Geophysical data along the northern Italian sector of the European Geotraverse

Tectonophysics,

167

176 (1990) 167-182

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

european

Geophysical data along the northern Italian sector of the European Geotraverse E. CASSANO,

L. ANELLI

and R. FICHERA

AGIP, P. 0. Box 12069, 20120 Milan (Iialy) (Received

March 251988;

revised version accepted

April 24,1989)

Abstract Cassano,

E., Anelli,

Geotraverse.

L. and Fichera,

In: R. Freeman

R., 1990. Geophysical

and St. Mueller

data

(Editors),

along

the northern

The European

Italian

Geotraverse,

sector

of the European

Part 6. Tectonophysics,

176:

167-182. Magnetic, northwards

gravimetric

and

from the Ligurian

geometry,

depth,

and nature

we attempt

a structural

anomalies,

the gravimetric

structures ophiolites,

volcanites

evolution Tertiary

reveal

Alpine

data provide

of the area. The study

by Permian

extensional

tary faults during characterized

structures, tectonics

the Mesozoic.

by large-scale

side, the panorama

bodies

Apenmne having

structural

evidence

area consists

basement

natures

slabs beneath

of the sedimentary

we depicted the central

coupled

with volcanism,

Beginning

in the Tertiary

uplifted

of localized

46”OO’ (Fig. 1). Our study consists of a series of maps and three geophysical-geological sections which all together furnish a tectonic-structural outline of the southern Alps-P0 Plain-northern Apennine system. Qualitative analysis of the data was performed with filtered maps, while the quantitative interpretation was obtained using 0 1990 Elsevier Science Publishers

sequence.

B.V.

From

By filtering and

a band

the presence

These

the southern

and

provide

crust).

system. High”

data indicate

and origins.

bodies

By integrating

geometries

involving

the Bouguer of translated to

Apetines.

seismic interpreta-

as well as the chronological zones, having

zone was affected

and dislocated

to gradual

the pre-Mesozoic

thick ophiolitic

on

these factors

of a substratum

and two erogenic

in the Triassic-Lias,

Pontremoli evidence

can be ascribed

Alps and northern

as a foreland

the area was subjected

Bouguer anomalies and the residual magnetic field were obtained for the PO Plain area. Part of these data were used for a synthesis of the area enclosed between longitudes 8”37’ and 10°07’E of Greenwich, and the N latitudes 44O 20’ and

Genoa

zone to the north and south. The central

overthrusting

by the presence

Apennine

the structural

of a rigid zone acting

which overthrust

between

(superficial

to the “Milan

runs. Magnetic

various

basement

data,

in the area

sector. Data and interpretation

relative

Introduction

0040-1951/90/$03.50

obtained

Basin-northern

anomalies

disrupted

Paleogene-Neogene

is complicated

Alps-P0

residual

geological

were

South-Alpine

down to the magnetic

and northern

and magnetic

and surface

compressive

data

of the southern

data

and tectonically

Seismic reflection tions with borehole

reflection

of the subsurface

outline

on the southern

with high susceptibility

seismic

coast to the Lombardy

by synsedimen-

sinking.

The fault zones are

basement.

On the Apennine

bodies.

graph systems checked by 2-D and 3-D computerized models (Talwani and Ewing, 1960; Talwani and Heirtzler, 1962; Talwani et al., 1964; Talwani, 1965). Borehole data and the PO Plain regional seismic sections (Pieri and Groppi, 1981) were integrated with the magnetic and gravimetric data by interactive modelling to obtain the geophysical-geological

sections.

Gravimetry The gravimetric survey, which was carried out by AGIP or by a subcontractor during hydrocarbon research operations, includes approxi-

E CASSANO

ET AL.

