Shear Zone Model Research Articles

A simple thermo-mechanical shear model applied to the Morcles fold nappe (Western Alps)

We present a one-dimensional shear zone model with a power-law flow law and a temperature dependent viscosity. The analytical solution for the velocity profile across the shear zone depends only on the single dimensionless parameter β. β depends on the activation energy of the applied flow law, the temperature at the base of the shear zone and the temperature difference across it. The solution can describe three types of shear deformation: (1) homogeneous simple shear for β<<1, (2) thrust-sheet emplacement on top of a thin ductile shear zone for β>>1 and (3) heterogeneous simple shear potentially generating fold nappes for β~5–10. We perform a systematic parameter search and determine the parameters that provide the best fit between a sheared model line and a reference line representing the first-order geometry of the frontal part of the Morcles nappe. We also apply a more elaborated pressure-driven model to test whether published flow laws for calcite and realistic pressure gradients provide realistic velocities and strain rates for the Morcles nappe. The value of β applicable to the Morcles nappe is estimated with three independent methods: (1) applying reported values for temperatures and activation energy to the analytical formula for β, (2) fitting the first-order geometry of the Morcles fold nappe and (3) comparing results of a velocity-driven (i.e. kinematic boundary condition) and pressure-driven (i.e. pressure source term and free surface) shear zone model. All three methods yield consistent estimates for β between 3 and 15. Our results show that previously published kinematic models, which explain the tectonic evolution of the Morcles fold nappe within a crustal shear zone, are physically feasible. The results further suggest that viscous heterogeneous simple shear played a dominant role during the formation of the Morcles fold nappe.

Modified Primary Shear Zone Analysis for Identification of Material Mechanical Behavior During Machining Process Using Genetic Algorithm

In the current work, an inverse analysis on the primary shear zone was introduced to determine the five constants in Johnson–Cook’s material constitutive equation under the conditions of metal cutting. Based on the detailed analysis on the boundary conditions of the velocity and shear strain rate fields, Oxley’s “equidistant parallel-sided” shear zone model was revisited. A more realistic nonlinear shear strain rate distribution has been proposed under the frame of nonequidistant primary shear zone configuration, so that all the boundary conditions can be satisfied. Based on the presented analysis, the shear strain, shear strain rate and temperature at the main shear plane were calculated. In conjugation with the measured cutting forces and chip thickness, a genetic algorithm (GA) based optimization program has been developed for the system identification. In order to verify the effectiveness of the developed algorithm, the obtained material constants were used in a forward analytical simulation. The acceptable agreement with experimental data validates the proposed method.

A phenomenological modeling of deformation during metal cutting: a modified thick shear zone analysis

In Oxley’s machining theory, efforts have been made to address work hardening and thermal softening effects and allow the material to flow continuously through an opened-up deformation zone. However, the strain, strain rate and temperature were calculated based on the average values. The distributions of these processing parameters, which are essential to describe the continuous flow, were not taken into account. In particular, the hodograph was adopted from the single shear plane model, in which the velocity field allowing for the continuous deformation was not described, and the issue of velocity discontinuity has not been resolved. In the current work, based on the detailed analysis on the boundary conditions of the velocity and shear strain rate fields, the thick ‘equidistant parallel-sided’ shear zone model was revisited. A more realistic nonlinear shear strain rate distribution has been proposed under the frame of non-equidistant primary shear zone configuration, so that all the boundary conditions can be satisfied. The obtained results have been compared with those obtained using the original Oxley analysis and the experimentally measured values. Although there was no significant difference in the force prediction, a clear improvement in the prediction of the deformation zone thickness was obtained.

