Highly Variable Latest Pleistocene‐Holocene Incremental Slip Rates on the Awatere Fault at Saxton River, South Island, New Zealand, Revealed by Lidar Mapping and Luminescence Dating

Abstract

Geomorphic mapping using high‐resolution lidar imagery and luminescence dating reveal highly variable incremental Holocene‐latest Pleistocene slip rates at the well‐known Saxton River site along the Awatere fault, a dextral strike‐slip fault in the Marlborough Fault System, South Island, New Zealand. Using lidar and field observations, we measured seven fault offsets recorded by fluvial terraces and bedrock markers. Improved dating of the offsets is provided by post‐IR‐IRSL225 luminescence ages. Incremental slip rates varied from <2 mm/yr to >15 mm/yr over intervals of thousands of years and tens of meters of slip, demonstrating order‐of‐magnitude temporal variations in rate at a single site. These observations have basic implications for earthquake fault behavior, lithospheric mechanics, discrepancies between geodetic and geologic slip rates, and probabilistic seismic hazard assessment.

Multimillennial Incremental Slip Rate Variability of the Clarence Fault at the Tophouse Road Site, Marlborough Fault System, New Zealand

“…Further episodes of incision and floodplain stability led to the formation of younger terrace treads (T2–T4; Figure c) separated by steep terrace risers, denoted here as the tread names separated by a slash (e.g., T1/T2 riser). Periods of high‐energy streamflow, during which Clarence River streampower was sufficient to laterally trim the adjacent risers, are consistent with coarse‐grained, pebble‐boulder gravels and sand deposits (Bull & Knuepfer, ; Cowgill, ; Lensen, ; Mason et al, ; Zinke et al, ). The gravels and sands are typically capped by younger silt horizons, which represent (1) the waning stages of terrace occupation, (2) overbank deposits from younger floodplains, and/or (3) loess.…”

“…The ages are reported in thousands of years before 2018 (kyb2018). We used a two‐step Bayesian age model (based on OxCal 4.3; Bronk Ramsey, , ; Rhodes et al, ; Zinke et al, ; Text S4) to refine the terrace and channel ages based on stratigraphic observations. The first step of our age model trimmed the gravel and silt ages in each pit according to their lithostratigraphic ordering.…”

“…To reconstruct the incremental slip history of the Clarence fault at Tophouse Road, we dated each offset geomorphic feature by considering its relation to the dated stratigraphic units at the site (e.g., Zinke et al, ). We evaluated whether the upper or lower terrace age is more appropriate to date each offset marker by considering its morphological and sedimentological context (Cowgill, ; Lensen, ).…”

“…We also determined the incremental slip rates between different offset markers to show how slip rate varied over different time periods (Figures c and d). We achieved this using a Monte Carlo sampling approach, in which physically realistic incremental slip rate estimates were obtained by rejecting sample paths resulting in negative (left‐lateral) slip rates (Gold & Cowgill, ; Zinke et al, ; Text S5 and Python Script S6). The displacement‐time markers at Tophouse Road characterize slip rates over four distinct intervals.…”

“…Furthermore, fault slip rates provide a basic input for most seismic hazard assessment strategies (e.g., Field et al, ; Stirling et al, 2012). Emerging evidence suggests that whereas some faults exhibit relatively constant slip rates over time (e.g., Gold & Cowgill, ; Kozacı et al, ; Salisbury et al, ; Van Der Woerd et al, ), others undergo periods of accelerated strain release spanning multiple earthquake cycles (e.g., Dolan et al, ; Gold & Cowgill, ; Mason et al, ; Ninis et al, ; Wallace, ; Weldon et al, ; Zinke et al, ). Constraining the incremental slip history of a fault requires a relatively rare set of geologic conditions (i.e., multiple progressively offset features sharing a common kinematic history).…”