Paleomagnetic studies of Mesozoic rocks from the Antarctic Peninsula ...
meters. We extended the profile to even lower elevations by sampling a small granitic nunatak under the northwest shoulder of Mount Black Prince and a previously unmapped granitic body at 71.68540 S 167.9676° E. Granite was also sighted at previously unmapped localities in bluffs to the west and north of this nunatak. We collected 23 samples in total, over an elevation range of 2,060 meters, from the summit of Mount Adam (4,010 meters) to the base of the previously unvisited nunatak (1,950 meters). Consistent joint patterns suggest that the Admiralty Mountains pluton has not been tilted significantly, permitting the data to be grouped in one structural block. We made the traverse back to the Boss Peak camp on 9 December. On 12 December the sampling profile was extended down to an elevation of 1,314 meters, four samples being collected from a ridge just east of the mouth of the Rastorfer Glacier (figure 2), a tributary to the Tucker Glacier. Under white-out conditions, we returned to camp safely with the aid of a Magellan global positioning system (GPS) instrument. Our scheduled pull-out date, 16 December, was fine, and we used the absence of an aircraft to visit the Inferno Peak pluton in the Victory Mountains to collect samples, close to, but not across any major faults. A blizzard descended upon NVL and, except for one brief spell of fine weather on the Navy safety standdown day, remained firmly inplace until 30 December. We were pulled outby LC-130 on 31 December. Soft snow precluded a simple field extraction; almost all field gear was left behind. Scientific samples and some field gear were retrieved by Twin Otter on 6 January 1992. The work done in the 1991-1992 austral summer, although not as complete as would have been desired, does allow the opportunity to address the question of Cretaceous uplift in the Admiralty Mountains of NVL. Returned samples are being prepared for analysis in the Fission Track Facility at Arizona State University. This work was supported by National Science Foundation grant DPP 90-17763. Thanks to UIAGM guide Charles Hobbs, who assisted throughout the season. Without the LC-130 crews of the US Navy squadron VXE-6 and the crew of the Ken Borek Air Twin Otter, neither the fieldwork nor subsequent laboratory analyses would have been possible.
Paleomagnetic studies of Mesozoic rocks from the Antarctic Peninsula: Implications for Weddell Sea opening A.M. GRUNOW Byrd Polar Research Center Ohio State University Columbus, Ohio 43210
West Antarctica's location with respect to East Antarctica after the Middle Jurassic breakup of the Gondwanaland supercontinent is important for an understanding of the opening history of the Weddell Sea. West Antarctica is composed of four major crustal blocks: the Antarctic Peninsula (AP), the Ellsworth-
14
References
Delisle, C., and K. Fromm. 1989. Further evidence for a Cretaceous therm event in north Victoria Land. Geologisches Jahrbuch, E38:143-151. Elliot, D. H., and K. A. Foland. 1986. Potassium-argon age determination, of the Kirkpatrick Basalt, Mesa Range. In E. Stump (Ed.), Geologica investigations in northern Victoria Land. (Antarctic Research Series. Washington, D.C.: American Geophysical Union. 279-288. Findlay, R. H., and H. Jordon. 1984. The volcanic rocks of Mt. Black Princ and Lawrence Peaks, north Victoria Land, Antarctica. Geologische Jahrbuch, B60:143-151. Fitzgerald, P.C. 1992. The Transantarctic Mountains in southern Victoria Land: The application of apatite fission track analysis to a rift shoulder uplift. Tectonics, 11:634-62. Fitzgerald, P. G., and A. J . W. Gleadow. 