GEOLOGICAL JOURNAL, VOL. 22, THEMATIC ISSUE, 107-118 (1987)

Lineaments in enhanced Landsat images from a portion of west Wales

R. Maude Department of Geology, University College of Wales, Aberystwyth, Dyfed, SY23 3DB, U.K. *

Remotely sensed lineaments are often used as indicators of fractures in near-surface rocks. Previous lineament interpretations of Wales have used aerial photographs and relatively unenhanced satellite images. New images of part of Wales have been produced from Landsat MSS data using several digital image processing techniques intended to enhance the visibility of linear features. A lineament interpretation of these images is presented and described. Five populations of lineaments are classified, primarily on the basis of their azimuths. The most numerically significant of these has a mean azimuth of 075” and is here termed the ‘Ystwyth Lineament Set’. Typically the lack of rock exposure on the line of lineaments makes direct geological investigation difficult. However, field evidence and literature have been used to shed light on the origin of the Ystwyth Lineament Set. It is concluded that this set of lineaments is the visual representation of a set of fractures which have had a long and tectonically varied history.

KEY WORDS Satellite images Wales Lineament populations

1. Introduction

This paper describes the results of a lineament interpretation of digitally enhanced Landsat MSS images of a portion of west Wales (Figures 1 and 2). The term ‘lineament’ was first used by Hobbs in his 1904 paper entitled ‘Lineaments of the Atlantic Border region’. He defined lineaments as ‘significant lines of landscape that reveal the hidden architecture of the rock basement’. O’Leary (1976) defined the term lineament as ‘A mappable, simple or composite linear feature of a surface, whose parts are aligned in a rectilinear or slightly curvilinear relationship and which differs distinctly from the patterns of adjacent features and presumably reflects some sub-surface phenomenon’. O’Leary’s definition is adopted here.

Early lineament intelpretations were made from aerial photographs. More recently satellite images have become available and are now extensively used for this purpose. Many authors have used lineament interpretations to delineate fractures and fracture systems in different parts of the world (e.g. Marrs and

*Present address: Department of Physics, University of Leicester, Leicester, LE1 7RH, U.K.

0072-1050/87/TI0107-12$06 .OO 0 1987 by John Wiley & Sons, Ltd

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OUTLINE OF T H E STUDY A R E A

7

5 3-

52'

Figure 1 . Location map showing the area covered by the survey and the structures discussed in the text.

Raines 1984; Cochrane and Tianfeng 1983). The technique has proved especially useful in those areas where drift cover inhibits the identification of fractures by other means, a problem well known to workers on the geology of Wales.

Thomas (1970) presented a geological interpretation of aerial photographs of Welsh upland areas. He drew attention to what he termed the 'structural grain': a locally parallel alignment of the microrelief. Most of the features that he observed were strike-ridges although some joint sets and faults were also observed.

Craig (1986) presented an interpretation based on Landsat and Seasat images of the whole of Wales. His interpretation, taken from relatively unprocessed images, provides a synoptic framework in the light of which he was able to interpret the detailed structure of several internal areas.

The method of image production and interpretation used in this survey is described in the following section. It is felt that this method reveals some important although subtle features that were missed in earlier work.

REMOTELY SENSED LINEAMENTS 109

Figure 2. Lineament interpretation of a portion of west Wales. Due to the map projection the scale and north pointer are only accurate in the area around Trawsfynydd.

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2. Image production

All the image production was carried out on an I2 S digital image processing system. The near infra-red Landsat MSS band 7 was used in this survey as it was found to reveal the most structural information in the images of Wales that were studied. Because low sun angles enhance the visibility of topographic variations, winter images are usually selected for this kind of work. The features that relate to fractures in west Wales are often subtle linear zones of topographic depression. It was noted that in local areas of high topographic relief, these zones tend to be masked in winter images which have low sun angles. Therefore summer scenes were selected to avoid this effect.

