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TM data processing techniques on Lesvos, Greece
Geomorphology 34 Ž2000. 101–109 www.elsevier.nlrlocatergeomorph
Identifying geomorphic features using LANDSAT-5rTM data processing techniques on Lesvos, Greece I.D. Novak a,) , N. Soulakellis b b
a Department of Geosciences, UniÕersity of Southern Maine, Gorham, ME 04038, USA Department of Human Geography, UniÕersity of the Aegean, 81100 Mytilene, LesÕos, Greece
Received 5 February 1999; accepted 5 December 1999
Abstract In order to delineate the geomorphic features of the island of Lesvos, Greece, fieldwork and digital satellite ŽLANDSAT-5rTM. data analysis were combined. The main image analysis techniques involved in this study were Ž1. principal component analysis ŽPCA. and Ž2. false color composite ŽFCC.. These techniques led to the creation of enhanced satellite images with respect to the topographic and geomorphologic characteristics of the island. The final digitally enhanced LANDSAT-5rTM images have been used to interpret and map the geomorphic features of Lesvos. Ground-truthing of image data led to the identification of the following geomorphic units: dissected metamorphic terrains, lava plateaus, colluvial foothills, alluvial plains Žincluding coastal and fluvial., and eroded regions underlain by ignimbrite and welded tuff. Lineaments detected in this study were coincident with: Ž1. mapped contacts or faults, Ž2. extensions of previously mapped faults or Ž3. previously undetected geologic features including faults, tectonic contacts or formational contacts. q 2000 Elsevier Science B.V. All rights reserved. Keywords: geomorphology; lineaments; remote sensing; Lesvos, Greece
1. Introduction This paper describes lineaments and geomorphic units delineated from digital satellite images of Lesvos island, Greece. The study is part of a larger effort to fully understand the tectonic regime and geomorphic responses that have shaped the island. We chose to use a conservative approach, limiting
) Corresponding author. Tel.: q1-207-780-5025; fax: q1-207780-5005. E-mail address: [email protected] ŽI.D. Novak..
our interpretation to macro-lineaments and to clearly recognizable geomorphic units. Lesvos is the third largest of the Greek Islands ŽFig. 1A.. The island comprises 1632 km2 , with a maximum length of 70 km and width of 45 km. The island is closer to the Turkish mainland than the Greek mainland, being only 14 km west of Turkey across the Aegean Sea. There is a general ginko leaf-like appearance to the island of Lesvos. This appearance derives from the two gulfs, which divide the island into three unequal parts ŽFig. 2.. Both the Gulfs of Yera, in the east, and the Gulf of Kalloni, in the west, have very narrow openings to the sea. The gulfs are, in all likelihood, a product of local re-
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I.D. NoÕak, N. Soulakellisr Geomorphology 34 (2000) 101–109
sponse, in the form of grabens, to regional tectonism. The island is interspersed with plains, though it is dominated by mountains. The two major peaks are Mt. Olympus Ž968 m. and Mt. Lepetimnos Ž968 m. in the northern part of the island. Given that the local terrain rises quickly from sea level to lava or tephra plateaus and then to the mountains, the local relief ranges from 150 m to 900 m depending upon the degree of dissection. Also known as the ‘‘emerald island’’ because of the lush green of the olive and pine trees, Lesvos is famous for its olive oil, ouzo, and soap. It has one of the largest petrified forests and one of the best preserved Roman aqueducts in the world.
2. Regional geology The geology of Lesvos is a part of the complex geology of Greece, the Aegean Sea, and the eastern Mediterranean. The rocks and structures of Lesvos ultimately reflect the tectonic framework and basin accretionary sequences of the eastern Mediterranean ŽFig. 1B.. The compound interaction of the closing of the Mediterranean in the north–south direction and the westward movement of Anatolia ŽTurkey. have contributed to the production, as well as uplift and exposure of metamorphosed rocks. Extrusive igneous materials overlie these, in turn. The rocks have also been subjected to lateral motion that produced low angle faults. Regional crustal uplift and tension produced a series of horsts and grabens bounded by near vertical faults. As the western Tethys was subducted, the intervening continental fragments attached to it collided with Eurasia to produce a Himalayan-style collisional orogen formed ŽDewey and Sengor, 1979; Sengor and Yilmaz, 1981.. The subduction zone
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migrated to the SW and this orogen began to collapse 20–25 Ma. The former thrust nappes ‘‘delaminated’’, i.e., thrust displacements were reversed in many places, and middle crustal metamorphic rocks from the core of the collisional orogen exposed in the footwalls of major low-angle normal faults all over the southern Balkans and possibly, Lesvos ŽDinter et al, 1995; Dinter, personal communication.. Eventually, continental crustal extension amounted to ) 100% and the Aegean Sea formed on the site of the former mountain chain. Finally, in mid-Pliocene time Ž; 3.5 Ma., the N. Anatolian Fault, associated with the westward Žlateral. escape of Anatolia from the Pontide suture, propagated into the northern Aegean, cross-cutting the older compressional and extensional structures and creating the present tectonic regime. In summary, sediments that had accumulated in the relatively shallow western arm of the Cimmeron basin of the western Tethys Sea were subjected to compressional forces related to subduction and collision. This caused deformation, metamorphism, and overthrusting with the superposition of multiple convergent events. Subsequently, crustal extension formed grabens, which are associated regionally with numerous hot springs and extrusive lava flows.
