Paleomagnetic study of upper Miocene rocks from northern Chile: Implications for the origin of Late Miocene‐Recent tectonic rotations in the southern Central Andes

Abstract

Paleomagnetic studies in the southern Central Andes have shown the widespread presence of clockwise vertical‐axis rotations. Rock units sampled in these studies, however, are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern Chile being from Mesozoic and lower Tertiary rocks, whereas most data in the southern Altiplano, Puna, and Cordillera Oriental are from upper Tertiary rocks. In this paper we present the results of a paleomagnetic study on upper Miocene sedimentary rocks and ignimbrites from the Precordillera of northern Chile (at 22°S). These rocks are coeval with the initiation of crustal shortening in the eastern foreland fold‐thrust belt which some tectonic models relate to oroclinal rotation of northern Chile. Primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites, suggesting the study area has acted as a single coherent block with respect to vertical‐axis rotational deformation. Although minor inadequate sampling of paleosecular variation can affect our data set, its time‐averaged paleomagnetic direction is indistinguishable from the expected late Miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 Ma. This suggests that late Miocene‐Recent oroclinal rotation of the northern Chilean forearc, if present, must be either very low or nonuniform. A direct implication of this result is that much of the unquestionable tectonic rotations detected from upper Miocene rocks in the southern Central Andes is of local origin. Available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the Andes.