JOURNAL OF THE BALKAN GEOPHYSICAL SOCIETY, Vol. 6, No. 4, November, 2003, p. 188 – 201.

Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin (NW Greece) by means of a Vertical Electrical Sounding (VES) survey

A. Atzemoglou (1) , P. Tsourlos (2) and S. Pavlides (2)

(1) Institute of Geology and Mineral Exploration, 1 Fragon str, 54626, Thessaloniki, Greece (E-mail: [email protected] ) (2) School of Geology, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece.

Abstract: A large-scale VES survey was conducted at the NW part of the Amynteon basin in order to study the tectonic setting of the area. Sixty-four soundings (AB/2= 1000 m) were measured on a near-regular grid and were processed with 1-D inversion algorithm. Several parameterisation patterns were tested before producing the final interpretations. Since the measured soundings centres are situated on a regular grid it was possible to combine the 1-D interpretation results and produce pseudo-2D and 3D depth slices. Interpretations are in very good agreement with the existing drilling and geological information and reveal a relatively detailed picture of the basin’s geological background. Further the results allowed us to verify the continuation of known faults and to obtain new structural information about the studied area. The results indicate that despite the increased use of 2D electrical surveys, 1-D VES measurements are still a very useful tool for large-scale applications.

Keywords: Tectonic structure, VES survey, Amynteon Basin, 1-D resistivity inversion.

INTRODUCTION 1989, Basokur, 1990, Basokur, 1999). One of the known problems of the 1-D VES A large-scale geoelectrical sounding interpretations is the inherent lack of solution survey was conducted at the Amynteon Basin uniqueness (Kunetz, 1966; Koefoed, 1979). (N. Greece) (Fig. 1). The surveyed area is Due to this problem, practice has shown, that situated near the two small lakes (Zazari and using fully automated inversion schemes, Chimaditis) and between the villages of without taking geological conditions into Pedino, Valtonera, Limnochori, Agrapidia and account, can result into interpretations, which Aetos in Florina prefecture in Western are mathematically correct but geologically Macedonia, Greece (Figure 2). The aim of this erroneous. In this framework, approaches survey was to obtain structural and geological involving interactive semi-automated information about the basin as part of a larger interpreting can become particularly useful in survey aiming to the investigation of the low taking both the mathematical and the enthalpy geothermal potential of the area. geological factors into consideration. The technique of VES for prospecting One popular way to incorporate 1-D structures is well established (Koefoed, geology into the VES automated interpretation 1979). A large number of fully automated 1-D is to provide some compact geological interpretation algorithms for VES can be found information (i.e. drill columns) by assuming in literature (Inman, 1975; Rijo, 1977; Zohdy, some certain properties of geoelectrical layers

188 Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin 189

FIG. 1. (a) Tectonic map and stratigraphic section of the Florina–Aetos–Ptolemais basin.

190 Atzemolglou et al.

FIG. 1.(b) Rose diagram showing the distribution of faults in Florina–Aetos–Ptolemais basin. (i) Strikes, showing the two main directions NW-SE and ENE-WSW. (ii) Dip direction indicating that the majority of the faults dip SSE and ENE. (iii) Circular histogram where the dip angles of the faults range from 45 ° to 90 ° while the majority are high angle dip slip faults (85 ° – 90 ° dip). (iv) Pitches of striation of 177 selected representative faults; the majority are dip slip typical normal faults (85 ° – 90 ° pitches) meso- and mega-scale and only few meso-scale accommodation structures show oblique slip sense of movements.

