Ductile Fault Research Articles

Exhumation processes during post-collisional stage in the Variscan belt revealed by detailed 40Ar/39Ar study (Tanneron Massif, SE France)

Detailed 40Ar/39Ar geochronology on single grains of muscovite was performed in the Variscan Tanneron Massif (SE France) to determine the precise timing of the post-collisional exhumation processes. Thirty-two plateau ages, obtained on metamorphic and magmatic rocks sampled along an east–west transect through the massif, vary from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous exhumation of the lower crust that lasted about 20 Ma during late Carboniferous. In the eastern part of the massif, the closure of the K–Ar isotopic system is at 311–315 Ma, whereas in the middle part of the massif it closes earlier at 317–321 Ma. These cooling paths are likely to be the result of differential exhumation processes of distinct crustal blocks controlled by a major ductile fault, the La Moure fault that separates both domains. In the western part of the massif, the ages decrease from 318 to 303 Ma approaching the Rouet granite, which provides the youngest age at 303.6 ± 1.2 Ma. This age distribution can be explained by the occurrence of a thermal structure spatially associated to the magmatic complex. These ages argue in favour of a cooling of the magmatic body at around 15 Ma after the country rocks in the western Tanneron. The emplacement of the Rouet granite in the core of an antiform is responsible for recrystallization and post-isotopic closure disturbances of the K–Ar chronometer in the muscovite from the host rocks. These new 40Ar/39Ar ages clearly outline that at least two different processes may contribute to the exhumation of the lower crust in the later stage of collision. During the first stage between 320 and 310 Ma, the differential motion of tectonic blocks limited by ductile shear zones controls the post-collisional exhumation. This event could be related to orogen parallel shearing associated with crustal-scale strike-slip faults and regional folding. The final exhumation stages at around 300 Ma take place within the tectonic doming associated to magmatic intrusions in the core of antiformal structures. Local ductile to brittle normal faulting is coeval to Upper Carboniferous intracontinental basins opening.

The kinematic history of the Khlong Marui and Ranong Faults, southern Thailand

The Khlong Marui Fault (KMF) and Ranong Fault (RF) are major NNE-trending strike-slip faults which dissect peninsular Thailand. They have been assumed to be conjugate to the NW-trending Three Pagodas Fault (TPF) and Mae Ping Fault (MPF) in Northern Thailand, which experienced a diachronous reversal in shear sense during India–Eurasia collision. It follows that the KMF and RF are expected to show the opposite shear sense and a slip sense reversal at a similar time to the TPF and MPF. New field data from the KMF and RF reveal two phases of ductile dextral shear separated by Campanian magmatism. Paleocene to Eocene post-kinematic granites date the end of this phase, while a brittle sinistral phase deforms the granites, and has exhumed the ductile fault rocks. The timing of these movements precludes formation of the faults in response to Himalayan extrusion tectonics. Instead, they formed near the southern margin of a Late Cretaceous–Paleocene orogen, and may have been influenced by variations in the rate of subduction ahead of India and Australia. North-south compression prior to reactivation of the subduction zone around southern Sundaland in the Eocene caused widespread deformation in the over-riding plate, including sinistral transpression on the KMF and RF.

A graphical–algebraic method for analysing shear zone displacements from observations on arbitrarily oriented outcrop surfaces

We present a practical graphical–algebraic method that enables one to achieve the true displacement and shape of deformed geological markers in fault or shear zones as observed on arbitrarily oriented outcrop surfaces. We use as our natural example deformed quartz veins that have been sheared across brittle–ductile faults in the Southern Alps of New Zealand. The technique is based on the assumption that simple shear has dominated the shear zone formation. For input data we require the strikes and dips of the outcrop surfaces, the offset markers, and the shear zones as well as the pitch of the simple shear vector in the plane of the shear zone. The paper develops a set of algebraic and graphical operations that allow one to convert photographs of faulted or sheared planar markers observed on an arbitrary outcrop surface into an equivalent view that is coincident with the movement ( m) plane of the fault or shear zone. This view displays the true displacement of the offset marker and delineates its deformed shape as seen in a section that is parallel to the slip vector.

