DOES FOREARC EROSION MIRROR SHORT-TERM MEGATHRUST EARTHQUAKE CYCLE DEFORMATION?

How the subduction zone earthquake cycle contributes to uplift, erosion, and permanent deformation of the overlying forearc remains largely unknown. The Hikurangi subduction zone (HSZ), along the east coast of the North Island of New Zealand, provides a unique location to examine the effects of plate coupling on forearc deformation over multiple millennia. There, the Wairarapa-Hawke’s Bay (W-HB) coast runs parallel to the HSZ and spans a transitional boundary between locked and freely slipping portions of the plate interface. Using digital topographic analysis and catchment-averaged erosion rates, we examined the geomorphology of the HSZ forearc to evaluate potential connections between plate coupling and forearc erosion and uplift. We calculated basin-averaged metrics such as channel steepness (k_sn), gradient, relief, and drainage area for 58 fluvial catchments along the W-HB coast. We compared these metrics to existing inventories of coastal uplift rates spanning the Late Pleistocene, and selected nine basins for cosmogenic ¹⁰Be erosion rate measurements. In general, our analyses show that coastal uplift varies at ~100-km wavelengths, similar to the length of upper plate faults that may splay at depth from the subduction interface. Basin-averaged k_sn and relief are positively correlated along the entire coastline. The greatest relief and steepest channels are present in the southernmost portion of the uplifted forearc. Channels there are almost entirely underlain by basement greywacke sandstones and display a strong, positive correlation between k_sn and coastal uplift rates. Elsewhere along the W-HP coast, coastal uplift rates and k_sn are not generally correlated, which may be due to variability in the type and durability of the underlying bedrock. Pending cosmogenic ¹⁰Be erosion rate measurements will allow us to examine the distribution of erosion rates along the W-HP coast, with the goal of understanding controls from the underlying plate interface on topographic and geomorphic development of the overlying forearc.