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How Overthrusts Occur

                                         by Glenn R. Morton
Copyright 1997, 1998 G.R. Morton. This may be freely distributed so long as no charge is made and no alterations to the text are made.

This is a note I posted to Walter Brown's Creation Listserve dated Fri Aug 22 21:14:34 1997. Walter Brown has made a career of saying that Overthrusts don't exist and can't occur. It was very shortly after this note was posted to his listserv that Walter shut it down and as far as I have been able to determine it has never been restarted. Other creationists like Henry Morris also make the same claim. Because of this, a discussion of overthrusts is needed.
The Aug. 22, 1997 post:
I did a literature search last week looking for an article I had seen several years ago which explained clearly how overthrusting occurs. The article is Raymond A. Price, "The Mechanical Paradox of Large Overthrusts," Geological Society of America Bulletin 100:1898-1908. The article discusses the overthrust model of Hubbert and Rubey, whose work, Walt bases his math upon. The article shows a clear mechanism for overthrust motion which does not exceed the strength of the rock.
The abstract says,
"The mechanical paradox of large overthrusts originates with a fallacious assumption in the conceptual model upon which mechanical analyses of overthrust faulting have been based. The notion that the maximum width of a sheet of rock that may be displaced along an overthrust fault can be determined by using the Mohr-coulomb failure criterion to compare the strength of the rocks with the stress required to overcome the total frictional resistance to slip (and the total cohesion or adhesion?) over the entire fault surface tacitly assumes that slip is initiated, and occurs, simultaneously over the entire fult surface. This unstated and untenable assumption, which is incompatible with what is known about real faults, is the real explanation for the 'mechanical paradox.' "A realistic model for overthrust faulting must be compatible with the nature of real overthrust faults, and particularly with the way in which displacements, including earthquake displacements, occur on real faults. Most large overthrusts are discrete shear surfaces within a coherent mass of rock that is physically continuous around their ends. Total displacement clearly varies from place to place over these discrete shear surfaces and, moreover, must be the cumulative result of many relatively small incremental displacements that also varied from place to place over the finite part of the fault surface on which they occurred. Each increment began as a local shear failure and propagated along the fault as a 'smeared-out' (Somigliana) dislocation at a velocity that was small considering the time required for propagation over the total area of the fault. During any individual displacement 'event,' whether a megathrust earthquake or a creep event, only a small part of the fault surface was in relative motion at any one time." Raymond A. Price, "The Mechanical Paradox of Large Overthrusts," Geological Society of America Bulletin, 100(Dec. 1988):1898-1908, p. 1898.
What this means is that when an earthquake occurs, the entire overthrust block is not moving at any given instant. The mathematics Walt uses assumes
1. instantaneous transmission of force across the entire fault 2. motion occurs across the entire thrust fault at the same time. 3. that the rock is perfectly rigid and has no elasticity at all. 4. Assumes that there is no alternative to moving the full fault in one big motion.
Assumptionn 1 violates the experimentally well verified law that nothing travels faster than light. Force is transmitted across a block at a finite speed of around 5 km/s. This has been measured in earthquakes. This velocity is also the approximate speed of sound in rock. This finite speed means that when the earthquake begins, the far extremities of the fault block are still motionless. It takes a few seconds for the signal to traverse a long distance.
Assumption 2 violates the observation that fault motion propagates with time.
Assumption 3 violates the laws of petrophysics. All rocks have a certain amount of elasticity. Some compression and rarefaction of rocks can occur without brittle failure.
Assumption 4 is weak because if a small portion of the fault begins and completes all motion before the next section moves, the force of motion never exceeds the strength of the rock. To put this in Walt's terms, the
Price writes:
"My basic objective in this paper is to critically review the origin, evolution, and widespread application of a mechanical model for overthrust faulting that is intuitively appealing, but fallacious, and consequently has impeded progress in understanding the mechanics of overthrust faulting. "The mechanical paradox of large overthrust faults originates with a fallacious assumption in the conceptual model upon which the mechanical analyses of overthrust faulting have been based. The model assumes, tacitly, that sliding is initiated, and occurs, simultaneously over the entire fault surface. Widespread use of this fallacious assumption ignores a large body of data from structural geology and from earthquake seismology that shows that slip on thrust faults involves the propagation of dislocations, each of which affects only a small portion of the fault at any one time,. This answer to the 'mechanical paradox' is not entirely new. Oldham pointed out in 1921 that 'the hypothesis of origin needs correction, not the facts of observation,' but his explanation 'that the thrusts did not move simultaneously over the whole of their extent, but partially, first in one part then in another, each separate movement involving an area limited by the strength of the rocks and their power to transmit or resist the effects of pressure...the movement would not be like that of a sledge, pushed bodily forward over the ground, but more akin to the crawl of a caterpillar which advances one part of its body at a time, and all parts in succession,' has been ignored by virtually everyone who has subsequently written about the phenomenon." Raymond A. Price, "The Mechanical Paradox of Large Overthrusts," Geological Society of America Bulletin, 100(Dec. 1988):1898-1908, p. 1898-1899
The Somigliana dislocation zones can be viewed as follows
t=0 x=0 x=10 km Stress field ready to move (/)
t=.001 sec. Shear failure begins (|= position of maximum motion)
t=.005 sec Stress field relieved, section finished moving (\)
t=.010 sec
t=.015 sec
etc til the end when the rock stress field is totally relaxed.
This is the "caterpillar" type of movement spoken of by Oldham.
Now when we come to the Alaskan Earthquake Price writes:
"Perhaps the best studied megathrust earthquake is the great Alaskan Earthquake of March 1964. It involved overthrust displacements ranging to at least 20 m on a shallow (5o-10o) north-dipping thrust fault over an area as much as 900 km long and 290 km wide (260,000 km^2) extending westward along the Aleutian arc-trench gap from near the intersection of the Queen Charlotte-Fairweather transform fault and the Aleutian trench. The hypocenter, at which the rupture began, was at a depth of between 20 and 50 km in the northeastern part of the area that ruptured. Although the rupture propagated initially in various directions, after an elapsed time of 44 s, it continued only in a southwesterly direction, mainly as a screw dislocation. The average velocity of propagation was 3.5 km/s, which means that only a very small part of the fault surface was in motion at any one time. The amount of displacement varied conspicuously and systematically over the portion of the fault that was affected by this particular earthquake and its associated aftershocks. The focus of maximum displacement was more than 100 km south of the epicenter. The overthrust mass underwent substantial distortion in conjunction with the