The Cretaceous Peninsular Ranges Batholith of Southern and Baja California Robert S

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Geoscience Canada Journal of the Geological Association of Canada Journal de l’Association Géologique du Canada

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California Robert S. Hildebrand and Joseph B. Whalen

Volume 41, Number 4, 2014 Article abstract Ever since the late 1960s when Warren Hamilton proposed that the great URI: https://id.erudit.org/iderudit/1062260ar Cordilleran batholiths of the western Americas are the roots of volcanic arcs DOI: https://doi.org/10.12789/geocanj.2014.41.059 like the Andes and were generated by longstanding eastward subduction, most geologists have followed suit, despite the evergrowing recognition that many See table of contents Cordilleran batholiths are complex, composite bodies that developed with intervals of intense shortening and exhumation between and during periods of magmatism. The Peninsular Ranges batholith of Southern and Baja California Publisher(s) provides a superb place to unravel the complexities because there is a lot of data and because it is longitudinally composed of two parts: an older western The Geological Association of Canada portion of weakly to moderately deformed, low-grade volcanic and epizonal plutonic rocks ranging in age from ~128–100 Ma; and a more easterly sector of ISSN deformed amphibolite grade rocks cut by compositionally zoned, mesozonal plutonic complexes of the La Posta suite, emplaced from 99–86 Ma. While 0315-0941 (print) plutons of the La Posta suite are generally considered to be the product of 1911-4850 (digital) continued eastward subduction, they are enigmatic, because they and their wall rocks were rapidly exhumed from as deep as 23 km and eroded during, Explore this journal and just after, their emplacement, unlike plutons in magmatic arcs, which are generally emplaced in zones of subsidence. Here we resolve the enigma with a model where westward-dipping subduction led to arc magmatism of the Cite this article western sector, the Santiago Peak–Alisitos composite arc, during the period ~128–100 Ma. Arc magmatism shut down when the arc collided with a Hildebrand, R. & Whalen, J. (2014). Arc and Slab-Failure Magmatism in west-facing Early Cretaceous passive margin at about 100 Ma. During the Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of collision the buoyancy contrast between the continental crust of the eastern Southern and Baja California. Geoscience Canada, 41(4), 399–458. block and its attached oceanic lithosphere led to failure of the subducting slab. https://doi.org/10.12789/geocanj.2014.41.059 The break-off allowed subjacent asthenosphere to upwell, adiabatically melt, and rise into the upper plate to create the large zoned tonalite–granodiorite–granite complexes of the La Posta suite. While compositionally similar to arc plutons in many respects, the examples from the Southern California and Baja segments of the batholith have geochemistry that indicates they were derived from partial melting of asthenosphere at deeper levels in the mantle than typical arc magmas, and within the garnet stability field. This is consistent with asthenosphere upwelling through the torn lower-plate slab. We identify kindred rocks with similar geological relations in other Cordilleran batholiths of the Americas, such as the Sierra Nevada, which lead us to suggest that slab failure magmatism is common, both spatially and temporally.

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PAUL F. HOFFMAN SERIES

complex, composite bodies that devel- abatically melt, and rise into the upper oped with intervals of intense shorten- plate to create the large zoned tonalite– ing and exhumation between and dur- granodiorite–granite complexes of the ing periods of magmatism. La Posta suite. While compositionally The Peninsular Ranges similar to arc plutons in many respects, batholith of Southern and Baja Cali- the examples from the Southern Cali- fornia provides a superb place to fornia and Baja segments of the unravel the complexities because there batholith have geochemistry that indi- is a lot of data and because it is longi- cates they were derived from partial tudinally composed of two parts: an melting of asthenosphere at deeper older western portion of weakly to levels in the mantle than typical arc Arc and Slab-Failure moderately deformed, low-grade vol- magmas, and within the garnet stability canic and epizonal plutonic rocks rang- field. This is consistent with asthenos- Magmatism in Cordilleran ing in age from ~128–100 Ma; and a phere upwelling through the torn Batholiths II – The more easterly sector of deformed lower-plate slab. We identify kindred amphibolite grade rocks cut by compo- rocks with similar geological relations Cretaceous Peninsular sitionally zoned, mesozonal plutonic in other Cordilleran batholiths of the Ranges Batholith of complexes of the La Posta suite, Americas, such as the Sierra Nevada, emplaced from 99–86 Ma. While plu- which lead us to suggest that slab fail- Southern and Baja tons of the La Posta suite are generally ure magmatism is common, both spa- California considered to be the product of con- tially and temporally. tinued eastward subduction, they are Robert S. Hildebrand1 and Joseph enigmatic, because they and their wall SOMMAIRE B. Whalen2 rocks were rapidly exhumed from as Depuis la fin des années 1960, Warren deep as 23 km and eroded during, and Hamilton a proposé que les grands 1Department of Earth and Planetary just after, their emplacement, unlike batholites de la Cordillère de l'ouest Sciences, University of California, Davis, plutons in magmatic arcs, which are des Amériques sont les racines d’arcs California 95616-8605, USA generally emplaced in zones of subsi- volcaniques andéens issus de la sub- E-mail: [email protected] dence. duction vers l'est de longue durée, et Here we resolve the enigma depuis la plupart des géologues ont 2Geological Survey of Canada with a model where westward-dipping emboîté le pas, bien qu’un nombre 601 Booth St., Ottawa, Ontario subduction led to arc magmatism of croissant d’indications montrent que de K1A 0E8, Canada the western sector, the Santiago Peak– nombreux batholites de la Cordillère Alisitos composite arc, during the peri- sont des entités composites complexes SUMMARY od ~128–100 Ma. Arc magmatism shut qui se sont développés lors d’inter- Ever since the late 1960s when Warren down when the arc collided with a valles intenses de contraction et d’ex- Hamilton proposed that the great west-facing Early Cretaceous passive humation, durant et entre les périodes Cordilleran batholiths of the western margin at about 100 Ma. During the de magmatisme. Americas are the roots of volcanic arcs collision the buoyancy contrast Le batholite Peninsular Ranges like the Andes and were generated by between the continental crust of the du Sud de la Californie et de Baja Cali- longstanding eastward subduction, eastern block and its attached oceanic fornia est un excellent endroit permet- most geologists have followed suit, lithosphere led to failure of the sub- tant de démêler les choses parce qu'il y despite the evergrowing recognition ducting slab. The break-off allowed a beaucoup de données et parce qu'il that many Cordilleran batholiths are subjacent asthenosphere to upwell, adi- est composé longitudinalement de

Geoscience Canada, v. 41, http://dx.doi.org/10.12789/geocanj.2014.41.059 © 2014 GAC/AGC® 400 deux parties: une partie occidentale batholites de la Cordillère des that the titanic Cordilleran-type plus ancienne, faiblement à modéré- Amériques, tel celles de la Sierra batholiths of western North America ment déformée, de roches volcaniques Nevada, ce qui nous amène à penser could be divided longitudinally into de faible métamorphisme et de roches que le magmatisme de cassure de parts: (1) a western zone of plutons plutoniques épizonales âgées d’environ plaque est commun, tant spatialement with dominantly dioritic and more 128 Ma à 100 Ma; et, d’un segment et temporellement. mafic compositions; and (2) a more plus à l'est de roches amphiboliques easterly tract of more siliceous and déformées recoupées par des roches de INTRODUCTION potassic-rich plutons, such as granodi- composition zonée des complexes Cordilleran batholiths are elongate orite and quartz monzonite (Budding- mésozonaux plutoniques de la suite de masses – many over 1000 km long – of ton 1927). By 1948 Larsen had also la Posta, mises en place entre 99 Ma et gregarious plutons emplaced within the realized that the more mafic bodies in 86 Ma. Bien que les plutons de la suite American Cordillera during the Creta- the Peninsular Ranges batholith of La Posta sont généralement considérés ceous (Larsen et al. 1958). They com- Southern and Baja California were comme le produit d’une subduction prise hundreds, if not thousands, of confined to the western parts; but soutenue vers l’est, ils posent prob- individual plutons, dykes and plugs despite these early forays into lème, parce qu'avec leurs roches encais- ranging in composition from gabbro to batholithic division, it took until 1959 santes, ils ont été rapidement exhumés granite and are related to many impor- for J.G. Moore to carry the observa- de profondeurs aussi grandes que 23 tant mineral deposits of the region. tions over the length of North Ameri- km, et érodées durant et juste après In his now classic report on ca, and name the boundary between leur mise en place, contrairement aux the geology of the US–Canada bound- the zones the quartz diorite line plutons des arcs magmatiques, qui sont ary region, Reginald Daly recognized (Moore 1959; Moore et al. 1961). généralement mis en place dans les that the batholiths were emplaced in The idea took root and iso- zones de subsidence. mountain belts (Daly 1912), but ulti- topic investigations by Kistler (Kistler Dans le présent article, nous mately it was Hamilton (1969a, b) who and Peterman 1973, 1978; Kistler proposons une solution à ce problème, associated the batholiths with subduc- 1990) led to recognition that the east- avec un modèle de subduction vers tion: specifically, long-lived eastward ern and western sectors of the Sierra l'ouest qui conduit à un magmatisme subduction of Pacific lithosphere Nevada batholith have different base- d'arc du secteur ouest, l'arc composite beneath the Americas. He argued, ments. Similarly, detailed field studies de Santiago Peak-Alisitos, durant la based on previous work elsewhere and geochronology within the Coast période d’environ 128 Ma à 100 Ma. (Hamilton and Myers 1967), that the batholith of Canada and southeastern Le magmatisme d’arc s’est arrêté batholiths were the roots of volcanoes Alaska led to the realization that it too lorsque l'arc est entré en collision avec built along the western margins of the could be readily divided into two parts, une marge passive à pendage ouest du Americas and were analogous to the there separated by the collapsed Nut- début du Crétacé, il y a environ 100 modern Andean volcanic arc. These zotin-Dezadeash-Gravina basin and Ma. Lors de la collision, le contraste de concepts provided a basis for the more related fold–thrust belt (Rubin et al. flottabilité entre la croûte continentale recent interpretation of Cordilleran 1990; Journeay and Friedman 1993; du bloc de est et la lithosphère batholiths as magmatic arc rocks Gehrels et al. 2009). And farther south, océanique qui y est rattachée a conduit emplaced during compressional defor- additional studies within the Peninsular à l'avortement de la plaque plongeante. mation and great crustal thickening, Ranges batholith demonstrated that it La cassure a entrainé la remontée de which is considered to be related to (1) could be readily divided into an older, l’asthénosphère sous-jacente, sa fusion voluminous influx of magmas from more westerly, magnetite-bearing suite adiabatique, et sa remontée dans la the mantle; (2) underthrusting of the and a younger, ilmenite bearing, high plaque supérieure pour former les crust in either a fore-arc or retro-arc Sr/Y suite to the east (Gastil et al. grands complexes zonés de tonalite- setting, commonly with shallow sub- 1975, 1990; Silver et al. 1979; Silver granodiorite-granite de La Posta. Bien duction; or (3) a combination of the and Chappell 1988; Kimbrough et al. que de composition similaire aux plu- two (Isacks 1988; Allmendinger et al. 2001; Tulloch and Kimbrough 2003). tons d'arc à bien des égards, les exem- 1997; Ducea 2001; Kay et al. 2005; So despite widespread recog- ples des segments de batholites de Cal- Grove et al. 2008; DeCelles et al. 2009; nition among Cordilleran geologists ifornie du Sud et de Baja ont une Paterson et al. 2012; Chin et al. 2012). that the major Cretaceous batholiths géochimie qui indique qu'ils provien- There are, however, complications. appeared to be constructed of two nent de la fusion partielle de Based on his work in Califor- halves (Fig. 1), our understanding of l’asthénosphère à des niveaux plus pro- nia’s Sierra Nevada, the tireless Cordilleran batholiths has barely pro- fonds dans le manteau que les magmas Swedish-American economic geologist, gressed since the late 1960s, largely d'arc typiques, à l’intérieur du domaine Waldemar Lindgren, knew in a general because most workers still follow de stabilité du grenat. Ce qui corre- way that irrespective of age, plutonic Hamilton’s original concept that the spond à une remontée d’asthénosphère compositions there graded from more Sierran and Peninsular Ranges à travers une dalle de plaque inférieure mafic in the west to more siliceous in batholiths were built entirely by east- cassée. Nous connaissons des roches the east (Lindgren 1915), but it was up ward subduction of oceanic litho- semblables avec les relations to the great American petrologist, sphere beneath the western margin of géologiques similaires dans d'autres Arthur Buddington, to clearly articulate the Americas (Todd et al. 1988, 2003; GEOSCIENCE CANADA Volume 41 2014 401

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Figure 1. Sketch maps illustrating the composite nature of three Cordilleran batholiths of North America as recognized by others. Peninsular Ranges modified from Kimbrough et al. (2001); Coast plutonic complex of British Columbia modified from Gehrels et al. (2009); central Sierra Nevada batholith adapted from Lackey et al. (2008).

Walawender et al. 1990; Kimbrough et batholiths there was a period of ure to elucidate the geological develop- al. 2001, 2014a; Ducea 2001; Schmidt intense deformation at ~100 Ma, ment of the batholith, test the viability et al. 2002, 2009; Grove et al. 2003; which occurred between the magma- of the model, identify possible geo- Tulloch and Kimbrough 2003; Wet- tism of the older western and the chemical characteristics of slab failure more et al. 2003; Busby 2004; Lee et al. younger eastern sectors. He outlined a magmas, and end by integrating our 2007; Gehrels et al. 2009; DeCelles et model in which the deformation was results with other similar magmatic al. 2009; Paterson et al. 2012; Kim- caused by a collision between the two suites. This is the second in a planned brough et al. 2014a; Shaw et al. 2014). sectors, and that plutons emplaced dur- series on Cretaceous batholiths of the In this paradigm, two subvariant mod- ing and after collision likely originated American Cordillera (Hildebrand and els, both with eastward-directed sub- by break-off and failure of the Whalen 2014) and is an outgrowth of duction, dominate the discussion: (1) descending oceanic lithosphere due to both authors’ long-standing interest in the younger, more siliceous and potas- the buoyancy contrast between conti- plutons and batholithic terranes (Hilde- sic easterly plutons are related to, and nental and oceanic lithosphere. As the brand 1981, 1984; Whalen 1985; thus document, progressive shallowing detached slab sank into the mantle, the Whalen and Chappell 1988). of eastward subducting oceanic litho- widening gap allowed the subjacent sphere with or without subduction of asthenosphere to upwell and melt adia- GEOLOGICAL SETTING the older arc and forearc sediments; or batically to create magmas that rose The Peninsular Ranges batholith, (2) the western sector was separated into, and interacted with, the overlying which comprises hundreds of distinct from the eastern sector by a narrow mantle lithosphere and lower crust. It plutons, extends over 1000 km from back-arc basin, and following closure, was those magmas that led to the just south of Los Angeles, California, subduction-related magmatism simply younger, post-collisional halves of at 34ºN to at least 28ºN near Guerrero continued to produce the eastern sec- Cordilleran batholiths. Negro (Fig. 2), located just south of tor of the batholiths. For this contribution then, we the frontier of Baja Norte de Califor- In his regional synthesis of examine the geology and geochemistry nia (Gastil et al. 1975). As the western the North American Cordillera, Hilde- of the Peninsular Ranges batholith of half forms a continuous linear belt of brand (2013) noted that within the Southern and Baja California in the magnetic anomalies visible beneath the Sierran and Peninsular Ranges context of arc, collision and slab fail- extensive Cenozoic volcano-sedimenta- 402

whereas to the east Upper Cretaceous 1¬ MagneticMagnetic FieldField 1 (nT)(nT) plutons intrude mainly Jurassic and CenozoicCenozoic vvolcanicolcanic covercover SGFSG GGF SAFSA UUpperpper CCretaceousretaceous cclasticslastics Paleozoic upper amphibolite grade F AAF 11611 720

666600 UUndatedndated pplutonslutons metasedimentary rocks that sit on <<100100 MMaa LaLa PostaPosta suitesuite NIF 660000 :¬ IF unexposed crystalline basement. 1130-9830-98 aarcrc pplutonslutons : 540 LM SSantiagoantiago PPeak-Alisitoseak-Alisitos Within the batholith itself, 480 vvolcanic-volcaniclasticolcanic-volcaniclastic EF 420 JJuro-Triassicuro-Triassic rrocksocks gabbroic plutons are mostly confined ¬ SJFS ¬ J PPrecambrianrecambrian & F 360 SAF PPaleozoicaleozoic rrocksocks to the western half (Larsen 1948; Kim- San AF EPRMZ 300 SanSan Diego DiegoDiego brough et al. 2014b) and so, according 240 La PostaPosta 180 pluplutonton to some workers, their eastern bound-

Ensenadaa 120 EnsenadaEnsenada ary defines a gabbro line (Walawender 6600 AguaAgua Blanca ABFA 1979; Smith et al. 1983). The gabbro B 0 faufaultlt F MainMain 30°30 -60-60 line coincides closely with the western ° MartirMartir SierraSierraS Sanan PPedroedro 118°11 -120-120 8° thrustthrust MMártirártir plutonpluton extent of gneissic Jurassic plutons -180-180 SanSan (Shaw et al. 2003). An I–S line, which SanS Quintinnt -240-240 QuintinQuintin AguaAgua R

o S B o largely reflects Jurassic plutonism, and s -300-300 CCalientealiente i a n s

r b o i e o FFaultault r a e -360-360 G an oxygen isotope gap, also closely

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o C l a u a

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f CataviñaCataviña n b o approximate the other lines (Taylor and

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pplutonluton r BBlanca ca Silver 1978; Todd and Shaw 1985). In ¬ ¬ CedrosCedros IIs.s. the west, Cretaceous plutons range in age from 128–100 Ma and have no sys- Vizcaino GGuerrerouerrero peninsula NNegroegro tematic age distribution; whereas to the ¬ V i z ¬ c HermosilloHermosillo east large nested tonalite–granodior- a i n o ite–granite complexes, called the La P e ElElA Arcorco n i n Posta suite, range in age from 99–92 26°26 s u G la u Ma and young progressively eastward l

Cenoz f (Silver and Chappell 1988; Walawender

oic volcanics f et al. 1990; Johnson et al. 1999b; Kim-

n C brough et al. 2001; Schmidt et al. a a e l i 2014). This distribution of ages led Sil- c f

o O r ver (Silver et al. 1979; Silver and Chap- ¬ n M 114°1 i 1 c pell 1988) to suggest that a magmatic 4 a a i g

f d a i l arc remained fixed in the west until e n c a

a ~105 Ma and then migrated eastward. Laa Paz P Geophysical data also support the two sector division in that the western 2222° ° more mafic part is dense, magnetic, has low heat flow, sparse seismicity, and CaboCabo SSanan 100 km LucasLucas 100 km seismic velocities > 6.25 km/s, where-

¬ ¬ as the more siliceous eastern sector is ¬ less dense, weakly and magnetically Figure 2. Aeromagnetic anomaly map of Southern and Baja California showing quiet, has higher heat flow, abundant the presumed continuation of the western part of the batholith southward beneath microseismicity, and seismic velocities Cenozoic volcanic cover (modified from Langenheim et al. 2014) and a geological < 6.25 km/sec (Langenheim et al. sketch and location map based largely on Kimbrough et al. (2001) but updated 2014). from various sources cited in the text. SAF–San Andreas fault, SJF–San Jacinto Due to the abundance of both fault, EF–Elsinore fault, ABF–Agua Blanca fault. pre- and post-deformational plutons, and cover related to the opening of the ry cover (Langenheim et al. 2014), it Todd et al. 1988; Silver and Chappell Gulf of California, the inferred probably extends another 400 km to 1988). Wall rocks in the west consist of boundary between the two zones is near Cabo San Lucas at the tip of the Lower Cretaceous volcanic and related difficult to locate, varies from worker peninsula. As mentioned briefly in the volcaniclastic rocks, typically little to worker, and is possibly different at introduction, the batholith and its wall metamorphosed, but generally depth than at the surface. In the Sierra rocks are traditionally divided into two deformed. The volcanic and sedimen- de San Pedro Mártir of Baja (Fig. 2) distinct zones based on different tem- tary rocks are collectively known as the there are two recognized boundaries, poral, lithological, geochemical, geo- Santiago Peak volcanics in the United both marked by significant faults: (1) a physical, and isotopic characteristics States and the Alisitos Group on most western fault known as the Main Már- that parallel the axis of the batholith of the Baja Peninsula, Mexico. They tir fault, which dips steeply eastward (Gastil 1975; Silver et al. 1979; presumably sit upon deformed and and places Jurassic and Lower Creta- DePaulo 1981; Baird and Miesch 1984; metamorphosed Triassic–Jurassic rocks ceous amphibolite grade gneiss, GEOSCIENCE CANADA Volume 41 2014 403 metavolcanic rocks of the Santiago east on the Mexican mainland. mates unreliable (Schoellhamer et al. Peak volcanics, and plutons on the east 1981). Fossiliferous limestone units over much lower grade rocks of the PRE-BATHOLITHIC ROCKS contain Callovian fossils, although Alisitos Group and plutons to the west The rocks on each side of the tradi- some geologists are concerned that the (Johnson et al. 1999b; Schmidt et al. tionally inferred crustal boundary (Fig. fossils are allochthonous and were 2009, 2014); and (2) a more easterly 3) are quite different, both in terms of resedimented, but nevertheless the fault, known as the Agua Caliente, that lithology and metamorphic grade. At rocks predate the Santiago Peak vol- dips moderately westward, and places the surface in the west, a Jurassic clas- canics (Silberling et al. 1961; Imlay high-grade migmatitic and ultramy- tic succession (Silberling et al. 1961), 1963; Moran 1976). lonitic gneiss on the west over an the Bedford Canyon Formation, is Just north of San Diego, a lat- amphibolite grade carbonate–quartzite unconformably overlain by the Creta- est Jurassic group of marine volcani- succession intruded by Jurassic gneissic ceous volcanic and volcaniclastic rocks clastic rocks, collectively named the plutons (Measures 1996; Schmidt et al. of the Santiago Peak volcanics and Peñasquitos Formation, was folded, in 2009, 2014). Farther south, in the Sier- Alisitos Group, which are themselves places even overturned, prior to depo- ra Calumague, the eastern boundary is unconformably overlain by upper Cre- sition of the Santiago Peak volcanics thought to be represented by ductile taceous sandstone and conglomeratic (Kimbrough et al. 2014a). Their detrital faults, which commonly, but not every- units. Basement to the volcanic and zircon data, plus similar fossils, suggest where, dip steeply eastward, and are volcaniclastic rocks appears to be dom- that these rocks correlate with other probably normal faults as they place inantly Jurassic and to comprise a volcaniclastic rocks on the Vizcaino lower grade rocks atop higher grade group of deformed and metamor- Peninsula of Baja, as originally suggest- rocks (Griffith and Hobbs 1993). phosed Jurassic plutons, known as the ed by Kimbrough and Moore (2003), Within the US segment of the Harper Creek and Cuyamaca Reservoir and possibly with the lowermost strata batholith, the contact is generally suites, that intruded Jurasso–Triassic of the Great Valley Group to the inferred to coincide with the geophysi- high-grade migmatitic schist and north. They are probably correlative cal, isotopic, and geochemical breaks, gneiss, known as either the Julian or with rocks of similar age and deforma- yet similar plutonic rocks appear on Stephenson Peak schist (Shaw et al. tion in Sonora, where they are known both sides of it, leading some workers, 2003; Todd 2004). as the Cucurpe Formation (Mauel et al. such as Todd et al. (2003) to argue that Supracrustal rocks in the east 2011). Cretaceous arc magmatism simply are scarce, not only due to the Preserved volcanic rocks, col- migrated eastward over a pre-existing emplacement of post-deformational lectively known as the Santiago Late Jurassic–Early Cretaceous crustal plutons, but also because they are Peak–Estelle Mountain volcanics, are boundary. They did not explain why buried by extensive volcanic and sedi- 128–110 Ma in age and consist of plutons and wall rocks older than mentary cover related to the Tertiary thousands of metres of basaltic to rhy- about 100 Ma are deformed and meta- opening of the Gulf of California. olitic lava and breccia, tuff, and vol- morphosed, whereas younger plutons Additionally, the structure is overprint- caniclastic rocks that sit uncon- are not. Unfortunately, post-deforma- ed by structures formed both during formably upon rocks of the Bedford tional plutons paid little heed to the the Laramide event (Hildebrand 2013) Canyon Formation and are overlain inferred crustal boundary and were and extensional denudation during unconformably by Turonian fanglom- also emplaced on both sides of the opening of the Gulf. Where they do erates of the Trabuco Formation boundary. outcrop, such as in the San Jacinto (Larsen 1948; Fife et al. 1967; Schoell- Whatever form it takes, and Mountains (Fig. 3), they are found to hamer et al. 1981; Tanaka et al. 1984; whether observed or not, the contact is be a sequence of interbedded quartzite Buesch 1984; Gorzolla 1988; Anderson considered by most workers to repre- and carbonate rocks of Paleozoic age 1991; Wetmore et al. 2003; Herzig and sent a suture zone related to either a and finer grained siltstone, chert and Kimbrough 2014). Although there is a closed marginal basin or a collision in a argillite, commonly assumed to repre- variety of more siliceous rocks, includ- doubly eastward-dipping subduction sent deeper water equivalents of the ing hypabyssal intrusions, porphyritic model (Todd et al. 1988; Johnson et al. quartzite–carbonate sequences (Gastil basaltic andesite, andesitic lava and 1999b; Schmidt and Paterson 2002; and Miller 1981; Gastil et al. 1991; related breccia are the most common Busby 2004; Lee et al. 2007; Schmidt et Gastil 1993). and widespread volcanic rocks in the al. 2014; Wetmore et al. 2014; Shaw et formation (Schoellhamer et al. 1981; al. 2014). Only Dickinson and Lawton Western Sector Herzig and Kimbrough 2014). Rocks (2001a) and a French group (Tardy et The Bedford Canyon Formation (Fig. in the west are less deformed and only al. 1992, 1994) proposed westward-dip- 3) comprises thousands of metres of weakly metamorphosed whereas to the ping subduction models. Here we will slightly metamorphosed argillite and east amphibolite grade and isoclinally argue that the contact, inferred to be slate, with lesser amounts of feldspath- folded tuff, breccia, and basaltic, an important crustal boundary is an ic sandstone, lenses of limestone, and andesitic, and dacitic lava flows occur unconformity between Jurasso–Triassic pebbly conglomerate (Larsen 1948). as concordant screens between and rocks and overlying Lower Cretaceous The base of the formation is unex- within Cretaceous plutonic sheets of volcanic and sedimentary rocks and posed and the rocks are probably iso- the western section and are the same that the critical suture lies well to the clinally folded making thickness esti- age as the Santiago Peak volcanics 404

