COEVAL CONTRACTION AND EXTENSION DURING GROWTH AND COLLAPSE OF OROGENIC PLATEAUX

During their evolution, orogens build gravitational energy, soften through heating and partial melting, and collapse. Contractional and extensional structures that are commonly interpreted as separate or sequential events, may be coeval and may not reflect far-field boundary conditions (plate motion). Two examples of such coeval contraction/extension domains are addressed by a series of numerical (Ellipsis) models.

1) The first concerns the transition between an orogenic plateau and its foreland. If the far-field (plate) boundaries of this system are stationary, extension in the plateau (formation of metamorphic core complex) is compensated by contraction in the foreland. The magnitude of this contraction/extension depends on i) the relative contributions of material transfer processes such as channel flow extrusion and erosion, ii) the rheology of the foreland, and iii) the buoyancy of the low-viscosity crust that underlies the plateau. Coeval contraction/extension may still occur if far field motion is convergent or divergent, up to a certain convergence/divergence rate, at which point both plateau and foreland are under general contraction or extension, respectively.

2) In the region of extension in the plateau, contraction at depth is coeval with extension in the shallow crust. Modeling results show that localized extension in the upper crust triggers Poiseuille flow of two channels in the lower crust that converge toward the extended zone, collide to form a steep contraction zone, then move upward to form migmatite subdomes in a metamorphic core complex. Coeval contraction and extension structures are dynamically coupled and develop spatially (laterally and vertically) and temporally in orogens. These simple results may help (re)interpret the complex deformation, metamorphic, and geochronologic histories of orogens.