Jurassic fault and dike patterns in the Beardmore Glacier area, central ...
Jurassic fault and dike patterns in the Beardmore Glacier area, central Transantarctic Mountains TERRY
J. WILSON
Byrd Polar Research Center
and
Department of Geological Sciences Ohio State University Columbus, Ohio 43210
Extensive Jurassic magmatism preceded fragmentation of the Gondwana supercontinent (Cox 1978; Dalziel etal. 1987). Active rifting during magmatism is well documented in Africa and South America whereas, in Antarctica, the presence of a rift system has been inferred from the apparent linear distribution of intrusive and extrusive rocks of the Ferrar Supergroup (Elliot 1975; Kyle, Elliot, and Sutter 1981; Elliot 1991) and from geophysical data in West Antarctica (e.g., Jankowski, Drewry, and Behrendt, 1983; Cooper, Davey, and Hinz 1991; Garrett 1991). Structural mapping in the Beardmore Glacier area in the 19901991 season demonstrated the widespread occurrence of Jurassic normal faulting and established the trends of Jurassic Ferrar dolerite dikes in the region. The orientation patterns of Jurassic faults and dikes in the Beardmore area correspond closely with those mapped in the 1989-1990 season in southern Victoria Land (Wilson 1990). This demonstrates that a regionally consistent Jurassic extensional strain regime extended over approximately 1,000 kilometers strike length in the Transantarctic Mountains. The field party, including Peter Braddock, Barry Muller, and Terry Wilson, was deployed to the Beardmore South Camp on 19 November 1990. One month was spent traversing the area by skidoo, examining localities in the Queen Alexandra Range at Montgomery Glacier, Tillite Glacier, and around Prebble Glacier, in the Colbert Hills at Coalsack Bluff and Mount Sirius, and in the Painted Cliffs at Dawson Peak and Mount Picciotto. The following month consisted of field work supported by Helicopters New Zealand. Localities visited during this period extended from The Gateway area and the Moore Mountains in the north, to the Marshall Mountains, Buckley Island, and the Dominion Range in the south. The helicopter support made it possible to document the orientation patterns of faults and dolerite dikes along an approximately 225-kilometer transect across the Transantarctic Mountains and to visit many otherwise inaccessible exposures around the Beardmore Glacier. The party left Beardmore South Camp for McMurdo on 19 January 1991. Thin Ferrar dolerite dikes cutting Beacon strata are common in the Beardmore region but are widely scattered throughout the area and do not form closely spaced dike swarms. The dikes are typically 20-300 centimeters thick and have a minimum total strike continuity of tens or hundreds of meters. In detail, individual dikes consist of a series of en echelon segments. This geometric form is typical of dikes that form as tensile hydraulic fractures (Pollard, Segall, and Delaney 1982). There is considerable dispersion in the Ferrar dike trends in the Beardmore area, but two well-defined dike sets with subperpendicular north-northwest and east-northeast trends occur consistently throughout the region (figure 1). Abutting relations between 1991 REVIEW
north-northwest and east-northeast dikes, where the dikes join and merge at T-junctions without crosscutting (figure 2), document contemporaneous emplacement of these subperpendicular dike sets. Striated mesoscopic fault planes are also prevalent throughout the Beardmore region. Offset of Beacon strata demonstrates that nearly all faults have normal displacement. Most faults measured in the study have displacement magnitudes of a few meters or less, but some faults in the area have offsets ranging up to hundreds of meters (Barrett and Elliot 1973; Elliot, Barrett, and Mayewski 1974; Collinson and Elliot 1984). Other relatively large-displacement normal faults are inferred from the presence of regionally developed, northeast-trending monoclines (Barrett and Elliot 1973) that are interpreted as extensional drape folds formed above faults cutting the crystalline basement beneath the Beacon sequence. Mesoscopic normal faults in the Beardmore area have northwest and northeast trends, and are commonly developed in conjugate sets (figure 3). The data for the whole area indicate that two subperpendicular sets of conjugate normal faults are developed in the region, although the complete conjugate pattern is not