1 The kinematic linkage of the Dent, Craven and related faults of … · 2017. 3. 8. · 1 1 The...

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The kinematic linkage of the Dent, Craven and related faults of Northern England 1

C W Thomas1 & N H Woodcock2 2

1British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, 3

United Kingdom 4

2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, 5

Cambridgeshire, CB2 3EQ, United Kingdom 6

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SUMMARY: 8

New mapping of the southern part of the Dent Fault reveals three 5 to 6 km long segments 9

overlapping at two left-stepping zones 1 to 2 km wide. The main fault strands probably dip 10

steeply WNW. A faulted footwall syncline in Carboniferous strata indicates reverse dip-slip, 11

with a stratigraphic throw of at least 750 metres. Locally-developed plunging folds and 12

imbricate fault duplexes developed at fault bends reveal a strike-slip component, indicated to 13

be sinistral from limited slickenline data. Silurian strata in the hanging wall lack the Variscan 14

folds observed further north. The northern overstep is problematic in hosting upfaulted slivers 15

of older Silurian and Ordovician rocks. The southern overstep zone hosts a younger faulted 16

block compatible with releasing kinematics in sinistral strike-slip. 17

The Dent Fault converges at its southern end with the Barbon Fault, with an upfaulted wedge 18

between them near the branch point. The two faults swing southeastward, joining the Craven 19

fault system via splays and linkages. Regionally, the Dent and Barbon faults form the 20

innermost pair of a fan of ~N-S trending faults splaying off the northwest end of the South 21

Craven – Morley Campsall Fault System around the southwestern corner of the Askrigg 22

Block. 23

The kinematics of the Dent, Barbon and Craven faults fit NNW-SSE orientated shortening 24

during late Carboniferous Variscan deformation. The rigid Askrigg Block forced 25

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displacements around its west and south margins where fault and fold orientations were 26

influenced by pre-existing structures, at least Acadian in age to the west and early 27

Carboniferous to the south. 28

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Zones of transpression in the Earth’s crust result in complex coupled fault and fold 30

architectures in which displacements are typically partitioned into domains dominated either 31

by strike- or dip–slip (Fossen & Tikoff 1993; Holdsworth et al 2002; Jones and Tanner 1995; 32

Sanderson and Marchini 1984; Woodcock & Rickards 2003a). Structures developed in such 33

zones vary along their length, particular complexities arising at restraining and releasing 34

bends and offsets and at the tips of major faults, where strain is often accommodated by 35

fanning arrays and duplexes of subsidiary faults (Woodcock & Fischer 1986). 36

Previous studies have demonstrated such complexity arising from Variscan sinistral 37

transpression along the northern sector of the Dent Fault of Northern England (Fig. 1). The 38

fault is of particular interest because it provides an easily accessible and locally well-exposed 39

example of a transpressive structure (Underhill et al 1988; Woodcock & Rickards 2003a) and 40

hosts informative fault breccias (Mort & Woodcock 2008; Tarasewicz et al 2005; Woodcock 41

& Mort 2008; Woodcock et al 2008). 42

The dominant architecture comprises a forced, east-facing monocline breached by the reverse 43

Dent Fault, the latter interpreted to be a reactivated basement fracture that propagated up 44

through the fold as deformation progressed. Weak clockwise-transecting cleavages in the 45

footwall syncline, clockwise en echelon arrangement of faults and fault-bounded folds, and 46

oblique slickenlines indicate a component of sinistral strike-slip. An assemblage of Variscan 47

folds and their bounding fault duplex forms positive flower structures developed within a 48

NNW-SSE transpressional regime (Woodcock & Rickards 2003a, fig. 7). 49

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In this study, we extend the work of Underhill et al (1988) and Woodcock & Rickards 50

(2003a), presenting new field evidence for transpressional deformation along the southern 51

sector of the Dent Fault, from Dentdale to its southerly termination against the Barbon and 52

Craven faults (Fig. 2). We demonstrate a broadly similar kinematic development, but 53

highlight several key structural differences in the Dent Fault zone in the south. We interpret 54

the available evidence to show that the Dent Fault represents the innermost component of a 55

10-kilometre-scale fault fan that extends off the northwest tip of the Craven Fault Zone, 56

particularly the South Craven Fault. We thereby reassess the geometrical and kinematic 57

relationship between the Dent, Craven and Barbon faults. Critically, several of these faults, 58

developed in the Silurian rocks of the Southern Lake District, have significant, pre-59

Carboniferous histories (Barnes et al 2006; Soper 1999; Soper & Woodcock 2003; 60

Woodcock & Rickards 2003a; Woodcock & Rickards 2003b), supporting interpretation of the 61

Dent Fault as a surface manifestation of a re-activated, pre-Carboniferous basement fracture 62

(Soper 1999; Woodcock & Rickards 2003a). Finally, we suggest that the wider system of 63

major faults that dominates the geology of Northern England reflects a structural template 64

developed during pre-Variscan deformation events. 65

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1. REGIONAL SETTING 67

One of the earliest major faults to be identified in Britain (Dakyns et al 1890; Sedgwick 68

1835), the Dent Fault forms the western margin of the Askrigg Block (Dunham & Wilson 69

1985). This buoyant region is cored by the Wensleydale Granite (Dunham 1974). Although 70

this granite currently has an Emsian Rb/Sr age, it is considered more likely on various 71

grounds to be late Ordovician in age (Millward 2002, 2006; Pharaoh et al 1997). The granite 72

is covered by a thin succession of cyclic sedimentary rocks of dominantly lower 73

Carboniferous (Visean – Bashkirian) age. To the west of the fault lie rocks largely of Silurian 74

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age, belonging to the Windermere Supergroup of the Southern Lake District (Figs 1, 2) 75

(Barnes et al 2006). At its southern termination, the Dent Fault links with the Craven Fault 76

system that bounds the southern margin of the Askrigg Block, separating the block ‘high’ 77

from the deep Craven Basin to the south. To the north of the Stockdale Disturbance (‘SD’, 78

Fig. 1), the Dent Fault passes into a linear zone of more complex and diffuse deformation 79

known as the Dent Line (Underhill et al 1988, figs. 1,2), ultimately linking to the Pennine 80

Fault that forms the western margin of the Alston Block (Fig. 1). Thus, the Dent Fault forms 81

a regionally SSW-trending structure linking two major northwesterly trending fault systems, 82

the Pennine Fault and the South Craven – Morley-Campsall Fault (Fig. 1). These three fault 83

systems are components of the fundamental structural framework defining the block and 84

basin architecture of the geology of northern England (e.g. Corfield et al 1996). 85

The relationship between the Dent and adjacent faults at its southern termination has been a 86

source of conjecture since first identified. Various interpretations have been proposed (Fig. 87

