Late-Pliocene timing of Corinth (Greece) rift-margin fault migration ...
Late-Pliocene timing of Corinth (Greece) rift-margin fault migration ...
Late-Pliocene timing of Corinth (Greece) rift-margin fault migration ...
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Earth and Planetary Science Letters 274 (2008) 132–141<br />
Contents lists available at ScienceDirect<br />
Earth and Planetary Science Letters<br />
journal homepage: www.elsevier.com/locate/epsl<br />
<strong>Late</strong>-<strong>Pliocene</strong> <strong>timing</strong> <strong>of</strong> <strong>Corinth</strong> (<strong>Greece</strong>) <strong>rift</strong>-<strong>margin</strong> <strong>fault</strong> <strong>migration</strong><br />
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<br />
a School <strong>of</strong> Environmental Sciences, University <strong>of</strong> East Anglia, Norwich, NR4 7TJ, UK<br />
b Department <strong>of</strong> Geological Sciences, New Mexico State University, Las Cruces, NM 88003, USA<br />
c NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, UK<br />
d Geochronology Research Laboratory, New Mexico Bureau <strong>of</strong> Mines and Mineral Resources, Socorro, NM 87801, USA<br />
e Paleomagnetic laboratory, University <strong>of</strong> Utrecht, Fort Ho<strong>of</strong>ddijk, Budapestlaan 17, 3584 CD Utrecht, Netherlands<br />
article<br />
info<br />
abstract<br />
Article history:<br />
Received 27 March 2008<br />
Received in revised form 3 July 2008<br />
Accepted 8 July 2008<br />
Available online 15 July 2008<br />
Editor: C.P. Jaupart<br />
Keywords:<br />
<strong>Corinth</strong> <strong>rift</strong><br />
Megara basin<br />
<strong>fault</strong> propagation<br />
extension rates<br />
Geochronological data for <strong>timing</strong> <strong>of</strong> Gulf <strong>of</strong> <strong>Corinth</strong> <strong>rift</strong>-<strong>margin</strong> <strong>fault</strong> <strong>migration</strong> is presented from the Megara<br />
basin, precursor to the active eastern <strong>Corinth</strong> <strong>rift</strong>. Fifteen sanidine separates from a thin tuff near the top <strong>of</strong><br />
the sedimentary basin fill analysed by laser-fusion give apparent 40 Ar– 39 Ar ages from 2.78 to 4.53 Ma. A<br />
weighted mean age <strong>of</strong> 2.82±0.06 Ma (at 2σ error) for the three youngest aliquots (range <strong>of</strong> mean ages 2.78–<br />
2.86 Ma) is regarded as a maximum probable age for ash eruption. Together with magnetostratigraphy<br />
results, this age constrains <strong>timing</strong> <strong>of</strong> Megara basin abandonment and likely propagation <strong>of</strong> the active South<br />
Alkyonides coastal <strong>fault</strong>s that presently bound the southern <strong>rift</strong>-<strong>margin</strong> to the late-<strong>Pliocene</strong>, ∼2.2 Ma.<br />
Initiation <strong>of</strong> the coastal <strong>fault</strong>s caused uplift, fluvial incision and calcrete formation on geomorphic surfaces<br />
over the abandoned Megara basin fill. A petrocalcic laminar horizon from a supermature calcrete<br />
unconformably capping the fill gives a U–Pb age <strong>of</strong> 0.77±0.08 Ma, dating a late stage in the history <strong>of</strong><br />
calcrete development. The new age for initiation <strong>of</strong> active <strong>fault</strong>ing in the eastern <strong>rift</strong> yields a low estimate <strong>of</strong><br />
long-term mean extension along the active South Alkyonides coastal <strong>fault</strong>s <strong>of</strong> ∼0.9–1.4 mm a −1 , consistent<br />
with previous geological data. This rate is less than the 100-year GPS-determined geodetic extension rate <strong>of</strong><br />
6±2.7 mm a − 1 measured along a ∼23°E meridional array just west <strong>of</strong> the Alkyonides gulf. It implies either<br />
that the geodetic rate declines rapidly over the ∼15 km distance into the eastern gulf or that the geodetic rate<br />
is unchanged but extra strain is taken up aseismically and/or along antithetic and unrecognised major<br />
intrabasinal <strong>fault</strong>s. In the full graben <strong>of</strong> the central <strong>rift</strong> displacements are ∼3.5 km along the Xylocastro and<br />
Antikyra <strong>fault</strong>s; the mean long term extension rate here since 2.2 Ma is ∼3.5 mm a − 1 , much less than current<br />
geodetic rates <strong>of</strong> ∼10 mm a − 1 . In the western <strong>rift</strong>, despite a lack <strong>of</strong> precise chronological data, geodetic and<br />
extension rates seem comparable. Overall, a late-<strong>Pliocene</strong> to early Pleistocene age is likely for initiation <strong>of</strong> the<br />
deep-marine <strong>Corinth</strong> <strong>rift</strong>, with no evidence for <strong>rift</strong> propagation, either eastwards or westwards. More<br />
generally, our results constrain <strong>timing</strong> <strong>of</strong> strain localisation and vertical axis rigid block rotation over the<br />
Aegean–Anatolian plate and demonstrate that intraplate deformation can be accomplished rapidly in<br />
response to regional-scale tectonic drivers.<br />
© 2008 Elsevier B.V. All rights reserved.<br />
1. Introduction, context and previous work<br />
The <strong>Corinth</strong> <strong>rift</strong> (Fig. 1) is one <strong>of</strong> the most tectonically-active and<br />
seismically-energetic areas in Europe. GPS studies (Clarke et al., 1997;<br />
Davies et al., 1997; Cocard et al., 1999; Briole et al., 2000; McCluskey<br />
et al., 2000; Avallone et al., 2004) and kinematic analysis (Goldsworthy<br />
et al., 2002) establish it as marking a major non-uniformity in<br />
surface velocity over the Aegea-Anatolia plate. Thus rapid acceleration<br />
<strong>of</strong> southern <strong>Greece</strong> with respect to central <strong>Greece</strong> (velocity increase <strong>of</strong><br />
∼13 mm a − 1 ) causes strain to be taken up on major normal <strong>fault</strong>s along<br />
the southern <strong>rift</strong> <strong>margin</strong> and <strong>of</strong>fshore (Stefatos et al., 2002; Moretti<br />
et al., 2003; McNeill et al., 2005; Lykousis et al., 2007; Bell et al., 2008).<br />
⁎ Corresponding author. Tel.: +44 1603 456161; fax: +44 1603 591327.<br />
E-mail address: m.leeder@uea.ac.uk (M.R. Leeder).<br />
South <strong>of</strong> the active <strong>fault</strong>s a ∼15 km wide terrane features apparently<br />
inactive <strong>fault</strong>s together with associated abandoned, uplifted and incised<br />
syn<strong>rift</strong> sediments (Fig. 1; Jackson et al., 1982; Ori, 1989; Leeder<br />
et al., 1991; Collier et al., 1992; Dart et al., 1994; Jackson,1999; Malartre<br />
et al., 2004; Rohais et al., 2007; Leeder and Mack, 2007; Bell et al.,<br />
