On the Hellenic subduction zone and the geodynamic evolution of Crete since the late Middle Miocene

Tecronop~~srcs.

146 (1988) 203-215

Elsevier Science Publishers

203

B.V.. Amsterdam

- Printed

in The Netherlands

On the Hellenic subduction zone and the geodynamic evolution of Crete since the late Middle Miocene J.E. MEULENKAMP,

M.J.R. WORTEL, and

Insiituie

W.A. VAN WAMEL,

E. HOOGERDUYN

W. SPAKMAN

STRATING

of Earth Scrences, Unraers~!, of Utrecht, Utrecht (The Netherlands) (Received

July 10, 1987; accepted

July 18. 1987)

Abstract Meulenkamp.

J.E., Wortel.

M.J.R..

subduction

zone and the geodynamic

(Editor).

The Origin

and Evolution

In recent syntheses its Initiation, Angelier

plays

system

are: (a) initiation

13 Ma ago. whereas and formation

subduction

giving

distribution

approximately

of the Aegean

Aegean Mantle

structure

to reconcile.

Assuming

contributing

to the separation

Hellenic

subduction greater

modification extension

importance

of the tectonic caused

In: F.-C. Wezel

the Hellenic

approximately regime

zone. and in particular

put forward

arc is inferred

by Le Pichon.

to have

5 Ma. and (b) migration

in the back-arc

region.

which

taken

place

of the trench led to general

the Aegean

a northward can

area indicate

dipping

not

he reconciled

400 km as an upper bound

that the former aspect requires

slab down

to a depth

with

of the present

for the effect of stretching

of Crete from the Eurasian

of at least 600 km

straightforward

mainland

initiation subduction

of zone

of the Aegean

Sea

we arrive at an age for the

zone of at least 26 Ma.

As to aspect (b): geological of much

model

of about 5 Ma for the time of initiation

and of other processes

E.. 1988. On the

Miocene.

subduction

comprehensive

along

underneath

are even more difficult

Strating.

Sea.

hypocentera

13 Ma ago. Estimates

area the Hellenic

of the recent

indicating

of earthquake

Hoogerduyn

146: 203-215.

rise to an extensional

of the Upper

W. and

of Crete since the late Middle

several others have suggested

The seismic velocity

the depth

Spakman.

of subduction

of the present-day

New data on the structure modification.

evolution

role. Two basic aspects

to the south-southwest.

subsidence

W.A..

evolution

of Arcs. Tecronophysics,

of the geodynamic

a central

and co-workers,

approximately

and

Van Wamel,

Hellenic

data of Crete lend strong than

evolution

by a migrating

concept,

the tectonics

hitherto

thought.

current trench

of the margins

models

for the Cretan system

ago is not attributed

to initiation

Crete is taken to be associated

of subduction with deformation

to the idea that (on Crete) compressional

Combining segment

(roll-hack)

of a region subject

In our model the documented

support

imply.

geophysical

of the Hellenic

in a land-locked

to stretching

fragmentation

and geological arc. Whereas

basin

remains

data

the basic

a very useful

process

of the roll-back

of the crust resulting

process.

from the overall

Compressional

tensional

is a of

and sound

(in this case Crete) may be more complicated

of Crete into several basins which started

hut to inception

tectonics we propose

about

than 12 Ma

tectonics

on

stress regime.

Introduction

ranean

Although the Aegean area (see Fig. 1) has been studied by various disciplines in earth sciences for many years, it has received renewed and intensified attention during the last 10 to 15 years. In particular since McKenzie’s attempt to apply plate tectonic models to the tectonics of the Mediter-

Kenzie, 1978a,b) that the Aegean Sea is an area subject to stretching of the lithosphere. much work has been dedicated to integrate pertinent regional geological and geophysical data. Especially the French team of Le Pichon, Angelier and coworkers have made an extensive study of the Aegean area, resulting in a series of papers dis-

0040-1951/88/$03.50

0 1988 Elsevier Sctence Publishers

B.V.

