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Bridget Scanlon, a hydrologist and senior research scientist at the University of Texas’ Bureau of Economic Geology, has been elected a member of the National Academy of Engineering (NAE), one of the highest professional honors accorded to engineers and scientists. “I’m extremely...

FEB
12

UTIG Seminar Series: Sarah Penniston-Dorland, University of Maryland
10:30 AM

UTIG Seminar Series: Sarah Penniston-Dorland, University of Maryland

  Start: February 12, 2016 at 10:30 am     End: February 12, 2016 at 11:30 am
 Location:PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758
 Contact:Laura Wallace, lwallace@ig.utexas.edu, 512-471-0324
 URL:Event Link
"Strength and Heat in the Subduction Channel: Evidence from Metamorphic Rocks and Geodynamic Models"

Abstract:

The thermal structure and flow of material within subduction zones are closely linked and are important for our understanding of seismicity within subduction zones and for the generation of arc magmas. This is a talk in two parts investigating evidence from metamorphic rocks for the thermal structure and degree of material flow within subduction zones. Evidence from natural rocks is compared to that generated from computational geodynamic models.

Part 1. Thermal structure: The maximum-pressure P-T conditions (Pmax-T) and prograde P-T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P-T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax-T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax< 2 GPa where only a few of the highest-T modeled paths overlap typical petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde P-T paths similarly indicate warmer subduction than typical models. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on P-T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models do not match the rock record, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.

Part 2. The Catalina Schist contains a spectacular, km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In this setting, mafic and ultramafic blocks ranging from cms up to 100s of m in diameter are surrounded by finer-grained matrix. All blocks throughout the mélange contain assemblages consistent with upper amphibolite-facies conditions, suggesting a relatively restricted range of depths and temperatures over which the mélange formed. This apparent uniformity contrasts with other mélanges, such as the Franciscan Complex, where rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. The Zr content of rutiles in samples from the amphibolite facies of the Catalina Schist were measured to determine peak metamorphic temperatures, identify whether these temperatures were different among blocks (within measurement error), and whether the spatial distribution of temperatures throughout the mélange was systematic or random. Resolvably different Zr contents are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730°C at an assumed pressure of 1 GPa. No systematic distribution of temperatures was found, however. Therefore material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a relatively restricted scale.

FEB
18

De Ford Lecture Series: Fiona Whitaker, University of Bristol (UK)
4:00 PM

UTIG Seminar Series: Sarah Penniston-Dorland, University of Maryland

  Start: February 12, 2016 at 10:30 am     End: February 12, 2016 at 11:30 am
 Location:PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758
 Contact:Laura Wallace, lwallace@ig.utexas.edu, 512-471-0324
 URL:Event Link
"Strength and Heat in the Subduction Channel: Evidence from Metamorphic Rocks and Geodynamic Models"

Abstract:

The thermal structure and flow of material within subduction zones are closely linked and are important for our understanding of seismicity within subduction zones and for the generation of arc magmas. This is a talk in two parts investigating evidence from metamorphic rocks for the thermal structure and degree of material flow within subduction zones. Evidence from natural rocks is compared to that generated from computational geodynamic models.

Part 1. Thermal structure: The maximum-pressure P-T conditions (Pmax-T) and prograde P-T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P-T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax-T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax< 2 GPa where only a few of the highest-T modeled paths overlap typical petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde P-T paths similarly indicate warmer subduction than typical models. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on P-T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models do not match the rock record, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.

Part 2. The Catalina Schist contains a spectacular, km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In this setting, mafic and ultramafic blocks ranging from cms up to 100s of m in diameter are surrounded by finer-grained matrix. All blocks throughout the mélange contain assemblages consistent with upper amphibolite-facies conditions, suggesting a relatively restricted range of depths and temperatures over which the mélange formed. This apparent uniformity contrasts with other mélanges, such as the Franciscan Complex, where rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. The Zr content of rutiles in samples from the amphibolite facies of the Catalina Schist were measured to determine peak metamorphic temperatures, identify whether these temperatures were different among blocks (within measurement error), and whether the spatial distribution of temperatures throughout the mélange was systematic or random. Resolvably different Zr contents are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730°C at an assumed pressure of 1 GPa. No systematic distribution of temperatures was found, however. Therefore material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a relatively restricted scale.

