Variscan Tectonics

The Variscan Orogeny is first recorded in syn-collisional sedimentation along the southern margin of the Gramscatho Basin dated to the Middle Devonian (Leveridge, 2008; Leveridge and Hartley, 2006) and the Lizard Complex is inferred by Leveridge et al. (1990) to have been tectonically emplaced in the Late Eifelian-Early Givetian. However, basin extension was still on-going to the north with several of the passive margin basins yet to form (Whittaker and Leveridge, 2011). Complete closure of the Gramscatho basin in the late Famennian to Early Carboniferous, represents the commencement of continental collision and basin inversion (Leveridge and Hartley, 2006; Leveridge et al., 1984, 1990; Wilkinson and Knight, 1989). Basin inversion is diachronous across the passive margin and allows for continued extension in the Mississippian Culm Basin (Isaac et al., 1982; Leveridge et al., 1984; Rattey and Sanderson, 1984).

The mode of deformation, either thin-skinned or thick-skinned, has been a matter of some debate since the early 1980s (e.g. Shackleton et al., 1982; Coward and McClay, 1983; Isaac et al., 1982; Sanderson, 1984; Shackleton, 1984). Both theories made the assumption that granite magmatism was Variscan; this was later disproven through a geochronology study by Darbyshire and Shepherd (1985). Thick-skinned deformation was preferred due to field evidence for inversion structures (Hartley and Warr, 1990; Leveridge et al., 2002; Selwood, 1990), however, the identification of a décollement surface in the middle crust from seismic reflection studies (Brooks et al., 1984) hindered its adoption. The solution, inferred by Shail and Leveridge (2009), is that “thick-skinned inversion … [occurs] above a regional, décollement surface at the base of the middle crust”.

Variscan Convergence

Variscan deformation is variable across the region, however, it is best characterised by structures in south Cornwall and south Devon. Notable areas that differ to those described here include the Pennsylvanian Culm Basin (Lloyd and Chinnery, 2002) and the margins of the Land’s End Granite (Hughes, 2018; Hughes et al., 2009).
Thrust tectonics led to the development of two deformation episodes, D1 and D2, first described by Smith (1965) and in detail by Rattey and Sanderson (1984) that were later reaffirmed in studies by Alexander and Shail (1995, 1996). Later extensional deformation reactivated many D1 and D2 structures and created new D3 structures (Alexander and Shail, 1996, 1995; Shail and Alexander, 1997). These events are summarised in Table 1.

Table 1 Deformation events and structures relating to Variscan convergence (D1 and D2) and eventual extension (D3) with inferred kinematics with a schematic chronology. Compiled from Rattey et al. (1984), Alexander and Shail (1995), Alexander and Shail (1996), Shail and Aleander (1997), Hughes et al. (2009)

Chronology		Structures and Orientations	Bulk Kinematics	Interpretation
D1	Low angle faults	Tight-isoclinal folds (F1) verge NNW, pervasive axial planar cleavage (S1) dips gently SSE	NNW-directed thrusting	Basin inversion and expulsion of thrust sheets (e.g. St Mellion Outlier)
	NW-SE faults	Steep NW-SE faults, local dextral transpression	NW-SE dextral shear	Transfer faults during locking of basins
D2		Open-close folds (F2) verge NNW, spaced axial planar cleavage (S2) dips moderately SSE	NNW-directed thrusting	“Out of sequence” thrusts associated with basin inversion closure
D3	Distributed shear	Localised folds (F3) verge SSE, axial planar cleavage (S3) dips moderately NNW	Top-to-the-SSE extension	Orogenic extension/collapse
	Detachments	Synthetic extensional faults
	Brittle listric faults	Extensional faults dip SSE, slickenlines plunge SSE

D1 structures are pervasive across the region displaying thrust-associated E-W- to ENE-WSW-trending folds (F1) where the axial planar cleavage (S1) is commonly sub-parallel to bedding (Alexander and Shail, 1995; Rattey and Sanderson, 1984). D1 deformation is ubiquitous with the first phase of Variscan deformation in the Gramscatho, Looe, South Devon and Tavy basins (Holder and Leveridge, 1986; Hollick et al., 2014; Isaac, 1985; Isaac et al., 1982; Leveridge and Hartley, 2006; Leveridge et al., 1990, 2002, 2003; Rattey and Sanderson, 1984; Shail and Leveridge, 2009). Further D1 structures include NW-SE transfer fault zones discussed in detail below.

