skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: LITHOLOGIC INFLUENCE ON PSEUDOTACHYLYTE-BEARING FAULT ZONES IN THE IKERTÔQ SHEAR ZONE, GREENLAND: AN NSF REU STUDY
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.  more » « less
Award ID(s):
1950839
PAR ID:
10526508
Author(s) / Creator(s):
; ; ; ;
Corporate Creator(s):
Publisher / Repository:
Geological Society of America
Date Published:
Format(s):
Medium: X
Location:
https://www.google.com/search?q=10.1130%2Fabs%2F2022AM-380427&client=firefox-b-1-e&sca_esv=f1620cbea5ce6b40&ei=zqyiZuHXJ5Op5NoP1IuG-A8&ved=0ahUKEwjh9tDK-8KHAxWTFFkFHdSFAf8Q4dUDCA8&uact=5&oq=10.1130%2Fabs%2F2022AM-380427&gs_lp=Egxnd3Mtd2l6LXNlcnAiGTEwLjExMzAvYWJzLzIwMjJBTS0zODA0MjdI8wxQggRYggRwAXgBkAEAmAE7oAE7qgEBMbgBA8gBAPgBAvgBAZgCAaACCMICChAAGLADGNYEGEeYAwCIBgGQBgiSBwExoAdN&sclient=gws-wiz-serp
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; (, Geological Society of America)
    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. 
    more » « less
  2. ; (, Tektonika)
    Kavanaugh, J. (Ed.)
    Quantifying shallow fault zone structure and characteristics is critical for accurately modeling the complex mechanical behavior of earthquakes as energy moves within faults from depth. We examine macro- to microstructures, mineralogy, and properties from drill core analyses of fault-related rocks in the steeply plunging ALT-B2 geotechnical borehole (total depth of 493 m) across the San Gabriel Fault zone, California. We use macroscopic drill core and outcrop-sample analyses, core-based damage estimates, optical microscopy, and X-ray diffraction mineralogic analyses to determine the fault zone structure, deformation mechanisms, and alteration patterns of exhumed deformed rocks formed in a section of the fault that slipped 5-12 million years ago, with evidence for some Quaternary slip. The fault consists of two principal slip zones composed of cohesive cataclasite, ultracataclasite, and intact clay-rich, highly foliated gouge within upper and lower damage zones 60 m and 50 m thick. The upper 6.5 m thick principal slip zone separates Mendenhall Gneiss and Josephine Granodiorite, and a lower 11 m thick principal slip is enclosed within the Josephine Granodiorite. Microstructures record overprinted brittle fractures, cohesive cataclasites, veins, sheared clay-rich rocks, and folded foliated and carbonate-rich horizons in the damage zones. Carbonate veins are common in the lower fault zone, and alteration and mineralization assemblages consist of clays, epidote, calcite, zeolites, and chloritic minerals. These data show that shallow portions of the fault experienced fluid-rock interactions that led to alteration, mineralization, and brittle and semi-brittle deformation that led to the formation of damage zones and narrow principal slip zones that are continuous down-dip and along strike. 
    more » « less
  3. ; (, CSUNPosium: Annual Student Research and Creative Works Symposium)
    null (Ed.)
    Subduction zones are sites where converging tectonic plates create magma that is transported upward by faults (acting as conduits) within the crust, incrementally building the continent over time. However, how faults and their deep, ductile counterparts (shear zones) transport magma across the entire thickness of the crust is not well understood. This is important to investigate because faults remain as zones of weakness and may trigger large magnitude earthquakes (Barnes et al., 2016). Fiordland, New Zealand, has a system of interconnected faults and shear zones that transect the world’s best-preserved section of crust produced in a subduction zone. We will focus on shear zone rocks from the middle crust, where the shear zone narrows in width as the composition of the crust changes. We hypothesize that the change in crustal composition acted as a strength boundary within the shear zone, changing the geometry of strain to allow for more localized movement of magma and deformation. 
    more » « less
  4. ; ; ; (, Geological Society of America abstracts)
    Throughout her career, Professor Sharon Mosher has been a pioneer in the structural analysis of polydeformed rocks and regions. Her work on the evolution of superposed rock fabrics in complexly deformed areas, for example, has greatly improved our ability to determine how faults, shear zones, and orogens evolve over time. Traditionally, sequences of foliations, mineral lineations, folds, and other structural elements have been interpreted in terms of discrete, multiphase deformation events. However, alternative interpretations where structural sequences result from a single, progressive event also are common, especially where changes in stress fields or flow parameters result in non-steady deformation. Here, in honor of Professor Mosher, we present examples of three different types of structural sequences that formed in large seismogenic faults and shear zones in SW New Zealand and southern California. These examples illustrate the different ways in which multiple generations and styles of rock fabrics develop and become preserved in zones of localized deformation. The first example is from a large fault zone located inboard of the Puysegur subduction zone in Fiordland, New Zealand. This zone displays several generations of superposed fabrics that record a history of repeated reactivations over a few tens of millions of years. A second set of examples, from both Fiordland and southern California, illustrates how non-steady deformation can result in parallel ductile and brittle fabrics, including veins of pseudotachylyte, that formed during a single, progressive shearing event. The third example, also from Fiordland, shows how parallel rock fabrics in a large, lower crustal shear zone formed diachronously across a large region as the inboard and outboard belts of the Mesozoic Median batholith converged. Each of these examples displays different structural relationships among rock fabrics in the field. To decipher their histories, we combined structural data with 40Ar/39Ar and U-Pb (zircon, titanite) geochronology. The examples illustrate the utility of combining field observations with both direct and indirect isotopic 
    more » « less
  5. ; ; ; ; (, Geological Society of America)
    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
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email