Greenschist Facies Mylonites Research Articles

Multi-stage carbonate veins at IODP Site U1504 document Early Cretaceous to early Cenozoic extensional events on the South China Sea margin

Recognition of the pre-spreading tectonic characteristics of the South China Sea (SCS) continental margin is key to understanding how the SCS opened. However, information on this subject is extremely scarce because of the lack of direct chronological constraints on deformation events. The International Ocean Discovery Program sampled greenschist-facies mylonite from the basement of the Outer Margin High at site U1504 in the SCS, which could provide information on the pre-spreading history. The microstructure analysis revealed that two episodes of extension had affected the mylonites, namely early ductile and late brittle deformation. Pre-mylonite, syn-mylonite and post-mylonite carbonate veins were identified on the basis of the intersecting relationships with the mylonite foliation. The pre-mylonite carbonate veins yielded U–Pb ages of 210 ± 20 and 195 ± 32 Ma, which might represent the age of the protolith. The age of the syn-mylonite carbonate vein is 135 ± 12 Ma. No effective ages were obtained for the post-mylonite carbonate veins using U–Pb dating. The δ13C, δ18O and 87Sr/86Sr compositions of the post-mylonite carbonate veins suggest that they were formed by hydrothermal fluid precipitation dominated by seawater. Considering the extensively developed marine environment in the northern continental margin after the late Eocene, the post-mylonite carbonate veins at site U1504 likely formed in the Eocene or later. Combining the microstructure, geochemistry and seismic profile, we hypothesize that post-mylonite carbonate veins were formed during early Cenozoic rifting. In reference to the geological background, we conclude that the basement of the SCS margin experienced at least two stages of extension before spreading: the first stage occurred during the Early Cretaceous and was caused by rollback of the Paleo-Pacific plate; the second occurred in the early Cenozoic because of passive rifting. These multiple extensional events of the active continental margin and of the passive margin collectively resulted in thinning the SCS continental margin.

Crustal deformation and exhumation within the India-Eurasia oblique convergence zone: New insights from the Ailao Shan-Red River shear zone

Abstract During the collision of India and Eurasia, regional-scale strike-slip shear zones played a key role in accommodating lateral extrusion of blocks, block rotation, and vertical exhumation of metamorphic rocks as presented by deformation on the Ailao Shan-Red River shear zone (ARSZ) in the Eastern Himalayan Syntaxis region and western Yunnan, China. We report structural, mica Ar/Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) data from the Diancangshan massif in the middle segment of the ARSZ. These structural data reveal that the massif forms a region-scale antiform, bordered by two branches of the ARSZ along its eastern and western margins. Structural evidence for partial melting in the horizontal mylonites in the gneiss core document that the gneiss experienced a horizontal shear deformation in the middle crust. Muscovite Ar/Ar ages of 36–29 Ma from the core represent cooling ages. Muscovite Ar/Ar ages of 25 and 17 Ma from greenschist-facies mylonites along the western and southern shear zones, respectively, are interpreted as recording deformation in the ARSZ. The AFT ages, ranging from 15 to 5 Ma, represent a quiescent gap with a slow cooling/exhumation in the massif. AHe results suggest that a rapid cooling and final exhumation episode of the massif could have started before 3.2 Ma, or likely ca. 5 Ma, and continue to the present. The high-temperature horizontal shearing layers of the core were first formed across the Indochina Block, locally antiformed along the tectonic boundaries, and then cooled through the mica Ar-Ar closure temperature during Eocene or early Oligocene, subsequently reworked and further exhumed by sinistral strike-slip movement along the ARSZ during the early Oligocene (ca. 29 Ma), lasting until ca. 17 Ma, then final exhumation of the massif occurred by dextral normal faulting on the Weixi-Qiaohou and Red River faults along the limbs of the ARSZ since ca. 5 Ma. The formation of the antiform could indicate local crustal thickening in an early transpressional setting corresponding to India-Asia convergence. Large-scale sinistral ductile shear along the ARSZ in the shallow crust accommodated lateral extrusion of the Indochina Block, and further contributed to the vertical exhumation of the metamorphic massif from the late Oligocene to the middle Miocene. Furthermore, the change of kinematic reversal and associated cooling episodes along the ARSZ since the middle Miocene or early Pliocene imply a tectonic transfer from strain localization along the major tectonic boundaries to continuous deformation corresponding to plateau growth and expansion.

