oblique continental rifting and long transform fault formation based on 3d thermomechanical numerical modeling abstract the length of oceanic transform faults is highly variable in nature and can range from zero offset fracture zones to longer than thousand kilometers the initiation and evolution of relatively short tens of km transform faults was successfully reproduced by previous thermomechanical models however the origin of long hundreds of km structures remains more enigmatic and requires further numerical modeling effort in this study we use a 3d thermomechanical model of oblique continental rifting and breakup followed by oceanic spreading to study the formation of long transform faults our starting model geometry consists of an initial oblique weak zone mobile belt located in the mantle lithosphere the obliquity of the weak zone the extension rate the mantle potential temperature the rheology of the lower crust and lateral boundary conditions are the main parameters examined by our experiments by varying these model parameters mature oceanic spreading with long transform faults was obtained we track the initiation and evolution of long transform faults from the initiation of oblique rifting to the point where steady state seafloor spreading is observed our model results suggest that 1 long transform faults with a length greater than 200 km are established gradually within a several million years long transition from oblique rifting to mature seafloor spreading 2 the initiation and development of opposite dipping large offset normal faults along the strike of a continental margin is a precursor process for the formation of these transform faults 3 large weak zone obliquity and slow boundary extension are favorable conditions for the establishment of long transform faults 4 extreme weak lower crust promotes the development of multiple ridge transform fault systems 5 other parameters the mantle potential temperature the rate of strain softening and lateral boundary conditions only play a secondary role on long transform faults development and 6 heat flux across mature long transform faults is relatively low

high resolution 3d numerical modeling of thrust wedges influence of decollement strength on transfer zones 1 the mechanics and dynamics of thin skinned compressible thrust wedges with prescribed offsets in the backstop i e transfer zones are investigated using a three dimensional finite difference numerical model with a visco brittle plastic rheology the main questions addressed are as follows i what is the influence of the initial length of the backstop offset and ii what is the effect of the frictional strength of the main decollement on the structural evolution of the brittle wedges along such transfer zones results show that the shorter the backstop offset the earlier these two thrust planes connect forming a curved frontal thrust along the entire width of the model younger in sequence thrusts are formed parallel to this curved shape long backstop offsets produce strongly curved thrust faults around the indenting corner simulations with a weak basal friction evolve toward almost linear frontal thrusts orthogonal to the bulk shortening direction increased basal drag in models with a strong decollement favors propagation of the backstop offset into a transfer zone up to the frontal thrust these simulations revealed that surface tapers of the wedge in front of the backstop promontory are larger than what the critical wedge theory predicts whereas the tapers on the other side of the transfer zone are smaller than analytical values this difference is amplified with increasing length of the backstop offset and or strength of the decollement modeled surface elevation schemes reproduce well the topographic patterns of natural orogenic systems such as the topographic low along the minab zendan transform transfer fault between the zagros and makran

crustal mechanics control the geometry of mountain belts insights from numerical modelling abstract continental collision forms mountain ranges that have shaped much of earth s topography yet the process by which material is transported and redistributed in collision zones remains debatable here we present a series of two dimensional thermo mechanical experiments on continent continent collision zones to investigate the role of crustal strength in terms of geometry deformation and exhumation depending on the crustal rheology rate of collision and initial temperature distribution continental collision may form double vergent orogens or result in continental subduction double vergent orogens are characterized by subduction of the lithospheric mantle diffuse fore and highly localized retro shears elevated topographies and exhumation of high grade metamorphic rocks in contrast continental subduction results in subduction of lower continental crust the formation of a wedge shaped moho a foreland propagating deformation zone lower topographic build up and exhumation of low grade metamorphic rocks it is the combination of strength variations and ambient conditions that determines the geometry of mountain belts strong rheological coupling of upper and lower crust forms double vergent orogens low rheological coupling of upper and lower crust results in continental subduction

a review of brittle compressional wedge models abstract fold and thrust belts and accretionary wedges form by compression of sedimentary sequences and basement rocks into forward and backward thrusts folds nappes and duplexes for over a century models have been used to investigate the essential characteristics of such brittle wedges here i review model studies of brittle thrust wedges in orthogonal compression focussing on critical taper theory analogue and numerical techniques for modelling brittle behaviour and the most commonly investigated variations in wedge model studies those in basal dip basal strength internal strength and surface processes many model results can be placed in the context of critical taper theory which provides analytical solutions for wedge taper angle and slip line orientations for a homogeneous material on the verge of failure throughout dynamic forward models have confirmed critical taper predictions of decreasing surface dip for increasing basal dip wider wedges with steeper forward thrusts and shallower backward thrusts for decreasing basal strength narrower wedges and enhanced exhumation for surface erosion and decreasing surface dip for increasing internal strength but analogue and numerical models have been able to take these results a step further by investigating the evolution of non critical thrust wedges and the effects of non homogeneous materials with decollement layers and strain weakening shear zones these results have highlighted the strong impact of heterogeneous materials on the internal structures of thrust wedges and raise the question whether the complexity of structures that are observed in many natural fold and thrust belts requires that lithological layering inherited faults or a wide range of rheologies need to be included in forward models of brittle thrust wedges

