High-strain Shear Zones Research Articles

Structure-controlled mineralization resulted from Paleoproterozoic orogenesis in the Dahenglu Cu-Co deposit, NE China

Paleoproterozoic orogenic-related (or orogenic-like) ore deposits play a significant role in the metal and non-metal metallogenic systems in the North China Craton. This study focuses on detailed field structural of the Dahenglu Cu-Co deposit to reveal relationship between structural controls and enrichment process of cobalt (Co)-bearing hydrothermal fluids in the metasedimentary rock. In the context of Paleoproterozoic orogenesis, at least three phases of deformation were identified within the ore-bearing country rocks: an early DPl-1, the peak-orogenic deformation phase called DPl-2 and a final exhumation stage of deformation designated as DPl-3. Through meticulous structural analysis, microscopic examination of drilling cores, and the application of the TESCAN Integrated Mineral Analyzer, we have discerned that thrust faults, operating at both outcrop-scale and microscale, function as substantial conduits for the migration of ore-forming hydrothermal fluids. Additionally, fractures within thrust faults, the core of folds, and the high strain micro-scale shear zone between overturned folds provided ideal spaces for the precipitate of Co-bearing hydrothermal fluid. Based on the regional geological correlations, the second phase of deformation (DPl-2) is characterized by S2 foliation, crenulation cleavage, and thrust fault, which played a crucial role in the enrichment of cobalt in minerals such as siegenite and pyrite. This cobalt enrichment is attributed to the peak-orogenic stage, DPl-2, which likely occurred between 1.95 and 1.90 Ga. The ore-forming geodynamic was associated with Paleoproterozoic orogenesis, characterized by intense deformation and metamorphism. This conclusion aligns with cobalt enrichment observed in the Paleoproterozoic Trans North China orogen in the central North China Craton and the Paleoproterozoic Peräpohja belt in northern Finland. The structures generated by significant tectonic events, such as Paleoproterozoic orogenesis, represent key controlling factors of Co-rich ore deposits.

Insights into the metamorphic history and origin of flake graphite mineralization at the Graphite Creek graphite deposit, Seward Peninsula, Alaska, USA

Graphite Creek is an unusual flake graphite deposit located on the Seward Peninsula, Alaska, USA. We present field observations, uranium-lead (U–Pb) monazite and titanite geochronology, carbon (C) and sulfur (S) stable isotope geochemistry, and graphite Raman spectroscopy data from this deposit that support a new model of flake graphite ore genesis in high-grade metamorphic environments. The Graphite Creek deposit is within the second sillimanite metamorphic zone of the Kigluaik Mountains gneiss dome. Flake graphite, hosted in sillimanite-gneiss and quartz-biotite paragneiss, occurs as disseminations and in sets of very high grade (up to 50 wt.% graphite), semi-massive to massive graphite lenses 0.2 to 1 m wide containing quartz, sillimanite, inclusions of garnet porphyroblasts, K-feldspar, and tourmaline. Restitic garnet, sillimanite, graphite, and biotite accumulations indicate a high degree of anatexis and melt loss. Strong yttrium depletion in monazite, high europium ratios (Eu/Eu*), and excursions of high strontium and thorium concentrations are consistent with biotite dehydration melting. Monazite and titanite U–Pb ages record peak metamorphism from ~ 97 to 92 million years ago (Ma) and a retrograde event at ~ 85 Ma. Raman spectroscopy confirms the presence of carbonaceous material and highly ordered, crystalline graphite. Graphite δ13CVPDB values of − 30 to − 12‰ and pyrrhotite δ34SVCDT values of − 14 to 10‰ are consistent with derivation from organic carbon and sulfur in sedimentary rocks, respectively. These data collectively suggest that formation of massive graphite lenses occurred approximately synchronously with high-temperature metamorphism and anatexis of a highly carbonaceous pelitic protolith. Melt extraction and fluid release associated with anatexis were likely crucial for concentrating graphite. High-temperature, graphitic migmatite sequences within high-strain shear zones may be favorable for the occurrence of high-grade flake graphite deposits.

