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High-resolution mapping of an extensive pseudotachylyte system in the Ikertôq shear zone of southwestern Greenland shows that the occurrence and style of interconnected pseudotachylyte-bearing faults are influenced by the lithology of the host rocks. The Ikertôq shear zone is a frontal structure of the Paleoproterozoic Nagssugtoquidian Orogen of which includes a 50 kilometer long pseudotachylyte system. Pseudotachylytes are vein-like rock melts that formed as a result of friction in shear zones, and are considered a proxy for paleo-earthquakes. These structures give insight into seismic behavior in the mid-upper crust. As part of an NSF REU, field observations were collected and laboratory analysis will be performed using electron microprobe, optical petrology, and scanning electron microscope (SEM) instrumentation. Our mapping of the lithologic units and boundaries along a transect of Sarfannguit Island identified lithologies varying from mafic intrusions, gabbroic pods, tonalite, felsic and intermediate gneisses, and metasedimentary rocks. Field observations such as rock unit descriptions, lithologic logs and maps show that the degree of foliation in a gniess has significant influence on the development of pseudotachylyte and fault geometry. Areas with well-foliated gneiss are characterized by a complex geometry of throughgoing pseudotachylyte-bearing faults, damage zones, imbricate wedges, and relay faults. In areas with less-foliated, thicker-banded gneiss, pseudotachylytes are less abundant and exhibit a less complex Riedel geometry. Our ongoing work will focus on gaining a better understanding of the petrology and tectonic setting of the transect.
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BRITTLE IMBRICATE WEDGES CONTROL OUTCROP-SCALE GEOMETRY OF PSEUDOTACHYLYTES IN THE IKERTÔQ SHEAR ZONE, GREENLAND: AN NSF REU STUDY
Previous studies have described the geometry of pseudotachylyte-bearing faults in the Ikertôq shear zone (ISZ) as “paired shears.” This study found these are more complex structures formed in preexisting, wedge-shaped, imbricate damage zones. As part of an NSF REU, this project conducted outcrop-scale mapping in part of the 50-km ISZ pseudotachylyte system on Sarfannguit Island in southwestern Greenland. The pseudotachylyte system is comprised of master oblique-reverse faults concordant to strongly foliated host gneisses linked through discordant strike-slip relay faults. Within the imbricate wedges, pseudotachylytes are complexly distributed around rotated and folded gneissic blocks between stacked systems of master reverse faults. This study mapped five imbricate wedges using high resolution drone imagery in map view and hand photography on vertical outcrops. This resulted in a new geometrical three-dimensional perspective. Wedges form where foliation is platy, typically between more coherent hanging wall and footwall blocks. Preliminary calculations indicate average rotations of eight to eighteen degrees within the damage zones. Field measurements suggest the upper fault in the imbricate wedge is approximately planar, while the lower fault splays off the upper fault at a twenty to thirty degree angle, creating a concave-up cuspate geometry. Both faults have the same shear sense, with fold axes and pseudotachylyte slickenlines indicating reverse oblique offset, usually with a component of dextral shear. Initial deformation and brecciation of the blocks is interpreted as forming prior to the pseudotachylyte-forming event.
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- Award ID(s):
- 1950839
- PAR ID:
- 10526514
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Flow connectivity between master and relay faults in the Ikertôq shear zone demonstrates that multiple ruptures during ancient earthquakes occurred during a single seismic event. The Ikertôq shear zone (ISZ) is part of the Paleoproterozoic Nagssuqtoqidian orogeny continental collision in West Greenland that includes a > 50 km pseudotachylyte system. As part of an NSF REU, this team mapped various faults throughout a 2 km transect on high-resolution UAV images of exhumed pseudotachylyte vein systems on the western end of Sarfannguit island to investigate the kinematics of multi-fault ruptures during individual seismic events. Pseudotachylyte veins exhibit a complex rupture geometry with linked kinematics between oblique reverse master faults striking approximately 240 and steep east-west relay faults dominated by strike-slip movement. Near complete exposure of veins provide a unique opportunity to document fault linkages and the partitioning of slip, including the interconnectivity of flow patterns of melt in pseudotachylyte veins, as well as angular ladders of melt. We measured the thickness of pseudotachylyte fault veins and injection veins along transects to examine slip partitioning between multiple reverse faults and strike-slip relay faults. Melt thickness is used as a proxy for earthquake slip since the pseudotachylyte melt occurred on faults that exhibit preexisting brittle displacement. The results of preliminary calculations from energy balance equations show that typical slip on some oblique reverse master faults was on the order of a meter or less, while typical slip on some east-west relay faults was cm scale. Our data clarify that most slip occurred on oblique reverse master faults with subsidiary slip on east-west relay faults.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1130/abs/2022AM-380430
