Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

282 PATA Days 2024 G E O M O R P H I C O F F S E T S The southeast-dipping Alpine Fault offsets alluvial markers (risers, channels) formed by the Maruia River at Marble Hill. Offsets were defined by Langridge et al. (2017) from an RTK-GPS survey (Fig. 2). T0 is unfaulted and he dextral and vertical offsets across terrace T1 are c. 1 m dextral and 0.25 m (Yetton, 2002). Channel and riser dextral offsets across terrace T2 are consistent and range from 7.2 ± 1 m to 13.2 ± 2 m, while vertical scarp heights range from 1 ± 0.2 m to 2 ± 0.4 m. On the higher (and older) T4 degradation surface channel and riser dextral offsets across terrace T2 are consistent and range from 6.4 ± 1.2 m to 9.6 ± 0.8 m, while vertical scarp heights range from 2.5 ± 0.5 m to 3 ± 0.3 m. This creates a conundrum because there are larger offsets across the lower T2 terrace. This cannot be completely explained by natural variability of displacement along the length of a rupture. Langridge et al. (2017) reasoned that the larger displacements on younger features could occur where there are en echelon, stepping fault traces, that each have peak displacements near the mid-point of a given trace. In addition, the average H:V ratio drops from 8.5 on T2 features to 3 on T4 features, meaning that additional partitioning of slip must be occurring at the location of terrace T4. A solution to this conundrum (i.e., less displacement farther from the river) is that degradation terraces T5 through T2 have all seen the same number of rupture events, discussed below in terms of avulsion and degradation processes. Fig. 2: The offset terraces at the Marble Hill locality. Four pits (0-3 in blue), and three trenches (1-3 in black), were excavated for slip rate and paleoseismic studies. Other abbreviations: w, Evison’s ‘fault creep’ wall; dh, drillhole. P I T S , I R S L D AT E S A N D S L I P R AT E Prior to paleoseismic trenching, four pits were excavated to log and date the alluvial terrace deposits, ultimately with a goal of estimating Alpine Fault slip rates at Marble Hill (Fig. 2). Four infrared stimulated luminescence (IRSL) dates were obtained from the UCLA Luminescence Lab; one from each pit. Two of these dates (CP31, CP42) were of Late Holocene age, reflecting