Shear Zone Core Research Articles

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.

Role of fluid in strain softening within the Main Central thrust in Sikkim: The origin of quartz-rich mylonites

The presence of fluids promotes strain softening, and profoundly affects the evolution of shear zones. The Main Central thrust (MCT) is a major shear zone that accommodated at least 90 km of shortening, and played a significant role in Cenozoic evolution of the Himalaya. Surprisingly, no information exists on the role of fluid in evolution of the MCT. This study integrates mineralogical, and geochemical analyses with mass-balance calculations to explore the role of fluids in evolution of the MCT shear zone from four exposures, which span ~40 km distance along the transport direction in Sikkim, eastern India. Our analyses reveal significant fluid-induced strain softening in the investigated exposures, attested by retrogression of biotite to chlorite, and feldspar to muscovite, and quartz. The retrogressed feldspar grains are surrounded by recrystallized quartz, and the muscovite, and chlorite produced due to retrogression define the mylonite foliations, which indicate fluid activity was syn-tectonic. We propose a three-stage conceptual model to analyze the role of fluid in the evolution of the MCT in Sikkim: First, the shear zone nucleated along pre-existing structurally weak, and compositionally distinct zones. Following which, the proto shear zone developed in closed-system conditions, and deformation localized by dynamic recrystallization, and dislocation creep resulting in formation of ~200–1000 m-thick protomylonites. Then, fluids infiltrated the shear zone, and induced retrogression, and element mobility. In the hinterland-most exposures, significant fluid-induced retrogression transformed the calc-silicate and paragneiss protoliths to mylonites resembling micaceous quartzites. In the frontal-most, and structurally intermediate exposures, the muscovite and chlorite grains underwent further dissolution, leaving behind a quartz-rich residue; thus, transforming tonalite orthogneiss, and paragneiss protoliths to mylonites resembling monomineralic quartzites. The deformation front narrowed through time, and became progressively localized within the ~120-410 m-thick strain-softened mylonites at the shear zone core, whereas, deformation ceased or continued slowly in the ~100-880 m-thick protomylonites at the margins. The study improves our understanding of the strain softening mechanisms operative within the MCT, and the origin of quartz-rich mylonites.

Quartz fluid inclusion abundance and off-fault damage in a deeply exhumed, strike-slip, seismogenic fault

Off-fault damage zones comprise highly fractured rocks surrounding the dynamic slip surface of faults. These damage zones modify fault-zone rheology and rupture dynamics by changing the bulk elastic properties and modulating fluid flow. Damage zones in the brittle upper crust, reaching widths >100 m, are commonly characterized by measuring fracture density, but identifying the extent of off-fault damage in deeply exhumed faults is challenging due to post- and inter-seismic viscous/plastic deformation. We measured fluid inclusion abundance in quartz deformed at ~400–500 °C from an ancient seismogenic strike-slip fault/shear zone to evaluate whether it can be a proxy for damage-zone width. In contrast to upper crustal fault zones displaying a high-low trend of healed microfracture density, the shear zone has a low-high-low trend of fluid inclusion abundance from the core toward the host rock. We find that pattern explicable through removal of fluid inclusions by recrystallization after co-seismic deformation, allowing us to use fluid inclusion abundance to measure damage zone extent, which is >~80 m (the low-high abundance region) from the shear zone core. Our findings indicate that extensive co-seismic damage zones may extend from Earth's surface to the base of the seismogenic zone, well within the frictional-to-viscous transition.

Evolution of a shear zone before, during and after melting

Partial melt in the deforming mid- or lower continental crust causes a strength decrease and drives formation of lithological heterogeneities. However, mechanisms of formation of syn-melt deformation zones and strain partitioning in partially molten rock remain poorly understood. We use field and microstructural observations to unravel the evolution of a partial melt shear zone, Seiland Igneous Province, northern Norway. The Øksfjord shear zone (ØSZ) is one of several paragneiss shear zones present within gabbros of the Seiland Igneous Province, formed by syn-intrusive deep crustal shearing during lithospheric extension related to continental rifting. Microstructures from the ØSZ show evidence for different deformation conditions. The first phase was active pre-melt and involved deformation at high subsolidus temperatures. This was followed by syn-melt deformation of the shear zone causing a relative strength increase towards the shear zone centre upon crystallization. The third phase nucleated two parallel shear zones at the edges of the ØSZ; melt textures are absent and microstructures indicate deformation at lower temperatures and higher stresses. In effect, melt migration towards the shear zone centre ultimately led to strengthening of the shear zone core, with post-crystallization deformation focusing along shear zone margins where significant heterogeneities are present. Supplementary material: Figures illustrating grain boundary dihedral angle data, EBSD maps, grain size distribution and additional pole figures is available at https://doi.org/10.6084/m9.figshare.c.4819494

