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Basement-cored uplifts are observed globally and remain an enigmatic feature of plate tectonics due to the fact that, in many cases, they occur distant from a plate boundary. The Laramide Bighorn Arch in Wyoming is an archetypal basement-involved foreland arch and provides an excellent setting for the investigation of such structures. Previous studies proposed diverse arch formation models; each of which predicts a unique crustal geometry. We use high-resolution crustal imaging from teleseismic P wave receiver functions to test these models. We obtained our data from 239 three-component seismometers deployed as part of the Bighorns Arch Seismic Experiment as well as coeval regional Transportable Array stations. A sequential, two-layer thickness VP/VS (H-κ) stacking algorithm constrains sediment and crustal structure. Receiver function Common Conversion Point stacking results in 2-D transect images across the arch. Our results define an upwarp of the crust beneath the central and northern arch that extends into the Powder River Basin, north-northeast of the arch. The lack of Moho-cutting faults or a Moho geometry mirroring the arch rules out most shortening models except a crustal detachment model where shortening was accomplished by fault-propagation folding on a thrust splay ramping off a midcrustal detachment fault. The mismatch between gentle, symmetric Moho and asymmetric Laramide arch geometries and their trends suggests a pre-Laramide origin for at least a part of the Moho high. This high, perhaps in combination with a lesser degree of Laramide lithospheric buckling, may have caused emergent Laramide thrusting and thus nucleated the Bighorn Arch. Our results suggest that midcrustal detachment can form basement-involved foreland arches and suggest the hypothesis that preexisting undulations in the Moho may have nucleated individual arches.

[1] 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.

[1] The basement rock and structures of the Ross Sea rift are exposed in coastal western Marie Byrd Land (wMBL), West Antarctica. Thinned, extended continental crust forms wMBL and the eastern Ross Sea continental shelf, where faults control the regional basin-and range-type topography at ∼20 km spacing. Onshore in the Ford Ranges and Rockefeller Mountains of wMBL, basement rocks consist of Early Paleozoic metagreywacke and migmatized equivalents, intruded by Devonian-Carboniferous and Cretaceous granitoids. Marine geophysical profiles suggest that these geological formations continue offshore to the west beneath the eastern Ross Sea, and are covered by glacial and glacial marine sediments. Airborne gravity and radar soundings over wMBL indicate a thicker crust and smoother basement inland to the north and east of the northern Ford Ranges. A migmatite complex near this transition, exhumed from mid crustal depths between 100–94 Ma, suggests a profound crustal discontinuity near the inboard limit of extended crust, ∼300 km northeast of the eastern Ross Sea margin. Near this limit, aeromagnetic mapping reveals an extensive region of high amplitude anomalies east of the Ford ranges that can be interpreted as a sub ice volcanic province. Modeling of gravity data suggests that extended crust in the eastern Ross Sea and wMBL is 8–9 km thinner than interior MBL (β = 1.35). Gravity modeling also outlines extensive regions of low-density (2300–2500 kg m−3) buried basement rock that is lighter than rock exposed at the surface. These regions are interpreted as bounded by throughgoing east-west faults with vertical separation. These buried low-density rocks are possibly a low-density facies of Early Paleozoic metagreywacke, or the low-density epizonal facies of Cretaceous granites, or felsic volcanic rocks known from moraines. These geophysical features and structures on land in the wMBL region preserve the record of middle and Late Cretaceous development of the Ross Sea rift. Thermochronology data from basement rocks and offshore stratigraphy suggest that the wMBL rift margin formed and most extension occurred in mid- and Late Cretaceous time, before seafloor spreading initiated between wMBL and the Campbell Plateau. The Cretaceous tectonic record in wMBL contrasts with the Transantarctic Mountains that form the western rift margin, where significant rift-flank relief developed in middle Tertiary time.

Antarctica is a key element in Earth's geodynamic and climatic systems. Nevertheless, on the eve of the 50th anniversary of the International Geophysical Year, we lack fundamental geologic and geophysical data from the deep interior of this vast continent. Meager exposures record the 3500-million-year history of a continent that participated in the formation and breakup of both the Rodinia and Gondwana super-continents. It continues to be tectonically active today although its kinematic relationship to the global plate circuit and its role as sub- strate to the world's major ice sheets remain in question.