Geophysical data collected in 1975 and 1976 reveal major crustal and tectonic elements of the continental margin of southern Baja California. Gravity, magnetic, seismic reflection and bathymetric data show seaward extension of the islands enclosing Magdalena and Almejas Bays. A seismic reflection profile, oriented approximately normal to the trend of the Baja peninsula, indicates normal faulting of the near surface sediment layers along the outer continental shelf. The reflection record also shows that sediment layers immediately above the acoustic basement dip toward the east at the base of the continental slope. A crustal and subcrustal cross section, oriented approximately parallel to the reflection profile and constrained by gravity, magnetic, bathymetric and seismic refraction data, indicates a maximum crustal thickness of approximately 21 km for Baja California, making it intermediate in thickness between normal continental and normal oceanic crusts. The section also indicates a low density zone in the mantle below the Gulf of California. Magnetic anomalies along the cross section require oceanic crust of the Pacific Plate to extend at least 50 km landward of the edge of the western continental shelf of Baja California. This suggests either a past period of oblique subduction of the Pacific Plate beneath Baja California or emplacement of Pacific Plate oceanic crust beneath the peninsula by descending spreading centers of the East Pacific Rise. / Graduation date: 1977

Seismic refraction, reflection and gravity data obtained across the Peru continental margin and Nazca Plate at 9° S. permit a detailed determination of crustal structure. Complex structures normal to the profile require the development of a ray trace technique to analyze first and later arrivals for eleven overlapping refraction lines. Other data integrated into the seismic model include velocities and depths from well data, near surf ac sediment structures from reflection profiles and velocities obtained from nearby common depth point reflection lines. Crustal and subcrustal densities and structures were further constrained by gravity modeling to produce a detailed physical model of a convergent margin. The western portion of the continental shelf basement consists of a faulted outer continental shelf high of Paleozoic or older rocks. It is divided into a deeper western section of velocity 5.0 km/sec and a shallower, denser eastern section of velocity 5.65 to 5.9 km/sec. The combined structure forms a basin of depth 2.5 to 3.0 km which contains Tertiary sediments of velocity 1.6 to 3.0 km/sec. In this area, near-surface sedimentary structure suggests truncated sinusoidal features caused by exposure to onshore-offshore bottom currents. The 3 km thick, 4.55 to 5.15 km/sec basement of the eastern shelf shoals shoreward. Together, this basement and the eastern section of the outer continental shelf high form a synclinal basin overlain by Tertiary sediments which have a maximum thickness of 1.8 km and a velocity range of 1.7 to 2.55 km/sec. The gravity model shows a large block of 3.0 g/cm³ lower crustal material emplaced within the upper crustal region beneath the eastern portion of the continental shelf. Refraction data indicates a continental slope basement of velocity 5.0 km/sec overlying a slope core material with n interface velocity of 5.6 km/sec. The sedimentary layers of the slope consist of an uppermost layer of slumped sediment with an assumed velocity of 1.7 to 2 km/ sec which overlies an acoustic basement of 2.25 to 3.6 km/ sec. The high velocities (and densities) of the slope basement suggest the presence of oceanic crustal material over lain by indurated oceanic and continental sediments. This slope melange may have formed during the initiation of subduction from imbricate thrusting of upper layers of oceanic crust. Once created, the melange forms a trap and forces the subduction of most of the sediments that enter the trench. A ridge-like structure within the trench advances the seismic arrival times of deeper refractions and supports the suggestion that it is thrust-faulted oceanic crust which has been uplifted relative to the trench floor. The model of the descending Nazca Plate consists of a 4 km thick upper layer of velocity 5.55 km/sec and a thinner (2.5 km) but faster 7.5 km/sec lower layer which overlies a Moho of velocity 8.2 km/sec. The gravity model indicates that the plate has a dip of 5° beneath the continental slope and shelf. West of the trench, the lower crustal layers shallow, which may represent upward flexure of the oceanic plate due to compressive forces resulting from the subduction process. The upper crustal layers of the 120 km long oceanic plate portion consist of a thin 1.7 km/sec sedimentary layer overlying a 5.0 to 5.2 km/sec upper layer. An underlying 5.6 to 5.7 km/sec lower layer becomes more shallow to the east within 60 km of the trench while a deeper 6.0 to 6.3 km/sec layer thickens to the east. The lower crustal model consists of a 7.4 to 7.5 km/sec high velocity layer which varies in thickness from 2.5 km to 4.0 km. The 8.2 km/sec Moho interface varies not more than ±0.5 km from a modeled depth of 10.5 km. / Graduation date: 1979 / Best scan available for figures.

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