The Role of Rheology and Fault Geometry on Fault Reactivation: A Case-Study from the Zsámbék-Mány Basin, Central Hungary

Abstract

In this study, we investigated the structural evolution of the Vértessomló (VT) Thrust and the Környe-Zsámbék (KZ) Fault, which are located in the Transdanubian Range in the center of the Miocene Pannonian back-arc basin. Our study is based on surface and well data. The Transdanubian Range was located on the Adriatic passive margin during the Late Triassic, where a thick succession of platform carbonates was deposited. Intercalations of intraplatform basin deposits occur in the eastern part of the study area. South-directed thrusting and the formation of the VT Thrust took place during the Cretaceous, related to the Austroalpine orogeny. Asymmetric half-grabens were formed during the Eocene in the hanging wall of the segmented dextral normal KZ Fault. The geometry and kinematics of the KZ Fault were influenced by the pre-existing VT Thrust located in the Mesozoic basement of the Paleogene sub-basins. These Eocene half-grabens suffered mild inversion due to the dextral reverse reactivation of the VT Thrust and the KZ Fault during the Oligocene–Early Miocene. The geometry of Miocene normal faults indicates that the VT-KZ Fault system was an active transfer fault during the Miocene extension of the Pannonian Basin, as well. We found a positive correlation between the rheology of the Triassic basement and the mode of Paleogene fault reactivation. Our results show that reactivation of the pre-existing thrust took place along that segment, where the Triassic basement is made up of homogeneous platform carbonates. In contrast, a diffuse fault zone developed, where the Triassic basement is represented by the weak layers of intraplatform basins.

1. Introduction

Fault reactivation is a frequent and extensively studied phenomenon, both along plate boundaries and in plate interiors, where different types of fault reactivation take place during the distinct stages of an orogenic cycle. Continental rift zones are frequently located in former orogens [1], where extensional reactivation or negative inversion of former thrusts occur [2,3]. Even the onset of intra-oceanic subduction might be localized by pre-existing detachment faults [4]. Subduction of former rifted margins resulted in the positive inversion of former extensional grabens and half-grabens and reverse reactivation of pre-existing normal faults. Extensive literature is available for positive inversion, which has been described in several orogens worldwide [5,6,7,8,9,10,11,12,13]. Lateral extrusion and/or extensional collapse [14,15] take place frequently at the final stage of collision. In some cases, the extrusion-related strike-slip faults reactivate pre-existing thrusts [16,17,18,19], particularly those that are steeply tilted at the backstop of the orogenic wedge. In this study, we describe the complex deformation history of the Vértessomló Thrust (VT) and Környe-Zsámbék (KZ) fault system running across the Zsámbék–Mány Basin, which is a young sub-basin of the Pannonian back-arc basin (Figure 1). We carried out a study of well and surface data, which permit the reconstruction of repeated reactivations of the VT-KZ Fault system during the different stages of the Alpine orogenic cycle, from the Triassic passive margin evolution through the Cretaceous nappe emplacement and Paleogene extrusion tectonics. We demonstrate how the original Triassic facies determined the original thrust system and its reactivation through the deformation history.

2. Geology of the Study Area

The Zsámbék–Mány Basin is situated in the northeastern part of the Transdanubian Range (TR), in the center of the Pannonian back-arc basin (Figure 1 and Figure 2). The Transdanubian Range is the uppermost nappe of the Austroalpine nappe-system [22], which represents a part of the Adriatic plate [23] (Figure 1). The post-Variscan evolution of the Adriatic plate was controlled by the opening and the closure of two distinct oceanic domains, the Neotethys and the Alpine Tethys [23]. The TR is built up by Variscan low-grade metasedimentary rocks and a Permian to Cenozoic non-metamorphic sedimentary succession.

