Competence Contrast Research Articles

Bushmanship: The explorers' silent partner

Bushmanship, or the competence to interact meaningfully with newly encountered environments in the achievement of predetermined goals, is a factor overlooked in analyses of land-based exploration. The importance of bushmanship is examined in this paper through a comparison of George Grey's inept exploration of north-western Australia in 1836-37 with Edward Eyre's increasingly successful overlanding ventures in south-eastern Australia at approximately the same time. Grey's impetuosity, inexperience and lack of competence contrast sharply with Eyre's growing confidence to handle unfamiliar conditions. The comparison reveals many of the factors that differentiate externally initiated expeditions such as Grey's from forays by settlers determined to wrest a living from newly settled territories.

Depth and modes of Pliocene–Pleistocene crustal extension of the Apennines (Italy)

Since early Pliocene times the Apenninic chain has been dissected by normal faults propagating towards the Adriatic foreland. In the Tyrrhenian Sea extension involved deep crustal sections, whereas in the ‘Central Apennines Downfaulted Area’ it affected the shallow crust. The Tyrrhenian back‐arc domain is connected to the overall flexural retreat of the Adriatic–Apulia plate in front of the Apenninic collisional wedge. In the outer Apenninic belt thin‐skinned delamination and gravitational collapse occurred in the hanging wall of a thickened wedge, overthrusting the uplifted, buoyant crust of the Apulia foreland. Differential sinking velocity of the foreland plate results from the inherited competence contrast between the rigid Gargano–Apulia carbonate platform to the south, and the low‐competence lithology of the pelagic sequence of the Adriatic basin to the north. During late Messinian–early Pliocene times this palaeogeographical boundary acted as a lithospheric tear, separating segments of the Apulia plate subjected to different subduction modes.

THE ROLE OF EARLY MIOCENE THRUST TECTONICS IN THE STRUCTURAL ARRANGEMENT OF THE VOLTRI GROUP (LIGURIAN ALPS, ITALY): EVIDENCE FROM THE BANDITA AREA

Geological mapping and structural analysis of the Bandita area in the northwestern sector of the Voltri Group (Ligurian Alps) revealed the occurrence of ductile shear zones and brittle thrust faults involving metamorphic rocks. This paper focuses on the brittle thrust faults and their role in the structural arrangement of a large metalherzolite body belonging to the Erro-Tobbio Unit. Layers of fault rocks, mostly banded cataclasites and crush breccia, are associated with the brittle thrust faults and show a consistent top to the east-northeast sense of shear. The features of these brittle thrusts match those of structures related to the Early Miocene post-collisional tectonic phase known as the D4 phase in the Voltri Group and “fase ligure III” (3rd Ligurian phase). In this area a major D4-related thrust fault is the basal contact between the metalherzolites and the underlying serpentinites. Similar structural relationships of the metalherzolites to other Voltri Group rock types also occur in adjoining areas. However, at the regional scale, structural and metamorphic evidence supports an early coupling (i.e. pre-D4) of the lherzolite with the serpentinite, metabasite and metasediment, indicating that the D4-related thrust surfaces rework a pre-existing structural arrangement. We suggest that the Erro-Tobbio metalherzolites are frequently involved in the D4 thrusting because the pre-existing contacts with the surrounding rock types, due to the competence contrast, acted as preferential sites for the nucleation of the D4 shear surfaces.

Ductile deformation of tonalite in the Suomusjärvi shear zone,south-western Finland

Different states of deformation of tonalite with a weak original contiguity of plagioclase were studied in the Suomusjärvi shear zone, south-western Finland. Weakly deformed tonalite is characterised by strain concentration in a conjugate network of narrow shears. Quartz and biotite deformed by crystal plasticity while plagioclase shows extensive fracturing. An interconnected weak layer structure is typical for this stage of deformation, with a densely packed strong phase, a relatively small proportion of weak matrix and a high competence contrast between the strong and weak phases. A high concentration of stress and strain rate in weak minerals produced continuous fracturing of relict plagioclase grains. This deformation stage was not accompanied by changes in the bulk chemistry of the rock. Increasing deformation led to a major change in deformation mechanism, expressed by granular flow in the ductile matrix and rigid body rotation of ovoid feldspar crystals. The size of the plagioclase crystals decreased by a combination of dynamic recrystallisation and chemical dissolution. This strain stage was also associated with fluid infiltration into the progressively narrowing shear zone. A steady-state fabric was probably attained in the core of the shear zone, which is characterised by a decrease in grain size and plagioclase aspect ratio, and a loss of shape-preferred orientation. A low stress concentration in a weak matrix in dynamic equilibrium is consistent with the proportion of relict plagioclase and size of the ovoid grains.

