New research could improve understanding of how today’s tremors release energy
By Nikk Ogasa
JUNE 7, 2022
These rocks at the Devil’s Punchbowl geologic formation near Los Angeles were uplifted by movement along the Punchbowl Fault, a now-inactive portion of the larger San Andreas Fault. NYARTSNWORDS/WIKIMEDIA COMMONS
Earthquakes have rocked the planet for eons. Studying the quakes of old could help scientists better understand modern tremors, but tools to do such work are scarce.
Enter zircons. Researchers used the gemstones to home in on the temperatures reached within a fault during earthquakes millions of years ago. The method offers insights into the intensity of long-ago quakes, and could improve understanding of how today’s tremors release energy, the researchers report in the April Geochemistry, Geophysics, Geosystems.
“The more we understand about the past, the more we can understand what might happen in the future,” says Emma Armstrong, a thermochronologist at Utah State University in Logan.
Armstrong and colleagues focused on California’s Punchbowl Fault. That now-quiet portion of the larger San Andreas Fault was probably active between 1 million to 10 million years ago, Armstrong says.
Heat from friction is generated in a fault when it slips and triggers an earthquake. Previous analyses of preserved organic material suggested that temperatures within the Punchbowl Fault peaked between 465° Celsius and 1065° C. The researchers suspected that zircons in rocks from the fault could narrow that broad window.
Zircons often contain the radioactive chemical elements uranium and thorium, which decay to helium at a predictable rate (SN: 5/2/22). That helium then builds up in the crystals. But when a zircon is heated past a temperature threshold — the magnitude of which depends on the zircon’s composition — the accumulated helium escapes.
Measuring the amounts of the three elements in zircons from the fault suggests that the most intense earthquake generated temperatures lower than 800° C. That roughly halves the range previously reported. The finding provides clues to the amount of heat released by quakes, something difficult to measure for modern tremors because they often occur at great depths.
Armstrong plans to continue studying zircons, in the hopes of finding more ways to exploit them for details about ancient quakes.
Questions or comments on this article? E-mail us at feedback@sciencenews.org
See: https://www.sciencenews.org/article...tm_medium=email&utm_campaign=Latest_Headlines
A Multi-Proxy Approach Using Zircon (U-Th)/He Thermochronometry and Biomarker Thermal Maturity to Robustly Capture Earthquake Temperature Rise Along the Punchbowl Fault, California
E. M. Armstrong, A. K. Ault, K. K. Bradbury, H. M. Savage, P. J. Polissar, S. N. Thomson
Earthquakes produce heat along faults from friction created as blocks of rock slide past each other. Identifying evidence of and quantifying these temperatures can pinpoint past earthquakes along faults and help us understand the physics of earthquakes. It is difficult to measure temperatures of modern earthquakes because the fault may be hot for less than a minute far below Earth's surface. Here we compare data from two techniques, heat-induced chemical changes in organic materials (biomarkers) and geochemical changes in minerals (zircon (U-Th)/He analysis), which are sensitive to short-lived, high temperatures. We apply zircon (U-Th)/He analysis to samples within and away from the Punchbowl fault, a strike slip fault in southern California and an ancient strand of the San Andreas fault system, where prior biomarker data showed evidence of past earthquake temperature rise in material within the fault. Zircon (U-Th)/He results, along with numerical models, reveal that earthquake temperatures were likely less than 725–800°C within the fault, similar to previous temperature estimates. Data also suggest earthquake temperatures may have been variable in space and time. Our work illustrates the agreement between these two geochemical methods and that this dual-approach can quantify earthquake-generated heat in other fault zones worldwide.
We resampled a subset of PF sample sites of Savage and Polissar (2019) exposed at Devil's Punchbowl Natural Area (Figure 1). We compare existing biomarker data with newly acquired ZHe thermochronometry data from a site with a well-defined PSZ and a second location characterized by a broader zone of fault core gouge, as well as from the adjacent crystalline basement and Punchbowl Formation (Fm) protoliths. We also acquired apatite (U-Th)/He (AHe) and apatite fission-track (AFT) thermochronometry for comparison with our ZHe results. We use thermal history modeling of material outside the PF to constrain the background thermal history of material within the PF. A suite of numerical modeling approaches is then used to constrain maximum coseismic temperatures along the PF and we compare these results with prior work.
