Segmented Coastal Uplift Along an Erosional Subduction Margin, Northern Hikurangi Fore Arc, North Island, New Zealand
Abstract
The Hikurangi subduction margin along North Island, New Zealand accommodates oblique convergence of the Pacific plate westward beneath the Australian plate at 45 mm/yr. Along the southern margin, frontal accretion and pronounced forearc uplift occur inboard of the subducting Hikurangi plateau. In the north, subduction erosion and segmented uplift occur inboard of subducting seamounts along the plateau flank. Prior workers have established a robust foundation for coastal terrace studies along the northern Hikurangi margin (e.g., Berryman et al., 1989; Ota et al., 1992; Berryman, 1993; Wilson et al., 2006, 2007; Clark et al., 2010; Litchfield et al, 2007, 2010). New field observations presented here provide additional constraints on terrace uplift along this erosional subduction margin. Along Raukumara Peninsula (north of Poverty Bay), multiple Holocene to late Pleistocene marine and fluvial terraces occur at varying elevations, recording differential uplift across six coastal segments from Gisborne to East Cape (Ota et al., 1992; Wilson et al., 2007). In this study, two to three late Pleistocene terraces were observed on rocky headlands within the first segment (Gisborne to Whangara) at elevations of 80-185 m above msl. Preliminary correlation with OIS 5a-e sea level high stands (80-125 ka) indicates net uplift at 1.2-1.5 m/ky. Uplifted Holocene wavecut platforms occur in steps along the seaward edge of these terraces, consistent with coseismic uplift. At Makorori Point, an uplifted bench occurs along the modern seacliff at 2.3 m above the cliff base. A fossil gastropod shell from paleo-beach gravels on the platform inner edge yielded a calibrated radiocarbon age of 1680 ×110 ybp. At Turihaua Point, a ≥1 m thick deposit of Holocene beach sands overlies an uplifted wavecut platform at ≥1.5 m above mean sea level. Carbonate-cemented beachrock at the base of the sand deposit yields a calibrated radiocarbon age of 2990 ×70 ybp. At Mahia Peninsula (between Poverty Bay and Hawke's Bay), a prominent flight of Holocene and late Pleistocene marine terraces (OIS 1-7) record outer forearc uplift at 1.0-2.5 m/ky above the upper-plate Lachlan thrust (Berryman, 1993). Five steps of discrete age within the Holocene terrace are interpreted as coseismic uplift events ranging in age from 250 - 4500 ybp. At Cape Kidnappers (south of Hawke's Bay), at least three late Pleistocene marine terraces were observed in this study at 50-120 m above msl. Preliminary correlation with OIS 5a-e sea level high stands indicates net uplift at 0.8-1.0 m/ky along the Kidnappers anticline. Adjacent Holocene shore deposits overlie a 6 m high uplifted wavecut platform that records coseismic uplift circa 2300 ybp (Hull, 1987). Tectonic uplift along the northern Hikurangi margin is the net result of a complex interaction between megathrust slip at depth and localized upper-plate contraction on steeply-dipping imbricate thrust faults. The segmented uplift pattern may reflect the focused impact of subducting seamounts and consequent variations in subduction erosion and downdip underplating of sediments and eroded debris.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T43E2710M
- Keywords:
-
- 8175 TECTONOPHYSICS Tectonics and landscape evolution;
- 8170 TECTONOPHYSICS Subduction zone processes;
- 3060 MARINE GEOLOGY AND GEOPHYSICS Subduction zone processes;
- 1824 HYDROLOGY Geomorphology: general