Ductile Zones Research Articles

Convective Flows in a TVZ-like Setting with a Brittle/Ductile Transition

This article presents a simple method for coupling the equations for fluid and heat flow in a porous medium with a rock mechanics model based on a simplified Mohr–Coulomb yield criterion. The aim is to investigate the effects of introducing a brittle–ductile transition zone (BDTZ) into models of the large scale hydrology of systems like the Taupo Volcanic Zone (TVZ) in New Zealand. The coupling between fluid/heat flow and rock mechanics is only partial in that it assumes that the local strain rate and lithostatic pressure are known a priori. A simple empirical relationship between the permeabilities of rock in its brittle and ductile states is also assumed, which mimics the effect of slow thermal creep in closing fluid pathways by reducing the effective permeability. This model is first applied to a number of simple situations where large scale convection of groundwater occurs above a depth of 20-km. These examples demonstrate the formation of a horizontal brittle–ductile transition zone under conductive conditions with uniform strain, and how this responds to local changes in pressure, temperature and strain rate. The presence of low permeability below the BDTZ effectively defines the greatest depth to which groundwater can steadily penetrate, providing a feedback to these models which influences the transport of heat and mass on all the length scales. The TVZ provides inspiration for the second model setting. Localised sources of water and heat, modelled on magmatic sill-like structures at a depth of 10-km, induce a BDTZ which shallows from 14-km to less than 10-km, consistent with geophysical estimates in the TVZ. A common feature of the models is that strong downflows of surface water occur in permeable regions adjacent to the heat sources, which depress the BDTZ by several kilometres between the geothermal areas. Water with near-surface temperatures can thus exist at great depths in these regions. Lastly, the models imply that the geothermal areas in the TVZ probably do not occur directly above any localised heat source, but are instead displaced towards the centre of the TVZ rift.

Local stress concentration in the seismic belt along the Japan Sea coast inferred from precise focal mechanisms: Implications for the stress accumulation process on intraplate earthquake faults

Spatial changes in the azimuth of the maximum principal stress (σ1) axis are found in the intraplate region of southwest Japan. It is inferred from stress inversions of numerous precise focal mechanisms that the azimuths of the σ1 axis in the seismic belt along the Japan Sea coast are oriented in N110°E–N130°E directions, while in the surrounding region they are aligned in almost the EW direction, N90°E–N100°E. Maximum horizontal compressional stresses in an EW direction are widely observed in the shallow crust in the inland plate along the Nankai Trough. However, the maximum horizontal stress in a WNW–ESE direction is seen only in the seismic belt. This spatial change in the stress field in and around the seismic belt is explained by the deformation of an aseismic fault or a ductile fault zone in the lower crust beneath the seismic belt.

Preliminary investigation of a major high-strain zone in the Caledonian Highlands, southern New Brunswick

A major ductile high-strain zone up to 5 km in width can be traced for at least 70 km diagonally across the Avalonian Caledonia terrane of southern New Brunswick. A study of the northeastern part of this zone from the Prosser Mountain area in the northwest to the Point Wolfe River area west of Fundy National Park shows that both the ca. 630-620 Ma Broad River Group and associated plutons and the 560-550 Ma Coldbrook Group contain similar structural elements, related to a largely shared deformational history. Some of this history is apparent also in the 560–550 Ma plutonic rocks. A pervasive foliation (S1) lies parallel to bedding (S0), and although evidently composite (S0-1) in the Broad River Group, this fabric is very heterogeneous in the younger Coldbrook Group, where low strain enclaves are widespread. No folds have been seen of an F1 generation, and no reversals of facing or vergence are apparent. A mineral lineation (L1m) is locally prominent. The plutonic rocks have early fabrics, including a foliation (S1) producing augen-gneiss with a prominent L-tectonite (L1m). S1 also includes a schistosity associated with the growth of white mica and breakdown of feldspar. Geometry suggests that S1 in the granites is related to S0-1 in the supracrustal rocks, and L1m in both units shares a common orientation. S1 and S0-1 are crenulated by a strong second cleavage (S2) axial planar to folds (F2), the large-scale expression of which is an asymmetric synform containing a belt of Coldbrook Group rocks. Kinematic indicators suggest an overall top-to-the-SE motion along thrusts that stack units of Broad River Group, Coldbrook Group, and plutonic rocks. Fabric development in the plutonic rocks implies a history of exhumation beginning under hot, anhydrous conditions, followed by hydration during retrogression as plutonic rocks were tectonically emplaced into this crustal stack. The age of these tectonic events is not yet well constrained, but could be as young as Carboniferous.
 
