September 15. Sonoma Orogeny

While Pangaea was more or less intact during the Triassic, together with the initial rifting we discussed a few days ago, there were places where Pangaea was still growing. Western North America was one such place.

Several long linear island arcs and zones of oceanic crust, and the sediment associated with them, was amalgamated to what is now northwestern Nevada and adjacent areas over millions of years, culminating in the Sonoma Orogeny in early Triassic time. The mountain-building event gets its name from the Sonoma Mountains in that part of Nevada.  

The Sonoma Orogeny was the second round of accretion, or adding of tectonic terranes, in this part of North America. We talked about the Antler Orogeny, in central Nevada, back in May, during the Devonian.

I visualize this something like a modern island arc, say, Japan, colliding with the nearby Asian continent. Slices of oceanic crust, piles of sediment, volcanic piles, and perhaps small bits of continents all became welded to the older North American continent. There is some controversy as to exactly when all this happened, and it did happen over millions of years. Some lines of evidence favor earlier or later culmination, but I believe the consensus is that it was complete by Early Triassic time, around 230 million years ago. It’s complicated by overprints of later tectonic activity as well as intrusive igneous and extrusive volcanic rocks, and a 2008 paper by Keith Ketner with the USGS even suggests that there wasn’t really a specific Sonoma Orogeny at all. The geology out there is complex enough that questions like this can be raised legitimately even now.

Sonomia Terrane in green (including parts covered by Cenozoic). Strontium 706 line in red.

The set of terranes in northwestern Nevada are usually combined together and called the Sonomia Terrane. The eastern edge of the Sonomia Terrane coincides fairly well with a geochemical boundary called the strontium 706 line. This value, 0.706, is the ratio between two isotopes of the element strontium, and it forms the western boundary of igneous rocks of continental origin, the main mass of North America to the east of that line. West of the Sonomia Terrane, the strontium isotope ratio reaches 0.704, indicating sources of magma in oceanic crust. Thus the Sonomia Terrane lies between the strontium 706 and 704 lines, a position that would be predicted for intermediate crust such as an island arc. All of this supports the idea that the rocks of the Sonomia Terrane came in from a considerable distance to be accreted, or attached, to the North American continent.

—Richard I. Gibson

Links:
Island arcs and oceanic crust  USGS Bull. 1857-B


Ketner 2008 paper

Map from Ron Blakey