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Abstract We explore the growth of lower-continental crust by examining the root of the Southern California Batholith, an ~500-km-long, paleo-arc segment of the Mesozoic California arc that lies between the southern Sierra Nevada Batholith and northern Peninsular Ranges Batholith. We focus on the Cucamonga and San Antonio terranes located in the eastern San Gabriel Mountains where the deep root of the Mesozoic arc is exhumed by the Quaternary Cucamonga thrust fault. This lower- to mid-crustal cross section of the arc allows us to investigate (1) the timing and rates of Mesozoic arc construction, (2) mechanisms of sediment incorporation into the lower crust, and (3) the interplay between mantle input and crustal recycling during arc magmatic surges. We use U-Pb detrital zircon geochronology of four quartzites and one metatexite migmatite to investigate the origin of the lower-crustal Cucamonga metasedimentary sequence, and U-Pb zircon petrochronology of 26 orthogneisses to establish the timing of arc magmatism and granulite-facies metamorphism. We find that the Cucamonga metasedimentary sequence shares broad similarities to Sur Series metasedimentary rocks in the Salinia terrane, suggesting that both were deposited in a late Paleozoic to early Mesozoic forearc or intra-arc basin marginal to the Southern California Batholith. This basin was progressively underthrust beneath the arc during the Middle Jurassic to Late Cretaceous and was metamorphosed during two high-grade (>750 °C), metamorphic events at ca. 124 Ma and 89–75 Ma. These metamorphic events were associated with 100 m.y. of arc magmatism that lasted from 175 Ma to 75 Ma and culminated in a magmatic surge from ca. 90 Ma to 75 Ma. Field observations and petrochronology analyses indicate that partial melting of the underthrust Cucamonga metasedimentary rocks was triggered by the emplacement of voluminous, mid-crustal tonalites and granodiorites. Partial melting of the metasedimentary rocks played a subsidiary role relative to mantle input in driving the Late Cretaceous magmatic flare-up event. 

We investigate the deformation conditions of coeval mylonites and pseudotachylytes (pst) exposed in the brittle-ductile transition (BDT) in the Black Belt Shear Zone (BBSZ) in the Southern California Batholith using SEM (Scanning Electron Microscope) imaging, and Electron Backscatter Diffraction (EBSD) analysis. We selected four representative samples along a strain gradient of the BBSZ. The BBSZ is a transpressional shear zone developed within hornblende and biotite tonalites and diorites. The shear zone is discontinuous over a ~ 1.5 - 2 km wide zone, and kinematic indicators show oblique top-to-SW, sinistral-reverse to thrust-sense motion. Metamorphic titanite grains aligned within the mylonitic fabric date the deformation to ~ 83 Ma. SEM and EBSD data show mm-thick seams of pst contained within and parallel to mylonitic foliation, and mutually overprinting relationships between brittle and plastic deformation. We observe a brittle overprint of mylonitic fabric in sample 46 and fractured porphyroclasts reworked into mylonitic fabric in samples 45 and 47. EBSD maps from sample 45 and 47 show decreasing modal percentages of hydrous mafic minerals (biotite and hornblende) in the mylonites with proximity to pst seams, suggesting these melted to form pst. In pst seams, there are embayed and rounded/elliptical plagioclase survivor clasts and acicular and aligned biotite microlites parallel to mylonitic fabric (45 & 47). EBSD maps show pst survivor clasts with the same shear sense as the mylonitic fabric, suggesting co-development. Pole figures show weak CPO in hornblende and plagioclase of sample 46. Samples 45 and 47 have no CPO present in plagioclase, however samples 45, 46, and 47 show strong CPO patterns for quartz that are consistent with prism slip. We interpret dislocation creep as the deformation mechanism accommodating plastic deformation in host mylonites. Quartz CPO patterns provide evidence of mylonitic deformation at temperatures ~ 600o C, and the presence of plagioclase survivor clasts as evidence of pst temperatures of ~1100oC. The kinematically consistent sense of shear between pst and host mylonitic fabrics suggests coeval development that indicate shifts from brittle to ductile deformation. Our results suggest periodic pst-generating events involving melting of hydrous mafic minerals aided the development of coeval mylonites and pst in the BDT. 

