Journal of the Geological Society, London, Vol. 163, 2006, pp. 1–14. Printed in Great Britain.

1

Testing thrust tectonic models at mountain fronts: where has the displacement

gone?

R. S. J. TOZER 1,2, R . W. H. BUTLER 1, M. CHIAPPINI 3, S . CORRADO 4,

S . MAZZOLI 5 & F. SPERANZA 3

1School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK2Present address: BP Exploration, Chertsey Road, Sunbury-on-Thames TW16 7LN, UK

(e-mail: Richard.Tozer@uk.bp.com)3Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy

4Dipartimento di Scienze Geologiche, Universita degli Studi di Roma Tre, Largo San Leonardo Murialdo 1,

00146 Rome, Italy5Dipartimento di Scienze della Terra, Universita di Napoli ‘Federico II’, Largo San Marcellino 10, 80138 Naples, Italy

Abstract: The alternative relationships that can exist between a mountain front and the adjacent foreland

basin have been recognized for many years. However, seismic reflection data from such areas are commonly

of poor quality and therefore structural models may contain large uncertainties. In view of scientific and

commercial interest in mountain belts, we have reviewed the methods for discriminating between alternative

interpretations using a case study from the Montagna dei Fiori in the central Apennines, Italy. In this area

Mesozoic and Tertiary carbonate sediments are juxtaposed with a foredeep basin containing up to 7 km of

Messinian and Plio-Pleistocene siliciclastic sediments. A new structural model for this area demonstrates how

the structures in this area form a kinematically closed system in which displacement is transferred from the

thrust belt to blind structures beneath the present-day foreland. Growth strata show that Pliocene shortening

was initially rapid (15 mm a�1) followed by slower rates during the final stages of deformation. Variations in

structural elevation indicate a component of basement involvement during thrusting, and this is further

supported by magnetic modelling. The results illustrate the interaction of thin- and thick-skinned structures in

the central Apennines, and the methods for discriminating between alternative structural models.

The outer parts of mountain belts are commonly defined by a

thrust carrying a frontal anticline that is evident as a topographic

high. In active orogens such as the Himalayas these structures

have proved useful for determining rates of tectonic processes by

linking dated landscape features or sediments to kinematic

descriptions of the deformation (e.g. Mugnier et al. 1992).

Anticlines at mountain fronts also trap important accumulations

of hydrocarbons (see Needham et al. 2004, and papers in the

same volume). For both of these and many other issues, it is

critical to resolve the relationship between the frontal anticline

and the adjacent foreland basin. In simple terms this ‘mountain

front problem’ (Vann et al. 1986) can be paraphrased as follows:

how is orogenic shortening that created the mountain front

anticline accommodated in younger strata and at the surface of

the Earth (Fig. 1)? In continental fold and thrust belts the

problem generally arises because although the anticline crest

may be exposed the critical lower parts of the forelimb are

commonly masked by young deposits. The solution lies in

integrating outcrop with seismic reflection data, but because the

forelimbs of folds are difficult to image, structural interpretations

have been model-driven rather than data-led. In the absence of

an emergent thrust (Fig. 1a), Vann et al. (1986) suggested four

general solutions to the problem (see also Morley 1986). One

solution is that a major backthrust is present at the mountain

front (creating the ‘triangle zone’ of Jones 1982; Fig. 1b).

Another explanation is that the thrust beneath the anticline was

formerly emergent but is now inactive, so that the younger

foredeep sediments onlap and bury the structure (Fig. 1c). A

geometric option is that the thrust carrying the anticline loses

displacement rapidly towards the foreland, with the displacement

gradient expressed as layer-parallel shortening in the overlying

sediments (e.g. Williams & Chapman 1983; Morley 1986; Fig.

1d). Alternatively, the anticline may not be the true mountain

front at all and the displacement beneath the frontal anticline

could be transferred out into the foreland basin (Fig. 1e).

In many active orogens the most obvious solution to the

mountain front problem, and the simplest geometry interpreted

from outcrop data alone, is to suggest that the thrust that is

inferred to carry the anticline simply emerges onto the synoro-

genic landscape (Fig. 1a). However, seismic reflection and well

data acquired from frontal anticlines commonly show that this

option is inappropriate. Our aim here is to describe how the

integration of seismic and well data together with outcrop

structural and stratigraphic data can limit the number of

geologically viable solutions to the mountain front problem. This

is illustrated with a case study from the Montagna dei Fiori in

the central Apennines of Italy. We review existing interpretations

of the structure, testing each against field and subsurface data

using the criteria of Vann et al. (1986). We then propose a new

kinematic model for this part of the thrust belt and examine its

implications.

Background

The Montagna dei Fiori area lies on the eastern side of the

central Apennines of Italy (Fig. 2). The region is a fold and

thrust belt that developed in the Neogene; during this time

compressional deformation migrated eastward so that the young-

est folds and thrusts are located on the eastern margin of the

Italian peninsula. The thrust belt has received much attention

over the past 20 years as it has become evident that the crustal

shortening it represents balances with crustal extension in the

orogenic interior (Malinverno & Ryan 1986; Lavecchia 1988;

Faccenna et al. 1996). A variety of tectonic models for the thrust

belt have been proposed, including very thin-skinned, high-

displacement versions (e.g. Bally et al. 1986). Orogen-wide

tectonic models lie outside the scope of this contribution, except

to say that they rely on appropriate choices of structural

geometry at the scale of the Montagna dei Fiori study area (see

Tavarnelli et al. 2004).

Stratigraphy

The stratigraphy of the Montagna dei Fiori and surrounding area

(Fig. 3) is well described and understood. Two distinct deposi-

tional cycles can be distinguished; the first is a broadly layer-

cake succession of Triassic to Miocene carbonate sediments.

