B - Early Paleozoic Rifting


 
Early Paleozoic Rifting
 
Earliest Cambrian; about 570 mya


rifting event is the opening of an ocean basin and the subsequent creation of a Divergent Plate Tectonic Boundary. Rifting events create new ocean basins that begin so narrow they barely exist (the Red Sea is a modern example), and may open to thousands of miles in width, like the Atlantic or Pacific oceans. Rifting events make up the first half of a Wilson Cycle and often begin within continental boundaries, where large continents are broken into smaller pieces, each forming a separate continent. The Virginia rifting episode describ ed here resulted in the breaking apart the supercontinent Rodinia, (from the Grenville orogeny) and the creation of the Proto-Atlantic ocean. We follow this rifting event through Stages B, C, and D.
 
The Demise of the Grenville Mountains
 
Mountains never survive long geologically. After the Grenville orogeny eastern North America became tectonically "quiet" for several hundred million years (from about 1.1 billion to .57 billion). "Africa" and North America were fused into a supercontinent (Rodinia), with Virginia located somewhere in the middle. No ocean existed along the east coast of Virginia from 1.1 to about 570 mya.
     
In this span of time, the Grenville mountains eroded down to sea level (a peneplain). A mountain can erode to sea level in about 20 milliion years. By the late Proterozoic (600-570 mya) the entire area was flat, featureless, and barren since neither land plants nor animals had evolved yet.  At that time, a person could have walked across what there was of Virginia (everything west of the Blue Ridge) and right onto "Africa".

Proto-Atlantic Rifting
 
During the latest Proterozoic and earliest Cambrian, North America and "Africa" started to rift apart forming the new Proto-Atlantic (also called Iapetus) ocean. The rifting event extended for thousands of miles up and down eastern North America. In Virginia, the axis of rifting runs down the middle of the Blue Ridge Province.
     
Rifting takes place when continental crust, heated from below by a hot spot, swells upward, thins, and stretches out like pulled taffy (notice the thinning in the cross section above). At depth, the rocks are hot and plastic, but at the surface they are cold and brittle, so when they stretch with the uplift, cracks form and develop into normal faults. Along the faults some blocks of the earth slide downward and form valleys called grabens; the adjacent blocks that move up to form mountains are called horsts (Cross Section). The relief, or difference in elevation, from the top of the horsts to the bottom of the graben can be many kilometers. 
     
In the center of the system there is the axial rift (or axial graben), a narrow, steep sided valley maybe only 50-100 kilometers wide but falling a kilometer or more below sea level. Early in its history it may be dry, but in time the sea spills in filling it. On either side of the axial rift, horsts rise abruptly two or more kilometer above sea level. These horst blocks bordering either side of the axial rift are the continental terraces, or hinge zones (Rift Section 1-2).

Today the remnants of this axial rift runs through the Charlottesville and the  Culpepper region. The sedimentary rocks (Lynchburg Group, see Cross Section) present indicate that during rifting the long, deep rift valley received coarse gravels and sandstones from the surrounding horst mountains on both sides. The western side of the rift graben is North America, but the eastern side, was "Africa". You can almost imagine this while standing at one of the east facing overlooks along Skyline Drive today, such as this picture looking east from the Buck Hollow overlook a few miles south of the Thornton Gap entrance to Skyline Drive. Looking east over the low lands below of the Blue Ridge and Piedmont provinces it is almost like you were standing on one side of the rift valley looking down into the graben. Out there in the distance is the Lynchburg group, sediments that filled the axial rift. And somewhere off in the distance, "Africa."
     

We can get ideas of how this may have looked by studying modern examples of rifts that are similar to the Proto-Atlantic rifting event. The photograph to the left, for example, is the continental terrace and axial rift of the East African rift system just west of Nairobi, Kenya, Africa. Notice how high the horst is, and how abruptly it drops into the rift. Virginia during the Proto-Atlantic rifting just before the sea invaded the rift probably resembled this.

To the right is a satellite view of the Gulf of Aden. The black area is the newly opening ocean Basin in the axial rift. The brownish rugged regions to the upper right are the horsts that compose the continental terrace. The light colored, almost white, areas are the alluvial fans and braided rivers carrying sediment down into the rift. 
     
The satellite image (below right) of the Red Sea in the Middle East shows a modern axial rift after the sea has invaded, and just as oceanic lithosphere begins to form and create a new ocean Basin.

It is difficult to fully imagine how things looked during Proto-Atlantic rifting because no plants or animals had evolved yet so the lushness of the African scene would not be Virginia. Imagine instead barren rock. Additionally, modern east Africa is in the tropics, but during the rifting episode in Virginia, there is much evidence that glaciers existed. So now one may imagine these horsts covered with ice streaming through valleys down to the rift floor, and then into the invading ocean waters. 

True, the horst and axial rift did exist down the axis of the Blue Ridge province in the Late Proterozoic, as the rock record tells us, but the geology in the Blue Ridge today is not a horst and graben system as we tried to imagine from Buck Hollow overlook (although a horst/graben system is preserved there), but an Overturned Anticline
     
The rock record is like a palimpsest, an old document where a previous text has been erased, but not completely, so that another text could be written over it. We clearly see the latest text, but beneath it is another text, largely hidden, that may be about something completely different. 
     
So now we have an axial rift, but in time the horsts will erode, and the axial rift fill with sediments. But before that process is finished the rift valley continues to spread apart, changing into a newly opening ocean Basin (Rift Section 3), which is explored more in Stage C. Or you can go back to see all the Stages of Geological Evolution
 
Contributed by Lynn Fichter 
Thursday, July 31, 2014
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