The Permian to early Middle Triassic sediments of the TR deposited in rift basins, which preceded the opening of the Neotethys Ocean. Continental sandstone, evaporate, and dolomite was deposited during the Late Permian in the NE part of the TR [29]. These deposits are covered by Early to early Middle Triassic mixed siliciclastic-carbonate succession, which formed on a shallow marine ramp [30].

The Anisian onset of the oceanic spreading of the Neotethys led to the partial drowning of the shallow marine carbonate ramp (Tagyon Formation, Figure 3): cherty limestones (Felsőörs F.) together with tuffs (Buchenstein Group) deposited in the tectonically controlled grabens and half-grabens [31,32], whereas the production of shallow marine carbonate continued on the elevated platform areas (Tagyon Fm., Budaörs Fm.) (Figure 3). These Middle Triassic basinal deposits were drilled by several wells in the Zsámbék–Mány Basin (Mány M-191, M-245, M-246; for location, see Figure 4) [33]. Similar tuffs, clays and coarse-grained volcanoclastic sediments (Inota Fm.) are exposed in the Strázsa Hill quarry at Zsámbék [34] (for location, see Figure 4). U-Pb geochronological data measured on zircons of the Inota Fm indicate that the volcanism was active until the Early Carnian in the northeastern part of the TR [35]. The late Anisian–Ladinian Budaörs Fm is made up of thick-bedded dolomite with cyclic stromatolite intercalations. The most important index fossil of this formation is the Diplopora annulata, which was found in the Strázsa Hill quarry [34] (for location, see Figure 4). The production of shallow marine dolomite continued during the Carnian (Gémhegy Formation) [36]. This formation crops out in the Góré Hill (near Gyarmatpuszta), where Carnian Megalodontids [37] and dasycladalea remains [34] were described (for location, see Figure 4). The Late Carnian basinal succession is represented by cherty dolomites and limestones of the Csákberény Fm. [36], which was documented in detail by Haas et al. [38] based on the Zsámbék Zs-14 well (for location, see Figure 4).

From the end of the Carnian, a broad carbonate platform developed, where the Hauptdolomit deposited during the Norian and the Dachstein Limestone in the Raethian (Figure 3). Both formations are characterized by a cyclic succession of thick-bedded shallow marine carbonates and stromatolites [30]. The boundary of the two formations is a transitional unit in which intertidal dolomites and subtidal limestones alternate (Fenyőfő Mb.) (Figure 3). The upper sections of the Mesozoic succession were eroded in the study area. In the following, we briefly summarize the Jurassic–Early Cretaceous succession of the northeastern TR, which provides important insights into the structural evolution of the study area.

The drowning of the Dachstein platform occurred at the Triassic-Jurassic boundary. The Jurassic succession is built up by condensed pelagic sediments, like ammonitico rosso type limestones, marlstones and radiolarite [39].

Intra-oceanic subduction was initiated in the Neotethys during the Middle Jurassic [23]. The Adriatic continental lithosphere entered into this subduction zone after the consumption of the lower oceanic lithosphere of the Neotethys. Shortening-related deformation progressively migrated on the Adriatic margin from Late Jurassic to the Early Cretaceous, which led to the formation of the Austroalpine nappe system. A foreland basin developed in the NE part of the TR (north of the study area) during the early stage of orogeny [22,40]. Early Cretaceous deep marine marl and upward coarsening turbiditic succession were deposited in this foreland basin, whereas Late Aptian shallow marine bioclastic limestone (Tata Fm) was deposited on the forebulge [41]. The TR suffered significant contractional deformation before, during or after the deposition of the Tata Limestone [41,42], which is manifested in variously oriented folds and thrusts in the NE part of the TR [43,44]. One of the most significant Cretaceous structures is the south-vergent Vértessomló (VT) Thrust [45,46,47,48,49], which crosses the study area in the E-W direction [34,50] (Figure 2 and Figure 4). Due to these folding phases, the Jurassic to Early Cretaceous sequences were eroded in the study area in the Late Cretaceous.

