The relationship between fracturing, asymmetric folding, and normal faulting in Lisburne Group carbonates: West Porcupine Lake Valley, northeastern Brooks Range, Alaska
This page is meant to be a potpourri of interesting figures and explanations of what I'm currently working on. I'll try to keep it up to date.
|
Here's a figure I put together that shows the general pattern of fracturing in one of the more "open" folds in my field area. Note the heavy swarming in the N-S oriented fractures (blue). Swarming in the N-S fractures seems to show up in the fracture density data, as the density of N-S fracturing is highly variable. Also note the pervasive dissolution cleavage in the hinge area of the anticline, and general lack of fracturing at that location. Box plots of fracture spacing in this fold are shown below. |
 |
|
The figure to the left shows box plots of fracture spacing for each sample location. Blue labeled box plots correspond to N-S and/or NW striking fractures. Red and orange labeled box plots correspond to E-W striking fractures at each sample location. Statistical tests show that E-W fracture spacing is uniform from the long, flat backlimb of the anticline to the hinge of the syncline. If these fractures are related to folding (and not normal faulting, which was observed in the field area) then this observation has important implications for fold kinematics. Using fracture distribution and character as a record of deformation in folds can aid in understanding how folds are forming. By comparing kinematic models to observed fracture distributions, I hope to better understand how folds are forming. |
 |
|
The drastic difference in structural style between the Phillip Smith
Mountains and the rest of the northeastern Brooks Range is an important
question that directly relates to the kinematics of these asymmetric folds.
The Lisburne Group in the Phillip Smith Mountains is over 800 meters thick,
with 100 meters of Kayak Shale (detachment unit) below it. Compare
that to around 500 meters of Lisburne overlying 100-200 meters of Kayak
Shale in upright symmetric detachment folds to the north. A thinner
detachment unit has very important implications for detachment folding,
which might favor early truncation of detachment folds, or even favor fault
propagation folding.
|
 |
|
Current studies are addressing whether these asymmetric folds might have formed initially as upright symmetric detachment folds, and were later modified to form asymmetric folds. Other possibilities include asymmetric detachment folding, fault propagation folding, or some combination of both. Using models developed by Paul Atkinson, it is possible to test whether there is enough Kayak Shale to produce detachment folds with observed geometries, or if thickening of the Lower Lisburne is an important process in fold development. Note the ratios of initial unit thickness to upper unit thickness in the animation to the right. With a 3:1 ratio of competent to incompetent thickness, the fold "bottoms out" at low inter limb angles. The ratio of competent to incompetent unit thickness in the Phillip Smith Mountains is more like 8:1, which might cause folds to lock up very early, or cause the Lisburne to thicken significantly for similar initial bed length (or hinge spacing). Animation courtesy of Paul Atkinson, completed in association with his
UAF master's thesis:
|
 |
UAF Tectonics and Sedimentation Research Group Home Page
UAF
Department of Geology and Geophysics
Geophysical
Institute