Extensional Veins Research Articles

Seismic cycles recorded in late Quaternary calcite veins: Geochronological, geochemical and microstructural evidence

Southwest Turkey is seismically active as a result of the Hellenic subduction process in the Eastern Mediterranean. We conducted high-resolution micro-sampling, high-precision U-series dating and microchemical analysis on an extensional vein system in a tectonically active but non-hydrothermal area. U/Th age data and microscopic observations provide evidence of repeated fracturing of a previously sealed crack system followed by a new increment of veining. Repeated injection of veinlets suggests that the vein system was formed by the crack-seal mechanism. Four major U/Th age groups for the emplacement of the vein system fall between 23.9 ± 0.2 ka and 23.2 ± 0.4 ka, 21.7 ± 0.4 ka and 19.2 ± 0.2 ka, 17.3 ± 0.1 ka and 16.2 ± 0.3 ka, and at 11.8 ± 0.2 ka. Stable and Sr isotope geochemistry of the calcite vein samples indicates that surface water interacting with the soil cover was the major component of the groundwater system from which the extensional veins precipitated. Trace element and O isotope data of the vein system are interpreted to reflect carbonate precipitation associated with seismic cycles involving fluids with different trace element compositions and CO 2 contents. Initial carbonate precipitation during a single seismic cycle occurred from CO 2-dominated fluids that were degassed from the original CO 2–water mixture. This was followed consecutively by carbonate precipitation from the remaining water, which was relatively impure with higher trace element contents. Millimetre to submillimetre-scale U-series dating in conjunction with geochemistry of carbonate veins related to active tectonism offers an innovative means of constraining the absolute timing of late Quaternary seismic and inter-seismic events.

Evolution of fluid chemistry and fluid-flow pathways during folding and faulting: an example from Taemas, NSW, Australia

Abstract In the Taemas area, New South Wales, Australia, a swarm of hydrothermal calcite and quartz veins is hosted in upright, open to close folded limestones and shales. Overprinting relationships and vein geometries demonstrate that the vein swarm formed progressively during fold growth and associated reverse faulting. Textures preserved in veins reveal that veins formed via hundreds to thousands of individual dilation and mineral precipitation events. Bedding-parallel flexural slip during fold growth was associated with laminated vein development, and limb-parallel stretching during fold growth was associated with the formation of bedding-orthogonal extension veins. The presence of subhorizontal extension fractures and severely misoriented reverse faults imply that fluid pressures exceeded lithostatic levels, at least transiently, during the development of the vein swarm. Vein δ 18 O compositions increase upwards through the Murrumbidgee Group in response to a progressive reaction of an externally derived, upwards-flowing low-δ 18 O fluid (of probable meteoric origin) with host limestones. Vein δ 18 O and 87 Sr/ 86 Sr compositions vary spatially and temporally within the same outcrop, and within individual veins. These variations are inferred to be caused by the ascent of packages of fluid along constantly changing flow pathways caused by multiple permeability creation–destruction cycles associated with fault slip and fault sealing. Vein trace and rare earth element (REE) concentrations are more variable, probably reflecting rapid rock buffering along fluid pathways on length scales of less than 10 m. Our results indicate that fluid-flow pathways change dynamically during crustal shortening, with pathways switching between states of low and high permeability during episodic fault slip and associated fracture development. Supplementary material: Two appendices are available at http://www.geolsoc.org.uk/SUP18492 .

Geology of the seismogenic subduction thrust interface

Abstract A microseismically active layer of underthrust sediments is commonly inferred along subduction thrust interfaces. The exhumed Chrystalls Beach Complex, in the Otago Schist, New Zealand, may be analogous to an actively deforming underthrust rock assemblage. The complex contains asymmetric competent lenses of sandstone, chert and basalt enclosed in a cleaved mudstone matrix. Continuous fabrics such as folds, boudins and asymmetric phacoids formed by distributed cataclasis and dissolution–precipitation creep. Discontinuous deformation is evident in an extensive fault-fracture mesh involving mutually cross-cutting subvertical extension veins and subhorizontal slickenfibre shear surfaces. The Hikurangi margin provides an example of along-strike variations in seismic style, possibly related to heterogeneous fluid-pressure state and interface geology. In both the ancient and active subduction-related shear zone, fluid-pressure state appears to be a critical control on frictional failure, which primarily occurs on weak, fluid-overpressured discontinuities. Continuous, aseismic deformation occurs where other mineral deformation mechanisms, such as dissolution–precipitation creep, are preferred. The geometry and composition of the underthrust rock assemblage appear to be first-order controls on megathrust fluid-pressure distribution, bulk rheology and dominant deformation mechanism, and thus may be significant controls on megathrust seismic style.

