Abstract: Paleomagnetism, Paleogeographic Origins, and Remagnetization of the Coast Range Ophiolite and Great Valley Sequence, Alta and Baja California
HAGSTRUM, JONATHAN T.
Reliable paleomagnetic
data for the Coast Range ophiolite (CRO) and Great Valley sequence (GVS)
of North America have been difficult to obtain. In Alta and Baja California,
characteristic remanent magnetizations for these rocks and their equivalents
often post-date folding, show only normal polarity
where both polarities
are expected, and tend to have directions similar to younger geomagnetic
field directions. These magnetizations are clearly secondary and may have
been acquired during deep subaerial weathering, burial and uplift, or low-temperature
chemical alteration. Similarly, Mesozoic oceanic rocks of the Franciscan
Complex have been extensively overprinted, but high-blocking temperature
components of magnetization showing
polarity
stratigraphies and equatorial
paleolatitudes have been found in hematite-bearing chert sequences from
northern California to Mexico. Upper Jurassic red tuffaceous chert above
the CRO at Stanley Mountain in southern California also has a high-temperature
component with dual polarities that is interpreted as a primary magnetization.
Separation of this primary component from an overprint component is best
shown by samples with negative-inclination primary directions, and a corrected
mean direction using only these samples indicates an initial paleolatitude
of 32 degrees plus or minus 8 degrees N. At Mount Diablo in northern California,
Middle to Upper Jurassic pillow lavas and diabase sills of the CRO have
two components of remanent magnetization. The characteristic component
shows two polarities as well, and its corrected direction indicates an
initial paleolatitude of 20 degrees plus or minus 12 degrees N. The general
agreement of these results suggests that the CRO in California has not
undergone large-scale northward transport with respect to cratonic North
America since its formation. Reliable estimates of paleolatitude for the
lower GVS remain unavailable.
Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California