ORIGIN OF THE STRUCTURE
In 1927, after a brief reconnaissance, W. D. Smith published a paper16 on the stratigraphy and structure of Steens and Pueblo Mountains, chiefly for the purpose of advancing a compressional interpretation of the origin of the former. Smith points out that a similar theory17 had been previously propounded to explain the structure of apparently analogous features in central Africa.
This hypothesis demands that the tilted fault blocks in the region owe their elevation to steep reverse faults either with or without horizontal movement. This displacement was thought to be due to shearing at approximately 45° to the direction of compression. A graben such as that forming Alvord Desert was considered to be due to mutually opposing thrust faults raising their blocks above an intervening flat floor which was thought to represent the undeformed prefaulting surface.
Without considering the many inaccuracies on which this theory is based, the writer will review briefly, or quote, the eight main arguments advanced by Fuller and Waters18 to indicate the lack of compressional faulting in the regional structure. A definite establishment of this basic principle is essential for the correct interpretation of many features in the volcanic history of the region, as well as in its physiographic expression.
No thrust fault has been discovered in southern Oregon, although a number of normal fault planes are well exposed. On Steens Mountain most of these are in zones near the scarp and roughly parallel to it. They result locally in the development of step faults. The step fault blocks cannot be confused with landslides, for they lack the characteristic reverse rotation, and where the faults are exposed the planes show no tendency to flatten even when they can be traced downwards for a thousand feet. Although definite fault surfaces are locally well exposed, the step faulting in this region usually is apparent only from physiographic evidence. Locally the thin blocks may be traced along the scarps for a mile or more. Several miles west of the main fault zone on Steens Mountain there are also some minor normal faults which dip into the scarp.
"The writers are of the opinion that the numerous step faults defining narrow step blocks which extend for a considerable distance along the fault scarps of southern Oregon are a direct proof of normal faulting and are inexplicable by the compressional hypothesis. Some advocates of the compressional hypothesis have explained these features as superficial phenomena consequent on the overhang produced by the emergence of a thrust along a valley side. A diagrammatic representation of this idea given by E. J. Wayland in his account of the Albertine Rift19 is reproduced by Smith20 in his paper on Steens Mountain, and has even been reproduced and the explanation quoted with approval in a standard textbook of structural geology.21 Therefore, it may be worth while to digress for a moment in order to point out some very obvious fallacies which it contains. The diagram in question is reproduced as fig. 3. In the sketch of the scarp due to thrusting, the overhanging portion of the emerged block is assumed to have fallen as two narrow step blocks so that a result is achieved very similar to the escarpment produced by normal faulting. These step blocks are, then, according to Wayland's hypothesis, nothing more than landslides. No explanation is offered as to the failure of these blocks to show the characteristic reverse rotation22 of ordinary landslides. Wayland states that movement on a thrust of this type could only be initiated by 'enormous pressure' and that 'tremendous reliefs' are necessitated in satisfaction of this pressure.23 A natural consequence of this pressure would be the extreme shearing, crushing, and granulation of the rocks adjacent to the thrust surface. When this crushed mass emerged overhanging the valley, one would expect it to give way and fall in an indiscriminate jumble of debris. No large blocks arranged in an orderly step-like fashion such as Wayland has drawn would be expected, and their occurrence could only be regarded as fortuitous. That blocks of this type could extend unbroken for long distances along the face of the escarpment is inconceivable.
"It is characteristic, however, that the narrow blocks bounded by step faults commonly extend for distances of a mile or occasionally several miles without marked disruption.24 Obviously the faults which bound them extend down parallel, or approximately parallel, to the main fault along which the maximum displacement of the range occurred and are not surficial features that stop at the valley floor.
"However, let us grant for the moment that step faults might be formed as assumed by Wayland, and inquire into the possibility that the lower blocks would still preserve their step-like relationship to those higher upon the escarpment. The diagerammatic sections in fig. 4 convey the writers' impressions of the necessary result. Upon a relatively small emergence of the thrust block, the first step (1) would form, further thrusting would overturn this block, and its lower part would be overridden by the advancing mass. The second step block (2), therefore, would not have a step-like relationship to the first, and this train of events would be continued as long as thrusting took place. The only way in which the relationship pictured by Wayland could occur would be to have the block thrust up unbroken to a position actually overhanging the valley (fig. 5), then to have step block (2) form, and this later split asunder and the outermost part dropped to form step block (1). The difficulties that such a hypothesis must encounter to explain a number of parallel step blocks extending unbroken for a considerable distance along a high fault scarp are too obvious to merit discussion.
"A necessary corollary to Wayland's method for the formation of step faults is that these faults are confined entirely to the main rift scarp and are not found on the back of the rift blocks or on the floor of the valley below. This is entirely out of accord with the evidence from southern Oregon, where subsidiary faults parallel to the main fault escarpment can be found, not only on the back of the blocks, but also on the floors of the grabens."
"The general conclusion is reached, therefore, that the narrow well-defined step blocks of considerable longitudinal extent, which are a common feature of normal fault scarps, are not to be confused with the irregular landslides and heterogeneous masses of debris that accumulate at the base of the scarps produced by steeply dipping thrusts."
