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\DOI{10.5802/crgeos.262}
\datereceived{2024-02-19}
\daterevised{2024-04-09}
\dateaccepted{2024-04-30}
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\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Structural conditions for relay ramp fault development on the
edge of the collision wedge: a~case study from the East Slovak Basin}

\alttitle{Conditions structurales pour le d\'{e}veloppement d'une
faille \`{a} rampe de relais sur le bord du coin de collision : une
\'{e}tude de cas du bassin slovaque oriental}

\author{\firstname{Stanislav} \lastname{Jacko}\CDRorcid{0000-0002-7568-9071}}
\address{Institute of Geosciences, Faculty of Mining, Ecology, Process
Control and Geotechnologies, Technical University of Ko\v{s}ice, Park
Komensk\'{e}ho 15, Ko\v{s}ice 040 01, Slovakia}
%\email{stanislav.jacko@tuke.sk}

\author{\firstname{Igor} \lastname{\v{D}uri\v{s}ka}\CDRorcid{0000-0002-8974-0800}\IsCorresp}
\addressSameAs{1}{Institute of Geosciences, Faculty of Mining, Ecology,
Process Control and Geotechnologies, Technical University of
Ko\v{s}ice, Park Komensk\'{e}ho 15, Ko\v{s}ice 040 01, Slovakia}
\email[I. \v{D}uri\v{s}ka]{igor.duriska@tuke.sk}

\author{\firstname{Juraj} \lastname{Jano\v{c}ko}\CDRorcid{0000-0003-0105-453X}}
\addressSameAs{1}{Institute of Geosciences, Faculty of Mining, Ecology,
Process Control and Geotechnologies, Technical University of
Ko\v{s}ice, Park Komensk\'{e}ho 15, Ko\v{s}ice 040 01, Slovakia}
%\email{juraj.janocko@tuke.sk}

\author{\firstname{Roman} \lastname{Farka\v{s}ovsk\'y}\CDRorcid{0000-0002-7795-318X}}
\addressSameAs{1}{Institute of Geosciences, Faculty of Mining, Ecology,
Process Control and Geotechnologies, Technical University of
Ko\v{s}ice, Park Komensk\'{e}ho 15, Ko\v{s}ice 040 01, Slovakia}
%\email{roman.farkasovsky@tuke.sk}

\author{\firstname{Alexander~Dean} \lastname{Thiessen}\CDRorcid{0009-0003-2485-1918}}
\addressSameAs{1}{Institute of Geosciences, Faculty of Mining, Ecology,
Process Control and Geotechnologies, Technical University of
Ko\v{s}ice, Park Komensk\'{e}ho 15, Ko\v{s}ice 040 01, Slovakia}
%\email{alexander.dean.thiessen@tuke.sk}

\shortrunauthors

\keywords{\kwd{Relay ramp}\kwd{Collision wedge}\kwd{Transform
faults}\kwd{Pannonian basin}}

\altkeywords{\kwd{Rampe de relais}\kwd{Angle de collision}\kwd{Failles
transformantes}\kwd{Bassin pannonien}}

\thanks{Slovak Grant Agency VEGA (grants 1/0585/20)}

\begin{abstract}
The Neogene evolution of the Pannonian extensional back-arc basin was
associated with the development of transform faults. These faults
significantly modified the sedimentary and structural evolution of the
basin. The key developmental factors were related to pull forces and to
the tilting of the subducted slab. The transitions from upper crustal
contraction of upper crustal extension affected the depositional space,
which had opened subparallel NW--SE normal faults along the southern
contact with the Carpathian collision zone. The entire transition
process was controlled by the closing of the rear part of the
accretionary space, which changed the dip of the subducted slab. Basin
subsidence, which occurred from west to east via N--S steps, generated
a system of subparallel-oriented faults, which were connected by relay
ramps. The relay ramps had four developmental phases: (a)~initial
crustal bending, (b)~intact ramp formation with structural integrity
disturbance, (c)~breaching with crustal breakage, and (d)~final fault
growth. The genesis of the relay ramps of the Eastern Slovak Basin was
closely linked to the initial strike-slip movements. The strike-slip
movements caused a counterclockwise rotation of the northern edge of
the Alpine--Carpathian--Pannonian block. This rotation was later
compensated by the tectonic subsidence of the basin, primarily during
the Middle Miocene.
\end{abstract}

\begin{altabstract}
L'\'{e}volution n\'{e}og\`{e}ne du bassin d'extension de
l'arri\`{e}re-arc pannonien a \'{e}t\'{e} associ\'{e}e au
d\'{e}veloppement de failles transformantes. Ces failles ont
consid\'{e}rablement modifi\'{e} l'\'{e}volution s\'{e}dimentaire et
structurale du bassin. Les principaux facteurs de d\'{e}veloppement
\'{e}taient li\'{e}s aux forces d'attraction et au basculement de la
plaque subduct\'{e}e. Les transitions de la contraction de la
cro\^{u}te sup\'{e}rieure \`{a} l'ex-tension de la cro\^{u}te
sup\'{e}rieure ont affect\'{e} l'espace de d\'{e}p\^{o}t, qui a ouvert
des failles normales NW--SE subparall\`{e}les le long du contact
m\'{e}ridional avec la zone de collision des Carpates. L'ensemble du
processus de transition a \'{e}t\'{e} contr\^{o}l\'{e} par la fermeture
de la partie arri\`{e}re de l'espace d'accr\'{e}tion, qui a modifi\'{e}
le pendage de la plaque subduct\'{e}e. La subsidence du bassin, qui
s'est produite d'ouest en est par des marches N--S, a
g\'{e}n\'{e}r\'{e} un syst\`{e}me de failles orient\'{e}es
subparall\`{e}lement, qui ont \'{e}t\'{e} reli\'{e}es par des rampes de
relais. Les rampes de relais ont connu quatre phases de
d\'{e}veloppement~: (a)~flexion initiale de la cro\^{u}te,
(b)~formation d'une rampe intacte avec perturbation de
l'int\'{e}grit\'{e} structurale, (c)~br\`{e}che avec rupture de la
cro\^{u}te, et (d)~croissance finale de la faille. La gen\`{e}se des
rampes de relais du bassin slovaque oriental est \'{e}troitement
li\'{e}e aux mouvements de glissement initial. Les mouvements de
glissement ont provoqu\'{e} une rotation dans le sens inverse des
aiguilles d'une montre de la bordure septentrionale du bloc
Alpin--Carpathien--Pannonien. Cette rotation a ensuite \'{e}t\'{e}
compens\'{e}e par la subsidence tectonique du bassin, principalement au
cours du Mioc\`{e}ne moyen.
\end{altabstract}

\maketitle
{\vspace*{1.6pc}}

\twocolumngrid

\end{noXML}

\section{Introduction}

The Pannonian back-arc extensional basin is associated with the
convergence of the Alpine orogenic system during the Upper Cretaceous
and Cenozoic \citep{Horvathetal2015}. During the Miocene, the Western
Carpathians were dominated by oblique intra-continental subduction
related to the slab roll-back effect and extruding crustal blocks
\citep{VanGelderetal2017}. Studies on recent back-arc areas
\citep{SdroliasMuller2006} describe the unexpected interplay of
convergence and extension, and the importance of subduction as the
driving mechanism for change. Subduction can occur in various ways,
such as in collision-related phenomena, oblique convergence
partitioning, and hinge roll-back and/or pull by the subducted slab.
Magmatism is not always associated with subduction and depends on the
movement of the hinge. Important extensions of the upper plate can be
perpendicular and parallel to the length of the subduction zone
\citep{Hall2002}. The development of back-arc basins is a gradual
process that may have many possible controls, such as rifting, back-arc
extension, subduction hinge rollback, regional strike-slip faulting,
mantle plume activity, basement structural influences, and differential
crust--lithosphere stretching \citep{MadonWatts1998,
WatcharanantakalMorley2000, Morley2001}.
In order to understand the basin's structure and geometry, the
individual characteristics of the basin must be determined, such as the
rate of spreading, asymmetry, magnetic anomaly identifiers, time of
activity \citep{Mulleretal2008}, and the inclination of the subducted
slab. Back-arc shortening is common \citep{Lallemandetal2005} for slabs
with a shallow dip of up to 32{\textdegree} (distance of 0--125~km),
whereas back-arc spreading commonly occurs in slabs with a steep dip of
up to 51{\textdegree} (distance of 0--125~km). Thus, angular
differences are key to the dynamics and further development of the
basin. Very shallowly inclined plates tend to close the upper part of
the crust, whereas steeply inclined slabs spread the upper crust.

The position of the Eastern Slovak Basin (ESB) along the margin of the
Pannonian back-arc basin is the result of structural--tectonic
evolution, which is related to the tectonic delamination of the
back-arc system. During the Middle Miocene, the maximum extension of
the Pannonian Basin was approximately 450~km
\citep{Ustaszewskietal2008}. The structural integrity of the basin was
mainly disrupted by the Mid-Hungarian fault zone (MHFZ). The MHFZ
originated from a transform fault where the
Alpine--Carpathian--Pannonian (ALCAPA) microplate overthrusted the
Tisza--Dacia Mega-Unit \citep{CsontosNagymarosy1998}. The results of
this thrusting produced variable amplitudes of extension, ranging from
78--180~km \citep{Ustaszewskietal2008}. The ESB structurally opened
within a tectonic ``triangle'' which is bounded by the Darn\'{o} fault,
the MHFZ, and the Miocene thrust belt. The interaction of individual
tectonics in the triangle caused extensional differences in the basin
which were related to strike-slip movements and extension, shortening,
block rotation \citep{Ratschbacheretal1993, Fodoretal1999, Marton2000,
MartonFodor2003, CsontosVoros2004, Martonetal2007, Tischleretal2007},
and retreating slabs [\citealp{Royden1988, WortelSpakman2000}, Sperner
and The CRC 461 Team, \citeyear{Sperner2005}].{\break} The collision
between the Eastern Alps/Western Carpathians and the Adriatic Plate
(ALCAPA Mega-unit) during the Miocene initiated a period of active
subduction, exhumation of the orogenic wedge, basin opening,
subsidence, and uplifted the Pannonian Basin \citep{Csontos1995,
Fodor1995, HorvathCloetingh1996, Badaetal2001, Konecnyetal2002,
Horvathetal2006, Kovacetal1998, Kovacetal2002, Kovacetal2016,
Kovacetal2017}. Evidence of Miocene unconformities
\citep[e.g.,][]{Gingeretal1993, TjiaLiew1996, Jardine1997, Higgs1999}
can be correlated to global sea levels \citep{Haqetal1987} the
geodynamic evolution of the ALCAPA block \citep{Ratschbacheretal1991,
Oszczypko1998, Oszczypko1999, Oszczypko2006, Oszczypkoetal2006,
Oszczypkoetal2012, Neubaueretal2000, SeghediDownes2011,
VanGelderetal2017, Kovacetal1997, Kovacetal2017}, and the northeastern
escape of the Western Carpathians. These geodynamic changes were
primarily related to the pulling forces of the subducted slab, which
were affected by asthenolite uplift. The extension between the volcanic
arc and the central subsidence region opened new tectonic sub-basins,
which were modified by the slab dip. During the back-arc extension
period, the sedimentation was controlled by volcanic activity, which
indicates different slab dips during the Miocene. The interpreted dips
of the subducted slab were approximately 50{\textdegree} and
75{\textdegree} in the Lower Serravallian and the Upper Serravallian,
respectively \citep{LexaKonecny1998}. \looseness=-1

This study focuses on the development of structural elements near the
collision zone of an extensional back-arc basin. These were controlled
by tectonic activity related to relay ramp structures, sedimentation
and volcanism. Hydrocarbon traps and evaporite deposits are directly
linked to these structural elements. 

\section{Geological and geodynamic relations} \label{sec2}

During its Miocene evolution, the Alpine--Carpathian orogeny generated
different types of deformation structures in its individual
palaeogeographic domains. The domains were associated with the
subduction of a retreating lithospheric slab \citep{Royden1988,
WortelSpakman2000} which lied beneath an upper plate formed by the
ALCAPA and Tisza--Dacia Mega-units (Figure~\ref{fig1}). The topography
of the European Foreland was deformed and modified by the thrusting of
the Outer Western Carpathian (OWC) Flysch Belt \citep{Royden1988}. This
thrusting reached the Dukla and Silesian units by the Lower Burdigalian
(Eggenburgian), approximately 20~MA \citep{Kovacetal1998,
Oszczypko2006}. However, accretion was not identical throughout the
seismic profile. The most significant spatial accretion occurred in the
fore-arc of the ESB. The structural predisposition enabled the
rearrangement of the peripheral units of the Inner Western Carpathians
(IWC) during the initial stage of basin formation, and during the
development of the Pannonian back-arc basin during the Oligocene
\citep{Horvathetal2015}. This collision model gradually produced the
zonal structure of the Western Carpathians and was associated with
several active sedimentation basins. The basin distribution was related
to the development of the orogenic system and its transverse division
into the fore-arc and back-arc basins \citep[sensu][]{Kovacetal2016,
Kovacetal2018}. The Pieniny Klippen Belt (PKB) is a mobile suture zone
along the edges of the ALCAPA microplate \citep{Nemcoketal2006,
Oszczypko2006, Ustaszewskietal2010, Rauch2013, Kovacetal2016}, along
which the Penninic ocean was shortened and engulfed. The PKB helps one
understand the overall geodynamic situation in the Western Carpathians.
The PKB crustal fragments were thrusted under the former basement, and
the flysch belt sediments were thrusted over the foredeep. The tectonic
regimes related to the PKB processes were (1)~lateral extrusion of the
Inner Western Carpathians and the North Pannonian domain (by
strike-slip faulting, collision, and orogenic wedge exhumation during
mountain development) \citep[e.g.,][]{Ratschbacheretal1991,
Csontosetal1992, Kovacetal1994, Kovacetal1997, Kovacetal1998,
Kovacetal2002, Schmidetal2008, Ustaszewskietal2008,
Ustaszewskietal2010, Matencoetal2013} and (2)~basin opening and crustal
subsidence \citep[e.g.,][]{Csontos1995, Fodor1995,
HorvathCloetingh1996, Fodoretal1998, Fodoretal1999, Fodoretal2005,
Badaetal2001, Badaetal2007, Konecnyetal2002, Cloetinghetal2005,
Cloetinghetal2006, Horvathetal2006, Minaretal2011, Matencoetal2013,
Kovacetal2016, Kovacetal2018}. \looseness=-1

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}(a)~The opening of the Pannonian back-arc basin is
related to the tectonic evolution of the microplates, which are bounded
by deep core faults. (b)~The Eastern Slovak Basin is a northern
peripheral basin and part of the Transcarpathian depression situated
between the southern edge of the collision wedge, the Mid-Hungarian
fault zone, and the Horn\'{a}d fault/linked with the Darn\'{o} fault
system in the west \citep[modified after][]{Ustaszewskietal2008,
Martonetal2007}.}
{\vspace*{-.15pc}}
\end{figure*}

The ESB primarily consists of siliciclastic deposits with lesser
amounts of evaporites. An important part of the fill is formed by
acidic volcanic rocks (the Lower Burdigalian to the Lower Langhian),
intermediary volcaniclastics (Langhian to Serravallian), and effusive
rocks. The volcanic activity culminated in the Langhian and
Serravallian periods. The maximum fill thickness is approximately 9~km,
but the data is inconsistent. The depocenters of the basin migrated
from the northwest to the southeast. In the northwestern and central
parts of the basin, the deposition culminated during the Upper
Burdigalian and Lower Serravallian, respectively. The most intensive
subsidence in the southeastern basin was recorded during the Middle
Serravallian. The basin lies on thin (approximately 27-km-thick) crust
that thickens to approximately 30~km towards the north and northwest.
The thickness of the lithosphere in the basin is approximately 80~km.
The basin contains several gravity anomalies, probably caused by the
occurrence of ultrabasic rock bodies. The largest anomalies are located
near Se\v{c}ovce, Zbudza, and in the southwestern part of the Moldava
Depression.

The Dynamic opening of the Panonian Basin towards NE was compensated by
perpendicular N--S oriented faults. These faults were concentrated in
the eastern part of the ESB and compensated the basin's rapid
subsidence towards southeast.

The Sl\'{a}nske Mts. volcanic chain, which is also oriented N--S,
divides this set of faults (such as the Horn\'{a}d, Albinov, and Ondava
fault). The volcanic chain also separates the ESB into the eastern
(Ko\v{s}ice) and western (Trebi\v{s}ov) depressions.

It therefore can be assumed that a transform fault exists along the
volcanic chain which was a channel for Miocene intermediate volcanism.
The interpretation of deep seismic profiles (705/92, 706/92,
Figure~\ref{fig2}) confirmed the existence of a system of normal step
faults ranging from the western margin of the ESB up to the volcanic
chain. In this 15~km wide ranging area, we can observe (as in the
example of the Middle Triassic Dolomites) tectonic subsidence up to
2500~m in depth.

\begin{figure*}
{\vspace*{-.3pc}}
\includegraphics{fig02}
{\vspace*{-.15pc}}
\caption{\label{fig2}Spatial distribution of the 2D seismic
cross-sections, 3D seismic cubes (A/cube---locality Vi\v{s}\v{n}ov,
B/cube---locality Trebi\v{s}ov, C/cube---locality Senn\'{e}-Stretava,
D/cube---locality Pavlovce) and exploration wells, realized during
hydrocarbon exploration and excavation by the Nafta Oil Company in the
Eastern Slovak Basin.}
{\vspace*{-.4pc}}
\end{figure*}

East of the volcanic chain there are dolomites buried at a depth of
4000~m (583/85, Figure~\ref{fig2}). The depth at which the dolomites
occur along the western and eastern edge of the volcanic chain differs
significantly. For this reason, it can be assumed that there is a
transform fault running along the length of the volcanic chain, which
would be tied to the above-mentioned subparalell N--S faults. The
faults mark the tectonic boundaries between the Miocene ESB and the
pre-Miocene IWC units, as well as several depositional subbasins which
developed towards the east (Ko\v{s}ice, Vranov, and Michalovce).
Additionally, the subducted slab was broken in this zone, and the
lithospheric plate became more inclined \citep{Csontos1995}. These
changes were controlled by increased sedimentation in the eastern
areas.

\section{Methodology} \label{sec3}

\looseness=-1
Relay ramp research was carried out using datasets collected from
surface analysis of Eastern Slovak Basin deposits. The datasets also
used the facies analysis of drill cores, the analysis of electricity
well logs, seismic data interpretations, seismostratigraphic analysis,
structural and geological research and modelling. Additionally
Spontaneous Potential (SP) responses were used and were compared to
seismic reflectors [Jacko in \citealp[][pp.~1--254]{Zecetal1997}; Jacko in
\citealp{Hruseckyetal2000}; Jacko in
\citealp{Janockoetal2006}; Jacko in \citealp{Salyetal2006},
\mbox{\citealp[][62~p]{Pachockaetal2010};}
Jacko in \citealp{Fricovskyetal2012,
Jackoetal2014,Jackoetal2021a}].
{\vspace*{-.5pc}}

\subsection{Seismic analysis and modelling} \label{sec3.1}

\looseness=-1
Hydrocarbon and (to a lesser extent) geothermal exploration during the
last few decades have prompted several drilling campaigns accompanied
by geophysical, structural, sedimentological, and geochemical
exploration. In this study, the results from 341 boreholes were added
to the database for further evaluation and modelling using Schlumberger
Petrel v.2010.1 software and Golden Software Surfer~12. Geophysical
campaigns from ESB Basin included 2D and 3D seismic exploration which
covered an area of 2000~km$^2$.\ We used 56 2D cross-sections
(Figure~\ref{fig2}) with varying orientations (mostly NW--SE; with
fewer NE--SW, N--S, and W--E campaigns from 1974--2000; with a total
length of 940~km). This study used four 3D seismic cubes covering an
area of 450~km$^2$: 3D Vi\v{s}\v{n}ov (8 ${\times}$ 8~km), 3D
Trebi\v{s}ov (12 ${\times}$ 12~km), 3D Senn\'{e}-Stretava (17
${\times}$ 11~km), and 3D Pavlovce (8~${\times}$~7~km).

The seismic cross-section processing yield results with reversed
polarity \citep{Brown1991}. Although the vertical scales of seismic
sections are in TWT, key observations (e.g., sediment thickness and
fault offsets) in the article were calculated in meters based on the
correlation of SP responses (SP tie) from wells in the study area. The
SP ties should be correct; however, inaccuracies may occur due to the
scarcity of check shots in the area. The age of the mapped seismic
horizons were identified based on 341 wells localized inside the
studied area. 

\subsection{Structural analysis} \label{sec3.2}

The deep seismic analyses were compared to the results of the
structural analysis.\ Temporal, \mbox{spatial} and structural
relationships were analysed and correlated with each other. Structural
research was carried out in a wide area of the ESB. This includes
several lithotectonic units, such as the Veporic Unit (crystalline
Paleozoic basement with Mesozoic cover), the Fatric Unit (Mesozoic
carbonates), the Inner Carpathian Paleogene Basin sediments
(Eocene-Oligocene/Eger), the Pieniny Klippen Belt
(Jurassic---Cretaceous sediments), the Carpathian Flysch Belt
(Paleogene sediments), and the ESB (Eggenburgian--Pannonian sediments
and volcanic rocks). Structural analysis and paleostress calculations
were completed using TectonicsFP software.

\subsection{Relay ramps} \label{sec3.3}

The relay ramps are typical structural patterns that involve
extensional tectonics and that control deposition and sedimentary fill
\citep{CorfieldSharp2000, GawthorpeLeeder2000, Gawthorpeetal1997,
Hamblin1965, RotevatnJackson2014, Schlische1995, Sharpetal2000,
Withjacketal2002, SerckBraathen2018}.
The development of relay ramp structures has been studied in rift
basins, where three stages of development have been identified:
(a)~fault initiation, (b)~fault intersection and linkage, and
(c)~through-going fault stage \citep{Cowieetal2000,
GawthorpeLeeder2000}. The basic geometric characteristics
\citep[e.g.][]{FossenRotevatn2016} are based on comparisons of the
fault length (fault overlap), the relay ramp width (fault separation),
and the relay ramp dip \citep{SolivaBenedicto2004, Hugginsetal1995,
Xuetal2011, RotevatnBastesen2012, Gibaetal2012, BastesenRotevatn2012}.
\citet{SolivaBenedicto2004} suggested a breaching criterion ($c^*$)
based on the relationship between the relay displacement ($D$, the sum
of the fault displacement) and the relay width (fault separation, $S$)
of the form $D = c^* S$, where $c^*$ varies from 1 ($D = S$) to 0.27
($D = 0.27S$) for relays that show evidence of incipient breaching.
\looseness=-1

