Development Of Shear Zones Research Articles

The collapse of the Caledonian orogen in SW Norway: Insights from quartz textures

Extensional collapse is a common late- to post-collisional feature of orogens. It is particularly prominent in the SW Scandinavian Caledonides, where extensional detachments formed progressively from the initial reactivation of the basal thrust zone to the formation of hinterland-dipping extensional shear zones. A mature stage, documented here, involves the development of bivergent (both hinterland- and foreland-dipping) shear zones and associated vertical basement mobilization. The main foreland-facing extensional shear zone in the study area is the Bergen Detachment – a until recently overlooked or misinterpreted structure. This detachment overprints top-to-W mylonitic fabrics related to the earlier Devonian extension stages and developed in response to the updoming of the Baltica basement west of Bergen (the Øygarden Complex) into a late core complex. Our microstructural and textural examinations suggest that for both the Hardangerfjord Shear Zone and the Bergen Detachment, strain was localized by activation of dislocation creep in quartz through the operation of multiple slip systems in the <a> direction, predominantly prism <a> and rhomb <a>. These examinations and existing radiometric age constraints suggest that the progressive shear zone development occurred over maybe as little as 5 million years, under upper to middle greenschist facies conditions. Synkinematic cooling brought both the Bergen Detachment and Hardangerfjord Shear Zone through the ductile-brittle transition zone. The main explanation for this prolonged collapse development is 1) that the early low-angle detachment became too low-angle for continued shearing, giving rise to the first hinterland-dipping set of shear zones, and 2) that the basement weakened rheologically and mobilized gravitationally with the formation of large upright folds with new detachments along their flanks (the bivergent stage), including the Bergen Detachment.

Kinematics of rift linkage between the Eastern and Ethiopian rifts in the Turkana Depression, Africa

AbstractRift initiation within cold, thick, strong lithosphere and the evolving linkage to form a contiguous plate boundary remains debated in part owing to the lack of time–space constraints on kinematics of basement‐involved faults. Different rift sectors initiate diachronously and may eventually link to produce a jigsaw spatial pattern, as in the East African rift, and along the Atlantic Ocean margins. The space–time distribution of earthquakes illuminates the geometry and kinematics of fault zones within the crystalline crust, as well as areas with pressurized magma bodies. We use seismicity and Global Navigation System Satellites (GNSS) data from the Turkana Rift Array Investigating Lithospheric Structure (TRAILS) project in East Africa and a new digital compilation of faults and eruptive centres to evaluate models for the kinematic linkage of two initially separate rift sectors: the Main Ethiopian Rift (MER) and the Eastern rift (ER). The ca. 300 km wide zone of linkage includes failed basins and linkage zones; seismicity outlines active structures. Models of GNSS data indicate that the ca. 250 km‐wide zone of seismically active en echelon basins north of the Turkana Depression is a zone, or block, of distributed strain with small counterclockwise rotation that serves to connect the Main Ethiopian and Eastern rifts. Its western boundary is poorly defined owing to data gaps in South Sudan. Strain across the northern and southern boundaries of this block, and an ca. 50 km‐wide kink in the southern Turkana rift is accommodated by en echelon normal faults linked by short strike‐slip faults in crystalline basement, and relay ramps at the surface. Short segments of obliquely oriented basement structures facilitate across‐rift linkage of faults, but basement shear zones and Mesozoic rift faults are not actively straining. This configuration has existed for at least 2–5 My without the development of localized shear zones or transform faults, documenting the importance of distributed deformation in continental rift tectonics.

Structural Evolution and Its Implication for the Emplacement of Gold Deposit in the Central Part of Burkina Faso, West Africa