Seismic cycle feedbacks in a mid-crustal shear zone

Mid-crustal fault rheology is controlled by alternating brittle and plastic deformation mechanisms, which cause feedback cycles that influence earthquake behavior. Detailed mapping and microstructural observations in the Pofadder Shear Zone (Namibia and South Africa) reveal a lithologically heterogeneous shear zone core with quartz-rich mylonites and ultramylonites, plastically overprinted pseudotachylyte and active shear folds. We present evidence for a positive feedback cycle in which coseismic grain size reduction facilitates active shear folding by enhancing competency contrasts and promoting crystal plastic flow. Shear folding strengthens a portion of a shear zone by limb rotation, focusing deformation and promoting plastic flow or brittle slip in resulting areas of localized high stress. Using quartz paleopiezometry, we estimate strain and slip rates consistent with other studies of exhumed shear zones and modern plate boundary faults, helping establish the Pofadder Shear Zone as an ancient analogue to modern, continental-scale, strike-slip faults. This feedback cycle influences seismicity patterns at the scale of study (10s of meters) and possibly larger scales as well, and contributes to bulk strengthening of the brittle-plastic transition on modern plate boundary faults.

Recrystallization fabrics of sheared quartz veins with a strong pre-existing crystallographic preferred orientation from a seismogenic shear zone

Microstructural investigations were carried out on quartz veins in schist, protomylonite, and mylonite samples from an ancient seismogenic strike-slip shear zone (Sandhill Corner shear zone, Norumbega fault system, Maine, USA). We interpret complexities in the microstructural record to show that: (1) pre-existing crystallographic preferred orientations (CPO) in the host rock may persist in the new CPO patterns of the shear zone and (2) the inner and outer parts of the shear zone followed diverging paths of fabric development.The host rocks bounding the shear zone contain asymmetrically-folded quartz veins with a strong CPO. These veins are increasingly deformed and recrystallized with proximity to the shear zone core. Matrix-accommodated rotation and recrystallization may position an inherited c-axis maximum in an orientation coincident with rhomb <a> or basal <a> slip. This inherited CPO likely persists in the shear zone fabric as a higher concentration of poles in one hemisphere of the c-axis pole figure, leading to asymmetric crossed girdle or paired maxima c-axis patterns about the foliation plane.Three observed quartz grain types indicate a general trend of localization with decreasing temperature: (1) large (>100μm), low aspect ratio (<~5) and (2) high aspect ratio (~5–20) grains overprinted by (3) smaller (<~80μm), low aspect ratio (<~4) grains through subgrain rotation-dominated recrystallization. In the outer shear zone, subgrain rotation recrystallization led to a well-developed c-axis crossed girdle pattern. In the inner shear zone, the larger grains are completely overprinted by smaller grains, but the CPO patterns are relatively poorly developed and are associated with distinctively different misorientation angle histogram profiles (“flat” neighbor-pair profile with similar number fraction for angles from 10 to 90°). This may reflect the preferential activation of grain size sensitive deformation processes in the inner-most part of the shear zone where transient deformation associated with the seismic cycle may have localized.

