GEOLOGICA CARPATHICA, DECEMBER 2010, 61, 6, 483—493 doi: 10.2478/v10096-010-0030-3
Strike-slip reactivation of a Paleogene to Miocene fold and
thrust belt along the central part of the Mid-Hungarian
and LÁSZLÓ CSONTOS
Eötvös Loránd University, Department of Geology, Pázmány Péter sétány 1/c, 1117 Budapest, Hungary; email@example.com
MOL PLC., Október huszonharmadika u. 18, 1117 Budapest, Hungary
(Manuscript received October 26, 2009; accepted in revised form November 8, 2010)
Abstract: Recently shot 3D seismic data allowed for a detailed interpretation, aimed at the tectonic evolution of the
central part of the Mid-Hungarian Shear Zone (MHZ). The MHZ acted as a NW vergent fold and thrust belt in the Late
Oligocene. The intensity of shortening increased westwards, causing clockwise rotation of the western regions, rela-
tively to the mildly deformed eastern areas. Blind thrusting and related folding in the MHZ continued in the Early
Miocene. Thrusting and gentle folding in the MHZ partly continued in the earliest Pannonian, and was followed by
sinistral movements in the whole MHZ, with maximal displacement along the Tóalmás zone. Late Pannonian inversion
activated thrusts and generated transpressional movements along the Tóalmás zone.
Key words: tectonics, fold and thrust belt, strike-slip faulting, Mid-Hungarian Shear Zone.
The WSW-ENE striking Mid-Hungarian Shear Zone (MHZ)
separates the ALCAPA and Tisza-Dacia mega-units (Cson-
tos & Vörös 2004). Its importance has been known for de-
cades, but the deformation history of this zone, lying
between the enigmatic Mid-Hungarian Line (MHL) to the
south and the well resolved Balaton—Tóalmás lines to the
north, is still debated.
Reviewing and extending tectonic studies based on 2D in-
dustrial seismic data (Csontos & Nagymarosy 1998), the
current study aims at the tectonic interpretation of three re-
cently shot overlapping 3D seismic cubes of MOL PLC.
) in the central part of the MHZ, southeast of
Budapest (Fig. 1). The Paleogene and Neogene tectonic evo-
lution is addressed, but this study does not focus on the Me-
sozoic evolution or on neotectonic events.
The idea that the MHL could be a mega-unit boundary was
suggested already decades ago (Géczy 1973). Based on paleo-
geographic evidence, Kázmér & Kovács (1985) suggested
450—500 km dextral strike-slip during the Eocene—Oligocene
that accompanied the extrusion of the Transdanubian Range
from the Alpine units. Following an initial model by Mészáros
(1984) and paleomagnetic data of Márton (1985), Balla (1984)
and Balla et al. (1987) introduced the basic and generally ac-
cepted concept: the Early-Middle Miocene counterclockwise
rotation of ALCAPA, in contrast to the clockwise rotation of
the Tisza-Dacia unit (for a summary see Csontos & Vörös
2004), accompanied by shearing of the Dinaric units between
the two major blocks. Dextral extrusion was accompanied by
east-oriented extension of both mega-units into the Car-
pathian embayment (Balla 1984; Tari 1994; Sperner et al.
2002) and is largely responsible for the bulk offset between
the two blocks.
Extrusion and rotations started already in the Eocene
(Fodor et al. 1992), but the bulk of the deformation took
place in the Oligocene—Early Miocene (Kázmér & Kovács
1985; Csontos et al. 1992; Fodor et al. 1992, 1998). The lack
of crustal thickening and formation of mountains in this de-
formed zone can be attributed to orogen-parallel extension,
perhaps facilitated by increased heat flux caused by large
amounts of Miocene syn-tectonic volcanics (Csontos &
Nagymarosy 1998; Kovács et al. 2007).
Between the ALCAPA Mega-unit (Csontos & Vörös 2004;
Schmid et al. 2008), consisting of the Eastern Alps, Western
Carpathians and Transdanubian Range, and the Tisza Mega-
unit, made up of the Mecsek, Villány-Bihar and Codru units,
an elongated zone with Dinaric-type rocks is found. In the
following, the MHZ is defined as the tectonic zone with Dina-
ric basement between the ALCAPA and Tisza mega-units,
bounded by the MHL to the south and the Balaton—Tóalmás
lines to the north.