In order to constrain the structural evolution in the central part of Burkina Faso and its implication for the emplacement of gold deposits, we undertook a structural mapping by coupling Landsat and aeromagnetic images interpretation to outcrop and core mapping followed by laboratory work. This approach reveals that the structural architecture in this locality mainly results from dextral transcurrent progressive deformation due to a NW-SE trending major stress. This architecture is similar to the Riedel-Tchalenko model. Initially, the dominant normal stress created an E-W constriction resulting in the development of N-S shear corridors. Subsequently, the tangential stress that took over this generated the progressive development of simple dextral shear zones with a NE-SW orientation that are locally taken up by ENE-WSW dextral shear bands associated with the Riedel's R structures. As a result of the cooling of the crust, we are witnessing the formation of sinistral NW-SE and dextral NNE-SSW strike-slip faults, respectively, which corresponds to Riedel's R' structures and Tchalenko's P structures. The development of NW-SE pre-Eburnean shear zones, particularly the N-S shear corridors, is synchronous with the circulation of gold-bearing fluids through the zone. However, the intersections of these directional corridors create zones suitable for gold concentration. Within these zones, ductile-brittle deformation following the emplacement of the shear bands has facilitated the remobilization and concentration of gold-bearing fluids within rocks with the appropriate rheological conditions. This is the case for Toega gold deposit.&#x0D; &#x0D;

Mantle Deformation Processes During the Rift‐To‐Drift Transition at Magma‐Poor Margins

AbstractThe rift‐to‐drift transition at rifted margins is an area of active investigation due to an incomplete understanding of the spatial and temporal evolution of physical and chemical processes at the ocean‐continent transition (OCT). Deep structures that characterize modern OCTs are often difficult to identify by seismic observations, while terrestrial exposures are preserved in fragments separated by tectonic discontinuities. Numerical modeling is a powerful method for contextualizing physical processes and observations relevant to rifted margin evolution. We synthesize results from geological observations of fossil OCTs preserved in ophiolites, a recent seismic experiment on the Ivorian margin, and numerical modeling to characterize mantle deformation and melt production for magma‐poor margins. Across varied surface heat fluxes, mantle potential temperatures, and extension rates, our model results show homologies with geological observations. We propose that the development of large shear zones in the mantle, melt infiltration, grain size reduction, and anastomosing detachment faults control the structure of OCTs. We also infer that a hot, upwelling, melt‐rich asthenosphere is an important control on the local stress environment. During the exhumation phase, continentward‐dipping shear zones couple with seaward‐dipping detachment faults to exhume the subcontinental and formerly asthenospheric mantle. The mantle forms core‐complex‐like domes of peridotite at or near the surface. The faults that exhume these peridotite bodies are largely anastomosing and exhibit magmatic accretion in their footwalls. A combination of magmatic accretion and volcanic activity derived from the shallow melt region constructs the oceanic lithosphere in the footwalls of the out‐of‐sequence continentward‐dipping detachment faults in the oceanic crust and subcontinental mantle.

Evolution of the strain localization and shear-zone internal structure in the granular material: Insights from ring-shear experiments

Shear zone is widely observed in natural faults and landslides as well as laboratory experiments on granular material. Understanding the shear-zone evolution process in granular materials is crucial in studying the dynamics of the landslides and faults. However, it is still not well understood. To this end, we conducted a series of ring-shear experiments to investigate the evolution of strain localization and shear-zone internal structure in cohesive and non-cohesive granular materials. The quantitative evaluation was conducted by using high-solution X-ray computed tomography (CT). The analyses included visualization of shear-zone internal structure and quantification of particle shapes, orientations, and grain-size distributions at different shear strain levels. We found that with the increase of shear displacement, the large particles inside the shear zone became more and more rounded, but without an orientation, wear and attrition resulted in an abundance of nanoparticles within the shear zones. We also found fine-particle layers (nanoparticle layers) formed on either side of shear zones during shear localization, implying that shear development progressed from a shear zone to an interface. These findings offer some new understanding of the evolution of shear zones in granular materials.

Constraining the evolution of shear zones in the Himalayan mid crust in Central–Western Nepal: implications for the tectonic evolution of the Himalayan metamorphic core

AbstractStructural analysis, petrochronology and metamorphic petrology enable identification and bracketing of the timing of a newly mapped high-temperature ductile shear zone (Jagat Shear Zone (JSZ)) in the Himalayan metamorphic core in Central-Western Nepal. In situ U-Th-Pb monazite petrochronology constrains the timing of top-to-the-S/SW shearing between 28–27 Ma and 17 Ma. Burial and prograde metamorphisms in footwall rocks were linked to thrust-sense movement along the JSZ, while the hanging wall rocks were retrogressed and exhumed. The identification and age of the JSZ (as part of a regional system of shear zones: the High Himalayan Discontinuity (HHD)) coupled with the localization and timing of activity of the Main Central Thrust (MCT) (i) fills a gap in tracing the HHD along orogenic strike, (ii) supports the identification of the position and timing of the long-debated MCT and (iii) helps to place the boundaries of the Himalayan metamorphic core and its internal architecture. Thus, our study is a significant step towards a precise identification of the burial, assembly and exhumation mechanisms of the Himalayan metamorphic core.

