Abstract. Oceanic Isostasy and Intraplate Stress · Oceanic Isostasy and Intraplate Stress James A....
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Oceanic Isostasy and Intraplate Stress James A. Conder Department of Geology, Southern Illinois University Carbondale, Illinois, 62901 ρ ice ρ L ρ L ρ L Ice mass (A) ρ 1 ρ 2 ρ 3 ρ air Pratt (B) ρ 1 ρ 2 ρ 3 ρ w Pratt +ocean mass (C) 0 10 20 30 40 50 60 70 80 90 100 0 0.01 0.02 0.03 0.04 0.05 isostatic equilibrium t c Mass increase rate (km/m.y.) Seafloor age (m.y.) (A) 0 10 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6 Seafloor age (m.y.) Mass flux (km 3 /m.y./km) (B) Abstract. Intraplate deformation is, by definition, unexplained by plate tectonics. Because intraplate strain rates are relatively small, dominant intraplate stresses driving observed deformation can derive from a number of different, non-mutually exclusive, sources. Driving processes include, but are not necessarily limited to, gravitational potential energy variations, glacial isostatic adjustment, and tractions at the base of the lithosphere from flow in the underlying asthenosphere. Tractions at the base of the lithosphere have long been suggested to contribute to plate motions as well as intraplate stresses. Any stationary asthenospheric flow field will contribute to plate driving or resistance, depending on whether asthenosphere is leading or lagging the overlying plate. Stationary flows that are also spatially variable will induce tractions imparting differential stress on the overlying lithosphere. One overlooked driver of asthenosphere flow at the asthenosphere-lithosphere boundary that could have implications for understanding intraplate stresses in the oceans is oceanic isostasy. In a manner similar to an icecap on a continent, the addition of ocean mass on top of subsiding lithosphere drives a small degree of flow in the asthenosphere to accommodate the excess mass accumulated on top. Typically, oceanic lithosphere is understood to cool and subside away from mid ocean ridges to a Pratt-like isostasy condition. However, the presence of seawater added on top of subsiding lithosphere necessitates an additional isostatic response that cannot be achieved through densification of lithosphere alone. The basic mathematics behind the isostasy- driven asthenospheric flow demonstrates that the flow is systematically from beneath younger seafloor towards older seafloor. The asthenosphere flux is variable, but systematic across the plate. The flow rate peaks beneath seafloor of about one-quarter the plate age and decreases to zero at the plate extremities. The maximum flow rate is a few tenths of a percent of the plate rate and superimposed on the regional asthenospheric flow field. The modification to the regional flow induces a small, but systematically positive drag (mantle push) component on the overlying lithosphere. Potentially importantly, the form of the drag predisposes young lithosphere to extensional stress and older lithosphere to compressional stress. While difficult to tie any particular intraplate observation to this mechanism, a statistical evaluation of intraplate stress indicators is consistent with the predicted pattern. Isostatically Driven Asthenosphere Flow References Conder, J.A., Non-Pratt component of oceanic isostasy, Lithosphere, 4, doi:10.1130/L229.1, 2012 Heidbach, O., Tingay, M., Barth, A., Reinecker, J., Kurfeß, D., and Müller, B., 2010, Global crustal stress pattern based on the World Stress Map database release 2008: Tectonophysics, v. 482, no. 1–4, p. 3–15, doi:10.1016/j.tecto.2009.07.023 Müller, R., Sdrolias, M., Gaina, C., and Roest, W., 2008, Age, spreading rates, and spreading asymmetry of the world’s ocean crust: G-cubed, 9, 1–19, doi:10.1029/2007GC001743. Parsons, B., and Sclater, J.G., 1977, An analysis of the variation of ocean floor bathymetry and heat flow with age: Journal of Geophysical Research, 82, p.803–827. Since its demonstration as a primary process in shaping cean basins (e.g., Parsons and Sclater, 1977), Pratt isostasy for oceanic lithosphere is typically viewed as a static process. However, cooling and contraction in the presence of a fluid ocean with non-negligible mass necessitatates mass exchange bewteen lithospheric columns to acheive isostatic equilibrium (Conder, 2012). Because fluid immediately fills any gap created by subsidence, a contracting column will gain mass (Upper left figure). Because the rate of mass gain depends on the subsidence