1988. Fission track geochronology, tectonics and structure of the Transantarctic Mountains in north em Victoria Land, Antarctica. Isotope Geoscience, 73:169-198. Fitzgerald, P. G., and E. Stump. 1991. Uplift history of the Transantarcti Mountains from Victoria Land (-700 S) to Scott Glacier (-.860 S) Evidence from fission track analysis. Sixth International Symposium o Antarctic Earth Sciences. 9-13 September 1991, Saitama, Japan. Ganovex Team. 1987. Geologic map of north Victoria Land, Antarctica 1:500,000. In F. Tessensohn and N. W. Roland (Eds.), German Antarctic North Victoria Land Expedition 1982183. Ganovex III, Vol. 2 Bundesanstalt für Geowissenschaften und Rohstoffe, Stuttgart. 7-79 Kreuzer, A., A. Hohndorf, L. Heinz, U. Vetter, F. Tessensohn, P. Muller H. Jordan, W. Harre, and C. Besang. 1981. K-Ar and Rb-Sr dating o igneous rocks from north Victoria Land, Antarctica. Geologisch Jahrbuch, B41:267-273. Roland, N. W., and F. Tessensobn. 1987. Rennick faulting an early phase o Ross Sea rifting. In F. Tessensohn and N. W. Roland (Eds.), Germa Antarctic North Victoria Land Expedition 1982183. Ganovex Ill, Vol. 2, Bundesanstalt für Geowissenschaften und Rohstoffe, Stuttgart, 275-302 Schmierer, K., and R. Burmester. 1986. Paleomagnetic results from th Cambro-Ordovician Bowers Supergroup, northern Victoria Land, Antarctica. In E. Stump (Ed.), Geological investigations in northern Victoria Land. (Antarctic Research Series.) Washington, D.C.: Amen can Geophysical Union. 69-90. Veevers,J.J. 1988. Seafloor magnetic lineation off the Otway/WestTasmania Basins: Ridge jumps and the subsidence history of the southwes Australian margins. Australian Journal of Earth Science, 35: 451-462.
Whitmore Mountains (EWM), the Thurston Island-Eights Coast (TI), and Marie Byrd Land (MBL) (Daiziel and Elliot 1982) (figur 1). This article describes new paleomagnetic data from th0 Antarctic Peninsula (combined with existing geologic and geo. physical data) that help to constrain the motion of the AP bloc$ relative to East Antarctica and hence increase understanding the opening history of the Weddell Sea. The paleomagnetic samples were obtained (along with sampl for uranium-lead geochronology by S. B. Mukasa and J.Tangeman from the University of Michigan and palynological samples collected by S. Fowell from Lamont-Doherty Geological Observatory) from the AP block during a 1-month cruise in May 1990, 2 weeks' camping on Byers Peninsula in December 1990, and a 1month cruise during January 1991. Most locations were visited by zodiak from the R/V Polar Duke, with the exception of remote field camps established on Byers Peninsula, Kopaitic Island, and Hope Bay (figure 1). Unusually deep snow cover prevented us from sampling the primary objectives at Byers Peninsula. The paleomagnetic samples were collected with a portable, gasoline-
4
ANTARCTIC JOURNAL
70°W
........... ..........
Antarctic Peninsula
South Shetland Islands
I I Sample Areas Byers Peninsula
0
Greenwich Is.
200km Is.. I I I Snow (President Head
ejtu,
C? Hope Bay Low IsJ' Kopaitic Is. i/ç de Islands Islands (Cape Wallace) Bone •.Danger Islands Camp Hill Joinville Is. Penola Strait
XCape S pring Charlotte Bay Andvord Bay Mt. Banck, Paradise Bay
(Cape Tuxen, Moot Pt, Rasmussen Is.) J Pitt Is $,
650
Watkins Is.[] C,
^ N
Antarctic EW1 Peninsula
EAST Weddell ANTARCTIC Sea CRATON
Lassiter Coast,
Avian Is. Stonington Is.
Horseshoe Is. Orville Coast East. Ellsworth U
Pine Is.
