The Ionian Abyssal Plain (central Mediterranean Sea): Morphology, subbottom structures and geodynamic history - An inventory

Abstract

In order to understand the structure and evolution of the Mediterranean Ridge accretionary complex, it is necessary to understand the structure and history of its foreland. The Ionian Abyssal Plain is one of the varying types of foreland. The state of knowledge for that is presented. Its contour and detailed relief are described for the first time. Based on published and hitherto unpublished seismic data, information on the thickness of the Plio-Quaternary and on the Messinian evaporites are presented. Of particular interest are data concerning the pre-Messinian reflectors. They indicate a pattern of tilted blocks and horst-like features created in pre-Messinian time by tensional tectonics. Varying subsidence continued, however, during Messinian time and controlled the thickness of evaporites. At some places (e.g. Victor Hensen Seahill) vertical tectonics seem to be still active. The main tectonic structures of the Ionian Abyssal Plain are not related to the process of the present accretion and subduction at the Africa/Eurasia plate boundary but are pre-existing and should influence the internal structure of the Mediterranean Ridge which is still growing at the expense of the foreland. As a consequence of our structural evidence, we favour the following interpretation: the Ionian Abyssal Plain is not a remainder of the Jurassic Tethyan ocean but originated by extensive attenuation of continental crust.

Full text

The Ionian Abyssal Plain (central Mediterranean Sea): Morphology,
subbottom structures and geodynamic history – an inventory
W. Hieke
1,
*, H. B. Hirschleber
2
& G.A. Dehghani
2
1
Lehrstuhl fu
¨r Ingenieurgeologie (formerly: Lehrstuhl fu
¨r Allgemeine, Angewandte und Ingenieur-Geologie),
Technische Universita
¨tMu
¨nchen, Arcisstr. 21, D-80290 Mu
¨nchen, Germany;
2
Institut fu
¨r Geophysik, Univer-
sita
¨t Hamburg, Bundesstr. 55, D-20146 Hamburg, Germany; (*Author for correspondence E-mail: werner.
hieke@tum.de)
Received 29 July 2004; accepted 05 August 2004
Key words: Ionian Abyssal Plain, Mediterranean Ridge accretionary complex, Messinian evaporites,
tensional tectonics, thinned continental crust
Abstract
In order to understand the structure and evolution of the Mediterranean Ridge accretionary complex, it is necessary to under-
stand the structure and history of its foreland. The Ionian Abyssal Plain is one of the varying types of foreland. The state of
knowledge for that is presented. Its contour and detailed relief are described for the first time. Based on published and hitherto
unpublished seismic data, information on the thickness of the Plio-Quaternary and on the Messinian evaporites are presented.
Of particular interest are data concerning the pre-Messinian reflectors. They indicate a pattern of tilted blocks and horst-like fea-
tures created in pre-Messinian time by tensional tectonics. Varying subsidence continued, however, during Messinian time and
controlled the thickness of evaporites. At some places (e.g. Victor Hensen Seahill) vertical tectonics seem to be still active. The
main tectonic structures of the Ionian Abyssal Plain are not related to the process of the present accretion and subduction at
the Africa/Eurasia plate boundary but are pre-existing and should influence the internal structure of the Mediterranean Ridge
which is still growing at the expense of the foreland. As a consequence of our structural evidence, we favour the following inter-
pretation: the Ionian Abyssal Plain is not a remainder of the Jurassic Tethyan ocean but originated by extensive attenuation of
continental crust.
Introduction
The ‘‘Ionian Abyssal Plain’’, located in the wes-
tern Ionian Sea between the Calabrian Rise, the
Mediterranean Ridge and the Medina Ridge has
been named in the literature as follows:
– ‘‘Ionische Tiefsee-Ebene’’ (Pfannenstiel, 1960a, b),
first map,
– ‘‘Messina Abyssal Plain’’ (Ryan and Heezen, 1965)
– ‘‘Sicilia plain’’ (Carter et al. 1971)
– ‘‘Ionian bathyal plain’’ (Morelli 1978)
– ‘‘Ionian Abyssal Plain’’ (The International
Bathymetric Chart of the Mediterranean ¼IB-
CM, Intergovernmental Oceanographic Com-
mission, 1981). Note that Ryan et al. (1970,
fig. 5) named a northwestern area of the Hel-
lenic Trench, off Zakynthos, ‘‘Ionian Abyssal
Plain’’.
– Catalano et al. (2000, 2001): confusing mixture
of undefined terms (Ionian Abyssal Plain,
Ionian basin, Ionian ocean, Western Ionian
Abyssal Plain, deeper abyssal plain, Eastern
Ionian Abyssal Plain) for the area between
the Malta and the Apulian Escarpments.
In some cases, the term ‘‘Ionian Abyssal
Plain’’ embraces the ‘‘Calabrian accretionary
wedge’’.
– Polonia et al. (2002) use the term ‘‘Messina
foredeep’’ (in relation to the Mediterranean
Ridge development).
– The list of geographical names of undersea fea-
tures (IHO/IOC, 1990) contains only the name
‘‘Ionian A.P.’’ without reference to the familiar
‘‘Messina A.P.’’ but with the remark ‘‘to be sub-
stituted in the later IBCM editions by Ionian
basin’’. Due to the horizontal seafloor, the reali-
Marine Geophysical Researches (2003) 24:279–310 Springer 2005
DOI 10.1007/s11001-004-2173-z

zation of that recommendation would be a
mistake.
Up to now, little has been known about the
recent shape and extension of the abyssal plain, its
sediment pile, its subbottom geology and its geo-
dynamic history. Therefore, the Ionian Abyssal
Plain has often been a matter of speculations, par-
ticularly when considered in plate tectonic models.
In the generally accepted plate tectonic sce-
nario, the IAP is part of the African Plate sub-
ducting beneath the Hellenic and the Calabrian
arcs as well. At the subduction zone towards the
east, the Mediterranean Ridge arises as an accre-
tionary complex and the IAP constitutes one of
the forelands. The detailed internal structure of
the MR is still mainly unknown, due to the poor
penetration. The nature of the already accreted
part of the foreland should have strongly influ-
enced the internal structure of the Mediterranean
Ridge. Therefore, it should be helpful for the
understanding of the entire area to know the nat-
ure of the foreland, in our case of the present IAP.
It is the aim of this paper to present an inven-
tory of IAP data embracing older mostly unpub-
lished observations, as well as those from recent
cruises. Basing on this data set, a hypothesis of
the geodynamic of the IAP will be presented.
Data source
The echosounding and/or seismic reflection data
compiled in this publication have been collected
during the following cruises: Chain 61 (1966),
Meteor 17 (1969), Marsili (1970, 1971 and 1979),
Meteor 50 (1978), Sonne 30 (1984), Valdivia 120
(1992, MEDRAC), Meteor 25/4 (1993, MEDRAC
II), project IMERSE (1994), Meteor 40/1 (1997;
MEDRAC III) and the Italian CROP project
(1991–1994). The tracks of these cruises are pre-
sented on Figure 1. For gravity and magnetics,
our data (cruises Valdivia 120, Meteor 25/4 and
40/1) have been combined with various older data
available from the GEODAS data bank. Some
data have already been published: Finetti and
Morelli (1973; Marsili), Hinz (1974; Meteor 17),
Avedik and Hieke (1981; Meteor 50), Finetti (1981
and 1982; Marsili), Hieke and Wanninger (1985;
Sonne 30), Hirschleber et al. (1994; Valdivia 120),
Hartmann (1995; Valdivia 120), Hieke et al. (1998,
Valdivia 120 and Meteor 25/4), Hieke and Deh-
ghani (1999; Valdivia 120, Meteor 25/4 and 40/1),
Catalano et al. (2000 and 2001; CROP), Reston
et al. (2002; IMERSE) and Polonia et al. (2002;
Marsili).
Data collected during a period of almost
30 years are different in quality, mainly in the
positioning. For a better evaluation of the preci-
sion, Table 1 indicates the systems for position-
ing, echosounding and seismic reflections as well
as gravity and magnetics (only cruises Valdivia
120, Meteor 25/4 and 40/1). In spite of technical
progress, the older data contribute as well to the
picture of the IAP which is presented in this
publication.
Morphology
Contour and area
The International Bathymetric Chart of the Medi-
terranean (IBCM, Intergovernmental Oceanogra-
phic Commission, 1981) suggests that the plain is
contoured by the 4000 m isobath. The deepest
point indicated is 4140 m (corr.).
1
The complete
and exact shape of IAP is still unknown. This
might be the background for the suggestion to
substitute the correct term ‘‘abyssal plain’’ by the
unprecise ‘‘basin’’ (IHO/IOC, 1990).
The characters of the borders of IAP are
influenced by varying sedimentary and structural
peculiarities which make a uniform definition dif-
ficult. We defined the borders at those points of
the echosounder lines where the horizontal,
slightly wavy or slightly raised seafloor changes
to a steeper slope or even to a break of the slope
(tracks on Figure 1). From that a preliminary
and incomplete shape of IAP has been derived
(Figure 2).
The Calabrian Rise and Mediterranean Ridge
borders are relatively well documented whereas
data from the western corner, the western part of
the Medina Ridge border and the northeastern
corner are rare. The quality of the navigation
varies, and is worst in the northeastern part of
the Calabrian Rise side.
The IAP is triangular as directed by the orien-
tations of Calabrian Rise, Mediterranean Ridge
and Medina Ridge. The area is roughly estimated
to 5000 km
2
. Appearance and orientations of the
rim sections vary considerably in detail according
280

to the respective rise area. The geographical
coordinates of the rim positions of Figure 2 are
listed in Table 2.
Calabrian Rise side
Dominant features at the lower Calabrian Rise
are subsymmetrical elevations rising 5–30 m
above the general trend of the seafloor, often
beginning as slight undulations of the almost
horizontal floor of the abyssal plain (Figure 1,
So30-2 and M17-24a; Figures 3a and b). In most
cases, the elevations produce a detailed relief
modifying the continuously rising slope of the
Calabrian Rise (Figure 1, 600; Figure 3c). The
general slope gradients vary between 030¢and
2. Between about 1745¢and 18longitude, a
smooth wavy rise without pronounced elevations
extends in front of the continuously rising slope
(Figure 2, position 5).
Mediterranean Ridge side
The situation in the northernmost part is quite
similar with that of the Calabrian Rise side: pro-
nounced elevations modify the general slope of
the lower MR. Near the contact with Calabrian
Rise and MR (Figure 1, M17-26b and M17-26a;
Figures 3d and e; ship’s positions not very pre-
cise), undulations of the plain seafloor can be
observed as on the Calabrian side. The general
slope gradients are very gentle (about 030¢).
Further in the south to about 3551¢latitude
(Figure 1, 1.13; Figure 3f), the slope gradients
vary between 110¢and 220¢; there, the occur-
rence of undulations is not proved. Between
Figure 1. Bathymetric map (IBCM) of Ionian Abyssal Plain area with ship tracks and positions of ESP 5 (De Voogd et al., 1992) and
DSDP Site 374. Track identification: 1.3–1.18 ¼Valdivia 120 (1992); 502–600 ¼Meteor 25 (1993); 101–213 ¼Meteor 40 (1997). Bold
lines: IAP rim features on Figure 3, arranged clockwise starting with A in the northwest. Dashed bold lines: SCS and MCS lines on
Figures 4 and 6–11.
281

