In deep horizons of the Quaternary sequence, attempts to reveal faults have been made by high-resolution seismic survey. In near-surface layers, faults and joints have been studied in outcrops and artificial exposures, including trenches. In geomorphic and geologic maps which characterize the E arth’s surface, there are a lot o f linear structures, they have been studied in satellite images.
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4.1. Deep horizons: high-resolution seismic sections
O f the high-resolution seismic sections surveyed in 1989 for detecting faults (Guthy and Hegedus 1990) the section Pa-8 produced results worth of disputing. It crossed fault B which in 1994-1995 was crossed by the profiles Pa-12, 13, 14 and 15 (Guthy and Janvari-Kantor 1994, Guthy 1995) as well.
A fter the loose, disintegrated, sandy cores from the Q uaternary sedim ents, Pannonian sedim ents appear in solid cores o f constant shape that is a significant, even visually well recognizable m echanical change occurs on the sequence boundary. In high-resolution seism ic sections, this boundary alone cannot be identified and can only be drawn putting borehole data in the section. T he reflection pattern o f the Q uaternary sedim ents is usually far o f that w hich could be described in geological term s. F or instance, the m ost genera] feature o f the fluvial sediments, i.e. the lenticular structure, is practically absent, and in the fragm ents with seem ingly good stratification there is no m arker horizon. This generate doubts concerning the idea that the reflectors in the sections everyw here coincide with layers, so that the most convincing criterion for tracing faults in sedim entary sequences, i.e.
displacem ent o f m arker horizons, seem s to be loosen.
In the structure o f the Quaternary sediments displayed in high-resolution seismic sections, two remarkable features are seen: a zone poor in signals (in stacking sections) and a trough-like depression (in migrated sections). In the sections Pa-8 = Pa-12, Pa-15 and P a-13, they occur together and follow in a map a filled-up Danube ox-bow (Figure 10). Here, the grounding conditions may get impoverished on the marshy ground, on the other hand, in a lens wedging out along the profile loose sediments, badly conducting seismic waves can be present. In both cases, local deficiency in energy - seismic shadow - is expectable, it is probably reflected by the vertical zone poor in signals.
Based on the static correction curve for the profile P a -12, it can be supposed that betw een 82 and 90 m a.s.l. there is a velocity low that points to a change in lithology. In the geoelectric sounding curve (Stickel and Zalai 1994), alo w -resistiv ity layer is detected at 8 4 -9 0 m a.s.l., and a high-resistivity layer, at 6 3 -7 5 m a.s.l. Both o f them point to the presence o f lenticular bodies w ithin the Quaternary sequence with lithologieal com position different o f that o f the surrounding rocks. T hese lenses may have seism ic velocities needed to consum e energy - In the sections Pa-15 and P a -13, the velocity anom aly is absent, this being only true for the layer rem oved from -above the section during the processing, and in the near-surface horizon o f the sections, this lens can be present. Local velocity low results in increasing travel times, and if it is not taken into account during the processing (this is the case with the sections in question) reflectors in deeper horizons will be pushed dow n. The trough-like depression is probably related w ith this phenom enon.
It can be concluded that there are unsolved problems in the seismic display of the Quaternary sediments, so that the direct structural interpretation of the reflection pattern is not convincing. Help can be given by the good stratification of the Pannonian sequence:
those of the faults within the Quaternary sediments can only be accepted which fall onto the direct upward continuation of the faults which displace Pannonian sediments. In the high-resolution seismic sections of the Paks area, however, no case o f this type can be proven.
4.2. Near-surface horizons: measurements o f joints
The first joints - 104 in 12 Pleistocene exposures - were measured by Chikan and Kokai (1989). During the geological mapping of the 30 km area around the nuclear power plant (Chikan 1992), 1379 joints were measured in 167 outcrops o f Quaternary sediments
(Dudko 1992). The results were processed and interpreted by Dudko (1992), Gem er (1993) and Balia (1994). In 1994, more 634joints were measured in 6 outcrops (Dudko and Maros
1994), so, the total data volume exceeded two thousand.
The surface of the joints is smooth or slightly undulating. For the geologists, this was an evident proof of the shear, i.e. tectonic origin o f the joints. In combination with the prevailingly steep, subvertical position o f the joint planes this fact suggested shear stress in early interpretations (Horvath et al. 1990, Balia 1991).
