Here's an example of what JonF means by an overthrust. First you have a region of geological layers, I'll give each layer a unique character. We're looking at the layers edge on:
------------------------------------------
++++++++++++++++++++++++++++++++++++++++++
==========================================
||||||||||||||||||||||||| 4;||||||||||||||||
Now the region gets compressed from both ends because of the motion of the continents, and this causes it to sheer (break) along a diagonal line:
--------------------\---------------------
+++++++++++++++++++++\++++++++++++++++++++
Thrust ====> ======================\=================== <==== Thrust
|||||||||||||||||||||||\|||� 124;||||||||||||||
Continued pressure from both ends causes one to slip along the fault and rise above the other:
\-------------------------
\++++++++++++++++++++++++
\=======================
\||||||||||||||||||||||
--------------------\*********************
Thrust ====> +++++++++++++++++++++\^^^^^^^^^^^^^^^^^^^^ <==== Thrust
======================\###################
|||||||||||||||||||||||\~~~~~~~~~~~~~~~~~~
The compression and pushing from each end continues and the layers that have risen are gradually pushed on top of the other layers, giving this result:
------------------------------------------
++++++++++++++++++++++++++++++++++++++++++
==========================================
||||||||||||||||||||||||| 4;||||||||||||||||
------------------------------------------
Thrust ====> ++++++++++++++++++++++++++++++++++++++++++ <==== Thrust
==========================================
||||||||||||||||||||||||| 4;||||||||||||||||
Now the youngest layer ("-") exists both at the top and several layers further down where it is just below the oldest layer ("|"). In reality many more layers are often involved, and subsequent erosion and deposition can make the situation very complex.
Sometimes the sheer lines still exist and can be identified, sometimes not. Geologic processes like erosion and subduction destroy a lot of evidence.
Much more complex overthrusts are common. Imagine a rug being pushed together from each end. It will gradually fold and bunch up, and bunched-up and folded portions can be folded again and can bunch up over other bunched up portions.
--Percy
Edited by Percy, : Minor correction.
Edited by Percy, : Rendering error somehow crept in - that's weird!
™ Version 4.2
