- Email: [email protected]
Accretionary processes at subduction zones in the eastern mediterranean
Tecronophysics, 112 (1985) 551-561 Elsevier Science Publishers
551
B.V., Amsterdam
- Printed
in The Netherlands
ACCRETIONARY PROCESSES AT SUBDUCTION ZONES IN THE EASTERN MEDITERRANEAN
YAIR
’ and ZVI BEN-AVRAHAM
ROTSTEIN
2
’ Institute {or Petroleum Research and Geophysics, P. 0. Box I 717, Holon 581 I7 (Israel) ’ Department of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69976 (Israel) (Revised
version received
September
13, 1984)
ABSTRACT
Rotstein,
Y. and
Ben-Avraham,
Mediterranean.
The tectonic the collision exist today
present
history
of the eastern
and accretion
of oceanic
in the eastern
the plate
complexity
Mediterranean plateaus
continental subduction trenches. western subduction
at subduction
Structures
zones
and Processes
Mediterranean
boundary,
particularly
of this subduction of a large
in the Hellenic Subduction parts.
Inner
plateau
in the eastern in Subduction
Many
other
arc area. Their presence by the seismicity
and Cyprean
an older
style are affected
results
and bathymetry
arcs, is probably
arc. This plateau
has been
arc takes place in the outer
segments
lithosphere
of these
outward
trenches
following
trenches,
particularly
in the Pliny and Ptolemy served
the collision
as the main of oceanic
plateaus
oceanic
plateaus
in the unusual patterns.
thrusted
subduction
The of the
over the
the two arcs. Active
in the Ionian trenches
by
oceanic
the consequence
and pieces of it are to be found in the region between
of Mediterranean
zones migrated
with
tectonic
zones. Small but distinct
away from the plate boundary. in the Hellenic
zone, as is evidenced
continental
mass of Anatolia
as well as the present
at subduction
two arcs system in this region, the Hellenic
collision
processes
and IS. Sacks (Editors),
Tecronophysics, 112: 551-561.
Zones.
crowd
Z., 1985. Accretionary
In: K. Kobayashi
and Strabo
is limited trenches
to their
before
the
plateaus.
INTRODUCTION
The eastern Mediterranean is a small ocean basin known for its unusual’ tectonic complexity. It includes a short segment of the convergence boundary between Africa and Eurasia. Subduction in this segment is along two very small arcs, the Hellenic and Cyprean arcs. In both arcs subduction has been documented using bathymetric, earthquake and other data (e.g. Rabinowitz and Ryan, 1970; McKenzie, 1970, 1972; Papazachos, 1973). The Hellenic arc is associated with backarc basin and volcanism, while the Cyprean arc is not. The bathymetric and seismic patterns in the eastern 0040-1951/85/$03.30
0 1985 Elsevier Science Publishers
B.V.
552
Mediterranean
seem to be significantly
type” subduction
zones. This apparent
models for this region (e.g. McKenzie, Pichon
and Angelier,
of agreement discuss
1979; Rotstein
on even
the tectonic
the geometry
framework
more complex complexity
than in most simple “pacific
has resulted
in numerous
1972, 1978; Nur and Ben-Avraham, and Kafka,
1982). These models express a lack
of the plate
of this region
tectonic 1978; Le
boundary.
try to present
Most
papers
a model
which
which
is
consistent with first-order observations. To date, relatively little effort has been made to propose models which are consistent with second-order observations in order to explain the complex bathymetric and seismic patterns. In this paper, we briefly describe some of the unusual but well-known characteristics of the bathymetric and seismic patterns in the eastern Mediterranean. We then proceed to propose a tectonic model in which the collision and accretion of oceanic plateaus are important factors which, together with subduction, explain these patterns. The occurrence of collision and accretion of oceanic plateaus in the eastern Mediterranean may be of special importance to the overall understanding of these processes. This area consists of small and accessible basins which appear to include a large number of oceanic plateaus in various stages of maturation; in addition, both these basins and the landmasses surrounding them are well known. Thus, the eastern Mediterranean may prove to be an area which is well suited for studying this process in general and the effects of various boundary in particular.
size oceanic
plateaus
on the shape of the plate
BATHYMETRY
Virtually all subduction arcs are characterized by a subduction trench, a single and mostly continuous ocean deep which marks the flexure of the descending oceanic lithosphere. In the eastern Mediterranean, however, the situation is markedly different. Subduction has been documented in this region, Hellenic arc, yet a single continuous trench cannot be traced.
particularly in the Figure 1 shows the
main bathymetric features in the eastern Mediterranean. The Hellenic arc is characterized by multiple, parallel and en echelon ocean deeps which follow the arc; in addition, numerous short deeps which are perpendicular to the trend of the arc are also present. Separate elevated blocks appear to be the elements causing the fragmentation of the longer deeps parallel to the arc. Generally, in the Hellenic arc, instead of one long subduction trench there are several deeps which could, probably, qualify as subduction trenches. This nontypical pattern is, probably, tectonically related and warrants an explanation. Several other bathymetric observations are of interest, For example, the different trenches are not equal in depth; in fact the outer, Strabo, trench is but a cleft in the ocean floor (Jongsma, 1977), whereas the one next to it, the Pliny trench, is much more distinct. Equally important may be the observation of a halo of small and, in places, deep
553
basins north of the subduction be argued
that
equivalent
to backarc
zone of the eastern
some of these, particularly basins.
The deep Finike
other hand, are much more difficult Anatolia
landmass,
Mediterranean.
the shallow
to account
It could possibly
ones around
and Rhodes
basins
Cyprus.
(Fig.
for. The Finike basin. adjacent
is an E--W trench which is as deep as the Mediterranean
plane;
however. in spite of its depth, it is not in the subduction
north
of a large elevated
block,
the Anaximander
Seamount
are
1). on the to the abyssal
zone since it appears and
since
it is not
associated with deep seismicity. Somewhat similarly, the Rhodes basin is a deep basin, even deeper than the abyssal plane, adjacent to the Anatolia landmass: moreover. it is characterized by a distinct, positive Bouger gravity anomaly (Woodside, 1976) which is similar to the gravity signature of the central parts of the eastern Mediterranean. Finally, the eastern Mediterranean includes a large number of elevated terranes particularly in the Ionian basin and the Hellenic arc complex. Some of the more distinct elevated terranes in the Ionian basin are the Medina Ridge, Medina Bank and the Cyrene, Epicharmos and Archimedes seamounts. Other prominent elevated terranes also appear within the Hellenic arc complex; however, whereas in the Ionian basin the elevated blocks are spread throughout the basin, in the Hellenic arc a large number of blocks crowds a small area causing the complex pattern of bathymetric
highs and lows.
