Jason Phipps Morgan | Royal Holloway, University of London (original) (raw)
Papers by Jason Phipps Morgan
Reproduces and explores, by analogue modelling, the effect of a releasing-restraining bend pair... more Reproduces and explores, by analogue modelling, the effect of a releasing-restraining bend pair geometry for the western part of the NAF. Presents crustal strain patterns and associated topographic changes obtained from analogue model PIV Analysis. Experimental results are compared with the actual structural/topographic evolution of the Marmara Sea region.
Tectonophysics, 2019
We present LaCoDe (Lagrangian Compressible Deformation), a MATLAB solver for the Stokes equations... more We present LaCoDe (Lagrangian Compressible Deformation), a MATLAB solver for the Stokes equations for compressible non-Newtonian visco-elastic in two dimensions, based on a Lagrangian formulation of the Finite Element Method. The incompressible Boussinesq approximation is a widespread assumption in numerical models of lithospheric deformation, thus potentially masking a significant contribution of mechanisms linked to volumetric changes that occur in the asthenospheric mantle and the lithosphere. LaCoDe employs a compressible formulation of the Stokes equations designed to address such volumechanging processes. First, we provide a description of the equations governing the deformation of Earth rocks and detailed overview of the algorithm, its numerical implementation, treatment of the non-linearities rising from the compressible formulation, and the remeshing algorithm that tracks and transfers the physical fields that describe the material deformation from a highly-distorted to a high-quality mesh. LaCoDe is then benchmarked by comparing numerical results to analytical solutions for the bending of a thin elastic beam under a constant uniform load, flow around a rigid inclusion, Rayleigh-Taylor instability, stress build-up in a visco-elastic Maxwell body, and Couette flow with viscous heating. The Rayleigh-Taylor instability test is further used to demonstrate the accuracy of the remeshing algorithm. The importance of including volumetric
Tectonics, 2019
Erosion and deposition redistribute mass as a continental rift evolves, which modifies crustal lo... more Erosion and deposition redistribute mass as a continental rift evolves, which modifies crustal loads and influences subsequent deformation. Surface processes therefore impact both the architecture and the evolution of passive margins. Here we use coupled numerical models to explore the interactions between the surface, crust, and lithosphere. This interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink. If transport is efficient, there are two possible outcomes: (1) Faulting within the zone of extension is longer lived and has larger offsets. This implies a reduction of the number of faults and the width of the proximal domain. (2) Efficient transport of sediment leads to significant deposition and hence thermal blanketing. This will induce a switch from brittle to ductile deformation of the upper crust in the distal domains. The feedbacks between these two outcomes depend on the extension history, the underlying lithosp...
Journal of Geophysical Research: Solid Earth, 2018
Cratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction s... more Cratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction since early Jurassic times (approximately 190 Ma). This is recorded in its episodic tectonic and magmatic history. In this time, its lithosphere changed thickness from approximately 200 km to <60 km. This change was associated with a peak time (approximately 120 Ma) of lithospheric thinning and magmatism that was linked with high surface heat flow recorded in rift basins. We believe that these records are best explained by a two‐stage evolutionary process. First, approximately 100 km of cratonic “keel” underlying a weak midlithospheric discontinuity layer (approximately 80–100 km) was rapidly removed in <10–20 Ma. This keel delamination stage was followed by a protracted (approximately 50–100 Ma) period of convective erosion and/or lithospheric extension that thinned the remaining lithosphere and continuously reworked the former cratonic lithospheric mantle. This study focuses on nu...
