ORCID

Abstract

The Bristol Channel Basin (BCB) is a Mesozoic continental rift basin and an importantanalogue for offshore reservoirs. Previous studies have used relative cross-cuttingrelationships and correlations with adjacent sedimentary basins to constrain itsdevelopment. For the first time, in-situ U-Pb carbonate geochronology has datedcalcite slickenfibres within normal, thrust, and strike-slip faults in the EastQuantoxhead and Kilve region of Somerset.Protracted N-S extension (ca. 150–120 Ma) formed normal faults, while subsequent NS shortening (ca. 50–20 Ma) involved (i) cross-cutting strike-slip faults, (ii) minor E-Wstriking thrust faults, and (iii) reactivation of pre-existing normal faults. DuringCenozoic contraction, σ2 and σ3 were periodically interchangeable due to local stressvariations and fluid pressure changes.Geochemical analyses indicate evolving fluid sources, with more saline waters duringextension transitioning to more freshwater during contraction. Larger normal faultsfacilitated the upward migration of saline fluids from the Mercia Mudstone Group(MMG), altering pore fluid composition and raising formation water temperatures inthe Blue Lias Group. These fluids also influenced fault mechanics, particularly along theEast Quantoxhead Fault (EQHF).Microstructural analysis of vein generations highlights fault weakening throughprogressive deformation. Celestine (SrSO₄), sourced from the MMG, localised strainand promoted reactivation during contraction, as evidenced by reverse-sense S-CIntroductionviii | P a g efabrics. In contrast, smaller faults lack celestine and exhibit simpler fluid histories,although crystal-plastic textures (e.g., GBM, SGR) are observed.Understanding fluid-driven changes in fault core composition and strength is criticalfor evaluating fluid flow partitioning along fractures. Constraining fault populationevolution within the BCB sheds light on regional stress dynamics and can informstudies of nearby basins. These insights have practical applications for managing fluidflow in modern reservoirs, such as Enhanced Geothermal Systems (EGS).

Awarding Institution(s)

University of Plymouth

Supervisor

Mark Anderson, Gregory Price, David Peacock, Catherine Mottram

Document Type

Thesis

Publication Date

2025

Embargo Period

2025-07-09

Deposit Date

July 2025

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Additional Files

Geochronology_sample_Mass_spec_data.xlsx (990 kB)

Sample_U-Pb_plots.pdf (9937 kB)

Supplamentary_table_2_European_Calcite_U-Pb_data.xlsx (21 kB)

Kinematics_supplamentary.pptx (19245 kB)

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