Table 1. Data source.
Cruise Year Positioning Echosounding Reflection seismics Gravimetry Magnetics
Chain 61 1966 various 100,000 joule sparker; Bolt
PAR 600 air gun
Meteor 17 1969 Loran C 30 kHz Schelfrandlot
15 kHz sonar (both
with stabilized
resonator)
SCS streamer
AQUATRONIX; 2–3 airguns
a
`0.16 l
Marsili
(MS-21 and
MS-21 ext.)
1970 Loran C MCS streamer, active length:
2400 m, coverage of data:
12-fold; Flexotir, 3 a
`50 g
dynamite
Marsili (MS-27) 1971 Loran C like Marsili cruise 1970
Meteor 50 1978 INDAS (Loran C
plus satellites)
30 kHz ‘‘Schelfrandlot’’
15 kHz sonar (both
with stabilized
resonator)
SCS streamer, active length:
150 m; TWG water gun: 1.3 l
Marsili
(MS-112)
1979 Loran C like Marsili cruise 1970
Sonne 30 1984 MAGNAVOX
satellite system
O.R.E 3.5 kHz
Valdivia 120
(MEDRAC)
1992 GPS ELAC 30 kHz MCS streamer, active length
2400 m, coverage of data:
24-fold; Airgun array:
6 (0.3–3.0 l), total volume:
7.9 l
KSS 30 ELSEC 7704
CROP 1991–1994 MSC streamer, active length
4500 m; 45-fold coverage of
data (Catalano et al., 2000)
Meteor 25/4
(MEDRAC II)
1993 GPS ATLAS Parasound KSS 30 GEOMETRICS G 811
IMERSE 1994 GPS MCS streamer, active length
4500 m; Airgun array: total
volume 80 l (Reston et al.,
2002).
Meteor 40/1
(MEDRAC III)
1997 GPS ATLAS Parasound KSS 30 GEOMETRICS G 813
282

3551¢and 3543.5¢latitudes the slope rises in
steps, forming ‘‘terraces’’ (Figure 1, 510, 513,
So30-1, 512 and M17-30a; Figures 3g–k).
The slope gradients vary between 030¢and
040¢. Also the southernmost presented record
(Figure 3l), again with rather subsymmetrical
elevations, is characterized by the smooth
gradient.
Medina Ridge side
Along this side the seafloor generally rises monot-
onously over a long distance (Figure 1, 515, 1.6
and M17-30b; Figures 3m, n and q) towards a
broad W–E oriented shoulder in front of Medina
Ridge (‘‘Medina Ridge Glacis’’; Hieke and Deh-
ghani, 1999). A different type of rim occurs where
‘‘Victor Hensen Seahill 2’’ (VHS-2; Hieke and
Dehghani, 1999) and the central finger of the
Medina Ridge at 1730¢E emerge. At VHS-2 the
seafloor first rises slowly about 10 m above the
plain level and then abruptly to 210 m above the
plain (Figure 1, 910 and 1617; Figures 3o and p;
Figure 2, positions 39 and 40). The situation at
the central finger is similar. However, the north-
eastern foreland of the finger shows a small-scaled
Figure 2. Bathymetric map (IBCM) of Ionian Abyssal Plain. Positions of the plain rim (numbers, see also Table 2) and approximate
contour of the plain (thin dashed line). Cross hatched: relief elements not recorded in IBCM (CF ¼northern tip of the central finger of
Medina Ridge; MRI ¼SW–NE oriented isobaths in the MR; VHS ¼Victor Hensen Seahill; VHS-2 ¼Victor Hensen Seahill
2)..¼maximum depth of IAP. Bold dashed lines (long): subbottom structures (VHStr ¼Victor Hensen Structure, NStr ¼Nathalie
Structure, VaStr ¼Valdivia Structure). Bold dashed line (short): axis of deepest position of TTL. Areas with waved lines (occurrence
of ‘‘Undulation Zone’’ (Hinz 1974): undulations restricted to pre-TTL layers (wide screen) and affecting both pre- and post-TTL layers
(narrow screen). Area with short lines ¼rising monoclines instead of undulations. *= position of subbottom prolongation of VHS-2
(Finetti and Morelli, 1972, fig. 11; 1973, fig. 29; Finetti, 1981, fig. 7; 1982, fig. 15 – see Hieke and Dehghani, 1999).
283

Table 2. Geographic coordinates.
Data name Coordinates Water depth
a
Rim positions (locations see Figure 2)
Against Calabrian Rise
(1) M40-101 3540.00¢N1720.01¢E 3950 m
(2) 1.18 3546.53¢N1728.47¢E 3960 m 3968 m
(3) So 30-2 3548.99¢N1732.53¢E 3955 m 3963 m
(4) M 17-24a 3550.0¢N1737.1¢E
b
3958 m 3966 m
(5) M 17-30b 3555.9¢N1738.9¢E
b
3971 m 3979 m
(6) 1.12 3553.23¢N1749.82¢E 3960 m 3968 m
(7) 1.8 3554.46¢N1752.80¢E 3965 m 3973 m
(8) 1.6 3559.35¢N1803.40¢E 3960 m 3968 m
(9) 1.7 3559.47¢N1803.62¢E 3970 m 3978 m
(10) 1.5 3559.29¢N1802.60¢E 3970 m 3978 m
(11) 600 3601.44¢N1805.71¢E 3975 m
(12) M 17-1a 3615.1¢N1818.6¢E
b
3958 m 3946 m
(13) M 17-26b 3615.2¢N1826.6¢E 3958 m
(14) M 17-26a 3612.5¢N1825.5¢E 3955 m
Against Mediterranean Ridge
(15) 1.13 3555.87¢N1835.67¢E 3968 m 3976 m
(16) 520/521 3554.43¢N1835.68¢E 3970 m
(17) 1.5 3553.28¢N1835.58¢E 3970 m 3978 m
(18) 1.14 3552.55¢N1835.58¢E 3969 m 3977 m
(19) 510 3551.54¢N1835.02¢E 3978 m
(20) 1.9 3547.06¢N1832.94¢E 3970 m
(21) 513 3547.04¢N1833.27¢E 3972 m
(22) So 30-1 3546.15¢N1835.19¢E 3962 m 3970 m
(23) 202 3546.50¢N1834.98¢E 3957 m
(24) 201 3546.44¢N1833.66¢E 3953 m
(25) 203 3545,00¢N1835.46¢E 3962 m
(26) 512 3543.76¢N1834.81¢E 3962 m
(27) 205 3543.32¢N1834.98¢E 3961 m
(28) M 17-30a 3543.3¢N1835.4¢E
b
3965 m 3973 m
(29) 206 3542,84¢N1835.46¢E 3962 m
(30) 1.11 3542.17¢N1834.52¢E 3953 m 3961 m
(31) 207 3541.82¢N1837.02¢E 3963 m
(32) 507/508 3541.29¢N1838.31¢E 3958 m
(33) 208 3540.80¢N1838.52¢E 3958 m
(34) 517 3540.15¢N1838.67¢E 3960 m
(35) 507 3529.78¢N1838.30¢E 3950 m
Against Medina Ridge
(36) 516 3531.03¢N1834.00¢E 3960 m
(37) 515 3534.80¢N1829.52¢E 3970 m
(38) 1.7 3534.79¢N1828.67¢E 3960 m 3968 m
(39) M 50 3537.05¢N1825.0¢E
b
3962 m 3970 m
(40) M 50 3535.2¢N1821.0¢E
b
3960 m 3968 m
(41) M 17-30a 3534.5¢N1758.0¢E
b
3957 m 3965 m
(42) M 17-30b 3538.3¢N1743.3¢E
b
3968 m 3976 m
(43) 102 3537.6¢N1740.6¢E 3970 m
(44) 102 3535.8¢N1736.8¢E 3971 m
(45) 102 3533.2¢N1733.8¢E 3971 m
(46) Chain 61 3535¢N1710¢E
b
?
284

