Guerrero-Alisitos-Vizcaino superterrane of western Mexico and its ties to the Mexican continental margin (Gondwana and SW Laurentia)

In 1998, Busby et al. proposed that Mesozoic oceanic arc–related rocks of Baja California formed in the upper plate of a single long-lived, east-dipping subduction zone beneath Mexico, and that they fringed the Mexican margin (i.e., they were not exotic). This model (Fig. 1) was proposed to be a general evolutionary trend for arcs facing large, old oceanic basins. A progression from strongly extensional oceanic arc through moderately extensional oceanic arc to contractional continental-margin arc was attributed to progressive younging of the very large Panthalassa oceanic plate that was being subducted, accelerated by increased convergence ca. 100 Ma (Busby et al., 1998; Busby, 2004). A similar model was proposed for the Guerrero composite terrane of western mainland Mexico by Centeno-García et al. (2011). A similar model has also been proposed for the Peninsular Ranges of southern California by Morton et al. (2014a, 2014b), referred to as subduction transitioning, i.e., from a Mesozoic extensional oceanic-arc setting to a continental-margin contractional arc setting. Finally, Martini and Ortega-Gutiérrez (2018) introduced the term “Mesozoic Pacific system” (Fig. 2). This term refers to Late Triassic to mid-Cretaceous oceanic arcs that evolved into a Late Cretaceous continental-margin arc (Martini and Ortega-Gutiérrez, 2018). In this chapter, I provide an overview of Triassic to mid-Cretaceous oceanic arc terranes of western mainland Mexico, Baja California, and adjacent southern California and review the evidence showing that these all have ties to each other, as well as the Mexican continental margin. I refer to this assemblage as the Guerrero-Alisitos-Vizcaino superterrane. The term “oceanic arc” is used herein to indicate a subduction-related volcanic-plutonic arc formed on a substrate of oceanic crust, recognized either in outcrop or by geochemical studies; this term is preferred herein in place of “island arc” because they lie largely below sea level.

There are two competing models for the Mesozoic oceanic-arc rocks of Baja California and western Mexico (Fig. 3). The first is the exotic arc model (Fig. 3A), where the oceanic-arc rocks represent distant archipelagos that formed above west-dipping subduction zones (Dickinson and Lawton, 2001). Two subduction zones are required if one accepts the Nazas province as a continental arc. The second model proposes that the Mesozoic continental arc of California, southern Arizona, and northern Sonora (Mexico) passed southward into the oceanic realm, where oceanic arcs fringed western Mexico (Fig. 3B). In this model, the Nazas province formed behind this arc as continental rift basins related to the breakup of Pangea (Centeno-García, 2017; Martini and Ortega-Gutiérrez, 2018; Busby and Centeno-García, 2022). Geophysicists have also proposed two competing models for the oceanic arc terranes of western Mexico, using mantle tomography. Sigloch and Mihalynuk (2013, 2017) proposed that the oceanic arcs of western Mexico grew in long-lived archipelagos 2000–4000 km west of Pangean North America. In this model, these were separated from North America by the Mezcalera Ocean, with westward subduction under the “Guerrero arcs” and eastward subduction under continental Mexico to produce the Nazas arc. This model places faith in a Nazas continental arc for which no evidence has been found (Centeno-García, 2017; Martini and Ortega-Gutiérrez, 2018; Busby and Centeno-García, 2022; Parolari et al., 2022). Furthermore, these workers proposed that the Alisitos oceanic arc formed on a separate plate from the “Guerrero arcs” (Clennett et al., 2020), but as summarized herein, they are tied together by geology, paleomagnetics, and detrital zircon. A second geophysical model ties the geometry of the eastward-subducted Cocos slab to geologic and paleomagnetic data from rocks of the upper plate in Mexico (Boschman et al., 2018a, 2018b). The Cocos slab is the longest subducted slab on the planet and is interpreted to record subduction under Mexico from ca. 220 Ma to present. In this second model, the Mesozoic oceanic-arc terranes (Guerrero-Alisitos-Vizcaino superterrane of this paper) fringed Mexico and formed above a single long-lived, east-dipping subduction zone.

This chapter summarizes the published geologic, paleomagnetic, and detrital zircon evidence showing that all of the oceanic-arc rocks of the Guerrero-Alisitos-Vizcaino superterrane are related to each other and were tied to the Laurentian/Gondwanan continental margin, so they cannot have formed as one or more distant archipelagos. In addition, many workers have recently gathered unpublished data that document detrital zircon ties between Mesozoic oceanic-arc rocks and the Laurentian/Gondwanan margin source rocks. The new data are reported in abstracts (Contreras-López et al., 2020a, 2021), a dissertation (Torres-Carrillo, 2016), and in written communications (e.g., Marty Grove, 2020, written commun.; Xóchitl Guadalupe Torres-Carillo, 2020, written commun.; Manuel Contreras Lopez, 2020, written commun.). Therefore, although this chapter briefly summarizes published detrital zircon data that tie these oceanic-arc rocks to Mexico, it is not an exhaustive treatment, because that data set is large and growing very quickly.

This chapter is divided into three parts. The first part provides a framework for the oceanic-arc terranes of Mexico in time slices, using map-view reconstructions, to provide context for the second section of the chapter. This first part starts with the Permian geologic framework of Mexico, which is needed to provide context for Mesozoic events. Then, the overview moves through Mesozoic time frames, from Early Triassic deep-sea fan sedimentation to Late Triassic subduction initiation to Jurassic extensional oceanic arcs to Early Cretaceous extensional oceanic arcs. The overview ends with the establishment of a contractional Late Cretaceous continental-margin arc in western Mexico, which migrated inboard with time accompanied by inboard expansion of contraction. The second part of the chapter provides more detailed descriptions of the Guerrero-Alisitos-Vizcaino superterrane for the interested reader, area by area, using lithostratigraphic columns. This is meant to document the evolution of the oceanic fringing arcs, and how they evolved into forearc basins or continental-margin arcs. The third part of the chapter returns to a big-picture view by using the detailed lithostratigraphic columns to construct a series of schematic cross sections through the Guerrero-Alisitos-Vizcaino superterrane. This should be of interest to all workers in convergent margins.

The continental building blocks of Mexico’s eastern spine were assembled from fragments that lay between Laurentia and Gondwana in Permian time (Fig. 4). Permian subduction occurred along the western margin of Pangea (Fig. 4). In Mexico, this subduction produced a Permian to earliest Triassic arc, with major volcanic centers and pluton suites, referred to as the East Mexico arc (Fig. 5; Torres et al., 1999; Rosales-Lagarde et al., 2005; Kirsch et al., 2012; Ortega-Obrégon et al., 2014; Coombs et al., 2020). At the same time, the continental margin of California was being truncated, and the Caborca block was being transported from the Mojave Desert region of California to Sonora, Mexico (Saleeby and Busby-Spera, 1992). This transform fault was termed the “California-Coahuila transform fault” by Dickinson (2000) and Dickinson and Lawton (2001). Earlier workers had interpreted displacement of the Caborca block to have occurred on a Late Jurassic Mojave-Sonora megashear (Anderson and Silver, 1979; Anderson and Schmidt, 1983), but that model has been rejected by many subsequent workers on geologic grounds (Dickinson, 2000; Dickinson and Lawton, 2001; Iriondo et al., 2004; Amato et al., 2009; González-León et al., 2009; Chapman et al., 2015; Levy et al., 2021).

In Early Triassic time, subduction under Mexico ceased, the paleo-Pacific side of Mexico became a passive margin, and large siliciclastic wedges formed along this margin (Fig. 6; Centeno-García et al., 2008; Schaaf et al., 2020). The presence of these large Triassic clastic wedges on the Mexican passive margin is crucial to understanding the prevalence of southwest Laurentian and Gondwanan detrital zircon in the fringing oceanic-arc terranes that developed next. The Triassic continental arc of California formed along the truncated margin, exploiting the California-Coahuila transform fault, but it did not extend south of southern California (Fig. 6). Instead, large siliciclastic fans were built on the passive continental margin of Mexico and spread out onto the oceanic realm. The Potosí fan dispersed detrital zircons onto the seafloor from Gondwanan sources, shown on Figure 6, and these were reworked into Jurassic to Early Cretaceous oceanic-arc rocks of the southern Mexican margin, as described further below (Silva-Romo et al., 2000; Centeno-García et al., 2003, 2011; Centeno-García, 2005; Barboza-Gudiño et al., 2010). In contrast, the northern Mexican margin contains Triassic siliciclastic deposits with detrital zircon derived from southwestern Laurentia (Fig. 6; Dickinson et al., 2010; Marty Grove, 2020, written commun.). Gastil (1993) was the first to propose that Triassic nonmarine to shelfal facies of Sonora (Antimonio and Barranca Formations) passed westward (outboard) into shelfal to deep-water facies of Baja California. I propose herein that these were supplied by a second large siliciclastic fan formed north of the Potosí fan, which I term the “Antimonio-Barranca fan” (Fig. 6). I infer that this siliciclastic wedge formed on the downthrown block of the California-Coahuila transform fault, while Permian plutons were exhumed on its upthrown block (Fig. 6). The Antimonio-Barranca basin cannot represent a forearc basin, as inferred by Dickinson et al. (2010), because the Triassic continental arc did not extend that far south (Fig. 6).