SEISMIC’ITY
Classical “pacific type” subduction ping seismic zone. Earthquakes in the have been thought to be in the form 1973). Figure 2 is an example of a subduction in the Hellenic arc. It is cross-section distribution
that,
although
of earthquakes
is characterized by a Benioff-Waddati dipHellenic arc reach a depth of over 160 km and of a dipping seismic zone (e.g. Papazachos, seismic cross-section which is used to infer clear from the distribution of events in this
a dipping
seismic
zone can indeed
is not a simple one. Normally,
be followed,
the dipping
the
seismic zone
is only several tens of kilometers thick, since it represents processes in the rigid upper part of the lithosphere. In the Hellenic arc, on the other hand, the zone of subcrustal
earthquakes
is unusually
thick, reaching
a thickness
of about
200 km,
which is over twice the estimated thickness of the lithosphere in this area (Papazachos, 1969). Such an observation raises the question of whether a simple two plate model can be applied in this area, The inconsistency of the simple plate model with the seismic data was noted earlier by Galanapoulus (1973) and has recently been investigated by several other workers (Leydecker et al., 1978; Richter and Strobach, 1978; Comninakis and Papazachos, 1980). They have re-analyzed the available data in an effort to determine whether the apparent complexity in the seismicity pattern is the result of poor data quality, particularly poor depth control. Their results generally support the observation that some kind of dipping seismic zone does exist
555
w*st km 410 0
CI
400 .I . . ..*
mm
250
300
350
A?C
SW 200
100
I50
0
50
SO
YeditrrronWll
*ll*nie
SodimWtlary ArC
VOICOtliC
turlwy Awwn
SW
Trmch IS0
IO0
200
300
250
0 IO0 -
Aatheiwsphwe
Asthmomphwo 2110 -
Fig. 2. Cross-section Papazachos, denote
intermediate
intermediate between
showing
the pattern
of deep focus earthquakes
1973). The plot is of focal depths against earthquakes
earthquakes
the distance
with M > 4.9 which occurred
with 4.5 6 M 4 4.8 and
shallow
between
earthquakes
1965 and 1971. Vertical and lateral errors were estimated,
km for data prior
across
the Hellenic
from a line following
1911 and 1971 and dots denote with M > 4.5, which
by the original
to 1965 and up to 25 km for more recent data.
arc (following
the arc. Triangles
See original
compiler, paper
occurred
to be up to 40
for details of data
sources.
20° 22' 24O 26* 28" 30° 32" 34O 36O 38O 4o" 42O 38O 4o" 36O 38'
36' 32O 34O 3o" 32' cl
3o"
5.0-
53
A
4.5-4.9
A
I;;,“,:
\ \
\1\
c
18O Fig. 3. Epicenters (following (hatched).
20°
of intermediate
Papazachos, Lateral
22O
24O and
1973) indicating
26*
deep focus
28O
earthquakes
a gap of seismicity
and vertical errors were estimated,
30° in the eastern
between
by the original
prior to 1965 and up to 25 km for more recent data. See original
32O
paper
Mediterranean
the Hellenic
compiler,
34O
and Cyprean
region arcs
to be up to 40 km for data
for details of sources.
556
in the Hellenic Particularly
arc, but that this zone is unusually interesting
lar to the Hellenic
arc which are presented
show that the dipping it is also discontinuous
wide and complex.
in this regard are the seismicity by Richter
cross-sections and Strobach
perpendicu(1978). These
seismic layer is not only thick and complex but that, in places, with a large and distinct
and lower parts of the dipping
seismic gap separating
seismic layer. A similar phenomenon
the upper
was observed
in
the northwestern corner of the Cyprean arc where it was assumed to be the result of collision and accretion processes (Rotstein and Kafka, 1982). Another observation which adds to the difficulty in using a simple subduction model in this area, is the obvious lateral discontinuity in the earthquake distribution. For example, in the Hellenic arc deep focus earthquakes extend eastwards as far as Rhodes. To the east, in the area which is thought to be the junction between the Hellenic and Cyprean arcs (e.g. McKenzie, 1972) deep focus earthquakes are missing (Papazachos, 1973 and Fig. 3). TECTONIC
SYNTHESIS
The first-order process in the eastern Mediterranean is undoubtedly the subduction of the African lithosphere beneath Eurasia (e.g., McKenzie, 1970, 1972). This simple, two plate model is still the most widely used in any tectonic analysis of this region; yet it clearly cannot account for the unusual complexity when small scale features are considered. Thus, a second-order process is possibly required to accompany the two plate model in order to account for the irregularities in the bathymetric and seismicity patterns. This process is the collision and accretion of oceanic plateaus at plate boundaries. The significance of this process has recently been described for several regions, particularly around the Pacific ocean (e.g., Coney et al., 1980; Ben-Avraham
et al., 1981).
It is very likely that oceanic plateaus have existed in the eastern Mediterranean and that they have had a significant role in shaping the present tectonic pattern in the area.
