Structural geology of the Lassiter Coast
Christoffel, D., and R. Falconer. 1972. Marine magnetic measurements in the southwest Pacific Ocean and the identification of new tectonic features. Antarctic Research Series, 19: 197-209. Daiziel, I. W. D., and D. H. Elliot. 1971. Evolution of the Scotia Arc. Nature, 233: 246-252. Gordon, A. L. 1967. Geostrophic transport through the Drake Passage. Science, 156: 1732-1734. Gordon, A. L. 1971. Oceanography of antarctic waters. Antarctic Research Series, 15: 169-204. Gordon, A. L. 1973. Physical oceanography. Antarctic Journal of the U.S., VIII(3): 61-69. Hayes, D. E., et al. 1973. Leg 28 deep-sea drilling in the southern ocean. Geotimes, 18(6) : 19-24. Hayes, D. E., and I. Ringis. 1973. Seafloor spreading in the Tasman Sea. Nature, 243(5407): 454-458. Kennett, J . P., et al. 1972. Australian-Antarctic continental drift, paleocirculation changes and Oligocene deep-sea erosion. Nature Physical Science, 239(91): 51-55. Weissel, J . K., and D. E. Hayes. 1972. Magnetic anomalies in the southwest Indian Ocean. Antarctic Research Series, 19: 165-196.
Structural geology of the Lassiter Coast KARL S. KELLOGG and PETER D. ROWLEY U.S. Geological Survey
Denver, Colorado 80225
A detailed structural analysis is being completed of the Lassiter Coast. This area at the base of the Antarctica Peninsula, bordering over 300 kilometers of the Weddell Sea, was mapped by the U.S. Geological Survey (usGs) during the 1969-1970, 19701971, and 1972-1973 field seasons. The Lassiter Coast is part of the Mesozoic and Tertiary Andean orogen; rocks of the Lassiter Coast (Williams and Rowley, 1971; Williams et al., 1972; Rowley, 1973) consist of marine slate and argillaceous sandstone of the Latady Formation, and overlying (unnamed) volcanics, mostly of Middle and Late Jurassic age (Erie G. Kauffman, personal communication, 1974). These rocks were folded and subsequently intruded by gabbroic to granitic stocks and batholiths that are middle Cretaceous, according to potassium-argon determinations (Mehnert et al., in press; A. H. Clark et al., personal communication, 1974). Plutonism was accompanied by metamorphism to grades as high as andalusite hornfels (Plummer, 1974). Plutonic rock and contact metamorphic features are more abundant toward the north; either plutons in the north were more voluminous or they intruded higher into the sedimentary rocks. Folds in the Latady Formation and overlying volcanics range from open to isoclinal, and the rocks display a well developed axial plane cleavage. The structural data demonstrate a 30° change in strike of the axial plans along the Lassiter Coast, 224
from about N.50°E. in the south to about N.U°E. in the north. This reflects the apparent orolinal bend of the Antarctic Peninsula. A continuation of this bend to the west agrees well with the westnorthwest structural fabric of eastern Ellsworth Land (Laudon, 1972). Many folds are asymmetric, and some are overturned; most axial planes dip northwest, indicating compression from the northwesi and yielding to the southeast. Most fold axes are horizontal or plunge gently northeast. A second period of folding, which caused disçlacement of the older folds, is due to forceful intrision of the plutons. Such folds are confined to distances of less than a kilometer from pluton contacts. This type of folding is much more common toward the northeast where plutonic rocks are abundant. Northwest compression, thought to be relatei to the Andean orogeny, also is reflected in the attitude of joints in both plutonic and sedimentary rods as well as in the attitude of the numerous and chemically varied dikes found all along the Lassiter Cast. Vertical joints define two major sets at right anghs to each other. The predominant set is oriented northwest, normal to the fold fabric, and is believed tc be extension jointing (e.g., formed normal to the nlinimum principal compressive stress). In many places these open joints contain fillings of epidote, iron oxides, and chlorite. The other major set of joints, oriented generally parallel to the northeast fold axes, mostly is barren of mineral fillings Numerous pegmatite and aplite dikes were emplaced into plutonic rock during late stages of plutonism. The