Late Triassic rift tectonics at the northernmost Andean margin (Sierra Nevada de Santa Marta)
Résumé
Break-up of western Pangaea initiated a back-arc type tectonic setting along the northwestern margin of Gondwana, which resulted in the accumulation of sedimentary and volcanic deposits in extensional basins during the Middle-Late Triassic. The Sierra Nevada de Santa Marta massif, at the northwestern termination of the Andes, exposes Upper Triassic syn-rift successions beneath a voluminous Lower Jurassic volcanic cover. In this contribution, we present a sedimentological and stratigraphical analysis of the Upper Triassic Los Indios and Corual formations, by integrating whole-rock geochemistry, zircon U-Pb geochronology, and a biostratigraphic revaluation of the conchostracan fossil content. We divide the Los Indios Formation into two sandy to conglomeratic and two muddy members. First and third members were deposited in a coarse-grained delta environment. The second and fourth members are interpreted to represent offshore deposits resulting from the encroachment of gravity-flow lobes. Volcanoclastic and volcanic rocks interbedded with sedimentary deposits of the Los Indios and Corual formations record bimodal basic-acid volcanism with a variably alkaline tendency. Based on their alkaline and calc-alkaline signature, and rare earth and trace element patterns, volcanic rocks display a back-arc affinity. With respect to the age of the Los Indios Formation, we recovered conchostracans diagnostic of Upper Carnian and Lower Norian fauna, bearing similarity with faunal assemblages of other Upper Triassic deposits exposed in the northern Andes. The Los Indios and Corual formations are considered as syn-rift successions in a back-arc extension setting. The development of this back-arc basin coincided with the Late Triassic separation of the Oaxaquia, Acatlán, and Chortis terranes during Pangaea break-up. Finally, the Early to Middle Jurassic evolution of the NW Gondwana margin is marked by the inception of a magmatic arc, which characterizes the establishment of Andean-type subduction coeval to ignimbrite flare-up.