The subduction of the Alpine Tethys was going on during the Late Cretaceous and the Paleogene, where the Adriatic plate (including the TR) represented the upper plate [23]. Prior to and during the collision of Adria and Europe a retro-wedge foreland basin, the Hungarian Paleogene Basin, developed on the upper plate [51]. The deposits of the Hungarian Paleogene Basin unconformably cover the folded Mesozoic beds. The Gánt Bauxite is overlain by a clastic coal-bearing succession of the Dorog Fm., which was deposited in fluvial and lacustrine, later in a marine swamp environment [52] (Figure 5). Due to the late Lutetian transgression, a mixed siliciclastic-carbonate succession of the Csernye Fm. deposited in a lagoon and shallow sea, laterally interfingering with the shallow marine carbonate ramp facies of the Szőc Limestone [52] (Figure 5). It contains fossils indicating normal salinity: primarily large foraminiferas, locally red algae in rock-forming quantity, as well as molluscs, echinoids, decapods, corals and vertebrate remains. The Eocene carbonate sequence interfingers with basinal marls toward the pelagic basin. Deeper marine (outer ramp) homogenous, well-bedded, grey clay marl and marl succession of the Csolnok Fm. represents partly heterotopic facies of the Szőc Fm. Fine to coarse-grained siliciclastic and marly succession (deltaic system deposit) of the Tokod Formation represent the youngest member of the Eocene succession in the study area [52].

The rest of the Eocene succession was eroded due to the Early Oligocene uplift and subaerial erosion. The Oligocene succession begins with bauxite (Óbarok Fm.), which is overlain by the terrestrial (fluvial) succession of the Csatka Fm. which is laterally interfingering with the shallow marine siliciclastic succession of the Törökbálint Fm eastward [52] (Figure 5). The deposits of a wide transitional zone between the fluvial and marine environments are represented by the Mány Mb., which is a characteristic sediment of the study area [52].

The subducted lithosphere of the Alpine Tethys tore off below the Eastern Alps after the Adria-Europe collision, and the slab pull forces retreated towards the East, under the Carpathians [53]. The combination of these processes resulted in the crustal-scale eastward lateral extrusion of a part of the Austroalpine nappes during the Late Oligocene–Early Miocene and the formation of the Pannonian back-arc basin during the Miocene [54,55,56].

The Early Miocene uplift and subaerial exposure of the study area is related to the extrusion tectonics, whereas the onset of late Early Miocene sedimentation migrated in time from NW to SE and took place in syn-sedimentary grabens and half-grabens, associated with the opening of the Pannonian back-arc basin [22,57]. Deep water marls (Baden and Kozárd Fm.) deposited in the grabens and half-grabens during the Miocene, whereas shallow-water limestone of the Tinnye Fm. deposited on the tectonically controlled elevated areas [58,59,60]. The Upper Miocene is represented by the clays and silts of the Csákvár Fm., which was deposited in swamps [52].

3. Methods and Data

The study area was the focus of intense exploration for Paleogene bauxites and coal deposits. Due to the raw material research, data for more than 1700 wells are available (for the locations of the wells, see Figure 4). Geological maps [24,25,26,27,28] were also used for the structural analysis of the study area.

We constructed the pre-Quaternary (Figure 4) geological map of the study area after filtering the topmost pre-Quaternary picks in every well. We took into account the exposed outcrops of pre-Quaternary formations on the mentioned maps during map construction.

We constructed the grids of major unconformities, namely the base of the Eocene, the base of the Oligocene and the base of the Miocene, by using Delaunay triangulation. The thickness map of the Eocene (for location see Figure 4) is based on the vertical difference between the base of the Eocene and the base of the Oligocene grids. Well data show that the Eocene succession is almost exclusively covered by Oligocene in the study area. Therefore only the Early Oligocene erosion decreased the thickness of the Eocene succession, and the pre-Middle Miocene erosion did not affect the Eocene thickness values. Consequently, recent Eocene thickness data reflect the sum of the original thickness of the Eocene and the amount of Early Oligocene erosion. These assumptions are not valid for a small area north of Nagyegyháza village (western boundary of the study area, Figure 4), where Eocene sediments are situated near/on the surface, and they suffered Quaternary erosion, as well.