Boudinage of a stretching slablet implicated in earthquakes beneath the Hindu Kush

As the fragments of Gondwana (Africa, Arabia, India and Australia) moved northward, arc-shaped belts with intervening basins formed in the Alpine–Himalayan mountain chain during and after collision. This was accompanied by subduction (or sinking) of the ancient Tethyan oceanic plate (or slab) into the underlying mantle. The arc-like shapes could in part be the end result of processes related to drips forming in the less-viscous mantle layer at the base of the Earth's rigid outer shell and then falling into the deeper mantle. Alternatively, the arcs could have formed because slabs constituted of intervening small ocean basins were independently subducted during convergence, and have now disappeared. The subducting slabs tend to stretch, tear and eventually break off, leaving behind thin, vertical strips of colder material that can easily be mistaken for mantle drips. Previous work indicates the presence of such remnant material beneath the Hindu Kush region, close to the collision zone between the Indian and Eurasian continental plates. Here, we analyse a cluster of intermediate-depth earthquakes beneath this region and suggest the existence of an elongate boudin, a lens-shaped feature bounded by ductile faults or shear zones. Our data do not support mantle drip and instead offer a snapshot into the process of break-off, as a thin strip of vertically stretching slab tears free before descending deeper into the underlying mantle.

U–Pb SHRIMP zircon geochronology and T– t– d history of the Kampa Dome, southern Tibet

Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia 206Pb/ 238U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D 1), predominately top-to-N ductile shearing and crustal extension (D 2), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D 3), and continued N–S contraction, E–W extension and doming (D 4). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.

Structural characterization of the Costa Rica décollement: Evidence for seismically-induced fluid pulsing

Ocean Drilling Program Legs 170 and 205 offshore Costa Rica provide structural observations which support a new model for the geometry and deformation response to the seismic cycle of the frontal sedimentary prism and décollement. The model is based on drillcore, thin section, and electron microscope observations. The décollement damage zone is a few tens of meters in width, it develops mainly within the frontal prism. A clear cm-thick fault core is observed 1.6 km from the trench. The lower boundary of the fault core is coincident with the lithological boundary between the frontal prism and the hemipelagic and pelagic sediment of the Cocos plate. Breccia clast distributions in the upper portion of the décollement damage zone were studied through fractal analysis. This analysis shows that the fractal dimension changes with brecciated fragment size, implying that deformation was not accommodated by self-similar fracturing. A higher fractal dimensionality correlates with smaller particle size, which indicates that different or additional grain-size reduction processes operated during shearing. The co-existence of two distinct fracturing processes is also confirmed by microscopic analysis in which extension fracturing in the upper part of the damage zone farthest from the fault core is frequent, while both extension and shear fracturing occur approaching the fault core. The coexistence of extensional and shear fracturing seems to be best explained by fluid pressure variations in response to variations of the compressional regime during the seismic cycle. During the co-seismic event, sub-horizontal compression and fluid pressure increase, triggering shear fracturing and fluid expulsion. Fractures migrate upward with fluids, contributing to the asymmetric shape of the décollement, while slip propagates. In the inter-seismic interval the frontal prism relaxes and fluid pressure drops. The frontal prism goes into diffuse extension during the interval when plate convergence is accommodated by creep along the ductile fault core. The fault core is typically a barrier to deformation, which is explained by its weak, but impermeable, nature. The localized development of a damage zone beneath the fault core is characterized by shear fracturing that appears as the result of local strengthening of the detachment.

The Monashee reflection: Re-examination of a Lithoprobe crustal-scale seismic reflection in the southern Canadian Cordillera

