Miocene paleotopography of the Central Alps

Reconstructing surface elevation, surface uplift, and relief histories is fundamental to understanding the growth of mountain ranges, to explore their topographic limits, and relate these to the interplay between geodynamic and Earth surface processes. Here, we aim to quantify Miocene paleoelevation of the Central European Alps through stable isotope paleoaltimetry. The novelty of our approach, which renders our analysis less sensitive to past climate change, is that stable isotope proxies of identical age are analyzed from both the high internal parts of the Alpine orogen and the adjacent foreland basin. Whereas the foreland basin was at or near sea level and traces the combined effects of upstream changes in rainfall amount and composition, the high Alpine site additionally records the effect of elevation on stable isotopes in precipitation. We compare hydrogen isotopic ratios (δD) in mica and chlorite that interacted with meteoric water along the Simplon detachment, a major normal fault that developed at high elevations, with meteoric water compositions deduced from carbonate-bearing paleosols of the North-Alpine foreland basin. ues of muscovite (−126‰) and chlorite (−135‰) from the brittle hanging wall and of recrystallized muscovite (−108‰) and biotite (−140‰) from adjacent footwall mylonites provide unequivocal evidence for localized syntectonic meteoric water interaction along the Simplon detachment. Detailed 40Ar/39Ar and fission track geochronology constrains the timing of isotopic exchange to ca. 14.5 Ma, when the footwall mylonites passed through the ductile to brittle transition. Age-equivalent oxygen isotope ratios (δ18O) measured within pedogenic carbonate from North-Alpine foreland paleosols that developed near Miocene sea level serve as our low-elevation point of reference. δ18O values in these paleosols, dated with ca. 100 ka precision, vary between +19 and +25‰ (SMOW) with average values of +20‰ at ca. 14.5 Ma. Using the relative differences between meteoric water compositions in the foreland basin and the high Alpine Simplon detachment, our isotope data are consistent with a mid-Miocene minimum average elevation difference of 2350 (+700/−500) m for the Simplon region. Our results indicate that Miocene Alpine elevations were at least comparable to those of today and are likely to have acted as an important barrier to Atlantic-derived moisture transport into central Europe and Eurasia since that time.