PALEOMAGNETIC EVIDENCE OF A CRETACEOUS AGE FOR THE CERRO SAN LUIS FORMATION, SIERRA SANTA ROSA, SONORA

Sierra Santa Rosa in northwest Sonora contains exposures Paleoproterozoic gneisses, Neoproterozoic strata, Lower Jurassic strata, and undated volcanic and volcanoclastic rocks assigned to the Cerro San Luis Formation. We collected ten paleomagnetic sites in rocks of the Cerro San Luis Formation. The NRM of the samples is multivectorial, with a low stability viscous component, a low laboratory unblocking temperature (< 400 ºC) component that is generally south directed and a high stability dual polarity magnetization. The magnetization is well resolved by alternating field demagnetization, but better resolution is obtained using thermal demagnetization. In tilt corrected coordinates, the mean magnetization is of Dec=353º - Inc=45.9º (n=9, k=10.1, and alpha95=16º). The Cerro San Luis Formation overlies concordantly Jurassic strata, and folding in the range is assigned to the Late Cretaceous. The magnetization passes a qualitative conglomerate test, suggesting a primary origin. Although better resolution is not possible, the mean direction rules out a Jurassic age for volcanoclastic and volcanic rocks in Sierra Santa Rosa and suggest that the rocs are of Cretaceous age.The mean direction indicates slight clockwise rotation with respect to the North America craton, probably resulting from shortening. The low unblocking temperature magnetization may be related to Late Cretaceous magmatism in the region. Cerro San Luis Formation may be correlative with Upper Cretaceous rocks of the Tarahumara Formation of northern Sonora.