Microstructures documenting Cenozoic extension processes in the northern continental margin of the South China Sea

ABSTRACT In order to investigate the thinning process of the northern continental margin of the South China Sea, petrographic and microstructural analysis were carried out on 20 greenschistfacies mylonite samples, which were obtained from Site U1504 of IODP Expedition 367/368 in the Outer Margin High of the region. The mineral assemblage of the greenschist-facies mylonite is chlorite + epidotite + albite (Ab = 94.7–99.9) + quartz, which contains 10-30% gravel components. Microstructural analysis indicates that the greenschist-facies mylonite experienced two episodes of deformation：early ductile deformation followed by a later stage of brittle deformatio. Both episodes of deformation suggest an extensional environment. The extensive development of bulging recrystallization (BLG) of quartz, microscopic fractures and fine granulation of albite suggest that the temperature of ductile deformation is about 300-400°C, compatiable with a ductile shearing at shallow crust levels (~5-10 km). Petrographic features suggest that the greenschist-facies mylonite might originate from volcanic sedimentary rocks or sedimentary rocks affected by the intrusion of mafic magma. Combined with seismic interpretation, we propose that the greenschist-facies mylonite might be formed by crustal exhumation after thick Mesozoic sediments were denuded by a major extension.

Weakness and mechanical anisotropy of phyllosilicate-rich cataclasites developed after mylonites of a low-angle normal fault (Simplon Line, Western Alps)

The Simplon Fault Zone is a late-collisional low-angle normal fault (LANF) of the Western Alps. The hanging wall shows evidence of brittle deformation only, while the footwall is characterized by a c. 1 km-thick shear zone (the Simplon Fault Zone), which continuously evolved, during exhumation and cooling, from amphibolite facies conditions to brittle-cataclastic deformations. Due to progressive localization of the active section of the shear zone, the thermal-rheological evolution of the footwall resulted in a layered structure, with higher temperature mylonites preserved at the periphery of the shear zone, and cataclasites occurring at the core (indicated as the Simplon Line). In order to investigate the weakness of the Simplon Line, we studied the evolution of brittle/cataclastic fault rocks, from nucleation to the most mature ones. Cataclasites are superposed on greenschist facies mylonites, and their nucleation can be studied at the periphery of the brittle fault zone. This is characterized by fractures, micro-faults and foliated ultracataclasite seams that develop along the mylonitic SCC′ fabric, exploiting the weak phases mainly represented by muscovite and chlorite. Approaching the fault core, both the thickness and frequency of cataclasite horizons increase, and, as their thickness increases, they become less and less foliated. The fault core itself is represented by a thicker non-foliated cataclasite horizon. No Andersonian faults or fractures can be found in the footwall damage zone and core zone, whilst they are present in the hanging wall and in the footwall further from the fault. Applying a stress model based on slip tendency, we have been able to calculate that the friction coefficient of the Simplon Line cataclasites was <0.25, hence this fault zone is absolutely weak. In contrast with other fault zones, the weakening effect of fluids was of secondary importance, since they accessed the fault zone only after an interconnected fracture network developed exploiting the cataclasite network.

Geological-structural map of the Orobic and Porcile thrust junction, central Southern Alps (N Italy)

The central Southern Alps retrobelt was built trough a polyphase tectonic evolution that developed in the hangingwall of the SE-directed Alpine Tethys subduction. In the northern sector of the central Southern Alps the earliest stages of shortening resulted in the inversion of structures inherited from the Permian and Triassic rifting phases, together with southward thrusting and large-scale folding. In the San Marco Pass area (N Italy) the Variscan basement overrides the Permian-Triassic sedimentary cover along regional thrusts. Here the E-W trending Orobic Thrust joins the NE-SE Porcile Thrust. Initial thrusting phases formed greenschist facies mylonites in the basement and S-verging folds in the sedimentary covers. The Porcile line, a feature likely inherited from the Permian extension, was re-activated as a steep mylonite shear zone. Fault activity continued at brittle conditions: cataclastic shear zones overprinted the mylonites. During this phase, shortening was preferentially accommodated along the Orobic Thrust. A final transpressive re-activation observed along both thrusts is linked to the Oligocene activity of the Insubric Fault.