\section{Results} \label{sec4}

The results of structural research have confirmed that the development
of ramp structures in the depositional space of the ESB depends on the
initial processes. These initial processes ``prepared'' the ramps for
their subsequent formation and development. In general, a ramp
structure development is associated with crustal extension.
Paradoxically, in the initial deformation stage of the ESB, the crust
was exposed to shortening in transpressional deformation
\mbox{conditions}. {\vspace*{-.5pc}}

\subsection{Basin opening on the edge of the collision zone (initial
stage), Wrench basins} \label{sec4.1}

The migration of the subduction boundary of the European platform
toward east initiated a number of shortening processes which were
compensated by thrusting in the accretionary zone of the OWC
\citep[e.g.][]{Kovacetal1998, Oszczypko2006}, during the Lower
Burdigalian (Eggenburgian). The major regional consequences of extreme
shortening in the accretionary wedge included: (a)~inclination changes
of the subducted slab, (b)~steepening of the wedge-top basement,
(c)~flysch belt space reduction compensated by overthrust and crustal
bending in the foredeep, (d)~south-vergent thrusts of pre-Miocene units
situated in the frontal part of the ESB and the Pieniny Klippen Belt,
(e)~exhumation of the Pieninic nappes, (f)~double or triple
(Figure~\ref{fig3}) the tectonic thickening of the pre-Miocene IWC
deposits, and (g)~strike-shear movements that compensated shortening
\mbox{activity} $[0, 0, 0]$.

\begin{figure*}
\includegraphics{fig03}
{\vspace*{.4pc}}
\caption{\label{fig3}Reverse south-vergent overthrusts along the edge
the collision zone (GPS coordinates of the location:
48{\textdegree}54$'$4.10$''$N 21{\textdegree}55$'$44.97$''$E) of the
Pieniny Klippen Belt, which enabled the movement and displacement of
the peripheral units (I\v{n}a\v{c}ovce--Kritchevo unit, Humenn\'{e}
Mesozoic unit, Inner Carpathian Paleogene unit) of the Western
Carpathians towards South. Tectonic thickening of the basal
conglomerates was confirmed by nummulite zones which occur in all
tectonic packets.}
\end{figure*}

Collisional processes along the boundary between the ESB and the
Pienniny Klippen Belt were compensated by dextral transpression which
formed a flower structure and splay wrench basins (Figure~\ref{fig4}).
Simultaneously a number of south-vergent isoclinal fold structures
formed in Paleogene/Lower Miocene sediments. These have fold axes that
are oriented in a WNW--ESE and a E--W direction. The folds are
associated with the thrusting and tectonic thickening of the basal
Paleogene sediments, which overthrust the Mesozoic Humenne Unit.

\begin{figure*}
\includegraphics{fig04}
{\vspace*{-.3pc}}
\caption{\label{fig4}(a)~The sedimentary log from the splay wrench
basin (approx. 500 ${\times}$ 250~m) at the northern margin of the
studied area in the ESB (GPS coordinates of the location:
49{\textdegree}03$'$18.2$''$N 21{\textdegree}22$'$13.5$''$E). The
vertical lithological logs documents the dynamics of the opening,
deposition, and closure stages of the basin. (b)~The location of
studied outcrop. (c)~Massive fine to medium-grained sandstones with a
monoclinic inclination of 20{\textdegree} to 35{\textdegree} to the
northeast with well-developed foreset beds. Sediments are segmented by
listric and planar normal faults with a NNW--NNE direction, and a steep
dip towards WNW. (d)~Cemented syn-sedimentary thrusts which developed
before the diagenesis of the sediment. The sandstone boudins in these
claystones are parallel to thrust faults. Middle Burdigalian shortening
on the northern edge is related to the uplift and sedimentary hiatus in
the ESB.}
\end{figure*}

This deformation stage initiated a sedimentary hiatus in the ESB during
the Middle Burdigalian. Additionally, Riedel shears formed, such as the
NNE--SSW sinistral and NNW--SSE. These have been observed in Mesozoic
carbonates of the Humenn\'{e} Unit along the NE edge of the ESB. This
entire structure was fixed in place by the overlying Serravallian
(Upper Badanian--Sarmatian) volcanics.

In the vertical log it is possible to observe the following sedimentary
facies: 
\begin{itemize}
\item Clast-supported, massive, and rarely positively graded
conglomerates. 
\item Matrix-supported, massive, and inversely graded conglomerates
with high matrix \mbox{content}.
\item Clast- to matrix-supported, parallel-laminated pebbles with a
mean clast size of 0.5~cm. The thickness of the laminae is determined
by the clast size and the lamination is formed by alternating coarse
and thin laminae, which consist of larger (maximum 1~cm) and smaller
clasts.\looseness=-1
\item Medium-grained sand with scattered granules.
\item Horizontally laminated fine-grained, muscovitic, dark-brown sand
and silt. 
\item Planar, cross-stratified, medium-grained sandstone with
occasional granules. Granules are arranged along the laminae and
emphasize cross-stratification. The mean dip of planar cross strata is
toward the NNW. The base and top of the beds are sharp, the bed
thickness is about 15~cm. This facies usually fills small erosional
scours, or it laterally passes into massive sandstones of facies~4. 
\item Thin layers of conglomerate are represented by well-rounded
clasts with a mean diameter of 6 cm, which mainly comprise of one-clast
thick beds.
\end{itemize}

Sedimentary facies described in the vertical log (Figure~\ref{fig4})
are interpreted as sediments deposited in an environment containing
delta fans or steeply inclined ramps. Shortening space in the collision
zone (Figures~\ref{fig3}, \ref{fig4}) caused crustal loading. This
loading was compensated by the occurrence of dominant depressions in
the N--S direction, which occurred after the sedimentary hiatus. The
depressions developed from the edge towards the center of the basin. A
new deposition center subsequently opened during the Karpathian period
\citep{Vassetal2000}. The sedimentary fill occurred nearly throughout
the entire basin and corresponds to the extension of the Pannonian
Basin.

\subsection{Basin differentiation linked to relay ramp development}
\label{sec4.2}

Structural changes in the Middle Miocene were related to modified
geodynamic conditions in the Pannonian Basin and were reflected in all
major crust fault lines, such as the MHFZ (including the Darn\'{o}
fault system, where the ALCAPA overthrusts the Tisza--Dacia Mega-unit)
\citep{CsontosNagymarosy1998}. The maximum basin spreading escalated
the crust delamination process, which created asymmetrically arranged
relief. The Burdigalian evolution of the ESB lacks structural field
data. Well and seismic sedimentary data shows us that the basin
depocenter was located in the axial part of the basin. The subsidence
was controlled by NE--SW extension along dextral oblique normal NW--SE
faults, when the northern edge of the basin was slightly uplifted. The
sedimentary fill at the base of the Burdigalian rocks begins with well
sorted clasts and sporadic andesite fragments. The clastic sediments
are covered by a lens shaped halite body 60--80~m thick and{\break}
3~km long. 

The advancing uplift in the north caused sea level regression, which
subsequently lagunar sedimentation up to 300~m thick. The evaporites
are tied to those sub-basins which have their axes oriented \mbox{N--S}
(Figure~\ref{fig5}). The step-arranged sub-basins gradually moved away
from the collision zone (sub-basin~A: 7~km, sub-basin~B: 10~km,
sub-basin~C: 23~km). The delamination of the crust (from the west to
the east) was controlled by subparallel faults systems, and relay ramps
connected to them. The axes of the main distribution channels are
oriented parallel to the direction of the fault networks and change
position depending on the amount of increasing subsidence. The most
striking changes are connected to breaching, which generated
perpendicular NE--SW extensional faults, that were later used as
pathways for fluid migration \citep{Jackoetal2021a}. This is supported
by current geothermal exploration in the area.

\begin{figure*}
\includegraphics{fig05}
\caption{\label{fig5}The arrangement of the elongated subbasins is
related to the bookshelf effect, the axially oriented distribution of
sediments, and strike-slip/normal fault development during the and
Lower Serravallian (Upper Badenian). The development of sub-basins was
gradual from West to East (from the oldest~A to~B and youngest~C). The
offset of the sub-basins is related to the dominant tectonics of N--S
direction. Sub-basin~C, (the youngest) is already under the partial
influence of the NW--SE system, which began to dominate from the Middle
Serravallian (Lower Sarmatian). A---Ko\v{s}ice depression, bordered in
the West by the Horn\'{a}d fault system (HF) and in the East by the
neovolcanic roks of the Slansk\'{e} vrchy. B---Vranov depression is
bounded in the West by the Albinov fault (AF) and in the East by the
Ondava fault (OF). C---Sliepkovce depression is tectonically bounded by
the Ondava fault (OF) and the Laborec fault (LF).}
{\vspace*{-1pt}}
\end{figure*}

\subsection{Relay ramp development} \label{sec4.3}

The relay ramps were studied in detail east of the Slanske Mts.
volcanic chain (Figures~\ref{fig6}, \ref{fig12}). The ramps are linked
to the Albinov fault system (AFS). The faults footwall was formed in
the N--S direction with a medium to steep inclination towards the~east.
The Podslansk\'{y} foothills were formed parallel to the fault as a
product of Miocene volcanism, which was associated with the ESB
development. AFS epizonal (multi-genetic) development continues into
the present, as demonstrated by earthquake epicenters with macroseismic
effects ranging from~3 to~5 MSK-64. The stratigraphic record along the
AFS is formed by the buried I\v{n}a\v{c}ovce--Kritchevo Unit which are
overlain by Neogene sediments of the ESB. The number of 3D seismic
surveys were implemented in the area to methodically solve the problem
of how relay ramp systems were formed.

\begin{figure*}
\includegraphics{fig06}
\caption{\label{fig6}The Albinov fault system (AFS) represents a
juxtaposition zone between the I\v{n}a\v{c}ovce--Kritchevo and the
Veporic basement (under Karpathian sediments) units. The AFS is a
shear-zone which compensated the counter-clockwise basin movements and
oblique normal fault with high subsidence during the Langhian and
Serravallian sedimentation. The result of the polystage evolution was
the formation of the flower structure, joined with the listric shape of
the fault.}
{\vspace*{.95pc}}
\end{figure*}

\subsubsection{Initial stage---crustal bending} \label{sec4.3.1}

The formation of the relay ramp fault system in the early stages of
back-arc extension was controlled by the tectonic transition at the
basins margin, which was a collision zone during the and Lower
Serravallian (Upper Badenian). The deltaic depositional systems
architecture with its distributional channels suggests there was an
increase in subsidence at the southern edge of the AFS. This subsidence
would have been related to the initial formation of the half-graben
structures at the southern margin of the basin. 

Most coarse-grained material was progressively deposited on the western
edge (later inboard fault) of the future AFS environment from the NNW
to the~S. The sigmoidal curvature of the depositional system
(Figure~\ref{fig10}) indicates a change in slab inclination with
subsidence towards the edge of the plate, that had not yet been
tectonically ruptured. The presence of separate individual segments
\citep{TrudgillCartwright1994,
Walshetal2003, FossenRotevatn2016} and embryonic faults was not
confirmed in the initial stage \citep[e.g.][]{Cowieetal2000,
SolivaSchultz2008}. The material was homogeneously deformed
\citep{McClayEllis1987, VendevilleCobbold1988, Wuetal2015} by a
flexural bend in the upper part of the crust, which compensated for the
massive subsidence that occurred at the southern margin of the basin.

As can be seen in Figure~\ref{fig7}, the subsidence created a
structural depression in the N--S direction that is well-documented by
the third distributary channel at the eastern margin of the ESB. This
distributary channel has a general NE--SW orientation and shows signs
of eastward migration (Figure~\ref{fig7}).

\begin{figure*}
\includegraphics{fig07}
\caption{\label{fig7}Initial stage: (left) The Upper Badenian (Lower
Serravallian) base horizon of the western margin of the ESB with no
signs of tectonic rupture. (Right) Orientation of the distributional
channels corresponding to the distribution of coarser sedimentary
material which was distributed from the north to the south.}
\end{figure*}

\subsubsection{Intact ramp formation---crust rupture} \label{sec4.3.2}

\vspace*{-1.5pt}

The dominant extension continued until the end of the Lower
Serravallian, where it fractured the \mbox{upper} crust through
flexural bending, and formed a fault system \citep{Vassetal2000}.
During this period, en-echelon subparallel faults were formed, that
consisted of individually growing faults. We assume that this phase was
associated with the rupturing crust and was isostatically compensated
for the ``unnatural'' crustal bending. This led to the retreat of the
distribution channel to the north. The vertical displacement of the
fault system reached 450~m along the southern margin. The
aforementioned events were likely accompanied by seismic shocks
\citep{Janocko1996, Rericha1992}. A narrow, dynamically meandering
deposition channel with a southward inclination opened parallel to the
fault system. Higher seismic amplitude at the Lower Serravallian
structural horizon (Figure~\ref{fig8}) documents the distributary
channel development. Sea regression and sedimentation occurred in a
dynamic delta environment \citep{Vassetal2000}. Dynamic subsidence
increased the ESB extension, and deposits of coarse-grained clastic
material transported by deltaic systems \citep{Janocko1990,
Reedetal1992, Rericha1992, Janocko1993, Janocko1996, HlavataKovac2010}
accumulated in the marginal N, NW, and W sides of the basin. 

\begin{figure*}
\includegraphics{fig08}
\caption{\label{fig8}The structural surface of the Upper Badenian
(Lower Serravallian) horizon points to the tectonic activity of the AFS
and the formation of en-\'{e}chelon subparallel fault. Already in this
period, the highest tectonic block was formed in the western part of
the depicted surface. The fault drops to a maximum of 450~m southward.}
\end{figure*}

Sediments deposited in the mouth bars of the terminal parts of the
deltaic distributional channels allowed the progradation of
coarse-grained sand bodies from the margin of the basin from NE and N
to the SW and~S. The western margin of the Albinov fault displays a
similar situation. The increasing subsidence gradually initiated
antithetical parallel-oriented faults, which completed the relay ramp
system. The faults are most visible in the northern part of the basin,
where four such identified systems reached developmental stage~B
\citep[sensu][]{PeacockSanderson1991, PeacockSanderson1994} without the
occurrence of a fault breach (Figure~\ref{fig8}).

\subsubsection{Relay ramp breaching} \label{sec4.3.3}

AFS activity in the Middle Serravallian was sufficient to interconnect
individual segments.\ The fault direction was generally N--S, and the
normal throw was compensated by antithetically and synthetically
oriented faults. The newly created ramp captured deposits in the
interdistributary area. This \mbox{development} coincides very well
with the tectonic development of the ESB, which contains documented
medium steep uplifts at the northern margin of the basin with a
collision edge. The uplifts primarily compensated subsidence in the
central area and the opposite margin of the basin, where the declines
were dominated by NW--SE-oriented planar and listric faults (documented
in 5 other locations). Well logs located on the AFS ramp indicate
lateral migration of the distribution channels, which were probably
affected by pulse ramp movements. This indicates breaching between the
inboard and outboard faults (Figure~\ref{fig9}).

\begin{figure*}
\includegraphics{fig09}
\caption{\label{fig9}The structural surface of the base of Middle
Serravallian (Lower Sarmatian) with the interpretation of the
distribution channels, with the deposition of sediments from the N and
NE to the S and SW. The sandiest parts are located West of the AFS on
the ramp. The longest distribution channel is situated on the ramp,
which is bounded by the antithetically oriented fault.}
{\vspace*{.52pc}}
\end{figure*}

\subsubsection{Geometry finalization} \label{sec4.3.4}

The geometry of the Albinov relay-ramp structures reached its
definitive and final form at the end of the Middle Serravallian. Normal
faulting determined surface morphology, sedimentary routing, and
stacking patterns \citep{CorfieldSharp2000, RotevatnJackson2014,
SerckBraathen2018}. The 3D seismic cross-section cube identified an
expansion of the faults related to planar surface breaching faults
(Figure~\ref{fig10}).\ The AFS is a planar structure with
conjugated-oriented faults. The maximum throw between the footwall and
the hanging wall was approximately 1100~m, and the hanging wall
horizons created wedge-shaped sedimentary packages
\citep[sensu][]{SerckBraathen2018}. The footwall in the cross-section,
parallel with the fault, is characterized by alternating high, medium,
and low amplitudes that are related to dynamic conditions present
during deposition (Figure~\ref{fig11}). The interpretation of the
depositional environment indicates an environment which was influenced
by deepening sedimentary basin conditions, during a period of
tectonic{\break} subsidence. 

\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}2D--3D spatial and geometric view of the fault
relay structure of the Albinov fault system.}
\end{figure*}

\begin{figure*}
\includegraphics{fig11}
{\vspace*{-.15pc}}
\caption{\label{fig11}The structural surface horizon of the Lower
Sarmatian (Middle Serravallian) period (on the left) with the relay
ramp structure and its four interpreted distributional channels (on the
right) \citep[sensu][]{Pickeringetal2014, Shepherd2009}. The channels
reveal migration and avulsion. High amplitudes---red, yellow, and
yellow-green document the sandiness of the environment.}
{\vspace*{-.55pc}}
\end{figure*}

\subsection{Relay ramp geometrical classification} \label{sec4.4}

The relay ramp geometric structure classification is primarily based on
half-graben geometry \citep[according to][]{Morleyetal1990,
Morley1995}. The comparison and application of the methods is limited
by the technical documentation, which is related to:
\begin{itemize}
\item The detail and quality of processing (geophysical or surface
data).
\item The rate of structural and sedimentary basin modification or
revisions.
\item Erosion (especially of surface sources).
\item Research horizons (that must correspond to one-time record).
\item The time to depth conversion.
\end{itemize}

During the ESB development, the area of subsidence migrated from the NW
to the SE. Changes in the inclination of the subducted slab, and
fault-related processes at the MHFZ caused: accelerated tectonic
activity, increased heat flow and initiated tectonically controlled
rapid subsidence. The response to this crustal effect was the evolution
of concave structures that predominantly compensated for the
south-oriented extension.\ In these \mbox{geodynamic} conditions,
dominant half-graben type faults were compensated by antithetically
oriented growth faults. The growth faults locally changed the
deposition trajectories.

Curved, synthetically oriented faults with an initial N--S direction
(Figure~\ref{fig12}) dominated the structural system. The initial
WNW--ESE and subsequent NE--SW extensional regimes were gradually
interconnected by the synthetically and antithetically oriented faults.
These were later transformed into transtensional faults that generated
the relay ramp geometry. 

\begin{figure*}
\includegraphics{fig12}
\caption{\label{fig12}The rapid subsidence along the northern edge (red
frame) of the Eastern Slovak basin was compensated by relay ramp growth
faults. The irregular fault geometry which increases with distance from
the major fault can be seen on the isohypse near the Middle
Serravallian (Lower Sarmatian) sedimentary horizon \citep[modified
after][]{Salyetal2006}.}
{\vspace*{-.02pc}}
\end{figure*}

The Middle Serravallian structural surface was geometrically classified
(Table~\ref{tab1}). The seismic cross-section was easily interpreted
due to clear sedimentary and tectonic structures, a sufficient dataset
source, and the absence of erosional surfaces. The individual segments
were geometrically analysed using a multi-criteria approach. The
variable width, length, and slope values of the ramp segments do not
affect the width or opening of the fault structure. The inclination of
the ramp varied from 2{\textdegree}--4{\textdegree}. Values from
5{\textdegree}--9.8{\textdegree} were deviations from the average. All
the surplus values were concentrated in a broad zone, in which the
structural links between the \mbox{N--S} and NW--SE systems were most
likely compensational. The thickness model of Upper Serravallian
deposits correlates very well with the before mentioned zone.\ However,
the relationship between ramp structures and the primary ``parent''
fault zone was confirmed.\ The ramp segments showed increasing slope
values with increasing distance from the primary fault.\ Paradoxically,
the width-to-length ratio{\break} is close to~1.

\begin{table*}%tab1
\fontsize{9.35}{11}\selectfont
\caption{\label{tab1}Geometry patterns measured from each fault 
limbs\vspace*{-4pt}}
\tabcolsep=4pt
\begin{tabular}{ccccccccc}
\thead
\xmorerows{1}{Fault No.} & \xmorerows{1}{Geometry} & \xmorerows{1}{Shape} &
\morerows{1}{\parbox[t]{1.25cm}{\centering Width (meters)}} &
\morerows{1}{\parbox[t]{1.25cm}{\centering Length (meters)}} &
\morerows{1}{\parbox[t]{1.55cm}{\centering Fault~heave (degrees)}} &
\morerows{1}{\parbox[t]{1.2cm}{\centering Height/{\break}length}} & 
\multicolumn{2}{c}{Inclination}\\\cline{8-9}
& & & & & & & Degree & Dip to ramp orientation\\
\endthead
1 & Synthetic & Curve & 500 & 1600 & 50 & 1:21.4 & 2.7 & Parallel\\
2 & Synthetic & Curve & 1025 & 3600 & 70 & 1:17.1 & 3.3 & Parallel\\
$2^*$ & Conjugate & Linear & 600--1200 & 4125 & 75--50 & 1:15 & 3.8 & Parallel\\
3 & Synthetic & Curve & 950 & 3625 & 50 & 1:22.7 & 2.5 & Parallel\\
$3^*$ & Conjugate & Curve & 600--1125 & 2625 & 45--60 & 1:14.6 & 3.9 & Parallel\\
4 & Synthetic & Curve & 960 & 2125 & 40 & 1:19.3 & 3 & Parallel\\
5 & Synthetic & Curve & 625 & 4475 & 75 & 1:26 & 2.2 & Parallel\\
6 & Synthetic & Curve & 750 & 4750 & 75 & 1:8.3 & 6.8 & Perpendicular\\
7 & Synthetic & Curve & 750 & 2700 & 50 & 1:5.8 & 9.8 & Perpendicular{\vspace*{2pt}}\\
\morerows{1}{$6^*$, $7^*$} & \morerows{1}{Conjugate} & \morerows{1}{Curve} & \morerows{1}{800--1500} & \morerows{1}{4750} & \morerows{1}{40} & 1:10 & 5.7 & \morerows{1}{Perpendicular parallel}\\
&&&&&& 1:11.4 & 5 &{\vspace*{2pt}}\\
8 & Conjugate & Linear & 780--1450 & 6500 & 45 & 1:24.2 & 2.4 & Parallel
\botline
\end{tabular}
\vspace*{-4pt}
\end{table*}