Ti distribution in quartz across a heterogeneous shear zone within a granodiorite: The effect of deformation mechanism and strain on Ti resetting

The study of a heterogeneous ductile shear zone that developed at ~500°C and 0.2GPa during post-magmatic cooling of a granodiorite has allowed the effect of strain and recrystallization on Ti re-equilibration of quartz to be assessed. Understanding this effect is critical for applying Ti-in-quartz thermobarometry to mylonites. Differently strained quartz across the shear zone shows a heterogeneous distribution of Ti concentrations ([Ti]) (measured by Secondary Ion Mass Spectrometry, SIMS) ranging between 2 and 45ppm. Quartz cathodoluminescence (CL) is proven by spectral analysis to be correlated with [Ti], allowing CL images to be calibrated as Ti maps using SIMS measurements. Coarse-grained weakly deformed domains consist of magmatic quartz extensively recrystallized by grain boundary migration (GBM) and mostly (65–75% area) contain 20–38ppm Ti. Resetting to lower [Ti] occurred locally: (i) in haloes surrounding titanite and biotite inclusions ([Ti] as low as 6ppm); (ii) along grain boundaries; and (iii) towards the interface of quartz domains with other mineral domains. With increasing strain, quartz underwent progressive grain size reduction and developed a bimodal microstructure with elongate grains (>100's μm long) surrounded by mantles of new grains (10–30μm in size) recrystallized by subgrain rotation (SGR). Dynamic recrystallization by SGR, associated with prism<a>slip, became increasingly dominant over GBM as strain increased towards the shear zone core. Significant resetting of Ti in quartz only occurred in high strain domains (at shear strain γ probably >>10) in the shear zone core where fine recrystallization amounts to 50–60% by area and coarser cores are strongly sub-structured. These domains are not compositionally homogeneous and still show a range of [Ti] mostly between 2 and 10ppm. In all strain facies of the shear zone quartz-filled pressure shadows associated with feldspar show an almost constant [Ti] of ~2ppm. The pristine Ti content of the magmatic quartz mylonitized in the shear zone core is therefore significantly reset and converges “asymptotically” towards the “equilibrium” 2ppm [Ti] shown by new quartz precipitated in pressure shadows. It is inferred that extensive recrystallization by SGR and repeated cycles of dislocation creep and rearrangement provided fluid access to quartz grain interiors, promoting chemical buffering and leading to partial re-equilibration to low [Ti]. These observations imply limitations on the use of the Ti-in-quartz thermobarometry to constrain ambient conditions of ductile deformation.

Mid-crustal shear zone formation in granitic rocks: Constraints from quantitative textural and crystallographic preferred orientations analyses

Abstract This paper presents quantitative microstructural and crystallographic preferred orientation (CPO) analyses of an Alpine amphibolite facies shear zone developed in the Fibbia metagranite (Gotthard massif, Central Alps). The weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse-grained monomineralic aggregates, derived from quartz and K-feldspar porphyroclasts, and softer fine-grained plagioclase-bearing shear bands. A characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates. CPOs and microtextures suggest that quartz and K-feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid-assisted diffusive mass transfer and grain boundary sliding. In the mylonite and ultramylonite shear zone core, the porphyroclasts-derived quartz and K-feldspar layers are broken-down to produce a polyphased matrix that is characterized by a homogeneous micron-scale grain size and regular/random distribution. Here, the deformation of the whole aggregate occurs via a fluid-assisted dissolution–precipitation creep and grain boundary sliding, referred as a fluid-assisted granular flow. We propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain. The lateral broadening of the shear zone is driven by (1) the increase in fluid pressure in permeable albite–oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates, and (2) the thermodynamic re-equilibration via metasomatic reactions of the shear zone walls.

Identifying deformed pseudotachylyte and its influence on the strength and evolution of a crustal shear zone at the base of the seismogenic zone

On a strand of the Norumbega fault system, a Paleozoic, subvertical, seismogenic fault system in northeastern New England, USA, we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite/phyllonite layers. We consider how those textural and mineralogical changes affected the rheology of the layer and how significant volumes of pseudotachylyte over time may have weakened the shear zone.The Norumbega fault system is characterized by a number of mylonitic shear zones exhumed from depths of ~10–15km, some of which preserve evidence for mutually-overprinting pseudotachylyte and mylonite. Along one of these, the Sandhill Corner shear zone, all stages of the pseudotachylyte to ultramylonite/phyllonite transformation are preserved, from (1) primary pseudotachylyte structures to (2) initial mineral crystallization, (3) grain coarsening and reactions, and (4) viscous deformation. Our observations show that ultramylonite layers exhibit identifying features that when present together are distinctive of a pseudotachylyte origin. Using these features, we estimate that ~5–50% of the rock volume in the Sandhill Corner shear zone (mean ~30%, locally >50%) is deformed pseudotachylyte, suggesting that deformed pseudotachylyte may be more prevalent than previously thought in faults exhumed from the base of the seismogenic zone.The Sandhill Corner shear zone localized along the contact between two rheologically-contrasting units. Mylonite fabric intensity and the occurrence of fresh and deformed pseudotachylyte increase with proximity to the contact and shear zone core, indicating that seismic rupture also localized there. A decrease in grain size promoting grain-size-sensitive creep and a progressively interconnected mica network associated with local basal slip within the deformed pseudotachylyte both worked to decrease the strength of those layers. The formation of multiple generations of weak, deformed pseudotachylyte layers at or near the lithologic contact may have played an important role in the spatiotemporal persistence of the shear zone core.

