Abstract

AbstractTemporary excavations during the construction of the Glendoe Hydro Scheme above Loch Ness in the Highlands of Scotland exposed a clay-rich fault gouge in Dalradian Supergroup psammite. The gouge coincides with the mapped trace of the subvertical Sronlairig Fault, a feature related in part to the Great Glen and Ericht–Laidon faults, which had been interpreted to result from brittle deformation during the Caledonian orogeny (c. 420–390 Ma). Exposure of this mica-rich gouge represented an exceptional opportunity to constrain the timing of the gouge-producing movement on the Sronlairig Fault using isotopic analysis to date the growth of authigenic (essentially synkinematic) clay mineralization. A series of fine-size separates was isolated prior to K–Ar analysis. Novel, capillary-encapsulated X-ray diffraction analysis was employed to ensure nearly perfect, random orientation and to facilitate the identification and quantification of mica polytypes. Coarser size fractions are composed of greater proportions of the 2M1 illite polytype. Finer size fractions show increasing proportions of the 1M illite polytype, with no evidence of 2M1 illite in the finest fractions. A series of Illite Age Analysis plots produced excellent R2 values with calculated mean ages of 296 ± 7 Ma (Late Carboniferous–Early Permian) for the oldest (2M1) illite and 145 ± 7 Ma (Late Jurassic–Early Cretaceous) for the youngest (1M) illite. The Late Carboniferous–Early Permian (Faulting event 1) age may represent resetting of earlier-formed micas or authigenesis during dextral displacement of the Great Glen Fault Zone (GGFZ). Contemporaneous WNW(NW)–ESE(SE) extension was important for basin development and hydrocarbon migration in the Pentland Firth and Moray Firth regions. The Late Jurassic–Early Cretaceous (Faulting event 2) age corresponds with Moray Firth Basin development and indicates that the GGFZ and related structures may have acted to partition the active extension in the Moray Firth region from relative inactivity in the Pentland Firth area at this time. These new age dates demonstrate the long-lived geological activity on the GGFZ, particularly so in post-Caledonian times where other isotopic evidence for younger tectonic overprints is lacking.

Highlights

  • Faults and fractures originate from strains that arise from stress concentrations around flaws, heterogeneities and physical discontinuities, on scales from the microscopic to continental, in response to lithostatic, tectonic and thermal stresses and high fluid pressures (National Research Council, 1996)

  • The importance of understanding basin thermal history in the hydrocarbon industry led to the development of the seminal Illite Age Analysis (IAA) technique (Pevear, 1992; Vrolijk et al, 2018) and its further application to fault dating to estimate hydrocarbon trap timing and with the potential for evaluating earthquake hazards (Pevear, 1992)

  • We suggest that the earlier age may either represent resetting of the wall rock-derived Sronlairig Fault micas or growth of authigenic 2M1 illite-muscovite during Late Carboniferous–Early Permian dextral displacement of structures related to the Great Glen Fault Zone (GGFZ)

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Summary

Introduction

Faults and fractures originate from strains that arise from stress concentrations around flaws, heterogeneities and physical discontinuities, on scales from the microscopic to continental, in response to lithostatic, tectonic and thermal stresses and high fluid pressures (National Research Council, 1996). The well-studied GGFZ is known to have experienced a protracted history of movement, responding to changing stress regimes from the Caledonian (Ordovician–Silurian) throughout the Late Paleozoic to Mesozoic development of the Moray Firth region, and continuing into the Paleogene. In detail, following on from the dominant sinistral strike-slip regime during the Caledonian (Dewey & Strachan, 2003; Mendum & Noble, 2010), the GGFZ is known to have experienced: extension during the Devonian (Séranne, 1992); dextral displacement in the latest Carboniferous to Early Permian (Speight & Mitchell, 1979); transtensional opening and expansion across the Moray Firth–Pentland Firth region from the Permian and through the Mesozoic (Underhill, 1991; Underhill & Brodie, 1993); and dextral oblique wrench in the Paleocene– Eocene as Atlantic Ocean opening progressed (Holgate, 1969; Watts et al, 2007)

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