ORIGINAL PAPER

Magnetic fabric of Pleistocene continental clays fromthe hanging-wall of an active low-angle normal fault(Altotiberina Fault, Italy)

Marco Maffione • Stefano Pucci • Leonardo Sagnotti •

Fabio Speranza

Received: 29 September 2010 / Accepted: 16 July 2011 / Published online: 21 August 2011

� Springer-Verlag 2011

Abstract Anisotropy of magnetic susceptibility (AMS)

represents a valuable proxy able to detect subtle strain

effects in very weakly deformed sediments. In compressive

tectonic settings, the magnetic lineation is commonly par-

allel to fold axes, thrust faults, and local bedding strike,

while in extensional regimes, it is perpendicular to normal

faults and parallel to bedding dip directions. The Altot-

iberina Fault (ATF) in the northern Apennines (Italy) is a

Plio-Quaternary NNW–SSE low-angle normal fault; the

sedimentary basin (Tiber basin) at its hanging-wall is in-

filled with a syn-tectonic, sandy-clayey continental suc-

cession. We measured the AMS of apparently undeformed

sandy clays sampled at 12 sites within the Tiber basin. The

anisotropy parameters suggest that a primary sedimentary

fabric has been overprinted by an incipient tectonic fabric.

The magnetic lineation is well developed at all sites, and at

the sites from the western sector of the basin it is oriented

sub-perpendicular to the trend of the ATF, suggesting that

it may be related to extensional strain. Conversely, the

magnetic lineation of the sites from the eastern sector has a

prevailing N–S direction. The occurrence of triaxial to

prolate AMS ellipsoids and sub-horizontal magnetic lin-

eations suggests that a maximum horizontal shortening

along an E–W direction occurred at these sites. The pres-

ence of compressive AMS features at the hanging-wall of

the ATF can be explained by the presence of gently N–S-

trending local folds (hardly visible in the field) formed by

either passive accommodation above an undulated fault

plane, or rollover mechanism along antithetic faults. The

long-lasting debate on the extensional versus compressive

Plio-Quaternary tectonics of the Apennines orogenic belt

should now be revised taking into account the importance

of compressive structures related to local effects.

Keywords AMS � Magnetic fabric � Detachment fault �Low-angle normal fault � Altotiberina fault � Central

Apennines

Introduction

The analysis of anisotropy of magnetic susceptibility

(AMS) is a non-destructive, relatively fast, and accurate

method for studying petrofabrics. It has long been estab-

lished that AMS data may represent a valuable strain proxy

and can be used to detect subtle strain effects in sediments

at the first stages of deformation (see reviews by Hrouda

1982; Borradaile 1988; Tarling and Hrouda 1993;

Borradaile and Henry 1997; Borradaile and Jackson 2004).

In particular, AMS is extremely sensitive to incipient strain

in fine-grained sediments well before other macro- and

mesoscopic strain features (such as cleavage) can be

observed in the field.

Therefore, the analysis of the AMS in weakly deformed

sediments has been increasingly used during the past few

decades to better constrain the structural setting of moun-

tain belts. It has been shown that if there is a stress field

acting on a sediment, causing progressive strain, the pri-

mary AMS fabric is modified according to the nature and

the extent of the deformation (e.g., Graham 1966; Sagnotti

and Speranza 1993; Sagnotti et al. 1998; Pares et al. 1999;

Pares 2004, among many others). Although no simple

M. Maffione � S. Pucci � L. Sagnotti � F. Speranza

Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy

Present Address:M. Maffione (&)

School of Geography, Earth and Environmental Sciences

(SoGEES), University of Plymouth, Plymouth, UK

e-mail: marco.maffione@plymouth.ac.uk

123

Int J Earth Sci (Geol Rundsch) (2012) 101:849–861

DOI 10.1007/s00531-011-0704-9

relation exists between degree of anisotropy and strain

magnitude, it has been demonstrated that the directions of

the principal axes of the magnetic susceptibility ellipsoid

are parallel to directions of the principal axes of the rock

strain ellipsoid, and that the AMS may be used to delineate

the preferred orientation of phyllosilicate grains in mud-

stones produced by compaction and tectonic processes

(Hrouda 1982; Tarling and Hrouda 1993). Such corre-

spondence makes AMS a fundamental tool to unravel the

tectonic setting of weakly deformed mudstones lacking

visible strain markers.

In Italy, there has been an extensive investigation of

mildly deformed marine clays of Oligocene to Pleistocene

age deposited both in the Apennine-Maghrebide belt

(Sagnotti and Speranza 1993; Mattei et al. 1997, 1999;

Sagnotti et al. 1998) and in the intraplate setting of Sardinia

(Faccenna et al. 2002). These studies have clearly dem-

onstrated that the magnetic lineation (i.e., the maximum

susceptibility axis) trends parallel to fold axes in foredeep

basins of the Apennine-Maghrebide chain, and it is thus

roughly orthogonal to the direction of maximum horizontal

shortening (Sagnotti et al. 1998; Speranza et al. 1999).

Conversely, the post-collisional syn-rift basins developed

at the rear of the Apennine chain yielded a magnetic lin-

eation sub-parallel to the direction of local extension

(Sagnotti et al. 1994; Mattei et al. 1999).

AMS studies in extensional settings of Italy were

systematically performed so far in basins bounded and cut

by high-angle (*60�) extensional faults, while recently, it

has been found that one of the most significant active

tectonic structures of the northern Apennines is a NE-

dipping low-angle (*20�) normal fault known as Altot-

iberina Fault (ATF, Brozzetti 1995; Barchi et al. 1998a;

Boncio et al. 1998, 2000; Collettini and Barchi 2002;

Brozzetti et al. 2009; Fig. 1). In this paper, we report on

an AMS study of lower Pleistocene continental clays

deposited in the northern sector of the Tiber basin (High

Tiber Valley, Central Apennines), which lies above the

ATF, and was formed (and strictly controlled) by the fault

activity.

Geologic and tectonic framework of the Altotiberina

Fault

During Miocene-Pliocene times, an eastward-migrating

compressive pulse built up the northern Apennines thrust-

and-fold belt, composed by a NE-verging imbricate system

(Lavecchia 1988; Ghisetti et al. 1993; Barchi et al.

1998c, among many others). During this phase, the Tuscan

allochthon (Paleogene-Early Miocene pelagites and turbi-

dites) and piggy-back Ligurian Unit (Late Jurassic ophiolites

and Cretaceous to Paleogene pelagites and turbidites)

overthrusted the inner Umbria-Marche domain (Triassic-

Jurassic shelf carbonates and Cretaceous to Miocene pel-

agites and turbidites).

Starting from the Late Pliocene-Early Pleistocene,

extension disrupted the compressive architecture of the

axial part of the northern Apennines by NW–SE trending

normal faulting, and possibly by minor normal to trans-

tensional features, oriented transversally to the major

faults. This last tectonic phase is responsible for the

arrangement of most Quaternary basins of the region. The

most important of these extensional basins in the study area

are the High Tiber Valley, to the west, and the intra-

mountain basins of Gubbio and Gualdo Tadino, to the east

(Fig. 1).

Field and seismic data pointed out the major role played

by a regional-scale, low-angle normal fault in the exten-

sional tectonics of the Umbria region: the Altotiberina

Fault, one of the best-studied active low-angle normal

faults in the world (Fig. 1; Brozzetti 1995; Barchi et al.