are known. These parameters are fixed pseudo2-D, 3-D images of the subsurface during the minimisation steps of the fitting resistivity. The resulted interpretations are in error (Rijo, 1977). However, this approach very good agreement with the existing drill effectively requires that prior geological hole information (Koutsinos et al, 1997) and knowledge should be transformed provide useful information about the successfully into compact geoelectrical geological and tectonic setting of the studied information and this is not always an easy area. task. Sixty-four VES situated on a near GEOLOGICAL SETTING regular grid were measured at the area of interest (Figure 2). Initial interpretations The pre-Neogene rocks of the were performed using the Zohdy’s algorithm Florina–Amynteon–Ptolemais broader basin (Zohdy, 1989). The IPI2WIN program and surrounding areas are part of the North (Bobatchev et al., 2001), which is based on a Pelagonian geotectonic zone, which consists non-linear optimisation algorithm using of Palaeozoic crystalline basement, Mesozoic Tikhonov’s regularized technique, was used carbonate cover and ophiolites (Mountrakis, as the main interpretation tool. The 1984). Some of the long faults of the area, interactive nature of the software as well as which show obvious neotectonic activity in its ability of simultaneous sounding the margins of the Neogene basin, were interpretation proved particularly helpful in initially created during Tertiary-folding to assuring that the interpretations are phases and reactivated in neotectonic times. geologically meaningful. The basin has tectonic origin and is created Final interpretations were produced along a pre-existed syncline. after finding a representative resistivity for The Neogene–Quaternary sediments, the bedrock and by fixing the resistivity of which fill up the Amynteon–Ptolemais basin, the half-space during the inversion procedure. Results were combined to form

190 Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin 191

Xino Nero

FL1 FL2 F L3 FL4 FL11 FL5 Aetos FL12 FL6 FL13 FL7 FL21 FL14 FL8 FL22 FL15 FL9 FL23 PedinoF L16 FL31 FL24 FL17 FL32 FL25 FL18 Agrapidia FL33 Drill4 FL26 FL19 F L42 FL34 FL27 FL35 FL28 FL43 FL36 Drill3 FL29 F L44 FL37 FL30 Drill6 F L38 F L45 FL39 FL46 FL40 Valtonera F L47 Drill1 FL41 FL48 FL52 FL49 FL53 Zazari Lake FL50 F L54 D rill2 F L51 LimnochoriFL57 FL55 FL58 FL56 FL61D rill5 FL59 FL62 FL60 F L63 FL64

Anargiri Chimaditis Lake

Probable fault Alluvial Deposits

Recent lacustrine deposits Normal Fault

Recent talus cones and scree Soundings

Drills Peat

Carbonate Rocks and Lake crystaline limestones

Schists-Gneisses Village

FIG. 2. Simplified map showing the VES centres and the main geological formations in the area of study.

192 Atzemolglou et al. overly uncomfortably both the Palaeozoic two categories (Pavlides, 1985; Pavlides and metamorphic rocks and the Mesozoic Mountrakis 1987). The first includes the crystalline limestone. They are continental faults observed only in the Plio–Quaternary sediments deposited in a lacustrine sediments of the basin (NE-SW to ENE- environment mainly. After detailed WSW); the second includes the faults stratigraphic work carried out by observed in the margins of the basin, and Anastopoulos and Koukouzas (1972), continue into the sedimentary layers of the Koukouzas et al. (1979) and others, these basin (NNW-SSE mainly). Most of these sediments have been divided into the faults are dip-slip typical normal structures following lithostratigraphic formations and some of them show oblique-slip normal (Pavlides and Mountrakis 1987; Koufos and fault character (Fig. 3). Pavlides 1988 and reference therein): a. The first (lower) formation consists at its base of conglomerates, containing pebbles of metamorphic rocks, which pass transitionally upwards into a marly layer consisting of marls, sandy marls, clays and some characteristics xylith beds. b. The second (middle) formation is an argillaceous one containing some thick lignite beds. Alternating with the lignite beds are clays, marls, sandy marls, sands and lacustrine calcareous marls. c. The third (upper) formation represents the Quaternary deposits, which consist mainly of fluvioterrestrial conglomerates, alluvial fans, scree and recent river FIG. 3 . Typical tectonic model of the studied deposits. area with high angle faults shaping dip slopes The dating the onset of the basin and "horst-graben" regime, as well as in formation by means of the oldest sediments some cases tilted blocks could be determined in the stratigraphical data. As mentioned earlier the oldest known As suggested by the geological map of sediments of the basin, mainly from borehole Figure 1, the large marginal NW-SE to samples and, secondly from outcrop NNW-SSE trending is difficult to be located observations, are the basal conglomerates. (see the Aetos NW-SE fault) as they are These pass transitionally upwards into marly covered by the sediments of the basin and layers which have been determined as of late can be detected mainly by boreholes or Miocene–early Pliocene age. Hence a Middle geophysical investigation. In the contrary the to late Miocene age for the initial creation of NE-SW trending faults dominate the the basin is the most likely (Anastopoulos topography and exhibited typical and Koukouzas 1972; Pavlides and geomorphological characteristics of recent Mountrakis 1987; Koufos and Pavlides neotectonic or either active faults. Some 1988). faults of the NE-SW direction have been Numerous faults affect the pre- activated recently by weak (Pavlides and Neogene rocks and intersect all the above- Simeakis 1988) and strong earthquakes mentioned Neogene–Quaternary formations. (Pavlides et al. 1995; Mountrakis et al. The studied faults could be subdivided into 1998). The chronology of faulting has been

