Geography Reference
In-Depth Information
fication, including mountain-front sinuosity, widths of valleys in the ranges, dissection
characters of fault scarp facets, and other features.
FIGURE 2.21 Generalized east-west cross section of the Lewis Overthrust in northern Montana. Older
(Pre-cambrian) rocks have been thrust over younger (Cretaceous) rocks along the low-angle fault
for at least 24 km (15 mi). Chief Mountain is an erosional remnant (outlier or klippe) of the over-
thrust sheet, which at one time was much more extensive. (After Plummer et al. 2003.)
The Sierra Nevada consists of several individual segments that have tilted more or
less in unison to present a block ∼650 km (∼400 mi) long by ∼80 km (∼50 mi) wide. This
huge block slopes gently to the west, whereas east-facing scarps rise abruptly along
eroded fault scarps, presenting the highest mountain front in the contiguous United
States. In a horizontal distance of less than 10 km (∼6 mi), a drop of ∼3,350 m (∼11,000
ft) occurs between the higher peaks of the Sierra Nevada and the graben floor of Owens
Valley. Similarly, the spectacular fault scarps of the Wasatch Range in Utah or the Grand
Tetons of Wyoming display abrupt mountain fronts rising as much as 1,800 m (∼6,000
ft) above the surrounding plains.
The cause of the tensional breakup of the Basin and Range Province after the prior
lengthy regime of compressional uplift and thrust faulting that had produced the North
American Cordillera has long been enigmatic. At first the application of a plate tectonics
model seemed to offer an explanation. The plate in the western Pacific, which had long
been subducting beneath western North America, was ultimately consumed and the
transform faults of its spreading center overridden. Disappearance of this slab allowed
hot mantle to rise and spread out beneath the province to cause horizontal extension of
the crust. Later evidence of the exact timing of all this, however, did not corroborate the
theory, and other explanations were sought. It is now thought that gravitational collapse
of the Basin and Range area occurred after the strong compressional phase produced
an overthickened collisional orogen that was gravitationally unstable. The best model of
collapse of the orogen involves production of simple horsts and grabens, tilted blocks,
and listric or domino-style faulting on a basal detachment surface with ductile deform-
ation at depth that would fill in voids otherwise required if the blocks behaved in an
entirely rigid fashion (Fig. 2.19; Strahler 1998; Stockli et al. 2003; Colgan et al. 2006).
Although normal faulting can be complex, the structures and landforms produced
through displacement and deformation of strata are relatively simple compared with
the bewildering disarray of high-angle reverse and gentle-angle thrust faults. These
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