Extensional veins induced by self-similar dissolution at stylolites: analytical modeling

Stylolites are surfaces of localized dissolution observed in rocks. Many stylolites share a common morphological feature: the amount of dissolved rock in the center of the seam is proportional to the stylolite's length. Field observations demonstrate that stylolites can locally influence formation of veins. The developed analytical model of the indirect solution of Eshelby's transformation problem calculates the stress field within and around the stylolite. The results suggest that growth of a stylolite with an elliptic dissolution pattern generates a local non-uniform stress field in the host rock, which is in turn accommodated by brittle Mode I fracturing in the stylolite midsection, further resulting in veins formation. Parametric analysis and qualitative comparison with "anticrack" model describing stress field within and around stylolite are conducted.

Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress

Some faults seem to slip at unusually high angles (>45°) relative to the orientation of the greatest principal compressive stress1, 2, 3, 4, 5. This implies that these faults are extremely weak compared with the surrounding rock6. Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface7, 8, 9, weakening from chemical reactions10, elevated fluid pressure11, 12, 13 or dissolution–precipitation creep14, 15. Here we describe shear veins within the Chrystalls Beach accretionary melange, New Zealand. The melange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins—compared with the direction of hydrothermal extension veins that formed contemporaneously—indicates that they were active at an angle of 80°±5° to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution–precipitation creep, may explain active slip on severely misoriented faults.

Brittle faulting in the Rawil depression: field observations from the Rezli fault zones, Helvetic nappes, Western Switzerland

The Helvetic nappes in the Swiss Alps form a classic fold-and-thrust belt related to overall NNW-directed transport. In western Switzerland, the plunge of nappe fold axes and the regional distribution of units define a broad depression, the Rawil depression, between the culminations of Aiguilles Rouge massif to the SW and Aar massif to the NE. A compilation of data from the literature shows that, in addition to thrusts related to nappe stacking, the Rawil depression is cross-cut by four sets of brittle faults: (1) NE-SW striking normal faults, (2) NW-SE striking normal faults and joints, (3) ENE-WSW striking and (4) WNW-ESE striking normal plus dextral oblique-slip faults. Fault set 1 was probably initiated during sedimentation and reactivated during nappe stacking, whereas the other fault sets formed after emplacement of the Helvetic nappes. We studied in detail two well-exposed parallel fault zones from set 4, the Rezli fault zones (RFZ) in the Wildhorn Nappe. They are SW-dipping oblique-slip faults with a total displacement across the two fault zones of ~200 m vertically and ~680 m horizontally. The fault zones crosscut four different lithologies: limestone, intercalated marl and limestone, marl and sandstone. The internal architecture of the RFZ strongly depends on the lithology in which they developed. In the limestones, they consist of extension veins, stylolites, cataclasites and cemented gouge, in the intercalated marls and limestones of shear zones, brittle fractures and chaotic folds, in the marls of anastomosing shear zones, pressure solution seams and veins and in the sandstones of coarse breccia, brittle faults and extension veins. Sharp, discrete fault planes within the broader fault zones cross-cut all lithologies. Fossil fault zones in the Rezli area can act as a model for studying processes still occurring at deeper levels in this seismically active region.

Talklüfte im Zentralen Aaregranit der Schöllenen-Schlucht (Kanton Uri, Schweiz)

