Patterns of strain localization in heterogeneous, polycrystalline rocks – a numerical perspective

The spatial and temporal patterns of strain localization in materials with pre-existing heterogeneities are investigated via a series of two-dimensional numerical models. Models include (i) a dynamic feedback process, to simulate rheological weakening in response to the transition from non-linear flow (dislocation creep) to linear flow (diffusion creep/grain boundary sliding), and (ii) a time dependent strengthening process, counteracting the weakening process. Different load bearing framework geometries with 20% weak component are used to evaluate the impact of geometry on the strength of the material and its ability to localize strain into an interconnected weak layer (IWL). Our results highlight that during simple shear, if dynamic weakening with or without strengthening feedbacks is present, strain is quickly localized into an IWL, where an increasing proportion of weak material increases the interconnections between the IWLs, thereby increasing the anastomosing character of the shear zones. We establish that not only bulk strain localization patterns but also their temporal patterns are sensitive to the dominance of the weakening or strengthening process. Consequently, shear zones are dynamic in time and space within a single deformation event. Hence, the pattern of finite strain can be an incomplete representation of the evolution of a shear zone network

Chemical Signatures of Melt–Rock Interaction in the Root of a Magmatic Arc

Identification of melt–rock interaction during melt flux through crustal rocks is limited to field relationships and microstructural evidence, with little consideration given to characterising the geochemical signatures of this process. We examine the mineral and whole-rock geochemistry of four distinct styles of melt–rock interaction during melt flux through the Pembroke Granulite, a gabbroic gneiss from the Fiordland magmatic arc root, New Zealand. Spatial distribution, time-integrated flux of melt and stress field vary between each melt flux style. Whole-rock metasomatism is not detected in three of the four melt flux styles. The mineral assemblage and major element mineral composition in modified rocks are dictated by inferred P–T conditions, as in sub-solidus metamorphic systems, and time-integrated volumes of melt flux. Heterogeneous mineral major and trace element compositions are linked to low time-integrated volumes of melt flux, which inhibits widespread modification and equilibration. Amphibole and clinozoisite in modified rocks have igneous-like REE patterns, formed by growth and/or recrystallisation in the presence of melt and large equilibration volumes provided by the grain boundary network of melt. Heterogeneities in mineral REE compositions are linked to localisation of melt flux by deformation and resulting smaller equilibration volumes and/or variation in the composition of the fluxing melt. When combined with microstructural evidence for the former presence of melt, the presence of igneous-like mineral REE chemical signatures in a metamorphic rock are proposed as powerful indicators of melt–rock interaction during melt flux

Zircon U‐Pb Dating of a Lower Crustal Shear Zone: A Case Study From the Northern Sector of the Ivrea‐Verbano Zone (Val Cannobina, Italy)

A geochronological study was performed on zircon grains from a middle‐lower crustal shear zone exposed in the northern sector of the Ivrea‐Verbano Zone (Southern Alps, Italy) for the first time. The shear zone developed at the boundary between mafic rocks of the External Gabbro unit and ultramafic rocks of the Amphibole Peridotite unit. It is ~10–20 m wide, can be followed along a NE strike for several kilometers, and consists of an anastomosing network of mylonites and ultramylonites. Zircon grains were studied in thin sections and as separates from three representative outcrops along the shear zone. Zircon grains are more abundant in the shear zone compared to wall rocks and are generally equant, rounded to subrounded with dimensions up to 500 μm. U‐Pb data are mainly discordant, and the apparent ²⁰⁶Pb/²³⁸U dates show a large variation from Permian to Jurassic. Isotopic data, combined with microstructural, morphological, and internal features of zircon, reveal an inherited age component and suggest partial zircon recrystallization under high‐temperature conditions during Late Triassic to Early Jurassic. High‐temperature deformation in the shear zone, at lower crustal levels, was coeval with amphibolite to greenschist facies mylonitic deformation at upper crustal levels and is inferred to be related to Mesozoic rifting processes at the Adriatic margin

Virtual Petrographic Microscope : a multi-platform education and research software tool to analyse rock thin-sections

We present a free, standalone Windows and Mac OSX desktop software tool designed to aid geoscience researchers, students and educators in rock thin-section analysis without the need for a petrographic microscope. Virtual Petrographic Microscope (VPM) allows a user to analyse prepared high-resolution images of rock thin-sections on a computer using traditional features familiar to users of microscopes including stage rotation, objective zoom and switching between plane-polarised light and crossed-polarised light. VPM includes a range of 'virtual' features not possible when analysing physical thin-sections, including auto-scaling grid overlays, and annotation of thin-section images with the ability to save, export and import annotation files for collaboration and education. A case study involved a trial of the software by an intermediate undergraduate geology class. Analysis of the final examination results shows that incorporation of the VPM tool into the class program improved skill at recognising common rock-forming minerals.7 page(s