A new analytical method based on unequal division shear zone model in orthogonal cutting

This paper presents an analytical method based on the unequal division shear zone model to study the machining predictive theory. The proposed model only requires workpiece material properties and cutting conditions to predict the cutting forces during the orthogonal cutting process. In the shear zone, the material constitutive relationship is represented using Johnson-cook model, and The material characteristics such as strain rate sensitivity, strain hardening and thermal softening are considered. The chip formation is supposed to occur mainly by shearing within the primary shear zone. The governing equations of chip flow through the primary shear zone are established by introducing a piecewise power law distribution assumption of the shear strain rate. The cutting forces are calculated for different machining conditions and flow stress data. Prediction results were compared to the orthogonal cutting test data from the available literature and found in reasonable agreement. In addition, an analysis of the deviation from experimental data for the proposed model was performed, the effects of cutting parameters and tool geometry were investigated.

Analytical prediction of cutting forces in orthogonal cutting using unequal division shear-zone model

This paper presents an analytical method based on the unequal division shear-zone model to study the machining predictive theory. The proposed model only requires workpiece material properties and cutting conditions to predict the cutting forces during the orthogonal cutting process. In the shear zone, the material constitutive relationship is described by Johnson–Cook model, and the material characteristics such as strain rate sensitivity, strain hardening, and thermal softening are considered. The chip formation is supposed to occur mainly by shearing within the primary shear zone. The governing equations of chip flow through the primary shear zone are established by introducing a piecewise power law distribution assumption of the shear strain rate. The cutting forces are calculated for different machining conditions and flow stress data. Prediction results were compared with the orthogonal cutting test data from the available literature and found in reasonable agreement. In addition, an analysis of the deviation from experimental data for the proposed model is performed, the effects of cutting parameters and tool geometry were investigated.

The role of bedding in the formation of fault–fold structures, Portalegre‐Esperança transpressional shear zone, SW Iberia

AbstractFold‐fault structures within a major transpressional shear zone of the SW Iberian Massif were investigated by combining geological mapping, cross‐section analysis and microtectonic studies. A significant example of contractional deformation is displayed in the Portalegre‐Esperança Shear Zone (PESZ) where a heterogeneous Ordovician stratigraphic sequence, showing a strong competence contrast between quartzites, slates and quartzo‐feldspathic rocks favoured strain localization and fault nucleation that controlled fold formation. The presence of pelitic layers within the thick‐bedded quartzites had probably provided weakness zones that were more favourable for the strain localization than the previous foliation present in the quartzites. The quartzites and the quartzo‐feldspathic rocks (granites and volcaniclastic rocks) accommodated heterogeneous high strain developing different degrees of mylonitization. The quartzites with protomylonitic textures are dominant and represent coarse‐grained siliciclastic sediments that suffered metamorphism and partial dynamic recrystallization. Ultramylonites occur within discrete high‐strain shear zones. It is probable that the strain localization in the PESZ involved both the effect of having layers of different competence and layers or stratigraphic contacts with rocks that experienced grain size reduction dominated by cataclasis and dislocation creep.Folds within quartzites with sub‐horizontal to gently plunging hinges vary from closed to open in thick‐bedded quartzites and from tight to closed in thin‐bedded quartzites. Observed changes in structural style of deformed quartzites, slates and quartzo‐feldspathic rocks are interpreted to result from the constraints imposed by the mechanical properties of the different lithologies.As the folds tightened during shortening, the alternating zones of contrasting competence favoured the disruption of the bedding. The deformation history in the PESZ was dominated by thrusting and strike‐slip faulting along incompetent layers that locally developed staircase geometry and transected the lower Ordovician stratigraphic sequence. The kinematic model proposed for the PESZ is consistent with the models of transpressional shear zones stretched along strike with the development of coeval strike‐slip and low angle oblique‐slip faults associated to active buckle folds with axes parallel to the principal extension direction. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Hot metamorphic core complex in a cold foreland

The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Visean/Namurian boundary (≤320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D1), all structural levels shared the same tectonic evolution: D2 (extension and exhumation), D3 (refolding) and post-D3 dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M2). M2 occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D2 effects defines a bulk prolate strain in a “pinched pull-apart” setting. Ductile D2 deformation during M2 dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D1 is only represented by the inverted position of bedding (overturned limbs of recumbent D1 folds) and by refolded D1 folds. U–Pb monazite and zircon ages and K–Ar muscovite ages are in accord with Ar–Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at ≥330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shear-zone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward propagating rift of the Palaeotethys ocean, which helped to destroy the Variscan orogen.

