Mapping of closely spaced, parallel extensional fractures in the Guinevere and Sedna Planitia regions of Venus reveals a concentric pattern of fractures around the edge of the large topographic rise of Western Eistla Regio. We have constructed 13 transects through these closely spaced parallel fractures (CSPF) and find a mean spacing of between 0.8 and 1.2 km. A two-dimensional, nonlayered, fracture mechanics computer model for the formation of CSPF is described. For cracks extended by a remote tensile stress the stress intensity factor controls the depth of penetration, which in turn, governs the spacing between adjacent cracks based upon the stress-shadow principle. The stress required to initiate cracks depends upon the strength of the material; thus spacing of CSPF depends only on the physical properties of the preexisting rock mass. Using a new fracture mechanics apparatus designed to simulate Venusian conditions (90 bar CO2, 450¿C) the fracture toughness of basalt was measured for confining pressures up to 20 MPa and for temperature up to 600¿C. Fracture toughness was found to increase from ~2.4 MPam1/2 at ambient pressure to ~3.0 MPam1/2 at 10 MPa confining pressure. Fracture toughness showed no clear trend with temperature. The experimental results for fracture toughness suggest that the preexisting rock mass that fits best with observations contains an inverse square distribution of flaws with maximum sizes of only ~20--80 cm and that a remote tensile stress of ~3--6 MPa is required to form the observed CSPF.