Tectonic Fabric Research Articles

Structure and internal deformation of thrust sheets in the Sawtooth Range, Montana: insights from anisotropy of magnetic susceptibility

Abstract Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques. In the absence of valid strain markers, it can be difficult to identify the presence of an early tectonic fabric development and resulting layer parallel shortening (LPS). In order to identify early LPS, we carried out anisotropy of magnetic susceptibility (AMS) analyses on Mississippian limestones from the Sawtooth Range of Montana. The Sawtooth Range is an arcuate zone of north-trending, closely spaced, west-dipping, imbricate thrust sheets that place Mississippian Madison Group carbonates above Cretaceous shales and sandstones. This structural regime is part of the cordilleran mountain belt of North America, which resulted from accretion of allochthonous terrains to the western edge of the North American continent. Although the region has a general east–west increase in thrust displacement and related brittle deformation, a similar trend in penetrative deformation or the distribution of tectonic fabrics is not observed in the field or in the AMS results. The range of magnetic fabrics identified in each thrust sheet ranges from bedding controlled depositional fabrics to tectonic fabrics at a high angle to bedding.

Combining finite strain analysis and illite crystallinity to examine strain variation in a shale detachment zone

Determining the physical conditions and material properties of low-grade (sub-green schist facies) deformation accurately is problematic due to the lack of adequate strain indicators and mineral equilibrium assemblages with established relationships to pressure and temperature conditions. For low-grade deformation, proxies such as illite crystallinity are often used to provide broad constraints on these conditions. The relationship between illite crystallinity and temperature remains qualitative, however, it has been suggested that illite crystallinity may be a more appropriate proxy for tectonic strain at low temperatures. Recent work on an exposed shale detachment zone in the Khao Khwang Fold-Thrust Belt has described strain partitioning into shale-dominated “shear domains” which delineate coarser-grained “fault domains”. This strain partitioning is a key influence in the development of the complex three-dimensional shear zone networks which characterise the multiple shale detachment zones in the area. Here we apply both the Fry Method of finite strain analysis and illite crystallinity by X-ray diffraction analysis to examine the potential strain variations between different shear and fault domains. Shear domains show less variable and “higher-grade” illite crystallinity values, while the Fry Method results show higher strain values in these shear domains. This is confirmed by QEMSCAN mineral mapping of each domain type, where shear domains show a far more developed tectonic fabric, indicating a much higher degree of strain is accommodated continually throughout these relatively thin shear zones. The fault domains they delineate, by contrast, show far less internal strain, which is accommodated discontinuously by discrete, low-offset, fault planes. Data from our results also show a remarkably strong correlation between illite crystallinity and finite strain, and while the small number of samples involved in this study leads us to caution the over-interpretation of these results, it points to a strong relationship between illite crystallinity and finite strain at low temperatures. If further investigated and quantified, this relationship could provide a powerful and inexpensive tool for strain determination in fine-grained rocks.

Tectonic strain recorded by magnetic fabrics (AMS) in plutons, including Mt Kinabalu, Borneo: A tool to explore past tectonic regimes and syn-magmatic deformation

Tectonic strain commonly overprints magmatic fabrics in AMS (Anisotropy of Magnetic Susceptibility) data for plutonic rocks produced by both compressional and extensional regimes. Mt Kinabalu, Borneo, is a composite pluton with an exceptional vertical range of exposure and clearly defined internal contacts. We show that tectonic fabrics are recorded pervasively throughout the intrusion, even near contacts, and present a workflow distinguishing compressive and extensional syn-magmatic deformation. At Mt Kinabalu this reveals a pervasive tectonic fabric indicating NW-SE Miocene extension in Borneo at 7.9–7.3 Ma, later than previously recognised, oriented NW-SE at 319° ±13.1°. Comparing data from Mt Kinabalu with data from globally distributed studies shows that tectonic strain is commonly recorded by plutons. Therefore, AMS fabric can be used to identify the syn-magmatic tectonic setting and combined with both geochronology and evidence for paleomagnetic rotation to provide a powerful tool for accurate determination of syn-magmatic tectonic regimes and strain orientations within temporal frameworks.

