Late Holocene evolution of the East Eliki fault, Gulf of Corinth (Central Greece)

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Quaternary International 115–116 (2004) 139–154

Late Holocene evolution of the East Eliki fault, Gulf of Corinth (Central Greece) S.B. Pavlidesa,*, I.K. Koukouvelasb, S. Kokkalasb, L. Stamatopoulosb, D. Keramydasa, I. Tsodoulosb b

a Department of Geology and Physical Geography, Aristotle University of Thessaloniki, Thessaloniki 54006, Greece Department of Geology, Division of Physical Marine Geology and Geodynamics, University of Patras, 26500 Patras, Greece

Abstract Trench stratigraphy and morphotectonic analysis was used to examine the recent evolution of the Helike (Eliki) Plain and Eliki Fault. The entire alluvial plain of the Kerynites and Vouraikos Rivers, which cross the Eliki Fault, has subsided at rate of 1.4 mm/yr, resulting in the burial of the Late Hellenistic-Roman occupation horizons under p3 m of fluvial and colluvial sediments. Along the eastern segment of the Eliki Fault five trenches have been excavated and their 5000-yr old fluvial and colluvial deposits have been studied in association with the fault strand imprinted on them. The Kerynites River crosses at high angle the Eliki Fault and appears to be sensitive in tectonic movements related with the Eliki Fault. Sedimentation in the alluvial plain and the fluvial geomorphology of the Kerynites River is important for understanding the fault evolution and the burial of the ancient (4th century BC) Helike city. The Kerynites River course highlights breaching deformation is the more likely to connect East and West Eliki Fault segments and that postseismic deformation is a significant mechanism for stress accommodation for part of the west end of the Gulf of Corinth. r 2003 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Extension-related deformations in continental regions are traditionally studied with structural studies, geodetic arrays and seismic networks. This is also the current practice within the 130-km long Gulf of Corinth (central Greece), which has been recognized as one of the most rapidly developing inter-continental rifts in the Mediterranean region (Fig. 1). Geodetic estimation of the extension in the Gulf indicates the southern shore of the Gulf is extending at an average of 1 cm/yr (Fig. 1) (Billiris et al., 1991; Clarke et al., 1998). However, although rivers are the next most sensitive tool capable for measuring river-course changes over periods of decades to centuries (see also Keller and Pinter, 2001), nothing more than passing references are known for the rate of river migration in the Gulf of Corinth (i.e. Koukouvelas 1998a). The Gulf trends NNW-SSE across the Hellenic Mountain range (up to 2 km high), approximately perpendicular to the structural grain. The deepest point *Corresponding author. Tel.: +30-310-998-494; fax: +30-310-998482. E-mail address: [email protected] (S.B. Pavlides).

of the Gulf is 900 m below sea level. The Gulf of Corinth is undergoing N–S extension (Sebrier, 1977; Papazachos et al., 1981; Jackson et al., 1982; King et al., 1985; Armijo et al., 1996), which is typical for the Aegean back-arc domain (McKenzie, 1978; Mercier et al., 1989). The majority of this extension is accommodated by active faults (Roberts and Jackson, 1991; Rigo et al., 1996; Bernard et al., 1997) (Fig. 1). Normal faults accommodate most of the extension in the Gulf and trend E–W to WNW–ESE, rupturing the upper crust (Fig. 1). The typical length of the faults ranges from 10 to 40 km (Jackson et al., 1982; Doutsos and Poulimenos, 1992; Roberts and Koukouvelas, 1996). The main active faults occur along the southern shore of the Gulf of Corinth (Northern Peloponnese) (Fig. 1C). These faults dip north typically at moderate angles (50–60
), but rarely at high angle (60–80
). The faults are segmented along their lengths in segments less than 10 km long. This segmentation is the dominant geometry for faults in the Gulf (Koukouvelas and Doutsos, 1996). The rift was developed in two stages (Ori, 1989; Doutsos and Piper, 1990). The first stage was characterised by a shallow fault-controlled basin, filled with fresh water sediments (1000–2000 m). The second stage was characterised by Gilbert-type fan deltas and coarse-grained

1040-6182/$ - see front matter r 2003 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/S1040-6182(03)00103-4

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Fig. 1. (Left) Map of the major normal faults (black heavy lines with ticks), main historical earthquakes (heavy circles) and the most reliable focal mechanisms of recent earthquakes of the broader Corinth Gulf area (modified from Bernard et al., 1997). (Right) Topographic map of the study area in the enlarged square, where the names of villages and localities are shown.

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terrestrial sediments that have a maximum thickness of 1200 m. Normal N–S or E–W trending faults, oriented obliquely to the subduction front-Hellenic trench system (Koukouvelas et al., 1996), dominate the Quaternary active faulting in the Peloponnese. These results have been based only on the detailed study of sediments, while the fault geometry is a hypothesis based on Jackson and McKenzie’s (1983) suggestion for the geometrical evolution of normal fault systems. Generally, the pre-rift sedimentary cover of the broader Peloponnese region is composed of Triassic evaporites and carbonates, Cretaceous limestone and Oligocene turbidites (flysch) and Lower Miocene (Burdigalian) marls and limestone. Geological, deep drilling and seismic profiles show that the syn-rift sediments are composed of middle Miocene to lower Pliocene marls, clays and Messinian evaporites, and Pliocene-Quaternary recent deposits. The evolution of the Tertiary basins is associated with the westward progressive thrust-foldpropagation of the Hellenides, during which asymmetric basins (foredeep) were formed in front of the thrust belt and piggyback basins on the propagating thrust sheets (Xypolias and Doutsos, 2000). Active deformation in the Gulf is suggested by uplifted shorelines, reversal of drainage pattern, earthquake induced land slides, both on land and offshore, faulted colluvial layers and by historical and recent seismic activity (Fig. 1). The oldest well described earthquake that occurred in this area happen in 373 BC. This was described later by philosophers, geographers and historians, among them Aristotle,

