1 The kinematic linkage of the Dent, Craven and related faults of Northern England
2 C W Thomas1 & N H Woodcock2
3 1British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA,
4 United Kingdom
5 2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge,
6 Cambridgeshire, CB2 3EQ, United Kingdom
7
8 SUMMARY:
9 New mapping of the southern part of the Dent Fault reveals three 5 to 6 km long segments
10 overlapping at two left-stepping zones 1 to 2 km wide. The main fault strands probably dip
11 steeply WNW. A faulted footwall syncline in Carboniferous strata indicates reverse dip-slip,
12 with a stratigraphic throw of at least 750 metres. Locally-developed plunging folds and
13 imbricate fault duplexes developed at fault bends reveal a strike-slip component, indicated to
14 be sinistral from limited slickenline data. Silurian strata in the hanging wall lack the Variscan
15 folds observed further north. The northern overstep is problematic in hosting upfaulted slivers
16 of older Silurian and Ordovician rocks. The southern overstep zone hosts a younger faulted
17 block compatible with releasing kinematics in sinistral strike-slip.
18 The Dent Fault converges at its southern end with the Barbon Fault, with an upfaulted wedge
19 between them near the branch point. The two faults swing southeastward, joining the Craven
20 fault system via splays and linkages. Regionally, the Dent and Barbon faults form the
21 innermost pair of a fan of ~N-S trending faults splaying off the northwest end of the South
22 Craven – Morley Campsall Fault System around the southwestern corner of the Askrigg
23 Block.
24 The kinematics of the Dent, Barbon and Craven faults fit NNW-SSE orientated shortening
25 during late Carboniferous Variscan deformation. The rigid Askrigg Block forced
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26 displacements around its west and south margins where fault and fold orientations were
27 influenced by pre-existing structures, at least Acadian in age to the west and early
28 Carboniferous to the south.
29 ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐
30 Zones of transpression in the Earth’s crust result in complex coupled fault and fold
31 architectures in which displacements are typically partitioned into domains dominated either
32 by strike- or dip–slip (Fossen & Tikoff 1993; Holdsworth et al 2002; Jones and Tanner 1995;
33 Sanderson and Marchini 1984; Woodcock & Rickards 2003a). Structures developed in such
34 zones vary along their length, particular complexities arising at restraining and releasing
35 bends and offsets and at the tips of major faults, where strain is often accommodated by
36 fanning arrays and duplexes of subsidiary faults (Woodcock & Fischer 1986).
37 Previous studies have demonstrated such complexity arising from Variscan sinistral
38 transpression along the northern sector of the Dent Fault of Northern England (Fig. 1). The
39 fault is of particular interest because it provides an easily accessible and locally well-exposed
40 example of a transpressive structure (Underhill et al 1988; Woodcock & Rickards 2003a) and
41 hosts informative fault breccias (Mort & Woodcock 2008; Tarasewicz et al 2005; Woodcock
42 & Mort 2008; Woodcock et al 2008).
43 The dominant architecture comprises a forced, east-facing monocline breached by the reverse
44 Dent Fault, the latter interpreted to be a reactivated basement fracture that propagated up
45 through the fold as deformation progressed. Weak clockwise-transecting cleavages in the
46 footwall syncline, clockwise en echelon arrangement of faults and fault-bounded folds, and
47 oblique slickenlines indicate a component of sinistral strike-slip. An assemblage of Variscan
48 folds and their bounding fault duplex forms positive flower structures developed within a
49 NNW-SSE transpressional regime (Woodcock & Rickards 2003a, fig. 7).
2
50 In this study, we extend the work of Underhill et al (1988) and Woodcock & Rickards
51 (2003a), presenting new field evidence for transpressional deformation along the southern
52 sector of the Dent Fault, from Dentdale to its southerly termination against the Barbon and
53 Craven faults (Fig. 2). We demonstrate a broadly similar kinematic development, but
54 highlight several key structural differences in the Dent Fault zone in the south. We interpret
55 the available evidence to show that the Dent Fault represents the innermost component of a
56 10-kilometre-scale fault fan that extends off the northwest tip of the Craven Fault Zone,
57 particularly the South Craven Fault. We thereby reassess the geometrical and kinematic
58 relationship between the Dent, Craven and Barbon faults. Critically, several of these faults,
59 developed in the Silurian rocks of the Southern Lake District, have significant, pre-
60 Carboniferous histories (Barnes et al 2006; Soper 1999; Soper & Woodcock 2003;
61 Woodcock & Rickards 2003a; Woodcock & Rickards 2003b), supporting interpretation of the
62 Dent Fault as a surface manifestation of a re-activated, pre-Carboniferous basement fracture
63 (Soper 1999; Woodcock & Rickards 2003a). Finally, we suggest that the wider system of
64 major faults that dominates the geology of Northern England reflects a structural template
65 developed during pre-Variscan deformation events.
66
67 1. REGIONAL SETTING
68 One of the earliest major faults to be identified in Britain (Dakyns et al 1890; Sedgwick
69 1835), the Dent Fault forms the western margin of the Askrigg Block (Dunham & Wilson
70 1985). This buoyant region is cored by the Wensleydale Granite (Dunham 1974). Although
71 this granite currently has an Emsian Rb/Sr age, it is considered more likely on various
72 grounds to be late Ordovician in age (Millward 2002, 2006; Pharaoh et al 1997). The granite
73 is covered by a thin succession of cyclic sedimentary rocks of dominantly lower
74 Carboniferous (Visean – Bashkirian) age. To the west of the fault lie rocks largely of Silurian
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75 age, belonging to the Windermere Supergroup of the Southern Lake District (Figs 1, 2)
76 (Barnes et al 2006). At its southern termination, the Dent Fault links with the Craven Fault
77 system that bounds the southern margin of the Askrigg Block, separating the block ‘high’
78 from the deep Craven Basin to the south. To the north of the Stockdale Disturbance (‘SD’,
79 Fig. 1), the Dent Fault passes into a linear zone of more complex and diffuse deformation
80 known as the Dent Line (Underhill et al 1988, figs. 1,2), ultimately linking to the Pennine
81 Fault that forms the western margin of the Alston Block (Fig. 1). Thus, the Dent Fault forms
82 a regionally SSW-trending structure linking two major northwesterly trending fault systems,
83 the Pennine Fault and the South Craven – Morley-Campsall Fault (Fig. 1). These three fault
84 systems are components of the fundamental structural framework defining the block and
85 basin architecture of the geology of northern England (e.g. Corfield et al 1996).
86 The relationship between the Dent and adjacent faults at its southern termination has been a
87 source of conjecture since first identified. Various interpretations have been proposed (Fig.
