The Brolo Island, a lentil in the “Ocean”

Abstract

The north-eastern Sicily coast reflects the effects of Holocene active tectonics associated to subduction system of Ionian crust beneath the Calabrian arc (CAPUTO et alii, 1970; WESTAWAY, 1993, DOGLIONI et alii, 1999). The latter, characterized by a stack of crystalline rock and its sedimentary cover, is the highest structural element in the Sicily chain. The Calabrian arc is a region that records one of the major Quaternary vertical tectonic movement in the whole Mediterranean basin. This uplift, well documented from Last Interglacial, is expressed as vertical variation of the height of the Quaternary marine terraces inner margin that characterize the north-eastern Sicily coast. The uplift can be divided into a "continuous" regional component and in an "episodic"coseismic component (FERRANTI et alii, 2007). The northwest Sicily coast shows as the uplift rates estimated for the Holocene have values greater than those estimated at the same point from Last Interglacial (ANTONIOLI et alii, 2006). At the regional scale it is possible to observe a decrease in uplift rates from Cape Peloro (1.1 mm / year) to Palermo and Capo S. Vito (stable). In north-eastern Sicily, between Capo d'Orlando and Capo Calavà, we studied the Brolo Stack, located in the central part of a small bay along the coast facing Brolo village. This area pertains to the northwestern sector of Peloritani Mountains (western portion of the Calabrian arc), which extend in E-W direction from Messina Straits to S. Agata di Militello village. In particular the tectonic style is characterized by tectonically overlapping bodies, generally dipping toward the northern quadrants. The Brolo coastal area is characterized by a wide coastal alluvial plain fed by two big rivers. On this plain rises a metamorphic salient, a sub-circular cylindrical structure, about 50 m high, placed inshore at 250 m from the coastline. In particular, the Brolo Stack is an outcrop of a residual block of impure, medium grain, marble interspersed with Paleozoic paragneiss and mica schists, which largely outcrops in the hinterland. These lithotypes are the crystalline substrate on which develops the current depositional sequence. The Brolo Stack constitutes a "lentil" of metamorphic rock emerging above the 14-18 m deep seafloor at 450 m from the coastline. The objective of this study is to calculate the potential vertical movements and assume when they could have occurred. To do this a detailed geomorphological subaerial and an underwater survey were conducted, which led to discovery a fossils bearing conglomerate (Fig. 1), in protected trays at 3.5 m from sea level, and well preserved lithophaga holes (Fig. 2) at about 70 cm from sea level. The Brolo stack show a particular morphological feature, different in the emerged and submerged parts (Fig. 3). The emerged portion strikes along the E-W trend for a length of about 43 m, being 29 m wide and about 15 m high. It has a mushroom shape with the submerged part more closely than the emerged, the result of combined physical and chemical processes triggered by both sea level rise and tectonic vertical movements. A system of schistosity gives to the white-to-gray stack an apparent stratification organized in decimetric beds with lying 005/60°. The whole structure is fractured along multiple directions often containing dark poorly consolidated volcanic rock. Large angular blocks are found on the seabed at the base of the submerged cliffs; their collapse is the result of mechanical erosion due to the high energy waves and the schistosity and intensely fractured structure of the rocks. The underwater survey allowed us to recognize little terraces at different depth recognized as structural surfaces. We used radiocarbon analysis on a gastropod found in the marine conglomerate (Fig. 4). We provide an age of 4745 +/- 59, ( 4965 years +-70 cal BP using Calib 5 program, Stuiver et al 2005). Based