52 research outputs found
Slab temperature evolution over the lifetime of a subduction zone: Model input files
No full textThe input files necessary for all of the numerical subduction models contained within Holt and Condit (in review at G-cubed). In addition to the input parameter (.prm) files, the repository contains an ASPECT 2.1.0 plugin (modified version of the visco_plastic rheology module) and python scripts (.py) to create the input geometries (.txt files) that are loaded into the model parameter files.
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Reference:
Holt, A. F., and Condit, C. B. Slab temperature evolution over the lifetime of a subduction zone. In revision for Geochem., Geophys., Geosys
Subduction Zone Blues: Laboratory and Field Constraints on the Rheology and Deformation Mechanisms of Mafic Blueschist at the Subduction Interface
No full textThesis (Ph.D.)--University of Washington, 2025Subduction zones are key drivers of plate tectonics, facilitating crustal recycling and deformation. The associated geological hazards of these convergent margins, including megathrust earthquakes and tsunamis, pose significant risks to society. Although these hazards are generated by frictional, seismic slip along the shallow subduction interface, aseismic, ductile deformation plays a critical role by modulating the strength of the subducting slab and transferring stress up-dip to load the seismogenic zone. During subduction, progressive metamorphic transformations of mid-ocean ridge basalt to blueschist and, ultimately, to eclogite alters the chemical and mechanical properties of the subducting crust through changes in its mineralogy, volatile content, density, and rheological strength. Field observations and limited experimental work have offered some insights into the evolution of subducting oceanic crust—such as a strength hierarchy estimating blueschist to be weaker than both gabbro (basalt) and eclogite—but a substantial knowledge gap persists. This thesis addresses this knowledge gap by investigating key questions regarding subduction zone dynamics including: (1) Does blueschist generate an observable seismic signal that can improve seismic imaging of blueschist along the subduction interface? (2) What mechanism(s) accommodates blueschist deformation along the subduction interface down-dip of the seismogenic zone and what are the implications for the seismic-aseismic transition? (3) How do chemical and mechanical changes at the blueschist-eclogite transition influence interface subduction zone dynamics during the subduction and exhumation of HP/LT lithologies?
Electron backscatter diffraction (EBSD)-based petrofabric analysis was applied to model the seismic anisotropy generated by a suite of mafic blueschists exhumed from a range of peak P-T conditions relevant to the ductilely deforming interface in active subduction zones. The blueschists displayed a broad range of P-wave anisotropies (AVp) up to 20%, correlating positively with the abundance and deformation-produced crystallographic preferred orientation (CPO) of the sodic amphibole mineral, glaucophane. Modeled AVp magnitudes were commonly ~10%, suggesting a common blueschist seismic anisotropy signal with potential to improve mapping of the extent and deformation of blueschists along the subduction interface.
To investigate the underlying deformation that generates observed seismic anisotropies, EBSD techniques—including a novel technique coupling weighted Burgers vector and misorientation analyses—were applied to glaucophane in a lawsonite blueschist from the Catalina Schist that was exhumed from P-T conditions down-dip of the seismogenic zone. The results of this investigation reveal deformation of blueschists at these conditions to be primarily accommodated by the dislocation creep mechanism. A suite of deformation experiments were performed on glaucophane aggregates to derive a constitutive, power-law relationship (flow law) for dislocation creep in glaucophane and, by extension, blueschist. Extrapolation of flow laws for glaucophane dislocation creep and related subduction lithology flow laws to natural conditions predict deformation by dislocation creep in the subducting slab initiates at ~350°C, evolving to grain-size-sensitive mechanisms with increasing temperatures and pore fluid pressures or finer grain sizes.
Detailed microstructural and petrological investigation were conducted on an exhumed blueschist-eclogite transition on Vroulidia Beach, Sifnos Island, Greece to investigate the complex interplay between deformation, metamorphism, and aqueous fluids in during the subduction and exhumation of blueschists and eclogites. This investigation demonstrates that fluid-deformation feedbacks promote zones of weakness that further localize deformation and fluid flow to enhance retrogression in such zones during exhumation. Together, these investigations advance our understanding of deformation and metamorphic processes in blueschists, eclogites, and at the blueschist-eclogite transition and subduction zone dynamics
Photomicrographs for "Structure and properties of the Cascadia plate interface: evidence from a newly-described exhumed paleomegathrust in the Olympic Subduction Complex"
No full textStraboMicro project associated with GSA Bulletin manuscript "Structure and properties of the Cascadia plate interface: evidence from a newly-described exhumed paleomegathrust in the Olympic Subduction Complex" by Anna M. Ledeczi, Harold J. Tobin, Tsai-Wei Chen, Sean R. Mulcahy, and Cailey B. Condit. DOI: 10.1130/B38335.