segmentation of the caledonian orogenic infrastructure and exhumation of the western gneiss region during transtensional collapse the ultra high pressure western gneiss region of the norwegian caledonides represents an archetypical orogenic infrastructure of a continent continent collision zone to test established exhumation models we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas migmatite u pb zircon samples date melt crystallization at c 405 ma in the oygarden complex expanding the spatial extent of devonian migmatization micas from shear zones in the oygarden and gulen domes yield 40ar 39ar ages mostly between 405 and 398 ma recording the exhumation of metamorphic core complexes on a larger scale the youngest ages of various geochronometers in different segments of the western gneiss region show abrupt breaks 10 30 myr across low angle detachments and sinistral transfer zones which also correspond to metamorphic and structural discontinuities we explain the segmentation of the orogenic infrastructure by partitioned post orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice strong cratonic foreland and orogenic wedge v soft infrastructure differential crustal stretching dragged out deep levels of the orogenic crust below low angle detachments and became progressively dominated by sinistral transfer zones collapse obliterated the syn collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments supplementary material complete analytical results are available at https doi org 10 6084 m9 figshare c 5241710

extensional polarity change in continental rifts inferences from 3 d numerical modeling and observations abstract extensional basins often show along strike variability in terms of fault geometries basement structures subsidence and thermal evolution this is particularly pronounced when extension reactivates preexisting suture zones with opposing dip directions that is opposite polarities which creates wide strike slip transfer zones we have studied this extensional variability by means of thermomechanical lithospheric scale 3 d numerical modeling we conducted a series of experiments to model the extension of a thick lithosphere simulating a young orogenic area containing segmented suture zones inherited from former opposing subduction polarities implemented as rheologically weak inclined layers with opposite dips numerical experiments demonstrate that the initial subduction sutures are reactivated by two long lived lithospheric scale detachment faults which remain active until the onset of oceanic spreading the opposite polarity of these faults causes their migration toward each other by asymmetric mantle lithosphere extension and thermal accretion crustal kinematics shows the formation of extensional detachment faults and high offset listric normal faults these structures undergo gradual tilting during their footwall exhumation while the rheological weak suture layers are redistributed beneath the crust the results demonstrate an active interaction along the strike of the system between the two opposite dipping weak zones where fault segments mechanically interact during extension the maximum horizontal fault offset switches the apparent sense of shear in the separating strike slip transfer zone this kinematics explain the rapid change in the sense of shear in major strike slip transfer zones or transform faults such as observed during the extension of the pannonian back arc basin

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

spreading rate dependence of gravity anomalies along oceanic transform faults gravity and bathymetry data from transform fault systems associated with mid ocean ridges that are spreading at different rates indicates that crustal thickening may occur under transform faults associated with intermediate and fast spreading ridges if confirmed this observation would require a change in our understanding of how magma accretion occurs along rapidly spreading mid ocean ridges mid ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge slow spreading mid ocean ridge segments exhibit significant crustal thinning towards transform and non transform offsets1 2 3 4 5 6 7 8 9 10 11 12 which is thought to arise from a three dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1 2 4 5 6 7 9 10 12 in contrast fast spreading mid ocean ridges are characterized by smaller segment scale variations in crustal thickness which reflect more uniform mantle upwelling beneath the ridge axis13 14 15 here we present a systematic study of the residual mantle bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate previous studies have shown that slow slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1 2 4 6 but our analysis reveals that intermediate and fast slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments this finding indicates that there is a mass deficit at intermediate and fast slipping transform faults which could reflect increased rock porosity serpentinization of mantle peridotite and or crustal thickening the most negative anomalies correspond to topographic highs flanking the transform faults rather than to transform troughs where deformation is probably focused and porosity and alteration are expected to be greatest indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed this finding in turn suggests that three dimensional magma accretion may occur near intermediate and fast slipping transform faults

seafloor expression of oceanic detachment faulting reflects gradients in mid ocean ridge magma supply abstract oceanic detachment faulting is a major mode of seafloor accretion at slow and ultraslow spreading mid ocean ridges and is associated with dramatic changes in seafloor morphology detachments form expansive dome structures with corrugated surfaces known as oceanic core complexes occs and often transition to multiple regularly spaced normal faults that form abyssal hills parallel to the spreading axis previous studies have attributed these changes to along axis gradients in lithospheric strength or magma supply however despite the recognition that magma supply can influence fault style and seafloor morphology the mechanics controlling the transition from oceanic detachment faults to abyssal hill faults and the relationship to along axis variations in magma supply remain poorly understood this study investigates this issue using two complementary modeling approaches the first consists of semi analytical two dimensional 2 d cross axis models designed to address the fundamental mechanical controls on the longevity of normal faults these 2 d model sections are juxtaposed in the along axis direction to examine the response of the plan view pattern of faults to along axis variations in magmatic accretion in the absence of along axis mechanical coupling the second approach uses three dimensional 3 d time dependent numerical models that simulate faulting and magma intrusion in a visco elasto plastic continuum the primary variable studied through both approaches is the along axis gradient in the fraction m of seafloor spreading that is accommodated by magmatism the 2 d and 3 d results predict different abyssal hill spacing and orientation however the plan view geometry of self emerging detachment faults predicted by the 3 d numerical models are well explained by the juxtaposed 2 d models this indicates a first order control by cross axis effects of changing values of m these models are also shown to explain the along axis extent and plan view curvature of the well developed 13 20 n and mt dent occs mid atlantic ridge and cayman rise in terms of quantifiable along axis gradients in magma emplacement rates