88 3). Broadly, the Dent Fault has been interpreted to be a continuation of the Craven fault
89 system (Fig. 3b) (Aveline et al 1872; British Geological Survey 2007; Stone et al 2010) or to
90 be separate, possibly linked to NNE-trending monoclinal structures, such as the Hutton
91 Monocline, that lie to the SW (Fig. 3a,c,d) (Aitkenhead et al 2002; Phillips 1836; Turner
92 1935).
93 In N England, pre-Carboniferous strata were strongly deformed and weakly metamorphosed
94 during late Early Devonian Acadian deformation at around 400 Ma (Woodcock & Soper
95 2006). Late Carboniferous Variscan deformation resulted in inversion of Carboniferous
96 basins and the development of fault-fold belts (Arthurton 1984; Corfield et al 1996;
97 Gawthorpe 1987). The influence of existing basement structures resulted in variable
98 orientation in Variscan fold and fault patterns, but an overall regional NNW-SSE shortening
99 direction is indicated from the kinematics (Corfield et al 1996; Underhill et al 1988;
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100 Woodcock & Rickards 2003a). As discussed by Underhill et al (1988) and Woodcock &
101 Rickards (2003a), this shortening vector results in sinistral transpression across the Dent
102 Fault. However, several other studies have presented evidence for geographically widespread
103 E-W shortening early on during Variscan deformation, including an analysis of fracture
104 patterns in the adjacent Alston Block (Critchley 1984).
105
106 2. STRUCTURAL GEOMETRY ALONG THE DENT FAULT
107 2.1 Overview
108 The entire southern sector of the Dent Fault and adjacent Carboniferous strata have been
109 mapped by CWT at 1:10 000-scale from Dentdale southwards to its junction with the Craven
110 Fault system (Fig. 2), as part of the revision of British Geological Survey 1:50 000 Sheet 49
111 (Kirkby Lonsdale). The Silurian strata west of the fault and north of gridline 85 were mapped
112 by NHW, with data from R.B. Rickards, on contract to the British Geological Survey.
113 Stereographic analysis was undertaken using StereoStat 1.6.1 (Rockware (R) © 1997-2012)
114 and Stereonet 8 (Allmendinger et al 2013; Cardozo & Allmendinger 2013).
115 In the study area, the Dent Fault comprises three gently concave-east segments, roughly 5 to
116 6 km long (Fig. 2). The segments meet at two overstep zones, 2 to 3 km long and about 1 km
117 wide. Overall, the fault trace becomes more southerly trending towards the south. Although
118 exposure is poor over the southernmost segment because of increasing superficial cover, the
119 evidence suggests that it is structurally simpler than the two northern segments.
120 The geometry of the main structures is described below from north to south.
121
122 2.2 Faults
123 2.2.1 North of the Dentdale overstep zone
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124 The complex deformation described by Woodcock & Rickards (2003a) to the northeast of
125 Sedbergh [SD 66 92] and Dentdale (Fig. 2), and bounded on the southeast by the Dent Fault,
126 is transferred across an overstep zone in Dentdale by a series of south-striking faults within
127 Silurian strata (A, Fig. 2). In their interpretation, dominantly strike slip displacement on the
128 Rawthey and the Branthwaite faults is transferred back on to the Dent Fault by two N-S
129 faults, the Helm Gill and Underwood faults (HGS, UF respectively; Figs 2, 4). The northern
130 end of the Underwood Fault branches from a strand designated as the Dent Fault in the sense
131 that it separates Silurian from Carboniferous rocks. The Helm Gill and Underwood faults
132 bound an inlier of late Ordovician Cautley Mudstone (Dent Group, CMU, C, Fig. 2; Figs 2, 4,
133 5a), discussed in section 3.2. A splay off the Helm Gill Fault trends SSW before losing
134 definition via a fan of minor faults in Silurian strata on Middleton Low Fell (Fig. 4). Offsets
135 in strata at the root of the fan around [SD 680 878] indicate sinistral displacement along this
136 splay.
137
138 2.2.2 Barbondale segment
139 Two main fault strands are developed through Barbondale (Figs 2, 4). These parallel
140 structures are roughly 200m apart, near vertical, and bound an elongate domain of very
141 steeply dipping or overturned Carboniferous strata (Figs 4, 5a). The easternmost of the two
142 strands separates very thick bedded massive limestones of the Great Scar Limestone Group
143 from those of the overlying heterolithic, thin- to thick-bedded Alston Formation (Yoredale
144 Group). East of this strand, the bedding dip changes rapidly to sub-horizontal in Alston
145 Formation strata and it is clear that this fault cuts through the hinge of a fault-parallel
146 footwall syncline analogous to the Fell End Syncline of Woodcock & Rickards (2003a) (Fig.
147 5b).
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148 At the northeastern end of the Barbondale segment, the two main strands converge closely
149 where the fault passes over the watershed into Dentdale, between Crag Wood [SD 687 872]
150 and Gawthrop [SD 694 875] (Fig. 4). They diverge to the southwest of Crag Wood due to a
151 marked southerly swing in the trace of the easternmost fault over c. 200m at Stone Rigg [SD
152 684 867], (Fig. 4). This fault is coupled with another just a few tens of metres east; between
153 them is a sliver of very steeply easterly dipping Alston Formation strata. In map view, this
154 fault-bound sliver has an open ‘S’ shape. Within the sliver, a prominent joint set dips at c. 55°
155 to the SW.
156 To the southeast of Crag Wood, and immediately south of the northwards bend in the Dent
157 Fault, a NW-divergent fan of normal faults with small NE-down displacements drops
158 Carboniferous strata progressively down into Dentdale, resulting in a broad dip-slope (Figs 4,
159 5b).
160 The development of these extensional structures in the footwall of the Dent Fault and at a
161 sharp bend in the fault is consistent with a sinistral component of displacement on the Dent
162 Fault; under sinistral displacement, this would be a releasing bend. The narrowness of the
163 Dent Fault zone in this area suggests that brittle fracture dominated over ductile forced
164 folding more than on other segments of the Dent Fault.