At present
there
are several
prominent
bathymetric
highs
or oceanic
plateaus in the eastern Mediterranean away from the zone of plate boundary. They appear mostly in the Ionian basin and are in the form of seamounts (e.g. Cyrene, Archimedes, Epicharmos, Eratosthenes) and other prominent highs (e.g. Medina Bank, Medina Rise). These structures are embedded in the floor of the eastern Mediterranean and move with it towards the zone of subduction. Their height, sometimes more than 1.5 km over the surrounding ocean floor, and at times their buoyancy will prevent subduction upon their arrival at the plate boundary. Instead, they can be expected to collide with the continental mass north of the plate boundary and eventually be accreted to it. This process of collision and accretion is probably associated with the emplacement of ophiolites. Ophiolites are found in Cyprus, Turkey and Greece (e.g., Gass and Masson-Smith, 1963; Robertson and Woodcock, 1981a; Smith and Woodcock, 1982). Their pres-
557
ence just north of the plate boundary clearly indicates previous occurrences of accretion of oceanic plateaus. Of particular interest is the Bey Daglari area in southern Anatolia. The ophiolites in this area (Pinar-Erden and Ilhan, 1977) have probably been emplaced by the collision of a large oceanic plateau which can be recognized today as an allochthonous terrane on land (Robertson and Woodcock, 1981b). Oceanic plateaus are thus found in this region both at sea and as accreted terranes on land. It is, therefore, reasonable to expect to find them also in the zone of plateau boundary; indeed, we propose that the many elevated blocks which crowd the Hellenic and Cyprean arcs are oceanic plateaus in various stages of collision and accretion. Some of the more prominent of these are the Gvados, Pliny and Strabo seamounts in the Hellenic arc, the Hecataeus Ridge in the Cyprean arc and the Anaximander Seamount between the two arcs (Fig. 4a). These larger structures and other smaller ones disrupted normal subduction in the area by their collision and we propose that this is the reason for the fragmented pattern of the subduction zone in the Hellenic area. It also resulted in multiple trenches, since new subduction zones tend to migrate behind the colliding oceanic plateaus. Hence, the outermost subduction trench, the Strabo trench, is the youngest and its features are the least developed. The same process can also be used to explain the main features of the seismicity pattern. New subduction behind a colliding and accreting block is associated with a new dipping seismic zone. The two dipping zones can result in the extraordinarily thick zone of deep seismicity observed in the Hellenic arc (Fig. 2). Under different circumstances, it could also appear as one dipping seismic zone with a distinct gap as observed, for example, in the northeastern corner of this arc (Richter and Strobach, 1978). A schematic history of collision and accretion in the eastern Mediterranean is outlined in Fig. 4; an important observation incorporated in this figure is the distinction between small and large oceanic plateaus. We assume that a small oceanic plateau which is accreted onto the continental mass has only a local effect on the shape of the arc. However, when a large oceanic plateau is involved, the situation is markedly different. A large oceanic plateau, pieces of which may possibly be found today in southern Anatolia, significantly affected the shape of the arc system. We suggest, in fact, that prior to the collision of this large oceanic plateau, the Hellenic and Cyprean arcs were parts of one subduction arc in which mostly normal (dip-slip) subduction took place. The collision of a large oceanic plateau created the present two arc system. By using this simple mechanism, it is possible to give a straightforward explanation for two important observations which are well documented but not yet accounted for. First is the observation that seismicity is no deeper than 60 km in the zone between the two arcs. This is an expected result if we assume that.this zone between the two arcs is occupied by an accreted block and that new subduction and deep earthquakes have migrated past this block to the south (Fig. 4c). Indeed,
558
J
.‘.., ,._.... ,i A
‘j.:. ...?
ATOLIA
N
A
N
ATOLIA
43
_,,,,.,(._ _.i”‘~‘~ -“%
. . ....
,..’
....
“.--.s .........___.... : ‘....- . .. . .. ..
,_.-. ..
‘,...._. :x.,,,_,_ -;,,._...... 2’
..’
A
NATOLIA
Papazachos (1973 and Fig. 3) documented subcrustal earthquakes south of the Anaximander Seamount near the new subduction trench, the Strabo trench and its extension eastward. Secondly, using the same sequence of events, the oblique subduction in the Hellenic arc east of Crete can be explained. In this zone the relative plate motion is almost parallel to the arc. This has led several investigators to suggest that this zone serves, in effect, as an inclined transform fault (McKenzie, 1978; Dewey and Sengor, 1979; Le Pichon and Angelier, 1979). However, the great depth of earthquakes associated with this inclined seismic zone supports the idea that it did originate as a normal subduction zone in which dipslip motion was dominant. We assume, therefore, that subduction in this region originated under normal circumstances. Only during a later period, the collision of a large oceanic plateau caused the division into a two arc system, as well as the changes in sense of relative motion along the shifted segments of these arcs. According to this model, active consumption of newly subducted African lithosphere occurs only at the outermost trenches. Thus, in the west Hellenic arc, the entire Ionian trench may be active but, in the central and eastern segments of the Hellenic arc, only those portions of the Ptolemy and Pliny trenches which face the open Mediterranean sea are active. Other trenches are now either partially or completely shut off. They are probably associated with sinking or laterally moving subducted slabs at depth, but have little or no recent surface subduction associated with them. The young Strabo trench and its even less developed eastward extension into the center of the Levant basin and the Cyprean arc, represent presently active subduction east of Crete and thus approximate the present zone of active plate boundary between Africa and Eurasia. The Rhodes and Finike basins are deep basins north of this plate boundary. They were an integral part of the Mediterranean floor (Fig. 4) as their unusual depth indicates. The observation that the Rhodes basin is characterized by a relatively large gravity anomaly may be another indication that they represent pieces of Mediterranean lithosphere which were trapped behind the plate boundary during the process of collision and accretion.
Fig. 4. Schematic history of collision and accretion of oceanic plateaus in the plate boundary zone of the eastern Mediterranean:
a. Before collision.
Several oceanic plateaus existed in the eastern Mediterranean.
The plate boundary consists of a continuous
and undisturbed
arc. b. During collision
and accretion. A
large plateau between Crete and Cyprus bends the arc and is possibly breaking up. c. Present day tectonic elements. The former single arc is now broken into two arcs. Remains of the large oceanic plateau north of the Anaximander
Seamount may now be appearing as accreted terranes on land in southwest Turkey.
Several smaller accreted terranes crowd the Hellenic arc. The consuming plate boundary is in the outmost trenches which face the African plate. Dotted coast lines in (a) and (b) are for reference. only. Heavy broken lines in (c) represent old zones of subduction-dipstip present motions.
as inclined
transform
in (a) and oblique in (b), which act at
faults with little or no active subduction.
