Open University Uranium-Series Laboratory  
Earth and Environmental Sciences, The Open University, Milton Keynes, UK

Localisation of Quaternary slip-rates in an active rift in ~105 years: an example from central Greece constrained by 234U-230Th coral dates from uplifted palaeo-shorelines. Prof John McArthur, Gerald Roberts, PGRA Sarah Houghton: IP/671/0900


Central Greece is amongst the most rapidly extending portions of the Earth’s continental crust, with 10-15 mm/yr of north-south extension localised within a < 30 km wide E-W zone associated with the Gulf of Corinth (Fig. 1a) (Billiris et al. 1991, Davies et al. 1997,  Clarke et al. 1998, Briole et al. 2000). When and how extension became localised in its present position is not well known due to lack of detailed fault-specific displacement histories with good age control. The timescale over which strain localises into the plate boundary is known for only a few examples, due to limitations in dating syn-rift stratigraphy (e.g. Cowie et al. 2005).

At the present-day, coastal uplift in the Gulf of Corinth occurs within the footwalls of high slip-rate active north-dipping normal faults. These faults have been ruptured by seven >Ms 6.0 normal faulting earthquakes since 1909, including two large normal faulting earthquakes in the eastern gulf (February 1981; Ms 6.9, 6.7) (Ambraseys and Jackson, 1990). The faults have clear geomorphic expressions and display evidence for repeated offsets in the upper Quaternary and Holocene.

Detailed mapping (Figure 1), dating and modelling of palaeo-shorelines uplifted in the footwall of the 1981 Gulf of Corinth earthquake fault (Ms 6.9-6.7), can be used to assess its slip-rate history relative to other normal faults in the area and study strain localisation. Uplifted Quaternary and Holocene palaeo-shorelines decrease in elevation towards the western tip of the fault, exhibiting larger tilt angles with age, showing that uplift is due to progressive fault slip.
  Perachora fault and shoreline map    
  Figure 1. Palaeo-shorelines, topography and active faulting on the Perachora Peninsula map with the active faults that ruptured in 1981, and locations and elevations of palaeo-shorelines. Co-seismic throws decrease from east to west.    
234U-230Th coral ages from Cladocora caespitosa date uplifted shoreface sediments and palaeo-shorelines from glacio-eustatic sea-level high-stands at 76 ka, possibly 100 ka, 125 ka, 175 ka, 200 ka, 216 ka, 240 ka and 340 ka. The 234U-230Th coral dates for palaeo-shorelines that formed during localisation of faulting show how Quaternary slip rates changed through time on a 105 y timescale (Figure 2). This allows us to constrain the mechanisms responsible for the localisation.  
  Perachora uplift curve    
Figure 2. Mapped and modelled palaeo-shorelines along Topographic Profile 1 (Figure 1). Palaeo-shorelines were identified using features including wave-cut platforms, shoreface sediments and lithophagid-bored coastal notches or marine-sub-aerial facies transitions.
Uplift rates were calculated to explain the elevations of mapped and dated palaeo-shorelines using initial sea-level elevations from the global sea-level curve (0.15 mm/yr until 175 ka followed by 0.51 mm/yr from 175 ka to present; R2 = 0.9939). These rates were used to calculate the expected elevations of other well-known sea-level high-stands. These modelled palaeo-shoreline elevations agree well with the mapped, but un-dated palaeo-shorelines. We interpret this to mean that the modelled ages are the ages of these un-dated palaeo-shorelines. Palaeo-shorelines from 76 ka, 100 ka, 125 ka, 200 ka, 240 ka and 340 ka are expected to be preserved and we have identified those. Palaeo-shorelines from 175 ka, 216 ka, 290 ka, 316 ka and 410 ka will be drowned by subsequent sea-level rises and subjected to marine erosion; they may not be preserved. With this uplift-rate scenario, the expected 216 ka, 175 ka, 316 ka and 410 ka palaeo-shorelines occupy similar elevations to the 125 ka, 200 ka, and 340 ka palaeo-shorelines, and the expected 290 ka palaeo-shoreline elevation is the same as one of our mapped but un-dated palaeo-shorelines (see Figure 2). We used the correlations between predicted and mapped but un-dated palaeo-shorelines to infer ages to un-dated shorelines on our maps.
Since 125 ka, uplift rates varied from 0.25 mm/yr to 0.52 mm/yr over a distance of 5 km away from the fault tip. Tilting was also occurring prior to 125 ka, but uplift rates were lower because the 125 ka palaeo-shoreline is at 77% of the elevation of the 240 ka palaeo-shoreline, despite being nearly half its age. Comparison of palaeo-shoreline elevations and sedimentology with the Quaternary sea-level curve show that slip-rates increased by a factor of 3.2 ± 0.2 at 175 ± 75 ka, synchronous with cessation of activity on a neighbouring normal fault at 382-112 ka. We suggest that the rapid localisation of up to 10-15 mm/yr of extension into the narrow gulf (c. 30 km wide) resulted from synchronous fault activity on neighbouring faults followed by localisation rather than sequential faulting, with consequences for the mechanism controlling localisation of extension.
Further discussion in : G. P. Roberts, S. L. Houghton, C. Underwood, I. Papanikolaou, P. A. Cowie, P. van Calsteren, T. Wigley, F. J. Cooper, J. M. McArthur. Localisation of Quaternary slip-rates in an active rift in 105 years: an example from central Greece constrained by 234U-230Th coral dates from uplifted palaeo-shorelines. Journal of Geophysiscal Research, 2009, in press.  
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Cowie, P.A., Underhill, J. R., Behn, M.D., Lin, J., Gill, C. E., Spatio-temporal evolution of strain accumulation derived from multi-scale observations of Late Jurassic rifting in the northern North Sea: A critical test of models for lithospheric extension, Earth and Planetary Science Letters, 234, 401-419, 2005    
Davies, R., England, P., Parsons, B., Billiris, H., Paradissis, D., Veis, G., Geodetic strain of Greece in the interval 1892-1992, Journal of Geophysical Research, 102, 24571-24588, 1997.    
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