A Silurian-Devonian active margin in the proto-Andes – new data on an old conundrum

Abstract

ABSTRACT The Devonian Terra Australis accretionary orogen along the proto-Andes is characterized south of 33°S by the eastward accretion of the Chilenia and Chaitenia terranes. From the middle Silurian to the early Carboniferous the evolution north of 33°S, in the southern central Andes, is enigmatic, and herein called the Silurian-Devonian lull. This lull is notable for the absence of magmatism, metamorphism and deformation while mature siliciclastic detritus accumulated in marginal basins governed by depositional and not by tectonic controls. Detrital zircon age spectra uniformly display a marked scarcity of Silurian and Devonian ages. No evidence of a Devonian active margin system has been put forward so far. Newly discovered volcanic arc lavas in the Devonian-early Carboniferous Compléjo Epimetamórfico Chañaral (CEMC) of northern Chile give evidence of Devonian to early Carboniferous intermediate arc magmatism in this area for the first time. The CEMC also includes mafic volcanic rocks as syndepositional intercalations and as tectonic accretions. They predominantly have E-MORB and OIB characteristics and very likely formed at an extensional volcanic rifted margin environment. The veiled presence of scarce arc magmatism during the Silurian-Devonian lull might be taken as evidence of flat-slab subduction. However, present-day flat-slab regimes produce strongly telescoped mountain belts with high topography and relief. The absence of any deformation during the lull attests against this interpretation. Assuming an extension of Chilenia into northern Chile is unlikely because of different stratigraphies and the absence of ophiolitic sutures. The Silurian-Devonian lull may be explained as a magmatic lull between the flare-ups of the Famatinian and Gondwanide orogenies (Cambrian-Ordovician and Carboniferous-Triassic, respectively) the latter of which is attributed to rapid trenchward migration of the volcanic axis. Magmatic lulls are characterized by less than 25% of magma production relative to flare-ups and occur at times of slow landward migration of an arc system.