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310-295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.

Separate Eocene–Early Oligocene and Miocene stages of extension and core complex formation in the Western Rhodopes, Mesta Basin, and Pirin Mountains (Bulgaria)

The Rhodope Metamorphic Province in Bulgaria and Northern Greece has been affected by significant extensional tectonics since the Middle to Late Eocene. An important fault system active in the Eocene and Early Oligocene includes the Ribnovo Fault on the eastern side of the Mesta Basin in Bulgaria and the Vertiskos–Kerdilion Fault in Greece. Together with several minor normal fault relicts identified during this study, these represent an originally west-southwest-dipping, low-angle (at least at the end of faulting) normal fault with greenschist-facies mylonites in the footwall and cataclasites along the fault plane, the Mesta–Kerdilion Detachment, exposed over ca. 150 km along strike and about 50 km parallel to the slip direction. During the intrusion of several plutons in the Pirin Mountains at ca. 32 Ma, the footwall of the fault was uplifted to form a large anticline parallel to fault strike, and the fault was offset by a system of antithetic, northeast-dipping normal faults along the northeastern flank of this anticline (Dobrotino and Breznitsa faults). The Mesta–Kerdilion Detachment was later, in the Miocene, again crosscut and offset by the southwest-dipping Strimon Valley Detachment which accommodated important, core complex-like exhumation to the South, strongly diminishing and finally ceasing towards north. This rotational activity of the Strimon Valley Detachment represents the onset of the extension that led to opening of the Aegean Basin. The Mesta–Kerdilion Detachment can be viewed as a precursor of this, but with slightly different kinematics (i.e. not involving significant vertical-axis rotation) and separated in time from the following events by a phase of relative tectonic quiescence in the Late Oligocene.

40Ar/ 39Ar dating of strike-slip motion on the Tan–Lu fault zone, East China

NE-trending, sinistral ductile shear belts in the southern part of the Tan–Lu fault zone, East China are exposed along the eastern edge of the northern Dabie belt and southern Zhangbaling belt. Three hornblende, three phengite and 12 biotite separates from greenschist facies mylonites in the shear belts were dated by the 40Ar/ 39Ar method in this study. A hornblende plateau age of 143.3 Ma is interpreted as representing the time of sinistral motion on the Zhangbaling part of the Tan–Lu fault zone. Seven biotite samples from the same part yield 40Ar/ 39Ar cooling ages from 137 to 125 Ma. A phengite plateau age of 138.8 Ma from the eastern edge of the Dabie belt is considered as being close to the time of deformation in the Tan–Lu strike-slip zone. Five biotite 40Ar/ 39Ar ages from 120 to 110 Ma and one phengite age of 121 Ma from the same area represent cooling ages after the sinistral faulting. It is concluded on the basis of these 40Ar/ 39Ar ages that the sinistral ductile shear belts in the southern part of the Tan–Lu fault zone were produced in earliest Early Cretaceous. This sinistral motion on the Tan–Lu fault zone is considered to be related to high-speed, highly oblique subduction of the Pacific plate at this time.

Strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab, Everest Massif, Tibet

