The “Nazas Arc” is a continental rift province: Implications for Mesozoic tectonic reconstructions of the southwest Cordillera, U.S. and Mexico

Abstract

Abstract We reject the notion of a Jurassic continental arc in eastern Mexico, termed the “Nazas arc,” on geologic grounds. Instead, we propose that the Jurassic continental arc of the SW Cordilleran U.S. and northern Sonora, Mexico, passed southward into the oceanic realm and is represented by Jurassic arc volcanic and plutonic rocks that fringed the Mexican paleo-Pacific margin, which are currently found in the western Peninsular Ranges of southern California, USA, and Baja California, the Vizcaino Peninsula of Baja California, and western mainland Mexico. To show this, we present a summary of the geologic features of a continental arc, using the geology of the southern end of the Jurassic continental arc, in southern Arizona and northern Sonora. These features include multi-kilometer–thick sections of volcanic rock; large volcanic centers, including silicic calderas; major eruptive units that can be correlated for distances of 100 km or more; abundant, large plutonic suites; and continuity of these features for distances of hundreds of kilometers along the length of the continental arc. Then we show that the “Nazas arc” consists of scattered, small continental rift basins with thin (meters to tens of meters thick) volcanic sections at the base of clastic sections that are hundreds of meters thick. Plutonic rocks are entirely absent from the “Nazas arc,” despite the fact that post-Jurassic tectonic events should have exposed them if they existed. This paper also presents a tabulation of all published U-Pb zircon dates in the Jurassic continental arc of southern Arizona, USA, and northern Sonora (Table 1A), and in the “Nazas arc” of eastern Mexico (Table 1B), with ages, methods, the rock type dated, and notes on geologic relations. We use this to detail the abundance of thick, laterally extensive volcanic sections and large plutonic suites in a continental arc (the Jurassic arc of southern Arizona–northern Sonora), which contrasts sharply with the “Nazas arc.” The term “Nazas arc” has been in widespread usage for volcanic rocks in eastern Mexico for decades in many dozens of papers, and it is portrayed as a 2000-km-long, 250-km-wide belt that extends from Sonora through eastern Mexico to Chiapas. It has been misunderstood to form a subduction-related silicic large igneous province (SLIP), and it has been proposed that the Gulf of Mexico formed as a backarc basin behind the “Nazas arc.” The “Nazas arc” model also requires an east-dipping subduction zone under Mexico, and a separate west-dipping subduction zone under the oceanic arc rocks of western Mexico, which those models portray as an exotic arc, despite the presence of abundant detrital zircon from the Mexican margin. We urge workers to abandon the term “Nazas arc” and replace it with “Nazas rift province,” which represents continental rift basins formed during the breakup of Pangea.