Date

6-2014

Degree Name

BS in Earth Sciences

Department

Natural Resources Management and Environmental Sciences Department

Advisor(s)

Scott Johnston

Abstract

The long-lived California section of the North American Cordillera is one of the most studied continental margins in the world; however, erosion and deformation in areas along the San Andreas Fault and the Sur-Nacimiento Fault has erased sections of the Cordillera which could provide insight in Cordilleran processes. The lost sections are partially preserved in detrital zircons deposited alongside the arc in fore-arc basins such as in the Toro Formation of the Nacimiento block which represents the coastal central California equivalent to the Great Valley Group, located in the Santa Ynez Mountains. Geochemical analysis of the detrital zircons from the Toro Formation suggests that a previous unmapped fault thrust Tithonian to Valanginian (133.9-152.1 Ma) sedimentary rocks over Albian (100.5-113.0 Ma) rocks. Additionally, a low of U/Yb at 150 Ma, decreasing Yb/Gd with time, and fluctuating Th/U levels with highs during magmatic pulses and lows during lulls correlates with previous work done with similarly aged sandstones in the McCoy Mountains which represents the retro-arc basin. Rising Gd concentration is the driver for the lowering of Yb/Gd while Yb concentrations remained relatively randomly stable which suggest presence of garnet throughout the arc’s lifetime and the appearance of titanite with age. Changes of the age distribution of the detrital zircon grains from pre-Cordilleran dominated Tithonian to Valanginian detritus to Cordilleran dominated Albian detritus suggest changes in the drainage networks. All together this study found records of geochemical and physical changes within the arc and physical changes to the Toro Formation on only a portion of the Nacimiento block. Additional studies up section into the Atascadero Formation would uncover additional records of the Cordillera as a whole.

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