transform marginal plateaus abstract numerous submarine plateaus form highstanding bathymetric highs at continent to ocean transitions due to their proximity to continents they have been frequently labelled marginal plateaus although this term has not been clearly defined or associated with a specific geology or geodynamic process until now these elevations have been interpreted as submerged thinned continental fragments detached from continents basaltic buildups formed by hotspots volcanic margins or oceanic plateaus many of these plateaus formed at transform margins connecting oceanic basins of contrasted ages we propose for the first time to define and review a class of marginal plateaus related to a specific tectonic setting transform marginal plateaus tmps based on a compilation of 20 tmps around the world we show that most of them have a polyphased history and have undergone at least one major volcanic phase our review highlights in particular a hitherto unrecognized close link between hotspots volcanic activity and transform margins we also propose that due to their polyphased history tmps may contain several successive basins and overlooked long lived sedimentary archives we finally highlight that because these tmps were transform plate boundaries perpendicular or oblique to surrounding rifts many of them were close to last contact points during final continental breakup and may have formed land bridges or bathymetric highs between continents therefore we discuss broader scientific issues such as the interest of tmps in recording and studying the onset and variations of oceanic currents or past biodiversity growth bio connectivity and lineage evolution

extremely asymmetric magmatic accretion of oceanic crust at the ends of slow spreading ridge segments we report the results of a deep towed magnetic survey of part of a mid atlantic ridge spreading segment analysis of the magnetic reversals indicates that for the past 0 7 m y magmatic accretion at the end of the segment has been effectively one sided with new crust being added only to the outside corner of the ridge offset eastern flank and not to the inside corner western flank spreading on the inside corner was accommodated by significant displacement on a single large fault the area between the fault and the axial volcanic ridge was effectively a thin static sliver at the plate boundary during this process in the short term asymmetric magmatic accretion was probably accomplished by progressively shifting the axial volcanic ridge to a new location at the inside corner western edge of the old one asymmetric spreading is unlikely to be sustainable as a steady state process the termination of a period of asymmetric spreading may be achieved either by establishing a new axial volcanic ridge to a position on the outside corner eastern plate thus isolating the old axial volcanic ridge on the inside corner plate or by simply arresting movement on the large fault and reverting to symmetric spreading at the axial volcanic ridge highly asymmetric accretion may be a common process at slow spreading segments particularly near discontinuities this asymmetry cannot be maintained for long periods and may be directly linked to intervals of spreading by tectonic extension

thermomechanics of mid ocean ridge segmentation the mechanics responsible for the initiation of the orthogonal pattern characterizing mid ocean ridges and transform faults are studied using numerical models the driving forces are thermal stresses arising from the cooling of young oceanic crust and extensional kinematic boundary conditions thermal stress can exert ridge parallel tension comparable in magnitude to spreading induced tension when selectively released by ridges and ridge parallel structure two modes of ridge segment growth have been identified in plan view an overlapping mode where ridge segments overlap and bend toward each other and a connecting mode where two ridge segments are connected by a transform like fault as the ratio of thermal stress to spreading induced stress increases the patterns of localized plastic strain change from the overlapping to connecting mode the orthogonal pattern marks the transition from one mode to the other besides the amount of stress from each driving force the rate of stress accumulation is crucial in determining the emergent pattern this rate dependence is characterized by the spreading rate normalized by a reference cooling rate pe when pe is paired with the ratio of thermal stress to the reference spreading induced stress they unambiguously define stability fields of the two modes the obliquely connecting the orthogonally connecting and the overlapping mode are similar to ridge transform fault intersections observed in ultra slow slow to intermediate and fast spreading centers respectively the patterns are also sensitive to the strain weakening rate fracture zones were created in part as a response to thermal stress

kinematic interpretation of the 3d shapes of metamorphic core complexes 1 metamorphic core complexes form dome shaped structures in which the ductile crust is exhumed beneath a detachment fault the 3d dome geometry inferred by mapping the schistosity in the exhumed crust can be either elongated normal to the stretching direction or along it in the first case the domes are interpreted as having formed during extension however in the second case they are interpreted either as strike slip transpressive or constrictive extensional structures depending on the geodynamic context numerical models of metamorphic core complexes published to date are all two dimensional and therefore theoretically only apply to domes which are elongated normal to the stretching direction here we explore by means of 3d thermomechanical modeling the impact of 3d kinematic extensional boundary conditions on the shape of metamorphic core complexes we examine the impact of a transtensional step over and of horsetail splay fault kinematics on the dynamics of exhumation finite strain and p t paths and compare them to cylindrical 3d models we show for the first time that domes formed in transtensional step over or at the tip of propagating strike slip faults display a finite strain field which can be interpreted as characteristic of a transpressive domes although no shortening was applied in the far field applying our models to the cyclades we propose that the coeval formation of domes elongated normal and parallel to the stretching during the miocene can be the result of horsetail splay fault kinematics which could correspond to the formation of a tear in the aegean slab

