Roger Buck - Academia.edu (original) (raw)
Papers by Roger Buck
Tectonics, 1988
Simple shear and pure shear extension of the lithosphere produce very different patterns of heat ... more Simple shear and pure shear extension of the lithosphere produce very different patterns of heat flow and topography. These differences are investigated using a numerical technique which solves for two-dimensional conductive and advective heat transport through time. Simple shear extension of the lithosphere is modeled as occurring along a straight shear zone. Two parameters define the simple shear model: the dip of the shear zone and its width. Likewise, the pure shear model is defined by two variables: the initial width of a vertical zone of pure shear extension and the rate of change of its width. These pairs of parameters are varied between calculations, as is the overall rate of extension. Each model results in distinct patterns of crustal thinning, lithospheric thermal structure, heat flow, thermal uplift, crustal subsidence, and topography. For the simple shear model, extension results in asymmetric uplift across the rift, while the total volume of uplift is limited by the total amount of extension. The peak heat flow and thermal uplift are centered over the intersection of the shear zone with the surface. Isostatic response to simple shear extension results in successive, formerly active shear zones being rotated into listtic faults which sole into a subhorizontal detachment. The pure shear results show that the surface heat flow is greater for smaller widths of the zone of extension. For the same overall 1Also at the Department of Geological Sciences, Columbia University.
Geophysical Monograph Series, 1998
... Belts: Models and Standards JF Lemaire, D. Heynderickx, and DN Baker (Eds.) 98 Magnetic Storm... more ... Belts: Models and Standards JF Lemaire, D. Heynderickx, and DN Baker (Eds.) 98 Magnetic Storms Bruce T. Tsurutani, Walter D. Gonzalez, Yohsuke Kamide, and John K. Arballo (Eds.) 99 ... ISBN 0-87590-089-5 1. Sea-floor spreading (Geology) 2. Faults (Geology) 3. Magmatism ...
Journal of Geophysical Research: Solid Earth, 2006
The best-studied dike intrusion events on a divergent plate boundary occurred along the Krafla se... more The best-studied dike intrusion events on a divergent plate boundary occurred along the Krafla segment of the northern rift zone in Iceland from 1975-1984. Seismic and geodetic measurements there showed that a central magma chamber fed dikes that propagated laterally many times the thickness of the lithosphere. The patterns of dike length, dike width, caldera subsidence, and lava extrusion strongly suggest that dike propagation is affected by tectonic stresses that change with each dike intrusion event and that magma pressures are linked to the dike opening. These observations have inspired us to develop a quantitative model for the lateral propagation of basaltic dikes away from a magma chamber. We assume dikes propagate as long as there is sufficient driving pressure, defined as the difference between magma pressure and tectonic stress at the dike tip. The opening dike and the magma chamber are treated as a closed system for a given dike intrusion event. During an event, magma pressure is reduced linearly with the magma volume withdrawn from the chamber. Relative tectonic tension in the lithosphere is reduced linearly as the dike width increases. A dike begins propagation when the driving pressure equals the ''breakout'' pressure needed to force the magma out of the chamber. It stops when the driving pressure reaches a minimum value. Generally, the dike width is proportional to this ''stopping'' pressure, and a reasonable value gives a width of 1 m. Besides the breakout and stopping pressures, the propagation distance depends on the initial distribution of tectonic stress and the thickness of the lithosphere cut by a dike. The intrusion of a dike changes the tectonic stress distribution so that subsequent dikes may propagate different distances and directions than the first dike. After a period of magma chamber refilling, a new dike can initiate if the breakout pressure is reached. For an idealized spreading segment the tectonic stress field evolves to produce a sequence of dikes propagating in one direction followed by a sequence of dikes propagating in the opposite direction. The first dike in each sequence should be the longest followed by successively shorter dikes. When tectonic stresses close to a magma chamber have been largely relieved, then extrusion of magma may start. The model pattern of dike propagation and extrusion is consistent with data from the Krafla episode. Magma chamber size should have a major effect on magmatic systems in other tectonic settings with larger magma chambers producing longer characteristic dikes.