The MHL is regarded as the boundary fault between the Tisza
and the MHZ. The approximate location of the line has been
known from borehole data (summarized by Fülöp & Dank
1987; Dank & Fülöp 1990; Haas et al. 2010) and geophysical
anomaly maps (e.g. Haáz & Komáromy 1966; Szabó &
Sárhidai 1989), but only a few studies have tried to actually
locate the boundary fault on seismic sections (Csontos &
Nagymarosy 1998 around the current study area; Csontos et
al. 2005 south of lake Balaton in the west). Technical difficul-
ties arise from the optimalization of industrial seismic pro-
cessing for the uppermost sedimentary infill, not for basement
structures below, as well as from imaging problems under
PALOTAI and CSONTOS
thick volcanic sequences. Due to the lack of exposures in the
majority of the Pannonian Basin, the exact surface position
and structural style of the MHL still raises questions. Only at
the Poiana Botizii area, the assumed eastern termination of
the MHL (Balla 1984; Csontos et al. 1992; Győrfi et al 1999;
Tischler et al. 2007) can the contact be directly observed: here
ALCAPA is thrust over Tisza-Dacia. The affiliation of the
zone near its assumed western end, close to Zagreb, is also de-
bated (Tari & Pamić 1998; Tomljenović & Csontos 2002).
The boundary of ALCAPA and the MHZ is much better de-
lineated: a steep fault or fault zone called the Balaton line was
recognized long ago. Considering paleomagnetic rotations
(Márton & Fodor 1995, 2003), paleogeographic (Kázmér &
Kovács 1985; Csontos & Vörös 2004) and geodynamic im-
plications (Kovács et al. 2007; Schmid et al. 2008; Kovács &
Szabó 2008; Ustaszewski et al. 2008), the Balaton line is the
continuation of the Periadriatic Line, separating the Eastern
and Southern Alps. Because the name comes from lake Bala-
ton (Fig. 1), this is a valid name in the western part of the
country. In a neotectonic study of the region (Fodor et al.
2005a), partly overlapping with the current study area, the east-
ern continuation of the Balaton line has been called the Tóalmás
line (or zone). As we deny the direct continuity of the strike-slip
system to the west (a separate study focusing on this issue is in
preparation), this latter name will be used in the following.
This study focuses on the MHZ. To overview the evolu-
tion of the area, however, a short description of the general
stratigraphic buildup in, and to both sides of the zone is giv-
The basement of the MHZ is greatly different from the one
found north and south of it. On ALCAPA, Paleozoic to Me-
sozoic sequences with Alpine affinities are found (Vörös et
al. 1990; Haas et al. 1995). Bükk-type and so Dinaric base-
ment rocks characterize the MHZ between the Mid-Hungari-
an and Balaton-Tóalmás lines (Wein 1969; Balla 1984;
Csontos et al. 1992) with a weak Cretaceous regional meta-
morphism (Árkai et al. 1995). As for crystalline rocks, only
some Variscan granites and Paleogene tonalites are found
along the Balaton line (Fülöp & Dank 1987). In contrast, on
the Tisza-Dacia block, high-grade metamorphic rocks
(Cserepes-Meszéna 1986) and late Variscan granites (Buda
1992) represent the basement of the non-metamorphosed,
Mesozoic sedimentary formations in Germanic facies (Géczy
1973; Kovács 1982; Vörös 1993).
Upper Cretaceous rocks are almost absent in the MHZ. To the
south on Tisza-Dacia, however, Upper Cretaceous deep marine
red marls, conglomerates and other clastics occur in several ma-
jor synforms (Szentgyörgyi 1989) in great thickness.
Fig. 1. Location map of the investigated area.
PALEOGENE—MIOCENE FOLD AND THRUST BELT OF THE MID-HUNGARIAN SHEAR ZONE
The Paleogene sequences also strongly differ on both sides
of the MHL. The variable sequences north of the line, in the
North Hungarian Paleogene Basin (Báldi & Báldi-Beke
1985; Fodor et al. 1992; Tari et al. 1993) comprise Upper
Eocene clastics, limestones, and deep marine marls, fol-
lowed by anoxic deposits and deep marine clays in the Oli-
gocene. This sedimentation terminated with an erosional event
ca. 25 Ma. Tuff horizons in the Paleogene deposits of the
Mid-Hungarian Zone indicate continuous volcanic activity.
South of the MHL, Paleogene rocks are restricted to the
Szolnok trough (Nagymarosy & Báldi-Beke 1993), where
marine clastics are topped by Oligocene shales. No such de-
posits are found in the immediate surroundings of the MHL.