Five overlapping full resolution 512 by 512 pixel Landsat MSS subscenes were digitally processed using destriping, deskewing and contrast-stretching routines. These are standard processes used for the correction of systematic distortions of the data, and to ensure an adequate range of grey-tone in the final images. Five different images were then produced from each subscene by different edge detec- tion processes (two dimensional spacial convolutions), one from a non-directional and four from directional filters. Three by three pixel kernels were used in all the edge enhancements. The kernels used for the directional edge detection had the following weighting factor arrangements:

-1-1-1 0 0 0 East-west enhancement 1 1 1

-1 0 1 -1 0 1 North-south enhancement -1 0 1 -1-1 0 - 1 0 1 Northeast-southwest enhancement

0 1 1 0-1-1 1 0- 1 Northwest-southeast enhancement 1 1 0

The five edge detection processes used tend to enhance the visibility of lin- eaments, the directional filters enhancing only lines trending near the specified azimuth (e.g. northeast, southwest). It is important that images enhanced in this way are not used in isolation as statistically biased interpretations would result. For further details of these processes reference should be made to Berhe and Rothery (1986) or a standard text on image processing.

3. Lineament interpretation

Some lineaments are only visible in individual enhanced images; such features are often artefacts of the processing. For this reason the following method of interpretation was adopted.

Each of the five images obtained for each of the five subscenes was interpreted separately. The five interpretations produced for each of the subscenes were then compared and all features present on more than one of the interpretations were traced onto a final interpretation. The result (Figure 2) is an interpretation that

REMOTELY SENSED LINEAMENTS 111

N . ‘ . I

I

S

Figure 3. Rose diagram to show the azimuthal frequency distribution of the lineaments in the interpretation.

includes only the most definite lineaments. This procedure is considered to be more objective than conventional approaches to interpretation.

4. Description of the lineament interpretation

It was noted by Nur (1981) that the surface of the crust is often ‘broken by 2 to 4 sets of subparallel lineaments, the uniformity and direction of which may persist for areas of up to 10,000 or more square kilometers’. The observation that lineaments occur in distinct azimuthal groups is corroborated by the lineament interpretation presented in this paper. The lineaments identified in this paper are therefore grouped into populations on this basis.

The term ‘set’ is used in this paper to indicate a group of lineaments that have similar azimuths and a distinct spatial distribution. This may imply some genetic relationship, although it is not in any way proved. The term ‘population’ is used in this paper to indicate the statistical significance of the distribution of lineament azimuth frequencies as shown in the rose diagram, Figure 3.

Due to the method of interpretation described above, visually distinct lineaments are shown as single continuous lines and the less distinct ones as composite structures composed of many short aligned portions. It is therefore unreasonable to produce a rose diagram by cumulating the frequencies of lineaments within different azimuth ranges by number, as this tends to bias the distribution towards the less visually distinct structures. The solution is to cumulate the total length of lineaments within given azimuth ranges. The rose diagram shown in Figure 3 was

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Table 1. Summary of lineament populations

Percentage of lineaments within Azimuth range specified azimuth range

("> (Yo)

Reference to Figure 4

000-010 020-040 050-060 065-080 150-170

6 11 13 42 14

Percentage of total lineaments included in above populations = 86

part b part c part d part e part f

taken from the lineament interpretation (Figure 2), and shows the azimuthal frequency distribution of lineaments calculated by length.

Lineaments are considered to be the visual effect of the variation in ground surface topography and reflectance, usually following the lines of geological struc- tures such as faults. Although many lineaments are easily related to known structures, the lineament and its parent structure are not synonymous. Where known faults have given rise to lineaments on the satellite images, the lineaments are named after the fault. For example, the Ystwyth fault has given rise to the Ystwyth Lineament.

On the basis of azimuth, five populations of lineaments are distinguished. These are summarized in Table 1 and are shown in Figure 4.

