Depositional and structural evolution of a foreland basin margin in a magnetostratigraphic framework: the eastern Swiss Molasse Basin

This integrated study of the sedimentology, magnetostratigraphic chronology and petrography of the mostly continental clastics of the Oligocene to Mio cene Swiss Molasse Basin underpins a reconstruction of facies architecture and delineates relationships between the depositional evolution of a forela nd-basin margin and exhumation phases and orogenic events in the adjacent o rogen. A biostratigraphically based high-resolution magnetostratigraphy pro vides a detailed temporal framework and covers nearly the whole stratigraph ic record of the Molasse Basin (31.5-13 Ma). Three transverse alluvial fan systems evolved at the southern basin margin. They are characterized by dis tinct petrographic compositions and document the exhumation and denudation history of the growing eastern Swiss Alps. Enhanced northward propagation o f the orogenic wedge is interpreted to have occurred between 31.5 and 26 Ma . During the period 24-19 Ma, intense in-sequence and out-of-sequence thrus ting took place as Molasse strata were accreted to the orogenic wedge. A th ird active tectonic phase, possibly caused by backthrusting of the Plateau Molasse, probably occurred between ca. 15 and 13 Ma. Fan head migration bet ween 31.5 and 13 Ma is probably controlled by the structural evolution of t he thrust front due to Molasse accretion and backthrusting.