(McKenzie,

1972) and his suggestion

(Mc-

204

~~~- ~_ 2:

Fig. 1. Sketch of the Aegean Angelier

24

area, with the Hellenic

Trench

system.

~~

~_~~ ~~ ! ~~ 7P

26

Triangles

indicate

O-3 Ma old volcanos.

After Le Pichon

and

(1979).

cussing the geodynamical including the underlying Angelier

evolution of the region mechanism. We refer to

et al. (1982) for a synthesis

McKenzie’s

suggestion

concerning

of this work. lithospheric

stretching was taken up by Le Pichon and Angelier and co-workers (Le Pichon and Angelier, 1979;

nature

and structure

along

the strike

Ionian

of the trench, however,

of the trench.

part is taking

up the convergence

the Aegean lithosphere (Eastern Mediterranean, Plate) lithosphere,

varies

The southwestern between

and the Ionian Basin a part of the African

whereas

the eastern

part of the

Angelier et al., 1982) who analyzed the bathymetry of the Aegean Sea in the light of the stretching

trench system rather acts as a transform fault systems (Le Pichon et al., 1979). Several distinct

hypothesis.

trenches

The results

the deformation incorporated

of an extensive

of Crete by Angelier in this analysis.

ted to be part of the region

study

of

(1979) were

Crete was interprein which stretching

is

and has been taking place. The uplift of Crete, which began in the Early Tortonian and became most pronounced in the Plio-Pleistocene, is not in agreement with stretching. Le Pichon and Angelier

can be distinguished

in this eastern

of the Hellenic

Trench

system,

and the Strabo 1.

trenches

(Jongsma,

part

such as the Pliny 1977). See Fig.

Two basic aspects of Le Pichon and Angelier’s recent model, both of which concern the Hellenic subduction zone, are: (a) initiation of subduction

(1981) explain this uplift by invoking underplating of Crete by subducted sediments (see also Ange-

along the Hellenic arc, inferred to have taken place approximately 13 Ma ago (Le Pichon and Angelier, 1979), and (b) migration of the trench

lier, 1981). In the geodynamical evolution of the Aegean region the Hellenic trench system plays an important role. The system marks the site of a subduction zone, as is evident from seismicity and focal mechanism data (McKenzie, 1978b). The

system to the south-southwest, giving rise to an extensional regime in the back-arc region, which led to general subsidence and formation of the present-day Aegean Sea (Le Pichon and Angelier, 1981). The latter process is part of the dynamics of a land-locked basin (Le Pichon, 1982) which

205

characterizes ranean Arabia

the

configuration

after continental and Turkey

sion between

collision

of the

Mediter-

lithosphere

(or rather

between

Africa/

lithosphere

involved)

by colli-

started

took place, preceded

Iberia

and North

Africa

(see Der-

court et al., 1986). In other studies (e.g. McKenzie, 1981) the age of the present zone was assigned Ma or somewhere

younger

Mantle current the

Hellenic

between

Aegean

about

Upper

reheating

Mantle

5

tually

to the extent

pendence

of the slab’s

reheating

is a strong

and interpretations

paper

the

evolution

contains

of

a brief

no earthquakes

1982). Through thickness

function

slab can be

the age-de-

the process

of

of the age of the

shows

resorption

slabs in the various

cooling

In the warm

of the slab, even-

slab. This is illustrated

account of our integrated approach. We analyze some fundamental aspects of the Hellenic subduc-

times

in Fig. 2, which

of the

subduction

lithospheric

zones as a function

*OI

tion zone. To investigate the relationship between the subduction process and the geology of the non-volcanic Hellenic arc we use field data from Crete.

centre.

descending the

and

of the downgoing

that

of the Aegean

Upper

of the

it has been

properties

(see Wortel,

for the geodynamic This

which

at a spreading

generated

of Crete lead us to reconsider area.

during

after formation

5 Ma and 10 Ma.

and new observations models

of the period

part

at the trench

since this age equals the length

affects the rheological

subduction

ages, usually

New data on the structure of the geology

1978a; Mercier,

to descend,

the particular arrived

18

16

i

1

1

;ii 14r.

Subduction

in the Hellenic

Subduction

zone seismicity

w 12zz F -

arc

z loo _ i= k*

The seismicity associated with the Hellenic subduction zone has played an important role in Le Pichon

and

Angelier’s

(1979,

and

much

evolution Wadatithe lithoSea these the sub-

lithosphere corresponds with subduction the last 13 Ma, with an uncertainty of as as 3 to 5 Ma. This conclusion

E6

44

subsequent

papers by these authors) analysis of the of the Aegean area. Assuming that the Benioff zone delineates the geometry of sphere subducted beneath the Aegean authors concluded that the length of ducted during

is

is based

on