relief which is the expression of subbottom struc-
tures (Figure 3r). Note that the central finger
reaches farther to the north than indicated in the
IBCM.
In summary, the morphological characteristics
of the IAP rims differ: the slopes of Calabrian
Rise and Mediterranean Ridge are complicated
by elevations superimposed on the general rise or
accentuating step-like ‘‘terraces’’, in contrast,
towards the Medina Ridge the seafloor generally
rises continuously and gently.
Bathymetry and relief
IAP is neither horizontal nor free of any relief. The
exact bathymetry is difficult to ascertain due to (a)
the diversity of sound velocities used for echo-
sounding and (b) the recording scale. Therefore,
the most recent data (Parasound of cruise Meteor
25/4, 1993) have been taken as the standard, and
the older data have been adjusted by 8–11 meters
2
.
In the southeastern part of the plain emerges
the Victor Hensen Seahill (VHS; Hieke and
Wanninger, 1985, and Figures 4a and 8). It is a
10.5 km long and 1.75–2.4 km wide elevation
with four summits in water depths between 3660
and 3740 m. The greatest elevation of VHS
above the plain is about 320 m. The four sum-
mits are arranged in a nearly straight line striking
about 040. In detail, the seahill shows a continu-
ous change of the orientations of slope sections.
These orientations are grouped as follows: (1)
013–027with a mean of 017, and (2) 064–085
with a mean of 072. The general orientation of
the seahill (040) corresponds to the bisectrix of
the angle between both orientations (Hieke and
Wanninger, 1985).
About 5 km NE of VHS rises a subbottom
structure above the seafloor (Figure 3g) at the
rim of the plain. It belongs obviously to the same
structure as VHS.
A second remarkable feature is a depression of
about 5 m maximum depth and 3 km width
northwest of the northeastern end of VHS
(Figure 4b). Its slopes are asymmetrical (north-
western one steeper than southeastern one).
VHS separates the main part of the plain from
the southeastern corner. The depth of the base of
VHS is 4 m deeper in the northwest (3978–
3979 m) than in the southeast (3974–3975 m) (sev-
eral M25 tracks and Hieke and Wanninger, 1985).
The maximum depth of the plain (3982 m) is
recorded in a shallow axial depression with SW–
NE orientation (Figure 2). From there, the sea-
floor rises generally towards the southeast, inten-
sified by the 4 m step along the VHS axis.
Southeast of VHS, the seafloor shows a depres-
sion of 2–3 m depth parallel to VHS (Figure 4e)
before rising almost continuously up to water
depths of 3970 m at the rim.
Towards NW the seafloor rises from the deep-
est area (3982 m) more or less continuously
(Figures 3a and 3c). Towards the northeastern
rim of the plain (track 521), the water depth
decreases in wavy rises to 3967 m. There, it is
difficult to define the boundary of the abyssal
plain against the Mediterranean Ridge. For that
part of IAP west of 18longitude, echosounding
data are insufficient for such a detailed bathyme-
try. On the Medina Ridge side, the seafloor
seems to be stable at 3976 m along line M17-30b
(Figure 3q) without any relief. Towards the wes-
tern corner (line M40-101) the seafloor rises con-
tinuously to 3950 m.
Table 2. Continued.
Data name Coordinates Water depth
a
Other positions
DSDP 374 3550.87¢N1811.78¢E 4078 m corr.
MS-27 VHS-2 prolongation 3524.4¢N1806.0¢E
b
So30/A 3543.36¢N1826.06¢E
513/A 3547.04¢N1817.80¢E
600/A 3549.28¢N1824.20¢E
660/B 2549.72¢N1823.52¢E
a
See foot-note
2
in the text.
b
Position not very exact.
285

Figure 3. Examples of seafloor and subbottom situations at the rims of Ionian AP (Positions see Figure 1). Calabrian Rise side: (a),
corresponding echosounder and 3.5 kHz lines, Sonne 30; (b), corresponding echosounder and SCS lines, Meteor 17, track 24a (identical
with fig. 14 of Hinz, 1974). (c), parasound line, Meteor 25, track 600; (d), corresponding echosounder and SCS line, Meteor 17, track
26b. Mediterranean Ridge side:(e), SCS line, Meteor 17, track 26a; (f), Echosounder line, Valdivia 120, track 1.13; (g) and (h):
Parasound lines, Meteor 25, tracks 510 and 513; (i), corresponding echosounder and 3.5 kHz lines, Sonne 30: (j), Parasound line,
Meteor 25, track 512, (k), corresponding echosounder and SCS line, Meteor 17, track 30a; (l), echosounder line, Valdivia 120, track
1.11. Medina Ridge side:(m), Parasound line, Meteor 25, track 515; (n), echosounder line, Valdivia 120, track 1.6; (o) and (p),
echosounder lines, Meteor 50, tracks 910 and 1617; (q), echosounder line, Meteor 17, track 30b; (r), Parasound line, Meteor 40, track
102. Seismic lines with depth scales in seconds TWT. AT = Augias turbidite; DTL = Deeper Transparent Layer; TTL = Thick
Transparent Layer.
286

The water depths of the plain rims vary. (1)
Calabrian Rise side: increase from 3968 m (in the
SW) to 3978 m (in the middle) and again
decrease to 3968 m (in the NE). (2) Mediterra-
nean Ridge side: variations between 3980 and
3970 m, not continuously but, with a general ten-
dency of decrease from N to S. (3) Medina Ridge
side: relatively stable between 3968 and 3970 m.
(4) western corner: a continuous rise of the sea-
floor can be supposed.
As a summary of the general relief situation:
the plain is not totally horizontal and at the same
level. The greatest depth (3982 m) is recorded
between VHS and Calabrian Rise (Figure 2).
Figure 3. Continued.
287

The rims have varying water depths (3968–
3978 m at the Calabrian Rise, 3970–3980 m at
the Mediterranean Ridge and 3968–3970 m at
the Medina Ridge sides). These observations
result in an asymmetrical concave sea-floor pat-
tern.
Sediments of the Ionian Abyssal Plain (IAP)
Direct information of the sediments comes from
some piston cores and Deep Sea Drilling Project
(DSDP) Site 374. DSDP Site 374, located at
about the center of the plain (Figure 1), pene-
Figure 3. Continued.
288

trated the Plio-Quarternary sequence and over
80 m into the Messinian (Upper Miocene) evapo-
rite formation. The thicknesses of Quaternary
and Pliocene amount to 300 m and 60–80 m,
respectively (Hsu
¨et al., 1978). The corresponding
mean sedimentation rates for Quaternary and
Pliocene are about 14 cm ·10
)3
years and about
2.1 cm ·10
)3
years, respectively. The high Qua-
ternary sedimentation rate is due to the domi-
nance of turbidite deposits (Mu
¨ller et al., 1978;
Hieke, 2000; Hieke and Werner, 2000).
A turbidite with a maximum thickness of
more than 12 m and a presumable age of
3500 years is recorded in piston cores (‘‘homoge-
nite’’ – Kastens and Cita, 1981, Cita et al., 1984;
‘‘Augias megaturbidite’’ – Hieke, 1984).
The large number of publications with inter-
preted seismic lines suggests a high level of infor-
mation on the pre-Messinian sequences. However,
the stratigraphic interpretations are mainly based
on extrapolations from neighbouring areas only.
The streamer was often not long enough to calcu-
late interval velocities from RMS data.
The largest part of published lines are
repeated interpretations of MS lines obtained by
the Osservatorio Geofisico Sperimentale, Trieste
(e.g. Finetti 1981, 1982). A more substantial
interpretation is that of seismic Flexotir lines
Figure 3. Continued.
289

MS-112 and GINA 4 by Casero et al. (1985,
fig. 7) who were able to include knowledge from
commercial wells on the Strait of Sicily platform.
Casero et al. distinguish a terrigenous pre-evapo-
ritic series of Tortonian to Messinian age, under-
lain by the ‘‘Gruppo Ragusano s.l.’’ of
Oligocene(?) to Serravallian age. Below this level,
the authors provide no interpretation.
Subbottom information
Seismic reflection
About 30 years of seismic reflection studies have
yielded data of various technics (subbottom pro-
filers, single and multi-channel seismic equip-
ments) and corresponding penetrations and
Figure 4. Seafloor and subbottom situations along Victor Hensen and Nathalie Structures. (a) and (c) from Avedik and Hieke (1981).
(a), SCS line Meteor 50 (1978), track M50-1112 in Figure 1. (b) and (d): Echosounder records of the seafloor depression on top of NStr.
(c), SCS line Meteor 50 (1978), track M 50-910 in Figure 1. (e), Parasound record Meteor 40 (1997), track 211A (not in Figure 1).
AT = Augias turbidite; BE = base of evaporites; DTL = Deeper Transparent Layer; M = top of evaporites; NStr = Nathalie
Structure; VHS = Victor Hensen Seahill; VHStr = Victor Hensen Structure.
290

resolutions (Table 1). The data are presented
here in three sections: (1) Plio-Quaternary with a
high resolution section in the uppermost 30–
60 m, (2) Messinian and (3) pre-Messinian.
Note that figures of seismic lines are errone-
ously indicated as ‘‘Ionian Abyssal Plain’’ on
‘‘Ionian Abyssal Basin’’ in following publications:
Finetti (1981, MS-69, fig. 5; and MS-60, fig. 13);
Finetti (1982, MS-60, fig. 18; MS-20, fig. 25; and
MS-33, fig. 26) and Finetti et al. (1996, MS-33,
fig. 2).
Seismic stratigraphy
The seismic stratigraphy is very detailed for the
uppermost 30–60 m part of the sediment pile
(subbottom profiler range). Below this interval,
resolution decreases with increasing penetration.
Figure 5 shows representative sections of our
records with identified reflectors. Parts of profiles
of Finetti and Morelli (1973), Catalano et al.
(2001) and Reston et al. (2002) are included for
comparison.
The subbottom profiler interval (Figures 5a–
b) is characterized by numerous reflectors with
short vertical distances which can be traced in
many cases over the researched area. According
to DSDP Site 374 all the reflectors are inter-
preted as the bases of turbidites (Mu
¨ller et al.,
1978). Conspicuous are three thick transparent
levels: (1) the sedimentologically identified Au-
gias megaturbidite (Hieke, 1984; Cita and Aloisi,
2000; Hieke and Werner, 2000); (2) the Deeper
Transparent Layer, with 7 m maximum thick-
ness and (3) the Thick Transparent Layer, with
35 m minimum thickness. DTL and TTL are
interpreted in analogy to AT as megaturbidites
as well. Their ages are very roughly estimated
to 235,000 and 650,000 years, respectively (Hie-
ke, 2000).
Records (Figure 5c) show also numerous par-
allel reflectors. TTL is the uppermost prominent
interval identified in SCS lines. A conspicuous
level recorded at about 100 m below the sea floor
near the southern rim of the plain and outside is
Figure 5. Examples of seismic resolution and stratigraphy depending on the methods. Note the different vertical scales. The sequence from
(a)to(e) documents increasing penetration. The profiles (f)to(h) presents published stratigraphies with the respective original inter-
pretations. (a) and (b), parasound (Meteor 25, tracks 513 and 515); (c), single channel seismic (Meteor 17, track 30a); (d), single channel
seismic (Meteor 50, track 910); (e), multi channel seismic (Valdivia 120, track 1.7); (f), multi channel seismic MS-27 (Finetti and Morelli,
1973, fig. 32); (g), multi channel seismic IM-1 (Reston et al., 2002, fig. 10); (h), multi channel seismic C9422 (Catalano et al., 2001, fig. 7).
AT = Augias turbidite, BE = base of evaporites, DTL = Deeper Transparent Layer, K = top of mesozoic carbonates, M = top of
evaporites, Mo = Moho, O = Top of oceanic crust; PM1)PM4 ¼pre-Messinian reflectors; TTL = Thick Transparent Layer.
291

