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.

In 1988, C.J. Busby published a paper titled “Speculative Tectonic Model for the Early Mesozoic Arc of the southwest Cordilleran United States” (Fig. 1). Previously, the term “Andean arc” had been applied to volcanic and plutonic rocks of this arc because it formed on continental crust (e.g., Hamilton, 1969; Burchfiel and Davis, 1972). This led to the widespread notion that this arc formed a contractional, high-standing region. Busby-Spera (1988) instead proposed that the early Mesozoic arc of the southwest U.S. and northern Sonora, Mexico, formed a deep, fault-bounded depression (Fig. 1A) and summarized the features of this extensional continental arc, including the fact that it acted as a trap, not a barrier, for craton-derived eolian sands (e.g., Wingate, Navajo, and Entrada sandstones; Fig. 1B). At that time, few modern analogs had been published (e.g., Central American arc; Burkart and Self, 1985; Kamchatka arc, Erlich, 1979). It is now known that other modern continental arcs are extensional (e.g., Taupo volcanic zone; Houghton et al., 1991). Many more are represented in the geologic record, for example, along the Mesozoic western edge of South America (Maze, 1984; Burke, 1988; Charrier et al., 2002, 2007; Cardona et al., 2020). Busby's 1988 model for the continental extensional arc has since been proven valid at numerous locations along the length of the arc in the southwest Cordilleran U.S. (Busby-Spera et al., 1990; Fisher, 1990; Wyld, 1990; Riggs and Busby-Spera, 1990, 1991; Solomon and Taylor, 1991; Saleeby and Busby-Spera, 1992; Schermer and Busby, 1994; Wadsworth et al., 1995; Barton and Johnson, 1996; Fackler-Adams et al., 1997; Busby et al., 2002; Schermer et al., 2002; Busby et al., 2005; Haxel et al., 2005, 2008a, 2008b; Saleeby, 2011; Clemens-Knott et al., 2013; Saleeby and Dunne, 2015; Chapman et al., 2015; Busby and Riggs, 2019; Clemens-Knott and Gavedon, 2019; Gevedon and Clemens-Knott, 2019; Lackey et al., 2019). Diagnostic features of the early Mesozoic extensional continental arc include multi-kilometer-thick sections of volcanic rock that span little time; syndepositional normal faults; giant continental calderas; high-level intrusions; local subsidence below sea level; rapid extensional unroofing of plutons; and penetration of meteoric water to mid-crustal levels (Fig. 1B).

The “Nazas arc” of central and eastern Mexico has been widely interpreted to represent the southern continuation of the early Mesozoic extensional continental arc described above (Fig. 2) (Grajales-Nishimura et al., 1992; Bartolini, 1998; Barboza-Gudiño et al., 1998, 1999; Dickinson and Lawton, 2001; Blickwede, 2001; Bartolini et al., 2003; Fastovsky et al., 2005; Barboza-Gudiño et al., 2008; Godínez-Urban et al., 2011; Mauel et al., 2011; Rubio-Cisneros and Lawton, 2011; Rubio Cisneros et al., 2011; Lawton et al., 2012; Zavala-Monsiváis et al., 2012; Eguiluz de Antuñano et al., 2014; Lawton and Molina-Garza, 2014; Villarreal-Fuentes et al., 2014; Lyons, 2016; Wengler et al., 2019; Coombs et al., 2020; Barboza-Gudiño et al., 2021; Molina Garza et al., 2020). The same feature has also been referred to as the “Cordilleran arc” (Leggett et al., 2007; Dickinson et al., 2010). The “Nazas arc” (Fig. 2) is commonly portrayed as a continuous 2000-km-long, 250-km-wide belt that runs southeastward from Sonora through the Chihuahua, Durango, Coahuila, Zacatecas, Nuevo León, San Luis Potosí, and Tamaulipas states, and reappears further southeast in Chiapas state (Fig. 2A) (e.g Grajales-Nishimura et al., 1992; Bartolini et al., 2003; Stern and Dickinson, 2010; Godínez-Urban et al., 2011; Lawton and Molina Garza, 2014; Villarreal-Fuentes et al., 2014; Wengler et al., 2019; Barboza-Gudiño et al., 2021; Coombs et al., 2020). We agree that the rocks of the purported “Nazas arc” were deposited in extensional basins, but these basins are dominated by sedimentary rocks, with minor Early to Middle Jurassic volcanic rocks that form thin (meters to tens of meters thick), laterally discontinuous sections. These “Nazas arc” rocks pass upward into clastic sedimentary sections, greater than hundreds of meters thick, that are widely interpreted to record continental rifting related to the breakup of Pangea (Salvador, 1987; Barboza-Gudiño et al., 2010; Rubio-Cisneros and Lawton, 2011; Barboza-Gudiño et al., 2014; Lawton and Molina Garza, 2014). We interpret both the “Nazas arc” rocks and the overlying clastic rocks to represent continental rift basin fill.

