Attempt to date Pleistocene normal faults of the Corinth-Patras Rift (Greece) by U/Th Method, and tectonic implications

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Attempt to date Pleistocene normal faults of the

Corinth-Patras Rift (Greece) by U/Th Method, and

tectonic implications

Nicolas Flotté, Valérie Plagnes, Denis Sorel, Antonio Benedicto

To cite this version:

Nicolas Flotté, Valérie Plagnes, Denis Sorel, Antonio Benedicto.

Attempt to date Pleistocene

normal faults of the Corinth-Patras Rift (Greece) by U/Th Method, and tectonic

implica-tions. Geophysical Research Letters, American Geophysical Union, 2001, 28 (19), pp.3769-3772.

�10.1029/2001GL012964�. �hal-03124803�

Attempt to date Pleistocene normal faults of the Corinth-

Patras Rift

(Greece) by U/Th

method, and tectonic

implications

Nicolas

Flott6

•, Va16rie

Plagnes

a, Denis

Sorel

•, Antonio

Benedicto

•

Abstract. The Rift of Corinth is a major intra-continental rill controlled by a Pleistocene and still active detachment fault.

Its hangingwall contains normal faults which have migrated as evidenced by geomorphologic studies and by the geometrical relations of the sedimentary series that they

controlled successively. However, because of the lack of

paleontological data in the continental synrift sediments, the timing of this migration is unknown. Theret0re, we attempted to date directly the faults using the U/Th method on calcite crystallizations. Preliminary results reveal that this method may be a useful tool in neotectonics. In this first attempt, we

studied the Xylokastro-Loutro fault and the Valimi fault. The

age obtained on post-tectonic calcite of the first fault shows

that it locked at least 112.4 + 0.4ka ago. Syntectonic calcite of

the Valimi fault yields an age of 382.0 + 31. I ka, showing that

the fault was still active at that time. Tectonic implications of

these results are discussed.

1. Introduction

The geometry of a rill can usually be precisely described on the basis of field observations and subsurthce geophysical

data. But the reconstruction of its structural evolution is more

problematic: one needs to know precisely the geometrical and chronological relationships of the rill sedimentary infiil, and an accurate palaeontological timing of these deposits. In the

Corinth-Patras rill, most of the older synritt series, which are

now uplitied in the northern Peloponnesus, are untOrtunately lacustrine and devoid of significant stratigraphic tOssils. TheretOre, most authors speculated that rifting might have started during the P!iocene or even the Miocene (e.g. Dufaure, 1975; OFF, 1989). Seismological studies in the western rill show that the major active structure is a low angle detachment fault (Rigo, 1994). The outcrop of this detachment in the northern Peloponnesus has been recognized (Sorel, 2000). During its evolution, steeper normal faults successively

branched on the detachment t¾om the south to the north,

localizing progressively more-northern basins. In the younger part of the series, marine intercalations occur. These marine ingressions have built a spectacular flight of stepped marine terraces (fig. l), which reach highs of up to 700m between Corinth and Xylokastro (e.g. S0btier, 1977; Dufaure and Zamanis, 1980). Using the altitudes of the marine terraces shorelines and their ages, Keraudren and Sorel (1987)

I UMR8616,

Universit6

Paris

XI-Orsay,

France

2Laboratoire

Sciences

du

elimat

et

de

l'environnement,

CNRS,

F-

Copyright 2001 by the American Geophysical Union. Paper number 2001GL012964.

0094-8276/01/2001GL012964505.00

calculated the upliti rate of the terraces. Assuming that the uplift rate was similar lot the oldest, more uplitted lacustrine synrift deposits, sedimentation in the early rill may have started roughly lMyr ago (Sorei, 2000). The age of the rift

can be estimated in an other way: the total throw on the

detachment is the sum of the throws on the successive normal

faults, such as on the cross-section of fig.2. If the mean extension rate during the rifting was similar to the present day rate determined by GPS measurements (e.g. Billiris, 1991; Clarke et al., 1998), this also indicates that the filling initiated

1Myr ago.

