90 STEPHEN TABER 



region. These arcs cut directly across the earher tectonic Hnes and 

 are everywhere marked by extremely precipitous slopes. 



The evidence outlined in the following pages indicates (i) that 

 the east-west arcs delineating the major relief features of the Greater 

 Antilles are zones of normal faulting developed in late geologic 

 time; (2) that this faulting has resulted in the formation of the 

 great troughs of the region; and (3) that the displacements are 

 continuing at the present time. 



CRITERIA OF FAULTING 



The fault-zone hypothesis rests partly on geologic evidence, but 

 largely, since most of the region is under water, on evidence that 

 is either topographic or seismologic. Topographic evidence of 

 faulting is unusually well preserved because the fault scarps are 

 mostly below sea-level where they have been protected from erosion. 

 The characteristics of these scarps which indicate an origin through 

 faulting are (i) great height; (2) extremely precipitous slopes; 

 (3) abrupt changes in slope at top and bottom; (4) rectihnear 

 course for long distances; and (5) subordinate depressions and 

 elevations (troughs and horsts) near the base of the main 

 scarps. 



The last-mentioned features develop within fault zones as a result 

 of the differential displacement of long narrow blocks or wedges 

 formed by the branching and intersection of nearly parallel faults. 

 The formation of these subordinate troughs and horsts is a common 

 accompaniment of normal faulting on a large scale and especially 

 of trough faulting. On land these minor topographic effects of 

 faulting are soon obhterated by erosion and by the accumulation 

 of rock-waste in the troughs; therefore the significance of these 

 criteria in the recognition and interpretation of faulting has been 

 generally overlooked. Because of their short life these topographic 

 features are seldom found on land except in arid regions and where 

 very recent displacements have occurred along old fault zones. 

 Beautiful examples of this type of fault topography on a small 

 scale have developed along the Wasatch fault zone in Utah as 

 a result of post-Pleistocene displacements. Similar topographic 

 effects were produced in California by the faulting that accompanied 