3. Geology of Lesvos The island is composed of metaclastic rocks, volcano-clastic formations and ophiolitic rocks of late Paleozoic and Triassic age ŽFig. 1C.. Post-Alpine volcanic, lacustrine, alluvial and colluvial deposits overlie more than one-half of the terrain. Pe-Piper and Piper Ž1993. and Pe-Piper et al. Ž1995. attribute the volcanic rocks of Lesvos and Chios Žthe next island south of Lesvos. to early Miocene back-arc
Fig. 1. ŽA. Location map of Lesvos, Greece Ždiagonal lines. and surrounding countries. ŽB. Regional tectonic map: AGS indicates the Aegean Graben System; AVA denotes the Aegean Volcanic Arc; HA refers to the Hellenic Arc Žbarbs in the direction of subduction.; and NAT is the Northern Anatolian Transform. Black-tipped arrows indicate the sense of plate motion relative to the fixed Eurasian plate; half-arrows indicate transformrstrike-slip faults. ŽB. is modified from Aksu et al Ž1992.. ŽC. Generalized geologic map of Lesvos. Stipples represent Quaternary and Neogene formations. Small ‘‘ v’’ denotes peridotites and serpentinites. Diagonals from upper-left to lower-right signify Triassic metabasites and metaclastic formations Žwith intercalations of crystalline carbonate rocks shown as narrow horizontal lines.; diagonals from upper-right to lower-left indicate Triassic schists and metasandstones Žwith intercalations of crystalline carbonate rocks shown by narrow diagonal lines drawn in opposite direction.. Wide horizontal lines represent Neopaleozoic schists and metasandstones Žwith intercalations of crystalline carbonate rocks shown by narrow diagonal lines drawn in the opposite direction.. ŽC. is modified from Hecht Ž1971–1974. and Katsikatsos et al. Ž1986..
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Fig. 2. Geomorphic units of Lesvos, Greece as interpreted from final RGB digital image; north is at the top. ŽA. Dissected tephra hills Žmoderate relief.. ŽB. Tilted Ž?. plains underlain by ignimbrite or lava. ŽC. Dissected hills underlain by schistrmarble Žhigh relief.. ŽD. Salt flats. ŽE. Alluvial plains. ŽF. Hills underlain by ophiolites. This black and white version was derived from the FCC. PC-1, PC-2, and TM-band 7 were combined to produce the final FCC image by assigning each image to a separate primary color ŽRGB..
extension. The ophiolites and eastern carbonates of Lesvos may have been part of the Neo-Tethys basement and shelf sediments, respectively. The ophiolites represent ocean basement rocks thrust upward by compression. Due to the most recent tectonic developments, Lesvos is located within one of the most seismically active regions in the world. From a geologic mapping perspective, there appears to be a general agreement on the distribution of rock types on Lesvos ŽHecht, 1971–1974; Katsikat-
sos et al., 1986.. Differences arise when specific areas are mapped at larger scales.
4. LANDSAT-5r r TM data processing techniques ERDAS ‘‘Imagine’’ software was used to analyze a sub-window of a LANDSAT-5rTM ŽThematic Mapper. image acquired on June 20, 1990 ŽRow: 33; Path: 181; center coordinates: Lat. 39.40190038,
I.D. NoÕak, N. Soulakellisr Geomorphology 34 (2000) 101–109
Long. 26.21107928.. The image covers 75 km by 53 km. Selected points from a previously rectified planimetric map of Lesvos were matched with the equivalent points on the unrectified satellite image. The computer ‘‘resampled’’ the whole image and ‘‘drew’’ a new rectified satellite image to match the planimetric map. Additional points were selected to reduce rectification error to a minimum. As a check on this approach, previously digitized and rectified geologic and topographic maps were overlain on the image with excellent equivalency. The image processing techniques employed in this study were adapted from Astaras and Soulakellis Ž1990. as applied in Greece and used standard methods as described in most remote sensing texts, e.g., Sabins Ž1987.. These methods include the following: Ž1. principal component analysis ŽPCA or Transformation. and Ž2. false color composite ŽFCC.. 4.1. PCA The PCA technique provides a systematic means of compressing the multi-spectral image data with the aim of reducing the redundancy in the different bands. The PCA has the following advantages: Ž1. most of the variance in a multi-spectral data set are compressed into one or two PC images; Ž2. noise may be relegated to the less-correlated PC images; and Ž3. spectral differences between materials may be more apparent in PC images than in individual bands ŽSabins, 1987.. Principal component ŽPC. images were prepared for the three TM visible and three reflected infrared ŽIR. bands of the available ŽLesvos. LANDSAT5rTM sub-scene. The first three PC images ŽPC 1, 2, 3. contain 97% of the variation of the original six TM bands, which is a significant compression of data. The last three PC images ŽPC 4, 5, 6. account for only 2.6% of the original variation ŽSabins, 1987.. The first PC image ŽPC-1. is a weighted positive sum of all the original bands and it represents a panchromatic view of the area containing 88% of the data variance. It is dominated by topography, expressed as highlights and shadows that are highly correlated in all six of the original TM bands. The second PC image ŽPC-2. represents a difference between the infrared and visible bands and as such, it serves to enhance any spectral differences
