Seamounts are sponges, and other insights from electromagnetic imaging at the northern Hikurangi Margin, New Zealand
Abstract
How does seafloor topography, that is seamounts, ridges, and other features that stick up above the sediment cover, affect subduction processes, particularly those related to megathrust fault slip? Before extensive geodetic networks and collocated seismic data were more widely available, it was hypothesized that subducting seamounts should act as rupture asperities whose size could generate large subduction earthquakes. Although some examples suggest that subducting topography may be associated with large megathrust events, more recent geophysical observations point to a link between such topography and aseismic creep, microseismicity, and slow earthquakes. Analogue models show that subducting topographic relief generates complex fracture networks in the overriding plate, which are unlikely to support earthquakes that rupture over large areas and are more likely to slip through a combination of small-earthquakes and creep. However, direct geophysical imaging of the complex fracture networks proposed and the hydrology of both the subducted topography and associated upper plate damage zones remains elusive. We will present results from magnetotelluric and controlled-source seafloor electromagnetic data collected at the northern Hikurangi Margin, New Zealand where active seamount subduction is occurring. We will show that the internal structure of a seamount on the incoming plate allows at least 3.24.7x more water than normal, unfaulted oceanic lithosphere to subduct. In the forearc, the data reveal a sediment-starved plate interface above a subducting seamount with similar electrical structure to the incoming plate seamount. A sharp resistive peak within the subducting seamount lies directly beneath a prominent upper plate conductive anomaly. The coincidence of this upper plate anomaly with the location of burst-type repeating earthquakes and seismicity associated with a recent slow slip event, directly links subducting topography to the creation of fluid-rich damage zones in the forearc that alter the effective normal stress at the plate interface by modulating fluid overpressure. In addition to severely modifying the structure and physical conditions of the upper plate, subducting seamounts represent an underappreciated mechanism for transporting water to the subduction system.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.T43A..06C