Paleomagnetic data from the Insular superterrane and related terranes in the western Canadian and northern US Cordillera argue for large-magnitude (~4000 km), northward translations along the western margin of the North American Cordillera in the Late Cretaceous (the Baja-BC hypothesis). This model postulates that initial collision of the Insular superterrane occurred in southern California and/or northern Baja Mexico prior to dextral translation along the western North American margin from 85-55 Ma. A major unresolved problem with the Baja-BC hypothesis is that faults that could have accommodated large-magnitude translation are missing or obscured by later Cenozoic faulting and/or sedimentary cover. Here, we investigate the deformation record of Late Cretaceous ductile shear zones in southern California with the goal of understanding the timing and kinematics of deformation at this time. We focus on the Alamo Mountain and Piru Creek shear zones, located within the central Transverse Ranges. We report new field observations and twenty-one U-Pb LA-ICPMS zircon ages from deformed and undeformed host rocks and dikes with the goal of documenting the timing of deformation. Our data show that the Alamo Mountain and Piru Creek shear zones were active at ~76-72 Ma and possibly included an earlier phase of deformation. Both shear zones record sinistral strike-slip to sinistral-normal motion in their present-day orientations. When Cenozoic block rotations are restored, we find that the Alamo Mountain and Piru Creek shear zones originated as NNW-SSE striking, moderately ENE dipping shear zones that formed at mid-crustal conditions (500-600C and 4 kbars). Structural analysis of the shear zones indicates that the dominant component of motion was sinistral strike-slip and that the dip-slip component of motion was minor. The timing and kinematics of deformation in the Alamo Mountain and Piru Creek shear zones are similar to other Late Cretaceous shear zones in the Southern California Batholith. When palinspastic reconstructions are considered, these shear zones comprise a regionally extensive shear zone system over 200 km long. The presence of this regionally extensive, sinistral shear zone system and the absence of dextral shear zones requires reevaluation of the Baja-BC hypothesis in southern California during the Late Cretaceous. 

We present a new method of linking microstructures, electron backscatter diffraction (EBSD)–derived crystallographic vorticity axis (CVA) analysis, and titanite petrochronology to directly link fabric development to specific deformation events in shear zone rocks with complex histories. This approach is particularly useful where overprinting is incomplete, such that it is unknown which fabric is being dated by the petrochronometer. Here, we compared single-phase CVA patterns of fabric-forming minerals with those of synkinematic petrochronometers (e.g., titanite) to associate the timing of fabric development with deformational events in the middle crust of the George Sound shear zone, Fiordland, New Zealand. The host rocks to the George Sound shear zone include the Carboniferous Large Pluton, where titanite petrochronology demonstrates an unequivocally Cretaceous age of metamorphic titanite growth within mylonitic foliation. However, the host rocks show two distinct CVA patterns: a transtensional deformation event recorded by quartz and plagioclase, and a pure-shear–dominated transpressional deformation event recorded by biotite and titanite. Therefore, the transpressional CVA pattern of the titanite, coupled with its Cretaceous age, shows that it cannot be used to date the quartz and plagioclase fabric developed in response to an older transtensional deformation event. These results demonstrate the necessity of combining EBSD and CVA analysis with petrochronology to demonstrate that synkinematic accessory phase petrochronometers show the same kinematic deformation geometry (i.e., CVA pattern) as the fabric being dated. 

Abstract 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. 

