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Structural Overview
The area of Porcupine Lake Valley in the northeastern Brooks Range (NEBR)
is at a major structural transition between symmetric detachment folds
that are characteristic of the NEBR proper, and asymmetric thrust truncated
folds resembling those along the main axis of the Brooks Range. Lisburne
Group carbonates in the western end of Porcupine Lake Valley are locally
folded into strongly asymmetric NE striking and plunging (detachment?)
folds characterized by short, steep to overturned forelimbs, and long (up
to 1 km) gently dipping backlimbs. Only one thrust fault was documented
in the NW end of the field area that places a long, relatively flat panel
of Lisburne Group carbonates above the Sadlerochit Group. NE and
NW striking normal faults with relatively small displacements cut folds
in West Porcupine Lake Valley. |
Cross section through the field area. Note fold asymmetry and
normal faulting.
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Some Preliminary Observations on Fracturing
Four major sets of extension fractures were documented in West Porcupine
Lake valley, the majority of which dip steeply between 60º-90º
in both directions: 1) a N-S striking set; 2) an E-W striking set; 3) a
N-S to NW striking set; and 4) a NE striking set. While the relative
timing of each of these fracture sets is unclear, some generalities can
be made. The NW set appears to be younger than the N-S and E-W sets.
E-W fractures terminate against N-S fractures at most sample locations.
However, the opposite relationship was documented elsewhere in the field
area, possibly suggesting multiple generations of N-S and E-W fracturing.
All three fracture sets were found in en echelon sets of extension fractures,
which indicate a component of shear during formation. Shear sense
on these sets was commonly normal or strike slip, suggesting that many
fractures are related to normal faulting in the area. The N-S and
NW striking fractures were often found in 3-5 meter wide swarms of en echelon
fractures, each swarm spaced approximately 10-20 meters apart. NE
striking fractures were well developed in the lower portions of one of
the major synclines in the area, although the timing of these fractures
is unclear. Other major mesoscopic-scale structures indicate some
period of penetrative semi-ductile deformation, including dissolution cleavage,
deformed crinoid stems, sheared stylolites, and elongated and transposed
chert nodules. |
Generalized mechanical stratigraphy of the Kayak Shale, Saderochit
and Lisburne Groups in the field area.
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Normal Faulting: A major complication?
Normal faulting in West Porcupine Lake Valley is atypical for the NEBR,
and may have influenced fracture character and distribution. Cross
cutting relationships suggest that NE striking faults occurred after thrusting,
whereas folds truncated by hinge sub-parallel normal faults suggest that
normal faulting may have occurred during folding, or may have significantly
modified fold geometries after a previous phase of compressional deformation.
Changes in fold geometry were observed across NW striking normal faults,
suggesting that either the normal faulting modified fold geometries, or
that these faults originated as transverse structures and developed a normal
sense of shear during or after folding.
Questions to be addressed
The goal of this project is to understand the relationship between fracturing,
faulting and folding in West Porcupine Lake Valley. Some important
questions to be addressed are: did folds in the field area form as detachment
folds or fault propagation folds, and how does each of these fold models
influence fracturing? Conversely, can we use fracture distribution
to understand the kinematics of fold formation? Another important
question is how normal faulting has affected fracturing and folding in
the area, and whether or not fractures related to folding can be distinguished
from those related to faulting. In order to answer these previous
questions, it will be important to understand how lithology and bed thickness
affect fracturing, since changes in these two variables affect fracture
spacing within the stratigraphy. |
Photograph of the stratigraphy in West Porcupine Lake Valley.
View is toward the northeast at the northwestern most Lisburne in the field
area. The full thickness of the upper Lisburne is not shown here.
Partial sections of the upper Lisburne are shown either side of the normal
fault. The upper and lower portions of the lower Lisburne tended
to be both recessive in their weathering patterns and mechanically weaker
than the upper Lisburne or the middle portion of the lower Lisburne.
Note the thrust, which places Lisburne and Kayak Shale above the Sadlerochit
group.
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Suppe and Medwedeff's (1990) model for fault propagation folding.
Note that rock layers in the anticlinal limbs must pass through the hinges.
Deformation is often localized in hinges, hence areas that have passed
through hinges might be expected to have higher fracture densities if hinge
migration occurred under brittle conditions.
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Simplified models for detachment folds. Detachment unit is stippled.
Fixed hinge folding requires that the limbs rotate with respect to relatively
fixed hinges. Migrating hinge folding requires that the hinges migrate
through the upper unit (circle shows a single point in the competent unit).
Note that the detachment volume must migrate to fill the core of the fold
in both cases. (Modified from: Homza and Wallace, 1995)
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Animation of fixed hinge detachment folding according to Homza
and Wallace's (1995) model. Note that hinges remain relatively fixed
with respect to the dark blue (competent) layer, although they do have
to migrate slightly.
Animation courtesy of Paul Atkinson, completed in association with his
UAF master's thesis:
(Masters Thesis: A Geometric Analysis of Detachment Folds in
the Northeastern Brooks Range, Alaska, and a Conceptual Model for Their
Kinematic Evolution, May 2001)
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UAF
Department of Geology and Geophysics
Geophysical
Institute