Geology and tectonics of the Baja California Peninsula and adjacent areas

Abstract

This volume honors Russell Gastil, emeritus professor at San Diego State University's Department of Geological Sciences, who served as professor there from 1959 to 1993. Gordon mapped and supervised the mapping of tens of thousands of square kilometers of Mexico, California, and Arizona. One of his many contributions was GSA Memoir 140, which included the first reconnaissance map of the northern half of the Baja California peninsula (Gastil et al., 1975). This long out-of-print opus probably is the most-cited work in all geo-literature pertaining to the peninsula and is jealously guarded by those who own a copy. Gordon and his students continued their studies in the region over the next two decades, and many of these analyses were collected in GSA Special Paper 279 (Gastil and Miller, 1993). Gordon's early interest in the geology and tectonics of the Peninsular Ranges in southern California and northern Baja California (e.g., Bushee et al., 1963; Gastil, 1968) expanded to include a wide variety of geologic and tectonic problems throughout northwestern Mexico. The regional focus of papers in this volume reflects his seminal work in the Peninsular Ranges, other parts of Baja, islands in the Gulf of California, and the Mexican states of Sonora, Sinaloa, Nayarit, and Jalisco (Fig. 1). The goals of this introductory paper are to (1) summarize our present understanding of the geology and tectonics of southernmost California and Baja California, the Gulf of California, and western mainland Mexico; (2) identify outstanding problems and suggest avenues of research that might address those problems; and (3) provide context for the other papers in this volume. The emphasis on tectonics-oriented issues reflects Gordon's interests as well as my own. Coverage of other topics, such as paleontology and modern sedimentary environments, is incomplete or absent. This paper first describes the salient aspects of the geography, modern tectonic setting, seismicity, and geophysical attributes of the region. It then summarizes the geology in terms of specific terranes and describes major batholiths throughout the region. The geology of western mainland Mexico is not covered systematically, though some components of it are addressed in later sections of the paper. The bulk of this paper is an analysis of major events in the Mesozoic and Cenozoic history of the Baja California peninsula and nearby regions. Three topics receive the most attention, reflecting the scope of other papers in this volume and the focus of tectonics-oriented research in the region in the last few decades: (1) the nature, origin, and development of the Peninsular Ranges batholith and its host rocks in southernmost and Baja California; (2) possible northward translation of part, most, or all of southernmost and Baja California in the Late Cretaceous, Paleogene, or both; and (3) late Cenozoic transtension, rifting, and formation of oceanic crust in the Gulf of California. Other tectonics-related issues are addressed, but in less detail.