Deep Critical Zone Architecture of Perpendicular Ridges in a Northern Maryland Piedmont Catchment as Imaged by Seismic Tomography

The Critical Zone is the thin layer of Earth's terrestrial surface where rock, water, soil, and atmosphere interact in a myriad of complex processes which support life. Characterizing deep critical zone architecture is important to understanding processes such as chemical weathering and subsurface fluid flow that ultimately shape our planet. Below the surface the critical zone is often characterized as a sequence of soil, saprolite, fractured rock, and unweathered bedrock. Here we study ridges and valleys within Oregon Ridge Park, including the Pond Branch Catchment, to infer controls on the critical zone architecture. Our study area is underlain by the Lock Raven Precambrian schist formation located in the Maryland Piedmont. This area is characterized by a series of weathered hillslopes where the relief ranges 20-40 meters. Observations of thick saprolite layers atop of ridges have been made here since the 1960s. Potential consequences of this deep weathering profile include large volumes of groundwater storage which may act to sustain baseflow in local streams. It has been recently shown that interactions between topographic and regional tectonic stresses can influence deep critical zone structure. In particular, the ratio between gravitational stresses and horizontal tectonic stress perpendicular to ridges determines if the weathering profile is surface-parallel or surface mirroring. At our study area, regional tectonic compressional stresses are oriented roughly N65°E. Using seismic methods to study a ridge oriented N0°E in the Pond Branch Catchment, it has been noted in a prior study that the interface between bedrock and regolith mirrors the topography. We compare this previous study with new survey results from a nearby ridge oriented N55°E, closer to the direction of ambient compressional stress. Our results show that the velocity structure here also mirrors topography. Additionally, we present preliminary results from an anisotropy survey within the Pond Branch Catchment that investigates the influence of foliation and fracture orientation on seismic p-wave velocities.