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The Late Cretaceous arc flare-up event from 90 to 70 in the Transverse Ranges of the Southern California Batholith was temporally and spatially associated with the development of a large contractional shear system that includes discontinuous segments of the Tumamait shear zone (Mt. Pinos), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone (Cucamonga terrane), and the Eastern Peninsular Ranges shear zone. The age and kinematics of these shear zones inform the tectonic setting of the continental arc in Southern California during the beginning of the Laramide orogeny and during postulated large-magnitude dextral translations along the margin (the Baja-BC hypothesis). The Mt. Pinos sector of the Southern California Batholith preserves the intra-arc, transpressional Tumamait shear zone and the ductile-to-brittle Sawmill thrust, both of which record Late Cretaceous deformation. The batholith and shear zone are hosted by Mesoproterozoic biotite gneisses and migmatites (1750-1760 Ma), Neoproterozoic biotite granites (660 Ma), Permo-Triassic granitic gneisses and amphibolite (260-250 Ma), and Late Jurassic granites and gneisses (160-140 Ma). Late Cretaceous rocks are variably deformed and include porphyritic granodiorite gneisses and peraluminous granites emplaced at 86 to 70 Ma. Mylonites of the Tumamait shear zone affect all rocks in the area and generally strike NW-SE and dip moderately to the NE and SW. Mineral stretching lineations plunge shallowly to the SE. Mylonitic fabrics are folded into a regional, SE-plunging synform that results in alternating bands of sinistral and dextral shear fabrics. Syn-kinematic titanites from 5 mylonitic samples give a 720-700°C temperature range, and lower-intercept 206Pb/238U dates of 77.0 Ma, 76.8 Ma, 75.1 Ma, 74.2 Ma, and 74.0 Ma. Subsequent folding of the mylonite is linked to N-directed motion on the Sawmill thrust. 40Ar-39Ar thermochronology ages of 67-66 Ma and onlapping Eocene shales indicate Latest Cretaceous activity on the thrust, prior to Eocene arc collapse. Based on the age of the Tumamait shear zone, we speculate that it is related to sinistral deformation observed in the nearby Alamo Mountain-Piru Creek and the Black Belt shear zones. We attribute the younger Sawmill thrust to collision of the Hess oceanic plateau with the Southern California Batholith after 70 Ma.
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Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith, 2022, Late Cretaceous arc flare up and sinistral intra-arc ductile deformation in the southern California batholith
We present >90 new igneous and metamorphic zircon and titanite petrochronology ages from the eastern Transverse Ranges of the Southern California Batholith (SCB) to investigate magmatic and tectonic processes in the frontal arc during postulated initiation of Late Cretaceous shallow-slab subduction. Our data cover >4000 km2 in the eastern Transverse Ranges and include data from Mesozoic plutons in the Mt. Pinos, Alamo Mountain, San Gabriel Mountain blocks, and the Eastern Peninsular mylonite zone. Igneous zircon data reveal 4 discrete pulses of magmatism at 258-220 Ma, 160-142 Ma, 120-118 Ma, and 90-66 Ma. The latter pulse involved a widespread magmatic surge in the SCB and coincided with garnet-granulite to upper amphibolite-facies metamorphism and partial melting in the lower crust (Cucamonga terrane, eastern San Gabriel Mountains). In this region, metamorphic zircons in gneisses, migmatites and calc-silicates record high-temperature metamorphism from 91 to 74 Ma at 9–7 kbars and 800–730°C. The Late Cretaceous arc flare-up was temporally and spatially associated with the development of a regionally extensive oblique sinistral-reverse shear system that includes from north to south (present-day) the Tumamait shear zone (Mt. Pinos), the Alamo Mountain-Piru Creek shear zone, the Black Belt shear zone (Cucamonga terrane), and the Eastern Peninsular Ranges shear zone. Syn-kinematic, metamorphic titanite ages in the Tumamait shear zone range from 77–74 Ma at 720–700°C, titanites in the Black Belt mylonite zone give an age of 83 Ma, and those in the eastern Peninsular Ranges mylonite zone give ages of 89–86 Ma at 680–670°C. These data suggest a progressive northward younging of ductile shearing at amphibolite- to upper-amphibolite-facies conditions from 88 to 74 Ma, which overlaps with the timing of the Late Cretaceous arc flare-up event. Collectively, these data indicate that arc magmatism, high-temperature metamorphism, and intra-arc contraction were active in the SCB throughout the Late Cretaceous. These observations appear to contradict existing models for the termination of magmatism and refrigeration of the arc due to underthrusting of the conjugate Shatsky rise starting at ca. 88 Ma. We suggest that shallow-slab subduction likely postdates ca. 74 Ma when high-temperature metamorphism ceased in the SCB.
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- Award ID(s):
- 2138734
- PAR ID:
- 10415241
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Programs
- Volume:
- 54
- Issue:
- 5
- Page Range / eLocation ID:
- 10.1130/abs/2022AM-381618
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Abstract Structural analyses combined with U-Pb zircon petrochronology show the influence of arc magmatism on the evolution of two transpressional shear zones in the deep root of the Late Cretaceous Southern California batholith. The mid-crustal Black Belt and lower-crustal Cucamonga shear zones (eastern San Gabriel Mountains, California, USA) formed at ca. 84 Ma, shortly after a large mass of tonalite and granodiorite intruded the lower crust. Both shear zones were active until at least ca. 74 Ma and probably until 72–70 Ma. In the mid-crustal shear zone, rheological contrasts between mingling magmas localized deformation at dike margins. The deformation began as hypersolidus flow in partially crystallized dikes and then transitioned to deformation below the solidus when alternations between viscous creep and brittle faulting produced interlayered pseudotachylyte, cataclasite, and mylonite. As the dikes solidified, strain hardening drove shear zone growth and created thin (10–30 m) high-strain zones and faults that are widely spaced across ~1 km. In contrast, the lower-crustal Cucamonga shear zone was magma-starved, lacks the variety of shear zone fabrics exhibited by its mid-crustal counterpart, and formed by the reactivation of a preexisting fabric that records pure reverse displacements at 124–93 Ma. The two shear zones created a partitioned style of intra-arc transpression where sinistral-reverse (mostly arc-parallel with some arc-oblique) displacements were accommodated on moderately dipping faults and shear zones and arc-normal shortening was accommodated by coeval folds. This study shows how a magmatic surge influenced the architecture and style of Late Cretaceous transpression in the Southern California batholith, including the evolution of high-strain zones that record alternating episodes of brittle, ductile, and hypersolidus deformation. The results illustrate how magmatism localizes strain on deep-crustal faults during orogenesis and oblique convergence.more » « less
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