While north of the MHL a shallow marine Lower Miocene
sandstone unit covers the Oligocene (Sztanó & Tari 1993),
in the south, rocks of similar age are missing. Isochronous
turbidites are only found in the Transylvanian prolongation
of the Szolnok trough (Nagymarosy & Báldi-Beke 1993).
Despite significant thickness changes close to the MHL, Mid-
dle and Upper Miocene deposits cannot be differentiated any
more on both sides. Ample Middle Miocene volcanics mark the
vicinity of the MHL. After an episode of shallow marine lime-
stone formation, the clastic infill of lake Pannon characterizes
the Late Miocene and Pliocene sedimentation of the area.
Seismic stratigraphy and interpretation
The following 3D seismic horizons (supplemented by
some 2D lines) were mapped using GeoProbe and Seis-
1. Base Pannonian: a regional unconformity at 11.6 Ma
(Piller et al. 2007), characterized by high amplitude positive
reflections. The Pannonian of the Central Paratethys includes
Upper Miocene as well as Pliocene standard stages, although
no well data have been examined to prove Pliocene ages in
the area. In the following, we will use the terms ‘Pannonian’
for the units above, and ‘Early to Middle’ or ‘pre-Pannonian’
Miocene for the units below this unconformity.
2. Base Miocene: negative reflections, mainly above Oli-
gocene clayey formations, often, but not always with an ero-
sional contact. Lower and Middle Miocene units were not
3. Top Eocene: positive reflections above carbonates. As at
some locations Oligocene formations are extremely thin, or
even missing, Eocene rocks may directly underlie the Miocene.
A model of the area was built with the Move Software of
Midland Valley Ltd. Because no significant difference be-
tween time and depth surface patterns was expected, no
depth conversion has been undertaken.
Deformation patterns were mainly obtained from two-way
travel (TWT) time maps of interpreted horizons (Figs. 3, 7
and 8). Please note that the maps are not northward oriented.
Individual maps show the superposition of (1) the paleoto-
pography at, and (2) the total deformation after the time of
formation. In the following, structures are discussed from
‘top to bottom’, that is beginning from the youngest, and
proceeding to older ones.
Fig. 2. Uninterpreted and interpreted seismic section with identified horizons and characteristic structures. Note small arrows for horizon termina-
tions. Horizon styles apply for all seismic lines shown later. For location see Fig. 1. The foredeep of NW-vergent thrusts in the central part is a
Pannonian structure, as Lower and Middle Miocene formations thicken only in the north-western part, indicating a shift of depocenters.
PALOTAI and CSONTOS
The Pannonian and post-Pannonian deformation pattern is
dominated by (1) the Tóalmás zone and (2) NW vergent
thrust propagation folds (Fig. 3).
The Tóalmás zone is a slightly bent, NE-SW trending
strike-slip system, perhaps terminating just at the eastern
margin of cube B. Here, at Tóalmás, an asymmetric strike-
slip pop-up complex (Fig. 4) creates a pronounced ridge.
Pannonian sedimentation starts on the north-western side of
the ridge: the earliest Pannonian reflectors onlap against the
elevated high, and are even normally offset. This indicates the
transtensional activity of the zone in earliest Pannonian times.
The Late Pannonian reflectors form a broad, asymmetrically
SE-wards tilted antiform above the larger strike-slip zone,
suggesting young, perhaps Pliocene or even Quaternary dom-
ing (see also Ruszkiczay-Rüdiger et al. 2007, 2009).
Two steep strike-slip fault segments form a right-stepping
stepover north of Tóalmás (Fig. 3). A small east-vergent re-
verse fault between them offsets top Eocene and base Mio-
cene horizons, and drags base Pannonian, thus creating a
restraining stepover. The geometry of this structure clearly
shows a sinistral character.
The above mentioned pop-up structure marks the eastern
end of the continuous Tóalmás zone. When trying to map the
fault zone on 2D seismic lines NE of 3D cube B, no clear ev-
idence for further prolongation of similar structures was
found. Even in 3D, fault splays around Tóalmás suggest the
ending of the strike-slip zone. However, the Darnó Zone, a
prominent strike slip and thrust zone with a Mesozoic-Paleo-
Fig. 3. Base Pannonian TWT time map. See text for details.
Fig. 4. Uninterpreted and interpreted seismic section in cube B. For horizon style see Fig. 2, for location Fig. 1. Note the asymmetrical
character of the strike-slip zone, forming a wide dome in the Late Pannonian (probably even younger). Lower and Middle Miocene forma-
tions are thickest in the vicinity of this zone, indicating post-Early to Middle Miocene uplift.