The Schöllenen Gorge in the Reuss Valley of the Central Swiss Alps (Figs. 1 and 2) is a famous tourist attraction and ideal location for the study of the properties and formation mechanisms of uplift and post-uplift unloading joints. The gorge is situated in the southern part of the Central Aar Granite, a granitic batholith which intruded about 300 million years ago. The magmatic fabric of this batholith (Fig. 4) has only been locally modified during Alpine tectonic and metamorphic overprinting, mainly in the vicinity of ductile-brittle shear zones. The up to 600 m deep gorge provides an ideal opportunity to study the complex fracture systems of the batholith, and tunnels of the Göschenen hydropower system allow the study of the fracture patterns below ground surface.Outcrop, tunnel and remote mapping of fractures in the study area lead to the recognition of two probably syntectonic (Oligocene-Miocene) joint sets (S and Q joints) and three generations of uplift and post-uplift joints (unloading joints). The frequent S joints run nearly parallel to the Alpine schistosity, i.e. striking approximately E–W and dipping steeply to the south (Figs. 5 and 7). The less frequent Q joints dip steeply to SW; the angle between the two joint sets ranges between 60 and 80 degrees. The first generation of uplift joints (called L- joints) is subhorizontal and probably related to Alpine extensional veins filled with fissure quartz (Zerrklüfte). These veins formed during the late Alpine (Miocene) uplift of the Aar Granite (Mullis 1996). A first generation of post-uplift joints (T1 joints) strikes parallel to the valley axes and dip with 30–45 degrees towards the valley bottom. This set probably formed during an earlier stage of glacial valley erosion in the Pleistocene (Figs. 9–11). The youngest generation of post-uplift joints (T2 joints) is orientated parallel to the present ground surface of the Schöllenen Gorge and to erosional surfaces with glacial striations (Figs. 9–11 and 21). The frequency and size of these joints seems to decrease with depth below the ground surface. In one tunnel, post-uplift joints could be observed within a horizontal and vertical distance from the ground surface of 150 and 80 meters. Post-uplift joints only form in granites with a primary fabric that has not been intensively overprinted by brittle or ductile Alpine tectonic deformations.Fractographic investigations, i.e. investigations of crack propagation markers on joint surfaces, confirm this relative age of the fracture sets and give valuable insights into the formation mechanisms of post-uplift joints. Post-uplift joints show intense and 5–10 meter long plumose markings and only rarely arrest lines (Figs. 18a and 20). It can be shown that sets of post-uplift joints join at pre-existing (uplift and syntectonic) fractures to form large (50–100 m sized) curved exfoliation structures (Fig. 19). The growth direction of the post-uplift joints is mainly in subhorizontal directions (Figs. 19 and 20).Fractographic markings, spatial and depth distributions as well as the relative size of post-uplift fractures are explained within the mechanical framework of uniaxial and biaxial compression tests on intact granite samples and samples with artificial flaws. Most of these experiments have been carried out in the framework of studies related to brittle failure (spalling and rockbursting) around deep mining drifts and tunnels in hard rock’s (e.g. Hoek & Bieniawski 1965, Read et al. 1998, Eberhardt et al. 1999). As suggested already by Holzhausen & Johnson (1979), post-uplift fractures form as extension fractures in a compressive stress field with small confining stress. Laboratory tests carried out on artificial Griffith cracks suggest that the macroscopic fracture size is mainly controlled by the ratio of the smallest to the largest principal stress (σ3/σ1), the so-called spalling limit. In steep slopes this ratio should increase with depth below ground surface (Fig. 24c), leading to smaller exfoliation fractures with increasing depth. The spatial occurrence of post-uplift fractures along the surface topography is a function of the deviatoric stress level (Fig. 24a) and/or the development of local tensile stresses (Fig. 24d). Preliminary numerical simulations of these failure criteria in a multistage glacial erosion model (Fig. 23) allow some of the observed patterns of post-uplift fracture distributions to be reproduced.post-uplift joints in steep glacial valleys play an important role in valley erosion and in connection with the risk of rock falls, the safety of traffic corridors, and the inflow of water to near-surface tunnels and hydropower caverns. The depth dependant sizes, frequencies and hydraulic conductivities of these fractures can be directly related to the occurrence and magnitudes of the corresponding hazards.

Structural control of low-sulfidation epithermal gold mineralization in the Rosario–Bunawan district, East Mindanao Ridge, Philippines

The Rosario–Bunawan district is situated about 200 km north of Davao City, the capital of the Mindanao Island, Southern Philippines. Gold is produced from the Co-O mine, containing about 2,034,000 t of ore at 10.9 g/t Au, and in numerous small-scale operations by local miners. Epithermal gold mineralization in the Rosario–Bunawan district and the Co-O mine is confined to narrow (0.2–4 m) low-sulfidation quartz–chalcedony–calcite veins in volcanic and volcaniclastic wall rocks. Three major vein orientations are distinguished: (1) the NNW–SSE-trending set with a sinistral strike-slip sense of deformation (Philippine Fault trend); (2) the ENE–WSW-trending dextral strike-slip set (Palawan trend) and associated veins in the Riedel geometry; and (3) the WNW–ESE-trending conjugate set (Co-O trend). Three structural stages are defined: (1) extensional shear or shear veins formed in the Co-O, the Philippine Fault, and Palawan trends during regional NW–SE compression and near vertical vein opening (D1); (2) reactivation of veins in the Philippine Fault, veins associated with the Palawan, and, to a lesser extent, the Co-O trends during E–W compression and near horizontal N–S-oriented vein opening (D2). New D2 extensional shear or shear veins formed in the Philippine Fault, and structures associated with the Palawan and associated Riedel trends; (3) the D3-stage block faulting subsequently displaced all of the auriferous veins. The auriferous Rosario–Bunawan district is situated between two splays of the Philippine Fault, which acted as a lateral ramp system during the oblique convergence of the Philippine Sea plate and the Eurasian plate. The oblique convergence resulted in a change from a compressional (D1) to a transpressional (D2) regime, which was a prerequisite for the two-stage vein opening and hydrothermal mineralization, leading to an economic gold enrichment. D1 compressional tectonics may have caused an elevated geothermal gradient in shallow crustal levels, forming the heat source for the fluid plumbing system, which is at variance to typical epithermal deposits formed in extensional zones. D2 thrusting of a limestone nappe together with syn-tectonic diorite intrusions may have further increased the geothermal gradient, maintaining the fluid plumbing system. The limestone nappe may, at the same time, have represented an aquitard forcing the hydrothermal fluids into the volcanic and volcaniclastic wall rocks, which is regarded as critical for the two-stage gold mineralization in the Rosario–Bunawan district.

