#7 - Deep-crustal Deformation: An Experimental and EBSD-based Study of Amphibole’s Seismic Anisotropy

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Abstract

Seismic anisotropy is a powerful tool for imaging the deep crust and identifying deformation structures at depth. However, interpreting these geophysical signals remains difficult due to a lack of geologic and mechanic constraints on their formation. Strong lattice preferred orientations (LPOs) of anisotropic minerals create significant seismic anisotropy, and the formation of these LPOs is intimately related to the style and conditions of deformation. We investigate anisotropy and LPO formation in amphibole, a common anisotropic deep crustal mineral, using deformation experiments and EBSD analysis. Two fine-grained, pure amphibole samples were experimentally hot-pressed for 16 hours at deep crustal conditions (1.5GPa, 750C). Subsequently, one sample was deformed using a Griggs-type apparatus at strain rates of while the other was recovered undeformed.

Microstructural observations show that both the undeformed and deformed samples exhibit strong LPOs. The undeformed sample shows minimal intragranular misorientations, suggesting that grains underwent rigid body rotation during-hot pressing, where elongated grained oriented themselves with the long [001] axis along the shear plane which developed a moderate LPO (J-Index: 2.39). However, the deformed sample has an even stronger LPO (J-Index: 2.75), and exhibited significant intragranular misorientations and subgrain formation, indicative of dislocation activity. Hence, it appears that the initial, rigid- body rotation induced LPO is subsequently modified by creep. The resultant calculated P-wave seismic anisotropy (AVp) for the deformed sample is 9.9%, while the undeformed sample’s AVp is 10.4%. In the deformed sample, LPO variation with grain size was also investigated. Large grains (>2µm) exhibit a stronger LPO and 10.6% AVp, while the fine grains have a moderate LPO and 8.1% AVp. This work demonstrates that amphibole is susceptible to rigid- body rotation induced LPO formation due to its crystal habit, with deformation modifying LPO only mildly under the explored conditions. In all, hot pressing and deformation of amphibole at deep crustal conditions lead to strong LPOs and seismic anisotropy, suggesting that deformed amphibole may be a significant contributor to deep crustal seismic anisotropy.

 
Nov 2nd, 10:20 AM Nov 2nd, 11:30 AM

#7 - Deep-crustal Deformation: An Experimental and EBSD-based Study of Amphibole’s Seismic Anisotropy

Cleveland Ballroom

Seismic anisotropy is a powerful tool for imaging the deep crust and identifying deformation structures at depth. However, interpreting these geophysical signals remains difficult due to a lack of geologic and mechanic constraints on their formation. Strong lattice preferred orientations (LPOs) of anisotropic minerals create significant seismic anisotropy, and the formation of these LPOs is intimately related to the style and conditions of deformation. We investigate anisotropy and LPO formation in amphibole, a common anisotropic deep crustal mineral, using deformation experiments and EBSD analysis. Two fine-grained, pure amphibole samples were experimentally hot-pressed for 16 hours at deep crustal conditions (1.5GPa, 750C). Subsequently, one sample was deformed using a Griggs-type apparatus at strain rates of while the other was recovered undeformed.

Microstructural observations show that both the undeformed and deformed samples exhibit strong LPOs. The undeformed sample shows minimal intragranular misorientations, suggesting that grains underwent rigid body rotation during-hot pressing, where elongated grained oriented themselves with the long [001] axis along the shear plane which developed a moderate LPO (J-Index: 2.39). However, the deformed sample has an even stronger LPO (J-Index: 2.75), and exhibited significant intragranular misorientations and subgrain formation, indicative of dislocation activity. Hence, it appears that the initial, rigid- body rotation induced LPO is subsequently modified by creep. The resultant calculated P-wave seismic anisotropy (AVp) for the deformed sample is 9.9%, while the undeformed sample’s AVp is 10.4%. In the deformed sample, LPO variation with grain size was also investigated. Large grains (>2µm) exhibit a stronger LPO and 10.6% AVp, while the fine grains have a moderate LPO and 8.1% AVp. This work demonstrates that amphibole is susceptible to rigid- body rotation induced LPO formation due to its crystal habit, with deformation modifying LPO only mildly under the explored conditions. In all, hot pressing and deformation of amphibole at deep crustal conditions lead to strong LPOs and seismic anisotropy, suggesting that deformed amphibole may be a significant contributor to deep crustal seismic anisotropy.