apparent in all of the subarea plots (figure 3). The subperpendicular northwest and northeast trends are also evident in the large-displacement faults mapped on stratigraphic grounds (Barrett and Elliot 1973), demonstrating that the mesoscopic faults mirror the geometry of the larger, regional structures and, thus, can be used to infer the large-scale displacement patterns in the area. Relations between both the northwest and northeast fault sets and Ferrar dike planes indicates that the faults are of Jurassic age. The occurrence of offset strata across dikes, of striated planes along and adjacent to dike margins, and of striated fault planes extending beyond dike terminations indicate that faulting predated or was synchronous with dike emplacement. The presence of clastic material, remobilized by fluids driven by the thermal effects of dolerite intrusion, along segments of mesoscopic normal fault planes indicates that the faults formed prior to or during dike emplacement. Both volcanic breccias and dikes occupy portions of faults associated with the regional monoclines, indicating that the monoclines are of Jurassic age. Normal faults cutting dolerite and Kirkpatrick Basalt are common, indicating that some faulting continued after magmatic activity ceased. The Jurassic structural trends in the Beardmore Glacier area are nearly identical to the Jurassic trends documented previously in southern Victoria Land (Wilson 1990), demonstrating a regionally consistent pattern of extensional strain during Ferrar magmatism. The subperpendicular trends of the contemporaneous Jurassic faults and dikes in both areas indicate that they formed in a triaxial strain field in which extension occurred in both northeast-southwest and northwest-southeast horizontal directions, and shortening occurred in the vertical direction. This documents Jurassic extension both perpendicular and parallel to the present trend of the Transantarctic Mountains. The principal extension direction is not directly resolved from the mesoscopic fault and dike data. The northwest orientation of the mountains and the offshore basins of the Ross embayment suggests that this structural trend may have originated during Jurassic rifting in response to dominant northeast-southwest extensional strain. If, however, the northeast-trending monoclines near Beardmore Glacier mark the major fault trend in that area, the dominant extension direction would have been northwest-southeast, perpendicular to the regional trend. This may indicate that the northeast-trending Jurassic rift structures formed on en echelon array along the 9
T(
°E
170°E I
I
PG
2sigma
P100
85° S
l' IZY
U. I. -
z sigma
50 km
Figure 1. Orientation patterns of Ferrar dolerite dike planes in the Beardmore Glacier area of the central Transantarctic Mountains (location shown by box on inset map). Lower hemisphere equal area projections show poles to Ferrar dike planes contoured by the Kamb method and great circles indicating average dike orientations. On the geologic sketch map of the study area, black denotes exposed basement rocks beneath the Kukri unconformity surface and stipple denotes Beacon and Ferrar Supergroup rocks. The boxes indicate subareas where data for each stereoplot were collected. (TG denotes Tillite Glacier. DP denotes Dawson Peak and Mount Picciotto. MS denotes Mount Sirius. MB denotes the Marshall Mountains and Buckley Island. DR denotes the Dominion Range. PG denotes Prebble Glacier km denotes kilometer.)
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ANTARCTIC JOURNAL
,, \
Figure 2. North-northwest trending dike abutting and joining eastnortheast trending dike without crosscutting it (arrow) at Barnes Peak, indicating that the subperpendicular dikes formed contemporaneously. The dikes are approximately 2 meters thick.
length of the northwest-trending Ferrar magmatic belt or, alternatively, that the northwest trend of the Ferrar belt is the product of preferential exposure along the structural trend associated with the Cenozoic uplift of the present Transantarctic Mountains. This research was supported by National Science Foundation grant DPP 88-16932.
References Barrett, P.J., and D.H. Elliot. 1973. Reconnaissance geologic map of the
Buckley Island Quadrangle, Transantarctic Mountains, Antarctica. (U.S.
Antarctic Research Program Map, Map A-3, U.S. Geology Survey.) Washington, D.C.: U.S. Government Printing Office.
