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The Guerrero terrane of western Mexico is characterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of arc affinity. The arc assemblage rests unconformably on partially metamorphosed rocks of possible Triassic-Jurassic age. These "basement units," the Arteaga and Placeres Complexes and the Zacatecas Formation, are composed of deformed turbidites, basalts, volcanic-derived graywackes, and blocks of chert and limestone. Sandstones from the basement units are mostly quartzitic and have a recycled orogen-subduction complex provenance. They have negative ᵋNdi (-5 to -7), model Nd ages of 1.3 Ga., and enrichment in light REE, indicating that they were supplied from an evolved continental crust. The volcanic graywackes are derived from juvenile sources (depleted in LREE and ᵋNd = +6), though they represent a small volume of sediments. Primary sources for these turbidites might be the Grenville belt or NW South America. Basement rocks in western North America are not suitable sources because they are more isotopically evolved. Igneous rocks from the basement units are of MORB affinity (depleted LREE and ᵋNdi = +10 to +6). The Jurassic(?)-Cretaceous arc volcanic rocks have ᵋNdi (+7.9 to +3.9) and REE patterns similar to those of evolved intraoceanic island arcs. Sandstones related to the arc assemblage are predominantly volcaniclastic. These sediments have positive ᵋNdi values (+3 to +6) and REE with IAV-affinity. The Guerrero terrane seems to be characterized by two major tectonic assemblages. The Triassic-Middle Jurassic "basement assemblage" that corresponds to an ocean-floor assemblage with sediments derived from continental sources, and the Late Jurassic-Cretaceous arc assemblage formed in an oceanic island arc setting. During the Laramide orogeny the arc was placed against nuclear Mexico. Then, the polarity of the sedimentation changed from westward to eastward, and sediments derived from the arc-assemblage flooded nuclear Mexico. This process marks the "continentalization" of the Guerrero terrane, which on average represents a large addition of juvenile crust to the western North American Cordillera during Mesozoic time.

The southernmost region of the Mexican Ridges extends from Bryant's gap near 22.5 N latitude to the Campeche Knolls near 19.0 N latitude. Analysis of 23,030 kilometers of sparker and CDP seismic data from six surveys reveals the existence of two separate areas of folding, Zones 4a and 4b. In the Zone 4a foldbelt symmetrical folds form a gentle salient which parallels the curved outline of Isla de Tuxpan. Structural relief often in excess of 500 meters is reflected by similar bathymetric relief. Fold wavelengths average 10-12 kilometers. A detachment or decollement is interpreted in a thick Upper Cretaceous to Lower Tertiary pelagic shale sequence by the existence of relatively undeformed reflectors below this interval. The 3 to 3.5 kilometer thick allochthonous sheet has experienced approximately 1% shortening and a maximum displacement of 1 to 2 kilometers. The Zone 4a foldbelt appears to be a massive gravity slide. Folded Plio-Pleistocene strata establish the youth of these folds. A large deep-rooted structure of questionable origin is observed on GLG 22. This structure, exhibiting roughly 1500 meters of bathymetric relief, acts as a foreland buttress against which the gliding allochthonous mass deforms. The tightly appressed thrust-faulted folds up dip from the buttress exhibit anomalously short wavelengths. While no folding is observed directly down dip from the buttress, folding is observed 30 to 50 kilometers basinward of this structure just a few kilometers to the south. The boundary separating Zones 4a and 4b is a linear feature oriented transversely to regional strike and may be a tear fault. Reflections at depth are not continuous across this feature. The Zone 4b foldbelt lies directly down dip from the Veracruz Basin. Structural relief commonly doubling that observed in Zone 4a is rarely expressed as bathymetric relief. Individual folds are asymmetric, having gently dipping landward flanks and either steeply dipping or growth-faulted seaward flanks. Fold cores appear to contain diapiric material. Fold growth due to gravity sliding began in the Middle Miocene. Subsequent loading by a thick Middle-Upper Miocene section gradually halted downslope movement and initiated flowage of plastic substrata from beneath loaded synclinal troughs into anticlinal cores. This deformation has continued to the present in some folds. / text

No description available.

