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A self-consistent regional-scale model of the crust and upper mantle for the southern Arabian Shield and Red Sea paar has been constructed from an integrated interpretation of seismic deep refraction, regional gravity, aeromagnetic, heat flow, and surface geologic data. The Shield consists of two 20-km-thick layers of crust with an average compressional wave velocity in the upper crust of about 6.3 km/s and in the lower crust of about 7.0 km/s. This crust thins abruptly to less than 20 km near the southwestern end of the transect, where Precambrian outcrops abut the Cenozoic rocks, and to 8 km beneath the Farasan Islands. The data over the Red Sea westward of Precambrian outcrop are fit satisfactorily by an oceanic crustal model. The major velocity discontinuities occur at about the same depth across the entire shield and indicate horizontal metamorphic stratification of the Precambrian crust. Several lateral inhomogeneities in bulk physical properties have been identified in both the upper and lower crust of the shield, indicating bulk compositional variations. The subcrustal portion of the model is composed of a hot, low-density lithosphere and asthenosphere beneath the Red Sea which is systematically cooler and denser to the northeast. This model provides a mechanism that explains the observed topographic uplift, regional gravity anomaly pattern, heat flow, and mantle seismic velocities. Such a lithosphere could be produced by upwelling of hot asthenosphere beneath the Red Sea which then flows laterally beneath the lithosphere of the Arabian plate. INTRODUCTION The major geologic province of western Saudi Arabia is a Precambrian shield bordered on the east by a platform of gently dipping Cambrian and younger sedimentary rocks and on the west by rocks of the Tertiary Red Sea paar. During the past 10 years, geophysical data have been collected or compiled at a regional scale along a 150-km-wide transect from the platform across the southern part of the Arabian Shield and into the Red Sea paar. A crustal section for southwestern Saudi Arabia has been constructed from an integrated study of these data and constrained by surface geological relations. Previous to this work, knowledge of the crustal structure of the Arabian Shield was limited to inference from surface geologic mapping [e.g., Schmidt et al., 1979], geochemical and isotopic studies [e.g., Stacey et al., 1980], and studies of Rayleigh waves from earthquakes [Niazi, 1968; Knopoff and Fouda, 1975]. The geophysical transect (Figure 1), extending from near Riyadh to the Farasan Islands in the southwest, includes aeromagnetic data [Andreasen et al., 1980], regional gravity data [Gettings, 1983], seismic deep re&action data [Blank et al., 1979; Healy et al., 1982], heat flow observations [Gettings and Showail, 1982], and regional-scale geologic information [U.S. Geological Survey and Arabian-American Oil Company, 1963; Brown, 1972]. The refraction profile extends for about 1000 km, approx-This paper is not subject to U.S. copyright. Published in 1986 by the A...

The Yellowstone plateau volcanic field is less than 2 million years old, lies in a region of intense tectonic and hydrothermal activity, and probably has the potential for further volcanic activity. The youngest of three volcanic cycles in the field climaxed 600,000 years ago with a voluminous ashflow eruption and the collapse of two contiguous cauldron blocks. Doming 150,000 years ago, followed by voluminous rhyolitic extrusions as recently as 70,000 years ago, and high convective heat flow at present indicate that the latest phase of volcanism may represent a new magmatic insurgence. These observations, coupled with (i) localized postglacial arcuate faulting beyond the northeast margin of the Yellowstone caldera, (ii) a major gravity low with steep bounding gradients and an amplitude regionally atypical for the elevation of the plateau, (iii) an aeromagnetic low reflecting extensive hydrothermal alteration and possibly indicating the presence of shallow material above its Curie temperature, (iv) only minor shallow seismicity within the caldera (in contrast to a high level of activity in some areas immediately outside), (v) attenuation and change of character of seismic waves crossing the caldera area, and (vi) a strong azimuthal pattern of teleseismic P-wave delays, strongly suggest that a body composed at least partly of magma underlies the region of the rhyolite plateau, including the Tertiary volcanics immediately to its northeast. The Yellowstone field represents the active end of a system of similar volcanic foci that has migrated progressively northeastward for 15 million years along the trace of the eastern Snake River Plain (8). Regional aeromagnetic patterns suggest that this course was guided by the structure of the Precambrian basement. If, as suggested by several investigators (24), the Yellowstone magma body marks a contemporary deep mantle plume, this plume, in its motion relative to the North American plate, would appear to be "navigating" along a fundamental structure in the relatively shallow and brittle lithosphere overhead. The concept that a northeastwardpropagating major crustal fracture controls the migration path of the major foci of volcanisim is at least equally favored by existing data, as Smith et al. (19) noted.

Introduction 4 Location of study area 4 Geologic setting 4 Acknowledgments 6 Part 1. Geological investigations 8 Data sources 8 Stratigraphy of the basin fill 8 Nogales Formation (lower basin fill) 9 Distribution 9 Age and contacts 9 General appearance and thickness 10 Clast composition 10 Depositional environment 11 Upper basin fill 11 Structure 12 Subbasins 12 Structural control of the upper Santa Cruz River 13 Upper Santa Cruz River gradient and Quaternary faulting 13 Summary 16 Part 2. Geophysical investigations 17 Data compilation 17 Separation of the basin gravity anomaly 17 Basin fill density functions 18 Drillhole information 24 Estimation of depth to bedrock 32 Estimation of thickness of upper basin fill 36 Summary 37 References 39

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