phase to help the qualitative analysis. The filtering operation can be done directly in the space domain by means

of a 2-D convolution

with the filter

operator,

main by means

of the product

trum (signal Fourier operator

spectrum

of the signal

or in the frequency

of the signal spec-

Transformation) (operator

do-

and the filter

Fourier

Transforma-

tion). Since the object the structures immediately

of this work is to investigate

of the sedimentary underlying

made using a Gaussian-type, a 52 km cutoff wavelength The amplitude-averaged

cover

basement,

and

a map

the was

high pass filter with (Fig. 3). radial

spectrum

of the

Bouguer anomalies and the dimensions of the studied area were taken into account when this cutoff wavelength was chosen. Therefore, the filtered map (Fig. 3) essentially represents the NNINES

components of the Bouguer anomalies with short and medium wavelengths. The Bouguer anomaly map (Fig. 2) shows a regional drop of 165 mGa1 between the Ligurian Sea (+ 15 mGa1) and the Apermine-Po Plain margin (-150 mGal), with a rise towards the pre-Alpine area where a maximum value of - 30 mGa1 is reached north of Bergamo. Certain sectors were differentiated by analyzing the anoma-

Fig. 1. Index map of the southern Alps-PO Plain-northern Apennine System area.

lies having Apennine

mately 20,000 stations equipped with highly sensitive gravimeters (0.01 mGa1) of the Western 4A, Worden 654, W. Wilde and Lacoste-Romberg type. Station densities vary from l/km* in the PO Plain to 0.5/km2 in the Apennines and O.l/km* in the Alps. The Bouguer anomaly map (fig. 2) was obtained using a density value of 2400 kg/m3 in the topngraphic and Bouguer reduction calculations. This map and a few elaborations were used to make a qualitative analysis and quantitative interpretation. The anomaly wavelength depends on the depth of the source; under certain conditions the filtered maps can be used, in a qualitative way, to identify and separate these sources from the regional context. Seismic, borehole data, and the quantitative interpretation itself are commonly used in this

shorter

wavelengths.

sector

This sector is characterized by almost-parallel isoanomalies having an average NW-SE trend (Fig. 2). The weak density contrasts between the geological formations do not cause isolated anomalies. The isoanomalies suddenly become denser at the longitude of Genoa and their trend rotates from NW-SE to E-W. This indicates that there might be an element with a NNE-SSW trend separating the two zones of the Apennine sector which appears homologous to the SestriVoltaggio Line. The anomalies with the shorter wavelengths (Fig. 3) have dispersed trends and are associated with frequent stratigraphic and tectonic contacts. PO Plain Sector This sector is characterized by a zone of minimum values (-150 mGa1) at the boundary with

GEOPHYSICAL

DATA

OF NORTHERN

8’37 1’

ITALIAN

SECTOR

OF EGT

169

’

10-07

BEG,G(O

1 ’

f

\

\+\

(

\

iL’bilO1~

-

\\L-GALSOMAGGI~RE

44’20’ -

CONTOUR

INTERVAL

50 mGal Y

CONTOUR

’

INTERVAL

5 mGsl

Fig. 2. Bouguer anomaly map of the study area.

\

1 11

170

E. CASSANO

44”20’ -

CONTOUR

INTERVAL

10 mGal \

CONTOUR

INTERVAL

2 mGal

Fig. 3. Bouguer anomaly map made using a highpass filter

ET AL.

GEOPHYSICAL

DATA

OF NORTHERN

ITALIAN

-

SECTOR

OF EG’I

CONTOUR

INTERVAL

50 nT \

CONTOUR

INTERVAL

5 nl DATUM

PLANE

= 4000

mt

a.s.1.

Fig. 4. Map of the residual magnetic field.

172

the outcropping Apenmne folds (Fig. 2). A series of short wavelength anomalies with amplitudes up to 10 mGal (Fig. 3) form an arc which goes from Cremona wards

to Casteggio

and

northwest.

These

the

gravimetric

expression

rato folds already The

area

acterized

structural “Milan

and Monfer-

(Cassano

A pseudocircular length

of about

nT, extends is connected ceptibility

which

Pavia has

a medium

of about

is attributed

is char30 mGa1

to a known called the

et al., 1986a). This high,

also depicted on the map of magnetic interpretation (Fig. 7), is confirmed by the reduced section encountered in the Battuda 1 well (Figs. 7 and 9).

Magnetometry

anomaly

which

having

and

high sus-

(Figs. 4 and 5).

SSW trend

in the Genoa

area with a NNE-

and an amplitude

of 900 nT (Figs. 4

and 5) is due both to a deep substratum susceptibility

and to the outcropping

the Voltri Group.

