continental rifting as a function of lithosphere mantle strength abstract the role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo mechanical finite element code in the lithosphere the mantle crust strength ratio s m s c that decreases with increasing moho temperature t m allows two strength regimes to be defined mantle dominated s m s c and crust dominated s m s c the transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for t m 700 c 2d numerical simulations for different values of s m s c show how the uppermost mantle strength controls the style of continental rifting a high strength mantle leads to strain localisation at lithosphere scale with two main patterns of narrow rifting coupled crust mantle at the lowest t m values and deep crustal decollement for increasing t m values typical of some continental rifts and non volcanic passive margins the absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust these numerical results are compared and discussed in relation with series of classical rift examples

pinch and swell structure and shear zones in viscoplastic layers abstract two dimensional finite element simulations are presented for the extension of stiff viscoplastic layers embedded in a weaker viscous matrix layers and matrix exhibit power law flow laws and the layers exhibit additionally a von mises yield stress the power law flow law applies to rock deformation in the diffusion and dislocation creep regime and the von mises plasticity to the low temperature plasticity regime e g dislocation glide simulations show that pinch and swell structure forms for small viscosity ratio i e 10 20 and typical power law stress exponents i e 1 5 the pinches in layers with initial random geometrical perturbation form consecutively i e not simultaneous in multilayers pinches on both the single layer and the multilayer scale develop furthermore shear zones develop due to the oblique linkage of pinches across the multilayer these shear zones have a stable position cause a normal drag geometry and exhibit significant displacement the numerical results and the importance of low temperature plasticity are supported by field observations microstructural observations and ebsd orientation maps for pinch and swell structure in calcite veins the presented models can explain strain localization by shear zone formation during bulk pure shear extension of viscoplastic multilayers without any material softening or feed back mechanism e g shear heating

half graben versus large offset low angle normal fault importance of keeping cool during normal faulting 1 we study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere two distinct fault patterns are seen depending on whether the brittle layer thickness hl is greater or less than a critical value hc for a thin layer hl hc a rift develops with multiple faults bounding an evolving set of grabens numerical results show that the value of hc depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension in the region where the displacement occurs along the fault the brittle lithosphere thins and we show that this promotes development of multiple faults in models with heat transfer only by rock motion and conduction the advective thinning effect can greatly reduce hc with rift velocities greater than 1 cm yr 1 hc is reduced by a factor of 5 to unreasonably small values however inclusion of simulated hydrothermal cooling increases hc this allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick extension of layers thicker than 10 km tends to produce a rift with multiple graben structures finally we compare the result of a simplified analytic model of extension to data we find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

the role of frictional strength on plate coupling at the subduction interface 1 at a subduction zone the amount of friction between the incoming plate and the forearc is an important factor in controlling the dip angle of subduction and the structure of the forearc in this paper we investigate the role of the frictional strength of sediments and of the serpentinized peridotite on the evolution of convergent margins in numerical models we vary thickness of a serpentinized layer in the mantle wedge 15 to 25 km and the frictional strength of both the sediments and serpentinized mantle friction angle 1 to 15 or static friction coefficient 0 017 to 0 27 to control the amount of frictional coupling between the plates with plastic strain weakening in the lithosphere our numerical models can attain stable subduction geometry over millions of years we find that the frictional strength of the sediments and serpentinized peridotite exerts the largest control on the dip angle of the subduction interface at seismogenic depths in the case of low sediment and serpentinite friction the subduction interface has a shallow dip while the subduction zone develops an accretionary prism a broad forearc high a deep forearc basin and a shallow trench in the high friction case the subduction interface is steep the trench is deeper and the accretionary prism forearc high and basin are all absent the resultant free air gravity and topographic signature of these subduction zone models are consistent with observations we believe that the low friction model produces a geometry and forearc structure similar to that of accretionary margins conversely models with high friction angles in sediments and serpentinite develop characteristics of an erosional convergent margin we find that the strength of the subduction interface is critical in controlling the amount of coupling at the seismogenic zone and perhaps ultimately the size of the largest earthquakes at subduction zones