De Ford Lecture Series: Fiona Whitaker, University of Bristol (UK)

  Start: February 18, 2016 at 4:00 pm     End: February 18, 2016 at 5:00 pm

FEB
19

UTIG Seminar Series: Demian Saffer, Penn State
10:30 AM

UTIG Seminar Series: Sarah Penniston-Dorland, University of Maryland

  Start: February 12, 2016 at 10:30 am     End: February 12, 2016 at 11:30 am
 Location:PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758
 Contact:Laura Wallace, lwallace@ig.utexas.edu, 512-471-0324
 URL:Event Link
"Strength and Heat in the Subduction Channel: Evidence from Metamorphic Rocks and Geodynamic Models"

Abstract:

The thermal structure and flow of material within subduction zones are closely linked and are important for our understanding of seismicity within subduction zones and for the generation of arc magmas. This is a talk in two parts investigating evidence from metamorphic rocks for the thermal structure and degree of material flow within subduction zones. Evidence from natural rocks is compared to that generated from computational geodynamic models.

Part 1. Thermal structure: The maximum-pressure P-T conditions (Pmax-T) and prograde P-T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P-T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax-T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax< 2 GPa where only a few of the highest-T modeled paths overlap typical petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde P-T paths similarly indicate warmer subduction than typical models. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on P-T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models do not match the rock record, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.

Part 2. The Catalina Schist contains a spectacular, km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In this setting, mafic and ultramafic blocks ranging from cms up to 100s of m in diameter are surrounded by finer-grained matrix. All blocks throughout the mélange contain assemblages consistent with upper amphibolite-facies conditions, suggesting a relatively restricted range of depths and temperatures over which the mélange formed. This apparent uniformity contrasts with other mélanges, such as the Franciscan Complex, where rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. The Zr content of rutiles in samples from the amphibolite facies of the Catalina Schist were measured to determine peak metamorphic temperatures, identify whether these temperatures were different among blocks (within measurement error), and whether the spatial distribution of temperatures throughout the mélange was systematic or random. Resolvably different Zr contents are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730°C at an assumed pressure of 1 GPa. No systematic distribution of temperatures was found, however. Therefore material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a relatively restricted scale.

De Ford Lecture Series: Fiona Whitaker, University of Bristol (UK)

  Start: February 18, 2016 at 4:00 pm     End: February 18, 2016 at 5:00 pm

UTIG Seminar Series: Demian Saffer, Penn State

  Start: February 19, 2016 at 10:30 am     End: February 19, 2016 at 11:30 am
 Location:PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758
 Contact:Peter Flemings/Laura Wallace, pflemings@jsg.utexas.edu/lwallace@ig.utexas.edu, 512-475-8738/512-471
 URL:Event Link
"Phyllosilicates, Friction, and Fluids: The Role of Clays in Plate Boundary Fault Processes"

Abstract:

Clay minerals, and hydrous smectite family clays in particular, have been hypothesized to control both the low absolute shear strength and the nature of slip along plate boundary faults, including subduction megathrusts, low-angle normal faults, and continental transforms. In addition to their effects on frictional behavior, hydrous clays are the largest and shallowest source of bound water in systems undergoing prograde diagenesis and metamorphism. For example, the transformation from smectite to illite at temperatures of ~80-150 °C releases ~20 wt% H2O. As a result, the reaction has the potential to generate fluid overpressures through expulsion of bound water; to change the elastic, hydraulic, and frictional properties of the rock or sediment via mobilization of quartz and associated cementation or vein-filling; and to generate geochemical signatures that yield information about fluid flow pathways from source regions at depth to observation points at the surface. Here, I summarize a set of integrated laboratory, numerical modeling, and field studies that address these linked processes, focusing on examples from the Costa Rican and Nankai (SW Japan) subduction zones.

