Late-Pliocene timing of Corinth (Greece) rift-margin fault migration ...

Earth and Planetary Science Letters 274 (2008) 132–141
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Earth and Planetary Science Letters
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Late-Pliocene timing of Corinth (Greece) rift-margin fault migration
M.R. Leeder a, ⁎, G.H. Mack b , A.T. Brasier a , R.R. Parrish c , W.C. McIntosh d , J.E Andrews a , C.E. Duermeijer e
a School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK
b Department of Geological Sciences, New Mexico State University, Las Cruces, NM 88003, USA
c NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, UK
d Geochronology Research Laboratory, New Mexico Bureau of Mines and Mineral Resources, Socorro, NM 87801, USA
e Paleomagnetic laboratory, University of Utrecht, Fort Hoofddijk, Budapestlaan 17, 3584 CD Utrecht, Netherlands
article
info
abstract
Article history:
Received 27 March 2008
Received in revised form 3 July 2008
Accepted 8 July 2008
Available online 15 July 2008
Editor: C.P. Jaupart
Keywords:
Corinth rift
Megara basin
fault propagation
extension rates
Geochronological data for timing of Gulf of Corinth rift-margin fault migration is presented from the Megara
basin, precursor to the active eastern Corinth rift. Fifteen sanidine separates from a thin tuff near the top of
the sedimentary basin fill analysed by laser-fusion give apparent 40 Ar– 39 Ar ages from 2.78 to 4.53 Ma. A
weighted mean age of 2.82±0.06 Ma (at 2σ error) for the three youngest aliquots (range of mean ages 2.78–
2.86 Ma) is regarded as a maximum probable age for ash eruption. Together with magnetostratigraphy
results, this age constrains timing of Megara basin abandonment and likely propagation of the active South
Alkyonides coastal faults that presently bound the southern rift-margin to the late-Pliocene, ∼2.2 Ma.
Initiation of the coastal faults caused uplift, fluvial incision and calcrete formation on geomorphic surfaces
over the abandoned Megara basin fill. A petrocalcic laminar horizon from a supermature calcrete
unconformably capping the fill gives a U–Pb age of 0.77±0.08 Ma, dating a late stage in the history of
calcrete development. The new age for initiation of active faulting in the eastern rift yields a low estimate of
long-term mean extension along the active South Alkyonides coastal faults of ∼0.9–1.4 mm a −1 , consistent
with previous geological data. This rate is less than the 100-year GPS-determined geodetic extension rate of
6±2.7 mm a − 1 measured along a ∼23°E meridional array just west of the Alkyonides gulf. It implies either
that the geodetic rate declines rapidly over the ∼15 km distance into the eastern gulf or that the geodetic rate
is unchanged but extra strain is taken up aseismically and/or along antithetic and unrecognised major
intrabasinal faults. In the full graben of the central rift displacements are ∼3.5 km along the Xylocastro and
Antikyra faults; the mean long term extension rate here since 2.2 Ma is ∼3.5 mm a − 1 , much less than current
geodetic rates of ∼10 mm a − 1 . In the western rift, despite a lack of precise chronological data, geodetic and
extension rates seem comparable. Overall, a late-Pliocene to early Pleistocene age is likely for initiation of the
deep-marine Corinth rift, with no evidence for rift propagation, either eastwards or westwards. More
generally, our results constrain timing of strain localisation and vertical axis rigid block rotation over the
Aegean–Anatolian plate and demonstrate that intraplate deformation can be accomplished rapidly in
response to regional-scale tectonic drivers.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction, context and previous work
The Corinth rift (Fig. 1) is one of the most tectonically-active and
seismically-energetic areas in Europe. GPS studies (Clarke et al., 1997;
Davies et al., 1997; Cocard et al., 1999; Briole et al., 2000; McCluskey
et al., 2000; Avallone et al., 2004) and kinematic analysis (Goldsworthy
et al., 2002) establish it as marking a major non-uniformity in
surface velocity over the Aegea-Anatolia plate. Thus rapid acceleration
of southern Greece with respect to central Greece (velocity increase of
∼13 mm a − 1 ) causes strain to be taken up on major normal faults along
the southern rift margin and offshore (Stefatos et al., 2002; Moretti
et al., 2003; McNeill et al., 2005; Lykousis et al., 2007; Bell et al., 2008).
⁎ Corresponding author. Tel.: +44 1603 456161; fax: +44 1603 591327.
E-mail address: m.leeder@uea.ac.uk (M.R. Leeder).
South of the active faults a ∼15 km wide terrane features apparently
inactive faults together with associated abandoned, uplifted and incised
synrift sediments (Fig. 1; Jackson et al., 1982; Ori, 1989; Leeder
et al., 1991; Collier et al., 1992; Dart et al., 1994; Jackson,1999; Malartre
et al., 2004; Rohais et al., 2007; Leeder and Mack, 2007; Bell et al.,
2008; Ford et al., 2008). The timing of northward fault migration is
unknown, apart from broad inferences using extrapolation of late
Quaternary uplift rates (Leeder et al., 1991; Collier et al., 1992),
biostratigraphy of synrift sediments (Malartre et al., 2004; Rohais
et al., 2007; Ford et al., 2008) and offshore sequence stratigraphy (Bell
et al., 2008). Absence of direct age-control hinders testing of hypotheses
for Corinth rift structural evolution and Aegean extensional
history (Armijo et al., 1996; Jackson, 1999; Duermeijer et al., 2000;
Hatzfeld et al., 2000; Goldsworthy et al., 2002; Leeder et al., 2003;
Mattei et al., 2004; Leeder and Mack, 2007). Although neither fault
0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2008.07.006

M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
133
Fig. 1. General location and tectonic summary maps for Gulf of Corinth, Central Greece and (inset) the Aegean context. Note the suite of inactive normal faults on the southern gulf
margin; these are associated with abandoned, uplifting and incised synrift basins. Topography and onshore and offshore faulting after Stefatos et al. (2002), McNeill et al. (2005),
Leeder et al. (2005) and Bell et al. (2008). Also shown in the Megara basin are sample localities and in the Corinth basin the Charalampos (CF) fault and the line of section (dashed line
A-B) for Fig. 7. Place names cited in the text are C — Corinth, M — Megara, A — Alepochori.