Stress/fault controls on the containment and release of overpressured fluids: Examples from gold-quartz vein systems in Juneau, Alaska; Victoria, Australia and Otago, New Zealand

The brittle carapace overlying a prograding metamorphic belt embraces the transition between a hydrostatically pressured near-surface fluid regime and the near-lithostatic fluid pressures characterising prograde metamorphism at depth, with the lower portion of the carapace acting as a low permeability barrier. During exhumation, the base of the carapace migrates downwards into the core of a cooling orogen. Mesozonal gold-quartz vein systems hosted in fault-fracture meshes (comprising low-displacement shears interlinked with hydraulic extension fractures) may develop towards the base of, or within, the brittle carapace. Such mesh structures, `self-generated' by the infiltration of overpressured fluids at pressures locally exceeding the least principal compressive stress (i.e. P f> σ 3), form high-permeability conduits for episodic large-volume fluid discharge by fault-valve action. These concepts are explored principally through comparison of gold-quartz vein systems hosted in fault-fracture meshes within the Bendigo–Ballarat zone of the Paleozoic Lachlan Fold Belt in Victoria, Australia, with those developed within the Mesozoic Otago Schists of southern New Zealand. The stress state within the carapace critically affects the containment of overpressured fluids within the mid-crust and the mode of fluid release. Fluids are most easily trapped beneath the carapace in compressional–transpressional regimes where the highest levels of overpressuring may be attained. Supralithostatic fluid pressures may then develop locally beneath permeability barriers over depth intervals determined by the tensile strength of sealing horizons. Mesh development is favoured by heterogeneity and high competence contrast between adjacent rock units but is inhibited by the presence of low-cohesion faults that are favourably oriented for reactivation within the carapace stress field, because such faults reactivate in shear at fluid pressure levels less than those required for hydraulic extension fracturing. Attainment of extreme overpressures ( P f> σ 3) and the formation of high-permeability meshes in any given stress regime therefore requires either intact crust (perhaps reconstituted through metamorphism), or crust where faults have become severely misoriented in the prevailing stress field through progressive deformation. In this latter case, the existing faults may be reactivated by the extreme overpressure and incorporated into the mesh structure. The combination of these circumstances explains why mesh structures associated with steep reverse faults promote particularly vigorous fault-valve action and form good mineralising sites. Major episodes of fluid loss and mineralisation may, however, also be effected by an abrupt transition from a compressional to an extensional stress field, as occurred locally within the Juneau gold-belt of SE Alaska.

Microdeformation of lacustrine laminite sequences from Late Miocene formations of SE Spain: an interpretation of loop bedding

Lacustrine laminated sediments (laminites) present in Late Miocene formations of the Híjar Basin, SE Spain, display well developed loop bedding, a structure consisting of bundles of laminae that are sharply constricted at intervals, giving a morphology of loops or links of a chain. The laminite sequences, which are interbedded with turbidite marlstones, were accumulated on the bottom of a permanently stratified lake developed in a rapidly subsiding basin limited by 010° and 105° normal faults. As deduced from both macro‐ and microdeformational analyses, the basin evolved under an extensional stress field throughout the Late Miocene. Four main types of loops, simple and complex loops with subcategories, have been recognized within the laminite sequence. Simple loops of type 1 show the best definite pattern, quite similar to ‘pinch and swell structures’, a type of boudinage typical of stretching of alternating beds where the competence contrast is not strongly marked. The remaining loop types display contortion and occasional breakage of laminae (microfaulted edges) indicative of microdeformation near the boundary between the ductile‐brittle deformational fields. The distribution of the various loop types across the laminite sequence reflects an interplay between progressive lithification of the laminites as sedimentation progressed and tectonic stresses which affected the sediment sequence. Accordingly, a mechanism of deformation under an extensional stress field, ultimately related to the creep movement of the main basin faults which resulted in successive seismic shocks of low magnitude, is proposed to explain the formation of loop bedding in the laminites.

