The Umbria-Marche Apennines as a Double Orogen

Ital. J. Geosci. (Boll. Soc. Geol. It.), Vol. 131, No. 2 (2012), pp. 258-271, 11 figs. (doi: 10.3301/IJG.2012.17)
© Società Geologica Italiana, Roma 2012
The Umbria-Marche Apennines as a Double Orogen:
Observations and hypotheses
MASSIMILIANO R. BARCHI (*), WALTER ALVAREZ (**) & DAVID H. SHIMABUKURO (***)
ABSTRACT
The Umbria-Marche Apennines, an arc-shaped fold and thrust
belt with eastward convexity and vergence, form the external part of
the Northern Apennines. In the middle 1980s, the Umbria-Marche
Apennines were interpreted by some as a classical thin-skinned foldthrust belt, with thrust sheets emplaced in an in-sequence, piggyback mode, from the interior to the exterior of the orogen, over a
main, basal detachment in the Triassic evaporites. In the mechanics
of this kind of edifice, the folds and thrusts would develop within a
prism or tapered wedge, bounded at the base by an undeformed
basement with a regional monocline dipping toward the hinterland,
and at the top by a topographic and structural slope generally dipping toward the foreland. Other authors saw the Umbria-Marche
Apennines as a more complex orogen, with basement involvement,
possible tectonic inversions, and out-of-sequence thrusts. In the present paper, the geometry, structure, and tectono-sedimentary evolution of the Umbria-Marche Apennines are compared with the classical thin-skinned model. We suggest that this fold-thrust belt can be
divided longitudinally into two sectors. The eastern part of the
chain, comprising the high Mesozoic carbonate anticlinal ridges of
the axial zone together with the Marche external folds, fits the classical model well, but the western part, comprising the Umbrian PreApennines, shows striking differences. The boundary between the
eastern and western parts of the Umbria-Marche Apennines is here
termed the Scheggia-Foligno Line (SFL). East of this line, the Eastern
Umbria-Marche Apennines show an eastward taper between the
undeformed basement, dipping gently west, and an upper surface in
which both topographic and structural elevation decrease toward
the east. West of the Scheggia-Foligno Line, by contrast, seismic
reflection profiles and subsurface data show basement involvement
in the thrusting at shallow depths, while both the topographic and
structural elevations are anomalously low compared to the more
easterly parts of the chain. There is also a notable discontinuity in
foredeep-basin evolution at the Scheggia-Foligno Line, with welldeveloped foredeep basins in the Umbrian Pre-Apennines and the
external Marche belt (the Marnoso-arenacea and Marche Plio-Pleistocene, respectively), whereas only thrust-top basins developed in
the axial zone, during the Tortonian-Messinian interval. Various
mechanisms, not all mutually exclusive, might be invoked to explain
the discontinuity at the Scheggia-Foligno Line. These possible
explanations include causes of local character, linked to the sedimentary and tectonic evolution of the region, involving episodic
departures from steady-state conditions, interrupting the regular
growth of the accretionary wedge. However, it also possible that the
Western and Eastern Umbria-Marche Apennines represent completely different orogenic systems, with different causes - possibly
with the former related to Corsica-Adria collision and the latter due
to slab rollback of Adriatic lithosphere. In either case, this study
demonstrates the complexity of evolution of the Northern Apennines, with adjacent zones showing abrupt variations in their history
(*) Dipartimento di Scienze della Terra, Piazza Università 06123 Perugia, Italy. Corresponding Author. Tel.: 075 5867168;
fax: 075 5852603; e-mail: [email protected]
(**) Department of Earth and Planetary Science, University of
California, Berkeley CA 94720, USA, and Osservatorio Geologico di
Coldigioco - 62021 Apiro (MC) Italy.
(***) Department of Earth and Planetary Science, University of
California, Berkeley CA 94720.
and style of deformation, which are difficult to incorporate in a single,
unified geodynamic model.
KEY WORDS: Thin-skinned tectonics, Foreland basins, UmbriaMarche Apennines.
INTRODUCTION
The Umbria-Marche Apennines (UMA) comprise a
curving fold-thrust belt with eastward convexity and vergence, representing the outer, eastern part of the Northern
Apennines in the Umbria and Marche Regions (fig. 1).
The evolution of the UMA has been complicated by late
extension, generally interpreted as involving an extensional front, moving eastward, about 100 km behind the
compressional front (ELTER et alii, 1975). In a very influential paper, BALLY et alii (1986) interpreted the UMA as
a typical thin-skinned fold-thrust belt, based largely on
the model of the Canadian Rockies (BALLY et alii, 1966).
In this view of the UMA, the thrust sheets were seen as
emplaced in a regular in-sequence, piggy-back mode, over
an undeformed basement whose upper surface was considered to dip gently southwest towards the hinterland. In
this interpretation, the basal décollement, or sole thrust,
separating the thrust sheets from the underlying basement, was placed within the 1500 m thickness of alternating
anhydrites and dolomites of the Late Triassic Burano
Formation (MARTINIS & PIERI, 1964).
The Bally synthesis was challenged by other authors
who objected, mostly on the basis of shallow structures
projected to depth, to a planar basal thrust, lack of basement involvement, and exclusively in-sequence thrusting
(e.g. SAGE et alii, 1991; BARCHI, 1991). The question was
debated in the literature, most notably in a discussion coauthored by Bally and four critics (GHISETTI et alii, 1993).
In the following years both thin-skinned and thickskinned models have been proposed, mainly by integrating
surface geology evidence and the interpretation of seismic
reflection profiles (e.g. BARCHI et alii, 1998b; SCISCIANI,
2009). These models have generally considered the UMA
to be a single, coherent tectonic unit, representing
the easternmost portion of the Northern Apennines
(e.g. CARMIGNANI et alii, 2001).
We now reopen the question on the basis of seismic
reflection profiles that image the subsurface character of
the thrust belt, together with new evaluations of stratigraphic data that constrain the time-space history of the
syntectonic foreland basins, and geomorphic observations
that reflect the evolution of the deforming hanging wall of
the thrust system.
THE UMBRIA-MARCHE APENNINES AS A DOUBLE OROGEN
259
Fig. 1 - Schematic geologic map of the UmbriaMarche Apennines: 1) Quaternary volcanics;
2) Quaternary sediments; 3) Plio-Quaternary of
the Adriatic foothills; 4) Thrust-top basins and
Laga formation; 5) Marnoso-Arenacea formation; 6) Jurassic-Paleogene Umbria-Marche carbonate sequence; 7) Tuscan sequence; 8) Ligurides; 9) thrust faults; 10) normal faults. The
traces of the sections of figs. 5, 7 and 9 are
also shown.
Our new analysis shows that the UMA need to be considered as two separate, adjacent orogens with different
characteristics. The younger, and probably still active,
Eastern UMA closely matches the classic model of foldthrust belts, of which the BALLY et alii (1966) study of the
Canadian Rockies was an early example. The older, and
now partially extended, Western UMA, departs quite
notably from the classic model. We define the “ScheggiaFoligno Line” (SFL) as the boundary between the older
and the younger parts of the UMA and we consider what
its tectonic significance may be.
CHARACTER OF THE CLASSIC FOLD-THRUST-BELT MODEL
There is, of course, no standard image of a fold-thrust
belt, which all such orogens can be expected to follow. Nevertheless, certain features are common to many fold-thrust
belts. These features have made their way into textbook
descriptions, they provide a reasonable starting point for
interpreting a poorly known fold-thrust belt (at least until
contradictory evidence is obtained), and they offer a satisfying way of interpreting the deformational structures in
terms of inferred driving forces. We suggest that the main
features of this classic fold-thrust belt model are as follows
(BALLY et alii, 1966; BOYER & ELIOTT, 1982; DAHLSTROM,
1970; DAVIS et alii, 1983; PRICE & MOUNTJOY, 1970):
(1) “thin-skinned”deformation, restricted to bedded
sedimentary rocks, not affecting the crystalline basement;
(2) a roughly planar, basal, master thrust surface that
dips gently toward the hinterland (regional monocline);
(3) complicated arrays of “ramps,” on which thrust
surfaces cut up through strong layers and generate ramp
anticlines, and “flats,” where thrust surfaces follow weak
layers for substantial distances, with each ramp-flat system
rooting in the basal master thrust (staircase trajectories);
(4) a history of repeated in-sequence jumping of the
active thrust toward the foreland in a way that progressively incorporates more of the undeformed footwall into
the deforming hanging wall;
(5) evidence for more intense shortening, such as
larger, tighter, more closely spaced anticlines, in the older
part of the fold-thrust belt, with shortening intensity
diminishing toward the foreland.
(6) deposition of clastic sediments in a migrating
foredeep in front of the active thrust, and their subsequent incorporation in the deforming hanging wall;
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M.R. BARCHI ET ALII
Fig. 2 - Seismic and mechanical stratigraphy of
the Umbria-Marche Apennines, showing the
main acoustic reflectors.
(7) deposition of additional sediments in small thrusttop or “piggy-back” basins in synclines between the ramp
anticlines of the deforming hanging wall.
In some fold-thrust belts, a “critical taper model”
(DAVIS et alii, 1983, DAHLEN et alii, 1984; DAHLEN, 1990)
can describe the internal geometry, the kinematics and
the mechanics and of thin-skinned thrust-fold belts and
accretionary prisms. In this model, fold-thrust belts
occupy a wedge shaped region, comprised between the
topographic surface, generally sloping towards the foreland, and the oppositely dipping main basal décollement,
separating the thrust sheets from the almost undeformed
basement. Within the tapered wedge of deforming material, applied and gravitational forces, rock strength, deformation geometry, and surface landscape are at a roughly
steady-state balance.
In the present analysis, we find that this classic model
(i.e. thin-skinned tectonics and tapered-wedge geometry)
well describes the Eastern UMA, east of a line passing
through Serravalle di Carda, Scheggia, Fossato di Vico,
Gualdo Tadino, Nocera Umbra, Foligno, and Spoleto,
which we here call the “Scheggia-Foligno Line” (SFL). On
the other side of this line, the Western UMA have a quite
different structure.
UMBRIA-MARCHE APENNINES
The UMA fold-thrust belt (fig. 1) is located between
the external front of the Tuscan Nappe (COSTA et alii,
1998) and the relatively undeformed Adriatic foreland
that separates the east-verging UMA from the west-verging
Dinarides.
The Jurassic and younger stratigraphy of the UmbriaMarche Apennines is well known because of the many
detailed studies that have extracted information about
Earth history from the pelagic carbonates in this sequence.
Syntheses of the stratigraphy have been published by
CANTALAMESSA et alii (1986), CENTAMORE et alii (1986),
CRESTA et alii (1989), MONTANARI & KOEBERL (2000),
and ALVAREZ (2009, 2010). From a structural point of
view, considering the mechanical properties of the components of the Umbria-Marche Apennines, we can recognize
the following major units, from top to bottom (fig. 2):
(5) Neogene synorogenic deposits, mainly the turbiditic Marnoso-Arenacea Formation, underlain by a marly
Oligocene to lower Miocene preflysch;
(4) a largely carbonate multilayer of pelagic origin,
representing the upper Liassic to the Eocene, with an AptianAlbian marly unit, the Marne a Fucoidi, which provides a
prominent seismic reflector; and with about 700 m of
massive, shallow-water lower Liassic platform limestone
(the Massiccio Formation) at the bottom;
(3) an Upper Triassic evaporitic unit (the Burano
Formation), exposed in a few outcrops in western Umbria
and drilled by many wells beneath the whole UMA;
(2) Permian to Middle Triassic clastics and metasedimentary rocks (Verrucano Group), reached by a few
wells in the UMA and exposed in Tuscany;
(1) Hercynian crystalline basement, neither exposed
nor drilled by any well.
Three different morpho-structural provinces have
been recognised in the UMA (BALLY et alii, 1986; DEIANA
& PIALLI, 1994), from west to east: the Umbria pre-Apennines, the Umbria-Marche Ridge and the Outer Marche
Foothills.
The Umbrian pre-Apennines, from the front of
Tuscan Nappe (Falterona unit) to the Monte Serra Maggio (= Monte Vicino) syncline, are characterised by extensive outcrops of Miocene Marnoso-Arenacea turbidites
and by relatively low elevation. The underlying JurassicPaleogene carbonates are exposed only in two arc-shaped
THE UMBRIA-MARCHE APENNINES AS A DOUBLE OROGEN
261
Fig. 4 - Discontinuity at the Scheggia-Foligno Line, and decreasing
deformational intensity toward the foreland in the Eastern UMA:
(a) Structural elevation (km) of the top Scaglia at the anticline crests;
(b) Spacing (km) between adjacent anticline crests. Both structural
elevation and spacing data were measured along the three northernmost cross-sections of BALLY et alii (1986). Straight lines fitted to the
data east of the Line are compatible with the classic tapered fold-thrustbelt model, but most of the data west of the Line is incompatible
with that model.
Fig. 3 - Topography of the Umbria-Marche Apennines (UMA) displayed
as a digital elevation model, showing the major topographic discontinuity at what we call the Scheggia-Foligno Line. This map covers
the same area as fig. 1.
compressional structures, i.e. the Perugia-Amelia-Narni
alignment and the Gubbio-Monte Subasio (Assisi)-Monti
Martani alignment.
East of the SFL, the Umbria-Marche Ridge is a belt of
major anticlines, with extensive exposures of CretaceousPaleogene pelagic limestones and local outcrops of Jurassic carbonates, within the broader province of the
Umbria-Marche Apennines where the turbiditic Neogene
is broadly exposed. This set of high anticlines forms the
major mountain range of the region. SCARSELLA (1951)
recognised that this range consists of two major ridges or
anticlinoria: the Inner Ridge (Ruga Interna, including
Monte Nerone, Monte Catria, and Monte Cucco) and the
Outer Ridge (Ruga Esterna, including Furlo and Monte
San Vicino). The two ridges are separated by an inner
synclinorium (the site of Acqualagna and Camerino),
characterised by the presence of minor, thrust-top basins
filled with Tortonian-Messinian turbidites.
The Outer Marche Foothills, from the external front
of the Umbria-Marche Ridge to the coastline, represent
the outer portion of the fold-thrust belt, characterized by
extensive outcrops of recent (Pliocene-Quaternary), shallow marine and continental clastic sediments.
These provinces represent the onshore portion of the
UMA, whose easternmost structures are submerged
beneath the western part of the Adriatic Sea.
In this paper, the Eastern UMA is formed by the
Outer Marche foothills and the Umbria-Marche Ridge,
whilst the Western UMA corresponds to the Umbria preApennines. To support our conclusion that the Eastern
and Western UMA, separated by the SFL, are very different
and should perhaps be considered two parallel orogens,
in the next two sections we review the ways in which the
Eastern UMA conform to the “Classic” model for foldthrust belts, and we show how the Western UMA depart
from the model.
MATCH OF THE EASTERN UMBRIA-MARCHE APENNINES
AND THE CLASSIC MODEL
TOPOGRAPHIC AND STRUCTURAL ELEVATION
East of the SFL, the topographic elevations of the
anticlinal mountain ridges decrease eastward (fig. 3) from
Monte Nerone (1525 m) to Montiego (975 m) and Furlo
(Monte Paganuccio, 976 m) to Monte della Cesana (648 m)
to the Serrungarina anticline (555 m), to 479 and 210 m
in the modest anticlines near the coast. Only the
Acqualagna anticline (556 m), between Montiego and
Furlo, is lower than expected (ALVAREZ, 1999, fig. 8).
The structural elevation also decreases in the same
direction, with Jurassic exposed at Monte Nerone, Montiego (Gorgo a Cerbara), and Furlo, Cretaceous in the
Monte della Cesana, Paleogene in the Serrungarina anticline, and Neogene from there to the coast (fig. 4a).
Again, only Acqualagna interrupts the sequence of northeastward decreasing structural elevations (ALVAREZ,
1999, fig. 8). The average dip of the structural slope can
be defined by interpolating between the anticlinal ridges
across the mountain belt. The structural slope is steeper
than the topographic slope because erosion in the higher
ridges and sedimentation in the foreland continually
reduces the topographic gradient, but does not affect the
structural profile.
262
M.R. BARCHI ET ALII
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Fig. 5 - Seismic line and geological cross-section across the mountains east of Foligno (trace on fig. 1). The intensely folded carbonates at the
surface contrast the gently dipping basement reflector at depth: this suggests a basal décollement within the Triassic evaporites. Modified after
MIRABELLA et alii (2008).
BASEMENT CONFIGURATION
In the UMA region, the basement and the basal thrust
are nowhere exposed, and available seismic profiles are
not simple to interpret. As a consequence, the basement
configuration and attitude are matter of much debate.
BALLY et alii (1986) considered the “regional monocline” to be located at the base of the Triassic evaporites,
where they rest on the Verrucano clastics and phyllites.
This level corresponds to a band of reflections that form a
good seismic marker, which we call the “acoustic basement”, generated by the velocity contrast between the
evaporites (>6 km/s) and the underlying Verrucano Group
(about 5 km/s) (fig. 2). In places this reflector shows a
gently west-dipping geometry, and the thrusts and folds
involving the succession above it (evaporites, carbonates
and locally turbidites) appear to be detached here on a
main décollement.
Beneath the Umbria-Marche Ridge, where the sedimentary cover exposed at the surface is intensely folded
and faulted, the underlying acoustic basement is flat or
gently dipping towards the hinterland: this implies the
existence of a major décollement above the acoustic basement. An example corresponding to this Bally-type view
can be seen on a seismic profile across Gualdo Tadino
(fig. 5), showing the basement reflectors dipping west
from about 3 to 4.5 sec two-way travel time (grossly corresponding to a depth of 8 to 12 km), beneath the Ruga
Interna folds and thrusts (MIRABELLA et alii, 2008). A similar geometry is observed in the folds submerged beneath
the Marche foothills (e.g. ORI et alii, 1986) or beneath the
Po Plain (e.g. PIERI & GROPPI, 1981). A review of the dip
of the regional monocline beneath the Northern Apennines is offered by MARIOTTI & DOGLIONI (2000).