Formation and structure of lithospheric shear zones with damage

Shear localization is an integral part of tectonics on Earth. We examine the role of two different forms of microstructural weakening (voids and grain size reduction) in the formation of shear zones as a function of depth. The evolution of grain size and voids employed in this study is determined within the framework of two-phase damage theory. The shear zone model is characterized by two-dimensional simple shear, which allows for lithostatic pressure to influence the evolution of void-generating damage. We consider cases with pure void-generating damage, pure grain size reducing damage, and combined void-generating and grain size reducing damage. The introduction of lithostatic pressure alters void evolution, and specifically leads to a suppression of void-generation at depth. Grain size reducing damage produces the most significant localization. Differences in the time scale and efficacy of the two different damage mechanisms result from the mechanical constraints of viscous compaction and dilation imposed on void-generating damage. Cases with combined void-generating and grain size reducing damage lead to a more complicated interaction, with grain size reduction driving porosity to the flanks of the central shear zone. This result is proposed to be relevant for interpreting the void and grain size microstructures observed in faults on Earth.

Heterogeneous shear zone evolution: The role of shear strain hardening/softening

Mathematical models are proposed to describe heterogeneous shear zones characterized by variation thickness with time. These models take into account several combinations of deformation type regime (simultaneous simple shear, pure shear, and volume change) and degree of strain localization. Increasing or decreasing shear zone thickness results, respectively, from strain hardening where deformation progressively diffuses into the host rock, and strain softening where deformation gradually localizes within a narrow zone. Shear zone models are made of a series of layers, each characterized by homogenous strain. A quantitative evaluation of displacement–thickness relationship and finite strain development may be obtained from each model. Temporal evolution depends on the following parameters: (i) rate of shear strain hardening/softening, (ii) initial shear strain increment imposed to the system, and (iii) thickness of homogenously deformed units within a layered shear zone. Based on data coming from natural shear zones, displacement rates may be obtained independently of shear zone thickness.

Permian magmatism and lithospheric deformation in the Altai caused by crustal and mantle thermal processes

New structural and petrological data have been obtained for the zone of Siberia-Kazakhstan oblique collision for Permian time. In terms of classical tectonics, the area coincides with the Zaisan folded area produced by closure of the Char paleo-ocean in the Late Carboniferous. However, the extent, structure, and composition of magmatism at the Carboniferous-Permian (280±10 Ma) and Permian-Triassic (250±5 Ma) boundaries require an active control from Morgan-type lower mantle plumes (Tarim and Siberian plumes). Structure formation in the lithosphere and heat sources of magmatism have been simulated in a 3D model including lithospheric strain rates (with regard to viscosity layering) and subcontinental upper mantle convection. According to our model, heat supply from slab break-off and/or delamination of lithosphere is insufficient to maintain large-scale mantle-crustal magmatism in the case of oblique collision between 80–100 km thick plates (“soft collision”). The Late Paleozoic-Early Mesozoic Altai is considered as a model of a large hot shear zone, a particular structure produced by interference of plate- and plume-tectonic processes. Special attention is given to structural and petrological markers of plume tectonics (reported for the case of the Altai collisional shear system), with their diagnostic features useful for understanding geodynamics of other similar regions.