Foreland Basement-Involved Structures

Basement-involved structures commonly occur as long, irregular chains of uplifts in foreland basins. These structures commonly contain significant hydrocarbon accumulations, with most major fields located on the broad crests of these structures. The search for complex traps in deeper targets and in subthrust structures requires an improved understanding of the geometry and evolution of these structures. Characteristic features of basement structures include deformation zones within the sedimentary cover that dissipate significant fault slip, and gently dipping frontlimbs and backlimbs. Fault slip in the basement is usually accommodated in the cover by a triangular, widening-upward deformation zone on the forelimb, with the nature of deformation controlled primarily by the mechanical stratigraphy. If the cover contains interlayered competent and incompetent units, the incompetent units are characterized by significant penetrative deformation, whereas the competent units are faulted after a relatively small amount of penetrative deformation. Depending on the competency contrast between the basement and cover, the nature of basement, and the physical conditions of deformation, the deformation zone may also propagate downward into the basement. Gently dipping backlimb and frontlimb panels are related to movement of the hanging wall over synclinal and anticlinal bends in the major fault, respectively. Many basement faults are characterized by a number of synclinal fault bends within the basement, which result in long and gently dipping backlimbs. Forelimb panels are related to anticlinal bends that typically occur at the basement-cover interface, as well as at one or more locations in the sedimentary cover. Case studies of well-constrained examples of structures from the Bighorn and Uinta basins and the Central Basin platform, demonstrate the development of these characteristic features and their strong dependence on the mechanical stratigraphy. These models and case studies will be useful in interpreting foreland basement structures in areas with poor or limited data.

The influence of mechanical anisotropy on the behaviour of the lower crust

The aim of this paper is to determine what can be deduced about the rheology of the lower crust undergoing extension, the state of stress existing there and how the finite strain pattern is developed, from a consideration of the theoretical analyses of layer-normal compression, analogue models and outcrops of rocks from the lower crust. It is concluded that the extended lower crust contains units with a significant competence contrast, has a high mechanical anisotropy and experiences bulk rheological changes during deformation. Based on a consideration of hydraulic fracturing in anisotropic materials it is argued that, depending on the magnitude differential stress, fluids will move through and out of the lower crust by fluid fracturing along the planar, horizontal stretching fabric and gently inclined normal kink-bands (extensional shears) or by the formation of vertical fractures.

Off‐scraping accretionary process under the subduction of young oceanic crust: The Shimanto Belt of Okinawa Island, Ryukyu Arc

The Eocene Kayo Formation exposed at the southwestern end of the Shimanto Belt in the Ryukyu Arc records the progressive development of off‐scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint. The formation consists almost wholly of coherent trench turbidites supplied by axial currents. Detailed structural analysis has revealed that two successive deformation stages are recognized in the Formation. The early deformation (stage 1) occurred by horizontal shortening related to the initial stage of off‐scraping accretion. In this stage, sands were water‐rich and nearly unlithified, and the competence contrast between sandstone and shale was low. The late deformation (stage 2) is marked by the formation of seaward verging imbricate fold‐and‐thrust system within the accretionary wedge. Compared with stage 1, sands in stage 2 were more dewatered and lithified, and the competence contrast between sandstone and shale increased. In accordance with this deformation history, structures caused by high pore fluid pressure indicate increased cohesion of sands during off‐scraping accretion. Studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the Formation became buried and suffered thermal imprint; pressure solution cleavage developed as an axial planar to the stage 2 folds. Thermal imprint on the Formation is characterized by high paleotemperatures at approximately 250°C–300°C in a region of the prehnite‐pumpellyite facies metamorphism. This thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate.

