Paleomagnetism of the Lodo Formation and kinematic evolution of the Vallecitos Syncline, Coast Ranges, Central California

Abstract:

The WNW-ESE-striking Vallecitos Syncline spans part of the California Coast Ranges, and is bounded by the Calaveras-San Andreas Fault System in the west and the Great Valley in the east. A northwest bend is observed as the syncline approaches the Calaveras and San Andreas Faults to the west, possibly a result of localized clockwise tectonic rotation. We present new paleomagnetic data from four sites across the syncline in an east-west transect to test this hypothesis. We sampled shale, sandstone, and siltstone from the Lodo Formation at Tumey Hills, Griswold Hills, New Idria, and Paicines. Using standard paleomagnetic methods, we conducted low temperature and thermal demagnetization. Data from some localities were generally scattered and difficult to interpret, suffering a major secondary chemical remanent magnetization. However the eastern end of the Vallecitos Syncline exhibited interpretable results. About a third of samples showed a strong reverse polarity at Griswold Hills and New Idria Mine, indicating that magnetization of the samples pre-date tilting of the outcrop and a secondary magnetization was acquired post deformation. Characteristic remanent magnetizations also exhibit inclination shallowing at three sites, likely the effect of compaction rather than poleward transport, since the fourth in the Tumey Hills exhibits minimal shallowing. Furthermore, results from Tumey Hills, and Griswold Hills indicate no localized tectonic rotation. Based on these findings, we tentatively conclude that the eastern end of the Vallecitos Syncline experienced no rotation, within error. Results from the western end remain inconclusive.