 RÉSUMÉ
 
 Il est possible de retracer une importante zone de forte contrainte ductile ayant jusqu’à cinq kilomètres de largeur sur une distance de 70 kilomètres en diagonale à travers le terrane avalonien de Caledonia, dans le Sud du Nouveau-Brunswick. Une étude de la partie nord-est de cette zone à partir du secteur du mont Prosser, dans le nord-ouest, jusqu’au secteur de la rivière Pointe Wolfe, à l’ouest du parc national du Canada Fundy, révèle que le groupe d’environ 630 à 620 Ma de la rivière Broad, les plutons connexes et le groupe de 560 à 550 Ma de Coldbrook abritent des éléments structuraux similaires, apparentés à des déformations passées largement partagées. Une certaine partie de ce passé est également apparente dans les roches plutoniques de 560 à 550 Ma. Une foliation intense (S1) se manifeste parallèlement à la stratification (S0) et, même si cette fabrique est nettement composite (S0-1) dans le groupe de la rivière Broad, elle est très hétérogène dans le groupe plus récent de Coldbrook, où les enclaves de faible contrainte sont répandues. On n’a observé aucun pli d’une production F1 et aucune inversion du regard ni de la vergence n’est apparente. Une linéation minérale (L1m), définie par des agrégats de biotite, est localement bien visible. Les roches plutoniques possèdent des fabriques qui se sont constituées pendant et peu après la cristallisation, notamment une foliation (S1) produisant du gneiss oeillé avec L-tectonite (L1m) en évidence. S1 à l’intérieur des roches plutoniques comporte en outre une schistosité associée à la croissance de mica blanc et à la décomposition de feldspath. La géométrie permet de supposer que S1 dans les granites est apparentée à S0-1 dans les roches supracrustales et que la linéation minérale (L1m) dans les deux unités partage une orientation commune. S1 et S0-1 sont crénelés par une seconde schistosité (S2) prononcée, de plan axial par rapport aux plis (F2), dont l’expression à grande échelle est une synforme asymétrique renfermant une ceinture de roches du groupe de Coldbrook. Les indicateurs cinématiques des structures F2 permettent de supposer un mouvement général du sommet vers le sud-est le long des chevauchements qui empilent les unités du groupe de la rivière Broad, du groupe de Coldbrook et des roches plutoniques. Le développement de la fabrique dans les roches plutoniques plus âgées suppose une exhumation passée ayant commencé dans des conditions très chaudes et anhydres pendant et peu après la cristallisation, vers 620 Ma, suivie par une hydratation pendant la rétrogression au moment où les roches plutoniques se sont tectoniquement mises en place à l’intérieur de cet éperon crustal. L’âge des événements tectoniques tardifs n’est pas encore bien circonscrit, mais ils pourraient remonter au Carbonifère. [Traduit par la redaction]

Intrusion of eccentric dikes: The case of the 2001 eruption and its role in the dynamics of Mt. Etna volcano

