Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 58

FIGURE 2.26 Cross section of convergent mountain belt at continent edge in continuous section from
beginning of (A) to end of (B). (A) Geoclinal sediments are metamorphosed, intruded with granite
batholiths, and volcanoes and horsts and grabens are produced on the surface. An accretionary
wedge of material scraped up at the submarine trench and above the subduction zone, a forearc
basin, and sedimentation in the grabens add to the complexity. (B) Further inland, past the crys-
talline core of the mountain range, a fold and thrust belt has developed and forced geoclinal rocks
over the stable interior craton. (After Plummer et al. 2003.)
Gravitational collapse of a range is much more important in the deformation of moun-
tains than previously thought, displaying a wide range of phenomena. In fact, degrada-
tion of mountains caused by gravity is scale-dependent, with a range of such phenom-
ena encompassing gravitational extension of a wide range, from the largest, slowest end
member; through sackung (German for “sagging”) collapse of a single mountain in the
middle range of the phenomena; down to smaller, rapid landslides (Shroder and Bish-
op 1998). Evidence of gravitational extension includes the fact that, whereas the most
intense rock metamorphism occurs near the axis of the range (commonly occupied by
a granitic-core batholith), the most complex folding is on the margins (Fig. 2.28). As
rock masses are transported downslope through gravity sliding (Fig. 2.29), the cent-
ral area typically undergoes isostatic uplift to compensate for loss of weight. When this
happens, the central core attains the highest point of relief, both because it undergoes
greater uplift and because the crystalline rocks of which it is composed are more res-
istant to erosion. Mount Denali (McKinley) in Alaska, for example, the highest mountain
in North America (6,188 m/20,300 ft), is carved from a granite batholith that under-
lies the Alaska Range (Hunt 1974). Gravity sliding can also account for displacement
of older strata over younger rock in overthrust faults. The Heart Mountain detachment
fault (Fig. 2.29), located in northwestern Wyoming near Yellowstone National Park, for
example, is a series of huge rock blocks that slid down a slight incline, probably as a res-
ult of ancient volcanism in early Cenozoic time, to lie unconformably on younger rocks