Gondwana Research, 2016
The relative rates of creation and destruction of continental crust at subduction zones are a key... more The relative rates of creation and destruction of continental crust at subduction zones are a key factor shaping the evolution of continental crust through time. Central America, arguably the best studied place where subduction erosion has been documented, is used here to assess past rates and modes of forearc recycling. Drilling from Guatemala to Costa Rica indicates that subduction erosion has been active since at least the early Miocene. Drilling also shows that the rates of subduction erosion have varied significantly both along strike and through time. The Integrated Ocean Drilling Program (IODP) Expedition 334 to southern Costa Rica documents unprecedented subduction erosion thereat rates larger than the fastest known rates of forearc accretion. In southern Costa Rica, accelerated subduction erosion of the upper plate initiated when the Panama Fracture Zone/Cocos Ridge, the latter being an over thickened aseismic ridge, arrived at the Middle America Trench. The forearc records this event with an unconformity at 2.2 ± 0.2 Ma. The recovered shelf sequence overlying the unconformity constrains a short (b2 Myr) interval of extreme subsidence (~1200 m) with a rapid pulse occurring during the first 0.3 Myr. This event removed an estimated 1.2 × 10 6 km 3 of forearc material at a rate of~1125 km 3 /Myr/km of trench during a time of rapid (~1035 m/Myr) contemporaneous shelf sediment accumulation. Detrital apatite fission-track thermochronology on the sediments above the unconformity indicates the pattern of surficial sediment transport during this subduction erosion event. The fission track data show that sediments from the extinct and exhumed volcanic arcthe Cordillera de Talamancawere able to immediately access the growing forearc basin after the onset of the 2.2 Ma subduction erosion event. The onset of subduction of an aseismic ridge as occurred at 2.2 Ma in southern Costa Rica is a fairly common tectonic event along a subduction margin. We suggest that similar rapid pulses of subduction erosion may punctuate the evolution of many margins, contributing disproportionately to crustal recycling at subduction zones. The (poorly) preserved geologic record of paleoforearcs needs to be reassessed with this mechanism in mind. It also implies that continental forearc material may be significantly consumed during short local bursts along a subduction margin, and furthermore, that margins abutting regions of frequent subduction of aseismic ridges, like the regions in the Western Pacific where the Darwin Rise currently subducts, should face disproportionate pulses of future subduction erosion and forearc recycling.
Geological Society of America eBooks, 2007
We have made a table of 57 hotspots, giving the track azimuths of their present motions and track... more We have made a table of 57 hotspots, giving the track azimuths of their present motions and track rates where there was age control. This electronic supplement (with 301 references) gives the supporting data for each entry in this table and has a discussion of the probable errors in azimuth/rate for each entry. It also contains a discussion of 'hotspot-like-features' considered not to show plate motion and omitted from the table. DISCUSSION OF EACH TRACK Eurasian Plate Eifel (50.2°N, 6.7°E) w= 1 az= 082° ± 8° rate= 12 ± 2 mm/yr This is the best defined track on the Eurasian plate. Also, seismic tomographic studies have shown a low velocity column beneath Eifel extending down to at least 400 km (e.g., Wüllner et al., 2006). We used the locations of volcanics from Lippolt (1983) to determine an azimuth (082°) and have estimated its uncertainty to be ± 8°. This track has a moderately well determined rate. Duncan et al. (1972) found a (poorly determined) rate of 23 mm/yr based on early published K-Ar data and stratigraphic estimates of the ages of different volcanics. (Most of their references were published between 1960 and 1971, although some were as early as the 1930's.) There was a large effort to date these volcanics by the K-Ar method in the late 70's and early 80's that is
Journal of Geophysical Research: Solid Earth, 1988
Systematic changes in seafloor depth, crustal structure, and crustal geochemistry can occur withi... more Systematic changes in seafloor depth, crustal structure, and crustal geochemistry can occur within 30 km of a fracture zone. The seafloor gradually deepens roughly 1 km within 30 km from the fracture zone, the crust may be thinner than crust created far from a fracture zone, and systematic compositional differences are observed between basalts erupted near and far from fracture zones. We call these effects the transform fault effect (TFE). To investigate the physical processes responsible for the TFE, a general numerical method is developed to solve for the three‐dimensional flow and thermal structure beneath a mid‐ocean spreading center. This model is applied to study an idealized spreading center consisting of a 100 km transform fault offsetting two ridge segments spreading at rates of 1, 2, and 4 cm/yr. Using an adiabatic melt relation and our flow and temperature calculations, we find the distribution of melt production beneath the spreading center. Finally, a porous flow model ...
