Mid-Cretaceous oblique rifting of West Antarctica: Emplacement and rapid cooling of the Fosdick Mountains migmatite-cored gneiss dome
Introduction
Extension of thickened and hot crust commonly leads to the formation of migmatite-cored metamorphic core complexes (MCC) (Coney and Harms, 1984, Lister and Davis, 1989, Whitney et al., 2004, Whitney et al., 2013). Within these crustal-scale structures, detachment zones record significant amounts of localized extension and preserve an interface between cool upper crust and hot middle to lower crust (Lister and Davis, 1989, Malavieille, 1993, Mulch et al., 2006). Studies in the Basin and Range (e.g. Crittenden et al., 1980), the northern Cordillera (USA and Canada) (e.g. Foster et al., 2001, Vanderhaeghe et al., 1999), and the Aegean region (e.g. Brichau et al., 2008, Denele et al., 2011) have shown a spatial and temporal link between extensional detachment tectonics and the emplacement of gneiss/migmatite domes and granite bodies. Field, thermochronologic, and numerical modeling studies suggest the intrusion of granites may initiate the formation of detachment zones (e.g. Foster et al., 2001, Lister and Baldwin, 1993, Tirel et al., 2006, Tirel et al., 2008) and the presence of a low-viscosity layer in the crust (partially molten) may enhance strain localization and the development of rolling-hinge detachment systems (Whitney et al., 2013).
In extending orogens, regions with migmatite-cored gneiss domes and MCCs typically record cooling rates that range from 30 °C/m.y. to > 100 °C/m.y. where granite intrusion and detachments may be linked (Whitney et al., 2013). This includes areas such as the northern Cordilleran core complexes (Fayon et al., 2004, Foster et al., 2001, Gordon et al., 2008, Kruckenberg et al., 2008, Norlander et al., 2002), the Basin and Range province (Foster et al., 1990, Foster et al., 1992), the Liaodong Peninsula in NE China (Charles et al., 2012, Yang et al., 2007), and the Aegean domain (Brichau et al., 2008, Lister et al., 1984). In addition, recent thermomechanical numerical models have shown that gneiss domes are exhumed over short timescales (a few m.y.) in cases of localized extension (Rey et al., 2009a, Rey et al., 2009b). These systems are characterized by localized upper crustal deformation, isothermal decompression, advection of heat (solidus) toward the surface, and crystallization of partially molten crust at shallow depths (Rey et al., 2009a, Rey et al., 2009b, Whitney et al., 2013).
In order to evaluate the timing of detachment tectonics, cooling, and exhumation of the Fosdick dome, we present new 40Ar/39Ar hornblende and biotite data combined with previously obtained U–Pb data on zircon, titanite, and monazite, 40Ar/39Ar data on hornblende, biotite, muscovite, and K-feldspar, and fission-track data on apatite from the Fosdick Mountains region. The new 40Ar/39Ar data clarify the Fosdick dome history from emplacement to cooling, expand understanding of the significant thermal event in the mid-Cretaceous West Antarctic region, assess the mid-crustal response to the initiation of West Antarctic rifting, and inform the cooling history of gneiss domes and MCCs.
Section snippets
Geologic setting
Cretaceous extension and crustal heating affected the wide accretionary zone developed in Paleozoic–Mesozoic time along the East Gondwana margin of West Antarctica and Zealandia (Davey and Brancolini, 1995, Luyendyk, 1995, Mortimer et al., 2006, Siddoway, 2008, Tulloch et al., 2006). A Jurassic–Cretaceous magmatic arc developed along the West Antarctica–Zealandia margin (Bradshaw et al., 1997, Mortimer et al., 1999). The magmatic arc included emplacement of the subduction-related Median
Fosdick Mountains migmatite-cored gneiss dome
The Fosdick Mountains preserve lithologies and structures associated with the Devonian-Carboniferous and Cretaceous crustal melting and deformation episodes. Cretaceous granites and migmatites are more voluminous than Devonian-Carboniferous granites and migmatites in the Fosdick Mountains and most fabrics and structures record Cretaceous deformation. However, kilometer-scale domains of migmatitic paragneiss, migmatitic orthogneiss, and granites associated with Devonian-Carboniferous crustal
Structures and kinematics within the Fosdick dome
Within the Fosdick Mountains, fabrics and structures are characterized relative to emplacement of the leucogranite sheets and movement along the SFD. These are: 1) “Early” syn-kinematic footwall/detachment fabrics and structures; 2) “Late” syn-kinematic footwall/detachment fabrics and structures; and 3) “Latest” post-kinematic hanging wall fabrics and structures.
Thermochronology
Previous studies concluded that the Fosdick dome sustained high temperatures during detachment tectonics and then underwent rapid cooling (McFadden et al., 2010a, Richard et al., 1994, Siddoway et al., 2004b). This study confirms and refines this rapid cooling history from zircon crystallization through biotite closure temperature in the Fosdick dome.
Titanium-in-zircon thermometry
Titanium concentrations from zircon grains were obtained by SHRIMP II at the Research School or Earth Sciences of the Australian National University (RSES-ANU) using methods similar to Hiess et al. (2008). Previous U–Th–Pb analytical spots (McFadden et al., 2010a) were lightly polished and then the same area within the grains was analyzed. The Ti concentrations were used to calculate the crystallization temperatures for zircon assuming TiO2-saturated conditions according to:T(°C) = [(4800 ±
Cooling of the Fosdick dome
In the Fosdick Mountains, temperature (T)–time (t) paths interpreted from combined geochronologic data indicate that the youngest suite of voluminous migmatites and granites crystallized between ca. 109 and 102 Ma and underwent rapid cooling between ca. 102 and 95 Ma (Fig. 9). Rapid cooling proceeded from granite crystallization through biotite closure temperature from ca. 102 to 99 Ma followed by slower cooling from biotite closure temperature through K-feldspar closure temperature from ca. 99 to
Summary
U–Pb zircon crystallization ages and 40Ar/39Ar hornblende and biotite cooling ages, over the time interval ca. 102–99 Ma, indicate cooling rates ranging from 85 to 155 °C/m.y. Rapid cooling followed a period of near-isothermal decompression and detachment tectonics that brought hot, deep migmatites and granites toward the surface. These migmatites and crustally-derived granites were emplaced at shallow levels in the crust, which led to rapid cooling owing to conductive heat loss. Below the
Acknowledgments
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
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