This paper presents quantitative data on strain, deformation temperatures and vorticity of flow at the top of the Greater Himalayan Slab. The data were collected from the Tibetan side of the Everest Massif where two low-angle normal faults bound the upper surface of the Greater Himalayan Slab, the earlier and structurally lower Lhotse Detachment and the later and structurally higher Qomolangma Detachment. Greenschist- to sillimanite-grade quartz-rich metasedimentary rocks exposed in the Rongbuk to North Col region of the Everest Massif are characterized by cross-girdle quartz c -axis fabrics indicating approximate plane strain conditions. Fabric opening angles progressively increase with depth beneath the overlying Lhotse Detachment, and indicate progressively rising deformation temperatures of 525–625 ± 50 °C at depths of 300–600 m beneath the detachment. Deformation temperatures of c . 450 °C are indicated by fabric opening angles in epidote amphibolite-facies mylonites located closer to the overlying detachment. A top down-to-the-north (normal) shear sense is indicated by the asymmetry of microstructures and c -axis fabrics, but the degree of asymmetry is low at distances greater than 400 m beneath the detachment, and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine, indicating that deformation occurred at close to peak metamorphic temperatures. These ‘quenched’ fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies. Mean kinematic vorticity numbers ( W m ) were independently calculated by three analytical methods. Calculated W m values range between 0.67 and 0.98, and indicate that although a simple shear component is generally dominant, particularly in greenschist-facies mylonites located between the Lhotse and overlying Qomolangma detachments, there is also a major component of pure shear in samples located at 400–600 m beneath the Lhotse Detachment (pure and simple shear make equal contributions at W k =0.71). Our integrated strain and vorticity data indicate a shortening of 10–30% perpendicular to the upper surface of the Greater Himalayan Slab and confirm that the upper surface of the slab is a ‘stretching fault’ with estimated down-dip stretches of 10–40% (assuming plane strain deformation) measured parallel to the flow plane–transport direction.

Extensional detachment faulting and core-complex formation in the southern Barberton granite–greenstone terrain, South Africa: evidence for a 3.2 Ga orogenic collapse

Abstract The Barberton greenstone belt in South Africa is an Early- to Mid-Archaean, very low-grade metamorphic supracrustal belt that is bordered in the south by a mid- to lower crustal gneiss terrain. Detailed mapping of the contacts between the supracrustal and gneiss domains along the southern margin of the greenstone belt shows that the supracrustal rocks are separated from the high-grade metamorphic gneiss terrain by an extensional detachment that is situated at and close to the base of the belt. The extensional detachment is approximately 1-km wide and its location corresponds with the heterogeneous, melange-like rocks of the Theespruit Formation. Within the detachment zone, two main strain regimes can be distinguished. Amphibolite-facies rocks at and below the granite–greenstone contacts are characterized by rodded gneisses and strongly lineated amphibolite-facies mylonites. The bulk constrictional deformation at these lower structural levels records, in a subhorizontal orientation, the vertical shortening and horizontal, NE–SW directed stretching of the mid-crustal rocks. The prolate, coaxial fabrics are overprinted by greenschist-facies mylonites at higher structural levels that cut progressively deeper into the underlying high-grade basement rocks. These mylonites have developed during non-coaxial strain and kinematic indicators consistently point to a top-to-the-NE sense of movement of the greenstone sequence with respect to the lower structural levels. This relationship between bulk coaxial NE–SW stretching of mid-crustal basement rocks and non-coaxial, top-to-the-NE shearing along retrograde mylonites at upper crustal levels is consistent with an extensional orogenic collapse of the belt and the concomittant exhumation of deeper crustal levels. The exhumation was initiated under amphibolite-facies conditions at depths of approximately 18 km. The extensional collapse is coeval with or shortly follows the main D2 collisional event in the Barberton greenstone belt at ca. 3230–3220 Ma. Voluminous plutonism at ca. 3225 Ma along the northern margin of the belt is possibly related to the orogenic collapse and associated decompression melting of lower crustal rocks. The extensional collapse coincides with the onset of the coarse-clastic Moodies Group sedimentation which suggests that the small, isolated Moodies basins formed as supradetachment basins in the collapsing hanging wall of the detachment. The steepening of lithologies and fabrics to their present-day vertical attitudes is ascribed to a late-stage solid-state diapiric component of the exhumed hot and buoyant basement gneisses that underlie the relatively cool and dense mafic and ultramafic supracrustal succession.

Basement‐Cover Sequences within the UHP unit of the Dabie Shan

AbstractField data define two lithologically distinct basement‐cover sequences within the ultrahigh‐pressure metamorphic (UHPM) unit of the Dabie Shan, eastern China. One of the cover units, the Changpu unit, comprises calc‐arenitic metasediments in stratigraphic contact with a basement consisting of gneisses of the Yangtze craton. The second cover unit, the Ganghe unit, consists of felsic‐intermediate metavolcanics and clastic metasediments. Basement exposure of the Ganghe unit is not known. Fold axes in the Ganghe unit are oblique to those of the Changpu unit, which are parallel to those of the Yangtze gneisses. Preservation of primary textures in some volcanic rocks, and tectonic separation from the Yangtze gneisses by a greenschist facies mylonite, support an interpretation of the Ganghe unit as a low‐strain domain. Protolith associations in the Ganghe and Changpu units are compatible with deposition in a rift setting and along a passive continental margin, respectively. A U–Pb single zircon age of 761 ± 33 Ma for volcanoclastic rocks of the Ganghe unit demonstrates a Neoproterozoic deposition age, concordant with inferred rifting at that time. Eclogite facies parageneses in the gneisses and both cover units, along with P–T data demonstrate regional UHPM in the Dabie Shan.

