Geological Society of America Bulletin

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Geological Society of America Bulletin, published online on 3 May 2012 as doi:10.1130/B30492.1
Geological Society of America Bulletin
Structural decoupling in a convergent forearc setting (southern Crete, Eastern
Mediterranean)
Eleni Kokinou, Alves Tiago and Kamberis Evangelos
Geological Society of America Bulletin published online 3 May 2012;
doi: 10.1130/B30492.1
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Copyright © 2012 Geological Society of America
Structural decoupling in a convergent forearc setting
(southern Crete, Eastern Mediterranean)
Eleni Kokinou1,†, Alves Tiago2, and Kamberis Evangelos3
1
Technological Educational Institute Crete, 3 Romanou Street Chalepa, Chania, Crete, GR 73133 Greece
3D Seismic Laboratory, School of Earth and Ocean Sciences, Cardiff University, Main Building-Park Place, CF10 3AT Cardiff, UK
3
Hellenic Petroleum (Exploration and Production of Hydrocarbons Division), 199 Kifissias Avenue, 15124 Maroussi, Athens, Greece
2
ABSTRACT
A multidisciplinary approach is used to investigate the structure of the southern Cretan
margin, which is located in one of the most
seismically active forearc regions in Europe.
Bathymetric, seismic-reflection, and fault
plane solution data were used to identify the
main tectonic features on the margin, correlating their evolution with the main sedimentary sequences recognized on Crete. In
contrast to the majority of forearc settings
in the Pacific and Indian Oceans, southern
Crete is a region of predominantly oblique
movement above well-defined detachment
zones. North-dipping thrust faults identified
on seismic-reflection profiles reveal significant crustal shortening during the Miocene
due to the westward propagation of the Hellenic fold-and-thrust system. In addition,
east-dipping thrust faults rooted on top of
pre-Neogene strata were also identified, but
only a few of these thrusts affect Neogene to
Holocene strata. Small-scale domes derived
from evaporitic (Messinian) intrusions deform Pliocene–Quaternary strata. West- and
east-dipping normal faults were also recognized within the Mesozoic and Cenozoic successions, and these are related to regional extension during forearc convergence. In such
a setting, the fault-bounded continental slope
of Crete effectively separates a region of
uplift (Crete) from subsiding troughs to the
south. Our work shows that structural segmentation at depth is complex, with multiple
crustal levels recording contrasting styles
of deformation and distinct moment-tensor
solutions. This complexity derives from the
oblique style of convergence recorded south
of Crete, which reactivates distinct crustal
levels depending on their rheology and relative degree of metamorphism inherited dur†
E-mail: [email protected]
ing Alpine compression. As a result, a strong
correlation between seafloor morphology and
transtensional movements is recorded in the
upper 10–15 km of the crust, while transpression prevailed after the Serravallian below
these depths.
INTRODUCTION
Forearc regions of convergent tectonic plates
are characterized by high seismicity and significant tectonic movements, with magmatic
activity being associated with deeper processes
of subduction and tectonic plate assimilation in
the upper mantle (Pindell and Kennan, 2007).
Discrete tectono-magmatic episodes are commonly associated with variations in subduction
velocity and geometry of the subducted slab
(Haschke et al., 2002). These same properties of subducted slabs are also responsible for
the geometric variability shown by convergent
margins in terms of their forearc dimensions,
relative importance of back-arc extension, and
tectono-magmatic evolutions (Hyndman et al.,
2005). Nevertheless, there are common features
on convergent margins around the world, including: (1) the presence of tectonic trenches in
forearc regions, which are themselves zones of
higher seismicity and tectonic movement as are,
for instance, parts of the Eastern Mediterranean
and Adaman Seas; (2) the generation of island
arcs in the regions of larger tectonic movements, often accompanying the exhumation of
basement terrains; and (3) intense, but relatively
restricted volcanism in back-arc regions, where
complex oblique movements accommodate the
shortening experienced in the forearc (Dasgupta and Mukhopadhyay, 1993; Hori, 2006;
Reilinger and McClusky, 2011).
By further investigating distinct forearc regions, Clift and Vannucchi (2004) proposed a
classification for convergent margins in which
the Mediterranean Ridge area between Libya
and Crete would be considered of the accretion-
ary type. According to the authors, accretionary
margins preferentially occur in regions of slow
convergence (<7.6 cm/yr) and/or trench sediment thicknesses >1 km. In contrast, tectonic
erosion is favored in regions where convergence
rates exceed 6 ± 0.1 cm/yr and where the sedimentary cover is <1 km. In the Eastern Mediterranean, however, the subduction movement
of the African plate beneath the Aegean plate
is diffuse, with head-on convergence in westcentral Greece changing into an oblique-slip
regime offshore Crete (Papazachos et al., 2000;
Bohnhoff et al., 2005; Shaw and Jackson, 2010).
This obliquity in late Cenozoic convergence vectors has been associated with rapid exhumation
of basement units (reaching depths <10 km and
temperatures of <300 °C at ca. 19 Ma; Thomson
et al., 1998) and significant uplift of the forearc
region where Crete, Gavdos, and Karpathos Islands are located (Le Pichon et al., 2002) (Fig.
1A). Contrasting with the rapid exhumation of
Crete, present-day convergence rates between
Libya and Crete reach 3 cm/yr, whereas maximum rates of only 1–2 cm/yr have been suggested by several studies (Meulenkamp et al.,
1988; Dewey et al., 1989; Thomson et al., 1998).
As a result of oblique convergence, tectonic
compression and uplift alternate offshore Crete
around a wide forearc region where developed
tectonic trenches record significant subsidence
(Mascle et al., 1986; Alves et al., 2007). This
particular geometry, and the fact that volcanism
is absent in Crete’s forearc region, makes the
area around the island an unusual convergent
margin. Arc volcanism is concentrated north of
Crete, with the island itself separating a region
of oblique convergence to the south from an
east-west basin (Cretan Sea) and adjacent volcanic island arc to the north (Fig. 1A). A wide
backstop region occurs immediately west of the
deeper parts of the Hellenic Trench (Le Pichon
et al., 2002), i.e., ~150 km southwest of our
study area (Fig. 1). In such a setting, sediment
thickness south of Crete can exceed 1 km,
GSA Bulletin; Month/Month 2012; v. 1xx; no. X/X; p. 1–13; doi: 10.1130/B30492.1; 7 figures.
For permission to copy, contact [email protected]
© 2012 Geological Society of America
1
Kokinou et al.