Using AMS and Palaeomagnetic Data to Assess Tectonic Rotation: A Case Study from Savannah Nickel Mine, WA

Recent research has shown that Ni-PGE mineralisation is typically associated with specific intrusion types, e.g., chonoliths, bladed-dykes and funnels that acted as high-throughput magma conduits within much larger magmatic provinces. The initial architecture of these systems is very different, but additional structural modification during orogenesis can render such intrusions very difficult to understand structurally, and hence make it very difficult to target the fertile zones within the system. In structural geology it is common to evoke shortening directions, which are assumed to apply to all rocks regardless of their rheology. This is not realistic. Furthermore, resolving the partitioning of strain is not straightforward for intrusive rocks, which tend not to develop visible tectonic fabrics, but act as rigid blocks that rotate to accommodate strain, rather than compress or shear, during deformation. Unfortunately there are few structural techniques that can be used to quantify such rotation. Intrusions near the Savannah Nickel Mine, East Kimberley, WA were observed to have different deformation histories, despite being temporally equivalent. In this study we measured anisotropy of magnetic susceptibility (AMS) and remanent magnetisation in two adjacent, temporally equivalent, intrusions. The observed K3 AMS vectors are typically normal to the magmatic layering in layered intrusions. Where K3 vectors sit along a great circle, the pole to that great circle indicates the rotation axis. Original palaeomagnetic vectors would be expected to be consistently oriented with respect to magmatic layering, and can be used similarly to test the consistency of the inferred rotation analysis. The rotations inferred for the intrusions tested were consistent between the two techniques. Savannah and Savannah North, were subjected to N-S, NE-SW and NW-SE shortening, consistent with the Halls Creek Orogeny. However, N-S shortening was dominant at Savannah and NE-SW dominant at Savannah North. Therefore, despite their equivalent emplacement ages and implied tectonic history, each intrusion has undergone very different deformation.

Auréola termal provocada pela intrusão do pluton Totoró em micaxistos do Grupo Seridó, Ediacarano da Província Borborema, NE do Brasil

A Província Borborema, NE do Brasil, é marcada por volumoso plutonismo ediacarano. Neste contexto geológico, o plutão Totoró, um corpo alongado NNE-SSW, compõe-se de uma sequência de tipos básicos a intermediários, incluindo dioritos e gabro-noríticos (mais precoces), granodioritos / tonalitos (predominante), biotita granitos equigranulares, biotita granitos porfiríticos e hololeucogranitos finos a médios. Dados químicos permitem estimar pressões de 1,6-2,7 kbar e temperaturas de 800-900oC para sua colocação. O calor aportado provocou uma auréola termal que se estende até 2 km do contato, constituída por: (i) xisto migmatizado imediatamente adjacente ao contato, com frequentes soleiras de leucossoma subconcordantes com a trama tectônica S2 da encaixante; (ii) uma zona intermediária com sillimanita (± cordierita + granada + biotita); (iii) uma zona com aparecimento de cordierita (± estaurolita ± andaluzita + granada + biotita); e (iv) uma zona externa com micaxisto de fina granulação. Modelamento baseado em petrofísica de rocha mostra que a temperatura na borda do plutão atingiu 688-756°C, diminuindo para 500oC a 1800 m do contato, com tempo de resfriamento estimado de 365.000 anos. Os minerais produzidos no efeito termal delineiam a xistosidade S2, de baixo ângulo (evento D2/M2 na fácies anfibolito ou piroxênio hornfels), os quais são afetados por dobras abertas a fechadas, normais, progredindo para forte transposição e milonitos em zona de cisalhamento NE-SW a N-S, representando o último evento dúctil, retrometamórfico (fácies xisto verde), na região (D3/M3). Os resultados obtidos revelam que pelo menos parte dos batólitos na região em tela se posicionou em níveis crustais relativamente rasos sob condições de alto gradiente geotérmico, porém ainda registrando a fase de arrefecimento da tectônica dúctil regional.

