Crustal Shear Zones Research Articles

Rheological Structure and Stress Triggered Megathrust Slip Constrained From the 2016 Mw 7.8 Kaikōura Crustal Earthquake

AbstractUnderstanding postseismic processes following the 2016 Mw 7.8 Kaikōura earthquake remains challenging due to the time‐dependent afterslip over the complex forearc system including crustal faults and megathrust, and the viscoelastic relaxation of the upper mantle. How the 2016 Mw 7.8 Kaikōura crustal earthquake interacts with the megathrust has yet to be better understood. Here we have derived the first 5‐year postseismic displacements from Global Positioning System (GPS) time series of 75 stations to study postseismic processes through a three‐dimensional viscoelastic finite element model. The optimal steady state viscosities of the crustal shear zone, megathrust shear zone, Australian upper mantle and Pacific upper mantle in the lowest‐misfit model among test models are 1018 Pa s, 4 × 1017 Pa s, 2 × 1019 Pa s and 1020 Pa s, respectively. The stress‐driven afterslip within the first 5 years after the earthquake is up to 80 cm over crustal faults, and up to 70 cm over the megathrust. A Kapiti slow slip sequence is probably promoted with a shorter interval by the 2016 earthquake, and is up to ∼11 cm within the first year after the earthquake. Afterslip over crustal faults and the megathrust are both required to reproduce the first‐order pattern of horizontal GPS observations. Coseismic rupture over the megathrust enhances shallow megathrust afterslip, which better fit the eastward postseismic displacement of sites near the rupture area. The southern end of Hikurangi megathrust may be activated during the 2016 earthquake and undergo continuous aseismic slip after the event.

Localized shear and distributed strain accumulation as competing shear accommodation mechanisms in crustal shear zones: constraining their dictating factors

Abstract. Understanding the underlying mechanisms of strain localization in the Earth’s lithosphere is crucial for explaining the mechanics of tectonic plate boundaries and various failure-assisted geophysical phenomena, such as earthquakes. Geological field observations suggest that shear zones are the most important lithospheric structures demonstrating intense shear localization at plate boundaries, accommodating a major portion of tectonic deformations. Despite extensive studies over the past several decades, the factors governing how shear zones accommodate bulk shear, whether via distributed strain (i.e. the development of macroscopic S (schistosity) foliations normal to the principal shortening strain axis) or via localized shearing (i.e. the formation of shear-parallel C bands, where C refers to the French “cisaillement” (shear)), remain largely unexplored. This study aims to address this gap in knowledge by providing observational evidence of varying S and C development in crustal shear zones from two geological terrains in eastern India. These field observations are complemented by 2D viscoplastic numerical simulations within a strain-softening rheological framework to constrain the factors controlling two competing shear accommodation mechanisms: distributed strain accumulation and shear band formation. The model-based analysis recognizes the bulk shear rate (γ˙b), initial viscosity (ηv), and initial cohesion (Ci) of a shear zone as the most critical factors determining the dominance of one mechanism over the other. For a given Ci value, low γ˙b and ηv values facilitate the formation of S foliation (uniformly distributed strain), which transitions to a C-dominated shear accommodation mechanism as ηv increases. However, increasing γ˙b facilitates shear accommodation through a combination of the two mechanisms, leading to S–C structures. The article finally discusses the conditions under which shear zones can significantly intensify rates of localized shear, producing rapid slip events, such as frictional melting and seismic activities.

Exploring the Eastern Abukuma Mountains: A field guide to high-pressure metamorphic rocks and strike-slip crustal shear zones in the Southern Kitakami Belt, NE Japan

Exploring the Eastern Abukuma Mountains: A field guide to high-pressure metamorphic rocks and strike-slip crustal shear zones in the Southern Kitakami Belt, NE Japan

Constraints on Growth and Stabilization of the Western Superior Craton From Inversion of Magnetotelluric Data