Deformation and kinematics in a melt-bearing shear zone from the Western Betic Cordilleras (Southern Spain)

This work deals with the structural evolution and kinematics of the basal dynamothermal aureole of the Ronda peridotites, located in the Sierra Bermeja massif of the Western Betic Cordilleras, in southern Spain. This aureole is characterized by: 1) a marked strain gradient towards the contact with the overlying peridotites; 2) the great angular dispersion, ≈90°, of stretching lineations; and 3) the intrusion of thin syn-kinematic granite sheets. These features are consistent with the aureole being a low-viscosity shear zone formed at temperatures high enough, ≈725 °C, to promote the partial melting of crustal metamorphic rocks. The high-temperature deformation is demonstrated by the activation of c-slip in quartz. We consider that the heat source came from the thrusting of the hot subcontinental mantle over metamorphic crustal rocks. The interaction of alternating events of melt-enhanced deformation and melt-enhanced embrittlement explains the heterogeneity of this ductile shear zone and the strong narrowing, to only 30 meters, of the deformed domain. The small thickness of this shear zone is in contradiction with classical conceptual models of shear zones suggesting a broadening of ductile shear zones with increasing depths and temperatures.

A major high-strain zone in the Lewisian Complex in the Loch Torridon area, NW Scotland: insights into deep crustal deformation

Abstract The Lewisian Complex is an Archaean-Proterozoic high-grade gneiss region with widespread amphibolite-facies fabrics. These fabrics have, on the 100 km scale, a subhorizontal enveloping surface and are interpreted as forming in gently dipping crustal-scale shear zones and steeper lateral ramp structures. The Scourie dyke swarm, of Proterozoic age, runs NW-SE subvertically outside such zones, but is transformed into subhorizontal concordant amphibolite sheets within them. The terms ‘Scourian’ and ‘Laxfordian’ are used to describe pre-dyke and post-dyke fabrics and events. In the southern mainland Lewisian at Loch Torridon, unmodified Scourian gneisses in the north pass southwards into lozenges bounded by anastomosing zones of Laxfordian deformation, and eventually to a region consisting entirely of Laxfordian fabrics. This geometry is compatible with a major Laxfordian shear zone that dips northeastwards beneath a Scourian hanging wall. Detailed maps show that structures on the metre to kilometre scale do not relate in a simple way to the crustal shear zone model for four reasons. First, many amphibolite-facies fabrics in the quartzo-feldspathic gneisses predate the dykes (they relate to a late Scourian or ‘Inverian’ event), so the dykes cannot be used to infer overall geometry and movement sense. A few structures along the Loch Roag Line, the most northeasterly high-strain belt in the Loch Torridon inliers, may indicate syntectonic dyke emplacement. Second, local shear with opposing senses is present in both the pre-dyke and post-dyke (Laxfordian) deformation. Third, foliation deflections on a variety of scales relate more to varying shear planes than to the deflection caused by increasing simple shear strain, and cannot be used to infer shear sense. Fourth, there is pure-shear stretching within the gneisses that masks the effects of simple shear. These structures can be incorporated into a crustal-scale shear zone model, but are not diagnostic of it. The scatter of foliation planes with a relatively constant lineation is a feature of other crustal-scale high-strain zones (e.g. Canisp, Nordre Strømfjord), although different explanations are available. Other high-strain zones (e.g. the Laxford Front) show a spatially varying lineation pattern, but this could relate more to a varying shear direction than to deflections of linear features into a single movement direction. The geometry of classic simple shear is not versatile enough to explain the kilometre-scale geometry of such high-strain zone, although it may work well in local subareas.

Styles of soft-sediment deformation on top of a growing fold system in the Gosau Group at Muttekopf, Northern Calcareous Alps, Austria: Slumping versus tectonic deformation

