USE OF SOILS AND COLLUVIAL DEPOSITS IN ANALYZING TECTONIC EVENTS - THE SOUTHERN ARAVA RIFT, ISRAEL

Buried soils in colluvium units developed on downthrown fault blocks have been presumed to form during periods of tectonic quiescence. This assumes however, that all colluvial units on a fault scarp are tectonically induced, particularly for a multiple-event fault scarp. To test this hypothesis we compared the colluvial stratigraphy and pattern of soil development on two non-tectonic scarps (terrace risers) and a nearby multiple-event fault scarp on the same alluvial fan surface. Both slopes developed under extremely arid climatic conditions. The colluvial stratigraphy and soil catenary relationship developed on terrace risers were found to be substantially different from those on the multiple-event fault scarp. The fault scarp colluvium contained several colluvial units separated by soils, whereas only two colluvial units without buried soils were found on the terrace risers. The colluvial units on the fault scarp were triangular in shape, while those on the terrace riser were concave and overlapped up the slope. Cumulic soils, indicating a continuous and low rate of deposition during pedogenesis, were formed along the terrace riser. In contrast, buried soils indicating episodic deposition occurred on the downthrown block of the multiple-event fault scarp. Accordingly, the colluvial stratigraphy developed on the fault scarp can be viewed as a record of distinct tectonic events with no climatically controlled deposition. The data indicate that soils and sedimentary stratigraphy can serve as useful indicators to distinguish terrace risers and single-event fault scarps from multiple-event fault scarps. These indicators can be helpful in distinguishing phases of tectonic activity and in evaluating recurrence intervals on a multiple-fault scarp.