However, regional seismic profiles across the Apennines show that at a larger scale the basement reflector
itself is involved in the major thrusts of the Apennines,
generating larger anticlinoria (Ruga Interna, Ruga
Esterna, Coastal Anticline of Monte Conero-Gabicce).
This conclusion, that at least the upper part of the
“acoustic basement” is involved in the major thrust
sheets, originally reached by SAGE et alii (1991) and
BARCHI (1991), was confirmed by the interpretation fo
the CROP03 profile, proposed by BARCHI et alii (1998b)
who termed it “multiple detachments”. In the Central
Apennines, the CROP11 profile also provided evidence of
basement involvement: BILLI et alii (2006) recognized the
presence at mid-crustal depth of a large anticline, associated to Late Messinian-Pliocene out-of-sequence thrusting.
These observations contrast with Bally’s view of a single, regionally extended basal décollement, located within
the Triassic evaporites. Beneath the main ridges of the
Apennines, seismic data suggest that the underlying Verrucano Group is involved in the major thrust sheets, and
incorporated into the tapered wedge, whose base is
located at approximately the same depth and with the
same dip as suggested by BALLY et alii (1986). A similar
structure has been proposed by PATACCA et alii (2008) for
the Central Apennines.
In general, we note that a tapered wedge does not
need to have a single planar décollement, located at a
constant stratigraphic level, e.g. the Triassic evaporites:
THE UMBRIA-MARCHE APENNINES AS A DOUBLE OROGEN
263
typically, the décollement of the basal thrust steps
downward toward the hinterland, so that progressively
deeper/older stratigraphic levels are incorporated into the
wedge (e.g. BALLY et alii, 1966; DAHLSTROM, 1970).
SHORTENING
The classic fold-thrust-belt model predicts the greatest
shortening at the hinterland margin of the orogen, with
shortening gradually diminishing toward the foreland and
reaching zero at the active thrust front. In the absence of
the subsurface information necessary to draw a careful
balanced cross section, we can note the general character
of the folds from west to east. The internal, western anticlines of the Eastern UMA are intensely deformed, with
steep to overturned dips on the flanks, and abundant solution cleavage, representing tens of percent shortening
(ALVAREZ et alli, 1976, 1978; GEISER, 1988), and adjacent
anticlines are driven over each other (e.g., the Monte Catria
anticline is driven over the anticline of Monte della Strega).
The eastern anticlines (Cesana, Serrungarina) have flanks
with gentle dips, and the easternmost anticlines, not yet
emerged from the Adriatic Sea, are almost imperceptible.
Restoration of the CROP03 profile (PIALLI et alii, 1998),
with continuous images of the Marne a Fucoidi reflector,
shows that the total shortening of the UMA is about 60 km
(about 30%). Most of the shortening (about 30 km) is concentrated in the Umbria-Marche Ridge, whilst the adjacent
provinces show a significantly lower value. CROP03 crosses
the northern termination of the Umbria-Marche arc (fig. 1)
and there is clear geological and geomorphological evidence
that shortening increases moving towards the south.
Unfortunately, shortening variations across the UMA
are not constrained by any other section with a comparable degree of confidence. This is mainly because the
shortening value depends strongly on the geometry
adopted for the deeper portion of the sections, where seismic interpretation is more ambiguous.
An indirect way to analyse the shortening variations is
to consider the different spacing of the anticlines and/or
the related thrusts. In fact, spacing of the crests of
the anticlines is strictly correlated with the amount of
shortening, as demonstrated by both field examples and
analogue models (e.g. MULUGETA & KOYI, 1987). The
spacing (i.e. wavelength) of the UMA anticlines can be
easily measured (see also MASSOLI et alii, 2006); in fact in
the Umbria Marche Ridge the structures are exposed at
the surface, whilst the anticlines beneath the Marche
Foothills are clearly imaged by a dense network of seismic reflection profiles (e.g. BALLY et alli, 1986; ORI et alli,
1986; SCARSELLI et alli, 2007). Data collected in the
northern portion of UMA (fig. 4b) show that the folds of
the Umbria-Marche Ridge are more closely spaced than
that of the Umbria-Marche foothills, indicating that the
amount of shortening diminishes towards the foreland.
Fig. 6 - Diagram showing the age of the foreland basins of the UmbriaMarche Apennines, from the Tiber Valley to the Adriatic foreland.
The small arrows indicate the main paleocurrent direction (Vertical
arrows: Alpine sources; horizontal arrows: Apennine sources).
ward migration of the advancing thrust front across the Italian Peninsula from Oligocene to Recent time, and this
includes the UMA. The piggy-back basins (“bacini minori”),
whose detailed study began in the mid-1970s (CENTAMORE
et alii, 1976, 1977, 1978), tell a similar story of eastward
propagation of the deformation at a more local scale.
The diagram of fig. 6 summarizes the available data
about the ages of the siliciclastic successions that overlie
the UMA Jurassic-Paleogene carbonates, along a transect
roughly corresponding to the CROP03 profile. There is a
clear eastward progression of the critical facies change
representing the transition between the forebulge environment (usually marls) and the foredeep environment
(usually turbiditic sandstones), which marks a major
change in the subsidence history, related to the flexing of
the foreland lithosphere (e.g. CASERO, 2004).
The diagram shows that the history of eastward propagation is not interrupted at the SFL (ALVAREZ, 1999, fig. 9),
where the Line runs northwest-southeast (immediately
northeast of the Monte Vicino Sandstone), so this line of
evidence, at least, does not provide support for our
hypothesis of a major difference between the Western and
Eastern UMA.
PROPAGATION HISTORY OF THRUSTS AND ANTICLINES
The propagation history of the UMA can be tracked both
by the age of the large-scale turbidite systems that filled the
foredeep basins in front of the advancing fold-thrust belt and
by the age of the small-scale turbidite systems that filled
piggy-back basins in synclines between ramp anticlines.
Since the time of MERLA (1951) it has been known that the
ages of the large-scale turbidite systems track a general east-
DRAINAGE EVOLUTION
The geomorphology of the Eastern UMA is marked by
deep canyons (e.g., Furlo, Burano, Frasassi) that cut
across the anticlinal ridges, giving a drainage pattern with
short, closely-spaced rivers that flow nearly straight northeastward to the Adriatic Sea and do not reflect the northwest-southeast grain of the Apennine folds. MAZZANTI &
264
M.R. BARCHI ET ALII
Fig. 7 - Seismic profile and its geological interpretation across the
Scheggia-Foligno Line (trace on fig. 1). Note that the structural step
down to the west across the Scheggia-Foligno Line, of about 3 km, is
much greater than the roughly 1 km topographic offset.
TREVISAN (1978) and ALVAREZ (1999) have interpreted
this drainage as the result of gradual emergence of the
folds from the sea in a smooth progression from southwest to northeast, just as predicted by the classic model of
the monotonically-advancing fold-thrust belt, although
this interpretation needs to be modified to take account of
the effects of the Messinian sea-level fall (SCARSELLI et alii,
2007). We will show below that this generally simple situation is not the case in the Western UMA.
DEPARTURE OF THE WESTERN UMBRIA-MARCHE
APENNINES FROM THE CLASSIC MODEL
TOPOGRAPHIC AND STRUCTURAL ELEVATION
If there is one single, overwhelming line of evidence for a major difference between the Western and
Eastern UMA, it is the discontinuity in topographic
and structural elevation across the SFL (fig. 3). The
topographic discontinuity is unmistakable, with the
anticlinorial complex of Monte Nerone (1525 m)-Monte
Catria (1701 m)-Monte Cucco (1566 m)-Monte Penna
(1432 m)-Monte Pennino (1571 m) looking down on
the foothills at around 500-800 m elevation just to the
west.
Furthermore, this step-down to the west is not merely
a topographic difference; the structural dislocation is
much greater (fig. 7). The top Majolica, a convenient
datum horizon that in places forms the crest of the anticlinal mountains (e.g., at Monte Petrano, 1100 m, between
the Bosso and Burano Gorges), is deeply buried beneath
Miocene turbidites immediately west of the SFL, to a
depth that may reach 3 km below sea level (MENICHETTI & P IALLI , 1986, fig. 2). The thickness of the
turbidite cover beneath Western UMA is also constrained by a network of good quality seismic reflection
profiles (e.g. BARCHI et alii, 1999; PAUSELLI et alii, 2002;
MIRABELLA et alii, 2004). Thus the vertical structural
displacement across the Line is around 4 km, which is
substantially greater than the topographic difference of
around 1 km.
Of course many mountain ranges have dramatic steps
in their topographic and structural profiles. Usually,
however, in fold-thrust belts such a step does not come at
the rear of the orogenic wedge, as it does in the UMA. A
wedge shaped fold-thrust belt is usually thought to result
from the advance of a massive driver block at the rear. In
the Eastern UMA there is a wedge, but there is no driver
behind it.
We are not aware that anyone has pointed out the
absence of a driver block west of the large ramp anticlines of the UMA. Ever since the influential paper
of ELTER et alii (1975), it has made sense to think of
the westward step-down across the SFL as marking an
advancing extensional front, moving eastward across the
Italian Peninsula about 100 km behind the compressional front now in the Adriatic Sea. This interpretation
of the SFL as a post-thrusting normal fault is shown on
the Structural Map of the Northern Apennines at
1:250,000 (BOCCALETTI & COLI, 1982, Sheet 2), and was
accepted by one of us (A LVAREZ , 1999), but we have
subsequently realized that this cannot be the correct
explanation for the difference in structural elevation
across the Line.
Three observations preclude attributing this structural and topographic offset to late extensional faulting
(fig. 8). First, if the area west of the SFL had formerly
been a block that was higher than the current UmbriaMarche Ridge so that its eastward motion could have
driven the thrusting farther east, its upper stratigraphy
should have been eroded away while it was high. This
is the case along the crest of the current Ridge, where
rarely is any rock younger than the Lower Cretaceous
Majolica preserved. In contrast, west of the SFL there
appears to be complete preservation of the CretaceousOligocene pelagic sequence, which is exposed in the
Gubbio half anticline and along the west flank of the
Umbria-Marche Ridge anticlinorium. We conclude that
the area west of the SFL was never high enough to
have been stripped by erosion, so it cannot be low
today simply because of late normal faulting along the
Line.
THE UMBRIA-MARCHE APENNINES AS A DOUBLE OROGEN
265
Fig. 8 - Observations (in boxes) incompatible
with the Scheggia-Foligno line being the
extensional front inferred by ELTER et alii (1975).
This conclusion is supported by the second observation, which concerns the structural deformation along the
SFL. If the topographic and structural offset across the
Line were due to normal faulting, one would expect to see
top-down-to-the west extensional structures there, but
these are not found. On the contrary, there is abundant
evidence for top-up-to-the-east shortening, including
parasitic folds and solution cleavage (FAZZINI, 1973;
ALVAREZ et alii, 1976, 1978; TAVARNELLI & ALVAREZ,
2002). CORSI & DEFEYTER (1991) have shown that this
situation is due to out-of-sequence thrusting. It is not the
result of extensional faulting, yet the problem remains
that there is no high driver block to the west.
A third line of evidence against the structural offset
being due to extensional faulting comes from the tectonics
of the area west of the SFL (ALVAREZ, 2001; SHIMABUKURO et alii, 2002). In an area that extends about 50 km
west from the SFL, there is a continuous cover of Miocene
turbidites (Cervarola and Marnoso-arenacea Formations)
which demonstrates that there can have been no major
anticlines in this area, which would have led to erosion of
the Miocene, as is the case along the Umbria-Marche
Ridge. The absence of major anticlines is also supported
by abundant, good-quality seismic data (BARCHI et alii,
1999; PAUSELLI et alii, 2002), showing that in the subsurface of the Umbria pre-Apennines the top of the carbonates is almost flat, with no evidence of the tight, closely
spaced anticlines that characterize the Umbria-Marche
ridge. Only isolated anticlines (Gubbio and Monte Suba-
sio) expose the Mesozoic in this hilly landscape, mostly
made of Miocene sandstone and marls (fig. 1).
BASEMENT CONFIGURATION
Basement never crops out in this region. What we
know about the deep structure mainly derives from seismic
reflection profiles, where the boundary between the Triassic evaporites of the Burano Fm. and the underlying Verrucano Group generates a major reflection that can be traced
on most profiles across the region (acoustic basement).
The seismic profiles show that there is a major step in the
depth of the acoustic basement across the SFL. West of the
SFL, a dense network of seismic reflection profiles constrain a detailed isobath maps of the top basement, located
at a depth of 4-6 km (MIRABELLA, 2002). East of the SFL, a
deeper basement (8-10 km) is imaged by seismic data
beneath the Ruga Interna folds (Mirabella et alii., 2008).
Seismic refraction data confirm the presence of a basement step, located beneath the SFL (PONZIANI et alii, 1995,
1998). The basement step-down to the east across the SFL
corresponds to a major SW-dipping thrust, involving the
acoustic basement reflector (fig. 9).
Summarizing, across the SFL shallow and deep structures show opposite features: at shallow levels, the western block is structurally lowered, whilst at deeper levels
the western block is uplifted. These features both differ
from the classic tapered wedge geometry, indicating that
the Eastern UMA tapered wedge is interrupted at the SFL.
266
M.R. BARCHI ET ALII
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Fig. 9 - Seismic profile and geological interpretation of the Ruga Interna structures at Gualdo Tadino (trace on fig. 1). The profile gives
seismic evidence for an offset in basement across the Scheggia-Foligno Line, with the basement shallower on the west. Modified after
MIRABELLA et alii (2008).
SYNTECTONIC
CLASTIC DEPOSITION AND PROPAGATION
HISTORY OF THRUST AND ANTICLINES
As we have seen in § 4.4, the age of the foreland
basins tracks the general eastward migration of the contractional deformation from Late Burdigalian (Tiber Valley) to the present time (Adriatic offshore), showing a
regular displacement of depositional environments, evolving
from forebulge basins to foredeeps to restricted thrust top
basins (e.g. DECELLES & GILES, 1996).
However, the migration is not completely regular. The
Marnoso-Arenacea and the Marche-Adriatic basins are
regionally developed foredeeps, related to the main imbrication events of the Tuscan Units (Late Burdigalian-Serravallian) and of the Umbria-Marche Units (Late MessinianPliocene), respectively. In the intervening time period
(Tortonian-Early Messinian) no proper foredeep was present,
but only minor basins, trapped between the growing UmbriaMarche folds. In other words, whilst the thrust-top basins of
the Marche foothills cover a previous, thick turbidite succession, deposited during a large-scale regional subsidence
phase, the inner Marche thrust-top basins directly cover forebulge marls or, in some cases, the carbonates.
The boundary between the Marnoso-Arenacea foredeep and the Inner Marche thrust-top basins is located at
the SFL, the same place where the topographic and structural anomalies described in the previous paragraphs
occur. When the Umbria Marche ridge started to rise
(Tortonian) it was higher than would be expected according to the wedge-shaped geometry of a classic thrustfold belt. During the progressive flexing of the Adriatic
lithosphere, the substratum of the Umbria-Marche ridge
remained higher than the adjacent regions.
DRAINAGE EVOLUTION
As noted in section 4.5, the drainage evolution in
the Eastern UMA is compatible with the Classic Model.
However, the Western UMA has a completely different
drainage style, showing a trellis pattern with northwestsoutheast river tracts following late extensional valleys
(Valle Umbra, Valle di Gubbio), and northeast-to-southwest tracts, like those of the Upper Tiber, that cut across
from one valley to the next. MAZZANTI & TREVISAN (1978)
and ALVAREZ (1999) attributed the trellis drainage of the
Western UMA to rearrangement of the drainage during
passage of the extensional front (ELTER et alii, 1975) that
has now reached approximately the position of the SFL.
Reconsidering that interpretation, we now point out
that passage of the extensional front cannot be the only
reason for the difference between the Western and
Eastern UMA and their strikingly different drainage patterns. As we previously noted in section 5.1, the Western
UMA cannot be simply an extensionally-collapsed equivalent of the Eastern UMA, as thick and extensive Miocene
turbidites (Marnoso-Arenacea Fm) are present, and the
Gubbio anticline preserves the complete Cretaceous
sequence (fig. 7). Thus, the Western UMA was never the
topographically high driver of the Eastern UMA.
There is an additional reason for rejecting the idea that
the different drainage patterns of Western and Eastern UMA
are the result of an extensional front advancing toward the
east, as proposed in the ELTER et alii (1975) model. As
shown in MAZZANTI & TREVISAN (1978, fig. 5), this model
implies that the abandoned canyons of rivers that formerly
flowed northeast to the Adriatic should be recognized as
notches in horst ridges west of the extensional front (fig. 10).
A place where these notches should be recognizable is
the ridge that forms the Apennine drainage divide,
extending about 50 km northwest from a point just east
of Gubbio. In the hypothesis of an advancing extensional
front, this ridge would have been at the western edge of
the tapered wedge when the Gubbio normal fault marked
the extensional front. The drainage-divide ridge would
have been cut through by canyons of rivers flowing to the
Adriatic, like the Burano and Bosso Gorges just to the
THE UMBRIA-MARCHE APENNINES AS A DOUBLE OROGEN
267
east. The drainage-divide ridge would now be left behind,
as the extensional front has migrated to the SFL, but the
canyons would still be visible. Fig. 11 shows that there is
no hint of such abandoned canyons on the very level summit of this part of the Apennine drainage divide.
This geomorphological argument strongly supports
our hypothesis that the Eastern and Western UMA are
fundamentally different; they are not simply two parts of
a single fold-thrust belt with its western part broken up by
late extension. The Eastern UMA fit the classic model of a
wedge-shaped fold-thrust belt; the Western UMA do not.
SUMMARY OF DIFFERENCES BETWEEN EASTERN
AND WESTERN UMA
To summarize, the departures of the UMA fold-thrust
belt from the classic model are due to the presence of a
major discontinuity in (1) the gradient of both topographic
and structural elevations across the fold-thrust belt, (2) the
depth to basement, (3) the migration history of the foredeep,
and (4) the drainage pattern. All these discontinuities occur
at the SFL, and they seem to reflect the same event, i.e. the
rise of the anticlinorial ridge of M. Nerone-M. CatriaM. Cucco (“Ruga interna”, SCARSELLA, 1951) that is higher
than might have been expected on the basis of the deformation that had previously occurred in the Western UMA.
