PROCESSES OF STRAIN WEAKENING: COMPARING SYNDEFORMATIONAL REACTION AND WEAK PHASE INTERCONNECTION

To determine the effect of syndeformation reaction in a rock undergoing ductile strain localization by interconnection of a weak phase, shear experiments have been performed using a fine-grained (d~50-100 mm) natural gneiss (13% Bt, 28% Pl, and 59% Qtz). The Bt grains are aligned but not interconnected and Qtz forms an interconnected framework. Samples with foliation parallel to the shear plane were deformed at two different temperature/strain rate combinations (800^oC, ~10^-5/s and 745^oC, ~10^-6/s) where the deformation mechanisms in the Qtz/Pl framework are the same, but the amount of Bt dehydration reaction varies (30% and <<1%, respectively).  Samples were deformed to low, medium and high strain (g~1, 1.5, and 3-4) at both sets of conditions; all samples underwent strain weakening, but samples with greater reaction had greater localization.

 All 745^oC samples yield at ~1350 MPa (g~1) and strain weaken.  Yielding occurs as stress concentrations at tips of Bt grains cause local grain-scale faults in Qtz/Pl, allowing interconnection of Bt by easy slip on (001). By g~1.5 samples are 20% weaker; there is local interconnection of Bt, but no through-going shear zone.  By g~2.1 the sample has fully weakened (by 35%); at g~3 there is a wide (~400 mm) through-going shear zone with an S-C' fabric and multiply interconnected Bt surrounding Qtz and Pl porphyroclasts. 

 The 800^oC samples have the same yield strength, and initial Bt interconnection occurs by the same mechanisms as in the 745^oC samples, with no reaction.  However, by g~3.2 the sample has fully weakened by 45% of its peak strength, and at g=4.3 there is a very narrow (100 mm) through-going shear zone with continuous layers of Bt (30% reacted) and discontinuous layers of recrystallized Qtz and Pl.  Reaction products (RP, 1-5 mm) in the Bt layers are thoroughly mixed with the Bt, indicating the operation of grain boundary sliding (GBS).  Strain partitioning is extreme (g=34 in the shear zone, 1.8 outside).     

 Although interconnection of Bt allows significant weakening of a Qtz/Pl rock, the degree of strain weakening and localization are much greater with concurrent reaction, indicating that GBS of fine-grained, mixed phase RP is weaker than (001) slip of Bt.  These results also indicate that plastic deformation enhances reaction kinetics.