Formation of mixed paragenesis diamonds during multistage growth – Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds

Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% − an unusually high proportion − contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth.Based on the δ13C–δ15N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; δ13C = –33.2 to −14.4‰ and δ15N = −5.3 to +10.1‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; δ13C = −6.0 to −1.1‰ and δ15N = −4.2 to +9.7‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; δ13C = −7.8 to −3.6‰ and δ15N = −7.9 to −2.1‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.

Amorphous materials and clay mineral formation in the coseismic gouge from a surface rupture of the Ms 8.0 Wenchuan earthquake

Microstructures from a fault gouge formed during the Ms. 8.0 Wenchuan earthquake in China were investigated in detail. In samples collected near the highest vertical surface offset, two types of amorphous materials were found in the high-strain shear zone: 1) material formed by creeping during the inter-seismic period, which is characterized by ground sandstone clasts; and 2) coseismic material, characterized by the comminution of the sandstone clasts of the former type. Interaction of amorphous material with fluids during creeping resulted in Fe-rich nano-sized grains in the pores, Mg- and Al-rich stratified zones near the pores, amorphous silica next to the stratified zones, as well as new clay minerals in the stratified zones. Feldspar and phyllosilicates were transformed into clay minerals and amorphous silica. We propose that amorphous materials and clay minerals contributed to the dynamic weakening of the Yingxiu–Beichuan fault.

U-Pb Isotope Geochronology of Syntectonic Granites from Hainan Island, South China: Constraints on Tectonic Evolution of the Eastern Paleo-Tethys Ocean

Since the recognition of the Indosinian orogeny in northern Vietnam, Triassic orogens have been widely identified around the western and southwestern boundaries of the South China block. The paleo-Tethys sutures stretch from west to southeast, from Jinshajiang-Ailaoshan via NE Vietnam to Hainan Island; these sutures exhibit and develop voluminous Permian-early Triassic magmatism and numerous high-strain shear zones. As ophiolites related to the paleo-Tethys are lacking on Hainan Island, the eastward extension of the Indosinian orogeny and subduction and closure time of the paleo-Tethys Ocean on Hainan Island remain controversial. Here, an integrated kinematic and geochronological study has been conducted on two shear zones, called the Xiaomei and Mangsan shear zones. U-Pb zircon dating yields an age of early Triassic (252–251 Myr) for Xiaomei syntectonic granites which formed in the same tectonic setting and presented the similar nanoparticles to the Indosinian granites from Ailaoshan ductile shear zone, including the strawberry-like and flower-like nanoparticles. The NE-trending Mangsan shear zone represented by the gneissic granites with middle Permian ages (264–262 Myr) formed in the same tectonic setting as the Wuzhi-shan granites that were proposed as I-type granites. These middle Permian gneissic granites with arc affinity may represent an arc setting related to the NW subduction of the paleo-Tethys. The analyses reveal that granites with late Triassic ages (235–232 Myr) in the Xiaomei shear zone have the characteristics of A-type granites. The late Triassic extensional events on Hainan Island may be related to the subduction of the paleo-Pacific Plate beneath the East Asian continent.

The role of deformation-reaction interactions to localize strain in polymineralic rocks: Insights from experimentally deformed plagioclase-pyroxene assemblages

In order to study the mutual effect of deformation and mineral reactions, we have conducted shear experiments on fine-grained plagioclase-pyroxene assemblages in a Griggs-type solid-medium deformation apparatus. Experiments were performed at a constant shear strain rate of 10−5 s−1, a confining pressure of 1 GPa and temperatures of 800, 850 and 900 °C. While the peak stress of plagioclase + orthopyroxene assemblages reaches values between those of the end-member phases, the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase (plagioclase). This weakening correlates with the coeval development of high-strain shear zones where new phases are preferentially produced, including new pyroxene, plagioclase and amphibole. The reaction products mostly occur as intimately mixed phases within fine-grained and interconnected shear bands, together with different compositions with respect to the starting material. This indicates that deformation significantly enhances the kinetics of mineral reactions, which in turn strongly weaken the deforming sample, here attributed to a switch to grain-size-sensitive diffusion creep through phase nucleation and grain size reduction. Such an interplay between deformation and mineral reactions may have strong implications for the initiation, development, and durability of shear zones in the lower crust.