We have compared the regional-scale features and the
time-space evolution of the UMA fold-thrust belt with the
classic model of thin-skinned tectonics, which describes
the steady-state growth of a wedge-shaped fold-thrust belt
advancing over an undeformed basement. Considering
both geological and geomorphological features, we find
that the UMA can be divided into two major regions, here
named the Eastern UMA and Western UMA, with a
boundary at the SFL. Our analysis shows that the Eastern
UMA, comprising the main Apennine ridge and the outer
(eastern) ridges and foothills near the Adriatic Sea, generally fits the classical wedge model. This is shown by the
fact that, moving from the foreland in the northeast
towards the hinterland in the southwest, both the structural and topographic elevations increase, while the top
of the acoustic basement deepens from about 5 to about
12 km. In contrast, the Western UMA is a topographically
and structurally low region, where Neogene turbidites
extensively crop out and the basement is much shallower
than below the mountain ridge just to the east.
We have also collected and analyzed stratigraphic and
sedimentological data from the syntectonic clastic wedges
of the UMA. These data show a major discontinuity in the
evolution and eastward migration of the foreland basins,
with a jump in the position of the foredeep, from the
Western UMA (Miocene Marnoso-Arenacea Formation)
across the high ridges to the Marche Foothills of the Eastern UMA (Pliocene-Recent sediments). This discontinuity
is related to the rise of the high Apennine ridge that
occurred during the Tortonian-Messinian time interval.
HYPOTHESES TO EXPLAIN THE DEPARTURE
FROM THE CLASSIC MODEL
In order to explain the departures of the Western
UMA from the Classic Model, various possible causal
mechanisms might be invoked, some of which may
Fig. 10 - This figure (modified after MAZZANTI & TREVISAN, 1978)
shows the notches (circled) that should be present in drainage divides
west of the Scheggia-Foligno Line if that area were a subsided region
that formerly had the structure of the Umbria-Marche Ridge. Fig. 11
shows that no such ridges are present along the modern Apennine
watershed just east of Gubbio, which argues that the Scheggia-Foligno
Line is not an extensional front.
have acted together. These mechanisms range from
local, internal causes related to the stratigraphy and
evolution of the thrust belt, to regional, or external,
causes, due to the geodynamic framework in which the
thrust belt formed. These considerations raise the possibility that the UMA should be interpreted as a double
orogen, rather than a single orogen showing local differences.
LOCAL CAUSES
Local causes for the differences might include: (1) the
effects of surface transport processes (syntectonic sedimentation and erosion) disturbing the regular (insequence) emplacement of thrust sheets from the hinterland to the foreland (e.g. SCARSELLI et alii, 2007); (2) the
presence of multiple décollements (BARCHI et alii 1998b)
generating multiple fold sets, producing irregularities in
the forward dipping structural slope; or (3) the effects of
tectonic inheritance (fault reactivation and/or inversion),
affecting the localization and/or amplification of the single thrusts (e.g. BUTLER et alii 2006).
(1) Surface transport processes: It is well known that
surface transport processes can influence the evolution
of a thrust belt, modifying its regular migration from
the hinterland to the foreland (e.g. MUGNIER et alii,
1997; LETURMY et alii, 2000; SIMPSON, 2006). When erosion prevails, the frontal thrust moves back, producing
out-of sequence thrusts, while an excess of sedimentation can force the frontal thrust to jump to a more
268
M.R. BARCHI ET ALII
Fig. 11 - The abandoned river canyons west of
the Scheggia-Foligno Line, predicted in fig. 10,
are not present along the Apennine watershed
just east of Gubbio, which argues against the
Scheggia-Foligno Line being an extensional front.
external position. For example, during the Messinian
salinity crisis (about 5.6 Ma) the Mediterranean sea
level experienced a drop of about 1500 m, followed by a
rapid early Pliocene marine ingression. These perturbations affected the rate of migration of the compressional
front (and the magnitude of shortening as well) in the
Marche-Adriatic region (SCARSELLI et alii, 2007). Similar effects have been documented in the Po Plain area
(CASTELLARIN et alii, 1985). Subcritical conditions of
the tapered wedge, possibly related to the Messinian
salinity crisis, may have also induced out-of sequence
thrusting in the rear of the Central Apennines wedge
(BILLI et alii, 2006).
Quaternary glaciations may also have had significant
effects on the late evolution of the thrust belt and of the
related basins.
(2) Multiple décollements: The style of deformation of
the UMA is characterized by a system of multiple décollements, where different sets of structures are generated at
different structural levels, linked to each other and developed in a hierarchical mode. The wavelength and amplitude of the structures depends on the depth of the
décollement to which they sole, so that the deeper the
décollement, the larger the structure (BARCHI et alii,
1998b; BARCHI, 2010). The interference between these different fold sets has produced irregularities in the structural slope (the upper surface of the tapered wedge),
dipping towards the foreland. The major ridges of the
Umbria-Marche belt (Ruga Interna and Ruga Esterna)
and the easternmost coastal anticline (BALLY et alii,
1986), culminating at M. Conero, represent the morphological and structural expression of the major thrusts,
detached at deeper levels. A low region (major syncline) is
developed at the back of the major “basement” thrusts.
This might explain why the region west of the SFL is
structurally and topographically low, but not why the
basement is shallow.
(3) Tectonic inheritance: The presence of pre-existing faults can affect the localization of the major thrust
ramps, which can be nucleated in positions that are different than those favored by the mechanical evolution
of the wedge. For example, major ridges can be located
in correspondence to previous, pre-orogenic structurally lowered basins (inversion tectonics). Since in
the Umbria-Marche region the Jurassic synsedimentary
tectonics is characterised by small fault blocks (ALVAREZ, 1989a, b; DE PAOLA et alii, 2007; SANTANTONIO
& CARMINATI, 2011), we would expect that this kind of
process did not produce relevant effects at the regional
scale.
