Arcuate Thrust Systems in Sandbox Experiments: A Comparison to the External Arcs of the Western Alps

Ford, Mary¹, Henry Lickorish², Judith Buergisser³, Peter R. Cobbold⁴ (1) Nancy School of Geology, ENSG-CRPG, Nancy, France (2) Rock Deformation Research, Calgery, Alberta (3) Dr. Henrich Jaeckli AG, Baden, Switzerland (4) Rennes University, Rennes, France

In the external western Alps two regional structural arcs were generated during Tertiary NW-directed collision between the Apulian indentor and the European passive margin. These arcs, distinguished by their geographic position and their age, are examined using a new compilation of structural data and comparing these to sandbox analog experiments. The principal western alpine (PWA) arc (late Eocene-early Miocene) comprises two orthogonal, synchronous thrust systems. Major shortening (105 km) was toward the NW-WNW with minor shortening (11 km) toward the SW. Shortening in each branch decreased toward the core of the arc. (2) During the late Miocene and Pliocene the Jura arc accommodated 35 km of NW shortening while 10.5 km of SW-SSW shortening was accommodated on the Digne thrust system.

Sandbox experiments investigate the role of the motion vector of a rigid rectangular indentor (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 suggest that the alpine indentor followed a slightly diagonal path with respect to the European margin from the Eocene to the early Miocene and curved anticlockwise by 10-15° in the mid-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 belt, Digne Thrust).