Journal of Geophysical Research: Solid Earth, 2000
We study the physical processes controlling the development and evolution of normal faults by ana... more We study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two-dimensional elastic-plastic (brittle) layer floating on an inviscid fluid. The yield stress of the layer is the sum of the layer cohesion and its frictional stress. Faults are initiated by a small plastic flaw in the layer. We get finite fault offset when we make fault cohesion decrease with strain. Even in this highly idealized system we vary six physical parameters: the initial cohesion of the layer, the thickness of the layer, the rate of cohesion reduction with plastic strain, the friction coefficient, the flaw size and the fault width. We obtain two main types of faulting behavior: (1) multiple major faults with small offset and (2) single major fault that can develop very large offset. We show that only two parameters control these different types of faulting patterns: (1) the brittle layer thickness for a given cohesion and (2) the rate of cohesion reduction with strain. For a large brittle layer thickness (> 22 km with 44 MPa of cohesion), extension always leads to multiple faults distributed over the width of the layer. For a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening: a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault. When the offset is very large, the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes.
Journal of Geophysical Research: Solid Earth, 2001
Mid-ocean ridge topography is modeled as the flexural response to loads using a thin plate approx... more Mid-ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow, but not require, a region of rapid cooling near the axis. Loads on the lithosphere arise from the presence of lowdensity melt, densification due to cooling with distance from the ridge axis, and thermal contraction stresses. We find two end-member classes of temperature and melt structure that can produce axial high topography and gravity observed at the East Pacific Rise (EPR). One class is very similar to previous models, requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis. The other is a new class, predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis, with melt which is contained primarily within the crust. The latter solution is consistent with tomography and compliance studies at the EPR which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation. This solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle. Model fits predict 0-30% melt in the lower crust, depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography. The model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis. This is consistent with previous correlations between large cross-sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime. For a slightly slower rate of lithospheric cooling at distances more than $5 km from the axis the model predicts a trough at the base of the axial high. Such troughs have been previously observed at the base of the high on the western flank of the southern EPR, where subsidence rates are anomalously low. Finally, thick axial lithosphere reduces the amplitude of the high, making it sometimes difficult to distinguish from long-wavelength subsidence. This morphology is comparable to that of some intermediate spreading ridges, where topography is relatively flat, suggesting a transition from fast to intermediate style morphology.
Geophysical Research Letters, 2009
Crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of di... more Crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of dikes, but their emplacement and the associated deformation are rarely observed. The few existing observations at subaerial rifts show that these diking events are followed by a decadal-scale period with extension rates faster than the secular divergent plate motion. This transient accelerated deformation has been explained by continued subsurface magma injection or by relaxation, in the viscoelastic mantle, of the stress changes imparted by dike opening. For the first time, GPS measurements were collected within a few months of a rifting event at a major plate boundary, the September 2005, 60 km-long dike intrusion in the Dabbahu segment, Afar, Ethiopia. Extension rates for the first 3 years greatly exceed the plate motion (Nubia-Arabia) secular divergence rate, even at sites located more than 60 km from the rift axis. Here we show that these observations are consistent with stress relaxation in a viscoelastic upper mantle with a viscosity of about 5 Â 10 18 PaÁs overlain by a 12-14 km-thick elastic crust. The alternative model of continued diking requires continuous opening well below the Moho and is therefore unlikely. Instead, magma injection in Afar since June 2006 has taken the form of smaller discrete diking events, tapping into a mid-crustal melt reservoir under the segment center.
