Emplacement conditions of igneous dikes in Ethiopian Traps

The emplacement conditions for 39 igneous dikes cutting basalts in northwestern Ethiopia are evaluated by analyzing their displacement–length scaling relations. Maximum opening displacements and lengths of the dikes demonstrate displacement–length scaling of the form Dmax = 0.088L0.48, consistent with other populations of dikes and veins and different than the power-law scaling relation typically found for faults. The dikes propagated through the thin Trap basalt sequence under conditions of constant fracture toughness, with values corrected for three-dimensional dike geometry of ~ 77–273 MPa m1/2. The large values of fracture toughness are likely associated with (1) the toughening effects of near-tip damage, (2) mixed-mode dike propagation, as shown by magma flow fabric analysis through anisotropy of magnetic susceptibility (AMS) and image analysis of thin sections, and (3) elevated temperature within the blocky and ductile basaltic host rock, evidence of which has been found in the field.