Brittle Field Research Articles

Structural Architectures of Precambrian Metamorphites in and Around Rongmil –Rongjeng Area, East Garo Hills Districts Meghalaya

The present work elucidates the Precambrian metamorphites in and around Rongmil-Rongjeng area in the light of their polydeformational episodes that witness four pfases of deformation showing multiple interference patterns in the ductlile to brittle fields. Field as well as microstructural study reveals that all the pre existing depositional fabrics have been affected by BD1, the first phase of deformation under progressive metamorphic phase leading to the formation of most dominant, highly penetrative schistose and gneissose fabrics of rocks of amphibolites facies. The second phase of deformation, BD2 forming NW-SE trending up and down facing folds of probable plane non cylindrical nature, was responsible for layer parallel shortening (buckling) of the mechanically active BS0 and BS1 fabrics in the NW-SE longitudinal compressive tectonics. The third phase folds, BF3 refolded the earlier set of folds into non –plane non cylindrical nature and occur as non coaxial, open warp type gently plunging with near vertical axial plane striking NW-SE direction. BD4, the fourth phase deformation is considered as brittle phase and represents the joints, fractures and fault structures.

Unveiling ductile deformation during fast exhumation of a granitic pluton in a transfer zone

Exhumation and cooling of upper crustal plutons is generally assumed to develop in the brittle domain, thus determining an abrupt passage from crystallization to faulting. To challenge this general statement, we applied an integrated approach involving meso- and micro-structural studies, thermochronology, geochronology and rheological modeling to the Miocene syn-tectonic Porto Azzurro pluton on Elba (Tuscan archipelago – Italy). This pluton is emplaced in an extensional setting, and we have realized that its fast exhumation is accompanied by localized ductile shear zones, developing along dykes and veins, later affected by brittle deformation. This is unequivocally highlighted by field studies and EBSD-aided microstructural analysis. To constrain the emplacement and exhumation rate of the Porto Azzurro pluton we performed U–Pb zircon dating and (U + Th)/He apatite thermochronology, which resulted in a magma emplacement age of 6.4 ± 0.4 Ma and an exhumation rate of 3.4–3.9 mm/yr. Thermo-rheological modeling established that localized ductile deformation occurred at two different time steps: within felsic dykes when the pluton first entered into the brittle field at 380 kyr, and along quartz-rich hydrothermal veins at c. 550 kyr after pluton emplacement. Hence, the major conclusion of our data is that ductile deformation can affect a granitic intrusion even when it is entered into the brittle domain in a fast exhuming extensional regime.

Transfer zones in Mediterranean back-arc regions and tear faults

Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat in back-arc regions. Because the space available for retreating slabs is limited in the Mediterranean realm, slab tearing during retreat has been a major ingredient of the evolution of this region since the end of the Eocene. The association of detailed seismic tomographic models and extensive field observations makes the Mediterranean an ideal natural laboratory to study these transfer zones. We review in this paper the various structures in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between high-angle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts. Simple, archetypal, crustal-scale strike-slip faults are in fact rare in these contexts above slab tears. Transfer zones are in general instead wide deformation zones, from several tens to several hundred kilometers. A partitioning of deformation is observed between the upper and the lower crust with low-angle extensional shear zones at depth and complex association of transtensional basins at the surface. In the Western Mediterranean, between the Gulf of Lion and the Valencia basin, transtensional strike-slip faults are associated with syn-rift basins and lower crustal domes elongated in the direction of retreat (a-type domes), associated with massive magmatic intrusions in the lower crust and volcanism at the surface. On the northern side of the Alboran Sea, wide E-W trending strike-slip zones in the brittle field show partitioned thrusting and strike-slip faulting in the external zones of the Betics, and E-W trending metamorphic core complexes in the internal zones, parallel to the main retreat direction with a transition in time from ductile to brittle deformation. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. Deep structures are not known there. In the Aegean-Anatolian region, two main tear faults with different degrees of maturity are observed. Western Anatolia (Menderes Massif) and the Eastern Aegean Sea evolved above a major left-lateral tear in the Hellenic slab. In the crust, the differential retreat was accommodated mostly by low-angle shear zones with a constant direction of stretching and the formation of a-type high-temperature domes exhumed from the middle and lower crust. These low-angle shear zones evolve through time from ductile to brittle. On the opposite side of the Aegean region, the Corinth and Volos Rift as well as the Kephalonia fault offshore, accommodate the formation of a dextral tear fault. Here, only the brittle crust can be observed, but seismological data suggest low-angle shear zones at depth below the rifts. We discuss the rare occurrence of pure strike-slip faults in these contexts and propose that the high heat flow above the retreating slabs and more especially above slab tears favors a ductile behavior with distributed deformation of the crust and the formation of low-angle shear zones and high-temperature domes. While retreat proceeds, aided by tears, true strike-slip fault system may localize and propagate toward the retreating trench, ultimately leading to the formation of new plate boundary, as shown by the example of the North Anatolian Fault.

