Page 180 - Global Tectonics

166 CHAPTER 7

120° 115° 110°
45° Idaho Montana
NORTH Oregon 45°
AMERICAN
PLATE Wyoming

125° California Nevada

40°
BASIN AND RANGE
40°
Utah
San

COLORADO

35° Andreas
PLATEAU 35°
PACIFIC Fault
PLATE
Arizona
120° 115° 110°
Extent of the Paleozoic Mesozoic
passive margin Batholiths
Physiographic
Sevier Thrust Belt
Provinces
Figure 7.12 Map of the western United States showing extent of Paleozoic passive margin sequences and the Sevier
thrust belt (after Niemi et al., 2004, with permission from Blackwell Publishing).

areas that record virtually no upper crustal degree that pieces of the middle crust are
strain. East–west extension beginning about exposed (Snow & Wernicke, 2000).
∼16 Ma has resulted in ∼250 km of extension One of the most enigmatic characteristics of
between the Sierras and the Colorado the Basin and Range Province involves local
Plateau (Wernicke & Snow, 1998). The relationships between large-scale extension in
intervening region responded to this the upper crust and the distribution of strain
divergence by developing a patchwork of in the lower part of the crust. Some studies
relatively unextended crustal blocks separated have shown that despite highly variable
by regions strongly deformed by extension, patterns of upper crustal strain, local crustal
strike-slip faulting, and contraction (Fig. thickness appears to be surprisingly uniform
7.13a). The heterogeneous distribution of (Gans, 1987; Hauser et al., 1987; Jones &
extension is illustrated in Fig. 7.13b, which Phinney, 1998). This result implies that large
shows estimates of the thickness of the pre- strains have been compensated at depth by
Miocene upper crust that remains after lateral fl ow in a weak lower crust, which
extension assuming an original thickness of acted to smooth out any Moho topography
15 km. In some areas, such as the Funeral (Section 7.6.3). Park & Wernicke (2003) used
and Black mountains, the upper crust has magnetotelluric data to show that this lateral
been dissected and pulled apart to such a fl ow and fl attening out of the Moho in the