The Physical and Mechanical Properties of Natural Fault Zones in Basaltic Rocks - appendices

   This digital appendix accompanies the thesis “The Physical and Mechanical Properties of Natural Fault Zones in Basaltic Rocks”. It contains the data from X-ray fluorescence (XRF) analysis of fault rock samples described in chapter 3, and the results from hydromechanical characterisation of fault rocks described in chapter 4.  XRF analysis was performed in the School of Geography, Geology and the Environment using a PANanalytical Axios Advanced wavelength dispersive spectrometer. The result file (XRF data.xlsx) contains the original measurements, the data recalculated to anhydrous composition, and standard deviations. This is accompanied by results of repeated analysis of international reference materials for calibration, “XRF calibration.xlsx”, and a text file used for calculations and plotting in Python, “batch1_recalc.txt”, containing the fault rock composition (recalculated to include LOI) as well as literature data of Faroese basalt composition from Soager & Holm (2011). Additional literature data of Faroese basalt composition by Holm et al. (2001) is contained in “Holm2001.txt”. The data was analysed and plotted in the Jupyter notebook ”massbalance_calculations.ipynb”, and a 1D volume change estimation across the Gotugjogv fault zone is presented in “mass_balance.xlsx”. Hydromechanical experiments were performed at the Rock Physics and Mechanics Lab of the British Geological Survey in Keyworth. The experimental procedure can be reproduced from the file “Test-procedure#2955v1.3.proc.gs”. Experimental results are grouped in folders and include the mechanical data recorded at intervals of 250 ms in “Time Test data.dat” and of 500 N in “specimen.dat”. Fluid pressure evolution is recorded in “SAMPLE-NAME_perm.txt”. The data was processed in a Jupyter notebook, “SAMPLE-NAME.ipynb”.    Holm et al., 2001, Chem. Geol., https://doi.org/10.1016/S0009-2541(01)00260-1 Soager & Holm, 2011, Chem. Geol., https://doi.org/10.1016/j.chemgeo.2010.11.017</p

The Physical and Mechanical Properties of Natural Fault Zones in Basaltic Rocks

Faults represent a critical heterogeneity in basaltic sequences, which are gaining importance as potential sites for geothermal heat extraction and carbon storage. The processes controlling the behaviour of such faults at depth remain poorly constrained. The purpose of this research is to develop an integrated model for upper-crustal fault evolution in basalts, based on detailed structural, petrological, and hydromechanical characterisation of passively exhumed fault zones in the Faroe Islands. Faults record strain localisation from incipient decametre-wide zones comprising networks of Riedel shears, into high strain, metre-wide fault cores. These contain multiple cataclastic shear bands enclosing low-strain lenses of hydrothermal breccias and/or tabular veins, indicating significant late-stage dilatation. Abundant overprinting fault rock assemblages record episodic fault zone reorganisation and reworking of fault rocks. Fluid-mediated alteration leads to replacement of the tholeiitic basalt protolith, dominated by plagioclase, pyroxene, and volcanic glass, by zeolite-smectite-pyroxene assemblages in and around the fault core. Alteration and mineralisation increase velocity-strengthening behaviour within the fault core, while frictional strength and permeability are reduced by alteration and enhanced by mineralisation. The damage zone is relatively strong and permeable, and encloses a fault core comprising weak and impermeable cataclastic shear bands and lenses of strong and more permeable hydrothermal breccias. Strength recovery between slip events is minimal. In such a configuration, shear on impermeable cataclasites may lead to transient fluid pressurisation within the slip zone, promoting episodic hydrofracture and triggering slow slip events. Acceleration to seismic slip rates is inhibited by velocity-strengthening behaviour in the fault core. Strengthening by cementation following hydrofracture may drive slip zone locking and migration into weaker structures. Shallow basalt-hosted faults in the Faroe Islands record hydrofracture events punctuating periods of aseismic cataclastic creep within a widening slip zone. These findings highlight the importance of constraining the natural heterogeneity and distribution of fault rock properties, as they have significant implications for fault behaviour.</p