3). Broadly, the Dent Fault has been interpreted to be a continuation of the Craven fault 88

system (Fig. 3b) (Aveline et al 1872; British Geological Survey 2007; Stone et al 2010) or to 89

be separate, possibly linked to NNE-trending monoclinal structures, such as the Hutton 90

Monocline, that lie to the SW (Fig. 3a,c,d) (Aitkenhead et al 2002; Phillips 1836; Turner 91

1935). 92

In N England, pre-Carboniferous strata were strongly deformed and weakly metamorphosed 93

during late Early Devonian Acadian deformation at around 400 Ma (Woodcock & Soper 94

2006). Late Carboniferous Variscan deformation resulted in inversion of Carboniferous 95

basins and the development of fault-fold belts (Arthurton 1984; Corfield et al 1996; 96

Gawthorpe 1987). The influence of existing basement structures resulted in variable 97

orientation in Variscan fold and fault patterns, but an overall regional NNW-SSE shortening 98

direction is indicated from the kinematics (Corfield et al 1996; Underhill et al 1988; 99

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Woodcock & Rickards 2003a). As discussed by Underhill et al (1988) and Woodcock & 100

Rickards (2003a), this shortening vector results in sinistral transpression across the Dent 101

Fault. However, several other studies have presented evidence for geographically widespread 102

E-W shortening early on during Variscan deformation, including an analysis of fracture 103

patterns in the adjacent Alston Block (Critchley 1984). 104

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2. STRUCTURAL GEOMETRY ALONG THE DENT FAULT 106

2.1 Overview 107

The entire southern sector of the Dent Fault and adjacent Carboniferous strata have been 108

mapped by CWT at 1:10 000-scale from Dentdale southwards to its junction with the Craven 109

Fault system (Fig. 2), as part of the revision of British Geological Survey 1:50 000 Sheet 49 110

(Kirkby Lonsdale). The Silurian strata west of the fault and north of gridline 85 were mapped 111

by NHW, with data from R.B. Rickards, on contract to the British Geological Survey. 112

Stereographic analysis was undertaken using StereoStat 1.6.1 (Rockware (R) © 1997-2012) 113

and Stereonet 8 (Allmendinger et al 2013; Cardozo & Allmendinger 2013). 114

In the study area, the Dent Fault comprises three gently concave-east segments, roughly 5 to 115

6 km long (Fig. 2). The segments meet at two overstep zones, 2 to 3 km long and about 1 km 116

wide. Overall, the fault trace becomes more southerly trending towards the south. Although 117

exposure is poor over the southernmost segment because of increasing superficial cover, the 118

evidence suggests that it is structurally simpler than the two northern segments. 119

The geometry of the main structures is described below from north to south. 120

121

2.2 Faults 122

2.2.1 North of the Dentdale overstep zone 123

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The complex deformation described by Woodcock & Rickards (2003a) to the northeast of 124

Sedbergh [SD 66 92] and Dentdale (Fig. 2), and bounded on the southeast by the Dent Fault, 125

is transferred across an overstep zone in Dentdale by a series of south-striking faults within 126

Silurian strata (A, Fig. 2). In their interpretation, dominantly strike slip displacement on the 127

Rawthey and the Branthwaite faults is transferred back on to the Dent Fault by two N-S 128

faults, the Helm Gill and Underwood faults (HGS, UF respectively; Figs 2, 4). The northern 129

end of the Underwood Fault branches from a strand designated as the Dent Fault in the sense 130

that it separates Silurian from Carboniferous rocks. The Helm Gill and Underwood faults 131

bound an inlier of late Ordovician Cautley Mudstone (Dent Group, CMU, C, Fig. 2; Figs 2, 4, 132

5a), discussed in section 3.2. A splay off the Helm Gill Fault trends SSW before losing 133

definition via a fan of minor faults in Silurian strata on Middleton Low Fell (Fig. 4). Offsets 134

in strata at the root of the fan around [SD 680 878] indicate sinistral displacement along this 135

splay. 136

137

2.2.2 Barbondale segment 138

Two main fault strands are developed through Barbondale (Figs 2, 4). These parallel 139

structures are roughly 200m apart, near vertical, and bound an elongate domain of very 140

steeply dipping or overturned Carboniferous strata (Figs 4, 5a). The easternmost of the two 141

strands separates very thick bedded massive limestones of the Great Scar Limestone Group 142

from those of the overlying heterolithic, thin- to thick-bedded Alston Formation (Yoredale 143

Group). East of this strand, the bedding dip changes rapidly to sub-horizontal in Alston 144

Formation strata and it is clear that this fault cuts through the hinge of a fault-parallel 145

footwall syncline analogous to the Fell End Syncline of Woodcock & Rickards (2003a) (Fig. 146

5b). 147

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At the northeastern end of the Barbondale segment, the two main strands converge closely 148

where the fault passes over the watershed into Dentdale, between Crag Wood [SD 687 872] 149

and Gawthrop [SD 694 875] (Fig. 4). They diverge to the southwest of Crag Wood due to a 150

marked southerly swing in the trace of the easternmost fault over c. 200m at Stone Rigg [SD 151

684 867], (Fig. 4). This fault is coupled with another just a few tens of metres east; between 152

them is a sliver of very steeply easterly dipping Alston Formation strata. In map view, this 153

fault-bound sliver has an open ‘S’ shape. Within the sliver, a prominent joint set dips at c. 55° 154

to the SW. 155

To the southeast of Crag Wood, and immediately south of the northwards bend in the Dent 156

Fault, a NW-divergent fan of normal faults with small NE-down displacements drops 157

Carboniferous strata progressively down into Dentdale, resulting in a broad dip-slope (Figs 4, 158

5b). 159

The development of these extensional structures in the footwall of the Dent Fault and at a 160

sharp bend in the fault is consistent with a sinistral component of displacement on the Dent 161

Fault; under sinistral displacement, this would be a releasing bend. The narrowness of the 162

Dent Fault zone in this area suggests that brittle fracture dominated over ductile forced 163

folding more than on other segments of the Dent Fault. 164

In the central part of the Barbondale segment, around Rowell Gill [Figs 2, 4; SD 67 85], there 165

is a marked swing in the trend of the Dent Fault from SW to SSW over just a few tens of 166

metres. Notably, the parallel eastern fault swings SSW some 500m NE of the swing in the 167

Dent Fault (Fig. 4). Between the two bends, two SW trending faults form a duplex within 168

which lie strata of the Great Scar Limestone Group deformed by strongly oblique folds (Figs 169

4, 5c). Immediately east of this duplex, two faults splay ENE off the Dent Fault zone. These 170

faults lose definition eastwards along their length in the more ductile shales and thin 171

sandstones of the Alston Formation (Fig. 4). However, between them, adjacent to the Dent 172