The Sperrgebiet Domain, Aurus Mountains, SW Namibia: A ∼2020–850 Ma window within the Pan-African Gariep Orogen

The Aurus Mountains in SW Namibia form a tectonic inlier (Sperrgebiet Domain) within the Neoproterozoic Gariep belt. We document the geological history from field mapping, whole-rock geochemical, metamorphic and U-Pb zircon isotope data. The calc-alkaline granodioritic orthogneisses of the Wasserkuppe Suite, dated at ∼2020Ma, are the predominant, oldest rocks, which are interpreted to form part of a ∼2Ga “Sperrgebiet arc”; the tectono-magmatic equivalent of the ∼250km wide Richtersveld arc to the southeast, but ∼100Ma older. ∼2Ga detritus is common throughout the western Namaqua belt, but its source was hitherto unknown. The suite intruded synchronously with amphibolite facies metamorphism and deformation, considered an early phase of the Palaeoproterozoic Orange River Orogeny. The Wasserkuppe Suite is intruded by alkali granite and microgranodiorite dated at ∼1885Ma, interpreted as equivalents of the Vioolsdrif Suite (Richtersveld arc). The Sperrgebiet Domain is thus seen as an early phase of a process of Palaeoproterozoic island-arc development at the margins of the Archaean Kaapvaal Craton/Rehoboth Province. It was cannibalised by the later Richtersveld arc which dominated the palaeogeography of this part of southern Africa 100m.y. later, and which forms the crustal infrastructure of much of Western Namaqualand.The Palaeoproterozoic arc rocks are overlain by a clastic metasedimentary sequence; the Aurus Schist. Detrital zircon geochronology indicates that these rocks were predominantly derived from both arcs, with minor Archean and younger ∼1800Ma detritus, of unknown source(s). Bodies of latest Mesoproterozoic (∼1000Ma) Warmbad Granite and pegmatite are mapped throughout the area and represent the only Namaquan rocks definitively identified. Evidence for Namaquan metamorphic or tectonic imprints are lacking, apart from a garnet-clinopyroxene metabasic inclusion in a quartz-feldspar leucogneiss which gave a typical “Namaquan” P-T estimate of ∼700–750°C at 6–7kbar. The leucogneisses, distributed throughout the area, were dated at ∼840Ma and are interpreted as metamorphosed felsic igneous rocks of the Richtersveld Suite, formed during early rifting of the proto-Gariep basin. The Sperrgebiet rocks are variably deformed by the Pan-African Gariep Orogeny that produced regionally-pervasive fabrics, predominantly partitioned into discrete high-strain shear zones in a NW-trending sinistral transpressive tectonic regime.

Seismic anisotropy in mid to lower orogenic crust: Insights from laboratory measurements of Vp and Vs in drill core from central Scandinavian Caledonides

Recent drilling of the first Collisional Orogeny in the Scandinavian Caledonides scientific borehole (COSC-1) near Åre, Sweden permitted a laboratory investigation of seismic anisotropy on high metamorphic grade and highly deformed core samples. The 2.5km deep borehole crosscuts the amphibolite-grade Lower Seve Nappe and intersects a high-strain shear zone in the lowermost 800m. Measurements of ultrasonic compressional (Vp) and shear (Vs) velocities are conducted at room temperature and pressures ranging from room conditions up to 260MPa on six core sections that represent the most abundant lithologies in the borehole. The core sections consist of two amphibolites, a calc-silicate gneiss, a felsic gneiss, an amphibole-rich gneiss, and a garnet-bearing micaschist from the shear zone. Three mutually perpendicular samples were taken to characterize the anisotropy induced by the clear foliation and lineation. The intrinsic (crack-free velocities) Vp0 and Vs0 in the direction perpendicular to foliation ranges from 5.51 to 6.67km/s and 3.31 to 4.13km/s, respectively. In the direction parallel to foliation the Vp0 and Vs0 ranges from 6.31 to 7.25km/s and 3.53 to 4.35km/s, respectively. Vp anisotropy ranges from 3% in the calc-silicate gneiss to 19% in the micaschist. In the upper crustal seismic reflection survey around the COSC-1 borehole, reflection coefficient analysis suggests that reflectors occur due to impedance contrast between commonly occurring amphibolites and gneisses in the upper 1800m and the micaschists below. When extrapolated to mid-crustal levels the analysis indicates that both rock types can produce reflection coefficients between adjacent lithologies in excess of 0.1. Similarities in lithologies, shear zone thickness, and reflectivity pattern in the Central Scandinavian Caledonides compared to other orogens (e.g., the Himalaya) demonstrate the importance of these measurements as a proxy for in-situ strongly anisotropic shear zones in the middle crust.

