The Crustal Evolution of Nemegt and Altan Uul, Southern Mongolia

Abstract

This thesis concerns the crustal evolution of Nemegt and Altan Uul in the Gobi Altai mountains of southern Mongolia. Nemegt and Altan Uul consist of polydeformed Palaeozoic rocks uplifted in the Cenozoic at a restraining bend along the active left-lateral Gobi-Tien Shan intra-continental fault system, one of several east-west trending left-lateral intra-continental transpressional fault systems associated with eastward continental extrusion tectonics in Central Asia. The tectonic evolution of southern Mongolia is of particular interest as it forms part of the Central Asian Orogenic Belt, which is the largest area of Phanerozoic continental growth on Earth, and is a natural laboratory for studying processes of continental growth and deformation including terrane accretion, ophiolite obduction, terrane amalgamation, terrane dispersal and crustal reactivation. The uplifted Palaeozoic rocks exposed in Nemegt and Altan Uul offer an opportunity to understand multiple phases of the crustal evolution of southern Mongolia. A series of cross-strike transects of Nemegt and Altan Uul were carried out to document the lithologies and structure of the ranges. Samples were taken along the transects and at several important localities, to constrain the metamorphic petrography of the rocks in the ranges. This data is used to define several distinct east-west trending litho-tectonic sequences in Nemegt and Altan Uul. The ranges have a systematic south to north litho-tectonic variation from greenschist grade meta-volcanic and volcaniclastic rocks, thrust north over a discontinuous ophiolite belt, which is thrust north over greenschist to epidote-amphibolite grade arkosic to mature meta-sedimentary rocks. Four phases of deformation are identified from cross-cutting field relationships and constrained by existing regional data: east-west trending south-dipping cleavage (D1), and north-vergent folds of cleavage and north-directed ductile thrust shear zones (D2) formed during late Carboniferous south to north arc-terrane accretion and ophiolite obduction. East-west and northeast-southwest trending D3 normal faults formed during Cretaceous basin extension. East-west and northwest-southeast trending D4 left-lateral oblique-slip and dip-slip thrust faults formed during Cenozoic transpressional deformation and define the modern mountain ranges. The structures identified are conservatively extrapolated to depth to suggest Nemegt and Altan Uul have a positive flower structure in cross-section. An evolutionary model of Nemegt and Altan Uul suggests that D1 and D2 structures and the ophiolitic rocks in the area may represent south-dipping east-west trending fabrics and rheological weaknesses that have been reactivated in a left-lateral transpressional sense in the Cenozoic