Localized vs distributed deformation associated with the linkage history of an active normal fault, Whakatane Graben, New Zealand

The deformation associated with an active normal fault is investigated at a high temporal resolution (c. 104 yr). The Rangitaiki Fault (Whakatane Graben, New Zealand) and its adjacent faults accommodated an overall extension of ?0.83% oriented at ?N324°E over the past 17 kyr. This is consistent along strike, but the pattern of faulting that accommodates this strain defines two different spatial domains. To the SW, one domain is characterized by a few large faults, with >80% of strain localized onto geometrically and kinematically linked segments of the main fault. This produces marked heterogeneity in the spatial distribution of strain across the graben. In contrast, to the NE, a domain of distributed faulting is characterized by numerous small faults contributing to the overall deformation, with only ?35% of strain localized onto the Rangitaiki Fault. The transition from distributed to localized deformation is attributed to an increase in linkage maturity of the Rangitaiki Fault. Progressive strain localization has been ongoing within the network over the last 17 kyr, with localization of fault activity increasing by ?12%, indicating this process occurs over kyr time periods that only reflect a few earthquake events

The importance of tectonic inheritance and reactivation in geothermal energy exploration for EGS resources in SW England

This is the author accepted manuscript. The final version is available from the World Geothermal CongressExploration for Enhanced Geothermal System (EGS) resources has long sought natural deep fractured reservoirs. The Upper Rhine Graben is the most studied area for these targets including the successful geothermal plants at Soultz-sous-Forêts, Bruchsal, Insheim, Landau and Rittershoffen (Genter et al., 2010; Vidal et al., 2017). Previous geothermal projects, such as the Hot Dry Rock project at Rosemanowes (1977-1991) in Cornwall (SW England), targeted areas that lacked major faults and fractures (Barker et al., 2000). The Upper Rhine Graben was incepted during Eocene-Oligocene rifting in a NNE-SSW orientation that underwent Miocene sinistral reactivation (Schumacher, 2002). However, structural inheritance from dextrally reactivated Variscan ENE-WSW faults during the Miocene may also contribute to the reservoir (Bertrand et al., 2017). Geothermal energy exploration is currently experiencing a revival in SW England, targeting EGS resources within deep granitehosted fault zones. Two deep wells have been drilled at the United Downs Deep Geothermal Power Project to depths of approximately 5.0 and 2.5 km near Redruth and Eden-EGS Energy have advanced plans for deep geothermal wells near St Austell. The geology of SW England comprises several E-W-trending Devonian-Carboniferous sedimentary basins that were deformed during the Variscan orogeny (Carboniferous) and intruded by the Cornubian Batholith (Early Permian). The region is characterized by a complex fault network including major NW-SE fault zones (first-order) that have traces lengths >10 km and usually comprise multiple faults across a broad zone of deformation. These are prospective EGS targets due to their inferred down-dip persistence, Cenozoic reactivation (Holloway & Chadwick, 1986; Cooper et al., 2012; Anderson et al., 2018), near parallelism with contemporary σH (Batchelor & Pine, 1986) and are known for low magnitude (6 µW-3 (Beamish & Busby, 2016). Granite emplacement post-dates the inception of both first- and second-order faults within the host rocks, but the granites demonstrate inheritance of both fault sets. Intra-granite structures indicate episodic fault reactivation from the Permian through to the Cenozoic (Holloway & Chadwick, 1986; Shail & Alexander, 1997). Furthermore, a persistent hydrothermal mineralization record in the region demonstrates that both first- and second-order faults have previously been episodically open for fluid flow and geothermal activity. Herein, the fault network is assessed across different plutons to demonstrate the ubiquity of first-order NW-SE faults and investigate the interaction of second-order faults. By analyzing data from Yeomans et al. (2019), different age plutons are shown to have variable lineament populations. Whilst initial research regarding tectonic inheritance requires further work, the available data appear to indicate an evolving stress regime in the Early Permian. We also present a proposed workflow for generating new EGS targets as part of a staged modelling process. A key aspect of this process is the incorporation of data from regional and outcrop scale modelling to determine if the regional structural framework is representative of the target reservoir. This will allow modelling to be adapted from regional to local scales to best identify and subsequently refine geothermal energy targets.Natural Environment Research Council (NERC

