GEOLOGICAL HISTORY OF THE CENTRAL MEDITERRANEAN BASIN: AN OUTLINE OF CENOZOIC EVENTS

Geologic features in the present-day Mediterranean essentially result from two major processes: the tectonic displacement caused by the subduction of the African plate underneath the Eurasian plate; and the progressive closure of the Mediterranean sea involving a series of submarine-insular sills.

The development of the Mediterranean basin begins with the breakup of the supercontinent Pangea in the Mesozoic. During this time, sea-floor spreading triggered the development of the Atlantic ocean in the Triassic period, which separated the African and Eurasian plates from the North American plate. Sea-floor spreading in another geographical location caused the development of the Tethys ocean, separating the African plate from the Eurasian. In the late Cretaceous period, these African and Eurasion plates began to converge, closing the Tethys ocean basin, and the remnants of this ancient ocean are now called the Mediterranean sea.

There are three major geomorphical settings within the Mediterranean basin; areas with stable margin characteristics, areas with unstable convergent margin charactericstics, and areas with extensional margin (rifting) characteristics. Thus the Mediterranean basin is a location of an intercontinental interplate system; with compressional and extensional events occurring within close proximity. Geologists have yet to come to a consensus about which plates in addition to the African and Eurasian ones, if any, are involved in Mediterranean tectonics. Subsidence-related and other vertical displacements are also found in compressional and extensional areas. A few notable events occurred during the Cenozoic which affected the entire Mediterranean; the Messinian "salinity crisis", when the closing off of the Mediterranean-Atlantic seaway caused complete isolation of the Mediterranean and thus widespread evaporation; and then the Pliocene "revolution", when the channel opened back up, causing reestablishment of marine conditions; and the Quaternary "transgressive raised terraces," of controversial geological origin; among others.

The Central portion of the Mediterranean basin exemplifies the juxtaposition of compressional and extensional tectonic activity in the area. The region bordered to the west by Sicily and to the east by Turkey's west coast (encompassing the Aegean, Ionian, and Adriatic seas) exhibit a particular set of features and will be the focus of my study.

There were four major periods of extension in this area. The first one occurred in the Mid-Upper Jurassic; evidence of this phase is seen in the Strepanosa Trough and Ionian plain. A second one occurred in the Mid-Late Triassic, opening up the Ionian sea and the Eastern Mediterranean. A third extensional phase occurred in the Mid-Upper Cretaceous, as evidenced by the stretched features of the Sirte Rise, a monocline with normal faults and tilted blocks. The fourth one, occurring in the Mid-Upper Miocene through to the Quaternary period, affected many areas of the Central Mediterranean. This extensional phase is closely associated with compressive motions; it is part of the reason for a counter-clockwise rotation of the Southern Appennine area which begins in the upper Cretaceous. All four of these extensional phases are the cause of geologic features found in the area, such as volcanic activity and rift-related sedimentary processes. Due to such extension, the oceanic crusts of the Central Mediterranean are considerably thinned in some places.

The Mediterranean Ridge or Outer Median Ridge is a sea-floor feature that marks the ustable (convergent) margin between two or more oceanic plates (geologists know that the African and Eurasian plates are involved, but which, if any, smaller plates are involved is a matter of debate). The first stages of the major collision between the North of the African plate and the South of the Eurasion plate is believed to have occurred in the lower-middle Miocene. This collision is also associated with the counter-clockwise rotation of the Appennine area, and both of these associations are exhibited in the Calabrian (Italy & Sicily) and Hellenic (Greece) orogenic arcs which are situated among both compressive and extensional dynamics. The ridge extends geographically from Sicily to Cyprus along a generally E/W strike. It is an extensive fold-fault system corresponding to recent uplift and folding of past abyssal plains.

The features in the Adriatic sea are results of this duality of compression as well as extension, and also from deposition-related subsidence on a deeply foundered foreland (on the shelf). The Sicily Channel Rift area is an example of the Miocene-Quaternary extensional phase. The Adriatic sea itself is relatively shallow, and almost all of the ocean floor (a thick carbonitic platform underlain by continental crust) exhibits compressional deformation structures, except for the Ionian Abyssal Plain, which is thought to be underlain by Paleoceanic crust. The history of the Alpine orogeny, constituting the northwestern portion of the Adriatic, really begins in the Mesozoic as well, for the sedimenary strata which constitutes most of its orogenic elements weas laid down in the continental margins of the ancient Tethys ocean. The Alpine orogeny and the Calabrian arc orogeny are both results of convergent plate margin movement between Africa and Europe, and display some vertical uplift associated with the subsidence of Mediterranean sea-floor deposits during the Cenozoic.

The Ionian sea perhaps experiences the major amounts of subsidence in the Central Mediterranean. The Ionian Abyssal Plain in this region is characterized by differentially sudbsiding areas but generally experiences more than adjacent regions, contributing greatly to the uplift associated with the Alpine orogeny and the Quaternary coastal blocks. The Hellenic trench (a thrust fault linked to the convergent activity in the Mediterranean ridge) began propagation in Miocene and continues today; it constitutes a major element of Ionian seafloor topography. The extensional features in the Ionian region are somewhat subdued, the dominant tectonic activity is convergent and/or related to vertical movement.

The Aegean sea experiences considerable amounts of extensional features as well, related to the suduction of the African plate underneath the Hellenic Arc. Subsidence in the late Miocene also had a grand affect on the region, resulting in the fragmentation of an Aegean landmass from vertical displacement. Extension in the Hellenic arc area runs generally N/S, and crustal shortening forms an E/W insular platform. Here the oceanic crust is thinned to almost 1/2 its original thickness The counter-clockwise motion is further expressed in the area by transcurrent faulting in the Northern Aegean, beginning in the fourth extensional phase of the Mid-Upper Miocene. The outer regions of the Hellenic zones, by conrtast, exhibit compressive geology.

All of the volcanic activity in the Central Mediterranean is related to one or more of these processes; subduction, back-arc extension, and/or other tectonic events throughout the Cenozoic.

OTHER LINKS OF INTEREST:

Volcanoes of Italy

Eolian and Aeolian island arcs

geothermal activity in Italian volcanoes

BIBLIOGRAPHY & WEB SOURCES

Image of tectonic map from web adress: http://geothermal.marin.org/map/tect_map.gif

Stanley & Wezel, Geological Evolution of the Mediterranean Basin, Springer-Verlag,1985

Berckhemer & Hsu, ed; Alpine-Mediterranean Geodynamics, American Geophysical Union,1982

Higgins & Higgins, A Geological Companion to Greece and the Aegean, Cornell University Press, 1996