The UNIVERSITY of WISCONSIN Geophysical & Polar Research Center DEPARTMENT OF GEOLOGY ANTARCTIC PENINSULA TRAVERSE GEOPHYSICAL RESULTS RELATING TO GLACIOLOGICAL AND GEOLOGICAL STUDIES by John C. Behrendt RESEARCH REPORT NO. 64-1-MARCH, 1964 Research Report Series Number 64-1 March 1964 ANTARCTIC PENINSULA TRAVERSE GEOPHYSICAL RESULTS RELATING TO GLACIOLOGICAL AND GEOLOGICAL STUDIES by John C. Behrendt The University of Wisconsin Geophysical and Polar Research Center 6021 South Highland Road Madison, Wisconsin 53705 SUMMARY This report presents the results of geophysical measurements relat- ing to glaciology and geology made during the Antarctic Peninsula Traverse of 1961-1962. Maps of snow surface and bedrock elevation, free-air and Bouguer anomalies, and total magnetic intensity anomalies are presented. The Antarctic Peninsula is an island separated from the Sentinel Range by a channel 1000 m below sea level and from the Eights Coast of Ellsworth Land by a channel 500 m deep. Seismic velocity variations in the upper 50 m of the firn,of approx- imately constant mean annual temperature, show correlations with the wide- ly ranging annual accumulation values; these relationships are used to obtain estimates of accumulation where only seismic data are available. A reflection horizon 450 m above the base of the ice near Eights Station was observed. Wide-angle reflections showed the existence of a low- velocity layer below this reflecting horizon which is interpreted as a result of moraine within the ice. Numerous high amplitude magnetic anomalies with sources close to the base of the ice characterize the southern Antarctic Peninsula. The Jones Mountains area also has many magnetic anomalies whose origin near the base of the ice is consistent with the shallow "magnetic basement" meas- ured in Marie Byrd Land to the southwest. The few magnetic anomalies in a central area of Ellsworth Land suggest a deeper basement and possibly a thick metasedimentary section associated with that of the Ellsworth Mountains. Several lines of evidence indicate that in spite of some structural similarities, the Antarctandean geology of the southern Ant- arctic Peninsula is noc continuous with the Ellsworth Mountains. Diurnal variations observed in the magnetic field have a standard deviation from diurnal variations measured at Eights Station of ± 15y. Standard devia- tions for individual days show a correlation with magnetic anomalies ob- served in the field. An 18 km refraction profile showed a velocity of 3.87 km/sec in the top 0.9 km of ice, a velocity of 4.4 km/sec in the next 0.2 km, a veloc- ity of 5.3 km/sec in the next 3L- km, and a possible velocity of 6.0 km/sec in the rock below. The rough subglacial topography precludes the use of gravity data to provide quantitative information on density variations in the underlying geology in most places. At one location on the Antarctic Peninsula where good subglacial topographic control exists from seismic soundings, theore- tical free-air anomaly profiles constructed by the line integral method show that more accurate bedrock elevations can be obtained from closely spaced gravity data than has previously been possible using infinite slab assumptions with the usual 6 km spacing. The method requires station spac- ing closer than the topographic "wave length" and seismic reflection sound- ings close enough to determine the regional gravity gradient. The mean free-air anomaly in Ellsworth Land is +11 mgals indicating that the area is approximately in isostatic equilibrium. Although high free-air anom- alies were observed on several crossings, the southern Antarctic Penin- sula is shown to be regionally compensated. Bouguer anomalies suggest several kilometers of crustal thickening beneath the Antarctic Peninsula when compared with Ellsworth Land. The data tabulated or presented in graphical form include: position, elevation of snow and rock surfaces, observed gravity, free-air anomaly, and total magnetic intensity at 6 km intervals; Bouguer anomalies at all seismic soundings or rock outcrops; detailed surface elevation and total magnetic intensity values at 0.75 km intervals; reflection travel times for each seismic sounding of ice thickness; geology observations at 11 nunataks; P-wave velocity in the firn vs. depth and distance data, and density vs. depth curves in the firn at all reflection stations; graphs of diurnal variation of magnetic total intensity for 20 days compared with Eights Station diurnal curves; and reproductions of portions of re- flection seismograms at all reflection stations. I I I I I I I I I PREFACE This report contains the geophysical results of the 1961-1962 Antarctic Peninsula Traverse. Discussions of many of the data in- cluded in the tabulations have previously been published by Behrendt (1963 and 1964). CONTENTS Page Introduction ........... Observation and Analysis of Data 5 Results........................... 