As shown below, the “Nazas arc” does not contain a single volcanic center or pluton, in contrast with all arcs, regardless of their tectonic setting (e.g., extensional, neutral, or contractional) or the type of crust on which they lie (continental, oceanic, or intermediate). Nonetheless, the “Nazas arc” has been widely cited as a major geotectonic element. Stern and Dickinson (2010) used the presence of the “Nazas arc” to propose that the Gulf of Mexico is a backarc basin. Kimbrough (2018, 2019) has referred to it as a “subduction-related silicic large igneous province (SLIP)” on the basis of thick rhyolite, ignimbrite-dominated volcanic/sedimentary sections with great aerial extent, and rapid emplacement during an arc magmatic flare-up at ca. 171–166 Ma. Yet, as we show below, there are no thick ignimbrite sections anywhere in the “Nazas arc.”

The acceptance of a “Nazas arc” has led to a widely accepted model for the tectonic evolution of Mexico (the double arc model, Fig. 3A), wherein the presence of a Jurassic continental arc in eastern Mexico requires two subduction zones, one that dips east under the continent and a second that dips west under a distant archipelago represented by oceanic arc rocks in western Mexico (e.g., Dickinson and Lawton, 2001; Sigloch and Mihalynuk, 2017). However, numerous publications show that the oceanic arc rocks of western Mexico have detrital zircons and shelfal to deep-water rocks that tie them to the Mexican margin (Gastil, 1993; Talavera-Mendoza et al., 2007; Centeno-García et al., 2011; Centeno-García, 2017; Busby, 2022). We present an alternative model (Fig. 3B): that the Jurassic continental extensional arc of California, Arizona, and northern Sonora continued southward off the continental margin and formed a fringing oceanic arc, termed the Guerrero–Alisitos–Vizcaino Superterrane (Busby, 2022). This arc formed above a single east-dipping slab, the Farallon/Cocos slab, which is the longest on the planet and has an interpreted age range of 220–0 Ma (Boschman et al., 2018; Boschman, 2020). The oceanic arc of western Mexico was a very robust arc with numerous large volcanic centers and many plutons. In our alternative model, the “Nazas rift province” represents a series of small, discontinuous continental rift basins that are related to the breakup of Pangea (Fig. 3B). This alternative model was first suggested by Centeno-García (2017) and Martini and Ortega-Gutiérrez (2018) and was hinted at in an abstract by Lawton (2017). The “Nazas rift province” model is supported by plate motion models (Müller et al., 2019) and low-temperature thermochronology (Villagómez et al., 2020) that show that rifting in eastern Mexico began at ca. 216 Ma.

Plutonic rocks are obviously an important component of a volcano-plutonic arc, and the Jurassic continental arc in southern Arizona and northern Sonora has many large plutonic suites (Table 1A, Fig. 2B), which are described further below. However, plutonic rocks are notably entirely absent from the “Nazas arc” (Table 1B, Fig. 2A). In fact, the review of the plutonic rocks of Mexico by Ortega-Gutiérrez et al. (2014) shows that Mesozoic plutonic rocks are restricted to western Mexico, where the arc was located in Jurassic time (Figs. 2 and 3B).

As is apparent from Figure 2, south of northern Sonora (the southern end of the Jurassic continental arc, Fig.2), there is a ca. 600-km-long gap in Jurassic igneous rocks, through southern Sonora and across Chihuahua, and the “Nazas arc” begins in central Mexico. Early workers in Mexico proposed that the Jurassic arc was displaced from northern Sonora to central Mexico along the Late Jurassic Mojave-Sonora megashear, during displacement of the Caborca block from California to Sonora (Anderson and Silver, 1979; Anderson and Schmidt, 1983). However, more recent work has provided firm evidence that the Caborca block could not have been displaced in Late Jurassic time and was instead displaced in Permian time along the California–Coahuila transform fault (Saleeby and Busby-Spera, 1992; Dickinson, 2000; Dickinson and Lawton, 2001; Amato et al., 2009; González-León et al., 2009; Chapman et al., 2015; Levy et al., 2020; Busby, 2022). Thus, we consider the large gap in magmatism between the Jurassic continental arc of southern Arizona–northern Sonora and the Nazas rift province of central Mexico to be evidence that the latter was not part of the Jurassic continental arc.

