STRUCTURAL EVOLUTION OF MCKINNEY HILLS LACCOLITH-HOST ROCK SYSTEM

Multiple tectonic events have been recorded in the McKinney Hills, Big Bend National Park, TX. The region is underlain by Cretaceous through Eocene shallowly southwest dipping limestones, sandstones, shales, and fluvial rocks, intruded by a 9 km by 3 km Oligocene-age alkali feldspar quartz syenite laccolith-sill complex. Detailed mapping, kinematic analysis of brittle faults, and analysis of ASTER/DEM images were used to discriminate structures into different tectonic events and evaluate pluton construction. High angle-reverse faults, open folds, and monoclines trending N-NW characterize the oldest structures. West of the structural aureole of the intrusion, these open folds are attributed to regional Late Cretaceous-Early Tertiary Laramide contraction seen elsewhere in the park. Intrusion of augite-hornblende-fayalite composite alkali feldspar quartz syenite occurred by multiple emplacement mechanisms. Concordant host rock doming occurred on the W- and NW- margins. The W-margin is defined by discordant contacts and ‘fingers' protrude from the main intrusion at km- to m-scales. A flat, shallowly-west-dipping floor of the intrusion is exposed on the E-margin. NW-trending Mode I and high-angle normal oblique faults characterize the youngest structures in the area and are ascribed to regional Miocene Basin and Range extension.

Field evidence indicates that the intrusive suite was emplaced by a series of inflated sills and connecting risers ascending and propagating along N-trending, preexisting Laramide structures. Complex contact relationships with host rocks represent multiple space-making processes. Along the NW margin, the intrusion concordantly intruded bedding planes of the Pen Formation and lifted Upper Cretaceous through Eocene overburden forming a laccolith geometry. Discordant intrusion-host rock contact relationships occur along the western and southern margins. Exposures display host rock xenoliths incorporated into the intrusion, discordant bedding planes cut by the intrusion, local magmatic breccias, and intrusive ‘fingering' at map to outcrop scales.

These results indicate that laccolith formation in the Big Bend region may occur by multiple mechanisms and that magmatism may in part be focused along pre-existing structures.