Microstructural evolution of the Seridó Belt, NE-Brazil: the effect of two tectonic events on development of c-axis preferred orientation in quartz

The polyphase evolution of the Seridó Belt (NE-Brazil) includes D 1 crust formation at 2.3–2.1 Ga, D 2 thrust tectonics at 1.9 Ga and crustal reworking by D 3 strike-slip shear zones at 600 Ma. Microstructural investigations within mylonites associated with D 2 and D 3 events were used to constrain the tectono-thermal evolution of the belt. D 2 shear zones commenced at deeper crustal levels and high amphibolite facies conditions (600–650 °C) through grain boundary migration, subgrain rotation and operation of quartz 〈 c〉-prism slip. Continued shearing and exhumation of the terrain forced the re-equilibration of high-T fabrics and the switching of slip systems from 〈 c〉-prism to positive and negative 〈 a〉-rhombs. During D 3, enhancement of ductility by dissipation of heat that came from syn-D 3 granites developed wide belts of amphibolite facies mylonites. Continued shearing, uplift and cooling of the region induced D 3 shear zones to act in ductile-brittle regimes, marked by fracturing and development of thinner belts of greenschist facies mylonites. During this event, switching from 〈 a〉-prism to 〈 a〉-basal slip indicates a thermal path from 600 to 350 °C. Therefore, microstructures and quartz c-axis fabrics in polydeformed rocks from the Seridó Belt preserve the record of two major events, which includes contrasting deformation mechanisms and thermal paths.

40Ar/ 39Ar age pattern associated with differential uplift along the Eastern Highlands shear zone, Cape Breton Island, Canadian Appalachians

The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende 40Ar/ 39Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The 40Ar/ 39Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The 40Ar/ 39Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones.

Kanairiktok shear zone: the boundary between the Paleoproterozoic Makkovik Province and the Archean Nain Province, Labrador, Canada.

The polyphase Kanairiktok shear zone (KNSZ) separates gneissic rocks of the Archean Nain craton from their reworked equivalents in the Paleoproterozoic Kaipokok domain of the Makkovik Province. In its early stages, the KNSZ bounded the Kaipokok domain as it was thermally softened by 1895-1870 Ma Andean-type magmatism, accompanied by dextral oblique convergence and resultant penetrative deformation. The amphibolite-facies tectonite that developed in this stage was widely overprinted by greenschist-facies mylonite. Laserprobe and spectral 40Ar/39Ar ages of recrystallized and porphyroclastic muscovite, from the greenschist-facies mylonite and from muscovite in a syntectonic quartz vein, bracket the age of deformation between 1740 and 1710 Ma with the best estimate at 1715 Ma. These ages are similar to those of A-type granites within the Makkovik Province and amphibole cooling ages from the province interior. Together with the petrological similarity of the greenschist-facies mylonite to localized low-grade shear zones elsewhere in the Makkovik Province, they are suggestive of a widespread, lithosphere-scale event. The 40Ar/39Ar data do not provide good constraints on the early activity of the KNSZ. However, preservation of relationships between granitoid sheets correlated with the 1895-1870 Ma Island Harbour Bay plutonic suite and early fabrics imply that the granites were emplaced syntectonically in the KNSZ. Thus, the KNSZ was a major, long-lived structure in the Makkovik Province that decoupled events in the reactivated Nain craton from an inert cratonic region.