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Eratosthenes and Herakleides in the writings of Strabo, Diodoros of Sicily, Pausanias, Ovid, Pliny and Aelian, and summarised also by Marinatos (1960), Mouyaris et al. (1992), Guidoboni (1994), Papadopoulos (1998) and Katsonopoulou (1998). Although the 373 BC earthquake was a strong event (s) there is no mention of surface rupture. Up to now this event was not documented either by archaeological or geological data (Koukouvelas et al., 2001; Katsonopoulou et al., 2002). Other known strong earthquakes within the western part of the Gulf occurred in 23 AD, 1402, 1748, 1817, 1861, 1888, 1889, 1965 (Ms 6.4), 1995 (M 6.3) (Ambraseys and Jackson, 1990; Guidoboni, 1994; Papazachos and Papazachou, 1997; Papadopoulos et al., 2000) (Fig. 1). Schmidt (1879) mapped the associated ground deformation (fractures and liquefaction) (Figs. 2 and 3) and the scarps from this rupturing of the 1861 earthquake, which are still evident (Fig. 4). Although structural, seismological and geodetic studies concerning the Gulf of Corinth are numerous, very few of them deal with the fluvial geomorphology within the Gulf and its Holocene evolution. The following detailed view of the Kerynites River course tries to redress this imbalance. It includes detailed structural mapping of the Eliki Fault that modifies the river course and provides data from five trenches (Fig. 2) that can send light on the course of the Kerynites River, in order to understand the active deformation of the area. The result is a Late Holocene map of the Kerynites River flow and estimations for the westward propagation of the East Eliki Fault.

Fig. 2. Tectonic map showing the east (thick black lines) and west (black lines) segments of the Eliki Fault and the fault array in the Eliki step-over zone (thick gray lines). The East Eliki Fault segment was broadly reactivated during the 1861 Eliki earthquake. Location of trenches and the Fig. 4 are also shown. Inset shows ruptures related with the 1861 Eliki (Schmidt, 1879) and 1995 Egion earthquakes (Koukouvelas and Doutsos, 1996; Koukouvelas, 1998b).

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Fig. 3. Aerial view showing the east and West Eliki Fault segments and the Eliki step-over zone. The Selinous (SEL), Kerynitis (KE) and Vouraikos (VOU) river courses are shown in the plain-coastal area. The area of the archaeological research is marked (Helicopter aerial photograph provided by the ‘‘Helike Society’’).

Fig. 4. Recent photograph showing part of the surface rupture associated with the 1861 Eliki earthquake (Locality is shown in Fig. 4). Black arrows denote the fault trace and white arrows active landslides, for locations see the Fig. 2. Width of photo is 200 m.

2. The geoarcheological problem of Helike In 373 BC, on a winter night, a very strong earthquake(s) devastated the ancient Greek city of Helike, resulting in extreme coastal and ground changes. Note that Helike is the proper spelling for the ancient homonym Greek city-state. Eliki Fault is a name used mainly in geological literature, while Eliki is the modern name of the homonym village. Helike was the most

important city in Achaia, capital of the Achean Dodekapolis (twelve city-states alliance). Considerable destruction was also caused in the neighbouring town of Bura. The intensity there, however, possibly did not exceed that of Helike. Some conservative calculations and re-examinations of the available data reveal that such an extreme event occurred and the co-seismic effects are similar to those of the well-known 1861 earthquake (Schmidt, 1879). Papazachos and

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Papazachou (1997) and Papadopoulos et al. (2000) judged that intensities of IX degrees (modified Mercalli (MM) scale) were felt in Helike city in 373 BC. Nevertheless, Helike vanished in a period of several hours, without leaving any trace. This destruction appears to be related to the subsidence of the coastal segment of the Helike alluvial plane that extended at maximum width from the hill-front area to the seashore for about 2.2 km. However, the exact length of the subsided segment is unknown. From that time on, many scientists have tried to locate the exact position of ancient Helike (Soter and Katsonopoulou, 1999 and citations therein). One of the most important reasons for this quest is that the sudden burial of the city may have preserved priceless knowledge about the ancient Greek culture. At the beginning of this search, it was thought that the city was submerged under the sea because of an earthquake and a tsunami (Marinatos, 1960; Papadopoulos, 1998). This hypothesis was strengthened by secondary phenomena following wellstudied historic earthquakes in the area, and the steep sea-bottom morphology which identifies the seaward area of the Kerynites alluvial plane as prone to submarine landslides that can induce tsunamis (Koukouvelas (1998a, b) and citations therein; Papatheodorou and Ferentinos, 1997; Papadopoulos, 1998). Thus the majority of the researchers tried to locate the submerged city somewhere under the sea or near the coast. In 1988 Soter and Katsonopoulou undertook a marine investigation, using sonar, but did not detect a submerged city (Soter, 1998). After these efforts, the search was redirected towards the coast itself and the plain between the towns of Aigion and Diakopto, using shallow coring (Soter and Katsonopoulou, 1999). This change of thinking was also due to the observation that in this area earthquakes induce large-scale liquefaction, which can cause devastating damage to any buildings in the area. Sediments prone to liquefaction consist of soft unconsolidated and poorly shorted deltaic deposits (Athanasopoulos et al., 1999). As a result it was then suggested that the city of Helike was probably devastated by successive strong shaking, accompanied by extensive liquefaction, followed by an induced submarine landslide. This helped in creating a tsunami, or a succession of tsunamis within a few hours of the earthquake. This hypothesis can also explain the creation of a lagoon or a lake in the former place of the city after its destruction, and is further supported by the fact that in 95 shallow boreholes throughout the region, 81 of which have been carried out between Selinous and Kerynites Rivers (Figs. 2 and 3), almost all the occupational horizons were located above present sea level (Soter and Katsonopoulou, 1999). These geological data, as well as the archaeological, lead to the conclusion that one of the most promising locations