The 2001 eruption represents one of the most studied events both from volcanological and geophysical point of view on Mt. Etna. This eruption was a crucial event in the recent dynamics of the volcano, marking the passage from a period (March 1993–June 2001) of moderate stability with slow, continuous flank sliding and contemporaneous summit eruptions, to a period (July 2001 to present) of dramatically increased flank deformations and flank eruptions. We show new GPS data and high precision relocation of seismicity in order to demonstrate the role of the 2001 intrusive phase in this change of the dynamic regime of the volcano. GPS data consist of two kinematic surveys carried out on 12 July, a few hours before the beginning of the seismic swarm, and on 17 July, just after the onset of eruptive activity. A picture of the spatial distribution of the sin-eruptive seismicity has been obtained using the HypoDD relocation algorithm based on the double-difference (DD) technique. Modeling of GPS measurements reveals a southward motion of the upper southern part of the volcano, driven by a NNW–SSE structure showing mainly left-lateral kinematics. Precise hypocenter location evidences an aseismic zone at about sea level, where the magma upraise was characterized by a much higher velocity and an abrupt westward shift, revealing the existence of a weakened or ductile zone. These results reveal how an intrusion of a dike can severely modify the shallow stress field, triggering significant flank failure. In 2001, the intrusion was driven by a weakened surface, which might correspond to a decollement plane of the portion of the volcano affected by flank instability, inducing an additional stress testified by GPS measurements and seismic data, which led to an acceleration of the sliding flanks.

40Ar/ 39Ar hornblende geochronology from the Forsmark area in central Sweden: Constraints on late Svecofennian cooling, ductile deformation and exhumation

At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82–1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of 40Ar/ 39Ar hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three 40Ar/ 39Ar hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between 40Ar/ 39Ar and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85–1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83–1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. One explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 °C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in hornblende throughout the area at 1.83–1.81 Ga. It is suggested that the regional exhumation at 1.83–1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains.

Tectonics and thermal structure of western Satpura, India

Increased seismicity and occurrences of hot springs having surface temperature of 36–58 °C are observed in the central part of India (74–81° E, 20–25° N), where the NE trending Middle Proterozoic Aravalli Mobile Belt meets the ENE trending Satpura Mobile Belt. Earlier Deep Seismic Sounding (DSS) studies along Thuadara-Sendhwa-Sindad profile in the area has showed Mesozoic Sediments up to around 4 km depth covered by Deccan Trap and the Moho depth with a boundary velocity ( P n ) of 8.2 km/s. In the present study, surface heat flow of 48 ± 4 mW m −2 has been estimated based on P n velocity, which agrees with the value of heat flow of 52 ± 4 mW m −2 based on Curie point isotherms estimates. The calculated temperature-depth profile shows temperature of 80–120 °C at the basement, which is equivalent to oil window temperature in Mesozoic sediments and around 570–635 °C at Moho depth of 38–43 km and the thermal lithosphere is about 110 km thick, which is comparatively higher than those of adjoining regions. The present study reveals the brittle–ductile transition zone at 14–41 km depth (temperature around 250–600 °C) where earthquake nucleation takes place.

The unroofing history of Naxos and Paros: Constraints and questions from thermochronology and thermal modeling

The Naxos/Paros extensional fault system (NPEFS) accommodated a significant portion of Miocene extension in the Aegean. The exposure is unusually complete, consisting of a thick lower-crustal ductile shear zone grading upwards into a thin, brittle detachment. Thermochronometric systems with closure temperatures documenting passage of the footwall through the brittle (apatite and zircon fission-track and (U-Th)/He) and ductile (hornblende K-Ar) zones provide important constraints on the unroofing history of this system. Brittle-zone slip rates based on fission-track data are generally in the 6–8 km/Myr range spanning times from 13–8 Ma, whereas inferred ductile-zone slip rates average ~5 km/Myr from 16–12 Ma. We used finite element thermal modeling to test the hypothesis that Naxos and Paros unroofed simultaneously and at similar rates, from ~16 Ma to ~8 Ma, with the only difference being a ~12 Ma granodiorite intrusion on Naxos. We found that a simplified model with uniform detachment fault dip (30°) and slip rate (6.5 km/Myr.) adequately explains all of the fission-track and some zircon (U-Th)/He data, although other (U-Th)/He data are problematic. The data are somewhat more fully reproduced if there was some acceleration during the latter stages of unroofing, but large uncertainties in slip rates preclude a firm conclusion. The thermal model also provides a reasonable mechanism for unifying inferred brittle-zone slip rates with slower ductile-zone ones, as the latter may have been rendered artificially low due to isotherm advance during the early stages of detachment faulting. At the same time, the modeling exercise forces a thorough assessment of the thermal budget of the crust before and during the initiation of detachment faulting. Other data have suggested that unroofing began as early as 20 Ma, and that early decompression was associated with heating. Insofar as unroofing is only in itself a cooling mechanism for the footwall, the source of this additional heat, and what happened to it, is a matter requiring further investigation before a complete picture of extension on the NPEFS can be formulated.