0500 50 EasternEasternP Peninsulareninsular Ranges Ranges InterstateInterstate 1010 mylonitemylonitez zoneone 34º kmkm CretaceousCretaceous PPeninsulareninsular n i s RangesRanges batholithbatholith B an a ning Fa B ult MyloniticMylonitic metasedimentarymetasedimentary

s rocks,rocks, anatexite,anatexite, ororthogneissthogneiss e l of Palm Palm Canyon Canyon Complex Complex e PalmPalm g S SSpringsprings PaleozoicPaleozoic DDesertesert Divide Group Groupp n a MSJMSJ n A C andand otherother mostlymostly upper PerrisPerris J o a a amphiboliteamphibolite gradegrade rrocksocks s c c h

o i e

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F S a a u n l R t C Z o o h n s o e a A c s n o d l P re a a a t c s e ifi S a M c l o O t u S o c C n n a t e n a a S t S SanSan i n an e n a a s J BorregoBorrego a JacintoJacinto c F SSpringsprings in a to u BBlocklock lt Fa u l CretaceousCretaceous sedimentary sedimentary t Z Z o o n rocks:rocks: TTrabuco,rabuco, LLusardi,usardi, A n e and Baker conglomerate conglomerate n e I a m LowerLower CretaceousCretaceous gabbrogabbro p e r LowerLower Cretaceous Cretaceous SSantiagoantiaggo i 33º M a PeakPeak and Estelle Estelle Mountain Mountaiin l PerrisPerris t volcanicvolcanic rrocksocks n V BBlocklock a JurassicJurassic BBedfordedford CanyonCanyon s l l . e metaturbiditesmetaturbidites aandnd alliedalliedd LaLa PPostaosta pplutoniclutonic y rocksrocks ccomplexomplex M-IM-I MMid-Lateid-Late JurJurassicassic ggneissicneissicc linene ElEl CentroCentro ggranite,ranite, migmatiticmigmatitic schistt InterstateInterstate 8 FigureFigure5 5 Julian schist: TTriassic-Jurassicriassic-Jurassic metasedimentarymetasedimentary rrocks,ocks, SSantaanta volcanicvolcanic rrocksocks & amphibolite amphiboolite SanSan AAnana U.S.A.U.SSA.A. JurassicJurassic mmigmatiticigmatitic scschisthist DiegoDiego BBlocklock MexicoMexico aandnd gneissgneiss ofof StephensonStephenson º 1 7 PeakPeak 1 1 6 1 º a

IsostaticIsostatic gravitygravity AeromagneticAeromagnetic ÝÝ ÝÝ ÝÝ ÝÝ ÝÝ ÝÝ ÝÝ ÝÝ mGalmGal nT

ÝÝ Ý 16 B'B 12 AA' -12 ÝÝ ÝÝ -16-16 A Ý Ý KM b KM c

Figure 3. (a) Geological sketch map modified from Jennings (1977), Morton and Miller (2006), Todd (2004), and Todd et al. (1988) illustrating the general geology of the Peninsular Ranges of Southern California, the location of Figure 5, as well as the M–I (magnetite–ilmenite) line, the δ18O step, and the eastern limit of gabbroic rocks (Todd et al. 1988). PV–Paloma Valley ring complex; SR–Searl Ridge, MSJ–Mount San Jacinto, A–Asbestos Mountain, M–Martinez Mountain, C–Coyote Mountain. (b) Iso- static gravity anomaly map and (c) aeromagnetic anomaly map, both modified from Langenheim et al. (2004). Note the change in gravity and aeromagnetic anomalies coincident with other signatures between western Santiago Peak volcano-sedimentary rocks and more easterly units. GEOSCIENCE CANADA Volume 41 2014 405

(Todd 2004; Shaw et al. 2014). bivalves define most of the Alisitos the Agua Blanca fault, ranges in age Based on the geochemistry of Group (Allison 1974). There, 2 km- from 170–149 Ma and is cut by gab- the volcanic rocks, which are altered thick piles of andesitic lava, breccia, broic and tonalitic plutons ranging in and dominantly metamorphosed to volcaniclastic rocks, and intrusive por- age from 132–100 Ma, typical of the greenschist grade, they represent both phyry, likely representing ancient stra- western sector (Schmidt et al. 2014). tholeiitic and subalkaline series, with tocones, are capped by reefal limestone The gneiss is most likely rock exhumed overall compositions similar to the bioherms. from deep beneath the Alisitos arc and contemporaneous plutonic rocks Farther south on the Baja if so, would provide a reasonably clear (Tanaka et al. 1984; Herzig and Kim- Peninsula, rocks of the formation are picture of the basement, which is con- brough 2014). Wetmore et al. (2003) mostly greenschist grade, but generally tinental, but young, consistent with used the Tanaka et al. (1984) data as lower grade in the west away from the primitive isotopic ratios obtained by well as those of Gorzolla (1988) and plutons, compositionally diverse, Weber and Lopéz Martinez (2006) at Herzig (1991) and, based on the large deposited in both marine and subaerial El Arco. number of rhyolitic analyses compared settings, and comprise andesitic strato- A steeply dipping panel of to basalt analyses – but without any cones, silicic ignimbrite and associated amphibolite-grade, highly strained volume estimates – as well as the pres- calderas, a variety of volcanclastic metavolcanic rocks of the Santiago ence of xenocrystic zircon grains, con- rocks, hypabyssal porphyritic rocks and Peak volcanics lies along the western cluded that the arc was built on conti- granitoid plutons of dominantly Creta- flank of the gneiss, but east of the nental crust, although the variety and ceous age (White and Busby-Spera Main Mártir fault (Schmidt et al. 2014). size of granitoid plutons demonstrate 1987; Schmidt et al. 2002; Busby et al. We speculate that the contact between that as well. Premo and Morton (2014) 2006). Volcanic rocks and associated the gneiss and volcanic rocks is most dated a number of detrital zircon plutons range from 116 to 100 Ma and likely a highly tectonized unconformity grains within metasedimentary rocks are not known to contain inherited zir- and that the gneiss lies beneath the and found abundant Triassic and con (Wetmore et al. 2003; Schmidt et volcano-sedimentary package. A care- 1740–1650 Ma grains, whereas high- al. 2014). ful search along this contact might grade gneiss units contained concor- A 330º-trending swarm of yield some evidence for this in the dant Proterozoic zircon and where dis- ~126 Ma basaltic to rhyolitic dykes, form of stretched pebbly conglomer- cordant, Proterozoic upper intercept known as the San Marcos dyke swarm ate and/or arkosic sand lenses. ages, similar to those reported from occurs north of the Agua Blanca fault Within the US sector of the volcanic rocks by Herzig and Kim- where it appears to be related to the batholith (Fig. 3) is another likely base- brough (2014). Santiago Peak volcanics (Farquharson ment assemblage, comprising the Julian Nd and Sr isotopic values for 2004). A similar swarm south of the Schist and associated Jurassic ortho- the volcanic rocks are mostly similar to fault, with the same major and trace and paragneiss. Rocks included in the plutons of the western sector of the element contents as those to the north, Late Triassic–Mid Jurassic Julian Schist batholith (Herzig and Kimbrough but at higher metamorphic grade, was are diverse and include predominantly 2014), and when coupled with the correlated with the northern swarm by schist and quartzite with subordinate overall intermediate to siliceous com- Schmidt et al. (2014), who used it to quantities of amphibolite, mafic schist, positions and xenocrystic zircon, clear- suggest continuity between the Santia- gneiss, metaconglomerate, calc-silicate ly indicate that the arc was built on go Peak and Alisitos segments, rock, quartzite, marble and talc schist continental, not oceanic, crust. although only one dyke there is dated, (Todd et al. 1988; Germinario 1993; Rocks included in the Santiago at 120 Ma. If Schmidt et al. (2014) Shaw et al. 2003). Although the rocks Peak volcanics continue for at least 300 were correct then the Agua Blanca are typically isoclinally folded, foliated km southward to just south of the fault is not a fault of great significance, and metamorphosed, holding porphy- Agua Blanca fault in Mexico (Fig. 2). at least after about 126–120 Ma. roblasts of sillimanite and/or There a NW-striking, steeply NE-dip- Basement outcrops other than andalusite, relict bedding is commonly ping mylonitic shear zone, 50–100 m the deformed Bedford siliciclastic observed (Germinario 1982). Todd et thick, separates the Santiago Peak vol- rocks were previously considered to be al. (2003) considered that the Julian canics from rocks of the Alisitos scarce, but we believe them to be more Schist was restricted to the western Group, which were deformed into common. Near the state line at El sector, but suggested that just to the southwest-vergent overturned folds Arco (Fig. 2) is a 6 km-thick section of east were similar rocks containing and thrusts prior to intrusion of 108 andesitic lava and breccia, and associat- greater amounts of marble and quartz- Ma plutons (Wetmore et al. 2005; ed pyroclastic and epiclastic rocks, all rich metasandstone. The Jurassic Alsleben et al. 2008). The zone is also cut by a monzodioritic body dated at metasedimentary rocks contain abun- the locus of a modern strike-slip fault ~165 Ma (Barthelmy 1975; Weber and dant Paleozoic and Proterozoic detrital with ~22 km of dextral separation Lopéz Martínez 2006). These rocks are zircon (Gastil and Girty 1993; Grove (Allen et al. 1960). South of the fault, probably basement to the Alisitos et al. 2008). in the type area, at least 7 km of Late Group, although the contact itself is A band of deformed Jurassic Aptian–Early Albian volcanic, volcani- not exposed. Jurassic orthogneiss (Fig. granitoid rocks intruded the Julian clastic rocks, and shallow marine sedi- 4) occurs throughout the central por- Schist and extends through the western mentary rocks with abundant Tethyan tions of the Baja peninsula south of part of the eastern zone from southern 406

B This are AguaAgua a not m appe CCalientealiente d in pplutonluton d t eta l il u a

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t h AlisitosAlisitos ararcc r u 2299 covercover basementbasement s t 118118 mainlymainly gabbrogabbro ultramyloniteultramylonite & hbd tonalite tonalite shearshear zonezone rocksrocks N mainlymainly ~128~128 MMaa volcanicvolcanic sheetedsheeted tonalitetonalite Muscovite-bearingMusccovite-bearing JurassicJurassic pluplutonton mainlymainly mmyloniticylonitic sedimentarysedimentary biotitebiotite ggraniteranite tonalitetonaalite SantiagoSantiago PPeakeak 5 kkmm metavolcanicsmetavolcanics orthogneissorthogneiss meta-carbonate,metaa-carbonate, quartzite,quartzite, and chertchert

100.9100.9 8 87.97.9 8 89.59.5 8 85.75.7 8 83.83.8 8 89.99.9 8 85.25.2 8 85.35.3 8 87.37.3 8 81.31.3 8 81.61.6 8 83.43.4 b biotiteiotite

ABB’A unconformity?unconformitty? A’A’ B B’ 16001600

Figure 4. Geological sketch map and section from Schmidt and Paterson (2002); Schmidt et al. (2014) showing the location and local geology in and around the Main Mártir thrust, the Agua Caliente fault, and the doubly vergent fan structure. Note that plutons older than 100 Ma occur east of the Main Mártir thrust, indicating that it cannot be the main suture. Similarly, 164 Ma Jurassic plutons occur on either side of the Agua Caliente thrust suggesting that it also is not the bounding suture. We infer, based on relations elsewhere as discussed in the text, that the contact between the metavolcanic rocks and Jurassic rocks east of the Main Mártir thrust is a tectonized unconformity.

California well into Baja California. tic zircon (Shaw et al. 2003, 2014). and Girty 1994). The nature and origin The plutons, dated to be 234–149 Ma, Large numbers of enclaves, as well as of the sheared rocks, which are cut by were subdivided into two suites, both exterior mantling by intensely a 104 Ma pluton, are obscure, as of which are dominantly ilmenite-bear- migmatitic and gneissic sheaths of deformed, but coherent and mappable, ing and strongly deformed: (1) the per- Stephenson Peak schist, are typical of Jurassic and Cretaceous plutons occur aluminous Harper Creek suite and (2) the Harper Creek suite (Todd and within the zone (Todd 2004). If the the Cuyamaca suite, which is transi- Shaw 1985). The plutons are elongate mylonitic fabrics in the wall rocks rep- tional in bulk composition between in the north-northwesterly direction resent a shear zone of sizable displace- metaluminous and peraluminous (Todd and are strongly foliated, gneissic, or ment it must have been active prior to and Shaw 1985; Shaw et al. 2003). mylonitic (Todd 2004). They are gener- emplacement of the Jurassic plutons. They have δ18O ranging from 12 to 20 ally considered to be within the Cuya- West of the San Jacinto fault per mil, initial 87Sr/86Sr ratios from maca–Laguna Mountain shear zone, zone (Fig. 3), the regional plunge ~0.706 to ~0.713, zircon Hf data rep- which is a 5–20 km wide polygenetic appears to be gently to the northwest, resenting model ages between 1200 feature that extends for about 50 km, as evidenced by the abundance of and 800 Ma, and Proterozoic xenocrys- and dips steeply eastward (Thomson strongly deformed and amphibolite- GEOSCIENCE CANADA Volume 41 2014 407 gradeJurassic plutons and gneiss and Mountains and western Nevada south- four dacitic clasts within one of the absence of volcanic rocks in the southward to the Sonoran Desert region conglomerate units yielded ages of east, and the abundance of green- (figure 42 in Hildebrand 2013). 108–106 Ma (Herzig and Kimbrough schist-grade volcanic rocks and A folded and now steeply 2014). absence of Jurassic plutons to the eastward-dipping unconformity In the San Diego and north- northwest. This fits with observations between basement and cover can satis- ernmost Baja areas Turonian–Campan- in the western part of the block, where fy the various elements, such as oxygen ian non-marine bouldery fanglomerate Shaw et al. (2003) found that plutonic isotopic step, aeromagnetic anomaly, and sandstone of the Trabuco, Baker, contacts of the Jurassic plutons are gravity, and I–M line that generally Lusardi, and Redonda formations (Fig. sharper, marginal facies more leuco- have been interpreted to represent a 3) sit unconformably above plutonic cratic, coarser grained, and with amphi- suture zone. Cretaceous plutons and volcanic bedrock on a surface of bole more common than biotite, intruded the basement, their own high relief and contain mostly clasts of whereas to the east, the plutons are cover, and the unconformity between plutonic and pre-batholithic rocks to much richer in metasedimentary them (Fig. 5). 10 m (Flynn 1970; Nordstrom 1970). enclaves, clots of mica, their outer On the Vizcaino Peninsula of The conglomerate is unconformably contacts tend to be more gradational, Baja (Fig. 2), over 10 km of turbidites overlain by Campanian–Maastrichtian up to a km across, and are strongly ori- of the Valles Group, interpreted to marine sandstone and shale (Kennedy ented parallel to gneissic foliation. represent a deep-sea fan complex 1975). Farther south in Mexico, the All of the data presented deposited on the edge of an ocean Turonian–Campanian section is uncon- above indicate that the Santiago Peak basin (Minch et al. 1976), have a formably overlain by Campanian to volcanics, or as they are sometimes diverse fauna indicating an Albian to Maastrichtian sedimentary rocks of called the Western metavolcanic rocks, Santonian age (Berry and Miller 1984). non-marine to shallow marine clastic sit unconformably upon the Julian They sit on older Jurasso–Triassic formations overlain by deep marine Schist and the associated plutons and ophiolitic, sedimentary, and volcanic fan deposits of the Rosario Formation migmatite. The contact is readily visible rocks (Smith and Busby 1993). Accord- (Morris and Busby-Spera 1990). on Figure 5, where 128–100 Ma plu- ing to Kimbrough et al. (2001), the tons intrude both sides of the contact. Valle Group is divided into three sub- Eastern Sector It is implausible that it is a fault as the basins containing sections of early Pre-batholithic rocks in the eastern half volcano-sedimentary package is the Cenomanian to middle Turonian of the batholith are highly varied, typi- same age as, or only very slightly older coarse clastic rocks enveloped in much cally strongly deformed, metamor- than, the plutons that intruded the finer grained shaly strata. The clastic phosed to amphibolite grade, and with- contact: it must be an unconformity. sections contain coarse conglomerate out exposed crystalline basement in the This implies that the Jurassic rocks beds and abundant sand-rich turbidites, Peninsular Ranges. Paleozoic clastic were exposed at the surface during the some of which are tightly dated by fos- and carbonate rocks dominate and, early Cretaceous, and that there was a sils as 92–91 Ma and contain abundant based on age and lithology, most work- significant basement complex beneath 100–90 Ma detrital zircon grains, ers interpreted them as part of North the Santiago Peak rocks. In fact, the which are interpreted to have been America. In the US sector Paleozoic presence of Proterozoic xenocrystic derived from the La Posta plutons metasedimentary rocks of the Desert zircon within the Harper Creek rocks (Kimbrough et al. 2001). Divide Group (Fig. 3) are well exposed indicates that the basement contained Another package of Upper along the spine of the San Jacinto Proterozoic rocks or sedimentary rocks Cretaceous sedimentary rocks extends Mountains, where it comprises over 4 containing Proterozoic detrital zircon. for over 500 km along the western side km of quartzose schist, paragneiss, As all the Jurassic basement rocks were of the Peninsular Ranges (Fig. 2), metacarbonate rock and quartzite, all at strongly deformed and metamor- where they unconformably sit on rocks amphibolite grade (Brown 1980, 1981). phosed prior to the deposition of the of the Santiago Peak volcanics, Alisitos Gastil et al. (1991) described the rocks Santiago Peak rocks, it is reasonable to Group and the western batholith (Bot- as compositionally similar to Neopro- infer that they were involved in a latest tjer and Link 1984). Outcropping to terozoic and Lower Paleozoic sedimen- Jurassic–earliest Cretaceous collision, the north above the unconformity, are tary rocks of central Nevada. likely the event responsible for the weakly consolidated sedimentary strata Just to the east, sitting struc- deformation of the Peñasquitos and of Turonian age comprising coarse turally above the Cretaceous plutons in Cucurpe formations, which is bracket- conglomerate with clasts up to 2 m, the Santa Rosa Mountains and rocks of ed to be between about 145 and 136 sandstone, and clayey siltsone (Pope- the Desert Divide Group (Fig. 3), is Ma (Peryam et al. 2012; Kimbrough et noe 1941; Schoellhamer et al. 1981). the Eastern Peninsular Ranges al. 2014a) more or less contemporane- According to Popenoe (1941), who mylonite zone and its hanging wall of ous with the widespread Nevadan counted clasts in several of the Late intensely deformed metasedimentary event (Schweickert et al. 1984). An Cretaceous conglomeratic units, the rocks of the Palm Canyon complex, older event, at ~160 Ma, postdated the clasts are dominantly porphyritic which itself sits structurally beneath emplacement of the Harper Creek and andesite with lesser quantities of grani- Cretaceous plutons (Todd et al. 1988). Cuyamaca Reservoir suites and affect- toid, metamorphic and sedimentary Within the mylonite zone itself, which ed Jurassic arc rocks from the Klamath rocks. U–Pb dating of zircon from has Late Cretaceous deformation and 408

Qu at CR ern ar y c ov floor of of er AAlpinelpine ttonaliteonalite

1 2 EC

roofroof of AlpineAlpine ttonaliteonalite DescansoDescanso AlpineAlpine floor of of Intersta AAlpinelpine te 8 ttonaliteonalite

< 10 0 Ma La Po sta -t yp e pl ut 5 on

CretaceousCretaceous JurassicJurassic LusardiLusardi Formation: Formation: LateLa L teC Cretaceousretaceous CorteCorte MaderaMadera mmonzogranite:onzogranite: JulianJulian SSchist:chist: possiblpossiblyy TTriassicriassic boulderybouldery fanglomeratefanglomerate 116-104116-104 MMaa inin papartrt ChiquitoChiquuitoP Peakeak monzogranite: monzogranite: GrGraniteanite Mtn ttonalite:onalitee: 126126-100-100 MMaa Gneiss of Harper Harper Creek:C reek:1 164 Ma Ma 113-114113-114 MMaa GranodioriteGranodiorite of CuyamacaCuyamacca JapatulJapatul VValleyalley TToTonalite:onalitte: 1102-10102-101 MMaa QtzQtz ddioriteiorite of EastEast Mesa:Mesa: 118118 MaMa Reservoir:Reservoir: 168-161168-161 MaMa PinePine ValleyValley monzogranite:monzogranite: MigmatiticMigmatitic schist & ggneissneisss CuyamacaCuyamaca gabbgabbro:ro: 1107-9907-99 MMaa 129-109129-109 MMaa of Stephenson Stephenson PeakPeak EarlyEarly CretaceousCretaceous metavolcanicmetavolcanic Las BaBancasncas ttonalite:onalite: 1109-10009-100 MMaa andand metasedimentarymmetasedimentary rrocks;ocks; SantiagoSantiaago PeakPeak equivalents,equivalents, N AlpineAlpine ttonalite:onalite: 1109-10709--107 MMaa 5km5km 128-110128-110 MMaa

Figure 5. Simplified geological map of part of the El Cajon map area (Todd 2004) showing the relationship of Early Creta- ceous metavolcanic and allied rocks (black) to the Jurassic rocks (blues). We interpret the relationship between the two as an unconformity because the contact is cut by plutons only slightly younger than the volcanic rocks. Note that metavolcanic rocks don’t occur east of, nor Jurassic rocks west of, the interpreted unconformity. This figure also shows that the Early Cretaceous plutons, which are characterized by wall-rock contacts that are generally concordant with bedding, were sill-like in their original form and are now complexly folded into a typical dome and basin interference pattern (see also Hildebrand 2013). The unconformity is overturned, dipping east (Langenheim et al. 2014), provides pre-deformational way-up to the west, and the map view is an oblique crustal section. One can readily see how the sheets (numbered 1–5 to denote their relative structural position), although irregular in detail, are concordant on a large scale and form concentric layers around cores of folds. Note that the floor of the Alpine tonalite is exposed again in the core of the WNW-trending synform between 4 and 5. Age data from Todd et al. (2003) and Shaw et al. (2014). CR –Cuyamaca, Reservoir, EC –El Capitan Reservoir. GEOSCIENCE CANADA Volume 41 2014 409 exhumation, recognizable sedimentary WE units are as thin as a 1 m thick bed of marble to huge westerly vergent imbri- 130 cate stacks and recumbent nappes of amphibolite grade marble, feldspathized quartzite, gneiss and Santa Ana suite metapelite more than 15 km thick – all 120 Premo et al. (2014b) overprinted by Miocene detachment Shaw et al. (2014) faulting and younger strike-slip defor- Wetmore et al. (2005) mation (Engel and Schultejann 1984). 110 The westward thrusting was likely related to the Laramide event at 80–75 Ma, as suggested by cooling ages of San Pedro de Grove et al. (2003) and Dokka (1984). 100 Age of deformation Mártir pluton Fossils are scarce, but conodonts from the carbonate units in the thrust stack at Coyote Mountain (Fig. 3) are Early (Ma) Age Ordovician in age (Miller and Dockum 90 1983). Similar rocks are recognized La Posta suite Premo et al. (2014b) southward through the eastern Baja Shaw et al. (2014) Peninsula to the Sierra de San Pedro 80 Gastil et al. (2014) Santa Rosa Mártir (Gastil et al. 1991). suite Still farther south at 29ºN, several km of Devonian–Mississippian U-Pb zircon ages with 2σ errors black chert, argillite, pillowed and mas- 70 sive alkali basalt were recumbently and isoclinally folded and are correlated Figure 6. U–Pb zircon ages with 2σ errors for the Peninsular Ranges batholith with the Golconda–Roberts Mountain plotted versus general longitude. Modified from Premo et al. (2014b) with addition- allochthons of Nevada (Gastil et al. al ages from Shaw et al. (2014), Gastil et al. (2014), and Wetmore et al. (2005). The 1991; Campbell and Crocker 1993; pluton ages prior to 100 Ma are not aligned by geography, but by age. Most work- Leier-Engelhardt 1993). ers have recognized that the western Santa Ana suite didn’t migrate with time (Sil- Rocks of the eastern block, ver and Chappell 1988; Shaw et al. 2014). which extend onto mainland Mexico, were correlated on the basis of litholo- Ma (Fig. 6), variably deformed, but series of sheets or sills (Fig. 5) as they gy and detrital zircon profiles with much more so to the east, range in have contacts that are concordant with rocks of the North American passive composition from gabbro to bedding in their wall rocks and with margin (Gastil et al. 1991; Gastil 1993; leucogranite, and show no directional foliation adjacent to both shallow and Alsleben et al. 2012). However, as they variation in composition with time (Sil- steep contacts (Todd 2004). They occur west of known exotic terranes ver and Chappell 1988; Clausen et al. appear to be more prevalent above such as Caborca and Guerrero (Sed- 2014; Premo et al. 2014b; Morton et al. (west of) the contact of the Jurassic lock 1993; Tardy et al. 1994; Centeno- 2014). We use the informal name Santa basement, where they intruded what is García et al. 2008;), locally contain Ana suite for these rocks after exten- assumed to be their own volcanic sequences and rocks similar to those of sive outcrops within the Santa Ana cover (Fig. 5). Recently acquired Hf in the Roberts Mountain and Golconda Mountains (Fig. 3). This suite includes zircon data from the western plutons, allochthons, during the Lower Creta- the more localized Escondido plutons including gabbro, indicate model ages ceous they were more likely to have recently studied by Clausen et al. of 900–250 Ma, which implies that the been part of Rubia, the Cordilleran (2014). original primitive mantle-derived melts Ribbon Continent of Hildebrand Overall, the Santa Ana suite is likely mixed or assimilated more than (2009, 2013), although their ultimate a calcic I-type suite with abundant pris- 50% crustal material (Shaw et al. 2014). origin was Laurentia. The presence of matic hornblende and lesser interstitial Gabbroic plutons are scarce in abundant Baltic and North Atlantic and anhedral biotite. In Mexico, a the east and common in the west (Figs. fauna in Ordovician carbonate rocks number of small (< 10 km), dominant- 3, 4, 5), where they tend to be large, just south of the international border ly epizonal gabbro–tonalite–trond- foliated and cumulate-layered plutons (Gastil and Miller 1981; Gastil et al. hjemite complexes are interpreted to of hornblende-bearing troctolite; 1991; Lothringer 1993) support this represent subvolcanic centres, ranging anorthositic gabbro; and conclusion. in age from 117–100 Ma (Johnson et amphibole–olivine gabbronorite, sur- al. 1999a, 2002; Tate and Johnson rounded by much smaller bodies of THE CRETACEOUS BATHOLITH 2000; Schmidt and Paterson 2002; finer-grained hornblende gabbro, all of Cretaceous plutonic rocks in the west- Schmidt et al. 2009). Many plutons of which have ambiguous temporal and ern sector of the batholith are 128–100 the Santa Ana suite were originally a mingled relations with granitic plutons 410

