Arcuate thrust systems in sandbox experiments: A comparison to the external arcs of the Western Alps

In the external Western Alps, two regional structural arcs were generated during Tertiary northwest-directed collision between the Apulian indenter and the European passive margin. These arcs, distinguished by their geographic position and their age, are examined by using a new compilation of structural data and comparing these to the results of sandbox-analogue experiments. The principal Western Alpine arc comprises two orthogonal, synchronous thrust systems. In the late Eocene–early Miocene, major shortening (105 km) was toward the northwest to west-northwest, and minor shortening (11 km) was toward the southwest. Shortening in each branch decreased toward the core of the arc. During the late Miocene and Pliocene, the Jura arc accommodated 35 km of northwest- directed shortening, while 10.5 km of southwest- to south-southwest–directed shortening was accommodated on the Digne thrust system. Sandbox experiments were used to investigate the role of the motion vector of a rigid rectangular indenter (orthogonal, diagonal, curved, or rotational paths) and the mechanical stratigraphy of the foreland in the evolution of upper-crustal arcuate systems (e.g., presence of a basal easy-slip [silicone] horizon). Comparison of experimental results with the external Alpine arc suggests that the indenter followed a slightly diagonal path with respect to the European margin from the Eocene to the early Miocene and curved counterclockwise by 10°–15° in the middle Miocene. Mechanical stratigraphy experiments support the hypothesis that thick Triassic evaporites played a primary role in the evolution of the Jura arc. The influence of mechanical stratigraphy was most prominent during weak deformation at the external boundaries of the Alpine orogen (Jura fold-and-thrust belt, Digne thrust).