By Late Triassic time, Proterozoic to Paleozoic rocks of Mexico formed a narrow neck of land adjacent to the North America craton, with rifting on its eastern side, related to the breakup of Pangea and the opening of the Gulf of Mexico, and subduction on its western side (Centeno-García et al., 2008; Martini and Ortega-Gutiérrez, 2018). The record of subduction initiation is preserved in the most outboard part of the Mesozoic Pacific system of western Mexico (Fig. 2), in Baja California (Figs. 7 and 8).

Subduction of the Cocos plate under Mexico was initiated at 221 Ma to form suprasubduction zone ophiolites of Baja California (Fig. 8, columns II and IV; Fig. 9). These are present across a broad region of Baja California, similar to the broad region occupied by the Eocene intra-oceanic subduction-initiation rocks of the Izu-Bonin-Marianas arc (Arculus et al., 2015a, 2015b; Reagan et al., 2017). In both cases, there are no older oceanic-arc rocks, so the ophiolite could not have formed by backarc spreading; instead, the ophiolite passes upward into younger arc rocks. The Late Triassic ophiolitic rocks are present in the Vizcaino Peninsula and the Peninsular Ranges of Baja California (Fig. 7; shown in purple on Fig. 8). Chromitites of the Vizcaino ophiolite contain Potosí fan detrital zircons that were subducted and then incorporated into the mantle (González-Jiménez et al., 2017a). Those workers inferred that this was accomplished on a west-dipping subduction zone beneath a Permian oceanic arc, but there is no evidence for a Permian arc. Instead, I propose that a west-dipping subduction zone is not necessary, and that Potosí fan detrital zircons were dispersed onto the seafloor before intra-oceanic subduction was initiated and were subducted eastward (Fig. 6). During the Late Triassic, turbidites continued to be shed off the Mexican continental margin, shown as Triassic to Jurassic turbidites of the eastern Peninsular Ranges (Fig. 8, column V).

Jurassic oceanic arcs developed atop the Late Triassic ophiolite in Baja California, from Cedros Island in the west through the Vizcaino Peninsula and eastward into the western Peninsular Ranges (Fig. 8, columns I through IV). However, the evolution of southernmost Baja California and southern mainland Mexico is more complex (Figs. 9, 10, and 11). There, the Jurassic oceanic arc is partially floored by the Arteaga accretionary complex, instead of an ophiolite (Fig. 11, columns B, C, and D). As described in more detail below, the Arteaga accretionary complex consists of deformed Potosí fan quartz-rich turbidites with local mélange containing blocks of chert and mid-ocean-ridge basalt (MORB) igneous rocks, and it is locally metamorphosed to blueschist facies (Fig. 11). The timing of the metamorphism and deformation is very poorly constrained to have occurred between ca. 232 Ma (youngest detrital zircon in the sedimentary protolith, the Potosí fan; Fig. 6; Centeno-García et al., 2003, 2011; Centeno-García, 2005; Barboza-Gudiño et al., 2010) and 180 ± 6 Ma (crosscutting gabbro intrusion; Centeno-García et al., 2003) to 160 Ma (oldest arc rocks that intrude and overlie the Arteaga accretionary complex; Fig. 11, column C). The cause of this deformation and metamorphism is not known. I speculate that a non-subductable object (such as an arc volcano, oceanic plateau, or continental sliver) entered the trench at this latitude and caused a temporary flip in subduction polarity in Early Jurassic time, with partial subduction of oceanic rocks and continentally derived turbidites (Fig. 9). However, this west-dipping subduction must have been short-lived, because Middle Jurassic oceanic-arc rocks developed on top of the Arteaga complex (Fig. 11, columns B, C, and D), recording resumed east-dipping subduction of the Cocos plate.

Slab rollback dominated the paleo-Pacific margin of Mexico and the adjacent southwestern United States in Jurassic time (Fig. 9). In the southwest United States and northern Sonora (Mexico), this produced the Jurassic continental arc, with deep fault-bounded basins and abundant silicic calderas (Busby-Spera, 1988a; see references in Busby and Centeno-García, 2022). South of northern Sonora, the continental arc extended southward into the oceanic realm, to produce Jurassic extensional oceanic-arc rocks (Fig. 9; shown in blue on Figs. 8 and 11). Paleomagnetic work on the Jurassic oceanic-arc rocks as well as the Triassic ophiolite shows that these formed at the same paleolatitude as continental Mexico (localities indicated by “PM” on Figs. 8 and 11; Vaughn et al., 2005; Torres-Carrillo et al., 2016; Boschman et al., 2018a, 2018b). Oceanic-arc extension in Baja California produced the Cedros Island ophiolite at 173 Ma (Fig. 8, column I). Detritus derived from the Gondwanan-Laurentian continental margin is present in Jurassic oceanic-arc sections on Cedros Island (Fig. 8, column I), the Vizcaino Peninsula (Fig. 8, column II), and the western Peninsular Ranges (Fig. 8, column IV), as well as most localities on the Mexican mainland (Fig. 11).

In mainland Mexico, two-mica granite (reflecting contamination from the underlying Arteaga complex) was emplaced in a section of deep-water rhyolite lavas with volcanogenic massive sulfides (VMS; Cuale terrane of the Guerrero composite terrane, Fig, 11, column C). The largest VMS deposit in the Guerrero composite terrane lies in the most inboard oceanic-arc terrane, the Teloloapan terrane (Fig. 11, column F), and the VMS formed ca 157–129 Ma (Martini et al., 2014). The presence of VMS deposits is consistent with an extensional tectonic environment (Huston et al., 2010).

Jurassic oceanic-arc rocks are less widespread in mainland Mexico than they are in Baja California (Figs. 8 and 11), but, as discussed below, the converse is true for Early Cretaceous oceanic-arc rocks. This shows that the main locus of magmatism migrated inboard with time.

In Early Cretaceous time, oceanic-arc magmatism was restricted to the most inboard part of Baja California (Alisitos oceanic arc) but was widespread in western mainland Mexico (shown in red on Figs. 8 and 11). The Alisitos oceanic arc shares many geological features with the Guerrero composite terrane (Tardy et al., 1994; Centeno-García et al., 2011; Centeno-García, 2017; Sarmiento-Villagrana et al., 2018). In the inboard parts of Baja California, Jurassic extensional oceanic-arc rocks are overlain by the Early Cretaceous Alisitos oceanic arc, but outboard of that, a large forearc basin formed on top of the Jurassic oceanic-arc rocks and their ophiolite substrate (Fig. 8, columns I, II, and III). Thus, the main locus of oceanic-arc magmatism moved inboard with time.

Late Jurassic oceanic-arc extension in southwest mainland Mexico produced an Early Cretaceous basin floored by MORB pillow lavas, referred to as the Arperos basin, described in detail below (Fig. 11, column E). It represents a backarc basin that formed by continental rifting, with continentally derived turbidites on its east (inboard) margin and a mafic submarine arc on the west (outboard) margin (Martini et al., 2011, 2014; Palacios-García and Martini, 2014). The turbidites on the eastern side have detrital zircons derived directly from the ancient Mexican margin. The detrital zircons on the western side were derived from the Guerrero arc and its Arteaga accretionary complex substrate (Martini et al., 2011, 2014; Palacios-García and Martini, 2014). The original width of the Early Cretaceous seafloor represented by the Arperos basin is not known, but it closed in Aptian time (Fig. 11). Since no major slab-like anomalies are found east of the Cocos slab, the amount of subduction-related lithosphere related to the closure of the Arperos basin must have been limited (Boschman et al., 2018a).

Earlier models attributed shortening in the 300-km-wide Mexico fold-and-thrust belt to accretion of the Guerrero composite terrane (Coney et al., 1980; Campa and Coney, 1983; Keppie, 2004; Talavera-Mendoza et al., 2007). However, thrusting did not begin in the western (outboard) part of the Mexico fold-and-thrust belt until ca. 83 Ma, ~30 m.y. after accretion of the Guerrero composite terrane (Martini et al., 2016). Instead, shortening propagated from the Pacific trench eastward (inboard) with time, perhaps due to an increase in convergence rate (Busby et al., 1998; Busby, 2004; Centeno-García et al., 2011; Martini and Ferrari, 2011; Martini et al., 2013, 2016). This shortening began with accretion of the Alisitos oceanic arc of the western Peninsular Ranges from ca. 115 to 105 Ma (late Albian; Fig. 8, column V; Alsleben et al., 2009). Shortening of the western Guerrero composite terrane began in late Cenomanian to Turonian time (Fig. 11, column D). Shortening of the eastern Guerrero composite terrane is recorded by deformation of Albian limestones that postdated accretion of the Arcelia terrane (Fig. 11, column E); these were not deformed until Campanian time (Martini et al., 2016). Then, the orogenic belt continued to expand eastward until Eocene time, in the Mexican fold-and-thrust belt (cf. Martini et al., 2016).

A Late Cretaceous continental arc was established on the orogenic belt. This included a major pulse of La Posta–type plutons in the eastern Peninsular Ranges at ca. 98 Ma (Fig. 8, column V, Kimbrough et al., 2001), the 85–80 Ma Puerta Vallarta batholith (Fig. 11, column B; Schaaf et al., 2020), and 85–71 Ma continental-arc volcanic rocks in the Zihuatenejo terrane (Centeno-García et al., 2011). The wealth of literature on the Late Cretaceous continental arc is beyond the scope of this paper.

