The Wilkes Land continent-ocean crustal boundary
1980b, 1983,1986; Webb in press). Savage (1982) and Savage and Ciesielski (1983) placed subunits 2A and 2B sediments in the Lower to Middle Miocene on the basis of diatom biostratigraphy. At site 272 a major decrease in foraminiferal abundance and diversity is noted between approximately 138 and 148 meters subbottom. This roughly corresponds to the subunit 2A/2B boundary and occurs at the level of a proposed 4-million-year hiatus which separates Lower and Middle Miocene sediments (Savage and Ciesielski 1983). Only sporadic occurrences of robust thick-walled forms such as Islandiella spp., Epistominella exigua (Brady), Globocassidulina suhglohosa (Brady), and Nonione!la iridea Heron-Allen and Earland occur below 147 meters (in subunit 2B) and much of the lower succession is barren of foraminifera. Absence of fauna is attributed to dissolution contemporaneous with deposition, or post-depositional diagenesis. Site 272 foraminiferal assemblages may have been subjected to glacial transport and redeposition and/or bottom current reworking. The wide size range of foraminifera at any stratigraphic level suggests size sorting is not a significant factor. This work was supported by National Science Foundation grant DPP 82-14174 to Peter-N. Webb; and Sigma Xi, American Association of Petroleum Geologists and Friends of Orton Hall (Ohio State University) grants to D. Mark Steinhauff. Thanks are extended to David Harwood for comments and suggestions during this laboratory study. References
D'Agostino, A.E. 1980.
Foraininiferal systematics, biostratigraphy and pal eoecology of DSt)P site 273, Ross Sea, Antarctica. (Masters thesis, North-
ern Illinois University, DeKalb, Illinois.)
The Wilkes Land continent-ocean crustal boundary STEPHEN L. EITTREIM U.S. Geological Survey Menlo Park, California 94025
Multichannel seismic data collected in 1984 on the east antarctic margin between 130° and 150° east have revealed the existence of a marginal rift basin that is proposed here to be the remnant of a Cretaceous rift valley similar to the present-day rift valleys of East Africa. A breakup unconformity that marks the change to a seafloor spreading regime in the rifting of Australia from Antarctica, overlaps the marginal rift basin and terminates at the oldest identified oceanic crust. Track lines of the 1984 U.S. Geological Survey investigation along which multichannel seismic, sonobuoy refraction, gravity and magnetics, 3.5- and 12-kilohertz echo sounding data were collected are shown in figure 1. Side-scan sonar data, 126
D'Agostino, A. E., and P.-N. Webb. 1980 Interpretation of mid-Miocene to Recent lithostratigraphy and biostratigraphy at osiiw site 273, Ross Sea. Antarctic Journal of the U.S., 15(5), 118-120. Hayes, D.E., L.A. Frakes, P.J. Barrett, D.A. Burns, P. Chen, A.B. Ford, A.G. Kaneps, E.M. Kemp, D.W. McCollum, D.J.W. Piper, R.E. Wall, and P.-N. Webb. 1975. 8. Sites 270, 271, 272. In D.E. Hayes, L.A. Frakes et al. (Eds.), Initial Reports of the Deep Sea Drilling Project (Vol.
28). Washington, D.C.: U.S. Government Printing Office. Leckie, R.M., and P.-N. Webb. 1980a. Foraminifera of DSDP 270 as indicators of the evolving Ross Sea in the late Oligocene/early Miocene. Antarctic Journal of the U.S., 15(5), 117-118. Leckie, R.M., and P.-N. Webb. 1980b. A provisional late Oligoceneearly Miocene foraminiferal zonation, Ross Sea continental shelf, Antarctica. Geological Society of America Abstracts with Program, 12, 469. Leckie, R.M., and P.-N. Webb. 1983. late Oligocene-early Miocene glacial record of the Ross Sea, Antarctica: Evidence from DSDP site 270. Geology, 11, 578-582. Leckie, R.M., and P.-N. Webb. 1986. Late Paleogene and Early Neogene foraminifers of Deep Sea Drilling Pro ject Site 270, Ross Sea, Antarctica. InJ.P. Kennett, C.C. von der Borch, et al., (Eds.), Initial Reports of the Deep Sea Drilling Project, (Vol. 90). Washington, D.C.: U.S. Government Printing Office. Savage, M . L. 1982. The glacial history of the Ross Sea region based on DSDP leg 28, sites 272 and 273. (Masters thesis, University of Georgia, Athens, Georgia.) Savage, ML., and P.F. Ciesielski. 1983. A revised history of glacial sedimentation in the Ross Sea region. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Canberra: Australian Academy of Science. Steinhauff, D.M. 1985. Paleoecologic and st ratigraphic analysis of Miocene foraminifera (Protozoa) from glacial-marine sediments, Ross Sea, Antarctica.
