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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 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 Recovering the time-evolving relationship between arc magmatism and deformation, and the influence of anisotropies (inherited foliations, crustal-scale features, and thermal gradients), is critical for interpreting the location, timing, and geometry of transpressional structures in continental arcs. We investigated these themes of magma-deformation interactions and preexisting anisotropies within a middle- and lower-crustal section of Cretaceous arc crust coinciding with a Paleozoic boundary in central Fiordland, New Zealand. We present new structural mapping and results of Zr-in-titanite thermometry and U-Pb zircon and titanite geochronology from an Early Cretaceous batholith and its host rock. The data reveal how the expression of transpression in the middle and lower crust of a continental magmatic arc evolved during emplacement and crystallization of the ∼2300 km2 lower-crustal Western Fiordland Orthogneiss (WFO) batholith. Two structures within Fiordland’s architecture of transpressional shear zones are identified. The gently dipping Misty shear zone records syn-magmatic oblique-sinistral thrust motion between ca. 123 and ca. 118 Ma, along the lower-crustal WFO Misty Pluton margin. The subhorizontal South Adams Burn thrust records mid-crustal arc-normal shortening between ca. 114 and ca. 111 Ma. Both structures are localized within and reactivate a recently described >10 km-wide Paleozoic crustal boundary, and show that deformation migrated upwards between ca. 118 and ca. 114 Ma. WFO emplacement and crystallization (mainly 118–115 Ma) coincided with elevated (>750 °C) middle- and lower-crustal Zr-in-titanite temperatures and the onset of mid-crustal cooling at 5.9 ± 2.0 °C Ma−1 between ca. 118 and ca. 95 Ma. We suggest that reduced strength contrasts across lower-crustal pluton margins during crystallization caused deformation to migrate upwards into thermally weakened rocks of the mid-crust. The migration was accompanied by partitioning of deformation into domains of arc-normal shortening in Paleozoic metasedimentary rocks and domains that combined shortening and strike-slip deformation in crustal-scale subvertical, transpressional shear zones previously documented in Fiordland. U-Pb titanite dates indicate Carboniferous–Cretaceous (re)crystallization, consistent with reactivation of the inherited boundary. Our results show that spatio-temporal patterns of transpression are influenced by magma emplacement and crystallization and by the thermal structure of a reactivated boundary. 

Abstract We investigated the interplay between deformation and pluton emplacement with the goal of providing insights into the role of transpression and arc magmatism in forming and modifying continental arc crust. We present 39 new laser-ablation–split-stream–inductively coupled plasma–mass spectrometry (LASS-ICP-MS) and secondary ion mass spectrometry (SIMS) 206Pb/238U zircon and titanite dates, together with titanite geochemistry and temperatures from the lower and middle crust of the Mesozoic Median Batholith, New Zealand, to (1) constrain the timing of Cretaceous arc magmatism in the Separation Point Suite, (2) document the timing of titanite growth in low- and high-strain deformational fabrics, and (3) link spatial and temporal patterns of lithospheric-scale transpressional shear zone development to the Cretaceous arc flare-up event. Our zircon results reveal that Separation Point Suite plutonism lasted from ca. 129 Ma to ca. 110 Ma in the middle crust of eastern and central Fiordland. Deformation during this time was focused into a 20-km-wide, arc-parallel zone of deformation that includes previously unreported segments of a complex shear zone that we term the Grebe shear zone. Early deformation in the Grebe shear zone involved development of low-strain fabrics with shallowly plunging mineral stretching lineations from ca. 129 to 125 Ma. Titanites in these rocks are euhedral, are generally aligned with weak subsolidus fabrics, and give rock-average temperatures ranging from 675 °C to 700 °C. We interpret them as relict magmatic titanites that grew prior to low-strain fabric development. In contrast, deformation from ca. 125 to 116 Ma involved movement along subvertical, mylonitic shear zones with moderately to steeply plunging mineral stretching lineations. Titanites in these shear zones are anhedral grains/aggregates that are aligned within mylonitic fabrics and have rock-average temperatures ranging from ∼610 °C to 700 °C. These titanites are most consistent with (re)crystallization in response to deformation and/or metamorphic reactions during amphibolite-facies metamorphism. At the orogen scale, spatial and temporal patterns indicate that the Separation Point Suite flare-up commenced during low-strain deformation in the middle crust (ca. 129–125 Ma) and peaked during high-strain, transpressional deformation (ca. 125–116 Ma), during which time the magmatic arc axis widened to 70 km or more. We suggest that transpressional deformation during the arc flare-up event was an important process in linking melt storage regions and controlling the distribution and geometry of plutons at mid-crustal levels.