Anomalies

lengths are present (Fig. 6).

throughout

with high ophiolites

having

shorter

the studied

In the Milano-Voipedo-Mortara anomalies have N-S and WNW-ESE plitudes Tertiary

of 100

to Volpedo-Mortara,

with a substratum

as

has a wave-

60 km and an amplitude

from Milan

The anomaly and

high in the PO Plain basement High”

the

1981). Milan

and an amplitude

2). This anomaly

1 toare

from seismic interpreta-

by an anomaly

wavelength (Fig.

between

Volpedo

anomalies

of the Emilia

known

tion (Pieri and Groppi,

from

a 52 km cutoff wavelength which was chosen the most representative for the study area.

of

wavearea

area, these trends, am-

of about 15 nT, and are associated volcanic bodies.

with

The anomalies in the Rapallo zone, with a NW-SE trend and an amplitude of 130 nT, indicate the presence

of thick ophiolitic

bodies.

The study area was surveyed by AGIP when the Magnetic Map of Italy was being prepared. A total of 15,000 km of survey lines were used for this study, flown at altitudes of 4800 ft, 5000 ft,

In the Pontremoli, Salsomaggiore and Bergamo zones, low-amplitude anomalies (lo-30 nT) indicate low-susceptibility sources related to cristalline basement wedges (Cassano et al., 1986b). The

8500 ft, and 13300 ft with various grids measuring 2 x 7.5 km, 5 x 5 km and 1.5 X 3 km. CFS and Varian magnetometers, with 0.01 and 0.02 nT

trends of the anomalies at Pontremoli and Salsomaggiore are “Apenninic” (i.e. NW-SE) and those at Bergamo are “South-Alpine” (E-W). The negative anomaly zones (Figs. 4, 5 and 6)

sensitivities were used to make the measurements. After making the appropriate corrections and sub-

are indicative of depressed basement largest of these zones has a NE-SW extends from Genoa to Lake Iseo.

areas. trend

The and

stracting the Geomagnetic Reference Field (IGRF 1978), the recorded values were used for plotting the residual magnetic field (RMF) anomaly map. These anomalies are either caused by susceptibility contrasts within the basement or by the presence of rocks with high susceptibility in the sedimentary cover. Although various values were recorded at different flight altitudes, an “upward continuation” operation was used to calculate the

We carried out a magnetic interpretation to (a) define the depth and structural setting of the magnetic basement and (b) identify the magnetic markers located inside the sedimentary cover. The

values for the same altitude, thus obtaining an homogeneous map (Fig. 4). Since the magnetic anomalies in the studied area were quite complex, the RMF map was treated with a “reduction to the pole” operator (Fig. 5). Therefore this map can be examined in a manner which is analogous to that of the gravimetric map. AC with the gravimetry, a filtered map was also made for the magnetometry using a highpass Gaussian-type filter with

term “magnetic basement” refers to the deepest marker revealed by the interpretation. The magnetic basement is generally characterized by relatively low susceptibility contrasts (125-377 X 10e5 S.I. units), with the Milan-Pavia high (“Milan High”) showing a local susceptibility increase to 2ooO X lop5 (S.I. units). The isobaths (Fig. 7) show a culmination in the Pavia zone (5000 m below sea level), which is

Structural map of the magnetic basement

GEOPHYSICAL

DATA

OF NORTHERN

ITALIAN

-

SECTOR

OF EGT

CONTOUR

173

INTERVAL

50 n-r -

CONTOUR

INTERVAL

5nT DATUM

PLANE

= 4000 mt a.s.I.

O’Pkm

Fig. 5. Map of the residual magnetic field reduced to the pole.

174

E. CASSANO

44”20’ -

CONTOUR

INTERVAL

20 I-IT -

CONTOUR

INTERVAL

2 nT DATUM

PLANE=

4000mt

S.S.I.

Fig. 6. Highpass filter map of the residual magnetic field reduced to the pole.

ET Al.

