Preliminary Estimate of Holocene Slip Rate on Active Normal Faults Bounding the Southern Coast of the Gulf of Evia, Central Greece
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Preliminary estimate of Holocene slip rate on active normal faults bounding the southern coast of the Gulf of Evia, central Greece R.T. Walker1, S. Claisse1, M. Telfer2, E. Nissen3, P. England1, C. Bryant4, and R. Bailey2 1Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR, UK 2Oxford University Centre for the Environment, South Parks Road, Oxford, OX1 3QY, UK 3Bullard Laboratories, Cambridge University, Madingley Road, Cambridge, CB3 0EZ, UK 4NERC Radiocarbon Facility (Environment), SUERC, Rankine Avenue, East Kilbride, G75 0QF, UK ABSTRACT through the Holocene. Both the Arkitsa and geomorphic expressions of this faulting are the Kamena Vourla faults are clearly active and Gulfs of Evia and Corinth, each of which is We investigate the late Quaternary history despite an absence of historical earthquakes bounded by footwalls of order 1 km in height of slip on the Kamena Vourla and Arkitsa on either fault, they should be considered (Fig. 1). GPS measurements show that the Gulf normal faults, which are segments of a fault to be a major hazard to local populations. of Corinth (e.g., Fig. 1 inset) is currently extend- system bounding the south coast of the Gulf However, further dating studies and palaeo- ing at a rate of ~12 mm/yr at its western end, of Evia in central Greece, and which we refer seismic investigations are required before the and 6 mm/yr in the east (Clarke et al., 1998; to as the Coastal Fault System. We examine slip rate and history can be fully quantifi ed. Avallone et al., 2004), and a variety of estimates two river terraces, near the village of Molos, of Quaternary and Holocene slip rates on the which are found within the uplifted footwall INTRODUCTION bounding faults are consistent with these rates of the Kamena Vourla fault. The upper ter- (e.g., Armijo et al., 1996; Leeder et al., 2003; race is ~20 m above the present river level Published estimates of slip rate on faults in Pirazzoli et al., 2004; Bell et al., 2008). In con- and appears to represent fan deposition into central Greece are rare, but are important both trast, the geomorphically similar Gulf of Evia the main river channel from surrounding for understanding the active tectonics of the is, according to geodetic measurements, extend- tributaries. The lower terrace, ~8 m above the region and for quantifying seismic hazard in ing at a rate of only 1–3 mm/yr; that is up to 10 present-day river bed, represents an interval this region, where population centers are con- times slower than the Gulf of Corinth (Clarke et of river-bed aggradation and correlates with centrated in the hanging walls of the normal al., 1998), raising the question of whether there the surface of a delta on the hanging-wall side faults. We present a quantitative estimate of slip may be a discrepancy between geodetic and of the fault. GPS profi les show a 6 ± 0.1 m rate across the system of faults that bounds the geological measurements of slip rate here. vertical offset of the lower terrace surface as southern coast of the Gulf of Evia (Fig. 1). This it crosses the fault. Preliminary dating of the Coastal Fault System, comprising the Lamia, ACTIVE FAULTING IN THE LOCRIS two terrace levels, using both optical lumi- Kamena Vourla, Arkitsa, and Atalanti fault seg- HALF-GRABEN nescence and radiocarbon methods, provides ments, is likely to accommodate the major pro- inconclusive results. The lower terrace, how- portion of the <3 mm/yr of geodetically mea- The Northern Gulf of Evia is bounded to its ever, grades toward the present-day sea level sured extension across the region (see section south by three north-dipping, and segmented, and correlates with the surface of a delta on Active Faulting in the Locris Half-graben) and fault systems (named the Coastal, Kallidromon, the hanging-wall side of the fault; it is, there- therefore poses a serious hazard to local popula- and Parnassos fault systems, Fig. 1) that form fore, likely to date from ~6 ka, when sea level tions. There is no record of recent or historical a series of half-graben, collectively known as stabilized at its present-day highstand. With destructive earthquakes on the Lamia, Kamena the Locris half-graben. The Kamena Vourla an age of ~6 ka, the 6 m vertical displacement Vourla, and Arkitsa faults, and there are no fault, which is the principal focus of our study, of the lower terrace yields an estimate of quantitative estimates of their average slip rates, is the central