Mount Stewart‐Halcombe Anticline: A look inside a growing fold in the Manawatu region, New Zealand

Abstract

A seismic reflection profile across the c. 25 km long, NNE‐trending Mount Stewart‐Halcombe Anticline in the southeastern Wanganui Basin, along with reinterpretation of its stratigraphy, reveals details of the structure and development of the fold. Two reverse faults (I and II), 1 km apart, bound the eastern side of the anticline and dip at 60— 65° beneath the anticline; a third reverse fault (III) dips 60° to the east towards the adjacent Feilding Anticline. The faults offset basement (Torlesse greywacke), Mangapanian, Nukumaruan, and lower Castlecliffian strata, but do not propagate through upper Castlecliffian ‐ Haweran strata to the surface; this upper part of the sedimentary section is folded but not faulted. Progressive deformation of the sedimentary sequence, as well as differences in sedimentary thickness across the faults, indicates that the Mount Stewart‐Halcombe Anticline has been growing from at least Mangapanian time (c. 3.1 Ma) through to the present. Topographic expression of the anticline, combined with seismic reflection data, shows that the anticline axis and its controlling faults curve from a northerly trend at Mount Stewart to an ENE trend 15 km farther south.

Correlations of horizons from the oil exploration well Santoft‐1A to Mount Stewart, using pre‐existing seismic data, as well as nearby occurrences of Castlecliffian sediments, demonstrate that the sedimentary sequence on Mount Stewart includes Castlecliffian strata, which had previously been thought to be absent. Based on correlations from Santoft‐1A, we date the oldest sediments on the flanks of Mount Stewart as Mangapanian, whereas on the crest of Mount Stewart, at the location of the oil exploration well Young‐1, onlap has cut out the Mangapanian, and Nukumaruan sediments lie on basement. “Upper Waitotaran” ages proposed by previous authors for the base of the sequence in Young‐1 are therefore too old.

Fault I, the westernmost fault, appears to be the only fault of the three to be currently active. Average dip‐slip rates over the period 2.6 to c. 0.6 Ma were between 0.1 and 0.2 mm/yr for faults I and II, whereas fault III was slightly slower at just under 0.1 mm/yr. Faults II and III have been inactive since c. 0.4 Ma, whereas fault I has moved faster over the period 0.2 Ma to the present, at c. 0.3 mm/yr. Based on a slip‐rate of 0.3 mm/yr for fault I, and assuming 1–4 m of dip‐slip displacement per earthquake, the return time for M6.5—7+ earthquakes on the Mount Stewart—Halcombe Anticline is at least several thousand years and may be longer than 10 000 years.

Keywords:

• Mount Stewart—Halcombe Anticline
• Manawatu
• Wanganui Basin
• seismic reflection
• active faults
• active folds
• slip rate
• Castlecliffian
• seismic hazard