We begin our description of the geology of continental arc volcanic rocks by describing the Jurassic arc in southern Arizona and northern Sonora. We use published figures to describe the geologic features of an extensional volcano-plutonic continental arc: abundant multi-kilometer-thick volcanic sections with little to no sedimentary rock, including major volcanic centers, and also many large plutonic suites. Then we go on to show that the Jurassic “Nazas arc” lacks the geologic features of an arc. We also tabulate all U-Pb zircon ages published to date on continental volcanic and plutonic rocks in southern Arizona and northern Sonora with brief rock descriptions (Table 1A) to contrast with the “Nazas arc” (Table 1B), which lacks plutons and has sparse, thin, widely spaced volcanic sections.

The Jurassic continental arc in southern Arizona and northern Sonora has numerous, closely spaced, large-volume volcanic centers that include five giant continental calderas (Fig. 2, Table 1A). This section begins with a description of volcanic rocks surrounding the Sawmill Canyon fault zone in southern Arizona (Fig. 4), where the products of individual, large-volume eruptions have been correlated for distances of up to 100 km (Busby et al., 2005). The rocks along the Sawmill Canyon fault zone and their ages are described in locations 1 through 18 of Table 1A. Then our description moves further west, to describe voluminous volcanic rocks of the Baboquivari, Comobabi, and Santa Rosa Mountains in southern Arizona (locations 18 through 26, Table 1A and Fig.2). Dated Jurassic continental arc volcanic rocks that occur in other parts of southern Arizona but are not as well studied will not be described further in this text, but they are described briefly in Table 1A: Ox Frame Volcanics of the Sierrita Mountains (location 14) and a rhyolite ignimbrite in the Tucson Mountains (location 19). Then our description moves to Jurassic continental arc plutonic rocks of southern Arizona, which are abundant and well dated (locations 1, 13, 15, 16, 17, 18, 21, 22, 23, 24, 25, and 26). Many of the volcanic and plutonic suites of southern Arizona have been correlated to northern Sonora, so we will describe northern Sonora last. We consider northern Sonora to represent the southern end of the Jurassic continental arc of Arizona and California (Fig. 3B).

Sawmill Canyon Fault Zone

The Sawmill Canyon fault zone of southern Arizona is surrounded by Jurassic arc volcanic rocks (Fig. 4). Five calderas have been mapped in this region. Three partially nested calderas are mapped by Lipman and Hagstrum (1992) at the southeast end of the Sawmill Canyon fault zone, in stratigraphic order: I—Montezuma caldera, II—the Turkey Canyon caldera, and III—the Parker Canyon caldera. Lipman and Hagstrum (1992) also interpreted the southern Dragoon Mountains to be the remnants of a Jurassic caldera (Dragoon caldera), but it is not shown on Figure 4 because it is very altered, and its eruptive products have not yet been correlated to any other ranges. Lastly, the very large Cobre Ridge caldera lies in the southern Pajarito Mountains and is filled with the Cobre Ridge Tuff (Fig. 4).

The map (Fig. 4A) and lithostratigraphic correlation diagram (Fig. 4B) show that Jurassic arc volcanic deposits are voluminous enough to be correlated over tens of kilometers to 100 km (Fig. 4). In Figure 4B, letters A, B, C, and so on indicate locations of Jurassic volcanic rocks and intrusions dated by U-Pb zircon (Table 1A). These columns are hung on the base of the Glance Conglomerate, which may be time transgressive. At most localities, the Glance Conglomerate rests on densely welded Jurassic ignimbrites with interstratified eolian quartz arenites, including the Mount Wrightson Formation, Cobre Ridge Tuff, and the Canelo Hills Volcanics (ignimbrites I, II, and III). The Mount Wrightson Formation (Fig. 5) has distinctive purple quartz-free welded ignimbrites that are correlated from the Santa Rita Mountains to northern Sonora (Fig 4B; Busby et al., 2005). The Cobre Ridge caldera (Fig. 6) is filled with the Cobre Ridge Tuff and straddles the Arizona-Sonora border. The Canelo Hills ignimbrites are correlated between five ranges (Busby et al., 2005) and consist of three distinctive members, all of which erupted from calderas (Lipman and Hagstrum, 1992; Busby et al., 2005): I is a crystal-rich, dacite ignimbrite that erupted from the Montezuma caldera (Fig. 4A). This ignimbrite, which is correlated between two ranges, is red-brown with ca. 30% crystals of plagioclase, biotite, and quartz. In the Mustang Mountains, it consists of outflow filling a paleocanyon that is cut into Permian limestone, where it has two cooling units separated by volcanic lithic sandstone and conglomerate. In the southern Huachuca Mountains, it forms the base of a continuous section of ignimbrites I, II, and III and represents thick caldera fill with carbonate megablocks up to 2 km long. The caldera is intruded by a possible resurgent dome (Huachuca quartz monzonite) dated at 168 Ma by K/Ar (Lipman and Hagstrum, 1992). Ignimbrite II is a crystal-poor rhyolite ignimbrite that erupted from the Turkey Canyon caldera (Fig. 4A). This ignimbrite is correlated across five ranges. It was previously mistaken for a rhyolite lava (Hayes, 1970) because it is an ultra-welded “lava-like” ignimbrite with extremely flattened and flow-lineated pumice banding, flow folds, ramp structures, and local domain and carapace breccias. It is also full of lithophysae and spherulites. The outflow lacks lithics, but the caldera fill has carbonate megablocks up to 1 km, and pyroclastic textures are well preserved where the intracaldera tuff cooled against the megablocks. This ignimbrite is not dated, and it would be poor in zircon because it has less than a few percent crystals. III is a quartz rhyolite tuff that erupted from the Parker Canyon caldera (Fig. 4A). This ignimbrite is correlated across four ranges (Fig. 4B) and is distinguished by its abundant large quartz phenocrysts. Its caldera fill has megablocks that include shattered blocks of ignimbrite II, and the caldera fill is cut by a co-genetic hypabyssal intrusion (Lipman and Hagstrum, 1992).

