PAST PROJECTS
FRACTURE MECHANICS AND JOINT FORMATION
Characterization of flaws and stresses that govern the formation of systematic joint sets is essential for understanding the process of fracturing in rocks. However, such flaws are rarely characterized in natural conditions. We studied the nucleation and growth path of mud cracks in clayey sediments and joint sets in dolostone and chalks by analyzing their diagnostic surface morphology. We demonstrated that stress amplification at flaw discontinuities and layer boundaries play a fundamental role during mud and dolostones fracturing. Relative velocities during joint propagation were analyzed in chalks based on field characteristics of the joint-surface morphology. The fundamental importance of the release of residual strain during rock exhumation and the consequence formation of systematic joints was demonstrated in the metamorphic rocks of Otago Schist, New Zealand. We analyzed and compared columnar joints in different rock types and setting.
FRACTURE AND FLUID MECHANICS OF CLASTIC DIKES
Clastic dikes form either by deposition of clastic material into pre-existing fissures or by fracturing and injection of clastic material during seismic shaking or passive overpressure. Because of their similar final geometry, the origin of clastic dikes is commonly ambiguous. Hundreds of clastic dikes whose origin was unclear crosscut the late Pleistocene Lisan Formation in the seismically-active Dead Sea basin. If originated as injection structure, they are quite important for paleo-seismic studies. We (T. Levi, Y. Eyal, S. Marco) conducted a multidisciplinary study, integrating field observations, AMS data, optically stimulated luminescence (OSL) dating and mechanical modelling in order to determine the origin and emplacement mechanism of these clastic dikes. A novel application of AMS analysis provides a petrofabric tool for distinguishing between injection structures and passive (filled) dikes. We further established analytical models based on field observations and experimental viscosity tests that constrained the pressures generated in the source layer and dikes and posed limitations on the injection velocities during dike emplacement. The clastic-dike studies demonstrate how to characterize the (paleo) seismic origin of clastic dikes, estimate the conditions during their emplacement (e.g., stress drop, velocities), and define hazardous regions along the DSF.
FAULTING IN POROUS SANDSTONE
Strain in highly porous rocks is accommodated by deformation bands rather than joints or slip surfaces. The deformation bands tend to enhance cohesion and reduce permeability compared to ordinary fractures, but yet, their kinematic evolution and practical effect on fluid flow is not fully clear. Together with Y. Katz and A. Aydin, we defined typical patterns of deformation zones (faults) in Navajo sandstone, Utah, and in the Israeli Negev, characterizing their basic structure, and analyzing their geometry at different scales. We suggested a kinematic explanation for the evolution of “ladder” (Riedel)-structure network, which related the network geometry to the progressive accumulation and localization of shear strain. Deformation bands are quite common next to magmatic dikes, forming unique patterns of deformation bands next to tips of dike segments. We (H. Gajst, E. Shalev, V. Lyakhovsky, S. Marco) combined meso-scale analysis of deformation zones and a numerical modeling (coupling poroelastic deformation with damage evolution) of deformed sandstone to study compaction bands.