On late miocene to recent vertical motions in the Cretan segment of the Hellenic arc

TECTONOPHYSICS ELSEVIER

Tectonophysics 234 (1994) 53-72

On Late Miocene to Recent vertical motions in the Cretan segment of the Hellenic arc J.E. Meulenkamp,

G.J. van der Zwaan, W.A. van Wamel

Department of Geology, Institute of Earth Sciences, University of Utrecht, Budapestlaan 4, 3485 CD Utrecht, The Netherlands

(Received June 17, 1992; revised version accepted January 5, 1994)

Abstract High-resolution foraminiferal and calcareous nannoplankton biostratigraphy and reliable paleobathymetry estimates based on plankton-benthos ratios in foraminifera make it possible to reconstruct in detail Late Neogene vertical motions along the Central Cretan segment of the Hellenic arc. These motions are considered to express the surficial effect of the roll-back process of the Hellenic subduction zone, which started about 12 Ma ago. In contrast to earlier views there is no sustained uplift since the late Middle Miocene. Successive paleotopographic profiles for central Crete show a predominance of subsidence, coupled with an increase of differential reliefs, from the latest Serravallian until the Messinian. Subsidence was most pronounced between the Tortonian-Messinian boundary interval and the early part of the Early Pliocene, locally up to a magnitude of more than 1000 m. A two-phased uplift history, coupled with tilting to the north or northeast, can be inferred from the late Early Pliocene-Recent record, separated by a short, early Late Pliocene episode of subsidence. Rates of uplift were highest in the Early Pliocene, up to about 125 cm/ ka. The Central Cretan Late Neogene to Recent paleotopographic profiles are compared with those reconstructed from seismic interpretations and drilling data from the Cretan Sea. The combined results show that paleotopographic configurations between the Cyclades and Crete were fairly similar to those on central Crete until Messinian time. Foundering of the Cretan Basin started in the course of the Early Pliocene, but subsidence rates were less than the contemporaneous uplift rates of Crete. The timing and magnitude of the vertical motions along the Cretan Sea-Central Cretan transect put detailed geological constraints on tectonophysical modelling of the surficial effects of the roll-back process. These vertical motions are discussed in view of the model of southward translation of a large supracrustal slab, resulting in the origin of the Cretan Sea and in thrusting and uplift in the frontal parts of the slab, where it thrusted over the northern limb of the subducted Ionian Plate. 1. Introduction The Late Neogene to Recent geodynamic evolution of the southern Aegean area is controlled by the effects of the roll-back process of the Hellenic subduction zone (Le Pichon and Angelier, 1981; Le Pichon, 1982; Meulenkamp et al.,

1988; Spakman et al., 1988). These effects caused extension in the overriding Aegean plate and

resulted in outward motions of the Hellenic arc (Fig. 1A). Meulenkamp et al. (1988) argued that the roll-back process started in latest Serravallian to earliest Tortonian time, i.e., about 12 to 11 Ma ago. They related the ensuing evolution of the southern Aegean to the generation of a southdipping, low-angle shear zone, which formed the detachment plane for a supracrustal slab sliding

0040-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved 0040-1951(94)00006-U

SSDI

J. fi.‘.Meulenkmnp

CI ul. / 7‘rctonophy,sics 234

(19941 5.7 - 72

----- .--latest

Middle

Miocene

Recent

1A 1

Santorini

S Sea

10

of Crete

1

0

I

0

10

10

20

20 30

30

: 150

\

I

Legend

300

:

200

M = Moho

100

Subducting

Sedlmentary

lithospheric

wedge

km

I

I

350km

0

slab

1B

Fig. l(A). Outward motions of the Hellenic arc since the latest Middle Miocene (simplified after Angelier et al., 1982). (B) Schematic section across the southern Aegean area to illustrate the origin of the Sea of Crete and the uplift of the Cretan segment of the Hellenic arc as a function of the southward translation of a supracrustal slab after the inception of the roll-back process (after Meulenkamp et al., 1988).

55

J.E. Meulenkamp et al. / Tectonophysics 234 (1994) 53-72

depths of more than 1800 m. Folding and thrusting in the frontal parts of the slab, rather than underplating of subducted sediments (Angelier et