The Huizachal-Peregrina Anticlinorium is a large NNW-trending structure in the front ranges of the Sierra Madre Oriental of Mexico (23° 45ʹ N; 99° 10ʹ W). The breached core of the anticlinorium exposes three major geologic terranes: (1) . A late Precambrian granulite terrane (Novillo Gneiss) remarkably similar in composition, appearance, grade and age of metamorphism to rocks of the Grenville Province, especially the Adirondacks; (2). A mid-Paleozoic low-grade metamorphic complex (Granjeno Schist) of volcano-sedimentary origin with ophiolite rock assemblages, that resemble rocks of the Ouachita-Appalachian inner zones; and (3). A strongly folded and faulted section of Paleozoic fossilifireous sedimentary strata, more than 1500 m thick, similar to the rocks of the Ouachita frontal zone exposed in the Marathon region of Texas. Except for an extensive subcrop terrain of Permo-Triassic granitic intrusives, the terranes in the area studied represent "unique samples" of the Pre-Mesozoic basement framework of eastern Mexico. These terranes belong to two ancient superposed orogenic systems: the Late Precambrian Oaxacan (Grenville) and the Paleozoic Huastecan (Ouachita-Appalachian) structural belts. Based on the geologic study of these pre-Mesozoic terranes at Huizachal-Peregrina, and compared with the widely spaced and limited outcrops (and sub-crops) of equivalent rock units of eastern Mexico, a tectonic model is proposed which interprets the granulite terrane as representative of continental crust, and the low-grade metamorphic terrane as rocks that accumulated on top of the ocean crust of a marginal basin. This latter sequence experienced a complex history of deformation and metamorphism as it was subducted towards the east, culminating 330 m.y. ago. The onset of Carboniferous-Permian orogenic flysch sedimentation is interpreted to have occurred when the marginal basin became closed by an arc-continent collision. The Permo-Triassic granitic rock terrane of the subsurface of the Gulf Coastal Plain represents the magmatic roots of that volcanic arc. The pervasive NW to NNW-trending structural grain, of the Oaxacan and Huastecan structural belts south of Huizachal-Peregrina through Oaxaca, when compared to the NE-trending distribution of Precambrian and Paleozoic terranes in the United States are compatible with the existence of a proposed zone of large left-lateral displacement across northern Mexico. / text

No description available.

Using gravity, magnetic, bathymetric and seismic refraction data, I have constructed a geophysical cross-section of the central part of the northern Gulf of California. The section exhibits a crustal thickness of 18 km and features an anomalous block of high density lower basement (3.15 g/cm³) which probably resulted from rifting processes during the opening of the Gulf. The magnetization of the upper basement ranges from 0.0005 to 0.0030 emu/cm³. Three different layers of sediments are modeled, ranging from unconsolidated (1.85 g/cm³) to compacted (2.50 g/cm³). I present a deconvolution method for automated interpretation of magnetic profiles based on Werner's (1953) simplified thin-dike assumption, leading to the linearization of complex nonlinear magnetic problems. The method is expanded by the fact that the horizontal gradient of the total field caused by the edge of a thick interface body is equivalent to the total field of a thin dike. Statistical decision making and a seven point operator are used to insure good approximations of susceptibility, dip, depth, and horizontal location of the source. After using synthetic models to test the inversion method, I applied it to the Northern Gulf of California using data collected in 1984 by the Continental Margins Study Group at Oregon State University. Fault traces, computed by the deconvolution, are plotted on a map. The faulting pattern obtained is in good agreement with that proposed by other workers using other methods. The depths to the top of the faults range from 4 to 5 km in the eastern part of the Gulf, where they may be interpreted as the top of the structural basement. Deeper estimates are obtained for the western part of the Gulf. / Graduation date: 1990

Assemblages of Paleozoic age and less significant Triassic and possibly Cretaceous-Tertiary volcanic rocks constitute the Sierra El Aliso, 186 km east-southeast of Hermosillo, Sonora. The Paleozoic section consists of approximately 2000 m of allochthonous Ordovician to Permian pelagic and hemipelagic deposits that accumulated in continental slope, continental rise and ocean floor (?) environments. The lower Paleozoic is characterized by graptolitic black shale and radiolarian chert, quartzite, argillite and local limestone. The upper Paleozoic is predominantly turbidite carbonates rich in benthonic foraminifera, and conodont faunas, subordinate bedded chert, siltstone, sandstone and chert-clast conglomerate. After Early Permian time, but prior to the deposition of the Late Triassic Barranca Group the Paleozoic section was imbricated along south-southeast vergent thrust faults. The Triassic rocks unconformably overlie the Paleo-zoic strata and all thrust faults. The Triassic and older rocks are overlain by the Cretaceous-Tertiary volcanics.