Schistosity Planes Research Articles

Archean turbidite hosted orogenic gold mineralization in the Gadag greenstone belt, Western Dharwar Craton, Peninsular India

Turbidite hosted orogenic gold mineralization in the Archean Gadag greenstone belt of the Western Dharwar Craton, forms a major auriferous zone (Central Auriferous Zone) extending over a strike length of about 12km in the Gadag duplex. The turbidite sequence comprises thick inter-bedded, medium to coarse grained lithic graywacke and thin laminated layers of fine grained carbonaceous phyllite. Gold bearing quartz veins impregnate preferentially along the en-echelon shear planes, fractures and schistosity planes. Auriferous quartz veins are enveloped by the altered wall rocks.Mineralogy of the auriferous zone is dominated by gangue minerals like quartz, ankerite, chlorite, sericite and carbonaceous matter, with subordinate plagioclase. Monazite and xenotime are the important accessory minerals. Arsenopyrite and pyrite are the major sulfide minerals, but pyrrhotite, chalcopyrite, sphalerite, galena and scheelite are also present. Gold in native state occurs within quartz, silicates and arsenopyrite.Notable distinctions in mineral assemblage, texture and in chemical compositions of altered wall rocks compared to the precursor host rock in the study area implies that the metasomatism and wall rock alterations are the results of pervasive infiltration and intense interaction between hydrothermal fluids and the surrounding host rocks over a prolonged period.Sulfides, carbonates, carbonaceous matter, K2O, MgO, CaO, Cr, Ni, Cu, Pb, Zn, As and higher values of gold (0.98–4.72ppm) are added into the altered wall rocks, immediately enveloping the auriferous quartz vein bodies. The chondrite normalized REE pattern of altered wall rocks exhibits enriched LREE (LaN/YbN=av. 9.54), with prominent negative Eu anomaly. The observed variation in geochemical characteristics and mineral assemblages in the alteration zones indicates differential response of the host rock and intensity of alteration depending on the composition of host rocks and hydrothermal fluids.The auriferous hydrothermal fluids were of low salinity (2.0 to 6.6wt.% NaCl), dominated by CO2–H2O (about 30mol% CO2) with moderate densities (0.7 to 1.04g/cm3), and gold deposition occurred over a wide temperature range between 175°C and 325°C. Gold deposition was influenced by fluid mixing, phase separation and redox reactions. Mixing between CO2–H2O fluids and more reduced fluids, which evolved during fluid reaction with adjacent carbonaceous wall rocks, was the key factor causing gold deposition.The formation of the Gadag duplex, deformation, folds and reverse strike slip faults (discontinuities) was caused by the compression associated with subduction related tectonic processes. During the initial period of intrusive magmatism (2,555±6Ma), regional metamorphism occurred in the entire greenstone belt, while during later period, hydrothermal fluids responsible for gold mineralization probably were derived from metamorphic processes as well as from intrusive granites. Such fluids channeled through the thrust in host turbidite sequence carrying dissolved gold, associated metals and sulfur, ultimately were precipitated in a reducing environment in the splays to the thrust in the Gadag duplex at about 2,522±6Ma, resulting in retrograde alteration assemblages.

Middle to late Triassic mélange exhumation along a pre-Andean transpressional fault system: coastal Chile (26°–42° S)