Titanite Petrochronology Records Secular Temperature and Fluid Evolution During Ductile Deformation: An Example From Late Cretaceous Shear Zones in the Eastern Transverse Ranges

AbstractQuantifying the timing and conditions of ductile deformation is essential for quantitative models of lithospheric deformation. Yet, directly constraining these variables and documenting how they change during a single deformation event remain difficult. We present titanite microstructural, zoning, trace‐element, and U‐Pb data from &lt;m‐scale mylonitic shear zones in the Eastern Transverse Ranges, CA (Joshua Tree National Park) that record (a) the timing and conditions of ductile deformation and (b) the evolution of deformation conditions within a single, punctuated ductile deformation event. Titanite grains in sheared plutonic rocks yield a bimodal distribution of dates (∼151 and ∼75 Ma) interpreted as the independently constrained pluton crystallization and shear zone deformation ages, respectively. Synkinematic titanite neocrystallization at ∼75 Ma documents early deformation at ∼640–710°C under hydrous fluid conditions. Recrystallized titanite rims with ∼75 Ma dates indistinguishable from the neocrystallized titanite record Zr‐in‐titanite temperatures of 600–610°C and higher F contents, suggesting that the fluid became a more halogen‐rich brine as deformation progressed and the system cooled. Chemical zoning related to fluid‐driven recrystallization occurs in tandem with lattice misorientations and partially to fully reset U‐Pb dates in bent titanite, implying that reactive fluid flow is the ultimate mechanism by which dates are reset in titanite deformed by dislocation creep. These data document a punctuated (e.g., ∼1–3 Myr) phase of high‐temperature deformation that accompanied local pluton emplacement and demonstrate that titanite petrochronology can capture multiple, discrete reactions associated with crustal weakening and variables that affect the rheological evolution of ductile shear zones.

On the Scale‐Dependence of Fault Surface Roughness

AbstractDefining roughness as the ratio of height to length, the standard approach to characterize amplitudes of single fault, joint and fracture surfaces is to measure average height as a function of profile length. Empirically, this roughness depends strongly on scale. The ratio is approximately 0.01 at a few mm but 10× smaller at a few tens of meters. Surfaces are rougher at small scales. However, these conclusions are metric‐dependent. If instead height is averaged over wavelength, roughness is nearly Brown spatial noise, having almost scale‐independent apparent surface height to wavelength ratio. The small deviation from scale‐independence is of the opposite sense than found using the standard metric; surfaces are slightly rougher at long wavelengths. Some natural surfaces may be Brownian within the measurement uncertainties. These contradictions are curiosities of surfaces that have Hurst exponents between 0.5 and 1, as natural fault surfaces do. The wavelength‐based analysis of roughness and how it changes with scale are straight‐forward; a normalized Fourier transform approximately preserves amplitude and its scale dependence in the wavelength domain. Among the conclusions from reconsideration of scale dependence are that the scale dependence is weak and much smaller than that of other fault and shear zone properties. Background and aftershock seismicity, jogs and step‐overs indicate strong localization (smoothing) with slip and scale. The lack of strong scale dependence to surface roughness suggests it is not the dominant control on brittle shear zone evolution.