displacement on the overthrust, and this deformation continued after the main earthquake. During the succeeding 9 months, there were more than 600 aftershocks of Richter magnitude 4.4 or greater, including six greater than magnitude 6, and these were distributed over the same area as that of the main rupture. A large area of the overthrust mass, was uplifted 3 to 4 m and locally as much as 10 m. A corresponding area over the downdip part of the overthrust subsided 1-2 m. There was substantial internal deformation within the overthrust mass, including horizontal shortening of 3- 6 m across Patton Bay fault, which is located approximately midway across the width of the area of the rupture and more than 150 km south of the epicenter. The Patton Bay fault, together with other subsidiary, northwest dipping reverse faults across which there was a total relative southeast displacement of 10 m of the hanging-wall rocks, would, in the currently fashionable jargon for thrust faulting, be called 'out-of sequence thrust faults." "The overthrust displacement associated with the great Alaskan Earthquake of March 1964 also differs fundamentally from the hypothetical faults in the mathematical models of Smoluchowski, Hubbert and Rubey, Hsu, and others. The overthrust mass was not displaced as a rigid block; it underwent substantial internal deformation in the course of a single episode of displacement on the overthrust. The displacement associated with the earthquake was not uniform and did not occur over the entire fault surface. it involved only part of the total fault surface, and within that part, it varied significantly in magnitude. It was a type of 'smeared' or Somigliana dislocation, and it represents one increment in the development of the total net slip over the whole overthrust fault, which is itself a very much larger Somigliana dislocation resulting from the superposition of many overlapping smaller Somigliana dislocations like that associated with the great Alaskan Earthquake of March 1964. "The mechanism of incremental displacement on the Aleutian arc-trench megathrust during the great Alaskan Earthquake of March 1964 provides a prototype for an actualistic process model for displacements on overthrusts in general. Displacement on overthrusts occurs as relatively small overlapping increments, each of which involves the propagation of a Somigliana dislocation over a discrete area that generally comprises only part of the fault surface. During each increment of slip, the displacement propagates outward from the initial point(s) of rupture at velocity that may range to as much as 4 or 5 km/s." Raymond A. Price, "The Mechanical Paradox of Large Overthrusts," Geological Society of America Bulletin, 100(Dec. 1988):1898-1908, p. 1904-1905
Slower propagation of fault movement has been observed on the San Andreas.
"King and others (1973) have monitored creep events propagating along the San Andreas fault at velocities of 10 km/day or less, which is roughly 5 orders of magnitude smaller than the corresponding rate of seismic faulting. The largest creep event recorded by them had a rupture length of 6 km and a maximum offset of 9 mm, which is comparable to the length and offset parameters of a shallow earthquake of magnitude 4.7."~Raymond A. Price, "The Mechanical Paradox of Large Overthrusts," Geological Society of America Bulletin, 100(Dec. 1988):1898-1908, p. 1905
Price concludes,
The idea that maximum length of the overthrust block is controlled by the total frictional resistance to slip, summed over the entire length of the fault surface, clearly implies that displacement is initiated, occurs simultaneously, and is equal in magnitude over the whole length of the fault surface. This is tantamount to assuming that the overthrust block is perfectly rigid. These tacit assumptions obviously lack any physical basis in reality and clearly are contradicted by what is known about real faults. Nevertheless, these assumptions have survived without serious challenge for a long time. It is appropriate to consider why." Raymond A. Price, "The Mechanical Paradox of Large Overthrusts," Geological Society of America Bulletin, 100(Dec. 1988):1898-1908, p. 1906
This is an article which should be read by anyone who believes that overthrusts can not occur. It provides a very plausible mechanism for fault motion. **
***end post***
I am also attaching another post on the same topic that discusses field evidence for overthrust faulting.
**begin post by Glenn Morton****
Sun Aug 24 21:34:38 1997
From: (Glenn Morton)
Subject: History of Overthrust Issue Cc: Bcc: X-Attachments:
The entire overthrust issue was generated by George McCready Price in the early part of this century. Price was a young-earth creationist and a Seventh-Day Adventist. Price believed that overthrusts demonstrated that the index fossils were not in an invariable order. If the fossils were more mixed up in the geologic record than geologists were willing to admit, then this would support the concept of a global flood. Price wrote:
"In other words, those who first examined and reported on these various cases thought that the rocks had of course been laid down as we find them. But afterwards, when it was discovered that the fossils were in the reverse of the 'invariable' order, they and their fellow geologists had to invent the theory of thrust faults, and say that the 'older' rocks must have been lifted up and pushed over bodily on top of the much 'younger' ones in order to save the reputation of their 'invariable' world time-table." George McCready Price, The Geological Ages Hoax, (New York: Fleming H. Revell, 1931), p. 79
Henry Morris took up Price's position and popularized it. He wrote:
"So we have done what we could, trying to concentrate our research efforts wherever the potential impact in terms of Christian witness would be greatest in relation to expenditures. To a large extent, this criterion has led us to focus on geological field studies, especially on so-called overthrust formations and on anomalous fossils. If it can be shown, by studies at the thrust planes, that enough of these 'thrust faults' are not really overthrusts but only normal sedimentary sequences, or if it can be shown that several supposed out-of place fossils have not really been displaced at all but have been normally deposited with the encasing sediments, then the standard geologic age system would eventually have to be abandoned in favor of the creation/flood model of earth history." "Teams have therefore been sent to study overthrusts in Colorado, Nevada, and other places. Other teams have studied anomalous fossil sites in Utah, Oklahoma, and other places. In each case, the evidence seemed strongly to favor the creation/flood model, but was not sufficiently compelling by itself to settle the question, and so we did not publish the results at the time." Henry M. Morris, A History of Modern Creationism, (San Diego: Master Book Publishers, 1984), p. 251.
The entire reason that young-earth creationists have fought against overthrusts has traditionally been with the aim of showing that the sediments in the overthrusted region must have been deposited in a flood. But the field relations along these thrust faults rules out what these men are saying. Consider the map. F is where the actual fault cuts the surface of the earth. - is the region where the rocks are overthrust.
B....................F.....................B' 0 km of thrusting
                     ---F                     Perpendicular to thrusting
W                    -----F                      ^
                     -------F                    |
                     ----------F ===> direction of thrusting C....................------------F.........C' 50 km of thrusting
The A...A' etc are the locations of the following cross-sections. A cross section is a vertical slice through the earth along the given line. When you cut the earth at the location of maximum thrusting the rock layers look like:

