Faults, fluids and geohazards at subduction zones

Abstract

Large magnitude earthquakes (MW 7.5) that occur within marine settings have the potential to generate powerful tsunamis with devastating consequences for coastal com munities. The 2004 MW 9.2 Aceh-Andaman megathrust earthquake and 2012 MW>8.0 intraplate earthquakes offshore North Sumatra served to highlight our limited understanding of the extent of seismogenesis associated with accretionary subduction zone margins, and the kinematics of oceanic intraplate deformation. International Ocean Discovery Program (IODP) drilling into the sediments on the incoming plate, in the Wharton Basin offshore North Sumatra in 2016 has provided new information on sediment depositional history, lithology and physical properties. These new data have been coupled with seismic reflection profiles and swath bathymetry to make detailed analyses of intraplate fault activity and to generate thermal models of the subduction zone input sediments. Pervasive faulting through the input sediments is the result of Riedel shearing in response to deeper left-lateral strike-slip deformation in the oceanic crust and upper mantle, which agrees with the complex rupture pattern of the 2012 earthquakes. Deformation and the timing of fault activity in the Wharton Basin is contemporaneous with activity throughout a diffuse plate boundary hosted by the wider Indian Ocean. The deformation is likely driven, and intensified, by an acceleration of the convergence between the Indian and Australian sub-plates around 7-9 Ma. Thermal models of the subduction zone input sediments indicate that high rates of accumulation and deep burial generates high temperatures, in excess of the requirements for thermally driven mineral dehydration, within the basal sediments before they reach the point of accretion/subduction. Fluids are liberated from the basal sediments since 1 Ma by the smectite-illite transformation in sufficient quantities to explain seismic reflection polarity reversal on the North Sumatra pre-decollement. The ´ interplay between lithology, sediment dehydration, faulting and hydrogeology within the incoming sediments control the position at which the decollement forms. ´ The southern Hikurangi margin is analogous to North Sumatra as an accretionary mar gin, exhibiting along-strike changes in: input sediment thickness, obliquity of convergence, and subduction velocity; with related variations in accretionary prism morphology and fault structure. Using widely spaced seismic reflection profiles (including new data acquired in 2017 by the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) processed in this study) that image the southern Hikurangi incoming plate and accretionary prism integrated with high resolution bathymetry, structural variations and their relationship to along-strike changes in the subduction zone con- figuration are described. The southern Hikurangi margin can be divided into three segments with contrasting characteristics: an eastern segment characterised by a moderately wide ( 70 km), low taper ( 5°) prism with rapid outward growth since 1 Ma, where seismically resolvable deformation accounts for 20% of crustal shortening, comparable with the central Hikurangi margin; a central segment characterised by a narrow ( 40 km), moderate taper ( 8°) prism, with similar shortening to the eastern segment, but taking place earlier (between 2 and 0.6 Ma), and where outward growth has effectively ceased; and a western segment where the prism widens (to 50 km) via rapid outward migration of the deformation front and utilisation of pre-existing in coming fault structure, accommodating very little to no crustal shortening since 2 Ma. These contrasting prism characteristics are interpreted to relate primarily to the westward increasing proximity of the Chatham Rise ancient accretionary complex, changing subduction velocities, and relative prominence of strike-slip faulting in the prism interior

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ORCID for Lisa Mcneill: orcid.org/0000-0002-8689-5882

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