Chemical Geology, 77 (1989) 375-398 375 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Crystalline and sedimentary rocks from the scarps of the Sicily-Sardinia Trough and Cornaglia Terrace
(southwestern Tyrrhenian Sea, Italy)" Paleogeographic and geodynamic implications
R. C O M P A G N O N I 1, E. M O R L O T T I 2 and L. T O R E L L I 2
1Dipartimento di Scienze della Terra, Universit~ di Torino, 1-10125 (Italy) 2Istituto di Geologia, Universit(~ di Parma, 1-43100 Parma (Italy)
{Accepted for publication June 7, 1989)
Abstract
Compagnoni, R., Morlotti, E. and Torelli, L., 1989. Crystalline and sedimentary rocks from the scarps of the Sicily- Sardinia Trough and Cornaglia Terrace (southwestern Tyrrhenian Sea, Italy): Paleogeographic and geodynamic implications. In: L. Beccaluva (Guest-Editor), Ophiolites and Lithosphere of Marginal Seas. Chem. Geol., 77: 375- 398.
The petrographic and stratigraphic analysis of crystalline and sedimentary rocks dredged in the west Sardinia Channel {southwestern Tyrrhenian Sea) led to the careful stratigraphic and structural reconstruction of a Lower Miocene submerged orogenic belt, previously analysed only on seismic sections. The deformed units, which include some flakes of the Sardinia margin, have a Hercynian-type crystalline basement of amphibolite-facies sillimanite- bearing metamorphic rocks, intruded by granitoids ranging in composition from granite to tonalite. This is quite different from the Sardinia basement because of the peraluminous character of the plutonic rocks; it can therefore be attributed to the generic Kabylo-Calabrian Domain, which may extend far beyond the investigated area. The crystal- line basement is directly overlain by a thick, Lower Miocene synorogenic sequence consisting of deep-sea arkosic turbidites which probably accumulated in a foreland basin or in minor basins migrating on a shortening continental margin.
Most of the rocks of the Kabylo-Calabrian Domain exhibit a common post-Hercynian structural and metamorphic evolution, which can be attributed to a low-grade tectono-metamorphic event corresponding to low greenschist-facies conditions; this possibly took place during the Early Miocene.
As regards the geodynamic setting, it should be pointed out that the penetrative structures and metamorphic re- equilibration of the Kabylo-Calabrian Units seem to result from an intracontinental collision which contempora- neously affected the Sardinia block, leading to the formation of a crustal shear zone. The Early Miocene age of the metamorphism and turbiditic sedimentation suggests that crustal shortening was probably connected with the drifting and rotational phases of the Corsica-Sardinia microplate.
1. Introduct ion
Recent geological and geophysical studies (Baticci et al., 1983, 1985; Barbieri et al., 1984; Beccaluva et al., 1984; Torelli et al., 1985) have shown that the structural framework of the
Sardinia Channel (Fig. 1 ) resulted from the in- teraction between the Oligo-Miocene compres- sive phases related to the continental collision between Europe and Africa and the more recent extensional features of the Tyrrhenian Basin.
~. \ ~ / ~ ~ . .~ - .~ ~j' ~,*. < x i - ~.' o ~ ~ s \ ~',, , ~ \ . ~ _
• ~ L~ " / " \ Ix" J ~ x 4 \
ALG':~, f / c. ~ g- .t ~'*' --// .(e "° \\\EGAD, ,
; , \ ~ AN ELLERIA i - - - 5)
Fig. 1. Physiographic map of the Sardinia Channel and adjacent areas with traces of the most significant seismic profiles and location of dredging sites (1 =crystall ine rocks; 2=carbonate (triangles) and clastic (dots) rocks; 3=igneous rocks; 4 = multichannel lines; 5 = single-channel lines; 6 = cross-section). The inset shows the investigated area ( see Fig. 2 ).
by the eastward displacement of the Corsica- Sardinia Block during Aquitanian to Burdigal- ian times (Cherchi and Montadert, 1982; Bur- rus, 1984) are generally recorded in the acous- tic basement. Extensional features, on the other hand, are reflected by the differential subsi- dence of the substrate and by the various types of sedimentation in syn- and post-rift Neogene to Quaternary times.
The effects of shortening phases have re- cently been assessed on multichannel seismic sections. Their analysis led to the identification of a large-scale, SE-verging imbricate struc- ture, characterized by low-dip thrust faults. A structural change from deep crustal deforma- tion in the western sector to thin-skinned thrusting and folding near the Strait of Sicily may also be observed.
In order to define the main paleogeographic domains represented in this fold/ thrust belt, a detailed sampling programme of the acoustic
basement was performed in the Sardinia Chan- nel during the last few years (Barbieri et al., 1984; Compagnoni et al., 1987). Lithological assessments of the acoustic basement at se- lected dredging stations, integrated with data from Colantoni et al. (1981), are summarized in Fig. 1. Rock samples taken from the eastern and western sectors show a remarkable differ- ence: the former consist of Jurassic-Paleocene carbonates, and are interpreted as the offshore extension of the mainland Panormide Domain (Catalano et al., 1985), whereas the latter con- sist mainly of Paleozoic plutonic and metamor- phic rocks. Minor clastic lithotypes were also observed: these derived from a crustal thrust system which includes flakes of the southern Sardinia margin.
This paper deals mostly with the mineral composition and the petrographic features of rocks dredged from a key sector of the Sardinia Channel {inset in Fig. 1 ).
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 377
The data collected at various times have been used for three main purposes:
(1) to reconstruct the geometric relation- ships between the inferred tectono-strati- graphic units;
(2) to make a comparison with onland sedi- mentary sequences;
(3) to try to evaluate the geological processes occurring during crustal shortening.
An interpretative cross-section across the Sardinia Channel (see Fig. 10) is presented on the basis of new and previous geophysical and geological data which have been reinterpreted.
2. Phys iographic and structural sett ing
The samples analysed in this study were re- covered from scarps of the Sicily-Sardinia Trough and the southern part of the Cornaglia Terrace during cruises BS 82 and MGC 83, 84 and 85 aboard the R /V "Bannock" of the Ital- ian C.N.R. (Consiglio Nazionale delle Ri- cerche).
The study area (as outlined by the bathy- metric map of Fig. 2) consists of deep depres- sions which separate the Sardinia margin from other parts of the Sardinia Channel, and pro- vide a deep-water connection between the Tyr- rhenian Sea and the Algerian-Balearic Basin.
The Cornaglia Terrace represents the north- ernmost morphological feature of the investi- gated area. It can be considered as an extension of the Tyrrhenian bathyal plain, and is char- acterized by a wide flat floor, ranging in depth from ~ 2500 to ~ 2700 m. The terrace shades southward into the Sicily-Sardinia Trough and into the Biserta Canyon. To the west it is di- vided from the peri-Tyrrhenian basins of the Sardinia margin by a broad, ESE-dipping steep slope.
To the southeast, the Cornaglia Terrace is connected with a shallow central plateau by a SW:NE-trending scarp ~ 100 km long (Fig. 2). This scarp displays a rather rough topography as a consequence of strong Neogene (mainly Miocene) tensional faulting; its upper bound-
ary coincides with a sharp morphological break at a depth of ~ 600 m. The average slope is ~ 7 °, although a value of 16 ° can be observed locally.
