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Master of Science (MS)
Amos, Colin B.
Larsen, Isaac James
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 subduction coupling on forearc deformation over multiple millennia. There, the Wairarapa coastline 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 from 10Be in fluvial sands, I examined the geomorphology of the HSZ forearc to evaluate potential connections between plate coupling and forearc erosion and uplift. I calculated basin-averaged metrics including normalized channel steepness (ksn), gradient, relief, and drainage area for 70 fluvial catchments along the Wairarapa coastline and selected nine of those basins for cosmogenic 10Be sampling. I compared these metrics to existing inventories of coastal uplift rates measured from Holocene – Late Pleistocene marine terraces, ranging from 0.3 - 3.7 mm/yr and varying at ~100 km wavelengths. Catchment-averaged erosion rates largely mirror coastal uplift rate and range from 0.5 - 3.4 mm/yr, indicating relatively fast erosion within each of the sampled basins. The highest rates (≥ 2 mm/yr) do not correlate strongly with uplift or other topographic metrics and likely represent delivery of sediment originating below the cosmogenic shielding depth through shallow landsliding or gullying. In general, the greatest relief and steepest channels occur in the Aorangi Range at the southernmost portion of the uplifted forearc. There, basins are formed in the oldest basement greywacke sandstones and lie directly above the locked portion of the megathrust. For basins spanning the entire Wairarapa coast, basin-averaged ksn shows a strong correlation with catchment-averaged slope, relief, and precipitation, but does not correlate as well with coastal uplift or erosion rate given the range and variability of rock types underpinning coastal basins. Examining these relationships for the Aorangi Range, where the underlying geology is comparatively uniform and plate coupling is strongest, reveals robust, positive correlations between ksn, uplift rate, and erosion rate. The strongly locked region is also where the highest topography, steepest channels, and greatest uplift rates are found. This relationship may indicate that the zone of coupling is stable over geological time and is driving the higher rates of uplift, erosion, and exhumation seen in the Aorangi Range over millennia. My results suggest that subduction coupling is a key driver of long-term forearc erosion and topographic development either through: (1) increased uplift during megathrust earthquakes in the strongly coupled region, or (2) through faster slip on upper plate faults driven by increased stresses from the underlying locked megathrust.
Geology, Tectonic geomorphology, Subduction coupling, Erosion rates, Hikurangi subduction zone, New Zealand, Forearc erosion, Megathrust earthquakes, Channel steepness, 10Be cosmogenic erosion analysis, Topographic analysis, Upper plate faulting
Western Washington University
Subject – LCSH
Morphotectonics--New Zealand--Wairarapa; Subduction--New Zealand--Wairarapa; Plate tectonics--New Zealand--Wairarapa; Coast changes--New Zealand--Wairarapa
Wairarapa (N.Z.)--Earthquake effects
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Jensen, Chantel E., "Forearc erosion in response to megathrust plate coupling along the Wairarapa Coastline, New Zealand" (2022). WWU Graduate School Collection. 1077.