L - Rifting of Pangaea

Rifting of Pangaea: Initiation of the Atlantic Ocean
Triassic and Lower Jurassic; 230 my to 175 mya

~The Unconformity~
An unconformity is a gap in the rock record where there is no information, typically marked by a surface of 1) no deposition or 2) erosion of the rock record. An unconformity followed the Grenville event at approximately 1.1 billion ybp (Years Before Present) (Stage B) and continued until the beginning of the Crossnore igneous activity at about 800 million ybp (Stage C). A second unconformity is noted following the Alleghanian orogeny.
From the final sediments deposited in southwest Virginia during the Pennsylvanian (Harlan Formation) through the late Triassic, there is no geologic record in the Mid-Atlantic region.  At that time, Virginia lay deep within the supercontinent Pangaea and was firmly connected to northwest Africa. During the Permian, the last of the Alleghanian Mountains eroded and, by the lower Triassic, Virginia was geologically inactive, and there was little topographic relief, although the areas was probably a high, broad, flat plateau since the great thickness of coal-containing sediments that had accumulated on the western side of the Alleghanian Mountains would have prevented erosion much below that level.
Rivers actively incised deep canyons into the plateau, as indicated by deep erosion (last unconformity in the stratigraphic column) into the Pennsylvanian sediments found in southwest Virginia, although exact timing of this erosion is unknown. The incision cut down to the Bluestone Formation, which indicates erosion on the order of over 2,000 meters of thickness. Most of North America to the west was land built up by rivers carrying the sediment eroded from the Allgehanian mountains into the Absaroka Sea. Some of these long, east-to-west flowing river systems likely had headwaters in Virginia and flowed for thousands of miles before finally entering the ocean through southwest Texas and Mexico. 
As erosion began, the landforms may have developed and looked similar to those today, since most of the structural geology that underlies today's topography was in place by this time. The Valley and Ridge, Blue Ridge, and Piedmont were all in their modern locations, with their modern structures, although not eroded as they are now. The Allehany Plateau would have possessed dendritic drainage, much as it does today. It is more difficult to guess what the Blue Ridge and Piedmont were like, but also perhaps not too different from what we see today. One major difference, however, is that where the Coastal Plain is today lay Africa, and the Triassic rift basins had not yet formed.
~Synopsis of the Atlantic Rifting Event~
By the late Triassic, a rich fossil record on both sides of the Atlantic exists, from northern Greenland to the tips of South America and Africa, recording the initiation of renewed geologic activity. The supercontinent Pangaea, built during the Alleghanian and Ouachita orogenies, began to rift apart quickly by the early Jurassic. Oceanic crust began to form and this marked the opening of the Atlantic Ocean.
Pangaea fragmented in stages. It first split along an east-west line running through the present Mediterranean Sea region (then called the Tethys Sea) forming a northern continent, Laurasia (North America, Europe, and Asia), and a southern continent, Gondwana (South America, Africa, Antarctica, India, and Australia). Then Laurasia and Gondwana each split, in stages, to the east and west to open the Atlantic Ocean.
A large number of triple junctions are associated with the opening of these ocean basins, and well over 100 grabens formed. From this evidence, the stages of opening can be dated: first, northwest Africa rifted from eastern North America (210-170 mya); then South America rifted from southwest Africa (145-125 mya); then Greenland rifted from North America (80 mya); and finally the Scandinavian countries split from Greenland (80-60 mya). Several other smaller episodes of rifting are also recognized, all resulting in the Atlantic Ocean basins' location today.

~The Rift Record~
All rifting events follow similar processes and produce similar results. An example of a rifting model  (Proto-Atlantic) is included in detail in Stages BC, and D.

All rifting events begin with formation of an axial rift system, the gash in the earth through which a new ocean basin develops. Associated with the axial rift, however, are a whole host of lateral half-grabens, running parallel to the axial rift, but up to 500 km inland on both sides from the axial rift. 
There is an excellent record of the axial rift located on the axis of the Blue Ridge (Blue Ridge Province) during the Proto-Atlantic, (Stage B) but not the lateral graben, which exists to the west as the Rome Trough buried deep in the subsurface under Kentucky, Ohio and western West Virginia. The situation in the later Atlantic-rifting is reversed; the lateral grabens are well exposed in the Mid-Atlantic, and the axial rift is buried far to the east (map below). The reasons for these differences are as follows: first, the Proto-Atlantic axial rift was thrust upward and westward during the Alleghanian orogeny, and subsequent erosion has unroofed it. If this had not occurred, the Proto-Atlantic axial rift would be as inaccessible today as the Atlantic axial rift (map below). 