D2 deformation displays E-W- to ENE-WSW-trending F2 folds, similar to D1 deformation, but with a steeper cleavage (S2) which crenulates S1 fabrics (Alexander and Shail, 1995). The coaxial nature with D1 structures can vary up to 30° anticlockwise resulting in transpressive fabrics when in proximity to earlier, oblique basinal or deformation structures (Leveridge and Hartley, 2006; Shail and Leveridge, 2009). D2 structures also include narrow, high-strain shear zones and are more intense to the south of the region (Alexander and Shail, 1996, 1995; Rattey and Sanderson, 1984).

Deformation associated with the D3 event comprises extensional brittle and ductile structures with top-to-the-SSE sense of movement. Ductile structures are most commonly described within the parautochthonous succession associated with the footwall of the Carrick Thrust (Alexander and Shail, 1996, 1995; Shail and Alexander, 1997). The timing of D3 deformation is constrained by the intrusion of a lamprophyre into an extensional fault zone at Toll Point [SW 7912 2722] suggesting that D3 initiated during the Stephanian indicating the cessation of collisional Variscan deformation (Alexander and Shail, 1996). Work by Dupuis et al. (2015) on lamprophyres in the Falmouth area corroborates the post-collisional interpretation by Alexander and Shail (1996) but indicates a Permian age, synchronous with granite magmatism.

Thrust Tectonics

Variscan convergence led to the tectonic emplacement of a number of nappes which were identified by Leveridge (1974) and further described by Leveridge et al. (1990, 2002) and Leveridge (2008). The base of each nappe is defined by a thrust fault, illustrated in Figure 1, the lowest of these is the Carrick Thrust and defines the limit of allochthonous terranes in SW England. Other thrusts include the Veryan, Dodman and Lizard thrusts in south Cornwall (Leveridge, 2008; Leveridge et al., 1990) and a thrust is inferred at Start Point in south Devon (Coward and McClay, 1983). Each thrust is associated with a nappe of a similar name comprising allochthonous rocks that are generally thought to be of Rhenohercynian affinity (Floyd et al., 1993; Shail and Leveridge, 2009). Of these nappes the most unusual are the Lizard and Start nappes which are characterised by “complexes”.

The Lizard Complex comprises a suite of mafic to ultramafic rocks, including serpentinised peridotites, gabbros, sheeted dolerite dykes (Cook et al., 2000; Roberts et al., 1993). It is considered to represent a section of obducted oceanic crust from within the Gramscatho Basin (Leveridge and Shail, 2011). The complex is also intruded by a granite, referred to as the Kennack Gneiss, which indicates that the evolution of oceanic crust may not be simple. The tectonic setting during ocean crust formation is still ambiguous and it may be that the Lizard Complex represents oceanic crust from the Armorican/Normannian microcontinents (Holder and Leveridge, 1986; Shail and Leveridge, 2009; Strachan et al., 2014).

The Start Complex consists of hornblende-schists but their protolith and structural affinity are uncertain. Floyd et al. (1993) attributes a mid-ocean ridge affinity to a basic igneous protolith whereas Shail et al. (2014) defines these as having a specific affinity to the Gramscatho Basin, albeit an allochthonous part. The Dodman and Start thrusts may be linked and indicate that the Devonian allochthonous rocks in the Dodman Formation (metasandstones and mudstones with a schistose component) underline a Rhenohercynian affinity (Hendriks, 1937; Leveridge, 2008), alternatively, the Start Thrust, as part of the Start-Perranporth Zone, may indicate proximity to the northern margin of the Gramscatho Basin.