We constructed geological sections (for location of the sections see Figure 4) based on well and surface data. We took a 100 m maximum projection distance for the wells, except the section of Figure 6A, where the maximum projection distance was 730 m.

4. Structure of the Zsámbék–Mány Basin

4.1. Structure of the Mesozoic Basement: Geometry of the Vértessomló (VT) Thrust

Triassic beds generally dip steeply northward (Figure 6A). In an early publication, Balla and Dudko [46] recognized several E-W trending belts of basin sediments within the platform carbonates that they interpreted as repetition due to strike-slip faulting. Although Budai et al. [34] supposed the south-directed imbrication of Triassic beds in the Zsámbék–Mány Basin, the exact geometry of the related structure has not been reconstructed yet. One of the most important thrusts, which is responsible for the northward tilting of the Triassic beds, is the Vértessomló (VT) Thrust, which is exposed in the Strázsa Hill quarry near Zsámbék town (Figure 4 and Figure 6) [34]. The Late Anisian to Ladinian Budaörs Dolomite and the Ladinian-early Carnian volcaniclastics of the Inota Fm. are thrust above the Norian Hauptdolomit in this quarry (Figure 6B,C). Based on our observations, the VT Thrust dips steeply (60°) towards the north in the quarry (Figure 6C). The Hauptdolomit dips moderately (25–30°) to the north in the footwall of the VT Thrust, so the cut-off angle of the thrust and the beds of the Hauptdolomit is approximately 30° (Figure 6C). The beds of the Inota Fm. are parallel to the VT Thrust (beds dip 60° toward the north) in the hanging wall of the thrust. The geometry of these beds indicates that the footwall ramp and the hanging wall flat of the VT Thrust are exposed in the quarry. Further north, the beds of the Inota Fm. turn to sub-horizontal, and the northernmost part of the quarry is made up of the sub-vertical beds of the Budaörs Dolomite (Figure 6C). The contact of the basinal beds and the Budaörs Dolomite is represented by a minor breakthrough thrust, which cuts across the hanging wall syncline of the VT Thrust (Figure 6C). The footwall of the VT Thrust is discordantly covered by terrestrial clay and clastics of the Upper Oligocene [61], whereas the hanging wall of Budaörs Dolomite is covered directly by a Middle Miocene shallow marine succession [62] (Figure 6C). The Oligocene beds turn to sub-vertical near the VT thrust (Figure 6C).

The subsurface continuation of the VT Thrust and the associated E-W striking belts of the tilted basement formations can be traced west of the Strázsa Hill quarry based on well data (Figure 7). We describe and interpret these belts from south to north. Most of the boreholes penetrated shallow marine dolomites below Paleogene in the southern part of the study area, except the B-2 well, which penetrated the Fenyőfő Member. Based on our interpretation, the shallow marine dolomites north of the B-2 well are overthrust above the Fenyőfő Mb. due to another south-vergent thrust; the Csabdi (Cs) thrust. Probably this thrust is responsible for the northward tilting of the footwall of, and the VT Thrust itself, in the Strázsa Hill quarry (Figure 6).

Further northwest–north of Csabdi village–another E-W striking belt of Fenyőfő Mb. is outlined based on four wells (e.g., M-20, M-71, Figure 7). This occurrence of the Fenyőfő Mb. represents the highest stratigraphic unit of the footwall of the VT Thrust. North of this belt, shallow marine dolomites were drilled below Paleogene sediments that represent the hanging wall of the VT Thrust. Four wells between Mány and Zsámbék villages (e.g., M-165, M-317) penetrated a basinal succession of cherty limestones, tuffitic clays and marls in an E-W striking belt below the Paleogene sediments (Figure 7). We interpret these rocks as occurrences of the late Anisian–Early Carnian basin deposits in the hanging wall of the VT Thrust. South of this belt, Triassic shallow marine dolomites are drilled below Cenozoic strata, which we interpret as Hauptdolomit in the footwall of the VT Thrust (Figure 7), based on the analogy of the Strázsa Hill quarry (Figure 6).

North of the belt of the late Anisian–Early Carnian basin deposits, shallow marine dolomites occur below the Paleogene deposits. Further north, there is another E-W striking belt where cherty dolomites and limestones occur below Paleogene (e.g., M-219, M-174, Zs-14 in Figure 7). This belt corresponds to the Csákberény Fm., which represents the upper interval of Carnian intraplatform basin deposits [36,38]. Shallow marine dolomites, which were drilled north of the belt of the Csákberény Fm., are interpreted as the overlying Hauptdolomit. Further north, the Fenyőfő Mb. underlies Paleogene based on the well Szomor K-3 (Figure 7).

This distribution of well data outlines a northward tilted continuous succession from Middle Triassic up to the Fenyőfő Mb., in the hanging wall of the VT Thrust, which is discordantly covered by Paleogene deposits (Figure 6A and Figure 7). Both the late Anisian to lower Carnian basin deposits and the Carnian Csákberény Fm. pinch out westward between Mány and Vasztély villages, that we explain as the lateral interfingering of the Triassic platform carbonates and the Carnian basin deposits (Figure 7). North of the Szomor K-3 well, drillholes penetrated shallow marine dolomite again below Cenozoic formations. We suppose that the contact of the Fenyőfő Mb. and the shallow marine dolomite is due to another south-directed thrust that we refer to as Vasztély (Va) Thrust (Figure 6A and Figure 7).

4.2. Basin-Bounding Faults of the Paleogene Deposits: The Környe-Zsámbék (KZ) Fault

The Környe-Zsámbék (KZ) Fault is clearly outlined on the Eocene thickness map (Figure 8): this fault represents the southern sharp contact between areas characterized by maximal Eocene thickness values and non-deposition areas [46]. The KZ Fault has three major segments in the study area. The WSW-ESE striking western segment of the KZ Fault coincides with the VT Thrust; therefore, we consider this segment as the reactivation of the VT Thrust (Figure 8).

The E-W striking middle segment of the KZ Fault runs parallel and approximately 1 km north of the VT Thrust (Figure 8 and Figure 9). Well data suggest the presence of a third segment, which represents the eastern segment of the KZ Fault (Figure 6A and Figure 8). This eastern segment runs 1 km north of the middle segment. Both the eastern and the middle segments of the KZ Fault follow stratigraphic contacts within the Triassic basement in map view. Namely, the middle and the eastern segments of the KZ Fault run along the lower and the upper stratigraphic contact of the Carnian Csákberény Fm., respectively (Figure 6 and Figure 7). The western tip-point of the middle segment of the KZ Fault is located exactly in the same area where the Csákberény Fm. pinches out and is laterally replaced by dolomite (Figure 8). West of this point, the Triassic basement is built up by a continuous succession of shallow marine dolomites in the hanging wall of the western segment of the KZ Fault.

The NNW-SSE striking cross-sections show a gradual thickening of Eocene succession toward the KZ Fault (Figure 9 and Figure 10). Overall, the Dorog Fm shows the most significant thickness variation, which indicates intense syn-sedimentary tectonics. The tectonic activity slowed down during the formation of coal-bearing layers (upper section of the Dorog Fm), and the rest of the Eocene succession onlaps onto the denudated Triassic towards the N (Figure 9 and Figure 10). The maximal thickness of Eocene succession is 200–350 m in the down-faulted block of the western segment. In most cases, the Eocene succession is missing, and the Triassic rocks are overlain directly by Oligocene deposits on the southern footwall block of the KZ Fault. This was already recognized by research on raw materials and some earlier maps [63]. An exception is the section of Figure 9a, where M-246 well- drilled thin remnants of the Eocene in the footwall of the KZ Fault. In this well, the coal-bearing layers (upper member of the Dorog Fm) deposited directly on the Triassic basement.