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Fault zone, there lies a complex set of folds; fold axial plane traces are parallel to the Dent 173

Fault, but axes have variable plunge. The fault duplex developed at Rowell Gill is inferred to 174

have developed at a restraining bend as the strike in the Dent Fault swings abruptly to the 175

SSW. 176

The parallel faults continue to the SSW into southern Barbondale (B, Figs 2; 6). Here they are 177

inferred to merge into a single structure that swings WSW along the northern side of Barkin 178

Beck, continuing westward through a narrow valley separating Barbon Low Fell from 179

Middleton Fell. Although the fault trace is imprecisely defined because of the similarities in 180

Silurian strata on either side, linear zones of brecciation and cataclasis broadly locate the fault 181

plane. 182

183

2.2.3. The Blindbeck overstep zone 184

The Barbondale fault strands lose displacement westwards into the Silurian, whilst the Dent 185

Fault itself is well located again about 1 km southeast of Blindbeck Bridge, crossing Great 186

Aygill and Hazel Sike (B, Fig. 2). The intervening area is an overstep zone analogous to that 187

in Dentdale. Underhill et al (1988) used VLF surveying to locate the trace of the Dent Fault 188

through this ground (their fig. 4a-c), contrasting their interpretation with an existing 189

Geological Survey interpretation that showed an easterly offset in the fault across an E-W 190

trending fault (their fig. 4c). Careful bedrock mapping across this ground has refined the trace 191

inferred by the VLF data, revealing a somewhat more complex arrangement of faults striking 192

both nearly N-S and E-W (Fig. 6), and confirming the easterly offset. 193

Within the transfer zone lies a small, fault-bound and folded outlying block of Alston 194

Formation strata (Fig. 6, at [SD 663 832]). The trace of the fault bounding the southern side 195

of this block is tightly constrained by adjacent outcrops of Silurian and Carboniferous strata. 196

Although surface exposure of Carboniferous strata is limited, underground evidence from 197

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large potholes immediately adjacent to the trace of the NNW-trending fault reveals extremely 198

broken ground (Hugh St. Lawrence, personal communication, 2009). 199

200

2.2.4. South of the Blindbeck overstep zone 201

South of the confluence of Hazel Sike with Great Aygill (Fig. 6; [SD 662 819]), the Dent 202

Fault strikes just W of S. Subsidiary sub-parallel faults account for displacements in Alston 203

Formation strata; sharp changes in dip and steep faulting parallel to the Dent Fault is 204

observed underground in Bull Pot of the Witches (Figs 6, 5d; [SD 662 813]). 205

South of Leck Beck Head (Fig. 2; [SD 662 801]), the Dent Fault strikes just E of S. Although 206

exposure is poor, available evidence suggests that the fault develops splays as it converges 207

with the North Craven and Barbon faults. The crop of Silurian strata at the southern end of 208

Barbon Low Fell narrows between the acutely convergent Dent and Barbon faults (Fig. 2). 209

The strata increase in age towards this tip and dip northwards. We interpret and discuss the 210

implications of this structure below. 211

Throughout the southern sector of the Dent Fault, the fault plane itself is not exposed; indeed, 212

unambiguously identifiable fault planes are very difficult to find. However, in Hazel Sike, 213

just a few tens of metres east of the confluence with Great Aygill, the plane of the Dent Fault 214

can be located to <5m in the stream section [Fig. 6; [SD 662 819]). Silurian strata become 215

increasingly brecciated eastwards until bedding is completely obscured and a coarse, 216

anastomosing, very steeply west-dipping fracture fabric is developed in brecciated rock. This 217

fabric is inferred to parallel the fault plane. Immediately east, Carboniferous strata are near 218

vertical (Fig. 6). 219

220

2.3 Folds 221

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In the sector of the Dent Fault north of our study area, en echelon fault/fold duplexes 222

characteristic of strike-slip deformation at constraining fault bends, are developed in Silurian 223

strata between the Rawthey and Dent faults (Woodcock & Rickards 2003a, figs. 3, 4). East of 224

the Dent Fault, a relatively simple footwall syncline (the Fell End Syncline) is developed. 225

Together, the duplexes and the Fell End Syncline are interpreted as a large-scale east-facing 226

monocline that developed above a reactivated basement fracture system. Ultimately, faults 227

propagating off this basement fracture pierced the monocline in an upwardly divergent fan or 228

flower. The Dent Fault is the most continuous strand, cutting the steep limb of the monocline. 229

The structural style is somewhat different south of Dentdale. Although the footwall syncline 230

persists along the southern sector of the Dent Fault, locally complex fault/fold duplexes are 231

developed in Carboniferous strata in the footwall but not observed in Silurian strata in the 232

hanging wall (Fig. 5), in contrast to the situation further north. 233

The contrast in response to deformation either side of the fault along the southern sector of 234

the Dent Fault is apparent from stereonet analysis of bedding orientation (Fig. 7). To the 235

west, Silurian strata are gently folded about Acadian hinges plunging gently between west 236

and northwest (Fig. 7a, b, c). They are little affected by Variscan deformation and en echelon 237

fault duplexes of the type that host the Taythes Anticline (Woodcock & Rickards 2003a, fig 238

3) are notably absent. In Carboniferous strata, poles to bedding reflect folds orientated 239

broadly parallel to the fault (Fig. 7d, e, f), particularly the persistent, fault-disrupted footwall 240

syncline (Fig. 5). Broader scatter indicated in stereonet 7d reflects the effects of the NW-241

trending fan of faults that intersects the southwesterly bend in the Dent Fault (Figs 2, 4). 242

Stereonet 7f reflects folds in line with the North Craven Fault, mostly within the fault duplex 243

at Tow Scar. 244

In general, the same Carboniferous stratigraphical level persists along the footwall of the 245

Dent Fault, indicating that the footwall syncline is grossly cylindrical and sub-horizontal (Fig. 246

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5). However, plunging structures are developed locally. The most evident of these is an 247

anticline hinge exposed over some 30m in Hazel Sike (Fig. 4, [SD 663 819]). This fold 248

plunges at about 25° towards the SW and is asymmetric, having a steeply-dipping NW limb 249

and shallow-dipping SE limb. This fold geometry is consistent with sinistral transpressional 250

deformation. 251

252

2.4 Estimates of displacement across the Dent Fault 253

The dip-slip component of Variscan displacement across the Dent Fault and its associated 254

monocline has been estimated by comparing the height of the base of the Carboniferous at 255

about two-kilometre intervals along the length of the fault, as shown in Fig. 8. The base 256

Carboniferous itself is infrequently exposed. Where higher Carboniferous units crop out in 257

the eastern footwall and between northing grid lines 90 to 95 and 01 to 15 in the 258

hangingwall, the base has been estimated using the nearest available stratigraphic thickness 259

of underlying units, as shown on the cross-sections here (Fig. 5) and in Woodcock & 260