tectonic reconstruction of the clipperton and siqueiros fracture zones evidence and consequences of plate motion change for the last 3 myr bathymetry side looking sonar and magnetics data from the northern east pacific rise have been analyzed to determine the tectonic history of the clipperton fracture zone cfz and the siqueiros fracture zone sfz over the last 2 3 m y results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at 2 5 ma 1 2 1 5 ma 1 2 and 0 5 ma 4 5 along the right stepping clipperton transform the most recent change in spreading direction has resulted in fracture zone normal compression which we propose has created a median ridge and transform parallel troughs along the active transform fault volcanic intersection highs located near the ridge transform intersections rti of the cfz are interpreted to be the result of fracture zone normal extension that has created pathways for magma emplacement into onto the older lithosphere across from the rti along the left stepping sfz we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges tectonic reconstructions indicate a single euler pole is unable to describe the motion of the pacific cocos plate boundary between the cfz and sfz during periods of spreading direction change since 3 ma transtensional transforms will adjust to a new spreading direction almost immediately while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

dynamical instability produces transform faults at mid ocean ridges transform faults at mid ocean ridges one of the most striking yet enigmatic features of terrestrial plate tectonics are considered to be the inherited product of preexisting fault structures ridge offsets along these faults therefore should remain constant with time here numerical models suggest that transform faults are actively developing and result from dynamical instability of constructive plate boundaries irrespective of previous structure boundary instability from asymmetric plate growth can spontaneously start in alternate directions along successive ridge sections the resultant curved ridges become transform faults within a few million years fracture related rheological weakening stabilizes ridge parallel detachment faults offsets along the transform faults change continuously with time by asymmetric plate growth and discontinuously by ridge jumps

a tectonic model for ridge transform ridge plate boundaries implications for the structure of oceanic lithosphere abstract the first order geologic and morphologic relationships at along and proximal to ridge transform ridge plate boundaries are used to construct an empirical and speculative tectonic model the distinctive but variable morphotectonic fabric and crustal structure of the transform domain are the product of a tectonic continuum that ranges from very slow rates of strain associated with very thick edges of lithosphere slowly slipping ridge transfrom ridge plate boundaries to extremely high rates of strain associated with very thin edges of lithosphere fast slipping ridge transform ridge plate boundaries the geometry of a ridge transform intersection necessitates the juxtaposition of a relatively cold thick edge of lithosphere against the truncated end of an accreting plate boundary the cold face of lithosphere cools the adjacent wedge of asthenosphere rising beneath the axis of accretion and restricts the amount of partial melting thus attenuating the volume of basaltic melt segregated from the asthenosphere per unit time the shallow level manifestation of this cold edge effect is a thinner oceanic crust at depth the evolving lithosphere thickens rapidly as the cold edge is approached creating changes in the properties of the young lithosphere and forming a weld of upper mantle material against the cold edge of truncating lithosphere the ridge transform weld of upper mantle material creates a shear couple in the lithosphere underlying the intersection resulting in the progressive reorientation of the maximum tensile stress from normal to the ridge axis at some distance from the ridge transform intersection to an oblique angle near the boundary the brittle carapace of oceanic crust that overlies the mantle weld will deform accordingly with the development of oblique dip slip faults the model predicts that the geologic expressions of this cold boundary effect will become more dramatic with increasing thickness of the truncating edge at very slow rates of accretion 100 km juxtapose thick 30 50 km edges of lithosphere against the accreting plate boundary all but nullifying the processes that lead to the emplacement of normal oceanic lithosphere in this environment a relatively strong mantle weld will form at the ridge transform intersection and the thick edges of opposing lithosphere along the length of the transform will tend to confine the strike slip tectonism to a relatively narrow and temporally stable principal transform displacement zone in contrast at very fast rates of accretion large offset transforms 100 km place relatively thin edges of lithosphere

comment on assessing discrepancies between previous plate kinematic models of mesozoic iberia and their constraints by barnett moore et al in their recent paper barnett moore et al 2016 reflect on current models of iberian plate motion in the jurassic and cretaceous as well as ongoing debates on the reliability of the various types of kinematic data that form independent constraints on iberia s motion relative to eurasia they question the validity of various marine geophysical seismic tomographic geological and paleomagnetic data sets from the bay of biscay central atlantic ocean and iberia for kinematic reconstruction of iberia and conclude that neither models invoking aptian albian transtension nor compression are consistent with currently available data an important element in their analysis is that they discard the large paleomagnetic data set from the jurassic and cretaceous from iberia based on perceived limitations of that data set in addition they argue that seismic tomographic images exclude a scenario of subduction in the aptian albian in the pyrenees and based on this question the validity of current plate reconstructions their constraints and geodynamic scenarios which are in support of this scenario e g vissers et al 2016 we welcome the discussion raised by barnett moore et al 2016 on the reliability and usefulness of paleomagnetic data as independent constraint for iberia s plate motion in the mesozoic taking these paleomagnetic data at face value vissers et al 2016 recently showed that these are consistent with an 40 counterclockwise rotation of iberia in the aptian requiring up to 500 km of aptian convergence across the pyrenees that is through subduction in this comment we aim to critically assess whether and how the concerns on the quality of paleomagnetic data raised by barnett moore et al 2016 may allow for an alternative explanation particularly one with a mesozoic rotation of iberia that is small enough so as to not requiring subduction we also reassess whether seismic tomographic images indeed refute subduction scenarios using 8 s wave and p wave tomographic models including those used in barnett moore et al 2016

