Multidecadal Basal Melt Rates and Structure of the Ross Ice Shelf, Antarctica, Using Airborne Ice Penetrating Radar

Basal melting of ice shelves is a major source of mass loss from the Antarctic Ice Sheet. In situ measurements of ice shelf basal melt rates are sparse, while the more extensive estimates from satellite altimetry require precise information about firn density and characteristics of near‐surface layers. We describe a novel method for estimating multidecadal basal melt rates using airborne ice penetrating radar data acquired during a 3‐year survey of the Ross Ice Shelf. These data revealed an ice column with distinct upper and lower units whose thicknesses change as ice flows from the grounding line toward the ice front. We interpret the lower unit as continental meteoric ice that has flowed across the grounding line and the upper unit as ice formed from snowfall onto the relatively flat ice shelf. We used the ice thickness difference and strain‐induced thickness change of the lower unit between the survey lines, combined with ice velocities, to derive basal melt rates averaged over one to six decades. Our results are similar to satellite laser altimetry estimates for the period 2003–2009, suggesting that the Ross Ice Shelf melt rates have been fairly stable for several decades. We identify five sites of elevated basal melt rates, in the range 0.5–2 m a⁻¹, near the ice shelf front. These hot spots indicate pathways into the sub‐ice‐shelf ocean cavity for warm seawater, likely a combination of summer‐warmed Antarctic Surface Water and modified Circumpolar Deep Water, and are potential areas of ice shelf weakening if the ocean warms

From Source to Sink: Petrogenesis of Cretaceous Anatectic Granites from the Fosdick Migmatite-Granite Complex, West Antarctica

This is a pre-copyedited, author-produced version of an article accepted for publication in Journal of Petrology following peer review. The version of record Brown, C. R., Yakymchuk, C., Brown, M., Fanning, C. M., Korhonen, F. J., Piccoli, P. M., & Siddoway, C. S. (2016). From Source to Sink: Petrogenesis of Cretaceous Anatectic Granites from the Fosdick Migmatite–Granite Complex, West Antarctica. Journal of Petrology, 57(7), 1241–1278 is available online at: https://doi.org/10.1093/petrology/egw039Anatectic granites from the Fosdick migmatite-granite complex yield U-Pb zircon crystallization ages that range from 115 to 100Ma, with a dominant grouping at 107-100 Ma, which corresponds to the timing of dome formation during the regional oblique extension that facilitated exhumation of the complex. The occurrence of leucosome-bearing normal-sense shear zones inmigmatitic gneisses indicates that suprasolidus conditions in the crust continued into the early stages of doming and exhumation of the complex. The structure allows access to variably oriented granites in networks of dykes at deeper structural levels and subhorizontal sheeted granites at shallower structural levels within the complex. This feature allows an evaluation of the mechanisms that modify the composition of granite melts in their source and of granite magmas during their ascent and emplacement using whole-rock major, trace element and Sr and Nd isotope compositions, zircon Hf and O isotope compositions, and phase equilibria modelling of potential source rocks. Geochemical variability within the granites is attributed to source heterogeneity and blending of melts, which themselves are consistent with derivation from regional metasedimentary and metaplutonic source materials. The granites typically contain coarse blocky K-feldspar and/or plagioclase grains within interstitial quartz, and have low Rb/Sr ratios and large positive Eu anomalies. These features are inconsistent with the composition of primary crustal melts derived from metasedimentary and metaplutonic source materials, but consistent with early fractional crystallization of feldspar and subsequent drainage of the fractionated melt. Processes such as peritectic mineral entrainment and accessory mineral dissolution, entrainment and crystallization did not have any significant influence on the major and trace element composition of the granites. The granites in the networks of dykes are interpreted to represent choking of magma transport channels through the middle crust as the rate of magma flow declined during doming and exhumation, whereas the sheeted granites record collapse of subhorizontal, partially crystallized layers of magma by filter pressing and melt exfiltration during vertical shortening associated with doming and exhumation. These processes separated feldspar-rich residues from evolved melt. Based on the results of this study, caution is urged in estimating melt proportion from the volume of granite retained in migmatitic gneiss domes, as the granites may not represent liquid compositions.US National Science Foundation [ANT0944615, OPP-0338279, OPP-0944600, EAR1032156]NISPLab at the University of MarylandGeological Society of AmericaNational Science and Engineering Research Council of Canad

Holocene Deglaciation of Marie Byrd Land, West Antarctica

Surface exposure ages of glacial deposits in the Ford Ranges of western Marie Byrd Land indicate continuous thinning of the West Antarctic Ice Sheet by more than 700 meters near the coast throughout the past 10,000 years. Deglaciation lagged the disappearance of ice sheets in the Northern Hemisphere by thousands of years and may still be under way. These results provide further evidence that parts of the West Antarctic Ice Sheet are on a long-term trajectory of decline. West Antarctic melting contributed water to the oceans in the late Holocene and may continue to do so in the future

Geological Insights from the Newly Discovered Granite of Sif Island between Thwaites and Pine Island Glaciers