Arrows
mark relative plate
The process of accretion of oceanic plateaus is consistent with the available bathymetric, seismicity and gravity data from the eastern Mediterranean. It is probably, in effect, the only presently available unified model with which the details of the data may be compared;
yet in order to further
model, additional
studies are required.
seismic refraction
and other methods
Crustal
establish
the validity
studies of the oceanic
are essential.
of this
plateaus
using
Makris et al. (1983), for example,
used seismic refraction to show that the Eratosthenes Seamount is indeed a continental fragment and similar studies of some other seamounts may reveal similar structures. In addition, a detailed comparison between the presently active Strabo and Ionic trenches and the inner inactive parts of the Pliny and Ptolemy trenches may also contribute to the examination of the accretion model. Finally, detailed seismicity studies of the plate margin using local seismic networks may yield an accurate
and detailed
structure
of the subducting
slabs in this region.
REFERENCES
Ben-Avraham, oceanic
Z., Nur,
plateaus
Comninakis,
P.E. and Papazachos,
earthquakes Coney,
in the Hellenic
J.F.
and
continuum
Sengor,
tectonics
Galanopoulos,
terranes.
B.C., 1980. Space and time distribution
A.M.C.,
Jongsma,
from
of the intermediate
focal depth
70: T35-T47.
1980. Cordilleran
Aegean
and
suspect
surrounding
terranes.
regions:
Nature,
188: 329-333.
complex
multiplate
and
zone. Geol. Sot. Am. Bull.. Part 1. 90: 84-92. in the area of Greece as reflected
D., 1963. The geology
R. Sot. London,
D., 1977. Bathymetry
Hellenic
and orogeny:
in the deep focus seismicity.
26 (1): 85-105.
Philos. Trans.
and gravity
anomalies
of the Troodos
Massif.
258: 417-467.
and shallow
structure
of the Pliny and Strabo
trenches
south
of the
arc. Geol. Sot. Am. Bull., 88: 797-805.
Le Pichon,
X. and Angelier,
evolution Leydecker,
J.W.H., 1979.
in a convergent
LG. and Masson-Smith,
Cyprus.
accretion
Science, 213: 47-54.
arc. Tectonophysics.
A.G., 1973. Plate tectonics
Ann. Geofis.,
of the eastern G., Berckhemer,
by precise
hypocenter
Appennines, Makris,
D.L. and Cox, A., 1981. Continental
P.J., Jones, D.L. and Manger,
Dewey,
Gass,
A., Jones,
to allochthonous
Hellenides.
J., Ben-Avraham,
Elesthriou,
J., 1979. The Hellenic Mediterranean.
H. and Delibasis, determinations.
system:
Int. Union Comm.
D. Roeder
of Geodynamics,
Z., Behle, A., Ginzburg,
and
in the Peloponnesus K. Schmidt
between
region
(Editors),
Alps.
Sci. Rep. No. 38.
A., Giese, P., Steinmetz,
profiles
a key to the neotectonic
60: l-42.
N., 1978. A study of seismicity In: H. Class,
S., 1983. Seismic refraction
J.R. Astron.
arc and trench
Tectonophysics,
Cyprus
L., Whitemarsh,
R.B. and
and Israel and their interpretation.
Sot., 75: 575-591.
McKenzie,
D.P., 1970. Plate tectonics
McKenzie,
D.P., 1972. Active tectonics
of the Mediterranean of the Mediterranean
region. Nature,
D.P., 1978. Active tectonics
of the Alpine-Himalayan
region.
226: 239-243.
Geophys.
J.R. Astron.
Sot., 30(2):
109-185. McKenzie, regions.
Geophys.
J.R. Astron.
Nur, A. and Ben-Avraham, collision. Papazachos, ranean
Tectonophysics,
belt: the Aegean
Sea and surrounding
Sot., 55: 217-254.
Z., 1978. The eastern
Mediterranean
and the Levant:
tectonics
of continental
46: 297-311.
B.C., 1969. Phase velocities sea. Pure Appl. Geophys.,
of Rayleigh
75: 47-55.
waves in southeastern
Europe
and eastern
Mediter-
561
Papazachos,
B.C., 1973. Distribution
tions. Geophys. Pinar-Erden,
J.R. Astron.
N. and Ilhan, E., 1977. Outlines
geology
of Cyprus.
Rabinowitz,
P.D. and
Mediterranean. Richter,
Ryan,
1970. Gravity
A.H.F.
Robertson,
Y. and
Mediterranean D.G.
and
J.H..
N.H.,
Kafka,
A., 1982. subduction,
Woodcock,
N.H.,
of Turkey,
implica-
with notes on the
New York, pp. 277-318.
and crustal
shortening
in the eastern
In: H. Class, D. Roeder
and K. Schmidt
Tectonic
1981b.
of Geodynamics, setting
of the Troodos
AIkir
Cay group,
Geol., 30: 95-131.
Seismotectonics
of the southern
collision
and arc jumping.
1982. Tectonic
Sci. Rep. No. 38. massif
in the east
pp. 36-49.
Sediment.
and K. Hsu (Editors),
synthesis
Antalya
complex,
boundary
J. Geophys.
of Anatolia,
vertical
tectonics
on the eastern
a
eastern
Res., 87: 7694-7706.
of the Alpine-Mediterranean
Alpine-Mediterranean
SW Turkey:
Geodynamics.
region: Am. Geophys.
Ser., 7: 15-38.
1976. Regional
sot., 47: 493-514.
1981a.
Symp. Cyprus,
margin.
region:
Geodyn.
anomalies
Int. Union Comm.
N.H.,
Woodcock, carbonate
review. In: H. Berckhemer Union,
Hellenides.
Woodcock,
and
Mesozoic
and tectonics
Vol. 4A. Plenum,
K., 1978. Benioff zones of the Aegean.
and
area and its tectonic
10: 585-608.
Proc. Int. Ophiolite
A.H.F.
deformed
Woodside,
W.F.B.,
Tectonophysics,
Mediterranean.
Smith,
Basins and Margins.
Alps, Apennines,
Robertson,
of stratigraphy
In: The Ocean
J. and Strobach,
(Editors),
Rotstein,
of seismic foci in the Mediterranean
Sot., 33: 421-430.