192 Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin 193 established by field evidence for successive values (few meters up to 400 meters). We fault motions using geometrical and can distinguish two distinct horizons that geomorphological criteria and by taking into differ in age and composition and they have account stratigraphic data. These were normal transition to each other. An intense created during the Pliocene. Numerous others period of tectonic movements was followed with a NE-SW trend cut through the whole after the deposition of the Pliocene Pliocene series and continue into the Lower sediments. This tectonism, which contributed Pleistocene fluvio-terrestrial conglomerates. to the huge rupture of the area, in correlation In addition, syn-sedimentary fault structures with the erosion, had as result the creation of were observed in Pleistocene deposits with rift tectonics ("horsts" and "grabens") of the the same NE-SW direction (Pavlides, 1985; sub-stratum and the creation of the Pavlides and Mountrakis 1987). Chimaditis – Zazari basin. Peats, peaty clay After the completeness of the Alpine and recent lacustrine deposits dominate in the orogenic cycle (late Oligocene-early vicinity of Zazari and Chimaditis lakes. Miocene) the typical continental crust of the These deposits thought that they constitute region failed into an NE-SW (σ3 trend) the impermeable cover to the regional extensional regime due mainly to the post- geothermal potential that is trapped between orogenic collapse. It has been shown by the bedrock and the basement conglomerate. quantitative tectonic analysis in the broader Water drill holes, made over the region (Pavlides, 1985; Pavlides and tertiary sediments, testify the existence of Mountrakis 1987; Caputo and Pavlides1993). large renewable and exploitable aquifers The result was the re-activation of the NW- (reservoirs) with large porosity at low depths SE trending mainly faults and the initial (Koutsinos et al, 1997). creation of the broader main basin. At the center of the Amynteon basin, A new extensional phase with a NW- both basin and aquifers becomes deeper with SE direction of extension (σ3) took place higher porosity. Near the basin’s margins, the during early to middle Pleistocene times and aquifers are found in the shallow depths and caused the most recent block – faulting in the consequently have lower porosity. area, which is particularly intensive in the Plio-Pleistocene sediments of the basin. It is MEASUREMENTS AND DATA very possible that this younger tectonism has PROCESSING continued until present times, as the very recent and unconsolidated sediments, which A total number of 64 Schlumberger have been affected by faults, indicate it. soundings were measured in the studied area The main faults that dominate the (Fig. 2) having a maximum AB/2 separation investigated area are the Petron–Xino Nero- of 1000 m. The sounding centers are 500 m Aetos-Nymfeo fault. This long fault divides apart and were situated in 8 parallel sections the broader basin into two sub-basins, the in a 3x4 km NW-SE grid while the intra- northern Florina and the Amynteon one. The section distance was 500 and 1000 m. studied area is the southwestern edge of Combined interpretations were carried out by Amynteon basin and lies between Zazari and considering almost collinear sounding Chimaditis lakes and Anargiroi, Nymfeon centers both with NW-SE (Figure 4a) and and Aetos villages (Fig. 1). Other faults of SW-NE (Figure 4b) orientations. NE-SW orientation, arising from the Data quality was particularly good as geological map, are the Vegoritis fault, the suggested by the low standard deviations Chimaditis – Anargiroi fault and some minor (0.5-1%) recorded in the field. The relatively structures. smooth behavior of all recorded sounding The studied area is occupied by curves indicate that the geology of the area Neogene and Quaternary (lower Pleistocene fits the assumptions of 1-D interpretation. to Holocene) sediments of various thickness This was also verified later by the low RMS