Mélanges occur as discontinuous, mappable, units along an extensive N–S-trending, steeply dipping zone of distributed shear in metamorphic complexes along the coast of central Chile. Large mélange zones, from north to south, near Chañaral, Los Vilos, Pichilemu, and Chiloé Island, contain variations in lithologic and structural detail, but are consistent in exhibiting cross-cutting fabric features indicating a progressive transition from earlier ductile to more brittle deformation. In the Infiernillo mélange near Pichilemu, Permian to Early Triassic, sub-horizontal schistosity planes of the Western Series schist are disrupted, incorporated into, and uplifted along high-angle, N–S- to NNE–SSW-trending brittle–ductile shears. Mylonitic and cataclastic zones within the mélange matrix indicate active lateral shear during cumulative exhumation from depths exceeding 12 km in some areas. Exotic lithologies, such as Carboniferous mafic amphibolite and blueschist, formed during earlier Gondwanide subduction, match well with similar rocks in the Bahia Mansa to Los Pabilos region 750 km to the south, suggesting possible dextral offset. The development of the Middle to Late Triassic, N–S=trending, near-vertical shear zones formed weaknesses in the crust facilitating later fault localization, gravitational collapse, and subduction erosion along the continental margin. The length and linearity of this zone of lateral movement, coincident with a general hiatus of regional arc magmatism during the Middle to Late Triassic, is consistent with large-scale dextral transpression, or possible transform movement, during highly oblique NNE–SSW convergence along the pre-Andean (Gondwana) margin. The resultant margin parallel N–S-trending shear planes may be exploited by seismically active faults along the present coastal area of Chile. The palaeo-tectonic setting during the transitional period between earlier Gondwanide (Devonian to Permian) and later Andean (Late Jurassic to present) subduction may have had some similarity to the presently active San Andreas transform system of California.

Invisible gold occurrence within the quartz reef pyrite of Babaikundi area, North Singhbhum fold-and-thrust belt, Eastern Indian Shield: Evidence from petrographic, SEM and EPMA studies

The Mesoproterozoic metasedimentary rocks of Chandil Formation belong to the northern part of Singhbhum Crustal Province, eastern India. The Chandil Formation lies within the North Singhbhum fold-and-thrust belt and is demarcated at its southern margin by the Dalma volcano-sedimentary belt. Tamar-Porapahar Shear Zone (TPSZ) separates the Chandil Formation from the Meso/Neoproterozoic Chhotanagpur Granite Gneissic Complex (CGGC) at its northern boundary. The area is characterized by quartz-mica-sericite-schist, quartzite, amphibolites and carbonaceous phyllites and is ubiquitously traversed by mineralized quartz reefs/veins. The field relationships and the petrographic characteristics of the lithounits of Chandil Formation indicate evidence of multi-phase deformations, exhibited by the presence of S–C fabric, S1–S2 planes of schistosity, alternate layers of coarse and thin quartz veins. The gold mineralization in the study area occurs along the east–west oriented Babaikundi-Birgaon axis, which has been established as a sheared/fractured contact running parallel to the Tamar Porapahar Shear Zone lying to the north. Gravity, magnetic, resistivity and induced polarization (IP) survey across the Babaikundi-Birgaon axis indicates the presence of sheared/fractured contact and sulfide mineralization. The gold mineralization within the area is confined to the quartz reef along the contact of mica schist and ambhibolites. Ore minerals observed in the area are chalcopyrite, pyrite, pyrrhotite, arsenopyrite and sphalerite. The majority of the sulfide mineralization is associated with the ferruginous quartz veins. Gold occurs in two distinct forms, as lodes/nuggets and as invisible gold within the pyrite mineralization.Field observations, reflected and transmitted light microscopic study, SEM analysis and electron probe micro analyzer (EPMA) were used to give a brief account of the mineral chemistry and presence of different forms of gold (including the first report of invisible gold within North Singhbhum fold-and-thrust Belt). The occurrence of gold in the crystal lattice of the pyrite may be attributed to the substitution and replacement of Fe ion within the pyrite aided by the hydrothermal activity.

Gold mineralization in Proterozoic black shales: Example from the Haoyaoerhudong gold deposit, northern margin of the North China Craton

The Haoyaoerhudong gold deposit in the northern margin of the North China Craton (NCC) is a large tonnage lower-grade deposit with a reserve of about 148 tons of gold. Gold mineralization is characterized by pyrite and pyrrhotite films and thin veins on the schistosity plane of the Proterozoic black shales. The orebodies, strictly controlled by a near EW-trending shear zone, are stratabound within carbonaceous phyllite and andalusite-garnet schist of the Bilute Formation. Hydrogen and oxygen isotopic data show that the ore-forming fluid was derived from a magmatic source and mixed with meteoric water. Sulfur and carbon isotope data indicate that most of the sulfur and carbon came from the black shale strata. Well-defined biotite Ar-Ar plateau age and inverse isochron age show that the deposit formed at ca. 270Ma, which suggests a probable link between Hercynian magmatism and gold mineralization. Studies on regional geology, ore geology, isotope geochemistry, and ore-forming age substantiate a complex evolutionary history of the deposit. The Proterozoic black shales rich in gold, sulfur, and organic matter, which were deposited in the Proterozoic continental margin rifts, comprised the source bed for gold mineralization. EW-ENE-structures, products of Palaeozoic orogenic process, provided pathways and mineralization space for ore-forming fluids. Hercynian tectono-magmatism and subsequent hydrothermal events remobilized gold and drove the ore-forming fluids to dilatational fracture zones. Related to postcollisional magmatic hydrothermal events, the Haoyaoerhudong gold deposit is considered as a special type of orogenic gold deposit formed in the compression–extension transition stage.

