THE TWO-STAGE EVOLUTION OF METAMORPHIC CORE COMPLEXES: EXAMPLES FROM THE NORTHERN BASIN AND RANGE

Ongoing studies on the chronology of the development in the Albion-Raft River-Grouse Creek Ranges, ID-UT (ARG), and Snake Range, NV, reveal a dichotomy in timing of development of the metamorphic core of the complexes (MCC) versus the extensional faults that exhume them (~10 to 15 Ma apart) leading to a two-stage interpretation of their origin: 1. Thinning and flow of metamorphic rocks during crustal melting and/or diapiric ascent of plutons 2. Large-offset rotational normal faults that result in the exhumation of MCC.

After regional magmatism initiated 42-38 Ma, the mid-crustal rocks in the ARG MCC underwent ductile thinning and shearing during the formation and rise of a series of broad domes. U-Pb geochronology of plutons and rocks involved in ductile thinning indicate that most of the high temperature fabrics developed during the rise and crystallization of plutons which were emplaced at 10-15 km depth, at 32-25 Ma. The normal fault which exhumes the eastern side of this complex is dated by deposition in the Raft River Basin ca.14-7 Ma based on U-Pb dating of detrital zircon suites and intercalated lavas. Sedimentation (and inferred slip on the fault) overlaps with Apatite Fission Track ages in footwall rocks (14-8 Ma; Wells et al., 2000; Egger et al., 2003).

Geologic relations and published geochronology/thermocronology in the Snake Range MCC are also compatible with a two-part evolution of the complex, but this distinction is not as clear as in the ARG: Formation of a broad dome with significant ductile thinning of the lower plate and brittle fault attenuation of the upper plate (separated by a ductile-brittle transition) occurred during a protracted event in the Eocene-Oligocene (35-25 Ma) followed by basin formation and rapid slip along a 150 km long normal fault system which exhumed the complex in the Miocene (~ 16 Ma).

These two examples give us insight into the possibility that two fundamental but distinct physical processes occur during continental extension: 1) Differential vertical rise of material during a diapiric-like process and, 2) Exhumation by high-angle, rotational normal faults. Recognizing these as separate process in the development of metamorphic core complexes using detailed thermo and geochronology is fundamental to resolving the low-angle and detachment fault controversy.