rate, the columns gain excess mass at different rates and thereby become out of equilibrium with their neighbors unless mass exchange takes place among the columns. For the oceans, this mass exchange must take place in the asthenosphere. The mass disequilibrium accumulation rate for subsiding seafloor is shown above in panel (A). As younger seafloor subsides more rapidly than older seafloor, young lithosphere gains excess more rapidly and must shunt equivalent asthenosphere mass beneath older seafloor. Seafloor of one-quarter the age of the oldest lithosphere is in balance with the average mass gain rate which I call the cross-over age (marked tc above). So, excess mass must be transferred from below seafloor younger than the cross-over age to beneath seafloor older than the cross-over age. This implies that the flow rate induced in the asthenosphere increases towards tc and decreases away from tc. Panel B shows the mass flux beneath seafloor with plate ages of 20, 40, 60, 80, & 100 Myr. As the plate ages (and grows), the cross-over age increases and the mass flux must also increase to accommodate the greater integrated mass excesses. The flow rate itself is rater slow - maybe 1% of the spreading rate, but has this systematic form shown above. Below the cross-over age, the flow rate increses towards older seafloor and decreases with age for seafloor older than the cross-over age. This form suggests that younger lithosphere may be biased towards extensional intraplate stress and older seafloor biased toward compressional stress. The asymmetry in both flow and lithospheric strength about the cross-over age also suggests that the effect is likely to be larger for younger seafloor. 5 10 15 20 30 210 60 240 90 270 120 300 150 330 180 0 Normal faulting −young seafloor σ 3 5 10 30 210 60 240 90 270 120 300 150 330 180 0 Normal faulting −old seafloor σ 3 5 10 15 30 210 60 240 90 270 120 300 150 330 180 0 Reverse faulting −young seafloor σ 1 5 10 15 30 210 60 240 90 270 120 300 150 330 180 0 Reverse faulting −old seafloor σ 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 20 40 60 80 Normal Intraplate Stress Observations (WSM) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 30 40 Reverse 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 20 40 60 Seafloor Normalized Age Strike−Slip 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.5 1 1.5 2 2.5 3 3.5 Seafloor Normalized Age Ratio NF/TF TF/NF −55 −50 −45 −40 −35 −30 −25 −8 −6 −4 −2 0 2 4 6 8 10 12 0 20 40 60 80 100 135 140 145 150 155 160 165 −75 −70 −65 −60 −55 −50 0 10 20 30 40 50 60 70 MAR SEIR Intraplate stress data. To look at possible signals of oceanic intraplate stresses arising from isostatic flow, data is utilized from the World Stress Map database (Heidbach et al., 2010). Data within 15 km of a plate boundary were removed along with data closely associated with diffuse boundaries (e.g., Indian Ocean), hotspots, microplates, or other identifiable tectonic influence. The remaining data comprises 110 extensional data, 106 compressional, and 149 strike slip data. The adjacent maps show the data examined here. Sigma1 directions are shown in blue and sigma3 directions are shown in red. Black arrows denote local spreading directions. Compressive vs Extensional Intraplate Stress. Isostatic flow drives asthenosphere from below young seafloor to beneath older seafloor. the maximum flow rate peaks at one-quarter of tIntraplate stress in the oceans is extremely variable. The histogram to the right shows counts of normal, reverse, and strike-slip data vs normalized age of the seafloor. Normalized age is the seafloor age divided by the maximum age in that particular spreading corridor (see intraplate stress data section). Most of the data of every type comes from young seafloor. However, the overall distribution differs with extensional data much more prevalent on young seafloor and compressional data having a larger presence on old seafloor. The graph at right shows the ratio of the normal and reverse histograms. A ratio of one and the cross over age (where a ratio of one would be predicted for isostatic flow stresses) are marked as dashed lines. The pattern is in strong agreement with the expected pattern induced from isostatic flow. The consistency of principle stress directions, sigma1 and sigma3, are more problematic. For semi-infinite plates, the flow direction will follow the spreading direction. So, sigma3 may be predisposed to follow the spreading direction at young