2008; Ford et al., 2008). The <strong>timing</strong> <strong>of</strong> northward <strong>fault</strong> <strong>migration</strong> is<br />
unknown, apart from broad inferences using extrapolation <strong>of</strong> late<br />
Quaternary uplift rates (Leeder et al., 1991; Collier et al., 1992),<br />
biostratigraphy <strong>of</strong> syn<strong>rift</strong> sediments (Malartre et al., 2004; Rohais<br />
et al., 2007; Ford et al., 2008) and <strong>of</strong>fshore sequence stratigraphy (Bell<br />
et al., 2008). Absence <strong>of</strong> direct age-control hinders testing <strong>of</strong> hypotheses<br />
for <strong>Corinth</strong> <strong>rift</strong> structural evolution and Aegean extensional<br />
history (Armijo et al., 1996; Jackson, 1999; Duermeijer et al., 2000;<br />
Hatzfeld et al., 2000; Goldsworthy et al., 2002; Leeder et al., 2003;<br />
Mattei et al., 2004; Leeder and Mack, 2007). Although neither <strong>fault</strong><br />
0012-821X/$ – see front matter © 2008 Elsevier B.V. All rights reserved.<br />
doi:10.1016/j.epsl.2008.07.006
M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141<br />
133<br />
Fig. 1. General location and tectonic summary maps for Gulf <strong>of</strong> <strong>Corinth</strong>, Central <strong>Greece</strong> and (inset) the Aegean context. Note the suite <strong>of</strong> inactive normal <strong>fault</strong>s on the southern gulf<br />
<strong>margin</strong>; these are associated with abandoned, uplifting and incised syn<strong>rift</strong> basins. Topography and onshore and <strong>of</strong>fshore <strong>fault</strong>ing after Stefatos et al. (2002), McNeill et al. (2005),<br />
Leeder et al. (2005) and Bell et al. (2008). Also shown in the Megara basin are sample localities and in the <strong>Corinth</strong> basin the Charalampos (CF) <strong>fault</strong> and the line <strong>of</strong> section (dashed line<br />
A-B) for Fig. 7. Place names cited in the text are C — <strong>Corinth</strong>, M — Megara, A — Alepochori.<br />
death nor birth are easy events to date directly, important age constraints<br />
on these events in other active <strong>rift</strong>s have come from the syn<strong>rift</strong><br />
sedimentary record (e.g. Suez <strong>rift</strong>, Gawthorpe et al., 1997; Rio<br />
Grande <strong>rift</strong>, Perez-Arlucea et al., 2000).<br />
In this contribution we present geochronological evidence relevant<br />
to the <strong>timing</strong> <strong>of</strong> <strong>rift</strong>-<strong>margin</strong> <strong>fault</strong> evolution from the inactive Megara<br />
basin (Fig. 1), south <strong>of</strong> the active eastern <strong>Corinth</strong> <strong>rift</strong>. Previously the<br />
age <strong>of</strong> sediments deposited adjacent to the basin-bounding Pateras<br />
normal <strong>fault</strong> have been poorly constrained (Theodoropoulos, 1968;<br />
Bentham et al., 1991). Our results constrain <strong>timing</strong> <strong>of</strong> strain localisation<br />
and vertical axis rigid block rotation over the Aegean–Anatolian<br />
plate and demonstrate that intraplate deformation can be accomplished<br />
rapidly in response to regional-scale tectonic drivers.<br />
2. Pagae Ash Member<br />
The newly-discovered Pagae Ash Member occurs in the topmost<br />
Louba Formation <strong>of</strong> the Alepochori Group (Fig. 2), part <strong>of</strong> the ∼1 km<br />
thick sedimentary fill to the Megara basin (Bentham et al., 1991). It<br />
occurs 45 m below the base <strong>of</strong> the Agia S<strong>of</strong>ia Formation and ∼100 m<br />
below the top <strong>of</strong> the fill. The ash (for location details see Appendix A)<br />
is a pale-coloured, 3–4 cm thick, airfall crystal tuff comprising submm<br />
sized sanidine and biotite crystals in a finer ash matrix. It occurs<br />
as a single distinctive, sharp-based and sharp-topped layer within<br />
gravel to mudrock grade sediments <strong>of</strong> fluviatile channel and floodplain<br />
origins much affected by arid-zone pedogenesis (Mack et al.,<br />
1993a). The airfall volcanic event evidently occurred over an inactive<br />
floodplain environment during an interval free from the activity <strong>of</strong><br />
combing shallow braided river channels. Subsequent channel avulsions<br />
led to sediment burial and partial erosion <strong>of</strong> the ash horizon. The<br />
source <strong>of</strong> the ash, whether from Aegean or Italian volcanic arcs will be<br />
the subject <strong>of</strong> future geochemical and stratigraphic research.<br />
3. 40 Ar/ 39 Ar analytical results<br />
A sanidine separate from the Pagae ash was analysed by laser fusion<br />
method. Multi-grain aliquots were analysed as the sanidine crystals<br />
were too small to be individually analysed. A small percentage <strong>of</strong> impurities,<br />
predominantly plagioclase, remained in the mineral separate.<br />
Abbreviated analytical methods and data are provided in Appendix A,<br />
Table 1. Fifteen fused multi-grain aliquots yielded apparent ages from<br />
2.78 to 4.53 Ma (Fig. 3) with radiogenic yields ranging from 44.6–99.8%.<br />
There is no correlation between radiogenic yield and age. K/Ca values are<br />
overall low for sanidine (3 to 26.5); a more typical sanidine K/Ca value<br />
would be in the 40–100 range, although some Ca-rich sanidines have K/Ca<br />
ratios as low as 10. A weighted mean age <strong>of</strong> 2.82±0.06 Ma is calculated<br />
from the three youngest aliquots and this is viewed as a maximum age for<br />
the eruption. The 1.75 Ma spread in overall ages suggests that the mineral<br />
separate is contaminated with xenocrysts and the possibility cannot be<br />
ruled out that all aliquots are contaminated to some degree. Low<br />
radiogenic yields (b∼95% for sanidine <strong>of</strong> this age), as obtained for 12 <strong>of</strong><br />
the analyses, are <strong>of</strong>ten indicative <strong>of</strong> alteration and possible Ar loss. A<br />
correlation between increasing apparent age and increasing radiogenic<br />
yield is commonly seen in samples that have undergone alteration. This<br />
correlation is not seen in the Pagae ash data so it is possible that the low<br />
radiogenic yields are a result <strong>of</strong> contamination by phases such as
134 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141<br />
Fig. 2. Stratigraphy <strong>of</strong> the topmost Megara basin sedimentary succession, with lithostratigraphy largely after Bentham et al. (1991) and magnetostratigraphy after Duermeijer and van<br />
Ho<strong>of</strong> (1996) and further correlations by the present authors. Paleomagnetic data were obtained by standard thermal demagnetization (see Duermeijer et al., 2000 for paleomagnetic<br />
