Description: Nature Geoscience 8, 793 (2015). doi:10.1038/ngeo2534 Authors: G. W. Bergantz, J. M. Schleicher & A. Burgisser Magma dominantly exists in a slowly cooling crystal-rich or mushy state. Yet, observations of complexly zoned crystals, some formed in just one to ten years, as well as time-transgressive crystal fabrics imply that magmas mix and transition rapidly from a locked crystal mush to a mobile and eruptable fluid. Here we use a discrete-element numerical model that resolves crystal-scale granular interactions and fluid flow, to simulate the open-system dynamics of a magma mush. We find that when new magma is injected into a reservoir from below, the existing magma responds as a viscoplastic material: fault-like surfaces form around the edges of the new injection creating a central mixing bowl of magma that can be unlocked and become fluidized, allowing for complex mixing. We identify three distinct dynamic regimes that depend on the rate of magma injection. If the magma injection rate is slow, the intruded magma penetrates and spreads by porous media flow through the crystal mush. With increasing velocity, the intruded magma creates a stable cavity of fluidized magma that is isolated from the rest of the reservoir. At higher velocities still, the entire mixing bowl becomes fluidized. Circulation within the mixing bowl entrains crystals from the walls, bringing together crystals from different parts of the reservoir that may have experienced different physiochemical environments and leaving little melt unmixed. We conclude that both granular and fluid dynamics, when considered simultaneously, can explain observations of complex crystal fabrics and zoning observed in many magmatic systems.

Description: Two major clay-rich fault zones (Betic Cordillera, SE Spain) exhumed under arid conditions were studied to establish the origin and nature of phyllosilicates in deformed mica schists and the role of fault zones in the genesis of rocks rich in clay minerals. Both areas contain wide zones (200 m across) of alternating fault gouge and protolith creating a complex arrangement of sheared lenses along fault strand cores. Minerals in schists were characterized by X-ray diffractometry and scanning and transmission electron microscopy, and two samples were selected for 40 Ar/ 39 Ar dating of four clay grain-size fractions each. Quartz, chlorite and K-micas are the principal phases in all samples (crystalline protolith and faulted schists). Paragonite and Fe and Ti-oxides are present in some samples and, additionally, most of the damaged samples have kaolinite, illite and gypsum. Backscattered electron images show irregularly oriented stacks of phyllosilicates with curved shapes in the fault rocks and quartz grains deformed by brittle fracturing. Clay fabrics in the damaged rocks are poorly developed. Lattice fringe images of faulted damaged samples revealed defect-free dioctahedral mica and Fe-rich chlorite packets which were generally with no more than several tens of nm thickness, although chlorite grains were usually thicker than micas. Chlorite showed more obvious deformation features. The calculated ages of the authigenic clay component in the two samples are 6.3 ± 0.8 Ma and 6.2 ± 0.4 Ma, while the ages calculated for the protolith clays are 22.8 ± 1.9 Ma and 11.7 ± 1.3 Ma. The age of the authigenic clay overlaps the range of ages from the nearby Cabo de Gata volcanic series (SE Spain). Textural and chemical results point toward that although the fault gouge is mostly a product of mechanical crushing of the protolith, the presence of quartz provided a strong and fragile behaviour to the faulted rocks increasing their permeability, and creating permeable paths through which low-temperature hydrothermal fluids circulated producing an intense leaching of the parent material and promoting crystallization of new authigenic minerals (illite, kaolinite, smectite and gypsum).

Description: Graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization. We examine processes of graphitic carbon emplacement and deformation in the active Alpine Fault Zone, New Zealand by analysing samples obtained from Deep Fault Drilling Project (DFDP) boreholes. Optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain. Raman spectroscopy allowed interpretation of the degree of graphite crystallinity, which reflects both thermal and mechanical processes. In the amphibolite-facies Alpine Schist, highly crystalline graphite, indicating peak metamorphic temperatures up to 640°C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions (500–600°C), resulting in clustered (in protomylonites) and foliation-aligned graphite (in mylonites). In cataclasites, derived from the mylonitized schists, graphite is most abundant (〈50% as opposed to 〈10% elsewhere), and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon. Tectonic–hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences. The increasing abundance of graphite towards the fault zone core may be a significant source of strain localization, allowing fault weakening.

Description: Fault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging-wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP-2). We present observational evidence for extensive fracturing and high hanging-wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP-2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging-wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off-fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation.

Description: During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.

Description: We present a new method for estimating the contribution of a pure clay fraction (i.e. devoid of organic matter) to the total effective rock stiffness. The method is based on published clay mineral stiffness data and on an original preferred clay mineral orientation data set obtained by X-ray texture goniometry (XTG) on 56 samples of Kimmeridgian and Devonian age from two North American shale plays. We find that (1) large variability in preferred orientation of clay results in moderate variability in effective clay elastic anisotropy, and (2) the effect of variations in preferred orientation on effective rock properties is small compared to the effects of variations in clay abundance. As a result, a single clay elastic tensor is computed to be used in effective medium models. Additionally, in order to account for various degrees of hydration, water is incorporated into the dry clay tensor through inclusion models. In situations where isotropic approximations are necessary, we also provide apparent bulk and shear moduli for a hydrated clay fraction as a function of porosity and propagation angle.