A single multi-scale and multi-sourced semi-automated lineament detection technique for detailed structural mapping with applications to geothermal energy exploration

This is the author accepted manuscript. The final version is available on open access from the Geological Society via the DOI in this recordData availability: The bathymetry data used in this study have been sourced from the UK Hydrographic Office and accessed via the Admiralty Marine Data Portal. The LiDAR data used in this study have been sourced from the Centre for Ecology and Hydrology. The British Geological Survey is thanked for making the BGS Geology 625k (DiGMapGB- 625), BGS Geology 250k (DiGMap250k) and BGS Geology 50k (DiGMapGB-50) data available on an Open Government Licence.Semi-automated algorithms incorporating multi-sourced datasets into a single analysis are increasingly common, but until now operate at a fixed pixel resolution resulting in multi-sourced methods being limited by the largest input pixel size. Multi-scale lineament detection circumvents this issue and allows increased levels of detail to be captured. We present a semi-automated method using a bottom-up Object-Based Image Analysis approach to map regional lineaments to a high level of detail. The method is applied to onshore LiDAR data and offshore bathymetry around the Land's End Granite (Cornwall, UK). The method uses three different pixel resolutions to extract detailed lineaments across a 700 km2 area. The granite displays large-scale NW-SE fault zones that are considered analogous to those being targeted as onshore deep geothermal reservoirs (2-5 km in depth). Investigation of the lineaments derived from this study show along-strike variations from NW-SE orientations within granite to NNW-SSE within slate and reflect structural inheritance of early Variscan structures within Devonian slates. This is furthered by analysing these major structures for reservoir potential. Lineaments proximal to these broadly NW-SE features indicate a damage zone approximately 100-200 m wide is present. These observations provide a preliminary understanding of reservoir characteristics for fault-hosted geothermal systems.Natural Environment Research Council (NERC

Characterization of catastrophic flood-related features in the English Channel

Megaflood flows are global-scale phenomena that can have a significant impact on Earth's landscapes and climate. The low frequency occurrence of these events prevents their direct observation. The analysis of ancient megaflood terrains and bedform associations are the primary resources for the understanding of catastrophic flows. Previous studies have proposed the palaeo-channel network present at the English Channel seafloor to have been carved by catastrophic flows. This thesis investigates the origin of the English Channel palaeovalley network, testing the hypothesis of its catastrophic formation and aiming to reconstruct the relative timing and magnitude of the events that formed the palaeo-channel system. Seismic reflection data reveal the presence of depressions up to 100 m-deep carved into bedrock at the proposed spill-point. New high-resolution bathymetric data show kilometre-scale channels and associated erosional bedforms that extends for more than 200 kilometres from the Dover Strait to the Central English Channel. The stratigraphic and geomorphologic analyses indicate the observed features are similar to other flood-generated features from other well-established megaflood terrains on Earth. The quantitative characterisation of the bedforms presented in this work, together with interpretation of cross-cutting relationships of the mapped erosional surfaces, allow for the reconstruction of the relative history of the catastrophic events that carved the palaeovalley network and led to the separation of Britain from Europe through the breaching of the rock ridge present at the Dover Strait. The detailed interpretation of the 100 m-deep bedrock depressions located at the proposed breach point allows for a reconstruction of the rapid erosion of the Dover Strait. Geomorphologic analyses of the palaeo-channel network and its associated bedforms and relation to bedrock geology are presented in this work thanks to available high-resolution bathymetry and seismic reflection data. Finally, the magnitude of the palaeo-flows, estimated through palaeo-hydraulic calculations, gives further evidence of the catastrophic nature of the flows. The proposed model and relative history of the English Channel megaflood is presented in agreement with palaeo-geographic and palaeontological studies previously carried out in the study area, contributing to the reconstruction of the Pleistocene palaeo-geography of north-western Europe.Open Acces