TI EW
MBL
Ross Ic e Ross Sea
f
NVL
70°S
o
1
Figure 1. Sample locations on the Antarctic Peninsula. Inset shows the west antarctic crustal blocks in their present-day positions with respect to east Antarctica. EWM, Ellsworth-Whitmore Mountains block; EM, Ellsworth Mountains; EW, Explora Wedge; MBL, Marie Byrd Land; NVL, North Victoria Land; TI, Thurston Island-Eights Coast block. Stippled lines show mountain (or basin) trends; slanted lines show location of ice shelves. 1992 REVIEW
15
Figure 2. Reconstruction for 170 to 155 million years ago. Between 170 and 155 million years ago the AP block rotated clockwise with respect to east Antarctica as a result of Initiation of seafloor spreading in the western Weddell Sea basin along the southern AP block margin. Inset: 175 million years ago reconstruction, before the start of Gondwanaland break-up. (See Grunow at at. 1991 for discussion of tectonic setting at 175 million years ago.) In reconstructions, the geographic south pole (+) is centered on the east Antarctic (eat) reference pole; the west antarctic paleopoles are located with asterisks. Latitude circles are in 150 increments. Abbreviations are as follows: AP, Antarctic Peninsula; ap, AP block A95 circle; BSB, Bryd subglacial basin; eat, east Antarctic A95; EW, Explora Wedge; EWM, Ellsworth-Whitmore Mountains; Fl, Faulkiand Islands; MBL, Marie Byrd Land; RVB, Rocas Verdes basin; TI, Thurston island-Eights Coast; ti, TI block A95 circle.) MBL is shown In a 100-million-years-ago position (Grindley and Oliver 1983) in the reconstructions.
powered, diamond-bit Pomeroy coring drill with a water/glycol mix as drill coolant. We collected 1,100 oriented drill cores and 20 oriented hand samples from 176 sites (six to seven cores per site) at 17 locations from the AP block. The samples were analyzed at the Lamont-Doherty Geological Observatory of Columbia University and the University of Oxford paleomagnetism laboratories. Pilot samples were pro16
gressively demagnetized with the use of step-wise alternating field (AF) or thermal techniques (TH) to determine the best demagnetization procedure for each site. Mean directions and paleopoles were determined using Fisher (1953) statistics. Samples collected from the Botany Bay Group sandstones and siltstones at Camp Hill (figure 1) proved to be weakly magnetized and unstable magnetically. Paleomagnetic cores were also colANTARCTIC JOURNAL
Figure 3. Between about 160 and 130 million years ago, opening began along the northern Antarctic Peninsula and Rocas Verdes basin of southern South America, effectively stopping opening in the southern AP block and causing counterclockwise rotation of the AP-TI blocks. Subduction of the older Jurassic Weddell seafloor (dashed spreading ridge) and transpressionai deformation occurred at the base of the AP block as the EWM block and east Gondwanaland moved southward. Inset: Reconstruction for 130 million years ago. At the start of South Atlantic basin opening (approximately 130 million years ago) the AP block was near its present-day position with respect to east Antarctica and opening stopped in the Rocas Verdes basin. Clockwise rotation of the TI-EWM block resulted in sinistral shearing between the EWM block and east Antarctica and dextral shearing between the TI-EWM block and the AP block (continuing upon Initiation of South Atlantic opening may have caused dextral shearing along the northernmost AP block). lected at Andvord Bay, Avian Island, Charlotte Bay, Horseshoe Island, the Lemaire Channel, the Meichoir Islands, the Pitt Islands, and the Watkins Island group (figure 1); but the rocks at these locations do not yet have radiometric age constraints. Samples of the structurally complex and poorly dated PermoTnassic Trinity Peninsula Group rocks were collected from Hope Bay, Paradise Bay, Cape Spring, and Kopaitic Island (figure 1).
1992 REVIEW
We obtained new uranium-lead ages from intrusions at Moot Point, Cape Tuxen, Bone Bay, Charlotte Bay, and Stonington Island, and a clast from a conglomerate on Horseshoe Island were obtained by Mukasa and Tangeman. A late Jurassic pole (about 155 million years ago) (130* E 67.5* A95=8.5*, N=llvirtual geomagnetic poles) was obtained from S, Oxfordian sedimentary rocks at Low Island (Thomson 1982) and
17
160-million-year-old granites at Bone Bay (R.J.Pankhurst, personal communication) (figure 1). The new data suggest that the AP block rotated clockwise between 170 and 155 million years ago because of early opening in the Weddell Sea Basin (figure 2). A new early Cretaceous (approximately 130 million years ago) paleomagnetic pole (178' E 76' S, A95= 7.2') (based on N = three locality mean poles from the Byers Peninsula mixed-marine member sedimentary rocks, the Mount Banck granite, and the Rasmussen Island granite—figure 1) indicates that the AP block was in or near its present-day position with respect to East Antarctica about 130 million years ago. Between 155 and 130 million years ago, counterclockwise rotation of the AP-TI blocks (Grunow et al. 1991) relative to East Antarctica may have caused subduction of Jurassic Weddell Sea ocean floor beneath the southern AP block and was probably the cause of the Palmer Land deformational event (figure 3). An approximately 130million-year-old pole from the TI block (Grunow et al. 1987) requires clockwise rotation of the TI and EWM blocks between 130 and about 110 million years ago, producing sinistral strikeslip motion between the EWM block and East Antarctica and dextral transpressional motion between the TI-EWM block and the southern Antarctic Peninsula (figure 3, inset). New paleopoles representing about 105 million years ago (from Moot Point and Vietor Rock on Byers Peninsula), about 85 million years ago (from Cape Tuxen), and ab'out 55 million years ago (from Snow Island), along with other equivalent age poles from the AP block (Wafts et al. 1984; Kellogg and Rowley 1989), are similar to the east antarctic reference poles and indicate that the AP block has been in its present-day position with respect to East Antarctica since about 105 million years ago. This work was supported by National Science Foundation grant DPP 90-22349.