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for the site of ancient Helike is the land area between the Selinous and Kerynites Rivers (Fig. 3).

3. Active deformation of the area The Eliki Fault is divided into two prominent segments, the western and the eastern segment, spaced about 2 km (Fig. 2). The lengths of the western and eastern segments are about 9 and 13 km, respectively. A series of remarkable morphologic and tectonic differences characterize the landscape and the structure of the two fault segments and some of them have particular interest for understanding both its evolution and seismic activity. At first, the morphology of the footwall differs significantly in the western segment, with a maximum elevation of 485 m, while in the eastern segment the maximum elevation is 721 m. Secondly, a narrow fault zone B100-m wide characterizes the West Eliki Fault, while at the eastern segment zone it is almost 400 m wide. Thirdly, the West Eliki Fault juxtaposes along more than 5.5 km of its length Cretaceous limestone or Jurassic chert in its footwall, while the east Eliki segment juxtaposes mainly Neogene fan-deltas. The Eliki Fault is a complex normal fault characterized by a composite fault scarp (sensu Stewart and Hancock, 1991). Detailed structural analysis of the Eliki Fault between the Kerynites and the Vouraikos Rivers suggests that two north dipping and E–W trending fault strands define the Eliki fault zone (Fig. 2). One is located along the range front, the other a smaller fault, is 400 m to the south. The southern fault strand defines the headwater area of a series of kilometer-scale streams draining the fault scarp. The western segment did not rupture during the 1861 event. Thus, in terms of its morphotectonic expression, the eastern Eliki Fault appears to be more active than the western Eliki Fault. Nevertheless the two fault segments dip north (synthetic faults) and their tips show an approaching configuration with no overlap and thus the rock mass between the two faults forms a transfer zone, called hereafter the Eliki zone (Figs. 2 and 3a). The topography of the Eliki zone includes a topographic ramp that shows tapering slip on the fault segments as they enter the transfer zone. According to the classification of Morley et al. (1990) (Fig. 1), the Eliki zone is an ‘‘approaching synthetic transfer zone’’. Estimates based on trench tectonostratigraphy indicate that the minimum vertical displacement and extension accommodated by the East Eliki Fault are as much as 1.5 and 1 mm/yr, respectively (Koukouvelas et al., 2001). McNeill et al. (2003) calculate higher sliprates in the period of Late Pleistocene–Holocene, for the same fault segment based on morphotectonic data. Many rivers flow into the western Gulf of Corinth. These rivers typically flow on a bed of their own detritus and thus are characterized as ‘‘alluvial rivers’’ following

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Schumm’s (1986) classification. Such rivers are sensitive to long-term and earthquake-related deformation that modifies both the river profile and course. River course shift towards or perpendicular to the fault is expected for alluvial fans and fan deltas in extensional settings (see Leader et al., 1988) due to sea level fluctuation and/ or tectonic activity. Especially for the Kerynites River, the control of the fault on the river is rather complex and further explanation is needed for the structural setting of the river course. Based on structural and morphological data as well as on data regarding coseismic ruptures during the 1861 Eliki earthquake, the river is expected to flow parallel with the East Eliki Fault segment, the most active segment of the fault. However, contrary to what is expected, at present the river flows around the Eliki zone and has flowed north since 1700 (Dokos and Panagopoulos, 1993). At its structural position the Kerynites River course is quite critical for two reasons. Firstly its course is not expected, and this requires an explanation, as river systems are sensitive in detecting deformation over decades to centuries. Secondly the analyzed stratigraphy within five trenches (Fig. 2) enables the analysis of a time interval of about 5000 yr, and thus almost the entire analyzed river flow (shift) corresponds broadly to tectonic activity. Significant data comes also from four topographic maps of the 19th century published for this area and three strong earthquake events (Koukouvelas et al., 2001; Pavlides et al., 2001) that enable one to analyze the changing river course in detail. Thus we can draw the long and short-term effect of tectonic movements in the area and the implication of the river course on the archaeological research.