1998a; Boncio et al. 1998, 2000; Collettini and Barchi

2002; Brozzetti et al. 2009). The ATF bounds the western

part of the High Tiber Valley and extends for at least

70 km from Sansepolcro to Perugia, with an average

NNW–SSE strike and an eastward dip ranging from 20� to

30�. The ATF surface expression, north of Perugia, is

composed of a set of normal faults that forms a ‘‘domino-

like’’ structure rooted on the east-dipping, low-angle fault

plane (Brozzetti 1995; Fig. 1b). Conversely, a high-angle

southwest-dipping faults array bounds the eastern side of

the Quaternary basins, showing a mean NW–SE strike that

diverges from the ATF trend. Surface expression of

the High Tiber Valley is composed of three en-echelon,

NW–SE-elongated sub-basins (Sansepolcro, Umbertide,

and Ponte Pattoli basins, from NW to SE, see Fig. 1a).

These basins are separated by morphological thresholds

and infilled by a continental depositional sequence that

unconformably overlays the Meso-Cenozoic marine sedi-

mentary formations of the northern Apennines (Fig. 1a).

The analysis of CROP 03 deep seismic profile, as well

as of commercial seismic reflection profiles, calibrated

using surface geology and borehole data with the aid of

geophysical data, including gravity, magnetics, heat flow

measurements, and tomography, gives a clear picture of

the ATF at depth (Boncio et al. 2000; Collettini 2002;

Collettini and Barchi 2002; Collettini et al. 2000; Pauselli

and Federico 2002; Pauselli et al. 2006). Its geometry has

been clearly shown by a well-marked reflector that dips

toward the east, interrupting the stratigraphic markers

down to about 13 km (Barchi et al. 1998b), and by the

distribution of microseismic activity (Piccinini et al. 2003;

Chiaraluce et al. 2007). Seismic reflection profiles show

that both synthetic NE- and antithetic SW-dipping normal

faults, exposed at the surface, splay out from the ATF,

850 Int J Earth Sci (Geol Rundsch) (2012) 101:849–861

123

forming an asymmetric graben. A 3D image of the ATF

provided by its isobath map (Collettini and Barchi 2002;

Chiaraluce et al. 2007) illustrates a pronounced staircase

trajectory and portrays some longitudinal bends, due to

attitude variations along its strike.

Although most of the fault splays at the surface show

displacements on the order of some hundreds of meters, the

total displacement on the ATF at depth is about 5 km

(Brozzetti et al. 2009; Collettini et al. 2000). The large

amount of extension is evidenced from the direct super-

position of the Umbria-Marche Miocene turbidites above

the Triassic evaporites, drilled in the Perugia 2 and San

Donato wells (Anelli et al. 1994). The geometry of the

seismic reflectors of the Quaternary High Tiber Valley

bottom suggests a syn-sedimentary tectonics related to the

ATF activity that would have produced up to 1,200-m thick

sedimentary infill in the Sansepolcro basin (Barchi and

Ciaccio 2009). The oldest exposed sediments indicate an

Early Pleistocene onset of extensional activity, though a

large part of the syn-tectonic succession is buried and

unknown in the central part of the basins. Considering the

age of sediments in the basin and the maximum offset of

the splay bounding the basin, an average slip rate of 1 mm/

year in the past 2 Ma can be estimated (Collettini 2002).

Local and regional GPS networks reveal a *3 mm/year

velocity of crustal extension accommodated between the

Tyrrhenian and Adriatic coasts. The majority of such

extension is concentrated across the High Tiber Valley,

with slip rate of 2.4 ± 0.3 mm/year, and subsidence rate

up to 3 mm/year (Hreinsdottir and Bennett 2009).

For the Sansepolcro basin, in particular, the seismic data

do not show any evidence of compressive structures

involving the Quaternary sequence. On the contrary, robust

evidence of normal faults propagating through the basin

sediments has been recognized: NW–SE-striking meso-

scopic normal faults are common within the Early Pleis-

tocene sediments, as well as NE-dipping faults displacing

and tilting the Pleistocene sequence (e.g., along the

Anghiari Ridge; Cattuto et al. 1995; Delle Donne et al.

2007; ISPRA-CARG Project, Foglio 289). Although

sparse, field evidence of normal fault activity has been also

detected in the Late Pleistocene–Holocene alluvial terraces

(Brozzetti et al. 2009).

The geometry and kinematics of these normal faults are

consistent with the focal mechanisms of the instrumental

earthquakes, indicating a SW–NE active extension,

Figure 2

N

Sansepolcro

Città diCastello

GualdoTadino

Gubbio

Perugia Assisi

TrasimenoLake

43.2°

43.4°

43.6°

12.0° 12.4°

A

A’

Sansepolcro basin

Umbertide basin

Ponte Pattoli basin

a

Main Thrusts ATF breakaway zone Normal faults

Umbria-Marche DomainTuscan allochton Ligurian Unit Quaternary deposit

Carbonate

15 km

dept

h (k

m)

GubbioA A’

20 km Altotiberina fault

Tibervalley

b

High Tiber Valley

Fig. 1 a Simplified geological

map of the High Tiber Valley

showing the main structural

features, b the Altotiberina

Fault imaged by a seismic

reflection profile (after Boncio

et al. 2000)

Int J Earth Sci (Geol Rundsch) (2012) 101:849–861 851

123

coherent with the regional seismotectonic framework, as

defined by both geological and seismological data (e.g.,

Lavecchia et al. 1994). This suggests that in the study area,

the extensional tectonics persisted throughout the Quater-

nary with spatial and temporal continuity, preserving the

same regional strain field.

The huge amount of geological and geophysical data

collected during the last decade (and discussed above)

seems to be at odds with the hypothesis that the Tiber

basin, as well as other hinterland basins of the central

Apennines, is a thrust-top basin (e.g., Boccaletti et al. 1992,

1997; Bonini 1998). According to these authors, the basins

were generated by Pleistocene compressive events and

their bowl shape is interpreted as a syncline disrupted by

Late Pleistocene extensional faulting.

Sampling and methods

We carried out an extensive AMS study within the three

sub-basins composing the High Tiber Valley. Here, a

complex fluvio-lacustrine Pleistocene sequence, exposed

up to a maximum elevation of 350 m above the present

valley, shows a progressive incision and disruption at the

hanging-wall of the ATF system. It consists of three

depositional cycles, bottom to top: the Early Pleistocene

Unit with a prevalent sandy clay sedimentation of marsh

to braided-river depositional environment; Early-Middle

Pleistocene Unit with prevalent conglomeratic sediments,

and some sandy interbodies of fluvial and alluvial deposi-

tional environment; the uppermost Middle-Late Pleisto-

cene Unit, with sparse outcrops of conglomeratic deposits

of alluvial-fan depositional environment. The lowermost

Early Pleistocene Unit is unconformably overlain by the

Early-Middle Pleistocene Unit, and both are dated based on

mammal faunas (Ambrosetti et al. 1978, 1987; Cattuto

et al. 1995; Petronio et al. 2002; Argenti 2004).

All sampling sites are from the Early Pleistocene Unit

and were selected considering the lithology (silty–clayey

sediments), the alteration degree (blue–gray colored sedi-

ments indicating a minor weathering), and their relative

location, trying to distribute them homogeneously

throughout the entire High Tiber Valley. Silty and clayey

sediments were sampled at twelve different localities

(Fig. 2), collecting a total amount of 129 standard cylin-

drical cores. Eight to thirteen samples, spaced in at least

two distinct exposures, were gathered at each site using a

petrol-powered portable drill cooled by water. Samples

were oriented using a magnetic compass corrected for the

local magnetic declination (forecasted at ca. 2� at the time

of sampling, Carta Magnetica d’Italia al 2005).