West C                              222222                     C' East
                               22222     33333            
top earth surface11111--------2        33/1111111111111111111                    
11111111111111111       222222      333/
             22222222222         333/
22222222222222                3333/22222222222222222222222222
   3333333333333333         / 333333333333333333333333333333333

In the above, Layer 1,2 and 3 have been pushed on top of their correlative strata to the east
As one walks or drives north and comes to the region of B..B' the cross section looks like:
West B                       111111                       B' East
                      1111111      11111
top earth surface11111         22       1111111111111111111
11111111111111111        222222  222222
              22222222222              22222
22222222222222                              222222222222222222
33333333333333333333333333333      333333333333333333333333333

The rocks are just deformed and pushed up a bit. There is no fault and all the rocks are in the correct depositional order.
If you go further north to A...A' the cross section is like:
West A                                                    A' East
top earth surface1111111111111111111111111111111111111111111



Now all the rocks are in proper order. The fact that one can find properly deposited rocks north or south of the thrust, and then follow these same layers into the thrust region, disproves what Morris and Price are saying about the thrusts.
Price writes of thrusts (directions shown in map above): "Real thrust faults differ fundamentally from the hypothetical faults in the mathematical models. In the models, the thrust sheets undergo rigid body translation, and the magnitude of the displacement is the same over the entire fault surface, but on real faults, the magnitude of the displacement changes conspicuously, both along the direction of displacement and along the direction perpendicular to the direction of displacement, and the rock mass is not rigid but deforms penetratively during displacement." Raymond A. Price, "The Mechanical Paradox of Large Overthrusts," Geological Society of America Bulletin, 100(Dec. 1988):1898-1908, p. 1901
Unfortunately, Christian apologists never talk about the extreme edges of overthrusts and what the rocks are doing there.

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