Earth and Planetary Sciences ETDs

Publication Date

Fall 9-21-2016

Abstract

The Rio Grande-Rio Chama (RG-RC) fluvial system has evolved dramatically over the last 8 Ma, undergoing channel migrations, drainage capture and integration events, volcanic damming, and carving and refilling of paleocanyons. Volcanism concurrent with the development of the river system provides a unique opportunity to apply multiple geochronometers to the study of its incision and drainage evolution. This paper reports 19 new 40Ar/39Ar basalt ages and 19 detrital mineral samples (zircon and sanidine) collected from RG-RC alluvium overlain by dated basalt flows in the context of a compilation of published 40Ar/39Ar basalt ages. The “run-out” geometry of 4.8 Ma basalts at Black Mesa suggests that the course of the northern Rio Grande connecting the San Luis to Española basins was established by then. Detrital zircon age spectra for ancestral Rio Grande alluvium underlying these basalt flows contain 10-12% of 37-27 Ma grains suggesting that the ~5 Ma Rio Grande had its headwaters in the San Juan Mountains. The 5-3 Ma accumulation of basalt flows on the Taos Plateau was accompanied by inset relationships downstream (near Black Mesa) documenting the v existence of a developing 5-2.5 Ma Rio Grande valley that provided downstream discharge. Coincident timing and pre-volcanic knickpoints suggest that surface uplift associated with the construction of the Taos Plateau volcanic field drove downward integration to the Palomas basin by 4.5 Ma. Changes in ancestral Rio Grande sediment provenance from 2.6 Ma to 1.6 Ma document a northward shift of the RG-RC confluence and indicate that surface uplift of the Jemez Mountains (Valles Caldera) likely drove further downstream integration. The Taos Plateau volcanic field reduced through-flowing surface drainage from the San Juan Mountains relative to the Sangre de Cristo Mountains until the ~0.69-0.44 Ma spillover of Lake Alamosa, but we view this event as a re integration, not initial integration, of upper Rio Grande drainage. Progressive downward integration of rift-aligned basins from 8 to 1 Ma was likely facilitated by waning rift extension that allowed aggradation to exceed subsidence. Downward integration events crudely mimic climate change “events” at 6 Ma (onset of the southwestern monsoon) and 2.5 Ma (global change toward glacial-interglacial climate). Magmatic influences include the 6-2.5 Ma building of the Taos Plateau volcanic field, Jemez Mountain caldera eruptions at 1.6 and 1.25 Ma, along with continued magmatism that developed constructional topography and Jemez lineament volcanism that may have been associated with mantle-driven epeirogenic uplift across northern New Mexico. Integration of the RG-RC system to the Gulf of Mexico by 0.8 Ma was facilitated by headwater uplift as well as increased frequency of ~100 ka high-amplitude glacial-interglacial cycles that contributed to higher discharge and bedrock incision rates during the Pleistocene. We conclude that magmatic and tectonic drivers dominated over the last ~8 Ma, but were amplified by climate change events to determine the fluvial evolution of the RG-RC system.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Karl Karlstrom

Second Committee Member

Laura Crossey

Third Committee Member

Matthew Heizler

Project Sponsors

Geological Society of America; ExxonMobil; New Mexico Geological Society

Language

English

Keywords

Rivers, geochronology, geomorphology, dynamic topography, sediment provenance

Document Type

Thesis

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