Progressive offset and surface deformation along a seismogenic blind thrust in the Po Plain foredeep...

©2014 American Geophysical Union. All rights reserved.

Progressive offset and surface deformation along a seismogenic

blind thrust in the Po Plain foredeep (Southern Alps, Northern

Italy)

Authors: Franz A. Livio, Andrea Berlusconi, Andrea Zerboni, Luca Trombino, Giancanio

Sileo, Alessandro M. Michetti, Helena Rodnight, Christoph Spötl

Affiliations:

(Livio F., Berlusconi A., Michetti A.M., Sileo G.) Dipartimento di Scienza e Alta Tecnologia,

University of Insubria. Via Valleggio 11 - 22100 Como (Italy).

(Sileo G., now at:) Università degli Studi della Basilicata - Scuola D'Ingegneria - Laboratorio

Di Analisi dei Dati Satellitari (LADSAT). Campus di Macchia Romana - 85100 Potenza

(Italy).

(Zerboni A., Trombino L.) Dipartimento di Scienze della Terra “A. Desio”, Università degli

Studi di Milano. Via L. Mangiagalli 34 - 20133 Milano (Italy).

(Rodnight H., Spötl C.) Institut für Geologie, Universität Innsbruck. Innrain 52 - 6020

Innsbruck (Austria).

Corresponding Author: Franz A. Livio, [email protected] Dipartimento di Scienza ed

Alta Tecnologia, University of Insubria. Via Valleggio 11 - 22100 Como (Italy)

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/2014JB011112


Abstract

Here we present, for the first time in the Po Plain foredeep (N Italy), the Middle-to-Late

Pleistocene growth history of an outcropping secondary fold and related faults, whose

progressive deformation over an intermediate time window (105

We trenched and logged an outcropping decametric secondary anticline, related to a deeper

blind compressional structure, which deforms fluvial sediments and an overlying loess-

paleosol sequence. Folded units were dated, using radiocarbon and OSL methods, to the Late

Pleistocene - Holocene and tentatively correlated with glacial-interglacial phases occurring

during the time interval from MIS 6 to the Present. A multistep retro-deformation of the fold

allowed us to calculate uplift rates for this secondary and shallow anticline, varying between

0.02 - 0.1 mm/yr since ca. 200 kyr. Trishear forward deformation modeling of the fold

indicates that the amplification of the observed fold could be caused by two shallow thrusts

formed through a break-backward activation. This generated a decametric surface fold

whose most recent growth was associated with bending-moment normal faulting in the

crestal and forelimb region.

yrs) is driven by an

underlying seismogenic blind thrust.

Our observations demonstrate that near-surface compressive tectonics can be caused by

blind thrusting, via a complex array of fault and folds: upward strain propagation and

generation of shallow low-angle thrust and related folding seem to be mainly due to

secondary fold-related faulting, according to an out-of-syncline thrusting mechanisms.

Keypoints: active tectonics, Po Plain foredeep, blind thrust, secondary surface faulting,

loess-paleosol sequence


1. Introduction

Foredeep basins are problematic settings for fault displacement hazard analysis since surface

faulting by blind thrusting can be elusive or unlikely, especially in regions characterized by

moderate deformation rates and/or seismic potential. Nevertheless, strain can propagate to the

surface via secondary folding and faulting, posing some important issues about fault

displacement hazard analysis but also offering the possibility to directly investigate

deformation due to blind thrusting seismic sources.

The Po Plain foredeep (N Italy) illustrates this point very clearly. While Late Pleistocene to

Holocene surface faulting in the piedmont belts of the N Apennines and S Alps has been

proposed by several authors [e.g., Serva, 1990; Castaldini and Panizza, 1991; Benedetti et al.,

2000; Picotti and Pazzaglia, 2008; Boccaletti et al., 2011; Bruno et al., 2011; Borgatti et al.,

2012], no agreed published record of coseismic or paleoseismic surface displacement exists

in the entire Po Plain. Conversely, evidence of broad and subtle surface deformations

affecting the drainage and landscape evolution of the plain area over a ≈ 10 4 yrs time window

has been frequently described in the literature [e.g., Wilson et al., 2009; Ponza et al., 2010;

Burrato et al., 2012; Gunderson et al., 2013; 2014]. Coseismic uplift of the ground surface

during blind thrusting earthquakes has been observed during the 2012 Emilia sequence (two

main shocks: 20th and 29th

As a matter of fact, a first evidence of paleoseismic ground rupture due to the growth of

surface decameter-scale folding in the Po Plain foredeep (Monte Netto site) is reported by

Livio et al. [2009] (see also Galadini et al., 2012). This has been interpreted as somehow

caused by the recent activity of a S-Alps blind backthrust imaged in seismic reflection data.

Still some issues remained unsolved about these shallow tectonic structures such as:

May, 2012, Mw 5.9 and 5.7, respectively [Scognamiglio et al.,

2012; Ventura and Di Giovanbattista, 2013]) with a maximum surface deformation of 10 –

15 cm over an area 10 km wide [Bignami et al., 2012].


a) a detailed stratigraphic description and dating of the outcropping deformed sequence

which would have allowed to define their rates of uplift/slip ;

b) a model describing their structural characteristics at the scale of tens to hundreds meters,

such as the geometry and detachment levels of the faults related to these folds, and

c) their structural relation with the seismic source at depth, that is, how strain can propagate

from underlying seismogenic thrusts to such shallow levels.

To solve these open questions we present a detailed structural analysis of the decameter-scale

folds and related bending-moment faults outcropping at Monte Netto site, located in the N

sector of the Po Plain, about 130 km NW of Modena (Fig. 1). Additional quarry works,

performed in 2013, exposed some new remarkable outcrops of these folded sediments,

offering a detailed 3D view of the local tectonic structures and of a loess-paleosol sequence

deposited during their progressive displacement. We conducted a detailed meso-structural

analysis of the new exposures, trenched the base of a quarry wall and logged it in order to

structurally resolve the outcropping anticline. Radiometric and optically stimulated

luminescence (OSL) dating on loess-paleosol sediments allows us to constrain a sequence of

depositional and pedogenic events correlated to mid-late Pleistocene glacial/interglacial

phases. Based on this new dataset it was possible to constrain the timing of the fold growth

and to develop a kinematic solution for the observed surface deformation, consistent with

shallow secondary thrusting, folding and normal faulting induced by a deeper buried thrust.

Finally, we model the geometric and kinematic characteristics of the fault related to the

observed fold, suggesting a possible structural solution linking these very shallow

compressive tectonic features with the underlying thrust. The structural relationships between

blind faulting at depth and coseismic secondary surface rupture illustrated at the Monte Netto

site provide a unique case study for the Po Plain foredeep, and with only few similar

analogues known in the literature for regions under active crustal shortening [e.g.,


Meghraoui et al., 1996; Champion et al., 2001; Heddar et al., 2013].

We argue that the insights obtained at Monte Netto are relevant for assessing the probability

of secondary displacement induced by strong blind thrusting earthquakes, and the maximum

expected magnitude for the Po Plain (one of the most industrialized and developed areas of

the world) and other similar seismically active foredeep settings.

2. Geological setting

The Po Plain foredeep is characterized by the convergence of two chains that face each other

beneath the Po River floodplain: the western Southern Alps (i.e. the conjugate retro-wedge of

the Alps) and the northern Apennines [e.g., Castellarin and Vai 1986, Castellarin et al., 1992,

Carminati et al., 2004, Fantoni et al., 2004]. The buried structural front of the western

Southern Alps is a structural belt characterized by an array of E-W trending thin-skinned

thrust sheets bound to the East by the Giudicarie and the Schio-Vicenza fault systems [e.g.

Castellarin and Cantelli, 2000; Castellarin et al., 2006; Pola et al., 2014; Fig. 1].

The opposite-facing structural fronts of the northern Apennines are buried beneath the Plio-

Pleistocene sediment fill of the Po Basin and show three main salients from west to east: the

Monferrato, Emilia and Ferrara arcs (Fig. 1). The latter generated the May 2012 seismic

sequence culminating in two Mw ≈ 6 thrust fault earthquakes.

The distribution and segmentation of the southalpine blind structures, in the area between

Lake Garda and Adda River (Fig. 2), have been previously defined by regional mapping,

using an extensive database of ca. 18,000 km of seismic profiles provided by ENI E&P

calibrated by several deep boreholes [Livio et al., 2009]. The Capriano del Colle Fault

System (Figs. 2 and 3) is a structural wedge composed by a main south-vergent forethrust

(Capriano del Colle forethrust) and an associated high-angle backthrust (Capriano del Colle

backthrust – CCB hereafter) whose structural culmination is marked by the presence of a


small isolated hill, Monte Netto, representing the topographic expression of the structural

relief of the underlying structures [e.g., Desio, 1965; Livio et al., 2009]. The thrust sheet

mainly involves a foreland and foredeep syntectonic sequence of terrigenous units that range

from Oligocene to Miocene in age, referred to as the Gonfolite Lombarda Group. Faults

typically ramp up from the top of the underlying Mesozoic carbonates [e.g., Fantoni et al.,

2004] and are associated with fault-related folds deforming the overlying infilling of the

basin. The deep geometry of thrusts and fault-related folds (Fig. 3) has been reconstructed

using constant-thickness kink band models [e.g.; Suppe and Medwedeff, 1990]. This clearly

illustrates the gentle folding of Quaternary beds, unconformably resting upon the CCB fault-

related fold, just below Monte Netto site; and the presence of a shallow south-vergent thrust

(CCα in Fig. 3) whose ramp sector is pinned, at its base, with the synclinal axial surface on

the CCB [Livio et al., 2009].