This section provides a more detailed summary of the Late Triassic to mid-Cretaceous oceanic-arc rocks of Baja California, area by area, with reference to the tectonostratigraphic columns (Fig. 8). Ongoing work has confirmed the model of Busby et al. (1998), in which these oceanic arcs are extensional features (phase 1 and phase 2 in Fig. 1), as summarized below. Most publications separate the Vizcaino terrane (columns I and II in Fig. 8) from the Alisitos terrane of the western Peninsular Ranges (column IV in Fig. 8), partly because the two regions are separated by the Vizcaino basin (Fig. 7), which lacks outcrops. However, geologic ties have long been proposed (Busby-Spera and Boles, 1986; see references in Busby, 2004) and have been greatly strengthened in recent years with recognition of Jurassic oceanic-arc rocks within the “Alisitos arc” (Nuevo Rosarito–El Arco oceanic arc; Fig. 8, column IV), described below, that correlate with Jurassic rocks in the Vizcaino terrane. Studies of cores in the Vizcaino basin (Fig. 8, column III) have confirmed correlations between the Vizcaino terrane and the Alisitos terrane (Helenes et al., 2019). In addition, Kimbrough and Ledesma-Vasquez (2008) reported a Late Triassic gabbro in the Peninsular Ranges (Fig. 8, column IV) that could be correlative to the Vizcaino ophiolite (Fig. 8, column II).

The Vizcaino subduction-initiation ophiolite (Fig. 8, column II) is dated at 221 ± 2 Ma by U-Pb zircon on plagiogranite in the northern Vizcaino Peninsula (Kimbrough and Moore, 2003). Chromitites in the ophiolite have high-Mg boninitic affinity, consistent with a suprasubduction zone origin, and the chromitites contain zircon xenocrysts derived from subduction of sediments with ancient Mexican margin detrital zircon (González-Jiménez et al., 2017a).

The Vizcaino ophiolite is overlain by Norian deep-water tuffs in the northern Vizcaino Peninsula (Fig. 8, column II; Barnes, 1982, 1984; Moore, 1985). In the southern Vizcaino Peninsula, the ophiolite is overlain by Norian cherts, which pass upward into limestone, olistostrome, and volcaniclastic rocks of the Early Jurassic San Hipolito Formation (Finch and Abbott, 1977; Pessagno, 1979; Barnes, 1982, 1984; Kimbrough and Moore, 2003; Whalen and Pessagno, 1984; Moore, 1985; Whalen and Carter, 2002). The San Hipolito Formation also has oceanic-arc pillow lavas (Gutierrez Cirlos Maraña and Centeno-García, 2007). The presence of overlying oceanic-arc lavas, tuffs, and volcaniclastic sandstones supports the interpretation that this is a suprasubduction zone ophiolite. Paleomagnetic work on the Vizcaino ophiolite shows that it lies on the paleolatitudinal path for North America, once the correction is made for the Neogene opening of the Gulf of California (Boschman et al., 2018a).

On Cedros Island (Fig. 8, column I), the oldest oceanic-arc rocks are the 173 Ma Cedros Island ophiolite and the 166 Ma Choyal oceanic-arc assemblage; these are overlapped by the 174–164 Ma Gran Canon Formation oceanic-arc apron with pillow lavas and silicic subaqueous pyroclastic flow deposits (Kimbrough, 1984, 1985; Moore, 1985, 1986; Busby-Spera, 1987, 1988b, 1988c; Critelli et al., 2002). The base of the Gran Canon Formation is altered where it rests on the Cedros Island ophiolite, indicating that the ophiolite was still hydrothermally active when the apron began to form (Busby-Spera, 1988b). This supports the interpretation that it is a suprasubduction zone ophiolite. The overlying Late Jurassic Coloradito Formation on Cedros Island is an olistostromal mélange that has volcanic blocks and Triassic chert blocks of local derivation, as well as blocks of Paleozoic limestone and quartzite that Boles and Landis (1984) attributed to a continental source. This is turn is overlain by the Late Jurassic Eugenia Formation. The lower part of the Eugenia Formation has abundant quartz, microcline, and muscovite and quartzite clasts, inferred by Boles and Landis (1984) to have a continental source. This passes upward into fresh volcanic detritus. The Eugenia Formation on Cedros Island (column I, Fig. 8) is correlated to the northern Vizcaino Peninsula (column II, Fig. 8; Kilmer, 1984).

Jurassic oceanic-arc rocks on the Vizcaino Peninsula include part of the San Hipolito Formation, described above, the Eugenia Formation, and plutons and volcanic rocks of Cerro El Calvario (Fig. 8, column II). The Eugenia Formation on the Vizcaino Peninsula is dominantly volcaniclastic, like that on Cedros Island, but it also contains pillow lavas (Lower Member; Hickey, 1984; Kimbrough and Moore, 2003; Kimbrough et al., 2014). The Upper Member of the Eugenia Formation contains boulders of biotite granite with minor garnet and muscovite, and inherited zircon with a lower-intercept age of 150 Ma and an upper-intercept age of 1.34 ± 0.08 Ga (Kimbrough et al., 1987). Paleomagnetic work on the Eugenia Formation on the Vizcaino Peninsula showed that it lies on the paleolatitudinal path for North America (Boschman et al., 2018a). The 168–135 Ma Cerro El Calvario oceanic-arc assemblage includes plutons, lavas, and volcaniclastic rocks (Fig. 8; Moore, 1984, 1985; Torres-Carrillo et al., 2016). Paleomagnetic work on its plutonic rocks showed that it lies on paleolatitudinal path for North America (Torres-Carillo et al., 2016).

All of the Triassic and Jurassic rocks in the Vizcaino-Cedros region became the substrate for a forearc basin complex when oceanic-arc magmatism migrated inboard to form the Early Cretaceous Alisitos oceanic arc (Fig. 8). These are described below, after an overview of the Peninsular Ranges is provided.

An overview of the Peninsular Ranges of Baja California and southern California is necessary before focusing on the Jurassic and Cretaceous oceanic-arc rocks of the western Peninsular Ranges.

Early work on the Peninsular Ranges of northwest Baja California and southern California recognized that this mountain belt is divided axially into a western belt, consisting of gabbro to monzogranite, and an eastern belt, consisting of granodiorite to granite (Silver et al., 1979; Silver and Chappal, 1988; Walawender et al., 1990). Plutons of the western belt yielded U-Pb zircon ages of 140–105 Ma (Early Cretaceous), with no systematic geographic distribution (“static arc”), whereas plutons of the eastern belt record an eastward-migrating linear locus of magmatism from 105 to 80 Ma (Late Cretaceous “migrating arc” of Silver, 1986). The boundary between the western and eastern batholithic belts was shown to coincide with a step in δ¹⁸O values (Taylor and Silver, 1978) and rare earth element abundances (Gromet and Silver, 1979), interpreted as the western margin of major continental crustal contribution to the batholith. Reconnaissance Nd and Sr isotopic work supported the interpretation that the western belt represents an oceanic arc (DePaolo, 1981). Todd et al. (1988) showed that the compositional, geochemical, and isotopic boundary coincides with a boundary defined by gravity and magnetic data, with more magnetic and dense rocks forming the western Peninsular Ranges batholith relative to the eastern Peninsular Ranges batholith. Additionally, pre-batholithic North American/Mexican continental-margin rocks were recognized in the eastern Peninsular Ranges, but not the western Peninsular Ranges (Fig. 2; Gastil and Miller, 1981; Gastil, 1993). The boundary between the western and eastern zones was shown to be largely coincident with a zone of shearing of variable width (Rangin, 1978; Gastil et al., 1975; Gastil and Miller, 1981; Todd et al., 1988; Walawender et al., 1990; Thomson and Girty, 1994; Johnson et al., 1999; Schmidt and Paterson, 2002), interpreted to have resulted from west-directed compression of the Cretaceous arc during ongoing subduction and arc plutonism (Johnson et al., 1999; Kimbrough et al., 2001; Johnson et al., 2003; Wetmore et al., 2003b, 2005; Tutak, 2008; Alsleben et al., 2014; Wetmore et al., 2014).

Recent work by Langenheim et al. (2014) demonstrated that the magnetic anomaly previously recognized in the western belt of the Peninsular Ranges batholith of southern California and northern Baja California extends all the way from the big bend of the San Andreas fault to the tip of the Baja California Peninsula (“Baja California anomaly”), although it is largely covered by Cenozoic strata in Baja California Sur (Fig. 1), and its northwest end is covered by sedimentary rocks of the Los Angeles Basin (Premo et al., 2014a, 2014b). The geophysical work by Langenheim et al. (2014) also showed that the western belt extends to at least midcrustal depths, and that it has acted as a relatively rigid, coherent crustal block along much of its ~1200 km length.

In Baja California, western belt batholithic rocks occur as diapirs or sheets shallowly emplaced into Early Cretaceous volcanic and sedimentary rocks, and plutonism appears to have waned as the oceanic arc was thrust beneath the Mexican margin (Tulloch and Kimbrough, 2003). Compressional deformation may have been the result of increased convergence and slab flattening between 115 and 100 Ma, with shortening commencing earlier and having greater magnitude in the north relative to the south (cf. Busby et al., 1998; Busby, 2004; Wetmore et al., 2014). This was followed by a short (98–92 Ma) and voluminous “burst” of plutonism in the eastern belt, referred to as the La Posta suite (Kimbrough et al., 2001). This “burst” was attributed to subduction of mantle-derived rocks of the western belt to a deep inboard location beneath the continental margin (Tulloch and Kimbrough, 2003) between 110 and 103 Ma (Johnson et al., 1999; Schmidt et al., 2014).

