Geological Society of America Memoirs

Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges, Baja California, Mexico

Paul H. Wetmore, Scott S. Hughes, Ciprian Stremtan, Mihai N. Ducea and Helge Alsleben

Geological Society of America Memoirs 2014;211;669-690 doi: 10.1130/2014.1211(21)

Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Memoirs Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society.

Notes

The Geological Society of America Memoir 211 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges, Baja California, Mexico

Paul H. Wetmore* Department of Geology, University of South Florida, Tampa, Florida 33620, USA

Scott S. Hughes Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA

Ciprian Stremtan Department of Geology, University of South Florida, Tampa, Florida 33620, USA, and Department of Geology, Faculty of Biology and Geology, Babes-Bolyai University, M. Kogalniceanu Str. 1, RO-400084, Cluj Napoca, Romania

Mihai N. Ducea Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, and Universitatea Bucuresti, Facultatea de Geologie Geoﬁ zica, Strada N. Balcescu Nr 1, Bucuresti, Romania

Helge Alsleben School of Geology, Energy and the Environment, Texas Christian University, Fort Worth, Texas 76129, USA

ABSTRACT

The Alisitos arc segment is the southernmost and only part of the western Penin- sular Ranges batholith accreted during the Cretaceous. Collision-related deformation is concentrated along the northern and eastern margins of the arc segment. While shortening within the Alisitos arc produced similar amounts of crustal thickening throughout the arc, suppression of parts of the lower crust of the Alisitos arc due to throw across the terrane-bounding faults varies substantially. Geobarometric change across the Main Mártir thrust suggests that ~15 km of additional crust was thrust onto the central Alisitos arc. Geochemical and geochronologic data from intrusive rocks of the Alisitos arc indicate arc magmatism was active before, during, and after collision. The data suggest that all Peninsular Ranges batholith intrusive rocks within the Alisitos arc were derived from a broadly similar primitive source, lacking interaction with evolved continental lithologies. Postcollisional intrusions from the central Alisitos arc adjacent to

*Corresponding author: [email protected].

Wetmore, P.H., Hughes, S.S., Stremtan, C., Ducea, M.N., and Alsleben, H., 2014, Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges, Baja California, Mexico, in Morton, D.M., and Miller, F.K., eds., Peninsular Ranges Batholith, Baja California and Southern California: Geological Society of America Memoir 211, p. 669–690, doi:10.1130/2014.1211(21). For permission to copy, contact [email protected]. © 2014 The Geological Society of America. All rights reserved.

669 Downloaded from memoirs.gsapubs.org on March 31, 2014

670 Wetmore et al.

the Main Mártir thrust yield trace elemental signatures suggesting melt derivation at depths where garnet would be a stable residual phase. The spatial and temporal coincidence of these intrusions with the Main Mártir thrust suggests that the increased pressure of anatexis inferred for the depth of generation of these melts was generated by displacement on this fault. Further, close temporal and spatial characteristics, and similar geochemical characteristics between the central Alisitos arc intrusions and La Posta intrusions east of the Main Mártir thrust suggest that the Alisitos arc intrusions represent precursors to the much larger ﬂ are-up event. This observation supports models suggesting collision as a cause of magmatic ﬂ are-ups in arcs.

INTRODUCTION (e.g., Todd et al., 1988; Thomson and Girty, 1994; Johnson et al., 1999a; Wetmore et al., 2002, 2003; Kimbrough and Grove, 2010; The southwestern margin of North America is defi ned, in Schmidt et al., this volume) and were subsequently contaminated large part, by the contractional deformation and magmatism through the assimilation of the evolved upper-crustal lithologies associated with a prolonged history of convergence between the that host the intrusions (e.g., Walawender et al., 1990; Tulloch North American and Farallon plates. The intrusive record for this and Kimbrough, 2003). phase of tectonism is dominated by bodies formed during punc- Some features temporally and spatially associated with the tuated, high-fl ux events known as fl are-ups (e.g., Coleman and La Posta intrusions of the central/transitional and eastern zones Glazner, 1997; Ducea, 2001; Ducea and Barton, 2007). Ducea of the Peninsular Ranges batholith are a suite of late syn- to post- and Barton (2007) argued, based on the compilation of thousands collisional intrusions of the western zone of the batholith (e.g., of radiogenic and stable isotopic data, that fl are-up arc magma- Johnson et al., 1999a, 1999b, 2003; Tate et al., 1999; Tate and tism is derived from upper-plate lithosphere. They further argued Johnson, 2000; Wetmore et al., 2003; Schmidt et al., this vol- that such events are intrinsically tied to crustal/lithospheric thick- ume). These intrusions are only known from the Alisitos arc seg- ening events such as subduction erosion, collision, or retro-arc ment, which comprises the western Peninsular Ranges batholith shortening (e.g., Sevier orogeny). south of the Agua Blanca fault, and is inferred to have been either In the southern and Baja Californian Peninsular Ranges a fringing (Gastil et al., 1975, 1981), or exotic island arc (John- batholith, such fl are-up magmatism is represented by the La Posta son et al., 1999a; Wetmore et al., 2002, 2003), collapsed against suite of intrusives (Walawender et al., 1990; Tulloch and Kim- the southwestern margin of North America between ca. 112 and brough, 2003), which form the bedrock along the backbone of 100 Ma (Schmidt et al., this volume). While late syn- to postcol- the Baja Peninsula. The La Posta event of the Peninsular Ranges lisional intrusions are distributed relatively uniformly along the batholith is represented by ~20 individual intrusive bodies length of the Alisitos arc segment, trace elemental chemistries of emplaced within the transitional and eastern zones of the batho- these intrusive rocks exhibit spatial variations that correlate with lith between 99 and 92 Ma, and it accounts for nearly 50% of similar spatial variations in the magnitude of crustal shortening. surface exposure of the Peninsular Ranges batholith (Kimbrough In the following sections, we will provide a brief background et al., 2001). The timing of the La Posta event overlaps that of of the tectonic evolution of the Baja Peninsula and the formation the Sierra Crest magmatic event of the Sierra Nevada (Coleman of the Peninsular Ranges batholith. This will be followed by a and Glazner, 1997), which has been associated with retro-arc relatively detailed description of the structural geology of the shortening of the Sevier orogeny (Ducea, 2001; DeCelles et al., Alisitos arc segment, including the timing of magmatism asso- 2009), and the high-fl ux emplacement of early Late Cretaceous ciated with this arc, followed by a description of the available intrusions (e.g., Ecstall suite) of the Coast Mountains batholith geochemistry for the Early Cretaceous intrusions of the Alisitos (west-central British Columbia, Canada), which are inferred to arc. We will conclude with a discussion of the tectonic and pet- be related to the collision of the Western magmatic belt (i.e., rogenetic implications for the structural and geochemical data. Alexander-Wrangellia terranes) with the Eastern magmatic belt (i.e., Stikine-Yukon-Tanana terranes) in the late Early Cre- CRETACEOUS TECTONICS AND MAGMATISM OF taceous (Gehrels et al., 2009; Wetmore and Ducea, 2011). The THE PENINSULAR RANGES BATHOLITH mechanism(s) responsible for the genesis of La Posta event magmas appears to be more consistent with that of the Coast The Baja Peninsula of southern and Baja California consists Mountains batholith (i.e., collision related). This follows from of a series of northwest-trending basement assemblages of vary- the observations that the La Posta melts were likely derived from ing ages and compositions, all intruded by the Peninsular Ranges the pre–Late Cretaceous assemblages from the western zone batholith. From west to east, these include the Late Cretaceous of the Peninsular Ranges batholith that underthrust the central/ forearc deposits of the Rosario Formation, the Early Cretaceous transitional to eastern zone of the Peninsular Ranges batholith arc volcanics/volcaniclastics of the Santiago Peak volcanics Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 671

and Alisitos Formation, Late Triassic through Jurassic turbidite Cretaceous intrusives within the Alisitos arc. These geochemical sequences of the Bedford Canyon complex, and deep- and shallow- data, and U/Pb zircon geochronology studies of these intrusives water portions of the early Paleozoic passive-margin assem- (e.g., Valencia et al., 2006) suggest that the basement to the Alisi- blages (see fi gure 1 in Schmidt et al., this volume; Gastil, 1993; tos arc is oceanic crust that lacks any clear interaction or proxim- Wetmore et al., 2003). The bedrock lithologies of the Peninsular ity to the North American continental margin prior to their col- Ranges batholith have been traditionally subdivided into western, lision in the latest Early Cretaceous (e.g., Alsleben et al., 2012). central or transitional, and eastern zones. That is, the Santiago The collapse of the Alisitos arc along the southwest margin Peak volcanics, and Alisitos and Rosario Formations comprise of North America during the latest Early Cretaceous appears to the western zone, the Bedford Canyon complex comprises the have dramatically affected both the magmatism and the geometry central/transitional zone, and the passive-margin assemblage of the continental margin south of the ancestral Agua Blanca fault. comprises the eastern zone. Similarly, intrusive rock petrologies/ Collision of the Alisitos arc occurred almost simultaneously with mineralogies and elemental/isotopic chemistries have often been the end of Santiago Peak arc magmatism, when suites of late syn- used to divide the Peninsular Ranges batholith into western and to postcollisional intrusive bodies were emplaced almost exclu- eastern zones (e.g., Taylor and Silver, 1978; Silver et al., 1979). sively within the Alisitos arc (Wetmore et al., 2005; Schmidt et During the early Mesozoic, the southwestern margin (pres- al., this volume). The collision resulted in the juxtaposition of ent-day coordinates) of North America was characterized by the the Alisitos arc with the Santiago Peak arc to the north across the eastward subduction of oceanic lithosphere of the Farallon plate ancestral Agua Blanca fault and with the central/transitional zone (e.g., Engebretson et al., 1985). The basement geology of the of the Peninsular Ranges batholith to the east along the Main continental margin at this point was largely defi ned by the Paleo- Mártir thrust. The Main Mártir thrust juxtaposes the Alisitos zoic passive-margin sequences, which may have been truncated arc against high-grade (amphibolite) metamorphic assemblages and transported southward in the late Pennsylvanian–Permian (e.g., Kopf et al., 2000; Rothstein and Manning, 2003; Melis, (e.g., Dickinson and Lawton, 2001). The associated arc during 2006), having protoliths from both the southern continuation of this phase was much further east (e.g., Coney and Reynolds, the Bedford Canyon complex and intrusives yielding ages that 1977) than the ultimate location of the future Cretaceous arc are the same as those of the Santiago Peak arc (e.g., Johnson et (i.e., the Peninsular Ranges batholith). However, the continen- al., 1999a; Wetmore et al., 2005; Melis, 2006; Schmidt et al., this tal margin was experiencing lateral growth through the accretion volume). Wetmore et al. (2005) argued that these observations of sediments largely derived from the continent. These accreted suggest that the majority of the Santiago Peak volcanics, which assemblages are represented by the Late Triassic through Jurassic appear to have continued along the continental margin south of Bedford Canyon complex located along the central/transitional the ancestral Agua Blanca fault, was tectonically eroded (i.e., zone of the Peninsular Ranges batholith (Gastil, 1993; Wetmore subducted) as a result of the collision in a manner similar to that et al., 2003). currently observed along the Middle America Trench in Costa Continued accretion and lateral growth of the continent Rica (e.g., Fisher et al., 2004). contributed to the westward migration of the arc during the Late Jurassic into the Early Cretaceous, when the arc began to Deformation of the Alisitos Arc intrude through and erupt onto the earlier-formed accretionary prism. This portion of the arc is preserved as the Santiago The remains of the Alisitos arc correspond to the Alisitos Peak arc, located in the western zone of the Peninsular Ranges Formation and entrained intrusions (Fig. 1) located between the batholith north of the ancestral Agua Blanca fault (aABF, Pacifi c coast and the Main Mártir thrust, which juxtaposes it with Fig. 1). Some intrusive rocks of this phase of magmatism are the intensely deformed and metamorphosed strata and intrusive also observed in the hanging wall of the east-dipping, west- rocks of the central/transitional zone of the Peninsular Ranges vergent Main Mártir thrust in the southern portion of the Pen- batholith to the east (Johnson et al., 1999a; Schmidt and Pater- insular Ranges batholith (Johnson et al., 1999a; Wetmore et al., son, 2002; Schmidt et al., this volume). To the north, rocks of the 2005; Schmidt et al., this volume). Alisitos arc are juxtaposed across the ancestral Agua Blanca fault The portion of the western zone south of the ancestral Agua with the Early Cretaceous Santiago Peak arc, a continental mar- Blanca fault is represented by the Alisitos arc, which has been gin arc built upon and through the Late Triassic through Jurassic interpreted to have been a fringing (Gastil et al., 1975, 1981) or North American accretionary prism (Wetmore et al., 2003). The exotic island arc (Johnson et al., 1999a; Wetmore et al., 2002, southern limit to the Alisitos arc is not known because it is buried 2003; Alsleben et al., 2008) that collapsed against the southwest- beneath Cenozoic volcanics and marine sediments; the southern- ern margin of North America during the latest Early Cretaceous. most exposures are the Middle Jurassic intrusions and hosting The true basement of the Alisitos arc is not known to be exposed, volcanic country rocks at El Arco (Weber and López-Martínez, but Late Jurassic intrusions, within the southernmost exposure of 2006; Valencia et al., 2006). the Alisitos at El Arco, located near the Baja Norte–Sur state line, The structural geology of the Alisitos arc is fundamentally yield island-arc elemental and isotopic compositions (Weber defi ned by a fold-and-thrust belt that mantles the northeastern and and López Martínez, 2006) similar to those observed from Early eastern sides of the arc terrane and that formed in response to the Downloaded from memoirs.gsapubs.org on March 31, 2014