Rickards (2003a, Fig. 4). In the remaining areas, between grid lines 77 to 90 and 95 to 01 in 261

the western hangingwall, the height of the topography in Silurian rocks provides only a 262

minimum constraint on the height of the now-eroded base Carboniferous. In the sector 263

opposite Taythes Gill and the River Rawthey (grid lines 95-01) the Silurian rocks on 264

topographic summits commonly preserve the secondary reddening characteristic below the 265

Carboniferous unconformity, and are unlikely to be more than about 100 metres below the 266

base-Carboniferous. Despite the uncertainties, the total throw across the Dent Fault zone must 267

exceed 1km down to the east along most of its length, with the majority of the displacement 268

accommodated on the Dent Fault itself. The displacement could decrease to about half its 269

maximum value at the south end of the fault as it joins the Barbon and Craven faults. 270

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Down-east displacement declines at the northern end of the fault until it switches to down-271

west throw of up to 700 m on the linked Argill Fault. There is an apparent decrease in the 272

throw where thick alluvial fan sediments of the Sedbergh Conglomerate Formation form the 273

base of the Carboniferous sequence in the hangingwall near the Clough River (grid 90-95). 274

However, these sediments may fill a deep palaeovalley into the sub-Carboniferous rocks, 275

questioning throw estimates that assume an originally sub-horizontal base to the 276

Carboniferous (Fig. 8). 277

278

3. KINEMATIC INTERPRETATION ALONG THE DENT FAULT 279

3.1 General kinematic model 280

Transpressional Variscan displacement on the Dent Fault due to NNW-SSE shortening is 281

well-established (Underhill et al 1988; Woodcock & Rickards 2003a) mostly from evidence 282

along the sector of the fault north of the study area. The new mapping along the southern 283

sector corroborates this gross kinematic model (Fig. 9) whilst revealing local contrasts. The 284

key differences include less complex Variscan deformation in Silurian strata in the hanging 285

wall, 1 – 2 km wide stepover zones in the Dent Fault (Fig. 2), and more structural complexity 286

within Carboniferous footwall rocks at fault bends (Figs 4, 5). 287

The few slickenline data for the Dent Fault (Fig. 10) come from central Barbondale 288

southwards (Fig. 2). Of these, three lineations plunge at between 60° and 70° on their 289

respective fault planes and indicate sinistral-oblique displacement on reverse faults. The 290

fourth, measured on a shallow west-dipping bedding plane, is consistent with sinistral strike 291

slip. Assuming that the host faults are all reverse, a kinematic analysis of these data using 292

FaultKin (based on Allmendinger et al 2013) yields a dominantly thrust solution with a 293

component of sinistral strike-slip on west or northwest dipping faults such as the Dent Fault. 294

The local shortening vector is predicted to be about 113° (Fig. 10). If this vector was proved 295

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to be correct by further data, much of the strike-slip component evidenced on the northern 296

segment of the Dent Fault would have to be partitioned on to the Barbon Fault in the study 297

area. However, locally-developed southwest-plunging folds and transverse structures, 298

including faults and joints, at fault bends along the Dent Fault indicate a component of strike-299

slip (Fig. 4). 300

301

3.2 Local kinematics along the Dent Fault 302

3.2.1 The Dentdale overstep zone 303

There are three zones in the study area that deserve further analysis, because they are both 304

problematic but instructive. The first such area is the overstep zone in Dentdale (Figs 2, 4). 305

This is interpreted as a left-stepping zone which should act, in sinistral strike slip, as a 306

releasing overstep. The faults that link across the overstep – the Helm Gill, Underwood and 307

Dent faults – would be expected to bound young Silurian rocks dropped down along normal 308

faults. Instead the faults bound slivers of older rocks; Cautley Mudstone Formation (upper 309

Ordovician) and Brathay Formation (mid-Silurian). 310

There are three possible explanations of this anomalous geometry. First, the slivers could 311

have been excised from the Dent Fault hangingwall further north, moved southwards and 312

obliquely upwards, and parked at the releasing overstep. Secondly, the old slivers could have 313

been thrust up by dominantly dip-slip displacements in the hangingwall, consistent with the 314

shortening vector from slickenline analysis (Fig. 6). In both these possibilities, under NNW-315

SSE compression, thrusting is possible at the southern ends of the inlying slivers where they 316

abut the Dent Fault (Fig. 11a). Thirdly, the old slivers could record deformation in a 317

restraining bend from a previous dextral phase on the Dent Fault. There is no evidence for 318

this third possibility, and we regard one of the first two explanations as more plausible. 319

320

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3.2.2. The Blindbeck overstep zone 321

The Blindbeck overstep zone (Fig. 5) is also left-stepping, and therefore also predicted to be a 322

releasing zone in sinistral strike-slip. Here the new mapping is more compatible with 323

releasing kinematics than in the Dentdale overstep. The overlapping strands of the Dent Fault 324

are linked mainly by NNW-striking and ENE dipping faults. They act as normal faults, with 325

net downthrow to the ENE, in sympathy with the releasing bend model. In contrast to the 326

fault-bounded slivers of older strata in the Dentdale overstep, these normal faults let down a 327

block of folded younger Alston Formation strata north of Fell House, surrounded by older 328

rocks. The fault planes are not exposed, but we assume them to be very steep. 329

330

3.2.3 The Barbon Low Fell fault wedge horst 331

At the southern end of Barbon Low Fell, the Dent Fault converges acutely with the 332

Barbon/South Craven fault system and is interpreted to link with the North Craven Fault via a 333

system of splays (Fig. 2, D). This linkage is likely to be more complex than shown as field 334

evidence is limited by poor exposure. The crop of the Wenlock (mid-Silurian) Brathay 335

Formation (Tranearth Group) is progressively narrowed between the Dent and Barbon faults. 336

The Silurian strata between the two faults increase in age southwards, dipping north. This 337

fault wedge contrasts with the Silurian strata west of the Barbon Fault, which young 338

southwards, and have a cover of basal Carboniferous conglomerates (Fig. 2). We attribute the 339

southwards increase in age in the faulted wedge to its uplift by transpression between the two 340

bounding faults, with dip-slip concentrated on the Dent Fault and strike-slip on the Barbon 341

Fault (Fig. 11b). The two faults must meet at depth as well as southwards, squeezing the 342

Silurian sliver upwards (Fig. 11b). 343

344

3.2.4 Linkage of the Barbon and Craven faults 345

15

Sinistral strike–slip displacement is apparent in the northern sector of the Barbon Fault 346

(Soper 1999; Woodcock & Rickards 2003b) and, based on wider mapping evidence, the fault 347

is interpreted to be continuous southward with the South Craven Fault (Figs 2, 12). However, 348

this continuity is geometric rather than kinematic, since it requires a switch from the sinistral 349

shear sense on the Barbon Fault to dextral shear recorded on the Craven Fault System 350

