MODELING THE RELATIONS AMONG PORPHYROBLAST GROWTH, PSEUDOMORPHISM AND STRAIN LOCALIZATION IN TURBIDITE SUCCESSIONS

Turbidites show marked gradients in bulk composition and modal mineralogy causing transient rheologic layering during metamorphism. At low metamorphic grades, more pelitic portions of turbidite couplets are weaker during shear due to the abundance of fine-grained phyllosilicates, compared with coarser-grained quartz and feldspar in the psammitic layers. At intermediate grades, rigid porphyroblasts grow preferentially in the pelitic layers, causing them to become strong relative to the porphyroblast-free psammitic layers. Porphyroblasts may then be pseudomorphed by less rigid minerals during prograde or retrograde metamorphism, causing the pelitic layers to once again become relatively weak. Strain will transiently partition into different layers depending on the timing of deformation relative to metamorphism in this sequence.

We present a field example from eastern New Hampshire that illustrates this evolving rheologic structure during syn-orogenic prograde metamorphism. The pervasive foliation in these rocks (S1) refracts into pelitic layers, suggesting that these layers were strong relative to psammitic layers. However, an overprinting crenulation cleavage (S4) refracts into the psammitic layers, suggesting that they were relatively stronger during this later deformation. We hypothesize that the change in relative strength is the result of the replacement of coarse-grained andalusite porphyroblasts by muscovite and fibrolitic sillimanite in pelitic layers after the first deformation.

We conducted numerical experiments to explore the relationship between porphyroblast growth and pseudomorphism and the evolving rheology of turbidite successions. Our experiments are constrained by the field example and consist of layered sequences with viscosity gradients defined by changes in grain size and mineralogy. We investigate the dependence of effective viscosity in pelite layers on porphyroblast abundance, size, shape and distribution. We also vary the timing of pseudomorph replacement between different layers to investigate the role of reaction timing relative to deformation on strain partitioning. Preliminary results suggest that the presence of porphyroblasts can lead to a three-fold increase in viscosity of pelitic layers relative to psammitic layers.