3 Understanding the Sierra Nevada Through New Regional Geologic and Geophysical Syntheses I

Wednesday, 12 May 2021
8:30 AM - 12:30 PM
UNR Centers for Transformative Monitoring Programs (CTEMPS) Room

COMPILATION OF THE PORTOLA 30’X60’ QUADRANGLE, A GEOLOGICALLY COMPLEX PART OF THE SIERRA NEVADA IN NORTHEASTERN CALIFORNIA

Paper number: 3-3
Program: Theme Sessions
Time: 9:10 AM – 9:30 AM

Authors

Abstract

A preliminary geologic map of the Portola 30’x60’ quadrangle integrates decades of published and unpublished maps in the northern Sierra Nevada. In conjunction with geophysical data, including more than 1000 new gravity measurements, the map seamlessly depicts a complex region at the intersection of the Sierra Nevada and Basin and Range provinces.

Paleozoic sedimentary and volcanic rocks of the Northern Sierra Terrane (NST) underlie much of the western part of the map. The NST is dominated by sedimentary rocks and mélange of the Shoo Fly Complex and overlying Paleozoic-Mesozoic arc-related strata, including rocks of the Sierra Buttes. Along the western edge of the Shoo Fly Complex, the Devil’s Gate amphibolite, part of the Feather River ultramafic belt, is associated with the highest gravity values in the map area.

Mesozoic arc magmatism resulted in the emplacement of plutons such as the Jurassic Haypress Creek pluton, which predates Cretaceous-aged plutons dominant elsewhere in the Sierras, and in the eastern half of the map. Gravity highs coincident with parts of the Haypress Creek pluton may indicate mafic compositions at depth.

Cenozoic magmatism produced numerous volcanic centers and deposits throughout the map area, such as the Dixie Mountain Volcano, a felsic dome complex flanked by extensive pyroclastic deposits. The Lovejoy Basalt, found as far away as the western edge of the Sacramento Valley, was first described at Lovejoy Creek in the northern part of the map, though its source is northeast of the map area.

Quaternary climatic fluctuations resulted in glaciers along the higher Sierra Nevada peaks, and pluvial lakes in Sierra and Mohawk Valleys. Glacial features such as moraines are mapped in the heavily forested landscape using extensive LiDAR coverage in the area.

The Mohawk Valley and Grizzly Valley faults are regarded as the northern end of the Walker Lane, a zone of dextral and normal faults accommodating 20-25% of the motion between the Pacific and North American plates. Pronounced gravity lows, attributed to > 1 km thick Cenozoic basin deposits, underlie Mohawk, Sierra, and Long Valleys. The basin shape beneath Mohawk Valley suggests it formed as a pull-apart basin by strike-slip faulting, whereas the geometry of basins beneath Long and Sierra Valleys suggest a combination of normal and strike-slip faulting.

Geological Society of America Abstracts with Programs. Vol. 53, No. 4

doi: 10.1130/abs/2021CD-363091

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.