Along-strike variations in porosity at the Hikurangi Margin, New Zealand from electromagnetic data

Abstract:

Pore fluids can affect the local stress state of a subduction margin by increasing fluid overpressure, which decreases effective normal stress and promotes stable sliding. This implies that a more complete understanding of the fluid budget of a margin will aid in regional hazard assessment. The Hikurangi Margin offshore the North Island of New Zealand exhibits a number of intriguing variations along its strike, including increased plate coupling to the south and the shallowing of slow slip event locations to the north. Prior studies have suggested that variations in the amount of fluids at the plate interface along the margin might play a dominant role in modulating plate coupling and promoting slow slip, therefore determining the amount of fluid subducted and where it is released are key to understanding this subduction system. To that end, in December 2018 - January 2019, we collected magnetotelluric (MT) and controlled-source electromagnetic (CSEM) data along three trench-crossing profiles at the Hikurangi Margin. Electromagnetic data are particularly sensitive to the presence of conductive fluids in pore spaces of more resistive material, and thus they are an ideal tool for mapping the margin’s porosity. Here we present resistivity models for the three profiles from joint inversion of the MT and CSEM data. Using Archie’s law, we convert those resistivities into porosity images and show that the incoming plate porosities are similar for the central and southern sections of the margin whereas the northern margin displays a more complex incoming plate porosity. Within the forearc, we observe distinct depth-porosity relationships for each profile. These observations suggest that processes occurring in the forearc and subducting slab may be more important for controlling fluid overpressure than along-strike variations in the incoming plate.