Time-strain evolution of shear zones from petrographically constrained Rb–Sr muscovite analysis

Some isotopic ratios in minerals like monazite, apatite, and mica can record the thermal response to mylonitization, but such systems may fail to track the time of low- to medium temperature fabric realignment (recrystallization). New analytical methods that allow spatially resolved measurement isotopic ratios in distinct microstructures offer the potential to characterize the time-strain evolution of mid-crustal shear zones over a temperature range conducive to capturing the growth timing of those structures. Here we present high spatial resolution electron backscatter diffraction (EBSD), mineral chemistry, and in situ Rb–Sr dates from two texturally distinct muscovite populations in a mid- to upper-greenschist facies granitic mylonite from Top Up Rise, Western Australia. Coarse-grained muscovite fish retain a primary magmatic composition, displaying microstructural evidence of mechanical (kink and folds) and crystal-plastic deformation. The muscovite fish yield a Rb–Sr isochron date of 1614±75 Ma that either records the crystallization of the granitic protolith or cooling from high-grade metamorphic overprinting as constrained by in situ garnet Lu–Hf dates in related metapelites (1696±43 Ma and 1670±36 Ma). Conversely, fine-grained muscovite texturally associated with shear bands is chemically distinct, display microstructural evidence of fluid-assisted dynamic recrystallization and yield a distinctly younger isochron date of 609±14 Ma. The Rb–Sr dates from the coarse-grained muscovite fish cannot be unequivocally linked to mylonitization as they are indistinguishable from regional metamorphic processes. Muscovite grains behaved as rigid porphyroclasts during later medium-temperature mylonitization, leading to fish geometries, whilst maintaining intact Rb–Sr systematics. However, recrystallized/neoblastic fine-grained muscovite records the timing of medium-temperature mylonitization, establishing a direct time–strain relationship. These results highlight the potential for significant diachronicity in mineralogical components of mylonitic fabrics, demonstrating the need to establish direct time-strain relationships in order to accurately reconstruct deformation histories.

Strain Partitioning and Frictional Behavior of Opalinus Clay During Fault Reactivation

The Opalinus Clay (OPA) formation is considered a suitable host rock candidate for nuclear waste storage. However, the sealing integrity and long-term safety of OPA are potentially compromised by pre-existing natural or artificially induced faults. Therefore, characterizing the mechanical behavior and microscale deformation mechanisms of faults and the surrounding rock is relevant for predicting repository damage evolution. In this study, we performed triaxial tests using saw-cut samples of the shaly and sandy facies of OPA to investigate the influence of pressure and mineral composition on the deformation behavior during fault reactivation. Dried samples were hydrostatically pre-compacted at 50 MPa and then deformed at constant strain rate, drained conditions and confining pressures (pc) of 5–35 MPa. Mechanical data from triaxial tests was complemented by local strain measurements to determine the relative contribution of bulk deformation and fault slip, as well as by acoustic emission (AE) monitoring, and elastic P-wave velocity measurements using ultrasonic transmissions. With increasing pc, we observe a transition from brittle deformation behavior with highly localized fault slip to semi-brittle behavior characterized by non-linear strain hardening with increasing delocalization of deformation. We find that brittle localization behavior is limited by pc at which fault strength exceeds matrix yield strength. AEs were only detected in tests performed on sandy facies samples, and activity decreased with increasing pc. Microstructural analysis of deformed samples revealed a positive correlation between increasing pc and gouge layer thickness. This goes along with a change from brittle fragmentation and frictional sliding to the development of shear zones with a higher contribution of cataclastic and granular flow. Friction coefficient at fault reactivation is only slightly higher for the sandy (µ ~ 0.48) compared to the shaly facies (µ ~ 0.4). Slide-hold-slide tests performed after ~ 6 mm axial shortening suggest stable creeping and long-term weakness of faults at the applied conditions. Our results demonstrate that the mode of fault reactivation highly depends on the present stress field and burial history.

Thermal evolution of a Variscan syn-orogenic intracontinental basin (Servoz basin, Western Alps): RSCM geothermometry and geochronology

