ABSTRACT This chapter expands upon a model, first proposed in 1998 by Busby and others, in which Mesozoic oceanic-arc rocks of Baja California formed along the Mexican continental margin above a single east-dipping subduction zone, and were extensional in nature, due to rollback of an old, cold subducting slab (Panthalassa). It expands on that model by roughly tripling the area of the region representing this fringing extensional oceanic-arc system to include the western third of mainland Mexico. This chapter summarizes the geologic, paleomagnetic, and detrital zircon data that tie all of these oceanic-arc rocks to each other and to the Mexican margin, herein termed the Guerrero-Alisitos-Vizcaino superterrane. These data contradict a model that proposes the oceanic-arc rocks formed in unrelated archipelagos some 2000–4000 km west of Pangean North America. Following the termination of Permian–Triassic (280–240 Ma) subduction under continental Mexico, the paleo-Pacific Mexico margin was a passive margin dominated by a huge siliciclastic wedge (Potosí fan) composed of sediments eroded from Gondwanan basement and Permian continental-arc rocks. I propose that a second fan formed further north, termed herein the Antimonio-Barranca fan, composed of sediment eroded from southwest Laurentian sources. Zircons from these two fans were dispersed onto the ocean floor as turbidites, forming a unifying signature in the Guerrero-Alisitos-Vizcaino superterrane. The oldest rocks in the Guerrero-Alisitos-Vizcaino superterrane record subduction initiation in the oceanic realm, producing the 221 Ma Vizcaino ophiolite, which predated the onset of arc magmatism. This ophiolite contains Potosí fan zircons as xenocrysts in its chromitites, which I suggest were deposited on the seafloor before the trench formed and then were subducted eastward. This is consistent with the geophysical interpretation that the Cocos plate (the longest subducted plate on Earth) began subducting eastward under Mexico at 220 Ma. The Early Jurassic to mid-Cretaceous oceanic arc of western Mexico formed above this east-dipping slab, shifting positions with time, and was largely extensional, forming intra-arc basins and spreading centers, including a backarc basin along the continental margin (Arperos basin). Turbidites with ancient Mexican detrital zircons were deposited in many of these basins and recycled along normal fault scarps. By mid-Cretaceous time, the extensional oceanic arc began to evolve into a contractional continental arc, probably due to an increase in convergence rate that was triggered by a global plate reorganization. Contraction expanded eastward (inboard) throughout the Late Cretaceous, along with inboard migration of arc magmatism, suggesting slab shallowing with time.

ABSTRACT A comprehensive correlation chart of Pennsylvanian–Eocene stratigraphic units in Mexico, adjoining parts of Arizona, New Mexico, south Texas, and Utah, as well as Guatemala, Belize, Honduras, and Colombia, summarizes existing published data regarding ages of sedimentary strata and some igneous rocks. These data incorporate new age interpretations derived from U-Pb detrital zircon maximum depositional ages and igneous dates that were not available as recently as 2000, and the chart complements previous compilations. Although the tectonic and sedimentary history of Mexico and Central America remains debated, we summarize the tectonosedimentary history in 10 genetic phases, developed primarily on the basis of stratigraphic evidence presented here from Mexico and summarized from published literature. These phases include: (1) Gondwanan continental-margin arc and closure of Rheic Ocean, ca. 344–280 Ma; (2) Permian–Triassic arc magmatism, ca. 273–245 Ma; (3) prerift thermal doming of Pangea and development of Pacific margin submarine fans, ca. 245–202 Ma; (4) Gulf of Mexico rifting and extensional Pacific margin continental arc, ca. 200–167 Ma; (5) salt deposition in the Gulf of Mexico basin, ca. 169–166? Ma; (6) widespread onshore extension and rifting, ca. 160–145 Ma; (7) arc and back-arc extension, and carbonate platform and basin development (ca. 145–116 Ma); (8) carbonate platform and basin development and oceanic-arc collision in Mexico, ca. 116–100 Ma; (9) early development of the Mexican orogen in Mexico and Sevier orogen in the western United States, ca. 100–78 Ma; and (10) late development of the Mexican orogen in Mexico and Laramide orogeny in the southwestern United States, ca. 77–48 Ma.

Abstract This chapter deals with the crystal structure of regulated and unregulated mineral fibres. The aim is to provide readers, both specialists and researchers broadly interested in environmental problems, with up-to-date information on a topic that is expanding daily. The chapter describes specifically the structure of the fibrous modification whenever available and outlines possible differences from the corresponding prismatic variety. Details of the experimental techniques used for structure determination/refinement are reported also, if appropriate, to outline the experimental difficulties faced due to the small dimensions, sensitivity and chemical complexity of mineral fibres.