3-D NUMERICAL MODELING OF ULTRAHIGH PRESSURE METAMORPHIC TERRANE EVOLUTION IN THE NORWEGIAN CALEDONIDES

We present the results of 3-D numerical modeling of the creation and exhumation of ultrahigh pressure metamorphic (UHPM) terranes. The Western Gneiss Region (WGR) of Norway likely represents the most well exposed UHPM terrane in the world, and as such, offers the best chance to constrain the physics of UHPM terrane evolution. Previous research in the WGR indicates that roughly 100 km of vertical motion has occurred to bring UHP materials to the surface; however, all exhumation related fabrics observed during this study are subhorizontal.

Recent fieldwork in the northern portion of the WGR explored a large (wavelength ca. 6 km in the field area) cylindrical, NE-SW oriented, orogen parallel syncline in the quartzofeldspathic orthogneisses of the Baltica basement (the Surnadal Syncline) that contains thin slices of overlying thrust sheets in its core. This syncline appears to be part of a major subduction related syncline that can be traced for roughly 100 km. A mineral lineation of very consistent orientation (trend 61° plunge 14°) overprints all other fabrics in the field area, with occasional reversals in plunge direction and is taken to be exhumation-related.

Coupled thermal/mechanical numerical models evaluate various stages of terrane evolution in the WGR. Specific attention is given to the separation of buoyant, crustal material from the subducting oceanic slab. The kinematic and mineralogical implications of Moho-parallel delamination and Moho-normal rupture are considered in terms of controlling (disequilibrium?) metamorphic reactions, strength of crust/mantle coupling, dynamic feedback mechanisms and trajectories through P-T-t space.

A major focus in our current modeling is the transition from subduction to exhumation during UHPM terrane evolution. A critical change in buoyancy-related driving forces occurs where the ratio of the thickness of continental crust to the thickness of underlying lithospheric mantle is approximately 1/10. Where the ratio is less than this, the net buoyancy of the slab is negative, and continental material can be subducted. Where the ratio is greater than 1/10, the net buoyancy of the slab is positive, and continental material is unable to be subducted. In a tectonic setting, this transition occurs during the subduction of passive continental margins, which may be critical for the development of UHPM terranes.