Fault Troughs

Fault troughs may be classed as superficial or profound. Superficial fault troughs result from (1) earthquakes, or (2) volcanic activity, or (3) tension. Tensional fault troughs are due to differential uplift, rotational strain, or the tilting of large fault blocks. The differential uplift may be caused by igneous intrusions and by the relative displacement of large rigid fault blocks underlying sedimentary formations. The faults associated with very large fault troughs extend to profound depths and reach the zone of flowage. Evidence bearing on their origin is less conclusive than that for superficial troughs. Several factors have probably entered into their formation, but tension, resulting in normal faulting and an upward tilting of the inclosing masses, is probably most important. Extrusion of lavas and increase in density, due to expulsion of gases and the crystallization of magmas, have played a more or less important part in some instances. Large troughs have probably developed in part along pre-existing faults. Illustrations are given of the different types of fault troughs.