The effects of fluid flow through faults in granite gneiss exhumed from seismogenic depths

Lawther, Susan E. M. (2012) The effects of fluid flow through faults in granite gneiss exhumed from seismogenic depths. PhD thesis, University of Glasgow.

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Fault zones are ubiquitous structures throughout the Earth’s crust and as a fault evolves it can significantly influence the rheological and hydrological properties of the crust. Fluid flow through fault zones is typically associated with fault-fluid-rock interactions and these interactions can affect the mineralogy, strength and evolution of a fault zone. In this study, field mapping is combined with petro-physical and stable isotopic analyses of the fault rock to evaluate the fault-fluid-rock interactions that occur within different fault zones, and the effects of these reactions on fault zone and fault population evolution.
At Passo Moro in the NW Itilian Alps, there are three sets of joints cross-cutting the granite gneiss and numerous faults have formed by reactivation of pre-existing joints. The distribution of faults at Passo Moro is ultimately controlled by the variability of joint density within the host rock and the pre-existing joint distribution also affects the likelihood of whether a fault will grow into a mature fault zone or not. Where the joint density is high, strain is unable to accumulate to significant levels to enable joint reactivation into faults, whereas where joint density is low, fault zones are isolated and thus there are no structures nearby to facilitate fault linkage.
At Passo Moro the fault population has evolved in a similar way as that described by Martel (1990) whereby small faults link to form simple faults which connect to form compound fault zones. The Virgin Fault and Spaghetti Fault would be considered as small fault zones and The Ciao Ciao Fault is equivalent to a compound fault zone. All three fault zones have different fault architectures and the small faults have been affected by different fluid-rock reactions compared to the larger fault. The small faults have experienced fault zone strengthening by K-feldspar precipitation, whereas the large fault has been weakened by muscovite precipitation.
The different reactions between the fault zones are primarily controlled by the water-rock ratio which in turn is governed by permeability and the volume of fluid that infiltrates the fault zone. The Virgin Fault is considered as a rock-dominated system (K-feldspar-rich) whereas The Ciao Ciao fault is a fluid-dominated system (muscovite-rich). However, stable isotopes from both fault zones record a low water-rock ratio signifying rock-dominated conditions. Therefore the mineralogy of the fault rock is not solely controlled by the permeability defined water-rock ratio. The fluid dominated conditions promoting muscovitization in the Ciao Ciao Fault were probably enabled by an open fluid system and large volumes of fluid flowing through the fault during its lifetime. Stable isotopes indicate that water-rock ratios got lower with time in the Virgin Fault implying a limited open system, whereas muscovitization and stable water-rock ratios in the Ciao Ciao Fault point towards open system behaviour. In the Ciao Ciao Fault quartz precipitation only occurs in the foliated cataclasite within the fault core. Quartz precipitation is typically associated with closed system behaviour and suggests that the foliated cataclasite periodically ceased to be open to fluids and hence experienced cycles of higher and lower permeability.
Stable isotopes show that the Virgin Fault records mineral precipitation from a metamorphic-like fluid, but after fault deactivation, the fault periodically transmitted progressively more meteoric-like fluids via a micro-fracture network. The Ciao Ciao Fault records mineral precipitation from a more meteoric-like fluid compared to the Virgin Fault, and flow through micro-fracture networks is dominated by an essentially meteoric fluid. The Ciao Ciao Fault therefore does not preserve evidence of fluids from its early history.
This study indicates that the geochemical reactions that occur within a fault are controlled in part by the evolutionary stage of the fault, the fault rock permeability and the volume of fluids that pass through the fault. These results have been used to propose models for how the hydraulic properties and strength of a fault (population) evolve with time. The models produced from this study help advance our understanding of the processes that occur during the timescale of the seismic cycle, and how a population of faults will evolve in terms of mineralogy, strength and fluid flow. Such information will be of use to those involved in mineralization and mining studies, the storage of nuclear waste in crystalline rock, and earthquake prediction studies.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Faults, fluid flow, mineralogy and microstructures, stable isotopes, fluid inclusions.
Subjects: Q Science > QE Geology
Colleges/Schools: College of Science and Engineering > School of Geographical and Earth Sciences
Supervisor's Name: Dempster, Dr. Tim, Shipton, Dr. Zoe and Persano, Dr. Cristina
Date of Award: 2012
Depositing User: Miss Susan E M Lawther
Unique ID: glathesis:2012-3374
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 15 May 2012
Last Modified: 21 Aug 2018 14:52

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