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Abstract

The fundamental features of the seismic-velocity structure of the crust of the Basin and Range province have been known for 30 years. Tatel and Tuve (1955) expressed surprise in finding that the crust is only 28–34 km thick in the Basin and Range/Colorado Plateau transition zone of Arizona, New Mexico, and Utah. From isostatic considerations they had expected the crust of this elevated region to be about 70 km thick. Carder and Bailey (1958) obtained similar results in the Basin and Range province in the vicinity of the Nevada Test Site. Later, Press (1960), Berg and others (1960), and Diment and others (1961) discovered the characteristic low velocity of the upper mantle of the province, although they regarded the 7.6–7.8 km/sec velocities they found at depths of 24–28 km as possibly too low for the upper mantle. Nevertheless, the unexpectedly thin crust and low-velocity upper mantle have been accepted by most investigators as characteristic features of the Basin and Range province since U.S. Geological Survey seismic teams demonstrated that the discontinuity below which the velocity is about 7.8 km/sec is the most prominent one in the province and argued that it should be regarded as the Mohorovičé discontinuity (Pakiser, 1963).

Our knowledge of the gross features of the crustal and upper-mantle structure of the Basin and Range province has changed little in the past 20 years, but many important details have been added. Major problems, however, remain to be resolved. Reexamination of the evidence for extremely low upper-mantle velocities in the vicinity of the Wasatch front (Smith, 1978) suggests that velocities there may not be very different from those in other parts of the province. New, more nearly definitive seismic-refraction and -reflection experiments and reanalysis of existing data using new analytical techniques will be required to resolve such problems.

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