4. Tectonostratigraphy of the trenches Five trenches were excavated along the range front fault of the eastern Eliki fault segment (Fig. 2). As a range front fault we define a fault that offsets for more than 400-m the Pliocene-Pleistocene fan delta deposits constituting mainly the footwall of the East Eliki Fault segment. According to Schmidt’s (1879) map (Fig. 2 inset) and our data (Fig. 4), the area was ruptured during the 1861 event. The trenches were excavated across the fault scarp, and their walls were logged at a scale of 1:20, and designated from west to east as Eli1 to Eli5 (Figs. 5–9). Each trench starts from the range front fault and extents northward. The wall that was mapped in each trench was chosen as preserving the best stratigraphy, and exhibiting faulting events with clear displacements. Samples were collected for sedimentological analysis and radiocarbon dating. In addition pottery fragments and their archaeometry helped to better date the sediments and tectonic events. Although the range front fault is almost rectilinear, detailed

mapping shows that between trenches Eli1 and Eli2 a step in morphology exists. Structural mapping in this area indicates that the step is due to the segmentation of the fault trace with the front fault (location of the Eli1 trench) apart almost 10 m north of the rear fault (location of trenches Eli2–Eli5). Overall, stratigraphy in the footwall consists of well-consolidated fan delta deposits. Colluvial stratigraphy in the hanging-wall varies significantly along strike and its structures and sedimentary units will be described below following the analysis of lithofacies by Nelson (1992). This procedure can be used to interpret tectonic and non-tectonic processes (see also McCalpin, 1996). 4.1. Trench Eli1 Trench Eli1 exposed well-bedded stratigraphy north of the range-front fault. The lowermost stratum, unit F2, is a bouldery-cobbly clast supported gravel, containing clasts regularly up to 50 cm in length. Unit F2 is interpreted as fluvial deposits within an active riverbed. Unit F3 is a well-bedded sequence divided into several sub-units according to variations in lithology and internal structures (Fig. 5). The entire sequence dips towards the fault and is characterized by common clast imbrication, having this imbrication to show a west to east average local flow direction. This unit is juxtaposed by a synthetic fault for more than 1-m north side down. This fault controls all mismatches in the upper part of the trench stratigraphy. The upper part of the trench includes two units a colluvial wedge that is separated based on its lithology (grain-size) and matrix into a lower part corresponding to a debris element association and an upper part classified as a wash-element association. Laterally and vertically accreted in the uppermost part of the trench and above the colluvial wedge was deposited a sandy unit including lenses of fine sand with cross-stratification. Age data from this trench is missing due to that almost all of the units are of fluvial origin and charcoals are not preserved. 4.2. Trench Eli2 Trench Eli2 is located close to the exit of a small gully and thus most of the exposed stratigraphy is dipping north highlighting the north dip of the alluvial fan that is formed by the gully. Nevertheless, within the trench are recognized two major structurally produced morphologic steps controlling most of the trench’s stratigraphy. Otherwise, the lithology of the unit F2 exposed as the lowermost stratum within the trench resembles the F2 unit in trench Eli1. Above this unit it is developed a sub-unit showing mostly granules and more than 10% sand matrix are also being rich in organics. Based on its lithology and that it covers a fault scarp, we interpret this unit as wash-facies colluvium. On top of this

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Fig. 5. Log of the east wall of the trench Eli 1 (initial mapping 1:20; for details and for location see text and Fig. 2, respectively). Colluvial stratigraphic data has been divided and interpreted following Nelson’s (1992) classification (see Table 1, p. 608 and Table 2, p. 612 in Nelson 1992).

Fig. 6. Log of the east wall of the trench Eli 2, (initial mapping 1:20; details in the text and symbols as in Fig. 5).

colluvium, gravel is deposited, including imbricated pebbles or cobbles with occasional lenses of crossstratified sands of fluvial origin. The uppermost units of the trench were controlled by the southernmost fault

scarp and include the sub-units C1, C2 and C4. The subunits C1 and C2 are of wedge-shaped geometry. Lithology in both sub-units is well-rounded pebbles and some intact blocks from the basement with

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Fig. 7. Log of the east wall of the trench Eli 3 (initial mapping 1:20; for details see text, for location see the Fig. 2). Photograph shows details regarding the offset associated to the 1861 event and fault strands as observed in the east wall of the Eli 3 trench (simplified from Koukouvelas et al., 2001).

Fig. 8. Log of the east wall of the trench Eli 4 (initial mapping 1:20; for details see text, for location see the Fig. 2). Rose diagram shows the orientation distribution of long axes of platy pebbles in the whitish-yellow fluvial formation. The parallelograms marked with a (X) show the positions where material for the dating was taken (BS34, BS24, C14 and BS14 see in Table 1 for details).

maximum length of about 20 cm. From bottom to the top an overall decrease in grain size and increase in matrix characterizes the two sub-units. Based on their lithology, we interpret these units as upper and lower

debris-elements. The sub-unit C4 shows a significant decrease in clast size and an increased matrix, which enables its interpretation as wash facies colluvium. Finally, the newest unit within the trench is gravel

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Fig. 9. Log of the west wall of the trench Eli 5 (initial mapping 1:20; for details see text, for location see the Fig. 2). The parallelograms marked ‘X’ show the positions where material for dating was collected (C15 and C25, Table 1). Photograph in the middle shows in detail the upper left corner of the log.

covering the southernmost fault. This gravel is composed mainly of loose angular debris and intact blocks of the scarp free face and as this has wedge geometry and is exposed in all units within the trench, we interpret it as a deposit probably related with the 1861-event. Age data from this trench is again missing due to incomplete sampling and poor preservation of dating materials within the sediments of the trench. 4.3. Trench Eli3 The trench Eli3 is almost the same as the trench description published by Koukouvelas et al. (2001), and all description and interpretations that appeared in this paper are valid also for the present work. The trench Eli3 is about 10 m east of the trench Eli2. However, the trench of Fig. 7 differs in that is more detailed than the Koukouvelas et al. (2001) section. Although the two trenches Eli2 and Eli3 are quite close, there are two significant differences in terms of stratigraphy between them. Sediment thickness increases dramatically from Eli2 to Eli3, and units in trench Eli2 are missing from Eli3 and vice versa. For example unit F3 in Eli2 is missing from Eli3, and unit F1 in Eli3 is missing from Eli2. Koukouvelas et al. (2001) explained these differences by fault geometry and deformation distribution along different fault strands. The lowermost exposed unit within the trench is a brick-orange unit which includes imbricated coarsegrained pebble and cobble gravels showing grain size reduction from south to north (away from the fault surface and from the bedrock) and from bottom to top (for more details see Koukouvelas et al. 2001). Of particular interest for understanding the possible location of the river course in the area is the segregation of tiles and human bones in the upper part of this unit (Fig. 7, F1,