Metamorphic response in orogens of different obliquity, scale and geometry

Investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets. To illustrate the problems in documenting flow vectors we present integrated structural-metamorphic datasets from two transpressional systems; the Kaoko Belt in Namibia and the Kalinjala Shear Zone in South Australia. These orogens experienced widely differing metamorphic responses to transpressional deformation. Integration of kinematic and metamorphic datasets from the Kaoko Belt indicate shallow up-plunging extrusion trajectories in the orogen core, and show that the maximum stretching direction pattern matches the inferred flow vectors. High-grade domains (800–840 °C and 7.0–8.0 kb) in the orogen core developed low-angle upward-verging maximum stretching direction trajectories, whereas a low-grade domain (575–600 °C and 5.0–5.5 kb) in the orogen core has downward-verging lineation trajectories. The barometric differential between these high-grade and low-grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high-grade domains that were extruded obliquely, for the amount of lateral shear estimated for the orogen core. The Kalinjala Shear Zone in South Australia contrasts strongly with the Kaoko Belt. In this example, the high-grade and high-strain shear zone core of the orogen, experienced high- T/high- P metamorphism with low thermal gradients of 21–26 °C/km and steep decompressive P– T paths. The lower-grade external domains experienced lower- T/lower- P metamorphism with high thermal gradients of 35–37 °C/km. Sub-horizontal maximum stretching directions do not match the vertical extrusional flow in the high-grade core that is indicated by the metamorphic data. This comparison shows that in general and on a gross scale, maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis. In some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow. Consequently, information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record. These two datasets must be used in concert to obtain realistic constraints on first-order material flow trajectories at orogenic scales. The horizontal component of flow is typically best recorded by structural fabrics (maximum stretching direction and sense of shear), whereas the vertical component is typically best recorded by metamorphic information, such as P– T paths, temperature over depth ratio (G) and metamorphic field gradients (i.e. Δ T, Δ P and Δ G) across the orogen.

Syntectonic development of a strain-independent steady-state grain size during mylonitization

Michibayashi, K., 1993. Syntectonic development of a strain-independent steady-state grain size during mylonitization. Tectonophysics, 222: 151–164. For a distance of greater than 50 km along the Median Tectonic Line, Japan, Syntectonic recrystallization of quartz has resulted in the development of a strain-independent steady-state grain size within the core of the Kashio Shear Zone, that is also independent of rock-type. The mean grain size of quartz in recrystallized aggregates progressively decreases towards the shear zone core, becoming stable at around approximately 37 μm, even though the strain increases within more strongly mylonitized rocks. Some variation of mean grain sizes may result from secondary effects due to hydrothermal activity associated with the Median Tectonic Line. Despite the development of a steady-state mean grain size, individual grains were not stable, as revealed by log-normal grain-size distributions. Consequently, dynamic recrystallization and grain growth were competing processes that achieved a balance and hence resulted in the development of the steady-state grain size. The actual grain size that resulted probably depended on both the water content and strain rate as well as the flow stress. Approaching the centre of the shear zone, mylonitic fabrics (e.g., S-C fabrics, ‘mica-fish’) were established well before a steady-state grain size was reached. A steady-state grain shape, shape orientation and crystallographic fabrics may not be developed until even more mylonitization had occurred. Variable grain sizes result from heterogeneous deformation as a consequence of deformation partitioning even under a stable stress prior to the development of a steady-state grain size. Consequently, it should be determined whether a steady-state grain size has been achieved within a shear zone, before the grain size is used as a paleopiezometer.