Mediterranean.
Geophys.
J.R. Astron.
a
551
B.V., Amsterdam
- Printed
in The Netherlands
ACCRETIONARY PROCESSES AT SUBDUCTION ZONES IN THE EASTERN MEDITERRANEAN
YAIR
’ and ZVI BEN-AVRAHAM
ROTSTEIN
2
’ Institute {or Petroleum Research and Geophysics, P. 0. Box I 717, Holon 581 I7 (Israel) ’ Department of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69976 (Israel) (Revised
version received
September
13, 1984)
ABSTRACT
Rotstein,
Y. and
Ben-Avraham,
Mediterranean.
The tectonic the collision exist today
present
history
of the eastern
and accretion
of oceanic
in the eastern
the plate
complexity
Mediterranean plateaus
continental subduction trenches. western subduction
at subduction
Structures
zones
and Processes
Mediterranean
boundary,
particularly
of this subduction of a large
in the Hellenic Subduction parts.
Inner
plateau
in the eastern in Subduction
Many
other
arc area. Their presence by the seismicity
and Cyprean
an older
style are affected
results
and bathymetry
arcs, is probably
arc. This plateau
has been
arc takes place in the outer
segments
lithosphere
of these
outward
trenches
following
trenches,
particularly
in the Pliny and Ptolemy served
the collision
as the main of oceanic
plateaus
oceanic
plateaus
in the unusual patterns.
thrusted
subduction
The of the
over the
the two arcs. Active
in the Ionian trenches
by
oceanic
the consequence
and pieces of it are to be found in the region between
of Mediterranean
zones migrated
with
tectonic
zones. Small but distinct
away from the plate boundary. in the Hellenic
zone, as is evidenced
continental
mass of Anatolia
as well as the present
at subduction
two arcs system in this region, the Hellenic
collision
processes
and IS. Sacks (Editors),
Tecronophysics, 112: 551-561.
Zones.
crowd
Z., 1985. Accretionary
In: K. Kobayashi
and Strabo
is limited trenches
to their
before
the
plateaus.
INTRODUCTION
The eastern Mediterranean is a small ocean basin known for its unusual’ tectonic complexity. It includes a short segment of the convergence boundary between Africa and Eurasia. Subduction in this segment is along two very small arcs, the Hellenic and Cyprean arcs. In both arcs subduction has been documented using bathymetric, earthquake and other data (e.g. Rabinowitz and Ryan, 1970; McKenzie, 1970, 1972; Papazachos, 1973). The Hellenic arc is associated with backarc basin and volcanism, while the Cyprean arc is not. The bathymetric and seismic patterns in the eastern 0040-1951/85/$03.30
0 1985 Elsevier Science Publishers
B.V.
552
Mediterranean
seem to be significantly
type” subduction
zones. This apparent
models for this region (e.g. McKenzie, Pichon
and Angelier,
of agreement discuss
1979; Rotstein
on even
the tectonic
the geometry
framework
more complex complexity
than in most simple “pacific
has resulted
in numerous
1972, 1978; Nur and Ben-Avraham, and Kafka,
1982). These models express a lack
of the plate
of this region
tectonic 1978; Le
boundary.
try to present
Most
papers
a model
which
which
is
consistent with first-order observations. To date, relatively little effort has been made to propose models which are consistent with second-order observations in order to explain the complex bathymetric and seismic patterns. In this paper, we briefly describe some of the unusual but well-known characteristics of the bathymetric and seismic patterns in the eastern Mediterranean. We then proceed to propose a tectonic model in which the collision and accretion of oceanic plateaus are important factors which, together with subduction, explain these patterns. The occurrence of collision and accretion of oceanic plateaus in the eastern Mediterranean may be of special importance to the overall understanding of these processes. This area consists of small and accessible basins which appear to include a large number of oceanic plateaus in various stages of maturation; in addition, both these basins and the landmasses surrounding them are well known. Thus, the eastern Mediterranean may prove to be an area which is well suited for studying this process in general and the effects of various boundary in particular.
size oceanic
plateaus
on the shape of the plate
BATHYMETRY
Virtually all subduction arcs are characterized by a subduction trench, a single and mostly continuous ocean deep which marks the flexure of the descending oceanic lithosphere. In the eastern Mediterranean, however, the situation is markedly different. Subduction has been documented in this region, Hellenic arc, yet a single continuous trench cannot be traced.
particularly in the Figure 1 shows the
main bathymetric features in the eastern Mediterranean. The Hellenic arc is characterized by multiple, parallel and en echelon ocean deeps which follow the arc; in addition, numerous short deeps which are perpendicular to the trend of the arc are also present. Separate elevated blocks appear to be the elements causing the fragmentation of the longer deeps parallel to the arc. Generally, in the Hellenic arc, instead of one long subduction trench there are several deeps which could, probably, qualify as subduction trenches. This nontypical pattern is, probably, tectonically related and warrants an explanation. Several other bathymetric observations are of interest, For example, the different trenches are not equal in depth; in fact the outer, Strabo, trench is but a cleft in the ocean floor (Jongsma, 1977), whereas the one next to it, the Pliny trench, is much more distinct. Equally important may be the observation of a halo of small and, in places, deep
553
basins north of the subduction be argued
that
equivalent
to backarc
zone of the eastern
some of these, particularly basins.
The deep Finike
other hand, are much more difficult Anatolia
landmass,
Mediterranean.
the shallow
to account
It could possibly
ones around
and Rhodes
basins
Cyprus.
(Fig.
for. The Finike basin. adjacent
is an E--W trench which is as deep as the Mediterranean
plane;
however. in spite of its depth, it is not in the subduction
north
of a large elevated
block,
the Anaximander
Seamount
are
1). on the to the abyssal
zone since it appears and
since
it is not
associated with deep seismicity. Somewhat similarly, the Rhodes basin is a deep basin, even deeper than the abyssal plane, adjacent to the Anatolia landmass: moreover. it is characterized by a distinct, positive Bouger gravity anomaly (Woodside, 1976) which is similar to the gravity signature of the central parts of the eastern Mediterranean. Finally, the eastern Mediterranean includes a large number of elevated terranes particularly in the Ionian basin and the Hellenic arc complex. Some of the more distinct elevated terranes in the Ionian basin are the Medina Ridge, Medina Bank and the Cyrene, Epicharmos and Archimedes seamounts. Other prominent elevated terranes also appear within the Hellenic arc complex; however, whereas in the Ionian basin the elevated blocks are spread throughout the basin, in the Hellenic arc a large number of blocks crowds a small area causing the complex pattern of bathymetric
highs and lows.
