Development of submarine canyon systems on active margins: Hikurangi Margin, New Zealand.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The development and activity of submarine canyons on continental margins is strongly influenced by temporal and spatial changes in sediment distribution associated with orbitally-forced sea-level cyclicity. On active margins, canyons are also strongly influenced by tectonic processes such as faulting, uplift and earthquakes. Within this framework the role of mass-wasting processes, including sediment failures, bedrock landslides and sediment gravity flows, are to: 1) transport material across the slope; 2) act as intra-slope sediment sources; and 3) shape seafloor morphology. In this project the seafloor-landscape signatures of tectonic and geomorphic processes are analysed to interpret the development of submarine canyon morphology on active margins. Datasets include high-resolution bathymetry data (Simrad EM300), multichannel seismic reflection data (MCS), high-resolution 3.5 kHz seismic reflection data, sediment cores, and dated seafloor samples. High-resolution bathymetric grids are analysed using techniques developed for terrain-roughness analysis in terrestrial landscapes to objectively map and interpret features related to seafloor mass-wasting processes. The Hikurangi subduction margin of New Zealand provides world-class examples of the control of tectonic and sedimentary processes on margin development, hosting multiple examples of deeply-incised canyon systems across a range of scales. Two main study sites, in Poverty Bay and Cook Strait, provide examples of canyon formation. From these examples conceptual and representative models are developed for the spatial and temporal relationships between active tectonic structures, geology, sediment supply, slope- and shelf-incised canyons, slope gully systems, and bedrock mass failures. The Poverty Bay site occurs on the subduction-dominated northern Hikurangi Margin, where the ~3000 km² Poverty re-entrant hosts the large Poverty Canyon system, the only shelf-break-to-subduction-trough canyon on the northern margin. The geomorphic development of the re-entrant is affected by gully development on the upper slope, and multi-cubic-kilometre-scale submarine landslides. From this site the study focuses on the initiation and development of upper-slope gullies and the role of deep-seated slope failure in upper-slope evolution. The Cook Strait site occurs on the southern Hikurangi Margin in the subduction-to-strike-slip transition zone. The 1800 km² Cook Strait Canyon incises almost 50 km into the continental shelf, with a multi-branching canyon head converging to a deeply slope-incised meandering main channel fed by multiple contributing slope canyons. Other medium-sized canyons are incised into the adjacent continental slope. Fluvial sediment supply to the coast is relatively low on the southern margin, but Cook Strait is subject to large diurnal tidal currents that mobilise sediment through the main strait area. Prior to the morphostructural analysis of the Cook Strait and Poverty study sites a revision of the tectonic structure was undertaken. In Cook Strait a revision of the available fault maps was undertaken as part of a wider, related tectonic study of the central New Zealand region. In Poverty Bay very limited prior information was available, and as part of this study the structure and stratigraphy of the entire shelf and upper slope has been interpreted. On active tectonic margins submarine canyons respond to tectonics at: 1) margin-setting scales relating to their ability to become shelf incised; 2) regional scales relating to canyon-incision response to base-level perturbations; and 3) local scales relating to propagating structures affecting canyon location and geometry. Interpretation of the spatial distribution of fluid vent sites, gully development and landslide scars leads to the conclusion that seepage-driven failure is not a primary control on the widespread instances of gully formation and landslide erosion affecting structurally-generated relief across the margin. Rather, the erosion of tectonic ridges is dominated by tectonics by: slope oversteepening; weakening of the rockmass in fault-damage zones; and triggering of slope failure by earthquake-generated cyclic loading. Deep-seated mass failures affect numerous aspects of submarine landscapes and play a major role in the enlargement of canyon systems. They enable the development of slope gully systems and represent a major intra-slope sediment source. Quantitative morphometric analysis together with MCS data indicate that landslides may evolve to be active complexes where landslide debris is remobilized repeatedly, analogous to terrestrial-earthflow processes. This process has not previously been documented on submarine slopes. A model is presented for the evolution of active margin canyons that contrasts highstand and lowstand canyon activity in terms of channel incision, sedimentary processes and slope-erosion processes. During sea-level highstand intervals, canyons become decoupled from their terrestrial/coastal sediment-supply source areas, while during sea-level lowstand intervals, canyons are coupled to fluvial and littoral sediment-supply sources, and top-down (i.e. shelf-to-lower-slope) sediment transport and channel incision is active. Canyon-head areas are incision dominated during the lowstand while mid to lower canyon reaches experience both a transient increase in sediment in storage and canyon-fill degradation and incision into bedrock. Tectonics influences the canyon landscape through both uplift-controlled perturbations to canyon base-levels and earthquake-triggering of mass movement. Following sea-level rise the sediment supply to canyon heads will be switched off at a certain threshold sea level. From this point canyon heads become aggradational. Mid to lower canyon reaches continue to incise due to continuing tectonic uplift and earthquake-triggered slope instability. Knickpoints are propagated up channel and excavate canyon and sub-canyon channels from the bottom up. Thus, while top-down infilling of non-coupled canyons occurs during sea-level highstands, the lower reaches of active margin canyons continue to incise due the influence of tectonic processes.