(Arthurton 1984; Gawthorpe 1987; Woodcock & Rickards 2003a) Local evidence for dextral 351

shear along the Craven Fault System is found at Tow Scar (Fig 2). Here, a fault duplex is 352

developed within the North Craven Fault zone, bounding an anticline-sycline pair. The en 353

echelon arrangement of the bounding faults and fold axial traces strongly suggests dextral 354

transpression. 355

The Craven Fault System and the fault and fold deformation developed in the Craven Basin 356

comprise a regional SE to ESE-striking dextral shear zone. The faults show a conjugate 357

strike-slip pattern facing a northwesterly maximum principal stress (Figs 9, 13), with the 358

North, Mid and South Craven faults as part of the dextral set. The folds in the Craven Basin 359

indicate NNW-directed shortening and have en echelon minor folds and faults indicating a 360

component of dextral strike-slip (Arthurton 1984).(cf. Woodcock & Fischer 1986, fig. 1) The 361

switch in shear sense from sinistral to dextral along the linked Barbon – Craven faults 362

requires a neutral shear point along their conjugate trace. This lies at about the tip of the 363

Barbon Low Fell fault wedge, as shown schematically on Figure 12. 364

365

3.4 Regional fault patterns: implications for fault history, kinematics and structural 366

architecture in NW England 367

When set in the wider context of Northwest England (Fig. 13), the Dent, Craven and Barbon 368

faults form part of a coherent system of faults that extends across the region. Readily 369

apparent is the swing in fault orientations from N-S in the Southern Lake District to WNW-370

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ESE in the Craven Basin. The first-order reason for this swing is the structural rigidity of the 371

Askrigg Block. The more plastic rocks beneath the Silurian Windermere Supergroup to the 372

west of the block and the Carboniferous Craven Basin to the south had preferentially to take 373

up the NNW-directed Variscan shortening. However, the width of the western and southern 374

deformation zones suggests other controls that require explanation. These controls are most 375

likely the influence on Variscan structures of Acadian basement structures (e.g. Moseley 376

1972; Turner 1949). 377

A number of the N-S faults in the Southern Lake District are known to have a pre-378

Carboniferous history and to partition strain during Acadian deformation in the late Silurian 379

(Moseley 1972; Soper 1999; Woodcock & Rickards 2003a; Woodcock & Rickards 2003b). 380

At least two of these faults, the Firbank and Kensgriff faults, are unconformably overlain by 381

Carboniferous strata (Fig. 13; (British Geological Survey 2007)). Fault breccia in the Wray 382

Castle Formation is hornfelsed (Soper, 1999) by the Shap Granite (397 ± 7 Ma; Millward 383

2002, and references therein), and two faults are cut by the pluton (British Geological Survey, 384

2007; Soper, 1999) (Fig. 13). An earlier history is further implied by apparent fault control on 385

Silurian sedimentation (Barnes et al 2006; Soper 1999, page 15) indicating that these faults 386

themselves likely represent older basement structures which were reactivated and propagated 387

up through the Silurian cover during Acadian deformation (Soper 1999). We pursue the 388

implications of such an older history with regard to the structural development of the 389

Caledonides of England and Wales more fully in a separate paper. 390

In the Craven (or Bowland) Basin, Gawthorpe (1987) used stratigraphic and gravity data to 391

identify major Dinantian (early Carboniferous) ENE-striking basin-bounding faults offset by 392

SE-striking strike-slip transfer faults. The ENE-striking structures parallel the Acadian 393

cleavage in the southern Lake District, suggesting an earlier control. The Variscan (late 394

Carboniferous) folds of the Craven Basin (Arthurton 1984) parallel the earlier basin-395

17

bounding faults and the Craven fault zone and other SE-striking faults in the basin parallel 396

the earlier transfer faults. 397

By at least 400 Ma (mid-Emsian, Early Devonian), and possibly much earlier, the Askrigg 398

Block had been structurally stabilised by the Wensleydale pluton. Although the upper margin 399

of the pluton in the Raydale borehole is cleaved, comparison with analogous exposed plutons 400

at Skiddaw and Shap suggests that the Acadian deformation (about 400-390 Ma; Woodcock 401

& Soper 2006) would not have pervaded the Wensleydale pluton and that earlier faults would 402

be sealed. The block therefore resisted both early Carboniferous extension and late 403

Carboniferous (Variscan) shortening. 404

The pattern of major faulting around the southwest corner of the Askrigg Block is 405

reminiscent of a leading extensional fan splaying off a dextral strike-slip fault, as shown 406

schematically by Woodcock & Fischer (1986, fig. 1). A subordinate but regular set of NNE-407

SSW trending conjugate faults is also developed, consistent with extensional displacement. 408

Given that the N-S faults in the Southern Lake District reflect basement fault patterns, and 409

given that the South Craven – Morley Campsall Fault is known to separate geophysically 410

different crust at depth in the sub-Carboniferous basement (Kirby et al 2000), we suggest 411

that, together, they reflect a structural architecture in the basement that is at least Acadian in 412

age. Indeed, this architecture may be considerably older, given that the Lake District is and 413

the Askrigg Block could be underlain by plutons of late Ordovician age. These plutonic 414

components are interpreted by some workers to be part of the supra-subduction arc 415

established in northern and eastern England during convergence of Avalonia with Baltica and 416

closure of the Tornquist Sea (e.g. Noble et al 1993; Pharaoh et al 1997; Pharaoh et al 1993). 417

Variscan deformation has resulted in the exploitation and reactivation of this basement 418

structural architecture, in response to the regional NNW-SSE shortening direction. The Dent, 419

18

Barbon and Craven faults and related fold and fracture patterns are readily explained as a 420

consistent, collective response to this tectonic event (Fig. 13). 421

In summary, the Variscan kinematic history of the region can be explained in terms of three 422

basement sectors of contrasting rheology. The Askrigg Block with its granite core acts as a 423

rigid body, with limited internal deformation. The Lower Palaeozoic rocks of the Southern 424

Lake District form a relatively rigid block, consolidated by ~E-W-trending Acadian 425

structures. The Craven Basin, with its thick fill of heterolithic Carboniferous strata, rich in 426

mudstones and siltstones, is more deformable than the Askrigg and Lower Palaeozoic blocks. 427

