Ship-to-shore seismic refraction investigation of the lithospheric ...
Gleadow, A.J.W., and P.C. Fitzgerald. 1987. Uplift history and structure of the Transantarctic Mountains: New evidence from fission track dating of basement apatites in the Dry Valleys area, southern Victoria Land. Earth and Planetary Science Letters, 82, 1-14. Harwood, D.M. 1983. Diatoms from the Sirius Formation. Antarctic Journal of the U.S., 18(5), 98-100. Harwood D.M. 1986. Recycled siliceous microfossils from the Sirius Formation. Antarctic Journal of the U.S., 21(5), 101-103. Lo Bello, Ph., C. Feraud, C.M. Hall, D. York, P. Lavina, and M. Bernat. 1987. 41 Ar/39Ar step-heating and laser fusion of a Quaternary pumice from Neschers Massif, Central France: The defeat of exnocrystic contamination. Chemical Geology, (isotope geoscience section), 66, 61-71. Lux, D.L. 1986. 40Ar/39Ar ages for minerals from the amphibolite dynamothermal aureole, Mont Albert, Gaspe, Quebec. Canadian Journal of Earth Science, 23, 21-26. McDougall, I., and T.M. Harrison. 1988. Geochronology and ther-
Ship-to-shore seismic refraction investigation of the lithospheric structure of the Transantarctic Mountain front
mochronology by the 40Ar/39Ar Method. New York: Oxford University Press. Steiger, RH., and E. Jaeger. 1977. Subcommission on Geochronology: Convention of the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359-362. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1983. Late Neogene and older Cenozoic microfossils in high elevation deposits of the Transantarctic Mountains: Evidence for marine sedimentation and ice volume variation on the East Antarctic Craton. Antarctic Journal of the U.S., 18(5), 96-97. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1984. Cenozoic marine sedimentation and ice-volume variation on the East Antarctic craton. Geology, 12, 287-291. Webb, P.-N., D.M. Harwood, B.C. McKelvey, M.C.G. Mabin, and J.H. Mercer. 1986. Late Cenozoic tectonic and glacial history of the Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 99-100.
Plateau icecap (figure 1). The array consisted of a 1.2-kilometer linear array of eight vertical seismographs, deployed at 150meter intervals away from the array center, and an equilateral (160-meter sides) triangular deployment of three-component
162.0 163.0 164.0 165.0 166.0
DANIEL R.H. O'CONNELL and RALPH R.B. VON FRESE
-74.
-74
Byrd Polar Research Center
and Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210 JOHN PASKIEVITCH -74.
74.5
U.S. Geological Survei Branch of Alaskan Geology Anchorage, Alaska 99508
As part of the 1988-1989 German Antarctic Northern Victoria Land Expedition V (GANOVEX V) offshore-onshore seismic retraction experiment, we established a seismic recording array in the Transantarctic Mountains to record a tuned airgun array operated in the Ross Sea. The objective was to determine the crustal structure of the transition zone between the Transantarctic Mountains and the Ross Sea by recording three onshore-offshore seismic refraction profiles across the Transantarctic Mountains near Terra Nova Bay. Unfavorable ice conditions in Terra Nova Bay reduced two onshore-offshore profile lengths and necessitated reorientation of the third profile. Other GANOVEX V groups deployed onshore and offshore seismographs to provide constraints on the crustal structure of the Ross Sea and inland Transantarctic Mountain portions of the profiles. An 11-element digital seismograph array was established 65 kilometers inland from the Terra Nova Bay on the Tourmaline 1989 REVIEW
-75
75
-7E
5 162.0 153.0 154.0 165.0 156.0
Figure 1. Map of the study area showing the position of the seismic recording array (triangle) in relation to seismic lines 1, 4, and 5 (circles). 33
seismographs around the array midpoint. The array recorded digital seismic waveforms generated by a 43.5-liter airgun array fired shots at 250-meter intervals in the Ross Sea. Data were recorded at offsets of 65-150 kilometers along three onshoreoffshore profiles. Data were also recorded during three marine seismic refraction experiments located more than 200 kilometers from the array. Onshore-offshore line 1 began 65 kilometers from the array and extended to 135 kilometers along an azimuth of S30°E. Line 1 provided a crustal profile between the Campbell and Priestley glaciers (figure 1). Onshore-offshore line 5 began 150 kilometers from the array at an azimuth of S21°E, and ended at a distance and azimuth of 85 kilometers and S52°E, respectively (figure 1). Line 5 constituted a "broadside shoot" with respect to the array and should provide constraints on the three-dimensional crustal velocity structure between the Campbell and Reeves glaciers. Onshore-offshore line 4 consisted of a sequence of stacking shots near Cape Washington 79 kilometers from the array at an azimuth of 552°E and continued as a marine profile 170 kilometers east of the coast (figure 