SEISMIC’ITY
Classical “pacific type” subduction ping seismic zone. Earthquakes in the have been thought to be in the form 1973). Figure 2 is an example of a subduction in the Hellenic arc. It is cross-section distribution
that,
although
of earthquakes
is characterized by a Benioff-Waddati dipHellenic arc reach a depth of over 160 km and of a dipping seismic zone (e.g. Papazachos, seismic cross-section which is used to infer clear from the distribution of events in this
a dipping
seismic
zone can indeed
is not a simple one. Normally,
be followed,
the dipping
the
seismic zone
is only several tens of kilometers thick, since it represents processes in the rigid upper part of the lithosphere. In the Hellenic arc, on the other hand, the zone of subcrustal
earthquakes
is unusually
thick, reaching
a thickness
of about
200 km,
which is over twice the estimated thickness of the lithosphere in this area (Papazachos, 1969). Such an observation raises the question of whether a simple two plate model can be applied in this area, The inconsistency of the simple plate model with the seismic data was noted earlier by Galanapoulus (1973) and has recently been investigated by several other workers (Leydecker et al., 1978; Richter and Strobach, 1978; Comninakis and Papazachos, 1980). They have re-analyzed the available data in an effort to determine whether the apparent complexity in the seismicity pattern is the result of poor data quality, particularly poor depth control. Their results generally support the observation that some kind of dipping seismic zone does exist
555
w*st km 410 0
CI
400 .I . . ..*
mm
250
300
350
A?C
SW 200
100
I50
0
50
SO
YeditrrronWll
*ll*nie
SodimWtlary ArC
VOICOtliC
turlwy Awwn
SW
Trmch IS0
IO0
200
300
250
0 IO0 -
Aatheiwsphwe
Asthmomphwo 2110 -
Fig. 2. Cross-section Papazachos, denote
intermediate
intermediate between
showing
the pattern
of deep focus earthquakes
1973). The plot is of focal depths against earthquakes
earthquakes
the distance
with M > 4.9 which occurred
with 4.5 6 M 4 4.8 and
shallow
between
earthquakes
1965 and 1971. Vertical and lateral errors were estimated,
km for data prior
across
the Hellenic
from a line following
1911 and 1971 and dots denote with M > 4.5, which
by the original
to 1965 and up to 25 km for more recent data.
arc (following
the arc. Triangles
See original
compiler, paper
occurred
to be up to 40
for details of data
sources.
20° 22' 24O 26* 28" 30° 32" 34O 36O 38O 4o" 42O 38O 4o" 36O 38'
36' 32O 34O 3o" 32' cl
3o"
5.0-
53
A
4.5-4.9
A
I;;,“,:
\ \
\1\
c
18O Fig. 3. Epicenters (following (hatched).
20°
of intermediate
Papazachos, Lateral
22O
24O and
1973) indicating
26*
deep focus
28O
earthquakes
a gap of seismicity
and vertical errors were estimated,
30° in the eastern
between
by the original
prior to 1965 and up to 25 km for more recent data. See original
32O
paper
Mediterranean
the Hellenic
compiler,
34O
and Cyprean
region arcs
to be up to 40 km for data
for details of sources.
556
in the Hellenic Particularly
arc, but that this zone is unusually interesting
lar to the Hellenic
arc which are presented
show that the dipping it is also discontinuous
wide and complex.
in this regard are the seismicity by Richter
cross-sections and Strobach
perpendicu(1978). These
seismic layer is not only thick and complex but that, in places, with a large and distinct
and lower parts of the dipping
seismic gap separating
seismic layer. A similar phenomenon
the upper
was observed
in
the northwestern corner of the Cyprean arc where it was assumed to be the result of collision and accretion processes (Rotstein and Kafka, 1982). Another observation which adds to the difficulty in using a simple subduction model in this area, is the obvious lateral discontinuity in the earthquake distribution. For example, in the Hellenic arc deep focus earthquakes extend eastwards as far as Rhodes. To the east, in the area which is thought to be the junction between the Hellenic and Cyprean arcs (e.g. McKenzie, 1972) deep focus earthquakes are missing (Papazachos, 1973 and Fig. 3). TECTONIC
SYNTHESIS
The first-order process in the eastern Mediterranean is undoubtedly the subduction of the African lithosphere beneath Eurasia (e.g., McKenzie, 1970, 1972). This simple, two plate model is still the most widely used in any tectonic analysis of this region; yet it clearly cannot account for the unusual complexity when small scale features are considered. Thus, a second-order process is possibly required to accompany the two plate model in order to account for the irregularities in the bathymetric and seismicity patterns. This process is the collision and accretion of oceanic plateaus at plate boundaries. The significance of this process has recently been described for several regions, particularly around the Pacific ocean (e.g., Coney et al., 1980; Ben-Avraham
et al., 1981).
It is very likely that oceanic plateaus have existed in the eastern Mediterranean and that they have had a significant role in shaping the present tectonic pattern in the area.
