Two different joint surface morphologies, plumes and rib marks, characterize joint surfaces, but the mechanical conditions that lead to the formation of either of the morphologies are not understood well. We studied two orthogonal joint sets that cut the same Santonian chalk beds in the Judea Desert, Israel. Joints of the J1 set are systematic, relatively long, characterized by almost exclusively by plumes and predate the shorter, non-systematic joints of the J2 set that are characterized by rib marks. Joints of the J1 set formed at high stress during deformation of the Syrian Arc folding in the Late Senonian. Joints of the J2 set formed at lower stress that occurred because of stress relaxation after the formation of the J1 joints. A mechanical analysis indicates that the J1 joints propagated at subcritical velocities several orders of magnitude faster than the J2 joints. Based on previously published data of laboratory tests, the plumes and the rib marks are semi-quantitatively placed on the subcritical part of the fracture velocity vs. stress-intensity factor diagram.
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