AbstractA data set consisting of 376 broadband and long‐period MT measurements was used to generate the first ever 3D resistivity model of the Archean western Superior Craton. The modeled resistivity structure is compared to coincident seismic reflection data. The observed geophysical signatures are interpreted within the context of the late stages of crustal growth and cratonization of the region via the progressive accretion of terranes against the initial cratonic core. The northern‐most terranes comprising the cratonic core exhibit a nearly homogenous highly resistive crust. The lower crust of the southern terranes contains largely continuous low resistivity bands which run subparallel to major terrane boundaries and corresponding fault systems. In some cases, low resistivity features are coincident with dense packages of sub‐horizontal to listric reflections within the mid‐ to lower crust. These resistivity structures are inferred to represent preserved geoelectric signatures of late to post‐orogenic magmatic pulses likely related to delamination of locally overthickened crust. Increased mantle heat flow resulted in partial melting of the lower crust and upper mantle and upward migration of CO2‐rich melts and fluids through crustal weak zones corresponding to shear and/or suture zones formed during terrane amalgamation. Thermal softening of the mid‐ to lower crust led to orogenic collapse and reactivation of the crustal shear zones, resulting in formation and interconnection of graphitic films which were preserved within the stable craton. These results have implications for the tectono‐magmatic history of the western Superior Craton, as well toward the understanding of the geodynamic regime of the Archean Earth.

Long‐Lived (180 Myr) Ductile Flow Within the Great Slave Lake Shear Zone

AbstractThe Great Slave Lake shear zone (GSLsz) is a type example for deeply eroded continental transform boundaries located in the Northwest Territories, Canada. Formed during the oblique convergence of the Archean Rae and Slave cratons, the GSLsz has accommodated up to 700 km of dextral shear. Here we present the results of in situ U‐Pb apatite and titanite geochronology from 11 samples that were collected across the strike of the shear zone. Both geochronometers record a near‐continuous history of ductile shear during crustal cooling and exhumation that spans ca. 1920–1740 Ma. By integrating the geochronological data with structural and metamorphic observations across the structure, we propose a tectonic model for the shear zone that consists of three stages. The first stage (ca. 1920–1880 Ma) is characterized by strain accommodation along two coeval fault strands. During the second stage (ca. 1880–1800 Ma), ductile shear ceases along the northernmost fault strand and the locus of strain migrates southwards toward the hinterland of the Rae cratonic margin. In the third stage (ca. 1800–1740 Ma), ductile strain localizes back along the southern of the two original fault strands, after which the present‐day surface level of the shear zone transitions to brittle shear. Our results highlight both the significance of the lateral migration of the zone of active deformation in major crustal shear zones as well as the localization of strain along existing crustal structures.

Multi-stage metamorphic and metasomatic imprints on apatite-monazite-xenotime assemblages in a set of small iron oxide-apatite (IOA) ore bodies, Prins Karls Forland, Svalbard

On Prins Karls Forland, Svalbard Archipelago, a set of small iron oxide-apatite (IOA) ore bodies have been discovered within a crustal shear zone, which deformed the polymetamorphosed Neoproterozoic metasedimentary rocks. The ores have various styles and grades of deformation and distinct mineral assemblages whose compositions record a multi-stage tectonothermal and metasomatic history. These IOA ore bodies can be subdivided into fluorapatite-bearing and predominant low-Th monazite in the upper section of the shear zone and F-Cl apatite-bearing and predominant high Th-monazite in the structurally lower higher-grade deformed part. The first stage of alteration for these ore bodies resulted in metasomatic alteration of the apatite and liberation of REE and P redeposited as monazite and xenotime. The transport of dissolved REE and P was likely enhanced by deformation. The second stage of alteration had a distinct impact on the individual ore bodies, which resulted in the Th-enrichment of a small subset of the monazite grains in the upper section of the shear zone. In the lower section of the shear zone most of the monazite was replaced by high Th monazite. Here the original fluorapatite is enriched in Cl, Mn, and Sr, most probably due to interaction with CaCl2-rich fluids enriched in Sr and Mn that was scavenged from the hosting metasediments and altered metagabbros. Contrasting textures, mineral assemblages, and the geochemistry of the ores from distinct localities reflect involvement of compositionally different fluids from the gabbroic rocks and surrounding metasedimentary rocks during the protracted tectonothermal evolution of Prins Karls Forland. Therefore, it is concluded that the IOA ore bodies most likely resulted due to the fractionation of Fe, P, Ca, and REE from hypersaline fluids associated with the gabbros. Once deposited, these IOA ore bodies were subsequently altered during at least one and perhaps two later metamorphic events.