Second, Atlantic rifting occurred much farther to the east than the Protoatlantic rifting. Axial rifts, those that will become an ocean basin, are always on the very edge of a continent, and as a result become deeply buried under the divergent continental margin as it develops (Rift Cross Section #4). On the map, the dark blue fields at the edge of the continental shelf are axial rifts. The axial rift is several hundred kilometers east of the exposed basins, buried under as much as 12 kilometers of continental margin sediments. 
Lateral grabens are well exposed and the U.S. has a rich record of these, beginning in New England and extending down into the Carolinas. For the most part they are scattered throughout the Piedmont Province, although a few lie under the Coastal Plain (Map Above). The exposed basins along the east coast include the Newark, Gettysburg, Culpepper, Taylorsville, Richmond, Farmville, Danville and Deep Water basins (Map). 

~Geology of the Exposed Basins (Lateral Graben)~
Exposed basins are typically half-grabens; that is a rift valley with a main, or border fault on one side, and the hinge zone on the other side. In the Mid-Atlantic the half grabens have the main fault on the west, and the hinge on the east. This is the expected orientation, but some of the New England grabens are reversed; the main fault is on the east and the hinge on the west. 
Most of the grabens are located in the Piedmont and so, for the most part, are faulted into igneous and metamorphic basement rocks, including Grenville basement on the far west, and the volcanic arcs and other terrane rocks which came to North America during the Paleozoic Taconic and Acadian orogenies (Piedmont Terrane Map).
The general development of a lateral graben begins with initiation of the border fault during which the horst lifts, the graben drops, and the hinge zone rotates to allow the half graben to form. The result is a long scarp along one side of a long valley, and the creation of considerable relief. The faults develop ultimately because the whole region is being heated by hot spots and uplifted into a broad plateau several kilometers above sea level, but as the area swells upward it also stretches at the surface. The brittle rocks crack, form normal faults, and the graben drops down to a lower elevation. This motion is not sudden, but takes place in stages over tens of thousands of years. 
Sediments begin eroding from the horsts almost immediately and flow into the graben (basin). Along the base of the fault alluvial fans and rivers deposit gravels and sands. The gravels are without structure implying they were deposited as debris or mud flows. The gravel in the Conglomerate is made of fragments of the horst basement rocks.
Events that occurred in the Triassic basins included simple scenarios of upward movement, formation of horsts and grabens, and deposition of sediment into the grabens; however, early in this basin history, coal deposits were fairly common in the Virginia (coal deposits), and even supported a coal mining industry for a while. They indicate that the paleoclimate was humid during this time, with abundant vegetation growing in the swamps and along the river edges. By the end of the rifting event, Virginia may have looked more like Nevada: dry and barren with only scattered scrub vegetation, as evaporite deposits accumulated in the basins. The evidence for this transition is discussed here:  Paleoclimates.

~Igneous Intrusions at the End of the Basin History~
 During the later stages of basin history, the Triassic and lower Jurassic sedimentary rocks were invaded by igneous magmas and lava flows. This picture of the Palisades (right) sill along the Hudson River across from New York City is an especially large intrusion, originally formed far below ground, but now exposed by erosion. Related, but smaller, intrusions are found throughout the Mid-Atlantic. 
The larger intrusions metamorphosed the surrounding sediments, baking them into red hornfels. The red crushed stone used in central Virginia comes from the Triassic sediments in the Culpepper region baked by a nearby igneous intrusion. Episodes of igneous invasion accompanied the initiation of Atlantic oceanic lithosphere formation as the axial rift, now buried in the subsurface off the modern coast, sheared and split, and from this point on the Atlantic ocean basin opened quickly. As opening progressed, the new continental margin cooled and sank below sea level, which led to the modern divergent continental margin (Coastal Plain); the Next Stage
Contributed by Lynn Fichter 
Friday, August 24, 2018
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