Differential Late Paleozoic active margin evolution in South-Central Chile (37°S–40°S) – the Lanalhue Fault Zone

The N–S oriented Coastal Cordillera of South Central Chile shows marked lithological contrasts along strike at ∼38°S. Here, the sinistral NW–SE-striking Lanalhue Fault Zone ( nomen novum) juxtaposes Permo-Carboniferous magmatic arc granitoids and associated, frontally accreted metasediments (Eastern Series) in the northeast with a Late Carboniferous to Triassic basal-accretionary forearc wedge complex (Western Series) in the southwest. The fault is interpreted as an initially ductile deformation zone with divergent character, located in the eastern flank of the basally growing, upwarping, and exhuming Western Series. It was later transformed and reactivated as a semiductile to brittle sinistral transform fault. Rb–Sr data and fluid inclusion studies of late-stage fault-related mineralizations revealed Early Permian ages between 280 and 270 Ma for fault activity, with subsequent minor erosion. Regionally, crystallization of arc intrusives and related metamorphism occurred between ∼306 and ∼286 Ma, preceded by early increments of convergence-related deformation. Basal Western Series accretion started at >290 Ma and lasted to ∼250 Ma. North of the Lanalhue fault, Late Paleozoic magmatic arc granitoids are nearly 100 km closer to the present day Andean trench than further south. We hypothesize that this marked difference in paleo-forearc width is due to an Early Permian period of subduction erosion north of 38°S, contrasting with ongoing accretion further south, which kinematically triggered the evolution of the Lanalhue Fault Zone. Permo-Triassic margin segmentation was due to differential forearc accretion and denudation characteristics, and is now expressed in contrasting lithologies and metamorphic signatures in todays Andean forearc region north and south of the Lanalhue Fault Zone.

A thermal test of the post‐subduction tectonic evolution along the California transform margin

The heat flow pattern along the transform margin of western North America reflects the nature of the tectonic evolution following the cessation of subduction. Whereas the high heat flow in the California Coast Ranges shows post‐subduction heating, the low heat flow in the Great Valley and Sierra Nevada still reflects the Cenozoic subduction thermal regime. We analyzed a combined heat flow dataset from these regions and compared the thermal implications of the proposed two end member tectonic models, namely the slab window and the stalled slab models. The results show that persistence of low heat flow in a wide region agrees better with the stalled slab model. The Coast Range thermal anomaly could be due to combined effects of thermal relaxation of the stalled slab; viscous heating in the ductile zone; and/or an apparent high surface heat flow due to rapid erosion.

Age and kinematics of ductile deformation in the Cerro Durazno area, NW Argentina: Significance for orogenic processes operating at the western margin of Gondwana during Ordovician – Silurian times

The Cerro Durazno Pluton belongs to a suite of Paleozoic granitoid intrusions in NW-Argentina, that are central for understanding the tectonic setting of the western margin of Gondwana in Ordovician and Silurian times. The pluton and its host rocks were tectonically overprinted by metamorphic mineral shape fabrics formed under middle greenschist-facies metamorphic conditions and associated with the nearby Agua Rosada Shear Zone. Kinematic analysis of the shear zone based on the geometric relationship between individual segments of the shear plane and principal axes of mineral fabric ellipsoids indicates reverse-sense of shear with a minor component of left-lateral displacement. This is compatible with the kinematics of other ductile deformation zones in this area, collectively forming a network, which accomplished orogen-parallel extension in addition to vertical thickening. Using the Rb–Sr isotopic system, an undeformed pegmatite dike of the Cerro Durazno Pluton was dated at 455.8 ± 3.6 Ma and mineral fabrics of the Agua Rosada Shear Zone formed at middle greenschist-facies metamorphism gave deformation ages of 437.0 ± 3.8 Ma and ⩽428.4 ± 4.5 Ma. Thus, tectonic overprint at low metamorphic grade occurred about 20–30 Ma after terminal magmatism in the Cerro Durazno area. Our data from the Cerro Durazno area and regional considerations suggest that the western margin of Gondwana was characterized by orogen-parallel extension in addition to crustal thickening as well as episodes of magmatism and ductile deformation that varied greatly in time and space.