the effect of material properties on growth rates of folding and boudinage experiments with wax models abstract the growth of unstable structures was studied experimentally in layered wax models the rheological properties of the two wax types were determined independently by a series of cylinder compression tests both waxes enhibited 1 a non newtonian stress vs strain rate relationship 2 strain softening and 3 temperature dependent viscosity the stress strain rate relationships approximated a power law with stress exponents of 5 for the microcrystalline wax and 1 8 for paraffin wax blocks of paraffin with a single embedded layer of microcrystalline wax were deformed in two dimensional pure shear with the layer oriented either parallel to the compressive strain axis so that it shortened and folded or perpendicular to that axis so that it would stretch and boundinage would form the growth rates of tiny initial disturbances were measured the growth rates for folding and boudinage were much higher than could be accounted for by theories assuming newtonian material properties theories taking non newtonian behaviour into account smith r b 1975 bull geol soc am 86 1601 1609 fletcher r c 1974 am j sci 274 1029 1043 better describe the folding growth rates boudinage however grew almost three times faster than would be predicted even by existing non newtonian theory a possible reason for this discrepancy is that the waxes do not exhibit steady state creep as assumed in the theory we therefore extend the theory to include strain softening the crucial step in this theory is the use of a scalar measure of the deformation as a state variable in the constitutive law in this way the isotropic manifestation of strain softening can be taken into account the analysis shows that strain softening can lead to greatly increased boudinage growth rates while having little influence on the growth rates of folds which is in agreement with the experiments

from ductile to brittle evolution and localization of deformation below a crustal detachment tinos cyclades greece 1 the cycladic oligo miocene detachment of tinos island is an example of a flat lying extensional shear zone evolving into a low angle brittle detachment a clear continuum of extensional strain from ductile to brittle regime is observed in the footwall the main brittle structures marking extension are shallow and steeply dipping normal faults associated with subvertical extensional joints and veins the earliest brittle structures are low angle normal faults which commonly superimpose on and reactivate earlier precursory ductile shear bands but newly formed low angle normal faults could also be observed low angle normal faults are cut by late steeply dipping normal faults the inversion of fault slip data collected within and away from the main detachment zone shows that the direction of the minimum stress axis is strictly parallel to the ne sw stretching lineation and that the maximum principal stress axis remained subvertical during the whole brittle evolution in agreement with the subvertical attitude of veins throughout the island the high angle of 1 to the main detachment suggests that the detachment was weak this observation together with the presence of a thick layer of cataclasites below the main detachment and the kinematic continuum from ductile to brittle leads us to propose a kinematic model for the formation of the detachment boudinage at the crustal scale induces formation near the brittle ductile transition of ductile shear zones near the edges of boudins shear zones are progressively exhumed and replaced by shallow dipping cataclastic shear zones when they reached the brittle field most of the displacement is achieved through cataclastic flow in the upper crust and only the last increment of strain gives rise to the formation of brittle faults the formation of the low angle brittle detachment is thus prepared by the ductile shear zone and the cataclasites and favored by the circulation of surface derived fluids in the shear zone

a back of the envelope approach to boudinage mechanics abstract smith has shown from linear hydrodynamic stability that pinch and swell boudinage is possible only in a non newtonian material by assuming uniaxial stress and strain and large viscosity contrast between the competent layer and the incompetent matrix smith s result can be derived in a very simple way the one dimensional force balance also predicts growth rates although not dominant wavelengths however we question the ability of even linear stability analysis to accurately predict boudin widths

structure of intraseasonal kelvin waves in the equatorial pacific ocean abstract previous studies have shown that intraseasonal kelvin waves are a prominent mode of variability in the eastern and central equatorial pacific these waves appear to be remotely forced by wind variations in the western pacific and propagate eastward at approximately first baroclinic mode phase speeds in this study moored temperature and velocity time series data from 1984 to 1987 between 110 and 140 w are studied to isolate the kelvin wave structures and to document their wave mean flow interactions with the south equatorial current equatorial undercurrent ruc system observed structures at periods of 59 125 days have rms amplitudes of about 10 cm s 1 for zonal velocity and about 1 c for temperature in the thermocline these structures exhibit significant departures from linear no mean flow theory zonal velocity is intensified below the euc core and temperature variability in the thermocline is weaker on the equator than 1 5 off the equator additionally the wave temperature signal at the

