A note comparing parameters controlling low-angle normal and thrust movement

A simple model for the initiation of normal and thrust motion along pre-existing low-angle faults shows that the parameters necessary for fault movement are nearly the same in both cases. The parameters controlling fault movement (which are tectonic tension and tectonic compression as well as fault friction or fault strength) are considered in terms of available observations of crustal stresses, and possible crustal stresses predicted from laboratory rock friction experiments. Because tectonic compression can exceed tectonic tension it is concluded that thrust movement along low-angle zones of weakness is somewhat more likely than normal movement; the relative difficulty of the different motions depends upon the ratio of tectonic compression to tectonic tension. If available observational data of maximum and minimum horizontal stresses in the crust represent reasonable upper and lower limits to tectonic compression and tectonic tension, then low-angle movement is greatly facilitated by low coefficients of fault friction, similar to values obtained in a number of recent studies of weak faults.