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Neotectonic investigations in the Pannonian basin based on satellite images
46. 1984 Adv. Space No.11, Printed in Res. Great Vol.4, Britain. All pp.139—1 rights reserved.
0273—1177/84 $0.00 + .50 Copyright © COSPAR
NEOTECTONIC INVESTIGATIONS IN THE PANNONIAN BASIN BASED ON SATELLITE IMAGES M. Poijak Geological Survey Ljubljana, Linharrova 9, Yugoslavia
61000 Ljubljana,
ABSTRACT Analysis of Landsat images of the Mura depression in the Pannonian basin enabled identification of a number of lineaments that, in most cases, were not known from previous geological investigations. Many of these lineaments act as real faults and they have neotectonic significance. Combination of Landsat and field data made possible construction of a structural model that can explain the neotectonic evolution of this area. The model is based on wrench—tectonics, where Sava, ~o~tanje and Labod faults represent the first order wrench faults, and Donat, 0rmo~ and Ljutomer the second order wrench faults. During this wrenching only a few accompanying structural forms were newly generated. Inmost cases they follow a pre—existing regional fracture pattern. INTRODUCTION Neotectonic structures in the Mura depression, that is in the northernmost part of the Pannonian basin that belongs to Yugoslavia, show a pattern that distincly differs from the basement structure. Physiographic features of this area are expressed by river valleys as Drava, Mura and several other smaller ones, and accompanying ridges both oriented NW—SE. The valleys are filled by Qiaternary proluvial and alluvial deposits up to 100 meter thick. This suggests a rapid recent erosion of surrounding hills and deposition of that material in lowlands. This is very probably caused by neotectonic processes, i.e. uplifting and subsidence. However, the geologic structures show a different pattern. It consists of a series of gentle folds oriented NE—SW, that is in the opposite direction than geomor— phologic forms. These folds in the Tertiary sedimentary cover could be related to the basement that consists of a series of horsts and grabens oriented NE—SW. According to various isopach maps of Neogene stratigraphic units (1), basement structures determined sedimentation during the Neogene time. It is generally thought that folds in Tertiary rocks in fact represent structural arches caused by vertical movement of basement blocks. However, the results of detailed field work suggest that a horizontal stress was present here, and that this stress represented the dominant factor in creating of Tertiary folds. The problem that remains to be solved then is, how to connect structures of the two Opposite directions, i.e. NE—SE. This paper represents an attempt to give an answer to that question. During this study, in addition to the field work, Landsat images were intensively studied. They showed to be of great help, because terrains in the Pannonian basin are not so suitable for geological mapping. They are generally covered by Quaternary deposits that mask geological structures. Thus, the satellite images enabled to detect numerous faults, basically neotectonic ones. These faults represented pieces of an incomplete mosaic, and their detecting enabled construction of a structural model that could explain the structural evolution of this area.
:39
140
M. Poljak
GEOLOGICAL SETTING The investigated area is placed among several large geotectonic units (Fig.l). The western border of the Mura depression are the Central Alps and Karavanken, on the south there are the Inner Dinariden, and towards the north and west, the Mura depression continues into the Pannonian basin of Austria and Hungary. The dominant structural elements here are the Labod fault, that separates the Central Alps from Karavanken (the Alpine—Dinaric transition zone), and the ~o~tanje fault between Karavanken and the Inner Dinariden. Both of them are large right lateral horizontal faults. Towards the east they terminate at the Zagreb—Balaton structural zone.
S
~
~
I I
I!U’:~
LL ::::
~D6
~7
1’
II
~
20
________________ -~ ~ii.
Fig. 1. Generalized geologic map of the Mura depression. 1. Pre—Tertiary rocks, 2. Tertiary rocks, 3. Quaternary sediments, 4. Fault and thrust, 5. Anticline and syncline, 6. Central Alps, 7. Alpine—Dinaric transition zone, 8. Inner Dinarides, 9. Labod fault, 10. ~o~tan.je fault, 11. Ljutomer fault, 12. Kungota fault. This zone represents in fact a paleotransform fault, according to ~iki6 (2), but today it is a system of faults and folds oriented NE—SW. It is proposed that along some faults of that zone, as the Donat, 0rmo~ and Ljutomer faults, there also exists right lateral horizontal movement. On the Fig. 1, general stratigraphic units can also be seen. They have not been divided in detail because only relationship between Tertiary and Quater— nary rocks and pre—Tertiary basement is relevant here. The boudary between the Central Alps and Dinarides is expressed also under the sedimentary cover in the Mura depression. Drilling data suggest that the Central Alps continue to the Ljutomer fault because the basement of this area is built up of crystalline rocks of Paleozoic age. Southwards of this fault, basement generally consists of Mesozoic carbonaceous rocks, thus this area is a part of Dinari— des. Regarding the Tertiary sediments, the oldest rocks belong to Middle Oh— gocene. Almost all Neogene stratigraphic units are present there, with numerous trans.. gressions and regressions that are related to the structural evolution of Paratethys
Neotectonic Investigations in the Pannonian Basin
Basement
141
Tectonics
Basement in the Mura depression shows a regular pattern. On Fig. 2 one can see a series of positive and negative structures oriented NE—SW. In the central part there is a prominent Uplift where the basement is on the depth of 500 meters. Southwestwards there is, however, a depression with an extraordinary depth of basement, more than 55oo meters.. The southern boundary of this depression is t~’&Ljutomer fault. As mentioned before, this fault is believed to -epresent tie border cetween the Alps and Dinarides.
