The Moorby Microgranite: a deformed high level intrusion of Ordovician age in the concealed Caledonian basement of Lincolnshire

Pharaoh, T. C.; Allsop, J. M.; Holliday, D. W.; Merriman, R. J.; Kimbell, G. S.; Rundle, C. C.; Brewer, T. S.; Evans, C. J.

doi:10.1144/pygs.51.4.329

Cited by 7 publications

(28 citation statements)

References 31 publications

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“…The undated provings occupy a NW-SE trending belt on Fig. 10, suggesting a tectonostratigraphic association, and were allocated to an 'Undated quartzite-phyllite association' by Pharaoh et al (2011). They appear to represent deep water deposits perhaps formed on the continental margin/rise of Avalonia.…”

Section: Basinal Sequences Of Megasequencementioning

confidence: 98%

“…Mapping of the Carboniferous basins of central and northern Britain and other structural analyses (e.g. Besly, 1988) helped define the northern and western margin of the ABM in greater detail (Fraser et al, 1990;Fraser and Gawthorpe, 2003;Smith et al, 2005;Pharaoh et al, 2011;Butler, in press) but as the ABM was regarded as unprospective, the seismic surveys do not extend far into its interior. In the late 1980's, more sophisticated three plate models were developed for the Caledonides (Cocks and Fortey, 1982;Soper and Hutton, 1984;Pickering et al, 1988) soon supported by more detailed combined palaeomagnetic/faunal studies (Scotese and McKerrow, 1990;Torsvik et al, 1992;Cocks and Torsvik, 1992).…”

Section: Previous Work and Terminologymentioning

confidence: 99%

“…Since that time, several other exploration boreholes have encountered such strata. Further details of these are presented in Pharaoh and Gibbons (1994); Table 5 and Pharaoh et al (2011). The following account is based on the examination of core, borehole records and geophysical log data in the National Geological Repository by the author.…”

Section: Basinal Sequences Of Megasequencementioning

confidence: 99%

“…On the Avalonian margin, the earliest subduction magmatism is of late Tremadocian age at Treffgarne and Rhobell in the Welsh Basin (Kokelaar, 1979;Bevins, 1982). Volcanic arcs developed elsewhere on the microcontinent through subsequent Ordovician time, and in Sandbian-Katian time a short-lived magmatic arc extended from the Lake District to East Anglia and the Brabant Massif of Belgium (Pharaoh et al, 1991;. This arc records the subduction of part of the Tornquist Sea basin beneath Avalonia Van Staal et al, 1998) (Fig.…”

Section: Introductionmentioning

confidence: 98%

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The Anglo-Brabant Massif: Persistent but enigmatic palaeo-relief at the heart of western Europe

Pharaoh

2018

Proceedings of the Geologists' Association

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The research collaborations established resulted in numerous further publications, including a special publication of the Geological Society (Winchester et al., 2002). One of the most significant results was the elucidation of the history of the Anglo-Brabant Deformation Belt (not to be confused with the ABM) which links the Caledonides of England and Belgium. Many of the same authors contributed to the Southern Permian Basin Atlas (SPBA) published by TNO and EAGE (Pharaoh et al., 2010; De Vos et al., 2010). 2. Crustal and lithospheric structure The crustal structure of the ABM is less well known than that of surrounding regions, e.g. northern England and the region south of the Variscan Front. It was recognised from the exploration boreholes drilled in the pre-seismic era (1940s-1950s) that although coal-bearing basins are locally present on its northern margins, the massif is poorly prospective for oil and gas (Lees and Taitt, 1946; Falcon and Kent, 1960). Thus the dense networks of seismic reflection data acquired since 1970 in northern and southern England, which are so informative about the upper crustal and sedimentary basin structure in those areas (e.g. Whittaker, 1985; Kirby et al., 2000; Smith et al., 2005; Pharaoh et al., 2011) are largely absent over the massif. Significant coverage is only present in the NW (Staffordshire), the Worcester Graben and Oxfordshire Concealed Coalfield areas (Butler, in press). A small number of seismic reflection profiles, gathered by the British Geological Survey (Chadwick, 1985; Chadwick and Smith, 1988) and academic institutions (e.g. Maguire, 1987) provide limited insights to crustal structure. The massif is crossed by only one long lithospheric-scale seismic refraction profile (Lithospheric Seismic Profile in Britain, LISPB) acquired in 1974 (Bamford et al., 1976). The GAMMA segment ends in Buxton, just beyond the northern edge of the massif (Barton, 1992). The DELTA segment extends from North Wales to the Isle of Portland and an interpretation was published by Maguire et al. (2011). The segment crosses the massif in its west central part, intersecting shorter refraction profiles in the Bristol Channel area (Mechie and Brooks, 1984). Deep seismic reflection data, recorded to 12 or 15 s Two-Way Travel Time (TWTT) by the British Institutions Reflection Profiling Syndicate (BIRPS), based at Cambridge University, cover only the offshore area (Klemperer and Hobbs, 1991). These profiles are nevertheless useful in assessing the tectonic milieu of the massif. Deep seismic reflection data from northern France (Cazes and Toreilles, 1988) and Belgium (Bouckaert et al., 1988; Meissner and Bortfeld, 1990) demonstrate a gently southward-dipping Variscan detachment, with the crust of the ABM forming a tapering wedge in the footwall (Blundell, 1993). The latter extends some 60 to 100 km south of the Variscan Front, and its nature is confirmed by the continuity of the South Central England Magnetic Anomaly beneath the Variscan nappes (Beamish et al., 2016). Interpretations of crustal...