The Sicily-Sardinia Trough is a NE-SW- trending graben, narrow and elongated, more than 2000 m deep in the axial part. It slopes into the Teulada Valley in a westward direction towards the 2500-m-deep floor of the Algerian- Balearic Basin (Gennesseaux and Stanley, 1984), and is separated from the Biserta Can- yon to the east by a random-shaped ridge, ex- tending from the Tunisian slope. The irregular floor of the Sicily-Sardinia Trough is bounded on both sides by broad, highly complex scarps, which can rise to ~ 200 m, as in the Ichnusa Seamount. Its northern flank is dissected by several canyons, whereas the steeper southern flank is characterized by bends clearly arranged in accordance with the main morphological trends of the area. A deep N-S-trending inci- sion interrupts the lateral continuity of the structure. The irregular topography of the Sic- ily-Sardinia Trough (including the Biserta Canyon) seems to have been caused by ero- sional processes and tensional and strike-slip faults, accompanied in some cases by promi- nent volcanic features.
From a structural point of view, the area - which is located between the pre-Alpine and Alpine Sardinian units and the Tertiary Sicil- ian and Tunisian deformed belts - shows a strong segmentation characterized by basin- and-ridge morphology, NE-SW in direction (Figs. 3 and 4). It is believed that segmentation of this area must have started during the Neo- gene extensional tectonic phases, which were responsible for the opening of the Tyrrhenian Sea (Kastens et al., 1986).
The high heat flow values (up to 120 mW m -2 ) observed in the central basin (Baticci et al., 1983, 1985) provide evidence to support the presence of a thinned continental crust ( ~ 15 km), and allow us to estimate thermal subsi- dence which took place ~ 11 Ma ago. Crustal thickness rapidly increases to 25-30 km to- wards the Sardinia and Sicilian continental
378
10 °
R. COMPAGNONI ET AL.
11 °
38°30 '- . 38 °30 '
10 ° 11 °
Fig. 2. B a t h y m e t r i c m a p of t he s tudy area w i th locat ion of dredging sites. Italics refer to dredging si tes of o the r ins t i tu t ions (see Table I ). Con tou r intervals: 100 m.
margins (Boccaletti et al., 1984). Basin geom- etry is controlled by planar and listric normal block-faults, which produce a southeastward tilting and the basement offset of some thou- sand meters. The extensional basins are in- filled by sediments more than 3000-3500 m thick in the Cornaglia Terrace, which consist of Plio-Quaternary and Messinian units, and of a poorly known pre-Messinian sequence. This last sequence, which does not appear to be older than Early Miocene, shows significant uncom- fortuities on seismic records (Barbieri et al., 1984). Diapiric structures are evident in the central part of the deeper basins, where thick Messinian salts are present {Fig. 5).
This Neogene sequence unconformably overlies an acoustic basement which is well ex- posed along the steep slopes of the study area {Figs. 5 and 6). The acoustic basement seems to have been deformed during the Late Oligo-
cene-Early Miocene period, when significant crustal shortening modified the paleogeogra- phy of the Western Mediterranean (Rehault et al., 1984). This contraction produced a fold/ thrust belt consisting of large-scale imbricated structures, involving several tectonostrati- graphic units (Torelli et al., 1985).
The main structural features of the region are two low-angle, SE-verging overthrusts, labelled respectively DTF {Drepano Thrust Front) and STF (Sardinia Thrust Front) (Fig. 3). The DTF, which is Burdigaiian in age, can be ob- served in the Central Plateau and can be traced for 200 km from the Biserta Canyon to Ustica Island with a WSW-ENE trend. The DTF is dissected by NW-SE-trending strike-slip faults, Late Miocene to Early Pliocene in age, and acts as a boundary between the previously mentioned Neogene fold/thrust belts and the deformed units of the Maghrebian Domain.
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 379
10" 11"
= / 1 1 i "C_ . : : / > , • . I
:¢ / ( / I ::;IIZi .<<,': "::"~-~,,-~-'- . i ~ ~ t f , , ~ L~=.~o.
- , - - I
0 10 20 k m ~ / / " "" "
. . . . . / J 10" 11'
Fig. 3. Structural map of the study area (1 =volcanic bodies; 2=ma jo r Neogene extensional basins; 3=norma l faults; 4 = overthrusts; 5 = strike-slip faults; STF, DTF= Sardinia Thrust Front and Drepano Thrust Front, respectively ).
On the other hand, the more westerly SW- NE-trending STF is difficult to identify, be- cause of a thick postorogenic cover. However, it was recognized on seismic line MS 98 south of the Ichnusa Seamount (Fig. 4): its presence along the Cornaglia Terrace is confirmed by geophysical and petrological evidence.
A crystalline basement (2-6 km deep) which becomes shallower to the southeast appears to be responsible for the low-frequency magnetic anomalies recorded in the area by A.G.I.P. (1981). These magnetic anomalies are charac- teristic of the Central Plateau (Compagnoni et al., 1987), where they suggest a sharp deepen- ing of the basement towards the Sicily-Sardi- nia Trough. Local high-frequency anomalies, occurring south of the Sardinia margin, may be caused by mafic intrusions along the alignment connecting the oceanic sectors of the North Al- gerian Basin with the Tyrrhenian bathyal plain.
3. Description of the dredged rocks
Although the dredging programme was strongly affected by the presence of a complex
grid of submarine cables, twenty dredging sta- tions were nonetheless performed, mostly along the southeastern slope of the Cornaglia Terrace (Barbieri et al., 1984). Twelve of these were empty, or recovered only Late Pleistocene mi- critic limestone and recent unconsolidated mud; they are therefore not considered here. The re- maining eight dredgings (Fig. 2) also contained a large amount of hard crystalline and sedimen- tary rocks, which are considered in situ because of their freshly broken surfaces and the fre- quent anchorings of the dredge during sam- pling operations. Additional lithostratigraphic information was provided by the microscopic examination of samples collected by Istituto di Geologia Marina, Bologna, dredging station T 75-34 and preliminarily studied by Savelli (1979).
Table I shows location, depth and main li- thology of the dredgings considered here, car- ried out from 1975 to 1985. Where present, Quaternary lithotypes are represented by mi- critic limestone with a generally thin, Fe-Mn crust and ochreous or grey-green unconsoli- dated mud. In particular, the micritic organic
c.o
(30
8
N
W.
DE
PT
H
2000
m
.
1
3 3
~4
4 O
3 5
5
6 6
,
m K~
,
MS
98 B
TU
NIS
IA C
ON
TIN
EN
TA
L
MA
RG
IN
SA
RD
INIA
C
ON
TIN
EN
TA
L
MA
RG
IN
~-~
---~
4-
1
Fig
. 4. I
nter
pret
ed n
orth
-sou
th m
ulti
chan
nel s
eism
ic s
ecti
on f
rom
Sar
dini
a to
Tun
isia
(af
ter
Tor
elli
et
al.,
1985
), s
how
ing
the
Sard
inia
bas
emen
t (S
B)
thru
st o
ver
the
Kab
ylo-
Cal
abri
an u
nit
s (K
CU
).
A =
Pli
o-Q
uate
rnar
y co
ver;
B =
pre
-Mes
sin
ian
uni
t; C
= O
ligo
(?)-
Mio
cen
e cl
asti
c un
it.
Mes
sini
an u
nit
is in
bla
ck.
M=
mul
tipl
e. F
or l
ocat
ion
see
Fig
. 1.