H ~
~7
OSC
20
Fig. 2. Basement map of the Mura depression. 1. Outcrops of basement rocks, 2. Fault, 3. Contour lines of basement with depths in meters (after /1/). The main orogeny in these areas was the Laramide orogeny at the end of Creta— ceous. After these events, the basement was exposed for a long time, which had to cause its flattening. So today’s big differences in the depth of basement were caused by later tectonic events. The isopachs of various Tertiary units suggest a continuous and differential subsidence and uplifting of basement in the Mura depression. These movements followed more or less the same pattern throughout thewhole Tertiary. Neotectonics Generally, the oldest Tertiary rocks in the Mura depression are of Tortonian age. This indicates the main subsidence and transgression over a large area. These events are beeing considered by many authors as the beginning of neo— tectonics in tnelPannonian basin. As sai~1 before, the Neogene sedimentation continued with numerous oscillations to the end of Pliocene. Sediments of Dalian age, mostly continental sands and gravels, lie horizontally over older deformed sediments. That means that in this area there occurs a Pliocene folding that compr~~edall Neogene beds. It has been generally thought that this originates from a differential vertical movement of particular basement blods that caused an arching of overlying sediments. However, taking into account events in the whole Pannonian area during Neogene, or even the circum — Mediterranean region, there existed obvious tangential stresses during that time that could have ~ised folding. For example Horvath et al. (3) and Royden et al. (4) described in detail extensional processes in the Pannonian basin during Tertiary. The idea of the~basically horizontal stresses was a premise upon which this study was done. In order to prove (or refute) this idea, numerous structural elements have been measured in various Tertiary rocks in the Mura depression. It was supposed that if the horizontal stress during Neogene had been present there, it has to be traced in bed, by systematic fracturing at the first place. On the map (Fig. 3) some of test sites have been marked. The southern row of test sites is situated in the area where folding is clearly expressed. In
142
M. Poljak
case that these folds have been formed by vertical stress, the resulting fractures should be non—systematic, or they should show a particular arrangement characteristic for the “force folding” as for example described by Stearns (5).
~
P
Fig.
3.
0
~
~
k
~ b
~Q0.
.~
Position map of field measured structural
Q~ 0
0
20
~
elements.
However, the detailed structural analysis suggests that numerous fractures, measured on particular outcrops are doubtlessly the systematic ones, and that the resulting stress is horizontal. Following, there are some results or’ structural measurements presented by Schxtdt diagrams (Fig. 4.).
