M3 - The Future


 
Past, Present, and Future 
 
~The human perspective~
 
It is difficult to think of geologic time in terms of a human lifetime because it is likely that nothing geologically significant enough to be recorded in the rock record will occur in the lifetime of a person. We are living in one geologically instantaneous moment of a still incomplete cycle which may require tens, or hundreds, or millions of years more to complete. Geologic changes occur so slowly that it is challenging to think that a location where we live today can someday be a submerged basin or an erupting volcano.
 
~Predicting the Future~
 
In the recent geological past, the east coast of North America underwent a rifting event (Stage L), and then became a stable, quiet divergent continental margin (Stage M). In the future, it is likely that nothing significant will change; that the present mountains will slowly erode down and the eastern coast will accumulate sediment. 
    
One of the most distinctive features of the geology of North America is it's pattern of cyclicity; the repetition of similar geologic events occurring at regular intervals. The geology of Virginia, extending back beyond 1.1 billion years, records the creation of an ocean basin (unnamed), and orogeny (Grenville), a rifting (Protoatlantic), two interim orogenies (Taconic and Acadian), a closing of the Protoatlantic (Alleghanian Orogeny), and another rifting event (Opening Of The Atlantic).
     
Repeated episodes of rifting and mountain building events are recorded around the world in even the oldest rocks on earth (3.96 billion years). Therefore, it is reasonable to surmise that cycles of rifting and mountain building will continue into the future. Geological activity will stop eventually because the Earth's internal heat driving the geological processes will cease to exist, but for perhaps the next 5 billion years, these cycles will continue.
    
Virginia's future is uncertain, however, by understanding that that nowhere in the world today exists an ocean basin much older than 200 million years (even though ocean basins must have existed since  4 billion ybp), one can  conclude that at some time in the future, the Atlantic ocean will  begin to close and disappear. The current conditions along the present-day Atlantic coast are dominated by a stable divergent continental margin of the opening half of a Wilson cycle that began with the Triassic rifting event. At some point in the future, the spreading ocean is going to reverse its direction, and the Wilson cycle will enter its closing phase. The closure of the Atlantic ocean will require a decoupling and the creation of a subduction zone to consume the Atlantic ocean crust. When this occurs, Europe/Africa, which are now drifting away from North and South America, will reverse directions and move back toward North and South America again. Predicting where and when the continents will come back together is impossible and there is no way of knowing where or when a decoupling will happen.
     
Below are included some feasible scenarios for the immediate future: 

Scenario One
 
A decoupling occurs under the edge of Europe and/or Africa. These regions then immediately experience volcanic (cordilleran) mountain building.  The eastern U.S. is quiet, except that in time the last remains of the Allegheny mountains erode down to a peneplain. 
     
A subduction zone on the eastern side of the Atlantic would consume oceanic crust, however, and the Atlantic ocean basin will shrink as it subducts. Finally Africa and/or Europe, with its cordilleran mountain building, will collide with the east coast, override its edge, and produces a major mountain range perhaps the size of the Himalayas. With this, Virginia's rocks will get shuffled, again.. This scenario is likely to produce a major Thrust Faulted System superposed over the present thrust faults system created during the Alleghenian orogeny.

Scenario Two
 
A decoupling occurs under the edge of North America. This results immediately in volcanic (cordilleran) mountain building in Virginia - which would resemble the Cascade volcanic mountains in Washington, Oregon, and northern California. These volcanoes could be active for perhaps 150-200 million years, the time required for the Atlantic ocean to close. During this time vast batholiths of Diorite and Granite would penetrate throughout the rocks of Virginia pushing them aside and metamorphosing them. The coast line will also change dramatically. The broad Coastal Plain and continental shelf would disappear, replaced by mountains rising abruptly out of the sea, and a deep trench would form just off the coast.
     
Finally, eastern North America will collide with Africa (or Europe), ride up over its edge, build Himalayan size mountains, and assemble another supercontinent. In this case Africa (or Europe) would be the foreland, and experience the major, Alleghanian-type Thrust Faulting
     
Also in this scenario, since North America and the Mid-Atlantic region would be the hinterland most of the Mid-Atlantic and Virginia would be destroyed in the ensuing erosion and subsequently spread across the foreland continent, just as parts of Africa in the Alleghanian orogeny were eroded and spread across the North American continent. There is no event in Virginia's known past comparable to this.

Scenario Three
 
A decoupling occurs within the ocean basin, creating a volcanic island arc. Now, the subduction zone could dip to the west, in which case the remnant ocean basin would lie to the east. In time the volcanic arc would collide with Europe or Africa and create a mountain. In this scenario Virginia does not change, at least for a long while. 
   
The subduction could dip east (as in the cross section above) in which case the remnant ocean basin would lie on the east between the volcanic arc and Virginia. In time the volcanic arc will collide with Virginia producing a Swiss Alps-type mountain range along its edge, and a foreland basin in the Blue Ridge, Shenandoah valley region. This would be similar to the Taconic and Acadian orogenies. 

These three simple scenarios illustrate possibilities for Virginia's future. In many respects the situation we are in now is similar to Virginia During The Cambrian
 
Contributed by Lynn Fichter 
Tuesday, August 19, 2014
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