The Monashee reflection is a major crustal-scale, crosscutting reflection that appears on two mutually perpendicular Lithoprobe seismic profiles in the southern Omineca belt of the Canadian Cordillera. It has previously been interpreted as the down-plunge extension of a regional ductile thrust fault, the Monashee decollement, which is said to separate the Monashee complex from the overlying Selkirk allochthon. We reinterpret the Monashee reflection as a shear zone with modest, normal displacement based on recent mapping, geometric analysis of the seismic profiles, and existing geochronological and metamorphic data. Recent mapping has demonstrated that this boundary is not a shear zone with thrust geometry, but rather it is a gradational boundary that has resulted from mutual folding and transposition of rocks ascribed to the Monashee complex and Selkirk allochthon. Overprinting the transition zone in different areas are three high-strain zones: the Green-bush Lake shear band zone, Slate Mountain shear zone, and a ductile shear zone associated with the Columbia River fault. We interpret these high-strain zones as segments of a single, high-strain zone that wraps around the margins of the Thor-Odin culmination. This marginal zone is a complex, outward-dipping, normal structure, which we name the Thor-Odin high-strain zone. Three alternative three-dimensional geometric models have been developed for the Monashee reflection in order to project the reflection to the surface. We favor a model in which the surface trace of the Monashee reflection coincides with the Thor-Odin high-strain zone. Normal shear sense kinematics are interpreted for the Monashee reflection based on: (1) the overall geometry and fault-drag–like relationship between the Monashee reflection and reflections in the hanging wall and foot-wall; (2) offset of metamorphic and geochronological gradients, which are consistent with an extensional zone rather than with a thrust fault interpretation; and (3) the crosscutting nature of the Monashee reflection, which is consistent with normal structures throughout the region.

Quartz Rheology and Short-time-scale Crustal Instabilities

We present numerical results of thermal-mechanical feedback in crustal quartz rheology and contrast this behavior to the vastly different character of an olivine mantle. In the numerical experiments quartz is found to have a very strong tendency for short-time-scale instabilities, while our numerical experiments show that olivine has a decisive tendency for a stable thermally lubricated slip. At the same time, olivine can also go through a transitional period of creep bursts, which are physically caused by multiple interacting ductile faults at various length and time scales which collocate quickly into a major shear zone. Since olivine has this strong propensity to self organize in a large apparently stable fault system, it lacks the dynamics of interacting ductile faults evident in other minerals. Quartz behaves totally different and keeps its jerky slip behavior for prolonged deformation. An example is shown here in which a 30×50 km piece of a wet quarzitic crust is extended for about 2 Ma. The associated total displacement field clearly shows the unstable slipping events, which have a characteristic time frame of one to several years. In contrast, olivine is very stable and has a much longer time scale for thermal, instability of 100 kyrs.

Dome formation and extension in the Tethyan Himalaya, Leo Pargil, northwest India

Metamorphic dome complexes occur within the internal structures of the northern Himalaya and southern Tibet. Their origin, deformation, and fault displacement patterns are poorly constrained. We report new fi eld mapping, structural data, and cooling ages from the western fl ank of the Leo Pargil dome in the northwestern Himalaya in an attempt to characterize its post‐middle Miocene structural development. The western fl ank of the dome is characterized by shallow, west-dipping pervasive foliation and WNW-ESE mineral lineation. Shear-sense indicators demonstrate that it is affected by east-west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting. Sustained top-to-northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation. Garnet and kyanite indicate that the Leo Pargil dome was exhumed from the mid-crust. 40 Ar/ 39 Ar mica and apatite fi ssion track (AFT) ages constrain cooling and exhumation pathways from 350 to 60 °C and suggest that the dome cooled in three stages since the middle Miocene. 40 Ar/ 39 Ar white mica ages of 16‐14 Ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation. AFT ages between 10 and 8 Ma suggest that ductile fault displacement had ceased by then, and AFT track-length data from high-elevation samples indicate that the rate of cooling had decreased signifi cantly. We interpret this to indicate decreased fault displacement along the Leo Pargil shear zone and possibly a transition to the Kaurik-Chango normal fault system between 10 and 6 Ma. AFT ages from lower elevations indicate accelerated cooling since the Pliocene that cannot be related to pure fault displacement, and therefore may refl ect more pronounced regionally distributed and erosion-driven exhumation.

