Fracture stresses at shallow depths during burial

Single-layer joints in Lower Eocene chalks near Beer Sheva, Israel, developed at shallow depths during burial prior to folding and uplift. Fracturing occurred in two stages. Initially, extensional joints marked by circular fracture-surface morphology formed at about 100 m depth when pore pressure P = 1.5 MPa, horizontal maximum effective stress \ ̄gs_y = 1.0 MPa, vertical effective stress \ ̄gs_z = 1.0 MPa, and horizontal minimum effective stress \ ̄gs_x = -0.5 MPa. Subsequently, hybrid joints that produced a dihedral angle of 35° and were marked by axial horizontal plumes formed at a burial depth of about 150 m, where P = 2.0 MPa, \ ̄gs_x ∼ -0.5 MPa, \ ̄gs_y ∼ 1.0 MPa, and \ ̄gs_z ∼ 1.8 MPa. Fracturing continued during further burial to depths of perhaps 200-400 m.