Abstract We present >500 zircon δ18O and Lu-Hf isotope analyses on previously dated zircons to explore the interplay between spatial and temporal magmatic signals in Zealandia Cordillera. Our data cover ~8500 km2 of middle and lower crust in the Median Batholith (Fiordland segment of Zealandia Cordillera) where Mesozoic arc magmatism along the paleo-Pacific margin of Gondwana was focused along an ~100 km wide, arc-parallel zone. Our data reveal three spatially distinct isotope domains that we term the eastern, central, and western isotope domains. These domains parallel the Mesozoic arc-axis, and their boundaries are defined by major crustal-scale faults that were reactivated as ductile shear zones during the Early Cretaceous. The western isotope domain has homogenous, mantle-like δ 18O (Zrn) values of 5.8 ± 0.3‰ (2 St.dev.) and initial εHf (Zrn) values of +4.2 ± 1.0 (2 St.dev.). The eastern isotope domain is defined by isotopically low and homogenous δ18O (Zrn) values of 3.9 ± 0.2‰ and initial εHf values of +7.8 ± 0.6. The central isotope domain is characterized by transitional isotope values that display a strong E-W gradient with δ18O (Zrn) values rising from 4.6 to 5.9‰ and initial εHf values decreasing from +5.5 to +3.7. We find that the isotope architecture of the Median Batholith was in place before the initiation of Mesozoic arc magmatism and pre-dates Early Cretaceous contractional deformation and transpression. Our data show that Mesozoic pluton chemistry was controlled in part by long-lived, spatially distinct isotope domains that extend from the crust through to the upper mantle. Isotope differences between these domains are the result of the crustal architecture (an underthrusted low-δ18O source terrane) and a transient event beginning at ca. 129 Ma that primarily involved a depleted-mantle component contaminated by recycled trench sediments (10–20%). When data showing the temporal and spatial patterns of magmatism are integrated, we observe a pattern of decreasing crustal recycling of the low-δ18O source over time, which ultimately culminated in a mantle-controlled flare-up. Our data demonstrate that spatial and temporal signals are intimately linked, and when evaluated together they provide important insights into the crustal architecture and the role of both stable and transient arc magmatic trends in Cordilleran batholiths. 

Abstract We present microbeam major- and trace-element data from 14 monzodiorites collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of evaluating processes involved in the production of andesites in lower arc crust. We focus on relict igneous assemblages consisting of plagioclase and amphibole with lesser amounts of clinopyroxene, orthopyroxene, biotite and quartz. These relict igneous assemblages are heterogeneously preserved in the lower crust within sheeted intrusions that display hypersolidus fabrics defined by alignment of unstrained plagioclase and amphibole. Trace-element data from relict igneous amphiboles in these rocks reveal two distinct groups: one relatively enriched in high field strength element concentrations and one relatively depleted. The enriched amphibole group has Zr values in the range of ∼25–110 ppm, Nb values of ∼5–32 ppm, and Th values up to 2·4 ppm. The depleted group, in contrast, shows Zr values <35 ppm and Nb values <0·25 ppm, and Th is generally below the level of detection. Amphibole crystallization temperatures calculated from major elements range from ∼960 to 830 °C for all samples in the pluton; however, we do not observe significant differences in the range of crystallization temperatures between enriched (∼960–840 °C) and depleted groups (∼940–830 °C). Bulk-rock Sr and Nd isotopes are also remarkably homogeneous and show no apparent difference between enriched (εNdi = 0·1 to –0·1; 87Sr/86Sri = 0·70420–0·70413) and depleted groups (εNdi = 0·3 to –0·4; 87Sr/86Sri = 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate that melts were highly fractionated (molar Mg# <50), andesitic to dacitic in composition, and were much more evolved than bulk lower continental crust or primitive basalts and andesites predicted to have formed from hydrous melting of mantle-wedge peridotite beneath an arc. We suggest that melts originated from a common, isotopically homogeneous source beneath the Malaspina Pluton, and differences between enriched and depleted trace-element groups reflect varying contributions from subducted sediment-derived melt and sediment-derived fluid, respectively. Our data demonstrate that andesites and dacites were the dominant melts that intruded the lower crust, and their compositions mirror middle and upper bulk-continental crust estimates. Continental crust-like geochemical signatures were acquired in the source region from interaction between hydrous mantle-wedge melts and recycled subducted sediment rather than assimilation and/or remelting of pre-existing lower continental crust.