Overlying and onlapping these are siliciclastic foredeep sedi-

ments of Messinian and Plio-Pleistocene age, as described below.

Pre-Messinian stratigraphy

The pre-Messinian stratigraphy of the Montagna dei Fiori is a

record of the evolution of the southern margin of the Tethys

Ocean during the Mesozoic and Tertiary (Parotto & Praturlon

1975). By analogy with the exposed stratigraphy in Tuscany (e.g.

Bortolotti et al. 1970), the succession is thought to begin with

Permo-Triassic continental siliciclastic sediments (the Verrucano

Group), followed by a thick succession of Triassic evaporites

(Burano Anhydrites), which record the evolution to marine

conditions (Fig. 4a). These are followed by platform carbon-

ates (Castelmanfrino Formation or Calcare Massiccio) of late

Triassic–Liassic age, the oldest formations known to be present

in this area both from outcrop and well data.

Rifting of the passive margin led to the establishment of a

pelagic basin during the Jurassic, within which both ‘complete’

(Corniola, Rosso Ammonitico, Salinello and Aptici Formations)

and ‘condensed’ (nodular limestones) pelagic successions are

present as a result of important normal faults of Liassic age

(Giannini 1960; Paradisi et al. 1969; Parotto & Praturlon 1975;

Mattei 1987). Thermal subsidence of the pelagic basin is

recorded by the deposition of fine-grained limestones and marls

from the Late Jurassic to the Mid-Miocene (Maiolica, Fucoidi

Marls, Scaglia Bianca, Scaglia Rosata, Scaglia Cinerea, Bisciaro

and Cerrogna Marls Formations). To the north (southern Marche)

and east (Adriatic Sea) at more marginal positions in the basin,

the sediments coeval to the Cerrogna Marls are neritic marls of

the Schlier Formation.

Messinian and Plio-Pleistocene stratigraphy

During the Late Miocene a NNW–SSE-trending foredeep basin

(the Laga Basin; Centamore et al. 1991) developed ahead of the

Fig. 1. Schematic illustrations of possible

explanations for the mountain front

displacement problem, after Vann et al.

(1986). For the purposes of the Montagna

dei Fiori case study, the pre-thrusting

stratigraphy equates to the pre-Messinian

carbonate succession and the foredeep basin

fill equates to the Messinian and Pliocene

siliciclastic stratigraphy.

Fig. 2. Simplified geological map of the Apennines showing the location

of the Montagna dei Fiori study area. After Funiciello et al. (1981).

R. S . J. TOZER ET AL .2

active thrust belt to the west. The inception of this basin is

recorded by a few tens of metres of hemipelagic marls (Orbulina

Marls; Fig. 4a). In the Messinian the basin was fed axially from

the north by siliciclastic turbidite flows that built up the Laga

Formation (Mutti et al. 1978; Chiocchini & Cipriani 1992). The

eastward-tapering geometry of the Laga Basin is clearly defined

by low-angle onlap (c. 38) of the formation onto the pre-

Messinian carbonate succession exposed in the Montagnone

Anticline to the south (Centamore et al. 1991; Vezzani &

Ghisetti 1998) and in seismic reflection profiles of the area to the

north (e.g. Bally et al. 1986). The Laga Basin never underwent

desiccation during the Messinian salinity crisis, an event re-

corded only by a 15–30 m thick horizon of resedimented gypsum

arenites, which is used to divide the formation into pre- and

post-evaporite members (Fig. 4a; Cantalamessa et al. 1986;

Roveri et al. 2001). To the east of the Montagna dei Fiori the

total thickness of the Laga Formation is almost 3000 m (Vezzani

& Ghisetti 1998), whereas coeval shales and gypsum deposited

on the foredeep margin (Gessoso-solfifera Formation) are

,100 m thick. These are penetrated by wells beneath the

Adriatic Sea to the east, and in the Marche region to the north

(Fig. 4b).

The foreland to the east of Montagna dei Fiori contains a thick

succession of Plio-Pleistocene sediments that record the final

episodes of the development of the central Apennine fold and

thrust belt (Fig. 4b). In keeping with previous studies of the area

(e.g. Crescenti et al. 1980; Paltrinieri et al. 1982; Cantalamessa

et al. 1986; Calamita et al. 1994), the informal subdivision of

the Pliocene into lower, middle and upper intervals is retained.

The succession begins with the lower Pliocene Teramo and

Cellino Formations (Sphaeroidinellopsis, Globorotalia (G.) mar-

garitae and G. puncticulata biozones), both of which comprise

siliciclastic turbidite deposits interbedded with clays and marls

deposited under several hundred metres of water (Artoni &

Casero 1997; Vezzani & Ghisetti 1998). These are followed by

the middle and upper Pliocene Castilenti Formation (G. crassa-

Fig. 3. Simplified geological map of the Montagna dei Fiori (MF), Acquasanta (AQ) and Montagnone (MO) Anticlines; ST denotes the Salinello Thrust.

Map compiled from Paradisi et al. (1969), Centamore et al. (1991) and Vezzani & Ghisetti (1998); it should be noted that the trace of the thrust fault to

the west of Teramo is interpretation-driven.

TESTING THRUST TECTONIC MODELS 3

formis and G. inflata biozones), which comprises similar turbi-

dite deposits intercalated with conglomerates and calcareous

sandstones derived from the emergent thrust belt. The sequence

is capped by Pleistocene sandstones and clays (Hyalinea (H.)

balthica and younger biozones) deposited in shallow marine

and continental environments, which complete the general

shallowing-upward pattern (Vezzani & Ghisetti 1998). The

thickness of the Plio-Pleistocene sediments varies greatly as a

result of syndepositional growth of the underlying structures, but

the maximum thickness, beneath the Adriatic coastline, is over

7000 m.

