K-Ar dating of recent thrusts: an application to the Tertiary clay gouges in the Northern Apennines of Italy

The Northern Apennines (NA) are a characteristic example of foreland fold-and-thrust belt (FTB) migrating towards its foreland. The progressive and quite regular eastward migration of the NA has been classically constrained in time by relying on the age of the syn-orogenic foreland basins, mainly determined by means of foraminifera and nannofossil biostratigraphy.

The well-known age of deformation makes the NA a perfect area where to test the reliability of the K-Ar illite dating applied to Cenozoic deformation involving siliciclatic deposits. In particular, we present the results of the first attempt to directly date, by K-Ar on illite separated from fault rocks, Neogene thrusts within the Trasimeno Tectonic Wedge (TTW), an imbricate thrust complex mainly made up of Tertiary siliciclastic rocks, located in the inner-central part of the NA, which represents the external front of the so-called Tuscan Nappe.

We sampled two WSW-dipping thrust faults, whose fault cores are composed of scaly gouge formed at the expense of the pelitic component of the host rocks. X-ray diffraction (XRD) and K-Ar isotopic analysis of multiple grain-sizes (from < 0.1 to 10 µm) allowed us to discriminate between syn-kinematic illite crystals formed during thrusting and detrital illite crystals inherited from the host rock. XRD data show a mineralogical association composed of quartz, calcite, albite, K-feldspar, chlorite, kaolinite and two populations of illite polytypes (1M_d and 2M₁). After the X-ray semiquantitative analysis, the results of the K-Ar dating of the two samples were regressed by the Illite Age Analysis (IAA) approach to assessing the effects of potential host rock contamination. Fault slip along the thrusts is then constrained to 15.2 ± 7.6 Ma and 15.4 ± 16.6 Ma.

Despite the large errors, the obtained dates are in excellent agreement with the timing of deformation along the base of the TTW, bracketed between the late Aquitanian and the latest Burdigalian – earliest Langhian and, more in general, with the proposed time evolution of the Northern Apennines.

Even if based on a limited dataset, our results suggest that the application of K-Ar dating of fault gouge can be extended to tectonic settings where independent constraints are not available and thus it becomes a valuable tool to study and constrain the time-space evolution of FTBs in recent and even active orogens.