Alps Nw Italy

Int J Earth Sci (Geol Rundsch) (2003) 92:758–778 DOI 10.1007/s00531-003-0351-x ORIGINAL PAPER Chiara Barbieri · Barbara Carrapa · Andrea Di Giulio · Jan Wijbrans · Glen R. Murrell Provenance of Oligocene synorogenic sediments of the Ligurian Alps (NW Italy): inferences on belt age and cooling history Received: 19 June 2003 / Accepted: 19 June 2003 / Published online: 11 September 2003 Springer-Verlag 2003 Abstract Minera l chemi stry, 40 Ar/ 39 Ar geoch ronolo gy on wh i te mi cas an d A pat it e Fi ss io n Tra ck Th er - moc hro nol ogy (AFTT) , are app lie d her e to study the provenance of the synorogenic Molare Formation (low- er most unit of the Te rti ar y Pi edmont Basin clastic sequence). The Molare Formation was deposited during transgression onto the Ligurian Alps nappe stack in the Ear ly Olig ocene. Dep osi tion al fac ies sho w tha t cla sti c dis trib ution remained tra nsv ers al, wit h loc al sou rce s located just landward from the coastline. Phengite mineral chemistry together with 40 Ar/ 39 Ar data clearly shows two distinctive source areas, each one mirroring the compo- si ti on of the ba se me nt di rect ly be ne at h the cl as ti c sequ ence. Amphi bole minera l chemistry allows second order provenance distinctions within each sector, reflect- ing heterogeneous metamorphic evolution of the bedrock complexes. Integrated 40 Ar/ 39 Ar dating and AFTT suggest that, following a fast cooling/exhumation episode of the Ligurian Alps during the Oligocene, very little net uplift has sinc e occ urr ed. This is due to a per iod of gen era l subs idenc e from the Oligoc ene– Late Mioce ne followed by comparable uplift from Late Miocene–Pliocene to the pr es ent. In ge ne ral our da ta pr ovide an image of the Ligurian Alps during the Oligocene, which is very similar to the present-day one. Keywords Ligurian Alps · Transgression · Provenance · Mineral chemistry · Thermochronology Introduction A multid iscip linary prove nance study inclu ding facie s analysis, white mica and amphibole chemistry, 40 Ar/ 39 Ar dat ing and Apa tite Fis sio n Tra ck The rmochr ono logy (AFTT) is performed to constrain the provenance of the Molare Formation, giving an image of the Ligurian Alps geology during the Oligocene, and providing data on the cooling/e xhu mat ion his tory of the bel t forming uni ts. Provenance studies of syno rogen ic sequ ence s are com- monly used to constrain the cooling/exhumation history of moun tain belts. This approach allows recogniti on of belt forming units that are different from what is observed today, due to the combined effects of erosion and tectonic unroofing which leads to near-surface tectonic units being dis man tle d and dee p uni ts coming to the sur fac e (e. g., Critelli and Le Pera 1994; Garzanti et al. 1996; Cibin et al. 2001). Minera l chemistry and thermo chron ology on detrita l minerals have been increasingly used in recent years as provenance tools (e.g., Najman et al. 1997; Copeland and Harrison 1990; von Eynatten and Gaupp 1999; Sircombe 199 9; Spi ege l et al. 2001; von Eynat ten and Wijb rans 2003). They can be used to complement traditional point- counting analyses on sandstone framework grain compo- sition and heav y minera l assoc iation s (Helle r and Frost 198 8, Ren ne et al. 199 0; Ibb eke n and Schle yer 199 1; Dallme ye r and Neub au er 1994 ; vo n Eyna tt en an d Wijbrans 20 03 ; Ha nd le r et al. 1997; Najman et al. 1997; Winkler et al. 1997; Dunkl et al.1998; Di Giulio et al. 1999). In pa rti cular, 40 Ar/ 39 Ar an d AF TT gi ve fundamental new information for the reconstruction of the tectonic-thermal evolution of eroded orogenic belts (e.g, Har riso n et al. 199 3). However, whe n usi ng this tech- niq ue, the following pos sible sou rce s of err or mus t be carefully con sid ered: (1) the dis trib uti on of spe cifi c minerals in sediments depends strictly on the occurrence of source rocks, which possibly provides only a partial picture of the source area; (2) the distribution of heavy min era ls and certa in light minera ls, suc h as mic as, is str ong ly inf lue nce d by the ir hyd rodyna mic beh avi or C. Barbieri Dipartimento di Scienze della Terra, Università di Pavia, 1 via Ferrata, 27100 Pavia, Italy B. Carrapa · J. Wijbrans · G. R. Murrell Faculteit der Aardwetenshappen, Vrije Universiteit, 1085 De Boelelaan, 1081 HV Amsterdam, The Netherlands A. Di Giulio ( ) ) Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy e-mail: [email protected] Tel.: +39-0382-505852 Fax: +39-0382-505890

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Alps Nw Italy