The Glance Conglomerate has abundant interstratified arc volcanic rocks along the Sawmill Canyon fault zone (Fig. 4). The “Glance Tuffs” in the southern Canelo Hills consist of ignimbrites interstratified with conglomerates that contain clasts of limestone and silicic volcanic rock, and the ignimbrites have not yet been correlated to other ranges. Although the Glance Conglomerate has been widely regarded as Early Cretaceous or Late Jurassic (Bilodeau, 1978; Anderson and Nourse, 2005; Mauel et al., 2011; Peryam et al., 2012), the “Glance Tuffs” are latest Early Jurassic to earliest Middle Jurassic in age (locations 9 and 10, Table 1A). The Glance Conglomerate in the Santa Rita Mountains is dominated by volcanic rocks, so it is referred to there as Glance Formation (Fig. 7). It has three volcanic units that can be correlated to other ranges (Fig. 4B), which are labeled: (1) nonwelded rhyolite ignimbrites, (2) nonwelded dacite block-and-ash-flow tuffs, and (3) andesite lavas, some of which occur as landslide megabreccias.

Three localities along the Sawmill Canyon fault zone (Fig. 4) are described in greater detail to give examples of the geology of a continental arc: the Mount Wrightson Formation, Cobre Ridge Tuff and the Cobre Ridge caldera, and the Glance Formation in the Santa Rita Mountains.

Mount Wrightson Formation

The Mount Wrightson Formation in the Santa Rita Mountains (Fig. 5) forms a >1.5-km-thick volcanic section, with its base intruded by the 188 ± 2 Ma Piper Gulch pluton (location 1, Table 1A), and its top is unconformably covered by the Glance Formation (Fig. 4B). The Mount Wrightson Formation in the Santa Rita Mountains is continuously exposed for a lateral distance of 22 km. This area lies along the northeast margin of the arc graben depression shown in Figure 1A and is referred to as a “multi-vent complex” because many small monogenetic volcanic centers were sited along numerous splays of the graben-bounding fault zone, which frequently leaked magmas to the surface. Vents were rapidly buried by eruptive products from other centers within the basin or outside of it, or by craton-derived eolian quartz sand, probably due to high rates of tectonic subsidence. Like the rapidly extending continental arc of the Taupo Volcanic Zone, fault scarps were buried too rapidly by pyroclastic debris to develop significant alluvial fans, and rapid subsidence prevented the development of erosional unconformities (Busby et al., 2005).

The Cobre Ridge Tuff and the Cobre Ridge Caldera

The Cobre Ridge Tuff (ignimbrite) fills the Cobre Ridge caldera (Fig. 6), where it is >3.0 km thick and 50 × 20 km in size. The northwest end of the caldera collapsed in two stages, which is recorded by deposition of eolian quartz sands during an eruptive hiatus of the Cobre Ridge Tuff (ignimbrite); the second collapse stage was followed by deposition of the accidentally ponded tuff of Brick Mine (ignimbrite; source unknown). The unusually large size, the rectilinear shape, the NW–SE strike of bounding structures, the complex collapse history, and the great thickness of the fill of this caldera indicate regional structural controls on its development (i.e., the arc graben depression, Fig.1). These features are similar to those of the Altiplano–Puna Volcanic Complex, where bigger eruptive centers are complex, large-scale structures influenced by tectonic grain.

