EGU21-791
https://doi.org/10.5194/egusphere-egu21-791
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

The timescale of the aseismic to seismic deformation in a cooling pluton: 40Ar-39Ar ages of the solid-state deformation in the Adamello (Southern Italian Alps)

Silvia Mittempergher1, Stefano Zanchetta2, Federico Caldiroli2, Andrea Bistacchi2, Andrea Zanchi2, and Igor Maria Villa3,4
Silvia Mittempergher et al.
  • 1Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, via G. Campi 103, 41125 Modena (Italy)
  • 2Dipartimento di Scienze dell'Ambiente e della Terra, Università di Milano Bicocca, Piazza della Scienza 4, 20126 Milano
  • 3Centro Universitario Datazioni e Archeometria, Università di Milano Bicocca, Piazza della Scienza 4, 20126 Milano
  • 4Institut für Geologie, Universität Bern, 3012 Bern

The northern Adamello is crosscut by ductile shear zones and pseudotachylyte-bearing faults, both compatible with the same stress field, with ductile shear zones crosscut by brittle faults. These relations are coherent with the re-equilibration of the pluton-related thermal anomaly to temperatures typical of the base of the seismogenic continental crust (T = 250 – 300°). Our new 40Ar-39Ar ages help to constrain the absolute age and duration of each deformation phase.

Samples included wall-rock biotite, bulk ultramylonites and pseudotchylytes. Before stepwise heating 40Ar-39Ar measurements, samples were characterized by microstructural, geochemical and petrological analyses.

The wall-rock biotite is 33.4±0.1 Ma old, independently of grainsize. Mylonites feature complex age spectra between 28-31 Ma, including biotite and altered feldspar. Four pseudotachylyte matrices are clustered around 30-31.5 Ma, and two samples have 25-26 Ma ages.

Ductile shearing active 2 Ma after wall-rock emplacement indicates either low strain rates, or a long-lasting thermal anomaly, which might be due to high emplacement depth, and/or the progressive assemblage of adjacent plutons through small magma pulses. Seismogenic faulting overlaps with mylonitization around 31 Ma; younger pseudotachylyte ages may be due to late-stage reactivation.

How to cite: Mittempergher, S., Zanchetta, S., Caldiroli, F., Bistacchi, A., Zanchi, A., and Villa, I. M.: The timescale of the aseismic to seismic deformation in a cooling pluton: 40Ar-39Ar ages of the solid-state deformation in the Adamello (Southern Italian Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-791, https://doi.org/10.5194/egusphere-egu21-791, 2021.

Corresponding presentation materials formerly uploaded have been withdrawn.