Page 7 - Maldonado_Stanley_1976

4 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

canoes (most are submarine cones) increase in the the Strait and associated volcanism in reiation to
northern sector of the Strait. subduction is presented by Akai (1972, fig. 16).
The islands of Linosa and Panteiieria reflect the That volcanic activity, fauit dispiacement, and
importance of Pliocene and Quaternary eruptions sedimentation have been concurrent is clearly of
in this part of the Mediterranean. Panteiieria, importance in this study. These Quaternary neo-
interpreted as a composite stratovolcano, rises from tectonic factors wiii be emphasized in the context
the 1300-m-deep Pantelleria Basin. The position of sedimentary processes and sedimentation rates
of other volcanic deposits, including some which in the Strait region. Physiographic and structurai
accumulated in historic time, are plotted by Zarud- considerations are considered in greater detail in
zki (1972, fig. 3) and Finetti and Morelli (l972a, later sections.
fig. 5); these are concentrated mostly in the north-
ern sector of the Strait. The presence of dike
H YDROGRAPHY
swarms or narrow lava streams are also suggested
on the basis of magnetic anomalies and appear The generai hydrographic and current trends in
aligned paraiiel to the principal tectonic prov- the centrai Mediterranean and Strait of Siciiy re-
inces. Some Mesozoic and early Tertiary intru-
gion are reasonabiy weii known (cf. Lacombe and
sions also have been penetrated by petroleum Tchernia, 1960, 1972; Wiist, 1961.) The dominant
exploratory weiis.
pattern is one of exchange of two weii-defined
In terms of regional Mediterranean-Alpine tec-
water masses, one flowing above the other in op-
tonics, the thick crustal sections of the platform
posite directions. This exchange across the broad,
are considered part of the African Plate, which shallow platform is simiiar to, but not as intense
underthrusts the Euro-Asiatic plate in the Ustica-
as, the one measured in the narrower Strait of
Lipari region of Sicily (Caputo et al., 1970). Gibraltar some 1280 km to the west. The water
Finetti and Morelli (I 972b, fig. 19) al so empha- fl.owing across the Strait of Siciiy must pass over a
size the role of compression but prefer to relate
seri es of sills ( eastern an d western sills a t opposite
plate motion to subduction of the African Plate ends of the Strait) and across a much Ionger and
below what they define as the Mediterranean Plate.
broader area than that at Gibraitar.
Like most geophysicists, these Iatter authors tend
Surface waters, primarily of Atiantic origin,
to agree that much of the Mediterranean-in par-
move toward the east-southeast at velocities of IO to
ticular the deep basins bounding the Strait-has
90 cmjsec (Frassettp, 1972). The lower part of
undergone considerable subsidence since the end of
this water mass extends to depths of 200 to 300 m
the Miocene. Benson (1972) has proposed that the
(depths increase in winter) and has a salinity of
Strait platform was deeper during the Piiocene
approximateiy 37.4%o; temperatures are seasonabiy
than at present. The development of vertical faults
variable (13° to over 23°C). Below this lies the
with offsets to l 000 m in the u p per crust is believed
Intermediate or Levantine water, which originates
to refl.ect isostatic adjustment foiiowing the main
by convective sinking of cooled surface water in
Alpine orogeny. Additionai structural offset may
the Levantine Basin east of the Strait. The Inter-
also be due to alternating phases of compression
mediate water, with an average salinity of 38.7%o
and distension. Zarudzki (1972) relates the gentle
and temperature of about l4°C, flows from the
folding of the more than 300 m of section in the
Ionian Basin across the sill toward the western
northwest end of the Pantelleria Trough, as ob-
Mediterranean. This water mass fills the deep
served in continuous seismic profiles, to the above-
Strait basins and is renewed rapidly as a result of
cited recent, postorogenic tectonic activity. The
the undercurrent (Morel, 1972). The exchange
fauit development, volcanism, and seismicity of
between Surface and Intermediate water is related
this region are not uniike those postuiated in some
to the higher evaporation rates in the eastern
subduction modeis (tension and rifting behind the
leading edge of a subducted piate margin, cf. Isacks Mediterranean which entrain the less saline waters
et al., 1968, fig. 7; Ninkovich and Hays, 1972, fig. as a replacement (outgoing versus ingoing flux
12, and others). An interpretative diagram show- calculations are provided by Morei, 1972). Deep
ing the origin of this modern rift-tension reiief in waters in the Ionian and Balearic basins are