In southern California, Morton et al. (2014a, 2014b) inferred that the western belt of the Peninsular Ranges batholith formed during an extensional subduction phase, on Mesozoic oceanic crust. They have shown that western belt plutons have more geochemical variation than the generally more felsic plutonic rocks to the east, ranging from olivine gabbro to high-silica monzogranites; silica contents range from 42.5% to 76.8% and average 64.8%. In southern California, plutons of the western belt of the Peninsular Ranges are isotropic from 126 Ma to 108 Ma, but dynamic emplacement occurred along its eastern side from 109 Ma to 98 Ma, resulting in foliated plutons formed by contraction (Morton et al., 2014a, 2014b), This is the same timing as the intra-arc deformation in Baja California described below. U-Pb zircon geochronology as well as thermochronology show that the transition from “western-type” oceanic-arc plutonism to “eastern-type” continental-arc plutonism occurred ca. 99–97 Ma (Premo et al., 2014a, 2014b; Miggins et al., 2014). Isotopic data from southern California show that the Peninsular Ranges batholith is a “progressively contaminated arc,” which formed initially as a west-facing oceanic arc built on Panthalassan lithosphere and evolved into a west-facing continental-margin magmatic arc (Kistler et al., 2014). This process was referred to as “subduction transitioning” by Morton et al. (2014a, 2014b), a term that signifies a process that takes place within a single arc, probably due to a change in subduction parameters. This contrasts with studies that view the western belt as an exotic arc accreted along a west-dipping subduction zone (Tardy et al., 1994; Dickinson and Lawton, 2001; Wetmore et al., 2002; Sigloch and Mihalynuk, 2013, 2017), rather than an arc that fringed North America above an east-dipping subduction zone (Busby et al., 1998; Busby, 2004; Busby et al., 2006; Alsleben et al., 2011; Centeno-García et al., 2011). Detrital zircon data support the continent-fringing west-facing arc interpretation, since it is unlikely that turbidites would have crossed an east-dipping subduction zone to become incorporated in an east-facing arc (cf. Kimbrough and Moore, 2003; Busby, 2004; Busby et al., 2006; Kimbrough and Grove, 2010; Kimbrough et al., 2014).

In Baja California, underthrusting and pluton emplacement resulted in rapid early Late Cretaceous uplift (~10–15 km) in the eastern belt, denuding plutonic rocks and eroding the Late Cretaceous volcanic cover, which was then deposited as coarse-grained sediment in early Late Cretaceous forearc basins (Busby-Spera and Boles, 1986; Busby et al., 1998; Kimbrough et al., 2001; Ortega-Rivera, 2003; Busby, 2004; Alsleben et al., 2008, 2014; Schmidt et al., 2014). In contrast, the western belt (in the lower plate) was not denuded and is dominated by volcanic rocks (Fig. 2). These volcanic rocks have long been referred to as “Alisitos Formation/Group” (Allison, 1974; Gastil et al., 1975), and we refer to them as the Alisitos arc because they include coeval plutons (Busby, 2004; Busby et al., 2006; Morris et al., 2019). This is similar to the terminology used for the Jurassic Bonanza arc of Vancouver Island (DeBari et al., 1999), the Jurassic Talkeetna arc of Alaska (Greene et al., 2006), the Paleocene Kohistan-Ladakh arc of Pakistan (Martin et al., 2020), and others.

Recent work on the Middle Jurassic to Early Cretaceous Nuevo Rosarito–El Arco oceanic arc of the southern Peninsular Ranges (Fig. 7) has major implications for the evolution of western Mexico (Valencia et al., 2006; Weber and Martínez, 2006; Peña-Alonso et al., 2012, 2015; Torres-Carrillo, 2016; Torres-Carrillo et al., 2016; Contreras-López et al., 2018, 2020a, 2020b, 2021; Torres-Carrillo et al., 2020). Only a brief summary is given here.

Discovery of Jurassic oceanic-arc rocks in the Nuevo Rosarito–El Arco region (Fig. 7; Fig. 8, column IV) began with the report of Middle Jurassic U-Pb zircon and Re-Os molybdenite ages on the El Arco porphyry copper deposit, which had previously been interpreted as part of the Late Cretaceous continental arc (Valencia et al., 2006). Pb, Sr, and Nd isotopic and chemical data were interpreted to represent a primitive “island arc” (herein referred to as an oceanic arc) (Weber and Martínez, 2006). El Arco was correlated with oceanic-arc rocks of the Vizcaino-Cedros region (Valencia et al., 2006; Weber and Martínez, 2006).

More recent studies have demonstrated that the Nuevo Rosarito–El Arco region preserves a continuous arc magmatic record from the Middle Jurassic to Late Cretaceous (166–90 Ma), with a switch from oceanic-arc to continental-arc magmatism at 100 Ma (Contreras-López et al., 2021). The discovery of additional Jurassic oceanic-arc rocks in the Nuevo Rosarito–El Arco region has strengthened the correlation with oceanic-arc rocks of the Vizcaino-Cedros region (Peña-Alonso et al., 2012; Torres-Carrillo et al., 2012; Contreras-López et al., 2020a, 2020b, 2021). This includes a 151.6 ± 2.6 Ma tonalite (Peña-Alonso et al., 2012). This tonalite and 139 ± 2 Ma volcaniclastic rocks were deformed and metamorphosed to greenschist to amphibolite facies under dextral transpression between 132 and 128 Ma (Peña-Alonso et al., 2012, 2015). This assemblage was deformed again in a 5–7-km-wide magma-assisted fan structure during emplacement of silicic plutons at ca. 108 Ma, during accretion of the Alisitos arc to the continental margin (Peña-Alonso et al., 2015).

Metasedimentary siliciclastic (not volcaniclastic) rocks in the Nuevo Rosarito–El Arco oceanic arc yielded isotopic data that indicate a continental provenance, similar to that of turbidites in the eastern Peninsular Ranges (Fig. 8, column V) and in western mainland Mexico (Fig. 11, columns B, C, and D; Contreras-López et al., 2020a). Detrital zircon work supports that interpretation (Torres-Carrillo, 2016; Xóchitl Guadalupe Torres-Carillo, 2020, written commun.). Furthermore, mantle-derived magmas of the oceanic arc show isotopic evidence of contamination by the metasedimentary rocks (Contreras-López et al., 2021). Paleomagnetic work on the Nuevo Rosarito–El Arco shows that it lies on the paleolatitudinal path for North America (Torres-Carrillo et al., 2016).

At the southernmost exposure of the Nuevo Rosarito–El Arco oceanic arc, two plutons straddle the suture zone between oceanic-arc rocks and continental-margin rocks (Contreras-López et al., 2018). The western pluton (Calmallí), outboard of the suture, is a pluton with oceanic-arc chemistry that is concentrically zoned from gabbro to quartz diorite–tonalite and was emplaced over a period of 5 m.y. (99.6 ± 1.7 Ma, 102 ± 1.3 Ma, and 104.8 ± 1.6 Ma; Contreras-López et al., 2018). It intrudes volcanic rocks correlated with Jurassic rocks at El Arco. The eastern pluton (Piedra Blanca), inboard of the suture, is a granite with a continental-arc signature, similar to Late Cretaceous La Posta–type intrusions found further north in the eastern Peninsular Ranges (Kimbrough et al., 2001); it is inferred to record crustal thickening and a transition to continental-arc magmatism (Contreras-López et al., 2018).

The Alisitos oceanic arc is exposed for a distance of 800 km in Baja California Norte (Fig. 7) and, as noted above, continues southward another 800 km through Baja California Sur beneath Cenozoic volcanic rocks (Langenheim et al., 2014). The Alisitos oceanic arc is largely Early Cretaceous but contains scattered localities of Jurassic oceanic-arc rocks, in addition to those found in the Nuevo Rosarito–El Arco region described above (Schmidt and Paterson, 2002; Alsleben et al., 2011, 2014; Schmidt et al., 2014; Wetmore et al., 2014). The Alisitos oceanic arc has abundant Early Cretaceous marine strata (Allison, 1955, 1974; Silver et al., 1963, 1979; Gastil et al., 1975; Beggs, 1984; Gastil, 1985; Busby-Spera and White, 1987; White and Busby-Spera, 1987; Fackler-Adams and Busby, 1998; Busby et al., 2006; Alsleben et al., 2008, 2009, 2011, 2014; Squires, 2018; Morris et al., 2019). Paleolatitudes determined by paleomagnetic work on plutons by Torres-Carrillo et al. (2016) and Molina-Garza et al. (2014) are concordant with North America. The Alisitos oceanic arc has syndepositional normal faults (i.e., it is extensional) and was separated from the Mexican continental margin by an extensional backarc basin (see phase 2 in Fig. 1; Busby et al., 1998; Busby, 2004). VMS deposits along in the inboard (eastern) margin of the Alisitos arc have been interpreted as backarc basin deposits (Camprubí, 2017).