Structural evolution of the Cruzeiro do Nordeste shear zone (NE Brazil): Brasiliano-Pan-African- ductile-to-brittle transition and Cretaceous brittle reactivation

The Borborema Province (NE Brazil) is characterized by the development of continental-scale transcurrent shear zones related to the Neoproterozoic Brasiliano-Pan-African Orogeny. These shear zones commonly border Cretaceous intraplate sedimentary basins. This work presents a structural and microstructural study of the Cruzeiro do Nordeste shear zone (CNSZ), which limits the northern border of the Jatobá Basin. The ductile deformation of the CNSZ is marked by high-angle, ENE-trending foliation bearing subhorizontal stretching lineation, with numerous kinematic indicators showing dextral shearing. We documented a continuous transition from high-temperature (high-T) to low-temperature (low-T) (c. 650 °C to c. 300 °C) ductile fabrics characterized, at the high-T end, by quartz recrystallization by grain boundary migration and feldspar recrystallization by subgrain rotation, and, at the low-T end, by bulging recrystallization of quartz and extensive fracturing of feldspars. The cooler semi-brittle to brittle deformation superimposed on the mylonites is characterized by conjugate pairs of strike-slip mesoscopic faults. The orientation of these faults (WNW-ESE, dextral, and N–S, sinistral) suggests they were formed under the same stress field than the ductile fabrics and thus evidence a continuum deformational from the ductile to the brittle field associated with exhumation during transcurrent tectonics. Brittle reactivation of the CNSZ is characterized by normal faults overprinting the mylonitic foliation. We report a U–Pb age from fault-hosted calcite slickenfibres of 135 ± 4.7 Ma, which provides constraints on the timing of brittle reactivation that can be associated with opening of the South Atlantic Ocean.

Discovery of Mega‐Sheath Folds Flooring the Liwan Subbasin (South China Sea): Implications for the Rheology of Hyperextended Crust

AbstractSheath folds recognized on kilometer scale are rare and only described in salt tectonics in seismic data. Here, we present mega‐sheath folds spectacularly imaged in a 3‐D seismic survey in the seismic basement flooring the hyperextended Liwan subbasin in the northern margin of the South China Sea. The sheath folds show eye structures delimited by anastomosing discontinuities. The axial planes of the sheath folds are subparallel to a rift‐related, extensional detachment surface separating hyperextended seismic basement from the syn‐rift sedimentary sequence. The sheath folds as well as the anastomosing discontinuities formed during hyperextension by ductile shearing of the forearc inherited and prestructured metasediments. The ductile shearing eventually resulted in the metasediments derived from midcrust unroofed at the footwall of an extension detachment fault. The discovery of mega‐sheath folds and ductile deformation within the basement conflicts with the general assumption that hyperextended domains are in the brittle field. The new seismic observations show the importance of forearc inherited, prestructured anisotropic basement controlling the crustal rheology during hyperextension.

XPS study of Cr segregation in a martensitic stainless steel

Cr martensitic steels are promising materials for structural applications in future nuclear fusion reactors. Because the embrittlement after tempering treatments can be a serious problem, the fracture mode of a steel with 10.5 wt% of Cr treated at 700°C for 18 h has been investigated through Charpy tests in the temperature range from −100°C to +150°C. X‐ray photoelectron spectroscopy (XPS) analyses carried out on the fracture surfaces evidenced the segregation of Cr in both ductile and brittle (quasicleavage) fields. The unexpected result indicates that Cr segregation weakens the atomic bonds; thus, the fracture path in both the cases corresponds to the zones with higher Cr content.

