[S-CG58] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG58-14] Experimental study on the brittle-plastic transition in quartz-feldspar aggregates

*古川 美穂¹、Verberne Berend A.²、澤 燦道¹、武藤 潤¹、高橋 美紀²、長濱 裕幸¹ (1.東北大学大学院理学研究科地学専攻、2.産業技術総合研究所地質調査総合センター)

キーワード：脆性－塑性遷移、剪断変形実験、石英－長石混合体、地殻強度、微細組織、グーテンベルグ・リヒター則のb値

The frequent occurrence of large continental earthquakes in the brittle-plastic transition zone of the upper crust highlights the importance of understanding the deformation mechanism of crustal rocks for unraveling earthquake nucleation. Based on rock mechanics, the crustal strength is estimated to increase with increasing depth in the brittle region while decreasing with depth in the plastic region. This gives the maximum strength at the brittle-plastic transition zone, although the microphysical process of controlling the crustal strength remains unclear, especially for the polyphase crustal rocks. Aiming to reveal the deformation mechanism of the brittle-plastic transition of the upper crust, we performed shear deformation experiments on quartz-feldspar aggregates. Using a Griggs-type solid medium apparatus installed at Tohoku University, a 1:1 quartz:albite mixture was sheared under temperatures T ranging from 210 to 900 °C and confining pressures P_c ranging from 185 to 870 MPa (i.e., at simulated depths (z) of 7 - 30 km assuming a geothermal gradient of 30 °C/km and a pressure gradient of ~ 27 MPa/km). The shear strain rate was sequentially stepped between ~10^-3 /s and ~10^-4 /s. For z < 18 km, the shear stress increased with increasing depth and reached a maximum at z = 18 km condition (T = 540 °C, P_c = 477 MPa) with ~ 1300 MPa. For z > 18 km, the shear stress decreased with increasing depth and showed a strain weakening behavior. Postmortem microstructural analysis illuminates a gradual transition from dominant brittle to plastic deformation with increasing depth. Specifically, for z < 10 km (T < 300 °C, P_c < 265 MPa), quartz and albite grains are pulverized to a size of a few hundred nm, forming gouges. For z = 13 and 18 km (T = 390 - 540 °C, P_c = 344 - 477 MPa), quartz grains are fractured, while albite grains display elongation in the major axis direction of the strain ellipsoid. At z = 24 km (720 °C, 750 MPa), quartz porphyroclasts surrounded by bent fine grains constitute an S-C fabric. At z = 30 km (900 °C, 870 MPa), polygonal-shaped quartz grains of ~ 1 μm in size are observed likely formed by bulging recrystallization. Moreover, albite grains in z = 13 to 30 km exhibit stripes in shades of brightness on a backscattered electron image, suggestive of flow. The flow structures in z = 24 and 30 km develop parallel to the shear direction, which may have caused the strain weakening. We used image analyses to characterize the crack geometry including the box-counting fractal dimension D₀, which showed a monotonical decrease with increasing T and P_c. The fractal dimension of the fragment size distribution (D_S) also decreases with increasing P_c, which is consistent with D_S of a granitoid gouge produced by deformation experiments at high T and P_c (Keulen et al., 2007). Using the correlation between D₀ of pre-existing fault system and the b value of the Gutenberg-Richter law given by Nanjo et al. (1998), the b values in this study decrease monotonically with increasing shear stress for z < 18 km. This is consistent with the b value of earthquakes decreasing linearly with differential stress (Scholz, 2015).
Our mechanical and microstructural results represent the brittle-plastic transition of the continental upper crust, gradually taking place at the microscopic scale even under conditions where mechanical behavior implies brittle deformation. Detailed deformation mechanisms and potential influence for earthquake nucleation will be subjected to future work.

References
Keulen et al., 2007, JSG, doi:10.1016/j.jsg.2007.04.003
Nanjo et al., 1998, Tectonophysics, doi:10.1016/S0040-1951(98)80067-X
Scholz, 2015, GRL, doi:10.1002/2014GL062863