Magnetic stratigraphy and a test for block rotation of sedimentary rocks within the San Andreas fault zone, Mecca Hills, southeastern California

Abstract

A 500-m section of the Palm Spring Formation in the southern Mecca Hills, located within the San Andreas fault zone in southeastern California, has been paleomagnetically sampled to determine possible tectonic rotation in this area and to establish time-stratigraphic control. This work was partly stimulated by the fact that 80 km farther south, previous studies demonstrated 35° of postdepositional rotation in the Palm Spring Formation of the Vallecito-Fish Creek basin east of the Elsinore fault. Several lines of evidence suggest that hematite is the main magnetic carrier of the Mecca Hills samples. Large anhedral hematite grains observed in magnetic extracts and a positive fold test imply a detrital origin of the remanence. The polarity reversal patterns, together with earlier vertebrate paleontologic studies, restrict the time span for deposition of this unit to the middle-late Matuyama chron (2.0–0.75 myr ago), thus of uppermost Pliocene and early Pleistocene age. Characteristic directions of best least-squares fit for 73 samples suggest little or no overall rotation, despite the severe late Quaternary tectonic activity demonstrated by the intense deformation of these strata.

Additional details