Mantle-driven differential uplift has been proposed to contribute to dynamic typography in the western United States. Neogene and ongoing mantle flow is postulated to be driven by upper mantle convection and to cause subtle broad-scale differential uplift that affects surface topography. This study focuses on the possible connection between mantle convection and surface topography along the Jemez lineament in New Mexico, and is motivated by tomographic images from the EarthScope experiment that show this zone is underlain by low-velocity mantle that potentially could drive uplift.
To test this possible connection, we constructed a longitudinal profile of the Rio San Jose that crosses the lineament at high angles, and use dated elevated river terraces to construct paleoprofiles through time. Our rationale is that rivers are sensitive gauges of changing landscapes and differential uplift. ⁴⁰Ar/³⁹Ar dating of basalts that overlie river gravels at sixteen locations provides a method of evaluating denudation rates and tracking landscape evolution in this region.
The average long-term incision rate of the Rio San Jose has been ~105 m/Ma over the last ~4 Ma but the study found that the incision history of the Rio San Jose has varied significantly spatially as well as temporally. Incision rates of ~177 m/Ma are calculated from ~3.7 to ~2.6 Ma basalts with fast rates likely reflecting the construction of Mount Taylor. From ~2.6 to today, average incision rates have been ~77 m/Ma.
The modern longitudinal profile shows several features that deviate from an equilibrium concave–up shape. First, the profile has convex central reaches, including bowing of the 348 ± 5 ka Laguna Pueblo flow. Second, seismic and drill core data indicate the presence of a graben in the El Malpais valley with sediment fill of ~200 m in the downthrown block. Within this graben, a drilled well core with three buried basalt flows were sampled ranging from 20, 41, and 61 m depth, with the oldest flow yielding an age of 340 ± 90 ka. Third, the boundary zone between the Colorado Plateau and Rio Grande rift is characterized by a migrating knickpoint that shows differential incision rates from 0 to 217 m/Ma since eruption of the 198 ± 5 ka Suwanee flow. This flow preserves a paleo-knickpoint downstream of the modern one, suggesting upstream migration of a transient knickpoint at a rate of ~20 km/Ma.
We suggest that mantle-driven uplift along the Jemez lineament may explain these observations as follows: (1) the El Malpais valley graben parallels the Jemez lineament and intersects the Rio San Jose in its central, convex, reaches, and the graben is interpreted to indicate extension due to upwarping above the Jemez lineament, and (2) the differential incision along the Suwanee flow is a migrating knickpoint and may be a result of a combination of base-level fall and headwater uplift broadly associated with the mantle anomaly.