Kinematic, Deformational, and Thermochronologic Conditions Along the Gossan Lead and Fries Shear Zones: Constraining the Western‐Eastern Blue Ridge Boundary in Northwestern North Carolina

AbstractThe fault boundary between the western and eastern Blue Ridge (WBR‐EBR) in the southern Appalachians separates Mesoproterozoic basement rocks and their cover from Neoproterozoic to Paleozoic accreted rocks. Several northeast striking faults delineate the boundary, including the Gossan Lead shear zone in northwestern North Carolina. Varying tectonic interpretations of WBR‐EBR boundary include a premetamorphic fault, an Acadian dextral strike‐slip fault, or an Alleghanian fault. We use field‐based, microstructural, and theromochronometric analyses to determine the conditions, kinematics, and timing of deformation, in order to distinguish among competing hypotheses for the Gossan Lead shear zone. This comprehensive approach has allowed us to attribute a number of new and previously observed tectonic fabrics to specific orogenic events; key relationships necessary to the study of multiply deformed tectonic margins. Detailed mapping and microstructural analysis of the Gossan Lead shear zone document a several kilometer‐wide mylonitic zone with kinematic indicators that record dominantly top‐to‐the‐NW thrust motion, with local strike‐slip and normal sense indicators. Dynamically recrystallized quartz and feldspar constrain a range of deformation conditions from amphibolite to greenschist facies. Two unaltered lineation‐forming amphiboles from mylonitic amphibolites record 40Ar/39Ar cooling ages of 347–345 Ma, and a mylonitized metagraywacke records a muscovite 40Ar/39Ar cooling age of 336 Ma. These data are consistent with dominantly NW directed thrusting along the Gossan Lead shear zone at amphibolite to greenschist facies conditions, and rapid cooling in the Middle Mississippian. We suggest these data support overprinting and/or reactivation of an earlier structure along this complexly deformed boundary by 336 Ma.

Novel anisotropic teleseismic body-wave tomography code AniTomo to illuminate heterogeneous anisotropic upper mantle: Part II – Application to data of passive seismic experiment LAPNET in northern Fennoscandia

Seismic anisotropy provides a unique constraint on the past and present dynamics of the lithosphere and sublithospheric mantle. To contribute to studies of large-scale tectonic fabric, we have developed code AniTomo for regional anisotropic tomography. AniTomo allows us to invert simultaneously relative traveltime residuals of teleseismic P waves for 3-D distribution of isotropic-velocity perturbations and velocity anisotropy in the upper mantle. Weak hexagonal anisotropy with the symmetry axis oriented generally in 3-D is considered. The first application of novel code AniTomo to data from passive seismic experiment LAPNET results in a model of anisotropic velocities of the upper mantle beneath northern Fennoscandia. We have opted for northern Fennoscandia for the first application because it is a tectonically stable Precambrian region with a thick anisotropic mantle lithosphere without significant thermal heterogeneities. We carefully analyse the distribution of the rays to limit the fully anisotropic inversion only to the volume with the sufficient directional ray coverage. Capability of the given inversion setup to reveal large-scale anisotropic structures in the upper mantle is documented by a series of synthetic tests. The strongest anisotropy and the largest velocity perturbations concentrate at depths corresponding to the mantle lithosphere, while in deeper parts of the tomographic model, the lateral variations are insignificant. We delimit regions of laterally and vertically consistent anisotropy in the mantle–lithospheric part of the model. We attribute the retrieved domain-like anisotropic structure of the mantle lithosphere in northern Fennoscandia to preserved fossil fabrics of the Archean microplates, accreted during the Precambrian orogenic processes.

The 2014 Cephalonia Earthquakes: Finite Fault Modeling, Fault Segmentation, Shear and Thrusting at the NW Aegean Arc (Greece)