672 Wetmore et al.

Legend SD U.S. Eastern PRb strata Tj Mexico M Western PRb (SPV) Western PRb (Alisitos) ancestral Agua Blanca and Main Mártir thrust faults 32° E Normal faults aABF Strike-slip faults ABF Northern ST Generalized contact between eastern and western strata SV SF Traces of upright folds 31° Ct MMT Steeply plunging folds Cl Central E Cities and towns SQ San Marcos dike swarm

Pacific Sea of Cortez-Gulf of California

O ER 30° cean

Southern C SC

29°N

EA Baja California Sur

050Km 116°W 115° 114° 113°

Figure 1. Partially schematic geologic map of central Baja California, Mexico, compiled based on maps produced by Gastil (1993), Johnson et al. (1999a), Schmidt and Paterson (2002), Wetmore et al. (2005), Alsleben et al. (2008), and regional mapping conducted by Paul Wetmore and Helge Alsleben. Abbreviations for cities are: SD—San Diego, Tj—Tijuana, M—Mexicali, E—Ensenada, ST—Santo Tomas, SV—San Vicente, Ct—Colonet, Cl—Camalu, SQ—San Quintin, SF—San Felipe, ER—El Rosario, C—Cataviña, SC—Sierra Calamajue, and EA—El Arco. Other abbreviations: aABF—ancestral Agua Blanca fault; MMT—Main Mártir thrust; PRb—Peninsular Ranges batholith; SPV—Santiago Peak volcanics. Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 673 latest Early Cretaceous collapse of the arc along the southwest- Mártir thrust juxtaposes the eastern portion of the Alisitos arc, ern margin of North America. The fold-and-thrust belt exhibits which exhibits no greater than greenschist-facies metamorphism variations along strike with respect to the relative position, the and Al-in-hornblende pressures of ~2 kbar (Schmidt and Pater- total amount of shortening due to both folding and ductile strains, son, 2002), with the westernmost portion of the transitional zone the structural throw across terrane-bounding structures, and the of the Peninsular Ranges batholith, which is characterized by style of contraction (Alsleben et al., 2008). Here, the deformation amphibolite-faces metamorphism and pressures as great as 6.4 of the Alisitos arc is described with respect to the northern (San kbar (Schmidt and Paterson, 2002). Vicente), central (Sierra San Pedro Mártir), and southern (Cal- The southern Alisitos arc is located between 30°40′N and amajue) regions of the arc as illustrated in Figure 1. 29°10′N (Fig. 1). The fold-and-thrust belt in this area is largely The northern Alisitos arc includes the area from the ancestral buried beneath a veneer of Late Cretaceous sediments (i.e., the Agua Blanca fault south to ~31°10′N, or ~10 km north of the Rosario Formation) and Pliocene lava ﬂ ows, except for that por- town of Colonet (Fig. 1). The fold-and-thrust belt in this portion within a few kilometers of the Main Mártir thrust (locally tion extends southwest from the ancestral Agua Blanca fault for known as the El Toro fault; Alsleben et al., 2008). The trend of more than 30 km to the Paciﬁ c coastline and trends at between the belt in this area is between N40–60°W, and, for the exposed N50–60°W. Deformation within this portion of the Alisitos arc section, the magnitude of horizontal shortening due to folding is dominated by short-wavelength and high-amplitude folding, and ductile strain is calculated to be 0.6 km and 0.8 km of crustal and ductile strain (10%–75% shortening in the z direction). Folds thickening, respectively. As a consequence of the limited expo- are increasingly tight, changing from gentle, upright folds in the sure, these values of shortening likely underestimate the actual southwest to southwest-vergent isoclinal folds within 5 km of magnitude greatly. However, based on a few scattered exposures the ancestral Agua Blanca fault. Horizontal shortening associ- of basement observable in satellite imagery of the area to the west ated with folding and ductile strain is calculated to be as great as of the Sierra Calamajue (Fig. 1), it appears that the actual width 10.2 km in the San Vicente portion, which Alsleben et al. (2008) of the fold-and-thrust belt may be comparable to that observed in calculated to have produced as much as 1.4 km of crustal thick- the northern areas described earlier herein. Faulting in the Sierra ening. Shortening due to thrusting is estimated to be less than a Calamajue area is much more complicated than that observed few kilometers, and vertical thickening is also assumed to be less in the two northern portions of the Alisitos arc. There, the main than ~5 km based on equivalent greenschist metamorphic facies boundary fault, referred to as the El Toro fault (Alsleben et al., on both sides of the ancestral Agua Blanca fault (Wetmore et al., 2008), between the Alisitos arc and the older Mesozoic turbidite 2005; Alsleben et al., 2008). sequences (i.e., the Bedford Canyon complex; Schmidt et al., this The central Alisitos arc includes the area immediately north volume) represents a modest jump from sub-greenschist-facies of the town of Colonet (i.e., south of 31°10′N) to 30°20′N, or rocks west of the fault to greenschist-facies rocks east of the ~30 km north of the town of El Rosario (Fig. 1). The fold-and- fault. Approximately 3 km further east, the metamorphic grade thrust belt in this portion of the Alisitos arc trends at N20–40°W, increases to lower-amphibolite facies across the east-dipping El and extends west from the Main Mártir thrust to the coastal plain, Molino fault (Alsleben et al., 2008). Alsleben et al. suggested that where it is covered by Late Cretaceous to Quaternary deposits. the total amount of vertical displacement across the combined El Folding and ductile strains throughout much of this part of the Toro and El Molino faults is less than ~6 km. Alisitos arc appear to be similar to those observed within the In summary, the deformation within the Alisitos arc associ- northern portion with respect to scale and magnitude. However, ated with its collision with the North American continental mar- within ~3 km of the Main Mártir thrust, folding and shear strain gin exhibits extreme variations along the length of the terrane. appear to be much more intense, as the folds there plunge paral- In the northern and southern portions, deformation appears to be lel to the shear direction and are commonly transposed within dominated by folding and minor vertical offset (3–5 km) of thrust the mylonites of the Main Mártir thrust (Johnson et al., 1999a; faults. As a result, crustal thickening along the eastern side of Schmidt and Paterson, 2002; Alsleben et al., 2008; mapping by the Alisitos arc in these two areas is estimated to be between 6 Wetmore and Alsleben, 2004, 2005, personal observations). The and 10 km. The deformation with the central portion of the Alisi- total magnitude of folding and ductile strain–related shortening tos arc appears to have resulted in much greater crustal thick- is calculated to be at least 3 km, with attendant crustal thickening, due primarily to the large amount of heave on the Main ening of ~2 km (Alsleben et al., 2008). However, these values Mártir thrust. While thickening due to folding and ductile strain clearly underestimate the total magnitudes due to uncertainties (~2.1 km) is only slightly higher than that observed in the north- within the region proximal to the Main Mártir thrust and the lim- ern and southern portions, the difference in calculated pressures ited scope of bedrock mapping completed in this portion of the across the Main Mártir thrust suggests as much as 14–16 km of Alisitos arc. Although shortening may ultimately be of a simi- additional crustal thickening within the eastern part of the central lar magnitude to that observed in the northern area, it is clear Alisitos arc. This amount of thickening is a maximum, however, that thrust faulting, particularly along the Main Mártir thrust, is as syncontractional erosion of the hanging wall may have been more prominent within the central Alisitos arc. In addition to a substantial during this phase of the collision, as it was during the kilometer-wide zone of mylonitization (Melis, 2006), the Main Late Cretaceous (e.g., Kimbrough et al., 2001). Downloaded from memoirs.gsapubs.org on March 31, 2014