Gold Mineralization at the Anomaly A Deposit, Clarence Stream Area, Southwestern New Brunswick: Distal Deposits of a Syn-Deformational Intrusion-Related Gold System?

Gold-bearing mineralization at Anomaly A occurs in polydeformed Ordovician turbidites of the Kendall Mountain Formation of the St. Croix belt in southwestern New Brunswick, and is hosted by WSW-trending, shallow-dipping, brittle–ductile high-strain zones associated with the latest stage of the second of four regional deformation events. Gold-bearing syn-deformational stockwork, massive, and multiply brecciated quartz-sulfide veined zones form irregular pods and sheets up to several meters thick, which contain arsenopyrite, pyrrhotite, pyrite, stibnite, and a variety of rare minerals that are gold-bearing. Mineralized zones are enveloped by locally auriferous, weakly to intensely altered wall rock. Vein paragenesis includes: (1) early (pre- to syn-D 2 ) quartz-chlorite veins that are largely the product of migration of quartz by pressure solution; (2) crosscutting, multiphase, vuggy quartz-sulfide veins that contain most of the gold and are interpreted to be structurally controlled; (3) late, crosscutting, nondeformed, non-gold-bearing veins containing laumontite, chlorite, muscovite, quartz, fluorite, and base metal sulfides. Carbonate-poor alteration associated with zones of gold mineralization is lithology-specific, and appears to reflect overprinting of multiple events. Major elements removed during alteration include Si, Na, Mn, and Mg. The minor and trace elements removed include Co, Li, Sc, and possibly Sr, Ni, and Zn. Minor and trace elements added include Au, Ag, As, Sb, S, and possibly F, Cu, Pb, and Zr. On the basis of proximity, time constraints, and a similar ore mineral assemblage, gold mineralization at Anomaly A is interpreted to be a distal equivalent of the intrusion-related gold mineralization at the nearby Clarence Stream Main zone deposit.

Relation between electrical resistivity and earthquake generation in the crust of West Anatolia, Turkey

In this paper, we present a relation between the earthquake occurrence and electric resistivity structures in the crust, in West Anatolia and the Thrace region of Turkey. The relationship between magnetotelluric georesistivity models and crustal earthquakes in West Anatolia, during a period from 1900 to 2000, is investigated. It is found that most of the large crustal earthquakes occurred in and around the areas of the highest electrical resistivity in the upper crust, although rare small magnitude earthquakes are observed in some parts of the conductive lower crust in West Anatolian extensional terrain. The high-resistivity zones may represent rocks that are probably mechanically strong enough to permit sufficient stress to accumulate for earthquakes to occur in western Anatolia and the Thrace region. However, some recent studies state that the generation of a large earthquake is not only a pure mechanical process, but is closely related to fluid existence. We also reviewed recent world-wide researches including results from the Anatolian data for the first time and discussed all general findings in combination. Our findings show that the boundary between the resistive upper crust and the conductive lower crust correlates well with the cutout depth of the seismicity in West Anatolia and Thrace. This boundary is also attributed to the fluid bearing brittle–ductile transition zone in world literature. Fluid migration from the conductive lower crust to the resistive upper crust may contribute the seismicity in resistive zones. Alternatively, the upper–lower crust boundary may act as a stress concentrator and fluids may help to release strain energy in brittle parts of lower crust, by small magnitude earthquakes, whereas they may help in focusing strain in mechanically strong and electrically resistive zones for large earthquakes to occur.

