Geomorphic Response to Differential Uplift: River Long Profiles and Knickpoints From Guadalcanal and Makira (Solomon Islands)

Abstract

Knickpoints have long been recognized 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/year 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 slopebreak 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 (k sn) scales with uplift, and K, coefficient of erosion, is in the range 1 × 10 −5-7 × 10 −6 m 0.1 yr −1 , while K can be estimated as 1 × 10 −5-5 × 10 −8 m 0.1 yr −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.

Quantifying the competing influences of lithology and throw rate on bedrock river incision

“…Brocard and Van der Beek (2006) also determined that for bedrock rivers in French Alpine rivers K should fall between 1.8 -4.7 x 10 -5 m 0.4 yr -1 for marl and between 1.1 -3.7 x 10 -5 m 0.4 yr -1 for limestone, whereas van der Beek and Bishop (2003) determined that for a river crossing crystalline basement rocks in SE Australia that K = x -7 m 0.4 yr -1 . K parameters determined for crystalline rocks on the Pacific Islands of Makira and Guadalcanal also fall in the range 1 x 10 -5 to x10 -8 m 0.1 yr -1 (Boulton, 2020). Similarly, the knickpoint retreat rate data of Whittaker and Boulton (2012) imply K of 10 -6 to 10 -7 yr -1 assuming m = 0.5.…”

Tectonic geomorphology of the Yatağan Fault (Muğla, SW Turkey): implications for quantifying vertical slip rates along active normal faults

“…Vertical-step knickpoints are mostly associated small-scale heterogeneities along river profile (e.g. lithological separation along a fault) and record no significant evidence about the uplift trends of the region (Wobus, Crosby and Whipple, 2006;Kirby and Whipple, 2012;Boulton, 2020). Conversely, slope-break knickpoints develop because of abrupt increases in channel steepness along a river profile towards downstream direction as a result of sustained base-level fall potentially resulting from tectonic perturbation Kirby and Whipple, 2012).…”

“…Tectonic forcing transforms river profile from steady-state to transient stage as a result in this change in the base-level (Kirby and Whipple, 2012). These differences allow the identification of differential rock uplift and initiation of previously unknown faults (Wobus et al, 2003;Wobus et al, 2005;Kirby and Whipple, 2012;Boulton, 2020). Thus, the analysis of slope-break knickpoints is critical to understand the pattern of regional-scale uplift Kirby and Whipple, 2012).…”

Incorporating Landscape Dynamics in Small-Scale Hydropower Site Location Using a GIS and Spatial Analysis Tool: The Case of Bohol, Central Philippines

“…Geomorphologists analyzed river profiles to extract information on landscape evolution and the processes that shape the topography of landscapes [24], linking climate, tectonic and surface processes across a landscape [25]. The geomorphic index such as the river steepness index [22,24] and the presence of knickpoints [26] are proxies of natural surface processes. As Boulton [26] noted, steeper channels and regions undergoing higher erosion and uplift rates, can be recognized with river profile analysis as indicated by the high river steepness index.…”

“…The geomorphic index such as the river steepness index [22,24] and the presence of knickpoints [26] are proxies of natural surface processes. As Boulton [26] noted, steeper channels and regions undergoing higher erosion and uplift rates, can be recognized with river profile analysis as indicated by the high river steepness index. Similarly, the study of Castillo et al [27] in western Mexico rivers found high river steepness index values that suggests high rock uplift and erosion rates that control the topography of river basins.…”