ages, while sigma1 would track better at older ages. Rose diagrams of stress direction deviations from the local spreading direction are shown at right (alignment = 0). The most curious signal is for young sigma1 directions. The diagram shows a mostly random fan, decreasing towards zero, with a large pie wedge near zero. Such a pattern would be predicted from a random set of stress axes perturbed by a small additional extensional stress oriented in the spreading direction. Those subparallel to the alignment become more aligned while those poorly aligned are largely unaffected. The distribution for sigma1 for older seafloor is more random. Isostatic flow is likely to be less beholden to the spreading direction at older ages as the differential subsidence rates of adjacent seafloor are smaller than at younger ages. So, other perterbations may affect flow directions. However, such deviations would still impart compressional stress albeit in the altered orientations. −150 −100 −50 0 50 100 150 −80 −60 −40 −20 0 20 40 60 80 Intraplate Strike−Slip Faulting Seafloor spreading corridors and data ages. The age of seafloor associated with each datum is found using the seafloor age compilation of Muller et al. (2008). The age of oldest seafloor on the particular seafloor corridor is found by manual inspection (examples below; colors are age, circle is stress datum location, x the location of the oldest spot on the seafloor spreading corridor, and pointer denotes local direction of spreading). −150 −100 −50 0 50 100 150 −80 −60 −40 −20 0 20 40 60 80 Intraplate Reverse Faulting −150 −100 −50 0 50 100 150 −80 −60 −40 −20 0 20 40 60 80 Intraplate Normal Faulting 2D models of isostatic flow. Finite element models of flow induced by differential mass excess accumulation are shown above (courtesy of Andrew Dombard). The flow models show the clear peak in flow rate beneath seafloor of 1/4 plate age. The thickness of the asthenosphere is an important parameter for the magnitude of induced intraplate stresses. While the mass flux is constant for all cases, the absolute flow rate (and it's derivative) increase with decreasing asthenosphere thickness. Isostatic flow accommodated by a thick channel will have a smaller effect on plate stresses than a thin asthenosphere channel. 100 km asthenosphere 300 km asthenosphere A conceptual model for the interplay of isostatic flow in the context of the observations is shown at right. Blue lines denote lithosphere and green vectors show isostatic flow. Beneath young seafloor the flow is primarily directed in the spreading direction. Towards older lithosphere where the isostatic flow is less strong, random perturbations may significantly deviate the flow directions. The perturbations have the effect of randomizing the compressive stress directions, despite keeping the overall compressive nature of the flow
Transcript of Abstract. Oceanic Isostasy and Intraplate Stress · Oceanic Isostasy and Intraplate Stress James A....
Oceanic Isostasy and Intraplate StressJames A. Conder
Department of Geology, Southern Illinois UniversityCarbondale, Illinois, 62901
ρice
ρL
ρL
ρL
Icemass
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ρ1
ρ2
ρ3
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Pratt
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ρ1
ρ2
ρ3
ρw
Pratt+oceanmass
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Abstract. Intraplate deformation is, by definition, unexplained by plate tectonics. Because intraplate strain rates are relatively small, dominant intraplate stresses driving observed deformation can derive from a number of different, non-mutually exclusive, sources. Driving processes include, but are not necessarily limited to, gravitational potential energy variations, glacial isostatic adjustment, and tractions at the base of the lithosphere from flow in the underlying asthenosphere. Tractions at the base of the lithosphere have long been suggested to contribute to plate motions as well as intraplate stresses. Any stationary asthenospheric flow field will contribute to plate driving or resistance, depending on whether asthenosphere is leading or lagging the overlying plate. Stationary flows that are also spatially variable will induce tractions imparting differential stress on the overlying lithosphere. One overlooked driver of asthenosphere flow at the asthenosphere-lithosphere boundary that could have implications for understanding intraplate stresses in the oceans is oceanic isostasy.