On the basis of industrial seismic reflection profiles and deep boreholes stratigraphy, the

overlying Plio-Pleistocene basin fill has been divided into four stacked bodies separated by

three sequence stratigraphic surfaces (Fig. 3) dated using calcareous nannofossils to ca. 1.6,

1.2 and 0.89 Myr [Carcano and Piccin, 2002]. These reflectors correspond to boundaries of

lithostratigraphic units firstly defined along the southern sector of the Po Plain subsurface

[Muttoni et al., 2003; Amorosi and Pavesi, 2010], as well as in the Apennines piedmont area

(Amorosi et al., 1998; Gunderson et al., 2014), that are, respectively, A) the base of Argille

Azzurre Fm. and B) the equivalent, in the northern Po Plain, of the base of the Sabbie di

Imola Fm. and Lower Emilia-Romagna synthem.

The resulting subdivision of the Plio-Pleistocene syn-growth strata allowed us to calculate

fold uplift and fault slip rates for the fault system, assuming a fill-to-the-top growth model

[e.g., Suppe et al., 1992; Masaferro et al., 2002]. The Capriano del Colle backthrust moved at

ca. 2.5 mm/yr between 1.6 and 0.89 Myr and slowed down to ca. 0.43 mm/yr during the most


recent resolvable time window, i.e. 0.89 Myr to present [Livio et al., 2009].

The ongoing compressional tectonics is also demonstrated by geodetic data indicating NNE-

SSW shortening rates in the range of 1.1 mm/yr at the longitude of Lake Iseo [e.g., Devoti et

al., 2011; Bennet et al., 2012; Michetti et al., 2012]. Historical and instrumental seismicity

records moderate-to-strong earthquakes, characterized by long recurrence intervals, often

unrelated with known seismogenic structures ([e.g., Magri and Molin, 1986; Guidoboni,

1986; Serva, 1990; Burrato et al., 2003; Guidoboni and Comastri, 2005]; Fig. 1). This was the

case for one of the strongest known historical earthquake of northern Italy, the Dec. 25, 1222,

Io IX-X (MCS) Brescia event [e.g., Guidoboni, 1986; Serva, 1990], whose epicenter is

located close to the study area (Fig. 1).

Monte Netto is an isolated hill, deeply eroded on its sides, almost rectangular in shape (5 km

in length and 2 km in width), trending 110 and standing ca. 30 m above the surrounding

alluvial plain. The latter is a vast outwash fan, developed during the Last Glacial Maximum

by a network of meltwater channels [e.g., Baroni and Cremaschi, 1986; Marchetti, 1996]

incised by the Holocene drainage. The vertical surface projection of the Capriano del Colle

backthrust is located just north of Monte Netto hill and the projected traces of the hinge zones

coincide with local structural and topographic relief (Fig. 3).

The core of the hill exposes a gently folded sequence of Lower-to-Middle Pleistocene fluvial

sediments (floodplain environment – FL1 and FL2 in Fig. 2). This oldest fluvial sequence is

made up of continental Lower Pleistocene greenish-grey fine sands and floodplain deposits

with abundant shell remains, tentatively correlated to similar sediments, 0.89 Myr old, drilled

in the Ghedi core [Carcano and Piccin, 2002; Scardia et al., 2006], ca. 80 m below the surface

and 10 km SE from the site. Younger fluvial sediments (Middle-Upper Pleistocene) onlap the

hill on both sides and locally fill small erosional valleys excavated across the hill (Fig. 3).

Loess strata interlayered by paleosols cover both the units on the top of the hill (Cremaschi,


1987). As already mentioned, chronological constraints available in the literature for the

above fluvial and loess sequence are poorly defined, which motivated our investigation for

new radiocarbon, OSL and pedostratigraphic dating, as described below.

3. Methods

We conducted an interdisciplinary study of the Monte Netto site by collecting and comparing

data from field mapping, structural analysis, pedostratigraphy and dating techniques. In the

following a brief overview of the methods applied for each approach is described.

3.1 Geologic survey and structural analysis

In order to reconstruct with refined resolution the recent (Middle to Late Pleistocene)

structural evolution of the Monte Netto, we first mapped the geology of the hilltop area at a

scale of 1:10,000, and conducted a detailed stereoscopic interpretation of aerial photo-

coverages available for the area (IGM photo-series, 1954, ca. 1:33.000 scale; Regione

Lombardia coverage, 1992, ca. 1:22.000 scale). The aim was to look for suitable outcrops, to

map the different lithostratigraphic units (following e.g., Boni et al., 1970; Fig. 2) and to

correlate surface geology with shallow drill logs and with the dated units recognized in the

relatively deep boreholes performed in the nearby area by Regione Lombardia down to ca.

200 meters of depth [e.g., Scardia et al., 2006].

This was followed by a detailed field survey of the study site, located in a quarry area,

mapping the stratigraphic units both in section and in plan-view. One of the quarry walls,

oriented perpendicular to the fold trend, was investigated in detail. We extended the

outcropping area downward by digging a trench at the base of the quarry wall. The resulting

surface was logged at a scale of 1:20 distinguishing both lithostratigraphic and

pedostratigraphic units.


The evolution of the folded sequence was reconstructed by progressively removing syn-

growth strata and unfolding the beds exposed in the quarry wall to a near-horizontal

depositional geometry. Fold limbs where translated and rotated using the Midland Valley

2DMOVE® structural geology software and considering constraints given by growth strata

onlap position or overlap. This allowed to calculate the uplift rates of the structure as

constrained by dated syn-growth strata and according to a fill-to-the-top model, following a

procedure already applied in similar tectonic settings [e.g., Leon et al., 2007, Streig et al.,

2007]. Finally, trishear modeling of the fold shape was used to obtain the best fitting

parameters of the thrust driving the fold growth (tip position, cumulated slip, ramp angle,

apical angle and propagation/slip ratio) [e.g., Erslev, 1991; Hardy and Ford, 1997].

Computations were made using the FaultFold 5.4 trishear code [e.g., Allmendinger, 1998;

Zehnder and Allmendinger, 2000] which used an inverse method for bed restoration,

assuming a reference bed and postulating its initial planar geometry. Calculations were made

performing a grid search over reasonable ranges of the considered parameters, resulting in

thousands of possible combinations for each grid search. Best fitting parameters were chosen

assuming that the restored reference bed is planar at the start of folding and then calculating a

simple least squares linear regression between the deformed and the reference bed (see

Supporting Information).

3.2 Pedostratigraphy

The interpretation of the pedostratigraphic features of the section mostly relied on the field

properties of strata. Field description of soil horizons followed the internationally accepted

guidelines proposed by FAO [2006], adapted to the interpretation of paleosols [sensu Zerboni

et al., 2011]. Main sedimentological and pedological properties are listed in Table 1, together

with the palaeoenvironmental significance of each layer. Field description was supported by

grain-size analyses, which permitted to better distinguish between fluvial, colluvial and


aeolian sediments, and pedological bodies originated by weathering these units. Grain-size

analyses [Gale and Hoare, 1991] were performed after drying samples and removing organics

by hydrogen peroxide (130 vol) treatment. Bulk samples were then air-dried and particle size

distribution analyzed by sieving the >63 μm fraction, while the grain-size distribution of fine

fraction (< 63 μm) was determined by the aerometer method. Sediments were wet sieved

(grain size 2000 to 63 µm), and the silt plus clay fraction (<63 mm) was determined using

Casagrande’s aerometer. Results are expressed as cumulative grain-size curves. Oriented and

undisturbed samples of sediment were collected in correspondence of some vertical fractures

filled with illuvial clay of different origin in order to understand the relation between seismic

events, faults opening and infilling of clay corresponding to subsequent weathering phases.

Thin sections (5x9 cm) were manufactured after impregnation by resin according to standard

methods [Murphy 1986]. The micromorphological study of thin sections employed a Leica

Laborlux 12pol petrographic microscope with an Olympus C4040 digital camera. Thin

sections were observed under plane-polarized light (PPL), cross-polarized light (XPL) and

oblique incident light. The terminology and concepts of Stoops [2003] and Stoops et al.,

[2010] were used in thin-section descriptions and interpretations.

3.3 Dating

Constraints on the age of the litho- and pedostratigraphic units was obtained by both

radiocarbon and OSL.

Organic-bearing sediment samples from the upper part of the pedosequence were submitted

to 14

The samples provided for OSL analysis were prepared in the laboratory at Innsbruck.

C dating by accelerator mass spectrometry at the CEDAD Laboratory (Lecce, Italy). The

uncalibrated dates were calibrated (2 sigma) according to INTCAL13 [Reimer et al., 2013]

using Calib 7.0 [Stuiver et al., 2013].


Approximately 20 g of each sample was used for water content measurement and dosimetric

analyses. The remainder was prepared for OSL analysis following standard techniques

[Wintle, 1997]; this involved initial pre-treatment with 10 % hydrochloric acid, followed by

20 volumes hydrogen peroxide to remove carbonates and organic matter prior to dry sieving.

Following initial preparation, 1 mg of the 4-11 µm polymineral fraction was obtained by

settling following Stokes’ Law for analysis. The cosmic dose rate was estimated for each

sample using the depth of overburden and the geomagnetic latitude, assuming a sampling

location at 45.5° N and 10.1° E and an elevation of 118 m a.s.l. [Prescott and Hutton, 1994],

with the assumption that the overlying sediment would have accumulated rapidly following

deposition, making a correction for changing overburden thickness unnecessary. The water

content for each sample was measured in the laboratory and calculated as the mass of water

divided by the mass of dry sediment, multiplied by 100 (Table 1). The containers that the

samples were collected in were airtight, and thus the water content measured in the laboratory

will have been close to the field value. The dose rate was calculated on the basis of the water

content value measured in the laboratory with an error of ± 5%.

The luminescence analysis was undertaken using the double single-aliquot regenerative-dose

(SAR) protocol [Banerjee et al., 2001; Roberts and Wintle, 2001] with a preheat of 220°C for

10 s, and a cutheat of 160°C.

The De

The D

values were obtained from the blue LED-stimulated measurements, and thus are

based on the quartz component of the polymineral fine-grain mixture.

e was measured for 24 aliquots of samples 8/CC01-03. From the De

dataset of 24

values for each sample, the burial dose was calculated using the Central Age Model

[Galbraith et al., 1999].