in southward direction (Fig. 1B). In this view, extension in the rear-end of the slab would have caused the Cretan Basin to subside, locally to

~~~

Sea of Crete

Neogene and Quaternary

4 T NORTHERN

IRiKLlON

BASIN ENTRAL

IRA/KLION

RIDGE

50km

Fig. 2(A). Schematic map of Crete showing the position of the Central Cretan transect. Shading refers to the distribution of Neogene and Quaternary sediments. (B) Present elevation of central Crete along the transect A-A’ of Fig. 2A.

J. E. Mruknkamp

56

el al. / Tectonophysicv 234 (1994) 53- 72

al., 19821, were held responsible for the overall uplift of the Cretan segment of the arc since the beginning of the Late Miocene. Parts of Crete were elevated to a height of about 2500 m since Late Miocene (Tortonian) time. These parts be-

Latest Middle Late Miocene

longed to a vast lowland-coastal plain area bordering the southern Aegean landmass to the south at the close of the Middle Miocene, i.e., in latest Serravallian time (Meulenkamp, 1985; Meulenkamp and Hilgen, 1986). This implies that the

and sediments

I-

j

Preneogene /

major

rocks

faults /

1 2 3 4 5 6 7 8

i---’

Kalithea Prassa Ag. Vlassios Finikia Veneraton Atsipadhes Apesokari Rizikas _-.i

/

PSILORITIS

Mts

ASTEROUSSIA

Mts

Fig. 3. Simplified geological map of central Crete and position of Pliocene sections studied for the bathymetrical SSW-NNE double lines delimit the position of the Central Cretan transect.

analyses.

J.E. Meulenkamp et al. / Tectonophysics 234 (1994) 53-72

present altitudes of about 2500 m are a fair measure for the (minimum) uplift of parts of Crete since the postulated initiation of the rollback process. Average rates of uplift based on these data are thus in the order of magnitude of 25 cm/ka (IX Pichon and Angelier, 1981). Geological evidence suggests, however, that the general uplift of Crete was most pronounced, or even began, during the Plio-Pleistocene. As a consequence, average uplift rate estimates for the entire latest Middle Miocene to Recent time span are of little use for a detailed reconstruction of vertical movements through time and might lead to erroneous conclusions on kinematic and dynamic aspects of the evolution of the Cretan segment of the Hellenic arc. Recently developed high-resolution stratigraphic scales (Hilgen and Langereis, 1988; Langereis and Hilgen, 1991; Zijderveld et al., 1991) and tested models to arrive at reliable paleo-depth estimates from plankton/benthos ratios in foraminifera (Van der Zwaan et al., 1990) make it possible to quantify subsidence and uplift patterns. In this paper we present the first results of such an exercise on temporal and spatial relations between uplift patterns inferred from the Pliocene sequences of central Crete. These results are combined with data on the Late Miocene in order to establish a tentative model for the nature and magnitude of the Late Miocene to Recent vertical movements of this area. Finally we will compare the results with data from the Cretan Sea in order to investigate the relationship between uplift and subsidence patterns along the ‘back-arc’ -arc transect. From this we will infer geological constraints on tectonophysical modelling of vertical movements along the Hellenic subduction zone.

51

profile perpendicular to the Cretan WNW-ESEtrending Alpine fold and thrust belt is shown in Fig. 2B. This profile represents the ‘end-member’ of a topographic evolution and mirrors the effects of overall tilting of Crete to the north or northeast. In central Crete this tilting started in the course of the Early Pliocene (Meulenkamp and Hilgen, 1986). The two more elevated parts of the section across central Crete represent the Asteroussia mountains and the Central Iraklion Ridge, respectively; the depression in between is the Messara plain. The Central Iraklion Ridge consists of a number of W-E-oriented culminations of pre-Neogene and Miocene rocks. A generalized geological map of central Crete and a set of simplified paleogeographic maps are represented in Figs. 3 and 4. The successive paleogeographic configurations reflect four major episodes in the evolution of reliefs along the Central Cretan transect. During the first (Late Serravallian) episode the area formed part of a lacustrine sedimentation realm bordering the southern Aegean landmass. The second and third episodes (latest Serravallian-Messinian) resulted in a paleogeographic setting where today’s culminations bordering and intersecting the Iraklion depression started to play a role in paleogeography and subbottom topography. During the Late Miocene the Iraklion depression proper developed and the W-E-orientied Central Iraklion ridge (Figs. 3, 4) started to separate the northern and southern Iraklion Basins. The fourth episode is characterized by the staggering uplift of the whole area in the course of the Plio-Pleistocene. The magnitude of vertical movements during the evolution of Late Neogene reliefs, will be treated in the following paragraphs.

3. Pliocene to Recent uplift history 2. The Central Cretan transect 3.1. The northern Iraklion Basin Neogene and Pleistocene sediments are widely distributed in the Iraklion depression, situated between the Psiloritis and Lassithi mountain chains (Fig. 2A), which reach altitudes of about 2500 and 2200 m, respectively. A topographic

The western and eastern margins of the Pliocene northern Iraklion Basin are formed by the Psiloritis/Talea Ori border faults and by the foothills of the Lassithi mountains, respectively.

.I.E. Meulenkump et al. / Tectonophysic,s 234 (I 994) 5% 72 I-~---~late

Early

D

Pliocene

Seaof Crate

Q Early

3.52 .----.

----~~----._

Ma

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Messinian

C sea

of

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6.10

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Tortonian

“Embryonic’

So4

of

Crete

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r-------latest

------I

Serravallian

Southern

Aegean

fluvio-lacustrine sediments

B

marine and non-marine terrigenous-elastic sequences

m

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El

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i

lundmass 12.00

p&

/

marine marls,

Ma

carbonates and locally evaporites

local onlap of platform carbonates over the preneogene basement m

;E;nc;;ine

marls

59

J.E. hfewlenkamp et al, f Tectmwphysics 234 (Ip94) 53-72

Kalithea

Ag.

0

Vlassios