(Walawender 1979; Todd 2004). Field isotropic plutonic complexes of the ized internally by inclusion-free horn- relations indicating consanguinous western zone such as the Paloma ring blende prisms up to 1 cm long, 0.5 cm emplacement were confirmed by Kim- complex (Fig. 3) appear to us to be titanite crystals, and books of biotite; brough et al. (2014b), who showed that folded sheets (Morton and Baird 1976) (2) a titanite–biotite granodiorite char- dated gabbroic plutons were emplaced especially considering that the geologi- acterized by poikilitic potassium over the same time interval as other cal map (Morton and Miller 2006) feldspar and hexagons of biotite up to plutons of the Santa Ana suite in that shows the Santiago Peak wallrocks to 1 cm across; and (3) an inner zone they range between, or are within ana- be folded. with small euhedral biotite, potassium lytic error of, 128–100 Ma. In Baja California, large pre- feldspar oikocrysts up to 5 cm across, Plutons of the Santa Ana suite 100 Ma plutons are also strongly and typically less than a couple percent within the southern California sector deformed with concordant contacts, muscovite (Walawender et al. 1990). are both normally and reversely zoned, transecting cleavage, and obvious folds Plutons of similar composi- isotropic to foliated or protomylonitic, (Murray 1979; Johnson et al. 1999b, tion, but slightly younger age (Fig. 6), sheeted plutonic complexes, highly 2003). Some plutons there, such as the sit structurally above the Eastern variable in composition ranging from Rinconada pluton (Johnson et al. Peninsular Ranges mylonite zone and tonalite through quartz diorite and gra- 2002), were recumbently folded (Fig. 4; are informally grouped as the Santa nodiorite to leucomonzogranite, locally see also figure 5 in Johnson et al. Rosa suite, after their location within with abundant wall rock screens and 2002). Overall, the data are compelling the Santa Rosa Mountains (Fig. 3). All mafic inclusions and containing varying that pre-100 Ma plutons of the Penin- three suites of plutons, Santa Ana, La proportions of clinopyroxene ± sular Ranges batholith are complexly Posta, and Santa Rosa, were recognized orthopyroxene ± biotite ± hornblende folded sills or sheets. This should be and dated from core recovered from and mafic inclusions (Todd et al. 2003; evident, for if the wall rocks were drill holes in the Los Angeles basin Todd 2004). Morton et al. (2014) noted recumbently folded at 100 Ma then the area farther north, demonstrating that that the westernmost plutons are pre-100 Ma plutons must have been as the batholith presently extends to the isotropic whereas those farther east are well. Transverse Ranges (Premo et al. foliated so that there is a megascopical- Plutonic rocks of the eastern 2014a), where similar rocks outcrop in ly visible deformation gradient from suite, named the La Posta plutons the Santa Monica Mountains just north west to east. This gradient might (Figs. 2 and 6), after a compositionally of Los Angeles (R. Powell, D. Morton, reflect the deeper levels of plutonic zoned plutonic complex that spans the personal communication 2014). Limit- emplacement to the east with the more international border (Walawender et al. ed Hf isotopic data from the La Posta westerly isotropic plutons emplaced 1990), range in age from 99–86 Ma plutons have model ages of about into their own cover at no more than (Premo et al. 2014b), possibly young 2300 Ma (Shaw et al. 2014). 2–3 kbar, whereas the foliated rocks slightly eastward (Ortega-Rivera 2003), Plutons of the La Posta–Santa were emplaced at pressures twice as and are dominated by large, concentri- Rosa suites probably extend eastward large or greater (Morton et al. 2014). cally zoned, mostly weakly foliated across the Gulf of California into Alternatively, the foliated plutons complexes comprising biotite and Sonora and Sinaloa, where a composi- might be closer to the deformational hornblende-bearing tonalitic marginal tionally similar suite of 101 ± 2 to ~90 front. phases grading inward over several tens Ma foliated to non-foliated tonalite Cumulate layering in gabbroic of metres to granodiorite and cored by plutons occurs within 50 km of the plutons is now mostly steeply dipping; granite, in places containing both coast (Henry et al. 2003). They were contacts of plutons are folded, in biotite and muscovite (Hill 1984; Silver intruded into Jurassic and older green- many places isoclinally, along with their and Chappell 1988; Walawender et al. schist-grade rocks common to the east- wall rocks; mineral foliation is steep 1990). While xenocrystic zircon is ern side of the Gulf (Gastil and and commonly transects plutonic con- scarce in both suites, local garnet-bear- Krummenacher 1977; Gastil 1979; tacts; and dykes of one pluton within ing monzogranite in the eastern suite Ramos-Velázquez et al. 2008). K–Ar another are isoclinally folded (Todd does contain it. Euhedral titanite is biotite and hornblende ages are also in and Shaw 1979). Todd and Shaw characteristic of the La Posta plutons the range 100–90 Ma (Henry et al. (1979) also pointed out that in thin (Silver and Chappell 1988). Plutons of 2003). section, Santa Ana plutons typically this suite were emplaced both above In the Sierra de San Pedro have broken phenocrysts, a lack of and below the basal unconformity Mártir of Baja California (Figs. 2 and oscillatory zoning in plagioclase, and (Johnson et al. 1999b; Schmidt and 4), which might provide the best both quartzo-feldspathic and mafic Paterson 2002; Shaw et al. 2014), exposed section across the boundary minerals recrystallized under strain. although most were emplaced farther between eastern and western sectors, These features, plus the map patterns east at greater depth within the base- the Main Mártir thrust, commonly (Fig. 5), clearly indicate that, prior to ment complex. considered by previous workers as the deformation at 100 Ma, many plutons The ilmenite-bearing 94 Ma suture between the two sectors, dips of the Santa Ana suite were flat sheets, La Posta intrusive complex comprises: eastward as the westerly part of a dou- sills, or laccoliths with dominantly con- (1) an outer titanite–hornblende– bly vergent fan structure (Goetz 1989; cordant contacts and were subsequent- biotite tonalite, that is locally banded Johnson et al. 1999b; Schmidt and ly recumbently folded. Even seemingly near the outer contact, but character- Paterson 2002; Schmidt et al. 2009, GEOSCIENCE CANADA Volume 41 2014 411

2014); however, there are many Lower suture then it must be located farther early La Posta bodies might have char- Cretaceous gabbro bodies, characteris- east, which barring structural complica- acteristics of the Santa Ana suite. It is tic of the western sector located east tions, implies that the arc basement also important to note that we are of the fault. This suggests that any contained Paleozoic metasedimentary referring here to deformation of the suture may lie farther east. rocks and very likely their basement, Alisitos–Santiago Peak arc, and not Jurassic orthogneiss and which we presume to be Precambrian. older deformations, such as the ~160 paragneiss, located within the more It also implies that the Baja Peninsula Ma Jurassic and 145–139 Ma post- central parts of the fan structure (Fig. contains the arc and significant quanti- Peñasquitos, both of which are con- 4), appear to be part of the same pack- ties of basement. We will return to this fined to the basement, nor the younger age as Jurassic rocks that we reason to topic and its implications later in the ~75 Ma Laramide deformation, as it is lie unconformably beneath the Santia- paper. the short-lived 100 Ma deformation go Peak rocks in the Santa Ana moun- that appears to coincide with a major tains of southern California (Fig. 5). AGE OF DEFORMATION change in Cretaceous magmatism. Plutons in both places are dated at 164 The best constraints on the age of Ma (Schmidt and Paterson 2002; Shaw deformation within the Cretaceous EXHUMATION et al. 2003). Thus, it is plausible, but Peninsular Ranges batholith (Fig. 6) The La Posta plutons (Fig. 6) represent perhaps difficult to document due to come from the detailed work of an intense magmatic pulse ranging in high strain, that the contact between Premo and Morton (2014) who dated a age from 99–85 Ma (Silver and Chap- higher grade and very strongly wide variety of rocks on Searl Ridge pell 1988; Walawender et al. 1990; deformed rocks of Santiago Peak on (Fig. 3) in the Perris block where they Kimbrough et al. 2001; Premo et al. the west (Schmidt et al. 2014), and the found and dated zircon in a pre- to 2014b). The plutons were emplaced at Jurassic gneissic rocks and pre-134 Ma syn-metamorphic diorite dyke to be depths of 5–23 km into upper green- rocks might be an unconformity (Fig. 103.3 ± 0.7 Ma and in a post meta- schist, but mainly amphibolite, grade 4). If correct, then the fan structure morphic pegmatite dyke to be 97.53 ± wall rocks that are in many places contains a tectonically compressed sec- 0.18 Ma, which they interpreted to migmatitic (Gastil et al. 1975; Ague tion of crust within the Alisitos–Santi- have been emplaced just after meta- and Brimhall 1988; Todd et al. 1988, ago Peak arc, much as the northwest morphism. Additionally, they dated 2003; Grove 1993; Rothstein 1997; plunge appears to do to the north in more than 30 hornblende separates Rothstein and Manning 2003; Gastil et the Southern California region. and determined that metamorphism al. 2014). The plutons were intruded One possible candidate for a took place at or before 100.1 ± 0.6 Ma. during a period of exhumation when suture appears to be the Agua Caliente This is consistent with older data col- rocks at depths of 15–23 km were thrust (Fig. 4), which is a major fault lected farther south in the Sierra de brought rapidly to the surface by that dips moderately westward and San Pedro Mártir, where the age of the detachment faulting and collapse along which migmatitic flysch, deformation is tightly constrained by (Krummenacher et al. 1975; George deformed Cretaceous plutons, and plutons. There, 100 Ma gabbro and the and Dokka 1994; Ortega-Rivera et al. sheeted gabbro plutons as young as 101 Ma gabbro–tonalite–trondhjemite 1997, Ortega-Rivera 2003; Grove et al. 100.1 ± 0.5 Ma, were transported east- Rinconada complex, are composition- 2003; Miggins et al. 2014). Large vol- ward over carbonate–quartzite ally linked to the western zone, strong- umes of sediment were eroded from metasedimentary rocks, prior to ly deformed, and folded (Johnson et al. the uplifted terrane, as documented by emplacement of the La Posta plutons 2002; Alsleben et al. 2008; Schmidt et abundant 100–90 Ma detrital zircon (Schmidt and Paterson 2002; Schmidt al. 2009), yet the 97–90 Ma La Posta- and feldspar deposited as part of a et al. 2009). The fault zone was recog- type Sierra de San Pedro Mártir plu- voluminous pulse of early Cenomanian nized by Measures (1996) who mapped tonic complex, which cuts the contact to Turonian coarse clastic sedimenta- a section across it and described it as a between the eastern and western section in basins located to the west 200–500 m thick migmatitic zone con- tors does not contain the same level of (Lovera et al. 1999; Kimbrough et al. taining imbricated slivers of amphibo- deformation (Ortega-Rivera et al. 1997; 2001). lite, Cretaceous orthogneiss, pre- Gastil et al. 2014). Similarly, the El In the Southern California batholithic schist, ultramylonite, and Potrero pluton, located just 1–2 km segment of the batholith the depth of S–C mylonitic gneiss with northwest west of the San Pedro Mártir body, is a emplacement of all plutons, based on over southeast shear at amphibolite- strongly deformed tonalite with U–Pb Al-in-hornblende barometry, increases facies conditions. The main problem zircon age of 102.5 ± 1.6 Ma and from about 2 kbar or less in the west, with selecting the Agua Caliente fault 40Ar/39Ar ages of 101 ± 5 Ma for to more than 5 kbar in the eastern as the suture is that the footwall con- hornblende and 94 ± 1.4 Ma for zone (Ague and Brimhall 1988; Todd tains a muscovite-bearing pluton dated biotite (Johnson et al. 1999b; Chávez et al. 2003). In the Sierra de San Pedro at 164 Ma that intruded the carbon- Cabello et al. 2006). Mártir, farther south (Fig. 4), emplace- ate–quartzite succession. As this is the We will use 100 Ma as the best ment depths are similar, except there is same age and similar composition to estimate for the age of deformation, a marked jump on the west from 2 dated plutons beneath the arc complex, but realize that some plutons of the kbar to 5 kbar that coincides with a this fault is unlikely to be the suture. If Santa Ana suite may still have been series of closely spaced, easterly dip- the Agua Caliente fault is not the crystallizing as late as 98 Ma or that ping, reverse faults (Schmidt et al. 412 º 2009). On the eastern side of the Gulf º Sa 117

n 116 of California rocks are dominantly 34º J SantaSannta RosaRosa suitesuite ac S in an LLaaP PostaPosta suite suite greenschist and we are unaware of any to A nd re SSantaannta Ana Ana suite suite geobarometric studies on the plutons as fau lt z there. on E e ls At a regional scale, K–Ar ages in o r fa e ul decrease eastward across the batholith t z on (Silver et al. 1979), but there has been e SanSan JacintoJacinto little agreement on the origin of the pattern, with some relating it to region- bbloblockck al tilt, others to eastward-migrating magmatism, and still others to two dis- SSantaanta M-IM-I f lineline crete events. Those who favour the au AAnana lt

zo idea of a regional tilt, based on shal- n blblockock e EasternEastern PPeninsulareninsular lower emplacement depths of plutons RRangesangesg mylonitemylonitlie in the east, generally relate the tilting to zzoneone opening of the Gulf of California P (Krummenacher et al. 1975), but based a gabbrogabbro PPerriserris c linel on modern seismic analyses the effects ifi bloblockck c of the Neogene uplift extend only O halfway across the Baja Peninsula c e (Lewis et al. 2001), so it is unlikely to a be the cause. Models that relate the n 33º younging to migrating magmatism δ 18 O (Ortega-Rivera 2003) don’t resolve the ststepep problem, for the K–Ar ages in the east are as much as 20 m.y. younger than La Figure 7. Map showing tectonic blocks (Santa Ana, Perris and San Jacinto), Posta magmatism. Using 40Ar/39Ar in bounding faults, and locations of geochemical samples in the northern Peninsular biotite and potassium feldspar collect- Ranges batholith of southern California based on figures 1 and 2 of Kistler et al. ed in a more focused area across the (2003) and figure 68 from Morton et al. (2014). central boundary, Grove et al. (2003) recognized two distinct periods of suite used to define the boundary results for whole rock Sr, 83 for bulk exhumation and cooling in rocks of between alkali-calcic and calcic pluton- rock oxygen and 103 for whole rock the Peninsular Ranges batholith: an ic suites on the SiO2–MALI diagram of Pb isotopes. This isotopic dataset, or early ~95–86 Ma period, which they Frost et al. (2001). His data set was portions thereof, was presented and ascribed to emplacement of the La also used to define the calcic trend on interpreted in several papers (Kistler Posta suite; and a < 78 Ma period, the normative Q’–ANOR diagram 1990, 1993). More recently, a subset of located farther east, which they related (Whalen and Frost 2013). 287 rocks from the Baird collection to the Laramide event. They clearly During the late 1970s, A.K. was re-analyzed by ICP–MS for major recognized two events and so Baird and coworkers collected 334 and trace elements by Lee et al. (2007). explained the regional timing, but their granitic rock samples on a uniform For this paper (Fig. 7), we uti- tectonic model failed to explain the grid from the northern Peninsular lized the Lee et al. (2007) data set, origin of the La Posta plutons as well Ranges batholith in Southern Califor- which included published sample loca- as the 19–23 km of uplift and exhuma- nia (Baird et al. 1979). To obtain repre- tion and initial Sr isotopic ratios tion during their emplacement. sentative compositions, they collected 8 (Kistler et al. 2003). To these data, we samples within a 400 x 400 foot (122 x added O and Pb isotopic data from GEOCHEMISTRY 122 m) square from each site. The Kistler et al. (2003) and more recent Sr The geochemistry of the Peninsular samples were then crushed and com- and Nd analyses from Premo et al. Ranges batholith is historically impor- bined to yield one homogeneous pow- (2014b). We also added whole rock tant because it is close to the Southern der from each sample site. To our geochemical data from a recent study California megalopolis and its geologi- knowledge, this represents the most of the largest La Posta-type pluton in cally rich university community, so that systematic and representative geochem- Baja California, the Sierra de San Pedro over the years many geochemical and ical sampling of a batholith carried out Mártir, by Gastil et al. (2014). petrological studies were completed. to date. Major element results from Instead of the purely geo- The early field and petrological studies this collection were discussed by Baird graphic division of plutonic rocks into by Larsen (1948) were highly regarded et al. (1979) and published (without a western and eastern half favoured by and strongly influenced several genera- MnO and P2O5 determinations) by most previous workers, we used a basic tions of petrologists studying Baird and Miesch (1984). Subsequently, two-fold subdivision: pre- and post- batholithic rocks. His geochemical data Kistler et al. (2003) published a compi- 100 Ma deformation, which does have set was employed for the ‘type’ calcic lation of isotopic data that contained some geographic connotations in that GEOSCIENCE CANADA Volume 41 2014 413 the pre-deformational plutons of the divided into two main groups based on (HREE) than the Santa Ana average Santa Ana suite are concentrated, but age. Rocks of the calcic Santa Ana over similar silica contents. Both the La not limited to, the west, and post-100 suite (Fig. 8a) span a broad silica range, Posta and Santa Ana suites have well

Ma plutons of the La Posta suite are from 48 to 77% SiO2, whereas the developed negative K, Nb, Zr, P, and concentrated in the east. We use this range of the La Posta suite is much Ti anomalies. In rocks with > 70 wt.% separation because we believe that the more limited, as all samples contain silica, the Santa Ana average is more deformation represents a regional tec- greater than 60% SiO2 (Fig. 8b). In enriched in all elements, except for Ba, tonic event and that magmatism before general, mainly metaluminous compo- P, and Ti than the felsic La Posta aver- and after are compositionally quite dif- sitions (Fig. 8e) and amphibole-bearing age, but has a pronounced negative Sr ferent, which suggests to us that they mineralogy indicate that both suites are anomaly. The felsic La Posta average is have distinct origins. Based on a new comprised of I-type granite (Silver and greatly depleted, and the Santa Ana is generalized geological map of the Chappell 1988; Chappell and Stephens enriched, in HREE relative to their northern Peninsular Ranges batholith 1988). Santa Ana granitoid rocks 60–70 wt.% silica range suite averages. showing Baird’s sample locations (fig- define the calcic trend in Figure 8a, Extended element-normalized compo- ure 68 in Morton et al. 2014), followed and lie in the calcic field on Figure 8c, sitional average patterns for the well up by discussions with Doug Morton, whereas La Posta samples exhibit a cal- studied Tuolumne intrusive suite of who has detailed location maps, we fil- cic to calc-alkalic affinity. Similarly, the Sierra Nevada (Fig. 11b) closely tered the Lee et al. (2007) data to samples from Santa Ana plutons plot match the patterns and overall elemen- exclude mylonitic, contact zone, sedi- almost exclusively in the medium-K tal abundances of the La Posta rocks. mentary and Jurassic plutonic samples, field (Fig. 8d), whereas members of We also plotted averages for the La so as to include only Santa Ana and La the La Posta suite spread upward into Posta suite, again over the 60–70% sili- Posta suite samples in this study. the high-K field at higher silica con- ca range, and compared them with As discussed earlier, we tents. Samples from both suites with compositional averages for other high- believe that the major break defined by < 70 wt.% SiO2 plot in the magnesian Al and La/Yb suites (Fig. 11c). Both various attributes on the surface (oxidized) field in Figure 8b, but cross adakite suites differ from the La Posta reflects the Cretaceous–Jurassic uncon- over into the ferroan (reduced) field at suite in that they are more depleted in formity (Fig. 5) rather than a steeply higher silica levels. On a SiO2 vs. Al2O3 large ion lithophile elements (LILE) dipping and fundamental crustal break plot (Fig. 9a), the rocks of both suites and HREE, but have strong positive Sr or suture, and that the contact was exhibit a high-Al trend (Barker and anomalies. On Figure 11d we compare more or less horizontal prior to defor- Arth 1976), but with rocks of the La rocks of the Santa Ana suite with com- mation at 100 Ma. It now dips steeply Posta suite defining a slightly higher Al positional averages for primitive to eastward (Langenheim et al. 2014) and trend than analyses from rocks of the evolved arc-type suites. There, the pat- may represent the eroded remnant of Santa Ana suite. For any given silica tern for the Santa Ana rocks more an overturned limb of a large nappe content, rocks of the Santa Ana suite closely resembles the continental arc formed during the deformation. have higher concentrations of transi- average, but is more depleted in the Within the post-deformational tion metals, such as V+Cr, (Fig. 9b) LIL elements to the left of Zr, where La Posta suite we recognize two sub- than samples from the La Posta suite. they define a trend about halfway groups: (1) the La Posta plutons prop- For samples with silica contents lying between the continental and oceanic er; and (2) plutons, which we call the between 60 and 70 wt.% SiO2, rocks of arc patterns. Santa Rosa suite, that are similar to the La Posta suite tend to exhibit sig- rocks of the La Posta, but slightly nificantly higher Ba and Sr and lower Y PREVIOUS MODELS younger, and which are exposed in the contents than samples from the Santa Silver et al. (1963; written in 1956) first Santa Rosa Mountains (Figs. 3, 6). Ana suite (Fig. 9c to 9e). While both suggested that the western part of the Santa Rosa plutons have a limited geo- suites have similar Rb contents, the Peninsular Ranges batholith represents graphical distribution in that they suites are readily divisible on a Rb–Sr a primitive island arc constructed on occur above the East Peninsular plot (Fig. 9f) due to contrasting Sr con- oceanic lithosphere. In a subsequent Ranges mylonite zone (Fig. 3), which is tents (Wallawender et al. 1990; Tulloch contribution, Silver and Chappell a Laramide-age, westerly vergent, shear and Kimbrough 2003). Rocks of the (1988) recognized the dual nature of zone (Todd et al. 1988; Morton et al. Santa Ana and La Posta suites also dis- the batholith; that the western batho- 2014). We cannot rule out the unlikely play marked differences in La/Yb, lith formed between 140–105 Ma as a possibility that the plutonic rocks of Gd/Yb, Nb/Y, and Sr/Y values and static arc above an eastward-dipping the Santa Rosa suite are not genetically these are readily apparent on his- subduction zone; that magmatism related to the La Posta plutons, so we tograms and Harker plots (Fig. 10). migrated eastward between 105–80 use different symbols to represent Extended element-normalized Ma; and that it was derived from a them on many of our geochemical plots of average compositions (Table 1 deeper, subcrustal eclogitic source to plots. and Fig. 11) show that the La Posta yield La Posta plutons. They consid- average over the 60–70 wt.% silica ered the batholith was a juvenile addi- General Geochemical Features range is more enriched in all trace ele- tion to the crust and formed where no As outlined above, samples from the ments to the left of Zr and more continental lithosphere existed. On the Peninsular Ranges batholith were sub- depleted in heavy rare earth elements other hand, based on Nd and Sr iso- 414

5050 topes, Allègre and Othman (1980) rec- a. SantaSanta Ana Ana SamplesSamples fromfrom the westernwestern ognized that plutons of the Peninsular suitesuite PeninsularPeninsuular RangesRanges babatholiththolith Ranges and Sierra Nevada batholiths

