Hillslope response to climate-modulated river incision in the Waipaoa catchment, East Coast North Island, New Zealand

Abstract

Quantifying how hillslopes respond to river incision and climate change is fundamental to understanding the evolution of uplifting landscapes during glacial-interglacial cycles. We investigated the interplay among uplift, river incision, and hillslope response in the nonglacial Waipaoa River catchment located in the exhumed inner forearc of an active subduction margin on the East Coast of the North Island of New Zealand. New high-resolution topographic data sets (light detection and ranging [lidar] and photogrammetry) combined with fi eld mapping and tephrochronology indicate that hillslopes adjusted to rapid latest Pleistocene and Holocene river incision through the initiation and reactivation of deep-seated landslides. In the erodible marine sedimentary rocks of the Waipaoa sedimentary system, postincision deep-seated landslides can occupy over 30% of the surface area. The ages of tephra cover beds identifi ed by electron microprobe analysis on 80 tephra samples from 173 soil test pits and 64 soil auger sites show that 4000-5000 yr after the initiation of river incision, widespread hillslope adjustment started between the deposition of the ca. 14,000 cal. yr B.P. Waiohau Tephra and the ca. 9420 cal. yr B.P. Rotoma Tephra. Tephrochronology and geomorphic mapping analysis indicate that river incision and deep-seated landslide slope adjustment were synchronous between main-stem rivers and headwater tributaries. Hillslope response in the catchment can include the entire slope, measured from river to ridgeline, and, in some cases, the interfl uves between incising subcatchments have been dramatically modified through ridgeline retreat and/or lowering. Using the results of our landform tephrochronology and geomorphic mapping, we derive a conceptual time series of hillslope response to uplift and climate change-induced river incision over the last glacial-interglacial cycle. Cretaceous to Early Tertiary interbedded mud and sandstoneMajor rivers and streams 1 10 0 0) ) ) W W Wa a ai i it t ta a an n ng g gi i i 1 11 1 1) ) ) U 1 18 8 8) ) ) W W Wh h ha a ar r re e ek k ko o op p pa a ae e e 1 19 9 9) ) ) T T To o ot t ta a an n ng g gi i i 2 20 0 0) ) ) W W Wa a ai i ik k ka a ak k ka a ar r ri i ik k ki i i 2 21 1 1) ) ) T T Te e e A A Ar r ra a ai i i 1 1) ) ) W W Wa a ai i ip p pa a ao o oa a a 2 2) ) ) W W Wa a ai i im m ma a at t ta a a 3 3) ) ) N N Ng g ga a ak k ko o or r ro o oa a a 4 4) ) ) U U Ur r ru u ut t ta a ar r ra a an n ng g ga a a 5 5) ) ) M M Ma a an n ng g ga a ao o oa a ai i i 6 6) ) ) W W Wa a ai i ih h ho o or r ra a a 7 7) ) ) W W Wa a ai i in n ng g ga a ar r ro o om m mi i ia a a 8 8) ) ) U Up p pp p pe e er r r W W Wa a ai i ip p pa a ao o oa a a 1 12 2 2) ) ) M M Ma a an n ng g ga a at t tu u u 1 13 3 3) ) ) M M Ma a an n ng g ga a am m ma a ai i ia a a 1 14 4 4) ) ) M M Ma a an n ng g ga a ap p pa a ap p pa a a 1 15 5 5) ) ) U U Ur r ru u uk k ko o ok k ko o om m mu u uk k ka a a 1 16 6 6) ) ) W W Wa a ai i ik k ka a ah h hu u u 1 17 7 7) ) ) W W Wa a ai i ih h hu u uk k ka a a

A source-to-sink perspective of the Waipaoa River margin

“…19 was nearly synchronous between mainstem river and headwater tributaries (Gomez and Livingston, 2012;Bilderback et al, 2014). Similar to rock uplift rates, cumulative post-LGM incision varies systematically in the Waipaoa, approaching 120 m in the headwaters and 55 m at the midpoint of the catchment, while lower sections near the coast experienced deposition driven by subsidence (Berryman et al, 2000).…”

“…to the Waipaoa since the post-LGM climate shift and in response to recent land-use change (Gage and Black, 1979;Marden et al, 2005Marden et al, , 2008bMarden et al, , 2014Page and Lukovic, 2011;Bilderback et al, 2014). Landslide style tends to correlate with rock-type variations throughout the Waipaoa (O'Byrne, 1967;Gage and Black, 1979;Reid and Page, 2002;Page and Lukovic, 2011;Bilderback et al, 2014) and thus geologic maps have been used to delineate the relative contribution of these different landslide styles.…”

“…Sediment production processes -During the LGM, sediment production in the Waipaoa catchment was likely driven by periglacial and mass-wasting processes as evidenced by inactive scree slopes and other characteristic surficial deposits found on low-gradient, relict hillslopes (Gage and Black, 1979;Bilderback et al, 2014). These slopes are typically found above hanging valleys and often grade to the extensive 17.6 ka Waipaoa 1 (W1) terrace.…”

“…Thus, in the absence of valley lowering, rock uplift accumulated, increasing the elevation of the valley network relative to the surrounding low-relief hillslopes (e.g., Bilderback et al, 2014).…”

“…Landslide style tends to correlate with rock-type variations throughout the Waipaoa (O'Byrne, 1967;Gage and Black, 1979;Reid and Page, 2002;Page and Lukovic, 2011;Bilderback et al, 2014) and thus geologic maps have been used to delineate the relative contribution of these different landslide styles. About 86% of the catchment consists of hilly terrain (slopes >20°), with the steepest angles occurring in the mid to lower parts of the catchment dominated by Miocene-Pliocene rocks.…”