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between those parts of the spectrum ŽCanas and Barnett, 1985.. Also, PC-2 is dominated by differences in albedo which also correlates from band to band because pixels that are bright in one TM band tend to be bright in adjacent bands ŽSabins, 1987.. 4.2. FCC An FCC image is arguably the most effective means of visual presentation of multi-channel Žmulti-band. image and survey data. A decision has to be made about the information content of the final color composite as a fraction of the total information available. Normally, workers choose three original bands in red, green, and blue ŽRGB.. A standard FCC image contains 73% of the available image variance, whereas the principal composite image contains 97% ŽCanas and Barnett, 1985.. On this basis, the first two PC images were employed in the present study in order to create FCC images. Some 20 combinations of original image bands andror principle components ŽPC. were tested for image enhancement qualities. Of these, six offered satisfactory enhancements in FCCs in red, green, and blue ŽRGB.. Of these six, two were the most acceptable using topography, drainage, and lineament definition as criteria for selection. One of the two was composed of original TM-band 3, PC-1, and PC-2; and the other one was composed of original TM-band 7, PC-1, and PC-2. The latter combination offers slight advantages because it includes the first and second axes of PC images along with band 7 which is the most appropriate band for geologic investigations. The two PC-images and TM-band 7 were combined to produce a final FCC image by assigning each image to a separate primary color ŽRGB.. This final digitally enhanced image was interpreted for geologic and geomorphic features, and was initially reported on by Novak and Soulakellis Ž1996..
5. Discussion 5.1. Geomorphology Analysis of satellite imagery has repeatedly demonstrated its value in deciphering the interrelation between lithology, structures, and landforms
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ŽKrishnamurthy and Srinivas, 1996.. A basic tenet of geomorphology is that geomorphic units can be expected to reflect underlying bedrock types and the processes that have been at work. Therefore, a geomorphic unit has a specific set of characteristics Žfor example, texture, tone, and reflectance. that determines its image signature. However, it is still possible that with the right combination of reflectance characteristics of soil and vegetation, the two different bedrock types might produce similar image expression. For this reason, we were cautious, and only delimited those geomorphic units that could be clearly differentiated from one another based on field observation and image processing techniques ŽFig. 2.. We believe the units shown here are primarily geomorphicrgeologic in origin and not simply vegetative. Based on field observations, and image tone and texture, we defined six different geomorphic units.
istics. Examples include areas to the north and northwest of the Gulfs of Kaloni and Yera Žsee Fig. 2..
ŽA. Dissected tephra hills Žmoderate relief. ŽB. Tilted Ž?. plains underlain by ignimbrite or lava ŽC. Dissected hills underlain by schistrmarble Žhigh relief. ŽD. Salt flats ŽE. Alluvial plains ŽF. Hills underlain by ophiolites
Lineament is the name given by geologists to lines or edges, of presumed geologic origin, visible on remotely sensed images ŽCampbell, 1987.. According to Campbell Ž1987, p. 439. ‘‘controversy arises when we attempt to judge the geological significance Žif any. of these features. There are sound reasons for assigning a geological meaning to some lineaments even if they do not always correspond to observable physical feature.’’ Not all faults are expressed topographically, but a fault plane may offer a preferred avenue to moisture, to vegetative growth, and may alter drainage patterns and therefore, the lineament may have structural origins. Given the sometimes-contentious history of lineament studies, we chose a conservative approach, electing to delineate only the most clearly discernible longer lineaments. Fig. 3A is a black and white version of the final RGB image. Fig. 3B shows the lineaments interpreted from the image and some limited field data. Some lineaments detected on this image were coincident with tectonic contacts or faults mapped by Hecht Ž1971–1974.. The lineaments of this type are shown as solid black lines ŽFig. 3B.. Some lineaments were hitherto undetected extensions of those mapped features. However, many lineaments detected in the analysis were previously unmapped and could be such features as faults, tectonic contacts or formational contacts.
5.1.1. Plains The plains are of two types: Ž1. those formed atop major lava flows ŽB. and Ž2. those formed near the coast by fluvial processes ŽE.. The lower Ž- 150 m. lava flows and ignimbrites could be considered plains; but they are ‘‘stacked’’ in such a way that those at higher elevations Ž) 150 m. are more correctly referred to as plateaus. Many of the surface lava flows andror ignimbrites appear to be tilted. We have not been able to determine whether this tilt is the original flow surfaces or if there was post-emplacement tilting due to tectonism. The plains near the coasts derive from floodplain andror deltaic deposits and may have been modified by coastal processes. These areas are often the vegetable growing regions of the island due to increased soil fertility, thickness, and water retention character-
5.1.2. Hills Post-Triassic to pre-Neogene intrusive rocks, the oldest on the island, are metamorphic in origin and underlie the most highly dissected hills in the east and southeast ŽC and F. ŽHecht, 1971–1974.. The rocks are highly sheared Žschists and ophiolites.; in some cases, highly soluble Žmarbles and slightly metamorphosed limestones., so that there are numerous structural weakness that have been exploited by weathering. The tephra hills ŽA. are underlain by Neogene rocks ŽPe-Piper et al, 1995. and have less inherent structural weaknesses. They are dissected much less and exhibit more moderate relief. 5.2. Lineaments
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Fig. 3. ŽA. Black and white version of FCC of Lesvos, Greece showing some of the major lineaments Žarrows.; north is at the top. ŽB. Lineament map interpreted from ŽA. and limited field data; north is at the top. Solid line represents previously mapped faults or tectonic contacts Žfrom Hecht, Ž1971–1974... Short dashes indicate extensions, observed on the image, of the previously mapped faults or tectonic contacts. Long dashes denote newly observed lineaments not associated with any previously mapped feature. Hachures show direction of down-dropped blocks along dipping normal faults.