"The prevalence of volcanic vents along the graben escarpments of southern Oregon . . . . with a predominant orientation parallel to a potential fault which later cut them, suggests that they occupied a line of tensional weakness even before the actual faulting had begun. Compression would close the fault fractures tightly and make them very unfavorable loci for vulcanism. In fact, some authors have stated as a general principle the theorem that magmas characteristically shun the thrust planes and tend to work inward toward the central portion of the deformed belt.25 Where magmas have been intruded into areas undergoing severe compression they usually form concordant bodies parallel to the schistosity or bedding, and not the roughly discordant types that are characteristic of volcanic feeders. Although some authors have considered the presence of volcanic vents to favor the compressional hypothesis, the occurrence of these features along definite thrust planes has been very rarely recorded. Their common association with normal faults, however, is now well established, and numerous examples are forthcoming from widely separated parts of the world.26 If we grant that the so-called "normal faults" are really thrusts we are at a loss to explain their common association with volcanic feeders, since in ordinary thrusts these are usually absent. On the other hand, if the normal faults are tensional this association is entirely logical."
"If roughly elliptical to circular depressions, exemplified by Summer Lake, Silver Lake, and the Upper Alvord playa are due to compressional faulting, then the forces must have acted centripetally like the closing of a camera shutter. Seemingly a dome would have been the more logical structure under these conditions. It is particularly difficult to understand how, in the case of the Upper Alvord playa, the deformation could have been restricted to a very small central area only about three miles in diameter, and yet was sufficiently severe to result in the walls of this tiny basin being thrust up as much as 1000 feet above the playa surface. The actual mechanism of the formation of features of this type is somewhat obscure, but regional tension allowing the release necessary for movement of such small units appears to be absolutely essential."
The rocks associated with steeply dipping thrust faults are practically always greatly buckled and folded, yet the blocks formed by the volcanic series in southern Oregon are astonishingly simple in structure. The most severe deformation should be found on the overthrust block adjacent to the thrust plane, yet the hundreds of thin sections from the lava samples collected on the face of the scarps show no evidence of granulation or crushing. Even vertical open bands of lithophysae in a rhyolite vent on the Steens escarpment have retained their most delicate structure intact, and the bedding of the highly incompetent waterlaid tuffs at the base of the mountain is still undisturbed (fig. 27).
"Certain more general features of the southern Oregon fault-block country have no obvious explanation if we assume that these structures are due to compression. From west to east across the faulted portion of the state is a distance of more than 200 miles, and we have no indication that the fault structure ceases short of the areas that Mansfield has mapped in southeastern Idahoa total distance of over 500 miles. In southern Oregon there are seven well defined north-south fault depressions of about equal magnitude. It would require a rather unusual distribution of stress, if we assume compression, in order to produce such uniform structures as these. Compressional stress generally tends to localize the failure in narrow zones, but in this district the failure is practically uniform over the entire area."
"Comparison with adjoining districts shows that there is no reason why the basalts of southern Oregon should not fold if subjected to compression. To the north the compression of the Columbia River basalt appears invariably to have resulted in folds27 rather than faults. . . . These anticlines are a very different kind of structural feature from that commonly observed in southern Oregon. Rising as long narrow ridges of from 1000 to 3000 feet in height and from two to twelve miles in width, they are striking contrast to the anticlines of southern Oregon (if we assume that the grabens represent the sunken keystones of anticlinal arches). These anticlines would be from 25 to 50 miles in width and would probably average around 4000 feet in height."
In later pages an additional argument of importance was advanced in consideration of Smith's suggestion that the northeasterly trend of the northern part of Steens Mountain and of the eastern scarp of Warner Valley is due to shearing in both the horizontal and the vertical section at 5° to the direction of compression.
". . . . If these mountains are considered to have risen on shears oriented 45° to the direction of pressure in both sections, there must have been considerable horizontal displacement along the fault planes. According to this interpretation, Steens Mountain moved northward in relation to the Alvord graben while the Bluejoint Rim moved southward relative to the adjacent Warner graben."
"This horizontal movement entails numerous difficulties. A long fault splinter, such as the Wildhorse Spur, which marks the position where one fault dies out and another continues the escarpment, somewhat offset from the first, would be sheared off by this horizontal movement; yet such features are common in southern Oregon. Horizontal movement of the Bluejoint Rim scarp appears to be impossible, for the scarp shows a pronounced series of zigzags and other irregularities which would lock the fault plane against lateral movement. Another obvious difficulty is found in explaining the horizontal movement for those faults which have uniform arc-like curves such as Winter Rim and the eastern scarp of Catlow Valley. If the strike of the plane of failure is normal to the direction of thrust, there would be no lateral movement. If, however, the strike were inclined, horizontal movement would occur which would necessitate subsidiary compressional effects in a salient of the thrust block and tensional effects in a re-entrant. No indication of these effects was observed in the blocks bounded by curving and zigzag faults."
Last Updated: 28-Mar-2006