1991 REVIEW
Collinson, J.W., and D.H. Elliot. 1984. Geology of Coalsack Bluff, Antarctica. In M.D. Turner and J.F. Splettstoesser (Eds.), Geology of the central Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Cooper, A.K., F.J. Davey, and K. Hinz. 1991. Crustal extension and origin of sedimentary basins beneath the Ross Sea and Ross Ice Shelf, Antarctica. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press. Cox, K.G. 1978. Flood basalts, subduction and the breakup of Gondwanaland. Nature, 274, 47-49. Dalziel, I.W.D., B.C. Storey, S.W. Garrett, A.M. Grunow, L.D.B. Herrod, and R.J. Pankhurst. 1987. Extensional tectonics and the fragmentation of Gondwanaland. In ME. Coward, J.E Dewey, and EL. Hancock (Eds.), Continental extensional tectonics. (Special publication.) London: Geological Society. Elliot, D.H. 1975. Tectonics of Antarctica: A review. American Journal of Science, 275-A, 46-106. Elliot, D.H. 1991. Triassic-Early Cretaceous evolution of Antarctica. In M.R.A., Thomson, J.A. Crame, and J.W. Thomson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press. Elliot, D.H., P.J. Barrett, and PA. Mayewski. 1974. Reconnaissance geo-
logic map of the Plunlet Point Quadrangle, Transantarctic Mountains, Antarctica. (U.S. Antarctic Research Program Map, Map A-4, U.S. Geological Survey.) Washington, D.C.: Government Printing Office. Garrett, S.W. 1991. Aeromagnetic studies of crustal blocks and basins in West Antarctica: A review. In M.R.A., Thomson, J.A. Crame, and J.W. Thomson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press. Jankowski, E.J., D.J. Drewry, and J.C. Behrendt. 1983. Magnetic studies of upper crustal structure in West Antarctica and the boundary with East Antarctica. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Canberra: Australian Academy of Science. Kyle, P.R., D.H. Elliot, and J.F. Sutter. 1981. Jurassic Ferrar Supergroup tholeiites from the Transantarctic Mountains, Antarctica, and their relationship to the initial fragmentation of Gondwana. In M.M. Cresswell and P Vella (Eds.), Gondwana Five. Rotterdam: Balkema. Pollard, D.D., P Segall, and P.T. Delaney. 1982. Formation and interpretation of dilatant echelon cracks. Geological Society of America Bulletin, 93, 1291-1303. Wilson, T.J. 1990. Mesozoic and Cenozoic structural patterns in the Transantarctic Mountains, southern Victoria Land. Antarctic Journal of the U.S., 25(5) 31-34.
11
T(
I. .2sigma
165°E
84°S- -
C.I. 2sigrna
N=38
I
N=22 G. 1. 2 sigma
C. - 2 sigma
50 km
Figure 3. Orientation patterns of Jurassic mesoscopic fault planes in the Beardmore Glacier area of the central Transantarctic Mountains (location shown by box on inset map). Lower hemisphere equal area projections show poles to fault planes contoured by the Kamb method and great circles indicating average fault orientations. Other symbols are defined in the caption for figure 1.
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ANTARCTIC JOURNAL
J. WILSON
Byrd Polar Research Center
and
Department of Geological Sciences Ohio State University Columbus, Ohio 43210
Extensive Jurassic magmatism preceded fragmentation of the Gondwana supercontinent (Cox 1978; Dalziel etal. 1987). Active rifting during magmatism is well documented in Africa and South America whereas, in Antarctica, the presence of a rift system has been inferred from the apparent linear distribution of intrusive and extrusive rocks of the Ferrar Supergroup (Elliot 1975; Kyle, Elliot, and Sutter 1981; Elliot 1991) and from geophysical data in West Antarctica (e.g., Jankowski, Drewry, and Behrendt, 1983; Cooper, Davey, and Hinz 1991; Garrett 1991). Structural mapping in the Beardmore Glacier area in the 19901991 season demonstrated the widespread occurrence of Jurassic normal faulting and established the trends of Jurassic Ferrar dolerite dikes in the region. The orientation patterns of Jurassic faults and dikes in the Beardmore area correspond closely with those mapped in the 1989-1990 season in southern Victoria Land (Wilson 1990). This demonstrates that a regionally consistent Jurassic extensional strain regime extended over approximately 1,000 kilometers strike length in the Transantarctic Mountains. The field party, including Peter Braddock, Barry Muller, and Terry Wilson, was deployed to the Beardmore South Camp on 19 November 1990. One month was spent traversing the area by skidoo, examining localities in the Queen Alexandra Range at Montgomery Glacier, Tillite Glacier, and around Prebble Glacier, in the Colbert Hills at Coalsack Bluff and Mount Sirius, and in the Painted Cliffs at Dawson Peak and Mount Picciotto. The following month consisted of field work supported by Helicopters New Zealand. Localities visited during this period extended from The Gateway area and the Moore Mountains in the north, to the Marshall Mountains, Buckley Island, and the Dominion Range in the south. The helicopter support made it possible to document the orientation patterns of faults and dolerite dikes along an approximately 225-kilometer transect across the Transantarctic Mountains and to visit many otherwise inaccessible exposures around the Beardmore Glacier. The party left Beardmore South Camp for McMurdo on 19 January 1991. Thin Ferrar dolerite dikes cutting Beacon strata are common in the Beardmore region but are widely scattered throughout the area and do not form closely spaced dike swarms. The dikes are typically 20-300 centimeters thick and have a minimum total strike continuity of tens or hundreds of meters. In detail, individual dikes consist of a series of en echelon segments. This geometric form is typical of dikes that form as tensile hydraulic fractures (Pollard, Segall, and Delaney 1982). There is considerable dispersion in the Ferrar dike trends in the Beardmore area, but two well-defined dike sets with subperpendicular north-northwest and east-northeast trends occur consistently throughout the region (figure 1). Abutting relations between 1991 REVIEW