21°00′E
22°00′E
23°00′E
24°00′E
25°00′E
26°00′E
27°00′E
28°00′E
Peloponnese
37°00′N
37°00′N
HE
Kos
Nisyros
LL
EN
36°50′N
IC
Rhodes
NC
H
Karpathos
BS
re
ec
e
Ionian
Basin
ia
Turkey
A
Crete
HE
n
EG
EN
IC
DSDP Site 378
ba
ck
st
op
Gavdos
LL
RHO
MRAC
34°00′N
21°00′E
35°50′N
LL
HE
G
Io
n
34°50′N
36°00′N
CRETAN SEA
Balkanic
Peninsula
35°50′N
36°50′N
Santorini
Kythira
E
TR
36°00′N
35°00′N
Yali
Milos
mv
KAR
top
ks
Koufonisi Isl.
bac
tine
Le v
22°00′E
an
TR
EN
CH
Levantine
Basin
23°00′E
24°00′E
Y TROUG
PTOLEM
35°00′N
H
GH
TROU
PLINY
25°00′E
26°00′E
BO
RA
T
S
27°00′E
OU
TR
GH
34°50′N
A
34°00′N
28°00′E
Figure 1 (on this and following page). (A) Bathymetry of the Cretan and Libyans Seas, surrounding the island of Crete. Structural pattern
of southern Cretan offshore is from Leite and Mascle (1982); ten Veen and Postma (1999); Le Pichon et al. (2002); Alves et al. (2007); and
from the bathymetric data in this work. (B) Seismotectonic map of Crete and its offshore surroundings. The distribution of the preNeogene nappe pile (dark gray) and Neogene and Quaternary sediments (gray) is according to ten Veen and Postma (1999). Available
focal mechanisms of earthquakes (stars) with M ≥ 5.0 are from Lyon-Caen et al. (1988), Taymaz et al. (1990), Pondrelli et al. (2002), and
Kiratzi and Louvari (2003). Onshore fault zones are compiled from the geological map of Greece (IGME), Armijo et al. (1992), Fassoulas
(2001), and Caputo et al. (2006, 2010). Dashed-shaded lines in the southern offshore area correspond to strike-slip faults (ten Veen and
Postma, 1999). The distribution of the events (black circles) used in the present study were reported by EMSC (Eastern Mediterranean
Seismological Centre) in the time period 2007/07/03 up to 2010/02/12 (03 July 2007 to 12 February 2010). Plate convergence motions are
taken from McClusky et al. (2000) and Reilinger and McClusky (2011). (C) Epicenter foci along the X-Y transect across western Crete
are taken from Papazachos et al. (2000).
mostly accumulated in transtensional basins dissimilar from the geometry proposed for accretionary margins by Clift and Vannucchi (2004).
As an example of a continental margin experiencing diffuse crustal convergence, the relative absence of geophysical data around Crete
has prevented, until now, a complete analysis
of the tectonic trenches and of the multiple
Alpine nappes extending toward the Libyan
Sea (Fig. 1). Moreover, the degree of structural
linkage between the upper and deeper crust has
not yet been investigated in southern Crete.
This results in a major gap in knowledge, since
four-dimensional variations in crustal properties offshore Crete, together with the degree and
relative angle of stretching experienced in transtensional basins, may have been responsible for
the generation of regions of high subsidence
during the late Cenozoic, similar to what is recorded in the North Aegean Trough (e.g., Angelier et al., 1982; Le Pichon et al., 1985). Similar
settings to the case study presented in this paper
2
are also poorly documented in the Pacific and
Indian Oceans, where complex subduction
movements extend through large areas of island
forearcs (Davis et al., 2006). Hence, this work
tries to address the following questions:
1. To what depth is the shallow crust influenced by deeper structures on the southern Cretan margin?
2. Is there any interaction between the lower
and uppermost crust in the study area?
3. How can variations in the velocity of
propagation of seismic waves relate to heterogeneities in forearc basins, such as those in the
Pacific and Indian Oceans?
Earlier studies proposed that, as a result of
plate convergence, two main nappes (the Hellenic Nappes) form the basement of middle
Miocene to Holocene tectonic troughs south of
Crete (Leite and Mascle, 1982). Onshore data
acknowledge the presence of low-grade metamorphic successions, overlaid by five post–midMiocene tectonic sequences (Kilias et al., 1985;
Postma et al., 1993; ten Veen and Postma, 1999)
(Fig. 2). However, no well data have so far confirmed, nor rejected, the presence of low-grade
metamorphic rocks offshore Crete.
Seismic data combined with bathymetric,
stratigraphic, and seismological information
from the southern Cretan margin are presented
in order to clarify the tectono-sedimentary evolution of forearc basins experiencing oblique
convergence (Figs. 1B and 1C). This paper
concludes that the geometry of tectonic troughs
south of Crete reflects a two-tier deformation
mechanism at depth, in which oblique extension predominates in the upper 10–15 km of
the crust, or even shallower southward, and
oblique compression predominates underneath
this limit.
We start with a description of the methods
utilized, followed by a summary of the regional
geology of Crete. Seismic-reflection profiles
imaging the upper ~5 km of the crust were
interpreted based on a compilation of velocity
Geological Society of America Bulletin, Month/Month 2012
Structural decoupling in a convergent forearc setting (southern Crete, Eastern Mediterranean)
- 36°N
24°E
25°E
Fault zone
Possible fault zone
Thrust fault
EMSC seismicity (2007-2010)
Earthquakes with focal mechanism
Velocity model referred to in the text
Y
Rethymnon
Ie
ra
p
ab etr
en a
Heraklion
gr
Sitia
17
50
0
21
- 35°N
HE
LL
E
m
a
b
47
29
45
B
40 39
30
Gavdos
NI
C
H
UG
RO
YT
M
LE
c
PTO
24
0
TR
E
Crete
50 km
50 km
Y
100 km
IN
PL
NC
Ierapetra
33
35°N -
Β΄
Koufonissi
C
Chrissi
C΄
Β
A
X
0
X
Α΄
H
UG
RO
T
Y
RA
ST
H
BO
O
TR
H
UG
100 km
thrust and
thrust strike-slip strike-slip normal
M
7.0
150 km
150 km
6.0
C
5.0
50 km
100 km
200 km
250 km
24°E
25°E
Legend:
Inner deformation
front
Direction of plate
convergence
Fault-plane
solutions
26°E
30 km
34°N -
B
Stratigraphic units on Crete:
Seismic lines
in text
Transects in text
X-Y transect in
inset C
AEG: Aegean Sea
BS: Black Sea
KAR: Karpathos island
MRAC: Mediterranean Ridge accretionary complex
RHO: Rhodes island
Quaternary, alluvium
Upper Miocene (Messinian) and Pliocene
Middle and Upper Miocene (Tortonian)
?Lower to Middle Miocene
Asterousia Nappe
Ophiolite
Pindos zone
Gavrovo-Tripolitza zone
Phyllite-Quartzite unit
Plattenkalk unit
Upper
Sequence
Lower
Sequence
Figure 1 (continued).
distribution as well as new information on the
regional geology and tectonics of southern Crete
published recently by Alves et al. (2007), Kokinou and Kamberis (2009), and Caputo et al.