segment of the Coastal Fault Sys- ~1.2–2.0 mm/yr for the Holocene rate of slip so the interval between earthquakes is unknown. tem. Several exposures of late Quaternary fault across the Kamena Vourla fault. This rate of The lack of constraint on the fault slip rates is scarps in limestone are observed along the slip is comparable with an estimated rate of due, at least in part, to the scarcity of suitable Coastal Fault System (e.g., Jackson and McK- ~0.7–2.0 mm/yr for the central (Arkitsa) seg- measurement sites within the steep, mountain- enzie, 1999). Jackson (1999) uses the geomor- ment of the Coastal Fault System, and with a ous topography. phology of the region to suggest that the Coastal 0.4–1.6 mm/yr slip rate measured on the east- Active normal faulting in Greece has a clear Fault System is both the most active, and also ernmost (Atalanti) segment. These estimates geomorphic expression. The uplifted footwalls the youngest, of the three fault systems in the of Holocene slip rates are consistent with the are frequently composed of resistant limestone, Locris half-graben. 1–3 mm/yr of present-day extension across and large sedimentary basins form in the hang- The only recorded large earthquakes in the the Gulf of Evia measured by GPS, arguing ing walls (e.g., Jackson, 1994; Goldsworthy entire Locris region during the past ~300 yr against large changes in rate of extension and Jackson, 2000). The two most prominent are two earthquakes in 1894 that ruptured the Geosphere; October 2010; v. 6; no. 5; p. 583–593; doi: 10.1130/GES00542.1; 5 fi gures; 2 tables. For permission to copy, contact [email protected] 583 © 2010 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/5/583/3341858/583.pdf by guest on 25 September 2021 Walker et al. A 41°N 1916 4.3.67 UL -426 AEGEAN Sperchios 40°N SKIROS basin SEA Kamena UL UL UL Gulf of Vourla Evia 1916 North Gulf of Evia UL UL UL 39°N UL UL UL 1894 UL UL 1894 38°N Gulf of EVIA Corinth 1870 551 1694 km 1708 0 50 100 37°N Kopais 1726 20°E 21°E 22°E 23°E 24°E basin UL 1874 Thiva 1914 South Gulf of Evia 1853 basin 1938 Thiva UL 1893 Platea-Kapparelli F. UL 8.4.70 25.2.81 7.3.81 1705 Gulf of 24.2.81 7.9.99 Corinth 1887 4.3.81 1928 5.3.81 Megara Athens basin SPERCHIOS EVIA B BASIN 1.5Ma Coastal Fault system Molos volcanics GULF OF EVIA (Lamia F.) (Kamena Vourla F.) Kamena Vourla RT 38° 48'N Renginion KNIMIS Arkitsa Basin (Arkitsa F.) Kynos Bralos Renginion UL drainage divide KALLIDROMON Kallidromon F. A Atalanti Scala G Atalanti F. Kifis os river S. limit of CHLOMON river incison Parnassos F. S. limit of N river incison MOUNT 38° 36'N PARNASSOS km 0 5 10 22° 24'E 22° 36'E 22° 48'E 23° 00'E Figure 1. (A) Shaded-relief topography of the Gulf of Evia and surroundings (adapted from Goldsworthy et al., 2002). Fault-plane solutions of instrumental earthquakes are from the Harvard CMT catalog (blue) and from body-waveform modeling (red). Epi- centers of historical earthquakes (represented by blue stars in white circles) are from (Goldsworthy et al., 2002, and references therein). Locations marked “UL” have lithophaga borings uplifted above sea level (Smith, 1994). Thick dotted lines mark loca- tions of elevated shorelines (Stiros et al., 1992). (B) Geological and geomorphic map of the study area (adapted from Goldsworthy and Jackson, 2001). Regions of Mesozoic limestone bed-rock exposure are colored blue. Flat-lying sediments are green. Regions of incised sedimentary cover are marked in white. Lithophaga borings uplifted above sea level at Kynos are marked “UL.” River ter- races near the village of Molos are marked by “RT.” 584 Geosphere, October 2010 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/5/583/3341858/583.pdf by guest on 25 September 2021 Holocene slip rate on faults of the Gulf of Evia, Greece Atalanti fault (Figs. 1A and 1B; Ambraseys and and Middleton, 2001). The mineralogy of the responds to a period of delta progradation into Jackson, 1990; Cundy et al., 2000; Pantosti et terrace fi ll refl ects the local geology, being pre- the Gulf of Evia. al., 2001, 2004; Ganas et al., 2006). The larger dominantly carbonate. The gradient of the present river is controlled of these two earthquakes had a magnitude of Two separate terrace levels were found. by numerous small concrete dams that prevented ~6.8 (Pantosti et al., 2001). The occurrence of The two terraces were mapped in the fi eld us from obtaining a complete riverbed profi le. these two earthquakes on the Atalanti fault raises and remotely using stereo aerial photographs The dams have also caused sediment to aggrade the question of what is the probable earthquake (Fig.