674 Wetmore et al.

Geochemistry of the Latest Early Cretaceous Intrusions of stration that host rocks were folded prior to emplacement the Alisitos Arc (Dos Tortas, El Tigre, and San Jacinto plutons). Folding is constrained to have been largely completed by the time the Latest Early Cretaceous magmatism related to the Alisitos Balbuena pluton was emplaced at ca.108 Ma, and the majority arc lasted at least from 118.4 ± 2.3 Ma (Wetmore et al., 2005; of subsequent contractional deformation was focused onto the Schmidt et al., this volume) until 102.5 Ma (Johnson et al., 2003) ancestral Agua Blanca fault and Main Mártir thrust (Wetmore based on the ages of volcanic and plutonic rocks, respectively. et al., 2005; Melis, 2006; Schmidt et al., this volume). The An earlier phase of magmatism (i.e., beyond the known range northern Alisitos intrusions are generally much larger, having from ca. 118 to ca. 102 Ma) is probable due to the observation exposure areas between 6 and 100 km2, and compositions that of excessively large crustal thicknesses for an island arc, which range from gabbros to tonalities; the latter composition com- are 35 ± 4 km (Lewis et al., 2001), even in regions away from prises more than 90% of nearly all mapped postcontractional known collision-related thickening. Limited knowledge of the intrusions (e.g., Tate and Johnson, 2000; Johnson et al., 2003; early history for the Alisitos arc notwithstanding, the available Wetmore et al., 2005). ages and structural constraints demonstrate that the majority of Geochemistry, including U/Pb geochronology, provided studied intrusions within the Alisitos arc are either precollisional in Tables 1 and 2, and discussed in the following sections, (emplaced immediately before collision), emplaced between derives from other studies (i.e., Tate et al., 1999; Tate and 118 Ma and ca. 110 Ma, or syn- to postcollisional, emplaced Johnson, 2000; Wetmore, 2003; Wetmore et al., 2005 [see between ca. 108 Ma and ca. 102 Ma (Johnson et al., 1999a, also Miller and Hughes, 2009]) and some additional data for 1999b, 2003; Tate and Johnson, 2000; Wetmore et al., 2005). the central Alisitos arc provided by Scott Johnson (University In the remainder of this report, we will refer to the late syn- to of Maine, 2010, written commun.); the latter were produced postcollisional intrusions simply as postcollisional. Geochemical together with those reported in Tate et al. (1999) and Tate and data for the Alisitos arc are essentially limited to those from the Johnson (2000). Readers are referred to those publications for northern and central areas, but they include some additional data descriptions of the methods used in producing these data. from Middle and Late Jurassic intrusions from the Sierra Cal- The geochemical data available from the Early Cretaceous amajue and the El Arco areas (ca. 144 Ma and 165 Ma, respec- intrusions of the Alisitos arc are quite limited in number, and, tively). In this section, we will focus only on the latest Early therefore, some of the observations that follow may need revision Cretaceous precollisional and postcollisional intrusions from the as additional data from a wider distribution, and more diverse northern and central Alisitos arc. suites of intrusions become available. This notwithstanding, Only a handful of precollisional intrusions associated with there are several observations made from the available data that the latest Early Cretaceous arc phase of magmatism within the we feel warrant review. Alisitos arc have been identiﬁ ed and studied to date. These Isotopic data for samples from all age (i.e., pre- and postcol- include the Arce pluton (118.4 ± 2.3 Ma U/Pb zircon) of the lisional) and regional (northern and central) groups of the Alisitos ε northern area, and the Ensinosa (110.4 Ma U/Pb zircon; Chávez- arc exhibit very similar primitive compositions, with Nd(100) rang- 87 86 Cabello, 1998), Zarza (115 ± 1 Ma U/Pb zircon; Tate et al., ing from 4.85 to 7.13, Sr/ Sr(100) ranging from 0.7022 to 0.7043, 1999), and Burro (114 ± 1 Ma U/Pb zircon; Tate and Johnson, and δ18O ranging from 7.7‰ to 8.9‰ (Table 2). In contrast to the 2000) intrusions of the central area. All four are relatively small isotopic data, pluton chemistry exhibits some systematic varia- intrusions (~1–10 km2) characterized by compositions that range tions between the groups. In general, samples from the precol- from gabbro to tonalite, but include some minor trondhjemite in lisional and postcollisional intrusions of the northern Alisitos the Zarza intrusive suite (Tate et al., 1999). arc and samples from the precollisional intrusions of the central Somewhat more postcollisional intrusions have been iden- area tend to have overlapping ranges and closely corresponding tiﬁ ed and studied from the northern and central portions of the mean values. These values are typically both higher or both lower Alisitos arc. These include the Balbuena (107.7 ± 3.6 Ma U/ than those samples from the postcollisional intrusions of the cen-

Pb zircon), Dos Tortas (undated), El Tigre (undated), Giganté tral Alisitos arc. For example, the range of SiO2 spans nearly (undated, but younger than the Piedra Rodada), Los Alamos 25 wt% for samples from both groups of northern intrusions and (undated, but younger than the Balbuena), Piedra Rodada the central area’s precollisional intrusions, whereas the range for (105.5 ± 3.4 Ma U/Pb zircon), San Jacinto (undated), and postcollisional samples from the central Alisitos arc intrusions

San Vicente (105.0 ± 3.4 Ma U/Pb zircon) plutons from the varies by only ~10 wt% (i.e., 60–70 wt% SiO2). Al2O3 and Na2O northern Alisitos arc, and the Potrero (102.5 ± 1.6 U/Pb zir- in samples from all the northern area intrusions and from the con; Johnson et al., 2003) and the San José (105.4 ± 1.1 Ma precollisional intrusions of the central area are on average lower U/Pb zircon; Johnson et al., 2003) plutons of the central Alis- than those from the postcollisional intrusions from the central itos arc. The assessment of timing of emplacement relative zone, while TiO2, Fe2O3, MnO, and K2O are all lower on average to collision for those intrusions that lack U/Pb zircon ages is in samples from the latter suite of intrusions. Similarly, average determined through contact relations with dated intrusions concentrations of several trace elements, including some of the (Giganté and Los Alamos plutons), and through the demon- rare earth elements (REEs), exhibit pronounced variations, such Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges) 675 39.3 56 ( Continued El Trigo Northern Postcollisional 0.83 0.3 0.43 1.36 2.32 2.51 0.38 0.19 0.2 1.42 0.968.310.182.21 5.46 0.94 0.245.28 1.754.36 5.34 2.77 0.25 4.72 1.63 2.65 5.37 35.4 39 26.6 18.5 18.1 31 51 61.1 66.8 66.4 15.5 14.9 14.7 13.6 15.3 15.5 224 360 155 345 480 535 160 185 140 Northern Dos Tortas Postcollisional 4 5 18 7.6 19 28.9 0.342.31 0.317 0.76 2.87 7.17 32.5 31.22 32 31 42 59 9 2 5 BDL 2 144 Northern Balbuena Postcollisional TABLE 1. ALISITOS ARC GEOCHEMISTRY ALISITOS 1. TABLE 6.9 22.8 9 7.4 0.1 0.1 0.21 0.11 0.08 0.08 0.12 6.17 10.4 15.19 14.3 12.4 15.03 11.2 33 22 17 41 165 120 114 140 185 161 165 470 238 267 525 495 448 355 310 245 295 165 175 155 160 0.53 0.72 1.2 0.8 0.51 0.53 1.7 2 7.8 42 2 9 20 Arce Northern Precollisional 0.56 0.5 1.97 2.64 1.67 1.04 0.81 0.72 1.97 1.84 1.65 1.93 0.54 0.4313.18 2.69 0.385.07 2.58 0.49 4.170.12 0.84 3.5 0.09 3.96 1.067 7.29 0.1 0.4 4 8.39 2.4 3.94 3.6 4.22 4.45 4.64 3.21 0.050.55 0.0552.64 0.62 2.56 0.06 0.67 2.65 0.05 0.97 0.16 3.57 3.03 0.173 6.38 3.12 0.05 6.13 0.71 0.07 2.92 0.55 0.081 2.66 0.14 0.52 2.5 2.43 5.83 A 7/3/01-J GW-01 GW-02 GW-09 6/9/01-F GW-03 GW-07 6/9/01-B 1/18/04-1 1/19/04-A 1/19/04-B 1/19/04-C 44.3 55.57 34.7 14.5 25.3 32.7 33 13.2 15 34 30 71.6 71.614.4 13.6 74.2 13.8 71.8 14.5 61.6 58.2 15.9 72.9 16.6 14.4 73.5 14.2 73 13.9 64.1 14.3 14.6 17.61 13.5 215 274 170 600 715 460 100.05 99.15 100.05 100.05 100.05 99.02 100.05 100.05 100.22 100.05 100.05 100.05 100.05 170 177 320 1/17/04- * 3 3 5 2 2 O O 2 O 2 O O 2 2 2 Area: Timing: Intrusion: Sample: SiO P Zr Nb Cs Ba La Ce Total Be Sc Al MnO MgO CaO Na K V Rb Sr Y5 TiO Fe Downloaded from memoirs.gsapubs.org on March 31, 2014