Styles of soft-sediment deformation in a syngrowth coarse-grained sediment gravity flow system are described from outcrops of the Upper Gosau Subgroup of the Northern Calcareous Alps. The deepwater sediments were deposited along the flanks and top of growing folds. Beside load and fluid escape structures, which are commonly found in sediment gravity flow systems, soft-sediment deformation is mainly related to fluidization of coarse-grained beds and downslope gliding of meter-thick sediment packages. Common structures affecting single or several beds are: hydroplastic folding of fine-grained beds in a coarse-grained fluidized matrix, symmetric or asymmetric mullions at the base of coarse-grained beds, and folding of heterolithic units into coarse-grained beds. Folds have varying geometries and styles, and fold axes are scattered. Soft-sediment folds also affect thick sediment packages. These folds are uniformly verging asymmetric metric to dekametric folds, formed at different degrees of lithification. Rheology of sandstones in such folds ranges from ductile to semi-brittle and brittle. Fold axes tightly cluster about one direction. The first group of deformational structures is attributed to slump-related deformation, whereas the second group represents folds related to tectonic deformation. A dislocation model and a shear zone model are used to interpret orientation of folds related to slump deformation. This considers processes taking place on the scale of the whole slump sheet and within the shear zone at the base of gliding units. Both models predict rotation of fold axes toward the transport direction of the slump. Field data support that fold axes scatter either about the downslope transport direction or about the strike of the slope.

The effects of three-dimensional structure on two-dimensional images of crustal seismic sections and on the interpretation of shear zone morphology

SUMMARY Crustal scale seismic images provide information on the geometry of subsurface structure. In this paper we examine shear zones as they provide geometrical constraints on the evolution of the crust and as they provide pathways for the migration of mineral-rich fluids from the lower crust. However, they typically appear in seismic images of the deep crust as laterally continuous bands of discontinuous reflections with individual reflections often having high amplitudes. Geological mapping of exposed shear zones show them to have a complex 3-D structure yet crustal-scale seismic reflection surveys use single or at the most only a few profiles, and therefore only create 2-D images of these structures. The processing and imaging of the multifold common midpoint (CMP) data assumes that the seismic energy comes entirely from within the plane of the section. In this paper, we use full-waveform 3-D synthetic data to consider the effects that 3-D topography on a reflector has on reflection character on a 2-D profile. We base our synthetics on an observed shear zone and test models with both a single layer and anastomizing layers. We show that topography on the reflector out of the plane of the section may cause spurious events both above and below the expected target depth. We derive the basic understanding using a simple isotropic homogeneous model, however, we then demonstrate that this is a robust phenomenon and is endemic on all 2-D sections even if the overburden is not homogeneous. We demonstrate that we obtain similar results with a velocity gradient or, for a more extreme case, with a 2-km-deep basin filled with low-velocity sediment. For crustal scale seismic profiles, in particular, the effect is pervasive as neither stacking nor migration can discriminate against out-of-plane energy and the 2-D stack represents the 3-D seismic response of a broad swath centred on the profile. However, we conclude that using the modelled data it is possible to identify qualitatively where there is significant contamination from out-of-plane topography and show examples from a shear zone in the Archaean Yilgarn Block in Western Australia.

Kinematic analysis of the southern Iberian shear zone and tectonic evolution of the Acebuches metabasites (SW Variscan Iberian Massif)

The Acebuches metabasites constitute a slice of oceanic crust located at the Variscan suture between the Ossa Morena and the South Portuguese zones (Iberian Massif). The emplacement of the metabasites onto the accretionary prism was associated with the activity of the southern Iberian shear zone. Careful measurements of the fabrics and structures related to this shear zone show consistent kinematic criteria pointing to the presence of a simple shear component, with an oblique predominantly left‐lateral sense of displacement. The data are consistent with existing models of triclinic or oblique shear zones considering coeval simple shear and pure shear components. These models provide a satisfactory explanation of the observed spatial variations in the foliation, lineation and fold hinge data. The kinematics of other shear deformations, older and younger than the southern Iberian shear zone, have also been established. These results offer new insights into the tectonic evolution of this suture of the Variscan belt, and show how complex transpressional strains can arise during oblique crustal deformation associated with the motions of tectonic plates.