Orogenic gold in the Blue Mountains, eastern Oregon, USA

At least 75% of gold produced in the State of Oregon came from the Blue Mountains. Most gold within the Blue Mountains was mined from orogenic gold veins and associated placer deposits. However, the geodynamic, regional, and deposit scale controls on orogenic gold mineralization within the Blue Mountains are not understood. Here, the mobilization, transport, deposition, and exposure of golds are linked to protracted tectonism of the western North American Cordillera through a scale-integrated mineral system. Gold production by mine (t) was estimated from public domain documents of production value (USD) and production time period (years). Lode endowment was then analyzed against criteria such as host rocks, vein structure, and location within the regional tectonic framework. Argillite, schist, and slate formed in accretionary wedge and fore-arc basins host the majority (∼60%; ∼29 t) of the total recorded gold production. Unique geochemical, geomechanical, and hydrological properties made these rock types excellent hosts. These properties were predetermined by both depositional setting (deep ocean basin) and metamorphic conditions (lower greenschist facies). Nearly half (50–55 %; ∼25.5 t) of recorded lode gold production was mined from lodes striking 030° ± 15° azimuth. A reconstructed paleo-stress field supports that these were left lateral faults. It is speculated that these were part of a margin-parallel shear zone that partitioned the strike slip component of oblique convergence between subducting oceanic plates and associated magmatic arcs. At both the district and the deposit scales, minor deviations in vein strike form local high-grade sites (i.e., dilatant zones). Such dilatant zones within major crustal shear zones could also explain the spatial, but not fluid-genetic association of granitic plutons with lode gold. The intersection lines between the veins, host rock fabrics, and secondary veins likely control the locations, thicknesses, and pitch of high-grade ore shoots.

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.

Fluid-enhanced grain-size reduction of K-feldspar from a natural middle crustal shear zone in northern Beijing, China

Grain-size reduction may lead to a transition from grain-size insensitive to grain-size sensitive deformation of rocks in the middle crustal level, thereby affecting the lithosphere's rheology significantly. However, the mechanisms of grain-size reduction of sheared rocks have long been debated. In this study, granitic mylonites from the Shuiyu shear zone of the Yunmengshan metamorphic core complex were sampled to investigate the mechanisms of grain-size reduction of K-feldspar during shearing in the middle crust. The mylonites are characterized by a typical microstructure of coarse-grained K-feldspar porphyroclasts surrounded by fine-grained matrix of oligoclase, K-feldspar, and quartz. Deformation temperatures are constrained from 400 °C to 550 °C, estimated using the feldspar and quartz microstructures, crystal-preferred orientation patterns of recrystallized quartz, and a two-feldspar geothermometer. Here, we show that the derivation of fine grains in the matrix from the K-feldspar porphyroclasts was accomplished via fluid-assisted subgrain rotation recrystallization. Walls of either well-organized or tangled dislocations constituted the boundaries of micron-scale subgrains along the margins of porphyroclasts. Hydrolytic weakening enhanced the dislocation motions to form subgrains. Furthermore, the subgrain rotation recrystallization promoted the replacement reaction of K-feldspar by sodium-rich fluid via increasing the areas of fluid/grain interfaces. The newly formed oligoclase and quartz were subsequently deformed by diffusion creep.

Seismic properties across an amphibolite- to greenschist-facies strain gradient (Neves area, eastern Alps, Italy): New considerations for shear zone imaging