Thermotectonic Evolution of the North Pyrenean Agly Massif During Early Cretaceous Hyperextension Using Multi‐mineral U‐Pb Thermochronometry

AbstractThe North Pyrenean Zone represents a fossil hyperextended passive margin, with limited orogenic overprinting, where the thermal and structural patterns associated with crustal thinning and mantle exhumation can be studied on rocks exposed at the surface. The Agly Massif is a tilted ~10‐km‐thick crustal section of Paleozoic and upper Proterozoic magmatic and greenschist to granulitic metamorphic rocks in the easternmost North Pyrenean Zone. We applied multi‐mineral geochronometry and thermochronometry on structural/crustal transects across the massif to understand the exhumation history of the upper and lower continental crust during extreme crustal thinning and mantle exhumation. Integration of zircon and apatite U‐Pb ages provides unprecedented constraints to understand the decoupled versus coupled extensional evolution, exhumation timing of the middle‐lower crust, and the age of juxtaposition of the upper crust granitic pluton with high‐grade gneisses. The Saint Arnac pluton was emplaced and cooled in the upper crust during the Carboniferous and remained at temperatures between 450 and 180 °C until the Late Cretaceous. The middle to lower crust was metamorphosed during the Carboniferous and remained at temperatures >450 °C until the Aptian, when it was rapidly exhumed along a midcrustal shear zone. Deformation was initially decoupled along a midcrustal ductile shear zone until the whole massif was in the brittle field, with structural juxtaposition of the units, and exhumation was coupled and controlled by a major southward dipping detachment fault at the southern border of the massif. The basement massif and synrift sedimentary rocks record significantly different thermal histories during rifting.

Structure of the Northern Moine thrust zone, Loch Eriboll, Scottish Caledonides

This paper reviews the geology of the Late Ordovician - Silurian Caledonian Moine Thrust zone in the Loch Eriboll region, NW Scotland. We present new detailed mapping and balanced/restored cross-sections to examine its structural evolution. This thrust zone comprises four major thrust slices, carried by the Moine, Lochan Riabhach, Arnaboll and Sole thrusts. Ductile mylonites were derived from lithologies in both the hangingwall (Moine schists) and the footwall (Lewisian basement gneisses, Cambrian quartzites, Ordovician dolomites) of the Moine Thrust. The mylonites are bounded by the Lochan Riabhach thrust below and the Moine thrust above. The Arnaboll thrust sheet comprises Lewisian basement gneisses and Cambrian-Ordovician sedimentary rocks over the Sole Thrust sheet, which exposes a spectacular imbricate sequence of Cambrian-Ordovician sedimentary rocks. Although thrusts sequentially propagated in-sequence from higher (Moine) to lower (Sole) thrusts with time, there is evidence for minor out-of-sequence thrusting and breakback thrusting. Thrusts evolve from deep ductile shears where strain is concentrated along mylonite zones up into the brittle field where they become discrete planar thrust faults. The Early Silurian Scandian metamorphism (~435–415 Ma) is related to a Himalayan style structure with SSE-directed subduction and WNW directed extrusion of a migmatitic core, which we liken to channel flow. We suggest that two major crustal-scale thrusts that extend down into the upper mantle imaged on seismic profiles across the foreland, the Outer Isles and Flannan thrusts, are unrelated spatially or temporally to the Moine thrust sequence.

Experimental identification of the transition from elasticity to inelasticity from ultrasonic attenuation analyses

The transition from recoverable elastic to permanent inelastic deformation is marked by the onset of fracturing in the brittle field. Detection of this transition in materials is crucial to predict imminent failure/fracturing. We have used an ultrasonic pulse transmission method to record the change in waveform across this transition during fracturing experiments. The transition from elastic to inelastic deformation coincides with a minimum in ultrasonic attenuation (i.e., maximum wave amplitude). Prior to this attenuation minimum, the existing microfractures close. After this minimum, new microfractures form and attenuation increases until peak stress conditions, at which point, larger fractures form leading to complete sample failure. In our experiments, velocity changes are not sensitive enough to be indicative for the transition from elastic to inelastic deformation. Analysis of attenuation, not velocity, may thus detect imminent failure in materials. Our results may help detect fracturing in borehole casings or the near-wellbore area, or they may help predict imminent release of energy by seismic rupture.

