The Queen Charlotte Fault system is a segment of the North America - Pacific
plate boundary. From 45 Ma - 5 Ma, plate motion has been primarily translational.
Since 5 Ma, transpression has been the dominant mode of interaction. The plate
boundary west of the Queen Charlotte Islands is characterized by an approximately 30-
km wide terrace, flanked to the west by a topographic trough and to the east by the
seismically active Queen Charlotte Fault. At 53.4°N the fault bends eastward and the
terrace becomes wider and discontinuous, forming triangular shaped highs and
intervening lows.
Approximately 300 km of multichannel seismic reflection and gravity data along
and across the Queen Charlotte Fault off Dixon Entrance were collected as part of the
ACCRETE experiment in 1994. Structural interpretation of the five new profiles
reveals the presence of faults and folds within the terrace, which form an angle of 20°
to the strike of the Queen Charlotte Fault. The direction of these structures
corresponds to the trend of the plate boundary south of the bend and west of the Queen
Charlotte Islands, implying that through complex compression and shear, material must
have been carried from south to north along the margin during oblique plate motion.
Based on this observation and on forward gravity modeling, which places limits on the
possible plate configuration at depth, a four-dimensional model has been developed to
explain the temporal and spatial evolution of structural styles in this region.
Considering the amount of shortening that must be accommodated within the
past 5 Ma (a maximum of 100 km), a model of an underthrusting Pacific plate is
preferred over one of pure upthrusting. About 5-6 Ma ago, when transpression began,
oceanic crust was flexed and thrust upward at the plate boundary to eventually reach a
steady-state configuration of a subducting slab. Fractured basement rock and
consolidated, deformed sediments underlie the terrace and form its foundation. As a
result of strain partitioning, the terrace is now decoupled and moves both parallel to the
continent and perpendicular to the underthrusting Pacific plate. North of the bend in the
Queen Charlotte Fault, underthrusting north of it occurs obliquely along preexisting
fractures at the base of the terrace. The repetitive pattern of triangular terrace slivers is
the result of continuing uplift and shear along these trends. Active tectonism influences
sediment dispersal and creates traps.
A N-S trending fault was also identified in the trough segment and possibly
involves oceanic basement. Its origin is thought to be due to distributed shear that was
transmitted across the plate boundary. Sea-floor spreading magnetic anomalies trend
north-south as well. Along these zones of weakness, synthetic strike-slip faults of a
transpressional strain ellipse could has been initiated during early stages of subduction.
Reactivation of such faults may occur when oceanic crust approaches the outer terrace
boundary, as is the case in the study region.
Gravity modeling confirmed the existence of thin (24 km) continental crust and
an increase in oceanic Moho dip beneath the terrace, which is topped by unconsolidated
sediments and underlain by material of near-basement densities. It could not be
determined using gravity modeling whether oceanic crust exists beneath the continent,
but if it does, it must be welded to the North American plate in shallow subduction. / Graduation date: 1998
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37218 |
| Date | 29 May 1997 |
| Creators | Scheidhauer, Maren |
| Contributors | Trehu, Anne M. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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