18
Special thanks to Sarah Fowell, Ann Hawthorne, Octavio Urbin, and especially Mickey Van Fossen for their assistance in collectirig the paleomagnetic samples and to Sam Mukasa, Robert Pankhurc. and Jean Tangeman for the radiometric age constraints. The reconstructions were made with the help of Lisa Gehagan using tFe PLATES system at the University of Texas at Austin.
References Dalziel, I. W. D. and D. H. Elliot. 1982. West Antarctica: Problem child of Gondwanaland. Tectonics, 1:3-19. Fisher, R. A. 1953. Dispersion on a sphere. Proceedings of the Royal Society I of London, Ser. A, 217:295-305. Grindley, G. W. and P. J. Oliver. 1983. Paleomagnetism of Cretaceous volcanic rocks from Marie Byrd Land. In R. L. Oliver, P. R. James, and J. B. Jago (Eds.), Antarctic Earth Science. Canberra: Australian Academy of Science, 573-578. Grunow, A. M., D. V. Kent, and I. W. D. Daiziel. 1987. Mesozoic evolution of West Antarctica and the Weddell Sea basin: New paleomagnetic constraints. Earth and Planetary Science Letters, 86:16-26. Grunow,A. M., D. V. Kent, and I. W. D. Dalziel. 1991. New pa1eomagneic data from Thurston Island and their implications for the tectonics of West Antarctica. Journal of Geophysical Research, 96:17,935-17,954. Kellogg, K., and P. D. Rowley. 1989. Structural geology and tectonics of the Orville Coast region, southern Antarctic Peninsula, Antarctica. United States Geological Survey Professional Paper, 1498. 25 pp. Pankhurst, R. J . , personal communication, July 1989. Thomson, M. R. A. 1982. Late Jurassic fossils from Low Island, South Shetland Islands. British Antarctic Survey Bulletin, 56:25-35. Watts, D. R., G. C. Watts, and A. M. Bramall. 1984. Cretaceous and Early Tertiary paleomagnetic results from the Antarctic Peninsula. Tectonics, 3:333-346.