(Masters thesis, Ohio State University, Columbus, Ohio.) Webb, P.-N. In press. Upper Oligocene-Holocene benthic foraminifera of the Ross Sea region. Revue de Paleohiologie.
sediment cores, and dredge samples were also collected in the area at the eastern edge of the map of figure 1. Reports on this data, including seismic records, most of which are processed through migration, are included in a volume of papers on the Wilkes Land margin (Eittreim and Hampton 1987). Four seismic sequences, A through ui, separated by the Unconformities T, Ki, and K2, have been recognized throughout most of the surveyed area (figure 2). The deepest sequence (D) consists of a block-faulted, moderate- to high-velocity layer with flat and parallel bedding. D is interpreted as a prerift sequence based on its block-faulted and parallel-bedded nature. The Beacon Supergroup (Barrett 1982), an antarctic lithologic unit of flat-bedded elastic sediments, with conformable sills of dolerite intrusions, may be represented in this sequence, with the dolerite sills seen as markedly high-amplitude reflecting horizons (Eittreim and Smith 1987). The top of this sequence is an erosional unconformity. C overlies this unconformity and is interpreted to be a synrift sequence that was deposited in basins produced during the crustal extension and subsidence associated with the extension of the Gondwana crust prior to the beginning of seafloor spreading at about 95 million years ago (Cande and Mutter 1982). Sequence C is topped by a breakup unconformity produced at the earliest generation of oceanic crust. C occurs only south of the edge of oceanic crust. The two remaining shallower sequences are interpreted as: B, a shelf or ANTARCTIC JOURNAL
136'
138'
140'
62'
Figure 1. Map showing multichannel seismic tracks and shot point numbers indicated for S.P. Lee cruise Li -84. Bathymetry from Chase et al. (1987). Tracks of Institut Français du Petrol (IFP) and Japan National Oil Company (JNoc) shown dashed. ("Km" denotes "kilometer.")
shallow-water deposit laid down post-breakup; and A, deepwater hemipelagic deposit laid down in an environment similar to the rise and slope of today but prior to the glacial onset. The southern edge of seismically observed oceanic crust is defined as the continent-ocean-boundary. Reflections from the base of the crust (Moho) can be followed across the continentocean boundary. Crustal thicknesses of about 7 kilometers are recorded for the oceanic crust, the Moho displays a smooth transition across the continent-ocean boundary, and then bends downward beneath continental crust to depths, beyond reflection range, of greater than 17 kilometers. A depression in basement reflectors occurs landward of the continent-ocean boundary that is bounded on the seaward side by a ridge of oceanic crust. This depression is termed a marginal rift basin, has a relief of about 2 kilometers, and averages about 40 kilometers wide. The synrift sequence C bounded by KI and K2 is thickest in this basin. Highly reflective, discontinuous layers floor the marginal rift basin and suggest the presence of volcanic flows restricted to within the boundaries of the basin. On line 3 the portion of sequence c that fills the marginal rift basin consists of dipping reflectors interpreted as a prograded clastic sequence about 2 kilometers thick. The marginal rift basin is therefore a 40-kilometer wide by 2kilometer thick basin on continental crust that abuts against the oldest oceanic crust of the southeast Indian Ocean. The basin is geometrically similar to the presently active East African rift valley (Bosworth, Lanbiase, and Keisler 1986). The steeply dipping (10°) prograded sediments in the marginal rift basin sug1987 REVIEW
gest an active tectonic environment, and the interpreted lava flows, restricted to the basin floor, imply that the basin was formed prior to volcanism. Based on the present great depth of K2, a significant amount of crustal thinning and postrift thermal subsidence is implied. By using the observed crustal thickness, and assuming an initial prerift thickness of 35 kilometers, a measured crustal thinning, or crustal 3-factor, is derived whose values are shown in figure 3. The age of continent breakup, or first generation of oceanic crust (95 million years old) and the McKenzie (1978) model are used to calculate the amount of thermal subsidence. The total tectonic subsidence, compensated for sediment load, is due to both thermal contraction and extensional thinning of the lithosphere, the latter denoted by P. A comparison between measured crustal thinning (13,.), the ratio of 35 kilometers to the present measured thickness (K2 to Moho) in kilometers, and lithospheric thinning calculated by using the McKenzie (1978) extension-subsidence model, applied to the depth of K2, compensating for sediment load ("inferred 13"), is shown in figure 3. Note that over most of the profiles, a fair agreement is obtained between the two kinds of 13-factors. Near the boundary with oceanic crust however, the values from the two methods diverge considerably with the 13c values much larger than those "inferred" from the McKenzie subsidence model. Both methods result in values greater than 4 over the marginal rift basin, which is higher than expected by theoretical considerations of LePichon and Sibuet (1981). They argue that 13 = 3.2 should be a limiting value of thinning on the basis of the relative buoyancy of new igneous oceanic crust 127