effect of plastic viscous layering and strain softening on mode selection during lithospheric extension 1 factors controlling the selection of deformation modes during continental extension are investigated using analytical and numerical methods we view the lithosphere as a laminate and examine a simple system with a uniform plastic layer overlying a uniform linear viscous layer the rate of energy dissipation is analyzed for pure shear ps symmetric plug sp and asymmetric plug ap extension modes and the analysis reveals that the primary control is the relative rate of dissipation in the two layers a basic difference is that the plastic layer yield strength is independent of the strain rate whereas the viscous stress depends on strain rate therefore dissipation scales linearly and quadratically with extension velocity for these respective layers when other parameters e g extension velocity and properties are held constant minimum dissipation predicts that the modes ap sp and ps will be selected in this order with increasing viscosity of the lower layer transition viscosities between modes t1 and t2 are 4 1021 pa s and 8 1022 pa s respectively for our parameters values numerical models confirm the analysis results inferred mode controls and order of mode selection when strain softening of the plastic layer occurs during extension implications for lithosphere that acts as a bonded plastic viscous laminate include the following 1 asymmetric extension ap mode is preferred when the extension rate and or effective viscosity is low or the viscous region temperature is high 2 symmetric sp mode extension is preferred for intermediate combinations of parameters and 3 overall pure shear ps mode may occur for opposite end member parameter combinations

the rheology of polymineralic rocks an approach deduced from empirical and theoretical work a model is proposed for the rheology of bi and polyphase rocks based on the rheology of the components their volume fractions and their geometrical distribution the model is formulated for the common case where some minerals deform by crystall plasticity at steady state and others stronger ones deform in the vincinity of the brittle ductile transition

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

networking of shear zones at the brittle to viscous transition cap de creus ne spain a crustal scale shear zone network at the fossil brittle to viscous transition exposed at cap de creus ne spain evolved by coeval fracturing and viscous mylonitic overprinting of an existing foliation initial fracturing led to mylonitic shearing as rock softened in ductilely deformed zones surrounding the fractures mylonitic shear zones widened by lateral branching of fractures from these shear zones and by synthetic rotation of the existing foliation between the fractures and shear zones shear zones lengthened by a combination of fracturing and mylonitic shearing in front of the shear zone tips shear zones interconnected along and across their shearing planes separating rhomb shaped lozenges of less deformed rock lozenges were subsequently incorporated into the mylonitic shear zones by widening in the manner described above in this way deformation became homogeneous on the scale of initial fracturing metre to decametre scale in contrast the shear zone network represents localisation of strain on a decametre length scale the strength of the continental crust at the time of coeval fracturing and viscous shearing is inferred to have decreased with time and strain as fracturing evolved to mylonitic shearing and as the shear zones coalesced to form a through going network subparallel to the shearing plane crustal strength must therefore be considered as strain and scale dependent

evolution of pinch and swell structures in a power law layer analytical and finite element fem solutions for the necking of a single power law layer up to large finite amplitude are obtained continuous necking of the layer produces pinch and swell structures the layer is either a free plate or embedded in a homogeneous medium an analytical solution for finite amplitude necking based on the assumption that plane sections remain plane psrp agrees well with the fem result for a layer power law stress exponent n 5 and for a ratio of layer to medium effective viscosities m 100 fem simulations for embedded layers verify that psrp for m 20 the presented numerical experiments generate localized necking and pinch and swell structures similar to natural ones for n 5 and m 20 additional weakening mechanisms such as strain softening although likely to be operative in nature are not required to generate natural pinch and swell structures fem experiments with random perturbation of the layer interfaces show that even with strong necking instability the layer is thinned at the swell as well as at the necks affecting strain estimation from pinch and swell geometry 2008 elsevier ltd all rights reserved

lithospheric boudinage in the western mediterranean back arc basin the western mediterranean is composed of irregular troughs formed as back arc basins in the hanging wall of the w directed apenninic subduction which retreated eastward during neogene and quaternary times the basins are progressively younger toward the east ageing from late oligocene early miocene valencia provencal alboran and algerian basins to middle miocene pleistocene tyrrhenian sea the basins isolated boudins of continental lithosphere the sardinia corsica block representing the largest the boudinage has a wavelength of 100 400 km and facilitated stretching of the continental lithosphere with formation of new oceanic crust in the provencal algerian vavilov and marsili basins the boudins developed both in the earlier alpine betic orogen alboran basin and in its foreland provencal and valencia troughs the extension appears clearly asymmetric due to its eastward polarity accommodated by e dipping master low angle normal faults moreover the thinning shows variations in boudinage wavelength and is characterized by several along strike transfer zones and heterogeneities the western mediterranean back arc setting is comparable with atlantic and western pacific back arc basins associated with w directed subduction zones that show similar large scale lithospheric boudinage