Interestingly, the thickness variation of the Oligocene succession shows an opposite trend with respect to the Eocene deposits. The Oligocene succession is gradually thinning from the north towards the KZ Fault on the NNW-SSE striking sections (Figure 9 and Figure 10); this feature is the most remarkable in the section of Figure 10A. In this section, the base-Eocene horizon is sub-horizontal, whereas the base-Oligocene horizon is north-dipping.

Based on the sections of Figure 9, the Oligocene is missing from the proximate hanging wall of the eastern part of the VT Thrust (south of the middle segment of the KZ Fault), and the Triassic is directly covered by Miocene. In contrast, the Triassic is overlain by thin Oligocene succession on the footwall of the VT Thrust (e.g., Zst-2 in Figure 9a, M-195 in Figure 9B).

4.3. Miocene Normal Faults

Our pre-Quaternary geological map (Figure 4) shows that the distribution of Triassic and Cenozoic sediments is strongly controlled by N-S to NNW-SSE striking normal faults (KH Fault, GH Fault, LF Fault), which represent the boundary between east-dipping blocks. These faults were active during the Late Miocene, evidenced by down-faulted Upper Miocene deposits next to Triassic rocks (Figure 4). Our WSW-ENE striking section (Figure 11) shows that the early Middle Miocene sediments occur mostly in the hanging wall blocks of these faults, whereas on the elevated blocks, the Middle Miocene is very condensed [52], or the Oligocene Mány Mb. is directly overlain by late Middle Miocene carbonates (Tinnye Fm.). Moreover, these NNW-SSE striking faults strongly controlled the depositional environment of the late Middle Miocene (Sarmatian) sediments: the uplifted blocks are dominated by the shallow-water carbonates of the Tinnye Fm. whereas mostly deep-water marls of the Kozárd Fm. deposited on the down-faulted blocks (Figure 11). Based on that, these N-S to NNW-SSE striking normal faults were active during the Middle to Late Miocene.

5. Discussion and Conclusions

5.1. Correlation of the VT Thrust and the KZ Fault: Comparison with Previous Works

The recent distribution of Mesozoic and Cenozoic formations is primarily determined by Miocene extension-related grabens and half-grabens in the northeastern part of the Transdanubian Range. Our observations are in accordance with the findings of previous studies [58,59,60], which recognized the fault control on late Middle Miocene (Sarmatian) half-grabens. These grabens are bordered by NW-SE to N-S striking Miocene normal faults [64]. However, E-W striking faults, like the VT Thrust and the KZ Fault, also played a fundamental role in the deformation of Mesozoic and Paleogene deposits [46,50](Figure 2). The age and kinematics of these E-W striking faults are strongly controversial in previous works [46,48,49,65]. Fault segments with different ages and kinematics were correlated by previous studies [46], which is misleading in many cases.

The VT Fault was known for a long time in the Vértes Hills (as the “Somlyó-Szár fault” [45]), west of the study area (Figure 2). Maros [65] considered the VT Fault as a sinistral strike-slip fault based on structural data measured near the fault. Balla and Dudko [46] followed this concept: based on the apparent displacement of NNW-dipping Mesozoic formations; they considered the VT “line” as a sinistral strike-slip fault in the Vértes Hills. In addition, Balla and Dudko [46] correlated the VT Fault with the Nagykovácsi Fault in the Buda Hills (For the location, see Figure 2), which shows opposite, apparent dextral offset based on NE dipping dissected Triassic beds. According to Balla and Dudko [46]) the VT and Nagykovácsi Faults are connected by the Zsámbék Fault, which represents the southern limit of Eocene deposits in the study area (it is equivalent to the KZ Fault of this study). Balla and Dudko [46] explained the evolution of the Vértessomló-Nagykovácsi “line” by a complex model, where Oligocene dextral shear was followed by the bending and Miocene sinistral reactivation of the Vértessomló-Nagykovácsi Fault. Bada et al. [58] interpreted the Vértessomló-Nagykovácsi Fault as a Paleogene–Early Miocene dextral strike-slip fault based on detailed fault-slip analysis. Furthermore, they considered its activation during the Cretaceous with a dextral and/or reverse sense of shear.