Detecting the presence and geometry of crustal shear zones by geophysical methods relies on our understanding of the intrinsic parameters controlling the seismic properties of these deformed rocks, over the range of pressure-temperature conditions expected in the Earth's crust. To this end, we aimed to track changes in P-wave propagation velocity (VP), anisotropy (AVP) and S-wave splitting (AVS) across a natural shear zone using experimental and electron backscatter diffraction methods. Five samples were collected across a meter-scale shear zone developed under amphibolite- to greenschist-facies in the Neves metagranodiorite (Tauern Window, Eastern Alps). With increasing strain, seismic properties show a non-linear evolution resulting from interactions among structural, textural and mineralogical changes. Geometry and magnitude of seismic anisotropy are controlled by the interference of strongly anisotropic phyllosilicates with less anisotropic but more abundant quartz and plagioclase. The amphibolite-facies shear zone margin (> 500 °C) is free of major mineral transformations, and rather characterized by the development of S-C structure, inducing an orthorhombic symmetry to the VP distribution and up to 12 % AVP, with the fast axis parallel to lineation. AVS is maximized (6 %) in the two directions parallel to the C-planes. Further deformation under greenschist-facies (450–500 °C) induces significant crystallization of muscovite, chlorite, and albite. A dominant hexagonal symmetry develops, with a fast VP plane and a fast S-wave polarization plane along the C-planes. AVP and AVS exceed 10 %. The computed elastic tensor of the whole shear zone emphasizes the importance of the strain gradient on the seismic signature of the overall structure. In our model, the bulk seismic properties are dominated by the orthorhombic symmetry of the protomylonitic zone rather than by the ultramylonite core with hexagonal symmetry (C-plane), which may explain why seismic studies of shear zones commonly involve a fast velocity plane oblique to the layer boundaries.

TRIASSIC-JURASSIC RIFT-RELATED DEFORMATION AND TEMPERATURE-TIME EVOLUTION OF THE FOSSIL ADRIATIC MARGIN: A REVIEW FROM OSSOLA AND STRONA DI OMEGNA VALLEYS (IVREA-VERBANO ZONE)

The direct observation and investigation of rift-related structures at the mesoscale is uncommon. Hence, detailed constraints on the evolution of the main faults and shear zones developed during crustal extension are not always available.The Ivrea-Verbano Zone, in the Italian Southern Alps, samples remnants of the former lower crust of the rifted margins surrounding the Alpine Tethys and therefore provides the opportunity to directly investigate rift-related tectonics. Here, several shear zones have been recognized and interpreted as related to Mesozoic rifting. However, even if there is a general agreement with this interpretation, the precise age of activity of many of those shear zones is not well constrained.In this paper we present a review of the Triassic-Jurassic geochronological and thermochronological data available for two sections of the Ivrea-Verbano Zone, the Strona di Omegna and Ossola valleys, where at least two extensional shear zones are exposed. Ductile deformation occurred under amphibolite-facies conditions and it was alternatively attributed to late Variscan deformation or to Triassic-Jurassic rifting-related tectonics.We discuss the available chronological data and the different interpretations provided for the shear zones considering also new geochronological studies on other lower crustal shear zones exposed in other sectors of the Ivrea-Verbano Zone. This review allows us to strengthen the more recent interpretations indicating that these shear zones are important tectonic structures related to Late Triassic-Jurassic deformation in the lower crust of the Adriatic margin.

Shape of plutons in crustal shear zones: A tectono-magmatic guide based on analogue models

Plutons in crustal shear zones may exploit inherited structures, interfere with strain localizing or be deformed passively. To constrain the relative timing of such tectono-magmatic relationships in natural settings is not always straight-forward. We here present sandbox-type analogue model experiments simulating magma emplacement into simple and transtensional crustal shear zones to test the diagnostic potential of pluton shape with respect to timing and setting. Observations based on surface deformation and intrusion shape exemplify the interplay between evolving and inherited tectonic structures and magma uprising. We observe markedly asymmetric intrusions in association with dikes reflecting the regional stresses, fault pattern and finite strain field. At the same time, the presence of an intrusion modifies the tectonic evolution, but only transiently, resulting in short-lived faults, reactivation and inversion. Diagnostic attributes include the pluton's aspect ratio, its orientation and amplitude as well as dike association. Accordingly, syn-tectonic intrusions show the highest pluton amplitudes, but only intermediate elongation compared to other scenarios. They are oriented parallel to Riedel shears in simple shear, respectively to the compression direction in transtension. Post-tectonic intrusions are least elongated, have medium amplitudes and exploit Riedel shears. Pre-tectonic intrusions are characterized by lowest amplitudes but the highest aspect ratios and are parallel to the finite elongation direction. Intrusions in transtensional shear zones are generally of less elongate than those in simple shear zones. Experimental results are tested against observations from natural examples validating the diagnostic potential of pluton shape for the timing and the tectonic setting of the emplacement.