194 Atzemolglou et al.

F L1 FL 1 FL2 F L2 F L3 F L3 FL 4 FL4 FL 11 FL1 1 F L5 FL5 F L12 FL6 FL 12 FL 6 FL13 F L7 F L7 FL 21 F L14 FL2 1 FL13 FL 14 FL 8 FL8 FL22 FL15 F L9 F L22 FL15 F L9 F L23 F L16 F L23 FL 31 SEC 1 F L16 F L24 F L17 FL3 1 F L24 F L32 F L25 F L17 F L18 FL32 F L25 F L18 FL3 3 F L26 FL 19 FL3 3 F L26 FL1 9 F L42 FL 34 F L27 FL3 4 FL35 SEC 2 F L42 F L27 F L43 FL 28 F L35 FL2 8 FL36 FL2 9 F L43 F L36 F L44 FL3 7 FL29 F L30 F L44 FL37 F L38 F L30 FL4 5 SEC 3 FL 38 F L39 FL45 FL39 F L46 F L40 F L46 FL40 F L47 FL 47 FL41 FL 41 F L48 FL 48 FL49 SEC 4 FL52 FL52 FL49 FL5 3 F L50 FL53 F L51 FL 50 FL54 FL54 FL 51 FL57 F L57 F L55 SEC 5 FL 55 F L58 FL5 6 F L58 FL 61 FL 59 FL6 1 FL56 SEC 6 FL5 9 F L62 FL60 F L62 FL 63 F L60 SEC 7 FL6 3 FL64 F L64 SEC 8

a b

FIG. 4. The interpreted geoelectrical sounding sections (a) sections with NW-SE orientation (b) sections with SW-NE orientation.

errors (<1%) obtained by all sounding area (6 test drills) in order to coincide with interpretations. the geological background (schist gneiss). The initial processing of the sounding Then, we invert data to retrieve the bedrock’s data involved interpretation using the Zohdy resistivity. Yet, the presence of a algorithm (Zohdy, 1989). Despite its conglomerate formation, of varying theoretical simplicity this algorithm is known composition and compaction, just above the to produce smooth models representative of bedrock suggest that this formation could the subsurface resistivity distribution. The locally behave geoelectrically similar to the individual sounding results were combined to bedrock. As a result, it was not possible to produce pseudo-2D sections of the define the bedrock limit exactly and subsurface resistivity. Such a section (SEC4 eventually we retrieved inversion results that of Figure 4a) is presented in Figure 5. produced quite varying resistivity values for We used the IPI2WIN program the bedrock. (Bobatchev et al., 2001) for the main Instead of using the drill information interpretation of the VES data. The software to constrain the inversion directly we used allows the user to present multiple sounding the following approach to overcome the interpretations in a single window and thus problem. A large number of VES, which interpretations of adjacent VES can be appeared to be affected by the bedrock, was represented as a single geological structure selected. The selection criteria involved the rather than a set of independent objects. shape of the sounding curve, the drill Further, it enables interactive interpretation information and the existing interpreted enabling the user to choose, from a set of resistivity sections obtained using the Zohdy possible models, the one that both fits the technique. Initially, we performed individual VES data and produces sectional unconstrained inversions, just to the selected interpretations that are geologically VES, by providing a simple initial model reasonable. extracted from the results of the Zohdy’s Initially we tried to fix the depth of technique. Inversion results produced quite the last layer, since there was relatively consistent resistivities for the bedrock that accurate geological information from the ranged between 1000 and 2000 ohm-m.

194 Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin 195

SEC 4

FL31 FL32 FL33 FL34 FL35 FL36 FL37 FL38 FL39 FL40 FL41

-25 ) -75 m (

-125

H -175 T -225 P

E -275

D -325 -375 -500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 DISTANCE (m)

RESISTIVITY (Ohm-m) 0 80 160 240 320 400 480

FIG. 5. Initial interpretation of the geoelectrical sounding section 4 (SEC4 of Figure 4a) using Zohdy’s algorithm.