Initiation of localized shear zones in viscoelastoplastic rocks

Shear localization is a process of primary importance for the onset of subduction and the evolution of plate tectonics on Earth. In this paper we focus on a model in which shear localization is initiated through shear heating. The rheology employed is linear Maxwell viscoelastic with von Mises plasticity and an exponential dependence of viscosity on temperature. Dimensional analysis reveals that four nondimensional (0‐D) parameters control the initiation of shear zones. The onset of shear localization is systematically studied with 0‐D, 1‐D, and 2‐D numerical models, both under constant stress and under constant velocity boundary conditions. Mechanical phase diagrams demonstrate that six deformation modes exist under constant velocity boundary conditions. A constant stress boundary condition, on the other hand, exhibits only two deformation modes (localization or no localization). Scaling laws for the growth rate of temperature are computed for all deformation modes. Numerical and analytical solutions demonstrate that diffusion of heat may inhibit localization. Initial heterogeneities are required to initiate localization. The derived scaling laws are applied to Earth‐like parameters. For a given heterogeneity size, stable (nonseismic) localization only occurs for a certain range of effective viscosities. Localization is inhibited if viscosity is smaller then a minimum threshold, which is a function of the heterogeneity size. The simplified rheological model is compared with a more realistic and more complex model of olivine that takes diffusion, power law, and Peierls creep into account. Good agreement exists between the models. The simplified model proposed in this study thus reproduces the main physics of ductile faulting. Two‐dimensional late stage simulations of lithospheric‐scale shear localization are presented that confirm the findings of the initial stage analysis.

Geodetic displacements and aftershocks following the 2001 Mw = 8.4 Peru earthquake: Implications for the mechanics of the earthquake cycle along subduction zones

We analyzed aftershocks and postseismic deformation recorded by the continuous GPS station AREQ following the Mw = 8.4, 23 June 2001 Peru earthquake. This station moved by 50 cm trenchward, in a N235°E direction during the coseismic phase, and continued to move in the same direction for an additional 15 cm over the next 2 years. We compare observations with the prediction of a simple one‐dimensional (1‐D) system of springs, sliders, and dashpot loaded by a constant force, meant to simulate stress transfer during the seismic cycle. The model incorporates a seismogenic fault zone, obeying rate‐weakening friction, a zone of deep afterslip, the brittle creep fault zone (BCFZ) obeying rate‐strengthening friction, and a zone of viscous flow at depth, the ductile fault zone (DFZ). This simple model captures the main features of the temporal evolution of seismicity and deformation. Our results imply that crustal strain associated with stress accumulation during the interseismic period is probably not stationary over most of the interseismic period. The BCFZ appears to control the early postseismic response (afterslip and aftershocks), although an immediate increase, by a factor of about 1.77, of ductile shear rate is required, placing constraints on the effective viscosity of the DFZ. Following a large subduction earthquake, displacement of inland sites is trenchward in the early phase of the seismic cycle and reverse to landward after a time ti for which an analytical expression is given. This study adds support to the view that the decay rate of aftershocks may be controlled by reloading due to deep afterslip. Given the ratio of preseismic to postseismic viscous creep, we deduce that frictional stresses along the subduction interface account for probably 70% of the force transmitted along the plate interface.

Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central Argentina

AbstractThe Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine‐grained biotite + plagioclase, that lacks a well‐defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1‐related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine‐grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser‐grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H+ and/or H2O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system.

Geology, fluid inclusions, isotope geochemistry, and geochronology of the Paishanlou shear zone-hosted Gold Deposit, North China Craton