450

550

650

750 m

Fig. 5. Three points moving average curves of paleo-depth estimates relative to Pliocene planktonic foraminiferal zones in sections Kalithea and Ag. Vlassios, northern Iraklion Basin, Crete.

To the south the basin is bordered by the Central Iraklion ridge, to the north it extends into the Cretan Sea. Submarine swells and elevated ridges, most of which act as culminations in the present

topography, intersected the basins in Pliocene time (Fig. 3). Pliocene sedimentation began with the accumulation of hemipelagic calcareous muds of the

Fig. 4. Successive steps in the paleogeo~aphic evolution of Crete since the latest Middle Miocene. (A) Latest Middle Miocene (latest Serravallian), showing extension of fluvio-Iacustrine sedimentation realm bordering the southern Aegean landmass. (B) Tortonian, showing position of basins during the early stage of break-up of the southern Aegean landmass. Vertical shading refers to areas which did not subside until Late Tortonian time. (0 Early Messinian. Note onlap of lowermost Messinian platform carbonates (indicated by oblique shading) over the margins of Tortonian basins. (D) Late Early Pliocene, showing the emergence of western and parts of central Crete ~rn~i~ed after Meulenk~p and Hilgen, 1986f.

hl)

J. E. Meulenkump et al. / Tecronophwcs 2.34 lI994) Xl- 72

‘Trubi’-type, when the sea re-invaded the basin after a short, latest Messinian episode of erosion (Meulenkamp et al., 1979a,b; Delrieu et al., 1993). This is evidenced by the occurrence of planktonic foraminiferal associations of the lowermost Pliocene Sphaeroidinellopsis Acme-zone in pebbles and in the matrix of mass flow deposits constituting the local base of the Pliocene in the northern Iraklion Basin. These ‘marl breccias’ unconformably overlie Lower Messinian platform carbonates or marls, or gypsum conglomerates, and contain admixtures of pre-Neogene rocks and greenish, unfossiliferous clays and greyish sandy limestones, which are not known from the Lower Messinian or older Neogene sequence. The latter sediments might represent remnants of the Upper Messinian. The mass flows mirror the effect of tectonic instability during the earliest part of the Pliocene, which caused the downslope sliding of packets of strata. They are overlain by marls or marly limestones of the Trubi-type again iKourtes facies of Meulenkamp et al., 1979a,b), which deposits still belong to the Sphaeroidinellopsis Acme-zone or to the next-higher Lower Pliocene Globorotalia margaritae Zone. The sediments of the Kourtes facies pass upwards into grey-bluish clays with sapropelitic interbeds (Finikia facies) of late Early Pliocene age. Field studies unambiguously show that the latter vertical transition from carbonate to clay-dominated sedimentation mirrors the initiation of subaereal erosion of the Central Iraklion ridge in the course of the Early Pliocene. Marls and diatomites of the Stavromenos faties, topped by intertidal limestones or near-shore deposited sands, form the upper part of the Pliocene sequences in the northern Iraklion Basin. These marls and diatomites are locally onlapping on the pre-Neogene or the Upper Miocene basement. At some places in today’s coastal area, along the margins of pre-Neogene culminations, a hiatus between the early Late Pliocene marl-diatomite sequence and the underlying late Early Pliocene clays of the Finikia facies can be observed (Jonkers, 1984). The lower part of the marls and diatomites of the Stavromenos facies represent the sedimentary expression of a short episode of relative sea level

rise in the early Late Pliocene, about 3 Ma ago. This relative sea level rise was superimposed on the overall shallowing that began in earliest Pliocene time and which locally resulted in emergence and erosion of part of the Lower Pliocene cover in the coastal area. Renewed shallowing trends, expressed by the transition from marls and diatomites to limestones or sands, witness of the final stage of regression in this part of Crete (Tsapralis, 1976; Meulenkamp et al., 1978). Sedimentation rates in the Pliocene northern Iraklion Basin were low, averaging about 4-5 cm/ ka, and the maximum thickness of the Pliocene sequences is not more than 125 m. If we consider that initial water depths were locally more than 1000 m, this suggests that sediment infill hardly played a part to account for the shallowing. Consequently, the shallowing has primarily to be interpreted in terms of the effect of tectonically controlled uplift. Since in the setting of the northern Iraklion Basin effects of sediment loading in the reconstruction of the uplift history may be considered negligible, the paleobathymetric evolution may be considered a fair measure for the uplift of the basin floor through time. In Fig. 5 we illustrate some of the data sets we employed. Here, three-points moving average values of paleobathymetry estimates are given relative to planktonic foraminiferal zones for sections Ag. Vlassios and Kalithea (for location, see Fig. 3). Both sections display clear shallowing trends with relatively minor fluctuations. To check the method used and the results obtained, we analyzed two more sections located in the vicinity of Ag. Vlassios and Kalithea, respectively (sections Finikia and Prassa, Fig. 3). By incorporating data from spot samples of the lowermost Pliocene in the vicinity of Ag. Vlassios-Finikia (not included in the sections of Fig. 3) and from the Pliocene of section Veneraton (situated about halfway Ag. Vlassios-Finikia and the Central Iraklion ridge) we completed our data set. All sets of samples were calibrated with the numerical time scales of Hilgen and Langereis (19881, Langereis and Hilgen (1991) and Zijderveld et al. (1991), which are based on first-order correlations between Mediterranean planktonic foraminiferal (Spaak, 1983) and calcareous

J.E. Meulenkump et al. / Tectotwphysics 234 (1994) 53-72

nannoplankton (Driever, 1988) zones and the polarity stratigraphy scale. Subsequently, average bathymetry values were calculated for a number of selected time-slices, delimited by biochronological horizons. The results obtained by this procedure are illustrated in the time-depth diagram of Fig. 6. Average water depths during the earliest Pliocene, between 4.85 and 4.65 Ma, were distinctly less in the present coastal area (600-700 m, sections Kalithea and Prassa) than in the more central parts of the northern Iraklion Basin (about 1000 m, sections Ag. Vlassios and Finikia). Since the ensuing bathymetric evolution is supposed to refleet mainly tectonic uplift, this evolution points

