We analyze a system of complex joints in thick dolomite layers within the western margins of the Dead Sea. These joints display two dominant features: "tree-like", hierarchial branching, and a gradual increase of density that leads to local fragmentation of the host layer. The development of these features were explored in laboratory experiments with brittle/ductile layered samples that were subjected to quasi-static extension. Three styles of fracturing appeared in the tests: Planar fractures, known from previous tests; branching fractures and clustering fractures, observed here for the first time in layered composites. Based on fracture morphology, we found that the branching and clustering fractures in the experiments, and the tree-like, dense joints in the field, propagated at dynamic, high-velocity growth rates. It is also suggested that dynamic propagation of joints is the likely mechanism for high density fracturing and fragmentation. We propose that fractographic features are reliable field criteria to recognize dynamic rates of rock fracturing. 

( J Structural Geology, in press Jan. 2001)

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Branching Fractures

Clustering Fractures