ELUCIDATION OF THE ROLE OF EXTENSION IN THE GENERATION OF DEEP DOWNWARPS IN CONTINENTAL CRUST

Abstract

The consolidated crust beneath many deep downwarps on the continents is much thinner that elsewhere. Most such downwarps contain normal faults, indicating extension of the crust during subsidence. The question arises, whether the extension in each downwarp was in itself sufficient to produce the observed crustal thinning and subsidence Most investigators believe that most of the deep downwarps in continental crust were formed by extension. An analysis of the structure of a large number of major downwarps, both present-day and ancient, has revealed, however, that only a small fraction of these underwent strong extension. Downwarps in which extension had occurred exhibited two main types of deformation. In the first case (synthetic extension) the basement in the blocks bounded by normal faults is horizontal or slopes toward the axis of the downwarp, as do the normal faults themselves. In the second case the planes of the normal faults in the basement dip in the opposite directions, i.e., antithetical extension. The paper considers both types, and presents an example analyses of the Fenwei grahen in central China.