Download Walker Creek right-lateral fault zone, central Rocky Mountains

Walker Creek right-lateral fault zone, central Rocky Mountains, British Columbia - southern
continuation of the Northern Rocky Mountain Trench fault zone
(M.E.McMechan, Geological Survey of Canada- Calgary, 3303 33rd St. N.W., Calgary, Alberta,
T3L 2A7; e-mail [email protected])
The Walker Creek fault zone (WCFZ) is a 2 km wide zone of right-lateral faulting exposed along the western
Rocky Mountains of central British Columbia near 54° (Fig. 1). In the Walker Creek area, the fault zone consists
of variably deformed, fault bounded slivers and lozenges of Neoproterozoic and Cambrian strata. Minor
structures, including subhorizontal stretching lineations, extensional shear bands, slickensides, displacement on
minor splay faults, and mesoscopic shearbands, found along and adjacent to individual block bounding faults
provide direct evidence of right-lateral displacement along many of the faults within the fault zone. Crenulation
cleavages have developed in rocks within and immediately adjacent to the Walker Creek fault zone. These indicate
right-lateral shear extended 500 m west of the westernmost mapped fault in the fault zone at Walker Creek. The
Walker Creek fault zone cuts obliquely across and is
younger than west-facing folds and a transverse tear
fault in the Snake Indian thrust sheet.
Offset
stratigraphic changes in correlative Neoproterozoic
strata indicate at least 60 km of right-lateral displacement
across the zone.
The Northern Rocky Mountain Trench is a prominent
physiographic feature that follows the trace of a major
right-lateral, strike-slip fault zone. South of Williston
Lake, the McLead Lake right-lateral strike-slip fault
splays with a more southerly trend from the Northern
Rocky Mountain Trench and fault zone (Fig. 1). Linear
drainages connect the most southerly exposures of the
Northern Rocky Mountain Trench fault zone and the
most northerly exposures of the Walker Creek fault
zone, and strongly suggest that the Walker Creek fault
zone is the southern continuation of the Northern Rocky
Mountain Trench fault zone. The Walker Creek fault
zone can be traced 100 km southward from Walker
Creek across the western edge of the Rocky Mountains
and into the floor of the southern Rocky Mountain
Trench. It shows that a through going, moderate
displacement, strike-slip fault system structurally links
the Northern Rocky Mountain Trench and the northcentral part of the Southern Rocky Mountain Trench. Strike-slip motion on the Walker Creek fault zone occurred
in the Upper Cretaceous to Early Eocene at the same time as northeast directed shortening in the fold and thrust
belt. Thus, oblique convergence in the eastern part of the south-central Canadian Cordillera was apparently
resolved into parallel, northwest trending zones of strike-slip and thrust faulting during the Upper Cretaceous to
Early Eocene. Net displacement of rocks southwest of the study area would be northerly. The Walker Creek
fault zone shows no direct evidence of strike-slip motion being transformed into increased shortening by thrust
faults. Splay faults along the eastern side of the Walker Creek fault zone are steeply dipping with little
stratigraphic separation. Crenulation cleavages developed adjacent to these faults indicate strike-slip displacement.
The southeasterly decrease in estimated strike-slip displacement and concurrent increase in fold and thrust belt
shortening from the central to southern Rocky Mountains (56° to 51° N) is probably the result of changes in
relative plate motions rather than transpressional kinematic conversion of strike-slip faulting to thrust faulting.
Biographical information: Margot McMechan has mapped in the Canadian Rocky Mountains and Foothills for over
20 years. She has published numerous papers on the structure and tectonics of the Rocky Mountains and
Proterozoic stratigraphy. Margot had the good fortune of doing her graduate work with Ray Price at Queen’s
University and then being employed by the Geological Survey of Canada. She has published 21 geological maps
that outline the wonderful geology of parts of the Canadian Rocky Mountains and Foothills.