Field Data for "Structure and properties of the Cascadia plate interface: evidence from a newly-described exhumed paleomegathrust in the Olympic Subduction Complex"
No full textStraboField project associated with GSA Bulletin manuscript "Structure and properties of the Cascadia plate interface: evidence from a newly-described exhumed paleomegathrust in the Olympic Subduction Complex" by Anna M. Ledeczi, Harold J. Tobin, Tsai-Wei Chen, Sean R. Mulcahy, and Cailey B. Condit. DOI: 10.1130/B38335.
DEFORMATION PARTITIONING ALONG AN IDEALIZED SUBDUCTION PLATE BOUNDARY AT DEEP SLOW SLIP CONDITIONS
No full textStratigraphy, structure, and plutonism in the Wiscasset-Dresden region of mid-coastal Maine
No full textin Berry, Henry N., IV, and West, David P., Jr., editors, Guidebook for field trips along the Maine coast from Maquoit Bay to Muscongus Bay: New England Intercollegiate Geological Conference, p. 165-182https://digitalmaine.com/mgs_publications/1029/thumbnail.jp
Slip partitioning along an idealized subduction plate boundary at deep slow slip conditions
No full textBelow the base of many subduction seismogenic zones, the plate interface periodically slips at rates 1 to 2 orders of magnitude faster than tectonic plate velocities. A number of competing hypotheses exist to explain the mechanisms for these slow slip events (SSEs), but they remain incompletely tested because we do not know how deformation is partitioned across the lithologically complex plate boundary interface. We use the deepest exposure of the Arosa zone, a ∼520 m-thick exhumed subduction interface, as a case study to evaluate the partitioning of strain between lithologic units throughout the SSE cycle. We review and synthesize published constitutive relations for the five lithologic units present to express shear stress as a function of deformation rate. We use these results to predict (1) the shear stress across the plate boundary as a function of slip velocity and (2) the partitioning of deformation among the different lithologic units for SSE and aseismic creep velocities. We conduct this analysis for pore fluid pressures from hydrostatic to near-lithostatic. Our results show that, at pore fluid pressure close to hydrostatic, aseismic creep and SSE velocities occur by viscous deformation of calcareous and quartzose units. However, once the pore fluid pressure increases above 80% of lithostatic, plate boundary slip migrates from the calcareous and quartzose rocks during aseismic creep to frictional deformation of talc schist during slow slip. This result is insensitive to differences in the thicknesses of metasedimentary units that may be present along subduction plate boundaries and, therefore, may apply to subduction plate boundaries in general
Fracturing, fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana, <scp>USA</scp>
No full textFRACTURING, FLUID FLOW, AND DEEP CRUSTAL SHEAR ZONE NUCLEATION IN PALEOPROTEROZOIC METAGABBRO, SW MONTANA
No full textDating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization
No full textWe present coupled textural observations and trace element and geochronological data from metasomatic monazite and zircon, to constrain the timing of high-grade Na-metasomatism (albitization) of an Archean orthogneiss in southwest Montana, USA. Field, mineral textures, and geochemical evidence indicate albitization occurred as a rind along the margin of a ~3.2 Ga granodioritic orthogneiss (Pl + Hbl + Kfs + Qz + Bt + Zrn) exposed in the Northern Madison range. The metasomatic product is a weakly deformed albitite (Ab + Bt + OAm + Zrn + Mnz + Ap + Rt). Orthoamphibole and biotite grew synkinematically with the regional foliation fabric, which developed during metamorphism that locally peaked at upper amphibolite-facies during the 1800–1710 Ma Big Sky orogeny. Metasomatism resulted in an increase in Na, a decrease in Ca, K, Ba, Fe, and Sr, a complete transformation of plagioclase and K-feldspar into albite, and loss of quartz. In situ geochronology on zoned monazite and zircon indicate growth by dissolution–precipitation in both phases at ~1750–1735 Ma. Trace element geochemistry of rim domains in these phases are best explained by dissolution–reprecipitation in equilibrium with Na-rich fluid. Together, these data temporally and mechanistically link metasomatism with high-grade tectonism and prograde metamorphism during the Big Sky orogeny.Funding for this work was provided by NSF grant (EAR1252295) to Maha
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