Cyclical salt efflorescence weathering: an invisible threat to the recovery of underground mine environment for tourist exploitation

Enhancing the value of an underground mine environment for tourist exploitation involves altering the physico-chemical balance of stone materials whose original mechanical properties guaranteed the structural stability of the site’s galleries and chambers. Humidity and temperature changes caused by the public exhibition of this kind of assets are the main causes of such disorders. After the intervention in the Agrupa-Vicenta mine in La Union (Spain) there were still runoff-water leaks into the mine. These water runoffs through the fault and schistosity planes of the enclosing rock mass give rise to salt precipitation over time. Adapting this mine and turning it into a museum have meant a decrease in relative indoors humidity and an increase in temperature. These variations have caused rocks, which were stable in the original conditions, to increase their rate of physico-chemical weathering due to the polycyclic supergene alteration of the metal sulfides they contain. The resulting release of sulfates into the solution and their subsequent precipitation as single and double salt efflorescence causes haloclasty, deteriorating the rock’s mechanical properties and diminishing the structural stability of the operation. This paper presents the results of characterizing the supergene mineral phases of salt efflorescence in the rock bed enclosing an underground sulfide mine value enhanced for tourist exploitation. Dangers for the structural stability of this type of architectural intervention, associated to the formation of efflorescences, are also identified; these efflorescences are caused by the weathering of rocks that make up its supporting structure.

Survival Conditions of Folding in Different Depth During Orogenesis‐Deformation in Texas Creek and Chester Dome, USA

AbstractA succession of 5 FIA trends (foliation intersection or inflection axes in porphyroblasts) preserved in high temperature‐low pressure regime PreCambrian rocks in the Texas Creek, Arkansas River region reflected by the fold axial plane traces and schistosity data in this region. Similar fold axial plane trace data measured in Palaeozoic rocks in Chester Dome, Vermont, which is high temperature to medium pressure regime, only preserve the effects of the youngest FIAs of the all 5 FIA sets that obtained in this region. The other three FIA sets have no equivalent fold axial planes. This difference from shallow to deeper orogenic regimes reflects decreasing competency at greater pressure with collapse and unfolding of earlier formed folds. The greater overlying load of rocks has tended to flatten all but the very largest early‐formed structures, preserving only those folds that were more recently developed.

Shear behavior of inherently anisotropic rocks

The effect of schistosity plane orientations on failure mechanism and effective shear strength response of inherently anisotropic rocks are dealt with in this paper, with an emphasis on shear parameters of the weak planes. The study employed an oblique shear apparatus to apply shear and normal stress on specimens at three predetermined shear inclinations. The concurrent application of confinement and shear loading was made to the apparatus. Both stresses were developed as a result of vertical loading, confined stress conditions and hence their failure mechanisms.The results indicate a transition state of shear strength values, fluctuating from a minimum to a maximum based on β angle, due to the anisotropic effects of schistosity plane orientation coupled with confinement stress. β is the angle between strike of weak planes and the predetermined shear loading plane. Hence, the anisotropic effect of weak planes orientation is a significant occurrence that must be noted in the analysis and failure mechanism studies. It was also observed that different shearing failure modes were built up depending on the orientations of, a family, weak planes with respect to confinement stresses as: (1) tensile fracture across the weak planes (non-sliding mode), (2) tensile-split along the weak planes (non-sliding mode), and (3) sliding failure along the weak planes and/or combination of these mechanisms. The experimental results demonstrated that, the failure modes were changed from non-sliding into sliding along weak planes when dip of weak planes increases. It was also noticed that due to the influence of confinement stresses, failure occurred through rock material together with tensile splitting of weak planes. This was more prominent for higher values of strike direction of weak planes (β). Investigations were extended to examine the influence of effective shear stresses on the failure mechanisms considering various dip and strike directions of weak planes.