During Variscan deformation, the Lower Palaeozoic block moved southwards (sinistrally) 428

relative to the Askrigg Block, which moved dextrally relative to the Craven Basin. The 429

convergence of the Lower Palaeozoic block with the Craven Basin was accommodated by 430

shortening and reverse faulting in the basin (Fig. 12). 431

432

4. CONCLUSIONS 433

Along its southern extent, south of Sedbergh, the Dent Fault comprises three gently east-434

concave strands, meeting at two 1-2 km wide overstep zones. 435

Structures developed along the fault demonstrate transpressional deformation. A persistent 436

faulted footwall syncline indicates strongly reverse dip-slip deformation, but locally 437

developed plunging folds and imbricate fault duplexes developed in the footwall at fault 438

bends indicate a strike-slip component along favourably orientated segments. 439

Silurian strata in the hanging wall lack the Variscan folds observed further north by Underhill 440

et al (1988) and Woodcock & Rickards (Woodcock & Rickards 2003a). However, horsts of 441

older Silurian and Ordovician strata at the Dentdale overstep and at the southern tip of the 442

Dent Fault are analogous to the excision up out of the hanging wall of the wedge of Silurian 443

strata in the Taythes Anticline (Woodcock & Rickards 2003a, fig. 3). The difference in style 444

19

probably reflects significant partitioning of sinistral strike-slip on to the Barbon Fault to the 445

west. 446

The Barbon Fault is considered to be contiguous with the Craven Fault system, particularly 447

the South Craven Fault. The Dent Fault merges with these two structures via splays and 448

linkages with the North Craven Fault. Regionally, the Dent and Barbon faults form the inner-449

most pair of an assemblage of ~N-S trending faults developed across the Southern Lake 450

District extending off the South Craven – Morley Campsall Fault System around the 451

southwestern margins of the Askrigg Block. 452

The kinematics of the Dent, Barbon and Craven faults, are consistent with NNW-SSE 453

orientated shortening during late Carboniferous Variscan deformation. The rigid Askrigg 454

Block partitioned Variscan displacements to its west and south margins. Here, Variscan fault 455

and fold orientations were strongly influenced by pre-existing structures, at least as old as 456

Acadian to the west and early Carboniferous to the south. 457

458

Acknowledgements. We wish to acknowledge the contributions of Jack Soper and the late 459

Barrie Rickards to our understanding of the geology of the Dent Fault and adjacent ground, 460

particularly of the adjacent Lower Palaeozoic rocks. Dave Millward is thanked for numerous 461

discussions about the Dent and related faults, and Northern England geology in general 462

during the course of this work, and for his encouragement. Dave Schofield and Tim Pharaoh, 463

Bob Holdsworth and Doug Holliday are thanked for their reviews of an original manuscript 464

that ultimately led to this significantly different paper. CWT wishes to acknowledge the help 465

given by Marion Dunn, Ray Duffy, Michael Hale, Andy Hall, Hugh St Lawrence and other 466

members of the Red Rose Cave & Pothole Club. CWT publishes with the permission of the 467

Executive Director, British Geological Survey (NERC). This paper is an output of the former 468

BGS Northern England Project. 469

20

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24

Captions 574

Figure 1. Summary of the regional geology of northern England, showing the major 575

stratigraphical subdivisions, the known or inferred extent of key plutonic rocks and the 576

distribution of the main fault systems, including the location of the Dent Fault and its 577

relationship to the regional geology. 578

579

Figure 2. The geology of the southern sector of the Dent Fault. Three NNE-striking segments 580

meet at two overstep zones in Dentdale and lower Barbondale. The southwestern limit of the 581

study of the northern sector of the Dent Fault by Woodcock & Rickards (2003a) is outlined, 582

together with the extent of figures 4 and 6. 583

584

Figure 3. Previously published models of the relationship between the Dent and adjacent 585

faults. NCF: North Craven Fault; MCF: Middle Craven Fault; SCF: South Craven Fault; SD: 586

Stockdale Disturbance. 587

588

Figure 4. The geology of the Dent Fault in upper Barbondale and Dentdale. 589

590

Figure 5. Simplified cross-sections across the Dent and adjacent faults, revealing the reverse 591

displacement and the moderate development of upward-divergent fault fans. 592

593

Figure 6. The geology of the Dent Fault in lower Barbondale. 594

595

Figure 7. Lower hemisphere equal area projections of poles to bedding from the Silurian 596

strata west of the Dent Fault (stereonets a to c) and Carboniferous strata (stereonets d to f) to 597

the east. 598

25

Figure 8. Estimated stratigraphic throw (vertical displacement) across the Dent Fault and its 599

associated monoclinal fold, using the base-Carboniferous as a datum. Data are plotted along 600

the whole length of the fault from its branch points with the Argill and Augill faults in the 601

north and the Barbon and Craven faults in the south. 602

603

Figure 9. Rose diagram of nominally straight-line segments of the Dent, Craven and Barbon 604

faults with respect to the inferred NNW-SSE shortening direction and the corresponding 605

strain fields that result. The Dent Fault lies dominantly in the sinistral displacement field, the 606

North Craven Fault wholly within the dextral field. 607

608

Figure 10. Lower hemisphere equal area projection of slickenline data and the surfaces on 609

which they were recorded, with results of kinematic analysis to yield best-fit shortening axis 610

using FaultKin (Allmendinger et al 2013). 611

612

Figure 11. 613

a) Kinematic interpretation of the horst of Ordovician Cautley Mudstone between the Helm 614

Gill (HGF) and Underwood UF) faults. 615

b) Block-diagram model for the uplifted wedge of older Silurian strata at the junction of the 616

Dent and the Barbon – South Craven faults. Dip-slip is partitioned increasingly on the Dent 617

fault as strike-slip continues on the Barbon Fault. The wedge of Silurian strata is pinched up 618

between these two converging faults (inferred also to connect at depth). 619

620

Figure 12. Summary of the kinematics around the junction of the Dent, Barbon and Craven 621

faults, highlighting the switch from sinistral to dextral shear around the southwest corner of 622

the Askrigg Block, and the partitioning of deformation around this rigid block. 623

26

Figure 13. Simplified map of fault arrays in N England to the west and south of the Askrigg 624

Block. Fault data are from British Geological Survey 1:50 000 Geological Sheets 39, 40, 625

41,49, 50, 51, 59, 60 & 61. Inset A: Location in the United Kingdom. Inset B: the resolved 626

best-fit shortening direction for Variscan deformation from Woodcock & Rickards (2003, fig. 627

7). Schematic strain ellipsoids, adapted from McClay (1987, fig. 6.16a), show 628

correspondence of observed fault and fold orientations to theoretical geometry of structures 629

developed in dextral and sinistral shear. 630

Proven or putativemainly buried plutons

Major faultsystems

SOLW

AY FIRTH

Hull

Leeds

SheffieldLiverpool

Manchester

FAULT

PENNINE

NORTH CRAVEN FAULT

ASKRIGGBLOCK

Gainsborough Trough

ALSTONBLOCK

STAINMORETROUGH

SOUTH CRAVEN - MORLEY-CAMPSALL FAULT

FLAMBOROUGH HEADFAULT ZONE

MARKET WEIGHTONBLOCK

PENNINE FAULT

DENT

FAUL

TLAKE DISTRICT BOUNDARY

FAULT

Ribblesdale

Fold Belt

Red

Rock

Faul

t

ClevelandBasin

CheshireBasin

East Irish SeaBasin

CravenBasin

STUBLICK - NINETY FATHOM FAULT

Key

Jurassic & younger

Permian & Triassic

Pennsylvanian

Middle Devonian - Mississippian

Late Upper Ordovician/Silurian/Lower Devonian

Lower, Middle and earlyUpper Ordovician

Solway Line (Iapetus Suture)

