Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps

摘要

Vitrinite and bituminite reflectance data, XRD studies on clay minerals, radiometric data, and geochemical data have been used to investigate metamorphism of very low and low grade sediments from the Lower Austroalpine and South Penninic nappes in the Oberhalbstein valley (central Alps). Numerical maturity modelling methods are calibrated with petrological, structural and new geochronological data to reconstruct temperature histories, even in nappe tectonic settings. The combination of low-grade data and maturity modelling shows that the duration of peak-temperature metamorphism plays an important role in the alteration of organic matter. Vitrinite reflectance (VR) often does not correlate with other low-grade parameters, but in the Oberhalbstein, equilibrium between the different parameters is evident because heating near the peak conditions for mure than 20 Ma took place. It is proposed to use the comparison to evaluate equilibrium conditions for the temperature parameters in general. In addition, from petrological work and VR modelling, the pressure conditions are estimated. The results yield an eroded overburden of 6 +/- 1 km and a temperature gradient of 30 +/- 2 degreesC km(-1). At the base of the nappe stack, pressure conditions of 4 +/- 1 kbar are determined and therefore no evidences for a high pressure event. The main heating is dated between 90 and 70 +/- 5 Ma and is attributed to nappe stacking in the upper crust. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 94]