Hinge migration and hinge replacement

The axial migration angle (ф) is defined as the angle between the current hinge line and a marker line which was parallel to the initial hinge line. It is generally assumed that the marker line is deformed in an entirely passive manner whereas the fold axis traces out the orientations of the long axis of the sectional strain ellipse. This concept is modified when the competence contrast between a layer and its embedding medium is taken into consideration. Large values of ф may be obtained in constrictional deformations; however, because of strong hinge line arcuations of the nonplane noncylindrical folds, the concept of hinge migration will not be meaningful. Hinge migration is greatly influenced by the competence of layers. In inextensible layers the magnitude of hinge migration may become very large even in flattening and plane strain. On the other hand, hinge migration will be very small or negligible if buckle folding is associated with large layer-parallel homogeneous strain. The concept of hinge migration and hinge replacement have been critically analysed for development of Types 0, 1 and 2 fold interference patterns.

Computer simulation of single-layer buckling

This paper aims to clarify the influence that initial perturbations have in controlling the buckling process. The questions are: how is the Biot—Ramberg dominant wavelength modified by the presence of finite initial perturbations? How is the shape of the resultant folds influenced by the initial geometry? In answering these questions we also revisit many of the results already embedded in the literature for viscous materials but contrast the behaviour of these materials with those of strongly pressure-dependent elastic—plastic materials; this paper represents the first time that the buckling behaviour of such materials has been reported. For layers with a series of initial small perturbations, the current results confirm that fold wavelength and growth rate are controlled by competence contrast ( R). Wavelength selection also occurs in a layer involving perfectly-sinusoidal small perturbations, resulting in a dominant wavelength different from the input one. For layers with an isolated initial perturbation, both R and initial perturbation geometries influence buckling. If the width of the initial perturbation is smaller than a critical width related to R, significant growth of the initial perturbation is possible. When the width of the initial perturbation is larger than the critical width, the simple growth of the perturbation is possible only for early stages. It then splits into two or more secondary perturbations according to R. These perturbations can all grow into finite folds in elastic—viscous models, but only some of them can do so in elastic—plastic models.

Physical models of regional fold superposition: the role of competence contrast

Physical models of superposed folds demonstrate that rheologic contrast strongly controls the style of fold interference. Rheologic contrast affects strain partitioning between layer-parallel shortening and buckling and affects the development of structural anisotropies parallel to first-generation folds. Laminates with insignificant competence contrast are characterized by circular to elliptical domes and basins (Type 1 interference). In addition to folding, the layers accommodate shortening by layer-parallel thickening, preferentially in the fold culminations. Laminates with significant competence contrast are characterized by buckled fold hingelines and axial surfaces (Type 2 interference). The less competent layers thicken in the hinges; the more competent layers maintain their initial thickness. In plan view, the F 1) hingelines are refolded in a lobate—cuspate to box style, whose axial traces form conjugate pairs. The models are dynamically scaled to represent folds with km-scale wavelengths. The evolution of surface structures, map patterns at different depths, and equal-area projections synthesized from structure contour maps were used for analysis, comparable to regional analysis common to field studies. Gravitational body forces effectively damp the vertical amplitudes of both the first and second generation folds, thereby enhancing the formation of Type 2 interference.

Physical models of regional fold superposition: the role of competence contrast

Physical models of regional fold superposition: the role of competence contrast

Emplacement features of Archaean TTG plutons along the southern margin of the Barberton greenstone belt, South Africa

The Barberton greenstone belt in South Africa and its surrounding granitoid terrane represents one of the type localities for Archaean granite-greenstone tectonics. Contact relationships between early Archaean granitoids and greenstone sequences along the southern margin of the greenstone belt illustrate multiple, simultaneously operating emplacement mechanisms for the plutons. These emplacement mechanism, which can be identified in individual plutons comprise ductile wall-rock deformation, stoping, assimilation, and the intrusion of ring dykes. The interaction between these various mechanisms is interpreted to be responsible for the complex and highly variable discordant and concordant contact relationships observed in the granite-greenstone terrane as well as the deformation observed within wall rocks and country-rock xenoliths. The absence of regionally consistent fabrics within the plutons suggests considerable competence contrasts between the tonalitic-trondhjemitic plutons and, in many cases, the hydrated greenstone cover-sequence during later subhorizontal crustal shortening which occurred parallel to the greenstone-granitoid interface. This competence contrast is interpreted to have resulted in the accentuation of cuspate-lobate folds along the granitoid-greenstone interface, being partly responsible for the typical arcuate, dome-and-keel geometry of the Barberton greenstone belt.