Glance Formation in the Santa Rita Mountains

The Glance Formation in the Santa Rita Mountains (Fig. 7) is 3 km thick and continuously exposed for a lateral distance of 22 km (Busby and Bassett, 2007). This formation has a high proportion of sedimentary rock (conglomerate) because it formed in response to sinistral–normal oblique faulting (Sawmill Canyon fault zone, Fig. 4A) rather than orthogonal rifting. Nonetheless, the basin fill is dominated by volcanic rock. The basin is asymmetric, with the dominant basin-bounding fault on one side (Sawmill Canyon fault zone) and a much less significant fault on the other side. Detailed stratigraphic analysis shows that the high-angle intrabasinal faults alternated between normal-slip and reverse-slip separation with time, and some faults with dip-slip separation were active synchronously with faults showing reverse-slip separation elsewhere in the basin; this is typical of strike-slip basins (see full discussion with references in Bassett and Busby, 2005). Uplift events produced numerous large-scale unconformities in the form of deep paleocanyons and huge landslide scars, while giant slide blocks of “cannibalized” basin fill accumulated in subsiding areas. Erosion on the uplifted side of the Sawmill Canyon fault zone yielded abundant sedimentary breccia and conglomerate to form talus cone and alluvial fan deposits, which are restricted to the “deep” end of the basin adjacent to the fault zone. These deposits include blocks or boulders up to 4 m across at distances of up to 2 km from the fault zone. Dacite domes sited on the Sawmill Canyon fault zone repeatedly shed block-and-ash flows into the deep end of the basin. The end of the basin adjacent to the Sawmill Canyon fault zone is “overfilled” due to high sediment supply from the fault as well as lava domes sited on intrabasinal faults. The “underfilled” end of the basin (distal from the Sawmill Canyon fault zone) provided accommodation for extrabasinally sourced ignimbrites and for fluvially reworked tuffs with abundant shards and euhedral-free crystals (previously mapped as sandstones by Drewes, 1971).

Topawa Group, Baboquivari Mountains

The Topawa Group in the Baboquivari Mountains, southern Arizona (Fig. 8; location in Fig.2) is another example of a robust Jurassic continental arc volcanic section. The 8-km-thick composite section accumulated in two adjacent, partially overlapping ca. 4-km-deep sub-basins in a timespan of only 5 m.y. or less and is constrained by a U-Pb zircon age at the base of the stratigraphic section and another on a pluton that cross-cuts the entire section (locations 20 and 21, Table 1A). Rhyolite ignimbrites and lavas constitute about three-fourths of the section. All contacts between formations and members are gradational, and there are no erosional unconformities, which is consistent with rapid basin subsidence and deposition.

From base to top, the Topawa Group includes (Fig. 8): (1) Rhyolite Member of the Ali Molina Formation, which consists of ignimbrites with interstratified thick, laterally continuous eolian quartz arenites. (2) Metaconglomerate Member of the Ali Molina Formation, a coarsening-upward, sandstone–conglomerate sequence that contains the only extrabasinal clasts in the section, which consist of well-rounded quartzite that was likely recycled from Paleozoic or Precambrian conglomerates in the region mixed with intraformational clasts. (3) The Pitoikam Formation, a 2-km-thick clastic section with no volcanic interbeds that represent alluvial fan–fluvial deposits shed from a synvolcanic fault (intruded by the synvolcanic Tinaja Springs Porphyry), and which contains largely intraformational volcanic clasts. (4) Mulberry Wash Formation, which consists largely of rhyolite lavas but also includes thin, minor lavas of comendite (bearing anorthoclase) and alkali basalt. All of the volcanic rocks in the section have calcalkaline chemistry except for the alkali basalts (intraplate), which compose much less than 1% of the section and are regionally unique (Haxel et al., 2005).

Plutonic Rocks in Southern Arizona

Jurassic continental arc plutonic rocks are abundant in southern Arizona. These include (Table 1A): (1) Piper Gulch quartz monzonite (location 1); (2) Las Guijas igneous suite, including Duranzo granite (location 12) and quartz latite porphyry (location 13); (3) Harris Ranch Monzonite, including quartz monzonite (location 15), Tascuela phase (location 16), and Ash Creek phase (location 17); (4) Sierrita granite (location 18); (5) Kitt Peak Plutonic Suite, including Kitt Peak Granodiorite (location 21), Aguirre Peak Quartz Diorite (location 22), and Pavo Kug Granite (location 23); and (6) Ko Vaya Suite, including perthite granite (location 24), Mt. Devine quartz syenite (location 25), and Ko Vaya Suite Granite (location 26). A brief description is given in Table 1A, and readers are referred to the references cited therein for much more detail. The main point of tabulating them herein is to show that exposures of plutonic rocks form a major component of the Jurassic continental arc, as they do in all other ancient arcs (but not the “Nazas arc”).