Before the AMS sampling, an extensive 1:10,000-scale

geological and geomorphological mapping along the High

Tiber Valley was carried out. The field mapping was based

on pre-existing geological maps, produced in the frame-

work of previous national (ISPRA-CARG Project, Fo. 288

and 289) and/or regional (Regione Umbria) projects, and

was aimed at producing an integrated and homogeneous

stratigraphic scheme of the continental deposits that have

been characterized also from a sedimentological and

structural point of view. The obtained map (Figs. 1a, 2)

was integrated with observations derived from 1:33,000-

scale aerial photographs (GAI), and 10- and 90-m resolu-

tion digital elevation models (DEMs).

All laboratory analyses were carried out in the paleo-

magnetic laboratory at the Istituto Nazionale di Geofisica e

Vulcanologia (INGV, Rome, Italy). The low-field AMS of

one specimen per core was measured with a Multi-Func-

tion Kappabridge (MFK1-FA, AGICO) using the spinning

specimen method. For each sample, the measurements

allowed to reconstruct the AMS tensor defined by three

eigenvalues (i.e., the maximum, intermediate, and mini-

mum susceptibilities) indicated as kmax C kint C kmin (or

k1 C k2 C k3). The magnetic susceptibility tensor may be

represented geometrically by a triaxial ellipsoid, whose

axes are parallel to the eigenvectors of the AMS tensor,

which are the maximum, intermediate, and minimum

principal susceptibility directions, along which the induced

magnetization is strictly parallel to the direction of the

applied field. For each specimen, the mean magnetic sus-

ceptibility (km) is defined as km = (k1 ? k2 ? k3)/3, while

the degree of anisotropy and the shape of the AMS ellip-

soid were evaluated using the following parameters (see

Jelinek 1981):

– Corrected degree of AMS (P0)= expH{2[(g1 - g)2 ? (g2 - g2) ? (g3 - g)2]}

– Magnetic lineation (L) = k1/k2

– Magnetic foliation (F) = k2/k3

– Shape parameter (T) = (2g2 - g1 - g3)/(g1 - g3)

where, g1 = ln k1, g2 = ln k2, g3 = ln k3, g = (g1 ? g2 ?

g3)/3.

Moreover, in order to characterize the main magnetic

mineral fraction contributing to the magnetic susceptibility,

we measured the hysteresis properties and analyzed the

acquisition and back-field demagnetization of an isother-

mal remanent magnetization (IRM) of a few selected

samples, using a Micromag magnetometer (model 2900,

Princeton Measurement Corporation) equipped with a

vibrating sample probe. From hysteresis loops, when not

representative of the paramagnetic fraction only, we

determined the saturation remanent magnetization (Mrs),

the saturation magnetization (Ms), and the coercivity (Bc),

after correction for the paramagnetic slope. The coercivity

of remanence (Bcr) was determined by the IRM backfield

demagnetization curves.

852 Int J Earth Sci (Geol Rundsch) (2012) 101:849–861

123

Results

The AMS ellipsoid and related parameters were evaluated

at 12 sites using Jelinek statistics (Jelinek 1978), and

results are reported in Table 1, Figs. 2 and 3. The mean

susceptibility values (km) are quite homogeneous and range

from 100 to 350 9 10-6 SI, with a main clustering around

120–130 9 10-6 SI (Fig. 3a).

12° 00’00”43

° 30

’00”

MON

MAI

TER

ANS

UMB

PIE

RESI

GRUC

CASTCAV

SANS

43°

20’0

0”

43°

30’0

0”43

° 20

’00”

12° 00’00”

12° 10’00” 12° 20’00”

12° 10’00” 12° 20’00”

SCAS

Fig. 2 Geological and structural map of the studied area showing the sampling sites with the kmax (magnetic lineation) directions (black arrows)

Int J Earth Sci (Geol Rundsch) (2012) 101:849–861 853

123

The AMS ellipsoids are well defined at all sites, with

eigenvectors well clustered and small confidence ellipses.

The shape of the AMS ellipsoid is predominantly oblate

(T [ 0.3, see Fig. 3b, Table 1). However, three sites (ANS,

CAST, GRUC) are characterized by a moderately oblate

AMS ellipsoid (0.1 \ T \ 0.2), one site (RESI) shows a

triaxial magnetic fabric (T & 0) and one site (MAI) even

has a moderately prolate magnetic fabric (T = -0.158, see

Table 1). The magnetic foliation was very well defined,

with a tight clustering of kmin axes, at all sites but MAI and

GRUC, as discussed below.

The anisotropy degree (P0) is relatively low, with site

mean values always lower than 1.025 (Table 1). Moreover,

in our data set, there is no correlation between the anisot-

ropy degree and the mean susceptibility of the individual

specimens (Fig. 3c). Conversely, the anisotropy degree

appears to be correlated to the shape factor (T) for the

specimens from site MAI (Fig. 4). At sites MAI and

GRUC, the kmax axes are tightly grouped and define a clear

N–S magnetic lineation, whereas the kint and kmin axes

show a tendency to be roughly scattered in the plane per-

pendicular to the kmax cluster (Fig. 4). Moreover, all the

sites (except site SCAS) are characterized by a well-

developed magnetic lineation, as documented by limited

dispersion of the kmax and kint axes in the magnetic foliation

plane [with the semi-angle of the 95% confidence ellipse

around the mean kmax axis in the kmax - kint plane (e12)

always lower than 30�; see Table 1]. In particular, at nine

sites, the e12 value is even lower than 20�, indicating a very

good clustering of kmax axes from the individual speci-

mens. The declinations of the kmax axes from all samples

cluster around a mean direction of N10� (Fig. 3d), though

at site level, it varies between directions N303� and N61�(Table 1 and Fig. 2). Since the magnetic foliation plane in

weakly deformed sediments is commonly sub-parallel to

the bedding plane, we used the kmin axes of the magnetic

susceptibility to infer the local bedding attitude at the

sampling sites (see Table 1). Only at site MAI, the bedding

orientation was clearly exposed, and an accurate mea-

surement of its orientation in the field was possible. The

directions of kmin axes for all sites indicate that the sam-

pling area is characterized by a general sub-horizontal

bedding attitude (Fig. 3d), as it can be roughly deduced

from field observations, except at sites SCAS and ANS,

where strata are markedly tilted (with the azimuth of dip

plunging 51� toward the SW and 30� toward the NE,

respectively).

The hysteresis data indicate that, as expected (see

Rochette 1987), the paramagnetic clay matrix dominates

the magnetic susceptibility of the samples with km values

lower than 250 9 10-6 SI (Fig. 5). These samples also

reach IRM saturation in low fields (estimated at 0.2–0.3 T)

and show Bcr values in the range 15–30 mT (Fig. 5). These

data indicate that the magnetic susceptibility and AMS of

these samples are mostly determined by paramagnetic

minerals of the clay matrix and that magnetite is the main

component of the subordinate ferromagnetic (sensu lato)

fraction. A significant ferromagnetic contribution was only

found for few samples with the highest magnetic suscep-

tibility values (only eight samples have km [ 250 9 10-6

SI, as shown in Fig. 3a, and five of them are from site

SANS). For these samples, the hysteresis loops show

Table 1 Anisotropy of magnetic susceptibility results from the High Tiber Valley

Site Coordinates (deg) Bedding

Az dip/dip

n/N km (10-6 SI) L F P0 T D (�) I (�) e12 (�)