the assumption that the thermal time constant for subducted lithosphere with an age > 70 Ma (which is pertinent for the Hellenic subduction zone) is much greater than 13 Ma (Le Pichon and Angelier, 1979). From a detailed study of all subduction zones for which adequate data on the age of the subducted lithosphere, the plate kinematics involved and the geometry of the subducted slab were available Wortel (1980, 1984) has shown that the latter assumption is not correct. For the subduction process the significant age of downgoing lithosphere is the age at the time the

2

1

I

20

40

80

60

100

I

r

I

140

120

160

1 1

180

’

I

200

AGE (Ma) Fig. 2. Resorption function

time of subducted

of lithospheric

tion times for the following 3 -Izu-Bonin; -Tonga;

9-Kermadec;

America; symbols plotted

I9-Aleutians.

slab started the present

tion

divided

potential ered

changes.

in age:

age at which

The other

part

are

of the

end of the bar bars are used

zones in which the age of the subducting 100 and 140 Ma. The two curves

by the convergence

temperature. and

12, 13

the circles

the deepest

times (that is, the maximum

as the real

compression

7 and 8

Bars on one side of the

variations

to descend.

Honshu;

16 and f7-Central

age at the trench. Two-sided

for some subduction the resorption

2 -N. Zealand;

Peru:

as a

the resorp-

6 -Java;

II-New

central

temporal

at the estimated

may vary between

I -Kuriles;

10 and

I8-Alaska;

lithosphere

indicate

5-Sumatra;

L-North

indicate

subducted indicates

zones:

4 -Ryukyus;

and Il-Chile;

oceamc

age. The symbols

Potential

temperature

rate)

downdip

of two isotherms

temperature minus

the contribution

indicate penetra-

the effect of latent

of

may be considof adiabatic

heat

of phase

206

of age. The resorption downdip from

as the

length of the seismic zone (S,,, measured

the

surface

earthquake normal

time t,,, is defined

to the

foci) divided increase

proximated

deepest

by the PDE in the years 1964-1984

this region. center

The accuracy

locations

of many

is known

the order

(u,). The graph shows

uncommon

for this region.

very poorly

determined

time with the age

lithosphere.

This

by a linear relationship

can

be ap-

for the resorp-

of several

tens

or 10 km of depth, concentration

The

which

of events

Errors

of kilometers

events

hypocenters

explains

the peculiar

at these levels. Whatever

errors are, there is a strong

f res= (0.12 * 0.03) X age

lation

the deeper

events

part of the slab. These events observed resorption time is

15 Ma. The Trench

lithosphere descending in the Hellenic is of Mesozoic age (see Dercourt et al.,

1986). and most likely about 100 Ma old. Compar13 + 3 Ma or 13 _t 5 Ma for the

duration of the present episode of subwith the resorption times in Fig. 2 for

lithosphere of Mesozoic age clearly shows that thermal resorption can explain the length of the Wadati-Benioff

zone. Thus, any inference

on the

duration of the ongoing drawn from the maximum

process of subduction depth of seismicity or

length of Wadati-Benioff ered with due caution.

zone should

Seismic

tomographic techniques for 3D-mapping of the earth’s seismic velocity structure. Figure 3 shows a Upper Mantle of the seismic

for P-waves.

high velocity zone (cross-hatched) the blurred image of the Aegean the cooling of oceanic lithosphere tion slab

known

The

dipping

is recognized as slab. It is due to prior to subduc-

and possibly chemical differences exhibits higher seismic velocities

are possibly

related

stresses

in the upper

the deepest

occurrence

whether

the slab

penetrates

than 600 km, which is the deepest of the tomographic Mantle

structure

mapping.

even significant

The imaged

of the Aegean

deeper level Upper

area is discussed

at greater length in Spakman et al. (1988). For the employed tomographic method we refer to Spakman and Nolet (1987). Analysis From

tomogrclphy

heterogeneity

corre-

the upper

of seismicity is not an indication for the maximum penetration depth of the Aegean slab. It is not

the tomographic

infer a minimum

cross section through the Aegean displaying the structure in terms

and

be consid-

The existence of a subducting slab that penetrates deep into the Aegean Upper Mantle, is demonstrated by Spakman (1985, 1986) using

velocity

of bending

part of the slab. Evidently

ing the estimated inferred duction

to the relaxation

of

are fixed at 33 km

the mislocation

Note that the muximum

on

are not

tion time:

between

for

of the hypo-

to be poor.

rate

in resorption

of the downgoing

of the

by the convergence

to the plate contact

a strong

depth

reported

that than

the the

ambient Upper Mantle and hence it can be made “ visible” by tomographic mapping. Spakman and Nolet (1987) argue that the slab’s outline is resolved with a spatial error between 50-100 km and that the velocity anomaly amplitudes are well resolved in their sign. Also indicated in Fig. 3 are the hypocenters of those events that have been