165 In the central part of the Barbondale segment, around Rowell Gill [Figs 2, 4; SD 67 85], there
166 is a marked swing in the trend of the Dent Fault from SW to SSW over just a few tens of
167 metres. Notably, the parallel eastern fault swings SSW some 500m NE of the swing in the
168 Dent Fault (Fig. 4). Between the two bends, two SW trending faults form a duplex within
169 which lie strata of the Great Scar Limestone Group deformed by strongly oblique folds (Figs
170 4, 5c). Immediately east of this duplex, two faults splay ENE off the Dent Fault zone. These
171 faults lose definition eastwards along their length in the more ductile shales and thin
172 sandstones of the Alston Formation (Fig. 4). However, between them, adjacent to the Dent
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173 Fault zone, there lies a complex set of folds; fold axial plane traces are parallel to the Dent
174 Fault, but axes have variable plunge. The fault duplex developed at Rowell Gill is inferred to
175 have developed at a restraining bend as the strike in the Dent Fault swings abruptly to the
176 SSW.
177 The parallel faults continue to the SSW into southern Barbondale (B, Figs 2; 6). Here they are
178 inferred to merge into a single structure that swings WSW along the northern side of Barkin
179 Beck, continuing westward through a narrow valley separating Barbon Low Fell from
180 Middleton Fell. Although the fault trace is imprecisely defined because of the similarities in
181 Silurian strata on either side, linear zones of brecciation and cataclasis broadly locate the fault
182 plane.
183
184 2.2.3. The Blindbeck overstep zone
185 The Barbondale fault strands lose displacement westwards into the Silurian, whilst the Dent
186 Fault itself is well located again about 1 km southeast of Blindbeck Bridge, crossing Great
187 Aygill and Hazel Sike (B, Fig. 2). The intervening area is an overstep zone analogous to that
188 in Dentdale. Underhill et al (1988) used VLF surveying to locate the trace of the Dent Fault
189 through this ground (their fig. 4a-c), contrasting their interpretation with an existing
190 Geological Survey interpretation that showed an easterly offset in the fault across an E-W
191 trending fault (their fig. 4c). Careful bedrock mapping across this ground has refined the trace
192 inferred by the VLF data, revealing a somewhat more complex arrangement of faults striking
193 both nearly N-S and E-W (Fig. 6), and confirming the easterly offset.
194 Within the transfer zone lies a small, fault-bound and folded outlying block of Alston
195 Formation strata (Fig. 6, at [SD 663 832]). The trace of the fault bounding the southern side
196 of this block is tightly constrained by adjacent outcrops of Silurian and Carboniferous strata.
197 Although surface exposure of Carboniferous strata is limited, underground evidence from
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198 large potholes immediately adjacent to the trace of the NNW-trending fault reveals extremely
199 broken ground (Hugh St. Lawrence, personal communication, 2009).
200
201 2.2.4. South of the Blindbeck overstep zone
202 South of the confluence of Hazel Sike with Great Aygill (Fig. 6; [SD 662 819]), the Dent
203 Fault strikes just W of S. Subsidiary sub-parallel faults account for displacements in Alston
204 Formation strata; sharp changes in dip and steep faulting parallel to the Dent Fault is
205 observed underground in Bull Pot of the Witches (Figs 6, 5d; [SD 662 813]).
206 South of Leck Beck Head (Fig. 2; [SD 662 801]), the Dent Fault strikes just E of S. Although
207 exposure is poor, available evidence suggests that the fault develops splays as it converges
208 with the North Craven and Barbon faults. The crop of Silurian strata at the southern end of
209 Barbon Low Fell narrows between the acutely convergent Dent and Barbon faults (Fig. 2).
210 The strata increase in age towards this tip and dip northwards. We interpret and discuss the
211 implications of this structure below.
212 Throughout the southern sector of the Dent Fault, the fault plane itself is not exposed; indeed,
213 unambiguously identifiable fault planes are very difficult to find. However, in Hazel Sike,
214 just a few tens of metres east of the confluence with Great Aygill, the plane of the Dent Fault
215 can be located to <5m in the stream section [Fig. 6; [SD 662 819]). Silurian strata become
216 increasingly brecciated eastwards until bedding is completely obscured and a coarse,
217 anastomosing, very steeply west-dipping fracture fabric is developed in brecciated rock. This
218 fabric is inferred to parallel the fault plane. Immediately east, Carboniferous strata are near
219 vertical (Fig. 6).
220
221 2.3 Folds
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222 In the sector of the Dent Fault north of our study area, en echelon fault/fold duplexes
223 characteristic of strike-slip deformation at constraining fault bends, are developed in Silurian
224 strata between the Rawthey and Dent faults (Woodcock & Rickards 2003a, figs. 3, 4). East of
225 the Dent Fault, a relatively simple footwall syncline (the Fell End Syncline) is developed.
226 Together, the duplexes and the Fell End Syncline are interpreted as a large-scale east-facing
227 monocline that developed above a reactivated basement fracture system. Ultimately, faults
228 propagating off this basement fracture pierced the monocline in an upwardly divergent fan or
229 flower. The Dent Fault is the most continuous strand, cutting the steep limb of the monocline.
230 The structural style is somewhat different south of Dentdale. Although the footwall syncline
231 persists along the southern sector of the Dent Fault, locally complex fault/fold duplexes are
232 developed in Carboniferous strata in the footwall but not observed in Silurian strata in the
233 hanging wall (Fig. 5), in contrast to the situation further north.
234 The contrast in response to deformation either side of the fault along the southern sector of
235 the Dent Fault is apparent from stereonet analysis of bedding orientation (Fig. 7). To the
236 west, Silurian strata are gently folded about Acadian hinges plunging gently between west
237 and northwest (Fig. 7a, b, c). They are little affected by Variscan deformation and en echelon
238 fault duplexes of the type that host the Taythes Anticline (Woodcock & Rickards 2003a, fig
239 3) are notably absent. In Carboniferous strata, poles to bedding reflect folds orientated
240 broadly parallel to the fault (Fig. 7d, e, f), particularly the persistent, fault-disrupted footwall
241 syncline (Fig. 5). Broader scatter indicated in stereonet 7d reflects the effects of the NW-
242 trending fan of faults that intersects the southwesterly bend in the Dent Fault (Figs 2, 4).
243 Stereonet 7f reflects folds in line with the North Craven Fault, mostly within the fault duplex
244 at Tow Scar.
245 In general, the same Carboniferous stratigraphical level persists along the footwall of the
246 Dent Fault, indicating that the footwall syncline is grossly cylindrical and sub-horizontal (Fig.
10
247 5). However, plunging structures are developed locally. The most evident of these is an
248 anticline hinge exposed over some 30m in Hazel Sike (Fig. 4, [SD 663 819]). This fold
249 plunges at about 25° towards the SW and is asymmetric, having a steeply-dipping NW limb
250 and shallow-dipping SE limb. This fold geometry is consistent with sinistral transpressional
251 deformation.
