Paleomagnetic study of upper Miocene rocks from northern Chile: Implications for the origin of late Miocene-Recent tectonic rotations in the southernCentral Andes

Paleomagnetic studies in the southern Central Andes have shown the widespre ad presence of clockwise vertical-axis rotations. Rock units sampled in the se studies, however, are heterogeneously distributed in stratigraphic age w ith most paleomagnetic data from northern Chile being from Mesozoic and low er Tertiary rocks, whereas most data in the southern Altiplano, Puna, and C ordillera 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 degrees S). Th ese rocks are coeval with the initiation of crustal shortening in the easte rn foreland fold-thrust belt which some tectonic models relate to oroclinal rotation of northern Chile. Primary magnetizations in rocks from widely di stributed sites in two ignimbrites indicate that no relative rotations have occurred between sites, suggesting the study area has acted as a single co herent block with respect to vertical-axis rotational deformation. Although minor inadequate sampling of paleosecular variation can affect our data se t, its time-averaged paleomagnetic direction is indistinguishable from the expected late Miocene reference direction indicating no paleomagnetically s ignificant 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 th is result is that much of the unquestionable tectonic rotations detected fr om upper Miocene rocks in the southern Central Andes is of local origin. Av ailable structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east si de of the Andes.