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DEFORMATION WITHIN A BLUESCHIST FACIES METACHERT ALONG A PALEOMEGATHRUST:��IMPLICATIONS FOR DEFORMATION MECHANISMS FOR DEEP SLOW SLIP

Meghomita Das (McGill U) [meghomita.das@mail.mcgill.ca]

Supervisor: Dr Christie Rowe (McGill U)

In Collaboration with: Dr Matty Mookerjee (SSU), Dr Jamie Kirkpatrick (McGill U)

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DEEP SLOW SLIP ALONG THE SUBDUCTION ZONE

Angel Island in the Franciscan Subduction Complex, hosts Early to

Late Cretaceous meta-sedimentary and metabasic blueschist facies rocks that

are faulted along a remnant of a paleomegathrust horizon in the Bay Area.

P~8 kbar, T~300-350°C

(Rowe and Griffith, 2015)

(Kirkpatrick et al., 2021)

What outcrop to grain scale structures might participate in tremor and slow slip in this environment?

Earthquakes exist across a range of Slip Rate and Rupture velocity. I will focus on the intermediate family of events termed as slow slip events. They happen in every active settings including SZ. However they happen at a depth that is inaccessible so we cannot study them directly. One way to look at them is to use exhumed rocks that come from that depth region and study them as an analogue to understand what is going on down at those depths.

For this study, we chose Franciscan subduction complex and specifically Angel Island where a part of the deep subduction plate interface is preserved with PT constraints of XX (quite cold). There is a lot of literature about structures that can generate slow slip in the rock record with this model summarizing the need for brittle-ductile coeval structures with different lithologies and length structures. However, even this model is a best candidate scenario and we should start by surveying the structures that we expect at this depth first.

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PERLES BEACH THRUST, ANGEL ISLAND: A PALEOMEGATHRUST

~5-10m prograde shear zone at blueschist facies stability conditions
Stilpnomelane-riebeckite chert mylonite
Sharp localized shear surface with a block-in-matrix structure

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Chloritic shear zone; ~5m

Serpentinized landslide deposit

Chert mylonite

Being in the quartz plasticity zone with chert as the main rock type, can we leverage the tools used for

quartz microstructures interpretations to get insights about the deformation environment of deep slow slip?

W

E

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FIELD EVIDENCE OF POTENTIAL SLOW SLIP STRUCTURES

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Mutually cross-cutting brittle ductile features such as boudinaged chert blocks wrapped by mylonitic foliation

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FIELD EVIDENCE OF POTENTIAL SLOW SLIP STRUCTURES

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Localised slip surfaces in the fault zone and along block contacts

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FIELD EVIDENCE OF POTENTIAL SLOW SLIP STRUCTURES

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En-echelon tension gashes oblique to foliation

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E

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DEFORMATION IN META-CHERT MYLONITES

Riebeckite meta-chert

Riebeckite-Stilpnomelane meta-chert mylonite

75cm

150 cm

225 cm

300 cm

Increasing fabric planarity toward shear zone core

Fault Zone

0 cm

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RIEBECKITE-STILPNOMELANE CHERT

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Grain size ~2-5 microns

10m away from the shear zone

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RIEBECKITE METACHERT

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Core-mantle textures in quartz (Qz)

Pinned Qz by riebeckite (Rbk)

Sub-grains and sutured grain boundaries in Qz

Rbk needles and aggregates

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RIEBECKITE STILPNOMELANE METACHERT MYLONITE

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Quartz (Qz) domain

Riebeckite (Rbk) domain

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EBSD ANALYSIS OF FABRICS IN METACHERT AND MYLONITE

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M-index: 0.0153

J-index: 1.2353

mean = 15.05

n = 918

M-index: 0.2098 J-index: 3.1060

Mean = 64.2

n =381

M-index: 0.0136

J-index: 1.3618

Mean = 65.49

n =1952

Reducing M & J indices and grain size

Weaker LPO

Switch to grain size sensitive creep from dislocation creep (?)

I have just started looking at the EBSD data from these samples and here we have a side to side comparison between the metachert and the mylonite. We did 2 zones in the metachert (one with qz and one with riebeckite and qz). We see a larger grains that are more strained and finer grains that are strain-free so some evidence of recrystallization there. Both show a strong maxima in the c-axis which could indicate dislocation creep in qtz. In the mylonite one, we see more low strain tiny grains and reduction in grain size relative to the metachert. The strength of LPO appears to become weaker in the mylonite which could indicate that other grain size sensitive creep mechanisms might be taking over from dislocation creep. However, we still need to probe more about what is the real trend in grain size from the main wall rock to the mylonite version.

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TAKEAWAYS AND FUTURE WORK

Co-eval mixed-mode deformation structures in exhumed blueschist plate boundary
Increase in fabric planarity towards the shear zone core with change in grain size.
Riebeckite and stilpnomelane appear to accommodate slip along the foliation
LPO stronger in the metachert hints at dislocation creep with a switch to other deformation mechanisms in the mylonite

Questions to be answered:

How did ductile deformation mechanisms change during mixed brittle-ductile slip?
Interplay between the grain size trend and pinning and dynamic recrystallization
Can we constrain the stress during tremor and slow slip events?

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REFERENCES

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ACKNOWLEDGEMENTS

Research permit granted by California State Parks. McGill is located on land which served as a site of meeting and exchange amongst the Haudenosaunee and Anishinaabeg nations. Angel Island is located on land which served as the site of hunting and gathering for Coast Miwok tribe. We honor, recognize, and respect these nations as the stewards of the lands and waters on which we meet today.

Questions?

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Wakabayashi, J., 1992. Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California. The Journal of Geology 100, 19–40.
Wakabayashi, J., 1992. Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Francis
Wahrhaftig, C., 1984. A streetcar to subduction and other plate tectonic trips by public transport in San Francisco, Rev. ed. ed. American Geophysical Union, Washington, D.C.
Wakabayashi, J., Rowe, C.D., 2015. Whither the megathrust? Localization of large-scale subduction slip along the contact of a mélange. International Geology Review 57, 854–870. https://doi.org/10.1080/00206814.2015.1020453
Lay, T., Kanamori, H., Ammon, C.J., Koper, K.D., Hutko, A.R., Ye, L., Yue, H., Rushing, T.M., 2012. Depth‐varying rupture properties of subduction zone megathrust faults. Journal of Geophysical Research: Solid Earth 117. https://doi.org/10.1029/2011JB009133
Cross, A.J., Prior, D.J., Stipp, M., Kidder, S., 2017. The recrystallized grain size piezometer for quartz: An EBSD-based calibration. Geophysical Research Letters 44, 6667–6674. https://doi.org/10.1002/2017GL073836
Stipp, M., Tullis, J., 2003. The recrystallized grain size piezometer for quartz. Geophysical Research Letters 30. https://doi.org/10.1029/2003GL018444
Rowe, C.D., Griffith, W.A., 2015. Do faults preserve a record of seismic slip: A second opinion. Journal of Structural Geology 78, 1–26. https://doi.org/10.1016/j.jsg.2015.06.006