Timing of deformation phases within the South Finland shear zone, SW Finland

The Palaeoproterozoic Svecofennian crust in southern and central Fennoscandia was established about 1.8 Ga ago after a prolonged history of accretion and intrusion. During late stages of the Svecofennian orogeny, deformation was partitioned into several crustal-scale shear zones in present-day Finland, Sweden and Estonia. One such major ductile deformation zone, ‘the South Finland shear zone’ (SFSZ) extends for almost 200 km through the Åland archipelago in southwestern Finland, and further along the southern and southwestern coast of Finland. This more than a kilometer wide transpressional zone appears to have been repeatedly reactivated. The deformation started with a period of regional, ductile dextral shearing of igneous rocks, producing striped granodioritic and tonalitic gneisses. The ductile phases are locally overprinted and followed by ductile to semi-ductile deformation evidenced by mylonite zones of variable width. The last stage of tectonic activity along the shear zone is recorded by pseudotachylites. Within this study, we dated zircons (SIMS U–Pb) and titanites (ID-TIMS U–Pb) from eight rock samples, and two pseudotachylite whole-rock samples ( 40Ar/ 39Ar) in order to reconstruct the deformation and (re)activation history of the shear zone. The results suggest that the medium-grained gneisses underwent three distinct deformation phases separated by time intervals without regional deformation. The ductile deformation within the study area initiated at ∼1.85 Ga. A second, more intensive deformation phase existed around 1.83 Ga, by which the shear zone was already well developed. Finally, the last ductile event is recorded by ∼1.79 Ga metamorphic titanites in relatively granoblastic granitoid gneisses that nevertheless already display protomylonitic textures, suggesting the initiation of large-scale mylonitisation around or soon after this time. The age of a pseudotachylite sample and, hence, the brittle deformation is bracketed between 1.78 and 1.58 Ga based on the age of pegmatites cut by pseudotachylites as well as 40Ar/ 39Ar minimum ages for the pseudotachylite, respectively. The data imply that the rocks within the study area entered the ductile–brittle transition zone due to rapid cooling and exhumation of the crust after ∼1.79 Ga.

Field rheology and structural evolution of the Homestake shear zone, Colorado

Proterozoic tectonites within the Homestake shear zone (HSZ) in the northern Sawatch Range, Colorado, record two major cycles of progressive deformation under contrasting P–T conditions. Field relations and microstructures in these tectonites can be used to constrain the geometry, kinematics, and rheology of rocks in the HSZ during these events. The first deformation cycle occurred ∼1.7 Ga at temperatures near the granite solidus and produced structures indicative of pervasive viscous flow at low differential stress levels. The second deformation cycle at ∼1.4 Ga resulted in localized plastic and brittle failure concentrated in relatively narrow shear zones and seismogenic faults. This second deformation occurred at temperatures of 350–450°C and produced mylonite, ultramylonite, and pseudotachylyte. Overprinting relationships between brittle-frictional faults and plastic shear zones suggest that both modes of deformation were active at about the same time. Thus, the latter deformation cycle probably occurred near the brittle-plastic transition. Simple calculations using standard rheologic relationships show that tectonic loads of 300–400 MPa—sufficient to drive brittle failure—could have been transmitted by ductile ultramylonite zones deforming at relatively high strain rates (on the order of 10 −9 –10 −11 ). Thus, plastic shear zones and brittle-frictional faults probably acted together as a dynamically coupled deformation system.

Applications of inclusion behaviour models to a major shear zone system: The Nordfjord-Sogn Detachment Zone in western Norway

