The Olivos Olistostrome: Remnant of a Late Permian Oceanic Basin along the Southwestern Margin of Laurentia, Chihuahua, Mexico

Abstract

We report on the finding of an olistostrome containing Late Permian basalts (257 ± 7 Ma K-Ar, whole-rock) with MORB affinity embedded within deep-water Jurassic sediments (Upper Bajocian—Middle Callovian) in south-central Chihuahua, northern Mexico, at the southwestern margin of the North American craton (Laurentia). The largest Paleozoic outcrops in northern Mexico occur in the Chihuahua, Caborca, and Cortez terranes; the Los Olivos area is a small outlier with no discernible relation with either of those outcrops. Study of the Sierra de Olivos, in south-central Chihuahua, offers key elements to decipher the Late Paleozoic and Mesozoic evolution of the southern North American craton. Elements of oceanic crust in the olistostrome of Los Olivos are massive and pillowed basalts. Their chemistry shows that they are K-poor (K2O ~0.1%) and TiO2 rich (~0.8%); they are extremely depleted in elements such as Rb, Sr, and Zr, and trace-element compositions place them in the N-MORB and IAT fields of tectonic discrimination diagrams. The basalts, together with associated granitic tectonic blocks exhibiting a well-developed mylonitic texture, lie at the southern margin of Laurentia, and south of the inferred trace of the Mojave-Sonora megashear. They also lie along one of the inferred traces of the Ouachita suture. The Permian basaltic slabs were tectonically emplaced within the Middle Jurassic sedimentary sequence, but we infer that they were deposited as gravity slides. Their contacts with less competent rocks were reactivated, facilitating tectonic displacement during the Hidalgoan orogeny. The deep-marine Middle Jurassic sequence is informally assigned to the Pelayo Formation, which is covered by deep-water Cretaceous sedimentary facies of the Mezcalera Formation and by Tertiary volcanic rocks of the Sierra Madre Occidental magmatic arc. If the radiometric age determination of the oceanic basalts correctly dates the emplacement of these lavas, our study provides evidence on the existence of a Late Paleozoic—Mesozoic oceanic plate, subducted collision of either Gondwanic or Pacific elements with Laurentia, or during accretion of the Guerrero terrane (e.g., the Mezcalera plate). Characteristics of the markedly epiclastic Middle Jurassic units that host the olistostrome supports the assumption of an active margin setting for Jurassic deposition at the southern extreme of the Laurentia craton.