Volcanic and Plutonic Rocks in Northern Sonora

Many of the U-Pb zircon dates in northern Sonora are multi-grain analyses of zircon that were dissolved with a flux in open beakers in large batches to overcome the lead blank, and they are only on two fractions, both of which are from between 1968 and 1978. Thus, these are referred to as “interpreted isotopic ages” (locations 27 through 39, Table 1A) by Anderson et al. (2005). However, the “interpreted isotopic ages” are consistent with those later determined by other workers on similar or the same rocks in southern Arizona (Anderson et al., 2005). To determine these ages, they used multi-grain analyses of smaller batches of zircons dissolved in Teflon bombs and analyzed multiple fractions (multi-grain analysis [MGA], Table 1). Correlations made by Anderson et al. (2005) include: (1) Gabino pluton (location 31) and the Plomo North porphyritic granite (location 33), which they correlated with the Kitt Peak Plutonic suite (locations 21, 22, and 23); (2) San Moises granite (location 35), which they correlated with the Ko Vaya plutonic suite (locations 24, 25, and 26); and (3) the Cubabi Rhyolite (location 34), the Las Avispas Formation (location 36), and the El Tunel quartz porphyry (location 37), which they correlated with the Cobre Ridge Tuff/Cobre Ridge caldera (location 11). We point out these correlations to bolster our argument that a continental arc should have magmatism that is robust enough to allow correlation of volcanic and plutonic rocks over large distances. Similarly, Calmus and Sosson (1995) made geologic correlations of Jurassic rocks between southern Arizona and northwestern Sonora. The “Nazas arc” does not have any widespread volcanic units, and it has no plutons.

Single-crystal U-Pb zircon analyses by SHRIMP in northern Sonora (locations 40–45, Table 1A) range from Early Jurassic to Late Jurassic (189 ± 1 Ma to 152 ± 2), which is consistent with the age span of Jurassic continental arc rocks in southern Arizona. This is a longer timespan than that covered by the “Nazas arc,” which was draped by a carbonate platform by Late Jurassic time after continental rifting succeeded to form the Gulf of Mexico (cf. Barboza-Gudiño et al., 2021).

In this section, we will show that, despite the fact that the “Nazas arc” volcanic rocks are inferred to occupy extensional basins similar to those of the Jurassic arc extensional basins described above, the “Nazas arc” basins are largely sedimentary. Most publications on the “Nazas arc” present stratigraphic correlation diagrams, which give no indication of the thicknesses of volcanic sections. Nor are geologic maps used to document any arc volcanic centers such as those shown for southern Arizona above. In this section, we focus on the publications that provide measured sections in the “Nazas arc,” and we argue that the geology is typical of scattered, small continental rift basins with minor volcanic sections and thick clastic sections (not a volcanic–plutonic arc). All locations of these sections are plotted in Figure 2, and ages are described by locality in Table 1B.

The “Nazas arc” measured sections shown in were made by Bartolini for his 1998 dissertation and simplified for publication (Bartolini et al., 2003). These show that the sections are dominantly sedimentary with nonmarine sandstones, siltstones, and conglomerates occurring in continuous sections up to 540 m thick. Volcanic sections are typically meters to tens of meters thick, and all are less than 200 m thick (with the possible exception of column 11, Fig.9). The volcanic rocks are described by Bartolini (1998) and Bartolini et al. (2003) as “rhyolite, andesite, basalt, and basalt breccia” (note that the “basalts” actually plot in the basaltic trachyandesite field on a Le Bas diagram), with lavas and flow breccias, as well as “tuff and ignimbrite.”

The “Nazas arc” measured sections shown in Figure 10 were published by Barboza- Gudiño et al. (2021) and again show that sedimentary rocks predominate, and volcanic sections are thin. Columns 3 and 5 rest on deformed clastic rocks, column 1 rests on schist, and the nature of the basal contact of the other three sections is unspecified. In all six columns, the Nazas Formation is overlain by the clastic sedimentary La Joya Formation (interpreted as rift clastic sedimentary rocks), which in turn is overlain by Oxfordian platform carbonate rocks. On these columns, the maximum thickness of volcanic rock is 820 m (column 1), but this contradicts the measured section at the same locality shown in Figure 9 (column 11) by Bartolini et al. (2003), which shows a much higher proportion of sedimentary rock. Additional apparent inconsistencies are the following: (1) column 3 shows a 175-m-thick volcanic section, while the section measured by Bartolini et al. (2003) shows a 134-m-thick section that is dominantly sedimentary (column 8, Fig.9), and (2) column 5 shows a 220-m-thick volcanic section (andesite lava), while the section measured by Bartolini et al. (2003) shows a 60-m-thick “tuff” in the middle of a 345-m-thick section that is otherwise sedimentary (column 6, Fig.9). Regardless of these inconsistencies, none of the volcanic sections shown here are typical of the great thicknesses of volcanic section (thousands of meters) that characterize arcs.