nucleation and evolution of ridge ridge ridge triple junctions thermomechanical model and geometrical theory abstract ridge ridge ridge triple junctions are among the most remarkable features of global plate tectonics but their nucleation and evolution remains incompletely understood here we employ 3d numerical models to study the processes of the nucleation and evolution of triple junctions induced by multi directional lithospheric extension the simulations show that two major classes of junctions develop i transient quadruple and triple plate rifting junctions formed by the initial plate breakup that are gradually converted into ii stable triple oceanic spreading junctions formed by the accretion of new lithosphere quadruple junctions break into two diverging triple oceanic spreading junctions connected by a linear spreading center lengthening with time this process gradually decreases the length of deforming boundaries between four diverging rigid plates and thus the integral mechanical resistance of these boundaries to the spreading the geometry of triple oceanic spreading junctions varies from asymmetrical t junctions to ideal 120 junctions the structure of these junctions includes two crucial tectonic elements oceanic ridges spreading centers and intra plate ranges triple junction traces dividing crust accreted from different spreading centers the geometrical steady state is achieved within several million years we propose a new geometrical theory of a migrating steady state triple junction which describes its structure as a function of the relative plate velocities in a moving triple junction reference frame the relative plate velocities define the orientations and growth rates of respective intra plate ranges orientations and lengthening rates of oceanic ridges are in turn given by the average relative velocity of two adjacent plates the steady state triple junction migration velocity satisfies the condition of minimal cumulative ridge lengthening rate weighted by the square root of the spreading velocity our analytical theory agrees well with the results of the numerical simulations and natural data

stress strength relationship in the lithosphere during continental collision lithospheric strength profiles generated for a shortening continental lithosphere generally predict excessively high differential stresses in the sub moho continental mantle this seems inconsistent with the relative scarcity of earthquakes at this depth this inconsistency was put forward as evidence for weak mantle rheology however this argument implicitly assumes that strength envelopes are valid in actively deforming regions we test this assumption on two end member model lithospheres having identical upper crustal rheologies but with 1 a weak lower crust and strong mantle and 2 a strong lower crust and weak mantle for this purpose we compare one dimensional 1 d with 2 d visco elastoplastic numerical models of continental shortening the 2 d models show that strongly heterogeneous deformation typically follows initially homogeneous deformation lithospheric scale buckle folds and shear zones result in strain rate variations of as much as three orders of magnitude differential stresses in the upper crust are close to yield as predicted by 1 d models stresses in deeper lithospheric regions however are significantly smaller than in 1 d models especially in actively deforming regions systematic numerical simulations as a function of temperature and deformation rate reveal that 1 d models are reliable in hot and or slowly deforming lithospheres only the relative scarcity of earthquakes at mantle levels should thus be interpreted as an intrinsic consequence of strong lithospheric deformation rather than as evidence for a weak upper mantle

evidence for weak oceanic transform faults 1 we present the results of a series of 3 d boundary element calculations to investigate the effects of oceanic transform faults on stress state and fault development at adjacent mid ocean ridge spreading centers we find that the time averaged strength of transform faults is low and that on time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness specifically mechanical coupling of only 5 best explains the observed patterns of strike slip and oblique normal faulting near a ridge transform intersection on time scales shorter than a typical earthquake cycle transient locked periods can produce anomalous reverse faulting similar to that observed at the inside corner ic of several slow spreading ridge segments furthermore we predict that extensional stresses will be suppressed at the ic due to the shear along the transform resisting ridge normal extension this implies that an alternative mechanism is necessary to explain the preferential normal fault growth and enhanced microseismicity observed at many ics index terms 3035 marine geology and geophysics midocean ridge processes citation behn m d j lin and m t zuber evidence for weak oceanic transform faults geophys res lett 29 24 2207 doi 10 1029 2002gl015612 2002

asymmetric lithospheric extension the role of frictional plastic strain softening inferred from numerical experiments plane strain thermomechanical finite element model experiments of lithospheric extension are used to investigate the effects of strain softening in the frictional plastic regime on the asymmetry of extension strain softening is considered in cases where the crust is either strongly or weakly coupled to the mantle and as the extension velocity varies from 0 3 to 30 cm yr in the absence of strain softening extension is symmetric ss mode when strain softening takes the form of a reduction in the internal angle of friction with increasing strain lithospheric extension may be asymmetric at a lithospheric scale aa mode or exhibit crustal asymmetry concomitant with mantle symmetry as mode the different styles depend on the relative control of the system by the frictional plastic and ductile layers which promote asymmetry and symmetry respectively high extension velocities and weak ductile crust mantle coupling tend to suppress the fundamental asymmetry induced by frictional strain softening this is because they respectively increase the effective strength of the lower lithosphere and decrease the control by frictional plasticity

