Table Of ContentRecent Evolution and Seismicity of
the Mediterranean Region
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Series C: Mathematical and Physical Sciences - Vol. 402
Recent Evolution and
Seismicity of
the Mediterranean Region
edited by
E. Boschi
Istituto Nazionale di Geofisica,
Rome, Italy
E. Mantovani
Dipartimento di Scienze della Terra,
Universita di Siena,
Siena, Italy
and
A. Morelli
Istituto Nazionale di Geofisica,
Rome, Italy
Springer-Science+Susiness Media, SV.
Proceedings of the NATO Advanced Research Workshop on
Recent Evolution and Seismicity of the Mediterranean Region
Erice, Italy
September 18-27,1992
Library of Congress Catatoging-in-Publication Data
Recent evolutl0n and selsmlclty of the Medlterranean reglon ! edited
by E. Bosch 1, E. Mantovan 1, A. More 111.
p. cm. -- (NATO ASI serles. Serles C, Mathematlcal and
physlcal SClences ; voi. 402)
ISBN 978-94-010-4891-0 ISBN 978-94-011-2016-6 (eBook)
DOI 10.1007/978-94-011-2016-6
1. Geology, Structural--Medlterranean Reglon--Congresses.
2. Geodynamlcs--Congresses. 3. Earthquakes--Medlterranean Regl0n-
-Congresses. 1. Bosch i, E. II. Mantovan 1, E. III. More 111, A.
IV. Series NATO ASI serles. Serles C, Mathematlcal and physlcal
sClences ; no. 402.
QE633.M43R43 1993 93-19532
ISBN 978-94-010-4891-0
Printed on acid-free paper
AII Rights Reserved
© 1993 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1993
Softcover reprint of the hardcover 1s t edition 1993
No part of the material protected by this copyright notice may be reproduced or
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TABLE OF CONTENTS
Introduction vti
List of Participants xiti
AM.C. ~engor, Some current problems on the tectonic evolution of
the Mediterranean during the Cainozoic 1
J. Jackson, Rates of active deformation in the Eastern Mediterranean 53
E. Mantovani, D. Albarello, D. Babbucci and C. Tamburelli, Post Tortonian
deformation pattern in the Central Mediterranean: a result of extrusion
tectonics driven by the Africa-Eurasia convergence 65
D. Albarello, E. Mantovani, D. Babbucci and C. Tamburelli, Africa-Eurasia
kinematics in the Mediterranean: an alternative hypothesis 105
M. Barazangi, D. Seber, T. Chaimov, J. Best, R. ~itak, D. Al-Saad and T. Sawaf,
Tectonic evolution of the Northern Arabian Plate in Western Syria 117
J.L. Mercier, D. Sorel, S. Lalechos and B. Keraudren, The tectonic regimes
along the convergent border of the Aegean Arc from the late Miocene
to the present; southern Peloponnesus as an example 141
E. Patacca, R. Sartori and P. Scandone, Tyrrhenian basin and Apennines.
Kinematic evolution and related dynamic constraints 161
A Argnani, Neogene basins in the Strait of Sicily (Central Mediterranean):
tectonic settings and geodynamic implications 173
A Morelli, S. Mazza, N.A Pino and E. Boschi, Seismological studies of upper
mantle structure below the Mediterranean with a regional seismograph network 189
D. Giardini, B. Palombo and E. Boschi, The determination of earthquake size
and source geometry in the Mediterranean Sea 213
G.A Papadopoulos, Tectonic and seismic processes of various space and time
scales in the Greek area 239
vi
D. Slejko, A review of the Eastern Alps - Northern Dinarides seismotectonics 251
A. Vdias and E. Bufom, Regional stresses in the Mediterranean region derived
from focal mechanisms of earthquakes 261
J. Drakopoulos and G.N. Stavrakakis, Source process of some large earthquakes
in Greece and its tectonic implication 269
M. Cocco, G. Selvaggi, M. Di Bona and A. Basili, Recent seismic activity and
earthquake occurrence along the Apennines 295
G.B. Cimini and A. Amato, P-wave teleseismic tomography contribution to the
delineation of the upper mantle structure of Italy 313
P. Favali, R. Funiciello and F. Salvini, Geological and seismological evidence of
strike-slip displacement along the E-W Adriatic-Central Apennines belt 333
M. Dragoni, Stable fault sliding and earthquake nucleation 347
E. Marton, Paleomagnetism in the Mediterraneum from Spain to the Aegean:
a review of data relevant to Cenozoic movements 367
G. Scalera, P. Favali and F. Florindo, Use of the Paleomagnetic databases for
geodynamical studies: some examples from the Mediterranean Region 403
Introduction
The Mediterranean is one of the most studied regions of the world. In spite of
this, a considerable spread of opinions exists about the geodynamic evolution and
the present tectonic setting of this zone. The difficulty in recognizing the driving
mechanisms of deformation is due to a large extent to the complex distribution in
space and time of tectonic events, to the high number of parameters involved in
this problem and to the scarce possibility of carrying out quantitative estimates
of the deformation implied by the various geodynamic hypotheses. However, we
think that a great deal of the present ambiguity could be removed if there were
more frequent and open discussions among the scientists who are working on this
problem.