Effect of strain-weakening on Oligocene–Miocene self-organization of the Australian-Pacific plate boundary fault in southern New Zealand: Insights from numerical modelling

Tectonic inheritance acquired from past geological events can control the formation of new plate boundaries. The aim of this paper is to explore the role of inherited NE and NW trending fabrics and their rheological influence on the propagation of Oligocene–Miocene strike-slip faulting that matured to become the Australian-Pacific plate boundary fault in southern New Zealand. Strain weakening plays a significant role in controlling the formation, growth and evolution of strain localization. In this study, three-dimensional thermo-mechanical models have been used to explore the effect of strain weakening on the Oligocene–Miocene self-organization of strain localization. Strain weakening is simulated through decreasing either the coefficient of friction of upper crust, its cohesion, or the rheological viscosity contrast between the inherited structures and their surrounding wall rocks. Viscosity contrast is obtained by varying the viscosity of inherited structures. Softening coefficient (α) is a measure of strain weakening. Our experiments robustly demonstrate that a primary boundary shear zone becomes mature quicker when softening coefficients are increased. Deformation is focused along narrow high-strain shear zones in the centre of the model when the softening coefficients are high, whereas the strain is more diffuse with many shear zones spread over the model and possibly some high-strain shear zones focused near one border at lower softening coefficients. Varying the viscosity contrast has less effect on the distribution of maximum finite strain.Under simple-shear boundary conditions, NW trending inherited structures make a major contribution to forming early zones of highly focused strain, up to a shear strain of about γ=3.7. During this process, most NE-trending structures move and rotate passively, accommodate less strain, or even be abandoned through time.

Magnetic fabrics and Pan-African structural evolution in the Najd Fault corridor in the Eastern Desert of Egypt

In order to assess the Pan-African structural evolution from early orogenic fabrics through Najd wrenching to the latest orogenic collapse/extension, the authors used field work, aided by aerial photographs and satellite images. This work is complemented by the study of the anisotropy of the magnetic susceptibility (AMS, or magnetic fabric). The Pan-African rock associations of the Um Gheig-Kadabora area can be divided into a lower tier composed mainly of amphibolite–migmatite and granitoid gneisses, and an upper tier of ophiolitic rocks, metavolcanics and their related volcaniclastics, and molasse-type Hammamat sediments. Both these units are intruded by late orogenic granitoid plutons and dykes. The lower tier is exposed in a domal structure in the El Sibai area, the upper tier forms a series of weakly to highly deformed thrust units, called Pan-African Nappes here, which are dissected by high strain shear zones. According to their age, these rock units are divided here into early and late-orogenic. The early orogenic rock association is characterized by medium–high metamorphic grades. The late orogenic rock association is characterized by low metamorphic grade. The rocks in the upper tier form a series of low angle thrust sheets, which are bounded by NW-striking high angle shear zones related to the Najd Fault System. The early orogenic rocks show a polyphase structural evolution with early folds, thrusts, and strike-slip shear zones. The late orogenic rocks show a relatively weaker deformation. The latest intrusives studied here are the dykes dissecting the late orogenic Kadabora granite. In the present work magnetic fabric data document the deformational features in detail and assess the role of the Najd Fault System in the deformational evolution. A strong variation in volume susceptibility of various rocks, due to their variations in mineral composition, is observed. Lower values are in the range of 10−6 SI units for late-orogenic alkaline granite and the dykes dissecting it, the highest susceptibilities exceed 7×10−2 SI units in magnetite-bearing serpentinite. Early orogenic rocks are characterized by relatively high anisotropies (P′ up to 1.7) and are deformed in numerous shear zones. Most of these shear zones can be related to the Najd Fault System. In contrast, late orogenic sediments and intrusives show mostly low anisotropies. However, magnetic lineations are still distinctly oriented parallel with the Najd Fault trend. The very latest Pan-African intrusives, the broadly N–S trending dykes crosscutting the Kadabora pluton, imply c. E–W directed extension. Such an extension is consistent with the magnetic fabric in some of the dykes. Therefore, the Kadabora dykes mark the end of Najd wrenching and a late stage of extension in this part of the Eastern Desert of Egypt. The other dykes display mostly primary fabrics, related to magma flow during their intrusion and are thus post-deformational with regard to the Pan-African orogeny.