200 100 FL25 FL38 Real Data Real Data 80 Model Data Model Data Resistivity Model (RMS=1.7%) Resistivity Model 100 60 ) m -

m 40 50 h O ( . s e R . p

A 20 20 D1 D4

10 10 12 5 10 20 50 100 200 500 1000 1 2 5 10 20 50 100 200 500 1000 AB/2 (m) AB/2 (m)

200 100 FL55 Real Data FL62 Model Data Real Data 100 Resistivity Model (RMS=1.8%) Model Data Resistivity Model (RMS=0.9%)

50 30

20

10 10 8 7 D2 D5

12 5 10 20 50 100 200 500 6 8 10 20 40 60 80 200 400 AB/2 (m) AB/2 (m)

100 1000 FL28 FL46 Real Data Real Data Model Data Model Data Resistivity Model (RMS=1.3%) Resistivity Model (RMS=2.1%) 300 50 100

30 20 10 8 D6 D3

10 12 5 10 20 50 100 200 500 12 5 10 20 50 100 200 500 AB/2 (m) AB/2 (m)

D? Drill Column Recent Sediments (Clay, sand, gravel)

Conglomerate

Schist-Gneiss

Granite

FIG. 6. Correlation of selected VES interpretation with the nearest test-drills columns (X-axis).

196 Atzemolglou et al.

We then performed individual inversions for Since geoelectrical interpretations were not all collected VES by fixing indiscriminately constrained a priori to fit the drill the resistivity of the half-space to be 1500 information, all six existing drills columns ohm-m. It is worth mentioning that this are presented against the produced approach did not create problems in areas for interpretations and fits of the VES that are which no high resistivity half space is the nearest to the test drills positions in order justified by data and geology. In these cases to test the validity of the interpretation inversion produced a depth for the resistive approach (Figure 6). The VES interpretations half-space, which is well below the VES are in very good agreement with the drillings depth of investigation (>400 m) and thus it as far as the location of the bedrock is can be safely ignored. concerned and this suggests that the final interpreted results can be considered reliable.

FL1 FL2 FL3 FL4 FL5 FL6 FL7 FL8 FL9 -50 -150 -250 SEC 1 -350 DEPTH (m) DEPTH -450 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m )

FL11 FL1 2 FL13 FL14 FL15 FL1 6 FL1 7 FL18 FL19 -50 -150 -250 SEC 2 -350 DEPTH (m) DEPTH -450 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m)

FL21 FL22 FL23 FL24 D4 FL25 FL26 FL27 FL28 D3 FL29 FL30 -50 -150 -250 SEC 3 -350 DEPTH (m) DEPTH -450 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m )

FL31 FL32 FL33 FL34 FL35 D4 FL36 FL37 FL3 8 D1 FL39 FL40 FL41 -50 -150 -250 SEC 4 -350 DEPTH (m) DEPTH -450 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m )

FL42 FL43 FL44 FL45 D6 FL46 FL47 FL4 8 FL49 FL50 FL51 -50 -150 -250 SEC 5 -350 DEPTH (m) DEPTH -450 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m)

FL52 FL53 FL54 D 2 FL55 FL56 0 -100 -200 SEC 6 -300

DEPTH (m) DEPTH -400 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m)

FL57 FL5 8 FL59 FL60 0

-200 SEC 7

DEPTH (m) DEPTH -400 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m)

FL6 1 D5 FL62 FL63 FL64 0 -100 -200 SEC 8 -300

DEPTH (m) DEPTH -400 -1000 -500 0 500 1000 1500 2000 2500 3000 3500 4000 DISTANCE (m)

10 100 1000 4000 Ohm-m

1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 log(Ohm-m)

FIG. 7. Pseudo-2D subsurface resistivity sections of the NW-SE sections SEC1-SEC8 (see also Figure 4a).