named ‘‘Southern Transparent Layer’’. It is
clearly older than TTL and cannot be identified
under the centre of the present plain (Figure 6b).
Succeeding well identifiable horizons (Figures 5d
and e) are reflector M (Ryan et al., 1970; top of
Messinian evaporites) and BE reflector (base of
evaporites).
Within the pre-Messinian, four prominent
reflectors have been observed on seismic profiles
of Valdivia cruise 120. They are named PM 1,
PM 2, PM 3 and PM 4 (Figure 5e). Reflectors
PM 1 and PM 3 can be correlated with the veloc-
ity/depth model of ESP 5 of de Voogd et al.
(1992): PM 1 corresponds to S2, PM 3 corre-
sponds to 2a (top of oceanic crust in the interpre-
tation of de Voogd et al.).
In two sections of profile MS-27 of the Italian
Marsili cruise, a reflector (K) at about 7.8 s
TWT has been interpreted as top of the Mesozoic
(Finetti and Morelli, 1973, fig. 32; Finetti, 1981,
fig. 7; Finetti, 1982, fig. 14) and a second (Z) at
about 9 s TWT as top of the basement (Finetti,
Figure 6. Subbottom situations. Single channel seismic lines from Meteor cruise 17 (1969): (a), M17-30a in Figure 1; (b), M17-30b in
Figure 1; (c), M17-24b, c, and part of M17-24/5 in Figure 1. Parasound lines from Meteor cruise 25 (1993): (d), 600 in Figure 1, with
positions 600/A and B; (e), 513 in Figure 1; with position 513/A. AT = Augias turbidite; DTL = Deeper Transparent Layer; M = top
of evaporites; STL = Southern Transparent Layer; TTL = Thick Transparent Layer.
292

1981, fig. 7; Finetti, 1982, fig. 14). Reflector K
corresponds with reflector PM 3.
The information given by Catalano et al.
(2000, 2001) is confusing: seismic profile M3
crosses completely the Calabrian Rise (not the
IAP) as also the larger part of profile C9422
does; only the southwestern part of the latter
touches the western/northwestern rim of the IAP.
From these lines, a seismic stratigraphy for the
‘‘Ionian deep basin’’ is derived by Catalano et al.
(2001, fig. 8). The level indicated as top of ‘‘oce-
anic crust?’’ in profile C9422 at about 8 s TWT
(l.c., fig. 7) corresponds in the crossing line MS-
27 (Finetti and Morelli, 1973, fig. 32) with the
above mentioned reflector K. Causing confusion,
the same profile C9422 (=M22) is interpreted by
Catalano et al. (2000, fig. 7) completely different:
the top of the crystalline basement is situated at
about 6 s TWT.
Reston et al. (2002) have published IMERSE
profile 1 which crosses the MR and terminates in
the southeastern corner of IAP. There the
authors establish between their reflector B (at
about 5.75 s TWT; corresponding with our BE)
and reflector K (at about 7 s TWT; interpreted
as top Mesozoic) their unit 3 (interpreted as Ter-
tiary clastics; weak reflections; some Mesozoic?),
and below K their unit 2 (interpreted as carbon-
ates; strong, low frequency reflections; some clas-
tics?). Strong reflectors like PM are not
recognized.
The Plio-Quaternary interval
The 3.5 kHz/Parasound range displays a gener-
ally horizontal layering. The thicknesses of layers
AT, DTL and the interval between them are
fairly constant with about 12, 6 and 13 m,
respectively, in the main part of the plain and 10,
5 and 11 m, respectively, in the southeastern cor-
ner. In detail and with increasing depth below
seafloor the structural picture is much less
monotonous:
(1) At a diffractional feature at position So30/A
(which is the southwestern prolongation of
the buried VHS, position see Table 2), DTL
lies on the southeastern side few meters
higher than on the northwestern one (Hieke
and Wanninger, 1985, Figure 3G).
(2) Figure 6d shows a syncline/anticline feature
which is most expressed in the subbottom
with an amplitude of 15 m, but with only
1 m at the seafloor.
(3) An anticline 50 m below the seafloor (Fig-
ure 6e) flattens increasingly to a slight mono-
cline at the seafloor.
The isolated features (2) and (3) are con-
nected with Nathalie Structure in the deeper
subbottom (see Section ‘‘The Messinian inter-
val’’).
(4) Near the centre of the IAP (Figures 3b and
6c), small undulations affect only seismic lay-
ers just above reflector M. Approaching the
northwestern and northeastern rims, undula-
tions affect increasingly younger layers, cause
slight bendings of the seafloor, and build
distinct elevations (Figures 3a–e and 6c). The
amplitude within each doming decreases ver-
tically (approaching the seafloor). In this
frame, the thicknesses of AT and DTL
decrease to 9 and 5 m just before the rim and
5 and 2 m, respectively, outside the plain
(Figure 3c). These features were called
‘‘Undulation Zone’’ by Hinz (1974). Its dis-
tribution is sketched on Figure 2.
Layering in the southeastern corner of the plain
differs considerably. Towards MR, there exists
no undulation but a pattern of rising monoclines
of the layers and corresponding seafloor steps
(Figures 3h, i and k).
Similar slight monoclinal bendings are
observed towards the Ionian Gap (Figure 3m).
There the thickness of AT decreases according
to the rising seafloor up to a water depth of
3930 m where its record disappears. The thick-
ness of DTL decreases earlier (from 5 to 3 m
at the rim), and its record ends at 3960 m pres-
ent water depth.
Detailed information from the deeper part of
the Plio-Quaternary interval is available from a
few tracks only. TTL (thickness in the order of
35 m; Hieke, 2000) might have been reached in
all the Parasound lines, but mostly without pene-
trating its basal reflector. TTL lies not horizontal
but dips generally from the rim (minimum depth
5.34 s TWT) towards the centre of the plain
(Figure 6). The axis of the largest depth below
seafloor (5.42 s TWT) coincides with the position
of the maximum depth of the seafloor (Figure 2).
TTL cannot be followed into the Calabrian Rise
because of deficiency of penetration. Towards the
Mediterranean Ridge, TTL is observed outside
293

the plain in the cobblestone-like features of track
513 (Figure 3h; top at 3962 m seafloor depth,
thickness 28 m) and of track 512 (Figure 3j;
3945, 25 m) as well as in the monoclinal rise near
the southeastern corner of the plain (Figures 3m
and 6a).
A special situation is observed near the
southern rim of the western part (Figure 6b).
There, TTL rises abruptly towards the south
and cannot be identified outside the plain.
Since the seafloor is almost level, the thickness
of the overlaying Quaternary sequence decreases
considerably. In contrast to that, STL is
recorded outside the plain, where the vertical
distance to the seafloor is almost constant. STL
dips under the plain and cannot be clearly
identified at that position where TTL begins to
rise.
Extraordinary features are V-shaped down-
bendings of the reflectors. They are best dis-
played on Figures 4a and c. The amplitude of
the bendings decrease upward, and at the sea-
floor they result in the 5 m deep depression
shown on Figures 4b and d.
The thickness of the Plio-Quaternary
sequence increases generally in a small degree
from SE to NW (Figures 7–11). Near the rim
towards the Calabrian Rise, however, the thick-
ness decreases abruptly (Figures 10 and 11)
when the thickness of the evaporites increases
(rise of reflector M).
The Messinian interval
The sequence of Messinian reflectors is inter-
preted in different ways. Hsu
¨et al. (1978, p. 194)
distinguish on seismic line OD 22 two layers and
interpret them as ‘‘upper evaporites’’ and ‘‘salt
layer’’. Kastens et al. (1992) assume a tripartition
of the evaporite sequence under the Ionian Sea,
which is documented by seismic records only for
the western Mediterranean (Montadert et al.,
Figure 7. MCS line Valdivia 120 (1992), track 1.11. Upper horizontal indications are CMP (common measurement points). NStr,
Nathalie Structure; VaStr, Valdivia structure; VHS, Victor Hensen Seahill; VHStr, Victor Hensen Structure. Interpretation insert: M,
reflector M (top of Messinian evaporites); BE, base of evaporites; PM 1–PM 4, pre-Messinian reflectors.
294

1978). De Voogd et al. (1992) subdivide the
evaporites into two layers due to different veloci-
ties. On the seismic records presented in this
paper, the Messinian interval shows only scarcely
internal reflectors. A bipartition can be observed
tentatively on Figures 7–11.
Figure 8. MCS line Valdivia 120 (1992), track 1.9. Abbreviations see Figure 7.
Figure 9. MCS line Valdivia 120 (1992), track 1.3. Abbreviations see Figure 7.
295