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Section: Basinal Sequences Of Megasequencementioning

confidence: 98%

Section: Previous Work and Terminologymentioning

confidence: 99%

Section: Basinal Sequences Of Megasequencementioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

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The Anglo-Brabant Massif: Persistent but enigmatic palaeo-relief at the heart of western Europe

Pharaoh

2018

Proceedings of the Geologists' Association

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“…The latter continues eastward into the Cleveland Basin, bounded to the south by the Flamborough Head -Vale of Pickering Fault Zone with the Market Weighton High in its footwall. The Askrigg Block is underlain by Caledonian granite; this is now thought to date from the Caradocian stage of the Ordovician (e.g., Pharaoh et al, 1997;Millward, 2006), rather than the Devonian age formerly adopted (e.g., Dunham, 1974;Dunham and Wilson, 1985). The Market Weighton High is inferred to also have a granite core from gravity studies (e.g., Bott et al, 1978;Bott, 1988).…”

Section: Figure Captionsmentioning

confidence: 99%

Integrating induced seismicity with rock mechanics: a conceptual model for the 2011 Preese Hall fracture development and induced seismicity

Westaway

2017

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By integrating multiple datasets with relevant theory, covering fluid injection and fracturing, a conceptual model has been developed for the fracture development and induced seismicity associated with the fracking in 2011 of the Carboniferous Bowland Shale in the Preese Hall-1 well in Lancashire, NW England. Key features of this model include the steep fault that has been recognized adjoining this well, which slipped in the largest induced earthquakes, and the presence of a weak subhorizontal 'flat' within the depth range of the fluid injection, which was 'opened' by this injection. Taking account of the geometry of the fault and the orientation of the local stress field, the model predicts that the induced seismicity was concentrated ~700 m SSE of the Preese Hall-1 wellhead, in roughly the place where microseismic investigations have established that this activity was located. A further key observation, critical to explaining the subsequent sequence of events, is the recognition that the fluid injection during stage 2 of this fracking took place at a high net pressure, ~17 MPa larger than necessary. As a result, the fluid injection 'opened' a patch of the 'flat', making a hydraulic connection with the fracture network already created during stage 1. Continued fluid injection thus enlarged the latter fracture network, which ultimately extended southward far enough to intersect the steep part of the fault and induce the largest earthquake of the sequence there. Subsequent fluid injection during fracking stages 3 and 4 added to the complexity of this interconnected fracture network, in part due to the injection during stage 4 being again under high net pressure. This model can account for many aspects of the Preese Hall record, notably how it was possible for the induced fracture network to intersect the seismogenic fault so far from the injection point: the interconnection between fractures meant that the stage 1 fracture continued to grow during stage 2, rather than two separate smaller fractures, isolated from each other, being created. Calculations indicate that, despite the high net pressure, the project only 'went wrong' by a narrow margin: had the net pressure been ~15 MPa rather than ~17 MPa the induced seismicity would not have occurred. The model also predicts that some of the smaller induced earthquakes had tensile or 'hybrid' focal mechanisms; this would have been testable had any seismographs been deployed locally to monitor the activity. The analysis emphasizes the undesirability of injecting fracking fluid under high net pressure in this region, where flat patches of fault and/or subhorizontal structural discontinuities are present. Recommendations follow for future 'best practice' or regulatory guidelines. Supplementary material: Background information on the stratigraphy, structural geology, rockmechanical properties of the study region, and its state of stress, as well as theory for fluid injection, hydraulic fracturing, and Coulomb failure analysis, is available at http:

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The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension?

Soper

Woodcock

2003

Geol. Mag.

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Illite crystallinity data from the Silurian slate belts of England and Wales indicate anchizone to low epizone metamorphism during the Acadian deformation in late Early Devonian time. This metamorphic grade implies a substantial overburden, now eroded, of Lower Devonian non-marine sediments of the Old Red Sandstone (ORS) magnafacies. A minimum 3.5 km pre-tectonic thickness of ‘lost’ ORS is estimated in the southern Lake District and comparable thicknesses in North Wales and East Anglia. Tectonically driven subsidence of the underlying Avalonian crust is required to accommodate such thicknesses of non-marine sediment. One proposed mechanism is flexure of the Avalonian footwall during convergence that continued from Iapetus closure in the Silurian until Acadian cleavage formation in the Emsian. The evidence for this model in the critical area of northwest England is reviewed and found to be unconvincing. An alternative model is developed following a recent suggestion that the Early Devonian was a period not of continued convergence but of orogen-wide sinistral transtension. Transtensional accommodation of the lost ORS is evidenced by Early Devonian extensional faults, by synchronous lamprophyric magmatism, and by compatibility with previously diagnosed sediment provenance patterns. A summary of Siluro-Devonian tectonostratigraphy for Britain south of the Highland Border emphasizes that, unlike the Scottish Highlands, this area was not affected by the Scandian Orogeny, but was by the Acadian. An important period of sinistral transtension in the Early Devonian (c.420–400 Ma) was common to both regions. This was a time of high heat flow, lamprophyric and more evolved magmatism, and major southward sediment transport, involving mainly recycled metamorphic detritus from the Highlands and from contemporaneous volcanicity. Old Red Sandstone, deposited in coalescing transtensional basins over much of Britain from the Midland Valley to the Welsh Borders, was largely removed and recycled southward during Acadian inversion.

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The Moorby Microgranite: a deformed high level intrusion of Ordovician age in the concealed Caledonian basement of Lincolnshire

Cited by 7 publications

References 31 publications

The Anglo-Brabant Massif: Persistent but enigmatic palaeo-relief at the heart of western Europe

The Anglo-Brabant Massif: Persistent but enigmatic palaeo-relief at the heart of western Europe

Integrating induced seismicity with rock mechanics: a conceptual model for the 2011 Preese Hall fracture development and induced seismicity

The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension?

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