We used the results of the geometric analysis to conclude that relay
ramp development depends on the following:
\begin{itemize}
\item The orientations of potential ramp structures in relation to the
paleo-stress field distribution.
\item The positions of particular faults in the overall structure. 
\item The position of the ramp in reference to the primary subduction
area, where the marginal structures show inclination differences (this
factor did not affect the width of the fault surface or the slope).
\item A wide range of structural zones correlated to heat flow
distribution. 
\end{itemize}

The asymmetric arrangement of structural elements significantly limits
the range of hydrocarbon deposits:
\begin{itemize}
\item Hydrocarbon structural traps are tectonically limited by a system
of listric fault structures, on which hydrocarbon migration also
\mbox{occurs}.
\item Structural zonality is thus an important factor for understanding
the distribution of underground deposits, whether hydrocarbons or
geothermal energy sources.
\end{itemize}

\subsubsection{Structural observations} \label{sec4.3.5}

By observing the geological structural framework, it is evident that
there is a relationship between an increasingly more dynamic upper
crust and increasing internal deformation and slab inclination. This is
connected to the formation of multiple faults and fractures, which are
dependent on several rheological and mechanical layer properties. In
general, it was presumed that the development of the ESB 
(Figure~\ref{fig13}) was primarily
linked to the development of fault \mbox{structures} NW--SE parallel to
the Pieniny Klippen Belt \citep{Jackoetal2021b}. However, the dominance
of this NW--SE system only starts in the Miocene because of rapid
subsidence. This was supported by crustal heating, and associated
volcanism. The subsidence transported the main depression towards SE.
The influence of the Horn\'{a}d Fault system 
(Figure~\ref{fig13}) along the western margin
of the ESB decreases, while the influence of subsidence and
antithetical shear structures (such as horse tail structures and later
relay ramps) increases. The formation of \mbox{relay} ramps in the ESB is
\mbox{therefore} connected to transform faults that are parallel to the
Horn\'{a}d fault system. These faults also restricted the CCW rotation
of the NE edge of the \mbox{ALCAPA} plate.\ Later, during the middle
Miocene, they compensated the tectonic subsidence of the basin
in the SE. 

\begin{figure*}
\includegraphics{fig13}
{\vspace*{.35pc}}
\caption{\label{fig13}The tectonic evolution of relay ramps in the ESB
is connected with the developmental phases of the Mid-Hungarian Fault
Zone and the back-arc basins development \citep[according
to][]{CsontosNagymarosy1998, Martonetal2007, Tischleretal2007,
Jackoetal2021b}. (a)~The compression during the Lower Miocene in the
ESB was related to the overall closing of the accretion wedge, where
the slope of the subducting plate was about 32{\textdegree}. These are
the deformation conditions where compression dominates in the upper
part of the crust, and it is manifested in a wider regional context. As
the shortening progressed from the north to the south, lateral dextral
transpressional movements were generated in the Pieniny Klippen Belt
zone, which also generated important structural fault systems in the
developing ESB. The progradation process of basins closure caused the
ALCAPA to be pushed onto Tisza--Dacia along the Mid-Hungarian fault
zone. (b)~In the Upper Badenian (Lower Serravallian), the tectonic
situation changed. The main cause was a change in the inclination of
the subduction plate. The deformation conditions changed where there
was an increase in the slab inclination. The upper crust began to be
dominated by extension, which accelerated the subsidence of the area
north of the MHFZ. This caused roll-back dips, offset by uplifts on the
opposite side of the basin (Figure~\ref{fig15}). The shortening and
thrusting of the MHFZ was transformed into sinistral movements which
caused the counterclockwise rotation of the ALCAPA. The rotation of the
plate was reduced along the N--S faults of the Horn\'{a}d fault system,
which separated the subducting plate into a western (shallow) and an
eastern (steep) segment. (c)~The transition from transpression to MHFZ
transtension supported the ongoing dynamic subsidence of ESB.
Syn-sedimentary oblique movements at faults parallel to the basin axis
(NW--SE) are offset by the emergence of oppositely oriented growth
faults/relay ramps. Abbreviations: Outer Western Carpathian (OWC),
Pieniny Klippen Belt (PKB), Inner Western Carpathian (IWC),
I\v{n}a\v{c}ovce--Kritchevo Unit (IKU), East Slovakian Basin (ESB),
Horn\'{a}d Fault (HF), Trebi\v{s}ov Fault (TF), Diojen\"{o} Fault (DF),
Albinov Fault (AF), Mid-Hungarian Fault Zone (MHFZ).}
\end{figure*}

If there is connection between the slab's inclination and the strained
state of the upper crust, then it is evident that the Horn\'{a}d fault
system is a genetically transform fault, with a high degree of heat
flow. Geothermal activity reveals a clear zonation trending E--W and
N---which is a result of a combination of structural (thickness and
spatial distribution of horizons), lithological (variations in
carbonates, siliciclastics, evaporites, andezite tuffites) and
geothermal features (approximated thermal conductivity, basal heat flow
densities, radiogenic heat production, geothermic gradient and basal
\mbox{temperatures}).

\section{Discussion} \label{sec5}
\subsection{Volcanism} \label{sec5.1}

Syndeposition volcanic and intrusive activity (Figure~\ref{fig14}) were
the main signs that accompanied the Neogene evolution of the Pannonian
basin and its peripheral areas. They are characterized by their spatial
distribution, and their relationships to tectonic phenomena such as
subduction, slab breakoff, tectonics and the block rotations of
microplate \mbox{segments}. Volcanism (from~21 to 0.01~Ma
\citep{Pecskayetal2006}) in this area is generally related to the
subduction and collision of that crust which underlies the former Outer
Carpathian flysch basins. 

\begin{figure*}
{\vspace*{-.1pc}}
\includegraphics{fig14}
{\vspace*{-.1pc}}
\caption{\label{fig14}Timeline of volcanic activity linked to
morphologically and tectonically active fault system of the ESB. The
volcanic sequence is marked by stars (red, green) and movement from
west to east is parallel to the oblique subduction
\citep[sensu][]{Pecskayetal2006}.}
{\vspace*{-.1pc}}
\end{figure*}

The ESB's centers of volcanism are concentrated in three areas
\citep{LexaKonecny1998}; the activity of which moved from west to east
over time. The first center was the westernmost, Slanske Mts. volcanic
chain. This chain is parallel with the N--S Horn\'{a}d fault system. A
system of nine linearly arranged pyroxene andesite stratovolcanoes
follow the same path. The volcanoes decrease in age as one moves from
north to south, with Middle Burdigalian stratovolcanoes in the north,
and Tortonian volcanoes in the south. Similarly, it is possible to see
environmental and geological changes between the north and south
regions. In the north the volcanoes are formed by diorite porphyre
stocks with epithermal mineralization. The meridional region is
dominated by shallow marine conditions with hydromagmatic pyroclastic
deposits
\citep{Lexaetal2010}.\ The second center is located along the southern
edge of ESB and is rimmed by andesite volcanic cones and effusive
complexes. Volcanic activity started at 14.5~Ma in a shallow marine
environment. This area is dominated by effusive andesite with complexes
which contain rhyollitic pumiceous tuff horizons, hyaloclastic
breccias, and hydromagmatic products.{\break} Later, the Tortonian
period (9.5~Ma) was related to the formation of large andesite
stratovolcanoes. The third and final center of volcanism is the
northern edge of ESB and partial accretionary wedge. The edge was
covered by products of the Vihorlat Mts. Volcanic Chain. The chain
consists of 5 large pyroxene andesite stratovolcanoes (12.5--9.1~Ma)
with pyroclastic rocks, dikes and sills.

