Timothy S. Paulsen

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, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. 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. 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 and atmospheric CO2 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 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 under conditions of elevated CO2.

Integrated tectonostratigraphic analysis of the Himalaya and implications for its tectonic reconstruction

Abstract The isotope geochronology of isochronously deposited Cambrian strata from different tectonostratigraphic zones of the Himalaya confirms new stratigraphic, sedimentological, and faunal evidence indicating that the Himalaya was a single continental margin prior to collision of India with Asia. Lesser, Greater, and Tethyan Himalaya represent proximal to distal parts of a passive continental margin that has been subsequently deformed during Cenozoic collision of India with Asia. Detrital zircon and neodymium isotopic data presented herein discount the prevailing myth that the Lesser Himalaya has a unique geochronologic and geochemical signature that is broadly applicable to modeling the uplift history of the Himalaya. The conclusion that all pre-Permian Lesser Himalaya strata lack young detrital zircons that are present in the Greater and Tethyan Himalaya underpins previous arguments that the Main Central Thrust forms a fundamental crustal boundary that separates the Indian craton from an accreted terrane to the north. The supposition that Himalayan lithotectonic zones differ in detrital zircon age populations has also been used to reconstruct the unroofing history of the Himalaya during foreland basin development in the Cenozoic. Our data conflict with the underlying assumptions implicit in these studies in that samples of similar depositional age from both the Lesser and Tethyan Himalaya contain detrital zircons with similar age spectra. Similarities between the Kathmandu Complex and the Tethyan Himalaya support stratigraphic continuity between the former and either age-equivalent Greater Himalayan protolith or the Tethyan. Assuming that the complex rooted along the Main Central Thrust, these strata would simply have escaped intense metamorphism during Cenozoic tectonism. Alternatively, the complex may represent a part of the Tethyan Himalaya that was emplaced during an early stage of movement along a south-directed thrust fault located near the present-day structural position of the South Tibetan Fault System.

Midcontinent U.S. fault and fold zones: A legacy of Proterozoic intracratonic extensional tectonism?

The U.S. continental interior (midcontinent) contains numerous fault and fold zones. Seismic and drilling data indicate that some of these zones first formed as Proterozoic-Eocambrian rift faults, but the origin of most remains enigmatic. We propose that the enigmatic fault and fold zones also began as Proterozoic-Eocambrian normal faults. We base our hypothesis on the following: (1) enigmatic zones parallel known rifts, (2) the structural style of enigmatic zones mirrors the structural style of known rifts, (3) the map pattern of some enigmatic zones (e.g., the La Salle deformation belt of Illinois) resembles the map pattern of contemporary rifts, and (4) it is easier to rupture an intact craton by normal faulting than by reverse or strike-slip faulting. These zones, along with known rifts, represent the legacy of widespread extensional tectonism that brittlely broke up the craton into fault-bounded blocks prior to deposition of Phanerozoic platform cover. Once formed, midcontinent fault and fold zones remained weak, allowing cratonic blocks to jostle relative to one another during the Phanerozoic, thereby inverting faults (and creating transpressional or transtensional structural assemblages), localizing seismicity, and channeling (or releasing) ore-generating fluids.

Cambrian stratigraphy and depositional history of the northern Indian Himalaya, Spiti Valley, north-central India

Recent work on Himalayan tectonics indicates that prior to the Cenozoic collision of India and Asia, an enigmatic Cambrian– Ordovician event may have strongly infl uenced the regional geology of the Himalaya. Stratigraphic and sedimentological analyses of well-preserved Cambrian deposits are critical for understanding the nature of this early tectonic event and its infl uence on the later tectonic evolution of the Himalaya. The Parahio Formation, defi ned herein, of the Parahio Valley, Spiti region, in the Tethyan Himalaya of India, is the best biostratigraphically resolved section of Cambrian strata in the entire Himalaya. This formation consists of >1350 m of dominantly siliciclastic deltaic deposits. The formation ranges from uppermost Lower Cambrian (Lungwangmiaoan Stage) to middle Middle Cambrian (Hsuchuangian Stage), representing a time span of ~5–10 m.y. It contains numerous mediumscale shoaling cycles that range from storminfl uenced offshore deposits to thick trough cross-bedded fl uvial facies. Many thin carbonate beds with abundant trilobite fossils directly overlie the fl uvial facies and represent transgressive systems tract deposits. The cycles are interpreted to have resulted from delta-lobe switching, based on a lack of systematic stratigraphic changes in cycle or facies thicknesses. This paleoenvironmental reconstruction contradicts previous interpretations of this unit that range from deep-sea fl ysch to shallow-marine tidalites. In addition, our paleoenvironmental analysis and paleocurrent data suggest that the uppermost Lower to Middle Cambrian deposits of the Lesser and Tethyan Himalaya are parts of the same ancient northward-prograding, fl uvial-deltaic depositional system of the paleo-Tethys margin of India. An angular unconformity with overlying Ordovician conglomeratic rocks has considerable local relief, with meter-scale scours and a valley fi ll >100 m thick. The scours have northeast-southwest orientations, which parallel both the paleocurrents in the underlying Parahio Formation as well as published paleocurrent readings from the coarse red beds of the overlying Ordovician strata. The Cambrian–Ordovician unconformity is of regional extent, and our recent biostratigraphic database indicates that the minimum hiatus associated with the unconformity in Spiti is ~15 m.y. Our sedimentological analysis and associated paleocurrent data from the Parahio Formation, along with additional data from units both above and below the unconformity, indicate that published models portraying foreland basin development at this time with southward-directed thrusting are problematic. An alternate possibility— that uplift took place south of the Tethyan Himalaya—is also problematic, because no published stratigraphic or structural evidence exists for such an uplift to the south for either the Greater or the Lesser Himalaya lithotectonic zones.