Larger exposedplutons

Faults/lineaments

Acadian folds

East Midlands Shelf

Acadian deformation

Variscan deformation

Sub-Windermere Supergroupunconformity in Lake District

CLOSEHOUSE - LUNEDALE FAULT

LAKEDISTRICT

1

2

3

4

5

6

1

2

2

2

21

3

3

3

3

3

3

3

3

3

3

34

3

4

4

4

4

45

5

55

5

6

6

66

53

4

SD Stockdale ‘Disturbance’

SD

Thomas & Woodcock. Figure 1

5

CMU-MDST

474000mE

3 600

00m

N3 6

0000

mN

474000mE

3 700

00m

N

90

3 700

00m

N

65

80 80

85 85

493000mE493000mE

86

87

88

89

81

82

83

84

76

75

77

78

79

76

75

77

78

79

81

82

83

84

86

87

88

89

61 62 63 64 66 67 68 69

91

92

6561 62 63 64 66 67 68 69

90

91

92

KNDL

KNDL

TNTH

AG

AG

SG

AG

AGSG

SG

CTG

STK

PCMGING

GSCL

AG

GSCL

GSC

L

GSCL

GSCL

GSCL GSCLGSCL

GSCL

GSCL

GSCL

A

C

FES

?

HA

CRA

CTG

SDCO

DNT

DENTDALEBA

RBO

NDAL

E

MIDDLETONFELL

BARBONLOW FELL

LECK

GSCL

LECK BECKHEAD

AG

SG

Raw

they

Fau

lt

Bran

thw

aite

Fau

lt

DEN

T FA

ULT

AYGILL

TOW SCAR

SEDBERGH

UF

HG

F

KNDL

KNDL

KNDL

KND

L

KNDL

SDCO

SDCO

SW limit of studyarea of Woodcock& Rickards (2003a)

B

D

CTG

CTG

SG

AG

BARBONDALE

KNDL

AG

AG

AG

BARB

ON

FA

ULT

DEN

T F

AULT

CRAVEN FAULTS

DENT F

AULT

FIRB

AN

K F

AU

LT

BK

PCMG

PCMG

BK

SDCO

CTG

SDCO

FES: Fell End Syncline

Figure 4

Figure 6


Ordovician &Silurian

Carboniferous

Kendal Group (KNDL)

Coniston Group (CTG)

Tranearth Group (TNTH)

Stockdale Group (STK)

Dent Group (DNT)

Stainmore Formation (SG)Alston Formation (AG)Great Scar Limestone Group(GSCL)

Key to stratigraphicalunits

Sedbergh Conglomerate Formation(SDCO)

Major fault zones

Related faults

PermianBrockram (BK)

Ingletonian (ING)

Lower Palaeozoic/Neoproterozoic?

British Geological Survey 2007including Aveline et al. 1872

a b

dc

Phillips 1836

Aitkenhead et al. 2002Turner 1935

Hutto

nM

onoc

line

SD

DEN

T FA

ULT

CRAVEN FAULT

DENT

LIN

E

DEN

T FA

ULT

(Pen

nine

Fau

ltof

Phi

llips

) DEN

T FA

ULT

BARB

ON

FAU

LT

SD

NCFSCF

AskriggBlock

MCF

MCF

NCFNCF SCF

SCF


Brown Beck

Geological boundary

AnticlineSyncline

Trace of Dent Faultstructures

Horizontal strata

Inclined strata; dip in degrees

Overturned strata;dip in degrees

Synclinal monoform

AG

AG

TNTH

DNTCTG

CTG

KNDL

KNDL

CTG

GSCL

DNT

GSCL

GSCL

GSCL

GSCL

GSCL

Minor streams

Roads

Track

500 m

75

8520

15

885

8585

25

33

18

28

501819

12

28

31

8

10

7

19

7

8

25

77

88

80

82

75

83

78

78

13

1010

17

10

5

8

5

5

65

5

5

510

20

5

14

14

8

420

15

10

6

10

10

4

10

1720 8

10

7

5

15

1048 2044

60

44

56

63 N

Key

Silurian Strata KNDL: Kendal Group CTG: Coniston Group TNTH: Tranearth Group

Ordovician Strata DNT: Dent Group

Carboniferous strata AG: Alston Formation

Limestones

GSCL: Great Scar Limestone Group

Other faults

5

78

Grid squares are 1 km

River Dee

Gawthrop

NorthLord’sLand

Crag Wood

Dentdale

Oliver

Gill

High Lathe Gill

Flin

ter G

ill

High G

ill

Dent Fau

lt

Dent Fault

Und

erw

ood

Faul

t

Hel

m G

ill F

ault

South Lord’sLand

StoneRigg

Hacra

368000 369000367000 370000

4880

0048

7000

4 860

004 8

5000

4890

00

RowellGill area


Rawridding

Barkin Beck

MiddletonLow Fell

c

d

?

?

Solid boundary

62 63 64 65 66 67 68 69

Easting grid line

70

SL

0.5

0.5

SL

SL

0.5

0.5

0.5

0.5

SL

0.5

0.5

section along northing grid line 87

section along northing grid line 89River Lune

River Lune

A683Southdale

Gill

Gale BeckCasterton Fell Ease Gill

Hangingstone Barbondale

LowFellside

Wrestle Gill Combe Top High Gill

Holme Knott River Dee Helmside Roantree



BF

BF

BF

BF

CHF

MF

MF DF

HKF HGF DF

HKF HGF DF

FF Firbank Fault

MF Millhouse Fault

CHF Casterton Hall FaultBF Barbon Fault

HKF Holme Knott FaultHGF Helm Gill Fault

DF Dent Fault

WEST EAST

DF

a

b

Fault

1 km

Ordovician &Silurian Carboniferous

71 72

Kendal Group (KNDL)Coniston Group (CTG)Tranearth Group (TNTH)Stockdale Group (STK)Dent Group (DNT)

Stainmore Formation (SG)Alston Formation (AG)Great Scar Limestone Formation (GSCL)Ravenstonedale Group (RG)

Yoredale Group

?

?

