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A seismic deep-refraction profile across the Arabian Shield provides new constraints for crustal models of the southwestern Shield and the structural transition from the Shield to Red Sea axial trough. The crust thins abruptly from 40 to 16 km at the eastern edge of the coastal plain, and further thins without any detected lateral disconformity to about 8 km (of which about half is composed of a Miocene evaporite sequence) beneath the Red Sea shelf. The upper crustal layer has a mean velocity of about 6.2 ± 0.2 kms/s on either side of the exposed Shield margin, and may be composed predominantly of diabase (analogous to the oceanic layer 2). The 16-km crustal thickness beneath the coastal plain and 6.2 km/s crustal velocity indicate that the crust is not typical oceanic crust, but the required total crustal thinning from 40 km on the Shield to 4 km (of non-evaporite crust) beneath the Red Sea shelf would seem to rule out a continental composition west of the Shield margin. This transitional crust, with an intermediate velocity, high density, and a thickness decreasing from that of attenuated crust (16 km) of an initial rift valley floor to that of a typical oceanic environment (4km), may be representative of crust produced during early stages of seafloor spreading. In general, such transitional crust is also a magnetic quiet zone, but, in the region traversed by the seismic refraction profile, magnetism was sufficiently localized with respect to a fixed axis of spreading to produce the distinct stripe anomalies. The seismic interpretation supports the hypotheses of (1) essentially shore-to-shore opening of the Red Sea between Aqaba and the Tihamat Asir; (2) two-stage extension; and (3) Miocene and younger crust beneath most of the Afar. INTRODUCTION A 1,000-km-long, deep-refraction seismic profile was recorded in early 1978 by the U.S. Geological Survey (USGS) and the Saudi Arabian Directorate General for Mineral Resources (DGMR) across southwestern Saudi Arabia. Its purpose was to provide estimates of crustal thickness and the bulk properties of the crust and upper mantle beneath the principal tectonic provinces of the Arabian Shield and its margins. The profile transects the Shield in a south and southwesterly direction, extending from Phanerozoic "cover rock" terrain of the Arabian platform, near Riyadh, to the JL/ U.S. Geological Survey, Menlo, Park, CA 2/ U.S. Geological Survey, Reston, VA outer edge of the Farasan Islands, in the southern Red Sea, less than 40 km from the edge of the Red Sea axial trough. Six shots were employed, and more than 500 recording stations were occupied. One hundred specially developed, portable, programmable instruments recorded the shots using 2-Hz vertical-component seismometers. A description of equipment, field j procedures, and initial results are contained in Blank and others (1979), and a general account of the results and their interpretation are given in Healy and others (1982$. The present report is concerned specifically with the interpretation of the southwesternmost 480 km of the recorded profile. The report focuses on the structural transition from Arabian Shield to oceanic crust of the central Red Sea and on the problem of the composition of the crust beneath the southern Red Sea shelf and coastal plain.

A Seismic refraction interpretation of the eastern margin of the Red Sea Depression, Southwest Saudi Arabia