orientations of these dikes nearly are random; this may reflect a nearly hydrostatic stress state in the hot, semiplastic plutonic rock. Syn- to post-plutonic rhyolitic to andesitic dikes, however, mostly are vertical and strike northwest parallel to the extension direction. Similarities in paleomagnetic (Kellogg and Reynolds, 1974), petrographic and chemical character between dikes and plutons, and limited potassium-argon determinations (A. H. Clark et al., personal communication, 193) indicate that the age of most dikes is mi die Cretaceous. The only known faults observed in the Lass ter Coast are high angle, northwest-striking shears of relatively minor, apparent dip-slip displacement. T ey are within plutons in the immediate vicinity o a small porphyry-type copper deposit in the central Lassiter Coast (Rowley et al., in preparation). These faults are associated with mid-Cretaceous plutonism and are interpreted to represent slippage, perhps along the predominant northwest joint planes, dtring late stages of magma emplacement. The overall picture of Lassiter Coast structure, therefore, is one of compression from the northwest ANTARCTIC JOURNAL
for 4n extended period of time, beginning with a singlemajor period of folding sometime after deposition lof Middle and Upper Jurassic sedimentary and volc.nic rocks and continuing until after middle Creticeous plutons were emplaced. Tliis study was supported by National Science Fourdation grant AG-187.
References Kellogg, K. S., and R. L. Reynolds. 1974. Paleomagnetic study of igneous rocks of the northern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(2) 38-40. Lau on, T. S. 1972. Stratigraphy of eastern Ellsworth Land. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, U iversitetsforlaget, Series B(1) 215-223.
Plutonic rocks of the Lassiter Coast PETER D. ROWLEY and PAUL L. WILLIAMS
U.S. Geological Survey Denver, Colorado 80225
Plutonic rocks underlie over a third of the 30,000square-kilometer Lassiter Coast area and were mapped by the U.S. Geological Survey (USGS) during the 1969-1970, 1970-1971, and 1972-1973 field seasons. Evaluation of petrographic, chemical, and field data indicates that these rocks belong to a single caic-alkaline igneous complex that is correlated with the Andean intrusive suite exposed elsewhere along the Antarctic Peninsula. The Lassiter Coast igneous complex intrudes tightly folded shale, siltstone, and sandstone of Middle and Late Jurassic age, as well as overlying ash flow tuff and andesitic lava flows of presumed Late Jurassic age (Williams et al., 1972). Some plutons reflect multiple intrusive events, and crosscutting relations prove that emplacement occurred at different times. Potassium-argon age determinations on biotite and hornblende from seven plutons in the southern and central Lassiter Coast range from 95 to 119 million years (A. H. Clark et al., personal communication; Mehnert et al., in press). The more felsic bodies generally are younger. Emplacement of each pluton was followed by intrusion of aplite and pegmatite and, in turn, by intrusion of mostly intermediate to mafic dikes. Hydrothermal alteration and copper mineralization occurred near the conclusion of igneous activity in some plutons (Rowley et al., in preparation). There are more than 50 known stocks and batholiths in which plutonic rocks occur. Most of the September-October 1974
Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. In press. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research. Plummer, C. C. 1974. Contact metamorphism of the Latady Formation, southern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(3) : 82-83. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., VIII (4) : 154-155. Rowley, P. D., P. L. Williams, D. L. Schmidt, A. B. Ford, R. L. Reynolds, A. H. Clark, E. Farrar, and S. L. McBride. In preparation. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Williams, P. L., and P. D. Rowley. 1971. Geologic studies of the Lassiter Coast. Antarctic Journal of the U.S., VI(4): 120. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula: preliminary report: In: Antarctic Geology and Geophysics. (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget, Series B(1): 143-148.
