Splay fault branching from the Hikurangi subduction shear zone: Implications for slow slip and fluid flow. (25th December 2016)

• Record Type: Journal Article
• Authors: Plaza‐Faverola, A.; Henrys, S.; Pecher, I.; Wallace, L.; Klaeschen, D.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 17:Number 12(2016:Dec.)
• Page Start: 5009
• Page End: 5023
• Publication Date: 2016-12-25
• Subjects: PSDM seismic -- P wave velocity -- décollement -- slow slip -- seepage -- underplating; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2016GC006563
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Prestack depth migration data across the Hikurangi margin, East Coast of the North Island, New Zealand, are used to derive subducting slab geometry, upper crustal structure, and seismic velocities resolved to ∼14 km depth. We investigate the potential relationship between the crustal architecture, fluid migration, and short‐term geodetically determined slow slip events. The subduction interface is a shallow dipping thrust at <7 km depth near the trench and steps down to 14 km depth along an ∼18 km long ramp, beneath Porangahau Ridge. This apparent step in the décollement is associated with splay fault branching and coincides with a zone of maximum slip (90 mm) inferred on the subduction interface during slow slip events in June and July 2011. A low‐velocity zone beneath the plate interface, updip of the plate interface ramp, is interpreted as fluid‐rich overpressured sediments capped with a low permeability condensed layer of chalk and interbedded mudstones. Fluid‐rich sediments have been imbricated by splay faults in a region that coincides with the step down in the décollement from the top of subducting sediments to the oceanic crust and contribute to spatial variation in frictional properties of the plate interface that may promote slow slip behavior in the region. Further, transient fluid migration along splay faults at Porangahau Ridge may signify stress changes during slow slip. Plain Language Summary: Advances in seismic hazard assessment depend on an understanding of how shallow portions of the subduction plate boundary interface change from strong seismic behavior to weak, aseismic behavior. However, the physical factors that control such transitions are still debated. Seismic reflection images can reveal the physical processes controlling subduction slip behavior across the central part of Hikurangi subduction margin, offshore New Zealand. This part of the margin is mostly stuck and accumulating stress likely to be relieved by future earthquakes. However, some regions of the plate boundary are also creeping, via episodic slow slip events, over days to weeks. Our seismic line crossed a region that experienced a slow slip sequence in 2011 and suggests that near the zone of maximum slip the plate boundary steps down and merges with major faults that extend from the plate interface to the seafloor. These features contribute to spatial variation in frictional properties of the plate interface that may promote slow slip. These structures may also provide pathways for fluid migration towards the surface and explain documented seafloor seeps. In which case, fluid seepage at the seafloor potentially provides a window into processes occurring on the subduction interface. Key Points: PSDM image reveals Hikurangi accretionary wedge architecture and P wave distribution to 14 km depth Plate interface step down from top of subducting sediments to oceanic crust may drive margin underplating Spatial variation in frictional properties of the plate interface may promote slow slip behavior