A mid-crustal contractional stepover zone in a major strike-slip system, North Cascades, Washington

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The segmentation of strike-slip faults at the Earth's surface is well known, but the geometry of these faults in the mid-crust is poorly understood, as are the intervening stepover zones. A mid-crustal example of a segmented strike-slip fault is present within the > 500-km-long Ross Lake fault system where the NW-striking Twisp River fault zone and Gabriel Peak tectonic belt occur as overlapping, left-stepping, dextral strike-slip shear zones. The contractional stepover between these structures is occupied by the Black Peak batholith, which is riddled by ductile shear zones that are typically 0.5–1.5-m thick and are restricted to the stepover. Gently to moderately NW-dipping, reverse-slip ductile shear zones in the center of the stepover strike at a high angle to the Twisp River fault zone and Gabriel Peak tectonic belt and lie in an appropriate position to transfer most of the displacement between the strike-slip segments.The orientation of the reverse-slip ductile shear zones relative to the strike-slip segments is broadly similar to that of folds and thrusts in upper-crustal stepovers. The stepover width is relatively large, however, in comparison to many active stepovers. Emplacement of the Black Peak batholith across an originally narrower stepover or bend in the fault system may have created a rigid buttress and subsequent deformation jumped to the mechanically weak southwest margin of the body. Because plutons are commonly intruded into strike-slip zones, such partitioning of later displacements may be widespread.