1). Virtually all the airgun shots comprising the three onshoreoffshore refraction profiles were recorded by all or part of the array. Complete waveform data from the entire array were obtained for the first 10 kilometers and last 25 kilometers of line 1. Due to intermittent recorder failures, waveform data are available at a 250-meter shot spacing from subsets of the array for the remainder of the line 1. Recording failures were reduced during lines 4 and 5, and more complete array recordings were obtained. Clear seismic signals from the three > 200 kilometers marine seismic profiles were not detected on single components of ground motion during our first review of the data. Excellent airgun signal quality and low background noise levels were a pleasant surprise considering the modest 43.5liter airgun array capacity. Spectral calculations indicate signalto-noise ratios exceeding 10 in the 2-30-hertz frequency range. Ambient noise levels in the 2-20-hertz band were less than 0.1 millimicrons during most of the recording period. First P-wave arrivals are clearly observed along the entire length of line 1. A portion of the vertical-component record section for line 1 is shown in figure 2. This portion of the record section contains lower crustal refracted first arrivals, wide angle PmP reflections, and Pn Moho refraction first arrivals which constrain the crustal thickness beneath the portion the Transantarctic Mountains near the shoreline of Terra Nova Bay to approximately 22 kilometers (figure 3). Large amplitudes produced by the Moho triplication and the lower crust Pg-Pn crossover produced excellent signal-to-noise on single seismograms. Sub-Moho refracted energy is indicated by a sharp increase in first-arrival apparent velocities (> 8 kilometers per second) at > 120-kilometer offsets on line 1. The Ross Sea portion of the crustal model shown in figure 3 was derived from the results of McGinnis et al. (1985); Cooper, Davey, and Behrendt (1987); and Cooper, Davey, and Cochrane (1987). Data obtained from line 1 indicate slow crustal thickening (1 kilometer thickening per 10 kilometer offset) beneath the Transantarctic Mountains between the coastline and 20 kilometers inland (figure 3). Constraints on the crustal structure beneath the Transantarctic Mountains further inland will be provided by data collected by the Hamburg GANOVEX V group. Preliminary analysis of the line 1 data suggests a gradual crustal thickening transition zone between the Transant34
w
U-) CD C) 7.)
a-) HU-) M 6
uJ F-J
uJ >
FT 4.
80.0
85.0 90.0 DIST (KM)
95.0
Figure 2. Vertical-component seismograms from a portion of the line 1 record section. Note the excellent signal strength, clear PmP Moho reflection at 5.3 seconds, and Pg-Pn crossover at 94 kilometers distance. (km denotes kilometer.)
arctic Mountains and the Ross Sea. Crustal thickness beneath the western portion of the Transantarctic Mountains near Terra Nova Bay is significantly less than the 40 kilometers assumed by Smithson (1972) and Davey and Cooper (1987) in the McMurdo Sound area. One explanation is that the Transantarctic Mountains crust is thinner than assumed in the gravity modeling of Smithson (1972) and Davey and Cooper (1987). Smithson (1972) assumed a Ross Sea crustal thickness of 28 kilometers; however, recent estimates of depth to Moho in the Ross Sea (McGinnis et al. 1985; Cooper, Davey, and Cochrane 1987; Davey and Cooper 1987; and ten Brink et al. 1989) suggest a crustal thickness of about 20 kilometers. Thus, the gravity data of Smithson (1972) could be explained by a gradual crustal thickening to only 30 kilometers beneath the western portion of the Transantarctic Mountains. Crustal thinning related to the southeast extension of the Rennick Grabin provides another explanation of thinner crust found beneath the Transantarctic Mountains in the study area. Line 1 is positioned along the southeast extension of the Rennick Graben. The crustal model in figure 3 supports the hypothesis of Cooper, Davey, and Behrendt (1987) and Fitzgerald and Gleadow (1988) that the granitic basement between the Rennick Graben and the Victoria Land Basin was thinned and uplifted. Whereas Davey and Cooper's (1987) gravity models postulate a rapid crustal thickening transition from 20 to 40 kilometers beneath the Transantarctic Mountains coastline in the southern Victoria Land Basin, their gravity model near Terra Nova Bay is consistent with the slowly thickening crustal transition model shown in figure 3. Thus, crustal attributes of the Transantarctic Mountains observed here may be atypical of the Transantarctic Mountains south of Terra Nova Bay. Complete support for seismic field work was generously provided by the Federal Institute for Geoscience and Natural Resources, Hannover, Federal Republic of Germany. This work was supported in part by National Science Foundation grant DPP 87-22536, ANI\tc11c JOURNAL.