Geology, 2007
The high elevation and considerable length of the Transantarctic Mountains have led to speculatio... more The high elevation and considerable length of the Transantarctic Mountains have led to speculation about their origin. To date, no model has been able to adequately reconcile the juxtaposition of the high, curvilinear Transantarctic Mountains with the adjacent West Antarctic Rift System, a broad region of thin extended continental crust exhibiting wide rift characteristics. We present a fi rst-order investigation into the idea that the West Antarctic Rift System-Transantarctic Mountains region was a high-elevation plateau with thicker than normal crust before the onset of continental extension. With major Cretaceous extension, the rift underwent a topographic reversal, and a plateau edge with thickened crust, representing the ancestral Transantarctic Mountains, remained. In the Cenozoic, minor extension and major denudation reduce the crustal root while simultaneously uplifting peak heights in the mountains. The Cretaceous stage of this concept is investigated using two-dimensional numerical models to determine under what conditions plateau collapse is plausible. Model results indicate that elevation of a remnant plateau edge decreases with increasing initial Moho temperature. Very cold initial Moho temperatures, <675 °C, under the plateau leave a thick plateau edge but do not exhibit wide rifting. A cold to moderate initial thermal structure, Moho temperatures of 675-850 °C, is needed to retain the plateau edge and still exhibit wide rifting in the middle of the plateau. We conclude that this plateau collapse concept is possible using these numerical experiments, and that application of this idea to the West Antarctic Rift System-Transantarctic Mountains system is also supported by geological and geophysical evidence.
Faulting and Magmatism at Mid-Ocean Ridges, 2013
ABSTRACT
Journal of Volcanology and Geothermal Research
Abstract One of the World's premier field geologists, Kristjan Saemundsson led immense geolog... more Abstract One of the World's premier field geologists, Kristjan Saemundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristjan began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristjan resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristjan's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude.
Int Geol Rev, 2003
... Many modelers have considered how such a broad region of extension could develop (eg, Fletche... more ... Many modelers have considered how such a broad region of extension could develop (eg, Fletcher and Hallet, 1983; Zuber et al., 1985, Hopper and Buck, 1996). One reason for this fascination with wide rifting is that many processes, like thinning of ...
Journal of Geophysical Research, 2001
ABSTRACT
Tectonics, 1988
Simple shear and pure shear extension of the lithosphere produce very different patterns of heat ... more Simple shear and pure shear extension of the lithosphere produce very different patterns of heat flow and topography. These differences are investigated using a numerical technique which solves for two-dimensional conductive and advective heat transport through time. Simple shear extension of the lithosphere is modeled as occurring along a straight shear zone. Two parameters define the simple shear model: the dip of the shear zone and its width. Likewise, the pure shear model is defined by two variables: the initial width of a vertical zone of pure shear extension and the rate of change of its width. These pairs of parameters are varied between calculations, as is the overall rate of extension. Each model results in distinct patterns of crustal thinning, lithospheric thermal structure, heat flow, thermal uplift, crustal subsidence, and topography. For the simple shear model, extension results in asymmetric uplift across the rift, while the total volume of uplift is limited by the total amount of extension. The peak heat flow and thermal uplift are centered over the intersection of the shear zone with the surface. Isostatic response to simple shear extension results in successive, formerly active shear zones being rotated into listtic faults which sole into a subhorizontal detachment. The pure shear results show that the surface heat flow is greater for smaller widths of the zone of extension. For the same overall 1Also at the Department of Geological Sciences, Columbia University.
Geophysical Monograph Series, 1998
... Belts: Models and Standards JF Lemaire, D. Heynderickx, and DN Baker (Eds.) 98 Magnetic Storm... more ... Belts: Models and Standards JF Lemaire, D. Heynderickx, and DN Baker (Eds.) 98 Magnetic Storms Bruce T. Tsurutani, Walter D. Gonzalez, Yohsuke Kamide, and John K. Arballo (Eds.) 99 ... ISBN 0-87590-089-5 1. Sea-floor spreading (Geology) 2. Faults (Geology) 3. Magmatism ...