Journal of Geophysical Research: Solid Earth, 1993
Segmentation and along-axis variations within individual segments indicate the inherently three-d... more Segmentation and along-axis variations within individual segments indicate the inherently three-dimensional nature of mantle upwelling and melting beneath oceanic spreading centers. Numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center. The experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher-viscosity mantle half-space. A periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets. Buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt. The relative magnitudes of the buoyant and plate-driven components of mantle flow are controlled by the spreading rate and mantle viscosity, with buoyant flow more important at lower spreading rates and viscosities. Buoyant flow beneath the spreading axis amplifies along-axis variations in upwelling near a ridge-transform intersection, and distributes the variations along the entire spreading axis. Buoyant flow may thus be responsible for the more threedimensional character of slow spreading centers. Away from the spreading axis, thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along-axis wavelength controlled by the prescribed thickness of the asthenosphere. However, the position and stability of rolls are influenced by the segmentation geometry. In cases where the spreading center geometry does not allow a stable configuration of rolls, the flow is time-dependent. Along-axis variations in upwelling cause variations in melt production, which imply large variations in crustal thickness that dominate the surface gravity signal. The crustal thickness distributions implied by these numerical experiments produce bulls-eye-shaped negative mantle Bouguer anomalies centered over spreading segments, as observed at several spreading centers. The amplitude of the anomaly increases with decreasing spreading rate. Phipps Morgan, 1992]. Axial topography and sea surface gravity are two properties of a spreading center that can be most easily measured and which show distinct spreading rate dependence. The depth of the axial valley and the Bouguer gravity anomaly both increase from the center of a segment toward ridge axis offsets. These variations are prominent at the slow spreading Mid-Atlantic Ridge [
![Research paper thumbnail of Correction [to “Hotspot melting generates both hotspot volcanism and a hotspot swell?” by Jason Phipps Morgan, W. Jason Morgan, and Evelyn Price]](https://attachments.academia-assets.com/106032598/thumbnails/1.jpg)
](Journal of Geophysical Research, 1996
Geophysical Research Letters, 1987
Geophysical Research Letters publishes short, concise research letters that present scientific ad... more Geophysical Research Letters publishes short, concise research letters that present scientific advances that are likely to have immediate influence on the research of other investigators. GRL letters can focus on a specific discipline or apply broadly to the geophysical science community ...
Earth and Planetary Science Letters, 1994
The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits... more The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits, at a constant spreading rate, almost the full range of the many geophysical and geochemical parameters characteristic of the 'slow' Mid-Atlantic Ridge and 'fast' East Pacific Rise. We ...
There is increasing speculation that the mantle of the downgoing oceanic plate at a subduction zo... more There is increasing speculation that the mantle of the downgoing oceanic plate at a subduction zone may become extensively serpentinized between the outer rise and the trench axis. As the incoming plate bends at the outer rise extensive normal faulting occurs. Seismic reflection data shows that some of these faults cut through the crust deep into the lithospheric mantle, thus potentially acting as conduits for seawater to reach and react with lithospheric mantle rocks to make serpentine. Plates> 20Ma in age will have at least 30km ...
Journal of Geophysical Research: Solid Earth, 2018
Cratons are generally observed to retain thick (>180 km) conductive keels for billions of year... more Cratons are generally observed to retain thick (>180 km) conductive keels for billions of years. However, some cratons have undergone keel removal, with well‐documented examples being the eastern North China Craton (NCC) and the Wyoming Craton (WC). These keelless subregions appear to have kept a lithospheric bottom at ~80–100‐km depths. This is also the depth range where modern cratons, including the remaining portions of the NCC and the WC, have seismically visible midlithospheric discontinuity layers (MLDLs). MLDLs are proposed to be regions of preferential accumulation of metasomatic minerals and/or anomalously wet (>1,000 ppm) peridotites, both of which would lead to a relatively weak rheology. We propose that the cratonic keels of the eastern NCC (ENCC) and the western WC (WWC) utilized this weak MLDL layer to delaminate from overlying lithosphere. We first explore this hypothesis with a lubrication‐theory‐based analytical model. This model suggests a close relationship ...
