Page 179 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 165

Utah (1962–1999) and 17 km for Nevada Mesozoic Sevier thrust belt (Fig. 7.12). This
(1990–1999) (Pancha et al., 2006). This deformation created a thick pile of weak
thickness of the seismogenic layer is similar sedimentary rocks that has contributed to a
to that displayed by most other rifts, delocalization of strain (Section 7.6.1) during
including those in East Africa, except that in Cenozoic extension (Sonder & Jones, 1999).
the Basin and Range it characterizes Some estimates place parts of the province at
thousands of square kilometers of crust. The a pre-rift crustal thickness of 50 km, similar
pattern implies that high geothermal to that of the unextended Colorado Plateau
gradients and crustal thinning have locally (Parsons et al., 1996). Others have placed it at
weakened a very large area. more than 50 km (Coney & Harms, 1984).
Because deformation is distributed over This pre-extensional history is one of the most
such a broad region, most of the major important factors that has contributed to a
faults in the Basin and Range have recurrence heterogeneous style of extensional deformation
times of several thousand years (Dixon in the Basin and Range.
et al., 2003). In the northern part of the The uniformity in size and spacing of normal
province, several hundred faults show faults in the Basin and Range, and the
evidence of slip since 130 ka, yet apparent uniform thickness of the

contemporary seismicity and large historical seismogenic layer, at first suggests that strain
earthquakes are clustered on only a few of and crustal thinning, on average, might also
them. This observation raises the possibility be uniformly distributed across the province.
that a significant portion of strain is However, this assertion is in confl ict with the

accommodated by aseismic displacements. results of geologic and geophysical surveys.
Niemi et al. (2004) investigated this possibility Gilbert & Sheehan (2004) found Moho
by combining geologic data from major depths ranging from 30 to 40 km beneath the
faults with geodetic data in the eastern Great eastern Basin and Range (Plate 7.1a), with
Basin. The results suggest that both data the thinnest crust occurring in northern
types defi ne a ∼350 km wide belt of east– Nevada and Utah (Plate 7.1b) and thicknesses
west extension over the past 130 ka. of 40 km in southern Nevada (Plate 7.1c)
Reconciling deformation patterns measured (Plate 7.1a–c between pp. 244 and 245). Louie
over different timescales is a major area of et al. (2004) also found signifi cant variations
research in this and most other zones of in Moho depths with the thinnest areas
active continental tectonics. showing depths of only 19–23 km beneath
the Walker Lane and northwest Nevada. This
2 Heterogeneous crustal thinning in previously southward thickening of the crust coincides
thickened crust. Wide rifts form in regions where with variations in the pre-Cenozoic
extension occurs in thick, weak continental architecture of the lithosphere, including
crust. In the Basin and Range and the Aegean differences in age and pre-extensional
Sea the thick crust results from a history thickness. Similar nonuniform variations in
of convergence and crustal shortening that crustal thickness occur beneath the Aegean
predates rifting. Virtually the entire western Sea (Zhu et al., 2006). These results illustrate
margin of North America was subjected to that crustal thinning in wide rifts is
a series of compressional orogenies during nonuniform and, like narrow rifts, is strongly
Mesozoic times (Allmendinger, 1992). These infl uenced by the pre-existing structure of
events thickened sedimentary sequences that the lithosphere.
once formed part of a Paleozoic passive The nonuniformity of crustal thinning in the
margin. The ancient margin is marked now by Basin and Range is expressed in patterns of
an elongate belt of shallow marine sediments faulting within the upper crust. The Death
of Paleozoic and Proterozoic age that thicken Valley region of eastern California contains
to the west across the eastern Great Basin and some of the youngest examples of large-
are deformed by thrust faults and folds of the magnitude extension in the world adjacent to