shortened by 300 km or more. Strain in the crust is accommodated by both
brittle and ductile mechanisms. The brittle upper crust can be compressed
and thickened by displacements on a series of thrust faults that form a thrust
belt such as that illustrated in Figure 1–49; each of the upthrust blocks
forms a mountain range. Sedimentary basins often form over the downthrust
blocks. In the Wyoming thrust belt these sedimentary basins are the sites of
major oil fields. Crustal thickening and shortening resulting from thrusting
during the collision of India and Asia are illustrated in Figure 1–48. In
some cases the entire brittle part of the continental crust is thrust over
the adjacent continental crust as a thrust sheet. Evidence indicates that a
thrust sheet in the southern Appalachian Mountains extends over hundreds
of kilometers. This structure is associated with the continental collision that
occurred when the proto-Atlantic ocean closed at about 250 Ma.
The crust can also be compressed by ductile deformation, one result of
which is folding. The convex upward or top of a fold is known as an anticline,
and the concave upward or bottom of a fold is known as a syncline. On a large
scale these are known as anticlinoria and synclinoria. Folding is illustrated
in Figure 1–50. When a region of large-scale folding is eroded, the easily
eroded strata form valleys, and the resistant stata form ridges. This type
of valley and ridge topography is found in Pennsylvania and West Virginia
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