61

to rapid uplift in the course of the Early Pliocene. Based on these paleobathymetrical estimates the average rates of uplift from 4.75 to 3.52 Ma are about 37 cm/ ka for Kalithea-Prassa and 57 cm/ ka for Ag. Vlassios-Fir&a. It is difficult to reconstruct in detail the Late Pliocene to Recent bathymetrical evolution, since the sedimentary record is incomplete. We know that the Early Pliocene phase of uplift was followed by a short episode of subsidence during the early Late Pliocene. Locally, Upper Pliocene sediments are onlapping on the pre-Neogene or Miocene basement. During this episode of relative subsidence, water depths were in the order of magnitude of 200-300 m both in the north and in m+ 500 400 300 I- 200

,>+; Ma

5.00

1

4.50

I

4.00

I

3.50

I

3.00

I

2.50

I

2.00

I

1.50

I.

/l

.o&Ci.>ys=f

A/

_-+

c I

100

_- -

sea

level

100 200 300 400 500 600

Prassa o Kalithea n Ag. Vlassios * Finikia A Veneraton l

700 800 900

1000 1100 1200

mFig. 6. Pliocene-Recent uplift curves for sections in the northern (Kalithea, Prassa), central (Ag. Vlassios, Finikia) and southern parts (Veneraton) of the northern I&lion Basin, based on averaged paleo-depth values for successive time spans. Numbers indicate uplift rate estimates in cm/Ka.

hl

J.E. Meulenkamp cr al. / Tectonophysic.s 234 (1994) SS7.2

the south. Renewed uplift, which started in the course of the Late Pliocene, resulted in the present elevation of the northern Iraklion Basin between about 50 and 200 m above sea level. From this we calculated average uplift rates between 9 and 15 cm/ka for the last 2.90 Ma. However, it should be emphasized that these numbers are rather meaningless, since they are merely based on the Late Pliocene and Recent end-members of the bathymetric evolution. In reality the Late Pliocene to Recent history will have been more complicated, also due to the effects of eustatic sea level changes and Pleistocene-Holocene tectonics. We summarized our data on the northern Iraklion Basin in a schematic time-distance diagram (Fig. 7), based on successive stages in the averaged uplift histories in the south (Finikia, Ag. Vlassios) and in the north (Kallithea, Prassa). By connecting time-equivalent stages we arrive at hypothetical uplift paths of the basin floor along a SW-NE profile. The diagram strongly suggests tilting to the north to be the mechanism that accounts for the temporal and spatial relationships of the uplift histories in various parts of the basin, which tilting started in the course of the Early Pliocene, around 4.40 Ma ago. Prior to 4.4 Ma and after deposition of the marl breccias the northern Iraklion Basin seems to have been subject to general uplift, as suggested by the roughly parallel 4.75 and 4.40 Ma isochrons. Extrapolation of the 4.40 and younger isochrons to the northeast reveals a remarkable convergence, suggesting the position of an intersection zone located in the Cretan Sea, some five to ten km offshore. Extrapolation to the southwest shows that the observations on section Veneraton (closed circles) are almost in line with the hypothetical, extrapolated values for this part of the basin. It is striking that the 3.0 Ma line does not fit with the reconstructed hinge line position probably due to the effects of the early Late Pliocene, short episode of subsidence. Tilting to the northeast being the mechanism behind the observed uplift patterns along the SW-NE profile intersecting the northern Iraklion Basin implies that the uplift rates inferred from the Ag. Vlassios and Finikia sequences do not

,

--_

I

/’

/’

,/’

/’ /’ 400 1’ /’ & ,/’ 600 700

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/’

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’

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/ /’ ,’

/’

’

900

1% bc

lOQ0

/ /I /r

1100 t

,’

b

b

1200

I

1Okm

Fig. 7. Distance-depth diagram showing successive stages in the uplift history along a SW-NE transect in the northern Iraklion Basin, based on the combination of data from sections Kalithea/Prassa and Ag. Vlassios/Finikia. Note the convergence of isochrons, suggesting northerly tilting from 4.40 Ma onward, and the close correspondence between hypothetical (dashed line) and real depths (closed circles) in section Veneraton. The 3.00 Ma isochron corresponds to the early Late Pliocene episode of subsidence, which interrupted the general uplift during the Early Pliocene-Recent time span.

necessarily reflect the maximum rates of uplift along the Central Cretan transect during the Early Pliocene and Late Pliocene to Recent time spans. To reconstruct estimates of maximum uplift and to trace the southward extension of the tilting block we analyzed some additional localities along

LE. ~eu~~ka~

the southern part of the transect, Central Iraklion ridge.

63

et al. / Tectonophysics 234 (1994)53-72

south of the

3.2. Southern Iraklion The most complete section in the southern Iraklion Basin, which basin is situated between the Central Iraklion Ridge and the Asteroussia mountains, is located near Atsipadhes, along the northern margin of the Messara plain. Additional samples were analyzed from the southern margin of the Messara plain (surroundings of Apesokari) and from section Rizikas, in the southern foothills of the Psiloritis mountains (Fig. 3). In all Iocalities deep marine marls or marly limestones of the Trubi-type overlie uppermost Miocene, lower

Messinian sediments. No marl breccias were found; in fact, lowermost Pliocene mass flow deposits seem to be absent south of the Central Iraklion Ridge. The Trubi-type sediments pass upwards into sequences characterized by the intercalation of sands and conglomerates. The upwards increasing number of elastic interbeds mirrors the shallowing trends during the Early Pliocene. In the Atsipadhes area the elastics were supplied from the Central Iraklion ridge, as shown by sedimentological data. The ridge emerged around 4.0 Ma ago. Paleobathymetrical estimates for the hemipelagic muds indicate that water depths during the early part of the early Pliocene (G~ob~~o~a~~a margaritae Zone) are in the order of magnitude

+ 500 400 300 200 100

Ma

m 100 200 300 400 500 600 700

Atsipadhes o Apesokari o Rizikas * Prassa l

salinity