Matrix mosaics, brittle deformation, and elongate porphyroclasts: granulite facies microstructures in the Striding–Athabasca mylonite zone, western Canada

The Late Archean Striding–Athabasca mylonite zone, western Canadian Shield, is a 400-km-long, linked system of granulite facies mylonite belts developed in the continental crust. The granulite facies mylonites (800–1000°C at 0.8–1.1 GPa) developed in subsets of the anhydrous mineral assemblage clinopyroxene–orthopyroxene–garnet–plagioclase–quartz±hornblende. Comparison of the microstructures of the Striding–Athabasca mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the Striding–Athabasca mylonites. In addition to dislocation creep and dynamic recrystallisation, extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism. The microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain-scale and aggregate-scale diffusion, grain boundary mobility, and mass transfer, at relatively slow strain rates. Despite their annealed appearance, the granoblastic matrix mosaics developed synkinematically; deformation-induced dilation may enhance metamorphic reactions where products are more voluminous than reactants. In the porphyroclast population, highly elongate (>20:1) monocrystalline orthopyroxenes appear to be fragments of dismembered, kinked parent grains, rather than stretched porphyroclasts. Granulite facies mylonites should not be treated as direct analogues of greenschist facies mylonites. In particular, it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained, high-temperature shear zone rocks.

Strain localization and fluid pathways in mylonite: inferences from in situ deformation of a water-bearing quartz analogue (norcamphor)

Simple shearing of polycrystalline norcamphor containing 10–15 vol% water, at near-atmospheric pore fluid pressure and a range of constant temperatures (3–35°C) and shear strain rates (5×10 −5–4×10 −4 s −1), induces localization of both strain and fluid flow. Prior to deformation, the water is located at grain triple junctions and pockets along grain boundaries. It forms an average dihedral angle of 46° with the surrounding norcamphor grains. During initial shearing ( γ≤1), grain boundaries oriented subparallel to the principal shortening direction dilate and fill with water. At 1< γ<2, these open grain boundaries interconnect to form water-filled dilatant shear surfaces at low angles (10–15°) to the shear zone boundary. These surfaces resemble shear bands or C′ surfaces in mylonitic rock and, depending on the temperature, accommodate displacement by cataclasis ( T<15°C) or dislocation creep ( T>15°C). The tips of the shear surfaces propagate alternately by intracrystalline plasticity and subcritical fracturing, concomitant with dynamic recrystallization in the rest of the sample. The episodic interconnection of dilatant shear surfaces is associated with short-term increases in displacement rate parallel to the surfaces. These surfaces coalesce to form a high-strain, fluid-filled network subparallel to the experimental shear zone. However, this network never spans the entire length of the shear zone at any given time, even after shearing to γ=8.5. The deformation is more homogeneous and fewer dilatant shear surfaces develop at higher deformational temperatures and/or lower strain rates due to the increased activity of dislocation creep. Prolonged stress annealing removes most microstructural evidence of the syntectonic fluid pathways. Dilatant shear surfaces in norcamphor resemble relics of mica-filled, synmylonitic fractures in dynamically recrystallized quartz from greenschist facies mylonite, suggesting that fluid played a similar mechanical role in nature and experiment. The coalescence of dilatant, fluid-filled shear surfaces represents a strain-dependent increase in pore connectivity within mylonitic shear zones. The experiments indicate that fluid flow along deep crustal mylonitic shear zones is probably limited by the rate at which the tips of the dilatant shear surfaces propagate subparallel to the shearing plane.

Orogen-parallel extension in the Southern Carpathians

A structural study, including a kinematic analysis based on sense of shear criteria recorded in fault rocks, is combined with fission-track dating. A two-stage Alpine tectonic evolution is proposed for the major tectonic units which constitute the Southern Carpathians of the Parang mountains area. Upper Cretaceous top-to-the-SSE nappe stacking was followed by WSW–ENE orogen-parallel extension in the Eocene. Top-to-the-ENE shearing in lower greenschist-facies mylonites from the eastern part of the Danubian window (Parang mountains) is associated with a low-angle detachment at the base of the brittlely deformed Getic nappe (the Getic detachment). Below the dome-shaped Getic detachment, east-dipping at the eastern termination of the Danubian window, the Danubian units were rapidly exhumed. Hence, the eastern part of the Danubian window represents a greenschist-facies core complex. As a working hypothesis, it is proposed that this orogen-parallel stretch was originally N–S-oriented and that it formed when the Rhodopean fragment (which includes the Getic and Supragetic nappes) moved northward into an oceanic embayment, past the western margin of Moesia. This Eocene extension was part of a process of oroclinal bending in the area of the Southern Carpathians and their continuation into the Balkan mountains. Extension was followed by about 50° clockwise rotation of the Southern Carpathians, associated with dextral wrenching along the northern boundary of Moesia and compression in the Moldavides of the Eastern Carpathians.

