NEOTECTONIC INFLUENCES ON THE EVOLUTION OF THE RIO GRANDE FLUVIAL SYSTEM OVER THE LAST 5 MA

The Rio Grande (RG) fluvial system preserves a record of southern Rocky Mountain erosion and sediment transport over the last 8 Ma, during which the river has undergone drainage capture and integration events, carving and refilling of paleocanyons, lake spill-overs, and reshaping of drainage divides. New ⁴⁰Ar/³⁹Ar basalt ages coupled with detrital zircon dating of fluvial sediments are helping to reconstruct and elucidate the processes that drove the dramatic evolution of the RG over the last 5 Ma. By evaluating interactions among tectonics, climate, and the RG fluvial system, this study addresses the ongoing debate over whether or not late Cenozoic mantle-driven surface uplift has occurred in the western U.S., and may benefit models for tectonic influences on fluvial systems in ancient orogens.

Detrital grain samples ranging in age from 8 to 0 Ma, as constrained by new ⁴⁰Ar/³⁹Ar ages for basalts overlying the deposits, were studied. Contrary to some models, detrital zircon age spectra for these samples suggest that the ancestral Rio Grande flowed through the Espanola Basin by 4.5 Ma and its sediment was sourced, at least in part, by the San Juan Mountains. Reconstructed paleoprofiles for the RG system show aggradation from 4.5 to 2.5 Ma, carving of km-deep paleocanyons until 1.6 Ma, subsequent filling, recarving, and refilling of paleocanyons by the Bandelier Tuff, which created short-lived lakes that influenced base level and created knickpoints in the RG profile. Surface uplift in the form of volcanic constructional topography re-shaped drainage divides from 2.6 to 1.2 Ma. Post-1 Ma RG terraces record bedrock incision at average rates of ~100 m/Ma, but offset of terraces and fault-slip measurements show that fault-dampened incision affected long profile development and caused spatially variable bedrock incision rates.

Temporal correlation of fluvial changes to magmatic events, and observed fault-dampened incision, implicate magmatic surface adjustment and fault tectonics as important influences on fluvial systems. Determining the relative roles of climatic changes, downstream base level fall, and upstream epeirogenic uplift to explain RG integration to central NM ~4.5 Ma and change to regional incision ~1 Ma requires improved temporal data, but current data favor mantle-driven surface uplift as a main driver.