Paleomagnetic evidence for a pre-early Eocene (∼50 Ma) bending of the Patagonian orocline (Tierra del Fuego, Argentina): Paleogeographic and tectonic implications

The southernmost segment of the Andes of southern Patagonia and Tierra del Fuego forms a ∼700 km long
orogenic re-entrant with an interlimb angle of ∼90° known as Patagonian orocline. No reliable
paleomagnetic evidence has been gathered so far to assess whether this great orogenic bend is a primary
arc formed over an articulated paleomargin, or is due to bending of a previously less curved (or rectilinear)
chain. Here we report on an extensive paleomagnetic and anisotropy of magnetic susceptibility (AMS) study
carried out on 22 sites (298 oriented cores), predominantly sampled in Eocene marine clays from the
external Magallanes belt of Tierra del Fuego. Five sites (out of six giving reliable paleomagnetic results)
containing magnetite and subordinate iron sulphides yield a positive fold test at the 99% significance level,
and document no significant rotation since ∼50 Ma. Thus, the Patagonian orocline is either a primary bend,
or an orocline formed after Cretaceous–earliest Tertiary rotations. Our data imply that the opening of the
Drake Passage between South America and Antarctica (probably causing the onset of Antarctica glaciation
and global climate cooling), was definitely not related to the formation of the Patagonian orocline, but was
likely the sole consequence of the 32±2 Ma Scotia plate spreading. Well-defined magnetic lineations
gathered at 18 sites from the Magallanes belt are sub-parallel to (mostly E–W) local fold axes, while they
trend randomly at two sites from the Magallanes foreland. Our and previous AMS data consistently show
that the Fuegian Andes were characterized by a N–S compression and northward displacing fold–thrust
sheets during Eocene–early Miocene times (50–20 Ma), an unexpected kinematics considering coeval South
America–Antarctica relative motion. Both paleomagnetic and AMS data suggest no significant influence from
the E–W left-lateral Magallanes–Fagnano strike–slip fault system (MFFS), running a few kilometres south of
our sampling sites. We thus speculate that strike–slip fault offset in the Fuegian Andes may range in the
lower bound values (∼20 km) among those proposed so far. In any case our data exclude any influence of
strike–slip tectonics on the genesis of the great orogenic bend called Patagonian orocline.

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