Mechanisms of faulting and permeability enhancement during epithermal mineralisation: Cracow goldfield, Australia

The geometries, kinematics, and failure mechanisms of epithermal fault-vein networks are examined at the Cracow goldfield, Queensland, Australia, and compared with observations of active geothermal areas. Quartz–carbonate cementation and precious metal mineralisation is confined to a network of steeply dipping faults (>70°). Breccia textures indicate fault rock formed from repeated events, involving large components of dilation during fracture, wall-rock fragmentation, and mineral precipitation. New fault rock tended to form on the margins of pre-existing fault rock, generating thick zones up to 10 m wide (fault cores) and accumulating up to 300 m normal offsets. The fault-vein networks are segmented, corrugated over tens of metres along strike, and contain complex fracture networks in step-over zones. Mineralisation is not associated with any specific fault location, but occurs along planar segments of faults, in step-over zones, at fault tips and where fault dips change. The wall rocks surrounding the faults contain networks of shear and extension veins, with a large range of orientations. Furthermore, kinematic indicators on the faults are broadly normal dip-slip, but vary to oblique and strike-slip with no obvious relationship to geometry or location along the fault. Inconsistent fault kinematics and the range of wall rock vein orientations are attributed to transient changes in stress state, due to intrusion of nearby dykes. As a result, permeability would have been enhanced by dilatancy in fault rock and wall rock fractures, when corrugated normal faults temporarily failed in oblique or strike-slip events. Other permeability enhancement mechanisms likely included failure driven by high fluid pressures, and failure where faults steepen (near the Earth's surface or at jogs). Mineralisation was associated with repeated, transient pulses of fluid flow rather than a steady-state process.

Quartz vein fabrics coupled to elevated fluid pressures in the Stawell gold deposit, south-eastern Australia

Shear and extensional veins formed during the reactivation of the Magdala shear system at Stawell in western Victoria, Australia, contribute to the formation of the auriferous Central and Basalt Contact lodes. Within this shear system is a range of fault rocks accompanied by steep-dipping (>65°) quartz-rich laminated shear veins and relatively flat-lying extensional veins. Both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses. The macro- and microstructures suggest that the dilatancy, that produced mineralized veins, formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime. A new microfabric analysis technique is used to investigate the quartz-rich veins, which allows rapid integration of the microstructure with the crystallographic preferred orientations (CPOs). Both the shear and extensional quartz veins have a random CPO with ∼120° dihedral angles between the quartz–quartz grains, which is typical of a metamorphic equilibrium microfabric. The microstructures indicate that the quartz has undergone extensive grain adjustment in the solid-state, with grain shape and size affected by interfacial solution (pressure solution) effects. These features are consistent with inferences from experimental rock deformation studies, where grain boundary migration is enhanced in a water-rich environment. The onset of solution-transfer processes (pressure solution) developed as the quartz microfabric stabilized and continued to modify the CPO and microstructure significantly. It is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes, related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures.

