Page 12 - An Atlas of Carboniferous Basin Evolution in Northern England

Chapter 2

Regional structural framework

Midlands (Turner 1949; Bolt 1967; Kent 1968; Le Bas 1972; Wills 1973, 1978; their subsequent inversion during the late Carboniferous. At a plate scale,
The Caledonides of northern England Evans 1979; Pharaoh et ul. 1987), which have influenced the deposition of the northern England lies within a complex orogenic belt with a long history of
overlying sediments. A number of exploration boreholes have encountered pre- continental collision from the early Palaeozoic Caledonian orogeny, through
The pre-Permian geology of northern England and Wales (Wills 1973, 1978; Carboniferous basement which is typically composed of deformed Silurian back-arc related extension reactivating the earlier Caledonian structural grain,
Whittaker 1985; BGS 1985) is divided by lineaments and faults into a series of phyllites or Ordovician Silurian volcanics and granites (Falcon & Kent 1960; to the accretion of micro-terranes culminating in the late Palaeozoic Variscan
major terranes (Fig. 4). Turner (1949) and Wills (1973, 1978) realized that old Le Bas 1972). Also late Precambrian sedimentary and volcanic rocks are orogeny (Fig. 6). The main tectonic models proposed to rationalize these events
Caledonian faults had exerted considerable influence on the geometry and exposed in Charnwood Forest. The gravity and aeromagnetic maps of the include: (i) dextral megashear, (ii) north-south back-arc extension, (hi) east-
orientation of subsequent tectonic features and recognized the presence of a region (BGS 1965) show important anomalies which outline the NW SE trend west Boreal rifting and (iv) dextral escape tectonics. These main models are
triangular shaped platform with a thin, flatlying undeformed Palaeozoic cover of this hidden Caledonian system, ft is suggested that many of the outlined below.
underlying the English Midlands (Figs 4 and 5). The platform is bounded on its Carboniferous and younger faults, which follow the same NW SE strike,
southern side by the Variscan thrust front, on its northwestern side by the represent rejuvenation of Caledonian tectonic lineaments. Most of the known The dextral megashear model. This model, first outlined by Arthaud & Matte
Longmynd Fault and to the northeast by a NW-SE-trending lineament which Carboniferous faults dip to the NE, suggesting that the Caledonian faults also (1977) and later developed by Ziegler (1981), Dewey (1982), Johnson (1982),
Turner (1949) and Wills (1973) placed in different locations. The Longmynd dipped in this direction. Examples of rejuvenated Caledonian faults are Badham (1982), Arthurton (1983) and Coward (1990), suggests that a wide
Fault, in the NW, separates the platform from the deformed and cleaved believed to include structures such as the Hoton, Cinderhill, Eakring and zone of right lateral shear affected Europe, particularly during the Carbonifer-
Palaeozoic sediments of the Welsh Caledonides. The NE boundary represents a Askern Spital Faults. Figure 4 also highlights the aeromagnetic anomalies ous as Gondwana docked with Laurasia. The northern England extensional
major lineament which separates the platform from a hidden East Midlands (BGS 1965) which are also thought to represent deeply buried Caledonian basins are interpreted as being formed by rifting associated with pull-apart
Caledonide belt, again with deformed and cleaved Lower Palaeozoic structures. However, not all the structures trend NW-SE. There are two basins in an east west-oriented zone of dextral shear, which linked the
sediments. In this study, the boundary of Turner (1949) was found to be the important cross-trending faults through the Wash and also localized structures Alleghenides in North America to the Urals in Russia. The minor inversion
more appropriate. This triangular platform is now referred to as the Midlands such as the Don Monocline that trend NE SW. These cross-cutting structures events which punctuated Carboniferous rifting are interpreted as reflecting the
Microcraton (Pharaoh et al. 1987). may represent old Caledonian lateral ramps and transfer zones, geometries evolution of the shear zone through a 'big bend' of the type evidenced along the
Turner (1949) compared the northern triangular apex of the microcraton typical of thrust fold belt terranes. San Andreas fault system in California.
with the Hindu Kush and suggested that the NE Caledonian trend in Wales Both Turner (1949) and Soper et al. (1987) suggested that the Lake District
could be traced in an arc around the apex into the East Midlands Caledonides contains an arcuate structure that links the East Midlands Welsh Caledonides Back-arc extension generated by northwards subduction of the Rheic Ocean. This
(Figs 4 and 5). This outer arc continues northwards as far as the lapetus suture, into a single belt. The link is mainly in the form of a cleavage arc (Fig. 4). In the model suggests that extension was driven by ductile creep of the lower crust
which separates the Welsh and East Midlands Caledonides from the SW of the Lake District the late Caledonian cleavage has a NE-SW trend. It towards a Rheic Ocean subduction zone (Leeder 1976), and as a result of back-