PALEOGENE—MIOCENE FOLD AND THRUST BELT OF THE MID-HUNGARIAN SHEAR ZONE
gene core is found on the trend and in the apparent continua-
tion of the Tóalmás zone (Fig. 9).
At Sülysáp the strike-slip fault splits into two parallel seg-
ments, creating a narrow trench in-between that runs west-
wards for ca. 25 km to the SW, with minor undulations in
the base Pannonian surface. Near the western end of cube C,
the southeastern fault segment turns SSW and then diminish-
es: it cannot be traced on 2D lines nearby.
A third, blind segment can be traced in cube C. Above this,
on the southern flank of the Tóalmás zone, one large (in the
immediate north-eastern vicinity of Fig. 6) and two smaller,
almost perpendicular incisions can be observed at the base of
Pannonian formations. We interpret these features as slump
scars on the faulted margin above the blind fault segment.
A steep SE dipping fault at M with relatively large normal
separation, and similar structures at the north-western
boundary of the Pánd High, between Pánd and M as well as
east of Tóalmás can be interpreted as Riedel shears to the
main sinistral strike-slip zone. These faults detach/diminish
in the Lower to Middle Miocene strata (Fig. 7B), and suggest
that strike-slip movements in the Pannonian were not re-
stricted to the Tóalmás zone s.str., but occurred in a broader
zone within the MHZ. Strike-slip motions along the north-
western boundary fault of the Örkény High cannot be ex-
cluded either, due to its steep dip (Fig. 6).
South-westwards increasing intensity of compressional
structures characterizes the MHZ. The N-S trending ridge at
Jászberény (Fig. 3) might be caused by the activity of a
W-vergent blind thrust below, although the Pannonian activ-
ity of this structure is unclear.
A larger footwall syncline with syntectonic Lower Pan-
nonian sedimentary infill is found in cube A (Fig. 3). Here,
and near the north-eastern margin of the Örkény High
(Fig. 6), normal faults accommodate space problems on syn-
cline limbs. The possible strike-slip component of the main
normal fault might be related to the movements of the
Tóalmás zone (see above).
The elevated ridge of the base Pannonian surface between
Sári and Farmos is a thrust propagation anticline that gently
plunges NE-wards with decreasing offset along faults. This
means that the southwestern areas are much more elevated than
those in the NE. A major step in the base Pannonian surface is
seen at Pánd. Another, shorter thrust (and related anticline) to
the NW of the mentioned one diminishes within cube C.
Fig. 5. Uninterpreted and interpreted seismic section in cube B. For horizon style see Fig. 2, for location Fig. 1. Note the increased Oli-
gocene thickness in the SE, perhaps related to relay structures – see Fig. 8.
Fig. 6. Uninterpreted and interpreted seismic section in cube C. For horizon style see Fig. 2, for location Fig. 1.
PALOTAI and CSONTOS
At the south-eastern margin of cube C, a slightly bent,
NNW-SSE oriented, rather diffusely imaged half-graben
separates the Bugyi—Sári high in the SW from all the MHZ
structures mentioned above.
NW of the Tóalmás zone (in cubes B & C) ENE dipping
planar normal faults offset Lower Pannonian formations,
creating a bookshelf structure (Tari 1992; Németh 1999;
Fodor et al. 2005; Ruszkiczay-Rüdiger et al. 2007). These
faults all terminate against the main strike-slip zone.
In the easternmost parts (Fig. 1), the margin of the Jászság
Basin was mapped: Early Pannonian delta systems prograde
into the SE-ward deepening basin.
Early and Middle Miocene
Lower and Middle Miocene formations were not distin-
guished during seismic mapping, thus any distinction between
tectonic events of different pre-Pannonian Miocene age is hy-
pothetical. The Early and Middle Miocene deformation pat-
tern is illustrated on a TWT time map (Fig. 7A) as well as on a
TWT thickness map (Fig. 7B). The Early and Middle Miocene
deformation is – to some extent – similar to the Pannonian
pattern, but also shows characteristic differences.
Pre-Pannonian Miocene structures are much more offset
on either side of the Tóalmás zone to be sufficiently correlat-
ed, although thickness variations at the restraining stepover
north of Tóalmás confirm that at least at some time within
the Early to Middle Miocene, sinistral transpression pre-
vailed in the zone.
NW of the Tóalmás zone NE dipping bookshelf-type
faults, similar to the bulk of Pannonian deformation, charac-
terize the deformation pattern; thickness variations indicate
ongoing faulting throughout the Miocene.
