PSEUDOTACHYLYTES AND RELATED BRITTLE FAULTS FROM THE IKERTÔQ THRUST ZONE, GREENLAND: FAULT GEOMETRY AND KINEMATICS

Pseudotachylytes from the Ikertôq thrust zone in western Greenland were first characterized and mapped by Grocott (1981) as dextral and sinistral strike-slip faults concentrated within three 4–5-km-long brittle fault zones. The work introduced the classic concept of the pseudotachylyte generation zone as a system of paired shears bounding a complex damage zone; the Ikertôq exposures were subsequently interpreted as a model for sidewall ripouts in strike-slip faults (Swanson, 1989). Our ongoing mapping of these faults on Sarfannguit and outlying islands in the Ikertooq fjord has extended the known length of the pseudotachylyte system to more than 13 km, defined a previously unknown fourth fault zone SE of the main system, and led to a revised kinematic model.

In the 2015 field season, we studied the southern three of the four fault zones. Offset dikes and slickenlines with steep rakes preserved in pseudotachylyte show that the faults primarily developed as dextral oblique reverse faults with top-to-SE displacement concordant with ~240/50˚NW straight-banded foliation in Archean and Paleoproterozoic host gneisses. In some vertical exposures, we observed m-scale, subhorizontal fault ramps that cut foliation and demonstrate these kinematic relationships. The NW-dipping faults are commonly linked by steep, dextral strike-slip relay faults that strike 280˚ for 1–4 m to accommodate a right-stepping fault geometry. Displacement on these faults controls the distribution of thick (4–15 cm) pseudotachylyte fault veins and spectacular 10–80-cm-thick pseudotachylyte breccia zones.

The pseudotachylyte-bearing faults are dispersed across each of the 100–250-m-wide fault zones with a typical density of 1–5 faults/10 m. Many faults are localized at the margins of 0.2–3-m-thick Paleoproterozoic Kangâmiut mafic dikes that are subparallel with gneissic foliation. On a broader scale, the four fault zones are localized within high-strain zones that strike 240˚ and are characterized by transposed foliation and mesoscopic isoclinal and sheath folds with moderately W-plunging axes. Gneisses between the high strain zones strike 220–230˚ with locally shallower dips (40˚). We also documented subordinate veins of older mylonitic pseudotachylyte indicating a long history of localization of seismicity in the Ikertôq zone.