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thermal springs and earthquake epicenters. Because remote-sensing techniques alone cannot provide a complete picture of the region, the methods specified above will be combined using geographic information systems ŽGIS..
Acknowledgements Discussions with Mark Swanson and David Dinter greatly contributed to an understanding of the tectonics of the region. We also thank Mary Snell and John Hatzopoulos for their generous support and encouragement.
Fig. 4. Rose diagram of the strike directions of lineaments on Lesvos and shown in Fig. 3B. Two major sets of lineaments were defined: one NE–SW and the other, NW–SE.
Analysis of the Lesvos image defined two major sets of lineaments: one NE–SW and the other, NW– SE. The strike directions of lineaments are illustrated in a rose diagram showing the two dominant sets ŽFig. 4. and suggest two dominant directions of tension. Nearly all of the faults mapped by Hecht Ž1971–1974. were oriented vertically. The few nonvertical fault planes are shown with hachures in Fig. 3B. The dip-direction of these faults suggests that northeast to southwest tension produced the grabens oriented NW–SE, and the faults oriented NW–SE were produced by northeast to southwest tension.
6. Future In order to refine the descriptive characteristics of the geomorphic units and precisely define the boundaries between them, the use of additional digital methods correlated with field observation is currently underway. These methods include the creation of an accurate digital elevation model ŽDEM., a slope map, and an aspect map. In order to more accurately determine the relationship between geomorphic units and bedrock types, a parallel effort to define drainage networks from the DEM will be undertaken. It is also our intention to explore the relationships between lineaments and the location of
References Aksu, A.E., Calon, T.J., Piper, D.J.W., Turgut, S., Izdar, E., 1992. Architecture of the late orogenic Quaternary basins in northeastern Mediterranean Sea. Tectonophysics 210, 191–213. Astaras, Th., Soulakellis, N., 1990. LANDSAT-TM data processing techniques for mapping geological and geomorphological features in the Central Macedonia area, N. Greece. In: Savascin, M.Y., Eronat, A.H. ŽEds.., Proceedings of the International Earth Sciences Congress on Aegean Regions Vol. II pp. 76–91. Campbell, J.B., 1987. Introduction to Remote Sensing. The Guilford Press, New York. Canas, A., Barnett, M., 1985. The generation and interpretation of false-colour composite principal component images. International Journal of Remote Sensing 6, 867–881. Dewey, J.F., Sengor, A.M.C., 1979. Aegean and surrounding regions: complex multi-plate and continuum tectonics in a convergent zone. Geological Society of America Bulletin 90, 84–92. Dinter, D.A., Macfarlane, A., Hames, W., Isachsen, C., Bowring, S., Royden, L., 1995. U–Pb and 40Arr39Ar geochronology of the Symvolon granodiorite: implications for the thermal and structural evolution of the Rhodope metamorphic core complex, northeastern Greece. Tectonics 14, 886–904. Hecht, J., 1971–1974. Geologic Map of Lesvos Island Ž5 sheets.. National Institute of Geological and Mining Research ŽIGME., Athens. Katsikatsos, G., Migiros, G., Triantaphyllis, M., Mettos, A., 1986. Geological structure of the internal Hellines Žeast Thessaly– southwest Macedonia, Euboea–Attica–northern Cyclades islands and Lesvos.. Institute of Geological and Mineral Exploration, Geology and Geophysical Research, 191–212, Special Issue. Krishnamurthy, J., Srinivas, G., 1996. Demarcation of the geological and geomorphological features of parts of Dharwar Craton, Karnataka, using IRS LISS-II data. International Journal of Remote Sensing 17, 3271–3288.
I.D. NoÕak, N. Soulakellisr Geomorphology 34 (2000) 101–109 Novak, I.D., Soulakellis, N., 1996. Mapping geologic and geomorphic features using LANDSAT-TM data processing techniques on Lesvos island. Geological Society of America 28, A301–A302, Annual Meeting, Abstracts with Programs. Pe-Piper, G., Piper, D.J.W., 1993. Revised stratigraphy of the Miocene volcanic rocks of Lesvos, Greece. Nues Jahrbuch fuer Geologie und Paleontologie 2, 97–110. Pe-Piper, G., Piper, D.J.W., Kotopouli, C.N., Panagos, A.G.,
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1995. Neogene volcanoes of Chios, Greece: the relative importance of subduction and back-arc extension. In: Smellie, J.L. ŽEd.., Volcanism Associated with Consuming Plate Margins. Geological Society. pp. 213–231, Special Publication No. 81. Sabins, F., 1987. Remote Sensing: Principles and Interpretation. 2nd edn. Freeman, New York. Sengor, A.M.C., Yilmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241.