Pinch and swell structures: evidence for strain localisation by brittle–viscous behaviour in the middle crust

The flow properties of middle crustal rocks are commonly represented by viscous flow. Examples of pinch and swell structures found in a high strain zone at St. Anne Point (Fiordland, New Zealand) and Wongwibinda (N.S.W., Australia) suggest pinch and swell structures may be initiated by brittle failure of the more competent layer in conjunction with subsequent material softening. On this basis we develop a numerical model where Mohr–Coulomb constitutive strain localising behaviour is utilised to initiate pinch and swell structure development. Results show that pinch and swell structures develop in a competent layer in both Newtonian and non-Newtonian flow, provided the competent layer has sufficient viscosity contrast and can localise strain to form shear bands. The flow regime and strain localising characteristics of the surrounding country rock appear not to impact pinch and swell structure formation. The degree of material softening after the initial strain localising behaviour is shown to impact pinch and swell characteristics, while extensive material softening causes the formation of thick necks between swells by limiting the focused localisation of strain into shear bands. To aid analysis of the structures and help derive the flow properties of rocks in the field, we define three stages of pinch and swell development and offer suggestions for measurements to be made in the field. <br><br> Our study suggests that Mohr–Coulomb strain localising behaviour combined with viscous flow is a viable alternative representation of the heterogeneous rheological behaviour of rocks seen in the middle crust. This type of mid-crustal rheological behaviour can have significant influence on the localisation of strain at all scales. For example, inclusion of Mohr–Coulomb strain localising behaviour with viscous flow in just some mid-crustal layers within a crustal-scale model can result in significant strain localisation, extending from the upper crust into the middle crust. This localisation also influences the development of near-surface structures

The Keepit arc: provenance of sedimentary rocks in the central Tablelands Complex, southern New England Orogen, Australia, as recorded by detrital zircon

<p>Detrital zircon from the Carboniferous Girrakool Beds in the central Tablelands Complex of the southern New England Orogen, Australia, is dominated by <i>ca</i> 350–320 Ma grains with a peak at <i>ca</i> 330 Ma; there are very few Proterozoic or Archean grains. A maximum deposition age for the Girrakool Beds of <i>ca</i> 309 Ma is identified. These data overlap the age of the Carboniferous Keepit arc, a continental volcanic arc along the western margin of the Tamworth Belt. Zircon trace-element and isotopic compositions support petrographic evidence of a volcanic arc provenance for sedimentary and metasedimentary rocks of the central Tablelands Complex. Zircon Hf isotope data for <i>ca</i> 350–320 Ma detrital grains become less radiogenic over the 30 million-year record. This pattern is observed with maturation of continental volcanic arcs but is opposite to the longer-term pattern documented in extensional accretionary orogens, such as the New England Orogen. Volcanic activity in the Keepit arc is inferred to decrease rapidly at <i>ca</i> 320 Ma, based on a major change in the detrital zircon age distribution. Although subduction continues, this decrease is inferred to coincide with the onset of trench retreat, slab rollback and the eastward migration of the magmatic arc that led to the Late Carboniferous to early Permian period of extension, S-type granite production and intrusion into the forearc basin, high-temperature–low-pressure metamorphism, and development of rift basins such as the Sydney–Gunnedah–Bowen system.</p

Cordillera Zealandia: A Mesozoic arc flare-up on the palaeo-Pacific Gondwana Margin

Two geochemically and temporally distinct components of the Mesozoic Zealandia Cordilleran arc indicate a shift from low to high Sr/Y whole rock ratios at c. 130 Ma. Recent mapping and a reappraisal of published Sr-Nd data combined with new 'in-situ' zircon Hf isotope analyses supports a genetic relationship between the two arc components. A reappraisal of geophysical, geochemical and P-T estimates demonstrates a doubling in thickness of the arc to at least 80 km at c. 130 Ma. Contemporaneously, magmatic addition rates shifted from ~14 km³/my per km of arc to a flare-up involving ~100 km³/my per km of arc. Excursions in Sr-Nd-Hf isotopic ratios of flare-up rocks highlight the importance of crust-dominated sources. This pattern mimics Cordilleran arcs of the Americas and highlights the importance of processes occurring in the upper continental plates of subduction systems that are incompletely reconciled with secular models for continental crustal growth

High-temperature–low-pressure metamorphism and the production of S-type granites of the Hillgrove Supersuite, southern New England Orogen, NSW, Australia

