Indenter growth in analogue models of Alpine-type deformation

A series of analogue experiments were carried out to simulate continental c onvergence, as seen in a profile through the Central Alps. A rigid indenter , representing the Adriatic plate, was driven laterally into a sand pack re presenting the brittle upper crust of Europe, detached and thickening above its subducting ductile lower crust. The rigid indenter advanced at the sam e steady rate in each experiment, but the dip of its front face was steepen ed in 15 degrees increments from 15 degrees to 90 degrees. Where the rigid indenter face dipped at 45 degrees or less, a sand wedge rose and was bound by a series of forekinks that nucleated at the toe of the indenter. Where the face of the rigid indenter dipped 60 degrees or more, the wedge was def ined by a single forekink and one or more backkinks that nucleated from a p oint advancing in front of the indenter toe. We interpret these results as indicating that slices of the sand pack and rising wedge are transferred ac ross kink bands to build an "effective" indenter with a frontal dip closer to that dictated by the changing shear strength of the sand pile, which thi ckens vertically as it shortens laterally. One of our models (with a rigid indenter dipping 75 degrees) simulates most of the major structures shown i n recent syntheses of surface geology and deep seismic data in the Central Alps, without the isostatic lithospheric depression. This model accounts fo r the late collisional stage (Oligocene to Present) complex strain and meta morphic histories in the core of the orogenic wedge, the rapid rise and ext rusion of small pips of Alpine eclogites, and the current passivity of the Insubric Line. It also emphasizes that lateral extension along gently dippi ng "thrusts" (orogen-normal horizontal escape) is confined to the extruded portion of the rising wedge.