Axial–transverse fluvial interactions in half‐graben: Plio‐Pleistocene Palomas Basin, southern Rio Grande Rift, New Mexico, USA

Accurate magnetostratigraphic dating of Plio‐Pleistocene alluvium in the Palomas half‐graben permits correlation of transverse and axial deposits, thus enabling analysis of the movement of alluvial facies belts in time and space for the first time. Northern areas show evidence for basinward progradation of footwall‐sourced Matuyama‐age alluvial fan deposits over axial channel belt deposits of the ancestral Rio Grande, despite both deposits having similar deposition rates. This gradual ‘forced’ westward migration of the axial belt was in opposition to ongoing eastward growth of hangingwall‐sourced fans and tectonic tilt imposed by the bounding Caballo normal fault. Fan growth was coincident with a recently proposed gradual climatic shift that may have increased sediment flux out of transverse catchments. It is also possible that continuing tectonic footwall uplift and divided retreat caused catchment areas to increase, contributing to these trends.

Southern areas of the Palomas half‐graben feature late Gilbert/early Gauss deposits indicative of rapid westwards progradation of large low‐gradient, footwall‐sourced fans over axial deposits. This ‘forced’ migration of the ancestral Rio Grande may have occurred due to footwall catchment and fan growth consequent upon initiation and growth of the Red Hills Fault. Subsequent eastward movement of the axial channel belt in late Gauss and Matuyama times overwhelmed these large fans. We attribute this to continued tilting on the Red Hills Fault and to development of the Jornada Fault to the south‐east, the axial river belt avulsing north and eastwards through a developing Red Hills/Jornada crossover transfer zone.

We conclude generally that facies architecture of axial and transverse elements in half‐graben must reflect both climatic influences and the effects of fault development. Careful field mapping, accurate dating and palaeoclimatic studies are all necessary to determine the relative importance of these controls. Although adequate as broad guides, previous purely ‘fixist’ tectonosedimentary models allow for no fault growth, decay or climatic modulation of facies trends and are thus generally inadequate to explain important aspects rift basin stratigraphy.