where the data are best

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

rheological stratification of the lithosphere a direct inference based upon the geodetically observed pattern of the glacial isostatic adjustment of the north american continent 1 a sequence of new analyses of the process of glacial isostatic adjustment gia is described the focus is upon the resolution of a recognized flaw in the currently prevalent model of this process that denoted ice 5g vm2 the flaw concerns a previously noted inability of the model to simultaneously reconcile the vlbi and gps measured rates of vertical and horizontal motion in the region of the north american continent that lay outboard and to the south of the laurentide ice sheet lis at last glacial maximum this characteristic misfit of the model to the data has been suggested to be reconcilable only by recourse to models that explicitly incorporate the influence of lateral viscosity heterogeneity it is demonstrated herein that on the contrary this flaw is entirely and unambiguously attributable to the rheological stratification of the lithosphere an influence not previously accounted for in global models of the gia process but which must exist on a priori grounds

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis