The CROP-11 deep seismic profile across the central Apennines, Italy, reveals a previously unknown, mid-crustal antiform here interpreted as a fault-bend fold-like structure. The seismic facies and gravity signature suggest that this structure consists of low-grade metamorphic rocks. Geomorphological, stratigraphic and tectonic evidence in the overlying shallow thrusts suggests that this structure developed in early to mid-Messinian time and grew out of sequence in late Messinian–
Pliocene time. The out-of-sequence growth may reflect a taper subcriticality stage of the Apenninic thrust wedge, which
induced renewed contraction in the rear

Billi, A.

Cavinato, G. P.

Cosentino, D.

Di Luzio, E.

Keller, J. V. A.

Kluth, C.

Orlando, L.

Parotto, M.

Praturlon, A.

Romanelli, M.

Storti, F.

Tiberti, M. M.

Wardell, N.

Earth-prints Repository

Journal of the Geological Society, London, Vol. 163, 2006, pp. 583–586. Printed in Great Britain.
583
S P E C I A L
First results from the CROP-
11 deep seismic profile,
central Apennines, Italy:
evidence of mid-crustal
folding
A . B I L L I 1, M . M . T I B E RT I 2,
G . P. C AV I NATO 3, D. C O S E N T I N O 1,
E . D I L U Z I O 3, J . V. A . K E L L E R 4,
C . K L U T H 5, L . O R L A N D O 6,
M . PA ROT TO 1, A . P R AT U R L O N 1,
M . RO M A N E L L I 7, F. S TO RT I 1 &
N . WA R D E L L 7
1Dipartimento di Scienze Geologiche, Universita` ‘Roma
Tre’, Largo S. L. Murialdo, 1, 00146 Rome, Italy
(e-mail: billi@uniroma3.it)
2Istituto Nazionale di Geofisica e Vulcanologia, 00143
Rome, Italy
3Istituto di Geologia Ambientale e Geoingegneria, CNR,
00185 Rome, Italy
4ChevronTexaco Energy Technology Co., San Ramon, CA
94583, USA
5Department of Geology and Geological Engineering,
Colorado School of Mines, Golden, CO 80401, USA
6Dipartimento di Idraulica, Trasporti e Strade, Universita`
‘La Sapienza’, 00184 Rome, Italy
7Istituto Nazionale di Oceanografia e di Geofisica
Sperimentale, 34010 Trieste, Italy
T
he CROP-11 deep seismic profile across the cen-
tral Apennines, Italy, reveals a previously un-
known, mid-crustal antiform here interpreted as a
fault-bend fold-like structure. The seismic facies
and gravity signature suggest that this structure consists
of low-grade metamorphic rocks. Geomorphological, strati-
graphic and tectonic evidence in the overlying shallow thrusts
suggests that this structure developed in early to mid-
Messinian time and grew out of sequence in late Messinian–
Pliocene time. The out-of-sequence growth may reflect a taper
subcriticality stage of the Apenninic thrust wedge, which
induced renewed contraction in the rear.
The Apennines (Italy) form a NW-trending, ENE-vergent, fold–
thrust belt (Fig. 1), which developed within the frame of
convergence between Africa and Eurasia during the Neogene
(Malinverno & Ryan 1986). During this time, contractional
deformations migrated eastward, mostly in a piggyback sequence
towards the foreland (Cipollari & Cosentino 1995) favoured by
the parallel retreat of the subduction zone toward the Adriatic
foreland (Malinverno & Ryan 1986).
Whereas the deep style of thrusting across the northern and
southern Apennines has been unveiled by, respectively, the
CROP-03 (Pialli et al. 1998) and CROP-04 (Scrocca et al. 2005)
deep seismic profiles, the deep tectonic architecture beneath the
central Apennines is still unconstrained because of the lack of
deep data. As a consequence, thin-skinned and thick-skinned
thrusting have been proposed as alternative models for the
tectonic accretion, these models leading to strongly contrasting
inferences about the amount and rate of shortening, and about
the tectonic evolution of the central Apennines (Ghisetti et al.
1993). By applying a thin-skinned criterion, for instance, Hill &
Hayward (1988) computed 157 km of shortening over a section
of 226 km length across the central Apennines. In contrast, by
applying a thick-skinned criterion over about the same section,
Tozer et al. (2002) obtained 37 km of shortening over a distance
of 158 km.
Below we present the central segment of the CROP-11 deep
seismic profile across the central Apennines (Fig. 1). These data
show the most prominent structure imaged in the entire profile,
i.e. a hitherto unknown mid-crustal antiform.
Results and interpretation. The CROP-11 profile runs approxi-
mately east–west from the Tyrrhenian Sea to the Adriatic Sea.
The location of the CROP-11 profile is the best compromise
between field accessibility and the occurrence of two structural
trends, namely north–south in the NW and NW–SE in the SE
(Fig. 1). The CROP-11 profile cuts across two major crustal
domains (Fig. 2a): the Adriatic domain (east), where the Moho is
imaged at c. 12 s two-way travel time (TWTT) (c. 32 km deep),
and the Tyrrhenian domain (west), where the crust has been
thinned by backarc extension (Malinverno & Ryan 1986) and the
Moho is imaged at c. 9.5 s TWTT (c. 25 km deep). In the
Adriatic domain, the CROP-11 profile includes flat or gently
inclined reflections, which image the front of the ENE-vergent
orogenic wedge lying above the regional foreland monocline.
Steeper reflections occur in the central and western segments of
the CROP-11 profile, where the core of the orogenic wedge is
imaged. In the central segment, strong reflections occur between
c. 5 and c. 8–9 s TWTT and define a wide, mid-crustal antiform
(Fig. 3). Two sets of upward-convex reflections image the relative
hinge zones. The antiform is associated with a ramp-flat-shaped
shear zone occurring between c. 7 and c. 10 s TWTT. Flat-lying
reflections are imaged in the footwall of the shear zone. Beneath
the Fucino and Marsica areas (Figs 1–3), shallow reflections
(,3 s TWTT) are inclined toward the east and are parallel to the
deep reflections in the forelimb of the antiform (Fig. 3b). In and
above the backlimb of the antiform, a set of parallel reflections
included between the ramp-flat-shaped shear zone and the
Olevano–Antrodoco thrust shows a constant westward inclina-
tion. The Olevano–Antrodoco thrust is imaged as a shallow
structure and is representative of an out-of-sequence event in the
central Apennines (Cipollari & Cosentino 1995).
To infer the physical attributes and lithology of the mid-crustal
antiform, we analysed the regional gravity anomalies obtained
through a ‘stripping off’ method. This method consisted in the
removal of the gravity effect produced by the geological bodies
shallower than 10 km from the Bouguer anomaly database
(which has a grid cell size of 3 km) of the Italian Geological
Survey (Tiberti et al. 2005). In the gravity profile (Fig. 2b), the
Tyrrhenian and Adriatic domains are characterized by gravity
highs, whereas a relative gravity low occurs in the central
Apennines beneath the Fucino basin and the Marsica area. A
second-order gravity low of c. 8 mGal occurs in the transitional
region between the Tyrrhenian high and the Fucino low. Its
location corresponds to the mid-crustal antiform imaged in the
seismic profile (Fig. 2a) and its wavelength indicates a source
depth between 10 and 20 km, a maximum thickness of c. 10 km,
and an average density of c. 2570 kg m3 (for details, see Tiberti
et al. 2005).
Discussion and conclusions. We interpret the antiform imaged
in the CROP-11 profile and the associated shear zone as a mid-
crustal, fault-bend fold-like structure (Suppe 1983). This inter-
pretation provides a first-order and possibly oversimplified
description of the actual tectonic architecture imaged in the
CROP-11 profile. More detailed analyses on specifically pro-
cessed segments of the CROP-11 profile are planned for the
future.
An immediate question raised by the occurrence of a mid-
Fig. 1. Geological map of the central Apennines. T., thrust sheet. Two grey stars on top of the Simbruini Mts show the location of exposed Triassic rocks,
i.e. the most elevated rocks of that age in the central Apennines. At the northeastern front of the Simbruini thrust sheet, the lower Messinian ‘Brecce della
Renga’ formation (i.e. pre-salinity crisis) constitutes the largest wedge of breccias in the central Apennines. Younger conglomerates (upper Messinian)
occur on top of and at the front of the Simbruini thrust. On the right, synthetic stratigraphy of the central Apennines compiled according to the Puglia-1
(41.058N, 16.208E, depth 7070 m) and Trevi-1 (41.888N, 13.208E; depth 3549 m) wells and surface data (Accordi & Carbone 1986).
Fig. 2. (a) Line drawing of the CROP-11 seismic profile. The relative location is shown in Figure 1. The 0 s datum corresponds to 500 m above sea level
(a.s.l.). Depths in kilometres of the Moho in the Tyrrhenian (west) and Adriatic (east) domains are known from two seismic refraction profiles (Cassinis et
al. 2003). A high-resolution image of the CROP-11 profile, the relative parameters and the station coordinates are available online at http://
www.geolsoc.org.uk/SUP18249. A hard copy can be obtained from the Society Library. (b) Regional gravity anomaly along the CROP-11 profile. In the
central sector, the gap (shaded area) between the observed regional trend (continuous line) and the hypothesized (unaffected) regional trend (dashed line)