around amount

length

results

(see Fig. 3) we

of the subducted

slab of

800 km. Any reasonable estimate of the of stretching which has taken place in the

Aegean Sea area would be smaller than 800 km in north-south direction. Even the full north-south dimension

of the present-day

Aegean Sea is smaller

(see Fig. 1). Therefore the observed subducted slab can not be attributed of the Eastern Aegean

Mediterranean

lithosphere

caused

latter. We, therefore, consider contributing to the slab

length of the to overriding

lithosphere

by the

by extension

of the

two types of processes length: (a) subduction

associated with Africa-Eurasia convergence and (b) subduction associated with extension of the Aegean,

including

rotation

of more

or less rigid

blocks, and with a component of the motion of the Turkish Plate taken up in the Hellenic subduction zone, and possible other processes on the boundary of the Eurasian plate. For simplicity, these processes will be loosely referred to as backarc processes.

207

Figure 4 schematically

displays

two types of contributions. “Liv.

the length

of the

data given in Livermore

and Smith (1985); it amounts

The solid line labelled

indicates

& Sm.”

basis of the plate rotation

the role of these

to 1.5 cm yr

lower solid line is based on the kinematic

sub-

ters given by Savostin

ducted slab produced by convergence between Africa and Eurasia as a function of duration of subduction process in the Hellenic subduction zone. This convergence rate is estimated on the

‘. The

parame-

et al. (1986). The average

convergence rate is around 1.0 cm yr- ‘. For any estimate of the total contribution of the back-arc processes, Fig. 4 enables us to derive the duration

distance&m) lon= azi=

+4% Fig. 3. A cross section through

the Aegean

longitude

The great

and 31.1’

geographxal

region.

latitude.

Dots indicate

lower panel shows a tomographic velocity

anomalies

measured

Upper

Mantle

circle segment

6561 earthquake

which is taken along the great circle segment has an azimuth

epicenters

image of the Aegean

in percentages

relative

Upper

which

of 14” and a length have been reported

Mantle

to the surrounding

model.

The scale IS given at the lower left. Dots indicate

the projection

located

between

panel.

the two dashed

lines shown

in the upper

in cross section. Upper

Mantle

23.9lat= 14.0 dis= A-B.

A corresponds

of 13”. The upper

reference

on the cross section

plane

indicates

velocrty.

to 23.9 o

panel displays

by the PDE m the years The hatching

31.1 13.0

196441984.

the inferred

the The

P-wave

i.e. the Jeffreys-Bullen

of those hypocentera

which

are

208

subduction

(also partially

the seismically

rect. Nevertheless, because

on the length

we will consider

ses in the light

not

such as dating

uniquely

a role in the analyses 1979; Mercier, Using 20 Ma BP

Fig. 4. Duration ducted

of subduction

slab in cross section

process

versus

length

of sub-

of Fig. 3. See text for explanation.

results

related

of changes

stress field, and ages of volcanics.

to slab

in the regional have also played

(see Le Pichon

and Angelier.

1981).

the upper

contribution

of

these hypothe-

of the new tomography

arguments

length,

based

active slab) are likely to be incor-

solid curve (in Fig. 4) for the

of Africa-Eurasia

convergence

we

analyse

the implications

cerning

initiation

of the hypotheses

Trench.

We note that by doing this and by assum-

of subduction

con-

in the Hellenic

ing that the Africa-Eurasia convergence is fully taken up in the Hellenic Trench system we arrive of the ongoing

subduction

process

required

to

at conservative

produce a subducted slab with a length of 800 km. For the combined back-arc processes we assume a

bution

contribution to the length of the subducted slab of 200 to 400 km. The lower bound is in agreement

started

with the amount basis

of crustal

of stretching thickness

and

calculated

on the

bathymetry

data

(see below). The value of 400 km is adopted as a safe upper bound including the effects of block rotations

and

motion

leaves 400 to 600 Africa-Eurasia plate

of the Turkish km to be convergence.

Plate.

This

explained by This range is

indicated by the light shading in Fig. 4. Then, the curve based on the rotation parameters given in Livermore

and Smith

(1985) yields

a duration

26 to 40 Ma. The lower curve, after Savostin (1986) estimates

gives a minimum for

the

of 40 Ma. By varying

contribution

of the

of

estimates

of back-arc

processes.

for the 800 km slab length slab length resulting total

convergence

In order to account

from back-arc rate

contri-

by subduction

5 Ma ago one needs to invoke averaged

which

720 km of

processes. over

The

the 5 Ma