In order to date more accurately the migration of the normal faults, we attempted to use the U/Th method directly on fault-

related calcites. It is assumed that fluids commonly circulate

and calcite crystallises within faults during their activity (syn- tectonic calcite) or after fault death (post-tectonic calcite). Measuring the U/Th isotopes in these calcites may give respectively an absolute precise age of activity or a minimum

age of fault death. This approach appears to be more direct

.•_ _ _!• ... L ß

than other geochronk, glcal •iudic• on ICI, LIIL• •LIL,,ii •,3 Co3•i•,-

ray exposure

dating

(rOBe,

26A1,

36C!)

of morphological

features displaced by the faults (e.g. Ritz et al., 1995), U- series nuclides adsorbed by detrital materials in fault (Szabo and Rosholt, 1989) or K/At dating of synkinematic illite in fault gouge (Zwingmann et ai., 2001). Here, we present the

first results obtained on two faults, and their tectonic

implications.

Faults are usually more or less thick zones containing several slip surfaces, which are not necessarily synchronous. In the studied area, the sampled faults are thin zones, often limited to one major slip surface; they .juxtapose relatively competent

Mesozoic limestones and much weaker Pleistocene sediments with a visible throw of several hundred meters to more than

one kilometer. TheretOre, most of the detOrmation occurred

on the major fault plane which has been sampled, and the age obtained is likely representative of the fault zone.

To date a small sample t?om such a zone and compare its age to the age of cessation of motion seems to be the more direct way to approach the absolute timing of the structural

evolution in continental settings.

2. Description of the sampled faults

Following a detailed fieldwork and structural mapping, we

sampled the Xylokastro-Loutro fault that we presume recent and that could be still active according to Armijo et al. (1996), and the Valimi fault located more to the south and likely older (fig.•).

The footwall of the Xylokastro-Loutro fault is a c.a. 1,000 m

high Mesozoic limestones mountain. Between the fault and

the shoreline of the gulf, its hangingwall consists of a thick series of Pleistocene white sandy marls, which accumulated

during the subsidence of the hangingwall. This series has been later uplifted, and is notched by three regressive stepped

marine terraces. The sampled segment (along the Loutro-

3770 N E S ion

GULF

CORINTH

l'l'l-l-l.l.l.l.l.

j

fault

•

Active

normal

fault

f

Detachment

fault

Xylokastro

marine

Xyloimstro-Loutro

Fig I. Structural sketch map of northern Peloponnese. Thick half<lotted line: emergence of the Gulf of Corinth detachment fault. Hatched lines: normal faults branching on the detachment. Grey areas: synrifi sediments; from dark to

light grey: older to recent deposits; dashed area: undivided synrifi series. White: mainly holocene alluvial fans. Dotted

stripe between Loutro and Corinth: middle and upper Pleistocene stepped marine terraces. 1: XylokastroLoutro fault. 2: Valimi fault. A-B: section of Fig 2.

Korfiotissa road) is about 5km long; the offset is more than I km and can be considered as one of the most representative.

A large exhumed fault plane, several hundred meters in size

(fig. l), is exposed near a lacustrine travertine limestones quarry. Large grooves and corrugations indicate a N-S trending extension. On a nearby outcrop, a karstic conduit filled with a 3cm thick crystalline calcite mat crosses the fault plane. This mat is characterised by fibrous undeformed calcite crystals perpendicular to the conduit's surface (Fig.3-a),

indicating that the fault was locked when the karstic conduit formed. Thus this calcite seals and post-dates the fault activity. Two samples were collected R)r U/Th analyses.