The dose rates were calculated using the conversion factors of Adamiec and Aitken [1998]

using the program ADELE [Kulig, 2005] and using an alpha effectiveness value for quartz of

0.03 ± 0.01 [Mauz et al., 2006].

4. Results

4.1 Stratigraphic and structural data

The trench site (see Figs. 2 and 4 for location) is located in the ‘Cava Danesi’ clay quarry, on

the top of the hill. The pre-existing N-S directed quarry wall, ca. 35 m long and 7 m high,

was logged and an explorative trench was excavated at the foot of the wall (Fig. 4a). Two

anticlines, N295 trending, characterized by a wavelength of tens of meters and an amplitude

varying between 4 and 9 m, were exposed along the deepened quarry wall. We focused on the

northernmost anticline (NAN herein after) logging at a scale of 1:20 an area of 32 x 13 m

centered on its southern forelimb (Fig. 4).

From a stratigraphic point of view, the studied section includes a fluvial fining-upward

sequence (Fig. 5), overlain by a loess-paleosol pedosequence. The latter wraps the fluvial

deposits and displays a lateral thickening of up to 5 m, levelling the structural relief created

by the folding of non-aeolian sediments. Stratigraphic and pedostratigraphic units are

distinguished by color, lithology and degree of weathering, and are labeled from oldest to

youngest. A description is summarized in Table 1, while sedimentological data are

summarized in Fig. 6.

Units FG1 to FG8 constitute the fluvial sequence. From the base: unit FG8 is a clast-

supported fluvial conglomerate, probably deposited in a paleo-channel of the Mella River.

The gravel is massive and intercalated with rare, thin lenses of medium to coarse sand.

Roughly imbricated clasts indicate a paleocurrent flow from the west. FG7 consists of a

fining-upward sequence of interbedded silt and medium sand representing settling of


suspended load in an overbank environment. Fining-upward coarse sands and conglomerates

of FG6 occupied a swale carved into FG7, probably representing a crevasse-channel. In the

lowermost part of the unit, just above an erosive surface, a lag deposit was present, consisting

in rip-up clasts belonging to FG7. FG5 is a 50 cm-thick overbank upward-coarsening

sequence. Sediments belonging to FG4, FG3 and FG2 represent three or more crevasse-splay

episodes. FG4 and FG2 showed a complete rising-waning flow sequence. FG1, the youngest

fluvial sediments in the sequence, consisted of levee deposits.

The upper part of the section is characterized by the occurrence of a loess cover, developed

during the Late Pleistocene and interlayered by buried paleosols, which displayed different

degrees of weathering. These cyclic loess-paleosol sequences typically reflect, in the Po

Plain, the alternation of cold/warm periods, with loess sedimentation under dry/cold climate

conditions and subsequent soil development during the following warmer and humid interval

[Cremaschi, 1987; Amit and Zerboni, 2013]. For a detailed description of the

pedostratigraphic and sedimentological characteristics of this sequence, which is beyond the

scope of the present work, please refer to the companion paper by Zerboni et al., [2014].

The loess sequence is exposed along the entire wall of the quarry and was marked by a

decreasing thickness toward the North, where the core of the anticline is located. As a

consequence, in the synclinal hinge, south of the anticline, the loess-paleosol cover reached

more than 5 m in thickness, in contrast to only ca. 2 m at the anticline culmination. At the

southernmost part of the section we observed, from the bottom, a silty to sandy layer (CL),

including a large amount of spherical Mn-concretions, interpreted as evidence of recycled

loess [e.g., Mroczek, 2013] after the colluviation of primary loess was deposited on the top of

the anticline. The CL unit is weathered and covered by PL3, which is the deeper primary

loess layer identified on the top of Monte Netto; its top is marked by the first soil developed

on loess (S3) and shows a moderate increase in clay content. Sedimentological analyses (Fig.


6) highlight the occurrence of PL2 at the top of S3; these two units were separated by a layer

rich in Mn-concretions. PL2 is shallow and, at its top, the S2 soil is present, richer in clay and

in Mn-concretions. At the top of the sequence the thicker loess cover (PL1) is present,

reaching up to 1.5 m; loess is poorly weathered and at its top the S1 unit includes a soil useful

for agriculture (Ap horizon). The same pedostratigraphic sequence is quite evident also in the

proximity of the gravity graben, but due to co-seismic deformation, the thickness of each

layer/soil differs. Furthermore, this part of the sequence is cut by vertical fissures related to

deformational stress, which we interpret as seismipedoturbations [sensu Previtali, 1992].

Fractures were filled by a clay-rich plasma, which displays, in thin section of undisturbed

blocks, different generations of clay infillings characterized by different colors (from red to

orange) and textures (from pure clay to silty/clay). Therefore, differences in appearance

indicate multiple infillings [Kühn et al, 2010], whose formation required a multi-step process

of fissure opening and illuviation by pedogenetic clay (Fig.8), i.e. coseismic movements.

Finally, at the top of the anticline a highly rubified, polycyclic buried paleosol (S4’; Fig. 7) is

present, and it is covered by a thin layer of poorly or unweathered loess. S4’ is a buried,

polycyclic paleosol that formed over a long time span and therefore shows a high degree of

reddening; it was shaped by the same pedogenetic processes responsible for the development

of the stacked S3 to S2 soils.

The NAN is a slightly conical fold showing a clear vergence to the south and a hinge

plunging 295/10 (cfr. Fig. 2b), thus suggesting a periclinal termination of a fold whose

structural culmination is located farther to the southeast, in an area now deeply re-worked by

quarry activity. The periclinal termination has been exposed in 2012 by the new quarry

excavations. High-angle reverse faults were identified at the foot of the exposed walls,

composed by two sub-vertical secondary splays, N130 trending and dipping to the NE. The

tip of these reverse faults is located in the stratigraphic unit FG1 (see Table. 1). Several


bending-moment faults offset the culmination of the anticline (Fig. 4) creating a main graben

structure and other secondary, mainly extensional, structures. The maximum offset (ca. 2 m)

was recorded on the S-dipping bending-moment fault of the gravity graben and its throw dies

out downward. The observed kinematics indicators (slickensides) show a pure dip-slip

movement. Joints are common, usually filled by illuvial clay. All faults and joints were

subdivided into two subsets, based on cross-cutting relationships with an erosive

unconformity located in the lowermost part of the section (Fig. 7). The oldest pre-

unconformity subset includes faults and joints deforming the pre-unconformity units and

clearly truncated upward by the erosive surface. The younger subset is composed of

structures cross-cutting the unconformity. The restoration of the pre-unconformity subset to

an horizontal datum, rotating the data according to the local bedding attitude, provides a good

match with the post-unconformity subset, thus indicating that the younger brittle deformation

mainly developed at the very end of fold amplification. The youngest subset also includes a

cluster of high-angle normal faults, ca. E-W trending and belonging to a normal fault system,

developed in the outer part of the fold. These structures are located at the fold hinge and are

not fully consistent with the retrodeformation of the oldest subset.

4.2 Chronological constraints

Eight samples were collected and analyzed from five units to constrain the age of the

sequence (Fig. 5 and Fig. 7). Radiocarbon and OSL results indicate that the pedostratigraphic

units in the trench exposures are Upper Pleistocene to Holocene in age (Fig. 5 and Table 2).

Radiocarbon dating was performed on the organic fraction of three samples, collected at the

top of the pedosedimentary sequence, at depths ranging from 1 to 0.4 m. The lowest sample

(CAP03), dated between 16241 and 15772 cal yr BP (2σ), corresponds to the upper and very

poorly weathered silt deposit, which is interpreted as a final phase of loess deposition,

probably dating to Lateglacial stadials preceding the Bølling-Allerød. Sample CAP03 was


collected from a weakly developed soil on loess and possibly represents the age of a soil

developed under glacial conditions in a tundra steppe environment with prolonged

accumulation of organics and microcharcoals and later isolated from the surface; it therefore

represents a mean value for the time of soil development [Zerboni et al., 2014]. This result is

comparable to the cumulic soil described at the top of the Val Sorda loess sequence

[Cremaschi et al., 1997; Ferraro, 2009], not far from Monte Netto. The dating CAP03

indicates a period almost contemporaneous with dust sedimentation recorded in other loess

sequences along the northern margin of the Po Plain Loess Basin [Cremaschi and Lanzinger,

1984, Cremaschi et al., 1987; Ferraro, 2009; Wacha et al., 2011]. Samples CAP01 and CAP2

gave Holocene ages for the upper part of the sequence (7483–7287 and 5574–5094 cal yr

BP). These young radiocarbon ages possibly reflect anthropogenic reworking of the loess and the

Holocene soil developed on it and post-date loess deposition (Zerboni et al., 2014).

OSL samples (Table 4), collected from the lower part of PL1, yielded ages between 27.5 and

15.8 kyr (Fig. 5), while samples coming from higher levels, inside PL1, yield considerably

younger ages. An OSL sample collected near the top of PL2 yielded an age of 49.8 to 39.0

kyr.

As stated above, PL2 deposition is constrained by OSL to ca. 44 kyr and radiocarbon and

OSL dating both bracket the age for PL1 deposition. Moreover, two archaeological findings

support the radiometric chronology of the sequence. Paleolithic finds from the top of CL do

not present a clear Levellois technology, and, on the basis of their similarity with other lithic

assemblages described in the region [Zorzi, 1959; Cremaschi, 1974], they are dated to the

Lower/Middle Paleolithic. On the contrary, at a depth of c. 1.2-1.5 m, we found a rich set of

Mousterian artifacts, which fit well with the OSL dating. Moreover, regional comparisons

also confirm a good correspondence between loess deposition and Mousterian frequentation

of the Po Plain [Cremaschi and Christopher, 1984; Peresani et al., 2008].