Japan Geoscience Union, 2017
Earth’s mantle and core are convecting planetary heat engines. The mantle convects to lose heat f... more Earth’s mantle and core are convecting planetary heat engines. The mantle convects to lose heat from secular cooling, internal radioactivity, and core heatflow across its base. Its convection generates plate tectonics, volcanism, and the loss of ~35 TW of mantle heat through Earth’s surface. The core convects to lose heat from secular cooling, small amounts of internal radioactivity, and the freezing-induced growth of a compositionally denser inner core. Until recently, the geodynamo was thought to be powered by ~4 TW of heatloss across the core-mantle boundary. More recent determinations of the outer core’s thermal conductivity (Pozzo et al., 2012; Gomi et al., 2013) would imply that >15 TW of power should conduct down its adiabat. Secular core cooling has been previously thought to be too slow for this, based on estimates for the Clapeyron Slope for high-pressure freezing of an idealized pure-iron core (cf. Nimmo, 2007).
One of the persistent enigmas of Plate Tectonics is why the old (>2.5Ga), cold, stable, viscou... more One of the persistent enigmas of Plate Tectonics is why the old (>2.5Ga), cold, stable, viscous cratonic lithosphere sometimes rifts, but most of the time does not. Recent suggestions include erosion by mantle-derived hydrous melts, back-arc extension, delamination etc. Here we plan to review and study the cracking of continents by considering two different aspects of this problem: (1) Continental Rifting including the origin of failed rifts and continent-scale diking events such as the McKenzie Dike Swarm and the long-term evolution of the East African Rift; (2) Continental thinning including the recent (<200Ma) loss of the North China Cratonic lithosphere. This project will combine an observational geophysical and petrological examination of these two types of geologic events with simple model explorations of (a) the effects of stresses associated with rift volcanism as well as (b) the intrusion and freezing of carbonatitic plume melts within cratonic lithosphere. Stresses w...
Reproduces and explores, by analogue modelling, the effect of a releasing-restraining bend pair... more Reproduces and explores, by analogue modelling, the effect of a releasing-restraining bend pair geometry for the western part of the NAF. Presents crustal strain patterns and associated topographic changes obtained from analogue model PIV Analysis. Experimental results are compared with the actual structural/topographic evolution of the Marmara Sea region.
Tectonophysics, 2019
We present LaCoDe (Lagrangian Compressible Deformation), a MATLAB solver for the Stokes equations... more We present LaCoDe (Lagrangian Compressible Deformation), a MATLAB solver for the Stokes equations for compressible non-Newtonian visco-elastic in two dimensions, based on a Lagrangian formulation of the Finite Element Method. The incompressible Boussinesq approximation is a widespread assumption in numerical models of lithospheric deformation, thus potentially masking a significant contribution of mechanisms linked to volumetric changes that occur in the asthenospheric mantle and the lithosphere. LaCoDe employs a compressible formulation of the Stokes equations designed to address such volumechanging processes. First, we provide a description of the equations governing the deformation of Earth rocks and detailed overview of the algorithm, its numerical implementation, treatment of the non-linearities rising from the compressible formulation, and the remeshing algorithm that tracks and transfers the physical fields that describe the material deformation from a highly-distorted to a high-quality mesh. LaCoDe is then benchmarked by comparing numerical results to analytical solutions for the bending of a thin elastic beam under a constant uniform load, flow around a rigid inclusion, Rayleigh-Taylor instability, stress build-up in a visco-elastic Maxwell body, and Couette flow with viscous heating. The Rayleigh-Taylor instability test is further used to demonstrate the accuracy of the remeshing algorithm. The importance of including volumetric
Tectonics, 2019
Erosion and deposition redistribute mass as a continental rift evolves, which modifies crustal lo... more Erosion and deposition redistribute mass as a continental rift evolves, which modifies crustal loads and influences subsequent deformation. Surface processes therefore impact both the architecture and the evolution of passive margins. Here we use coupled numerical models to explore the interactions between the surface, crust, and lithosphere. This interaction is primarily sensitive to the efficiency of the surface processes in transporting mass from source to sink. If transport is efficient, there are two possible outcomes: (1) Faulting within the zone of extension is longer lived and has larger offsets. This implies a reduction of the number of faults and the width of the proximal domain. (2) Efficient transport of sediment leads to significant deposition and hence thermal blanketing. This will induce a switch from brittle to ductile deformation of the upper crust in the distal domains. The feedbacks between these two outcomes depend on the extension history, the underlying lithosp...