dependence of mid ocean ridge morphology on spreading rate in numerical 3 d models abstract the morphology of natural mid ocean ridges changes significantly with the rate of extension full spreading rate on earth varies over more than one order of magnitude ranging from less than 10 mm yr at the gakkel ridge in the arctic ocean to 170 mm yr at the east pacific rise the goal of this study is to reproduce and investigate the spreading patterns as they vary with extension rate using 3 d thermomechanical numerical models the applied finite difference marker in cell code incorporates visco plastic rheology of the lithosphere and a crustal growth algorithm the evolution of mid ocean ridges from nucleation to a steady state is modelled for a wide range of spreading rates with increasing spreading rate four different regimes are obtained a stable alternating magmatic and amagmatic sections 10 mm yr b transient features in asymmetrically spreading systems 20 mm yr c stable orthogonal ridge transform fault patterns 40 mm yr and d stable curved ridges 60 mm yr modelled ultraslow and slow mid ocean ridges share key features with natural systems abyssal hills and oceanic core complexes are the dominant features on the flanks of natural slow spreading ridges numerically very similar features are produced both generated by localised asymmetric plate growth controlled by a spontaneous development of large offset normal faults detachment faults asymmetric accretion in our models implies a lateral migration of the ridge segment which might help explaining the very large offsets observed at certain transform faults in nature

the seismic cycle at subduction thrusts insights from seismo thermo mechanical models 1 the underestimation of the size of recent megathrust earthquakes illustrates our limited understanding of their spatiotemporal occurrence and governing physics to unravel their relation to associated subduction dynamics and long term deformation we developed a 2 d continuum viscoelastoplastic model that uses an eulerian lagrangian finite difference framework with similar on and off fault physics we extend the validation of this numerical tool to a realistic subduction zone setting that resembles southern chile the resulting quasi periodic pattern of quasi characteristic m8 m9 megathrust events compares quantitatively with observed recurrence and earthquake source parameters albeit at very slow coseismic speeds without any data fitting surface displacements agree with gps data recorded before and during the 2010 m8 8 maule earthquake including the presence of a second order flexural bulge these surface displacements show cycle to cycle variations of slip deficits which overall accommodate 5 of permanent internal shortening we find that thermally and stress driven creep governs a spontaneous conditionally stable downdip transition zone between temperatures of 350 c and 450 c ruptures initiate above it and below the forearc moho propagate within it interspersed by small intermittent events and arrest below it as ductile shearing relaxes stresses ruptures typically propagate upward along lithological boundaries and widen as pressures drop the main thrust is constrained to be weak due to fluid induced weakening required to sustain regular subduction and to generate events with natural characteristics fluid pressures of 75 99 of solid pressures the agreement with a range of seismological geodetic and geological observations demonstrates the validity and strength of this physically consistent seismo thermo mechanical approach

from continental rifting to seafloor spreading insight from 3d thermo mechanical modeling abstract continental rifting to seafloor spreading is a continuous process and rifting history influences the following spreading process however the complete process is scarcely simulated using 3d thermo mechanical coupled visco plastic numerical models we investigate the complete extension process and the inheritance of continental rifting in oceanic spreading our modeling results show that the initial continental lithosphere rheological coupling decoupling at the moho affects oceanic spreading in two manners 1 coupled model a strong lower crust mechanically couples upper crust and upper mantle lithosphere generates large lithospheric shear zones and fast rifting which promotes symmetric oceanic accretion i e oceanic crust growth and leads to a relatively straight oceanic ridge while 2 decoupled model a weak ductile lower crust mechanically decouples upper crust and upper mantle lithosphere generates separate crustal and mantle shear zones and favors asymmetric oceanic accretion involving development of active detachment faults with 3d features complex ridge geometries e g overlapping ridge segments and curved ridges are generated in the decoupled models two types of detachment faults termed continental and oceanic detachment faults are established in the coupled and decoupled models respectively continental detachment faults are generated through rotation of high angle normal faults during rifting and terminated by magmatism during continental breakup oceanic detachment faults form in oceanic crust in the late rifting early spreading stage and dominates asymmetric oceanic accretion the life cycle of oceanic detachment faults has been revealed in this study