252
253 2.4 Estimates of displacement across the Dent Fault
254 The dip-slip component of Variscan displacement across the Dent Fault and its associated
255 monocline has been estimated by comparing the height of the base of the Carboniferous at
256 about two-kilometre intervals along the length of the fault, as shown in Fig. 8. The base
257 Carboniferous itself is infrequently exposed. Where higher Carboniferous units crop out in
258 the eastern footwall and between northing grid lines 90 to 95 and 01 to 15 in the
259 hangingwall, the base has been estimated using the nearest available stratigraphic thickness
260 of underlying units, as shown on the cross-sections here (Fig. 5) and in Woodcock &
261 Rickards (2003a, Fig. 4). In the remaining areas, between grid lines 77 to 90 and 95 to 01 in
262 the western hangingwall, the height of the topography in Silurian rocks provides only a
263 minimum constraint on the height of the now-eroded base Carboniferous. In the sector
264 opposite Taythes Gill and the River Rawthey (grid lines 95-01) the Silurian rocks on
265 topographic summits commonly preserve the secondary reddening characteristic below the
266 Carboniferous unconformity, and are unlikely to be more than about 100 metres below the
267 base-Carboniferous. Despite the uncertainties, the total throw across the Dent Fault zone must
268 exceed 1km down to the east along most of its length, with the majority of the displacement
269 accommodated on the Dent Fault itself. The displacement could decrease to about half its
270 maximum value at the south end of the fault as it joins the Barbon and Craven faults.
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271 Down-east displacement declines at the northern end of the fault until it switches to down-
272 west throw of up to 700 m on the linked Argill Fault. There is an apparent decrease in the
273 throw where thick alluvial fan sediments of the Sedbergh Conglomerate Formation form the
274 base of the Carboniferous sequence in the hangingwall near the Clough River (grid 90-95).
275 However, these sediments may fill a deep palaeovalley into the sub-Carboniferous rocks,
276 questioning throw estimates that assume an originally sub-horizontal base to the
277 Carboniferous (Fig. 8).
278
279 3. KINEMATIC INTERPRETATION ALONG THE DENT FAULT
280 3.1 General kinematic model
281 Transpressional Variscan displacement on the Dent Fault due to NNW-SSE shortening is
282 well-established (Underhill et al 1988; Woodcock & Rickards 2003a) mostly from evidence
283 along the sector of the fault north of the study area. The new mapping along the southern
284 sector corroborates this gross kinematic model (Fig. 9) whilst revealing local contrasts. The
285 key differences include less complex Variscan deformation in Silurian strata in the hanging
286 wall, 1 – 2 km wide stepover zones in the Dent Fault (Fig. 2), and more structural complexity
287 within Carboniferous footwall rocks at fault bends (Figs 4, 5).
288 The few slickenline data for the Dent Fault (Fig. 10) come from central Barbondale
289 southwards (Fig. 2). Of these, three lineations plunge at between 60° and 70° on their
290 respective fault planes and indicate sinistral-oblique displacement on reverse faults. The
291 fourth, measured on a shallow west-dipping bedding plane, is consistent with sinistral strike
292 slip. Assuming that the host faults are all reverse, a kinematic analysis of these data using
293 FaultKin (based on Allmendinger et al 2013) yields a dominantly thrust solution with a
294 component of sinistral strike-slip on west or northwest dipping faults such as the Dent Fault.
295 The local shortening vector is predicted to be about 113° (Fig. 10). If this vector was proved
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296 to be correct by further data, much of the strike-slip component evidenced on the northern
297 segment of the Dent Fault would have to be partitioned on to the Barbon Fault in the study
298 area. However, locally-developed southwest-plunging folds and transverse structures,
299 including faults and joints, at fault bends along the Dent Fault indicate a component of strike-
300 slip (Fig. 4).
301
302 3.2 Local kinematics along the Dent Fault
303 3.2.1 The Dentdale overstep zone
304 There are three zones in the study area that deserve further analysis, because they are both
305 problematic but instructive. The first such area is the overstep zone in Dentdale (Figs 2, 4).
306 This is interpreted as a left-stepping zone which should act, in sinistral strike slip, as a
307 releasing overstep. The faults that link across the overstep – the Helm Gill, Underwood and
308 Dent faults – would be expected to bound young Silurian rocks dropped down along normal
309 faults. Instead the faults bound slivers of older rocks; Cautley Mudstone Formation (upper
310 Ordovician) and Brathay Formation (mid-Silurian).
311 There are three possible explanations of this anomalous geometry. First, the slivers could
312 have been excised from the Dent Fault hangingwall further north, moved southwards and
313 obliquely upwards, and parked at the releasing overstep. Secondly, the old slivers could have
314 been thrust up by dominantly dip-slip displacements in the hangingwall, consistent with the
315 shortening vector from slickenline analysis (Fig. 6). In both these possibilities, under NNW-
316 SSE compression, thrusting is possible at the southern ends of the inlying slivers where they
317 abut the Dent Fault (Fig. 11a). Thirdly, the old slivers could record deformation in a
318 restraining bend from a previous dextral phase on the Dent Fault. There is no evidence for
319 this third possibility, and we regard one of the first two explanations as more plausible.
320
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321 3.2.2. The Blindbeck overstep zone
322 The Blindbeck overstep zone (Fig. 5) is also left-stepping, and therefore also predicted to be a
323 releasing zone in sinistral strike-slip. Here the new mapping is more compatible with
324 releasing kinematics than in the Dentdale overstep. The overlapping strands of the Dent Fault
325 are linked mainly by NNW-striking and ENE dipping faults. They act as normal faults, with
326 net downthrow to the ENE, in sympathy with the releasing bend model. In contrast to the
327 fault-bounded slivers of older strata in the Dentdale overstep, these normal faults let down a
328 block of folded younger Alston Formation strata north of Fell House, surrounded by older
329 rocks. The fault planes are not exposed, but we assume them to be very steep.
330
331 3.2.3 The Barbon Low Fell fault wedge horst
332 At the southern end of Barbon Low Fell, the Dent Fault converges acutely with the
333 Barbon/South Craven fault system and is interpreted to link with the North Craven Fault via a
334 system of splays (Fig. 2, D). This linkage is likely to be more complex than shown as field
335 evidence is limited by poor exposure. The crop of the Wenlock (mid-Silurian) Brathay
336 Formation (Tranearth Group) is progressively narrowed between the Dent and Barbon faults.
337 The Silurian strata between the two faults increase in age southwards, dipping north. This
338 fault wedge contrasts with the Silurian strata west of the Barbon Fault, which young
339 southwards, and have a cover of basal Carboniferous conglomerates (Fig. 2). We attribute the
340 southwards increase in age in the faulted wedge to its uplift by transpression between the two
341 bounding faults, with dip-slip concentrated on the Dent Fault and strike-slip on the Barbon
342 Fault (Fig. 11b). The two faults must meet at depth as well as southwards, squeezing the
343 Silurian sliver upwards (Fig. 11b).
344
345 3.2.4 Linkage of the Barbon and Craven faults
14
346 Sinistral strike–slip displacement is apparent in the northern sector of the Barbon Fault
347 (Soper 1999; Woodcock & Rickards 2003b) and, based on wider mapping evidence, the fault
348 is interpreted to be continuous southward with the South Craven Fault (Figs 2, 12). However,
349 this continuity is geometric rather than kinematic, since it requires a switch from the sinistral
350 shear sense on the Barbon Fault to dextral shear recorded on the Craven Fault System
351 (Arthurton 1984; Gawthorpe 1987; Woodcock & Rickards 2003a) Local evidence for dextral
352 shear along the Craven Fault System is found at Tow Scar (Fig 2). Here, a fault duplex is
353 developed within the North Craven Fault zone, bounding an anticline-sycline pair. The en
354 echelon arrangement of the bounding faults and fold axial traces strongly suggests dextral
355 transpression.