The meeting of ERICE was organized to provide an opportunity in this sense. In
making this effort, we were prompted by the conviction that each step towards the
understanding of the Mediterranean evolution is of basic importance both for its
scientific consequences and for the possible implications for society. It is well known,
for instance, that the knowledge of ongoing tectonic processes in a given region and
of their connection with seismic activity may lead to the recognition of middle-
long term precursors of strong earthquakes. The few cases of tentative earthquake
prediction in the world occurred where information on large scale seismotectonic
behavior was available. This led to identify the zones prone to dangerous shocks,
where observations of short-term earthquake precursors were then concentrated.
The choice of participants to the ERICE meeting was made so as to set up a
multidisciplinary group of experts in many fields of earth sciences, able to discuss
in detail all types of possible implications of the proposed geodynamic hypotheses.
Presentations and discussions were mainly focused on some crucial problems, with
particular reference to the Africa-Eurasia relative motion in the Mediterranean and
to geodynamic processes and structural-seismotectonic features in the peri-Adriatic
and eastern Mediterranean regions.
As concerns the first problem, the main object of debate was the direction of the
Africa- Eurasia convergence. So far, this information has been determined through
the analysis of kinematic indicators in the North Atlantic. By these data it is
possible to estimate the Africa-North America and Eurasia-North America relative
motions from which the Africa- Eurasia motion can be determined. The results
obtained by this type of approach suggest that Africa has moved N to NW with
respect to Eurasia during the last 5-10 My. This hypothesis, however, encounters
some difficulties in explaining the post-Tortonian deformation pattern in the Cen-
tral Mediterranean. Furthermore, it is not easily reconcilable with reconstructions
of previous (pre-Tortonian) Africa-Eurasia kinematics,which provide a NEward con-
vergence between these two blocks. This implies that around 9-10 My the drifting
trend of Africa must have undergone a change of roughly 90°, passing from NE to
NW.
VII
viii
The above difficulties led some authors to check the reliability of the assump-
tions which were used in previous analyses of North Atlantic data. One crucial
assumption is the internal coherence of the Africa and Eurasia blocks. This hy-
pothesis cannot easily be reconciled with the occurrence of significant deformation
and seismic activity in the region comprised between the Rhine Graben system
and the Azores Gibraltar belt. If this intraplate deformation is not negligible, the
kinematic indicators observed in the Atlantic ridges just north of the Azores and
along the Azores-Gibraltar belt cannot be straightforwardly used to constrain the
Eurasia-North America and Africa-Eurasia relative motions. This may have impor-
tant consequences, especially if one considers that the other kinematic data in the
North Atlantic can be reconciled with Africa-Eurasia relative motions significantly
different from the ones previously obtained. This would mean, for instance, that a
NE to NNEward direction of the Africa-Eurasia convergence, which seems to best
account for the post-Tortonian deformation pattern in the Central Mediterranean,
is not necessarily in contrast with North Atlantic data (see, Albarello et al.).
However, if one decides to explore this last kinematic hypothesis, there remains
the problem of understanding the tectonic mechanisms through which part of the
Atlantic spreading is absorbed by deformation in the western European region and
the adjacent oceanic area. Moderate seismicity affects the Rhine Graben system and
the Pyrenees, while stronger activity is observed in the southern part of Iberia and
in the oceanic zone offshore of Portugal and Morocco. Is this deformation sufficient
to absorb part of the spreading which develops in the Mid Atlantic ridge lying
just north of the Azores? Finding an answer to this question may have important
consequences for understanding Africa-Eurasia kinematics in the Mediterranean
area, while we are waiting for the time when geodetic measurements will help to
solve this basic problem.
Another argument which attracted attention and triggered lively discussion is
the formation of the Tyrrhenian basin and the structural evolution of the other
peri-Adriatic regions (see, Argnani, Mantovani et al., Mercier et al., Patacca et al.,
Sengor, Slejko). The main difficulty in explaining these tectonic events is due to
the fact that the framework of evidence does not fit satisfactorily with any simple
known geodynamic process. Some years ago, the formation of the Tyrrhenian basin
was generally explained as a consequence of oceanization processes connected with
mantle plumes or as back-arc spreading in response to the active subduction of the
Ionian lithosphere beneath the Calabrian Arc. The difficulties that these models
encountered in accounting for some major features led many authors to look for al-
ternative interpretations. This search has been considerably influenced by the fact
that a much better knowledge is now available on the structural-tectonic evolution
,.f the Tyrrhenian-Apennines system. In particular, an important constraint on
the geodynamic evolution of the whole central Mediterranean region is that crustal
stretching in the Tyrrhenian basin and major deformations in the adjacent thrust
belt-foredeep Apenninic system have occurred in three main phases, well differenti-
ix
ated in space and time. Another piece of evidence which may have very important
implications for the understanding of the rheological behavior of the lithosphere-
asthenosphere system in the region considered, is that a large part of the continental
Adriatic foreland has disappeared, since the upper Miocene, beneath the Apenninic
chain (see, Patacca et al.).
