Phase stability and metamorphic reactions related to intraplate seismicity in the Nazca Plate

Subduction zones are complex geotectonic environments where multiple processes interact resulting in different kinds of seismicity. Among them is intermediate depth intraplate seismicity, which occurs within the subducting plate in conditions that should favor ductile shear rather than fragile faulting. A high variability in focal mechanisms and spatial distribution has been observed globally for this kind of events. In some subduction zones a double seismicity zone develops while in others not. Moreover, these earthquakes may occur in dry and hydrated conditions. Therefore, there is no consensus on the process that originate them.

In the context of hydrated subductions, such as is the Chilean case, the influence of fluids liberated through the metamorphism of the slab is generally considered as the main triggering factor. It is therefore important to know at what pressure and temperature and between which mineral associations these reactions occur.

Hacker et al. (2003) compiled information on average mineralogy and whole rock composition to create phase diagrams which have allowed the study of dehydration reactions and intraplate seismicity around the world. However, their work is based on data from the FAMOUS area in the Atlantic Ridge for the MORB and the Semail ophiolite for ultramafic rocks, which do not correlate to compositions in the Nazca Plate.

To better constrain the conditions on which dehydration reactions take place within the Nazca slab, we used PERPLE_X to calculate pseudosections with geochemical data more representative of it. We created a simple model of the plate consisting of a top layer with MORB compositions from drilled and dredged samples for the crust and a bottom layer with ultramafic rock compositions obtained from ophiolites from a geotectonic context consistent with that of the Nazca Plate. We then coupled the pseudosections with a kinematic thermal model of the Chilean subduction zone to create profiles of stable mineral associations and hydration gradient along the subducted slab.

We observe that, for constant PT, hydrated mineral stability is mainly controlled by the initial (pre-subduction) slab hydration percentage and in a much lesser extend by slab composition. For areas where slab hydration is constrained by geophysical data, we tested different slab compositions and found that modelling with data from Nazca Plate layer 2 basalts and a mid ocean ridge type ophiolites provides the best fit to seismic data. It appears that intraplate seismicity nucleates along areas with strong hydration gradients, i.e. where dehydration reactions occur. We then extrapolated these compositions to the rest of the plate and with the assumption that the correlation observed between hydration gradient and intraplate seismicity hypocenters is maintained along the margin, we estimated hydration percentages along 5 latitudinal profiles.  Although further work remains to improve our seismic catalogues and spatial resolution of the thermal model, preliminarily it seems that the Nazca Plate is more hydrated in northern Chile (~2.5%) and less to the south (~1%) and that the Chilean double seismicity zone only occurs where hydration is above ~2%.