Origin of “Reverse Drag” on the Downthrown Side of Normal Faults

Abstract

“Reverse drag,” also called “downbending” or “turnover,” was first recognized by Powell in the Colorado Plateau and subsequently was found to be an important structure associated with “down-to-basin” faulting in the Gulf Coast region. Many conflicting hypotheses have been proposed to explain this structure, most of which are based upon subsurface studies or observations in local areas. Detailed field studies of reverse drag made in the western Colorado Plateau reveal that the flexure extends practically the entire length of most normal faults in the region. It is characterized by a broad, asymmetrical arc on the downthrown block, approximately 1 mile wide, with maximum dips of more than 30 degrees near the fault plane. Normal drag is common adjacent to the fault on both the upthrown and downthrown blocks. Reverse drag has been formed repeatedly during recurrent movement along the Hurricane and Grand Wash faults, clearly indicating that it is genetically related to faulting. The magnitude of the flexure is roughly proportional to displacement, and the trend of the fold closely parallels the trend of the fault. In many places reverse drag passes both vertically and laterally into antithetic faults. Observations in the Grand Canyon reveal that the dip of the faults with which reverse drag is associated decreases with depth. It is concluded that reverse drag results from an alternate response to the same forces that produce antithetic faults and develops because of curvature of the fault plane at depth. Normal movement along a curved fault plane, in effect, tends to pull the blocks apart as well as to displace them vertically. Adjustments to fill the incipient gap by rupture produces antithetic faults, whereas failure by flexing develops reverse drag.