The Variscan basement of the Aiguilles-Rouges massif (Western Alps) exposes the Servoz syncline which consists of a metavolcano-sedimentary sequence composed of (i) a volcanic unit of unknown age and origin, (ii) Early Carboniferous sedimentary series affected by the Variscan orogeny and intruded by the Montées-Pélissier pluton, and (iii) a Late Carboniferous late-orogenic sedimentary sequence. We combined field investigations, Raman Spectroscopy on Carbonaceous Material geothermometry, and LA-ICPMS U-Th-Pb geochronology on zircon in order to reappraise the sedimentary sequence of the Servoz syncline. Our results allow us to identify three distinct sedimentary formations (F1, F2 and F3). The F1 formation is composed of metagreywackes, bimodal volcanic and magmatic rocks formed during basin opening at an early rifting stage (370–350 Ma) within a back-arc geodynamic setting. This extensional regime was responsible for a high thermal event recorded by a ca. 115 °C/km apparent geothermal gradient. Local anatexis of the basement rocks under the basin is dated at 351 ± 5 Ma. Basin inversion occurred between 350 and 330 Ma in response to oblique collision, with the development of large-scale dextral shear zones and syn-kinematic 340–330 Ma granite intrusions. Subsequent dextral transtension was responsible for the opening of a pull-apart basin between ca. 330 and 310 Ma with the deposition of the F2 phyllite formation that was later deformed by the ongoing dextral transcurrent Variscan tectonics at temperatures between 200 and 350 °C. Finally, the F3 terrigenous sedimentary rocks deposited at ca. 310–290 Ma in a late-orogenic extensional basin. The Alpine-related tectonic event overprinted all the temperatures below 350 °C. Although similar basins have been recognized in other External Crystalline Massifs of the Alps, the Servoz syncline is the first example that allows a major part of the polyphase tectonic evolution, since the early stages of the Devonian, to be recognized. Comparison with similar back-arc basins from the French Central massif, the Vosges massif and the Bohemian massif suggests that the External Crystalline Massifs initially belonged to the Moldanubian hinterlands of the Variscan belt.

Shear zone evolution and the path of earthquake rupture

Abstract. Crustal shear zones generate earthquakes, which are at present unpredictable, but advances in mechanistic understanding of the earthquake cycle offer hope for future advances in earthquake forecasting. Studies of fault zone architecture have the potential to reveal the controls on fault rupture, locking, and reloading that control the temporal and spatial patterns of earthquakes. The Pofadder Shear Zone exposed in the Orange River in South Africa is an ancient, exhumed, paleoseismogenic continental transform which preserves the architecture of the earthquake source near the base of the seismogenic zone. To investigate the controls on earthquake rupture geometries in the seismogenic crust, we produced a high-resolution geologic map of the shear zone core mylonite zone. The core consists of ∼ 1–200 cm, pinch-and-swell layers of mylonites of variable mineralogic composition, reflecting the diversity of regional rock types which were dragged into the shear zone. Our map displays centimetric layers of a unique black ultramylonite along some mylonite interfaces, locally adding to thick composite layers suggesting reactivation or bifurcation. We present a set of criteria for identifying recrystallised pseudotachylytes (preserved earthquake frictional melts) and show that the black ultramylonite is a recrystallised pseudotachylyte, with its distribution representing a map of ancient earthquake rupture surfaces. Pseudotachylytes are most abundant on interfaces between the strongest wall rocks. We find that the geometry of lithologic interfaces which hosted earthquakes differs from interfaces lacking pseudotachylyte at wavelengths of ≳ 10 m. We argue that the pinch-and-swell structure of the mylonitic layering, enhanced by viscosity contrasts between layers of different mineralogy, is expected to generate spatially heterogeneous stress during viscous creep in the shear zone, which dictated the path of earthquake ruptures. The condition of rheologically layered materials causing heterogeneous stresses should be reasonably expected in any major shear zone, is enhanced by creep, and represents the pre-seismic background conditions through which earthquakes nucleate and propagate. This has implications for patterns of earthquake recurrence and explains why some potential geologic surfaces are favored for earthquake rupture over others.

