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

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 parallel 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 Altotiberina 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 infilled with a syn-tectonic, sandy-clayey continental succession. 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 lineations
suggests that a maximum horizontal shortening along an E–W direction occurred at these sites. The presence 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.

KeywordsAMS–Magnetic fabric–Detachment fault–Low-angle normal fault–Altotiberina fault–Central Apennines

Full text

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ORIGINAL PAPER
Magnetic fabric of Pleistocene continental clays from
the 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
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
Anisotropy of magnetic susceptibility (AMS)
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
Low-angle normal fault ? Altotiberina fault ? Central
Apennines
AMS ? Magnetic fabric ? Detachment fault ?
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; Borradaile1988; Tarling
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
and Hrouda1993;
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
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DOI 10.1007/s00531-011-0704-9

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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)andpiggy-backLigurianUnit(LateJurassicophiolites
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,
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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 (Hreinsdo ´ttir 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, indicatingaSW–NE activeextension,
Figure 2
N
Sansepolcro
Città di
Castello
High Tiber Valley
Gualdo
Tadino
Gubbio
Perugia
Assisi
Trasimeno
Lake
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 Domain Tuscan allochtonLigurian Unit Quaternary deposit
Carbonate
15 km
depth (km)
Gubbio
A
A’
20 km
Altotiberina fault
Tiber
valley
b
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)
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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,tryingto distribute
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
them homogeneously
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 kmaxC kintC kmin(or
k1C k2C 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.
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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 10-6SI, with a main clustering around
120–130 9 10-6SI (Fig. 3a).
12° 00’00”
43° 30’00”
MON
MAI
TER
ANS
UMB
PIE
RESI
GRUC
CAST
CAV
SANS
43° 20’00”
43° 30’00”
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)
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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 kminaxes, 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 kmaxaxes are tightly grouped and define a clear
N–S magnetic lineation, whereas the kintand kminaxes
show a tendency to be roughly scattered in the plane per-
pendicular to the kmaxcluster (Fig. 4). Moreover, all the
sites (except site SCAS) are characterized by a well-
developed magnetic lineation, as documented by limited
dispersion of the kmaxand kintaxes in the magnetic foliation
plane [with the semi-angle of the 95% confidence ellipse
around the mean kmaxaxis in the kmax- kintplane (e12)
always lower than 30?; see Table 1]. In particular, at nine
sites, the e12value is even lower than 20?, indicating a very
good clustering of kmaxaxes from the individual speci-
mens. The declinations of the kmaxaxes 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 kminaxes 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 kminaxes 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 kmvalues
lower than 250 9 10-6SI (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/Nkm(10-6SI)
LFP0
TD (?)
I (?)
e12(?)
Latitude NLongitude E
SANS 43.523045 12.100983 298/2 10/112661.004 1.0151.0190.529 240.60.420.4
SCAS43.44404012.228146 121/5112/12 1111.0011.0161.020 0.862207.7 4.450.5
CAST43.43368912.266249178/19/101141.0081.0111.0210.221179.40.8 4.3
CAV 43.426679 12.261508259/4 10/111271.004 1.0181.0240.563346.9 0.214.5
GRUC43.409705 12.274222 255/1111/11 1571.0041.0061.010 0.231173.5 1.6 12.7
MAI 43.403004 12.270098 065/1510/101791.0041.0031.008-0.158355.04.7 27.0
TER43.357442 12.278236 058/510/13209 1.004 1.0161.0210.646039.5 5.07.5
MON 43.37192312.318925 192/37/81011.001 1.009 1.0110.732123.11.327.8
ANS43.347642 12.311456069/309/112301.0051.0061.010 0.109354.08.113.5
UMB43.31821812.365007 266/1210/111491.0011.0031.0040.655346.72.1 17.1
PIE43.262574 12.403740079/107/111201.006 1.0121.018 0.350161.81.2 5.4
RESI43.20042712.434114279/1710/10 1721.0101.0111.0210.073 188.52.79.5
Geographic coordinates are in WGS84 datum. Bedding attitude is inferred from kminaxes 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. P0and 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). e12is the semi-angle of the 95% confidence ellipse around the mean kmaxaxis in the kmax- kintplane
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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
10-6SI), 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 Jana `k 1976; Hrouda
and Kahan 1991). The P0values 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
kmax
Kmin
1.045
P’
-1
1
T
1.000
400
0
km (10-6 SI)
1.000
1.044
P’
Fraction
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
100 2003004000
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
kmaxand kminaxes of the AMS ellipsoid (in situ coordinates) and their
mean values (larger symbols)
Int J Earth Sci (Geol Rundsch) (2012) 101:849–861855
123

Page 8

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 P0parameters observed at site MAI (Fig. 4); and (3)
the scattering of the kint and kmin axes around a plane
perpendicular to the cluster of kmaxaxes 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 kintand kminaxes 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 kintand kmaxdispersed
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 provento
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 Jana `k 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
kmaxaxes 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; Hreinsdo ´ttir 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).
856Int J Earth Sci (Geol Rundsch) (2012) 101:849–861
123

Page 9

N
90
180
270
K1
K2
K3
N
90
180
270
K1
K2
K3
N
90
180
270
K1
K2
K3
N
90
180
270
1.000
1.016
P’
-1
1
T
N
90
180
270
K1
K2
K3
1.000
1.025
P’
-1
1
T
N
90
180
270
K1
K2
K3
1.0001.020
P’
-1
1
T
1.000 1.032
P’
-1
1
T
N
90
180
270
K1
K2
K3
N
90
180
270
K1
K2
K3
1.000
1.043
P’
-1
1
T
N
90
180
270
K1
K2
K3
1.0001.044
P’
-1
1
T
1.000
1.025
P’
-1
1
T
N
90
180
270
K1
K2
K3
N
90
180
270
K1
K2
K3
1.0001.025
P’
-1
1
T
N
90
180
270
K1
K2
K3
1.000
1.006
P’
-1
1
T
1.014
P’
-1
1
T
1.000
1.028
P’
-1
1
T
1.000
1.033
P’
-1
1
T
1.000
K1
K2
K3
Fig. 4 AMS stereoplots and relative P0–T diagrams for all sampled sites
Int J Earth Sci (Geol Rundsch) (2012) 101:849–861857
123

Page 10

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
-0.20.20.40.6 0.81
B (T)
M (Am2)
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 (Am2)
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 0.80.60.4 0.20.20.40.60.81
B (T)
4.0E-06
3.0E-06
2.0E-06
1.0E-06
M (Am2)
-4.0E-06
-3.0E-06
B (T)
M (Am2)
-1
-0.8
-0.6
-0.4
-0.20.20.40.6 0.81
-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
-0.20.20.40.60.81
Bcr = 53.3 mT
SANS04
M (Am2)
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 10-6SI (upper plots), and a
representative sample with km[250 9 10-6SI (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

Page 11

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-
tocenetectonics.While
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.
theextensional strainis
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 datafromthe High Tiber Valleyare 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
to the volume celebrating the scientific career of Prof. Frantis ˇek
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.
It is a pleasure and an honor for us to contribute
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