Geomorphic response of submarine channels to active deformation, Niger Delta Geological Society of America Bulletin, v. 1XX, no. XX/XX 1 ABSTRACT The interaction between submarine chan- nels and active seabed deformation controls sediment delivery to the deep sea. Here, we combined seismic and geomorphic tech- niques to investigate quantitatively how the gravity-driven growth of thrust-related folds in the deep-water Niger Delta has influenced the morphology of four Pleistocene to Holo- cene submarine channels with present-day geomorphic expression. We extracted the bathymetric long profile of each of these modern seabed channel systems, and we evaluated the down-system evolution of channel widths, depths, and slopes as they have interacted with growing seabed struc- tures. This information was used to derive estimates of bed shear stresses and velocities, to infer morphodynamic processes that have sculpted the channel systems through time, and to evaluate how these channels have re- sponded to actively growing structures in the toe of the delta. The long profiles of these channels are relatively linear, with concavity from -0.08 to -0.34, and an average gradient of ~1°. They are characterized by small knick- points that are apparent near mapped structures and therefore implicitly reflect variations in substrate uplift rate. Channel incised depths increase significantly near the active structures, leading to entrench- ment, but there is little change in the down- system distribution of channel width, in contrast to rivers crossing active faults, or buried submarine channel complexes. Re- GSA Bulletin; Month/Month 2017; v. 129; no. X/X; p. 000–000; doi: 10.1130/B31544.1; 10 figures. § Corresponding author: a.whittaker@imperial .ac.uk † Present address: Department of Geology, Ahmadu Bello University, Zaria, Nigeria. Quantifying the geomorphic response of modern submarine channels to actively growing folds and thrusts, deep-water Niger Delta Byami A. Jolly † , Alexander C. Whittaker § , and Lidia Lonergan Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London, SW7 2AZ, UK 2003; Gerber et al., 2009; Amblas et al., 2011). However, our ability to “read” submarine channel morphology, similarly to the way in which rivers can be used to decode fault move- ment (Mitchell, 2006; Whittaker et al., 2007; Whittaker, 2012), is currently hampered by the facts that we have (1) a limited understanding of the time-integrated (as opposed to event- based) erosivity of submarine channels (Clark and Pickering, 1996; Pirmez and Imran, 2003; Talling et al., 2013, 2015); and (2) limited data sets that document the geomorphic response of submarine channels to faulting where the tim- ing, magnitude, and locus of deformation can be constrained independently (e.g., Pirmez et al., 2000; Gee and Gawthorpe, 2006; Clark and Cartwright, 2009; Mayall et al., 2010; Geor- giopoulou and Cartwright, 2013). Addition- ally, our understanding of modern submarine channel systems is limited because we have few direct observations of turbidity current velocities, erosion processes, and sediment concentrations (cf. Talling et al., 2015). These measurements are difficult to obtain in the sub- marine environment because of the intermittent nature of turbidity current events. Additionally, it is not clear how to scale up measurements from flume tank experiments, or the limited direct observations, to geologic time periods (e.g., Mitchell, 2006; Talling et al., 2012, 2015; Konsoer et al., 2013; Peakall and Sumner 2015). Improved constraints on the responses of submarine channels to tectonic deformation over geological time periods therefore require the use of high-quality seismic data sets and seabed digital elevation models that allow us to resolve the time-integrated rate and distri- bution of deformation with respect to channel form at a high spatial resolution. In this paper, we address this research challenge by constraining the spatial interac- tions between deep-water channels and active faults and folds using three-dimensional (3- D) seismic reflection data from the toe-thrust constructed bed shear stresses near faults are estimated to lie in the range of 100–200 Pa, which would be associated with turbi- dite flow velocities of 2–4 m/s. A comparison of the magnitude and distribution of struc- tural uplift since 1.7 Ma and the distribu- tion of channel incision over this time shows that three of these channels have been able to keep pace with the time-integrated uplift since 1.7 Ma and have likely reached a local topographic steady state. Entrenchment of the submarine channels upstream of grow- ing folds helps to drive this process, and we estimate that bed shear stresses of >100 Pa are sufficient to keep pace with structural strain rates of ~4 × 10 –3 m.y. –1 . INTRODUCTION Study Aims An understanding of the behavior and evo- lution of submarine channels is vital for con- straining the pathways and delivery of sedi- ment from the shelf edge to deep water (Mayall et al., 2010; Jolly et al., 2016). The response of submarine channels to a tectonic deformation, such as active faulting or folding, depends on their erosional dynamics. This is significantly influenced by their channel morphology and their long profile form (e.g., Heiniö and Davies, 2007; Peakall and Sumner, 2015; Talling et al., 2015) Consequently, the hydraulic geometry and longitudinal profiles of submarine chan- nels are a manifestation of the competing pro- cesses of sedimentation, erosion, and deforma- tion (Pirmez et al., 2000; Huyghe et al., 2004; Ferry et al., 2005; Heiniö and Davies, 2007; Covault et al., 2011). Since channel geometry can record the time-integrated history of ero- sion and sedimentation, it follows that channel responses to ongoing deformation may also contain information about long-term incisional process and tectonic rates (Pirmez and Imran, For permission to copy, contact editing@geosociety.org © 2017 Geological Society of America