sample B1). This archaeological data suggest that although this unit is of fluvial origin and was formed by 100 BC, the unit was not incorporated in an active river channel. Above this unit was deposited an almost 2-m thick unit of colluvial deposits rich in organics, charcoal and pottery shreds dipping north at about 40
. This colluvial wedge can be differentiated in terms of its lithology and matrix into three sub-units. The middle unit (C2) includes matrix supported and poorly sorted granule gravels with pottery shreds. This is included between two matrix supported pebbly gravels with tiles (C1 and C3). On the top of the sub-units C2 and C3 and at the northernmost part of the trench interfingering with the colluvium, unit F4 was deposited, composed of well-rounded cobble and pebble gravels with sandy matrix of fluvial origin. The newest unit within the trench, composed of loose angular debris and intact blocks of the scarp free face is identical with unit C5 in the trench Eli2. Dating of C5 comes from a tile collected from the contact between units C3 and C5 recognized as Ottoman age and dated with thermoluminescence at 1200–1311 (Fig. 7, sample P1 and Table 1). Since this unit is newer than this age range and its thickness is controlled by a surface rupturing event and covers the produced scarp we interpret this as the debris–element association related with the 1861 event. From the trench Eli3, five stratigraphically consistent ages are available (Table 1) including the P1 with three of them dated with the thermoluminescence method of tiles (Fig. 7, samples P1-P3), two human bones (Fig. 7, sample B1), and a wood fragment (Fig. 7, sample W1). The top of the unit F1 must be younger than 3600 BC, the oldest part of the age range at 2s; and may be as young as AD 700. However, this age range is reduced significantly by the archaeological data, indicating ca. 100 BC as a possible age for the onset of colluvial deposition.

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Table 1 Laboratory and calibrated radiocarbon and thermoluminescence (TL) ages of samples from Trenches Eli3 to Eli5 Trench

Ref. no.

Layer

Dated material

Measured age TL, or 14C Age

Eli3b

P1 P2 P3 B1

C3 Top of C1 Base of C1

Pottery Pottery Pottery Bone

TL 745756 TL 1079785 TL 20497150 14 C 30207880

W1

F4

Wood fragment

14

C 3007120

BS34 BS24 C14 BS14

Base of C3 Top of F3 Base of F3 Top of F2

Soil Charcoal Charcoal Soil

14

870740 1000740 2170740 4840740

C15

Top of C3

Charcoal

14

C 130740

C25

Top of F3

Charcoal

14

C 340740

Eli4c

Eli5b

C C 14 C 14 C 14

Age rangea

2400–100 BC (0.68) 3600 BC700 AD (0.95) 1440–1680 AD (0.62) 1770–1810 AD (0.05) 1930–1950 AD (0.01) 1030–1260 AD (0.95) 970–1160 AD (0.95) 380–90 BC (0.95) 3710–3620 BC (0.57) 3600–3520 BC (0.38) 1670–1770 1790–1960 1400–1850 1900–1950

AD AD AD AD

(0.39) (0.56) (0.94) (0.01)

a

The s errors are presented in terms of probabilities ð0:94 ¼ 94%Þ based on Oxcal 3.5 (Stuiver and Reimer, 1993). University of Patras finds. c CORSEIS finds. b

Other consistent ages for the onset of accumulation of colluvial deposits come from thermoluminescence dating of tiles from the colluvial deposits (Fig. 7, samples P1-P3 and Table 1) and the unit F4. The middle of the unit F4 must be younger than AD 1400, the oldest part of the age range at 2s; and may be as young as AD 2000. However, since the Venetian map of AD 1700 and all other known maps for the area show that practically the river from AD 1700 and onwards is located at its present day course, the deposition of F4 happened closer to AD 1400. 4.4. Trench Eli4 Trench Eli4 exposed well-bedded stratigraphy north of the range front fault, which is juxtaposed and controlled by two basin-ward fault strands. In detail, unit F2, which is similar in lithology to the F2 unit in Eli1 and Eli2 trenches, fines upward and contains clasts up to 30-cm in diameter. The upper part of the unit is matrix supported pebble gravel. The southernmost part of the unit as exposed in the Eli4 is composed mainly of coarse sands with cross bedding including sparse granules and lenses of fine sand or granule gravel which can be interpreted as a point-bar deposit. The rest of the F2 unit is interpreted as a riverbed deposit. However, significant for the overall interpretation of the unit along the range front fault is the systematic decrease of the unit’s grain size from west to the east. Based on the grain size reduction and given that imbrication in the F2 unit indicates flow from west to east, it is fair to suggest that

