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

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 palcomagnetic 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 palcomagnetic study on upper Miocene sedimentary rocks and ignimbrites from the Precordillera of northern Chile (at 22øS). These rocks are coeval with the initiation of crustal shortening in the eastern foreland fold-thnmt belt which some teetome 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 palcosecular variation can affect our data set, its time-averaged palcomagnetic direction is indistinguishable from the expected late Miocene reference direction indicating no palcomagnetically significant rotation of the study area since circa 11 Ma. This suggests that late Miocene-R•nt oroclmal rotation of the northern Chilean forearc, if present, must be either very low or nonuniform. A direct implication of this restfit is that much of the unquestionable teetome 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.

New paleomagnetic data from the northern Puna and western Cordillera Oriental, Argentina: a new insight on the timing of rotational deformation

“…Eastward of the Cordillera Oriental, shortening in the external part of the orogen (Subandean belt) largely started at times of SJOS formation and is still going on. Relevant paleomagnetic data from the 13-12 Ma Quebrada Honda section (MacFadden et al, 1990), located in the easternmost border of the Cordillera Oriental, indicate clockwise rotation of about 20 • (Table 4) associated with a transfer zone in the Subandean belt (Somoza et al, 1999). The development of this transfer zone, and by inference, of the associated rotation in the Quebrada Honda section, is likely younger than the development of the SJOS (Kley, 1996).…”

Tectonic rotations and internal structure of Eocene plutons in Chuquicamata, northern Chile

“…Our understanding of the spatial distribution and timing of the CARP has improved in recent years (e.g. Arriagada et al, 2000Arriagada et al, , 2003Randall et al, 1996Randall et al, , 2001Roperch et al, 2000Roperch et al, , 2006Roperch et al, , 2011Rousse et al, 2003;Somoza and Tomlinson, 2002;Somoza et al, 1999;Taylor et al, 2007). In particular, the timing of rotations in northern Chile has been constrained by Somoza and Tomlinson (2002), who showed the presence of unrotated lower Miocene strata that unconformably lie on rotated Cretaceous rocks.…”

Implications for crustal rotation and tectonic evolution in the central Andes fore arc: New paleomagnetic results from the Copiapó region of northern Chile, 26°–28°S

“…Recent studies have shown that while the observed rotations in the currently deforming magmatic to retro arc region are indeed compatible with Neogene deformation those located in the present fore arc are unrelated to and must predate this Neogene deformation [Arriagada et al, 2003;Somoza et al, 1999;Somoza and Tomlinson, 2002].…”

“…The average rotation of all localities in this area is 30.8 ± 17.9° [ Arriagada et al, 2006] which is similar to that observed in the Copiapó region. Like the Copiapó region the timing of rotation is believed to be synpost-Paleocene to Eocene, with the younger bound being constrained by the nonrotation of Oligocene-Miocene sediments and ignimbrites [Arriagada et al, 2003;Somoza et al, 1999;Somoza and Tomlinson, 2002].…”