Fold Hinges Research Articles

Structural evolution of the Agnew–Wiluna greenstone belt, Eastern Yilgarn Craton and implications for komatiite-hosted Ni sulfide exploration

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is a narrow package of complexly deformed Archean supracrustal rocks that hosts two of the world's largest komatiite-hosted nickel sulfide deposits, the Mt Keith and Perseverance deposits. These deposits and several others in the belt are centred on thick lenses of adcumulate-textured komatiite interpreted to represent areas of channelised magma flow. The large nickel sulfide deposits are located in parts of the belt associated with ca 2720 to 2700 Ma felsic volcanism (e.g. the Leinster and Mt Keith nickel camps). In these areas, felsic to intermediate volcanic rocks are intercalated with syn-volcanic massive sulfides of inferred exhalative origin. While these primary magmatic features are clearly first-order controls on the distribution of Ni sulfide deposits in the belt, several regional-scale deformation events have significantly complicated the interpretation of primary stratigraphic relationships. The earliest recorded deformation events (D1,2,3) resulted in an east–west trending greenstone belt with recumbent isoclinal folds and ductile shear zones. Subsequent west-southwest–east-southeast shortening during the D4 event at ca 2666 Ma involved the refolding of the tectono-stratigraphy to produce belt-scale, north- to north-northwest-trending upright folds, a pervasive axial planar schistosity in all rocks, and the present-day steeply dipping, overturned supracrustal sequences, and emplacement of granitoids in major antiformal fold hinges. Polyphase folding of supracrustal rocks produced Type 2 fold interference patterns with multiple facing reversals at various scales across the belt. West-southwest–east-southeast extension during the D5 event at ca 2665 Ma triggered the development of terrestrial basins (i.e. Scotty Creek and Jones Creek) in areas flanking major antiforms, resulting in the deposition of the Jones Creek Conglomerate. Subsequent west-southwest–east-southeast shortening during the D6 event resulted in the folding of the Jones Creek Conglomerate and formation of gold-bearing veins in the Agnew gold camp. Belt-wide relaxation in east–west shortening during the D7 event caused open, recumbent F7 folding of the steeply dipping stratigraphy. Broadly east–west shortening during the D8 to D10 events resulted in the tightening of existing fold hinges, the dismemberment and displacement of panels of supracrustal rocks by sinistral (e.g. Perseverance shear zone) and then dextral (Waroonga) shear zones. The Agnew–Wiluna belt displays (para)autochthonous associations within the belt, with district-scale heterogeneities caused by primary volcano-sedimentary facies changes combined with polyphase deformation. Importantly, nickel sulfide-bearing sequences identified in nickel camps can potentially be traced to different parts of the belt by unravelling the effects of polyphase deformation.

High resolution reflection seismic profiling over the Tjellefonna fault in the Møre-Trøndelag Fault Complex, Norway

Abstract. The Møre-Trøndelag Fault Complex (MTFC) is one of the most prominent fault zones of Norway, both onshore and offshore. In spite of its importance, very little is known of the deeper structure of the individual fault segments comprising the fault complex. Most seismic lines have been recorded offshore or focused on deeper structures. This paper presents results from two reflection seismic profiles, located on each side of the Tingvollfjord, acquired over the Tjellefonna fault in the southeastern part of the MTFC. Possible kilometer scale vertical offsets, reflecting large scale northwest-dipping normal faulting, separating the high topography to the southeast from lower topography to the northwest have been proposed for the Tjellefonna fault or the Baeverdalen lineament. In this study, however, the Tjellefonna fault is interpreted to dip approximately 50–60° towards the southeast to depths of at least 1.3 km. Travel-time modeling of reflections associated with the fault was used to establish the geometry of the fault structure at depth, while detailed analysis of first P-wave arrivals in shot gathers, together with resistivity profiles, were used to define the near surface geometry of the fault zone. A continuation of the structure on the northeastern side of the Tingvollfjord is suggested by correlation of an in strike direction P-S converted reflection (generated by a fracture zone) seen on the reflection data from that side of the Tingvollfjord. The reflection seismic data correlate well with resistivity profiles and recently published near surface geophysical data. A highly reflective package forming a gentle antiform structure was also identified on both seismic profiles. This structure could be related to the folded amphibolite lenses seen on the surface or possibly by an important boundary within the gneissic basement rocks of the Western Gneiss Region. The fold hinge line of the structure is parallel with the Tjellefonna fault trace suggesting that the folding and faulting may have been related.