We present refined finite fault models (FFM) for the 2014 Cephalonia (Keffalinia, Kefalonia) seismic sequence (Mw ~ 6.0), at the NW edge of the Aegean Arc (Ionian Sea). The area represents the seismically most active part of Europe and a continental promontory in which fault modeling is a challenge because of structural complexity and poor coverage by seismological, GPS and InSAR data. Inversion was based on GPS data and a new algorithm permitting fusion of slip vectors of individual earthquakes and of their cumulative dislocation and accepting constraints and collocation-type analysis of uncertainties. Computed FFM, which correspond to an essentially strike-slip fault and a blind, shallow oblique slip thrust, were assessed by sensitivity analysis and InSAR data and are consistent with the tectonic fabric of the area. They can also explain the observed extreme peak ground accelerations. The 2014 faults, in combination with FFMs of the 2003 and 2015 Leucas (Lefkada, Lefkas) earthquakes farther NE and of the 1983 M7.0 earthquake farther SW, constrain a > 100 km long immature, strike-slip fault zone along/close to the Cephalonia–Leucas coasts. This fault pattern, previously regarded as a poorly documented Cephalonia Transform Fault, consists of occasionally overlapping oblique slip segments with variable geometric and kinematic characteristics in a shear zone landwards of the plate interface, as evidence from seismic profiles reveals. This pattern may explain the enigmatic superimposition of shear and compression in the NW edge of the Aegean Arc.

Volcanic and hydrothermal processes in submarine calderas: The Kulo Lasi example (SW Pacific)

The study area is located at the transition between the northern end of the Tonga Trench and the North Fiji fracture zone, where tectonic movements are reputed to be the fastest in the world. To the southeast of Futuna Island, a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a NE-SW oriented volcanic graben and the N-S oriented Alofi ridge. In 2010, the active volcano Kulo Lasi, which represents the most recent volcanic episode in the Futuna area, was discovered in the center of this extensive volcanic zone. Kulo Lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor. It is composed of basaltic to trachy-andesitic lava with no obvious geochemical affinity with the Tonga subduction that occurs 500 km to the east. The central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m. Diving operations with the submersible Nautile and high-resolution AUV mapping, have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera. Four tectono-volcanic stages can be distinguished at Kulo Lasi caldera. In stage 1, the shield volcano is built. Annular reverse faults develop at the summit and control circulation of water/rock-dominated hydrothermal fluids and high-temperature alteration of rocks along the nascent normal faults. Mixing of hydrothermal fluids with seawater is favored along normal superficial faults, leading to the formation of low-temperature Fe/Mn mineralization at the summit of the volcano. During stage 2, the caldera collapse, gradually revealing outcrops of the altered and mineralized zones formed during Stage 1. As the magma chamber cools and collapses, less heat is available. As a result, medium to low-temperature (<100 °C) Fe/Si deposits form on the floor of the caldera. In stage 3, refilling and ascent of the magma chamber at depth promote the uplift of a central resurgent dome devoid of recent lava emission. The depth of the magma chamber under the dome is estimated to be 1.6 km. Annular dykes feeding the new lava flows are controlled by the deep reverse faults at the outer rim of the caldera floor. The eruptive events are accompanied by short-lived emission of magmatic fluids, rich in SO2, materialized by the presence of native sulfur depositions on the surface of the most recent lava flows. During stage 4 (present day), black smoker sulfide chimneys, controlled by the deep reverse faults, form at the surface of the most recent lava flows at the outer part of the caldera floor. The fluids emitted result from the mixing of a deep fluid of the water/rock reaction type and of shallow seawater superheated in contact with the hot dykes. Hydrogen is generated during reaction of seawater with the hot dikes and H2 is enriched in the mixed fluids. Active (43 °C) siliceous chimneys at the base of the caldera walls and low-temperature vents (6 °C) perched on the caldera walls suggests a decrease in the venting temperature at a distance from the floor of the caldera. In chimneys, the sphalerite/pyrrhotite/isocubanite/barite paragenesis, probably linked to the high H2 concentrations in the fluids, indicate unusual reducing conditions for a back-arc. The sulfide mineralization also has unusual trace element concentrations. The concentration in Pb and Ba appears characteristic of back-arc environments while the enrichment in Co, Se, and Sn is more common to mineralization associated with basalt in mature back-arcs and associated with ultramafic rocks on mid ocean ridges.

Shear Tectonic Fabric in the Atlantic Ocean and Its Relation to the Geodynamic Condition of the Upper Mantle and Intraplate Deformations

The tectogenesis of the Atlantic Ocean segments is complicated by the axial difference in spreading half-velocities, which causes additional shear displacements between the lithospheric blocks along the transform faults. The intensity of these processes and density of the fault zones iis related to the presence of “cold” sublithospheric lenses along the MAR at a depth of 500 km.