196 Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin 197

INTERPRETATION AND DISCUSSION initial inversion model it vanished in the inversion results. A possible explanation is Based on the produced sounding that in areas of low compaction this interpretations the following correlation formation has resistivities similar to the between the geological and geoelectrical overlaying sediments but in cases of higher formations can be considered: The topsoil compaction it behaves electrically similarly layer (0-3 m) has a varying resistivity due to to the bedrock. its highly variable composition. The interpretation results were The main sedimentary formation combined to form pseudo-2D subsurface (Pleistocene to Holocene) consists of clays resistivity sections of the studied area. and alternations of relatively thin layers of Figures 7,8 depict NW-SE sections (SEC1- sands and gravels, which effectively cannot SEC8, Fig. 4a) and SW-NE sections be identified as individual geoelectrical (VSEC1-VSEC9, Figure 4b) respectively. layers. In Figure 9 the interpretation results The geoelectrical formation, which were combined to produce pseudo-3D depth corresponds to the sedimentary series, has a slices for depths ranging from 50 m to 450 resistivity ranging from 15 to 60 ohm-m and m. can be distinguished into two sub-formations Finally, in Figure 10 a 3-D image of of higher and lower resistivity ranging from the surveyed area depicting the relative 30-60 ohm-m (top formation) and 15-30 depths of the geological background is ohm-m (bottom formation) respectively. This shown. Note, that the results of Figure 10 are resistivity reduction could be attributed in very good agreement with the typical mainly to a lithological variation since the tectonic model of the studied area (Figure 3) formation is becoming more argillaceous and fully verify the tectonic structure with depth, and partly due to the increase of proposed by Pavlides (1985) and Pavlides the geothermal gradient (Koutsinos et al, and Moundrakis (1987). 1997). The latter is suggested by geothermal As it can be seen from the results of exploration in the area, which indicates an the geophysical interpretation two main average increase of 15 oC in temperature at a faulting systems can be identified. The first depth of 300 m. This differentiation between has a NW-SE direction and it appears to be the above mentioned sedimentary series is between the Aetos-Valtonera axis and the partly arbitrary. It concerns geoelectrical Pedino village. This appears to be associated layers with overlapping range of resistivity with the known tectonic contact (Pavlides, values. 1985) between the schists and marbles to the The bedrock formation is clearly North of the Aetos village. This faulting identified as a geoelectrical layer and was system appears to be responsible for the fixed to be of resistivity 1500 ohm-m. No lowering of the bedrock at the eastern part of distinction of the bedrock lithology (schist- the studied area. gneiss or granite) can be made due to the The second faulting system has similar range of resistivities that these almost a N-S direction (Pedino-Valtonera formations present. axis) and it sets the western limit of the The drill information suggests the bedrock prior to its dipping. existence of a conglomerate formation of At the southern part of the studied varying composition, thickness and region it is worthwhile mentioning the compaction, which is intermediate between existence of a localized dipping of the the sediments and the bedrock (Koutsinos et bedrock, which appears to be associated to al, 1997). This formation cannot be the formation of the Zazari Lake. recognized within the interpreted VES results as an individual geoelectrical unit. Whenever we tried to incorporate this layer into the

198 Atzemolglou et al.

50m

100m

150m

200m

250m

300m

350m

400m

10 100 10 00 400 0 Oh m-m

1 1.5 2 2.5 3 3.5 log(Ohm -m )

450m

FIG. 9. Interpreted resistivity results presented in a 3D layout as depth slices from a relative depth of 50 m to 450 m.

198 Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin 199

-50 -100 -150 -200 -250 -300 -350 -400 -450 -500 -550

Bedrock Depth (m)

FIG. 10. 3-D image of the surveyed area depicting the relative depths of the geological background .

CONCLUSIONS Geophysical results can’t be accepted alone for a final interpretation of the Interactive interpreting of VES and Amynteon-Zazari-Chimaditis sub-basin, but careful consideration of the existing in association with geological, geological information leads to increasingly geomorphological and borehole data, in order reliable VES interpretations. Further, by to establish the type and the dimension of combining the 1-D interpretation results and faulted area and basin's floor. The structure presenting them into pseudo-2D and 3D of the sub-basin, which is the structure of the subsurface resistivity image certainly helps greater basin in smaller scale, from surface into forming a clearer picture about the geological data, boreholes and mainly from studied area. the geoelectric ones are characterized by rift We suggest that for large-scale tectonics and normal faults activated into two surveys, which target to intermediate depths distinct tectonic phases. They are high angle of investigation (100-300 m), VES faults shaping dip slopes and "horst-graben" measurements remain a practical and regime, as well as in some cases tilted effective subsurface imaging tool. Compared blocks. The resulted geophysical image is in to the increasingly popular 2-D resistivity very good agreement with the existing surveys VES at low-resolution scale and geological and tectonic information for the intermediate investigation depths is certainly area and indicates the ability of geophysical less costly and almost equally effective. surveying to aid and verify the geological