0
Neotectonic Investigations in the Pannonian Basin
143
0
Fig. 4. Schmidt diagrams (upper hemisphere) of structural elements measured in the field. V — dip of strata, R — tensional fractures, R — compressive fractures, R~ and R 4— shear fractures. a) Locality 1 on t~e position map, b) bc. 2, d) and e) bc. 5, f) bc. 7, g) bc. 8, h) boo. 11. 4.a) represents the distribution of fractures in Obigocene tuff deposits in Fig.Sava folds. The diagram is the upper hemisphere of mathematical means of several tens of fractures measured on one outcrop. Fracture distribution has obviously a systematic character, where R~ represents tensional, ft~, compr~sive and R shear forms. Positign of the aip CV) is perpendicular to the resulting stz~ess direction of 34o . That confirms the premise of horizontal stress as a causing mechanism for folding. This example represents one typical case among many measurements which all show a similar pattern. Figures4.o) and 4.c) represent examples of distinctly folded areas. In the first case the measured outcrop was in Helvetian marla, and in second in Tor— tonian limestone. In both cases, the relationship between tensional (R 1), compressive CR2) and shear (82, 84) fractures and the dip position (V) suggests systematic character of fractures and the NW—SE oriented horizontal stress. In the area where folding is not clearly expressed (Fig. .d) and 4.c) , fractures also show a regular pattern. In these two particular cases (Pliocene marls), only the stress direction is not constant; in the first one it is oriented NE—SW and in the second NW—SE. On the northern rim of the investigated area, strata are generally horizontaL Some recent geological maps of this area show geological structures (folds) that are oriented NW—SE. Here, on the contrary, was proposed that geological structures have the same southward direction, i.e. NE—SW. After detailed structural analysis, this premise was confirmed; strata dip to NW and SE at bow angles and fractures have a systematic character with resulting stress direction that is also oriented NW—SE. Some typical examples are presented on Fig. 4.t~) and Fig. 4.g), where the first one represents Helvetian and the second one Sarmatian beds. In the northernmost part of the investigated area, structural elements were measured in basaltic tuff deposits of Middle Pliocene age (Fig. U.h). Here, gently folded strata farm an anticline oriented NW—SE. Numerous fractures are clustered into several groups. Mathematical means of their positions have been also plotted on the upper hemisphere of Schmidt diagram. Their distribution suggests a systematic character with horizontal stress oriented NE—SW. LINEAMENTS In spite of the fact that geological mapping and structural analysis has given us the basic answer about the origin of the Mura depression, a complete structural framework of this area still remains to be solved. It was said before that terrains in the Pannonian basin are usually covered with Quaternary deposits. Therefore, in such a situation just satellite ima— ges represent a very usefull tool in geologic research. The lineamerot map seen on Fig. 5 represents a result of Landsat images analysis of the Mura depression. Here it has to be said that, without going into the discussion of the lineament theory, the author strongly believes in the existence of planetary fracturing which manifests as lineaments. However, they should be studied with respect to their geological meaning and significance. In this
144
M. Poijak
study, a number of bineaments showed to be “true” faults, defined by horizontal or vertical offset. We have to seek their origin in local geological conditions where parts of an already existing fracture grid were re—activated. In addition to this, some new structural forms were also generated.
~
Fig.
5.
Lineament map of the Mura depression based on
—
Landsat images.
The dominant features on the presented map are the ~o~tanje and Labod faults accompanied with several smaller faults genetically related to the main ones. The next prominant group are lineaments oriented generally east-west that coincide with faults; folds and lithobogical boundaries in Sava folds. Group of lineaments oriented ENE—WSW in most cases coincide with faults of the Zagreb— Babaton structural zone. Perpendicular to these, there is a group of linea— ments which are well expressed in the Gori~ko area. Eruption of Pliocene basaltic tuffs in this area is usually rebated to faults of this direction. Otherwise, these lineaments crosscut geobogical and geomorphobogical structures with no visible influence. On the contrary, lineaments oriented NW—SE determine the physiography of this area. Finally, we can notice on the map the same additional groups of lineaments or single ones with different azimuths, that could not be related to any geological structure of this area. SYNTHESIS On the basis of all presented data, an attempt was made to explain origin and rebationship of structures in the Mura depression. Some constants were the basis upon which the structural model shown in Fig. 6 was constructed. These parameters are: right — lateral horizontal movement along Sava, Labod and ~o5tanje faults; compressive Sava folds; tensional faults in Gori~ko with tuff eruption; numerous normal faults along extentional Drava and Mura valieys, right — lateral horizontal movement along Donat, Ormo~ and Ljutomer faults; and finally, the distinct NW—SE oriented horizontal stress ito the whole Mura depression. On the first sight these facts lead to the idea of wrench tectonics as a causing mechanism for the structural framework of this area. This idea was applied for the Pannonian basin by Royden et al. (4). They proposed that the main wrench fault here is the Periadriatic line and that, among others, Sava folds represent the consequence of this fault. Furthermore, they proposed numerous other horizontal faults in the whole Pannonian basin that caused formatiOn of several extentional (pull—apart) basins. Although this model is rather hypothetical, it seems to be applicable for the Mura depression. Thus, the model on Fig. 6 is based on wrench tectonics in the sense proposed by Wilcox et ab. (6). The dominant wrench faults are the Sava, ~ogtanje and Labod faults. Resulting compressive forms (A—A) are folds, faults and thrusts
Neotectonic Investigations in the Pannonian Basin
145
in the Sava folds, as well as folding area in Sbovenske gorice and Haboze (Ormo~—Sebnica anticline). A number of pre—existing bineaments belonging to the Zagreb—Balaton zone, which is believed to be of Paleozoic age (2), were reactivated, and they became compresaive forms too. Tensional forms (B—B) are perpendicular to the compressive ones and they are represented by bineaments oriented NNW—SSE. Extention in Gori~ko and eruption of basalt tuff is the most evident effect on the tension. Shear forms C—C and D—D have an angle of 600, and they are represented by two systems of bineaments. According to the wrench theory, along these shear forms only horizontal movements should occur. However,just lineaments oriented NW—SE that correspond to the shear forms C—C show distinct tensional tendency, specially during Quaternary. This fact, together with some other indications, suggests one additional stress system to be present here.
