Collaborative Research: Making the San Andreas Fault at the Mendocino Triple Junction

一个密集的宽带地震仪阵列正在被用来调查门多西诺三重连接点的结构，门多西诺三重连接点是加利福尼亚州著名的圣安德烈亚斯断层系统的最北端。研究区域从加利福尼亚-俄勒冈海岸延伸到西经120度，北纬37度到43度。门多西诺三重结合部是北美板块、太平洋板块和戈达板块交汇的地方，相对于固定的北美参考框架，随着时间的推移向北迁移。与迁徙中的门多西诺三联结有关的一系列相互作用过程，从地表延伸到软流圈水平，将北美岩石圈从卡斯卡迪亚的俯冲杂岩修改到圣安德烈亚斯转换板块边界，将加利福尼亚州沿海抛在后面。地幔通过控制板块强度和运动学以及从软流圈到北美板块的热量和质量通量，对MTJ附近发生的过程进行了主导控制。1993-94年广泛的活动震源研究结果在MTJ地区提供了很好的地壳控制。地震研究提供了对MTJ地区地幔和下地壳的结构、组构、地震和非地震变形的改进成像。戈尔达地壳和上面的沉积物俯冲，并与卡斯卡迪亚俯冲带的增生地体合并，随着这些块体流入新兴的转换边缘，并构造成圣安德烈亚斯走滑系统。与此同时，戈尔达板块南面的板块缝隙开口导致软流圈上升和减压熔融，这神奇地使圣安德烈亚斯断层附近的北美板块下沉。再加工和膨胀的岩石圈增厚，形成了一个小型的门多西诺角造山高原，而侵蚀则沿着受构造控制的北向河流将上地壳的大部分流向俯冲带。在附近，戈尔达-胡安德富卡俯冲导致了喀斯喀特火山，这也有助于大陆地壳的生长、分离和再循环。作为该项目的一部分，将开发的地壳和上地幔的三维地震速度模型将包括在加利福尼亚州北部和俄勒冈州南部的事件和强地面运动描述中，这两个地区可能发生毁灭性的大地震。

A dense array of broadband seismographs are being used to investigate the structure of the Mendocino Triple Junction, the northernmost terminus of California's famed San Andreas fault system. The study area extends from the California-Oregon coast to 120o W longitude, and from 37 to 43o North latitude. The Mendocino Triple Junction, the site where the North America, Paclific and the Gorda plates meet, migrates northward with time relative to a fixed North American reference frame. A set of interacting processes associated with the migrating Mendocino Triple Junction, extending from the surface to asthenospheric levels, act to modify North America lithosphere from a subduction complex at Cascadia to the San Andreas transform plate boundary, leaving coastal California in its wake. The mantle imparts a dominant control on the processes occurring near the MTJ through its control on plate strength and kinematics and the flux of heat and mass from the asthenosphere to the North American plate. Results of an extensive active source seismic study in 1993-94 provide excellent crustal control in the MTJ region. The seismic investigations provide improved imaging of the structure, fabric, and seismic and aseismic deformation of the mantle and lower crust in the MTJ region.. Gorda crust and overlying sediment subduct and become incorporated with accretionary terranes from the Cascadia subduction zone as this mass flows into the emerging transform margin and becomes structured into the San Andreas strike-slip system. Simultaneously, slab gap opening south of the Gorda slab causes asthenospheric ascent and decompression melting, which magmatically underplates North America near the San Andreas fault. The reprocessed and inflated lithosphere thickens to create a small Cape Mendocino orogenic plateau, while erosion fluxes large fractions of the upper crust back to the subduction zone along tectonically controlled north-trending rivers. Nearby, Gorda-Juan de Fuca subduction results in the Cascade volcanoes, which also contribute to continental crust growth, segregation and recycling. The 3-D seismic velocity models of the crust and upper mantle, which will be developed as part of this project, will be included in event and strong ground motion characterization for northern California and southern Oregon, a site of potentially devastating great earthquakes.