THE LINK BETWEEN ORDOVICIAN – EARLY DEVONIAN TERRANE COLLISION AND TRANSLATION AND THE FLARE-UP AND SUDDEN CESSATION OF THE FAMATINA ARC

The Paleozoic Gondwana margin of Argentina records widespread tectonism and multiple continental arc magmatic events. Numerous studies in the last 20+ years have focused on accretion of the Precordillera and Chilenia terranes and their role in driving large-scale crustal shortening in the proposed overriding plate, which contains the Cambro-Ordovician Famatina arc of the western Sierra Pampeanas. This long-lived and regionally extensive collisional system preserves a variable history along and across strike with the result that studies focused on one aspect have led to contradictions on what is driving some of the major processes. We provide here a perspective that incorporates results from our transects as well as published research from along and across strike, from the Precordillera to the eastern Sierra Pampeanas.

Increased precision of isotopic ages reveal that several distinct changes in the regional depositional, structural, metamorphic, and magmatic history are coeval within as little as 3-5 my. The correlation of these changes with specific structural fabrics support the formerly proposed concept of distinct phases within the Famatina orogeny. The clearest indication of a major change in tectonic setting occurs at 470-465 Ma. Peak magmatism in the Famatina arc followed by abrupt shutdown of the arc system coincides with drowning of the Precordillera carbonate platform, deep crustal burial of the Famatina forearc, and development of shear zones in the retroarc. A simple orthogonal collision model is complicated by the existence of some blocks between the Precordillera and Famatina arc that show little to no evidence of this Ordovician event. Instead, these blocks preserve only major events at ~1.2-1.0 Ga and ~435-405 Ma, or in some cases little or no regional metamorphism. Some of these current juxtapositions may be explained by out-of-sequence thrusting accompanied by higher structural level extension. Some of these blocks, however, are bounded by Late Silurian – Early Devonian sinistral transpressional to transtensional ductile shear zones that are part of regionally extensive localized shear zones in both the retro- and forearc regions. The excellent exposure of the region provides abundant opportunity for study of the whole-crustal response to long-lived continental collision.