Geology and structure of a sulfide-rich gold deposit; an example from the Mouska gold mine, Bousquet District, Canada

The Mouska mine in the Bousquet mining district represents a sulfide-rich gold deposit hosted by the Archean mafic metavolcanites of the Blake River Group, southern Abitibi greenstone belt. Mineralization shows both lithological and structural control and coincides with variably oriented high angle-reverse ductile and brittle-ductile shear zones. Host shear zones are superimposed on the regional foliation, and both appear to have formed from the same bulk stress field. Gold mineralization is composed of sulfide and quartz lodes and is associated with three distinct ore zones; each differs from the others by its style of veining and the nature of tectono-metamorphic overprint. Sulfide mineralization in this area varies from semimassive to massive sulfide lodes and dissemination. The strain features and age relationships of ore-bearing rocks and vein material throughout the ore zones display evidence of earlier sulfide mineralization with gold. This mineralization is pretectonic (with respect to the development of the foliation) and is interpreted as hydrothermal-synvolcanic. The auriferous quartz sulfide veins correspond to a multistage syn- to late tectonic mineralization and clearly crosscuts massive sulfide bodies. In the Mouska deposit, strain features and structural setting of gold mineralization show interactions between lithological factors, such as contacts, competency contrast and rock-body geometry, and the presence of pretectonic sulfides. In several parts of the mine volcanic sequence, the shear zone nucleation and development were controlled by both rock heterogeneities and the presence of soft sulfide bodies. The sulfides, in particular, were responsible for the structural instability recorded in some volcanic contacts and their activation as shear zones. In other parts, the sulfides have variably influenced the local slip movement. The shapes of ore shoots throughout the ore zones are variable and interpreted as reflecting the initial geometry of anisotropic interfaces involved in the deformation history.

Kinematic behaviour of an interface and competence contrast: analogue models with different degrees of bonding between deformable inclusions and their matrix

In a system composed of a ductile matrix with a particle included in it, the deformation of the matrix, the displacement field, and the rotation and the deformation of the block are related to the viscosity ratio between block and matrix and also to the degree of bonding between the matrix and the block. In this paper, belemnites from the Lower Lias of the Alps provide a natural example of moderately deformed objects included in a slaty matrix. They are compared with analogue models made of paraffin, in which a long block of a more competent paraffin is included. Depending on the degree of bonding between matrix and object, the same block appears to be deformable or quite rigid. With a high degree of bonding, the strain refraction observed corresponds to the viscosity ratio between the block and the matrix. When the degree of bonding is weak, the block is quite undeformed, its rotation is great and the deviation of the strain in the matrix is analogous to that observed around a fault. The kinematic conditions appear to have a greater effect on the finite strain than the viscosity ratio of the materials.

Competence contrasts in ductile deformation as illustrated from naturally deformed chert-mudstone layers

A sample of chert-mudstone layers provides a rare opportunity for quantitative analyses of competence contrasts in naturally deformed rocks. This sample contains deformed quartz veins, possibly formed subperpendicular to bedding during initial bedding-normal compaction, and abundant radiolarians as strain markers whose viscosity during deformation is approximated by that of chert. Under several assumptions these allow quantitative analyses of two types of competence contrast in ductile deformation, layer competence contrast and inclusion/matrix competence contrast. Layer competence contrast results in strain and cleavage refraction through the layers, while inclusion/matrix competence contrast is illustrated by the difference in strain state between radiolarians and their matrix. A quartz vein presumed to be initially perpendicular to bedding provides estimates of layer-parallel shear strains and chert/mudstone layer viscosity ratios which were also used as radiolaria/matrix viscosity ratios in different layers. Radiolarian shapes are factorized into cleavage strains and pre-cleavage shapes. Cleavage strains in matrix are then determined from radiolarian strains applying Eshelby's equations, and their variations as well as the refraction of cleavage through the layers are compared with Treagus's theoretical modeling of strain refraction. With decreasing layer competence, cleavage refracts toward bedding and increases in intensity so that cleavage-bedding intersections vary slightly but systematically through the layers, suggesting a threedimensional strain refraction. All three principal axes of radiolarian and matrix cleavage strain ellipsoids estimated indeed refract from layer to layer. Strain magnitude systematically increases with decreasing layer competence. These results are consistent with the strain refraction theory. However, no systematic change in strain ellipsoid shape associated with strain refraction is recognized.