The Paishanlou gold deposit lies along the northern margin of North China Craton gold province, the third largest Au province in China. The deposit is mainly hosted in Archean metamorphic rocks of the Jianping Group and is structurally controlled by two sets of ductile shear zones. Gold mineralization is closely associated with intense hydrothermal alteration along the ductile shear zones, with a typical greenschist facies alteration assemblage of sericite + chlorite + calcite + biotite + quartz and a distinct alteration zoning of inner pyrite-sericite zone, middle carbonate zone, and outer epidote-biotite-chlorite zone from orebody to wall rock. Fluid inclusion petrography and microthermometric results suggest that four types of fluid inclusions are present at the deposit. Type 1 CO 2-rich inclusions have homogenization temperatures of 22.4–27.9 °C. Type 2 H 2O–CO 2 inclusions show salinities of 4.6–12.3 wt.% NaCl equivalent and Th-tot of 242 to 337 °C. Type 3a primary aqueous inclusions have salinities of 5.9–11.2 wt.% NaCl equivalent and Th-tot of 187–289 °C, while Type 3b secondary aqueous inclusions have salinities of 3.1–8.3 wt.% NaCl equivalent and Th-tot of 128–239 °C. Type 4 multiphase inclusions have salinities ranging from 31.4 to 35.2 wt.% NaCl equivalent and Th-tot of 370–447 °C. The close association of CO 2-rich inclusions and H 2O-rich inclusions in groups and along the same trail suggests the presence of fluid immiscibility. Estimated P– T values range from 1.4 to 1.9 kbar at 322–417 °C. The δ 18O values of the ore-forming fluids are calculated to range from 3.8‰ to 7.4‰ and the δD values from − 87.3 to − 116.2‰, indicating a mixed source of deep-seated magmatic water and shallower meteoric water. The δ 34S values (0.3–7.5‰) of sulfide ore minerals are similar to those of both pyrite from metamorphic country rocks and whole rock sulfur of metamorphic country rocks. The 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb of sulfide ores range within 16.40–17.0, 15.21–15.37, and 36.69–37.38, respectively, with the model ages of 397–727 Ma, consistent with those of Late Mesozoic granites and lying between Archean metamorphic rock lead and the Proterozoic dolomitic marble lead. These sulfur and lead features reflect the fact that the ore-forming materials originated mainly from multiple wall-rock sources. The well-defined Ar/Ar plateau age (126.6 ± 1.1 Ma) of biotite from the NNE ductile shear zone provides a possible temporal link between Au mineralization, magmatism and deformation at the Paishanlou deposit. The concordance of the biotite age with other well-constrained mineralization ages for the deposit, and the intrusion age of the granite, suggests that Au mineralization was essentially contemporaneous with late Mesozoic granitic magmatism and the second stage of mylonitization. In terms of its genetic linkage with the subduction process at the leading northern edge of the North China Craton, the Paishanlou deposit can be favored as an orogenic gold deposit. Its formation benefited from the combined action of protracted activity of ductile fault zones, hydrothermal circulation associated with granitoid intrusion and preferred structural setting favoring gold deposition. The major geodynamic switch towards generalized extensional tectonics that occurred during uplift, and thinning of the upper crust after the collisional stages in the northern edge of the North China Craton, may be the key factors controlling Au concentration and enrichment in the Paishanlou deposit.

THE REGIONAL VARIATION IN THE MANTLE ROCKS OF THE NORTHERN UAE-OMAN OPHIOLITE

THE REGIONAL VARIATION IN THE MANTLE ROCKS OF THE NORTHERN UAE-OMAN OPHIOLITE

On the thermodynamics of listric faults

We investigate a novel fully coupled thermal-mechanical numerical model of the crust in order to trace the physics of interaction of its brittle and ductile layers. In a unified approach these layers develop in a natural transition as a function of the state variables pressure, deviatoric stress, temperature and strain-rate. We find that the main storage of elastic energy lies in the domain where brittle and ductile strain-rates overlap so that shear zones are attracted to this zone of maximum energy dissipation. This dissipation appears as a local heat source (shear heating). The brittle-ductile transition zone evolves through extreme weakening by thermo-mechanical feedback. The physics of the weakening process relies on repeated breaching of a critical energy flux threshold for feedback within this sub-horizontal brittle-ductile flow layer, thus developing unstable slipping events at postand pre-seismic strain-rates. The width- and the temperature domain of the feedback layer is controlled by the activation enthalpy Q of the material. For olivine rheology (Q ∼ 500 kJ/mol) the layer can be extremely thin <500 m and adheres to the 875 K isotherm. For quartz (Q ∼ 135 kJ/mol) the width of the feedback layer fans out into multiple interacting ductile faults covering a temperature domain of 450–600 K. The weakening by thermal-mechanical feedback entirely controls the location and rejuvenation of upper crustal shear zones propagating from the detachment upwards in the form of listric faults. Within the detachment shear layer we identify an astonishing rich dynamics featuring distinct individual creep bursts. We argue that the rich ductile dynamics holds the key to earthquakes in the brittle field.

What controls the occurrence of shallow intraplate earthquakes?

We simulated the occurrence of both intraplate and interplate earthquakes in a simple spring-slider-dashpot model that includes both types of earthquake faults. We found that recurrence intervals of intraplate earthquakes are not controlled by the velocity of the relative plate motion but by the viscosity in the ductile fault zone, the coefficients of friction of both the interplate and intraplate earthquake faults, and the stress drop of intraplate earthquakes, when the stress drop of intraplate earthquakes is assumed to be only a small part of the total shear stress. These findings may open new avenues for the physics-based long-term forecasting of intraplate earthquakes.

