Hikurangi Margin: Geology, Flow Rates, Water-Rock Interaction and Relative Fluid Ages

Abstract:

The subaerial part of the Hikurangi Accretionary Prism (HAP) has nearly 330 sources of saline waters (2000-26000 mg/kg Cl), SO₄-rich waters, CH₄-rich gases and occasional oil seeps discharging from springs, mud volcanoes and gas vents from -37.5^o to -41.3^o latitude, 80-100 km from the subduction margin. Discharge areas, occupying <0.01 to 11 ha each, form a band about 500 km in length with widths ranging from 25 km in the south to 75-100 km in the north and centre. All fluid discharges are cold except for two hot springs in the central and N segments and another 6 on the W margin of the HAP. The total flow rate along the HAP is at least 10 x 10⁸L/a, with nearly 50% contributed by cold discharges.

Most fluid manifestations emerge along NE-trending faults and minor structures such as anticlines and synclines. However in the N, where fluids are also discharged along NW- and EW-trending structures within the allochton, manifestations are relatively more abundant (>150 sites), discharging nearly 45% of the annual surface fluid volume along the HAP.

Fluid discharges in the HAP, based on chemical and isotopic fluid compositions, consist of subducted waters derived from clay water of hydration (<50%) and seawater that had interacted at varying degrees with organic-containing sedimentary rocks at depth at different temperatures. Discharges from the central segment exhibit the lowest degree of water-rock interaction and have the highest inferred subsurface temperatures (median: 110^oC compared to S at 80^oC and N at 90^oC), suggesting (1) more recent influx of subducted waters and (2) more rapid upflow of fluids from depth, relative to the S and N. Fluids in the S segment exhibit the highest degree of water-rock interaction suggesting longer residence time at depth, or slower fluid movement to the surface due to thicker crust or greater fluid channel tortuosity. The high concentration of isotopic He in the central and N fluids can be due to deep and highly permeable faults that channel mantle gases and also allow waters to rise very rapidly from as deep as the fracture zone between the overriding and down going plates. Fluid compositions indicate that the youngest fluids are rapidly expelled in the central and N segments although the latter is complicated by the presence of inherited allochton fluids.