EXAMPLES OF NARROW STRIKE-SLIP FAULTS THROUGH THE CRUST INDICATED BY GEOLOGIC AND GEOPHYSICAL EVIDENCE

One of the major debates over the three-dimensional geometry of major strike-slip faults focuses on width of the deformation zone with depth, particularly below the seismogenic zone. The traditional view of very localized slip on anastomosing strands in the brittle zone and progressively wider diffuse deformation with depth implies strain does not remain highly localized below the brittle-plastic transition. Another debate is how oblique slip is accommodated at depth: does the fault remain subvertical with oblique displacement or do low-angle high strain zones develop to transfer the oblique component away from the strike-slip fault? We integrate geologic studies with geophysical results from major strike-slip faults in Alaska and Argentina to examine these questions.

Our structural, metamorphic and geochronologic studies document that amphibolite facies rocks exhumed in ancient and active strike-slip fault systems show narrow (<100 m) sub-vertical mylonite-ultramylonite zones at the boundaries between rock units with strongly contrasting tectonic history. We interpret the ultramylonite zones as the subvertical continuation of the main slip strand of the brittle fault. Zones of low-angle faults that have similar metamorphic and thermochronologic histories often are closely associated with the subvertical fault but are not found to offset the main strand. Other examples show narrow (< 2 km wide) lozenges of amphibolite grade rocks with mylonitic boundaries juxtaposed along brittle faults within an overall brittle fault zone. These examples require a narrow zone of focused high strain connecting the mid-lower crust directly with the anastomosing fault strands in the brittle crust.

Several lines of geophysical results support the interpretation that the faults can remain subvertical through the crust. The main lines of evidence include truncated magnetic anomalies and offset Moho boundaries that occur directly beneath the surface expression of major high-angle faults. These are more common on active systems but also occur beneath ancient lineaments, indicating that these crustal-scale vertical offsets can persist in the lithosphere over 100s of millions of years.