Timing and spatial variation of deformation along the Kanggur-Huangshan shear zone in the Chinese Tianshan: Implications for regional differential uplift and mineralization

The Chinese Tianshan experienced large-scale transcurrent tectonics, synkinematic emplacement of ultramafic to felsic intrusions, and the formation of various mineral deposits during late Paleozoic accretionary orogenesis. The relationships among the spatial variation of deformation, the distribution of Permian orogenic Au and magmatic Ni-Cu sulfide deposits, and the kinematic evolution of crustal-scale shear zones, however, remain ambiguous. To address these ambiguities, the spatial variation in the degree of deformation in the Kanggur-Huangshan shear zone in the Chinese Tianshan was characterized using detailed structural measurements and zircon U-Pb and muscovite 40Ar/39Ar age data. The new structural data indicate that a prominent spatial variation exists in the style of deformation throughout the Kanggur-Huangshan shear zone; intense ductile deformation structures are dominant in the east, while brittle structures become progressively more dominant toward to the west. Zircon U-Pb and muscovite 40Ar/39Ar age data for syn- and postkinematic intrusions along the Kanggur-Huangshan shear zone indicate that dextral strike-slip shearing occurred between 279 Ma and 249 Ma. The spatial variation in the degree of deformation and exhumation along the Kanggur-Huangshan shear zone was potentially caused by regional differential uplift induced by the collision of the Tianshan and Beishan regions; this was likely responsible for the predominant occurrence of magmatic Ni-Cu sulfide deposits in the eastern portion of the Kanggur-Huangshan shear zone and orogenic Au deposits in the western portion. The identified spatio-temporal relationship between deformation and distribution of orogenic Au and magmatic Ni-Cu sulfide deposits is crucial to the future success of mineral exploration in the Central Asian orogenic belt.

Origin of subhorizontal shear zones: Examples from a syntectonic laccolith within a metamorphic core complex

The origins of subhorizontal shear zones in the tensile middle crust are unclear. Structures in the syntectonic Congjia laccolith (128–126 Ma) in the Linglong metamorphic core complex (MCC; 137–108 Ma) of eastern China provide an opportunity to investigate the formation of such subhorizontal shear zones. Heterogeneous uplift of the MCC triggered SE-directed subhorizontal magmatic flow to solid-state deformation within the syntectonic laccolith, leading to the development of top-to-the-SE subhorizontal high-to low-temperature shear zones at 128–125 Ma. The magmatic flow and solid-state deformation were driven by gravity and facilitated by intrusion-induced thermal weakening. As the laccolith was exhumed, magmatic fluid overpressure and corresponding cyclical changes in the principal stress axis led to the development of alternating vertical and subhorizontal hydrofracturing within the brittle-ductile regime. Some of the subhorizontal hydrofractures were overprinted by top-to-the-SE brittle-ductile shear zones during the period from 125 to 123 Ma, and subsequent top-to-the-SE brittle faults formed between 123 and 108 Ma, indicating persistent subhorizontal shearing in the ductile, brittle-ductile, and brittle regimes. The structural evolution of the Congjia laccolith demonstrates that gravity-driven subhorizontal magmatic flow can trigger the development of subhorizontal shear zones in middle crust under a regional tensile setting.

Strain localization at brittle-ductile transition depths during Miocene magmatism and exhumation in the southern Basin and Range

Feldspar dominates the middle and lower continental crust. Models of crustal rheology depend on our knowledge of feldspar deformation mechanisms. To learn more about natural feldspar deformation, we conducted a multidisciplinary investigation of a brittle-ductile transition (BDT) hosted within a Cretaceous pluton in the Colorado River Extensional Corridor, southern Nevada. Here, a distinct BDT was observed at ∼10 km paleo-depth, where brittle faulting transitioned to discrete, localized ductile shearing. Field relationships confirm this BDT to have deformed and exhumed in the Miocene, temporally associated with the intrusion of two surrounding ca. 16 Ma plutons and subsequent footwall rotation in an east-directed normal-fault system. We established the overall structural framework, strain rates, and thermal histories through field traverses, aluminum-in-hornblende barometry to reconstruct paleo-depth, zircon (U–Th)/He thermochronology (ZHe) to constrain temperature-time histories, and 1D thermal models to further resolve deformation temperatures. Localized shear zones within the BDT formed ca. 16.2–15.5 Ma at temperatures of ∼500–600 °C, which continued deforming as they cooled both conductively and advectively to ∼200 °C by ca. 14 Ma, as constrained by ZHe dates. Strain was concentrated in fine-grained (7–10 μm) feldspar-rich ultramylonites that were interpreted to have primarily deformed via grain-size-sensitive diffusion creep. Grain-size reduction that allowed activation of diffusion creep likely resulted from brittle cataclasis, fluid-assisted fracturing and neocrystallization, and dislocation creep mechanisms, thus emphasizing the importance of these processes to localize deformation at relatively strong mid-crust conditions to facilitate the development of diffusion-creep shear zones. Grain-size piezometers suggest stresses of ∼50 MPa, which are significantly weaker than peak strength in quartz-rich BDTs that deform via dislocation creep. This integrated process allows relatively low viscosities (∼1018–1019 Pa s) at lower crust temperatures and explains a coherent process of strain localization at near-BDT conditions in feldspar-dominated lithologies. We suggest that this naturally deformed feldspar shear zone was preserved due to the unique geologic history with fast heating and exhumation, which froze both brittle vs crystal-plastic structures to provide valuable insights into mechanisms of feldspar deformation.

