IMMEDIATE SCIENCE REPORT
K114: Geophysical investigations of the tectonic, sedimentary and volcanic history of the Victoria Land Basin: Site surveys for ANDRILL Project
Antarctica New Zealand
1 Popular Summary of Scientific Work Achieved
- Identification of up to 1 km of sediment under the McMurdo Ice Shelf, in the "moat" east of Ross Island that has accumulated in the last 5 million years.
- Up to 500m thickness of sediment beneath the floor of the Taylor Valley in New Harbour.
- Imaging of the distribution volcanic and igneous rock beneath the McMurdo ice shelf.
- An understanding of the timing and nature of deformation associated with volcanic loading of the crust and its effect of stratigraphic geometry.
- New insights into the basement structure of the southern Victoria Land Basin.
- New estimates of bathymetry under the McMurdo Ice Shelf and in New Harbour
- Greatly improved understanding of the water depth, stratigraphy and rates of ice movement likely to be encountered at proposed ANDRILL drill sites.
2 Proposed Programme
Our proposed scientific programme had two aims. Firstly, to accurately define the structural and stratigraphic architecture of the southern Victoria Land Basin in McMurdo Sound for the location of proposed ANDRILL Project drill sites. Secondly, to use this information to test and develop models of rift evolution and uplift of the Transantarctic mountains. Both objectives were to be achieved through an integrated geophysical survey involving a seismic reflection, ground-based gravity and magnetic measurements and an aeromagnetic survey. Collectively, the imaging/remote sensing of geological properties and Earth models derived from these techniques can provide information on the subsurface distribution, geometry and structure of (1) the basement floor of the basin, (2) rift-related and intraplate volcanoes, and (3) sediments infilling the basin.page 2
Recent geological drilling, glacial geology and geophysical investigations demonstrate that understanding the evolution of sedimentary basins beneath the Ross Sea and Ross/McMurdo Ice Shelf is key to deciphering the evolution of the West Antarctic Rift, uplift of the Trans Antarctic Mountains (TAM) and Cenozoic ice cover history. The Victoria Land Basin has developed in response to the propagation of a rift system through west Antarctica and its sediments should record the mechanism and timing of uplift of the TAM together with its influence in Antarctic climate and ice sheet development.
This year, the programme acquired 40km of seismic reflection data in the New Harbour area and on the Ross/McMurdo ice shelf between Hut Point Peninsula and Ross Island. Three, 80 km line grids (New Harbour, Windless Bight/McMurdo Ice Shelf, Southern McMurdo Sound/McMurdo Ice Shelf) were traversed with magnetometer and gradiometer for gravity and magnetic measurements. An area of 5000km2 was covered by a helicopter-borne aeromagnetic survey between Hut Point Peninsula, White Island and Minna Bluff/Brown Peninsula. Additionally differential GPS was used to estimate ice movement in the vicinity of proposed drill sites.
Our geophysical programme (of which year 1 is complete) is already providing information that can directly address some of the above issues. However, the value of this geophysical work will be fully realised once integrated with drill core records to be recovered by the ANDRILL project. Major progress has been made towards addressing the immediate goals of the research which are two-fold: (1) Use data to design a safe and successful drilling programme, and (2) to publish a geophysical account of the internal stratigraphic architecture of the Victoria Land Basin in southern McMurdo Sound region, particularly beneath the Ross/McMurdo Ice Shelf.
- University of Oxford, UK. NERC-funded "Evolution of Sedimentary Basins in Southern McMurdo Sound: Implications for Antarctic tectonic and climate history".
- British Antarctic Survey, UK - use of seismic equipment
- University of Nebraska, USA - NSF-funded "Seismic and stratigraphic data acquisition and integration for Cenozoic tectonic and paleoenvironmental analysis in McMurdo Sound"
- Geological and Nuclear Sciences, NZ – FRST-funded "Neogene Global Cooling – ANDRILL Site Survey Task'
- Victoria University of Wellington, Antarctic Research Centre – Internal funding
- Bundesanstalt fuer Geowissenscaften und Rohstoffe, Germany
- Webster Drilling and Exploration (Sub-contract)
3 Scientific Endeavours and Achievements
- Accurately define the structural and stratigraphic architecture of the Victoria Land Basin southern in McMurdo Sound for the location of proposed ANDRILL Project drill sites.page 3
- Test and develop models of rift evolution and uplift of the Transantarctic Mountains.
Figure 1. Composite Landsat image of southern McMurdo Sound showing the Transantarctic Mountains, Dry Valleys, southern McMurdo Sound volcanoes, and the major survey areas for the K114 01/02 field season.
