DEFORMATION LOCALIZATION DURING SHORTENING OF HETEROGENEOUS CRUST

Orogens are characterized by the presence of large scale heterogeneities such as granitic plutons, lateral changes in sedimentary or tectonic cover sequence thickness, and in special cases spatial variation in upper crustal thickness due to impact cratering processes. We report on 3D isothermal scaled analogue experiments and 2D numerical experiments that explore the behavior of individual competent tabular plutons, spatially distributed intrusions, and upper crustal lateral (orogen parallel) and localized (i.e., impact structures) thickness variations. These heterogeneities are incorporated in rheologically layered crust (frictional/brittle over ductile/viscous) subjected to regional shortening. Individual tabular plutons respond to shortening by developing domal antiforms whose wavelength and amplitude are controlled by density and viscosity contrasts with the host material, with strain localization occuring on dome boundaries. Spatially distributed tabular intrusions control the location of major doubly plunging antiforms and shorter wavelength synforms in the intervening crust. Strain accumulation on pluton margins leads to localization of thrusts that eventually link to form orogen subparallel curvilinear transpression zones. Both the style of folding and the shear zone distribution are geometrically similar to structural patterns in Archean granite-greenstone belts. Shortening of crust with orogen-parallel variation in brittle sedimentary cover thickness results in rhomb-shaped deformation domains whose boundaries consist of an anastomosing network of kinematically coupled, sinistral and dextral transpressive deformation zones. A similar segmentation of the upper crust into rhomb-shaped deformation domains, often forming sedimentary basins, is a ubiquitous characteristic of wide orogenic belts, notably the central Andes and Tibet. Shortening of crust containing a large, circular impact structure excavated to mid-crustal levels results in the development of an asymmetric doubly plunging basin with a reverse fault nucleated on the steep flank. The result is geometrically similar to the post-impact deformation of the Sudbury impact structure and the surrounding Huronian cover and Archean basement.