ANTARCTIC JOURN
7
Paleomagnetic studies of Mesozoic rocks from the Antarctic Peninsula: Implications for Weddell Sea opening A.M. GRUNOW Byrd Polar Research Center Ohio State University Columbus, Ohio 43210
West Antarctica's location with respect to East Antarctica after the Middle Jurassic breakup of the Gondwanaland supercontinent is important for an understanding of the opening history of the Weddell Sea. West Antarctica is composed of four major crustal blocks: the Antarctic Peninsula (AP), the Ellsworth-
14
References
Delisle, C., and K. Fromm. 1989. Further evidence for a Cretaceous therm event in north Victoria Land. Geologisches Jahrbuch, E38:143-151. Elliot, D. H., and K. A. Foland. 1986. Potassium-argon age determination, of the Kirkpatrick Basalt, Mesa Range. In E. Stump (Ed.), Geologica investigations in northern Victoria Land. (Antarctic Research Series. Washington, D.C.: American Geophysical Union. 279-288. Findlay, R. H., and H. Jordon. 1984. The volcanic rocks of Mt. Black Princ and Lawrence Peaks, north Victoria Land, Antarctica. Geologische Jahrbuch, B60:143-151. Fitzgerald, P.C. 1992. The Transantarctic Mountains in southern Victoria Land: The application of apatite fission track analysis to a rift shoulder uplift. Tectonics, 11:634-62. Fitzgerald, P. G., and A. J . W. Gleadow. 1988. Fission track geochronology, tectonics and structure of the Transantarctic Mountains in north em Victoria Land, Antarctica. Isotope Geoscience, 73:169-198. Fitzgerald, P. G., and E. Stump. 1991. Uplift history of the Transantarcti Mountains from Victoria Land (-700 S) to Scott Glacier (-.860 S) Evidence from fission track analysis. Sixth International Symposium o Antarctic Earth Sciences. 9-13 September 1991, Saitama, Japan. Ganovex Team. 1987. Geologic map of north Victoria Land, Antarctica 1:500,000. In F. Tessensohn and N. W. Roland (Eds.), German Antarctic North Victoria Land Expedition 1982183. Ganovex III, Vol. 2 Bundesanstalt für Geowissenschaften und Rohstoffe, Stuttgart. 7-79 Kreuzer, A., A. Hohndorf, L. Heinz, U. Vetter, F. Tessensohn, P. Muller H. Jordan, W. Harre, and C. Besang. 1981. K-Ar and Rb-Sr dating o igneous rocks from north Victoria Land, Antarctica. Geologisch Jahrbuch, B41:267-273. Roland, N. W., and F. Tessensobn. 1987. Rennick faulting an early phase o Ross Sea rifting. In F. Tessensohn and N. W. Roland (Eds.), Germa Antarctic North Victoria Land Expedition 1982183. Ganovex Ill, Vol. 2, Bundesanstalt für Geowissenschaften und Rohstoffe, Stuttgart, 275-302 Schmierer, K., and R. Burmester. 1986. Paleomagnetic results from th Cambro-Ordovician Bowers Supergroup, northern Victoria Land, Antarctica. In E. Stump (Ed.), Geological investigations in northern Victoria Land. (Antarctic Research Series.) Washington, D.C.: Amen can Geophysical Union. 69-90. Veevers,J.J. 1988. Seafloor magnetic lineation off the Otway/WestTasmania Basins: Ridge jumps and the subsidence history of the southwes Australian margins. Australian Journal of Earth Science, 35: 451-462.
Whitmore Mountains (EWM), the Thurston Island-Eights Coast (TI), and Marie Byrd Land (MBL) (Daiziel and Elliot 1982) (figur 1). This article describes new paleomagnetic data from th0 Antarctic Peninsula (combined with existing geologic and geo. physical data) that help to constrain the motion of the AP bloc$ relative to East Antarctica and hence increase understanding the opening history of the Weddell Sea. The paleomagnetic samples were obtained (along with sampl for uranium-lead geochronology by S. B. Mukasa and J.Tangeman from the University of Michigan and palynological samples collected by S. Fowell from Lamont-Doherty Geological Observatory) from the AP block during a 1-month cruise in May 1990, 2 weeks' camping on Byers Peninsula in December 1990, and a 1month cruise during January 1991. Most locations were visited by zodiak from the R/V Polar Duke, with the exception of remote field camps established on Byers Peninsula, Kopaitic Island, and Hope Bay (figure 1). Unusually deep snow cover prevented us from sampling the primary objectives at Byers Peninsula. The paleomagnetic samples were collected with a portable, gasoline-
4
ANTARCTIC JOURNAL
70°W
........... ..........
Antarctic Peninsula
South Shetland Islands
I I Sample Areas Byers Peninsula
0
Greenwich Is.
200km Is.. I I I Snow (President Head
ejtu,
C? Hope Bay Low IsJ' Kopaitic Is. i/ç de Islands Islands (Cape Wallace) Bone •.Danger Islands Camp Hill Joinville Is. Penola Strait
XCape S pring Charlotte Bay Andvord Bay Mt. Banck, Paradise Bay
(Cape Tuxen, Moot Pt, Rasmussen Is.) J Pitt Is $,
650
Watkins Is.[] C,
^ N
Antarctic EW1 Peninsula
EAST Weddell ANTARCTIC Sea CRATON
Lassiter Coast,
Avian Is. Stonington Is.
