龙门山断裂带大地震孕震环境的岩石磁学证据

地震断裂带的孕震环境对于研究地震的发生至关重要.本文以汶川地震断裂带科学钻探2号(WFSD-2)钻孔岩心中的假玄武玻璃、碎裂岩及其围岩为研究对象,通过岩石磁学测试,并结合显微结构观察探讨龙门山断裂带大地震的孕震环境.WFSD-2岩心碎裂岩中的假玄武玻璃是龙门山断裂带曾经发生过大地震活动的岩石学证据,假玄武玻璃具有高磁化率特征,而碎裂岩的磁化率值与围岩相似,假玄武玻璃中的新生磁铁矿是其高磁化率值异常的重要原因之一.假玄武玻璃中较少量的新生磁铁矿暗示了假玄武玻璃的生成环境为含氧量较低的高温还原环境.大地震断裂的摩擦热是改变龙门山断裂带中假玄武玻璃岩石磁学特征的主导因素,流体作用较弱.无高磁化率异常的碎裂岩经历的温度小于300℃,推测假玄武玻璃的生成深度较深.WFSD-2岩心中20余层假玄武玻璃脉体证明映秀—北川断裂带是一条长期活跃的断裂带,龙门山断裂带上曾经发生了多次大地震断裂活动,这些大地震可能发生在深度较大、流体作用较弱的还原性孕震环境中.

The seismogenic setting of earthquake fault zones is important for seismological research. To investigate the seismogenic environment of earthquakes in the Longmen Shan fault zone, we carried out microstructural and rock-magnetic analyses of pseudotachylyte, cataclasite and its wall rocks within cores from the Wenchuan earthquake Fault Scientific Drilling borehole 2 (WFSD-2). Pseudotachylyte, observed in a cataclasite zone, attests to that large earthquakes have occurred in the Longmen Shan fault zone. It has higher magnetic susceptibility values than cataclasite, while cataclasite has similar values as its wall rocks. The newly-formed magnetite contributes to the higher values of magnetic susceptibility of pseudotachylyte. In addition, a little newly-formed magnetite in the pseudotachylyte indicates that pseudotachylyte was formed at high temperature and a reducing environment. Frictional heating is the main factor which changes the rock magnetic properties of pseudotachylytes in the Longmen Shan fault zone, while fluids play a minor role. The cataclasite has never been subjected to heating over 300 degrees C in nature, which implies that the pseudotachylye was formed at depth. More than 20 layers of pseudotachylytes demonstrate that the Yingxiu-Beichuan fault zone is a long-term active fault, and ancient powerful earthquakes have occurred repeatedly in the Longmen Shan fault zone. These earthquakes most likely occurred in deep seismogenic environment, with strong reducing and weak fluid activity.