Neogene indentation of the Adriatic plate into Europe led to major
modifications of the Alpine orogenic structures and style of deformation in
the Eastern and Southern Alps. The Giudicarie Belt is a prime example of
this, as it offsets the entire Alpine orogenic edifice; its activity has
been kinematically linked to strike-slip faulting and lateral extrusion of
the Eastern Alps. Remaining questions on the exact role of this
fold-and-thrust belt in the structure of the Alpine orogen at depth
necessitate a quantitative analysis of the shortening, kinematics, and depth
of decoupling beneath the Giudicarie Belt and adjacent parts of the Southern
Alps. Tectonic balancing of a network of seven cross sections through the
Giudicarie Belt parallel to the local NNW–SSE shortening direction reveals
that this belt comprises two kinematic domains that accommodated different
amounts of shortening during overlapping times. These two domains are
separated by the NW–SE-oriented strike-slip Trento-Cles–Schio-Vicenza
fault system, which offsets the Southern Alpine orogenic front in the south
and merges with the Northern Giudicarie Fault in the north. The SW kinematic
domain (Val Trompia sector) accommodated at least ∼ 18 km of
Late Oligocene to Early Miocene shortening. Since the Middle Miocene, this
domain experienced at least ∼ 12–22 km shortening, whereas the
NE kinematic domain accommodated at least ∼ 25–35 km
shortening. Together, these domains contributed an estimated minimum of
∼ 40–47 km of sinistral strike-slip motion along the Northern
Giudicarie Fault, implying that most offset of the Periadriatic Fault is due
to Late Oligocene to Neogene indentation of the Adriatic plate into the
Eastern Alps. Moreover, the faults linking the Giudicarie Belt with the
Northern Giudicarie Fault reach ∼ 15–20 km depth, indicating a
thick-skinned tectonic style of deformation. These fault detachments may
also connect at depth with a lower crustal Adriatic wedge that protruded
north of the Periadriatic Fault and are responsible for N–S shortening and
eastward, orogen-parallel escape of deeply exhumed units in the Tauern
Window. Finally, the E–W lateral variation of shortening across the
Giudicarie Belt indicates internal deformation and lateral variation in
strength of the Adriatic indenter related to Permian–Mesozoic tectonic
structures and paleogeographic zones.
Introduction
The fold-and-thrust belt of the eastern Southern Alps formed due to
indentation of the Adriatic plate into the Eastern Alps (Schönborn,
1992, 1999; Picotti et al., 1995; Frisch et al., 1998, 2000; Castellarin and
Cantelli, 2000; Linzer et al., 2002; Rosenberg et al., 2007; Pomella et al.,
2011, 2012; Favaro et al., 2017). Indentation is defined as the
post-collisional penetration of a relatively stiff part of an upper plate
into a weaker, already existing orogenic edifice (e.g. Tapponier et al.,
1986). In the Alps, indentation modified the early Tertiary nappe structure
of the orogen (Schmid et al., 2004), including post-nappe folding and
exhumation of Alpine metamorphic units in the Tauern Window (Rosenberg et
al., 2018), eastward lateral extrusion of the Austroalpine nappes
(Ratschbacher et al., 1991; Rosenberg et al., 2007; Scharf et al., 2013), and
south-directed folding in the Southern Alps (Doglioni and Bosellini, 1987;
Castellarin et al., 2006b). The Giudicarie Fault, subdivided into northern
(NGF) and a southern (SGF) branches, is a prominent structure that sinistrally offsets
the Periadriatic Fault (PF) by ∼ 75 km in map view
(Fig. 1). It ends to the north at the western boundary of Neogene post-nappe
folding of the Tauern Window and lateral escape of the Eastern Alps (Frisch
et al., 2000; Linzer et al., 2002; Scharf et al., 2013). Previous studies
have established a direct kinematic link between sinistral motion along the
NGF and Neogene exhumation in the Tauern Window (Ratschbacher et al., 1989;
Frisch et al., 1998, 2000; Linzer et al., 2002; Rosenberg et al., 2007;
Scharf et al., 2013; Schmid et al., 2013; Handy et al., 2015; Favaro et al.,
2015, 2017; Hülscher et al., 2021). This complex response of the
orogenic crust to indentation calls for detachment of both sedimentary cover
and metamorphic units above one or more detachments located at or above the
Moho Discontinuity (Oldow et al., 1990). To the south, the Giudicarie
fold-and-thrust belt trends obliquely to the dominantly south-verging
thrusts in the Southern Alps and consists of (1) N–S-to-NNE–SSW-trending
thrusts and strike-slip faults and (2) E–W-trending thrusts (e.g. the Valsugana
Fault and Southern Alps orogenic front; Fig. 1). The question arises as to how
oblique sinistral shortening accommodated by the Giudicarie Belt is linked
to Neogene displacements on the Giudicarie Fault, the Periadriatic Fault, and
in the Tauern Window. In particular, the kinematic link of this belt to the
northern and southern segments of the Giudicarie Fault remains poorly
constrained and is the focus of this study.
(a) Tectonic provenance map of the Alps (Handy et al.,
2010 simplified from Schmid et al., 2004, 2008). The Southern Alps and
Dinarides are accreted units derived from the Adriatic plate (brown). The
Southern Alps are bounded to the north by the Periadriatic Fault (PF), which
is sinistrally offset by the Northern Giudicarie Fault (NGF). The PF is
divided into a western segment (Tonale and Canavese Lines, TL and CL,
respectively) and eastern segment (Pusteria-Gailtal Line, PG). (b) Geological map of our study area (Giudicarie Belt) in the eastern Southern
Alps indicated by a black square in (a) (TW is the Tauern Window). Fault abbreviations are as follows: BA stands for Bassano, CAL stands for Calisio,
FO stands for Foiana, MA stands for Marana, MC stands for Mezzocorona,
MO stands for Montello, MS stands for Monte Stivo, PA stands for Paganella,
SGF stands for Southern Giudicarie, SV stands for Schio-Vicenza,
TC stands for Trento-Cles, TH stands for Thiene-Bassano,
TR stands for Truden, TT stands for Tremosine-Tignale, VD stands for Val di
Sella, VS stands for Valsugana, and VT stands for Val Trompia.
The Giudicarie Fault also coincides with significant changes in the mantle
structure, as imaged by tomographic P-wave models that show a SE-dipping
slab anomaly beneath the Central Alps and a steeply dipping enigmatic slab
anomaly beneath the Eastern Alps (Lippitsch et al., 2003; Mitterbauer et al.,
2011; Karousová et al., 2012). The model of Lippitsch et al. (2003) shows
that the slab anomaly beneath the Eastern Alps dips towards the NNE, which
would suggest a subduction of the Adriatic plate, with a length varying from
50 km in the west to 210 km in the east. However, the exact dip, length, and plate
tectonic affinity (Adriatic, European, or mixed) of this slab anomaly remains
under strong debate (Mitterbauer et al., 2011; Handy et al., 2015;
Qorbani et al., 2015; Kästle et al., 2020). Any amount of Adriatic
subduction should be reflected in both Neogene shortening estimates across
the eastern Southern Alps and Neogene estimates of lateral extrusion of the
Eastern Alps. Proposed estimates for the amount of shortening across the
Giudicarie Belt and Southern Alps range from 30 to 100 km (Laubscher, 1990;
Doglioni, 1992; Roeder, 1992; Schönborn, 1992, 1999; Picotti et al.,
1995; Castellarin and Cantelli, 2000; Nussbaum, 2000). This wide range
reflects the complexity in the kinematics and strain partitioning between
thrusts and strike-slip faults within the Giudicarie Belt. Therefore, this
study aims to provide new constraints on the kinematics of shortening within
the Giudicarie Belt, as well as on the depth of deformation within the
Adriatic indenter, to discuss its relationships to Neogene exhumation in the
Tauern Window and lateral extrusion in the Eastern Alps. To do so, we first
review the Permian to Jurassic pre-existing structures and paleogeography of
the eastern Southern Alps that affect the kinematics and partitioning of the
deformation related to the Alpine Orogeny (Sect. 2). Structural observations
on the field (Sect. 3) and deformation ages (Sect. 4) along the Giudicarie
belt are then used to construct a series of balanced geological
cross sections (Sect. 5). Based on the calculated amount of shortening, we
propose a kinematic sub-division of the Giudicarie Belt (Sect. 6) and
discuss the relationship between Neogene shortening along the Giudicarie
Belt and Neogene motion along the NGF, a topic which has fascinated many
researchers in the past (Castellarin and Vai, 1981; Picotti et al., 1995;
Prosser, 1998, 2000; Müller et al., 2001; Viola et al., 2001; Linzer et
al., 2002; Stipp et al., 2004; Pomella et al., 2011, 2012).
Geological settingThe eastern Southern Alps
The Southern Alps are a predominantly south-vergent fold-and-thrust belt, at
the leading edge of the Adriatic plate indenting the European plate to the
north. The Giudicarie Fault is the boundary between eastern and western
parts of the Southern Alps (Schönborn, 1999; Castellarin and Cantelli,
2000). In the Late Cretaceous, the tectonic regime shifted in the Alps from
E–W extension to N–S shortening due to convergence between Adria and Europe
(e.g. Dewey et al., 1989). The western Southern Alps record a Late
Cretaceous compressional phase (documented by the so-called Lombardian
Flysch, e.g. Doglioni and Bosellini, 1987; Bernoulli and Winkler, 1990; as
well as thrusting that pre-dates the Paleogene Adamello intrusive body; see
Brack, 1981) followed by Oligocene to Middle Miocene shortening (e.g.
Schönborn, 1992; Schmid et al., 1996). Only the western Southern Alps
have recorded this Late Cretaceous deformation (Castellarin et al., 1992),
as within the eastern Southern Alps evidence of this phase is lacking. The
eastern Southern Alps comprise actively accreting cover units of the
Adriatic plate and are bounded to the north by the PF, to the west by the
NGF and SGF, to the east by Dinaric thrusts and active strike-slip faults,
and to the south by the Po Plain (Fig. 1). Paleostress analyses (Castellarin
et al., 1992; Castellarin and Cantelli, 2000; Zampieri et al., 2003) and
structural analysis of the NGF (Prosser, 1998, 2000; Viola et al., 2001)
show that during the Middle to Late Miocene, the eastern Southern Alps were
dominantly affected by NNW–SSE shortening, which reactivated many
Permo-Jurassic faults. This Neogene NNW–SSE shortening was preceded by a
minor phase of Eocene E–W extension related to the opening of a volcanic
graben, which was only recorded locally in the southernmost part of the
Trento Platform (Zampieri, 1995).
Directly north of the Giudicarie Belt and Periadriatic Fault, nappes in the
Tauern Window were affected by kilometre-scale upright folding and orogen-parallel
extrusion towards the Pannonian Basin in Miocene time (Ratschbacher et al.,
1991; Frisch et al., 1998; Scharf et al., 2013). The northern Alpine front
west of Munich (northwest of the Tauern Window) was active until the
Pliocene (Von Hagke et al., 2012), whereas east of Munich it ceased
propagating northwards after ∼ 20 to 15 Ma (Ortner et al.,
2015). This cessation coincides broadly with the onset of sinistral
transpression along the Giudicarie Belt, as well as south-directed thrusting
in the eastern Southern Alps (Castellarin and Cantelli, 2000; Schmid et al.,
2013, and references therein). During this time, rapid exhumation of Penninic
and Subpenninic nappes in the Tauern Window commenced (Fügenschuh et
al., 1997), although slower exhumation in the Tauern Window may already have
started earlier, in the late Oligocene time (Pomella et al., 2012; Scharf et
al., 2013; Favaro et al., 2015). Taken together, these events have been
interpreted to indicate that Adriatic indentation triggered the
orogen-parallel extrusion of the Eastern Alps towards the Pannonian Basin
(Ratschbacher et al., 1989; Frisch et al., 1998, 2000; Linzer et al., 2002;
Rosenberg et al., 2007; Favaro et al., 2017). Contrasting estimates have
been proposed for the amount of lateral extrusion, ranging from 14–62 km
(Rosenberg and Garcia, 2011; disputed by Fügenschuh et al., 2012), 65–77 km (Favaro et al., 2017), 100 km (Scharf et al., 2013), to as much as 120 km
(Linzer et al., 2002) and 160 km (Frisch et al., 2000).
The NGF and its northern prolongation, the Meran Mauls Fault, sinistrally
offset the PF by 75 km, dividing the PF into the Tonale Line (western
segment) and the Pusteria-Gailtal Line (eastern segment; Fig. 1). Discussion
has focused on whether the NGF is an exclusively Neogene fault that
sinistrally offsets a formerly straight PF, or if the NGF formed a
pre-Neogene bend of the PF that was reactivated under transpression during
the Neogene (Castellarin and Vai, 1981; Picotti et al., 1995; Prosser, 1998,
2000; Müller et al., 2001; Viola et al., 2001; Linzer et al., 2002;
Stipp et al., 2004; Pomella et al., 2011, 2012). The latter hypothesis is
supported by Picotti et al. (1995), who estimated that the amount of
strike-slip (30–40 km) along the NGF, extrapolated from the amount of
Neogene shortening calculated within the southwestern part of the Giudicarie
Belt, was less than the 75 km offset of the PF. In addition, several studies
(e.g. Doglioni and Bosellini, 1987; Castellarin et al., 2006b) have
interpreted the lateral termination of the Cretaceous pre-Adamello
deformation phase along the NGF, as a late Cretaceous lateral ramp. However,
the interpretation of a pre-Neogene offset PF is incompatible with the
proposed 100–150 km of Paleogene dextral strike-slip along this fault
(Laubscher, 1991; Schmid and Kissling, 2000) based on offset correlative
features on either side of it, i.e. the Ivrea and Pejo zones (Laubscher,
1991) and the Sesia-Dent Blanche and Margna units (Laubscher 1991; Schmid et
al., 1996; Handy et al., 2005). In addition, a paleomagnetic study of
Pomella et al. (2011) convincingly shows that the Periadriatic granitoid
intrusions along the NGF underwent anticlockwise rigid-body rotation into
their current NNE–SSW-striking orientation in Neogene time, suggesting the
NGF can be interpreted as a rotated segment of the PF (Pomella et al., 2011,
2012). A paleomagnetic study across the Southern and Eastern Alps
possibly indicates a larger-scale coherent Oligocene to Miocene rotation
rather than a local reorientation (Thöny et al., 2006). Potential links
between Neogene shortening in the Giudicarie Belt and strike-slip motion
across the NGF and adjacent Tonale and Pusteria-Gailtal fault segments are
therefore of great relevance for a better understanding of Neogene collision
processes in the Eastern and Southern Alps.
To assess the amount of Neogene shortening within the eastern Southern Alps,
it is important to constrain the role of inherited Permian to Mesozoic
structures. Indeed, many of the Neogene fault structures were inherited from
earlier tectonic events, which include Permian to Jurassic rifting phases
(Handy, 1987; Handy and Zingg, 1991; Bertotti et al., 1993; Selli et al.,
1996; Picotti and Cobianchi, 2017; Le Breton et al., 2021).
Permian to Jurassic paleogeography
Based on Permo-Mesozoic stratigraphic variations, we divide our study area
into six different paleogeographical zones (Fig. 2) which, as we show below,
exert important controls on the partitioning of Neogene shortening. Within
the eastern Southern Alps, the Atesina Volcanic Complex (Fig. 2) is a good
example of a Permian basin filled with volcanic sediments, with the
Valsugana Fault interpreted as a dextral Permian feature (Zampieri et al.,
2003) and the Calisio and Truden faults recording a Permian normal faulting
history (Selli et al., 1996). Another minor unit in the eastern Southern
Alps containing evidence of this interpreted Permian transtensional phase
(Zampieri et al., 2003) is exposed in the hanging wall of the Foiana Fault
(Fig. 1).
Triassic to Jurassic paleographic basinal (1, Lombardian
Basin; in violet) and platform zones (2, Trento Platform in blue). Based on
Mesozoic thickness variations, the Trento Platform is subdivided into units
2a–d. Note that within zone 2c no Mesozoic sediments are exposed and the
thickness of the eroded Mesozoic sequence is extrapolated from zone 2b,
based on the occurrence of Miocene conglomerates with Mesozoic clasts
unconformably overlying Mesozoic strata in zone 2d. Zone 2 also includes a
Cenozoic sequence unconformably overlying Mesozoic strata outcropping at
Monte Brione (see discussion Sect. 4). Simplified
stratigraphic columns (lower panel) for each paleogeographic zone 1–2 d.
From the Late Triassic to Jurassic, a rifting related to the opening of the
Alpine Tethys west and north of Adria (Piemont-Liguria Ocean) affected the
Adriatic plate (e.g. Handy, 1987; Handy and Zingg, 1991; Bertotti et al.,
1993; Picotti and Cobianchi, 2017; Le Breton et al., 2021). The rifting of
Alpine Tethys led to the formation of carbonate platforms and basins bounded
by N–S-trending faults that are still well preserved within the Southern
Alps (Bernoulli and Jenkyns, 1974; Winterer and Bosellini, 1981). The main
N–S-trending faults divided the area into four different zones from west to
east (Bernoulli and Jenkyns, 1974; Winterer and Bosellini, 1981): the
Lombardian Basin, the Trento Platform, the Belluno Basin, and the Friuli
Platform (Fig. 2). The Lombardian Basin recorded the most Triassic to
Jurassic subsidence (Winterer and Bosellini, 1981; Bertotti et al., 1993),
reaching ∼ 5 km of sedimentary thickness, and is bounded in
the east by the Ballino-Garda and Tremosine-Tignale Faults (Figs. 1 and 2).
Differential subsidence caused significant stratigraphic thickness
variations within the Jurassic paleogeographic zones (Picotti et al., 1995).
Within the Trento Platform, thicknesses decrease from ∼ 3 to
∼ 2 km both towards the SE and NE, dividing the Trento Platform
itself into sub-zones: the Trento Platform (numbered 2 on Fig. 2), the
Reduced Recoaro High (2a), the Reduced Trento Platform (2b), the Atesina
Volcanic Complex (2c), and the Asiago Plateau (2d). These thickness
variations broadly coincide with Neogene faults, including the Trento-Cles
Fault, Truden Fault, Calisio Fault, Valsugana Fault, and Schio-Vicenza Fault
(Figs. 1 and 2). The Atesina Volcanic Complex forms the most prominent
Permian paleogeographical zone with a 2 km thick Permian sequence, with no
overlying Triassic to Cretaceous sedimentary cover outcropping (Fig. 2). It
is assumed that the Atesina Volcanic Complex (zone 2c) had a similar
Mesozoic cover to the nearby and paleogeographically comparable Reduced
Trento Platform (2b) (Selli, 1998), which was later eroded in the Miocene.
This assumption is based on the observation of Mesozoic clasts found in
Miocene conglomerates in the footwall of the Valsugana Fault (Castellarin et
al., 1992). Another paleogeographic zone containing thinner Permian volcanic
successions (up to ∼ 1 km thick; Fig. 2) is the Reduced Trento
Platform. Further small-scale thickness variations across Mesozoic faults
occur within the paleogeographic zones defined here (Franceschi et al.,
2014). However, these are beyond the aim of this paper and are neglected in
order to simplify the construction and balancing of cross sections (Sect. 5).
Structural observations and fault-slip analysis
Structural data from the eastern Southern Alps were collected as input for
fault-slip analysis to calculate the main Neogene shortening direction. This
was necessary to estimate the amount of shortening parallel to the
shortening direction in the balanced cross sections. Special attention was
paid to fault systems coinciding with paleographic variations (the
Trento-Cles, Calisio, Valsugana, and Schio-Vicenza Faults; Figs. 1, 2, 3 and
4) to test if these features coincide with variations in structural style
and amount of shortening.
(a) Steepened Lower Cretaceous strata (Majolica),
which are commonly found in the forelimb of the Monte Grappa ramp anticline
forming the hanging wall of the blind Bassano Thrust. The dashed black lines
indicate axial planes of chevron folds (formed due to flexural slip). (b) The Marana Thrust at Zanconati emplaces Norian Dolomia Principale on Lower
Jurassic Calcari Grigi. (c) The Schio-Vicenza Fault at Passo della Borcola,
offsetting Norian Dolomia Principale and Eocene mafic dykes. Striations on
fault surfaces indicate oblique sinistral down-dip motion. (d) Older
striations on the Borcola Pass Fault Zone (BPFZ; after Fondriest et al.,
2012) indicating a sinistral sense of motion. (e) Younger striations on the
BPFZ indicating oblique downthrow of the NE block of the BPFZ. (f) Dextral
strike-slip slickenfibres at the eastern end of the hanging wall of the
Valsugana Fault (near Pieve di Cadore) where the Valsugana Fault branches
into several fault splays and becomes transpressive. The uninterpreted field
photographs can be found in Verwater et al. (2021).
Structural map of the eastern Southern Alps showing the
distribution of fault slip data from this study (in red; locations a–e
indicated by stars on the map) and from the literature (in black). The
geological map is a compilation of own field data and published geological
maps (Bartolomei et al., 1969; Bosellini et al., 1969; Castellarin et al.,
1968, 2005; Braga et al., 1971; Dal Piaz et al., 2007; Avanzini et al.,
2010). See Fig. 1 for fault abbreviations. The field data
include fold axes plotted on their fold axial planes for the Bassano Fault (b) and fault-slip analyses with local paleostrain directions for the Marana (a), Valsugana (f), and Borcola (c–e) faults. Two distinct stereoplots are
shown for Borcola as two generations of striations are observed here (see
discussion in Sect. 3). Regional (Castellarin and Cantelli, 2000; Zampieri
et al., 2003) and local (Prosser, 1998, 2000; Viola et al., 2001)
paleostrain analyses from the literature are indicated in black. The data
indicate a dominant NNW–SSE shortening direction.
Structural analysis in the Giudicarie-Valsugana sector of the eastern
Southern Alps included measuring fault surfaces, their striations and
shear-sense, fold-axial planes, and bedding planes near folds and
faults. These data formed the input for fault-slip analysis, which was
performed in the WinTensor software version 5.8.8 (Delvaux and Sperner,
2003). An angle of 30∘ between the maximum principal stress and
the shear plane based on the Mohr–Coulomb failure criterion was assumed.
However, given the proximity of the measured fault striations to major fault
structures (Fig. 4), this angle might have been different due to strain
complexities. The maximum principal stress 1σ was calculated in the
WinTensor software for various fault systems, assuming that structures
measured in the field reflect the principal incremental strain direction and
principal stress direction 1σ. The final stereoplots with results of
the fault-slip analysis calculated with WinTensor were drawn using FaultKin
8.0.0 software (Fig. 4; Marrett and Allmendinger, 1990).
Bedding and striation measurements (Fig. 3) were collected along the Bassano
(Fig. 3a) and Marana (Fig. 3b) thrust faults on either side of the
Schio-Vicenza Fault (Fig. 3c, d and e) to test for variations in
structural style. The Schio-Vicenza Fault represents an inherited Mesozoic
feature associated with stratigraphic thickness variations (Fig. 2). Along
the Borcola Pass Fault zone, a branch of the Schio-Vicenza Fault, steeply
dipping NW–SE-to-N–S-trending fault surfaces containing sinistral striations
cut Eocene mafic dikes (Fig. 3c and d). Striation steps indicating
sinistral motion related to local NNW–SSE incremental strain (Fig. 3d)
parallel to the main shortening direction (Castellarin and Cantelli, 2000)
indicate that the Schio-Vicenza Fault was reactivated during post-Eocene
time. These striations are overprinted by younger striations indicating
downthrow of the NE block (Fig. 3e) that are interpreted to be related to
flexural extension of the foreland bulge of the Apennines (Pola et al.,
2014). Contrasting deformation styles east and west of the fault may
indicate that the Schio-Vicenza Fault acted as a transfer fault. Indeed,
directly east of the Schio-Vicenza Fault, strata in the hanging wall of the
Bassano Fault (Fig. 1) are steeply dipping, strongly folded, and form the
forelimb of the ENE–WSW-trending Monte Grappa ramp anticline (Fig. 3a),
whereas to the west, the Marana thrust emplaces cataclastically deformed
Norian Dolomia Principale on Hettangian to Pliensbachian Calcari Grigi (Fig. 3b). We note that in the hanging wall of the Marana thrust, a deeper
structural level is exposed than in the hanging wall of the Bassano Fault,
implying that the observed variation in structural style may partly be due
to a different erosional level. Variability in the geometry at depth of the
Bassano and Marana Faults (see discussion in Sect. 5.2) are the cause of the
observed contrasting structural styles. The deeper level of exposure along
the Marana thrust and the southern part of zone 2a (Fig. 2) has been
interpreted to indicate that this area represents a structural high
(prolongation of the Adige embayment in Laubscher, 1990). No thrust
associated with the Monte Grappa ramp anticline is exposed; the anticline is
therefore interpreted as a fault-propagation fold above the blind Bassano
Fault, in agreement with earlier studies (Doglioni, 1990; Roeder, 1992;
Pilli et al., 2012; Pola et al., 2015). The Schio-Vicenza Fault is only
observed as far north as the Adige Valley. However, previous studies argued
for either a continuous Trento-Cles–Schio-Vicenza (TC-SV) fault system
(Semenza, 1974) or a contractional strike-slip step-over, with the Calisio
Fault connecting the Trento-Cles and Schio-Vicenza Faults (Zampieri et al.,
2003, 2021). The TC-SV fault system will be further
discussed in Sects. 5 and 6.
Strike-slip deformation along the Schio-Vicenza Fault ends towards the NW in
the vicinity of the Valsugana Fault (Fig. 1). This fault was reactivated as
a Neogene thrust and emplaced Variscan basement onto Mesozoic sediments of
the Trento Platform in its western part and Triassic onto Jurassic strata in
the east. Striations in the field measured along faults conjugate to the
main trend of the Valsugana show thrust motion with components of dextral
(Fig. 3f) and sinistral strike-slip. Fault-slip analysis on these striations
(Fig. 4) is consistent with NNW–SSE shortening, exactly perpendicular to the
main Valsugana Fault trend. Therefore, we interpret that there was no
significant component of Neogene strike-slip motion along the Valsugana
Fault. The lateral ramp of the Valsugana Fault is the Calisio Fault (Fig. 1), which merges with the Trento-Cles Fault, thereby inverting a major
Permian paleographic boundary (Fig. 2). Striations from the Valsugana,
Marana, Bassano, and Schio-Vicenza faults; ENE–WSW-trending axial planes and
axes of the Monte Grappa ramp anticline; and striations in the footwall of
the Marana Fault and results from a literature compilation (Fig. 4) are all
consistent with NNW–SSE shortening.
Age of deformation in the eastern Southern Alps
Individual thermochronological dated faults within the eastern Southern Alps
include the NGF, SGF, Valsugana, and Val Trompia faults, which are broadly
coeval with stratigraphic age constraints indicating Neogene deformation
(Fig. 5). Deformation in the eastern Southern Alps initiated during late
Oligocene time (27.8–23.0 Ma), indicated by a detailed seismic survey
beneath the Po Plain that documented Chattian to Burdigalian deposits
affected by Late Oligocene to Neogene thrusts sealed by Messinian strata
(e.g. Fantoni and Franciosi, 2010, and references therein).
Compilation of stratigraphic and AFT age data in the
eastern Southern Alps from Luciani and Silvestrini (1996), Martin et al. (1998), Castellarin and Cantelli (2000), Viola et al. (2001), Stipp et al. (2004), Pomella et al. (2011, 2012), and Heberer et al. (2016). Purple boxes
mark biostratigraphic ages of deformed sediments (Castellarin et al., 1992).
The inset map outlines the Adamello-Giudicarie area at the junction of the
NGF and TC faults, with the locations of many tightly spaced age data. See
Fig. 1 for fault abbreviations.