676 Wetmore et al. ) 1.2 1.1 8.9 ( Continued El Trigo Northern Postcollisional 0.6 2 6.82 6.12 6.12 0.37 0.433.03 5.56 0.46 5.22 1.911.29 1.16 1.1 1.25 6.63 9.284.5 6.18 8.2 0.85 0.97 0.9 4.340.59 4.06 0.63 4.2 0.6 5.73 7.04 7.88 3.23.13 2.030.84 3.772.41 0.62 2.14 2.54 3.69 0.66 2.7 14.6 10.3 11 27.6 24.5 28.3 25.37 31.37 34.52 118 72 107 5.92 17.2 17.1 Northern Dos Tortas Postcollisional ) 1.7 3.48 0.3 5.5 6.4 0.84 3.8 4 2.55 0.41 0.99 0.8 6.47 8 15 0.5 1.2 2 1.6 4.3 0.44 0.45 0.72 1 42 71 82 3.2 6.1 1.04 3.97 4.8 5.17 3.11 0.47 0.1 6 2.2 3 BDL 18 10.32 Northern Balbuena Postcollisional TABLE 1. ALISITOS ARC GEOCHEMISTRY ( Continued ARC GEOCHEMISTRY ALISITOS 1. TABLE 0.23 0.5 34.5 0.8 2.2 1.11 0.99 1.54 0.96 0.94 1.016.6 1.05 15.5 0.27 0.34 0.44 0.4 1.27 0.86 0.98 0.78 0.78 0.83 0.54 73 63 93.4 61 0.45 0.341.3 3.2 0.27 0.3 0.37 0.36 0.4 0.29 5.65 2.3 2.2 2 5.62 4.41 4 5.19 5.83 3.045.2 3.5 4.81 5.44 4.8 5.65 1.75 7.01 6.99 8.16 5.29 0.8 0.2 1.7 3.7 3.42 13 10.8 17 Arce Northern Precollisional 0.94 0.95 1.05 1.02 0.84 0.93 1 1.21 1.33 0.86 0.454.93 0.66 7 0.87 0.69 0.69 0.66 1.36 6.11.64 6.94 1.5 4.394.87 2.2 5.437.350.62 8 3.39 0.8 3.541.4 4.633.1 4 1.66 2.3 3.85 2.15 3.7 2.97 3.8 2.72 6.11 3.13 2.89 5.3 0.69 0.84 0.5 2.18 2.18 2.79 2.38 3.2 3.6 3.68 2.93 3.48 1.61 30.8 3.24 0.65 3.81 0.7 3.72 4.84 5.11 5.26 3.96 5.19 2.11 A 7/3/01-J GW-01 GW-02 GW-09 6/9/01-F GW-03 GW-07 6/9/01-B 1/18/04-1 1/19/04-A 1/19/04-B 1/19/04-C 27.5 28.8 20.4 11.4 16.4 17.29 19.6 19.7 16.36 23.1 48 80.4 61 41.1 40.61 34.07 76.18 22.88 17.59 36.71 39.92 29.81 31.7 1/17/04- O 7.04 6.81 5.62 5.04 4.76 4.32 6.19 6.29 6.29 5.14 O 2.58 1.58 2.38 3.86 4.84 5 2.14 2.42 2.81 1.67 2 2 N O/K 2 O+Na 2 Pr Nd Tb Pb Th W Ga A/CNK Sm Eu Gd Dy Ho Er Tm Yb Lu Hf Ta Tl U Cr Co Ni Zn Ge Sn K Area: Timing: Intrusion: Sample: Na Sr/Y La/Yb Ba/La Eu/Eu La/Lu ∑ REE 100.91 118.02 78.9 37.76 59.62 75.62 75.52 73.5 71.42 80.84 91.55 91.87 96.37 Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular) Ranges 677 ( Continued 3.99 4.79 Northern San Jacinto Postcollisional 1.3 1.1 2.1 1.25 0.78 0.79 9.3 9.8 8.9 30.7 16.7 25.3 53.3 22.1 36 67 66 114 2.31 2.22 2.3 2.57 3.97 4.16 4.49 3.78 3.55 3.63 3.1 2.7 3.28 3.34 2.9 2.68 0.12 0.14 0.09 0.1 1.6 1.1 9.6 3.73 5 0.06 0.06 0.06 0.04 0.47 0.52 0.45 0.4 0.940.83 1.08 0.72 1.04 0.81 0.97 0.74 7.32 22.3 7.84 17.4 70.3 69.6 70.8 73 13 21 14.9 15.2 15.1 14 77 100.05 100.05 100.05 100.05 585 595 570 615 250160 290 160 195 165 190 160 7 2 5.2 5.62 1.17 5 5.43 33 57 503 209 159 100 1.16 1.25 Piedra Rodada 29.3 34.6 23.88 15.4 15.6 15.4 89 65 ) 1.73 2.56 1.85 1.88 3.55 3.36 3.35 3.51 5.37 3.99 4.68 4.68 5.822.42 4.54 1.86 6.04 2.37 6.07 2.3 0.12 0.1 0.14 0.14 0.93 1.2 0.94 1.1 0.68 0.63 0.72 0.68 1.13 1.08 0.79 0.94 0.96 5.32 4.72 6.17 5.32 0.12 0.09 0.12 0.11 64.2 67.5 65.2 62.9 19.2 13 19.4 21.5 26.2 33.6 22.16 16 13 34.1 69.5 77.5 48.11 50 420 640 445 210 220 205 140 140 175 Postcollisional Northern 0.36 0.37 105 115 Los Alamos TABLE 1. ALISITOS ARC GEOCHEMISTRY ( Continued ARC GEOCHEMISTRY ALISITOS 1. TABLE 3.3 11.99 4.38 8.93 1.07 5.6 1.67 4.47 1.89 0.99 1.59 0.64 4.06 2.34 4.14 3.31 3.36 9.85 4.46 7.62 0.45 1.11 0.52 1.01 0.3 0.06 0.13 0.14 1.090.84 0.67 0.48 1.08 0.87 0.98 0.64 0.51 4.72 1.69 5.1 3.09 0.08 0.22 0.1 0.19 8.8 29.1 13.4 25.6 70.615 48.6 67.2 19.3 56.7 15.9 17 39 8.8 39 19.7 14.4 3.83 15.7 7.5 20.2 5.5 22.6 13.4 42 30 32 13 100.05 100.05 100.05 100.05 100.05 100.05 100.05 98.7 500 157 395 233 185125 280 200 275 1/18/04-A 1/18/04-B 1/18/04-C 1/18/04-D 1/17/04-B 1/17/04-C 1/17/04-D 7/1/01-F 1/16/04-A1 1/16/04-A2 1/16/ 04-B 1/16/04-C Northern Postcollisional 21 18.1 22 Gigante Northern Postcollisional 0.67 0.98 0.11 0.17 1.170.88 1.19 0.82 0.21 0.7 4.53 3.92 0.12.24 0.14 3.19 0.64 0.98 3.86 3.54 9.96 10.3 6.01 6.92 64.516.6 58.5 17.9 18 26.319 25.8 18 320 425 300155 320 180 * 5.34 7.77 3 3 5 2 2 O O 2 O 2 O O 2 2 2 Timing: Intrusion: Sample:SiO 1/18/04-E 1/18/04-F Area: P K TiO Fe Al Total 100.05 100.05 Be Sc Eu Gd Tb Dy Ho La Cs Ba Ce Pr Nd Sm Yb Nb Sr Zr Rb V MnO MgO CaO Na Downloaded from memoirs.gsapubs.org on March 31, 2014

678 Wetmore et al. ) ( Continued Northern San Jacinto Postcollisional 8.2 6.2 6 2.2 0.7 0.9 10.5 3.2 5 59 52 34 3.53 3.15 3.63 3.74 6.28 6.38 6.8 6.35 0.51 0.45 0.51 0.53 1.712.07 1.87 7.08 1.95 2.16 1.47 4.65 0.96 0.96 0.97 1 0.54 0.62 0.49 0.54 4.87 5.33 6.07 5.5 9.16 10.4 7.15 6.37 7.6 1.5 0.74 0.72 0.78 0.67 1.49 5.2 1.61 3.43 9 50.86 116.69 56.61 89.47 79.92 26.68 72.7 35.34 58 5.39 1.8 1.2 2.6 5.3 3.6 0.56 0.3 7 6.41 17 85.6 Piedra Rodada 2.2 1.5 9.1 11.9 78 76 ) 3.75 3.52 4.28 3.61 5.28 5.92 5.2 0.52 0.46 0.55 0.54 3.47 8.32 8.08 6.62 2.06 1.31 1.81 1.87 0.92 0.99 0.97 0.94 0.38 0.44 0.37 0.4 5.47 5.45 5.75 5.2 5.46 15.7 15.1 12 1.2 0.66 0.69 0.63 0.72 2.58 6.6 6.48 4.58 Continued 15.2 12.5 14.5 80.4 135.49 158.8 105.84 14.6 32.31 21.84 12.86 21.04 63 Postcollisional Northern Los Alamos TABLE 1. ALISITOS ARC GEOCHEMISTRY ( ARC GEOCHEMISTRY ALISITOS 1. TABLE 1.01 0.84 0.95 0.85 3.725.81 1.29 1.36 3.65 4.65 2.14 2.82 6.24 0.26 5.26 1.11 7.1 38.1 9.8 23.9 0.53 0.22 0.5 0.34 3.87 2.97 4.3 3.49 2.14 2.35 2.6 5.19 5.95 3.33 5.74 3.94 0.54 0.04 0.34 0.2 1.46 0.4 9.3 1.6 15.1 0.2 45 0.73 0.98 0.68 0.9 2.84 1.84 3.27 2.3 59 33 93 85 84.5 22.54 88.5 47.75 34.72 40.99 25.16 31.23 1/18/04-A 1/18/04-B 1/18/04-C 1/18/04-D 1/17/04-B 1/17/04-C 1/17/04-D 7/1/01-F 1/16/04-A1 1/16/04-A2 1/16/ 04-B 1/16/04-C Northern Postcollisional 78 13.4 Gigante Northern Postcollisional 2.84.69 2.43 4.25 0.92 2.14 0.40.21 0.32 0.2 0.5 0.3 0.78 0.87 2.57 3.31 3.56 4.24 77 26 13.7 19.2 O 4.51 4.52 O 5.99 3.62 2 2 N O/K 2 O+Na 2 Ni Zn Ge Sn W Ga A/CNK 0.92 0.92 K Sample:Er 1/18/04-E Tm Yb 1/18/04-F Lu Hf Ta Area: Timing: Intrusion: Tl Pb Th U Cr Co ∑ REE 65.04 62.88 Sr/Y La/Yb Ba/La 32.13 41.26 Eu/Eu La/Lu Na Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 679 ) 9 3 Continued ( 6 22 5 86 3.060.77 1.33 0.07 1.94 0.45 0.34 0.1 0.2 0.05 0.03 0.05 2.43 0.78 1.49 5.670.85 4.28 0.42 5.86 0.64 3.73 3.16 3.39 2.91 5.31 4.18 0.09 0.03 0.03 6.57 5.69 7.04 0.89 0.72 1.06 94 6 45.6 34.6 58.91 99.26 99.57 99.96 24.1 17.3 26.05 71.613.8 75.8 12.6 74.5 13.1 18.5 14.5 24.58 36 67 88 82 130 55 72 173 83 135 667 560 583 Zarza Central Precollisional 5.09 1.46 0.68 0.1 3.98 7.58 1.35 4.52 1.6 0.17 1.42 8.79 5 7 3 35 27.5 99.84 22.4 10.3 66.6 15.1 59 169 176 423 2.34 4.98 1.02 0.08 7.06 5.33 1.6 5.34 0.58 7 0.19 7.65 1.16 2 5 3 62.3 15.5 303 143 125 125 34 ) 2 10 3 1 7.94 10.41 02 53 62 81 Central Ensinosa Precollisional TABLE 1. ALISITOS ARC GEOCHEMISTRY ( Continued ARC GEOCHEMISTRY ALISITOS 1. TABLE 2 2 2 4 22 3 53 4.31 3.97 4.17 3.86 2.8 3.82 5.33 3.17 2 43 39 78 39 27 22 3 114 70 152 195 188 126 182 214 157346 176 521 128 313 90 187 46 138 55 51 100 48 44 126 209 193 219 288 232 400 389 221 0.76 1.82 2.6 1.58 1.15 0.76 0.18 0.24 0.56 0.53 0.8 0.520.13 0.88 0.13 0.97 0.68 0.13 1.21 0.13 0.19 1.18 0.79 0.11 0.44 0.21 0.13 0.1122.265.57 0.14 2.04 4.93 0.07 1.48 3.3 0.19 2.83 0.2 5.9 3.59 0.16 7.4 6.08 0.07 10.96 3.54 13 0.11 3.29 0.19 4.94 2.09 6.99 5.92 6.31 5.84 3.63 5.92 6.15 4.27 3 0.36 1.03 0.83 0.95 0.92 0.55 0.89 0.97 0.63 0.7 8.44 17.8 16.3 18.9 14.1 10.2 17.4 15.8 9.79 17.6 3.7 2 6 2.170.79 5.56 1.2 4.7 1.03 5.35 1.25 4.43 1.2 2.67 0.88 4.82 1.71 4.88 1.73 3.01 0.8 7.38 11.2 10.1 10.5 7.66 5.58 10.8 8.76 5.13 10.63 2.11 23 1.94 89 18 62.716.7 63.4 15.7 66.5 14.9 60.4 16.3 55.3 17.8 51.4 18.1 57.3 17.7 56.1 52.4 16.2 17.8 65 15.62 27.2 24.5 26.8 20.2 14.9 27.2 21.9 13.37 26.9 278 327 * 5.64 6.17 5.57 7.26 8.7 7.6 2.53 8.8 P8.53 3 3 5 2 2 O O 2 O 2 O O 2 2 2 Na MnO MgO CaO K P TiO Fe TotalBe Sc 99.55V Rb 100.39 100.27 100.64 100.05 99.93 100.49 100.08 100.25 99.99 Al Dy Nb Sm Eu Gd Tb Y1 Zr Cs Ba La Ce Pr Nd Sr Timing: Postcollisional Area:Intrusion: San Vicente Northern Sample:SiO 1/12/03-A EP-1 EP-2 EP-3 EP-4 EP-5 EP-6 EP-7 Gabbro Cone sheets Pink tonalite Pink tonalite Pink tonalite Trondhjemite Downloaded from memoirs.gsapubs.org on March 31, 2014