Gold mineralisation throughout about 45�Ma of Archaean orogenesis: protracted flux of gold in the Golden Mile, Yilgarn craton, Western Australia

The Golden Mile deposit was discovered in 1893 and represents today the largest Archaean orogenic lode gold system in the world (50 M oz produced gold). The Golden Mile deposit comprises three major styles of gold mineralisation: Fimiston, Oroya and Charlotte styles. Fimiston-style lodes formed at 250 to 350 °C and 100 to 200 MPa and are controlled by brittle–ductile fault zones, their subsidiary fault zone and vein networks including breccias and open-cavity-infill textures and hydrothermally altered wall rock. Fimiston lodes were formed late D1, prior to D2 regional upright folding. Hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite, an increase in sericite and in Fe content of carbonates. Lodes contain siderite, pyrite, native gold, 17 different telluride minerals (Au–Ag tellurides contain ~25% of total gold), tourmaline, haematite, sericite and V-rich muscovite. Oroya-style lodes formed at similar P–T conditions as the Fimiston lodes and are controlled by brittle–ductile shear zones, associated dilational jogs that are particularly well developed at the contact between Paringa Basalt and black shale interflow sedimentary rocks and altered wall rock. The orebodies are characterised by micro-breccias and zones of intense shear zone foliation, very high gold grades (up to 100,000 g/t Au) and the common association of tellurides and vanadian mica (green leader). Oroya lodes crosscut Fimiston lodes and are interpreted to have formed slightly later than Fimiston lodes as part of one evolving hydrothermal system spanning D1 and D2 deformation (ca. 2,675–2,660 Ma). Charlotte-style lodes, exemplified by the Mt Charlotte deposit, are controlled by a sheeted vein (stockwork) complex of north-dipping quartz veins and hydrothermally altered wall rock. The Mt Charlotte orebody formed at 120 to 440 °C and 150 to 250 MPa during movement along closely spaced D4 (2,625 Ma) and reactivated D2 faults with the quartz granophyre in the Golden Mile Dolerite exerting a strong lithological control on gold mineralisation. Veins consist of quartz–carbonate–minor scheelite, and wall-rock alteration comprises chlorite destruction and growth of ferroan carbonate–sericite–pyrite–native gold. Pyrite–pyrrhotite is zoned on the scale of vein haloes and of the entire mine, giving a vertical temperature gradient of 50–100 °C over 1,000 vertical metres. The structural–hydrothermal model proposed consists of four major stages: (1) D1 thrusting and formation of Fimiston-style lodes, (2) D2 reverse faulting and formation of Oroya-style lodes, (3) D3 faulting and dissecting of Fimiston- and Oroya-style lodes, and (4) D4 faulting and formation of Mt Charlotte-style sheeted quartz vein system. The giant accumulation of gold in the Golden Mile deposit was formed due to protracted gold mineralisation throughout episodes of an Archaean orogeny that spanned about 45 Ma. Fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing, efficiency and duration. In addition to the long-lasting fluid plumbing system, the wide variety of transient structural and geochemical traps, multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world.

SHEAR ZONE-HOSTED POLYMETALLIC SULFIDES IN THE SOUTH LIMOUSIN AREA, MASSIF CENTRAL, FRANCE: REMOBILIZED SULFIDE DEPOSITS RELATED TO VARISCAN COLLISIONAL TECTONICS AND AMPHIBOLITE FACIES METAMORPHISM