period would be 16 cm yr-‘. The length of 720 km and the very high convergence rate makes it extremely

unlikely

that initiation

place 5 Ma ago. Initiation of subduction

of subduction

took

13 Ma ago requires

600

km of slab length produced by back-arc processes. The total convergence rate averaged over the 13 Ma period convergence

would

be 6 cm yr-‘.

rate may be possible

et al.

above

estimate

the

bound

to the contribution

back-arc

for the required

Stretching

Although

this

we consider

the

of 400 km as a realistic

factors

for

of back-arc the

Aegean

upper

processes. are

crudely

processes and using other values for the average Africa-Eurasia convergence rate one can investi-

estimated to be about 2 by McKenzie (1978b). On the basis of a detailed analysis of the bathymetry

gate the family of possible solutions. Le Pichon and Angelier (1979) that subduction along the Hellenic

Angelier et al. (1982) arrived at values of about 1.6 to 1.8 for the Sea of Crete, and of 1.1 to 1.4 for

hypothesized Trench was

initiated 13 Ma ago, with an uncertainty of as much as 3 to 5 Ma. In their model the downdip end of the Wadati-Benioff seismic zone indicates the depth reached by lithospheric material during the last 13 Ma. Their assumption that this maximum hypocentral depth corresponds with the leading edge of the subducting plate, however, is not in agreement with the tomographic results. Similarly, estimates of about 5 Ma for initiation of

most of the central and northern Thus it appears that the required

Aegean region. 600 km implicit

in the 13 Ma hypothesis is too high to be accounted for (see also Fig. 1 for present-day dimensions). In addition, the seismicity data are not in support of initiation of subduction as recent as 13 Ma (or 5 Ma) ago. The corresponding young ages of the subduction zone would imply that several hundreds of kilometers of slab material subducted during the last 13 Ma (or even 5 Ma) are not

209

seismically pletely

active. This would be a situation

unknown

subduction

from all other

zones.

as a possibility

because

such as the Hellenic

the

dynamics

is not

ably

older

seismicity because

data

migration 13

Ma-as

would

the downdip

not

We conclude seismicity

we

any

problem, activity

with resorption time zones (see Fig. 2).

the combined indicate

that

tomography subduction

(Meulenkamp,

of

1979).

interval

to another

changes

in basin

patterns. curred

The

transitions

were marked

configuration

One of the most in the Late

boundary

between

mainland, transformed event

connecting

started

one

and sedimentation dramatic

changes

Serravallian-Early

interval

mass, hitherto

from

by pronounced

about

oc-

Tortonian 12 Ma and

Ma ago. At that time the Southern

depressions

suggest-the

end of the seismic

that data

sys-

zone is consider-

cause

would be in full agreement data from other subduction and

by the roll-back of the trench

the subduction

than

zone

to differ

zones where plate

dominated

(oceanward

tem). If, however,

of (and

in) a subduction

zone are expected

from those of other subduction process

well-documented

Still this may not be excluded

hence the stress distribution

convergence

com-

Aegean

11

land-

Crete with the European

to break up: Crete itself became

into

a mosaic

(Meulenkamp

was connected

of culminations and Hilgen,

by Le Pichon

and Angelier

(1979) with the beginning

of the Hellenic

tion.

targets

One of the primary

and

1986). This subduc-

of our renewed

investigation of the Neogene basins was to unravel the tectonic processes which controlled the paleogeographic

revolution

in the latest

Serraval-

lithospheric material did not start approximately 5 Ma nor approximately 13 Ma ago. On the basis of

lian to Early Tortonian and the ensuing repeated changes in basin configuration from the Early

the data and results available now we arrive at a minimum estimate of 26 Ma. In addition to the

Tortonian onward. These processes ultimately led to the uplift of parts of Crete to a height of about

possibilities of higher ages discussed above, it should be kept in mind that the observed length of 800 km constitutes a minimum length because at

2500 m. For this purpose we investigated the orientation and nature of the folds and faults which controlled the Neogene basin development and the continuations of these structures in the

present no information is available on the structure of the mantle deeper than the maximum depth of the cross section in Fig. 3. Vertical movements the arc: Crete The available

and tectonic stress field along

relatively

detailed

knowledge

pre-Neogene

basement.

From an extensive analysis of the properties of the fault systems of Crete, Angelier (1979) and Angelier

et al. (1982) postulated

an overall

exten-

of

the structure of the Aegean Upper Mantle and of the geology of Crete provide an excellent opportunity to study the relationship between the process of subduction beneath the Hellenic arc and the geological

