干旱区生态环境知识资源中心

In this paper, a reconstruction of the long-term geomorphological evolution focused on weathering and exhumation history is proposed for a tectonically active area in a typical upland Mediterranean environment, the Sila Massif (Calabria, southern Italy). Spheroidal boulders and associated landforms are investigated in detail to estimate their depth of formation, usually not well known, to fix the time constraints of their development and subsequent exhumation, and to evaluate corresponding volumes of eroded rock and long-term erosion rates.

Field work aimed at a detailed survey of boulders morphology and their relationships with the soils of the Sila upland plateaus. SEM-EDS analyses were performed to assess the possible occurrence, provenance, and age of volcanic input in a widespread andic soil, correlated with a pedostratigraphic marker, to obtain some chronological constraints. Based on the spatial distribution and the calculated surface area of boulder outcrops in the study area, minimum budgets of eroded materials were roughly estimated as presumed rock volumes ("missing volume") removed by erosion processes between the present topography and a presumed paleotopography (before complete boulder exhumation) marked by the recurrent boulder heights and the presumed thickness of weathering and soil profiles. Respective denudation rates were calculated relative to two time ranges assumed for the onset of erosion phases, and compared to published cosmogenic Be-10 erosion rates.

Exhumed spheroidal boulders occur as boulder fields or associated to bornhardt and tor morphologies, often showing flared slopes and slope breaks across their outline, which testify for a multistep exhumation through time. An andic soil including a parts per thousand currency sign42 ka rhyolitic pumice grains diffusely covers the base of exposed boulders, indicating that boulders had been already exhumed before the volcanic input and corresponding soil formation. The deep spheroidal weathering probably developed during the planation of the high-standing landscape under prolonged geomorphological stability and partly warm-humid climate (lower Miocene and late Pliocene-early Pleistocene), followed by tectonic uplift and erosion at about 800 and 400 ka. The denudation rates calculated for these time spans compared to those obtained by Be-10 suggest a clear underestimation based on the missing volume method.

Cosmogenic nuclide erosion rates permitted to recalculate reliable thicknesses of material eroded from top of the exhumed boulders in the range of about 40-80 m that could be assumed as their potential depths of formation in the Sila Massif.