‘~‘~
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~
/
/
___
/
/( ~Dn.
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/
+
~-“-~-~-~
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o
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,.-(
,~
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,—~
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Fig. 6. Structural model of the Mura depression, based on wrench—style ~tonics. 1. Main wrench faults, 2. Faults genetically related to main faults, 3. Compressive faults, 4. Tensional faults, 5. Shear faults, 6. Extension, 7. Subsidence, 8. Anticbine or folded areas, 9. Stress direction measured on the field, 9a. Non—defined lineaments, 10. Fault, lb. ~o~tanje fault, 12. Labod fault, 13. Donat fault, 14. Ormo~ fault, 15. Ljutomer fault, 16. Ormo~—Selnica anticline. Sava, ~o~tanje and Labod faults represent the first order wrench faults, and Donat, Or— mo~ and Ljutomer faults represent the second order wrench faults. It has been proposed by some authors, as for example Premru (7) that the other dominant group of faults in the Mura depression, i.e. the Donat, Ormo~ and Ljutomer faults have also horizontal character. Premru placed the activity along this faults on the end of Cretaceous time, but more probably they are neotectonically active faults. On the Fig. 6 one can see right lateral Sava, ~o~tanje and Labod faults. They are proposed to be the first order faults. Tectonic transport of large blocks changed direction in contact with the Zagreb—Balaton structural zone. This resulted in re—orientation of the stress too. At this moment an existing group of faults (Donat,0rmo~ and Lju— tomer) started to behave as horizontal faults; in this case as right—lateral ones of the second order (it has to be mentioned here that this terminology should not be misused with Moody and Hill’s (8) structural models, where the geometry of wrenching is quite different). The direct consequence of this second order
wrenching was the formation of
146
M. Poijak
folds in Lendavske gorice and Gori~ko, and transformation of shear forms C—C into tensional ones (S~—B). This caused the formation of extentional Drava and Mura valleys. From the lineament map (Fig. 5) we can obviously conclude, that during these tectonical events only a few structures were newly generated; almost all Other represent only one re—activated pre—existing fracture pattern.
CONCLUSION During neotectonic investigations in the Pannonian basin, specifically in the Mura depression, satellite images showed to have a dominant role. General structural framework of this area was kt’own before, mainly from geological mapping, geophysics and drilling. But there were just satellite images that enabled recognition of many additional structural elements. At that moment, it was possible to construct ore plausible model for the neotectonic genesis of the Mura depression. Of course, we should not consider this model as the finite one. More advanced techniques in future will give us new data of be~ter quality and research will continue. REFERENCES 1.
N. Kisovar, Contribution to the solution of the structural relationship in our part of the Mura depression, Znanst. savjet za naftu JAZU, Zbor. radova, Novi Sad 1977, vol. 1, pp. 3lb—322 (in Serbocroatian).
2.
D. ~iki6, Deep fault of the Zagreb zone. pp. 251—263, (1978) (in Serbocroatian).
3.
F. Horvath, et. al., Models of Mediterranean back—arc basin Phil. Trans. R.Soc. London, vol. A 300, pp. 383—4o2 (1981).
4.
L.H. Royden et al., Carpathian Pannonian (1982).
5.
D. Stearns, Faulting and forced folding in the Rocky Mountains foreland, Geol. Soc. Amer. Memoir 151, pp. 1—37 (1978).
6.
R.E. Wilcox et al., Basic Wrench Tectonics, Bull., v. 57,no 1, pp. 74—96 (1973).
7.
U. Premru, Tectonic evolution of Slovenia during the time interval from the Upper Cretaceaus to the Tertiary period, Symposium on problems of Danian in Yugoslavia, Ljubljana 1981, pp. 147—154 (in Sbovenian).
8.
J.D. Moody v. 67, pp.
Geol.
vjesnik,
vol.
30/1,
formation,
Transform faulting, extension, and subduction in the region, Geob. Soc. Amer. Bull., vol 93, pp. 717—725
Amer. Assoc.
and M.J. Hill, Wrench—fault tectonics, l2o7—l246 (1956).
