Watching a continental split
Interested in waterfront property in Southern California? A new study of a continental schism running east of Los Angeles offers a clear “buy” signal for the long-term investor: The North American continent is splitting apart along a rift, and if you got the patience, we have the real-estate-appreciation potential!
In just a few million years, as the North American continent sunders in a weak zone called the Salton Trough, the Gulf of California will stretch further north.
On our unstable Earth, not even the continents are rock solid. Instead, they shift around like blocks of sea ice that join, fissure and separate once again — over millions of years.
Geologists know the process is occurring in the Southern California desert, and we’ve just read a sophisticated analysis that finds an ominous thinning of the strong crustal layer in the Salton Trough.
Ominous, that is, unless you are planning a waterfront resort here, with a grand opening in, say, 2,002,011.
The study helps to fill a gap in our understanding of the earth, says first author Vedran Lekic, a National Science Foundation post-doctoral fellow at Brown University. “The main question is, how do continents come to break apart? This process is really fundamental to shaping how the Earth looks; if not for rifting, once Pangaea formed, it would never have broken apart and we would have only one continent.”
Pangaea is a giant agglomeration of continents that broke up about 150 million years ago, creating our current collection of continents.
Cross section of Salton Trough, California
Scoping out the Earth
The lithosphere, Earth’s crust and the rigid rock beneath it, essentially floats on the asthenosphere, the soft and hot outer layer of the mantle that is located tens of kilometers belowground.
As a continental rift grows, one would expect to find a thinned lithosphere at the Salton Trough. But Lekic says the actual thinning was more dramatic than expected — as much as a 50 percent reduction compared to adjacent areas.
By studying earthquake waves passing through Earth, Lekic and colleagues measured the thickness of the lithosphere by locating its lower border. They knew that one type of wave converts to a faster wave type as it passes up from the asthenosphere into the lithosphere, so the conversion could be used to mark the base of the lithosphere.
It turned out that the lithosphere measured about 40 kilometers thick beneath the Salton Trough, compared to 60 to 80 kilometers on nearby areas. That thinning translates into a weakening that will eventually allow open water into the Trough, and myriad real-estate opportunities along the new shoreline.
Previous efforts to estimate the lithosphere’s depth have relied mainly on surface data, says Lekic, and that limited our knowledge of how the continental splitsville takes place. From relying on “surface observations of faults, topography, heat flow, and some studies of the crustal structure, we have not been able to image the detailed topography of the base of the tectonic plate, as it looks during rifting.”
Although the study relied on the interest in Southern California seismology that is a response to extreme seismic activity, the finding says little about earthquake probabilities.
But earthquakes are not the only tectonic game in town, says Eugene Humphreys, a professor of geophysics at the University of Oregon. “While most people know southern California is being sheared by the San Andreas and related faults, most people are not aware that the region also is being pulled apart as the Pacific plate also moves slowly away from North America. These researchers have imaged the deep structure of the plate where it is being torn apart by this process, and contrary to what many have thought, the tears go through the entire plate right where the surface expression of this rifting is seen. It’s exciting work.”
The study provides insight into deep structure and processes of fluid migration up into the plate, says Humphreys. “These lower-plate interfaces were not expected to exist at all, and the scientific community is excited but struggling to determine what could create relatively sharp interfaces.”
Although Earth warms with depth, that is unlikely to explain the weakness, Humphreys says, “so the search for other causes is on. By associating the position and shape of these interfaces with a specific deformation history, this study provides important information on the origin of these interfaces.”
Lekic, who worked with co-author Karen Fischer of Brown, on the study, says that “Even at great depth, we see the same stretching and deformation that we see near the surface. At the bottom of the lithosphere, there is this persistent weakness, in a zone that runs more or less vertically, and that’s surprising.”
But as scientists wrestle with the geological goulash that is Southern California, we suggest you send a down payment to Rift ‘n Grift Realty on the ocean-front lot of your dreams – and wait a few million years!
–David J. Tenenbaum