Journal of Geophysical Research: Solid Earth, 2018
Cratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction s... more Cratonic lithosphere beneath the eastern North China Craton has undergone extensive destruction since early Jurassic times (approximately 190 Ma). This is recorded in its episodic tectonic and magmatic history. In this time, its lithosphere changed thickness from approximately 200 km to <60 km. This change was associated with a peak time (approximately 120 Ma) of lithospheric thinning and magmatism that was linked with high surface heat flow recorded in rift basins. We believe that these records are best explained by a two‐stage evolutionary process. First, approximately 100 km of cratonic “keel” underlying a weak midlithospheric discontinuity layer (approximately 80–100 km) was rapidly removed in <10–20 Ma. This keel delamination stage was followed by a protracted (approximately 50–100 Ma) period of convective erosion and/or lithospheric extension that thinned the remaining lithosphere and continuously reworked the former cratonic lithospheric mantle. This study focuses on nu...
Gondwana Research, 2016
The relative rates of creation and destruction of continental crust at subduction zones are a key... more The relative rates of creation and destruction of continental crust at subduction zones are a key factor shaping the evolution of continental crust through time. Central America, arguably the best studied place where subduction erosion has been documented, is used here to assess past rates and modes of forearc recycling. Drilling from Guatemala to Costa Rica indicates that subduction erosion has been active since at least the early Miocene. Drilling also shows that the rates of subduction erosion have varied significantly both along strike and through time. The Integrated Ocean Drilling Program (IODP) Expedition 334 to southern Costa Rica documents unprecedented subduction erosion thereat rates larger than the fastest known rates of forearc accretion. In southern Costa Rica, accelerated subduction erosion of the upper plate initiated when the Panama Fracture Zone/Cocos Ridge, the latter being an over thickened aseismic ridge, arrived at the Middle America Trench. The forearc records this event with an unconformity at 2.2 ± 0.2 Ma. The recovered shelf sequence overlying the unconformity constrains a short (b2 Myr) interval of extreme subsidence (~1200 m) with a rapid pulse occurring during the first 0.3 Myr. This event removed an estimated 1.2 × 10 6 km 3 of forearc material at a rate of~1125 km 3 /Myr/km of trench during a time of rapid (~1035 m/Myr) contemporaneous shelf sediment accumulation. Detrital apatite fission-track thermochronology on the sediments above the unconformity indicates the pattern of surficial sediment transport during this subduction erosion event. The fission track data show that sediments from the extinct and exhumed volcanic arcthe Cordillera de Talamancawere able to immediately access the growing forearc basin after the onset of the 2.2 Ma subduction erosion event. The onset of subduction of an aseismic ridge as occurred at 2.2 Ma in southern Costa Rica is a fairly common tectonic event along a subduction margin. We suggest that similar rapid pulses of subduction erosion may punctuate the evolution of many margins, contributing disproportionately to crustal recycling at subduction zones. The (poorly) preserved geologic record of paleoforearcs needs to be reassessed with this mechanism in mind. It also implies that continental forearc material may be significantly consumed during short local bursts along a subduction margin, and furthermore, that margins abutting regions of frequent subduction of aseismic ridges, like the regions in the Western Pacific where the Darwin Rise currently subducts, should face disproportionate pulses of future subduction erosion and forearc recycling.