Geol.
Petrol.
Soc.
Geob.
Amer. Bull.,
0273—1177/84 $0.00 + .50 Copyright © COSPAR
NEOTECTONIC INVESTIGATIONS IN THE PANNONIAN BASIN BASED ON SATELLITE IMAGES M. Poijak Geological Survey Ljubljana, Linharrova 9, Yugoslavia
61000 Ljubljana,
ABSTRACT Analysis of Landsat images of the Mura depression in the Pannonian basin enabled identification of a number of lineaments that, in most cases, were not known from previous geological investigations. Many of these lineaments act as real faults and they have neotectonic significance. Combination of Landsat and field data made possible construction of a structural model that can explain the neotectonic evolution of this area. The model is based on wrench—tectonics, where Sava, ~o~tanje and Labod faults represent the first order wrench faults, and Donat, 0rmo~ and Ljutomer the second order wrench faults. During this wrenching only a few accompanying structural forms were newly generated. Inmost cases they follow a pre—existing regional fracture pattern. INTRODUCTION Neotectonic structures in the Mura depression, that is in the northernmost part of the Pannonian basin that belongs to Yugoslavia, show a pattern that distincly differs from the basement structure. Physiographic features of this area are expressed by river valleys as Drava, Mura and several other smaller ones, and accompanying ridges both oriented NW—SE. The valleys are filled by Qiaternary proluvial and alluvial deposits up to 100 meter thick. This suggests a rapid recent erosion of surrounding hills and deposition of that material in lowlands. This is very probably caused by neotectonic processes, i.e. uplifting and subsidence. However, the geologic structures show a different pattern. It consists of a series of gentle folds oriented NE—SW, that is in the opposite direction than geomor— phologic forms. These folds in the Tertiary sedimentary cover could be related to the basement that consists of a series of horsts and grabens oriented NE—SW. According to various isopach maps of Neogene stratigraphic units (1), basement structures determined sedimentation during the Neogene time. It is generally thought that folds in Tertiary rocks in fact represent structural arches caused by vertical movement of basement blocks. However, the results of detailed field work suggest that a horizontal stress was present here, and that this stress represented the dominant factor in creating of Tertiary folds. The problem that remains to be solved then is, how to connect structures of the two Opposite directions, i.e. NE—SE. This paper represents an attempt to give an answer to that question. During this study, in addition to the field work, Landsat images were intensively studied. They showed to be of great help, because terrains in the Pannonian basin are not so suitable for geological mapping. They are generally covered by Quaternary deposits that mask geological structures. Thus, the satellite images enabled to detect numerous faults, basically neotectonic ones. These faults represented pieces of an incomplete mosaic, and their detecting enabled construction of a structural model that could explain the structural evolution of this area.
:39
140
M. Poljak
GEOLOGICAL SETTING The investigated area is placed among several large geotectonic units (Fig.l). The western border of the Mura depression are the Central Alps and Karavanken, on the south there are the Inner Dinariden, and towards the north and west, the Mura depression continues into the Pannonian basin of Austria and Hungary. The dominant structural elements here are the Labod fault, that separates the Central Alps from Karavanken (the Alpine—Dinaric transition zone), and the ~o~tanje fault between Karavanken and the Inner Dinariden. Both of them are large right lateral horizontal faults. Towards the east they terminate at the Zagreb—Balaton structural zone.
S
~
~
I I
I!U’:~
LL ::::
~D6
~7
1’
II
~
20
________________ -~ ~ii.
Fig. 1. Generalized geologic map of the Mura depression. 1. Pre—Tertiary rocks, 2. Tertiary rocks, 3. Quaternary sediments, 4. Fault and thrust, 5. Anticline and syncline, 6. Central Alps, 7. Alpine—Dinaric transition zone, 8. Inner Dinarides, 9. Labod fault, 10. ~o~tan.je fault, 11. Ljutomer fault, 12. Kungota fault. This zone represents in fact a paleotransform fault, according to ~iki6 (2), but today it is a system of faults and folds oriented NE—SW. It is proposed that along some faults of that zone, as the Donat, 0rmo~ and Ljutomer faults, there also exists right lateral horizontal movement. On the Fig. 1, general stratigraphic units can also be seen. They have not been divided in detail because only relationship between Tertiary and Quater— nary rocks and pre—Tertiary basement is relevant here. The boudary between the Central Alps and Dinarides is expressed also under the sedimentary cover in the Mura depression. Drilling data suggest that the Central Alps continue to the Ljutomer fault because the basement of this area is built up of crystalline rocks of Paleozoic age. Southwards of this fault, basement generally consists of Mesozoic carbonaceous rocks, thus this area is a part of Dinari— des. Regarding the Tertiary sediments, the oldest rocks belong to Middle Oh— gocene. Almost all Neogene stratigraphic units are present there, with numerous trans.. gressions and regressions that are related to the structural evolution of Paratethys
Neotectonic Investigations in the Pannonian Basin
Basement
141
Tectonics
Basement in the Mura depression shows a regular pattern. On Fig. 2 one can see a series of positive and negative structures oriented NE—SW. In the central part there is a prominent Uplift where the basement is on the depth of 500 meters. Southwestwards there is, however, a depression with an extraordinary depth of basement, more than 55oo meters.. The southern boundary of this depression is t~’&Ljutomer fault. As mentioned before, this fault is believed to -epresent tie border cetween the Alps and Dinarides.