processes

at and near

the surface.

Please note in the following that differences-for example between Le Pichon and Angelier (1979) and our work-in dating of the same geological events arise from the different time scales used. We refer to Meulenkamp and Hilgen (1986) for information on the time scale used in the present study. Stratigraphic basin analysis of the Neogene made it possible to reconstruct eleven successive intervals for the evolution of the Cretan area from the Middle Miocene to the Late Pliocene

Fig. 5. Stereographic projection) fault planes, n = 35.

showing carrying

diagram

(Schmidt

the distribution striations

net, lower hemisphere

of the poles to the normal

with pitches

larger

than

80°.

210

sive regime since the end of the Middle Expressed

into a triaxial

operating

principal

u2 = intermediate these authors

orthogonal

stresses stress

vertical orientation, horizontal, during Although Angelier

(u, = maximal

stress,

u3 = minimal

stress)

and

concluded

that

whereas

Miocene.

a,

sion would have occurred

system of the

had

Fig.

6. Highly

projection)

schematic

structural

depressions,

Dashed

lines indicate

downthrown

block;

plane;

5 = oblique-slip

planes,

single barbed

8 = fold-axis

map

insets give the preliminary

from oblique-slip

synclinal

geological

the Neogene

uncertainties. 4 = reverse

with the direction

and

fault. Barbs are drawn

fault. Barbs are drawn arrows

sequences

I = Neogene

indicate

and amount

till

lateral

of plunge.

of

the

Pliocene

et al. (1982), we observed

features

all over Crete.

containing

of the analysis

culminations

planes of the nappes block,

block, double

movement;

(see Fig. 5) (1979) and

compressional

reverse

faults

and

the preliminary results of our structural geological (plotted in stereographic of o,, ez and cr3 orientations

2 = pre-Neogene;

in the downthrown

Folds,

and early

faults were found, deforming Neogene and the pre-Neogene basement

the hinge zones of anticlinal

as well as the thrust Quatemary;

features

the

by Angelier

Angelier

oblique-slip sequences

in the downthrown

the relative

transition

end

which were also mentioned

of Crete, results

faults. The axial traces indicate

deforming

the Miocene-Pliocene

duing the Quaternary.

Apart from the extensional

cr2 and (Jo were about

approximately the last 13 Ma. (1979) reported the existence of

The circular

ably during

an almost

compressional features on Crete, he qualified these as slight and of subordinate importance. Compres-

investigations.

only locally, most prob-

3 = normal

the arrow

barbed

6 = axial trace

arrows

and the trough

within

indicates indicate

of an anticline;

the pre-Neogene

fault.

Barbs

zones of the basement.

are drawn

in the

the dip direction

of the fault

the dip directions

of the fault

7 = axial trace of a syncline;

211

throughout

the island. The first attempts

the orientation slip faults

of the active

u, from the oblique-

(see also Angelier,

existence

of CJ, orientations

to analyse

1979) revealed with inclinations

tween 0 o and 54 o and declinations

between

sults graphic

be-

periods

006 o

sinistral

( a3) caused maximum

extension

slight

NNW-SSE

(see Figs. 6 and 8~). Some

folds measured orientations

from the Neogene

of the fold-axes

tions between

088’

to

rocks revealed

with

plunge

direc-

and 126 O, as well as between

278” and 322”, with plunges

up to 20” (see Figs.

7 and 8d). Combination

of these structural

geological

results

the Neogene

folds

directions

known

the

and 060 ‘. as well as between 173” and 243” (see Figs. 6 and 8a). Coeval minimal principal stresses in ESE-WNW

with

re-

stratigraphic

(Meulenkamp,

with

slightly

inclined

and

dextral

oblique-slip

above.

ready in late Serravallian folding

Aegean

faults

and

the

appears

that

al-

12 Ma ago)

in the development the margin

the large basin landmass

time. The ensuing

the

and the anticlinal

within,

from

to date the

u,, generating

It thus

tions, which in turn defined as “swells”

paleogeo1985)

time (around

resulted

depressions

southern

1979,

offers a good opportunity

mentioned

synclinal

and

of the culmina-

of, as well

bordering

to the south

gravitational

sliding

the

at the

of the Pre-

Neogene and Neogene slabs from the culminations into the basin in the latest Serravallian (Fortuin,

1977; Meulenkamp

well be associated At the beginning

and

Hilgen,

to this early period of the Tortonian

1986) may of folding.

(approximately

11 Ma ago) folding and/or faulting controlled uplift of the pre-Neogene massifs in the Cretan area proper already had begun. During the Early Pliocene (approximately 4 Ma ago) the general northward

tilting

of the island,

accompanied

with

N

I’ #’

c

:_::~

N

Cl ,’

,’ ..:.. .y...