¢3
O
Z O z >
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 381
N W S E
,, .k-~:;~; ~ ' ~ i ~ : ' i
w DEPTH 2400 rn ~ } ~ . ~ . # ~ ; ~ . _ ~
:#
PM 24 Eixl ~6 6 k m ; ~ . ±-~
× ~ -2
-5
Fig. 5. Interpreted single-channel seismic section across the Cornaglia Terrace showing the post-orogenic sedimentary cover and the acoustic basement ( 1 = Plio-Pleistocene cover; 2 = Messinian evaporites; 3 = Mess inian salts; 4 = pre-Mes- s inian clastic unit ; 5 = acoustic basement: A = Early Mio- cene clastic unit; B = crystalline basement ) . For location of profile see Fig. 1.
limestone is generally encrusted with aherma- typic corals. Late Pleistocene-Holocene plank- tonic foraminifera and pteropods were also rec- ognized in thin sections. Rocks of Pliocene age are represented by creamy and white, more or less consolidated marlstones, very similar to the Trubi facies, a foraminiferal ooze widely out- cropping in mainland Sicily. They contain for- aminiferal assemblages, whose ages range from Earliest to Early Pliocene. Little is known about the Messinian limestones of dredge T 75-34 (Colantoni et al., 1981), which were assigned to the Messinian evaporitic complex on the ba- sis of the occurrence of small, acicular gypsum aggregates.
The significant characteristics of pre-Mes- sinian sedimentary and crystalline rocks are described below with special emphasis on their composition, age, and deformational features. The minerals of the crystalline samples, to- gether with the main alteration products, are summarized in Table II.
SE NW
e km P M 2 2 Fix. 24-13 X X X i I
~Y ^x2"~ \ ~ b \ ~ \ ~
3- ~,~ X x , ~
Fig. 6. In terpre ted single-channel seismic section showing the extensional features of the Cornaglia Terrace scarp. Dredging s ta t ions T 75-34 and BS 82-28 are projected on th is section. For legend see Fig. 4.
3.1. Dredge T 75-34 (Fig. 6)
Hand specimens: several fragments, up to 10X7X3 cm, of metamorphic and plutonic rocks, with a thin Fe-Mn-oxide coating (R. Selli's dredge description, unpublished data, 1975).
Ten samples were studied (Table II), and consist mainly of plagioclase or two-feldspar gneisses with biotite, white mica and silliman- ite. Almost all gneisses are medium- to coarse- grained, and contain only a small amount of mica; their massive structure, with poorly de- veloped rough foliation, makes them similar to granitoid rocks.
Different lithologies were found only in two cases: a fine-grained plagioclase-muscovite- biotite gneiss containing completely sericitized fibrolite, and a gneissic mica-schist character-
OO
t,D
TABLE I
Synoptic
table of
the dredging sites
Dre
dge
Lo
cati
on
L
atit
ud
e L
on
git
ud
e D
epth
L
ith
oty
pes
( °
'N)
(° 'E)
(m)
T 75
-4 *~
S
E s
lope
of
the
38 °
44.6
' -3
8 °
43.
7'
10 °
47.0
' -1
0 °
48.
3'
1,8
02
-1,4
40
C
orn
agli
a T
erra
ce
T 76
-13
*2
Ich
nu
sa S
eam
ou
nt
38 °
46.6
' -3
8 °
47.9
' 09
° 52
.2'
-09
° 5
1.3'
1
,80
0-1
,30
0
BS
82-
27
SE
slo
pe o
f th
e 38
° 4
4.1
0'-
38
° 4
3.20
' 10
° 4
5.5
8'-
10
° 4
5.50
' 1
,98
2-1
,57
4
Co
rnag
lia
Ter
race
BS
82
-28
S
E s
lope
oft
he
38 °
39
.50
'-3
8 °
38.
45'
10 °
43
.73
'-1
0 °
39.
91'
2, 1
79
-1,9
43
C
orn
agli
a T
erra
ce
MG
C 8
3-1
SE
slo
pe o
f th
e 38
° 4
2.6
4'-
38
° 4
5.00
' 10
° 2
2.6
7'-
10
° 2
0.34
' 2
,42
8-1
,88
1
Co
rnag
lia
Ter
race
MG
C 8
4-4
SE
slo
pe o
f th
e 38
° 4
5.5
9'-
38
° 4
3.05
' 10
° 4
4.9
7'-
10
° 4
6.04
' 2,
39
3-1
,65
9
Co
rnag
lia
Ter
race
MG
C 8
5-3
SE
fla
nk
of
the
38 °
22
.97
'-3
8 °
22.
47'
09 °
35
.13
'-0
9 °
35.
94'
2, 0
39
-1,6
20
S
icil
y-S
ard
inia
T
rou
gh
MG
C 8
5-4
SE
fla
nk
of
the
38 °
21
.49
'-3
8 °
22.
04'
09 °
39
.59
'-0
9 °
38.
20'
1,7
36
-1,2
98
M
GC
85-
5 S
icil
y-S
ard
inia
T
rou
gh
MG
C 8
5-6
SE
fla
nk
of
the
38 °
21
.50
'-3
8 °
22.
04'
09 °
40
.20
'-0
9 °
38.
81'
1,7
22
-1,6
03
S
icil
y-S
ard
inia
T
rou
gh
Ho
loce
ne
och
reo
us
mud
; E
arly
Pli
oce
ne
wh
ite
mar
ls;
Mes
sin
ian
lim
esto
nes
; p
luto
nic
an
d m
etam
orp
hic
ro
cks
Lat
e P
leis
toce
ne-
Ho
loce
ne
mic
riti
c li
mes
ton
es; p
luto
nic
ro
cks
Ho
loce
ne
och
reo
us
mud
; L
ate
Ple
isto
cen
e-
Ho
loce
ne
mic
riti
c li
mes
ton
es; p
luto
nic
an
d m
etam
orp
hic
ro
cks
Ho
loce
ne
och
reo
us
mud
; L
ate
Oli
goce
ne
(?)-
Ear
ly M
ioce
ne
silt
ites
, san
dst
on
es
and
fin
e-g
rain
ed c
on
glo
mer
ates
; plu
ton
ic
and
met
amo
rph
ic r
ock
s
Ho
loce
ne
och
reo
us
mud
; p
luto
nic
ro
cks
Ho
loce
ne
och
reo
us
mud
; E
arly
Pli
oce
ne
wh
ite
mar
ls;
Lat
e O
ligo
cene
(?)
-Ear
ly
Mio
cen
e si
ltit
es, s
and
sto
nes
an
d f
ree-
g
rain
ed c
on
glo
mer
ates
; plu
ton
ic a
nd
m
etam
orp
hic
ro
cks
Lat
e P
leis
toce
ne-
Ho
loce
ne
mic
riti
c li
mes
ton
es; p
luto
nic
ro
cks
Holo
cene
ochreous mu
d; plutonic a
nd
meta
morp
hic rocks
met
amo
rph
ic r
ock
s C
~ o
*~D
ata
from
Co
lan
ton
i et
al.
(19
81
).
*~D
ata
from
Bo
rset
ti e
t al
. (1
979a
, b).