death nor birth are easy events to date directly, important age constraints
on these events in other active rifts have come from the synrift
sedimentary record (e.g. Suez rift, Gawthorpe et al., 1997; Rio
Grande rift, Perez-Arlucea et al., 2000).
In this contribution we present geochronological evidence relevant
to the timing of rift-margin fault evolution from the inactive Megara
basin (Fig. 1), south of the active eastern Corinth rift. Previously the
age of sediments deposited adjacent to the basin-bounding Pateras
normal fault have been poorly constrained (Theodoropoulos, 1968;
Bentham et al., 1991). Our results constrain timing of strain localisation
and vertical axis rigid block rotation over the Aegean–Anatolian
plate and demonstrate that intraplate deformation can be accomplished
rapidly in response to regional-scale tectonic drivers.
2. Pagae Ash Member
The newly-discovered Pagae Ash Member occurs in the topmost
Louba Formation of the Alepochori Group (Fig. 2), part of the ∼1 km
thick sedimentary fill to the Megara basin (Bentham et al., 1991). It
occurs 45 m below the base of the Agia Sofia Formation and ∼100 m
below the top of the fill. The ash (for location details see Appendix A)
is a pale-coloured, 3–4 cm thick, airfall crystal tuff comprising submm
sized sanidine and biotite crystals in a finer ash matrix. It occurs
as a single distinctive, sharp-based and sharp-topped layer within
gravel to mudrock grade sediments of fluviatile channel and floodplain
origins much affected by arid-zone pedogenesis (Mack et al.,
1993a). The airfall volcanic event evidently occurred over an inactive
floodplain environment during an interval free from the activity of
combing shallow braided river channels. Subsequent channel avulsions
led to sediment burial and partial erosion of the ash horizon. The
source of the ash, whether from Aegean or Italian volcanic arcs will be
the subject of future geochemical and stratigraphic research.
3. 40 Ar/ 39 Ar analytical results
A sanidine separate from the Pagae ash was analysed by laser fusion
method. Multi-grain aliquots were analysed as the sanidine crystals
were too small to be individually analysed. A small percentage of impurities,
predominantly plagioclase, remained in the mineral separate.
Abbreviated analytical methods and data are provided in Appendix A,
Table 1. Fifteen fused multi-grain aliquots yielded apparent ages from
2.78 to 4.53 Ma (Fig. 3) with radiogenic yields ranging from 44.6–99.8%.
There is no correlation between radiogenic yield and age. K/Ca values are
overall low for sanidine (3 to 26.5); a more typical sanidine K/Ca value
would be in the 40–100 range, although some Ca-rich sanidines have K/Ca
ratios as low as 10. A weighted mean age of 2.82±0.06 Ma is calculated
from the three youngest aliquots and this is viewed as a maximum age for
the eruption. The 1.75 Ma spread in overall ages suggests that the mineral
separate is contaminated with xenocrysts and the possibility cannot be
ruled out that all aliquots are contaminated to some degree. Low
radiogenic yields (b∼95% for sanidine of this age), as obtained for 12 of
the analyses, are often indicative of alteration and possible Ar loss. A
correlation between increasing apparent age and increasing radiogenic
yield is commonly seen in samples that have undergone alteration. This
correlation is not seen in the Pagae ash data so it is possible that the low
radiogenic yields are a result of contamination by phases such as

134 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
Fig. 2. Stratigraphy of the topmost Megara basin sedimentary succession, with lithostratigraphy largely after Bentham et al. (1991) and magnetostratigraphy after Duermeijer and van
Hoof (1996) and further correlations by the present authors. Paleomagnetic data were obtained by standard thermal demagnetization (see Duermeijer et al., 2000 for paleomagnetic
sampling details and procedures). Geomagnetic polarity timescale from Cande and Kent (1995). The Divide palaeomagnetic section is measured down from a zero reference datum close
to the local top of the Louba Formation. The Pagae Ash Member and New Road paleomagnetic sections are measured down from the base of the easily-mappable Agia Sofia Formation.
plagioclase with low radiogenic yields and similar ages, rather than being
caused by alteration and accompanying Ar loss. However, the possibility
of Ar loss cannot be completely ruled out and must be considered when
using the assigned age for ash eruption. It is also possible that low-K
phases have common ‘old’ argon, either as excess argon in numerous
plagioclase phenocrysts, or by sanidine xenocrysts providing old argon
and the plagioclase providing little radiogenic argon, but abundant Ca to
lower the K/Ca ratios.