0
GD U) III
LD
FH
H—
Cu
TAN
I
Ross Sea
DISTANCE (KM)
20 40 60 80 100 120 140 160 180
0
I I
I I
0
10
0 w
Moho
1olo Cr)
0
Figure 3. A model of the Transantarctic Mountains/Ross Sea transition structure is shown at the bottom with P-wave ray paths for line 1. The C denotes the position of the coastline. Synthetic seismograms are plotted at the top using a reduction velocity of 8 kilometers per second. The synthetic seismograms approximately reproduce the P-wave first arrival patterns of the observed data. (km denotes kilometer. s denotes seconds.)
References Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratigraphy and structure of the Victoria Land Basin, western Ross Sea, Antarctica. In A.K. Cooper and F. J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 27-76.
Cooper, A.K., F.J. Davey, and G.R. Cochrane. 1987. Structure of extensionally rifted crust beneath the western Ross Sea and Iselin Bank, Antarctica, from sonobuoy data. In A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics
of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 93-118. Davey, F.J., and A.K. Cooper. 1987. Gravity studies of the Victoria Land Basin and Iselin Bank. In A.K. Cooper and F.J. Davey (Eds.),
1989 REVIEW
The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 119-137.
Fitzgerald, PG., and A.J.W. Gleadow. 1988. Fission-track geochronology, tectonics and structure of the Transantarctic Mountains in Northern Victoria Land, Antarctica. Chemical Geology (isotope geoscience section), 73, 169-198. McGinnis, L.D., R.H. Bowen, J.M. Erickson, B.J. Allred, and J.L. Kreamer. 1985. East-west Antarctic boundary in McMurdo Sound. Tectonophysics, 114: 341-356. Smithson, S.B. 1972. Gravity interpretation in the Transantarctic Mountains near McMurdo Sound, Antarctica. Geological Society of America Bulletin. 83, 3437-3442. ten Brink, U., T. Stern, I. Paintin, B. Beaudoin, T. Hefford, and J. McGinnis. 1989. Seismic investigation of lithospheric flexure within the Ross Embayment, Antarctica. EOS, 70(15), 468.
35
Ship-to-shore seismic refraction investigation of the lithospheric structure of the Transantarctic Mountain front
mochronology by the 40Ar/39Ar Method. New York: Oxford University Press. Steiger, RH., and E. Jaeger. 1977. Subcommission on Geochronology: Convention of the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359-362. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1983. Late Neogene and older Cenozoic microfossils in high elevation deposits of the Transantarctic Mountains: Evidence for marine sedimentation and ice volume variation on the East Antarctic Craton. Antarctic Journal of the U.S., 18(5), 96-97. Webb, P.-N., D.M. Harwood, B.C. McKelvey, and L.D. Stott. 1984. Cenozoic marine sedimentation and ice-volume variation on the East Antarctic craton. Geology, 12, 287-291. Webb, P.-N., D.M. Harwood, B.C. McKelvey, M.C.G. Mabin, and J.H. Mercer. 1986. Late Cenozoic tectonic and glacial history of the Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 99-100.
Plateau icecap (figure 1). The array consisted of a 1.2-kilometer linear array of eight vertical seismographs, deployed at 150meter intervals away from the array center, and an equilateral (160-meter sides) triangular deployment of three-component
162.0 163.0 164.0 165.0 166.0
DANIEL R.H. O'CONNELL and RALPH R.B. VON FRESE
-74.
-74
Byrd Polar Research Center
and Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210 JOHN PASKIEVITCH -74.