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

controls of inherited lithospheric heterogeneity on rift linkage numerical and analog models of interaction between the kenyan and ethiopian rifts across the turkana depression inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts the turkana depression in the east african rift connects the main ethiopian rift to the north with the kenya rift in the south this region is characterized by a nw se trending band of thinned crust inherited from a mesozoic rifting event which is cutting the present day n s rift trend at high angle in striking contrast to the narrow rifts in ethiopia and kenya extension in the turkana region is accommodated in sub parallel deformation domains that are laterally distributed over several hundred kilometers we present both analog experiments and numerical models that reproduce the along axis transition from narrow rifting in ethiopia and kenya to a distributed deformation within the turkana depression similarly to natural observations our models show that the ethiopian and kenyan rifts bend away from each other within the turkana region thus forming a right lateral stepover and avoiding a direct link to form a continuous n s depression the models reveal five potential types of rift linkage across the pre existing basin three types where rifts bend away from the inherited structure connecting via a 1 wide or 2 narrow rift or by 3 forming a rotating microplate 4 a type where rifts bend towards it and 5 straight rift linkage the fact that linkage type 1 is realized in the turkana region provides new insights on the rheological configuration of the mesozoic rift system at the onset of the recent rift episode

rifted margins ductile deformation boudinage continentward dipping normal faults and the role of the weak lower crust the stunningly increased resolution of the deep crustal levels in recent industrial seismic profiles acquired along most of the world s rifted margins leads to the unraveling of an unexpected variety of structures it provides unprecedented access to the processes occurring in the middle and lower continental crust we present a series of so far unreleased profiles that allows the identification of various rift related geological processes such as crustal boudinage ductile shear and low angle detachment faulting and a rifting history that differs from the classical models of oceanward dipping normal faults the lower crust in rifted margins appears much more intensely deformed than usually represented at the foot of both magma rich and magma poor margins we observe clear indications of ductile deformation of the deep continental crust along large scale shallow dipping shear zones these shear zones generally show a top to the continent sense of shear consistent with the activity of continentward dipping normal faults cdnf observed in the upper crust this pattern is responsible for a migration of the deformation and associated sedimentation and or volcanic activity toward the ocean we discuss the origin of these cdnf and investigate their implications and the effect of sediment thermal blanketing on crustal rheology in some cases low angle shear zones define an anastomosed pattern that delineates boudin like structures the maximum deformation is localized in the inter boudin areas the upper crust is intensely boudinaged and the highly deformed lower crust fills the inter boudins underneath the boudinage pattern controls the position and dip of upper crustal normal faults we present some of the most striking examples from the margins of uruguay west africa south china sea and barents sea and discuss their implications for the time temperature history of the margins

the effect of bimineralic composition on extensional processes at lithospheric scale we investigate how lithospheric scale compositional heterogeneities affect kilometric scale deformation processes to this end we perform numerical experiments of lithospheric extension in which we vary the moho temperature and the mineralic composition of the mantle and the crust in both the crust and the mantle we use an explicit bimineralic composition by randomly distributing two mineral phases in the materials comparison of our models to simulations using an implicit bimineralic composite one average viscous flow laws for a two phase aggregate crust and mantle demonstrates that an explicit bimineralic composition assimilated to heterogeneities succeeds in explaining observations related to the formation of rifted margins such a 1 the absence of a sharp deformation zone at the brittle ductile transition bdt 2 the initiation of the rifting process as a wide delocalized rift system with multiple normal faults dipping in both directions 3 the development of anastomosing shear zones in the middle lower crust and the upper lithospheric mantle similar to the crustal scale anastomosing patterns observed in the field or in seismic data 4 the preservation of undeformed lenses of material leading to lithospheric scale boudinage structure and resulting in the formation of continental ribbons as observed along the iberian newfoundland margin