Constraining the ductile deformation mechanisms of garnet across pressure-temperature space

Garnet is a common mineral at elevated pressures, and how it deforms plays an important role in the strength of lower crustal shear zones, subduction thrusts, and the mantle. Strong shape preferred orientations in elongate garnets at elevated temperatures and pressures attests to garnet's ability to deform through ductile mechanisms; however, how individual garnets deformed is frequently ambiguous or disputed. Garnet microstructures from the Morin shear zone and the Sulu ultra-high pressure terrane are revisited using fine-scale electron backscattered diffraction and wavelength dispersive spectroscopy mapping. The dominant deformation mechanism at each site is re-interpreted, and we show that garnet deformed through dissolution-precipitation creep at Sulu and through dislocation-assisted diffusion creep at Morin. These observations are integrated into a compilation of ductile deformation microstructures for garnet which reveals domains where dissolution-precipitation creep, recovery-accommodated dislocation creep (through subgrain rotation dynamic recrystallization), and dislocation-assisted diffusion creep dominate in P-T space. Garnet may deform through ductile deformation at temperatures as low as ~500 °C and deforms under low differential stresses in both eclogite and granulite facies conditions. The efficacy of deformation indicates that 1) garnet may not be the strongest phase in these environments, and 2) that at elevated temperatures, inclusion barometry may be affected by garnet's inability to maintain high differential stress.

Vent distribution and structural inheritance in an embryonic rift: The example of the Chyulu Hills off-rift magmatic range (South Kenya)

In the East African Rift System (EARS) tectonic inheritance rules the location, the shape and the development of rift segments. Such lithospheric/crustal-scale inherited structures may be exploited by magma favouring its ascent to shallow crustal level and, possibly, to the surface producing volcanism. In the eastern branch of the EARS in southeast Kenya, the Pleistocene Chyulu Hills volcanic field (CHVF) is a volcanic range formed east of the main rift along a NW-SE trending Proterozoic crustal-scale shear zone (the Emali-Voi shear zone). The volcanic range consists of hundreds (at least 514) of scoria cones of basaltic composition, often coalescent and well-aligned along the trace of the crustal shear zone. Morphostructural and spatial analyses of vents and fractures in the CHVF show that vent density and clustering change moving south-eastward along the ridge mimicking the age pattern of volcanism with the youngest activity in the southeast of the CHVF. Notably, the parameters defining the vent self-similar clustering indicate a decreasing thickness of the upper brittle crust along the CHVF from about 12 km in the north-west to about 7 km in the southeast, in agreement with the crustal structure of the area as imaged by seismic and geophysical data sets. Here we propose an evolutionary model of the CHVF occurring at spatial and time scales quite different from that of magma-assisted rifting processes, where CHVF is an example of an embryonic rift stage with a focussed magma injection into the continental lithosphere localized along a weakness discontinuity represented by an inherited crustal shear zone.

Crustal structure of the Indian plate underneath the alluvial plains of the central Ganga basin by broadband magnetotellurics

The Ganga basin consists of thick Cenozoic sediments that mask the tectonic fabric of the underlying Indian plate. Despite the availability of fairly good information on the intra-basin structure, especially at the foothills of the Himalaya due to hydrocarbon exploration activities, very little is known about the basement and the crustal/lithospheric structure although composition and thermomechanical properties of the underthrusting Indian plate are expected to have a significant role in controlling the deformation and seismicity along the Himalayan arc. Here, we present a two-dimensional geoelectric model of the crustal structure along a 285-km-long profile across the central Ganga basin. The results were obtained by inversion of decomposed TE- and TM-mode impedance tensors derived from broadband magnetotelluric data recorded at 45 sites along the profile. The results reveal three main features: (i) the highly resistive Bundelkhand massif (BKC) extends northward beneath the sediments up to Lucknow; (ii) the massif is constituted of two blocks separated by a 10-km-wide moderately resistive nearly vertical zone; and (iii) a super-deep sedimentary basin constituted of highly conductive sedimentary succession exists in the northern segment of the profile. We report the presence of an electrical Moho at 40–45 km depth for the buried BKC and a crustal shear zone within the craton masked by the alluvial sediments. These results shall facilitate in furthering our understanding of the evolution of the BKC as well as the lithospheric/flexural deformation of the Indian continental lithosphere.