Latitude N Longitude E

SANS 43.523045 12.100983 298/2 10/11 266 1.004 1.015 1.019 0.529 240.6 0.4 20.4

SCAS 43.444040 12.228146 121/51 12/12 111 1.001 1.016 1.020 0.862 207.7 4.4 50.5

CAST 43.433689 12.266249 178/1 9/10 114 1.008 1.011 1.021 0.221 179.4 0.8 4.3

CAV 43.426679 12.261508 259/4 10/11 127 1.004 1.018 1.024 0.563 346.9 0.2 14.5

GRUC 43.409705 12.274222 255/11 11/11 157 1.004 1.006 1.010 0.231 173.5 1.6 12.7

MAI 43.403004 12.270098 065/15 10/10 179 1.004 1.003 1.008 -0.158 355.0 4.7 27.0

TER 43.357442 12.278236 058/5 10/13 209 1.004 1.016 1.021 0.646 039.5 5.0 7.5

MON 43.371923 12.318925 192/3 7/8 101 1.001 1.009 1.011 0.732 123.1 1.3 27.8

ANS 43.347642 12.311456 069/30 9/11 230 1.005 1.006 1.010 0.109 354.0 8.1 13.5

UMB 43.318218 12.365007 266/12 10/11 149 1.001 1.003 1.004 0.655 346.7 2.1 17.1

PIE 43.262574 12.403740 079/10 7/11 120 1.006 1.012 1.018 0.350 161.8 1.2 5.4

RESI 43.200427 12.434114 279/17 10/10 172 1.010 1.011 1.021 0.073 188.5 2.7 9.5

Geographic coordinates are in WGS84 datum. Bedding attitude is inferred from kmin axes orientation, except for site MAI (see text for

‘‘Discussion’’). n/N, number of samples used for the computation of the AMS tensor/number of studied samples at a site. km, L, and F, are mean

magnetic susceptibility, magnetic lineation, and foliation values, respectively. P0 and T are the corrected anisotropy degree and shape factor,

respectively, according to Jelinek (1981). D and I are the in situ site mean declination and inclination, respectively, of the maximum suscep-

tibility axis (kmax). e12 is the semi-angle of the 95% confidence ellipse around the mean kmax axis in the kmax - kint plane

854 Int J Earth Sci (Geol Rundsch) (2012) 101:849–861

123

features characteristic for assemblages of single-domain

(SD) ferrimagnetic grains, with Mrs/Ms ratios of about 0.5,

the saturation of IRM reached in fields of about 0.3–0.5 T

and the Bcr values in the range of 50–80 mT (Fig. 5).

These properties indicate that the ferromagnetic fraction

most likely consists of SD greigite (see Sagnotti and

Winkler 1999; Roberts et al. 2011), and its contribution to

mean susceptibility and AMS is not negligible.

Discussion

The low value of mean magnetic susceptibility (km = 162 9

10-6 SI), besides the shape of the hysteresis loops, indi-

cates a main contribution of the paramagnetic matrix.

Consequently, the AMS fabric is mostly determined by the

preferred orientation of clay minerals (Borradaile et al.

1986; Rochette 1987; Averbuch et al. 1995; Sagnotti et al.

1998; Speranza et al. 1999). Also, the low variability of the

magnetic susceptibility values indicates that the magnetic

mineralogy of our samples does not change substantially.

This is also confirmed by the lack of correlation between P0

and km (Fig. 3c), being the AMS anisotropy degree of a

rock and also a function of the intrinsic susceptibility of the

composing minerals (e.g., Rochette et al. 1992; Borradaile

and Jackson 2004). In fact, the AMS anisotropy degree (P0)in deformed sediments may also be related to the strain

degree of a rock acquired during progressive deformation

and metamorphism (e.g., Hrouda and Janak 1976; Hrouda

and Kahan 1991). The P0 values observed in our samples

(ranging between 1.004 and 1.024) are within the typical

range of variability for completely undeformed sediments

or for sediments at the very early stages of deformation

(e.g., Hrouda 1982; Pares 2004). These values are generally

lower than those measured for other weakly deformed

clays in the Italian peninsula (Sagnotti and Speranza 1993;

Sagnotti et al. 1994, 1998; Mattei et al. 1999; Maffione

et al. 2008). A prevalent oblate shape of AMS ellipsoids

indicates that the sampled sediments mostly preserve a

predominant sedimentary compactional AMS fabric (e.g.,

Tarling and Hrouda 1993). Actually, in low-energy envi-

ronments such as the marshes where the studied clayey

sediments were deposited, gravity-driven sedimentation

brings platy grains to lay with their longer dimensions

statistically parallel to the bedding-compaction plane.

Then, with further sediment burial, the effect of diagenetic

compaction on platy minerals by pressure and water

expulsion reinforces the parallelism between the magnetic

foliation and the bedding plane. A depositional-compac-

tional AMS fabric is consistent with field observations,

since no clear macroscopic pervasive deformation was

observed at the sampling sites. The clustering of the kmax

axes observed at all sites defines a distinct magnetic

lineation, which in absence of water currents, cannot be

related to sedimentary processes. Even though the effect of

water currents on magnetic susceptibility of fine-grained

a

b

c

d

N

90

180

270

kmaxKmin

1.045P’

-1

1T

1.000

4000km (10-6 SI)1.000

1.044P’

Fra

ctio

n

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

100 200 300 4000

Fig. 3 Results from the 115 specimens used for the computation of

the AMS tensor. a Frequency distribution of the mean susceptibility

(km) values for all the measured specimens. b Shape factor (T) versus

anisotropy degree (P0). c Anisotropy degree (P0) versus mean

susceptibility (km). d Lower hemisphere equal-area projection of the

kmax and kmin axes of the AMS ellipsoid (in situ coordinates) and their

mean values (larger symbols)

Int J Earth Sci (Geol Rundsch) (2012) 101:849–861 855

123

sediments has been tested by laboratory analog experi-

ments (e.g., Rees 1961), we think that the real depositional

conditions (i.e., sedimentation rate, depth of the basin,

water current velocity, etc.) of fine-grained sediments are

hardly reproducible in laboratory. More likely, grain size of

silty-clayey sediments is clearly indicative of a very low-

energy depositional environment, where water currents are

either absent or not strong enough to interfere with the

sedimentation processes. Furthermore, no studies about

paleocurrent directions were carried out so far from the

coarse-grained lithologies of the sampled formations, thus

we cannot test the existence of correlation between them

and the observed magnetic lineation directions. Con-

versely, analogously to other weakly deformed mudstones

in various compressive and extensional settings, the mag-

netic lineation that we found in the silty-clayey units may

rather be interpreted as the first effect of a superimposed

stress field. This hypothesis is supported by three lines of

evidence: (1) the triaxial to prolate AMS ellipsoid shapes

of sites MAI, ANS, and RESI; (2) the correlation between

T and P0 parameters observed at site MAI (Fig. 4); and (3)

the scattering of the kint and kmin axes around a plane

perpendicular to the cluster of kmax axes observed at sites

GRUC and MAI (Fig. 4). These features are not compati-

ble with an original ‘‘sedimentary’’ fabric and cannot be

related to the action of paleocurrents. In particular, the

dispersion of the kint and kmin axes is a clear tectonic effect

that occurs when mineral rotations are developed enough to

bring clay flakes to a high angle to the shortening direction

and to impart a magnetic fabric typical of the ‘‘pencil

structure’’ stage (e.g., Graham 1966; Pares 2004; Cifelli

et al. 2005). On the contrary, the oblate shape of the AMS

ellipsoid at site SCAS (T = 0.8, and kint and kmax dispersed

into a plane perpendicular to the kmin) is strongly indicative

of a pure sedimentary fabric, with no effect of water cur-

rent, as expected for these fine-grained lithologies.