In the MHZ, structures in the SW are more elevated than
those in the NE: shortening seems to increase westwards. The
transition zone between the relatively high and low regions,
lies between Pánd and Sülysáp. The most prominent feature is a
thrust propagation syncline with a thick Early/Middle Miocene
syntectonic infill (Fig. 7B) in the foreland of the Sári—Farmos—
Jászberény thrust and fault propagation anticlines. These anti-
clines, with their Early/Middle Miocene activity best seen on
the thickness map (Fig. 7B), are composed of three segments.
In the SW, two parallel anticlines are found that largely corre-
late with Pannonian-age structures, indicating ongoing, NW
vergent shortening. The southern fault propagation anticline
continues from Sári to the NE through Pánd to Farmos, where
it turns NNE to form the WNW vergent Jászberény segment,
and gradually diminishes nortwards. East of the Jászberény
ridge no compressional structures are observed.
The location of the thickest pre-Pannonian Miocene forma-
tions (Fig. 7B) lies SE of the topographic depression in the base
Miocene surface (Fig. 7A). This means that the Tóalmás ridge
experienced great uplift in the Pannonian, inverting the depo-
centre of the pre-Pannonian Miocene basin. This asymmetrical
uplift is shown on Fig. 4, whereas Fig. 7 demonstrates thickness
variations and depression geometry in this zone.
In the footwall syncline in cube A (Fig. 3), normal faults ac-
comodate space problems on fold limbs. A similar situation
might exist in the southern part of cube C, where a narrow
trench was formed in front of normal faults (Fig. 6) at the very
margin of the 3D cube: the thrust behind can only be assumed.
All observed structures: thrusts, related folds and faults termi-
nate against a NNW-SSE oriented, ENE dipping half-graben in
the south-western part of cube C (Fig. 7), coincident with the
Pannonian-age structure above it that bounds the Bugyi-Sári
high to the SE. A closer look shows that the master fault is seg-
mented into two parts with slightly different dip directions. The
north-western segment is cut by the Tóalmás zone in the north.
The architecture of the relay structure between the two seg-
ments is below mapping resolution. On the western side of this
fault, no mappable Early to Middle Miocene-age features were
found in the marginal zone of the 3D cube.
Paleogene deformation is shown on a top Eocene TWT
time map (Fig. 8A) and a seismic thickness map for the Oli-
gocene (Fig. 8B). To investigate Eocene tectonics, the map-
ping of the pre-Tertiary (typically Triassic carbonate)
basement would be necessary. A preliminary study, however,
showed no major difference in the topography of top Eocene
and the top of the Mesozoic basement – the interpretation
of the latter one is thus omitted here.
Again, the structural style of both sides of the Tóalmás zone
greatly differs, indicating significant Neogene movements and
inhibiting direct correlation of the two compartments.
In the MHZ a large number of flat, mainly NW vergent
thrusts are found that clearly offset top Eocene, showing that
they were active in the Oligocene. Towards the SW, thrust off-
set greatly increases on individual faults (compare Figs. 2, 4
and 5 with Fig. 6), also increasing uplift in these regions. In
the NE two, in the SW three major thrust sheets were mapped,
with a number of smaller transfer faults. Filling synsedimenta-
ry synclines, Oligocene formations reach their greatest thick-
ness in the foreland of the south-westernmost thrusts.
Oligocene sequences are very thin, or (in the Jászberény-
south area) even missing on the thrusted Sári—Pánd—
Jászberény high, suggesting its Paleogene activity.
A step within the uplifted ridge of Oligocene formations is
seen just north of the Pánd area. However, this is much
sharper than the similar structure in the Miocene above it.
This implies an increase in Oligocene thickness in the zone
between the steps in base Miocene and top Eocene (Figs. 5
and 8B), perhaps caused by fault transfer geometries.
N and NW dipping normal faults also occur within the
main thrust-related basin in cube A. These faults were hard
to map, and seem to be cut by thrusts, thus pre-dating the
compressional phase. An alternative solution would be that,
because they are basically perpendicular to thrusts, they are
related to them.
In the central and eastern parts, namely in cubes A & B,
two larger antithetic (SW vergent) thrusts were mapped at,
and east of Tóalmás. These might have initiated as a single
thrust that was offset later, or, alternatively, a relay structure
might connect them. The Tóalmás and Nagykáta Highs were
interpreted as pop-ups.