After integrating OSL and radiocarbon age constraints with archeological finds, observations

from other pedosequence in the area [Cremaschi, 1987] and with data on the Pleistocene

evolution of the northern sector of the Po Plain, a tentative correlation between the described

loess-paleosol sequence and the Marine Isotope Stages (MIS, e.g., Waelbroeck et al., 2002]

can be proposed. These correlations indicate that the Monte Netto sequence represents a long

stratigraphic recording of cold/warm climate stages (Fig. 5). Radiometric dating are limited to

the upper part of the stratigraphic section, but a tentative model illustrating the evolution of

the whole sequence can be summarized as follow:

- The lowermost part of the sequence corresponds to fluvial to fluvio-glacial sediments

deposited in the Po Plain in the Early and Middle Pleistocene [e.g., Muttoni et al.,

2003; Scardia et al., 2006]. The top of this part of the sequence possibly suffered

intense weathering during an interglacial of the Middle Pleistocene, which triggered

the formation of a rubified soil.

- After the initial deformation (uplift) of the top of the hill, the upper part of the fluvial

to fluvio-glacial sequence and the soil at its top underwent colluviation and the CL

unit formed. Subsequently, the upper part of the CL unit underwent strong

pedogenesis under interglacial conditions (formation of soil S4). On the basis of the

properties of the S4 paleosol and comparison with other sites of the Po Plain [e.g.,

Ferraro, 2009], the formation of this soil can be dated back to the MIS 5e interglacial,

under warm conditions. As a consequence, colluvial processes were attributed to a

glacial phase preceding the MIS 5e, possibly the MIS6. A correlation with an older

glacial (for instance MIS8) is also possible, but it would imply a significant hiatus in

the loess/paleosol sequence for which we see no evidence in the exposed quarry wall

(no unconformity, no erosional surfaces), and thus we consider unlikely.

- During the cold and arid period following the Eeemian MIS 5e, the soil S4 was buried


by a thick accumulation of loess (PLs 3 to 1), which formed in a long polycyclic cold

phase encompassing (according to radiometric dating) the MIS5d – 2. Each one of the

three loess strata so long identified is capped by a thin soil, whose formation requires

environmental conditions typical of the upper Pleistocene interstadials.

- At Monte Netto the loess sedimentation, which in the Po Plain generally lasted up to

the end of the Last Glacial Maximum, continued up to the stadial preceding the

Bølling-Allerød interstadial; after this phase a soil (S1) developed during the

Lateglacial interstadials and in the Early and Middle Holocene.

5. Structural evolution of the fold

The structural crest of the NAN roughly matches the topographic culmination of the hill,

standing ca. 30 m above the surrounding alluvial plain. A direct correlation between the

highest uplifted fluvial unit (FG1) and correlative sediments buried in the plain cannot be

easily done due to the lack of dated material in the surrounding borehole stratigraphic logs for

intervals younger than 0.89 Myr. However, the pedogenetic overprint of the colluvial unit

(CL) marked the first emergence of a morphological relief in the plain, preserved from

erosional process. This in turn allowed the subsequent deposition of loess on the hilltop. The

minimum uplift rate of the hill, considering the height difference between CL and the

surrounding alluvial plain (ca. 30 m) and a time window from CL deposition to the present, is

ca. 0.16 mm/yr.

Based on trench data it is possible to reconstruct the Middle to Late Pleistocene evolution of

these secondary structures, placing some important constraints on the activity of the main

blind thrust fault. Uplift values were calculated according to a fill-to-the-top model, where

systematic changes in unit thickness are due to contemporary gain in structural relief of

underlying structures; onlapping of strata can thus provide only a minimum value for


contemporary uplift. Uplift rates were then calculated considering uplift differences over two

successive dated units.

Fig. 9 shows a 6-step reconstruction of folding-faulting and deposition on the NAN. The

southern termination of the fold is considered as a local datum for the calculation of structural

relief and tilting. In step 1 the top of FG3 was restored to a horizontal datum and the resulting

strata geometry clearly shows bed thinning toward the south. This geometry is consistent with

the growing of a shallow thrust fault (Fault 1 in Fig. 9), whose propagating tip reaches the

surface at step 2, soon after the deposition of units FG 2-1 which are partially displaced by

the fault. Structural relief, cumulated during the deposition of FG7 to FG3, is ca. 2.7 m.

Thinning of FG7 is uncertain, since the base of overlying FG6 is erosive. Several secondary

normal faults develop in the fault hangingwall, accommodating deformation in the incipient

fold crest. These faults have a limited length and cross-cut only units FG 5–3 while

displacement rapidly dies out upward not allowing to pre-date faults movement.

Nevertheless their back-rotated orientation at this step is consistent with the accommodation

of extension in the fold crest and thus a principal stress vertically orientated; a later

activation, conversely, would imply a low angle normal faulting not consistent with the local

stress field and with a contemporary development of the main gravity-graben.

From step 2 onward, most of the deformation is due to folding of the whole sequence with a

structural culmination placed at the northern termination of the logged wall. This anticline is

a fault-propagation fold, associated with an underlying buried thrust fault, whose tip is

concealed below the base of the section. Fault 1 is not growing anymore (excepting a re-

activation at step 5 along a secondary splay), as proved by growth-strata geometry and

shifting of the fold structural culmination toward the north. Fold vergence is to the south, as

suggested by the asymmetry. From this step on the fold can be approximately divided into

four kink-bands that experienced differential tilting during fold amplification. These are,


from north to south, anticline crest, forelimb I, forelimb II and forelimb III (Fig. 9). Uplift

occurred contemporary with the deposition of unit CL and of the overlying stacked bodies of

loess cover.

From step 5 on, forelimb I and II were separated by a collapse structure made of a main

south-dipping synthetic normal fault and an antithetic one. These are bending-moment faults

that accommodated stretching in the outer part of the fold, causing collapse into the

developing graben. This non-seismogenic surface faulting, characterized by a decimeter

offset, is nevertheless caused by the fold amplification which, in turn, is driven by the slip on

the underlying seismogenic thrust. In a simple rigid plate model (Fig. 10), the equivalent area

of the graben (A) should be equal to the void space created by differential tilting (A’) where

the plate height (r) is equal to the length of the main normal fault of the graben, measured

considering the downward dying out of the graben displacement.

Since A’ can be approximated to a circle sector area of radius r and angle α:

The resulting value of α is ca. 9o and is consistent with the total differential tilting between

forelimb I and II, which is equal to 12o

Uplift rates of the anticline were calculated for each interval between step 3 and 6 (cfr. Fig.

9), considering the cumulative uplift recorded at the end of each sedimentary phase (based on

the correlation with the MIS and/or absolute dating) and according to a fill-to-the top model

(Fig. 11). It is important to note that these values can be affected by uncertainties in the

numerical ages and correlations and to errors in evaluating the unit thickness (colluviation,

soil formation). CL correlation with MIS, as stated before, is uncertain and so two end-

member uplift rate values were considered, based on different bracketing ages.

.


Differential uplift rates vary between ca. 0.02 and 0.1 mm/yr. An increase in the rate with

time is highlighted by the data, with an average uplift value, over the entire time window

considered, of ca. 0.03 mm/yr. Calculated anticline uplift rates are also typically lower than

the medium-term Monte Netto uplift rate (ca. 0.16 mm/yr) and consistent with secondary

structures linked to a deeper fault-related fold, driving the growth of the hill. The uplift rate

of the Capriano del Colle backthrust, in fact, calculated over the last 0.89 Ma, is 0.22 mm/yr

[Livio et al., 2009].

A consistent structural solution for the thrust generating this anticline can be obtained

through a kinematic restoration of the fold in order to restore the fold frontlimb to a straight

horizontal datum. The fold geometry was approximated to the top of FG3, considered as a

good proxy for pre-growth strata, and six different parameters (including fault ramp

geometry, total slip, trishear apical angle and P/S ratio) were modeled using the FaultFold 5.4

trishear code [e.g., Allmendinger, 1998; Zehnder and Allmendinger, 2000].

The best-fitting combination of the considered parameters (R2 = 0.96) included a 24o

6. Discussion

north-

dipping thrust, a fault tip position ca. 20 m below the present-day topography and a

cumulated slip of 35 m (Fig. 12a). The calculated strain field (Fig. 12b) shows a good

orientation fitting between LNFE (Lines of No Finite Elongation, considered as good proxies

for shear plane orientation and shear sense) and the observed faults, supporting the modeled

fold geometry. Moreover, the south-dipping bending-moment fault (Fig. 12b) fits well with

the LNFE and is consistent with a near-vertical σ1 stress orientation, thus strengthening the

interpretation of a late-stage formation of the gravity graben.

6.1 Structural Interpretation

The detailed stratigraphic analysis of the loess paleosol sequence exposed at Monte Netto


site, together with dating and the structural reconstruction of the NAN, allowed to pose some

important constraints on the local tectonic evolution.

The growth of the NAN is characterized by a progressive and continuous deformation over

the entire time window considered (ca. 200 kyr) which is younger than the most recent period

investigable in this region through industrial seismic reflection data. Uplift rates calculated

for both the NAN and Monte Netto hill are consistent, considering a longer time window

(0.89 Ma – present; Livio et al. [2009]), with uplift and slip values calculated for the

underlying backthrust, CCB. We can infer a genetic relationship between the observed

surface structures and CCB whose medium-term uplift rates can therefore be extended over

younger time windows. In extrapolating on deeper structures activity trends obtained from

shallow ones, however, some restrictions are necessary. The NAN uplift rates show, for

example, an apparent increase in the last 50 kyr (Fig. 11) but this trend could be equally

possibly due to a) a contemporary increase in the activity of the underlying main thrust; b)

changes in the partitioning of the strain propagating to the surface, which, due to the

kinematic evolution of the underlying system, had started to be preferably expressed at the

site; c) a combination of the two previous causes.