Significance of first-order faults in folding mechanically isotropic layers: Evidence from the Sudbury Basin, Canada

The Sudbury Basin is a non-cylindrical fold basin demarcated by the layered Sudbury Igneous Complex (SIC), the eastern part of which is transected by prominent curved faults. Folding of the SIC and adjacent rock units occurred in the brittle field and is peculiar due to its petrographically distinct, but initially mechanically similar layers. Overall, the layers are characterized by low levels of solid-state strain raising the question how the layer contacts acquired their curvature. We addressed this question by developing a G.I.S.-based workflow to analyze the orientation and slip vectors of the faults. Slip vectors form clusters of normal and reverse slip along a given fault. The clustering is best interpreted in terms of successive slip events during folding of the SIC. As the faults formed most likely as planar reverse faults prior to folding of the SIC they subsequently served as mechanically anisotropic elements to fold the SIC. The results contribute to (1) better understand the folding mechanisms of thick melt sheets in the upper crust, (2) explain apparently incompatible principal strain axes during progressive deformation, and (3) efficiently analyze the orientation and kinematics of fault zones close to the Earth's surface.

The Ikaria high-temperature Metamorphic Core Complex (Cyclades, Greece): Geometry, kinematics and thermal structure

This work attempted at clarifying the structure of Ikaria using primarily intensive geological mapping combined with structural analysis and a geothermometry approach of Raman spectrometry of carbonaceous material. Foliation over the whole island defines a structural dome cored by high-grade to partially molten rocks. Its exhumation was completed by two top-to-the-N ductile extensional shear zones, operating in the ductile and then the brittle fields, through a single extensional event coeval with progressive strain localization. The thermal structure of the dome with regard to position of ductile shear zones was retrieved using the Raman spectroscopy of carbonaceous material. Peak-metamorphic temperatures range from 390°C in the upper parts of the structure down to 625°C in the core of the dome in the vicinity of migmatites and S-type granite. Pioneer in situ U–Th–Pb analyses on monazite performed on the leucosome parts of these rock yielded a 15.7±0.2Ma age. Ikaria Island thus completes the series of Miocene migmatite-cored Metamorphic Core Complex in the central part of the Aegean domain where a genuine high-temperature zone can be defined as the central Aegean HT zone. There, the extreme stretching of the continental crust is associated with dominantly top-to-the-N kinematics.

The influence of porosity and vesicle size on the brittle strength of volcanic rocks and magma

Volcanic rocks and magma display a wide range of porosity and vesicle size, a result of their complex genesis. While the role of porosity is known to exert a fundamental control on strength in the brittle field, less is known as to the influence of vesicle size. To help resolve this issue, here, we lean on a combination of micromechanical (Sammis and Ashby's pore-emanating crack model) and stochastic (rock failure and process analysis code) modelling. The models show, for a homogenous vesicle size, that an increase in porosity (in the form of circular vesicles, from 0 to 40 %) and/or vesicle diameter (from 0.1 to 2.0 mm) results in a dramatic reduction in strength. For example, uniaxial compressive strength can be reduced by about a factor of 5 as porosity is increased from 0 to 40 %. The presence of vesicles locally amplifies the stress within the groundmass and promotes the nucleation of vesicle-emanating microcracks that grow in the direction of the applied macroscopic stress. As strain increases, these microcracks continue to grow and eventually coalesce leading to macroscopic failure. Vesicle clustering, which promotes the overlap and interaction of the tensile stress lobes at the north and south poles of neighbouring vesicles, and the increased ease of microcrack interaction, is encouraged at higher porosity and reduces sample strength. Once a microcrack nucleates at the vesicle wall, larger vesicles impart higher stress intensities at the crack tips, allowing microcracks to propagate at a lower applied macroscopic stress. Larger vesicles also permit a shorter route through the groundmass for the macroscopic shear fracture. This explains the reduction in strength at higher vesicle diameters (at a constant porosity). The modelling highlights that the reduction in strength as porosity or vesicle size increases is nonlinear; the largest reductions are observed at low porosity and small vesicle diameters. In detail, we find that vesicle diameter can play an important role in dictating strength at low porosity but is largely inconsequential above 15 % porosity. Vesicle clustering and stress lobe interaction are implicit at high porosity, regardless of the vesicle diameter. In the case of an inhomogeneous vesicle size, the microcracks grow from the largest vesicles, and brittle strength is closer to that of the largest vesicle end-member. The results of this study highlight the important role of vesicle size, and the complex interplay between porosity and vesicle size, in controlling the brittle strength of volcanic rocks and magma.