Identifying geomorphic features using LANDSAT-5rTM data processing techniques on Lesvos, Greece I.D. Novak a,) , N. Soulakellis b b
a Department of Geosciences, UniÕersity of Southern Maine, Gorham, ME 04038, USA Department of Human Geography, UniÕersity of the Aegean, 81100 Mytilene, LesÕos, Greece
Received 5 February 1999; accepted 5 December 1999
Abstract In order to delineate the geomorphic features of the island of Lesvos, Greece, fieldwork and digital satellite ŽLANDSAT-5rTM. data analysis were combined. The main image analysis techniques involved in this study were Ž1. principal component analysis ŽPCA. and Ž2. false color composite ŽFCC.. These techniques led to the creation of enhanced satellite images with respect to the topographic and geomorphologic characteristics of the island. The final digitally enhanced LANDSAT-5rTM images have been used to interpret and map the geomorphic features of Lesvos. Ground-truthing of image data led to the identification of the following geomorphic units: dissected metamorphic terrains, lava plateaus, colluvial foothills, alluvial plains Žincluding coastal and fluvial., and eroded regions underlain by ignimbrite and welded tuff. Lineaments detected in this study were coincident with: Ž1. mapped contacts or faults, Ž2. extensions of previously mapped faults or Ž3. previously undetected geologic features including faults, tectonic contacts or formational contacts. q 2000 Elsevier Science B.V. All rights reserved. Keywords: geomorphology; lineaments; remote sensing; Lesvos, Greece
1. Introduction This paper describes lineaments and geomorphic units delineated from digital satellite images of Lesvos island, Greece. The study is part of a larger effort to fully understand the tectonic regime and geomorphic responses that have shaped the island. We chose to use a conservative approach, limiting
) Corresponding author. Tel.: q1-207-780-5025; fax: q1-207780-5005. E-mail address: [email protected] ŽI.D. Novak..
our interpretation to macro-lineaments and to clearly recognizable geomorphic units. Lesvos is the third largest of the Greek Islands ŽFig. 1A.. The island comprises 1632 km2 , with a maximum length of 70 km and width of 45 km. The island is closer to the Turkish mainland than the Greek mainland, being only 14 km west of Turkey across the Aegean Sea. There is a general ginko leaf-like appearance to the island of Lesvos. This appearance derives from the two gulfs, which divide the island into three unequal parts ŽFig. 2.. Both the Gulfs of Yera, in the east, and the Gulf of Kalloni, in the west, have very narrow openings to the sea. The gulfs are, in all likelihood, a product of local re-
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sponse, in the form of grabens, to regional tectonism. The island is interspersed with plains, though it is dominated by mountains. The two major peaks are Mt. Olympus Ž968 m. and Mt. Lepetimnos Ž968 m. in the northern part of the island. Given that the local terrain rises quickly from sea level to lava or tephra plateaus and then to the mountains, the local relief ranges from 150 m to 900 m depending upon the degree of dissection. Also known as the ‘‘emerald island’’ because of the lush green of the olive and pine trees, Lesvos is famous for its olive oil, ouzo, and soap. It has one of the largest petrified forests and one of the best preserved Roman aqueducts in the world.
2. Regional geology The geology of Lesvos is a part of the complex geology of Greece, the Aegean Sea, and the eastern Mediterranean. The rocks and structures of Lesvos ultimately reflect the tectonic framework and basin accretionary sequences of the eastern Mediterranean ŽFig. 1B.. The compound interaction of the closing of the Mediterranean in the north–south direction and the westward movement of Anatolia ŽTurkey. have contributed to the production, as well as uplift and exposure of metamorphosed rocks. Extrusive igneous materials overlie these, in turn. The rocks have also been subjected to lateral motion that produced low angle faults. Regional crustal uplift and tension produced a series of horsts and grabens bounded by near vertical faults. As the western Tethys was subducted, the intervening continental fragments attached to it collided with Eurasia to produce a Himalayan-style collisional orogen formed ŽDewey and Sengor, 1979; Sengor and Yilmaz, 1981.. The subduction zone
103
migrated to the SW and this orogen began to collapse 20–25 Ma. The former thrust nappes ‘‘delaminated’’, i.e., thrust displacements were reversed in many places, and middle crustal metamorphic rocks from the core of the collisional orogen exposed in the footwalls of major low-angle normal faults all over the southern Balkans and possibly, Lesvos ŽDinter et al, 1995; Dinter, personal communication.. Eventually, continental crustal extension amounted to ) 100% and the Aegean Sea formed on the site of the former mountain chain. Finally, in mid-Pliocene time Ž; 3.5 Ma., the N. Anatolian Fault, associated with the westward Žlateral. escape of Anatolia from the Pontide suture, propagated into the northern Aegean, cross-cutting the older compressional and extensional structures and creating the present tectonic regime. In summary, sediments that had accumulated in the relatively shallow western arm of the Cimmeron basin of the western Tethys Sea were subjected to compressional forces related to subduction and collision. This caused deformation, metamorphism, and overthrusting with the superposition of multiple convergent events. Subsequently, crustal extension formed grabens, which are associated regionally with numerous hot springs and extrusive lava flows.
