Paleomagnetic Determination of Vertical-Axis Block Rotation and Magnetostratigraphy in the Coachella Valley, California

Abstract:

Here, we report new paleomagnetism and magnetostratigraphy data from the Palm Spring Fm of the Mecca Hills, the Pleistocene conglomeratic sandstone in Desert Hot Springs and the Plio-Pleistocene San Timoteo beds from Live Oak Canyon.

From the Mecca Hills, new data are from 29 sites and 112 samples. The paleomagnetic results yielded well-defined components of magnetization- defining seven polarity zones within the Ocotillo and upper Palm Spring Fm. Correlation to the geomagnetic polarity timescale, using the Bishop Ash near the top of the section as a tie point, places the Brunhes-Matuyama boundary near the base of the Ocotillo Fm, and the Jaramillo, Olduvai, and Reunion normal polarity sub-chrons in the upper Palm Spring Fm. This indicates the upper Palm Spring Fm in the Mecca Hills was deposited between 2.3 and 0.9 Ma. Sites from the Mecca Hills section have mean directions of D = 343, I = 53, α₉₅ =11.3 N = 5 for normal sites, D = 175, I = -50, α₉₅ = 4.9 N = 24 for reverse sites, and normal and reverse sites combined have a mean direction of D = 353, I = 51 α₉₅ = 4.4. This indicates modest (7 degrees) CCW rotation of the section.

Results from 19 sites (53 samples) of the Pleistocene conglomeritic sandstone from Desert Hot Springs have very well-defined paleomagnetic components. Six of the sites have normal polarity- 13 sites have reverse polarity. Sites with normal polarity have a mean direction of D = 358, I = 45, α₉₅ = 13 and reverse sites have a mean of D = 182, I = -50, α₉₅ = 6.6.The combined mean direction (in tilt-corrected coordinates) is D = 0.7, I = 49, α₉₅ = 5.6 and indicates that 3.1° ± 2.3° of CW rotation has occurred at this location since ~1 to 1.5 Ma.

Results from 8 sites (35 samples) of the upper-most San Timoteo beds from Live Oak Canyon also have well-defined paleomagnetic components for 6 sites. All of the results have normal polarity, and one site has a direction that is >40 degree from the other sites- the mean of the remaining 5 sites is D = 11, I = 49, k=51, a95=11.

The paleomagnetic results from the Coachella Valley collectively indicate that this area has experienced modest (in most cases less than 10 degrees) CW or CCW rotation during the past 1-2 Ma. The lack of variation in amount or sense of rotation as a function of age suggest that rotation has been relatively recent (during the past ~ 1 Ma).