o z
SCARPS OF T HE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 383
TABLE II
Main petrographic features of igneous and metamorphic rocks from dredging sites T 75-34, BS 82-27. BS 82-28, MGC 83-1, MGC 84-4, MGC 85-3, MGC 85-4,
MGC 85-5 and MGC 85-6
Minerals Primary minerals Accessory Metamorphic Late Veins Alteration Rock type minerals minerals minerals
V cataclastic pegmatite V W cataclastic pegrnatite V M leucogranite W W •plite? M M cataclastic pegnmtite
W M tonalitic gneiss W W tonalitic gneiss W W tonalitic gneiss W W tonalitic gneiss W W dioritic gneiss
W W dioritic gneiss W M tonalitic gneiss WW dioritic/tonalitic gneiss M W pegmatite W M dioritic gneiss
W W two-mica sillimanite granite W M dioritic gneiss W M dioritic gneiss M W tonalitic gneiss W W biotite-sillimanite granite
W W tonalitic gneiss S W two-mica siUimanite gneiss M W two-mica sillimanite gneiss S M granite or granitic gneiss WW two-mica granite
W W two-mica granite M W leucogranite W M tonalitic gneiss W M biotite tonalite W M two-mica siUimanite granite S S pegmatite W M dioritic gneiss S M two-mica granite V W graphic pegmatite
tonalitic gneiss
3 8 4
T A B L E II (continued)
R. C O M P A G N O N I E T AL.
Minerals Primary minerals Accessory Metamorphic Late Veins Alteration Rock type minerals minerals minerals
S am p le s
• " . ~ o~ ~ =.~'~ g os.~ •
BS82-27/36c o o o • • • • M M tona l i t i cgne i s swi thqua r t z -p l ag ioc l a seve in
BS82-27/37 o o o o o o • • • • W t w o - m i c a t w o - f e l d s p a r s i U i m a n i t e g n e i s s
BS82-28/63 o o A • • • B A W W tona l i t i c gneiss cu t by p e g m a t i t e
BS82-28/74 o o o A o • • • C A t w o - m i c a m i c r o c l i n e g ran i t e
B S 82-28/75 o o o A o • • • C A two-mica microc l ine g ran i t e
BS82-28/77 o o o A o e • • C g ran i t e or g ran i to id gneiss
B S 82-28/78 o o o A o • • • g ran i t e or g ran i to id gneiss
B S 82-28/80 o o A o • • • A two-mica tona l i t e
B S 82-28/81 o o o A o • • B A two-mica microcl ine g ran i t e
BS82-28/83 c • o o • • • A b io t i t e g r a n i t o i d g n e i s s
B S 82-28/84 o o o o • • B A two-mica g ran i to id gneiss
B S 82-28/87 o o ? • S ca tac las t i c p e g m a t i t e
BS82-28/89 o o o A o • C A t w o - m i c a m i c r o c l i n e g r a n i t e
M G C 83-1/5 o o o A o ? • • M two-mica g ran i te '~
MGC83-1/6 o o o o • • • • A W b io t i t e g ran i to id gneiss
MGC83-1/7 o o o o • • • • • • A S b io t i t e g ran i t e
MGC83-1/9 o o o o • • • • • • A S b io t i t e g ran i te
MGC84-4/3 o • o o o • • A two-mica g ran i t e or gneiss
M G C 84-4/5 o o o o o • • A S W two-mica g ran i te or gneiss
M G C 85-3 o o o o • • • • • • • W W M b io t i t e g ran i t e or gran i t ic gneis
MGC85-3bis o o o o • • • • W t w o - m i c a m i c r o c l i n e g r a n i t e
M G C 85-4/1 o o o o • • • • • • • • B M b i o t i t e - a m p h i b o l e t o n a l i t e
MGC85-4/2 o o o o • • • • • B S S schis tose b i o t i t e - a m p h i b o l e t o n a l i t e
MGC85-4/3 o o o • • • • • • B S M M t o n a l i t i c g n e i s s
MGC85-5/1 o • o A • • • • • • • S W two-mica g ran i t e or gneiss
MGC85-6/1 o o o A • • • • • • • A A W M t w o - m i c a g n e i s s
M G C 8 5 - 6 / 2 o o o A o • • • • • A S W M two-mica gneiss
M G C 8 5 - 6 / 2 b i s o o o A o • • • • • A A S W W two-mica gneiss
For s i te loca t ions see Fig. 2. Minerals: A = comple te ly a l te red; o = m a i n c o m p o n e n t ; e = mino r or accessory componen t . Deformation: W = weak; M = modera te ;
S = s t rong; V = very s t rong. Metamorphism: W = only ra re occurrence of m e t a m o r p h i c minera l s ; M = c o m m o n occurrence of m e t a m o r p h i c minera l s ; S = widespread
occur rence of m e t a m o r p h i c mine ra l s w i t h t h e deve lopmen t of a m ine ra l fol ia t ion. Alteration: Bio t i t e - A = a l t e red to chlor i te + sphene _+ adular ia ; B = a l t e red to
whi te mica + sphene; C = a l tered to colourless chlor i te + l imonite; Plagioclass- A = a l tered to sericite; B = a l tered to saussurite; carbonate replacement- W = inc ip i en t ;
M = modera te ; S = advanced.
ized by biotite-muscovite-fibrolite-bearing layers alternating with irregular pockets of mostly quartz and feldspar.
3.2. Dredge BS 82-27 (Fig. 6)
Hand specimens: seventy-six angular frag- ments, up to 35 × 17 × 12 cm, of metamorphic and plutonic rocks, all with a thin Fe-Mn coat-
ing. The prevailing metamorphic rocks are gen- erally unaltered, and show (only in some cases) a thin alteration rim. They also display marked schistosity and deformation features.
Tonalitic and dioritic gneisses prevail among the metamorphic samples (Table II; Plate I, B; Plate II, A; Plate III, C). Tonalitic gneisses consist of quartz, plagioclase, biotite, and mi- nor muscovite, and may contain garnet (Plate III, B). Dioritic gneisses contain plagioclase,
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 385
biotite+muscovite, whereas quartz and K- feldspar are generally accessory.
Dioritic gneisses are characterized by the oc- currence of zoned plagioclase with an oligoclase core surrounded by a narrower, more sodic, rim (Plate II, B). Two-feldspar, two-mica_+ fibrolitic sillimanite gneisses were also ob- served (Plate III, A).
Plutonic rocks (Table II) are represented by granite, tonalite and pegmatite. Two-mica granite usually displays a medium-grained, equigranular structure. Some porphyritic or fine-grained types with a general leucogranitic affinity possibly derived from dykes (Plate I, C) are also present.
Two-mica granite with both fibrolitic and prismatic sillimanite, generally replaced by white mica pseudomorphs, was recognized in two samples.
Biotite or two-mica tonalite is rare; micro- cline pegmatite - sometimes with an apparent graphic structure (Plate I, D) - is more abun- dant, and generally shows a strong cataclastic texture.
3.3. Dredge BS 82-28 (Fig. 5)
Hand specimens: (1) twenty-four fragments of fresh deformed
metamorphic and plutonic rocks, partly cov- ered by Fe-Mn coating and Upper Pleistocene- Holocene micritic limestone crust;
(2) a large amount of semi-consolidated po- lymictic conglomerate fragments, supported in a matrix of reddish altered mudstone. The clasts are mainly rounded plutonic fragments, pebble sized or smaller;
(3) several poorly altered fragments of bed- ded brown-grey laminated sandstone, grey- green siltstone, and pelitic layered rocks cov- ered by Fe-Mn coating and ahermatypic corals. One large sandstone sample has groove sole marks.
Microscopic analysis indicates the presence of deformed lithotypes probably deriving from the local crystalline basement. Most samples
consist of cataclastic pegmatite. Deformed fragments of a two-mica granitoid gneiss, a ton- alitic gneiss cut by a cm-sized igneous vein, and foliated rocks (which could be interpreted as either gneiss or strongly deformed granite) were also found.
The rounded crystalline pebbles of the con- glomerate consist of undeformed fine-grained microcline granite and minor two-mica tonalite.
Petrographic study* was also carried out on several sandstone fragments. Among these three fresh samples were selected for microscopic analysis; methods suggested by Gazzi (1966) and Dickinson (1970) were used. The three samples reveal the following textural characters:
The three resulting detrital modes are fairly homogeneous. The mean parameters are Q59, F37, L4, where Q=total quartz grains, F= to ta l feldspar grains and L-- fine-grained ( < 62/~m ) lithic fragments (Fig. 7). The accessory grains, which amount to 10-20% of the rock, are mainly very deformed white mica and biotite.
A pseudomatrix (which represents only a small percentage of the total sandstone) may be the result of alteration of mica and unstable minerals. The original fine detritus is mostly recrystaUized to sheet silicates.
