TRIPLE JUNCTIONS AND RIDGE COLLISIONS: NORTH AMERICA

The western margin of North America preserves a record of multiple triple junction interactions since 60Ma. The best-studied is the fault-fault-trench Mendocino Triple Junction (MTJ), presently located in northern California. Since the interaction of the East Pacific Rise with the subduction margin around 30Ma, the MTJ has migrated northward with the Pacific plate by more than 1000km. Passage of the MTJ is associated with a change from subduction to transform tectonics as the San Andreas fault system lengthens in its wake. Subduction-related volcanism ceases, and is replaced with relatively short-lived pulses of bimodal volcanic activity that track the past motion of the triple junction. Seismic data collected onshore and offshore the MTJ region allow elucidation of some of the active processes, but also raise questions about the system. The subducting slab is observed as expected to the north of the MTJ, and its edge can be identified in the seismic velocity structure, in gravity data, and by changes in seismicity. More surprisingly, the seismic velocity structure to the south shows low velocity (<6km/s) Franciscan terrane accretionary rocks overlaying a high velocity (~7km/s) lower crustal layer: Simple models for a slab window suggest that the accretionary terrane rocks should directly overlay a mantle with reduced seismic velocity, whereas evidence for low upper mantle velocities is also equivocal. Thickening of the crust around the southern edge of the slab has been explained by drag of mantle circulation, but may equally be due to the complex nature of the San Andreas plate boundary: strike-slip motion is distributed on three faults over almost 100km onshore of the main San Andreas Fault, implying north-south convergence near the MTJ. Since such complexities arise from inherited features, and from the three-dimensionality of the tectonic processes, they are likely at other triple junctions also.