Seismic imaging evidence that forearc underplating built the accretionary rock record of coastal North and South America

The submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time.

Morphological Exposure of Rocky Platforms: Filling the Hazard Gap Using UAVs

Rock platforms are dangerous environments commonly subject to high wave energy on the open coast. Platform morphology is central to understanding what makes one stretch of coastline more hazardous than another, and it can be used to create site-specific morphological exposure hazard indices to assess the relative risk of being washed into the sea, assisting coastal managers in an effort to reduce the number of injuries and drowning incidents. This paper describes the use of an unmanned aerial vehicle (UAV) to derive morphological parameters for two data-poor rock platforms along the Illawarra coast of southern New South Wales, to fill the gap using an easily replicable site-specific hazard index, developed previously, that can be applied to other microtidal wave-dominated settings. The approach is based on the subdivision of the terrestrial seaward edge of platforms into segments, classified according to mean elevation, orientation and edge type, to model different weighting scenarios of predominant southeasterly and northeasterly wave direction. UAV-derived results were deemed satisfactory for all study sites, and a comparison of results derived from LiDAR for two platforms suggested that UAV data can be successfully used to guide risk policy on rock coasts, despite differences in the delimitation of the seaward edge due to tidal level during survey acquisition.

Sediment Transport Calculation Considering Laminar and Turbulent Resistance Forces Caused by Infiltration/Exfiltration and Its Application to Tsunami-Induced Local Scouring

A sediment transport calculation was proposed, which consistently considered the influence of laminar and turbulent resistance forces caused by infiltration/exfiltration. From a comparison of the nondimensional bed–load sediment transport rate, it was found to be essential to consider both laminar and turbulent resistance forces when formulating the influence of infiltration/exfiltration in sediment transport calculations. A three-dimensional coupled fluid–structure–sediment interaction model was improved using the proposed sediment transport calculation, and applied to tsunami-induced local scouring around an inland structure. Numerical results showed that consideration of infiltration/exfiltration improved the computational accuracy of the prediction of a scour hole formed around the seaward edge of the structure, and accordingly the improved model could capture the evolution of the scour hole with sufficient accuracy. This suggests that the improved model should be a useful tool for assessing tsunami-induced local scouring.

Sediment Transport Calculation Considering Laminar and Turbulent Resistance Forces due to Infiltration/Exfiltration and its Application to Tsunami-Induced Local Scouring

A sediment transport calculation which consistently considers the effect of laminar and turbulent resistance forces due to infiltration/exfiltration was proposed. From a comparison of the non-dimensional bed-load sediment transport rate, it is found to be essential to consider laminar resistance force as well as turbulent resistance force when formulating the effect of infiltration/exfiltration in sediment transport calculations. The proposed sediment transport calculation was incorporated to improve a three-dimensional coupled fluid-structure-sediment interaction model, and the improved model is applied to tsunami-induced local scouring around an inland structure. Numerical results show that the consideration of infiltration/exfiltration improves the computational accuracy of a scour hole formed around the seaward edge of the structure, and accordingly the improved model can capture the evolution of the scour hole with sufficient accuracy. This suggests that the improved model is expected to be a useful tool for assessing tsunami-induced local scouring.

Rock debris in an Antarctic ice shelf

We have discovered a band of stones and coarse sand in the Ronne Ice Shelf, Antarctica, some 60 m above the ice shelf’s base, 40 km from its seaward edge and 420 km from the point where the ice originally went afloat. A study of ice-sounding radar data from across the Ronne Ice Shelf has revealed other areas likely to contain debris in significant quantities. It appears that basal debris at the margins of ice streams feeding the ice shelf can be buried in the ice shelf by sea water freezing-on at the ice-shelf base. These findings are evidence for a mechanism active in a present-day ice-sheet/shelf system, which enables icebergs to transport large volumes of ice-rafted debris, and which also provides a potential mechanism for the formation of ice rises near ice fronts. We anticipate that a seismics study of debris melted from the ice shelf and deposited beneath will provide a valuable control on the history of ice-shelf–ocean interactions.