) define thethe ttypeypec calcicalcic trtrendend

n 4040 were formed from mantle melts plus A LLaa PostaPosta significant quantities of continental + suitsuitee b crust. calc c--aalk In their landmark book on the

r+A 30 kaalic c 30 reconnaissance geology of Baja, Gastil +O LaLaP Postaosta et al. (1975, submitted in 1971) were Q 20 ( SantaSantta 20 RosaR understandably coy in their tectonic 10 SSPMSSPM e postpossts interpretation, as plate tectonics had c n=122n=1222 10010000 Ma Ma enc 0

rr 50 60 70 80 SantaSanta Rosa only recently “come on land” (Hoff-

cur c 20 suitesuite a

- 100*Q/ man 2013); yet in the time it took to lc ’ 10 SantaSanta AnaAna i % Occurrence %Oc c 1010 SierraSieerra SanSan publish the book, Gastil created and Q’ - 100*Q/(Q+Or+Ab+An) Q’ Q nn=123=123 PedroPeddroM Mártirártir pre-100pre-10000M Ma 0 LaLa Posta-typePosta-type published a short paper waxing poeti- 50 60 70 80 SiO2SiO2 (wt%)wt% plutonpluton in BBajaaja cally on a simple eastward-dipping sub- 0 0204060801000 20 40 60 80 100 duction model in which “great welts of tonalitic magma accumulated near the base of ANORANOR - 100*100*(An/(Or+An))(An/(Or+An)) the pre-existing crust . . . spawned plutons that rose as diapirs . . . then bled upward into 1 8 d.d. shallow plutons and extrusive masses” b.b. PostPost 100 MMaa )) .9 n oa SantaSaanta RosaRosa O r fer SantaSanta Rosa (Gastil 1975). By 1981, he had devel- g suitsuitee LaLa PostaPosta M 6 SSSSPMSPM .8 oped a more complex model involving + l e* a Fe*F t ta PPrere 1100 MMaa o t to SSantaaanta AAnana shoshoniticshoshonitic two eastward-dipping subduction .7.7 ) eO LaLa PostaPosta zones, one beneath a more westerly /(F l

wt% 4 a

susuiteite ( t ta

o totalt to total

.6 O fringing arc and the other to the east

SierraSierraS Sanan 2

PedroPedro MMártirártir K2O (wt%) K K2 high-Khigh-K eO beneath the continental margin (Gastil (F .5 sian = gne 2 * ma et al. 1981). Following closure of the e Fe* = (FeOFe* F .4 /(FeO + MgO)) SantaSanta AAnana eastern basin, which led to crustal suitesuite mmedium-Keedium-K low-KlKlowloww-K-K-K thickening and uplift, eastward subduc- 10 0 cc.. 5500 55 60 65 70 75 80 tion continued and a shallowing slab LaLaP Postaosta SiO 2 (wt %) 5 suitsuitee 1.1.55 led to arc magmatism farther east. ) lc ali-calcic PostPost 100 MMaa aO alkali-calcicalkka Based largely on differences in )

C SantaSanta Rosa - 0 pera LaLaP Postaosta lum O SierraSierra SanSaan i 2 n degree of deformation between the PedroPedroM Mártirártir SSPSSPMM ou K lic s kalicaal + -a-alkl alcaallcc PrePre 110000 MMaa Bedford Canyon and Santiago Peak O c 22 2 -5 llccici ca SSantaanta AAnana a SantaSanta Rosa *P + Na K 1.01.0 rocks, Todd et al. (1988) also utilized a 7 N

(N suitsuitee PostPost 1100 MMaa -10-10 SSantaannta Rosa LaLa PostaPosta two-arc model but argued that “Colli- - 1.6 LI = a

A SSPSSPMPM C

( sion of the Santiago Peak–Alisitos arc with MALI = (NaMALI M O+K O-CaO) SantaSanta AAnanaa / --1515 suitesuite PPrere10 10000 MMaa l SSantaannta AnaAna - 1.67*P + Na K) Al/(Ca A m etaluminous western North America in the Early Creta- 00.5.5 e.e. 6 5 4 3 2 1 ceous resulted in folding of the continental- 45 50555 60 65 70 75 80 margin deposits and eventual underthrusting SiO (wt %) Al/(NaAl/(Na + K)K) 2 of the arc beneath the continental margin.” Figure 8. (a) Representative samples from the two main suites of the Peninsular Contemporaneously with sub- Ranges batholith plotted on the normative Q’ (100*(Q/ (Q+Or+Ab+An)) versus duction of the western island arc, mag- ANOR (100*(An/ (Or+An)) classification diagram (Streckeisen and LeMaitre matism migrated slightly eastward 1979). We use the Whalen and Frost (2013) compositional trends. SiO2 histograms where it produced melts from the sub- are for the Santa Ana and La Posta suites. Sample subdivision is shown in the sym- ducted arc crust. Fifteen years later, bol legend and discussed in the text. (b) Peninsular Ranges batholith samples plot- Todd et al. (2003) concluded that “a total total ted on the: FeO / (FeO + MgO) (or Fe*) vs. SiO2; the boundary between fer- single Cretaceous arc migrated from west to roan and magnesian plutons has been modified as suggested by Frost and Frost east across a preexisting Late Jurassic–earliest (2008); (c) Na2O+K2O–CaO (or MALI) vs. SiO2 granitic rock classification dia- Cretaceous lithospheric boundary.” grams of Frost et al. (2001). Ranges for alkali-calcic, calc-alkalic, and calcic rock Meanwhile Busby et al. (1998) series, based on the alkali–lime classification of Peacock (1931). Together with alu- also developed a fringing arc model minum saturation index (ASI) (Shand 1943) (Fig. 8e), these plots comprise a three- based on their extensive work in Baja. tiered geochemical scheme for granitoid rock classification. The horizontal dashed They posited that a Jurassic arc, which line at ASI= 1.1 in the Shand plot, modified after Maniar and Piccoli (1989), is the developed offshore, grew into a more dividing line between I- and S-type granites (Chappell and White 1992); and (d) a mature fringing arc separated from SiO2 vs. K2O plot with suite subdivisions: low-, medium-, and high-K fields from North America by a back-arc basin, LeMaitre (1989) and high-K, shoshonitic fields after Peccerillo and Taylor (1976). and that, “an increase in plate convergence Analyses from Lee et al. (2007) and Gastil et al. (2014). collapsed the fringing arc against the continent GEOSCIENCE CANADA Volume 41 2014 415

a. d. Post 100 Ma Sierra San Pedro Mártir La Posta Santa Rosa Santa Ana suite 600 La Posta suite SSPM La Posta suite 20 Hi-Al Pre 100 Ma Sierra San Santa Ana Pedro Mártir 400

Post 100 Ma Sr (ppm) Santa Rosa

Al2O3 (wt%) La Posta 200 15 Santa Ana SSPM Santa Rosa Lo-Al Santa Rosa suite suite Pre 100 Ma suite Santa Ana 0 e. Santa Ana Post 100 Ma Santa Rosa suite suite b. 40 Santa Rosa 500 Santa Ana La Posta suite SSPM 30 Pre 100 Ma Santa Ana Y (ppm)

Santa Rosa 20 100 suite La Posta suite 10

Cr+V (ppm) Sierra San Pedro Mártir Post 100 Ma 0 Santa Rosa La Posta La Posta suite 45 50 55 60 65 70 75 80 SSPM SiO2 (wt%) 10 Pre 100 Ma Sierra San 700 Santa Ana Pedro Mártir Sierra San 600 f. Pedro Mártir Post 100 Ma Santa Rosa 2000 La Posta c. La Posta Post 100 Ma suite 500 SSPM Santa Rosa Pre 100 Ma La Posta 1600 Santa Rosa 400 Santa Ana SSPM suite

Pre 100 Ma Sr 1200 Santa Ana 300 Santa Rosa La Posta suite suite Ba (ppm) 800 Santa Ana 200 suite

400 100 Sierra San Santa Ana Pedro Mártir suite 0 0 45 50 55 60 65 70 75 80 0 40 80 120 160 200 240 SiO2 (wt%) Rb (ppm)

Figure 9. Harker variation diagrams for (a) Al2O3; (b) Cr + V; (c) Ba; (d) Sr; (e) Y; and (f) Sr–Rb for Peninsular Ranges batholith sample groups. In (a) the dividing line of Barker (1979) between high- (> 15 wt.%) and low-Al tonalite suites at SiO2 = 70 wt.% is shown. Sample subdivision is shown in the symbol legend and discussed in the text. Note that on most of these plots pre- and post-deformational plutons are compositionally distinct. and caused the reverse faulting and uplift”. crustal thickening whereas continued crust and mantle to produce the La Based on geological mapping easterly subduction led to the younger Posta magmas. and dating Johnson et al. (1999b) magmatism. In an important paper, Kim- invoked a double eastward subduction Several researchers developed brough et al. (2001) tied together many model with two eastward-dipping sub- single eastward subduction models critical elements, such as the post- duction zones, largely because they based largely on the composition and deformational nature of the La Posta believed that Nd and Sr isotopic val- characteristics of the La Posta suite. suite, the rapid exhumation, and coeval ues, coupled with the absence of Walawender et al. (1990) devised a sim- sedimentation to the west, which they inherited zircon in the Alisitos and ple model where increased conver- viewed as the fore-arc region, but in related plutonic rocks, indicated an gence caused the eastward-dipping the end they attributed the La Posta absence of continental input. Collision subducting slab to shallow, buckle and suite to nothing more than a transient of the arc with a more easterly arc break, so that continued subduction episode of high-flux magmatism. Tul- built on western North America led to was able to melt the overlying oceanic loch and Kimbrough (2003) expanded 416

7070 AdakitesAdakites 20 20 n ==122122 SantaSanta RosaRosa ssuiteuite LaLa PPostaosta susuiteite SanSan PPedroedro ddee n ==1221122 10 6060 10 MártirMártir pplutonluton 0 60 4455 30 15 0 0 0 SanSan PPedroedro de

e 0 0.505 1.0101.0 1.5151.5 60 50 Mártir pplutonluton enc

10 50 1.01.0 40 4

5050 = curr La PosPostasta suite b Y 40 30 / Y=0. % Occurrence %Oc La/Yb=10 La 30 20 b/

Nb/Y=0.4N nn=87=87 4040 10 b SaSantanta Rosa suite 20 n=87n=87 /Y 0 Nb/Y=0.4Nb/Y=0.4

/Y 10 0 0.5 1.0

b 1.5 Nb/YNb/Y Nb/YN La/Yb La 3030 0 60 445 30 15 0 0 Santa Ana suite La/YLa/Y 20

La/Yb=10La/Yb=10 0.10.1 1010 Arcs Santa Ana suite MORBMORB a.a. c.c. 0 012345670 1 2 3 4 5 6 7 4545 50 55 60 65 70 75 80 YbYb (ppm(ppm)) SiO2SiO2 ((wt%)wt%) 200200 20 50 La Posta suite San Pedro de 0 40 .0 MáMártirrti r pplutonl ut on

e n ==122122 10

=2 SantaSanta 55.0.0 30 b San Pedro ode de enc RRosaosa /Y nn=87=87 Mártir pluplutonton 0 20 d ssuiteuite curr 80 60 40 20 0 c Gd/Yb=2.0 G 150150 e O

10 0 % Occurrence 40 enc

0 Y=1

1 4 / 020 2 3 5 6 7 curr r Gd/YbGd/Yb Sr/Y=10 S AdakitesAdakites 30 % Occurrence %Oc b

/Y 100 20

Gd/Yb=2Gd/Yb=2 r d/Y Sr/Y S Laa Posta suite nn=87=87 Gd/Yb G 10

0 80 60 4400 20 0 Sr/YSr/Y 1.01.0 50 SantaSanta 30 RRosaosa 20 n ==122122 ssuiteuite SantaSanta AAnana suite 10 SantaSanta AAnana suite 0 MORBM ArcsArcs 0 1 2 3 4 5 6 7 b. d. 0.50.5 0 4550 55 6 600 65 70 7 755 8 800 0100 100 20 30 40 SiO2SiO2 ((wt%)wt%) Y ((ppm)ppm)

Figure 10. Some trace element ratio plots for Peninsular Ranges plutonic suites. (a) La/Yb versus Yb plot and La/Yb histograms; (b) Gd/Yb versus SiO2 and Gd/Yb histograms; (c) SiO2 versus Nb/Y plot and Nb/Y; (d) Sr/Y versus Y plot and Sr/Y histograms. In all insert histograms and 10a and 10d X-Y plots, only samples with > 60% silica are plotted, whereas in 10b and 10c X-Y plots all samples are plotted, as in Figures 8 and 9. Fields are shown for adakite, arc andesite–dacite–rhyolite and MORB from Richards and Kerrich (2007), as is the average medium-K arc andesite (blue cross in a). In the histograms, ver- tical lines approximately separate pre- and post-deformational granitoid rocks. on the earlier model by recognizing ing and subduction of forearc and tion built a near-margin or fringing that the La Posta suite was a high Na, accretionary prism rocks, such as the extension arc built entirely on oceanic Sr and low Y suite and so created a Catalina schist. In their model, a hypo- crust, and that as arc magmatism model in which the older, western and thetical deep lithospheric mantle root migrated eastward the tectonic regime low Sr, Y Santiago Peak–Alisitos arc had formed beneath the La Posta belt went from extensional to compression- was underthrust beneath the arc during but was removed by Laramide shallow al, which caused thrusting and exhu- slab-flattening, which shut off normal subduction, so that it no longer exists mation of the western arc rocks arc magmatism and generated the burst beneath the region. between 105 and 98 Ma. They then of La Posta magmatism. Grove et al. Ortega-Rivera (2003), noting stated that between 98 and 91 Ma large (2003) related the pre-90 Ma exhuma- the general eastward younging of plu- amounts of highly enriched magma tion to the emplacement of the La tonic ages, advanced a model of long- were formed and that it was the Posta suite. Subsequently, Grove et al. lived eastward subduction and varia- emplacement of the magma that led to (2008) developed a complex model – tions in dip and velocity of the sub- the exhumation of the eastern region. based largely on detrital zircon, their ducting plate to generate the magma- The reader was left to imagine what Th/U ratios, and whole rock Pb iso- tism. Miggins et al. (2014) developed a caused the events. topes of the plutonic rocks – of east- vague model, based on that of Grove Schmidt et al. (2014) summa- ward-directed subduction, slab-flatten- et al. (2008), where eastward subduc- rized over a decade of work by stu- GEOSCIENCE CANADA Volume 41 2014 417

Table 1. Average compositions of Peninsular Ranges Batholith plutonic suites. dents at USC and concluded that the Santiago Peak rocks were deposited Group Santa Ana Suite La Posta Suite upon Jurassic basement whereas the SiO2 Range 60-70% 1 >70% 1 60-70% 1 >70% 1 Alisitos rocks were established upon No. Samples 39 41 110 12 oceanic crust. In their model, the Alisi- Major Elements (wt.%) tos and Santiago Peak arcs collided SiO2 66.48 8.46 76.47 0.77 65.93 2.14 71.54 0.84 TiO 0.61 0.25 0.16 0.06 0.71 0.16 0.29 0.07 along the ancestral Agua Blanca and 2 Main Mártir thrusts prior to 108 Ma Al2O3 16.08 2.27 12.64 0.33 16.77 0.78 15.86 0.44 Total Fe2O3 3.09 2.05 1.87 0.35 4.27 2.00 2.01 0.94 and the resulting contractional defor- MnO 0.07 0.03 0.05 0.02 0.08 0.03 0.04 0.02 mation and crustal thickening “culminat- MgO 1.25 0.62 0.22 0.11 1.47 0.48 0.46 0.17 ed in a major pulse of arc magmatism that CaO 4.01 1.27 1.14 0.41 4.52 0.67 2.91 0.48 formed the 99–92 Ma La Posta suite.” Na2O 3.89 0.73 3.59 0.28 3.93 0.61 4.49 0.44 Numerous difficulties with this model, K2O 2.36 0.77 3.79 0.77 2.21 0.55 2.45 0.78 which was based largely on work by P2O5 0.18 0.06 0.07 0.02 0.20 0.04 0.09 0.03 Wetmore et al. (2002), were summa- Trace Elements (ppm) rized by Busby (2004), and include the Cr 17 32 12 9 16 9 6 4 similar basements and the presence of Co 6 5 21 74 12inherited zircon for both groups of Sc 7 4 64 73 52rocks, as well the presence of abun- V49268655192115dant siliceous volcanic rocks within the Cu 12 11 8 8 14 11 3 3 Alisitos arc, including ignimbrite sheets Zn 66 19 30 11 73 12 52 13 (Busby et al. 2006), all cut by numerous W 0.3 0.2 0.4 0.2 0.3 0.2 0.0 0.1 intermediate–siliceous composition Mo 1.8 1.4 3.8 1.7 1.8 1.2 0.2 0.4 plutons, which are atypical of arcs built Rb 80 30 123 47 77 24 70 22 on oceanic crust. Cs 2.3 1.0 2.9 2.1 2.3 0.8 2.2 0.7 Ba 899 317 809 316 920 302 962 328 Another group of researchers Sr 465 145 83 34 515 67 459 61 linked the Alisitos–Santiago Peak arc Ta 0.8 0.5 0.8 0.4 0.8 0.6 0.4 0.2 to the Guerrero composite terrane Nb 8.5 4.5 6.4 1.8 9.3 4.6 4.8 1.4 located on the Mexican mainland (Cen- Hf 4.4 1.2 4.3 1.3 4.6 1.1 3.2 0.5 teno-García et al. 2008). A French Zr 160 44 124 35 171 40 128 20 group (Tardy et al. 1992, 1994) also Y14727913583argued that the Alisitos arc was part of Th 10.8 7.4 15.8 7.4 10.8 7.5 6.7 2.9 the Guerrero composite terrane but U 1.9 1.2 3.2 1.6 1.9 1.1 0.8 0.2 reasoned that it collided with the west- La 26.8 12.1 19.4 5.2 28.8 12.3 20.1 6.6 Ce 55.8 25.1 43.7 11.1 60.0 25.6 37.8 12.0 ern margin of North America over a Nd 25.0 10.1 19.0 3.8 27.0 9.9 15.4 4.3 westward-dipping subduction zone. Sm 5.1 2.0 4.7 1.4 5.4 1.9 2.9 0.8 Dickinson and Lawton (2001a) pre- Eu 1.2 0.4 0.7 0.5 1.3 0.4 0.8 0.3 sented a model in which the Santiago Gd 3.7 1.4 4.2 1.3 3.8 1.3 2.1 0.6 Peak volcanics represented an arc Tb 0.6 0.3 0.8 0.3 0.6 0.2 0.3 0.1 accreted to the western margin of the Yb 1.4 0.9 3.5 1.2 1.3 0.6 0.7 0.2 Caborca block, but that the contact Parameters was obscured by younger plutons. Q' 26.6 6.5 39.8 1.4 25.1 4.3 29.9 2.6 In an important contribution, ANOR 55.8 15.9 19.4 9.0 61.5 8.2 49.8 11.3 Centeno-García et al. (2011) noted the MALI 2.2 1.8 6.2 1.0 1.6 1.1 4.0 1.1 Fe* 0.74 0.10 0.89 0.04 0.73 0.03 0.80 0.03 strong ties between the history of Baja Zr Temp.(C) 748 89 766 23 760 31 726 16 California and the Guerrero composite ASI1 0.99 0.12 1.05 0.02 1.00 0.03 1.04 0.02 terrane, as well as the strong detrital Alkali Index 1.79 0.32 1.27 0.09 1.90 0.17 1.59 0.12 zircon ties of both to North America, Isotopes and so speculated that the arc fringed 18O‰ 6.7 0.7 6.4 1.8 9.3 0.7 10.5 3.0 the continent and was separated from i 87Sr/86Sr 0.7050 0.0054 0.7040 0.0006 0.7070 0.0001 0.7055 0.2720 it by a marginal basin. They argued that i206Pb/204Pb 18.772 0.092 18.867 0.110 19.211 0.181 18.931 the closure of the basin started in the i207Pb/204Pb 15.603 0.020 15.612 0.017 15.682 0.024 15.626 208 204 west during the Cenomanian, migrated i Pb/ Pb 38.446 0.138 38.496 0.118 38.787 0.125 38.657 eastward, and ended prior to deposi- Ratios tion of Santonian sediments between La/Yb 5.86 2.96 6.62 3.18 26.80 19.22 31.90 14.07 Gd/Yb 1.15 0.41 1.25 0.35 3.23 0.74 3.13 0.63 about 93–84 Ma. They stated that “Arc Sr/Y 4.45 4.33 3.43 1.77 49.26 30.88 67.22 32.07 extension ended by the Albian–Cenomanian Ba/Y 27.7 13.3 31.9 13.3 86.1 61.0 147.0 102.6 boundary in Baja California, when the Early Nb/Y 0.22 0.07 0.25 0.05 0.71 0.21 0.64 0.19 Cretaceous Alisitos fringing arc underthrust 1. ASI = Al saturation index. the Mexican continental margin and the crust 418

Peninsular Ranges batholith La Posta 60-70% SiO2 average with Comparison Compositions 100 La Posta Suite averages 100 La Posta Suite avg (N=110) 60-70% SiO2 (N=110) Taiwan Unit KYS (N=6) >70 % SiO2 (N=12)

10 10

Santa Ana suite averages Cenozoic Adakite (N=140)

60-70% SiO2 (N=39) post-Archean Adakite & a. High-Al TTG (N=394) >70 % SiO2 (N=41) c. 1 1 Tuolumne Intrusive Suite averages Santa Ana 60-70% SiO2 average TIS 70-75% SiO2 average (N=40) with Comparison Compositions 100 100 TIS 60-70% SiO2 average (N=112) Continental arc: andesite TIS <60% SiO2 average (N=19) dacite-rhyolite (N=815) Santa Ana Suite (N=39) Oceanic arc pluton (N=5) Sample/Primitive Mantle Sample/Primitive

10 10

La Posta suite 60-70% SiO2 (N=110) average b. d. 1 1

Figure 11. Primordial mantle-normalized extended element plots for: (a) Peninsular Ranges batholith plutonic suite compositional averages from Table 1 for 60–70 and > 70 wt.% silica ranges; (b) Tuolumne Intrusive Suite compositional range averages compared to La Posta 60–70% SiO2; (c) La Posta 60–70% SiO2 average compared to other similar units including KYS from Taiwan (Wang et al. 2004), Cenozoic adakite bodies, post-Archean adakite bodies, and high-Al TTG suites; and (d) Santa Ana suite 60–70% average, oceanic arc pluton, and continental arc. Comparison compositions in (c) and (d) from Drummond et al. (1996) and Whalen (1985); data in (b) are based on a compilation of data from various sources including Memeti (2009). Pri- mordial mantle-normalizing values are from Sun and McDonough (1989). was greatly thickened” without explaining post-collisional plutonic rocks in order for it is atypical of continental arcs and how the arc ended up on the lower to better understand the sources of the so, many geologists, also strongly influ- plate and the continental margin the magmatism. To end – but prior to pre- enced by the supposedly deep Sierran upper. senting our final conclusions – we con- crustal root (Lawson 1936; Bateman et cisely compare the development of the al. 1963; Bateman and Wahrhaftig INTERPRETATION Peninsular Ranges batholith with some 1966; Christiansen 1966; Dodge and In what follows we show how the vari- other Cordilleran batholiths of the Bateman 1988), assumed that ous components of the Cretaceous Americas. Cordilleran batholiths were generated Peninsular Ranges batholith can be in zones of great crustal thickening combined into a new, dynamic, but Arcs (Bateman 1992; Ducea 2001; Ducea entirely actualistic, model. We do this Although Coats (1962) clearly argued and Barton 2007; Grove et al. 2008; first by succinctly examining some crit- that subduction of oceanic crust creat- DeCelles et al. 2009). ical attributes of modern arc terranes, ed the volcanoes of the Aleutian arc, it One only has to survey conti- then we move on to examine the sub- was Hamilton (1969a, b) who hypothe- nental arcs worldwide to recognize that duction polarity for the Santiago Peak– sized that the only modern volcanic there are problems in using a thick Alisitos arc. This leads us directly into fields of comparable size to great crust Andean-type model, for, contrary a discussion of lithospheric slab failure Mesozoic batholiths of the western to popular belief, thick sequences of during collision and how it can parsi- Americas were Andean-type volcanic arc rocks erupted and deposited within moniously explain major features in fields, where he suggested, “that a great a subsiding basin are the norm, not the development of the batholith. late Cenozoic batholith, comparable in size volcanoes sitting on high-standing Next we look at the exhumation of the and composition to the late Mesozoic thick crust. Most continental arcs form batholith and touch on the formation batholiths of western North America, has within subsiding depressions or basins of the doubly vergent fan structure. formed beneath the young volcanic pile of the on crust of average or below average We then interpret the geo- central Andes.” In many ways it is unfor- thickness (Levi and Aguirre 1981; chemistry of the pre-collisional and tunate that Hamilton chose the Andes Hildebrand and Bowring 1984; Busby- GEOSCIENCE CANADA Volume 41 2014 419

