# P–T-t conditions of successive deformation stages during exhumation of orogenically thickened crust; a case study from the Aar Massif, Central Alps, Switzerland

## TL;DR

This study uses mineral data to track the pressure, temperature, and timing of crustal exhumation in the Swiss Alps, revealing how faulting and fluid flow shaped the region's geological history.

## Contribution

The paper introduces a novel integration of hydrothermal mineral data and multiple thermometric techniques to precisely constrain the timing and conditions of deformation events during crustal exhumation.

## Key findings

- Hydrothermal quartz fissures formed between 15 and 7 Ma under varying P–T conditions, linked to faulting events.
- Stress regime changes at 12–11 Ma shifted faulting to dextral strike-slip, with CO2-rich fluids altering mineral assemblages.
- Exhumation rates and cooling history were estimated without relying on traditional thermochronology methods.

## Abstract

Obtaining precise pressure–temperature-time constraints on the history of exhumation of orogenically thickened crust using rock-forming minerals of greenschist-facies rocks can be a challenging task. Rare examples exist where structurally distinct hydrothermal mineralisations have been used to pin-point specific stages during this evolution. This study combines hydrothermal fissure-quartz fluid and solid inclusion data with Ti-in-quartz thermometry, solute thermometry, and fissure monazite-(Ce) Th-Pb ion probe dating in order to establish a link between hydrothermal mineral crystallisation and major faulting events in the Grimsel Pass study area, central Aar Massif, Switzerland. Six fluid inclusion populations in quartz are distinguished in the older, steeply NNW-dipping fissure at the well-known Zinggenstock locality, four can be identified in quartz in younger, vertical fissures. All data together constrain formation and subsequent stepwise growth and evolution of the fissures to a P–T-t range of 450 °C/440 MPa and 300 °C/240 MPa between c. 15 and 7 Ma. In quartz zones containing rutile-whiskers in fluid inclusions, Ti-in-Qtz thermometry yields temperatures comparable to fluid inclusion solute thermometry. The combined data indicate that the oldest cleft quartz generation formed c.15 Ma ago during reverse faulting at 450 °C/440 MPa. A major change in the direction of the regional stress field linked with onset of dextral strike-slip movements along the Rhone-Simplon-Centovalli fault system then led to predominant dextral strike-slip faulting starting at c. 12–11 Ma, at P–T conditions between 375 °C/320 MPa and 330 °C/230 MPa. At Zinggenstock, the original cleft becomes overprinted by sinistral shear zones, and fluid advection at 330 °C/230 MPa. This CO2-bearing fluid led at the Zinggenstock location to the formation of white mica (muscovite-ferriphengite) at the expense of chlorite. At Oberaar, renewed dextral strike-slip reactivation occurred between c. 10 and at 7 Ma at conditions of 330 °C/230 MPa to 300 °C/240 MPa. Our data document variable stress regimes, locally associated with focused fluid flow, across an approximate depth interval of 16.3–8.5 km (~ 440 to 230 °C) during unroofing of the orogenically thickened crust. Hydrothermal mineral formation ages precisely constrain the chronology of successive deformation events, thus offering valuable constraints for unravelling the mechanisms of tectonically and buoyancy-driven exhumation of peripheral domains of the NW European Alps. Together, these data permit to estimate exhumation and cooling rates independent of thermochronology.