This paper investigates the relationship between deformation and mineralization developed in amphibolite facies shear zones in the south Limousin area using structural, microstructural, and textural analyses. The investigated shear zone is a ductile thrust fault that stacked low-grade metamorphic rocks onto a highly metamorphosed basement, with both being folded during a late orogenic tectonic phase. The shear zone hosts sulfide-bearing mylonitic quartzites composed of dynamically recrystallized quartz, syntectonic pyrite, pyrrhotite, sphalerite, chalcopyrite, and late covellite. Mylonites display northwest-southeast–trending lineation and asymmetric microstructures, including sigmoidal quartz aggregates and shear bands, that indicate a northwestward movement along the shear zone. Sulfides occur mainly along shear bands and exhibit primary, undeformed textures, attesting to their syntectonic emplacement. Based on petrographic analyses of the metamorphic host rocks, mineralization formed at P = 5.0 to 5.5 kbars, and T = ~300° to 600°C, consistent with P-T conditions for recrystallization of metapelites and metabasites involved in the shear zone. All data indicate that ductile deformation and emplacement of mineralization were coeval. The occurrence of similar pretectonic sulfide deposits in the hanging wall of the mineralized shear zone suggests that the sulfides in the thrust fault were remobilized from preexisting mineralization by fluid that migrated toward the thrust. This is the first evidence for thrust zone-hosted mineralization within the Massif Central and attests to the role of crustal-scale thrust faults as occasional pathways for mineralized fluids during the upper Paleozoic continental collision. These results suggest that ductile thrust faults may be important sites of ore deposition in the Variscan belt of western Europe and in similar collisional belts elsewhere.

Positive feedback of interacting ductile faults from coupling of equation of state, rheology and thermal-mechanics

We present a fully coupled energy approach to investigate the important problem of ductile faulting within the lithosphere. Our starting setup consists of a 70 km×106 km segment of a model lithosphere containing a random set of thermal perturbations. This allows the development of multiple sets of interacting ductile faults. The perturbations diffuse through conduction while the model is subject to constant velocity pure shear boundary conditions. The entire lithosphere has two domains, one governed chiefly by the Peierls mechanism (cold top part) and another by power law creep (hot lower part). In the calculations, the primary creep mechanisms are not assigned but activated by their dominance through adding power and Peierls creep strain-rates. The water content is always below solubility, hence brittle fracture is not considered in this model. Extreme localization through thermal–mechanical feedback is observed in interacting ductile faults inverse cascading from short- to long-length scales. Another outstanding effect of interacting faults is the remarkable ability to reduce the time scale leading to finite amplitude-instability. Both Peierls- and power law-domains are affected, but the Peierls-domain exhibits true, ductile, faults while the power law domain develops unstable slipping events akin to brittle shear faults. This affinity to a seismic response is due to the exponential temperature dependence of power law creep leading to short creep-bursts with time scales less than a day. The addition of water can stabilize the brittle like response into a more ductile one through its effect on the activation energy. The main storage of elastic energy lies in the Peierls domain so that shear zones are attracted to an isotherm 875–1100 K, thus marking the transition between both creep regimes in the p– T–strain-rate space. Criteria for stability and accuracy in the presence of thermal feedback are developed. These show that extremely high mesh resolution (∼100–500 m local resolution) is required for accurately assessing the potential for shear localization. The potential for upscaling schemes based on critical Peclet and dissipation numbers is discussed and the critical energy quantity (dissipation number) for spontaneous symmetry breaking is introduced.

Regional deformation of the Sierra de San Luis, Argentina: Implications for the Paleozoic development of western Gondwana

Metamorphic and ductile deformation fabrics within the Sierra de San Luis, central Argentina, provide evidence for the Early to Middle Paleozoic development of the paleo‐Pacific margin of Gondwana. Presumed Vendian‐aged metasedimentary rocks within both the Sierra de San Luis and the Sierras de Córdoba preserve early pressure solution cleavage development. Cambrian peak metamorphism in the Sierras de Córdoba and Ordovician peak metamorphism in the Sierra de San Luis indicate that juxtaposition of these two terranes did not occur prior to the Late Ordovician. Peak metamorphism and regional folding of the Sierra de San Luis metasedimentary rocks is equated with early stages of the development and shortening of the Famatinian magmatic arc along the western margin of Gondwana. Extensive, NNE trending zones of upper greenschist‐ to amphibolite‐facies ductile deformation within the Sierra de San Luis and along the western margin of the Sierra de Comechingones record the dextral oblique juxtaposition of the Sierra de San Luis terrane against the southwestern margin of the Sierras de Córdoba, coincident with, or closely followed by, the suture of the exotic Precordillera terrane to the western margin of Gondwana by the Early Devonian. Middle to Late Devonian restricted, lower greenschist‐grade reactivation of ductile faults may record the accretion of the Chilenia terrane, the final stage of Paleozoic convergent tectonism along this segment of western Gondwana.