Rate‐ and strain‐dependent brittle deformation of rocks

We develop a unifying framework to quantify rate‐dependent deformation in the brittle field and establish links between the microscale time‐dependent crack growth processes and the macroscopically observed rate dependency. Triaxial deformation experiments have been performed under both constant strain rate and constant stress (creep) conditions on three types of sandstone. The measured relative evolution of P wave speeds as a function of inelastic axial strain is similar for both types of test, despite differences in strain rate of up to 3 orders of magnitude. This similarity indicates that there exists a direct, time‐independent link between the microstructural state (as reflected by the variations in P wave speed) and the inelastic axial strain. Comparison of applied stresses between constant strain rate and creep experiments as a function of inelastic strain indicates that creep deformation requires less mechanical work to bring the sample to failure. This energy deficit corresponds to a stress deficit, which can be related to a deficit in energy release rate of the microcracks. We establish empirically that the creep strain rate is given by , where ΔQ is the stress deficit (negative) and σ∗ is an activation stress. This empirical exponential relation between creep strain rate and stress deficit is analogous to rate‐and‐state friction law. We develop a micromechanical approach based on fracture mechanics to determine the evolution of an effective stress intensity factor at crack tips during creep deformation and estimate the activation volume of the stress corrosion reaction responsible for brittle creep.

Constraints on brittle field exhumation of the Everest‐Makalu section of the Greater Himalayan Sequence: Implications for models of crustal flow

New apatite and zircon fission track (FT) data from the summit slopes of Everest and along the Barun, Arun, Dudh Kosi, and Kangshung valleys that drain the Everest and Makalu massifs cover a vertical sample transect of almost 8000 m of the Eastern Nepal Greater Himalayan Sequence (GHS). Apatite FT ages range from 0.9 ± 0.3 Ma to 3.1 ± 0.3 Ma in the GHS with ages increasing systematically with elevation. Apatite FT ages in the Everest Series and summit Ordovician limestones are much older, up to 30.5 ± 5.1 Ma. Zircon FT ages from the GHS range from 3.8 ± 0.4 Ma to 16.3 ± 0.8 Ma. The brittle exhumation rates calculated from these data show the GHS was exhumed, since ∼9 Ma, at an average rate of 1.0 ± 0.2 mm/a. Pliocene exhumation rates are higher at 1.7 ± 0.3 mm/a. These values are not significantly different from the estimate of ductile exhumation rates of 1.8 mm/a recorded by metamorphic minerals undergoing decompression between 18.7 and 15.6 Ma but are well below the values (up to 10 mm/a) used by thermomechanical models for ductile channel flow in the GHS. If representative of the GHS these models will therefore require further ‘tuning’. Higher exhumation rates in the Pliocene have also been observed in other parts of the Himalaya and points to a regional cause, likely increased erosion due to the onset of late Pliocene–Pleistocene glaciation of the high Himalaya.

An experimental study of the brittle‐ductile transition of basalt at oceanic crust pressure and temperature conditions