Analysis of very-high-resolution Galileo images and implications for resurfacing mechanisms on Europa

The young (<100 Ma) surface of Jupiter's icy satellite Europa raises the key questions: (1) what are the resurfacing mechanisms for creating Europa's young surface, and (2) how have these processes evolved through time? To address these questions we analyze the nine high-resolution frames obtained by the Galileo Solid State Imager (SSI)—eight at 16 m/pixel and one at 8 m/pixel (commonly quoted at the planned 6–12 m/pixel as in Greeley et al., 2000)—during the E12 flyby of Europa in Dec. 1997. This dataset is now two decades old, but it has not been analyzed in detail until this work. Despite the largely different viewing and lighting conditions, we mosaic these high-resolution frames into the 220 m/pixel regional context frame. We then perform geomorphologic mapping of the high-resolution image mosaic and the regional image frame, for comparison, and we also create a structural map of the high-resolution image mosaic. The units in the geomorphologic map are defined by surface texture, landform shape (morphology), dimension, and relative albedo. The structural map units include interpretations from the geomorphologic map units and their interpretation implies potential kinematic processes for the formation of particular structures. Our primary mapping observations include the regular spacing and gentle slopes of the ridge-and-trough terrain, the sharp boundaries and preserved structures of the chaos terrain, and the symmetry but irregular size of double ridges. We then evaluate proposed formation mechanisms for these and other mapped features. The high-resolution images also reveal an abundance of small (<100 m) pits, the presence of a newly identified high-albedo smooth material, and potential tectonic fabric, all of which have possible implications for the surface history. The mapping and structural analyses lead to the key finding that local-scale resurfacing mechanisms have transitioned from distributed deformation expressed by the formation of the ridged plains to discrete deformation characterized by the formation of chaos and isolated fractures. This finding is consistent with simultaneous ice-shell thickening and cooling occurring as the ice-shell deformed.

The Strike‐Slip West Wishbone Ridge and the Eastern Margin of the Hikurangi Plateau

AbstractThe West Wishbone Ridge (WWR), east of New Zealand, has previously been interpreted as a Cretaceous plate boundary, although both the nature and timing of motion along this feature have been disputed. Here, using recently acquired seismic reflection data from the region of the intersection of the WWR with the Chatham Rise, we show that the WWR was primarily a dextral strike‐slip fault during the Late Cretaceous. The WWR first propagated along the eastern margin of the Hikurangi Plateau after the collision of the Hikurangi Plateau with the Chatham Rise, in response to the slowing of spreading at the Osbourn Trough while spreading continued unabated east of the East Wishbone Ridge, from ca. 105 Ma. Reorientation of spreading at the Osbourn Trough at this time resulted in short‐lived, oblique subduction beneath the southern extent of the WWR. Motion along the WWR likely ceased with the cessation of Osbourn Trough spreading. Using our seismic reflection data and gravity modeling across those same profiles, we locate, for the first time, the eastern boundary of the Hikurangi Plateau between 42–43°S. We also find anomalously thick oceanic crust (∼12 km thick) north of the Hikurangi Plateau. Matching tectonic fabric and fracture zone links between the East Wishbone Ridge and the De Gerlache Gravity Anomaly near Ellsworth Land on the West Antarctic margin indicates that these features were linked or parallel during the Late Cretaceous, and fed strike‐slip motion southward into the Gondwana interior, contributing to the onset of Gondwana rifting ca. 85 Ma.

Separation of Diamagnetic and Paramagnetic Fabrics Reveals Strain Directions in Carbonate Rocks

