Online

Pidgeon, Jarret William

Geology

2023

M.S.

The Mongol-Okhotsk Suture Zone (MOSZ) is a continental-scale structure that signifies the closure of a Paleozoic ocean basin. Though suturing of the Siberian Craton and southern Mongolian terranes is thought to have occurred in the Mesozoic Era, the exact timing and mechanisms remain debated. This prolonged period of orogenic activity is believed to be responsible for forming much of the Central Asian Orogenic Belt (CAOB) that encompasses essentially all of Mongolia, part of China, Siberia, and Kazakhstan. Also permeated throughout the CAOB are intra-continental, post-suturing structures. In the Late Jurassic--Early Cretaceous, a significant, crustal-scale extensional episode affected central and eastern Asia and resulted in rift basin and metamorphic core complex formation, including those that formed along the MOSZ. This study examines the geology of the Ulz Gol region of the MOSZ in the Khentii Province of northeastern Mongolia. The region is of interest for several reasons: 1) the presence of topographic lineaments defined by NE-SW (and lesser E-W) ridgeline and drainage trends, 2) rock units mapped as Precambrian that may record younger ages when geochronology is applied, and 3) its proximity to the Muron Shear Zone (a left-lateral shear zone <60 km east of Ulz Gol), and the Ereendaava Metamorphic Core Complex (EMCC; an Early Cretaceous NW-SE extensional structure <100 km northeast of Ulz Gol). The primary objectives of this research are to use data from field geology, microstructural analysis, and 40Ar/39Ar dating to determine: (1) the temperature, kinematics, and timing of deformation at singular outcrops, and (2) the general patterns that establish relationships between the outcrops studied and the greater MOSZ. The results of this work show that Ulz Gol Region displays structural complexity in rocks units previously mapped as old as Precambrian and as young as Triassic. Several samples of rocks mapped as Precambrian have mylonitic fabrics that are, rather, Mesozoic in age, which is more consistent with suturing and later extension. The oldest 40Ar/39Ar ages were obtained from the NW flank of the Ulz Gol valley. There, regional folding about an E-W axis is supported by data from outcrops scattered across the region; however, the timing of folding is not well constrained. Mylonitic rocks dated on this side of the valley yield Early Jurassic ages. The youngest ages were obtained in mylonitic rocks along the SE flank of the Ulz Gol river valley. There, multiple samples record ENE-WSW-directed motion, with a primarily reverse-oblique relationship, and yield ages as young as ~100 Ma. This area is juxtaposed with Cretaceous-Tertiary sediments and is along strike of the EMCC. Several samples in the field area record evidence for brittle deformation overprints (e.g., microfractures) and yield Cenozoic minimum 40Ar/39Ar ages. These data suggest a younger system of faults likely reactivated Mesozoic structures. Overall, the Ulz Gol valley appears to be a long-lived structural corridor along the northeastern MOSZ trend, with more components of the suture likely buried beneath Quaternary alluvial fill in the valley.