Large-scale geological structures have controlled the long-term development of the bed and thus the flow of the West Antarctic Ice Sheet (WAIS). However, complete ice cover has obscured the age and exact positions of faults and geological boundaries beneath Thwaites Glacier and Pine Island Glacier, two major WAIS outlets in the Amundsen Sea sector. Here, we characterize the only rock outcrop between these two glaciers, which was exposed by the retreat of slow-flowing coastal ice in the early 2010s to form the new Sif Island. The island comprises granite, zircon U-Pb dated to ~177–174 Ma and characterized by initial ɛNd, 87Sr/86Sr and ɛHf isotope compositions of -2.3, 0.7061 and -1.3, respectively. These characteristics resemble Thurston Island/Antarctic Peninsula crustal block rocks, strongly suggesting that the Sif Island granite belongs to this province and placing the crustal block's boundary with the Marie Byrd Land province under Thwaites Glacier or its eastern shear margin. Low-temperature thermochronological data reveal that the granite underwent rapid cooling following emplacement, rapidly cooled again at ~100–90 Ma and then remained close to the Earth's surface until present. These data help date vertical displacement across the major tectonic structure beneath Pine Island Glacier to the Late Cretaceous

Paleozoic tectonism on the East Gondwana margin: Evidence from SHRIMP U-Pb zircon geochronology of a migmatite-granite complex in West Antarctica

The Fosdick Mountains migmatite-granite complex in West Antarctica records episodes of crustal melting and plutonism in Devonian-Carboniferous time that acted to transform transitional crust, dominated by immature oceanic turbidites of the accretionary margin of East Gondwana, into stable continental crust. West Antarctica, New Zealand and Australia originated as contiguous parts of this margin, according to plate reconstructions, however, detailed correlations are uncertain due to a lack of isotopic and geochronological data. Our study of the mid-crustal exposures of the Fosdick range uses U-Pb SHRIMP zircon geochronology to examine the tectonic environment and timing for Paleozoic magmatism in West Antarctica, and to assess a correlation with the better known Lachlan Orogen of eastern Australia and Western Province of New Zealand. NNE-SSW to NE-SW contraction occurred in West Antarctica in early Paleozoic time, and is expressed by km-scale folds developed both in lower crustal metasedimentary migmatite gneisses of the Fosdick Mountains and in low greenschist-grade turbidite successions of the upper crust, present in neighboring ranges. The metasedimentary rocks and structures were intruded by calc-alkaline, I-type plutons attributed to arc magmatism along the convergent East Gondwana margin. Within the Fosdick Mountains, the intrusions form a layered plutonic complex at lower structural levels and discrete plutons at upper levels. Dilational structures that host anatectic granite overprint plutonic layering and migmatitic foliation. They exhibit systematic geometries indicative of NNE-SSW stretching, parallel to a first-generation mineral lineation. New U-Pb SHRIMP zircon ages for granodiorite and porphyritic monzogranite plutons, and for leucogranites that occupy shear bands and other mesoscopic-scale structural sites, define an interval of 370 to 355 Ma for plutonism and migmatization. Paleozoic plutonism in West Antarctica postdates magmatism in the western Lachlan Orogen of Australia, but it coincides with that in the central part of the Lachlan Orogen and with the rapid main phase of emplacement of the Karamea Batholith of the Western Province, New Zealand. Emplaced within a 15 to 20 million year interval, the Paleozoic granitoids of the Fosdick Mountains are a product of subduction-related plutonism associated with high temperature metamorphism and crustal melting. The presence of anatectic granites within extensional structures is a possible indication of alternating strain states ('tectonic switching') in a supra-subduction zone setting characterized by thin crust and high heat flow along the Devonian-Carboniferous accretionary margin of East Gondwana

The geological history and evolution of West Antarctica

West Antarctica has formed the tectonically active margin between East Antarctica and the Pacific Ocean for almost half a billion years, where it has recorded a dynamic history of magmatism, continental growth and fragmentation. Despite the scale and importance of West Antarctica, there has not been an integrated view of the geology and tectonic evolution of the region as a whole. In this Review, we identify three broad physiographic provinces and present their overlapping and interconnected tectonic, magmatic and sedimentary history. The Weddell Sea region, which lays furthest from the subducting margin, was most impacted by the Jurassic initiation of Gondwana break-up. Marie Byrd Land and the West Antarctic rift system developed as a broad Cretaceous to Cenozoic continental rift system, reworking a former convergent margin. Finally, the Antarctic Peninsula and Thurston Island preserve an almost complete magmatic arc system. We conclude by briefly summarizing the geologic history of the West Antarctic system as a whole, how it provides insight into continental margin evolution and what key topics must be addressed by future research

Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth.

The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon

Origin and emplacement of a middle Cretaceous gneiss dome, Fosdick Mountains, West Antarctica

The Fosdick Mountains, West Antarctica, form an 80 × 15 km migmatite dome comprising massive paragneisses that exhibit polyphase fabrics, nappe-scale folds that involve granodiorite to leucogranite intrusions, and diatexite. High strain zones developed

Cretaceous oblique extensional deformation and magma accumulation in the Fosdick Mountains migmatite-cored gneiss dome, West Antarctica

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