450 MILLON YEARS OF EPISODIC CO-AXIAL FOLDING, CENTRAL ANDES, FAMATINA, ARGENTINA

The Famatina mountain range extends for 400 km between 27º-31º S in the Central Andes, Argentina. It trends N-S along the boundary between the fold-and-thrust belt and the broken foreland of the Sierras Pampeanas. Famatina is interpreted to have originated as an Ordovician magmatic arc on the western border of Gondwana, accreted during the docking of the Laurentian-derived Precordillera terrane.

The structure of central Famatina, from W to E, consists of: 1) hinterland thrusts involving Ordovician granites thrust onto the foreland wedge with a mylonite zone at the base; 2) several E-vergent thrust sheets involving Ordovician, upper Paleozoic and older Tertiary rocks; 3) an E-vergent fault-propagation anticline in Ordovician to Tertiary rocks (Los Colorados Anticline); 4) E-vergent thrusts, the easternmost of which has Neogene on Quaternary rocks; and 5) thick skinned W-vergent thrusts carrying Precambrian basement and mylonitized granites over Tertiary rocks.

The folding events are recorded in the following major unconformities on the flanks of the Los Colorados Anticline: 1) between Early Ordovician marine volcano-sedimentary rocks and Late Ordovician volcanic rocks; 2) inside an Early Silurian? marine clastic interval; 3) below Late Carboniferous fluvial sandstones; 4) below Permian redbeds; and 5) below the 3000 m - thick Cenozoic foreland section. In addition, syntectonic unconformities recording fold growth occur within the earliest (pre-Neogene) Cenozoic rocks. The angle between rocks below and above the unconformities opens westward on the W flank of the fold and eastward on the E flank. For each unconformity the angularity is greater on the E, suggesting episodic and progressive E-verging growth. Poles to bedding show similar northward axial trends in older and younger rocks but a decrease in plunge in the younger rocks (50º in the Early Ordovician, 30º in the Carboniferous, and 10º in the Cenozoic). The fold interlimb angle decreases from 100º at the base of the Cenozoic, to 75º in the Carboniferous, and 45º in the Ordovician.

In Famatina, at least 20 % of fold shortening is pre-Cenozoic, and most of this is in the Late Ordovician and Late Carboniferous. This early folding may be a widespread phenomena in the Andes, where most of the shortening is considered to be Cenozoic.