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

Abstract

A linear surface process model is used to examine the effect of different patterns of rock uplift on the evolution of the drainage network of the Swiss Alps. An asymmetric pattern of tectonic forcing simulates a phase of rapid retrothrusting in the south of the Swiss Alps (‘Lepontine’‐type uplift). A domal pattern of tectonic forcing in the north of the model orogen simulates the phase of the formation of the ‘Aar massif’, an external basement uplift in the frontal part of the orogenic wedge (‘Aar’‐type uplift). Model runs using the ‘Lepontine’‐type uplift pattern result in a model mountain chain with a water divide in the zone of maximum uplift and orogen‐normal rivers. Model runs examining the effect of ‘Lepontine’‐type uplift followed by ‘Aar’‐type uplift show that the initially formed orogen‐normal river system and the water divide are both very stable and hardly affected by the additional uplift. This indifference to changes in tectonic forcing is mainly 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 with 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 times and with the modern pattern observed in the Coast Range of British Columbia; in both cases river incision occurred across a zone of rapid uplift in the lower course of the rivers. The model behaviour does not, however, explain the modern drainage pattern in the Alps with its orogen‐parallel rivers. When the model system is forced to develop two locally independent main water divides (simultaneous ‘Lepontine’‐ and ‘Aar’‐type uplift), a zone of reduced erosional potential forms between the two divides. As a consequence, the divides approach each other and eventually merge. The new water divide remains 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.

Post-collisional sediment budget history of the Alps: tectonic versus climatic control

“…Erodibility of exposed rock will also play an important role as lower plate (Penninic) metapelites became exposed during the Miocene (see Kuehni and Pfiffner 2001;Schlunegger et al 2001;Kuehni et al 2002).…”

Possible environmental effects on the evolution of the Alps-Molasse Basin system

“…However, because the establishment of the orogen-parallel drainages (Rhein and Rhône valleys) was most likely contemporaneous with the ca. 15 km-northward shift of the Ticino headwaters (Kühni & Pfiffner, 2001a), and since evidence of stream piracy and drainage reorganization is still preserved in the Ticino basin (Fig. 10C), we assign a relatively young, possibly Pliocene, age for the initiation of this third stage.…”

“…This modification appears to have occurred as Alpine palaeorivers were rerouted around the hinge of the growing Aar massif. Schlunegger et al (2001) and Kühni & Pfiffner (2001a) postulated that exhumation and exposure of granites and gneisses of the Aar massif (which caused a strong contrast in the pattern of bedrock erodabilities according to Pfiffner & Kühni (2001b) was sufficient to initiate this reorganization of the drainage network. It is unclear at the moment when this third phase started.…”

Characteristics of steady state fluvial topography above fault‐bend folds

“…[62] Past modeling studies have shown that topographic profile asymmetry can also arise owing to spatial gradients in rock uplift rate [e.g., Kuhni and Pfiffner, 2001;Schlunegger et al, 2001], internal shortening [Willett et al, 2001], orographic precipitation [Beaumont et al, 1992;Batt and Braun, 1999;Willett, 1999;Herman and Braun, 2006], and from the internal frictional properties and fault geometries of critical wedges [Koons, 1990;Carena et al, 2002;Whipple and Meade, 2004]. Therefore the cause of asymmetry in real mountain ranges must be interpreted with caution.…”