.‘.‘.‘::::: .‘.‘.‘. ........... ............ ........... ............ ........... ............ ........... ............ ...........

800 900 1000 1100 1200

Fig. 8. Generalized Late Miocene-Recent subsidence and uplift patterns in some sections in the northern and southern Iraklion Basins. Note the effects of the early Late Pliocene episode of subsidence in section Atsipadhes and the large-scale subsidence in the latest Miocene Kate Messinianj-Early Pliocene time span.

J.E. Meulenkamp et al. / Tectonophysic.r 234 (1994) 53-72

h4

of 950 to 1100 m. Poor biostratigraphic control for the more elastic part of the sequence, due to the scarcity of index fossils, impede to follow in detail the bathymetric patterns through time. However, the available data sets suggest rapid uplift and even emergence in the course of the later part of the Early Pliocene at either side of the Messara depression. Around Atsipadhes water depths had decreased to about 200 m in the later part of the Early Pliocene and the area emerged about 3.5 Ma ago. These findings for the

Early Pliocene are illustrated in Fig. 8. They point to Early Pliocene uplift rates of more than 100 cm/ ka. Late Pliocene open marine marls and diatomites witness of an intra-Pliocene episode of deepening of about 300 m in the Atsipadhes area. Uplift-controlled shallowing started again in the course of the Late Pliocene, and at present the Atsipadhes area is located 350 m above sea level. From these data an average uplift rate of 19 cm/ ka can be inferred for the Late Pliocene to

Northegl-aklion

I

I \

Re: I S

4.40

\

\

\

I

I

-1600 -1600

\

Ma 50km

*

N

Fig. 9. Diagram’ showing real and hypothetical uplift patterns along the Central Cretan transect since the beginning of general tilting to the north or northeast, about 4.40 Ma ago. Note the close correspondence between hypothetical and real amounts of uplift for the late Early Pliocene (4.40-3.52 Ma) and the discrepancy between hypothetical and real total amounts of uplift (4.40 Ma-Recent) if Central Crete is taken to have reacted as one single block throughout the Plio-Pleistocene. Numbers in italics refer to real and hypothetical, extrapolated (between brackets) rates of uplift in cm/Ka. Solid square indicates amount of uplift of section Rizikas, located in the southern foothills of the Psiloritis. For further explanation, see text.

J.E. Meulenkamp et al. / Tectonophysics 234 (1994) 53-72

Recent time span. Although the data on the bathymetric evolution of the southern Iraklion Basin are as yet limited, it seems justified to

65

conclude to a two-phased uplift history, which conclusion is in line with those inferred for the northern Iraklion Basin.

600 500 400 300

200 100 h_

-

-1 loo -: 100

100 -1

kO0

-L $00 -t 500 -; WCI -6 loo -S ml -1IO00 -1 1100 -1 I200 -1 I300 ,“-

Fig. 10. Tentative reconstruction of the evolution of latest Middle Miocene-Recent reliefs along the Central Cretan transect. Numbers refer to MaBP; symbols indicate reference points, based on paleobathymetrical analyses (closed circles), extrapolation of paleo-depth estimates (open circles), or on the combination of qualitative paleontological and sedimentological data and field evidence (asterisks). The 12.0 Ma isochron is taken to correspond to sea level and indicates the latest Middle Miocene coastal lowland topography. For further explanation, see text.

3.3. Temporal and spatial relationships

In order to investigate if central Crete responded as one single block to the northerly tilting since the late Early Pliocene, we plotted the data presently available on the uplift history along the Central Cretan transect in a time-distance diagram, using 4.40 Ma as a reference level (Fig. 9). By extrapolation to the south of the angles of tilting reconstructed for successive episodes in the evolution of the northern Iraklion Basin, hypothetical amounts of uplift are obtained for the southern parts of the transect. The comparison of hypothetical and real amounts of uplift of southern Iraklion displays a striking correspondence for the Early Pliocene phase of uplift (3.52 Ma isochron in Fig. 9). From this we might infer that the Early Pliocene uplift patterns of southern Iraklion corroborate the single-block hypothesis, which implies overall tilting of central Crete to the north at an angle of about 1.5” between 4.40 and 3.52 Ma ago. If correct, the highest amount of uplift during the Early Pliocene tilting phase (about 1400 m) would have been achieved in the southernmost part of the transect, in the present Asteroussia mountains. Here, rates of uplift between 4.40 and 3.52 Ma ago might have been in the order of magnitude of 160 cm/ ka. If the total, Early Pliocene-Recent amounts of uplift are considered, a differentiation can be seen between the north and the south. In the single-block hypothesis the total uplift amount in southern Iraklion should have been about 2000 m along the margins of the Messara plain and even more than 3000 m in the Asteroussia mountains (Fig. 9). However, in reality the total amounts of uplift along the margins of the Messara plain are not more than 1100-1400 m. This means that the southern and northern Iraklion Basins had their own uplift histories since the latest Early Pliocene, i.e., since about 3.52 Ma ago. In the northern Iraklion Basin the net-amounts of uplift since 3.52 Ma ago vary from 300 m in the coastal area to more than 700 m north of the Central Iraklion ridge; values of 600-800 m can be calculated for the southern Iraklion Basin. The combination of values obtained for the Early

Pliocene and Late Pliocene-Recent phases of uplift indicates that the maximum amount of uplift along the Central Cretan transect was locally in the order of magnitude of 2000 m since the beginning of the Pliocene.

