5:15 PM - 7:15 PM
[SSS10-P15] Seismic Fault Repetition Estimated from Pseudotachylyte in Eidsfjord, Northern Norway
Introduction: Pseudotachylytes, are fault rocks that record seismogenic fracturing and deformation structures, and thus their formation processes provide crucial insights into earthquake generation processes. Since pseudotachylyte is likely to have overlapped different late-stage events, such as retrograde metamorphism and brittle and/or plastic deformation, a comprehensive understanding of the formation and development process requires careful observation of the fracturing and deformation structure. In the Eidsfjord region of Northern Norway, the lower crust is widely exposed, and pseudotachylytes associated with crustal-scale detachment faults have been identified (e.g., Markl, 1998). This study aims to clarify the microstructural and chemical characteristics of minerals in anorthosites containing multiple pseudotachylyte injection veins, elucidate their formation process and consider the deformation processes in the seismogenic regions based on estimating the formation temperature conditions of the pseudotachylyte veins.
Methods: The details of microstructures in the deformed anorthosites were observed using a polarization microscope and a scanning electron microscope (SEM). Mineral chemical composition analysis was conducted using an energy-dispersive X-ray spectrometer (EDS). The formation temperature of the pseudotachylyte veins was estimated mainly using pyroxene chemistry based on pyroxene thermometer (Lindsley, 1983). Furthermore, the phase equilibrium calculation program MELTS was used to model the melting process associated with pseudotachylyte formation.
Results: The constituent minerals of the host anorthosite include plagioclase, orthopyroxene, clinopyroxene, orthoclase, quartz, biotite, magnetite, ilmenite, and rutile. In the pseudotachylyte, plagioclase, clinopyroxene, orthoclase, and magnetite which are considered to be crystallized from the melt are observed. Three types of pseudotachylyte (Pt-1, Pt-2, Pt-3) with significantly different mineral sizes and morphologies, as well as a fine-grained zone with notable fracturing, are identified. No significant differences were found in the chemical composition of the matrix portions of the three types of pseudotachylyte. Pt-1, which exhibits a decussate texture indicative of rapid crystal growth, cuts through the fine-grained zone, while Pt-2 truncates both. Additionally, Pt-1, Pt-2, and the fine-grained zone are displaced along the same fracture, while Pt-3 is found along fractures or near the boundary between Pt-2 and the host rock. The pyroxene thermometer results show that the crystallization temperatures of pyroxene in the host rock and pseudotachylyte are ~600 °C and ~1000 °C, respectively.
Discussion: The presence of three different pseudotachylytes in the anorthosite sample, which differ significantly in mineral size and morphology, and their structural features, which show a clear order of formation, suggest that at least three melt injections have occurred. However, the lack of significant differences in the chemical composition of the matrix portions of the pseudotachylyte implies that fracturing and melting repeatedly occurred within the brittle regime of the shear zone. The crystallization temperature of pyroxene in the pseudotachylytes (~1000 °C) represents the temperature at which pyroxene crystallized from the pseudotachylyte melt, meaning that the melt formation temperature must have been higher. Using MELTS, the energy required to melt the host rock was estimated to be at least 27.14 MJ/m3. We will discuss earthquake magnitude and other factors based on the estimated melting energy.
References: Markl G (1998) NGU-Bull 434, 53-75; Lindsley DH (1983) American Mineralogist 68, 477-493.
Methods: The details of microstructures in the deformed anorthosites were observed using a polarization microscope and a scanning electron microscope (SEM). Mineral chemical composition analysis was conducted using an energy-dispersive X-ray spectrometer (EDS). The formation temperature of the pseudotachylyte veins was estimated mainly using pyroxene chemistry based on pyroxene thermometer (Lindsley, 1983). Furthermore, the phase equilibrium calculation program MELTS was used to model the melting process associated with pseudotachylyte formation.
Results: The constituent minerals of the host anorthosite include plagioclase, orthopyroxene, clinopyroxene, orthoclase, quartz, biotite, magnetite, ilmenite, and rutile. In the pseudotachylyte, plagioclase, clinopyroxene, orthoclase, and magnetite which are considered to be crystallized from the melt are observed. Three types of pseudotachylyte (Pt-1, Pt-2, Pt-3) with significantly different mineral sizes and morphologies, as well as a fine-grained zone with notable fracturing, are identified. No significant differences were found in the chemical composition of the matrix portions of the three types of pseudotachylyte. Pt-1, which exhibits a decussate texture indicative of rapid crystal growth, cuts through the fine-grained zone, while Pt-2 truncates both. Additionally, Pt-1, Pt-2, and the fine-grained zone are displaced along the same fracture, while Pt-3 is found along fractures or near the boundary between Pt-2 and the host rock. The pyroxene thermometer results show that the crystallization temperatures of pyroxene in the host rock and pseudotachylyte are ~600 °C and ~1000 °C, respectively.
Discussion: The presence of three different pseudotachylytes in the anorthosite sample, which differ significantly in mineral size and morphology, and their structural features, which show a clear order of formation, suggest that at least three melt injections have occurred. However, the lack of significant differences in the chemical composition of the matrix portions of the pseudotachylyte implies that fracturing and melting repeatedly occurred within the brittle regime of the shear zone. The crystallization temperature of pyroxene in the pseudotachylytes (~1000 °C) represents the temperature at which pyroxene crystallized from the pseudotachylyte melt, meaning that the melt formation temperature must have been higher. Using MELTS, the energy required to melt the host rock was estimated to be at least 27.14 MJ/m3. We will discuss earthquake magnitude and other factors based on the estimated melting energy.
References: Markl G (1998) NGU-Bull 434, 53-75; Lindsley DH (1983) American Mineralogist 68, 477-493.