Evoluzione morfotettonica quaternaria del bacino intermontano di Sanza (Appennino meridionale)

Abstract

Di Leo P., et al., Quaternary morphotectonic evolution of the Sanza intermontane basin (southern Apennines, Italy. (IT
ISSN 0394-3356, 2009).
Opening kinematics and morphotectonic evolution of the Sanza catchment, a Quaternary basin of the southern Apennines axial zone,
have been studied. Besides geomorphological classical analyses, an interdisciplinary approach has been used here joining structural
analysis of brittle deformation, mineralogical and chemical analyses of paleosoils and sediments, anisotropy of magnetic susceptibility
(AMS), and tephra radiometric dating. Several Pleistocene morpho-sequences have been outlined, also comparing them with a wider
scenario on a regional scale.
The Sanza basin is hosted in an E-W trending narrow lacustrine trough. This morpho-structural low is a lateral branch of the wider Vallo
di Diano basin, characterized by a multi-stage morphotectonic evolution. Red clays and associated palaeosols from the Sanza basin
have been studied from a geomorphological viewpoint and mineralogical and chemical characteristics have been analyzed to set the
palaeoclimate and palaeoenvironmental conditions during middle to late Pleistocene time-span. The Pleistocene morpho-sequence of
the study area has been outlined, also comparing the stratigraphic succession of the Sanza basin with those from the adjacent Vallo di
Diano basin, mainly on the grounds of tephra radiometric dating.
Survey and mapping of remnants of land surfaces, organized in six altimetric classes, permitted us to reconstruct the Pleistocene geomorphological evolution of the relief. Using the surface at the top of the Sanza deposits as a reference marker with regard to the other orders of terraced surfaces, it has been possible to include the genesis and evolution of the basin in a larger time-span and to relate its structural history with regional and local tectonic episodes. In particular, the E-W trending trough was generated in a tectonically-controlled narrow transversal strip due to the activity of a transfer zone of the chain. The changing tectonic regime was responsible for the endorheic conditions reached by the depression during mid-Pleistocene times. In such a basin, lacustrine red clays and fan conglomerates were deposited. To the top, weathered horizons and palaeosols with interbedded tephra have been recognized and analysed.
The mineralogy and geochemistry of 14 pelitic sediments allowed us to delineate the climate condition which characterized the sedimentation in this basin. The clay minerals association observed in the analyzed sediments, mainly constituted by illite/smectite - chlorite/smectite mixed layers, vermiculite, kaolinite+halloysite and goethite+hematite, coupled to high values of CIA (Chemical Index of Alteration; values > 85), are compatible with an high degree of weathering. The increase of mineral phases such as kaolinite, goethite and hematite and decrease of clay minerals abundances such as vermiculite and illite/smectite - chlorite/smectite mixed layers in the younger sediments (late Pleistocene) with respect to the older ones (middle Pleistocene) are indicative of a palaeoclimate scenario characterized by temperate conditions with dry-wet periods which evolves toward warm and humid conditions.
The presence of 7Å halloysite exhibiting mainly a spheroidal habitus and a large amount (up to 5%) of feldspars are indicative of the
presence of a volcanoclastic input in the Sanza basin. Major/trace element ratio relationships and multivariate statistical analysis
(Principal Component Analysis method) of geochemical variables strongly support the presence of the volcanoclastic input.

Mineralogical and geochemical comparison of the samples with altered middle Pleistocene volcanic products from the first cooling unit
of the Roccamonfina volcanic complex (630-385 ka, Roccamonfina Supersynthem), used as an indirect dating, allowed us to temporally confine the fluvial-lacustrine Sanza succession. Moreover, 40Ar/39Ar radiometric dating of sanidine crystals from tephra layers interbedded into the nearby Vallo di Diano lacustrine succession - correlated to the fluvio-lacustrine Sanza succession - allows us to chronologically constrain the top and the bottom of the Sanza succession (706.3±8.1 ka e 106.7±1.6 ka).
The Sanza basin can be assimilated to a morphostructural trough coinciding with a long-term transfer zone of the chain, in which a
severe change in stress field, uplift rate, climate conditions, and relief production occurred during Pleistocene times. In particular, it
can be stated that the study area underwent a climate shift from temperate to warm/humid conditions. Furthermore, the fluvio-lacustrine Sanza succession age is univocally set to the middle-late Pleistocene.