5. The Ystwyth Lineament Set

A significant proportion of the lineaments within the area of the interpretation trends at azimuths between 065" and 080". Although they are ubiquitous, they are most frequent in the area south of the Bala Fault. Prominent amongst them is a lineament coincident with the line of the Ystwyth fault. For this reason this structure is here termed the 'Ystwyth Lineament' and features of this azimuth, the 'Ystwyth Lineament Set'.

The following structures (Figure 1) are correlated with this set of lineaments: 1. The Ystwyth and Llyfnant faults. 2. The Pennel and Cwm Sylwi faults. 3. Some mineralized normal faults in the Plynlimon Dome area. 4. Some veins of the Dolgellau Gold Belt. 5. Numerous ENE-trending mesofractures observed in north Ceredigion.

5a. The Ystwyth and Llyfnant faults These faults were described by Jones and Pugh (1915) and Jones (1922). They

were distinguished from the numerous mineralized veins of the area (which have approximately the same azimuth) on the basis of their greater lengths and displacements. Those authors demonstrated that the Llyfnant fault has a dextral strike-slip displacement of up to 975 m, and vertical displacement of up to 188 m. The lack of primary mineralization and the presence of brecciated fragments of vein material within the gouge of the Ystwyth fault indicated to Jones (1922) that

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I

I f ' I

/

d

Figure 4. (a) of population of population

b I C

/

All lineaments in the interpretation; (b Lineaments of population A; ( c ) Lineaments B; (d) Lineaments of population C; (e! Lineaments of population D; (f) Lineaments E.

there was movement on the Ystwyth and Llyfnant faults subsequent to the formation of the local mineral veins.

James and James (1969) demonstrated significant thickness and facies changes within the Ashgillian and Llandoverian rocks of west Wales. These they ascribed to synsedimentary movement on basement fractures and suggested that the Ystwyth and Llyfnant faults were two of the structures responsible. This implies Lower Palaeozoic movement on the faults.

Cave (1979) showed that the sense of displacement on the Ystwyth fault (downthrow to the north) is incompatible with the theory of James and James.

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The movement on the Ystwyth fault could not have accommodated the large observed sediment thickness increase to the south. This makes questionable the only evidence for pre-Caledonian fault movements.

5b. The Pennel and Cwm Sylwi faults These long ENE-trending faults lie just north of the Dovey Estuary. Fault

displacements are not certain although significant downthrows to the north are indicated by Jones and Pugh (1915). These two named features form part of a complex zone of parallel faults and shatter belts over 3 km wide and of undeter- mined length.

5c. Mineralized normal faults in the Plynlimon Dome area These faults, the sites of lead-zinc mineralization extensively exploited during

the 19th Century, were described in detail by Jones (1922), both in terms of historical mine outputs and geological setting. The mineralization occurs in pods within wide fault breccia zones. The displacement on the faults is dominantly normal although dextral strike-slip movement may occasionally be inferred on some of the larger faults (e.g. the Camdwr Lode). Vertical displacements com- monly exceed 200 m.

Moorbath (1962) examined lead isotope abundances in galena from four Cardi- ganshire mines and indicated an age of 430 k 40 Ma. This would imply an Ordovi- cian or Silurian age for vein formation.

Phillips (1972) has shown that the mineralization occurred during fault initiation and propagation in a tensile environment. The mineralizing fluids had the effect of lowering the stresses required to cause tensile failure and caused the hydraulic brecciation of the country rocks.

Raybould (1976) demonstrated that the mineralized faults locally deviate to follow the trend of the a-c relaxation joints of the area. As well as adding supporting evidence for the ideas suggested by Phillips, this demonstrates that the veins post-date the local joint set. The formation of the joints probably occurred during the relaxation phase at the end of the Caledonian deformation (Price 1962). The formation of the mineralized faults must post-date this event and, according to Fitches (1987), are probably due to early Variscan extension.