Reflector M lies generally horizontal. BE reflec-
tor is recorded mainly subparallel with M but in
different depths below M. Exceptions are: (1) In
Figure 7 (CMP 5200-5500) BE dips towards the E
and is thus subparallel to deeper reflectors. (2)
Considerable ‘‘domings’’ of BE can be joined to
Figure 10. MCS line Valdivia 120 (1992), track 1.13. Abbreviations see Figure 7.
Figure 11. MCS line Valdivia 120 (1992), track 1.7. Abbreviations see Figure 7.
296

three elongated structures where pre-Messinian
rocks stand up (Figure 2): Victor Hensen Struc-
ture ¼VHStr and Nathalie Structure ¼NStr
(Hieke, 1978; Avedik and Hieke, 1981; Hirschleber
et al., 1994) and one recorded during Valdivia
cruise 120 in 1992. It is much less dominant than
VHStr and NStr, well developed to some extent
only in the southern part of IAP (Figure 7) and
named now ‘‘Valdivia Structure’’ (VaStr).
Within VHS the BE reflector rises extremely
and interrupts the level of reflector M
(Figures 4a and 8). West of NStr, the depth of
M is about 0.1 s TWT deeper than east of it
(Figures 7–9, 11). The increase of depth of reflec-
tor M is connected with monoclines or V-shaped
down-bendings of the Plio-Quaternary reflectors.
Though generally horizontal, M is wavy in detail
west of NStr.
Figures 7, 8 and 11 illustrate that upstanding
pre-Messinian rocks (‘‘thresholds’’) bound
‘‘basins’’ in which variable maximum thicknesses
of evaporites have accumulated. The respective
maximum thickness of basins increases corre-
spondingly with the depth of reflector M from
SE to NW (the deeper reflector M, the thicker
the underlaying evaporites). The maximum thick-
ness of the evaporites in the southeastern corner
is only one third of that in the area NW of
NStr.
The primarily relatively low thicknesses on
the thresholds have been reduced obviously often
by a subsequent dissolution of evaporites, which
produced V-shaped down-bendings. It is conspic-
uous that the ‘‘V’’s are situated asymmetrically
above the thresholds, according to the step-wise
increase of the depth of reflector M.
Near the rim towards Calabrian Rise, the
thickness of evaporites increases abruptly by ris-
ing of reflector M whereas the BE reflector stays
almost horizontally (Figures 10 and 11).
The Pre-Messinian interval
In contrast to the previously described units, the
pre-Messinian interval shows a more varying and
dynamic picture.
Beneath the ‘‘domings’’ of BE, no (VHStr)
or only a few reflectors (NStr) can be recog-
nized. Between VHStr and NStr, the not fully
discernible reflector sequence seems to display a
concave, slightly asymmetrical feature (Figures 7
and 11). West and northwest of NStr, the
sequence shows an asymmetrical pattern with
eastward or southeastward dipping. The reflec-
tors are jointly interrupted and antithetically
tilted several times, producing a repetition of
similar units. Comparable easterly dipping pre-
Messinian reflectors are observed in the north-
eastern corner of IAP in profile MS-112 (Polo-
nia et al., 2002, fig. 4).
The vertical distance between the reflectors is
rather constant in the PM2/PM3 and PM3/PM4
intervals. PM1 is mostly dipping less than PM2
which results in a wedge-like increase of the
PM1/PM2 interval thickness from the west
(northwest) to the east (southeast) (best displayed
in Figure 7). More conspicuous is the thickness
variation of the BE/PM1 interval between the
dominantly horizontal BE and the dipping PM1
reflectors.
The antithetical displacements of the PM
reflectors are normally not paralleled by that of
BE reflector. The exception is VaStr (Figure 7)
where the high position of the reflector sequence
coincides with a westward declining of BE and a
V-shaped down-bending of reflector M. East of
that exceptional VaStr, there is one more excep-
tion: the BE reflector dips towards the east (par-
allel with the underlying PM1).
Finetti (1982, fig. 14) has interpreted in MCS
line MS-27 around 3540¢N and 1745¢E some
lenticular reflector groups as volcanic layers
indicating volcanic activity in pre-Messinian
Miocene time. They correspond with the toe of
the easternmost finger of Medina Ridge and the
subbottom VHStr (both crossed in MS-27, Fi-
netti, 1982, fig. 15; interpreted by Finetti as vol-
canic bodies as well). In Finetti’s opinion, VHS
(named ‘‘Marconi Seamount’’ by Finetti, 1982)
is a large subcircular volcanic body. However,
VHS is, as has been proved, of non-volcanic
nature, and there are no arguments supporting
the volcanic nature of the above mentioned fea-
tures.
Inconsistent and confusing opinions are pre-
sented in the following publications: Catalano
et al. (2001, fig. 7) give divergent and not data-
based interpretations for a ‘‘half-lense-shaped
body’’ in the southwestern segment of line C
9422: (1) a basaltic flow (which is not supported
by magnetic anomalies, e.g. Finetti and Morelli,
1973, pl. XI, and IOC, 2000); (2) a deep-water
clastic fan sourced from North Africa (which is
297

unrealistic because of the slightly rising toe of the
Medina Ridge, as the relief shows).
The boundary ‘‘Tertiary/Mesozoic pelagic’’ of
Catalano et al. (2001, fig. 7) corresponds in our
lines 1.8 (crossing C9422) and 1.13 (Figure 10)
with reflector BE.
The position of the top of the crystalline base-
ment varies considerably if it is indicated at all. At
the crossing of seismic lines M22 (=C9422) and
M23, Catalano et al. (2000, pl. II and fig. 7) place
it at about 6.6 s TWT. In contrast, Catalano et al.
(2001, fig. 7) indicate that level at about 8 s TWT
(base of the ‘‘clastics ?’’). Moreover, lines M22
and M23 have been ‘‘calibrated’’ by Catalano
et al. (2000) using the ESP data of de Voogd et al.
(1992), however, there is no coincidence in the
respective interpretations. So, the referred inter-
pretations are highly questionable.
Seismic refraction
A first complete crust model based on seismic
refractions was presented by Hinz (1974). It indi-
cates the following profile:
The velocity-depth model did not show a first
order discontinuity between the crust and the
mantle. A velocity of 8 km/s corresponds with a
depth of about 19 km. The reinterpretation of
Weigel (1974) results in three alternating models
with a crust–mantle discontinuity at a depth of
16–17 km, where the velocity changes from 6.4 to
8.1 km/s. The crust thickness of this model is con-
sidered both by Hinz (l.c.) and Weigel (l.c.) to be
between those of typically continental and oceanic
ones and interpreted as developed by rifting or
oceanization of a continental crust.
A second model was given by Makris et al.
(1986). Along a profile from Sicily to IAP, data
from three ocean bottom seismographs (OBS)
were connected to a model with the boundary
between 7.2 and 8.1 km/s at a depth of about
18 km northwesterly outside IAP (l.c., fig. 9).
Unfortunately, the OBS from the abyssal plain
itself had no data. Therefore, only unreversed data
recorded at the rim of the basin are available. The
extrapolated model results in a crustal thickness of
11 km for the IAP (6 km of sediments and 5 km
of crystalline rocks). The crust–mantle boundary
(Moho) was found as a first order discontinuity in
15 km depth, where the observed velocity changes
from 7.2 to 8.1 km/s. Makris et al. believe that
such a thin igneous crust can be of oceanic origin
or severely stretched continental, intruded by
upper mantle material.
Cloetingh et al. (1980) and Calcagnile et al.
(1982) presented extreme interpretations of crust
thicknesses of 35–40 and 35–51 km, respectively,
inferred from Raleigh wave dispersion. Calcagnile
et al. (l.c.) argue that the discrepancy with the
model of Hinz (1974) may derive from a misinter-
pretation of a high-velocity layer within the crust
(as generally postulated by Mueller, 1977) as the
beginning of mantle material. Ferrucci et al.
(1991) gave evidence of reduced crustal thickness
in the middle of the Ionian basin, based on refrac-
tion seismic survey using OBSs.
The most recent survey was made in 1988
(Pasiphae cruise; De Voogd et al., 1992), carrying
out expanding spread profiles (ESP). ESP 5 was
measured in the IAP in SW–NE orientation. The
velocity-depth model shows the top of the igne-
ous crust at 9 km and the beginning of the man-
tle (8.5 km/s) at 17 km. From this, a thickness of
the igneous crust of 8 km results. De Voogd
et al. interpret the crust as oceanic.
During Valdivia cruise 120 (1992), some seis-
mic refraction measurements were done by
OBSs. The results confirm more or less the veloc-
ity-depth model of De Voogd et al. (l.c.) (Weigel,
personal communication).
Gravity
Maps of free air and/or Bouguer gravity anoma-
lies were published by Finetti and Morelli (1973),
Morelli et al. (1975), IOC (1989) and Catalano
et al. (2001) with different scales but similar
information. In the free air gravity map, there
are figured Medina Ridge as well as a SW–NE
oriented chain of positive anomalies. The
Bouguer maps show only one strong positive
anomaly (+310 mGal; in the map of Catalano
et al. +270 mGal). Its core trends SW–NE and
4 km water
0.5 km Plio-Quaternary
1.5 km vp = 4.0–4.5 km/s Messinian evaporites
1.4 km vp = 2.2 km/s
4 km Increasing vp from
5.0 to 6.8 km/s
Below Increasing vp from
6.8 to 8.5 km/s
298

coincides with the VHS area. The escorting iso-
lines outline a triangle which extends to the
southeast as well.
The strong positive anomaly has been inter-
preted as follows:
– Finetti and Morelli (1973, p. 333): combination
of thinning of the igneous crustal layer and of
the upper Mantle density anomaly.
– Finetti (1981, p. 482): intermediate or oceanic
crust with sialic fragments.
– Finetti (1982, p. 276): very consistent thinning
of the Mesozoic sequence and uplifting of the
basement, prominent volcanic activities with a
seamount (meant is VHS).
– Boccaletti et al. (1984, p. 231): thinned crust,
high-density intruded magmas, cooling of man-
tle material.
Gravity modelling supports the interpretation
as a pre-Messinian tectonic structure probably
constituted by shales and/or carbonates of
Paleogene-Mesozoic age (Della Vedova and
Pellis, 1989).
Our compilation of measurements made dur-
ing cruises Valdivia 120 (1992), Meteor 25/4
(1993) and Meteor 40/1 (1997) as well as of data
from GEODAS (Geophysical Data System for
Marine Geophysical Data) gave a free-air gravity
map, presented on Figure 12a. There, two areas
of positive anomalies are most conspicuous: a
W–E oriented chain at about 35latitude (coin-
ciding with the Medina Ridge) and a SW–NE
oriented chain starting from the eastern finger of
the Medina Ridge and including VHS. Hieke and
Dehghani (1999) published free-air and Bouguer
maps for the eastern part of the present
Figure 12a. The Bouguer map is repeated as Fig-
ure 12b. It shows that the free-air anomaly coin-
ciding with the eastern finger of the Medina
Ridge is caused by the relief only, whereas the
SW–NE oriented anomaly is caused by subbot-
tom density anomalies.
The negative values in the northwestern part
of Figure 12a indicate the lower slope of the Cal-
abrian Rise as well as the relative flat area
between IAP and Malta Escarpment. IAP does
not become apparent in the gravity map.
Bouguer data for the western part of
Figure 12 are not available since the quality of
bathymetric data is not sufficient to correct the
gravity data. Nevertheless, considering the
general relief situation, distinct gravity anomalies
are not to expect in that area.
Magnetics
An aeromagnetic survey presented by Vogt and
Higgs (1969, fig. 2 and 6) shows residual
magnetic anomalies with negative values, which
coincide with the Medina Ridge. The authors
assume that the anomalies may reflect young vol-
canism.
Also the map of total magnetic intensity of
Finetti and Morelli (1973, pl. XI) shows anomalies
which coincide with the Medina Ridge. They are
much less spectacular than those along the Malta
Escarpment where the occurrence of basalts has
been observed (Scandone et al., 1981; Biju-Duval
et al., 1982). Hieke and Dehghani (1999) demon-
strated that the eastern finger of the Medina Ridge
is characterized by strong magnetic anomalies (up
to 180 nT), whereas no considerable anomalies
can be identified in the area of VHS.
The map of magnetic anomalies (IBCM-M;
International Oceanographic Commission, 2000)
does indicate neither the VHS nor the eastern fin-
ger of the Medina Ridge by anomalies.
Our map of residual magnetic anomalies
(Figure 12c) results from measurements of cruises
Valdivia 120, Meteor 25/4, Meteor 40/1 and data
of different source (GEODAS). It shows little
correlation with the gravity map. The negative
anomalies along 3550¢N might be the result of
modern measurements contrasting to older ones
(see tracks on Figure 12c).
VHS cannot be identified by an anomaly.
Oceanic type magnetic anomalies have never
been recorded.
Heat flow
A simplified heat flow map of the Eastern Mediter-
ranean on the base of C
ˇermak and Hurtig (1979)
was published by Makris and Stobbe (1984). It
shows mainly low heat flow values (<0.9
HFU ¼37.7 mWm
)2
) and a W–E oriented zone
with values of 0.9–1.6 HFU (=37.7–67 mWm
)2
)
in the area north of the Medina Ridge.
Erickson et al. (1977) report a generally low
average value (0.74 ± 0.30 HFU ¼31±12.6
mWm
)2
) in the Eastern Mediterranean and no
299