Laboratory shearing experiments on a suite of synthetic and natural fault gouges demonstrate that clay- and mudstones sampled from major fault zones, including the San Andreas Fault and the Costa Rican and Nankai subduction megathrusts, are frictionally weak, with friction coefficients in the range µ = ~0.08-0.35. These fault rocks exhibit rate-strengthening behavior, which should lead to only aseismic creep. However, they become increasingly rate-weakening (and therefore potentially able to host unstable slip) with increasing quartz abundance, raising the possibility that clay transformation, primarily through the by-product of silica mobilization, may play a role in controlling the upper aseismic-seismic transition and its spatial variability or “patchiness” at some subduction margins.

Numerical models describing the kinetics of clay dehydration and associated fluid expulsion show that at both the Costa Rican and Nankai margins, earthquakes nucleate only in areas down-dip of the reaction, after it has peaked and is near completion. This behavior is robust, and it parallels variations in thermal structure along the strike of the Costa Rican margin. Although the number of observations is limited, the correlation further supports the idea that the onset of unstable slip is linked to clay transformation, likely through the combination of changes in rock frictional properties and the dissipation of the bound water source that allows effective normal stress to increase. Interestingly, the rupture areas of large (M>6-7) events extend into the zone of dehydration, suggesting the possibility that nucleation may be restricted to depths where the clay reaction has progressed, but that slip may propagate up-dip into weak and potentially overpressured regions where the dehydration reaction is peaking. Finally, these models can also be used to predict the evolution of pore fluid composition within subducting and accreting sediments due to the release of bound (fresh) water and the desorption of fluid-mobile elements like B and Li. Comparison of simulated pore fluid compositions in these source regions with observations at seafloor seeps and in shallow drillholes provides evidence for long-distance (10’s of km) up-dip fluid migration along fault zones.

FEB
25

De Ford Lecture Series: G. Burch Fisher
4:00 PM

UTIG Seminar Series: Sarah Penniston-Dorland, University of Maryland

  Start: February 12, 2016 at 10:30 am     End: February 12, 2016 at 11:30 am
 Location:PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758
 Contact:Laura Wallace, lwallace@ig.utexas.edu, 512-471-0324
 URL:Event Link
"Strength and Heat in the Subduction Channel: Evidence from Metamorphic Rocks and Geodynamic Models"

Abstract:

The thermal structure and flow of material within subduction zones are closely linked and are important for our understanding of seismicity within subduction zones and for the generation of arc magmas. This is a talk in two parts investigating evidence from metamorphic rocks for the thermal structure and degree of material flow within subduction zones. Evidence from natural rocks is compared to that generated from computational geodynamic models.

Part 1. Thermal structure: The maximum-pressure P-T conditions (Pmax-T) and prograde P-T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P-T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax-T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax< 2 GPa where only a few of the highest-T modeled paths overlap typical petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde P-T paths similarly indicate warmer subduction than typical models. Our compilation and comparison suggest that exhumed high-P rocks provide a more accurate constraint on P-T conditions within subduction zones, and that those conditions may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models do not match the rock record, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.

Part 2. The Catalina Schist contains a spectacular, km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In this setting, mafic and ultramafic blocks ranging from cms up to 100s of m in diameter are surrounded by finer-grained matrix. All blocks throughout the mélange contain assemblages consistent with upper amphibolite-facies conditions, suggesting a relatively restricted range of depths and temperatures over which the mélange formed. This apparent uniformity contrasts with other mélanges, such as the Franciscan Complex, where rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. The Zr content of rutiles in samples from the amphibolite facies of the Catalina Schist were measured to determine peak metamorphic temperatures, identify whether these temperatures were different among blocks (within measurement error), and whether the spatial distribution of temperatures throughout the mélange was systematic or random. Resolvably different Zr contents are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730°C at an assumed pressure of 1 GPa. No systematic distribution of temperatures was found, however. Therefore material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a relatively restricted scale.

