Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone

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

How can we characterise graphite via electron microscopy?

Graphite is one of the most electrically-conductive and mechanically weak minerals commonly encountered in crustal rocks, so its presence affects geophysical properties and rheology. It also behaves as a buffer for oxidation reactions, can record peak temperature through crystallinity if undeformed, has been encountered as a minor ‘pinning’ phase affecting grain size, and can impact the ability of dissolved species to permeate along grain boundaries. However, because our typical rock sample preparation methods involve use of resins containing carbon, and because we typically coat samples with carbon to ensure conductivity in electron beam instruments, there have been comparatively few attempts to map and describe its distribution in metamorphic rocks. We have attempted to characterize graphite using a wide variety of sample preparation and analytical methods, and have found: (i) Specimens for scanning electron microscopy (SEM), electron microprobe (EMP), and transmission electron microscopy (TEM) analyses should be prepared as polished billets rather than thin sections without use of carbon-containing compounds. Analyses should target freshly polished surfaces where there would be no residual/smeared epoxy/plastic. (ii) Mechanical polishing, even with colloidal silica, does not yield a sample surface suitably crystalline to be analysed using EBSD but broad ion beam (BIB)-polishing does. (iii) Element maps made using a TEM as well as an EMP with wavelength and energy dispersive spectroscopy (WDS and EDS) detectors optimized for light elements, and soft X-ray detectors (SXES) can reliably demonstrate the presence of C even if it is disseminated on grain boundaries in layers only tens of nm thick. If SXES is to be employed, we found it useful to first map a wider variety of elements as well as using a large area WDS pseudocrystal optimized for light elements (e.g. the JEOL LDE6L) to narrow down the target regions and then acquiring SXES data from the same areas. This is because SXES map acquisition is currently several times slower than WDS in stage mapping mode and still slightly slower using beam scanning. High vacuum Ir-coating is preferred; a thin 1 nm coat does the job well and allows BSE imaging with only slight extraneous peaks in the spectra. Field emission source X-ray mapping at lower kV (e.g. 10 kV and lower) and lower current (e.g. 10 nA) reduces the C Ka X-ray range and keeps the electron beam compact. (iv) The crystalline and molecular-scale structure of carbon can be characterized using electron loss spectroscopy (EELS) on a TEM. (v) Raman spectroscopy provides a good alternative to electron beam methods to map C at grain scale. These methods have allowed us to demonstrate that carbon fills quartz triple junctions and lies along grain and phase boundaries in <50nm thick layers in quartz-feldspar mixtures typically encountered in Alpine Fault Zone mylonites

How can we characterise graphite via electron microscopy?

Graphite is one of the most electrically-conductive and mechanically weak minerals commonly encountered in crustal rocks, so its presence affects geophysical properties and rheology. It also behaves as a buffer for oxidation reactions, can record peak temperature through crystallinity if undeformed, has been encountered as a minor ‘pinning’ phase affecting grain size, and can impact the ability of dissolved species to permeate along grain boundaries. However, because our typical rock sample preparation methods involve use of resins containing carbon, and because we typically coat samples with carbon to ensure conductivity in electron beam instruments, there have been comparatively few attempts to map and describe its distribution in metamorphic rocks. We have attempted to characterize graphite using a wide variety of sample preparation and analytical methods, and have found: (i) Specimens for scanning electron microscopy (SEM), electron microprobe (EMP), and transmission electron microscopy (TEM) analyses should be prepared as polished billets rather than thin sections without use of carbon-containing compounds. Analyses should target freshly polished surfaces where there would be no residual/smeared epoxy/plastic. (ii) Mechanical polishing, even with colloidal silica, does not yield a sample surface suitably crystalline to be analysed using EBSD but broad ion beam (BIB)-polishing does. (iii) Element maps made using a TEM as well as an EMP with wavelength and energy dispersive spectroscopy (WDS and EDS) detectors optimized for light elements, and soft X-ray detectors (SXES) can reliably demonstrate the presence of C even if it is disseminated on grain boundaries in layers only tens of nm thick. If SXES is to be employed, we found it useful to first map a wider variety of elements as well as using a large area WDS pseudocrystal optimized for light elements (e.g. the JEOL LDE6L) to narrow down the target regions and then acquiring SXES data from the same areas. This is because SXES map acquisition is currently several times slower than WDS in stage mapping mode and still slightly slower using beam scanning. High vacuum Ir-coating is preferred; a thin 1 nm coat does the job well and allows BSE imaging with only slight extraneous peaks in the spectra. Field emission source X-ray mapping at lower kV (e.g. 10 kV and lower) and lower current (e.g. 10 nA) reduces the C Ka X-ray range and keeps the electron beam compact. (iv) The crystalline and molecular-scale structure of carbon can be characterized using electron loss spectroscopy (EELS) on a TEM. (v) Raman spectroscopy provides a good alternative to electron beam methods to map C at grain scale. These methods have allowed us to demonstrate that carbon fills quartz triple junctions and lies along grain and phase boundaries in <50nm thick layers in quartz-feldspar mixtures typically encountered in Alpine Fault Zone mylonites

