Magmatic surge requires two-stage model for the Laramide orogeny

The Laramide orogeny is a pivotal time in the geological development of western North America, but its driving mechanism is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith (SCB) which caused the angle of subduction beneath the continent to shallow and led to shut-down of the arc. Here, we use over 280 zircon and titanite Pb/U ages from the SCB to establish the timing and duration of magmatism, metamorphism and deformation. We show that magmatism was surging in the SCB from 90 to 70 Ma, the lower crust was hot, and cooling occurred after 75 Ma. These data contradict plateau underthrusting and flat-slab subduction as the driving mechanism for early Laramide deformation. We propose that the Laramide orogeny is a two-stage event consisting of: 1) an arc ‘flare-up’ phase in the SCB from 90-75 Ma; and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma that is linked to subduction of an oceanic plateau.


Introduction
One of the most important periods in the development of western North America occurred at ca. 90 − 80 million years (m.y.) ago when a series of deeply rooted thrust faults began to uplift and imbricate slices of continental lithosphere located hundreds to thousands of kilometers inland from the coast (Fig. 1A) [1][2][3][4] .This major thick-skinned tectonic event, called the Laramide orogeny, lasted ~ 40 m.y., and resulted in mountain building, the formation of foreland basins, and the development of ore mineralization from Canada to northern Mexico and as far east as the Black Hills of South Dakota [4][5][6] .Nevertheless, despite its widespread impact on the tectonic development of western North America, the exact mechanisms that caused this major tectonic event remain controversial 3,[7][8][9][10] .
Most widely cited models argue that the cause of the Laramide orogeny was the at-slab subduction of a thick oceanic plateau beneath the Southern California Batholith (present-day Transverse Ranges in Southern California) at 90 − 80 Ma resulting in shutdown of arc magmatism and cooling of the upperplate crust 9,[11][12][13][14][15][16][17] .These 'amagmatic' models resemble the present-day central Andean orogen (27-33°S)   where a attened subducted oceanic slab is associated with thick-skin deformation and relative magmatic quiescence in the Sierras Pampeanas 18 .In contrast, other models propose that this tectonic transition was caused by the outboard collision of off-shore arc terranes 19 , or increased lithospheric coupling 7,20 .Important features of these con icting models are that they make different predictions regarding the timing and duration of magmatism, cooling and uplift during the development of the Laramide orogeny.
To resolve these con icting models, we examined the frontal arc of the Southern California Batholith (SCB) which represents an ~ 500 km-wide, paleo-arc segment of the Mesozoic California arc where prior studies place the initial collision between the conjugate Shatsky plateau and the western North American margin (Fig. 1B) 14,16 .This arc segment lies between the southern Sierra Nevada batholith (SNB) and northern Peninsular Ranges Batholith (PRB) and is now represented by fault-bounded structural blocks that make up the central and eastern Transverse Ranges and the western Mojave province.This region experienced varying degrees of faulting and rigid block rotation related to development of the San Andreas transform plate boundary beginning in the Miocene 21,22 .We focus on the frontal arc of the Southern California Batholith (SCB) in the Transverse Ranges because of its unique location as the proposed site of initial collision.As such, the frontal arc of the SCB is the key location to test and resolve con icting models for the structural, magmatic and metamorphic response to tectonic changes that affected continental lithosphere in western North American in the Late Mesozoic.

