The emplacement of the Cuyania terrane against the Western Gondwana margin: main suture and foreland shear zones, Argentina.

MARTINO, R.D., GUERESCHI, A.B., VUJOVICH, G.I., OTAMENDI, J.E., TIBALDI, A.M., CRISTOFOLINI, E.A.

Buzios

Congreso; Gondwana 14; 2011

Universidad de Rio de Janeiro

The Cuyania terrane was accreted against the pacific margin of Gondwana at ~455 Ma, culminating its emplacement at ~400 Ma. Two main provenance models have been postulate. (A) In a Laurentian allochthonous model, the Cuyania terrane would have detached at ~515 Ma of the Ouachita embayment and emplaced in three modes: (1) a collision in the medium Ordovician of eastern Laurentia with western Gondwana leaving a “tectonic tracer”, (2) a microcontinent detached from Laurentia in the early-medium Cambrian, drifted for the Iapetus ocean, being assembled by subduction and later collision with the Gondwana margin, and (3) the collision of an hypothetical laurentian promontory during the clockwise rotation and moving away of Laurentia from Gondwana. (B) In a Gondwana parautochthonous model, the proposed emplacement mode for the Cuyania terrane was: (4) strike-slip accretion of a microcontinent (~1000 x ~200 km), which affected the Gondwana margin lasting ~115 Ma. Contrasting dextral-sinistral strike-slip or thrusting movements were proposed for this process. The deformation was weakly in the eastern Cuyania terrane and strongly in the Gondwana margin. In the latter, a localized deformation with west vergence ductile thrusting has taken place; associated backthrusting were located in the Puna, Sierras Pampeanas transition or inside minor blocks on these provinces. This deformation stylke is recognized in a large area (~1500 x ~500 km; 22-37ºS, 64-68ºW). Protomylonites to ultramylonites, with a planar foliation striking NNW to NNE and dipping toward the east with variable angles and leves with sheath and oblique folding are recognized. A stretching lineation, pitching near 90º ± 30º, is associated. Kinematic indicators show up to the west movements. All this evidence is interpreted as a dominant orthogonal contractional strain. The huge NNW sinistral shear zone named Valle Ancho- Jagué, combining a lateral ramp with a duplex zone, acted as the northern limit of the Cuyania terrane. This shear zone merges with the Valle Fértil Lineament, outlining the possible suture with the Gondwana margin, marked by strongly mylonitized marbles, anorthosites and orthogneisses. The southern limit proposed here could be the Limay lineament and/or the largely debated Huincul faul zone in the Neuquina basin. The age of the localized ductile deformation is medium to late Ordovician, later deformed near the ductile-fragile transition during the early Silurian up to the early Devonian. This is the time of the definitive amalgamation of the Cuyania terrane and the waning of the magmatism, associated with the subduction and later collision. Later on, reactivation or development of new shear zones around the Devonian-Carboniferous boundary are the expression of the Chanica orogeny related to the emplacement of the Chilenia terrane against the Gondwana.