Rigid inclusion models have reached a stage where one should be able to use them to obtain quantitative values from ductile shear zones. We used natural data collected in three sites and combined analogue and theoretical modelling to assess vorticity, strain, nature of rigid inclusion/matrix interface and confinement in the large-scale, ductile Nordfjord-Sogn Detachment Zone (NSDZ) of the Caledonides of western Norway. Our study shows that: (1) the observed shape preferred orientation (SPO) at higher structural levels of the NSDZ at Site 1, Gjervika, can be explained by simple shear (pure shear/simple shear ratio S r = 0) associated with a slipping inclusion/matrix interface. (2) The observed SPO at deeper structural levels of the NSDZ at Site 2, near Sandane can be produced by simple shear associated with a significant amount of shortening across the shear zone ( S r ≈ 1), acting upon rigid inclusions in slipping contact with the enclosing matrix. (3) Observed back rotated boudins deeper in the NSDZ at Site 3, Biskjelneset, can form in confined flow associated with a considerable amount of shortening across the shear zone ( S r ≥ 0.4). (4) The observed tails of porphyroclasts indicate a minimum (at least local) strain of ca. γ ≈ 20. (5) The clasts in the studied shear zones strongly depart from Jeffery's model [Jeffery, G. B., 1922. The motion of ellipsoidal particles immersed in a viscous fluid. Proceedings of the Royal Society of London A102, 161–179]. The large scale extensional NSDZ under investigation shows evidence of strain partitioning: rocks vary from protomylonites to ultramylonites, and the simple shear and pure shear components are heterogeneously distributed. Therefore, we conclude that flow in the NSDZ was very heterogeneous both at the kilometre and the metre scale. However, the present study suggests that the amount of shortening across the shear plane throughout the NSDZ increases with depth, and the flattening component contributes to exhumation of the eclogite facies rocks in its footwall.

Crustal melting and melt extraction, ascent and emplacement in orogens: mechanisms and consequences

Significant volume of wet melting requires an influx of H 2 O-rich volatile phase. In hydrate-breakdown melting, initial melt accumulation is diffusion-controlled and melt accumulates around peritectic phases in low-pressure sites. As the melt-bearing rock weakens, it becomes porous at a few volume per cent melt, initiating an advective flow regime; as melt volume reaches the melt connectivity transition, melt may be lost from the system in the first of several melt-build-up–melt-loss events. Using mineral equilibria modelling, major and accessory phase controls on melt chemistry are evaluated. In residual migmatites and granulites, microstructures indicate the former presence of melt whereas leucosome networks record melt extraction pathways. Sites of initial melting nucleate shear instabilities; strain and anisotropy of permeability control the form of millimetre- to centimetre-scale inferred melt channels and strong anisotropy promotes high fluid focusing. Focused melt flow occurs by dilatant shear failure of low melt volume rocks, which leads to the formation of melt flow networks allowing accumulation and storage of melt, and forming the link for melt flow from grain boundaries to ascent conduits. Melt ascent is via ductile fractures, which may propagate from dilation or shear bands. Fractures are characterized by blunt tips; also they may exhibit zigzag geometry close to the tips and petrographic continuity between leucosome in the host and granite in the dyke. Horizontal tabular and wedge-shaped intrusions commonly are associated with the brittle–ductile transition zone. Vertical lozenge-shaped intrusions represent congelation of magma in ascent conduits and blobby plutons record lateral expansion localized by instability; these intrusion types characterize emplacement at deeper levels in the crust.

Genesis of the Hongzhen metamorphic core complex and its tectonic implications

The Hongzhen metamorphic core complex is situated in the Yangtze plate to the east of the Dabie orogenic belt. Its ductile detachment zone in the foot wall overprints on the metamorphic complex of the Proterozoic Dongling Group. The present profile of the ductile shear zone with consistent SW-dipping mineral elongation lineation shows antiform and reversed S-shape from northeast to southwest respectively. Exposure structures, microstructures and quartz C-axis fabric all indicate top-to-SW movement for the ductile shear zone. Recrystallisation types of quartz and feldspar in the mylonites demonstrate that the shear zone was developed under the amphibolite facies condition and at mid-crust levels. The metamorphic core complex formed in the Early Cretaceous with a muscovite plateau age of 124.8±1.2 Ma. Regional NE-SW extension along a SW-dipping, gentle detachment zone was responsible for formation of the core complex. Intrusion of the Hongzhen granite with a biotite plateau age of 124.8±1.2 Ma rendered the ductile shear zone curved, uplifted and final localization of the core complex. The Hongzhen metamorphic core complex suggests that the Early Cretaceous magmatism in this region took place under the condition of regional extension and the eastern Yangtze plate also experienced lithospheric thinning.