Geological Society of America eBooks, 2007
We have made a table of 57 hotspots, giving the track azimuths of their present motions and track... more We have made a table of 57 hotspots, giving the track azimuths of their present motions and track rates where there was age control. This electronic supplement (with 301 references) gives the supporting data for each entry in this table and has a discussion of the probable errors in azimuth/rate for each entry. It also contains a discussion of 'hotspot-like-features' considered not to show plate motion and omitted from the table. DISCUSSION OF EACH TRACK Eurasian Plate Eifel (50.2°N, 6.7°E) w= 1 az= 082° ± 8° rate= 12 ± 2 mm/yr This is the best defined track on the Eurasian plate. Also, seismic tomographic studies have shown a low velocity column beneath Eifel extending down to at least 400 km (e.g., Wüllner et al., 2006). We used the locations of volcanics from Lippolt (1983) to determine an azimuth (082°) and have estimated its uncertainty to be ± 8°. This track has a moderately well determined rate. Duncan et al. (1972) found a (poorly determined) rate of 23 mm/yr based on early published K-Ar data and stratigraphic estimates of the ages of different volcanics. (Most of their references were published between 1960 and 1971, although some were as early as the 1930's.) There was a large effort to date these volcanics by the K-Ar method in the late 70's and early 80's that is
Journal of Geophysical Research: Solid Earth, 1988
Systematic changes in seafloor depth, crustal structure, and crustal geochemistry can occur withi... more Systematic changes in seafloor depth, crustal structure, and crustal geochemistry can occur within 30 km of a fracture zone. The seafloor gradually deepens roughly 1 km within 30 km from the fracture zone, the crust may be thinner than crust created far from a fracture zone, and systematic compositional differences are observed between basalts erupted near and far from fracture zones. We call these effects the transform fault effect (TFE). To investigate the physical processes responsible for the TFE, a general numerical method is developed to solve for the three‐dimensional flow and thermal structure beneath a mid‐ocean spreading center. This model is applied to study an idealized spreading center consisting of a 100 km transform fault offsetting two ridge segments spreading at rates of 1, 2, and 4 cm/yr. Using an adiabatic melt relation and our flow and temperature calculations, we find the distribution of melt production beneath the spreading center. Finally, a porous flow model ...
Journal of Geophysical Research: Solid Earth, 1993
Segmentation and along-axis variations within individual segments indicate the inherently three-d... more Segmentation and along-axis variations within individual segments indicate the inherently three-dimensional nature of mantle upwelling and melting beneath oceanic spreading centers. Numerical convection experiments are used to explore the effects of local buoyancy forces on upwelling and melt production beneath a segmented spreading center. The experiments are conducted in a region consisting of a thermally defined rigid lithosphere and a uniform viscosity asthenosphere overlying a higher-viscosity mantle half-space. A periodic plate boundary geometry is imposed consisting of spreading segments and transform offsets. Buoyancy forces are caused by thermal expansion and the compositional density reduction due to the extraction of partial melt. The relative magnitudes of the buoyant and plate-driven components of mantle flow are controlled by the spreading rate and mantle viscosity, with buoyant flow more important at lower spreading rates and viscosities. Buoyant flow beneath the spreading axis amplifies along-axis variations in upwelling near a ridge-transform intersection, and distributes the variations along the entire spreading axis. Buoyant flow may thus be responsible for the more threedimensional character of slow spreading centers. Away from the spreading axis, thermal buoyancy drives convective rolls that align with the direction of plate motion and which have an along-axis wavelength controlled by the prescribed thickness of the asthenosphere. However, the position and stability of rolls are influenced by the segmentation geometry. In cases where the spreading center geometry does not allow a stable configuration of rolls, the flow is time-dependent. Along-axis variations in upwelling cause variations in melt production, which imply large variations in crustal thickness that dominate the surface gravity signal. The crustal thickness distributions implied by these numerical experiments produce bulls-eye-shaped negative mantle Bouguer anomalies centered over spreading segments, as observed at several spreading centers. The amplitude of the anomaly increases with decreasing spreading rate. Phipps Morgan, 1992]. Axial topography and sea surface gravity are two properties of a spreading center that can be most easily measured and which show distinct spreading rate dependence. The depth of the axial valley and the Bouguer gravity anomaly both increase from the center of a segment toward ridge axis offsets. These variations are prominent at the slow spreading Mid-Atlantic Ridge [
Journal of Geophysical Research, 1996
Geophysical Research Letters, 1987
Geophysical Research Letters publishes short, concise research letters that present scientific ad... more Geophysical Research Letters publishes short, concise research letters that present scientific advances that are likely to have immediate influence on the research of other investigators. GRL letters can focus on a specific discipline or apply broadly to the geophysical science community ...