Folding of granite and Cretaceous exhumation associated with regional-scale flexural slip folding and ridge subduction, Kitakami zone, northeast Japan

The early Cretaceous granitic plutons intrude the Kitakami zone, northeast Japan, whose southern and northern regions consist of forearc basin and accretionary complex rocks, respectively. All country rock of the Kitakami zone exhibit prominent pressure-solution cleavage and associated folds formed during shortening with a small component of sinistral shear, whereas most plutons show only igneous textures. The Kesengawa granite and some other plutons, however, have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks. We document a tectonic fold with axial planar foliation in part of the Kesengawa granite. The metamorphic minerals associated with the contact aureole and country rocks of the Kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled. The Kesengawa pluton and country rocks of the southern Kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern Kitakami zone are overturned and verge to the east. The Kitakami basement is not bounded by normal or reverse faults, so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions, nor is the exhumation associated with extensional doming. Instead, the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines. The regional shortening of accretionary prism, forearc basin, and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation. The adakitic magmatism and forearc shortening resulted from subduction of the buoyant Izanagi–Kula ridge, a regional event known as the Oshima orogeny.

Development of Transverse Fault along North Almora Thrust, Kumaun Lesser Himalaya, India: A Study Based on Field and Magnetic Fabrics

Abstract Along the North Almora Thrust (NAT) in the Kumaun Lesser Himalaya, a zone of mylonitic rocks has developed due to strain localization during the tectonic emplacement of the Almora Nappe over the Lesser Himalayan Sequence. This zone is referred here as the NAT zone (NATZ) that is dissected by faults, which are transverse to the Himalayan orographic trend and are known as seismically active structures. Trending NNW-SSE these are the Chaukhutiya and Raintoli faults. Two E-W oriented subsidiary brittle faults across the Chaukhutiya Fault are also recognized. Based on the field study and magnetic fabric analysis an attempt has been made to evaluate the deformation and kinematic history of northeastern margin of the Almora Nappe superposed by the Chaukhutiya faulting that coincides with northeastern margin of the NAT. Field study reveals brittle-ductile and brittle regimes of deformation along the Chaukhutiya Fault. Away from the NAT variable attitudes (E-W or ENE-WSW with gentle dip) of field foliation and axial planes of folds are observed, whereas at and near the NAT the attitudes of beds, including curved lithounits, are steeply dipping and are oriented parallel with the NNW-SSE trending NAT. Curvature in fold hinge line and discontinuous occurrence of lithounits are observed along the fault. Anisotropy of Magnetic Susceptibility (AMS) study of the rocks was carried out. It reveals that the anisotropy is controlled mostly by paramagnetic minerals, whereas ferromagnetic minerals have played a minor role. High temperature demagnetization through hysteresis curves has yielded negligible contribution of ferromagnetic minerals. The steep magnetic foliation is interpreted to be on account of NE-SW oriented progressive regional compression. Moreover, the magnetic foliation is inferred to represent a secondary foliation due to superimposition on primary foliation. Analyses of AMS and structural data indicate that the Chaukhutiya Fault has a strike-slip right lateral displacement including rotational component. AMS results reveal high anisotropy value (Pj) at the surface trace of the NAT and low anisotropy (Pj) away from it. Variable style of deformation is observed along the transverse fault length.