evidence for any regional heat flow anomaly
associated with the Mediterranean Ridge, nor
with the basin and trench provinces located north
or south of it. In the central part of IAP a value
of 0.80 ± 0.10 HFU (33.5 ± 4.2 mWm
)2
) has
been determined at DSDP Site 374 (Erickson and
Von Herzen, 1978).
The only detail survey has been carried out
along two profiles by Della Vedova and Pellis
(1989): (a) A transect crossing the VHS (almost
coinciding with profile IV of Avedik and Hieke,
1981) shows maximum values of 36.9±4.3
mWm
)2
(NW) and 36.3 ± 3.5 mWm
)2
(SE) and a
minimum of 30.6 ± 0.6 mWm
)2
near the rim of
VHS and an average observed value of
35.1 ± 2.6 mWm
)2
. (b) A profile parallel to a
seismic line along the Medina Ridge Glacis (Ave-
dik and Hieke, 1981, I and II in fig. 1). Eight
Figure 12. Maps of gravity and magnetic anomalies of the Ionian Abyssal Plain area and corresponding track charts. Relief infor-
mation in the anomaly maps: long-dashed line ¼contour of the Ionian Abyssal Plain, short-dashed line ¼3800 m isobath. Track
maps: solid lines = data from GEODAS; dashed lines = data collected by the authors (Valdivia 120, Meteor 25 and Meteor 40). (a):
Free-air gravity anomalies (based on the 1967 formula). Bold lines = 50 mGal intervals; thin lines = 10 mGal intervals; gray = areas
with extreme positive values (exceeding + 30 mGal). (b): Bouguer gravity anomalies (Hieke and Dehghani, 1999, fig. 6). Gray = areas
with extreme values (exceeding 190 mGal). Bouguer data for the western part of (a) are not available due to insufficient quality of
bathymetric data. (c): Residual magnetic anomalies. For all measurements, there have been calculated the corresponding regional fields
(using IGRF and DGRF, respectively) and subtracted from the total field. Bold lines = 250 nT intervals; thin lines = 50 nT intervals;
gray = areas with extreme intensities (exceeding + 100 nT and )100 nT).
300

measurements near the western end exhibit an
average value of 44.5 ± 2.8 mWm
)2
, whereas the
two measurements located near VHS-2 result in an
average value of 34.9 ± 2.2 mWm
)2
which is con-
sistent with that observed on the VHS profile. A
modelling suggests that VHS is in thermal equilib-
rium with the deep structures.
All reported values are below the mean heat
flow for the earth (50–63 mWm
)2
; Sheriff, 1984).
They obviously signalize stable thermal condi-
tions though recent vertical movements are
observed in the IAP.
Discussion
The main contrast in the interpretation of the
characteristics and the history of the IAP is
defined with the question ‘‘Is the IAP a remnant
of an old Tethys ocean which opened in Jurassic
time, or is it a thinned part of the African conti-
nental crust (and Adria is then to be considered
as an African promontory)?’’
The following discussion will try to find out
how the observations can be interpreted and
which of the conflicting models of IAP structure
and history can be corroborated by them.
Subbottom information
Four types of structural elements observed in the
seismic reflection records are conspicuous: (A)
antithetical, southeastward (in W–E oriented pro-
files apparently eastward) dipping pre-Messinian
reflectors, (B) domings of pre-Messinian rocks,
(C) a non-uniform Plio-Quaternary sequence in
the southern IAP, and (D) the ‘‘Undulation
Zone’’ (Hinz, 1974).
Pre-Messinian, southeastward dipping reflectors
The pre-Messinian reflectors PM2 to PM4 show
a pattern of repeated parallel inclinations (tilted
blocks). PM1 is sometimes also parallel with the
deeper ones (Figures 7–10), sometimes it dips
with a smaller angle (Figures 7–9, 11) or even in
the opposite direction (Figures 9–11). The BE
reflector lies normally almost horizontal except in
Figure 7 where it is subparallel with the PMs.
These variations result in wedge-like increases of
the PM2/PM1 and PM1/BE interval thicknesses
(synrift wedges).
The geometry of the PM reflectors signalize
that the area was stable up to young pre-Messini-
an times. Then, the whole area was affected by tec-
tonic deformations. The width of the blocks varies
considerably as can be obtained e.g. from Fig-
ures 7 and 8 (the horizontal scale is fairly identi-
cal).
Domings of pre-Messinian rocks
The domings (see Section ‘‘The Messinian inter-
val’’) uncover their internal structures in varying
degrees. The most prominent VHStr gives the
worst information; reflectors are just indicated if
at all. Within NStr, there can be discerned a
reflector (PM2?) which is tilted towards the west
(Figure 7) as well as towards the east (Figures 8
and 11). VaStr (Figure 7) presents a well devel-
oped sequence of three reflectors (PM2, PM2a,
PM3) which dip eastward.
The identification of the domings in the respec-
tive seismic profiles was made using the VHStr as
the guide line because of its great vertical ampli-
tudes which result in seafloor elevations. NStr and
VHStr are more or less parallel SW–NE oriented
(Figure 2). The domings separate ‘‘basins’’ of
Messinian evaporites. Their maximum thicknesses
increase from SE to NW. The basins must have
pre-existed at the beginning of evaporation and
have been modified in detail during the Messinian.
A vertical difference of about 0.1 s TWT of the
M reflector above NStr indicates post-Messinian
movements. VHStr is part of the Medina-Victor
Hensen Structure (M-VHStr; Hieke and Dehgha-
ni, 1999). The latter is displaced along its course
several times vertically (seahills) and horizontally
(by left lateral faults) as well. Figure 2 shows a
modified course compared with Hieke and Deh-
ghani (1999). The faulting cannot be dated. How-
ever, a recent vertical offset of the seafloor is
evident above VHStr.
Length and degree of deformation of the
domings correlate positively: The most extended
M-VHStr is that one most affected by deforma-
tion.
There are obviously close relations between
the tilted blocks and the domings, as VHStr dem-
onstrates. Where VHStr is only a subbottom
structure, it is very similar to NStr. That again is
comparable with the unspectacular VaStr. As a
consequence, one can consider the domings and
the highest parts of the tilted blocks as parts of a
301

common tectonic pattern with different age of
deformation and intensity: The tilting of the
blocks happened in pre-Messinian time. Some
are inactive since the beginning of the evaporite
deposition or became inactive before the end of
the Messinian. The domings, originally the risen
edges of the rotated blocks, became later the
places of normal faults (with a horizontal compo-
nent?) which did not produce visible vertical
rotations. VaStr, NStr and VHStr document (in
this order) increasingly longer periods and inten-
sities of deformation as well. Thus, we can
deduce that the deformation pattern, character-
ized by a general SW–NE orientation, has been
laid out in pre-Messinian time. Only some of the
tectonic elements stayed active in post-Messinian
time. Thus, they became the dominating ones
(NStr, VHStr).
The continuation of VHStr into the eastern
finger of the Medina Ridge is obvious (M-VHStr;
Hieke and Dehghani, 1999). A similar but less
spectacular situation is supposed to exist north
of the central finger of the Medina Ridge
(Figure 2), though there is not a clear dominance
of the SW–NE orientation of the relief but a
mixing with a NW–SE orientation as represented
by the Malta Escarpment (insert of Figure 1). A
broad and slight doming of BE reflector is
recorded on seismic line MS-27 (Finetti 1982,
fig. 14, SP 770–790) at about 3540¢Nand1745¢
E. It can be speculated whether it represents the
prolongation of the central finger of Medina
Ridge.
Reston et al. (2002, fig. 10) have recorded
bendings of their reflector B (=BE) which they
interpret as folding. It is conspicuous that the
bendings do not correspond with those of the
overlying reflector M which shows much more
bendings. Thus their geometry should have origi-
nated independently which contradicts a common
folding event. Those few bendings of B might be
minor domings compared with VHStr.
None of the subbottom structures show pecu-
liarities of folds as has been assigned to the
VHStr by Jongsma et al. (1987).
V-shaped down-bendings of sediments on top
of NStr and VaStr (Figures 7–9; Avedik and
Hieke, 1981, fig. 10 and 12) may be caused by a
combination of post-Messinian faulting and con-
nected subsolution of a thin evaporite cover of the
domings (Avedik and Hieke, 1981).
Plio-Quaternary sequence in the southern IAP
The post-Messinian sedimentary filling of IAP is
not uniform. Figure 6b demonstrates that the
maximum thickness of the pre-STL sequence is
situated outside the southern rim of the plain.
The STL–TTL interval cannot be observed very
well since both reflectors are not identifiable
jointly over the whole record. For the post-TTL
interval, however, the maximum thickness is
recorded beneath the present plain.
According to the turbidite dominance during
the Quaternary, we have to explain the situation
as follows: During the earlier Plio-Quaternary,
the relief was deepest (or the subsidence was
highest) near the southern rim of IAP and out-
side. At a not identifiable time before the sedi-
mentation of TTL (about 650,000 years?), the
zone of maximum subsidence shifted towards the
north where the present plain is situated. That
subsidence is continuing as the concave seafloor
indicates.
The Undulation Zone
This occurs generally NW of the domings (i.e.
in the area facing the Calabrian Rise) and in
part along the rim towards the MR (Figure 2).
The undulations affect the complete post-evapo-
ritic sequence in a sector paralleling the rims
whereas, with increasing distance from the rims,
the undulation effects fade away downwards
(Figures 3 and 6c; see also Polonia et al., 2002,
profile MS-112 on fig. 4). The occurrence of
the Undulation Zone is linked with the shape
of the top of evaporites: (1) Above the evapo-
rite basin with the largest width (NW of NStr
and VaStr, respectively; Figures 7–9, 11) reflec-
tor M is rough. (2) This large basin is that one
with the greatest maximum thickness of evapor-
ites. (3) It continues towards the NW under
the Calabrian Rise. There, the thickness of
evaporites increases though BE reflector is more
or less horizontal. Such an increase can be
interpreted only by lateral pressure and the
resulting plastic evasion of evaporites (incipient
diapirism). The effects of these processes do
obviously not extend beyond NStr or VaStr
towards the SE.
At the MR side of the plain, the situation is
more complicated. Along the northern part of
the rim, the undulation pattern is almost identi-
cal with that on the Calabrian side (affecting
302

