Collision-orogen provenance (Western Alps): Detrital signatures and unroofing trends

The Alps are perhaps the best studied thrust belt formed during continent-continent convergence (collision orogen). Shallow, intermediate, and deep structural levels can be distinguished within the Alpine thrust stack, each shedding sediments with distinct petrographic and mineralogical signatures. In modern first-cycle sands carried by Alpine rivers, bulk composition, rank of metamorphic lithic grains (metamorphic index [MI]), and dense mineral assemblages all faithfully mirror the tectono-metamorphic history of exposed basement and cover units. The shallow structural level, widely preserved in the Ligurian Alps, consists of remnant ocean turbidites, shedding sedimentary to very low rank metasedimentary detritus (MI < 180). The few dense minerals include recycled ultrastables and garnet. The intermediate structural level, widely exposed in the Western Alps from the Simplon Fault to the Sestri-Voltaggio Zone, consists of high-pressure nappes retrogressed to greenschist facies. Detritus includes medium-rank metasedimentary lithic grains from cover units, quartz and feldspars from continental basements, or metabasite and foliated serpentine-schist lithic grains from subducted ophiolites. Dense minerals are epidote dominated and include high-pressure minerals (e.g., carpholite, Mg-chloritoid, glaucophane); Austroalpine and Penninic massifs supply abundant garnet. The deep structural level, unroofed in the Lepontine Dome, consists of amphibolite facies granitoid gneisses shedding very high rank quartzofeldspathic detritus (MI > 380). Dense minerals are hornblende dominated. The hornblende/epidote ratio increases, and the color of hornblende grains (hornblende-color index [HCI]) changes from dominantly blue green to green and green brown, from the periphery (HCI < 10) to the core (HCI 25-50) of the Lepontine Dome. Before reaching foreland basins on both sides of doubly vergent collision orogens, detritus from axial nappes mixes with detritus from basement and cover units uplifted in external thrust belts, and further modifications occur as a result of recycling of accreted molasse and alluvial plain deposits. This case study, which emphasizes both the complexities involved in provenance studies of foreland basin fills and their potential, provides detailed petrographic and mineralogical data on modern orogenic sands. Such actualistic information is the key for reconstructing paleotectonic and erosional evolution of orogenic source areas from detrital modes of ancient clastic wedges.