Fractured pebbles - a new stress indicator

Well-organized patterns of tensile fractures were found in pebbles of young conglomerates in the Salton Trough, California, and in the Dead Sea rift, Israel. The fractures are subparallel to each other in a single pebble and are within numerous pebbles in an outcrop. It is shown that intrapebble tension could develop due to the amplification of the stresses inside a competent pebble embedded within a compliant matrix; furthermore, tensile stresses may form in the pebbles, even under compressive tectonic stresses. The regional trends of the fractures are consistent in each of the study areas, and they appear to be excellent indicators of the tectonic stresses. The derived angles between σ _hmax (the largest horizontal compression) and the dominant local strike-slip fault is about 40° in the En Yahav region of the Dead Sea rift, and about 75° in the Indio Hills area of the Salton trough; these angles are in agreement with other stress data. -Authors