At many localities, the Alisitos oceanic arc contains richly fossiliferous biostromal limestones previously reported as either late Aptian (Allison, 1955) or early Albian (Allison, 1974), updated to middle Albian (Squires, 2018). A geochemical study of Alisitos limestones showed that they were deposited in lagoonal to shallow-marine depositional environments, with some yielding seawater compositions and showing involvement of hydrothermal fluids, and others showing evidence for contamination by mafic and felsic volcanic sources (Madhavaraju et al., 2017). However, the Alisitos arc also contains abundant nonmarine and deep-marine strata (White and Busby-Spera, 1987; Busby-Spera and White, 1987; Fackler-Adams and Busby, 1998; Busby, 2004; Busby et al., 2006; Morris et al., 2019).

The Alisitos arc in general is characterized by more silicic volcanic rocks, and some plutons with higher average silica contents, than are considered typical of oceanic arcs (Beggs, 1983, 1984; Wetmore et al., 2003a; Busby, 2004; Busby et al., 2006; Morris et al., 2019; Busby et al., 2023). In the northern Alisitos arc, this has been attributed to the local presence of an unusually thick oceanic-arc crust during magmatism, related to thickening during the onset of accretion at ca. 115 Ma (Tate and Johnson, 2000; Wetmore et al., 2003a). In contrast, at the same time (112 Ma) in the Rosario segment of the southern Alisitos arc, Busby et al. (2006) demonstrated synvolcanic extension and attributed the silicic volcanism to extension, which commonly produces silicic calderas in modern oceanic arcs (e.g., Kermadec arc—Wright and Gamble, 1999; Izu-Bonin arc—Fiske et al., 2001; New Hebrides arc—Greene et al., 1988). The extensional oceanic-arc setting of the Rosario segment of the Alisitos arc makes it a perfect field analog to the Izu-Bonin arc. Recent work in the Rosario segment of the Alisitos arc showed that its geochemistry most perfectly fits the active rift just behind the Izu arc front, which is distinct from the arc front as well as the rear arc behind the active rift (Morris et al., 2019). Silicic calderas are abundant in the Izu active rift (see overview in Busby et al., 2017).

The Rosario segment of the Alisitos arc (denoted by El Rosario in Fig. 7) forms a structurally intact, unmetamorphosed, spectacularly well-exposed, gently tilted, upper- to middle-crustal section that is 50 km long and 7 km thick (Busby et al., 2006). The Rosario segment of the Alisitos oceanic arc is divided into three subsegments: a central subaerial edifice, a southern volcano-bounded basin (dominantly shallow marine), and a northern fault-bounded basin (dominantly deep marine), each underpinned by a separate pluton. Using a combination of published and new geochronological data (Busby et al., 2006; Busby et al., 2023), we infer that the time span represented by the arc crustal section is ~1.5 m.y., at 111–110 Ma. Volcanic and plutonic samples show a continuum from basalt/basaltic andesite to rhyolite, have low to medium K, and are transitional tholeiite to calc-alkaline in character (Morris et al., 2019; Busby et al., 2023). Hf isotopic data from zircons indicate primitive arc magma, consistent with the whole-rock isotopic data of Morris et al. (2019). The volcanic stratigraphy has been correlated across all three subsegments using the tuff of Aguajito, a distinctive rhyolite welded ignimbrite that fills a 15-km-wide, >3.6-km-deep caldera on the central subaerial edifice. Additionally, a second caldera is preserved below the tuff of Aguajito in the northern fault-bounded basin, floored by a large rhyolite sill complex, up to 700 m thick with a lateral extent of >7 km. Up section from the tuff of Aguajito, there is an abrupt shift to dominantly mafic volcanism that we correlated across all three subsegments of the Rosario segment, dividing the section into two distinct parts (phase 1 and phase 2). The initial emplacement of the pluton beneath the central subaerial edifice occurred during the formation of the caldera for the tuff of Aguajito, during phase 1. The plutons beneath the northern fault-bounded basin and the southern volcano-bounded basin were emplaced during phase 2. However, final emplacement of the pluton beneath the central subaerial edifice continued into phase 2, resulting in tilting, uplift, and envelopment of both phase 1 and phase 2 strata there. We attribute this to resurgent magmatism beneath the caldera for the tuff of Aguajito (Busby et al., 2006; Busby et al., 2023).

Evidence for ancient continental Mexican zircons in the Early Cretaceous Alisitos oceanic arc comes from three localities. First, in the Rosario segment of the southern Alisitos arc, three different ignimbrites have Proterozoic grains (Busby et al., 2006; Busby et al., 2023). Second, in the northern Alisitos arc, one sandstone yielded Proterozoic grains, and another yielded Proterozoic and Archean grains (Alsleben et al., 2011). The third locality consists of “Cretaceous sedimentary and continental volcanic arc units” (see figure 3, p. 697, of Alsleben et al., 2014) that are thrust westward over southern Alisitos arc rocks along its inboard margin. This unit is referred to as a “continental-margin arc” by Alsleben et al. (2014, p. 698) because it has silicic welded ignimbrites, but silicic ignimbrites are abundant in the Alisitos oceanic arc, as described above. This unit also contains sandstones with detrital zircons that range from 2740 Ma to 105 Ma, including Permian, Pan African, and Grenville zircon (Alsleben et al., 2011). I interpret these to be reworked from the Triassic Potosí fan (Fig. 6). The youngest age cluster in these sandstones is ca. 112 Ma (Alsleben et al., 2011), which is the same age as the ignimbrites in the Rosario segment to the west (described above). I therefore interpret this thrust sheet as a remnant of a backarc basin (not a continental arc) that received Alisitos arc pyroclastic flows from the west and continentally derived sandstone from the east, as previous workers have done (phase 2 in Fig. 1; Gastil et al., 1978; Beggs, 1983; Griffith and Hoobs, 1993; Phillips, 1993; Busby et al., 1998; Busby, 2004). This is very similar to the asymmetry in provenance documented for the Arperos backarc basin in the Guerrero composite terrane of mainland Mexico (Martini et al., 2014), described below, and it provides another tie between the Alisitos oceanic arc and the Mexican continental margin.

The Alisitos arc began underthrusting pre-batholithic rocks of the Mexican margin between 115 and 105 Ma (phase 3 in Fig. 1; Fig. 8, columns IV and V). However, there is a profound contrast in the magnitude of shortening between the northern Alisitos arc and the southern Alisitos arc. The northern part of the Alisitos arc began deforming at 115 Ma and was intensely deformed in a 25-km-wide zone of deformation that increases eastward in the Alisitos arc toward the suture, the west-directed Main Mártir thrust, which puts pre-batholithic continental margin rocks over the Alisitos arc rocks (Johnson et al., 1999). The Main Mártir thrust has a vertical component of displacement of at least 16 km (Kopf and Whitney, 1999; Kopf et al., 2000; Wetmore et al., 2014). In contrast, the southern part of the Alisitos arc continued to undergo extension until ca. 105 Ma, and an oceanic backarc basin lay behind it (Busby et al., 1998, 2006; Busby, 2004; Morris et al., 2019). The backarc basin closed at 105 Ma, but shortening occurred in a much narrower belt than in the northern Alisitos arc, with a zone of deformation less than 5 km wide, and a vertical component of displacement of only 6 km (Alsleben et al., 2014).

North of the Alisitos arc lies the Santiago Peak arc of southern California and northernmost Baja California (Fig. 7), which is also an Early Cretaceous oceanic arc, but it is traditionally divided from the Alisitos arc because it lacks marine fossils (cf. Herzig and Kimbrough, 2014). The Santiago Peak arc was not thrust beneath continental-margin rocks; instead, Late Jurassic to Early Cretaceous forearc rocks were thrust beneath the Santiago Peak arc, producing the high-grade portions of the Catalina Schist (Grove et al., 2008; Platt et al., 2020). Wetmore et al. (2003a, 2003b, 2014) proposed that the ancestral Agua Blanca fault formed a kinematic link between the Santiago Peak arc and the Alisitos arc. More specifically, underthrusting of the forearc along the Santiago Peak arc segment was coeval and kinematically linked to intra-arc thrusting documented in the Alisitos arc (Marty Grove, 2021, written commun.).

The extreme deformation in southern California and northern Baja California can be attributed to the presence of a continental backstop there. The Sr_i = 0.706 line is well defined in the Peninsular Ranges batholith, along the inboard (east) margin of the Santiago Peak oceanic arc and the inboard margin of the northern Alisitos oceanic arc, indicating that pre-batholithic continental crust was present inboard of these arcs (Figure 1 of Busby et al., this volume). This is consistent with the presence of the Caborca block in the northern Peninsular Ranges (Fig. 5). In contrast, the Sr_i = 0.706 line does extend southward to the southern Peninsular Ranges of Baja California, indicating that the pre-batholithic rocks there were oceanic. This is consistent with the presence of a Late Triassic ophiolite in the southern Peninsular Ranges (Fig. 8, column IV), and the presence of Jurassic oceanic-arc rocks inboard of them (western mainland Mexico; Fig. 9B). Therefore, the along-strike changes in arc-forearc deformation patterns can be attributed to inherited irregularities in the edge of the continental margin and do not require other mechanisms, such as local terrane collisions or along-strike variation in subduction parameters.