Journal of Geophysical Research: Solid Earth, 2006
The best-studied dike intrusion events on a divergent plate boundary occurred along the Krafla se... more The best-studied dike intrusion events on a divergent plate boundary occurred along the Krafla segment of the northern rift zone in Iceland from 1975-1984. Seismic and geodetic measurements there showed that a central magma chamber fed dikes that propagated laterally many times the thickness of the lithosphere. The patterns of dike length, dike width, caldera subsidence, and lava extrusion strongly suggest that dike propagation is affected by tectonic stresses that change with each dike intrusion event and that magma pressures are linked to the dike opening. These observations have inspired us to develop a quantitative model for the lateral propagation of basaltic dikes away from a magma chamber. We assume dikes propagate as long as there is sufficient driving pressure, defined as the difference between magma pressure and tectonic stress at the dike tip. The opening dike and the magma chamber are treated as a closed system for a given dike intrusion event. During an event, magma pressure is reduced linearly with the magma volume withdrawn from the chamber. Relative tectonic tension in the lithosphere is reduced linearly as the dike width increases. A dike begins propagation when the driving pressure equals the ''breakout'' pressure needed to force the magma out of the chamber. It stops when the driving pressure reaches a minimum value. Generally, the dike width is proportional to this ''stopping'' pressure, and a reasonable value gives a width of 1 m. Besides the breakout and stopping pressures, the propagation distance depends on the initial distribution of tectonic stress and the thickness of the lithosphere cut by a dike. The intrusion of a dike changes the tectonic stress distribution so that subsequent dikes may propagate different distances and directions than the first dike. After a period of magma chamber refilling, a new dike can initiate if the breakout pressure is reached. For an idealized spreading segment the tectonic stress field evolves to produce a sequence of dikes propagating in one direction followed by a sequence of dikes propagating in the opposite direction. The first dike in each sequence should be the longest followed by successively shorter dikes. When tectonic stresses close to a magma chamber have been largely relieved, then extrusion of magma may start. The model pattern of dike propagation and extrusion is consistent with data from the Krafla episode. Magma chamber size should have a major effect on magmatic systems in other tectonic settings with larger magma chambers producing longer characteristic dikes.
Journal of Geophysical Research: Solid Earth, 2000
We study the physical processes controlling the development and evolution of normal faults by ana... more We study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two-dimensional elastic-plastic (brittle) layer floating on an inviscid fluid. The yield stress of the layer is the sum of the layer cohesion and its frictional stress. Faults are initiated by a small plastic flaw in the layer. We get finite fault offset when we make fault cohesion decrease with strain. Even in this highly idealized system we vary six physical parameters: the initial cohesion of the layer, the thickness of the layer, the rate of cohesion reduction with plastic strain, the friction coefficient, the flaw size and the fault width. We obtain two main types of faulting behavior: (1) multiple major faults with small offset and (2) single major fault that can develop very large offset. We show that only two parameters control these different types of faulting patterns: (1) the brittle layer thickness for a given cohesion and (2) the rate of cohesion reduction with strain. For a large brittle layer thickness (> 22 km with 44 MPa of cohesion), extension always leads to multiple faults distributed over the width of the layer. For a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening: a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault. When the offset is very large, the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes.
Journal of Geophysical Research: Solid Earth, 2001
Mid-ocean ridge topography is modeled as the flexural response to loads using a thin plate approx... more Mid-ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow, but not require, a region of rapid cooling near the axis. Loads on the lithosphere arise from the presence of lowdensity melt, densification due to cooling with distance from the ridge axis, and thermal contraction stresses. We find two end-member classes of temperature and melt structure that can produce axial high topography and gravity observed at the East Pacific Rise (EPR). One class is very similar to previous models, requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis. The other is a new class, predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis, with melt which is contained primarily within the crust. The latter solution is consistent with tomography and compliance studies at the EPR which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation. This solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle. Model fits predict 0-30% melt in the lower crust, depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography. The model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis. This is consistent with previous correlations between large cross-sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime. For a slightly slower rate of lithospheric cooling at distances more than $5 km from the axis the model predicts a trough at the base of the axial high. Such troughs have been previously observed at the base of the high on the western flank of the southern EPR, where subsidence rates are anomalously low. Finally, thick axial lithosphere reduces the amplitude of the high, making it sometimes difficult to distinguish from long-wavelength subsidence. This morphology is comparable to that of some intermediate spreading ridges, where topography is relatively flat, suggesting a transition from fast to intermediate style morphology.
