IGNIMBRITE-FILLED PALEOVALLEYS: KEY MARKERS FOR THE STRUCTURAL DEVELOPMENT OF THE PACIFIC-NORTH AMERICAN PLATE BOUNDARY

Voluminous Cenozoic ash-flow tuffs erupted from a belt of calderas in central Nevada and along much of western North America to the southern Sierra Madre Occidental during the late Eocene through early Miocene (~37-19 Ma). This Ignimbrite Flare-up was the most intense magmatism in western North America, and the widespread tuffs are key units for analyzing the tectonic and magmatic evolution of the Great Basin and Sierra Nevada. Our detailed and reconnaissance mapping, geochemistry, geochronology, and paleomagnetic investigations show that the tuffs erupted onto mountainous country with deep paleovalleys that drained westward to the Pacific Ocean, in the Great Valley at the time. The tuffs were channelized into these paleovalleys, which were as much as 1.2 km deep and 8 to 10 km wide in Nevada but shallowed westward into the Sierra Nevada where they are the famed gold-bearing gravel channels. The most widespread tuffs almost certainly reached the ocean, and ash was reworked extensively in the then coastal plain.

The distribution of paleovalleys and ash-flow tuffs reveal that the Basin and Range – Sierra Nevada structural and topographic boundary did not exist before 23 Ma, the age of the youngest tuff we currently can correlate across the boundary. What is now relatively low Basin and Range country was at higher elevation than the Sierra Nevada at the time. Although major normal (Basin and Range) and strike-slip faulting (the Walker Lane and eastern California shear zone) has been proposed to have begun in western Nevada at about 25-26 Ma, any faulting before 23 Ma was insufficient to disrupt the paleodrainages other than temporarily.

Offset of tuff-filled paleovalleys, the best piercing points to determine displacement, reveal that the northern Walker Lane (north of ~39.5°) has ~25 km total dextral slip (~10 km on individual faults). Numerous tuff-filled paleovalleys also cross the central Walker Lane (~39-37.5°) and can be used to determine total displacement, which is so far poorly constrained. Tuff distribution has been used to determine displacement across the northern Gulf of California and is applicable all along the Gulf. Although access to some regions is difficult, delineation of paleovalleys and correlation of specific tuffs potentially can provide precise displacements along much of the transform boundary.