The Valimi fault is an inactive t3.ult, located farther south

(fig. I). Its footwall consists of Mesozoic limestones. In its hangingwall have accumulated more than I km of syntectonic

series of freshwater sandy marls and conglomerates. To the

north, this series is offset by younger faults, likely synchronous with the Xylokastro-Loutro fault. Moreover, the

s 5 km Valimi fault Helike GULF OF CORINTH N

Fig 3. Pictures of the dated samples. 3a: post-tectonic calcite of Xylokastro-Loutro fault. Stalagmite-

like elongated calcite crystals perpendicular to the conduit surface. Sub•amples were collected in the less coloured parts in order to avoid detrital contamination. S l-a is close to the wall. Its age should be closer to the fault lock. S I-b is located higher and thus should be younger than S I-a. This is confirmed by U/Th dating. 3b: syn-tectonic calcite sampled on the Valimi fault. It is mixed with

brecciated limestones of the •)otwall.

series is discordantly overlain by conglomerates related to these younger faults. The Valimi fault is theretbre older than the Xylokastro-Loutro lhult. It is a major simple thult

characterised by a fault breccias made of clasts of substratum cemented by white isometric calcite (tbllowing the definition

of Ramsey and Huber, 1983) (Fig.3-b). The limestones of the tbotwall are devoid of calcite veins, so that the crystallisations are clearly associated with the damage zone. A microscopic study indicates that the calcite is twinned and a slightly

broken, indicating that the t•ult was still active atier the

crystallisation of the calcite. Thus the sampled calcite is synchronous with the fault activity. Patches of this syntectonic calcite stuck on the t•ult plane have been sampled and calcite grains have been carefully sorted t¾om Mesozoic limestones clasts for U/Th analysis.

3. U/Th method & results

The U/Th dating method, described in Ivanovich and Harmon (1992), is usually applied on middle and upper Pleistocene

corals (Barnes et al., 1956), mollusks shells (Kaut•nan et al.,

1971), or speleothems (Rosholt and Antal, 1962). A detailed

descripti•)n of the analytical process used in this work is given

in Part 6.

Isotopic ratios were measured using a Finnigan 262 Thermo- Ionisation Mass Spectrometer (TIMS) in the Laboratoire des

Sciences du Climat et de l'Environnement of the CEA/CNRS

(Gif-sur-Yvette, France). Analytical results are presented in

Table I.

The two sub-samples of post-tectonic calcites ti'om the Xylokastro-Loutro thult (SI-a and S l-b) give homogeneous U

contents

(0.55-0.89

ppm)

and

234U?38U

activity

ratios

(2.12-

2.13). Respective calculated ages are: ! ! 2.4 + 0.4 ka and 108.2 + !.0 ka. S l-a is older than S l-b, these ages being consistent with the growth of the calcite mat (Fig.3a). Consistency of these results suggests a geochemical U-Th system remained closed t?om the time of crysta!lisation. Therefore, calculated ages may be considered to be of geological significance.

Syn-tectonic calcite t¾om the Valimi fault (S2) yields an age

of 382.0 + 31. ! ka. In the lack of a second sample, we cannot

ascertain that the geochemical system remained closed.

4. Tectonic implications

Our results suggest that the Xylokastro-Loutro thult is locked

since at least ! 12 ka. This is in agreement with geological and geomorphological observations along this t•ult system. For instance, above Kamares village, the c.a. 200m strandline of the highest marine terrace notches the fault plane and cuts into

the footwall limestones. This terrace, which seals the fault

activity, is laterally equivalent to one of the stepped terraces east of Xylokastro that has been attributed to the 5.5 marine isotopic stage (125 ka) by Keraudren and Sorel (1987). Seismology and field observations indicate that the normal faults located south of the Xylokastro-Loutro t•ult are

inactive. The recent and active extension across the Corinth

rill, at a rate of 1-1.5 cm.yr

'• (Clarke

et al., 1998),

should

Table 1. TIMS U-Th results

and

age

estimates

from

calcite

samples.

& is the apparent

23øTh/234U

age.

Errors

are

calculated

by error

propagation

and

given

at 20 level.23øTh/234U

ratios

are

activity

ratios

calibrated

to HU! assumed

to be

2•0

at secular

equilibrium.

Ages

Ao

have

been

corrected

ibr ' Th-excess

due

to detrital

material

mixed

with

calci'te,

even

if

2_t 232

all -øTh/

Th

activity

ratios

are

higher

than

100.

