Synclinal-horst basins: examples from the southern Rio Grande rift and southern transition zone of southwestern New Mexico, USA
In areas of broadly distributed extensional strain, the back-tilted edges of a wider than normal horst block may create a synclinal-horst basin. Three Neogene synclinal-horst basins are described from the southern Rio Grande rift and southern Transition Zone of southwestern New Mexico, USA. The late Miocene-Quaternary Uvas Valley basin developed between two fault blocks that dip 6-81 toward one another. Containing a maximum of 200m of sediment, the Uvas Valley basin has a nearly symmetrical distribution of sediment thickness and appears to have been hydrologically closed throughout its history. The Miocene Gila Wilderness synclinal-horst basin is bordered on three sides by gently tilted (101, 151, 201) fault blocks. Despite evidence of an axial drainage that may have exited the northern edge of the basin, 200-300m of sediment accumulated in the basin, probably as a result of high sediment yields from the large, high-relief catchments. The Jornada del Muerto synclinal-horst basin is positioned between the east-tilted Caballo and west-tilted San Andres fault blocks. Despite uplift and probable tilting of the adjacent fault blocks in the latest Oligocene and Miocene time, sediment was transported off the horst and deposited in an adjacent basin to the south. Sediment only began to accumulate in the Jornada del Muerto basin in Pliocene and Quaternary time, when an east-dipping normal fault along the axis of the syncline created a small half graben. Overall, synclinal-horst basins are rare, because horsts wide enough to develop broad synclines are uncommon in extensional terrains. Synclinal-horst basins may bemost common along the margins of extensional terrains, where thicker, colder crust results in wider fault spacing.