PSEUDOTACHYLYTES FROM THE IKERTÔQ STEEP BELT, GREENLAND: FIELD RELATIONS AND MICROSTRUCTURE

Pseudotachylyte-bearing faults of the Ikertôq steep belt in SW Greenland were first mapped and described by Grocott (1981; J. Struct. Geol.). We revisited the exposures in 2013 and documented the field relations and microstructure of the fault rocks. Thin sections of pseudotachylyte exhibit ample evidence for high-temperature melt generation including the presence of microlites and lithic clasts with melt-filled, intragranular fractures. Microlites include plagioclase feldspar crystals up to 1 mm in length, and 1-5 micron opaque octahedral iron oxides. The microlite domains in the matrix are zoned subparallel to the wall rock and are concentrated in the center of the veins suggesting the presence of chilled vein margins. In the field, pseudotachylyte is closely associated with cataclasite and brittle faults in at least three NE-striking, 100-200-m-wide fault zones hosted by granodioritic gneisses with ~ 240/60°N foliation and subparallel mafic dikes and pegmatites. Pseudotachylytes within these fault zones were previously inferred to be dextral strike slip with a possible vertical component of slip. This interpretation was derived from geologically limited observations of separations on 2-dimensional surfaces and by consideration of an inferred Riedel fracture geometry. We observed structural trends that mirror those of Grocott (1981), including the observation that most pseudotachylyte zones are concordant with the host-rock foliation. However, we also noted the presence of slickenlines on barren fault surfaces and embedded into the walls of some pseudotachylyte-bearing faults. Preliminary analysis of these, along with separations noted in the field indicate that the primary pseudotachylyte zones are top-to-S reverse faults with a component of dextral oblique slip, rather than strike-slip faults. The primary pseudotachylyte-bearing faults are connected by subvertical, E-W-striking dextral relay faults that cross-cut foliation. Overall, most brittle deformation appears to be localized at lithologic contacts, especially at dike margins where they are in contact with apparently rigid gneisses, suggesting a rheologic and possible mineralogic control on generation of frictional melts.