Late Cretaceous, synorogenic, low-angle normal faulting along the Schlinig fault (Switzerland, Italy, Austria) and its significance for the tectonics of the Eastern Alps

The Schlinig fault at the western border of theÖtztal nappe (Eastern Alps), previously interpreted as a west-directed thrust, actually represents a Late Cretaceous, top-SE to -ESE normal fault, as indicated by sense-of-shear criteria found within cataclasites and greenschist-facies mylonites. Normal faulting postdated and offset an earlier, Cretaceous-age, west-directed thrust at the base of theÖtztal nappe. Shape fabric and crystallographic preferred orientation in completely recrystallized quartz layers in a mylonite from the Schlinig fault record a combination of (1) top-east-southeast simple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directed shortening during the Early Tertiary, also recorded by open folds on the outcrop and map scale. Offset of the basal thrust of theÖtztal nappe across the Schlinig fault indicates a normal displacement of 17 km. The fault was initiated with a dip angle of 10° to 15° (low-angle normal fault). Domino-style extension of the competent Late Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting. The Schlinig fault belongs to a system of east- to southeast-dipping normal faults which accommodated severe stretching of the Alpine orogen during the Late Cretaceous. The slip direction of extensional faults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip sense is reversed and the normal faults are slightly steeper than the thrusts. In the western Austroalpine nappes, extension started at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of Austroalpine Gosau basins with fluviatile to deep-marine sediments. West-directed rollback of an east-dipping Piemont-Ligurian subduction zone is proposed to have caused this stretching in the upper plate.

Structural evolution and tectonic significance of the Eastern Highlands shear zone in Cape Breton Island, the Canadian Appalachians

The Eastern Highlands shear zone in Cape Breton Island of the Canadian Appalachians is characterized by an amphibolite-facies deformation zone over 5 km wide overprinted by a greenschist-facies mylonite zone about 1 km wide. Deformation zones in both metamorphic grades dip steeply to the southeast with movement direction pitching steeply to the southwest, and shear sense indicators indicate the same sense of shear, that is, an east-over-west dip-slip movement with minor sinistral strike-slip component. Deformation in both conditions is constrained to the Late Silurian to Early Devonian (mainly Late Silurian). It is suggested that the greenschist-facies deformation represents the last stage of a single episode of deformation that occurred initially under amphibolite-facies conditions. The west-vergent shearing along the shear zone is antithetic to the westward subduction that led to the Silurian continent-continent collision, which is interpreted by tectonic wedging in this part of the Canadian Appalachians.

What brought them up? Exhumation of the Dabie Shan ultrahigh-pressure rocks

Metamorphic coesite and diamond in the Dabie Shan, eastern China, testify to subduction of continental crust to >100 km depth. Exhumation of these ultrahigh-pressure rocks through the crust encompassed two stages. (1) South-dipping foliation, southeast-plunging stretching lineation, lineation-parallel isoclinal folds, and boudins indicate extreme subhorizontal shortening and subvertical extension during top-to-northwest shearing at 200–180 Ma. Syntectonic recrystallization occurred at eclogite and amphibolite facies temperatures and at pressures below coesite stability. (2) Northwest-southeast subhorizontal extension from 133 to 122 Ma was concentrated within an asymmetric structural dome in a magmatic complex that forms the northern half of the Dabie Shan. Pluton cores at deep structural levels have weak hypersolidus fabrics, and pluton carapaces are mylonitic gneisses formed at upper amphibolite facies conditions. Deformation is concentrated in greenschist facies mylonites and ultramylonites along the Xiaotian-Mozitang detachment fault at the northern topographic limit of the Dabie Shan. Our preferred exhumation model involves two stages: Triassic indentation—vertical extrusion and erosion—followed by Cretaceous plate margin transtension.