Geologic controls on fluid flow and seismic imaging of faults in carbonate rocks : Insights from quantitative outcrop analysis and reflection seismic modeling

Utforsking av undergrunnen har historisk sett hovudsakleg vore drive av økonomiske insentiv gjennom leiting etter hydrokarbon og andre geologiske ressursar. I nyare tid, derimot, har geologiens rolle i det grøne skiftet fått aukande fokus. Undergrunnen er svært viktig for blant anna CO2- og hydrogenlagring, grunnvassressursar og geotermisk energi, og avgjerande for alle disse bruksområda er strukturar som forkastingar og sprekkenettverk, og deira kontroll på væskestraum i undergrunnen. Hovudfokuset i denne avhandlinga er å forbetre forståinga av undergrunnen gjennom to overordna tema; (i) kontrollen forkastings- og sprekkenettverk har på væskestraum og reaksjonar mellom væsker og bergartar, og (ii) seismisk avbilding av slike forkastingar og deira væske-bergart reaksjonsprodukt i undergrunnen. Sjølv om det er generell einigheit i at geologiske strukturer utøver sterk kontroll på væskestraum, har få forsøkt å kvantifisere dette forholdet. I samsvar med tema (i) ovanfor byrjar denne avhandlinga difor med å kvantifisere og visualisere forholdet mellom væskestraum og strukturell kompleksitet (Artikkel 1). Ved hjelp av sementerte haugar med lav porøsitet som bevis for paleo-væskestraum, nyttast topologi for å karakterisere og visualisere strukturell kompleksitet i lokaliserte område, og kvantitativt dokumentere deira korrelasjon med områder for lokalisert paleo-væskestraum. Artikkel 2 gir ei oversikt over geologiske kontrollar på væskestraum i sedimentære bergartar, basert på ein kombinasjon av litteraturanalyse og studie av eksempel frå felt der jernoksidutfelling blir brukt som bevis for paleo-væskestraum. Artikkelen dokumenterer eit vidt spenn av kontrollar på væskestraum og inkluderer alt frå enkle geologiske heterogeniteter til meir komplekse nettverk av strukturar, og dokumenterer og kvantifiserer topologien til både sprekkenettverk og hybridnettverk beståande av ein kombinasjon av sprekker og sedimentære avsetningsstrukturar som fungerer som leiarar for væskestraum. Eit viktig verktøy for kartlegging av slike væskeleiande strukturar i undergrunnen er seismiske refleksjonsdata. I tråd med Tema (ii), undersøkjer Artikkel 3 effekten av småskala forkastingar (nær eller under seismisk oppløysing) på seismiske bilde ved hjelp av seismisk modellering. Hovudmålet i denne artikkelen er å dokumentere effekten som forkastingar under- eller på grensa til seismisk oppløysing kan ha på seismiske data. For eksempel illustrerer artikkelen korleis subtile forandringar i ein refleksjon kan vere resultat av små forkastingar i undergrunnen, noko som er viktig ny innsikt og som mogleg kan bidra til å forbetre tolking av strukturar i seismikk. I tillegg til avbilding av forkastingar i seg sjølv, er også produkta av væske-bergart interaksjon av interesse, og det er derfor viktig å også forstå deira seismiske signatur. Ved hjelp av seismisk modellering av forkastingskontrollerte dolomittar i ei kalksteinsdominert sone i Suezbukta, blir seismisk avbilding av både massive og stratigrafisk kontrollerte dolomittkroppar undersøkt med varierande geologiske og geofysiske parametrar i Artikkel 4. Resultata i denne artikkelen gir ny innsikt i korleis forkastingskontrollerte hydrotermale dolomittkroppar blir avbilda i seismiske data; artikkelen dokumenterer at dei generelle trendane og nærværet av dolomitt i undergrunnen er mogleg å slutte frå seismiske data, men identifisering av individuelle, stratigrafisk kontrollerte dolomittkroppar er ikkje stadfesta mogleg. Funna frå denne avhandlinga fremjar den grunnleggande forståinga av sentrale geologiske kontrollar på væskestraum i undergrunnen, og avbilding av potensielt strøymingskontrollerande forkastingar og