Laurentian Caledonides of Scotland and the northern part of Ireland. The curves round continuously into an east-west trend where it is cut off against the arc extension from a northwards dipping subduction zone in southern France
lapetus suture was located using the positions suggested by Dewey (1971, Dent Fault (Moseley 1972). East of the fault, the Palaeozoic rocks of the (Leeder 1982, 1987a). The northern England rift system is interpreted within
1982), Soper & Hutton (1984), Beamish & Smythe (1986) and Klemperer & Craven inlier contain WNW ESE-trending cleavage, passing into the East this setting as essentially back-arc in nature, with the inversion events linked to
Matthews (1987). Midlands aeromagnetic anomalies and presumed NW SE Caledonide trend. initial pulses of the Variscan orogeny which intensified towards final collision
The Malvern line, which runs north-south through the apex of the Late Caledonian granites, intruded around 390 Ma, are exposed in the in the late Carboniferous-early Permian.
microcraton is also a major lineament (Figs 4 and 5). It may continue to the Southern Uplands and the Lake District, and are also known in the subsurface
north within the Caledonides following the Red Rock and Pennine-Dent fault in deep boreholes in Weardale and Wensleydale (Bott 1967). Granites also exist East-west extension derived from southwards propagation of the Boreal rift. This
zones. Wills (1973, 1978) also suggested that the Malvern Fault continues further south around Charnwood Forest (Le Bas 1972) where they have older model, with its origins based on observations in the Midland Valley of
south of the Variscan thrust front along the 'Hawkins Line' (Fig. 5). radiometric ages of around 540 Ma. The granites had a significant impact on Scotland, emphasizes the progressive southwards propagation of the Boreal rift
The Welsh Caledonides exhibit a strong NE SW structural grain, as the style and location of subsequent tectonics because they formed isostatically during the Carboniferous. This resulted in east-west extension, generating
evidenced by the trend of the Dinorwic, Bala, Pontesford and Church Stretton buoyant areas which generally form the footwalls to subsequent extensional crustal thinning, basin formation, syn-tectonic ore deposition and volcanicity,
Faults (Fig. 4). Many cover anticlines and synclines run parallel to these faults faults (Bott 1967; Leeder 1982). and eventually leading to the generation of oceanic crust in the region of the
and curve in an arc, along with the associated cleavage, into an east west trend There has always been a problem explaining these late Caledonian intrusives present day Rockall Trough, towards the end of the Carboniferous (Haszeldine
where the Caledonian rocks plunge eastwards beneath younger cover in terms of lapetus subduction because they occur on both sides of the suture 1984, 1988, 1989; Haszeldine & Russell 1987).
sediments. Shackleton (1954) described stratigraphic variations in fades and zone. Subduction related volcanics should only occur on the subducted side of
thickness in the Cambro-Silurian in terms of a horst and graben model whereas a Benioff zone. However, recognition of the Mid European Caledonides Large-scale dextral escape tectonics. In the 1990s, several authors have focused
Coward & Siddans (1979) interpreted the structure as a thin-skinned thrust (Ziegler 1982) suggests that these northern England Caledonian intrusives can on strike-slip extension models (e.g. Coward 1993; Maynard et al. 1997). This
belt. The latter model fails to provide a full explanation of the marked easily have been generated by a northward-dipping subduction zone which hypothesis asserts that late Devonian-Carboniferous tectonics in NW Europe
stratigraphic variations across the main faults and a modification of the extended from the Mid European Caledonides underneath the Midlands can be explained in terms of a continuation of Caledonide events, with the
Shackleton model is offered here. The horst and graben structure reflects a half- Microcraton. northward movement of an Arcadian indentor causing eastwards (dextral)
graben geometry with down-to-the-SE border faults. The surface folds and extrusion of a triangular shaped block comprising northern England, the
thrusts can thus be explained in terms of end-Caledonian inversion of these North Sea and Baltica. During the early Carboniferous, the extruding crust
half graben. Variscan plate cycle was able to extend in a north-south direction as it moved eastwards. Closure of
The buried Caledonian of the East Midlands is interpreted to have a strong the Ural Ocean and subsequent Ural plate collision during the late
NW-SE structural grain (Fig. 4). Several authors have described the presence Several tectonic models have been proposed to explain the origin of the late Carboniferous, reversed the sense of shear along the extruded block margins
of pre-Carboniferous Caledonian rocks buried at depth beneath the East Devonian-early Carboniferous extensional basins of northern England and as it was pushed back between the Acadian collision zones causing inversion of