In deformed sediments, the magnetic fabric has proven to

serve as a valuable strain proxy when visible strain markers

are not recognizable in the field, being the magnetic linea-

tion parallel to the maximum elongation axis (e1) of the

strain ellipsoid (Hrouda and Janak 1976; Hrouda and Kahan

1991). Previous works carried out on weakly deformed

clays from different Italian localities have found that during

the incipient stages of deformation, the magnetic foliation

maintains parallel to the bedding of the strata, whereas the

kmax axes tend to cluster parallel to the direction of the fold

axes in compressive settings (Sagnotti and Speranza 1993;

Averbuch et al. 1995; Mattei et al. 1997; Sagnotti et al.

1998; Speranza et al. 1999), and perpendicular to the nor-

mal faults (i.e., parallel to the stretching direction) in

extensional basins (Sagnotti et al. 1994; Mattei et al. 1997,

1999; Cifelli et al. 2004). Consequently, in tilted sequences,

the magnetic lineation should trend parallel or orthogonal to

the local bedding strike, when forming after compressive or

extensional tectonics, respectively. Therefore, the angle

between magnetic lineation and local bedding dip (or strike)

direction is a fundamental parameter that has the potential

to discriminate the compressive versus extensional settings

(e.g., Mattei et al. 1997). Furthermore, being the magnetic

fabric sensitive to the syn-sedimentary tectonics (e.g.,

Mattei et al. 1997; Sagnotti et al. 1998), AMS data provide

relevant information about the local tectonic regime acting

during sedimentation (in our case, during the detachment

fault formation).

Relying on the assumed parallelism between the mag-

netic foliation and the bedding planes, five out of twelve

sites have a sub-horizontal bedding attitude (dip B 5�) (see

Fig. 4, Table 1). Consequently, the relationship between

the orientation of the magnetic lineation and the bedding

attitude (which may discriminate the compressive versus

extensional origin of the magnetic lineation) can be eval-

uated for the only six sites (MAI, ANS, UMB, PIE, RESI,

GRUC) characterized by a well-defined magnetic lineation

(e12 \ 30�), and strata dip [ 5�. At five out of these six

sites (excluding site MAI), the magnetic lineation, trending

NNE to NNW (*N–S on average), is sub-parallel to the

respective strike of magnetic foliation planes, as always

observed in folds of compressive tectonic settings (e.g.,

Lowrie 1989). This evidence has three important implica-

tions: (1) the sediments of the High Tiber Valley deposited

above the ATF are affected by local folding, which is either

incipient or presents long wavelength, resulting hardly

visible in the field; (2) fold axes are predominantly N–S-

trending and roughly parallel to the local trend of the ATF;

(3) folds, which are routinely considered as proofs for

shortening and associated to thrust-sheet emplacement,

developed here in the purely extensional regime of the

ATF. The magnetic lineations of the remaining five sites

with sub-horizontal bedding attitude and e12 \ 30� trend

both NW to NNW (sites CAST, CAV, and MON) and NE

(sites SANS and TER; Table 1; Figs. 2, 4). Being the angle

between the magnetic lineation and the bedding strike not

inferred at these sites, we interpreted the origin of their

magnetic lineation on the basis of its relative direction with

respect to the regional stress field, which is extensional and

NE–SW-oriented according to microseismic and geodetic

data (Lavecchia et al. 1994; Boncio et al. 1996; Chiaraluce

et al. 2007; Hreinsdottir and Bennet 2009; D’Agostino

et al. 2009). In view of that, an extensional magnetic lin-

eation is inferred at sites SANS and TER, where it is

parallel to the stretching direction, and orthogonal to the

ATF. Conversely, a compressive magnetic fabric is infer-

red at sites CAST, CAV, and MON, since their magnetic

lineations are roughly orthogonal to the stretching direc-

tion, and parallel to the local strike of the ATF and to the

magnetic lineation found at the other tilted sites (Fig. 2).

856 Int J Earth Sci (Geol Rundsch) (2012) 101:849–861

123

N

90

180

270

K1K2K3

N

90

180

270

K1K2K3

N

90

180

270

K1K2K3

N

90

180

270

1.000 1.016P’

-1

1T

N

90

180

270

K1K2K3

1.000 1.025P’

-1

1T

N

90

180

270

K1K2K3

1.000 1.020P’

-1

1T

1.000 1.032P’

-1

1T

N

90

180

270

K1K2K3

N

90

180

270

K1K2K3

1.000 1.043P’

-1

1T

N

90

180

270

K1K2K3

1.000 1.044P’

-1

1T

1.000 1.025P’

-1

1T

N

90

180

270

K1K2K3

N

90

180

270

K1K2K3

1.000 1.025P’

-1

1T

N

90

180

270

K1K2K3

1.000 1.006P’

-1

1T

1.014P’

-1

1T

1.000

1.028P’

-1

1T

1.000

1.033P’

-1

1T

1.000

K1K2K3

Fig. 4 AMS stereoplots and relative P0–T diagrams for all sampled sites

Int J Earth Sci (Geol Rundsch) (2012) 101:849–861 857

123

More in detail, a pattern of the magnetic lineation is

observed across the High Tiber Valley. In fact, the sites

from the western flank of the valley (i.e., SANS and TER)

show a NE–SW- to ENE–WSW-trending magnetic linea-

tion, while the sites from the eastern flank are characterized

by a general N–S lineation (see Fig. 2). We propose an

extensional magnetic lineation for sites SANS and TER

(i.e., the only two sites with a *NE–SW-trending linea-

tion) and an compressive magnetic lineation for the

remaining sites. This suggests that portions of the hanging-

wall that are located to the west, adjacent to the main

NE-dipping fault, were more susceptible to the *NE–SW-

directed regional extension controlling the ATF activity,

while the eastern domains that are close to the antithetic

SW-dipping fault array recorded an *E–W-directed

compression. Consequently, while the stretching direction

inferred by AMS analysis at sites SANS and TER (i.e.,

NE–SW) is compatible with the regional stress field con-

trolling fault activity, the E–W compression inferred at the

remaining sites is at odds with the fault regional kinemat-

ics, and seems to be related to local fault plane directions.

In summary, our data reveal a coexistence of both

compressive and extensional stress field affecting the

coeval Early Pleistocene deposits lying over the shallow

hanging-wall of the ATF. The existence of compressive

structures in extensional settings characterized by low-

angle extensional faults has been already documented

elsewhere and studied in detail in the Basin and Range

Province (Schlische 1995; Janecke et al. 1998). These

authors suggested the possibility to have folds in exten-

sional regime through eight different mechanisms. These

mechanisms are sorted on the basis of the geometry of the

resulting fold, which can be characterized by axis parallel

(‘‘longitudinal’’), orthogonal (‘‘transverse’’), or ‘‘oblique’’

to the associated normal fault. Models concerning the

formation of oblique folds (Janecke et al. 1998) consider

(1) the local effects related to a non-planar fault surface,

(2) the transtensional strain, or (3) a simultaneous effect of

the longitudinal and transverse mechanisms, as possible

driving mechanisms. Furthermore, recent numerical mod-

eling on extensional faults demonstrated that the uppermost

part (*4 km) of the hanging-wall block can be dominated

-1 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8 1B (T)

M (

Am

2 )

2.0E-05

1.5E-05

1.0E-05

5.0E-06

-5.0E-06

-1.0E-05

-1.5E-05

-2.0E-05

ANS03

a

Bcr = 29.0 mT

B (T)

M (

Am

2 )