356 The Craven Fault System and the fault and fold deformation developed in the Craven Basin
357 comprise a regional SE to ESE-striking dextral shear zone. The faults show a conjugate
358 strike-slip pattern facing a northwesterly maximum principal stress (Figs 9, 13), with the
359 North, Mid and South Craven faults as part of the dextral set. The folds in the Craven Basin
360 indicate NNW-directed shortening and have en echelon minor folds and faults indicating a
361 component of dextral strike-slip (Arthurton 1984).(cf. Woodcock & Fischer 1986, fig. 1) The
362 switch in shear sense from sinistral to dextral along the linked Barbon – Craven faults
363 requires a neutral shear point along their conjugate trace. This lies at about the tip of the
364 Barbon Low Fell fault wedge, as shown schematically on Figure 12.
365
366 3.4 Regional fault patterns: implications for fault history, kinematics and structural
367 architecture in NW England
368 When set in the wider context of Northwest England (Fig. 13), the Dent, Craven and Barbon
369 faults form part of a coherent system of faults that extends across the region. Readily
370 apparent is the swing in fault orientations from N-S in the Southern Lake District to WNW-
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371 ESE in the Craven Basin. The first-order reason for this swing is the structural rigidity of the
372 Askrigg Block. The more plastic rocks beneath the Silurian Windermere Supergroup to the
373 west of the block and the Carboniferous Craven Basin to the south had preferentially to take
374 up the NNW-directed Variscan shortening. However, the width of the western and southern
375 deformation zones suggests other controls that require explanation. These controls are most
376 likely the influence on Variscan structures of Acadian basement structures (e.g. Moseley
377 1972; Turner 1949).
378 A number of the N-S faults in the Southern Lake District are known to have a pre-
379 Carboniferous history and to partition strain during Acadian deformation in the late Silurian
380 (Moseley 1972; Soper 1999; Woodcock & Rickards 2003a; Woodcock & Rickards 2003b).
381 At least two of these faults, the Firbank and Kensgriff faults, are unconformably overlain by
382 Carboniferous strata (Fig. 13; (British Geological Survey 2007)). Fault breccia in the Wray
383 Castle Formation is hornfelsed (Soper, 1999) by the Shap Granite (397 ± 7 Ma; Millward
384 2002, and references therein), and two faults are cut by the pluton (British Geological Survey,
385 2007; Soper, 1999) (Fig. 13). An earlier history is further implied by apparent fault control on
386 Silurian sedimentation (Barnes et al 2006; Soper 1999, page 15) indicating that these faults
387 themselves likely represent older basement structures which were reactivated and propagated
388 up through the Silurian cover during Acadian deformation (Soper 1999). We pursue the
389 implications of such an older history with regard to the structural development of the
390 Caledonides of England and Wales more fully in a separate paper.
391 In the Craven (or Bowland) Basin, Gawthorpe (1987) used stratigraphic and gravity data to
392 identify major Dinantian (early Carboniferous) ENE-striking basin-bounding faults offset by
393 SE-striking strike-slip transfer faults. The ENE-striking structures parallel the Acadian
394 cleavage in the southern Lake District, suggesting an earlier control. The Variscan (late
395 Carboniferous) folds of the Craven Basin (Arthurton 1984) parallel the earlier basin-
16
396 bounding faults and the Craven fault zone and other SE-striking faults in the basin parallel
397 the earlier transfer faults.
398 By at least 400 Ma (mid-Emsian, Early Devonian), and possibly much earlier, the Askrigg
399 Block had been structurally stabilised by the Wensleydale pluton. Although the upper margin
400 of the pluton in the Raydale borehole is cleaved, comparison with analogous exposed plutons
401 at Skiddaw and Shap suggests that the Acadian deformation (about 400-390 Ma; Woodcock
402 & Soper 2006) would not have pervaded the Wensleydale pluton and that earlier faults would
403 be sealed. The block therefore resisted both early Carboniferous extension and late
404 Carboniferous (Variscan) shortening.
405 The pattern of major faulting around the southwest corner of the Askrigg Block is
406 reminiscent of a leading extensional fan splaying off a dextral strike-slip fault, as shown
407 schematically by Woodcock & Fischer (1986, fig. 1). A subordinate but regular set of NNE-
408 SSW trending conjugate faults is also developed, consistent with extensional displacement.
409 Given that the N-S faults in the Southern Lake District reflect basement fault patterns, and
410 given that the South Craven – Morley Campsall Fault is known to separate geophysically
411 different crust at depth in the sub-Carboniferous basement (Kirby et al 2000), we suggest
412 that, together, they reflect a structural architecture in the basement that is at least Acadian in
413 age. Indeed, this architecture may be considerably older, given that the Lake District is and
414 the Askrigg Block could be underlain by plutons of late Ordovician age. These plutonic
415 components are interpreted by some workers to be part of the supra-subduction arc
416 established in northern and eastern England during convergence of Avalonia with Baltica and
417 closure of the Tornquist Sea (e.g. Noble et al 1993; Pharaoh et al 1997; Pharaoh et al 1993).
418 Variscan deformation has resulted in the exploitation and reactivation of this basement
419 structural architecture, in response to the regional NNW-SSE shortening direction. The Dent,
17
420 Barbon and Craven faults and related fold and fracture patterns are readily explained as a
421 consistent, collective response to this tectonic event (Fig. 13).
422 In summary, the Variscan kinematic history of the region can be explained in terms of three
423 basement sectors of contrasting rheology. The Askrigg Block with its granite core acts as a
424 rigid body, with limited internal deformation. The Lower Palaeozoic rocks of the Southern
425 Lake District form a relatively rigid block, consolidated by ~E-W-trending Acadian
426 structures. The Craven Basin, with its thick fill of heterolithic Carboniferous strata, rich in
427 mudstones and siltstones, is more deformable than the Askrigg and Lower Palaeozoic blocks.