In a manner similar to an icecap on a continent, the addition of ocean mass on top of subsiding lithosphere drives a small degree of flow in the asthenosphere to accommodate the excess mass accumulated on top. Typically, oceanic lithosphere is understood to cool and subside away from mid ocean ridges to a Pratt-like isostasy condition. However, the presence of seawater added on top of subsiding lithosphere necessitates an additional isostatic response that cannot be achieved through densification of lithosphere alone. The basic mathematics behind the isostasy-driven asthenospheric flow demonstrates that the flow is systematically from beneath younger seafloor towards older seafloor. The asthenosphere flux is variable, but systematic across the plate. The flow rate peaks beneath seafloor of about one-quarter the plate age and decreases to zero at the plate extremities. The maximum flow rate is a few tenths of a percent of the plate rate and superimposed on the regional asthenospheric flow field. The modification to the regional flow induces a small, but systematically positive drag (mantle push) component on the overlying lithosphere. Potentially importantly, the form of the drag predisposes young lithosphere to extensional stress and older lithosphere to compressional stress. While difficult to tie any particular intraplate observation to this mechanism, a statistical evaluation of intraplate stress indicators is consistent with the predicted pattern.
Isostatically Driven Asthenosphere Flow
ReferencesConder, J.A., Non-Pratt component of oceanic isostasy, Lithosphere, 4, doi:10.1130/L229.1, 2012
Heidbach, O., Tingay, M., Barth, A., Reinecker, J., Kurfeß, D., and Muller, B., 2010, Global crustal stress pattern based on the World Stress Mapdatabase release 2008: Tectonophysics, v. 482, no. 1–4, p. 3–15, doi:10.1016/j.tecto.2009.07.023
Muller, R., Sdrolias, M., Gaina, C., and Roest, W., 2008, Age, spreading rates, and spreading asymmetry of the world’s ocean crust: G-cubed, 9,1–19, doi:10.1029/2007GC001743.
Parsons, B., and Sclater, J.G., 1977, An analysis of the variation of ocean floor bathymetry and heat flow with age: Journal of GeophysicalResearch, 82, p.803–827.
Since its demonstration as a primary process in shaping cean basins (e.g., Parsons and Sclater, 1977), Pratt isostasy for oceanic lithosphere is typically viewed as a static process. However, cooling and contraction in the presence of a fluid ocean with non-negligible mass necessitatates mass exchange bewteen lithospheric columns to acheive isostatic equilibrium (Conder, 2012). Because fluid immediately fills any gap created by subsidence, a contracting column will gain mass (Upper left figure). Because the rate of mass gain depends on the subsidence rate, the columns gain excess mass at different rates and thereby become out of equilibrium with their neighbors unless mass exchange takes place among the columns. For the oceans, this mass exchange must take place in the asthenosphere. The mass disequilibrium accumulation rate for subsiding seafloor is shown above in panel (A). As younger seafloor subsides more rapidly than older seafloor, young lithosphere gains excess more rapidly and must shunt equivalent asthenosphere mass beneath older seafloor. Seafloor of one-quarter the age of the oldest lithosphere is in balance with the average mass gain rate which I call the cross-over age (marked tc above). So, excess mass must be transferred from below seafloor younger than the cross-over age to beneath seafloor older than the cross-over age. This implies that the flow rate induced in the asthenosphere increases towards tc and decreases away from tc. Panel B shows the mass flux beneath seafloor with plate ages of 20, 40, 60, 80, & 100 Myr. As the plate ages (and grows), the cross-over age increases and the mass flux must also increase to accommodate the greater integrated mass excesses. The flow rate itself is rater slow - maybe 1% of the spreading rate, but has this systematic form shown above. Below the cross-over age, the flow rate increses towards older seafloor and decreases with age for seafloor older than the cross-over age. This form suggests that younger lithosphere may be biased towards extensional intraplate stress and older seafloor biased toward compressional stress. The asymmetry in both flow and lithospheric strength about the cross-over age also suggests that the effect is likely to be larger for younger seafloor.
