INITIATION AND EVOLUTION OF A TRANSFORM PLATE BOUNDARY: ONE SLAB WINDOW BECOMES TWO, MIGRATING TRANSPRESSION-TRANSFORM PROGRESSION, SHUFFLING BY MULTIPLE FAULT STRANDS

The transition from eastward subduction to a dextral transform plate boundary recorded by the geology of coastal California from ca. 30 Ma to the present has been well studied, but some details have received less attention. Collectively the details of this transition may help identify similar transform initiation events in the history of various orogenic belts. The transition began with the arrival of the Pacific-Farallon spreading center at the Farallon-North America trench, owing to the tangential trajectory of the Pacific Plate to the western margin of North America. The arrival of the spreading center resulted in creation of a slab window with associated elevated geothermal gradients and magmatism. The transform margin formed and lengthened at the expense of the subduction zone with northward migration of a fault-fault-trench triple junction and the southward migration of a ridge-trench-fault triple junction. Simply envisioned, this resulted in a progressively-growing triangular slab gap, but the lack of magmatism in the central part of this gap suggests that upwelled asthenosphere in the window cooled with time to form new lithosphere separating two distinct active slab windows, one associated with each triple junction. The southern window is associated with the stalling of a ridge (ie eastern margin of Pacific Plate) at the trench, whereas the northern one is associated with the northward migration of the southern edge of the subducting Gorda (formerly part of the Farallon) slab. If the transform plate boundary formed as a single dextral fault, the original spatial-temporal distribution of slab window magmatism would be preserved, but the dextral plate boundary has evolved with multiple parallel strands which moved older slab window products north of and outboard (west) of younger ones. The multi-stranded fault system has resulted in a large-scale northward migrating restraining step-over that transfers slip from the northward-propagating tip regions of the inboard dextral faults to the triple junction. This suggests that transpressional deformation (superimposed on earlier subduction-related structures), precedes more purely strike-slip kinematics. The multiple dextral faults subsequently shuffle the spatial-temporal deformation pattern.