Structure

The Montagna dei Fiori area is limited to the west by the north–

south-trending Sibillini front, a major line of thrust faulting

along which sediments of the Umbria–Marche pelagic basin are

thrust over the Laga Formation to the east. The southern limit of

the area is defined by the east–west-trending Gran Sasso thrust

belt, along which sediments deposited on the margin of the

Latium–Abruzzi Carbonate platform override north–south-

trending structures in the Laga Basin. This basin has been

deformed into two major anticlines, from west to east the

Acquasanta and Montagna dei Fiori Anticlines (Fig. 3). Seismic

reflection profiles show that two major structures are also present

beneath the foreland to the east of these anticlines.

Surface structures

The surface geology of the area is dominated by the Acquasanta

and Montagna dei Fiori Anticlines, which have a wavelength of

15 km. The Acquasanta Anticline is an asymmetric, east-vergent

structure with a gently west-dipping western limb and a sub-

vertical eastern limb. The oldest sediments exposed in the core

of this structure are the Scaglia Rosata and Scaglia Cinerea

Formations. To the east of this structure lies a west-dipping

minor thrust (the Acquasanta Thrust; Fig. 3); at the surface, the

hanging wall and footwall of this fault are both composed of the

pre-evaporites member of the Laga Formation, which is dis-

placed by a maximum of a few hundred metres. This thrust dies

out to the south and north and has a maximum length of

,40 km.

In contrast to the Acquasanta Anticline, the greater structural

elevation in the centre of the Montagna dei Fiori Anticline has

resulted in the exposure of limestone formations, which are

resistant to erosion (Fig. 3). Therefore, the zone of maximum

structural elevation is also the area of maximum topographic

elevation. The Montagna dei Fiori Anticline plunges in both

directions along its axis; to the south the equivalent structure is

known as the Montagnone Anticline and has a similar asym-

metric geometry to the Acquasanta Anticline.

Within the core of the Montagna dei Fiori Anticline an

important minor thrust, the Salinello Thrust, is exposed (Giannini

1960; Mattei 1987; Calamita et al. 1998; Fig. 3). This has an

antiformal geometry and separates the east-dipping upper limb of

the Montagna dei Fiori Anticline from the overturned lower limb.

The horizontal shortening accommodated by this fault is only

750 m, as shown by the separation between the hanging wall and

footwall cutoff of the top Fucoidi Marls (Mattei 1987). Although

this thrust has been interpreted to emerge on the eastern side of

the anticline by Paradisi et al. (1969) and Vezzani & Ghisetti

(1998), these maps do not agree on its location further to the

south where field relationships are less clear. To avoid this

problem, seismic reflection data have instead been used in this

study to constrain the structure in this area.

The western side of the Montagna dei Fiori Anticline is cut by

a major normal fault (Fig. 3); this dips steeply to the WSW, is

c. 15 km in length and has a maximum displacement of at least

1400 m (Paradisi et al. 1969; Centamore et al. 1991; Vezzani &

Ghisetti 1998). There is evidence that this fault was active in the

Early and Mid-Miocene (Calamita et al. 1998; Mazzoli et al.

2002; Scisciani et al. 2002; Fig. 4a) and during the Pleistocene

(Ghisetti & Vezzani 2000); that is, both before and after

compressional deformation. In the southwestern part of the study

area lies a second major fault with a similar orientation, the

Monte Gorzano normal fault (Ghisetti & Vezzani 2000; Fig. 3).

Fig. 4. Simplified stratigraphic columns: (a) the pre-Messinian carbonate

succession and Messinian Laga Formation, based on exposure in and

around the Montagna dei Fiori Anticline; (b) the Plio-Pleistocene

sediments beneath the present-day foreland, based on wells drilled on the

crest of the Costiera Structure. Ash bed fission-track dating of outcrop

samples is from Bigazzi et al. (2000).

R. S . J. TOZER ET AL .4

This is a Pleistocene structure, which has a maximum displace-

ment in excess of 1000 m and juxtaposes the Laga Formation in

the hanging wall against the Cerrogna Marls in the footwall.

Blind structures

The area beneath the Acquasanta and Montagna dei Fiori

Anticlines is covered by very few seismic profiles and these are

uncalibrated by wells; because of confidentiality restrictions

these data are not presented. The most important observation is

that beneath the Acquasanta Anticline a repetition of the seismic

stratigraphy can be recognized, indicating that a major thrust

underlies this structure. The footwall ramp to this thrust appears

to lie beneath the western limb of the surface anticline where the

footwall cutoff of the top Calcare Massiccio is located 11 km

west of the outcrop position of the Acquasanta Thrust. Given that

this thrust accommodates a maximum of a few hundred metres

shortening at the surface, most of the displacement must remain

in the subsurface and continue beneath the Montagna dei Fiori

Anticline. Seismic data covering the southern part of this

structure confirm that this thrust is present (T1 in Fig. 5a). In this

area a type example of the seismic stratigraphy of the pre-

Messinian carbonate succession can be recognized beneath the

surface anticline; this is believed to connect to the footwall

beneath the Acquasanta Anticline.

Two major structures are present beneath the foreland to the

east of the Montagna dei Fiori Anticline (Fig. 5b). The more

internal of these, which we indicate as the Bellante Structure, is

characterized by a zone c. 5 km wide of very poor quality

seismic data. However, imbricates can be recognized within the

Messinian and Pliocene sediments, and older on younger tectonic

contacts have been reported for at least two of the wells drilled

on this structure. In contrast, seismic data provide spectacular

images of the most external structure beneath the foreland, the

Costiera Structure. This is an east-vergent thrust-propagation

anticline composed of two or more imbricates of Pliocene

sediments (Artoni & Casero 1997).