Rheological evolution of a pseudotachylyte-bearing deep crustal shear zone in the western Canadian shield

Crustal-scale continental strike-slip fault systems are typically thought to be thermally segregated into an upper seismogenic expression and a lower zone of ductile deformation likely extending into the lower crust. The ability of the lower crust to sustain stress is fundamental to understanding the mechanical behavior of the lithosphere, yet significant debate exists regarding the strength of the ductile crust. The Cora Lake shear zone in the western Churchill Province of the Canadian Shield is a 4–7 km wide zone of granulite-to upper amphibolite-facies mylonite which hosts synkinematic pseudotachylyte, suggesting a locally strong lower crust. Kilometer-scale mapping suggests that the shear zone localized at a major lithological and terrane boundary, with the pseudotachylyte network centered within the ca. 1 km-wide more intensely deformed, finer-grained ultramylonitic core. In this contribution, we use optical petrography and electron-backscatter diffraction to describe microstructure across a transect of the Cora Lake shear zone. By combining these observations with grain size piezometry, experimental flow laws, and regional pressure-temperature paths, we conclude that stress amplification from a relatively common type of km-scale lithological heterogeneity and cooling history is the most reasonable explanation for the deep crustal pseudotachylyte.

Reversely zoned plagioclase in lower crustal meta-anorthosites: An indicator of multistage fracturing and metamorphism in the lower crust

AbstractThis paper describes the formation mechanism of reversely zoned plagioclase, which has been observed frequently in lower crustal shear zones and is indicative of multistage fracturing and meta-morphism in the lower crust, by studying the microstructural and chemical characteristics of plagioclase in sparsely fractured anorthosites and anorthositic mylonites from the Eidsfjord shear zone, Langøya, northern Norway. Based on the field relationship between sparsely fractured anorthosite and anorthositic mylonite, the fracturing of anorthosite occurred before the formation of mylonite. In sparsely fractured anorthosites, transgranular fractures are observed; hydration-reaction products, including Na-rich plagioclase, occur within cracks and fractures, suggesting that hydration reactions occurred during or after fracturing. The hydration reactions in sparsely fractured anorthosites are estimated to have occurred at higher-pressure (P) amphibolite-facies conditions (~0.9–1.0 GPa and ~550–700 °C). In anorthositic mylonites, which are considered to have initiated by fracturing and subsequent hydration metamorphism at lower-P amphibolite-facies conditions (~0.7 GPa and ~600 °C), recrystallized plagioclase grains often show compositional zoning with an Na-rich core and a Ca-rich rim. Because the compositions of metamorphic plagioclase grains in the sparsely fractured anorthosites and those of the Na-rich cores of the reversely zoned plagioclase in anorthositic mylonites are similar to each other, the Na-rich cores of the matrix plagioclase in the anorthositic mylonites have recrystallized under higher-P amphibolite-facies conditions and then been overgrown or replaced by the Ca-rich rims under lower-P conditions. Consequently, the reversely zoned plagioclase observed frequently in lower crustal shear zones is an indicator of multistage brittle fracturing and subsequent hydration metamorphism during exhumation, providing information relevant to understanding the deep rupture process caused by repeated seismicity alternating with aseismic creep below the seismogenic zone.

Geochronologic constraints on deformation and metasomatism along an exhumed mylonitic shear zone using apatite U-Pb, geochemistry, and microtextural analysis