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<br />
to the local top <strong>of</strong> the Louba Formation. The Pagae Ash Member and New Road paleomagnetic sections are measured down from the base <strong>of</strong> the easily-mappable Agia S<strong>of</strong>ia Formation.<br />
plagioclase with low radiogenic yields and similar ages, rather than being<br />
caused by alteration and accompanying Ar loss. However, the possibility<br />
<strong>of</strong> Ar loss cannot be completely ruled out and must be considered when<br />
using the assigned age for ash eruption. It is also possible that low-K<br />
phases have common ‘old’ argon, either as excess argon in numerous<br />
plagioclase phenocrysts, or by sanidine xenocrysts providing old argon<br />
and the plagioclase providing little radiogenic argon, but abundant Ca to<br />
lower the K/Ca ratios.<br />
4. Megara Calcrete Member<br />
Calcretes are common in Quaternary continental sediments <strong>of</strong><br />
central <strong>Greece</strong> (Brasier, 2007). The Megara Calcrete Member occurs as<br />
a discontinuous caprock lying unconformably on the topmost Louba<br />
Formation in the Megara basin (Leeder et al., 2005; Leeder and<br />
Gawthorpe, 2007; Leeder and Mack, 2007). The unit is a 3–5 m thick<br />
Stage V calcrete in the terminology <strong>of</strong> Gile et al. (1981) and Machette<br />
(1985). Field attributes <strong>of</strong> pedogenic (soil) origin are supported by<br />
detailed petrological characteristics and stable isotopic composition<br />
(Brasier, 2007). It is closely analogous to the Upper and Lower La Mesa<br />
calcretes that feature as duricrusts capping geomorphic surfaces <strong>of</strong> the<br />
Rio Grande <strong>rift</strong>, SW USA (Gile et al., 1981; Mack et al., 1993b, 1994).<br />
5. U–Pb dating <strong>of</strong> the Megara Calcrete Member<br />
Although chronology for late Quaternary calcretes can be provided<br />
by either radiocarbon or U/Th techniques, dating calcretes in deeper<br />
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<br />
135<br />
Table 1<br />
40 Ar/ 39 Ar analytical data<br />
ID<br />
40 Ar/ 39 Ar<br />
37 Ar/ 39 Ar<br />
36 Ar/ 39 Ar<br />
(x10 − 3 )<br />
39 Ar K<br />
(x10 − 15 mol)<br />
Louba Ash, 10–20 sanidine crystals, J =0.0008996+0.05%, D=1.002+0.001, NM-196H, Lab#=56266<br />
01 3.838 0.0491 7.213 7.594 10.4 44.6 2.78 0.05<br />
07 2.408 0.0503 2.267 5.618 10.2 72.4 2.83 0.06<br />
15 2.192 0.0497 1.472 8.226 10.3 80.3 2.86 0.04<br />
# 10 1.825 0.0192 0.0147 2.189 26.5 99.8 2.95 0.15<br />
# 02 3.183 0.0217 4.590 2.195 23.5 57.5 2.97 0.16<br />
# 14 2.792 0.0697 3.258 3.859 7.3 65.7 2.98 0.09<br />
# 04 2.456 0.0206 1.966 2.958 24.8 76.4 3.04 0.12<br />
# 06 2.865 0.0569 3.359 2.499 9.0 65.5 3.04 0.14<br />
# 09 1.983 0.0707 0.3465 3.174 7.2 95.1 3.06 0.11<br />
# 12 1.958 0.0208 0.0468 2.975 24.5 99.4 3.16 0.11<br />
# 13 3.579 0.1271 5.467 7.261 4.0 55.1 3.20 0.05<br />
# 03 3.985 0.1073 6.430 1.669 4.8 52.5 3.39 0.21<br />
# 11 2.965 0.1413 2.920 2.323 3.6 71.3 3.43 0.15<br />
# 08 3.356 0.0433 2.922 3.079 11.8 74.4 4.05 0.12<br />
# 05 3.542 0.0199 2.534 2.182 25.6 78.9 4.53 0.16<br />
K/Ca<br />
40 Ar⁎<br />
(%)<br />
Age<br />
(Ma)<br />
+1σ<br />
Mean age <strong>of</strong> youngest 3 samples N 3 MSWD 0.69 K/Ca 10.3+0.2 Age 2.82 +2σ 0.06<br />
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<br />
error only, without interfering reaction or J uncertainties. 3) Mean age is weighted mean age <strong>of</strong> Taylor (1982). Mean age error is weighted error <strong>of</strong> the mean (Taylor, 1982), multiplied by<br />
the root <strong>of</strong> the MSWD where MSWDN1, and also incorporates uncertainty in J factors and irradiation correction uncertainties. 4) Decay constants and isotopic abundances after Steiger<br />
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<br />
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,<br />
( 40 Ar/ 39 Ar) K =0+0.0004.<br />
difficult. Recent advances in methodology have enabled U–Pb dating<br />
to extend more widely to Pleistocene terrestrial carbonate, opal or<br />
phosphate (Richards et al., 1998; Neymark et al., 2000; Walker et al.,<br />
2006; Woodhead et al., 2006; Brasier, 2007). Importantly, the<br />
recognition <strong>of</strong> the influence <strong>of</strong> initial uranium activity on the interpretation<br />
<strong>of</strong> U–Pb data has been shown to be crucial to self-consistent<br />
interpretations (Neymark et al., 2000).<br />
Megara calcrete sample MRL15903-2 (Fig. 4) was studied petrographically<br />
and sub-sampled for analysis. Aliquots for analysis were selected<br />
from a distinct centimetric-scale lamina present in the petrocalcic<br />
horizon (Fig. 4). Initially, several analyses were performed to determine<br />
if sufficient U was present and to establish whether a range <strong>of</strong> U/Pb ratios<br />
were likely to be encountered. After this initial screening was deemed<br />
successful, subsequent sample aliquots were measured that came from<br />
similar petrographic layers <strong>of</strong> the specimen (Fig. 4). Other parts <strong>of</strong> the<br />
sample appear to have formed or incorporated common Pb with a<br />
different composition and as a result were not useful or used in any<br />
regressions.<br />
The measurement <strong>of</strong> residual disequilibrium <strong>of</strong> 234 U/ 238 U resulted<br />
in present day activity ratios <strong>of</strong> 1.180±0.005 and 1.187±0.005 for the<br />
relevant portion <strong>of</strong> the sample (Table 2). The initial activity ratio can<br />
be calculated using equations in Woodhead et al. (2006) for a specified<br />
time in the past. For example if this sample were 1 Ma old, its initial U<br />
activity ratio would be 4.8, itself an unusually high value for surface<br />
waters. This alone suggests that the sample may be less than a million<br />
years old. From the isotope dilution measurements, the Pb isotopic<br />
composition <strong>of</strong> the samples and U/Pb isotope ratios were determined<br />
and are given in Table 3. The U contents <strong>of</strong> the aliquots varied from 95–<br />
450 ppb. We have used a 238 U/ 207 Pb v. 206 Pb/ 207 Pb ‘isochron’ diagram<br />
(Fig. 5) to illustrate the data spread and reasonable coherence. A 238 U/<br />
208 Pb v. 206 Pb/ 208 Pb diagram would have demonstrated similar spread<br />
and co-linearity.<br />
The most coherent sub-samples are those from layer A <strong>of</strong> Fig. 4, and<br />
include those C2-5 to 8, C3-2 and MRL15903-2b (data in Table 3).<br />
