Rheological transitions in high-temperature volcanic fault zones
Rheological transitions in high-temperature volcanic fault zones
复制标题
高温火山断层带的流变转变
DOI：
10.1002/2014jb011532
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发表时间：
2015
影响因子：
--
通讯作者：
Osamu Sasaki
中科院分区：
文献类型：
--
作者：
Satoshi Okumura;Kentaro Uesugi;Michihiko Nakamura;Osamu Sasaki
硅质岩浆在上升过程中经历了剪切诱导的脆性破裂，导致在导管边缘形成岩浆断层。一旦断裂形成，断裂的摩擦行为控制着岩浆的上升过程。利用同步辐射X射线照相技术，在800℃和900℃的温度下，对岩浆断层泥的扭转变形进行了原位观测。扭转变形速率设置为0.110 rpm，对应的等效滑移速度为2.27RPM× × 10−5-1.74 × 10−3M −1，剪切应变率为0.014-1.16 S−1，所用正应力为1，5和10 Mpa。岩浆断裂在玻璃化转变温度以上仍表现为摩擦滑动和粘性流动。摩擦滑动和粘性流动之间的转换取决于温度、变形速率和断层上的法向应力。在90 0℃时，断层在10 Mpa的法应力下表现为粘性变形，而在80 0℃时以摩擦滑动为主，提出了断层愈合与变形时间尺度之比作为摩擦滑动与粘性流动过渡的判据。实验标定的判据推断，在爆炸喷发过程中，大约500 m深度以摩擦滑动为主，这可能解释了岩浆在没有有效排气的情况下快速上升的原因。摩擦加热反过来会促进断层愈合，导致从摩擦滑动到粘性流动的反向转变，随后岩浆上升的减速。因此，摩擦滑动和粘性流动之间的循环转变可能是对熔岩喷流循环行为的一种解释。
Silicic magma experiences shear‐induced brittle fracturing during its ascent, resulting in the formation of a magmatic fault at the conduit margin. Once the fault is formed, frictional behavior of the fault controls the magma ascent process. We observed torsional deformation of a magmatic fault gouge in situ at temperatures of 800 and 900°C using synchrotron radiation X‐ray radiography. The torsional deformation rate was set at 0.1–10 rpm, corresponding to equivalent slip velocities of 2.27 × 10−5–1.74 × 10−3m s−1and shear strain rates of 0.014–1.16 s−1. The normal stresses used were 1, 5, and 10 MPa. The magmatic fault showed frictional sliding as well as viscous flow even above the glass transition temperature. The transition between frictional sliding and viscous flow depends on temperature, deformation rate, and normal stress on the fault. At 900°C, the fault showed viscous deformation at a normal stress of 10 MPa, while frictional sliding was predominant at 800°C. We propose the ratio of timescales of fault healing and deformation as a criterion for transition between frictional sliding and viscous flow. The experimentally calibrated criterion infers that frictional sliding is predominant from ~500 m in depth during explosive eruption; this may explain rapid magma ascent without efficient outgassing. Frictional heating would in turn enhance fault healing, resulting in the reverse transition from frictional sliding to viscous flow, followed by deceleration of magma ascent. Therefore, cyclic transitions between frictional sliding and viscous flow are a possible explanation for the cyclic behavior of lava effusion.