AbstractWe present a new procedure for separating magnetic fabrics in coccolith‐bearing chalk samples, demonstrated in the case studies of three sites located within the Dead Sea Fault (DSF) plate boundary. The separation is achieved by combining measurements of room temperature and low‐temperature anisotropy of magnetic susceptibility (RT‐AMS and LT‐AMS, respectively) with anisotropy of anhysteretic remanence magnetization (AARM). The LT‐AMS, measured at ~77 K, enhances the fabric of paramagnetic clay minerals. The AARM represents the fabric of ferromagnetic Fe oxides. By subtracting the paramagnetic and ferromagnetic fabrics from the RT‐AMS, the diamagnetic fabric is separated. In the studied samples, we found that the ferromagnetic contribution to the bulk magnetic fabric is negligible and could be excluded from the subtraction procedure. Our analysis indicates that in chalks with a negligible ferromagnetic contribution, diamagnetic fabric predominates the rock bulk magnetic fabric, if the mean susceptibility is &lt;−6 × 10−6 SI, whereas with a mean susceptibility &gt;11 × 10−6 SI, paramagnetic fabric predominates. In the studied rocks, the paramagnetic clay minerals preserve the original depositional fabric, whereas the diamagnetic minerals show a tectonic fabric. We propose a mechanism by which coccolith rotation under tectonic strain contributes to the development of the diamagnetic fabric parallel to the shortening direction. We infer that the diamagnetic fabrics of the studied rocks indicate strain regime of approximately N‐S horizontal shortening near strands of the DSF system. This suggests a deflection of the regional principal strain axes near the DSF. The diamagnetic fabric is more sensitive to tectonic strain than paramagnetic fabric in chalks and provides a valuable strain indicator near major faults.

N–S crustal shear system in the Bundelkhand massif: A unique crustal evolution signature in the northern Indian peninsula

The Bundelkhand massif, located in the northern part of the Indian shield, is a poly-deformed and poly-metamorphic terrain. This paper reports a new shear system developed throughout the massif in the form of N–S trending quartz veins that are sometimes quartzo-feldspathic and rarely granitic in composition. The veins are vertical and commonly occur in conjugate sets. This tectono-magmatic event appears to represent the youngest shear system of the massif as it cross-cuts all the earlier shear systems (E–W, NE–SE and NW–SE). Emplacement of this N–S vein system may have taken place due to extensional processes that developed some cracks along which siliceous magma was vertically emplaced. The complete absence of signature of the N–S event from the surrounding sedimentary cover of Vindhyan Supergroup, Bijawar and Gwalior Groups suggests that this shear system is pre-tectonic to the nearly E–W trending passive basins developed at the margins of the Bundelkhand craton. Further, several workers have considered the Bundelkhand massif as a part of the Aravalli craton. However, due to the absence of N–S, as well as the other (i.e., E–W, NW–SE and NW–SE), tectonic fabrics of the Bundelkhand massif in other cratons of the Peninsular India, and vice versa, makes the Bundelkhand block a separate and unique craton of its own and is not part of the Aravalli craton.

Spatial variability of the Purbeck–Wight Fault Zone—a long-lived tectonic element in the southern UK

New seamless onshore to offshore bedrock (1:10k scale) mapping for the Lyme Bay area is used to resolve the westward termination of the Purbeck–Wight Fault Zone (PWFZ) structure, comprising one of the most prominent, long-lived (Variscan–Cimmerian–Alpine) structural lineaments in the southern UK. The study area lies south of the Variscan Frontal Thrust and overlays the basement Variscide Rhenohercynian Zone, in a region of dominant E-W tectonic fabric and a secondary conjugate NW-SE/NE-SW fabric. The PWFZ comprises one of the E-W major structures, with a typical history including Permian to early Cretaceous growth movement (relating to basement Variscan Thrust reactivation) followed by significant Alpine (Helvetic) inversion. Previous interpretations of the PWFZ have been limited by the low resolution (1:250k scale) of the available offshore BGS mapping, and our study fills this gap. We describe a significant change in structural style of the fault zone from east to west. In the Weymouth Bay area, previous studies demonstrate the development of focussed strain associated with the PWFZ, accompanied by distributed strain, N-S fault development, and potential basement uplift in its hangingwall. In the Lyme Bay area to the west, faulting is dominantly E-W, with N-S faulting absent. Comparison of the newly mapped faulting networks to gravity data suggests a spatial relationship between this faulting variation and basement variability and uplift.