%’ ,::

1’

,’

e

Fig. 7. Stereographic projection)

of the poles

the Neogene, Platanias

diagrams

(Schmidt

to folded

with reconstructed

area, (b) Agia Galini

net. lower hemisphere

sedimentary

planes

fold axes (encircled area, (c) region

ion. (d) Mirtos area, (e) Goudouras

area.

within

dots). (a)

south of Irakl-

Fig. 8. Stereographic projection)

showing

diagrams

n2 and ej and the fold-axes, orientations, tions,

analysed

net, lower hemisphere

of the orientations during

n = 15, (b) cr2 orientations,

n = 15, all analysed

Fig. 6. the circular (compare

(Schmidt

the compilations

Fig. 7)

from

n = 15. (c) LT)orienta-

oblique-slip

insets), (d) orientations

of n,,

this study. (a) (1, faults

(compare

of fold-axes,

n = 5

212

the folding

and/or

faulting

controlled

the pre-Neogene

massifs,

lift of the Cretan

area rapidly

Early

Pliocene;

since

started.

that

uplift

of

The rate of up-

increased

during

moment

the

uplift of Crete and the main foundering

the

general

of the Sea

tion of al) have been alternated

with periods

approximately

N-S

oriented

ing (slightly

inclined

opinion

rectly related

The

The age of the Hellenic strong

implications

the relation

Trench

for current

between

the geological

subduction

zone has

ideas

concerning

the subduction

processes

process

at and near

and

the surface.

retreating

all extensional

regime

system

of the

processes

and the generation zones

an over-

lithosphere.

of lithospheric

of south-dipping

low

et al., 1984)

sche-

Le Pichon and Angelier (1979) relate the important changes (see above) which occurred in the

matically

Late

supracrustal

slab sliding

driven by supracrustal

gravity. In the rear-end of the slab active continental extension (Lis-

Serravallian-Early

terval

(12-11

present

Hellenic

Ma

ago)

these

changes

boundary

to the initiation

subduction

mate of a minimum modification of this that

Tortonian zone.

in-

of the

Our new esti-

age of 26 Ma requires a interpretation. We propose are not

associated

with

ini-

shear

indicated

(see Lister

motion

generated

in the Aegean

This might have initiated shear

of the Sea

the existence

(south-southwestward) Trench

stretching

origin

is di-

to its south.

of the Hellenic

angle

In our

on Crete

to the extensional and

shorten-

of u,).

situation

to its north

Hellenic Interpretation

orientation

this particular

of Crete

of Crete took place.

to NE-SW

with

zones

in Fig. 9. One of the low angle

formed

the detachment

plane

in southward

ter et al., 1984) led to the generation

of a

direction,

of the Sea of

Crete. In the frontal parts of the supracrustal slab compression may be generated, especially during

tiation of subduction but with the inception of the roll-back process, leading to the south-southwest-

periods

ward migration

As

up to the surface within

the Hellenic

it changed the boundary conditions for the Aegean lithosphere, the beginning of roll-back must have

of Crete. In this frontal

part this shear zone dips

of the Hellenic

Trench

system.

significantly affected the stress field (see Wortel and Cloetingh, 1986) in particular along the arc, and turned it into a tensional regime. We tentatively relate the beginning of the roll-back process to

the

final

stages

of

the

collision

Africa/Arabia and Turkey. A comparison between the results

between

of tectonic

slab. In our opinion

of the supracrustal

the low angle shear zone cuts

north, as it meets the bulge ducted Ionian Plate.

Trench

of the former

south sub-

The tectonic setting just described elucidates many of the apparent contradictions in the geology of Crete. The position of the island at the southern front of a supracrustal slab, sliding in southward

of the analy-

transport

tion

direction,

of extensional

would

explain

the combina-

and compressional

structures

sis of structural features of Angelier (1979) and Angelier et al. (1982) on one hand and our results on the other learns that there is a strong agree-

found. During periods of tectonic transport within the supracrustal slab compressional structures could be generated. Due to NE-SW and

ment concerning the importance of extensional processes during the Neogene of Crete. Also the orientation of maximal extension (= orientation

NNE-SSW

of ai) is quite comparable in both studies. The difference of opinions lies in the appreciation of the importance of compression during the Neogene of Crete. Contrary to Angelier and co-workers, we attribute an important role to the compressional processes during the Neogene history of Crete. Our preliminary results indicate that during the Neogene of Crete periods with almost universallly directed extension (approximately vertical orienta-

oriented

compressional

forces

(u,

slightly inclined) the pre-Neogene basement has been weakly folded, accompanied by reverse faulting. In the meantime conjugate oblique-slip faults with an E-W dextral

