When geologist Elizabeth Miller began mapping a fault system in Death Valley, she questioned the origin of some sedimentary rocks that were previously believed to have originated in the area. Now the analysis has revealed where they really came from: central Nevada, indicating that part of today’s Great Basin was the highest land in North America 40 to 20 million ago. years.
The journey of these sediments southward through river systems flowing into the ancient Pacific Ocean tells a story about Earth’s history and the much-discussed formation of the Sierra Nevada mountain range, the impressive backbone. of eastern California which encompasses three national parks: Sequoia, Kings Canyon and Yosemite.
The ancestral Sierra Nevada began as a volcanic chain over 100 million years ago, at a time when dinosaurs still roamed the Earth. New work published as chapters in a Special document of the Geological Society of America on paleogeography and topography of the western United States suggests that the mountains later “died” – meaning they were eclipsed by a vast plateau – in a scale volcanic eruption. the region about 40 to 20 million years ago. Then, they were “reborn” about 10 million years ago, rising to the panoramic heights we know today.
“40 to 20 million years ago, the highest points were in central Nevada. Then the basin and chain faults came in and shattered everything, and now the Sierra Nevada is the most westerly or the last of those major fault blocks, ”Miller said. “As a mountain range, it has had three completely different stories. “
Findings from Miller and Jens-Erik Lund Snee, who conducted the research while they were doctoral students at Stanford, suggest that the continental divide – which is generally seen as static – has experienced a gradual eastward shift. The rift, which separates watersheds that empty into the Pacific Ocean from those that drain east, remained in the ancestral Sierra Nevada in eastern California for tens of millions of years before moving into the central Nevada when volcanism that began 40 million years ago lifted the Earth. surface in a wave migrating south.
The articles describe how a surge in region-wide volcanic activity in southern Idaho, Nevada and Utah caused the interior plateau to rise above the ancestral Sierra. This upheaval of the Earth’s mantle and crust created whole new river systems, some of which carried sediment south, forming the layers Miller studied near Death Valley with co-author Mark Raffrey, a former student. diploma.
“The material from these volcanoes reached the Pacific side of the Sierra Nevada – this is how we know the area of central Nevada where the eruptions occurred was higher than everything else,” Miller said, noting that previous documents have mapped the ancient rivers that carried volcanic material. “Our work builds on this previous work in that we argue that the volcanism itself actually caused a large increase in topography because there was so much hot material coming from below the continent.”
For tens of millions of years after the rise of the plateau, the ancestral range of the Sierra was “simply the ramp from the high country of Nevada to the paleo-ocean in what is now the Central Valley,” said Lund Snee. It was also around this time that much of California’s famous gold was deposited in ancient rivers that flowed west from central Nevada to the Central Valley. Then, about 10 million years ago, the new Sierra Nevada emerged when the western United States was chiseled by basin and chain faults, which involved uplift and extension – a process that didn had very little to do with its previous history, according to Miller.
“There has been a lot of debate lately about when the Sierra Nevada became a mountain range, and our work suggests that the two dominant views are right – it’s old and also young for completely different tectonic reasons. “said Lund Snee, who is now a Mendenhall Research Fellow at the US Geological Survey.
When the ancestral Sierra Nevada first appeared over 100 million years ago, the mountains marked the edge of the North American continent, bordered by the Pacific Ocean to the west. East of this area, geologists have long believed that the earth’s crust thickens and becomes unstable, ultimately causing the continent to spread and the current basin and range topography to form.
But Miller and Lund Snee found that the region east of the ancestral Sierra was relatively low, supported by a thinner, more stable crust until the wave of volcanic eruptions 40 to 20 million ago. years has raised the plateau higher than the ancestral chain. The eruptions came from dozens of Yellowstone-type supervolcanic calderas in addition to hundreds of smaller volcanoes – an event that blanketed some areas with thousands of feet of lava.
The research paints a picture of the topographical evolution of the western United States, which has been the subject of debate since the region was first explored by geologists in the 1800s and flooded by researchers from gold seeking fortune in the western foothills of the Sierra Nevada. It also impacts our understanding of how plants and animals evolved and dispersed across the West; In order to understand migration, biologists need a clear understanding of how the landscape is changing.
The authors refined the geological maps and used radiometric dating of the minerals zircon and feldspar to assess the timing of eruptions and changes in topography. They also revised the ages of previous estimates of elevation and climate from stable isotope analyzes of calcite in sediments deposited before and after volcanic rock.
“You have to know when things happened and how long it took for things to happen to really understand them in the geological context,” Miller said. “It’s an evolving story, and as we pick up more pieces the story starts to get tighter and tighter.”