Geology of the Entracque-Colle Di Tenda Area (Maritime Alps, NW Italy)
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AperTO - Archivio Istituzionale Open Access dell'Università di Torino Geology of the Entracque-Colle di Tenda area (Maritime Alps, NW Italy). This is the author's manuscript Original Citation: Availability: This version is available http://hdl.handle.net/2318/1525678 since 2017-05-16T11:27:14Z Published version: DOI:10.1080/17445647.2015.1024293 Terms of use: Open Access Anyone can freely access the full text of works made available as "Open Access". Works made available under a Creative Commons license can be used according to the terms and conditions of said license. Use of all other works requires consent of the right holder (author or publisher) if not exempted from copyright protection by the applicable law. (Article begins on next page) 23 September 2021 This is an author version of the contribution published on: Questa è la versione dell’autore dell’opera: Journal of Maps, 2015, DOI: 10.1080/17445647.2015.1024293 ovvero Barale et al., Taylor & Francis, 2015, pagg.1-12 The definitive version is available at: La versione definitiva è disponibile alla URL: http://www.tandfonline.com/doi/full/10.1080/17445647.2015.1024293 1Geological map of the Entracque–Colle di Tenda area (Maritime Alps, NW Italy) 2Luca Baralea*, Carlo Bertoka, Anna d’Atria, Luca Martirea, Fabrizio Pianab, Gabriele Dominic 3a) Dipartimento di Scienze della Terra, Università degli Studi di Torino, Via Valperga 4Caluso 35 - 10125 Torino, Italy. [email protected], [email protected], [email protected], [email protected] 6b) Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche, Via Valperga 7Caluso 35 - 10125 Torino, Italy. [email protected] 8c) Via degli Olmi 4 - 10023 Chieri (TO), Italy. [email protected] 9* Corresponding author. 10 11Acknowledgements 12The research was supported by funds from the CNR–IGG (Unità di Torino). Alessia Musso 13is acknowledged for sharing field work in the early phases of the research. We would like 14to thank the Associate Editor Claudio Riccomini, and the Referees Riccardo Bersezio, 15Silvio T. Hiruma and Thomas Pingel, whose useful suggestions really improved both the 16manuscript and the geological map. 1 1 2 17Abstract 18The 1:25,000 geological map of the Entracque–Colle di Tenda area covers an area of 19about 130 km2 in the Italian Maritime Alps, between the Gesso and Vermenagna valleys. 20The map area is of great relevance since the Alpine units of this region sampled a 21geological nodal point in the Mesozoic, at the transition between two different 22sedimentation domains of the Alpine Tethys European paleomargin (the Dauphinois basin 23to the NW and the Provençal platform to the SE). During the Cenozoic, this 24paleogeographic hinge was progressively incorporated along multiple shear-zone systems 25developed at the southern termination of the Western Alps arc. 26The realization of the map was made possible by studies concerning both the stratigraphic 27and structural setting of the area, which have been (or are being) published in several 28international journals. 29 30Keywords: Provençal Domain, Dauphinois Domain, Argentera Massif, Limone–Viozene 31Zone, Maritime Alps 32 331. Introduction 34The 1:25,000 geological map of the Entracque–Colle di Tenda area is the result of original 351:10,000 scale surveys performed in the last ten years at the southern termination of the 36Western Alps Arc (NW Italy) over an area significantly wider than the one here 37represented. This document is thus the first published detailed map in the frame of a 38scientific program focusing on the revision of the stratigraphic and tectonic setting of a 39large area encompassing, from NW to SE, the Italian side of the Maritime Alps and the 40westernmost part of the Ligurian Alps, the main results of which were just published 41(Barale et al., 2013; Bertok, Martire, Perotti, d’Atri, & Piana, 2011, 2012; Martire, Bertok, 3 2 4 42d’Atri, Perotti, & Piana, 2014; Perotti et al., 2012; Piana et al., 2009, 2014; d’Atri, Piana, 43Barale, Bertok, & Martire, submitted). 44The map area extends from the left side of the Gesso valley (NW) to the Colle di Tenda 45area (SE) (see Structural Scheme in the Map). It is composed of several tectonic units, 46separated by NW–SE striking Alpine tectonic contacts. Their geometrical position has 47been classically interpreted to reflect the paleogeographic position along the Mesozoic 48European paleomargin of the Tethys, thus the lowest units were attributed to the more 49external Dauphinois and Provençal domains, whereas the overlying units to the more 50internal Subbriançonnais Domain (Carraro et al., 1970). On the whole, they presently form 51a SE–NW trending narrow belt, comprised between the Argentera Massif to the SW and 52the more internal Briançonnais and Western Ligurian Helminthoides Flysch units to the 53NE. However, Barale (2014) and d’Atri et al. (submitted) showed that the stratigraphic 54succession of the uppermost tectonic unit of our map area (Colle di Tenda unit sensu 55Carraro et al., 1970; Roaschia unit in this work, see section 3) has striking similarities with 56the Provençal ones (see Chapter 4). For this reason, in this paper the Roaschia Unit has 57been attributed to the Provençal Domain, in agreement with Maury and Ricou (1983) 58which questioned the existence, in the study area, of the Subbriançonnais 59paleogeographic domain. In the map area the transition between the Dauphinois and 60Provençal domains occurs across paleoescarpments placed transversally to the contacts 61between the main tectonic units as, for instance, within the Entracque Unit (see Chapter 625). 