An isotopic age determination more credible than that obtained by Moorbath (1962) was obtained by Ineson and Mitchell (1975). They used K-Ar techniques on wallrock alteration products in the vicinity of some of the veins. An age of 356 2 7 Ma was obtained in this way. This estimate fits the age constraint given by Raybould whereas the age determination supplied by Moorbath does not.

In conclusion it can be said that the mineralized normal faults were initiated by hydraulic fracturing after the formation of a-c relaxation joints at the end of the Caledonian deformation. The theory suggested by Phillips and corroborated by Raybould is not compatible with ideas of formation by re-activation of previously existing structures.

5d. Veins of the Dolgellau Gold Belt On the southeast flank of the Harlech Dome, there is a series of ENE to NE-

trending fractures that are in places mineralized with base metals and gold. Some of these mineralized fractures are visible in the satellite imagery and form part of the Ystwyth Lineament Set. Some published estimates of the age of formation of the veins are as follows:

REMOTELY SENSED LINEAMENTS

Author Dewey and Eastwood (1925) Dunham (1947) Archer (1959) Moorbath (1962) Ineson and Mitchell (1975) Shepard and Allen (1985) Fitches (1987)

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Postulated age of formation Post-Carboniferous-Pre-Triassic Caledonian Post-Carboniferous-pre-Triassic 430 2 4 Ma (Silurian) Between 368 and 397 Ma (Devonian) End of Caledonian deformation Early Variscan

Ashton (1981), in a comparison with the veins of the Plynlimon Dome area, showed that both sets of veins were formed in a tensile regime, but that the Dolgellau veins were formed at a lower structural level. Shepard and Allen (1985) have shown how the mineralization may have occurred during a period of hydraulic fracturing in a tensile environment at the end of the Caledonian deformation. There is'a striking similarity between this interpretation and the preferred theory of the formation of the mineralized normal faults of the Plynlimon Dome area. The postulated tectonic environment, mode of formation, and age are all compatible.

5e. ENE mesofractures observed in the field in north Ceredigion Attempts were made to investigate the geological nature of the lineaments of

this set by direct field observation but these were severely restricted by the lack of outcrop in the neighbouring areas. Only in rare cases is there outcrop within 200 m of the mapped position of the lineaments and in none of these cases was a study of that outcrop in any way illuminating. The only conclusion drawn from this initial field 'ground truthing' exercise was that there is a direct correlation between the visual prominence of a lineament on the image and the width of the zone of low percentage exposure around it on the ground.

Thus major lineaments tend to be clearly visible not only because of their length but because of the wide zone of subdued exposure they induce. Conversely, structures less than 500 m in length are usually invisible on the images partly because of their relatively small effect on the percentage of exposure. It was decided therefore that the best opportunity for observing geological factors that cause lineaments was to examine the exposures associated with structures that are themselves too small to appear on the images but that are thought to be related to the larger structures and show the same features. For this reason a field based fracture analysis was carried out with the intention of testing whether or not a statistically significant population of mesofractures exists that has the same azi- muthal distribution as the lineament set. The area chosen for the study was a 7 km stretch of wave-cut platform in north Ceredigion with greater than 85 per cent rock exposure. Within that study area every fracture measurable with an accuracy of greater than ?3' of azimuth and dip with a displacement of more than 3 mm was observed and systematically described. In addition several fractures with less than 3 mm displacememts were recorded in this way.

The investigation showed the existence of several fracture sets, the most numeri- cally important of which has an azimuthal trend of between 068" and 080". These fractures cross-cut and therefore post-date the local a-c relaxation joints and cleavage, both thought to be Caledonian structures (Raybould 1976; Price 1962). The fractures observed commonly have displacements between 10 and 100 cm. Those with displacements over 5 m are seldom exposed in outcrop although they are demonstrably present below several gullies that separate areas of relatively complete exposure.

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Figure 5 . Rose diagram to show the distribution of slickenside lineation plunges in ENE-trending mesofractures in north Ceredigion.

