The Lewis Thrust

by Tim Van Dijk

Figure 1. This photograph illustrates the Lewis thrust fault.  The white Paleozoic limestone has been thrust over the brown Mesozoic shales.  (http://uregina.ca/~sauchyn/geog221/191.html)

 

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Abstract

The Lewis Thrust is one of the world's great thrust sheets.  It forms a 450 km long fault that runs between Alberta and Montana.  This geologic phenomenon reveals the thrusting of Paleozoic rock over top of much younger Cretaceous rock.  This large sheet was displaced by approximately 100 km at Waterton and decreasing distances towards the edges.  The Lewis thrust has a duplex structure that exhibits the geometries of both a hinterland-dipping duplex and an antiformal stack.  This webpage provides information on the development and structural geology of the Lewis Thrust.
 

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Introduction

Thrust faults are low-angle reverse faults in which older rocks are generally emplaced over younger rocks.  The Lewis fault is a low-angle thrust fault in which older Precambrian limestone has been thrust over younger Cretaceous shales.  This has resulted in some of the oldest exposed sedimentary rock in the Canadian Rockies, approximately 1.5 billion years old.  The image below depicts the folding and thrusting that has occurred in the Front Ranges.
 

Figure 2. A cross-section showing the kinematics of the  fold and thrust belt of the Canadian Rockies. (from Boyce, 2002)
 
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Lewis Thrust Location


The Lewis thrust is located in the Frontal Ranges of the southern Canadian Rockies and extends for approximately 450 km from Mount Kidd, near Calgary, to Steamboat Mountain, Montana.
 


Figure 3. Location of the Lewis thrust, which begins in the bottom right of this
diagram and runs northwest. (from Twiss and Moores, 1992)
The greatest displacement of the Lewis thrust occurred in the region of Waterton, Alberta.  Waterton is located just north of the Canada-US border near the mid-point of the Lewis thrust.
 

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Formation of the Lewis Thrust

The oldest rocks in the region formed as sedimentary rock in the ancient Belt Sea, as far back as 1.5 billion years.  Because of its age, there are few fossils found in this rock.

About 75 million years ago, the rocks started buckling under compressional forces caused by plate collision to the west of the region.

Eventually, the layers cracked and moved as essentially one large unit to the northeast.  This slab measured approximately 160 km long and 6 km thick.  The largest displacement of 100 km occurred near Waterton and decreased towards the edges of the thrust.  This displacement took about 15 million years.
 


Figure 4.  A low-angle thrust fault that exemplifies the uplift of the Lewis Thrust. (from Twiss and Moores, 1992)


Erosion and weathering have since removed much of the material but the Lewis overthrust still remains.  Chief Mountain is a good example of a Klippe.  Although it is part of the hanging wall, it has been isolated by erosion.
 


Figure 5.  Chief Mountain in Montana has been isolated by erosion.  (http://www.talkorigins.org/faqs/lewis/#chief)
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Geometry of the Lewis Thrust System

The Lewis thrust is classified as a thrust duplex.  This means that it contains inclined and stacked thrust horses that are bounded by the main fault traces.   Being a thrust duplex, it is necessarily contained within the stratigraphic section.  Figure 6 shows that the Lewis thrust appears as the floor thrust and the Mount Crandell thrust appears as the roof thrust.
 

Figure 6. A cross-section showing the current Lewis thrust sheet and the restored structure.  (from Boyce, 2002)


There are several different types of geometries seen in thrust duplexes.  The Lewis thrust has elements of both a hinterland-dipping duplex and an antiformal stack.  The higher thrust faults in the duplex are folded over lower faults ramps and their associated horses.  This indicates that slip on the higher thrusts must have occurred before the lower ones became active.  Therefore, the formation of thrusts progressed downward and toward the foreland.

The Lewis thrust surface is a low-angle thrust fault with ramp-flat geometry.  This means that it essentially moved horizontally, and 'stepped' upwards through stratigraphic layers.
 


Figure 7.  A block diagram showing the geometry of the Lewis thrust surface. (from Twiss and Moores, 1992)


The thrust sheet is brought up the surface over the frontal ramp, as shown in Figure 7.  The sidewall ramp near Marias Pass, Montana is parallel to the direction of sheet movement.  On the other side, the direction of movement is not parallel to the lateral wall, so it is called an oblique ramp.
 

Figure 8 shows that the Lewis thrust forms an arcuate belt that is convex toward the foreland, which is northeast of the thrust.  This typically results in culminations, or topographically high areas along the thrust.


Figure 8.  Map of the Lewis thrust before and after erosion.  The thrust moved northeast (up in this figure) and dips to the southwest, the direction from which it came.  (from Twiss and Moores, 1992)

 

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What makes the Lewis Thrust so unique?


Figure 9. Chief Mountain (http://talkorigins.org/faqs/lewis/)

References


Boyce, J.  Lecture 16: Fault Systems.  BSB/330.  McMaster University.  [GEO 3Z03].  March 26, 2002.

Davis, G.H., and S.J. Reynolds.  1996.  Structural Geology of Rocks and Regions.  2nd ed.  Toronto: John Wiley & Sons.

Shankar, M., and G.W. Fisher, ed. 1992.  Structural Geology of Fold and Thrust Belts.  Baltimore: John Hopkins University Press.

Suppe, J.  1985.  Principles of Structural Geology.  Englewood Cliffs: Prentice-Hall.

Twiss, R.J., and E.M. Moores.  1992.  Structural Geology.  New York: W.H. Freeman and Company.

Solum, J.G.  (2002, Feb. 7).  Thrust Faults.  [Online].  The Talk.Origins Archive.  Available: http://talkorigins.org/faqs/lewis/ [2002, March 20].

Geology & Geomorphology.  [Online].  Waterton Park Information Services.  Available: http://watertoninfo.ab.ca/r/geology.html[2002, April 4].