- Anomaly timescales for the last 90 million years, derived from
marine magnetic profiles and published prior to mid-1979, are summarized,
illustrated for comparison, and critically reviewed. A revised timescale
is constructed using calibration points which fix the ages of anomalies
2.3', 5.5, 24, and 29. An equation is presented for converting K-Ar
dates that is consistent with the recent adoption of new decay and
abundance constants. The calibration points used in the revised timescale,
named NLC-80, are so converted, as are the boundary ages of
geologic epochs within the range of the timescale.
NLC-80 is then used, along with recently acquired and rigorously
navigated underway geophysical data from the region of the mouth of the
Gulf of California, to prepare detailed bathymetric, gravimetric, and
seismo-tectonic maps of the area. The basement ages at DSDP Leg 63
drilling sites 471, 472, and 473 are estimated from magnetic anomalies
fit to timescale NLC-80. The estimates agree with biostratigraphically
determined basement ages and support the proposal that an aborted ridge
of about 14 MY age has left a small fragment of the Farallon Plate
beneath the Magdalena Fan. Several large inactive faults are identified
on the deep-sea floor west of the tip of the peninsula of Baja California.
Additional magnetic anomaly profiles and bathymetric profiles across
the Rivera Ridge are interpreted. These contradict the existence of a
3.5 MY old aborted spreading center on the Maria Magdalena Rise.
Instead, it is proposed that an episode of subduction of the Pacific
Plate beneath the southeastern tip of Baja California, concomitant with
strike-slip faulting west of the peninsula, occurred and that this subduction
may be responsible for the uncentered location of the Rivera
Ridge within the mouth of the Gulf of California.
A single magnetic anomaly profile obtained northeast of the Tamayo
Fracture Zone is used to determine that the rate of Pacific/North American
plate motion, for the last 3 MY is 68 km/MY at this location. This result,
if correct, indicates that the peninsula of Baja California is separating
from mainland Mexico faster than the Rivera Ridge is generating oceanic
crust in the wake of opening in the gulf. This, in turn, requires that
either slow diffuse extension is occurring presently across the Maria
Magdalena Rise, or across the Cabo Corrientes-Colima region, or that the
portion of North America south of the trans-Mexican volcanic belt is
moving right-slip with respect to the North American Plate at a rate of
Large horsts and many smaller continental fragments are found within
the southern gulf. Several of them have active seismic boundaries, while
others have apparently foundered.
The gulf began to open approximately 14-15 MY ago with slow, diffuse
block-faulting and the deposition of the Maria Magdalena Fan at the
mouth of the gulf. Oceanic crust was exposed in the gulf by about 9-10 MY,
at the same time that the Rivera Ridge began reorienting by clockwise rotation.
Strike-slip motion along the Tosco-Abreojos Fault took up some
of the Pacific/North American motion with the remainder occurring within
the gulf itself. During this period the Pacific Plate forming within
the gulf was slowly subducting beneath Baja California. By 4-5 MY
subduction ceased and all of the Pacific/North American plate motion
was shifted to the Gulf of California fault system.
The gulf and peninsula of California are still in the process of
adjusting to the change from Pacific/Farallon to Pacific/North American