Role of inherited compositional and structural heterogeneity in shear zone development at mid-low levels of the continental crust (the Anzola shear zone; Ivrea-Verbano Zone, Southern Alps)

The formation of shear zones is crucial to understand the deformation of the crust and the evolution of rifted margins. However, despite their intrinsic importance, a detailed characterization of the compositional and structural patterns of shear zones is often lacking, resulting in poorly constrained models of shear initiation. In this contribution, we reconstruct the pre-shearing lithological, structural and metamorphic proprieties of rocks forming a major, rift-related shear zone with the aim to assess the conditions promoting the strain localization. We focus on the Anzola shear zone, a major extensional structure from one of the best-preserved cross-sections through the middle to lower continental crust of a fossil passive margin, the Ivrea-Verbano Zone (Southern Alps, Italy). Until now, the Anzola shear zone is believed to have developed within a rheologically hard and isotropic gabbro rather than in the surrounding weaker and anisotropic volcano-sedimentary metamorphic sequence. New geological mapping shows that significant pre-existing heterogeneities related to the composition and deformation structures, characterize the Anzola shear zone. Field relationships and geochemistry reveal that the (ultra-)mylonitic rocks overprinted a multi-lithological sequence that have already experienced Variscan deformation and late Variscan High-Temperature metamorphism, at the boundary between amphibolite and granulite facies. Our in-depth trace elements study is shown to be a powerful tool in reconstructing the pre-shearing relationships between wall rocks and mylonites and determining the protoliths of tectonites. Estimated P-T conditions indicate that mylonitic deformation started at high temperature (~820 °C) with presence of melt and continued as solid-state deformation down to amphibolite facies (~650 °C), following a retrograde path. We argue that strain localization was promoted by the combination of rheological boundaries derived from pre-existing conditions, including: i) compositional and structural anisotropies of the volcano-sedimentary metamorphic sequence contrasted by ii) the close intrusion of a nearly isotropic gabbro and iii) the presence of melt within the metamorphic boundary depicted by the transition between granulites (dominated by anhydrous minerals) and amphibolite facies (dominated by hydrous minerals). Our findings finally suggest that pre-existing significant heterogeneities relate to rock composition, deformation and metamorphism represent the preferential loci for strain localization controlling the initiation and development of rift-related structures in the mid to lower crust of passive margins. • Field mapping and chemical analysis of an extensional shear zone from middle/lower crust. • Trace element analysis reveals composite protoliths of the mylonites. • Anisotropic and/or hydrated rocks are folded with isotropic and/or anhydrous layers. • Mylonites postdates folding and upper-amphibolite-granulite facies metamorphism. • Pre-existing contrasted rheological boundaries promoted strain localization.