4. Late Miocene evolution of reliefs No detailed data sets on the paleobathymetric evolution of the late Middle to Late Miocene sections along the Central Cretan transect are available as yet. The tentative reconstruction of reliefs since the latest Middle Miocene, summarized in Fig. 10, is based on the combination of qualitative paleontological and sedimentological data and field evidence. The latest Middle Miocene (latest Serravallian) to Late Miocene (Early Messinian) record mirrors the development of an irregular relief in the course of the Late Miocene in response to the break-up of the southern Aegean landmass and the transformation of Crete into a mosaic of small-sized culminations and depressions (Fig. 4). In latest Serravallian time (12.0 Ma) the southern Aegean landmass was bordered to the south by a vast lowland area (Meulenkamp, 1985) without any pronounced relief; prevailingly lacustrine sedimentation kept pace with subsidence. The margin of the latest Serravallian basin was located close to the present northern coastline (Fig. 4). Differentiation of reliefs along the Central Cretan transect started in the Serravallian-Tortonian boundary interval (10.9 Ma, Figs. 4 and 10) and became more accentuated in the course of the Late Tortonian (6.5-6.0 Ma). Simultaneously with the development of the northern Iraklion Basin, the basin bordering the Central Iraklion ridge to the south deepened. Both this ridge and the Asteroussia block started to play a pronounced role in the sedimentary history by supplying elastics to the adjacent basins. The depth of these receiving basins was generally not more than 50-100 m and sedimentation was characterized by the accumulation of fluvio-lacustrine, brackish and shallow marine sands and clays. Only in the area bordering the Central Iraklion

J.E. Meulenkamp et al. / Tectonophysics 234 (1994) 53-72

ridge to the south some blocks subsided more strongly to depths of a few hundred meters. The relief became more differentiated from the earliest Messinian onward (Fig. 10). This more pronounced differentiation was superimposed upon the general subsidence affecting the Cretan area at the time (Meulenkamp, 1985). This subsidence caused the submergence of the Central Iraklion ridge and of large parts of the present Asteroussia mountains (Fig. 4). Paleodepths during the earliest Messinian did not exceed that of the photic zone on the submarine ridges (accumulation of reefal limestones); in the depressions local depths of the basin floor were probably not more than 200-300 m. Unfortunately, no sound information is available on the evolution of reliefs during the later part of the Messinian. This episode was characterized by subaerial erosion, as witnessed by the local findings of fluviatile conglomerates. Elsewhere in Crete (Meulenkamp et al., 1979a) there is evidence of intra-Messinian tectonics resulting in a conspicuous rejuvenation of reliefs. For the Central Iraklion transect we can only conclude that the total amount of differential movements between the Early Messinian and an ill-defined horizon in the Early Pliocene was more than 1000 m of subsidence. This is clearly evidenced by data from the northern Iraklion Basin, where paleodepths during Late Tortonian and/or Early Messinian time were not more than 50-100 m at some places, whereas those of the Early Pliocene (Sphaeroidineflopsis Acme-zone) were 1000-1200 m at the same localities. Similar amounts of subsidence resulting from Late Messinian to Early Pliocene (Globorotalia margaritae Zone) tectonics can be inferred from localities along the northern margin of the Asteroussia and the southern foothills of the Asteroussia. Our present data from the northern and southern Iraklion Basins do not allow to reconstruct the post-Early Messinian subsidence history more precisely. It may be argued that thick Late Messinian sequences should have been deposited if the differential movements in the Miocene-Pliocene boundary interval were most pronounced in Late Messinian time. If so, we have to conclude that the net-result of subsidence

67

of more than 1000 m was largely due to earliest Pliocene tectonics, superimposed upon the effects of the Early Pliocene flooding. New findings of Delrieu et al. (1993) seem to corroborate the latter hypothesis.

5. Interpretation There is a striking difference between the Late Miocene and the Pliocene-Recent evolution of reliefs along the Central Cretan transect. From latest Serravallian until Messinian time this evolution was marked by an increased differentiation between culminations and depressions. The netresult of differential movements was the origin of shallow basins separated by (slightly) emerged blocks. These processes are related to the fragmentation of the southern Aegean landmass, which started in the Serravallian-Tortonian boundary interval. The presence of angular unconformities in the Upper Miocene sequences suggests that successive steps in the formation of reliefs were related to differential block movements. Regional, large-scale vertical displacements, however, do not seem to have occurred until the Miocene-Pliocene boundary interval and no general uplift took place in Late Miocene time. In fact, culminations which had emerged in the course of the early Late Miocene, submerged during the Early Messinian episode of general subsidence and in the early part of the Early Pliocene the northern and southern Iraklion basins had subsided to depths of about 1000 m. The effects of the Messinian salinity crises were superimposed upon this development. In the course of the Early Pliocene the upliftsubsidence pattern changed completely. At that time the overall uplift of central Crete began, coupled with the inception of large-scale tilting to the north or northeast along an intersection zone located in the Sea of Crete, about 5-10 km north of the present coastline, from about 4.40 Ma onward. This uplift continued through most of the Pliocene-Recent time span; it was interrupted only by a short episode of subsidence in the early Late Pliocene. We will attempt to explain the characteristics

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of the Late Miocene and Pliocene-Recent evolution of reliefs in terms of the surficial expression of the effects of the roll-back process. In the model proposed by Meulenkamp et al. (19881 the inception of the roll-back process, some 12 Ma ago, is considered to have generated the detachment of a large, supracrustal slab, which slided in southward direction along low-angle shear zones (Fig. 1). Extension in the rear-end of the supracrustal slab would account for the origin and foundering of the Sea of Crete. Folding and thrusting in the frontal parts of the slab were taken to be the cause of the development of anticlinal culminations and synclinal depressions. The transformation of the Late Serravallian

IA

SEE FK.

12

234 ( I9941 5% 72

