6.7: Wilson Cycles
- Page ID
- 22632
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)It is critical for historical geologists to recognize that the dynamics of a given plate boundary (divergent, convergent, etc.) are not eternal. A given spot on Earth’s surface may be divergent for some time, then convergent, then divergent again. Just because a given place is tectonically inactive today, that doesn’t mean it was not active in the past. Things change!
From the perspective of continents, another way of putting this is to say: supercontinents form, and then break up. A given spot on a continent may be a site of divergence and rifting (opening up a new ocean basin next door), and later a site of convergence and mountain-building (as that ocean basin is closed and the continents on either side collide). Tuzo Wilson recognized these signatures and described them in a classic 1966 paper about the opening and closing of ocean basins. Because he was first to publish the idea that the change of tectonic setting could be cyclical, we often refer to supercontinent cycles as “Wilson cycles” today.
Mid-Atlantic Wilson Cycles
The eastern margin of the United States offers a classic example of how a given place can shift from convergent to divergent to passive to convergent to divergent to passive again. These sequential shifts accompanied the construction of two subsequent supercontinents (Rodinia and Pangaea) and their subsequent break-up. These repetitive “cycles” of supercontinent formation and destruction are particularly well recorded in the Appalachian mountain belt.
At locations up and down the east coast of North America, there are four major batches of active tectonism, interspersed with times of passive margin conditions:
1. From 1.2 to 1.0 Ga, mountain-building dominated, marked by granites, metamorphism, and deformation. This made the supercontinent Rodinia.
2. From 700 Ma to 565 Ma, rifting dominated, marked by mafic volcanism and immature sedimentary deposits. This broke Rodinia up into fragments, and opened the Iapetus Ocean between them.
3. Subsidence and passive margin sedimentation occurred through the Cambrian and early Ordovician (~550 to ~480 Ma).
4. From 300 to 250 Ma, mountain-building dominated, marked by granites, metamorphism, and deformation. This made the supercontinent Pangaea.
5. From 200 to 180 Ma, rifting dominated, marked by mafic volcanism and immature sedimentary deposits. This broke Pangaea up into fragments, and opened the Atlantic Ocean between them.
6. Passive margin sedimentation resumed, from the Cretaceous (~100 Ma) until today.
The Grenville Orogeny
The oldest rock on the east coast of North America is intrusive. Much of it is felsic in composition, reflecting the partial melting of older source rocks. Dating zircons from these granites and granite gneisses of the “basement complex” gives ages of 1.2 to 1.0 Ga. In general, the older units are more foliated than the younger units, suggesting they were around for a longer period of time during mountain building, with more time to build up a pervasive compressional fabric. Basement rocks of this age are found from Texas to Newfoundland, including the anorthosites making up the Adirondack Mountains in New York, and the granites of the Blue Ridge province in North Carolina and Virginia.
Iapetan rifting
When Rodinia broke up, it was marked by the intrusion of mafic dikes and the eruption of large volumes of mafic lava. The resulting flood basalts bear vesicles, xenoliths, and cooling columns. Normal faulting led to rift valleys that filled with immature clastic sediment, including gravel, arkosic sand, and mud. Some of these rift basins connected up, becoming “the weakest link” between ancestral North America (Laurentia) and the various Proterozoic continental fragments that drifted away from it. Seafloor spreading took over, filling in the gap between continents with fresh injections of basalt. The Iapetus Ocean was born. Inch by inch, this ocean basin widened for hundreds of millions of years, adding a “skirt” of oceanic crust to the edge of the ancestral North American plate.
Appalachian mountain-building
The Appalachian Mountains mark the suture between ancestral North America (Laurentia) and ancestral Africa (the leading edge of Gondwana). They represent the collision between these two large continental landmasses, formerly separated by the Iapetus Ocean. Over the course of the Paleozoic, the Iapetus began to close (resulting in two earlier, smaller orogenies), and the temporary “skirt” of oceanic crust was recycled into the mantle via subduction. This brought Africa and North America ever closer, until eventually they merged in a collisional mountain belt. When the Appalachians were young, they ran through the heart of the Pangaean supercontinent on a scale that may have dwarfed the modern Himalayas. Based on the age of the rocks deformed by compressional stresses and the dating of granites and metamorphic rocks, it appears that the final orogeny occurred in the late Paleozoic, starting around 300 Ma (in the Pennsylvanian) and wrapping up by about 250 Ma (in the Permian). This compressional event resulted in the folding and faulting of older strata in what is today the Valley & Ridge province, metamorphism of the rock in the Blue Ridge and Piedmont province, intrusion of granite plutons, and suturing of African crust to the North American crust.
Atlantic rifting
When Pangaea began to break up in the Triassic, the tectonic extension was first marked by the intrusion of mafic dikes. Normal faulting resulted in the opening of numerous parallel rift basins up and down the east coast. Some of these rift basins connected up, becoming “the weakest link” between North America and Africa/Europe. They stretched wide enough apart that seafloor spreading took over, filling in the gap between continental fragments with fresh injections of basalt. Inch by inch, the Atlantic basin has been widening ever since. Numerous “failed” rift basins remain in the Piedmont geologic province, sort of like “stretch marks” in the crust. They are dominated by two kinds of rocks: immature clastic sediments, and mafic volcanics. The immature clastic sediments include conglomerates and arkose sandstone near the margin of the basins, but also lake deposits in the basins’ centers.
Did I Get It? - Quiz
Which of the following happened during the Mesoproterozoic on the east coast of North America?
a. Rifting of Pangaea
b. Grenville Orogeny
c. Appalachian Orogenies
d. Rifting of Rodinia
- Answer
-
b. Grenville Orogeny
Which of the following happened during the Triassic on the east coast of North America?
a. Rifting of Rodinia
b. Passive margin sedimentation
c. Appalachian Orogenies
d. Grenville Orogeny
e. Rifting of Pangaea
- Answer
-
a. Rifting of Pangaea
Which of the following was a signature of the rifting of Rodinia during the Neoproterozoic on the east coast of North America?
a. Intrusion of granite
b. Eruption of floods of basalt
c. Thrust faulting
d. Deposition of limestone and quartz sandstone
- Answer
-
b. Eruption of floods of basalt
Which of the following was a signature of Appalachian mountain-building during the Paleozoic on the east coast of North America?
a. Deposition of limestone
b. Eruption of floods of basalt
c. Intrusion of granite
d. Normal faulting
- Answer
-
c. Intrusion of granite