It seems unlikely that such a large feature as the 4-km
eastward step-up of the structural elevation across the
SFL (section 5.1), which extends longitudinally for more
than 100 km (fig. 1), would result from any local irregularity within the fold-thrust belt. We think a regional
cause is more likely.
REGIONAL CAUSES
THE UMBRIA-MARCHE APENNINES AS A DOUBLE OROGEN
Three regional causes are worth consideration: (1) The
effects of late extensional tectonics, superimposed on the
earlier compressional structures (ELTER et alii, 1975),
could explain the low topography at the back of the main
ridge, but not the shallow basement with its complete
Jurassic-Miocene cover. (2) LAVECCHIA et alii (2003) proposed the presence at depth of major thrusts affecting the
whole crust; in such thick-skinned model, thin-skinned
thrusts would only be local, shallow structures, and the
SFL could reflect a crustal-scale thrust at depth. (3) In a
model with discontinuous retreat of the Adriatic slab, as
described by FACCENNA et alii (2001), the main foredeeps
of the Marnoso-Arenacea and Outer Marche could correspond to major pulses of tectonic activity, separated by
relatively quiet time intervals.
(1) Effects of late extension: In the inner part of the
Northern Apennines, late extensional tectonics severely
displace the previous compressional structures (ELTER et
alii, 1975; COLLETTINI et alii, 2006), disrupting the original wedge geometry. A simple hypothesis is that the SFL
is the boundary between the extended Western UMA
and the not-(yet)-extended Eastern UMA, where the taper
geometry is still preserved. However, as we have previously discussed (see § 6.1) the SFL is a compressional feature. On the contrary, robust geological, geophysical and
seismological evidences indicate that the extensional
front (i.e. the position of the easternmost extensional
structures) is a NNW-SSE alignment of still active normal
faults (from Sansepolcro to Gubbio, Colfiorito and Norcia), obliquely crossing the arc-shaped SFL (fig. 1). The
position of this active extensional system is also marked
by the distribution of the historical and instrumental seismicity of the region (e.g. CHIARALUCE et alii, 2005; CIACCIO et alii, 2006; PONDRELLI et alii, 2006).
(2) Thick skinned tectonics: Thin-skinned models are
strictly related to subduction scenarios, where the portion
of the continental crust not involved in the thrust belt is
sinking in the mantle. In a non-subduction scenario,
LAVECCHIA et alii (2003) propose a thick-skinned model,
where each major thrust affects the entire crust and possibly the lithosphere, whilst the thin-skinned thrusts,
observed at the surface and/or imaged by the seismic profiles, are only local, shallow features. Such a structural
setting would not require the existence of a regionally
extended wedge-shaped thrust-fold belt: the SFL would be
simply a boundary between two different lithospheric
thrust sheets. However, this model does not explain why a
large region (Eastern UMA) strictly follows the geometrical features expected for a thin-skinned belt. Moreover,
no geophysical evidence of thrusting, displacing the
Adriatic Moho, is available at the moment (e.g. PAUSELLI
et alii, 2006).
(3) Pulsating subduction: The evolution of the Apennines compressional belt is strictly related to the contemporaneous opening of the Tyrrhenian Sea (SCANDONE, 1979; MALINVERNO & RYAN, 1986). Considering
the time evolution of the Tyrrhenian-Apennines system,
some Authors (e.g. FACCENNA et alii, 2001; NICOLOSI et
alii, 2006) noted remarkable irregularities in the opening of the Tyrrhenian Sea: these intermittent pulses of
back-arc extension might be related to irregularities in
the sinking and retreating of the Adriatic lithosphere,
not to be considered as a continuous, almost steady-
269
state process, but a discontinuous one. In this view, the
SFL could mark a prominent pause in the construction
of the thrust belt.
ARE THE UMBRIA-MARCHE APENNINES A DOUBLE OROGEN?
Finally, DOGLIONI et alii (1998) and BARCHI et alii
(1998a) have hypothesized that the Northern Apennines
consist of two adjacent thrust belts, generated by different
driving mechanisms. The earlier thrust belt in the west
(Etruscan belt), would have been generated, concurrently
with the rotation of the Corsica block, as a response to
Oligocene-Early Miocene collision between Corsica and
the Adriatic lithosphere. The younger thrust belt in the
east (Umbria-Marche belt) would be related to the subsequent roll-back and retreat of the Adriatic lithosphere. We
propose that in this scenario, the SFL would represent the
border between these two different mountain belts of different origins. If correct, this double-orogen model would
radically change how the Umbria-Marche Apennines are
understood.
In a recent paper, Thompson et alii (2010) discussed
the wedge kinematics and the exhumation history of the
northernmost part of the Northern Apennines (i.e. at the
boundary between Tuscany and Emilia-Romagna regions),
mainly using thermo-chronological data. Their study supports the existence of a transversal discontinuity (the “Sillaro Line”) separating two segments of the Northern
Apennines, which would have experienced a quite different
recent (i.e post-Late Miocene) tectonic history.
In the region of our study, no comparably dense set of
thermochronological data is presently available (ZATTIN
et alii, 2002) to constrain the recent kinematics and
exhumation history. However, using a different approach
and different datasets, our study concludes that also in
the southern part of the Northern Apennines two adjacent
segments of the orogen (Western and Eastern UMA) experienced a markedly different tectonic history: in our case,
the discontinuity (i.e. the Scheggia-Foligno Line) is parallel to the orogen. Both studies suggest that the Northern
Apennines are a complex orogen, characterized by a large
variability, both along and across the mountain belt,
where different geodynamic processes may have acted at
different times in different segments of the orogen, and
where no single model is likely to explain the long-term
wedge kinematics.
ACKOWLEDGMENTS
The central idea of this paper started in our discussions more
than a decade ago with Giampaolo Pialli about the differences
between what he called the Etruscan Belt and the Umbrian Belt. We
thank ENI E&P for the permission of reproducing the seismic lines
in figs. 5, 7 and 9. We also thank the constructive suggestions of two
anonymous reviewers, who helped to improve the paper. MRB’s stay
in Berkeley was financed by Regione Umbria grants.
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Manuscript received 22 July 2011; accepted 2 April 2012; editorial responsability and handling by C. Faccenna.