Quartz occurs in fragments of both coarse- grained plutonic and fine-grained low-grade metamorphic rocks. Mean value of the plagio- clase/K-feldspar ratio of 0.54 is only indicative, because of the marked alteration of feldspar grains.
Fine-grained lithic clasts are poorly repre- sented (micas 4 vol.% ), and mainly consist of quartz-mica foliated types.
Both detrital mode and main and accessory
*The petrographic study was performed by M.A. Calzolari (Istituto di Petrografia, Parma).
PLA
TE
I
Oo
O~
~. M
GC
85-
3bis
. Mic
rocl
ine
gran
ite
show
ing
inci
pien
t def
orm
atio
n ex
empl
ifie
d by
wav
y ex
tinc
tion
in th
e qu
artz
gra
ins
(upp
er le
ft)
and
by b
andi
ng
tnd
kink
ing
of b
ioti
te. N
otic
e se
rici
tiza
tion
of p
lagi
ocla
se.
15 X
, cr
osse
d po
lari
zers
. B.
MG
C 8
5-4~
Ibis
. Mod
erat
ely
defo
rmed
bio
tite
-am
phib
ole
tona
lite
. The
dar
k in
clus
ions
in a
mph
ibol
e (u
pper
left
) su
gges
t an
orig
in fr
om p
rim
ary
:lin
opyr
oxen
e. N
otic
e th
e th
in f
ilm o
f neo
blas
tic
whi
te m
ica
rim
min
g th
e pl
agio
clas
e po
rphy
rocl
ast w
ith
bent
tw
in la
mel
lae.
15
X, c
ross
ed p
olar
izer
s.
5. B
S 82
-27/
3. S
tron
gly
shea
red
leuc
ogra
nite
sho
win
g fe
ldsp
ar p
orph
yroc
last
s se
t in
a m
atri
x of
rib
bon
quar
tz. N
otic
e th
e de
velo
pmen
t of a
rou
gh
Yol
iatio
n (u
pper
hal
f) m
arke
d by
neo
blas
tic
whi
te m
ica.
15
X,
cros
sed
pola
rize
rs.
D. B
S 82
-27/
33.
Cat
acla
stic
peg
rnat
ite
wit
h re
mna
nts
of d
efor
med
gra
phic
qua
rtz
pres
erve
d in
a m
icro
clin
e-pe
rthi
te p
orph
yroc
last
. 40
X,
cros
sed
3ola
rize
rs.
PL
AT
E I
I c~
O
*q
c~ z O
c~
O
Z
A. B
S 82
-27/
17a.
Dio
riti
c gn
eiss
, re
pres
enta
tive
of d
redg
ed m
etam
orph
ic r
ocks
. 15
X, c
ross
ed p
olar
izer
s.
B. B
S 82
-27/
19. D
etai
l of
a d
iori
tic
gnei
ss s
how
ing
slig
htly
def
orm
ed p
lagi
ocla
se w
ith
shar
p zo
ning
. R
ed-b
row
n bi
otit
e is
fri
nged
by
very
fin
e-
grai
ned
gree
n bi
otit
e. 4
0 X
, cro
ssed
pol
ariz
ers.
C
. BS
82-2
7/27
. Str
ongl
y sh
eare
d gr
anit
ic r
ock
show
ing
deve
lopm
ent o
f a r
ough
met
amor
phic
folia
tion
mar
ked
by n
eobl
asti
c w
hite
mic
a an
d gr
een
biot
ite.
15
X, c
ross
ed p
olar
izer
s.
D. B
S 82
-27/
8d. T
onal
itic
gne
iss
show
ing
red-
brow
n bi
otit
e pa
rtly
rep
lace
d by
fine
-gra
ined
neo
blas
tic
whi
te m
ica.
40
X, c
ross
ed p
olar
izer
s.
OC
PL
AT
E I
II
A. B
S 82
-27/
37a.
Tw
o-m
ica
two-
feld
spar
sil
lim
anit
ic g
neis
s w
ith
plag
iocl
ase
cont
aini
ng s
mal
l nee
dles
of
fibr
olit
e m
ainl
y al
tere
d to
ser
icit
e. 1
5 X
, cr
osse
d po
lari
zers
. B
. B
S 82
-27/
8b.
Mod
erat
ely
defo
rmed
ton
alit
ic g
neis
s co
ntai
ning
xen
obla
stic
gar
net
(ope
n ar
row
sho
ws
frac
ture
s fi
lled
wit
h fi
ne-g
rain
ed w
hite
m
ica)
. 15
×,
cros
sed
pola
rize
rs.
C. B
S 82
-27/
17.
Poo
rly
defo
rmed
bio
tite
-bea
ring
dio
riti
c gn
eiss
. 15
×,
cros
sed
pola
rize
rs.
D. B
S 82
-27/
17.
Det
ail o
f (C
) sh
owin
g re
d-br
own
biot
ite
frin
ged
by n
eobl
asti
c gr
een
biot
ite
(ope
n ar
row
). 4
0 ×
, cr
osse
d po
lari
zers
.
O
O
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 389
2o BS 8 2 - 2 8 / 7 6
\ / \
F ~o ;o 20 2"o L
Fig. 7. Detrital mode of three representative sandstone samples from dredge BS 82-28 plotted in the quartz (Q)- feldspars (F)-Ethic fragments (L) diagram. For compari- son, the compositional fields of Oligo-Miocene arenaceous formations from the Northern Apennines are reported (from Valloni and Zuffa, 1984, modified).
(e.g., micas) mineral grains are very similar to those of the Northern Apennine Oligo-Miocene turbidites (Valloni and Zuffa, 1984). This af- finity is further supported by textural features, namely high values of packing, presence of a pseudomatrix, recrystallization of the original fine detritus, and moderate overgrowth of au- thigenic quartz on quartz clasts.
Twenty-nine samples of sandstone, siltstone and shale were examined either in this section or in washed residues for biostratigraphic purposes.
Sandstones are often barren, or yield only rare and doubtful ghosts of foraminifera. In a few cases some planktonic genera were identified (i.e. Globigerinoides sp. and Globorotalia sp. ).
Siltstone, silty shale and shale samples are almost always more or less fossiliferous. They locally display an abundant fauna of well-pre- served agglutinated foraminifera, radiolarians, fish teeth, and siliceous sponge spicules. Plank- tonic foraminifera are often corroded, and usu- ally very rare; among them, the following taxa were identified: Catapsydrax dissimilis, Catap-
sydrax sp., Globigerina cf. angiporoides, G. prae- bulloides, G. tripartita, G. venezuelana, Globiger- ina sp., Globigerinoides cf. immaturus, Globoquadrina cf. dehiscens, Globorotalia cf. ku- gleri, G. mayeri, G. opima nana and Globorota- loides suteri. The joint occurrence of Globo- quadrina dehiscens plexus - whose first appearance is recorded within the N 4 Zone of Blow (1969) - and of Catapsydrax dissimilis, which disappears at the top of the N 6 Zone (Blow, 1969; Bolli and Saunders, 1985) sug- gests an Aquitanian to Early-Middle Burdigal- ian age.
Although the above-mentioned assemblage could be more indicative of Aquitanian age, the wider range is preferred because of the possi- bility of selective dissolution of some solution- sensitive taxa as suggested by the bad preser- vation of fossils.