again the evaporite basin with the greatest maxi-
mum thickness). In that area, where NStr and
VHStr come in contact with the MR deformation
front, undulations are almost missing but mono-
clinal flexures (Figure 3i and k) occur. Further to
the south, undulation features occur again
(Figure 3j) but less expressed and mixed with
flexural ones. Reston et al. (2002, fig. 10) inter-
preted bendings of the Plio-Quaternary sequence
in IMERSE profile 1 (SPs 300-350), which corre-
spond to our Undulation Zone, as folding: The
top of Messinian ‘‘appears folded even out in the
Messina Abyssal Plain [=IAP]. This folding must
predate the deposition of the Plio-Quaternary
sequence. (l.c., p. 74).’’
From the Medina Ridge side, undulation fea-
tures are unknown.
The distribution of the Undulation Zone can
be explained as follows: The suggested direction
of subduction or accretion under the Calabrian
Rise is normal to the orientation of the subbot-
tom structures (‘‘domings’’). A lateral stress
could affect a wide area between the Calabrian
Rise and the domings which act as a barrier.
The evasion of the evaporates decreased towards
the barrier. At the MR side in contrast, the
deformation front of the MR accretionary com-
plex as well as the subduction direction are ori-
ented in angles of about 45to VHStr and
NStr. Moreover, the evaporite basin between
both structures is of smaller width and maxi-
mum thickness as well. This might be the case
also for that basin following SE of VHStr. Its
southeastern rim is already incorporated into
the accretionary complex and, therefore,
unknown. This situation should be unfavourable
for the unhindered propagation of lateral stress.
Rim features and tectonics
The distribution of the Undulation Zone and
other subbottom and bottom features indicate that
the rims of the IAP cannot simply be explained as
the deformation fronts of accretionary complexes
with outward directed upthrow faulting.
Where the undulations reach the seafloor,
they cause a small-scaled rough relief (‘‘cobble-
stones’’). This is sometimes arranged in ‘‘ter-
races’’. At the MR side, the situation shows
more variations since, additionally to the pure
Calabrian type, also step-like rims (tensional?)
occur. It is necessary to interpret the outer
boundaries of the Calabrian Rise and the MR (if
defined with the rim of IAP) as primarily vertical
reactions of the plastic evaporites and not as out-
ermost effects of upthrow faulting (deformation
front). Real accretionary shortening may act
inside the accretionary complexes in a distance
from their morphologically defined outer bound-
aries.
In contrast, the rim towards the Medina
Ridge is characterized by flexures and steps
obviously caused by normal faults being com-
mon in the Medina Ridge Glacis (Hieke and
Dehghani, 1999), which itself seems to be the
product of down-faulting compared with the
Medina Ridge.
Nature of the crust underneath the Ionian Sea and
structural implications
An evaluation of the literature (Hieke and
Dehghani, 1999), yielded a wide spectrum of con-
troversial interpretations including oceanic and
continental crusts. They can be very roughly cat-
egorized as follows (some authors gave ambiva-
lent interpretations):
Continental or thinned continental crust
Finetti and Morelli (1973), Cloetingh et al.
(1980), Farrugia and Panza (1981), Baldi et al.
(1982), Calcagnile et al. (1982), Mantovani
(1982), Mantovani and Boschi (1982), Boccaletti
et al. (1984), Makris et al. (1986), Leister et al.
(1986), Ferrucci et al. (1991), Cernobori et al.
(1996), Mantovani et al. (2002).
Neither typically continental nor typically
oceanic crust
Hinz (1974), Weigel (1974), Morelli (1978)
Oceanic or intermediate type of oceanic crust;
uplifting of the basement and volcanic activities
Finetti (1981, 1982), Makris et al. (1986), Leister
et al. (1986), de Voogd et al. (1992), Finetti et al.
(1996), Catalano et al. (2001).
Geological and tectonic maps demonstrate
with their inconsistency the scarcity of exact crust
data:
– Khain and Leonov (1979): oceanic crust is
restricted to the IAP.
303

– Choubert and Faure-Muret (1987): boundary
continental/oceanic crust crosses the Medina
Ridge.
– Bogdanov and Khain (1994): the highly ele-
vated Medina Ridge is traversed by the bound-
ary between oceanic crust and a stripe of
thinned continental crust which embraces
water depths between 3500 and 1000 m.
The geodynamic concept of an Ionian Ocean of
Catalano et al. (2001) is based on paleogeo-
graphic and plate tectonic considerations in the
neighbourhood of the Ionian Sea. There is no
new and verifiable information from the Ionian
Sea itself. Main arguments are the Malta and
Apulia Escarpments which are considered as old
conjugating passive margins. From that, the exis-
tence of an ocean between them seems inevitably
to result. Catalano et al. (2001) continued to find
evidence for a (NW–SE trending) midoceanic
ridge just in the middle between the passive mar-
gins. They found it in their seismic profile C9434
(fig. 6) on the Calabrian Rise with its generally
rough topography: An ‘‘anomalous depression’’
connected with converging reflectors should be
witness for that feature. A look at a modern
bathymetric map (e.g. IOC, 1981) or at a multi-
beam imagery (e.g. Loubrieu et al., 2000) will
easily show, how many such ‘‘anomalous depres-
sions’’ occur on the Calabrian Rise.
Bosellini (2002) presents strong arguments
(different geological history) against the passive
margin nature of the apparently ‘‘conjugating’’
Malta and Apulia Escarpments. Moreover, from
the occurrence of dinosaurs on the Apulia car-
bonate platform, Bosellini (l.c.) concludes that
Apulia was connected to Africa (not separated
by deep water) and the Late Jurassic – Early
Cretaceous Ionian Sea region was a ‘‘cul-de-sac’’
-type basin enclosed by shallow-water carbonate
banks.
Hieke and Dehghani (1999) demonstrated that
the VHStr is connected with the eastern finger of
the Medina Ridge. The resulting ‘‘Medina – Victor
Hensen Structure’’ (M-VHStr) is at least 155 km
long, can be followed to the lower slope of the MR
and is intersected by left lateral faults. The Medina
Ridge at the southwestern end of the M-VHStr is
commonly accepted to be underlain by thinned
continental crust (e.g. Finetti, 1982). In the case of
Khain and Leonov (1979), the narrow M-VHStr
would extend from thinned continental to old oce-
anic crust surmounting a maximum depth differ-
ence of 2500 m. In the cases of Choubert and
Faure-Muret (1987) and Bogdanov and Khain
(1994) one would have a respective depth differ-
ence within an area underlain by oceanic crust.
The VHStr has been considered by several
authors to be the continuation of the Cefalonia
Fault (Hieke and Dehghani, 1999). That again is
interpreted as a long transform fault which easily
could run from an oceanic to a (thinned) conti-
nental area. However, two facts take the VHStr
its value as an unique feature (even if it is the
most prominent one) and the considerations on
the Cefalonia Fault/VHStr relation become less
obtruding: (a) the pattern of several subbottom
structures more or less parallel to the VHStr
(Figures 7–11) and (b) the piercing of the central
finger of the Medina Ridge into the IAP near its
western end (Figure 2) which suggests a situation
similar to that of the M-VHStr but not so
expressed.
The geological and geophysical observations
presented by Hieke and Dehghani (1999) and
those presented in this paper let tend us to a
strongly thinned continental character of the IAP
crust.
Peculiarities of the IAP – expressions of its geody-
namic history
We can summarize the observations as follows:
– IAP is characterized by a SW–NE trending
strong positive gravity anomaly.
– There are no magnetic anomalies indicating
large magmatic bodies as supposed by some
authors.
– Heat flow is low with minor variations.
– The type of the crust (interpretations based on
seismic refraction and Raleigh wave dispersion
measurements) is contested (old oceanic,
thinned continental, thick continental).
– The deepest recognized seismic reflector lies in
a depth of 9 s TWT.
– Most of the pre-Messinian seismic reflectors
are parallel and inclined to the southeast.
– The uppermost pre-Messinian reflector interval
shows wedge-like thickness variations because
of the horizontal position of BE reflector. This
dates the vertical rotation of the PM reflectors
as mainly pre-Messinian.
304

