Chaotic breccia along the Dent Fault, NW England: implosion or collapse of a fault void?

Journal of the Geological Society

doi: 10.1144/0016-764905-067 2006, v.163; p431-446.Journal of the Geological Society

N.H. Woodcock, J.E. Omma and J.A.D. Dickson of a fault void?Chaotic breccia along the Dent Fault, NW England: implosion or collapse

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Journal of the Geological Society, London, Vol. 163, 2006, pp. 431–446. Printed in Great Britain.

431

Chaotic breccia along the Dent Fault, NW England: implosion or collapse of a

fault void?

N. H. WOODCOCK, J. E. OMMA & J. A. D. DICKSON

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

(e-mail: [email protected])

Abstract: A body of chaotic breccia along the reverse-oblique Dent Fault zone is ascribed to hanging-wall

collapse into persistent voids created by geometric mismatch of fault walls, although some implosion into

transient voids is a possibility. The breccia comprises a 20 m wide body of hanging-wall lithologies, with a

chaotic clast-supported fabric that contrasts with the fitted-fabric breccias typical of the Dent Fault damage

zone. The breccia body has crude bedding defined by clast shape and size contrasts. The void fill is cut by

Variscan fault strands, which, together with its ferroan calcite and barite cement, prove its late Carboniferous

rather than recent age. It is shown that any fault void, transient or persistent, had a smaller aperture than the

final width of the breccia body, and no more than 5 m; a span that can be supported to depths of 2 or 3 km.

However, cement zonation in the breccia fill suggests that the void opened in multiple increments, each of an

aperture compatible with the maximum displacement in any one event along the Dent Fault. The Dent Fault

example highlights the possible general importance of fault-void collapse but also the problems in

distinguishing it from implosion processes.

Many upper crustal fault zones contain, or are fringed by,

significant volumes of brecciated wall rock. Understanding the

nature and genesis of these breccias is important both economic-

ally and scientifically. Unsealed fault breccias provide permeabil-

ity that can allow both cross-fault and cross-stratal migration of

fluids, which may include economically important water, hydro-

carbons, or mineralizing hydrothermal fluids. Brecciated fault

damage zones preserve a rich historical record of seismic

faulting; a record that is yet to be fully studied and understood

(e.g. Sibson 1986, 1989; Roberts 1994; Cowan 1999; Mick-

lethwaite & Cox 2004; Woodcock et al. 2007).

Fault breccias are the coarsest member of the predominantly

random-fabric fault rock series that also includes gouge, catacla-

site and pseudotachylite (Sibson 1977; Passchier & Trouw 1996;

Snoke et al. 1998; Killick 2003). Precise non-genetic classifica-

tion of this fault rock series has proved problematic, with

continuing debate on the utility of fragment size, fragment-

matrix proportion, presence or absence of foliation, and particu-

larly of the distinction between cohesive and incohesive fault

breccias (e.g. Jebrak 1997). An alternative approach has been to

include fault breccias in genetic classifications, either of tectonic

breccias alone (Sibson 1977; Killick 2003), or as part of wider

schemes embracing hydrothermal, magmatic or karstic breccias

(e.g. Sillitoe 1985; Laznicka 1988; Genna et al. 1996; Jebrak

1997; Kosa et al. 2003). Of concern in this paper are breccias

with a mineral cement rather than fine-grained matrix between

the fragments, and with textures varying from a tight, fitted-

fabric geometry to more open and chaotic packing. Such fault

rocks are the implosion breccias of Sibson (1986), reflecting the

view that they form by ‘the sudden creation of void space and

fluid pressure differentials at dilational fault jogs during earth-

quake rupture propagation’ (Sibson 1986, p. 159). This implo-

sion, bursting or hydraulic fracture mechanism, first detailed by

Phillips (1972), is thought to form many fitted-fabric to chaotic

fault breccias, often cemented by ore-bearing hydrothermal fluids

(e.g. Sibson 1987; Pavlis et al. 1993; Jebrak 1997; Williams et

al. 2000; Clark & James 2003; Labaume et al. 2004). The

implosion hypothesis envisages that faulting-induced voids are

transient and filled coseismically by a dilated mass of rock

fragments. Another hypothesis, much less commonly invoked, is

that large fault voids might remain open after fault slip, and only

later be filled by downward and inward collapse of the fault

walls. Such collapse breccias were first proposed as the fill of

ore-bearing breccia pipes (MacDiarmid 1960; Mitcham 1974;

Park & MacDiarmid 1975) such as that in the Bristol Mine,

Nevada (Fig. 1). Other possible examples have been noted from

the Jebel Aouam Pb–Ag deposit, Morocco (Jebrak 1997) and

from the Cirotan gold deposit, West Java (Genna et al. 1996).

However, collapse breccias along faults are more commonly

thought to fill voids created by later solution-widening (Wilcox

et al. 1973; Kosa et al. 2003) rather than by primary mismatch

of fault walls.

In this paper, we describe a chaotic breccia body, along the

Dent Fault of NW England, that shows permissive though not

conclusive evidence for collapse into a fault void. The difficult

distinction of collapse from implosion breccias is discussed. We

suggest that collapse into open fault voids should be more

generally recognized along other faults in the top few kilometres

of the crust.

Geological setting of the Dent Fault

The Dent Fault forms the southeastern edge of the Lake District

Massif, NW England, faulting its deformed and metamorphosed

Lower Palaeozoic rocks against the flat-lying Carboniferous

cover to the Askrigg Block (Fig. 2a). A precursor to the fault

probably influenced deposition of Silurian turbidites (Rickards &

Woodcock 2005) and Lower Devonian continental sediments

(Soper & Woodcock 2003). It then separated panels of Acadian

(mid-Devonian) folding (Woodcock & Soper 2006), before acting

as a transfer fault during Early Carboniferous extension (Under-

hill et al. 1988). However, the main displacements at the present

exposure level are Variscan (late Carboniferous, 300–290 Ma) in

age, one strand of the fault being unconformably overlain by

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Permian sediments 12 km north of the study area (British

Geological Survey 1997).

The Dent Fault is one of a grid of reactivated basement faults

in the foreland to the Variscan Orogen, which lies some 300 km

further south (Fig. 2a and b). The Dent Fault itself is about

30 km long, but some of its displacement is taken up north-

eastward in a 15 km long monoclinal fold belt, the Dent Line

(Underhill et al. 1988). Similarly, the Dent Fault may be linked

southwestward with the Hutton Monocline (Turner 1935; Aitken-

head et al. 2002). Variscan shortening across the Dent system

was NNW–SSE, parallel to that in the main orogen (Fig. 2a;

Woodcock & Rickards 2003). This shortening vector resulted in

sinistral reverse displacement across the Dent Fault zone. There

was nearly 1700 m of missing Carboniferous cover above the

Dent Fault zone at the time of its Variscan activity. This estimate

is obtained by adding the locally measured thickness of the

Wensleydale Group (460 m) to that of the overlying Stainmore

Group (380 m) and Coal Measures Group (300 m exposed,

500 m missing) to the NE (Institute of Geological Sciences 1974;

British Geological Survey 1997).

The chaotic breccia to be described here occurs about halfway

along the Dent system, where Taythes Gill crosses the fault zone

[SD 709 952]. Previous records of the breccia have described it

merely as a fault breccia (Dakyns et al. 1891; Underhill et al.

1988). Along this segment of the Dent Fault, its main strands dip

steeply west-northwestward and cut the overturned limb of a

Variscan monocline, the sense of which is in sympathy with the

reverse displacement on the faults (Fig. 2c and d). The synclinal

element of the monocline returns the footwall Carboniferous

rocks to horizontal in a few hundred metres east of the fault.

More complex domal outcrop patterns occur in the hanging wall,

as a result of interference of the anticlinal element of the

monocline with pre-existing east–west Acadian (400–390 Ma)

folds. The hanging wall is also cut by steep faults forming a

duplex geometry in map view (Fig. 2c). Woodcock & Rickards

(2003) reported that these duplex faults took up much of the

Variscan strike-slip, with most of the dip-slip being partitioned

onto the main Dent Faults. Along the Taythes Gill transect, the

dip-slip component totalled across the faults and monocline is at

least 1000 m, although the displacement on the Dent Fault itself

is probably less than half this. The strike-slip component across

the Dent zone is probably of the same kilometre-scale order but

is poorly constrained. The Taythes Gill locality lies at a point

where the Dent Fault zone branches southwards into a number of

strands (Fig. 2c), a geometry possibly responsible for the

volumetric incompatibility and void formation here.

Dent fault breccia characteristics

The faults in this central segment of the Dent zone typically

comprise 1–10 cm thick fault cores, marked by cataclasite or

gouge, and hosting most of the displacement, fringed by 10–

100 m thick damage zones made up of fault breccias (terminol-

ogy after Caine et al. 1996; Tarasewicz et al. 2005). Field

observation of the textures and cements of these breccias has

been augmented by study of stained thin sections and acetate

peels. The stain of combined alizarin red-S and potassium

ferricyanide permits distinction of calcite from dolomite and

from their ferroan variants (Dickson 1965), allowing cement

histories to be derived.

Texturally, the damage zone breccias comprise a randomly

oriented fracture mesh defining highly angular fragments with a

fitted-fabric texture (Fig. 3a and c). The fracture mesh is

dilational, occupies 10–15% of the rock volume, and is sealed

with crystalline carbonate cement rather than matrix. Veins,

showing some degree of preferred orientation, may predate or

post-date the main brecciation. The dilation breccias probably

formed during the seismic phases of movement on the Dent Fault

strands, but many were resealed in the interseismic phase

between successive earthquake events (Tarasewicz et al. 2005;

Woodcock et al. 2007). This reseal seems to have strengthened

the fractured rock, so that rebrecciation of the same volume is

uncommon, except where repeated dilational strains are focused

at fault bends or jogs.

The common dilation breccia textures from along the Dent

Fault contrast strongly with the chaotic breccias from Taythes

Gill (Fig. 3b and d). In these breccias, the simple fitted-fabric

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Fig. 1. (a) Isometric drawing of a mineralized breccia pipe, Bristol Mine,

Nevada; (b, c) schematic development of voids by displacement across

an irregular fault surface. Both figures modified from Park &

MacDiarmid (1975, fig. 4-9).

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textures are lost and more evolved breccias predominate, with

interclast porosity typically reaching 20–30%. Clast size varies

widely, but is mostly in the range 10–100 mm. Clasts have been

separated and redistributed, so as to have no obvious match with

adjacent clasts. Although all the clasts seem to be of dolostone,

subtle lithological contrasts also emphasize the mismatch be-

tween adjacent clasts. Some clasts have lost their extreme

angularity, but only to become subangular; redistribution of clasts

clearly did not involve more than local transport and attrition.

The pores in these evolved breccias sporadically contain fine-

grained matrix, but most pore space is filled with cement.

Cement composition is more varied than in the dilation breccias,

with pink barite conspicuous in the field in addition to abundant

carbonate. The cements will be detailed in a later section.

The spectrum of breccia from fitted-fabric dilation breccia to

more evolved types is not catered for by existing classifications

of either fault breccias or hydrothermal breccias. The non-genetic

fault breccia schemes (e.g. Sibson 1977; Snoke et al. 1998;

Killick 2003) subdivide breccias only on fragment size and

whether or not they are cohesive. Schemes for hydrothermal

breccias (e.g. Laznicka 1988; Jebrak 1997) favour genetic classes

with complex and typically overlapping geometrical criteria. The

dilation breccia spectrum is, significantly, covered better by

schemes to classify cave breccias (Fig. 4a: Loucks 1999; Loucks

et al. 2004) using the predominance of fractures, clasts, and

interclast sediment. In this scheme, the Taythes Gill fitted-fabric

breccias would be crackle or mosaic breccias, and the more

evolved breccias would be chaotic breccias, with small amounts

of matrix sediment content. Faced with both tectonic and karstic

breccias along solution-widened faults, Kosa et al. (2003) used

the terms tight, loose and chaotic for the tectonic equivalents

respectively of crackle, mosaic and chaotic.

A modified version of Loucks’ diagram is proposed here (Fig.

4b), based rigorously on the proportion of fragments and the

ratio of cement to matrix. For a rock with negligible remaining

porosity, the corners of the new triangular diagram are effectively

fragments, cement and matrix. It is beyond the scope of the

present paper to develop the potential of this diagram much

further. However, we note that real examples of fault-related

breccia can be found over much of this triangular field. There are

no obvious gaps in breccia distribution, as proposed by Loucks

(1999) for cave breccias. Even extreme cement-supported brec-

cias can be generated by polyphase tectonic brecciation, and

indeed veins plot at or near the cement corner. Gouge and

Fig. 2. Geological setting of Dent Fault (a) within NW England and (b) within Britain and Ireland. (c) Geological map of the central part of the Dent

Fault zone. (d) Cross-section across the fault zone near Taythes Gill. Modified from Woodcock & Rickards (2003).

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Fig. 3. Field photographs (a, b) and simplified sketches (c, d) of the contrasting textures of normal dilation breccia of the Dent Fault damage zone and the

chaotic breccia of the proposed void fill. Photograph (a) is taken perpendicular to relict bedding in the Penny Farm Gill Dolomite, in the hanging wall to

the void fill in Taythes Gill. Photograph (b) is taken within the middle part of the void fill: locations are shown in Figure 6.

Fig. 4. (a) Classification scheme for breccias in cave systems (Loucks 1999). (b) A more quantitative version of the same scheme based on the percentage

of fragments, and the relative proportion of cement and matrix. Veins, gouge and cataclasite can be plotted on this diagram. Terms in italics apply only to

cave collapse settings.



cataclasite both plot towards the matrix corner, although the

genetic implications of these terms compared with cave sediment

is evident.

Cave breccias originate as the roof and walls of a cave

passage, typically produced by limestone solution, progressively

collapse onto the passage floor (Loucks 1999). The resemblance

of the Taythes chaotic breccias to these collapse breccias cannot,

of course, be taken as proof that voids along the Dent Fault zone

filled in this way. However, it prompts consideration of two

competing hypotheses for the origin of the Taythes body: was it

due to coseismic implosion of fault walls into a transient void, or

to gravity collapse into a more persistent void? In this paper,

these hypotheses will be tested against broader evidence: the

overall structure of the fault void and its fill, and the fine texture

and cementation history of this fill. In considering the unconven-

tional collapse origin for the breccia, several subsidiary questions

also need to be answered.

(1) Could the chaotic breccias merely fill a recent solution

cavity related to the widespread Holocene glaciokarst (Waltham

et al. 1996) rather than to Variscan faulting?

(2) If the chaotic breccias are of broadly Variscan age, was the

void that they fill produced rapidly by fault displacement, by

mismatch of fault walls, or more slowly by subsequent solution

along the fault zone?

(3) In either case, could an open void, like the Taythes

example, have been supported at its likely formation depth

during Variscan deformation?

The Dent Fault zone in Taythes Gill

Carboniferous footwall to the chaotic breccia

The Dent Fault zone in Taythes Gill is about 200 m wide (Fig.

5a). It is flanked to the SE by the sandstones, shales and

limestones of the Wensleydale Group, steeply dipping and over-

turned in the central limb of the monocline associated with the

Dent zone. The Wensleydale Group is returned to horizontal

across the Fell End Syncline, some 400 m east of the fault zone.

A plot of bedding poles on a transect across the Wensleydale

Group (Fig. 5e) shows their simple dispersion about a subhor-

izontal fold axis trending about 0258. This folding kinematically

reflects the west-up dip-slip component across the Dent Fault

zone.

Followed northwestward, down its inverted stratigraphy to-

wards the Dent Fault, the Wensleydale Group includes three

limestone units, taken to be the Simonstone, Hardraw Scar and

Hawes limestones (Dakyns et al. 1891). The most conspicuous of

these, the Hawes Limestone, forms a coherent slab dipping NW

at 50–708 (Fig. 5d). A few metres of sandstone and shale then

separate the Hawes Limestone from first a matrix-rich fault

breccia and then a zone containing isolated exposures of the

Great Scar Limestone Group (Fig. 6). Bedding in these exposures

is gently rotated with respect to the Hawes Limestone, suggest-

ing, together with the discontinuous nature of the exposures, that

the Great Scar unit is structurally disrupted here adjacent to the

bedding-parallel fault strand to its east. This disrupted zone then

forms the footwall to the chaotic breccia body, which occupies

the next 20 m across strike (Figs 5a and 6) and which has a near-

random orientation of larger bedded fragments (Fig. 5c).

Carboniferous hanging wall to the chaotic breccia

The hanging wall to the chaotic breccia body is formed by

medium-bedded dolostones with thin shale partings, part of the

Penny Farm Gill Formation (Figs 5a and 6). This panel of

dolostones is pervasively brecciated, but with fitted-fabric,

crackle to mosaic, textures in which clasts have not been

substantially redistributed (Fig. 3a). The brecciation is typical of

the dilation breccia described widely from the Dent zone

(Tarasewicz et al. 2005) and ascribed to coseismic or postseismic

brittle fracture of the fault damage zone (Woodcock et al. 2007).

Despite the brecciation, bedding in the dolostone units is still

visible (Figs 5a and 6). Although the bedding orientation is

comparable with that in the footwall, both strike and dip are

more variable. In particular, in southwestern exposures of the

hanging wall, the dip steepens and the strike swings from SW to

south and even to SE. A stereoplot (Fig. 5b) shows bedding poles

dispersed about a moderately plunging NW-trending fold axis,

probably a response to the strike-slip component of displacement

on the Dent Fault. The bedding in this fold is strongly truncated

at the margin of the fault-void breccia body, showing that this

margin has its origin as a fault surface

Lower Palaeozoic NW of the Dent Fault

Followed across strike to the NW, the Penny Farm Gill dolos-

tones are interbedded with a sandstone bed, and then become

increasingly fractured and poorly exposed before being faulted

against cleaved and shattered mudstones of the Browgill Forma-

tion (Llandovery, lower Silurian). This fault is the main strand of

the Dent zone, striking about 0308 and with steep but uncertain

dip. Further fault strands introduce first a sliver of black shales

of the Skelgill Formation (Llandovery), then the Ashgill (upper

Ordovician) Cautley Mudstone Formation (Fig. 5a). Against the

fault zone, the Cautley bedding is NW-dipping and overturned in

the monocline steep limb. To the NW, bedding reverts to right

way up then folds over the Taythes Anticline. However, com-

pared with Carboniferous rocks, bedding patterns are more

dispersed in these Lower Palaeozoic units as a result of

interference of the Variscan monocline with earlier Acadian

structures.

Structure of the Taythes chaotic breccia

Margins of the breccia body

The volume of the Dent Fault zone occupied by chaotic breccia

is enlarged in map and cross-sectional views in Figure 7. The

NW bank of Taythes Gill is well enough exposed (Fig. 6) to

show that both the footwall and hanging wall to the proposed

void-fill continue for some 30 m along strike. There, two minor

SE-dipping reverse faults displace the footwall limestone over

the breccia body, reducing the width of the void fill (Figs 6 and

7a). The upper fault clearly cuts the breccia body as a thin shear

zone in which clasts are realigned. This phase of reverse faulting,

presumably Variscan, therefore post-dated the formation of the

chaotic breccias. Reverse faults with smaller displacements also

cut the hanging-wall contact and adjacent chaotic breccia else-

where on the north bank of Taythes Gill (Figs 6 and 8a, b).

The plane of contact of the chaotic breccia with its margin is

mostly unremarkable. However, at a few places, this contact

preserves slickenfibres showing sinistral strike-slip. This evi-

dence is compatible with the void margins being primarily faults,

although fault reactivation of karstic solution surfaces cannot be

ruled out. Some other present-day surfaces in the Great Scar

Limestone, not at the void margin, show horizontal solution

ledges and vertical solution pipes with a veneer of subangular to

subrounded cemented limestone gravel (Fig. 8c). These features



are interpreted as products of the Quaternary, post-glacial down-

cutting of Taythes Gill and match identical features being

actively formed in the stream bed.

The SW side of the valley is poorly exposed and here the

course of the void walls is less certain. However, an exposure of

coherently bedded dolostone south of the stream forms a lateral

wall to the void fill, and this dolostone is postulated to occupy

most of the space between footwall and hanging wall in this area

(Fig. 7a). Bedding character within this wedge suggests that it is

a detached and strongly rotated part of the hanging-wall

dolostones, now isolated within the same fault sliver as the void-

fill breccia.

As delimited in Figure 7a, the chaotic breccia body measures

about 20 m across strike, at least 60 m along strike and 15 m in

height, a minimum volume of about 18 000 m3.

Internal structure of the chaotic breccia

At first sight, the chaotic breccia body appears internally

disorganized at all scales. However, three subtle internal struc-

tures can be discerned. First, there are two places where slabs of

relict-bedded dolostones, one 7 m and the other 15 m long, lie

surrounded by breccia (Fig. 7a). The disrupted bedding in the

slabs parallels that in the hanging-wall dolostones above, as if

Fig. 5. (a) Map of Taythes Gill locality; (b–e) lower hemisphere equal-area stereonets of bedding poles from Carboniferous rocks in four separate

subareas. Open circles, squares and diamonds are respectively the first, second and third eigenvectors of the bedding pole distribution.



they are slabs locally detached from this hanging wall. Second,

there are places where crude bedding is defined by variations in

clast size and orientation (Fig. 6b). This bedding dips in the

same direction as, but slightly less steeply than, the bedding in

the footwall limestones. At the roof of the breccia body, the

bedding can be seen to evolve from progressive breakdown of

beds in the hanging-wall dolostones (Fig. 8d). Third, there are

variations in cement composition, conspicuous even in the

outcrop. In particular, pink barite cement occurs in a dipping

zone (Fig. 7a and b) that crudely parallels the breccia bedding.

Whatever its structural significance, the mere occurrence of

barite cement is important. The barite is almost certainly the

product of the well-constrained latest Carboniferous or early

Permian mineralization of the northern Pennines (Dunham &

Wilson 1985). It is thought to have formed at temperatures

between 50 8C and 130 8C (Sawkins 1966) precluding a Quatern-

ary origin for the chaotic breccia.

Cementation of the breccia and margins

Tarasewicz et al. (2005) found that the damage zone breccias

along the Dent Fault typically preserve a cement sequence from

early calcite to main-phase dolomite to late ferroan calcite. This

sequence provides a useful benchmark for comparing the ce-

ments of the chaotic breccia body and its margins.

In thin sections and peels, the limestones of the Great Scar

and Wensleydale groups in the void footwall are confirmed as

calcitic limestones with little brecciation, but with some ferroan

calcite veining, presumably formed late in the fault zone history.

An even later episode of ferroan dolomite veining is present

locally, such as in a sandstone bed within the Wensleydale Group

and in the bedding-parallel zone of fault breccia a few metres

into the void footwall.

By contrast to the footwall, the dolostones in the hanging wall

show pervasive crackle breccias mostly sealed by dolomite

cement (Fig. 9a). This dolomite is locally post-dated by saddle

dolomite, often with a limonite rim. A later phase of ferroan

calcite cement and veining typically shows compositional band-

ing caused by varying amounts of iron. This cement phase, late

in the regional history, is clearly associated (Fig. 9a) with the

breakdown of the crackle breccias, through mosaic breccias into

the chaotic breccias of the void fill.

The fragments in the chaotic breccia are formed of the

dolomite-cemented crackle breccia derived from the hanging

wall. However, the dominant interclast cement is the late phase

ferroan calcite, often compositionally zoned (Fig. 9b). Not all the

late cements are ferroan calcite, however. Barite cements (Fig.

9c) dominate in the discrete 10 m thick zone that parallels the

crude bedding in the void fill (Fig. 7a and b). The barite and

ferroan calcite cements seem spatially exclusive, and their

relative age has not been determined. Barite has not been

observed in the crackle breccias along the Dent Fault damage

zone, and its concentration in the chaotic breccia suggests a

correlation with the site, timing and perhaps the formation

mechanism of this breccia.

Two important observations link all the cements in the fault

zone in Taythes Gill. First, there are no pendant or meniscus

cements suggestive of vadose cementation. All cements were

apparently deposited below the water table, an observation

inconsistent with cementation of the chaotic breccia at the

present Quaternary erosion level. The second observation is that

all the cements grew as granular aggregates into open porosity

rather than as fibrous growths that kept pace with incremental

Fig. 6. (a) Panorama of the north bank of Taythes Gil; (b) location of the main geological features around the chaotic breccia. Dashed features are only

approximately located. Ringed numbers are locations of photographs in other figures.



opening of breccia pore space. Woodcock et al. (2007) have

argued in detail that such granular fills in tectonic dilation

breccias imply their rapid fragmentation during the seismic

phases of the earthquake cycle, followed by slower interseismic

reseal. The same argument applies to the chaotic breccia,

although with the additional possibility that some or all of the

fragmentation happened by interseismic void collapse rather than

by coseismic fragmentation.

Fig. 7. (a) Detailed map of the void fill. (b) Inferred cross-section of void fill along the line XX9 shown on the map.



Hypotheses for chaotic breccia formation

In this section, we consider possible formation mechanisms for

the Taythes chaotic breccia. Some mechanisms are disproved by

the field evidence, but implosion into a transient fault void or

collapse into a persistent void both remain possibilities. These

two mechanisms are compared, and a case is made for favouring

the collapse hypothesis for the Taythes example. The difficulty in

distinguishing the products of these two mechanisms highlights

the continuum of processes operating in natural fault zones.

Fill of a Quaternary glaciokarstic solution cavity? In NW

England, thousands of solution hollows or dolines, locally known

as shake-holes or swallow-holes, have formed where a veneer of

Pleistocene till overlies solution-widened fissures in Carbonifer-

ous limestone (Waltham et al. 1996). The occurrence of recent

solution pipes and ledges at various points on the study outcrop

prompts the hypothesis that the Taythes breccia is the infill of a

large doline. This possibility is discounted by: (1) the monomict

clast content, lacking locally available lithologies other than

those from the void hanging wall; (2) the scarcity of fine-grained

matrix in the breccia introduced from the overlying soil profile;

(3) the presence of a barite cement to the breccia, known only

from the late Carboniferous–early Permian mineralization event

(Dunham & Wilson 1985); (4) the ubiquitous phreatic cements;

(5) reverse faults, presumably Variscan, that post-date the breccia

fill.

Fill of a Variscan karstic solution cavity? Accepting the Variscan

age for the chaotic breccia, could it result from collapse of a

void produced by solution along a pre-existing fault weakness

rather than primarily by faulting itself? A solution void might

preserve, at its margins, evidence of fluted or pitted solution

surfaces or of speleothem deposits. No unambiguous evidence

has been found of such features either at the margins of the

Taythes void or in the breccia into which such evidence may

have collapsed. However, a solution component to void-widening

is difficult to discount entirely, even if the primary void was

formed by displacement of mismatched walls.

Tectonic attritional fault breccia? Random-fabric fault rocks,

including breccia, cataclasite and gouge, can form by cataclastic

breakdown of wall rocks, the shearing off of asperities on all

scales and progressive rock fragmentation and comminution (e.g.

Sibson 1977; Snoke et al. 1998; Watterson 1998) However, this

origin seems to be precluded for the Taythes body by: (1) the

presence of angular rather than rounded, abraded fragments; (2)

the predominance of interclast cement rather than fine-grained

comminuted matrix; (3) the lack of polyphase rebrecciation and

reseal of the same rock volume; (4) the absence of internal shear

surfaces within the breccia body.

Implosion brecciation into a transient fault void? Fault displace-

ment across dilational fault jogs and some fault branches can

Fig. 8. Field photographs from the Taythes chaotic breccia body: locations are shown in Figure 6. (a) Reverse fault cutting breccia–hanging-wall contact;

(b) slickensided surface on small fault cutting from footwall Great Scar Limestone into chaotic breccia; (c) recent solution pipe in footwall Great Scar

Limestone; (d) bedded Penny Farm Gill Dolomite passing down into chaotic breccia.



create a transient zone of lowered normal stress and pore fluid

pressure into which wall rocks can implode. In water-saturated

rocks, this tensile failure occurs when the transient pore pressure

difference between the void and the wall rock exceeds the tensile

strength of the rock (Sibson 1986). Sibson calculated that, for

initial hydrostatic pore pressures, hydraulic implosion is possible

at any depth greater than about 0.5 km. This constraint is

compatible with implosion at the estimated 1.7 km depth on the

Dent Fault, although strong enough wall rocks might resist such

implosion.

A check list of morphological characteristics (Table 1) ex-

pected from implosion breccias has been derived particularly

from the descriptions of Sibson (1986, 1987) and Jebrak (1997).

The Taythes example matches many of these criteria, the main

exception being that the chaotic breccia shows higher dilation

and clast rotation than is characteristic of implosion breccias.

The genetic significance of breccia texture is crucial yet

debatable. Sibson (1986) noted that ‘a frequently observed

feature of such [implosion] breccias is that they possess an

exploded-jigsaw texture with the clasts showing little evidence of

frictional attrition.’. Jebrak (1997) observed that ‘fluid assisted

brecciation generates in situ fragmentation textures (mosaic

breccias) in a jigsaw puzzle pattern without significant rotation

of the fragments, although rotation can often be observed in

critical brecciation [implosion] because the fragments generally

collapse immediately following the fragmentation.. . . An absence

of rotation indicates that critical brecciation did not occur

extensively, and that the dilation process in the vein was a

transient phenomenon with a limited amount of open space.’

These last quotations clearly highlight the expected continuum of

process between implosion and collapse in breccia formation,

and consequently a difficulty in always discriminating between

the two.

An important constraint on the interpretation of the Taythes

chaotic breccia is the evidence for only very limited rebreccia-

tion of each breccia volume. Most fragments comprise dolostone

that has suffered the dolomite-cemented crackle brecciation

common to much of the Dent zone (Tarasewicz et al. 2005).

However, once incorporated into the chaotic breccia, these

fragments and their new ferroan calcite or barite cement have

not been refractured. The high dilation ratio in the chaotic

breccias therefore arose in one implosion or collapse episode and

not by multiple phases of brecciation. An instructive contrast

with the Taythes breccia is provided by a body of polyphase

tectonic breccia (Fig. 9d) from Dovecote Gill 3 km further south

along the Dent Fault (location shown in Fig. 2c). This body

shows multiple phases of brecciation and reseal by varying

compositions of ferroan calcite cement, textures conspicuously

absent from the Taythes chaotic breccia. The Dovecote breccia is

interpreted as the result of repeated brecciation and veining by

Fig. 9. Cement characteristics. (a) Dolostone crackle breccia (top) passing down into mosaic breccia cemented by zoned ferroan calcite (Penny Farm Gill

Dolomite hanging wall). (b) Chaotic breccia cemented by zoned ferroan calcite. (c) Chaotic breccia cemented by barite. (d) Polyphase fault breccia from

the Dent Fault 3.5 km south of Taythes Gill (Dovecote Gill, Fig. 2b).



implosion localized at a dilational releasing bend on the Dent

Fault (Woodcock et al. 2007). The Taythes chaotic breccia, if

formed by implosion at all, must therefore reflect lower volu-

metric strain and less prolonged activity.

Collapse into a persistent fault void? The final alternative is that

the chaotic breccia results from collapse of the hanging wall of

one or more persistent fault voids produced by geometric

mismatch between opposing fault walls. The voids were pro-

duced below the water table and possibly as deep as 1700 m in

the fault zone. The collapse breccias were cemented by minerals

from hydrothermal fluids that exploited the new but transient

permeability created in the fault zone.

A comparison of characteristics of the Taythes chaotic breccia

with those to be expected in collapse breccias (Table 1) reveals

some important matches. The tendency for fragments to be

subangular rather than completely angular is compatible with

their corners being abraded during the impacts associated with

collapse. The presence of a weak clast fabric and bedding, at a

plausible angle of rest for scree debris yet at an angle to the fault

walls, is most easily explained by the collapse hypothesis. The

lower brecciation state in the void footwall is also more

compatible with collapse than implosion. However, the Taythes

breccia lacks several features that would be conclusive proof of

persistent voids. In particular, Genna et al. (1996) recorded

speleothem deposits, particularly stalactites and stalagmites, and

‘cockade’ cements formed in relict void space above collapse

breccias in the Cirotan gold system, West Java. The stalactites

and stalagmites at least would not be expected in the phreatic

conditions of the Taythes void, and their absence is therefore not

significant in this example.

On balance, we favour the fault-void collapse model for the

Taythes deposits. This model is explored in more detail in the

next section, but always with the possibility in mind that some of

the void space was transient not persistent between fault slip

events. The hypothesis raises a number of supplementary ques-

tions. How was such a void formed, when not at a conspicuous

releasing bend in the Dent system? Was the void ever the full

20 m width of its breccia fill? If not, what was the incremental

history of void forming and filling? Could open voids of the

implied size exist under the deduced overburden?

Model for fault-void formation and infill

Void formation

Formation of voids, or potential voids, along an irregular fault

plane is inevitable, as any displacement causes geometric

mismatch between fault walls. This principle has long been

recognized as an important control on the width of individual

hydrothermal mineral veins (e.g. Newhouse 1940) and of vein

networks or hydrothermal breccia zones (Sibson 1987). The

demonstration that fault roughness is self-similar on scales,

measured along the fault, from millimetres to kilometres (Brown

& Bruhn 1996) implies that large potential voids are to be

expected on natural faults. At the confining stresses present at

depth in the brittle upper crust, such voids are likely to implode

as fast as they form (Phillips 1972). However, data from cave

systems at shallow crustal depths show that open voids up to

about 8 m in height or width are common near the surface and

may survive to depths of 2 or 3 km (Loucks 1999). Voids are

infrequently more than 8 m and rarely more than 30 m in size.

That large voids can survive in the upper crust is most directly

proved by deep mines in South Africa penetrating to more than

3.5 km depth.

There is insufficient local kinematic evidence to decide

whether the Taythes void was formed by strike-slip or dip-slip

displacement, or by a combination of the two. A strike-slip

component (sinistral on regional kinematic evidence) is compa-

tible with the steeply plunging fold in the hanging-wall dolomite

succession (Fig. 5b). However, the weak bend in the Dent Fault

system south of Taythes Gill (Fig. 2c) would be transpressive in

sinistral strike-slip and it seems unlikely that the voids opened as

a result of this geometry. The Dent Fault also branches into

several strands here (Fig. 2c), and the right combination of

relative displacement on these strands could produce dilation

near the branch point, in the manner well known in other strike-

slip fault systems (Crowell 1974; Christie-Blick & Biddle 1985).

Alternatively, the void could have formed by reverse dip-slip

displacement across a jog, as seen in cross-section (Fig. 10).

Given the evidence (Woodcock & Rickards 2003) that dip-slip

displacements were preferentially partitioned onto the Dent Fault,

this is the geometry explored here, despite the lack of any field

evidence for the void shape at depth.

Table 1. Comparison of expected characteristics of implosion and collapse breccias along faults

Hydraulic implosion breccias Gravity collapse breccias

Breccia texture Crackle to mosaic Mosaic to chaoticFragment rotation Low to moderate; fitted-fabric typically preserved Moderate to high; fitted-fabric lostDilation Low to moderate Moderate to highFragment shape Angular Angular to subangularFragment fabric Random Random to weakly oriented or beddedFragment composition Local wall rock; transport , void width Mainly local wall rock; transport may be . void widthExotic material No exotic fragments or matrix Possible exotic fragments or matrix if fault void provides

cross-stratal connectivityInterfragment material Cement Mainly cement; possibly some fine-grained matrixFragment support Single-phase breccias are fragment-supported; possible

fragment separation through rebrecciationSingle-phase breccias are fragment-supported; rolledfragments may become cement-rimmed to give cockadebreccias

Hanging-wall features Possible speleothem, e.g. stalactites, grown into vadosecavities

Footwall features Footwall typically brecciated like hanging wall Footwall may be undeformed, or less deformed than hangingwall

Shared characters Decreasing dilation into hanging wall through mosaic andcrackle breccia; little internal clast deformation



Whatever the fault kinematics, void formation does depend on

adequate flexural strength of the footwall and, particularly, the

hanging-wall carbonates. Whereas the footwall limestones to the

Taythes breccia body are, with the exception of the discrete

bedding-parallel fault breccia (Fig. 7a and b) only weakly

brecciated, the extent of dilation brecciation in the hanging wall

might be expected to have compromised its strength and

precluded its support to a persistent void. However, more general

evidence from the dilation breccias along the Dent Fault

(Tarasewicz et al. 2005; Woodcock et al. 2007) suggests that

resealed dilation breccias are actually stronger, not weaker, than

the unbrecciated rock, making flexural support of voids more,

not less, plausible.

A schematic model for void formation and filling is shown

Fig. 10. Schematic history of sequential formation and infill of fault voids by hanging-wall collapse, shown in cross-section view. The void geometry

cannot be verified from the limited exposure of the Taythes void, but the infill architecture and cementation matches that observed. Voids can form in

strike-slip as well as dip-slip. Complex geometries might develop depending on how later faults cut through earlier breccias, and if coseismic implosion

contributed to void fill.



(Fig. 10) for reverse dip-slip displacement, but the same

principles apply whatever the kinematics of the fault. The initial

volume of the fault void (Fig. 10b) is shown as considerably less

than the volume of the resulting breccia body (Fig. 10e). Loucks

(1999) has highlighted the important fact that void collapse

necessarily produces a substantially larger volume of breccia

than the volume of the original void. If the wall rocks have zero

initial porosity and collapse one-dimensionally into a tabular

void, the width Wb of the breccia body is related to the width Wv

of the fault void by Wv ¼ �bWb, where �b is the porosity of the

collapse breccia. With �b ¼ 0:3, a 5 m aperture void would

generate the observed 16.5 m structural thickness (20 m width)

of chaotic breccia. This estimate applies equally well to implo-

sion breccias, where the void is transient rather than persistent.

However, a 5 m void aperture is comfortably within the limits

known to be stable in present cave systems to at least the 1700 m

depth at which the Taythes void was formed.

A further important consideration is that a 5 m aperture is

greater than the likely maximum displacement in a single earth-

quake on the Dent Fault. Even if the whole 30 km length of the

Dent Fault ruptured at once, then, using a displacement/length

ratio of about 10�4 (Wells & Coppersmith 1994; Scholz 2002),

the maximum displacement in any one event would be about

3 m. Allowing also that void aperture will, depending on the

local geometry at a jog or bend, be less than the fault displace-

ment, it is an unavoidable conclusion that the Taythes void must

have formed over a number of seismic events. As discussed

below, the evidence is certainly that it filled in several incre-

ments.

Void fill

The filling of a fault void is shown schematically in cross-

sectional view in Figure 10c-e. The hanging wall, relieved of the

confining stress at its free surface, weakens by gravity-induced

crackle brecciation, probably accelerated at times by minor

seismic activity. Sections of the hanging wall, from small

fragments to slabs some metres long, then fail under gravity,

falling through the water-filled void. The hanging-wall fragments

impact with the footwall below, fragment further, have some

corners and edges abraded, and move down dip to build sheets of

scree (Fig. 10c). These sheets naturally dip in the same direction

as the footwall but, at around 308, less than the average footwall

dip of about 558. The scree sheets build upwards through the

void, their toes abutting the lower edge of the hanging wall,

progressively inhibiting its further collapse. Hydrothermal flow

through the breccia sheets begins to cement the fragments.

Collapse continues into the remaining void space as it moves

upwards, a form of stoping (Fig. 10d). Eventually, the void is

completely choked with collapse breccia, more or less cemented,

which supports the crackle breccias of the hanging wall (Fig.

10e). At any time, it is possible that a new seismic event may

cause major implosion of the hanging wall, and total filling of

the void.

A new increment of void opening is shown in the schematic

model (Fig. 10f). The new fault strand is shown at the hanging

wall of the void fill, between the chaotic and crackle breccias, a

common site for slip in brecciated fault zones (e.g. Genna et al.

1996). The new void then fills and cements in the same way as

that from the first opening phase (Fig. 10g). The zonation of

cements in the Taythes breccia suggests a minimum of three

phases of opening. In the lower scree sheets, the cement was

ferroan calcite; the second phase marked a switch to barite-

depositing fluid, before a return to ferroan calcite cementation in

the third phase. It is important that, even with three slip

increments to the 5 m total opening, any one increment is of the

right order to be generated by the likely maximum seismic event

on the Dent Fault. In practice, a larger number of opening

increments was probably responsible for the void fill.

More complex opening histories are possible, both in theory

and in the Taythes example. For instance, the fault causing the

new voids need not have followed the margin of the chaotic

breccia (Fig. 10f): it could have cut through either the chaotic or

crackle breccias. In the first case, cemented chaotic breccias

would form the roof to the new void, and then be found as

redeposited fragments in its fill; not something observed in the

Taythes void. In the second case, slivers of crackle breccia would

be isolated in the footwall to the new void, then overlain by fresh

chaotic breccias. This is an alternative origin, other than roof-

fall, for the slabs of less brecciated dolostone now preserved

within the void fill (Fig. 7a and b).

Tarasewicz et al. (2005) showed that ferroan calcite was

deposited late in the brecciation and reseal history of Dent Fault

zone, and was preceded by dolomite cements and, earlier still, by

calcite. The predominance of ferroan calcite cements in the

chaotic breccia suggests that void formation was also late in the

regional deformation history. By this stage, a substantial over-

burden might have been eroded from the upthrown hanging wall

of the Dent Fault. The void may therefore have formed under a

lower overburden than the 1700 m of the total missing stratigra-

phy. After the void fill was complete, it was cut by the east-

dipping reverse faults, a yet later phase in the final shortening

across the Dent zone.

Discussion

The formation and filling of the Taythes void in a number of

increments implies that the aperture of the open void at any time

may have been only a fraction of the width of the final chaotic

breccia body. Indeed, as discussed above, the possibility cannot

be ruled out that some or all of these voids were transient and

filled coseimically by implosion of the hanging wall. We have

argued against this possibility for the Taythes void because of

the strong textural contrast between the chaotic breccia of the

void fill and the crackle or mosaic breccias of the hanging wall.

We also suggest that (1) the degree of clast disaggregation in the

chaotic breccia requires a void aperture at least as wide as its

largest clasts, 0.1 m or so, and (2) incremental fill and cementa-

tion even of voids of this size would produce a more pronounced

pseudo-bedding in the breccia than is observed. We therefore

suggest that void sizes in the Taythes body were of the order of

1 m rather than 0.1 m. However, there is no reason why different

scales of void fill should not dominate in other fault zones.

Importance of fault-void breccias

Surface-linked void fills

Although close analogues of the Taythes collapse breccia are

difficult to find in the literature, some related breccias are rather

commonly described. These are ‘fissure-fills’ thought to have

formed by downward percolation of unconsolidated sediment

along fault or joint planes, and often classed as ‘sedimentary

dykes’. Such fissure-fills are a common, almost ubiquitous,

feature of karst terrains, where the fissures are widened by near-

surface solution of limestones and filled by surface sediment

typically rich in the insoluble residues of the same dissolved

limestones (e.g. Ford & Williams 1989). More relevant here are



examples where the void formed by extension across a fault or

joint. This non-karstic origin is clear where the fissure formed in

a submarine setting, and was partly infilled by marine sediment.

Apposite examples occur in Carboniferous limestones in SW

England, cut by Triassic to early Jurassic extensional faults and

filled by contemporaneous sediment (Wall & Jenkyns 2004).

More extensively documented, however, are widespread examples

in the Triassic of the Tethyan Belt (Fuchtbauer & Richter 1983),

where transitions from crackle breccia to matrix-rich chaotic

breccia are common.

In general, breccias in fissures filled by surface sediment are

matrix-rich and often matrix-supported, with a high proportion of

clasts exotic to the wall-rock composition. By contrast, the

subsurface fault-void collapse breccias in the Taythes body are

mostly clast-supported, cement-rich and lacking in exotic clasts.

These criteria allow the distinction of voids with sedimentologi-

cal linkage to the Earth surface from those, like the Taythes void,

deep enough to have no sedimentological connection to the

surface. The critera should be generally applicable (Fig. 11).

Subsurface void fills

Amongst the few detailed descriptions of fault-related collapse

breccia, the closest geometric match to the Taythes deposit are

the ore-bearing breccias of the Cirotan gold deposit, West Java

(Genna et al. 1996). These breccias have volcanic fragments in a

quartz cement, in places in ‘cockade’ encrustations that dominate

the breccia fabric. Despite these compositional contrasts to the

Taythes example, the incremental void-fill mechanism suggested

for the Cirotan body is very similar to that deduced in this paper:

graded ‘scree’ sheets were produced within voids, the persistence

of which over decades or more is proved by stalagmites and

stalactites preserved at their hanging wall. Jebrak (1997) cited

other possible fault-related collapse breccias, particularly from

Jebel Auoam (Morocco) and Maine (France). Kosa et al. (2003)

described collapse breccias in low-displacement syndepositional

faults on the Capitan reef front, of west Texas and and New

Mexico, where the contribution of solution widening rather than

primary fault gape is difficult to determine.

Another variety of subsurface fault-void collapse is repre-

sented by that at Bristol silver mine, Nevada (Fig. 1), described

by MacDiarmid (1960; Park & MacDiarmid 1975). Such breccia

pipes (a term applied irrespective of their steep or gentle

inclination) are a very common host to chaotic breccias ce-

mented by hydrothermal minerals. However, despite their eco-

nomic importance and consequent intensive study, these breccias

are typically attributed to one of five other processes: (1)

implosion into incipient voids at dilational jogs or fault intersec-

tions (Phillips 1972; Sibson 1987); (2) hydraulic brecciation,

without need for fault displacement, by superheated steam

(Barrington & Kerr 1961), sometimes volcanically sourced

(Williams 1936); (3) collapse into epigenic voids, that is, those

dissolved by meteoric or marine water (Palmer 1991); (4)

aggressive hypogenic, that is, hydrothermal, solution of carbo-

nates, followed or accompanied by collapse of the resultant void,

a mechanism of mineralization stoping (Locke 1926); (5) one of

the interlinked magmatically mediated mechanisms for breccia

formation reviewed by Sillitoe (1985)

Numerous examples exist in the literature of breccia bodies

that might have a fault-void collapse component. Reviews of

Mississippi Valley type (MVT) lead–zinc deposits (Leach &

Sangster 1993) have emphasized the predominance of ore-

hosting in breccias assigned either to collapse of solution voids

or to fault-related processes: faults are often seen as localizing

later solution voids, but not as producing primary voids by wall

mismatch. An active example of this debate, in an area relevant

to the Taythes void, concerns the breccias in the MVT deposits

of the Irish Midlands (e.g. Hitzman et al. 1998; Lee & Wilkinson

2002), where some breccias are interpreted as forming by yet

another process, sedimentary debris flow. Examples of miner-

alized fault-related breccias abound in other mineralization

provinces. For instance, the gold–quartz deposit at Rain Mine,

Nevada (Williams et al. 2000) has collapse breccias into a

solution void along one of the ore-localizing faults.

We suggest that some of the examples in the large mining

literature would repay re-examination in the light of new

evidence that some voids may be primarily produced by

geometrical mismatch of fault walls. A good test of the primary

origin of some fault voids by mismatch of walls would be if they

can be verified in non-carbonate rocks, where solution voids

should be much less common. By comparison with the two-stage

process of epigenic or hypogenic solution along an earlier fault,

the fault-void collapse hypothesis then provides a simpler, one-

stage mechanism for void formation. The hypothesis therefore

has the potential to simplify the interpreted geological history of

some economically important areas, and to enhance the predic-

tive value of such histories in frontier areas.

Conclusions

(1) A distinctive body of chaotic breccia, 20 m wide and at least

18 000 m3 in volume, occurs within and parallel to the Dent Fault

zone, NW England. It contrasts in texture with less-evolved

crackle and mosaic breccias that formed in the rest of the fault

damage zone.

(2) The chaotic breccias are clast-supported, with a carbonate

or barite cement deposited below the water table. The subangular

clasts entirely comprise material from the hanging wall of the

breccia body.

(3) The breccia body has a crude bedding defined by clast

shapes and size contrasts, and a subparallel zonation in cement

type. The body is cut by Variscan fault strands; this feature,

Fig. 11. The breccia classification diagram from Figure 4b, with

suggested fields of surface-linked fissure fills and subsurface fault-void

collapse breccia lacking connectivity to surface sediment supply.



together with the barite cement, proves that the breccia is late

Carboniferous rather than recent in age.

(4) The chaotic breccia is thought to have formed by progres-

sive hanging-wall collapse into primary fault voids, formed by

geometrical mismatch of fault walls rather than by later solution.

(5) A case is made that these voids were persistent for some of

the tens to thousands of years between major earthquakes,

although the possibility cannot be ruled out that they were

transient and filled coseismically by implosion of wall rock.

(6) The initial fault voids had smaller apertures than the width

of the breccia body they generated. Apertures were certainly no

more than 5 m, an open span that is commonly observed in

recent cave systems to 2 or 3 km depth.

(7) The cement zonation of the breccia fill suggests that the

void opened in a minimum of three increments, each of an

aperture compatible with the likely 3 m maximum displacement

on the Dent Fault.

(8) Although fault fissures filled from the ground surface have

been commonly described, examples caused by subsurface

collapse of persistent rather than transient voids are less well

documented. The Taythes example prompts a re-examination of

chaotic breccias in fault zones for signs that fault-void collapse

may be a more common process than previously thought.

The 1:10 000 mapping of the Dent Fault zone by N.H.W. was partly done

under contract to the British Geological Survey (Contract GA/98E/44).

Detailed study of the chaotic breccias formed an undergraduate research

project by J.E.O. G. Foreman is thanked for assistance with rock

preparation. The manuscript was much improved by the suggestions of

the referees, S. Cox and C. Bonson.

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Received 10 May 2005; revised typescript accepted 1 September 2005.

Scientific editing by Tim Needham



Documents

Chaotic breccia along the Dent Fault, NW England: implosion or collapse of a fault void?