Analysis of drainage-basin symmetry as a rapid technique to identify areas of possible Quaternary tilt-block tectonics: An example from the Mississippi Embayment
There is a coincidence of seismicity and major river valleys in the southwestern Mississippi Embayment that suggests control of the drainage pattern by northwest-trending faults. Pleistocene river terraces reveal a strong southwestward preferred river migration in this region that may be a result of fault-block tilting. A rapid new technique of drainage-basin analysis is herein presented that quantifies transverse topographic basin symmetry. These data permit discrimination of random stream migrations and regionally preferred stream migrations and identify the direction of maximum migration. If bedrock dip can be shown to be a negligible influence on stream migration, then the direction of regionally preferred migration implies a period of ground tilting in that direction. Drainage-basin asymmetry vectors (bearing and degree of asymmetry) calculated from transverse topographic profiles of 271 4-km basin segments in the study area suggest preferred stream migration in response to west-southwest-ward ground tilting (bearing 259°). The eastward regional dip of bed-rock horizons only locally influences migration. River terrace distributions reveal a chronology of stream-migration episodes. Southwestward migration in the Ouachita, Saline, and Arkansas river basins occurred during early to middle Pleistocene time in the southwestern Mississippi Embayment. A Wisconsin/early Holocene southwestward migration episode is evidenced along southeast-flowing reaches of the Ouachita and Arkansas Rivers but not along the southeast-flowing reach of the intervening Saline River, suggesting that these rivers overlie separate northwest-trending tilt blocks. Although this technique does not provide direct evidence of ground tilting, it provides a quick-look method of identifying possible tilting elements in neotectonic regions using only topographic maps. This technique is of greatest utility where active faults are concealed or poorly exposed, such as in the Mississippi Embayment.