A• are

corrected

ages,

us•ing

the

hypothesis

of an

initial

-' Th/- Th

= I

i...-. :..: .... : :-' -..

'

...

...

.

....

:•?.Si•-

....

...:"'•_

....

1.• '0'.•6•)•.0006

2,!227•-0.•39

0.69•/•"0022

112,907

1.40 . 1•2•? 0,412

';•4.;•.•

....

:'8•)5•:•;'

l.$, •0.5485+/-0,0007...2.1312+/-0.0085...0.6½88+/-0.0052

!08.535 208

.... 1•17:•, 0.979

3772 FLOTTI• ET AL.' ATTEMPT TO DATE PLEISTOCENE NORMAL FAULTS OF THE CORINTH-PATRAS RIFT

therefore be accommodated on an active thult located thrther

north (eventually the ofl•;hore eastward prolongation of the

Helike thult). This is also supported by the uplitt of the Kamares and Xylokastro terraces for the last 125 ka at least, during which they are located in the tbotwall of this active

normal fault.

The 382.0 + 31.1 ka age obtained on the Valimi fault agrees

with the data indicating a Pleistocene age of the Corinth-

Patras rill recalled above, rather than with older ages

proposed tbr the initiation of titling by different authors (e.g. Dut3,ure, 1975; Ori, 1989, Armijo et al., 1996).

5. Discussion

The preliminary results presented here suggest that the dating of Pleistocene faults by TIMS based on the U-series can be a strong useful tool in neotectonics analysis, especially in continental settings where paleontological data are scarce.

However, care must be taken in using ages obtained for the

syntectonic calcite as absolute tracers of the fault activity or death because syntectonic calcite may have /brmed at any

time between the initiation and the lock of the thult. The throws on the faults of the Corinth-Patras rift can reach l km

or more. But calcite which formed early during the thult activity may be deslroyed during the later slip. It is more likely that the syntectonic calcite patches samples tbrmed lately during the fault activity. If we assume that'(l) the 1.5

cm/yr extension rate measured by GPS method (Clarkeet al., 1998) has been constant during the rifting, and (2) total

horizontal extension calculated from cross-sections on the

Valimi thult system is 4km, then the Valimi thult should have initiated 670-700 ka ago.

The results presented here are being completed by new datings on the Xylokastro-Loutro thult, on the Valimi thult and other faults. In parallel, a microstructural analysis and fluid characterisation are being performed. Applying this

method to several t•tults of the rill should enable us to

determine more precisely the timing of the thult migration,

lateral

thult propagations,

and theretbre

to better

understand

the rifting evolution.

6. Annex: Analytical U/Th procedure

Samples of less than 2-grams were combusted at 900øC for i

hour to oxydise all organic and mineral compounds present in the calcite. Next, the samples were dissolved in HC! (6N) in

teflon

beakers

containing

a measured

amount

of mixed

233U-

236U-22øTh

spike already

dried. The sample-spike

mixture

(with a carrier FeCI3) was !eti on a hot plate overnight to ensure complete ionic equilibration with the spike solution. Then, coprecipitation with NH4OH (pH = 7) separates U and Th from most of Ca. The development of the precipitates was ensured overnight. U and Th of these precipitates were separated using Dowex anion exchange resin (IX8) conditioned with HC! 6N. Then, U and Th were purified using

Eichrom resins (Uteva and Teva respectively) conditioned by HNO3 3N. U and Th tYactions were loaded onto pre-outgassed

single thenlure filaments with graphite coating and the isotope ratios were measured in the spectrometer.

Acknowledgments We thank C. Causse for her active contribution to the analysis and to the ideas developed in this work.

References

Armijo, R., B. Meyer, G.C.P. King, A. Rigo, D. Papanastassiou, Quaternary evolution of the Corinth Rift and its implications for

the late Cenozoic evolution of the Aegean,Geophy. J. lnt., 126, ! !-53, 1996.

