The onset and evolution of the Dead Sea transform are re-evaluated based on new in situ U-Pb dating and strain analyses of mechanically twinned calcites. Direct dating of 30 syn-faulting calcites from 10 different inactive fault strands of the transform indicates that the oceanic-to-continental plate boundary initiated between 20.8 and 18.5 Ma within an ~10-km-wide distributed deformation zone in southern Israel. Ages from the northern Dead Sea transform (17.1-12.7 Ma) suggest northward propagation and the establishment of a welldeveloped >500-km-long plate-bounding fault in 3 m.y. The dominant horizontal shortening direction recorded in the dated twinned calcites marks the onset of left-lateral motion along the evolving plate boundary. The observed changes in the strain field within individual fault strands cannot be simply explained by local "weakening effects" along strands of the Dead Sea transform or by gradual changes in the Euler pole through time.

S U M M A R YThe pervasive damage of rocks by microcracks and voids stron ly affects their macroscopic elastic properties. To evaluate the damage effects, w d derive here the macroscopic stress-strain relations for a 3-D elastic solid with nohinteracting cracks embedded inside a homogeneous matrix. The cracks considered are oriented either perpendicular to the maximum tension axis, or perpendicular to the maximum compression axis. In the first case they dilate during loading and in the second case they contract during loading. We derive a solution for the elastic energy of this rock following the self-consistent scheme of Budiansky & O'Connell ( 1976). The solution describes the stress-strain relations in terms of Ad and pd, which are the modified Lame constants, and an additional parameter y. The latter accounts for the non-linear behaviour of the solid and is related to crack density. The solution predicts a non-linear elastic rheology even for an infinitesimal strain of ~<0.001, and abrupt change in the elastic moduli when the loading reverses from uniaxial compression to uniaxial tension.We use the derived solution to analyse rock-mechanics experiments in which beams of Indiana limestone were deformed under four-point loading. This configuration provides simultaneously the apparent tensile and compressive moduli for small strains. The apparent moduli fit the effective elastic moduli calculated with the present damage model well, including the differences between tensile and compressive moduli.

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