FAULTING, BASEMENT CONTROLLED

BLOCK-FAULTED AND RIFTED REGIONS
The youngest structures in many regions are systems of approximately parallel faults with dip-slip displacements measured in hundreds or thousands of meters. Well-known examples are faults of the Basin and Range Province in the western United States, the system of rift valleys in East Africa, the Rhine graben in Germany, and the Baikal rift in Russia. Others of special interest, because of their continuity with oceanic rifts (Section 10.2.1), are the fault troughs of Iceland and the Red Sea.
The major faults may be concentrated in narrow zones either side of a central down thrown block, or they may occur quite evenly spaced over a wide area and isolate numerous parallel blocks (Fig. 9.3). All transitional patterns are known between these two extremes. The physiographic trough over the downthrown block in the first case is referred to as a rift or rift valley. The structure in both cases can be referred to as block faulting, a general descriptive term for all situations involving differential vertical movements of large, fault-bounded blocks. Individual fault blocks range in width from several tens of kilometers to upwards of 100 km. In regions like the Basin and Range Province the blocks may be displaced with or without tilting of initially horizontal surfaces. In rifted regions, the rift valleys tend to be situated on the crests of regional arches, and originally horizontal surfaces dip gently away from the central block [see lilies (1970)].
This classic interpretation of block-faulted regions is that they have been extended perpendicular to the strike of the faults [e.g., Gregory (1921)]. This interpretation is sometimes carelessly expressed by referring to such regions as "tensionaL" Note, however, that tension describes stress whereas extension refers to strain. Even if the evidence [()rextension is compelling, this does not necessarily mean that any of the principal stresses were tensile.
The evidence for extension of block-faulted regions is strong, particularly if we include the evidence for spreading across oceanic ridges. Present-day extension across block-faulted regions has been measured using geodetic methods in the Basin and Ranf1e Province [Tompson (1966)] and in Iceland [Decker and Einarsson (1971). Rates of continental extension appear lower by one or two orders of magnitude than typical sea floor spreading; oceans spread at rates of centimeters per year whereas the Rhine graben as extended 4.8 km in 45 m.y. [according to Illies (1970)] and the Dixie valley graben in Nevada extended 3 km in 10-15 my.