Magnetostratigraphic constraints on relationships between evolution of the central Swiss Molasse basin and Alpine orogenic events

Abstract

Magnetostratigraphic chronologies, together with lithostratigraphic, sedimentological, and petrological data enable detailed reconstruction of the Oligocene to Miocene history of the North Alpine foreland basin in relation to specific orogenic events and exhumation of the Alps. The Molasse of the study area was deposited by three major dispersal systems (Rigi, Hohronen, Napf). Distinguished by characteristic heavy mineral suites, conglomerate clast populations, and the presence of key clasts, these systems record three major phases of denudation of the Alpine edifice. The Rigi system eroded the Austroalpine and Penninic nappes of eastern Switzerland from 30 to 25.5 Ma as a result of backthrusting and uplift of these units along the Insubric Line. Subsequent uplift of the Aar massif some 40 km to the north appears to have controlled the duration of the Hohronen and Napf dispersal systems, spanning 24–22 Ma and 21.5–15 Ma, respectively. They record downcutting into the crystalline cores of the Penninic and Austroalpine nappes of eastern (Hohronen) and western (Napf) Switzerland. High-resolution reconstruction of the structural and geometrical evolution of the proximal Molasse reveals in-sequence and out-of-sequence thrusting events at the Alpine front and incorporation of the Molasse into the orogenic wedge by in-sequence thrusting and underplating. Furthermore, it reveals close relationships between periods of rapid denudation in the central Alps and phases of increased sediment accumulation rates at the proximal basin border. An initial increase in Molasse accumulation rates to >1 km/m.y. occurred between 30 and 25.5 Ma and coincides with the Insubric phase of backthrusting along the eastern Insubric Line, where >10 km of vertical displacement is interpreted. During the same time span, the Alpine wedge propagated forward along the basal Alpine thrust, as indicated by the coarsening- and thickening-upward megasequence and by occurrence of bajada fans derived from the Alpine border. The end of this tectonic event is marked by a basinwide unconformity, interpreted to have resulted from crustal rebound after initial loading. A subsequent increase in accumulation rates to >1 km/m.y. between 23 and 21.5 Ma coincides with initial uplift of the eastern Aar massif by at least 4 km. This phase of high accumulation rates is associated with incorporation of early Chattian conglomerates into the orogenic wedge. The third advance of the Alpine wedge between 21 and 15.5 Ma caused underplating of Molasse deposits, resulting in synsedimentary backthrusting of previously deposited Molasse sequences and in the development of a progressive unconformity. A rapid increase in accumulation rates from 0.35 to >1 km/m.y. between 15.5 and 15 Ma marks the final loading event in the wedge, which may be caused by further major displacement and loading of the Aar massif. This deformation is coeval with out-of-sequence thrusting of the Helvetic border chain and of the piggyback stack of North Penninic and Ultrahelvetic Flysch nappes along the basal Alpine thrust.