Geophysical Research Letters, 2009
Crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of di... more Crustal accretion at divergent plate boundaries typically occurs via the periodic intrusion of dikes, but their emplacement and the associated deformation are rarely observed. The few existing observations at subaerial rifts show that these diking events are followed by a decadal-scale period with extension rates faster than the secular divergent plate motion. This transient accelerated deformation has been explained by continued subsurface magma injection or by relaxation, in the viscoelastic mantle, of the stress changes imparted by dike opening. For the first time, GPS measurements were collected within a few months of a rifting event at a major plate boundary, the September 2005, 60 km-long dike intrusion in the Dabbahu segment, Afar, Ethiopia. Extension rates for the first 3 years greatly exceed the plate motion (Nubia-Arabia) secular divergence rate, even at sites located more than 60 km from the rift axis. Here we show that these observations are consistent with stress relaxation in a viscoelastic upper mantle with a viscosity of about 5 Â 10 18 PaÁs overlain by a 12-14 km-thick elastic crust. The alternative model of continued diking requires continuous opening well below the Moho and is therefore unlikely. Instead, magma injection in Afar since June 2006 has taken the form of smaller discrete diking events, tapping into a mid-crustal melt reservoir under the segment center.
Geology, 2007
The high elevation and considerable length of the Transantarctic Mountains have led to speculatio... more The high elevation and considerable length of the Transantarctic Mountains have led to speculation about their origin. To date, no model has been able to adequately reconcile the juxtaposition of the high, curvilinear Transantarctic Mountains with the adjacent West Antarctic Rift System, a broad region of thin extended continental crust exhibiting wide rift characteristics. We present a fi rst-order investigation into the idea that the West Antarctic Rift System-Transantarctic Mountains region was a high-elevation plateau with thicker than normal crust before the onset of continental extension. With major Cretaceous extension, the rift underwent a topographic reversal, and a plateau edge with thickened crust, representing the ancestral Transantarctic Mountains, remained. In the Cenozoic, minor extension and major denudation reduce the crustal root while simultaneously uplifting peak heights in the mountains. The Cretaceous stage of this concept is investigated using two-dimensional numerical models to determine under what conditions plateau collapse is plausible. Model results indicate that elevation of a remnant plateau edge decreases with increasing initial Moho temperature. Very cold initial Moho temperatures, <675 °C, under the plateau leave a thick plateau edge but do not exhibit wide rifting. A cold to moderate initial thermal structure, Moho temperatures of 675-850 °C, is needed to retain the plateau edge and still exhibit wide rifting in the middle of the plateau. We conclude that this plateau collapse concept is possible using these numerical experiments, and that application of this idea to the West Antarctic Rift System-Transantarctic Mountains system is also supported by geological and geophysical evidence.
Faulting and Magmatism at Mid-Ocean Ridges, 2013
ABSTRACT
Journal of Volcanology and Geothermal Research
Abstract One of the World's premier field geologists, Kristjan Saemundsson led immense geolog... more Abstract One of the World's premier field geologists, Kristjan Saemundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristjan began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristjan resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristjan's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude.
Int Geol Rev, 2003
... Many modelers have considered how such a broad region of extension could develop (eg, Fletche... more ... Many modelers have considered how such a broad region of extension could develop (eg, Fletcher and Hallet, 1983; Zuber et al., 1985, Hopper and Buck, 1996). One reason for this fascination with wide rifting is that many processes, like thinning of ...
Journal of Geophysical Research, 2001
ABSTRACT