Burial and exhumation of the Alpine Foreland Basin constrained by low-temperature thermochronometry and stylolite morphology

Frings, Kevin Alexander; Kukla, Peter (Thesis advisor); von Hagke, Christoph (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023

Abstract

Foreland basins are important as they may contain resources as hydrocarbons, geothermal systems suitable for energy production, or they host potential sites for radioactive waste disposal. Scientifically, understanding foreland basins and their burial and exhumation history provides information on the underlying processes, and foreland basins witness the tectonic history of the adjacent orogen. Reconstructing the evolution of foreland basins that experienced late exhumation is often challenging due to an incomplete sedimentary record. The northern Alpine Foreland Basin of the Central European Alps is a well-studied example of a foreland basin subjected to late-stage exhumation, where the magnitude and timing of this exhumation remain disputed. In such cases, thermochronometry is a powerful tool, but subject to uncertainties. Consequently, previous studies provided different exhumation estimates. In this thesis, a new approach using low temperature apatite (U-Th-Sm)/He thermochronometry is used with the aim to unravel exhumation magnitude and timing and to reconcile previous studies. In addition, the obtained thermochronometric dataset is analyzed for factors responsible for the age spread and the specific age-depth pattern observed. Thermochronometry is complemented by maximum burial stress analysis using stylolite morphology, which could provide an independent estimate for exhumation since maximum burial. The new apatite (U-Th-Sm)/He (AHe) data measured on samples of the Bülach-1 well shows ages of 4 to 30 Ma in the upper 500 meters and ages of 3 to 80 Ma below 1300 meters. This pattern is counterintuitive, because the depth trend of ages from the shallow samples indicates total reset at depths exceeding approximately 600 m. Thermal modeling with different software is used to arrive at a thermal history explaining the data. In particular, the influence of different provenance histories is tested using the tool PyBasin, which can also differentiate between changes in heat flow and changes in exhumation. With this approach 1050 m +/- 100 m of exhumation are determined, starting slowly at 13 Ma, and accelerating at 9 Ma. Coinciding with exhumation is a rise of heat flow that leads to peak temperatures at 5 Ma. 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. Modeling results additionally allow to infer post-Miocene hydrothermal flux in the Neogene sediment fill and to conclude driving processes behind exhumation. A 5 Ma climatic event can be ruled out and faulting is minor in the region, leaving deep seated processes related to mantle dynamics as main drivers. Following modeling success, a precise analysis of the dataset with the aim to find reasons for the age spread and pattern of older ages at the bottom of the sampled profile is conducted. Typically suspected influences of mother isotope concentration (effective uranium, eU) and grain volume on (U-Th-Sm)/He ages are shown to be weak for the dataset and are thus not sufficient to explain the age spread. Instead, the age spread is proposed to root in mixing of different provenance populations. These are based on different provenance thermal histories and respond differently to reheating and cooling in a basin. Consequently, variations of closing temperatures (Tc) between the different grain age populations are expected. By combining effective uranium (eU) and helium concentration derived Tc, separation of populations of grains reacting similarly to temperature exposure over time is reached in this thesis. It is shown that using this method, quantitative estimates of Tc for individual populations can be made and using these separated and quantified thermochronometric populations, consistent exhumation scenarios can be drawn from the counterintuitive datasets measured for this thesis. For the attempt to use burial stress at stylolite formation to complement results from thermochronometry, stylolites from the 800 m long Mesozoic carbonate containing sequence of the Bülach-1 well are analyzed. Macroscopic and microscopic investigation, as well as analysis of closely spaced parallel sections of the same stylolite reveal a large variety of geometries and host rock properties. Two complementary approaches are chosen to derive depth dependencies of parameters and provide an estimate for maximum burial depth the stylolites witnessed. For the first approach, over 150 stylolites sampled along the well are measured for wavelengths and associated peak heights to compile a large statistical dataset. Using this dataset, factors controlling the wavelength - peak height relation are examined. The results are that stylolite type and the share of non-carbonate minerals in the host rock are no controls, while increasing porosity apparently leads to lower peak heights. Depth and lithology affect wavelength - peak height correlations, but results are ambiguous, and it is not possible to quantify the effects. For the second approach, frequency spectra of single stylolites are analyzed for a change in scaling of wavelength and amplitude using a method introduced by previous studies. With this approach, maximum stress at stylolite formation can be calculated. With the derived stress, burial depth at formation and, consequently, exhumation magnitude can be derived. However, the obtained value suggests about 700 m of burial since stylolite formation and is on average 1700 m below results from apatite (U-Th-Sm)/He thermochronometry. Many factors oppose uncertainties to the method and the calculation, some of these prone to systematic errors. Most important are the great variability of stylolites and host rocks, biases from the process of stylolite mapping, uncertainties in rock mechanical parameters and instabilities of the determination algorithm. Furthermore, it can be debated whether the stylolite morphology is dominated by peak stress at maximum burial at all. In summary, this thesis provides a unique thermochronologic dataset and uses a new method to constrain thermal history, as well as new insights into the apatite helium system and how it is affected by provenance populations. Using stylolites as complementing stress and maximum burial depth indicators is a method of high potential. However, it requires elimination of systematic errors, quantification and reduction of uncertainties and more understanding of the many factors discussed in this thesis to arrive at robust constraints.

Institutions

• Division of Earth Sciences and Geography [530000]
• Chair of Geology and Paleontology and Geological Institute [531110]