200 Atzemolglou et al. interpretations. Further the results allowed us Koufos, G. and Pavlides, S., 1988, to verify the continuation of known faults Correlation between the continental and to obtain new structural information deposits of the lower Axios valley about the studied area. The results indicate and Ptolemais basin: Bull. Geol. Soc. that despite the increased use of 2D electrical Greece, 22 , 9-19. surveys, 1-D VES measurements are still a Koukouzas, C., Kotis, T., Ploumidis, M. and very useful tool for larger scale applications. Metaxas, A., 1979, Coal exploration of Anargiri area, Aminteon (W. REFERENCES Macedonia), Mineral Deposit Research. IGME, Athens, No 9, p. 1- Anastopoulos, J., and Koukouzas, C., 1972, 69, (in Greek). Economic geology of the southern Koutsinos, S., Kolios, N., Karidakis, G. and part of Ptolemais lignite basin Atzemoglou, A., 1997, Preliminary (Macedonia, Greece): Geol. And results of geothermal research in lake Geoph. Research, IGME, Athens, No Zazari and Chimatidis region, Florina 16, p. 1-189, (in Greek). prefecture, Macedonia, Greece. Basokur, A.T., 1990, Microcomputer IGME Report (in Greek). program for the direct interpretation Kunetz, G., 1966, Principles of direct current of resistivity sounding data: resistivity prospecting: Gebruder Computers & Geosciences, 16 , 587- Borntaeger, Berlin. 601. Mountrakis, D., 1984, Structural evolution of Basokur, A T., 1999, Automated 1D the Pelagonian zone in North - interpretation of resistivity soundings Western Macedonia. The Geological by simultaneous use of the direct and Evolution of the Eastern iterative methods: Geophysical Mediterranean, Special Publication of Prospecting, 47 , 149-177. the Geological Society, No 17, p. Bobatchev, A., 1992, Electrical sounding of 581-590. geological media: Moscow Mountrakis, D., Pavlides, S. Zouros, N., University Edition, part 2, 200 pp. (in Astaras, TH. and Chatzipetros, A., Russian). 1998, Seismic fault geometry and Bobatchev, A, Modin, I. and Shevnin, V., kinematics of the 13 May 1995 2001, IPI2WIN v.2.0, User’s manual. western Macedonia (Greece) Caputo, R. and Pavlides, S., 1993, Late earthquake: J. Geodyn. Spec. Issue Cenozoic geodynamic evolution of Pavlides & King (Eds), 26, 2-4, 175- Thessaly and surroundings (central – 196. northern Greece): Tectonophysics, Pavlides, S., 1985, Neotectonic evolution of 223 , 339-362. the Florina - Vegoritis – Ptolemais Ghosh, D. P., 1971, The application of linear basin (W. Macedonia, Greece), PhD filter theory to the direct thesis, Aristotle University of interpretation of geoelectrical Thessaloniki, 265 pp. (in Greek with resistivity sounding measurements: Engl. abs). Geophysical Prospecting, 19 , 192- Pavlides, S. and Simeakis, K., 1988, 217. Neotectonics and active tectonics in Inman, J. R., 1975, Resistivity inversion with low seismicity areas of Greece: ridge regression: Geophysics, 40 , Vegoritis (NW Macedonia) and 798-817. Melos isl. Complex – Comparison, Koefoed, O., 1979, Geosounding principles, Ann. Geol. des Pays Hellen., 33, pp. 1, Resistivity Sounding 191-176. Measurements: Elsevier, Amsterdam. Pavlides, S. and Mountrakis D, 1987, Extensional tectonics of North-

200 Investigation of the Tectonic Structure of the NW Part of the Amynteon Basin 201

western Macedonia, Greece, since the Rijo, L., 1977, Modelling of Electric and late Miocene. Journal of Structural Electromagnetic data: Ph.D. Thesis. Geology, 9, 385-392. University of Utah. Pavlides, S., Zouros, N., Chatzipetros, A. Zohdy, A. A. R., 1989, A new method for the Kostopoulos, D. and Mountrakis, D., interpretation of Schlumberger and 1995, The 13 May 1995 western Wenner sounding curves: Macedonia, Greece (Kozani- Geophysics, 54 , 245-253. Grevene) earthquake; preliminary results: Terra Nova, 7, 544-549.