More Like this

1. 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

   Schwartz, J. ; Lackey, J.S. ; Miranda, E. ; Klepeis, K. ; Mora-Klepeis, G. ; Robles, F. ; Bixler, J. ( October 2022 , Geological Society of America Abstracts with Programs)

   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. 

   more » « less
2. Constraining the timing and conditions of magmatism and metamorphism in the lower crust of the Southern California batholith using U-Pb zircon geochronology and Ti-in-zircon thermometry

   Robles, F. ; Schwartz, J. ; Miranda, E. ; Klepeis, K.A. ; and Mora-Klepeis, G. ( October 2022 , Geological Society of America Abstracts with Programs)

   The Southern California batholith contains a geologic record that can help clarify the timing of events that occurred during the Late Cretaceous (100-65 Ma) along the western margin of the North American Cordillera. The subduction of the oceanic conjugate Shatsky plateau beneath North America is postulated to have ended active magmatism in the arc at 88-70 Ma; however, the timing of this event is poorly constrained in Southern California. We use U-Pb laser ablation zircon petrochronology to document the timing and conditions of magmatism and metamorphism in the lower crust of the Cretaceous arc. We focus on the Cucamonga terrane in a part of the Southern California batholith located northeast of Los Angeles in the southeastern San Gabriel Mountains. These rocks contain exhumed lower crustal (7-9 kbar) rocks predominantly composed of granulite-facies metasedimentary rocks, migmatites, charnockite and dioritic to tonalitic gneiss. We report 20 new zircon dates from 11 samples, including 4 mafic biotite gneisses, 3 mylonitic tonalites, 2 charnockites, a quartzite, and a felsic pegmatite dike crosscutting granulite-facies metasedimentary rocks. New 206Pb/238U ages show that magmatism occurred in the Middle Jurassic (ca. 172-166 Ma), the Early Cretaceous (ca. 120-118 Ma), and the Late Cretaceous (88-86 Ma) at temperatures ranging from 740 to 800 oC. Granulite-facies metamorphism and partial melting of these rocks occurred during the 88-74 Ma interval at temperatures ranging from 730°C to 800oC. Our data indicate that high-temperature arc magmatism and granulite-facies metamorphism continued through the Late Cretaceous and overlapped in timing with postulated subduction of the conjugate Shatsky plateau from previous models. We speculate that termination of arc activity and cooling of the lower crust in response to plateau subduction must postdate ca. 74 Ma. 

   more » « less
3. Tectonic and magmatic construction of lower crust in the southern California batholith

   Schwartz, J.J. ( May 2024 , Geological Society of America Abstracts with Programs)

   We explore the growth of lower-continental crust by examining the root of the Southern California Batholith, a ~ 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 detrital zircon geochronology of 4 quartzites and paragneisses to investigate the origin of the lower-crustal Cucamonga paragneiss sequence, and U-Pb petrochronology of 26 orthogneisses to establish the timing of arc magmatism and granulite-facies metamorphism. We find that the Cucamonga paragneisses share 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. This basin was progressively underthrust beneath the arc during the Middle Jurassic to Late Cretaceous and was metamorphosed during two high-grade (>750°C) migmatization events at ca. 124 and 89–75 Ma. These metamorphic events were associated with 100 m.y. of arc magmatism that lasted from 175 to 75 Ma and culminated in a magmatic surge from ca. 90–75 Ma. Field observations and petrochronology analyses indicate that partial melting of the underthrust Cucamonga metasedimentary rocks was triggered by 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. Our observations demonstrate that tectonic incorporation of sediments into the lower crust led to structural, compositional and rheological changes in the architecture of the arc including vertical thickening. These structural changes created weak zones that preferentially focused deformation and promoted present-day reactivation along the Cucamonga thrust fault. 