SL

0.5

0.5

Km

Thomas & Woodcock Figure 5

RG

GSCL

AF

SG

SG

SG

SG

AF

AF

AF

GSCL

GSCL

GSCL

RG

RG

RG

TNTH

TNTH

KNDL

KNDL

KNDL

CTG

CTG

TNTH

DNT

DNT

DN

TKNDL

KNDL

KNDL

KNDL

CTG

GSCL

GSCL

GSCL

GSCL

GSC

L

Lower Palaeozoic stratawest of the Dent Fault,

undivided(see Figure 2)

Bullpot Farm

Barbondale

Barkin Beck

Bull Pot ofthe Witches

Cow Pot

AG

AG

AG

AG

Fell House

20

65

65

42

7535

27

556233

Dent F

ault

Dent Fault

Dent Fau

lt

Den

t Fau

lt

BlindbeckBridge

32

3229

22

61

37

10

32

80

43

7

3875

10

8

5

10

82

60

50

8

5

6

5

5

Geological boundary

Trace of Dent Fault (s.s)structures; tick ondownthrown side

Horizontal strata

Inclined strata; dip in degrees

Minor streams

Roads

Track

KeyCarboniferous strata(succession youngs to east)

AG: Alston Formation

Limestones

GSCL: Great Scar Limestone Group

Other related faults

5

Grid squares are 1 km

Path

Barkin Beck

Major sink hole

GSCL

N

500 m

4810

0048

2000

4830

00

366000 367000

BarbonLow Fell

Hazel S

ike

Great Aygill


o beddingmean bedding pole

t-fit fold axist-fit girdle to bedding poles

poles to bedding in Carboniferous rocks east of the Dent Fault

poles to bedding in Ordovician and Silurian rocks between the Barbon and Dent Faults

pole t

besbes

ilometr

N

0 1 2 k es

475

485

480

365

BARB

ON

FAU

LT

DEN

T FA

ULT

North Craven Fault

South Craven Fault

DENT

FAUL

T

DEN

TFA

ULT

1

2

3

4

5

6


a

b

c

d

e

f

stra

tigra

phic

thro

w (m

etre

s)

National Grid line (kilometres)

east

dow

nthr

oww

est d

ownt

hrow

800

600

400

200

0

1000

800

600

400

200

1400

1200

75 80 85 90 95 00 05 10 15

Fe d fe ot ldsall el r E dnn a ed n ilSync

ent F tD a luArgill Fault

Den

tdal

e ov

erst

ep

Blin

dbec

k ov

erst

ep

Sto

ckda

le

Faul

ts

cut b

y A

ugill

Fau

lt

join

s B

arbo

n an

d C

rave

n fa

ults

throw underestimated due to Sedbergh Conglomerate palaeovalley fill?

Clo

ugh

Riv

er

Tayt

hes

Gill

Riv

er

Raw

they

constraints on base Carbon-iferous depth west of fault

extrapolation from Carbon-iferous horizon at outcrop

constraint only by topo-graphic height of sub-Carboniferous

SSE NNW

Thomas & Woodcock Figure 8

Dent Fault Segments;n = 39

N

Barbon Fault Segments;n = 4North Craven Fault Segments;n = 5

Sinistral

Dex

tral


Best-fit regional shorteningdirection during VariscanCompression (after Woodcock& Rickards, 2003a)

1

2

3

1

Slickenline orientation

Bedding plane

Fault plane


Orientation of principal axesBest estimate shortening direction:13/113 (axis 3)

Relationshipsconjectural

Rigid Askrigg Block

Barbon Low FellView to NE

A

North Craven Fault

South Craven Fault

Barbon Fault

Dent Fault

Silurian strata squeezedup at tip where faults converge

A

A’

B

B’

C

C’

A A’A B B’ C C’

BarbonFault

DentFault

DentFault

BarbonFault Splay

S CravenFault

N CravenFault

View N View NW View W

Schematiccross-sections

B

11b

11a

Riedelshears

Hor

stThrust

ExtensionalFault

ExtensionalFaults

Den

t Fau

ltBran

thw

aite

Faul

t

Raw

they

Fau

lt

330° - 345°


Variscanshorteningdirection

SIN

ISTR

AL

DEXTRAL

Relatively rigid Block:Lower Palaeozoic rockswith exisiting ~ E-WAcadian structuralgrain

Lower Palaeozoic Blocktranslated south relativeto Askrigg Block

Askrigg BlockRigid, cored by granite

Barb

on F

ault

Den

t Fau

lt

Den

t Fau

lt

South Craven Fault

North Craven Fault

Variscanshorteningdirection

TaythesAnticline


Shortening

of the Craven Basin under

NNW-SSE compression

Sedbergh Fault

Raw

they

Fau

lt

DeformableCraven Basin

Neutral shearpoint

Craven Basin

Variscan Deformation

‘Front’

South Craven - Morley-Campsall

Fault Zone

To Flamborough Head Fault Zone

Askrigg Block

Den

t Fau

lt

Den

t Fau

lt

N Craven Fault

N Craven Fault

Barb

on

Fau

lt

Mid Craven Fault

CarbDevn

Sedbergh

Fault

Firb

ank

Fau

lt

Stockdale Disturbance

Craven and Bowland Basins

South Craven Fault

N

~ 10 km

Best-fitshorteningdirection

NCFS

DLDFRFBFN

SCF

dextra

l

sinistralB

Schematic strike-slip strain ellipsoids for sinistral anddextral strain induced by NNW-SSE shortening

A

South Craven -

Morley-Campsall

Fault Zone

Askrigg Block

SouthernLake District

Den

t Fau

lt

Den

t Fau

lt

N Craven Fault

N Craven Fault

Bar

bo

n F

ault

Mid Craven Fault

CarbDevn

Sedbergh

Fault

Firb

ank

Fau

lt

Hut

ton

Mon

oclin

e

Que

rnm

ore

Faul

t

Stockdale Disturbance

Fig

ure

2

~10 km

N

Den

t Lin

e

Raw

they

Fau

lt


Orientation of folds, Ribblesdale Fold Belt

Orientation ofextensional faults

Reverse faults,thrusts

Ken

sgri

ff

Fau

lt

Orientation of R1Riedelshears (dextral strain)or R2 Riedel shears(sinistral strain)

Thrusts

Exte

nsio

nal

faul

ts

Extensional

faults

R1

R1

R2

R2

P

P

Fold

s

X

X

Thrusts

σ1

σ1

σ3

σ3

Sinistralstrain

Thrusts

Thrusts Extensional

faults

Extensional

faults R1

R1

R2

R2X

X

P

P

Folds

σ1

σ1

σ3

σ3

Dextralstrain

CravenBasin

Orientation of R1Riedelshears (sinistral strain)or R2 Riedel shears(dextral strain)

Orientation of major faultsand folds in NW England

Dent & BarbonFaults

CravenFaults

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