Abstract The numerous cruises of Scripps Institution vessels to the Gulf of California between 1957 and 1963 have provided thousands of miles of new sounding lines that have made possible an extensive revision of the earlier bathymetric charts. The fault scarps suggested by earlier soundings are now well confirmed, and also the indications of right-lateral movement with resulting tensional fractures. Several newly discovered northeast-trending deeps have been discovered. En echelon faults diagonal to the general trend of the Gulf are apparently a southward extension of the San Andreas system. Displacements of the order of 160 miles appear to have taken place between the two sides of the Gulf, with a break in the crust forming the central Gulf with its near-oceanic depths. The end of the Baja California Peninsula is thought to have been located originally in the vicinity of Band eras Bay on the mainland. Two types of elevations occur in the Gulf, (1) those forming islands and banks along the western margin and consisting largely of “granite,” and (2) central highs, most of which do not reach the surface and contain basic volcanics. The continental shelves are very narrow or nonexistent on the west side of the Gulf but fairly broad on the east, especially to the south of Mazatlan. The eastern shelves appear to be of a depositional character.

Abstract The geology of the Gulf of California region is discussed as it pertains to developing a hypothetical structural model presented to describe Gulf evolution. The Gulf of California supposedly evolved as fractured plates of crustal material moved northwestward and Pacific-ward by gravitational sliding, on extremely gentle slopes, from the regions of western Mexico uplifted by batholithic intrusions. The source of the uplift and westward tilting, and perhaps the formation of the intrusions, is ascribed to the development of the East Pacific Rise. This rise is the present expression of a subcrustal welt that reaches the North American continent near the south end of the Gulf of California, as demonstrated by the work of Menard and others.

During the Capricorn Expedition of 1952-1953 the research vessels H orizon and S pencer F. B aird spent two weeks in, the Tonga Trench and Archipelago making echo-sounding, coring, seismic-refraction and towed-magnetometer studies. The Tonga Trench is a slightly arcuate furrow convex to the east over most of its length. Between Tafahi and Samoa it bends sharply to the northwest, appearing to follow the andesite line. It is continuous at depths greater than 7000 m for 1100 km, including in this segment three elongate basins deeper than 9000 m and one deeper than 10,000 m. The deepest basin explored lies at about 23° 15′ S., near a sharp bend in the axial trend, where bomb sounding gave a depth of 10,800 ± 200 m. A typical west-to-east profile, 350 km long, across the central Tonga Islands, crosses a chain of active volcanic islands at Kao, crosses the narrow Tofua Trough of 1700 m depth to the coralline island Lifuka, then down the west flank of the trench to a maximum depth of 9000 m in a steep-walled gorge 1–5 km wide. The east flank rises more gently to a broad ridge rising about 350 m above the general level of the sea bottom eastward. At 26° 00′ S. Lat. and at 18° 45′ S. Lat. the trench is constricted by large seamounts on the east flank. The latter seamount caps a smooth rise from 7500 m to 410 m and appears to be a guyot tilted westward. Scattered bottom samples indicate that, except for the calcareous deposits found on the main Tongan ridge, the sea floor in the area is largely volcanic sand and gravel, probably ejected from the near-by active volcanoes. Seismic-refraction profiles, parallel to the axis of the trench, indicate that the sediment foundation throughout the area is a rock, probably volcanic, of compressional wave velocity 5.2 ± 0.2 km/sec. The sediment thickness is about 2 km in the Tofua Trough, very thin, <200 m on the average, in the inner gorge of the trench, and about 400 m on the east flank, 50 km from the trench axis. Beneath the 5.2 km/sec basement the crustal velocity is 7 km/sec in Tofua Trough, 6½ km/sec at the trench axis and also on the east flank. The Mohorovičić discontinuity, characterized by a velocity of 8.1 ± 0.1 km/sec, is estimated to be at a depth of 20 km below sea level at the trench axis, and 12 km below sea level on the east flank. This velocity was not observed beneath Tofua Trough, where the highest velocity was 7.6 km/sec, estimated to occur at a depth of 12 km below sea level. Two profiles of total magnetic field strength, recorded across the trench, have been used to estimate profiles of sediment thickness. When the effects of basement topography are eliminated, a deep-seated asymmetry remains that could indicate either that the Curie point isothermal surface is deeper or that the magnetic susceptibility is greater on the west side than on the east side of the trench.