These 6 subsamples yield an age <strong>of</strong> 1.7±0.4 Ma (95% confidence) on<br />
the 238 U– 206 Pb isochron referenced to 207 Pb (Fig. 5). If the initial activity<br />
ratio is calculated using this age, then an unrealistic value <strong>of</strong> 23.3<br />
results.<br />
Table 2 shows combinations <strong>of</strong> measured current activity ratio,<br />
‘isochron’ ages, and initial activity ratios calculated using such ages.<br />
The older the age for a given measured activity ratio, the more<br />
unrealistic the initial calculated activity is. Conversely if the isochron<br />
age was much younger the age correction would be affected in the<br />
opposite direction. An optimum value results for an age <strong>of</strong> about<br />
765,000 y with an initial activity ratio <strong>of</strong> 2.59.<br />
The common Pb-corrected age <strong>of</strong> a young sample can be calculated<br />
and the degree <strong>of</strong> concordance between 238 U/ 206 Pb and 207 Pb/ 206 Pb<br />
ages visualised on a Tera–Wasserburg plot (Fig. 6). The concordia<br />
curve on such a plot is affected by the initial 234 U/ 238 U disequilibrium,<br />
and this must be taken into account, i.e. a different concordia location<br />
corresponds to each initial activity ratio. When 238 U/ 206 Pb vs 207 Pb/<br />
206 Pb molar isotope ratios from the subsamples are plotted on a Tera–<br />
Wasserburg diagram calculated to be consistent with an initial 234 U/<br />
238 U activity ratio <strong>of</strong> 2.59 (equivalent to a measured value <strong>of</strong> 1.18 and<br />
an age <strong>of</strong> 765 ka), the discordia line intercepts the disequilibrium<br />
concordia line at 760 ka with an uncertainty <strong>of</strong> approximately 80 ky. If<br />
an age much greater than 760 ka is assumed in the calculation <strong>of</strong> the<br />
initial 234 U/ 238 U activity ratio, the (constant) discordia line actually<br />
Fig. 3. Age probability distribution diagram <strong>of</strong> the Pagae Ash sanidine. Shaded distribution<br />
and data points are those <strong>of</strong> the three youngest aliquots. All errors quoted at 2<br />
sigma. MSWD=mean sum weighted deviate value.
136 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141<br />
6. Discussion: <strong>timing</strong> <strong>of</strong> <strong>fault</strong>ing, <strong>Corinth</strong> <strong>rift</strong> evolution and wider<br />
implications<br />
6.1. Megara Basin evolution, abandonment and uplift<br />
Fig. 4. Megara basin calcrete specimen MRL 15903-2, with some <strong>of</strong> the different fabrics<br />
separated by dashed lines. Subsamples from a single petrographically distinct layer that<br />
were used in dating are shown in the zone labelled A, comprising subsamples C2-5 to<br />
C2-8, MRL15903-2b and C3-2 (data in Appendix B). Other subsamples from adjacent or<br />
other layers did not have the same common Pb composition and therefore were not<br />
used in regressions.<br />
intercepts the modified concordia line at a younger age than assumed<br />
in the calculation <strong>of</strong> the initial 234 U/ 238 U activity ratio (see Table 2).<br />
Together, the iteratively calculated initial activity ratio and measured<br />
isotopic composition <strong>of</strong> the aliquots suggests that the Megara basin<br />
calcrete subsample is approximately 765±80 ka in age. In these<br />
calculations the initial 230 Th/ 234 U activity ratio is assumed to be zero<br />
but in terms <strong>of</strong> the sensitivity <strong>of</strong> this assumption, if the initial 230 Th/<br />
234 U activity ratio was 1.0, this would only have the effect <strong>of</strong> reducing<br />
the calculated age by ∼10%.<br />
The Pagae Ash Member, maximum likely age 2.82 Ma, is the key<br />
time marker in the Megara basin. It establishes the majority <strong>of</strong> the<br />
underlying basin fill and probably also the thin (b80 m) overlying<br />
succession (with the exception <strong>of</strong> the Megara Calcrete Member, see<br />
below) as <strong>Pliocene</strong> in age. The radiometric age requires ash fallout<br />
during the late Gauss chron (see geomagnetic timescale <strong>of</strong> Cande and<br />
Kent, 1995). This is consistent with previously determined palaeomagnetic<br />
results (Duermeijer et al., 2000; Duermeijer and van Ho<strong>of</strong>,<br />
1996) from separate sections in the general area <strong>of</strong> the ash exposure<br />
(Fig. 2). These show reversed polarity above the stratigraphic level <strong>of</strong><br />
the ash with a normal polarity interval at the very top <strong>of</strong> the measured<br />
succession below the level <strong>of</strong> the Megara Calcrete Member. Below the<br />
stratigraphic level <strong>of</strong> the ash there is normal polarity, preceded by<br />
another reversed interval and then further reversed intervals from<br />
samples <strong>of</strong> unknown stratigraphic position (due to <strong>fault</strong>ing). The palaeomagnetic<br />
results and stratigraphic architecture are consistent<br />
with sedimentation and ash fallout during the upper part <strong>of</strong> the Gauss<br />
normal chron interval. The reversed Matuyama chron is represented<br />
above, terminating in what is probably the first normal Matuyama<br />
subchron, the Reunion (see Fig. 2B). It is likely that the Kaena reversed<br />
subchron <strong>of</strong> the Gauss chron is that represented below the normal<br />
polarity ash-bearing interval. Given the ∼1.2 km total thickness <strong>of</strong> the<br />
Megara basin sediments it is probable that the syn<strong>rift</strong> fill may extend<br />
well into the Gilbert reversed chron (age 5.89–3.58 Ma), with its many<br />
normal subchrons. In support <strong>of</strong> this there are both reversed and<br />
normal magnetised sections in the Harbour Ridges Formation and<br />
normal sections in the Rema Mazi Formations below (Duermeijer<br />
et al., 2000; Duermeijer and van Ho<strong>of</strong>, 1996).<br />
In summary, geochronologic, paleomagnetic and stratigraphic<br />
constraints suggest that the uppermost sediments below the Megara<br />
Calcrete Member are <strong>of</strong> Reunion subchron age and date from 2.15 Ma<br />
(Cande and Kent, 1995; Fig. 2). This gives a likely maximum age for<br />
basin abandonment. Assuming regional extensional strain in the<br />
eastern <strong>rift</strong> is due to steady Aegean plate motion over the past ∼2.2 My<br />
it seems reasonable to infer that Megara basin <strong>fault</strong> abandonment was<br />
closely followed by initiation <strong>of</strong> the presently active coastal <strong>fault</strong>s<br />
bounding the Alkyonides basin. The subsequent truncation, subsidence<br />