4. Megara Calcrete Member
Calcretes are common in Quaternary continental sediments of
central Greece (Brasier, 2007). The Megara Calcrete Member occurs as
a discontinuous caprock lying unconformably on the topmost Louba
Formation in the Megara basin (Leeder et al., 2005; Leeder and
Gawthorpe, 2007; Leeder and Mack, 2007). The unit is a 3–5 m thick
Stage V calcrete in the terminology of Gile et al. (1981) and Machette
(1985). Field attributes of pedogenic (soil) origin are supported by
detailed petrological characteristics and stable isotopic composition
(Brasier, 2007). It is closely analogous to the Upper and Lower La Mesa
calcretes that feature as duricrusts capping geomorphic surfaces of the
Rio Grande rift, SW USA (Gile et al., 1981; Mack et al., 1993b, 1994).
5. U–Pb dating of the Megara Calcrete Member
Although chronology for late Quaternary calcretes can be provided
by either radiocarbon or U/Th techniques, dating calcretes in deeper
geological time has until recently (Rasbury et al., 1997) been very

M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
135
Table 1
40 Ar/ 39 Ar analytical data
ID
40 Ar/ 39 Ar
37 Ar/ 39 Ar
36 Ar/ 39 Ar
(x10 − 3 )
39 Ar K
(x10 − 15 mol)
Louba Ash, 10–20 sanidine crystals, J =0.0008996+0.05%, D=1.002+0.001, NM-196H, Lab#=56266
01 3.838 0.0491 7.213 7.594 10.4 44.6 2.78 0.05
07 2.408 0.0503 2.267 5.618 10.2 72.4 2.83 0.06
15 2.192 0.0497 1.472 8.226 10.3 80.3 2.86 0.04
# 10 1.825 0.0192 0.0147 2.189 26.5 99.8 2.95 0.15
# 02 3.183 0.0217 4.590 2.195 23.5 57.5 2.97 0.16
# 14 2.792 0.0697 3.258 3.859 7.3 65.7 2.98 0.09
# 04 2.456 0.0206 1.966 2.958 24.8 76.4 3.04 0.12
# 06 2.865 0.0569 3.359 2.499 9.0 65.5 3.04 0.14
# 09 1.983 0.0707 0.3465 3.174 7.2 95.1 3.06 0.11
# 12 1.958 0.0208 0.0468 2.975 24.5 99.4 3.16 0.11
# 13 3.579 0.1271 5.467 7.261 4.0 55.1 3.20 0.05
# 03 3.985 0.1073 6.430 1.669 4.8 52.5 3.39 0.21
# 11 2.965 0.1413 2.920 2.323 3.6 71.3 3.43 0.15
# 08 3.356 0.0433 2.922 3.079 11.8 74.4 4.05 0.12
# 05 3.542 0.0199 2.534 2.182 25.6 78.9 4.53 0.16
K/Ca
40 Ar⁎
(%)
Age
(Ma)
+1σ
Mean age of youngest 3 samples N 3 MSWD 0.69 K/Ca 10.3+0.2 Age 2.82 +2σ 0.06
Notes: 1) Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions. 2) Errors quoted for individual analyses include analytical
error only, without interfering reaction or J uncertainties. 3) Mean age is weighted mean age of Taylor (1982). Mean age error is weighted error of the mean (Taylor, 1982), multiplied by
the root of the MSWD where MSWDN1, and also incorporates uncertainty in J factors and irradiation correction uncertainties. 4) Decay constants and isotopic abundances after Steiger
and Jäger (1977). 5) # symbol preceding sample ID denotes analyses excluded from mean age calculations. 6) Ages calculated relative to FC-2 Fish Canyon Tuff sanidine interlaboratory
standard at 28.02 Ma. 7) Decay constant (LambdaK (total))=5.543e−10/a. 8) Correction factors: ( 39 Ar/ 37 Ar) Ca =0.000676+ 4e−06, ( 36 Ar/ 37 Ar) Ca = 0.000277+ 2e−06, ( 38 Ar/ 39 Ar) K =0.0126,
( 40 Ar/ 39 Ar) K =0+0.0004.
difficult. Recent advances in methodology have enabled U–Pb dating
to extend more widely to Pleistocene terrestrial carbonate, opal or
phosphate (Richards et al., 1998; Neymark et al., 2000; Walker et al.,
2006; Woodhead et al., 2006; Brasier, 2007). Importantly, the
recognition of the influence of initial uranium activity on the interpretation
of U–Pb data has been shown to be crucial to self-consistent
interpretations (Neymark et al., 2000).
Megara calcrete sample MRL15903-2 (Fig. 4) was studied petrographically
and sub-sampled for analysis. Aliquots for analysis were selected
from a distinct centimetric-scale lamina present in the petrocalcic
horizon (Fig. 4). Initially, several analyses were performed to determine
if sufficient U was present and to establish whether a range of U/Pb ratios
were likely to be encountered. After this initial screening was deemed
successful, subsequent sample aliquots were measured that came from
similar petrographic layers of the specimen (Fig. 4). Other parts of the
sample appear to have formed or incorporated common Pb with a
different composition and as a result were not useful or used in any
regressions.
The measurement of residual disequilibrium of 234 U/ 238 U resulted
in present day activity ratios of 1.180±0.005 and 1.187±0.005 for the
relevant portion of the sample (Table 2). The initial activity ratio can
be calculated using equations in Woodhead et al. (2006) for a specified
time in the past. For example if this sample were 1 Ma old, its initial U
activity ratio would be 4.8, itself an unusually high value for surface
waters. This alone suggests that the sample may be less than a million
years old. From the isotope dilution measurements, the Pb isotopic
composition of the samples and U/Pb isotope ratios were determined
and are given in Table 3. The U contents of the aliquots varied from 95–
450 ppb. We have used a 238 U/ 207 Pb v. 206 Pb/ 207 Pb ‘isochron’ diagram
(Fig. 5) to illustrate the data spread and reasonable coherence. A 238 U/
208 Pb v. 206 Pb/ 208 Pb diagram would have demonstrated similar spread
and co-linearity.