Spera 1988b; Busby 2004, 2012). Mod- As stated earlier, the general accretionary complex rocks (Minch et ern examples include the Cascades, paradigm for generation of the Santia- al. 1976; Boles 1986; Busby-Spera and where the volcanoes sit in a half go Peak–Alisitos arc and the Peninsular Boles 1986; Sedlock 1993). graben (Williams and McBirney 1979; Ranges batholith – and in fact all the While most workers relate the Mooney and Weaver 1989), the low- Cordilleran batholiths of the American ophiolite–blueschist facies rocks–tur- standing Alaskan Peninsula (Burk 1965; Cordillera – has been east-directed sub- bidite basin triad to an eastward-dip- Fliedner and Klemperer 2000) where duction beneath the western margin of ping subduction zone and forearc volcanoes such as Augustine are par- North America. In what follows, we basin related to the Alisitos arc–Penin- tially submerged in Cook Inlet (Power provide evidence that indicates that at sular Ranges batholith, the Vizcaino et al. 2010), the Kamchatka Peninsula least part of the Santiago Peak–Alisitos ophiolite is 221 ± 2 Ma, the Cedros of easternmost Russia where towering arc was constructed by magmatism Island ophiolite 173 ± 2 Ma, and the stratovolcanoes sit in huge fault- arising from westerly, not easterly, sub- arc plutons, 165–135 Ma (Kimbrough bounded troughs (Erlich 1968, 1979; duction. and Moore 2003) – all considerably Levin et al. 2002a, b), to New Zealand older than the 128–100 Ma Alisitos arc. where the Taupo zone sector of the Polarity of Subduction Given the age difference and the wide arc is actively extending as calderas and As indicated previously, nearly all and flat Desierto de Vizcaino and the stratocones erupt within it (Houghton researchers who have presented mod- Llano de Magdalena (Fig. 2), both et al. 1991; Harrison and White 2006; els for the origin of the Peninsular extensive areas without bedrock out- Downs et al. 2014), and the Central Ranges batholith have assumed an crop, separating the Peninsular Ranges American arc where volcanoes are eastward-dipping subduction regime. batholith and its wall rocks from the aligned in a long linear depression Yet as pointed out by Todd et al. Sierra de San Andres, there is no com- (Williams et al. 1964; Williams and (1988), it was only the presence of pelling reason that the two areas have McBirney 1969; Burkhart and Self scattered occurrences of blueschist- any direct relationship to one another, 1985; MacKenzie et al. 2008). facies metamorphic rocks and ophiolite nor that they were in close proximity Furthermore, the stratigraphy located to the west of the Peninsular during the Early Cretaceous. Other within pendants and wall rocks of Ranges batholith in the California bor- sequences, such as those on Catalina Cordilleran batholiths provides no evi- derlands which suggested that subduc- Island and related rocks of the south- dence of thick crust as they too sat at tion was eastward. These rocks out- ern California borderlands (Grove et or below sea level during volcanism. crop in the Sierra de San Andres on al. 2008), are of Early Cretaceous age The Sierra Nevada was clearly low- the Vizcaino Peninsula and on Cedros in part, but are separated from the standing during magmatism, for it con- Island (Fig. 2). There, a sequence of batholith by deep water and/or an tains marine rocks deposited at about volcanic and sedimentary rocks, gener- unknown number of major faults, such 100 Ma (Nokleberg 1981; Saleeby et al. ally interpreted to represent a magmat- as the Oceanside (Abbott and Smith 2008; Memeti et al. 2010). So was the ic arc, active from at least 166 ± 3 Ma 1989; Crouch and Suppe 1993; Bohan- western Peninsular Ranges arc, where until 160 Ma and again during the Cre- non and Geist 1998), so that there is the Santiago Peak and Alisitos vol- taceous, sits atop a Late Triassic and no direct link to the batholith. Addi- canics are interbedded with sedimenta- mid-Jurassic supra-subduction ophi- tionally, as documented by Grove et al. ry rocks containing marine fossils (Fife olitic basement, which in turn, sits (2008), most of the units within the et al. 1967; Allison 1974; Phillips 1993; structurally above a blueschist-bearing thrust stack on Catalina Island are Griffith and Hoobs 1993; Wetmore et accretionary complex and is separated younger than the age of deformation al. 2005; Busby et al. 2006; Centeno- from it by a serpentinite mélange con- within the Peninsular Ranges batholith García et al. 2011). In South America, taining high-grade blocks (Rangin and are generally considered to be part the 9 km thick Casma arc volcanic 1978; Kimbrough 1985; Moore 1985, of the Franciscan accretionary com- rocks, the wall rocks for the younger 1986; Kimbrough and Moore 2003; plex, which, along with rocks of the Coastal batholith of Peru, are domi- Sedlock 2003). The blocks in the Coast Range ophiolite and Great Valley nantly marine (Cobbing 1978, 1985; mélange are typically eclogite and group, were unlikely to have been Atherton et al. 1985) as are many of amphibolite, with ages in the 170–160 located west of the Sierran–Peninsular the thick Jurasso–Cretaceous arc rocks Ma range, and blueschist-facies blocks Ranges arc prior to 100 Ma (Wright within the Ocoite arc of northern ranging in age from 115 to 95 Ma and Wyld 1997; Hildebrand 2013). Chile (Levi and Aguirre 1981; Åberg et (Baldwin and Harrison 1989, 1992). However, because the upper al. 1984). Even ancient arcs, such as The upper contacts of the blueschist- 92–91 Ma Turonian sandstone of the the Paleoproterozoic Great Bear mag- facies belts are interpreted as normal Valle Group is dominated by 100–90 matic zone of the Wopmay orogen, faults and reflect their probable Late Ma detrital zircon, presumably derived were low standing zones of subsidence Cretaceous–Paleogene exhumation from the La Posta plutons (Kimbrough during magmatism (Hoffman and (Sedlock 1996, 1999). Perhaps 10 km et al. 2001), one can reasonably argue McGlynn 1977; Hildebrand and of upper Albian–Cenomanian to that rocks of the group were close to Bowring 1984). Where arcs are con- Coniacian–Maastrichtian siliciclastic the Peninsular Ranges batholith by the cerned the Andes are the outlier – sim- turbidites, known as the Valle Group Late Cretaceous, but the nature of the ply atypical of modern and ancient and described earlier, sit uncon- basin and its relationship to the Alisi- arcs. formably on the older arc–ophiolite– tos arc are obscure. Busby-Spera 420

(1988a) argued that the ophiolite and genic flysch during the Maastricht- by U–Pb as 105 Ma, are strikingly simi- its overlying volcaniclastic apron on ian–Danian (Fourcade et al. 1994), and lar to those of the Alisitos Group in Cedros Island represented a remnant overthrust by ultramafic nappes. Rocks that they are shallow marine sedimen- of a back-arc basin of Mid-Jurassic of the lower-plate crystalline basement tary rocks, volcaniclastic rocks, and a age. Given that arc magmatism there is were metamorphosed to eclogite at 76 wide variety of subaerial–submarine dated to be as young as 135 Ma (Kim- Ma, which implies that part of the intermediate composition volcanic brough and Moore 2003), it is conceiv- North America margin was subducted rocks with reefal limestone of able in a west-dipping scenario that to greater than 60 km depth at about middle–late Albian age towards the top this magmatism represents an older that time and exhumed to amphibolite (Tardy et al. 1992; Centeno-García et segment of the Alisitos arc and that grade levels a million years later al. 2003, 2011). Martini and Ferrari the arc migrated eastward due to slab (Martens et al. 2012), presumably by (2011) showed that the carbonate rocks rollback. Alternatively, the 135 Ma plu- slab failure. and their basement were folded during tonic package could be part of a pre- Some 25–30 m.y. earlier near the Cenomanian prior to the 94 Ma Alisitos arc. the end of the Albian, much of east- start of deposition of clastic sedimen- If the subduction polarity central Mexico, such as Oaxaquia, Cen- tation. The results from more detailed wasn’t necessarily eastward, then we tral and Mixteca terranes, formed a detrital zircon studies of sandstone in must examine the region to the east on coherent block and was covered by a the area show – in addition to clusters the other side of the arc. Unfortunate- west-facing Lower Cretaceous carbon- at 106–110 Ma, ~250 Ma, 480–650 ly, due to the effects of the opening of ate platform (Fig. 12), known as the Ma, and 1.0–1.3 Ga – strong 160–162 the Gulf of California, the relations of Guerrero–Morelos platform, or locally Ma peaks, presumably reflecting the the arc to the lower plate are not par- the El Doctor platform, in the south Jurassic basement, and in two younger ticularly well exposed in Southern and and the Sonoran shelf in the north rocks, huge peaks at 99–97 Ma, which Baja California; but as Baja is readily (LaPierre et al. 1992a; Monod et al. might represent La Posta type intru- restored back to the Mexican mainland 1994; Centeno-García et al. 2008; sions (Centeno-García et al. 2011). (Oskin and Stock 2003), the relation- González-Léon et al. 2008; Martini et Given that when Peninsular ships might be clearer there. In fact, al. 2012). In the south, the platform Baja is restored to its pre-Gulf of Cali- several workers (Tardy et al. 1994; was built upon ~1000 m of Lower fornia paleogeographical position (Fig. Dickinson and Lawton 2001a; Cen- Cretaceous red beds, alluvial sandstone 12), the Alistos arc is on strike with, teno-García et al. 2008, 2011; Schmidt and conglomerate with thick evaporite and just north of, the Zihuatanejo ter- et al. 2014) suggested that the Alisitos deposits and an older metamorphic rane and its lower Cretaceous arc, cou- arc formed part of the Guerrero basement (Fries 1960). Locally, a suite pled with the striking similarities in superterrane of the western Mexican of Late Jurassic–earliest Cretaceous Jurassic arc and Jurasso–Triassic accre- mainland. volcanic and volcaniclastic rocks pre- tionary complexes in the basement, The Guerrero superterrane is dated the platform (Monod et al. their overall stratigraphic packages and a composite terrane made up of sever- 1994). settings, as well as their similar ages, al different terranes that were assem- The west-facing carbonate they almost certainly represent the bled during the Mesozoic (Centeno- platform was uplifted, eroded, pulled same arc complex (Centeno-García et García et al. 2003, 2008) and, along down to deeper water, and buried by al. 2011). In the Zihuatanejo terrane with other amalgamated terranes to the orogenic deposits (Fig. 13), known the relations are clear: the leading edge east such as Oaxaquia, collided with locally as the Mexcala flysch, during of a west-facing Albian carbonate plat- North America during the Laramide the latest Albian–early Cenomanian at form sitting atop older rocks was event at about 75 Ma. The Laramide about 100 Ma (Monod et al. 2000). pulled beneath a Lower Cretaceous arc docking of the arc-bearing superter- During drowning the platform shed and its Jurasso–Triassic basement, at rane (Tardy et al. 1994; Centeno-García carbonate blocks up to 2 m across into about 100 Ma (Monod et al. 1994), et al. 2011) is documented by a well- the lower parts of the flysch to the consistent with relations inferred for developed thin-skinned fold and thrust west (Monod et al. 1994). This drown- the Santiago Peak–Alistos arc in South- belt and associated foredeep, located in ing was caused by the attempted sub- ern and Baja California. The collision extreme eastern Mexico along the east- duction of the easterly block and its marks the closure of an ocean along ern margin of Oaxaquia, just west of cover beneath a Lower Cretaceous arc the western margin of the Cordilleran the Gulf of Mexico (Busch and Gavela complex, now located in the Zihu- Ribbon Continent some 20–25 m.y. 1978; Tardy et al. 1992, 1994; Eguiliz atanejo terrane, but which was built prior to the Laramide event, which de Antuñano et al. 2000; Salinas-Prieto upon Triassic–Jurassic basement com- reflects terminal collision of the rib- et al. 2000; Nieto-Samaniego et al. prising a Triassic–Jurassic accretionary bon continent with North America 2006; Pérez-Gutiérrez et al. 2009; complex and associated 164–154 Ma (Hildebrand 2013, 2014). Hildebrand 2013). Where best exposed Jurassic magmatic rocks (Salinas-Prieto Far to the north in Sonora is at the southern end of the Maya block, et al. 2000; Bissig et al. 2008; Martini et the Sonoran shelf (Fig. 12), another a southwest-facing carbonate-dominat- al. 2010; Centeno-García et al. 2011; segment of the west-facing Albian ed platform sitting on basement of the Martini and Ferrari 2011). The Lower platform or ramp located in western Maya block was drowned during the Cretaceous arc rocks, now folded and Mexico, this one dominated by carbon- uppermost Campanian, buried by oro- cut by plutons, one of which was dated ate rocks of the Mural Formation GEOSCIENCE CANADA Volume 41 2014 421

California P 0 500 Baja Norte ? Arizona New Mexico km

T New Mexico ? Bisbee basin core D Texas ia C EP b Llano Cenozoic volcanic cover lo C C uplift h Marathon a i uplift b h u Albian platformal carbonate o Sonoran a r h O c re u u u a shelf a t a c hita su H n Bisbee basin clastics t r o Burro Ch u C g o h Alisitos-Santiago Peak arc rt e S s S a b i i e na Inferred Alisitos-Santiago r C s o r ah b a u a s i n ila USA Peak arc M p Mexico lat a for d m Grenville basement r T e ah of Oaxaquia u O ~100 Ma e c c i Guerrero d e suture Laramide n t T thrust belt a co a i l P m ex LuisValles-San platform p M ic a o f - o M

i f s l c a n u t Baja l i G a G b fi a restored ED s c in Tr Zi ans- hu Mexica elt O at n volcanic b an Z e on jo go c lic elt e a fold-thrust b a n Zihuatanejo section of arc Guerrero-Morelos platform

Figure 12. Geological sketch map illustrating the extent of the Guerrero–Alisitos–Santiago Peak arc, its terranes, and Albian carbonate platforms to the east. The Laramide thrust belt and related foredeep are located even farther east as shown. ED–El Doctor platform; G–Guanajuato; H–Hermosillo, C–Caborca, T–Tucson, P–Phoenix, EP–El Paso, Ch–Chihuahua. Map data from Dickinson and Lawton (2001a); Martini et al. (2012); Hildebrand (2013); restoration of Baja from Oskin and Stock (2003). within the Bisbee Basin (Warzeski (Mauel et al. 2011). Hildebrand (2013) both unconformably overlain by 1987; Lawton et al. 2004; González- interpreted rocks of the Ko Vaya suite 130–125 Ma rocks; in the east the Cur- Léon et al. 2008). The shelf was part to represent slab failure magmas curpe is overlain by rocks of the Bis- of a complex, west and southwest-fac- formed during and immediately follow- bee margin, and to the west, rocks of ing passive margin that developed fol- ing the 160 Ma collision, whereas the Peñasquitos Formation are overlain lowing rifting within the Cordilleran Dickinson and Lawton (2001b) consid- by the Santiago Peak volcano-sedimen- Ribbon Continent during the Late ered them to be magmatism associated tary sequence (Fig. 14). Kimbrough et Jurassic (Dickinson and Lawton 2001a, with what they termed the Border Rift al. (2014a) noted that another b). – an extensional system extending sequence, the Mariposa Formation of Beneath the rocks of the Bis- from the Gulf of Mexico to Califor- the western Sierra Nevada (Fig. 14), is bee Basin and sitting unconformably nia. also of the same age (Snow and Ernst atop Jurassic arc rocks, which extend Rocks of the Cucurpe Forma- 2008), has a similar detrital zircon pro- from the Klamath Mountains to south- tion might correlate with the Tithonian file, and was intruded by 125–120 Ma ern Arizona and were deformed during Peñasquitos Formation of the western plutonic rocks of the Sierran batholith a 160 Ma collision, are isoclinally fold- Peninsular Ranges (Kimbrough et al. (Lackey et al. 2012a, b). ed Oxfordian to Tithonian marine sed- 2014a), as both formations have similar Coarse clastic sedimentation imentary rocks of the Sonoran basements and contain similar rocks of and eruption of bimodal volcanic Cucurpe Formation (Fig. 14), which the same age (Fig. 14). Furthermore, rocks in the Bisbee basin are common- were largely derived from magmatic both sequences were deformed ly thought to have started around 150 rocks of the bimodal Ko Vaya suite between about 145 and 139 Ma and are Ma, following the Early to Mid-Jurassic 422

oldero yyoungerounger about 750 km wide. We call the basin west-facingwest-ffaacing ememergencemerrggencg e & ererosionosion the Arperos–Bisbee Sea, and the colli- carbonatecarbonate aas platformlf passepassedd mamarginrrgging susubsidingubsidingg as it trtrenchench rimmedrimmed shelfshelf ooverver outerouter swswellellll wwawasas pulledpullell dd downdod wn intoi intotr trenchench ssesedimentationdimentation sion the Oregonian event, which was the name used by Rangin (1986) for WEW coarsecoarrse limestone limestone condensedconddensed horhorizonizon E the Albian–Cenomanian deformational bbrecciareccia in MMn-richn-rich of LateLate AAlbianlbian calcacalcareousareous mamatrixtrix AmmonitesAmmmonites and event in western Mexico. rudistrudist fragments fragments mmanganeseanganese disconformitydisconffoormity ccoatingsooatings Between the two areas around pelagicpelagicc mudstonemudstonne MexcalaMexcala Guanajuato (Fig. 12) is another likely flyschflysch piece of the arc. There, imbricated lat- coarse,coarse, massivemassiive bbioclasticioclastic limestonelimesstone est Jurassic and Lower Cretaceous vol- well-beddedwelll-bedded well-beddedwell-bedded calcarenitescalcarenites calcarenitescalcarrenites canic and plutonic rocks with an and carcarbonaterbonate tuturbiditesrbiidites uppermost section of calc-alkaline basalt and basaltic andesite, overlain by volcaniclastic rocks and Albian reefal Figure 13. Detailed cross section of the uppermost few meters of the west-facing carbonate rocks, sits on an ophiolitic Guerrero–Morelos carbonate platform showing the rapid transition from carbonate basement (LaPierre et al. 1992b). shelf to orogenic deposits. Interpretation of stratigraphic units at top. Hoffman Pieces of Lower Cretaceous (2012) presents an excellent overview of the process of platform foundering at the seamounts, presumably off-scraped beginning of orogenesis. Figure modified from Monod et al. (2000). into the accretionary prism occur locally (Ortiz-Hernández et al. 2003). Based on the location of the arc magmatism (Bilodeau et al. 1987; The tectonic subsidence was suture, it appears that the basement to Krebs and Ruiz 1987; Lawton and caused by west-to-east overthrusting the Santiago Peak–Alisitos arc was McMillan 1999; Dickinson and Lawton during the Cenomanian when Paleo- more extensive and varied than previ- 2001b), but the oldest sedimentary zoic platformal rocks unconformably ously thought with the Caborca, rocks within the basin were recently overlain by Jurassic and Lower Creta- Cortes, Tahue and Zihuatanejo ter- shown by detrital zircon studies and ceous volcanic rocks were placed atop ranes all forming part of the basement dating of intercalated volcanic rocks to the western margin of the carbonate for the arc (Fig. 12). Uppermost Trias- have been deposited between 136 Ma platform (Pubellier et al. 1995). The sic and Lower Jurassic arc magmatism and 125 Ma (Peryam et al. 2012). With- Cintura Formation is overlain in Sono- appears to have occurred along the in the Bisbee Basin, the lowermost ra by conglomerate of the Cocóspera length of the arc as it is preserved clastic rocks have bimodal NE–SW Formation interbedded with andesitic within the arc basement. Therefore, we paleocurrents and reflect shelf, lagoon- lava dated by the 40Ar/39Ar method to entertain a model of arc rifting to sep- al, tidal flat, and fluvial environments be 93.3 ± 0.7 Ma (González-León et arate the arc terrane from the western (Klute 1991), but pass upwards into an al. 2011). Anderson et al. (2005) also margin of the Cordilleran Ribbon eastward-transgressive sequence of fin- described the belt in some detail, and Continent during the Jurassic. Others ing-upwards fluvial to shallow marine based on the age of a little deformed have noted the presence of Jurassic arc deposits (Peryam et al. 2012). The pluton that cut mylonite of the zone, rocks beneath the rifted margin on the overlying carbonate platform had a pointed out that the deformation was ribbon continent (Lawton and McMil- well developed reefal rim along the older than 84 Ma and attributed it to lan 1999), and Dickinson and Lawton southwest side (González-Léon et al. deformation along the Mojave–Sonora (2001b) called upon slab rollback to 2008). Beginning in the Late Albian the megashear. create the extension leading to the Bis- platform experienced rapid tectonic Taken in its entirety, the evi- bee basin. subsidence and during the Cenomanian dence in western Mexico suggests that The idea for two discrete and Turonian was buried by at least the Alisitos–Santiago Peak arc collided mid–late Cretaceous deformational 1500 m of clastic sediment of the Cin- with a west-facing passive margin at events in western Mexico has become tura/Mojado formations shed in a about 100 Ma. The polarity of the sub- more popular (for example: Pubellier et more or less easterly direction and duction was clearly westward and the al. 1995; Martini and Ferrari 2011; deposited in a flexural basin (Mack western edge of the passive margin Hildebrand 2013) and here we note 1987; Gonzalez-Léon and Jacques- was partially subducted beneath the that the two events, the ~100 Ma Ore- Ayala 1988). The most southwestern arc. The basin was apparently a mar- gonian and ~75 Ma Laramide, devel- exposures of Cintura Formation are in ginal basin open for about 30 m.y. and oped on opposite sides of Oaxaquia excess of 2000 m thick and are over- was of unknown width, although it (Fig. 12). The ~125–105 Ma Sevier lain gradationally by latest Albian– must have been sufficiently wide to be deformation of the western US and its Early Cenomanian fluvio-deltaic sand- floored by oceanic crust in order to hinterland, now located in the Canadi- stone holding pebbles of quartzite and drive the 100 Ma collision. If we an Cordillera (Hildebrand 2014), does limestone and overthrust from the assume that half of the 30 m.y. interval not appear to have affected rocks in southwest by plutonic rocks (Jacques- was spreading, then at average spread- Mexico. Ayala 1992; T. Lawton, personal com- ing and convergence rates of 5 Late Cretaceous volcanic rocks munication 2014). cm/year, the basin would have been interbedded with coarse conglomerate GEOSCIENCE CANADA Volume 41 2014 423

WestWest EastEast Paleogene P e PeninsularPeninsular NV-AZ-NMNVV--AZ-NM SouthernSouthern a l n RRangesanges & SSierraierra e e SonoraSonora MMexicoexico o g NNevadaevada g o e e n MaMa l MaMa a e Paleogene P 60 LaramideLaramide 60 plutonsplutons TarahumaraTarahumara RingboneRingbone volcanicsvolcanics SSkunkkunk RancRanchh deformationdeformation LaL Unión Cretaceous 80 ZZihuatanejoihuatanejo Cocóspera-ECocóspera-Ell CChanatehanate C s 80 Altar r e u SierranSierran LaLa t o CCrestrest PostaPosta a

e pluplutonstons plplutonsutons CinturaCinturaf foredeeporedeep M Mexcalaexcala flflyschysch c

c OregonianOregonian eventevent e 100 a

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c SSonoranonoran plaplatformtform Guerrero-Guerrero- r n s g os an C er t

c MorelosMorelos SevierSevier eveventent r iago i t i kar k k n t ee s ar arc of westernw estteern US US er li est clasticsclastics & i eak platformplatffoorm an S Sierran Alisitos- A P Peak arc Peak

120 sbe 120 Western W S Santiago volcanicsvolcanics i

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140 NevadanNevvaadan eventevent 140 TeloloapanTTeeloloapanp MariposaMariposa PeñasquitosPeñasquitos IDS-KoIDS-Ko VayaVaya CucurpeCucurpe magmatismmagmatismi Luning-FencemakerLuning-Fencemaker eeventvent Jurassic 160 J 160

KittKitt PPeakeak u c SierraSierraNe Nevadavaada BC

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Figure 14. Regional correlation chart illustrating temporal relations between many of the rock units and their relationships to regional deformational events. BC–Bedford Canyon Formation. units that post-date the Oregonian glomeratic wedges that were interpret- unconformably upon folded rocks of event and predate the Laramide appear ed to have been shed from the west the older Albian limestone and vol- to be relatively common within west- and deposited in an elongate NW-SE canic rocks are coarse volcanogenic ern and central Mexico (Fig. 14). For thrust-front basin (Jacques-Ayala conglomerate and sandstone layers example, conglomerate of the 1999). Bouldery to cobbly polymictic intercalated with tuff beds and Cocóspera Formation of Sonora is conglomerate of the Altar Formation, andesitic lava flows and breccia of the intercalated with 93 Ma andesitic lava located to the northeast of Caborca in La Unión and Zihuatanejo formations, (González-León et al. 2011). Generally the Sierra El Batamote, is also dated to be Late Cenomanian to San- considered correlative with the interbedded with volcanic rocks tonian in age (Martini et al. 2010; Mar- Cocóspera Formation is the upper Cre- (Nourse 2001) and contains Turonian- tini and Ferrari 2011). These Late Cre- taceous El Chanate Group, which out- age detrital zircon (T. Lawton, personal taceous volcanic rocks, mostly andesite crops just east of Caborca (Fig. 12), communication 2014). To the south in to rhyolite, might represent a younger and contains thick volcanogenic con- the Zihuatanejo terrane, and sitting period of arc magmatism erupted on 424 the amalgamated Guerrero–Oaxaquia 119º119º 115º115º 111º block and generated by westerly subduction prior to the terminal Laramide 36º336º C collision. The coarse conglomerate M o o lo might be debris shed from the ja ra v do exhumed Oregonian collision zone or e P la Tr d te be alluvial fans on the flanks of volca- an e au sv se noes, or both. er ? r Caborca terrane – located just se t to the west of the suture, the Sonora SSanan GGabrielabriel platform (Fig. 12) and the thrust belt – MountainsMountains 34º334º contains distinctive Neoproterozoic R and Paleozoic strata that are a detailed SanSan BBernardinoernardino P a PhoenixPhoenixx MMountainsountains n match for strata found today in eastern e g n e MohawkMohawk i s MountainsMountains California–western Nevada (Stewart n YumaYuma 2005) and in the San Bernardino P s a u GilaGilaa Mountains near Los Angeles (Cameron c l MtnsMtns S i a o fi 1982; Stewart et al. 1984). Crystalline r SierraSierraL LosLos AjoAjo n TTucsonucson c o AlacranesAlacrannes basement is also exposed within the O r 32º332º

a terrane west of the thrust belt in Sono- c R CerroCerro n e a LosLos OOjosjos ra, where it comprises 1725–1696 Ma a d n n e orthogneiss, tabular granitoid bodies g QuitovacQuitovac NoNogalesgales S s (Nourse et al. 2005), and anorthositic e e e s a r t complexes dated at around 1100 Ma o CaborcaCaborca

(Espinoza et al. 2003). In the San f MagdalenaMagdalena ProterozoicProterozoic basemebasementnt C

