The Monashee Complex is a structural culmination which exposes rocks from the
lowest stratigraphic levels of the Canadian Cordillera. The Monashee Complex is
subdivided into two lesser structural culminations; the Frenchman Cap and Thor–Odin
culminations. The lithostratigraphic succession of the Thor–Odin Culmination is
completely transposed by penetrative isoclinal folds with amplitudes from microscopic
(<1 mm) to regional (10’s km). Lower structural levels are occupied by Proterozoic
gneisses and migmatites of the Monashee basement assemblage. These are infolded with
overlying metasedimentary rocks of the Monashee cover assemblage, which are
Proterozoic to possibly Paleozoic in age. The basement and cover assemblages were
subsequently intruded by Eocene granitic pegmatite, aplite and lamprophyre dykes.
Regional metamorphism of the basement and cover assemblages reached upper
amphibolite to lower granulite facies.
The northeastern portion of the Thor–Odin Culmination of the Monashee
Complex contains a suite of structures and fabrics, which are classified into four sets,
based on their interpreted kinematic significance. These are: 1) transposition related
structures (DT); 2) open, upright folds (DO); 3) exhumation related structures (DE); and 4)
brittle faults (DB). Each successive set of structures exerted a control on the geometry of
the next set. The large-scale geometry of the culmination is an interference structure
between DT folds, a DE arch and high-strain zones, and a DB brittle horst.
Early, DT fold style varies from intrafolial isoclinal “mature” style folds to
upright or inclined asymmetric “immature” folds. This continuum of fold styles, along
with evidence of anticlockwise rotation (looking down a vertical axis toward the shear plane) of fold axes and lineations is interpreted as being a result of penetrative triclinic
non-coaxial flow. DO upright, symmetrical folds overprint early structures and fabrics,
but are only preserved at low structural levels in the culmination where the DE coaxial
stretching overprint is weak. DE normal shear bands and boudins overprint all earlier
structures. A complex high-strain zone, the Thor–Odin High-Strain Zone, outcrops at
high structural levels and along the margins of the culmination. The Thor–Odin High-
Strain Zone developed as a result of material moving away from the crest of the
culmination, outwards toward the flanks.
Eocene brittle faults (DB) and fractures within the Thor–Odin Culmination of the
Monashee Complex are divisible into three distinct sets. Initial 340–010º trending strikeslip
faults (Set 1) were locally overprinted and reactivated by normal faults with a 325–
020º trend (Set 2). A third set of 255–275º trending fractures (Set 3) are interpreted as
conjugates to Set 1, reactivated as transfer faults to the Set 2 normal faults. Large
regional faults weather recessively forming topographic lineaments that transect the
Monashee Complex. The Victor Creek Fault defines one such lineament. Detailed
mapping within the northern Thor–Odin Culmination, reveals piercement points (fold
hinges) on the east side of the fault, which are not readily matched on the west side. The
minimum displacement required on the Victor Creek Fault to down-drop the fold hinge
below the level of exposure on the west side is 1370 m, assuming normal down-to-the
west displacement. However, the geometry of the fault is consistent with a Set 1 dextral
strike-slip fault. Matching the piercement points in the study area with possible
equivalents to the north indicates 55–60 kms of dextral strike-slip displacement.
The Monashee Reflection (MR) is a major crustal-scale, cross-cutting reflection
appearing on two mutually perpendicular Lithoprobe seismic profiles in the southern
Omineca Belt of the Canadian Cordillera. It has previously been interpreted as the downplunge
extension of an arched regional ductile thrust fault, the Monashee Décollement,
and is described as separating the Monashee Complex from the overlying Selkirk
Allochthon. Recent mapping demonstrates that this boundary is not a discrete ductile
thrust, but rather transposed and gradational. Overprinting the transition zone is a
complex, outward-dipping, normal, structure; the Thor−Odin High-Strain Zone.
Three alternative 3-D geometric models have been developed for the MR in order
to project the reflection to the surface. The favoured model correlates the surface trace of
the Thor−Odin High-Strain Zone with MR.
Normal shear sense kinematics are interpreted for the MR based on: 1) the
overall geometry and asymptotic relationship between the MR and reflections in the
hanging wall and footwall; 2) offset of metamorphic and geochronological gradients,
consistent with an extensional zone, rather than with thrust fault interpretation and 3) the
cross-cutting nature of the MR is consistent with normal structures throughout the region.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:UNB.1882/941 |
| Date | January 2007 |
| Creators | Kruse, Stefan |
| Contributors | Williams, Paul F. |
| Publisher | University of New Brunswick |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation, Thesis or Dissertation |
| Format | 132159286 bytes, application/pdf |
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