The most coherent sub-samples are those from layer A of Fig. 4, and
include those C2-5 to 8, C3-2 and MRL15903-2b (data in Table 3).
These 6 subsamples yield an age of 1.7±0.4 Ma (95% confidence) on
the 238 U– 206 Pb isochron referenced to 207 Pb (Fig. 5). If the initial activity
ratio is calculated using this age, then an unrealistic value of 23.3
results.
Table 2 shows combinations of measured current activity ratio,
‘isochron’ ages, and initial activity ratios calculated using such ages.
The older the age for a given measured activity ratio, the more
unrealistic the initial calculated activity is. Conversely if the isochron
age was much younger the age correction would be affected in the
opposite direction. An optimum value results for an age of about
765,000 y with an initial activity ratio of 2.59.
The common Pb-corrected age of a young sample can be calculated
and the degree of concordance between 238 U/ 206 Pb and 207 Pb/ 206 Pb
ages visualised on a Tera–Wasserburg plot (Fig. 6). The concordia
curve on such a plot is affected by the initial 234 U/ 238 U disequilibrium,
and this must be taken into account, i.e. a different concordia location
corresponds to each initial activity ratio. When 238 U/ 206 Pb vs 207 Pb/
206 Pb molar isotope ratios from the subsamples are plotted on a Tera–
Wasserburg diagram calculated to be consistent with an initial 234 U/
238 U activity ratio of 2.59 (equivalent to a measured value of 1.18 and
an age of 765 ka), the discordia line intercepts the disequilibrium
concordia line at 760 ka with an uncertainty of approximately 80 ky. If
an age much greater than 760 ka is assumed in the calculation of the
initial 234 U/ 238 U activity ratio, the (constant) discordia line actually
Fig. 3. Age probability distribution diagram of the Pagae Ash sanidine. Shaded distribution
and data points are those of the three youngest aliquots. All errors quoted at 2
sigma. MSWD=mean sum weighted deviate value.

136 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
6. Discussion: timing of faulting, Corinth rift evolution and wider
implications
6.1. Megara Basin evolution, abandonment and uplift
Fig. 4. Megara basin calcrete specimen MRL 15903-2, with some of the different fabrics
separated by dashed lines. Subsamples from a single petrographically distinct layer that
were used in dating are shown in the zone labelled A, comprising subsamples C2-5 to
C2-8, MRL15903-2b and C3-2 (data in Appendix B). Other subsamples from adjacent or
other layers did not have the same common Pb composition and therefore were not
used in regressions.
intercepts the modified concordia line at a younger age than assumed
in the calculation of the initial 234 U/ 238 U activity ratio (see Table 2).
Together, the iteratively calculated initial activity ratio and measured
isotopic composition of the aliquots suggests that the Megara basin
calcrete subsample is approximately 765±80 ka in age. In these
calculations the initial 230 Th/ 234 U activity ratio is assumed to be zero
but in terms of the sensitivity of this assumption, if the initial 230 Th/
234 U activity ratio was 1.0, this would only have the effect of reducing
the calculated age by ∼10%.
The Pagae Ash Member, maximum likely age 2.82 Ma, is the key
time marker in the Megara basin. It establishes the majority of the
underlying basin fill and probably also the thin (b80 m) overlying
succession (with the exception of the Megara Calcrete Member, see
below) as Pliocene in age. The radiometric age requires ash fallout
during the late Gauss chron (see geomagnetic timescale of Cande and
Kent, 1995). This is consistent with previously determined palaeomagnetic
results (Duermeijer et al., 2000; Duermeijer and van Hoof,
1996) from separate sections in the general area of the ash exposure
(Fig. 2). These show reversed polarity above the stratigraphic level of
the ash with a normal polarity interval at the very top of the measured
succession below the level of the Megara Calcrete Member. Below the
stratigraphic level of the ash there is normal polarity, preceded by
another reversed interval and then further reversed intervals from
samples of unknown stratigraphic position (due to faulting). The palaeomagnetic
results and stratigraphic architecture are consistent
with sedimentation and ash fallout during the upper part of the Gauss
normal chron interval. The reversed Matuyama chron is represented
above, terminating in what is probably the first normal Matuyama
subchron, the Reunion (see Fig. 2B). It is likely that the Kaena reversed
subchron of the Gauss chron is that represented below the normal
polarity ash-bearing interval. Given the ∼1.2 km total thickness of the
Megara basin sediments it is probable that the synrift fill may extend
well into the Gilbert reversed chron (age 5.89–3.58 Ma), with its many
normal subchrons. In support of this there are both reversed and
normal magnetised sections in the Harbour Ridges Formation and
normal sections in the Rema Mazi Formations below (Duermeijer
et al., 2000; Duermeijer and van Hoof, 1996).
In summary, geochronologic, paleomagnetic and stratigraphic
constraints suggest that the uppermost sediments below the Megara
Calcrete Member are of Reunion subchron age and date from 2.15 Ma
(Cande and Kent, 1995; Fig. 2). This gives a likely maximum age for
basin abandonment. Assuming regional extensional strain in the
eastern rift is due to steady Aegean plate motion over the past ∼2.2 My
it seems reasonable to infer that Megara basin fault abandonment was
closely followed by initiation of the presently active coastal faults
bounding the Alkyonides basin. The subsequent truncation, subsidence
and onlap of Megara basin infill by Pleistocene syn-rift sediments
is recorded from seismic reflection data from the Alkyonides
basin (Sakellariou et al., 2007).
