Neotectonics and Paleoseismicity of a Major Junction Between Two Strands of the Awatere Fault, South Island, New Zealand

Abstract

In northeastern South Island, New Zealand, obliquely-convergent relative motion between the Pacific and Australian plates is accommodated by slip across active dextral-oblique faults in the Marlborough fault system. The Awatere Fault is one of four principal active strike-slip faults within this plate boundary zone, and includes two sections (the eastern and Molesworth sections) that have different strikes and that join across a complex fault junction in the upper Awatere Valley. Detailed mapping of the fault traces and measurement of 97 geomorphic displacements along the Awatere Fault in the vicinity of the fault junction show that the eastern and Molesworth sections of the fault intersect one another at a low angle (10-15º), at the eastern end of an internally faulted, elongate, ~15 km long and up to 3 km wide fault wedge or sliver. The region between the fault sections is split by a series of discontinuous, en-echelon scarps that are oriented from ~10º to 20-30º clockwise from the principal fault sections. Based on other observations of discontinuities in strike-slip earthquake ruptures around the globe, this low-angle intersection geometry suggests that the junction between these fault sections may not act as a significant barrier to earthquake rupture propagation. This interpretation of the mechanical significance of the fault junction to earthquake ruptures is counter to previous suggestions, but is supported by new paleoseismic data from four paleoseismic trenches excavated on each side of the junction. In a new paleoseismic trench on the Molesworth section at Saxton River, 18 km to the west of the junction, up to ten surface-rupturing events in the past ~15 ka are recognised from 12 radiocarbon ages and 1 optically stimulated luminescence age. In two new trenches on the eastern section near to Upcot Saddle, 12 km northeast of the fault junction, five events took place in the past 5.5 ka, based on 21 radiocarbon ages. This chronology from Upcot Saddle is combined with data from two previous trenches located ~55 km to the northeast at Lake Jasper, to infer nine events on the eastern section since 8330-8610 cal. years B.P. These well-dated events on the eastern section are compared to those on the Molesworth section to the west of the fault junction. At 95% confidence, five events on both sections have occurred with statistical contemporaneity since ~6 ka B.P. These five events may have ruptured both the eastern and Molesworth sections simultaneously, in accordance with the interpretation that the fault section junction does not arrest rupture propagation. Alternatively, these events may have been separate earthquakes that occurred within the statistical resolution provided by radiocarbon dating. The most recent event to rupture the eastern section was the Mw ~7.5 1848 Marlborough earthquake. The coseismic slip distribution and maximum traceable length of this surface rupture are calculated from the magnitude and distribution of small, metre-scale geomorphic displacements attributable to this earthquake. These data suggest this event ruptured >100-110 km of the eastern section, with mean surface displacement of 5.3 ±1.6 m. Based on these parameters, the moment magnitude of this earthquake would be Mw 7.4-7.7. This magnitude estimate is indistinguishable from previous calculations that were based on attenuation of shaking intensity isoseismals that were assigned from contemporary historical accounts of that earthquake. On the basis of similar rupture lengths and coseismic displacements, it is inferred that the penultimate event had a similar moment magnitude to the 1848 earthquake. Horizontal displacement of a flight of 6 fluvial terraces at Saxton River by the Molesworth section of the Awatere Fault is constrained to have occurred at a nearconstant rate of 5.5 ±1.5 mm/a since ~15 ka B.P. These rates are based on two new optically stimulated luminescence ages for the highest terrace treads of 14.5 ±1.5 and 6.69 ±0.74 ka B.P. These rates are indistinguishable from recent strike-slip rate estimates for the eastern section of 5.6 ±1.1 and 6 ±2 mm/a. Comparing the magnitudes and ages of the terrace riser displacements at Saxton River to the timing of paleoearthquakes on the Molesworth section implies a mean per-event displacement of 4.4 ±0.2 m since ~15 ka. The new terrace ages also record two periods of aggradation that post-date the Last Glacial Maximum.