Weakening the lower crust: conditions, reactions and deformation

The impact of fluid infiltration on the deformation mechanisms that facilitate the development of lower-crustal ductile shear zones is evaluated through a multiscale structural, geochemical, and thermobaric analysis undertaken across a shear zone/wall-rock interface exposed on the island of Radøy in the Bergen Arcs (western Norway). At the outcrop scale, the shear zone is characterized by a strain gradient reflected in the progressive evolution from weakly-deformed coronitic gabbroic anorthosite to finer-grained foliated amphibolite characterized by a distinct mineral lineation, shear bands, and σ-type porphyroblasts. Electron backscattered diffraction (EBSD) crystallographic orientation data from the coronitic gabbroic anorthosite define an initial stage of shear localization whereby most of the deformation is accommodated by crystal plasticity within plagioclase accompanied by grain size reduction through subgrain rotation recrystallization. As deformation proceeds, complementary to increasing fluid-rock interaction, the replacement of the anhydrous mineral assemblage results in strain partitioning and the development of a heterogeneous ductile shear zone. At the grain scale, the distinct CPO of amphibole, epidote and kyanite suggests deformation being dominated by crystal plastic mechanisms. U-Pb age data obtained from zircon grains within the Caledonian shear zone cluster at 883 ± 3 Ma consistent with ages derived from the granulite facies assemblage. Phase equilibria modelling indicates conditions of deformation within the shear zone at ~600 °C and ~11 kbar, consistent with mid-crustal levels at amphibolite facies conditions. Conversely, geochemical data from garnet of the shear zone characterized by the absence of Eu anomaly, point to mineralogical reactions having initially occurred at higher pressure conditions. This study highlights the key role of fluid infiltration and metamorphic reactions on strain localization processes which can facilitate the ductile deformation of the original assemblage and ultimately contribute to the rheological weakening of an anhydrous and refractory lower crust. • Fluid infiltration into an anhydrous rocks promotes mineral reactions. • Strain localization in the lower crust is controlled by hydration processes. • Fluid-induced recrystalizzation contributes to ductile shear zones development.

Dynamic pressure variations in the lower crust caused by localized fluid-induced weakening

Metamorphism of dry, lower crust within orogens is often localized in shear zones composed of mechanically weaker rocks1–4. Several field-based studies suggest shear zone development is preceded by brittle faults which provide the fluid pathways required for metamorphism and weakening4–12. However, a unifying model which couples long-term geological deformation to fluid migration and metamorphic reactions does not exist. Here, we present a visco-elasto-plastic model where the most pertinent features observed in transformed lower crust emerge from basic mechanical principles during the deformation of a coherent rock volume with associated fluid introduction. These include a strikingly dynamic and heterogeneous pressure distribution in the reacting and deforming rock volumes. Lower crustal pressure variations may reach 1 GPa at the same depth. This will have first order effects on the pattern of fluid migration in the lower crust, and may explain the apparent discrepancies between the relevant tectonic settings and petrologically-inferred burial depths.

Strain partitioning in the Variscan deformation of the Marão domain (Central Iberian zone; northern Portugal)

The formation of the Ibero-Armorican Arc, a major structure in the SW Variscan orogeny, induced in most of its southern Iberian branch a heterogeneous sinistral transpressive regime. The presence of pre- and syn-orogenic steep planar anisotropies subparallel to the main WNW-ESE structural trend, gave rise to an important strain partitioning of the Variscan deformation. This lead to the lateral juxtaposition of narrow blocks where simple shear dominated transpression predominates, with wider ones where the pure shear to pure shear dominated transpression is pervasive. This partitioning of deformation is observed from the orogenic macroscale to the mesoscopic and even microscopic scales.The detailed structural mapping of the well exposed Lower Ordovician Armorican quartzites in one of the simple shear dominated sectors (the Marão Mountain in Northern Portugal), coupled with microstructural observations of chosen thin sections, emphasize the coeval development of WNW-ESE sinistral shear zones and subparallel folds. The interference between different Variscan structures, with particular geometries and kinematics, help to understand how transpressive regimes related to the major early oblique orogenic collisions could evolve to late intracontinental orthogonal collision.When the Variscan transpressive regime of Marão is interpreted in the context of the Iberian Variscides, it becomes clear that independent sectors of the same fold belt could compensate the orogenic shortening by different amounts of vertical thickening versus horizontal lateral escape. This explains why tectonic domains where the structures are related with a stretching subparallel to the a kinematic axis (X ≡ akin), could exist in the vicinity of domains where the stretching is subparallel to the b kinematic axis (X ≡ bkin).

Constraining the Timing of Evolution of Shear Zones in Two Collisional Orogens: Fusing Structural Geology and Geochronology

In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust.