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The importance of shear zones in naturally deformed wet sediments
Tectonophysics,
163
145 (1988) 163-175
Elsevier Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
The importance of shear zones in naturally deformed wet sediments ALEX Department
J. MALTMAN
of Geology, University College of Wales, Aberystwyth, (Received
December
Dyfed SY23 3DB (Great Britain)
1, 1986; revised version accepted
March
31, 1987)
Abstract Maltman,
A.J.,
1988. The importance
of shear
zones
in naturally
deformed
wet sediments.
Tectonophysics,
145:
163-175. The production
of narrow
shear zones was recently
sediments
deform
in the laboratory.
sediments
deform,
that this mode of deformation
Examples
cores;
and in deformed
gravitationally-induced
conditions
disturbances
sediments such
shown
illustrates,
is of widespread
importance
to the experimentally
that are now lithified.
as slumping,
and
their
of recognizing possible
of the sediment
the natural
use as shear-sense
shear
zones
possible
are mentioned,
indicators,
a variety
by which wet argillaceous of circumstances
in which
in nature.
produced
features,
in glaciogenic
The latter examples
include
tectonically-formed
features
sediments
the results of both such
as occur
in
and
for example,
the possibility
that
their influence they
may
indicate
on dewatering the physical
deformation.
Introduction The deformation of wet argillaceous sediments in laboratory experiments has shown that they change shape, not by pervasive homogeneous flow - as has sometimes been surmised in the past, but by intense slippage within very narrow discrete zones of shear. These experimentally-produced shear zones have recently been described in detail (Maltman, 1987). Macroscopically, they are shiny, finely-grooved narrow zones of slip; microscopically they appear as approximately planar commonly complex zones of pronounced particle reorientation. The shear zone mode of deformation was found to be dominant in specimens tested under a range of physical conditions (e.g., strain rate, confining pressure, consolidation ratio) and with water contents between about 15% and 65’%, 0040-1951/88/$03.50
to be the chief mechanism by considering
complexes.
Implications behaviour,
article
are given of shear zones, highly similar
and D.S.D.P. accretionary
This
0 1988 Elsevier Science Publishers
B.V.
corresponding to a considerable range of sediment burial depths. This laboratory work prompts the obvious question of whether or not the shear zone process operates in nature. Although the experimental conditions were designed to reasonably simulate natural circumstances, it is, of course, possible that some critical factor has been overlooked and an artificial situation has been inadvertently created. The purpose of this article is to illustrate that the shear zone mode of deformation is indeed important in nature. Occurrences are noted here of structures, extremely similar to the experimental shear zones, in sediments which have been deformed naturally in a wide variety of circumstances. Some remarks are made about the geological implications of recognizing shear zones in natural sediments.
164
Shear zones in existing sediments Engineering
materials
shearing.
The structures
oriented
clay particles
surfaces,
which
those produced Shear zones of the kind produced in the experiments have been recognized for some time by civil engineers
interested
in soil mechanics.
technicat
behaviour
of sediments
gineering
parlance)
is greatly
presence
of mechanical
ties”, for example, p. 315). That
influenced
anisotropies
see Attewell
gineering
Hence
in enby the
(“discontinui-
shire (pers. commun.,
in
disturbed
“zones
glacial
of localized
all the structures
cate-
to displacement
context
to investigate
experimentally (references the engineering literature natural examples.
in the en-
the shear
zones
in Maltman, 1987) and contains references to
The instances reported by civil engineers from argillaceous sediments are direct analogues of those produced in the laboratory. Examples are the disturbed silty clays at the Cod Beck dam, England (compare Morgenstern and Tchalenko, 1967, fig. 1, with Maltman, 1987, fig. 4f); the mudflows at Sevenoaks, England (compare Morgenstern and Tchalenko, 1967, fig. 11, with Maltman, 1987, fig. 5d); and the landslide at Wahon’s Wood, England (compare Tchalenko, 1968, fig. 9, with Maltman, 1987, fig. 4~). All these examples are presumably the result of various kinds of gravitational slipping, but Skempton (1966) has described clays in a dam foundation at Mangla, Pakistan, in which the shear zones appear to be associated with recent tectonic folding of Plio-Pleistocene sediments. Because of the repercussions for the bulk mechanical properties of sediments, Skempton (1966, p. 335) regards the presence of such shear zones in engineering materials as ‘a matter of extreme significance in the design of engineering works. Glaciogenic sediments The presence of shear zones in recent sediments has received much less attention outside civil engineering. However, Derbyshire et al. (1985a, b) have reported the existence in glacial tills of polished shear surfaces, perhaps due to sub-glacial
very
similar
to
work. E. Derby-
1985) feels that such shear
and Van Loon (1983) have discussed
1976,
by Skempton
seem
and grooved
zones are common in disrupted tills, although they may only be visible ~croscopi~ally. Brodzikowski
and Farmer,
there have been efforts
therefore
in the laboratory
shear zones are an important
gory of these has been demonstrated (1964).
The geo-
(“soils”
are narrow zones of highly with polished
perimental (1976, shear
deposits, visco-plastic
them
flowage”.
as
Almost
they illustrate
appear
to be due
along localized
planes,
as the ex-
Davies
and Cave
work would
predict.
figs. 8a, 8b) illustrate zones
shear zones
viewing
in a glacial
what
clay from
appear
Machynlleth,
Wales, drawing attention to the restriction deformation to the narrow zones. The physical conditions at the glaciogenic sediments were deformed,
to be of the
time these as with the
engineering materials discussed earlier, may well differ from those of the submarine sediment burial that the laboratory experiments were meant to simulate; but the presence of shear zones in these materials indicates that this mode of deformation is of widespread occurrence. D. S.D. P. cores
The experimental investigations have shown that shear zones are important in argillaceous materials with water contents of up to at least 60%. Presumably at some higher value, perhaps 70% or so, this mode of deformation
is supplanted
by pervasive inter~anular slip. Thus subaqueous sediments which are presently very near the sediment-water interface, where it is conceivably possible to observe their deformation directly, will probably have too high a water content to develop shear zones. However, after normal water loss equivalent to burial of a few tens of metres, shear zones are likely to form on disturbance of the sediment, and should persist until lithification has progressed sufficiently to curb the necessary grain slippage. Hence the kinds of sediment depths typically encountered by cores of the Deep Sea Drilling Project (D.S.D.P.) are likely to be appropriate for shear zone production. So far only the D.S.D.P. cores from the forearc
165
areas of active plate margins have received detailed ~crost~~tur~ study (for example, Lundberg and Moore, 1981, 1986). Many of the microstructural features reported from such cores have similarities with the shear zones produced in experiment (compare, for example, &ripe, 1986, fig. 12, with Maltman, 1987, fig. Sd). Some structures are directly comparable. Figure IA shows a discrete shear zone in Miocene mud from the Japan Trench. This material, recovered from a sediment depth of about 760 m, lay upslope from the trench
Fig. 1. Photornicrographs
of shear zones in D.S.D.P.
and so may have been subject either to downslope slippage or to the contractions forces that are part of the accretion process. Whatever the cause of the movement, it has been accomplished by producing shear zones such as the one shown. This example compares well with some of those produced expe~ment~y (Maltm~, 1987, e.g., figs. 4a, b). In another part of the same D.S.D.P. sample, minor conjugate shear zones have developed (Fig. lB), analogous to some experimental specimens (Maltman, 1987, fig. 5d).
samples. A. Discrete shear zone in Miocene mud, Japan Trench.
B.
Closer-spaced shear zones with development of minor conjugate set. Same material as A. Both specimens photographed between crossed-nicols: matrix appears dark, highly-oriented clays within shear zones appear light. Sample 87-584-56-2-132-139; kindly provided by Neil Lundberg.
thin-sections
166
The laboratory work was confined to bulk strains of less than 30%, but led to the suggestion that with continuing strain additional shear zones would have to be generated (Maltman, 1987, p. 12). This is borne out in a D.S.D.P. sample from a zone of high shear strain, namely that in which Miocene muds are tectonically brought over Pliocene sediments at the deformation front of the Barbados ridge (Moore et al., 1982; Cowan et al., 1984). Figure 5 of the latter article shows an interweaving array of shear zones in the material at the junction. The thrusting of the sediments has apparently taken place by phyllosihcate sliding and reorientation within shear zone systems, rather than by fracture or pervasive gram sliding. Shear zones in se&rents
now li~fi~
Sediments deformed by ~~urn~i~g
It follows from the thesis being considered here that sediments which were deformed in the geological past before they were lithified may preserve the shear zones produced at the time of disturbance. Among the best described rock sequences which were un~uiv~ally disturbed before lithification is the Namurian of County Clare, West Ireland (Gill, 1979). Some of the materials in that sequence have disaggregated in various degrees, implying movement while still very waterrich, while others have moved as generally intact sheets. The latter contain many examples of the kind of shear zones produced in the experiments, such that they must have played an important part in the translation of the sediment. A good example is provided by the slump sheet at Fisherstreet Bay, Co. Clare. Much of the sediment is argillaceous and comprises a sliding mass at least 20 m thick (Gill, 1979) so that the ccmditions at the time of movement would almost certainly fall within the range of the experiments. Gill remarked on the host of features seen at the microscopic scale in the Fisherstreet slide, and many of these are variations and mo~fications of shear zones. They are sufficiently strongly and abundantly developed in some parts to be striking even in outcrop. Where the rocks are finely laminated, the shear zones are evident as narrow
zones across which the laminations are offset. The zones may be in conjugate or in en-echelon sets (Fig. 2A). Many of the outcrop surfaces show arrays of crenulations and offsets (Fig. 2B) which result from intersection of the shear zones with the surface. These kinds of features were illustrated, without discussion, by Gill (1979), figs. 36 and 37), and have been reported from other areas of disturbed sediments (e.g., Williams and Prentice, 1957, pl. 6). A range of similar ~crost~ctures also occurs in the Silurian rocks of the Denbigh Moors area (Warren et al., 1984) which are generally supposed to have undergone considerable slumping. These particular shear zones will be discussed more elaborately in a separate article. Shear zones were recorded by Woodcock (1976, e.g., fig. 10b) in slumped units in the Powys Trough, Wales, and they have now been recognized in other parts of the Welsh Basin where for independent reasons there are thought to have been early sediment movements. They occur, for example, in the lower Silurian rocks at Traeth-~-ynys Lochtyn, north of Cardigan where the preservation of some of them in early diagenetic concretions testifies to their pre-~t~fication origin {Craig, 1986, pl. 6.10). M. Smith (1987) has marshalled evidence that the deformation of the Cambro-Ordovician sequence around Tremadoc, North Wales, which was previously interpreted as due to post-lithification thrusting, is the result of sediment disturbance, and consistent with this is the occurrence in some of the argillaceous rocks of arrays of shear zones. Thomson (1973) recorded shear zones, which he called soft-sediment micro-faults, from the Tesnus Formation of the Marathon region of Texas and suggested that they indicate the orientation of the palaeoslope on which the sediments were slipping. Sediments deformed by gkxciai movements
Bell (1981) described a suite of pre-lithification structures in the mid-Palaeozoic Cape Supergroup, South Africa. A submarine sliding origin had been suggested by previous workers but Bell argued that the structures were associated with the Dwyka glacial movements. Because many of the features illustrated by Bell appear to have arisen through
167
the shear zone mode of deformation, the structures provide an example of glacially-induced shear zones in the geological past. In addition to the sets of shear zones, which show normal and reverse displacements, a fold is illustrated (Bell, 1981, fig. 2) which has an array of narrow shear zones producing the effect of a spaced axial planar fabric. Intersection of the different kinds of shear zones with the layering and other planar surfaces produces lineations which Bell regarded as “the most striking and persistent feature of the defor-
mation”. Clearly the production of shear zones has been paramount in the glacial deformation of these sediments. Sediments
deformed by tectonic faulting
Laville and Petit (1984), in discussing the role of basement faults in controlling the development of Triassic sedimentary basins in Morocco, noted that the tectonic stresses were taken up in the sediments by the production of discrete shear
Fig. 2. Shear zones in outcrop, Namurian siltstones, Fisherstreet Bay, Co. Clare, West Ireland. A. En-echelon array of narrow shear zones displacing fine bedding laminations. B. Bedding plane showing intersection of shear zone arrays. Conspicuous set runs from top left to bottom right of photograph, but other orientations are present also.
168
zones,
particularly
The zones appear the experiments, common,
in the argillaceous overall
very similar
and bear
additional
such as sub-fabrics,
sediments.
the laboratory
features
induce
a grooved
in
“patina”
on the shear zone surfaces,
and Riedel-type
splaying
shear zones (Maltman,
off the principle
shears
1987, figs. 5a, 3b, and 3a). Sediments
deformed in a regime of tectonic com-
It is now known
Fig. 3. Displacement attenua Ition of bedding o‘f inclined
supported
that stresses generated
tectoni-
along shear zones. B. Low-strain
shear zones. Pigeon Point Formation,
is likely
such settings.
1984), and these
parts.
on accretion
to be hindered overprints
should This
is
(Carson
natural
shear
In parts
sediment
(now lithified)
Pigeon Point, central
zones
reason have
re-
from geologists.
(now lithified)
California.
in rocks from
the main
in sediments
of the low-grade
showing
by the tectonic
inherent
This is probably
ceived little attention
A. Highly-strained sediment
that
in the higher
by experiments
and metamorphic
up the sedi-
effect of shear zones in outcrop.
zones
in Maltman,
predicts
and Berglund, 1986). However, the recognition of shear zones in on-land hthified contractional
why
cally at depth can be communicated
work
shear
margins
pression
array
ment pile (see discussion
to those of
displacement
Franciscan
with extreme of bedding
assemb-
displacement laminations
and along
169
lage of California, which developed at a compressional plate margin, shear zones are abundant and appear to have been highly significant in the deformation of the argillaceous sediments. In the highly deformed rock shown in Fig. 3A, the beds are attenuated and sheared along numerous narrow zones of ductile displacement. These shear zones therefore play an important part in the scenario of pre-lithification events envisaged by Aalto (1982). Moreover, as Aalto (pers. commun., 1985) has remarked, the features are precisely similar to those described by Cowan (1982) in the Franciscan of the South California coast ranges and also interpreted to be due to deformation of unlithified sediment. A further, less strained, example is shown in Fig. 3B. Lash (1985) has considered the deformation of argillaceous sediments in the early Palaeozoic accretionary complex of the central Appalachians. The shear zone mechanism would be expected to have been important there, and the commonly developed fabric referred to by Lash as “scaly cleavage” seems to be identical with the experimental shear zones. The fabric consists of “domains of preferred grain orientation” with “little or no change in grain size. . . away from the zones, whose surfaces are characterized by microstriations and microsteps”. Subsequent burial and tectonic effects appear to have been insufficient to mask the nature of these early zones of sediment shearing. All these examples formed in an overall setting of regional tectonic compression, and although in some cases the shear zones are probably due to local gravitationally induced extension in the sediments overlying the accreting sedimentary wedge, as argued by Kleist (1974) and Cowan (1982), in other cases the shear zones may represent the response of the sediment to the tectonic compressive stresses. Continuing laboratory work on the characterization of shear zone geometry under different physical conditions may allow in future a distinction between the different levels within an accretionary complex. Sediments
deformed
in a regime of tectonic tension
The tectonic deformation of unlithified sediments in tensional settings has yet to be recorded.
Shear deformation of sediments by gravitational slumping can, of course, abound in these settings, and this hinders identification of any tectonicallyinduced shear zones. The Carboniferous “shrimp bed” at Cheese Bay, southeast Scotland, contains a variety of structures which result from the development of spaced shear zones forming early in the post-sedimentary history of the materials (e.g., Hesselbo and Trewin, 1984, pl. 2). Although these may be reflecting down-slope extension, the fine preservation of organisms in very thin laminae suggests a fairly static sedimentary environment in which unstable slopes are unlikely. Moreover, the orientation of the shear zones parallels the tensional tectonic pattern of the region (J. Cater, pers. commun., 1986) and so the shear zones may be due to tectonic tensile stresses acting on the sediments. More data are needed to substantiate this connection, but there seems no reason in principle why regional tensile stresses, as with compressive stresses, should not produce shear zones in waterrich sediments. Some implications of natural shear zones Sediment
dewatering
The experimental results suggest that shear zones in nature will form at water contents upwards of 208, so the circumstance will commonly arise where a sediment containing the zones will continue to lose water as it is further buried. The presence of the shear zones will almost certainly influence dewatering behaviour, although this does not seem to have been taken into account in previous discussions on sediment drainage. Conceivably the intense clay fabric within the zones could form poorly-permeable barriers which inhibit dewatering, but there is some evidence that the opposite is the case: that water moves preferentially along the parallel-aligned particles. It seems that although the porosity in the shear zones is presumably reduced, the uniaxial permeability along them is greater than in the relatively poorly aligned host sediment. Shear zones have developed in parts of the Pigeon Point Formation of the central California
170
coast as a result of pre-lithification disturbance. As discussed by Tobisch (1984) the water content of the sediments at the time of deformation was probably considerable. In some cases, displacements of la~ations record the locations of shear zones in the sediments but the zones themselves have lithologies different from those of the host, appearing as incipient sedimentary dykes (Fig. 4). It seems that sediment became entrained in the water as if discharged preferentially, and perhaps rapidly, along the displacement zones. Similar oc-
currences of sediment entrained along early shear zones have been noted in the Denbigh Moors area, Wales, and at Fisherstreet, Co. Clare, Ireland. A less striking but possibly more common record of shear zones acting as channels for water movement is provided by subtle mineralogical changes preserved within the zones. Concentrations of minerals such as epidote and sphene within the shear zones of the Powys Trough, Wales (Fig. 5A) reflect an increased water flux in these areas (R. Wintsch, pers. commun., 1983) and ac-
Fig. 4. Shear zones acting as dewateting channels. A. Conjugate array of shear zones, inclined around vertical, displacing bedding laminations. Sediment entrained along shear zones indicates water flux. B. Similar feature as (A) on set of inclined shear zones. Photo~aph of hand specimen, half actual size. Both examples from Cretaceous Pigeon Point Formation, Pigeon Point, central California.
171
cumulations of very fine, unidentified dark grains in other examples may also indicate an “enhanced diagenesis”, as it were, in the shear zones. Similar effects occur in a D.S.D.P. sample of Miocene sediments from the Japan Trench (Fig. 5B). Recognition of shear zones in sediments and the likelihood that they act as fluid conduits could be significant for the numerous situations were fluid entrapment and migration are important.
The stepped nature of brittle fracture surfaces in rocks has long been known, although its explanation and use for deducing displacement direction has been the subject of discussion (e.g., Norris and Barron, 1969). Comparable stepped surfaces have been described from partly lithified sediments from the Japan Trench (Moore, 1978)
Fig. 5. Photomicrographs
A. Shear zones (running
bedding
laminations
showing
(horizontal)
within zones. Plane-polarized and contain kindly
concentrations
provided
effect of shear zones on diagenesis. and contain
light. Silurian, of dark minerals.
by Neil Lundberg.
Shear direction
concentrations
of epidote,
Ring Hole, Powys, Silty clay, Nankai
sphene,
top left to bottom
etc., thought
to represent
right) give offsets of
greater
water movement
Wales. B. Shear zones give offsets
of poorly-defined
Through,
57-44OB-43-3-30-33;
Japan,
D.S.D.P.
Sample
bedding
fabric
thin section
I IL
and
from Triassic
rocks
in Morocco
thought
to have been deformed
(Lavilie
and
Petit,
the kinematic steps
surfaces
1984). In these materials
interpretation
produced
are also
seems
unclear.
The
on the experimental
shear
zone
were viewed
Riedel-type
which
before lithification
as the result
of subsidiary
Moore’s invocation
sediments
and
Petit (1984,
fig.
significance of the stepped in experimentally deformed
is currently
being examined
At present the use of the steps shear direction is precarious.
sion fractures
Arenaceous
more fully.
as indicators
of
triggered
by incipient
shear”,
and
of overall shear opposite
Fig. 6. Shear zones in very slightly
indurated
sand, Cardigan
of coarse clasts. B. Shear zones giving displacement
to be due to increased
water movement
sediments
The experimental
of the steps being due to “ ten-
would lead to a direction
thought
by Laville
3A). The kinematic shear zone surfaces
shears ~~r~i~ the shear zone intersect-
ing with the shear zone surface (Maltman, 1987, p. 17, fig. 5a). This interpretation differs from
re-alignment
to that deduced
work
(1987) and the analogies
in nature
have been largely confined
sand pit, Cardigan, of bedding,
within the zones. Hand
West Wales.
and standing
lens (arrowed)
reported
by Maltman discussed
to argillaceous
A. Shear zone, made
out in relief because gives scale.
of greater
so far materi-
apparent
by
induration,
173
als. Some extrapolation to arenaceous sediments may be possible, however, for there are indications that in some circumstances, at least, these materials may deform in a shear zone mode. Mandl et al. (1977) have investigated shear zones in sands deformed in the laboratory; they found that the appearance and genesis of the zones have much in common with the argillaceous equivalents. Brodzikowski and Cegla (1981, e.g. fig. 2.2.) have illustrated shear zones in sandy glacial sediments, and the very slightly indurated glaciogenic sands near Cardigan, Wales (Helm and Roberts (1975) have deformed along discrete shear zones (Fig. 6A). Moreover, in the last example the material within the shear zones tends to be iron-strained and more indurated (Fig. 6B), with lower porosity (Helm and Roberts 1975, p. 142). It seems that these shear zones have acted as channels for water movement, analogous to the suggestion made earlier for shear zones in argillaceous materials. Scaly clays
Scaly clays are common lithologies in D.S.D.P. cores recovered from active margins and in on-land mobile belts, but the origin of their characteristic attribute, the “scaly foliation”, seems not to have been studied. The appearance of the fabric has
Fig. 7. Photomicrograph
of scaly clay, showing complex cross-cutting
D.S.D.P. sample 87-58680-2-132-139,
been described, for instance by Lundberg and Moore (1981); an example is illustrated in Fig. 7. It is suggested here that each foliation surface is a shear zone, of the type being discussed in this article. The complex intertwining array, typical of scaly clays, is the result of the high bulk strains the sediment is likely to experience in a setting such as an accretionary prism. The cross-cutting nature of the different sets of shear zones reflects the progressive locking of shear zones and initiation of replacements, as envisaged by Maltman (1987). The laboratory tests carried out so far are restricted by the equipment design to bulk strains less than 30%, but the prediction of progressive nucleation of new zones with continuing strain and its implications for the mechanical behaviour of scaly clays will be the subject of future work in collaboration with Utrecht State University. This scaly lithology also forms the matrix to some melanges, such as the “Type III” melange of Cowan (1985). The shear zone array, in addition to producing the scaly foliation, may be at least partly responsible for the disruption and separation of some of the clasts of the melange. Conditions of shear zone formation The experiments indicated that there is some variation in appearance of the shear zones formed
arrays of closely-spaced
shear zones, due to high bulk strains.
Japan Trench; thin section kindly provided by Neil Lundberg.
174
under different test conditions, and in particular the water content of the clay. In nature the water content will depend chiefly on the depth of burial; in the top few tens of metres of the sediment, trapping of pore fluid is unlikely to have become very efficient. Hence, with a systematic knowledge of the variation with water content of the width, shape, spacing, etc. of the expe~ment~ly produced shear zones it may be possible to deduce from the natural equivalents the depth at which they were produced. It is consistent with this thinking that in an area of slumped sediments such as west Co. Clare, Ireland (&ll, 1979) the shear zones are best developed in the thickest slump sheets, for instance that at Fisherstreet Bay, while they appear to be absent from sheets which can be seen to be thin and therefore probably had too high a water content. At Llangranog, West Wales, shear zones can be discerned only in places, where they are broad and form a closely-spaced curving array suggestive of a relatively high water content and a burial depth of no more than a few metres. This is consistent with the opinion of M. Anketell (pers. co~un., 1986), who has investigated the sedimentology of the Llangranog area, that the slumped horizons were open-cast and of a few metres thickness. The possibility therefore arises of using the appearance of shear zones in areas of poorly exposed deformed sediments to deduce the burial depths at which the deformation took place,
and
deformational
sediments Sudetica, Carson,
history
of unconsolidated
of the Jaroszow
zone (Sudetic
Quaternary
Foreland).
Geol.
18: 123-195.
B. and
associated
Berglund,
P.L.,
1986.
Sediment
with subduction-accretion:
dewatering
experimental
results.
Geol. Sot. Am., Mem., 166: 135-150. Cowan,
D.S., 1985. Structural
melanges
in the western
styles in Mesozoic Cordillera
and Cenozoic
of North
America.
Bull.
Geoi. Sot. Am., 96: 451-462. Cowan,
D.S., Moore,
Lucas,
SE.,
front
J.C., Roeske,
1984. Structural
of the Barbados
Project
Leg
78A.
Drilling
Project.
S.M., Lundberg
features
Ridge Complex,
In:
Initial
Deep Sea Drilling
Reports
U.S. Goverment
N. and
at the deformation of the
Printing
Deep
Sea
Office, Washing-
ton, D.C., Vol. 78, pp. 535-545. Craig,
J., 1986. Tectonic
and Cardigan,
evolution
Dyfed,
of the area between
W. Wales.
Ph.D.
Thesis,
Borth
Univ.
of
Wales (unpub~sh~). Davies,
W. and
termined Derbyshire,
R., 1976.
Folding
sedimentation.
and
cleavage
de-
Sed. Geol., 15: 89-133.
E., Edge, M.J. and Love, M.A., 1985a. Soil fabric
variability tor),
Cave,
during
in some glacial
Glacial
diamicts.
Tills ‘85. Proc.
Int.
In: M.C. Forde Conf.
Glacial Tills and Boulder Clays. Engineering Edinburgh, Derbyshire,
(Edi-
Construction Technics
in Press,
pp. 41-59. E., Love, M.A. and Edge, M.J. 1985b. Fabrics
probable
segregated
ground
cores from the North
ice origin
Sea Basin. In: J. Boardman
Soils and Quaternary
Landscape
of
on some sediment
Evolution.
(Editor),
Wiley, London,
pp. 261-280. Gill, W.D., 1979. Syndepositional West Clare Namurian
sliding and slumping
Basin, Ireland.
in the
Geol. Surv. Ireland,
Spec. Pap., 4; 31 pp. Hesselbo,
J. and Trewin,
structures
N., 1984. Deposition
of the Cheese Bay shrimp
erous, E. Lothian.
by soft-sediment
R.J.,
1986. Faulting
extension
in
Complex,Geology, 2: 501-504.
the Coastal belt, Franciscan examples
and
Scott. J. Geol., 20: 281-296.
Kleist, J.R., 1974. Deformation Knipe,
Diagnosis
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mechanisms
from Deep Sea Drilling
in slope
Project
sediments:
cores. Geol. Sot.
Am., Mem., 166: 45-54.
The improvements to the manuscript suggested by Peter Cobbold are gratefully acknowledged.
Lash, G.G., (early
1985. Accretion-related Paleozoic)
orogen. Laville,
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Trench-Forearc
London,
10: 419-432.
Atteweli,
and
Petit,
faults
In: J.K.
Spec. Publ.,
I.W., 1976. Principles and Hall, London,
Bell, C.M., 1981. Soft-sediment
Geol.
See.
glaciation
deformation in South Africa.
of Engineer-
of sandstone Sedimentology,
re-
Van Loon,
A.J.,
1983. Sedimentology
Appalachian
J.P.,
1984.
Role
in the formation
of syn-sedimentary
of Moroccan
Triassic
12: 424-427. features
of the
America
Mexico,
Deep
Sea Drilling Washington,
Trench
Project Project. DC,
in D.S.D.P.
Slope off southern
Leg 66, In: Initial Reports U.S.
Government
of the Deep
Printing
Office,
Vol. 66, pp. 793-814.
N. and Moore,
features
J.C., 3986. Macroscopic
cores from forearc
structural
regions.
Geol. Sot.
Am., Mem., 166: 13-44. Maltman,
K. and
of an ancient
central
N. and Moore, J.C., 1981. Structural
Middle
Lundberg,
1045 pp.
28: 321-329. Brodrikowski,
Lundberg,
Leggett
Sea Drilling
Chapman
lated to Dwyka
of northernmost
Geohw.
P.B. and Farmer,
ing Geology.
Complex tectonics.
deformation deposit,
Bull. Geol. Sot. Am., 96: 1167-1178.
Basins. Geology, Aalto,
trench-fill
J.
A.J., 1984. On the term soft-sediment
StNCt.
Geol., 6: 589-592.
deformation.
175
Maltman, A.J., 1987. Shear zones in argillaceous sediments an experimental study. In: M.E. Jones and R.M.F. Preston (Editors), Deformation of Sediments and Sedimentary Rocks. Geol. Sot. London, Spec. Pub]., 29: 77-87. Mandl, G., De Jong, L.N.J. and MaItha, A., 1977. Shear zones in granular material. Rock Mech., 9: 95-144. Moore, G.W., 1978. Slickensides in deep sea cores near the Japan Trench, Leg 57, Deep Sea Drilling Project. In: Initial Reports of the Deep Sea Drilling Project. U.S. Government Printing Office, Washington, DC., Vol. 57, pp. 1107-1115. Moore, J.C. and 17 others, 1982. Offscraping and underthntsting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A. Geol. Sot. Am. Bull. 93: 1065-1077. Morgenstem, N.R. and TchaIenko, J.S., 1967. Microstructural observations on shear zones from slips in natural clays. Proc. Geotech. Conf. Oslo, 1: 147-152. Norris, D.K. and Barron, K., 1969. Structural analysis of features on natural and artificial faults. In: A.J. Bear and
D.K. Norris (Editors), Conference on Research in Tectonics. Geol. Sot. Can., Pap., 68-52: 136-174. Skempton, A.W., 1964. Long-term stability of clay-slopes. Geotechnique, 14: 77-101. Skempton, A.W., 1966. Some observations on tectonic shear zones. Proc. Int. Congr. Rock Mech., 1st Lisbon, 1: 329-335. TchaIenko, J.S., 1968. The evolution of kink-bands and the development of compression textures in sheared clays. Tectonophysics, 6: 159-174. Thomson, A., 1973. Soft-sediment faults in the Tesnus Formation and their relationship to palaeoslope. J. Sediment. Petrol., 43: 525-528. Tobisch, O., 1984. Development of foliation and fold interference patterns produced by sedimentary processes. Geology, 12: 44-444. Warren, P.J., Price, D., Nutt, M.J.C. and Smith, E.G., 1984. Geology of the country around Rhyl and Denbigh. Mem. Br. Geol. Surv. H.M.S.O., London, 108 pp.
163
145 (1988) 163-175
Elsevier Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
The importance of shear zones in naturally deformed wet sediments ALEX Department
J. MALTMAN
of Geology, University College of Wales, Aberystwyth, (Received
December
Dyfed SY23 3DB (Great Britain)
1, 1986; revised version accepted
March
31, 1987)
Abstract Maltman,
A.J.,
1988. The importance
of shear
zones
in naturally
deformed
wet sediments.
Tectonophysics,
145:
163-175. The production
of narrow
shear zones was recently
sediments
deform
in the laboratory.
sediments
deform,
that this mode of deformation
Examples
cores;
and in deformed
gravitationally-induced
conditions
disturbances
sediments such
shown
illustrates,
is of widespread
importance
to the experimentally
that are now lithified.
as slumping,
and
their
of recognizing possible
of the sediment
the natural
use as shear-sense
shear
zones
possible
are mentioned,
indicators,
a variety
by which wet argillaceous of circumstances
in which
in nature.
produced
features,
in glaciogenic
The latter examples
include
tectonically-formed
features
sediments
the results of both such
as occur
in
and
for example,
the possibility
that
their influence they
may
indicate
on dewatering the physical
deformation.
Introduction The deformation of wet argillaceous sediments in laboratory experiments has shown that they change shape, not by pervasive homogeneous flow - as has sometimes been surmised in the past, but by intense slippage within very narrow discrete zones of shear. These experimentally-produced shear zones have recently been described in detail (Maltman, 1987). Macroscopically, they are shiny, finely-grooved narrow zones of slip; microscopically they appear as approximately planar commonly complex zones of pronounced particle reorientation. The shear zone mode of deformation was found to be dominant in specimens tested under a range of physical conditions (e.g., strain rate, confining pressure, consolidation ratio) and with water contents between about 15% and 65’%, 0040-1951/88/$03.50
to be the chief mechanism by considering
complexes.
Implications behaviour,
article
are given of shear zones, highly similar
and D.S.D.P. accretionary
This
0 1988 Elsevier Science Publishers
B.V.
corresponding to a considerable range of sediment burial depths. This laboratory work prompts the obvious question of whether or not the shear zone process operates in nature. Although the experimental conditions were designed to reasonably simulate natural circumstances, it is, of course, possible that some critical factor has been overlooked and an artificial situation has been inadvertently created. The purpose of this article is to illustrate that the shear zone mode of deformation is indeed important in nature. Occurrences are noted here of structures, extremely similar to the experimental shear zones, in sediments which have been deformed naturally in a wide variety of circumstances. Some remarks are made about the geological implications of recognizing shear zones in natural sediments.
164
Shear zones in existing sediments Engineering
materials
shearing.
The structures
oriented
clay particles
surfaces,
which
those produced Shear zones of the kind produced in the experiments have been recognized for some time by civil engineers
interested
in soil mechanics.
technicat
behaviour
of sediments
gineering
parlance)
is greatly
presence
of mechanical
ties”, for example, p. 315). That
influenced
anisotropies
see Attewell
gineering
Hence
in enby the
(“discontinui-
shire (pers. commun.,
in
disturbed
“zones
glacial
of localized
all the structures
cate-
to displacement
context
to investigate
experimentally (references the engineering literature natural examples.
in the en-
the shear
zones
in Maltman, 1987) and contains references to
The instances reported by civil engineers from argillaceous sediments are direct analogues of those produced in the laboratory. Examples are the disturbed silty clays at the Cod Beck dam, England (compare Morgenstern and Tchalenko, 1967, fig. 1, with Maltman, 1987, fig. 4f); the mudflows at Sevenoaks, England (compare Morgenstern and Tchalenko, 1967, fig. 11, with Maltman, 1987, fig. 5d); and the landslide at Wahon’s Wood, England (compare Tchalenko, 1968, fig. 9, with Maltman, 1987, fig. 4~). All these examples are presumably the result of various kinds of gravitational slipping, but Skempton (1966) has described clays in a dam foundation at Mangla, Pakistan, in which the shear zones appear to be associated with recent tectonic folding of Plio-Pleistocene sediments. Because of the repercussions for the bulk mechanical properties of sediments, Skempton (1966, p. 335) regards the presence of such shear zones in engineering materials as ‘a matter of extreme significance in the design of engineering works. Glaciogenic sediments The presence of shear zones in recent sediments has received much less attention outside civil engineering. However, Derbyshire et al. (1985a, b) have reported the existence in glacial tills of polished shear surfaces, perhaps due to sub-glacial
very
similar
to
work. E. Derby-
1985) feels that such shear
and Van Loon (1983) have discussed
1976,
by Skempton
seem
and grooved
zones are common in disrupted tills, although they may only be visible ~croscopi~ally. Brodzikowski
and Farmer,
there have been efforts
therefore
in the laboratory
shear zones are an important
gory of these has been demonstrated (1964).
The geo-
(“soils”
are narrow zones of highly with polished
perimental (1976, shear
deposits, visco-plastic
them
flowage”.
as
Almost
they illustrate
appear
to be due
along localized
planes,
as the ex-
Davies
and Cave
work would
predict.
figs. 8a, 8b) illustrate zones
shear zones
viewing
in a glacial
what
clay from
appear
Machynlleth,
Wales, drawing attention to the restriction deformation to the narrow zones. The physical conditions at the glaciogenic sediments were deformed,
to be of the
time these as with the
engineering materials discussed earlier, may well differ from those of the submarine sediment burial that the laboratory experiments were meant to simulate; but the presence of shear zones in these materials indicates that this mode of deformation is of widespread occurrence. D. S.D. P. cores
The experimental investigations have shown that shear zones are important in argillaceous materials with water contents of up to at least 60%. Presumably at some higher value, perhaps 70% or so, this mode of deformation
is supplanted
by pervasive inter~anular slip. Thus subaqueous sediments which are presently very near the sediment-water interface, where it is conceivably possible to observe their deformation directly, will probably have too high a water content to develop shear zones. However, after normal water loss equivalent to burial of a few tens of metres, shear zones are likely to form on disturbance of the sediment, and should persist until lithification has progressed sufficiently to curb the necessary grain slippage. Hence the kinds of sediment depths typically encountered by cores of the Deep Sea Drilling Project (D.S.D.P.) are likely to be appropriate for shear zone production. So far only the D.S.D.P. cores from the forearc
165
areas of active plate margins have received detailed ~crost~~tur~ study (for example, Lundberg and Moore, 1981, 1986). Many of the microstructural features reported from such cores have similarities with the shear zones produced in experiment (compare, for example, &ripe, 1986, fig. 12, with Maltman, 1987, fig. Sd). Some structures are directly comparable. Figure IA shows a discrete shear zone in Miocene mud from the Japan Trench. This material, recovered from a sediment depth of about 760 m, lay upslope from the trench
Fig. 1. Photornicrographs
of shear zones in D.S.D.P.
and so may have been subject either to downslope slippage or to the contractions forces that are part of the accretion process. Whatever the cause of the movement, it has been accomplished by producing shear zones such as the one shown. This example compares well with some of those produced expe~ment~y (Maltm~, 1987, e.g., figs. 4a, b). In another part of the same D.S.D.P. sample, minor conjugate shear zones have developed (Fig. lB), analogous to some experimental specimens (Maltman, 1987, fig. 5d).
samples. A. Discrete shear zone in Miocene mud, Japan Trench.
B.
Closer-spaced shear zones with development of minor conjugate set. Same material as A. Both specimens photographed between crossed-nicols: matrix appears dark, highly-oriented clays within shear zones appear light. Sample 87-584-56-2-132-139; kindly provided by Neil Lundberg.
thin-sections
166
The laboratory work was confined to bulk strains of less than 30%, but led to the suggestion that with continuing strain additional shear zones would have to be generated (Maltman, 1987, p. 12). This is borne out in a D.S.D.P. sample from a zone of high shear strain, namely that in which Miocene muds are tectonically brought over Pliocene sediments at the deformation front of the Barbados ridge (Moore et al., 1982; Cowan et al., 1984). Figure 5 of the latter article shows an interweaving array of shear zones in the material at the junction. The thrusting of the sediments has apparently taken place by phyllosihcate sliding and reorientation within shear zone systems, rather than by fracture or pervasive gram sliding. Shear zones in se&rents
now li~fi~
Sediments deformed by ~~urn~i~g
It follows from the thesis being considered here that sediments which were deformed in the geological past before they were lithified may preserve the shear zones produced at the time of disturbance. Among the best described rock sequences which were un~uiv~ally disturbed before lithification is the Namurian of County Clare, West Ireland (Gill, 1979). Some of the materials in that sequence have disaggregated in various degrees, implying movement while still very waterrich, while others have moved as generally intact sheets. The latter contain many examples of the kind of shear zones produced in the experiments, such that they must have played an important part in the translation of the sediment. A good example is provided by the slump sheet at Fisherstreet Bay, Co. Clare. Much of the sediment is argillaceous and comprises a sliding mass at least 20 m thick (Gill, 1979) so that the ccmditions at the time of movement would almost certainly fall within the range of the experiments. Gill remarked on the host of features seen at the microscopic scale in the Fisherstreet slide, and many of these are variations and mo~fications of shear zones. They are sufficiently strongly and abundantly developed in some parts to be striking even in outcrop. Where the rocks are finely laminated, the shear zones are evident as narrow
zones across which the laminations are offset. The zones may be in conjugate or in en-echelon sets (Fig. 2A). Many of the outcrop surfaces show arrays of crenulations and offsets (Fig. 2B) which result from intersection of the shear zones with the surface. These kinds of features were illustrated, without discussion, by Gill (1979), figs. 36 and 37), and have been reported from other areas of disturbed sediments (e.g., Williams and Prentice, 1957, pl. 6). A range of similar ~crost~ctures also occurs in the Silurian rocks of the Denbigh Moors area (Warren et al., 1984) which are generally supposed to have undergone considerable slumping. These particular shear zones will be discussed more elaborately in a separate article. Shear zones were recorded by Woodcock (1976, e.g., fig. 10b) in slumped units in the Powys Trough, Wales, and they have now been recognized in other parts of the Welsh Basin where for independent reasons there are thought to have been early sediment movements. They occur, for example, in the lower Silurian rocks at Traeth-~-ynys Lochtyn, north of Cardigan where the preservation of some of them in early diagenetic concretions testifies to their pre-~t~fication origin {Craig, 1986, pl. 6.10). M. Smith (1987) has marshalled evidence that the deformation of the Cambro-Ordovician sequence around Tremadoc, North Wales, which was previously interpreted as due to post-lithification thrusting, is the result of sediment disturbance, and consistent with this is the occurrence in some of the argillaceous rocks of arrays of shear zones. Thomson (1973) recorded shear zones, which he called soft-sediment micro-faults, from the Tesnus Formation of the Marathon region of Texas and suggested that they indicate the orientation of the palaeoslope on which the sediments were slipping. Sediments deformed by gkxciai movements
Bell (1981) described a suite of pre-lithification structures in the mid-Palaeozoic Cape Supergroup, South Africa. A submarine sliding origin had been suggested by previous workers but Bell argued that the structures were associated with the Dwyka glacial movements. Because many of the features illustrated by Bell appear to have arisen through
167
the shear zone mode of deformation, the structures provide an example of glacially-induced shear zones in the geological past. In addition to the sets of shear zones, which show normal and reverse displacements, a fold is illustrated (Bell, 1981, fig. 2) which has an array of narrow shear zones producing the effect of a spaced axial planar fabric. Intersection of the different kinds of shear zones with the layering and other planar surfaces produces lineations which Bell regarded as “the most striking and persistent feature of the defor-
mation”. Clearly the production of shear zones has been paramount in the glacial deformation of these sediments. Sediments
deformed by tectonic faulting
Laville and Petit (1984), in discussing the role of basement faults in controlling the development of Triassic sedimentary basins in Morocco, noted that the tectonic stresses were taken up in the sediments by the production of discrete shear
Fig. 2. Shear zones in outcrop, Namurian siltstones, Fisherstreet Bay, Co. Clare, West Ireland. A. En-echelon array of narrow shear zones displacing fine bedding laminations. B. Bedding plane showing intersection of shear zone arrays. Conspicuous set runs from top left to bottom right of photograph, but other orientations are present also.
168
zones,
particularly
The zones appear the experiments, common,
in the argillaceous overall
very similar
and bear
additional
such as sub-fabrics,
sediments.
the laboratory
features
induce
a grooved
in
“patina”
on the shear zone surfaces,
and Riedel-type
splaying
shear zones (Maltman,
off the principle
shears
1987, figs. 5a, 3b, and 3a). Sediments
deformed in a regime of tectonic com-
It is now known
Fig. 3. Displacement attenua Ition of bedding o‘f inclined
supported
that stresses generated
tectoni-
along shear zones. B. Low-strain
shear zones. Pigeon Point Formation,
is likely
such settings.
1984), and these
parts.
on accretion
to be hindered overprints
should This
is
(Carson
natural
shear
In parts
sediment
(now lithified)
Pigeon Point, central
zones
reason have
re-
from geologists.
(now lithified)
California.
in rocks from
the main
in sediments
of the low-grade
showing
by the tectonic
inherent
This is probably
ceived little attention
A. Highly-strained sediment
that
in the higher
by experiments
and metamorphic
up the sedi-
effect of shear zones in outcrop.
zones
in Maltman,
predicts
and Berglund, 1986). However, the recognition of shear zones in on-land hthified contractional
why
cally at depth can be communicated
work
shear
margins
pression
array
ment pile (see discussion
to those of
displacement
Franciscan
with extreme of bedding
assemb-
displacement laminations
and along
169
lage of California, which developed at a compressional plate margin, shear zones are abundant and appear to have been highly significant in the deformation of the argillaceous sediments. In the highly deformed rock shown in Fig. 3A, the beds are attenuated and sheared along numerous narrow zones of ductile displacement. These shear zones therefore play an important part in the scenario of pre-lithification events envisaged by Aalto (1982). Moreover, as Aalto (pers. commun., 1985) has remarked, the features are precisely similar to those described by Cowan (1982) in the Franciscan of the South California coast ranges and also interpreted to be due to deformation of unlithified sediment. A further, less strained, example is shown in Fig. 3B. Lash (1985) has considered the deformation of argillaceous sediments in the early Palaeozoic accretionary complex of the central Appalachians. The shear zone mechanism would be expected to have been important there, and the commonly developed fabric referred to by Lash as “scaly cleavage” seems to be identical with the experimental shear zones. The fabric consists of “domains of preferred grain orientation” with “little or no change in grain size. . . away from the zones, whose surfaces are characterized by microstriations and microsteps”. Subsequent burial and tectonic effects appear to have been insufficient to mask the nature of these early zones of sediment shearing. All these examples formed in an overall setting of regional tectonic compression, and although in some cases the shear zones are probably due to local gravitationally induced extension in the sediments overlying the accreting sedimentary wedge, as argued by Kleist (1974) and Cowan (1982), in other cases the shear zones may represent the response of the sediment to the tectonic compressive stresses. Continuing laboratory work on the characterization of shear zone geometry under different physical conditions may allow in future a distinction between the different levels within an accretionary complex. Sediments
deformed
in a regime of tectonic tension
The tectonic deformation of unlithified sediments in tensional settings has yet to be recorded.
Shear deformation of sediments by gravitational slumping can, of course, abound in these settings, and this hinders identification of any tectonicallyinduced shear zones. The Carboniferous “shrimp bed” at Cheese Bay, southeast Scotland, contains a variety of structures which result from the development of spaced shear zones forming early in the post-sedimentary history of the materials (e.g., Hesselbo and Trewin, 1984, pl. 2). Although these may be reflecting down-slope extension, the fine preservation of organisms in very thin laminae suggests a fairly static sedimentary environment in which unstable slopes are unlikely. Moreover, the orientation of the shear zones parallels the tensional tectonic pattern of the region (J. Cater, pers. commun., 1986) and so the shear zones may be due to tectonic tensile stresses acting on the sediments. More data are needed to substantiate this connection, but there seems no reason in principle why regional tensile stresses, as with compressive stresses, should not produce shear zones in waterrich sediments. Some implications of natural shear zones Sediment
dewatering
The experimental results suggest that shear zones in nature will form at water contents upwards of 208, so the circumstance will commonly arise where a sediment containing the zones will continue to lose water as it is further buried. The presence of the shear zones will almost certainly influence dewatering behaviour, although this does not seem to have been taken into account in previous discussions on sediment drainage. Conceivably the intense clay fabric within the zones could form poorly-permeable barriers which inhibit dewatering, but there is some evidence that the opposite is the case: that water moves preferentially along the parallel-aligned particles. It seems that although the porosity in the shear zones is presumably reduced, the uniaxial permeability along them is greater than in the relatively poorly aligned host sediment. Shear zones have developed in parts of the Pigeon Point Formation of the central California
170
coast as a result of pre-lithification disturbance. As discussed by Tobisch (1984) the water content of the sediments at the time of deformation was probably considerable. In some cases, displacements of la~ations record the locations of shear zones in the sediments but the zones themselves have lithologies different from those of the host, appearing as incipient sedimentary dykes (Fig. 4). It seems that sediment became entrained in the water as if discharged preferentially, and perhaps rapidly, along the displacement zones. Similar oc-
currences of sediment entrained along early shear zones have been noted in the Denbigh Moors area, Wales, and at Fisherstreet, Co. Clare, Ireland. A less striking but possibly more common record of shear zones acting as channels for water movement is provided by subtle mineralogical changes preserved within the zones. Concentrations of minerals such as epidote and sphene within the shear zones of the Powys Trough, Wales (Fig. 5A) reflect an increased water flux in these areas (R. Wintsch, pers. commun., 1983) and ac-
Fig. 4. Shear zones acting as dewateting channels. A. Conjugate array of shear zones, inclined around vertical, displacing bedding laminations. Sediment entrained along shear zones indicates water flux. B. Similar feature as (A) on set of inclined shear zones. Photo~aph of hand specimen, half actual size. Both examples from Cretaceous Pigeon Point Formation, Pigeon Point, central California.
171
cumulations of very fine, unidentified dark grains in other examples may also indicate an “enhanced diagenesis”, as it were, in the shear zones. Similar effects occur in a D.S.D.P. sample of Miocene sediments from the Japan Trench (Fig. 5B). Recognition of shear zones in sediments and the likelihood that they act as fluid conduits could be significant for the numerous situations were fluid entrapment and migration are important.
The stepped nature of brittle fracture surfaces in rocks has long been known, although its explanation and use for deducing displacement direction has been the subject of discussion (e.g., Norris and Barron, 1969). Comparable stepped surfaces have been described from partly lithified sediments from the Japan Trench (Moore, 1978)
Fig. 5. Photomicrographs
A. Shear zones (running
bedding
laminations
showing
(horizontal)
within zones. Plane-polarized and contain kindly
concentrations
provided
effect of shear zones on diagenesis. and contain
light. Silurian, of dark minerals.
by Neil Lundberg.
Shear direction
concentrations
of epidote,
Ring Hole, Powys, Silty clay, Nankai
sphene,
top left to bottom
etc., thought
to represent
right) give offsets of
greater
water movement
Wales. B. Shear zones give offsets
of poorly-defined
Through,
57-44OB-43-3-30-33;
Japan,
D.S.D.P.
Sample
bedding
fabric
thin section
I IL
and
from Triassic
rocks
in Morocco
thought
to have been deformed
(Lavilie
and
Petit,
the kinematic steps
surfaces
1984). In these materials
interpretation
produced
are also
seems
unclear.
The
on the experimental
shear
zone
were viewed
Riedel-type
which
before lithification
as the result
of subsidiary
Moore’s invocation
sediments
and
Petit (1984,
fig.
significance of the stepped in experimentally deformed
is currently
being examined
At present the use of the steps shear direction is precarious.
sion fractures
Arenaceous
more fully.
as indicators
of
triggered
by incipient
shear”,
and
of overall shear opposite
Fig. 6. Shear zones in very slightly
indurated
sand, Cardigan
of coarse clasts. B. Shear zones giving displacement
to be due to increased
water movement
sediments
The experimental
of the steps being due to “ ten-
would lead to a direction
thought
by Laville
3A). The kinematic shear zone surfaces
shears ~~r~i~ the shear zone intersect-
ing with the shear zone surface (Maltman, 1987, p. 17, fig. 5a). This interpretation differs from
re-alignment
to that deduced
work
(1987) and the analogies
in nature
have been largely confined
sand pit, Cardigan, of bedding,
within the zones. Hand
West Wales.
and standing
lens (arrowed)
reported
by Maltman discussed
to argillaceous
A. Shear zone, made
out in relief because gives scale.
of greater
so far materi-
apparent
by
induration,
173
als. Some extrapolation to arenaceous sediments may be possible, however, for there are indications that in some circumstances, at least, these materials may deform in a shear zone mode. Mandl et al. (1977) have investigated shear zones in sands deformed in the laboratory; they found that the appearance and genesis of the zones have much in common with the argillaceous equivalents. Brodzikowski and Cegla (1981, e.g. fig. 2.2.) have illustrated shear zones in sandy glacial sediments, and the very slightly indurated glaciogenic sands near Cardigan, Wales (Helm and Roberts (1975) have deformed along discrete shear zones (Fig. 6A). Moreover, in the last example the material within the shear zones tends to be iron-strained and more indurated (Fig. 6B), with lower porosity (Helm and Roberts 1975, p. 142). It seems that these shear zones have acted as channels for water movement, analogous to the suggestion made earlier for shear zones in argillaceous materials. Scaly clays
Scaly clays are common lithologies in D.S.D.P. cores recovered from active margins and in on-land mobile belts, but the origin of their characteristic attribute, the “scaly foliation”, seems not to have been studied. The appearance of the fabric has
Fig. 7. Photomicrograph
of scaly clay, showing complex cross-cutting
D.S.D.P. sample 87-58680-2-132-139,
been described, for instance by Lundberg and Moore (1981); an example is illustrated in Fig. 7. It is suggested here that each foliation surface is a shear zone, of the type being discussed in this article. The complex intertwining array, typical of scaly clays, is the result of the high bulk strains the sediment is likely to experience in a setting such as an accretionary prism. The cross-cutting nature of the different sets of shear zones reflects the progressive locking of shear zones and initiation of replacements, as envisaged by Maltman (1987). The laboratory tests carried out so far are restricted by the equipment design to bulk strains less than 30%, but the prediction of progressive nucleation of new zones with continuing strain and its implications for the mechanical behaviour of scaly clays will be the subject of future work in collaboration with Utrecht State University. This scaly lithology also forms the matrix to some melanges, such as the “Type III” melange of Cowan (1985). The shear zone array, in addition to producing the scaly foliation, may be at least partly responsible for the disruption and separation of some of the clasts of the melange. Conditions of shear zone formation The experiments indicated that there is some variation in appearance of the shear zones formed
arrays of closely-spaced
shear zones, due to high bulk strains.
Japan Trench; thin section kindly provided by Neil Lundberg.
174
under different test conditions, and in particular the water content of the clay. In nature the water content will depend chiefly on the depth of burial; in the top few tens of metres of the sediment, trapping of pore fluid is unlikely to have become very efficient. Hence, with a systematic knowledge of the variation with water content of the width, shape, spacing, etc. of the expe~ment~ly produced shear zones it may be possible to deduce from the natural equivalents the depth at which they were produced. It is consistent with this thinking that in an area of slumped sediments such as west Co. Clare, Ireland (&ll, 1979) the shear zones are best developed in the thickest slump sheets, for instance that at Fisherstreet Bay, while they appear to be absent from sheets which can be seen to be thin and therefore probably had too high a water content. At Llangranog, West Wales, shear zones can be discerned only in places, where they are broad and form a closely-spaced curving array suggestive of a relatively high water content and a burial depth of no more than a few metres. This is consistent with the opinion of M. Anketell (pers. co~un., 1986), who has investigated the sedimentology of the Llangranog area, that the slumped horizons were open-cast and of a few metres thickness. The possibility therefore arises of using the appearance of shear zones in areas of poorly exposed deformed sediments to deduce the burial depths at which the deformation took place,
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