(2006, 2010). Bathymetric data were used to
trace the main geomorphologic structures on the
margin. Later in the paper, we combine: (1) recorded velocity variations with seismic activity data for specific time periods, and (2) focal
mechanisms of the stronger events affecting the
study area in the past 40 yr to justify the presence
of a two-tier deformation mechanism offshore
south Crete (Fig. 1B). In particular, we show
how the determination of focal depths, derived
using the present velocity models, confirms that
an upper tier is located in the upper 10–15 km of
the crust. In the discussion section, we explain:
(1) the significance of two-tier deformation in
southern Crete to the tectono-sedimentary evolution of convergent margins; and (2) the inter-
action between lower and upper crust, and how
this interaction differs from other convergent
margins in the Pacific and Indian Oceans.
METHODOLOGY
The following methodology was implemented
using commercial and free software such as
Matlab, Surfer 8, Origin Pro 8, FP Tectonics,
Microsoft Excel, ArcGIS, and GMT system
(Wessel and Smith, 1998).
Velocity Model
Velocity models were implemented using
data from seismic-reflection experiments undertaken on the southern Cretan margin (e.g., Kokinou et al., 2006; Kokinou and Kamberis, 2009).
The majority of the seismic data was acquired
during the 1980s and 1990s (Western Geophysi-
cal Company). The acquisition system used a
high-pressure air-gun tuned array. In the majority of the seismic lines, the receiver group interval was 25 m, and the minimum offset was
250 m. The recording length, the sampling rate,
and the shooting interval were 8 s, 4 ms, and
25 m, respectively. The data-processing sequence included geometrical spreading correction, deconvolution, velocity analysis, stacking,
and time-variant filtering.
Common depth point (CDP) velocity data
were used in this work to construct onedimensional (1-D) velocity models for the
upper 15–20 km of the continental crust. Root
mean square (RMS) velocities were converted
to interval velocities according to the Dix Equation (1955). Previous velocity models were also
used in order to correlate our models and to extend them to a depth of 35 km (e.g., Makris and
Stobbe, 1984; Bohnhoff et al., 2001).
3
Kokinou et al.
SW
NE
Cretan Sea
Libyan Sea
A
Upper Sequence
24°E
23°E
36°N
25°E
26°E
27°E
36°N
Gavrovo - Tripolitza
Zone
Karpathos
Upper TriassicOligocene
Profile in A
Gavdos
35°N
0
Crete
23°E
24°E
Plattenkalk Series
Chrissi
25°E
26°E
Middle TriassicLower Oligocene
Ophiolites and
metamorphic rocks
Uppermost JurassicLowest Cretaceous
Lower Sequence
35°N
Koufonissi
30 km
Pindos-Ethias
Series
Tripali Series
Phyllite - Quarzite
Unit
Upper Triassic L. Jurassic
Permian-Triassic
27°E
B
Permian - Eocene
Figure 2. Sketch showing the distribution of the pre-Neogene Hellenic nappes (A) on the island of Crete (B). A nonmetamorphic succession reaching 10 km in thickness overlies the metamorphic sequence of the Plattenkalk Series and Phyllite-Quartzite (PQ) Unit. 1—Parauthochtonous
Plattenkalk Series (Ionian zone: Ida Sequence) characterized by high-pressure, low-temperature metamorphism; 2—Tripali Series, white
stromatolitic dolomite and limestones metamorphosed at high-pressure, low-temperature conditions; 3—Phyllite-Quartzite Series, showing high-pressure, low-temperature metamorphism; 4—Tripolitza Limestone Series, a dark limestone unit, part of the Gavrovo-Tripolitza
Zone and Eocene flysch; 5—Pindos-Ethias Series, including deep-water limestones, chert, and shales; 6—Asterousia and ultramafic (UM)
series, including systems of ophiolites, limestones, and flysch units, locally intruded by granitic to granodioritic rocks (Asterousia) and metabasalts, andesite, and peridotites (UM). Figure is modified from Kilias et al. (1985) and Postma et al. (1993).
Earthquake Data
In order to test the velocity models derived
by the procedure previously described, we used
earthquake data in which events of magnitude M ≥ 3.0 occurring in southern Crete from
2007/07/03–2010/02/12 (03 July 2007 to 12 February 2010) were selected from the Eastern
Mediterranean Seismological Centre (EMSC)
database. These data were plotted in Figure 1B.
For the calculation of the focal depths, we assumed a flat Earth model (Snoke and Lahr, 2001)
and used an algorithm (Kokinou et al., 2009)
implemented on a LINUX platform by using the
gcc compiler.
Information on fault plane solutions of strong
seismic events (M ≥ 5.0) that occurred in the
study area in the past 40 yr was taken either
from published studies (Papazachos, 1973;
Papadopoulos et al., 1986; Lyon-Caen et al.,
1988; Taymaz et al., 1990; Papazachos et al.,
1991, 2000; Papadimitriou, 1993; Papazachos
and Papazachou, 1997; Pondrelli et al., 2002;
Kiratzi and Louvari, 2003; Benetatos et al.,
4
2004) or from global catalogs (U.S. Geological Survey; Instituto Nazionale di Geofisica e
Vulcanologia [INGV] MEDNET 2001–2004;
Schweizer Erdbeben Dienst). These data were
plotted in order to investigate the geodynamic
status of the crust at depths greater than 5–6 km,
and to emphasize the three-dimensional (3-D)
distribution of the depth-relocated events.
quisition used an Elict Triton Delphseismic©
system providing data with vertical resolutions between 3.5 m (immediately below seafloor) and 7.5 m (at depth). The seismic data
were consistently filtered onboard for low-pass
(40 kHz) and high-pass (700 kHz) frequencies.
Postcruise processing included band-pass filtering (10–150 Hz).
Seismic-Reflection Data
GEOLOGICAL SETTING
A database of seismic-reflection data was
created and reinterpreted (Hsü et al., 1978;
Maldonado et al., 1981; Leite and Mascle,
1982; Peters and Huson, 1985; Mascle et al.,
1986; Finetti et al., 1991; Limonov et al., 1996;
Karvelis, 1996; Chaumillon and Mascle, 1997;
Bohnhoff et al., 2001; Le Pichon et al., 2002;
Tay et al., 2002; Polonia et al., 2002; Costa
et al., 2004; Kopf et al., 2006; Alves et al.,
2007; Kokinou and Kamberis, 2009).