and onlap <strong>of</strong> Megara basin infill by Pleistocene syn-<strong>rift</strong> sediments<br />
is recorded from seismic reflection data from the Alkyonides<br />
basin (Sakellariou et al., 2007).<br />
Table 2<br />
Calculated initial 234 U/ 238 U activity ratios, together with the sample age assumed in<br />
calculation <strong>of</strong> these ratios, and the ages at which the fixed discordia line (with its<br />
associated error) intersects Tera–Waserburg concordia curves modified by these<br />
calculated initial 234 U/ 238 U activity ratios<br />
Fig. 5. 238 U/ 207 Pb vs 206 Pb/ 207 Pb plot <strong>of</strong> selected data (Appendix B) from Megara Basin<br />
calcrete MRL 15903-2. A number <strong>of</strong> other analyses from other zones (listed in Brasier, 2007)<br />
plot well above the line shown with 206 Pb/ 207 Pb values between 1.16 and 1.21 and 238 U/<br />
207 Pbb15. These are excluded from consideration since they are likely to have an entirely<br />
different, more radiogenic common Pb composition inherited from the depositing fluids.<br />
Current 234 U/<br />
238 U activity<br />
ratio (2σ<br />
absolute error)<br />
Age assumed in<br />
calculation <strong>of</strong> initial<br />
234 U/ 238 U activity<br />
ratio (years)<br />
Calculated initial<br />
234 U/ 238 U activity<br />
ratio (2σ absolute<br />
error)<br />
Age calculated from<br />
modified Tera–<br />
Wasserburg diagram using<br />
initial 234 U/ 238 U activity<br />
ratio (years)<br />
1.183 ± 0.007 740,000 2.48 (± 0.06) (± 90)<br />
790,000<br />
1.183 ± 0.007 750,000 2.52 (± 0.06) (± 80)<br />
770,000<br />
1.183 ± 0.007 760,000 2.57 (± 0.06) (± 80)<br />
770,000<br />
1.183 ± 0.007 770,000 2.61 (± 0.06) (± 80)<br />
760,000<br />
1.183 ± 0.007 780,000 2.66 (± 0.07) (± 80)<br />
750,000<br />
1.183 ± 0.007 1,000,000 4.08 (± 0.12) (± 70)<br />
520,000<br />
1.183 ± 0.007 1,700,000 23.25 (± 0.89) (± 10)<br />
150,000<br />
The uncertainty in calculated initial 234 U/ 238 U activity ratio results from measurement<br />
uncertainty, and contributes an error <strong>of</strong> b10 ka to all samples. The bulk <strong>of</strong> the error is<br />
thus derived from the uncertainty associated with the discordia line.
M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141<br />
137<br />
Table 3<br />
U/Pb ICP-MC-MS data, with 2σ (%) error; analyses used in regression to obtain the Megara calcrete age<br />
Method<br />
Conc<br />
nitric<br />
Conc<br />
nitric<br />
Conc<br />
nitric<br />
Conc<br />
nitric<br />
Conc<br />
nitric<br />
Conc<br />
nitric<br />
206 Pb/<br />
208 Pb<br />
%<br />
stdev<br />
238 U/<br />
208 Pb<br />
%<br />
stdev<br />
206 Pb/<br />
204 Pb<br />
%<br />
stdev<br />
238 U/<br />
204 Pb<br />
%<br />
stdev<br />
238 U/<br />
207 Pb<br />
%<br />
stdev<br />
206 Pb/<br />
207 Pb<br />
%<br />
stdev<br />
238 U/<br />
206 Pb<br />
%<br />
stdev<br />
208 Pb/<br />
204 Pb<br />
207 Pb/<br />
204 Pb<br />
238 Uppb 208 Pb<br />
ppb<br />
subsample<br />
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<br />
2B<br />
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<br />
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<br />
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<br />
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<br />
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<br />
A further looser control on tectonic basin history is provided by the<br />
U–Pb age <strong>of</strong> 0.77 Ma for the Megara Calcrete Member. Mature<br />
calcretes elsewhere record passage <strong>of</strong> long time intervals, for example<br />
the Lower La Mesa calcrete <strong>of</strong> the Rio Grande <strong>rift</strong>, SW USA, dates back<br />
some 800 ky (Gile et al., 1966, 1981; Mack et al., 1993b, 1994). In this<br />
area, mature calcretes cap stable geomorphic surfaces left behind after<br />
regional fluvial incision. The Megara Calcrete Member has a similar<br />
context and must have been initiated well before the radiometric age<br />
<strong>of</strong> its laminar petrocalcic horizon, since similar horizons in Stage IV–VI<br />
calcretes form after the pr<strong>of</strong>iles ‘plug-up’ from below as impermeable<br />
Stage III nodular carbonate horizons (Gile et al., 1966; Machette, 1985).<br />
Over the Megara basin, we assume that initiation <strong>of</strong> calcrete formation<br />
followed landscape stability as remnant geomorphic surfaces developed<br />
under climatic conditions suitable for calcrete formation. These<br />
surfaces formed as the uplifting, now-inactive Megara basin was<br />
backtilted to the SE by the new coastal <strong>fault</strong>s, causing reversal <strong>of</strong><br />
formerly NW-flowing Louba Formation drainages channels (Bentham<br />
et al., 1991) and partial incision <strong>of</strong> the slowly backtilting dip slope<br />
(Fig. 7).<br />
6.2. Slip and extension rates in the eastern <strong>rift</strong><br />
The inferred <strong>timing</strong> <strong>of</strong> <strong>fault</strong> propagation in the eastern <strong>rift</strong> at around<br />
2.2 Ma places the major change to <strong>fault</strong> activity in the eastern <strong>Corinth</strong><br />
<strong>rift</strong> much earlier than the ∼1 Ma previously deduced by extrapolation<br />
<strong>of</strong> late Pleistocene (post-MIS 5e) footwall uplift rates (Leeder et al.,<br />
1991; Collier et al., 1992), <strong>fault</strong> modelling (Armijo et al., 1996) or much<br />
younger estimates <strong>of</strong> b0.5 Ma from active <strong>Corinth</strong> <strong>rift</strong> subsidence rates<br />
using <strong>of</strong>fshore subsurface seismic stratigraphy (Lykousis et al., 2007;<br />
Sakellariou et al., 2007). It supports the previous suggestion <strong>of</strong> Hatzfeld<br />
et al. (2000) that regional tectonics in western <strong>Greece</strong> and in the<br />
eastern <strong>rift</strong> underwent significant changes in the Upper <strong>Pliocene</strong>.<br />
Total vertical stratigraphic separation (aka vertical <strong>fault</strong> displacement<br />
or throw) on the active eastern <strong>Corinth</strong> <strong>rift</strong> <strong>fault</strong>s along the<br />
southern <strong>margin</strong> to the Alkyonides Gulf is a maximum <strong>of</strong> 2–3 km<br />
(Armijo et al., 1996; Collier et al., 1998; Leeder et al., 2005; Lykousis<br />
et al., 2007; Sakellariou et al., 2007). Our new geochronological data<br />
for initiation <strong>of</strong> this active <strong>fault</strong>ing yields a low estimate <strong>of</strong> long-term<br />
mean extension rate <strong>of</strong> ∼0.9–1.4 mm a − 1 . The new data is consistent<br />
