Geology topics

P. Kyle

Publications and source records attributed to P. Kyle.

3 recordsLinked to original sources

USGS2009

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth's orbital geometry control the ice ages 1 , fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles 2 . Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the 'warmer-than-present' early-Pliocene epoch ( 5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming 3 . Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, 40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth's axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to 3 °C warmer than today 4 and atmospheric CO 2 concentration was as high as 400 p.p.m.v. (refs 5 , 6 ). The evidence is consistent with a new ice-sheet/ice-shelf model 7 that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt 8 under conditions of elevated CO 2 .

Nature↗

USGS2002

Draped aeromagnetic survey in Transantarctic Mountains over the area of the Butcher Ridge igneous complex showing extent of underlying mafic intrusion

A draped aeromagnetic survey over the area surrounding the Butcher Ridge igneous complex (BRIC), Transantarctic Mountains, was acquired in 1997–1998 as part of a larger Transantarctic Mountains Aerogeophysical Research Activity survey. The BRIC is a sill-like hypoabyssal intrusion ranging in composition from tholeiitic basalt to rhyolite. An 40 Ar/ 39 Ar age of 174 Ma and the chemical character of the basaltic rocks show the BRIC to be part of the widespread Jurassic Ferrar suite of continental tholeiitic rocks, that extends for 3500 km across Antarctica. The aeromagnetic survey shows a horseshoe-shaped pattern of anomalies reaching amplitudes as great as 1900 nT generally associated with the bedrock topography where it is exposed. It is apparent that the high-amplitude anomaly pattern is more extensive than the 10-km-long exposed outcrop, first crossed by a single 1990 aeromagnetic profile. The highest-amplitude anomalies appear south of the profile acquired in 1990 and extend out of the survey area. The new aeromagnetic data allow determination of the extent of the interpreted Butcher mafic(?) intrusion beneath exposures of Beacon sedimentary rock and ice in the area covered, as well as beneath the small BRIC exposure. The magnetic anomalies show a minimum area of 3000 km 2 , a much greater extent than previously inferred. Magnetic models indicate a minimum thickness of ∼1–2 km for a horizontal intrusion. However, nonunique models with magnetic layers decreasing in apparent susceptibility with depth are consistent with of a 4- to 8-km-thick layered intrusion. These magnetic models indicate progressively deeper erosion of the interpreted mafic-layered body from the south to north. The erosion has removed more magnetic upper layers that mask the magnetic effects of the lower less magnetic layers. The probable minimum volume of the intrusion in the area of the survey is ∼6000 km 3 . An alternate, but less likely, interpretation of a series of dikes can also fit the observed magnetic anomalies.

Journal of Geophysical Research B: Solid Earth↗

USGS1991

Middle to late Cenozoic magmatism of the southeastern Colorado Plateau and central Rio Grande rift (New Mexico and Arizona, U.S.A.): A model for continental rifting

The region of the present Rio Grande rift and southeastern Colorado Plateau underwent lithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and erupted to the surface. Study of the igneous rocks erupted or intruded during this deformation yields insights into processes of magmatism associated with extension of continental lithosphere. Magmatic rocks associated with an early (late Oligocene-early Miocene) ductile phase of extension are dominantly basaltic andesites and related, calc-alkaline intermediate to silicic derivative rocks. Mafic magmas were probably derived from isotopically “enriched” lithospheric mantle. Igneous rocks associated with a later (middle Miocene-Holocene), more brittle phase of extension include widespread basaltic rocks and localized central volcanoes of intermediate to silicic composition. Isotopic compositions of mafic rocks, which include both tholeiitic and alkalic basalts, correlate strongly with tectonic setting and lithospheric structure. Basalts erupted in areas of greatest crustal extension, such as the central and southern rift and Basin and Range province, were derived from isotopically “depleted” (correlated with “asthenospheric”) mantle. Also, isotopic compositions of Pliocene to Holocene basalts are slightly more depleted than those of Miocene basalts, suggesting that subcrustal lithospheric mantle was thinned during late Miocene extension. Intermediate rocks of the central volcanoes formed by a complex combination of processes, probably dominated by fractional crystallization and by assimilation of upper and lower crust in isolated, small magma chambers. The petrologic, geochemical, and isotopic data are compatible with a model, derived first from geophysical data, whereby lithosphere is thinned beneath the central rift and southeastern Colorado Plateau, with greatest thinning centered beneath the axis of the rift. A lithospheric model involving uniform-sense simple shear does not appear compatible with the data as presently understood.

Arizona, New Mexico↗