intrusions are oval shaped in plan with long axes commonly oriented in a northerly direction. The plutons range in size from less than 1 kilometer to a nearly continuous 20 kilometers wide strip extending from the central Lassiter Coast north about 150 kilometers into the Black Coast. In the southern third of the area most bodies are stocks with lengths
10
35
65
Ploqioclase K-feldspar (Including pert hite)
Figure 1. Plutonic rock classification triangle with long axes of fields of modal quartz and feldspar for those plutons in the Lassiter Coast defined by five or more modal analyses. Plots of rocks from associated aplite, pegmatite, or other dikes are not shown. This figure represents 221 modal analyses. Solid lines are plutons of northern Lassiter Coast (generally north of Swann Glacier); dashed lines are plutons of central Lassiter Coast; dotted lines are plutons of southern Lassiter Coast (south of Wetmore Glacier).
225
Structural geology of the Lassiter Coast KARL S. KELLOGG and PETER D. ROWLEY U.S. Geological Survey
Denver, Colorado 80225
A detailed structural analysis is being completed of the Lassiter Coast. This area at the base of the Antarctica Peninsula, bordering over 300 kilometers of the Weddell Sea, was mapped by the U.S. Geological Survey (usGs) during the 1969-1970, 19701971, and 1972-1973 field seasons. The Lassiter Coast is part of the Mesozoic and Tertiary Andean orogen; rocks of the Lassiter Coast (Williams and Rowley, 1971; Williams et al., 1972; Rowley, 1973) consist of marine slate and argillaceous sandstone of the Latady Formation, and overlying (unnamed) volcanics, mostly of Middle and Late Jurassic age (Erie G. Kauffman, personal communication, 1974). These rocks were folded and subsequently intruded by gabbroic to granitic stocks and batholiths that are middle Cretaceous, according to potassium-argon determinations (Mehnert et al., in press; A. H. Clark et al., personal communication, 1974). Plutonism was accompanied by metamorphism to grades as high as andalusite hornfels (Plummer, 1974). Plutonic rock and contact metamorphic features are more abundant toward the north; either plutons in the north were more voluminous or they intruded higher into the sedimentary rocks. Folds in the Latady Formation and overlying volcanics range from open to isoclinal, and the rocks display a well developed axial plane cleavage. The structural data demonstrate a 30° change in strike of the axial plans along the Lassiter Coast, 224
from about N.50°E. in the south to about N.U°E. in the north. This reflects the apparent orolinal bend of the Antarctic Peninsula. A continuation of this bend to the west agrees well with the westnorthwest structural fabric of eastern Ellsworth Land (Laudon, 1972). Many folds are asymmetric, and some are overturned; most axial planes dip northwest, indicating compression from the northwesi and yielding to the southeast. Most fold axes are horizontal or plunge gently northeast. A second period of folding, which caused disçlacement of the older folds, is due to forceful intrision of the plutons. Such folds are confined to distances of less than a kilometer from pluton contacts. This type of folding is much more common toward the northeast where plutonic rocks are abundant. Northwest compression, thought to be relatei to the Andean orogeny, also is reflected in the attitude of joints in both plutonic and sedimentary rods as well as in the attitude of the numerous and chemically varied dikes found all along the Lassiter Cast. Vertical joints define two major sets at right anghs to each other. The predominant set is oriented northwest, normal to the fold fabric, and is believed tc be extension jointing (e.g., formed normal to the nlinimum principal compressive stress). In many places these open joints contain fillings of epidote, iron oxides, and chlorite. The other major set of joints, oriented generally parallel to the northeast fold axes, mostly is barren of mineral fillings Numerous pegmatite and aplite dikes were emplaced into plutonic rock during late stages of plutonism. The orientations of these dikes nearly are random; this may reflect a nearly hydrostatic stress state in the hot, semiplastic plutonic rock. Syn- to post-plutonic rhyolitic to andesitic dikes, however, mostly are vertical and strike northwest parallel to the extension direction. Similarities in paleomagnetic (Kellogg and Reynolds, 1974), petrographic and chemical character between dikes and plutons, and limited potassium-argon determinations (A. H. Clark et al., personal communication, 193) indicate that the age of most dikes is mi die Cretaceous. The only known faults observed in the Lass ter Coast are high angle, northwest-striking shears of relatively minor, apparent dip-slip displacement. T ey are within plutons in the immediate vicinity o a small porphyry-type copper deposit in the central Lassiter Coast (Rowley et al., in preparation). These faults are associated with mid-Cretaceous plutonism and are interpreted to represent slippage, perhps along the predominant northwest joint planes, dtring late stages of magma emplacement. The overall picture of Lassiter Coast structure, therefore, is one of compression from the northwest ANTARCTIC JOURNAL
for 4n extended period of time, beginning with a singlemajor period of folding sometime after deposition lof Middle and Upper Jurassic sedimentary and volc.nic rocks and continuing until after middle Creticeous plutons were emplaced. Tliis study was supported by National Science Fourdation grant AG-187.