Part of the seismic-reflection data used in this
work comprises a grid of single-channel lines
(fig. 1 in Alves et al., 2007). Seismic data ac-
Structural Evolution of Crete
The Mediterranean Sea is an E-W–trending
ocean basin formed during the breakup of the
supercontinent Pangea. The island of Crete lies
in the forearc region of the Hellenic subduction zone, and it is located in the transition zone
between the African and Eurasian plates (Fig.
1A). Convergence between Africa and Eurasia
was initiated in the Late Jurassic, leading to the
total closure of the Neotethys around the Levantine region in the Late Cretaceous (Dewey and
Şengör, 1979). East-west–oriented features
were later deformed together with Apulia to
create the Aegean arc and related (Hellenic)
tectonic nappes (Kissel and Laj, 1988). These
tectonic nappes, usually named as Hellenic
Nappes, are the dominant sequences within the
upper crust of Crete. Two major successions are
distinguished within the Hellenic Nappes, the
pre-Neogene and the Neogene successions (Fig.
1B). The pre-Neogene succession consists of
a pile of nonmetamorphosed rocks (Upper Sequence, Fig. 2), which overlies metamorphosed
units (Lower Sequence, Fig. 2). The Lower Sequence is composed of the Phyllite-Quarzite
Unit, the Tripali Series, and parautochthonous
rocks of the Plattenkalk series (Fig. 2) (Creutzburg et al., 1977; Kilias et al., 1985).
Present-day subduction of African crust
under the Aegean microplate is suggested by a
north-dipping Wadati-Benioff zone extending
beneath Crete to a depth of ~200 km (Caputo
et al., 1970; Le Pichon and Angelier, 1979). During the Holocene, the Hellenic arc experienced
moderate arc-parallel extension and strong
compression perpendicular to it (Kahle et al.,
1998). The Hellenic subduction zone appears
to have operated continuously since ca. 26 Ma
and likely back to 40 Ma (Spakman et al., 1988).
According to tomographic studies, present-day
seismicity around Crete results from the movement of a cold lithospheric slab extending
through a transition zone into the lower mantle
below the Aegean microplate (Spakman et al.,
1988). Offshore, the subduction of oceanic crust
created an accretionary complex, the Mediterranean Ridge accretionary complex, consisting of accumulated sediments of the subducted
African plate (Fig. 1A). Between Crete and the
Mediterranean Ridge, there is a series of E-NE–
trending depressions or troughs (e.g., Hellenic,
Ptolemy, Pliny, Strabo; Fig. 1A).
The island of Crete is situated on an emergent
structural high at the forearc of the subduction
system. North of the island, tomography data
show a thinned continental crust surrounding
the volcanic arc region (Cretan Sea; Makris and
Stobbe, 1984) (Fig. 1A). The volcanic arc, positioned ~100 km north of Crete, is represented
by the islands of Santorini, Milos, Kos, Nisyros,
and Yali (Fig. 1A). Global positioning system
(GPS) and seismic studies (Jackson, 1994)
show that Crete and the southern Aegean are
moving together as a coherent block. In parallel, the divergent motion between the Aegean
microplate and mainland Europe is indicated by
an extension zone in the northern Aegean, with
Crete and the Aegean microplate diverging from
mainland Europe at a rate of ~3 cm/yr. Africa is
moving northward relative to Europe at a rate of
~1–2 cm/yr (McClusky et al., 2000; Reilinger
and McClusky, 2011).
Seismic-Stratigraphic Record of
Plate Convergence
Sedimentation has occurred on Crete since
the Miocene. Meulenkamp et al. (1979) divided
strata on Crete into six formations (Prina, Tefeli,
Vrisses, Hellenikon, Finikia, and Agia Galini)
and the undifferentiated Pleistocene. Offshore,
three main units can be identified and correlated with Deep Sea Drilling Project (DSDP)
Site 378 data (Hsü et al., 1978), with outcrop
information (ten Veen and Postma, 1999; van
Hinsbergen and Meulenkamp, 2006), and with
seismic profiles from the Cretan Basin (Kopf
et al., 2006; Alves et al., 2007). Fault-related
subsidence occurred in two principal phases:
(1) a mid-late Miocene phase resulting in the
onset of subsidence on the southern Cretan margin, and (2) a later (latest Messinian–earliest
Pliocene) episode of subsidence synchronous
with a second phase of margin segmentation
(Alves et al., 2007). The two stages are illustrated on seismic data by the lateral migration
of depocenters over deeper Miocene subbasins
filled with strata older than the latest Serravallian (Unit 3, figs. 4b and 4c in Alves et al.,
2007). Additionally, tectonic uplift on Crete
was accompanied by enhanced subsidence and
relative deepening of offshore tectonic troughs
after the earliest Pliocene. This setting is consistent with the dissection of post-Miocene
units by active faults (Sector 3, figs. 4b and 4c
in Alves et al., 2007), in the same region where
halokinetic and/or fluid-escape structures are
observed. Such faults apparently follow a trend
similar to WSE-ENE transtensional fault sets
recognized onshore (ten Veen and Kleinspehn,
2003). Tortorici et al. (2010) considered some
of these structures to be the result of a persisting compressional regime on Crete. In addition, sinistral strike-slip faulting was initially
reported by Le Pichon and Angelier (1979)
and tenVeen and Kleinspehn (2003) on N70°E
sinistral faults, interpreted to play a major role
within the tectonic reorganization of the Hellenic subduction zone at ca. 3.4 Ma.
DATA ANALYSIS
One-Dimensional Velocity Distribution
Representative examples of one-dimensional
(1-D) velocity distribution in south Crete are
shown in Figure 3. Generally, velocity increases
with depth, but velocity reversals are also observed at depths of 2.2–5.5 km. Thus, we divided the Cretan crust in five units based on the
velocity revealed and the geological character:
1. The upper layer shows a velocity range
between 1.5 and 2.2 km/s, corresponding to
uppermost post-Alpine strata of mainly Pleistocene-Holocene age.
2. The second layer reveals a velocity between 2.3 and 4.4 km/s and represents the lower
post-Alpine strata, flysch units, and parts of the
upper Alpine successions.
3. The third layer shows velocity values of
4.5–6.0 km/s, and represents the lower successions, which mainly comprise metamorphosed
rocks of the Phyllite-Quarzite Unit and the parautochthonous rocks of the Plattenkalk series.
4. A fourth layer indicates an approximate
velocity range between 6.1 and 6.4 km/s, most
likely comprising the lowermost carbonate succession and the Paleozoic(?) metamorphosed
sequence.
