Deciphering the 3-D distribution of fluid along the shallow Hikurangi subduction zone using P- and S-wave attenuation
Abstract
We use local earthquake velocity spectra to solve for the 3-D distribution of P- and S-wave attenuation in the shallow Hikurangi subduction zone in the North Island of New Zealand to gain insight into how fluids control both the distribution of slip rate deficit and slow-slip events at the shallow plate interface. Qs/Qp gives us information on the 3-D distribution of fluid saturation, which we can compare with the previously determined 3-D distribution of Vp/Vs, which gives information on pore fluid pressure. The Hikurangi margin is unusual, in that a large igneous province (the Hikurangi Plateau) is being subducted. This plateau has had two episodes of subduction-first at 105-100 Ma during north-south convergence with Gondwana, and currently during east-west convergence between the Pacific and Australian plates. We find that in the southern part of the subduction zone, where there is a large deficit in slip rate at the plate interface, the plate interface region is only moderately fluid-rich because the underlying plateau had already had an episode of dehydration during Gondwana subduction. But fluid pressure is relatively high, due to an impermeable terrane in the upper plate trapping fluids below the plate interface. The central part of the margin, where the slip rate deficit is very low, is the most fluid-rich part of the shallow subduction zone. We attribute this to an excess of fluid from the subducted plateau. Our results suggest this part of the plateau has unusually high fracture permeability, on account of it having had two episodes of bending-first at the Gondwana trench and now at the Hikurangi Trough. Qs/Qp is consistent with fluids migrating across the plate interface in this region, leaving it drained and producing high fluid pressure in the overlying plate. The northern part of the margin is a region of heterogeneous deficit in slip rate. Here the Hikurangi Plateau is subducting for the first time, so there is less fluid available from its dehydration than in the central region. Fluid pressure in the overlying plate is high, but Qs/Qp indicates that it is not uniformly fluid-rich. This heterogeneity is consistent with the rough topography of the plateau, including seamounts which entrain fluid-rich sediments. Deep slow-slip events in the southern part of the margin occur where the Moho of the overlying plate meets the plate interface, as typically seen in other deep slow-slip events worldwide. But in the central and northern parts of the margin, the locations of shallow slow-slip events appear to be controlled by a shallow brittle-viscous transition within the fluid-rich upper plate. There is also evidence that a major fault zone in the overlying plate might bleed off some of the high fluid pressure promoting slow-slip events.
- Publication:
-
Geophysical Journal International
- Pub Date:
- November 2017
- DOI:
- 10.1093/gji/ggx348
- Bibcode:
- 2017GeoJI.211.1032E
- Keywords:
-
- New Zealand;
- Crustal imaging;
- Seismic attenuation;
- Seismic tomography;
- Subduction zone processes