Earth and Planetary Sciences ETDs

Publication Date

Fall 11-15-2021

Abstract

This study addresses the evolution of the San Juan River system and its confluence with the Colorado River, ~ 100 km above the regionally important Lees Ferry knickzone. The San Juan River is a 600-km-long continental-scale tributary of the Colorado River. From its headwaters in the San Juan Mountains in Colorado, the San Juan River flows across the Colorado Plateau, and into the Colorado River upstream of Grand Canyon. Published apatite fission track and apatite (U-Th)/He thermochronologic data show that rocks in Marble Canyon, as well as in middle and upper reaches of the San Juan River, were >75 °C and hence buried by 1.5-2 km of overlying strata until rapid cooling after ~5 Ma. This ~5 Ma age marks the birth of the San Juan system and its integration with the Colorado River as we know it. The longitudinal profile of the San Juan River is generally concave-up, with no major knickpoints. This apparent “equilibrium” river profile contrasts with the Colorado and Little Colorado rivers that have major bedrock-influenced knickzones coincident with the Kaibab Limestone. This paper evaluates incision rates through time across the Lees Ferry knickzone and in the lower, middle, and upper reaches of the San Juan River to evaluate alternative models for river evolution. Incision rates are constrained using new 40Ar/39Ar detrital sanidine (DS) dating of river terraces. We dated over 5000 grains, from 27 terraces, yielding ~ 2% young (< 2 Ma) grains that give important new maximum depositional ages (MDAs) and minimum incision rates for 18 of the terraces. The most common youngest grains were ca. 0.63, 1.2, and 1.6 Ma and, in most reaches, the terraces containing 0.63 Ma grains are ~half as high above the river as the terraces containing 1.2 Ma grains suggesting steady average incision over the past 1.2 Ma. However, age-correlative terraces are very different heights and yield different incision rates reach to reach: 143-160 m/Ma for eastern Grand Canyon and Marble Canyon, 219-270 m/Ma along the Colorado River in Glen Canyon above Lees Ferry knickzone, 97-128 m/Ma in the San Juan River as it crosses the Colorado Plateau, and up to 237 m/Ma in the San Juan Mountain headwaters. Steady incision argues against punctuated climatic events or transient knickpoint migration in the past 1.2 Ma and suggests differential uplift. Adding to the previously proposed ~150 m/Ma surface uplift of eastern Grand Canyon relative to the Gulf of California based on downstream differential incision, our differential incision magnitudes imply an additional 100 m/Ma uplift of paleoriver profiles across the broad, ~200-km-wide, Lees Ferry knickzone and ~100 m/Ma uplift of the San Juan Mountains headwaters relative to the Colorado Plateau. If incision has been steady over 5 Ma, with thermochronologic data inferring more rapid exhumation in the 5-2 Ma timeframe after integration, this suggests that the Lees Ferry knickzone region is the edge of the uplifting southwestern Colorado Plateau region that has given rise to Grand Canyon, and that the San Juan Mountains region has simultaneously been uplifted ~ 500 m relative to the Colorado Plateau over the past 5 Ma. The rate changes are spatially associated with sharp mantle velocity transitions and we infer that they reflect mantle-driven differential epeirogenic uplift.

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

Fourth Committee Member

Mary Gillam

Language

English

Keywords

Incision, Colorado Plateau, Geochronology, Tectonics

Document Type

Thesis

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