MARINE GEOLOGY AND GEOPHYSICS
Tectonic setting
Intense tectonic compression between
Africa and Europe has broken the European tectonic plate into a
number of smaller segments known as microplates. The dominant
collisional force is directed towards the NNE by Africa. Each
microplate responds with movement in a variety of directions (Figure
1). The largest of these, the Aegean microplate, is heading
SSW, exactly opposite to the incoming African plate. This push
for the Aegean microplate is a consequence of, first, a push from
the north by the westward-moving Anatolian microplate, and, second,
from a mechanism known as “roll-back”. The former occurs due
to the Levantine microplate moving northerly thus forcing the
Anatolian microplate aside to the west. The latter, “roll back”,
describes crustal plates that, at the surface, are stretched and
move exactly opposite to plate closure resulting in extension and
foundering of the crust overlying the subduction zone (thus the
formation of the Aegean Sea some 1.8 million years ago). This
increases closure rates between the continents and magnifies
compressional forces: Africa is moving north at about 1 cm/year; the
Aegean is moving south at about 4 cm/year. The closure rate is
on the order of 5 cm/year.
The consequence is a highly deformed and disturbed Mediterranean sea floor (Figures 2 and 3). Caught in the tectonic squeeze, thick accumulations of soft sediments are compressed, folded, and reshaped. This is reflected in the ridges, hills, hummocks, troughs, and basins that characterize the eastern Mediterranean sea floor and its margins. Deep burial of water-rich sediment, combined with the tectonic crush, squeezes sediment leading to the upward migration of both fluids and mud. If the sediment contains relict organic material that has undergone partial conversion to hydrocarbons, methane gas rises to the surface as well. The results are mud volcanoes and methane seeps on the sea floor; frozen methane (clathrates) may also form in the sediment.
During the latest Miocene epoch (five million years ago) the Mediterranean dried-up after, first, the Levantine area had been uplifted to disconnect the sea from its eastern connection to the global ocean, then, later, a narrow seaway across southern Spain was uplifted to disconnect the sea from the Atlantic Ocean. The consequence was evaporation of seawater, dessication, and the accumulation of a thick layer of salts. This layer forms a slippery surface deep beneath the few kilometers of sediment that have accumulated since the Miocene, facilitating sediment deformation and movement. In addition, tectonism and deep burial have mobilized the salt and its fluids to rise and form diapiric hills as well as brine seeps and pools.
Uplift of the sea floor produces slopes that exceed angles of stability for water-rich sediments. This results in slumps, landslides, and avalanches of muds as turbidity currents, all of which reshape the sea floor through erosion and redeposition of enormous volumes of sediment. In an active seismic zone like the Aegean island arc, remobilization events are common. There is also evidence for redeposition of abyssal sediments by the passage of tsunami.
Maps created by side-scan sonar surveys provide dramatic images of sea floor features created by this variety of geological processes. Large segments of the eastern Mediterranean sea floor are outlined that are almost in themselves microplates – tectonic blocks - caught in the tectonic compression and its consequences. It is these features that were surveyed in this, and other, studies.
Survey area
A large plateau occurs on the eastern Mediterranean sea floor south
of central and eastern Crete (Figure 3 and 4). The western
portion is known as the Ptolemy Mountains; the larger eastern
portion, as yet unnamed, was the survey area for this study.
The plateau is bordered to the north by the Ptolemy Trench (trough,
more properly), to the south by the Pliny Trench (trough, more
properly), and to the east by the Cretan-Rhodos Ridge.
This plateau represents a tectonic block created by the African-Aegean plate convergence. Its boundaries are active faults: the northern boundary (Ptolemy trench/trough) is a left-lateral strike-slip fault extending onto Crete as the spectacular Ierapetra fault (with significant normal-fault motion on land); the southern boundary (Pliny trench/trough) is also a left-lateral strike-slip fault that continues to the north between Crete and Kasos islands to form an eastern boundary; the western boundary is a series of normal faults.
Both strike-slip faults are the consequence of the compressional stress regime created by the African-Aegean convergence. The normal fault to the west represents crustal breakage due to differential movement along both strike-slip faults.
The result in the survey area is an abyssal physiography of NW-SE ridges along the southern area of the plateau, a prominent seamount in the central area, an insular rise to the north containing Galdhouronisi Island. The central seamount is partially surrounded by a semi-circular depression resembling a moat. It is difficult to relate these physiographic features to tectonic factors without additional geophysical data. Minimum depth, 488 m, occurs on the central seamount. Maximum depths occur in the flanking trenches/troughs, 2617 m (north) and 3950 m (south).
The southern survey zone was positioned over the central seamount.
The northern survey zone focused on the insular margin south of Galdhouronisi Island. The seafloor here is dissected by a series of submarine canyons with a steep escarpment in the western portion of the survey area. Shallowest depths here are 460 m; deepest depths are 740 m. In general, slopes are considerably steeper than in the northern survey area.
Survey Techniques and
Equipment
Previous bathymetric and swath mapping surveys for the eastern
Mediterranean Sea were incomplete over this area. Detailed
SWATH bathymetric surveys in the survey area were previously done by
HCMR, as part of Project Hermes – South Cretan Margin (V. Likousis,
Chief Scientist), using the SeaBeam 2120 system (20 kHz) (Figure 3).
This mapping provided details not seen in regional side-scan surveys
done previously (Figure 2). Data from the DANAOS cruise 2007
has contributed to the South Cretan Margin survey particularly for
the area where bathymetric data were lacking.
Track lines for side-scan surveys were constructed for the northern and southern zones of the survey area that were 300 m apart, oriented east-west, with swath widths of 150 m (at 100 kHz) (Figure 4). Concurrently, 3.5 kHz profiles were collected for inferences on bottom characteristics (sediment types, rock outcrops, subsurface reflectors, etc.). Acoustic reflections that presented good prospective targets for archaeological investigation were identified, then selected for further investigation by remotely operated vehicles (Max Rover ROV) or manned submersible (Thetis).
Excellent weather conditions allowed uninterrupted surveys except for high winds during the last few days. This allowed virtually uninterrupted surveys and submersible dives – the excellent quality of the data reflects this and the professionalism of the HCMR scientists and crew.
Dr. Floyd McCoy, Marine Geologist, Dept. of Natural Sciences, University of Hawaii – Windward