Baja California has the most complete and undisrupted suite of forearc basin rocks and subduction zone rocks of the Guerrero-Alisitos-Vizcaino superterrane (shown in orange and yellow in columns I through IV, Fig. 8). Cenozoic tectonism disrupted the forearc of the Peninsular Range in southern California, leaving behind only the largely Campanian–Maastrichtian Peninsular Ranges forearc basin complex shown in yellow on Figure 8. The forearc basin and subduction zone rocks for the southern three-fourths of the Guerrero composite terrane in mainland Mexico are not preserved (Figs. 10 and 11). Although the topic of the forearc basins is beyond the scope of this paper, which focuses on the oceanic-arc terranes, the forearc basins form overlap assemblages on the oceanic-arc terranes of Baja California, so they are described briefly here.

The oldest forearc sedimentary rocks are represented by the Aptian–Albian Asunción Formation of the southern Vizcaino Peninsula, which formed in the forearc of the Alisitos oceanic arc (Fig. 8, column II; Busby, 2004). Like the Alisitos arc, it is extensional, with syndepositional normal faults. The normal fault scarps are fringed by thick, coarse-grained slope apron deposits that built outward from coastal normal fault scarps onto graben floors at bathyal water depths (Busby-Spera and Boles, 1986). Fresh volcanic detritus sourced from the Alisitos arc was able to make its way into each subbasin, so the forearc grabens must have stepped downward toward the trench (Busby-Spera and Boles, 1986). This is also a characteristic of modern extensional forearcs, such as those of Peru (Moberly et al., 1982), the Marianas (Wessel et al., 1994), and northern Chile (Buddin et al., 1993).

The Perforada Formation is the basal formation of the Valle Group in the northern Vizcaino Peninsula, and it consists of Aptian to Albian mudstones and sandstones with lesser ash deposits (Fig. 8, column II; Barnes, 1982, 1984). Paleomagnetic work on the Perforada Formation showed that it lies on paleolatitudinal path for North America (Vaughn et al., 2005). The Perforada Formation is overlain by Albian fine-grained distal turbidites of the Lower Valle Group on the northern Vizcaino Peninsula (Fig. 8, column II), which also occurs on Cedros Island (Fig. 8, column I; Moore, 1984, 1985; Patterson, 1984; Smith and Busby, 1993).

In Cenomanian time, the forearc basin on the Vizcaino Peninsula and Cedros Island was flooded with deep-water boulder and cobble conglomerates derived from uplift of the eastern Peninsular Ranges due to underthrusting of the Alisitos oceanic arc and emplacement of the La Posta tonalite-granodiorite suite (Fig. 8, columns IV and V). This is referred to as the Upper Valle Group (Fig. 8, columns I and II; Barnes, 1984; Patterson, 1984; Smith and Busby, 1993). These conglomerates contain three types of quartzite clasts, classified by their detrital zircon provenance (Kimbrough et al., 2006): (1) Peace River Arch, derived from the Ordovician San Felipe quartzite of the Caborca block in the eastern Peninsular Ranges, which is in turn correlated with the Eureka quartzite of the Cordilleran miogeocline (Gehrels et al., 2002); (2) southwest North America clast types, derived from the “pre-batholithic flysch” of Gastil (1993), which I interpret as the Triassic Antimonio-Barranca fan (Fig. 6); and (3) quartzite clasts that may have either southwest Laurentian or Gondwanan provenance. Deposition of the Upper Valle Group was accompanied by extensional unroofing of the structurally underlying blueschist subduction complex of the Western Baja terrane of Vizcaino and Cedros Island (see “Structure and Metamorphism I and II,” at the left edge of Fig. 8; Sedlock, 1988a, 1988b; Smith and Busby, 1993). Conglomerates of the Valle Group on Cedros Island fill a deep-marine half-graben structure that formed by reactivation of a Jurassic fault between rifted arc and ophiolite basement (Cedros Island ophiolite and Choyal arc; Fig. 8, column I; Smith and Busby, 1993). Faulting and seismicity resulted in oversteepened and unstable slopes that generated numerous submarine landslide scars, producing apron deposits of chaotic mudstone-sandstone and soft-sediment folds. This extension resulted in exhumation of the blueschists in the lower plate beneath detachment faults (Sedlock, 1988a, 1988b; Baldwin and Harrison, 1989; Sedlock, 2003).

The Valle Group rests on the Alisitos oceanic arc in the Vizcaino Basin (Fig. 8, column III), providing yet another tie between the Alisitos terrane and the Vizcaino terrane (Helenes et al., 2019).

As the Late Cretaceous continental-arc magmatism migrated eastward, forearc basin sedimentation began on top of the western Peninsular Ranges oceanic-arc rocks of Baja California and southern California, referred to as the Rosario Group (Fig. 8, column IV; Kilmer, 1963; Gastil and Allison, 1966; Morris, 1981, 1992; Nilsen and Abbott, 1981; Bottjer and Link, 1984; Yeo, 1984; Boehlke and Abbott, 1986; Cunningham and Abbott, 1986; Morris and Busby-Spera, 1988, 1990; Morris et al., 1989; Filmer and Kirschvinck, 1989; Renne et al., 1991; Abbott et al., 1993; Fulford and Busby-Spera, 1993; Morris and Busby, 1996; Busby et al., 1998, 2002; Busby, 2004; Kane et al., 2009; Peecook et al., 2014; McArthur et al., 2016; Hansen et al., 2017; Kneller et al., 2020; Fastovsky et al., 2020). The Rosario Group is largely Campanian and Maastrichtian, although it has poorly dated basal red beds (Fig. 8, column IV). It is largely deep marine, but it has shallow-marine and nonmarine sections, including a dinosaur bone locality in a nonmarine section at El Rosario (location on Fig. 7; cf. Fastovsky et al., 2022).

The Rosario Group near El Rosario has excellent exposures and is structurally intact and unaltered, so it has been studied intensively. The basin was tectonically active, with rapid vertical alternation of nonmarine and bathyal marine strata indicating alternating uplift and downdropping of the basin, and it has synsedimentary faults with reverse-slip separation and normal-slip separation, as well as synsedimentary grabens and folds, respectively; these features are attributed to strike-slip tectonics (Busby, 2004). The Rosario area also preserves a Cretaceous-Tertiary boundary section, both in deep-marine facies (Morris and Busby-Spera, 1988) and in coastal lithofacies (Busby et al., 2002). The Rosario area also includes the first-recognized Pacific margin stratigraphic sequence containing evidence for catastrophic landsliding attributed to Cretaceous-Tertiary boundary bolide impact-induced seismicity (Busby et al., 2002).

This section of the paper provides an overview of Late Triassic to Late Cretaceous oceanic-arc rocks in western mainland Mexico (Figs. 10 and 11), focusing mainly on oceanic-arc rocks, but also describing their substrates (where preserved), timing of accretion events, and timing of shortening/orogenesis during establishment of the Late Cretaceous continental arc.

The Tahue terrane (Fig. 11) is the northernmost terrane of the Guerrero composite terrane, and it contains the oldest rocks in the Guerrero composite terrane (Mullan, 1978; Campa and Coney, 1983; Gastil et al., 1991; Roldán-Quintana et al., 1993; Poole and Perry, 1998; Arredondo-Guerrero and Centeno-García, 2003; Centeno-García, 2005; Keppie et al., 2006; Centeno-García et al., 2008). These include metamorphosed and deformed Ordovician oceanic-arc rocks, unmetamorphosed but deformed Pennsylvanian–Permian deep-marine sedimentary rocks, and the 206 Ma Francisco Gneiss. The Francisco Gneiss has a protolith of interleaved rhyolites and continental rift tholeiites (Keppie et al., 2006). These rocks are the same age as the Barranca Group immediately to the north, suggesting that both formed in a rift environment (Keppie et al., 2006). This is consistent with the paleogeographic map shown in Figure 6, which portrays the Mexican margin as a rifted/passive margin in Triassic time.

Early Cretaceous marine arc volcanic rocks, and the mafic and ultramafic intrusions that cut the Paleozoic rocks, are interpreted as part of the Guerrero composite terrane, because these rocks have oceanic-arc geochemical signatures (Ortega-Gutiérrez et al., 1979; Henry and Fredrikson, 1987; Roldán-Quintana et al., 1993; Freydier et al., 1995; Gastil et al., 1999; Arredondo-Guerrero and Centeno-García, 2003; Centeno-García et al., 2008). However, Sedlock et al. (1993) proposed that the Tahue terrane should not be included in the Guerrero terrane/composite terrane because of its old basement, and Sarmiento-Villagrana et al. (2018) used geochemical data to support that interpretation, suggesting that the northern limit of the Guerrero terrane/composite terrane needs reevaluation. I include it in the Guerrero composite terrane pending further evaluation.

