THE EFFECT OF SYN-TECTONIC HYDRATION REACTIONS ON THE STRENGTH OF MAFIC ROCKS

Field studies demonstrate that strain localization in mafic lithologies often occurs where syn-tectonic reactions change the mineralogy of the host rock. Fluid infiltration in the lower continental crust may enhance these reactions that consume water and stabilize hydrous phases, such as amphibole, creating a strength contrast between wet and dry lithologies and promoting strain localization. Thus, lower crust deformation may be accommodated in highly localized zones of amphibole-bearing rock. Hydration reactions in basaltic compositions have been well studied; however, the effect of these reactions on the strength of mafic rocks remains unquantified. We hypothesize that the formation of amphibole during deformation contributes to strain localization and may dramatically decrease the strength of mafic lithologies. To investigate the role of this syn-tectonic reaction on strain localization, we are conducting general shear deformation experiments in a Griggs solid-medium apparatus on powdered basalt at pressure and temperature conditions similar to those at the base of the continental crust and within the amphibole stability field (800ºC and 1GPa). The primary modes in the basalt starting material are plagioclase, pyroxene, olivine, and possibly minor glass. We powdered, sieved, and water settled this basalt to obtain grain sizes of ~5 to 15 µm. At axial strain rates of 10^-5 and 10^-6/s, basalt with 1 wt% water added reaches a peak strength of ~450 MPa at 0.8 to 1 γ, after which it begins to weaken. Initial microprobe analyses show that there is more Al and Mg in the matrix of the deformed samples, which indicates that amphibole may have formed during deformation. A comparison of these experiments to those conducted on amphibolite will allow us to examine the strength difference between a rock forming hydrous phases during deformation versus one possessing pre-existing abundant hydrous phases. This study is important because rocks of mafic to intermediate composition comprise much of the lower crust where amphibole may be the most stable and abundant hydrous mineral. The results of this study have important implications regarding strain accommodation along plate interfaces, deformation and flow of polyphase rocks, the fluid budget in the lower crust, and the rheology of rocks at depths where slow slip/tremor occur.