13 Discussion of Glaciological Results 19 Snow Densification ......... 23 Basal Low Velocity Layer . 31 Discussion of Geological Results 37 Summary of Surface Geology . 38 Magnetic Anomalies......... 41 Long Refraction Profile . 55 Gravity Anomalies ...... 59 Acknowledgements ...... 65 References ............ 67 Appendix I: Tabulated Data . 71 Table A: General Data Summary 72 Table B: Detailed Elevation an d Magnetic Data........ 76 Table C: Distance, Velocity, and Depth from Seismograms 84 Appendix II: Reproductions of Seismograms ......... 87 ILLUSTRATIONS Page Figure 1. Map of Antarctica .... a 0 a 0 • . • 2 Figure 2. Surface Elevation Map . Figure 3. Velocity-Depth Curves . Figure 4. Profiles of Data, Stations 0-320. .......... 12 Figure 5. Profiles of Data, Stations 320-940. ......... 14 Figure 6. Profiles of Data, Stations 432-700. ......... 15 Figure 7. Profiles of Data, Stations 700-940. ......... 16 Figure 8. Bedrock Elevation Map . .a17 ... Figure 9. Density-Depth Curves 20 Figure 10. Density-Depth Curves 20 ... Figure 11. Density-Depth Curves . .021 ... Figure 12. Density-Depth Curves . 6. .. 21 ... Figure 13. Density-Depth Curves . .022 ... Figure 14. Accum. vs. Density at h = 40m. ........... 27 Figure 15. Accum. vs. V at X = 50 m 27 p Figure 16. Accum. vs. V at X = 00 m 28 p Figure 17. Accum. vs. V at X = 200 m 28 p Figure 18. p vs.-ln p at h = 40 m 29 1 -cosh-vs. Figure 19. -In p at h 40m............. 29 A G Figure 20. Eights Station (432) Trave 1-Time Curve ....... 30 X2 . 2 Figure 21. x vs. T , Station 432 . .032 ... Figure 22. Temperature vs. Depth, Sta tion 432... ..........32 Figure 23. Station 432 Profile .4.9. 32 Figure 24. Magnetic Anomaly Map .. 42 Page Figure 25. AK vs. AD........................43 Figure 26. Magnetic Anomaly Model . ... ............... .43 Figure 27. Magnetic Anomaly Model..................... 43 Figure 28. Diurnal Magnetic Variation Curves. ............ 49 Figure 29. Diurnal Magnetic Variation Curves. .......... .. 50 Figure 30. Diurnal Magnetic Variation Curves.. ........... 51 Figure 31. Diurnal Magnetic Variation Curves........... 52 Figure 32. Diurnal Magnetic Variation Curves........... 53 Figure 33. a vs. Distance from Eights.............. 54 Figure 34. a vs. A . ...... .......................... 54 Figure 35. Travel Time Curve, Station 796.. ........... 56 Figure 36. Profile, Station 796....................58 Figure 37. Free-Air Anomaly Map..... ................ 61 Figure 38. Bouguer Anomaly Map . ... ................. .62 Figure 39. Bouguer Anomaly Section AA'.........................64 Table 1. Reflection Travel Time Data........ 8 Table 2. Accumulation Determined from Seismic Data and Near Surface Glaciology . 26 Table 3. Summary of Rocks Collected .. 40 INTRODUCTION Oversnow traverses in West Antarctica before the 1961-1962 austral summer showed great irregularities in the subglacial topography (Bentley, 1962). The sub-sea-level Byrd Basin separates the Ellsworth Mountains from the mountains of contrasting geology in northern Marie Byrd Land. The Antarctic Peninsula appeared to extend southward to connect with the Ellsworth Mountains (Bentley et al., 1960) separating the Ross and Filch- ner Ice Shelves. A rather critical gap remained in the part of Ellsworth Land and the part of the Antarctic Peninsula south of George VI Sound. Consequently a seven-man party, airlifted to the jones Mountains with vehicles, carried out 1700 km of geophysical, glaciological and limited geological traverse observations between 30 November, 1961 and 5 February, 1962. The map of Antarctica (Fig. 1) shows the region studied. (The name "Antarctic Peninsula" as used herein refers to the grounded ice or rock above sea level, east and north of Eights Station.) This report presents the results including the reduced data, of the altimetric, seismic, gravimetric, and magnetic observations. Deductions about the glaciology and crustal geology of the area are attempted using these data and other glaciologic, topographic, geophysical, and geological information available. The route is shown in Figure 2, a map of snow surface elevation con- structed from surface altimetry observations made on the Antarctic Penin- sula traverse (APT) and on the Ellsworth Highland traverse (EHT) (Bentley, 1962), using the usual interval system common to the antarctic traverses. Measurements were also taken on two flights using radio and aneroid altimeters, which provided additional control. Maximum surface relief in the area is 2 km. North and west of Eights Station is a featureless snow surface about 1000 m in elevation; south of the station the surface drops down