deira bergreaktive produkt i refleksjonsseismiske data. Den nye innsikta presentert i avhandlinga har eit breitt spekter av implikasjonar og nytteområde, for eksempel for lokalisering av økonomisk betydelege førekomstar av nyttige mineral, i risikovurdering for CO2- og hydrogenlagring, grunnvasshandtering, lokalisering av forureining, og meir.Exploration of the sub-surface has historically largely been driven by economic incentives through potential hydrocarbon recovery, but more recently the role of subsurface geology in the green shift has gained more focus. The subsurface are highly important for CO2- and hydrogen storage, freshwater resources, and geothermal energy to name a few, and crucial for all these are structures such as faults and their conjugate damage zones, veins, joints, and deformation bands, as well as the depositional architecture of the host rock. This thesis aim to improve our understanding of the sub-surface through two overarching themes, (i) fault and fracture networks and their controls on fluid flow and fluid-rock interaction, and (ii) the seismic imaging of such faults and their fluid-rock reactive products in the subsurface. Despite the consensus that structures exert strong control on fluid flow, few have attempted to quantify this relationship. In accordance with theme (i) above, this thesis therefore starts by quantifying and visualizing the relationship between fluid flow and structural complexity; using low-porosity cemented mounds as a record of paleo-fluid flow and topological characterization, zones of past fluid flow and areas of structural complexity are correlated and documented. Further, an overview of controls on fluid flow is given through literature review and documentation of outcrops using iron oxide precipitates as proxy for paleo-fluid flow; the fluid flow controls are ranging from simple geological heterogeneities to more complex networks of structures, and through topological characterization, the connectivity of fracture networks and hybrid networks are documented. The structures controlling flow can be found at all scales, but as the most commonly used tool for subsurface investigation is seismic imaging many of these will fall below seismic resolution. Following theme (ii), we investigate the effect of sub-seismic faults on seismic imaging using synthetic seismic from outcrop analogues and prove that under the right conditions, faults with throw as low as 5 m still produce distortion of reflections. But not only fault and fractures but also the products of fluid-rock interaction are of interest to explore in the subsurface, and hence it is important to understand their seismic signature. Using seismic modelling of fault-controlled dolomites in a limestone-dominated succession in the Golf of Suez it is found that the presence of both stratabound and massive dolomite bodies can be inferred, although only the massive dolomite can be distinguished individually. The findings form this thesis have implications for the fundamental understanding of how fluids are controlled in the subsurface, and present examples of how these controls might be imaged in reflection seismic data. This is in turn critical for a variety of processes and areas such as locating economically significant ore deposits, hydrocarbon seal risk assessment, prediction of earthquakes, groundwater management, and more. Due to the vast number of processes that are controlled by sub-surface fluid flow it is of both environmental, societal, and economic interest to be able to predict and understand fluid flow pathways, how they interact to form networks, and how they and their fluid-rock interactive products might be portrayed in reflection seismic data.Doktorgradsavhandlin

Structural and sedimentological controls on the evolution of carbonate platforms on equatorial margins

