High Elevation Synclines?

Words by L. Whalen

Models by P. Prince

How do you end up with synclines – troughs or U-shaped folds – as the highest parts of a mountain belt? Synclines are easier to imagine as valleys – not ridges.


image: wikipedia.org

The answer has to do with one of the strongest rock units in the Appalachians – the Tuscarora sandstone. Found in Virginia, West Virginia, Maryland and Pennsylvania, this unit forms many of the summits of the Appalachian range.


The view from McAfee Knob in Virginia, a prominent ridge of Tuscarora sandstone. Photo: L.Whalen

In this 4 part series we explore how rock strength has affected the topography of the Appalachian chain.


Basin Inversion

Text by L. Whalen

Models by P. Prince

In this video continental crust is first pulled apart during extension and then tectonic forces change direction and the crust is put under compression. What results is first a basin and then an inverted basin.

Relative timing of CAMP, rifting, continental breakup, and basin

Figure of basins along the East coast of North America from Schlische and Withjack (2003)

Similar structures can be found all along the east coast of North America. These rift basins formed during the breakup of Pangea ~200 Ma. The basins later experienced compression when the North American plate transitioned from rifting to seafloor spreading.


Strike-slip deformation with erosion and cross sections


In this model we show how different subsequent types of faults form in relationship to a strike-slip fault like the San Andreas fault.

Images: geology.com and wikipedia.org

First to form are Riedel faults (R faults). The first set forms at ~15 degree angles to the main strike-slip fault. In the video these faults break up the main anticline into smaller anticlines. The second set of faults (R’ – not shown explicitly in the video) form at ~75 degree angles and link the R faults together.


Image: wikipedia.org

If you examine the R faults in cross-section you see they are made up of lots of smaller little faults that either have a normal or reverse sense of motion. If the smaller faults are normal then they form “negative” flower structures, but if they’re reverse they form “positive” flower structures.


image: wikipedia.org