5. Finally, the layer with P-wave velocities of
6.5–7.8 km/s most likely represents the lowermost part of the continental crust and the subducted slab of the oceanic crust (e.g., Bohnhoff
et al., 2001).
The velocity models in this work correlate
partly with the seismic velocity structure across
the Middle American land bridge in northern
Costa Rica (Sallarès et al., 1999). On both margins, the velocity distributions are similar for
the intermediate and deep crust but differ for the
upper crust. On the Cretan margin, a possible
low-velocity zone is locally present at depths of
2.2–5 km (Fig. 3), while on the northern Costa
Rica margin, the velocities generally increase
with depth, even for the very shallow crust.
Distribution of Hypocenters
Figure 4 shows the 3-D distribution of depthrelocated seismic events in the study area. A
shallow seismogenic zone is detected up to a
depth of 15–20 km, while a second zone occurs at depths ranging from 30 km to 40 km.
A notable difference between the distribution
of shallow and intermediate events is that the
majority of intermediate earthquakes occurs at
latitudes ranging from 23.5°E to 25.5°E, i.e.,
offshore southwest Crete. This work also demonstrates the presence of intense shallow seismicity (depth h < 20.0 km) across the southern
part of the Cretan crust (see also Papazachos
et al., 2000).
In the study area, moment-tensor solutions
for the earthquakes in Figures 1B and 4 indicate:
1. Structural decoupling at 15–20 km is in
agreement with other transtensional/extensional
basins in Greece, where the principal structures
decouple at 15–20 km (see Moretti et al., 2006).
2. There is a clear structural boundary on
the continental slope of Crete. Transtensional
movements occur to the south of the continental
slope, with higher seismicity being associated
with these movements.
5
Kokinou et al.
Velocity (km/s)
0.0
0.0
2.0
a
4.0
Velocity (km/s)
6.0
8.0
0.0
0.0
b
a
c
10.0
d
Depth (km)
30.0
4.0
6.0
8.0
b
c
d
10.0
20.0
2.0
20.0
Velocity models (group 1)
Model 17
Model 21
Model 24
Model 25
Model 29
Model 30
e
Velocity models (group 2)
Model 31
Model 33
Model 39
Model 40
Model 45
Model 47
e
30.0
24°E
23°E
25°E
26°E
27°E
36°N
36°N
Kythira
Karpathos
Crete
35°N
35°N
Gavdos
Chryssi
Koufonissi
0
23°E
24°E
25°E
30 km
26°E
27°E
Figure 3. One-dimensional (1-D) velocity models (locations in Fig. 1B and in the map below) for the study area and the five units (a, b, c, d, e)
recognized (details in the text) according to the velocity range and the geological character. The models highlight the reversal in velocities
observed between depths of 2.2 km and 5.5 km, in the upper crust. Group 1 (17–30) corresponds to velocity models for western Cretan
Trough, while group 2 (31–47) corresponds to velocity models for Gavdos and eastern Crete.
3. There is a second set of hypocenters clustering at 35–45 km, where they become predominantly compressive.
Focal plots of earthquake data highlight this
bimodal distribution of hypocenters, with a region around 15–20 km separating two distinct
clusters (Fig. 4). Similar hypocenter distributions are indicated in margins where distinct
tectonic movements are recorded with sufficient depth accuracy, such as offshore Alaska
(Wiemer and Wyss, 2000), in the Cascadia subduction zone (Mazzotti and Adams, 2004), or in
the continental collision zone of the Himalayas
(Monsalve et al., 2006), but they occur at deeper
crustal levels than south of Crete. This discrepancy in hypocenter distributions is a topic that
is further developed in the Discussion section.
6
Structural Transects across the Southern
Cretan Margin
Offshore Crete, the uppermost sedimentary sequence is characterized by transparent
seismic facies, mostly representing Pliocene–
Quaternary strata (Figs. 1 and 5A–5C). Its
thickness varies between 0.2 s and 0.54 s twoway traveltime (TWT) (Figs. 5A–5C). The
Pliocene–Quaternary unit is unconformable
over Upper Miocene–Lower Pliocene (Ms-Pli)
strata, which are shown as a dense pattern of
continuous, parallel, thin-bedded reflections.
The Upper Miocene–Pliocene sequence shows
a thickness of 0.64 s to 1.25 s TWT. Locally,
greater thicknesses could be attributed to the
presence of older Middle Miocene sediments,
partly a result of the compressional tectonics
persisting until the late Pliocene–early Pleistocene (Tortorici et al., 2010). Upper Miocene–
Pliocene sediments blanket pre-Neogene strata
and show characteristic onlap structures onto
older units (Fig. 5B).
Thrust faults control the pre-Neogene morphology, which is dominated by anticlines and
synclines at depth (Fig. 5A; and in Mascle et al.,
1986; Karvelis, 1996). Thrust faults also affect
part of the Miocene deposits. The strike of major
thrusts is assumed to be approximately eastwest, subparallel to the southern coast of Crete.
On seismic sections, these thrusts are shown as
low-angle features across the margin (Fig. 5A;
and in Mascle et al., 1986; Karvelis, 1996). The
structural thickening of Miocene sediments
Depth (km)
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
36.2
36.0
35.8
35.6
35.4
L
North atit 35.235.0
ud
34.8
e(
34.6
°N
34.4
)
24.0
23.5
23.0
South
27.0
26.5
26.0
25.5
)
25.0
°E East
(
24.5
e
Lo
West
it
ng
ud
View from SW
Figure 4. Focal mechanisms for earthquakes recorded on the southern Cretan margin (see
Fig. 1B for location of epicenters). This figure highlights the depth-relocated events shown
in Figure 1B in a three-dimensional volume seen from the SW.
mainly results from thrust faults. Small-scale
possible diapirs and/or gas-charged sediment,
developed on preexisting normal faults, are locally present (Fig. 5C).
Normal faults, many of which seem to penetrate the late Miocene to Pliocene–Quaternary
strata, form complex structural steps within the
southern Cretan tectonic troughs (Figs. 5A and
5B). Younger (Quaternary) normal faults create
sea-bottom scarps; with a maximum of 0.12 s
TWT in offset. Divergent geometries in nearseafloor strata confirm that sediment infilling
was simultaneous with tectonic subsidence during the Pliocene and Quaternary.
Based on the seismic-reflection database in this
work, and on recent stratigraphic evidence from
the Ierapetra region, southeast Crete (Postma
et al., 1993; Zachariasse et al., 2008; Alves and
Lourenço, 2010), we produced three transects (or
geomodels) for offshore Ierapetra (Fig. 1B).