Table 2
Calculated initial 234 U/ 238 U activity ratios, together with the sample age assumed in
calculation of these ratios, and the ages at which the fixed discordia line (with its
associated error) intersects Tera–Waserburg concordia curves modified by these
calculated initial 234 U/ 238 U activity ratios
Fig. 5. 238 U/ 207 Pb vs 206 Pb/ 207 Pb plot of selected data (Appendix B) from Megara Basin
calcrete MRL 15903-2. A number of other analyses from other zones (listed in Brasier, 2007)
plot well above the line shown with 206 Pb/ 207 Pb values between 1.16 and 1.21 and 238 U/
207 Pbb15. These are excluded from consideration since they are likely to have an entirely
different, more radiogenic common Pb composition inherited from the depositing fluids.
Current 234 U/
238 U activity
ratio (2σ
absolute error)
Age assumed in
calculation of initial
234 U/ 238 U activity
ratio (years)
Calculated initial
234 U/ 238 U activity
ratio (2σ absolute
error)
Age calculated from
modified Tera–
Wasserburg diagram using
initial 234 U/ 238 U activity
ratio (years)
1.183 ± 0.007 740,000 2.48 (± 0.06) (± 90)
790,000
1.183 ± 0.007 750,000 2.52 (± 0.06) (± 80)
770,000
1.183 ± 0.007 760,000 2.57 (± 0.06) (± 80)
770,000
1.183 ± 0.007 770,000 2.61 (± 0.06) (± 80)
760,000
1.183 ± 0.007 780,000 2.66 (± 0.07) (± 80)
750,000
1.183 ± 0.007 1,000,000 4.08 (± 0.12) (± 70)
520,000
1.183 ± 0.007 1,700,000 23.25 (± 0.89) (± 10)
150,000
The uncertainty in calculated initial 234 U/ 238 U activity ratio results from measurement
uncertainty, and contributes an error of b10 ka to all samples. The bulk of the error is
thus derived from the uncertainty associated with the discordia line.

M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
137
Table 3
U/Pb ICP-MC-MS data, with 2σ (%) error; analyses used in regression to obtain the Megara calcrete age
Method
Conc
nitric
Conc
nitric
Conc
nitric
Conc
nitric
Conc
nitric
Conc
nitric
206 Pb/
208 Pb
%
stdev
238 U/
208 Pb
%
stdev
206 Pb/
204 Pb
%
stdev
238 U/
204 Pb
%
stdev
238 U/
207 Pb
%
stdev
206 Pb/
207 Pb
%
stdev
238 U/
206 Pb
%
stdev
208 Pb/
204 Pb
207 Pb/
204 Pb
238 Uppb 208 Pb
ppb
subsample
15903- 0.4769 0.05 48.3366 0.75 18.2739 1.11 1852.2663 1.16 117.4983 0.72 1.1592 0.08 101.3613 0.82 38.3201 15.7642 95.88 1.78
2B
C2-5 0.4708 0.07 15.3532 2.41 17.5473 0.40 572.1818 2.70 37.0401 2.34 1.1359 0.14 32.6079 2.50 37.2680 15.4477 389.08 25.16
C2-6 0.4710 0.08 14.2599 2.63 17.8337 0.62 539.9694 2.99 34.4096 2.55 1.1365 0.15 30.2780 2.72 37.8663 15.6924 424.44 29.87
C2-7 0.4704 0.09 13.3548 2.62 17.7743 0.58 504.6570 2.98 32.2110 2.54 1.1345 0.17 28.3925 2.71 37.7885 15.6672 442.33 33.22
C2-8 0.4703 0.04 6.4164 1.01 17.7164 0.24 241.7252 1.15 15.4690 0.98 1.1337 0.08 13.6442 1.05 37.6728 15.6264 452.08 65.05
C3-2 0.4724 0.02 17.5273 0.27 17.7375 1.02 652.6991 0.48 48.5685 0.29 1.1393 0.03 42.6297 0.27 37.5497 15.5686 290.63 14.49
A further looser control on tectonic basin history is provided by the
U–Pb age of 0.77 Ma for the Megara Calcrete Member. Mature
calcretes elsewhere record passage of long time intervals, for example
the Lower La Mesa calcrete of the Rio Grande rift, SW USA, dates back
some 800 ky (Gile et al., 1966, 1981; Mack et al., 1993b, 1994). In this
area, mature calcretes cap stable geomorphic surfaces left behind after
regional fluvial incision. The Megara Calcrete Member has a similar
context and must have been initiated well before the radiometric age
of its laminar petrocalcic horizon, since similar horizons in Stage IV–VI
calcretes form after the profiles ‘plug-up’ from below as impermeable
Stage III nodular carbonate horizons (Gile et al., 1966; Machette, 1985).
Over the Megara basin, we assume that initiation of calcrete formation
followed landscape stability as remnant geomorphic surfaces developed
under climatic conditions suitable for calcrete formation. These
surfaces formed as the uplifting, now-inactive Megara basin was
backtilted to the SE by the new coastal faults, causing reversal of
formerly NW-flowing Louba Formation drainages channels (Bentham
et al., 1991) and partial incision of the slowly backtilting dip slope
(Fig. 7).