The Timmins-Porcupine Gold Camp, Ontario: Anatomy of an Archean Greenstone Belt and Ontogeny of Gold Mineralization

New structural and geochronological data and interpretations from the Timmins-Porcupine gold camp, Abitibi subprovince, more precisely define the stratigraphy and the generation, timing, and effects of deformation. Tisdale (basalts) and Porcupine (volcaniclastic and turbiditic sedimentary rocks) assemblage rocks exhibit three major foliations (S2, S3, S4, plus a late constrictional fabric), of which S3 and S4 are also present in younger Timiskaming assemblage sedimentary rocks. Pre-Timiskaming folding has long been recognized in Tisdale assemblage rocks, but no foliation had previously been attributed to such an event. D1 regional uplift and extension produced a low-angle unconformity between the Tisdale and Porcupine assemblages, with partial excision of upper Tisdale stratigraphy and perhaps of the Blake River assemblage. D2 involved imbrication of a set of south-over-north thrust sheets rooted in the Porcupine-Destor deformation zone. D3 en echelon folds along the northern flank of the Porcupine-Destor deformation zone are a result of left-lateral strike-slip along this zone. The Timiskaming basin, lying along the curvilinear trace of the Porcupine-Destor deformation zone, formed as a D3 transtensional half graben, and the Dome fault formed its faulted margin. S4 foliation formed during subsequent right-lateral strike-slip movement and folding of S3 foliation. Intense D5 constrictional strain deformed earlier lineations, as the previously dilatational jog that hosts Timiskaming assemblage rocks acted as a compressional jog during right-lateral movement along the Porcupine-Destor deformation zone. These D2 to D5 phases represent an overall thrusting–strike-slip transpressional regime. There were several phases of gold mineralization in the Timmins-Porcupine gold camp. Clasts of ankerite veins occur in Timiskaming assemblage conglomerate at the Dome mine, and D3 foliation and folds crosscut early ankerite veining and alteration and some quartz veining. Thus, low-grade mineralization and possibly the quartz-fuchsite veins predate Timiskaming sedimentation. However, the bulk of gold postdates Timiskaming sedimentation. Quartz-carbonate-tourmaline-gold veins (Hollinger-McIntyre, Hoyle Pond, Dome, Aunor–Delnite mines) formed within Timiskaming assemblage and Tisdale assemblage rocks. These gold deposits developed largely as oblique slip and extensional vein arrays formed during north-south shortening and local strike-slip faulting. Younger deposits (Pamour) are associated with D4 dip-slip faulting and were weakly deformed during late D4. Thus, the largest known Archean lode gold camp formed throughout protracted orogenesis consisting of a broadly transpressional regime with unusually well developed early thrusting.

Gold-bismuth-telluride-sulphide assemblages at the Viceroy Mine, Harare-Bindura-Shamva greenstone belt, Zimbabwe

AbstractGold mineralization at the Viceroy Mine is hosted in extensional veins in steep shear zones that transect metabasalts of the Archaean Arcturus Formation. The gold mineralization is generally made up of banded or massive quartz carrying abundant coarse arsenopyrite. However, most striking is a distinct suite of Au-Bi-Te-S minerals, namely joseite-A (Bi4TeS2), joseite-B (Bi4Te2S), hedleyite (Bi7Te3), ikunolite (Bi4S3), ‘protojoseite’ (Bi3TeS), an unnamed mineral (Bi6Te2S), bismuthinite (Bi2S3), native Bi, native gold, maldonite (Au2Bi), and jonassonite (AuBi5S4). The majority of the Bi-Te-S phases is characterized by Bi/(Se+Te) ratios of >1. Accordingly, this assemblage formed at reduced conditions at relatively low fS2 and fTe2. Fluid-inclusion thermometry indicates depositional temperatures of the main stage of mineralization of up to 342°C, in the normal range of mesothermal, orogenic gold deposits worldwide. However, melting temperatures of Au-Bi-Te phases down to at least 235°C (assemblage (Au2Bi + Bi + Bi7Te3)) imply that the Au-Bi-Te phases have been present as liquids or melt droplets. Furthermore, the close association of native gold, native bismuth and other Bi-Te-S phases suggests that gold was scavenged from the hydrothermal fluids by Bi-Te-S liquids or melts. It is concluded that a liquid/melt-collecting mechanism was probably active at Viceroy Mine, where the distinct Au-Bi-Te-S assemblage either formed late as part of the main, arsenopyrite-dominated mineralization, or it represents a different mineralization event, related to rejuvenation of the shear system. In either case, some of the gold may have been extracted from pre-existing, gold-bearing arsenopyrite by Bi-Te-S melts, thus leading to an upgrade of the gold ores at Viceroy. The Au-Bi-Te-S assemblage represents an epithermal-style mineralization overprinted on an otherwise mesothermal (orogenic) gold mineralization.

