Luminescence age estimates of Pleistocene marine terrace and alluvial fan sediments associated with tectonic activity along coastal Otago, New Zealand

Abstract

Six luminescence age estimates have been obtained from raised beach sands and loess, and alluvial fan sediments in the outboard zone of the active Otago reverse fault province, coastal Otago, New Zealand. These age estimates constrain the timing of movement of the Akatore Fault and the Titri Fault System. Five of the samples were dated by optical dating of K‐feldspar in the polymineral fine silt fraction; the remaining sample was dated using thermoluminescence (TL) emitted from quartz (fine sand fraction). Raised beach sands from the lowest Pleistocene marine terrace on the upthrown side of the reverse faults (Taieri Beach) yielded ages of 117 ± 13 ka (optical dating, K‐feldspar) and 117 ± 12 ka (TL dating, quartz), indicating terrace formation during the peak sea‐level highstand of the last interglacial, marine oxygen isotope substage (MISS) 5e (c. 125 ka). The K‐feldspar optical age has not been corrected for anomalous fading, but its consistency with the quartz TL age indicates that anomalous fading is insignificant in this sample. Samples from a raised beach sand unit and an underlying loess unit that bracket a marine terrace that has previously been associated with the c. 125 ka sea‐level highstand, situated 10 km northeast of the effects of faulting (Warrington), yielded optical age (K‐feldspar) estimates of 97 ± 11 and 96 ± 5 ka, respectively. These ages have not been corrected for anomalous fading and therefore the true ages might be slightly older. Two loess units within alluvial fans along the Titri Fault System in the Waihola region yielded optical age estimates of 92 ± 5 and 151 ± 5 ka (both K‐feldspar), which bracket the last period of faulting along the Titri Fault System; the latter age result is consistent with geomorphic evidence that suggests fan formation during the glacial period MIS 6 (186–128 ka); this consistency also implies that there was no significant anomalous fading in these samples. Together with published radiocarbon ages, our results imply that no activity has occurred on the Akatore Fault in the period c. 125–3.8 ka and only localised activity (at Moneymore) on the Titri Fault System post‐150 ka.