lowland area bordering the southern Aegean landmass in the Late Serravallian into a configuration of culminations and depressions in the early part of the Late Miocene, may be attributed to the (surficial) expression of slight folding in the frontal parts of the supracrustal slab during the early stage of transport to the south. The pronounced increase in differential reliefs between the culminations and depressions in the course of the later part of the Late Miocene possibly mirror the effects of an increase of shortening by folding within the frontal parts of the southward sliding supracrustal slab. The latter increase is to be expected if we consider the shift from subhorizontal to a northward dip of the

-1 B

Fig. 11. Present topography and tentative reconstruction of Late Miocene and Pliocene reliefs along a transect across the southern Aegean area, between the Libyan Sea and Santorini. Numbers indicate MaBP. The inset shows the Pliocene-Recent subsidence path of the Cretan Basin, DSDP-Site 378. A-B corresponds to the location of the profile of Fig. 12.

J.E. Meulenkamp et al. / Tectonophysics 234 (1994) 53-72

back thrust where the slab met the northerly inclined limb of the bulge of the formerly subducted Ionian Plate (see Fig. 1B). At the present stage of our investigation we cannot detect the details of the relation between the presumed increase in the effects of folding in the course of time and successive steps in the Late Miocene evolution of reliefs. Field evidence suggests, however, that folding did play a part, for instance in the generation and evolution of the Central Iraklion ridge. In latest Tortonian-Early Messinian time folding seems to have defined slopes and facies distribution patterns upon and at either side of the ridge. The inception of large-scale tilting to the north or northeast, coupled with the beginning of overall uplift in the Early Pliocene, may have been related to the accentuation of shortening and the development of thrusts within the frontal parts of the southward migrating supracrustal slab at the time.

6. Uplift of the arc and the evolution of the Cretan Sea We extended the Central Cretan transect to a section across the southern Aegean area, from the island of Santorini in the north until the Libyan Sea in the south (Fig. 11). The combination of land data and Recent bathymetry suggests that Crete forms part of a large, asymmetrical, southern Aegean block, the southern limit of which is formed by the steeply inclined southern margin of Crete. The break in subbottom topography in the Cretan Sea marks the fault-bounded northern margin of the Cretan Basin (Iraklion Basin of Bartole, 1983), where, according to Angelier et al. (1982) the maximum amount of lithospheric stretching occurred. About 1000 m of Late Neogene to Holocene sediments accumulated in the more rapidly subsiding parts of the Cretan Basin; sediment thicknesses between this basin and Crete are much less, about 400 m at the maximum (Jongsma et al., 1977). Seismic data of Jongsma et al. (1977) and Bartole (1983) suggest the local presence of preevaporitic sediments, although this has not been

69

proved by drilling. Messinian evaporites are more widely distributed. They have a maximum thickness of about 350 m in the centre of the Cretan Basin and consist predominantly of gypsum, deposited in shallow marine environments (Hsii et al., 1978; Bartole, 1983). No halite occurs in the basins intersected by our profile. The Early Pliocene to Holocene sequence, drilled in DSDP-Sites 378 and 378 A, reflects a pronounced deepening trend from about 600 m to 1835 m (Fig. 11). Sedimentation rates increased from 0.9 cm/ ka in the Early Pliocene to 9.6 cm/ ka in the late Early Pliocene to Early Pleistocene time span (Hsue et al., 1978). It should be emphasized that the paleobathymetry estimates we used are based on a semi-quantitative interpretation (Wright, 1978) of the available data. The diagram shows that the total subsidence was about 1500 m since the Early Pliocene (4.6 Ma), the tectonic subsidence (i.e., after correction for sediment loading) was in the order of magnitude of 1400 m. Comparison of the bathymetric evolution and subsidence patterns of the Cretan Sea with the evolution of reliefs in central Crete, suggests a close correspondence for the Messinian, whereas opposite patterns evolved since the Early Pliocene. In Messinian time the paleogeographic configuration along the Santorini-Libyan Sea profile seems to have been characterized by shallow marine basins, separated by submarine swells and ridges. Sediment thicknesses were in the same order of magnitude (350 m at the maximum, Bartole, 1983) as in central Crete. The evaporitic sequences are composed of selenitic gypsum and anhydrite, and no halite was formed, neither on Crete, nor in the Cretan Basin. This correspondence probably ended in the early part of the Early Pliocene. Late Messinian and/or earliest Pliocene tectonics resulted in local subsidence of more than 1000 m in central Crete, whereas there is no hard evidence for vertical movements of comparable magnitude in the Cretan Basin at the time. From the Early Pliocene onward the vertical movements of the Cretan segment of the Santorini-Central Cretan transect were primarily defined by uplift and large-scale tilting to the north,

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probably in response to the thrusting of the supracrustal slab over the northerly dipping limbs of the subducted Ionian Plate. The corresponding Pliocene to Recent uplift of central Crete of about 2000 m had its counterpart in the increased subsidence to the north (Fig. 11 I, i.e., in the foundering of the Cretan Basin. This relation between uplift and subsidence patterns is visualized in Fig. 12. The diagram suggests that rates of uplift in central Crete were invariably higher than subsidence rates of the Cretan Sea. Consequently, uplift was not balanced by subsidence. From this we infer that no simple rotation took place. Possibly the discrepancy between uplift and subsidence may, at least in part, be explained by the thrusting and piling up of sediments after the frontal parts of the supracrustal slab thrusted over the northern limb of the subducted Ionian Plate. It should be emphasized that our conclusions concerning the timing and magnitude of vertical movements are pertinent to the Santorini-Central

m+

CRETE

+---

\

2000

53-7-7