Metamorphism, Fluid Flux, and Fluid Evolution Relative to Gold Mineralization in the Hutti-Maski Greenstone Belt, Eastern Dharwar Craton, India

We evaluate the metamorphic conditions (P, T, and fluid composition), resultant fluid flux, and gold mineralization in the Hutti-Maski greenstone belt, eastern Dharwar craton, southern India, from the integrated study of three working mines (Hutti, Uti, and Hira-Buddini). In the observed D1 to D5 deformation sequence at Hutti, the proximal biotite alteration zones, containing isoclinally folded quartz veins (D2) and the laminated fault-fill veins (D3) are auriferous. The absence of extension or extensional shear veins implies that the pore fluid pressure was ≤σ 3 + T, where T is the tensile strength of the greenstones. Important auriferous vein structures of the Hira-Buddini mine are the fault-fill veins that run along the steeply dipping, reverse, brittle-ductile shear zone and shallow-dipping, sigmoidal extension veins (Pf ≤σ 3 + T). Metabasites from Hutti record amphibolite facies conditions (3–5 kbars and ~650°C) as the probable peak metamorphic P-T. A clockwise P-T-t path could be established for the Uti region, from garnetiferous amphibolite and garnet biotite schist, with peak P-T reaching amphibolite facies conditions (~6 kbars and 650°–700°C). The deduced path can be explained by a subduction-related compressional to transpressional tectonic setting, invoked for the Dharwar craton. The estimated average enthalpy change for the relevant dehydration and decarbonation is about 75 kJ, which is necessary to release one mol of H2O + CO2. By assuming volatile release between 400° and 600°C, the total heat required to metamorphose a kilogram of an average mafic rock is ~235 kJ. Furthermore, by considering 3 percent volatile loss during metamorphism, the maximum rate of volatile production is 28.98 kg·cm−2my−1. Gold mineralization at Hutti took place on the metamorphic retrograde path beginning with initial alteration (and sulfidation) at upper greenschist facies. After a protracted phase of fluid evolution, the mineralization culminated with the formation of auriferous laminated quartz veins at lower greenschist facies. The ore fluid had a ∑S content ~0.1 m, and a decrease in both f O2 (and pH) caused the precipitation of gold in the proximal biotite zone from a hydrothermal solution containing Au(HS2−). Fluid inclusions in D2 quartz veins within the proximal zone comprise a unique assemblage of five distinct types of carbonic inclusions containing variable proportions of CO2, CH4, graphite daughter products, and H2O. Precipitation of thin films of graphite in the inner walls of carbonic inclusions is interpreted to be the result of reaction between CO2 and CH4 (CO2 + CH4 = 2C + 2H2O) within those inclusions that were trapped at >400°C and contained sufficient CH4. Entrapment of these carbonic and aqueous inclusions followed phase separation of the initial aqueous-carbonic fluid during a decompression event. Simultaneous entrapment of coeval and cogenetic aqueous and carbonic inclusions in D3 auriferous laminated quartz veins was due to phase separation of fluid of broadly similar composition but at lower temperature. Accordingly, gold precipitation in these veins may have been a result of decrease in ∑S content of the aqueous fluid rather than the wall-rock sulfidation and f O2 decrease, as in the biotite zone. Thus, gold precipitation at Hutti, in the proximal alteration zone and laminated veins occurred over a range of temperatures and by several mechanisms. As in the Hutti deposit, mineralization at Uti occurred on the meta-morphic retrograde path. Mass-balance calculations indicate introduction of SiO2, K2O, S, As, and Zr and depletion of CaO in the mineralized portion. Gold at Hira-Buddini is recovered both from the wall-rock mylonites and from the fault-fill and sigmoidal extension veins. The sigmoidal extension veins contain numerous aqueous as well as carbonic inclusions in close association, showing large density variation within single clusters. Such variation points to pressure cycling or operation of a fault-valve during the formation of these extensional veins. Occurrence of gold within these veins implies that phase separation during sudden pressure drops caused gold precipitation. Thus, a striking characteristic of this belt is the variability in the style of mineralization and fluid evolution among the deposits.

Optimally oriented “fault‐valve” thrusts: Evidence for aftershock‐related fluid pressure pulses?