A major point to make clear is how the underlying steeplydipping backthrust can cause

shallow S-verging thrusting and related folding. Fig. 12c shows in cross-section the

outcropping Southern and Northern anticlines, the modeled Fault 2 and, below these

structures, the shallowest fault interpreted by Livio et al. [2009] (CCα). Fault 2 ramp

enucleates from the triangle zone indicating growth strata of CCα fault-bend fold. No

evidence of a downward projection of Fault 2 ramp can be found neither in seismic reflection

data (whose signal-quality is by-far too low in such shallow sectors) nor from fault modeling.

We have thus to suppose that Fault 2 is not rooted at the CCα fault plane; its formation can

be the result i.e. of localized strain accommodation connected to CCα synclinal active axial


surface (Fig. 12c). The structural reconstruction of the NAN highlights a break backward

activation of reverse and thrust faults generating the fold (i.e., Fault 1 and, successively, Fault

2): this is consistent with a progressive passive movement of the limb through the fixed

active axial surface, and a consequent progressively northward migration of strain through

activation of i.e out-of-syncline thrusting. Further interpretations cannot unfortunately be

drawn basing solely on these data lacking high-resolution geophysical investigations filling

the gap between typical resolution of commercial seismic reflection data and the scale of

surveyed structures.

A summary reconstruction that best matches these data and modeling results offering an

explanation of upward strain propagation starts with a coseismic activation of the deep blind

CCB, considered as a seismogenic source [Livio et al., 2009; Galadini et al., 2012]. Upward

strain accommodation is partly granted by shallow structures, possibly multibend flexural-

slip faults, like CCα, whose activity is driven by synclinal axial surface of the CCB-related

fold. Finally, in turn, CCα causes near-surface localized deformation, expressed as out-of-

syncline thrusting and related folding. This model implies that a major role in upward strain

propagation through compressive structures can be played by synclinal axial surfaces pinned

to changes in ramp angles. Timing and sequence of fault/fold activation (i.e. break-forward

vs -backward activation) can help in distinguish between different structural solutions and

models for surface deformations induced by deeper structures. The results obtained confirm

that restoration models provide a powerful tool in documenting the growth history of

compressive structures [e.g. Suppe et al., 1992; Shaw and Suppe, 1994; Allmendinger and

Shaw, 2000].

6.2 Surface faulting at Monte Netto site and paleoseismic potential.

The structural evolution of the NAN and, in general, the shallow tectonic deformations

observed at Monte Netto site offer new data on the tectonic evolution of the area and


significantly constraint the debate about surface faulting along shallow blind thrust systems.

Modes of strain partitioning and upward propagation of deformation is a major challenge for

studies on fault displacement hazard analysis, especially regarding blind structures and this

case study proves that very shallow compressive tectonics was caused by upward strain

propagation from underlying blind thrust through both decametric-scale folding and shallow

thrusting. Fold-related faulting can play an important role in causing localized surface

displacement; in this sense, even if this second- or higher-order structure cannot be easily

related and structurally linked through a balanced solution to the underlying main structure,

the calculated uplift rates and the evidence of progressive offset and deformation on the

outcropping structure can be considered as a proxy for the activity of the main thrust at depth

[i.e. Yeats, 1986; Galadini et al., 2012; Michetti et al., 2012]. Moreover secondary structures,

such as bending-moment faults, can provide good paleoseismological records of the

underlying blind structures. Paleoseismic analyses of the events recorded by the bending-

moment faults at the Monte Netto site are in progress; paleoseismic evidence at this site,

which is clearly related to the growth of a recent compressional structure, is significant to

assess the threshold for coseismic surface rupture along compressive capable faults in the Po

Plain foredeep and raises some important issues about the assessment of fault displacement

hazard. Indeed, environmental effects similar to those recorded at the Monte Netto site are

typically associated with macroseismic intensities > IX on the MCS (Mercalli Cancani

Sieberg; Sieberg, 1930) and ESI 2007 intensity scales [Michetti et al., 2007]. In fact, the ESI

2007 scale has been calibrated with the MCS scale, commonly used in Italy [Michetti et al.,

2004]. It is possible, therefore, to infer an epicentral intensity for the earthquake accompanied

by the surface faulting effects observed at Monte Netto of Io = IX MCS or greater .

Following the empirical relations between Io and Mw for shallow crustal earthquakes that

are currently used in Italy,


we obtain an Mw = 5.65 – 6.33 for an earthquake with Io = IX. This equation has been used

in the most recent compilation of the catalogue of Italian earthquakes [Gruppo di Lavoro

CPTI, 2004] in order to evaluate the seismic hazard on the Italian peninsula.

During the 2012 seismic sequence in Emilia, only few tens of km SE of Monte Netto, the two

Mw 5.9 and 5.8 (Io VIII MCS) main shocks generated ca. 10 – 15 cm of surface cumulative

uplift along the causative blind anticlines, accompanied by severe liquefaction effects, but no

surface faulting. We argue that the threshold for surface faulting earthquakes in the Po Plain

might be on the order of Mw 6.5 and Io IX-X; such conditions were observed perhaps only

during the Jan. 13, 1117, Verona, and Dec. 25, 1222, Brescia events. This is consistent with

Maximum Expected Magnitudes in the range of Mw 5.9 – 6.8 calculated by Livio et al.

(2009) and Michetti et al. (2012) for the Capriano del Colle fault system based on fault

rupture area and fault slip, considering different rupturing scenarios.

7. Conclusions

Logging and trenching of a quarry wall in the Monte Netto site (Brescia, N Italy), together

with radiocarbon and OSL age constraints on the exposed loess layers, allow us to unravel the

fold growth history of a secondary fault-related fold, linked to a blind backthrust in the Po

Plain whose activity has been previously documented through seismic reflection data (Livio

et al. 2009).

In summary from above, the following conclusions can be drawn from this study:

- recent uplift rates of the Monte Netto hill can be constrained as ca. 0.16 mm/yr from

MIS6 to present, considering ca. 30 m of uplift of the entire hill;

- the uplift rates calculated for a secondary fold on the hill (Northern anticline - NAN)

indicate values ranging between 0.02 and 0.1 mm/yr;


- the reconstructed structural evolution of the NAN highlights that two shallow thrusts

formed through a break-backward activation and resulting in a fold whose later growth

was associated with bending-moment normal fault in the crestal and forelimb region;

- the modeling of a trishear kinematic solution for the NAN indicates that the fold-

causative thrust has a ramp sector rooted at ca. 70 m below surface and I characterized

by a high P/S ratio, thus promoting upward strain propagation;

- upward strain propagation and generation of shallow low-angle thrust and related folding

seem to be mainly due to secondary fold-related faulting, like out-of-syncline thrusting

mechanisms.

- secondary near-surface structures offer a good opportunity to investigate through a

paleoseismological approach the activity of the underlying seismogenic source.

Acknowledgments

Data on grain-size curves, uplift rates, structural data and trishear model methods and results

are respectively available as in Supporting Information Table S1, Table S2, Table S3 and

TRISHEAR modeling files.

We thank the municipality of Capriano del Colle, Brescia Province administration and

Fornaci Laterizi Danesi S.p.A. and for permission to site access and trenching. In particular

we would like to thank Dr. Febbrari, Dr. Zamboni and Alessandro at F.lli Arici S.r.l. for their

logistic support.

This work has been in part funded by grants from the Operational Programme Cross Border

Cooperation IT / CH 2007-2013 - project "SITINET: census, networking and development of

geological and archaeological sites" ID 7621984. This research has also benefited from

funding provided by the Italian Presidenza del Consiglio dei Ministri - Dipartimento della

Protezione Civile (DPC). Scientific papers funded by DPC do not represent its official


opinion and policies. An Academic License of MOVE® suite software was provided by

Midland Valley and was used for fault/fold restoration. Many thanks to the municipality of

Capriano del Colle (BS) and Soprintendenza per i Beni Archeologici della Lombardia for

their support.

References

Adamiec, G. and M. Aitken (1998), Dose-rate conversion factors: update, Ancient TL, 16, 37 - 49.

Allmendinger, R. (1998), Inverse and forward numerical modelling of trishear fault-propagation folds, Tectonics, 17(4), 640-656.

Amit, R. and A. Zerboni (2013), Report on the INQUA-AEOMED field-trip workshop ‘Reconsidering Loess in Northern Italy’ (Po Plain, 1-3 July 2013), AMQ – Alpine and Mediterranean Quaternary, 26(2), xi-xv.

Amorosi, A., M. Barbieri, F. Castorina, M. L. Colalongo, G. Pasini and S.C. Vaiani (1998), Sedimentology, micropaleontology, and strontium-isotope dating of a lower-middle Pleistocene marine succession (" Argille Azzurre") in the Romagna Apennines, northern Italy, Bollettino della Società geologica italiana,117(3), 789-806.

Amorosi, A. and M. Pavesi (2010), Aquifer stratigraphy from the middle-late Pleistocene succession of the Po basin, Mem. Descrittive Carta Geol. Ital. XC, 7-20.

Banerjee, D., A. Murray, L. Bøtter-Jensen and A. Lang (2001), Equivalent dose estimation using a single aliquot of polymineral fine grains, Radiation Measurements, 41, 73 - 94.

Baroni, C. and M. Cremaschi (1986), Geologia e pedostratigrafia della Collina di Ciliverghe (Brescia), Natura Bresciana, Ann. Mus. Civ. Sci. Nat., 23, 55-78.

Benedetti, L., P. Tapponier, C. King, B. Meyer and I. Manighetti (2000), Growth folding and active thrusting in the Montello region, Veneto, Northern Italy, Journ. Geophys. Res., 105(B1), 739-766.

Bennet, R. A., E. Serpelloni, S. Hreinsdóttir, M.T. Brandon, G. Buble, T. Basic, G. Casale, A. Cavaliere, M. Anzidei, M. Marjonovic, G. Minelli, G. Molli and A. Montanari (2012), Syn-convergent extension observed using RETREAT GPS network, northern Apennines. Italy, Journal of Geophysical Research–Solid Earth, 117, B04408, doi:10.1029/2011JB008744.