Beyond Swinging: Hinged Dissections that Twist or Fold

A geometric dissection is a cutting of a geometric figure into pieces that can be rearranged to form another figure. Some dissections can be connected with hinges so that the pieces form one figure when swung one way on the hinges, and form the other figure when swung another way. In addition to using “swing hinges”, which allow rotation in the plane, we can use “twist hinges”, which allow one piece to be flipped over relative to another piece via rotation by 180° through a third dimension. Furthermore, we can use “fold hinges”, which allow rotation along a shared edge, a motion that is akin to folding. This talk will introduce a variety of twist-hinged and fold-hinged dissections of regular polygons and stars, and other figures such as polyominoes. The emphasis will be on both appreciating and understanding these fascinating mathematical recreations. I will employ algorithmic and tessellation-based techniques, as well as symmetry and other geometric properties, to design the dissections. The goal will be to minimize the number of pieces, subject to the dissection being suitably hinged. Animations and video will be used to demonstrate the hinged dissections, in addition to actual physical models. Bio: Greg N. Frederickson is a Professor of Computer Science at Purdue University, in West Lafayette, Indiana. His primary area of research is the design and analysis of algorithms, and he has served on the editorial boards of SIAM Journal on Computing, SIAM Journal on Discrete Mathematics, Algorithmica, and IEEE Transactions on Computers. He also pursues interests in mathematical recreations, specifically geometric dissection. On this topic he has published three books and a number of articles. He has twice won the George Polya Award from the Mathematical Association of America.

L tectonites

Rocks with a pure linear fabric, or L tectonites, often indicate nearly perfect constrictional deformation. This paper assimilates published data, models, and interpretations to understand the forcing mechanisms that can form L tectonites. Most noncoaxial kinematic geometries that can result in constrictional deformation involve vorticity-parallel shortening. Local variations in external boundary conditions that localize components of constriction include releasing and restraining bends in shear zones, linear channels in shear zone boundaries, intersections between shear zones, and foliation triple points between ballooning diapirs. Internally, L tectonites are often localized in fold hinge zones, and rheologic variations partition constriction into discrete domains.The most common external kinematic framework that can form L tectonites involves simultaneous transport-perpendicular shortening in two directions. Hence, large domains of L>S and L tectonites are a common feature of orogen-parallel elongation. In every case, external variations in boundary conditions and/or internal variations in structural setting and rheology localize constriction to form L tectonites. External boundary conditions are important in density-driven vertical tectonics. Elsewhere, internal variations in structural setting and rheology are more important. The most common are the formation of L tectonites in fold hinge zones and in compositionally homogeneous rocks while heterogeneous rocks accommodate constriction by folding.

Fracture abundance and patterns in the Subandean fold and thrust belt, Devonian Huamampampa Formation petroleum reservoirs and outcrops, Argentina and Bolivia

Opening-mode fractures in Devonian sandstones in outcrop and in several fields in the Subandean Ranges of southern Bolivia and northern Argentina compose two fracture sets, I and II that strike west–northwest and north–northeast, respectively. Abundant Set I fractures are at a high angle to local Andean structural trends, and Set II is aligned with fold axes. Crosscutting relations and quartz textures in fractures suggest that, although Set I is locally older, these sets may have opened partly contemporaneously. Sets comprise both macroscopic fractures and more abundant, millimeter-scale microfractures. Fractures with opening displacement of less than 0.1 mm are typically sealed with quartz, but wider fractures are lined with quartz and contain connected fracture porosity. Microfractures are more abundant than macrofractures, and size distributions can be interpreted to approximate power laws. Microfracture strain is an efficient method of quantifying fracture abundance. Both sets record small fracture strains of 0.00016–0.0083. In backlimbs Set I strain is higher in brittle quartz-rich sandstones. Set II strain varies markedly and is generally high in hinges and steep forelimbs of asymmetric anticlines. For individual samples, Set I–II strains in fold hinges and forelimbs are comparable, consistent with concurrent Set I–II growth. Open fractures that could augment permeability are present both on and off structure, but microfractures document fracture abundance that varies with structural position and sandstone quartz content that could account for variations in production outcomes.

Analysis of folding and deformation within layered evaporites in Blocks BM-S-8 & -9, Santos Basin, Brazil

Abstract The São Paulo Plateau in the deepwater Santos Basin is the site of numerous recent pre-salt petroleum discoveries. The area is characterized by a thick sequence of layered evaporites comprising primarily halite, with subordinate anhydrite and carnalite and trace amounts of other minerals. The sequence is divided into six stratigraphic packages: three relatively competent beams containing the bulk of the stronger anhydrite and three relatively weak detachment layers. Observed structural styles range from the simple to the complex, including: upright open folds, inclined thrusted folds, recumbent isoclinal folds, sheath folds and superposed folds. Multiple detachments lead to polyharmonic folding, disharmonic folding and overtightened folds. Major anticlinal structures contain acoustically transparent material surrounding disrupted, highly deformed pieces of the lower two beams. The deformation is non-coaxial, with anticlines forming a polygonal pattern and fold hinges that are highly curvilinear. The São Paulo Plateau is a contractional province that formed in response to proximal extension at the Albian Gap during convergent gravity gliding/spreading of the margin. Shortening possibly began during the waning stages of evaporite deposition, but the bulk of the movement occurred during the Santonian–Mid-Eocene. The evaporite sequence shortened much more than the cover because of extreme updip attenuation and consequent basinwards flow beneath the cover; deeper levels of evaporite exhibit more shortening due to strain partitioning across internal detachments.