frictional viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength abstract a theoretical composite flow law is presented for mylonite containing interconnected layers of a weak mineral undergoing power law creep and porphyroclasts of a stronger mineral undergoing fracture and frictional sliding such mylonite termed clastomylonite is said to undergo frictional viscous fv mylonitic flow its bulk strength is expressed as a function of bimineralic composition temperature effective pressure and shear strain rate as well as the material parameters for the constituent minerals the fv flow law predicts that the rheology of clastomylonite is predominantly non linear viscous and only slightly pressure sensitive for most bimineralic compositions fv mylonitic flow is shown to occupy a depth interval between cataclastic flow involving adhesive wear in the upper crust and fully viscous mylonitic flow at greater depths the transition from adhesive wear to fv mylonitic flow is related to the onset of dislocation creep glide plus climb in the weakest phase and is inferred to coincide with a crustal strength maximum a peak strength of about 80 mpa is calculated with a combination of byerlee s constants for frictional sliding of granite and the fv flow law for granitic clastomylonite 30 vol quartz at elevated fluid pressures this value falls within the range of flow stresses independently obtained from quartz palaeopiezometry in many greenschist facies granitic mylonite zones

a mechanism to thin the continental lithosphere at magma poor margins crustal thinning where continental plates break apart can be accomplished by a system of conjugate concave downward faults instead of multiple normal faults through exhumation of mid crustal and mantle material where continental plates break apart slip along multiple normal faults provides the required space for the earth s crust to thin and subside1 after initial rifting however the displacement on normal faults observed at the sea floor seems not to match the inferred extension2 here we show that crustal thinning can be accomplished in such extensional environments by a system of conjugate concave downward faults instead of multiple normal faults our model predicts that these concave faults accumulate large amounts of extension and form a very thin crust 10 km by exhumation of mid crustal and mantle material this transitional crust is capped by sub horizontal detachment surfaces over distances exceeding 100 km with little visible deformation our rift model is based on numerical experiments constrained by geological and geophysical observations from the alpine tethys and iberia newfoundland margins3 4 5 6 7 8 9 furthermore we suggest that the observed transition from broadly distributed and symmetric extension to localized and asymmetric rifting is directly controlled by the existence of a strong gabbroic lower crust the presence of such lower crustal gabbros is well constrained for the alpine tethys system4 9 initial decoupling of upper crustal deformation from lower crustal and mantle deformation by progressive weakening of the middle crust is an essential requirement to reproduce the observed rift evolution this is achieved in our models by the formation of weak ductile shear zones

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

ductile extensional shear zones in the lower crust of a passive margin we describe and interpret an unpublished set of ion geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the uruguayan volcanic rifted margin we distinguish two main groups of reflectors in the lowermost continental crust a first group at depths ranging from 32 km below the continent to 16 km in the continent ocean transition comprises reflectors continuous over tens of kilometers peculiarly visible near the mantle crust boundary a second group of reflectors dipping toward the ese oceanward is widely distributed in the lower crust these reflectors are slightly curved and tend to merge and become sub parallel with the first group of reflectors together they draw the pattern of thick shallow dipping top to the continent shear zones affecting the lower continental crust such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn tectonic volcanic formations sdr a major portion of the continental crust behaved in a ductile manner and recorded a component of top to the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle

the generation of internal waves by tidal flow over stellwagen bank the generation mechanism of internal waves by tidal flow over stellwagen bank in massachusetts bay is qualitatively clarified first in order to demonstrate the continuous generation process of internal waves a numerical simulation is carried out for the hydrographic condition during a field study by chereskin 1983 the resulting internal waveforms agree well with the acoustic images obtained at several stages of the tidal flow next the generation mechanism for these internal waves is analyzed via a physical interpretation of the calculated result this analysis is made by use of characteristics which describe the propagation of the first and second mode internal waves it is shown that each mode internal wave with an upstream phase propagation is efficiently amplified while being carried downstream toward the location where the maximum tidal flow becomes critical with the decrease of the tidal flow the internal wave thus amplified begins to propagate upstream in previous studies these internal waves were interpreted as quasi steady lee waves the present analysis however provides a satisfactory explanation for the time dependent features that are shown in numerical simulation this indicates that the internal waves over the bank are transient waves formed in response to a time varying tidal flow and not quasi steady lee waves