Horseshoe Is. Orville Coast East. Ellsworth U
Pine Is.
TI EW
MBL
Ross Ic e Ross Sea
f
NVL
70°S
o
1
Figure 1. Sample locations on the Antarctic Peninsula. Inset shows the west antarctic crustal blocks in their present-day positions with respect to east Antarctica. EWM, Ellsworth-Whitmore Mountains block; EM, Ellsworth Mountains; EW, Explora Wedge; MBL, Marie Byrd Land; NVL, North Victoria Land; TI, Thurston Island-Eights Coast block. Stippled lines show mountain (or basin) trends; slanted lines show location of ice shelves. 1992 REVIEW
15
Figure 2. Reconstruction for 170 to 155 million years ago. Between 170 and 155 million years ago the AP block rotated clockwise with respect to east Antarctica as a result of Initiation of seafloor spreading in the western Weddell Sea basin along the southern AP block margin. Inset: 175 million years ago reconstruction, before the start of Gondwanaland break-up. (See Grunow at at. 1991 for discussion of tectonic setting at 175 million years ago.) In reconstructions, the geographic south pole (+) is centered on the east Antarctic (eat) reference pole; the west antarctic paleopoles are located with asterisks. Latitude circles are in 150 increments. Abbreviations are as follows: AP, Antarctic Peninsula; ap, AP block A95 circle; BSB, Bryd subglacial basin; eat, east Antarctic A95; EW, Explora Wedge; EWM, Ellsworth-Whitmore Mountains; Fl, Faulkiand Islands; MBL, Marie Byrd Land; RVB, Rocas Verdes basin; TI, Thurston island-Eights Coast; ti, TI block A95 circle.) MBL is shown In a 100-million-years-ago position (Grindley and Oliver 1983) in the reconstructions.
powered, diamond-bit Pomeroy coring drill with a water/glycol mix as drill coolant. We collected 1,100 oriented drill cores and 20 oriented hand samples from 176 sites (six to seven cores per site) at 17 locations from the AP block. The samples were analyzed at the Lamont-Doherty Geological Observatory of Columbia University and the University of Oxford paleomagnetism laboratories. Pilot samples were pro16
gressively demagnetized with the use of step-wise alternating field (AF) or thermal techniques (TH) to determine the best demagnetization procedure for each site. Mean directions and paleopoles were determined using Fisher (1953) statistics. Samples collected from the Botany Bay Group sandstones and siltstones at Camp Hill (figure 1) proved to be weakly magnetized and unstable magnetically. Paleomagnetic cores were also colANTARCTIC JOURNAL
Figure 3. Between about 160 and 130 million years ago, opening began along the northern Antarctic Peninsula and Rocas Verdes basin of southern South America, effectively stopping opening in the southern AP block and causing counterclockwise rotation of the AP-TI blocks. Subduction of the older Jurassic Weddell seafloor (dashed spreading ridge) and transpressionai deformation occurred at the base of the AP block as the EWM block and east Gondwanaland moved southward. Inset: Reconstruction for 130 million years ago. At the start of South Atlantic basin opening (approximately 130 million years ago) the AP block was near its present-day position with respect to east Antarctica and opening stopped in the Rocas Verdes basin. Clockwise rotation of the TI-EWM block resulted in sinistral shearing between the EWM block and east Antarctica and dextral shearing between the TI-EWM block and the AP block (continuing upon Initiation of South Atlantic opening may have caused dextral shearing along the northernmost AP block). lected at Andvord Bay, Avian Island, Charlotte Bay, Horseshoe Island, the Lemaire Channel, the Meichoir Islands, the Pitt Islands, and the Watkins Island group (figure 1); but the rocks at these locations do not yet have radiometric age constraints. Samples of the structurally complex and poorly dated PermoTnassic Trinity Peninsula Group rocks were collected from Hope Bay, Paradise Bay, Cape Spring, and Kopaitic Island (figure 1).
