Abstract

We use palaeomagnetic methods to decipher the thermal histories of a succession of massive to weakly stratified debris flow deposits (Ngatoro and Te Popo formations) of late Holocene age located on the eastern lower flanks of Mt. Taranaki/Egmont Volcano, western North Island, New Zealand. Results from two sites, Vickers Quarry and Surrey Road Quarry, both c. 9.6 km from the present-day summit, enable us to distinguish between clast incorporation temperatures of about 400 degrees C and emplacement temperatures between 150 and 200 degrees C, consistent with observation of superficial charring and desiccation of outer podocarp-hardwood tree trunks at Vickers Quarry. Analysis of palaeomagnetic directions and lithofacies architecture suggest that these deposits were likely initiated as a closely-spaced succession of block-and-ash flows (BAFs) that rapidly cooled as they descended the volcano flanks. Radiocarbon chronology and the widespread occurrence of a palaeosol between the products of the preceding Inglewood eruptive phase, c. 3.4 cal. ka B.P., and the overlying Ngatoro Formation suggest that these two events are temporally unrelated. Certainly, there is no field evidence of contemporaneous explosive volcanic activity that might be related to the emplacement of Ngatoro Formation. However, we suggest that these low-temperature deposits might either relate to collapse of a small emergent lava dome or a cooling dome remnant, possibly emplaced in the aftermath of the Inglewood eruption. How collapse was initiated remains uncertain: the remnant dome may have been rendered unstable by volcano-tectonic or tectonic seismic events and/or by adverse meteorological events. Nevertheless, this study demonstrates that edifice collapse events generating potentially hazardous debris flows can occur independent of specific eruptive activity. (C) 2018 Elsevier B.V. All rights reserved.