TOPOGRAPHIC STRESS AND ROCK FRACTURE IN THE DEEP CRITICAL ZONE

Theoretical calculations imply that stresses produced by gravity acting on topography may be large enough in some scenarios to fracture rock. The stress perturbation extends to a depth that scales with the topographic relief and depends sensitively on the shape of the overlying topography, such that topographic stresses can differ dramatically beneath ridges and valleys. This suggests a view of the deep critical zone in which rocks experience an evolving stress field as they are exhumed, potentially developing fractures at depth that reduce rock resistance to weathering, disaggregation and eventual entrainment by fluvial or mass wasting processes. Several studies have examined stress fields beneath idealized topographic profiles and considered the consequences for rock fracture, but few have tested whether the stresses predicted for specific landscapes are consistent with fracture patterns observed at depth. We use a boundary element model to calculate plane stresses beneath measured topographic profiles in the presence of an ambient tectonic stress field, and compare the stresses with fractures observed in boreholes. Calculations for a range of valley shapes demonstrate that the dominant fracture mode in compressive tectonic regimes is expected to be shear fracture. We consider two endmember proxies for rock damage: one based on the instantaneous potential for shear fracture as a function of a rock parcel's present depth, and another based on the cumulative potential for shear fracture as a rock parcel is exhumed. These proxies predict different patterns of fracture abundance as a function of depth and position along a valley cross-section. At the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, optical images of boreholes in the valley bottom reveal conjugate shear fractures that decline in abundance with depth, consistent with (but not uniquely diagnostic of) either damage proxy. We show how future efforts to explore the deep critical zone could further test these predicted effects of topographic stress on patterns of rock weathering and fracture.