The Zihuantanejo terrane (Fig. 11) is the largest of all the terranes that form the Guerrero composite terrane, and it has the most protracted record of arc magmatism (Centeno-García et al., 2008, 2011; Centeno-García, 2017). For this reason, it is represented by three columns (Fig. 11, columns B, C, and D). It is divided into four tectonostratigraphic assemblages: (1) Triassic–Early Jurassic Arteaga accretionary complex with ancient detrital zircons of the Gondwanan Mexican margin; (2) Jurassic to earliest Cretaceous extensional oceanic-arc assemblage (163–145 Ma); (3) late Early Cretaceous extensional oceanic-arc assemblage (peaks at 129–123 Ma and 109 Ma). Extensional tectonics and erosion of normal fault scarps during deposition of tectonostratigraphic assemblages 2 and 3 led to repeated recycling of zircons from tectonostratigraphic assemblages 1, 2, and 3. Assemblages 1 through 3 were then folded between Cenomanian and Santonian time and overlain by (4) Late Cretaceous (Santonian–Maastrichtian) compressional arc assemblage (Centeno-García et al., 2011). More information on the Zihuatenejo terrane and its Gondwanan detrital zircon is provided in Centeno-García et al. (2003, 2008, 2011), Talavera-Mendoza et al. (2007), and Martini et al. (2009). Paleomagnetic data from Upper Aptian (ca. 119–112 Ma) limestones and from andesitic red beds with a U-Pb zircon maximum depositional age of 118 Ma yielded paleolatitudinal plate-motion history concordant with North American (Fig. 11, column D; Boschman et al., 2018b). This contradicts alternative paleogeographic models in which the Guerrero terrane is considered to be exotic to the North American continent (Lapierre et al., 1992; Tardy et al., 1994; Freydier et al., 1996; Dickinson and Lawton, 2001; Umhoefer, 2003; Hildebrand, 2013; Sigloch and Mihalynuk, 2013, 2017).

The Arteaga accretionary complex (Fig. 11, columns B, C, and D) has been given different names in different areas, including Taray formation, El Chilar complex, Zacatecas Formation, Rio Placeres Formation, Arteaga complex, and Las Ollas complex (Centeno-García et al., 1993; Centeno-García and Silva-Romo, 1997; Talavera-Mendoza, 2000; Anderson et al., 2005; Dávila-Alcocer et al., 2013). It consists of strongly deformed quartzose sandstone, black shale, radiolarian chert, and megablocks of pillow lava, gabbro, chert, and rare limestone, metamorphosed to greenschist to amphibolite facies (Centeno-García, 2005). Its protolith age is poorly constrained, with Triassic (Ladinian–Carnian) fossils reported in an abstract (Campa et al., 1982), a maximum depositional age of 250 Ma obtained by U-Pb detrital zircon analysis on the sandstone matrix (Centeno-García et al., 2011), and a minimum depositional age provided by a crosscutting gabbro dated by U-Pb zircon analysis at 180 ± 6 Ma (Centeno-García et al., 2003). Additionally, one sample from rocks described as petrographically similar to rocks of the Arteaga complex yielded detrital zircon ages from 2781 Ma (Archean) to 200 ± 8 Ma, with the youngest cluster and largest peak at 247 Ma (Tzitzio sample; Talavera-Mendoza et al., 2007). Mafic volcanic rocks of unknown age in the Arteaga complex are mainly normal mid-ocean-ridge basalts (N-MORBs), although a few have island-arc tholeiite compositions (Centeno-García et al., 2003). U-Pb dating of detrital zircon of the matrix of the Arteaga accretionary complex in the Zihuatenejo terrane (Fig. 11, column C) showed it has a Gondwanan provenance, with peaks at 1.0 Ma (Grenville), 650–480 Ma (Pan-African), and 260 Ma (East Mexico arc) (see basement source rocks on Fig. 6). This shows that the accretionary complex formed along the Mexican margin (Centeno-García et al., 2011), rather than representing an exotic terrane as proposed by Dickinson and Lawton (2001).

At Puerta Vallarta (Fig. 11, column B), the Yalapo-Chimo metamorphic complex includes (Schaaf et al., 2020): (1) paragneisses and schists (sedimentary protolith) correlated with the Arteaga accretionary complex, and (2) paragneiss and amphibolite (mafic to intermediate granitoid protolith) of oceanic-arc composition with 136–128 Ma U-Pb zircon ages (Chimo arc), correlated with the Alisitos arc of Baja California (Fig. 8, columns III and IV; Schaaf et al., 2020). The timing of metamorphism of the Yalapo-Chimo metamorphic complex is constrained by 120–112 Ma U-Pb zircon rim ages and U-Pb monazite ages in the paragneisses (Schaaf et al., 2020). This correlates roughly with the timing of accretion of the Alisitos arc in Baja California (Fig. 8, column IV). Continental-margin granitoids were emplaced at 85–80 Ma, interpreted to represent the southeast extension of the continental-margin arc of the eastern Peninsular Ranges (Schaaf et al., 2020), which migrated eastward with time (Fig. 8, column V). Schaaf et al. (2020) correlated the Arteaga complex rocks at Puerta Vallarta with metasediments in the Cabo block at the southern tip of Baja California Sur, as well as metasediments in a Gulf of California island that lies between Puerta Vallarta and the Los Cabos block (Isla San Juanito; Fig. 11, column B).

At Cuale (Fig. 11, column C), 160 Ma two-mica granites (Bissig et al., 2008) intrude a volcanosedimentary greenschist sequence that is included in the Arteaga complex (Centeno-García et al., 2008, 2011). The Arteaga complex is unconformably overlain by Late Jurassic unmetamorphosed phyric rhyolite lavas and hyaloclastites that host VMS deposits (Bissig et al., 2008), possibly related to the two-mica granites (Schaaf et al., 2020). U-Pb zircon ages include those from rhyolite hyaloclastite (157.2 ± 0.5 Ma), aphyric rhyolite lava (154.0 ± 0.9 Ma), and a rhyolite dike (155.9 ± 1.6 Ma). These host a VMS that is older than others of the Guerrero composite terrane (Mortensen et al., 2008).

The Guanajuato terrane (central Mexico) and the Arcelia terrane (southern Mexico) are herein grouped together (Fig. 11, column E), even though they are separated by the Neogene Trans-Mexican volcanic belt, because they both contain vestiges of the Arperos backarc basin, and they have the same depositional architecture (Martini et al., 2014). The Arperos basin is important because previous workers have inferred that it represents a large ocean basin (Mezcalera plate) that subducted westward under a distant archipelago that included all of the oceanic-arc terranes discussed in this paper (Fig. 3A; Dickinson and Lawton, 2001). Others consider it to have been an oceanic backarc basin that developed along the continental margin of Mexico (Fig. 3B; Cabral-Cano et al., 2000; Elías-Herrera et al., 2000; Centeno-García et al., 2008; Martini et al., 2011; Centeno-García et al., 2011; Martini et al., 2014; Centeno-García, 2017). As summarized herein, U-Pb detrital zircon geochronology supports the latter interpretation.

Rocks of the Guanajuato and Arcelia terranes are characterized by suprasubduction ophiolite assemblages, MORB, oceanic-island basalt (OIB), and island-arc basalt (IAB) (Ortiz and Lapierre, 1991; Mendoza and Guerrero, 2000; Ortíz-Hernández et al., 2003; Talavera-Mendoza et al., 1995, 2007; Centeno-García et al., 2008; Martini et al., 2011). Additionally, in Guanajuato, the inboard part of the terrane has volcanic rocks and strata formed by backarc continental rifting (Martini et al., 2011).

Recent petrographic and detrital zircon work has shown that turbidites of the Arperos Basin have a marked sandstone provenance asymmetry, with a Mexican continental provenance to the east and a Guerrero arc provenance (including Arteaga complex) to the west (Martini et al., 2011, 2014). The assemblages are schematically divided into three blocks on column E of Figure 11, representing the eastern continental-margin assemblage (block 1), the western oceanic-arc assemblage (block 2), and the ocean-floor assemblage (block 3).

The oldest rocks of the Guanajuato-Arcelia terrane (Fig. 11, column E, block 1) are Tithonian continentally derived quartzose sandstones and mudstones (Martini et al., 2011, 2014). These have zircon grains that range from 1557 to 184 Ma, with Triassic, Permian, and Grenville peaks and subordinate Ordovician–Silurian and Pan-African populations. The lack of Middle and Late Jurassic detrital zircons indicates that the sandstones were deposited on the continental side of the Arperos basin, far from the influence of the Guerrero arc (Martini et al., 2011). These sandstones are intruded by Late Jurassic rhyolite and dacite dikes with peperite margins (U-Pb zircon age of 150.7 ± 0.8 Ma), and the sandstones have interstratified peraluminous rhyolite and dacite lavas (U-Pb zircon age of 145.4 ± 1.1Ma), with nine inherited grains ranging from 663 to 151 Ma (Martini et al., 2011, 2014). This assemblage is interpreted to record continental rifting in the early stages of the opening of the Arperos basin (Martini et al., 2014). Turbidites with a Mexican continental-margin source were deposited through the Aptian, interbedded with felsic and mafic lavas (Martini et al., 2011, 2014; Palacios-García and Martini, 2014).

The next youngest rocks of the Guanajuato-Arcelia terrane (Fig. 11, column E, block 2) consist of a mafic submarine arc. In the Arcelia terrane, these include fine-grained volcaniclastic sandstone that yielded two older grains (1343 ± 42 and 869 ± 37 Ma; Talavera-Mendoza et al., 2007). In the Guanajuato terrane, sandstones from the mafic submarine arc yielded no zircon, as expected in a mafic arc setting, but a tonalite yielded a U-Pb zircon age of ca. 144 Ma, with Paleozoic and Precambrian inherited zircon. These data indicate that the arc on the outboard side of the Arperos basin was not exotic to Mexico (Martini et al., 2011).