At present
there
are several
prominent
bathymetric
highs
or oceanic
plateaus in the eastern Mediterranean away from the zone of plate boundary. They appear mostly in the Ionian basin and are in the form of seamounts (e.g. Cyrene, Archimedes, Epicharmos, Eratosthenes) and other prominent highs (e.g. Medina Bank, Medina Rise). These structures are embedded in the floor of the eastern Mediterranean and move with it towards the zone of subduction. Their height, sometimes more than 1.5 km over the surrounding ocean floor, and at times their buoyancy will prevent subduction upon their arrival at the plate boundary. Instead, they can be expected to collide with the continental mass north of the plate boundary and eventually be accreted to it. This process of collision and accretion is probably associated with the emplacement of ophiolites. Ophiolites are found in Cyprus, Turkey and Greece (e.g., Gass and Masson-Smith, 1963; Robertson and Woodcock, 1981a; Smith and Woodcock, 1982). Their pres-
557
ence just north of the plate boundary clearly indicates previous occurrences of accretion of oceanic plateaus. Of particular interest is the Bey Daglari area in southern Anatolia. The ophiolites in this area (Pinar-Erden and Ilhan, 1977) have probably been emplaced by the collision of a large oceanic plateau which can be recognized today as an allochthonous terrane on land (Robertson and Woodcock, 1981b). Oceanic plateaus are thus found in this region both at sea and as accreted terranes on land. It is, therefore, reasonable to expect to find them also in the zone of plateau boundary; indeed, we propose that the many elevated blocks which crowd the Hellenic and Cyprean arcs are oceanic plateaus in various stages of collision and accretion. Some of the more prominent of these are the Gvados, Pliny and Strabo seamounts in the Hellenic arc, the Hecataeus Ridge in the Cyprean arc and the Anaximander Seamount between the two arcs (Fig. 4a). These larger structures and other smaller ones disrupted normal subduction in the area by their collision and we propose that this is the reason for the fragmented pattern of the subduction zone in the Hellenic area. It also resulted in multiple trenches, since new subduction zones tend to migrate behind the colliding oceanic plateaus. Hence, the outermost subduction trench, the Strabo trench, is the youngest and its features are the least developed. The same process can also be used to explain the main features of the seismicity pattern. New subduction behind a colliding and accreting block is associated with a new dipping seismic zone. The two dipping zones can result in the extraordinarily thick zone of deep seismicity observed in the Hellenic arc (Fig. 2). Under different circumstances, it could also appear as one dipping seismic zone with a distinct gap as observed, for example, in the northeastern corner of this arc (Richter and Strobach, 1978). A schematic history of collision and accretion in the eastern Mediterranean is outlined in Fig. 4; an important observation incorporated in this figure is the distinction between small and large oceanic plateaus. We assume that a small oceanic plateau which is accreted onto the continental mass has only a local effect on the shape of the arc. However, when a large oceanic plateau is involved, the situation is markedly different. A large oceanic plateau, pieces of which may possibly be found today in southern Anatolia, significantly affected the shape of the arc system. We suggest, in fact, that prior to the collision of this large oceanic plateau, the Hellenic and Cyprean arcs were parts of one subduction arc in which mostly normal (dip-slip) subduction took place. The collision of a large oceanic plateau created the present two arc system. By using this simple mechanism, it is possible to give a straightforward explanation for two important observations which are well documented but not yet accounted for. First is the observation that seismicity is no deeper than 60 km in the zone between the two arcs. This is an expected result if we assume that.this zone between the two arcs is occupied by an accreted block and that new subduction and deep earthquakes have migrated past this block to the south (Fig. 4c). Indeed,
558
J
.‘.., ,._.... ,i A
‘j.:. ...?
ATOLIA
N
A
N
ATOLIA
43
_,,,,.,(._ _.i”‘~‘~ -“%
. . ....
,..’
....
“.--.s .........___.... : ‘....- . .. . .. ..
,_.-. ..
‘,...._. :x.,,,_,_ -;,,._...... 2’
..’
A
NATOLIA
Papazachos (1973 and Fig. 3) documented subcrustal earthquakes south of the Anaximander Seamount near the new subduction trench, the Strabo trench and its extension eastward. Secondly, using the same sequence of events, the oblique subduction in the Hellenic arc east of Crete can be explained. In this zone the relative plate motion is almost parallel to the arc. This has led several investigators to suggest that this zone serves, in effect, as an inclined transform fault (McKenzie, 1978; Dewey and Sengor, 1979; Le Pichon and Angelier, 1979). However, the great depth of earthquakes associated with this inclined seismic zone supports the idea that it did originate as a normal subduction zone in which dipslip motion was dominant. We assume, therefore, that subduction in this region originated under normal circumstances. Only during a later period, the collision of a large oceanic plateau caused the division into a two arc system, as well as the changes in sense of relative motion along the shifted segments of these arcs. According to this model, active consumption of newly subducted African lithosphere occurs only at the outermost trenches. Thus, in the west Hellenic arc, the entire Ionian trench may be active but, in the central and eastern segments of the Hellenic arc, only those portions of the Ptolemy and Pliny trenches which face the open Mediterranean sea are active. Other trenches are now either partially or completely shut off. They are probably associated with sinking or laterally moving subducted slabs at depth, but have little or no recent surface subduction associated with them. The young Strabo trench and its even less developed eastward extension into the center of the Levant basin and the Cyprean arc, represent presently active subduction east of Crete and thus approximate the present zone of active plate boundary between Africa and Eurasia. The Rhodes and Finike basins are deep basins north of this plate boundary. They were an integral part of the Mediterranean floor (Fig. 4) as their unusual depth indicates. The observation that the Rhodes basin is characterized by a relatively large gravity anomaly may be another indication that they represent pieces of Mediterranean lithosphere which were trapped behind the plate boundary during the process of collision and accretion.
Fig. 4. Schematic history of collision and accretion of oceanic plateaus in the plate boundary zone of the eastern Mediterranean:
a. Before collision.
Several oceanic plateaus existed in the eastern Mediterranean.
The plate boundary consists of a continuous
and undisturbed
arc. b. During collision
and accretion. A
large plateau between Crete and Cyprus bends the arc and is possibly breaking up. c. Present day tectonic elements. The former single arc is now broken into two arcs. Remains of the large oceanic plateau north of the Anaximander
Seamount may now be appearing as accreted terranes on land in southwest Turkey.
Several smaller accreted terranes crowd the Hellenic arc. The consuming plate boundary is in the outmost trenches which face the African plate. Dotted coast lines in (a) and (b) are for reference. only. Heavy broken lines in (c) represent old zones of subduction-dipstip present motions.
as inclined
transform
in (a) and oblique in (b), which act at
faults with little or no active subduction.