Transect A-A′
Transect A-A′ is located 4 km west of Chryssi
Island, trending north-south (Figs. 1B and 6A).
In the southern part of transect A-A′, a structural
high is covered by relatively thin layers of Miocene and Pliocene–Quaternary sediments with
a maximum thickness of 0.37 s TWT. Chryssi
Island is the superficial expression of this structural high, which trends east-west. We consider
the island to be a thrust-related structure, as inferred by the presence of thrust faults affecting
its southern flank. In particular, north-dipping
thrust faults could be recognized at a depth of
1.8–2.0 s TWT in the southernmost part of transect A-A′, probably affecting deeper horizons in
the Alpine basement.
Normal faulting, active up to present, influences the hanging-wall anticline, forming a
small-scale graben in its central part. In addition, a structure similar to a diapir is expected
to occur at the northern end of this structure, affecting the overlying post-Alpine sediments, as
well as the sea-bottom topography. We note that
igneous rocks outcropping at the western part of
Chryssi Island are possibly associated with this
same dome-shaped structure.
In the northern part of transect A-A′, a faultcontrolled basin occurs, where thick amounts
of post-Alpine sediments were accumulated.
Its southern boundary is characterized by an
en-echelon fault pattern. In particular, structural
steps were formed by subvertical north-dipping
normal faults with significant offset. The major
fault also seems to display a horizontal movement component, since it brings in contact
different lithological units. The very rapid subsidence in this part of the basin resulted in the
accumulation of up to 1.8 s TWT of sediment
in the proximity of basin-bounding faults. A
strike-slip component can be inferred for certain
faults, due to a noticeable difference between
sediment thickness on hanging-wall and adjacent footwall blocks. As a result, the uppermost
Pliocene–Quaternary sequence possibly reveals
a thickness of ~0.4 s TWT, while the underlying Messinian–Pliocene sequence comprises
up to 0.8 s TWT of sediment. In addition, a
zone of subvertical south-dipping normal faults
bound the basin to the north, possibly affecting the Miocene and the basal part of Pliocene
sediments.
Transect B-B′
Transect B-B′ is located 2 km west of Koufonissi Island, trending N-S (Figs. 1B and 6B).
A structural high is observed in the northern part
of this transect. A thin (0.5–1.0 s TWT) postAlpine sequence of Miocene and Pliocene–
Quaternary age drapes this same high, which is
intersected by a series of north-dipping parallel
thrust faults affecting the pre-Neogene formations to the south and both Alpine and Miocene
strata near Koufonissi. Younger normal faults
are superimposed on the structural high, resulting in the gradual deepening of the base of
Neogene strata toward the south. As a result, a
deep basin filled by thick (~1.5 s TWT) Miocene sediments is observed in the southern part
of transect B-B′.
A major south-dipping normal fault delimits
the basin to the north, dropping the base of Neogene strata to a depth of ~3.0 s TWT. An offset
of ~0.75 s TWT is inferred for the fault, probably in association with a strike-slip component
of movement in its adjacent hanging wall.
Subvertical normal faults located in the northern part of this transect are characterized by their
small throws, reaching no more than 0.1 s TWT.
Some of these faults are active at present, especially in the northern part of transect B-B′ where
they control seafloor topography. The seafloor at
the northern part of the Transect is possibly deformed by a diapir, which is presently active. A
thin layer of Messinian evaporites, corresponding to a thickness of ~0.1 s TWT, is also displaced at the top of the Miocene pile toward the
south (Fig. 6B).
On the southern limb of the anticline in
transect B-B′, two depositional patterns are
7
Kokinou et al.
Distance (km)
0.0
Distance (km)
0.0
20.0
S
20.0
S
N
N
0.0
0.0
P-Q
MsPli
1.2
1.2
PM
TWT (s)
Distance (km)
0.0
12.0
3.2
NE
SW
TWT (s)
A
Distance (km)
0.0
12.0
3.2
NE
SW
3.4
3.4
3.6
3.6
3.8
3.8
Onlap
4.0
4.0
Base of P - Q
MsPli
4.2
4.2
PM
4.4
4.4
TWT (s)
TWT (s)
B
Distance (km)
0.0
12.0
S
Distance (km)
2.5 0.0
N
12.0
S
2.5
N
3.0
3.0
3.5
3.5
P-Q
?
4.0
PM
MsPli
4.0
4.5
TWT (s)
4.5
TWT (s)
C
Figure 5. Reinterpreted seismic sections (locations in Fig. 1B) highlighting key structures in southern Crete. (A) N-S–
trending profile from the region south of Agia Galini, western Crete (in Karvelis, 1996). (B) NE-SW high-resolution
(single-channel) profile imaging Neogene strata deposited south of Agia Galini (in Alves et al., 2007). (C) N-S singlechannel profile across the Ptolemy Trough showing evidence of local thrusting and possible halokinesis. TWT—twoway traveltime.
distinguished. The lower part of the Miocene
sedimentary pile is characterized by aggradation, indicating a balance between sediment supply and accommodation. In contrast, sediment
progradation predominates in Upper Miocene
strata, as revealed by downlapping geometries
8
onto older deposits. This character suggests that
the sediment supply exceeded accommodation
during the Upper Miocene. This observation
agrees with the assumption that the area close to
Koufonissi Island underwent rapid uplift during
the Miocene.
Transect C-C′
Transect C-C′ is located ~2 km south of Koufonissi Island (Figs. 1B and 6C). A structural
high predominates in its western part, marking
the development of a thrust-related anticline. A
thrust fault is recognized at the back limb of this
0.0
0.0
S
Distance (km)
8.25
Chrissi Island
N
P-Q
Ms-Pli
0.0
1
Distance (km)
0.0
2.0
3.0
3.0
TWT (s)
2.0
P-Q
Ms-Pli
1.0
Messinian salt
Pre-Neogene
2.0
A
-A′
TRANSECT A-A
0.0
Distance (km)
8.0
0.0
S
Koufonissi
0.0
1.0
Md-Me (?)
1.0
TWT (s)
9.5
E
W
Koufonissi
2
Dome?
1.0
16.5
0.0
2.0
C
TRANSECT C-C
-C′
16.0
0.0
N
Legend:
P-Q: Pliocene - Quaternary
Messinian evaporites
TWT (s)
Md-Me: Middle - Lower Miocene
Mesozoic or flysch
Pre-Neogene
Basement
P-Q
Messinian salt
Pre-Neogene
Ms-Pli
2.0
2.0
Me
Normal faults
Md
Me or flysch ?
