Struttura, esumazione ed evoluzione morfologica del nucleo mesozoico del Monte Motola (Cilento, Italia meridionale)

Bollettino- Societa Geologica Italiana (Impact Factor: 1). 01/2008; 127(3):477-493.


Il Monte Motola è un rilievo formato da carbonati di mare basso prevalentemente mesozoici e circondato da un'aureola di terreni argillosi e calcareo-marnosi più facilmente erodibili e tormentati da un diffuso dissesto. È posto a cavallo della porzione più interna del promontorio del Cilento (Campania, Italia meridionale) e della depressione del Vallo di Diano ed è perimetrato da un complesso sistema di faglie transpressive ed estensionali certamente attive in tempi plio-quaternari che, unitamente all'azione erosiva del reticolo idrografico, hanno consentito l'esumazione del nucleo mesozoico del rilievo. Lo studio morfotettonico ha permesso di stimare a circa 0.6 mm/a il tasso medio di sollevamento della struttura durante il Quaternario, in buon accordo con quanto noto per la zona assiale dell'Appennino meridionale. L'area del Monte Motola si è rivelata una zona di concentrazione della deformazione nel contesto della transpressione tardo-pliocenica operante su un'area ben più vasta dell'Appennino meridionale, e tale sembra essere rimasta anche nella successione dei regimi deformativi quaternari. Tale peculiarità appare ereditata dallo scenario paleogeografico mesozoico in cui l'elemento corrispondente all'attuale dorsale del Monte Motola-Monte Vivo doveva già rappresentare un moderato alto strutturale, capace tuttavia di generare condizioni di anisotropia atte a condizionare l'intera storia tettono-evolutiva dell'area.

ABSTRACT: Geology, geomorphology, and exhumation modalities of the Monte Motola structure, Cilento region, southern Italy. Southern Italy underwent a poly-phase tectonic history from late Oligocene to Quaternary times, after the positive inversion of the Apulian segment of the African Mesozoic passive margin. «External» structural units derived from this important palaeogeographic domain and their deformation started in the early Miocene, when the Liguride ophiolite-bearing accretionary prism and the Sicilide Complex (i.e. the assemblage of the «Internal units») overthrust the westernmost portion of the continental margin. This was articulated in a series of platforms and basins where also more restricted palaogeographic elements, such as emerged zones of a platform, structural highs, seamounts and narrow troughs, were present in different times. Monte Motola is a sharp and narrow positive morphostructure made of Mesozoic shallow-water carbonates surrounded by Cretaceous to Miocene «internal» clayey units, largely affected by landslides. This area is located in the inner part of the Cilento promontory and bordered to the east by the Quaternary intermontane basin of the Vallo di Diano. Although Monte Motola has lithology and elevations similar to the calcareous massifs located to the
north (Alburni Mts) and to the south (Mt. Cervati), it is morphologically separated from the other mountains by two narrow troughs in which shaly and marly units crop outs, forming a contrasting soft landscape. The calcareous ridge of Monte Motola-Monte Vivo is in fact completely bordered by upper Pliocene to Quaternary transpressional and extensional faults that – together with the activity of the fluvial net – allowed the exhumation of its Mesozoic core. The progressive lateral migration of part of an ancient hydrographic network, expressed by a meandered and weakly intrenched stream and marked by several other relics of fluvial channels, related to the organization of erosional land surfaces in different orders as the erosion base level changed, allowed to reconstruct the
main steps of exhumation and morphological evolution of the ridge and to calculate the uplift/erosion rates during the last 2.5 Ma. The mean value is ca. 0.6 mm/a, in good agreement with those calculated in previous contributions for the axial zone of the southern Apennines. This study also revealed that Monte Motola is a zone of strain
concentration in the context of the late Pliocene transpression which affected a lager sector of the southern Apennines. Quaternary tectonics inherited such a peculiarity, by amplifying the uplift and the separation of the mountain by the rest of the relief. The anisotropy acquired during Cretaceous times and caused by palaeotectonic extensional faulting seems to be the reason for the differential behaviour of the study area with respect to the adjacent massifs. Indeed, in the Mesozoic palaeogeographic scenario Monte Motola
could have represented a minor structural high bordered by normal (or transtensional) faults, probably characterized by moderate offsets but yet able to generate a textural grain that since influenced the successive local tectonic history.

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