Paleostress evolution during the exhumation of high-P marbles, Samaná Complex, northern Hispaniola

The marble of the Samaná complex presents a widespread foliation formed during its exhumation following a general decompressive strain path from high pressure (2.0&gt;P&gt;0.7 GPa) and low temperature (&lt;500 ºC) conditions. The foliation is plano-linear and blastomylonitic. Deformation distribution is highly heterogeneous. Calcite preferred orientation is poor, even though the marble has a well-defined tectonic fabric. The blastomylonitic fabric is masking an earlier tectonic fabric. Cathodoluminescence images reveal that intense fracturing formed prior to foliation development in the marbles. The thermodynamic modelling of mineral phase transformations during prograde metamorphism indicate an increase in water content (1.2%&lt;w.t.H2O&lt;1.8%) that may have involved an increase in fluid pressure and triggered rock embrittlement and subsequent exhumation. Stress drops after a cataclastic event, as well as grain-size reduction by abrasion, may have activated dissolution-precipitation processes along cataclastic bands. Differential stress |σ1-σ3| increased as exhumation progressed after the cataclastic event. Estimates of paleostress based on calcite mechanical twinning indicate values of |σ1-σ3| &gt;350 MPa during deformation. In contrast, mean flow stress during grain-boundary migration is estimated in |σ1-σ3| &lt;150 MPa. The high paleostress record and microstructures of the marble are consistent with the high exhumation rate calculated (&gt;110 MPa Ma-1). All of these data suggest that exhumation always occurred near the brittle-ductile regime of deformation.

An evaporite‐bearing accretionary complex in the northern front of the Betic‐Rif orogen

AbstractThe Guadalquivir Accretionary Complex forms a largely oblique prism at the northern edge of the Betic‐Rif orogen, where Miocene sediments plus allochthonous evaporite‐bearing units were accreted during the displacement of the Alborán Domain toward the west. Traditional interpretations end the tectonic structuring of the Betic Cordillera at the present topographic front, beyond which gravitational and/or diapiric processes would predominate. However, this study shows pervasive tectonic deformation in the outer prism with coherent oblique shortening kinematics, which is achieved through an alternation of roughly N‐S arcuate thrust systems connected by E‐W transfer fault zones. These structures accord well with the geophysical models that propose westward rollback subduction. The main stage of tectonic activity occurred in the early‐middle Miocene, but deformation lasted until the Quaternary with the same kinematics. Evaporite rocks played a leading role in the deformation as evidenced by the suite of ductile structures in gypsum distributed throughout the area. S‐ and L‐ gypsum tectonites, scaly clay fabrics, and brittle fabrics coexist and consistently indicate westward motion (top to 290°), with subordinate N‐S contraction almost perpendicular to the transfer zones. This work reveals ductile tectonic fabrics in gypsum as a valuable tool to elucidate the structure and deformational history of complex tectonic mélanges involving evaporites above the décollement level of accretionary wedges.

Pre-Himalayan tectono-magmatic imprints in the Darjeeling-Sikkim Himalaya (DSH) constrained by 40Ar/39Ar dating of muscovite

The Lower Lesser Himalayan Sequence (L-LHS) in Darjeeling-Sikkim Himalaya (DSH) displays intensely deformed, low-grade meta-sedimentary rocks, frequently intervened by granite intrusives of varied scales. The principal motivation of our present study is to constrain the timing of this granitic event. Using 40Ar/39Ar geochronology, we dated muscovite from pegmatites emplaced along the earliest fabric in the low grade Daling phyllite, and obtained ∼1850Ma Ar-Ar muscovite cooling age, which is broadly coeval with crystallization ages of Lingtse granite protolith (e.g., 1800–1850Ma U-Pb zircon ages) reported from the L-LHS. We present here field observations to show the imprints (tectonic fabrics) of multiple ductile deformation episodes in the LHS terrain. The earliest penetrative fabric, axial planar to N-S trending reclined folds, suggest a regional tectonic event in the DSH prior to the active phase of Indo-Asia collision. Based on the age of granitic bodies and their structural correlation with the earliest fabric, we propose that the L-LHS as a distinct convergent tectono-magmatic belt, delineating the northern margin of Indian craton in the framework of the ∼1850Ma Columbia supercontinent assembly.