The influence of crenulation cleavage development on the bulk elastic and seismic properties of phyllosilicate-rich rocks

The anisotropy of seismic wave propagation is strongly influenced by the mineralogy and microstructure of rocks. Phyllosilicates are elastically highly anisotropic and are therefore thought to be important contributors to seismic anisotropy in the continental crust. Crenulation cleavage is one of the most common microstructural fabrics found in multiply-deformed, phyllosilicate-rich, crustal rocks. We calculated the bulk elastic properties and resulting wave velocities for rock samples that preserved three different stages of crenulation cleavage development: an initial planar foliation, a moderately-developed crenulation cleavage, and a well-developed crenulation cleavage. Mineral orientation maps were obtained using electron backscatter diffraction and calculations were made using asymptotic expansion homogenization combined with the finite element method. The difficulties involved with sample preparation and data acquisition of phyllosilicate-rich rock samples are also discussed. We compare our results to more conventional methods for calculating an aggregate stiffness matrix from a mineral orientation map, namely Voigt and Reuss averages. These averages do not account for grain-scale interactions and therefore deviate from the results calculated using asymptotic expansion homogenization. Our results show that the rocks characterized by a planar foliation and a moderately developed crenulation cleavage are highly anisotropic, with P-wave anisotropies up to 30.9% and S-wave anisotropies up to 34.2%, whereas the rock characterized by a well developed crenulation cleavage is only mildly anisotropic, with a P-wave anisotropy of 15.5% and S-wave anisotropy of 10.7%. Progressive development of the fabric also causes the orientations of P- and S-wave velocity maxima and S-wave polarization directions to change markedly. Despite the high anisotropy imparted by a planar schistosity, the variety of folds and fabrics typically found in phyllosilicate-rich rocks within larger-scale crustal volumes will tend to mute the anisotropy, possibly to the point of appearing nearly isotropic.

Slate flexural and anchorage strength considerations in cladding design

This paper describes an experimental investigation on the flexural and anchorage strength of slate for cladding. The study has been conducted on sawed slate specimens, all showing the same surface finishing. Slate flexural strength was compared for two distinct situations: (i) using a 3-Point flexure loading configuration in batches of materials with larger cross-sectional specimen dimension (50 × 30 mm 2); and (ii) using a 4-Point flexure loading configuration in the same batch of materials but with smaller cross-sectional dimensions (30 × 25 mm 2). The 4-Point bending specimens were tested in three different directions considering slate anisotropy planes. Load was applied along the direction perpendicular to the planes of schistosity; and also along two directions parallel to the planes of schistosity. Slate anchorage strength has been determined on slate slabs with 400 × 200 × 30 mm 3 with dowel anchorage in 8 mm diameter cylindrical holes with 35 mm depth. Test load was applied perpendicularly to the schistosity planes. Cladding stone in building facades and its supporting systems must be compatible with the behaviour and performance of other interfacing systems, such as curtain walls and superstructure frames. In this sense, a properly executed dimensional stone cladding should be designed and installed within the capabilities and limitations of the slate’s support system to resist all active forces or actions. The results of this work reveal the importance of complementary characterization techniques for dimension stone cladding, particularly for anisotropic rocks as slates. From the results it is possible to conclude that schistosity planes have an utter influence on either anchorage or flexural strength.