Folding intensity generally increases SW-wards. Whereas
in the NE only gentle synclines are characteristic (Figs. 4, 5),
PALEOGENE—MIOCENE FOLD AND THRUST BELT OF THE MID-HUNGARIAN SHEAR ZONE
Fig. 7. Base Miocene TWT
time map (A) and TWT
time thickness map (B).
Note pattern differences to
conclude for pre-Pannonian
Miocene (B) or Pannonian
(A) age deformation. Struc-
tures present on both maps
were active throughout both
TWT time map (A) and
TWT time thickness
map (B). Note pattern
differences to conclude
for Oligocene (B) or
Early to Middle Mio-
cene (A) age deforma-
tion. Structures present
on both maps were active
throughout both stages.
Note that no correlation
across the Tóalmás zone
PALOTAI and CSONTOS
fold amplitude and tightness is much higher in the SW, cul-
minating in the north-western foreland of Sári (Fig. 6).
NW of the Balaton zone NW and NE dipping normal
faults were assumed, but the vicinity of 3D cube margins in-
hibited a detailed observation of these structures.
Structural evolution: evidence and novelties for kinematics
The Oligocene of the MHZ is characterized by mainly
NW vergent thrusts and related folds, generated in a stress
field with NW-SE compression. The amount of shortening
increases westwards, with a major intensity change in the
Pánd area. The thrust system starts to build up in cube A, a
feature first described here. The main strike of structures is
NE-SW in the west, and turns gradually to N-S eastwards.
This change in apparent strike might be due to many factors,
but we suggest that it is mainly on the account of different
amounts of thrusting. Since the folds and thrusts seem to
fade away towards the east, it is proposed here that the origi-
nal shortening direction was WNW—ESE (still preserved in
the orientation of the easternmost structures), but due to rota-
tions induced by increasing shortening towards the west it
changed apparently to NW-SE there (Fig. 9).
Although the Paleogene shortening directions in the re-
gion suggested by Csontos & Nagymarosy (1998) are similar
to our ideas, the interpretation of recently shot high quality
3D seismic surveys showed thrust vergencies opposite to the
south-eastern ones proposed by those authors.
The large-scale model of Fodor et al. (1999) supposes
NW-SE to WNW-ESE oriented compression, NW to WNW
vergent thrusting and related folding in the MHZ of the cur-
rent study area between the Middle Eocene and the late Early
Oligocene. This is roughly in accordance with our results.
In the Buda Mts, Fodor et al. (1994) demonstrated WNW-
ESE compression in the Late Paleogene and Early Miocene,
with ESE vergent blind thrusting and E-W oriented dextral
transpressional strike-slip zones at the range margins. The
main shortening directions are identical to our supposed
original (i.e. eastern) ones, therefore it is probable that no
large scale structures separated the MHZ from the Buda Mts
in these times. In other words the Tóalmás zone, as a strike-
slip belt, was not yet active, an idea also supported by short-
ening directions in the western parts of the study area being
perpendicular to the zone (Fig. 9). However, the different
orientations of structures in the west suggest compartmental-
ization along the strike of the belt.
The zone north of the MHZ shows signs of NW-SE as well
as NE/ENE-SW/WSW oriented tension. The assumed NW-SE
oriented extensional regime does not correlate with known
stress fields – but this is not fully constrained because it is
at the margin of the study area.
The pre-Pannonian Miocene in the MHZ generally fol-
lows Paleogene patterns, with some important differences.
Compression is apparently E-W/NNW-SSE oriented in the
east, but NW-SE in the western parts; this could be ex-
plained by the rotation model, similarily to the Paleogene
Fig. 9. Work hypothesis for the Late Paleogene of the region showing possible connec-
tions of tectonic units. Dashed boxes show source of maps: Buda Mts – Fodor et al.
1994; Darnó Zone – Fodor et al. 2005b; MHZ – this study. For details see text.
situation (see above). The model for the
MHZ Early Miocene (Eggenburgian—Ottnan-
gian) of Fodor et al. (1999) nicely fits these
Syntectonic depocentres in thrust fore-
deeps, shown in detail for the first time in
our work (Figs. 7—8B), indicate increased
shortening in the central-western zones; the
cube C region underwent the largest shorten-
ing. The lack of thick foreland deposits can
be attributed to the (present day) vicinity of
the Tóalmás zone. Normal faults on syncline
limbs have a geometrical/gravity reason and
possibly do not relate to any regional stress
ENE-WSW oriented tension of unclear
Miocene age can be derived from the half-
graben bounding the Bugyi-Sári high. Mio-
cene structures terminate against this
structure. Based on 2D data, Csontos &
Nagymarosy (1998) mapped N-S striking
post-Pannonian normal faults around the
Bugyi High, but their position shows slight
differences from the structure seen in the
high resolution 3D seismics, and also has op-
posite dips (W in the previous, E in the cur-
rent work). The bent character (in the
Pannonian), and assumed relay structure be-
tween mapped segments (pre-Pannonian
PALEOGENE—MIOCENE FOLD AND THRUST BELT OF THE MID-HUNGARIAN SHEAR ZONE
Miocene) are also considered as novelties. The evaluation of
the Bugyi-Sári high will be the subject of a separate paper.