Fault rock lithologies and architecture of the central Alpine fault, New Zealand, revealed by DFDP-1 drilling

The first phase of the Deep Fault Drilling Project (DFDP-1) yielded a continuous lithological transect through fault rock surrounding the Alpine fault (South Island, New Zealand). This allowed micrometer- to decimeter-scale variations in fault rock lithology and structure to be delineated on either side of two principal slip zones intersected by DFDP-1A and DFDP-1B. Here, we provide a comprehensive analysis of fault rock lithologies within 70 m of the Alpine fault based on analysis of hand specimens and detailed petrographic and petrologic analysis. The sequence of fault rock lithologies is consistent with that inferred previously from outcrop observations, but the continuous section afforded by DFDP-1 permits new insight into the spatial and genetic relationships between different lithologies and structures. We identify principal slip zone gouge, and cataclasite-series rocks, formed by multiple increments of shear deformation at up to coseismic slip rates. A 20−30-m-thick package of these rocks (including the principal slip zone) forms the fault core, which has accommodated most of the brittle shear displacement. This deformation has overprinted ultramylonites deformed mostly by grain-size-insensitive dislocation creep. Outside the fault core, ultramylonites contain low-displacement brittle fractures that are part of the fault damage zone. Fault rocks presently found in the hanging wall of the Alpine fault are inferred to have been derived from protoliths on both sides of the present-day principal slip zone, specifically the hanging-wall Alpine Schist and footwall Greenland Group. This implies that, at seismogenic depths, the Alpine fault is either a single zone of focused brittle shear that moves laterally over time, or it consists of multiple strands. Ultramylonites, cataclasites, and fault gouge represent distinct zones into which deformation has localized, but within the brittle regime, particularly, it is not clear whether this localization accompanies reductions in pressure and temperature during exhumation or whether it occurs throughout the seismogenic regime. These two contrasting possibilities should be a focus of future studies of fault zone architecture

Drilling reveals fluid control on architecture and rupture of the Alpine Fault, New Zealand

Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200–400 yr earthquake cycle, reveal a &gt;50-m-thick “alteration zone” formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a &lt;0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10?20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ?10?14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles

Transpression and tectonic exhumation in the Heimefrontfjella, western orogenic front of the East African/Antarctic Orogen, revealed by quartz textures of high strain domains

The metamorphic basement of the Heimefrontfjella in western Dronning Maud Land (Antarctica) forms the western margin of the major ca. 500 million year old East African/East Antarctic Orogen that resulted from the collision of East Antarctica and greater India with the African cratons. The boundary between the tectonothermally overprinted part of the orogen and its north-western foreland is marked by the subvertical Heimefront Shear Zone. North-west of the Heimefront Shear Zone, numerous low-angle dipping ductile thrust zones cut through the Mesoproterozoic basement. Petrographic studies, optical quartz c-axis analyses and x-ray texture goniometry of quartz-rich mylonites were used to reveal the conditions that prevailed during the deformation. Mineral assemblages in thrust mylonites show that they were formed under greenschist-facies conditions. Quartz microstructures are characteristic of the subgrain rotation regime and oblique quartz lattice preferred orientations are typical of simple shear-dominated deformation. In contrast, in the Heimefront Shear Zone, quartz textures indicate mainly flattening strain with a minor dextral rotational component. These quartz microstructures and lattice preferred orientations show signs of post-tectonic annealing following the tectonic exhumation. The spatial relation between the sub-vertical Heimefront Shear Zone and the low-angle thrusts can be explained as being the result of strain partitioning during transpressive deformation. The pure-shear component with a weak dextral strike-slip was accommodated by the Heimefront Shear Zone, whereas the north–north-west directed thrusts accommodate the simple shear component with a tectonic transport towards the foreland of the orogen