– Rotation of the PM reflectors produced some
depressions and elevations which are elongated
with a SW–NE orientation.
– The depressions have been filled by evaporites
during the Messinian (evaporite basins).
– The maximum thicknesses of the evaporite
basins increase from SE to NW.
– The elevations (domings of the pre-Messinian)
between the basins (VHStr, NStr, VaStr) rise
to different levels.
– The most extended of the domings (VHStr)
owns the largest vertical differences. It rises at
some places above the present seafloor. Its axis
is shifted several times by left lateral faults (i.e.
clock-wise rotation). The trend of the faults
might be about 110–120as can be constructed
between VHS-2 and the Medina Ridge finger.
– Reflector M shows a vertical offset above or
on both sides of VHStr and NStr, which indi-
cates post-Messinian tectonics.
– The Plio-Quaternary sedimentary cover shows
V-shaped down-bendings above some domings.
They are interpreted as the result of a combi-
nation of graben-like faulting triggering subso-
lution of evaporites.
– The drastic change of the facies in the present
IAP area at about the Pliocene/Quaternary
boundary (normal hemipelagic to turbidite-
dominated) might indicate a strong subsidence
which gave the relief for turbidite accumulation.
– The thickness of the Plio-Quaternary sequence
(which means mainly of the turbidite-domi-
nated Quaternary) is controlled by (a) the sub-
sidence of the respective area and (b) the
secondary increase of the thickness of evapor-
ites near the Calabrian Rise.
– Thickness variations within the (Plio-)Quater-
nary sequence document that the maximum
subsidence was located during the pre-STL
time near the southern rim of the IAP and
changed in post-STL time towards the north.
– Subsidence/vertical tectonics are still active at
present time.
These peculiarities provide the following concept
of the tectonic and sedimentary evolution of the
IAP.
In Mesozoic (and early Tertiary?) time, the
area seems to have been tectonically quiescent.
Then, the area was broken in elongated blocks of
varying widths. They were antithetically tilted (rif-
ting?) during late pre-Messinian times. Declining
movements may have continued during the Mes-
sinian. A general subsidence, varying in detail,
controlled the evaporite thicknesses within basins
which were separated by thresholds (the highest
parts of the tilted blocks). The alternative idea of
a fixed pre-existing relief which has been simply
filled during the Messinian evaporation phase is
rather unlikely since it would describe a persis-
tence which is not in accordance with the general
dynamic situation.
The thresholds (domings) may have been
active (relative elevation) also during Messinian
time. In any case, normal faulting was active in
post-Messinian time (vertical offset of reflector
M). Shifting of the place of maximum subsidence
from the south to the north and faulting con-
trolled the thickness of Quaternary sediments
which mainly consist of turbidites.
The continuation of subsidence and normal
faulting up to the present produced the slightly
concave seafloor and seafloor steps, which could
be soon balanced by the frequent turbidites.
The V-shaped down-bendings of the Plio-Qua-
ternary layers on top of NStr and VaStr are still
active. They might be caused by a combination
of graben-forming faults on top of a horst and
the subsolution of the evaporite cover on the
threshold supported by water circulating along
faults.
A completely different process caused the
decreasing thickness of Quaternary sediments
towards the Calabrian Rise rim: the lateral immi-
gration of evaporites from the Calabrian Rise
produced a more or less continuous rising of the
seafloor and the undulation layering of parts of
the plain filling. This process played obviously a
minor role on the MR side, where the primary
thickness of evaporites is smaller.
IAP in the frame of plate tectonics
Published conceptions of the role of IAP in the
plate tectonic history (synopsis see Hieke and
Dehghani, 1999) are contrary and supported very
differently. Scandone et al. (1981) listed six
hypotheses on the origin of the Ionian Basin. All
the ideas seem to be based on arguments which
cannot be easily disregarded. In this situation, it
would be helpful to be able to rule out some of
the hypotheses by established facts:
305

The concept of the occurrence of large volcanic
bodies in the IAP (Biju-Duval and Montadert,
1977; Finetti, 1982, Marconi Seamount instead of
VHStr) is not valid.
Medina Ridge is not part of a right-lateral
wrench zone as postulated by Jongsma et al.
(1987). The northern part of the Medina Ridge is
characterized by SW–NE and SSE–NNW trend-
ing faults. There are no SW–NE trending folds
in the IAP. The horizontal offsets of M-VHStr
indicate left-lateral faults.
Cephalonia Fault cannot be considered simply
as a large SW–NE trending strike-slip fault (it
matters little whether left or right lateral) extend-
ing to IAP, correlatable with M-VHStr (Della
Vedova and Pellis, 1989) or as still separating the
Adriatic microplate from the African Plate
(Anzidei et al., 1996). M-VHStr is (a) not only a
fault but a horst-like elevation of pre-Messinian
rocks and (b) intersected by SE–NW trending
left-lateral faults.
Active subduction of IAP under the Calabrian
Arc is not well proved by data (Mantovani et al.,
1985, p. 69). The upper part of the Calabrian
Rise consists of chaotic masses (Rossi and Sar-
tori, 1981; Morlotti et al., 1984) which may have
slumped from the Calabrian Arc which under-
went a considerable uplift since Pliocene time
(Ghisetti and Vezzani, 1982). The lower slope of
the Calabrian Rise and its detailed relief are
determined by the secondary increase of the
thickness of evaporites which were squeezed
towards the Ionian foreland. The evaporite
migration caused the diapir-like undulations of
the covering Plio-Quaternary sediments.
The tectonic activities we obtained from the
seismic records of IAP start in late pre-Messinian
times. That means they are mainly contempora-
neous with the opening of the Tyrrhenian Basin.
Therefore, it suggests itself to consider the his-
tory of IAP since late pre-Messinian times in
very close connection with that of the Tyrrhe-
nian. The causes for the opening of the Tyrrhe-
nian have been discussed by Calcagnile et al.
(1981) and Mantovani et al. (1990). Sartori
(1990) has described the tectonic history of the
Tyrrhenian Sea and peri-Tyrrhenian areas based
on the results of ODP Leg 107. From Sartori’s
diction one could get the impression that the rif-
ting in the Tyrrhenian Sea caused the anticlock-
wise rotation of the Apennines (and implicitly of
the Adriatic block). Mantovani et al. (1990) pre-
sented the hypothesis that the opening of the
Tyrrhenian was caused by the anticlockwise rota-
tion of the Adriatic block which was always in
connection with the African Plate. Driving force
for the rotation was the northeastward push of
Africa against Adria.
Malinverno and Ryan (1986) characterized in
their concept of the extension of the Tyrrhenian
Sea the area between the present Malta and Apu-
lian Escarpments as ‘‘deep basin in the Africa-
Adriatic domain’’ (fig. 10). The authors abstain
from identifying that domain as an oceanic
realm, following the arguments of D’Argenio
et al. (1980) for the post-Triassic evolution of the
Tethyan continental margin in the Apennines
and in Sicily-North Africa. Malinverno and Ryan
suggested that the Ionian Sea crust (thinned con-
tinental or possibly oceanic) is involved in the
subduction. This suggestion is based on the con-
siderations of Molnar and Gray (1979) that con-
tinental lithosphere can theoretically be
subducted if it is pulled into the asthenosphere
by oceanic lithosphere coupled to it, and/or if the
continental crust is thin enough.
The model of Malinverno and Ryan (1986)
has some similarity with the suggestion of Leister
et al. (1986) that the Ionian Sea crust is stretched
continental and that a very old passive mar-
gin was subjected to subsidence over large peri-
ods.
We are far from a clear concept of how to
integrate the IAP scenario, which we deduced
from our inventory, into the hypothesis of
Mantovani et al. (1990). However, we believe
that this hypothesis is the right way to find an
explanation for what happened in this relatively
small but very complicated area, instead of those
models which operate with IAP as a remainder
of the Jurassic Tethys.
The Malta Escarpment has been considered
by Carbone et al. (1984) to be rejuvenated and
by Scandone et al. (1981) to be not necessarily
coinciding with a continent–ocean transition. The
correspondence of the orientations of Malta
Escarpment and of internal structures of the wes-
tern part of Medina Ridge is obvious. The east-
ern part of the Medina Ridge is dominated by
the SW–NE orientation which is also characteris-
tic for the subbottom structures of IAP.
M-VHStr demonstrates a direct structural con-
306

nection between Medina Ridge and IAP. Manto-
vani et al. (1997) assume a clockwise rotation
and westward motion of the ‘‘Southern Adriatic
Plate’’ which is allowed by a left lateral shear
zone (Medina Ridge) decoupling the Adriatic/
Northern Ionian area from northnortheast-ward
drifting Africa. This concept is indeed in contrast
to the interpretation of Medina Ridge as a right
lateral wrench zone (Jongsma et al., 1987), but
fits well with our observation of the left lateral
shearing of M-VHStr.
Many questions are still open. For example
what was the stress field which caused the SE
dipping pre-Messinian blocks? Why is IAP char-
acterized by very low heat flow, though the plain
is affected by young faulting and subsidence?
We will leave the stage of hypotheses and
arrive at the stage of a well founded concept for
the history of the central Ionian Sea not by spec-
ulations and extrapolations but only by collecting
more data from the area itself and its neighbour-
hood.
Acknowledgements
We acknowledge the work and help of the cap-
tains, ship’s and scientific crews of the cruises
Meteor 50 (1978), Valdivia 120 (1992), Meteor
25/4 (1993) and Meteor 40/1 (1997). We are also
grateful to K. Hinz and F. Fabricius as well as
Woods Hole Oceanographic Institution for mak-
ing available unpublished data, K. Brodbeck for
computer assistance, J. Hartmann for processing
of seismic data. M.B. Cita and two anonymous
reviewers gave helpful comments.
Notes
1
All water depths without indication are uncor-
rected with regard to the sound velocity.
2
Recorded differences on crossings of Parasound
and older records: Track 513 (Parasound) with
3978 m versus track 910 (Meteor 50, 1978) with
3970 m; track 513 (Parasound) with 3979 m ver-
sus track 1.7 (Valdivia 120, 1992) with 3971 m.
Comparisons between Parasound and Sonne 30
data show that the Sonne 30 depths are 10–11 m
less than those of Parasound. Therefore, it seems
to be justified to correct the depths of cruises
Meteor 17, Meteor 50 and Valdivia 120 by addi-
tion of 8 m and the Sonne 30 depths by addition
of 11 m.
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