ANS03

8.0E-08

6.0E-08

4.0E-08

2.0E-08

-8.0E-08

-6.0E-08

b

Bc = 38.8 mT

Mrs = 1482 nAm2

Ms = 2838 nAm2

SANS04

1 0.8 0.6 0.4 0.2 0.2 0.4 0.6 0.8 1B (T)

4.0E-06

3.0E-06

2.0E-06

1.0E-06

M (

Am

2 )

-4.0E-06

-3.0E-06B (T)

M (

Am

2 )

-1 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8 1-5.0E-06

-1.0E-05

-1.5E-05

-2.0E-05

5.0E-06

1.0E-05

1.5E-05

2.0E-05

c

-1 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8 1

Bcr = 53.3 mT

SANS04

M (

Am

2 )

B (T)

2.0E-06

1.5E-06

-2.0E-06

d

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

-1.0E-06

-1.5E-06

-5.0E-07

5.0E-07

1.0E-06

-4.0E-08

-2.0E-08

-1.0E-06

-2.0E-06

Fig. 5 Hysteresis measurements (a, c) and IRM acquisition/backfield

remagnetization curves (b, d) for a representative sample with mean

magnetic susceptibility (km) \ 250 9 10-6 SI (upper plots), and a

representative sample with km [ 250 9 10-6 SI (lower plots). In this

latter case, the hysteresis parameters have been computed after

subtraction of the paramagnetic contribution (the uncorrected hyster-

esis measurement is shown in the inset on the bottom right of the

diagram)

858 Int J Earth Sci (Geol Rundsch) (2012) 101:849–861

123

by a widespread, horizontal, compressive stress (e.g., Bott

2009).

In the past, evidence of compressive tectonics docu-

mented in the High Tiber Valley led to the formulation of

two incompatible theories about the Plio-Pleistocene tec-

tonics of the northern Apennines extensional basins: the

first considered the Plio-Pleistocene sediments as deposited

in purely extensional post-collisional syn-rift basins

(Barchi et al. 1999; Collettini et al. 2000; Barchi and

Ciaccio 2009), while the second proposed compressive

pulses punctuating the extensional tectonics (Bernini et al.

1990; Boccaletti et al. 1992, 1994, 1997; Bonini 1998;

Bonini and Tanini 2009).

Our AMS data from the shallow hanging-wall of the

ATF revealed the presence of synchronous local exten-

sional and compressive strain, resulted after Early Pleis-

tocene tectonics. While the extensional strain is

compatible with the regional tectonics of the ATF, the

compressive strain features may be produced by two

possible mechanisms. A first mechanism could be the

motion of the ATF hanging-wall block above a shallow

undulated fault surface, which generated gently folds

parallel to the irregularities of the fault plane at depth (i.e.,

*NS-trending). This is in agreement with the model of

Janecke et al. (1998) for folds formation in extensional

regimes and is supported by seismic reflection profiles,

which portray a staircase trajectory of the ATF plane at

depth (Collettini and Barchi 2002; Chiaraluce et al. 2007;

Barchi and Ciaccio 2009). A second mechanism could be

the accommodation of the hanging-wall during exten-

sional activity, which resulted in the formation of rollover

anticlines. This latter hypothesis is supported by the

observation that the magnetic lineation in the western

domains of the High Tiber basin is sub-parallel to the

local direction of the antithetic faults (Fig. 2). Accord-

ingly, we suggest that the evidence for compressive strain

should not be interpreted as the result of late Miocene-

Quaternary compressive events (as proposed in the past),

but they may rather reflect local effects in the framework

of a regional extensional regime. The same interpretation

could be applied to similar compressive structures

observed in other sedimentary basins at the rear of the

Apennine chain lying on top of low-angle extensional

faults (Ambrosetti et al. 1978; Bernini et al. 1990;

Boccaletti et al. 1992, 1994; Collettini et al. 2006). Fur-

ther AMS studies of the Plio-Quaternary sediments from

the High Tiber Valley and similar basins of the central

Apennines would be needed to unravel the regional versus

local origin of such compressive structures, and possibly

reconstruct the regional stress field acting in areas where

low-angle detachment faults occur.

Conclusion

The lower Pleistocene continental clayey sediments sam-

pled in the High Tiber Valley are characterized by a well-

developed magnetic fabric. The AMS data collected in our

study indicate that the original sedimentary-compactional

fabric of these sediments has been partly overprinted by

strain effects linked to the activity of the Altotiberina low-

angle normal fault. As concluding remarks of our study, we

stress that:

• AMS data from the High Tiber Valley are consistent with

both structural evidence gathered in the past from the

Basin and Range detachment fault system and numerical

modeling, and clearly document that the hanging-wall of

a low-angle extensional fault can be characterized by

folds, developed in function of the non-planarity of the

detachment fault at depth, or by the development of

rollover anticlines along antithetic faults.

• Lack of suitable outcrops of the clayey sediments of the

High Tiber Valley limits a clear detection of these

compressive structures in the field. Thus, our results

confirm the potentiality of AMS to reveal hidden

tectonic features characterizing mildly deformed sedi-

ments, which appear undeformed at the outcrop scale.

• According to our interpretation, the long-standing

opposite views on the late Miocene-Pleistocene tecton-

ics of the internal northern Apennines (extensional

versus compressive) can now be reconciled by consid-

ering the occurrence of compressive features, as local

effects linked to the activity of low-angle normal faults.

Acknowledgments It is a pleasure and an honor for us to contribute

to the volume celebrating the scientific career of Prof. Frantisek

Hrouda, in recognition of his fundamental contribution to the research

on the magnetic anisotropy of rocks and its application in geophysics.

We are grateful to the reviewers Martin Chadima (also associate

editor) and Massimo Mattei for their very constructive comments,

which helped to considerably improve the original manuscript. This

work has been carried out in the framework of the INGV-DPC 2007–

2009 Seismological Projects, S5 ‘‘Test-sites’’: ‘‘Altotiberina normal

fault’’ WP 1.5, and have benefited of INGV funds.

References

Ambrosetti P, Carboni MG, Conti MA, Costantini A, Esu D, Gadin A,

Girotti O, Lazzarotti A, Mazzanti R, Nicosia U, Parisi R,

Sandrelli F (1978) Evoluzione paleogeografica e tettonica dei

bacini tosco-umbro-laziali nel Pliocene e nel Pleistocene infe-

riore. Mem Soc Geol Ital 19:573–580

Ambrosetti P, Bosi C, Carraro F, Ciaranfi N, Panizza M, Papani G,

Vezzani L, Zanferrari A (1987) 1:500, 000 Neotectonic map of

Italy. CNR Progetto Finalizzato Geodinamica, Litografia Artis-

tica Cartografica, Florence

Int J Earth Sci (Geol Rundsch) (2012) 101:849–861 859

123

Anelli L, Gorza M, Pieri M, Riva M (1994) Subsurface well data in

the northern Apennines (Italy). Mem Soc Geol Ital 48:461–471

Argenti P (2004) Plio-quaternary mammal fossiliferous sites of

Umbria (Central Italy). Geol Rom (2003–2004) 37:67–78

Averbuch O, Mattei M, Kissel C, Frizon de Lamotte D, Speranza F

(1995) Cinematique des deformations au sein d’un systeme

chevauchant aveugle: l’exemple de la ‘‘Montagna dei Fiori’’ (front

des Apenins centraux, Italie). Bull Soc Geol France 5:451–461

Barchi MR, Ciaccio MG (2009) Seismic images of an extensional

basin, generated at the hanging-wall of a low-angle normal fault:

the case of the Sansepolcro basin (Central Italy). Tectonophysics

479:285–293

Barchi MR, Minelli G, Pialli G (1998a) The CROP 03 profile: a

synthesis of results on deep structures of the Northern Apen-

nines. Mem Soc Geol Ital 52:383–400

Barchi MR, De Feyter A, Magnani MB, Minelli G, Pialli G, Sotera

BM (1998b) Extensional tectonics in the Northern Apennines

(Italy): evidence from the CROP03 deep seismic reflection line.