Bernardino Mountains the basement to wwithinithin Peeninsularninsular Ranangesges b o r the metasedimentary section is 1750 ± babatholithictholithic blocblockk a t t e C 30º330º 15 Ma (Silver 1971). In fact, the base- h z ab orc ? ment west of the presumed suture UndividedUndivided ProterozoicProterozoic o a basementbasemento of NorNorthth li contrasts sharply with other Protero- AmericaAmerica and andC Cordilleranordilleran th zoic basement provinces of the south- RibbonRibbon ContinentConntinent 100 km km western US, and is termed the Mojave basement province (Bennett and Figure 15. Sketch map modified from Kistler et al. (2014) showing distribution of DePaolo 1987; Wooden and Miller Proterozoic rocks, locations of rocks in Figure 17, and likely basement rocks in the 1990). These rocks form a band of Peninsular Ranges batholithic block. similar Proterozoic rocks from the Caborca terrane northwestward formed during the 100 Ma collision. ments were incorporated within the through southwest Arizona and the The collisional belt also likely contin- Cordilleran Ribbon Continent, only to Transverse Ranges of California (Fig. ues up along the eastern side of, or be rifted off during the Late Jurassic 15). They probably represent the more even into, the Sierra Nevada, which and return some 30–40 m.y. later. easterly basement to the now dismem- probably led to its present juxtaposi- When considered together, all the evi- bered arc. tion nearly orthogonal to structures dence indicates that subduction was Fragments of the northeast- and facies in central Nevada; but that westward beneath the Alisitos arc, vergent fold and thrust belt separating takes us far afield, so additional discus- occurred within a broad marginal sea, the arc rocks and their basement from sion of this interesting topic is with- and that the lower plate during the col- the Albian platformal rocks may con- held for a subsequent contribution. lision was located well to the east and tinue to the north in the Big Maria Nevertheless, pieces of the contained a west-facing passive margin Mountains (Hamilton 1982) and the presumed North American passive capped by an Albian carbonate plat- Clark, Ivanpah, and Mescal mountains margin are included within the arc and, form. (Walker et al. 1995) of eastern Califor- along with other Neoproterozoic– As arc magmatism in the San- nia, where strongly tectonized strati- Lower Paleozoic sequences in the base- tiago Peak–Alisitos shut down at 100 graphic sections, similar to those of ment, likely accumulated on the south- Ma during the collision, and the rocks the southwestern US continental mar- western continental edge of North were rapidly deformed, it was the gin, occur with Mesozoic volcanic and America as that margin has long been active pre-collisional arc. The central plutonic rocks. Cross-cutting thrusts, known to be truncated and missing its area of the arc underwent extreme younger than those of the 125–105 Ma passive margin (Hamilton and Myers exhumation, uplift, and erosion during Sevier fold-thrust belt in southern 1966; Burchfiel and Davis 1972). and immediately following La Posta Nevada and eastern California (Pavlis When, and precisely where, the pieces magmatism, which is atypical for mag- et al. 2014), are of approximately the were torn off North America is open matic arcs, so something unique hap- right age and orientation to have been to speculation, but apparently frag- pened during the collision that led to GEOSCIENCE CANADA Volume 41 2014 425 the emplacement of the voluminous and the lower plate is freed of its Cloos et al. 2005; Hildebrand 2009). La Posta suite and simultaneous exhu- oceanic anchor, rocks of the partially Several factors are important mation characteristic of the region. subducted continental margin rise due in slab failure and govern where and If arcs are not zones of to buoyancy forces (Duretz et al. when during the collision the slab will crustal thickening why did the eastern 2011). The failure also allows asthenos- rupture. First, the age of the subduct- Peninsular Ranges batholith have such phere to upwell through the tear, melt ing lithosphere is perhaps most impor- intense exhumation and uplift? We sug- adiabatically, and rise into the collision tant because young lithosphere is gest that the attempted subduction of zone, where it interacts especially with weaker and therefore break-off will be the eastern block beneath the western subcontinental lithosphere and crust of fast, commonly less than 1 m.y. after arc effectively doubled the thickness of the upper plate. The resulting magmas, initial collision (Duretz et al. 2012), and the crust and that due to the buoyancy which form linear arrays above tears in will occur at shallow levels, whereas contrast between subducting oceanic the descending slab, are upwellings with older, thicker and stronger litho- lithosphere and the difficulty to flowing through the breach in the slab sphere, break-off is slower and the subduct continental lithosphere of the (Macera et al. 2008). They commonly continental edge is subducted deeper lower eastern plate, the lower oceanic overlap the terminal stages of defor- into the mantle. The depth of break- portion of the slab broke off and sank mation. If the magmas intrude the off largely controls the width of the into the mantle allowing the collision upper plate, they may form a linear orogen, for it is the rebound of the zone to rise. Next we look at the belt atop or alongside the old arc and partially subducted continent that will process of slab failure and show how it appear temporally continuous with the lead to the region of intense uplift and can generate not only the exhumation, older magmatism, and so be readily exhumation (Duretz et al. 2011, 2012; but also the post-collisional La Posta confused with it. Magmas might also Duretz and Gerya 2013). Thus, shallow magmatism and other features. intrude rocks of the foredeep and/or break-off creates narrow orogens, the shortened passive margin of the lower-grade metamorphism, and Slab Failure lower plate, or both (Hoffman 1987; intense, rapid and higher rates of exhu- Because the continents are very old Hildebrand and Bowring 1999; Hilde- mation, whereas deep break-off creates and oceanic lithosphere young by com- brand et al. 2010). broad orogens with higher grades of parison, it is obvious that every colli- There is now a burgeoning lit- metamorphism and slow, more sub- sion that entails the closure of an erature on the effects of slab failure, dued rebound (Duretz et al. 2011). In ocean basin wide enough to drive colli- ranging from currently active to Pre- the case of deep failure the subducted sion, must involve break-off of the cambrian (Hildebrand and Bowring margin might be sufficiently buoyant subducting slab, for the alternatives are 1999; Teng et al. 2000). The effects of that it initially rises to the Moho, where for continents to be subducted or for slab failure are important, diverse, and it might stall until the over-thickened slabs to dangle off continental margins may be responsible for features such as crust collapses by extension and/or into the mantle. Neither is observed. rapid uplift (Chatelain et al. 1992), syn- denuded by erosion (Walsh and Hacker Therefore, slab failure, or break-off as collisional magmatism (Davies and von 2004). it is sometimes called, is an integral Blanckenburg 1995; Keskin 2003; Mac- The depth of break-off likely component of plate tectonics and a era et al. 2008), tomographic gaps in also controls the volume of slab failure natural consequence of subduction the descending slab (Wortel and Spak- magmatism because in shallow failure (Roeder 1973; Price and Audley- man 1992), thick-skinned foreland the asthenosphere upwells to shallower Charles 1987; Sacks and Secor 1990; deformation (Cloos et al. 2005), seis- depths, which generates greater vol- Davies and von Blanckenburg 1995; mic discontinuities (Wortel and Spak- umes of melt due to adiabatic melting Davies 2002; Atherton and Ghani man 1992), crustal recycling (Hilde- (McKenzie and Bickle 1988). Addition- 2002; Cloos et al. 2005). brand and Bowring 1999), transitory ally, during shallow failure the Over 40 years ago, seismolo- pulses of mafic magmatism (Ferrari upwelling asthenosphere, say at 35–50 gists suspected that when the edges of 2004) doubly vergent orogens (Regard km depth, creates an advective heat large continental masses are partially et al. 2008), plateau uplift (Rodgers et source capable of generating melts in subducted, their buoyancy leads to fail- al. 2002), ultra-high pressure exhuma- the lithospheric mantle, the asthenos- ure of the subducting slab (McKenzie tion (Anderson et al. 1991; Babist et al. pheric mantle and the crust (van de 1969; Isacks and Molnar 1969). This is 2006; Xu et al. 2010), changes in plate Zedde and Wortel 2001). Thus, shallow because the buoyancy forces resisting motion (Austerman et al. 2011); sub- break-off will create larger quantities the subduction of continental litho- horizontal swarms of deep earth- of magma and they are likely to be sphere are as large as those pulling quakes (Chen and Brudzinski 2011), compositionally varied. Furthermore, oceanic lithosphere downward (Cloos lateral shifts in foredeep sedimentation because the asthenospheric mantle et al. 2005). Eventually, the greater (van der Meulen et al. 1998), opening source region is highly variable (Men- density of the oceanic lithosphere of small ocean basins (Carminati et al. zies et al. 1987; Foley 1992), and causes the lower plate to break, pre- 1998), switchover from foredeep flysch because the rising magmas can assimi- dominantly by viscous necking (Duretz to orogenic molasse (Sinclair 1997; late between 10% and 75% crust- et al. 2012) at its weakest point, and Wilmsen et al. 2009), and porphyry derived materials during their rise sink into the mantle. copper and other mineralization through the crust (McDowell et al. Once the subducting slab fails (Solomon 1990; de Boorder et al. 1998; 1996; Housh and McMahon 2000; 426

McMahon 2000, 2001; Chung et al. coarse cobbly and WE 2003), the resultant magmas can be bouldery molasse heterogeneous, ranging from pure during the early pre-collision Albian carbonate platform asthenospheric melts, mixtures involv- Cenomanian (Kim- Precambrian ing lithospheric mantle melts, to com- brough et al. 2001). Juro-Trias Paleoz plex crustal melts and, of course, mix- These are all charac- & Paleozoic Alisitos-SP tures of the three (e.g. Hart et al. teristics of shallow arc Ribbon continent 2004). break-off and so When the break-off is located suggest that the beneath the arc, or nearly so, upwelling oceanic lithosphere asthenosphere rises into the region of was not particularly little or no lithospheric mantle, and the old, but based on resulting magmas might look very thermomechanical much like those of an arc, but where models, older than the break-off occurs adjacent to the about 30 million Alisitos-Santiago La Posta slab-failure arc the magmas might be quite differ- years at the time Peak arc plutons ent in composition because the (Duretz et al. 2011). arc deformed asthenosphere would impinge first on volcanics plutons post-collisional plutons the subcontinental lithospheric mantle. Exhumation and 128 100 86 Ma And given that those mantle regions Origin of the have different properties, such as com- Doubly Vergent Fan Structure position, depending on their age (Jor- Mesozoic basement PC & Paleozoic basement dan 1978; Pearson and Nowell 2002; Once the subducting Poupinet and Shapiro 2009), magmas oceanic slab fails, the created in those areas might have dif- leading edge of the foreland ferent compositions from place to lower plate is no 85 Ma Cenomanian basin place and orogen to orogen. Magma- longer being pulled molasse tism might start out reflecting melting down beneath the within enriched lithospheric mantle upper plate, and due and quickly revert to asthenospheric to buoyancy forces, melts as the lithosphere is thermally will rise, typically at a thinned and transected by deeper level rapid rate, limited melts (Perry et al. 1987). Additionally, only by the erosion given that the margin breaks off at rate (Burbank 2002) depth beneath and more or less parallel and strength of the to the arc, it might be common for the lithosphere (Avouac Figure 16. Tectonic model showing the development of asthenosphere to rise, at least in part, and Burov 1996). At the Alisitos–Santiago Peak arc above a westward-dipping beneath the transition from arc to con- the same time, subduction zone and its collision with the western margin tinental lithosphere, which would gen- asthenospheric man- of the Cordilleran Ribbon Continent and its west-facing erate additional compositional varia- tle is streaming Albian carbonate platform as discussed in the text. The tions, and might even appear as a upward through the collision was followed by slab failure of the lower plate and smooth transition between two entirely tear and creating emplacement of the La Posta plutons during a period of different magma types. buoyant melts that major exhumation. In Southern and Baja Califor- will also rise into the nia, the shutdown of arc magmatism at collision zone, the exact location magmas might span tectonic bound- ~100 Ma marked the collision and, depending on how and where the aries in the resultant orogen. Also, within 1–2 m.y. of collision, the earliest descending plate failed. For example if changes in the subduction regime can plutonic complexes of the 99–86 Ma the tear is at or near the oceanic–conti- happen abruptly along strike and have La Posta suite were emplaced (Kim- nent interface, then diachronous tear- huge impacts on the form of deforma- brough et al. 2001; Premo et al. ing (Wortel and Spakman 2000; Schild- tion and magmatism (Ely and Sandi- 2014b). The magmatism was especially gen et al. 2014), or perhaps basal trac- ford 2009). voluminous and magmas appear to tion (Alvarez 2010), would cause con- In a somewhat similar fashion, have been emplaced during a period of tinued convergence, either or both of the leading edge of the lower plate is immediate exhumation close to and which would pull the subducting slab pulled beneath the overriding plate and overlapping with the upper plate arc over the rising asthenospheric welt and how far depends on the buoyancy con- (Fig. 16). They are contemporaneous cause slab failure magmas to rise just trast and strength of the plate. As stat- with deposition of thick Cenoman- inboard of the tear. Similarly, re- ed earlier, younger lithosphere will ian–Turonian clastic successions to the entrants and promontories in the lower break more quickly, in less than 1 m.y., west, which involved a rapid change plate might fail at different places and in those cases the continental edge from flysch-type sedimentation to along strike so that the slab failure isn’t subducted deeply but instead is GEOSCIENCE CANADA Volume 41 2014 427 barely tucked beneath the arc. In the Due to the release of the critical observations, which the current case of shallow break off, as soon as downward pull of the oceanic litho- long-lived eastward-dipping subduction the slab fails it rises and immediately sphere, and perhaps fueled by models fail to do. In our model, the begins to exhume the overriding plate. upwelling asthenosphere and rising westward-dipping subduction led to Thus, the time between initial collision magma, the orogenic root began its partial subduction of the eastern block and initiation of exhumation and mag- buoyant upheaval, lifting the upper and its west-facing passive margin matism due to slab failure is very short. plate on its back. However, the upper beneath the arc, which caused its The rapid response to slab plate was simply not strong enough to young oceanic lithosphere to break off failure is important but given that the withstand the forces so it failed more and generate slab failure magmatism of lower plate typically represents a for- or less above the distal edge of the the La Posta suite within 1 m.y. of the merly thinned section of continental lower plate. As the dynamically rising collision. Rebound of the eastern, par- lithosphere, it is unlikely to rebound to hot hinterland was elevated it spread tially subducted plate caused rapid sea level but instead will have a cara- laterally over the Alisitos arc and per- uplift and exhumation during the pace or veneer of upper plate rocks haps eastward as well. In this interpre- emplacement of the plutons. atop it. Depending on the crustal tation, the western part of the doubly thickness, strength profile, and perhaps vergent fan structure marks the Detrital Zircon rate of exhumation of the upper plate approximate western limit of the lower Our recognition that basement to the as governed by rainfall (Hoffman and plate at depth. Good seismic profiles Santiago Peak–Alisitos arc rifted from Grotzinger 1993), it might get lifted should show the upper plate within the rocks to the east at about 139–136 Ma, gently to form a broad dome, or alter- hinterland as a fairly thin sheet sitting and that it was composed of a wide natively it might fail and there will be a above a gently inclined to broadly variety of Jurasso–Triassic rocks, as major fault in the upper plate separat- warped, reflector representing the colli- well as a nearly complete section of ing the hinterland from the relict arc. sional suture. Paleozoic and Neoproterozoic sedi- In some instances a bivergent orogen, The uplift, exhumation, and mentary rocks built on Precambrian such as the Alps, might form (Regard consequent erosion also created huge crystalline basement, provides a ready et al. 2008). volumes of coarse debris, much of outboard, or offshore, source for a Following exhumation, uplift which was shed to the windward, wet wide detrital zircon spectrum in sedi- and erosion, one should find a zone of side of the rising orogenic welt, which mentary rocks of the arc complex and higher grade upper plate rocks between in this case would have been westward. precludes that they must have been the upper plate arc rocks and the lower And that is what is observed as flysch derived directly from North America plate. The grade should rise gradually sedimentation to the west was abruptly as argued by some researchers (Gehrels towards the hinterland in the lower swamped during the early Campanian et al. 2002; Kimbrough et al. 2006, plate, then increase rapidly at the by voluminous coarse debris (Kim- 2014a). This conclusion, coupled with suture as the base of the upper plate is brough et al. 2001). This is similar to the likelihood that fragments of Lau- encountered. In the case of a faulted the transition from flysch to molasse in rentia such as the Antler platform of upper plate, the grade should remain the Alpine belt, which is thought to the Great Basin region and Caborca high, perhaps decreasing gently due to have been caused by slab failure (Sin- terrane in Sonora (Ketner 1986; Gastil the thinner lower plate, until the fault clair 1997). et al. 1991; Stewart 2005; Hildebrand is crossed, where the grade will drop In many ways, parts of the 2009, 2013; Premo et al. 2010) were precipitously in the upper plate, which Peninsular Ranges orogen are similar probably derived from the southwest was never buried. to the Alps of Europe, in that both corner of Laurentia in what is the If one assumes that subduc- orogens are doubly vergent with a present day Mexico region and incor- tion prior to collision was westward, metamorphosed crystalline core and porated within the Cordilleran Ribbon then the Sierra de San Pedro Mártir much lower grade thrust sheets on the Continent, possibly during the transi- cross-section (Fig. 4) of the Peninsular external flanks. The main differences tion from Pangea B to A (Irving 1977, Ranges batholith displays the features are the amount of magmatism and the 2004; Kent and Muttoni 2003), serves described above that are characteristic width of the orogen, which could be to complicate correlations and detrital of slab failure with a faulted upper directly related to the depth of break- zircon studies. Terranes derived from plate. In this case the leading edge of off. The Main Mártir thrust, which the southwest corner of North Ameri- the eastern block was partially subduct- placed amphibolite-grade rocks of the ca should contain North American ed beneath the Alisitos arc and slab arc and its basement westward over flora and fauna, but also large quanti- failure occurred at 100 Ma. Once the lower grade arc rocks, is more or less ties of Grenville age zircon reflecting slab failed, magmas rising from the an equivalent structure to the Insubric their proximity to that belt, which was upwelling asthenosphere rose into the Line, an antithetic thrust fault, which rich in Grenvillian basement (Hoffman crust to produce the La Posta suite. placed amphibolite-grade rocks of the 1989). Although the plutons intruded both upper plate over the Southern Alpine Also, it appears as though the eastern and western sectors, they were nappes to the south. western Sierra Nevada batholith has a mostly emplaced into the deeper east- In the preceding sections we remarkably similar history to the Santi- ern sector. The fan structure might be developed a coherent, actualistic, and ago Peak–Alisitos arc, including an a root zone (Roeder 1973). testable model that explains all of the older arc complex in the west, and 428 compositionally zoned, post-deforma- that even if it was possible to get the upon crust assembled mainly during tional, plutonic complexes, such as the crust beneath them, by Ducea’s (2001) the Jurassic, whereas the eastern half Tuolumne, Whitney, John Muir, and own admission there is no obvious contains large compositionally zoned Sonora, of the Sierran Crest magmatic mechanism to melt such a large vol- tonalite–granodiorite–granite complex- event (Coleman and Glazner 1998) to ume of continental crust; (3) that es, such as the Tuolumne, Whitney, the east. The Snow Lake pendant (Fig. because the bulk composition of the John Muir, Domelands, and Sonora 17), located along the north edge of batholiths is so close to that of bulk (Fig. 17), emplaced during the 98–86 the Tuolumne intrusive complex in middle and lower crust (Ducea 2002; Ma Sierran Crest magmatic event Yosemite National Park is yet another Rudnick and Gao 2003), there would (Coleman and Glazner 1998; Saleeby et fragment of the distinctive Neopro- be very little restite to drip into the al. 2008). And just like the Peninsular terozoic–Early Paleozoic sequence mantle, which means that the crust Ranges batholith, rocks of the Sierra found in the Death Valley and Caborca should be very much thicker than Nevada contain evidence of an areas (Stewart 1970, 2005; Lahren et al. observed; (4) because the thrusting ~103–100 Ma deformational event that 1990; Memeti et al. 2010) and, like the took place at the scale of the litho- postdated all known sedimentary and Peninsular Ranges, it represents part of sphere, the Ducea model provided no volcanic wall rocks within the western the basement to the arc. The original actualistic mechanism to dispose of arc (Peck 1980; Nokleberg and Kistler location of the sections remains uncer- the sub-continental lithospheric man- 1980; Bateman et al. 1983; Saleeby et tain. We now look a bit more closely at tle; and (5) that the so-called flare-up al. 1990; Bateman 1992; Wood 1997; the Sierra Nevada to ascertain if there may not be beyond the bounds of Memeti et al. 2010; Hildebrand 2013) are features there that might inform magmatic flux in young arcs. yet predated, or was partly coeval with, our understanding of the Peninsular Ducea (2001) also noted that the compositionally zoned plutonic Ranges batholith and related rocks. since it took time for the isotherms to complexes of the Sierran Crest mag- rebound after crustal thickening, the matic event (Greene and Schweickert Comparison with Other Cordilleran melting event, however it was caused, 1995; Coleman and Glazner 1998; Batholiths postdated the thickening by ~15–25 Davis et al. 2012). The plutons were Just over a decade ago Ducea (2001) m.y. However, such an early thickening emplaced more or less simultaneously proposed an eastward-directed subduc- is countermanded by the deposition of with development of mylonitic shear tion model for the origin of Californ- shallow marine sedimentary rocks at zones, a rapid increase in cooling rates ian Cordilleran batholiths to explain about 100 Ma within both the Sierran between about 90–87 Ma, and what he thought was a period of and Alisitos–Santiago Peak arc ter- increased sedimentation in the basin to unusually high magmatic flux, from ranes. As these rocks were deposited the west during the Turonian (Mans- 100–85 Ma, within the Sierra Nevada. immediately prior to the regional field 1979; Renne et al. 1993; Tobisch In his model, eastward directed retro- deformation and within a couple of et al. 1995). Thus, by analogy with the arc, thin-skinned thrusting in the Great million years of the emplacement of Peninsular Ranges batholith, we sug- Basin area was balanced by westward the post-deformational plutons, there gest that the western half of the Sier- underthrusting of middle to lower is no evidence of crustal thickening ran batholith was an arc generated by crust, which as a result was shoved within the arc prior to the ~100 Ma westward subduction, and that the arc beneath the Sierran arc, melted to pro- deformational event. collided at ~103 Ma with the western duce the 100–85 Ma magmatism, and As touched upon in the early margin of the Cordilleran Ribbon all the excess material, or restite, sink- part of this contribution, the major Continent. Slab failure of the partially ing into the mantle. Others soon Cordilleran batholiths of North Amer- subducted eastern block led to volumi- expanded on the model (Ducea and ica are clearly composite bodies that nous magmatism of the Sierran Crest Barton 2007; DeCelles et al. 2009), share significant similarities. The Sierra magmatic event. whereas some workers, mindful of a Nevada has long been known to be There is support for the gen- likely sedimentary component to the composed of two halves: an older eral slab failure model in Sierran xeno- magmatism, proposed that large vol- western sector and a younger eastern liths, which also provide feedback that umes of crustal material were subduct- sector separated by an apparent break can be applied to the Peninsular ed from the west beneath the Peninsu- in the lithosphere, as defined by geo- Ranges batholith. First, ultramafic lar Ranges in order to thicken the crust chemistry, magnetic susceptibility, age, xenoliths collected from much younger (Todd et al. 1988; Grove et al. 2008; radiometric and stable isotopes, wall volcanic rocks (Fig. 18) suggest that Miggins et al. 2014; Premo and Mor- rock provenance, and basement types the xenoliths are dominantly residual ton 2014). (Nokleberg 1983; Kistler 1990, 1993; cumulates remaining after extraction of Hildebrand (2013) pointed Chen and Tilton 1991; Bateman et al. partial melts from both upper mantle out: (1) that it is difficult, if not impos- 1991; Coleman and Glazner 1998; and subcontinental lithosphere at sible, to get the requisite 400 km of Saleeby et al. 2008; Lackey et al. 2008, depths of at least 32–18 kb North American cratonic lithosphere 2012a, 2012b; Chapman et al. 2012). (Mukhopadhyay and Manton 1994; beneath the Sierra Nevada–Peninsular Just as with the Peninsular Ranges Chin et al. 2012), consistent with Ranges batholiths because it is simply batholith, the western sector of the upwelling mantle and adiabatic melting too buoyant unless attached to nega- Sierra Nevada represents an ~125–100 resulting from slab failure. Ducea and tively buoyant oceanic lithosphere; (2) Ma arc (Bateman 1992) constructed Saleeby (1998) showed that the cumu- GEOSCIENCE CANADA Volume 41 2014 429

Plutonic Rocks 120 124° 122° 120° Lake Susanville Sierra Nevada batholith; Almanor mainly 125 Ma to 82 Ma purples are <100 Ma Medford OREGON 42° eastern zoned complexes K CALIFORNIA Kc l 40 a ~130 Ma m Modoc M a t Plateau t n h C Eureka s ~140 Ma Redding Chico dgo

HC ~150 Ma 40° Basin Chico

P & Range ~160 Ma a C Truckee o S LT c a s i and Tuttle Lake and Sailor Canyon Fms. LT t Sacramento e i G NE r V AD ~170 Ma Lake r Marysville LT fi r A LT a e Tahoe 38° c a 39 N San t ~200 Ma e Francisco R V v a a n a d O g l e l a > ~370 Ma Snow Lake s e c y pendant e S S a n A n d r e a s F a u l t ali ? ? ? a n Unclassified 36° ian n 119 bloc So no k Tethyan fossil fauna (Permian) ra Bridgeport McCloud fossil fauna (Permian) T ? u Mono ? o 38 ? l ? ? u Lake m n Other Rocks & Terranes e ?