with 1) lower limits to displacement rates determined from late-<br />
Holocene palaeoseismology <strong>of</strong> the active Skinos <strong>fault</strong> (Collier et al.,<br />
1998), 2) mean Holocene displacement across the active Psatha <strong>fault</strong><br />
(Leeder et al., 2002) and 3) estimates <strong>of</strong> basin tilt due to the Skinos–<br />
East Alkyonides <strong>fault</strong>s (Leeder et al., 2005). It confirms previous<br />
suspicions (Leeder et al., 2005) that the geological extension rate <strong>of</strong><br />
the eastern <strong>rift</strong> since 2.2 Ma may be less than the 100-year GPSdetermined<br />
geodetic extension rate <strong>of</strong> 6 ±2.7 mm a − 1 measured along<br />
a ∼23°E meridional array just west <strong>of</strong> the Alkyonides Gulf (Clarke<br />
et al., 1997). This implies that either the geodetic rate declines rapidly<br />
over the ∼15 km distance to the eastern gulf (Fig. 1; see also Goldsworthy<br />
et al., 2002) or the geodetic rate is unchanged but extra<br />
extension is taken up along antithetic and other hitherto unrecognised<br />
intrabasinal <strong>fault</strong>s.<br />
6.3. Fault propagation and extension rates in the wider <strong>Corinth</strong> <strong>rift</strong><br />
In the wider <strong>Corinth</strong> <strong>rift</strong> basin, the <strong>Pliocene</strong> Charalampos tilt block<br />
basin east <strong>of</strong> the Isthmus (Fig. 1) was abandoned after eruption <strong>of</strong><br />
andesitic volcanics, whose biotite and hornblende phenocrysts are K–Ar<br />
dated as 3.62±0.18 and 4.0±0.4 Ma (Collier and Dart, 1991). Palaeomagnetic<br />
results (Duermeijer et al., 2000; Duermeijer and van Ho<strong>of</strong>,<br />
1996) indicate normal polarities with modern declinations in the<br />
andesite unit. Taking into account the errors on the two ages, eruption<br />
took place either at the very base <strong>of</strong> the Gauss normal chron at a little less<br />
than 3.58 Ma (consistent, within errors, with both radiometric dates) or<br />
in the Cochiti normal subchron <strong>of</strong> the Gilbert reversed chron between<br />
4.18 and 4.29 Ma (consistent with the older radiometric date only).<br />
Recent field work in the eastern <strong>Corinth</strong> basin (Mack et al., in press)<br />
establishes that the Saros <strong>fault</strong>s (Fig. 1) are inactive, since the degraded<br />
<strong>fault</strong> scarps and <strong>fault</strong> planes are everywhere onlapped by alluvial fan<br />
sediments. A U-series age <strong>of</strong> 304 +44/−30 ka (P. Rowe pers. comm.) for<br />
the laminar cap to a calcrete palaeosol developed on the youngest<br />
onlapping sediment indicates that <strong>fault</strong> death occurred well before this<br />
time. The record <strong>of</strong> progressive <strong>of</strong>flap <strong>of</strong> shallow marine to littoral<br />
sediments from the central horst in the <strong>Corinth</strong> Canal section (Collier,<br />
1990) indicates that uplift has affected the basin fill since at least 400 ka.<br />
The likely Milankovich eccentricity scaling <strong>of</strong> the <strong>Corinth</strong> terraces to the<br />
west towards Xylocastro (Fig.1), together with marine nann<strong>of</strong>ossils from<br />
NN20 zone (0.46–0.45 Ma) indicates uplift here since at least 0.5 Ma<br />
(Keraudren and Sorel, 1987; Armijo et al., 1996).<br />
Fig. 6. Tera–Wasserburg diagram <strong>of</strong> 238 U/ 206 Pb vs 207 Pb/ 206 Pb, with the concordia curve<br />
modified for an initial 234 U/ 238 U activity ratio <strong>of</strong> 2.59 (assuming a sample age <strong>of</strong> 765 ka).<br />
The fixed discordia line (labelled line) is drawn through samples C2-5 to C2-8, C3-2 and<br />
MRL15903-2b) crosses this concordia curve at an age <strong>of</strong> 760 ka. The lines parallel to the<br />
discordia line represent the errors associated with it. Lines labelled with ages in ka and<br />
ma are isochrons.
138 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141<br />
Fig. 7. Schematic section along the line A–BinFig. 1 to show the young, active, <strong>of</strong>fshore Psatha/East Alkyonides <strong>fault</strong> bounding the Alkyonides Gulf and the abandoned, uplifting and<br />
incising Pleistocene Megara basin. The inactive Pateras <strong>fault</strong> is projected into the line <strong>of</strong> section for illustration purposes. We show major morphological features useful in<br />
parameterisation <strong>of</strong> rates <strong>of</strong> uplift and subsidence associated with displacements along the active <strong>fault</strong>s (see Leeder et al., 2005).<br />
Along the central <strong>rift</strong> flank, former movement on the now-inactive<br />
onshore Xylocastro–Loutro <strong>fault</strong> in the footwall to the active <strong>of</strong>fshore<br />
Xylocastro (<strong>Corinth</strong>) <strong>fault</strong> (Fig. 1; Stefatos et al., 2002) has been dated<br />
by U-series analysis <strong>of</strong> syntectonic calcite to around 1 Ma (Causse<br />
et al., 2004), whilst the inactive Valimi <strong>fault</strong> (Fig. 1) was active<br />
between 0.38–1Ma(Flotté et al., 2001; Causse et al., 2004). Regarding<br />
the age <strong>of</strong> basin-fill sediments adjacent to inactive north Peloponessian<br />
<strong>fault</strong>s, in western areas <strong>of</strong> the <strong>rift</strong> around Patras, the earliest<br />
syn<strong>rift</strong> fluvio-deltaic units associated with these are dated as Middle<br />
to <strong>Late</strong> <strong>Pliocene</strong> (Kontopoulos and Doutsos, 1985; Frydas, 1987, 1989;<br />
Zelididis, 1998). Palynological taxa collected from Vouraikos fan delta<br />
deposits that formed by subsidence in the hangingwall to the Pirgaki–<br />
Mamoussia <strong>fault</strong> (Malartre et al., 2004; Fig. 1) indicate a Lower<br />
Pleistocene age (i.e. 2.6 to ∼1 Ma). This is consistent with ‘Calabrian’<br />
ages (∼1.8–0.8 Ma) for mammalian fossils in equivalent units closer to<br />
Patras (Simeonidis et al., 1987). Early Pleistocene dates are not<br />
consistent with hypotheses for onset <strong>of</strong> very young deepening <strong>of</strong> the<br />
proto-Gulf (Sorel, 2000) or <strong>of</strong> any rapid, young tectonic ‘phase’ (Armijo<br />
et al., 1996; Hinsbergen et al., 2005).<br />
Despite a lack <strong>of</strong> precise chronological data in the central and<br />
western <strong>rift</strong>, we conclude that a late-<strong>Pliocene</strong> to early Pleistocene age,<br />
around 2 Ma, is likely for modern, deep-marine, <strong>Corinth</strong> <strong>rift</strong> initiation.<br />
Before this a wide, mainly fluvio-lacustrine, syn<strong>rift</strong> province developed<br />