Mem Soc Geol Ital 52:527–538

Barchi M, De Feyter A, Magnani MB, Minelli G, Pialli G, Sotera BM

(1998c) The structural style of the Umbria-Marche fold and

thrust belt. Mem Soc Geol Ital 52:557–578

Barchi MR, Paolacci S, Paeselli C, Pialli G, Merlini S (1999)

Geometria delle deformazioni estensionali recenti del bacino

dell’Alta Val Tiberina fra S. Giustino Umbro e Perugia: evidenze

geofisiche e consideraioni geologiche. Boll Soc Geol Ital

118:617–625

Bernini M, Boccaletti M, Moratti G, Papani G, Sani F, Torelli L

(1990) Episodi compressivi neogenico-quaternari nell’area

estensionale tirrenica nord-orientale Dati in mare e a terra.

Mem Soc Geol Ital 45:577–589

Boccaletti M, Cenina-Feroni A, Martinelli P, Moratti G, Plesi G, Sani

F (1992) Late Miocene-Quaternary compressive events in the

Tyrrhenian side of the Northern Apennines. Ann Tectonicae

4:214–230

Boccaletti M, Cerrina Feroni A, Martinelli P, Moratti G, Plesi G, Sani

F (1994) L’area tosco-laziale come dominio di transizione tra il

bacino tirrenico e i thrusts esterni: rassegna di dati mesostruttu-

rali e possibili relazioni con le discontinuita del ‘‘Ciclo

Neoautoctono’’. Mem Descr Carta Geol Ital 49:9–22

Boccaletti M, Granelli G, Sani F (1997) Tectonic regime, granite

emplacement and crustal structure in the inner zone of the

northern Apennines (Tuscany, Italy): a new hypothesis. Tectono-

physics 270:127–143

Boncio P, Brozzetti F, Lavecchia G (1996) State of stress in the

Northern Umbria-Marche Apennines (Central Italy): inferences

from microearthquakes and fault kinematics analyses. Ann

Tecton 10:80–97

Boncio P, Brozetti F, Ponziani F, Barchi M, Lavecchia G, Pialli G

(1998) Seismicity and extensional tectonics in the northern

Umbria-Marche Apennines. Mem Soc Geol Ital 52:539–555

Boncio P, Brozzetti F, Lavecchia G (2000) Architecture and

seismotectonics of a regional low-angle normal fault zone in

central Italy. Tectonics 19:1038–1055

Bonini M (1998) Chronology of deformation and analogue modelling

of the Plio-Pleistocene ‘‘Tiber Basin’’: implications for the

evolution of the Northern Apennines (Italy). Tectonophysics

285:147–165

Bonini M, Tanini C (2009) Tectonics and quaternary evolution of the

Northern Apennines watershed area (upper course of Arno and

Tiber rivers, Italy). Geol J 44:2–29. doi:10.1002/gj.1122

Borradaile GJ (1988) Magnetic susceptibility, petrofabrics and strain.

Tectonophysics 156(1–2):1–20. doi:10.1016/0040-1951

Borradaile GJ, Henry B (1997) Tectonic applications of magnetic

susceptibility and its anisotropy. Earth Sci Rev 42(1–2):49–93.

doi:10.1016/S0012-8252

Borradaile GJ, Jackson M (2004) Anisotropy of magnetic suscepti-

bility (AMS): magnetic petrofabrics of deformed rocks. In:

Martın-Hernandez F, Luneburg C, Aubourg C, Jackson M (eds)

Magnetic Fabric methods and applications. Geol Soc Lond Spec

Publ 238:299–360

Borradaile GJ, Mothersill J, Tarling D, Alford C (1986) Sources of

magnetic susceptibility in a slate. Earth Planet Sci Lett 76:336–

340

Bott MHP (2009) Stresses in planar normal faulting: shallow

compression caused by fault-plane mismatch. J Struct Geol

31:354–365

Brozzetti F (1995) Stile deformativo della tettonica distensiva

nell’Umbria Occidentale: l’esempio dei Massicci Mesozoici

Perugini. Stud Geol Camerti 1995/1:105–119

Brozzetti F, Boncio P, La Vecchia G, Pace B (2009) Present activity

and seismogenic potential of a low-angle normal fault system

(Citta di Castello, Italy): constraints from surface geology,

seismic reflection data and seismicity. Tectonophysics 463:31–46

Carta Magnetica d’Italia al (2005) CD-ROM, Istituto Nazionale di

Geofisica e Vulcanologia (INGV), Rome, Italy

Cattuto C, Cencetti C, Fisauli M, Gregori L (1995) I bacini

pleistocenici di Anghiari e Sansepolcro nell’alta valle del

Tevere. II Quat 8:119–128

Chiaraluce L, Charabba C, Collettini C, Cocco M (2007) Architecture

and mechanics of an active low-angle normal fault: Alto

Tiberina Fault, northern Apennines, Italy. J Geophys Res

112:B10310. doi:10.1029/2007JB005015

Cifelli F, Mattei M, Hirt AM, Gunther A (2004) The origin of tectonic

fabrics in ‘‘undeformed’’ clays: the early stages of deformation

in extensional sedimentary basins. Geophys Res Lett 31(9):

L09604. doi:10.1029/2004GL019609

Cifelli F, Mattei M, Chadima M, Hirt AM, Hansen A (2005) The

origin of tectonic lineation in extensional basins: combined

neutron texture and magnetic analyses on ‘‘undeformed’’ clays.

Earth Planet Sci Lett 235:62–78

Collettini C (2002) Hypothesis for the mechanics and seismic

behaviour of low-angle normal faults: the example of the

Altotiberina fault Northern Apennines. Ann Geophys 45:683–697

Collettini C, Barchi MR (2002) A low-angle normal fault in the

Umbria region (Central Italy): a mechanical model for the related

microseismicity. Tectonophysics 359:97–115. doi:10.1016/S0040-

1951(02)00441-9

Collettini C, Barchi MR, Paeselli C, Federico C, Pialli G (2000)

Seismic expression of active extensional faults in northern

Umbria (central Italy). In: Cello G, Tondi E (eds) The resolution

of geological analysis and models for earthquake faulting

studies. J Geodyn 29:309–321

Collettini C, De Paola N, Holdsworth RE, Barchi MR (2006) The

development and behaviour of low-angle normal faults during

Cenozoic asymmetric extension in the Northern Apennines. Italy

J Struct Geol 28:333–352

D’Agostino N, Mantenuto S, D’Anastasio E, Avallone A, Barchi M,

Collettini C, Radicioni F, Stoppini A, Fastellini G (2009)

Contemporary crustal extension in the Umbria–Marche Apen-

nines from regional CGPS networks and comparison between

geodetic and seismic deformation. Tectonophysics 476:3–12.

doi:10.1016/j.tecto.2008.09.03

Delle Donne D, Piccardi L, Odum JK, Stephenson WJ, Williams RA

(2007) Highresolution shallow reflection seismic image and

surface evidence of the Upper Tiber Basin active faults

(Northern Apennines, Italy). Boll Soc Geol Ital 126(2):323–331

Faccenna C, Speranza F, D’Ajello Caracciolo F, Mattei M, Oggiano G

(2002) Extensional tectonics on Sardinia (Italy): insights into the

arc-back-arc transitional regime. Tectonophysics 356:213–232

Ghisetti F, Barchi M, Bally AW, Moretti I, Vezzani L (1993)

Conflicting balanced structural sections across the central

860 Int J Earth Sci (Geol Rundsch) (2012) 101:849–861

123

Apennines (Italy): problems and implications. In: Spencer AM

(ed) Generation, accumulation and production of Europe’s

hydrocarbons III. Eur Assoc Petrol Geol Spec Publ 3:219–231,

Springer-Verlag, Berlin

Graham JW (1966) Significance of magnetic anisotropy in Appala-

chian sedimentary rocks. In: Steinhart JS, Smith TJ (eds) The

Earth beneath the continents. Geophysical Monograph Series 10.