1992 REVIEW
We obtained new uranium-lead ages from intrusions at Moot Point, Cape Tuxen, Bone Bay, Charlotte Bay, and Stonington Island, and a clast from a conglomerate on Horseshoe Island were obtained by Mukasa and Tangeman. A late Jurassic pole (about 155 million years ago) (130* E 67.5* A95=8.5*, N=llvirtual geomagnetic poles) was obtained from S, Oxfordian sedimentary rocks at Low Island (Thomson 1982) and
17
160-million-year-old granites at Bone Bay (R.J.Pankhurst, personal communication) (figure 1). The new data suggest that the AP block rotated clockwise between 170 and 155 million years ago because of early opening in the Weddell Sea Basin (figure 2). A new early Cretaceous (approximately 130 million years ago) paleomagnetic pole (178' E 76' S, A95= 7.2') (based on N = three locality mean poles from the Byers Peninsula mixed-marine member sedimentary rocks, the Mount Banck granite, and the Rasmussen Island granite—figure 1) indicates that the AP block was in or near its present-day position with respect to East Antarctica about 130 million years ago. Between 155 and 130 million years ago, counterclockwise rotation of the AP-TI blocks (Grunow et al. 1991) relative to East Antarctica may have caused subduction of Jurassic Weddell Sea ocean floor beneath the southern AP block and was probably the cause of the Palmer Land deformational event (figure 3). An approximately 130million-year-old pole from the TI block (Grunow et al. 1987) requires clockwise rotation of the TI and EWM blocks between 130 and about 110 million years ago, producing sinistral strikeslip motion between the EWM block and East Antarctica and dextral transpressional motion between the TI-EWM block and the southern Antarctic Peninsula (figure 3, inset). New paleopoles representing about 105 million years ago (from Moot Point and Vietor Rock on Byers Peninsula), about 85 million years ago (from Cape Tuxen), and ab'out 55 million years ago (from Snow Island), along with other equivalent age poles from the AP block (Wafts et al. 1984; Kellogg and Rowley 1989), are similar to the east antarctic reference poles and indicate that the AP block has been in its present-day position with respect to East Antarctica since about 105 million years ago. This work was supported by National Science Foundation grant DPP 90-22349.
18
Special thanks to Sarah Fowell, Ann Hawthorne, Octavio Urbin, and especially Mickey Van Fossen for their assistance in collectirig the paleomagnetic samples and to Sam Mukasa, Robert Pankhurc. and Jean Tangeman for the radiometric age constraints. The reconstructions were made with the help of Lisa Gehagan using tFe PLATES system at the University of Texas at Austin.
References Dalziel, I. W. D. and D. H. Elliot. 1982. West Antarctica: Problem child of Gondwanaland. Tectonics, 1:3-19. Fisher, R. A. 1953. Dispersion on a sphere. Proceedings of the Royal Society I of London, Ser. A, 217:295-305. Grindley, G. W. and P. J. Oliver. 1983. Paleomagnetism of Cretaceous volcanic rocks from Marie Byrd Land. In R. L. Oliver, P. R. James, and J. B. Jago (Eds.), Antarctic Earth Science. Canberra: Australian Academy of Science, 573-578. Grunow, A. M., D. V. Kent, and I. W. D. Daiziel. 1987. Mesozoic evolution of West Antarctica and the Weddell Sea basin: New paleomagnetic constraints. Earth and Planetary Science Letters, 86:16-26. Grunow,A. M., D. V. Kent, and I. W. D. Dalziel. 1991. New pa1eomagneic data from Thurston Island and their implications for the tectonics of West Antarctica. Journal of Geophysical Research, 96:17,935-17,954. Kellogg, K., and P. D. Rowley. 1989. Structural geology and tectonics of the Orville Coast region, southern Antarctic Peninsula, Antarctica. United States Geological Survey Professional Paper, 1498. 25 pp. Pankhurst, R. J . , personal communication, July 1989. Thomson, M. R. A. 1982. Late Jurassic fossils from Low Island, South Shetland Islands. British Antarctic Survey Bulletin, 56:25-35. Watts, D. R., G. C. Watts, and A. M. Bramall. 1984. Cretaceous and Early Tertiary paleomagnetic results from the Antarctic Peninsula. Tectonics, 3:333-346.
ANTARCTIC JOURN
7