The volcanic evolution of the ESB occurred alongside sedimentary,
tectonic and geodynamic events which were important in the Middle
Miocene \citep{Tomeketal1987, Kovacetal1989, Kovacetal1993,
Kovacetal1994, Markoetal1990, Markoetal1991, Fodoretal1999}. Crustal
bending and later crustal breaching was recorded not only in the
sediments, but also in the volcanic activity, and subduction. These
relationships emphasize the observable volcanic migration, not just
geographically, but also in the magmatic composition, from felsic to
intermediate alkaline rocks. The last stage of volcanism is linked to
the Vihorlat Mts. Chain. Here, the melting of steeply inclined crust
created stratovolcanoes on the border with the accretionary wedge, this
is linked to slab break-off. 

{\vspace*{-1pt}}

\subsection{Geodynamic aspects} \label{sec5.2}

{\vspace*{-1pt}}

The geodynamic factors in the subduction zone are linked to back-arc
extension and the rollback of the subduction hinge. Experiments
\citep{Faccennaetal2001, Schellartetal2003} have shown that rapid hinge
rollback is caused by an increase in buoyancy forces and by the
steepening of the subducting slab \citep{Halletal2003}. The total
dynamic conditions for the subducting slab are defined as a combination
of: (a)~surface kinematics, (b)~properties of the descending slab and
the (c)~effect of lateral mantle flow on the subducting slab
\citep{SdroliasMuller2006}.

\begin{figure*}
\includegraphics{fig15}
{\vspace*{.15pc}}
\caption{\label{fig15}Spatial timeline and architectural model. A view
of the interaction between upper crust forces which opened the Eastern
Slovak Basin, and ramp structures which developed as the result of
gradual increases in slab inclination, from west to east.}
{\vspace*{.15pc}}
\end{figure*}

The ESB evolution reflects specific conditions which are related to the
asymmetrical spreading of the Pannonian basin, which was predominantly
opened towards the NE. This opening was made possible by block
movement.\ The block movements caused the rotation of microplates, which
were linked to transpression along the MHFZ. This process involved
marginal peripheral areas, that were relatively tectonically
autonomous, and which were accelerated by subduction and slab
inclination changes. \looseness=-1

Relay ramps are a good ``litmus test'' for tracking geodynamic
processes in an extensional environment. Their structural evolution
sensitively records conditional changes and sedimentation. Current
published knowledge from the ESB has so far not included relay ramp
structures, or their importance for the tectonic structure and basin
evolution. New seismic data, models, and new deep boreholes for
hydrocarbon and geothermal prospection show the importance of relay
ramps. Even though their development mainly took place in the Middle
Miocene, they occur in the tectonic record younger periods, even up
until the present.\looseness=-1

The development of the relay ramp is closely tied to slab geometry
(Figure~\ref{fig15}), which has been often discussed
\citep{Jolivetetal2013, Lallemandetal2005, PapanikolaouRoyden2007,
Taymazetal2004}. In the Middle Miocene slab inclination start to change
with direct effect to increasing sedimentation (from Langhian to
Serravallian, 14--11~Ma). If we correlate the slab dip changes to the
present slab dips of the back-arc basins, we cannot find large
differences.

Basins which are dominated by back-arc spreading have steep dips larger
than 51{\textdegree}. The steep slabs were observed in more than 42\%
of the peripheral basins \citep{Lallemandetal2005}. These are
acceptable tectonic conditions for relay ramp development. A good
example is from the Aegean back-arc of the Hellenic subduction zone.\ 
This zone has a complex{\break} tectonic history, which describes the
geometry of slab retreat and fragmentation \citep{Jolivetetal2013,
PapanikolaouRoyden2007, Taymazetal2004}. The relay ramps were formed
paralel to major faults and later coincided with steepening of the
slab. Later they were modified into connected bridges between transfer
faults and were used as pathways for magma intrusion
\citep{Hooftetal2017}. Active seismicity and relay ramp geometry in the
Aegean basin are related to each other. The parts of the fault with the
highest normal fault displacement rate are seismically active, whereas
the opposite arm is relatively stable. This example is also applicable
in the ESB. The high subsidence area of the ESB is concentrated on the
SE edge of the basin which is currently the lowest point (98~m.a.s.l.)
in the whole of the Western Carpathians. This same point, where all
three main fault systems converge (NW--SE, N--S and NE--SW), is
associated with the last earthquake from October 2023{\break} $M =
5.1$. 

\looseness=-1
Lateral paleomantle flow on the subducting slab is difficult to
determine in the post active areas. However, it is possible to
interpret it from the paleoreconstruction of sedimentary and volcanic
activity, and their relationship to the Moho. Further relationships
arise when one compares the locations of the ESB's fault systems with
the thicknesses of the Moho depth map isolines \citep{Bieliketal2010,
Janiketal2011}. The Darn\'{o} Fault Zone follows an approximately 27~km
Moho depth isoline and bounds the southwestern limb of overheated
NE--SW Moho antiform structure. This structure continues towards the NE
section of the ESB and is parallel to the highest thermal flow area of
the Pannonian basin. The Slansk\'{e} Mts. located directly along the
N--S axial zone of the dome continuously preserves both mentioned
courses of the Moho dome. The semiplastic Moho model formed during
active rifting conditions was supported by simultaneously increasing
thermal flow, which resulted in the undulated shape of the thinned
Moho. The spatial relationships between the surface thermal flow
isolines, the main faults, and the Slansk\'{e} Mts. neovolcanic chain
connect the apical parts of undulated Moho morphology in the East
Slovakia region. This clearly indicates that these structures are
mutually interconnected. These relationships are especially sensible in
the Eastern Slovak Basin territory. Here, maximal thermal flow isolines
(which are linked to the parts of the region with the thinnest crust)
are spatially controlled by two NW--SE and N--S dominating fault sets.
It is worth mentioning that the max.---e.g., 120~{\textdegree}C thermal
flow isoline, located at the connecting point of the N--S and the
NE--SW faults, has an opposite---NW--SE course \citep{Jackoetal2021a}.

\section{Conclusions} \label{sec6}

The opening of a depositional space at the collision zone boundary, to
accommodate the Eastern Slovak Basin, was generated by the closure of
an accretionary wedge. The wedge formed a transpressional environment
which generated a boundary wrench, and tensile compensation structures.
The structures were later used as pilot faults for basin formation,
which were related to the subsequent deformation processes.

The transition from upper crust compression to upper crust extension
accelerated tilting progradation.\ The increased slab inclination
generated \mbox{fractures} which were parallel to the collision zone,
and which were perpendicular to the basin opening direction. This
significantly influenced the basin's subsidence, shape, and the
migration of the depositional environment. The core breakage, caused by
the transform fault parallel with Horn\'{a}d Fault System, was a
crucial moment for the later evolution of the basin. The N--S fault
direction is connected with a wedge of antithetic reader shears. Along
the fault linked to slab inclination volcanism, relay ramp development,
depositional changes, rapid and deep subsidence of the basin (up to
8~km) are observed.

The deep subsidence, up to 60~km in length, would not have been
possible without the existence of the relay ramp system. The four
evolutionary stages were controlled by crust mechanical conditions,
which were supported by thermal heating. The relay ramps created an
effective system of listric normal faults, where negative movement on
one side of the fault was compensated by uplift on the opposite side of
the fault (close to the accretionary wedge). This arm fault mechanism
is well-preserved up until now, as evidenced by the earthquake from
October 2023 $M = 5.1$.\looseness=-1

Relay ramp structural zonality in the basin is important for the
understanding of deep hydrocarbon and geothermal deposits.

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that could \mbox{benefit} from this article, and
have declared no affiliations other than their research organizations.

\section*{Funding}

This research was funded by the Slovak Grant Agency VEGA, grants
1/0585/20.

\CDRGrant[VEGA]{1/0585/20}

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