indicates the effect of the mid-crustal antiform imaged on the CROP-11 profile. The gravity low is entirely compensated by assuming this structure is as
thick as c. 10 km and as dense as c. 2570 kg m3.
A. BILLI ET AL.584
crustal antiform beneath the central Apennines concerns the
nature of the material involved. The seismic facies of the
hanging-wall panels and the relative gravity data suggest that
the rocks involved in the antiform are probably layered, very
low-grade metamorphic rocks. Of the rock types known in the
central Apennines, the Permian–Triassic argillites (Fig. 1) best
fit these physical properties. The sonic log from the Puglia-1 well
shows that the Permian–Triassic argillites have a strong acoustic
impedance contrast with the overlying carbonates. This physical
attribute and the possible occurrence of pressurized fluids
entrapped in the mid-crustal antiform may explain the high
reflectivity of the rocks forming this structure. In the northern
Apennines, seismic reflections similar to those forming the mid-
crustal antiform of Figure 3 correspond to the top of a phyllitic
basement underlying the Triassic argillites (Pialli et al. 1998).
Mid-crustal folding in the central Apennines must have
significantly influenced the topographic and tectonic architectures
of the thrust wedge. The evidence that the entire rock multilayer
in the hanging wall is involved in the crustal antiform (Figs 2
and 3) suggests that this structure was active in an out-of-
sequence fashion since late Messinian times. This age is inferred
from the timing of the Marsica–Morrone thrusts (Messinian–
early Pliocene, Fig. 4), which are at present tilted toward the east
and are roughly parallel to the forelimb of the mid-crustal
antiform. The presence on top of the hanging wall of the lower
Messinian ‘Brecce della Renga’ breccias, which are the largest
and thickest deposits of Neogene conglomerates in the central
Apennines (Figs 1 and 4), suggests that the underlying mid-
crustal antiform was already active during early Messinian time,
Fig. 3. (a) Central segment of the CROP-11 profile (i.e. between the Tiber valley and the Fucino basin; see Fig. 2a). (b) Interpretation of first-order
tectonic structures. The upper boundary of the hanging-wall area is drawn where a major change of seismic facies occurs.
Fig. 4. Schematic diagram showing the time–space migration of
thrusting and the distribution of Messinian clastic deposits. Data are
plotted along the profile D–E across the central Apennines (the location
is shown in Fig. 1). Columns of conglomerates correspond, by number, to
outcrops shown in Figure 1. ‘A’ is the approximate altitude for the
outcrops of upper Messinian conglomerates. The location and linear
extension of the antiform shown in Figure 3b is displayed in the top left
of the diagram. Two grey stars show the location (corresponding to the
crest of the mid-crustal antiform) along the profile D–E of exposed
Triassic rocks on top of the Simbruini thrust sheet (see Fig. 1). This
diagram shows that the central Apennines consist of an imbricate fan of
shallow, carbonate, thrust sheets accreted in a piggyback sequence
directed towards the foreland with episodic out-of-sequence thrusting.
MID-CRUSTAL FOLDING IN THE APENNINES 585
possibly causing localized uplift and associated subaerial erosion.
Such a localized uplift is also supported by the occurrence of
exposed Triassic rocks (i.e. the oldest rocks exposed in the
central Apennines) only in correspondence to the crest of the
mid-crustal antiform (Figs 1 and 4). Moreover, in the same area,
upper Messinian polygenetic conglomerates rest unconformably
on eroded Mesozoic carbonates and synorogenic Miocene depos-
its (Figs 1 and 4). Significant vertical separations occur between
the upper Messinian conglomerates at present lying atop the
crestal region of the mid-crustal antiform and those preserved
atop the forelimb (Fig. 4). These deposits suggest that the mid-
crustal antiform was active during late Messinian time, whereas
their vertical displacements suggest a post-late Messinian
growth.
In synthesis, the available geophysical and geological data
suggest, for the mid-crustal antiform imaged in the CROP-11
profile, an early Messinian–Pliocene evolution. From the loca-
tion of the mid-crustal antiform and the time–space migration
of thrusting in the central Apennines (Fig. 4), we infer that the
mid-crustal antiform grew as an out-of-sequence structure since
late Messinian time. The out-of-sequence growth of this struc-
ture can be explained by assuming that the Miocene–Pliocene
sequence of thrusting breaking towards the foreland (Fig. 4)
