Rock erodibility and the interpretation of low-temperature thermochronologic data

Rocks vary significantly in strength and erodibility. Here we evaluate if rock erodibility variations should be considered when interpreting thermochronologic datasets. We do this by applying 1D thermo-kinematic numerical models that exhume two lithologies of contrasting erodibility. For thick layers (>2 km), soft over hard layering causes earlier cooling and therefore older thermochronologic dates than no layering, with the opposite true for hard over soft layering. In some circumstances, even 2-10x erodibility contrasts substantially influence the results, and a 10x erodibility contrast can be nearly as important as contrasts several orders of magnitude greater. Thinner alternating layers (<0.5 km) dramatically reduce the effect. The results imply that rock erodibility variations should not substantially influence thermochronologic data from most continental sedimentary packages, which are dominated by lithologic layering <0.5 km-thick. However, the effect may be important for data from basement samples exhumed beneath softer sedimentary rocks. For example, the abrupt cooling and erosion rate decrease recorded by thermochronologic data from Rocky Mountain basement uplifts of the western U.S. coincides with when erosion-resistant Precambrian basement was exposed after removal of softer sedimentary cover. These data may largely record a change in exposed rock type erodibility rather than a dramatic change in external erosional forcing. Our results suggest that in some cases, variations in rock erodibility should be considered when interpreting cooling and erosion histories from thermochronologic datasets.