north-northwest and east-northeast dikes, where the dikes join and merge at T-junctions without crosscutting (figure 2), document contemporaneous emplacement of these subperpendicular dike sets. Striated mesoscopic fault planes are also prevalent throughout the Beardmore region. Offset of Beacon strata demonstrates that nearly all faults have normal displacement. Most faults measured in the study have displacement magnitudes of a few meters or less, but some faults in the area have offsets ranging up to hundreds of meters (Barrett and Elliot 1973; Elliot, Barrett, and Mayewski 1974; Collinson and Elliot 1984). Other relatively large-displacement normal faults are inferred from the presence of regionally developed, northeast-trending monoclines (Barrett and Elliot 1973) that are interpreted as extensional drape folds formed above faults cutting the crystalline basement beneath the Beacon sequence. Mesoscopic normal faults in the Beardmore area have northwest and northeast trends, and are commonly developed in conjugate sets (figure 3). The data for the whole area indicate that two subperpendicular sets of conjugate normal faults are developed in the region, although the complete conjugate pattern is not apparent in all of the subarea plots (figure 3). The subperpendicular northwest and northeast trends are also evident in the large-displacement faults mapped on stratigraphic grounds (Barrett and Elliot 1973), demonstrating that the mesoscopic faults mirror the geometry of the larger, regional structures and, thus, can be used to infer the large-scale displacement patterns in the area. Relations between both the northwest and northeast fault sets and Ferrar dike planes indicates that the faults are of Jurassic age. The occurrence of offset strata across dikes, of striated planes along and adjacent to dike margins, and of striated fault planes extending beyond dike terminations indicate that faulting predated or was synchronous with dike emplacement. The presence of clastic material, remobilized by fluids driven by the thermal effects of dolerite intrusion, along segments of mesoscopic normal fault planes indicates that the faults formed prior to or during dike emplacement. Both volcanic breccias and dikes occupy portions of faults associated with the regional monoclines, indicating that the monoclines are of Jurassic age. Normal faults cutting dolerite and Kirkpatrick Basalt are common, indicating that some faulting continued after magmatic activity ceased. The Jurassic structural trends in the Beardmore Glacier area are nearly identical to the Jurassic trends documented previously in southern Victoria Land (Wilson 1990), demonstrating a regionally consistent pattern of extensional strain during Ferrar magmatism. The subperpendicular trends of the contemporaneous Jurassic faults and dikes in both areas indicate that they formed in a triaxial strain field in which extension occurred in both northeast-southwest and northwest-southeast horizontal directions, and shortening occurred in the vertical direction. This documents Jurassic extension both perpendicular and parallel to the present trend of the Transantarctic Mountains. The principal extension direction is not directly resolved from the mesoscopic fault and dike data. The northwest orientation of the mountains and the offshore basins of the Ross embayment suggests that this structural trend may have originated during Jurassic rifting in response to dominant northeast-southwest extensional strain. If, however, the northeast-trending monoclines near Beardmore Glacier mark the major fault trend in that area, the dominant extension direction would have been northwest-southeast, perpendicular to the regional trend. This may indicate that the northeast-trending Jurassic rift structures formed on en echelon array along the 9
T(
°E
170°E I
I
PG
2sigma
P100
85° S
l' IZY
U. I. -
z sigma
50 km
Figure 1. Orientation patterns of Ferrar dolerite dike planes in the Beardmore Glacier area of the central Transantarctic Mountains (location shown by box on inset map). Lower hemisphere equal area projections show poles to Ferrar dike planes contoured by the Kamb method and great circles indicating average dike orientations. On the geologic sketch map of the study area, black denotes exposed basement rocks beneath the Kukri unconformity surface and stipple denotes Beacon and Ferrar Supergroup rocks. The boxes indicate subareas where data for each stereoplot were collected. (TG denotes Tillite Glacier. DP denotes Dawson Peak and Mount Picciotto. MS denotes Mount Sirius. MB denotes the Marshall Mountains and Buckley Island. DR denotes the Dominion Range. PG denotes Prebble Glacier km denotes kilometer.)