The brittle to ductile transition (BDT) in rocks may strongly influence their transport properties (i.e., permeability, porosity topology…) and the maximum depth and temperature where hydrothermal fluids may circulate. To examine this transition in the context of Icelandic crust, we conducted deformation experiments on a glassy basalt (GB) and a glass‐free basalt (GFB) under oceanic crust conditions. Mechanical and micro‐structural observations at a constant strain rate of 10−5 s−1 and at confining pressure of 100–300 MPa indicate that the rocks are brittle and dilatant up to 700–800°C. At higher temperatures and effective pressures the deformation mode becomes macroscopically ductile, i.e., deformation is distributed throughout the sample and no localized shear rupture plane develops. The presence of glass is a key component reducing the sample strength and lowering the pressure of the BDT. In the brittle field, strength is consistent with a Mohr‐Coulomb failure criterion with an internal coefficient of friction of 0.42 for both samples. In the ductile field, strength is strain rate‐ and temperature‐dependent and both samples were characterized by the same stress exponent in the range 3 < n < 4.2 but by very different activation energy QGB = 59 ± 15 KJ/mol and QGFB = 456 ± 4 KJ/mol. Extrapolation of these results to the Iceland oceanic crust conditions predicts a BDT at ∼100°C for a glassy basalt, whereas the BDT might occur in non‐glassy basalts at deeper conditions, i.e., temperatures higher than 550 ± 100°C, in agreement with the Icelandic seismogenic zone.

A continuum mechanics approach to quantify brittle strain on weak faults: application to the extensional reactivation of shallow dipping discontinuities

A number of field observations suggest that sliding on fault planes may occur at very shallow dip in the brittle field. The existence of active low angle normal faults is much debated because (1) the classical theory of fault mechanics implies that faults are locked when the dip is less than 30° and (2) shallow dipping fault planes do not produce large earthquakes (M > 5.5). To reconcile observations and theory, we propose a new model for fault reactivation by introducing an elasto-plastic frictional fault gouge as an alternative to the classical dislocation models with frictional properties. Contrary to the classical model which implies that the dilation angle ψ equals the friction angle φ, our model accounts for ψ < φ and permits ψ < 0 in the fault gouge as deduced from laboratory and field observations. Whilst the predicted locking angles differ in most cases by less than 10° from the classical model, a significant amount of plastic strain (strain occurring in elasto-plastic regime) is predicted to occur on badly oriented faults prior to locking in a strain-hardening regime. We describe four modes of reactivation which include complete/partial reactivation with/without tensile failure in the surrounding medium. This paper presents analytical solutions to separate those four different modes as well as numerically computed estimates of the amount of plastic strain accumulated before locking. We conclude that plastic strain on badly oriented faults is favoured by compaction of the fault gouge and that those faults are probably aseismic because this strain always occurs in a hardening regime.

Earthquake-induced soft-sediment deformation structures in Upper Jurassic open-marine microbialites (Neuquén Basin, Argentina)

Penecontemporaneous decimetre-scale soft-sediment deformation structures are reported from the basal part of the Upper Jurassic–Lower Cretaceous Vaca Muerta Formation, in the Malargüe–Las Leñas area of the back-arc Neuquén Basin (Mendoza Province, Central Andes). The deformed interval (Amarillas bed) is only 0.3 to 0.9 m thick but occurs in a wide area, larger than 1500 km 2. Its age, determined by ammonite biostratigraphy, is Early Tithonian. The soft-sediment deformation structures were generated in finely laminated, partially consolidated, organic-rich, carbonate microbialites that were deposited in open-marine, poorly oxygenated settings, apparently devoid of any significant slope. Those structures include boudins of different sizes and complexity, a variety of folds, normal (listric) dm-scale faults, sub-horizontal detachment surfaces and other features, which are part of several larger-scale, complex slump structures. Deformation was dominantly plastic but near to the ductile–brittle field transition. On the basis of the observed soft-deformation structures, their geographic distribution, their lateral homogeneity, and the geodynamic framework of the basin in which it was generated, the Amarillas bed can be tentatively attributed to a large, intermediate-depth earthquake that occurred within the plate that subducted beneath the Andean continental margin and the Neuquén back-arc basin.

S.-I. Karato 2008. Deformation of Earth Materials. An Introduction to the Rheology of Solid Earth. x + 463 pp. Cambridge, New York, Melbourne: Cambridge University Press. Price £45.00, US $90.00 (hard covers). ISBN 9780 521 84404 8.