thermal stress and the spacings of transform faults bathymetric charts are used with satellite altimeter profiles to locate major ridge transform intersections along five spreading ridges the ridges are the mid atlantic ridge the east pacific rise the chile rise the pacific antarctic rise and the southeast indian ridge analysis of these data show spacings between transform faults w increase linearly with spreading rate w 6 28 m y this linear correlation is explained by a thermoelastic model of a cooling strip of lithosphere spreading at a rate the traction free boundaries of the thin elastic strip simulate cracks in the lithosphere at transform faults a two dimensional thermoelastic solution for the in plane stress shows the largest stress component is tensional and parallel to the ridge stresses are zero at the ridge and increase as age to a maximum value at an age of w 4 all stress components are small for ages greater than w when the transform spacing is large compared with the spreading rate w 100 m y thermal stresses exceed the strength of the lithosphere for ages between 0 and 30 ma the observed maximum ratio of transform spacing to spreading rate w 10 m y results in low thermal stresses that only exceed the strength of the lithosphere for ages less than 1 ma thus transform faults relieve most of the thermal stress model predictions also agree with earthquake studies showing that normal faults in young lithosphere have tensional axes aligned with the ridge moreover oceanic intraplate earthquakes rarely occur in lithosphere older than 30 ma as predicted by the model these and other geophysical observations confirm turcotte s hypothesis that transform faults are thermal contraction cracks

control on seafloor spreading geometries by stress and strain induced lithospheric weakening abstract seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults few regions exhibit different patterns such as the east pacific rise epr which additionally displays overlapping spreading centres oscs and microplates we introduce a dynamical model using two independent scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins the elastic modulus that is affected by the damage determines the type of localized deformation damage reducing the bulk modulus tends to result in tensile fractures while a reduction in shear modulus leads to shear fractures the damage source determines the fracture orientation material weakening in tension results in fractures perpendicular to the most tensile principal stress while shear weakening results in two conjugate fractures at 45 relative to the applied stress strain or energy dependent damage results in propagating localized fractures stress dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones starting from small perturbations with reduced elastic moduli as nucleation points all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress the orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones oscs form when the shear damage leading to transform faults is suppressed while microplate formation requires additional reduction of the shear modulus by tensile energy oblique spreading at 45 recently discovered along ultraslow spreading ridges occurs when both moduli are weakened by shear energy a parameter regime exists in which ridge transform patterns develop at low applied tension while microplates form at higher stresses these results indicate that at least three different micromechanical processes operate with different evolution rates oscs and oblique spreading require different material properties

the initiation of pull apart basins and transform continental margins results from numerical experiments of kinematic partitioning in divergent settings we present a set of numerical models to investigate how transform faults initiate in pull apart basins or transform continental margins the model represents an elastic plate with three aligned weak spots applying strain on the edge of the model firstly propagates extensional cracks from the pre existing damaged spots the subsequent evolution depends on the angle obliquity between the applied strain and the alignment of the damaged spots for obliquity 60 transtensional transfer zones connect the divergent cracks and no transform fault forms these experiments suggest that pull apart basins do not form as previously assumed by connection of overlapping transform faults but that transform faults form as connections of extensional areas

evidence for asymmetric accretion and low angle planar faults in slow spreading oceanic crust new seismic reflection data in the eastern atlantic have enabled two large scale dipping reflectors imaged within mesozoic ocean crust to be mapped in three dimensions and placed in their sea floor spreading context one of the reflectors dips to the west and extends from a step in the igneous basement to the moho the other reflector dips to the east and extends between 4 and 10 km below basement both reflectors are oriented approximately parallel to isochrons lie just north of a 5 km offset third order discontinuity are planar in cross section and dip at 30 we interpret both these reflectors as lithospheric scale faults formed within the median valley at the axis we suggest that the lower part of the eastward dipping fault was captured from the north american plate possibly by rapid propagation of an active volcanic rift across the discontinuity

how does the initial configuration of oceanic ridge segments affect their interaction insights from analogue models 1 oceanic ridge segments interact forming different types of structures of which transform faults and overlapping spreading centers represent the most common end members this study uses analogue models for testing the role of the initial configuration of the ridges on the geometry and kinematics of the resulting interaction zones the models run in a centrifuge consist of a lighter silicone lds rising within a denser silicone hds below a brittle layer simulating the oceanic lithosphere precut fractures in the brittle layer simulate the initial configuration of oceanic ridges the rise and lateral spreading of the lds induce the propagation widening interaction and linkage of the fractures we varied the offset and overlap between the fractures obtaining 10 distinctive types of interaction variations in fracture overlap and offset define several interaction types increase of fracture overlap leads to interaction zones of lower aspect ratio with fractures propagating at lower angles to the mean extension direction increase of offset leads to the elongation of the interaction zones with the new fractures propagating subparallel to the extension direction the comparison with several examples of natural ridges shows close geometric similarities confirming the existence of predominant types of ridge interaction among these ridges with smaller offsets develop interactions similar to overlapping spreading centers whereas ridges with larger offsets have geometries reminiscent of transform zones the comparison between experimental and natural examples suggests that the observed wide spectrum of ridge interaction types in nature results from the initial configuration of the divergent plate boundary