(Rohais et al., 2007; Bell et al., 2008; Ford et al., 2008; Fig. 8). In<br />
this was deposited the Lower Group onshore in the west (Rohais et al.,<br />
2007) with possible <strong>of</strong>fshore equivalents (Bell et al., 2008), the<br />
Charalampos sediments <strong>of</strong> the <strong>Corinth</strong> basin (Collier and Dart, 1991)<br />
and the Megara basin sequences in the east. There is no geochronological<br />
evidence for rates <strong>of</strong> subsidence or extension in these basins. In<br />
western areas there are at least two episodes <strong>of</strong> major <strong>rift</strong>-bounding<br />
<strong>fault</strong>s, 1) the now-dead Pirgaki–Mamoussia <strong>fault</strong>s and lateral equivalents<br />
active in the interval 0.8–1.8 Ma (Malartre et al., 2004) and 2)<br />
the presently active West and East Helike <strong>fault</strong>s (McNeill and Collier,<br />
2004; Bell et al., 2008; Fig. 1) and the youngest <strong>of</strong>fshore intrabasinal<br />
Eratini <strong>fault</strong>s at 0.5 Ma (Bell et al., 2008). In western areas, geological<br />
extension rates along active onshore and <strong>of</strong>fshore <strong>fault</strong>s are compatible<br />
with GPS-determined geodetic extension rates (McNeill et al.,<br />
2005; Bell et al., 2008).<br />
Finally, in the full graben <strong>of</strong> the central <strong>rift</strong>, maximum vertical<br />
displacement is ∼3.5 km along the main, basin-bounding and 45°dipping<br />
Xylocastro (<strong>Corinth</strong>) <strong>fault</strong> (Stefatos et al., 2002; Moretti et al.,<br />
2003; Lykousis et al., 2007; Fig.1). It is probably similar along the major<br />
antithetic Antikyra intrabasinal <strong>fault</strong> system mapped by Stefatos et al.<br />
(2002; Fig. 1). We compute the mean long term extension rate here<br />
since 2.2 Ma as ∼3.5 mm yr − 1 , much less than current geodetic rates <strong>of</strong><br />
∼10 mm yr − 1 (Clarke et al.,1997; Davies et al.,1997; Cocard et al.,1999;<br />
Briole et al., 2000; McCluskey et al., 2000; Avallone et al., 2004). It<br />
Fig. 8. Sketches to show sequential development <strong>of</strong> the <strong>Corinth</strong> <strong>rift</strong> by northward<br />
<strong>migration</strong> <strong>of</strong> active <strong>fault</strong>ing during the <strong>Pliocene</strong>–Pleistocene according to the new<br />
geochronological data presented in this paper and our discussion <strong>of</strong> previously<br />
published results.
M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141<br />
139<br />
remains to be determined how much <strong>of</strong> this discrepancy may be taken<br />
up aseismically or on other <strong>of</strong>fshore <strong>fault</strong>s.<br />
6.4. Wider implications for continental <strong>rift</strong> evolution and eastern<br />
Mediterranean tectonics<br />
Apart from establishing possible discrepancies in the central<br />
<strong>Corinth</strong> <strong>rift</strong> between observed geodetic and longer term geological<br />
extension rates, the results presented above have general implications<br />
for both the evolution <strong>of</strong> continental <strong>rift</strong>s and eastern Mediterranean<br />
tectonics.<br />
1) Our geochronological data and discussion <strong>of</strong> its relation to previous<br />
biostratigraphic results for the <strong>Corinth</strong> <strong>rift</strong> establish that strain<br />
localisation and deep <strong>rift</strong> basin development was synchronous and<br />
did not propagate through time, either eastwards or westwards, as<br />
stated previously in the literature (e.g. Doutsos and Poulimenos,<br />
1992; Armijo, 1996).<br />
2) Regionally the <strong>Corinth</strong> <strong>rift</strong> is the locus <strong>of</strong> the majority <strong>of</strong> active strain<br />
within the Aegean–Anatolian plate. Abandoned <strong>rift</strong>s and normal<br />
<strong>fault</strong>s, like those <strong>of</strong> the Megara basin and the southern <strong>Corinth</strong> <strong>rift</strong><br />
flank, occur broadly over <strong>Greece</strong> and western Turkey (Jolivet, 2001;<br />
Goldsworthy et al., 2002) and make it is clear that internal strain in<br />
the plate was once more widely distributed. The major change in<br />
internal plate dynamics that must have occurred in order to focus<br />
the majority <strong>of</strong> strain along the <strong>Corinth</strong> <strong>rift</strong> happen around 2.2 Ma.<br />
3) In addition to the general clockwise rotation <strong>of</strong> western <strong>Greece</strong> with<br />
respect to the Balkans since the Miocene (LePichon and Angelier,<br />
1979; Hinsbergen et al., 2005), plate re-organisation involved a<br />
local counter clockwise rotation <strong>of</strong> Peloponnissos with respect to<br />
central <strong>Greece</strong> (Boetia–Locris–Evia) along the meridional <strong>Corinth</strong><br />
<strong>rift</strong> (Goldsworthy et al., 2002; Mattei et al., 2004) about a pole<br />
located in eastern <strong>Greece</strong>. This caused extension in the western <strong>rift</strong><br />
to be greater than that in the eastern <strong>rift</strong>. Our results constrain the<br />
<strong>timing</strong> <strong>of</strong> localisation <strong>of</strong> vertical-axis rigid block rotation over the<br />
Aegean–Anatolian plate. They demonstrate that changing intraplate<br />
deformation can be accomplished rapidly as block <strong>margin</strong>s<br />
migrate in response to regional scale tectonic drivers.<br />
4) Regarding possible dynamic drivers, large-scale <strong>fault</strong> <strong>migration</strong> and<br />
strain focussing from wide to narrow extensional zones is a marked<br />
feature <strong>of</strong> other active continental <strong>rift</strong> provinces. Good examples are<br />
<strong>migration</strong> during the last 10 My <strong>of</strong> active <strong>fault</strong>ing from the wider<br />
Basin and Range to more concentrated <strong>rift</strong> zones like the Rio Grande<br />
<strong>rift</strong> and Central Nevada seismic belt (Buck 1991). Such <strong>migration</strong><br />
was originally attributed to temporal changes brought about by<br />
decreasing heat flux on crustal and mantle rheology. We have no<br />
evidence that geotherms changed in <strong>Greece</strong> in the late-<strong>Pliocene</strong> and<br />
recent work (Jackson et al., 2008) indicates that previous assumptions<br />
about rheology used in extensional modeling (Buck 1991) are<br />
probably in error.<br />
5) It is thus more likely that kinematic drivers are responsible for strain<br />
localisation in the Aegean plate. Candidates have previously been<br />