3.0
Ms-Pli: Upper Miocene - Lower Pliocene
1.0
1.0
Reverse faults
.X
3
TRANSECT B-B
-B′
B
Strike-slip faults
3.0
.x
Figure 6. Transects (locations in Fig. 1B) for the area offshore Ierapetra, southeast Crete. (A) Transect A-A′ from
the region west of Chryssi Island. (B) Transect B-B′ located south of Gouduras, SE Crete. (C) Transect C-C′ showing the area south of Xerocambos, SE Crete.
anticline, which is divided upward into distinct
fault splays affecting the top of the Miocene deposits. A thin Pliocene–Quaternary sequence (of
~0.1–0.2 s TWT) characterizes the uppermost
strata in transect C-C′, unconformably draping the Miocene units. A lensoid body is well
delineated at the eastern part of transect C-C′,
between Pliocene–Quaternary and Miocene
strata. It likely consists of channel-fill deposits
or Messinian evaporites deposited in a restricted
basin, bounded by channel overbank deposits.
The axis of the feeder channel seems to have approximately N-S direction, perpendicular to the
orientation of the transect.
A fault-controlled basin is observed in the
eastern part of transect C-C′, where a thick pile
of Miocene sediments is deposited. Subvertical
normal faults bound the basin to the west, affecting the basal part of Miocene deposits. To
the east, a normal fault is recognized, affecting
the top of Miocene deposits.
It is worth mentioning that the thickness of
both Pliocene–Quaternary and Miocene sequences clearly changes across the anticline.
Close to the anticline axis, both sequences are
relatively thin, totaling ~0.15 s and ~0.4 s TWT.
These same Pliocene-Quaternary and Miocene
sequences become thicker to the east and to the
south, reaching 0.24 s and 0.9 s TWT, respectively. Thickness variations result from regional
uplift around Koufonissi Island occurring from
the middle Miocene to the present day.
DISCUSSION
Effect of Oblique Convergence on Margin
Structure and Segmentation Offshore Crete
A key finding in this work is that a strong
correlation exists between seafloor morphology and normal faulting offshore southern Crete
(Figs. 5 and 6). Normal faults affect the upper
3–4 km of the crust, offsetting strata as old as
the Miocene and, locally, the Alpine basement
(Figs. 5 and 6). Normal-fault orientations are
predominantly NNW-SSE and NNE-SSW. In
western Crete, NW-trending faults are scarce.
North-northeast-trending faults become more
important and have controlled, since the late
Miocene, the evolution of offshore and onshore
basins (Mascle et al., 1986).
Earthquakes generated at the subduction interface by low-angle thrusts show some variation
in depth with position around the arc (Fig. 4).
Well-determined earthquake depths occur
at 37–45 km, with some shallower events at
15–20 km, which Shaw and Jackson (2010)
associated with reverse-faulting earthquakes
with steeper dips (>30°). These are presumably
connected with splay faults that merge at depth
and accommodate some of the convergence
in the subduction zone. In addition, low-angle
thrusts are deeper, reaching a maximum depth
of 40–45 km (Fig. 4). It was one of such lowangle thrusts that Shaw et al. (2008) suggested
was responsible for the catastrophic A.D. 365
earthquake and tsunami.
Shallow fault mechanisms in southern Crete
are due to interplate seismicity, indicating
N-NNE–dipping thrust faulting up to ~20 km
depth. The overall set of interplate events indicates a N-NNE–trending direction of relative
motion between the Aegean and African plates
that is uniform along the Hellenic arc. Thrust
faulting affects the Alpine basement and the
sedimentary cover up to Lower Pliocene strata,
leaving the Pliocene–Quaternary sequence
mostly unaffected. In contrast, focal mechanisms below 20 km are related to seismicity
within the subducting African lithosphere, a
process reflecting slab pull as the dominant
deformation mechanism. Our data show that the
upper crust (up to 12.5–15.0 km) of southern
Crete is dominated by transtension and minor
thrusting, a character contrasting with predominant E-W extension in western Crete (see LyonCaen et al., 1988; Hatzfeld et al., 1993). Thus,
the extensional domains on western Crete and
around Karpathos are decoupled from the sinistral transtensional fault zones (i.e., Ptolemy,
Pliny, and Strabo deep-sea depressions) located
south of central and eastern Crete. Previous
models for oblique plate convergence, generally thought to be resolved into orthogonal subduction and trench-parallel strike slip, suggest
a change from oblique convergence to oblique
thrusting and trench-parallel strike slip (Fitch,
9
Kokinou et al.
nental crust, limited by the Wadati-Benioff
zone, is extended from 20 km in depth up to
~45 km. These limits are comparable to that
of Andean subduction (Delouis et al., 1996;
Fig. 7B). The corresponding zone in Andean
subduction is characterized by underthrusting
and localized reverse faulting. In contrast, on
the Cretan margin, compressional features are
present at depths lower than 20 km, but affecting all sequences up to Lower Pliocene strata.
McGinty et al. (2000) studied the Hikurangi
subduction zone (New Zealand) by inversion of
focal mechanisms and identified the least compressive stress to be closely aligned with the dip
of the subducting plate. Similar results were
obtained for the Copiapo (northern Chile) part
of the Andean subduction zone (Comte et al.,
2002) and for the Alaska subduction zone by
1972). For instance, in the Aleutian Arc ~60%
of the trench-parallel component is partitioned
into strike-slip faulting (Ekström and Engdahl,
1989). Oblique subduction of 60°–70° was also
reported for central Crete by Bohnhoff et al.
(2005), increasing further to the east toward
Karpathos-Rhodes, where it reaches angles of
40°–50° (Fig. 1A).
Comparison to Other Convergent
Forearc Settings
In Figure 7A, we present an integrated model
of the Cretan margin and compare it to other
convergent margins (Delouis et al., 1996; Lu
et al., 1997; McGinty et al., 2000; Hasegawa
et al., 2000; Comte et al., 2002; Christova et al.,
2004; Strasser et al., 2009). The deep conti-
S
0.0
Lu et al. (1997), who also found σ1 to be parallel along strike of the plate boundary. Christova et al. (2004) observed a downdip direction
for σ3 in the Vanuatu (New Hebrides) WadatiBenioff zone, but it was limited to the upper
60 km. Strasser et al. (2009) presented a model
of the Kumano Basin (Fig. 7C) in the Nankai
accretionary wedge, showing some similarities
to our model (Fig. 7A).
The concentration of shallow seismicity (less
than 40 km) in southern Crete is comparable
to that of the NE Japan subduction zone along
or in the vicinity of the boundary between the
subducted Pacific plate and the overriding continental plate, especially along the volcanic arc.