The suggested age is also confirmed by the second-order biostratigraphic information pro- vided by the agglutinated foraminifera. Their assemblage is in fact characterized by the abun- dant occurrence of Haplophragmoides walteri, Cyclammina rotundidorsata, C. acutidorsata and Bathysiphon filiformis. Haplophragmoides com- pressa, Ammodiscus siliceus, A. aff. peruvianus, Psammosphaera fusca, Dendrophyra excelsa, Recurvoides sp. and Glomospira sp. are also present. Cyclammina acutidorsata, Haplo- phragmoides compressa and H. walteri are re- corded in the Late Oligocene of Site 338 and in the Late Oligocene-Early Miocene of Site 348 (Leg 38, Norwegian-Greenland Sea) by Ver- denius and Van Hinte (1983); the same au- thors, following Cicha and Zapletalova (1963), state that Cyclammina rotundidorsata ranges up to the Burdigalian.
The depositional environment of the studied samples seems to be very deep. Calcareous specimens of planktonic foraminifera are very rare, poorly preserved, and show signs of dis- solution. On the other hand agglutinated fora- minifera, radiolarians, fish teeth and siliceous sponge spicules are always present, well pre- served, and often quite abundant.
390 R. COMPAGNONI ET AL.
3.4. Dredge MGC 83-1 (Fig. 8)
Hand specimens: four angular fragments, up to 10 X 4 X 3 cm, of plutonic rocks, covered with a thin Mn-Fe coating.
The studied samples (Table II) consist of a cataclastic biotite granite with porphyritic K- feldspar and large zoned zircons; a two-mica granite with aggregates of muscovite and minor biotite, probably pseudomorphic after original cordierite; a granitoid rock with biotite par- tially replaced by K-feldspar; a granitoid rock of probable hypabyssal origin, characterized by red-brown biotite phenocrysts with orthoclase cores.
3.5. Dredge MGC 84-4 (Fig. 6)
Hand specimens: (1) several large fragments of strongly de-
formed plutonic and metamorphic (?) rocks;
(2) very small rounded crystalline clasts, completely altered;
(3) minor bed fragments of barren, brown, altered, fine-grained sandstone and siltstone.
Among the sharp-edged samples, several types of cataclastic pegmatite prevail; the strong deformation, which affects all the recovered rocks, makes it difficult to assign some minor lithologies either to deformed two-mica granite or to gneiss.
The strong alteration affecting the other lithotypes prevented the execution of more sig- nificant analyses; nevertheless they seem to be very similar to the sedimentary complex re- covered by dredge BS 82-28.
3.6. Dredge MGC 85-3
Hand specimens: two large angular frag- ments, up to 20 X 13 X 3 cm, of plutonic rocks, covered with a thin Mn-Fe coating.
The studied samples (Table II) are a micro-
w
a 3 03
4 4
6 km B S 21 E Fix 19 1
• x ~ ~ - . ~ x x'. ~ x~, , , ; ,% x ~ / : ~ : / = x,,(x ~ "~\ /x x \ , .,e~
~ • ? x - - \ f x X -"
3- X\~ . ~ x X~ X \ X ~ , ~ / /~ 'C/X \ X ' ~ - 3 ^/ \ 4 - ~ ' X x ~ ] ~ "" x x X - - ~ " ~ ~'~"
Fig. 8. Interpreted single-channel seismic section across the Sicily-Sardinia Trough showing tilting of the faulted blocks along the Sardinia margin (1 -- Plio-Pleistocene cover; 2--- Messinian evaporites; 3-- pre-Messinian clastic unit; 4 = acoustic basement; 5 = erosional surface). Dredging station MGC 83-1 is projected.
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 391
cline microgranite with a melanocratic charac- ter (Plate I, A), and a strongly cataclastic bio- tite microgranite.
3.7. Dredge MGC 85-4
Hand specimens: four fresh fragments, up to 31 X 20 X 4 cm, of deformed plutonic and meta- morphic rocks, all with a thin Mn-Fe coating.
Three plutonic samples (Table II) consist of strongly deformed biotite-amphibole tonalite, characterized by locally porphyritic zoned pla- gioclases with saussuritized core. Green horn- blende, sometimes rich in preferentially ori- ented sphene inclusions, suggests their probable derivation from an original pyroxene (Plate I, B ). Allanite, with a clinozoisitic rim, is typical among accessory minerals.
The only metamorphic sample is a fine- grained biotitic gneiss characterized by thin phyllonitic levels.
3.8. Dredge MGC 85-5
Only one sample was collected in this station, namely a granitic gneiss or a strongly cataclas- tic two-mica granite, characterized by the pres- ence of stilpnomelane sheaves within recrystal- lized quartz pockets.
3.9. Dredge MGC 85-6
Hand specimens: three fresh broken frag- ments, up to 20 × 10 X 5 cm, of metamorphic rocks, covered with a thin Fe-Mn coating and ahermatypic corals.
All the samples can be classified as two-feld- spar two-mica gneisses (Table II); one of them is particularly mylonitized, whereas another one is cataclastic and contains small garnets.
It is interesting to note that a further dredge (MGC 85-1) (Fig. 2) recovered a cm-sized fragment of vesicular lava embedded in abun- dant Holocene mud. This fragment, referable to a magma of intermediate composition, shows chilled structures such as networks of skeletal
plagioclases and needle-like orthopyroxene, set in a pale greenish-yellow glass. Its source area could be one of the volcanic structures shown in Fig. 3.
4. General remarks
The above petrographic descriptions, to- gether with the data of Table II, suggest that the crystalline substrate of the study area is a basement composed of sillimanite-bearing am- phibolite-facies, metamorphic rocks intruded by granitoids ranging in composition from granite to tonalite, and by a swarm of felsic dykes with widespread pegmatites.
Most of these plutonic rocks have a peralu- minous character, as suggested by the wide- spread occurrence of muscovite and the fairly common presence of sillimanite.
Radiometric datings are so far lacking; nevertheless, the most probable geological framework is a "Hercynian" crystalline base- ment, such as the one well known all over Southern Europe (Vai et al., 1984; Vai and Co- cozza, 1986). A similar crystalline basement outcrops widely in the Corsica-Sardinia Block (Carmignani et al., 1982), the Calabrian-Pe- loritan Arc (Amodio-Morelli et al., 1976; Atzori et al., 1984) and Little and Great Kabylie (Bouillin, 1984). However, it is important to emphasize that a series of K/At ages (ranging from 295 to 190 Ma) was obtained by different authors from both fresh and altered crystalline rocks (e.g. dredge T 76-13 of Fig. 2) of the Sar- dinia continental slope, attributed to the Her- cynian orogenic cycle (Borsetti et al., 1979a, b; Savelli, 1979).
Most of the plutonic and metamorphic rocks show a common post-Hercynian structural and metamorphic evolution, characterized by a se- vere cataclastic-mylonitic overprint. This is particularly evident in the quartz, which devel- ops ribbons with a very marked wavy extinc- tion, wrapping around the feldspar pheno- clasts. Deformation is accompanied locally by the development of a rough foliation marked by
392 R. COMPAGNONI ET AL.
sericite films + fine-grained green or yellowish- green biotite (Plate II, C). Even when true fol- iation is lacking, deformation is commonly ac- companied by the development of small sericite and green biotite flakes, both growing as fringes on Hercynian red-brown biotite (Plate II, D; Plate III, D). These generally ubiquitous min- erals are sometimes accompanied by Fe-chlo- rite, clinozoisite, sphene, and - in one particu- lar case - by stilpnomelane as well.
The very same deformation pattern over- prints both magmatic and metamorphic Her- cynian parageneses, though with variable in- tensity from one sample to another. As a whole, it reveals characteristics referable to a low-grade tectono-metamorphic event, corresponding to low greenschist-facies conditions. It is interest- ing to note that, of all the crystalline clasts from dredge BS 82-28 (unlike the pegmatite frag- ments, which very likely belong to the base- ment) , none show any apparent cataclastic de- formation or low-grade metamorphic re- equilibration. This suggests a probable origin from a portion of the basement which was not affected or was only slightly affected by the low- grade tectono-metamorphic event described above.
