Diopside microfabric development in lower-crust oceanic detachment fault zones

Exhumation of oceanic core complexes occurs through large-scale extensional shear zones that expose parts of the deformed gabbroic lower crust. However, it is not well understood how these high-temperature shear zones nucleate and develop. Since diopside is traditionally described as a load bearing phase in deforming systems, its microstructures may record the deformation mechanisms involved in the progressive stages of shear zone development. In this study, we focus on the fabrics of diopside in both the host coarse-grained gabbro and the adjacent high-temperature shear zone from the Atlantis Bank (IODP Exp 360), in order to better constrain the role of diopside during strain localization in deep crustal detachment fault zones.

In the host rock directly in contact to the shear zone, diopside porphyroclasts display microfractures filled with fine-grained diopside (~ 65 µm) and minor amounts (~10%) of plagioclase, amphibole and Fe-Ti oxides with grain size ~ 30 µm that occur as interstitial phases. Diopside grains in the microfractures have little internal deformation and are interpreted as “new” grains. On the other hand, fragments of the host diopside within the fracture are distinguished by their larger diameters of ~200 µm and dominant cleavage planes that is systematically missing in the new grains. These microstructures indicate cataclastic deformation with later precipitation of plagioclase, amphibole and Fe-Ti oxides. Other diopside porphyroclasts in the host rock show undulatory extinction, low-angle grain boundaries and new grains with crystallographic orientations controlled by the host, indicating dislocation creep.

Diopside porphyroclasts within the shear zone show undulatory extinction as well as bent cleavage planes and exsolution lamellae. New grains of diopside (~35 µm) that occur rimming the porphyroclasts - concentrated at sites of strong undulatory extinction - have long axes correlating the orientation of the bent cleavages within the host. These new grains have a crystallographic orientation with poles of (100) planes close to the X-axis and [001] axes close to the Z-axis, and high angle boundaries (>140º) with misorientation axes clustered between [001] and [100]. We propose that these new grains are a result of dislocation glide and growth due to bending of the host diopside during the early stages of shear zone nucleation.

In the strain shadow of the porphyroclasts within the shear zone, new grains of diopside (~20µm) occur together with amphibole, plagioclase and Fe-Ti oxides. They are rounded, strain free, have random orientations and the amount of diopside decreases with distance from the host. These grains are interpreted to have precipitated from the pore fluid during ongoing deformation of the shear zone.

We suggest that diopside in the host rock was deformed by cataclasis associated with dislocation glide during nucleation of the shear zone at probably high stress, as indicated by the similar microfabric of diopside porphyroclasts in the shear zone compared to those in the host rock. Unlike, ongoing deformation localized within the shear zone is due to dissolution and precipitation, as indicated by the polyphase aggregates in the strain shadows and in the matrix.