3. Geology of Lesvos The island is composed of metaclastic rocks, volcano-clastic formations and ophiolitic rocks of late Paleozoic and Triassic age ŽFig. 1C.. Post-Alpine volcanic, lacustrine, alluvial and colluvial deposits overlie more than one-half of the terrain. Pe-Piper and Piper Ž1993. and Pe-Piper et al. Ž1995. attribute the volcanic rocks of Lesvos and Chios Žthe next island south of Lesvos. to early Miocene back-arc
Fig. 1. ŽA. Location map of Lesvos, Greece Ždiagonal lines. and surrounding countries. ŽB. Regional tectonic map: AGS indicates the Aegean Graben System; AVA denotes the Aegean Volcanic Arc; HA refers to the Hellenic Arc Žbarbs in the direction of subduction.; and NAT is the Northern Anatolian Transform. Black-tipped arrows indicate the sense of plate motion relative to the fixed Eurasian plate; half-arrows indicate transformrstrike-slip faults. ŽB. is modified from Aksu et al Ž1992.. ŽC. Generalized geologic map of Lesvos. Stipples represent Quaternary and Neogene formations. Small ‘‘ v’’ denotes peridotites and serpentinites. Diagonals from upper-left to lower-right signify Triassic metabasites and metaclastic formations Žwith intercalations of crystalline carbonate rocks shown as narrow horizontal lines.; diagonals from upper-right to lower-left indicate Triassic schists and metasandstones Žwith intercalations of crystalline carbonate rocks shown by narrow diagonal lines drawn in opposite direction.. Wide horizontal lines represent Neopaleozoic schists and metasandstones Žwith intercalations of crystalline carbonate rocks shown by narrow diagonal lines drawn in the opposite direction.. ŽC. is modified from Hecht Ž1971–1974. and Katsikatsos et al. Ž1986..
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Fig. 2. Geomorphic units of Lesvos, Greece as interpreted from final RGB digital image; north is at the top. ŽA. Dissected tephra hills Žmoderate relief.. ŽB. Tilted Ž?. plains underlain by ignimbrite or lava. ŽC. Dissected hills underlain by schistrmarble Žhigh relief.. ŽD. Salt flats. ŽE. Alluvial plains. ŽF. Hills underlain by ophiolites. This black and white version was derived from the FCC. PC-1, PC-2, and TM-band 7 were combined to produce the final FCC image by assigning each image to a separate primary color ŽRGB..
extension. The ophiolites and eastern carbonates of Lesvos may have been part of the Neo-Tethys basement and shelf sediments, respectively. The ophiolites represent ocean basement rocks thrust upward by compression. Due to the most recent tectonic developments, Lesvos is located within one of the most seismically active regions in the world. From a geologic mapping perspective, there appears to be a general agreement on the distribution of rock types on Lesvos ŽHecht, 1971–1974; Katsikat-
sos et al., 1986.. Differences arise when specific areas are mapped at larger scales.
4. LANDSAT-5r r TM data processing techniques ERDAS ‘‘Imagine’’ software was used to analyze a sub-window of a LANDSAT-5rTM ŽThematic Mapper. image acquired on June 20, 1990 ŽRow: 33; Path: 181; center coordinates: Lat. 39.40190038,
I.D. NoÕak, N. Soulakellisr Geomorphology 34 (2000) 101–109
Long. 26.21107928.. The image covers 75 km by 53 km. Selected points from a previously rectified planimetric map of Lesvos were matched with the equivalent points on the unrectified satellite image. The computer ‘‘resampled’’ the whole image and ‘‘drew’’ a new rectified satellite image to match the planimetric map. Additional points were selected to reduce rectification error to a minimum. As a check on this approach, previously digitized and rectified geologic and topographic maps were overlain on the image with excellent equivalency. The image processing techniques employed in this study were adapted from Astaras and Soulakellis Ž1990. as applied in Greece and used standard methods as described in most remote sensing texts, e.g., Sabins Ž1987.. These methods include the following: Ž1. principal component analysis ŽPCA or Transformation. and Ž2. false color composite ŽFCC.. 4.1. PCA The PCA technique provides a systematic means of compressing the multi-spectral image data with the aim of reducing the redundancy in the different bands. The PCA has the following advantages: Ž1. most of the variance in a multi-spectral data set are compressed into one or two PC images; Ž2. noise may be relegated to the less-correlated PC images; and Ž3. spectral differences between materials may be more apparent in PC images than in individual bands ŽSabins, 1987.. Principal component ŽPC. images were prepared for the three TM visible and three reflected infrared ŽIR. bands of the available ŽLesvos. LANDSAT5rTM sub-scene. The first three PC images ŽPC 1, 2, 3. contain 97% of the variation of the original six TM bands, which is a significant compression of data. The last three PC images ŽPC 4, 5, 6. account for only 2.6% of the original variation ŽSabins, 1987.. The first PC image ŽPC-1. is a weighted positive sum of all the original bands and it represents a panchromatic view of the area containing 88% of the data variance. It is dominated by topography, expressed as highlights and shadows that are highly correlated in all six of the original TM bands. The second PC image ŽPC-2. represents a difference between the infrared and visible bands and as such, it serves to enhance any spectral differences
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between those parts of the spectrum ŽCanas and Barnett, 1985.. Also, PC-2 is dominated by differences in albedo which also correlates from band to band because pixels that are bright in one TM band tend to be bright in adjacent bands ŽSabins, 1987.. 4.2. FCC An FCC image is arguably the most effective means of visual presentation of multi-channel Žmulti-band. image and survey data. A decision has to be made about the information content of the final color composite as a fraction of the total information available. Normally, workers choose three original bands in red, green, and blue ŽRGB.. A standard FCC image contains 73% of the available image variance, whereas the principal composite image contains 97% ŽCanas and Barnett, 1985.. On this basis, the first two PC images were employed in the present study in order to create FCC images. Some 20 combinations of original image bands andror principle components ŽPC. were tested for image enhancement qualities. Of these, six offered satisfactory enhancements in FCCs in red, green, and blue ŽRGB.. Of these six, two were the most acceptable using topography, drainage, and lineament definition as criteria for selection. One of the two was composed of original TM-band 3, PC-1, and PC-2; and the other one was composed of original TM-band 7, PC-1, and PC-2. The latter combination offers slight advantages because it includes the first and second axes of PC images along with band 7 which is the most appropriate band for geologic investigations. The two PC-images and TM-band 7 were combined to produce a final FCC image by assigning each image to a separate primary color ŽRGB.. This final digitally enhanced image was interpreted for geologic and geomorphic features, and was initially reported on by Novak and Soulakellis Ž1996..