extension of continental crust at the margin of the eastern grand banks newfoundland seismic and gravity observations from the rifted margin of the eastern grand banks newfoundland support a new model for extension of the continental crust from the shelf edge to odp site 1277 where mantle rocks are exhumed we find that the largest decrease in crustal thickness from about 28 km to 6 km occurs beneath the continental slope of the grand banks over a distance of just 20 km this rapid decrease in crustal thickness coincides with anomalously high seismic velocities 7 0 7 2 km s 1 in the lower crust of the shelf edge the thin crust of the continent ocean transition cot in this area has a smooth basement surface void of upper crustal blocks and prerift sediments we compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust the geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins beneath the continental shelf of the grand banks the moho and the strong lower crust rotated upwards toward to a 50 dip without visible internal deformation the presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward with ongoing extension some high angle normal faults may have rotated to a sub horizontal orientation which would explain the lack of brittle deformation visible in the seismic reflection data the two geodynamic models produce different amounts of extension of continental crust in the distal margins the hot rifting model localizes strain much more rapidly leaving narrow zones of extended continental crust and it produces a relatively large amount of melt 30 in the final stages of rifting continental breakup may occur rapidly in hot lithosphere 5 myr on the other hand a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin similar to what we inferred at the eastern grand banks the cold lithospheric model requires about 23 myr of extension before continental breakup and it predicts much less melting in the mantle 13 the long rift duration wide zones of thinned continental crust and small amount of magmatism make the cold rifting model the most applicable to newfoundland iberia rift

a review of cretaceous smooth slopes extensional basins along the iberia eurasia plate boundary how pre rift salt controls the modes of continental rifting and mantle exhumation this article points out for the first time a striking correlation between the paleogeography of upper triassic deposits and the mode of crustal stretching around and inside the northern iberia plate during the cretaceous transtensional event first we propose a review of the architecture of the basins which opened during the mid cretaceous times along the iberia eurasia plate boundary similarly to the emblematic parentis basin these basins exhibit a peculiar synclinal shaped profile and are devoid of prominent block faulting the top of the basement is characterized by gentle slopes which dip symmetrically toward the center of the basins as revealed by recent comparisons with geologically constrained rifting models established from the north pyrenean zone this architecture results from the thinning of a heterogeneous continental crust under greenschist facies conditions basement deformations are thus dominantly ductile and are characterized by large scale boudinage hyper thinning and subsequent lateral extraction bulk deformation of the upper and middle crust leads to the formation of anastomosed shear zones and the development of mylonitic fabric tectonic lenses consisting of crustal material remain welded on top of the exhuming mantle the common character shared by all the pre rift sequences of the studied basins is the presence of a thick low strength upper triassic evaporites and clays layer belonging to the keuper group i e pre rift salt and clay unit in the studied basins efficient decollement along the pre rift salt and clay unit triggers mechanical decoupling and gliding of the pre rift cover that remains in the center of the basin thus during the early rifting phase the basement undergoes thinning while the pre rift cover remains preserved in the basin center in response to hyper thinning and horizontal extraction of the continental crust hot mantle is exhumed beneath the pre and syn rift cover subsequent thermal exchange i promotes ductile deformation of the basement and ii induces the development of ht lp metamorphic conditions in the pre rift sediments and at the base of the syn rift flysch levels this thermal event is well recorded in the axial portion of the pyrenean realm future north pyrenean zone as well as in the pre rift sediments of the cameros basin northern spain the architecture of the smooth slopes type basins thus contrasts with the structure of iberia newfoundland atlantic margins which are characterized by i top basement detachment faults accommodating crustal extension through rotation and translation of undeformed basement blocks and ii the individualization of continental extensional allochthons tectonically emplaced over exhumed lower crust or mantle rocks finally using recent paleogeographic reconstructions we show that the distribution of the pre rift salt and clay unit remarkably matches the distribution of the pyrenean and peri pyrenean smooth slopes type basins this allows for the first time to propose a genetic link between the distribution of evaporite bearing pre rift sedimentary formations and the development of smooth slopes basins