GEOPHYSICAL

DATA

OF NORTHERN

ITALIAN

SECTOR

175

OF EGT

S. CQLOMSANO

n

4

,

,

44’20’ +

+

+

STRUCTURAL

HIGH

AXIS

-

-

-

STRUCTURAL

LOW

AXIS

-r -

MAGNETIC -

BASEMENT DEPTH CONTOUR IN km BELOW THE SEA LEVEL TUSCAN

DISCONTINUITV

FAULT OVERTHRUST

.3-

.q?y

LIGURIAN

D.

Fig. 7. Map of the magnetic basement structure.

BASEMENT UNITS

E. CASSANO

176

46”OC

MORTARA

1

44”20’ ‘4s. 3/ .d**..

DEPTH CONTOUR IN km BELOW THE SEA LEVEL ~Di~ERENliAlE~ MAGNETIC LAYERS

/...

\

\

I

,,“Ey 0 .“2p.

Fig. 8. Map of the intrasedimentary

* MAGNETIC

CLASTIC

PERMIAN

VOLCANITE

TERTIARY

,uv++-ww OPHlOLlTE

... ..

&w #x*-*x.

3 WELL 30km

magnetic

markers.

VOLCANITE

BODY

ET AL

I

0 A

SOUTH

Miocene.

Surface idealized

the south,

where strong

structural

model indicated

data show that the Apennines

towards

and subsurface

that thin basement

tilting

The Garlasco-Sartirana in the thrust detachment.

Apemtines. volcanites

opposite

(dacites)

with

polarity.

The

date from the

to the Tertiary

A basement

NORTH

is thin compared faults having

section

Alps to the northern The Mesozoic

the PO Plain area with low-angle

are present.

are present.

from the southern

sheets are involved

Alps overthrust

and regional and the southern

subsidence

volcanites

cross section magmatic-stage

PO Plain geological

sector where Permian

western

LEGEND

no. 4 (see Fig. 1 for location):

is seen in the Sartirana-Lachiarella

section

20 km

of the latter increases

susceptibility

the thickness

Early-Middle

section;

high magnetic

Fig. 9. Geophysical-geological

a

3 loo-

178

E. CASSANO

dislocated by faults and magnetic discontinuities having a NNW-SSE trend. There is a very deep depression on the eastern side of this high zone, which extends from Genoa to Lake Iseo where the basement reaches its maximum depth (over 15,000 ml. The Milan High is bordered on the north and on the south by a few magnetic horizons, which have been associated with the South-Alpine and Apennine basements respectively. Those horizons bordering the high on the north develop with an E-W trend in the Bergamo area at a depth that varies between 0 and 8OfKl m. Their structural geometry is compatible with the tectonics of the sedimentary cover. Those horizons bordering the high on the south develop with an average NW-SE trend in the Pontremoli area at a depth of 30004000 m, and in the Salsoma~ore-Ponte dell’olio area at a depth of 10,000 to 12,000 m. Faults and magnetic d&continuities with NNE-SSW and SE-SW trends involving these sectors pertain to the tectonic alignments that have developed in the region. A few of the detineated structures have been confirmed by wells drilled in the area. The Monza 1 and Battuda 1 wells have cut into a Carboniferous crystalline basement, which is analogous to the outcropping “Serie dei Laghi” basement consisting of Carboniferous schists with a Permian volcanic cover. The Pontremoli 1 well on the other hand, reached a Carboniferous basement underlying a Tuscan-type succession. Map of ~~~irnen~~

ET AL

10m5 (S.I. units). These volcanites were investigated by the Monza 1 and Battuda 1 wells. The Permian volcanites are masked locally by volcanites (mostly Tertiary) that have higher susceptibility. Tertiary volcanites (dacites) in the Mortara area (1250-2880 X 10e5 S.1. units), located at a depth of 5~0-80~ m (Mortara 1 well), cover the western sector of Milan-Pavia structural high and mask the underlying Permian volcanites detected by the Battuda 1 well. The northern Apennine area is characterized by frequent ~gh-susceptib~ity markers (1250-15,000 x lo-” S.I. units) that could be related to the ophiolites of the Ligurian Units. In the LiguriaEmiiia Apennines, these ophiolites either outcrop or are located at shallow depths (2000 m); in the Rapallo area, they are located at depths ranging between 2000 and 6000 m. The area north of the “Voltri Group” is characterized by a magnetic response which is complicated by the conspicuous presence of magnetic sedimentary levels, most of which are the result of erosion of the Ligu~an-Apen~ne ophiolite and crystalline lithotypes (Oligocenic conglomerates and sandstones of the Molare Formation). These lithotypes increase in depth in the direction of the Piedmont Tertiary Basin, deepening from 1000 to 6000 m (see Fig. 9). Other magnetic sedimentary levels were also detected inside the Miocene sequences in the area southwest of Cremona at depths ranging between 5000 and 7000 m (see also Fig. 11).