5. Discussion 5.1. Geomorphology Analysis of satellite imagery has repeatedly demonstrated its value in deciphering the interrelation between lithology, structures, and landforms
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ŽKrishnamurthy and Srinivas, 1996.. A basic tenet of geomorphology is that geomorphic units can be expected to reflect underlying bedrock types and the processes that have been at work. Therefore, a geomorphic unit has a specific set of characteristics Žfor example, texture, tone, and reflectance. that determines its image signature. However, it is still possible that with the right combination of reflectance characteristics of soil and vegetation, the two different bedrock types might produce similar image expression. For this reason, we were cautious, and only delimited those geomorphic units that could be clearly differentiated from one another based on field observation and image processing techniques ŽFig. 2.. We believe the units shown here are primarily geomorphicrgeologic in origin and not simply vegetative. Based on field observations, and image tone and texture, we defined six different geomorphic units.
istics. Examples include areas to the north and northwest of the Gulfs of Kaloni and Yera Žsee Fig. 2..
ŽA. Dissected tephra hills Žmoderate relief. ŽB. Tilted Ž?. plains underlain by ignimbrite or lava ŽC. Dissected hills underlain by schistrmarble Žhigh relief. ŽD. Salt flats ŽE. Alluvial plains ŽF. Hills underlain by ophiolites
Lineament is the name given by geologists to lines or edges, of presumed geologic origin, visible on remotely sensed images ŽCampbell, 1987.. According to Campbell Ž1987, p. 439. ‘‘controversy arises when we attempt to judge the geological significance Žif any. of these features. There are sound reasons for assigning a geological meaning to some lineaments even if they do not always correspond to observable physical feature.’’ Not all faults are expressed topographically, but a fault plane may offer a preferred avenue to moisture, to vegetative growth, and may alter drainage patterns and therefore, the lineament may have structural origins. Given the sometimes-contentious history of lineament studies, we chose a conservative approach, electing to delineate only the most clearly discernible longer lineaments. Fig. 3A is a black and white version of the final RGB image. Fig. 3B shows the lineaments interpreted from the image and some limited field data. Some lineaments detected on this image were coincident with tectonic contacts or faults mapped by Hecht Ž1971–1974.. The lineaments of this type are shown as solid black lines ŽFig. 3B.. Some lineaments were hitherto undetected extensions of those mapped features. However, many lineaments detected in the analysis were previously unmapped and could be such features as faults, tectonic contacts or formational contacts.
5.1.1. Plains The plains are of two types: Ž1. those formed atop major lava flows ŽB. and Ž2. those formed near the coast by fluvial processes ŽE.. The lower Ž- 150 m. lava flows and ignimbrites could be considered plains; but they are ‘‘stacked’’ in such a way that those at higher elevations Ž) 150 m. are more correctly referred to as plateaus. Many of the surface lava flows andror ignimbrites appear to be tilted. We have not been able to determine whether this tilt is the original flow surfaces or if there was post-emplacement tilting due to tectonism. The plains near the coasts derive from floodplain andror deltaic deposits and may have been modified by coastal processes. These areas are often the vegetable growing regions of the island due to increased soil fertility, thickness, and water retention character-
5.1.2. Hills Post-Triassic to pre-Neogene intrusive rocks, the oldest on the island, are metamorphic in origin and underlie the most highly dissected hills in the east and southeast ŽC and F. ŽHecht, 1971–1974.. The rocks are highly sheared Žschists and ophiolites.; in some cases, highly soluble Žmarbles and slightly metamorphosed limestones., so that there are numerous structural weakness that have been exploited by weathering. The tephra hills ŽA. are underlain by Neogene rocks ŽPe-Piper et al, 1995. and have less inherent structural weaknesses. They are dissected much less and exhibit more moderate relief. 5.2. Lineaments
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Fig. 3. ŽA. Black and white version of FCC of Lesvos, Greece showing some of the major lineaments Žarrows.; north is at the top. ŽB. Lineament map interpreted from ŽA. and limited field data; north is at the top. Solid line represents previously mapped faults or tectonic contacts Žfrom Hecht, Ž1971–1974... Short dashes indicate extensions, observed on the image, of the previously mapped faults or tectonic contacts. Long dashes denote newly observed lineaments not associated with any previously mapped feature. Hachures show direction of down-dropped blocks along dipping normal faults.
