Holocene tectonic uplift patterns in northeastern Sicily: evidence from marine notches in coastal outcrops

Marine notches are groove-like features formed in rocky coastal outcrops in relation to past sea levels, and are widely used as indicators of rates and patterns of tectonic activity. Examination of uplifted notches in the complex plate boundary setting of northeast Sicily reveals the need for considerable care in their interpretation. Present notch-forming processes are dominated by carbonate dissolution, aided by microbial activity, as waves surge and recede at the base of the cliffs, and produce notches approximately 0.5 m above present biological mean sea level (i.e. above an organic constructional rim of coralline algae, used to define sea level). At sea level, dissolution is restricted through saturation and the bedrock protected by the outbuilding algal rim. This assemblage varies in response to fetch exposure and uplift rate, typically producing a complex band of uplifted notch features exposed above sea level. Correlations (based on height) of the shallow and discontinuous notch indentations within this band are unreliable. However, the upper margin of the band, marked by the roof of the uppermost notch, is a prominent and laterally persistent feature easily recognised as the boundary between subaerial and marine processes. We propose that this is an approximately coeval datum formed when post glacial sea-level rise decelerated in the mid Holocene to match the rate of tectonic uplift, a notion strengthened by the available dates of about 5 ka for this feature. Subsequent uplift outpaced later sealevel rise, taking the notches above sea level. This proposed datum varies in height from about 2 m at Capo Milazzo (on the north coast) to about 5 m on the east coast bordered by the Messina fault system. The height differential is consistent with the respective elevations of nearby marine terraces dating from the last interglacial at about 125 ka, but also exhibits shorter term variations suggesting Holocene increases in activity rates on more localised structures affecting the localities involved. Regionally, northeast Sicily is characterised by long term differential uplift rather than behaving as a coherent block. The highest uplift rate, probably exceeding 2 mm a(-1) (calculated using the sea-level curve from Fairbanks, R.G., 1989. A 17000-year glacio-eustatic sea level record; influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation. Nature 342, 637-642), is suggested for a notch locality on the upthrown side of the Malta Escarpment. Rates of about 2 mm a(-1) are indicated by the 5 m notch at localities bordered by the Messina fault system, and a rate close to 1 mm a(-1) is suggested for the (undated) 2 m notch at Capo Milazzo. (C) 2000 Elsevier Science B.V. All rights reserved.