Barnes, J.W., E.J. Lang, H.A. Potratz, Ratio of ionium to uranium

coral limestones, Science, 124, 175-176, 1956.

Billiris, H., D. Paradissis, G. Veis, P. England, W. Featherstone, B.

Parson, P. Cross, P. Rands, M. Rayson, P. Sellers, V. Ashkenazi,

M. Davison, J. Jackson, N. Ambraseys, Geodetic determination of tectonic deformation in central Greece from 1900 to 1988,Nature, 350, 124-129, 1991.

Causse, C., J.S. Vincent, Th/U disequilibrium dating of middle and

late Pleistocene wood and shelf from Banks and Victoria islands,

Arctic Canada, Canadian journal of Earth Sciences, 26, 2718-

2723, 1989.

Clarke, P.J., R.R. Davies, P.C. England, B. Parsons, H. Bi!!iris, D. Paradissis, G. Veis, P.A. Cross, P.H. Denys, V. Ashkenazi, R.

Bingley, H.G. Kahle, M.V. Muller, P. Briole, Crustal strain in

central Greece from repeated GPS measurements in the interval 1989-1997, Geophys. ,1. lnt., 135, 195-214, 1998.

Dufaure, J.J., Le relief du P61oponn•se, Thesis, Universit6 Paris IV, 1422 pp, 1975.

Dufaure, J.J., A. Zamanis, Styles n6otectoniques et 6tagements de niveaux matins sur un segment d'arc insulaire, !e P61oponn•se, Proc. Colloquium on Niveaux matins et tectonique quaternaire

dans !'aire mdditerran6enne, University of Paris 1,1980.

Kaufman, A., W.S. Broecker, T.L. Ku, D.L. Thurber, The status of U-series methods of dating molluscs, Geoch. et Cosmoch. Acta, 35, 1155-1183, 1971.

Keraudren, B., D. Sorel, The terraces of Corinth (Greece)- a detailed record of eustatic sea-level variations during the last 500,000

years, Marine Geology, 77, 99-I 070, 1987.

Ori, G.G., Geologic history of the extensional basin of the Gulf of Corinth (?Miocene-Pleistocene), Greece, Geology, 17, 918-921,

1989.

Ramsey, J.G., M.I. Huber, in The techniques of modern structural geology, Strain analysis, vol. 1, Academic Press, London, 1983. Rigo, A., Etude sismotectonique et g6od6sique du golfe de Corinthe

(Gr•:ce), Ph.D. Thesis, University of Paris VII-IPGP, 281 pp,

1994.

Ritz, J.F., E.T. Brown, D.L. Bourl•:s, H. Philip, A. Schlupp, G.M. Raisbeck, F. Yiou, B. Enkhtuvshin, Slip rates along active faults estimated with cosmic-ray exposure dates: Application to tohe

Bogd fault, Gobi-A!ta¾, Mongolia, Geology, 23, 1019-1022, 1995.

Rosholt, J.N., P.S. Antal, Evaluation of the 231Pa/U-230Th/U

method for dating Pleistocene carbonate rocks, US Geol. Survey

Professional Paper, 450-E, 108-11 I, 1962.

S6brier, M., Tectonique r6cente d'une transversale /• !arc 6g6en. Le golfe de Corinthe et ses r6gions p6riph6riques, Ph.D. Thesis•

Universit6 Paris XI-Orsay, 137 pp, 1977.

Sorel, D., A Pleistocene and still-active detachment fault and the

origin of the Corinth- Patr,as riff, Greece, Geology, 28, 83-86,

2000.

Szabo, B.J., J.N. Rosholt, Uranium-series nuclides in the Golden fault, Colorado, U.S.A.: dating latest thult displacement and measuring recent uptake of radionuclides by faultzone materials, Appl. Geochem., 4, ! 77-182, 1989.

Zwingmann, H., W. Tanja, O. Robin, K/At dating of synkinematic

Illite in fault gouge, EUGXI, 2001.

(Received February 2, 2001' revised May 21, 2001' accepted June