Evolution of structures in a continental subduction channel: an example from the Sierra de Pie de Palo, Cuyania terrane.

VAN STAAL, C.R., VUJOVICH, G.I., DAVIS, W.

Mendoza

Conferencia; Gondwana 12 "Geological and Biological Heritage of Gondwana"; 2005

Asociación Geológica Argentina

Early structures formed in the subduction channel of continental collision zones are commonly masked by later deformation and collision-related magmatism. Orogens involving the accretion of microcontinents, such as the Alps and the Ocloyic of the Cuyania terrane in Argentina, seem to have escaped much of the over-printing generally associated with such events, at least locally, and hence may be used as a proxy for the early development of continental collision zones (A-subduction). The collision complex preserved in the Sierra de Pie de Palo is an exceptionally well exposed example and complications introduced as a result of the development of oblique structures are also minimized. We focused our structural investigations on the Mesoproterozoic Pie de Palo Complex in the immediate hanging wall of the Las Pirquitas thrust and the highly deformed Lower Palaeozoic metasediments of the Caucete Group in the footwall. The Caucete Group has a tripartite stratigraphy with dolomite and poorly sorted red micaceous sandstone of the El Desecho Formation separating arenite (El Quemado/La Paz formations) and limestone (Angaco Formation). The regional distribution of these units suggests that imbrication of the rocks started early in the deformation history, prior to formation of large west-verging recumbent folds. Mesoscale interleaving of rocks of the Pie de Palo Complex and arenites of the El Quemada Formation is probably also due, at least in part, to early imbrication. The earliest penetrative structures developed in these rocks comprise a very strong bedding-parallel foliation in the metasediments and a domainal schistosity in the mafic igneous rocks. This early foliation is not axial planar to the earliest formed recumbent folds, but is instead consistently folded by them. The earliest isoclines (F1) are penetrative throughout the rock sequence on all scales and generally co-linear with a well-developed stretching lineation. The F1 structures and early thrusts have been refolded by large recumbent to strongly westerly overturned F2 structures, which are associated with a second generation of thrust-related shear zones. Meso-scale F2 folds are generally curvilinear and large-scale sheath folds have been mapped. Metamorphic minerals in the Pie de Palo Complex record relatively high pressure amphibolite facies conditions, consistent with an A-subduction zone setting. The structurally underlying Caucete Group records lower metamorphic temperatures and pressures. Hence, the Las Pirquitas thrust accommodated movements that postdate peak metamorphism in the Pie de Palo Complex, consistent with the observed retrogression along this structure. Metamorphic zircon rims dated at 455±10 Ma and significant discordance in the U–Pb system with lower intercepts between 460 and 490 Ma indicates a significant Middle-Late Ordovician tectonometamorphic event, supporting a Gondwana-Cuyania collision at this time. We interpret the bedding-parallel foliation to have formed mainly in response to imbrication accompanying lowtemperature subduction of the Caucete Group. Possibly these rocks were too cold and stiff to develop folds during this stage. F1 and F2 are typically buckle folds, with numerous parasitic folds on the limbs of the nappes. Hence, it is unlikely that they formed as a result of drag in shear zones. We interpret these folds to have developed originally as upright structures, possibly as a result of increased traction forces in the subduction channel. Continued underthrusting-related shear subsequently modified these folds into intrafolial, recumbent structures.