EGU2020-2196
https://doi.org/10.5194/egusphere-egu2020-2196
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Fast cooling of normal-fault footwalls: rapid fault slip or thermal relaxation?

Reinhard Wolff1, Ralf Hetzel1, István Dunkl2, Aneta A. Anczkiewicz3, and Hannah Pomella4
Reinhard Wolff et al.
  • 1University of Münster, Geology and Palaeontology, Structural Geology, Münster, Germany (rwolff@uni-muenster.de)
  • 2Institut für Sedimentologie und Umweltgeologie, Universität Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany
  • 3Institute of Geological Sciences, Polish Academy of Sciences, Senacka 1, 31-002 Kraków, Poland
  • 4Institute of Geology, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria

Rapid rock exhumation in mountain belts is often associated with crustal-scale normal faulting during late-orogenic extension. The process of normal faulting advects hot footwall rocks towards the Earth's surface, which shifts isotherms upwards and increases the geothermal gradient. When faulting stops, this process is reversed and isotherms move downwards during thermal relaxation. Owing to these temporal changes of the geothermal gradient, it is not straightforward to derive the history of faulting from mineral cooling ages (Braun, 2016). Here, we combine thermochronological data with thermokinematic modeling to illustrate the importance of syntectonic heat advection and posttectonic thermal relaxation for a crustal-scale normal fault in the European Alps. The N–S trending Brenner fault defines the western margin of the Tauern Window and caused the exhumation of medium-grade metamorphic rocks during Miocene orogen-parallel extension of the Alps (Rosenberg & Garcia, 2011; Fügenschuh et al., 2012). We analyzed samples from a 2-km-thick crustal section, including a 1000-m-long drillcore. Zircon and apatite (U-Th)/He ages along this transect increase with elevation from ~8 to ~10 Ma and from ~7 to ~9 Ma, respectively, but differ by only ~1 Myr in individual samples. Thermokinematic modeling of the ages indicates that the Brenner fault became active 19±2 Ma ago and caused 35±10 km of crustal extension, which is consistent with independent geological constraints. The model results further suggest that the fault slipped at a total rate of 4.2±0.9 km/Myr and became inactive 8.8±0.4 Ma ago. Our findings demonstrate that both syntectonic heat advection and posttectonic thermal relaxation are responsible for the cooling pattern observed in the footwall of the Brenner normal fault.

References

Braun, J., 2016, Strong imprint of past orogenic events on the thermochronological record: Tectonophysics, v. 683, p. 325–332.

Fügenschuh, B., Mancktelow, N., Schmid, S., 2012, Comment on Rosenberg and Garcia: Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps: Int. J. Earth Sci., v. 101, p. 1451–1455.

Rosenberg, C.L., Garcia, S., 2011, Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps: Int. J. Earth Sci., v. 100, p. 1129–1145.

Wolff, R., Hetzel, R., Dunkl, I., Anczkiewicz, A.A., Pomella, H. 2020, Fast cooling of normal-fault footwalls: rapid fault slip or thermal relaxation? Geology, v. 48, doi:10.1130/G46940.1.

How to cite: Wolff, R., Hetzel, R., Dunkl, I., Anczkiewicz, A. A., and Pomella, H.: Fast cooling of normal-fault footwalls: rapid fault slip or thermal relaxation?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2196, https://doi.org/10.5194/egusphere-egu2020-2196, 2020

Comments on the presentation

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Presentation version 2 – uploaded on 23 Apr 2020
Here, I added the comments to the slides. RW
  • CC1: Comment on EGU2020-2196, Martin Reiser, 07 May 2020

    I think my comment got buried in the chat, so I post a slightly extended version here:

    You chose a dip angle of 35° for your pecube model. According to the map on page 7, most dip angles of the mylonitic foliation are between 20-28°, which also produce the small shoulders above the valley in your profiles. Did you also try lower angles as input parameters? To what extent does the amount of extension change if you apply an angle of say 25° in your pecube model?

    • AC1: Reply to CC1, Reinhard Wolff, 07 May 2020

      Dear Martin,

      we constructed a PECUBE model with a fault dipping 35°, because it is a value intermediate between the gentle dip of the mylonites and the slightly steeper dip of the brittle fault; Fügenschuh et al., 1997, 2012; Rosenberg and Garcia, 2011). Of course, a different fault dip will change the exhumation rate. A lower dip than our preferred value of 35° will result in lower rates of exhumation and heat advection. Additional model runs revealed that for a fault dip of 30°, the slip rate predicted by our PECUBE model is higher (~4.8 instead of ~4.2 mm/yr); conversely, for a steeper fault dip of 40° the slip rate is lower (~3.7 mm/yr). Nevertheless, the predicted thermochronological ages remain unchanged, because the vertical slip rate derived from the inversion is the same as for the best-fit model.

      In conclusion, fault dips of 30° or 40° (instead of 35°) lead to extension and slip-rate values that are 11% higher or lower, respectively. The other model results, in particular the predicted cooling ages, the vertical slip rate, and the amount of exhumation, remain unchanged.

      All the best, Reinhard

      • CC2: Reply to AC1, Martin Reiser, 07 May 2020

        Dear Reinhard,

        thank you for the clarification and also for the extra effort that you put in recording your presentation! I enjoyed watching it!

        Cheers

        Martin

Presentation version 1 – uploaded on 17 Apr 2020 , no comments