Bignami, C., P. Burrato, V. Cannelli, M. Chini, E. Falcucci, A. Ferretti, S. Gori, C. Kyriakopoulos, D. Melini, M. Moro, F. Novali, M. Saroli, S. Stramondo, G. Valensise and P. Vannoli (2012), Coseismic deformation pattern of the Emilia 2012 seismic sequence imaged by Radarsat-1 interferometry, Annals of Geophysics, 55(4), 789 – 795.

Boccaletti, M., G. Corti and L. Martelli (2011), Recent and active tectonics of the external zone of the Northern Apennines (Italy), International Journal of Earth Sciences, 100(6), 1331-1348.


Boni A., G. Cassinis and S. Venzo (1970), Note illustrative della Carta geologica d’Italia alla scala 1:100000. Foglio 47 Brescia. – Geological Map of Italy, 1:100.000 scale. Serv. Geol. d’Italia, 93 pp., Poligrafica & Cartevalori, Ercolano (Napoli).

Bruno, P.P.G., F.J. Pazzaglia, and V. Picotti (2011), Evidence for active folding and faulting at the northern Apennines mountain front near Bologna, Italy from high resolution seismic reflection profiling, Geophysical Research Letters, 38(16).

Burrato, P., F. Cucci, and G.Valensise (2003), An Inventory of river anomalies in the Po Plain, Northern Italy: evidence for active blind thrust faulting, Annals of Geophys., 46(5), 865-882.

Carcano, C. and A. Piccin (2002), Geologia degli acquifi Padani della Regione Lombardia. Firenze: S.EL.CA.

Carminati, E., C. Doglioni and D. Scrocca (2004), Alps vs. Apennines, Special volume of the Italian Geological Society for the IGC 32, Florence 2004, 141-151.

Castaldini D. and M. Panizza (1991), Inventario delle faglie attive tra i fiumi Po e Piave e il Lago di Como (Italia settentrionale), Il Quaternario, 4 (2), 333-410.

Castellarin, A. and L. Cantelli (2000), Neo-Alpine evolution of the Southern Eastern Alps, Journal of Geodynamics, 30, 251-274.

Castellarin, A., L. Cantelli, A.M. Fesce, J.L. Mercier, V. Ricotti, G.A. Pini, G. Prosser and L. Selli (1992), Alpine compressional tectonics in the Southern Alps; relationships with the N-Apennines, Annales Tectonicae, 6 (1), 62-94.

Castellarin, A. and G.Vai (1986), Southalpine versus Po Plain Apeninic Arcs, in C. W. (Editor), The origin of Arcs, p. 253-280, Elsevier Science Publisher B. V., Amsterdam.

Castellarin, A., G.Vai and L. Cantelli (2006), The Alpine evolution of the Southern Alps around the Giudicarie faults: a Late Cretaceous to Early Eocene transfer zone, Tectonophysics, 414, 203-223.

Champion, J., K. Mueller, A. Tate and M. Guccione (2001), Geometry, numerical models and revised slip rate for the Reelfoot fault and trishear fault-propagation fold, New Madrid seismic zone, Engineering Geology, 62, 31-49.

Cremaschi, M. (1974), Manufatti del Paleolitico medio - inferiore, provenienti da Monte Netto di Brescia e loro rapporti con i depositi quaternari del Colle. Natura Bresciana - Ann. Mus. Civ. St. Nat. Brescia, 11, 41-57.

Cremaschi, M. (1987), Paleosol and vetusols in the Central Po Plain (Northern Italy) a study in Quaternary geology and soil development, Ed. Unicopli, Milan.

Cremaschi M. and C. Christopher (1985), Palaeolithic settlement and environment in the Middle Pleistocene of Northern Italy: the Ghiardo site. The Third Conference of Italian Archaeology, vol I, the environment, B.A.R. London, pp. 87-104.

Cremaschi, M. and M. Lanzinger (1984), La successione stratigrafica e le fasi pedogenetiche del sito epigravettiano di Andalo, i loess tardiglaciali della Val d'Adige, Preistoria Alpina, 19, 179 - 188.

Cremaschi, M., M. Alessio, L. Allegri, C. Azzi, G. Calderoni, C. Cortesi, V. Petrone and C. Spezzi Bottiani (1987), Una data radiocarbonica del paleosuolo su loess incluso nella successione stratigrafica della Val Sorda (sistema morenico del Garda), Rediconti della Società Geologica Italiana, 10, 29 - 32.


Cremaschi, M., A. Zerboni, C. Nicosia, F. Negrino, H. Rodnight and C. Spötl (2014), Age, soil-forming processes, and archaeology of the loess deposits at the Apennine margin of the Po Plain (northern Italy). New insights from the Ghiardo area. Quaternary International, DOI: 10.1016/j.quaint.2014.07.044, in press.

Desio, A. (1965), I rilievi isolati della Pianura Lombarda ed i movimenti tettonici del Quaternario. Rend Ist Lom Acc Sc Lett, Sez A, 99, 881-894.

Devoti, R., A. Esposito, G. Pietrantonio, A. Pisani and F. Riguzzi (2011), Evidence of large scale deformation patterns from GPS data in the Italian subduction boundary, Earth Planet. Sci. Lett., 311, 230 - 241.

Erslev, E. (1991), Trishear fault-propagation folding, Geology, 19, 617-620.

Fantoni, R., R. Bersezio and F. Forcella (2004), Alpine structure and deformation chronology at the Southern Alps-Po Plain border in Lombardy, Bollettino della Società Geologica Italiana, 123(3), 463-476.

Ferraro, F. (2009), Age, sedimentation and soil formation in the Val Sorda loess sequence, Northern Italy, Quaternary International, 204, 54 - 64.

Galadini F., E. Falcucci, P. Galli, B. Giaccio, S. Gori, P. Messina, M. Moro, M. Saroli, G. Scardia, and A. Sposato (2012), Time intervals to assess active and capable faults for engineering practices in Italy, Engineering Geology, 39–140, 50-65, ISSN 0013-7952, 10.1016/j.enggeo.2012.03.012.

Galbraith, R., R. Roberts, G. Laslett, H. Yoshida and J.Olley (1999), Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: Part I, Experimental design and statistical models, Archaeometry, 41, 339 - 364.

Gale, S.J. and P.G. Hoare (1991), Quaternary sediments. New York: Belhaven Press.

Guidoboni, E. (1986), The earthquake of December 25th, 1222: analysis of a myth, Geol. Appl. Idr., 21, 413-424.

Guidoboni, E. and A. Comastri (2005), Catalogue of earthquakes and tsunamis in the Mediterranean area from the 11th to the 15th century, SGA Storia Geofisica Ambiente srl., Bologna.

Gunderson, K.L., D.J. Anastasio, F.J. Pazzaglia and V. Picotti (2013), Fault slip rate variability on 104–105

Gunderson, K.L., F.J. Pazzaglia, V. Picotti, D.A. Anastasio, K.P. Kodama, T. Rittenour, K.F. Frankel, A. Ponza, C. Berti, A. Negri and A. Sabbatini (2014), Unraveling tectonic and climatic controls on synorogenic growth strata (Northern Apennines, Italy), Geological Society of America Bulletin, B30902-1.

yr timescales for the Salsomaggiore blind thrust fault, Northern Apennines, Italy, Tectonophysics,608, 356-365.

Hardy, S. and M. Ford (1997), Numerical modeling of trishear fault-propagation folding and associated growth strata, Tectonics, 16, 841-854.

Heddar, A., C. Authemayou, H. Djellit, A. Yelles, J. Déverchère, S. Gharbi, A. Boudiaf and B. Van Vliet Lanoe (2013), Preliminary results of a paleoseismological analysis along the Sahel fault (Algeria): New evidence for historical seismic events. Quaternary International, 302, 210–223.

Kühn, P., J. Aguilar and R. Miedema (2010), Textural pedofeatures and related horizons. In:


Stoops, G., Marcelino, V., Mees, F., (Eds.), Interpretation of micromorphological features of soils and regoliths. Elsevier, Amsterdam, 217-250.

Kulig, G. (2005), Erstellung einer Auswertesoftware zur Altersbestimmung mittels Lumineszenzverfahren unter spezieller Berücksichtigung des Einflusses radioaktiver Ungleichgewichte in der 238U-Zerfallsreihe. Freiberg: Unpublished BSc thesis, Technical University Bergakademie Freiberg.

Leon, L., S. Christofferson, J. Dolan, J. Shaw and T. Pratt (2007), Earthquake-by-earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: implications for fold kinematics and seismic hazard, Journal of Geophysical Research, 112 (B3), DOI: 10.1029/2006JB004461.

Livio, F., A. Berlusconi, A.M. Michetti, G. Sileo, A. Zerboni, L. Trombino, M. Cremaschi, K. Mueller, E. Vittori, C. Carcano and S. Rogledi (2009), Active fault-related folding in the epicentral area of the December 25, 1222 (Io = IX MCS) Brescia earthquake (northern Italy): seismotectonic implications, Tectonophysics, 476, 320-335.

Magri, G. and D. Molin (1986), I terremoti del 3 Gennaio 1117 e del 25 Dicembre 1222. ENEA-RTI-PAS-ASPGEOL-LO, 2.

Marchetti, M. (1996), Variazioni idrodinamiche dei corsi d'acqua della Pianura Padana centrale connesse con la deglaciazione, Il Quaternario, 9, 465-472.

Masaferro, J.L., M. Bulnes, J. Poblet and G.P. Eberli (2002), Episodic folding inferred from syntectonic carbonate sedimentation: the Santaren anticline, Bahamas foreland. Sedimentary Geology, 146(1), 11-24.

Mauz, B., S. Packman and A. Lang (2006), The alpha effectiveness in silt-sized quartz: New data obtained by single and multiple aliquot protocols, Ancient TL, 24, 47 - 52.

