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The thermal history of the gambier embayment, Western Otway basin, Australia, from apatite fission track analysis
ABSTRACTS
Fractured granitic plutons of western Alaska, conduits for modern and paleo.hydrothermal systems: evidence from apatite fission track studies John M. Murphy (Geology Dept, La Trobe University, Bundoora, VIC 3083 Australia) Modem hot springs in western Alaska are associated with fractured Cretaceous granitic plutons or fault zones. These springs, with surface temperatures of up to 60oc, are mainly meteoric, with maximum predicted temperatures of -170°C at depths of -5Km. In these springs the top of a local partial annealing zone intersects the earths surface, whereas nearby the top of the 'normal' partial annealing zone is kilometers below the surface. Apatite fission track studies have identified areas were 'younger' ages occur at higher elevations than 'older' ages. Since no volcanic overlaps or intrusions occur at such occurrences, paleohydrothermal systems are considered to be responsible for some of the measured fission track age and track length distributions. The plutons of southeastern Seward Peninsula (SP) and western Yukon-Koyukuk plutonic belt have overlapping K-Ar (69-112 Ma) and apatite fission track ages (AFT; 54-105 Ma). Although the older AFT ages suggest fast initial cooling, lowered mean lengths of samples (M.L. 12-14p.) and their broad track length distributions (S.D. 1.82.4gt ) indicate a more complex thermal history. AFT ages of 5,1-58 Ma (M.L.-12g; S.D. >31.1.)may have incurred partial age mducfinn from local hydrothermal systems. Each of the eastern Yukon Koyukuk plutons has a different low-temperature thermal history, with K-Ar ages of 78-84 Ma and AFT ages of 36-85 Ma. AFT ages of 54-79 Ma in Wheeler Creek pluton (M.L.11.9-13.01.t ; S.D.-3R) contrast with ages of 36-42 Ma in the Zane Hills pluton (M.L.11.7-13.6~t ; S.D.~3tt). Classical fission track interpretations would place a young-side-up fault between them, but instead these plutons could have been differently effected by hydrothermal waters. A single AFT age of -55 Ma (M.L. 14.4; S.D. 1.5) from Wheeler Creek pluton shows complete annealing followed by rapid cooling at aboat that time. AFT ages of the Indian Mtn. pluton range from 58-85 Ma and cluster around 60 Ma. Their bimodal length distributions suggest that the younger ages resulted from partial reserdng and cooling after 60 Ma. Two periods of local heating by hydrothermal waters coincide with episodes of regional volcanism at <40 Ma and 56-60 Ma, however overall the volcanic rocks and associated dikes are not directly responsible for fission track age reductions. Daring and following periods of increased heat in the upper crust during volcanic episodes, circulating meteoric waters were 'piped' up conduits provided by fractured granitic plutons and faults. Many conduits continue to be episodically active.
The thermal history of the Gambier Embayment, Western Otway Basin, Australia, from apatite fission track analysis. Melinda M. Mitchell, Kevin C. Hill, and David A. Foster (VEIPS, Dept of Geology, LaTrobe University, Bundoora, Vic 3083 Australia). The Gambier Embayment is the most westerly section of the Otway Basin of southeastern Australia. The sub-basin is bounded on the east by the Merino Uplift and on the north by the Padthaway-Dundas Ridge. Apatite fission-track (AFT) analysis obtained from samples from the Mocumburm-1, Penola-1, Beachport-1 and Geltwood Beach-1 wells have significant implications for basin evolution. Mccumburra-1 is drilled on the structural high of the Merino Uplift in western Victoria and bottoms in Palaeozoic basement at -1430 m. The sample from 14.=30m gives an AFT age of 45+4 Ma and has a bimodal track length distribution (X 8.9 microns;S.D.2.5) with relic short tracks probably inherited from the original cooling of the basement source. The single grain ages from this sample vary between 0 and 130 Ma, in contrast to published AFT ages of 300-400 Ma from exposed basement to the north of the basin. The AFT results from this well suggest that the Merino Uplift has undergone a complex thermal history which is consistent with Early Cretaceous heating followed by Late Cretaceous and Tertiary age fault reactivation and cooling. The AFT results from the Penola-1 well record the dewitai ages of the apatite grains which are of mixed provenance with both rift contemporaneous volcanism (-100 Ma) and Palaeozoic basement (300-400 Ma) ages represented. Estimated geothermal gradients based on presem well temperatures, vitrinite reflectance and gradients in AFT parameters indicate that this well has been denuded by -1100 m and has cooled recently, probably in the mid-Tertiary. Scruples analyzed from Beachport-1 exhibit moderately annealed mean lengths, 13-14 microns, and AFT ages from 156-106 M a . A component of short tracks in the confined track distribution suggests that this well has been at higher temperatures (-100°C) in the past and then cooled relatively rapidly. The results suggest that the deepest sample (1120m) has experienced heating to temperatures significantly higher than at present. The deepest sample from the Geltwood Beach-1 well gives an age of 41+14 Ma (X 10.8 microns) which is substantially younger than its Lower Cretaceous stratigraphic age. From present day well temperatures in the area, we estimate that the well is presently at its maximum temperature. These results suggest that there has been major structural inversions on both •the Merino Uplift and the basement fault bounded sections of the Gambler Embayment. Data from three of the four wells indicates mid-Tertiary to recent cooling (denudation?).