Timing and conditions of poly-phase metamorphism within the Twelve Mile Bay shear zone: implications for the evolution of mid-crustal decollement zones and western Grenville tectonics

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (˜10–11 kbar and ˜840°C) metamorphism overprinted by amphibolite facies (˜5–7 kbar and ˜650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and 207Pb/206Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with 207Pb/206Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.

Exhumation of (ultra-)high-pressure terranes: concepts and mechanisms

Abstract. The formation and exhumation of high and ultra-high-pressure, (U)HP, rocks of crustal origin appears to be ubiquitous during Phanerozoic plate subduction and continental collision events. Exhumation of (U)HP material has been shown in some orogens to have occurred only once, during a single short-lived event; in other cases exhumation appears to have occurred multiple discrete times or during a single, long-lived, protracted event. It is becoming increasingly clear that no single exhumation mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. Subduction zone style and internal force balance change in both time and space, responding to changes in width, steepness, composition of subducting material and velocity of subduction. In order for continental crust, which is relatively buoyant compared to the mantle even when metamorphosed to (U)HP assemblages, to be subducted to (U)HP conditions, it must remain attached to a stronger and denser substrate. Buoyancy and external tectonic forces drive exhumation, although the changing spatial and temporal dominance of different driving forces still remains unclear. Exhumation may involve whole-scale detachment of the terrane from the subducting slab followed by exhumation within a subduction channel (perhaps during continued subduction) or a reversal in motion of the entire plate (eduction) following the removal of a lower part of the subducting slab. Weakening mechanisms that may be responsible for the detachment of deeply subducted crust from its stronger, denser substrate include strain weakening, hydration, melting, grain size reduction and the development of foliation. These may act locally to form narrow high-strain shear zones separating stronger, less-strained crust or may act on the bulk of the subducted material, allowing whole-scale flow. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Future research directions include identifying temporal and spatial changes in exhumation mechanisms within different tectonic environments, and determining the factors that influence those changes.

Depressurized Cavities within High-strain Shear Zones: their Role in the Segregation and Flow of SiO2-rich Melt in Feldspar-dominated Rocks

We observe void growth and coalescence into cavity-bearing shear bands during deformation of wet synthetic anorthite aggregates containing53 vol. % silica-enriched melt. Samples were deformed in the Newtonian creep regime to high strain during torsion experiments at 11008C and 400MPa confining pressure. Localized cavity-bearing shear bands show an S^C’-geometry: the bands (C’) are oriented at about 308 to the compression direction of the imposed simple shear and the internal foliation (S) of the bands is rotated towards the horizontal external shear plane. Cavity-bearing shear bands started to nucleate in the sample periphery above a shear strain threshold of 2. Quartz crystallized from the water-saturated SiO2-rich melt within large cavities inside these bands, which requires that the melt is decompressed by4200MPa during their formation. The dynamically evolving cavities are sites of locally reduced pressure that collect the melt distributed in the adjacent matrix.Therefore, cavitation damage under ductile conditions may result in the development of an efficient melt channelling system controlling SiO2-rich melt flow in the lower crust. Electron backscatter diffraction analysis shows that the quartz inside the cavity bands has a crystallographic preferred orientation (CPO). The development of the CPO is explained by the preferred dissolution of crystals oriented with the rhombohedra and trigonal dipyramids orthogonal to the compression direction and by preferential growth of crystals aligned with their500014axis in the extension direction of the externally applied simple shear deformation.