6.2. Slip and extension rates in the eastern rift
The inferred timing of fault propagation in the eastern rift at around
2.2 Ma places the major change to fault activity in the eastern Corinth
rift much earlier than the ∼1 Ma previously deduced by extrapolation
of late Pleistocene (post-MIS 5e) footwall uplift rates (Leeder et al.,
1991; Collier et al., 1992), fault modelling (Armijo et al., 1996) or much
younger estimates of b0.5 Ma from active Corinth rift subsidence rates
using offshore subsurface seismic stratigraphy (Lykousis et al., 2007;
Sakellariou et al., 2007). It supports the previous suggestion of Hatzfeld
et al. (2000) that regional tectonics in western Greece and in the
eastern rift underwent significant changes in the Upper Pliocene.
Total vertical stratigraphic separation (aka vertical fault displacement
or throw) on the active eastern Corinth rift faults along the
southern margin to the Alkyonides Gulf is a maximum of 2–3 km
(Armijo et al., 1996; Collier et al., 1998; Leeder et al., 2005; Lykousis
et al., 2007; Sakellariou et al., 2007). Our new geochronological data
for initiation of this active faulting yields a low estimate of long-term
mean extension rate of ∼0.9–1.4 mm a − 1 . The new data is consistent
with 1) lower limits to displacement rates determined from late-
Holocene palaeoseismology of the active Skinos fault (Collier et al.,
1998), 2) mean Holocene displacement across the active Psatha fault
(Leeder et al., 2002) and 3) estimates of basin tilt due to the Skinos–
East Alkyonides faults (Leeder et al., 2005). It confirms previous
suspicions (Leeder et al., 2005) that the geological extension rate of
the eastern rift since 2.2 Ma may be less than the 100-year GPSdetermined
geodetic extension rate of 6 ±2.7 mm a − 1 measured along
a ∼23°E meridional array just west of the Alkyonides Gulf (Clarke
et al., 1997). This implies that either the geodetic rate declines rapidly
over the ∼15 km distance to the eastern gulf (Fig. 1; see also Goldsworthy
et al., 2002) or the geodetic rate is unchanged but extra
extension is taken up along antithetic and other hitherto unrecognised
intrabasinal faults.
6.3. Fault propagation and extension rates in the wider Corinth rift
In the wider Corinth rift basin, the Pliocene Charalampos tilt block
basin east of the Isthmus (Fig. 1) was abandoned after eruption of
andesitic volcanics, whose biotite and hornblende phenocrysts are K–Ar
dated as 3.62±0.18 and 4.0±0.4 Ma (Collier and Dart, 1991). Palaeomagnetic
results (Duermeijer et al., 2000; Duermeijer and van Hoof,
1996) indicate normal polarities with modern declinations in the
andesite unit. Taking into account the errors on the two ages, eruption
took place either at the very base of the Gauss normal chron at a little less
than 3.58 Ma (consistent, within errors, with both radiometric dates) or
in the Cochiti normal subchron of the Gilbert reversed chron between
4.18 and 4.29 Ma (consistent with the older radiometric date only).
Recent field work in the eastern Corinth basin (Mack et al., in press)
establishes that the Saros faults (Fig. 1) are inactive, since the degraded
fault scarps and fault planes are everywhere onlapped by alluvial fan
sediments. A U-series age of 304 +44/−30 ka (P. Rowe pers. comm.) for
the laminar cap to a calcrete palaeosol developed on the youngest
onlapping sediment indicates that fault death occurred well before this
time. The record of progressive offlap of shallow marine to littoral
sediments from the central horst in the Corinth Canal section (Collier,
1990) indicates that uplift has affected the basin fill since at least 400 ka.
The likely Milankovich eccentricity scaling of the Corinth terraces to the
west towards Xylocastro (Fig.1), together with marine nannofossils from
NN20 zone (0.46–0.45 Ma) indicates uplift here since at least 0.5 Ma
(Keraudren and Sorel, 1987; Armijo et al., 1996).
Fig. 6. Tera–Wasserburg diagram of 238 U/ 206 Pb vs 207 Pb/ 206 Pb, with the concordia curve
modified for an initial 234 U/ 238 U activity ratio of 2.59 (assuming a sample age of 765 ka).
The fixed discordia line (labelled line) is drawn through samples C2-5 to C2-8, C3-2 and
MRL15903-2b) crosses this concordia curve at an age of 760 ka. The lines parallel to the
discordia line represent the errors associated with it. Lines labelled with ages in ka and
ma are isochrons.

138 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
Fig. 7. Schematic section along the line A–BinFig. 1 to show the young, active, offshore Psatha/East Alkyonides fault bounding the Alkyonides Gulf and the abandoned, uplifting and
incising Pleistocene Megara basin. The inactive Pateras fault is projected into the line of section for illustration purposes. We show major morphological features useful in
parameterisation of rates of uplift and subsidence associated with displacements along the active faults (see Leeder et al., 2005).