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Fractured granitic plutons of western Alaska, conduits for modern and paleo.hydrothermal systems: evidence from apatite fission track studies John M. Murphy (Geology Dept, La Trobe University, Bundoora, VIC 3083 Australia) Modem hot springs in western Alaska are associated with fractured Cretaceous granitic plutons or fault zones. These springs, with surface temperatures of up to 60oc, are mainly meteoric, with maximum predicted temperatures of -170°C at depths of -5Km. In these springs the top of a local partial annealing zone intersects the earths surface, whereas nearby the top of the 'normal' partial annealing zone is kilometers below the surface. Apatite fission track studies have identified areas were 'younger' ages occur at higher elevations than 'older' ages. Since no volcanic overlaps or intrusions occur at such occurrences, paleohydrothermal systems are considered to be responsible for some of the measured fission track age and track length distributions. The plutons of southeastern Seward Peninsula (SP) and western Yukon-Koyukuk plutonic belt have overlapping K-Ar (69-112 Ma) and apatite fission track ages (AFT; 54-105 Ma). Although the older AFT ages suggest fast initial cooling, lowered mean lengths of samples (M.L. 12-14p.) and their broad track length distributions (S.D. 1.82.4gt ) indicate a more complex thermal history. AFT ages of 5,1-58 Ma (M.L.-12g; S.D. >31.1.)may have incurred partial age mducfinn from local hydrothermal systems. Each of the eastern Yukon Koyukuk plutons has a different low-temperature thermal history, with K-Ar ages of 78-84 Ma and AFT ages of 36-85 Ma. AFT ages of 54-79 Ma in Wheeler Creek pluton (M.L.11.9-13.01.t ; S.D.-3R) contrast with ages of 36-42 Ma in the Zane Hills pluton (M.L.11.7-13.6~t ; S.D.~3tt). Classical fission track interpretations would place a young-side-up fault between them, but instead these plutons could have been differently effected by hydrothermal waters. A single AFT age of -55 Ma (M.L. 14.4; S.D. 1.5) from Wheeler Creek pluton shows complete annealing followed by rapid cooling at aboat that time. AFT ages of the Indian Mtn. pluton range from 58-85 Ma and cluster around 60 Ma. Their bimodal length distributions suggest that the younger ages resulted from partial reserdng and cooling after 60 Ma. Two periods of local heating by hydrothermal waters coincide with episodes of regional volcanism at <40 Ma and 56-60 Ma, however overall the volcanic rocks and associated dikes are not directly responsible for fission track age reductions. Daring and following periods of increased heat in the upper crust during volcanic episodes, circulating meteoric waters were 'piped' up conduits provided by fractured granitic plutons and faults. Many conduits continue to be episodically active.
The thermal history of the Gambier Embayment, Western Otway Basin, Australia, from apatite fission track analysis. Melinda M. Mitchell, Kevin C. Hill, and David A. Foster (VEIPS, Dept of Geology, LaTrobe University, Bundoora, Vic 3083 Australia). The Gambier Embayment is the most westerly section of the Otway Basin of southeastern Australia. The sub-basin is bounded on the east by the Merino Uplift and on the north by the Padthaway-Dundas Ridge. Apatite fission-track (AFT) analysis obtained from samples from the Mocumburm-1, Penola-1, Beachport-1 and Geltwood Beach-1 wells have significant implications for basin evolution. Mccumburra-1 is drilled on the structural high of the Merino Uplift in western Victoria and bottoms in Palaeozoic basement at -1430 m. The sample from 14.=30m gives an AFT age of 45+4 Ma and has a bimodal track length distribution (X 8.9 microns;S.D.2.5) with relic short tracks probably inherited from the original cooling of the basement source. The single grain ages from this sample vary between 0 and 130 Ma, in contrast to published AFT ages of 300-400 Ma from exposed basement to the north of the basin. The AFT results from this well suggest that the Merino Uplift has undergone a complex thermal history which is consistent with Early Cretaceous heating followed by Late Cretaceous and Tertiary age fault reactivation and cooling. The AFT results from the Penola-1 well record the dewitai ages of the apatite grains which are of mixed provenance with both rift contemporaneous volcanism (-100 Ma) and Palaeozoic basement (300-400 Ma) ages represented. Estimated geothermal gradients based on presem well temperatures, vitrinite reflectance and gradients in AFT parameters indicate that this well has been denuded by -1100 m and has cooled recently, probably in the mid-Tertiary. Scruples analyzed from Beachport-1 exhibit moderately annealed mean lengths, 13-14 microns, and AFT ages from 156-106 M a . A component of short tracks in the confined track distribution suggests that this well has been at higher temperatures (-100°C) in the past and then cooled relatively rapidly. The results suggest that the deepest sample (1120m) has experienced heating to temperatures significantly higher than at present. The deepest sample from the Geltwood Beach-1 well gives an age of 41+14 Ma (X 10.8 microns) which is substantially younger than its Lower Cretaceous stratigraphic age. From present day well temperatures in the area, we estimate that the well is presently at its maximum temperature. These results suggest that there has been major structural inversions on both •the Merino Uplift and the basement fault bounded sections of the Gambler Embayment. Data from three of the four wells indicates mid-Tertiary to recent cooling (denudation?).
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