Paleomagnetically defined rotations from the Precordillera of northern Chile: Evidence of localized in situ fault-controlled rotations

A paleomagnetic study of Paleocene, Cretaceous, and Jurassic units in the P recordillera of northern Chile (26 degreesS to 27 degreesS) demonstrate a c omplex pattern of crustal rotations. The region was selected to investigate the pattern of deformation associated with the Sierra Castillo-Agua Amarga Fault and its associated structural subdomains, which form part of the Dom eyko Fault System, the fault system that controls much of the structure of the Chilean Precordillera. Paleocene lavas from the center of the study are a pass a fold test, indicating the primary nature of the remanence in these rocks. A second group of lavas from the south has a similar formation mean inclination after tilt correction for a uniform dip, which coupled with th e presence of reversals, suggests that these lavas also carry a primary rem anence. The data indicate clockwise rotations of similar to 35 degrees and 42 degrees respectively, for the two sampling localities. Three geographica lly separate areas of Lower Cretaceous red beds also reveal primary remanen ces based on the presence of sites with reversed polarity at two of the loc alities and an inclination-only fold test of the three locality mean direct ions for this formation. These sampling areas yield remanence directions th at reveal 30 degrees of counterclockwise and 31 degrees of clockwise rotati on as well as an area without statistically significant rotation. These dat a include the first well-constrained counterclockwise rotation to be recove red from rocks in the southern central Andes, although mapping suggests the block involved is of limited geographic extent. Jurassic strata failed to yield any reliable results. Overall, the data indicate a greater variabilit y in the rotation pattern than has previously been observed in most compara ble areas south of the Arica Deflection. The variation cannot be explained by large-scale rotation mechanisms, such as oroclinal bending or domino-sty le block rotation by widely separated transandean faults. Instead, the rota tions are consistent spatially, and temporally and in their sense and magni tude with the Eocene transpressional deformation associated with the Domeyk o Fault System; the deformational event that generated most of the importan t structures in the studied area. The data emphasize the significance of lo cal tectonics in controlling rotations in this part of the south central An des and the importance of understanding the local structure of regions samp led for paleomagnetic study.