428 During Variscan deformation, the Lower Palaeozoic block moved southwards (sinistrally)
429 relative to the Askrigg Block, which moved dextrally relative to the Craven Basin. The
430 convergence of the Lower Palaeozoic block with the Craven Basin was accommodated by
431 shortening and reverse faulting in the basin (Fig. 12).
432
433 4. CONCLUSIONS
434 Along its southern extent, south of Sedbergh, the Dent Fault comprises three gently east-
435 concave strands, meeting at two 1-2 km wide overstep zones.
436 Structures developed along the fault demonstrate transpressional deformation. A persistent
437 faulted footwall syncline indicates strongly reverse dip-slip deformation, but locally
438 developed plunging folds and imbricate fault duplexes developed in the footwall at fault
439 bends indicate a strike-slip component along favourably orientated segments.
440 Silurian strata in the hanging wall lack the Variscan folds observed further north by Underhill
441 et al (1988) and Woodcock & Rickards (Woodcock & Rickards 2003a). However, horsts of
442 older Silurian and Ordovician strata at the Dentdale overstep and at the southern tip of the
443 Dent Fault are analogous to the excision up out of the hanging wall of the wedge of Silurian
444 strata in the Taythes Anticline (Woodcock & Rickards 2003a, fig. 3). The difference in style
18
445 probably reflects significant partitioning of sinistral strike-slip on to the Barbon Fault to the
446 west.
447 The Barbon Fault is considered to be contiguous with the Craven Fault system, particularly
448 the South Craven Fault. The Dent Fault merges with these two structures via splays and
449 linkages with the North Craven Fault. Regionally, the Dent and Barbon faults form the inner-
450 most pair of an assemblage of ~N-S trending faults developed across the Southern Lake
451 District extending off the South Craven – Morley Campsall Fault System around the
452 southwestern margins of the Askrigg Block.
453 The kinematics of the Dent, Barbon and Craven faults, are consistent with NNW-SSE
454 orientated shortening during late Carboniferous Variscan deformation. The rigid Askrigg
455 Block partitioned Variscan displacements to its west and south margins. Here, Variscan fault
456 and fold orientations were strongly influenced by pre-existing structures, at least as old as
457 Acadian to the west and early Carboniferous to the south.
458
459 Acknowledgements. We wish to acknowledge the contributions of Jack Soper and the late
460 Barrie Rickards to our understanding of the geology of the Dent Fault and adjacent ground,
461 particularly of the adjacent Lower Palaeozoic rocks. Dave Millward is thanked for numerous
462 discussions about the Dent and related faults, and Northern England geology in general
463 during the course of this work, and for his encouragement. Dave Schofield and Tim Pharaoh,
464 Bob Holdsworth and Doug Holliday are thanked for their reviews of an original manuscript
465 that ultimately led to this significantly different paper. CWT wishes to acknowledge the help
466 given by Marion Dunn, Ray Duffy, Michael Hale, Andy Hall, Hugh St Lawrence and other
467 members of the Red Rose Cave & Pothole Club. CWT publishes with the permission of the
468 Executive Director, British Geological Survey (NERC). This paper is an output of the former
469 BGS Northern England Project.
19
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23
574 Captions
575 Figure 1. Summary of the regional geology of northern England, showing the major
576 stratigraphical subdivisions, the known or inferred extent of key plutonic rocks and the
577 distribution of the main fault systems, including the location of the Dent Fault and its
578 relationship to the regional geology.
579
580 Figure 2. The geology of the southern sector of the Dent Fault. Three NNE-striking segments
581 meet at two overstep zones in Dentdale and lower Barbondale. The southwestern limit of the
582 study of the northern sector of the Dent Fault by Woodcock & Rickards (2003a) is outlined,
583 together with the extent of figures 4 and 6.
584
585 Figure 3. Previously published models of the relationship between the Dent and adjacent
586 faults. NCF: North Craven Fault; MCF: Middle Craven Fault; SCF: South Craven Fault; SD:
587 Stockdale Disturbance.
588
589 Figure 4. The geology of the Dent Fault in upper Barbondale and Dentdale.
590
591 Figure 5. Simplified cross-sections across the Dent and adjacent faults, revealing the reverse
592 displacement and the moderate development of upward-divergent fault fans.
593
594 Figure 6. The geology of the Dent Fault in lower Barbondale.
595
596 Figure 7. Lower hemisphere equal area projections of poles to bedding from the Silurian
597 strata west of the Dent Fault (stereonets a to c) and Carboniferous strata (stereonets d to f) to
598 the east.
24
599 Figure 8. Estimated stratigraphic throw (vertical displacement) across the Dent Fault and its
600 associated monoclinal fold, using the base-Carboniferous as a datum. Data are plotted along
601 the whole length of the fault from its branch points with the Argill and Augill faults in the
602 north and the Barbon and Craven faults in the south.
603
604 Figure 9. Rose diagram of nominally straight-line segments of the Dent, Craven and Barbon
605 faults with respect to the inferred NNW-SSE shortening direction and the corresponding
606 strain fields that result. The Dent Fault lies dominantly in the sinistral displacement field, the
607 North Craven Fault wholly within the dextral field.
608
609 Figure 10. Lower hemisphere equal area projection of slickenline data and the surfaces on
610 which they were recorded, with results of kinematic analysis to yield best-fit shortening axis
611 using FaultKin (Allmendinger et al 2013).
612
613 Figure 11.
614 a) Kinematic interpretation of the horst of Ordovician Cautley Mudstone between the Helm
615 Gill (HGF) and Underwood UF) faults.
616 b) Block-diagram model for the uplifted wedge of older Silurian strata at the junction of the
617 Dent and the Barbon – South Craven faults. Dip-slip is partitioned increasingly on the Dent
618 fault as strike-slip continues on the Barbon Fault. The wedge of Silurian strata is pinched up
619 between these two converging faults (inferred also to connect at depth).
620
621 Figure 12. Summary of the kinematics around the junction of the Dent, Barbon and Craven
622 faults, highlighting the switch from sinistral to dextral shear around the southwest corner of
623 the Askrigg Block, and the partitioning of deformation around this rigid block.
25
624 Figure 13. Simplified map of fault arrays in N England to the west and south of the Askrigg
625 Block. Fault data are from British Geological Survey 1:50 000 Geological Sheets 39, 40,
626 41,49, 50, 51, 59, 60 & 61. Inset A: Location in the United Kingdom. Inset B: the resolved
627 best-fit shortening direction for Variscan deformation from Woodcock & Rickards (2003, fig.
