Geological Society of America Special Paper 378 2004 Detecting provenance variations and cooling patterns within the western Alpine orogen through 40Ar/ 39Ar geochronology on detrital sediments: The Tertiary Piedmont Basin, northwest Italy B. Carrapa* J. Wijbrans* G. Bertotti* Vrije Universiteit Amsterdam, Faculteit der Aardwetenschappen, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands ABSTRACT The Tertiary Piedmont Basin is a synorogenic basin located on the internal side of the Western Alps. Because of its key position, the Tertiary Piedmont Basin represents an important record of processes occurring in the Alpine retrowedge for over the last 30 m.y. 40Ar/39Ar geochronology has been applied to detrital white micas as a prov- enance tool and to derive information on cooling and exhumation patterns within the surrounding orogen. The age distribution in the detritus shows that in the Oligocene the clastic sediments were fed mainly from a southern source area (Ligurian Alps) that widely records high pressure (HP) Alpine metamorphism (40–50 Ma) and, in part, Variscan metamorphism (ca. 320 Ma). From the Miocene, the main source area gradually moved from the south to a western Alpine provenance characterized by strong Late Cretaceous (70 Ma) and Early Cretaceous (120 Ma) signals. This enlarge- ment in the source is likely linked to an evolution of the main paleodrainage system into the basin. From the Serravallian, Variscan ages reappear; this is attributed to the exposure of the Argentera Massif as a new source for the Tertiary Piedmont Basin. The lack of thermal overprinting of the main detrital signals through time suggests that the western Alpine orogen has been regulated by episodic fast cooling and exhu- mation events followed by periods of slower erosion. Also, detrital 40Ar/39Ar Creta- ceous signals in Miocene and Present sediments suggest the presence of real Eoalpine events in the Alps. Keywords: Western Alps, provenance, 40Ar/39Ar geochronology, cooling, exhumation. INTRODUCTION consequence, clastic sediments are the only remaining direct evidence of the original source rocks outcropping at the time Synorogenic clastic sediments contained in sedimentary of sediment deposition. Therefore, they provide a record of the basins preserve a record of the exhumation kinematics of original setting of mountain belts through time. an orogen. Because of erosional and tectonic processes, the The Tertiary Piedmont Basin, located within the internal original rocks outcropping in the orogen no longer exist. As a western Alpine Arc (retrowedge; Beaumont et al., 1996) in northwest Italy (Fig. 1), contains up to 4 km of clastic sedi- *Present address, Carrapa (corresponding author)—Universität Potsdam, ments (Fig. 2). The Tertiary Piedmont Basin and the western Institut für Geowissenschaften, Postfach 601553, 14415 Potsdam, Germany, Alpine arc formed as a result of the Tertiary European-African [email protected]. E-mails: Wijbrans—[email protected]; Ber- totti—[email protected]. plate collision (e.g., Platt et al., 1989). The Tertiary Piedmont Carrapa, B., Wijbrans, J., and Bertotti, G., 2004, Detecting provenance variations and cooling patterns within the western Alpine orogen through 40Ar/39Ar geochro- nology on detrital sediments: The Tertiary Piedmont Basin, northwest Italy, in Bernet, M., and Spiegel, C., eds., Detrital thermochronology—Provenance analysis, exhumation, and landscape evolution of mountain belts: Boulder, Colorado, Geological Society of America Special Paper 378, p. 000–000. For permission to copy, contact [email protected]. © 2004 Geological Society of America. 69 70 B. Carrapa, J. Wijbrans, and G. Bertotti Figure 1. Geological map of the study area; TPB—Tertiary Pied- mont Basin (study area), VG—Vol- tri Group, LA—Ligurian Alps, AGM—Argentera Massif, DM— Dora Maira, GP—Gran Paradiso, SL—Sesia Lanzo. Basin is well suited for investigation of a substantial part of the of white micas (350–420 °C; e.g., Hames and Bowring, 1994; complex Alpine orogen because it is one of the main sedimen- Kirschner et al., 1996; Kohn et al., 1995; von Blanckenburg et tary basins collecting clastic sediments produced by cooling and al., 1989) is high enough to avoid substantial overprinting due exhumation and erosion of the internal Western Alps (including to short-lived thermal disturbances and sedimentary burial. Also, the Ligurian Alps). The Tertiary Piedmont Basin stratigraphy is 40Ar/39Ar dating of white mica is a well-tested technique that well preserved, exposed, and documented (e.g., Gnaccolini et al., records the time in which the investigated minerals pass through 1998, and references therein; Fig. 2). These data, together with a closure temperature of 350–420 °C (Najman et al., 1997; von paleocurrent directions data (Fig. 3A) on the study area, allowed Eynatten and Wijbrans, 2002; White et al., 2002). a good sample strategy (Fig. 3B). Research in the Himalayas (e.g., Harrison et al., 1993; For the present study, we have applied 40Ar/39Ar geochronol- Najman et al., 2001; White et al., 2002), in the Central Alps ogy to detrital white micas derived from