LAUSR

Facets are major topographic features built over several 100 k.y. above active normal faults. Their development integrates cumulative displacements over a longer time frame than many other geomorphological markers, and they are widespread in diverse extensional settings. We have determined the 36Cl cosmogenic nuclide concentration on limestone faceted spurs at four sites in the Central Apennines (Italy), representing variable facet height (100–400 m). The 36Cl concentration profiles show nearly constant values over the height of the facet, suggesting the facet slope has reached a steady-state equilibrium for 36Cl production. We model the 36Cl buildup on a facet based on a gradual exposure of the sample resulting from fault slip and denudation. Data inversion with this forward model yields accurate constraints on fault slip rates over the past 20–200 k.y., which are in agreement with the long-term rate independently determined on some of those faults over the past 1 m.y. 36Cl measurements on faceted spurs can therefore constrain fault slip rate over time spans as long as 200 k.y., a time period presently undersampled in most morphotectonic studies.