Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 54

(Keller and Pinter 2002). The western section (the Pacific Plate, on which Los Angeles is
located) is moving northward at the rate of ∼5.8 cm (∼2.3 in.) per year. Past movement
is estimated to have been ∼l,100 km (∼700 mi), and if the present trend continues, Los
Angeles will one day—several million years from now—be a suburb of San Francisco.
Although movement along the fault is primarily horizontal, some buckling and vertical
displacements have taken place, resulting in mountainous terrain (e.g., the San Gabriel
and San Bernardino Mountains).
The manner in which strike-slip or transform faults produce mountains is called
transpression, and the result may be a flower structure (Fig. 2.22). The uplift mechan-
ism occurs when two plates move past each other along a fault that has a curve or a
jog in it. Where the jog causes the moving rock to be restrained, rock is then squeezed
up and out along associated high-angle reverse faults on the strike-slip fault. The Alpine
Fault of New Zealand is a primary example of transpression, wherein the Southern Alps
are the uplifted result of the oblique convergence of the Indian (Australian) and Pacific
Plates (Kneupfer 1992; Tippett and Hovius 2000).
A similar sort of faulting that can produce major mountains is also exemplified by
Nanga Parbat (8,125 m, 26,660 ft), a pop-up structure in the western Himalaya (Sch-
neider et al. 1999). In this case, major erosional unroofing caused a tectonic aneurysm
in which the mountain rose like a cork along two inward-facing, high-angle, reverse
faults to produce the ninth highest mountain in the world (Zeitler et al. 2001a, 2001b).
FAULT AND FAULT-LINE SCARPS
Fault and fault-line scarps are characteristic of most mountain ranges, yet they are no-
toriously difficult to interpret. Most of the features have a long history of formation, with
concomitant erosion that may so alter the original structural landform as to obscure its
origins (Fig. 2.23). Once a fault scarp is produced on the landscape it undergoes erosion
by either downwearing or parallel retreat, generally as a function of lithology. For ex-
ample, crystalline rocks tend to decline in slope angle through time, whereas some sed-
imentary rocks will tend to promote erosional maintenance of similar slope angles. In
either case, if the fault undergoes renewed movement, a composite fault scarp (upper
eroded, lower uneroded) results. If erosion is continued it can remove the original fault
scarp and produce a new, entirely erosional escarpment, near the location of the ori-
ginal fault. This is a fault-line scarp, and it can occur with the scarp facing in the same
direction as the original (resequent fault-line scarp), or in the opposite direction (ob-
sequent fault-line scarp). A fault scarp buried in sediment and then exposed at a later
time is an exhumed fault scarp. Deciphering the sequence of fault and erosional events
in any particular complex faulted area is commonly a difficult task.