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Normal faulting−old seafloor
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Reverse faulting−young seafloor
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Reverse faulting−old seafloor
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MAR SEIR
Intraplate stress data. To look at possible signals of oceanic intraplate stresses arising from isostatic flow, data is utilized from the World Stress Map database (Heidbach et al., 2010). Data within 15 km of a plate boundary were removed along with data closely associated with diffuse boundaries (e.g., Indian Ocean), hotspots, microplates, or other identifiable tectonic influence. The remaining data comprises 110 extensional data, 106 compressional, and 149 strike slip data.
The adjacent maps show the data examined here. Sigma1 directions are shown in blue and sigma3 directions are shown in red. Black arrows denote local spreading directions.
Compressive vs Extensional Intraplate Stress. Isostatic flow drives asthenosphere from below young seafloor to beneath older seafloor. the maximum flow rate peaks at one-quarter of tIntraplate stress in the oceans is extremely variable. The histogram to the right shows counts of normal, reverse, and strike-slip data vs normalized age of the seafloor. Normalized age is the seafloor age divided by the maximum age in that particular spreading corridor (see intraplate stress data section). Most of the data of every type comes from young seafloor. However, the overall distribution differs with extensional data much more prevalent on young seafloor and compressional data having a larger presence on old seafloor. The graph at right shows the ratio of the normal and reverse histograms. A ratio of one and the cross over age (where a ratio of one would be predicted for isostatic flow stresses) are marked as dashed lines. The pattern is in strong agreement with the expected pattern induced from isostatic flow.
The consistency of principle stress directions, sigma1 and sigma3, are more problematic. For semi-infinite plates, the flow direction will follow the spreading direction. So, sigma3 may be predisposed to follow the spreading direction at young ages, while sigma1 would track better at older ages. Rose diagrams of stress direction deviations from the local spreading direction are shown at right (alignment = 0). The most curious signal is for young sigma1 directions. The diagram shows a mostly random fan, decreasing towards zero, with a large pie wedge near zero. Such a pattern would be predicted from a random set of stress axes perturbed by a small additional extensional stress oriented in the spreading direction. Those subparallel to the alignment become more aligned while those poorly aligned are largely unaffected. The distribution for sigma1 for older seafloor is more random. Isostatic flow is likely to be less beholden to the spreading direction at older ages as the differential subsidence rates of adjacent seafloor are smaller than at younger ages. So, other perterbations may affect flow directions. However, such deviations would still impart compressional stress albeit in the altered orientations.
−150 −100 −50 0 50 100 150−80
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Intraplate Strike−Slip Faulting
Seafloor spreading corridors and data ages. The age of seafloor associated with each datum is found using the seafloor age compilation of Muller et al. (2008). The age of oldest seafloor on the particular seafloor corridor is found by manual inspection (examples below; colors are age, circle is stress datum location, x the location of the oldest spot on the seafloor spreading corridor, and pointer denotes local direction of spreading).
−150 −100 −50 0 50 100 150−80
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Intraplate Reverse Faulting
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Intraplate Normal Faulting
2D models of isostatic flow. Finite element models of flow induced by differential mass excess accumulation are shown above (courtesy of Andrew Dombard). The flow models show the clear peak in flow rate beneath seafloor of 1/4 plate age. The thickness of the asthenosphere is an important parameter for the magnitude of induced intraplate stresses. While the mass flux is constant for all cases, the absolute flow rate (and it's derivative) increase with decreasing asthenosphere thickness. Isostatic flow accommodated by a thick channel will have a smaller effect on plate stresses than a thin asthenosphere channel.
100 km asthenosphere 300 km asthenosphere
A conceptual model for the interplay of isostatic flow in the context of the observations is shown at right. Blue lines denote lithosphere and green vectors show isostatic flow. Beneath young seafloor the flow is primarily directed in the spreading direction. Towards older lithosphere where the isostatic flow is less strong, random perturbations may significantly deviate the flow directions. The perturbations have the effect of randomizing the compressive stress directions, despite keeping the overall compressive nature of the flow