A maximum age for the onset of thrusting within the Giudicarie Belt itself
is given by the youngest sediments below a thrust that accommodated NNW–SSE
shortening. At Monte Brione, at the northern end of Lake Garda (Fig. 1), the
base of an arenitic–pelitic sequence dated at 21.5 Ma (Luciani and
Silvestrini, 1996) shows westward divergent on-laps suggestive of growth in a
footwall syncline (Castellarin et al., 2005). Directly east of Monte Brione,
similar stratigraphic age constraints are found within deformed sediments of
28–16 Ma in the footwall of the Monte Stivo thrust (Fig. 5). Yet further to
the east in the footwall of the Valsugana Fault, deformed Serravallian
(∼ 14–12 Ma) sediments date south-directed thrusting
(Castellarin et al., 1992; Selli, 1998). Similar ages (17–9 Ma) were
obtained with apatite fission tracks from the hanging wall of the Valsugana
Fault and several faults along the Giudicarie Belt (Pomella et al., 2012;
Heberer et al., 2016). This main phase of Neogene shortening has been termed
the Valsugana Phase and was coeval with shortening along the Giudicarie
Belt (Castellarin et al., 1992; Castellarin and Cantelli, 2000). Several
former Mesozoic structures were reactivated during the Valsugana phase,
including the Schio-Vicenza Fault (Fig. 2). The final pulse of exhumation of
the Valsugana phase occurred between 10 and 8 Ma, as shown by a study of
Reverman et al. (2012) using (U-Th-Sm)/He and fission track thermochronology
on apatites from the Adamello batholith.
A younger phase of Messinian to Pliocene shortening has been recognized
along the Bassano and Montello faults of the Southern Alpine orogenic front
(Massari et al., 1986; Castellarin et al., 1992). Late Tortonian to Early
Pliocene fossils (Dal Piaz, 1912; Massari et al., 1986) are found in
mudstones interbedded with conglomerates in a syn-tectonic wedge in the
hanging wall of the Bassano and Montello Faults (Massari et al., 1986).
Historical seismicity (e.g. Galadini et al., 2005) and clustering of
(micro-)seismic hypocentres (Romano et al., 2019; Anderlini et al., 2020;
Jozi Najafabadi et al., 2021) along these fault systems and morphological
evidence of fault surface rupture (Moratto et al., 2019; Romano et al.,
2019) indicate that this younger phase of shortening is still active.
The Adamello granitoids, the Variscan basement, and overlying Permian
volcanics are better suited for geochronological studies, as reflected in
the clustering of age data within these rock units (Fig. 5). Early to Late
Miocene thermochronological ages along the NGF (Viola et al., 2001; Pomella
et al., 2011, 2012) and within the Val Trompia sector of the Giudicarie Belt
(Heberer et al., 2016; VT in Fig. 5) fall within the same age interval as
the stratigraphic constraints above, indicating the coeval evolution of the
NGF and Giudicarie Belt.
In addition, pre-Oligocene ages of shortening have been reported in the
eastern Dolomites (Doglioni and Bosellini, 1987) based on similarities with
Dinaric thrust directions and Oligocene deposits unconformably overlying
Mesozoic strata (Doglioni and Bosellini, 1987; Keim and Stingl, 2000).
However, these faults only occur east of our study area.
West of the SGF, a pre-Adamello phase of shortening is recognized along the
Val Trompia sector of the Giudicarie Belt (Brack, 1981; Doglioni and
Bosellini, 1987; Picotti et al., 1995; Castellarin et al., 2006b). Relative
age constraints are provided here by the observation that the Gallinera
Fault is cross-cut by the western side of the Adamello batholith (Brack,
1981; Picotti et al., 1995; Castellarin et al., 2006b), which indicates a
pre-Adamello (possibly Cretaceous) age of deformation. However, within our
study area east of Adamello, no such indications for pre-Adamello
deformation can be found. This suggests the presence of a Cretaceous lateral
ramp between the western and the eastern Southern Alps (Doglioni and
Bosellini, 1987; Castellarin et al., 2006b).
Shortening estimates and depth of detachment in the eastern Southern AlpsAssumptions for structural analysis and cross section balancing
All kinematic models of the Giudicarie Belt agree that deformation involved
Neogene transpression with strain partitioning between strike-slip and
thrust faults (Picotti et al., 1995; Prosser, 1998, 2000). To obtain
reliable estimates of shortening across the belt, it is necessary to account
for rock volumes moving into and out of 2D section traces. Seven cross
sections were constructed parallel to the NNW–SSE Neogene direction of the
principal shortening direction obtained from fault-slip analysis above (Fig. 3e). These cross sections straddle the Schio-Vicenza, Trento-Cles, and
Ballino-Garda faults (Fig. 6), which, as we have shown above, coincide with
the main boundaries of paleogeographic zones (Fig. 2). In addition, seven
perpendicular (orogen-parallel) cross sections were constructed to assess
stratigraphic variations across these faults (Fig. 6). Stratigraphic and
tectonic contacts were projected into the sections using structural contours
constructed from published geological maps (Bartolomei et al., 1969;
Bosellini et al., 1969; Castellarin et al., 1968, 2005; Braga et al., 1971;
Dal Piaz et al., 2007; Avanzini et al., 2010). In addition, publicly
available borehole data for the southern part of the studied area beneath
the Po Plain (Paese, Scaldaferro, Travettore and Villaverla wells;
VIDEPI-database: https://www.videpi.com/videpi/videpi.asp, last access: 11 May 2021) constrain the thickness of the sedimentary cover. Thickness
variations within the paleogeographic zones of Fig. 2 are neglected and only
variations across these major paleogeographic boundaries are incorporated in
the cross sections. Thicknesses within the profiles were extrapolated to
depth and above the current erosional level using the kink-band and
dip-domain methods (Boyer, 1986).
Traces of cross sections constructed and balanced in this
study. Details of profile reconstructions for profiles 1, 5, and 6 (thick
black lines) are shown on Figs. 7, 8, and 9, respectively.
Profiles not shown in Figs. 7, 8, 9, and 10 are available in
Verwater et al. (2021). Abbreviations are as follows: BG stands for Ballino-Garda Fault,
PF stands for Periadriatic Fault, SV stands for Schio-Vicenza Fault, and TC stands for Trento-Cles
Fault.
Line-length balancing was performed with the Move software package (provided
by Petex) using the flexural slip algorithm for unfolding the horizons. We
assumed a flat top Cretaceous horizon and used this as a marker to estimate
the fault displacements. This is admittedly a simplification given the
lateral facies and thickness variations in Cretaceous strata (Doglioni and
Bosellini, 1987). Modelled fault geometries at depth were tested using a
trial and error, forward modelling approach until a best fit with the
present-day surface geology was found. We assumed that all the observed
folds along the profiles are fault-related and employed a fault-bend-fold
algorithm (Suppe, 1983) in Move. This algorithm treats faults as brittle,
discrete discontinuities, even at depths below ∼ 10 km where
ductile behaviour is expected in natural systems. The fault-bend-fold
algorithm maintains line length and area in forward modelling, in contrast
to its alternative, fault-parallel flow, which only maintains line length
when a correct angle of angular shear is specified. We avoided
fault-parallel flow modelling because this angle is not well constrained in
the studied area. It is inferred that the Northern Giudicarie Fault (NGF)
forms the backstop of the south-vergent Neogene deformation of the eastern
Southern Alps and that all interpreted detachments are connected at depth
with the NGF. This is in line with the absence of any significant Neogene
deformation in the Austroalpine basement to the NW of the NGF. It is clear
that sinistral strike-slip motion of up to 75 km along the NGF itself was
coeval with thrusting in the Giudicarie Belt (e.g. Pomella et al., 2011),
but transpressional movement cannot be incorporated in the
fault-bend-folding algorithm of Move. Thus, in forward modelling of all
cross sections, the NGF is progressively offset as a passive marker along
the fault detachments (Figs. 7, 8 and 9).
(a) Profile 1 across the Lombardian Basin and Trento
Platform. (b) Profile 1 retro-deformed to a layer-cake stratigraphy. (c) Results of forward modelling. Colours in the legend are modified following the
International Chronostratigraphic Chart, with the pre-Permian basement beneath in
white. Black dots indicate earthquake hypocentres during 2017–2018 (SWATH-D
Network; Jozi Najafabadi et al., 2021; vertical bars indicate the average
error range on hypocentre depths), projected from a swath covering 6 km on
either side of the profile. Dashed black lines below ∼ 10 km
indicate ductile deformation.
(a) Profile 5 across the Trento Platform and its
structural Recoaro High. (b) Profile 5 retro-deformed to a layer-cake
stratigraphy. (c) Results of forward modelling. Colours in the legend are modified
following the International Chronostratigraphic Chart, with the pre-Permian basement
beneath in white. Black dots indicate hypocentres of seismic events during
2017–2018 (SWATH-D Network; Jozi Najafabadi et al., 2021; vertical bars
indicate the average error range on hypocentre depths), projected from a
swath covering 6 km on either side of the profile. Dashed black lines below
∼ 10 km indicate ductile deformation.
(a) Profile 6 across the Reduced Trento Platform, Atesina
Volcanic Complex, and the Asiago Plateau (AFT age for Valsugana Fault from
Heberer et al., 2016). (b) Profile 6 retro-deformed to a layer-cake
stratigraphy. (c) Results of forward modelling. Colours in the legend are modified
following the International Chronostratigraphic Chart, with the pre-Permian basement
beneath in white. Black dots indicate hypocentres of seismic events during
2017–2018 (SWATH-D Network; Jozi Najafabadi et al., 2021; vertical bars
indicate the average error range on hypocentre depths), projected from a
swath extending to 6 km on either side of the profile. Dashed black lines
below ∼ 10 km indicate ductile deformation.
The 3D Move database, including all 14 cross sections, as well as
descriptions of selected cross sections 1, 5, and 6 and details on the forward
modelling approach, are available in Verwater et
al. (2021; see data availability section below). Regardless of the balancing
methods employed here, all estimates of shortening must be regarded as
minima. There are several basic reasons for this: (1) the cutoff lines of
strata in the hanging wall of many thrusts have been eroded, such that
markers for determining the full amount of displacement are lacking. This is
particularly true of the highest detachments in a thrust system, where erosion
is greatest. (2) Many detachments in the deeper levels of thrust systems are
“blind”, i.e. the displacement at their tips tends to zero at depths
where constraints on the geometry of their propagation structures (e.g.
folds) are poorly constrained or even unconstrained. This is particularly the case where
boreholes or seismic information are unavailable, as in the foreland of some
of the thrust systems reported below. (3) In extreme cases, thrust systems
can “cannibalize” their own deepest detachments, removing them to depths
beyond detection with available seismic information. This type of tectonic
erosion, especially in basement units with low impedance contrasts, will
always lead to an underestimation of shortening. (4) Out-of-section motion,
for example the sinistral strike-slip component in the Giudicarie Belt,
necessarily renders 2D areal and line balancing inadequate for determining
the true amount of shortening. (5) Alternative structural interpretations
involving duplexes and/or triangle zones at depth increasing the amount of
shortening (see discussion in Sect. 5.2 and Verwater et al., 2021). We have attempted to compensate for this innate
bias by balancing shortening in the aforementioned network of closely spaced
cross sections, and in fact we use the differences in minimum shortening
estimates to judge the degree of transpression within the Giudicarie Belt
(Sect. 6 below). Despite these limitations, the minimum shortening estimates
allow us to place a lower limit on the amount of shortening parallel to the
bulk shortening direction within the Giudicarie Belt.
Results of shortening estimates from cross section balancing
Amounts of shortening in orogen-normal profiles vary laterally along the
Giudicarie Belt (Fig. 6). To the southwest (profiles 1 and 2), balancing
yield 18–22 km of shortening. Similarly, to the northeast (profiles 6 and
7), shortening estimates range from 17 to 25 km. However, shortening is only
8 to 12 km in the central part of the Giudicarie Belt (profiles 3, 4, 5).
The largest variation in shortening occurs between profiles 5 (11 km) and 6
(25 km) across the TC-SV fault system (Figs. 6, 8 and 9). We infer a
dominantly thick-skinned style of deformation, with detachments reaching
down to ∼ 15 km depth based on exposure of basement in the
hanging wall of steep thrusts, recent seismicity clustering at depths of 10
to 20 km (Viganò et al., 2015; Jozi Najafabadi et al., 2021) and forward
modelling in Move (Figs. 6, 7, 8 and 9; Verwater et al., 2021).
These minimum shortening estimates are fairly well constrained, as the fault
displacement observed at the surface provides the most important constraint
for the amount of shortening, and uncertainties of fault structures at depth
have errors of 2–3 km (see discussion in Verwater et al., 2021). Although the
Giudicarie Belt exposes several long décollements within the sedimentary
cover (e.g. Tremosine-Tignale, Paganella and Tosa Faults; Figs. 7 and 8),
field observations show that, with the exception of the Mezzocorona
antiformal stack along profile 6 (Fig. 9a), there are no large-scale
duplexes exposed. This suggests that kinematic models with several putative
duplexes at depth are inconsistent with field observations. However, the
geometries in the footwall of the Paganella (profile 5; Fig. 8) and
Valsugana Faults (profile 6; Fig. 9) allow for alternative interpretations
in which possible duplexes at depth are linked with shortening at the
surface. These duplexes would increase shortening along profiles 5 and 6 by
∼ 2 to 3 km (see alternative interpretations of profiles 5 and
6 in the data repository of Verwater et al., 2021). The 3 km uncertainty in
shortening along profile 5 is 27 % of the total minimum amount of
shortening observed along this profile and can also be taken as a percentage
of uncertainty in the shortening estimates of the other profiles (Fig. 6).
The presence of eroded hanging wall cutoffs along our profiles (Figs. 7, 8
and 9) could yield additional errors in our shortening estimates, further
increasing the 27 % uncertainty. In addition, the clustering of seismic
events along modelled fault structures in the basement propagating southward
in profiles 1, 5, and 6 (Figs. 7, 8, and 9; Jozi Najafabadi et al., 2021) is
consistent with thick-skinned tectonics featuring ramps that root in the
basement units.
Forward modelling of fault detachments at depth
Sub-surface faults in profiles 1–7 (Figs. 7, 8, 9 and 10) were determined
from forward modelling of an initial undeformed state involving layer-cake
stratigraphy (Figs. 7b, 8b and 9b). This assumption is necessary for
balancing, though in light of the aforementioned Mesozoic rifting in this
area (Sect. 3), layer-cake stratigraphy is only an approximation of the
pre-Neogene structure. All the NNW–SSE profiles were tested with one, two,
or three basement nappe slices involved in thrusting. Within the Lombardian
Basin and Trento Platform along profiles 1 and 5, the surface geology was
best reproduced with two basement thrust sheets (Figs. 7c and 8c). We
interpret the Mesozoic Ballino-Garda Fault (Fig. 2) as ramping down to
pre-Permian basement and connecting at depth with a basement detachment (Fig. 7c). However, for the Reduced Trento Platform and Atesina Volcanic Complex
along profile 6 (Fig. 9c), the surface geology was best reproduced with
three basement thrust sheets. The 10–12 km depth of the lower detachment in
profile 1 suggests a possible connection with the lower detachment in
profile 5 and the middle detachments of profile 6, both situated at 10–12 km
depth (Figs. 7, 8 and 9). A possible correlation between the bottom
detachments of profiles 5 and 6 is suggested by similar detachment
geometries and depths down to 15 to 20 km. Lateral jumps of a few kilometres in
detachment level (Fig. 10) are interpreted to occur along major steep
Permo-Mesozoic faults. These fault jumps could be an artefact produced by
the uncertainty of the fault geometries at depth within the forward
modelling in Move. Such a deep basement level is required in forward
models to reproduce the structures associated with the Bassano Fault west of
the Schio-Vicenza Fault.
(a) Profile 9 across the Trento-Cles Fault separating the
Trento Platform and Reduced Platform. (b) Profile 12 across the
Ballino-Garda Fault between the Lombardian Basin and Trento Platform. Dashed
lines at and below ∼ 10 km indicate ductile deformation.
Recently, the SWATH-D seismic network (Heit et al., 2021) provided accurate
hypocentres in the study area (average vertical and horizontal resolutions
of ∼ 1.7 km and of ∼ 0.5 km, respectively; Jozi
Najafabadi et al., 2021), basically confirming existing seismicity patterns
(e.g. Viganò et al., 2015). The seismicity indicates that the most
frontal fault systems within the eastern Southern Alps are active (Figs. 7a,
8a and 9a). Plotted onto the cross sections, the hypocentres provided an
additional independent constraint on the depth of basement fault nappes and
indicate a possible blind thrust system propagating southward in profiles 1–7 (Figs. 7a, 8a and 9a), (Jozi Najafabadi et al., 2021). Note that
seismicity does not necessarily delineate all active faults, as deformation
may occur aseismically (Sect. 6.2) or occur along different structures than
the main basement thrust. We note that some hypocentres are plotting below
the modelled fault detachments (Figs. 7, 8 and 9), possibly indicating the
development of an even deeper southward-propagating thrust sheet (Jozi
Najafabadi et al., 2021). This is in agreement with focal mechanisms of
recent earthquakes, indicating a thrusting regime (Petersen et al., 2021).
Along profile 6 (Fig. 9), this deeper deformation may be linked with the
Thiene-Bassano Fault (Galadini et al., 2005; Burrato et al., 2008), whose
surface expression is the Montecchio-Precalcino Hill (Viganò et al.,
2018), situated in isolation on the Po Plain south of the Bassano Fault (Fig. 1). At this location, the Villaverla well (VIDEPI-database: https://www.videpi.com/videpi/videpi.asp, last access: 11 May 2021) crosses a gentle
anticline fold, indicating a development in the hanging wall of a blind
thrust (Viganò et al., 2018). This may introduce an additional amount of
shortening to profile 6 (Fig. 9), although we do not expect this be more
than ∼ 1–2 km. In addition, no offset sediments can be
observed at the Montecchio-Precalcino and therefore it is indicated as a
dashed line on Fig. 9. Along profile 5 no deep blind faults have been
reported in the literature; however, deep seismic events (Fig. 8) suggest the
presence of a blind thrust sheet.
DiscussionKinematic divisions of the Giudicarie Belt
The lateral variation in shortening described above follows Permian–Mesozoic
paleogeographic zones (Fig. 6), whose boundaries were reactivated as
strike-slip faults during the Neogene. This is consistent with field
observations along the Ballino-Garda, Schio-Vicenza, and Trento-Cles faults
and fault-slip analysis indicating strike-slip motion along these faults
(Fig. 4). We interpret the TC-SV fault system to act as a major sinistral
transfer zone (Fig. 11), accommodating the largest variation of shortening
along the Giudicarie Belt (between profiles 5 (11 km) and 6 (25 km); Fig. 6)
and therefore subdividing the area into two kinematic domains (1) and (2),
west and east of the TC-SV fault system, respectively (Fig. 11).
Lateral variation of shortening coinciding with
paleogeographic zones (Fig. 2), bounded by Mesozoic faults
(BG, SV, TC) that were reactivated as Neogene strike-slip faults. The TC-SV
fault system accommodated the largest variation of shortening and subdivides
the area into two kinematic domains west and east thereof, domain (1) and (2),
respectively. Each profile is furthermore subdivided into a northern and
southern segment based on the age of deformation, with the northern domains
indicating older deformation of the Giudicarie or Valsugana phase, and southern
domains showing a younger, active phase of shortening along the Southern
Alpine orogenic front (labelled as the Marana (MA) and Bassano Faults (BA);
see Fig. 12). See Fig. 1 for fault
abbreviations.
Based on age constraints (Sect. 4), we further subdivide the
shortening phases along each cross section into southern (actively
deforming) and northern (inactive) segments. We estimate the amount of
strike-slip motion along the TC-SV fault system by comparing shortening on
either side of the fault (Fig. 11), which results in minimum estimates of 4 km along the SV and 9 km along the TC. Strike-slip motion along the
Ballino-Garda Fault is estimated to be no more than 6 km (Picotti et al.,
1995). As profile 5 straddles the SV and profiles 3 and 4 intersect the
Ballino-Garda Fault, the amount of shortening along these profiles may be
underestimated by a few kilometres due to movement of rock volume out of the 2D
section trace.
The difference in shortening within the three sectors is attributable to
changes in stratigraphy across these faults that are inherited from
Permo-Mesozoic time. For example, in the western sector within the
Lombardian Basin, most shortening is taken up by the thin-skinned
Tremosine-Tignale Fault, which has a long detachment at the base of the
Calcare Di Zu and consequently stacks the Dolomia Principale (Fig. 7a). The
Ballino-Garda Fault is its lateral ramp (Picotti et al., 1995) and therefore
does not continue to the Trento Platform. The sedimentary cover is much
thicker in the Lombardian Basin than in the Trento Platform (Fig. 2) and
comprises a thick Jurassic sequence of slope and deep-water deposits that
favour the development of extended décollements. In the Trento Platform,
similar stacking of the Dolomia Principale can be observed along the Tosa,
Mezzocorona, and Paganella Faults (Figs. 8 and 9). However, their fault
displacements are much smaller, possibly due to the presence of different
décollement layers within the sedimentary cover.
Another example is in the eastern sector (northeast of TC-SV fault system),
where most shortening is taken up by the Valsugana Fault and the inversion
of a Permian graben containing the Atesina Volcanic Complex. The Calisio
Fault represents the lateral ramp of the Valsugana Fault, which is connected
with the Trento-Cles Fault (Fig. 1; Selli et al., 1996), indicating the
Valsugana Fault cannot be traced to the Trento Platform. Although the
Valsugana Fault exhumes pre-Permian basement in its hanging wall, the
sedimentary cover of the Atesina Volcanic Complex is much thicker than that
of the Trento Platform and contains competent volcanic and pyroclastic
deposits of the Lower Permian Formazione di Ora and Formazione di
Gargazzone, which are less abundant within the Trento Platform. We propose
that the thick sedimentary cover of the Atesina Volcanic Complex favoured a
different tectonic style with a contrasting amount of shortening due to the
reactivation of former Permian normal faults as steep Neogene thrusts.
We argue that these local variations in shortening in the eastern Southern
Alps are related to inherited paleogeographic features from Permian to
Jurassic times with the majority of the deformation focused within the
Lombardian Basin and the Atesina Volcanic Complex, which have a thicker
sedimentary cover than the Trento Platform, which is composed of mainly
competent carbonate platforms. Moreover, internal fragmentation of the
Adriatic indenter along the NGF and TC-SV fault system played a major role
in strain partitioning (discussed in Sect. 6.2)
Lateral variations in Neogene shortening across the eastern Southern Alps
The presented model of dominantly thick-skinned tectonics with three
basement thrust sheets is in agreement with the cross section across the
Dolomites of Schönborn (1999). However, Schönborn (1999) estimated
twice the shortening in his profile across the Dolomites (> 50 km; orange line on Fig. 12). Although his cross section transects the
Belluno Thrust, which is not considered in our study, the 4–5 km of
displacement along this thrust (Schönborn, 1999) is insufficient to
explain the difference in shortening (ca. 25 km). In fact, shortening
estimated by Schönborn (1999) along the Valsugana-Belluno system (22 km)
is similar to our estimate of 18 km along this same system (Fig. 9). The
difference between Schönborn (1999) and our study is mainly due to
different interpretations of the Bassano Fault, which Schönborn
associated with 33 km of shortening, while we only propose 7 km.
The 33 km estimate of Schönborn (1999) is based on hinterland-dipping panels
above footwall ramps, which must be matched with hanging wall geometries.
Other profiles transecting the Bassano Fault published by Selli (1998)
(close to the TC-SV fault system; parallel to our profile 6) and Castellarin
and Cantelli (2000) (parallel to the section of Schönborn, 1999; orange
section in Fig. 12) indicate 33 and 35 km of Neogene shortening,
respectively, of which 14 and 19 km were accommodated by the Bassano
Fault. Along profile 6 (Fig. 9), surface measurements of layer dips indicate
that several hinterland-dipping strata of Schönborn (1999) are not
observed at the surface in the hanging wall of the Bassano Fault. Therefore,
we prefer the 7 km shortening estimate in our study area (Fig. 9) and the 19 km toward the east from Castellarin and Cantelli (2000) to the 33 km of
Schönborn (1999).
Kinematic map of the eastern Southern Alps, divided into
two domains east and west of the TC-SV fault system (dashed grey). These
made contrasting and partly overlapping kinematic contributions to sinistral
strike-slip offset along the NGF. The subdivision into domains 1a, 1b, 2a and 2b is
based on the age of deformation (see the legend and Sect. 4).
Seismicity in domains 1b and 2b indicates that shortening is still ongoing.
Profile traces from this study (green), Picotti et al. (1995, purple),
Schönborn (1999, orange, parallel to the cross sections of Castellarin
and Cantelli, 2000), Nussbaum (2000, blue), Favaro et al. (2017) (pink),
and the inferred surface trace of a putative Adriatic crustal wedge
(Gebrande et al., 2002, dashed blue) are indicated. Triangle
A1BC depicts the geometrical relationship between
Late Oligocene to Early Miocene shortening in kinematic domain 1 of the
Giudicarie Belt (vector BC) and motion along the NGF (vector A2B), which is
estimated to be ∼ 18–25 km. This was obtained by projecting 18 km of shortening along the Val Trompia sector of the Giudicarie Belt
(Picotti et al., 1995, purple). Triangle A2BC depicts
the geometrical relationship between Middle Miocene shortening in kinematic
domain 2 of the Giudicarie Belt (vector A2C) and
motion along the NGF (vector AB), which is estimated to be at least
∼ 35–50 km. This was obtained by projecting 25–35 km (green)
of minimum shortening across the Giudicarie Belt east of the TC-SV fault
system. Fault abbreviations are as follows: BA stands for Bassano, MA stands for Marana,
NGF stands for Northern Giudicarie Fault, SV stands for Schio-Vicenza, and
VS stands for Valsugana.
More to the east, Nussbaum (2000) presented cross sections with several
basement thrust sheets and an estimated minimum of 50 km shortening (blue
line on Fig. 12). To the west of the Giudicarie Belt, Picotti et al. (1995)
obtained shortening estimates of 30 to 40 km (purple line on Fig. 12). The
main difference between cross sections h, g, and f of Picotti et al. (1995)
and cross sections 1 to 7 of this study is the major thin-skinned component
of shortening beneath the Po Plain west of Lake Garda (Picotti et al., 1995;
and references therein); this shortening decreases towards its lateral ramps
up to the Adige Valley. Such buried thin-skinned shortening is not present
further to the east, as shown by a seismic survey across the Po Plain
directly east of the Schio-Vicenza Fault (Pola et al., 2014).
In addition, the TC-SV fault system seems to be the lateral boundary for
most thrust systems, as thrusts merge with it and cannot be traced across.
It also coincides with contrasting (though partly overlapping) ages of
shortening (Fig. 11), which forms the basis for a distinction between
kinematic domains 1 and 2 (Fig. 12). Kinematic domain 1 contains the Val
Trompia sector of the Giudicarie Belt with ∼ 18 km of Late
Oligocene to Early Miocene shortening along thrusts that merge with the NGF
and SGF (Picotti et al., 1995; Fig. 12). The same thrusts then accommodated
at least ∼ 12–22 km of Middle Miocene to recent shortening. In
kinematic domain 2, a minimum of 25 km of post-Middle Miocene shortening
(Figs. 6, 9, 11 and 12) was accommodated directly east of the TC-SV fault
system, broadly coeval with shortening in kinematic domain 1. Given the
proximity of profile 6 to the TC-SV fault system, a major strike-slip
corridor, significant out-of-plane motion along these profiles may have
occurred. Therefore, we have included the estimated 35 km of Neogene
shortening of Castellarin and Cantelli (2000) in the Neogene displacement
vector triangle of domain 2 (Fig. 12).