Geological discontinuities, main flow path and chemical alteration in a marly hill prone to slope instability: Assessment from petrophysical measurements and borehole image analysis

AbstractClay shale landscapes are sensitive to landslide, erosion, and re‐sedimentation processes. In this context, tectonized clay shales have peculiar hydrological and mechanical behaviours. In order to improve our understanding of the processes involved in such systems, an experimental site has been settled in the Draix Observatory (ORE DRAIX) on a black marl hill slope of the Southern Alps (France) in the framework of the ECOU‐PREF project. The site is a natural laboratory to study the evolution of clay shale landscapes at several scales (regional to µm). In this study, we aim to characterize the internal structure and fissure patterns of a stable interfluve, in order to locate preferential water flows within the system, and anticipate its evolution. This work is based on a series of mutli‐disciplinary approaches including petrophysical analyses, well pulse injection tests and downhole geophysical measurements. Borehole geophysical results were combined with laboratory measurements on core plugs (permeability, density, porosity, and acoustic velocity). Optical and acoustical images have been used as an original tool for internal discontinuities characterisation and potential active flow path detection. They allowed the identification of three main lithological units interpreted as several stages of shale alteration. In addition, several main structural plane discontinuities (open fractures, schistosity planes, and fractures infilled with calcite or clays) were identified from the images and their potential transmissivity discussed with regard to piezometric and tracer measurements. In all, the integration of measurements leads to propose a simple scenario of fluid circulation and chemical alteration of the interfluve. Copyright © 2011 John Wiley & Sons, Ltd.

Influence of the geometric characteristics of wedges on the safety of Vrasna tunnel in Egnatia highway, N. Greece

The present paper concerns the influence of the geometric characteristics of the potential wedges on tunnels safety, which are supported by shotcrete and rock bolts, during the excavation of poor and medium quality rock mass, in accordance to RMR classification system. The geological and tectonic data which were used in our estimations were collected in situ during the excavation of Vrasna's tunnel. According to shear test along discontinuities planes, friction angle was considered 21° on schistosity planes and 35° on joint planes. Furthermore, no cohesion was taken into account, as the fractures were, more or less, opened. The orientation and spacing of discontinuities were taken into account for estimating tunnel stability, given that they affect the strength and the quality of the rock mass during the construction. The collected data and the obtained, after elaboration, results were correlated statistically and power regressions were determined

Syngenetic origin for the sediment-hosted disseminated gold deposits in NW Sichuan, China: ore fabric evidence

Sediment-hosted disseminated gold deposits in NW Sichuan China have many features in common with the well-known Carlin-type deposits in the western United States. They are hosted by Middle–Upper Triassic turbidites composed of 1300–4300 m of rhythmically interbedded, slightly metamorphosed calcareous sandstone, siltstone, and slate. The ore bodies are typically layer- or lens-like in shape and generally extend parallel to the stratification of the host sedimentary rocks, with a strike length of tens to several hundreds of meters. The immediate host rocks consist mainly of calcareous slate and siltstone characterized by high contents of organic matter and diagenetic pyrite. The main primary ore minerals associated with gold mineralization include pyrite, arsenopyrite, realgar, and stibnite. Gangue minerals comprise mostly quartz, calcite and dolomite. Gold is extremely fine-grained, usually less than 1 μm, and cannot be seen with an electron microscope. Two types of ore mineralization have been recognized in the deposits. The stratiform ores are composed of rhythmical interbeds of sulfides (e.g., pyrite, arsenopyrite, realgar, stibnite) interpreted to be authigenic and detrital quartz, quartzite, sericite, and graphite of allogenic origin. They were folded and deformed concordantly with host rocks, and grade both vertically and laterally into normal country rocks. Another type of ore forms a network of numerous gold-bearing veins and veinlets of quartz–calcite–sulfides of millimeter-, centimeter-, decimeter-, and even meter-scale in width. The network ore randomly fills fissures, microfissures, and cleavages, but still is stratabound in character. Detailed studies on ore fabrics show abundant evidence for synsedimentary origins, although subsequent diagenesis, metamorphism, tectonic deformation, and epigenetic hydrothermal activity have significantly remolded the primary fabrics. Primary fabrics are shown either by rhythmical interbeds of different mineral components parallel to the bedding, or by the change of grain size of the same minerals such as pyrite, realgar, and stibnite. The layer inhomogeneity of the stratiform ore is clarified by parallel overprints of later schistosity planes, resulting in distinct grain orientation and elongation, aggregate polarization, and undulating extinction of ore minerals, especially of mechanically and chemically extremely mobile ones, such as realgar and stibnite. It is proposed that the stratiform ores in these Chinese deposits were most probably formed concurrently with their host Middle–Upper Triassic turbidites in submarine, hot spring environments, while the network mineralization was formed as a result of complicated processes such as diagenesis, weak metamorphism, tectonic deformation, and epigenetic hydrothermal activity, responsible for the remobilization or reworking of the pre-existing stratiform ores. Geochemical data also support this genetic model.