In NE Japan, the abundance of shallow microearthquakes with low P-wave velocities, occurring together with significant crustal shortening
35.1°E
34.7°E
P-Q
Ms-Pli
5
4
5.0
3
1
Depth (km)
PM
35.8°E
P -Q
Detachment
?
Strike s
lip
2
10.0
15.0
N
35.5°E
Detachment
20.0
25.0
30.0
35.0
40.0
45.0
50.0
71°W
70°W
69°W
0.0
68°W
67°W E
SE
NW
0
Depth (km)
Depth (km)
W
A
4
6
8
Forearc basin
Kumano
sediments
Forearc
high
Older accretionary
prism
lt
au
yf
pla
s
ga
Me
10
50.0
B
Imbricate thrust zone Frontal Trench
thrust
Slope sediments
zone
Decollement
Shikoku basin
sediments
Subducting oceanic crust
5 km
C
Figure 7. (A) Schematic model of the Cretan margin and the distribution of events reported by EMSC (Eastern Mediterranean Seismological
Centre) for the time period 2007/07/03 up to 2010/08/30 (03 July 2007 to 30 August 2010). (B) Model of Andean subduction (Delouis et al.,
1996). (C) Model of the Nankai Trough (Strasser et al., 2009).
10
and topography uplift, suggests that a significant
portion of the deformation is caused by diffuse
deformation in the upper brittle seismogenic
zone. The relatively low, but highly diffuse, seismicity promotes topographic uplift on the margin and mountain building onshore, a process
that also explains the present-day structure of
the Cretan margin up to a depth of 40 km.
A compilation of data from 13 accretionary margins undertaken by Clift and Vannucchi
(2004) recognized that accretionary or erosive
margins can, in any given system, alternate in
time and space in a subduction zone. Apart from
basal tectonic erosion of the forearc crust, other
tectonic processes explaining the subsidence
observed in many forearc regions include:
(1) extension of forearc wedges because of gravitational collapse of an unstable steep tapered
wedge (Platt, 1986); and (2) reduction in basal
friction along the plate interface, aiding gravitational collapse (Aubouin et al., 1984). In addition, accretionary plate margins tend to be areas
of rapid sediment input, often from large rivers
draining mountainous areas. Thus, rapid trench
sedimentation is a phenomenon long associated
with subduction accretion (von Huene and
Scholl, 1991; Clift et al., 2010). On accretionary
margins, gravity and seismological data are commonly used to constrain the large-scale crustal
structure of the orogen, especially the depth and
geometry of the main detachment on which the
thrust sheets (representing deposition on older
passive-margin settings) are displaced (e.g.,
Chen et al., 2004).
In this work we show that deep-rooted compressional structures observed offshore Crete in
an otherwise extension-dominated upper crust
are detached from overburden rocks by thick
ductile units, either evaporites or mud-rich
strata, as suggested by the presence of potential
halokinetic and/or gas-escape structures (fig. 4c
in Alves et al., 2007). We therefore postulate
that Alpine and pre-Alpine terrains with distinct
degrees of metamorphism, thus presenting different rheological properties, can potentially
form secondary décollement zones at depth.
Intrabasin segmentation after the late Miocene
is a strong evidence for the accommodation of
horizontal and vertical deformation by extensive zones of décollement, resulting in the formation of large basin-scale transtensional and
transpressional structures.
In essence, the structural setting in this work
adds key information to the classification of Clift
and Vannucchi (2004) by demonstrating that
further complexity can occur in oblique accretionary margins. Structural decoupling was suggestively initiated during rapid exhumation of
the Hellenic Nappes, recorded on Crete around
the mid-Miocene (ca. 19–15 Ma; Thomson
et al., 1998, 1999). Consequently, transtension
has dominated the evolution of southern Crete
from the mid-late Miocene up to the present
day: a minimum of 4000 m of subsidence have
been recorded since 19–15 Ma in the tectonic
troughs bordering the island (Figs. 5B and
5C). The setting documented in south Crete
contrasts with documented areas in Japan, Sumatra, southern Alaska, and Chile, where structural (and seismic) decoupling occurs closer to
the subduction slab (Armijo and Thiele, 1990;
Dasgupta and Mukhopadhyay, 1993; Delouis
et al., 1996; Gorbatov and Kostoglodov, 1997;
Hasegawa et al., 2000).
CONCLUSIONS
The aim of this analysis was a multidisciplinary approach to study Crete’s shallow and
deep structure to elucidate seismotectonic features of the Hellenic subduction zone. We summarize our results as follows:
1. Strong evidence for the existence of SWverging and NE-dipping thrusts has been found.
Thrusting is related to the propagation of the
Hellenic fold-and-thrust system. This process
occurred during the Oligocene. The thrust front
affects the sedimentary sequences up to the
Messinian or Lower Pliocene, with possible
reactivation in the early Pleistocene (Tortorici
et al., 2010).
2. NNE-SSW and approximately E-W faults
intersect Neogene sediments in southern Crete.
These sediments are likely Tortonian or younger.
NNE-SSW and E-W faults possibly affected the
entire southeastern Aegean, and remain active in
the study area.
3. The top of pre-Neogene strata in southern
Crete is represented by a very strong reflector
ranging at 0.8–4.0 s TWT. Small-scale domes
derived from magmatic or evaporitic (Messinian)
intrusions deform Pliocene–Quaternary strata.
4. Shallow compressional structures are interpreted to occur at depths of ~20 km.
5. Intense extensional (locally oblique) tectonics is recognized in the upper 10–15 km of
the crust, while oblique compression predominates below these depths.
ACKNOWLEDGMENTS
Special thanks go to Professors of Seismology
Papadimitriou Eleftheria and Karakostas Vassilis
for their valuable suggestions. RMS (root mean
square) velocity data were available in the context of
“Pythagoras” funded by the EPEAEK (Operational
Programme for Education and Initial Vocational
Training), Geophysics Department contribution 689
in 2006. V. Lykousis, D. Sakellariou, and the Hellenic Centre of Marine Research (ELKETHE) are
acknowledged for providing seismic-reflection data
on the tectonic troughs south of Crete. We also thank
the scientific and technical crew of the RV Aegeao for
their support during the 2005 Hermes-1 and Hermes-2
cruises (Cretan margin). We are grateful to Professor
Caputo Riccardo and an anonymous reviewer for the
critical review and constructive comments.
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SCIENCE EDITOR: NANCY RIGGS
ASSOCIATE EDITOR: VICTOR A. RAMOS
MANUSCRIPT RECEIVED 13 FEBRUARY 2011
REVISED MANUSCRIPT RECEIVED 7 DECEMBER 2011
MANUSCRIPT ACCEPTED 8 DECEMBER 2011
Printed in the USA
13

Geological Society of America Bulletin

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