bathymetric gradients of lineated abyssal hills inferring seafloor spreading vectors and a new model for hills formed at ultra fast rates abstract abyssal hill morphology provides a preliminary measure of the direction and rate of seafloor spreading however additional information e g magnetic anomaly data or a nearby mid ocean ridge is usually required to verify these estimates previous attempts to identify a unique spreading rate proxy from abyssal hill dimensions e g height length width have largely failed due to the relatively large scatter of data or the non linear character of spreading rate trends we present a new stand alone method of determining both spreading rate and spreading direction using the distribution of azimuths for slopes facing toward and away from the ridge axis the spreading rate is determined with the peak height parameter defined as the difference in the height maximum frequency of the two dominant modes observed in the azimuthal histograms this parameter exhibits a clear nearly linear spreading rate trend and allows half spreading rates to be estimated to within 10 20 km myr the spreading direction is determined with the peak width parameter which compares the average width of the two dominant modes in the azimuthal histograms the wider distribution of slope azimuths is oriented away form the paleo ridge axis for all spreading rates and thus spreading direction can be determined the trends in the peak height and width parameters are used to constrain a new model of abyssal hill formation at ultra fast spreading rates which require greater off axis extensional faulting resulting in a few large throw faults on the outward facing hillsides and many smaller throw faults on the inward facing hillsides

numerical models for continental break up implications for the south atlantic we propose a mechanism that explains in one unified framework the presence of continental break up features such as failed rift arms and high velocity and high density bodies that occur along the south atlantic rifted continental margins we used 2d and 3d numerical models to investigate the impact of thermo rheological structure of the continental lithosphere and initial plume position on continental rifting and break up processes 2d experiments show that break up can be 1 central mantle plume induced and directly located above the centre of the mantle anomaly 2 shifted mantle plume induced and 50 to 200 km shifted from the initial plume location or 3 distant self induced due to convection and or slab subduction delamination and 300 to 800 km off set from the original plume location with a 3d perfectly symmetrical and laterally homogeneous setup the location of continental break up can be shifted hundreds of kilometres from the initial position of the mantle anomaly we demonstrate that in case of shifted or distant continental break up with respect to the original plume location multiple features can be explained its deep seated source can remain below the continent at one or both sides of the newly formed ocean this mantle material glued underneath the margins at lower crustal levels resembles the geometry and location of high velocity high density bodies observed along the south atlantic conjugate margins impingement of vertically up welled plume material on the base of the lithosphere results in pre break up topography variations that are located just above this initial anomaly impingement this can be interpreted as aborted rift features that are also observed along the rifted margins when extension continues after continental break up high strain rates can relocalize this relocation has been so far attributed to rift jumps most importantly this study shows that there is not one single rift mode for plume induced crustal break up

stress fields associated with oceanic transform faults abstract although their formation is commonly attributed to shear stresses oceanic transform faults contain many structural elements indicating extension across them examples include transform parallel tension fractures normal faults grabens dykes and small scale spreading centres at the junctions between transform faults and adjacent ridge segments there are commonly sets of oblique fractures a detailed field study of oblique fractures in the vicinity of a junction between a mid ocean ridge and an oceanic fracture zone in north iceland shows that they are mostly tension fractures and normal faults this paper presents the results of a boundary element study of the stress field associated with the junction between an oceanic transform fault and its ridge segments the results show that uniaxial tensile loading plate pull parallel with the transform fault tends to lock it but also gives rise to shear stress concentration in a zone that coincides with the fault this shear stress is primarily responsible for the strike slip faulting and associated earthquakes in the transform fault to generate the transform parallel fracture zone graben normal faults tensile fractures dykes and spreading centres however requires tensile loading parallel with the ridge axis the results suggest that biaxial ridge parallel and ridge perpendicular tensile loading generates a stress field that 1 unlocks and makes slip easier on the transform faults and 2 explains the oblique fractures at the ridge transform junctions

dating of a regional hydrothermal system induced by the 1850 ma sudbury impact event new u pb geochronologic data from the sudbury structure directly link extensive hydrothermal activity in crater fill breccias with the 1850 ma impact event and constrain complex impact induced processes to 4 m y semiconformable alteration of breccias in the onaping formation includes silicification albitization chloritization calcitization and complex feldspathization which directly underlies zn cu pb ore deposits geochronologic data demonstrate a middle to lower crustal source for the crater fill deposit that was subsequently affected by the hydrothermal event at 1848 4 3 8 1 8 ma these and other data confirm that the crater fill breccia hydrothermal system sudbury igneous complex and its sublayer ni ore bearing units were emplaced as a result of impact the ages of events associated with the sudbury structure are now more closely constrained than in any other meteorite impact structure

toward 4d modeling of orogenic belts example from the transpressive zagros fold belt abstract latest developments in computer power and computational solutions open new ways to envision complex natural systems advances in high resolution 3d geodynamic modeling in particular allow investigating tectonic processes such as transpression which involve normal and tangential kinematics with respect to the bulk orogenic trends we demonstrate that implementing the 4 dimensions space and time provides and constrains new answers to long lasting discussions as case study we have chosen the zagros simply folded belt in iran which exhibits spatial variations of structural patterns from the homogeneously shortened wide fold belt in the fars to the narrower higher elevated area in the izeh high resolution 4d numerical modeling shows how a low viscosity decollement becoming frictional toward the dezful embayment influences the exposed fold patterns results also emphasize the importance of an oblique backstop to produce en echelon arranged folds where convergence related and backstop controlled folds are mingled in the transpressional orogen