680 Wetmore et al. ) 5 4 2 ( Continued 6 7 1.28 0.6 0.81 0.52 0.35 0.38 6.64 8.47 7.57 4.42 5.9 6.61 3.33 1.15 1.14 4.19 2.46 3.72 0.68 1.02 1.05 1.4 1.5 1.68 1.01 1.02 1.02 2.82 1.48 2.43 7 8 13 14 15 36.05 38.62 23.72 12 20 25 191 222 Zarza Central Precollisional 2.83 0.58 6.12 6.17 2.96 1.42 0.98 2.12 0.92 0.93 5 3 8 14 48.12 76.19 100.71 78.74 124.76 183 9.21 0.86 5.92 4.25 7.21 2.5 0.66 1.63 0.7 1.67 3 3 17 10 178 ) 42 7 2 5.57 3.73 9 29 Central Ensinosa Precollisional TABLE 1. ALISITOS ARC GEOCHEMISTRY ( Continued ARC GEOCHEMISTRY ALISITOS 1. TABLE 71 3 2 6 0 3 3 3 5 8 13 14 48 15 0.56 0.62 0.5 0.42 0.28 0.36 0.48 0.35 5 4 13 12 15 17 13 20 20 15 11 144 164 189 124 352 159 66 298 1 6 BDL . . 4 2.2 0.42 1.48 1.27 1.34 1.19 0.73 1.19 1.25 0.9 3.74 2.08 1.69 2.18 1.89 2.07 3.11 1.89 1.52 1.25 0.2 1.310.2 0.2 3.97 2 3.94 3.59 2.78 1.91 2.54 3.27 2.35 1.3 3.8 19 27.23 6.15 5.51 6.84 10.29 14.5 16 12.97 9.61 78.6 O 4.46 6.13 6.57 5.75 5.01 3.56 4 O 4.87 2.37 1.53 2.64 3.36 3.68 21.22 22.21 5.66 2 2 N O/K 2 O+Na 2 ∑ REE 36.27 68.45 62.02 67.84 51.71 36.97 65.72 57.79 35.43 68.13 Sr/Y La/YbBa/LaEu/EuLa/Lu 5.65 37.69 1.11 2.87 30.89 0.68 51.58 2.56 0.7 29.81 2.92 24.41 0.74 2.76 24.73 0.8 2.92 4.72 4.25 0.93 5.48 1.01 24.56 2.68 2.18 0.99 11.76 0.79 Na Er Tm Yb Lu Hf Ta Tl Pb Th U0 Zn Ge Sn W1 Co Ni Sample:Ho 1/12/03-A EP-1 EP-2 EP-3 EP-4 EP-5 EP-6A/CNK EP-7K Gabbro 0.98 Cone sheets Pink tonalite 0.87 Pink tonalite Pink tonalite Trondhjemite 0.97 0.84 0.85 0.71 0.59 0.69 0.72 Cr Ga Intrusion: San Vicente Timing: Postcollisional Area: Northern Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 681 3 Continued ) ( 2 Jose 61.7 63.1 64.2 Jose2 San Jose San Jose3 2 4.78 4.35 4.92 5.15 9.3 8.36 10.8 10.4 2 66 70 63 56 18 22 23 24 San Jose1 San Postcollisional San 0.07 0.07 0.08 0.06 0.08 3.66 4.57 4.32 3.94 3.57 4.98 6.28 6.18 5.41 5.18 4.7 2.44 2.42 2.54 2.73 2.69 0.14 0.16 0.15 0.15 0.12 2.27 2.97 2.9 2.56 2.13 0.78 0.71 0.82 0.74 0.92 0.55 0.68 0.65 0.63 0.55 1.03 0.76 0.79 0.94 0.98 4 22.3 19.4 19.2 21.9 23.5 11.2 10.2 11.3 12.9 13.2 17.4 18.5 18.2 17.3 17.4 64.1 60.5 82 106 101 86 79 64 24 476 332 347 407 454 617 674 691 647 682 ) 2 3 2 Continued 60.5 61.7 61.1 61.4 2 66 70 70 60 18 22 19 16 2 Central Central Potrero Postcollisional 2 TABLE 1. ALISITOS ARC GEOCHEMISTRY ( ARC GEOCHEMISTRY ALISITOS 1. TABLE 2.31 2.56 2.69 2.42 2.54 2.42 2.39 8.57 10.5 9.96 9.3 8.36 8.1 9.29 10.3 0.15 0.13 0.17 0.16 0.15 0.16 0.14 2 95 90 98 106 101 108 102 10.2 11.1 11.9 10.2 11.3 10.2 11 62 65 58 18 18 20 347 379 379 332 347 358 315 669 712 624 674 691 652 704 41 0.77 0.93 0.63 0.76 0.83 0.92 0.71 0.82 0.78 0.81 2.28 2.79 2 2.65 2.3 0.81 2.67 2.33 2.66 2.97 2.9 2.96 2.8 0.085.93 0.07 6.35 0.05 3.42 0.06 5.71 0.07 5.6 0.07 5.64 0.07 6.28 0.08 0.08 6.18 0.08 6.1 6.38 4.91 4.93 5.23 4.74 4.9 4.93 4.78 4.35 4.68 4.79 6.38 9.788.77 11.3 10.6 11 2 2 2 0.63 0.69 0.260.13 0.62 0.14 0.6 0.1 0.61 0.68 0.65 0.66 0.65 4.14 4.57 1.89 4.14 3.73 4.18 4.57 4.32 4.44 4.43 0.8 0.72 0.69 0.81 0.8 0.84 0.76 0.79 0.76 0.68 99.2 100.34 99.85 100.13 100.29 99.82 100.08 100.12 99.92 100.83 99.74 100.08 99.28 99.08 100.58 63 70 86 15.7 21.6 23.6 18.4 22.9 22.7 19.4 19.2 17.8 19.2 61.118 61.4 70.6 18.5 16.3 62.1 18.1 63.4 17.8 62.3 17.7 18.5 18.2 18 18.6 17 15 20 360 382 526 671 712 549 102 * 3 3 5 2 2 O O 2 O 2 O O 2 2 2 SiO Area: Timing: Sample: PP-1 PP-2 PP-4 PP-5 PP-7 PP-8 PP-9 PP-11 PP-12 PP-13 PP-14 Intrusion: Eu Sm Cs Ba La Pr Nd Ce Fe TiO Sr Y755557776757776 Zr Total Be Sc V Rb Nb Al MgO MnO CaO Na P K Downloaded from memoirs.gsapubs.org on March 31, 2014

682 Wetmore et al. 3 2 0.84 1.03 3 1 Jose 4 0 San Jose2 San Jose San Jose3 0.4 0.35 0.48 0.39 5 1 26 26 22 19 22 19 21 20 San Jose1 Postcollisional Central San 0.97 0.92 0.94 0.91 0.92 0.09 0.08 0.07 0.07 0.07 0.352.17 0.35 2.13 0.34 2.09 0.44 2.69 0.38 2.13 0.98 0.92 1 0.63 0.63 0.63 0.75 0.51 0.4 5.73 5.54 5.14 5.86 6.13 4.56 6.29 5.51 5.23 5.26 3 3 22 20 318 235 245 221 243 ) 4 3 2 0 0 2 Continued 5 1 26 26 24 25 22 19 21 22 3 2 Central Potrero Postcollisional 2 1 TABLE 1. ALISITOS ARC GEOCHEMISTRY ( ARC GEOCHEMISTRY ALISITOS 1. TABLE 4 1 24 21 21 21 22 18 341 225 215 235 245 226 224 0.91 0.9 1.05 0.95 0.93 0.92 0.92 0.94 0.92 0.92 0.08 0.07 0.08 0.1 0.07 0.09 0.08 0.07 0.08 0.15 2.350.44 2.26 0.44 1.97 0.36 2.28 0.4 2.3 0.42 2.13 0.35 2.13 0.4 2.09 0.35 2.05 0.37 2.73 0.52 0.39 0.4 0.31 0.39 0.37 0.36 0.35 0.34 0.37 0.44 7.880.99 17.16 18.23 1.01 0.96 11.58 19.44 0.97 15.09 0.99 14.76 1.04 13.27 0.92 13.28 8.68 1 16.35 0.98 0.98 14.76 13.27 14.4 20.39 4 2 7 0.81 0.57 0.62 0.74 0.54 0.66 0.63 0.63 0.61 1.07 1 1 2 8.28 14.5 14.66 8.9 15.57 11.49 12.07 12.4 10.51 6.43 11.88 12.07 12.4 16.01 15.42 23 12 7 22 22 19 56.43 39.06 46.55 40.49 36.1 38.05 35.7 41.51 44.2 33.91 46.21 35.7 41.51 37.69 43.65 95.86 142.4 109.8 133.8 142.4 89.14 96.29 98.71 108.67 100.57 123.4 96.29 98.71 92.43 113.67 218 120 165 O 5.71 5.65 5.92 5.55 5.7 5.77 5.54 5.14 5.44 5.47 O 6.14 6.85 7.58 5.85 6.13 5.87 6.29 5.51 6.16 7.04 2 2 below detection limits. BDL — below REE—rare earth element; N O/K 2 O+Na Note: 2 Co Ni Zn Ge Sn W Ga U Cr A/CNK K Th Area: Lu Hf Ta Tl Pb Sample: PP-1Er PP-2Tm Yb PP-4 PP-5 PP-7 PP-8 PP-9 PP-11 PP-12 PP-13 PP-14 Timing: Intrusion: Dy Ho Gd Tb La/Lu Ba/La Eu/Eu Sr/Y La/Yb Na ∑ REE 35.18 46.74 49.54 41.47 48.87 49.28 43.09 43.26 40.36 44.35 48.09 43.09 43.26 50.33 51.67 Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 683 TABLE 2. ISOTOPE GEOCHEMISTRY AND GEOCHRONOLOGY OF ALISITOS ARC INTRUSIVE ROCKS 143 144 87 86 87 86 206 238 Position Setting Sample Sample Nd/ Nd(100) Sr/ Sr(0) Sr/ Sr(100) Pb/ U Errors region age (Ma) 1/17/04-A 118.4 2.3 Precollisional Arce 7/3/01-J 0.512804 0.704018 0.703324 118.4 2.3 GW-01 108.6 3.96 20-WG 6.801 69.3 90-WG 6.801 69.3 Balbuena 6/9/01-F 0.5128199 0.704335 0.70323 108.6 3.96 30-WG 7.701 6.3 70-WG 7.701 6.3 6/9/01-B 0.5128258 0.703534 0.703301 107.7 3.6 Dos Tortas 1/18/04-I 1/19/04-A El Trigo 1/19/04-B 1/19/04-C 1/18/04-E Gigante 1/18/04-F Postcollisional

Northern 1/18/04-A Los 1/18/04-B Alamos 1/18/04-C 1/18/04-D 1/17/04-B 105.5 3.4 Piedra C-40/71/1 5.501 4.3 Rodada D-40/71/1 5.501 4.3 7/1/01-F 0.5127829 0.704575 0.7037 105.5 3.4 1/16/04-A1 San 1/16/04-A2 Jacinto 1/16/04-B 1/16/04-C San 1/12/03-A 0.5128169 0.703641 0.703423 105 3.4 Vicente EP-1 110.4 1.6 2-PE 4.011 6.1 3-PE 4.011 6.1 Ensinosa 4-PE 4.011 6.1 5-PE 4.011 6.1 6-PE 4.011 6.1 Precollisional EP-7 110.4 1.6 Gabbro (12) 0.7039 115 1 Cone sheets (9) 115 Zarza Pink tonalite 1 0.7043 115 2 etilanot kniP etilanot 2 511 3 etilanot kniP etilanot 3 511 Trondhjemite 0.7022 115 PP-1() 102.5 2.6