We note that the shortening directions in the eastern Southern Alps shifted
from NNE–SSW during the Late Oligocene to Early Miocene towards NNW–SSE
during the Middle Miocene. The Late Oligocene shortening direction has been
determined with mesostructural analysis and tectonic reconstructions
(Castellarin et al., 1992; Picotti et al., 1995) based on field data
(Schönborn, 1992; Picotti et al., 1997) and subsurface data (e.g.
Fantoni and Franciosi, 2010). This shift in shortening directions is an
important constraint on drawing displacement vector triangles (Fig. 12), as
the hypotenuse of these should be parallel to the motion direction.
Consequently, the hypotenuse of the Late Oligocene displacement triangle is
oriented NNE–SSW (Fig. 12; parallel to NGF and oblique to the profile from
Picotti et al., 1995), whereas the hypotenuse of the Middle Miocene
displacement triangle is oriented NNW–SSE (Fig. 12; parallel to profiles 1
to 7). Using this triangular approach, we estimate 40–47 km of Late
Oligocene to Middle Miocene displacement along the NGF, a calculation we
discuss step by step in Sect. 6.4.
Based on stratigraphic and radiometric age criteria (Sect. 4), we further
divide domains 1 and 2 into sub-domains a and b (Fig. 12), with ongoing
shortening in domain b indicated by seismicity. Low instrumental seismicity
and high strain rates in the central part of domain 2b (Fig. 13; Serpelloni,
2016) might indicate a seismic gap (Anselmi et al., 2011). Note that strain
rates are also relatively high in the Friuli area, where there is a prominent
cluster of seismicity (Fig. 13; see, e.g. Bressan et al., 2016). This could
be due to its junction with the Dinaric system and/or an increase of
shortening towards the east (Nussbaum, 2000) due to anticlockwise
rotation of Adria with respect to Europe (e.g. Ustaszewski et al., 2008; Le
Breton et al., 2017; Mazzoli and Helman, 1994). We note that in the
reconstruction of Mazzoli and Helman (1994), the relative motion vectors of
Africa relative to Europe from the Late Oligocene to Middle Miocene shift
from NNE–SSW to NNW–SSE, similar to the observed reorientation of shortening
directions in the eastern Southern Alps (e.g. Castellarin et al., 1992). This
may suggest that the shortening directions in the eastern Southern Alps
coincide with large-scale plate motions, a point previously made by Caputo
et al. (2010).
Epicentres of seismic events from 2017–2018
(ML≥1;
the size of the black dots indicates earthquake magnitudes up to
ML 4.2) compiled from the SWATH-D Network (Jozi
Najafabadi et al., 2021) superposed on strain rates determined from GPS
velocities (following Serpelloni et al., 2016). Note that the area in the centre
with the highest strain rates (vicinity of the Montello Thrust) coincides
with lower seismic activity, although clustering of low-magnitude events has
been reported (Moratto et al., 2019; Romano et al., 2019).
FS stands for Fella-Sava Fault; see Fig. 1 for the other
fault abbreviations.
Kinematic link between shortening along the Giudicarie Belt and
sinistral motion along the NGF
Variations in shortening along strike of the Giudicarie Belt may be
attributed to large-scale strain partitioning, especially near the TC-SV
fault system (Fig. 12). This system merges with the NGF, thereby transferring
differential shortening between domains 1 and 2 to the NGF and contributing
to the sinistral offset along the NGF (Figs. 12 and 14). Figure 14 shows a
reconstruction from Late Oligocene to present and uses displacement vector
triangles for different time slices to show how shortening contributed to
the observed sinistral offset along the NGF. During the Late Oligocene to
Early Miocene, 18 km of shortening in domain 1 was transferred to a minimum
of 22 km of motion along the NGF (see displacement vector triangles in Fig. 14). Subsequently, coeval shortening in domains 1 and 2 since the Middle
Miocene contributed at least 18 to 25 km of additional motion along the NGF
(Fig. 14). The range of 18 to 25 km motion along the NGF since the Middle
Miocene reflects the uncertainty in minimum shortening estimates in
kinematic domains 1 and 2 related to the presence or absence of putative
duplexes at depth along profiles 5 and 6 (Figs. 8 and 9; see discussion in
Verwater et al., 2021). Combining the
estimates of 22 km of Late Oligocene to Early Miocene sinistral slip and
another 18 to 25 km of Middle Miocene to recent slip on the NGF yields a
total of 40 to 47 km of sinistral offset along this fault since the Late
Oligocene. This minimum 40 to 47 km of Oligocene to Neogene strike-slip
displacement along the NGF may be insufficient to solve the question as to whether
the observed 75 km of sinistral displacement on the NGF is exclusively
Oligocene, such that the dextral PF would acquire its interpreted straight
pre-Oligocene geometry, as discussed above in Sect. 1 (Laubscher, 1991;
Schmid and Kissling, 2000; Handy et al., 2015).
Kinematic evolution of the eastern
Southern Alps from the present day (a) to Late Oligocene time (c). (a) Present configuration of the eastern Southern Alps with ∼ 75 km sinistral offset of the PF along the NGF. There are two kinematic
contributions to the ∼ 75 km of strike-slip motion: (1) 25–35 km of Middle Miocene NNW–SSE shortening across kinematic domain 1 and 2
(Fig. 12), which when projected onto the NGF, corresponds to
∼ 18–25 km of sinistral strike-slip (displacement vector
triangle A2BC), and (2) 18 km of Chattian to Middle
Miocene, NNE–SSW to NNW–SSE shortening west of the TC-SV fault system, which
when projected onto the NGF corresponds to ∼ 22 km of
sinistral strike-slip (see discussion in Sect. 3). (b) At
∼ 14 Ma the NGF sinistrally displaced the PF by
∼ 50–57 km (both minimum values). Since the Middle Miocene
(≥14 Ma), ∼ 20–35 km of shortening both east and west of the TC-SV fault system were
transferred to motion along the NGF at ≥14 Ma which is therefore at this time only displaced by 50–57 km.
The minimum ∼ 50–57 km offset along the NGF at ∼ 14 Ma is partly attributable to a minimum 18 km of Late Oligocene to Middle
Miocene shortening along the Val Trompia-Giudicarie Belt (“kinematic step 1”
of Picotti et al., 1995). (c) At ∼ 28 Ma the Periadriatic
Fault was only modestly offset (∼ 28–35 km). If one assumes a
mean value within this range of offsets, the PF restores to a relatively
straight fault showing only a minor bend north of the Adamello batholith.
We note that some previous workers regard the NGF to have accommodated no
more than 30–40 km of sinistral displacement (Picotti et al., 1995;
Castellarin et al., 2006b) or even less (15 km, Viola et al., 2001), which
is significantly lower than our minimum estimate for displacement on the
NGF. These authors used small-scale markers and only considered motion along
parts of the Giudicarie Fault System (the Passeier fault in Viola et al.,
2001; the SGF in Castellarin et al., 2006b) or used slightly lower
shortening estimates (Picotti et al., 1995). These studies further based
their view on Mesozoic paleogeographical variations on either side of the
NGF (Castellarin and Vai, 1981; Picotti et al., 1995; Prosser, 1998) that
are at a low angle with the NNW-striking, early Mesozoic facies change going
from the Trento Plateau to the Lombardian Basin. They interpret this
near-linear coincidence as evidence for a pre-Alpine (Mesozoic) offset, thus
rendering the map-view offset along the PF to be partly an artefact of
Mesozoic rift tectonics. In addition, Müller et al. (2001) estimated a
shortening of more than 40 km based on jumps in metamorphic grade across the
NGF, which (if valid) would suggest only ∼ 40 km of Paleogene
dextral motion along the PF and allow a pre-Oligocene offset of the NGF.
However, this is incompatible with the interpreted late Paleogene dextral
motion on the PF of some 100–150 km (Laubscher, 1991; Schmid and Kissling,
2000), which would have been accommodated by a roughly equivalent amount of
the late Paleogene E–W shortening at high angles to the NGF. E–W shortening
of such a magnitude along the NGF has not been observed.
Transfer of motion along the NGF to the Eastern Alps
The minimum 40 to 47 km of sinistral motion on the NGF must have been
transferred into the Alpine orogenic lithosphere north of the
Pustertal–Gailtal part of the PF and south of the Northern Calcareous Alps.
