P-wave velocity of weathering profiles from a basalt climosequence: Implications for weathering on the mechanical properties of the critical zone

Abstract

We characterize the critical zone structure across a basaltic climosequence in Kohala, Hawaii from shallow seismic refraction data. First-arrival P-waves surveys were tomographically inverted from an active source to create 2D profiles from eight locations of varying mean annual precipitation (MAP). P-wave velocities are interpreted as a weathering profile that is constrained by natural and manmade vertical exposures. Using MAP as a proxy for weathering magnitude on similar age exposures of Pleistocene-aged basalts, we evaluate differences in weathering as a primary control on critical zone architecture. The velocity-depth gradient decreases by a factor of 3 between 500 and 1000 mm/yr MAP and does not significantly change further with increasing precipitation between 1000 and 3000 mm/yr. Depth-to-unweathered bedrock increases between sites of 500 mm/yr to 1000 mm/yr MAP and may not be resolvable by the maximum depth of investigation on wet-side profiles (~ 15 m). Observed interbedded paleosols within exposures of the volcanic flow sequence at sites with MAP > 1000 mm/yr may explain both lower velocities at equivalent depths to dry sites, and the uniformity of velocity gradient with increasing MAP above a threshold value for soil development. Interbedded weak horizons within the depth profile may result in near-surface low velocity to much greater depth than would be predicted by a weathering profile produced by top-down progressive alteration from infiltration of surface water. Hence the main effect of climate is manifested not in the deepening of a near surface altered layer, but rather in the integrated exposure time and precipitation amount between eruptive phases leading to weathering horizons deep within the rock profile. Vii The mechanical implications of weathering influence hillslope stability, hillslope gradient, and the capacity for sediment transport by a variety of geomorphic processes.