PALEOMAGNETISM AND PETROFABRIC OF THE CUATE GRAY PLUTON, SONORA: IMPLICATIONS FOR THE EXTENSIONAL HISTORY IN THE GULF OF CALIFORNIA

Cuate Gray, in northwest Sonora, is a Laramide pluton (ca. 72 Ma) that intrudes Permian granodioritic gneiss, and is non-conformably overlain by Miocene volcanic rocks (ca. 22 Ma) tilted by a NW trending extensional fault. It crops out in the NW extreme of Sierra Los Tanques, near the Mexico-USA border about 15 km SW of the Quitobaquito Hills. We collected 17 paleomagnetic sites in an area ~4 km2 in order to understand the pluton fabrics in relation to mylonites in the eastern portion of Sierra Los Tanques, proposed to be of Jurassic age. Cuate Gray is a megacrystic monzo- to syeno-granite with K-feldspar > quartz > plagioclase + biotite and muscovite that includes various co-genetic dioritic dikes. Samples of the Cuate Gray intrusive complex are characterized by well-defined magnetic fabrics, with mean susceptibility of the order of 5x10-3 SI, degrees of anisotropy greater than 1.2, well grouped minimum susceptibility axis, and NW-SE to E-W magnetic foliations that dip mostly south, and oblate fabrics. This fabric is parallel to megascopic fabrics in the pluton and the host rock, they thus appear to be syntectonic. Correcting for paleomagnetically determined tilt the foliations are sub-vertical. A characteristic remanent magnetization (ChRM) was isolated in 9 of 17 sites sites. In 4 sites the ChRM is a near univectorial magnetization defined in the range of temperatures from 200 to 560ºC or 15 to 40 mT, in 5 sites partial overlap of an overprint in the direction of the recent dipole field and the ChRM required combination of SEP and great circles for the site-mean calculation. In the rest of the sites the ChRM was not recovered because either the demagnetization behavior is erratic, or a north directed overprint is the only stable remanence. The ChRM is WNW directed and of positive inclination or ESE directed and of negative inclination, and the accepted sites yield a mean direction of D= 291.1º, I= 20.5º (k=25.8, alpha95=10.3º, n=9). Corrected for the attitude of overlying Miocene volcanic rocks the direction is D= 305.4º, I= 25.4º. The expected direction assuming stability with respect to North America is D= 345.8º, I= 59.4º, suggesting that about 45º of additional tilt of the pluton occurred prior to deposition of the Miocene volcanic rock. The additional rotation occurred between about 70 and 22 Ma. We interpret the rotation as related to extension rather than Laramide deformation, suggesting that extension in the Gulf of California started before 22 Ma.