Anatomy of a 10 km scale sheath fold, Mount Hay ridge, Arunta Region, central Australia: The structural record of deep crustal flow

Mount Hay ridge, Arunta Region, central Australia, exposes a 10 km scale, antiformal sheath fold formed in the deep crust (P ≈ 0.6–0.9 GPa) in Proterozoic time. Tilting and exhumation during the Devonian Alice Springs orogeny resulted in an oblique cross section, such that the once subhorizontal sheath fold now has a moderately dipping, gently folded axial plane and overturned southern limb. Fabric types (e.g., S = L, L > S) were mapped in the field, and finite strain data were collected from a widely distributed gabbroic granulite unit. Fabric and finite strain data sets exhibit remarkable correspondence and confirm the common assumption that fabric type is directly related to finite strain type for deep crustal rocks. Both data sets reveal a distinct pattern of oblate strain on fold limbs grading into prolate strain in the fold hinge. There is no field evidence for significant strain localization, and measured strain magnitudes are consistent across the fold. Thus, we infer that different structural domains record different though coeval strain paths and that strain compatibility was maintained across domain boundaries. The uniformity of foliation, lineation, and mesoscale fold orientations across Mount Hay and the consistent magnitude of maximum stretch across structural domains indicate that the sheath fold and subsidiary structures formed during a single, progressive deformational event. Based on its geometry, the inferred scale of the enclosing shear zone (≥8 km in thickness), and regional tectonic correlations, we infer that the Mount Hay sheath fold formed in a subhorizontal, crustal‐scale flow zone during the Paleoproterozoic Strangways orogeny (1740–1690 Ma). The record of deformation is consistent with fold formation either on one side of a channel or through detachment flow.

The Middle Riphean volcanogenic complex of Kokchetav massif (Northern Kazakhstan): Structural position and age substantiation

The volcanogenic Kuuspek Formation is a well-defined part of the succession of the Pre-Vendian complexes of the Kokchetav massif (Northern Kazakhstan). The formation is built up of mildly metamorphosed acid lavas, tuffs, and tuffaceous sandstones. At the reference site to the west of the Kokchetav Mountains, the rocks of the Kuuspek Formation compose hinges of small anticlinal folds with sericite-quartz schists of the Late Riphean Sharyk Formation forming the limbs. The Kuuspek Formation lavas are high-alumina rhyolites of high-potassium calc-alkaline series. The U-Pb zircon age of the rhyolites is 1136 ± 4 Ma, thus referring to the Middle Riphean. The Kuuspek rhyolites form the basal part of the Precambrian sedimentary cover of the Kokchetav massif. The cover also comprises schists, limestones, and dolomites of the Sharyk Formation, and quartzites and quartzitic schists of the Late Riphean Kokchetav Formation.

Kinematic and temporal relationships between parallel fold hinge lines and stretching lineations: A microstructural and crystallographic preferred orientation approach

Folds with hinge lines subparallel to stretching lineation are common features in shear zones and represent extreme cases of fold modification. However, their microstructures and in particular their kinematic significance remain poorly understood. Here we describe the meso- and micro-scale structures and crystallographic preferred orientation (CPO) of quartz and muscovite from a metric-scale shear-modified fold from Plattjen, Saas Fee, Western Internal Alps. The studied fold is of class-2 and the samples collected around the fold have essentially the same microstructures. Quartz [c]-axes plotted in the tectonic coordinate system exhibit an apparent reverse obliquity on opposing limbs. Muscovite fabrics are controlled essentially by the orientation of {001} planes. If the CPO data are plotted in a common reference frame (e.g. either fold or geographical), all samples exhibit essentially the same patterns for both minerals. In particular, relative to the fold axis and axial surface, the reversal of quartz CPO obliquity on opposing limbs is no longer evident. The small obliquity (~ 15°) between the intersection lineation of the two planes of {001} of muscovite and the fold axis results in the apparent reversal of shear sense on opposite limbs when viewing the CPO related to the macroscopic fabric. The homogeneity of the microstructures and CPOs of quartz and muscovite across the fold support the interpretation that the fold developed during simultaneous activation of heterogeneous and homogeneous simple shear, with the former responsible for fold development and the latter responsible for fold rotation and the overwhelming of the heterogeneous deformation. These processes led to the observed parallelism between fold hinge line and local stretching lineation.