Tensional directions at the eastern margin of the
Jászberény ridge comply with late Middle Miocene (Sarma-
tian) data of Fodor et al. (1999) in the Jászság Basin. The ob-
tained NE-SW oriented tension north of the MHZ complies
with stress field directions in the Middle Miocene (Karpa-
tian—Badenian) of the Zagyva half-graben in the northern
continuation of the study area (Benkovics 1991). Bearing in
mind that this zone is at the margin of our study area, with
only small fault segments to map north of the Tóalmás zone,
the existence of the late Middle Miocene (Sarmatian) E-W to
NW-SE tension of the Zagyva half-graben (Benkovics 1991;
Fodor et al. 1999) was not proven in the study area.
In the Pannonian, bulk shortening continued, but individ-
ual thrusts were hard to recognize: gentle folds suggest the
uplift or subsidence of broader areas. The – in detail previ-
ously unpublished – syncline in cube A (Fig. 3) has a thick
Lower Pannonian syntectonic infill proving its activity.
Anticlines in the western area also show Early Pannonian
NW-SE compression, slightly different from the idea of
Fodor et al. (1999), who suggested NNW-SSE oriented com-
pressional directions for this period.
The sinistral character of the Tóalmás zone in the Early Pan-
nonian (compatible with the ideas of Csontos & Nagymarosy
(1998), Fodor et al. (1999) and Ruszkiczay-Rüdiger et al.
(2007)) is constrained by the restraining step-over at Tóalmás
(Fig. 2), first imaged in great detail here. Steep NNE-SSW
striking faults as synthetic Riedel shears at M, Pánd and in be-
tween, as well as east of Tóalmás, also suggest widespread
left-lateral movements. All these features corroborate the –
previously not supposed – idea that sinistral displacement
was not restricted to the Tóalmás zone, but distributed in a
wider deformation belt, presumably including the whole
MHZ. This event clearly post-dates folding mentioned above,
but is still constrained to the Early Pannonian.
Although still a problem to be solved, the splayed character
of the Tóalmás zone near Tóalmás (Fig. 2), proves that the
eastern termination of the zone has to be close to the present
study area. So far, no clear correlation with structures further
to the E/NE, such as the Darnó Zone (Fodor et al. 2005b), has
been found, but similarities in the Paleogene tectonic style and
shortening directions of the Darnó Zone and the eastern parts
of the mapped area suggest that these regions could have been
connected to each other (Fig. 9), and were separated only later,
most likely during the Miocene.
NE-SW oriented normal faulting north of the MHZ is
compatible with structures in the Zagyva Graben (Benkovics
1991; Fodor et al. 1999), indicating the existence of a single
northern unit. The same applies to the Buda Mts, where E-W
to NW-SE oriented tension prevailed from the Middle Mio-
cene until the Pliocene (Fodor et al. 1994), indicating a sin-
gle unit with only minor internal rotations.
The last documented phase affects even Upper Pannonian
deposits. Thrusts as well as segments of the Tóalmás zone
were reactivated during this event. This is corroborated by
doming of the Upper Pannonian at Tóalmás, as already ob-
served by Fodor et al. (2005a) and Ruszkiczay-Rüdiger et al.
(2007). As no detailed sedimentological mapping for the
Pannonian has been undertaken, the absolute age of this
phase remaines to be solved.
Almost no evidence is seen regarding the pre-Pannonian ac-
tivity of the Balaton—Tóalmás zone. Stress field correlations
(see above) suggest that the MHZ and ALCAPA were not sep-
arated by large-scale structures, so no great offset is expected.
