Earth Structure An Introduction to Structural Geology and Tectonics

8. Faults and Faulting

Imagine a miner in a cramped tunnel crunching eastward through a thick seam of coal. Suddenly, his pick hits hard rock. The miner chips away a bit more only to find that the seam which he's been following for the past three weeks abruptly terminates against a wall of sandstone. He curses, "My flippin' seam's cut off -- there's a fault with it!" From previous experience, the miner knew that because of the fault, he would have to waste precious time digging a shaft up or down to intersect the coal seam again. Geologists adopted the term fault, but you'll find that they use the term in different ways in different contexts. In a general sense, a fault is any surface or zone in the Earth across which measurable slip (shear displacement) develops. In a more restricted sense, faults are fractures on which slip develops primarily by brittle deformation processes. This second definition serves to distinguish 'faults' (sensu stricto) from 'fault zones' and 'shear zones'. We use the term fault zone for brittle structures in which loss of cohesion and slip occurs on several faults within a band of definable width. Displacement in fault zones can involve formation and slip on many small, subparallel brittle faults, or slip on a principal fault off which many smaller faults diverge (fault splays), or slip on an anastamosing array of faults. Shear zones are ductile structures, across which a rock body does not lose macroscopic cohesion, so that strain is distributed across a band of definable width. In ductile shear zones, rocks deform by cataclasis, a process involving fracturing, crushing and frictional sliding of grains or rock fragments, or by primarily crystal plastic deformation mechanisms. We describe cataclastic shear zones in this chapter, but delay discussion of other processes in ductile shear zones until Chapter 12.

Faults occur at all scales in the lithosphere, and geologists study them for many reasons:

• Faults control the spatial arrangement of rock units, so their presence creates puzzles that challenge even the most experienced geologic mappers.
• Faults affect topography and modify the landscape.
• Faults affect the distribution of economic resources (e.g., oil fields and ore bodies).
• Faults control permeability of rocks and sediments, properties which, in turn, control fluid migration.
• Faulting creates deformation (strain ± rotation ± translation) in the lithosphere during plate interactions and intraplate movements.
• Faulting may cause devastating earthquakes.