10
ANTARCTIC JOURNAL
,, \
Figure 2. North-northwest trending dike abutting and joining eastnortheast trending dike without crosscutting it (arrow) at Barnes Peak, indicating that the subperpendicular dikes formed contemporaneously. The dikes are approximately 2 meters thick.
length of the northwest-trending Ferrar magmatic belt or, alternatively, that the northwest trend of the Ferrar belt is the product of preferential exposure along the structural trend associated with the Cenozoic uplift of the present Transantarctic Mountains. This research was supported by National Science Foundation grant DPP 88-16932.
References Barrett, P.J., and D.H. Elliot. 1973. Reconnaissance geologic map of the
Buckley Island Quadrangle, Transantarctic Mountains, Antarctica. (U.S.
Antarctic Research Program Map, Map A-3, U.S. Geology Survey.) Washington, D.C.: U.S. Government Printing Office.
1991 REVIEW
Collinson, J.W., and D.H. Elliot. 1984. Geology of Coalsack Bluff, Antarctica. In M.D. Turner and J.F. Splettstoesser (Eds.), Geology of the central Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Cooper, A.K., F.J. Davey, and K. Hinz. 1991. Crustal extension and origin of sedimentary basins beneath the Ross Sea and Ross Ice Shelf, Antarctica. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press. Cox, K.G. 1978. Flood basalts, subduction and the breakup of Gondwanaland. Nature, 274, 47-49. Dalziel, I.W.D., B.C. Storey, S.W. Garrett, A.M. Grunow, L.D.B. Herrod, and R.J. Pankhurst. 1987. Extensional tectonics and the fragmentation of Gondwanaland. In ME. Coward, J.E Dewey, and EL. Hancock (Eds.), Continental extensional tectonics. (Special publication.) London: Geological Society. Elliot, D.H. 1975. Tectonics of Antarctica: A review. American Journal of Science, 275-A, 46-106. Elliot, D.H. 1991. Triassic-Early Cretaceous evolution of Antarctica. In M.R.A., Thomson, J.A. Crame, and J.W. Thomson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press. Elliot, D.H., P.J. Barrett, and PA. Mayewski. 1974. Reconnaissance geo-
logic map of the Plunlet Point Quadrangle, Transantarctic Mountains, Antarctica. (U.S. Antarctic Research Program Map, Map A-4, U.S. Geological Survey.) Washington, D.C.: Government Printing Office. Garrett, S.W. 1991. Aeromagnetic studies of crustal blocks and basins in West Antarctica: A review. In M.R.A., Thomson, J.A. Crame, and J.W. Thomson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press. Jankowski, E.J., D.J. Drewry, and J.C. Behrendt. 1983. Magnetic studies of upper crustal structure in West Antarctica and the boundary with East Antarctica. In R.L. Oliver, P.R. James, and J.B. Jago (Eds.), Antarctic earth science. Canberra: Australian Academy of Science. Kyle, P.R., D.H. Elliot, and J.F. Sutter. 1981. Jurassic Ferrar Supergroup tholeiites from the Transantarctic Mountains, Antarctica, and their relationship to the initial fragmentation of Gondwana. In M.M. Cresswell and P Vella (Eds.), Gondwana Five. Rotterdam: Balkema. Pollard, D.D., P Segall, and P.T. Delaney. 1982. Formation and interpretation of dilatant echelon cracks. Geological Society of America Bulletin, 93, 1291-1303. Wilson, T.J. 1990. Mesozoic and Cenozoic structural patterns in the Transantarctic Mountains, southern Victoria Land. Antarctic Journal of the U.S., 25(5) 31-34.
11
T(
I. .2sigma
165°E
84°S- -
C.I. 2sigrna
N=38
I
N=22 G. 1. 2 sigma
C. - 2 sigma
50 km
Figure 3. Orientation patterns of Jurassic mesoscopic fault planes in the Beardmore Glacier area of the central Transantarctic Mountains (location shown by box on inset map). Lower hemisphere equal area projections show poles to fault planes contoured by the Kamb method and great circles indicating average fault orientations. Other symbols are defined in the caption for figure 1.
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ANTARCTIC JOURNAL