Central 2-PP 5.201 6.2 4-PP 5.201 6.2 5-PP 5.201 6.2 7-PP 5.201 6.2 Potrero 8-PP 5.201 6.2 9-PP 5.201 6.2 Postcollisional 11-PP 5.201 6.2 21-PP 5.201 6.2 31-PP 5.201 6.2 PP-14 102.5 2.6 1 esoJ naS esoJ 1 6.501–4.301 San Jose 2 0.7035 103.4–105.6 San Jose esoJ naS esoJ 6.501–4.301 San Jose 3 103.4–105.6 Downloaded from memoirs.gsapubs.org on March 31, 2014

684 Wetmore et al. as Sr, which has an average of 230 ppm for the postcollisional Tate and Johnson, 2000). Radiogenic and stable isotopes intrusions from the northern Alisitos arc, but has an average of (Table 2) have limited variation throughout the Alisitos arc 87 86 δ18 665 ppm in the postcollisional intrusions of the central area. (e.g., Sr/ Sr(100): 0.7022–0.7043; O: 7.7‰–8.9‰) and Ytterbium, which has average concentrations ranging from 3.21 clearly indicate that the source is primitive, consistent with the to 5.15 ppm for samples from all groups from the northern area observations of several regional isotopic studies (e.g., Taylor and the precollisional intrusions from the central area, averages and Silver, 1978; Silver et al., 1979; DePaolo, 1981). Several only 0.67 ppm for samples from the postcollisional intrusions of workers (e.g., Gastil et al., 1975, 1981; Johnson et al., 1999a; the central Alisitos arc. Wetmore et al., 2002, 2003, 2005; Schmidt et al., this volume) In summary, the Early Cretaceous intrusions of the north- have suggested that the basement to the Alisitos arc is oce- ern and central regions of the Alisitos arc have broadly similar anic lithosphere, based on the geochemical data represented petrologic classiﬁ cations, relatively primitive radiogenic and here, as well as those from studies of other igneous rocks stable isotopic compositions, and broadly similar elemental throughout the arc terrane (e.g., Valencia et al., 2006; Weber geochemistries. However, in detail, while intrusions of all ages and López Martínez, 2006). The age of this lithosphere is not from the northern region and the precollisional intrusions of the constrained, but, based on the presence of Middle Jurassic central region all have very closely corresponding composi- intrusions within pre-Alisitos arc stratigraphy near Sierra Cal- tions, the postcollisional intrusions from the central zone are amajue and the El Arco areas, we can say that it is no younger distinct from the rest in that they are characterized by a very than ca. 165 Ma (Valencia et al., 2006). The isotopic geo- restricted range of silica concentrations, overall higher Al2O3, chemistry from throughout the Alisitos terrane (i.e., including

Na2O, and Sr compositions, and lower TiO2, Fe2O3, MnO, K2O, the Middle Jurassic intrusions) also suggests that there is no and Yb concentrations. evolved continental material present at depth within the strati- graphic framework of the Alisitos arc basement (e.g., Tate et DISCUSSION al., 1999). Wetmore et al. (2003, 2005), Alsleben et al. (2012), and Schmidt et al. (this volume) have further argued that there The structural data from the Alisitos arc and adjacent cen- is no demonstrable association with continental crust prior to tral/transitional zone of the Peninsular Ranges batholith reveal the initiation of collision of the Alisitos arc terrane with the that by the end of the Early Cretaceous, collision between the arc North American continental margin at ca. 112 Ma. terrane and the North American continental margin had locally Geochemistry of the northern and central Alisitos arc plu- produced large crustal thicknesses in the central Alisitos arc tons further supports the suggestion that the basement to the adjacent to the Main Mártir thrust. Geochemical data reveal that arc terrane is essentially the same throughout the area of study. postcollisional intrusions in the central Alisitos arc possess com- Figure 2 illustrates that the geochemical data from pre- and positions that are unique within the Alisitos terrane. Their com- postcollisional intrusions from the northern Alisitos arc over- positions overlap those of the La Posta intrusions of the central/ lap those of the precollisional intrusions of the central zone, transitional and eastern zones of the Peninsular Ranges batholith defi ning trends that are typical of the calcic suite of Peninsular (e.g., Tulloch and Kimbrough, 2003), formed during the La Posta Ranges batholith intrusives as originally described by Larsen fl are-up event that followed collision and the emplacement of the (1948). The most salient differences between the data sets postcollisional intrusions of the western zone by just a few mil- defi ning the zone and temporal divisions are the presence of lion years. a few more low-silica samples from the central precollisional In this section we will: (1) revisit the geochemical data to intrusions and the presence of a distinct group of samples argue that all of the Early Cretaceous melts of the Alisitos arc from that group (including those with low silica) that have derive from a source of similar composition; (2) argue that the lower overall A/CNK values. Trace-element data (Figs. 2–4) unique geochemical signature of the central Alisitos arc’s post- reveal additional differences between these groups, including collisional magmas are the direct result of Main Mártir thrusting, a few samples with slightly higher negative Eu anomalies in resulting in the depression of the crustal source rocks for these the precollisional intrusions from the central Alisitos arc and melts to depths in excess of 35 km; and (3) show that the postcol- consistently higher values (2–4 vs. 3–8) for La/Yb ratios for lisional intrusions of the central Alisitos arc exhibit a temporal the postcollisional intrusions from the northern Alisitos arc. and spatial relationship to the La Posta intrusions, suggesting that Most of these differences may result from the limited number they represent the earliest stages of magmatic fl are-ups in arcs of data available and possible sampling biases. However, the accompanying collision. consistently higher La/Yb values of the postcollisional central Alisitos arc (see Fig. 4) seem to be real and may refl ect minor Uniform Source for the Alisitos Arc pressure differences at the site of melt generation between these two intrusive suites. Differences notwithstanding, these The isotopic and the elemental chemistry of all of the three intrusive suites appear to have shared a common source, late Early Cretaceous intrusions of the Alisitos arc suggest and to have evolved along similar lines of liquid descent to that these melts derive from a broadly similar source (e.g., produce the degree of overlap observed in the elemental data. Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 685

Figure 2. Harker diagrams of elemental data presented in Table 1. All oxides are weight percentage, Sr and Yb are in parts per million. Open symbols represent precollisional samples, and ﬁ lled symbols represent postcollisional samples. Triangles are for samples from the northern Alisitos arc, while circles are from the central Alisitos arc. These symbols are also employed in Figures 3 and 4. Downloaded from memoirs.gsapubs.org on March 31, 2014

686 Wetmore et al.

Figure 3. Plot of Sr/Y ratio to SiO2 (wt%) illustrating the inﬂ uence of garnet on samples of the postcollisional intrusions from the central zone relative to all other samples from the Alisitos arc terrane. See Figure 2 for symbol iden- tiﬁ cation.

Collision-Related Crustal Thickening and Alisitos arc is estimated to be ~10 kbar or a depth of ~35 km Postcollisional Magmatism (Tate and Johnson, 2000). Large postcollisional intrusions having geochemical sig- In contrast with the northern intrusions and the precolli- natures consistent with derivation from depths of 35 km in the sional intrusions of the central Alisitos arc, the geochemical data central part of the Alisitos arc suggest that collision-related defor- from the postcollisional intrusions of the central part of the arc mation produced a degree of crustal thickening in this part of suggest notably higher pressures of melt generation and very lim- the Alisitos arc that is not apparent elsewhere in the arc terrane. ited evolution of the melt during ascent and emplacement. While The Alisitos arc is inferred to have been a relatively long-lived the elemental data from the postcollisional intrusions of the cen- (mature?) island arc, active for more than 60 m.y. (e.g., Schmidt tral Alisitos arc do partially overlap the data from the rest of the et al., this volume), and the shortening related to folding, fault- samples described earlier (Fig. 2), they are systematically more ing, and ductile strains within the Alisitos arc is inferred to be aluminous and sodic, have lower concentrations of FeOt, K2O, relatively consistent along the length of the arc (Alsleben et al., and heavy (H) REEs (e.g., Yb), and yield pronounced Sr enrich- 2008). However, the magnitude of throw across the terrane- ments. Tate and Johnson (2000) argued that these variations bounding structures (e.g., Main Mártir thrust and ancestral Agua reﬂ ect dehydration melting of an amphibolitic source at depths Blanca fault) appears to exhibit substantial along-strike vari- where plagioclase melts incongruently in the presence of garnet. ability. For example, whereas the metamorphic grade does not As a consequence, little, if any, plagioclase is left in the residue, change across the ancestral Agua Blanca fault, suggesting no and the melts derived from this environment yield higher initial more than a few kilometers of throw across the fault (Wetmore et

Al2O3 concentrations and virtually no Eu anomalies relative to al., 2003, 2005), the determined pressures in the central Alisitos all other melts in the central and northern Alisitos arc (Fig. 4). arc increase by more than 4 kbar from the footwall in the Alisi- In addition, they show highly fractionated REE patterns, as the tos arc to the hanging wall in the central/transitional zone of the heaviest elements are strongly partitioned into residual garnet Peninsular Ranges batholith (Schmidt and Paterson, 2002). This (e.g., Hildreth and Moorbath, 1988). The pressure of melt gen- difference would have placed the lower crust of the eastern and eration that produced the postcollisional intrusions of the central central Alisitos arc adjacent to the Main Mártir thrust at a depth Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 687

Figure 4. Plot of Eu/Eu* against La/ Yb ratios. See Figure 2 for symbol identification.

possibly as much as 15 km greater than that of other portions of low Y and are inferred to have been generated through the partial the terrane. A potential causative relationship between the unique subduction of the magmatically thickened western zone (Tull- chemistry of the postcollisional intrusions of the central Alisitos och and Kimbrough, 2003). In the petrogenetic model proposed arc and the terrane-bounding Main Mártir thrust is further sup- by Tulloch and Kimbrough (2003), the chemically similar (dis- ported by the close spatial relationship between the intrusions cussed next) postcollisional intrusions of the central Alisitos arc and the faults. The postcollisional intrusions in the central Alisi- formed as a direct consequence of melting of crust that had thick- tos arc appear to be conﬁ ned to the easternmost part of the ter- ened as the result of magmatic accretion of a long-lived arc. This rane adjacent to the Main Mártir thrust, whereas in the northern is an unlikely scenario for the formation of these melts based Alisitos arc, intrusions of the same age are emplaced across the on the close spatial and temporal coincidence of the timing of entire width of the terrane (>25 km). collision-related deformation described herein. We further suggest that the postcollisional intrusions represent the initial stages Relationship to the La Posta Flare-Up Magmatism or precursors to the La Posta ﬂ are-up event. Comparison of the postcollisional intrusions of the central Flare-up magmatism in the form of the La Posta intrusive Alisitos arc and the La Posta magmas reveals several similari- suite (e.g., Walawender et al., 1990; Tulloch and Kimbrough, ties, including a restricted range in SiO2 (60–70 wt%), lower iron

2003) followed the end of magmatism in the entire western and HREEs, and higher Na2O, light (L) REEs, and Sr when zone of the Peninsular Ranges batholith and after all discernible compared to the geochemical data from the remainder of the motion had ceased on the terrane-bounding faults (e.g., Wetmore Alisitos arc and the western zone as a whole (e.g., Tulloch and et al., 2005; Melis, 2006; Schmidt et al., this volume). The La Kimbrough, 2003). However, there are signiﬁ cant differences in Posta magmatic event is concentrated in the central/transitional isotopic compositions between the postcollisional intrusions of zone of the Peninsular Ranges batholith and lasted from 99 to the central Alisitos arc and the La Posta suite of intrusions. La 92 Ma (Kimbrough et al., 2001), initiating ~4 m.y. after the Posta suite intrusions are more evolved in both radiogenic and youngest of the postcollisional intrusions of the Alisitos arc. The stable isotopes when compared to all western zone intrusions, La Posta intrusions are characterized by high Na, Al, and Sr, and reﬂ ecting their interaction (assimilation?) with older, continental Downloaded from memoirs.gsapubs.org on March 31, 2014