Flow variations in power-law multilayers: implications for competence contrasts in rocks

An idealized bilaminate multilayer of alternating ‘competent’ and ‘incompetent’ layers with different proportional thicknesses is used to model theoretically the manner in which flow in rocks may vary from layer to layer during deformation. The layers are assumed to flow according to a ‘power law’, with stress exponent ( n) of 1 (=Newtonian flow), 3 or 5, which is in accordance with current experimental data for flow in rocks. The theoretical analysis concentrates first on incompressible plane flow, and is then extended to some forms of incompressible three-dimensional flow. Strain-rate variations from competent to incompetent layers are illustrated for two-dimensional flows on ‘rheology graphs’ of log stress vs log strain-rate. Three types of bulk multilayer deformation are illustrated: layer-parallel pure shearing (LPPS), layer-parallel simple shearing (LPSS) and layer-oblique straining (in different orientations). The degree of flow partitioning varies with the bulk flow, layer thicknesses, and layer n values. For all systems in plane (two-dimensional) flow, the greatest partitioning occurs for 45° bulk shortening (LPSS), and is most extreme for high n values, such that the strain-rate is virtually zero in competent members. However, for three-dimensional flows, preliminary results suggest that the value of n may play a less significant role in flow partitioning, particularly for bulk pure flattening or constrictional flows. These theoretical flow variations provide some meaning to ‘competent contrast’ in rocks, and reveal possible links between relative competence and deformation history. Such flow variations in space and time may provide the driving mechanism for folding in ductile multilayered systems.

Crystallographic preferred orientation development in a buckled single layer: a computer simulation

The development of crystallographic preferred orientation in a single-layer buckling assembly has been numerically simulated using an explicit, finite difference computer code, FLAC. The model treats the materials in the single competent layer and the embedding less competent matrix as a polycrystalline aggregate with one slip system, which exhibits elastic-perfectly-plastic behaviour, and assumes dislocation glide as the dominant strain accommodation mechanism. The results show that the preferred orientation of slip planes is much better developed in the less competent matrix than in the buckled competent layer. This correlates with the low strains developed in the layer and the high strains in the matrix. The competency contrast in the model also influences the fabric development, since changing the contrast effectively alters the final buckle size and strain distributions. The patterns of the preferred orientations vary throughout the assembly, showing different orientation relationships to the fold axial plane. The slip planes are generally preferably oriented approximately parallel to the local principal extension direction.

Enhanced Fluid Flow Resulting from Competency Contrast within a Shear Zone: The Garnet Ore Zone at Gore Mountain, NY

The occurrence of extraordinarily large garnet crystals (many are 15 cm in diameter) on Gore Mountain is generally attributed to an influx of water that caused retrograde metamorphism along the margin of a granulite facies olivine meta-gabbro body. We propose that the mechanism responsible for this influx of fluids was a high-temperature shear zone that crossed contacts between rocks with contrasting rheologies. Adjacent to the hydrated border of the olivine meta-gabbro are two feldspar-rich lithologies, a meta-syenite and gabbroic meta-anorthosite. At amphibolite facies temperatures, such feldspar-rich lithologies deform by dominantly ductile processes, while deformation of mafic rocks, such as the olivine meta-gabbro, is dominantly brittle. The juxtaposition of rocks of contrasting plastic strengths produces unusually high strain rates along the margin of a competent body, where it is pulled along passively by the weaker plastically deforming lithologies. Resulting fractures and microfractures in the border of the competent body provide conduits for fluid flow, and increase rock-fluid contact surfaces, providing ideal conditions for retrograde metamorphic reactions. These metamorphic reactions would increase ion diffusion, allowing for enhanced mineral growth, fracture annealment and plasticity, and result in a plastic deformation overprint along the boundary of the initially more competent unit.