Carbonate platforms are common features on Cenozoic Equatorial Margins. The growth and development of carbonate platforms and their associated depositional settings depend on a series of controlling factors. This thesis analyses the structural and sedimentological factors controlling four different study areas with carbonate platforms, utilising a variety of datasets. Study areas include the Vulcan Sub-Basin, Bonaparte Basin (Northwest Shelf of Australia), the Cariatiz carbonate platform in the Sorbas Basin (SE Spain), the Pernambuco Basin (Eastern Brazil), and the Pará- Maranhão Basin (Equatorial Brazil). Datasets include 2D and 3D seismic data, wellbore data, airborne LiDAR maps, outcrop maps and multispectral satellite imagery, spanning multiple scales of observation. This thesis aims to improve the current understanding of shallow- and deep-water carbonate depositional and structural settings, aiding industry and academia in prospect identification and reservoir characterisation. A comprehensive analysis of fault evolution and its relationship with the distribution of isolated carbonate platforms is investigated in the Vulcan Sub-Basin, Northwest Australia, using 3D seismic and borehole data. Detailed fault-throw measurements along arrays of normal faults were completed to generate throw- depth (T-Z) and throw-distance (T-D) profiles, as well as fault-throw maps. The results obtained were useful to determine the fault styles and timing(s) of fault initiation in the Vulcan Sub-Basin, and data were compared to the growth rates of isolated carbonate platforms (ICPs). Three types of ICPs were defined: one in which fault-throw is larger than carbonate productivity (type 1), a second type in which fault-throw is equal or lower than carbonate productivity (type 2), and ICPs where fault-throw postdates the growth of carbonate platforms (type 3). An integrated method to characterise fracture networks and their scale relationships is proposed using multi-scale datasets from the Cariatiz and Pernambuco carbonate platforms. Small fractures are obtained via detailed outcrop mapping, while intermediate-scale fractures are mapped from airborne LiDAR imagery. Large-scale fractures are measured from 3D seismic data. Geometrical and topological data are acquired to demonstrate that fracture properties behave differently depending on their size, and that particular fracture types correlate to specific scales of observation. The key result in this Chapter is that small-scale fractures strike in all directions, and are highly connected in the two study areas. However, intermediate- and large-scale fractures strike predominantly parallel to the platform margin and have lower connectivity rates than small-scale fractures. Understanding sub-seismic fracture networks is theefore critical to quantify fluid flow and permeability in carbonate reservoirs. Toward the end of this thesis, deep-water depositional settings from the Pará- Maranhão Basin, Equatorial Brazil, are studied utilising 2D and 3D seismic, borehole and multispectral satellite data to better understand platform-to-basin sedimentary processes. Neogene calciclastic submarine fans and channel-levee systems are analysed, and a comprehensive geomorphologic analysis is undertaken with the ultimate aim of finding similarities (or major contrasts) with their siliciclastic counterparts. Mixed calciclastic and siliciclastic sediment was transported from shallow waters into deep and ultra-deep waters by turbidity flows. Of importance is the confirmation that the pre-existing palaeotopography - such as terraces and gullies - was key to funnel sediment and create distinct types of channel-levee systems in Equatorial Brazil. Three types of channels are recognised: channels related to calciclastic submarine fans (type 1), low-sinuosity, aggradational channels (type 2), and high-sinuosity channels (type 3)

The effect of lithology, sub-bed scale heterogeneities, and mechanical stratigraphy on fault and fracture properties in coal bearing sequences

