Complex multi-fault earthquake rupture recorded by pseudotachylytes from the Ikertôq shear zone, western Greenland

Recent seismological observations demonstrate that earthquakes can accommodate failure on multiple fault strands during a single event. For example, the 2010 M_w7.2 El Mayor-Cucapah earthquake exhibited both source complexity attributed to multi-fault rupture, and a system of kinematically diverse surface ruptures (Fletcher et al. 2016). A fundamental understanding of geologic controls on complex rupture at the source is limited since the record of incremental strain in exhumed faults is usually erased. Here, we examine the geometry and kinematics of pseudotachylyte (pst) in the Ikertô q shear zone (ISZ) at the outcrop and multi-km scales as an exhumed record of rupture interaction with pre-existing structural heterogeneities in a high-grade gneiss terrane.

The ISZ is a >15-km-wide zone of high-strain, N-NW-dipping charnockites, orthogneisses, and metasedimentary rocks deformed in the Paleoproterozoic Nagssugtoqidian orogen in western Greenland. Our ongoing geologic mapping shows pst to be concentrated within at least six ~100-m-scale fault zones distributed across a width of 10 km at a length scale >34 km. Individual earthquakes are recorded by pst fault-vein systems consisting of fault veins (foliation parallel; ~240˚/52˚) and accompanying dm- to m-scale damage zones recording off-fault melt injection. Offset dikes, slickenlines, and brushlines document top-S-SE kinematics as reverse and dextral oblique reverse faults (~47˚/305˚ plunge/trend). These are spaced at 2-5 fault-vein systems/10 m, and connected by ~280˚/80˚ strike-slip lateral ramps (slickenlines ~07˚/313˚ and 14˚/104˚). Melt connectivity shows earthquake rupture generated multi-story stacking of at least 2-6 reverse faults at a time connected in 3D by m- to dam-scale lateral ramps.

Complexity of the fault-vein systems, as well as the positioning of pst breccias and lateral ramps is correlated to fabric heterogeneity in the host. Examples include variations in layer thickness and gentle to close folding of competent hanging-wall gneisses. These observations show earthquake rupture can be kinematically diverse and geometrically complex at the source. This indicates faults in initially heterogeneous host rock do not require aseismic tectonic wear to create through-going planar structures prior to hosting earthquakes.