The Murchison Greenstone Belt, South Africa: Accreted slivers with contrasting metamorphic conditions

This paper presents new petrological and geochronological data for the ∼3.09–2.92Ga Murchison Greenstone Belt (MGB), located in South Africa's Kaapvaal Craton, and discusses their geotectonic implications. The MGB is made of three tectono-metamorphic units: the Silwana Amphibolites, the Murchison Unit and the La France Formation. They underwent contrasting clockwise pressure–temperature–deformation (P–T–D) histories, and are separated from each other by relatively narrow, high-strain shear zones, with a sinistral, transpressive top-to-the-south movement, consistent with the deformation patterns observed throughout the belt. These patterns are explained by a N–S compressional stress field, affecting the Murchison Belt between 2.97 and 2.92Ga. Results of new petrological investigations indicate that ultramafic to felsic volcano-sedimentary rocks of the Murchison Unit underwent a greenschist- to lower-amphibolite-facies metamorphism at maximum P–T conditions of 5.6±0.6kbar at 570°C, along a relatively hot, minimum apparent geotherm of ∼30°C/km. In contrast, the Silwana Amphibolites and the La France Formation were metamorphosed at much higher peak metamorphic conditions of 8.7–10kbar, 630–670°C, and 8–9kbar, 600–650°C, respectively, and require a colder apparent geotherm of ∼20°C/km. A retrograde, nearly isothermal–decompression P–T path followed by isobaric cooling is also inferred for the La France Formation. The timing of the structural–metamorphic overprint is bracketed between 2.97 and 2.90Ga, which is constrained by U–Pb zircon ages of a syn-deformation granite within the Murchison Unit and the post-deformation Maranda granite, respectively. Monazite and xenotime from La France metapelites yield much younger ages of ca. 2.75Ga, with few inherited components at 2.92 Ga. They point to a later activation of the MGB, perhaps related with tectono-thermal events in the Rooiwater Complex and the Pietersburg Greenstone Belt. The relatively cold apparent geotherms recorded in the Silwana and La France rocks, the contrasted peak P–T conditions between the different units, and the near isothermal decompression of the La France Formation indicate that the Kaapvaal craton crust must have been cold enough to enable significant crustal thickening and strain localisation along narrow shear zones and, as a consequence, fast tectonic juxtaposition of rocks metamorphosed at different crustal depths. These features are similar to those observed along Palaeozoic or modern day, oblique subduction–collision zones, but different to those of hot Archaean provinces. We therefore interpret the MGB as representing part of an oblique collision-zone between two terrains of the Kaapvaal craton: the Witwatersrand and Pietersburg terrains.

Modelling of deformation around magmatic intrusions with application to gold-related structures in the Yilgarn Craton, Western Australia

This study simulates rock deformation around high temperature granite intrusions and explores how gold bearing shear zones near intrusions were developed in the Yilgarn, using a new continuum damage mechanics algorithm that considers the temperature and time dependent elastic-visco-plastic constitutive behaviour of crustal materials. The results demonstrate that strain rates have the most significant effects on structural patterns for both extensional and compressional cases. Smaller strain rates promote the formation of narrow high-strain shear zones and strong strain localisation along the flank or shoulder areas of the intrusion and cold granite dome. Wider diffuse shear zones are developed under higher strain rates due to strain hardening. The cooling of the intrusion to background temperatures occurred over a much shorter time interval when compared to the duration of deformation and shear zones development. Strong strain localisation near the intrusion and shear zone development in the crust occurred under both extensional and compressional conditions. There is always clear strain localisation around the shoulders of the intrusion and the flanks of the “cold” granitic dome in early deformation stages. In the models containing a pre-existing fault, strain localisation near the intrusion became asymmetric with much stronger localisation and the development of a damage zone at the shoulder adjacent to the reactivated fault. At higher deformation stages, the models produced a range of structural patterns including graben and half graben basin (extension), “pop-up” wedge structures (compression), tilted fault blocks and switch of shear movement from reverse to normal on shear zones. The model explains in part why a number of gold deposits (e.g. Wallaby and Paddington deposits) in the Yilgarn were formed near the flank of granite-cored domes and deep “tapping” faults, and shows that the new modelling approach is capable of realistically simulating high strain localisation and shear zone development.