While sub-surface extraction of coal in the UK has ceased, renewed interest into the internal structure and growth of faults cutting coal measures exists due to applications for mine geothermal projects and analogues for natural gas plays in the Southern North Sea. In this study three field sites are used with detailed field observations, geological mapping, and sedimentological classification undertaken to understand the role lithology and subbed scale heterogeneity plays in the deformation of UK Carboniferous Coal Measure. This study demonstrates, and suggests methods to limit, geologists’ own biases during fracture data collection that can influence the data collected, and hence the derived statistics used for fracture modelling. Biases are also introduced when the temporal evolution and connectivity of individual sets of fault-fracture networks are not considered when assessing connectivity. Unlike binary mechanically layers sequences (e.g. limestone/marl), we find the presence of shale inter-beds and the abundance of sub-bed scale sedimentary heterogeneities (e.g. channel coals) to strongly affect the development fault and fractures. Where jointing exists at the time of faulting, fault-growth was found to be restricted by favourably orientated structures. The thickness and composition of fault-rock is controlled by lithological juxtaposition, with organic fragments found along principle displacement zones of faults of all sizes, even where organic rich layers have apparently not been cut. This study also shows faults that form following the collapse of Pillar and Stall mine workings are strongly affected by mechanical stratigraphy, with the height disruption controlled by a combination of the width of the stall, and the distribution and thickness of competent sandstone layers. Collapse leaves a clay-rich anthropogenic sedimentary layer which will retard the flow of groundwater in abandoned pillar and stall workings. These findings will improve our ability to assess geo-technical risk in ex-coal mining areas and de risk shallow mine geothermal projects.While sub-surface extraction of coal in the UK has ceased, renewed interest into the internal structure and growth of faults cutting coal measures exists due to applications for mine geothermal projects and analogues for natural gas plays in the Southern North Sea. In this study three field sites are used with detailed field observations, geological mapping, and sedimentological classification undertaken to understand the role lithology and subbed scale heterogeneity plays in the deformation of UK Carboniferous Coal Measure. This study demonstrates, and suggests methods to limit, geologists’ own biases during fracture data collection that can influence the data collected, and hence the derived statistics used for fracture modelling. Biases are also introduced when the temporal evolution and connectivity of individual sets of fault-fracture networks are not considered when assessing connectivity. Unlike binary mechanically layers sequences (e.g. limestone/marl), we find the presence of shale inter-beds and the abundance of sub-bed scale sedimentary heterogeneities (e.g. channel coals) to strongly affect the development fault and fractures. Where jointing exists at the time of faulting, fault-growth was found to be restricted by favourably orientated structures. The thickness and composition of fault-rock is controlled by lithological juxtaposition, with organic fragments found along principle displacement zones of faults of all sizes, even where organic rich layers have apparently not been cut. This study also shows faults that form following the collapse of Pillar and Stall mine workings are strongly affected by mechanical stratigraphy, with the height disruption controlled by a combination of the width of the stall, and the distribution and thickness of competent sandstone layers. Collapse leaves a clay-rich anthropogenic sedimentary layer which will retard the flow of groundwater in abandoned pillar and stall workings. These findings will improve our ability to assess geo-technical risk in ex-coal mining areas and de risk shallow mine geothermal projects

How do we see fractures? Quantifying subjective bias in fracture data collection

The characterisation of natural fracture networks using outcrop analogues is important in understanding subsurface fluid flow and rock mass characteristics in fractured lithologies. It is well known from decision sciences that subjective bias can significantly impact the way data are gathered and interpreted, introducing scientific uncertainty. This study investigates the scale and nature of subjective bias on fracture data collected using four commonly applied approaches (linear scanlines, circular scanlines, topology sampling, and window sampling) both in the field and in workshops using field photographs. We demonstrate that geologists' own subjective biases influence the data they collect, and, as a result, different participants collect different fracture data from the same scanline or sample area. As a result, the fracture statistics that are derived from field data can vary considerably for the same scanline, depending on which geologist collected the data. Additionally, the personal bias of geologists collecting the data affects the scanline size (minimum length of linear scanlines, radius of circular scanlines, or area of a window sample) needed to collect a statistically representative amount of data. Fracture statistics derived from field data are often input into geological models that are used for a range of applications, from understanding fluid flow to characterising rock strength. We suggest protocols to recognise, understand, and limit the effect of subjective bias on fracture data biases during data collection. Our work shows the capacity for cognitive biases to introduce uncertainty into observation-based data and has implications well beyond the geosciences