A thrust‐vein network from the Triumph gold deposit, Western Australia, is explained in terms of an extremely high rate of fluid‐pressure increase, prior to failure, relative to the rate of stress increase. The thrust fault is a small‐displacement fault characterized by a thick, fault‐parallel shear vein, plus multiple low‐angle extension veins, with orientations that demonstrate the thrust was optimally oriented relative to the locally imposed crustal stresses. Large extension veins have irregular margins, are dominantly composed of coarse milky quartz with no obvious laminations or solid inclusion trails, and are regularly spaced along the thrust (1–2 m). The fault‐vein geometries indicate the Triumph thrust is a rare candidate for “fault‐valve” failure of an optimally oriented thrust, and it is possible the structure formed in a small number of failure events, during load weakening of the thrust. An analysis using the Coulomb criterion shows that load weakening of a thrust occurs when fluid pressure increases relative to tectonic stress by a factor dependent on the orientation of the thrust. Thrust and reverse faults in dry crust load strengthen prior to failure, but the poroelastic behavior of sealed, fluid‐saturated crust is enough to induce load weakening in compressive environments; thus poroelastic load weakening is expected to be an important failure mechanism in hydrothermal environments. However, in the case of the Triumph thrust, dilatant shear failure necessitates a fluid pressure increase which is an order of magnitude larger still. The observations and results are consistent with a pulse of high fluid pressure migrating up through fault or fracture networks that have elevated permeability relative to the wall rock, under conditions of transiently low differential stress. Fluid pressure differences resulted between the fault and wall rock, leading to extension fracture and fault failure. Such conditions may occur when adjacent large earthquakes induce damage, breach overpressured fluid reservoirs, and generate fluid‐pressure driven aftershocks.

Relationships between faults, extension fractures and veins, and stress

Faults are commonly related to extension fractures, defined here as including extension veins. Extension fracturing is integral to fault initiation and propagation, and extension fractures also form after fault slip. In these situations, fault planes contain the intermediate principal stress σ 2, and slip is perpendicular to the line of intersection between the fault and the extension fractures. However, for reactivated faults, multiple fault sets, and faults formed according to the Healy theory, σ 2 is not necessarily within the fault plane. A theoretical analysis shows that the trace of an extension fracture on a fault can make angles from 0° to 90° with the maximum resolved shear stress. The angle depends on the fault orientation relative to the principal stresses, and the ratio between the stresses. Extension fractures only intersect faults perpendicular to the maximum resolved shear stress on faults containing the maximum or intermediate principal stresses, or when their magnitudes are equal ( σ 1 = σ 2). Field observations show that extension fractures can intersect faults along lines at oblique angles to slip directions, as predicted by the theory. Such angles may be indicators of fault reactivation, multiple sets of faults, or Healy theory faulting.

OCEANIC RELICT TEXTURES IN THE MOUNT AVIC SERPENTINITES, WESTERN ALPS

The Mount Avic serpentinites derive from mantle peridotites that have been involved in several geodynamic episodes, from their formation in the Jurassic Tethys, to the subduction event and suture in the Alpine chain. The main steps of their tectono-metamorphic evolution are inferred by investigating the texture and mineralogy of selected samples. Some serpentinite samples show relict textures and mineralogy related to the emplacement in the oceanic realm and to the early Alpine evolution. Olivine and clinopyroxene porphyroblasts preserve relict mantle textures. Magnetite crystals include relict chromite at their core. Abundant pseudomorphs of antigorite on former porphyroblasts are present. Mineral composition of the pseudomorphic serpentine partly inherits the composition of former minerals (e.g., relatively Ti-rich antigorite replacing pyroxene). These observations support the hypothesis that the Mount Avic serpentinites derive from original mantle porphyroclastic peridotites. An early formation of Ti-clinohumite during oceanic serpentinization of mantle peridotites at relatively high temperature conditions, followed by HP subduction-related Alpine recrystallization, is proposed. The late orogenic history is characterised by transposition accompanied by greenschist facies minerals crystallization. In these samples, antigorite represents the main mineral phase, marking a pervasive, often mylonitic, foliation. The last generation of serpentine, that occurs as yellow fibres filling late extension veins, was formed during a Neo-Alpine brittle-ductile deformation stage.

Fault and fluid interaction in a rifted margin: integrated study of calcite-sealed fault-related structures (southern Corinth margin)

Abstract This work integrates microstructural, petrologic, microthermometric and geochemical analysis of calcite-sealed fault-related structures of the major faults of the southern margin of the Corinth rift (Greece). It draws attention to the main deformational processes and fault–fluid interaction in fault zones along the whole margin. Data analysed come from the compact breccia located immediately adjacent to major fault planes. Samples were collected at different positions alongside different faults juxtaposing synrift continental deposits (hanging wall) against the prerift Pindus limestones (footwall). This implies hectometric fault displacements and that the fault zones have recorded a relatively long evolution of deformation. Our study identifies four main deformation features common to all studied faults: calcite cemented breccia, extensional veins, shear veins and fault slip surfaces. The integrated data analysis shows that deformation structures and fault rocks that coexist in the same outcrop formed at different depths and times. During inferred upward fault propagation, the style of deformation evolved from distributed (brecciation) to localized (slip surfaces) and from a closed system with fluid–rock equilibrium during brecciation to a more open one with limited influx of meteoric water during extensional and shear fracturing. This implies relative uplift of the footwall during fault propagation and fault interaction with different water aquifers as the fault propagates upwards. The results demonstrate similar fault evolution and fault–fluid interaction through the entire rifted margin and during the whole rifting period.