oblique-slip

faults

with

sets of sinistral orientation and a

NNE-SSW

orientation developed. Synclinal bending of the pre-Neogene basement as well as the normal faulting and the oblique slip faulting strongly controlled the generation of the Neogene basins. The irregularity of the fold pattern, of the traces of the axial planes and the plunge directions

213

slab’s

volume-between

warmer

the

surrounding

Upper

Wortel

and

tiation

of subduction,

Cloetingh,

slab has insufficient force associated

cold

1986).

and

Shortly

however, length,

slab

Mantle the

the

(see

also

after

ini-

descending

and hence insufficient

with it, to overcome

the resistive

forces acting on the slab. At this stage the roll-back can not yet start the rather

inferred

by Le Pichon

the present

Fig. 9. A schematic Hellenic

Trench,

N-S with

slab. The position indicated

transect a possible

of the Moho

schematically.

from Ios, over Crete, model

of the supracrustal

in the Aegean

For further

to the

explanation

lithosphere

is

see text.

to operate.

whether

short

dipping

seismic

uplift

within

the

southern

slab have caused

of Crete,

being

stronger

extreme the recent

of

the

overall

in the south,

cou-

pled to a northward tilting of large parts of the island (see Fig. 9). During periods without tectonic transport of the supracrustal slab, almost vertical ui (gravity) generates almost universally oriented extension within the supracrustal slab. Discussion The presence

as

(1979) from

zone (see also Fig. 3)

is sufficiently

long to produce

large enough

to overcome

a gravitational

the resistive

force

forces.

In

our model subduction has been active for a considerably longer time (> 26 Ma). Thus, by the time

of the

Late

Serravallian-Early

ate, there was already

supracrustal

of the slab

and Angelier

Tortonian

boundary interval (about 12 to 11 Ma ago), when in our model the roll-back process starts to oper-

of the fold axes (see Fig. 6) is in agreement with a gravitational origin of these folds. The folding and thrusting

It is even doubtful

length

in our model of several hundreds

of (hitherto undiscovered) kilometers of subducted slab deep in the Upper Mantle solves a problem attached to the hypothesis put forward by Le Pichon and Angelier (1981) and Le Pichon (1982) regarding the retreating motion of the Hellenic Trench system. This roll-back process is assigned a key-role in the dynamics of the Eastern Mediterranean, and may well be responsible for the extension in the Aegean lithosphere. We agree with Le Pichon and Angelier that this process is a physically sound explanation for the fundamental aspects of the geodynamics of the Eastern Mediterranean region. The retreating of the trench system is caused by the gravitational force associated with the density difference-integrated over the

to generate

a subducted

the retreating

system. We have proposed

slab “available”

motion

of the trench

that the southward

transport

of the detached supracrustal slab (of the Aegean lithosphere) may be accompanied with compression in the frontal

parts

of the slab. Folding

and

thrusting within this slab are taken to be the cause of the uplift of Crete. At this stage of our investigations this southward sliding is primarily corroborated by the timing a-Id nature of the geological processes on Crete. Whether the potential energy available in the Aegean lithosphere under tension can adequately

account

nism or whether

large scale processes

Mantle provide (part yet to be analysed.

for the proposed

mecha-

in the Upper

of) the necessary

energy

is

Conclusions In view of the unique availability cal and geological data the Hellenic

of geophysiarc provides

an important opportunity to study the evolution of a non-volcanic arc within the framework of a landlocked basin. In the light of new tomographic results concerning the length of the slab descending beneath the Hellenic arc we present a revision of previous estimates of the age of the Hellenic subduction zone. We arrive at a minimum age of 26 Ma. This implies that the geological processes

214

around

12 Ma ago. such as the fragmentation

Crete into several basins, to the initiation Trench. ated

we propose

with the inception

(south-southwestward

hitherto that almost

tectonics

process

of the

of Crete

is more

of Crete

directed

with periods oriented

the stretching

results

than indicate

A.R..

compression. of the Aegean

In:

N-S

Academic

K.

trench

forms a part, compression Folding and thrusting within

may be generated. the southern extreme

of the supracrustal slab have uplift of Crete. In this way

caused the recent compression (and

uplift) along the arc is a corollary process.

of the stretching

Trans.

of

Crete.

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