Figure 2. Location map of New Harbour showing orientation of seismic lines and the gravity/magnetic survey grid.
Multifold, normal-incidence seismic measurements were made along two, 10km profiles in the New Harbour region (Fig. 2) and one 19km line in the McMurdo Ice Shelf (Fig. 3). The data were recorded on a 48-channel Bison digital recording seismograph (On loan from the British Antarctic Survey). Shots, 1 every 100 m, consisting of 1.2 kg (3 × Anzomex PPP Primers) and 3.2-4.0 kg (8-10 × Anzomex PPP Primers) of explosive were placed at 5m below sea-ice in New Harbour and 18m depth in the ice shelf. 40 Hz geophone receivers will be placed every 50-m to give a 12 fold geometry for the profile. Shots were placed in front of the spread in off-end geometry with an offset range of 100 to 2450 m. Geophones were drilled into the sea-ice to ensure effective coupling. Shot hole drilling operations carried under contract by Webster Drilling and Exploration with supervision provided by Alex Pyne (VUW). Differential GPS was used for shot point and peg surveying. Preliminary processing was carried out in the field and more detailed processing will be undertaken back at Geological and Nuclear Sciences using Claritas software and Oxford University using Promax software. Processing of the ice shelf line will form part of a Natalie Balfour's B.Sc.(hons) dissertation at VUW, and will be supervised by Stephen Bannister at GNS. Acquisition and processing parameters are summarised in a preliminary report (Bannister and Naish, 2001). Procedures utilised involve muting noisy traces, optimum filter and deconvolution parameters will be chosen following tests on interactive displays of shot records. Pre-stack traces balanced using AGC. Velocity analysis by constant velocity stack.
Gravity and Magnetics
Gravity and magnetic data were acquired along each seismic profile, as well as over a selected detailed grid in New Harbour and McMurdo Ice shelf sites. Additionally, a reconnaissance trip led by Gary Wilson acquired gravity and magnetic records over a grid in Southern McMurdo Sound on the ice shelf between Minna Bluff-Black Island-Brown Peninsula. Gravity measurements were acquired with a Lacoste-Romberg geodetic meter. Magnetic measurements were acquired with a Barringer Research GM122 proton magnetometer. Gravity measurements were "tied-in" to temporary base stations in the survey region which themselves were tied to the gravity base station located as Scott Base. The seismic, gravity and magnetic data were positioned using differential GPS. Some sites were re-occupied and their position measured over several weeks (by GPS to dcm accuracy) to establish ice movement rate and direction. The gravity and magnetic data were reduced in the field Smoothed data were gridded using minimum curvature techniques. The data were then modelled using both indirect (e.g. line integral) and direct (e.g. matrix inversion) techniques. Particular emphasis was placed on the determination of the geometry of the gravity and magnetic basement and, hence, the distribution of the sedimentary depocentres and the rim thicknesses of volcanic islands of the Ross archipelago.
A helicopter-borne aeromagnetic survey was carried out over the McMurdo Ice Shelf using an optically pumped Cs-magnetometer housed in a "bird" flown 25 m below the Helicopters New Zealand Bell 212. Survey flights operated from McMurdo Station with an average duration of 3 hours. Navigation was by GPS waypoint and a 500m-spaced E-W line grid was flown at a 300m dropped elevation (Fig. 4). Grid tie lines were flown N-S at the same dropped elevation but with a 2.5 km spacing. Total field and 3 dimensional position was collected on-board and processed against two proton magnetometer base stationed page 7 positioned on the Pegasus road and behind Black Island, respectively. Standard post processing was applied to all flight lines (repositioning of flight paths, diurnal variation removal, IGRF correction and levelling). The resulting magnetic anomaly data was then gridded by applying a weighted averaging procedure.page 8 page 9
Figure 8. Gravity anomaly map for the McMurdo Ice Shelf gravity surveys with the moat axes and White Island ridge extension indicated.
The event operated as 4 "tiger teams" with clear objectives - seismic team (K114N), drilling team (K114P) gravity and magnetics team (K114W), aeromagnetics team (K114D).