Golden Bar gold deposit as an extension of the Hyde‐Macraes Shear Zone, east Otago, New Zealand

The Golden Bar gold mine pit near Macraes, east Otago, was developed in Textural Zone (TZ) 3 of the Otago Schist, at least 100 m structurally up‐section from the Hyde‐Macraes Shear Zone. This study tests the hypothesis that the Golden Bar mineralised zone was an extension of the major Hyde‐Macraes Shear Zone mineralisation system. The Golden Bar rocks have been extensively, but subtly, hydro‐thermally altered. Principal hydrothermal alteration effects are replacement of titanite by rutile, replacement of epidote by siderite and kaolinite, and addition of pyrite. Mineralised (gold‐bearing) rocks within this hydrothermal alteration zone are volumetrically minor and include micaceous schist with disseminated pyrite and arsenopyrite, silicified micaceous schist, and quartz veins. The structurally lowest mineralised rocks occur in a well‐defined foliation‐parallel zone of black sheared rocks with abundant hydrothermal graphite and sulfide minerals. The top of the mineralised zone is defined by a duplex shear zone, c. 10 m thick, containing abundant silicified and sulfide‐bearing schist that has been deformed into an imbricate stack. Between the lower shear and the upper duplex zone, shallow‐dipping thick (metre scale) mineralised extensional quartz veins cut across schist foliation and extend for up to 60 m laterally. The Golden Bar mineralised rocks were emplaced during contractional deformation and this resulted in WNW‐directed thrusting that controlled the formation of ore‐grade zones. The deformation and mineralisation style at Golden Bar is essentially identical to that of the Hyde‐Macraes Shear Zone that structurally underlies the Golden Bar zone. This inferred relationship between the Hyde‐Macraes Shear Zone and Golden Bar mineralisation extends the overall structural thickness of this gold‐bearing deformation zone to >250 m.

Numerical modelling of mechanical controls on coeval steep and shallow dipping auriferous quartz vein formation in a thrust zone, Macraes mine, New Zealand

The Hyde-Macraes Shear Zone (HMSZ) is a regionally continuous, low-angle, NE dipping (~15°) late-metamorphic thrust zone in the Mesozoic Otago Schist. The shear zone, which is host to large volumes of mineralised schist, consists of foliated fissile schist with some massive schist pods. Two sets of quartz veins are found within the HMSZ: thrust-related, shallowly dipping veins that were emplaced parallel or sub-parallel to the shears and swarms of steeply dipping extensional veins, which cut across the metamorphic foliation. The latter are restricted to the massive schist pods. Mutual cross-cutting relationships occur between steep extensional veins and shallow-dipping veins, suggesting that they formed contemporaneously. The co-existence of these two vein types locally implies local rotation of the principal stress axes to produce extensional veins within a regional thrust setting. The steep extensional veins are spatially related to lateral and oblique ramps within the HMSZ. Three-dimensional mechanical models show that these lateral or oblique ramps can produce favourable conditions for extensional vein formation when combined with a high fluid pressure and oblique convergence. Mechanical requirements include a reduced differential stress, a positive volumetric strain and an increase in the horizontal shear stress. Our models show that under certain conditions, it is possible for extension-related structures to form during shortening because of local changes in the stress state without the need for a regional scale switch in the imposed stress field. The convergence direction across the HMSZ during formation of the steep extensional veins was ~WNW.

Arc-parallel extension and fluid flow in an ancient accretionary wedge: The San Juan Islands, Washington

Structural analysis of the Lopez Structural Complex, a major Late Cretaceous terrane-bounding fault zone in the San Juan thrust system, reveals a sequence of events that provides insight into accretionary wedge mechanics and regional tectonics. After formation of regional ductile flattening and shear-related fabrics, the area was crosscut by brittle structures including: (1) southwest-vergent thrusts, (2) extension veins and normal faults related to northwest-southeast extension, and (3) conjugate strike-slip structures that record northwest-southeast extension and northeast-southwest shortening. Aragonite-bearing veins are associated with thrust and normal faults, but only rarely with strike-slip faults. High-pressure, low-temperature (HP-LT) minerals constrain the conditions for brittle deformation to ≥20 km and