title: Reconnaissance geology of Lake McRae, Marlborough
title: Landslides, lake sediments and paleo-earthquakes
titleType: Subtitle
Creators: name: (:Tba)
Publisher: name: GNS Science
Year: 2015
Subjects: Paleoseismicity
Landslides
Lake sediments
Clarence Fault

description: Lake McRae is a small alpine (0.65 km2, 900 m a.s.l.) landslide-dammed lake nestled within the Inland Kaikoura Ranges in Marlborough. Results from cosmogenic dating indicate that the lake formed near the end of the last ice age when two very large landslides (LS1, LS2) blocked an east-west fault-parallel valley fed mainly from Goat Valley Stream. We conducted a reconnaissance investigation of the lake and its environs, looking at the central section of the Clarence Fault adjacent to the lake, the morphology and age of the landslides, and the lake bathymetry and stratigraphy. The Clarence Fault was mapped within the study area. The fault cuts both landslides, crosses the floor of Lake McRae and traverses a bedrock slope above the lake. A hand-dug trench into deposits at the toe of a scarp along this shutter ridge yielded colluvial deposits that are probably related to surface faulting events. Based on two radiocarbon dates, these deposits are suggestive of 1 or perhaps 2 faulting events during the last 1900-2000 years. We estimate a long term slip rate for the central section of the Clarence Fault as c. 2.8 ± 1.4 mm/yr. Bathymetric mapping of the lake floor shows that at its western end part of landslide LS1 is submerged and draped with sediment. Shallow seismic sections indicate a layered sedimentary fill and also help to identify the Clarence Fault within the lake. Shallow (1-m) cores taken from the deepest parts of the lake penetrate the upper part of the lake section. Three radiocarbon dates help define the age structure of the uppermost lake section. Calculation of sediment accumulation rates indicates that the lake has been filled for at least 10,000 years, which is consistent with the cosmogenic ages obtained from landslide LS1. No slump or turbidite deposits were identified in the 1-m lake cores, which suggests that there has been no shaking of Modified Mercalli (MM) intensity IX during the last 1000 years. This means that it is unlikely that the nearby Clarence or Elliott faults have ruptured during this period. Increased sediment flux into the lake has been recognised in the uppermost part of the shallow lake cores. This phenomenon is more likely related to historic farming practice (burning, clearance) and rabbit infestation during the mid-19th century, rather than being linked to a specific earthquake event such as the 1848 M 7.5 Marlborough earthquake. Our results indicate Lake McRae formed at the end of the last ice age (15.2 [+3.8/-2.7] thousand years ago) when two very large landslides blocked an east-west fault parallel valley. The lake has since been accumulating sediments derived mainly from Goat Valley Stream. The preliminary record from Lake McRae indicates that it has significant potential for further integrated landslide, paleoearthquake, ground motion, and lacustrine studies. In light of the recent November 14, 2016 Mw 7.8 Kaikoura earthquake sequence, further work on the active faulting and sedimentary record of this important small basin is vital.