The seismic and drilling teams arrived at Scott Base on the 11th of October and after a period of preparing the drilling and seismic systems at Scott Base left by tractor train to New Harbour on the 17th of October. Following a series of shot tests to establish charge size and depth below sea-ice, drilling operations were carried out in New Harbour from the 20th until completion on the 24th of October. 120 shots were drilled and loaded. All were successfully detonated. Drilling teams surveyed and flagged the grid lines for gravity and magnetic surveys. Shot and peg surveys and ice movement surveys were undertaken using UNAVCO provided differential GPS equipment. Ice survey sites were subsequently reoccupied by the gravity/magnetic team and surveyed with NERC differential GPS equipment. Seismic operations ran between the 20th and 30th of October. Progress was initially slow as operations systems were streamlined, but primarily because the shot firing electronics did not perform well in the cold temperatures (<−20°C). The return trip to Scott Base of the seismic team, on the 30th of October was incredibly slow (took 22 hours) as inadequate plant was used to tow the train (D3 bulldozer). The gravity and magnetics team arrive at Scott base on 26th of October and after a period of preparation and base station measurements deployed for New Harbour on 31st of October. A GPS and Gravity base station was established at the DVDP-11 drillsite in the mouth of Taylor Valley. A 20km by 20km grid (Fig. 2) was surveyed around the predefined seismic lines. Surveying was undertaken during the afternoon to early morning hours local time as base station measurements at Scott Base had indicated that these were times of optimum stability of the ambient magnetic field. This timeframe of operation meant returning to camp between 2 and 5 am and made field operations quite onerous as this was always the coldest time of day. Gravity measurements were time consuming and difficult because of vibrations within the sea ice cover. The magnetics and gravity survey was completed on November 10th and the team returned to Scott Base by tractor train towed by Hagglunds,
McMurdo Ice Shelf, Hut Point Peninsula to White Island & Black Island to Brown Peninsula
After 4 days at Scott base re-supplying the drilling team began setting up and test drilling on the ice shelf. The 19km long seismic line was surveyed and shot holes and pegs flagged in stages. Operational drilling was completed between the 2nd and 12th of November. The seismic survey was completed between the 6th and 13th of November. Shot tests were carried out to establish charge size. Although 10 PPP primers were used per shot it was decided that an optimal charge size of 8 primers would be used in future surveys. The coupling of source and receiver was excellent due to extension spikes on geophones and shot placement at 18-20m depth. Source was high-frequency and a high-resolution producing high-frequency record with 5m resolution to 1.5 seconds (see Bannister and Naish, 2002 for details). After a helicopter reconnaissance to locate any crevasses in the study area and establish a safe operating environment, the magnetics and gravity team deployed out to the McMurdo Ice Shelf camp. A local gravity base station was established at the camp and six days of surveying saw a 20km by 20km grid (Fig. 3) of magnetic and gravity measurements. Again, surveying was undertaken during the afternoon to early page 12 morning hours local time. Gravity measurements were much more straightforward as the Ice Shelf provided a very stable surveying platform. The K114 camp was returned to Scott Base by tractor train, and an additional gravity and magnetics survey undertaken over Hut Point peninsula from Scott Base. K114W redeployed to NZ in early December.
On December 30th K114W deployed a second time to undertake a reconnaissance survey of the southern McMurdo Ice Shelf between Black Island and Brown Peninsula, Mount Discovery and Minna Bluff (Fig. 1). A camp and GPS and gravity base station was established on the SW corner of Black Island. Over an 8-day period ice movement was monitored by GPS and a preliminary gravity survey was undertaken by snowmobile. The weather was fine and unseasonably warm and large melt ponds and lakes developed over much of the McMurdo Ice Shelf preventing a complete survey and making travel extremely slow and difficult. Because of this, a few critical additional gravity measurements were made later in January from helicopter. From January 12th, K114D began the aeromagnetic survey as outlined above. Survey flights were undertaken in the evening and early morning hours, the window of stable ambient magnetic field behaviour was between 4pm and 2 am local time. Flying began as soon as the helicopter had completed its daytime flying often this was quite late (7-9pm) which sometimes only allowed a single flight each night. The survey flights departed and returned to the McMurdo helicopter pad and required hand launching and catching of the magnetometer "bird" beneath the helicopter. In late January, unusual weather including low cloud cover and ice shelf fog prevented flying on many occasions. The survey was delayed and not completed until January 30th. Navigational GPS data was hand entered by the pilot for each day which was time consuming. A more streamlined system needs to be employed for and future helicopter-borne aeromagnetic surveys.
New Harbour Seismics
Figure 5 shows an example of the brute stack (preliminary data) form the NHS-1 line. The seafloor is seen to dip down to the east reaching a maximum water-depth pf c. 340m at the eastern end of the line. A brute stack of the NHS2 data (Fig. 6) shows a seafloor high at the western end of the line at about c. 180m depth with the sea-floor depth increasing to c. 280m to the east. Sub-seafloor strata dip east. The quality of the seismic data is not as good as "normal" land-seismic data, but is the best expected given the complex physical (ice-water-sediment interface) environment. It is widely acknowledged that flexure of sea-ice significantly complicates the seismic arrival pattern. Major noise arrivals include: large amplitude sea-floor multiples, airwave arrivals, side scatter reflections of local valley topography, and a multiple due to either a very low velocity layer below the sea-floor, or else a complex P-SV converted wave. Sedimentary reflections are visible at the eastern ends of both of the New harbour lines, with stacking velocities of 1800 m/s and above the multiple, up to 0.4ms.