In contrast to the model of Balla and Dudko [46], Császár [47] considered the VT Fault as a thrust based on the Vst-8 well (for location, see the western part of Figure 2), which penetrated the Aptian Tata Limestone juxtaposed to early Albian sediments. This interpretation was confirmed by the structural analysis of Fodor and Bíró [48]. Moreover, these authors found that the VT Thrust is covered by Eocene or Oligocene sediments in the Vértes Hills, and the displacement of Paleogene deposits does not occur along the VT Thrust but another fault that is running north at a low angle to the VT Thrust [21] (Figure 2). This fault was defined as the Környe-Zsámbék (KZ) Fault by Fodor [49]. The geometry of the VT Thrust and the KZ Fault is very similar in the Vértes Hills and in the eastern part of our study area, namely the eastern and middle segment of the KZ Fault–that controlled the Paleogene sedimentation–developed on the hanging wall of the pre-Eocene VT Thrust. In contrast, the western segment of the KZ Fault reactivates the VT Thrust in the study area (Figure 10).

The N-S striking sections (Figure 9 and Figure 10) show that the KZ Fault underwent a complex deformation history during the Paleogene. We explained the southward thickening of the Eocene beds by a syn-sedimentary normal slip of the KZ Fault (in section view) during the Eocene. This interpretation is in accordance with the section of Végné et al. [63], who illustrated the southern boundary fault of the Eocene deposits (equivalent to the KZ Fault) as a normal fault. Previous studies marked the KZ Fault as one single surface; in contrast, our study shows that the fault is laterally segmented. In addition to the E-W striking KZ Fault, a few NW-SE striking faults are outlined based on the thickness map of the Eocene (Figure 8). These faults were already mentioned by Véghné et al. [63], who considered these structures as the major basin-controlling faults of the Eocene sedimentation. Similar fault patterns with E-W and NW-SE striking syn-sedimentary faults were reconstructed in the Eocene Basin near Tatabánya (north of Vértes Hills) [66,67,68]. Bada et al. [58] reconstructed a strike-slip stress field for the Eocene, which can be characterized by WNW-ESE compression and perpendicular tension. Taking into consideration these fault-slip data [58], we suppose that the KZ Fault was acting as a dextral normal fault during the Eocene. In this stress field, the NW–SE striking faults could have normal kinematics.

According to Bada et al. [58], the Oligocene–Early Miocene stress field can be marked by NW-SE compression and NE-SW tension. In our opinion, the slight change of the maximal horizontal stress axis (Sh max) between Eocene and Oligocene resulted in the transition from transtension to transpression along the KZ Fault. We suggest that the gradual thinning of the Oligocene beds toward the KZ Fault is due to the dextral reverse reactivation of the KZ Fault, which occurred before the onset of Middle Miocene sedimentation. The geometry of the Eocene deposits outlines an asymmetric anticline in the section of Figure 10A, where the maximal thickness occurs in the core of this anticline. We interpret this anticline as an inversion anticline or harpoon structure (following the term of McClay [69]), which came into being during or after the deposition of the Oligocene but before the onset of Middle Miocene sedimentation. The lack of Oligocene on the hanging wall of the eastern part of the VT Thrust indicates its mild inverse reactivation during Oligocene–Early Miocene times. Taking into consideration the paleo stress data of Bada et al. [58], the VT Thrust acted as a dextral reverse fault during the Oligocene–Early Miocene. According to Véghné et al. [70], the Triassic is thrust over Paleogene in several wells west of the study area. They interpreted this juxtaposition as a south-directed post-Oligocene thrust (Csordakút Thrust), which probably belongs also to this deformation phase.