Strike-slip Faulting

Major oblique strike-slip faulting, first identified by Dearman (1963) as wrench-faulting, is considered a D1 structure by Leveridge et al. (2002) and Leveridge and Hartley (2006). The primary orientation of these structures are NW-SE, with a subordinate NE-SW component, and have been identified as a prominent feature of SW England geology (Dearman, 1970, 1963; Holloway and Chadwick, 1986; Jackson et al., 1989; Leveridge et al., 2002; Peacock, 2004; Sanderson and Dearman, 1973; Scrivener, 2006; Walsh et al., 1987). Regional mapping of these features is inconsistent and often shows sporadic faults; a compilation of regional fault mapping is presented in Figure 2.

It has been postulated by Le Gall (1991) that the structures may relate to earlier Caledonian wrench structures, but the position of SW England within the Rhenohercynian Zone at this time may preclude this theory. Alternatively, NW-SE dextral faults may relate to the Bristol Channel-Bray Fault Zone representing locking of the basins and are associated with ductile tectonic reworking around 320 Ma (Leveridge and Hartley, 2006; Matte, 1986). The structures have been regularly reactivated and influence granite intrusions (Hughes, 2018; Powell, 2004), Triassic mineralisation (Craig, 2018; Morris, 2016; Scrivener et al., 1994) and later Cenozoic deformation (Holloway and Chadwick, 1986).

The distribution of these NW-SE structures is poorly understood and many maps are inconsistent. Mapping of faults in the 1:50 000 and 1:625 000 BGS digital mapping products share few common structures and maps presented in other works such as Dearman (1963, 1970), Holdsworth (1989), Jackson et al. (1989) and Scrivener (2006) are inconsistent. More detailed studies have been compiled but only at a local scale and these have not been correlated to regional structures (e.g. Powell, 2004; Craig, 2018; Hughes, 2018; Morris, 2016; Nixon et al., 2011).

The most studied NW-SE fault is the Sticklepath-Lustleight Fault Zone which cuts the Dartmoor Granite and has been correlated through the Celtic Sea and southern Ireland (Holloway and Chadwick, 1986; Le Gall, 1991). The fault zone comprises several traces of NW-SE faults that demonstrate right-stepping and left-stepping traces and are associated with the Cenozoic strike-slip basins of the Bovey and Petrockstow basins. The mapping of several fault traces to other NW-SE faults has not been detailed in other works, however, a map presented in Stanley and Criddle (1990) represents a number of NW-SE faults as broad fracture zones across the region. It is unlikely that single fault traces are truly representative of strike-slip faulting in SW England given the presence of strike-slip basin formation and the long strike extent inferred by Holloway and Chadwick (1986) and Le Gall (1991). It is, however, difficult to correlate other NW-SE faults and reconcile these to distinct fault zones such as the postulated Porthtowan Fault Zone (Craig, 2018; Dearman, 1963) and scope for reappraisal of these faults remains.

Variscan Extension

Extension of the Variscan Orogen, initiated in the Late Carboniferous with D3 deformation, continued into the Early Permian (Alexander and Shail, 1996, 1995; Shail and Alexander, 1997). Detailed descriptions of extensional structures are made in Alexander and Shail (1995, 1996) and are analysed to define three different stress regimes exhibited from onshore rocks of south Cornwall (Shail and Alexander, 1997; Shail and Wilkinson, 1994). These regimes are summarised in Table 2. Some of the changes in stress field described by Shail and Alexander (1997) correlate with those inferred for the development of the offshore Plymouth and Melville basins (Harvey et al., 1994; Hillis and Chapman, 1992). Furthermore, the development of D4, D5 and D6 deformation structures through the Permian and into the Triassic is detailed in Figure 3.

