Hillslope response to climate-modulated river incision and the role of deep-seated landslides in post-glacial sediment flux: Waipaoa Sedimentary System, New Zealand

摘要

Quantifying how hillslopes respond to river incision and climate change is fundamentalto understanding the geomorphic evolution of tectonically uplifting landscapes duringglacial-interglacial cycles. Hillslope adjustment in the form of deep-seated bedrocklandslides can account for a large proportion of the regional sediment yield anddenudation rates for rapidly uplifting landscapes. However, the timing and magnitude ofthe response of hillslopes to climatic and tectonic forcing in moderate uplift temperatemaritime catchments characteristic of many active margins worldwide is not wellquantified. This study seeks to investigate how hillslopes respond to climate-modulatedriver incision and to quantify the magnitude of the sediment flux from this response in atypical active margin setting.The non-glacialWaipaoa Sedimentary System (WSS) on the East Coast of the NorthIsland of New Zealand consists of river catchments, coastal foothills to upliftingmountain ranges, and terrestrial and marine sediment depocentres collectively underlainby relatively young (Cretaceous and younger) sedimentary rocks within a tectonicallyactive setting and temperate maritime climate. These attributes make theWSS similar tomany coastal catchments on oceanic-continental convergent margins worldwide.However, because of widespread destruction of primary forests for conversion to pasturelands by the mid 20th Century, theWSS is currently a globally significant source ofsediment to the world’s oceans. Because of these factors, theWSS was selected as one oftwo global study sites for the international, NSF supported, MARGINS Source-to-Sinkinitiative designed to investigate the transfer of sediment from terrestrial source tomarine sink. Previous studies on theWSS have shown a strong link between climatechange and geomorphic response in the system. River incision since the last glacialcoldest period has generated a significant amount of topography, leaving small remnantsof the ca.18,000 cal. yr BP last glacial aggradation terrace scattered up to 120 m abovemodern rivers.In this study, the hillslope response to river incision is quantitatively examined using newhigh resolution topographic data sets (lidar and photogrammetry) in combination with3field mapping and tephrochronology. Hillslopes are found to be coupled to river incisionand adjusted to rapid incision through the initiation and reactivation of deep-seatedlandslides. In the erodible marine sedimentary rocks of the terrestrialWSS, post-incisiondeep-seated landslides can occupy over 30% of the surface area. Many of these slidesshow evidence of multiple “nested” failures and landslide reactivation. The ages oftephra cover beds identified by electron microprobe analysis show that following aninitial 4,000 to 5,000 year time lag after the initiation of river incision, widespreadhillslope adjustment started between the deposition of the ca. 13,600 cal. yr BPWaiohautephra and the ca. 9,500 cal. yr BP Rotoma tephra. Tephrochronology and geomorphicmapping analysis indicates that river incision and deep-seated landslide slope adjustmentis synchronous between mainstem rivers and headwater tributaries. Tephrochronologyfurther shows that many slopes have continued to adjust to channel incision into the lateHolocene. Hillslope response in the catchment can involve the entire hillslope from riverto ridgeline, with some interfluves between incising sub-catchments being dramaticallymodified through ridgeline retreat and/or lowering. Using the results of the landformtephrochronology and geomorphic mapping, a conceptual time series of hillsloperesponse to uplift and climate change-induced river incision is derived for a timeframeencompassing the last glacial-interglacial cycle.Using the same high resolution topography datasets, in-depth field analysis, andtephrochronology, the 18,000 year sediment yield from terrestrial deep-seated landslidesin theWSS is estimated in order to investigate the magnitude of hillslope response toclimate-modulated, uplift driven river incision. This completes one of the first processbasedmillennial time-scale sediment budgets for this class of temperate maritime, activemargin catchments. Fluvial and geomorphic modelling is applied to reconstruct pre18,000 cal. yr BP topography in 141 km2 of detailed study area and the resultingvolumetric estimates from 207 landslides are used to estimate deep-seated landslidesediment flux for the broader system. An estimated 10.2 km3 of deep-seated landslidederivedsediment with a multiplicative uncertainty of 1.9 km3 (+9.2 km3, -4.8 km3) wasdelivered to terrestrial and marine sinks. This accounts for between 10 and 74% of thetotal mass of the terrestrialWSS budget of ca. 91,000 Mt (+37,000 Mt, -26,000 Mt).Combining the deep-seated landslide results with other studies of terrestrial sedimentsources and terrestrial and shelf sinks, the estimated terrestrial source load ranges from4Abstract1.2 to 3.7 times larger than the mass of sediment sequestered in terrestrial and shelfdepocentres. This implies that off-shelf transport of sediment is important in this systemover the last 18,000 cal. yr BP, as it is today for anthropogenic reasons. Based on thederived sediment budget, the denudation rate for the terrestrialWSS of 0.8 mm yr-1 (+0.3mm yr-1, -0.2 mm yr-1) is indistinguishable from the average terrestrialWaipaoa lateQuaternary uplift rate, indicating an approximate steady-state balance betweendenudation and uplift. This thesis provides a quantitative analysis of the role of deepseatedlandslides in an active margin catchment that is used to improve theunderstanding of landscape and terrestrial source-to-marine-sink sediment transferdynamics.