identified as plate boundary strike-slip <strong>fault</strong> propagation along the<br />
North Anatolian <strong>fault</strong> (Armijo et al., 1999; 2003; Flerit et al., 2004),<br />
intra-plate strike slip <strong>fault</strong> <strong>migration</strong> causing local block rotations<br />
(Goldsworthy et al., 2002) and motion <strong>of</strong> Aegea–Anatolia over<br />
shallow subducting African plate (Leeder and Mack, 2007). Now that<br />
the <strong>timing</strong> <strong>of</strong> tectonic change is better established, future work<br />
should set out to decide which <strong>of</strong> these processes, if any, caused the<br />
late-<strong>Pliocene</strong> to Early Pleistocene événement in the <strong>Corinth</strong> <strong>rift</strong>.<br />
Acknowledgements<br />
We thank Rebecca Bell, Martin Brasier, Richard Collier, Mary Ford,<br />
Rob Gawthorpe, James Jackson, Lisa McNeill, Pete Rowe and Jenni<br />
Turner for discussions and help at various times. ATB acknowledges<br />
award <strong>of</strong> NERC Studentship grant NER/S/A/2003/11221. Matt Horstwood<br />
<strong>of</strong> the NERC Isotope Geosciences Laboratory is thanked for<br />
assistance with U and Pb analysis by ICP-MC-MS funded by NIGL and<br />
Lisa Peters for 40 Ar– 39 Ar analyses by laser fusion at the New Mexico<br />
Geochronology Research Laboratory. We also thank the IGME, Athens,<br />
for permissions to do fieldwork and collecting in central <strong>Greece</strong>.<br />
Appendix A. Pagae Ash Member and 40 Ar/ 39 Ar analytical methods<br />
The Pagae (pronounced Pahee) Ash member is named after the<br />
ancient Greek settlement at modern day Alepochori that was the<br />
major <strong>Corinth</strong>ian gulf port <strong>of</strong> embarkation to Delphi. It outcrops in two<br />
NW–SE orientated roadcut faces (38° 4.163'N 23° 13.246'E) located<br />
5.6 km from the T-junction at the beginning <strong>of</strong> the main road to<br />
Megara out <strong>of</strong> Alepochori and 0.4 km uphill on a straight from the<br />
wayside shrine to St Jerome located on the left after a sharp right bend<br />
at a prominent culverted arroyo. There is ample parking space beside<br />
the outcrops. The Pagae ash sample was initially prepared by crushing,<br />
elutriating clay-sized ash and cleaning with HF. The sanidine was<br />
separated by standard heavy liquid, magnetic separator and handpicking<br />
techniques. A small percentage <strong>of</strong> impurities, predominantly plagioclase,<br />
remained in the mineral separate. The separate was loaded<br />
into an aluminium disc and irradiated for 8.85 h at the Nuclear Science<br />
Center in College Station, Texas. Neutron flux monitor was Fish Canyon<br />
Tuff sanidine (FC-2), age 28.02 Ma. Fifteen 10–20 sanidine crystal<br />
aliquots <strong>of</strong> Pagae Ash were subsequently analysed in a Mass Analyzer<br />
Products 215–50 mass spectrometer on line with automated all-metal<br />
extraction system. Samples and flux monitor were fused by a 50 W<br />
Synrad CO2 laser. Reactive gases were removed during a 2-minute<br />
reaction with two SAES GP-50 getters, one operated at ∼450 °C and<br />
one at 20 °C. The gas was also exposed to a W-filament operated at<br />
∼2000 °C and a cold finger operated at 140 °C.<br />
Appendix B. Megara calcrete and U–Pb analytical methods<br />
The locus typicus for the Megara calcrete (Leeder and Gawthorpe,<br />
2007) is above Alepochori village in a roadcut along the main Megara<br />
highway at 38°03′45.9″N, 23°13′42.2″E. The samples used in the present<br />
radiometric dating are subsampled from a single hand specimen<br />
<strong>of</strong> a laminated petrocalcic horizon (Fig. 4) taken from a natural outcrop<br />
<strong>of</strong>f the same road closer to Megara town at 38°02′9.0″N, 23°18′<br />
16.5″E. U–Pb analysis was made with a VG Elemental Axiom multicollector<br />
at the NERC Isotope Geoscience Laboratories, Keyworth.<br />
For the chemical procedure, samples <strong>of</strong> carbonate were dissolved<br />
using nitric acid. Centrifuging <strong>of</strong> the sample to separate undissolved<br />
residue followed dissolution. Most samples were spiked with a mixed<br />
205 Pb– 233-235 U tracer prior to centrifuging. Chemical purification and<br />
separation <strong>of</strong> U and Pb used UTEVA and AG1X8 ion exchange resins for<br />
U and Pb purification, respectively with nitric and hydrochloric acids.<br />
Purified U and Pb were taken up in ∼1 ml <strong>of</strong> 2% HNO 3 for mass<br />
spectrometry. Mass spectrometry for both instruments employed a<br />
mixed multiple collector array using both Faraday cups and ion<br />
counting secondary electron multiplier(s) using peak switching and<br />
multiple ion detection. Mass bias was monitored using CRM950 natural<br />
uranium solutions, and for the Axiom, the secondary electron<br />
multiplier was calibrated for gain and non-linearity using a CRM U010<br />
solution <strong>of</strong> varying concentration. Data were reduced in part <strong>of</strong>f line<br />
for isotope age calculation. Mass bias for Pb measurements was done<br />
via external Pb isotope standard normalisation. All analyses were<br />
blank corrected for U and Pb using the mean <strong>of</strong> duplicate blank measurements<br />
for each batch <strong>of</strong> chemistry.<br />
Specimen MRL 15903-2 (Fig. 4) is a thick laminar calcrete with a<br />
complex history. It is possible that these different generations <strong>of</strong> calcrete<br />
may span much <strong>of</strong> the history <strong>of</strong> the basin-capping Megara calcrete<br />
deposit. Following initial pilot sub-sample analyses, detailed sample<br />
selection involved scanning the specimen at a resolution <strong>of</strong> 600 dots per
140 M.R. Leeder et al. / Earth and Planetary Science Letters 274 (2008) 132–141<br />
inch (dpi) on a flatbed scanner, and cut with a wire saw. Subsamples<br />
were taken from a single confined area <strong>of</strong> the specimen (zone A <strong>of</strong> Fig. 4)<br />
that differed slightly in colour from white to brown, probably reflecting a<br />
variable organic content. Avariety <strong>of</strong> sample aliquots in addition to those<br />
used in the final regressions were measured, in all cases these were from<br />
layers other than that with the most coherent data. This additional data<br />
do not form any coherent pattern and are not used. It is likely that they<br />
incorporated common Pb <strong>of</strong> variable and much more radiogenic<br />
composition than the analyses from layer A <strong>of</strong> our sample (Fig. 4).<br />
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