MARGINAL RIFT OCEANIC CRUST
CONTINENTAL CRUST
BASIN K1 K2 20KM
LINE 1 /
KM
Detachment fault'
LINE 3 T
I,
K KM 10 15 20
LINE 5
M
K1 50 KM
KM 1
T
:K2
15 20
LINE 7
M I
COB
Figure 2. Line drawings of depth sections of four westernmost lines, located in figure 1, registered along the continent-ocean boundary. Vertical exaggeration 2:1. Depth computation based on velocity model generated from smoothed stacking velocities. The four sequences discussed in text, A, B, c, and care bounded, from top downward, by the sequence boundaries T, 1(1, and 1(2. See Eittreim and Smith (1987) for uninterpreted records. ("Km" denotes "kilometer?')
versus typical continental crustal material. Our observed apparent "over-thinning" of crust may argue for non-uniform thinning, with the upper lithosphere including the crust thinned more than the lower lithosphere in the marginal rift basin region. This could be accomplished by a throughgoing low-angle detachment fault of the style advocated by Wernicke (1981). Perhaps the most critical interpretation in the seismic section is that of the breakup unconformity. Ki, interpreted as the breakup unconformity, is given an age of 95 million years, and beds below are presumed older, that is, synrift and prerift. Validation of these ages is important for building a tectonic model of the Gondwana rifting event which is displayed so distinctly in these seismic data. Drilling this margin as part of the Ocean Drilling Program has been proposed by French and U.S. workers, with the identification of age and lithology of these seismic sequences and their bounding unconformities a primary objective. 128
This work was supported by the U.S. Geological Survey. Logistic support during the data collection in 1984 was provided by the National Science Foundation and the U.S. Coast Guard.
References Barrett, P.J. 1982. History of the Ross Sea region during the deposition of the Beacon Supergroup 400-180 million years ago. Journal of the Royal Society of New Zealand, 11, 447-458. Bosworth, W., J . Lanbiase, and R. Keisler. 1986. A new look at Gregory's rift: The structural style of continental rifling. EQS. 67, 577-583. Cande, S., and J.C. Mutter. 1982. A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica. Earth and Planetary Science Letters, 58, 151-160. Chase, T.E., B.A. Seekins, J.D. Young, and S.L. Eittreim. 1987. Marine topography of offshore Antarctica. In S.L. Eittreim and M.A. Hampton (Eds.), The Antarctic continental margin: Geology and ANTARCTIC JOURNAL
3000
CDP 4000
0
LINE 1 2000
1000
10
(I) Cr w LLJ
-
le 10
-..---..-- - -
0 20 KM
$ inferred
V.E2.1
M
20 0 CDP
Or
1000
LINE 3
2000
3000
Rc
measured"
ci) II
w 10 2 -----.--
/3 inferred
M
20
LINE 5 COP 3000 2000 0 - - - - - -
Ir LU
-- -
1000
PC
LIJ
20
Figure 3. Depth sections of the three westernmost lines, with lithospheric thinning factor, 3, "inferred" by the Mckenzie (1978) model and crustal thinning factor, 11,, measured by taking the observed K2-to-MOhO thickness divided into the assumed initial crustal thickness of 35 kilometers. ("Km' denotes "kilometer.") geophysics of offshore Wilkes Land. (Circum-Pacific Council for Energy
and Mineral Resources Earth Science Series, Vol. 5A.) Houston, Texas. Eittreim, S. L., and M.A. Hampton. 1987. The Antarctic continental margin: Geology and geophysics of offshore Wilkes Land. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5A) Houston, Texas. Eittreim, S.L., and G.L. Smith. 1987. Seismic sequences and their distribution on the Wilkes Land margin. In S.L. Eittreim and M.A. Hampton (Eds.), The Antarctic continental margin: Geology and
1987 REVIEW
geophysics of ofshore Wilkes Land. (Circum-Pacific Council for Energy
and Mineral Resources Earth Science Series, Vol. 5A.) Houston, Texas. LePichon, X., and J.C. Sibuet. 1981. Passive margins: A model of formation. Journal of Geophysical Research, 86, 3708-3720. McKenzie, D.P. 1978. Some remarks on the development of sedimentary basins. Earth Planetary Science Letters, 40, 25-32. Wernicke, B. 1981. Low-angle normal faults in the Basin and Range province: Nappe tectonics in an extending orogen. Nature, 21, 645-648.
