Dating tectonic activity in the Lepontine Dome and Rhone-Simplon Fault regions through hydrothermal monazite-(Ce)

Abstract

Abstract. Zoned hydrothermal monazite-(Ce) from Alpine-type fissures and clefts is used to gain new insights into the tectonic history of the Lepontine Dome in the Central Alps and the timing of deformation along the Rhone-Simplon Fault zone on the dome's western end. Hydrothermal monazites-(Ce) (re)crystallization ages directly date deformation that induces changes in physicochemical conditions of the fissure or cleft fluid. A total of 480 secondary ion mass spectrometry (SIMS) spot analyses from 20 individual crystals, including co-type material of the monazite-(Nd) type locality, record ages for the time of ∼19 to 2.7 Ma, with individual grains recording age ranges of 2 to 7.5 Myr. The combination of these age data with geometric considerations and spatial distribution across the Lepontine region gives a more precise young exhumation history for the area. At the northeastern and southwestern edges of the Lepontine Dome, units underwent hydrothermal monazite-(Ce) growth at 19–12.5 and 16.5–10.5 Ma, respectively, while crystallization of monazite-(Ce) in the eastern Lepontine Dome started later, at 15–10 Ma. Fissure monazite-(Ce) along the western limit of the dome reports younger ages of 13–7 Ma. A younger age group around 8–5 Ma is limited to fissures and clefts associated with the Simplon normal fault and related strike-slip faults such as the Rhone Fault. The data set shows that the monazite-(Ce) age record directly links the fluid-induced interaction between fissure mineral and host rock to the Lepontine Dome's evolution in space and time. A comparison between hydrothermal monazite-(Ce) and thermochronometric data suggest that hydrothermal monazite-(Ce) dating may allow us to identify areas of slow exhumation or cooling rates during ongoing tectonic activity.

Bias and error in modelling thermochronometric data: resolving a potential increase in Plio-Pleistocene erosion rate

“…Recent field studies (Persaud and Pfiffner, 2004;Malùsa et al, 2005;Egli and Mancktelow, 2013) find no significant post-Miocene motion of the Penninic Line Fault of any sense (normal or thrust). Egli and Mancktelow (2013) (Bergemann et al, 2020). Identified normal faults in the western and central Alps are small, steep, and interpreted to be gravity-driven features with small displacements, with the possible exception of the Simplon detachment, which has a local effect due to its orientation with respect to the regional transcurrent motions.…”

Variations in the P‐T‐t of Deformation in a Crustal‐Scale Shear Zone in Metagranite

“…Our estimates for the timing of shearing are also consistent with independent constraints: our oldest relict deformation is indirectly dated at ∼24.5 Ma, consistent with deformation following ∼32.5–25 Ma peak metamorphism (Vance & O'Nions, 1992). Several studies have dated brittle fracturing, vein filling, and gouge formation in the SSZ footwall to ∼16.5–5.7 Ma, based on the ages of hydrothermal monazite in fissures, vein‐hosted muscovite, and gouge illite (Bergemann et al., 2020; Hetherington & Villa, 2007; Pettke et al., 1999; Zwingmann & Mancktelow, 2004). However, the transition from viscous to frictional deformation in the exposed footwall was likely diachronous from north to south.…”

“…The SSZ is well studied, with over 170 years of publications (e.g., Bergemann et al, 2020;Gerlach, 1869;Schardt, 1903;Studer, 1851). A significant body of knowledge is thus available for the SSZ, including detailed structural mapping highlighting a zonation in the footwall, from a broad zone of rock deformed under amphibolite-facies conditions, through a narrowing zone of greenschist-facies mylonites, to the highly localized brittle Simplon Line (e.g., Campani et al, 2014;Mancktelow, 1985Mancktelow, , 1987; detailed accounts of quartz microstructure, recrystallization mechanisms, and Ti-in-quartz contents as they vary with distance into the footwall (Haertel et al, 2013;Haertel & Herwegh, 2014); and thermochronology and thermokinematic modeling, which has provided estimates on the timing and rates of exhumation (Campani et al, 2010a(Campani et al, , 2010bGrasemann & Mancktelow, 1993).…”

Extensional reactivation of the Penninic frontal thrust 3 Myr ago as evidenced by U–Pb dating on calcite in fault zone cataclasite

“…However, until now, no direct dating of the tectonic shift from compression to extension on the PFT has been obtained, which leads to many possible geodynamic scenarios. At the present day, a large range of ages for this transition have been hypothesized, from ∼ 12-5 Ma (Tricart et al, 2006) to only a few tens of thousands of years (Larroque et al, 2009), which shows the lack of direct dating of brittle deformation (Bertrand and Sue, 2017). In this study, we applied the laser ablation U-Pb dating method on secondary calcites from a cataclasite fault zone that testify to the extensional deformation of an exhumed palaeo-normal fault during the PFT inversion.…”

“…This transition is marked by shear zone development in greenschist facies conditions and recrystallization during burial of the Alpine external zone in the PFT footwall compartment (Rossi et al, 2005;Sanchez et al, 2011;Bellahsen et al, 2014). The early ductile PFT activity is dated at 34-29 Ma by 40 Ar/ 39 Ar dating of syn-kinematic phengite from shear zones in the Pelvoux and Mont Blanc external crystalline massifs (Seward and Mancktelow, 1994;Rolland et al, 2008;Simon-Labric et al, 2009;Bellanger et al, 2015;Bertrand and Sue, 2017) and by U-Pb on allanite (Cenki-Tok et al, 2014). The age of the PFT hanging wall tectonic motion and joint erosion is highlighted by the exhumation of the Briançonnais units constrained by apatite fission tracks (AFTs) at 26-24 Ma (Tricart, 1984;Tricart et al, 2001Tricart et al, , 2007Ceriani and Schmid, 2004).…”