By ANDREW
BRIDGES, AP Science Writer
LOS
ANGELES (AP) - Baja California, the 760-mile-long peninsula off the northwestern
coast of Mexico, was wrenched from mainland Mexico by a series of earthquakes
that began 6.5 million years ago, researchers say.
Geologists have long
known that movement along the boundary separating two plates in the Earth's crust
tore Baja California from the rest of Mexico, opening up what is now the Gulf
of California.
But the timing had been in question. Whether the movement
began as much as 12 million years ago and proceeded gradually, or happened abruptly
in more recent times, remained unknown.
Michael Oskin, a graduate student
in geology at the California Institute of Technology, said he found and matched
identical volcanic rocks on opposite sides of the gulf that allowed him to pinpoint
the size, timing and rate of movement.
Oskin concluded that Baja California
started pulling away 12.5 million years ago but that most of the movement took
place within the past 6.5 million years.
That supports the theory of a
relatively abrupt and recent origin.
The break separating the peninsula
from the mainland is the southern extension of the earthquake-prone San Andreas
fault system.
The findings were published in the May issue of the journal
Geology.
The Baja Peninsula pushed 160 miles to the northwest, creating
a gulf that varies in width from 30 to 150 miles.
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 at Los Angeles. |