Along the central rift flank, former movement on the now-inactive
onshore Xylocastro–Loutro fault in the footwall to the active offshore
Xylocastro (Corinth) fault (Fig. 1; Stefatos et al., 2002) has been dated
by U-series analysis of syntectonic calcite to around 1 Ma (Causse
et al., 2004), whilst the inactive Valimi fault (Fig. 1) was active
between 0.38–1Ma(Flotté et al., 2001; Causse et al., 2004). Regarding
the age of basin-fill sediments adjacent to inactive north Peloponessian
faults, in western areas of the rift around Patras, the earliest
synrift fluvio-deltaic units associated with these are dated as Middle
to Late Pliocene (Kontopoulos and Doutsos, 1985; Frydas, 1987, 1989;
Zelididis, 1998). Palynological taxa collected from Vouraikos fan delta
deposits that formed by subsidence in the hangingwall to the Pirgaki–
Mamoussia fault (Malartre et al., 2004; Fig. 1) indicate a Lower
Pleistocene age (i.e. 2.6 to ∼1 Ma). This is consistent with ‘Calabrian’
ages (∼1.8–0.8 Ma) for mammalian fossils in equivalent units closer to
Patras (Simeonidis et al., 1987). Early Pleistocene dates are not
consistent with hypotheses for onset of very young deepening of the
proto-Gulf (Sorel, 2000) or of any rapid, young tectonic ‘phase’ (Armijo
et al., 1996; Hinsbergen et al., 2005).
Despite a lack of precise chronological data in the central and
western rift, we conclude that a late-Pliocene to early Pleistocene age,
around 2 Ma, is likely for modern, deep-marine, Corinth rift initiation.
Before this a wide, mainly fluvio-lacustrine, synrift province developed
(Rohais et al., 2007; Bell et al., 2008; Ford et al., 2008; Fig. 8). In
this was deposited the Lower Group onshore in the west (Rohais et al.,
2007) with possible offshore equivalents (Bell et al., 2008), the
Charalampos sediments of the Corinth basin (Collier and Dart, 1991)
and the Megara basin sequences in the east. There is no geochronological
evidence for rates of subsidence or extension in these basins. In
western areas there are at least two episodes of major rift-bounding
faults, 1) the now-dead Pirgaki–Mamoussia faults and lateral equivalents
active in the interval 0.8–1.8 Ma (Malartre et al., 2004) and 2)
the presently active West and East Helike faults (McNeill and Collier,
2004; Bell et al., 2008; Fig. 1) and the youngest offshore intrabasinal
Eratini faults at 0.5 Ma (Bell et al., 2008). In western areas, geological
extension rates along active onshore and offshore faults are compatible
with GPS-determined geodetic extension rates (McNeill et al.,
2005; Bell et al., 2008).
Finally, in the full graben of the central rift, maximum vertical
displacement is ∼3.5 km along the main, basin-bounding and 45°dipping
Xylocastro (Corinth) fault (Stefatos et al., 2002; Moretti et al.,
2003; Lykousis et al., 2007; Fig.1). It is probably similar along the major
antithetic Antikyra intrabasinal fault system mapped by Stefatos et al.
(2002; Fig. 1). We compute the mean long term extension rate here
since 2.2 Ma as ∼3.5 mm yr − 1 , much less than current geodetic rates of
∼10 mm yr − 1 (Clarke et al.,1997; Davies et al.,1997; Cocard et al.,1999;
Briole et al., 2000; McCluskey et al., 2000; Avallone et al., 2004). It
Fig. 8. Sketches to show sequential development of the Corinth rift by northward
migration of active faulting during the Pliocene–Pleistocene according to the new
geochronological data presented in this paper and our discussion of previously
published results.

M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
139
remains to be determined how much of this discrepancy may be taken
up aseismically or on other offshore faults.
6.4. Wider implications for continental rift evolution and eastern
Mediterranean tectonics
Apart from establishing possible discrepancies in the central
Corinth rift between observed geodetic and longer term geological
extension rates, the results presented above have general implications
for both the evolution of continental rifts and eastern Mediterranean
tectonics.
1) Our geochronological data and discussion of its relation to previous
biostratigraphic results for the Corinth rift establish that strain
localisation and deep rift basin development was synchronous and
did not propagate through time, either eastwards or westwards, as
stated previously in the literature (e.g. Doutsos and Poulimenos,
1992; Armijo, 1996).
2) Regionally the Corinth rift is the locus of the majority of active strain
within the Aegean–Anatolian plate. Abandoned rifts and normal
faults, like those of the Megara basin and the southern Corinth rift
flank, occur broadly over Greece and western Turkey (Jolivet, 2001;
Goldsworthy et al., 2002) and make it is clear that internal strain in
the plate was once more widely distributed. The major change in
internal plate dynamics that must have occurred in order to focus
the majority of strain along the Corinth rift happen around 2.2 Ma.
3) In addition to the general clockwise rotation of western Greece with
respect to the Balkans since the Miocene (LePichon and Angelier,
1979; Hinsbergen et al., 2005), plate re-organisation involved a
local counter clockwise rotation of Peloponnissos with respect to
central Greece (Boetia–Locris–Evia) along the meridional Corinth
rift (Goldsworthy et al., 2002; Mattei et al., 2004) about a pole
located in eastern Greece. This caused extension in the western rift
to be greater than that in the eastern rift. Our results constrain the
timing of localisation of vertical-axis rigid block rotation over the
Aegean–Anatolian plate. They demonstrate that changing intraplate
deformation can be accomplished rapidly as block margins
migrate in response to regional scale tectonic drivers.
4) Regarding possible dynamic drivers, large-scale fault migration and
strain focussing from wide to narrow extensional zones is a marked
feature of other active continental rift provinces. Good examples are
migration during the last 10 My of active faulting from the wider
Basin and Range to more concentrated rift zones like the Rio Grande
rift and Central Nevada seismic belt (Buck 1991). Such migration
was originally attributed to temporal changes brought about by
decreasing heat flux on crustal and mantle rheology. We have no
evidence that geotherms changed in Greece in the late-Pliocene and
recent work (Jackson et al., 2008) indicates that previous assumptions
about rheology used in extensional modeling (Buck 1991) are
probably in error.
