Bornhardts and associated fracture patterns

Abstract

Bornhardts are bald domical hills. In plan they are defined by systems of steeply-dipping fractures, in profile by arcuate-upward sets. They occur in multicyclic landscapes. They have formed through much of geological time, for some date from the Late Archaean and are represented in most subsequent periods. They have been explained as due to tectonism, or structure or environment, but most workers interpret them either as the last remnants surviving long-distance scarp retreat (monadnocks de position), or as two-stage or etch forms which have survived because of their massive structure (monadnocks de résistance). The field evidence suggests that though bornhardts originate in various ways, on balance most appear to be of etch origin. They were initiated at the base of the regolith by differential structurally-controlled subsurface weathering. Many have been exposed in stages. Differential weathering is based in variations in fracture density which is attributed to shearing and fracture propagation. Similarly recurrent shear stresses are responsible for the arcuate fractures characteristic of bornhardts.