Striae and crystal lineations are often present on slickenside surfaces. Where the vector of displacement is demonstrable (e.g. by matching an identifiable bed within a fold on each side of the fracture), it is invariably corroborated by the slickenside data. The frequency distribution of slickenside surface feature plunges (Figure 5 ) indicates a relatively constant sense of oblique (normal/dextral) displace- ment.

The fractures are spatially grouped in high fracture density swarms, with most areas remaining comparatively unaffected. This grouping suggests that the frac- tures were formed in zones of weakness possibly near the propagating edge of larger faults. These larger faults would not usually be seen in the field due to the action of preferential erosion.

The mesofractures may be in the same orientation as the postulated parent faults. The zone of weakness would be partially due to the movement of connate fluids concentrated by the increasingly fractured and therefore permeable rock.

Another possibility is that the mesofractures are Riedel shears formed in the shearing conditions that must exist around the propagating edge of a larger fault. The shears would be formed with an orientation rotated 15" clockwise from the parent fault. The azimuth difference between the Riedel shears and the parent fault would be less than 15" as the movement involved is not purely strike-slip.

6. Origin of the Ystwyth Lineament Set

As discussed in the preceding sections, the various fault systems that fall within this same azimuthal range are of differing character and possibly of disparate ages. An explanation is presented here for the formation of this group of structures, including a plausible chronology of events.

REMOTELY SENSED LINEAMENTS 117

The idea that the mineral veins of the Plynlimon Dome area were formed through the re-activation of earlier fractures is inconsistent with the work of Phillips (1972). If his theory is accepted then a significant set of fractures of similar azimuth could not have existed before the formation of the veins. For this reason it is suggested that the first event in the genesis of the lineament set in that area was the formation of mineral veins by hydraulic fracturing. The NNW-SSE crustal tension required could also have been responsible for the formation of veins by similar processes in the Harlech Dome area. This event must have been post- Caledonian because the veins cross-cut the Caledonian cleavage and a-c relaxation joints. It is possible that the Pennal and Cwm Sylwi faults were also formed at this time.

The Llyfnant and Ystwyth faults have significant amounts of strike-slip displace- ment and were therefore not formed during the same early tensile stage. The re- activation and propagation of earlier normal faults is the most likely explanation of their origin. The ENE-trending mesofractures of north Ceredigion are probably Riedel shears developed near faults being propagated during this event. The tectonic environment was probably tensile as the vertical component of displace- ment on the fractures is consistently normal. The tension would have been oriented NE-SW to cause dextral displacements.

Some of the large faults display sinistral displacements which cannot have been formed during either of the two events described. These movements must have occurred in subsequent episodes by re-activation.

7. Conclusions

Digitally enhanced satellite images are a source of geological information that is complementary to traditional methods of investigation. The interpretation pre- sented in this paper shows sets of lineaments of which many individuals were previously unidentified. The nature of the geological structures that have given rise to them is in many cases uncertain but their existence is not questionable. Although no genetic relationship between the members of any one set is proved, the existence of so many features with such a high degree of parallelism is a. strong indication of a related origin.

The Ystwyth Lineament Set was formed by a several stage process: 1. The formation of the mineralized veins of the Plynlimon and Harlech Domes

and of several other normal faults such as at Pennal and Cwm Sylwi. This was a post-Caledonian event and was the result of NNE-SSW tension.

2. The formation of the Ystwyth and Llyfnant faults by reactivation and propa- gation of faults formed during stage 1, and the formation of numerous mesofractures in north Ceredigion, probably as Riedel shears near larger propagation faults. This event was caused by crustal extension in a NE-SW direction.

3. Re-activation of the larger faults during at least one further event in which sinistral movements occurred.

Acknowledgements. I wish to express thanks to the following people for their help during the production of this manuscript: Dr. A.J. Maltman, Dr. W.R. Fitches, Miss S.M. Jennison, Mr. J . Arch, Mrs. N.P. Maude, Dr. J.M. Mitchell, and Dr. J. James. This work was financially supported by a research grant from the University of Wales.

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