The Budaörs right lateral shear zone (Fodor et al. 1994) has a
similar trend to the Tóalmás zone, suggesting that the latter
one could also have acted as a dextral strike-slip fault zone
during the Paleogene, or, more likely, in the Early Miocene
(Fig. 9). Its Paleogene activity is debated, as – based upon
thrust dips – Paleogene shortening directions in the western
parts of the study area are basically perpendicular to the
Tóalmás zone. We thus assume that the activity of the
Tóalmás zone started in the Early Miocene. Along this zone
only a (most likely late Middle Miocene) sinistral transpres-
sional event could be documented. In the Early Pannonian, a
possibly large, but still undetermined amount of sinistral
movement was determined, as discussed already by Csontos
& Nagymarosy (1988) and Ruszkiczay-Rüdiger et al. (2007).
Associated structures suggest that after the cessation of NW-SE
compressive folding, probably the whole MHZ acted as a sin-
istral strike-slip zone in the Early Pannonian, with maximal
displacement along the Tóalmás zone.
As an inter-plate deformation zone during the Paleogene
and Early Miocene, the MHZ shows signs of significant
shortening. Balla (1984) and Csontos et al. (1992) suggested
that the ALCAPA and Tisza blocks underwent opposite rota-
tions. The idea was based on paleomagnetic data (Márton
1985; Márton & Márton 1989; Márton & Fodor 2003). Bear-
ing the sheared character of the MHZ in mind, the above
mentioned changes in Paleogene and Early Miocene tectonic
directions along the study area strike can easily be regarded
as internally rotating blocks within the MHZ.
Because no mountain chain is found in the zone, and even the
crust is strongly thinned (Kilényi & Šefara 1989), Csontos &
Nagymarosy (1998) proposed significant orogen-parallel, that
is NE-SW oriented stretching. Detailed mapping in this study,
however, failed to detect such large scale extensional structures
in the Paleogene and Early Miocene: only small normal faults,
mainly perpendicular to thrusts, were found, suggesting that
the loci of Late Paleogene—Early Miocene stretching were
elsewhere (see also Tomljenovic & Csontos 1999; Csontos et
al. 2005). In any case, this issue needs further investigations.
As the MHL runs south of the study area proper, the geo-
metry and deformation along the MHL itself remains to be
solved. The general NW vergent character of Paleogene and
Miocene thrusting, however, suggests that – at least in this
part of the orogen – the MHZ, intermittent between the Tisza
and ALCAPA, likely overrode ALCAPA, in contrast to the
idea of Csontos & Nagymarosy (1998) and Schmid et al.
(2008). As an alternative, the described thrusts in the present
work may form a larger backthrust zone to the main, SE
verging thrust of the MHL.
The activity of the Tóalmás zone is likely to have started
only after the Paleogene, but its correlation with the Balaton
PALOTAI and CSONTOS
and Periadriatic lines is a question to be further analysed. It
is possible that the belt currently defined as the Tóalmás
strike-slip zone is a reactivation of a Paleogene tectonic
zone, an idea also supported by the fact that basement rocks
essentially differ on both sides of the zone, while facies di-
versity diminishes in the younger formations.
On the basis of detailed maps of horizons for top Eocene,
base Miocene and base Pannonian, as well as on fault geo-
metries from 3D seismics, the structural evolution of the
studied segment of the MHZ can be described as follows.
1. The MHZ acted as a generally NW vergent fold and
thrust belt at least in the Late Oligocene, beginning probably
already in the late Early Oligocene.
2. The intensity of shortening generally increased west-
wards, with a major step at Pánd. It is likely that the gradual
change in thrusting directions to the west was caused by dif-
ferential rotation within the shear zone between the ALCAPA
and Tisza blocks, with the original shortening directions pre-
served in the relatively mildly deformed eastern parts.
3. Transport directions in the Early Miocene were similar
to those of the Oligocene, but instead of hard-linked faults,
blind thrusts and related folds prevailed.
4. The Tóalmás zone (possibly as a reactivation of a Paleo-
gene thrust belt) initiated most probably in the Early Mio-
cene as a dextral strike-slip zone.
5. Top NE thrusting and gentle folding in the MHZ partly
continued in the earliest Pannonian, and was followed by
sinistral movements in the whole zone (with maximal dis-
placement along the Tóalmás zone). The latest observed tec-
tonic event was the Pliocene-Quaternary inversion of the
Acknowledgments: The authors appreciate the support of
MOL PLC., especially that of A. Király and I. Czeller. This
study was supported by the Hungarian Scientific Research
Fund (OTKA) 81530. Discussions with M. Kajári and A.
Milánkovich during seismic mapping were indispensable.
The 3D model was built using the Move Software of Mid-
land Valley Ltd. The thorough reviews of S. Schmid and
L. Fodor are also appreciated.
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