The values in the summary rose diagram for the joints measured in 1992 (Figure 11) are dispersed between 3.75 and 7.70%, i.e. within a very narrow interval around the 5.56% expectable for the completely uniform distribution, not showing any definitely preferred orientation. That is why the data were filtered for the tectonic interpretation:
removed were joints qualified already in the field as ’’slum p-derived” (Dudko 1992) or those which could be related to the slope and outcrop orientation (Gerner 1993) also recorded in the field. The remaining data assemblage showed three maxima, in a WSW- ENE, W NW-ESE and NNE-SSW directions.
A lthough this induced attem pts in tectonic interpretation (Dudko 1992, G em er 1993), the fact that joints are not concentrated in a som e km w ide strip above the main fault generated heavy doubts concerning the tectonic origin o f the w hole o f the jo in t assem blage o r even o f its significant portion (D udko 1992).
The angles between the three maxima (Figure 11) are roughly 6 0 °, and the weight of the three m axima is approximately the same. This depicts a hexagonal system which is similar to that originated from the contraction o f sheet-like rock masses, e.g., in cooling of basalt lava flows (Balia 1994). 165 of the 167 outcrops and 1335 o f the 1379 joints were in loess, consequently, the measurements first o f all characterize loess. It seemed to be possible that the hexagonal system is related with the desiccation of the loess.
In order to throw light upon the problem, in 1994 all the outcrops were documented in details: the exposure bottom line and the position o f the joints were fixed in plans. In the outcrops studied, each 1-3 m a joint occurred, their frequency being changeable, mostly as a function of the exposition. 83% of the 634 joints measured in 1994 were in two neighboring outcrops (Figure 12) with different distribution. Since the orientation maxima are different even in the outcrops located in some hundreds o f meters, they cannot characterize a stress field stable in space. The dom inating in 1992 directions could not be reproduced even in outcrop groups within some km .
The rather high frequency, depending on the exposition features, the low represen- tativity and the bad reproducibility are in harmony with the non-tectonic origin. In the framework of this idea both the frequency and the preferred orientation are due to local factors: most o f the joints are planes bordering slumped blocks or desiccation fissures. The joints originated from slumping limit rectangular blocks. Hexagonal jointing was nowhere observable directly, its existence could only be inferred from rose-diagrams and still needs to be confirmed by data beyond the statistics.
In 1992, an attempt was made to reveal faults by trenching. Trenches were excavated after two-stage geophysical survey in the sites which seemed to be most suitable (Mesko et al. 1993). In the approximately 1800 m 2 rock surface exposed in the 500 m trenches, there were absolutely no joints or faults (Chikan-Jedlovszky et al. 1992). This fact is in sharp contrast with the fact that in the outcrops usually in each 1-3 m there was a joint. It seems therefore probable that the joints are not present as distinct planes within the original rock mass and only appear subsequently, near the earth’s surface that is possible
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due to both desiccation or disintegration/loosen but as a consequence of tectonic stresses.
In some o f the outcrops, joints which were well visible in the wall disappeared within the rock mass when the wall was being demolished, so that the near-surface origin o f the joints could be directly proven.
4.3. E arth's surface: lineaments on satellite images
The advantage of the lineaments in satellite images consist in’they can be studied evenly whereas their disadvantage is due to the fact they can be originated in various ways.
Interpretation of the lineaments in satellite images for the Paks area can be found in works by Horvath et al. (1990, 1993) and Bodrogi et al. (1994), here the latter is used.
Numerous attempts in the field o f automatic processing of digital satellite images Landsat TM and SPOT P were made in order to reveal possibilities to detect tectonic lineaments. In the highly variable pictures obtained, the faults known from the seismic sections are usually not recognizable. In some areas, the question o f the relationships with some o f the faults can be put (Figure 13), but the connection is highly uncertain. This is best reflected by the fact that none of the pictures alone or any combination of them would form basis for delineating concrete faults: there are too many similar ’’lineaments” far of any of the faults.
4.4. Conclusions
In the high-resolution seismic section Pa-8 = P a-12, a zone poor in signals is visible above the fault B in the Pannonian sediments; primarily it was regarded as a proof of the Quaternary rejuvenation o f the fault. Later sections made clear that the zone in question , follows a Danube ox-bow and its origin is probably due to grounding problem s and lithological screening. The reflection pattern of the Quaternary sediments is rather uncer
tain in seismic sections, convincing features in favor o f faults - first of all, displacements of marker horizons - are not available.
Distribution o f the joints in exposures and lineaments in satellite images is similar:
only some of the preferred orientations can be related to the known faults, not the spatial position and distribution o f them. M ost of the concrete lineaments in satellite images and joints in exposures conform to the elements of the present day topography. Consequently, the fact that their tectonic origin is full of doubts confirms at the same time the reservations concerning the direct relationships between the topography and tectonics (Balia et al.
1993).