H ~
~7
OSC
20
Fig. 2. Basement map of the Mura depression. 1. Outcrops of basement rocks, 2. Fault, 3. Contour lines of basement with depths in meters (after /1/). The main orogeny in these areas was the Laramide orogeny at the end of Creta— ceous. After these events, the basement was exposed for a long time, which had to cause its flattening. So today’s big differences in the depth of basement were caused by later tectonic events. The isopachs of various Tertiary units suggest a continuous and differential subsidence and uplifting of basement in the Mura depression. These movements followed more or less the same pattern throughout thewhole Tertiary. Neotectonics Generally, the oldest Tertiary rocks in the Mura depression are of Tortonian age. This indicates the main subsidence and transgression over a large area. These events are beeing considered by many authors as the beginning of neo— tectonics in tnelPannonian basin. As sai~1 before, the Neogene sedimentation continued with numerous oscillations to the end of Pliocene. Sediments of Dalian age, mostly continental sands and gravels, lie horizontally over older deformed sediments. That means that in this area there occurs a Pliocene folding that compr~~edall Neogene beds. It has been generally thought that this originates from a differential vertical movement of particular basement blods that caused an arching of overlying sediments. However, taking into account events in the whole Pannonian area during Neogene, or even the circum — Mediterranean region, there existed obvious tangential stresses during that time that could have ~ised folding. For example Horvath et al. (3) and Royden et al. (4) described in detail extensional processes in the Pannonian basin during Tertiary. The idea of the~basically horizontal stresses was a premise upon which this study was done. In order to prove (or refute) this idea, numerous structural elements have been measured in various Tertiary rocks in the Mura depression. It was supposed that if the horizontal stress during Neogene had been present there, it has to be traced in bed, by systematic fracturing at the first place. On the map (Fig. 3) some of test sites have been marked. The southern row of test sites is situated in the area where folding is clearly expressed. In
142
M. Poljak
case that these folds have been formed by vertical stress, the resulting fractures should be non—systematic, or they should show a particular arrangement characteristic for the “force folding” as for example described by Stearns (5).
~
P
Fig.
3.
0
~
~
k
~ b
~Q0.
.~
Position map of field measured structural
Q~ 0
0
20
~
elements.
However, the detailed structural analysis suggests that numerous fractures, measured on particular outcrops are doubtlessly the systematic ones, and that the resulting stress is horizontal. Following, there are some results or’ structural measurements presented by Schxtdt diagrams (Fig. 4.).