The Salado River fault: reactivation of an Early Jurassic fault in a transfer zone during Laramide deformation in southern Mexico

The Salado River fault (SRF) is a prominent structure in southern Mexico that shows evidence of reactivation at two times under different tectonic conditions. It coincides with the geological contact between a structural high characterized by Palaeozoic basement rocks to the north, and an ∼2000 m thick sequence of marine and continental rocks that accumulated in a Middle Jurassic–Cretaceous basin to the south. Rocks along the fault within a zone up to 150 m across record crystal-plastic deformation affecting the metamorphic basement of the Palaeozoic Acatlán Complex. Later brittle deformation is recorded by both the basement and the overlying Mesozoic sedimentary rocks. Regional features and structural textures at both outcrop and microscopic scale indicate two episodes of left-lateral displacement. The first took place under low-to medium-grade P-T conditions in the late Early Jurassic (180 Ma) based on the interpretation of 40Ar/39Ar ratios from muscovite within the fault zone; the second occurred under shallow conditions, when the fault served as a transfer zone between areas with differing magnitudes of shortening north and south of the fault. In the southern block, fold hinges were dragged westward during Laramide tectonic transport to the east, culminating in brittle deformation characterized by strike–slip faulting in the Mesozoic sedimentary rocks. North of the fault, folds are not well defined, and it is clear that the fold hinges observed in the southern block do not continue north of the fault. Although the orientation and kinematics of the SRF are similar to major Cainozoic shear zones in southern Mexico, our new data indicate that the fault had become inactive by the time of Oligocene volcanism.

Structural analysis of Turtle Mountain: origin and influence of fractures in the development of rock slope failures

Abstract Large slope failures in fractured rocks are often controlled by the combination of pre-existing tectonic fracturing and brittle failure propagation in the intact rock mass during the pre-failure phase. This study focuses on the influence of fold-related fractures and of post-folding fractures on slope instabilities with emphasis on Turtle Mountain, located in SW Alberta (Canada). The structural features of Turtle Mountain, especially to the south of the 1903 Frank Slide, were investigated using a high-resolution digital elevation model combined with a detailed field survey. These investigations allowed the identification of six main discontinuity sets influencing the slope instability and surface morphology. According to the different deformation phases affecting the area, the potential origin of the detected fractures was assessed. Three discontinuity sets are correlated with the folding phase and the others with post-folding movements. In order to characterize the rock mass quality in the different portions of the Turtle Mountain anticline, the geological strength index (GSI) has been estimated. The GSI results show a decrease in rock mass quality approaching the fold hinge area due to higher fracture persistence and higher weathering. These observations allow us to propose a model for the potential failure mechanisms related to fold structures.

The development of cavities and clastic infills along fault-related fractures in Tertiary basalts on the NE Atlantic margin

Field-based geological observations have revealed the hitherto unrecognised development of post-magmatic, brittle deformation structures cutting Tertiary volcanic rocks in the Faroe Islands. These faults and fractures are characteristically associated with different styles of clastic sedimentary infill including: 1) 0.3–1.0 m thick clastic units infilling open fractures formed along pre-existing steeply-dipping to sub-vertical faults; 2) 0.1–0.6 m thick sub-horizontal clastic units displaying internal features consistent with deposition from flowing water passing through complex open subterranean cavity systems within fractured basalts; 3) Anastomosing mm-scale and planar dm-scale clastic intrusion features mobilised and emplaced during transient, fault-related overpressuring events along pre-existing fractures cutting the surrounding volcanic units. The infill features provide evidence for the existence of sustained open cavities in the sub-surface. The clastic materials are commonly internally affected by later fault-related deformation and lack mineralisation, unlike all preceding faulting episodes in the Faroes region, perhaps reflecting their near-surface development. We believe structures equivalent to these features may occur widely in other parts of the NE Atlantic margin, particularly along the outer arcs of gentle regional-scale fold hinges. The uncemented fracture-hosted clastic infills potentially represent important fluid migration pathways within the otherwise low permeability Cenozoic volcanic sequences of the NE Atlantic region.