De Ford Lecture Series: Fiona Whitaker, University of Bristol (UK)

  Start: February 18, 2016 at 4:00 pm     End: February 18, 2016 at 5:00 pm

UTIG Seminar Series: Demian Saffer, Penn State

  Start: February 19, 2016 at 10:30 am     End: February 19, 2016 at 11:30 am
 Location:PRC, 10100 Burnet Road, Bldg 196, Rm 1.603, Austin, TX 78758
 Contact:Peter Flemings/Laura Wallace, pflemings@jsg.utexas.edu/lwallace@ig.utexas.edu, 512-475-8738/512-471
 URL:Event Link
"Phyllosilicates, Friction, and Fluids: The Role of Clays in Plate Boundary Fault Processes"

Abstract:

Clay minerals, and hydrous smectite family clays in particular, have been hypothesized to control both the low absolute shear strength and the nature of slip along plate boundary faults, including subduction megathrusts, low-angle normal faults, and continental transforms. In addition to their effects on frictional behavior, hydrous clays are the largest and shallowest source of bound water in systems undergoing prograde diagenesis and metamorphism. For example, the transformation from smectite to illite at temperatures of ~80-150 °C releases ~20 wt% H2O. As a result, the reaction has the potential to generate fluid overpressures through expulsion of bound water; to change the elastic, hydraulic, and frictional properties of the rock or sediment via mobilization of quartz and associated cementation or vein-filling; and to generate geochemical signatures that yield information about fluid flow pathways from source regions at depth to observation points at the surface. Here, I summarize a set of integrated laboratory, numerical modeling, and field studies that address these linked processes, focusing on examples from the Costa Rican and Nankai (SW Japan) subduction zones.

Laboratory shearing experiments on a suite of synthetic and natural fault gouges demonstrate that clay- and mudstones sampled from major fault zones, including the San Andreas Fault and the Costa Rican and Nankai subduction megathrusts, are frictionally weak, with friction coefficients in the range µ = ~0.08-0.35. These fault rocks exhibit rate-strengthening behavior, which should lead to only aseismic creep. However, they become increasingly rate-weakening (and therefore potentially able to host unstable slip) with increasing quartz abundance, raising the possibility that clay transformation, primarily through the by-product of silica mobilization, may play a role in controlling the upper aseismic-seismic transition and its spatial variability or “patchiness” at some subduction margins.

Numerical models describing the kinetics of clay dehydration and associated fluid expulsion show that at both the Costa Rican and Nankai margins, earthquakes nucleate only in areas down-dip of the reaction, after it has peaked and is near completion. This behavior is robust, and it parallels variations in thermal structure along the strike of the Costa Rican margin. Although the number of observations is limited, the correlation further supports the idea that the onset of unstable slip is linked to clay transformation, likely through the combination of changes in rock frictional properties and the dissipation of the bound water source that allows effective normal stress to increase. Interestingly, the rupture areas of large (M>6-7) events extend into the zone of dehydration, suggesting the possibility that nucleation may be restricted to depths where the clay reaction has progressed, but that slip may propagate up-dip into weak and potentially overpressured regions where the dehydration reaction is peaking. Finally, these models can also be used to predict the evolution of pore fluid composition within subducting and accreting sediments due to the release of bound (fresh) water and the desorption of fluid-mobile elements like B and Li. Comparison of simulated pore fluid compositions in these source regions with observations at seafloor seeps and in shallow drillholes provides evidence for long-distance (10’s of km) up-dip fluid migration along fault zones.

De Ford Lecture Series: G. Burch Fisher

  Start: February 25, 2016 at 4:00 pm     End: February 25, 2016 at 5:00 pm

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