McMurdo Ice Shelf Seismics
Figure 7 shows the brute stack of the data. At the north-western end of the line the data is similar to that observed by Bannister (1993) and Melluish et al. (1995) from the Hut Point peninsula seismic line (HPP-1; Fig. 3). The seafloor is relative flat for 4000m and rises from c. 900m to the SE, to less than 320m depth over the northern bathymetric extension of White Island, before deepening again to 660m at the end of the line. High-frequency page 13 sedimentary reflections with up to 5m resolution are visible to 1.5 s below the sea floor. Sea floor multiples are observed for most of the seismic line. These multiples become a problem only at the SE end of the line.
McMurdo Ice Shelf Gravity
Figure 8 illustrates the gravity anomalies measured in the survey areas between Ross and White Islands and South and West of Black Island, respectively. Both surveys indicate the presence of "moats" associated with the respective volcanic loads. A deep moat is indicated subparallel to and with its axis 10km East of Hut Point Peninsula. It shallows towards Ross Island and then deviates east over a ridge representing a 30km northwards extension of White Island. A similar, but shallower moat is indicated west and south of Black Island encompassing an oxbow shaped basin between Black Island and Mount Discovery. A shallow sea floor is indicated to the south of Black Island with a 10km northward extension of Minna Bluff beneath the McMurdo Ice Shelf and additional volcanic edifices on the sea floor between Black Island and Minna Bluff.
McMurdo Ice Shelf Aeromagnetism
Figure 9 illustrates the corrected and partly levelled magnetic anomaly map acquired during the January aeromagnetic survey. The map is annotated to indicate initial interpretations. Strong red colours indicate strong positive magnetic anomalies, and strong blue colours indicate strong negative magnetic anomalies. The high frequency anomaly patters Brown Peninsula, Mount Discovery, Black Island, White Island and Hut Point Peninsula indicate strongly magnetised normal and reversed volcanic flows at or near the surface. Reduced intensities and slightly extended wavelengths north of White Island indicate similar flows but progressively deeper below the ice shelf. The magnetic anomaly profiles indicate that the Black Island volcanic massif is centred 10km further SW than the current topographic expression of Black Island. The northern peninsula of Black Island is clearly indicated as a separate but smaller volcanic massif that may extend beneath the ice shelf to the North. Large magnetic anomalies beneath the ice shelf to the north of Minna Bluff and between Black Island and Hut Point Peninsula indicate deep but strongly magnetic igneous or volcanic provinces. Between Brown Peninsula and Black Island a strong positive magnetic anomaly indicates a substantial volume of volcanic sediments close to the surface of the ice shelf.
Bannister, S & Naish, T, 2002. ANDRILL Site Investigations, New Harbour and McMurdo Ice Shelf, Southern McMurdo Sound, Antarctica. Institute of Geological and Nuclear Sciences report 2002/01. 24p
- Wilson G. S et al. (in prep). Initial results from gravity and magnetic surveys.
- Wilson et al. (in prep) and Bannister et al. (in prep) Papers to be submitted to international journal
- Results will be incorporated into the ANDRILL Science Logistics Implementation Plan,
- BSc(hons)Thesis at VUW (Natalie Balfour)
- BSc(hons) Thesis University of Oxford
Funding support is acknowledged from NERC (Wilson), NZFRST (Naish and Bannister), NSF (Harwood and Whitehead), BGR (Damaske), British Antarctic Survey (Lavelle) and VUW Antarctic Research Centre (VUW Students and Pyne). Logistical support was provided by Antarctica New Zealand. We also acknowledge the support of our home institutions.
Ed King, British Antarctic Survey (BAS), is kindly thanked for his advise and support in loaning and operating the seismic recorder. BAS management is thanked for its support of the project. Webster Drilling and Exploration are thanked for their professional drilling services. Charlie O'Reilly (GNS) is thanked for his help in setting up seismic equipment. Antarctica New Zealand staff in Christchurch and Scott Base are thanked for their considerable effort and support. Special acknowledgement is made to Jim Cowie for handling K114 - an event that kept increasing in size and complexity, which because of his operational management has extremely successful.