The mountain front problem at the Montagna dei Fiori

The seismic reflection data reveal that a major thrust lies beneath

the study area; the separation between the footwall ramp beneath

the Acquasanta Anticline and the hanging wall cutoff beneath the

Montagna dei Fiori Anticline (Fig. 5a) shows that this thrust has

a displacement of at least 25 km. What happens to this displace-

ment at the mountain front on the eastern side of the Montagna

dei Fiori? The purpose of the following analysis is to review the

possible answers to this question using the framework of Vann et

al. (1986; Fig. 1). Several of the alternative explanations have

already been proposed in published cross-sections and are

reviewed where appropriate.

Emergence of a major thrust

This option was explicitly omitted from the original work by

Vann et al. (1986) which instead focused on situations where it

is demonstrably not the case. However, in several previous

models for the area a major thrust has been interpreted to emerge

on the eastern side of the Montagna dei Fiori Anticline (Ghisetti

et al. 1993; Ghisetti & Vezzani 1997, 2000; Alberti et al. 1998).

This fault has c. 12 km of displacement and places the Mesozoic

and Tertiary carbonate sediments over the Messinian Laga

Formation (e.g. Fig. 1a). Along much of the eastern side of the

Montagna dei Fiori Anticline it is unclear if this thrust is

emergent as much of this area is covered by drift (Fig. 3).

However, projecting this fault 10 km along strike to the NNW

into an area of better exposure, geological maps show that the

members of the Laga Formation have little (Paradisi et al. 1969)

or no (Centamore et al. 1991) stratigraphic separation at this

position.

In addition, seismic reflection data from the southern part of

the Montagna dei Fiori Anticline do not support this option (Fig.

5a). In this area the forelimb dips moderately east and is

distinctly resolved; it is very clear that the thrust beneath the

anticline does not cut across this limb to emerge at the surface.

Presence of a major backthrust at the mountain front

The idea that large displacements beneath the Montagna dei Fiori

anticline are accommodated by a major backthrust at the base of

the Laga Formation (e.g. Fig. 1b) has been proposed by several

workers (Bally et al. 1986; Bigi et al. 1995; Barchi et al. 2001).

However, kinematic fabrics at the mountain front close have

unambiguous top to the east shear sense (Fig. 6; see also

Averbuch et al. 1995) related to the presence of an important

pre-existing detachment at the base of the Laga Formation

(Koopman 1983). In addition, backthrusts are not shown on any

of the geological maps of this area (Paradisi et al. 1969; Mattei

1987; Centamore et al. 1991; Vezzani & Ghisetti 1998), no

growth strata related to a major backthrust are evident in seismic

data (e.g. Fig. 5a), and backthrusting of the Laga Formation over

the Montagna dei Fiori and Acquasanta Anticlines raises serious

mechanical questions (Vann et al. 1986). Another possibility is

that a major backthrust is located at the base of the Pliocene as

suggested by Bally et al. (1986), but there is no map, field or

stratigraphic evidence for this structure and those workers

admitted that this is a ‘major problem’.

Burial of a formerly emergent thrust

The pre-Messinian carbonate succession is ubiquitously folded

and faulted and clearly predates contractional deformation. No

westward onlap of the Laga Formation onto the Montagna dei

Fiori Anticline can be observed in seismic data (e.g. Fig. 5a) or

in the field. In addition, in the external central–northern

Apennines, the spread of palaeomagnetic rotations with respect

to the Adriatic–African foreland for both the pre-Messinian

carbonate succession and the Messinian Laga Formation is very

similar (Dela Pierre et al. 1992; Mattei et al. 1995; Speranza et

al. 1997), indicating that rotations occurred after Messinian

times. Therefore there is no evidence for large pre-Pliocene

displacements.

Westward onlaps onto the Montagna dei Fiori Anticline are

present in some Pliocene units, but if this is the blanketing unit

then all of the displacement (.25 km) would have had to have

occurred almost instantaneously. This is because the post-

Messinian rock-record to the east of the Montagna dei Fiori

Anticline is up to 7 km thick and has accumulated almost

continuously. Indeed, the Costiera Structure (Fig. 5b) provides a

spectacular illustration of the depositional geometries that result

from contractional deformation during sediment deposition.

A good example of the problem with burial of an emergent

thrust is provided by the balanced cross-section of Calamita et

al. (1994, plate 1c). In this model, the main thrust beneath the

Montagna dei Fiori Anticline is shown to be buried at the

mountain front beneath middle Pliocene sediments, yet there is

c. 15 km separation of the top of the lower Pliocene along this

TESTING THRUST TECTONIC MODELS 5

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R . S . J. TOZER ET AL .6

fault. This implies that thrusting was almost instantaneous at the

end of the early Pliocene.

Loss of displacement into a zone of distributed strain

There is no field evidence for major zone(s) of distributed strain

at the mountain front (e.g. Fig. 1d), and none are reported in the

literature (e.g. Mattei 1987). In addition, measurements of the

anisotropy of magnetic susceptibility (AMS) in the Laga Forma-

tion show a magnetic fabric typical for weakly deformed

sediments, with the plane of magnetic foliation always coincident

with bedding (Sagnotti et al. 1998). The magnetic fabric at the

sites just east of the mountain front is similar to that at the other

sites, documenting no local variation related to an increase of

shortening (Averbuch et al. 1995).

Propagation of the thrust front into the foreland

Almost all published cross-sections for the area show that the

blind structures beneath the foreland are detached within the pre-

Messinian carbonate succession (Paltrinieri et al. 1982; Bally et

al. 1986; Ghisetti et al. 1993; Calamita et al. 1994; Bigi et al.

1995; Ghisetti & Vezzani 1997, 2000; Alberti et al. 1998;

Scisciani et al. 2002). This precludes these structures from

absorbing displacement along the major thrust beneath the

Acquasanta and Montagna dei Fiori Anticlines (Fig. 5a), because

the seismic data show that this thrust runs along the top of the

pre-Messinian carbonate succession.