magnetic markers

The interpretation of the magnetic anomalies discloses many intrasedimentary magnetic markers (Fig. 8). A tentative determination of the nature, lithostrati~ap~c position, and age of these markers were attempted through the comparison and correlation with rocks recognized and sampled both in surface and subsurface. In some places the seismic data helped to locate their position within the stratigraphic sequence. Extensive volcanic tuffs of Permian age, outcropping in the Lakes region, develop in the milan-Pavia area at a depth of 4000-7000 m and show a low susceptibility contrast of 500-700 X

The three sections represent our depiction of the geological configuration of the area. These sections tie in most of the exploration wells drilled in the western PO Plain (Fig. 1). Seismic reflection, gravimetric, and magnetometric data, integrated with the available field and borehole geological data, occur and delineate the main structural elements that characterize the area. These are, from north to south, the south Alpine thrust, the central high (Milan High), and the northern Apennine thrust. The reliability of these sections has been checked by two-dimensional (2D) gravimetric and magnetic modelling. Since the mapped seismic,

HI&ION

311,ow

3lklo1d

a3N”sv3yY

(131mw03

ClN393-l

- - -

w oz

0

10 km

PROFlLE

_

_

MEASURED PROFILE _ COMPUTED

_

FUGHT *LTI*Yoc 1500m

NORTH

are also identified in the Apennine side based on their age of deformation.

and no volcanites are present in the area. Locally sedimentary magnetic levels occur in the Miocene sequence. Two main thrust units

section no. 6 (see Fig. 1 for location): central PO Plain geological cross section. The external Apennine unit overlies the southern Alps thrust complex.

The magnetic basement shows low susceptibility

Fig. 11. Geophysical-geological

0 .

SOUTH i I 120-j ”

F

? 2

GEOPHYSICAL

DATA

gravimetric

OF NORTHERN

and magnetic

tend in the direction modelling The

is considered units

basement

SECTOR

features

normal

South-Alpine

thrusted

ITALIAN

sufficiently

ex-

to the profiles,

2-D

thrust

is involved

consists

in

in the thrust.

This

by early

faulting.

sequences

are simply

tion. Volcanites Tertiary consist

draped

of Permian,

in Tertiary

Deeper

ten-

and Tertiary

over the lower possibly

Triassic,

Apennine

flysch

units.

occur within

high-susceptibility

and

Layers

the Tertiary bodies

sec-

thrust

elastic units, locally overlain

Ligurian

susceptibility

quence.

character.

Mesozoic

Mesozoic

age occur. The northern

allochthonous higher

The Upper

the

The central

with foreland

area was disrupted

several

carbonates;

high has a rigid behaviour sional

by with

7, 9, 10, 11). In the southern deepens

se-

any reliable

further

m under

evidence

the

involved

Apennines

differs

the Apennines

without

of being

involved

Towards

in thrust-

area (Fig. 7). The

in the internal

thrust

units

of

Pontremoli

1 well,

Fig.

7)

(see

in susceptibility

ment beneath

the

a depth

ing, except in the Salsomaggiore basement

sectors,

until it reaches

contrast

from

the base-

the PO Plain. the west,

the PO Plain

basement

re-

aches a depth of over 10,000 m. The structural relationships with the Ivrea-Verbano zone are not yet clear. Towards extends

the east, in the depression

from Lake Garda

basement

reaches

a depth

to eastern

Liguria,

that the

of 14,000 m.

under-

neath the thrusted units overlie the basement and could be tentatively identified as basement wedges or Tertiary volcanics. It is apparent from the sections that the age of the south-Alpine thrust is pre-Messinian, while the northern Apenmne thrust continued to develop until the Plio-Pleistocene.