Arrows
mark relative plate
The process of accretion of oceanic plateaus is consistent with the available bathymetric, seismicity and gravity data from the eastern Mediterranean. It is probably, in effect, the only presently available unified model with which the details of the data may be compared;
yet in order to further
model, additional
studies are required.
seismic refraction
and other methods
Crustal
establish
the validity
studies of the oceanic
are essential.
of this
plateaus
using
Makris et al. (1983), for example,
used seismic refraction to show that the Eratosthenes Seamount is indeed a continental fragment and similar studies of some other seamounts may reveal similar structures. In addition, a detailed comparison between the presently active Strabo and Ionic trenches and the inner inactive parts of the Pliny and Ptolemy trenches may also contribute to the examination of the accretion model. Finally, detailed seismicity studies of the plate margin using local seismic networks may yield an accurate
and detailed
structure
of the subducting
slabs in this region.
REFERENCES
Ben-Avraham, oceanic
Z., Nur,
plateaus
Comninakis,
P.E. and Papazachos,
earthquakes Coney,
in the Hellenic
J.F.
and
continuum
Sengor,
tectonics
Galanopoulos,
terranes.
B.C., 1980. Space and time distribution
A.M.C.,
Jongsma,
from
of the intermediate
focal depth
70: T35-T47.
1980. Cordilleran
Aegean
and
suspect
surrounding
terranes.
regions:
Nature,
188: 329-333.
complex
multiplate
and
zone. Geol. Sot. Am. Bull.. Part 1. 90: 84-92. in the area of Greece as reflected
D., 1963. The geology
R. Sot. London,
D., 1977. Bathymetry
Hellenic
and orogeny:
in the deep focus seismicity.
26 (1): 85-105.
Philos. Trans.
and gravity
anomalies
of the Troodos
Massif.
258: 417-467.
and shallow
structure
of the Pliny and Strabo
trenches
south
of the
arc. Geol. Sot. Am. Bull., 88: 797-805.
Le Pichon,
X. and Angelier,
evolution Leydecker,
J.W.H., 1979.
in a convergent
LG. and Masson-Smith,
Cyprus.
accretion
Science, 213: 47-54.
arc. Tectonophysics.
A.G., 1973. Plate tectonics
Ann. Geofis.,
of the eastern G., Berckhemer,
by precise
hypocenter
Appennines, Makris,
D.L. and Cox, A., 1981. Continental
P.J., Jones, D.L. and Manger,
Dewey,
Gass,
A., Jones,
to allochthonous
Hellenides.
J., Ben-Avraham,
Elesthriou,
J., 1979. The Hellenic Mediterranean.
H. and Delibasis, determinations.
system:
Int. Union Comm.
D. Roeder
of Geodynamics,
Z., Behle, A., Ginzburg,
and
in the Peloponnesus K. Schmidt
between
region
(Editors),
Alps.
Sci. Rep. No. 38.
A., Giese, P., Steinmetz,
profiles
a key to the neotectonic
60: l-42.
N., 1978. A study of seismicity In: H. Class,
S., 1983. Seismic refraction
J.R. Astron.
arc and trench
Tectonophysics,
Cyprus
L., Whitemarsh,
R.B. and
and Israel and their interpretation.
Sot., 75: 575-591.
McKenzie,
D.P., 1970. Plate tectonics
McKenzie,
D.P., 1972. Active tectonics
of the Mediterranean of the Mediterranean
region. Nature,
D.P., 1978. Active tectonics
of the Alpine-Himalayan
region.
226: 239-243.
Geophys.
J.R. Astron.
Sot., 30(2):
109-185. McKenzie, regions.
Geophys.
J.R. Astron.
Nur, A. and Ben-Avraham, collision. Papazachos, ranean
Tectonophysics,
belt: the Aegean
Sea and surrounding
Sot., 55: 217-254.
Z., 1978. The eastern
Mediterranean
and the Levant:
tectonics
of continental
46: 297-311.
B.C., 1969. Phase velocities sea. Pure Appl. Geophys.,
of Rayleigh
75: 47-55.
waves in southeastern
Europe
and eastern
Mediter-
561
Papazachos,
B.C., 1973. Distribution
tions. Geophys. Pinar-Erden,
J.R. Astron.
N. and Ilhan, E., 1977. Outlines
geology
of Cyprus.
Rabinowitz,
P.D. and
Mediterranean. Richter,
Ryan,
1970. Gravity
A.H.F.
Robertson,
Y. and
Mediterranean D.G.
and
J.H..
N.H.,
Kafka,
A., 1982. subduction,
Woodcock,
N.H.,
of Turkey,
implica-
with notes on the
New York, pp. 277-318.
and crustal
shortening
in the eastern
In: H. Class, D. Roeder
and K. Schmidt
Tectonic
1981b.
of Geodynamics, setting
of the Troodos
AIkir
Cay group,
Geol., 30: 95-131.
Seismotectonics
of the southern
collision
and arc jumping.
1982. Tectonic
Sci. Rep. No. 38. massif
in the east
pp. 36-49.
Sediment.
and K. Hsu (Editors),
synthesis
Antalya
complex,
boundary
J. Geophys.
of Anatolia,
vertical
tectonics
on the eastern
a
eastern
Res., 87: 7694-7706.
of the Alpine-Mediterranean
Alpine-Mediterranean
SW Turkey:
Geodynamics.
region: Am. Geophys.
Ser., 7: 15-38.
1976. Regional
sot., 47: 493-514.
1981a.
Symp. Cyprus,
margin.
region:
Geodyn.
anomalies
Int. Union Comm.
N.H.,
Woodcock, carbonate
review. In: H. Berckhemer Union,
Hellenides.
Woodcock,
and
Mesozoic
and tectonics
Vol. 4A. Plenum,
K., 1978. Benioff zones of the Aegean.
and
area and its tectonic
10: 585-608.
Proc. Int. Ophiolite
A.H.F.
deformed
Woodside,
W.F.B.,
Tectonophysics,
Mediterranean.
Smith,
Basins and Margins.
Alps, Apennines,
Robertson,
of stratigraphy
In: The Ocean
J. and Strobach,
(Editors),
Rotstein,
of seismic foci in the Mediterranean
Sot., 33: 421-430.
Mediterranean.
Geophys.
J.R. Astron.
a