Unraveling the burial and exhumation history of foreland basins using the spread of apatite (U-Th-Sm)/He single grain ages
Kevin Alexander Frings1,2,Elco Luijendijk3,István Dunkl4,Peter Kukla5,Nicolas Villamizar-Escalante2,Herfried Madritsch6,a,and Christoph von Hagke2Kevin Alexander Frings et al.Kevin Alexander Frings1,2,Elco Luijendijk3,István Dunkl4,Peter Kukla5,Nicolas Villamizar-Escalante2,Herfried Madritsch6,a,and Christoph von Hagke2
Received: 23 Nov 2022 – Discussion started: 05 Dec 2022
Abstract. Reconstructing the evolution of foreland basins that experienced late exhumation is challenging due to an incomplete sedimentary record. Thermochronometry has been applied successfully to reconstruct basin evolution, but the method is subject to uncertainties. For the Swiss Molasse Basin, a wide range of exhumation magnitude and timing has been proposed based on thermochronometry. We aim to reduce uncertainty by dating larger numbers of grains and samples, to obtain statistically more robust data. New apatite (U-Th-Sm)/He (AHe) data from a single borehole shows ages of 4 to 30 Ma in the upper 500 meters and ages of 3 to 80 Ma below 1300 meters. This is counterintuitive as a total reset is expected at depths exceeding approximately 600 m. To arrive at a single consistent thermal history including our and previously published data, we conduct thermal modeling with different software. In particular we test the influence of different provenance histories and distinguish between cooling associated with changes in heat flow vs changes in exhumation.
We determine 1050 m +/- 100 m of exhumation, starting slowly at 13 Ma and accelerating at 9 Ma. Coinciding with exhumation, heat flow begins to rise sharply, causing heating until 5 Ma, despite ongoing exhumation. We show that this discrepancy between start of exhumation and start of cooling is the main reason for differing estimates for the burial and exhumation history of the basin. We suggest that the remaining misfit between modeled and measured Molasse AHe ages can be explained by post-Miocene hydrothermal flux in the Neogene sediment fill above a sealing layer, potentially the Opalinus Clay or Triassic evaporites.
In summary, we show that a single consistent model for basin exhumation relies on large sets of grains and samples, as well as inclusion of provenance ages in the models. With timing of the main exhumation phase constrained to start at 9 Ma, we can rule out a 5 Ma climatic event as exhumation driver. As the region is not affected by extensive faulting, deep seated processes related to mantle dynamics remain as exhumation driving process.
This paper potentially provides new thermochronological data that could help in the clarification of thermal history of Swiss Molasse. I could appreciate the efforts made to explain the complexity of the dataset, including the use of different modelling approaches. However, I found some important issues that need to be properly addressed before considering this paper for publication. Detailed comments are on the annotated pdf but they can be summarized as follows.
Your discussion on possible causes for overdispersion of AHe data is rational but you did not consider implantation from surrounding host U-Th rich minerals. As it has been well demonstrated (e.g. Spiegel et al., 2009, EPSL; Murray et al., 2014, Chem. Geol.), implantation results in anomalous “old” ages. This could be the case of your BST and RL samples, above all given that you describe the presence of iron coating. Other possible causes (radiation damage, zonation) cannot be discarded a priori but their effect on the AHe system is less important. Thermal history of BST and RL is characterized by a very long permanence at shallow depths (i.e. at temperatures mostly lower than 100°C) and this enhances different kinetics (whatever is the cause).
Discussion of HeFTy and QTQt results is not always correct, especially when you refer to the lack of sensitivity to heat flow variations. These programs are not designed to take into account petrophysical properties and heat flow as they only consider mineral kinetic and time-temperature data as inputs.
Vitrinite data are not properly shown and discussed. They represent usually the most reliable tool for paleotemperature reconstructions so I think that they give a strong constraint to the thermal history. I have not the exact values but I played a bit with another software and other solutions are possible. By the way, both HeFTy and QTQt give the possibility to model VR data. Why did you not use them? Finally, the idea of possible inherited VR values is plausible but it should be verified by analysis of raw data.
I stopped my reading at chapter 5.3 as I think that a serious re-evaluation of data is necessary before going into discussion of geological implications.
We use apatite (U-Th-Sm)/He thermochronologic on detrital grains sampled from a well to unravel the exhumation history of the northern Swiss Molasse Basin and reconcile seemingly contradicting previous studies. With single grain ages and provenance ages, we achieve to narrowly constrain exhumation magnitude and timing and embed previous results into a single consistent thermal history. This includes proof for hydrothermal activity and a contribution to the discussion on exhumation drivers.
We use apatite (U-Th-Sm)/He thermochronologic on detrital grains sampled from a well to unravel...