63Since middle Eocene–early Oligocene the paleo-European continental margin was 64progressively involved in the on-going formation of the Alpine belt. All the studied 65successions underwent at least three deformation events well recorded at regional scale: a 66first event with S- and SW-ward brittle–ductile thrusting and superposed folding, a second 5 3 6 67event with NE-vergent folding and a third event with southward brittle thrusting and flexural 68folding (Brizio et al., 1983; Piana et al., 2009). The overall regional kinematic was 69displayed, at least for the two last deformational stages, in a transpressional regime with 70important strain partitioning of contractional vs. strike-slip related structural associations 71(Molli et al., 2010; Piana et al., 2009; d’Atri et al., submitted), as evidenced by the 72occurrence of the Limone Viozene Zone (LiVZ, a post-Oligocene NW–SE Alpine 73transcurrent shear zone extending for several kilometres from Tanaro valley to the eastern 74margin of the study area) and of the E–W strike-slip shear zone system generally known 75as Stura Fault (Ricou & Siddans,1986) and here referred as Demonte–Aisone shear zone 76(DAZ). 77The study area was previously mapped in the Demonte sheet of the Geological Map of 78Italy at 1:100,000 scale (Abiad et al., 1970, explanatory notes by Crema, Dal Piaz, Merlo, 79& Zanella, 1971), and in the Geological Map of the Argentera Massif at 1:50,000 scale 80(Malaroda, 1970, explanatory notes by Carraro et al., 1970). 81The aims of the map are: 82 to represent in detail the lithological and structural features of the stratigraphic 83 successions at the northeastern side of the Argentera Massif; 84 to characterize and represent the stratigraphic transition between the Dauphinois 85 and Provençal successions; 86 to identify and map the tectonic structures connecting the LiVZ (occurring to the E 87 of the study area) with the DAZ (occurring to the W); 88 to map in detail the carbonate rock bodies affected by dolomitization or 89 recrystallization. 90 912. Methods 7 4 8 92The geological map is drawn at 1:25,000 scale and covers an area of approximately 130 93km2. Data were collected through fieldwork at 1:10,000 scale, stored in a GIS database 94(Coordinate System WGS 1984 UTM, Zone 32N), and represented on a vector 95topographic map, which for the most part derives from the CTRN (Vector Regional 96Technical Map) of Piedmont, vector_10 series, edition 1991–2005 (1:10,000 scale) (see 97Main Map for further details). 98Geological survey of the main lithostratigraphic units has been integrated by observations 99on mesoscale geological features and by microscale analysis of rocks in thin sections, 100whose results are fully reported in Barale (2014) and Barale et al. (2013). Orthophoto 101analysis ( 2012 AGEA–Agency for Agricultural Supply orthophoto; web map service 102available at http://wms.pcn.minambiente.it/ogc? 103map=/ms_ogc/WMS_v1.3/raster/ortofoto_colore_12.map) was also performed to precisely 104map the boundaries of Quaternary deposits. A tectonic sketch map at 1:150,000 scale, 105attached to the map, shows the main tectonic units of the study and adjoining areas (see 106also d’Atri et al., submitted). The Permian–Lower Cretaceous units, which had no formal 107names in the literature, have been labeled referring to toponyms within or adjoining the 108study area, whereas, for the Aptian–Upper Cretaceous succession and the Alpine 109Foreland Basin succession, the names existing in the previous literature have been 110utilized. 111 1123. Structural setting 113The two main macroscale tectonic features of the map area, both NW–SE striking, are: the 114Serra Garb Thrust (SGT) in the northwestern part and the Limone–Viozene Zone (LiVZ) in 115the eastern and southeastern parts (see Structural Scheme in the Map). 9 5 10 116The SW-vergent Serra Garb Thrust caused the superposition of the Roaschia Unit, 117characterized by a Provençal succession, onto the Entracque Unit (section A–A’ in the 118Map) that consists of two distinct stratigraphic successions: Dauphinois in the 119northwestern sector and Provençal in the southeastern sector. These two sectors are 120separated by a Early Cretaceous paleoescarpment (probably active since the Jurassic), 121partially preserved in the Caire di Porcera locality (see Chapter 5) despite its relative 122closeness to the multiple shear zones developed in the footwall of the SGT (section B–B’ 123in the Map).