THE EFFECT OF MAGMATIC UNDERPLATING ON RHEOLOGICAL PROPERTIES OF THE UPPER MANTLE: THEORY AND OBSERVATION

Magmatic underplating at the base of the crust leads to heating of the upper mantle, which has a profound influence on its strength characteristics. Such thermal perturbations of the mantle are numerically modeled and combined with olivine flow laws to establish a range of feasible viscosities for a differentially stressed lithosphere. The results demonstrate that, dependent on the thickness and duration of underplating, the upper 10–20 km of mantle is substantially weakened relative to deeper levels. The effects of heating are illustrated by mantle xenoliths from the 4 Ma Kozákov volcano, which is located on the southern flank of the Cenozoic Eger Graben (Czech Republic) in a region that experienced Neogene magmatic underplating. Spinel peridotite xenoliths reveal a km–scale layered mantle architecture, where an upper, 15–km thick, fine–grained equigranular layer (EL) is underlain by a coarse–grained protogranular layer (PL). The deeper PL was not significantly deformed during Neogene heating and rifting, as indicated by the preservation of pyroxene–spinel symplectite pseudomorphs after garnet. In contrast, the EL was strongly deformed during and after Neogene heating, resulting in well–developed olivine fabrics that indicate a combination of (010)[100] and (0kl)[100] slip systems, in which [100] is oriented at a high angle to the axis of the Eger Graben. M–index values are as high as 0.35, which are associated with calculated seismic anisotropies of ∆V_P = 8–15% and ∆V_S = 5–9%. Closure temperatures for Ca diffusion in pyroxene in the EL are above those obtained from two-pyroxene geothermometry, requiring deformation–induced recrystallization below Ca closure temperatures in order to maintain chemical equilibrium. In the context of the time-temperature-viscosity model, it appears that the EL continued to be weaker than underlying mantle in the PL, despite its eventual lower temperatures and conventionally calculated greater strength based on olivine flow laws. We postulate that the crystallographic fabric in the EL produced a highly anisotropic viscosity that is not adequately expressed by common olivine flow laws, which may account for this apparent discrepancy.