688 Wetmore et al.

components of the central/transitional zone’s crustal column brating to its new position beneath the additional ~15 km of the (Tulloch and Kimbrough, 2003). central/transitional zone now overlying the easternmost Alisitos The interpretation that the La Posta magmatic event is arc crust. This may explain the apparent early arrival of the high- genetically linked to the eastward partial subduction of western pressure, La Posta–like melts. The fact that at the present level litho-tectonic terranes has been proposed in recent studies (e.g., of exposure the ascending melt would have passed through only Tulloch and Kimbrough, 2003; Grove et al., 2008). Tulloch and the Alisitos arc terrane lithologies and, therefore, would not have Kimbrough (2003) argued that the La Posta ﬂ are-up was driven interacted with the more-evolved rocks in the hanging wall above by the juxtaposition of the western zone lithologies with the base the Main Mártir thrust, explains the primitive isotopic composi- of the central/transitional zone crust through the contraction of tions relative to those seen in true La Posta intrusions. the entire Peninsular Ranges batholith. More recently Grove et al. (2008) and Kimbrough and Grove (2010) have suggested that CONCLUSIONS subduction erosion of the accretionary prism placed the Catalina Schist into the zone of melt generation, particularly in the Pen- Structural and geochemical data sets from the Alisitos arc of insular Ranges batholith north of ~31°30′N latitude. However, western Baja California Norte, Mexico, reveal the existence of radiogenic and stable isotopic data from the southern part of the a laterally extensive, though spatially variable, collision-related La Posta intrusive suite are less evolved than to the north, an deformation and magmatism event that spanned the time interval observation that has been attributed to changes in the lithologies from before to after the completion of this deformation. Colli- of the bedrock of the central/transitional zone along the north- sion of the Alisitos arc with the southwestern margin of North south axis of the Peninsular Ranges batholith (e.g., Tulloch and America resulted in the formation of a broad (15–>25 km wide) Kimbrough, 2003). An alternative interpretation is that while fold-and-thrust belt within the arc terrane, as well as the juxtapo- subduction erosion of the Early Cretaceous accretionary prism sition of relatively shallow-level portions of the arc (i.e., depths was taking place to the north, collision-related forearc removal of 5–7 km) with midcrustal levels of the central/transitional of the southern continuation of the Santiago Peak arc, and partial zone of the Peninsular Ranges batholith across the Main Mártir subduction of the eastern portion of the Alisitos arc was taking thrust. Juxtaposition of the Alisitos arc with rocks of the central/ place to the south (e.g., Wetmore et al., 2005; Schmidt et al., this transitional zone of the Peninsular Ranges batholith was accom- volume). The lithologic transition from miogeoclinal passive- plished through the removal of a substantial portion the Santiago margin sequences to more deep-water sequences is consistent Peak arc south of the ancestral Agua Blanca fault by subduction along the trace of the Peninsular Ranges batholith, but the true erosion. As a result of this deformation, the easternmost portion boundary is reported to be near ~30°N latitude (e.g., Gastil, 1993; of the Alisitos arc would have been placed beneath an additional Schmidt and Paterson, 2002). The observed isotopic transition ~15 km of crustal section in its position in the footwall of the at ~31°30′N latitude is tantalizingly close to the ancestral Agua Main Mártir thrust. Blanca fault and the boundary between the accreted Alisitos arc Isotopic and elemental geochemistry demonstrates that terrane and the Santiago Peak arc that was built on and through the source for all intrusions of the Alisitos arc, regardless of the Late Triassic through Jurassic North American accretionary the timing of melt generation relative to that of the collision- prism (i.e., the Bedford Canyon complex; Wetmore et al., 2003). related deformation, was broadly similar and primitive. Postcol- As discussed herein, and in detail in Schmidt et al. (this vol- lisional intrusions of the central Alisitos arc are characterized ume), the same structure (e.g., Main Mártir thrust) that presently by distinct trace-elemental compositions that indicate derivation juxtaposes the Alisitos arc with the central/transitional zone of from depths (~35 km) where plagioclase melted incongruently the Peninsular Ranges batholith accommodated the erosional and completely, as indicated by Sr and Eu/Eu*, and that garnet removal of the Santiago Peak arc and formed the suture between would be a stable phase in the residuum, as indicated by strongly North America and the Alisitos terrane. As a consequence of this depleted HREE abundances. The timing of these melts postdates process, the southern continuation of the Santiago Peak arc south principal slip on the Main Mártir thrust, and the proximity of the of the ancestral Agua Blanca fault would have been forced into intrusions to that structure strongly suggests that the increased the zone of melting along with the easternmost edge of the Alisi- pressure inferred from their trace-elemental chemistries resulted tos arc terrane. Melt generation within the Alisitos arc at the end from the increased load placed onto the eastern part of the cen- of, or even after, collision likely represents the last gasp of tradi- tral Alisitos arc by the throw across the Main Mártir thrust. The tional arc magmatism in this part of the system, with the only real correlation of this locally anomalous magmatism with collision- exception being that in the region immediately adjacent to the related crustal thickening in the outboard magmatic arc contrasts Main Mártir thrust, where the bottom of the arc crust had already with earlier suggestions that it formed as a consequence of crustal been driven to depths where pressures were sufﬁ ciently high to thickening associated with the longevity of arc magmatism in the stabilize garnet and destabilize plagioclase. Furthermore, in addi- Alisitos terrane. tion to the thermal input from the mantle, which was driving tra- The short pause in magmatism, strong elemental (major and ditional arc magmatism throughout the Alisitos, the geothermal trace) overlap, and close spatial association between the postcol- gradient in the crust beneath the Main Mártir thrust was reequili- lisional intrusions of the central Alisitos arc and the La Posta Downloaded from memoirs.gsapubs.org on March 31, 2014

Tectonic implications of postcontractional magmatism of the Alisitos arc segment of the Peninsular Ranges 689 intrusions of the central/transition zone of the Peninsular Ranges Dickinson, W.R., and Lawton, T.F., 2001, Carboniferous to Cretaceous assem- batholith indicate similar conditions of melt generation, and the bly and fragmentation of Mexico: Geological Society of America Bul- letin, v. 113, p. 1142–1160, doi:10.1130/0016-7606(2001)113<1142 former magmas probably represent the initial phases of La Posta :CTCAAF>2.0.CO;2. magmatism in this part of the Peninsular Ranges batholith. The Ducea, M., 2001, The California arc: Thick granitic batholiths, eclogite resi- locally anomalous conditions of melt generation for the postcol- dues, lithospheric-scale thrusting, and magmatic fl are-ups: GSA Today, v. 11, p. 4–10, doi:10.1130/1052-5173(2001)011<0004:TCATGB>2.0 lisional magmas of the central Alisitos arc arose because of the .CO;2. partial subduction of the eastern part of the arc terrane beneath Ducea, M.N., and Barton, M.D., 2007, Igniting fl are-up events in Cordilleran the central/transitional zone of the Peninsular Ranges batholith arcs: Geology, v. 35, p. 1047–1050, doi:10.1130/G23898A.1. Engebretson, D.C., Cox, A., and Gordon, R.G., 1985, Relative Motions along Main Mártir thrust. This subduction was responsible for Between Oceanic and Continental Plates in the Pacifi c Basin: Geologi- the removal of the portion of the Santiago Peak arc where the cal Society of America Special Paper 206, 60 p., doi:10.1130/SPE206-p1. Alisitos arc now resides. Both entities, the eastern part of the Fisher, D.M., Gardner, T.W., Sak, P.B., Sanchez, J.D., Murphy, K., and Vannucchi, P., 2004, Active thrusting in the inner forearc of an ero- Alisitos arc and eroded Santiago Peak arc, were placed into the sive convergent margin, Pacifi c coast, Costa Rica: Tectonics, v. 23, zone of melting. Melts generated in the lower crust of the east- doi:10.1029/2002TC001464. ern Alisitos arc never interacted with evolved continental crust Gastil, R.G., 1993, Prebatholithic history of Peninsular California, in Gastil, R.G., and Miller, R.H., eds., The Prebatholithic Stratigraphy of Peninsular along their line of ascent through the crust, at least to the level of California: Geological Society of America Special Paper 279, p. 145–156. present-day exposure. Gastil, R.G., Morgan, G.J., and Krummenacher, D., 1981, The tectonic history of peninsular California and adjacent Mexico, in Ernst, W.G., ed., The Geotectonic Development of California: Englewood Cliffs, New Jersey, ACKNOWLEDGMENTS Prentice-Hall, p. 284–306. Gastil, R.G., Phillips, R., and Allison, E., 1975, Reconnaissance Geology of the The research presented in this contribution was primarily State of Baja California: Geological Society America Memoir 140, 170 p. Gehrels, G., Rusmore, M., Woodsworth, G., Crawford, M., Andronicos, C., funded through student grants from the Geological Society of Hollister, L., Patchett, J., Ducea, M., Butler, R., Klepeis, K., Davidson, C., America, Sigma Xi, and the Department of Earth Sciences at Friedman, R., Haggart, J., Mahoney, B., Crawford, W., Pearson, D., and the University of Southern California to Helge Alsleben and Girardi, J., 2009, U-Th-Pb geochronology of the Coast Mountains batholith in north-central British Columbia: Constraints on age and tectonic Paul Wetmore. Thanks go to Scott Johnston and Fred Miller evolution: Geological Society of America Bulletin, v. 121, p. 1341–1361, for their review of our manuscript and to Doug Morton for doi:10.1130/B26404.1. his patience. We would also like to thank Lawford Anderson, Grove, M., Bebout, G.E., Jacobson, C.E., Barth, A.P., Kimbrough, D.L., King, R.L., Haibo Zou, Lovera, O.M., Mahoney, B.J., and Gehrels, G.E., 2008, Scott Paterson, David Kimbrough, George Gehrels, Jorge The Catalina Schist: Evidence for middle Cretaceous subduction erosion Ledesma, Francisco Suarez-Vidal, and Jeff Ryan for many of southwestern North America, in Draut, A.E., Clift, P.D., and Scholl, D.W., eds., Formation and Application of the Sedimentary Record in important discussions about Peninsular Ranges batholith Arc Collision Zones: Geological Society of America Special Paper 436, geology and geochemistry. p. 335–361. Hildreth, W., and Moorbath, S., 1988, Crustal contributions to arc magmatism in the Andes of central Chile: Contributions to Mineralogy and Petrology, REFERENCES CITED v. 98, p. 455–489, doi:10.1007/BF00372365. Johnson, S.E., Tate, M.C., and Fanning, C.M., 1999a, New geologic map- Alsleben, H., Wetmore, P.H., Schmidt, K.L., Paterson, S.R., and Melis, E.A., ping and SHRIMP U-Pb data in the Peninsular Ranges batholith, Baja 2008, Complex deformation during arc-continent collision: Quantify- California, Mexico: Evidence of a suture?: Geology, v. 27, p. 743–746, ing fi nite strain in the accreted Alisitos arc, Peninsular Ranges batho- doi:10.1130/0091-7613(1999)027<0743:NGMASU>2.3.CO;2. lith, Baja California: Journal of Structural Geology, v. 30, p. 220–236, Johnson, S.E., Paterson, S.R., and Tate, C.M., 1999b, Structure and emplace- doi:10.1016/j.jsg.2007.11.001. ment history of a multiple-center, cone-sheet–bearing ring complex: Alsleben, H., Wetmore, P.H., Gehrels, G.E., and Paterson, S.R., 2012, Detrital The Zarza intrusive complex, Baja California, Mexico: Geological zircon ages in Paleozoic and Mesozoic basement assemblages of the Pen- Society of America Bulletin, v. 111, p. 607–619, doi:10.1130/0016 insular Ranges batholith, Baja California, Mexico: Constraints for depo- -7606(1999)111<0607:SAEHOA>2.3.CO;2. sitional ages and provenance: International Geology Review, v. 54, no. 1, Johnson, S.E., Fletcher, J.M., Fanning, C.M., Paterson, S.R., Vernon, R.H., and p. 93–110, doi:10.1080/00206814.2010.509158. Tate, M.C., 2003, Structure and emplacement of the San Jose tonalite Chávez-Cabello, G., 1998, Mecanismos de Emplazamiento y Evolucion Mag- pluton, Peninsular Ranges batholith, Baja California, Mexico: Journal of matica de Varios Plutones al Oeste de la Sierra San Pedro Martir, Baja Structural Geology, v. 25, p. 1933–1957. California, México [Master’s thesis]: Ensenada, Baja California Norte, Kimbrough, D.L., and Grove, M., 2010, The case for rapid crustal recycling in México, Centro de Investigación Científi ca y de Educación Superior de arcs based on zircon inheritance patterns in the eastern Peninsular Ranges Ensenada (CICESE), 165 p. batholith of southern and Baja California: Geological Society of America Coleman, D.S., and Glazner, A.F., 1997, The Sierra Crest magmatic Abstracts with Programs, v. 42, no. 5, p. 666. event: Rapid formation of juvenile crust during the Late Cretaceous Kimbrough, D.L., Smith, D.P., Mahoney, J.B., Moore, T.E., Grove, M., Gastil, in California: International Geology Review, v. 39, p. 768–787, R.G., Ortega-Rivera, A., and Fanning, C.M., 2001, Forearc-basin sedimen- doi:10.1080/00206819709465302. tary response to rapid Late Cretaceous batholith emplacement in the Penin- Coney, R.J., and Reynolds, S.J., 1977, Cordilleran Benioff zones: Nature, sular Ranges of southern and Baja California: Geology, v. 29, p. 491–494, v. 270, p. 403–406, doi:10.1038/270403a0. doi:10.1130/0091-7613(2001)029<0491:FBSRTR>2.0.CO;2. DeCelles, P.G., Ducea, M.N., Kapp, P., and Zandt, G., 2009, Cyclicity in Kopf, C.F., Whitney, D.L., and Schmidt, K.L., 2000, Distinct P-T domains Cordilleran orogenic systems: Nature Geoscience, v. 2, p. 251–257, across the Peninsular Ranges batholith in northern Baja California, Mex- doi:10.1038/ngeo469. ico, evidence for a long-lived inter-arc convergent zone: Geological Soci- DePaolo, D.J., 1981, A neodymium and strontium isotopic study of the Meso- ety of America Abstracts with Programs, v. 32, p. 296. zoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Larsen, E.S., 1948, Batholith and Associated Rocks of Corona, Elsinore, and Ranges, California: Journal of Geophysical Research, v. 86, p. 10,470– San Luis Rey Quadrangles, Southern California: Geological Society of 10,488, doi:10.1029/JB086iB11p10470. America Memoir 29, 182 p. Downloaded from memoirs.gsapubs.org on March 31, 2014