129
D'Agostino, A.E. 1980.
Foraininiferal systematics, biostratigraphy and pal eoecology of DSt)P site 273, Ross Sea, Antarctica. (Masters thesis, North-
ern Illinois University, DeKalb, Illinois.)
The Wilkes Land continent-ocean crustal boundary STEPHEN L. EITTREIM U.S. Geological Survey Menlo Park, California 94025
Multichannel seismic data collected in 1984 on the east antarctic margin between 130° and 150° east have revealed the existence of a marginal rift basin that is proposed here to be the remnant of a Cretaceous rift valley similar to the present-day rift valleys of East Africa. A breakup unconformity that marks the change to a seafloor spreading regime in the rifting of Australia from Antarctica, overlaps the marginal rift basin and terminates at the oldest identified oceanic crust. Track lines of the 1984 U.S. Geological Survey investigation along which multichannel seismic, sonobuoy refraction, gravity and magnetics, 3.5- and 12-kilohertz echo sounding data were collected are shown in figure 1. Side-scan sonar data, 126
D'Agostino, A. E., and P.-N. Webb. 1980 Interpretation of mid-Miocene to Recent lithostratigraphy and biostratigraphy at osiiw site 273, Ross Sea. Antarctic Journal of the U.S., 15(5), 118-120. Hayes, D.E., L.A. Frakes, P.J. Barrett, D.A. Burns, P. Chen, A.B. Ford, A.G. Kaneps, E.M. Kemp, D.W. McCollum, D.J.W. Piper, R.E. Wall, and P.-N. Webb. 1975. 8. Sites 270, 271, 272. In D.E. Hayes, L.A. Frakes et al. (Eds.), Initial Reports of the Deep Sea Drilling Project (Vol.
28). Washington, D.C.: U.S. Government Printing Office. Leckie, R.M., and P.-N. Webb. 1980a. Foraminifera of DSDP 270 as indicators of the evolving Ross Sea in the late Oligocene/early Miocene. Antarctic Journal of the U.S., 15(5), 117-118. Leckie, R.M., and P.-N. Webb. 1980b. A provisional late Oligoceneearly Miocene foraminiferal zonation, Ross Sea continental shelf, Antarctica. Geological Society of America Abstracts with Program, 12, 469. Leckie, R.M., and P.-N. Webb. 1983. late Oligocene-early Miocene glacial record of the Ross Sea, Antarctica: Evidence from DSDP site 270. Geology, 11, 578-582. Leckie, R.M., and P.-N. Webb. 1986. Late Paleogene and Early Neogene foraminifers of Deep Sea Drilling Pro ject Site 270, Ross Sea, Antarctica. InJ.P. Kennett, C.C. von der Borch, et al., (Eds.), Initial Reports of the Deep Sea Drilling Project, (Vol. 90). Washington, D.C.: U.S. Government Printing Office. Savage, M . L. 1982. The glacial history of the Ross Sea region based on DSDP leg 28, sites 272 and 273. (Masters thesis, University of Georgia, Athens, Georgia.) Savage, ML., and P.F. Ciesielski. 1983. A revised history of glacial sedimentation in the Ross Sea region. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Canberra: Australian Academy of Science. Steinhauff, D.M. 1985. Paleoecologic and st ratigraphic analysis of Miocene foraminifera (Protozoa) from glacial-marine sediments, Ross Sea, Antarctica.