5) It is thus more likely that kinematic drivers are responsible for strain
localisation in the Aegean plate. Candidates have previously been
identified as plate boundary strike-slip fault propagation along the
North Anatolian fault (Armijo et al., 1999; 2003; Flerit et al., 2004),
intra-plate strike slip fault migration causing local block rotations
(Goldsworthy et al., 2002) and motion of Aegea–Anatolia over
shallow subducting African plate (Leeder and Mack, 2007). Now that
the timing of tectonic change is better established, future work
should set out to decide which of these processes, if any, caused the
late-Pliocene to Early Pleistocene événement in the Corinth rift.
Acknowledgements
We thank Rebecca Bell, Martin Brasier, Richard Collier, Mary Ford,
Rob Gawthorpe, James Jackson, Lisa McNeill, Pete Rowe and Jenni
Turner for discussions and help at various times. ATB acknowledges
award of NERC Studentship grant NER/S/A/2003/11221. Matt Horstwood
of the NERC Isotope Geosciences Laboratory is thanked for
assistance with U and Pb analysis by ICP-MC-MS funded by NIGL and
Lisa Peters for 40 Ar– 39 Ar analyses by laser fusion at the New Mexico
Geochronology Research Laboratory. We also thank the IGME, Athens,
for permissions to do fieldwork and collecting in central Greece.
Appendix A. Pagae Ash Member and 40 Ar/ 39 Ar analytical methods
The Pagae (pronounced Pahee) Ash member is named after the
ancient Greek settlement at modern day Alepochori that was the
major Corinthian gulf port of embarkation to Delphi. It outcrops in two
NW–SE orientated roadcut faces (38° 4.163'N 23° 13.246'E) located
5.6 km from the T-junction at the beginning of the main road to
Megara out of Alepochori and 0.4 km uphill on a straight from the
wayside shrine to St Jerome located on the left after a sharp right bend
at a prominent culverted arroyo. There is ample parking space beside
the outcrops. The Pagae ash sample was initially prepared by crushing,
elutriating clay-sized ash and cleaning with HF. The sanidine was
separated by standard heavy liquid, magnetic separator and handpicking
techniques. A small percentage of impurities, predominantly plagioclase,
remained in the mineral separate. The separate was loaded
into an aluminium disc and irradiated for 8.85 h at the Nuclear Science
Center in College Station, Texas. Neutron flux monitor was Fish Canyon
Tuff sanidine (FC-2), age 28.02 Ma. Fifteen 10–20 sanidine crystal
aliquots of Pagae Ash were subsequently analysed in a Mass Analyzer
Products 215–50 mass spectrometer on line with automated all-metal
extraction system. Samples and flux monitor were fused by a 50 W
Synrad CO2 laser. Reactive gases were removed during a 2-minute
reaction with two SAES GP-50 getters, one operated at ∼450 °C and
one at 20 °C. The gas was also exposed to a W-filament operated at
∼2000 °C and a cold finger operated at 140 °C.
Appendix B. Megara calcrete and U–Pb analytical methods
The locus typicus for the Megara calcrete (Leeder and Gawthorpe,
2007) is above Alepochori village in a roadcut along the main Megara
highway at 38°03′45.9″N, 23°13′42.2″E. The samples used in the present
radiometric dating are subsampled from a single hand specimen
of a laminated petrocalcic horizon (Fig. 4) taken from a natural outcrop
off the same road closer to Megara town at 38°02′9.0″N, 23°18′
16.5″E. U–Pb analysis was made with a VG Elemental Axiom multicollector
at the NERC Isotope Geoscience Laboratories, Keyworth.
For the chemical procedure, samples of carbonate were dissolved
using nitric acid. Centrifuging of the sample to separate undissolved
residue followed dissolution. Most samples were spiked with a mixed
205 Pb– 233-235 U tracer prior to centrifuging. Chemical purification and
separation of U and Pb used UTEVA and AG1X8 ion exchange resins for
U and Pb purification, respectively with nitric and hydrochloric acids.
Purified U and Pb were taken up in ∼1 ml of 2% HNO 3 for mass
spectrometry. Mass spectrometry for both instruments employed a
mixed multiple collector array using both Faraday cups and ion
counting secondary electron multiplier(s) using peak switching and
multiple ion detection. Mass bias was monitored using CRM950 natural
uranium solutions, and for the Axiom, the secondary electron
multiplier was calibrated for gain and non-linearity using a CRM U010
solution of varying concentration. Data were reduced in part off line
for isotope age calculation. Mass bias for Pb measurements was done
via external Pb isotope standard normalisation. All analyses were
blank corrected for U and Pb using the mean of duplicate blank measurements
for each batch of chemistry.
Specimen MRL 15903-2 (Fig. 4) is a thick laminar calcrete with a
complex history. It is possible that these different generations of calcrete
may span much of the history of the basin-capping Megara calcrete
deposit. Following initial pilot sub-sample analyses, detailed sample
selection involved scanning the specimen at a resolution of 600 dots per

140 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141
inch (dpi) on a flatbed scanner, and cut with a wire saw. Subsamples
were taken from a single confined area of the specimen (zone A of Fig. 4)
that differed slightly in colour from white to brown, probably reflecting a
variable organic content. Avariety of sample aliquots in addition to those
used in the final regressions were measured, in all cases these were from
layers other than that with the most coherent data. This additional data
do not form any coherent pattern and are not used. It is likely that they
incorporated common Pb of variable and much more radiogenic
composition than the analyses from layer A of our sample (Fig. 4).
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