Drainage patterns and tectonic forcing: a model study for the Swiss Alps

A linear surface process model is used to examine the effect of different p atterns of rock uplift on the evolution of the drainage network of the Swis s Alps. An asymmetric pattern of tectonic forcing simulates a phase of rapi d retrothrusting in the south of the Swiss Alps ('Lepontine'-type uplift). A domal pattern of tectonic forcing in the north of the model orogen simula tes the phase of the formation of the 'Aar massif', an external basement up lift in the frontal part of the orogenic wedge ('Aar'-type uplift). Model runs using the 'Lepontine'-type uplift pattern result in a model moun tain chain with a water divide in the zone of maximum uplift and orogen-nor mal rivers. Model runs examining the effect of 'Lepontine'-type uplift foll owed by 'Aar'-type uplift show that the initially formed orogen-normal rive r system and the water divide are both very stable and hardly affected by t he additional uplift. This indifference to changes in tectonic forcing is m ainly due to the requirement of a high model erosion capacity for the river systems in order to reproduce the exhumation data (high-grade rocks in the south of the Swiss Alps point to removal of a wedge-shaped nappe stack wit h a maximum thickness of about 25 km). The model behaviour is in agreement with the ancestral drainage pattern of the Alps in Oligocene and Miocene ti mes and with the modern pattern observed in the Coast Range of British Colu mbia; in both casts river incision occurred across a zone of rapid uplift i n the lower course of the rivers. The model behaviour does not, however, ex plain the modern drainage pattern in the Alps with its orogen-parallel rive rs. When the model system is forced to develop two locally independent main wat er divides (simultaneous 'Lepontine'- and 'Aar'-type uplift), a zone of red uced erosional potential forms between the two divides. As a consequence, t he divides approach each other and eventually merge. The new water divide r emains fixed in space independent of the two persisting uplift maxima. The model results suggest that spatial and temporal changes in tectonic forcing alone cannot produce the change from the orogen-normal drainage pattern of the Swiss Alps in Oligocene-Miocene times to the orogen-parallel drainage observed in the Swiss Alps today.