Meghraoui, M. and F. Doumaz (1996), Earthquake-induced flooding and paleoseismicity of the El Asnam, Algeria, fault-related fold, Journal of geophysical research, 101(B8), 17617-17.

Michetti A.M., E. Esposito, L. Guerrieri, S. Porfido, L. Serva, R. Tatevossian, E. Vittori, F. Audemard, T. Azuma, J. Clague, V. Comerci, A. Gürpinar, J. Mc Calpin, B. Mohammadioun, N.A. Mörner, Y. Ota and E. Roghozin (2007), Environmental Seismic Intensity Scale 2007 – ESI 2007. In: Vittori, E. and L. Guerrieri (eds.) Memorie Descrittive della Carta Geologica d’Italia, LXXIV. Servizio Geologico d’Italia, Dipartimento Difesa del Suolo, APAT, SystemCart Srl, Roma, Italy, 7–54.

Michetti, A.M., F. Giardina, F. Livio, K. Mueller, L. Serva, G. Sileo, E. Vittori, R. Devoti, F. Riguzzi, C. Carcano, S. Rogledi, L. Bonadeo, F. Brunamonte and G. Fioraso (2012), Active compressional tectonics, Quaternary capable faults and the seismic landscape of the Po Plain (Northern Italy), Annals of Geophysics, 55(5), 969 - 1001.

Mroczek, P. (2013), Recycled loesses: A micromorphological approach to the determination of local source areas of Weichselian loess, Quaternary International, 296, 241-250.

Murphy C.P. (1986), Thin section preparation of soils and sediments, AB Academic Publishers, Berkhamsted, Herts.

Muttoni, G., C. Carcano, E. Garzanti, M. Ghielmi, A. Piccin, R. Pini, S. Rogledi and D. Sciunnach (2003), Onset of major Pleistocene glaciations in the Alps, Geology, 31(11), 989-992.

Peresani, M., M. Cremaschi, F. Ferraro, C. Falguères, J.J. Bahain, G. Gruppioni, E. Sibilia,


G. Quarta, L. Calcagnile and J.M. Dolo (2008), Age of the final Middle Palaeolithic and Uluzzian levels at Fumane Cave, Northern Italy, using 14C, ESR, 234U/230Th and thermoluminescence methods. Journ. Arch. Sci, 35, 2986-2996,

Picotti, V. and F.J. Pazzaglia (2008), A new active tectonic model for the construction of the Northern Apennines mountain front near Bologna (Italy), Journal of Geophysical Research: Solid Earth (1978–2012) 113.B8 (2008).

Pola, M., A. Ricciato, R. Fantoni, P. Fabbri, & D. Zampieri (2014), Architecture of the western margin of the North Adriatic foreland: the Schio-Vicenza fault system, Italian Journal of Geosciences, 133(2), 223-234.

Ponza, A., F.J. Pazzaglia and V. Picotti (2010), Thrust-fold activity at the mountain front of the Northern Apennines (Italy) from quantitative landscape analysis, Geomorphology, 123(3), 211-231.

Prescott, J. and J. Hutton (1994), Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term variations, Radiation Measurements, 23, 497 - 500.

Previtali, E. (1992), Seismipedoturbations in Volcanic Soils in North-Eastern Ecuador, CATENA 19, 441-450.

Reimer P.J., E. Bard, A. Bayliss, J.W. Beck, P.G. Blackwell, C. Bronk Ramsey, C.E. Buck, R.L. Edwards, M. Friedrich, P.M. Grootes, T.P. Guilderson, H. Haflidason, I. Hajdas, C. Hatté, T.J. Heaton, D.L. Hoffman, A.G. Hogg, K.A. Hughen, K.F. Kaiser, B. Kromer, S.W. Manning, M. Niu, R.W. Reimer, D.A. Richards, M. Scott, J.R. Southon, R.A. Staff, , C.S.M. Turney and J. van der Plicht (2013), INTCAL13 and MARINE13 radiocarbon age calibration curves 0-50,000 years cal BP, Radiocarbon, 55, 1869-1887.

Roberts, H. and A. Wintle (2001), Equivalent dose determinations for polymineralic fine-grains using the SAR protocol: application to a Holocene sequence of the Chinese Loess Plateau, Quaternary Science Reviews, 20, 895 - 863.

Rovida, A., R. Camassi, P. Gasperini and M. Stucchi (2011), CPTI11, la versione 2011 del Catalogo Parametrico dei Terremoti Italiani. http://emidius.mi.ingv.it/CPTI. Milano, Bologna.

Scardia, G., G. Muttoni and D. Sciunnach (2006), Subsurface magnetostratigraphy of Pleistocene sediments from the Po Plain (Italy): Constraints on rates of sedimentation and rock uplift, Geological Society of America Bulletin, 118(11/12), 1299-1312.

Scognamiglio L., L. Margheriti, F.M. Mele, E. Tinti, A. Bono, P. De Gori, V. Lauciani, F.P. Lucente, A.G. Mandiello, C. Marcocci, S. Mazza, S. Pintore and M. Quintiliani (2012), The 2012 Pianura Padana Emiliana seimic sequence: locations, moment tensors and magnitudes. Annals of Geophysics, 55(4)), doi: 10.4401/ag-6159.

Serva, L. (1990), Il ruolo delle Scienze della terra nelle analisi di sicurezza di un sito per alcune tipologie di impianti industriali; il terremoto di riferimento per il sito di Viadana (MN), Boll. Soc. Geol. Ital., 109, 375-411.

Shaw, J. H., and Suppe, J. (1994), Active faulting and growth folding in the eastern Santa Barbara Channel, California. Geological Society of America Bulletin, 106(5), 607-626.

Sieberg A. (1930), Geologie der Erdbeden, in Handbuch der Geophysik, 2, 4,550-555.


Stoops, G. (2003), Guidelines for analysis and description of soil and regolith thin sections. Soil Science Society of America, Madison, WI.

Stoops, G., V. Marcellino, and F. Mees (2010), Interpretation of micromorphological features of soil and regoliths. Elsevier, Amsterdam.

Streig, A., C.W.S. Rubin, Y. Chen, L. Lee, S. Thompson, C. Madden and S. Lu (2007), Evidence for prehistoric coseismic folding along the Tsaotun segment of the Chelungpu fault near Nan-Tou, Taiwan. Journal of Geophysical Research, 112 (B3), DOI: 10.1029/2006JB004493.

Stuvier, M. and H. Polach (1977), A discussion and reporting of 14

Stuvier, M., P.J. Reimer and R. Reimer (2013): Calib Radiocarbon Calibration version 7.0. – http://calib.qub.ac.uk/calib/

C data, Radiocarbon, 19, 355 - 363.

Suppe, J., and D. Medwedeff (1990), Geometry and kinematics of fault-propagation folding, Eclog. Geol. Helv., 83, 409-454.

Suppe, J., G. Chou and S. Hook (1992), Rates of folding and faulting determined from growth strata, in K. McClay, Thrust tectonics (p. 105-120), Chapman & Hall, London.

Ventura, G. and R. Di Giovambattista (2013), Fluid pressure, stress field and propagation style of coalescing thrusts from the analysis of the 20 May 2012 ML 5.9 Emilia earthquake (Northern Apennines, Italy), Terra Nova, 25(1), 72-78.

Yeats, R.S. (1986), Active faults related to folding. Active tectonics, 63-79.

Wacha, L., S. Pavlakovic, A. Novothny, M. Crnjakovic and M. Frechen (2011), Luminescence dating of Upper Pleistocene loess from the Island of Susak in Croatia, Quaternary International, 234, 50 - 61.

Waelbroeck, C., L. Labeyrie, E. Michel, J. Duplessy, J. McManus and K. Lambeck (2002), Sea-level and deep water temperature changes derived from benthic foraminifera isotopic records, Quaternary Science Reviews, 21, 295 - 305.

Wilson, L.F., F.J. Pazzaglia and D.J. Anastasio (2009), A fluvial record of active fault-propagation folding, Salsomaggiore anticline, northern Apennines, Italy, Journal of Geophysical Research: Solid Earth (1978–2012), 114(B8).

Wintle, A. (1997), Luminescence dating: laboratory procedures and protocols, Radiation Measurements, 27, 769 - 817.

Zehnder, A.T. and R.W. Allmendinger (2000), Velocity field for the trishear model. Journal of Structural Geology, 22(8), 1009-1014.

Zerboni, A., L. Trombino, C. Frigerio, F. Livio, A. Berlusconi, A.M. Michetti, H. Rodnight and C. Spötl (2014), The loess-paleosol sequence at Monte Netto: a record of climate change in the Upper Pleistocene of the central Po Plain, northern Italy, Journal of Soils and Sediments. DOI: 10.1007/s11368-014-0932-2. – in press

Zorzi, F. (1959), Un’Amigdala Acheuleana scoperta a Lughezzano di Valpantena nel quadro del paleolitico inferiore e medio Veronese, Memorie del Museo Civico di Storia Naturale di Verona, vol. III.


Table 1: Stratigraphic and pedostratigraphic units

Unit Description Pedo-sedimentary interpretation

S1 Poorly weathered coarse and medium silt deposit (unimodal distribution), very low sand content, moderate content of organic matter.

Soil + Ap horizon

PL1 Coarse and medium silt deposit (unimodal distribution), very low sand content; at the base, common to frequent sandy Mn-concretions (Mn-pisolites).

Loess

S2 Coarse and medium silt deposit with significant clay content (but lower than S1) and pedofeatures related to clay illuviation and weathering.

Paleosol developed on loess

PL2 Coarse and medium silt deposit (unimodal distribution), common to frequent sandy Mn-concretions (Mn-pisolites).

Loess

S3 Coarse and medium silt deposit with significant clay content and pedofeatures related to clay illuviation and weathering.

Paleosol developed on loess

PL3 Coarse and medium silt deposit (unimodal distribution), very low sand content.