Preliminary investigation results on fabrics and related physical properties of an anisotropic gneiss

In order to evaluate the fabric-dependent anisotropy of a particular gneiss type, we assessed the quartz lattice and shape preferred orientations as well as the microcrack pattern statistics. In the rock mechanics laboratory, several strength and deformability tests on drilled rock samples were run and the seismic wave propagation properties along the rock’s principal strain axes were determined. In the mechanical tests it turned out that despite a distinct stretching lineation with initially extreme grain elongation, the rock performs for the larger part as a transversely isotropic material with the schistosity as plane of isotropy. The anisotropy in seismic wave propagation within the plane of schistosity is attributed primarily to a highly anisotropic microcrack pattern, the distinct gneissose banding and to a lower degree to the LPO of quartz.

Displacement transfer and forced folding in the Maritimes basin of Nova Scotia, eastern Canada

Abstract Field and seismic data in northern Nova Scotia, eastern Canada, demonstrate that displacement transfer from steep basement faulting to bedding-parallel detachment is necessary in the development of forced folds. Lateral translation of the strata above horsted and down-dropped blocks generates a monoclinal structure which, as the faults are kinematically linked, evolves in a manner similar to fault-bend folds in thrust-and-fold belts. In the case of partial transfer of displacement a breached drape syncline is developed. The breached syncline is characterized by steep upturned beds against the fault that truncates them. In the Nova Scotia example the detachment horizon is located at the base of a Visean evaporitic sequence, and is exposed in the study area showing shearing structures within the evaporites. Brittle fault rock types (gouge and cataclasite) and meso- to microstructures were formed, including stretching lineation, principal schistosity plane and secondary shear planes, as well as intrafolial to upright asymmetrical folds. The regionally extensive weak evaporitic layer was remarkably effective in transferring displacement between the two faults, with mechanical decoupling of the strata above the evaporitic detachment being observed in the horsted block 70 km away from the steep basement fault. Moreover, displacement was also transferred as much as 40 km onto the down-dropped block at the frontal part of the system.

Measurements of the relationship between Sonic wave velocities and tensile strength in Anisotropic rock

Abstract This work investigates relationships between the mechanical behaviour under tension of a building stone widely used in Italy and the compressional and shear-wave velocities within the stone. The stone investigated is the Antigorio orthogneiss, which is characterized by an orthotropic petrographical anisotropy. This rock was sampled in three orthogonal directions two of which lie in the plane of schistosity, the first parallel and the second perpendicular to the elongated structures in that plane, with the third sampling direction oriented normal to the plane of schistosity. In these directions rock cylinders were cored co-axially to provide hollow cylinders of 14cm outer diameter together with their inner solid cores of approximatly 5 cm diameter. These inner cores were used for ultrasonic wave velocity measurements ( V p and polarized V 5 ) oriented at 90° to the core axis, and conducted at intervals around their circumference so as to obtain wave velocities through the gneiss in different orientations. The hollow cylinders were cut into hoops and tested by the hoop tension test to provide measurements of tensile strength in the same directions as those in which wave velocities had been measured. Thus nondestructive velocity tests and destructive tensile tests were conducted in the same orientations. The paper describes the results so obtained using dry samples.

Radar experiments in isotropic and anisotropic geological formations (granite and schists)1

AbstractIn order to understand various aspects of radar wave propagation, a survey of electromagnetic wave behaviour relative to the geological characteristics of the formations prospected was undertaken. The sites chosen for the tests were a granite quarry and an underground schist working. By investigating an electrically resistive isotropic site and a conductive anisotropic site, it was demonstrated that non‐conventional use of a radar system (antennae raised, various orientations of the transmitter/receiver, etc.) could improve data quality, and could allow information other than reflector depth to be collected (volume scattering intensity, isotropy, etc.). By studying wave propagation velocities, we underlined the difficulties encountered in establishing a velocity versus depth law, despite recourse to seismic data processing, such as NMO corrections. The results of field experiments, complemented by laboratory measurements of dielectric permittivities, clearly showed anisotropy effects: in the case of a path that is perpendicular to the schistosity plane, an electromagnetic wave propagates more slowly and is more attenuated than a wave parallel to the schistosity plane.

