Phreatic overgrowths on speleothems: a useful tool in structural geology in littoral karstic landscapes. The example of eastern Mallorca (Balearic Islands)
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Along the eastern coast of Mallorca, many littoral caves partly filled with brackish waters occur. The most peculiar aspect of these caves is the presence of abundant phreatic overgrowths formed on pre-existing supports located at the underground pools' water table, which corresponds to the present sea level. Besides a specific geomorphological interest, these subaqueous speleothems provide an excellent record of Quaternary sea level stands. The clear relation between phreatic speleothem growth and the contemporary sea level allows the control of the tectonic evolution of an area, by comparing speleothems' ages and heights with the regionally established eustatic curves. In the studied region different altimetric positions of coeval phreatic speleothems suggest the existence of a recent tectonic activity. The characteristics and chronology of this tectonic event are the objectives of this paper, pointing out at the same time the potential of phreatic speleothems in structural geology investigations. Along the coastline of the studied area, alignments of phreatic speleothems attributed to high sea stands 5a, 5c and 5e are recorded at increasing elevations northwards. This is an evidence of a significant tectonic tilting that took place, at least partially, after substage 5a because phreatic speleothems of this substage are now located at different altitudes. Considering that tectonic tilting has been continuous from post-substage 5a (approximately 85 ka) until now, and that normal displacement is approximately of 1.5 m, the average minimum velocity of the tilting can be estimated about 0.02 mm/year in the southern part with respect to the north end. Data obtained from phreatic speleothems have been compared with other regional, stratigraphical, geomorphological and tectonic evidence that all together point to the same existence of the postulated tectonic tilting. Consequently, phreatic speleothem investigation results in a new method that allows the quantification of average velocities of tilting as well as other