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

A preliminary geologic map of the Portola 30’x60’ quadrangle integrates decades of published and unpublished maps in the northern Sierra Nevada.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.