Geochemical Characterisation of the Alpine Fault Zone from the DFDP Boreholes

Abstract:

The Alpine Fault of the South Island, New Zealand marks the active transpressional boundary between the Australian and Pacific plates. Phase one of the Deep Fault Drilling Project (DFDP1) drilled two holes that sample the Alpine Fault zone (DFDP1A and DFDP1B) in the near surface. Two distinct principal slip zones (PSZ) were recovered in these cores (one in DFDP1A and two in DFDP1B) enabling investigation of chemical and mineralogical changes throughout the fault’s hangingwall and footwall rocks.

Here we use geochemical analyses to identify fault rock protoliths, alteration styles, and mass changes in the fault zone to test the control of chemical alteration on fault rock material properties and compare with distal parts of the fault zone sampled in the second phase of DFDP (DFDP2). ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopes, and immobile trace element ratios identify protolith lithology contributions. We show that cataclasites above the upper principal slip zone in holes DFDP1A and DFDP1B contain a mixture of hangingwall Alpine Schist and radiogenic granitic and metasedimentary footwall lithologies indicating physical mixing of material up to ~25 m above the PSZ. In DFDP1B between upper and lower PSZs cataclasites distinctly resemble granitic footwall rocks, and below the lower PSZ radiogenic strontium isotope ratios identify porphyroclastic ultramylonite breccias as Australian plate Palaeozoic metasediments. Lithological mixing is overprinted by alteration of primary minerals to clays and infilling of pore spaces and fractures by calcite and chlorite. As proximity to the upper PSZ increases permeability decreases corresponding to an increase in volatile content (LOI). LOI peaks in the PSZ where permeability is lowest and clay content and carbonate cementation are greatest. Local, meteoric-derived spring waters are saturated in secondary minerals documented in the Alpine Fault zone and fault zone secondary mineral δD compositions indicate formation from meteoric waters. These findings imply that chemical alteration resulting from the circulation of meteoric waters modifies the fault zone’s permeability structure by redistributing mass and changing the material properties of the plate boundary, and therefore plays an important role in the evolution of the fault zone which may prime it for repeated rupture.