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

zooming on northern cap de creus shear zones abstract the northern cap de creus shear belt provides fine examples of shear zones developed at different scales in rocks bearing pre existing penetrative foliations the shear zones developed under retrograde metamorphic conditions and they are preferentially located in crystalline schists lower grade metasedimentary rocks accommodated deformation by folding the shear zones are linked in an anastomosing framework with self similar properties they are interpreted to have formed as the result of a progressive wrench dominated deformation affecting anisotropic rocks a progressive non coaxial deformation regime is considered responsible for the development of the complex kinematic pattern wherein shear zones with opposite senses of shear may come to lie in close parallelism in such a framework individual shear zones are characterised by marked strain gradients not only across the shear zone but also along it these features together with the geometrical analysis of shear zone marginal domains indicate that these structures developed with a component of shortening across the shear zones the shear zones are interpreted to have nucleated as buckling instabilities that gradually evolved to become shear zones approaching the simple shear model microstructural and quartz c axis fabric analyses indicate that shear zone formation is associated with strong strain partitioning on all scales

magnitude of weakening during crustal scale shear zone development we describe and apply a field based approach for calculating the bulk strength of a heterogeneous material to a crustal scale shear zone defining the margin of the ma parry sound domain in the grenville province of southeastern ontario using a numerical method we calculate bulk strength defined as effective viscosity as the ratio between the surface traction needed to deform a square block in simple shear and the velocity gradient across that block we use natural shear zone geometries to define the internal block structure and assign internal relative viscosities based primarily on textural criteria the margin of the parry sound domain developed into the km scale twelve mile bay shear zone accommodating several tens of km of transport while the domain interior remained rigid fracturing and fluid infiltration drove development of an amphibolite facies meter scale shear zone network that evolved into the twelve mile bay structure we analyzed three sites across the w5 km wide strain gradient from near the granulitic domain to the large scale shear zone the rocks at the shear zone margin weakened by approximately 30 those in the core weakened by at least 77 and probably by an order of magnitude these values lie between but differ substantively from the isostress and isostrain rate bounds indicating that a numerical approach such as presented here markedly improves the accuracy of bulk strength calculations

stretching instabilities and lithospheric boudinage a competent layer with a nonlinear rheology can under extension exhibit pinch and swell instabilities such instabilities can explain small scale regular deformations of rock recently they have also been invoked in relation to the distribution of basins and ranges fletcher and hallet 1983 and the undulation of the bouguer map froidevaux 1986 in the western part of north america for the case of a simple stratified structure different authors have expressed mathematical solutions describing these instabilities in this paper we elucidate the general physical meaning of these nonhomogeneous deformations in the simplest case where the competent layer is fully embedded in a softer material a fundamental mode develops with a preferred wavelength equal to 4 times the thickness of a competent layer the novelty is the occurrence of overtones with alternate symmetrical and antisymmetrical deflections of the interfaces between layers these modes have a large amplitude when the rheology tends to perfect plasticity the presence of a free surface like the earth s surface for the upper crust is shown to double the preferred wavelength of the fundamental mode and to forbid one overtone out of two this new effect is caused by gravity which inhibits the growth of surface topography another shift is of opposite sign to that caused by a free surface numerical solutions for multilayer structures can elucidate the nature of couplings between various layers the case of a three layer lithosphere having a soft lower crust and overlying a mantle asthenosphere has been analyzed in some detail it shows that in the range of wavelengths larger than 30 km up to five preferred modes may be superimposed this should encourage further analysis of the western u s topography and gravity pattern which is known to contain at least two wavelengths about 40 km for the individual basins and ranges and about 200 km for the bouguer troughs in nevada and their associated broad topographies

controls on the thermomechanical evolution of hyperextended lithosphere at magma poor rifted margins the example of espirito santo and the kwanza basins high quality long offset seismic data from many distal rifted margins show evidence for hyper extended 10 km thick crust direct observation of such domains is challenging as they lie at great water depth buried beneath thick sedimentary sequences and formed by rock assemblages that are hydrated and geophysically indistinguishable only a few drill holes have penetrated basement at ultradistal rifted margins these observations together with outcrops of preserved analogs exposed in collisional orogens suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or in the serpentinized mantle controls the formation of the ocean continent transition while depth dependent thinning controls the early phases of rifting conforming to classical rift models we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup here we develop a rheological parameterization to simulate the formation of and slip on large offset normal faults rooted in growing brittle to ductile shear zones the evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle we also propose a simplified formulation to simulate magmatic underplating and seafloor spreading the resulting numerical models provide a self consistent picture for the evolution of magma poor rifted margins from initiation of rifting to seafloor spreading the model results are compared with first order observations of the kwanza and espirito santo conjugate margins in the south atlantic as well as of magma poor margins globally