Influence of the antiformal setting on the kinematics of a large mass movement: the Passo Vallaccia, eastern Italian Alps

The article examines the influence of antiformal settings on the evolution of alpine slopes using 2D numerical modelling. The northern slope of the Upper Vizze Valley, which follows a huge antiform involving meta-granitic and meta-sedimentary rocks, is affected by a complex gravitational phenomenon mainly controlled by the attitude of the foliation. A preliminary geological model based on field surveys was refined using the results of 2D distinct element numerical modelling. In the crown area, the sub-horizontal meta-sedimentary sequence at the fold hinge is cut by sharp planes, whilst in the middle zone the sequence was passively dragged downwards and finally, in the lowest part of the slope, corresponding to the steeply dipping antiformal flank, the deformation propagated across the entire meta-sedimentary succession through strain partitioning along multiple shear surfaces.

Discussions on the character and interpretation model of Kelasu deep structures in the Kuqa area

Abstract The character and interpretation model of deep structures in the Kelasu belt are discussed from growth strata, thrusting sequence, salt accumulation, and imbricate patterns between the shallow and deep structures. Seismic data shows that the fold hinges of growth strata in the south limb migrate toward north from deep to shallow, the growth axial surface connected along these fold hinges dips southward, and strata thickness in its south are thicker than that in the north. These characteristics of growth strata are completely different from that of basement-involved structures and salt structures. By the kinematic analysis of faults on the southern and northern sides of the Kela-2 structure, the northern fault is considered to thrust northward and form Kela-2 structure firstly, and then be cut through by the southern fault. Strong thrusting of the southern fault caused the Kela-2 structure to uplift several kilometers, and made the peripheral plastic rocks passively flow and accumulate in the footwall, this course is very similar to the pump suction. Based on this model, the sub-salt and sub-coal thrusting systems are interpreted in the deep Kelasu belt. In the sub-salt structural layer, the remaining lower part of Kela-2 structure in the footwall of its southern fault is the northern Keshen anticlinal belt. The discovery of the northern Keshen anticlinal belt and the Jurassic sub-coal thrusting structures greatly widens the exploration scope of the Kelasu belt, showing an enormous exploration potential.

Crustal-Scale Fluid Pathways and Source Rocks in the Victorian Gold Province, Australia: Insights from Deep Seismic Reflection Profiles

