Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

<p>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.</p

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

International audienceFault 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

Micro- And nano-porosity of the active Alpine Fault zone, New Zealand

Porosity reduction in rocks from a fault core can cause elevated pore fluid pressures and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 %–0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples' microstructural organization, where nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data imply that (i) the porosity of the fault core is very small and not connected; (ii) the distribution of clay minerals controls the shape and orientation of the associated pores; (iii) porosity was reduced due to pressure solution processes; and (iv) mineral precipitation in fluid-filled pores can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of very low porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture

Structural disorder of graphite and implications for graphite thermometry

Graphitization, or the progressive maturation of carbonaceous material, is considered an irreversible process. Thus, the degree of graphite crystallinity, or its structural order, has been calibrated as an indicator of the peak metamorphic temperatures experienced by the host rocks. However, discrepancies between temperatures indicated by graphite crystallinity versus other thermometers have been documented in deformed rocks. To examine the possibility of mechanical modifications of graphite structure and the potential impacts on graphite thermometry, we performed laboratory deformation experiments. We sheared highly crystalline graphite powder at normal stresses of 5 and 25  megapascal (MPa) and aseismic velocities of 1, 10 and 100 µm s−1. The degree of structural order both in the starting and resulting materials was analyzed by Raman microspectroscopy. Our results demonstrate structural disorder of graphite, manifested as changes in the Raman spectra. Microstructural observations show that brittle processes caused the documented mechanical modifications of the aggregate graphite crystallinity. We conclude that the calibrated graphite thermometer is ambiguous in active tectonic settings

Revil archive

Spectral induced polarization data of rocks and porous media with graphit

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

Evolution of microporosity and permeability of quartzofeldspathic rocks during changes in crustal conditions and tectonite fabric

Most crustal rocks contain some microporosity that can host fluids and allow them to permeate, which in turn impacts their physical properties and rheology. Using electron microscopic methods we have examined the nature of microporosity in quartzofeldspathic rocks recovered from surface outcrops and boreholes that are representative of those actively deforming at up to 35km depth beneath New Zealand’s Southern Alps. In these exhumed samples the main types of microporosity are: (i) dilatant grain boundaries, (ii) grain boundary dislocation etch pits, (iii) intragranular fluid inclusions of various types, (iv) dilatant sites on phyllosilicate basal planes. We observe changes in the distribution and nature of porosity with proximity to the mylonitic shear zone down-dip of the active Alpine Fault, and these may be related to changes in the nature of the tectonite fabric. Our measurements of anisotropy of experimentally measured electrical conductivity and elastic wave propagation, and its change with increasing confining pressure (Pconf), provide insights into the relationship of microfracture porosity and mineral orientation. For example, electrical and elastic wave anisotropy (ρk/ρ⊥ and vpk/vp⊥) is high but decreases rapidly with increasing Pconf in samples with the strongest foliations, comprising foliation domains of quartz+feldspar with planar and through going phyllosilicate microlithons. Conversely, most weakly foliated samples where the same phases are well-mixed are less anisotropic and display less change in electrical and elastic wave anisotropy with increasing Pconf. This suggests linked type (iv) pore spaces parallel to foliation, which can host saline fluids, are preferentially closed with increasing Pconf. These types of changes are likely to only be significant at low Pconf, i.e., brittle conditions. At greater depth, in the creeping part of the shear zone, changes in the geometric arrangement of microporosity are more likely to result from differential thermal expansion and/or fluid-rock interactions. The TESA toolbox (https://umaine.edu/mecheng/vel/software/tesa_toolbox/) allows us to predict how thermal contraction may yield anisotropic grain boundary porosity for real microstructures. We can then validate these predictions by relating evidence of limited fluid-rock reactions to equilibrium phase diagrams for real bulk compositions

A Blended Learning Approach to Structural Field Mapping: Combining Local Geology, Virtual Geology, and Web-based Tools

In September 2020, the Corona crisis offered us an opportunity to develop and test a blended real and virtual interdisciplinary field mapping class, as well as revealing the need for, and stimulating development of new web-based tools for structural interpretation. Universität Mainz’ usual Master’s advanced field mapping, and Universität Tübingen’s usual Bachelor’s mapping classes were replaced with combinations of (i) virtual field mapping of Jurassic-Cretaceous sedimentary units at Molinos, Teruel Province, Spain, and (ii) field mapping of metamorphic rocks in the Mittelrhein Gorge and the Arh Valley, and outcrops of sedimentary rocks near Tübingen, Germany, which the students were mostly able to access on day trips using public transport or by bicycle. For the Molinos part of the exercise both groups were offered hand specimens containing distinctive fossils, linked to locations (and pseudo-locations) by google .kmz files, a variety of structural measurements also linked via .kmz files, and detailed satellite imagery within which mappable geological units display distinct characteristics. Introductions to the stratigraphy were made in three virtual outcrop sections examined in Google Street View from within Google Earth, and via web-based photogrammetric 3D outcrop models made available on the V3Geo virtual 3D geoscience platform. The students then extrapolated this stratigraphy based on the satellite imagery and .kmz file information. Our perception, validated by student feedback, is that the real parts of both field excursions were very important since they allowed us to teach and refine mapping and compass methodology and best demonstrate how to analyze 3D geometries of geological structures. Universität Mainz students particularly benefited from being able to visit locations where we had already made 3D outcrop models and offered a digital excursion, in the Ahr Valley (Rhenish Massif). They were able to compare real structural measurements with those derived from the precisely georeferenced 3D models, which enhanced their ability to subsequently obtain such information solely from the models. Although final student maps were of comparable quality to those produced in the field, structural interpretations were hampered by a lack of field measurements. In many cases, the Google Earth DEM is of too low resolution and ways should be found to include higher-resolution DEMs in web-based data sets. Overall, we think there were advantages compared to traditional field mapping, such as (i) enhanced evidence that methods like ‘structure contouring’ were used in all mapping, (ii) we were stimulated to teach the students to use digital methods to acquire field data, such as StraboSpot and Stereonet11 Apps. We observed these tools, and others we were unaware of, being used in combination with traditional paper and compass during the real mapping exercise. We hope to continue to employ this blended teaching approach even when the Corona crisis passes. This will be facilitated by our development of further 3D outcrop models, .kmz files with key information about outcrops in the Mittelrhein, and especially, web-based (rather than PC-based) tools to extract structural data such as plane and line orientations from 3D outcrop models and enable collaborative work on one data set