628 7). Schematic strain ellipsoids, adapted from McClay (1987, fig. 6.16a), show
629 correspondence of observed fault and fold orientations to theoretical geometry of structures
630 developed in dextral and sinistral shear.
26
2 5 3 3 STUBLICK - NINETY FATHOM FAULT 5
5 PENNINE ALSTON PENNINE FAULT BLOCK 4 3 FAULT Solway Line (Iapetus Suture) 3 6 4 1 SOLWAY FIRTH CLOSEHOUSE - LUNEDALE FAULT 6 STAINMORE LAKE 3 TROUGH LAKE DISTRICT BOUNDARY DISTRICT 3 SD 2 Cleveland 6 FAULT Basin DENT FAULT ASKRIGG 3 BLOCK 3 3 4 NORTH FAULT CRAVEN FLAMBOROUGH HEAD Craven FAULT ZONE 5 4 Basin 3 5 Ribblesdale Fold Belt SOUTH CRAVENLeeds - MORLEY-CAMPSALLMARKET FAULT WEIGHTON BLOCK Hull East Irish Sea Basin Gainsborough Trough 5 4 Manchester 4 6 Sheffield Liverpool 5 3 Cheshire East Midlands Shelf 1 2 2 4 3 Fault Basin Red Rock 3
Key 6 Proven or putative Jurassic & younger mainly buried plutons
5 Larger exposed Permian & Triassic plutons Variscan deformation Major fault 4 Pennsylvanian systems Faults/lineaments 3 Middle Devonian - Mississippian Acadian folds Acadian deformation
2 Late Upper Ordovician/ SD Stockdale ‘Disturbance’ Silurian/Lower Devonian Sub-Windermere Supergroup unconformity in Lake District 1 Lower, Middle and early Upper Ordovician
Thomas & Woodcock. Figure 1 N N 000m 000m 70 60 3
3 61 62 63 64 65 66 67 68 69 493000m E 493000m E CTG SDCO Key to stratigraphical KNDL SDCO AG units 92 SEDBERGH 92 KNDL KNDL Permian Brockram (BK) Rawthey Fault 91 ? 91 Branthwaite Fault Carboniferous
CTG FES Stainmore Formation (SG) DENT FAULT Alston Formation (AG) 90 90 Great Scar Limestone Group (GSCL)
SW limit of study GSCL Sedbergh Conglomerate Formation (SDCO)
area of Woodcock B HACRA 89 89 & Rickards (2003a) Ordovician & DNTCMU-MDST AG Silurian Kendal Group (KNDL)
KNDL UF 88 88
HGF TNTH A Coniston Group (CTG) KNDL DENTDALEGSCL Tranearth Group (TNTH) Stockdale Group (STK) 87 87 Dent Group (DNT)
BARBON FAULT BARBON GSCL 86 BARBONDALE 86 Lower Palaeozoic/ MIDDLETON Neoproterozoic? FELL Ingletonian (ING) KNDL AG 85 KNDL 85 DENT FAULT KNDL Figure 4
FIRBANK FAULT Major fault zones
84 GSCL 84
BARBONDALE SG Related faults SDCO C 83 83 FES: Fell End Syncline CTG AYGILL SDCO 82 82 AG CTG
81 81 BARBON Figure 6 DENT FAULT LOW FELL
80 SDCO GSCL LECK BECK 80 HEAD AG CTG 79 79
GSCL STK GSCL SG GSCL GSCL D 78 AG 78 SG PCMG
AG GSCL AG AG AG 77 77 LECK SG SG CRAVEN FAULTS TOW SCAR GSCL 76 76 PCMG BK
75 75 BK ING PCMG GSCL
474000m E 474000m E N N 61 62 63 64 65 66 67 68 69 000m 000m 70 60 3 3
Thomas & Woodcock. Figure 2 a b
LT U A F T T L N U E SD A F D
T N
E of Phillips) (Pennine Fault BARBON FAULT Askrigg D C RA NCF Block VEN FA SCF ULT
MCF Phillips 1836 British Geological Survey 2007 including Aveline et al. 1872 c d
LT U SD A F E T
N N I L E
T D N DE
SCF NCF SCF NCF Hutton
Monocline MCF
Turner 1935 Aitkenhead et al. 2002
Thomas & Woodcock. Figure 3 367000 368000 369000 370000
000 GSCL 85
89 DNT 20 4 75 Rawridding
Dent Fault AG River Dee North 15 Lord’s 85 25 8 Land CTG 85 Hacra Underwood Fault 85 Dentdale 000 88
4 25 TNTH GSCL 7 KNDL CTG DNT GSCL
Helm Gill Fault Gill Helm 18 Gawthrop 75 15 8 Stone Rigg 5 8 33 High Gill 7 14 28 Crag Wood Oliver Gill
000 82 18 14 87 19 4 50 4 5 12 77 20 10 10 28 83 88 5 20 80 10 Middleton KNDL 78 Low Fell 17 GSCL 4 20 8 5 31 Brown Beck Dent Fault 15 5 10 10 000 7 6 86 High Lathe Gill
4 8 GSCL 10 19 South Lord’s 5 CTG Land 5 6 AG 78 10 10 8 Rowell 60 17 Flinter Gill Gill area 13 56 10 5 10 10 44 48 20 44 5 Barkin Beck 000
85 N 4 63
GSCL 500 m
Key Geological boundary 78 Overturned strata; dip in degrees Carboniferous strata Trace of Dent Fault Silurian Strata structures Minor streams KNDL: Kendal Group AG: Alston Formation Other faults CTG: Coniston Group Anticline Roads TNTH: Tranearth Group Limestones Syncline Track Ordovician Strata GSCL: Great Scar Limestone Synclinal monoform Grid squares are 1 km DNT: Dent Group Group Horizontal strata 5 Inclined strata; dip in degrees
Thomas & Woodcock. Figure 4 WEST EAST section along northing grid line 89 River Lune Holme Knott River Dee Helmside Roantree BF HKF HGF DF ? a KNDL SG 0.5 KNDL DNT AF SL GSCL ? RG 0.5
section along northing grid line 87 River Lune Wrestle Gill Combe Top High Gill BF MF HKF HGF DF ? SG b KNDL 0.5 AF KNDL
DNT SL KNDL ? GSCL CTG TNTH RG 0.5
section along northing grid line 85 Low Southdale Hanging A683 Fellside Gill stone Barbondale CHF BF MF DF SG c KNDL KNDL AF 0.5
GSCL SL CTG TNTH RG 0.5 0.5
SL Km section along northing grid line 81 Casterton Fell Ease Gill BF Gale Beck DF SG d CTG AF 0.5 0.5
TNTH GSCL SL DNT RG 0.5 62 63 64 65 66 67 68 69 70 71 72
Easting grid line 1 km Ordovician & BF Barbon Fault Solid boundary Silurian Carboniferous CHF Casterton Hall Fault Fault Kendal Group (KNDL) Stainmore Formation (SG) DF Dent Fault Yoredale Group Coniston Group (CTG) Alston Formation (AG) FF Firbank Fault Tranearth Group (TNTH) Great Scar Limestone Formation (GSCL) HGF Helm Gill Fault Stockdale Group (STK) Ravenstonedale Group (RG) HKF Holme Knott Fault Dent Group (DNT) MF Millhouse Fault