Deep seismic reflection data collected across the western Lachlan orogen of southeast Australia have provided important insights into crustal-scale fluid pathways and possible source rocks in one of the richest orogenic gold provinces in the world. The profiles span three of the most productive structural zones in Victoria: the Stawell, Bendigo, and Melbourne zones. zone-scale variations in the age and style of gold deposits correspond with differences in crustal structure and composition. The bilateral distribution of gold production in the Stawell and Bendigo zones is related to the V-shaped crustal-scale geometry of the two zones in cross section. Major first-order faults, like the east-dipping Moyston fault and a set of west-dipping listric faults within the Bendigo zone, were probably major fluid conduits in the lower to middle crust during gold mineralization. First-order faults in the Stawell and Bendigo structural zones appear to have accommodated large-scale thickening down to the lower crust. The faults converge in a region beneath the western Bendigo zone where a thick section of mafic volcanic rocks and lesser sedimentary rocks are identified as a likely common source of gold-bearing fluid that was largely generated by Late Ordovician to Early Silurian metamorphism. The areas with the greatest gold endowment lie above lower crustal regions that have preserved the thickest succession of “fertile” mafic igneous rocks up to about 25 km thick. They also correspond to a region of thin or absent Precambrian lithosphere. Mafic rocks in the Stawell zone and far western Bendigo zone were probably partly consumed by Cambrian west-dipping subduction. Similar rocks in the rest of the Bendigo zone lay outside the influence of Cambrian subduction-accretion and were deformed later, probably beginning in the Late Ordovician-Early Silurian Benambran orogeny during crustal-scale imbrication. This imbrication preserved much of the mafic rocks to form the lower to middle crust. First-order listric faults in the Bendigo zone are interpreted as major controls on the locations of goldfields, even though they are largely unmineralized near the present surface. The shallow-dipping segments of first-order listric faults were favorably oriented for reactivation at the time of gold mineralization and acted as major fluid conduits in the lower to middle crust. In contrast, the upper steeply dipping segments of first-order listric faults were unfavorably oriented for reactivation and were poor fluid conduits. The seismic data show that the transition from predominantly shallow- to steeply dipping fault segments occurs in the middle to upper crust near the boundary between thick imbricated metavolcanic rocks that lie immediately below 6 to 15 km of folded metasedimentary rocks. This transition may have coincided with fluid escape zones that aided the transfer of permeability away from first-order faults and into the overlying fold-dominated turbidites. This transfer of permeability was enhanced by the growth of subvertical, fold-related fault and fracture meshes in the upper-crustal turbidites. The fault and fracture meshes consisted of bedding-parallel faults, limb thrusts, and tension vein arrays that developed along fold hinges. In the Bendigo zone, individual fold hinges and regional fold culminations were important controls on the distribution of fluid flow. Fluid flow was partly syndeformational but overlapped into the immediate postdeformational period of the Benambran orogeny. Later reactivation of first-order faults in the Late Silurian to Early Devonian and again in the Late Devonian led to further, although less important, mineralizing events as fluids exploited the preexisting fault architecture. Deeply penetrating, north-dipping listric faults in the less gold rich Melbourne zone cut into inferred Proterozoic basement and may have been fluid conduits during a Late Devonian mineralizing event.

Towards a volcanic–structural balance: relative importance of volcanism, folding, and remobilisation of nickel sulphides at the Perseverance Ni–Cu–(PGE) deposit, Western Australia

Perseverance is a world-class, komatiite-hosted nickel sulphide deposit situated in the well-endowed Leinster nickel camp of the Agnew–Wiluna greenstone belt, Western Australia. The mine stratigraphy at Perseverance trends north-northwest (NNW), dips steeply to the west, and is overturned. Stratigraphic footwall units lie along the western margin of the Perseverance Ultramafic Complex (PUC). The PUC comprises a basal nickel sulphide-bearing orthocumulate- to mesocumulate-textured komatiite that is overlain by a thicker, nickel sulphide-poor, dunite lens. Hanging wall rocks include rhyodacite that is texturally and compositionally similar to footwall volcanic rocks. These rocks separate the PUC from a second sequence of nickeliferous, E-facing, spinifex-textured komatiite units (i.e. the East Perseverance komatiite). Past workers argue for a conformable stratigraphic contact between the PUC and the East Perseverance komatiite and conclude that the PUC is extrusive. This study, however, clearly demonstrates that these komatiite sequences are discordant, implying that the PUC may have intruded rhyodacite country rock as a sill with subsequent structural juxtaposition against the East Perseverance komatiite. Early N–S shortening associated with the regional DI deformation event (corresponding to the local DP1 to DP3 events at Perseverance) resulted in the heterogeneous partitioning of strain along the margins of the competent dunite. A mylonite developed in the more ductile footwall rocks along the footwall margin of the PUC, while isoclinal F3 folds, such as the Hanging wall limb and Felsic Nose folds, formed in low-mean stress domains along the fringes of the elongated dunite lens. Strata-bound massive and disseminated nickel sulphides were passively fold thickened in hinge areas of isoclinal folds, whereas basal massive sulphides lubricated fold limbs and promoted thrust movement along shallowly dipping lithological contacts. Massive sulphides were physically remobilised up to 20 m from their primary footwall position into deposit-scale fold hinges to form the 1A and Felsic Nose orebodies. First-order controls on the geometry of the Perseverance deposit include the thermomechanical erosion of footwall rocks and the channelling of the mineralised komatiitic magma. Second- or third-order controls are several postvolcanic deformation events, which resulted in the progressive folding and shearing of the footwall contact, as well as the passive fold thickening of massive and disseminated sulphide orebodies. Massive sulphides were physically remobilised into multiple generations of fold hinges and shear zones. Important implications for near-mine exploration in the Leinster camp include identifying nickeliferous komatiite units, defining their three-dimensional geometry, and targeting fold hinge areas. Fold plunge directions and stretching lineations are indicators of potential plunge directions of massive sulphide orebodies.