0
Neotectonic Investigations in the Pannonian Basin
143
0
Fig. 4. Schmidt diagrams (upper hemisphere) of structural elements measured in the field. V — dip of strata, R — tensional fractures, R — compressive fractures, R~ and R 4— shear fractures. a) Locality 1 on t~e position map, b) bc. 2, d) and e) bc. 5, f) bc. 7, g) bc. 8, h) boo. 11. 4.a) represents the distribution of fractures in Obigocene tuff deposits in Fig.Sava folds. The diagram is the upper hemisphere of mathematical means of several tens of fractures measured on one outcrop. Fracture distribution has obviously a systematic character, where R~ represents tensional, ft~, compr~sive and R shear forms. Positign of the aip CV) is perpendicular to the resulting stz~ess direction of 34o . That confirms the premise of horizontal stress as a causing mechanism for folding. This example represents one typical case among many measurements which all show a similar pattern. Figures4.o) and 4.c) represent examples of distinctly folded areas. In the first case the measured outcrop was in Helvetian marla, and in second in Tor— tonian limestone. In both cases, the relationship between tensional (R 1), compressive CR2) and shear (82, 84) fractures and the dip position (V) suggests systematic character of fractures and the NW—SE oriented horizontal stress. In the area where folding is not clearly expressed (Fig. .d) and 4.c) , fractures also show a regular pattern. In these two particular cases (Pliocene marls), only the stress direction is not constant; in the first one it is oriented NE—SW and in the second NW—SE. On the northern rim of the investigated area, strata are generally horizontaL Some recent geological maps of this area show geological structures (folds) that are oriented NW—SE. Here, on the contrary, was proposed that geological structures have the same southward direction, i.e. NE—SW. After detailed structural analysis, this premise was confirmed; strata dip to NW and SE at bow angles and fractures have a systematic character with resulting stress direction that is also oriented NW—SE. Some typical examples are presented on Fig. 4.t~) and Fig. 4.g), where the first one represents Helvetian and the second one Sarmatian beds. In the northernmost part of the investigated area, structural elements were measured in basaltic tuff deposits of Middle Pliocene age (Fig. U.h). Here, gently folded strata farm an anticline oriented NW—SE. Numerous fractures are clustered into several groups. Mathematical means of their positions have been also plotted on the upper hemisphere of Schmidt diagram. Their distribution suggests a systematic character with horizontal stress oriented NE—SW. LINEAMENTS In spite of the fact that geological mapping and structural analysis has given us the basic answer about the origin of the Mura depression, a complete structural framework of this area still remains to be solved. It was said before that terrains in the Pannonian basin are usually covered with Quaternary deposits. Therefore, in such a situation just satellite ima— ges represent a very usefull tool in geologic research. The lineamerot map seen on Fig. 5 represents a result of Landsat images analysis of the Mura depression. Here it has to be said that, without going into the discussion of the lineament theory, the author strongly believes in the existence of planetary fracturing which manifests as lineaments. However, they should be studied with respect to their geological meaning and significance. In this
144
M. Poijak
study, a number of bineaments showed to be “true” faults, defined by horizontal or vertical offset. We have to seek their origin in local geological conditions where parts of an already existing fracture grid were re—activated. In addition to this, some new structural forms were also generated.
~
Fig.
5.
Lineament map of the Mura depression based on
—
Landsat images.
The dominant features on the presented map are the ~o~tanje and Labod faults accompanied with several smaller faults genetically related to the main ones. The next prominant group are lineaments oriented generally east-west that coincide with faults; folds and lithobogical boundaries in Sava folds. Group of lineaments oriented ENE—WSW in most cases coincide with faults of the Zagreb— Babaton structural zone. Perpendicular to these, there is a group of linea— ments which are well expressed in the Gori~ko area. Eruption of Pliocene basaltic tuffs in this area is usually rebated to faults of this direction. Otherwise, these lineaments crosscut geobogical and geomorphobogical structures with no visible influence. On the contrary, lineaments oriented NW—SE determine the physiography of this area. Finally, we can notice on the map the same additional groups of lineaments or single ones with different azimuths, that could not be related to any geological structure of this area. SYNTHESIS On the basis of all presented data, an attempt was made to explain origin and rebationship of structures in the Mura depression. Some constants were the basis upon which the structural model shown in Fig. 6 was constructed. These parameters are: right — lateral horizontal movement along Sava, Labod and ~o5tanje faults; compressive Sava folds; tensional faults in Gori~ko with tuff eruption; numerous normal faults along extentional Drava and Mura valieys, right — lateral horizontal movement along Donat, Ormo~ and Ljutomer faults; and finally, the distinct NW—SE oriented horizontal stress ito the whole Mura depression. On the first sight these facts lead to the idea of wrench tectonics as a causing mechanism for the structural framework of this area. This idea was applied for the Pannonian basin by Royden et al. (4). They proposed that the main wrench fault here is the Periadriatic line and that, among others, Sava folds represent the consequence of this fault. Furthermore, they proposed numerous other horizontal faults in the whole Pannonian basin that caused formatiOn of several extentional (pull—apart) basins. Although this model is rather hypothetical, it seems to be applicable for the Mura depression. Thus, the model on Fig. 6 is based on wrench tectonics in the sense proposed by Wilcox et ab. (6). The dominant wrench faults are the Sava, ~ogtanje and Labod faults. Resulting compressive forms (A—A) are folds, faults and thrusts
Neotectonic Investigations in the Pannonian Basin
145
in the Sava folds, as well as folding area in Sbovenske gorice and Haboze (Ormo~—Sebnica anticline). A number of pre—existing bineaments belonging to the Zagreb—Balaton zone, which is believed to be of Paleozoic age (2), were reactivated, and they became compresaive forms too. Tensional forms (B—B) are perpendicular to the compressive ones and they are represented by bineaments oriented NNW—SSE. Extention in Gori~ko and eruption of basalt tuff is the most evident effect on the tension. Shear forms C—C and D—D have an angle of 600, and they are represented by two systems of bineaments. According to the wrench theory, along these shear forms only horizontal movements should occur. However,just lineaments oriented NW—SE that correspond to the shear forms C—C show distinct tensional tendency, specially during Quaternary. This fact, together with some other indications, suggests one additional stress system to be present here.
