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Marine geophysical data from the continental margin of Baja California and the Gulf of California, and geological and geophysical data from the Baja California Peninsula and mainland Mexico, outline the major geologic and tectonic features of the Baja California Peninsula and the surrounding areas from 24.5° N. Lat. to 27.5° N. Lat. A crustal and subcrustal cross section consistent with observed gravity and magnetic anomalies, and constrained by seismic refraction stations and the mapped surface geology shows major variations of density and magnetization in these areas. A geologic interpretation of the cross section indicates the rocks of the Pacific continental margin are composed of unconsolidated and semiconsolidated sediments. Tertiary sedimentary rock, metamorphic rock of Franciscan type, and continental crystalline rock probably corresponding to the Peninsular Range batholith. The depth to mantle under the Baja California Peninsula is postulated to be 20 km. In the Gulf of California a section of low-density mantle beginning at a depth of 11 km is necessary to fit the observed gravity values and accounts for the low seismic velocities associated with the mantle in the Gulf. The correlation between the observed magnetic anomalies on the Pacific continental margin of the Baja California Peninsula and the theoretical magnetic anomalies expected from a spreading center shows that the youngest identifiable remanent anomaly on the Pacific side of south central Baja California is anomaly 3' formed at 6 my B.P. The remanent magnetic anomalies extend 50 km landward from the western edge of the continental slope. / Graduation date: 1978

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The composition and spatial distribution of Quaternary volcanism in the Trans-Mexican Volcanic Belt (TMVB) exhibit some remarkable correlations with the seismicity, age, and structure of ocean lithosphere being consumed at the Middle America Trench. In the west, the TMVB is related to aseismic subduction of the Rivera plate (2 cm/yr) and in the east to a moderately dipping (20-30°) rapidly subducting Cocos plate (6-9 cm/yr). These contrasting arc segments are bounded by the Colima Graben, a zone of high-angle faulting and contemporaneous alkaline/calc-alkaline volcanism, situated above a sinistral transform fault (4 cm/yr) developed in the downgoing slab at the Cocos/Rivera juncture. Geologic mapping and K-Ar dating of the lavas of Iztaccíhuatl, a major calc-alkaline volcano in the TMVB, have established two main phases of eruptive activity that began prior to 0.9 Ma. The substructure of Iztaccíhuatl (>0.6 Ma) is composed principally of two-pyroxene andesites and dacites (300 km³) erupted from Llano Grande and Ancestral Pies volcanoes. The second stage of cone construction (<0.6 Ma) involved horn- blende dacites and andesites (150 km³) extruded from NNW-SSE oriented vents to form the modern summit region. The earliest glacial deposits date prior to 0.27 Ma but moraine complexes on the flanks are Wisconsin to Neoglacial. The phenocryst mineralogy and chemistry of the Younger Andesites and Dacites indicate that magma mixing has played an important role in the evolution of Iztaccíhuatl magma chambers. Mixed lavas characterized by disequilibrium phenocryst assemblages involving magnesian olivine and in the crust and olivine-phyric basaltic magma ascending from depth. Mixing and homogenization are effected by liquid blending and dynamic fractional crystallization in turbulently convecting hybrid liquids. The whole-rock geochemistry of mixed lavas and hornblende dacites is used to derive the composition of each batch of basaltic magma periodically replenishing crustal magma chambers. Basaltic end-members exhibit significant variations in large-ion lithophile elements and Sr isotopic composition which are attributed to heterogeneities in mantle source regions. The primitive compositions of these magmas are compatible with an origin involving partial melting of fertile peridotite under hydrous high-pressure conditions. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

The geologic map of El Cuervo area shows distribution of stratigraphic units ranging in age from Jurassic (?) to Recent. Outcropping strata are principally Cretaceous in mountainous areas and Cenozoic in bolson areas. From the Late Jurassic Epoch until the Late Cretaceous Epoch the Chihuahua Trough was a negative feature with respect to adjacent platforms and 10,000 to 18,000 feet of Jurassic-Comanchean sedimentary rock, including a thick basal evaporate sequence, accumulated in it; whereas on the adjacent Diablo Platform of Texas about 3,000 feet of sedimentary rock accumulated. The lower part of the Mesozoic sedimentary record shows a gradual transgression, interrupted by numerous minor regressions, from the Chihuahua Trough onto the Diablo Platform. The upper part of the record shows a regression. The eastern edge of a Jurassic (?) evaporite basin is in the eastern part of the area. Neocomian-Aptian formations are dominantly siliciclastic. Middle Albian formations are dominantly siliciclastic in the eastern part of the area, but are predominantly shallow-water carbonate to the west. Late Albian-early Cenomanian formations are limestone with subordinate shale. During the Cenomanian Epoch siliciclastic deposition again became dominant and the medial Cenomanian-Senonian formations record a transition from marine to continental deposition. Mesozoic and Paleozoic "basement" rocks were deformed during the Late Cretaceous-Early Tertiary Laramide orogeny. Jurassic (?) evaporites acted as a décollement zone between Mesozoic and Paleozoic rocks. Major thrust faults, over-thrust toward the east, developed along the eastern edge of the evaporite basin in El Cuervo area. During thrust-faulting, evaporites were diapirically injected into younger rocks along thrust- and tear-fault zones. As evaporites flowed into diapirs and cores of anticlines, blocks settled differentially into space abandoned and chaotic patterns of normal faults resulted. Olivine-diabase sills and dikes and amphibole-rich rocks (where olivine diabase was intruded into and contaminated by evaporites) may have formed during early stages of Laramide deformation. During and after Laramide deformation denudation created a surface of erosion in the area. During Early Tertiary erosion as evaporites were removed, collapse structure developed over diapirs, which had been injected along tear-fault zones. At several places in near- vertical beds on flanks of folds, as erosion removed nonresistant beds, gravity developed flaps and detached flaps in adjacent resistant beds of limestone. During late Eocene-early Oligocene time flow rocks, ignimbrites, and associated sedimentary rocks were deposited in widely scattered, topographically low areas. Volcanic and associated rocks, deposited in collapse features, were deformed as erosion of evaporites continued and they foundered into evaporites. Two porphyritic andesite intrusions are associated with Laramide faults; one is in the core of a large anticline. In the southeastern part of the area several trachyte intrusions along east-trending joints formed dikes. Subsequent to vulcanism, the region was uplifted from elevations near sea level to thousands of feet above sea level and Late Tertiary block-faulting was superimposed on Laramide structure in the eastern part of El Cuervo area. Intrusion of olivine-biotite "peridotite" may have accompanied faulting or immediately followed the main episode of faulting. Thick sections of bolson fill were deposited in the Presidio and Benigno grabens as a consequence of block-faulting. Although some Laramide faults may have been reactivated during Tertiary block-faulting, major Tertiary faulting did not take place west of the eastern front of the easternmost range of the Chihuahua Tectonic Belt. / text

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