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thermal springs and earthquake epicenters. Because remote-sensing techniques alone cannot provide a complete picture of the region, the methods specified above will be combined using geographic information systems ŽGIS..
Acknowledgements Discussions with Mark Swanson and David Dinter greatly contributed to an understanding of the tectonics of the region. We also thank Mary Snell and John Hatzopoulos for their generous support and encouragement.
Fig. 4. Rose diagram of the strike directions of lineaments on Lesvos and shown in Fig. 3B. Two major sets of lineaments were defined: one NE–SW and the other, NW–SE.
Analysis of the Lesvos image defined two major sets of lineaments: one NE–SW and the other, NW– SE. The strike directions of lineaments are illustrated in a rose diagram showing the two dominant sets ŽFig. 4. and suggest two dominant directions of tension. Nearly all of the faults mapped by Hecht Ž1971–1974. were oriented vertically. The few nonvertical fault planes are shown with hachures in Fig. 3B. The dip-direction of these faults suggests that northeast to southwest tension produced the grabens oriented NW–SE, and the faults oriented NW–SE were produced by northeast to southwest tension.
6. Future In order to refine the descriptive characteristics of the geomorphic units and precisely define the boundaries between them, the use of additional digital methods correlated with field observation is currently underway. These methods include the creation of an accurate digital elevation model ŽDEM., a slope map, and an aspect map. In order to more accurately determine the relationship between geomorphic units and bedrock types, a parallel effort to define drainage networks from the DEM will be undertaken. It is also our intention to explore the relationships between lineaments and the location of
References Aksu, A.E., Calon, T.J., Piper, D.J.W., Turgut, S., Izdar, E., 1992. Architecture of the late orogenic Quaternary basins in northeastern Mediterranean Sea. Tectonophysics 210, 191–213. Astaras, Th., Soulakellis, N., 1990. LANDSAT-TM data processing techniques for mapping geological and geomorphological features in the Central Macedonia area, N. Greece. In: Savascin, M.Y., Eronat, A.H. ŽEds.., Proceedings of the International Earth Sciences Congress on Aegean Regions Vol. II pp. 76–91. Campbell, J.B., 1987. Introduction to Remote Sensing. The Guilford Press, New York. Canas, A., Barnett, M., 1985. The generation and interpretation of false-colour composite principal component images. International Journal of Remote Sensing 6, 867–881. Dewey, J.F., Sengor, A.M.C., 1979. Aegean and surrounding regions: complex multi-plate and continuum tectonics in a convergent zone. Geological Society of America Bulletin 90, 84–92. Dinter, D.A., Macfarlane, A., Hames, W., Isachsen, C., Bowring, S., Royden, L., 1995. U–Pb and 40Arr39Ar geochronology of the Symvolon granodiorite: implications for the thermal and structural evolution of the Rhodope metamorphic core complex, northeastern Greece. Tectonics 14, 886–904. Hecht, J., 1971–1974. Geologic Map of Lesvos Island Ž5 sheets.. National Institute of Geological and Mining Research ŽIGME., Athens. Katsikatsos, G., Migiros, G., Triantaphyllis, M., Mettos, A., 1986. Geological structure of the internal Hellines Žeast Thessaly– southwest Macedonia, Euboea–Attica–northern Cyclades islands and Lesvos.. Institute of Geological and Mineral Exploration, Geology and Geophysical Research, 191–212, Special Issue. Krishnamurthy, J., Srinivas, G., 1996. Demarcation of the geological and geomorphological features of parts of Dharwar Craton, Karnataka, using IRS LISS-II data. International Journal of Remote Sensing 17, 3271–3288.
I.D. NoÕak, N. Soulakellisr Geomorphology 34 (2000) 101–109 Novak, I.D., Soulakellis, N., 1996. Mapping geologic and geomorphic features using LANDSAT-TM data processing techniques on Lesvos island. Geological Society of America 28, A301–A302, Annual Meeting, Abstracts with Programs. Pe-Piper, G., Piper, D.J.W., 1993. Revised stratigraphy of the Miocene volcanic rocks of Lesvos, Greece. Nues Jahrbuch fuer Geologie und Paleontologie 2, 97–110. Pe-Piper, G., Piper, D.J.W., Kotopouli, C.N., Panagos, A.G.,
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1995. Neogene volcanoes of Chios, Greece: the relative importance of subduction and back-arc extension. In: Smellie, J.L. ŽEd.., Volcanism Associated with Consuming Plate Margins. Geological Society. pp. 213–231, Special Publication No. 81. Sabins, F., 1987. Remote Sensing: Principles and Interpretation. 2nd edn. Freeman, New York. Sengor, A.M.C., Yilmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241.