Latest Pleistocene–Holocene Incremental Slip Rates of the Wairau Fault: Implications for Long‐Distance and Long‐Term Coordination of Faulting Between North and South Island, New Zealand

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 22; no. 9
• Main Authors: Zinke, R., Dolan, J. F., Rhodes, E. J., Van Dissen, R. J., Hatem, A. E., McGuire, C. P., Brown, N. D., Grenader, J. R.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.09.2021; Wiley
• Subjects: Earthquakes; Fault lines; Geological hazards; Hazard assessment; Holocene; incremental slip rates; Lasers; Lidar; Lithosphere; Luminescence; Marlborough fault system; Plate boundaries; Pleistocene; Probability theory; Rivers; Sedimentology; Seismic activity; Seismic hazard; slip rate variability; Statistical methods; Terraces; New Zealand
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2021GC009656

We use high‐resolution lidar microtopographic data and luminescence dating to constrain incremental Holocene–latest Pleistocene slip rates for the Wairau fault, a major dextral strike‐slip fault in the Marlborough Fault System, South Island, New Zealand. Our data come from two closely spaced study areas along the structurally simple, central portion of the fault: The well‐known Branch River terrace flight, and a previously undated series of offset risers and channel features several km to the east that we refer to as the Dunbeath site. Field work and mapping using lidar‐derived topography yields revised or novel measurements of nine fault offsets. We date those features using a post‐IR50‐IRSL225 infrared stimulated luminescence dating method, and a stratigraphically informed Bayesian age model. The dated slip history of the Wairau fault is further constrained using newly cataloged offset measurements collected along a ∼35 km stretch of the fault, and available paleoseismic data. Incremental slip rates are precisely computed using a Monte Carlo resampling scheme. Our results provide a nearly earthquake‐by‐earthquake record of incremental slip, with pronounced variations in incremental slip rate spanning multiple millennia and tens of m of slip. These extreme, multi‐millennial variations in fault slip rate have basic implications for earthquake occurrence, plate boundary lithosphere behavior, and probabilistic seismic hazard assessment. Plain Language Summary Fault slip rates reflect the myriad mechanical processes operating at depth, and are an important input in seismic hazard analyses. In this study, we measure how the Wairau fault in South Island, New Zealand, sped up and slowed down over multiple earthquake cycles throughout the past 15 thousand years. Using lidar laser topographic scans and field geologic techniques, we precisely measure a series of earthquake fault displacements ranging from ∼5 to 60 m, recorded in river terraces and other landscape features. We then use luminescence dating to determine when the faulted features were deposited and last exposed to sunlight. Employing fundamental principles of sedimentology, we accurately date the measured displacements, constructing a fault slip rate history. This slip history shows factor‐of‐three variations in fault slip rate spanning multiple earthquakes, implying the action of poorly understood mechanisms at depth. Interestingly, the slip history of the Wairau fault appears to correspond to that of the nearby Wellington fault, located >150 km away on the other side of the structurally complicated Cook Strait, suggesting that the two faults influence each other within the plate boundary system. Key Points Incremental slip rates of the Wairau fault, New Zealand, have varied by a factor of three or more since latest Pleistocene time Slip history of the Wairau fault may be coordinated with that of the Wellington fault, 150 km distant along‐strike in southern North Island Sustained and coordinated slip rate variations imply the influence of one or more as‐yet poorly understood shear zone mechanisms