Only very weak dynamo-metamorphic defor- mation effects were observed in samples from dredge T 75-34. This is probably due either to their peculiar massive structure or to a more competent response to deformation, because of their structurally higher position in the meta- morphic building.
Most samples, regardless of low-grade meta- morphic re-equilibration, display a fairly strong alteration of biotite to a mineral with the opti- cal characteristics of chlorite (Mg-chlo- rite! )-t-a semi-opaque, reddish product (limo- nite and/or anatase? ) _ adularia, generally re- mobilized in veins.
It is also worth pointing out that most sam- ples have been affected by a replacement pro- cess on the part of a mangano trigonal carbon- ate (N~--1.54) or more rarely an oxidized Fe- bearing carbonate. This process, seemingly
more developed in the more deformed rocks, begins as a thin intergranular carbonate film, which progressively replaces silicate minerals along a network of veins.
The Aquitanian-Burdigalian arkosic sand- stones collected by dredges BS 82-28 and MGC 84-4 seem to belong to a deep turbiditic se- quence with a proximal character. This se- quence probably includes a coarser part, rep- resented by the paraconglomerates with barren reddish pelitic matrix. We exclude that the red- dish matrix could belong to subaerial deposits (e .g., of Messinian age ) on account of the abun- dant matrix, plus the roundness and composi- tion of clasts. In fact clasts, as previously de- scribed, consist mostly of crystalline lithotypes without any traces of cataclastic deformation or low-grade metamorphic overprinting. The turbiditic sequence (whose thickness can reach 1000-1500 m) appears to directly overlie the Hercynian basement (Fig. 5 ). It is truncated by an irregular erosional surface, and is uncon- formably overlain by a thin Plio-Quaternary sedimentary cover (Compagnoni et al., 1987). In some seismic sections, this sedimentary unit is wedge-shaped, and shows syndepositional deformation (Fig. 9) probably connected with the compressional phases of the substrate.
5. Discuss ion and conclusions
Petrographic and seismic data indicate that the Hercynian crystalline basement crops out in apparent continuity for more than 100 km along the scarps of the Cornaglia Terrace and Sicily-Sardinia Trough. Southwest of the Bis- erta Canyon, the basement shows a marked tec- tonic uplift and probably lacks any sedimen- tary cover. To the southeast, it can be recognized as magnetic basement at a depth of 2-3 km as far as the DTF. East of the DTF it quickly de- scends to a depth of 7-8 km (Cassano et al., 1986; Compagnoni et al., 1987). Based on their metamorphic grade, the dredged lithotypes can be related to the amphibolite-facies complex (sillimanite zone), which is exposed in north-
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 393
SARDINIA
25 kr. M S 9 7 SP 190-30
2 - ~ " ~ - _ ~ - - -
Fig. 9. Interpreted multichannel seismic section across the Sardinia Channel southeast of the Cornaglia Terrace showing a deep-sea elastic unit overlying the crystalline basement, both deformed by NW-dipping thrust planes and/or reverse faults (1 = Plio-Quaternary cover; 2 = Early Miocene deep-sea elastic unit; 3 = crystalline basement).
eastern Sardinia (Carmignani et al., 1982). This complex is part of the Hercynian metamorphic basement, which shows a prograde regional zonation from the anchimetamorphic condi- tions of the Iglesiente (to the southwest). This zonation results from strong crustal shorten- ings related to ensialic subduction (Carmig- nani et al., 1987).
A marked compositional difference exists within the plutonic rocks of the two basements. In fact, whereas the granitoids dredged in the Sardinia Channel frequently exhibit a peralu- ruinous character, those of the Corsica-Sardi- nia batholith (Ghezzo and Orsini, 1982) and those dredged across the Sardinia continental slope are mesoaluminous (Borsetti et al., 1979a, b). This pronounced diversity is further sup- ported by the random character of marine sam- pling, which is generally conditioned by sea- floor morphology and consequently unable to detect uncommon lithologies.
Hence petrographic data suggest that the crystalline basement of the Sardinia Channel
is much more similar to the uppermost units of the Calabrian nappe pile. These units - which are extensively exposed in the Sila, Aspro- monte and Peloritani Mountains - consist of Hercynian medium- to high-grade gneisses and late Hercynian granitoids, very rich in peralu- minous lithotypes (Bonardi et al., 1984). Such a similarity may also exist between the inferred crystalline basement of the Sardinia Channel and of Little and Great Kabylie. This basement could, therefore, be referred to as one Kabylo- Calabrian Domain. Its western boundary would be located approximately along the morpho- logical axes of the Sicily-Sardinia Trough and Cornaglia Terrace, and would probably extend to the northeast along the Raimondo Selli Line (former Central Fault) of the Tyrrhenian bathyal plain (Sartori, 1987). Taking into ac- count also the xenoliths erupted by the volca- noes of the Eolian Islands, the paleogeographic Kabylo-Calabrian Domain can be widely ex- tended outside the investigated area from the La Galite Island in the Tunisian Platform to
394 R. COMPAGNONI ET AL.
the Flavio Gioia Mountain in the central Tyr- rhenian Basin (Dal Piaz et al., 1983). Such a domain, then, defines a complex arcuate struc- ture, which was strongly deformed during the Neogene Tyrrhenian extensional phases.
The lithostructural features of the Lower Miocene sedimentary sequence represent fur- ther, more significant facts to distinguish the Kabylo-Calabrian Domain from the other con- tiguous paleogeographic realms.
As previously outlined, the recovered litho- types belong to synorogenic turbiditic deposits, which probably accumulated in a foreland basin or in minor basins migrating on a shortening continental margin. The main sources of these deposits were in the adjacent fold/thrust belt. Therefore this sedimentary sequence cannot be correlated with the Oligo-Miocene deposits un- conformably overlying the western Sardinia basement. In fact, the latter consists widely of syn- and post-rift continental to shallow-water sediments, which were deposited in graben-like basins related to the eastward displacement of the Corsica-Sardinia microplate (Cherchi and Montadert, 1982).
Age, petrographic composition, lithofacies and stratigraphic position of the turbiditic se- quence seem, on the other hand, to be perfectly equivalent to the Stilo-Capo d'Orlando For- mation exposed in the Serre, Aspromonte and Peloritani Mountains (Bonardi et al., 1980).
The Aquitanian to Langhian (?) Stilo-Capo d'Orlando Formation belongs to the Oligo-Mio- cene Flysch group of the Calabrian-Maghreb- ian belt (Wezel, 1974). It is a thick conglom- eratic-pelitic-arenaceous (arkose to lithic arkose) sequence with turbiditic characters, which unconformably overlies the high- and medium-grade metamorphic units and, some- times, their Mesozoic sedimentary cover as well. It partly reflects a proximal sedimentation de- veloped in orogenic basins, occurring over a fold/thrust belt characterized by strong crustal shortenings. The source area is represented by the structurally uppermost crystalline units of
the Calabrian-Peloritani Arc (Carmisciano and Puglisi, 1982).
The deep-sea arkosic turbidites of the Sar- dinia Channel can be seismically traced to the east up to the DTF; southeast of the DTF, they are replaced by the classical quartz arenites of the Numidian Flysch (Compagnoni et al., 1987). Although, to the west, these turbidites are hard to identify because of the presence of a thick Neogene-Quaternary sedimentary cover, they seem to be absent in the Sardinia continental margin. In fact, the few Oligo-Mio- cene lithotypes recovered show facies typical of the Sardinian Domain (Borsetti et al., 1979a, b).
