LOS ANGELES — Baja California was wrenched from mainland Mexico 6 million years ago by a series of earthquakes, starting in earnest the peninsula’s 160-mile push to the northwest, a study says.
Geologists have long known that movement along the boundary separating the North American and Pacific plates tore Baja California from the rest of Mexico, opening up what is now known as the Gulf of California. But the timing had been a question.
The break separating the peninsula from the mainland is the southern extension of the San Andreas fault system that runs nearly the length of California.
But whether the movement began in a gradual process as many as 12 million years ago, or more abruptly in more recent times, remained unknown.
Michael Oskin, a graduate student in geology at the California Institute of Technology, said he has found and matched identical volcanic rocks on opposite sides of the gulf that allowed him to pinpoint the size, timing and rate of the movement between the two plates. The results are published in the May issue of the journal Geology.
By correlating the different tie-points – now separated by the roughly 160 miles of slip that has taken place along the fault system, but closely joined in the distant past – Oskin said the study he co-authored shows that Baja California started pulling away 12.5 million years ago, but the bulk of the the peninsula’s movement has taken place within the last 6.5 million years.
“We have now concrete evidence that the motion history of the gulf can be very well divided around this 6.5 million-year-old time interval,” Oskin said.
One expert in the geologic history of Baja California said the study further refines the chronology of the peninsula’s evolution.
“This is fairly important in that it’s by far the most accurate matching point across the gulf to date, and it’s certainly the youngest that you can come up with,” said Gary Axen, an assistant professor of geology at the University of California, Los Angeles.
Continental rifting probably first opened up what is now the Gulf of California more than 12 million years ago when subduction off the shore of northwest Mexico ground to a halt.
The Gulf opened even farther apart some 6 million to 7 million years after that, when Baja California shifted onto the Pacific plate from the North American plate and the San Andreas fault system plunged farther southward.
If it weren’t for barriers, the Gulf of California would stretch farther north, reaching the depression now occupied by California’s Salton Sea, which is well below sea level. However, the troughlike depression is blocked by the delta of the Colorado River, which has steadily poured sediments into the Gulf of California over millions of years.