Triassic high-strain shear zones in Hainan Island (South China) and their implications on the amalgamation of the Indochina and South China Blocks: Kinematic and 40Ar/ 39Ar geochronological constraints

A kinematic and geochronological study has been carried out on the Triassic high-strain shear zones in Hainan Island, the southern South China Block. There are WNW- and NE-trending high-strain shear zones with greenschist- to amphibolite-facies metamorphism in this island. Kinematic indicators suggest a dextral top-to-the-NNE thrust shearing for the WNW-trending high-strain shear zones and a sinistral top-to-the-SE thrust shearing for the NE-trending shear zones. The quartz c-axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system. The timing of shearing for these shear zones has been constrained by the 40Ar/ 39Ar dating analyses of synkinematic minerals. Middle Triassic (242–250 Ma) and late Triassic–early Jurassic (190–230 Ma) have been identified for the WNW- and NE-trending shear zones, respectively. A synthesis of these kinematic and thermogeochronological data points to a two-stage tectonic model for Hainan Island, that is, top-to-the-NNE oblique thrusting at 240–250 Ma followed by top-to-the-SE oblique thrusting at 190–230 Ma. In combination with the available data from the southern South China and Indochina Blocks, it is inferred that South Hainan and North Hainan have affinity to the Indochina and South China Blocks, respectively. The tectonic boundary between South Hainan and North Hainan lies roughly along the WNW-trending Changjiang–Qionghai tectonic zone probably linking to the Song Ma and Ailaoshan zones. The middle Triassic structural pattern of Hainan Island is spatially and temporally compatible with those of the South China and Indochina Blocks, and thus might be a derivation from the amalgamation of the Indochina with South China Blocks in response to the closure of the Paleotethys Ocean and subsequent subduction/collision.

Low-angle normal faults in the compressional Himalayan orogen; Evidence from the Annapurna–Dhaulagiri Himalaya, Nepal

The Annapurna Detachment (AD) is a low-angle (∼20°–30° dip), north-dipping normal fault and ductile high-strain shear zone in calc-mylonites, and forms part of the South Tibetan Detachment (STD) that runs along 1800 km length of the Himalaya. The AD separates kyanite and sillimanite grade gneisses and tremolite + clinopyroxene ± hornblende-bearing marble–calc-silicates of the Greater Himalayan Sequence (GHS) below from unmetamorphosed Palaeozoic–Mesozoic sedimentary rocks of the Tethyan sedimentary zone above. It was active at ca. 22–18 Ma during south-vergent ductile extrusion (channel flow) and exhumation of the Himalayan mid-crust footwall. Restoration of the STD system suggests around 80–100 km of southward extrusion of the footwall gneisses relative to the Tethyan hanging-wall rocks. Folds in the hanging wall of the AD were formed prior to normal faulting, but axial planes are curved into alignment with the shear zone suggesting extrusion of the metamorphic footwall rocks beneath a passive roof fault. North-vergent recumbent backfolds in the Nilgiri–Tukuche peaks were enhanced by backsliding during footwall extrusion, although this does not indicate “orogenic collapse,” lowering of surface elevation, or decreasing crustal thickness because new material was continually being underthrust from the south. Axial planes of backfolds are curved and progressively rotate from subvertical in the north to subhorizontal immediately above the AD. Low-angle normal faults in the Himalaya were active during the Early Miocene, concomitantly with thrusting at deeper structural levels along the Main Central Thrust (MCT) zone. The passive normal faults and ductile shear zone were initiated at low angles aided by partial melting and ductile flow within the GHS. They do not indicate alternating periods of extension and compression but were active in a wholly compressional environment. The Channel Flow model for the Greater Himalaya and the passive roof fault model for the low-angle normal faults adequately explain all geological field structural and metamorphic criteria.

