Abstract A 1500-km transect from offshore southern California to central New Mexico portrays the transition from oceanic crust to stable North American craton (Figure 1). Three sheets of maps and sections cover segments of offshore and onshore southern California, southwestern Arizona adjacent to the international border, as well as southeastern Arizona and central New Mexico. The western edge of the offshore segment is Long. 122°W and the corridor lies between Lat. 33°N and Lat. 34°N. A northward extension, which continues to Lat. 35°N, includes the western Tranverse Ranges. After crossing the Peninsular Ranges, the corridor bends sharply toward the international border to pass through southeastern California and southwestern Arizona to Long. 111°W near Tucson, where the transect bends northeastward and terminates near Socorro, New Mexico at Long. 106°W between Lat. 33°30′N and 34°30′N. The east-west cross section generally is about halfway between the boundaries of the transect. A north-south section approximately along Long. 119°30′W, crosses the western Transverse Ranges and adjacent northern Channel Islands. A brief tectonic history of the continental margin of the southwestern United States is presented using the concept of tectonostratigraphic terranes. This history is supported by the Appendix, in which the stratigraphy of each terrane is described. Geologic relations depicted deeper than a few kilometers are inferential and somewhat subjective. Nevertheless, this summarization of contemporary tectonic interpretations is expected to stimulate interest and further research in this region. The following text embellishes the Chronology of Terrane Assembly diagram and Tectonostratigraphic Columns shown on sheet 1.

Abstract DNAG Transect C-3. Part of GSA’s DNAG Continent-Ocean Transect Series, this transect contains all or most of the following: free-air gravity and magnetic anomaly profiles, heat flow measurements, geologic cross section with no vertical exaggeration, multi-channel seismic reflection profiles, tectonic kindred cross section with vertical exaggeration, geologic map, stratigraphic diagram, and an index map. All transects are on a scale of 1:500,000.

Characteristically lineated and foliated rocks of middle to late Mesozoic(?) age crop out in ranges in north-central Sonora throughout an area of 15,000 km 2 , between lat 31° 307′N and 30° 30′N. The terrane consists predominantly of layered sedimentary, volcanic, and volcaniclastic units, commonly metamorphosed to greenschist facies. Associated plutons, which are unambiguously intrusive, consist mainly of biotite- or biotite-muscovite–bearing granite. The layered suite and intrusive rocks are distinguished by subhorizontal, penetrative lineation, commonly defined by smeared mineral grains, which consistently trend northeast. Foliation is also predominantly low dipping. South of lat 30° 30′N, sporadic occurrences of lineated granitic and metamorphic rocks suggest the existence of this deformational fabric at least to Sierra Mazatan, which lies east of Hermosillo near lat 29°00′N. Cogenetic suites of zircon from one undeformed and three deformed plutons yield U-Pb ages from 75 to 55 m.y. These intrusive bodies are elements of a widespread, time-transgressive, late Mesozoic magmatic suite. They are not known to be affected by folds and faults commonly related to the Laramide orogeny. The apparent ages of the plutons are interpreted to be crystallization ages and therefore indicate that lineation and foliation formed, at least locally, later than 55 m.y. ago. Outcrops of distinctively deformed rocks appear to crudely define a north-trending belt. Rocks outside of this general zone are composed of sedimentary, volcanic, and volcaniclastic rocks of Precambrian and Mesozoic age which locally have been strongly folded and metamorphosed to greenschist or higher facies. However, postdeformational pegmatites and intrusive rocks indicate pre-Tertiary minimum ages for related episodes of deformation and metamorphism.