Discussion of: Casini, G., Decandia, F. A. & Tavarnelli, E. (2007). Analysis of a mesoscopic duplex in SW Tuscany, Italy: implications for thrust system development during positive tectonic inversion. In: Ries, A. C., Butler, R. W. H. & Graham, R. H. (eds.) Deformation of the Continental Crust. The Legacy of Mike Coward. Geological Society, London, Special Publications, vol. 272, pp. 437-446. Casini et al. (2007) describe and interpret the deformation of the Poggio al Carpino Formation at its type-section in the Monticiano-Roccastrada Area of S Tuscany, Italy. The latter is part of the Mid-Tuscan Ridge, which consists of a linear belt of several discretely exhumed, low to medium grade metamorphic core complexes of Tertiary age and contains the Verrucano Group and its Palaeozoic substratum of sedimentary and volcanic rocks. The Poggio al Carpino Formation was interpreted as storm influenced siliciclastic inner shelf deposits, which form the proximal part of highly tectonized relicts of late Palaeozoic continental margin deposits of the Adriatic Microplate (Engelbrecht 2008). Casini et al. state that at the type section of the Poggio al Carpino Formation a late Tertiary duplex contains evidence for an earlier (Triassic?) phase of extension that resulted in boudinage. In their introduction, Casini et al. state that the Monticiano-Roccastrada Area is intersected by a shallow, west-dipping thrust. According to my observations (Engelbrecht 1997: supplement I), this structure is in fact a steeply east-dipping normal fault, because of: 1) the mapped surface termination directions, 2) its association with Quartz-Antimony pocket-mineralization and Pliocene marine basin fragments, both of which are typically associated with normal faults in S-Tuscany (Arisi-Rota et al. 1971; Martini et al. 2001). The tectonic profile and the columnar section published by the authors, are highly simplified and seem incorrect in several points: according to my own observations, the geology is far more complex. The large-scale overturned sequence in their profile is not present and the thrusts indicated are not necessary (Engelbrecht 1997: supplements I, II). Furthermore, Casini et al. state that the Verrucano and its substratum went during late Oligocene through low-grade (greenschist-facial) regional metamorphism. However, according to own observations, there are at least three differently tectonized and metamorphosed subzones, and so this is an over-simplification (Engelbrecht 2008). The authors attribute the Poggio al Carpino Formation into the late Permian to early Triassic. According to its observed heteropy with nearby exposed outer shelf - slope - basin units (Carpineta- and Farma Formations) containing age diagnostic marine faunas, the age of the Poggio al Carpino Formation is regarded as Carboniferous (Engelbrecht 2008). See http://www.umweltgeol-he.de/discussion1.htm. Casini et al. state that "a well developed bedding-parallel foliation with the features of a coarsely spaced stylolytic cleavage is present within the arenaceous layers of the duplex." According to my own observations, the majority of these foliations originated on omnipresent inter- and intrastratal angular unconformities: separation planes representing primary sedimentary layering and sedimentary lamination (Engelbrecht 2000). Evidently the orientations of these transformed layer- and lamination-surfaces occur not only bedding-parallel, as stated by Casini et al., but at various inclinations to the sedimentary layering. The authors further wrote that this foliation is systematically associated with the development of boudinage structure. Boudins can originate in extensional zones, if the effective differential stress is oriented subvertically on alternating strata characterized by differing degrees of competence. This may be realized at I Canaloni, where meta-quartzsandstone is interlayered with schist. Because the latter forms only a minor constituent (ca. 4.5%, Engelbrecht 2000), one basic requirement for the origination of boudins is substantially restricted. Instead, long-wavy pinch-and-swell structures, reflecting faint competence-differences between the quartzsandstone-layers should prevail, impossible to identify in laterally rapidly changing layer thicknesses and sedimentary texture due to unstable depositional conditions with high diversity of energy indexes. The coeval origination of folds and boudins (Zulauf & Zulauf 2005) is not addressed by Casini et al.. According to my own observations, no Triassic systematic boudinage is present at I Canaloni. Within coeval, but regularly interbedded lithologies of the Carpineta Formation ca. 6,5 km east of I Canaloni, cm- to dm-sized boudins are present, which I interprete as middle- to late Tertiary in age. The authors state that "the close relationship between ubiquitous bedding-parallel foliation and boudinage structures indicates that the Poggio al Carpino Formation experienced extensional deformation after deposition ... and that ... the age of that extension is probably Triassic....." Casini et al. neither analyzed the type of boudinage (Mandal et al. 2000) nor estimated the magnitudes of the geological variables acting on the Poggio al Carpino Formation in the assumed passive margin setting of the late Triassic Tuscan Domain: the intensities of the (1) effective differential stress, (2) geothermal heat flow and (3) fluid flow. There is only indirect information about the thickness of the sediments deposited above the Poggio al Carpino Formation: according to published data, the pile measures ca. 2300m. But the fact that recent stress-plunge-directions in equivalent tectonic settings gained by analyses of borehole breakouts and focal mechanisms - documented in the World Stress Map http://dc-app3-14.gfz-potsdam.de/ - scatter widely, the first essentials for the origination of boudins are not favourable. Further, it is not explained, how these Triassic boudins can be distinguished from younger boudins, because extension in the N-Apenninic sedimentary basin continued during Jurassic and early Cretataceous times (drifting stage, documented by the Ligurian and Subligurian Domain). Neptunian Dykes - e. g. in the Devonian Cephalopode-Limestone and late Triassic Dachstein-Limestone - and disintegration of the basin fill into horsts and grabens reflect the extensional movements in rift settings. According to own observations, the stacked tectonic thrusts plotted in the Fig. 4 of Casini et al. 2007 do not exist: no thrust-related tectonic offsets are visible and the trailing branch point X is not present; see Figure below:
Therefore the postulated Tertiary duplex at I Canaloni is not present; instead the tectonic structure under consideration is seen as a Tertiary ramp fold overprinted by subsequent tectonic dilatation. It is maintained that the tectonometamorphic structures mentioned above – foliation, folds and boudins - developed during Oligocene-Miocene subduction-related thrusting, tectonic burial to ca. 20km and subsequent exhumation (Engelbrecht 2008). A better tectonic interpretation of the situation at I Canaloni will be presented here soon. Therefore the theses and opinions presented in Casini et al. (2007) must be rejected. It made no sense to publish that again - slightly modified and expanded - in the BSGI (Italian Journal of Geosciences). References:
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