蛇紋岩斷層帶內的橄欖石與頑火輝石可為地震破裂指標;Olivine and Enstatite in the Serpentinite-Bearing Fault Zone as a Seismic Indicator

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Title:	蛇紋岩斷層帶內的橄欖石與頑火輝石可為地震破裂指標;Olivine and Enstatite in the Serpentinite-Bearing Fault Zone as a Seismic Indicator
Authors:	吳惟馨;Wu, Wei-Hsin
Contributors:	地球科學學系
Keywords:	橄欖石;頑火輝石;蛇紋岩;脫水作用;旋剪;排水;地震指標;Olivine;Enstatite;Serpentinite;Dehydroxylation;Rotary-shear;Fluid drainage;Earthquake indicator
Date:	2022-07-06
Issue Date:	2022-10-04 11:11:33 (UTC+8)
Publisher:	國立中央大學
Abstract:	蛇紋岩斷層帶中，橄欖石（與頑火輝石）在前人研究中被視為曾經發生地震的指標。本研究為了解淺部隱沒帶環境中，蛇紋岩脫水作用（dehydroxylation）以及其對斷層強度的影響，我們分別在排水與不排水條件下，對含飽和水的蛇紋岩粉末施加10個百萬帕的正向應力，並以地震滑移速度進行旋剪摩擦試驗。另外，我們同時在距離滑動面約1.5毫米處插入熱電偶來測量實驗中斷層泥的溫度演化。結果顯示，在不排水條件下，斷層泥伴隨著壓縮，其視摩擦係數（apparent friction coefficient）會從峰值（peak）約0.32-0.33降至穩態（steady-state）約0.09-0.12，並在實驗結束時其溫度的最高測量值達到180°C。而在排水條件下，斷層泥伴隨著膨脹與溫度的上升（測量值從350°C上升到450°C），其視摩擦係數也會從一個平坦狀（plateau-like）的峰值約0.37-0.42降至穩態約0.19-0.32。然後，隨著斷層泥的壓縮與溫度的持續上升（測量值在實驗結束時達到最高溫約為635°C），其視摩擦係數會再上升到約0.29-0.55。綜合偏光顯微鏡、熱場發式掃描式電子顯微鏡、同步輻射X光繞射儀，以及聚焦離子束系統－穿透式電子顯微鏡的分析結果，只有在排水條件的斷層泥主要滑動帶（principal slip zone）裡，才能發現熔融（frictional melt）與蛇紋石的脫水產物。因此，我們推論在不排水條件下，孔隙水的熱增壓作用（thermal pressurization）除了會導致斷層泥的弱化，也會抑制溫度的上升使得蛇紋岩的脫水作用無法發生。而在排水條件下，伴隨著溫度的上升，蛇紋岩脫水作用會促成力－熱－化學增壓作用（mechanical-thermal-chemical pressurization）並導致弱化。隨著水份排出主要滑動帶，形成高黏滯度的熔融並促使斷層泥摩擦係數的再強化（frictional restrengthening）。故本研究的結論認為，蛇紋岩同震的脫水作用只有在排水條件下可以發生，並可以橄欖石與頑火輝石的形態被保存下來，成為我們判斷蛇紋岩斷層帶古地震的指標（paleoseismic indicators）。此外，脫水所導致的熔融，在地震破裂傳遞的過程中可能成為一個摩擦係數上的屏障（frictional barrier），阻止繼續破裂的繼續發生。;Olivine (and enstatite) have been regarded as evidence of paleo-earthquakes within serpentinite shear zones. To investigate the serpentinite dehydroxylation and its influences on the fault strength in shallow subduction zone earthquake-like environments, we performed rotary-shear friction experiments on water-saturated serpentinite powders at a seismic slip rate at 10 MPa normal stress, either in fluid-drained or undrained conditions. Gouge temperature (T) was measured by a thermocouple ~1.5 mm from the slipping surface. Results showed that, in undrained experiments, the apparent friction coefficient dropped from a peak value of ~0.32-0.33 to a steady-state value of ~0.09-0.12, accompanied by gouge compaction and reached a max. T of ~180°C at the thermocouple by the end of the experiment. Drained experiments also displayed a drop from a plateau-like peak value of ~0.37-0.42 to a steady-state value of ~0.19-0.32, associated with gouge dilation and an increase of temperature from ~300°C to ~450°C. The friction coefficient then restrengthened to a value of ~0.29-0.55 with gouge compaction, and recorded a max. T of ~635°C at the thermocouple by the end of the experiment. Integrate microanalytical results from the polarizing microscope, thermal-emission field scanning electron microscope, focused ion beam-transmission electron microscope, and synchrotron X-ray diffraction showed frictional melts and serpentine dehydroxylated products only formed in the principal slip zone (PSZ) of drained experiments. As a result, we suggest that for undrained conditions, thermal pressurization of the pore fluid would lead to frictional weakening and buffered the temperature rise to below that required for serpentine dehydroxylation. However, in drained conditions, with increasing temperature, dehydroxylation of serpentine would first cause mechanical-thermal-chemical pressurization of pore fluid and lead to frictional weakening. As water moves out of the PSZ, frictional recovery would happen by the formation of viscous melts. We conclude that frictional melts (mixed with olivine and enstatite) generated behind rupture fronts, could not only likely become frictional barriers for the ongoing seismic slips but also be preserved as paleoseismic indicators, helping the recognition of paleoseismic events.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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