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The Fosdick Mountains, West Antarctica, expose a 15 x 80 km migmatite‐cored gneiss dome consisting of migmatitic gneisses, diatexite migmatite, and subhorizontal leucogranite sheets. The Fosdick dome was emplaced and exhumed in the mid‐Cretaceous due to oblique extension associated with the West Antarctic Rift system along the West Antarctic–New Zealand segment of East Gondwana. The dome is bounded to the south by a dextral oblique detachment structure and to the north by an inferred dextral strike‐slip fault. Within the Fosdick dome and in the detachment zone, granite occupies leucosomes, dikes, sills, and dilatant and shear structures. The pattern of kilometer‐scale domains of migmatite and granite suggest that lithologic variations and heterogeneous deformation (boudinage) resulted in pressure gradients that enhanced melt flow and magma accumulation in the Fosdick dome. Steep foliations are overprinted, folded, and transposed by subhorizontal fabrics. The crosscutting relationship is interpreted as a transition from wrench deformation to oblique divergence. Steep structures in the dome host concordant, subvertical leucosome and granite sheets yielding SHRIMP U‐Pb zircon ages between ca. 117 and 114 Ma. Prevalent subhorizontal domains host large volumes of subhorizontal diatexite migmatite and granite sheets that yield U‐Pb zircon ages between ca. 109 and 102 Ma. These ages indicate a timescale for melt influx of approximately 15 Ma and that the transition from wrench to oblique divergence may have occurred in as little as 5 Ma. Granites with crystallization ages between ca. 109 and 102 Ma were also emplaced in the South Fosdick Detachment zone, indicating that the detachment was active during oblique divergence. SHRIMP U‐Pb titanite ages between ca. 102 and 97 Ma for late‐ to post‐tectonic diorite dikes are interpreted as emplacement ages and give a minimum age for gneissic foliation development during detachment faulting. The Fosdick Mountains preserve a record of the middle to lower crustal response to a transition from wrench to oblique extensional deformation. Overprinting structural relationships show that a change in the angle of oblique extension can induce accumulation of subhorizontal magma sheets and lead to initiation of a detachment zone.

The Fosdick migmatite-granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian-Carboniferous and another in the Cretaceous. The Hf-O isotope composition of zircons from Devonian -Carboniferous granites is explained by mixing of material from two crustal sources represented by the high-grade metamorphosed equivalents of a Lower Palaeozoic turbidite sequence and a Devonian calc-alkaline plutonic suite, consistent with an interpretation that the Devonian-Carboniferous granites record crustal reworking without input from a more juvenile source. The Hf-O isotope composition of zircons from Cretaceous granites reflects those same two sources, together with a contribution from a more juvenile source that is most evident in the detachment-hosted, youngest granites. The relatively non-radiogenic 1Hf isotope characteristics of zircons from the Fosdick complex granites are similar those from the Permo-Triassic granites from the Antarctic Peninsula. However, the Fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of Gondwana, including the Tasmanides of Australia and the Western Province of New Zealand, where the wider range of more radiogenic 1Hf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis. These new results highlight prominent arc-parallel and arc-normal variations in the mechanisms and timing of crustal reworking v. crustal growth along the former active margin of Gondwana.

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The Fosdick Mountains form an E-W trending migmatite dome in the northern Ford Ranges of Marie Byrd Land, Antarctica. Pervasively folded migmatites derived from lower Paleozoic greywacke and middle Paleozoic plutonic rocks constitute the dome. New field research documents a transition from melt-present to solid-state deformation across the south flank of the dome, and a mylonitic shear zone mapped for 30 km between Mt. Iphigene and Mt Richardson. Kinematic shear sense is dextral normal oblique, with top-to-the-SW and-WSW transport. A U-Pb age of 107 Ma, from a leucosome-filled extensional shear band, provides a meltpresent deformation age, and a U-Pb age of 96 Ma, from a crosscutting granitic dike, gives a lower age limit for deformation. The shear zone, here named the South Fosdick detachment zone, forms the south flank of the migmatite dome and was in part responsible for the exhumation of mid-crustal rocks.