Critical taper analysis of the North Alpine Thrust Front, SE Germany – Influence of fluid overpressure on fault strength in a subaerial orogenic wedge

Weak detachments below active thrust fronts of subaerial orogenic wedges are mostly controlled by wedge taper geometry. A common method to investigate wedge properties is critical taper analysis. According to critical taper theory fault strength in a mechanically homogeneous wedge can be constrained from surface slope angle α and the angle of inclination of the basal detachment β. However, the influence of fluid overpressure on fault strength is often underestimated. Here, we present a simplified 3D wedge taper model of the North Alpine Thrust Front (SE Germany) between Lake Constance and the Inn Valley which is used for critical taper analysis. Different scenarios with varying input parameters are considered with special emphasis on fluid overpressure ratios.

Critical taper analysis shows that frictional sliding resistance within the wedge varies both, laterally and longitudinally. Small fault strength values are predominant in the inner wedge, whereas the outer wedge is characterized by a higher fault strength due to a steepening detachment. Frictional sliding resistance in general decreases towards the hinterland likely resulting from lower surface slope values. For the western part of the North Alpine Thrust Front, our model shows low fault strength which is probably controlled by pore fluid overpressure and a flat detachment angle. Comparatively high fault strength in the eastern part however might be controlled by a steeper basal detachment, which is problay caused by underplating and a more complex structural framework. Finally, we use acquired data to discuss the influence of laterally varying sliding friction within the basal detachment on observed pore fluid overpressure in adjacent tectonic units, i.e. the Subalpine Molasse and the Foreland Molasse in Bavaria.