Anisotropic strength properties of a quartz schist saprolite

Strength anisotropy in a quartz schist saprolite is examined via shear tests in which the inclinations of the planes of schistosity to the compression axis were varied. Results obtained are anomalous when compared with the general trend of results obtained for other schistose formations. The cementation of the soil particles along the planes of schistosity by the goethite present in the saprolite is deemed responsible for the anomaly.

Archean regional transpression and paleomagnetism in northwestern Ontario, Canada

The Archean metamorphic rocks of the Superior province of the Canadian Shield occur in lithologically defined belts or subprovinces. The tectonically more stable interiors of belts possess consistent primary components of magnetic remanence. In the case of the Quetico belt, these stable directions are tightly grouped about 005°/55° with some minor dispersion and most were acquired during the cooling that followed syntectonic recrystallisation. This study examines the directions of primary remanence components for rocks along the margins of the Quetico belt, within 4 km of the strongly deformed vertical, ENE-trending boundaries. The boundaries are known to have experienced dextral transpression involving penetrative single-phase deformation which out-lasted metamorphism. Within a few kilometres of the belt boundaries, the primary remanence components are re-distributed along a vertical ENE-trending, great-circle girdle which is nearly parallel to the plane of transpressive shear and regional schistosity. It is suggested that the effects of transpression have mechanically deflected the components of primary remanence toward this plane.

Metamorphism and uplift of Alpine schist in the Franz Josef–Fox Glacier area of the Southern Alps, New Zealand

Geothermometry and geobarometry of 10 garnet–oligoclase zone schists in the Franz Josef–Fox Glacier area, Southern Alps, New Zealand, give temperatures ranging from 415 to 625°C and pressures from 5.2 to 9.2 kbar, indicating a T–P array of about 50°C/kbar and inferred peak temperature conditions over a c. 15‐km‐thick section at depths between c. 20 and 34 km. The present‐day distribution of the schist samples implies that only about one‐third of the original crustal section is now exposed.The garnet–oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late Palaeozoic–early Mesozoic subduction along the Gondwana continental margin. Partial uplift (c. 0.2 m/Ma) and erosion of the complex during Jurassic–Cretaceous times (Rangitata uplift) was synchronous with D2 deformation and recrystallization, as recorded by the P–T array. Cenozoic (Kaikoura) uplift and exhumation of the schist since c. 30 Ma to form the Southern Alps was associated with oblique‐slip movement on the Alpine Fault. The present‐day position and steep eastward dip of isograds and D2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two‐thirds of the original schist sequence, largely due to simple shear along schistosity planes. As the schist generally shows only incipient greenschist facies retrograde recrystallization, an apparently complete (although contracted) prograde mineral sequence has been preserved by rapid uplift (>5 km/Ma) of hot rock and the effects of limited shear heating near the Alpine Fault.

Strain analysis method using the maximum frequency of unimodal deformed orientation distributions: applications to gneissic rocks : J Struct Geol V13, N3, 1991, P329–335

Abstract The effect of varying strain ratio ( R a ) on initially uniform and non-uniform orientation distributions of passive marker lines is calculated. The frequency graphs for uniform distributions have a unimodal form ( Gaussian-like distribution ), centered in the finite extension direction ( X ) of the strain ellipse. The maximum frequency is related to the strain. Using non-uniform distributions the frequency graph has a different unimodal pattern. The difference between the measured strain ratio ( R ) with the maximum frequency and the true strain ratio ( R a ) to the initial distribution is quantified for different types of distribution. The method is applied to samples of gneiss from southern Spain, using the orientation of tourmaline and feldspar lying in the schistosity plane, with respect to the stretching lineation. The strain of the marker distribution has been quantified, and after distinguishing the initial type of distribution, the true strain ratio of the whole rock in the schistosity plane can be estimated.