30 PROCEEDINGS OF THE CHICAGO MEETING 



EVOLUTION OF A/iCUATE MOUNTAINS 

 BY W. H. HOBBS 



{Ahstract) 



Following Suess, it has been a general practice to regard the folds within 

 mountain arcs as overthrust outward from within the arc. In 1912 the w^riter, 

 in a series of three papers which appeared in the Journal of Geology, showed 

 that the mechanics of the folding process in rocks requires that the folds be 

 underthrust from without inward, and that this is borne out by the character 

 of the deformation within the folded strata. 



Since the arcs are convex toward the oceans, this requires that the active 

 force which produced the folding be exerted from the sea toward the land^ 

 not from the land area toward the surrounding seas. The study is here ex- 

 tended to a consideration of the relationship of each mountain range to the 

 sea on the border of which it was pushed up. Many of the serious difficulties 

 which Suess encountered in attempting to fit his theory to observed facts 

 when regarding the seaward slide of the land-masses now disapijear com- 

 pletely. The problem is further greatly simplified for the reason that we can 

 delimit the areas covered by former seas through mapping their deposits, 

 whereas former land-masses have often left little trace of their existence 

 beyond the mass of their migrated deposits. 



Presented without mamiscript. 



Discussion 



Prof. A. C. Lawson : I am glad that Mr. Hobbs is pursuing his studies of 

 the origin of arcuate mountains. I am inclined to agree with him that under- 

 thrusting is far more common than overthrusting. The important thing for 

 geology, from a general point of view, is the cause of this underthrusting or 

 the force which operates to produce it. According to Mr. Hobbs, this cause is 

 to be found, in the case of the arcuate mountains of Asia, in the collapse of 

 the Pacific arc and the consequent thrust from the deepening oceanic basin 

 toward the continent. I do not believe that this is the cause. Even if the 

 Pacific arc were to collapse, as suggested, the force could not be propagated 

 for such long distances. The crust would be sheared infinitely before the 

 effect of the thrust could be felt in Asia. We must recognize another force 

 applied to the under side of the crust, a subcrustal current which carries the 

 crust with it, creating strains which find expression in the arcuate mountains. 

 What, then, is the cause of this subcrustal current? It is to be found in the 

 application of the principle of isostasy. If a continental region is subjected 

 to erosion, the load is transferred seaward. Compensation is ellected by a 

 subcrustal flow from the region of loading to the region of degradation. This 

 current carries the crust with it, and the newly deposited sediments are folded 

 when they are crowded against the passive region toward which the current 

 is flowing. This process is repeated from age to age, giving us the sequence of 

 arcuate mountains from north to south, growing seaward by underthrust from 

 the early Paleozoic to the late Tertiary. 



