The rheology of the lower continental crust profoundly affects our understanding of crustal dynamics, lithospheric curst/mantle coupling, and seismic hazards. However, the effect of processes associated with syntectonic hydration and metamorphism, deformation in polyphase rocks, and the evolution of a strong mineral fabric on lower crust rheology are not well constrained. To examine these processes, we conducted field and experimental investigations of deformation processes in hydrated, mafic lithologies. Field observations from a lower crust gabbro shear zone show that over ~1 meter the gabbro progresses from nominally undeformed to highly sheared where it is adjacent to a hydrous pegmatite. Phase diagrams, the presence of hydrous reaction products (amphibole), and deformation mechanism maps indicate that the water activity during deformation must have been high (~1) in the sheared gabbro compared to the non-hydrated, surrounding host gabbro. These observations indicate that fluid intrusion into mafic lower crust initiates syn-deformational, water-consuming reactions, creating a rheological contrast between wet and dry lithologies that promotes strain localization. Thus, deformation of lower continental crust can be accommodated in highly localized zones of enhanced fluid infiltration. To examine the effect of these hydration reactions on mineral fabric development and mafic rock strength, we conducted deformation experiments on powdered basalt with added water at lower crust conditions (800 ºC, 1 GPa). Amphibole formed during deformation exhibits both a strong shape preferred orientation (SPO) and lattice preferred orientation (LPO). With increasing strain, amphibole LPO rotates and strengthens with [001] maximum aligned sub-parallel to the flow direction and SPO. At low effective strain rates (10^−5 to 10^−6/s), the stress exponent is consistent with deformation accommodated by diffusion creep. The correlation of the SPO and LPO coupled with the rheological evidence for diffusion creep and progressive rotation of amphibole [001] in the direction of shear with increasing strain indicates that amphibole LPO forms via passive grain rotation. Additionally, the rheology of these amphibolites is comparable to that predicted using flow laws for wet anorthite, indicating that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust.
Getsinger, Amanda Jean,
"The Rheology of Amphibolite"
(2015).
Earth, Environmental and Planetary Sciences Theses and Dissertations.
Brown Digital Repository. Brown University Library.
https://doi.org/10.7301/Z04J0CH3