118 Great Valley Group; upper Cretaceous

n B o

Jurassic volcanic arc units: Kettlerock and Mt. Jura W a w

P s Bishop L o h

s a s sequences, and Tuttle Creek and Sailor Canyon Fms. k s J L i it e b o Ow a - h M l k e n e

e o p M s j a t a l e Jurassic volcanic and sedimentary rocks Merced o v e u e o n n t ir a r a s l i c t e e fa Roof pendants in Sierra Nevada batholith; ult 37 Paleozoic to mid-Cretaceous SC V 117 a

l l e Smartville complex Fresno y Central belt: Includes Fiddle Creek, Slate Creek, KINGS I M n t Lake Combie, & American River terranes, Tuolumne . L y W o h n o ophiolitic melange, Jasper Point Fm., Peñon Blanco Fm., i e s t n P and serpentinite melange e i y n

Soda Ravine terrane; slaty argillite with S E Q tectonic blocks of Permian and Late Triassic limestone Visalia U K e E r n

N C C a K n E E y R

o N n Red Ant Schist; Triassic(?) metamorphic age P 36 L

A A D T r o E g A u m U s

R e P Calaveras terrane; late Paleozoic and/or a f l E n a a N g u n l D e t Triassic chert-argillite melange d A & N s T P S E Feather River terrane: mostly serpentinized peridotite N D A and related ultramafic and mafic rocks; includes Devils p N r T o t s Gate ophiolite S o tn - M

f o

a s u a

l l t E Don Pedro terrane; pre-Jurassic melange overlain fault

by Jurassic volcanic and sedimentary rocks Bakersfield Garlock

Kings-Kaweah ophiolitic melange 35 Mojave 117

Permian volcanic arc sequence San Andr eas fault 118

Taylorsville sequence: Late Devonian to Pennsylvanian; 119 depositional on Shoo Fly complex Shoo Fly Complex; includes four thrust slices of Ordovician(?) to pre-Late Devonian rocks

Proterozoic to Mesozoic sedimentary rocks 0 20 20406080 40 60 80 100 km of the White-Inyos and other ranges east of the Sierra Nevada

Figure 17. Generalized geological map of the Sierra Nevada batholith and environs, showing the basement terranes, location of the Snow Lake pendant, and the distribution of major plutonic complexes of the post 100 Ma Sierran Crest magmatic suite (modified from Irwin and Wooden (2001) with additional data from Dunne et al. (1978), Saleeby et al. (1978), Bateman (1992), and Saleeby and Busby-Spera (1993). Distribution of post 100 Ma suite modified from Van Buer and Miller (2010). 430

T (ºC) ages for the cores of detrital zircon, and Hf isotopic ratios like those from 0 600º 1200º 1800º Proterozoic basement of the eastern 5 Present da Sierra Nevada – appeared to indicate gabbro that rocks of the North American pas- Liquid sive margin were transported deep y Sierran geother beneath the arc and metamorphosed Spinel websterite close to 100 Ma; yet they had no viable mechanism for getting them there in 10 an eastward-directed subduction G ar model. The presence of the quartzite m net w xenoliths, their ages and isotopic char- eb st acteristics thus provide an unexpected er ite test and confirmation of our west- 15 ward-dipping subduction model. Because of the paradigm that Spine l perid otit the Sierran batholith was built on the e western margin of North America, Gar net other researchers used the eclogite-

facies quartzite xenoliths, with their

North American isotopic signatures, as 20 p evidence that the sub-batholithic crust Lee et al. e r

P (kb) 2001 id was at least 70 km thick and was sim- o t ply thickened North American crust i t e (Ducea and Saleeby 1998; Ducea 2001). They also used the presence of Chin et al. garnet peridotite xenoliths to suggest 25 2012 Mukhopadhyay that the mantle lithosphere was at & Manton 1994 ~120 km depth. In our model, the western margin – whether you believe it was Sierra North America or the ribbon conti- 30 nent – was subducted beneath the arc Nevada (Fig. 14), which readily accounts for the quartzite xenoliths, the eclogite- facies metamorphism and their isotopic signatures. It also provides a more actualistic method of getting 35 mantle lithosphere to the inferred depths and, given that the xenoliths are Figure 18. P–T grid illustrating the final equilibrium conditions calculated for vari- restite or cumulate from removal of ous groups of mantle xenoliths from the Sierra Nevada. Although the arrays are melt along an adiabatic geotherm from artifacts of the different reaction kinetics of the geothermometers and geobarome- ~3.5–1.7 GPa (Mukhopadhyay and ters, the points represent minimum P values (Chin et al. 2012). These high pres- Manton 1994), they confirm the con- sures (~3 GPa) support the model presented here in that the La Posta and Sierran cept of mantle rising through the torn Crest magmatic suites resulted from melting at great depth caused by slab failure, slab and melting adiabatically to pro- upwelling of asthenosphere, and melting within the garnet stability field. duce the slab failure magmas. This was corroborated by two other detailed late rocks are the same age as Sierran ern margin of the Cordilleran Ribbon studies of peridotite xenoliths (Fig. 18), granitoid magmatism. The idea of Continent beneath the arc readily the first by Lee et al. (2001), who, as upwelling mantle and adiabatic melting resolves the difficulties of Chin et al. mentioned, found two types of xeno- is supported by the detailed work of (2013) when trying to interpret gran- liths with contrasting thermal histories Lee et al. (2001), who also studied Sier- ulitic quartzite xenoliths from a and concluded that hot asthenosphere ran mantle xenoliths and found evi- Miocene diatreme in the central Sierra and cold Proterozoic lithospheric man- dence for a cool subcontinental litho- Nevada. They were baffled because tle “were suddenly juxtaposed, a feature con- sphere of Proterozoic age underplated their extensive data from the quartzite sistent with the aftermath of rapid lithospher- by hot asthenosphere during the Meso- xenoliths – T = 700–800ºC, P = 7–10 ic removal or sudden intrusion of asthenos- zoic. kb, ~103 Ma mean metamorphic age pheric mantle into the lithosphere” during Second, our model for west- from U–Pb analyses in zircon, Protero- the Mesozoic. The second by Chin et ward-dipping subduction of the west- zoic and Archean U–Pb crystallization al. (2012), showed that peridotite GEOSCIENCE CANADA Volume 41 2014 431 underwent shallow melt depletion at southerly batholiths. A group of post- 2014). The Santa Ana plutons have a 1–2 GPa and was re-fertilized at deformational plutons (Gehrels et al. continuous compositional range from depths of about 3 GPa, which could 2009) are likely slab failure bodies. gabbro to granite, with 47%–77% SiO2 readily be accomplished by taking par- In this contribution we have (Fig. 8) typical of arcs, yet Clausen et tially subducting depleted lithosphere shown how the Peninsular Ranges al. (2014) suggested that the mafic then re-fertilizing it by upwelling batholith comprises two distinct peri- magmas rising from the mantle into a asthenosphere. Thus, the quartzite and ods of magmatism – separated by a lower crust dominated by peridotite xenoliths, as well as their fea- deformational event – that are simply gabbro/amphibolite created the spec- tures and histories – so difficult to and logically explained by a period of trum of melts by partial melting, explain in static eastward subduction arc magmatism followed by an magma mixing and fractional crystal- models – are readily accounted for in arc–continent collision, which caused lization. The main problem with a an actualistic westward subduction– rapid shutdown of the arc, thickening lower crustal scenario, which they rec- collision–slab failure model. of the subjacent crust as the continen- ognized, is that the overall process Similar types of xenoliths are tal edge was partially subducted, and would create huge volumes of high- known from the Pamirs and Tibetan consequent failure of the westward- density ultramafic restite in the lower- Plateau where they are generally inter- dipping slab to produce slab failure most crust for which there is no geo- preted to represent fragments of sub- magmas (Fig. 16). The consilience of physical, or other, evidence. ducted continental crust (Hacker et al. so many independent lines of evi- Based on comparison with the 2000, 2005; Ducea et al. 2003). dence, from isotopic to structural data, La Posta plutons, they suggested that Detailed study of samples from the stratigraphy to plate kinematics, mantle there is something peculiar about the Pamir found much older detrital zircon xenoliths and xenocrystic zircon to composition of plutons within the that appears to have had a Gond- rapid exhumation gives us great confi- Santa Ana suite in that they have initial wanan, as opposed to Indian, source dence in our overall model for west- Sr ratios lower than 0.704 and SiO2 and so suggests a subduction polarity ward-dipping subduction beneath the contents > 55%, but they compared (Hacker et al. 2005), just as do the Sier- 130–100 Ma Alisitos–Santiago Peak apples to oranges because they con- ran examples. and Sierra Nevadan arc terranes. Not trasted them with the slab failure-type As is well known, the western only does the model tie together many plutons (Clausen et al. 2014). If they Sierran arc has a basement comprising disparate bits of geology, it also places had compared them to other arc suites numerous Jurassic and Paleozoic ter- them within a modern and actualistic of the western US they would have ranes (Saleeby 1981; Irwin and Wood- plate tectonic framework. found them to be rather typical. For en 2001; Hildebrand 2013). Late Juras- example, the western, or arc portion, sic sedimentary rocks of the Mariposa Petrogenesis of the Sierra Nevada contains plutons, Formation (Fig. 14), commonly inter- Given that the Cordilleran batholiths such as the multiply folded sheets of preted to represent forearc deposits to described here are composed of two the Stokes Mountain complex (Fig. 1), a more easterly arc are part of this compositionally distinct magmatic with the same characteristics (Clemens basement (Snow and Ernst 2008) and suites separated by a period of defor- Knott 1992). Furthermore, nearly all are consistent with similar age rocks mation, it might be enlightening to the Quaternary volcanic rocks of the containing comparable detrital zircon look briefly at their petrogenesis. Both Cascades from 51ºN to 41ºN have suites along the west side of the Penin- suites would seem to involve partial 87Sr/86Sr lower than 0.704 independent sular Ranges batholith (Kimbrough et melts of asthenosphere and variable of SiO2 content and amount of con- al. 2014a) and in Sonora (Mauel et al. amounts of mantle lithosphere and tamination by varied continental crust 2011). crust, yet they are quite different in and mantle lithosphere (Hildreth The Coast plutonic complex composition. Here we present a few 2007). It is the slab failure plutons that of British Columbia is another com- ideas and constraints on their petroge- are markedly different from the arc posite batholith built of two blocks: nesis. bodies as seen on our various geo- 190–110 Ma to the east and 160–100 The presence of basaltic lavas chemical and isotopic figures. Ma to the west (Gehrels et al. 2009). A and gabbroic intrusions, coupled with Overall, there can be little west-vergent fold-thrust belt, that their overall calcic nature (Fig. 8a, b), doubt that batholiths and associated developed around 100 Ma (Rubin et al. within the Santiago Peak–Santa Ana siliceous volcanic rocks are generated 1990; Haeussler 1992), places high- arc indicate that mafic melts were ulti- in continental crust, for in volcanic grade rocks of the eastern belt over mately responsible for the magmatism arcs built on oceanic crust there are no lower grade rocks of the in the arc, although there appear to be batholiths or plutonic complexes like Gravina–Dezadeash–Nutzotin belt to no primary mantle melts present there those observed in western North form a thrust stack that is of higher today. Recently, the most detailed stud- America, (Waters 1948; Bateman 1981; metamorphic grade upwards (Lynch ies to date on the Santiago Peak–Santa Whalen 1985). One has only to study 1992, 1995; Journeay and Friedman Ana arc concluded that the arc was the differences in magmatism in the 1993; Crawford et al. 2000; McClelland built upon oceanic crust (Clausen et al. Kurile–Kamchatka and Aleutian–Alas- and Mattinson 2000). Although the 2014), or a heterogeneous Jurassic ka Peninsula arcs where the transition deformation is about the same age, the basement deposited at or near a conti- between the two crustal types is readily polarity is the reverse of the more nental margin (Herzig and Kimbrough observed. On continental crust, mag- 432

P 10 MORB e Peninsular Ranges Pre-100 Ma n Santa Rosa plutonic suite SP western in arc rocks s La Posta plutonic suite 120 Ma Sierran u Santa Ana plutonic suite 5 GL Stokes Mt. arc la Drill holes-greater LA area rocks r Zarza intrusive complex R Santiago Peak volcanics a GP n Sierra Nevada g CHUR 0 Onion Sierra Nevada (DePaolo) Valley Post 100 Ma e eastern slab s Tuolumne intrusive complex Sahwave failure rocks complex Lamarck granodiorite Nevada 86 Ma Santa Mt. Whitney intrusive suite Rosa suite -5 Sierra YC, Taft, Sentinal, El Capitan i Nevada GP Sierran xenolith E & W 120 Ma Stokes Mtn complex Nd Post 103 Ma GG eastern Sierran

ε 10 slab failure rocks Alacranes Mohawk G nora Mohawk Jurassic and Late Cretaceous Quitovac So -15 plutonic rocks of Transverse Quitovac Los Ojos Ranges and northern Sonora SG t wes Quitovac -20 th Gila Nor

0.706 0.710 0.714 0.718

Sri

Figure 19. εNd versus Srinitial of various suites within the Peninsular Ranges batholith showing the evolved nature of the plutons and their similarities to Mesozoic rocks of the Transverse Ranges; eastern, post 103 Ma plutons of the Sierra Nevada (inverted triangles), and likely basement rocks of the northwest Caborca terrane (purple triangles). Modified from Premo et al. (2014b). Peninsular Ranges samples from drill holes in greater Los Angeles area from Premo et al. (2014a); Zarza intrusive complex, a Santa Ana arc pluton in Baja, from Tate et al. (1999); Santiago Peak volcanics from Herzig and Kimbrough (2014); Sierran xenoliths: SP–spinel peridotite; GP–garnet peridotite; GG–garnet granulite; G–granulite. 120 Ma Stokes Mountain plutons of western Sierran arc from Clemens Knott (1992); Sierran xenolith data from Domenick et al. (1983); post 103 Ma plutons of eastern Sierra Nevada: Sierra Nevada from DePaolo (1981); Tuolumne intrusive suite from Memeti (2009); 94 Ma Lamarck granodiorite from Coleman et al. (1992); 88–83 Ma Mt. Whitney intrusive suite from Hirt (2007); 103–95 Ma Yosemite Creek (YC), Taft, El Capitan, and Sentinal from Ratajeski et al. (2001); Onion Valley, a 92 Ma hornblende gabbroic sill complex, from Sisson et al. (1996); 93–89 Ma Sahwave intrusive suite–a post 100 Ma zoned complex, located in western Nevada, and similar in composition, zoning and petrography to Sierran Crest plutons, from Van Buer and Miller (2010). mas are richer in silica and incompati- early on within the Santiago Peak arc plutons assimilating such young crust ble elements, intermediate–siliceous as indicated by early eruption of volu- have low initial Sr (Fig. 19). However, ignimbrites are common, and large minous quartz porphyritic siliceous tuff Santa Ana–Santiago Peak arc magma- composite batholiths are emplaced into that predated the andesitic eruptions tism exhibits an initial εNd range of the volcanic suprastructure (Hilde- (Herzig and Kimbrough 2014). Given +8.2 to –0.6 and depleted mantle brand and Bowring 1984). that the immediate crust beneath the model ages (TDM) ranging from 0.31 to Crustal melts were generated arc was Jurassic, it is not surprising that 1.03 Ga and are clearly not directly GEOSCIENCE CANADA Volume 41 2014 433

15.7515.75 DMM-derived magmas. Thus, we see a. SantaSanta Rosa =9 ~1.75~1 75 MMaa BamorBamorii M S parparagneissagneiss susuiteite up 15.715.7 0.0 an ra the Santiago Peak–Santa Ana arc to be 00.2 Ga t younger c le ru 0.4 Ga - st lik a 0.6 Ga 15.70 e l a rather typical arc suite (Fig. 11d). so so u 1.7 GGaaC Cerroerro u rc

b r e RajonRajon ggraniteranite a c s M

The La Posta plutons have P e 15.6 MojaveMojave s 00M b ProvinceProvince 04 20

P older 100 more evolved Nd, Sr, Pb and O iso- / 15.6515.65 04 b oon 100M 2 r LaLaP Postaosta topic signatures than the Santa Ana P suitesuite b/ b eochre

PostPostt 100 MMaa 207Pb/204Pb 207 GeochrGGe

P 15.515 5 suite (Figs. 19, 20, 21) and clearly 7 CentralCentral SantaSanta Rosa

207Pb/204Pb 20 15.60 dmm ArizonaArizona LaLaP Postaosta SantaSanta AnaAna suitesuite PostPost 100 MMaa involved crustal contamination. Over- 00.2 Ga PrePre 110000 MMaa 15.415.4 0.0 SSantaanta AAnana a. SSantaanta Rosa all, the Pb isotopic dataset appears to =8.5 15.5515.55 LaLaP Postaosta be a mixing trend between a more M SantaSanta RRosaosa PrePre 100 MMaa b.b. an suitesuite SSantaanta AAnana t 39 le primitive, less radiogenic end-member, MojaveMojave - lik .708.708 e 207 204 ProvinceProvvince represented by the lowest Pb/ Pb so LaLaP PostaPosta u r suitesuuite r c 208 204 4 e and Pb/ Pb samples forming the = S s b /U/U P hh/ /86 .706.706 Santa Ana suite, and the consistently =8.5=8.5 ThT 204 0.0 Ga 7Sr

8 s e l most radiogenic or evolved samples of b/ 38 rc a P u ~1.75 MMaa BBamoriamori so 08 0.2 Ga iti 2 208Pb/204Pb paragneissparagneiss .704.704 al the Santa Rosa suite (Fig. 20). Elevated n t Initial 87Sr/86Sr I SantaSanta Ana Ana us 207 204 suitsuitee cr 2 ra Pb/ Pb signatures, relative to DMM = Sup dmm h//UU 1.7 GGaaC Cerroerro ThT b. CentralCentral .702.702 and especially the most radiogenic end- RajonRajon ggraniteranite ArizonaArizona member Santa Rosa suite, resulted 5 7 9 1111 13 37 WholeWholeR Rockock δ18O δ18O vsmowvsmow from decay of the almost extinct 235U 207 206Pb/204Pb206Pb/204Pb parent of Pb, a mostly Archean .710.710 process. Thus, these 207Pb/204Pb signa- Figure 20. Plots of whole rock (a) LaLa PostaPosta suitesuite tures indicate very long-lived source 207Pb/204Pb; and (b) 208Pb/204Pb vs. .708.708 differences and substantiate input from 206 204 Pb/ Pb for Peninsular Ranges SantaSanta AnaAna 86Sr suitesuite / an ancient upper crustal component, in .706.706 SantaSanta RoRosaosa batholith plutonic groups compared to susuiteite 87Sr both the La Posta and Santa Ana l PostPost 100 MMaa Proterozoic basement of the Mojave a

ti SSantaanta Rosa suites. Although input from ancient i

n .704.704 LaLaP Postaosta

province and central Arizona and two Initial 87Sr/86Sr I crust within the Santa Ana suite may basement samples from the Caborca PrePre 110000 MMaa have been volumetrically minor, the cc.. SSantaanta AAnana region from Farmer et al. (2005) and .702.702 207 204 observed Pb/ Pb signatures are model Th/U of 4 and 2 for 1.7 Ga incompatible with derivation solely reservoirs from Iriondo et al. (2004). 207Pb/204Pb207Pb/204Pb from juvenile DMM-like sources with- Single stage reference growth curves Figure 21. Plots of (a) 207Pb/204Pb; and in an isolated intra-oceanic arc setting. 238 204 for U/ Pb ratios (μ) of 8.5 and 9, a (b) initial 87Sr/86Sr vs. δ18O Initial Pb–Sr and Nd ratios 232 204 Th/ Pb ratio of 40.5 and the 87 86 from the Proterozoic basement west of (VSMOW)(‰); and (c) initial Sr/ Sr geochron were calculated using 4.5 Ga vs. 207Pb/204Pb for Peninsular Ranges the suture zone within the Caborca ter- for the age of the Earth and Canyon rane (Iriondo et al. 2004; Farmer et al. batholith plutonic groups. Isotopic Diablo troilite (Tatsumoto et al. 1973) data from Kistler et al. (2003, 2014). 2005; Nourse et al. 2005) are a good fit for initial values. for the continental crustal component of the La Posta plutonic rocks (Kistler bined Proterozoic–Paleozoic–Mesozoic when mantle basalt is interacting with et al. 2014). SHRIMP analyses docu- metasedimentary component; (2) a continental crust, especially when mented xenocrystic cores of Protero- Neoproterozoic constituent; and (3) an hybridization is incomplete. We com- zoic zircon within La Posta plutons element with a model age of about pared the 0.2 Ma Kuanyinshan basalt that also support the probability that 2300 Ma (Shaw et al. 2014). When the (8.4–5.8% MgO) from Taiwan with the the Proterozoic terranes or sedimenta- Hf model ages are combined with the La Posta plutons on Figure 11c. As can ry rocks derived from them could have overall low δ18O isotope data (Fig. 21) be seen, the trace elements are similar been the crustal component in the plu- from the Santa Ana suite and their low yet major elements differ widely. This tons (Kistler et al. 2014). This fits well initial Sr ratios the best fit for a crustal suggests to us that the rocks from Tai- with recently acquired data from plu- source is a large ion lithophile element- wan spent little time in contact with tons on Searl Ridge (Fig. 3) that show, depleted lower crust of Neoprotero- crust prior to eruption, whereas the based on detrital zircon grains and zoic age because the low δ18O values presumably much larger volumes of La Pb–Sr–Nd isotopic data, a strong rule out a metasedimentary source Posta magma were able to assimilate involvement of older crust, likely Pro- (Shaw et al. 2014). much greater volumes of crustal mate- terozoic (Premo and Morton 2014). An important point to remem- rial, but that nearly all of the transfer Hafnium data from both ber is that, based both on models of trace elements took place very early Jurassic and Cretaceous plutons that (Reiners et al. 1995) and experiments on with very small amounts of partial cut the Jurassic basement suggest that, (Watson 1982), large changes in trace melting. This early hybridization in addition to the primitive mantle element content and isotopic composi- favours increasing asymmetry of Sr component, at least three additional tions can take place with very little and Nd isotopic ratios because the dif- components are required: (1) a com- change in major element chemistry fusion rates for Sr are faster than those 434 for Nd (Lesher 1994). Thus, there is a.a b b.. little doubt that rocks of the Santa Ana 10001000 pospost-COLGt-COLG 100 WPWPGG suite had significant crustal input. And WPGWPG given the even more evolved isotopes ssyn-COLGyn-COLG in the La Posta rocks, there can be little doubt that they too involved a sig- ) slab failure faailure 100 plutonsplutons

nificant crustal component. (ppm) b (ppm 1010 RbR (ppm)

The main question concerns b the origin of the mafic component ris- NbN (ppm) VVAVAGAG ing into the crust from the mantle. The ORGORG rare earth element (REE) concentra- 1010 tions of the two suites are different as ORG arcarc pplutonslutons originally noted by Gromet and Silver 1010 100100 10 100100 (1987), who suggested that rocks of NbNbb+ + Y ((ppm)ppm) Y ((ppm)ppm) the Santa Ana arc were derived from partial melting of plagioclase-rich Figure 22. (a) Rb vs. Y + Nb; and (b) Nb vs. Y plots for Peninsular Ranges sources whereas La Posta rocks were batholith plutonic groups with fields from Pearce (1996). In (b), we divided the for- derived from garnet-bearing assem- mer volcanic arc plus collisional granite field of Pearce et al. (1984) into arc and blages. They also pointed out that gen- slab failure fields, based on data presented here. eration of La Posta rocks involved a higher δ18O source. Silver et al. (1979) of melts (Ducea and Saleeby 1998; almost all the Santa Ana and about half noted that the extremely heavy nature Lackey et al. 2005). As the xenoliths of the La Posta samples fall in the of the oxygen in the La Posta plutons provide minimum pressure bounds overlapping fields of VAG and post- (Fig. 21a, b) required that they had a (Chin et al. 2012), they clearly were collisional granites (Fig. 22a). On the prior history of access to surface formed within the garnet peridotite Nb–Y diagram of Pearce et al. (1984), waters. As the La Posta plutons were field (Fig. 18), and as La Posta plutons all samples plot in the VAG plus syn- emplaced into the mid and lower crust, have REE patterns of residual garnet collisional granite field; however, due an unlikely source region for surface (Gromet and Silver 1987), it is reason- to differences in Nb/Y between the fluids, the source of fluids was more able to assume that the slab failure Santa Ana and La Posta suites (Fig. likely to have been within the mantle basalt magma originated from rising 10c), there is quite good separation source region. As the Santa Ana and asthenosphere at deeper levels, say in between these two groups on this plot, La Posta plutons do not have the same the garnet stability field (Grove et al. though not into the fields defined by oxygen signature they must have had 2013), than the precursors to typical Pearce et al. (1984). In our opinion different mantle sources. arc magmas, which appear to be these tectonomagmatic diagrams can- In the Sierra Nevada, deep- formed at shallower levels by melting not be reliably employed for the geo- seated plutons contain abundant evi- in the plagioclase and spinel stability chemical identification of slab failure dence for young supracrustal contami- fields (Till et al. 2012). versus normal arc-type granitoid mag- nation, even in gabbroic plutons, based We cannot evaluate whether mas. However, there is the possibility on high δ18O in zircon values (Lackey or not subcontinental mantle was based on our few examples, that vol- et al. 2005). These, along with similar involved in slab failure magmatism, but canic arc granite and slab-failure gran- values in La Posta rocks (Fig. 21), it is possible that it was. Given that ite can be distinguished as on Figure might indicate that parts of the leading both Nd and Sr isotopes and trace ele- 22b. edge of the subducted plate melted or ment patterns within the plutons are Although we firmly believe at least dehydrated to produce suffi- similar to early lithospheric mantle that the best indicator of slab-failure cient water to contaminate the mantle melts of the Rio Grande rift (Perry et magmatism is its post-collisional tim- melts before they ever reached the al. 1987) or even oceanic islands (Sis- ing, we tried to discriminate between upper plate crust. Later melts of the son et al. 2002) we cannot separate arc and slab-failure magmas by utilizing Santa Rosa suite have lower δ18O values them on the basis of isotopic composi- the differences in trace element con- (Fig. 21), which suggest that the source tion. It might be that only the earliest centrations shown on histograms (Fig. region was depleted in the fluid phase La Posta magmas involved a lithos- 10). We did this empirically by observ- by about 86 Ma. pheric mantle component, but dating is ing the separation of trace element Mafic and ultramafic xenoliths not yet sufficient to evaluate this possi- concentrations to arrive at values of in the Sierra Nevada shed some light bility. La/Yb, Gd/Yb, Nb/Y, and Sr/Y that on the mantle melts involved in slab separate the largest numbers of pre- failure, for the xenoliths, dated to be Geochemical Identification of Slab and post-deformational samples. the same age as the plutons of the Failure-Related Granitoid Magmas As can be seen from Figure eastern slab failure plutons, have simi- On the Rb–Y + Nb diagram of Pearce 23, the pre- and post-deformational lar Nd and Sr isotopic values along (1996), all samples from the Peninsular rocks of the Peninsular Ranges with elevated δ18O, and are arguably Ranges batholith plot in the volcanic batholith fall predominantly into two cumulates left behind after extraction arc granite (VAG) field within which discrete groups on all three plots. We GEOSCIENCE CANADA Volume 41 2014 435 0 0.4 =1 = b ger a /Y b/Y La La/Yb=10 0.2 Ma M N Nb/Y=0.4 5 . 100 SW SW a 0 - oung M Ma 2 . younger y 0.2-0.5 0 a

