Jeffrey H. Marsh

Kinematic vorticity analysis and evolving strength of mylonitic shear zones: New data and numerical results

The kinematic vorticity number is an important quantity in structural geology and tecton- ics, giving a nonlinear ratio of simple shear to pure shear deformation. We use natural obser- vations and numerical models to show how rigid clast methods for determining the kinematic vorticity number (W k ) are compromised where strain localization occurs at the matrix-clast interface. Our numerical results show that the critical shape factor cutoff between perma- nently rotating and stable clasts, used to determine W k , is highly sensitive to coupling between the clast and the matrix. This fi nding provides an elegant explanation for the fact that rigid clast methods tend to underestimate W k relative to other methods. We present numerically determined envelopes for clast behavior across a range of kinematic vorticity numbers, clast shape factors, and matrix-clast coupling. Our numerical models show that the shape-pre- ferred orientations of feldspar clasts trend toward the positions of mica fi sh with increasing localization at the clast boundary, suggesting that mica fi sh behave as highly lubricated clasts. Our data and numerical results show that the clast-matrix interface may be several orders of magnitude weaker than the surrounding matrix and that weak interfaces can lead to a marked drop in the bulk shear strength of faults and shear zones.

Timing and conditions of poly-phase metamorphism within the Twelve Mile Bay shear zone: implications for the evolution of mid-crustal decollement zones and western Grenville tectonics

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (˜10–11 kbar and ˜840°C) metamorphism overprinted by amphibolite facies (˜5–7 kbar and ˜650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and 207Pb/206Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with 207Pb/206Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.