The youngest rocks of the Guanajuato-Arcelia terrane (Fig. 11, column E, block 3) formed by seafloor spreading in a backarc basin. In the Arcelia terrane, fine-grained volcaniclastic sandstones interstratified with backarc pillow lavas yielded 15% older grains (Paleoproterozoic to Permian; Talavera-Mendoza et al., 2007). On its western margin, the Arperos basin has Aptian volcaniclastic turbidites with ca. 160–118 Ma detrital zircons sourced from the Guerrero terrane arc rocks and Middle Triassic to Mesoproterozoic grains sourced from the Arteaga complex (Martini et al., 2012, 2014).

The Guanajuato terrane has at least five significant VMS deposits in the arc rocks (Fig. 11, column E; Mortensen et al., 2008). A U-Pb zircon multigrain analysis on a rhyolite lava from the footwall of the El Gordo deposit gave an interpreted crystallization age of 146.1 ± 1.1 Ma, with a Grenville upper-intercept age (Mortensen et al., 2008).

The Arcelia terrane contains a mafic-ultramafic suprasubduction zone complex with chromitite deposits that yield Re-Os model ages of ca. 300 and 130 Ma, interpreted to record an earlier mantle-melting event followed by Early Cretaceous precipitation of the chromitites (González-Jiménez et al., 2017b).

The Arperos basin was accreted in the late Aptian (ca. 115 Ma; Fig. 11, column E) and overlain by reefal limestones (Martini et al., 2016).

The Teloloapán terrane (Fig. 11, column F) is the easternmost (most inboard) terrane of the Guerrero composite terrane in southern Mexico. It lies east of the Arcelia terrane; it does not occur inboard of the Guanajuato terrane in central Mexico. It consists of a 3000-m-thick Tithonian to Aptian submarine oceanic-arc succession, with calc-alkaline mafic lavas, radiolarian cherts, and locally interbedded 150–139 Ma rhyolite lavas hosting VMS deposits (Talavera-Mendoza, 1994; Talavera-Mendoza et al., 1995, 2007; Mendoza and Guerrero, 2000; Monod et al., 2000; Guerrero-Suástegui, 2004; Mortensen et al., 2008; Centeno-García et al., 2011; Martini et al., 2014). Martini et al. (2014) inferred that it did not originally lie between the Arcelia terrane and the Mexican margin, but was tectonically transported over the Arperos basin rocks (Arcelia terrane) by thrusting. They considered it correlative with similar oceanic-arc rocks of the same age that lie on the west side of the Arperos basin in both the Arcelia and Guanajuato terranes (Fig. 11, column E, block 2).

The Teloloapán terrane has the largest concentration of VMS deposits in the Guerrero composite terrane (Mortensen et al., 2008). U-Pb zircon ages from the Teloloapán terrane range from 150.2 Ma to 138.8 Ma, and all have inherited zircon (Mortensen et al., 2008).

The arc volcanic rocks of the Teloloapán terrane are gradationally overlain by Aptian–Albian volcanic lithic sandstones and limestones with a ca. 115 Ma detrital zircon cluster, which are in turn overlain by Turonian turbidites with Paleoproterozoic detrital zircon derived from the Acatlan complex (Fig. 11, column F; Talavera-Mendoza et al., 1995, 2007; Guerrero-Suástegui, 2004; Centeno-García, 2017).

A series of schematic cross sections illustrates the evolution of the Guerrero-Alisitos-Vizcaino superterrane (Fig. 12), constructed using lithostratigraphic columns presented in Figure 8 (columns I through V) and Figure 11 (columns A through F), and references cited in the text above. Each time frame covers millions to tens of millions of years, and the arc front likely shifted positions at time scales shorter than that, so not all of the volcanoes portrayed in any one time frame were active at the exact same time:

(A) The Triassic time frame shows the passive margin of west Mexico. After Permian to earliest Triassic subduction ceased (Figs. 4 and 5), large submarine fans (Potosí and Antimonio-Barranca) shed ancient Gondwanan and Laurentian detrital zircon, as well as zircon from the Permian arc, onto the Panthalassa seafloor (Fig. 6).

(B) The Late Triassic time frame shows subduction initiation in an intra-oceanic setting (Fig. 9A). A suprasubduction zone ophiolite was generated before the onset of arc volcanism, so it is interpreted herein as a subduction-initiation ophiolite. The trench formed inboard of the outer margin of the submarine fans, and ancient Mexican zircons were subducted to be recycled into chromitites of the ophiolite.

(C) The Early Jurassic time frame is divided into two transects: a northern transect through the Vizcaino Peninsula–Peninsular Ranges region (Fig. 7) and a southern transect through the southern tip of Baja California through southwest mainland Mexico (Fig. 10). In the northern transect, extensional oceanic arcs formed on top of the ophiolite. In the southern transect, the Potosí fan turbidites were deformed in a subduction complex that incorporated seafloor rocks (Arteaga complex; Fig. 11, columns B, C, D). A speculative reconstruction is shown, where an obstacle enters the trench and causes a brief flip in subduction polarity. In both transects, rift basins formed on the continent (Nazas continental rift province) and were filled with clastic sedimentary rocks and minor volcanic rocks, interpreted to be related to the breakup of western equatorial Pangea (Busby and Centeno-García, 2022).

(D) The Middle Jurassic time frame shows extensional oceanic-arc magmatism and generation of an intra-arc ophiolite in the most outboard region (Vizcaino-Cedros). Sedimentation of turbidites with detrital zircon derived from the ancient Mexican continental margin, as well as recycling of those zircons from normal fault scarps, was ongoing. Clastic sedimentation and minor volcanism in continental rift basins (Nazas continental rift province) also continued.

(E) The Late Jurassic time frame shows widespread extensional oceanic arc magmatism, atop oceanic lithosphere in the outboard region, and atop Arteaga complex in the inboard region. Not all of the volcanoes shown in this time frame were necessarily active at the exact same time. Recycling of ancient Mexican continental detrital zircons from normal fault scarps continued. The Arperos backarc basin began to open by rifting of the continental margin, generating new seafloor and receiving turbidites derived from the ancient Mexican continental margin on its inboard side and turbidites derived from the oceanic arc and its Arteaga complex substrate (with ancient zircons) on its outboard side. The Nazas continental rift province became inactive.

(F) The Early Cretaceous time frame shows inboard migration of the extensional oceanic arc, to form the Alisitos arc in Baja California and correlative rocks in western mainland Mexico. Meanwhile, a forearc basin formed atop the Triassic–Jurassic extensional arc-ophiolite assemblages in the outboard position. This forearc basin is well preserved in western Baja California but is missing from the outboard part of southwest mainland Mexico, suggesting it was removed by later tectonic processes (e.g., subduction erosion or strike-slip tectonics). Recycling of ancient Mexican continental detrital zircons from fault scarps continued. The Arperos backarc basin closed at 115 Ma, producing a “relatively narrow” suture/fold-and-thrust belt (Martini et al., 2016). Accretion was not responsible for the development of the very broad (~500 km) Mexican fold-and-thrust belt, as proposed by many previous workers, because the Mexican fold-and-thrust belt did not begin to form until 83 Ma (Martini et al., 2016).

(G) The Late Cretaceous time frame portrays the development of the contractional continental arc. This began with mid-Cretaceous thrusting of the Alisitos arc under Mexican margin rocks during ongoing subduction and arc magmatism, resulting in crustal thickening and development of a high-standing arc with high magma flux (La Posta–type tonalite-granodiorite suite; Kimbrough et al., 2001). The arc shed Cenomanian to middle Turonian conglomerates into the forearc basin, and outboard of that, a subduction complex with blueschists formed. This likely resulted from an increase in convergent rate and attendant slab flattening (phase 3 in Fig. 1; Busby et al., 1998; Busby, 2004). Increased convergence may have resulted from a worldwide plate organization event that occurred at 105–100 Ma due to the cessation of an ~7000-km-long subduction zone on East Antarctica, which broadly corresponds to the southern margin of the Pacific Basin (Matthews et al., 2012; Seton et al., 2012). This timing corresponds to widespread intra-arc deformation, followed by magmatic arc flareup, along the length of the North American Cordillera, from the Yukon through northwest Washington, western Idaho, the Sierra Nevada batholith, and the Peninsular Ranges batholith (see discussion in Busby et al., this volume). After the contraction and arc flareup in Baja California, contraction expanded progressively inboard with time, and the locus of arc magmatism migrated inboard, suggesting progressive slab flattening. The most inboard contraction, in the Mexican fold-and-thrust belt, began at 83 Ma (Martini et al., 2016).

In summary, all of the oceanic-arc rocks included here in the Guerrero-Alisitos-Vizcaino superterrane were extensional and formed from Late Triassic to mid-Cretaceous time. They share many geologic features in common, share the same paleomagnetic path as Mexico, and have detrital zircons derived from the Mexican continent. Thus, they formed as oceanic arcs that fringed the Mexican continental margin, all forming above a single east-dipping subduction zone. These were collapsed against the Mexican continental margin in Late Cretaceous time due to a global plate reorganization that caused increased convergence along the entire North American Pacific margin.

This paper benefited from many discussions with Elena Centeno-García over the past 20 years. I thank Scott Johnson, Nancy Riggs, and Alex Zagorevski for their very helpful formal reviews. Special thanks go to Janice Fong, professional illustrator and artist in the Department of Earth and Sciences at the University of California–Davis, for her beautiful work on the figures. I am also grateful to the undergraduates, graduate students, and postdoctoral researchers who have worked with me in Mexico for the last 37 years. Support from National Science Foundation grant EAR-1917361 is gratefully acknowledged.