The Nagykovácsi Fault in the Buda Hills (for location, see Figure 2) is considered the eastern continuation of the VT Fault by several authors (“Vértessomló-Nagykovácsi line” [46]). Field observations indicate Late Eocene syn-sedimentary dextral transpressional movement on the Nagykovácsi Fault [25].

5.2. The Role of Fault Geometry and Rheology on Fault Reactivation

The 3D orientation of a fault plane relative to the principal stress axes has a fundamental role in fault reactivation [71,72,73]. Further controlling factors, such as the strain rate [12], the pore-fluid pressure in the fault zone [74], the frictional coefficient and the cohesion of the fault, also have a significant impact on fault reactivation [75,76]. Our data show that the high-angle dip of the VT Thrust (~60° in the Strázsa Hill quarry) contributed to both its transtensional and transpressional reactivation during the Eocene and Oligocene–Early Miocene, respectively.

Based on our results, the mode of fault reactivation is strongly controlled by the mechanical properties of the Triassic basement. Namely, the Eocene transtensional and Oligocene –Early Miocene transpressional deformation was localized along one single fault—the reactivated VT Thrust—in the western part of the study area, where the Triassic basement is represented by a homogenous succession of competent shallow marine dolomites (Figure 10 and Figure 12). In contrast, the Paleogene deformation took place in a wide zone of strike-slip faults (including the eastern and middle segment of the KZ Fault and the reactivated VT Thrust) in the eastern part of the study area, where the Triassic basement is more heterogeneous due to the intercalations of the incompetent Triassic intraplatform basin deposits (Figure 9 and Figure 12). Our observations are in accordance with the studies of Ferill et al. [77] and Libak et al. [78], which are based on field observations and seismic interpretation. According to these authors, the deformation of competent rocks resulted in a narrow, less segmented fault zone, which is manifested in one single laterally long fault. In contrast, the deformation of a layer-cake disposition of competent and incompetent rocks produces a wider, intensively segmented fault zone, where the individual fault segments are laterally short, and the segments have minor displacements. It is important to note that these observations are based on extensional normal faults which deform successions of horizontal beds [77,78]. Nevertheless, this relationship between fault geometry and rheology remains crucial in the study area, where the different mechanical properties of imbricated Triassic basement rocks (platform vs. intra-platform basin) resulted in the various geometries of Paleogene strike-slip faults (single fault vs. segmented fault-zone) even when crossing the already tilted successions and reactivating pre-existing faults.

The lateral segmentation of the KZ Fault–which formed during the Eocene–had a significant impact on the oblique (dextral) inversion of the KZ Fault during the Oligocene–Early Miocene (Figure 9A); the Eocene sediments were folded into a local inversion anticline during this time span. This inversion anticline is most remarkable above the relay ramp that connects the western and middle segments of the KZ Fault (Figure 10A). This stepover formed a restraining bend during the dextral transpressional reactivation of the KZ Fault, which supports the formation of this local inversion anticline. Similar strike-slip reactivation of segmented normal fault zones has been described by several authors, e.g., [79,80,81]. One of the most important features of inverted relay ramps that they form narrow (laterally short) zones of folds and thrusts [8,79], just like the inversion anticline of Figure 10a, which cannot be followed on the parallel sections (Figure 9B and Figure 10B).

The Middle to Late Miocene normal faults of the study area are almost perpendicular to the VT Thrust and the KZ Fault. However, this inherited fault system localized the relay ramps and lateral tip points of Miocene normal faults (Figure 4). Therefore, the VT Thrust and the KZ Fault influenced fault nucleation during the Miocene extensional deformation, too. Most probably, these inherited faults played as an active transfer fault during the Miocene.