American Geophysical Union, Washington, DC, pp 627–648

Hreinsdottir S, Bennett RA (2009) Active aseismic creep on the Alto

Tiberina low-angle normal fault, Italy. Geology 37:683–686.

doi:10.1130/G30194A.1

Hrouda F (1982) Magnetic anisotropy of rocks and its application in

geology and geophysics. Surv Geophys 5(1):37–82. doi:10.1007/

BF01450244

Hrouda F, Janak F (1976) The changes in shape of the magnetic

susceptibility ellipsoid during progressive metamorphism and

deformation. Tectonophysics 34:135–148

Hrouda F, Kahan S (1991) The magnetic fabric relationship between

sedimentary and basement nappes in the High Tatra Mts (N

Slovakia). J Struct Geol 13:431–442

ISPRA-Carg Project, Fo. 288 ‘‘Arezzo’’ of the Carta Geologica

d’Italia, in press by APAT; pre print at http://www.apat.gov.

it/Media/carg/index.html

ISPRA-Carg Project, Fo. 289 ‘‘Citta di Castello’’ of the Carta

Geologica d’Italia, in press by APAT; pre print at http://www.

apat.gov.it/Media/carg/index.html

Janecke S, Vandenburg CJ, Blankenau JJ (1998) Geometry, mech-

anisms and significance of extensional folds from examples in

the Rocky Mountain Basin and Range province, USA. J Struct

Geol 20(7):841–856

Jelinek V (1978) Statistical processing of magnetic susceptibility on

groups of specimens. Stud Geophys Geod 22:50–62

Jelinek V (1981) Characterization of the magnetic fabric of rocks.

Tectonophysics 79:63–67

Lavecchia G (1988) The Tyrrhenian-Apennines system: structural

setting and seismotectogenesis. Tectonophysics 147:263–296

Lavecchia G, Brozzetti F, Barchi MR, Keller JVA, Menichetti M

(1994) Seismotectonic zoning in east-central Italy deduced from

the analysis of the Neogene to present deformations and related

stress fields. Geol Soc Amer Bull 106:1107–1120

Lowrie W (1989) Magnetic analysis of rock fabric. In: James DE (ed)

Encyclopedia of Solid Earth Geophysics. Van Nostrand Rein-

hold Company, New York, pp 698–706

Maffione M, Speranza F, Faccenna C, Cascella A, Vignaroli G, Sagnotti

L (2008) A synchronous Alpine and Corsica-Sardinia rotation.

J Geophys Res 113:B03104. doi:10.1029/2007JB005214

Mattei M, Sagnotti L, Faccenna C, Funiciello R (1997) Magnetic

fabric of weakly deformed clay-rich sediments in the Italian

peninsula: Relationship with compressional and extensional

tectonics. Tectonophysics 271:107–122

Mattei M, Speranza F, Argentieri A, Rossetti F, Sagnotti L, Funiciello

R (1999) Extensional tectonics in the Amantea basin (Calabria,

Italy): a comparison between structural and magnetic anisotropy

data. Tectonophysics 307:33–49

Pares J (2004) How deformed are weakly deformed mudrocks?

Insights from magnetic anisotropy. Geol Soc Lond Spec Publ

238:191–203. doi:10.1144/GSL.SP.2004.238.01.13

Pares JM, Van der Pluijm BA, Dinares-Turell J (1999) Evolution of

magnetic fabrics during incipient deformation of mudrocks

(Pyrenees, northern Spain). Tectonophysics 307:1–14

Pauselli C, Federico C (2002) The brittle/ductile transition along the

CROP03 seismic profile: relationship with the geological

features. Boll Soc Geol Ital 1:25–35

Pauselli C, Barchi MR, Federico C, Magnani MB, Minelli G (2006)

The crustal structure of the Northern Apennines (Central Italy):

an insight by the CROP03 seismic line. Am J Sci 306:428–450

Petronio C, Argenti P, Caloi L, Esu D, Girotti O, Sardella R (2002)

Updating Villafranchian mollusc and mammal faunas of Umbria

and Latium (Central Italy). Geol Rom (2000–2002) 36:369–387

Piccinini D, Cattaneo M, Chiarabba C, Chiaraluce L, De Martin M, Di

Bona M, Moretti M, Selvaggi G, Augliera P, Spallarossa D,

Ferretti G, Michelini A, Govoni A, Di Bartolomeo P, Romanelli

M, Fabbri J (2003) A microseismic study in a low seismicity area

of Italy: the Citta di Castello 2000–2001 experiment. Ann

Geophys 46(6):1315–1324

Rees AI (1961) The effect of water currents on the magnetic

remanence and anisotropy of susceptibility of some sediments.

Geophys J Royal Astron Soc 5:251–635

Roberts AP, Chang L, Rowan CJ, Horng CS, Florindo F (2011)

Magnetic properties of sedimentary greigite (Fe3S4): an update.

Rev Geophys 49:RG1002. doi:10.1029/2010RG000336

Rochette P (1987) Magnetic susceptibility of the rock matrix related

to magnetic fabric studies. J Struct Geol 9:1015–1020

Rochette P, Jackson MJ, Aubourg C (1992) Rock magnetism and the

interpretation of anisotropy of magnetic susceptibility. Rev

Geophys 30(3):209–226

Sagnotti L, Speranza F (1993) Magnetic fabric analysis of the Plio-

Pleistocene clayey units of the Sant’Arcangelo basin, southern

Italy. Phys Earth Planet Int 77:165–176

Sagnotti L, Winkler A (1999) Rock magnetism and palaeomagnetism

of greigite-bearing mudstones in the Italian peninsula. Earth

Planet Sci Lett 165:67–80

Sagnotti L, Faccenna C, Funiciello R, Mattei M (1994) Magnetic

fabric and structural setting of Plio-Pleistocene clayey units in an

extensional regime: the Tyrrhenian margin of Central Italy.

J Struct Geol 16:1243–1257

Sagnotti L, Speranza F, Winkler A, Mattei M, Funiciello R (1998)

Magnetic fabric of clay sediments from the external northern

Apennines (Italy). Phys Earth Planet Inter 105:73–93

Schlische RW (1995) Geometry and origin of fault-related folds in

extensional settings. AAPG Bull 79:1661–1678

Speranza F, Maniscalco R, Mattei M, Di Stefano A, Butler RWH,

Funiciello R (1999) Timing and magnitude of rotations in the

frontal thrust systems of southwestern Sicily. Tectonics

18(6):1178–1197

Tarling DH, Hrouda F (1993) The magnetic anisotropy of rocks.

Chapman and Hall, London 217 pp

Int J Earth Sci (Geol Rundsch) (2012) 101:849–861 861

123