caused the superficial taper of the orogenic wedge to shallow.
The taper consequently became subcritical and contractional
deformations were resumed at the rear of the thrust wedge,
causing the out-of-sequence growth of the mid-crustal antiform.
In the study area, post-orogenic normal faulting is a young
process (,4–5 Ma) that has produced significant crustal exten-
sion only in the first 2 s TWTT (Cavinato et al. 2002), and is
therefore negligible for the mid-crustal, structural architecture.
From the CROP-11 profile, the vertical and horizontal dis-
placements on the ramp-flat-shaped shear zone at the base of the
mid-crustal antiform can be estimated at c. 2.5 s TWTT (i.e. c.
5 km) and c. 30 km, respectively. The vertical displacement is
about consistent with the thickness of the eroded sedimentary
cover on top of the antiform crest, where Triassic rocks are
exposed (i.e. 4–5 km of eroded Jurassic–Palaeogene limestones
on top of the Simbruini Mts; Fig. 1).
The discovery of a mid-crustal antiform beneath the central
Apennines compels: (1) further research into the superficial
geology of this region to properly constrain the effects and
chronology of the mid-crustal antiform; (2) further geophysical
prospecting to constrain the 3D geometry and vergence of the
mid-crustal antiform; (3) the critical revision of previously
proposed tectonic models for this region, possibly abandoning
the oversimplified contraposition between thin-skinned and thick-
skinned end-member templates.
We thank all colleagues who have worked with us during the planning,
acquisition, processing and interpretation of CROP-11. We thank M.
Barchi and R. Tozer for insightful reviews.
References
Accordi, G. & Carbone, F. (eds) 1986. Lithofacies map of Latium–Abruzzi and
neighbouring areas. Consiglio Nazionale delle Ricerche, Rome.
Cassinis, R., Scarascia, S. & Lozej, A. 2003. The deep crustal structure of Italy
and surrounding areas from seismic refraction data. A new synthesis.
Bollettino della Societa` Geologica Italiana, 122, 365–376.
Cavinato, G.P., Carusi, C., Dall’Asta, M., Miccadei, E. & Piacentini, T.
2002. Sedimentary and tectonic evolution of Plio-Pleistocene alluvial and
lacustrine deposits of Fucino Basin (central Italy). Sedimentary Geology, 148,
29–59.
Cipollari, P. & Cosentino, D. 1995. Miocene unconformities in the central
Apennines: geodynamic significance and sedimentary basin evolution.
Tectonophysics, 252, 375–389.
Ghisetti, F., Barchi, M., Bally, A.W., Moretti, I. & Vezzani, L. 1993.
Conflicting balanced structural sections across the Central Apennines (Italy):
problems and implications. In: Spencer, A.M. (ed.) Generation, Accumula-
tion and Production of Europe’s Hydrocarbons III. Special Publication of the
European Association of Petroleum Geoscientists, 3, 219–231.
Hill, K.C. & Hayward, A.B. 1988. Structural constraints on the Tertiary plate
tectonic evolution of Italy. Marine and Petroleum Geology, 5, 2–16.
Malinverno, A. & Ryan, W.B.F. 1986. Extension in the Tyrrhenian Sea and
shortening in the Apennines as result of arc migration driven by sinking of
the lithosphere. Tectonics, 5, 227–254.
Pialli, G., Barchi, M. & Minelli, G. (eds) 1998. Results of the CROP 03 deep
seismic reflection profile. Memorie della Societa` Geologica Italiana, 52,
1–647.
Scrocca, D., Carminati, E. & Doglioni, C. 2005. Deep structure of the southern
Apennines, Italy: thin-skinned or thick-skinned? Tectonics, 24, TC3005,
doi:10.1029/2004TC001634.
Suppe, J. 1983. Geometry and kinematics of fault-bend folding. American Journal
of Science, 283, 648–721.
Tiberti, M.M., Orlando, L., Di Bucci, D., Bernabini, M. & Parotto, M. 2005.
Regional gravity anomaly map and crustal model of the Central–Southern
Apennines. Journal of Geodynamics, 40, 73–91.
Tozer, R.S.J., Butler, R.W.H. & Corrado, S. 2002. Comparing thin- and thick-
skinned tectonic models of the central Apennines, Italy. In: Bertotti, G.,
Schulmann, K. & Cloetingh, S.A.P.L. (eds) Continental Collision and the
tectono-sedimentary evolution of forelands. European Geophysical Union
Stephan Mueller Special Publication Series, 1, 181–194.
Received 13 January 2006; revised typescript accepted 23 March 2006.
Scientific editing by Ken McCaffrey
A. BILLI ET AL.586


First results from the CROP-11 deep seismic profile, central Apennines, Italy: evidence of mid-crustal folding

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First results from the CROP-11 deep seismic profile, central Apennines, Italy: evidence of mid-crustal folding

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