Monday, March 31, 2008
Incredible New Research in Central California
A new paper being published tomorrow in the Journal of California Geophysics by Pompilius Eostur-Monath and Numa Huhtikuu includes some eyebrow-raising findings concerning the origin of California's Central Valley and Coast Ranges. They relate their findings to the rapid uplift of the adjacent Sierra Nevada in the last few tens of millions of years and their apparent lack of mountain roots that has caused consternation and confusion among researchers for years.
The last decade or so has seen the rise of the "mantle drip" hypothesis in which the once-present roots detached from the base of the crust, and sank into the mantle. The Sierra then rose as hot mantle material surged into the gap created by the sinking mass. What has been less clear has been the position and role of the Central Valley in the process.
Paleomagnetic data from deep bore holes in the deepest parts of the valley between Bakersfield and Modesto indicate that the sediments of the Central Valley accumulated 40 miles east of their present location, and that they moved as a coherent unit with very little internal deformation. In essense, they are the upper plate of a huge detachment system. Although they didn't come right out and say it, the researchers suspect that the movement may have been nearly catastrophic in scale, similar say, to the giant landslides that have originated on the Hawaiian Islands that are implicated in giant Pacific Ocean tsunamis. In a personal communication, one of the researchers suggested to me that the movement may have resulted from a single huge seismic event, possibly in the range of a moment magnitude of 10 or 10.5 (normally faults can't store this kind of stress, but the unusual nature of the contact zone and the four-hundred-mile length allows for much higher levels of energy release). In other words, the sediments of the Central Valley didn't originate from erosion of the Sierra Nevada...they were deposited on top of the Sierra, and later catastrophically slid off!
And here's the most unlikely part of the story: why didn't the Central Valley sediments continue westward, sliding off the continental margin into the Pacific, causing an ocean-wide mega-tsunami? Believe it or not, the San Andreas fault! Because of the shifting the granitic terrane of the Salinian Block north nearly 200 miles, a ridge of solid rock stood in the path of the shifting mass, blocking its westward movement, and crumpling the sediments of the valley floor into the the folded sediments of the eastern Coast Ranges!
So, in one stunning paper, the researchers have explained the origin of the Sierra Nevada, the Central Valley and the Coast Ranges. They do caution that the events they describe are so unlikely that they have no fears that there will be a repeat of this activity in the event of a large earthquake on the San Andreas fault.
I hope I explained this clearly. It was difficult to describe some of the more esoteric parts of the paper when I was working under tomorrow's deadline for new posts....