On thrusting, regional unconformities and exhumation of high-grade greenstones in Neoarchean orogens. The case of the Waroonga Shear Zone, Yilgarn Craton

During the Neoarchean, the dominant tectonic style progressively changed from an episodic-overturn/stagnant-lid regime to modern-style plate tectonics. The Neoarchean strengthening of continental lithosphere changed the style of deformation of orogenic belts. The case study presented here provides insights into how such transition in tectonic style occurred, a matter that is generally controversial.We present structural and metamorphic data from the c. 2660Ma Waroonga Shear Zone (WSZ) in the Neoarchean Yilgarn orogen (Western Australia). The WSZ contains a syntectonic pluton and older, high-grade greenstones. The tectonic fabric in the pluton developed during melt-present thrusting, followed by syn-cooling wrench-dominated transpression.Mafic greenstones preserve three metamorphic assemblages. The M1 assemblage (Grt–Cpx–Qtz) records peak P–T conditions of 12±1kbar and 800±50°C, followed by isothermal decompression to ~9kbar (M2). These anhydrous assemblages might predate the WSZ. Greenstones then underwent decompression at c. 2660Ma (3–4kbar; 600–650°C), defined by the amphibole-rich M3 assemblage, synkinematic with the tectonic fabric in the WSZ. We show that shearing along the WSZ exhumed these greenstones by at least 10km, inducing major uplift and erosion.Archean accretionary orogens developed on weak lithosphere, where deformation suppressed crustal thickening, orogenic relief and synorogenic exhumation of orogenic roots. However, our study indicates a genetic link between (i) strain localization along contractional structures, inducing large-scale uplift; (ii) exhumation of high-grade greenstones; (iii) development of inverse metamorphic gradients; (iv) establishment of a regional unconformity, with clastic sediments fed by the uplifted terrane; (v) incorporation of portions of the newly-formed orogenic basins into the footwall of the WSZ. These features imply that the Yilgarn orogenic lithosphere at c. 2660Ma was stiff enough to allow tectonic processes analogous to those that dominate modern-style orogenic belts, representing an intermediate stage between Archean- and modern-type orogens.

Radiogenic heating and craton‐margin plate stresses as drivers for intraplate orogeny

ABSTRACTThe Proterozoic belts that occur along the margins of the West Australian Craton, as well as those in intraplate settings, generally share similar geological histories that suggest a common plate‐margin driver for orogeny. However, the thermal drivers for intraplate orogenesis are more poorly understood. The Mutherbukin Tectonic Event records a protracted period of Mesoproterozoic reworking of the Capricorn Orogen and offers significant insight into both the tectonic drivers and heat sources of long‐lived intraplate orogens. Mineral assemblages and tectonic fabrics related to this event occur within a 50 km‐wide fault‐bound corridor in the central part of the Gascoyne Province in Western Australia. This zone preserves a crustal profile, with greenschist facies rocks in the north grading to upper amphibolite facies rocks in the south. The P–T–t evolution of 13 samples from 10 localities across the Mutherbukin Zone is investigated using phase equilibria modelling integrated with in situ U–Pb monazite and zircon geochronology. Garnet chemistry from selected samples is used to further refine the P–T history and shows that the dominant events recorded in this zone are prolonged D1 transpression between c. 1,320 and 1,270 Ma, followed by D2 transtension from c. 1,210 to 1,170 Ma. Peak metamorphic conditions in the mid‐crust reached &gt;650°C and 4.4–7 kbar at c. 1,210–1,200 Ma. Most samples record a single clockwise P–T evolution during this event, although some samples might have experienced multiple perturbations. The heat source for metamorphism was primarily conductive heating of radiogenic mid‐ and upper crust, derived from earlier crustal differentiation events. This crust was thickened during D1 transpression, although the thermal effects persisted longer than the deformation event. Peak metamorphism was terminated by D2 transtension at c. 1,210 Ma, with subsequent cooling driven by thinning of the radiogenic crust. The coincidence of a sedimentary basin acting as a thermal lid and a highly radiogenic mid‐crustal batholith restricted to the Mutherbukin Zone accounts for reworking being confined to a discrete crustal corridor. Our results show that radiogenic regions in the shallow to mid crust can elevate the thermal gradient and localize deformation, causing the crust to be more responsive to far‐field stresses. The Mutherbukin Tectonic Event in the Capricorn Orogen was synchronous with numerous Mesoproterozoic events around the West Australian Craton, suggesting that thick cratonic roots play an important role in propagating stresses generated at distant plate boundaries.