Can the size of K-feldspar phenocrysts be used to map the occurrence of mylonitic shear zones? Results from the Wyangala Granite, Eastern Lachlan Fold Belt, New South Wales

We report the results of mapping the high-strain (shear) zones in the Wyangala Granite. The high-strain zones are part of the Wyangala Fault System and primarily occur along the eastern margin of the Wyangala Granite, Eastern Lachlan Fold Belt in New South Wales. The aim of this study was to determine if the size of K-feldspar phenocrysts could be used to map variations in strain intensity in the Wyangala Granite and map the location of these high-strain shear zones. The high-strain zones were classified into components of relative strain intensity on the basis of K-feldspar phenocryst grainsize, the presence of S and C fabrics (and the angle between the foliations) and the width of mylonite and ultramylonite outcrops. Our results suggest that there is no correlation between the size of K-feldspar phenocrysts and recognised indicators of high strain, such as the acute angle between S and C fabrics, and thickness of bands of mylonite/ultramylonite. This may indicate that the size of K-feldspar in the Wyangala Granite was not homogeneous after crystallisation and prior to deformation. We therefore suggest that the size of K-feldspar phenocrysts is not a good guide to the strain intensity. However, the high-strain shear zones in the Wyangala Granite can be mapped using the acute angle between S and C fabrics and the intensity and thickness of mylonites/ultramylonites. This work suggests that the high-strain zones are not continuous along strike.

Magnetic Fabric Study and Its Tectonic Significance of Suture Zones in Joint Area of Qinling and Qilianshan

AbstractTangzang‐Guanzizhen‐Wushan and Xinyang‐Yuanlong tectonic zones are the major suture zones in joint area of Qinling and Qilianshan mountains. Structure analysis and the anisotropy of low‐field magnetic susceptibility (AMS) of 168 samples from 41 sites was measured to determine the strain and fabric of the high strain shear zone in the suture zones. Most of the mean susceptibilities (Km) are low; magnetic ellipsoid analyses display that most of the samples are dominated by shear and oblate compressional fabric, and have high magnitude values of anisotropy which increases with the intensity of deformation. Caution should be exercised when use T and P' to deduce the shape of magnetic ellipsoid and the deformation intensity for they are also affected by rock types. The directions of principle susceptibilities are closely related to deformation; the magnetic foliations defined by Kmax and Kint are divided into two conjugate groups that imply the gridding configuration of high strain shear zone; clustered foliation and more scattered lineation are attributed to the shearing and flattening regime. Most of the samples have steep magnetic foliations and subhorizontal magnetic lineations and others have steep lineation, which demonstrate the transpressional characteristics of the shear zone. The shear sense of complex high strain zone inferred from magnetic fabric may be unreliable or even wrong for variation of the foliation attitude across the shear zone and great angle between Kmin and the striking of shear zone.

Metamorphic response in orogens of different obliquity, scale and geometry

Investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets. To illustrate the problems in documenting flow vectors we present integrated structural-metamorphic datasets from two transpressional systems; the Kaoko Belt in Namibia and the Kalinjala Shear Zone in South Australia. These orogens experienced widely differing metamorphic responses to transpressional deformation. Integration of kinematic and metamorphic datasets from the Kaoko Belt indicate shallow up-plunging extrusion trajectories in the orogen core, and show that the maximum stretching direction pattern matches the inferred flow vectors. High-grade domains (800–840 °C and 7.0–8.0 kb) in the orogen core developed low-angle upward-verging maximum stretching direction trajectories, whereas a low-grade domain (575–600 °C and 5.0–5.5 kb) in the orogen core has downward-verging lineation trajectories. The barometric differential between these high-grade and low-grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high-grade domains that were extruded obliquely, for the amount of lateral shear estimated for the orogen core. The Kalinjala Shear Zone in South Australia contrasts strongly with the Kaoko Belt. In this example, the high-grade and high-strain shear zone core of the orogen, experienced high- T/high- P metamorphism with low thermal gradients of 21–26 °C/km and steep decompressive P– T paths. The lower-grade external domains experienced lower- T/lower- P metamorphism with high thermal gradients of 35–37 °C/km. Sub-horizontal maximum stretching directions do not match the vertical extrusional flow in the high-grade core that is indicated by the metamorphic data. This comparison shows that in general and on a gross scale, maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis. In some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow. Consequently, information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record. These two datasets must be used in concert to obtain realistic constraints on first-order material flow trajectories at orogenic scales. The horizontal component of flow is typically best recorded by structural fabrics (maximum stretching direction and sense of shear), whereas the vertical component is typically best recorded by metamorphic information, such as P– T paths, temperature over depth ratio (G) and metamorphic field gradients (i.e. Δ T, Δ P and Δ G) across the orogen.