The structural evolution of the McKinney Hills Laccolith, Big Bend National Park, Texas

Abstract

Multiple tectonic events are recorded in the McKinney Hills, Big Bend National Park, TX. The region is underlain by Cretaceous through Eocene shallowly southwest dipping-limestones, sandstones, and shales, 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/aerial orthoimages were used to discriminate structures into different tectonic events and evaluate pluton construction as well as discriminate gross lithologies. 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. 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.

Mechanical modeling indicates the laccolith was emplaced at a near neutrally buoyant level and magma overpressure, on the order of 32 MPa, was used to lift a maximum of 1400 meters of overburden. These results indicate that laccolith formation elevation and form in the Big Bend region may strongly be controlled by buoyancy and not local horizontal anisotropies. Results also indicate that magmatism may in part be focused along pre-existing structures. A comparison of fingered sheet peripheries at the Shonkin Sag laccolith, Montana and Henry Mts., Utah with McKinney Hills finger forms is attributed to not only viscosity contrasts with magma and host rocks as Pollard (1975) suggested, but the length of time which the magma has to crystallize.