FAULTS OF GREAT BASIN RANGES 



03 



librium after it had been disturbed. It deserves more consideration than it 

 has received. The case of compression was first stated : 



"In some regions, such as the Appalachian, overthrusts and folds testify to 

 great reduction in the horizontal extent of rocks at the surface, the reduction 

 having been accomplished in a small fraction of geologic time. If the sub- 

 jacent portion of the nucleus had been correspondingly forced into narrower 

 space, there would have resulted an enormous mountain range, but the actual 

 uprising was of moderate amount. Plausible explanations of the phenomena 

 necessarily include horizontal movements of the upper rocks without corre- 

 sponding movements of the nucleus, and thereby imply mobility of an inter- 

 vening layer." 



The case of extension is then taken up : 



"In certain block-mountain districts of the West the master faults are anti- 

 thetic in type to the overthrust and demonstrate pronounced extension of the 

 upper part of the crust. The nucleal tract beneath could not share in this 

 extension without creating an enormous depression, which does not exist ; and 

 the interpretation of the phenomena involves horizontal shear in material more 

 mobile than the visible upper rocks." 2 



Figure 1. — An Overthrust, O, leaving a Cavity, G, behind it 



Figure 2. — An Overthrust, O, emerging from beneath the Crust, AB, tohieh is displaced 



and extended by "XJnderdrag" 



If this interpretation be correct, we may be led to regard the area of the 

 Basin ranges as filling a long-felt want in representing the "other side of an 

 overthrust," and as therefore showing the effects of what may be called, for 

 want of any other name, an "underdrag." This may be hypothetically if not 

 fantastically illustrated by figures 1 and 2. On the right, VV is a passive part 

 of the crust that has been covered by an overthrust, O. Let the overthrust 

 mass be followed back toward its source. It should presumably be found to 

 rest on a slanting undersurface, MN, on which the advancing ascent of the 

 overthrust for one or two score miles, as represented by long arrows, took 

 place. As to the upper surface of the overthrust, had its backward extension 

 not dipped underground, it must have somewhere left a vast gap, G, figure 1. 

 where it withdrew from that part of the crust which was not involved in the 

 overthrusting ; and as no such gap is anywhere known, it appears advisable 

 to change the supposition that the backward extension of the upper surface 

 of the overthrust did not dip underground into the opposite supposition that 

 it did dip underground, as in figure 2. Thus interpreted, the advancing, over- 



2 Interpretation of anomalies of gravity 

 1913, p. 35. 



IT. S. Geol. Survey, Professional Paper 85-C, 



