Structural geology . drying of asediment may be in more or less regular sets. Relaxational settlingafter a period of compressive faulting or folding may develop nor-mal or steeply dipping joints and faults in intersecting sets. Thesystems in these cases are not likely to be uniform and yet for smallareas may have a considerable regularity of arrangement. Another explanation is that one of the vertical sets may be 40 STRUCTURAL GEOLOGY tensional and the intersecting set may be compressional, in themanner determined experimentally (see p. 16). REVERSE OR THRUST FAULTS Sections in the southern Ap
Structural geology . drying of asediment may be in more or less regular sets. Relaxational settlingafter a period of compressive faulting or folding may develop nor-mal or steeply dipping joints and faults in intersecting sets. Thesystems in these cases are not likely to be uniform and yet for smallareas may have a considerable regularity of arrangement. Another explanation is that one of the vertical sets may be 40 STRUCTURAL GEOLOGY tensional and the intersecting set may be compressional, in themanner determined experimentally (see p. 16). REVERSE OR THRUST FAULTS Sections in the southern Appalachian folios show thrust faultsassociated with overthrust folds. The fault planes may be in-ferred to have the relations to stress indicated on pp. 20-21, inrotational or shearing compressive strains. The inference isusually made that there is an overthrust, and therefore shortening,of so many feet. This is true in the plane of the section. It tellsus nothing of the movements inclined to the plane of the section,. Fig. 26. Overthrust faulting localized by tension fracture break thrust. AfterWillis. 1. Shows break in the massive limestone bed which determines theplane of the break thrust along which the displacement shown in 2 takes place. which may have been fully as great. The association of thrustfaults with overthrust folds usually indicates compression, but notso much compression as a two-dimensional cross section mightindicate. Consideration of many cross sections is the same in effectas considering the fault in three dimensions, and leads to closerestimates of actual shortening. An examination of the United States Geological Survey foliosbrings out this interesting fact that in the southern Appalachians83% of the thrust faults, as indicated on the cross sections, aredefinitely related to overthrust folds. Willis1 classifies them as(1) break thrusts where the thrust fault plane follows a previouslyformed tension fracture on the crest of the anticline; (2) shear orstret
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