   more » « less
4. TECTONIC AND MAGMATIC CONSTRUCTION OF LOWER CRUST IN THE SOUTHERN CALIFORNIA BATHOLITH

   Schwartz, J.J. ( May 2024 , Geological Society of America abstracts)

   We explore the growth of lower-continental crust by examining the root of the Southern California Batholith, a ~ 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 detrital zircon geochronology of 4 quartzites and paragneisses to investigate the origin of the lower-crustal Cucamonga paragneiss sequence, and U-Pb petrochronology of 26 orthogneisses to establish the timing of arc magmatism and granulite-facies metamorphism. We find that the Cucamonga paragneisses share 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. This basin was progressively underthrust beneath the arc during the Middle Jurassic to Late Cretaceous and was metamorphosed during two high-grade (>750°C) migmatization events at ca. 124 and 89–75 Ma. These metamorphic events were associated with 100 m.y. of arc magmatism that lasted from 175 to 75 Ma and culminated in a magmatic surge from ca. 90–75 Ma. Field observations and petrochronology analyses indicate that partial melting of the underthrust Cucamonga metasedimentary rocks was triggered by 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. Our observations demonstrate that tectonic incorporation of sediments into the lower crust led to structural, compositional and rheological changes in the architecture of the arc including vertical thickening. These structural changes created weak zones that preferentially focused deformation and promoted present-day reactivation along the Cucamonga thrust fault. 

   more » « less
5. What we can learn in the kitchen sink: an example from garnet granulite

   Stowell, H.H. ( January 2021 , America Geophysical Union)

   Interpretation of geochronological and petrological data from partially-melted granulite is challenging. However, integration of multiple chronometers and mineral assemblage diagrams (MAD) can be used to estimate the nature and duration of processes. Excellent lower-crustal exposures of garnet granulite from the Malaspina Pluton, Fiordland New Zealand provide an ideal place to employ this kitchen sink approach. We use zircon U-Pb ages from LA-ICPMS, SHRIMP-RG, and CA-TIMS, garnet Lu-Hf and Sm-Nd ages, and MAD in order to evaluate local partial melting vs. melt injection, equilibrium volumes, P-T conditions, and the duration of lower crustal thermal events. Host diorite (H), garnet-clinopyroxene reaction zones (GRZ), coarse garnet selvages, and tonalite veins provide a record of intrusion and granulite facies partial melting. Zircon U-Pb ages range from 123 to 107 Ma (all); LA-ICPMS ages contain the entire range; CA-TIMS ages range from 118.30±0.13 to 115.7±0.18 Ma; and SHRIMP-RG ages range from 121.4±2 to 109.8±1.8 Ma. The latter two techniques are interpreted to indicate primary igneous crystallization from ~119 to ~116 Ma and the youngest ~110 Ma ages are interpreted as metamorphic zircon growth. Garnet ages for ~1 cm grains are ~113 Ma (Lu-Hf & Sm-Nd) and record metamorphic growth, and <0.3 mm grains with Sm-Nd ages from 113 to 104 Ma reflect high temperature intracrystalline diffusion and isotopic closure during cooling to amphibolite facies. Zircon trace-element compositions indicate 2 distinct crystallization trends reflecting evolution of primary magma batches. MAD indicate that garnet was not in equilibrium with sampled rock compositions. Instead, garnet shows apparent equilibrium with a modeled mixture of the GRZ and the H and grew in equilibrium with an effective bulk composition that shifted toward the leucosome. This would produce the observed increase in garnet grossular content. We conclude that: Malaspina rocks from Crooked Arm preserve evidence for 2 igneous layers which evolved as discrete magmas, igneous crystallization lasted 2 to 3 m.y., granulite metamorphism peaked ~ 3 m.y. after intrusion, metamorphism lasted ≥3 m.y., cooling occurred at ~20°C/m.y., and granulite minerals equilibrated with a mixture of solid phases and melt at ~14 kbar and 920°C (based on garnet compositions and MAD). 

   more » « less