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/
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Fig. 6. Structural model of the Mura depression, based on wrench—style ~tonics. 1. Main wrench faults, 2. Faults genetically related to main faults, 3. Compressive faults, 4. Tensional faults, 5. Shear faults, 6. Extension, 7. Subsidence, 8. Anticbine or folded areas, 9. Stress direction measured on the field, 9a. Non—defined lineaments, 10. Fault, lb. ~o~tanje fault, 12. Labod fault, 13. Donat fault, 14. Ormo~ fault, 15. Ljutomer fault, 16. Ormo~—Selnica anticline. Sava, ~o~tanje and Labod faults represent the first order wrench faults, and Donat, Or— mo~ and Ljutomer faults represent the second order wrench faults. It has been proposed by some authors, as for example Premru (7) that the other dominant group of faults in the Mura depression, i.e. the Donat, Ormo~ and Ljutomer faults have also horizontal character. Premru placed the activity along this faults on the end of Cretaceous time, but more probably they are neotectonically active faults. On the Fig. 6 one can see right lateral Sava, ~o~tanje and Labod faults. They are proposed to be the first order faults. Tectonic transport of large blocks changed direction in contact with the Zagreb—Balaton structural zone. This resulted in re—orientation of the stress too. At this moment an existing group of faults (Donat,0rmo~ and Lju— tomer) started to behave as horizontal faults; in this case as right—lateral ones of the second order (it has to be mentioned here that this terminology should not be misused with Moody and Hill’s (8) structural models, where the geometry of wrenching is quite different). The direct consequence of this second order
wrenching was the formation of
146
M. Poijak
folds in Lendavske gorice and Gori~ko, and transformation of shear forms C—C into tensional ones (S~—B). This caused the formation of extentional Drava and Mura valleys. From the lineament map (Fig. 5) we can obviously conclude, that during these tectonical events only a few structures were newly generated; almost all Other represent only one re—activated pre—existing fracture pattern.
CONCLUSION During neotectonic investigations in the Pannonian basin, specifically in the Mura depression, satellite images showed to have a dominant role. General structural framework of this area was kt’own before, mainly from geological mapping, geophysics and drilling. But there were just satellite images that enabled recognition of many additional structural elements. At that moment, it was possible to construct ore plausible model for the neotectonic genesis of the Mura depression. Of course, we should not consider this model as the finite one. More advanced techniques in future will give us new data of be~ter quality and research will continue. REFERENCES 1.
N. Kisovar, Contribution to the solution of the structural relationship in our part of the Mura depression, Znanst. savjet za naftu JAZU, Zbor. radova, Novi Sad 1977, vol. 1, pp. 3lb—322 (in Serbocroatian).
2.
D. ~iki6, Deep fault of the Zagreb zone. pp. 251—263, (1978) (in Serbocroatian).
3.
F. Horvath, et. al., Models of Mediterranean back—arc basin Phil. Trans. R.Soc. London, vol. A 300, pp. 383—4o2 (1981).
4.
L.H. Royden et al., Carpathian Pannonian (1982).
5.
D. Stearns, Faulting and forced folding in the Rocky Mountains foreland, Geol. Soc. Amer. Memoir 151, pp. 1—37 (1978).
6.
R.E. Wilcox et al., Basic Wrench Tectonics, Bull., v. 57,no 1, pp. 74—96 (1973).
7.
U. Premru, Tectonic evolution of Slovenia during the time interval from the Upper Cretaceaus to the Tertiary period, Symposium on problems of Danian in Yugoslavia, Ljubljana 1981, pp. 147—154 (in Sbovenian).
8.
J.D. Moody v. 67, pp.
Geol.
vjesnik,
vol.
30/1,
formation,
Transform faulting, extension, and subduction in the region, Geob. Soc. Amer. Bull., vol 93, pp. 717—725
Amer. Assoc.
and M.J. Hill, Wrench—fault tectonics, l2o7—l246 (1956).
Geol.
Petrol.
Soc.
Geob.
Amer. Bull.,