The possibility of an impact-related origin of quasi-conglomerate pseudotachylytes on Barra, Outer Hebrides, Scotland, an integrated study

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Awarding institution
  • University of Strathclyde
Date of award
  • 2021
Thesis identifier
  • T15830
Person Identifier (Local)
  • 201267856
Qualification Level
Qualification Name
Department, School or Faculty
  • This study investigates the origin of the quasi-conglomerate pseudotachylytes in the Lewisian Gneiss on Barra, Outer Hebrides, Scotland. These exceptionally large pseudotachylytes have either been ascribed to the Outer Hebrides Fault Zone (OHFZ) or considered to be at a distance from the OHFZ. The Barra quasi-conglomerates visually resemble the Sudbury Breccia and the Vredefort pseudotachylytes, both of which are situated in and related to two of the largest impact structures known on Earth. This thesis examines the evidence for an impact related origin of these enigmatic rocks.;A nested multi-scale mapping method was developed to capture the extent and structures of the pseudotachylytes. On the macro scale, outcrops in one square kilometre were mapped using a colour code for the pseudotachylyte mode of occurrence. Pseudotachylyte veins, networks and quasi-conglomerates were distinguished. Meso scale maps covered 10 x 10 cm, 30 x 30 cm or 1 x 1 m and were used to calculate the amount of pseudotachylyte (sensu stricto) versus clasts and host rock, with a resolution of up to a millimetre, and to document and illustrate structures and clast rotation. Thin sections were used for microstructural analyses by optical and electron beam microscopy. Stereographical data was collected to document a potential relationship between pseudotachylyte veining and pre-existing anisotropies. Geochemical analyses of the Sm / Nd and Rb / Sr isotopes were performed to exclude a fluid assisted process of quasi-conglomerate formation or a remobilisation during the later intrusion of the abundant dykes.;In the mapped area, all outcrops show some degree of pseudotachylyte occurrence. The quasi-conglomerates are found in two thirds of all outcrops and seem to occur in three quasi-continuous, foliation parallel zones. However, due to the outcrop situation, the preferential exposure of quasi-conglomerates on vertical outcrops, and their complicated three-dimensional nature and lack of a clear boundaries, it was not possible to map these zones with the methods available. Between 1 and 14% of the mapped square kilometre consists of pseudotachylyte sensu stricto. The meso and micro structures point to a formation of pseudotachylyte in situ rather than intrusion from an external generation locus.;There is a markable lack of finite shear displacement, apart from clast rotation. Pseudotachylyte formation appears to have been initiated by two separate processes: mafic minerals melted (though seemingly not by contact melting), producing a dark, opaque fluid characteristic for pseudotachylyte, and felsic minerals deformed cataclastically, forming a granular fluid. The two visually and geochemically different fluids mingled, creating schlieren or flow structures, and eventually mixed. The initial processes of mafic melting and felsic cataclasis continued to be initiated during progressive deformation. The results of the isotopic analyses confirm the formation of the quasi-conglomerates in a closed and dry system without later remobilisation.;The observations suggest that the quasi-conglomerates formed by seismic shaking rather than frictional (heat induced) melting along a (super) fault. Visually, in terms of size, and in terms of amount of pseudotachylyte sensu stricto, the quasiconglomerates are more similar to the pseudotachylytes from two of the largest impact structures on Earth than to any documented fault related ('tectonic') pseudotachylytes. There is a strong likelihood that the quasi-conglomerates formed during an impact cratering event.
Advisor / supervisor
  • Shipton, Zoe
Resource Type
Date Created
  • 2021
Former identifier
  • 9912970891502996