S.-I. Karato 2008. Deformation of Earth Materials. An Introduction to the Rheology of Solid Earth. x + 463 pp. Cambridge, New York, Melbourne: Cambridge University Press. Price £45.00, US $90.00 (hard covers). ISBN 9780 521 84404 8. - Volume 147 Issue 2

Rapid exhumation of deep crust in an obliquely convergent orogen: The Kaoko Belt of the Damara Orogen

The exhumation of deep crustal rocks and juxtaposition of structural‐metamorphic domains from different depths in a transpressional orogen may occur during the prograde evolution of the orogen by vertical extrusion or during the retrograde evolution of the orogen via extension. Metamorphic petrology, kinematics, and thermochronology of strike‐slip shear zones in the Kaoko Belt of the Damara Orogen are used to evaluate extrusion and extension processes in this transpressional orogen. Mineral assemblages and deformation mechanisms record shearing at pressures of 4–6 kbar and temperatures of ∼550°C for the Three Palms mylonite zone, 600–650°C for the Purros mylonite zone, and ∼630–700°C for the Village and Khumib mylonite zones. The Three Palms mylonite zone, which separates the accreted Coastal Terrane from the former passive margin of the Congo Craton, exhibits progressive deformation during decreasing temperatures through lower greenschist facies and into the brittle field, with consistent oblique normal shear indicators. Lower‐temperature fabrics and brittle features also overprint the Village mylonite zone. The207Pb‐206Pb (titanite) and40Ar/39Ar (hornblende, muscovite, and biotite) data indicate rapid cooling, at rates of 30–100°C/Ma, of all high‐grade structural domains in the core of the Kaoko Belt between circa 535 and 525 Ma, which is about 20–30 Ma younger then peak metamorphism. The40Ar/39Ar data from synkinematic muscovite fish in the retrograde shear zone assemblages indicate that the Khumib, Village, and Three Palms mylonite zones were actively deforming at temperatures below 350°C at circa 530–524 Ma. These data indicate that the high‐grade metamorphic rocks of the Kaoko were rapidly exhumed and juxtaposed after the main transpressive deformation by oblique extension. Reactivation of the shear zones and tectonic exhumation of high‐grade structural domains from beneath the accreted Coastal Terrane was caused by collision in the Damara Belt and terminal accretion of Gondwana.

Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and its tectonic significance

The Jurassic Pindos and Vourinos ophiolites in the Western Hellenides of Greece are part of the Mesohellenic mafic-ultramafic slab underlying the Eocene–Miocene sedimentary basin (Mesohellenic Trough). The tectonic incorporation of this oceanic slab into the western edge of the Pelagonian subcontinent occurred via trench – passive margin collision in the late Jurassic. Much of the tectonic architecture of these ophiolites, particularly Vourinos, was acquired during progressive inhomogeneous deformation associated with the initial displacement of the Jurassic oceanic crust from its original igneous environment of formation, and its subsequent tectonic emplacement eastward onto the Pelagonian margin. The heterogeneous deformation in the mantle and crustal sequences of the Pindos–Vourinos ophiolites occurred in ductile, ductile-brittle, and brittle fields synchronously, as the Jurassic oceanic lithosphere was translated eastward; it also resulted in differential horizontal rotations within the displaced oceanic slab. Areas retaining high-temperature (diapiric) mantle fabric appear to have been “passively” translated by lower temperature ductile shearing, commonly along mylonite zones. Individual dunite bodies in the harzburgite tectonites indicate movement distances of at least kilometer scale. Pervasively mylonitic domains within the upper mantle peridotites suggest elongation on the order of five to ten times in the direction of ophiolite emplacement. Seafloor-spreading related, high-temperature mantle structures within the Pindos and Vourinos ophiolites are sub-parallel. Imprinted ductile kinematic indicators and ductile shear zones pervasive to the mantle and lower crustal sections in both ophiolites are also sub-parallel, consistent with the direction of tectonic vergence, and appear traceable across the sedimentary overburden of the Mesohellenic Trough. These geometric relations imply that the Pindos–Vourinos ophiolites retain their relative orientations from the time of cessation of ductile deformation within the ophiolitic slab. More than 90° anticlockwise horizontal rotation of the Vourinos ophiolite seems to have taken place during progressive simple shear deformation associated with the eastward emplacement of the Jurassic oceanic lithosphere. This kind of rotational deformation postdating the igneous accretion of ancient oceanic crust may be common in many ophiolites and should be carefully distinguished from tectonic extension-related vertical rotational deformation developed during their seafloor spreading evolution.