Postseismic deformation and the strength of ductile shear zones

A good understanding of the rheology and strength of the whole crust is needed to obtain a physics-based earthquake prediction models. However, geodynamics-based and laboratory-based strength estimates disagree. Geodynamics tend to indicate that the actual strength of the plastic crust is less than deduced from laboratory experiments. Here, I evaluate lower crust strength from observations of transient postseismic deformation. Fault motion during an earthquake produces only a small stress perturbation, but that perturbation is sufficient to significantly affect the deformation rate of the aseismic levels of the crust, as observed by space geodesy. Even considering the non-linearity of plastic flow in geological materials, one cannot escape the conclusion that the pre-earthquake stress on the region where transient postseismic deformation occurs is not more than an order of magnitude larger than earthquake-induced stress perturbations. Using a simple shear zone model and assuming wet quartzite rheology, I show that such stress levels are not compatible with a km-scale shear zone, in spite of the geological evidence for localized deformation in the plastic crust. This implies that in plastic shear zones, rock strength is reduced. Possible explanations for the strength reduction include structural effects such as reduced grain size and/or a localized thermal anomaly associated with the shear zone.

Controls of shear zone rheology and tectonic loading on postseismic creep

Postseismic deformation is well documented in geodetic data collected in the aftermath of large earthquakes. In the postseismic time interval, GPS is most sensitive to creep in the lower crust or upper mantle activated by earthquake‐generated stress perturbations. In these regions, deformation may be localized on an aseismic frictional surface or on a ductile shear zone. These two hypotheses imply specific rheologies and therefore time dependence of postseismic creep. Hence postseismic creep constitutes a potential probe into the rheology of aseismic regions of the lithosphere. I present a simple shear zone model of postseismic creep in which the rheology of the creeping element can be varied. In the absence of tectonic loading during the postseismic time interval, the displacement history of the shear zone obeying a power law rheology with stress exponent n follows an analytical relaxation curve parameterized by 1/n. For a frictional surface, postseismic creep follows the same relaxation law in the limit 1/n → 0. A rough estimate of the apparent stress exponent can be obtained from continuous GPS records. Application to data collected after the 1994 Sanriku earthquake yields 1/n ∼ 0.1, which is consistent with dislocation creep mechanisms. However, the records of two other subduction zone events, the 2001 Peru event and the 1997 Kronotski earthquake, and a continental strike‐slip earthquake, the 1999 İzmit earthquake, require negative 1/n. Rather than characterizing the shear zone rheology, these negative exponents indicate that reloading of the shear zone by tectonic forces is important. Numerical simulations of postseismic deformation with nonnegligible reloading produce curves that are well fit by the generalized relaxation law with 1/n &lt; 0, although the actual stress exponent of the rheology is positive. While this prevents rheology from being tightly constrained by the studied GPS records, it indicates that reloading is important in the postseismic time interval. In other words, the stress perturbation induced by an earthquake is comparable to the stress supported by ductile shear zones in the interseismic period.

Upper-mantle reflectors: modelling of seismic wavefield characteristics and tectonic implications

SUMMARY Deep seismic experiments on continental lithosphere generally reveal marked reflectivity from structures in the crust and a significant decrease in reflectivity from the upper mantle. However, reflected and refracted energy from mantle lithosphere are observed in both near-normal incidence and wide-angle data. The origin of the reflective structures is a matter of debate. Hypotheses include remnant subduction zones, shear zones, fluids and seismic anisotropy. Through analytical and numerical modelling studies, including full wavefield modelling, we investigate seismic characteristic signatures generated from a variety of geologically plausible models. We have found that both upper-mantle shear zones of reduced velocity and density and remnant subduction slabs containing high-density eclogites may contain sufficient seismic impedance contrasts to normal mantle peridotites to generate near-normal incidence reflectivity. Wide-angle energy originates from subduction slabs containing either high- or lowvelocity eclogites, whereas intermediate-velocity eclogites are unlikely to produce significant wide-angle phases. In general, energy of seismic phases originating from upper-mantle zones of anomalous seismic velocities and densities is significantly increased if homogeneous zones are replaced by zones of inhomogeneous petrophysical properties resulting from constructive interference. Maximum wavefield anomalies are generated from sub Moho dipping slabs of incomplete transformation of low-velocity/low-density crustal material to high-velocity/highdensity eclogites. Localized shear zones generated in mantle peridotite generally do not produce significant wide-angle energy. Only if highly inhomogeneous structures containing material of marked (ca 10 per cent) velocity and density reduction are present, may shear zones be observed in wide-angle data. Analyses of two specific deep-seismic data sets (MONA LISA data) from the North Sea and (BABEL data) from the Baltic Sea, show good agreement between observations and modelling results for dipping remnant subduction slabs containing small-scale inhomogeneities associated with incomplete transformation. Our modelling results improve our possibilities of distinguishing between two often contrasting tectonic interpretations for dipping upper-mantle seismic reflectors, the remnant subduction and shear zone models.