However, the position of the detachment beneath the most

external blind structure (the Costiera Structure) has been reas-

sessed. Detailed analysis of this structure by Artoni & Casero

(1997) showed that it is detached from the carbonate succession

along the Messinian Gessoso-solfifera Formation. Therefore,

several kilometres of the displacement along the major thrust

beneath the surface anticlines could be accommodated by this

structure (e.g. Fig. 1e). Accounting for the remaining displace-

ment is more difficult. In view of the evidence against any of the

other solutions, the only viable interpretation is that large

amounts of shortening are accommodated by the remaining

structure beneath the foreland, the Bellante Structure (Fig. 5b).

This interpretation is supported by the recognition of imbricates

within the Laga and Teramo Formations in seismic reflection

data (Fig. 5b). These also appear to be detached from the top of

the pre-Messinian carbonate succession. In addition, thrust faults

are reported in several wells drilled on this structure.

A new structural model for the Montagna dei Fiori

Data and methods

The cross-section presented in Figure 7 was constructed at a scale of

1:100 000 using geological maps of the area (Paradisi et al. 1969; Mattei

1987; Centamore et al. 1991; Vezzani & Ghisetti 1998) and additional

structural data collected in the field. Where sufficient outcrop and

topographic relief allows, structure contours have been used to constrain

the precise geometry of geological boundaries.

In the subsurface, the cross-section has been completed using seven

exploration wells and 38 km of commercial seismic reflection profiles

(e.g. Fig. 5); a further 18 wells and 350 km of time-migrated seismic data

in the immediately surrounding area were used to determine the seismic

stratigraphy and calibrate these lines. Wells were projected a maximum

distance of 3.0 km and then tied to the relevant seismic line by converting

the depths of key horizons to two-way travel time using time–depth

curves or interval velocities from the closest common depth point on the

seismic line. In the eastern part of the area the seismic data covering the

Costiera Structure are of sufficient density and quality to allow loop

tying. However, to the west of this correlations based on seismic facies

are required because the data are more sparse and of lower quality.

Throughout the area there is much variation in the seismic signature of

fault planes; these are therefore interpreted both where discordant seismic

reflectors are present and where compound reflector terminations occur.

Following interpretation, key seismic horizons have been depth converted

with interval velocities calculated using the method of Dix (1955) and

assuming vertical raypaths.

The cross-section has been restored using 2DMove software; two

restoration algorithms have been used in particular. The pre-Messinian

carbonate succession that forms the base of the Adriatic monocline has

been restored using vertical shear, a type of deformation that is envisaged

to have been induced by the loading of Plio-Pleistocene sediments.

Further west, the dominant deformation mechanism within the thrust belt

Fig. 6. Photograph, field sketch and kinematic data (lower hemisphere,

equal area projection) illustrating pervasive S–C fabric developed in the

uppermost part of the Cerrogna Marls on the eastern side of the

Montagna dei Fiori Anticline (GR 42847.999N, 13837.229E; refer to Fig.

3 for location). The east-vergent kinematics at this outcrop strongly

suggests that a backthrust previously interpreted in this position cannot

be present. (See also Averbuch et al. (1995) for a similar interpretation.)

TESTING THRUST TECTONIC MODELS 7

R. S. J. TOZER ET AL .8

is flexural slip, as evidenced by ubiquitous slickenfibres parallel to

bedding and perpendicular to fold axes (e.g. Mattei 1987). These

structures were therefore restored using the flexural-slip algorithm using

pins located along zones of no interbed slip; that is, parallel to axial

surface traces in the plane of cross-section (Dahlstrom 1969). A zone of

distributed strain (undefined imbricates and/or ductile deformation) is

also interpreted where the Bellante Structure is located; this has simply

been area balanced with the corresponding area in the restored cross-

sections.

Assumptions

In our 2D restoration we assume that there is no movement of material in

or out of the section plane. To determine the correct orientation for such

a section of plane strain it was therefore necessary to analyse the

kinematics of minor structures in the field. The results of this are shown

in Figure 3. Using a combination of minor structures (S–C fabrics,

slickensides and duplexes) the mean transport direction at 13 locations

(180 striae) throughout the area has been determined. Although there is

some variation in these directions between individual sites, the mean

displacement vector of 0708N agrees well with previously published

kinematic analyses in this area in which values of 0758N (Mattei 1987;

249 striae) and 0628N (Averbuch et al. 1995; 107 striae) were reported.

The pin lines within the thrust belt are not vertical in the present-day

and partially restored cross-sections and therefore it is necessary to take

all measurements from a single horizon (key bed) that is representative of

the overall structure. The top of the Calcare Massiccio was selected for

this because it is composed of .1 km of platform carbonates, which have

probably controlled the style of deformation to a large extent. In addition,

its position (and therefore length) in the subsurface is commonly clearly

defined by seismic reflection data.

Results

In contrast to published cross-sections, which generally cross the

central part of the Montagna dei Fiori anticline in the area of

greatest structural complexity, the new section (Fig. 7) crosses an

area in which the surface structures are relatively simple. The

advantages of this line of section are that the surface strati-

graphic relationships and subsurface seismic data are much

clearer.

The westernmost structure is the Monte Gorzano Pleistocene

normal fault, along which the Laga Formation in the hanging

wall is juxtaposed against the Cerrogna Marls. The line of

section then runs to the south of the southern tip of the

Acquasanta Thrust; in this area only a broad syncline is present

in the Laga Formation, an arrangement that clearly demonstrates

that the Acquasanta thrust is a low-displacement structure. East

of this, only the uppermost formations in the carbonate succes-

sion (Cerrogna and Orbulina Marls) are exposed in the Montagna

dei Fiori Anticline, which has an open geometry defined by

outcrop and seismic data. The eastward onlap of the Laga

Formation onto the pre-Messinian carbonate succession below is

clearly shown by the cross-section.