Origin of the Uinta recess, Sevier fold–thrust belt, Utah: influence of basin architecture on fold–thrust belt geometry

Abstract Structural trends in the Sevier fold–thrust belt define a pronounced concave-to-the-foreland map-view curve, the Uinta recess, in north-central Utah. This recess separates two convex-to-the-foreland curves, the Provo salient on the south and the Wyoming salient on the north. The two limbs of the recess comprise transverse zones (fault zones at a high-angle to the regional trend of the orogen) that border the flanks of the east–west-trending Uinta/Cottonwood arch. Our structural analysis indicates that the transverse zones formed during the Sevier orogeny, and that they differ markedly from each other in structural style. The Charleston transverse zone (CTZ), on the south side of the arch, initiated as a complex sinistral strike-slip fault system that defines the abrupt northern boundary of the Provo salient. The Mount Raymond transverse zone (MRTZ), on the north side of the arch, represents the region in which the southeast-verging southern limb of the gently curving Wyoming salient was tilted northwards during the Laramide phase of uplift of the Uinta/Cottonwood arch. In effect, the MRTZ represents an oblique cross section through a thrust belt. The contrasting architecture of these transverse zones demonstrates how pre-deformation basin geometry influences the geometry of a fold–thrust belt. Analysis of isopach maps indicates that, at the time the Sevier fold–thrust belt formed, the area just north of the present site of the Uinta/Cottonwood arch was a basement high, with a gently dipping north flank, and a steeply dipping south flank. Thus, predeformational sediment thickened abruptly to the south of the high and thickened gradually to the north of the high. As illustrated by sandbox models, the distance that a fold–thrust belt propagates into the foreland depends on the thickness of the sedimentary layer being deformed, so the shape of the salient mimics the longitudinal cross-sectional shape of the sedimentary basin. Where basins taper gradually along strike, the thrust belt curves gently, but where basins taper abruptly along strike, the thrust belt curves so tightly that it disarticulates and becomes bounded laterally by a strike-slip accommodation zone. The geometry of the Uinta recess provides a field example of this concept. Differential movement of Sevier thrusts led to formation of gradually curving thrusts on the north side of the high, because of the gradual slope of the highs north flank, but led to the along-strike disarticulation of thrusts on the south side of the high, because of the steep slope of the highs south flank. In effect, therefore, thrust belt map-view geometry provides insight into predeformational basin geometry.

The Tethyan Himalaya: palaeogeographical and tectonic constraints from Ordovician palaeomagnetic data

Abstract: To test whether the Tethyan Himalaya were part of the northern margin of India in the early Palaeozoic we have produced the first primary palaeomagnetic data (bedding-corrected declination 267.5°, inclination 63.0°, α95 = 10°; pole latitude 20.2°N, longitude 28.6°E) from low metamorphic grade Ordovician red beds in the Tethyan Himalaya (Shian Formation). The palaeomagnetic data are of excellent quality, and a statistically positive fold test combined with a comparison with late Cambrian–Ordovician Gondwana poles suggests a primary hematite-bearing magnetization, acquired between 470 and 500 Ma. This is in excellent agreement with stratigraphic, faunal and provenance age estimates, and the palaeomagnetic data demonstrate that the Tethyan Himalaya must have been located in proximity to the Indian craton during early Ordovician times, and are therefore consistent with a continuous margin at that time. The Shian Formation pole overlaps with 470–500 Ma Gondwana poles, but an even better fit can be obtained by invoking a post-Ordovician clockwise rotation of 13° ± 4°. Such a rotation is similar in both sense and magnitude to clockwise rotations recorded in primary Triassic sequences as well as Palaeogene palaeomagnetic overprint data from the Tethyan Himalaya: rotations of the Tethyan Himalaya compared with cratonic India are thus probably all of post late Eocene age. Triassic and Early Ordovician data do not imply any crustal shortening between Tethyan Himalaya and cratonic India. However, in the Early Ordovician, India was rotated 90° compared with its present orientation, and any enlargement of India would not be detected by palaeomagnetic data.

Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene

Significance New information from the ANDRILL-2A drill core and a complementary ice sheet modeling study show that polar climate and Antarctic ice sheet (AIS) margins were highly dynamic during the early to mid-Miocene. Changes in extent of the AIS inferred by these studies suggest that high southern latitudes were sensitive to relatively small changes in atmospheric CO2 (between 280 and 500 ppm). Importantly, reconstructions through intervals of peak warmth indicate that the AIS retreated beyond its terrestrial margin under atmospheric CO2 conditions that were similar to those projected for the coming centuries. Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23–14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3–4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2. These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

Airborne laser scanning for high‐resolution mapping of Antarctica

In order to evaluate the potential of airborne laser scanning for topographic mapping in Antarctica and to establish calibration/validation sites for NASAs Ice, Cloud and land Elevation Satellite (ICESat) altimeter mission, NASA, the U.S. National Science Foundation (NSF), and the U.S. Geological Survey (USGS) joined forces to collect high-resolution airborne laser scanning data. In a two-week campaign during the 2001–2002 austral summer, NASAs Airborne Topographic Mapper (ATM) system was used to collect data over several sites in the McMurdo Sound area of Antarctica (Figure 1a). From the recorded signals, NASA computed laser points and The Ohio State University (OSU) completed the elaborate computation/verification of high-resolution Digital Elevation Models (DEMs) in 2003. This article reports about the DEM generation and some exemplary results from scientists using the geomorphologic information from the DEMs during the 2003–2004 field season.

Structure and age of volcanic fissures on Mount Morning: A new constraint on Neogene to contemporary stress in the West Antarctic Rift, southern Victoria Land, Antarctica

Antarctica is characterized by a unique combination of active processes, including active crustal deformation, volcanism, and glacial loading and unloading, but little is known about the Neogene to contemporary geodynamic state of the Antarctic plate. This paper presents new data on the structure and timing of volcanism on the Mount Morning shield volcano, with the purpose of defining Pleistocene stress directions concomitant with volcanism and rifting in the southern portion of the Victoria Land rift basin. Elongate vents and vent alignments indicate parasitic volcanism has predominantly occurred along a primary set of NE fissures. Parasitic basaltic cinder cones yield 40 Ar- 39 Ar cooling ages, presented herein, that range from essentially zero to ca. 3.5 Ma, though mostly of Pleistocene age. The systematic NE trend of the fissures on Mount Morning records magmatically induced fracturing of the volcano flanks controlled by a regional N31°E maximum horizontal stress (S H ) that dominated the Mount Morning area during the Pleistocene and, probably, also the Pliocene. Minor volcanism occurred along shorter NW alignments that have no discernible age difference from the NE fissures. The Pleistocene age of the parasitic volcanism suggests that the stress direction documented at Mount Morning represents the contemporary differential stress field in the area. This NE S H direction differs from the N15°W S H direction measured in a borehole at Cape Roberts, 100 km to the north, and the S H orientations appear to track the changing trend of the segmented rift boundary. The exact cause for the variability in the S H directions is uncertain, but the change in S H could record different stress provinces within the rift system, or it could reflect stress reorientation along the major lithospheric boundary of the rift. The new contemporary stress datum at Mount Morning is consistent with a neotectonic normal-fault to strike-slip fault regime within the Terror Rift, which was active in Pliocene-Pleistocene times and could remain active today. This stress regime still dominates the contemporary geodynamic state of this sector of the West Antarctic Rift system.

A simple method for orienting drill core by correlating features in whole-core scans and oriented borehole-wall imagery

Abstract Assessing the regional significance of fractures in drill cores requires the collection of oriented core intervals. Direct orientation of cores during drilling is possible, but is commonly precluded because of expense and time requirements. A simple and accurate method of core reorientation is presented where high-resolution imagery of drill cores (whole-core scans) are directly compared with oriented borehole imagery. Core intervals are reoriented by aligning features (i.e. fractures, bedding, and clasts) in whole-core scans with correlative, oriented features in borehole-wall imagery. Unlike other core orientation techniques, the direct side-by-side comparison of core scans and borehole-wall imagery can identify core segments that were mismatched due to undetected rotation between two portions of core. The combined analysis of core-based fracture data, whole-core scans and borehole imagery in this method optimizes data integration to improve structural interpretations.