The “Nazas arc” measured section shown in Figure 11A was published by Molina Garza et al. (2020), who divide the section with volcanic rocks into Lower Member and Upper Member of the Boca Formation. These were divided into two formations by Fastovsky et al. (2005), because they are separated by an angular unconformity, and the lower formation has more volcanic rocks than the upper formation. The Boca Formation is overlain by clastic sedimentary rocks of the La Joya Formation, which also lies above Nazas Formation at all localities in Figure 10. The U-Pb zircon age of 189 ± 0.2 Ma on tuff (Fastovsky et al., 2005), described at location 34 (Table 1B, Fig.3), comes from the base of the unit referred to here as Upper Member Boca Formation. Two U-Pb detrital zircon samples from the Lower Member have a maximum depositional age of 184–183 Ma, and two detrital zircon samples from the Upper Member have a maximum depositional age of 167–163 Ma (Rubio-Cisneros and Lawton, 2011). Rubio-Cisneros and Lawton (2011) suggested that the multi-grain analysis U-Pb zircon age of 189 ± 0.2 reported by Fastovsky et al. (2005) had older zircon grains. However, the vertebrate fossils from the base of the Upper Member are Early Jurassic in age (Fastovsky et al., 2005). Not shown in the Figure 11 measured section of Rubio-Cisneros and Lawton (2011) is a rhyolite intrusion that cross cuts both members of La Boca Formation, which is mentioned in the text. The La Boca Formation, combined with the overlying La Joya Formation, was interpreted to be the fill of a “rift basin formed during the breakup of Pangea” (Rubio-Cisneros and Lawton, 2011, p. 167), although numerous other workers, including Fastovsky et al. (2005), Molina Garza et al. (2020), and Barboza-Gudiño et al. (2021), have considered it part of the “Nazas arc.” An arc interpretation is inconsistent with the fact that volcanic rocks total <100 m of the entire Early and Middle Jurassic section (Fig. 11A).

The “Nazas arc” measured section shown in Figure 11B was published by Godínez-Urban et al. (2011) from the Sierra homocline of west-central Chiapas (location P, Fig.2). The section rests on Permian basement. The La Silla Formation is 150 m thick, roughly half clastic rock and half volcanic rock (Fig. 11B), with a U-Pb zircon age of 191 ± 3 Ma on a quartz hornblende potassium feldspar plagioclase andesite (Table 1B). The La Silla Formation is interpreted as “Nazas arc” by Godínez-Urban et al. (2011), but the volcanic section is very thin (ca. 75 m). This is overlain by the El Diamonte Member of the Todos Santos Formation, which is 250 m thick and almost entirely clastic except for a single 20-m-thick lava (Fig. 12). It has a detrital zircon maximum depositional age of ca. 171 Ma. The El Diamonte Member is interpreted as an “intra-arc basin” by Godínez-Urban et al. (2011), but the distinction between that and the “Nazas arc” is unclear. The overlying Jericó Member of the Todos Santos Formation is a 550-m-thick section of clastic sedimentary rocks (not shown in Fig. 11B) with a detrital zircon maximum depositional age of 175 ± 4 Ma. It is interpreted as “continental rift” by Godínez-Urban et al. (2011). We consider the entire 950-m-thick section, including La Silla Formation, El Diamante Member, and Jericó Member, to represent continental rift basin fill, less than 10% of which is volcanic.

We base our rejection of the “Nazas arc” on its geology: it has none of the geologic features of an arc. Several authors have used whole rock major and trace element geochemistry (no isotopic analyses) to conclude that it is an arc (Bartolini et al., 2003; Fastovsky et al., 2005; Zavala-Monsiváis et al., 2012; Villarreal-Fuentes et al., 2014; Barboza-Gudiño et al., 2021; Molina Garza et al., 2020), but as we know from numerous modern settings, geochemistry can be misleading. For example, the “Nazas arc” lies on thick, ancient crust (e.g., see Ortega-Gutiérrez et al., 2018; Rodríguez-Dominguez et al., 2019) that can lead to crustal contamination, the mantle rocks have seen at least two episode of subduction (Permian and early Paleozoic), and it lacks the true basalts that are most useful for tectonic discrimination. Furthermore, as discussed by Centeno-García (2017) and Martini and Ortega-Gutiérrez (2018), the “Nazas” continental rift basins lie close to the robust Jurassic arc of western Mexico, so their geochemistry could be influenced by that proximity. This is because the Jurassic Mexican continent formed a very narrow spine, with Pacific margin subduction on the west and Atlantic margin rifting on the east.