Thomas & Woodcock Figure 5 366000 367000
GSCL 5 Lower Palaeozoic strata 27 AG west of the Dent Fault, 5562 33 undivided GSCL (see Figure 2) GSCL42
Dent Fault 65 20 AG
65 35 75 Barbondale 000 Dent Fault 83 4 29 32
Barkin Beck 32 Fell House 5
GSCL AG 22Blindbeck Bridge 6 Hazel Sike
61
Dent Fault 37 Great Aygill
7
000 32 38 82
4 75 10 Barbon 80 10 Low Fell 43 8 GSCL
AG
Bullpot Farm
5 Bull Pot of the Witches 10 82
Dent Fault 000 81 4 60
N 50 Cow Pot
8
5 500 m GSCL
Key Carboniferous strata Geological boundary Barkin Beck (succession youngs to east) Trace of Dent Fault (s.s) Minor streams structures; tick on AG: Alston Formation downthrown side Roads Other related faults Track Limestones Horizontal strata Path 5 Inclined strata; dip in degrees GSCL: Great Scar Limestone Grid squares are 1 km Group Major sink hole
Thomas & Woodcock. Figure 6 3 a 65
DENT d
FAULT
1
2
485 DENT FAULT e
b 3 BARBON FAULT BARBON
4
DENT FAULT
480
c f
5
South Craven Fault
6
North Craven Fault 475
poles to bedding in Ordovician poles to bedding in and Silurian rocks between the Carboniferous rocks east Barbon and Dent Faults of the Dent Fault
pole to bedding N mean bedding pole best-fit girdle to bedding poles
best-fit fold axis 0 1 2 kilometres
Thomas & Woodcock. Figure 7 stratigraphic throw (metres) Thomas & Woodcock Figure 8 1000 1400 west downthrow east downthrow 1200 800 600 400 400 600 800 200 200 0 75 SSE joins Barbon and Craven faults 80 palaeovalley fill? Conglomerate due toSedbergh throw underestimated Blindbeck overstep 85
Dentdale overstep 90 National Grid line(kilometres) National Grid
Clough
F
River e
l
l
E
n
d
S y 95
n D Taythes Gill c
e l
i
n n t e
River F a
a n
Rawthey u d
l t r e
l a
t 00
e
d
f
o
l d Stockdale s Faults 05 iferous depthwestoffault constraints onbaseCarbon-
Carboniferous graphic heightofsub- constraint onlybytopo- iferous horizonatoutcrop extrapolation fromCarbon- Argill Fault Argill 10 NNW 15 cut by Augill Fault N Sinistral
Dent Fault Segments; n = 39 Barbon Fault Segments; n = 4 North Craven Fault Segments; n = 5
Best-fit regional shortening Dextral direction during Variscan Compression (after Woodcock & Rickards, 2003a)
Thomas & Woodcock. Figure 9 1
3
2
Slickenline orientation
1 Orientation of principal axes Best estimate shortening direction: 13/113 (axis 3)
Bedding plane
Fault plane
Thomas & Woodcock. Figure 10 11a
330° - 345° Fault
Branthwaite
Rawthey Fault
Dent Fault
Variscan shortening Extensional direction Fault
Riedel Horst shears
Thrust
Extensional Faults
11b
View to NE A Barbon Low Fell
Rigid Askrigg Block Dent Fault A’ B’ C’ North Craven Fault South Craven Fault
C A Barbon Fault
B
Relationships Silurian strata squeezed conjectural up at tip where faults converge
B View N View NW View W A A’ B B’ C C’ Barbon Dent Barbon Dent S Craven N Craven Fault Fault Fault Fault Splay Fault Fault
Schematic cross-sections
Thomas & Woodcock. Figure 11 Relatively rigid Block: Lower Palaeozoic rocks with exisiting ~ E-W Acadian structural grain
Rawthey Fault Variscan
Dent Fault shortening Lower Palaeozoic Block direction translated south relative Taythes to Askrigg Block Anticline
Sedbergh Fault
Barbon Fault
SINISTRAL
Askrigg Block Rigid, cored by granite
Dent Fault Neutral shear point
Craven Basin Variscan Deformation‘Front’ DEXTRAL South Craven Fault North Craven Fault
Shortening
of the Craven Basin under NNW-SSE compression
Deformable Craven Basin
Thomas & Woodcock. Figure 12 NN Devn Dent Line
Carb Fault Southern Kensgriff Firbank Fault Stockdale Disturbance ~ ~1010 kmkm
Lake District Rawthey Fault Askrigg Block A
Sedbergh Dent Fault Barbon Fault Fault B sinistral
N BF RF DF DL
dextral
Dent Fault NCFS
SCF Figure 2 Figure N Craven Fault
Hutton Best-fit Monocline shortening direction
N Craven Fault
Craven Mid Craven Fault Basin
Quernmore Fault
South Craven - Morley-Campsall Fault Zone
Orientation of major faults Schematic strike-slip strain ellipsoids for sinistral and and folds in NW England dextral strain induced by NNW-SSE shortening
Devn Orientation of folds,Carb σ1 Sinistral strain Ribblesdale Fold Belt Stockdale Disturbance R1 P Extensional σ Firbank Fault Thrusts 3 Orientation of R2 faults Dent & Barbon extensional faults X Faults Sedbergh Dent Fault X Fault
Orientation ofBarbon Fault R1Riedel faults R2
shears (dextral strain) Extensional Thrusts σ P Askrigg3 Block or R2 Riedel shears Folds R1
Dent Fault (sinistral strain) σ N Craven Fault 1 Orientation of R1Riedel shears (sinistral strain) σ1 Dextral strain or R2 Riedel shears South Craven Fault R2 N Craven Fault X Craven and Bowland Basins σ3 (dextral strain) Extensional Thrusts To Flamborough Head Fault Zone R1 faults Craven Mid Craven Fault
Reverse faults, P Faults Folds P thrusts Extensional
faults
South Craven - Morley-CampsallR1
Thrusts σ X Fault Zone 3 R2
σ1
Thomas & Woodcock. Figure 13