The Physical and Mechanical Properties of Natural Fault Zones in Basaltic Rocks

Faults represent a critical heterogeneity in basaltic sequences, which are gaining importance as potential sites for geothermal heat extraction and carbon storage. The processes controlling the behaviour of such faults at depth remain poorly constrained. The purpose of this research is to develop an integrated model for upper-crustal fault evolution in basalts, based on detailed structural, petrological, and hydromechanical characterisation of passively exhumed fault zones in the Faroe Islands. Faults record strain localisation from incipient decametre-wide zones comprising networks of Riedel shears, into high strain, metre-wide fault cores. These contain multiple cataclastic shear bands enclosing low-strain lenses of hydrothermal breccias and/or tabular veins, indicating significant late-stage dilatation. Abundant overprinting fault rock assemblages record episodic fault zone reorganisation and reworking of fault rocks. Fluid-mediated alteration leads to replacement of the tholeiitic basalt protolith, dominated by plagioclase, pyroxene, and volcanic glass, by zeolite-smectite-pyroxene assemblages in and around the fault core. Alteration and mineralisation increase velocity-strengthening behaviour within the fault core, while frictional strength and permeability are reduced by alteration and enhanced by mineralisation. The damage zone is relatively strong and permeable, and encloses a fault core comprising weak and impermeable cataclastic shear bands and lenses of strong and more permeable hydrothermal breccias. Strength recovery between slip events is minimal. In such a configuration, shear on impermeable cataclasites may lead to transient fluid pressurisation within the slip zone, promoting episodic hydrofracture and triggering slow slip events. Acceleration to seismic slip rates is inhibited by velocity-strengthening behaviour in the fault core. Strengthening by cementation following hydrofracture may drive slip zone locking and migration into weaker structures. Shallow basalt-hosted faults in the Faroe Islands record hydrofracture events punctuating periods of aseismic cataclastic creep within a widening slip zone. These findings highlight the importance of constraining the natural heterogeneity and distribution of fault rock properties, as they have significant implications for fault behaviour.</p

The Physical and Mechanical Properties of Natural Fault Zones in Basaltic Rocks - appendices

   This digital appendix accompanies the thesis “The Physical and Mechanical Properties of Natural Fault Zones in Basaltic Rocks”. It contains the data from X-ray fluorescence (XRF) analysis of fault rock samples described in chapter 3, and the results from hydromechanical characterisation of fault rocks described in chapter 4.  XRF analysis was performed in the School of Geography, Geology and the Environment using a PANanalytical Axios Advanced wavelength dispersive spectrometer. The result file (XRF data.xlsx) contains the original measurements, the data recalculated to anhydrous composition, and standard deviations. This is accompanied by results of repeated analysis of international reference materials for calibration, “XRF calibration.xlsx”, and a text file used for calculations and plotting in Python, “batch1_recalc.txt”, containing the fault rock composition (recalculated to include LOI) as well as literature data of Faroese basalt composition from Soager & Holm (2011). Additional literature data of Faroese basalt composition by Holm et al. (2001) is contained in “Holm2001.txt”. The data was analysed and plotted in the Jupyter notebook ”massbalance_calculations.ipynb”, and a 1D volume change estimation across the Gotugjogv fault zone is presented in “mass_balance.xlsx”. Hydromechanical experiments were performed at the Rock Physics and Mechanics Lab of the British Geological Survey in Keyworth. The experimental procedure can be reproduced from the file “Test-procedure#2955v1.3.proc.gs”. Experimental results are grouped in folders and include the mechanical data recorded at intervals of 250 ms in “Time Test data.dat” and of 500 N in “specimen.dat”. Fluid pressure evolution is recorded in “SAMPLE-NAME_perm.txt”. The data was processed in a Jupyter notebook, “SAMPLE-NAME.ipynb”.    Holm et al., 2001, Chem. Geol., https://doi.org/10.1016/S0009-2541(01)00260-1 Soager & Holm, 2011, Chem. Geol., https://doi.org/10.1016/j.chemgeo.2010.11.017</p

Petrological Evolution and Mass Redistribution in Basaltic Fault Zones: An Example From the Faroe Islands, North Atlantic Igneous Province

Fault rock petrology exerts an important influence on the permeability structure and mechanical properties of fault zones. Slip‐related deformation on upper‐crustal faults in basaltic rocks is closely associated with fluid‐rock interaction, altering the distribution of physical properties within the fault. Here, we present quantitative descriptions of the geochemical and petrological evolution of basalt‐derived fault rocks from three passively exhumed fault zones in the Faroe Islands. Fault‐rock petrology is determined by optical petrography and automated phase identification based on micrometer‐scale chemical maps from scanning electron microscope X‐ray spectroscopy. Geochemical evolution is assessed from major and trace element composition measured by X‐ray fluorescence. The fault rocks show intense fluid‐mediated alteration from a tholeiitic basalt protolith in the damage zones, and mechanical mixing in the fault cores. Pervasive alteration occurs early during fault zone evolution, with incipient fault damage increasing permeability and allowing along‐fault percolation of carbonated meteoric water, increasing fluid‐rock ratios. Our results suggest that the only mobile species within the fault zones are Ca, Si, and Al, which are leached during the hydrolysis of volcanic glass and plagioclase, and CO2, which is added by percolating waters. These species are transported from the damage zones into the fault cores, where they precipitate as zeolite and calcite cement in veins and hydrothermal breccias. We propose that solutes are replenished by cement dissolution through pressure‐solution during cataclastic creep, during repeated cycles of hydrofracture and cementation. The fault zones are natural reactors for fluid‐mediated alteration by CO2 and water, while other species are redistributed within the fault zones.</p