Abstract
Knickpoints have long been recognised as key geomorphic features that can be used to reveal the landscape evolution of a region. In particular, mobile knickpoints resulting from relative base-level fall record a landscape in the process of change and can encode information about the timing and rate of landscape response. Here, digital elevation model analysis is undertaken to; a) identify topographic lineaments related to active faulting, and b) extract geomorphic metrics and document associated knickpoints for rivers on Guadalcanal and Makira (San Cristobal) part of the Solomon Island chain. These islands have been experiencing uplift of up to 2 mm/yr since at least the mid Holocene on the upper (Pacific) plate of the San Cristobal Trench of the Solomon Island Forearc. For Guadalcanal, 23 out of 53 studied rivers exhibit slope-break knickpoints, characteristic of base-level fall, and 27 new topographic lineaments with ~E-W orientation are identified. By contrast, on Makira 14 of 41 studied rivers have slope-break knickpoints, where the rivers are steeper below the knickpoint than above. In addition, 76 new lineaments are inferred, trending NE-SW and likely to be extensional faults. For both Guadalcanal and Makira there is a good correlation between knickpoint elevation/catchment area and distance upstream from the sea, and a weak correlation between relief and knickpoint elevation. There are no clear relationships between the knickpoints and the new topographic lineaments. These data indicate that both islands are undergoing active river incision caused by regional tectonic uplift along an active subduction zone. On Makira, river steepness (ksn) scales with uplift, and K, coefficient of erosion, is in the range 1 x 10-5 – 7 x 10-6 m0.1yr-1, while K can be estimated as 1 x 10-5 – 5 x 10-8 m0.1yr-1 for Guadalcanal. Calculation of K for steady-state rivers demonstrates a rock strength control on the fluvial response and highlights the importance of lithology on river evolution. Furthermore, the distinct landscape response of the two islands supports the hypothesis that there are different arc segments present along the Solomon Arc and suggests that the Holocene uplift rates for Guadalcanal may not be representative of long-term uplift.
Highlights
IntroductionResearch into quantitative landscape evolution has undergone a revolution over the last 40 years, with the advent of high-quality global digital elevation models (DEMs) (Finnegan et al, 2005; Pipaud et al, 2015; Harel et al, 2016;), the development of sophisticated computer models of landscape evolution (van der Beek et al, 2002; Whipple and Tucker, 2002; Sklar and Dietrich, 2006; DiBiase et al, 2010) and advances in geochronology (Gosse and Phillips 2001; Balco et al, 2008).In particular, the study of fluvial geomorphology has been a major focus of the landscape evolution community because bedrock rivers transmit base-level changes to the entire watershed and set the hillslope angle; controlling erosion and sediment deposition (e.g., Snyder et al, 2000; Whipple 2004; DiBiase et al, 2010).One application of fluvial geomorphic analysis has been the study of regional uplift and faulting, where the location and slip rate of individual active faults can even be determined, through the recognition of features indicative of rivers responding to changing boundary conditions, for example an increase in uplift rate or a fall in relative base-level (e.g., Kirby and Whipple, 2001; Boulton and Whittaker, 2009; Kent et al, 2017)
Lineament analysis is consistent with existing mapping, showing that both islands have previously unrecognised NE-SW and ESE-WNW striking faults, likely to be extensional in nature
Rivers flowing to the north were overall less steep and longer than the rivers flowing to the south coast and northern rivers were more likely to contain slope-break knickpoints
Summary
Research into quantitative landscape evolution has undergone a revolution over the last 40 years, with the advent of high-quality global digital elevation models (DEMs) (Finnegan et al, 2005; Pipaud et al, 2015; Harel et al, 2016;), the development of sophisticated computer models of landscape evolution (van der Beek et al, 2002; Whipple and Tucker, 2002; Sklar and Dietrich, 2006; DiBiase et al, 2010) and advances in geochronology (Gosse and Phillips 2001; Balco et al, 2008).In particular, the study of fluvial geomorphology has been a major focus of the landscape evolution community because bedrock rivers transmit base-level changes to the entire watershed and set the hillslope angle; controlling erosion and sediment deposition (e.g., Snyder et al, 2000; Whipple 2004; DiBiase et al, 2010).One application of fluvial geomorphic analysis has been the study of regional uplift and faulting, where the location and slip rate of individual active faults can even be determined, through the recognition of features indicative of rivers responding to changing boundary conditions, for example an increase in uplift rate or a fall in relative base-level (e.g., Kirby and Whipple, 2001; Boulton and Whittaker, 2009; Kent et al, 2017). The identification, quantification and analysis of rivers and knickpoints, and other features linked to landscape rejuvenation, routinely utilizes global DEM datasets to investigate regional trends in fluvial geomorphology. This remote approach to landscape analysis is especially useful in areas that were previously lacking data owing to either accessibility issues or the subtlety of landscape expression (e.g., Oguchi et al, 2003; Ganas et al, 2005; Marliyani et al, 2016; 62 Menier et al, 2017)
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