(Masters thesis, Ohio State University, Columbus, Ohio.) Webb, P.-N. In press. Upper Oligocene-Holocene benthic foraminifera of the Ross Sea region. Revue de Paleohiologie.
sediment cores, and dredge samples were also collected in the area at the eastern edge of the map of figure 1. Reports on this data, including seismic records, most of which are processed through migration, are included in a volume of papers on the Wilkes Land margin (Eittreim and Hampton 1987). Four seismic sequences, A through ui, separated by the Unconformities T, Ki, and K2, have been recognized throughout most of the surveyed area (figure 2). The deepest sequence (D) consists of a block-faulted, moderate- to high-velocity layer with flat and parallel bedding. D is interpreted as a prerift sequence based on its block-faulted and parallel-bedded nature. The Beacon Supergroup (Barrett 1982), an antarctic lithologic unit of flat-bedded elastic sediments, with conformable sills of dolerite intrusions, may be represented in this sequence, with the dolerite sills seen as markedly high-amplitude reflecting horizons (Eittreim and Smith 1987). The top of this sequence is an erosional unconformity. C overlies this unconformity and is interpreted to be a synrift sequence that was deposited in basins produced during the crustal extension and subsidence associated with the extension of the Gondwana crust prior to the beginning of seafloor spreading at about 95 million years ago (Cande and Mutter 1982). Sequence C is topped by a breakup unconformity produced at the earliest generation of oceanic crust. C occurs only south of the edge of oceanic crust. The two remaining shallower sequences are interpreted as: B, a shelf or ANTARCTIC JOURNAL
136'
138'
140'
62'
Figure 1. Map showing multichannel seismic tracks and shot point numbers indicated for S.P. Lee cruise Li -84. Bathymetry from Chase et al. (1987). Tracks of Institut Français du Petrol (IFP) and Japan National Oil Company (JNoc) shown dashed. ("Km" denotes "kilometer.")
shallow-water deposit laid down post-breakup; and A, deepwater hemipelagic deposit laid down in an environment similar to the rise and slope of today but prior to the glacial onset. The southern edge of seismically observed oceanic crust is defined as the continent-ocean-boundary. Reflections from the base of the crust (Moho) can be followed across the continentocean boundary. Crustal thicknesses of about 7 kilometers are recorded for the oceanic crust, the Moho displays a smooth transition across the continent-ocean boundary, and then bends downward beneath continental crust to depths, beyond reflection range, of greater than 17 kilometers. A depression in basement reflectors occurs landward of the continent-ocean boundary that is bounded on the seaward side by a ridge of oceanic crust. This depression is termed a marginal rift basin, has a relief of about 2 kilometers, and averages about 40 kilometers wide. The synrift sequence C bounded by KI and K2 is thickest in this basin. Highly reflective, discontinuous layers floor the marginal rift basin and suggest the presence of volcanic flows restricted to within the boundaries of the basin. On line 3 the portion of sequence c that fills the marginal rift basin consists of dipping reflectors interpreted as a prograded clastic sequence about 2 kilometers thick. The marginal rift basin is therefore a 40-kilometer wide by 2kilometer thick basin on continental crust that abuts against the oldest oceanic crust of the southeast Indian Ocean. The basin is geometrically similar to the presently active East African rift valley (Bosworth, Lanbiase, and Keisler 1986). The steeply dipping (10°) prograded sediments in the marginal rift basin sug1987 REVIEW
gest an active tectonic environment, and the interpreted lava flows, restricted to the basin floor, imply that the basin was formed prior to volcanism. Based on the present great depth of K2, a significant amount of crustal thinning and postrift thermal subsidence is implied. By using the observed crustal thickness, and assuming an initial prerift thickness of 35 kilometers, a measured crustal thinning, or crustal 3-factor, is derived whose values are shown in figure 3. The age of continent breakup, or first generation of oceanic crust (95 million years old) and the McKenzie (1978) model are used to calculate the amount of thermal subsidence. The total tectonic subsidence, compensated for sediment load, is due to both thermal contraction and extensional thinning of the lithosphere, the latter denoted by P. A comparison between measured crustal thinning (13,.), the ratio of 35 kilometers to the present measured thickness (K2 to Moho) in kilometers, and lithospheric thinning calculated by using the McKenzie (1978) extension-subsidence model, applied to the depth of K2, compensating for sediment load ("inferred 13"), is shown in figure 3. Note that over most of the profiles, a fair agreement is obtained between the two kinds of 13-factors. Near the boundary with oceanic crust however, the values from the two methods diverge considerably with the 13c values much larger than those "inferred" from the McKenzie subsidence model. Both methods result in values greater than 4 over the marginal rift basin, which is higher than expected by theoretical considerations of LePichon and Sibuet (1981). They argue that 13 = 3.2 should be a limiting value of thinning on the basis of the relative buoyancy of new igneous oceanic crust 127
MARGINAL RIFT OCEANIC CRUST
CONTINENTAL CRUST
BASIN K1 K2 20KM
LINE 1 /
KM
Detachment fault'
LINE 3 T
I,
K KM 10 15 20
LINE 5
M
K1 50 KM
KM 1
T
:K2
15 20
LINE 7
M I
COB
Figure 2. Line drawings of depth sections of four westernmost lines, located in figure 1, registered along the continent-ocean boundary. Vertical exaggeration 2:1. Depth computation based on velocity model generated from smoothed stacking velocities. The four sequences discussed in text, A, B, c, and care bounded, from top downward, by the sequence boundaries T, 1(1, and 1(2. See Eittreim and Smith (1987) for uninterpreted records. ("Km" denotes "kilometer?')