A clastic unit overlying the crystalline base- ment can be recognized all along the northern margin of the Tunisian Plateau (Auzende et al., 1974; Torelli et al., 1985 ). It can be only tenta- tively correlated with the arkosic turbidites of the Kabylo-Calabrian Domain, due to lack of sampling data. However, if this correlation is correct, this clastic unit would also be related to the "transgressive" Oligo-Miocene deposits overlying the Little Kabylie basement (Bouil- fin, 1979 ).
The post-Hercynian low-grade metamorphic overprint, identified in the dredged crystalline rocks, is reported from most of the basement of the Kabylo-Calabrian Domain. So far it has not been recognized either to the west of the Sicily- Sardinia Trough and Cornaglia Terrace or on- land in the Sardinia basement. Its presence all along a wide continuous belt, more than 100 km long but well defined paleogeographically, leads us to formulate several related questions. What is the age of this event, and in which geody- namic framework did it take place?
Dating is made difficult by lack of systematic radiometric measurements, and by the signifi- cant stratigraphic gap between the basement and its sedimentary cover.
A 96.8 + 3.4-Ma K/Ar biotite data, obtained on a mica-schist from dredging station T 75-34 (Savelli, 1979) could suggest an Eo-Alpine age for this tectono-metamorphic re-equilibration.
SCARPS OF THE SICILY-SARDINIA TROUGH AND CORNAGLIA TERRACE 395
Fig. 10. Geological cross-section of the Sardinia Channel from Sardinia to Sicily (SB = Sardinia basement; K C U = Kabylo- Calabrian units; M U = Maghrebian units; S T F and D T F as in Fig. 3). For location see Fig. 1.
However, it is more probable that this single dating represents a mixed age between a Her- cynian and a Tertiary metamorphic event. This hypothesis seems to be supported by the fact that some arenaceous lithotypes belonging to the Aquitanian-Burdigalian turbiditic se- quence show in thin section a slight low-grade metamorphic overprint, confirmed by defor- mation and development of neoblastic white mica. Consequently the post-Hercynian meta- morphic event may have occurred approxi- mately during the Early Miocene.
A polyphase tectono-metamorphic green- schist facies re-equilibration of Alpine age was recognized in the Aspromonte Unit of Southern Calabria (Bonardi et al., 1984), in the medium- grade crystalline basement of the Peloritani Mountains (Messina et al., in prep.), and in most Alpine and Apenninic units of northern Calabria (Scandone, 1982). In northern Ca- labria, numerous K / A t whole-rock determina- tions (Paglionico, 1985, with references) high- lighted an isotopic re-equilibration of Oligo- Miocene age, to be related to the overthrusting of the Calabrian Alpine units over the African margin. Finally, Alpine blastomylonitic events giving radiometric ages from 38 to 22 Ma (Peu- cat and Bossi~re, 1981 ) were recognized in the Great Kabylie basement and with less certainty in Little Kabylie (Bouillin, 1982, 1984).
One of the best known low-grade tectono- metamorphic events in the Apenninic area is that which affected both the basement and the
sedimentary cover of the Apuane Alps (Car- mignani et al., 1978; Kligfield et al., 1986). This event, ranging in age from 27 to 11 Ma, was in- terpreted as the result of a large-scale intracon- tinental low-angle overthrust shear zone re- lated to the collision of Corsica with Italy (Kligfield, 1979).
In our opinion, a simple shear kinematic model can also be used to explain the structural evolution of the Lower Miocene orogenic belt detected in the central and western Sardinia Channel. Unlike the northern Apennines and the northern part of the Calabrian-Peloritani Arc, this orogenic segment does not contain any oceanic slices. This suggests that the Sardinia and Kabylo-Calabrian Domains in this sector were juxtaposed at least during Cretaceous to Eocene consumption of the Tethyan oceanic lithosphere.
It is, therefore, very probable that the pene- trative deformation and low-grade metamor- phic re-equilibration observed both in the dredged samples and in the mainland Kabylo- Calabrian Domain are the result of a large-scale intracontinental collision. This collision af- fected the Sardinia Block, which overthrust the Kabylo-Calabrian Domain along a WNW-dip- ping low-angle surface, previously detected on seismic sections (Torelli et al., 1985). The de- velopment of the crustal shear zone, whose present boundaries are the STF and DTF (Fig. 3 ), led to the formation of a basin, whose main
396 R. COMPAGNONI ET AL,
source area was represented by the Sardinia basement.
The Early Miocene age of both metamor- phism and turbiditic sedimentation suggests that crustal shortening occurred at the same time as the geodynamic evolution of the Alge- rian-Provenqal Basin (Rehault et al., 1984). In particular, it seems to be connected with the drifting and rotation of the Corsica-Sardinia microplate - dated between 23-24 and 19 Ma ago (Cherchi and Montadert, 1982; Burrus, 1984 ) - which involved continental margin col- lision and westward ensialic subduction of the African lithosphere (Scandone, 1979). The well-known calc-alkaline Oligo-Miocene vol- canism and the Aquitanian and intra-Burdi- galian deformation phases, recently detected in Sardinia (Letouzey et al., 1982; Cherchi and Tremoli~res, 1984), confirm this chronological and structural framework, which is partly to be correlated with the main deformation phases (D1) of the Apuane Alps (Kligfield et al., 1986).
Our ideas and hypotheses are summarized in the cross-section of Fig. 10. This is based on multichannel seismic sections which are nor- mal to the overthrust fronts, and represents a tentative structural synthesis of the Sardinia Channel, based on all geophysical and geologi- cal data available.
This cross-section highlights the width and complexity of the crustal shear zone, plus the change in structural style at the boundary be- tween Kabylo-Calabrian and African margin units (MU). In this respect, the Maghrebian fold/thrust belt is characterized by thrust sheets of Meso-Cenozoic sedimentary cover, which were detached from the basement and thrust towards southwest during the Middle to Late Miocene; they thereby formed generally imbri- cated fans with a ramp-flat geometry (Cata- lano et al., 1989).
Tensional and strike-slip tectonics, active at lithospheric levels, generally took place during the Neogene evolution of the Tyrrhenian Sea (Beccaluva et al., 1984 ). However, one impres- sive extensional feature, the Cornaglia Basin,
seems to have been developed close to the shear zone during pre-Messinian times (L. Torelli, unpublished data, 1988), i.e. before the central Tyrrhenian rifting (Kastens et al., 1986). This basin displays a complex detachment geometry along one sole fault, which may represent a ten- sional reactivation of the former overthrust plane between the Sardinia margin and the Ka- bylo-Calabrian units.
Acknowledgements
The cooperation of the Captain and the crew of R/V "Bannock" during the oceanographic cruises is sincerely appreciated.
R. Nicolich (Trieste) and the Istituto di Geo- logia Marina of C.N.R. (Bologna) provided multi-channel and single-channel seismic pro- files. G. Gabbianelli (Bologna) made available thin sections of dredging station T 75-34.
S. Cornini and F. Marchetti (Parma) con- tributed to the sample dredging and seismic profile interpretations.
Discussions on stratigraphy and petrology of sedimentary rocks with F. Barbieri and R. Val- loni (Parma) have been greatly beneficial to us.
E. Masini (Parma) drew the maps and profiles.
D. Sodero (Calgary, Canada) significantly improved the English text.
The authors are greatly indebted to the above persons and institutions.
This paper was financially supported by M.P.I. 40% ("Evoluzione dei Margini Conti- nentali" and "Analisi strutturale dei thrusts Neogenico-Quaternari dell 'Appennino e del Maghreb") through grants 1985 and 1986 to Professor F. Barbieri ( Parma University).
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