Earth and Planetary Science Letters, 1994
The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits... more The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits, at a constant spreading rate, almost the full range of the many geophysical and geochemical parameters characteristic of the 'slow' Mid-Atlantic Ridge and 'fast' East Pacific Rise. We ...
There is increasing speculation that the mantle of the downgoing oceanic plate at a subduction zo... more There is increasing speculation that the mantle of the downgoing oceanic plate at a subduction zone may become extensively serpentinized between the outer rise and the trench axis. As the incoming plate bends at the outer rise extensive normal faulting occurs. Seismic reflection data shows that some of these faults cut through the crust deep into the lithospheric mantle, thus potentially acting as conduits for seawater to reach and react with lithospheric mantle rocks to make serpentine. Plates> 20Ma in age will have at least 30km ...
Journal of Geophysical Research: Solid Earth, 2018
Cratons are generally observed to retain thick (>180 km) conductive keels for billions of year... more Cratons are generally observed to retain thick (>180 km) conductive keels for billions of years. However, some cratons have undergone keel removal, with well‐documented examples being the eastern North China Craton (NCC) and the Wyoming Craton (WC). These keelless subregions appear to have kept a lithospheric bottom at ~80–100‐km depths. This is also the depth range where modern cratons, including the remaining portions of the NCC and the WC, have seismically visible midlithospheric discontinuity layers (MLDLs). MLDLs are proposed to be regions of preferential accumulation of metasomatic minerals and/or anomalously wet (>1,000 ppm) peridotites, both of which would lead to a relatively weak rheology. We propose that the cratonic keels of the eastern NCC (ENCC) and the western WC (WWC) utilized this weak MLDL layer to delaminate from overlying lithosphere. We first explore this hypothesis with a lubrication‐theory‐based analytical model. This model suggests a close relationship ...
Japan Geoscience Union, 2017
Earth’s mantle and core are convecting planetary heat engines. The mantle convects to lose heat f... more Earth’s mantle and core are convecting planetary heat engines. The mantle convects to lose heat from secular cooling, internal radioactivity, and core heatflow across its base. Its convection generates plate tectonics, volcanism, and the loss of ~35 TW of mantle heat through Earth’s surface. The core convects to lose heat from secular cooling, small amounts of internal radioactivity, and the freezing-induced growth of a compositionally denser inner core. Until recently, the geodynamo was thought to be powered by ~4 TW of heatloss across the core-mantle boundary. More recent determinations of the outer core’s thermal conductivity (Pozzo et al., 2012; Gomi et al., 2013) would imply that >15 TW of power should conduct down its adiabat. Secular core cooling has been previously thought to be too slow for this, based on estimates for the Clapeyron Slope for high-pressure freezing of an idealized pure-iron core (cf. Nimmo, 2007).
One of the persistent enigmas of Plate Tectonics is why the old (>2.5Ga), cold, stable, viscou... more One of the persistent enigmas of Plate Tectonics is why the old (>2.5Ga), cold, stable, viscous cratonic lithosphere sometimes rifts, but most of the time does not. Recent suggestions include erosion by mantle-derived hydrous melts, back-arc extension, delamination etc. Here we plan to review and study the cracking of continents by considering two different aspects of this problem: (1) Continental Rifting including the origin of failed rifts and continent-scale diking events such as the McKenzie Dike Swarm and the long-term evolution of the East African Rift; (2) Continental thinning including the recent (<200Ma) loss of the North China Cratonic lithosphere. This project will combine an observational geophysical and petrological examination of these two types of geologic events with simple model explorations of (a) the effects of stresses associated with rift volcanism as well as (b) the intrusion and freezing of carbonatitic plume melts within cratonic lithosphere. Stresses w...