S-arY Basement

Figs.

basement

of over 14,000

adequate.

cored by Mesozoic

181

OF EGT

behaviour and characteristics

The examined PO Plain sector is characterized by a magnetic basement which is identified - in almost every location - with the pre-Mesozoic substratum. This substratum is correlated to the “Serie dei Laghi” crystalline basement which outcrops in the northwestern southern Alpine area. Permian and Tertiary volcanic bodies have been identified, the latter located in the Mortara area (for example the dacites in the Mortara 1 well, Fig. 8). A few bodies at depth of 8000-12,000 m under the outermost Apenninic thrusts (southern part of sections nos. 4, 5, 6 in Figs. 9, 10, 11) could be interpreted as being wedges of basement or magnetic bodies relating to the Tertiary volcanics (Volpedo-Casteggio-Ponte dell’olio zone in Fig. 7). In the Battuda area (Milan High) the basement is 5000 to 6000 m deep. Towards the southern Alps, the basement deepens to over 8000 m and it is locally involved in thrusted units (i.e. Cernusco, Seregna, Malossa, Brignano zones in

Time evolution The basement comprises the lowermost part of a Triassic-Liassic carbonate sequence characterized mostly by condensed sequences with sedimentary hiatuses (Battuda area in Fig. 9). The post-Liassic Mesozoic sequence evolves into basinal lithofacies in the Jurassic-Cretaceous. In the Tertiary, the whole PO Plain area deepens gradually until it reaches a very considerable depth. Subsidence is accompanied by thick elastic deposits (Oligocene-Miocene) in the south-Alpine and Apennine

foredeeps

(Figs. 9, 10, 11). During

the Oligocene-Miocene late-evolutionary, continental-erogenic, talc-alkaline magmatism occurs at least in one particular sector of the area (Mortara zone in Fig. 8, Ottobiano zone in Fig. 9). Tectonics The central-western part of the PO Plain is an area of rigid behaviour where opposite polarity south Alpine and Apennine structures, increasingly younger towards the PO Plain foreland, face each other (Figs. 9, 10, 11). The south Alpine structures are sealed by the Messinian sediments, while the Apennine structures remain tectonically active until the Plio-Pleistocene. References Cassano, E., AnelIi, L., Fichera, R. and CappeIIi, V., 1986a. Pianura Padana. Interpretazione integrata di dati geofisici e

182

E. CASSANO

geologici.

73”

Congr.

Naz. Sot. Geol.

Ital.,

1986, Rome,

25: 27 pp. Cassano,

E., Fichera,

Aeromagnetico CNR

Gruppo

R. and Arisi

d’Italia:

Alcuni

Naz. Geofis.

Rota,

F., 1986b.

Risultati

Terra

Solida,

Rilievo

Interpretativi. Atti 5”

Conv.,

of the PO plain, namica,

G., 1981. Subsurface Italy.

Sottoprogetto

CNR

geological

Progetto

“Modello

Finalizzato

Strutturale”,

structure Geodi-

Pubbl.

414:

Talwani,

M., 1965. Computation

computer

of magnetic

trary shape. Geophysics,

anomalies

with

the help

caused

30(5): 797-817.

of a digital

by bodies of arbi-

Ewing,

attraction

trary shape.

Geophysics,

Talwani,

for the magnetic

of polygonal

Obs. Columbia Talwani, gravity

M., 1960.

Rapid

computation

of three-dimensional

M. and Heirtzler,

pression

cation

l-13.

M. and

gravitational

body

Rome, II: 939-962. Pieri, M. and Groppi,

Talwani,

25(l): J.R.,

ET AL.

of

bodies of arbi-

203-225. 1962. The mathematical

anomaly

cross-section.

ex-

over a two-dimensional Lamont.

Doherty

Geol.

Univ., Tech. Rep.. 6.

M., Worzel, calculation

J.L. and

to the Mendocino

phys. Res.. 64(l):

Landisman,

for two-dimensional 49-59.

submarine

M., 1964. Rapid bodies

fracture

with appli-

zone. J. Geo-