74.5
U.S. Geological Survei Branch of Alaskan Geology Anchorage, Alaska 99508
As part of the 1988-1989 German Antarctic Northern Victoria Land Expedition V (GANOVEX V) offshore-onshore seismic retraction experiment, we established a seismic recording array in the Transantarctic Mountains to record a tuned airgun array operated in the Ross Sea. The objective was to determine the crustal structure of the transition zone between the Transantarctic Mountains and the Ross Sea by recording three onshore-offshore seismic refraction profiles across the Transantarctic Mountains near Terra Nova Bay. Unfavorable ice conditions in Terra Nova Bay reduced two onshore-offshore profile lengths and necessitated reorientation of the third profile. Other GANOVEX V groups deployed onshore and offshore seismographs to provide constraints on the crustal structure of the Ross Sea and inland Transantarctic Mountain portions of the profiles. An 11-element digital seismograph array was established 65 kilometers inland from the Terra Nova Bay on the Tourmaline 1989 REVIEW
-75
75
-7E
5 162.0 153.0 154.0 165.0 156.0
Figure 1. Map of the study area showing the position of the seismic recording array (triangle) in relation to seismic lines 1, 4, and 5 (circles). 33
seismographs around the array midpoint. The array recorded digital seismic waveforms generated by a 43.5-liter airgun array fired shots at 250-meter intervals in the Ross Sea. Data were recorded at offsets of 65-150 kilometers along three onshoreoffshore profiles. Data were also recorded during three marine seismic refraction experiments located more than 200 kilometers from the array. Onshore-offshore line 1 began 65 kilometers from the array and extended to 135 kilometers along an azimuth of S30°E. Line 1 provided a crustal profile between the Campbell and Priestley glaciers (figure 1). Onshore-offshore line 5 began 150 kilometers from the array at an azimuth of S21°E, and ended at a distance and azimuth of 85 kilometers and S52°E, respectively (figure 1). Line 5 constituted a "broadside shoot" with respect to the array and should provide constraints on the three-dimensional crustal velocity structure between the Campbell and Reeves glaciers. Onshore-offshore line 4 consisted of a sequence of stacking shots near Cape Washington 79 kilometers from the array at an azimuth of 552°E and continued as a marine profile 170 kilometers east of the coast (figure 1). Virtually all the airgun shots comprising the three onshoreoffshore refraction profiles were recorded by all or part of the array. Complete waveform data from the entire array were obtained for the first 10 kilometers and last 25 kilometers of line 1. Due to intermittent recorder failures, waveform data are available at a 250-meter shot spacing from subsets of the array for the remainder of the line 1. Recording failures were reduced during lines 4 and 5, and more complete array recordings were obtained. Clear seismic signals from the three > 200 kilometers marine seismic profiles were not detected on single components of ground motion during our first review of the data. Excellent airgun signal quality and low background noise levels were a pleasant surprise considering the modest 43.5liter airgun array capacity. Spectral calculations indicate signalto-noise ratios exceeding 10 in the 2-30-hertz frequency range. Ambient noise levels in the 2-20-hertz band were less than 0.1 millimicrons during most of the recording period. First P-wave arrivals are clearly observed along the entire length of line 1. A portion of the vertical-component record section for line 1 is shown in figure 2. This portion of the record section contains lower crustal refracted first arrivals, wide angle PmP reflections, and Pn Moho refraction first arrivals which constrain the crustal thickness beneath the portion the Transantarctic Mountains near the shoreline of Terra Nova Bay to approximately 22 kilometers (figure 3). Large amplitudes produced by the Moho triplication and the lower crust Pg-Pn crossover produced excellent signal-to-noise on single seismograms. Sub-Moho refracted energy is indicated by a sharp increase in first-arrival apparent velocities (> 8 kilometers per second) at > 120-kilometer offsets on line 1. The Ross Sea portion of the crustal model shown in figure 3 was derived from the results of McGinnis et al. (1985); Cooper, Davey, and Behrendt (1987); and Cooper, Davey, and Cochrane (1987). Data obtained from line 1 indicate slow crustal thickening (1 kilometer thickening per 10 kilometer offset) beneath the Transantarctic Mountains between the coastline and 20 kilometers inland (figure 3). Constraints on the crustal structure beneath the Transantarctic Mountains further inland will be provided by data collected by the Hamburg GANOVEX V group. Preliminary analysis of the line 1 data suggests a gradual crustal thickening transition zone between the Transant34
w
U-) CD C) 7.)
a-) HU-) M 6
uJ F-J
uJ >
FT 4.