The northern Alpine Front is not offset by an equivalent amount. Therefore,
obvious structures to accommodate this displacement are in the Tauern
Window, as already proposed by numerous authors (e.g. Laubscher, 1991;
Frisch et al., 1998; Linzer et al., 2002; Scharf et al., 2013). Based on 2D
map view kinematic reconstructions, Favaro et al. (2017) estimated at least
25 km of N–S Neogene shortening (Fig. 12) associated with kilometre-scale upright,
post-nappe folding in the Western Tauern Window, located just north of the
angular junction of the NGF and PF defining the tip of the eastern Adriatic
indenter. This implies that a remaining amount of N–S shortening was
transferred into orogen-parallel, lateral escape of the Eastern Alps towards
the Pannonian Basin (Ratschbacher et al., 1989; Frisch et al., 1998, 2000;
Linzer et al., 2002; Rosenberg et al., 2007; Scharf et al., 2013; Handy et
al., 2015). Estimates of lateral extrusion vary between 65–77 km (Favaro et
al., 2017) and 160 km (Frisch et al., 2000). The latter amount is probably
an overestimate because the authors assumed that Austroalpine units exposed
at the western and eastern ends of the Tauern Window were in contact prior
to Neogene extension, effectively neglecting the contribution of kilometre-scale
post-nappe upright folding to denudation (see discussion in Favaro et al.,
2017). The amount of eastward motion can be roughly estimated by balancing
the amount of shortening within the Southern Alps with shortening within the
Tauern Window, although the amount of lateral escape as a function of
indentation also depends on the depth of detachment underneath the Tauern
Window, which is unknown. Our minimum estimates for Middle Miocene to recent
NNW–SSE shortening within the Giudicarie Belt (∼ 25 km)
combined with Late Oligocene to Early Miocene NNE–SSW shortening estimates
across the Val-Trompia fold-and-thrust belt (Picotti et al., 1995) would
suggest less lateral extrusion than the 160 km proposed by Frisch et al. (2000). We therefore favour the estimate of 65 to 77 km (Favaro et al., 2017)
of E–W-directed, orogen-parallel extension of the Eastern Alps since 21 Ma
(Favaro et al., 2017). Assuming that 40 to 47 km motion along the NGF was
transferred to the Tauern Window (Laubscher, 1990) and resulted in 25 km N–S
shortening in the Tauern Window (Favaro et al., 2017), a total of 65 to 72 km N–S Neogene Alpine shortening can be estimated. This range of shortening
estimates is clearly less than Adria–Europe Neogene plate convergence
proposed in plate tectonic reconstructions (∼ 100 km, Van
Hinsbergen et al., 2020; ∼ 135 km, Le Breton et al., 2017,
2021). This either suggests that the plate tectonic reconstructions are
overestimating the Neogene Adria–Europe plate convergence or, alternatively, that
additional sources of shortening may have contributed to the overall plate
convergence. This discrepancy could be attributed to three possible factors:
(1) alternative interpretations of profiles across the Giudicarie Belt (see
discussion in Sect. 5.1 and 5.2), (2) “buried” shortening underneath the Po
Plain (see discussion in Sect. 5.3), or (3) additional N–S shortening
associated with upright folding and lateral extrusion rooting in the Tauern
Window, possibly associated with an Adriatic crustal wedge, as we discuss in
Sect. 6.5 below.
Transfer of motion at lower crustal – mantle depths
The contribution of shortening along the Giudicarie Belt to offset along the
NGF requires that the basal detachment of the former roots in the latter.
Alternatively, the basal detachment of the Giudicarie Belt and NGF occupy
different depths, implying the absence of a kinematic link between
shortening along the Giudicarie Belt and motion along the NGF. Additional
shortening could be accommodated by a lower crustal Adriatic wedge, which
according to reflection seismic studies extends 25 km north of the PF
beneath the Tauern Window (Fig. 12; Gebrande et al., 2002; Lüschen et
al., 2004; Castellarin et al., 2006a). This would necessitate decoupling
between relatively weak orogenic crust and rigid denser lower Adriatic
crust. The shortening associated with this sub-Tauern wedge can be no more
than its 25 km length measured in a N–S direction, which is equal or less
than the minimum amount of N–S shortening in our sections and along other
transects of the eastern Southern Alps (Schönborn, 1999; Castellarin and
Cantelli, 2000; Nussbaum, 2000). Our results show that along-strike variations
in Neogene shortening within the Giudicarie Belt coincide with pre-existing
structures, indicating a main detachment horizon exists beneath the
Giudicarie Belt. This detachment horizon could either be linked at depth
with the NGF, PF, and a lower crustal Adriatic wedge underneath the Eastern
Alps (sub-Tauern wedge; Lüschen et al., 2004; Castellarin et al., 2006a)
or alternatively could extend northwestward of the NGF and linked with the
Adriatic lower crustal wedge beneath the central Alps. We consider the
latter hypothesis to be rather unlikely given the wide E–W extent of this
wedge towards the Western Alps and the modest amount of shortening along the
Giudicarie Belt. The Eastern Alpine crustal wedge may be shorter than the
lower crustal Adriatic wedge in the Central Alps (Schmid et al., 1996;
Rosenberg and Kissling, 2013). Nevertheless, its relationship with Neogene
shortening across the eastern Southern Alps remains enigmatic. Possibly, the
amount of shortening observed at the surface and within the Adriatic crust
is an expression of lateral variations in the strength of the Adriatic upper
and lower crust, as also proposed for the Central Alpine lower crustal
Adriatic wedge (Rosenberg and Kissling, 2013). To test this hypothesis,
future studies using local earthquake tomography are necessary to image and
delineate a potential lower Adriatic crustal wedge beneath the Tauern Window
and Eastern Alps. Such studies may also constrain the dip of the NGF at
depth, which in existing Moho maps does not appear to offset the Moho (e.g.
Spada et al., 2013).
Previous studies have also suggested the presence of an Adriatic slab
underneath the Eastern Alps (Lippitsch et al., 2003; Handy et al., 2015).
Recent seismic tomography studies using the dense AlpArray seismic network
indicate, however, that the slab beneath the Eastern Alps is completely
detached from the orogenic crust and extends down to (and locally beyond)
the 410 km discontinuity that marks the top of the mantle transition zone
(Handy et al., 2021). This slab length is far greater than the 25–50 km
estimates of shortening in the eastern Southern Alps (this study and
Schönborn 1999), which represent accreted units of the Adriatic plate.
This discrepancy strongly favours a European origin of the slab and
effectively rules out an Adriatic origin as proposed in earlier studies
(e.g. Lippitsch et al., 2003; Handy et al., 2015).
Conclusion
This study focuses on constraining its Neogene kinematics and depth of
deformation, in particular the kinematic link between shortening in the
Giudicarie Belt and motion along the NGF, and their connection to deeper
structures. Fault-slip analysis confirms that the main Neogene shortening
direction is NNW–SSE. Balancing of seven geological cross sections within the
Giudicarie Belt indicates a dominant thick-skinned structural style of
deformation, as shown by ramp anticlines exposing the pre-Permian basement in
the hanging wall of faults and clustering of recent seismicity at ca. 15 to
20 km depth within the basement. This thick-skinned tectonic structure implies a
modest amount of shortening (8–25 km) within the studied area. The
variations in Neogene shortening coincide with major Permian–Mesozoic
paleogeographic domains that divide the studied area into two kinematic
domains, west and east of the Trento-Cles–Schio-Vicenza transfer fault
system that accommodated about 13 km of sinistral slip. The lateral
variation in shortening reflects competence contrasts and sedimentary
thickness variations across the paleogeographic zones. Zones of thickest
sedimentary cover, comprising relatively incompetent slope deposits, have
accommodated more Neogene shortening (17–25 km), whereas zones of thinnest
sedimentary cover, with competent platform carbonates, accommodated the
least amount of shortening (8–12 km).
Projecting the amount of shortening onto the NGF yields a minimum of 40 to
47 km of sinistral slip along this fault since the Late Oligocene. This
amount may be insufficient to fully explain the 75 km of sinistral offset of
the Periadriatic Fault, although a possible additional source of Neogene
shortening could be a potential Adriatic lower crustal wedge. Future work
using local earthquake tomography is necessary to test this hypothesis and
determine its potential lateral extent and relationship to shortening within
the upper crust. Here, we follow the interpretation that Adriatic
indentation into the Eastern Alps is responsible for most of the 75 km
sinistral offset along the NGF, triggering lateral escape of the Eastern
Alps. Furthermore, we interpret the eastward increase of shortening, from 11 to 25 km, within our study area to reflect lateral variations in strength
of the Adriatic indenter due to inherited Permian to Mesozoic tectonic
structures and paleogeographic zones.
Data availability
All reconstruction and forward modelling files, as well as the complete set
of cross sections of this study, are available from the following data
repository website: https://doi.org/10.5880/fidgeo.2021.006 (Verwater et al., 2021).
Author contributions
The project was conceived by MRH, ELB, and CH as part of the German Research
Priority Program (SPP-2017) “Mountain Building in 4-Dimensions”, an
interdisciplinary arm of the European AlpArray Project. Fieldwork, fault-slip
analysis, drawing of stratigraphic columns, and construction and balancing of
cross sections were performed by VFV under the supervision of ELB and MRH and
with the advice of VP. VFV wrote the manuscript with major contributions from
ELB and MRH. AJN and CH provided the earthquake catalogue and participated in
the discussion on seismicity and active structural domains of the Southern
Alps.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “New insights into the tectonic evolution of the Alps and the adjacent orogens”. It is not associated with a conference.
Acknowledgements
We would like to thank Petex for granting the FU Berlin educational licenses of the MOVE suite. This work greatly
benefited from stimulating conversations with Philipp Balling, Jan Pleuger, Philip Groß, and Julian Hülscher, as
well as a visit in the field with Alfio Viganò (ICAMO-Trento) and discussions with Giuliana Rossi, Alessandro Vuan,
and Stefano Parolai at the OGS in Trieste.
Financial support
This research has been supported by the Deutsche Forschungsgemeinschaft (grant nos. BR 4900/3-1, HA 2403/22-1, and HA 3326/5-1).
Review statement
This paper was edited by Giancarlo Molli and reviewed by Christoph von Hagke and Dario Zampieri.
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