Fig. 1 - Simplified geologic map modified from California Geological Survey overlain on a digital elevation model showing the Punchbowl fault in the Devil's Punchbowl Natural Area and San Andreas fault, San Gabriel Mountains, CA. Biomarker (Savage & Polissar, 2019) and new thermochronometry (this study) site locations are shown.
Developing new geochemical approaches for documenting cryptic coseismic temperature rise and thus past earthquakes along exhumed faults is important because textural and mineralogical evidence for seismic slip, including the presence of pseudotachylyte, can be overprinted by subsequent deformation and/or fluid-rock interaction. Robustly quantifying this coseismic temperature rise requires intermethod comparison of paleothermometers with different kinetics. Here we leverage prior biomarker evidence for friction-generated heat along the Punchbowl fault and acquire new ZHe and complementary AHe and AFT thermochronometry data from the same sample locations. We infer that analyzed zircon grains are low accumulated radiation damage because of their limited visual metamictization, low to moderate eU, and likely Phanerozoic age. Zircon chemistry reveals that grains entrained within the PSZ and gouge are derived from both the crystalline basement and Punchbowl Fm adjacent to the PF.
Zircon (U-Th)/He data patterns suggest friction-generated heat from past seismic slip on the PF was insufficient to completely reset ZHe dates in the PSZ and gouge. It is permissible that some PSZ zircon grains may have experienced up to ∼75% He loss (or partial resetting) during coseismic temperature rise, but more data is required to evaluate this as well as rule out effects of U and Th zonation on ZHe dates. Thermal history forward models of the Punchbowl Fm bracket the background, long-term thermal history characterized by two burial events, including the development of the Punchbowl basin. Three different numerical modeling approaches, each with different inputs and assumptions, in conjunction with ZHe date-eU patterns, collectively suggest the temperature rise along the PF was <725–800°C for 90% (i.e., near complete) He loss. These peak temperatures are similar to the temperatures recorded by biomarkers, reflecting broad compatibility between the two systems.
See: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GC010291
Superb articles on the quantification of coseismic temperature rise which required an intermethod comparison of paleothermometers with different kinetics. Prior biomarker evidence was leveraged for friction-generated heat along the Devil's Punchbowl fault in California.
Hartmann352
By Nikk Ogasa
JUNE 7, 2022
These rocks at the Devil’s Punchbowl geologic formation near Los Angeles were uplifted by movement along the Punchbowl Fault, a now-inactive portion of the larger San Andreas Fault. NYARTSNWORDS/WIKIMEDIA COMMONS
Earthquakes have rocked the planet for eons. Studying the quakes of old could help scientists better understand modern tremors, but tools to do such work are scarce.
Enter zircons. Researchers used the gemstones to home in on the temperatures reached within a fault during earthquakes millions of years ago. The method offers insights into the intensity of long-ago quakes, and could improve understanding of how today’s tremors release energy, the researchers report in the April Geochemistry, Geophysics, Geosystems.
“The more we understand about the past, the more we can understand what might happen in the future,” says Emma Armstrong, a thermochronologist at Utah State University in Logan.
Armstrong and colleagues focused on California’s Punchbowl Fault. That now-quiet portion of the larger San Andreas Fault was probably active between 1 million to 10 million years ago, Armstrong says.
Heat from friction is generated in a fault when it slips and triggers an earthquake. Previous analyses of preserved organic material suggested that temperatures within the Punchbowl Fault peaked between 465° Celsius and 1065° C. The researchers suspected that zircons in rocks from the fault could narrow that broad window.
Zircons often contain the radioactive chemical elements uranium and thorium, which decay to helium at a predictable rate (SN: 5/2/22). That helium then builds up in the crystals. But when a zircon is heated past a temperature threshold — the magnitude of which depends on the zircon’s composition — the accumulated helium escapes.
Measuring the amounts of the three elements in zircons from the fault suggests that the most intense earthquake generated temperatures lower than 800° C. That roughly halves the range previously reported. The finding provides clues to the amount of heat released by quakes, something difficult to measure for modern tremors because they often occur at great depths.
Armstrong plans to continue studying zircons, in the hopes of finding more ways to exploit them for details about ancient quakes.