from the trench Eli1 towards the trench Eli4 locations are proximal and distal, respectively. For this unit we have only one age coming from a bulk soil sample which gives evidence that the top of the unit must be younger than BC 3710, the oldest part of the age range at 2s; and may be as young as BC 3520. In the trench Eli4, unit F2 is capped by aD1 m stratum thick gravel that is similar to the fluvial gravels in all other trenches. In addition, the fact that the unit maintains its thickness over all fault strands indicates that almost all of the juxtapositions have happened after its deposition or that the onset of the deposition of F3 coincides with an overall tectonic quiescence. However, the top of the F3 unit appears to bury the northernmost fault within the trench or changes its thickness a meter south of the median fault. A significant juxtaposition of the unit had happened during the final stages of the unit’s deposition. Significant for the stratigraphy of the trenches and the understanding of the river course is that after this event within the trench active fluvial sedimentation ended earlier than it did at Eli3 (Table 1). Three samples collected from unit F3 one north of the northernmost fault and two south of it (Fig. 8 and Table 1) enable the definition of the age of both its deposition and the river’s counterclockwise migration. The age range for the deposition of the unit is defined with two samples one at the bottom of the unit and the other on its top to cover a deposition period of 1120–1640 yr (Fig. 8 and Table 1). In the trench Eli4 the uppermost unit are colluvial deposits that can be differentiated based upon their

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lithology into three sub-units. The base of the unit must be younger than AD 1030, the oldest part of the age range at 2s; and may be as young as AD 1260. More dating is beyond the scope of this work. However, based on the lithology and structures of the colluvial wedge, we can correlate its sub-units with C4 and C5 wash- and debris-elements within the other trenches. 4.5. Trench Eli5 Trench Eli5 exposed completely identical stratigraphy and lithology with the trench Eli4. Based on these similarities, we are able to compare all units, especially in the last three sections. Fault strands exposed within the trench show a more complex array of predominantly north dipping faults than in all other trenches, especially regarding the median fault which appears to be splaying at least in four branches, some of them characterized by open space. This median fault juxtaposed strongly the fluvial sediments of units F2 and F3 into two steps with maximum displacement of about 1 m. Important for the purposes of this paper is the northernmost fault where the wealth of charcoal within the strata and the progradation of the colluvial wedge towards north indicate a good area for looking for the counterclockwise river migration. For defining the process of river migration we collected samples from units F3, C3 and C4 (Fig. 9 and Table 1). Other units in the trenches can not be dated due to absence of charcoal, or stratigraphic correlation is difficult due to combined fault and river erosion modification. The top of unit F3 must be younger than AD 1400, the oldest part of the age range at 2s; and may be as young as AD 1850 with the older age as the most probable for the onset of the counterclockwise river migration. This result is consistent with age data from both the other trenches. All other ages are consistent in that the river migration was followed by the accumulation of colluvial deposits. Finally, remarkable is that the counterclockwise river migration appears to develop after AD 1400 when a strong earthquake is inferred to have ruptured the Eliki Fault (Koukouvelas et al., 2001) although up to now nothing is known about surface ruptures during this event.

5. Tectonics and sedimentation Drainage entering the western part of the Gulf of Corinth is related to footwall lithology, sea-level changes, and changes of plain morphology after floods and faulting, with the latter factor being the most important. Specifically the rock type and its resistance to erosion are also of great importance, both for the drainage pattern and morphology and the divide retreat. In particular in the Gulf of Corinth rift zone drainage is developed in a variety of lithologies including calcium

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carbonated sequences of the Pindos and Tripolis isopic zones, flysch deposits and unconsolidated Neogene deposits. Steep, small drainage basins have developed in Mesozoic carbonate sequences (i.e. the upper reaches of the Kerynites River), large basins with dendritic drainage within unconsolidated sediments (i.e. the rivers draining the eastern part of the Gulf of Corinth; Zelilidis, 2000) and larger basins in flysch deposits. Divide retreat, accomplished mostly by rockfalls and slide at the head-cuts of deep gorges, is faster within unconsolidated sediments rather than in the more resistant Mesozoic carbonates. Faulting controls kilometric-scale drainage and regional drainage patterns. In particular drainage off the steep fault faces within limestones is minor, as karstic drainage is significant (see also Goldsworthy and Jackson, 2000). In addition, rivers crossing the northern Peloponnesus show juvenile, anteceded and re-established drainage (sensu Zelilidis, 2000) with steep gorges being common across the Gulf while some rivers found their way between fault segments (i.e. Collier and Gawthorpe, 1995; Koukouvelas et al., 1999). The Kerynites River finds its own way between the east and west Eliki Fault segments, flowing through the Eliki zone before entering its alluvial plain and thus its course is primarily controlled by the fault activity. In the following analysis we will try to outline how the structural mapping and the trench tectonostratigraphy shed light on the archaeological problem of Helike. We will also show how the river course is modified by the move of the East Eliki Fault during surface breaking earthquakes (Fig. 10). The stratigraphy of all the excavated sites along the East Eliki Fault segment indicates that during the Holocene period fluvial and colluvial sedimentation were fault controlled. Pebble imbrication in the fluvial sediments suggests that most of the long axes of pebbles are oriented at high angles to the foothill front, and thus indicates a river flowing from west to east. Channel deposits of this river became thicker to the west, while cobbles within these deposits diminishing from 0.5 m in diameter in the west (Eli1 trench) to 0.2 m in the east (Eli5 trench). In addition, from the Eli1 to the Eli5 trenches there is an overall prominent decrease in grain size of the unit A, suggesting that the easternmost trench was the most distal position for this river after it enters its alluvial plane. All these data indicate that the river flowing along the East Eliki Fault segment is the PalaeoKerynites River. The present day morphology in the area south of Zachloritika village is characterized by a series of abandoned channels trending north–south in a depression just east of trench Eli5. As there are no significant catchments to the south of abandoned channels in the hanging wall area of the fault, it is reasonable to suggest that these channels were probably related to the PalaeoKerynites and/or the Vouraikos Rivers.

