STRUCTURAL GEOLOGY
Lecture 25
The Geometry of Strike-Slip Faulting
(A look at the San Andreas Fault System)

While thrust faulting is characteristic of convergent margins of continental crust and normal faults are characteristic of divergent margins where rift basins have formed, strike-slip faults are characteristic of margins where lithospheric plates slide past each other.  Some of the most significant strike-slip margins are associated with large strike-slip faults including the San Andreas Fault (Figure 25-1), the Alpine Fault (New Zealand), Chaman Fault (Pakistan), the Motagua Fault (Guatemala), the Dead Sea Transform (Israel-Jordan), the Altyn Tagh Fault (Tibet), Kunlun Fault (China), Herat Fault (Afghanistan), and the Anatolian Fault (Turkey).

Figure 25-1:  The San Andreas (adapted from Twiss and Moores, 1992).

Strike-slip faults are rarely planar.  This is best illustrated by the big bend on the San Andreas Fault just north of Los Angeles.  The San Andreas is a right lateral strike-slip fault with Pacific Plate moving north relative to the North American Plate.  This means that Los Angeles and San Francisco on the Pacific Plate are moving north relative to the rest of North America.  The big bend north of Los Angeles is called a restraining bend largely because the westward jump in the fault when tracing the fault to the north, leaves rocks in the way for further slip (Figure 25-2).  These ‘extra’ rocks must be moved out of the way for further slip.  Depending on the shape of the irregularities along strike-slip faults relative to the motion along the fault, there may also be releasing bends.  These are places where cavities open because opposite sides of an irregularity move away from each other (Figure 25-2). 

Figure 25-2:  A.  A planar strike-slip fault.  B. Strike-slip movement along irregularly curved faults producing gaps at releasing bends and crowding at restraining bends (adapted from Crowell, 1974).

Restraining and releasing bends produce diagnostic structures.  The restraining bend produces a ‘flower structure’ which is a series of vertical faults of compressional duplexes that are subparallel to the parent fault.  The vertical faults divide upthrown blocks the spread near the surface in a manner resembling a flower.  The upthrown blocks are sometimes called popups. 

Figure 25-3:  Restraining bend with a compressional duplex (adapted from Twiss and Moores (199)).

Releasing bends produce pull-apart basins.  Like the restraining bend, a series of faults that form subparallel to the parent fault produce a structure that in cross section is called a ‘flower structure’ (Figure 25-4).

Figure 25-4:  Extensional duplexes at releasing bends (adapted from Twiss and Moores, 1992).

An example of a restraining bend is found along the San Andreas Fault where the contraction expected at a left bend in a dextral fault is seen as the block uplifts on the Transverse Ranges (Figure 25-5).  The restraining bend of the San Andreas is reflected in the blocks bounded by east-west thrust faults.  Extensional basins are also present including the Salton Sea trough between the San Andreas and San Jacinto faults.

Figure 25-5:  The San Andreas Fault system showing the block uplifts of the Transverse Ranges of southern California (adapted from Twiss and Moores, 1992).