2 M an m km Y

0.2 Ma 0.

/ 000 r

a ~1000k Sr/Y S M

1 1 Ma aiw 2.6 Ma 2.6 M T Ta

. Y=10 / r S Sr/Y=10 10 a d t 100 Ma zoned complex, 2.6 Ma M e E ds margin (Wang et al. 2004). margin (Wang i NE N c 2 older older o y g = g t stern Anatolia, Turkey ailur s y (Marchev et al. 2013). (d) Sam- (Marchev i v an b F lv bs y c t bs l ls n nsional Y ck ar o ocks ta Rosa slab failure plutonic ta Rosa w r (2010). (c) Analyses from / TlsT KysK Ky Ttvg Tt Klvg K PcyP Mhy M Mh Kbs K Sbs Sb rocks r an en ac t d Back arc B lab aiw x gr granitoids G Gd/Yb=2 e extensional Slab Failure S Taiwan T Ta from Gastil et al. (2014). (b) Samples 1 10 100 cs i an ons c l t /Y=0.4 o b ons v

100 Nb/Y=0.4N a y a J olian plu an plut t an i lk r na a I Y B

a xamples / aA a r M e M M Sr/Y S

r 0

19 M e - -4

10 r/Y= Sr Sr/Y= 10 10 Sr/Y= 10 h 7.5-2.5 Ma Irian Jaya volcanics 7.5-2.5 Ma Irian Jaya plutons 24-19 Ma Anatolian 24 56-40 Ma Balkan56 plutons t O e

r . 2 u 10 c. Otherc. c examplesTaiwan d. il oids = = b fa b b10

Y anit ab d/ l granitoids gr slab failure s La/Yb=10 Gd/Yb=2 La/Y G 1 10 100 1 ed z i t y < a e ll ks m es Y=0.4 c ca i / k so a 2 o uit ok b/ r ypi y

t t etas 2 2

c N Nb/Y=0.4 b 100 m ada d -ar i a/R an S x SiO % SiO B Ba/Rb typically < 1 lu v Fluid metasomatized Fluid F e l % 70 e - mp 60 a Sierr N Y o >70% SiO2 SiO 60 60-70% SiO2 < <60% SiO2

/ es uolumne su r it Tuolumne suite Tuolumne T tn c l & Sr/Y S

p 120 Ma Sierran Stokes 120 Ma Sierran M rocks Mtn complex-arc M A Aplites as

2 0 r/Y=1 S r Sr/Y=10 10 2 SiO e suit e % Sie r v

ds 2 i a . 10 lu 70 = i - o = w t a b b. Sierras & Nevada Sierras b. b b F Y

>70% SiO2 SiO 60 60-70% SiO2 ni ah a d/ a/Y r ab Sahwave suite Sahwave S la La/Yb=10 Gd/Yb=2 La G gr granitoids Slab Failure S 1 10 100 1 r e i on e t .0 10 t t i ons = ár =2 b b10 su M b

e Y / d na

ype plu La/Yb=10 La/Y d t o - A e G Gd/Yb=2.0 sta plut 100 dr o ta Rosa suit ta ) Peninsular Ranges Batholith plutonic samples plotted on Nb/Y, La/Yb, and Gd/Yb vs. Sr/Y diagrams. Dashed lines are ratio Sr/Y diagrams. and Gd/Yb vs. La/Yb, Ranges Batholith plutonic samples plotted on Nb/Y, ) Peninsular e 2 anges P osta an an aP P

R SantaS Rosa suite plutons La Posta L suite S Santa Ana an a San Pedro de Mártir SanS Pedro pluton L La Posta-type r onic suit t a lu l p /Y

r c

Sr/Y S s su arc plutonic suite arc a ar

Y=10 / r S Sr/Y=10 10 10 100 nin e

P e 4

. ds i 0

o a. Peninsular Ranges a. Peninsular t (a) Felsic (>60% SiO (a) Felsic ni a b/Y= 0 4 Nb/Y=N 0.4 gr granitoids slab failure failur 1

1 0 .

10 50 10 10

5 1.0 0.5 5.0 1.0 0.5 1.0 0.1 50

b/Y Nb/Y N b b Y / a La/Yb L b Y / d Gd/Yb G values obtained from histograms (Figure 10b, c, and d) that separate most Santa Ana arc-type plutonic samples from La Posta–San c, obtained from histogramsvalues (Figure 10b, rocks. Analyses from the Sierra San Pedro Mártir pluton, a La Posta-type pluton in Baja California, Mártir are also plotted using data Analyses from the Sierra pluton, a La Posta-type San Pedro rocks. complex of intrusive from the 94–84 Ma Tuolumne suite–a pos intrusive the Sierra from Memeti (2009) and 93–89 Ma Sahwave Nevada Buer and Mille from Van and similar in composition, zoning petrography to Sierranlocated in western Nevada, Crest plutons, from we from McMahon (2000, 2001); 24–19 Ma post-collisional plutonic rocks 7.5–2.5 Ma post-collisional magmatism in Irian Jaya et al. 2012); and analyses from the 56–40 Ma post-collisional Rhodope Massif(Altunkaynak of southern Bulgaria–northern Greece ofples from a 1000 km-long swath along the Asian continental extending northeastward from northern 2.5–0.2 Ma volcanoes Taiwan Figure 23. 436 were encouraged by the differences in dominated by samples with > 60% magnesian basaltic magmas, which trace element ratios and so plotted a SiO2, which might explain why the appear to have resulted from very few more suites in order to see if the Irian Jaya samples cluster near the bot- small degrees of partial melting of discrimination based on the Peninsular tom of the slab-failure field on two of metasomatized SCLM, have trace ele- Ranges worked for other post-colli- the plots. ment concentrations nearly identical to sional magmatic suites inferred to have Samples from western Anato- those of the seemingly much more formed by slab failure magmatism. lia, where 24–19 Ma magmatism voluminous La Posta suite (Fig. 11), We plotted post-Sierran arc formed following the collision of the perhaps providing a real world case of plutons of the well-studied Tuolumne Sakarya and Anatolide–Tauride blocks early assimilation of trace elements intrusive complex on the same figure (Altunkaynak et al. 2012), also fall during partial assimilation. (Fig. 23), along with 120 Ma rocks of within the slab failure field on our dis- It is important to understand the Stokes Mountain complex, repre- crimination diagrams (Fig. 23) and that these are not universal discrimina- sentative of arc magmatism in the have similar Nd and Sr isotopic com- tion plots, for some arc suites, such as westernmost Sierran batholith positions to the La Posta suite. Also the Bodie Hills of eastern California (Clemens Knott 1992). Modern and within the Alpine belt, and plotted on (John et al. 2012; du Bray et al. 2013), complete analyses of the older Sierran Figure 23, likely slab-failure magma- which sit atop the eastern Sierran slab- rocks are scarce, and even the data set tism occurred in the Macedonian– failure rocks, likely scavenged sufficient from the Stokes Mountain complex Rhodope–North Aegean zone of the subjacent crustal material so that they does not include complete REE analy- southern European Balkans, where too would plot in the upper right on all ses. Nevertheless, samples from the Early–Middle Eocene magmatism was these plots. Even the bimodal post 103 Ma Tuolumne intrusive com- bookended by Late Cretaceous arc Plio–Pleistocene volcanic rocks of the plex, inferred here to be a result of magmatism and Oligocene extensional Aurora Volcanic field, located in the slab failure, fall mostly within our pro- magmatism (Marchev et al. 2013). extensional Basin and Range province posed slab failure field, whereas the They list Nd and Sr isotopic composi- just north of Mono Lake, have similar bulk of the arc rocks do not. Both Nd tions similar to the other examples geochemical signatures that also sug- and Sr isotopic compositions from the cited here. gest involvement of plutonic rocks of Tuolumne and other post-deforma- Our last example (Fig. 23) of the Sierra Nevada (Kingdon et al. tional plutons of the eastern Sierra post-collisional magmatism is also the 2014). Nevada are nearly identical to those of youngest and while one would think it the La Posta–Santa Rosa suite (Fig. 19), therefore the simplest, they would be Criteria for Identification of Slab whereas Stokes Mountain samples have mistaken. Volcanic rocks ranging in age Failure-Related Plutonic Rocks isotopic compositions that plot with from 2.5 Ma to 0.2 Ma occur sporadi- We showed earlier in the paper that the the Santiago Peak–Santa Ana arc rocks cally in a linear band extending from batholith was likely the upper plate in a (Fig. 19). Rocks of the Sahwave com- northern Taiwan for about 1000 km collision with an Albian carbonate plat- plex (Van Buer and Miller 2010), a along the Asian margin. According to form located to the east and that the Tuolumne-like intrusive complex of Wang et al. (2004), from whom this western edge of the eastern continen- similar age located in northwestern description was extracted, the oldest tal margin was pulled beneath the arc. Nevada, also plot within our proposed magmas, from Mienhuayu and Sek- The timing of the collision was ~100 slab failure fields. We note that rocks ibisho (MHY and SBS) have: (1) Mg Ma and following collision, or perhaps with greater than 70% SiO2 may have contents of ~6–8%; (2) the highest in part overlapping it, major magma- significant interaction with a volatile 143Nd/144Nd and relatively elevated tism of the La Posta suite took place component and do not plot within the 87Sr/86Sr; and (3) were derived from over the length of the batholith. It is slab failure field. asthenospheric sources that were the timing of magmatism during and We also plotted (Fig. 23) sam- fluxed by slab fluids during opening of directly after collision, coupled with ples from Irian Jaya, where 7.5–2.5 Ma the Okinawa back-arc basin. In con- the strong exhumation, that provides magmatic rocks are interpreted to have trast, the youngest rocks, Tsaolingshan the most compelling evidence that the formed as the result of collision of the (TLS) and Kuanyinshan (KYS), which La Posta plutons are slab-failure bod- northern margin of Australia with the were both erupted on the northwest- ies, not the geochemistry. That is not south-facing 20–9 Ma Marimumi arc ern part of the island of Taiwan at to say that our discrimination diagrams (Cloos et al. 2005). The rocks are dom- about 0.2 Ma have: (1) Mg0 contents do not work, for they do seem to sepa- inantly intermediate calc-alkaline, high- of about 15%; (2) the lowest Nd and rate slab-failure magmas from arc K shoshonitic and syenitic with rela- highest Sr isotopic compositions; (3) rocks in the cases that we investigated. tively unradiogenic Nd and moderately model Nd ages falling between 2.5 and However, it must be noted that early radiogenic Sr isotopic compositions 2.0 Ga, which suggest derivation in slab-failure magmas might be more (Housh and McMahon 2000) broadly part from Proterozoic domains in the arc-like as the asthenospheric melts similar to those of the La Posta suite, subcontinental lithospheric mantle interact with the subcontinental lithos- but with lower εNd reflecting the (SCLM) or the crust; and (4) Kuanyin- pheric mantle. Following slab failure in Archean basement. The rocks shown shan rocks typically have Ba/Rb <1, many collisional orogens, a new sub- in the plot are dominantly andesite indicating significant fluid metasoma- duction zone commonly starts up, but whereas most of the other suites are tism. Remarkably, these young highly with opposite polarity. The new arc GEOSCIENCE CANADA Volume 41 2014 437 may form atop the pre-collisional arc have a slab-failure component, this matism might typify the shallow and the post-collisional slab-failure mechanism adds significantly to and deep break-off end members magmas to form a complex amalgama- our understanding of how conti- respectively, as reflected in their tion of deformed and little deformed nental crust formed, grew, and was orogenic width, proximity to the magmatic rocks. This, coupled with recycled over Earth history (Hilde- suture, and differences in timing coincidental opening of a marginal brand and Bowring 1999). A possi- relative to the collision. That said, basin, such as the Okinawa trough, ble check on our hypothesis would the mantle and crust are sufficient- behind the Ryukyu arc during collision, be to investigate whether major ly heterogeneous that it is likely can make a mockery of simplistic pulses or peaks in crust formation that no discrimination plot can be models and serves as a cautionary tale. during Earth history postdate unfailingly diagnostic. major ocean-closure events, such 3. We believe that our model for the Some Implications of Our Model as supercontinent amalgamation. A Peninsular Ranges batholith is gen- 1. Lee et al. (2007) employed geo- recent study of δ18O in zircon on a erally applicable to other orogenic chemical data from the Peninsular worldwide basis suggests that sub- belts, and so ‘paired’ plutonic belts Ranges batholith to examine link- duction of sediment and surficially (Tulloch and Kimbrough 2003) ages between arc magmatism and weathered rocks during collisions consisting of arc-related magma- Phanerozoic continental crust for- has led to a long-term record of tism, followed by post-tectonic mation. They concluded that supercontinent amalgamation as slab failure type plutonism, could refinement into a felsic crust documented by the heavy nature represent substantive evidence for occurred after oceanic arc accre- of the δ18O of zircon (Spencer et the presence of major tectonic tion during ‘the continental arc al. 2014). sutures in much more deeply erod- stage’ when an accretion-thickened 2. Geologists studying the geochemi- ed terranes, much as do remnants crustal and lithospheric column cal characteristics of plutonic and of suprasubduction ophiolites pre- was thermally reworked by volcanic associations with high served in less denuded terranes. It emplacement of subduction-gen- Sr/Y and La/Yb values have gen- is worth emphasizing that in cases erated basaltic magmas. There are erated an abundant literature over of deep break-off that slab-failure several problems with their model. the last 20 years as they attempted magmatism may occur a significant First, the Santiago Peak–Alisitos to classify and understand the pet- distance from the suture and long arc was not a primitive arc built on rogenesis and tectonomagmatic after the initial collision. oceanic crust, for it contains far implications of rocks such as 4. In an attempt to reconcile juxta- too large a volume of felsic rocks, adakite (Defant and Drummond posed basement blocks and their and its basement, where exposed, 1990; Sajona et al. 2000; Macpher- cover, such as the abrupt north- comprises abundant Jurassic son et al. 2006; Moyen 2009), eastern edge of the Caborca ter- orthogneiss and Paleozoic Archean tonalite–trondhjemite– rane in Sonora, Silver and Ander- metasedimentary rock. Second, granodiorite (TTG) suites (Martin son (1974) hypothesized that Late there is no evidence that the La 1986, 1987, 1994; Smithies 2000; Jurassic sinistral motion along a Posta suite was derived from sub- Kamber et al. 2002; Whalen et al. major strike-slip fault, which they duction of oceanic lithosphere, 2004), Archean sanukitoid suites termed the Mojave–Sonora megas- and third, the crustal materials (Whalen et al. 2004; Martin et al. hear, separated rocks of Caborca involved were likely the older Pro- 2005, 2009) and adakitic granite from similar rocks in the Death terozoic–Paleozoic basement of (Wang et al. 2007; Whalen et al. Valley region. Our rifting and colli- the batholith as demonstrated by 2010). To our knowledge, only one sional model presents a viable xenocrystic cores in zircon grains of the published petrogenetic alternative to the Mojave–Sonora and Pb–Sr–Nd isotopes (Premo models for these various igneous megashear hypothesis (Anderson and Morton 2014; Kistler et al. suites link them to possible slab and Silver 2005) because in our 2014), augmented by fluids and failure (Whalen et al. 2010), yet model lithological units, consid- possibly melts from the subducted they are similar chemically to the ered to be offset along the megas- continental margin. La Posta suite, as well as Cenozoic hear, may have originally formed In our model, the thermal examples, such as northernmost more coherent blocks that were event that refined mantle materials Taiwan, where comparable mag- separated by rifting and break-up into more evolved continental mas were erupted as the exhumed leading to widening of the Bis- crust was directly related to shal- mountain belt collapsed within a bee–Arperos sea. When the sea low slab failure and consequent few m.y. of collision (Chung et al. closed, some 30 m.y. later, those upwelling of asthenosphere, which 2001; Wang et al. 2002, 2004) and fragmented blocks, such as Cabor- was the advective heat source Tibet, where enigmatic magmatism ca, simply did not return to their responsible for the major, short- accompanied uplift and exhuma- original locations; but instead were lived, and focused thermal pulse tion (Turner et al. 1993, 1996; accreted farther south. required to drive the crustal melt- Mahéo et al. 2002; Kohn and 5. Previously, paleogeographic recon- ing and reworking process. Parkinson 2002). These two exam- structions of the pre-160 Ma Because every collision should ples of probable slab failure mag- Jurassic arc of the Klamath Moun- 438

tains and northern Sierra required Peñasquitos–Mariposa rocks. This many Cordilleran batholiths are it to bifurcate southeast to head situation would have been analo- composed of two magmatic phas- into southern Arizona and south- gous to Taiwan where the Oki- es, arc and slab failure, might be west to Baja California. Our model nawa trough opened as the result used to reconstruct widely separat- simplifies the paleogeography in of collision, slab failure and sub- ed terranes. For example, within that the Jurassic arc of Baja was duction reversal (Viallon et al. the American Cordillera there are rifted from the eastern strand at 1986; Teng et al. 2000) or varia- two batholithic terranes recognized about 139–130 Ma as the Bisbee tions on this theme as seen in as out-of-place orphans: the Salin- margin formed. numerous other examples ian block (Ross 1978), located just 6. Another peculiarity that might be (McCabe 1984; Wallace et al. west of the San Andreas fault in resolved by our model, or at least 2009). If it was a marginal sea then central California; and the Coastal partially so, is the Klamath Moun- there should have been an arc batholith of Peru with its Are- tains block, which contains many behind which the sea could open. quipa–Antofalla basement (Loewy similar rock packages as the Sierra However, there was little time et al. 2004). In our accompanying Nevada, including a Jurassic arc between the deformation of the contribution (Hildebrand and that is generally correlated with the Cucurpe–Peñasquitos–Mariposa Whalen 2014: this volume) we northern Sierran rocks (Irwin and rocks, bracketed to be 145–139 briefly describe the geology of Wooden 2001), but lacks the Sier- Ma, and the deposition of the those terranes, hypothesize that ran Cretaceous arc and slab failure unconformably overlying rocks of the two were formerly joined, and magmatism. Because the western the Bisbee basin dated to be that the high-grade Salinian block Klamaths were deformed during 136–125 Ma (Mauel et al. 2011; with its 100–82 Ma plutons the ~145 Ma Nevadan event, it Peryam et al. 2012; Kimbrough et formed the opposing block to the thus seems likely that it was sepa- al. 2014a). Perhaps the 141–135 dominantly lower grade, mainly rated from both rocks to the east Ma volcanic and plutonic rocks of Albian, arc complex of the Coastal and the Sierran block during the the Vizcaino Peninsula (Kim- batholith. 139–130 Ma rifting event. brough and Moore 2003) represent 10. A major unanswered question is 7. We now recognize that the Bis- a remnant of the arc, as might the where is the arc that is inferred to bee–Arperos sea had an eastern swarm of 143–140 Ma plutons in lie to the east of the Franciscan boundary that trended more or the western Sierra Nevada (Irwin mélange, Coast Ranges ophiolite, less southeast from southwest Ari- and Wooden 2001; Day and Bick- and Great Valley Group? Based on zona to southern Mexico (Fig. 12) ford 2004). In any case, possible deformation (Hildebrand 2013) yet Dickinson and Lawton (2001b) arc rocks of this age are not par- and detrital zircon (Wright and suggested that the Bisbee basin ticularly common. If the basin Wyld 2007), it seems clear that formed part of what they termed opened as a marginal sea then those rocks were not adjacent to the Border rift system, which was there was also an unidentified the Sierra Nevada prior to the 100 an intracontinental rift extending event that caused the subduction Ma collision. A possible candidate from the Gulf of Mexico north- to step into the basin and reverse is the Xolapa terrane of southern west through the Sabinas basin polarity to dip westward. Whether Mexico, which contains Late Juras- and Chihuahua trough to at least this was the attempted subduction sic and Early Cretaceous plutons the Bisbee basin (Fig. 12). If active of an oceanic plateau or a collision (Herrmann et al. 1994; Ducea et at the same time they likely of terranes is unknown but a rea- al. 2004), yet has long been known formed oceanic margins on both sonable candidate might be the to lack a forearc and accretionary the eastern and western sides of proposed arc–arc collision complex (Karig et al. 1978). the Oaxaquian terrane. Closure of between the Alisitos and Santiago 11. Arcs appear on their face to be the Arperos–Bisbee sea on the Peak blocks (Alsleben et al. 2008; very simple elements, and during west occurred during the Oregon- Schmidt et al. 2014). They subduction probably are relatively ian event at about 100 Ma and an described a narrow SW-vergent so, at least at a regional scale. unnamed sea – but possibly the 110–108 Ma fold-thrust belt, However, appearances are often last vestiges of the Panthalassic which extends southward from the deceiving, and as arcs commonly ocean – formerly located along the Agua Blanca fault through at least collide with other tectonic ele- eastern margin of Oaxaquia, van- the northern Baja Peninsula (Fig. ments, the subducting slab tears ished during the Laramide event at 2). The proposed suture placed the off to allow different magmas to about 75 Ma. Santiago Peak block atop the Alisi- rise into or adjacent to the arc, and 8. An unresolved question is what tos block (Alsleben et al. 2008). new oppositely dipping subduction caused the Bisbee–Arperos sea to However, given the likely large- can start up very quickly, typically open? Presumably, it was subduc- scale meridional migration of out- within a million years, to create a tion reversal following the defor- board Cordilleran terranes, it is complex and commonly confusing mation of the Nevadan collisional also possible that the arc rocks are magmatic tableau (Dewey 2005; event responsible for the strong now located far to the north. Huang et al. 2006; van Staal et al. deformation of the Cucurpe– 9. The model proposed here, that 2007; Hildebrand et al. 2010). Fur- GEOSCIENCE CANADA Volume 41 2014 439

thermore, as these events take plate was subducted. nections often run deep in geology and place at continental margins and 5. Plutons of the La Posta suite were Paul held the prestigious Sturgis Hoop- within ribbon continents, the pos- emplaced during a period of major er chair at Harvard University for 15 sibility that these complex arc- exhumation, which reflects the years whereas another Canadian, Regi- bearing terranes might be torn rebound of the lower plate follow- nald A. Daly, author of Igneous Rocks apart by rifts and/or strike-slip ing slab break-off. and their Origin and who recognized that faults and strewn laterally along 6. The switchover from flysch to batholiths were emplaced in mountain the margin, only to have new arcs molasse in basins to the west of belts, was Sturgis Hooper Professor built atop them, must be consid- the batholith occurred during the from 1912–1942. ered. On a world covered by mov- early Cenomanian and it reflected Dave Kimbrough, Keegan ing plates, simplicity is not neces- the rapid uplift and erosion of the Schmidt, Helge Alsleben, Paul Wet- sarily correct. rising orogenic welt following slab more, Wayne Premo, and Vicki Lan- failure. genheim generously shared important CONCLUSIONS 7. Shallow slab break-off during the preprints with us as we wrote the initial 1. The Peninsular Ranges batholith is 100 Ma collision provides a simple draft of this paper before publication a composite batholith comprising and actualistic model that ties of GSA Memoir 211 on the Peninsular a western arc complex, the Santia- together the voluminous magma- Ranges batholith. We are especially go Peak–Alistos–Guerrero arc, tism of the La Posta suite, the nar- grateful to Schmidt and Langenheim, that formed between 128–100 Ma row orogen, and its rapid exhuma- as well as Jade Star Lackey, who pro- on Jurassic, Paleozoic and Precam- tion. vided vector versions of their figures brian crust dominantly above a 8. Although they have similar com- so that we could readily modify them westward-dipping subduction positions and are ultimately to suit our needs and include them zone, and a 99–86 Ma suite of derived from asthenospheric here. Ongoing discussions with Tim post-deformational plutons that source material, slab-failure mag- Lawton clarified many questions on were emplaced after the arc collid- matism appears to form at greater the geology of Sonora and the south- ed with a composite terrane locat- depths than does arc magmatism western US. Doug Morton helped ed to the east. and is governed by melting in the locate geochemical samples so that we 2. The basement within the arc com- presence of garnet as opposed to were able to remove samples collected plex appears to have rifted from mantle-derived magmas of arcs, from contact or highly altered zones. the Cordilleran Ribbon Continent which appear to form in the We thank C-T. Lee for helping us bet- between about 139 and 130 Ma. spinel–plagioclase regime. ter understand the ins and outs of the 3. The Santiago Peak–Alistos–Guer- 9. We note similar relations in other Sierran mantle xenoliths. Keegan rero arc collided at about 100 Ma Cordilleran batholiths such as the Schmidt, Doug Morton, and Tim Law- with a Lower Cretaceous passive Sierra Nevada and suggest that ton reviewed earlier versions of the margin, located to the east and paired batholiths might generally manuscript. Reviews by A. Zagorevski topped by a west-facing Albian represent both pre-collisional arc and J.B Murphy were helpful. The sen- carbonate platform terrace, which and syn- to post-collisional slab ior author especially thanks Sam was pulled into the trench, buried failure magmatism. Bowring for 30+ years of stimulating by orogenic flysch, and overthrust 10. Arcs are dominantly extensional discussions about arcs and batholithic by exotic allochthons containing and are not regions of thickened terranes. This is more self-funded slices of the arc and its basement. crust unless they are built on older research. NRCan contribution This platform is intermittently collisional terranes. 20140247. exposed from Caborca, where it is 11. Arcs are not generally deformed known as the Sonoran shelf, to unless they collide with another REFERENCES Zihuatanejo, where it is known as block. It is collision and attempted Abbot, P.L., and Smith, T.E., 1989, Sonora, the Guerrero–Morelos platform. subduction of the lower plate that Mexico, source for the Eocene Poway 4. During the short-lived collision, thickens the crust. 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