Displacement history of a limestone normal fault scarp, northern Israel, from cosmogenic 36Cl

Abstract

The abundance of cosmogenic 36Cl, measured in 41 limestone samples from a 9 m high bedrock fault scarp, allows us to construct the 14 kyr fault displacement history of the Nahef East normal fault, northern Israel (300 m above sea level, N33° latitude). The Nahef East fault is one of a series of fault scarps located along the 700 m high Zurim Escarpment, a major geomorphic feature. Samples at the top of the scarp have the highest nuclide concentrations (79 x 104 atoms (g rock)-1); samples at the base have the lowest (11 x 104 atoms (g rock)-1). Using chemical data from the samples, Nahef East fault scarp geometry, and surface and subsurface production rates for the 36Cl-producing reactions, we have constructed a numerical model that calculates 36Cl accumulation on a scarp through time, given a series of unique displacement scenarios. The resulting model 36Cl concentrations are compared to those measured in the scarp samples. Faulting histories that result in a good match between measured and modeled 36Cl abundances show three distinct periods of fault activity during the past 14 kyr with over 6 vertical meters of motion occurring during a 3 kyr time period in the middle Holocene. Smaller amounts of displacement occurred before and after the period of most rapid motion. The episodic behavior of the Nahef East fault indicates that the average displacement rate of this fault system has varied through time. Copyright 2001 by the American Geophysical Union.