Cretan transect only. More data are needed from the Sea of Crete and from transects across westcm and eastern Crete to arrive at a more comprehensive, regional picture of the temporal and spatial relationships between vertical movements related to the effects of the roll-back process. In fact, our model would imply conspicuous differences in the uplift history across Crete, from the west to the east, as a consequence of the oblique motion of Crete relative to the position of the retreating Hellenic trench (M.J.R. Wortel. pers. commun., 1992).

7. Conclusions Two major stages in the latest Middle Miocene to Recent evolution of reliefs can be inferred from the sedimentary record of the transect intersecting the Cretan Sea and central Crete. The first stage, latest Middle Miocene to Late Miocene was marked by the fragmentation of the hitherto

SEA

OF

CRETE

\ \

1600 1200 800 400

A

B 400 800 1200

, 0

1

10

20km

- 1600 - 2000 - 2400 - 2800 - 3200 m-

Fig. 12. Diagram showing the relation between Pliocene-Recent uplift and subsidence patterns along a profile across Central Crete and the Sea of Crete (A-E in Fig. ll), using 4.40 Ma as a reference level. Note that amounts of uplift of Central Crete are higher than those of subsidence in the Sea of Crete.

J.E. Meulenkamp et al. / Tectonophysics 234 (1994) 53-72

existing southern Aegean landmass. This resulted in the origin of culminations and shallow marine basins, on central Crete as well as in the present Sea of Crete. Differential (subbottom) topography increased in the course of the Late Miocene, coupled with overall subsidence in Early Messinian time. Subsidence was most pronounced in the Miocene - Pliocene boundary interval in the central Cretan part of the southern Aegean transect. Intra-Messinian to earliest Pliocene tectonics locally caused vertical movements of more than 1000 m between the Early Messinian and an ill-defined level in the early part of the Early Pliocene. The second stage of the latest Middle Miocene to Recent evolution of reliefs along the southern Aegean transect was marked by the foundering of the Cretan Basin and the staggering uplift of Crete. These processes started in the course of the Early Pliocene. The central Cretan record suggests a two-phased uplift, interrupted by a short, early Late Pliocene episode of general subsidence. In the course of the Early Pliocene uplift became coupled with tilting to the north or northeast, the maximum uplift in Plio-Pleistocene time was in the order of magnitude of 2000 m. Our results indicate unambiguously that there was no sustained uplift of Crete since the late Middle Miocene, i.e., since the supposed beginning of the roll-back process of the Hellenic subduction zone. In fact, the reconstruction of vertical movements puts fairly detailed geological constraints on tectonophysical modelling of timing and magnitude of vertical motions pertinent to the roll-back process. Any model has to account for an initial stage in which this expression was marked by the predominance of subsidence all along the Cretan Sea-Central Cretan transect, culminating in the collapse-like subsidence across the Miocene-Pliocene transition, which subsidence was almost instantaneously followed by rapid uplift of Crete and foundering of the Cretan Sea. Acknowledgements

The authors gratefully acknowledge the assistance of P.J. van Eijck and G. Ittman in the

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micropaleontological analyses. The illustrations were made by T. van Hinte. Thanks are due to P. Meijer and M.J.R. Wortel for suggestions and critical reading of the manuscript and to anonymous reviewers for their valuable comments. Part of the research was supported by a grant from the Netherlands Organization of Scientific Research (NWO).

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nental collision: the Eastern Mediterranean as a case example. In: K. Hsii (Editor), Mountain Building Pro ccsses. Academic Press, London, pp. 201-211. Le Pichon, X. and Angeher, J., 1981. The Aegean Sea. Philos. Trans. R. Sot. London, Ser. A, 300: 3.57-372. Meulenkamp, J.E.. 1985. Aspets of the Late Cenozoic evolution of the Aegean region. In: D.J. Stanley and F.C. Wezel (Editors), Geological Evolution of the Mediterranean Basin. Springer, New York N.Y.. pp. 3077321. Meulenkamp, J.E. and Hilgen, F.J.. 1986. Event stratigraphy, basin evolution and tectonics of the Hellenic and Calabro-Sicilian arcs. In: F.C. Wezel (Editor), The Origin of Arcs. Elsevier, Amsterdam, pp. 327-350. Meulenkamp. J.E., Schmidt, R.R.. Tsapralis, V. and Van der Zwaan, G.J., 1978. An empirical approach to paleoenvironmental analysis. I. Foraminifera, calcareous nannoplankton and ostracodes from the Pliocene of section Prassa, Crete. Proc. Kon. Ned. Akad. Wetensch., B, 81: 3399363. Meulenkamp, J.E., Dermitzakis, M., Georgiadou-Dikeoulia, E.. Jonkers, H.A. and Boger, H.. 1979a. Field guide to the Neogene of Crete. Publ. Dep. Geol. Paleontol. Univ. Athens, 1: l-32. Meulenkamp, J.E.. Jonkers, H.A. and Spaak. P., 1979b. Late Miocene to Early Pliocene development of Crete. Proc. 6th Coil. Geol. Aeg. Reg., Athens, 1: 137-149. Meulenkamp, J.E., Wortel, M.J.R., Van Wamel, W.A., Spakman, W. and Hoogerduijn Strating, E., 1988. On the Hellenic subduction zone and the geodynamic evolution of

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C.‘rete since the late Middle Miocene. Tectonophysics. 146. 203-215. Spaak, P., 1983. Accuracy in correlation and ecological aspects of the planktonic foraminiferal zonation of the Mediterranean Pliocene. Utrecht Micropaleontol. Bull.. 28, 160 pp. Spakman, W., Wortel, M.J.R. and Vlaar, N.J., 1988. The Hellenic subduction zone: a tomographic image and its geodynamic implications. Geophys. Res. Lett., IS: 60-63. Tsapralis, V., 1976. Ostracode associations and paleoenvironmental analysis of the Pliocene of Section Prassa, Crete. Greece. Proc. Kon. Ned. Akad. Wetensch., Ser. B. 79 (4): 300-311. Van der Zwaan, G.J., Jorissen, F.J. and De Stigter, H.C., 1990. The depth dependency of planktonic/ benthic foraminiferal ratios: constraints and applications. Mar. Geol., 95: l- 16. Wright, R., 1975. Neogene paleobathymetry of the Mediterranean based on benthic fora minifers from DSDP Leg 42A. In: K.J. Hsii, L. Montadert et al. (Editors), Initial Reports of the Deep Sea Drilling Project, Vol. XLB. Scripps Inst. Oceanogr., pp. 837-846. Zijderveld, J.D.A., Hilgen, F.J., Langereis, C.G., Verhallen, P.J.J.M. and Zachariasse, W.J., 1991. Integrated magnetostratigraphy and biostratigraphy of the upper Pliocenelower Pleistocene from the Monte Singa and Crotone areas in Calabria, Italy. Earth Planet. Sci. Lett., 107: 697-714.