In the subsurface at the western end of the cross-section, the

position of the main footwall thrust ramp is located beneath the

Monte Gorzano normal fault. East of this, beneath the Montagna

dei Fiori Anticline, lies a major thrust (T1), which has an

antiformal geometry (note that this is not the equivalent of the

more minor Salinello Thrust described previously); in the hang-

ing wall the pre-Messinian carbonate succession on the eastern

limb of the Montagna dei Fiori Anticline is cut off against this.

At the top of the footwall to this thrust (T1) is a condensed

succession of the Schlier and Bisciaro Formations, thickening

westward. Below this the rest of the carbonate succession has

normal thickness. However, on the eastern side of the subsurface

anticline a pre-existing normal fault is interpreted; this is similar

to those exposed in the Montagna dei Fiori Anticline (e.g.

Giannini 1960; Mattei 1987; Calamita et al. 1998). To the east of

this fault the Jurassic succession is condensed.

The footwall beneath the Montagna dei Fiori Anticline is itself

duplicated along a deeper thrust (T4), which dips moderately

west; there is c. 5.5 km of displacement along this. Thrusts T1

and T4 branch together in the subsurface to the east of the

Montagna dei Fiori Anticline along a major horizontal detach-

ment at the top of the pre-Messinian carbonate succession at

6100 m. Above the detachment, dips in the Laga Formation

immediately east of the Montagna dei Fiori Anticline decrease

from subvertical at the surface to gently east in the subsurface.

This defines a minor hanging-wall syncline in the Laga and

Teramo Formations. Beneath the syncline a gently west-dipping

thrust fault (T2) with c. 3.0 km of displacement branches from

the main detachment and separates this syncline from the

Bellante Structure to the east.

The Bellante Structure has an antiformal geometry and is

limited on its eastern side by a moderately west-dipping thrust

fault (T3), which branches from the main detachment below. The

internal structure is not defined in the cross-section, but com-

prises stacked slices of the Laga and Teramo Formations. The

structure has associated growth strata of early Pliocene age and

is sealed within the lower Pliocene ii unit of the Costiera

Structure.

Beneath the foreland and coastline the geometry of the

Costiera Structure is defined by a pre-kinematic unit of earliest

Pliocene age (lower Pliocene i unit). This has been deformed into

a major east-vergent anticline as a result of movement along an

underlying thrust fault (T5), which branches from the main

detachment. Associated with this structure are growth strata of

early Pliocene to late Pliocene age (LPii to UPQ). This structure

accommodates 4.5 km of shortening. Below the Adriatic Sea to

the east, the pre-Messinian carbonate succession dips gently west

and is penetrated by exploration wells 17 km offshore. Above

this, the Pliocene sediments onlap the top of the carbonates and

thin to the east.

The total shortening determined by restoration of this cross-

section is 32.6 km.

Shortening rates

In the new model almost all of the displacement is transferred to

the blind structures beneath the foreland (i.e. the portion of the

thrust system that has been analysed is kinematically closed).

Therefore the timing of deformation revealed by the blind

structures must equally relate to more internal structures. Seismic

stratigraphic analysis of the blind structures suggests that the

main period of contractional deformation began after deposition

of the Teramo Formation and during deposition of the Cellino

Formation, in the interval containing both G. margaritae and

Fig. 7. Present-day (top) cross-section covering the southern part of the Montagna dei Fiori Anticline and Acquasanta Syncline (refer to Fig. 3 for the line

of section). The restored cross-section shows the onlapping geometry of the Laga foredeep basin at the end of the Messinian; also shown are intermediate

steps in the restoration. Total shortening revealed by restoration is 32.6 km. Triangles represent hydrocarbon exploration wells; dashed arrows are pin

lines. The main thrusts are numbered T1–T5.

TESTING THRUST TECTONIC MODELS 9

G. puncticulata. The youngest structure (the Costiera Structure)

is sealed within sediments of late Pliocene age (G. inflata

biozone) and below Pleistocene sediments (H. balthica biozone).

Many of the first and last occurrences (FO and LO) of the

Pliocene Foraminifera have been calibrated (� 0.01 Ma) using

precession-controlled sedimentary cycles exposed in Sicily (Lou-

rens et al. 1996). It is therefore possible to calibrate this relative

time scale with absolute time (Fig. 8). Although no absolute

dating is available for H. balthica, at the type section for the

Plio-Pleistocene boundary in Calabria it is synchronous with first

occurrence of the calcareous nannofossil Gephyrocapsa spp.

.5.5 �m (Pasini & Colalongo 1997; Rio et al. 1997), which is

dated at 1.61 Ma (Lourens et al. 1996). Using this chronology,

deformation is estimated to have started at 4.3 Ma (� 0.2 Ma)

and ended at 1.7 Ma (� 0.1 Ma), a total period of 2.6 Ma

(� 0.3 Ma). The average shortening rate for the new model is

therefore 12.5 (� 1.5) mm a�1.

It is possible to break down further the deformation chronology

into two main steps corresponding to the periods of growth of the

Bellante and then Costiera Structure (Fig. 8). The start and end

dates discussed above relate to these two structures respectively;

additional seismic stratigraphic analysis shows that the Bellante

Structure was sealed during the later part of the early Pliocene (G.

puncticulata biozone; c. 2.5 (� 0.2) Ma) and that the Costiera

Structure began to develop just before this but within the same

interval (c. 2.7 (� 0.1) Ma). Using this refined chronology, the

initial 27.3 km shortening, accommodated by the Bellante Struc-

ture, occurred at a rate of 15.2 (� 3.5) mm a�1 (27.3 km between

4.3 and 2.5 Ma). This was followed by a period of much less rapid

shortening at a rate of 5.3 (� 1.1) mm a�1 (5.3 km between 2.7

and 1.7 Ma), as recorded by the Costiera Structure.

