Drainage integration in extensional tectonic settings

The development of geomorphic theory regarding fluvial-system reorganization and drainage basin evolution, resulting from drainage integration, has been slow to progress since the abandonment of Davisian geomorphology in the mid-twentieth century. Central to the development of this theory is an understanding of the processes that allow rivers to cut across topographic and/or structural barriers that separate neighboring drainage basins. Barrier-crossing rivers are termed transverse drainages. Development of this geomorphic theory also includes an understanding of the basin-wide geomorphic and sedimentologic response to the establishment of a transverse drainage. Since at least the eighteenth century, geomorphic scholars such as J. Hutton, J. Playfair, J.W. Powell, G.K. Gilbert, C.E. Dutton, W.M. Davis, E. Blackwelder, C.B Hunt, and T.M. Oberlander described transverse drainages using a variety of terms like: water gaps, transverse valleys, transverse gorges, transverse river gorges, drainage anomalies, transverse trunk valleys and boxes. A resurgence of drainage-integration research in the past few decades produced a consensus that four generalized processes, and variations therein, result in drainage integration and transverse drainage establishment: Antecedence; Superimposition; Piracy/Capture; and Overflow/Spillover. Antecedence occurs when a river maintains its position through sufficient erosive power during tectonic uplift. This results in a river that has cut through the uplifted terrain. Superimposition occurs when a river incises through erodible materials, or a cover mass, and becomes locked in place across an exhumed bedrock high. Both Antecedence and Superimposition require a river that is older than the most recent exposure of the topographic and/or structural feature it now cuts through. Piracy, or Capture, occurs when a river shifts to a new and steeper gradient path, resulting from the capture and rerouting of the original drainage. Overflow, or Spillover, takes place when a basin fills up with sediment and water sufficiently to breach the lowest point in the basin divide and subsequently spills out into a neighboring drainage basin or outlet. Both Piracy and Overflow require the river to be younger than the topographic and/or structural feature the river transverses. The timing of this special issue's publication aligns with a resurgence in interest and awareness of the importance of transverse drainages in economics, cultural history, establishment of surface and groundwater resources, distribution of aquatic and riparian biology and ecosystems, and even in understanding the history of Martian landscapes and climate. Therefore, we put forth this special issue to elucidate on our current understanding of drainage integration and the establishment of transverse drainages along with new insights into basin to basin and basin-wide response to transverse-drainage development. This special issue includes comprehensive literature reviews and original research in tectonic settings of regional extension where through-flowing transverse drainages exist, but also suggests that similar integration processes occur in a variety of settings globally.