The Early Stages of the Alpine Collision: an Image Derived from the Upper Eocene–Lower Oligocene Record in the Alps–Apennines Junction Area
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Sedimentary Geology 171 (2004) 181–203 www.elsevier.com/locate/sedgeo The early stages of the Alpine collision: an image derived from the upper Eocene–lower Oligocene record in the Alps–Apennines junction area B. Carrapaa,*, A. Di Giuliob, J. Wijbransa aDepartment of Isotope Geochemistry, Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, Amsterdam 1081 HV, The Netherlands bDipartimento di Scienze della Terra, Universita` di Pavia, Via Ferrata 1, Pavia 27100, Italy Received 15 October 2003; received in revised form 1 March 2004; accepted 12 May 2004 Abstract The upper Eocene–lower Oligocene sediments deposited in the eastern part of the Tertiary Piedmont Basin in northern Italy provide a complete record of the unroofing of the Alpine orogenic prism during the early stages of exhumation in the Ligurian sector. From late Priabonian till late Rupelian time, the sediments in the study area were derived from two different sources, one characterised by white micas with Sib6.5 pfu and Permian 40Ar/39Ar ages (270 Ma), and the other characterised by white micas with SiN7 pfu and Eocene–Oligocene 40Ar/39Ar ages (32–50 Ma). The first source is considered to be indicative of low-pressure metamorphic rocks that covered the HP rocks of the Ligurian Alps, and were completely eroded by Chattian time. From this time on, the study area started to record the first input from western Alpine sources characterised by a larger span of ages with a more frequent Eoalpine signal. Thus, sediments deposited in the eastern part of the Tertiary Piedmont Basin contain the only available evidence of rocks belonging to high crustal levels in the Alpine orogenic prism that were not affected by the Alpine overprint. These data also provide time constraints to the poorly dated first conglomerates deposited in this area. 40Ar/39Ar geochronology reveals a minimum age of 33F1.4 Ma for the Pianfolco Conglomerates in the type locality, and of 31.4F3.5 Ma for the Borbera Conglomerates. D 2004 Elsevier B.V. All rights reserved. Keywords: Provenance; Ligurian Alps; 40Ar/39Ar geochronology; Cooling/exhumation; Paleogeography 1. Introduction q * Corresponding author. Present address: Institut f r Geo- Examining the provenance of clastic sediments wissenschaften, Universit7t Potsdam, Karl-Liebknecht-Str. 24/H25, 14476 Golm, Potsdam 14415, Germany. Tel.: +49 331 977 5078; derived from orogenic belts is a classical tool for fax: +49 331 977 5060. unravelling the evolution of collisional systems E-mail address: [email protected] (B. Carrapa). (Dickinson, 1974; Dickinson, 1985). Substantial 0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2004.05.015 182 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 advancements have been made in this field through The area has been extensively studied, mostly with the the application of mineral chemistry and geochronol- aim of unravelling the tectonic evolution of this ogy to clastic minerals, as these can provide informa- geologically complex region (Cavanna et al., 1989; Di tion on the cooling and exhumation paths of the Giulio, 1996; Di Giulio and Galbiati, 1995; Mutti et eroded rock units (Heller and Frost, 1988; Copeland al., 1995; Vanossi et al., 1994). and Harrison, 1990; Renne et al., 1990; Harrison et Recently, the provenance of clastic sediments in al., 1993; Najman et al., 1997; von Eynatten and the eastern TPB has been systematically investigated, Gaupp, 1999; Najman et al., 2001; Sherlock, 2001; in order to improve models of paleogeographic White et al., 2002; von Eynatten and Wijbrans, 2003). evolution of the orogenic system following collision Recently, this approach has been applied successfully (Cibin et al., 2001, 2003; Di Giulio and Galbiati, to clastic sediments deposited in the southern part of 1995; Gnaccolini, 1974; Gnaccolini and Rossi, 1994; the Piedmont Tertiary Basin (TPB) in northwestern Martelli et al., 1998). Sandstone petrography in the Italy (Fig. 1; Barbieri et al., 2003; Carrapa et al., 2003, study area suggests a possible low-pressure source for 2004). these sediments with south Alpine affinity, which The current study focuses on the eastern margin of were not affected by late Alpine metamorphism. the TPB, which is located on the tectonic junction Presumably, these sediments were once located on between the Ligurian Alps and the northern Apen- top of the Ligurian Alps and are presently completely nines (Fig. 1). Here, the clastic succession uncon- missing (Di Giulio, 1991). However, the lack of formably covers the Ligurian Alps to the south and thermochronological data has so far limited the contains the oldest sediments deposited in the TPB. validity of this proposition. If correct, this would Fig. 1. Geological map of the Alps (modified from Polino et al., 1990). A: Adula nappe; Ad: Adamello; AU: eastern Austroalpine cover and basement nappes; B: Bergell; DI: Dinarides; EW/TW/RW: Engadina, Tauern, and Rechniz windows; HE: Ultrahelvetic, Helvetic, and Dauphinois units; LA: Ligurian Alps; LPN: lower Penninic nappes; MR/GP/DM/S: upper Penninic Monte Rosa, Gran Paradiso, Dora Maira and Suretta nappes; NCA: northern calcareous Alps; PF: Penninc front; SA: southern Alps; SB: Gran St. Bernard nappe; SC: Subalpine chains; SL/ DB: western Austroalpine Sesia Lanzo and Dent Blanche nappes; VG: Voltri Group. TPB: Tertiary Piedmont Basin; inset square: study area reported in Fig. 2. B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 183 mean that the sediments deposited in the eastern part the eastern part of the TPB. Such a maximum of the TPB record the unique signal of rocks once estimate of the depositional age can be obtained exposed at the top of the Alpine edifice, which would (e.g. Najman et al., 2001) under the assumption have important paleogeographic implications. Also, that the depositional age of sediments cannot be the sediments preserved in the eastern part of the TPB greater than the 40Ar/39Ar ages of the detrital are the oldest sediments preserved in the study area, micas. This will be the case when no alteration meaning that their investigation would provide and/or resetting of the micas occurred after information on the paleogeography of the belt during deposition. the earliest, late Eocene steps of belt evolution after (2) To attempt a paleogeographic reconstruction of collision. Ultimately, continental conglomerates of the study area during the late Eocene–early uncertain ages deposited in the eastern TPB are here Oligocene. Provenance discrimination of the analysed with the aim of assessing a maximum investigated sediments is made in order to depositional age for these sediments (Najman et al., confirm the presence of sources with south 1997, 2001). Alpine affinity as previously proposed from Detrital mineral chemistry and 40Ar/39Ar thermo- sandstone petrography (Di Giulio, 1991). This chronology has been performed on continental to aim is pursued by looking at the white mica transitional and shallow marine sediments of the geochemical signal together with the 40Ar/39Ar Molare Formation which form the base of the south- detrital populations recorded by the studied ern part of the TPB (Barbieri et al., 2003). Detrital sediments. Differences in major element geo- 40Ar/39Ar ages in these sediments suggest two local chemistry and in 40Ar/39Ar age families reflect sources located in the Ligurian Alps. The first is the contribution in composition and ages present characterised mainly by high-pressure (HP) rocks and in the original source area surface at the time of Eocene–Oligocene 40Ar/39Ar ages (32–45 Ma) sediment deposition. recording the exhumation of deep crustal levels of the original orogenic prism. The second is charac- These objectives will be met through the integrated terised by low-pressure (LP) rocks and Carboniferous study of mineral chemistry and 40Ar/39Ar thermo- ages (Barbieri et al., 2003). In particular, the youngest chronology of clastic white micas, sampled in the 40Ar/39Ar detrital signal suggests a fast episodic lowermost part of the succession in the eastern part of cooling event occurring sometime in the Oligocene the TPB, where late Eocene sediments occur at the Ligurian belt (Barbieri et al., 2003; Carrapa et al., very base. 2003). However, due to a lack of paleontological markers in the mainly continental sediments of the Molare 2. Stratigraphic framework and sample strategy Formation, this formation has only a loosely defined early Oligocene age (Gnaccolini, 1974; Barbieri et al., The TPB is an episutural basin located in a complex 2003 and referenced therein) which consequently tectonic area that represents the boundary between the prevents a detailed provenance discrimination. On Alpine and the Apennine thrust belts (Fig. 2). The the other hand, the mainly marine sediments preserved stratigraphy of the area is complex, compounded by in the easternmost part of the TPB (e.g. Ranzano inconsistency in the published literature (Fig. 3). In Formation and Rigoroso Marls) are biostratigraphi- this study, we will use the stratigraphic scheme of Di cally well dated (Di Giulio et al., 2002; Mancin and Giulio (1991) integrated with other studies reported in Cobianchi, 2000; Mancin and Pirini, 2001; Martelli et Fig. 3. Biostratigraphic ages of the formations consid- al., 1998), allowing a more robust constraint on the ered in this study are given using works reported in time of cooling of the source area. Table 1 and the geological timescale of Haq and Van The aims of this study are: Eysinga (1998). In the western sector of the eastern TPB, (1) To better constrain the time of sedimentation of sedimentation was perceived to have started in the the poorly dated conglomerates outcropping in upper Eocene–early Oligocene, with continental to 184 B. Carrapa et al. / Sedimentary Geology 171 (2004) 181–203 Fig. 2. Sample locations with specification of the stratigraphic columns studied by Di Giulio (1991) reported in Fig. 3.