26.6: The end of subduction
- Page ID
- 22804
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Subduction of the Farallon Plate continued until the around 25 million years ago. By the point, the faster rate of subduction had caught up to the slower rate of seafloor spreading, which consumed the last bit of young, fresh Farallon crust, and then the oceanic ridge itself. This brought the North American Plate into contact with the Pacific Plate for the first time. This “moment” was the birth of the San Andreas Fault, the modern transform plate boundary between North America and the Pacific Plate. The shape of the Farallon Plate was such that it was narrowest in the middle, and wider both north and south of that. So the first spot the subduction zone gobbled up the spreading center was in the middle of the Farallon Plate. This broke it into two. Over time, the Farallon’s remnants (the Juan de Fuca Plate in the north, and the Cocos Plate in the south) continued to subduct, but they get smaller and smaller with each passing millennium. Between them, the San Andreas Fault continues to get longer and longer. Here’s an animation showing how this has played out:
That covers what happened horizontally after the end of subduction, but there were also vertical motions of the crust. With the cessation of the drag of the subducting slab, the accretionary wedge was no longer pulled downward. It was allowed to rebound upward, and eventually rose to poke up above sea level, making the California Coast Ranges. Similarly, the forearc basin uplifted to become the flat-as-a-pancake Great Central Valley, and erosion of the volcanoes of the continental volcanic arc revealed their deeper plumbing system: the plutons of granite and granodiorite that we now see in the Sierra Nevada range.
Anderson, Don (2007). “The Eclogite Engine,” Chapter 5 in New Theory of the Earth (book), Cambridge University Press.
Atwater, Tanya (1970). “Implications of plate tectonics for the Cenozoic tectonic evolution of western North America,” Bull. Geol. Soc. Amer., v. 81, p. 3513-3536. DOI: 10.1130/0016-7606(1970)81[3513:IOPTFT]2.0.CO;2
Kirkpatrick, Jamie; Rowe, Christie; Bentley, Callan; Blisniuk, Kim, and Wakabayashi, John (2019). Streetcar 2 Subduction, digital field trip guide the geology of the San Francisco Bay Area. American Geophysical Union.
Warhaftig, Clyde (1984). A Streetcar to Subduction, and other plate tectonic trips by public transportation in San Francisco. American Geophysical Union: book.