<p>The high-temperature–low-pressure Wongwibinda Metamorphic Complex of the southern New England Orogen is bound by S-type granite plutons of the Hillgrove Supersuite to the north, east and south. New U–Pb geochronology of five samples of the Hillgrove Supersuite demonstrates that plutonism in the complex involved two pulses: <i>ca</i> 300 Ma and <i>ca</i> 292 Ma. This indicates that plutonism partially overlaps the age of high-<i>T</i>–low-<i>P</i> metamorphism (296.8 ± 1.5 Ma), but also postdates it. Zircon grains identified as xenocrysts based on age (≥310 Ma) have U–Pb–Hf isotopic character that largely overlaps detrital grains in the host Girrakool Beds, indicating that accretionary complex crust is the likely source of these xenocrysts. The ¹⁷⁶Hf/¹⁷⁷Hf initial character for zircon for the <i>ca</i> 300 Ma plutons (three samples) is less radiogenic than those in the <i>ca</i> 292 Ma plutons (two samples). The progression in ¹⁷⁶Hf/¹⁷⁷Hf initial character for zircon infers an increasing mantle component in the Hillgrove Supersuite with time. These data are evidence of a rift tectonic setting, where mantle-derived magmas are predicted to more readily migrate to shallower crustal levels as the crust thins and becomes hotter. Additionally, early episodes of partial melting in the system melt-depleted the metasedimentary sources, thus reducing the S-type component as anatexis progressed. The evolution of the Hillgrove Supersuite coincides with a period of early Permian slab roll back and extension accompanied by crustal rifting and thinning, leading to high-<i>T</i>–low-<i>P</i> metamorphism, anatexis and S-type granite production and the development of rift basins such as the Sydney–Gunnedah–Bowen system.</p

Crustal differentiation in a thickened arc-evaluating depth dependences

Cretaceous plutonic and volcanic rocks of the c. 126–112 Ma Western Fiordland Orthogneiss (WFO) and related Separation Point Suite (SPS), New Zealand, reflect a major flux of arc magmatism along the Late Cretaceous Gondwana margin and provide an opportunity to evaluate processes controlling magmatic differentiation in thickened arc crustal settings. Trace element mineral fractionation models are evaluated for key parts of the WFO based on relict crystal chemistry and cumulate material in garnet-bearing monzodioritic plutons. Garnet pyroxenite (garnet–diopside) adcumulate formed in the Malaspina Pluton at 1·2 GPa, together with two-pyroxene, garnet-absent monzodioritic components and xenoliths of hornblendite. Adcumulate garnet–omphacite (eclogite), garnetite and clinopyroxenite material crystallized in the Breaksea Orthogneiss protoliths at 1·8 GPa. Mineral rare earth element (REE) characteristics of garnet and clinopyroxene are remarkably similar between all cumulate layers and both host-rock plutonic bodies, despite distinctions of inferred emplacement depth and clinopyroxene type. Fractionation of magmatic garnet pyroxenite and/or eclogite can account for a diverse (40–75% SiO₂) majority of WFO and SPS rocks, and control crustal differentiation in overthickened continental arcs (P >1·2 GPa, >40 km). However, the natural REE data do not clearly discriminate between fractionation controlled by garnet pyroxenite or eclogite in thickened arc settings. The structural level of differentiation is dependent on the lower limit of plagioclase stability, which acts as a buffer forcing most accumulation towards the arc base.26 page(s

Identifying relic igneous garnet and clinopyroxene in eclogite and granulite, Breaksea Orthogneiss, New Zealand

Eclogite and omphacite granulite occur interlayered on centimetre to decametre scales to form the Cretaceous Breaksea Orthogneiss, with assemblages reflecting peak conditions of P ≈ 1.8GPa and T ≈ 850°C. It is the highest-grade part of the c. 125-115 MaWestern Fiordland Orthogneiss, New Zealand. A gneissic fabric in the host omphacite granulite truncates igneous layering in decametre-scale, coarsely layered clinopyroxenite and garnetite inclusions. Field and microstructural relationships, together with rare earth element (REE) characteristics across all rock types, support the conclusion that most garnet has a common igneous origin; geochemical data alone do not discriminate whether it is igneous or metamorphic. Igneous diopside persists in coarse-grained, weakly deformed samples of clinoyroxenite. Garnet cores in garnetite, late-formed garnet rims in garnetite and delicate garnet films in clinopyroxenite have a range of REE contents interpreted to reflect cumulate processes involving progressive grain growth isolated from the parent magma. Clear rims on inclusion- and Ca-Tschermakite-rich diopside in clinopyroxenite, of a composition that is distinct from grain cores, are interpreted as recrystallization features. Garnet in omphacite granulite occurs in three textural settings, the most commonType 1 garnet having REE characteristics identical to garnet in eclogite, but depleted in heavy REE (HREE) relative to garnet in garnetite. Type 2 garnet in omphacite granulite forms metamorphic coronae on omphacite in contact with plagioclase, has a pronounced positive Eu anomaly and is depleted in HREE compared with Type 1 garnet. Type 3 garnet in omphacite granulite migmatite is indistinguishable from Type 1 garnet, consistent with its formation through magma injection. Cumulate eclogite, pyroxenite and garnetite formed at c. 1.8 GPa, exposing an unusually deep and coherent arc section.18 page(s