Table 2 Summary table of Permian to Triassic deformation based on work by Alexander and Shail (1996) and Shail and Alexander (1997) which defined three reactivation episodes (D3, D4 and D5). Hughes (2018) infers continued deformation of D3 is key to accommodate granite emplacement with short-lived D4 before D5 deformation and a further D6 episode.

Chronology	Structures and Orientations	Bulk Kinematics	Interpretation
D3 (Continued)	Extensional faults dip NNW, slickenlines plunge NNW	NNW-SSE extension	Extension coeval with granite emplacement
D4	ESE- and NE-striking high angle faults, ENE-striking subvertical tensile fractures, NNW-striking subvertical cleavage sporadically developed	Dextral slip on NE-striking faults, sinistral slip on ESE-E striking faults	Conjugate fractures relating to ENE-WSW shortening
	NW- and NNE-striking high angle faults, NNW- to N-striking subvertical tensile fractures and veins, sporadic E-striking cleavage	Dextral slip on NW-striking faults, sinistral slip on NNE-striking faults	NNW-SSE shortening
D5	NNW-striking high angle faults, subvertical tensile fractures/veins	NNW-striking steep tensile fractures with extensional pull-aparts	NNW-SSE shortening and/or ENE-WSW extension
D6	NW-SE reactivation and inception of N-S faults with downthrow to the west	ENE-WSW extension	Extension relating to cross-course mineralisation

Continued D3 deformation into the Permian developed high angle faults coinciding with granite emplacement (Alexander and Shail, 1996, 1995; Shail and Alexander, 1997). These structures are important in accommodating the granite batholith. The development and reactivation of NW-SE structures during the D3 episode also control pluton geometries (Hughes, 2018).

The change in stress field during D4, from NNW-SSE extension to ENE-WSW shortening, can be constrained from mineralisation relating to the Camborne-Redruth Sn-Cu-W mineralisation (286.2 $\pm$ 0.5 Ma and 279.9 $\pm$ 0.8 Ma, Ar-Ar muscovite; Chen et al., 1993). Sn-W mineralisation (272.3 $\pm$ 0.9 Ma and 268.4 $\pm$ 0.8 Ma, Ar-Ar muscovite; Chesley et al., 1993) has been inferred to be later around Land’s End, however, this is in contrast to work by Tapster et al. (2017) who suggests Sn-W mineralisation at Hemerdon (SW Dartmoor) to be polyphase and earlier. The D4 episode generated ENE-trending dilatational structures with a subset of strike-slip NE-SW and WNW-ESE faults (locally referred to as “caunter lodes”) which crosscut all previous structures (Shail and Alexander, 1997).

D5 NNW-striking structures relate to the reactivation of Variscan transfer faults during NNW-SSE shortening. The D5 episode is considered to relate to intra-plate tectonics which may then extend into the mid-Triassic (Shail and Alexander, 1997). Work on mineralisation in the St Just Mining District by Hughes (2018) suggests that D5 was active in the Middle Permian (c. 270 Ma). Field evidence from Bosigran (west Cornwall) suggests the regional stress field changes rapidly from D3, through D4 and into D5 (c. 2-3 Ma). Further evidence for early D5 deformation can be drawn from the magmatic-hydrothermal mineralisation system in the St Just Mining District which contains NW-SE lode structures (Hughes, 2018). Cross-cutting relationships at Ding-Dong Mine can be observed between earlier ENE-WSW lodes and later NW-SE lodes; mineralisation has yet to be dated at this locality.

Based on interpretations by Hughes (2018), ENE-WSW extension during the Triassic is considered D6; constrained by mid-Triassic Pb-Zn-Ba mineralisation (236 $\pm$ 3Ma, Sm-Nd; Scrivener et al., 1994). Extension generated approximate N-S faults with downthrow to the west and is considered to be temporally associated with the Ladinian-Carnian boundary (Scrivener et al., 1994).

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