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摘要
In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of approximate to 2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit's interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98Ma. Rare fossils had indicated that this unit was mostly 151-137Ma, but it is mostly much younger. (2) Central Belt: SNB; ca. 103 to 53Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50Ma. Much of the Yager's detritus was shed during major core complex extension and erosion in Idaho that started 53Ma. An Eocene Princeton River-Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, +/- SNB, +/- nearby Cascade arc, +/- Nevada Cenozoic ignimbrite belt; 52 to <32Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16-14Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by progressive accretion. From ca. 118 to 70Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37-32Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.
- 出版日期2015-6-11