The Jurassic “Nazas arc” has been widely accepted as a major geotectonic element in Mexico by many dozens of workers over the past few decades. Yet the geology of this province does not in any way resemble that of a volcano-plutonic arc. We use the Jurassic arc in southern Arizona and northern Sonora to demonstrate the geologic features of a continental arc, which extends northward all the way through California. The “Nazas arc” lacks the geologic features of an arc, which includes major eruptive centers and plutonic suites (Table 1), all with great aerial extent (Fig. 4). Furthermore, although it is widely inferred to be extensional, it lacks the multi-kilometer–thick volcanic sections typical of an extensional continental arc, which are described herein (Figs. 1, 5, 6, 7, and 8). Instead, it consists of scattered, thin volcanic sections at the base of thick clastic intervals that are interpreted to record continental rifting during the breakup of Pangea (Figs. 911). Too much reliance has been placed on a calc-alkaline geochemistry of the volcanic rocks; this can be produced by other means such as crustal contamination, enrichment of the mantle lithosphere during earlier subduction episodes, and close geographic proximity of the Atlantic–Gulf of Mexico divergent margin to the paleo-Pacific subduction margin. Furthermore, volcanic rocks may occur in continental rift basins, although they generally do not dominate, as is the case with the “Nazas arc.”

We infer that the Jurassic continental arc of Arizona and California passed southward into the oceanic realm (Figs. 2A and 3B). The oceanic realm consists of Late Triassic to Middle Cretaceous oceanic arc–ophiolite–turbidite assemblages that are referred to as the Guerrero–Alisitos–Vizcaino superterrane (Busby, 2022). Some workers have inferred that these oceanic arc rocks represent archipelagos that formed above one or more west-dipping subduction zones thousands of kilometers west of Pangean North America (Fig. 3A; e.g., Dickinson and Lawton, 2001; Sigloch and Mihalynuk, 2013, 2017). However, U-Pb detrital zircon age data from the Guerrero–Alisitos–Vizcaino superterrane show that it is firmly tied to the Gondwanan to southwestern Laurentian margin of Mexico (Centeno-García et al., 2011; Centeno- García, 2017; Busby, 2022). The Guerrero–Alisitos–Vizcaino superterrane consists of very robust oceanic arc rocks, with abundant plutons, abundant large volcanic centers (including stratovolcanoes and calderas), laterally extensive eruptive products that can be correlated up to 100 km, and multi-kilometer–thick volcanic sections with little to no sedimentary rock (for a complete description, see Busby, 2022). There are multiple modern analogs for continental arcs passing laterally into oceanic arcs, including the Alaskan arc (Vallier et al., 1994) and the Sunda arc (Kurnio, 2019). An ancient example is the early Mesozoic arc of California and Oregon, which transitioned northward from a continental arc in the Mojave Desert–Southern Sierra Nevada region to an oceanic arc in the northern Sierra Nevada–Klamath Mountains–Blue Mountains region (cf. LaMaskin et al., 2011).

We conclude that the “Nazas arc” should be referred to as the “Nazas rift province” and understood to consist of small, isolated Early to Middle Jurassic continental rift basins with thick clastic successions and minor volcanic rocks that formed in response to the breakup of Pangea.

A big thanks goes to Jon E. Spencer for helping C.J. Busby with compiling some of the data presented in Table 1A. Busby also thanks Diane Clemons-Knott for inviting her to speak on the Jurassic arc at the GSA annual meeting in 2019 and the organizers of the 2020 GSA meeting for inviting her to speak in the Pardee session “Assembling Laurentia,” because these invitations stimulated her to return to the Jurassic after a 15 year hiatus of publishing on this topic. We thank Andrew Barth for his constructive review. Busby also thanks Jason Saleeby for his mentorship and for sharing his insights into tectonic processes over the past 45 years. Support for Busby was provided by the National Science Foundation's Division of Earth Sciences (1917361). Support for Centeno-García was provided by the National Autonomous University of Mexico's Research and Technological Innovation Projects Support Program (IN117517).

Science Editor: Shanaka de Silva
Associate Editor: Eric H. Christiansen