80.0
85.0 90.0 DIST (KM)
95.0
Figure 2. Vertical-component seismograms from a portion of the line 1 record section. Note the excellent signal strength, clear PmP Moho reflection at 5.3 seconds, and Pg-Pn crossover at 94 kilometers distance. (km denotes kilometer.)
arctic Mountains and the Ross Sea. Crustal thickness beneath the western portion of the Transantarctic Mountains near Terra Nova Bay is significantly less than the 40 kilometers assumed by Smithson (1972) and Davey and Cooper (1987) in the McMurdo Sound area. One explanation is that the Transantarctic Mountains crust is thinner than assumed in the gravity modeling of Smithson (1972) and Davey and Cooper (1987). Smithson (1972) assumed a Ross Sea crustal thickness of 28 kilometers; however, recent estimates of depth to Moho in the Ross Sea (McGinnis et al. 1985; Cooper, Davey, and Cochrane 1987; Davey and Cooper 1987; and ten Brink et al. 1989) suggest a crustal thickness of about 20 kilometers. Thus, the gravity data of Smithson (1972) could be explained by a gradual crustal thickening to only 30 kilometers beneath the western portion of the Transantarctic Mountains. Crustal thinning related to the southeast extension of the Rennick Grabin provides another explanation of thinner crust found beneath the Transantarctic Mountains in the study area. Line 1 is positioned along the southeast extension of the Rennick Graben. The crustal model in figure 3 supports the hypothesis of Cooper, Davey, and Behrendt (1987) and Fitzgerald and Gleadow (1988) that the granitic basement between the Rennick Graben and the Victoria Land Basin was thinned and uplifted. Whereas Davey and Cooper's (1987) gravity models postulate a rapid crustal thickening transition from 20 to 40 kilometers beneath the Transantarctic Mountains coastline in the southern Victoria Land Basin, their gravity model near Terra Nova Bay is consistent with the slowly thickening crustal transition model shown in figure 3. Thus, crustal attributes of the Transantarctic Mountains observed here may be atypical of the Transantarctic Mountains south of Terra Nova Bay. Complete support for seismic field work was generously provided by the Federal Institute for Geoscience and Natural Resources, Hannover, Federal Republic of Germany. This work was supported in part by National Science Foundation grant DPP 87-22536, ANI\tc11c JOURNAL.
0
GD U) III
LD
FH
H—
Cu
TAN
I
Ross Sea
DISTANCE (KM)
20 40 60 80 100 120 140 160 180
0
I I
I I
0
10
0 w
Moho
1olo Cr)
0
Figure 3. A model of the Transantarctic Mountains/Ross Sea transition structure is shown at the bottom with P-wave ray paths for line 1. The C denotes the position of the coastline. Synthetic seismograms are plotted at the top using a reduction velocity of 8 kilometers per second. The synthetic seismograms approximately reproduce the P-wave first arrival patterns of the observed data. (km denotes kilometer. s denotes seconds.)
References Cooper, A.K., F.J. Davey, and J.C. Behrendt. 1987. Seismic stratigraphy and structure of the Victoria Land Basin, western Ross Sea, Antarctica. In A.K. Cooper and F. J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 27-76.
Cooper, A.K., F.J. Davey, and G.R. Cochrane. 1987. Structure of extensionally rifted crust beneath the western Ross Sea and Iselin Bank, Antarctica, from sonobuoy data. In A.K. Cooper and F.J. Davey (Eds.), The Antarctic Continental Margin: Geology and geophysics
of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 93-118. Davey, F.J., and A.K. Cooper. 1987. Gravity studies of the Victoria Land Basin and Iselin Bank. In A.K. Cooper and F.J. Davey (Eds.),
1989 REVIEW
The Antarctic Continental Margin: Geology and geophysics of the western Ross Sea. Houston: American Association of Petroleum Geologists. Earth Science Series, 5B, 119-137.
Fitzgerald, PG., and A.J.W. Gleadow. 1988. Fission-track geochronology, tectonics and structure of the Transantarctic Mountains in Northern Victoria Land, Antarctica. Chemical Geology (isotope geoscience section), 73, 169-198. McGinnis, L.D., R.H. Bowen, J.M. Erickson, B.J. Allred, and J.L. Kreamer. 1985. East-west Antarctic boundary in McMurdo Sound. Tectonophysics, 114: 341-356. Smithson, S.B. 1972. Gravity interpretation in the Transantarctic Mountains near McMurdo Sound, Antarctica. Geological Society of America Bulletin. 83, 3437-3442. ten Brink, U., T. Stern, I. Paintin, B. Beaudoin, T. Hefford, and J. McGinnis. 1989. Seismic investigation of lithospheric flexure within the Ross Embayment, Antarctica. EOS, 70(15), 468.
35
