INSIGHTS ON THE TRANSITION FROM SLAB ROLLBACK TO SLAB DETACHMENT USING DETRITAL ZIRCON U-PB GEOCHRONOLOGY IN THE MARNOSO-ARENACEA AND LAGA FORELAND BASINS, NORTHERN AND CENTRAL APENNINES, ITALY

Deepwater marine turbidite systems of the Marnoso-arenacea (MAF) and Laga formations preserve an extensively studied record of orogenesis and drainage development associated with topographic development and exhumation of the Apennine mountains in northern and central Italy. Presented here are new U-Pb detrital zircon ages (N=15, n=3875) from the Langhian-Tortonian MAF and the Messinian Laga Fm.

Major peak ages across the stratigraphic intervals of interest span from 270-287 Ma, 430-460 Ma, 560-680 Ma and overlap in age with Variscan, Caledonian, Cadomian sources, respectively. Mesozoic and Precambrian zircons are rare and comprise less than 10% of the sample population. Most samples have noteworthy (15-20% of the sample population) contributions from peri-Adriatic sources with major peak ages of 33 Ma. Sedimentary mixing models suggest the MAF was receiving detritus primarily from Alpine sources (80%) with minor contributions derived from the northern Apennines during the earliest (Langhian) phase of sedimentation. Serravallian strata record an upsection transition to increased detritus derived from the northern Apennines (60%) with decreased contributions from Alpine sources (40%). The youngest (Tortonian) stratigraphic interval in the MAF records the largest contributions from Alpine source regions (90%) coupled with decreased contributions from northern Apennine sources (10%). In contrast, mixing models in the Laga Fm. suggest that detritus was primarily derived from recycled strata of the MAF (80%) with minor contributions from other bedrock sources.

Mixing models and detrital zircon age trends outlined above support a provenance shift from primarily Alpine sources during the Langhian, to Apennine sources during Serravallian, and then back to Alpine sources during the Tortonian for the MAF. During deposition of the Laga Fm., sediment was primarily derived from recycling of strata in the MAF. These trends may reflect surface processes that include initial subsidence followed by uplift that results from slab detachment of the Adriatic slab. Surface uplift of the Apennines and MAF in response to migration of the detaching slab may have led to the development of new source areas for the MAF which were later exhumed and cannibalized into the Laga Fm.