Paleomagnetism and Neogene clockwise rotation of the Northern Cady Mountains, Mojave Desert of southern California

The early to middle Miocene Hector Formation of the northern Cady Mountains consists of about 400 m of syntectonic alluvial fan and volcaniclastic sediments deposited in the central Mojave desert. Paleomagnetic samples were collected from 60 sites throughout this unit. Samples from each site were treated using both alternating field and thermal demagnetization. These studies and isothermal remanent magnetization experiments indicate that the natural remanent magnetization (NRM) is complex and that characteristic directions can be carried in either high coercivity or continuous unblocking temperature spectra depending upon the sample. A positive reversals test and concordant directions of the alternating field (AF) and low and high temperature components suggest a stable magnetization. Although a Unique magnetostratigraphic correlation to the time scale is difficult, the section studied may represent chrons C6B to C5C. After screening of individual components and calculation of site means using Fisher statistics, 48 of the original 60 sites were used to calculate an overall formation mean direction, i.e., declination 18.6°, inclination 45.4° (α95 = 6.0°). Using a 20-m.y. paleomagnetic pole for North America [Irving and Irving, 1982] the Hector Formation has undergone 20.6°±7.6° (R±ΔR) of clockwise rotation around a vertical axis. This amount of rotation is considerably less than reported for some early Miocene rocks elsewhere in the Mojave Block. Least squares regression of the 48 site mean declinations versus stratigraphic position indicates no trend throughout the section, suggesting that there was no syndepositional rotation of this unit. The late Cenozoic structural history of the Mojave Block is considerably more complex than was originally believed, and currently available paleomagnetic data indicate that many blocks within this region have experienced different degrees of rotation. There were at least two phases of tectonic rotation in this region: (1) early Miocene (about 25 Ma), probably related to the initiation of extension within the Mojave Rift; and (2) post middle Miocene-Pliocene (between about 16 Ma to the present), possibly related to movement along principal structural features such as the San Andreas fault zone. The exact timing of the younger phase of tectonic activity cannot be better constrained until paleomagnetic data become available for late Cenozoic rocks within the Mojave Block.

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