Closely spaced parallel lineations in the plains of Venus are interpreted as extensional rubble-filled fractures because they show no structure and are radar-bright irrespective of look angle. Their formation was investigated using a new methodology which combines material science and fracture mechanics principles together with experimental measurement and the analysis of Magellan data. Mapping in the Guinevere and Sedna Planitia regions shows that the closely spaced parallel fractures (CSPF) follow a concentric pattern around the edge of the large topographic rise of Western Eistla Regio. 13 spacing profiles show that most of the CSPF have spacings of between 0.8 and 1.2km. Using a new fracture mechanics apparatus designed to simulate Venusian surface conditions (90bar of CO2, 450°C), the fracture toughness of basalt was measured from atmospheric to 200 bar confining pressure and from room temperature to 600°C. 1 /2 Fracture toughness was found to increase from -2.4 MPam1/2 at ambient pressure to about -3.0 MPam1/2 at 50 bar confining pressure. Higher confining pressures have no further effect. Fracture toughness shows no clear trend with temperature, rising from an ambient level of -2.4MPam1/2 to -3.0MPa1/2 at 150°C and returning to -2.4MPam1/2 at higher temperatures. A new, two-dimensional model based upon fracture mechanics is described. The depth of the CSPF is controlled by the stress intensity factor but their spacing is controlled by the initiation of new cracks. Application of a faulting criterion to limit the conditions under which the CSPF can form shows that the spacing is consistent with a regional tensile stress of 5.5-8.5MPa. This stress could have resulted from uplift of Western Eistla Regio by -2km
