Page 182 - Global Tectonics

168 CHAPTER 7

Basin and Range probably occurred during compatible with evidence of high heat fl ow,
the Miocene. By contrast, other regions, such elevated geotherms, and shallow
as the Aegean Sea and the D’Entrecasteaux asthenosphere. Pre-rift volcanism is mostly
islands (Section 7.8.2), do not show this calc-alkaline in composition. Magmatism
relationship, implying a more viscous lower that accompanied extension is mostly
crust that resists fl ow beneath highly basaltic. Basalts from Nevada have an
extended areas. isotopic signature suggesting that they were
derived from sublithospheric mantle. This
3 Thin mantle lithosphere and anomalously high heat pattern matches evidence of mantle
fl ow. Like most wide rifts, the Basin and Range upwelling beneath the rift (Savage &
is characterized by high surface heat fl ow, Sheehan, 2000).
negative long-wavelength Bouguer gravity
anomalies, and low crustal P n and S n velocities 4 Small- and large-magnitude normal faulting. Large
(Catchings & Mooney, 1991; Jones et al., 1992; extensional strains and thinning of the crust
Zandt et al., 1995; Chulick & Mooney, 2002). in wide rifts is partly accommodated by slip
Regional topography in the Basin and Range on normal faults. Two contrasting patterns
also is unusually high with an average of 1.2 km are evident. First, the deformation can involve
above mean sea level. Low seismic velocities are distributed normal faulting where a large
discernible down to 300–400 km depth. Seismic number of more or less regularly spaced
tomographic models indicate that adiabatic normal faults each accommodate a relatively
mantle temperatures of 1300°C occur as small amount (<10 km) of the total extension.
shallow as 50 km under most of the province. Second, the strain may be highly localized onto
For comparison, temperatures at 50–100 km in a relatively small number of normal faults that
the cratonic mantle beneath the stable eastern accommodate large displacements of several
part of North America are on average 500°C tens of kilometers. Both patterns are common
cooler than under the Basin and Range. All of and may occur during different stages of rift
these characteristics indicate a shallow evolution.
asthenosphere and very thin, warm upper Many of the range-bounding normal faults in
mantle (Goes & van der Lee, 2002). the Basin and Range record relatively small
Temperatures at 110 km depth inferred offsets. These structures appear similar to
from seismic velocity models suggest the those that characterize narrow rift segments.
presence of small melt and fl uid pockets in Asymmetric half graben and footwall uplifts
the shallow mantle beneath the Basin and are separated by a dominant normal fault
Range (Goes & van der Lee, 2002). Warm, that accommodates the majority of the
low-density subsolidus mantle also may strain. The morphology of these features is
contribute to the high average elevation and governed by the elastic properties of the
large-scale variations in topography of the lithosphere (Section 7.6.4) and the effects of
region. Other factors contributing to the high syn-rift sedimentation and erosion. The
elevations probably include isostatic effects asymmetry of the half graben and the dips
caused by previously thickened continental of the range-bounding faults also commonly
crust and magmatic intrusions. However, a change in adjacent basin-range pairs. Many
lack of correlation between crustal thickness of the tectonically active faults maintain
variations and surface topography indicates steep dips (>45°) that may penetrate through
that simple Airy isostasy is not at play and the upper crust. However, unlike the border
the high elevations across the southwestern faults of East Africa, some of the range-
United States must involve a mantle bounding faults of the Basin and Range
component (Gilbert & Sheehan, 2004). exhibit geometries that involve low-angle
Volcanic activity is abundant, including extensional detachment faults. A few of these
eruptions that occurred both before low-angle normal faults accommodate very
and during extension. This activity is large displacements and penetrate tens of