Loess

S4 Silty sand with a high degree of clay mobilization and accumulation of Fe-Mn

Paleosol developed on CL

CL Fine sandy to silty deposit with common sandy Mn-concretions (Mn-pisolites).

Colluvial deposit

S4’ highly rubified, polycyclic buried paleosol developed mainly on fluvioglacial sediments.

Paleosol developed mainly on FG2 & FG1

FG1 3 cm of gray plastic clay overlying a grayish-yellow silty fine sand, closely alternating with gray clayey silt.

Levee

FG2 Fining-upward coarse sand with small discontinuous lenses of microconglomerates.

Crevasse-splay

FG3

Matrix-supported conglomerate made of centimetric deeply weathered clasts in a reddish-brown coarse sandy matrix and with a slightly erosive basal contact.

Crevasse-splay

FG4

From the base: medium sand passing upward to grayish clayey silt; a 40 cm-thick layer of coarse sand with hard Fe-Mn concretions; yellowish medium to fine sand passing upward to grayish clayey silt including a lens of coarse sand, pinching out southward.

Crevasse-splay


FG5 Upward-coarsening brownish-green silt passing to silty sand.

Levee

FG6

Coarse sand and matrix-supported microconglomerates passing upward to brownish fine sand. Fe-Mn concretions concentrate locally at the upper interface of intercalated silty and fine sand lenses. The base is marked by an erosive surface.

Crevasse-channel

FG7

From the base: a thin lens of fining-upward brownish-yellow silty fine sand with intercalated medium sand lenses. Soft Fe-Mn pisolitic concretions are present. Silt and greenish-grey clay, at the top, are truncated by an erosive surface.

Levee and floodplain fines

FG8

Clast-supported conglomerate in a sandy reddish-brown matrix; rounded and flatted clasts (max 5 – 6 cm). Both crystalline and carbonate clasts are deeply weathered and mm-thick coatings are common.

Channel facies; outwash braided plain


Table2: Bulk radiocarbon analyses of samples collected in trenches.

Sample Code

Stratigraphic unit

Depth below

ground surface (m)

Radiocarbon age (yr BP)*

δ13 Calibrated age C (‰)

yr BP†

CAP01 PL1 (upper part) 0.4 4635 ± 45 -22.5 ±

0.3 5574 - 5094

CAP02 PL1 (upper part) 0.5 6485 ± 50 -28.0 ±

0.3 7483 - 7287

CAP03 PL1 (lower part) 1.0 13313 ± 75 -33.7 ±

0.5 16241 - 15772

*Quoted in radiocarbon years BP using the Libby half-life of 5568 yr and following the conventions of Stuiver and Polach [1977]; uncertainty ±1σ.

† Calibrated radiocarbon ages (±2σ) calculated using Calib 7.0 [Stuiver et al., 2013] and INTCAL 13 (Reimer et al., 2013).


Table 3. Details of dosimetry calculations

Sample code

Grain size

(µm)

Water content

(%) K (%) U (ppm) T (ppm)

Cosmic dose-rate

(mGy/kyr)

Total dose-rate (Gy/kyr)*

CC01 4 - 11 9.5 1.99 ± 0.10 15.9 ± 0.8 4.5 ± 0.2 176 ± 18 7.01 ± 0.83

CC02 4 - 11 7.3 1.95 ± 0.10 16.6 ± 0.8 4.8 ± 0.2 176 ± 18 7.37 ± 0.88

CC03 4 - 11 6.3 1.65 ± 0.08 16.8 ± 0.8 4.9 ± 0.2 161 ± 16 7.22 ± 0.89

* The total effective dose-rate from the environment to quartz grains 4-11 µm in diameter was calculated taking into account the alpha efficiency factor.


Table 4: Results for OSL dating

Sample code

number of

samples

Burial dose (Db

(Gy) )*

Environmental dose-rate (Gy/kyr)

Stratigraphic unit Age (kyr)

CC02 24 146 ± 3 7.37 ± 0.88 PL1 (middle part) 19.9 ± 2.3

CC01 24 173 ± 4 7.01 ± 0.83 PL1 (few cm upon S4’) 24.6 ± 2.9

CC03 24 321 ± 9 7.22 ± 0.89 PL2 44.4 ± 5.4

*Calculated using the central age model.


Fig. 1: Structural regional framework and historical and instrumental regional seismicity from

the CPTI catalogue [Rovida et al., 2011]; focal mechanisms of the 2004 Salò earthquake and

the two main shocks of the 2012 Po Plain seismic sequence are shown. WSA: Western

Southern Alps; ESA: Eastern Southern Alps.


Fig. 2: a) Map of buried thrusts (modified after [Livio et al., 2009]) as revealed by the

analysis of an extensive database of seismic reflection lines (courtesy of ENI E&P) and

historical and instrumental seismicity data (CPTI catalogue; [Rovida et al., 2011]); b)

geological sketch map of the Monte Netto area and (inset) detail on a topo survey of the

quarry area, acquired in 2008: the hinge traces of the outcropping folds underline that

structural relief is closely mimicked by topography; c) 3D perspective of the Monte Netto

hill elaborated from a 20 m digital elevation model. The white arrow indicates the location of

the studied quarry.


Fig. 3: a) Interpreted seismic reflection profile (for line trace see Fig. 2); b) close up to the

Monte Netto hill area (see Fig. 2b for section trace). Modified after [Livio et al., 2009].


Fig. 4: Photographic documentation of the quarry area at Monte Netto. The map of the quarry

walls and the point of view of the photos are shown in h). a) N-S oriented quarry wall

showing the south-verging anticline analyzed in detail. Arrows mark the bending-moment

faults offsetting the anticline crest and the white box indicates the area logged in Fig. 7; b) a

lateral perspective of the same wall and of other cuts offering well-oriented cross-sections of

the deformed sequence; c), d) and e) highlight the exposure of the loess cover along different

quarry walls thus allowing a 3D reconstruction of the pedo-sedimentary (e) and tectonic; f)

detail of a fault plane: slickensides are clearly visible indicating a dip-slip movement. Note

the thick infilling of rubified illuvial clay smeared along the fault plane; g) sand and gravel

injection dike, ca. 80 cm high, indicating a coseismic triggering contemporary to fold

development.


Fig. 5: a) stratigraphic column of the analyzed sequence exposed at the Monte Netto quarry.

A brief description of the Units is provided in Table 1; for the complete Log of the section

see Fig. 7. Dating positions and the archeological findings are reported; b) the inferred

correlation with the chronological scale, the marine isotope curve [Waelbroeck et al., 2002]

and the paleomagnetic chronology is given for comparison.


Fig. 6: Grain-size curves for the stratigraphic sequence at Monte Netto; data are referred to

the deepest sequence observed south of the anticline core. In the graph: solid green curve is

CL/FG; dashed green curve is weathered CL/FG; solid orange curve is the poorly weathered

loess PL3; dashed orange is the highly weathered soil on loess S3; solid red curve is the mean

value for PL2/S2 units (moderately weathered loess); solid blue curve is the mean value for

PL1/S1 units (poorly weathered loess); the solid and dashed black lines represent the

theoretic curves illustrating the typical grain size for respectively fresh and weathered loess in

the Po Plain [according to Cremaschi, 1987].


Fig. 7: a) panoramic view of the logged quarry wall; b) Detailed log of the western trench

wall across the anticline. Stratigraphic units FG8 to PL1 are described in Table 1. Position of

samples collected for radiocarbon and OSL dating are indicated (values are given in Tables 2,

3 and 4 and Fig. 5; AMS-14

C ages are expressed in cal yr BP). Stereoplot of the brittle

structures affecting the fold are also drawn: c) pre-unconformity structures; d) post-

unconformity structures; e) poles and density contouring of the (c) and (d) subset; f) the pre-

unconformity data were back-rotated according to the local bedding attitude; g) poles and

density contouring of (d) and (f) subset.


Fig. 8: a) Scan of a thin section of sediment sampled from of a system of sub-vertical

fractures deforming fine-sand fluvial sediments below the main fault of the graben (evidence

of seismipedoturbations). Fractures are filled by illuvial clay originating from the paleosol

developed on top of the loess; b) b') and c) c'): photomicrographs of a system of sub-vertical

fractures filled by illuvial clay (b, c: plain-polarized light; b', c': cross-polarized light ). Note

the occurrence of an external coating of red clay (black dashed lines), filled by yellow clay

(white dashed lines) in the close-up (d: plain-polarized light; d': cross-polarized light ).


Fig. 9: Progressive forward deformation of the NAN. Fold limbs were progressively uplifted

and rotated in order to observe the structural relief constraints given by the growth strata.

Faults are red-colored while moving and chronological constraints are given by datings and

correlations. More than 9 meters of vertical uplift and an inferred shortening of ca. 1.5 meters

is obtained.


Fig. 10: Simplified model for area balancing of the moment-bending faults affecting the

anticline: void space, created by the gravity graben, along a differential limb rotation (α), is

approximately equal to the area (α’) subtended by the circle sector described by a rigid-plate

tilting.


Fig. 11: Uplift measured along the anticline; calculated uplift rates are indicated for each

considered step.


Fig. 12: a) Fault parameters obtained from the anticline modeling, using a Trishear velocity

field code [e.g., Allmendinger, 1998]; b) detail of the modeled forelimb: strain ellipses and

LFNE (Lines of No Finite Elongation) are indicated; pale-pink lines are the observed

geometry of units, thick grey lines represent modeled unit horizons. Note the good orientation

correlation between LNFE and brittle deformation on the fold frontlimb; c) comparison

between a detail of the shallowest part of the interpreted seismic line (see Fig. 3b for

location) and the modeled fault ramp: northern and southern outcropping anticlines are

indicated as blue lines.

Progressive offset and surface deformation along a seismogenic blind thrust in the Po Plain foredeep...

Documents

Transcript of Progressive offset and surface deformation along a seismogenic blind thrust in the Po Plain foredeep...