Discussion

General points

In the new cross-section the displacement beneath the Acqua-

santa and Montagna dei Fiori Anticlines is transferred to the

Bellante and Costiera Structures beneath the foreland. The key

to this interpretation is the presence of a major detachment at

a stratigraphically shallower level than the main detachment

within the Burano evaporites. This second detachment lies at

the base of the Laga and Pliocene Formations and below the

blind structures under the foreland. Thus it is no longer

Fig. 8. Range chart for key Pliocene and

Pleistocene Foraminifera in the Marche–

Abruzzi Apennines and Adriatic Sea; the

biostratigraphic ranges have been calibrated

in absolute time using dating by Lourens et

al. (1996). Using these dates it is possible

to interpret the deformation chronology for

the Bellante and Costiera Structures; this is

shown on the right of the figure. FCO, first

common occurrence; FO, first occurrence;

G., Globorotalia; LCO, last common

occurrence; LO, last occurrence; mP,

middle Pliocene.

R. S . J. TOZER ET AL .10

necessary to have an emergent thrust, backthrust, high-strain

zone or buried thrust to the east of the Montagna dei Fiori

Anticline.

East-vergent folds are present further east beneath the Adriatic

Sea, but these clearly involve the entire pre-Messinian carbonate

succession regardless of whether they are interpreted to have

been generated by thin- (Bally et al. 1986; Ori et al. 1986) or

thick-skinned thrusts (Argnani & Gamberi 1995). Therefore they

cannot accommodate any of the displacement along the main

thrust beneath the Montagna dei Fiori Anticline. For this reason,

the shallow structures in the new cross-section form a kinemati-

cally closed system; that is to say, shortening on one structure is

balanced by shortening on another elsewhere within the cross-

section.

The difference in structural style of the blind structures can be

correlated with a change in detachment composition. In the

Bellante Structure imbrication is the main shortening mechan-

ism; this structure is detached towards the base of the Laga

Formation. In contrast, thrust-propagation folding along one

dominant thrust characterizes the Costiera Structure; this is

detached along the Gessoso-solfifera Formation, an easy slip

horizon composed of marl and gypsum. Specific predictions of

the models are therefore compatible with the geological reality.

The origin of the Laga Formation’s onlap around the Acqua-

santa Syncline along the line of section is debatable. A similar

relationship is well mapped and described on the Montagnone

Anticline adjacent to the south of the Montagna dei Fiori

Anticline (e.g. Centamore et al. 1991; Vezzani & Ghisetti 1998;

Scisciani et al. 2001). It might seem coincidental that the marker

bed exposed along the cross-section pinches out on the crest of

the Montagna dei Fiori Anticline (Fig. 7); was this a pre-existing

structural high at the time of Laga deposition? The lack of

carbonate conglomerates and/or breccias (derived from structural

highs) within this formation suggests not, and if an onlapping

foredeep basin were to be folded, some of the onlaps would

naturally coincide with the crests of anticlines.

It is tempting to interpret that the location of the Monte

Gorzano Normal Fault has been controlled by the T1 thrust ramp

below (Fig. 7). However, if the position of this thrust ramp is

mapped using additional seismic data, serial cross-sections and

palaeomagnetic constraints (Speranza et al. 2003), it strikes

north–south beneath the Laga Mountains, a trend that is oblique

to the strike of the normal fault (Fig. 3). The coincidence of

these two structures is therefore simply a quirk of the section

line.

The shortening value for the Costiera Structure (4.5 km) in

this study is likely to represent only a minimum, as shortening

by distributed strain is not considered and the internal structure

is simplified. More detailed restoration of the structure by Artoni

& Casero (1997) suggests that this structure accommodates

7.4 km shortening. This is because their interpretation contains

five imbricates in contrast to the simplified structure shown in

Figure 7. In addition, a greater amount of shortening was

determined by Artoni & Casero (1997) as a result of their

assumption that deformation commenced during the earliest

Pliocene. In contrast, the analysis in this study shows that the

sediments deposited during this time have a pre-kinematic

geometry and therefore predate compressional deformation.

Comparison with published models

Many existing models fail because of their interpretations of

emergent thrusts, which are incompatible with geological map

patterns for the Acquasanta Anticline (Bally et al. 1986; Ghisetti

et al. 1993 (thin-skinned model); Calamita et al. 1994; Scisciani

et al. 2002) or the Montagna dei Fiori Anticline (Ghisetti et al.

1993 (thick-skinned model); Ghisetti & Vezzani 1997, 2000;

Alberti et al. 1998). There are also problems with the two

alternative explanations that have been presented for the ‘miss-

ing’ displacement on the eastern side of the Montagna dei Fiori

Anticline. Burial of a formerly emergent thrust below the

unconformity at the base of the middle Pliocene introduces the

‘instantaneous thrusting’ problem (Calamita et al. 1994; Sciscia-

ni et al. 2002) and there is no evidence reported for a major

backthrust in this area (Bally et al. 1986; Ghisetti et al. 1993

(thin-skinned model); Barchi et al. 2001). None of these

structures are present in the new models so these do not suffer

from the problems described above.

The key difference between the new model and those

described above is that none of these solutions are required

because all of the displacement has been transferred to the blind

structures below the foreland. The amount of shortening east of

the Sibillini mountains determined by restoration of the cross-

section (32.6 km) is therefore much less than previously esti-

mated (e.g. 52 km, Calamita et al. 1994; c. 60 km, Bigi et al.

1995). This is because the blind structures are kinematically

linked to the more internal structures.

