In Marie Byrd Land, West Antarctica, the Fosdick Mountains migmatite-cored gneiss dome was exhumed from mid- to lower middle crustal depths during the incipient stage of the West Antarctic Rift system in the mid-Cretaceous. Prior to and during exhumation, major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle-derived diorite in dikes. A succession of melt- and magma-related structures formed at temperatures in excess of 665. ±. 50. °C based on Ti-in-zircon thermometry. These record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique South Fosdick Detachment zone. Solid-state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels.To determine the duration of exhumation and cooling, we sampled granite and gneisses at high spatial resolution for U-Pb zircon geochronology and 40Ar/39Ar hornblende and biotite thermochronology. U-Pb zircon crystallization ages for the youngest granites are 102Ma. Three hornblende ages are 103 to 100Ma and 12 biotite ages are 101 to 99Ma. All overlap within uncertainty. The coincidence of zircon crystallization ages with 40Ar/39Ar cooling ages indicates cooling rates >100°C/m.y. that, when considered together with overprinting structures, indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting. Orientations of structures and age-constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the Marie Byrd Land margin. The rotation may be a result of localized extension arising from unroofing and arching of the Fosdick dome, extensional opening within a pull-apart zone, or changes in plate boundary configuration.The rapid tectonic and temperature evolution of the Fosdick Mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension/transtension, and accords with numerous studies of migmatite-cored gneiss domes in transcurrent settings.
Bibliographical noteFunding Information:
Research was funded by the National Science Foundation Office of Polar Programs grants NSF-OPP 0337488 to C. Teyssier and NSF-OPP 0338279 to C.S. Siddoway. We thank Mike Roberts, Forrest McCarthy, and Allen O'Bannon for field coordination and safety. For logistical support, we thank employees of Raytheon Polar Services, ANG 109th, and Kenn Borek Air crews. Reviews by B. Reno and N. Charles greatly improved the quality of this manuscript. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
© 2015 Elsevier B.V.
- <sup>40</sup>Ar/<sup>39</sup>Ar thermochronology
- Gneiss domes
- Rapid cooling