Directly dating and characterizing deformation and metasomatism in fault zones is critical to understanding fault zone processes and lithospheric evolution. Integrated apatite U-Pb geo-thermochronometry, geochemistry, and microtextural analysis has potential to directly date and characterize deformation and fluid-rock interactions in shear zones using a single mineral phase. Integrated apatite U-Pb, microtextural, and trace and rare earth element results from an exhumed mylonitic shear zone, the Main Mylonitic Band in the Northern Pyrenees, constrain timing of deformation and metasomatism along the shear zone as Early Cretaceous, synchronous with ductile deformation and exhumation of middle-lower crustal gneisses in the footwall, and significant mineralization and metasomatism in the brittle hanging wall. Samples from the hanging wall preserve magmatic apatite U-Pb crystallization ages that overlap with zircon and monazite U-Pb ages of ∼300 Ma, granitic geochemical trends, and primary magmatic growth zoning. Footwall samples below the MMB record dynamic recrystallization of apatite associated with mylonitization and exhumation of the gneisses in the Early Cretaceous. Apatite along the MMB record partial recrystallization via dissolution-reprecipitation associated with shearing and circulation of deeply sourced fluids along the shear zone evident by mixed ages (300-100 Ma), geochemical signatures enriched in light rare earth elements, and patchy microtextures. The MMB likely acted as a conduit for deeply sourced fluids that affected the brittle-ductile transition, strain localization, and fluid pathways through the continental crust. This study demonstrates that the integration of apatite U-Pb geo-thermochronology, geochemistry, and microtextural analysis is a powerful approach to dating deformation and metasomatism, characterizing grain scale deformation and (re)crystallization processes, and geochemically characterizing fluid-rock interactions in exhumed middle crustal shear zones.

The effect of hydrous mineral content on competitive strain localization mechanisms in felsic granulites

This study investigates the grain-scale mechanisms that facilitated strain localization in the lower crust within the East Athabasca Mylonite Triangle of northern Saskatchewan, Canada. There, two distinct lithotectonic units, the Upper Deck and Northwestern Domain, are juxtaposed across a km-scale shear zone that has been previously interpreted to record thrust and/or normal-sense displacement. Our study reports new quartz c-axis opening angle, Ti-in-biotite and Ti-in-quartz geothermometry and amphibole-plagioclase geothermobarometry within syn-kinematic phases indicating that both units were deformed and metamorphosed at upper amphibolite to granulite facies (~725 °C and ~0.8 GPa). While sharing similar conditions, the dominant mechanisms of deformation across the two units are strongly linked to modal proportion of syn-kinematic hydrated mineral phases. Detailed microstructural and crystallographic preferred orientation (CPO) analyses reveals that specimens poor in hydrated minerals (<10%) deformed mainly by dislocation creep and dynamic recrystallization allowing it to develop a strong quartz CPO, while in specimens rich in hydrated minerals (>10%) pressure solution and (re)precipitation processes enhanced phase mixing and promoted grain boundary sliding resulting in weakly developed or absent quartz CPO. These results indicate a strong mineralogical and fluid control on deformation and, therefore, which strain softening process (es) may dominate in lower crustal shear zones.

Mesoscopic to microcropic structures associated with the Wanni Complex/Highland Complex Boundary Shear Zone in Sri Lanka

In this study, we report mesoscopic and microscopic structures in sheared rocks associated with the Wanni Complex/Highland Complex boundary shear zone (WHBSZ) which represents a deep part of a granulite-facies, crustal-scale ductile shear zone in the Proterozoic basement of Sri Lanka. Normal khondalitic mylonites, spinel-bearing khondalitic mylonites, semipelitic mylonites and metagranitoid mylonites (striped gneisses) found along the WHBSZ show mesoscopic to microscopic scale shear sense indicators such as S-C' fabrics, rotated garnet porphyroblasts, δ-type and σ-type asymmetric objects, asymmetric folds and sheath folds. These asymmetric mesoscopic to microscopic structures indicate that the dominant deformation along the WHBSZ is non-coaxial ductile flow and that the WC has been emplaced from NNW (present orientation) on top of the HC. The presence of mylonites and ultramylonites along the WHBSZ suggests severe grain size reduction during the shearing. All the mylonites studied along the boundary shear zone have developed a mylonitic foliation, defined by quartz plates and ribbons, elongated and recrystallized fine-grained aggregates of feldspar, and a very strong stretching lineation mostly defined by c-axis of sillimanite and stretched quartz and elongated aggregates of other minerals. All the mesoscopic and microscopic structures suggest that the WHBSZ has formed during the second phase of D2 non-coaxial deformation accompanying high-temperature (HT) to ultrahigh temperature (UHT) granulite facies meta-morphism related to the first collision between the WC and the HC. This study emphasizes that careful study of granulite facies mylonites may provide reliable shear sense indicators for understanding the nature and kinematics of deep crustal shear zones.