Abstract
The Brenner normal fault bounds the Tauern Window to the west and accommodated a significant portion of the orogen-parallel extension in the Eastern Alps. Here, we use zircon (U–Th)/He, apatite fission track, and apatite (U–Th)/He dating, thermokinematic modeling, and a topographic analysis to constrain the exhumation history of the western Tauern Window in the footwall of the Brenner fault. ZHe ages from an E–W profile (parallel to the slip direction of the fault) decrease westwards from ~ 11 to ~ 8 Ma and suggest a fault-slip rate of 3.9 ± 0.9 km/Myr, whereas AFT and AHe ages show no spatial trends. ZHe and AFT ages from an elevation profile indicate apparent exhumation rates of 1.1 ± 0.7 and 1.0 ± 1.3 km/Myr, respectively, whereas the AHe ages are again spatially invariant. Most of the thermochronological ages are well predicted by a thermokinematic model with a normal fault that slips at a rate of 4.2 km/Myr between ~ 19 and ~ 9 Ma and produces 35 ± 10 km of extension. The modeling reveals that the spatially invariant AHe ages are caused by heat advection due to faulting and posttectonic thermal relaxation. The enigmatic increase of K–Ar phengite and biotite ages towards the Brenner fault is caused by heat conduction from the hot footwall to the cooler hanging wall. Topographic profiles across an N–S valley in the fault footwall indicate 1000 ± 300 m of erosion after faulting ceased, which agrees with the results of our thermokinematic model. Valley incision explains why the Brenner fault is located on the western valley shoulder and not at the valley bottom. We conclude that the ability of thermokinematic models to quantify heat transfer by rock advection and conduction is crucial for interpreting cooling ages from extensional fault systems.
Highlights
IntroductionLow-temperature thermochronological data record the cooling history of rocks on their way to the Earth’s surface and are routinely used to quantify rates of rock exhumation and slip rates of normal faults (e.g., Fitzgerald et al 1993; Foster and John 1999; Brady 2002; Brichau et al 2010; Buscher1 3 Vol.:(0123456789)International Journal of Earth Sciences (2021) 110:2955–2977 during thermal relaxation decreases the geothermal gradient in the entire footwall block and may cause rapid posttectonic cooling (Braun 2016; Wolff et al 2020)
We describe our thermochronological data set and the thermokinematic model (Fig. 6A) that we use to predict the cooling ages obtained from the three different thermochronometers (ZHe, apatite fission-track (AFT), and AHe)
We describe the cooling ages obtained from the WSW–ENE profile and the age-elevation profile at Olperer mountain, followed by the age results at the sites A and B near the Brenner fault
Summary
Low-temperature thermochronological data record the cooling history of rocks on their way to the Earth’s surface and are routinely used to quantify rates of rock exhumation and slip rates of normal faults (e.g., Fitzgerald et al 1993; Foster and John 1999; Brady 2002; Brichau et al 2010; Buscher1 3 Vol.:(0123456789)International Journal of Earth Sciences (2021) 110:2955–2977 during thermal relaxation decreases the geothermal gradient in the entire footwall block and may cause rapid posttectonic cooling (Braun 2016; Wolff et al 2020). The position of active faults in mountain belts is often marked by valleys, as brittle fault rocks such as fault gouge and cataclasite have relatively higher erodibility than less deformed rocks adjacent to the fault (e.g., Koons et al 2012) This is important for normal faults, where the process of footwall uplift causes a relative downward movement of the hanging wall, which causes the flow of water or ice to be focused at or near the fault trace. We present new structural, topographic, and thermochronological data from the Brenner normal fault in the European Alps These data complement a recent study that applied thermokinematic modeling to a thermochronological data set from a vertical transect (including a 1000-m-long drill core) and showed that posttectonic thermal relaxation affected the rock cooling history in the fault footwall (Wolff et al 2020). Our new data and thermokinematic modeling provide improved constraints on the evolution of the Brenner normal fault, the cooling history of its footwall (the western Tauern Window), and the amount of erosion and valley incision after faulting stopped
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