Structure of the Mountain Front Flexure along the Anaran anticline in the Pusht-e Kuh Arc (NW Zagros, Iran): insights from sand box models

Abstract The Mountain Frontal Flexure shows a single step along the front of the Pusht-e Kuh Arc with about 3 km of structural relief. This front has been interpreted as being formed by a basement monocline above a blind crustal-scale and low-angle thrust with a ramp–flat geometry (the ramp dips 12–15° towards the inner part of the orogen and cuts the entire crust). The Anaran anticline on top of the Mountain Frontal Flexure shows an irregular geometry in map view and consists of four segments with diverse directions of which the SE Anaran, the Central Anaran and the NW Dome are culminations. The North–South Anaran segment may form a linking zone developed during the rise and amplification of single culminations, the NW Dome and the Central Anaran, above the Mountain Frontal Flexure. The asymmetric Anaran anticline is characterized by the existence of multiple normal faults, some of them with significant dip-slip displacements of up to 1000 m. These faults limit grabens located along the crests of the anticline segments. Cross-cutting relationships show that the normal faults along the Central Anaran are older than along the North–South Anaran, reinforcing the temporal constraints on the later growth of this segment of the anticline. The geometry of the Anaran anticline is asymmetric with the subvertical forelimb very little exposed. This forelimb is cut above and below by a thrust system that seems to develop along the fold hinges. The lower thrust, with a ramp–flat geometry, carries the entire anticline towards the foreland on top of slightly deformed rocks in the footwall. The thrust flattens in the Gachsaran evaporitic level forming a typical triangular zone filled with evaporites, which produce a strong fold disharmony between the overburden (Passive Group) and the underlying rocks (Competent Group). The growth of the Anaran anticline lasted for about 6 Ma and was the consequence of detachment folding that was subsequently thrust, rotated and uplifted above the Mountain Frontal Flexure with coeval reactivation of earlier crestal layer-parallel extension normal faults to accommodate the large increase of structural relief between the foreland and the tectonic arc. Three main results from analogue modelling have been combined with field data to resolve the geometry of the Anaran anticline as well as its evolution: (1) a thickening of intermediate evaporites (Gachsaran Formation) is produced above the flat segment of the thrust carrying the anticline on top of foreland strata; (2) growth strata deposited in the adjacent syncline modify the geometry of the anticline by increasing the dip and the length of its forelimb; (3) coeval erosion to anticline growth, as well as thick growth strata deposition, increases fold amplification rather than foreland propagation of deformation. The proposed fold model may be applied to other anticlines on top of this major basement-related thrust, such as the Siah Kuh and Khaviz anticlines in the Pusht-e Kuh Arc and Dezful Embayment domains.

Progressive fold and fabric evolution associated with regional strain gradients: a case study from across a Scandian ductile thrust nappe, Scottish Caledonides

Abstract Fold and fabric patterns developed within a major Caledonian thrust nappe in NW Scotland reflect a progressive increase in regional D 2 strain towards the basal ductile detachment. Within the upper greenschist to lower amphibolite facies thrust sheet, the main gently east-dipping foliations and SE-plunging transport-parallel lineations maintain a broadly similar orientation over c . 600 km 2 . Associated main phase, thrust-related folds (F 2 ) are widely developed, and towards the base of the thrust sheet display progressive tightening and increasing curvilinearity of fold hinges ultimately resulting in sheath folds. Secondary folds (F 3 ) are largely restricted to high-strain zones and are interpreted as flow perturbation folds formed during non-coaxial, top-to-the-NW ductile thrusting. These features are consistent with a structural model that incorporates plane strain pure-shear flattening with a superimposed and highly variable simple shear component focused into high-strain zones. The increase in strain over a distance of 30 km across strike is similar to the increasing deformation observed when structures are traced along strike to the north, and which are apparently related to proximity to basement-cover contacts. A U–Pb zircon age of 415±6 Ma obtained from a syn-D 2 meta-granite confirms that regional deformation occurred during the Scandian phase of the Caledonian orogeny.