During the meeting, the discussion about the driving mechanism of the Central
Mediterranean deformation pattern mainly concerned two kinds of mechanism. One
hypothesizes that the Tyrrhenian opening was caused by the gravitational sinking
of the Adriatic-Ionian subducted margin, which was responsible for the progres-
sive E-SEward migration of the rift-thrust belt-foredeep system in the Tyrrhenian-
Apenninic region (see, Patacca et al.).
The other model (see, Mantovani et al.) suggests that the retreat of the Adriatic-
Ionian foreland and the outward migration of the Apenninic belt were the effects
of the compressional regime which affected the central Mediterranean region in
connection with a NNE to NEward convergence between Africa and Eurasia. The
great amount of light crustal material belonging to the Alpine and Apenninic oro-
genic belts, which occupied the future Tyrrhenian region around the late Miocene,
reacted to this compressional framework through a lateral extrusion towards the
Adriatic foreland. This collisional pattern caused the scraping off of the Adriatic
Mesozoic platform and the accumulation of thrust units along the external front
of the Apennines. Once decoupled from its buoyant crustal material, the Adriatic
lithosphere offered much less resistance to sinking and underwent downward bend-
ing in response to two possible dynamic causes: negative buoyancy with respect
to the surrounding mantle and horizontal compressional forces connected with the
Africa-Adriatic convergence. Afterwards, some role in driving the roll- back mech-
anism might have also been played by the pull of the downgoing slab.
As concerns the eastern Mediterranean area, the debate mainly centered on
whether the SoN extension in the Aegean zone has been closely connected with
the westward extrusion of Anatolia from the Arabia-Eurasia collisional border (see,
Jackson, Papadopoulos, ~engor). This geodynamic hypothesis seems to be the
most likely, on the basis of the available evidence but some outstanding problems
still exist. In particular, it appears opportune to check the geological information
which indicate that the Aegean extension started before the westward migration of
Anatolia.
Considerable ambiguity still surrounds the starting time of the ongoing subduc-
tion process beneath the Hellenic trench. Some authors suggested a recent acti-
vation of this mechanism, 5-6 My ago, whereas other geodynamic reconstructions
hypothesize a Miocenic starting time, 13-16 My ago. The huge amount of sub-
ducted material beneath the Aegean zone identified by tomographic investigations
could imply an even longer duration of the subduction process.
Another peculiar feature whose geodynamic implications should be better under-
stood is the noticeable difference between the convergence rate (at least 6 cm/yr)
x
predicted along the Africa-Aegean boundary (Hellenic trench) by current kinematic
models and the seismic energy release, which can only account for a relative mo-
tion slower than 1 cm/yr (see, Jackson). This discrepancy has been tentatively
explained as due to the occurrence of aseismic slip in the Hellenic trench, but other
explanations should be explored. In particular, it could be useful to verify the
reliability of the assumption that all extension in the Aegean region is taken up
along the Hellenic trench. For example, a Northward drifting of the Balkan system
with respect to stable Eurasia, driven by the combined pushes of the Adriatic and
Anatolian blocks, might playa non-negligible role in determining extension in the
North Aegean trough.
More generally, it becomes ever clearer that understanding the tectonic pro-
cesses which are taking place in the Mediterranean region can give very important
insights into the structural and tectonic evolution of continental collision zones. Pa-
leomagnetic measurements would represent precious indications on the past block
kinematics, to be compared with other pieces of evidence. Unfortunately, however,
the studies and interpretations of this type of data in some Mediterranean zones
have often provided different and even contrasting results. A review of the presently
available observations, accompanied by a description of the main possible causes of
uncertainty, is included in this volume (see Marton, Scalera et al.).
A fundamental contribution to the growth of knowledge about the structure and
kinematics of the complex Mediterranean area comes from seismological studies.
The area is characterized by strong seismic activity which we need to understand
better in order to improve our view of the active tectonics. At the same time, the
occurrence of earthquakes also provides invaluable information about the structure
of the area. A general scheme of the regional stress pattern can be delineated by
using available source mechanisms for the largest earthquakes (see Udias et al.).
More particular studies are carried out to better characterize several key areas,
such as the Hellenic subduction trench (see Jackson), or the northern border of the
Adriatic (see Slejko), Apenninic structures (see Cocco et al.), and the Arabian plate
(see Barazangi et al.).
Source mechanisms computed from the distribution of polarity of first motion
may be affected by errors due to unknown calibration of stations if bulletin reports
are used. The discussion stressed the importance of referring to the original wave-
form data. Data availability then becomes an important issue. For strong events,
well recorded worldwide, long period analyses allow very reliable reconstruction of
the source mechanisms, whereas broadband studies can image in detail the time
evolution of the earthquake process (see Drakopoulos et al.). In the Mediterranean
region, however, most of the interesting earthquakes are of moderate size, a fact
which calls for the presence of local stations and modification of the computation
algorithms in order to deal with events at regional distances (see Giardini et al.).
A detailed experimental knowledge of source processes is also needed to test theo-
retical models of fault slip and earthquake nucleation (see Dragoni).
