300-7 Characterizing sediment sources of Jurassic strata in the Western Sierran Foothills, CA: Preliminary zircon U-Pb and Lu-Hf isotopic results from the Clipper Gap Formation

Session: Reconstructing Earth Surface Processes in Orogenic Systems (Posters)

Poster Booth No.: 140

Presenting Author:

Brandon Botha

Authors:

Botha, Brandon Joaquim Louis¹, Orme, Devon A.², Quigley, Nathan³, Craig, Lucas⁴, Shaffer, Cate⁵, Sikes, Wiley⁶, Surpless, Kathleen DeGraaff⁷

(1) Montana State University, Bozeman, MT, USA, (2) Montana State University, Bozeman, MT, USA, (3) Montana State University, Bozeman, MT, USA, (4) Trinity University, San Antonio, TX, USA, (5) Trinity University, San Antonio, TX, USA, (6) Trinity University, San Antonio, TX, USA, (7) Trinity University, San Antonio, TX, USA,

Abstract:

The direction of subduction polarity along the North American Cordillera during Jurassic-Early Cretaceous time remains debated, with models ranging from a single east- or west-directed to double-sided subduction systems. This study tests models of subduction polarity at the latitude of the Western Sierra Nevada Foothills of California through provenance and age investigation of the Clipper Gap Formation. The Clipper Gap Formation is a mélange unit that is estimated to be Late Triassic to Early Jurassic in age and is primarily composed of sandstone, chert, siltstone, conglomerate, pillow lava, blocks of limestone, serpentinite, and granitic dikes. One metasandstone and one cross-cutting granitic dike from the South Fork of the Yuba River and clasts of tuff from a conglomerate near the town of Colfax were collected and analyzed for zircon U-Pb geochronology and Lu-Hf isotope geochemistry. The tuffaceous sample yielded 50 concordant dates with a primary age mode of ~164.5 Ma. Lu-Hf analyses on a subset of 25 grains yield ƐHf values ranging from +8 to +15. Detrital zircon from the metasandstone yielded 85 concordant dates with age populations of ~176 Ma, ~263 Ma, ~350, ~420 Ma, and a scattering of Precambrian grains. A subset of 25 grains have ƐHf values of +4 to +15 for grains <400 Ma, with grains ~420 Ma plotting between +10 and -15. The cross-cutting granitic dike has 44 concordant dates with an age population at ~167 Ma, and ƐHf values of +9 to +14. The youngest age population from the tuff and granitic dike are younger (middle Jurassic) than previous dates from the Clipper Gap Formation. The presence of Precambrian zircon in the metasandstone supports the Clipper Gap being deposited on the North American margin. Pre-Mesozoic ƐHf values show a broad range similar to North American signatures such as zircons derived from Appalachia or recycled from Paleozoic-Triassic deposits. Jurassic ƐHf signatures are exclusively positive for all three samples, overlapping with bedrock and detrital records of the Jurassic Sierra Nevada arc and sedimentary deposits such as the Mariposa Formation, but with a more restricted range. We interpret the U-Pb and Lu-Hf signatures of these samples to reflect deposition and emplacement between ~176 and ~ 164 Ma, respectively, on the North American plate during east-directed subduction.

 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7259

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