Zircon geochronology
To test models for the Laramide orogeny, we compiled > 260 Pb/U ages, including 79 new zircon and titanite dates to establish the timing and duration of magmatism, metamorphism and deformation in the Southern California Batholith with the goal of resolving con icting models.Areal addition rates were calculated from integrating igneous ages with pluton areas determined from digitized geologic maps of Southern California (see Supplementary le).Our compilation and new data encompass all major blocks in the SCB including the San Gabriel, Pine Mountain, Alamo-Frazier Mountain, Little San Bernardino, San Bernardino, and Salinian blocks (Fig. 1B).Igneous zircon ages reveal 4 discrete pulses of Late Paleozoic to Mesozoic magmatism at 260-210 Ma, 160-140 Ma, 120-118 Ma, and 90-70 Ma (Fig. 2A).The latter pulse culminated in an arc are-up event which peaked at 85-75 Ma and was associated with widespread, voluminous plutonism throughout the SCB (Fig. 1B, 2B).These Late Cretaceous plutons have typical Cordilleran arc magma geochemistry: most are magnesian, metaluminous to weakly peraluminous, and calc-alkalic with strong crustal a nities that re ect mixtures of Proterozoic crust with juvenile (Mesozoic) sources 23,24 .Although the cause of the 85-75 Ma are-up remains unclear, magmatism persisted in the frontal arc of the SCB until ca.65 Ma and our data show that most igneous activity had ceased by ca.70 Ma (Fig. 2C; Supplementary le).
An analysis of metamorphic zircon and titanite shows that the batholith records high temperatures during the Late Cretaceous transition to thick-skinned, Laramide shortening.The data demonstrate that the arcare up event in the SCB was also coeval with high-temperature metamorphism at garnet-granulite to upper amphibolite-facies metamorphism and partial melting in the lower crust of the arc.Evidence for high-temperature metamorphism is preserved in the Cucamonga terrane (eastern San Gabriel Mountains) where metamorphic zircons in gneisses, migmatites and calc-silicates give dates ranging from 86 to 76 Ma at 9 − 7 kbars 25,26 (Supplementary le).Ti-in-zircon analyses on metamorphic rims indicate temperatures of 800 to 713°C and garnet-quartz oxygen isotope thermometry yields similar metamorphic temperatures of 835 to 777°C.These data are consistent with mineral exchange thermometry which give temperatures of 800 − 775°C 26 .In the Coast Ridge Belt (Santa Lucia Mountains, Salinia block), Kidder et al. 27 also report peak metamorphic pressures of 800°C at 7.5 kbars at 81 to 76 Ma.These results are signi cant because they show that the root of the arc was hot and partially molten through ca.76 Ma.
Our regional mapping of mylonitic ductile-shear zones in the SCB documents that the Late Cretaceous arc are-up was temporally and spatially associated with syn-plutonic development of a regionally extensive, transpressional shear system.This system includes, from north to south (present-day coordinates): the Nacimiento fault 28 , the Tumamait shear zone (Pine Mountain block), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone 25,29 , and the Eastern Peninsular Ranges shear zone 30 .Kinematic indicators from the shear zones generally give oblique top-to-west, sinistral reverse-to thrust-sense motion (present-day geometry).Syn-kinematic, metamorphic titanite dates and Zr-in-titanite temperatures in the Tumamait shear zone range from 77 to 74 Ma at 750°C, titanites in the Black Belt shear zone give an age of 83 Ma, and those in the eastern Peninsular Ranges shear zone give ages of 89-78 Ma at 750°C 31 (Supplementary le).In the Salinian block, Kidder et al. 27 document extensive thickening and high-temperature ductile deformation from 93 − 81 Ma, and Singleton and Cloos 32 show that the juxtaposition of the Salinian block against the Nacimiento block occurred between ca.75 Ma and 60 Ma along sinistral strike-slip and/or thrust faults.Thus, these dates and temperatures indicate that the frontal arc of the SCB experienced sinistral-reverse, transpressional deformation during the Late Cretaceous are-up event (Fig. 2C).Therefore, the collective data show simultaneous magmatism and high-temperature intra-arc shortening during the Late Cretaceous transition to thick-skinned deformation.
Implications for Late Cretaceous high-temperature arc processes in the SCB Our compilation of igneous zircon dates from the frontal arc of the SCB document a widespread surge of Late Cretaceous magmatism from 90 − 70 Ma.This surge occurred throughout all major structural blocks which make up the ca.500-km wide segment of the Late Cretaceous arc system (Fig. 1B, 2).Outside of the frontal arc, coeval magmatism also occurred in the back-arc which is now represented by the adjacent Mojave Desert 33 .The widespread occurrence of plutonic rocks throughout the SCB and their arc-like geochemical features 24,34 suggests that mechanisms for generating large volumes of melt were still active through the Late Cretaceous.Arc are-ups similar in magnitude and duration also occurred in adjacent sectors of the arc slightly before the SCB event (e.g., southern SNB at 110 − 90 Ma 35 and northern PRB at 99 − 91 Ma 36 (Fig. 2B).Importantly, our data from the SCB are in direct con ict with existing amagmatic models that invoke underthrusting of the conjugate Shatsky plateau beneath the SCB from 88 − 75 Ma and removal of the lower crust and lithospheric mantle during the beginning of the Laramide orogeny 9,14,16 .