The August 2002 earthquake sequence in north Afar: Insights into the neotectonics of the Danakil microplate

In August 2002, there was high seismic activity in Afar concentrated at the plateau margin of the northern Ethiopian rift east of Mekele, near the western part of the Danakil microplate. The spatial and temporal distributions of this seismic activity over four weeks indicate the NNW propagation of the Gulf of Aden rift across the Afar Depression towards the western Ethiopian plateau. Fault plane solutions for six larger earthquakes from the August 2002 sequence are estimated from moment tensor inversion of local broadband waveform data. The results show only normal faulting on NNW trending and NE dipping faults, which agree with tectonics of the area and distribution of aftershocks. No strike-slip component is observed in any of our fault plane solutions or those of other workers including Harvard CMT solutions in the region. Such motion would be indicative of oblique-slip deformation between the Nubian plate and the Danakil microplate consistent with counter-clockwise rotation of the microplate. Hypocentral depths of well-constrained events are 5–7 km, which is the approximate elastic plate thickness in the Main Ethiopian rift, possibly indicating the depth to the brittle–ductile transition zone in this part of the Afar Depression. The shallowness of the depth estimates agree with the macroseismic reports available from a wide area in northern Ethiopia. Potential future shallow crustal deformation may cause significant loss of human life and damage to property in the densely populated highland region around Mekele unless measures are taken in improving building standards. The b-value for this sequence is estimated to be 0.66 using a least squares fit, while it is 0.67 ± 0.16 from a maximum-likelihood approach. This estimated b-value is low or the frequency of occurrence of relatively larger magnitude events is high indicating that it is a highly stressed region as evidenced by the recent increase of the seismicity in the area.

Thermomechanical analysis of strain localization in a ductile detachment zone

At the latitude of the Thor‐Odin dome (British Columbia) the Columbia River Detachment defines the eastern margin of the Shuswap metamorphic core complex and localizes in a 1 km thick muscovite‐bearing quartzite mylonite. We present a combined40Ar/39Ar, (micro)structural, and oxygen isotope study of the deformation history in the detachment and evaluate the spatial and temporal relationships between microstructure formation and localization of strain. High‐precision40Ar/39Ar geochronology from different levels in the mylonite delineates a pattern of increasingly younger (49.0 to 47.9 Ma) deformation ages in deeper levels of the mylonitic footwall. The correlation of40Ar/39Ar ages with decreasing deformation temperatures (∼550°–400°C) in the top 200 m of the mylonite indicates that deformation migrated downward from the contact with the hanging wall. Strain localization was diachronous in progressively deeper levels of the footwall and was likely controlled by fluid‐assisted strain hardening due to advective heat removal and contemporaneous reaction weakening due to dissolution‐reprecipitation of white mica. The observed constant high‐stress microstructures across the entire detachment indicate that flow stress was buffered by the interplay of strain rate and temperature, where high strain rates at elevated temperature produced the same microstructure as lower strain rates under decreasing temperature conditions. The combined data suggest that the complex interplay among temporally nonuniform rates of footwall exhumation, heat advection, and embrittlement by meteoric fluids strongly determines the thermomechanical behavior of extensional detachments.

Microstructure of low alloyed steel close to the fracture surface

A transmission electron microscopy study of a tempered bainitic nuclear reactor pressure vessel steel was carried out. Cross-section thin foils from nickel electroplated Charpy V-notch specimens fractured in impact and quasi static loading at −30 °C were prepared in the ductile tearing and cleavage regions of the fracture surface. In the ductile tearing zone, the microstructure was very heterogeneous. Dislocation cells, shear bands, and fine heavily deformed subgrains were found. The deformation in the impact specimen was localized only in the vicinity of the fracture surface, where long thin subgrains situated along the fracture surface were often observed. In the quasi static three-point bend specimen, the strain localization was found also in deeper areas under the fracture surface. There were shear bands (bundles of long thin subgrains) mostly aligned in parallel to the fracture surface. Numerous areas of the shear band intersections (at ∼45° to the main shear band direction) were also observed. In the vicinity of cleavage zones of both types of specimens, no significant differences in the microstructure with respect to the bulk material were observed. Only an increased dislocation density was found.