samples selected from (e.g., Bernet et al., 2001; Spiegel et al., 2001; von Eynatten and the entire Tertiary Piedmont Basin stratigraphy (Lower Oligo- Wijbrans, 2002), and in western North America (e.g Heller et cene–Upper Miocene). To extend the covered time interval from al., 1992) has demonstrated the potential of the geochronologi- the oldest Tertiary Piedmont Basin sediments to the present, we cal approach for both provenance and tectonothermal evolution sampled sands from three of the main rivers currently draining studies. In particular, studies on the Central Alpine sedimentary the internal side of the southwestern Alps, which are the main record and on the exhumation of crystalline rocks from the Alpine sources of Tertiary Piedmont Basin sediments (Fig. 3B). orogen have suggested rapid episodic exhumation (e.g., Hurford Detrital mineral thermochronology on present-day river et al., 1991) followed by a relatively steady state of exhumation sands has been largely applied to different tectonic settings to (e.g., Bernet et al., 2001; Schlunegger and Willett, 1999). So far, characterize the present river drainage pattern (e.g., Heller et al., however, little data exist (Carrapa et al., 2003) on the depositional 1992). Dating minerals from present-day river sediments pro- counterpart of the western Alpine erosion. Our data set of over vides new information on the geochronological signal currently 500 individual white mica analyses is well constrained due to recorded at the surface in the southwestern margin of the Dora the proximity of the source to the basin and sheds new light on Massif, the Argentera Massif, and on the Ligurian Alps. the reconstruction of the thermotectonic evolution of the western The major objective of this work is to obtain new informa- Alpine Arc for a time period of over 30 m.y. tion on: (1) Provenance of the clastic sediments; (2) Cooling patterns due to unroofi ng of the original source rocks related to GEOLOGICAL SETTING past denudation and/or tectonic and erosional processes; and (3) potential information on the timing of HP and ultra-high pressure The Tertiary Piedmont Basin is located on the boundary (UHP) metamorphism in the Western Alps. between the internal Alpine domain, consisting of deep crustal White micas are well suited for this geochronological rocks, and the Apennine domain, constituted mainly by upper approach because of their high K content, their widespread Cretaceous–Eocene fl ysch (Fig. 1). The Tertiary Piedmont Basin occurrence in different lithologies, and their resistance against is fl anked to the south by the Ligurian Alps and to the west and mechanical breakdown. Furthermore, the closure temperature north by Plio-Quaternary sediments, which in turn are bounded Detecting provenance variations and cooling patterns 71 to the west by metamorphic units belonging to the internal west- Alps (sensu stricto) to the west. The western Alpine arc contains ern Alpine domain (Fig. 1). HP and UHP rocks that experienced deep burial during the Alpine orogeny and subsequent rapid exhumation, such as the The Orogen Surrounding the Tertiary Piedmont Basin Dora Maira and Gran Paradiso massifs to the west (Hurford et al., 1991; Hurford and Hunziker, 1989; Rubatto and Hermann, 2001) The western Alpine arc surrounding the Tertiary Piedmont and the Voltri Group to the south (Brouwer, 2000). All these Basin includes the Ligurian Alps to the south and the Western rocks constitute potential sources of sediments for the Tertiary Piedmont Basin. Figure 2. (continued on next page) A: Geological framework of the study area modifi ed after Gnaccolini et al. (1998). 1—Pliocene to Recent deposits; 2—Messinian deposits; 3—Langhian to Tortonian siliciclastic and carbonate shelf to slope deposits; 4—Late Burdigalian to Tortonian mainly turbidite succession (only Burdigalian in the eastern sector of the fi gure); 5—Late Oligocene to Burdigalian turbidite systems and hemi- pelagic mudstones; 6—Late Eocene to Early Oligocene deposits: (a) alluvial to coastal conglomerates, shallow marine sandstones and hemipe- lagic mudstones, (b) slope and base-of-slope, resedimented conglomerates, and (c) mainly turbidites; 7—Late Eocene to Tortonian siliciclastic deposits of the northwestern Apennines–Monferrato–Torino Hill wedge; 8—Alpine and Apennine allochthonous units; 9—depocenter axis of the Plio-Quaternary basins; 10—buried thrust-front of the Torino Hill–Monferrato–northwestern Apennine wedge; 11—buried, southvergent back- thrusts of the Monferrato, active from Messinian onward; 12—buried, pre Burdigalian backthrusts of the Western Alps (as inferred from Roure et al., 1990); 13—faults: SV—Sestri-Voltaggio; VVL—Villalvernia-Varzi line; I: Bagnasco–Ceva–Bastia Mondovì transect; II: Millesimo–Mo- nesiglio–Somano transect; III: Dego–Torre Bormida–Alberetto della Torre transect; IV: Spigno Monferrato–Cessole transect; V: Montechiaro d’Acqui; VI: Cavatore; VII: Visone. The square corresponds to the area of the paleocurrent study of Gnaccolini and Rossi (1994). B: Oligo-Mio- cene depositional sequences (A, B1–B6, C1–C6) in the study area after Gelati et al.