Mountain ranges in the southern Rocky Mountains, first uplifted during the early Cenozoic Laramide orogeny, have followed separate landscape evolutionary pathways in the late Cenozoic. We present a model that reconstructs the post-Laramide tectonic and geomorphic history of Sierra Nacimiento and the Taos Range, two nearly adjacent rift-flank ranges in north-central New Mexico that serve to illustrate the various processes shaping landscapes across the southern Rocky Mountains. The Sierra Nacimiento landscape reflects the exhumation of hard Precambrian rocks from beneath a softer Phanerozoic sedimentary cover. The exhumation is continuous, but not steady, being driven by distal base-level fall. Downstream diverging river terraces in the Jemez River valley on the eastern flank of Sierra Nacimiento and late Pliocene to Holocene fluvial deposits on the western Sierra Nacimiento piedmont document the base-level fall. The timing and contemporary rates of incision from these river systems suggest that exhumation is being propagated from south to north as knickzones work their way headward from the Rio Grande. In contrast, the Taos Range landscape reflects alternating active stream incision and aggradation astride, and throttled by, an active range-front normal fault. The distinction between the exhumation-dominated and tectonic-dominated mountain front is best quantified by analyses of first-order stream gradients and a watershed metric we call the drainage basin volume to drainage basin area ratio (R va). Gradients of first-order streams in the exhumation-dominated Sierra Nacimiento have a mode of 6.8 degrees, significantly less than the 17.7 degrees obtained from a comparable data set of Taos Range first-order streams. The distinct stream gradient and R va populations hint at an important change in the processes shaping hillslopes and low-order channels, which is supported by the lack of slope-clearing landslides in the Sierra Nacimiento landscape and the presence of such landslides in the Taos Range. Analogue and numeric models find that steep, rugged, faceted topography associated with tectonically active mountain fronts like the Taos Range can only be produced and maintained by creep and landslides where the sediment flux scales as a power law with respect to average hillslope or low-order channel gradient. Here, the fingerprint of active tectonics is recorded by both high R va values and steep modal channel gradients. By comparison, the Sierra Nacimiento landscape is shaped primarily by creep where the sediment flux has a linear relationship to average hillslope and low-order channel gradient. In this situation, the signatures of distal base-level fall are low R va values and relatively gentle modal channel gradients.

Two Oligocene conglomeratic units, one primarily nonvolcaniclastic and the other volcaniclastic, are preserved on the west side of the Jemez Mountains beneath the 14 Ma to 40 ka lavas and tuffs of the Jemez Mountains volcanic field. Thickness changes in these conglomeratic units across major normal fault zones, particularly in the southwestern Jemez Mountains, suggest that the western margin of the Rio Grande rift was active in this area during Oligocene time. Furthermore, soft-sediment deformation and stratal thickening in the overlying Abiquiu Formation adjacent to the western boundary faults are indicative of syndepositional normal-fault activity during late Oligocene–early Miocene time. The primarily nonvolcaniclastic Oligocene conglomerate, which was derived from erosion of Proterozoic basement-cored Laramide highlands, is exposed in the northwestern Jemez Mountains, southern Tusas Mountains, and northern Sierra Nacimiento. This conglomerate, formerly called, in part, the lower member of the Abiquiu Formation, is herein assigned to the Ritito Conglomerate in the Jemez Mountains and Sierra Nacimiento. The clast content of the Ritito Conglomerate varies systematically from northeast to southwest, ranging from Proterozoic basement clasts with a few Cenozoic volcanic pebbles, to purely Proterozoic clasts, to a mix of Proterozoic basement and Paleozoic limestone clasts. Paleocurrent directions indicate flow mainly to the south. A stratigraphically equivalent volcaniclastic conglomerate is present along the Jemez fault zone in the southwestern Jemez Mountains. Here, thickness variations, paleocurrent indicators, and grain-size trends suggest north-directed flow, opposite that of the Ritito Conglomerate, implying the existence of a previously unrecognized Oligocene volcanic center buried beneath the northern Albuquerque Basin. We propose the name Gilman Conglomerate for this deposit. The distinct clast composition and restricted geographic nature of each conglomerate suggests the presence of two separate fluvial systems, one flowing south and the other flowing north, separated by a west-striking topographic barrier in the vicinity of Fenton Hill and the East Fork Jemez River in the western Jemez Mountains during Oligocene time. In contrast, the Upper Oligocene–Lower Miocene Abiquiu Formation overtopped this barrier and was deposited as far south as the southern Jemez Mountains. The Abiquiu Formation, which is derived mainly from the Latir volcanic field, commonly contains clasts of dacite lava and Amalia Tuff in the northern and southeastern Jemez Mountains, but conglomerates are rare in the southwestern Jemez Mountains.