690 Wetmore et al.

Lewis, J.L., Day, S.M., Magistrale, H., Castro, R.R., Astiz, L., Rebollar, C., ed., Metamorphism and Crustal Evolution of the Western United States: Eakins, J., Vernon, F., and Brune, J.N., 2001, Crustal thickness of the Pen- Englewood Cliffs, New Jersey, Prentice-Hall, p. 894–937. insular Ranges and Gulf Extensional Province in the Californias: Journal Tulloch, A.J., and Kimbrough, D.L., 2003, Paired plutonic belts in conver- of Geophysical Research, v. 104, p. 29,419–29,438. gent margins and the development of high Sr/Y magmatism: Peninsu- Melis, E.A., 2006, Structural, Temporal, and Metamorphic Evolution of the lar Ranges batholith of Baja California and Meridian batholith of New Main Martir Thrust, Baja California, Mexico [Ph.D. dissertation]: Orono, Zealand, in Johnson, S.E., Paterson, S.R., Fletcher, J.M., Girty, G.H., Maine, University of Maine, 337 p. Kimbrough, D.L., and Martín-Barajas, A., eds., Tectonic Evolution of Miller, M.L., and Hughes, S.S., 2009, Mixing primitive and evolved olivine Northwestern Mexico and the Southwestern USA: Geological Society of tholeiite magmas in the eastern Snake River Plane, Idaho: Journal of Vol- America Special Paper 374, p. 275–295. canology and Geothermal Research, v. 188, p. 153–161, doi:10.1016/j Valencia, V.A., Barra, F., Weber, B., Ruiz, J., Gehrels, G., Chesley, J., and .jvolgeores.2009.06.004. Lopez Martinez, M., 2006, Re/Os and U/Pb geochronology of the El Arco Rothstein, D.A., and Manning, C.E., 2003, Geothermal gradients in continental porphyry copper deposit, Baja California Mexico; implications for the magmatic arcs: Constraints from the eastern Peninsular Ranges batholith, Jurassic tectonic setting: Journal of South American Earth Sciences, v. 22, Baja California, México, in Johnson, S.E., Paterson, S.R., Fletcher, J.M., p. 39–51, doi:10.1016/j.jsames.2006.08.005. Girty, G.H., Kimbrough, D.L., and Martín-Barajas, A., eds., Tectonic Walawender, M.J., Gastil, R.G., Clinkenbeard, J.P., McCormick, W.V., East- Evolution of Northwestern Mexico and the Southwestern USA: Geologi- man, B.G., Wernicke, R.S., Wardlaw, M.S., and Smith, B.M., 1990, Ori- cal Society of America Special Paper 374, p. 337–354. gin and evolution of the zoned La Posta–type plutons, eastern Peninsular Schmidt, K.L., and Paterson, S.R., 2002, A doubly vergent fan structure in the Ranges batholith of southern and Baja California, in Anderson, J.L., ed., Peninsular Ranges batholith: Transpression or local complex ﬂ ow around The Nature and Origin of Cordilleran Magmatism: Geological Society of a continental margin buttress?: Tectonics, v. 21, p. 14-1–14-19. America Memoir 174, p. 1–18, doi:10.1130/MEM174-p1. Schmidt, K.L., Wetmore, P.H., Alsleben, H., and Paterson, S.R., 2014, this vol- Weber, B., and López Martínez, M., 2006, Pb, Sr, and isotopic and chemical ume, Mesozoic tectonic evolution of the southern Peninsular Ranges batho- evidence for a primitive island arc emplacement of the El Arco porphyry lith, Baja California, Mexico: Long-lived history of a collisional segment copper deposit (Baja California, Mexico): Mineralium Deposita, v. 40, in the Mesozoic Cordilleran arc, in Morton, D.M., and Miller, F.K., eds., p. 707–725, doi:10.1007/s00126-005-0028-4. Peninsular Ranges Batholith, Baja California and Southern California: Wetmore, P.H., 2003, Investigation into the Tectonic Signiﬁ cance of Along Geological Society of America Memoir 211, doi:10.1130/2014.1211(20). Strike Variations of the Peninsular Ranges Batholith, Southern and Baja Silver, L.T., Taylor, H.P.J., and Chappell, B.W., 1979, Some petrological, geo- California [Ph.D. thesis]: Los Angeles, University of Southern California, chemical, and geochronological observations of the Peninsular Ranges 199 p. batholith near the international border of the U.S.A. and Mexico, in Wetmore, P.H., and Ducea, M.N., 2011, Geochemical evidence of a near- Abbott, P.L., and Todd, V.R., eds., Mesozoic Crystalline Rocks: San surface history for source rocks of the central Coast Mountains batholith, Diego, California, San Diego State University, Department of Geological British Columbia: International Geology Review, v. 51, p. 1–26. Sciences, p. 83–110. Wetmore, P.H., Schmidt, K.L., Paterson, S.R., and Herzig, C., 2002, Tectonic Tate, M.C., and Johnson, S.E., 2000, Subvolcanic and deep-crustal tonalite gen- implications for the along-strike variation of the Peninsular Ranges esis beneath the Mexican Peninsular Ranges: The Journal of Geology, batholith, southern and Baja California: Geology, v. 30, p. 247–250, v. 108, p. 721–728, doi:10.1086/317948. doi:10.1130/0091-7613(2002)030<0247:TIFTAS>2.0.CO;2. Tate, M.C., Norman, M.D., Johnson, S.E., Fanning, C.M., and Anderson, J.L., Wetmore, P.H., Herzig, C., Schultz, P.W., Alsleben, H., Sutherland, M., Pater- 1999, Generation of tonalite and trondhjemite by subvolcanic fraction- son, S.R., and Schmidt, K.L., 2003, Mesozoic tectonic evolution of the ation and partial melting in the Zarza intrusive complex, western Penin- Peninsular Ranges of southern Baja California, in Johnson, S.E., Paterson, sular Ranges batholith, northwestern Mexico: Journal of Petrology, v. 40, S.R., Fletcher, J.M., Girty, G.H., Kimbrough, D.L., and Martín-Barajas, p. 983–1010, doi:10.1093/petroj/40.6.983. A., eds., Tectonic Evolution of Northwestern Mexico and the Southwest- Taylor, H.P.J., and Silver, L.T., 1978, Oxygen isotope relationships in plutonic ern USA: Geological Society of America Special Paper 374, p. 93–116. igneous rocks of the Peninsular Ranges batholith, southern and Baja Cal- Wetmore, P.H., Alsleben, H., Paterson, S.R., Ducea, M.N., Gehrels, G.E., and ifornia, in Zartman, R.E., ed., Short Papers of the Fourth International Valencia, V.A., 2005, Field Trip to the Northern Alisitos Arc Segment: Conferences on Geochronology: U.S. Geological Survey Open-File Ancestral Agua Blanca Fault Region: Field Conference Guidebook for Report 78-701, p. 423–426. the VII International Meeting of the Peninsular Geological Society: Thomson, C.N., and Girty, G.H., 1994, Early Cretaceous intra-arc duc- Ensenada, Baja California, Mexico, Peninsular Geological Society, 39 p. tile strain in Triassic–Jurassic and Cretaceous continental margin arc rocks, Peninsular Ranges, California: Tectonics, v. 13, p. 1108–1119, doi:10.1029/94TC01649. Todd, V.R., Erskine, B.G., and Morton, D.M., 1988, Metamorphic and tectonic evolution of the northern Peninsular Ranges batholith, in Ernst, W.G., MANUSCRIPT ACCEPTED BY THE SOCIETY 11 JUNE 2013

Printed in the USA