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Fig. 10. Photomosaic showing the East end of the West Eliki Fault segment, the Eliki step-over zone and the East Eliki Fault. The Kerynitis courses are also marked through the last 2000 years or so.

In the Eli1 trench two fault strands were mapped. One is recognized as the range front fault, across which Pliocene–Pleistocene conglomeratic fan-delta deposits are vertically offset by more than 400 m. The second recognized fault in the unconsolidated deposits vertically displaces the coarse grained fluvial deposits by a total offset of 1.3 m (Fig. 5). Further to the east in the Eli4 trench, a north dipping fault strand displaces a fluvial channel by 1.4 m (Fig. 8). In the Eli 3 trench the fluvial compartments of the succession in the trench are located about 6 m basinwards from the boundary fault overlapping a colluvium wedge. In addition, the lower part of the fluvial unit and the brick orange deposits are offset by 1.4 m. Similarly, in the trench Eli5 (Fig. 9) two different fault strands juxtapose the F2 subunit into two steps, with displacement totaling about 2.5 m. Although available dating exists only for the trench Eli3 (Fig. 7 and Table 1), and it is thus difficult to date the displacement, it is worth noting that the offset appears to be systematic for about 1 km along the fault trace. The absence of specific dates from trench to trench allows two possible explanations for the displacement over the last 2000 years: (a) throughout the fault length the offset occurred as discrete events is the same, or (b) the accumulated displacement along the faults for longer periods is remarkably constant. Data for a

longer period are poorly preserved only in the trenches Eli3 and Eli4. Especially in the trench Eli4 age data indicate sediments 1.5 m below the surface being dated at 3710–3620 BC. Given that this age is expected at a depth of about 4 m below surface we suggest that probably at this point sediment accumulation and subsidence ended earlier in the vicinity of Eli4 than it did at Eli3 (Fig. 8). Thus there is strong evidence that the PalaeoKerynites River appears to have been strongly affected by tectonic movements related to the westward propagation of the East Eliki Fault segment and retreat towards the Eliki transfer zone. Finally, the Kerynites River was captured in front of the foothills at the Eliki zone altering its course for 140
ca. 1700 AD (Soter and Katsonopoulou, 1998, based on the 1700 Venetian map of the area) (Figs. 3 and 10). This northwest-ward migration is estimated at about 0.2
/yr, an order of magnitude faster than the 0.02
/yr clockwise rotation of the Selinous River in the west (Koukouvelas, 1998a). However, from published data (Stewart, 1996; Koukouvelas et al., 2001) an earthquake probably occurred in the Helike area at about 600 AD and the last significant river deposits in the area were deposited at about 1000 AD. Thus the time interval for the retreat of the PalaeoKerynites River course lasted almost 700 yr. By the time

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of the Venetian map (1700) (Dokos and Panagopoulos, 1993), the river had completed a retreat totaling 140
, for a rate of 0.2
/yr. It is significant to analyze if the data suggests that the river retreat was episodic or continuous, because this has structural and archaeological implications for the area. Topographic maps (Fig. 10, comparison between the 1700 river course with that of Schmidt’s (1879) map after the 1861 earthquake) indicate a clockwise migration of the river after the 1861 event towards the central portion of the East Helike fault segment (Fig. 10). This style of migration is quite similar to that in the PisiaSkinos fault easternmost end of the Gulf of Corinth (see Collier et al., 1998). In that case a small river draining the near tip area (as also does the Kerynites River which drains the west tip of the East Eliki fault segment) retreated due to the 1981 Corinth Gulf earthquake towards the fault’s central portion (Leader et al., 1988). During the 1861 event this retreat was about 30
of clockwise migration, and then the river started retreating to its present course, which is similar to the 1700 course, for another 30
of counterclockwise rotation. Thus the 5th to 6th century palaeoearthquake (600 sensu lato), which is recognized in the trenches (Koukouvelas et al. 2001 and this study), in notches to the east (Stewart, 1996) and in the archaeological excavations in the west (Soter and Katsonopoulou, 1999), was an event rupturing the East Eliki Fault. Since this event initiated the river retreat, we suggest that it was probably the first event that caused surface rupture of the fault westward of the Kerynites River. If this suggestion is correct, this event onset the linkage of the west and East Eliki Fault segments. Modeling for fault linkage by Crider and Pollard (1998) indicates that the prominent behavior of Kerynites River in flowing around the step-over zone attests to high deformation between the strong earthquakes at the step-over zone and a progressive hard linkage between the two fault segments.