References Kellogg, K. S., and R. L. Reynolds. 1974. Paleomagnetic study of igneous rocks of the northern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(2) 38-40. Lau on, T. S. 1972. Stratigraphy of eastern Ellsworth Land. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, U iversitetsforlaget, Series B(1) 215-223.
Plutonic rocks of the Lassiter Coast PETER D. ROWLEY and PAUL L. WILLIAMS
U.S. Geological Survey Denver, Colorado 80225
Plutonic rocks underlie over a third of the 30,000square-kilometer Lassiter Coast area and were mapped by the U.S. Geological Survey (USGS) during the 1969-1970, 1970-1971, and 1972-1973 field seasons. Evaluation of petrographic, chemical, and field data indicates that these rocks belong to a single caic-alkaline igneous complex that is correlated with the Andean intrusive suite exposed elsewhere along the Antarctic Peninsula. The Lassiter Coast igneous complex intrudes tightly folded shale, siltstone, and sandstone of Middle and Late Jurassic age, as well as overlying ash flow tuff and andesitic lava flows of presumed Late Jurassic age (Williams et al., 1972). Some plutons reflect multiple intrusive events, and crosscutting relations prove that emplacement occurred at different times. Potassium-argon age determinations on biotite and hornblende from seven plutons in the southern and central Lassiter Coast range from 95 to 119 million years (A. H. Clark et al., personal communication; Mehnert et al., in press). The more felsic bodies generally are younger. Emplacement of each pluton was followed by intrusion of aplite and pegmatite and, in turn, by intrusion of mostly intermediate to mafic dikes. Hydrothermal alteration and copper mineralization occurred near the conclusion of igneous activity in some plutons (Rowley et al., in preparation). There are more than 50 known stocks and batholiths in which plutonic rocks occur. Most of the September-October 1974
Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. In press. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research. Plummer, C. C. 1974. Contact metamorphism of the Latady Formation, southern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(3) : 82-83. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., VIII (4) : 154-155. Rowley, P. D., P. L. Williams, D. L. Schmidt, A. B. Ford, R. L. Reynolds, A. H. Clark, E. Farrar, and S. L. McBride. In preparation. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Williams, P. L., and P. D. Rowley. 1971. Geologic studies of the Lassiter Coast. Antarctic Journal of the U.S., VI(4): 120. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula: preliminary report: In: Antarctic Geology and Geophysics. (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget, Series B(1): 143-148.
intrusions are oval shaped in plan with long axes commonly oriented in a northerly direction. The plutons range in size from less than 1 kilometer to a nearly continuous 20 kilometers wide strip extending from the central Lassiter Coast north about 150 kilometers into the Black Coast. In the southern third of the area most bodies are stocks with lengths
10
35
65
Ploqioclase K-feldspar (Including pert hite)
Figure 1. Plutonic rock classification triangle with long axes of fields of modal quartz and feldspar for those plutons in the Lassiter Coast defined by five or more modal analyses. Plots of rocks from associated aplite, pegmatite, or other dikes are not shown. This figure represents 221 modal analyses. Solid lines are plutons of northern Lassiter Coast (generally north of Swann Glacier); dashed lines are plutons of central Lassiter Coast; dotted lines are plutons of southern Lassiter Coast (south of Wetmore Glacier).
225