Questions or comments on this article? E-mail us at feedback@sciencenews.org
See: https://www.sciencenews.org/article...tm_medium=email&utm_campaign=Latest_Headlines
A Multi-Proxy Approach Using Zircon (U-Th)/He Thermochronometry and Biomarker Thermal Maturity to Robustly Capture Earthquake Temperature Rise Along the Punchbowl Fault, California
E. M. Armstrong, A. K. Ault, K. K. Bradbury, H. M. Savage, P. J. Polissar, S. N. Thomson
Earthquakes produce heat along faults from friction created as blocks of rock slide past each other. Identifying evidence of and quantifying these temperatures can pinpoint past earthquakes along faults and help us understand the physics of earthquakes. It is difficult to measure temperatures of modern earthquakes because the fault may be hot for less than a minute far below Earth's surface. Here we compare data from two techniques, heat-induced chemical changes in organic materials (biomarkers) and geochemical changes in minerals (zircon (U-Th)/He analysis), which are sensitive to short-lived, high temperatures. We apply zircon (U-Th)/He analysis to samples within and away from the Punchbowl fault, a strike slip fault in southern California and an ancient strand of the San Andreas fault system, where prior biomarker data showed evidence of past earthquake temperature rise in material within the fault. Zircon (U-Th)/He results, along with numerical models, reveal that earthquake temperatures were likely less than 725–800°C within the fault, similar to previous temperature estimates. Data also suggest earthquake temperatures may have been variable in space and time. Our work illustrates the agreement between these two geochemical methods and that this dual-approach can quantify earthquake-generated heat in other fault zones worldwide.
We resampled a subset of PF sample sites of Savage and Polissar (2019) exposed at Devil's Punchbowl Natural Area (Figure 1). We compare existing biomarker data with newly acquired ZHe thermochronometry data from a site with a well-defined PSZ and a second location characterized by a broader zone of fault core gouge, as well as from the adjacent crystalline basement and Punchbowl Formation (Fm) protoliths. We also acquired apatite (U-Th)/He (AHe) and apatite fission-track (AFT) thermochronometry for comparison with our ZHe results. We use thermal history modeling of material outside the PF to constrain the background thermal history of material within the PF. A suite of numerical modeling approaches is then used to constrain maximum coseismic temperatures along the PF and we compare these results with prior work.
Fig. 1 - Simplified geologic map modified from California Geological Survey overlain on a digital elevation model showing the Punchbowl fault in the Devil's Punchbowl Natural Area and San Andreas fault, San Gabriel Mountains, CA. Biomarker (Savage & Polissar, 2019) and new thermochronometry (this study) site locations are shown.
Developing new geochemical approaches for documenting cryptic coseismic temperature rise and thus past earthquakes along exhumed faults is important because textural and mineralogical evidence for seismic slip, including the presence of pseudotachylyte, can be overprinted by subsequent deformation and/or fluid-rock interaction. Robustly quantifying this coseismic temperature rise requires intermethod comparison of paleothermometers with different kinetics. Here we leverage prior biomarker evidence for friction-generated heat along the Punchbowl fault and acquire new ZHe and complementary AHe and AFT thermochronometry data from the same sample locations. We infer that analyzed zircon grains are low accumulated radiation damage because of their limited visual metamictization, low to moderate eU, and likely Phanerozoic age. Zircon chemistry reveals that grains entrained within the PSZ and gouge are derived from both the crystalline basement and Punchbowl Fm adjacent to the PF.
Zircon (U-Th)/He data patterns suggest friction-generated heat from past seismic slip on the PF was insufficient to completely reset ZHe dates in the PSZ and gouge. It is permissible that some PSZ zircon grains may have experienced up to ∼75% He loss (or partial resetting) during coseismic temperature rise, but more data is required to evaluate this as well as rule out effects of U and Th zonation on ZHe dates. Thermal history forward models of the Punchbowl Fm bracket the background, long-term thermal history characterized by two burial events, including the development of the Punchbowl basin. Three different numerical modeling approaches, each with different inputs and assumptions, in conjunction with ZHe date-eU patterns, collectively suggest the temperature rise along the PF was <725–800°C for 90% (i.e., near complete) He loss. These peak temperatures are similar to the temperatures recorded by biomarkers, reflecting broad compatibility between the two systems.
See: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GC010291
Superb articles on the quantification of coseismic temperature rise which required an intermethod comparison of paleothermometers with different kinetics. Prior biomarker evidence was leveraged for friction-generated heat along the Devil's Punchbowl fault in California.
Hartmann352
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