versus typical continental crustal material. Our observed apparent "over-thinning" of crust may argue for non-uniform thinning, with the upper lithosphere including the crust thinned more than the lower lithosphere in the marginal rift basin region. This could be accomplished by a throughgoing low-angle detachment fault of the style advocated by Wernicke (1981). Perhaps the most critical interpretation in the seismic section is that of the breakup unconformity. Ki, interpreted as the breakup unconformity, is given an age of 95 million years, and beds below are presumed older, that is, synrift and prerift. Validation of these ages is important for building a tectonic model of the Gondwana rifting event which is displayed so distinctly in these seismic data. Drilling this margin as part of the Ocean Drilling Program has been proposed by French and U.S. workers, with the identification of age and lithology of these seismic sequences and their bounding unconformities a primary objective. 128
This work was supported by the U.S. Geological Survey. Logistic support during the data collection in 1984 was provided by the National Science Foundation and the U.S. Coast Guard.
References Barrett, P.J. 1982. History of the Ross Sea region during the deposition of the Beacon Supergroup 400-180 million years ago. Journal of the Royal Society of New Zealand, 11, 447-458. Bosworth, W., J . Lanbiase, and R. Keisler. 1986. A new look at Gregory's rift: The structural style of continental rifling. EQS. 67, 577-583. Cande, S., and J.C. Mutter. 1982. A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica. Earth and Planetary Science Letters, 58, 151-160. Chase, T.E., B.A. Seekins, J.D. Young, and S.L. Eittreim. 1987. Marine topography of offshore Antarctica. In S.L. Eittreim and M.A. Hampton (Eds.), The Antarctic continental margin: Geology and ANTARCTIC JOURNAL
3000
CDP 4000
0
LINE 1 2000
1000
10
(I) Cr w LLJ
-
le 10
-..---..-- - -
0 20 KM
$ inferred
V.E2.1
M
20 0 CDP
Or
1000
LINE 3
2000
3000
Rc
measured"
ci) II
w 10 2 -----.--
/3 inferred
M
20
LINE 5 COP 3000 2000 0 - - - - - -
Ir LU
-- -
1000
PC
LIJ
20
Figure 3. Depth sections of the three westernmost lines, with lithospheric thinning factor, 3, "inferred" by the Mckenzie (1978) model and crustal thinning factor, 11,, measured by taking the observed K2-to-MOhO thickness divided into the assumed initial crustal thickness of 35 kilometers. ("Km' denotes "kilometer.") geophysics of offshore Wilkes Land. (Circum-Pacific Council for Energy
and Mineral Resources Earth Science Series, Vol. 5A.) Houston, Texas. Eittreim, S. L., and M.A. Hampton. 1987. The Antarctic continental margin: Geology and geophysics of offshore Wilkes Land. (Circum-Pacific Council for Energy and Mineral Resources Earth Science Series, Vol. 5A) Houston, Texas. Eittreim, S.L., and G.L. Smith. 1987. Seismic sequences and their distribution on the Wilkes Land margin. In S.L. Eittreim and M.A. Hampton (Eds.), The Antarctic continental margin: Geology and
1987 REVIEW
geophysics of ofshore Wilkes Land. (Circum-Pacific Council for Energy
and Mineral Resources Earth Science Series, Vol. 5A.) Houston, Texas. LePichon, X., and J.C. Sibuet. 1981. Passive margins: A model of formation. Journal of Geophysical Research, 86, 3708-3720. McKenzie, D.P. 1978. Some remarks on the development of sedimentary basins. Earth Planetary Science Letters, 40, 25-32. Wernicke, B. 1981. Low-angle normal faults in the Basin and Range province: Nappe tectonics in an extending orogen. Nature, 21, 645-648.
129