Basement involvement and magnetic modelling

The new cross-section is completed only to the base of the

Calcare Massiccio–top of the Burano evaporites. This horizon

corresponds to the deepest reflector that can be identified with

confidence in the seismic data; its position can also be estimated

with knowledge of the thicknesses of the units above. The

horizon therefore provides the best guide to the depth of the top

of the basement below; in the footwall below the Acquasanta

Syncline it lies .1 km shallower than below the foreland despite

the reduced thickness of the pre-Messinian carbonate succession

below the foreland. Is this difference in elevation due to a pre-

existing structural high or to involvement of the basement in the

thrust belt? Given that the Jurassic and Miocene pelagic

carbonate sediments are more condensed in the region of lower

basement elevation (below the foreland), the currently more

elevated area cannot have been a pre-existing high at the time of

their deposition. Although it is not possible to rule out the

presence of a pre-existing high at the eastern margin of the Laga

Basin, it seems more likely that there is a component of

basement involvement in the thrust belt at this position. This

hypothesis is supported by the most recent magnetic anomaly

map of Italy (Chiappini et al. 2000), which shows that a þ53 nT

magnetic anomaly is centred below the southern part of the

Montagna dei Fiori Anticline (Fig. 9). The line of section runs

across the southern part of this anomaly.

To test the hypothesis of basement involvement a magnetic

model was constructed for the line of section (Fig. 10a).

Assuming a constant geothermal gradient of 25 8C km�1 along

the line of section (della Vedova et al. 2001), the 600 8C

isotherm (corresponding to the Curie temperature of magnetite)

was placed at the base of the model at 24 km; above this, a top

basement horizon was added 1600 m below the base of the

Calcare Massiccio. Although there are no susceptibility measure-

ments available for the Verrucano, it is thought to have moderate

susceptibility by analogy with similar formations (Speranza &

Chiappini 2002); a value of 10 000 �SI was therefore assigned to

the basement (including Verrucano and underlying basement

down to 24 km depth). This value represents a mix between

susceptibility values of the Verrucano and similar low-grade

TESTING THRUST TECTONIC MODELS 11

metamorphic rocks (c. 1000 �SI), and high-grade and lower

crustal rocks, which are known to be very magnetic (up to

100 000 �SI, e.g. Rochette 1994). The Laga–Plio-Pleistocene

below the foreland were also added as a simplified polygon with

a susceptibility of 100 �SI (e.g. Sagnotti et al. 1998). The

Triassic to Miocene carbonate succession is known to be

essentially non-magnetic (Speranza & Chiappini 2002).

The results of this first model are shown in Figure 10a. In the

eastern part of the cross-section the positive anomaly exceeding

50 nT represents the northernmost part of a regional intense

(100–200 nT) positive anomaly characterizing the whole external

central Apennines (Chiappini et al. 2000; Fig. 9) and interpreted

to arise from intrusions at 13–18 km depth within the basement

(Speranza & Chiappini 2002); the target of the modelling was

not this but the more subtle positive anomaly over the Montagna

dei Fiori Anticline. The top basement horizon was therefore

modified to obtain a match with this; the result is shown in

Figure 10b. A good fit is achieved between the observed and

modelled anomaly by adding a major top basement step (from

�8.3 to �15.0 km) below the Montagna dei Fiori Anticline and

deepening the basement to the west of this; there can be

confidence in this solution because a change in only one

parameter is necessary to obtain agreement. The basement step

lies directly below the anticline, a geometry that is similar to

idealized models of reactivated half-grabens (e.g. Williams et al.

1989). However, we do not rule out the possibility that the

displacement on thrust T4 could also be the expression of

thrusting or distributed shortening within the half-graben. In

summary, there is evidence for a component of basement influ-

ence to the surface structure in the Montagna dei Fiori area.

Conclusions

A repetition of the seismic stratigraphy at depth below the

Acquasanta and Montagna dei Fiori Anticlines indicates that a

major thrust lies beneath these structures. A new model has

therefore been constructed to illustrate a possible solution to the

‘missing’ displacement on the eastern side of the Montagna dei

Fiori Anticline. In the absence of data showing a major emergent

thrust, backthrust, buried thrust or high-strain zone, the solution

is to balance the displacement with shortening of the blind

Bellante and Costiera Structures beneath the foreland. These are

detached at the base of the Pliocene, a shallower stratigraphic

level than the Acquasanta and Montagna dei Fiori Anticlines,

and this is therefore a viable solution. The main implication is

that the Bellante Structure accommodates a large amount of the

total shortening, .20 km. The total amount of shortening for the

new model is 32.6 km.

The secular shortening rate for the new model is 12.5 mm a�1.

This can be broken down into two periods corresponding to

growth of the blind Bellante and Costiera Structures; early

shortening took place at c. 15 mm a�1, followed by shortening at

one-third of this rate during the final stages of contractional

deformation in the central Apennines.

The base of the footwall to the Acquasanta and Montagna dei

Fiori Anticlines is shallower than the equivalent horizon beneath

the foreland, indicating a component of basement-involved uplift

of these structures. Modelling of magnetic anomalies along the line

of section indicates that a major top-basement step (lower in the

west) lies directly below the Montagna dei Fiori Anticline, further

supporting a composite thin- and thick-skinned uplift mechanism.

Financial support from the NERC (award reference GT 04/99/ES/118)

and BG is gratefully acknowledged (R.S.J.T.). BG generously provided

seismic and well data, and L. Vezzani kindly supplied geological maps of

the area. Figure 9 was prepared by F. D’Ajello Caracciolo. Cross-section

restoration was carried out using 2DMove provided by Midland Valley

Exploration. Constructive reviews by P. Shiner and E. Tavarnelli helped

to improve the manuscript and figures.

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Received 5 November 2004; revised typescript accepted 30 May 2005.

Scientific editing by Ian Alsop

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