Alpine deformation and ⁴⁰Ar/³⁹Ar geochronology of synkinematic white mica in the Siviez‐Mischabel Nappe, western Pennine Alps, Switzerland

Abstract

Abstract. We explore the timing of deformation and exhumation of the Siviez-Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents 4øAr/39Ar ages obtained on both synkinematic white mica from Permo-Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez-Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation.Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez-Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported 4øAr/39Ar white mica ages from cover units of the

Geochronological evidence for continuous exhumation through the ductile‐brittle transition along a crustal‐scale low‐angle normal fault: Simplon Fault Zone, central Alps

“…Muscovite MC283 from the hanging wall has an apparent age of 22.5 Ma, but with a release spectrum for which no properly defined plateau age could be calculated. The result probably reflects a mixed age from the two generations of white mica (S 1 and S 2 ) actually observed in the sample, but could also be due to partial loss of 40 Ar during the second deformation D2 or to thermal advection from the exhuming footwall of the SFZ, as proposed by Markley et al [1998].…”

“…In the Helvetic nappes north of the Rhone Valley and in the interpreted prolongation of the SFZ footwall, Kirschner et al [2003] also applied 40 Ar/ 39 Ar and Rb/Sr methods to directly date synkinematic white micas related to nappe thrusting and established that activity or reactivation lasted until 25-15 Ma. They proposed that the Rhone (Simplon) Line (Figure 1), which crosscuts the "root" of the Helvetic nappes, was therefore active at around 17 Ma, which is close to the 19 Ma age proposed for movement on the same structure by Markley et al [1998].…”

“…The age of mylonitic fabrics older than the Simplon mylonites in footwall and hanging wall is relevant for distinguishing between the two proposed models. Markley et al [1998] used 40 Ar/ 39 Ar geochronology to date synkinematic white mica from a greenschist facies mylonite related to the emplacement of the Siviez-Mischabel nappe in the hanging wall of the SFZ, at about 41-36 Ma. They noted that fine-grained white micas (20 mm) from the hanging wall close to the SL detachment were partially reset at 22 Ma.…”

Abandonment of the South Penninic–Austroalpine palaeosubduction zone, Central Alps, and shift from subduction erosion to accretion: constraints from Rb/Sr geochronology

“…the Middle Penninic domain) yields synkinematic white mica ages systematically younger than 52-48 Ma (Thöni 1981, Thöni 1988, Markley et al 1998, Cartwright and Barnicoat 2002, Reddy et al 2003, Meffan-Main et al 2004Rosenbaum & Lister 2005, and references therein), indicating that subduction-related deformation had already shifted into the footwall of the South Penninic subduction mélange at about 48 Ma (Fig. 9).…”

“…towards NW in modern coordinates; Fig. 4), is indicated by isotopic ages between ~52 Ma and ~30 Ma for units exposed in the footwall of South Penninic units in the Western and Central Alps (Markley et al 1998, Cartwright and Barnicoat 2002, Reddy et al 2003, Meffan-Main et al 2004Rosenbaum & Lister 2005, and references therein). This age interval for subduction is again consistent with the sedimentation history.…”