Shear zone versus fold geometries at the Cannington Ag–Pb–Zn deposit: implications for the genesis of BHT deposits

Existing structural analysis of drill-core and sulphide zonation were taken as evidence for a large-scale synform that dominates the geometry of the Cannington Ag–Pb–Zn deposit—a Broken Hill-type (BHT) deposit. Underground mapping in this study has found that northwesterly directed thrusting took place during peak-temperature D 1, and that S 1 gneissosity is continuous around an amphibolite mega-boudin. D 2 sillimanite–biotite shear zones contain asymmetric folds previously interpreted to be parasitic folds, but in this study they are interpreted as due to easterly-directed extension of S 1. S 1 and S 2 indicate that shear zones were continuous and not folded into the current deposit geometry. D 2 is associated with garnet–pyroxene alteration that proved to be a favourable host for low-temperature sulphide precipitation. Retrograde deformation post-D 2 controlled lode geometry. Such a concept of high-temperature shear zone formation can be applied to other BHT deposits such as the Broken Hill Main Lode and the Aggeneys–Gamsberg base metal deposits in South Africa. In the shear zone model, Cannington and other BHT deposits are situated within high-temperature thrust shear zones that formed in conjunction with regional large-scale folding.

Geometria e cinemática da aba sul da estrutura divergente do Rio Paraíba do sul ao longo da Seção Areal-Três Rios, Rio de Janeiro

The Rio Paraiba do Sul structural divergence is a regional scale structure in which the mylonitic foliation delineate a fan geometry, those of the northern limb dipping to SE, and those of the southern limb dipping to NW. On the core of the structure occurs a large dextral ductile transpressional shear belt. The present work deals with the geometry and kinematics of structures in a section cross-cutting the southern limb of this structure. At the base of section, occurs a high-grade banded mylonite granitoids, with mylonitic foliation dipping moderately to NW and stretching lineation plunging to NNE. Northwards, the same geometry of ductile structures is seen in mylonitic S-type granitoids and paragneisses. In this region, however, the foliation steepens and the lineation increases in plunge with a slightly rotation towards the NE. Flattening strains leads to the formation of S and SL high-grade tectonites. At the base of section shear-sense indicators (S-C foliations, s -porphyroclasts and shear-bands) suggest a dextral component in horizontal planes and down-dip normal motion in a monoclinic shear-zone model. Towards the top of section, however, the asymmetry of the structures is not clear, but one oblique shearing with top-to-SW is also present. To the north the foliation have slow dips, and the stretching lineation plunges shallower to NE. In this domain the shearsense indicators suggest top-to-SW sense of shear parallel to the orogenic trend. Field relations suggests that ganitoids sheets were intruded lit-par-lit during this stage of the tectonic movements. In this domain also occur open to tight folds with axis sub-parallel to the stretching lineation in the same outcrop and axial surfaces with high dips preferentially to NW. This folding is interpreted as resulted from a shortening component at high angle to the regional shear-zones during transpressional strain, coeval with transpressive dextral shear-zones and thrust (ductile/ruptile) with top-to-SE sense of shear. The structural and field data suggests that the early movements were parallel and oblique to the belt in this region, and occurred near the peak temperatures of regional metamorphism accompanied by widespread crustal anatexis. This early stage of motion would be interpreted as horizontal and vertical extrusion of a partially molten crust during unconfined transpression.