6. Discussion

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In particular the counterclockwise rotation indicated from trench Eli4 starts with the deposition of the unit C3. Unit C3 yielded a calibrated age of about AD 870 (2s range is AD 1030–1260). Confidence interval of the age of the unit C3 is consistent with the age range for the top of unit F3 of about AD 1000 (Table 1), and thus we suggest as secure these ages for interpreting a changing river course towards north. Additional information for the counterclockwise rotation came from trench Eli5 and the base of the colluvium unit C3 yielding a calibrated age AD 370 (2s range is AD 1440–1640). This age is consistent with the age of the top of the underlying unit F3 suggesting post 1440–1640 migration of the river towards north. In the trench Eli3 the last fluvial deposits yielded a calibrated age of about AD 300 (1s range confidence interval 62% is AD 1440–1640). In addition this rotation based on the age data from trench Eli4 appears to start changing its course from about 1200 BP until 1500 BP but the significant migration is after 1600 BP. Thus all ages for river migration fit quite well with the Venetian map showing the river at 1700 flowing at its present day course. A second offlapping structure is recognised in the lower northern part of Eli4 trench. This observation is mainly based on a small-scale colluvial wedge unit in the top of the F2 yielding a calibrated age of about AD 4840 (2s range is BC 3710–3620) and the age of the base of F3 (AD 2170). As these two units show remarkable differences in lithology and chronology we interpret this as an offlapping event. However, as this evidence comes only from one trench the age of the offlap is poorly constrained with these chronological data. Regarding the clockwise rotation evidence comes from the Eli3 and Eli4 trenches. Within the Eli4 trench stratigraphy shows a significant onlap of river deposits toward the fault scarp starting sometime before the deposition of the base of the unit F3. The base of this unit yielded a calibrated age of about AD 2170 (2s range is BC 380–390). Evidence from trench Eli3 suggests that this onlapping process lasted over a long time until 100 BC. After this period the location of the Eli3 trench was used as a cemetery and thus can not be an active river bed (see also Koukouvelas et al., 2001).

6.1. Interpretation of Kerynites River course change 6.2. The Kerynites River in its tectonic setting Two events of clockwise and one of counterclockwise rotation of the Kerynites course are recognised in the upper 3 m of stratigraphy in the trenches. As mentioned above stratigraphic units are truncated, thicken or change character across the fault, presumably due to deposition against scarps. Scarp-controlled colluvial deposits in the present work are used here as indicator of counterclockwise rotation of the river away from the main fault scarp, while fluvial deposits are interpreted as indicators of clockwise rotations of the river against the main scarp.

As most of the rivers in the study area are of the alluvial type and thus changes of their courses are expected during the recent past, the detailed analysis of the Kerynites River course may provide clues for other similar rivers. The Kerynites River course is also critical for understanding of the evolution the East Eliki Fault, the tectonostratigraphy along the fault scarp and the geoarchaeology of ancient Helike. A change of river course has implications for the sediment dispersal over its alluvial plain and the depth of burial of ancient ruins.

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In addition the area east and west of the present day Kerynites course is expected to be at least at the territories of the ancient town (Soter and Katsonopoulou, 1998). Thus the definition of the sediment dispersal is critical since in most cases significant ruins, even of the Roman period, are buried under at least 1.5 m of sediments, primarily of fluvial origin of almost equal thickness to the sediments accumulated in our study area. In addition trench results combined with archaeological studies support the repeated destruction of the buildings due to strong earthquakes offsetting the surface for a maximum of 1.4 m and an equivalent maximum magnitude of the order 6.8 (see also Koukouvelas et al., 2001). There is evidence for at least three strong earthquakes associated with the East Eliki Fault during the past 2000 years, including the 1861 event (Koukouvelas et al., 2001; Pavlides et al., 2001; Pantosti et al., 2002; McNeill et al., 2003). The vertical displacement ranges from 0.5 to 1.4 m, as measured at all sites on the fault strands affecting sediments and colluvial layers. From the archaeological point of view, rivers are traditionally associated with borders of cities (Katsonopoulou, personal communication, 2002). Thus, Helike was probably located between the Selinous and Kerynites, both of which are retreating towards the Eliki step-over zone. Thus the evidence that Helike after the 373 BC event never regained its glory and that all ruins in the area are buried is the result of co-seismic subsidence and fluvial activity.

Although the rate of fluvial retreat is uncertain, four major conclusions of geological and archaeological importance can be drawn. (a) There is a prominent process for the Kerynites River to migrate northwest, in the area of the Eliki step-over zone, where subsidence due to fault segment interaction, overcomes subsidence during earthquake events along the East Elike fault segment. (b) The long-term counterclockwise migration of the river is interrupted by seismically induced clockwise migration. (c) The two fault segments during the future events will be breached and a zigzagging fault pattern will develop. (d) Most of the ancient ruins which lie in the area between the present day Kerynites course and the along fault river course is very probably eroded.

7. Conclusion

References

Age data and offsets in the trenches indicate that three events in a time interval of about 2000 years produced a total stratigraphic offset of about 340 cm, which can be separated into 273 cm of vertical displacement or throw and 200 cm of extension or heave. Stratigraphy in the trenches varies from trench to trench in relation to the position of each trench and the river course changes. Although we have no information about ground hazards in the area, it is fairly probable that some of them, if not all were related to surface ruptures (Stewart, 1996; Soter and Katsonopoulou, 1998; Soter, 1999; Koukouvelas et al., 2001). Thus it is probable also that some of these events were related to the rupture propagation of the East Helike fault to the west. From this point of view the Kerynites River was a strongly migrating river, and this has implications for the eastern end of the Helike, as well as for the depth of burial and the preservation of the ancient architectural sites below fluvial deposits. The location of ancient Helike can be extended as far as modern Zachloritika, or west of the present day Kerynites River flow.

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Acknowledgements This paper, which is a result of the co-operation of the two geological departments of the Thessaloniki and Patras universities, could not have been realised without the additional support of the CORSEIS (Contract No. EVG1-1999-00002) project. We thank colleagues S. Soter (NY, USA), D. Katsonopoulou (Heliki Society), D., Pantosti, P. De Martini (INGV Rome), R. Collier (Leeds), L.C. McNeilll (UK) and the following students D. Agrafiotis, G.-A., Alexandris, S. Sboras, V. Zigouri, S. Verroios, for their help in the field. Thanks are also due to Liew Ping-Mei, Y. Ota and M. Stirling, Guest Editors of the Special Issue and one anonymous reviewer.

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