Metamorphic ages and thermometry in the Cucamonga and Salinian granulites also demonstrate that the lower crust of the SCB was hot and partially molten through 75 Ma, and these features cannot be explained by existing amagmatic models.The presence of this hot arc root provides further evidence that high-temperature arc processes were operating until 75 − 70 Ma and shut-down of the frontal arc did not occur until after 70 Ma.This observation is illustrated in Fig. 3 which shows a compilation of time-temperature pro les derived from mineral thermochronology from the major structural blocks in the SCB.These data highlight two important features of the SCB: 1) the Late Cretaceous are-up in the SCB was coeval with intra-arc, oblique sinistral-reverse thrusting and high-temperature metamorphism, and 2) termination of arc magmatism in this region was associated with an abrupt phase of rapid regional cooling of the SCB below 350°C at ca. 75 − 70 Ma (Fig. 3).This regionally extensive and rapid cooling event signi es the onset of regional refrigeration of the SCB due to at-slab subduction involving the cold, conjugate Shatsky plateau and tectonic underplating of trench sediments beneath the SCB after 75 − 70 In the SCB, the presence of underplated schists is commonly cited as evidence for at-slab subduction 38 , and our geo-and thermochronological results also support a post-75 Ma emplacement model for the schists.Underplated schists in the Transverse Ranges have zircon age distributions with maximum depositional ages ranging from 75 − 68 Ma (Pelona Schist 38,39 ) and amphibole and muscovite 40 Ar/ 39 Ar metamorphic ages from ma c schists are no older than 60 Ma 38,40,41 .Field observations in the Pelona schist also show no evidence for partial melting or intrusion by Cretaceous plutons.These textural and temporal constraints indicate that underplating of schists beneath the SCB took place after the termination of the SCB are up event after 70 Ma.Therefore, these data are also consistent with at-slab subduction after 70 Ma.Spatial and temporal trends in sedimentary provenance within California forearc sediments also show a pronounced and sudden in ux of continent-derived detritus to the southern California margin at ca. 75 Ma.Sharman et al. 42 argued that this sudden in ux re ects the development of a geomorphic breach within the Cretaceous arc and an associated rapid migration of forearc drainages into the continental interior.The timing of this breach is ~ 15 Myr younger than postulated plateau underthrusting in prior models, but is compatible with our model for the arrival of the conjugate Shatsky after 75 − 70 Ma (e.g., Fig. 3).These data are also consistent with eastward migration of magmatism away from the coastal arcs after 75 Ma which has been interpreted to re ect shallowing of the subduction angle over time 4,43,44 .We illustrate these features in our model (Fig. 4) which shows the tectonomagmatic evolution of the SCBarc segment from 85 − 50 Ma.
A two-stage, model for the Laramide orogeny Data from the SCB provide critical new information that allows us to resolve the debate about the link between upper-plate deformation in the western North American Cordillera and the kinematics and geometry of the down-going plate at the beginning of the Laramide orogeny.Our results show clearly that arc magmatism was robustly active through ca.70 Ma, and underthrusting of the conjugate Shatsky plateau must have occurred after 75 − 70 Ma.Therefore, at-slab subduction of the conjugate Shatsky plateau cannot explain the transition from thin-to thick-skin deformation at 90 − 80 Ma.This conclusion is supported by a number of sedimentological and thermochronological studies on Mesozoic sediments in SW Montana which provide evidence for the early onset of Laramide-style deformation well before 80 Ma 10,45,46 .These data are problematic from the standpoint of a single at-slab event because the timing of basin formation predates the arrival of the conjugate Shatsky plateau in all at-slab models 9,14,16 .
Moreover, the southwest Montana basins are well outside the commonly cited corridor of Laramide deformation caused by at-slab underthrusting 17 .These relationships coupled with our data from the SCB lead us to the conclusion that the Laramide orogeny cannot have a single driving mechanism.
Therefore, we propose that the Laramide orogeny is a composite tectonic event consisting of two distinct stages of paired mountain building and basin formation: 1) an early phase at 90 − 75 Ma, which was active during are-up activity in the SCB, and 2) a more-widespread phase of basement-involved thrusting and basin formation in the continental interior from 75 − 40 Ma.In this model, the driving mechanism for initial Laramide deformation is not related to at-slab subduction, but is closely linked to orogen-scale dextral transpression from ca. 100 − 85 Ma along the US Cordillera 19,47,48 .We attribute the second stage to at-slab subduction of the conjugate Shatsky plateau beneath the SCB following the termination of are-up magmatism in the SCB.These new data from the SCB show that multiple driving mechanisms are required to explain the diverse and previously con icting datasets for the development of Laramide orogeny from 90 − 40 Ma.

Ma 37 .
Post 75-70 Ma underthrusting of the conjugate Shatsky plateau One of the key results of our work is that at-slab subduction beneath the SCB post-dates the beginning of Laramide deformation by ~ 15 Myr.Consequently, oceanic plateau underthrusting cannot be called upon as the driver for thick-skin deformation in the western US prior to ca. 75 Ma.However, several existing data sets support a link between Laramide deformation and at-slab subduction after 75 Ma.For example, the general timing of major thick-skin, basement-cored thrusting and basin development in Utah, Colorado, Wyoming and SW Montana occurred from 70 − 50 Ma 4 which agrees well with underthrusting of the conjugate Shatsky after 75 Ma.In addition, zircon ages in peridotite xenoliths from the Colorado Plateau are consistent with derivation from the lithosphere at the base of the SCB and ~ 500 km of lateral displacement inboard to the Colorado Plateau transition zone after 70 Ma 9 .

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