Abstract

The onset of the Laramide orogeny is of great tectonic significance to the geologic history of the US, but the timing and nature of the shift between Sevier and Laramide tectonics remains enigmatic. The eastern Transverse Ranges of southern California provide the opportunity to observe the effects of Laramide tectonics on the mid-crust. Wide Canyon is a north/south-trending canyon in the northern Little San Bernardino Mountains of the eastern Transverse Ranges. Al-in-hornblende thermobarometry of Needy et al. (2009) yields a projected paleodepth depth of ~20 km for Wide Canyon where Cretaceous granitoids intrude metamorphic country rock of Proterozoic age in a regional NW-trending antiform.

U/Pb geochronology of five igneous samples from Wide Canyon reveals two zircon growth events at 88-84 Ma (32 analyses) and 76-72 Ma (30 analyses). Granite, granodiorite and gabbro samples contain zircons with 88-84 Ma cores and 76-72 Ma rims. One granite sample yielded a unimodal age of 74.1 ± 1.6 Ma. U and Th concentrations among 88-84 Ma zircon cores span three and four orders of magnitude in concentration, respectively, as would be expected from samples that range from granite to gabbro. In contrast, 76-72 Ma zircon rims yield U-Th concentrations that are within an order of magnitude.

Syndeformational structures such as a regional synmagmatic shear zone, melt-filled parasitic folds and a melt-filled field area-scale antiform are constrained by the reported 76-72 Ma zircon ages. Shear sense indicators in the southwest-dipping shear zone (S/C fabrics, sigma and delta clasts and asymmetric boudins) yield strong top-to-the-northeast kinematics.

The bimodal age distributions coincide with the shift from Sevier to Laramide tectonics during the Late Cretaceous. 88-84 Ma zircon ages are interpreted as a pulse of arc-magmatism during the Sevier Orogeny. Mafic rocks of synchronous age found in the arc-derived Teutonia batholith bear compositional similarities to mafic rocks of Wide Canyon, indicative of similar magmatic sources from 88-84 Ma.

Zircon ages of 76-72 Ma zircon ages are interpreted as recording a shearing event, the emplacement of some granite bodies, and widespread zircon overgrowth. Top-to-the-northeast kinematics in the southwest-dipping synmagmatic regional shear zone are interpreted as backthrusting during northwest/southeast regional contraction that is well constrained by U/Pb zircon geochronology. This contraction is interpreted as the onset of Laramide tectonics between 76-72 Ma. The timing for the onset of flat slab subduction in the Mojave section of the Cordilleran arc is much later than is presumed by proposed tectonic models.Two zircon growth events are observed in igneous rocks of Wide Canyon at 88-84 Ma and 76-72 Ma. Bimodal zircon ages, are widespread in granite, granodiorite and gabbro across the field area, exhibit 88-84 Ma cores and 76-72 Ma rims. One granite sample yielded a unimodal age population from 76-72 Ma. Trace element concentrations among 88-84 Ma zircon cores are highly diverse, whereas 76-72 Ma zircon rims yield much more homogeneous trace element geochemistry.

Syndeformational structures such as a regional synmagmatic shear zone, melt-filled parasitic folds and a melt-filled field-area-scale antiform are constrained by the reported 76-72 Ma zircon ages. Shear sense indicators in the southeast-dipping shear zone (S/C fabrics, sigma and delta clasts and asymmetric boudins) yield strong top-to-the-northeast kinematics across the synmagmatic shear zone.

The bimodal age distributions coincide with the shift from Sevier to Laramide tectonics during the Late Cretaceous. 88-84 Ma zircon ages are interpreted as a record of a pulse of arc-magmatism during the Sevier Orogeny. Mafic rocks of synchronous age found in the Teutonia batholith bear compositional similarities to mafic rocks of Wide Canyon, implying that initial compositions from 88-84 Ma emplacement are preserved despite the later zircon-growth event.

Zircon ages of 76-72 Ma zircon ages are interpreted as recording a shearing event, the emplacement of some granite bodies, and widespread zircon overgrowth. Top-to-the-northeast kinematics in the southwest-dipping synmagmatic regional shear zone are interpreted as backthrusting during northwest/southeast regional contraction that is well constrained by U/Pb zircon geochronology. This contraction is interpreted as the onset of Laramide tectonics between 76-72 Ma. This precise observation of the onset of flat slab subduction in the Mojave section of the Cordilleran arc must be taken into account by future tectonic modelers.

Degree Date

Summer 8-2018

Document Type

Thesis

Department

Earth Sciences

Advisor

Rita Economos

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

GeologicmapWideCanyon.pdf (30498 kB)
Geologic Map of Wide Canyon, Eastern Transverse Ranges, southern California

geochron.zip (472 kB)
Geochronology results

XRF.zip (1245 kB)
XRF geochemistry methods and results

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